Title of Invention

"A CU-CONTAINING STEEL MATEIRAL AND A METHOD OF PRODUCTION OF THE SAME"

Abstract

A method of production of a Cu- containing steel material by heating a steel material containing Cu by mass%, of 0.05% to 3% of a base material in a heating furnace, then starting hot rolling, said method of production of a Cu-containing steel material excellent in surface properties characterized by, forming an atmosphere of an oxygen concentration P02 (vol%) shown in the equation as herein described (low oxygen concentration atmosphere condition) when heating by the heating furnace, in the entire region or partial region in the heating furnace where the temperature of the surface of the steel material becomes at least 1080°C so as to cause the formation of an oxide scale made of wustite, thereby to make the amount of Cu concentration Ecu (ug.cm-2) per unit surface area of the Cu-containing steel material concentrated near the oxide scale/metal interface as herein described, wherein, when the effective thickness of the steel material after hot rolling is obtained by dividing a sectional area s of a cross-section of the steel material vertical to the rolling direction by its circumferential length 1 is d (mm).

Full Text

The present invention relates to a cu-containing steel material and a method of production of the same. TECHNICAL FIELD The present invention relates to a Cu-containing steel material excellent in surface properties produced by hot rolling and a method of production of the same. More specifically, the present invention relates to a Cu-containing steel material excellent in surface properties able to prevent hot shortness of the steel material by suppressing the concentration of Cu in the surface of the steel material at the time of heat treatment of the steel material applied preceding the hot rolling and a method of production of the same. BACKGROUND ART Many steel material scraps are recycled as a source of iron of a ferrous metal material. In recycling these hteel material scraps, when the steel material scraps ontain Cu, the removal of the Cu by smelting is cifficult. Therefore, the Cu remains mixed in the steel rriaterial and sometimes causes a problem in the hot xolling etc. Namely, Cu is concentrated at the interface between the oxide scale and iron at the time of the heating of the steel material preceding the hot rolling. When the amount of this Cu concentration is large, the problem of hot shortness causing cracking in the surface cf the steel material will occur. In order to avoid this problem, under actual circumstances, the amount of usage of such steel material scraps containing Cu is restricted. However, when considering the energy consumption waen producing the steel material from iron ore and the increase of the amount of the steel material scraps built up , it is desired to use a larger amount of steel material scraps as the source of iron in the future. Development of a method of production of a steel material not causing hot shortness even if Cu is contained is strongly desired. In general, in the production of a steel material by hot rolling, the steel material is loaded into a heating furnace preceding the hot rolling, heated for 1 to 4 hours by a combustion gas, removed from the heating furnace at a temperature of about 1100 to 1300°C, is cleared of oxide scale (descaled) with high pressure water, then is hot rolled. Usually, the combustion gas supplied into the heating furnace contains an oxidizing gas such as oxygen, water vapor, and carbon dioxide, therefore an oxide scale layer is produced on the surface of the steel material heated to a high temperature in the heating furnace. This oxide scale layer is mainly comprised of iron oxide and generally is comprised of the three layers of hematite (Fe2O3), magnetite (Fe3O4) , and wustite (FeO) from the surface layer. Then, when the iron is oxidized by the oxidizing gas in the combustion gas at a high temperature, if metals more precious than iron, for example Cu and Ni, is contained, these metals are not oxidized and become concentrated at the interface between the oxide scale layer and the steel material. In the case of Cu, the solubility in the -iron is only a few percent. If the amount of Cu concentration becomes more than this, the Cu appears as a metal phase. The melting point of Cu is 1080°C. The heating of the steel material before the usual hot rolling is carried out at a temperature higher than this. Therefore, a liquid phase of Cu in a melted state is produced at the oxide scale/metal interface. This invades the grain boundary of the steel material. Due to this, the shearing stress and a tensile stress at the time of hot rolling can no longer be withstood, so surface cracking, that is, hot shortness, occurs. It is known that the addition of Ni in amount substantially equal to the Cu concentration or more is effective in order to prevent the hot shortness caused by Cu. This is due to the fact that the dissolution limit of the Cu in the y-xxon increases by adding Ni and the melting point of the Cu concentration phase becomes high, therefore the appearance of Cu at the oxide scale/metal interface can be suppressed (refer to for example Japanese Unexamined Patent Publication (Kokai) No. 7-242938). Further, Japanese Unexamined Patent Publication No. (Kokai) No. 6-297026 deems that the addition of Si has an effect of prevention of red shortness. When Si is added, firelite is produced near the oxide scale/metal interface, reacts with the wustite in the oxide scale at 1170°C or more, and forms an oxide having a liquid phase. The liquid phase of the Cu is incorporated into this liquid phase, so the invasion of the liquid phase Cu into the iron grain boundary of Cu is suppressed. In the method of 'adding Ni to prevent hot shortness caused by the Cu, however, since the expensive metal Ni is used, there is the problem of an increase of costs. Further, Ni promotes grain boundary oxidation at the time of heating. Therefore, even if hot shortness caused by Cu can be prevented, there arises the problem of oxide scale defects being produced by inhibiting the peeling property of the oxide scale. Further, in the method of adding Si to prevent hot shortness caused by Cu, there are the problems that the peeling property of the oxide scale is poor in a steel material containing Si and that the oxide scale still remains even after descaling by high pressure water before rolling, so the surface properties are degraded, for example, the surface of the steel material becomes red. Further, thereafter, when there is a pickling step, the oxide scale is easily dissolved by the picking, therefore there also exists the problem that the cost of the pickling step increases and, at the same time, the productivity drops. DISCLOSURE OF THE INVENTION Therefore, an object of the present invention is to provide a Cu-containing steel material excellent in surface properties able to suppress hot shortness of the steel material due to the Cu when hot rolling the Cu-containing steel material preferably without changing the steel ingredients such as adding Ni or Si, more specifically advantageously suppress the concentration of Cu on the surface of the steel material at the time of the heating of the steel material containing 0.05 to 3 mass% of Cu and avoid hot shortness, and a method of production of the same. In order to attain the above object, the present invention has as its gist the following (1) to (9). (1) A Cu-containing steel material having an oxide scale on its surface, said Cu-containing steel material excellent in surface properties characterized in that the Cu concentration Ccu (mass%) of a base material is 0.05% to 3% and, when the effective thickness of the steel material obtained by dividing a sectional area s_ of a cross-section of the steel material vertical to the rolling direction by its circumferential length 1 is d (mm), the amount of Cu concentration Ecu (µg.cm-2) per unit surface area concentrated near the oxide scale/metal interface has the relationship of the following Equation (1) : (Equation Removed) (2) A Cu-containing steel mate::ial having an oxide scale on its surface, said Cu-containing steel material excellent in surface properties characterized in that a Cu concentration Ccu (mass%) of a base material is 0.05% to 3% and, when an effective thickness of the steel material obtained by dividing a sectional area s_ of a cross-section of the steel material vertical to the rolling direction by its circumferential length 1 is d (mm) and a total of the concentrations of the base materials of elements inducing hot shortness which are more precious than iron for oxidation in a temperature range of XOOCC to 1300°C and having a melting point of 1300°C or less, that is, a total concentration of base materials of hot shortness inducing elements, is Ci, the total of the amounts of concentration of the hot shortness inducing elements per unit surface area concentrated near the oxide scale/metal interface, that is, the total amount of concentration Ei, (µg.cm-2) of the hot shortness inducing elements, has the relationship of the following Equation (2): (Equation Removed) (3) A Cu-containing steel material excellent in surface properties as set forth in the above (2), wherein one type of hot shortness inducing element is Cu, and the rest is one type or two or more types of Sn, Sb, and As. (4) A Cu-containing steel material excellent in surface properties as set forth in any one of the above (1) to (3), wherein the relationship between a base material Ni concentration CNi (mass%) and a base material Cu concentration Ccu (mass%) satisfying the following Equation (3): (Equation Removed) (5) A Cu-containing steel material excellent in surface properties as set forth in any one of the above (1) to (3), wherein the Cu-containing steel material contains at least one type or two or more types of 0.01 to 0.15 mass% of Ti, 0.01 to 0.15 ma3s% of Nb, and 0.01 to 0.15 mass% of V and further contains one type or two or more types of 0.010 to 0.100 mass% of P, 0.010 to 0.050 mass% of S, and 0.002 to 0,150 mass% of an REM. (6) A Cu-containing steel material excellent in surface properties as set forth in the above (5), wherein the Cu-containing steel material contains a precipitate comprised of a carbide, nitride, or carbonitride of one type or two or more types of Ti, Nb, and V and having a particle size of 10 nm to 1 µm and particle density of 105/mm2 or more. (7) A method of production of a Cu-containing steel material heating a Cu-containing steel material in a heating process, then starting hot rolling, said method of production of a Cu-containing steel material excellentin surface properties characterized by making the Cu content Ccu (mass%) of the steel material 0.05 % to 3% and, when heating at the heating furnace, forming an atmosphere of an oxygen concentration po2 (vol%) shown in the following Equation (4) or less (low oxygen concentration atmosphere condition) in the entire region or partial region in the heating furnace where the temperature of the surface of the steel material becomes 1080°C or more so as to cause the formation of an oxide scale made of wustite and thereby make the amount of Cu concentration Ecu (µg.cm-2) per unit surface area of the Cu-containing steel material concentrated near the oxide scale/metal interface less than 18.6Ccu x d: (Equation Removed) when making an effective thickness of the steel material obtained by dividing a sectional area £ of a cross-section of the steel material vertical to the rolling direction after the end of the hot rolling by its circumferential length 1 d (mm), where kp is a parabolic rule speed constant (g2-cm-4-s-1) , specifically, (Equation Removed) (kpo = 0, 60g2-cm-4-s-1) . Note that E is an activation energy (E=140 kJ.mol-1.K-1) , R is a gas constant, and T is a temperature (K). Further, w is mass grain by oxidation (g.cm-2), and ki is the linear rate constant (k1 =9.6 x 10-6g-cm-2.%-1.s-1). (8) A method of production of a Cu-containing steel material heating a Cu-containing steel material in a heating process, then starting hot rolling, said method of production of a Cu-containing steel material excellent in surface properties characterized by making the Cu content Ccy (mass%) of the steel material 0.05 % to 3% and applying treatment for taking out the oxide scale formed on the surface of the steel material twice or more after the taking out of the steel material from the heating furnace and before the hot rolling to thereby make the amount of Cu concentration Ecu (µg.cm-2) per unit surface area of the Cu-containing steel material concentrated near the oxide scale/metal interface less than 18.6Ccu x d when making an effective thickness of the steel material obtained by dividing a sectional area s of a cross-section of the steel material vertical to the rolling direction after the end of the hot rolling by its circumferential length 1 d (mm). (9) A method of production of a Cu-containing steel material by heating a Cu-containing steel material in a heating process, then starting hot rolling as set forth in (7), said method of production of a Cu-containing steel material excellent in surface properties characterized by applying treatment for taking out the oxide scale formed on the surface of the steel material twice or more after the taking out of the steel material from the heating furnace and before the start of the hot rolling. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing a situation of a surface of a steel material cracking due to hot shortness at the time of hot rolling and a relationship between an amount of Cu (amount of Cu concentration) per unit surface area of the steel material concentrated near the oxide scale/metal interface after hot rolling and an effective thickness of the steel material. FIG. 2 is a graph for explaining a method of finding by the GDS analysis results the amount of Cu (amount of Cu concentration) per unit surface area of the steel material concentrated near the oxide scale/metal interface from a distribution of concentration of the Cu in a depth direction from the surface of the steel material having the oxide scale on the surface. FIG. 3 is a graph of the relationship between a particle density of a precipitate having a particle size of 10 nm to 1 µm and a surface cracking depth. FIG. 4 is a diagram schematically showing a preferred embodiment of a facility from a heating furnace to a hot rolling mill for working a first method of production of the present invention and schematically showing a situation of formation an oxide scale layer on the surface of the steel material in this embodiment. FIG. 5 is a diagram schematically showing a preferred example of a facility from a heating furnace to a hot rolling mill for working a second method of production of the present invention and an example of heat treatment conditions by the same and a situation of formation of an oxide scale layer on the surface of a steel material at the time of treatment. BEST MODE FOR WORKING THE INVENTION Conventionally, it has been considered that the Cu inducing hot shortness is concentrated at only the interface between the oxide scale produced on the surface of the steel material at the time of heating preceding the hot rolling and the iron. However, the inventors engaged in many experiments and verifications and consequently newly found the fact that the Cu behaved as follows in addition to the concentration of the Cu at this oxide scale/metal interface. (a) The Cu appearing as the liquid phase at the oxide scale/metal interface easily moves at the grain boundary of the oxide scale. (b) In the case of oxide scale not containing magnetite, that is, the case of oxide scale made of a wustite layer, the Cu of the liquid phase from the oxide scale/metal interface moves in the oxide scale (grain boundary), reaches the surface of the oxide scale, and evaporates and scatters as a vapor of Cu or CuO. (c) When an oxide scale made of three layers of. hematite, magnetite, and wustite is formed, the Cu of the liquid phase from the oxide scale/metal interface'moves in the oxide scale (grain boundary), and the Cu dissolves into the magnetite layer. When considering the phenomenon when steel material containing Cu is heated and oxide scale is produced based on such a new discovery, first, at the oxide scale/metal interface, the steel material is oxidized, while the Cu more noble than the steel material is not oxidized and is concentrated. A certain amount of this concentrated Cu stays at the oxide scale/metal interface as considered hitherto. The remaining amount of Cu will exhibit one or two or more behaviors among the above newly found behaviors. In this case, the amount of Cu contained inside the steel material consumed by the oxidation becomes equal to the sum of the amount of Cu concentrated at the oxide scale/metal interface, the amount of Cu moving in the grain boundary of the oxide scale and volatilized from the oxide scale surface, and the amount of Cu dissolved into the magnetite layer. Therefore, in order to avoid hot shortness by reducing the amount of Cu concentrated at the oxide scale/metal interface, the present inventors came up with the idea that an increase of the amount of the volatilized Cu and the amount of Cu dissolved into the magnetite layer would be useful. They engaged in further studies and thereby completed the present invention. Namely, in order to get the Cu to volatilize from the surface of the oxide scale, as described above, an oxide scale made of wustite must be produced. In the present invention, that condition is obtained by heating this under a low oxygen concentration atmosphere condition as will be explained in detail later. Further, in order to make the Cu dissolve into the magnetite layer of the oxide scale, as described above, it is necessary to form oxide scale made of three layers of hematite, magnetite, and wustite. This condition can be obtained by heating under a high oxygen concentration atmosphere condition as will be explained in detail later. Note that the Cu content of the steel material covered by the present invention is made 0.05 mass% to 3 mass%. This is because if the Cu content is less than 0.05 rriass%, the hot shortness caused by Cu does not occur even if heating in a usual heating furnace. Further, if the Cu concentration exceeds 3 mass%, the effects of the dissolution of the Cu into the magnetite layer in the oxide scale and the volatilization of the Cu from the surface as newly discovered can no longer be fully expected, and red shortness occurs at the time of the rolling. The ingredients of the steel by which the present invention exhibits these effects will be explained next. As explained hitherto, the present invention provides a steel material wherein the action of Cu is utilized when producing the oxide scale so as to reduce the amount of Cu concentrated at the oxide scale/metal interface and a method of production of the same and is effective within a range where the composition and structure of the oxide scale produced on the steel do not change. Specifically, this is effective within the range of 1 mass% or less of C, 3 mass% or less of Si, 10 raass% or less of Mn, 0.1 mass% or less of P, 0.1 mass% or less of S, 5 mass% or less of Cr, 3 raass% or less of Al, and 1.5 mass% or less of Ni. First, an explanation will be given of the invention disclosed in claim 1. The inventors cast, heated, and hot rolled steel materials having various Cu contents under conditions of various steel material thicknesses after casting (50 min to 250 mm) and various effective thickness of the steel materials after rolling (1 mm to 100 mm). The heating at that time was performed by heating by combustion of LNG. The heating temperature was mad© llOCC to 1300°C, and the oxygen concentration of the heating atmosphere was made 0 to 5 vol%. By changing the oxygen concentration of the heating atmosphere, for example, by setting a low oxygen concentration atmosphere condition, an oxide scale made of a wustite layer was formed. The amount of Cu concentration was changed to various amounts, for example, the amount of Cu (amount of Cu concentration) at the oxide scale/metal interface was reduced. For evaluation under these conditions, the existence of cracking at the surface of the steel material due to hot shortness after the hot rolling was checked. Further, the amount of Cu (amount of Cu concentration) per unit surface area of the steel material concentrated near the oxide scale/metal interface after the hot rolling was checked. The results are shown in FIG. 1. In this graph, steel materials suffering from hot shortness degrading the outer appearance to an extent becoming a problem in the final product, are indicated by the "+" marks, steel materials suffering from light hot shortness but not degrading the outer appearance are indicated by the "A": marks, steel materials wherein hot shortness was not confirmed by observation by the naked eye, but wherein very light hot shortness was confirmed by examination under a microscope are indicated by the marks, and steel materials wherein hot shortness was not confirmed even by the observation under a microscope are indicated by the "O" marks. It is seen from this graph that the amount of Cu concentration has a good correlation with the product of the Cu concentration of the base material and the effective thickness of the steel material first. Further, it is also seen that hot shortness which becomes a problem in outer appearance does not occur with an amount of Cu concentration within the range shown in Equation (1): (Equation Removed) where, Ecu: Amount of Cu per unit surface area concentrated near the -oxide scale/metal interface (µg.cm-2) Ccu: Cu concentration of base material (mass%) d: Effective thickness of steel material (mm) The effective thickness of the steel material d used here is obtained by dividing the steel material sectional area s of the cross-section of the steel material vertical to the rolling direction at the time of the hot rolling by its circumferential length 1 and is defined as in the following Equation (6): (Equation Removed) When there are a plurality of circumferential lengths such as an inner circumference and outer circumference like a pipe material, the inner circumference and the outer circumference are added to obtain the circumferential length. Further, by using the effective thickness of the steel material defined in this way, even in the case of a wire material, rod material, pipe material, rail material, and steel shape other than a plate material, the conditions for avoiding the hot shortness shown in Equation (1) can be equally evaluated. This effective thickness of the steel material corresponds to a rough plate thickness in the case of a plate material, corresponds to a radius in the case of a wire material, and corresponds to a wall thickness in the case of a pipe. The hot shortness is induced by Cu concentrated at the oxide scale/metal interface, therefore, the smaller the amount of Cu concentration, the more preferred in view of the prevention-of hot shortness. The lower the value of the constant of Equation (1), the better. As seen from the result of FIG. 1, in order to suppress light hot shortness which does not cause a problem in the outer appearance, the coefficient of Equation (1) is preferably 9.3 or less, and in order to completely suppress hot shortness that can be observed only in a micro field of vision like observation under a microscope, the coefficient of Equation (1) is further preferably 4.5 or less. The invention disclosed in claim 1 has the above Equation (1) as the main structure of the invention. Here, a measurement method preferred for measuring the amount of Cu concentration at the oxide scale/metal interface will be explained. For measuring the amount of Cu concentration, a mean concentration of a surface area of an area of 0.01 mm2 or more must be measured. This is because the Cu concentrated at the oxide scale/metal interface appears as metal Cu having a size of about 100 nm to 1 µm, and a correct amount of concentration cannot be found if a sufficient surface area does not exist. As an easy and convenient technique, there is the method of finding the distribution of concentration in the depth direction of the steel material by glow discharge optical emission spectrometry (GDS). By this method, the mean Cu concentration of an area of about several mm2 can be measured in the depth direction from the surface of the steel material. This analysis method is explained in detail in for example the Japan Metal Society edited, Kaitei 6-Ban Klnzoku Binran (Revised Sixth Edition Metal Handbook), p. 4 71. FIG. 2 shows an example of finding the amount of concentration of Cu from the GDS analysis results. In the graph, the distributions of concentration of the Cu (copper), 0 (oxygen), and Fe (iron) are shown with respect to the distance in the depth direction from the surface of the steel material. It is seen that the concentration of 0 is high near the surface of the steel material, so the oxide scale is on the surface. The concentration of 0 becomes lower when the distance from the surface of the steel material is 3 µm to 7 µm. The vicinity of this is the oxide scale/metal interface. There is a peak of Cu in the vicinity of this oxide scale/metal interface. The Cu concentration of the base material is 0.195% (background Cu concentration). By integrating the Cu concentration of the portion higher than this in the depth direction (finding the area of a hatching portion of FIG. 2), the amount of concentration of Cu per unit area can be found. Here, it is necessary to multiply the density for converting the volume to the weight. The density of iron, i.e., 7.8 6 g-cm-3, is used as this density. When the surface is not flat like a wire material having a small diameter and CDS analysis is difficult, it is also possible to use a method of surface analysis by X-ray microanalysis (Electron Probe X-ray Microanalyzer: EPMA) of vertical cross-sections of the oxide scale and steel material. In this case, Cu appears as metal Cu having a size of about 100 nm to 1 µm, therefore, a field of vision of analysis of at least 100 µm or more is necessary in the width direction as the direction parallel to the surface of the steel material. From the result, the Cu concentration is averaged in the width direction to find the mean distribution of Cu concentration in the depth direction as the direction vertical to the surface of the steel material. The amount of concentration of Cu can be found in the same way as the case of CDS analysis. This analysis method is explained in detail in Japan Metal Society edited, Kaitei 6-Ban Kinzoku Binran (Revised Sixth Edition Metal Handbook),pp. 462 to 465. Next, the invention disclosed in claim 2 will be explained. Hot shortness is mainly induced by Cu, but there are elements promoting this. Namely, in the same way as Cu, these are elements more noble than iron for oxidation in the temperature range of 1000°C to 1300°C and having a melting point of 1300°C or less. In the present invention, these elements including Cu are defined as "hot shortness inducing elements". These hot shortness inducing elements appear as a liquid phase at the oxide scale/metal interface at the time of formation of the oxide scale. Then, in the same way as the case of Cu alone, the hot shortness inducing elements exhibit the behavior of (a) moving in the oxide scale by passing through the grain boundaries, (b) volatilizing from the surface of the oxide scale in the case of tfxide scale made of wustite, and (c) dissolving into the magnetite in the case of oxide scale having a three-layer structure of hematite, magnetite, and wustite. Accordingly, more strictly, in place of the amount of Cu concentration (Ecu) shown in Equation (1), preferably, use is made of a total amount of concentration (Ei) of hot shortness inducing elements obtained by adding up the amounts of concentration per unit surface area of the hot shortness inducing elements, more noble than iron for oxidation in the temperature range of 1000°C to 1300°C and having a melting point of 1300°C or less, concentrated near the oxide scale/metal interface and, in place of the Cu concentration (Ccu) of the base material, use is made of a total concentration (Ci) of the base materials of the hot shortness inducing elements obtained by adding up the concentrations of base materials of elements more noble than iron for oxidation in the temperature range of 1000°C to 1300°C and having a melting point of 1300°C or less. An invention defining as hot shortness inducing elements not only Cu, but also Sn, Sb, and As is the invention disclosed in claim 3. In this case, it is possible to make the hot shortness inducing elements Cu, Sn, Sn, and As and find the total amount of concentration (El) of hot shortness inducing elements and the total concentration (Ci) of the base materials of the hot shortness inducing elements. Next, an explanation will be given of the invention disclosed in claim 4. As is conventionally known, Ni has the action of raising the solubility of Cu in -Fe to suppress the hot shortness caused by Cu. Anticipating this action, conventionally, in order to suppress the hot shortness caused by Cu, an amount of Ni of 1/2 of the Cu content of the base material to almost the same amount in terms of mass% was added. On the other hand, in a steel material reduced in amount of Cu concentration at the oxide scale/metal interface according to the present invention, it is possible to sufficiently suppress hot shortness even by an amount of addition of Ni smaller than that in the past mentioned above and further even with no addition-of Ni. In the present invention as well, the addition of Ni can further reduce the degree of occurrence of hot shortness and is a preferred embodiment. The invention disclosed in claim 4 define.6 the amount of addition of Ni (concentration of base material Ni) when the Ni is added in this way by the relationship with the concentration of the base material Cu. Namely, even with a concentration of the base material Ni smaller than that in the past, by adding the Ni within the range of the condition of Equation (3), the hot shortness can be more advantageously suppressed. Further, when the concentration of Ni of the base material exceeds 1.5%, the surface of the steel material becomes easily flawed and the outer appearance is degraded, therefore desirably the concentration of Ni is 1.5 % or less: O.OeiCcu' + 0.32Ccu + 0.0035 where, CN1: concentration of Ni of base material (mass%) Ccu: concentration of Cu of base material (raass%) Next, the invention disclosed in claim 5 will be explained. First, as elements added for preventing the cracking flaws caused by Cu, Ti, Nb, and V can be mentioned. When heat treating a steel material containing suitable amounts of these elements, many fine precipitates of carbides, nitrides, or carbonitrides of Ti, Nb, and v can be formed in the steel. Due to this, the grain growth can be obstructed, and the austenite grain size can be kept in the micro state. Accordingly, many grain boundaries can be formed per unit surface area of the oxide scale/metal interface, therefore the invasion of the liquid phase Cu into the austenite grain boundary can be dispersed and it becomes possible to advantageously prevent the hot shortness caused by Cu. Nb and V, other than the above actions, can promote the incorporation of the liquid phase Cu into the oxide scale since their oxides form oxides of iron and oxides of low melting points (melting point of oxides containing Mb: 1190°C, melting point of the oxides containing V: 635°C) and can prevent the hot shortness due to Cu by reducing the amount of Cu concentration existing at the oxide scale/metal interface. Further, as elements added for preventing hot shortness caused by Cu, P and REM can be mentioned. Both of these elements are elements segregated to the grain boundaries. The grain boundary energy is lowered due to the segregation to the grain boundaries. Due to this, it becomes possible to suppress the invasion of the Cu concentration phase into the austenite grain boundaries, so hot shortness caused by Cu can be advantageously prevented. P and REM have also the action of making the austenite particle si2e finer. As a mechanism thereof, P is segregated to dendrites, so can obstruct the grain growth, while an REM can form many fine precipitates of carbides, nitrides, or carbonitrides in the steel, so can obstruct the grain growth of the austenite, whereby both elements can advantageously prevent the hot shortness caused by Cu. P, in addition to the above action, can promote the incorporation of the liquid phase Cu into the oxide scale since its oxides form oxides of iron and oxides of low melting points (melting point of oxides containing P: 960°C) and can prevent the hot shortness due to Cu by reducing the amount of Cu concentration existing at the oxide scale/metal interface. Ti, Nb, and V, both if used alone and if used combined in a plurality of types simultaneously, can exhibit their effects when the content is 0.01 mass% or more, but the effects thereof are saturated when the content exceeds 0.15 mass%, therefore 0.15 mass% is defined as the upper limit. Further, simultaneously with this, it is necessary to use one type or two or more types of P, S, and an REM. P exhibits the above effects when the content is 0.010 mass% or more, but when the P concentration becomes over 0.100 mass%, the processability and rolling property are deteriorated, so the upper limit is made 0.100 mass%. Further, an REM exhibits the above effects when the content is 0.002 mass% or more, but the effects are saturated when the content exceeds 0.150 mass%, so 0.150 mass% is made the upper limit. On the other hand, S is more noble than iron, therefore concentrates at the oxide scale/metal interface at the time of the high temperature heating. Further, S forms a sulfide having a low melting point with Cu (melting point of CuS: 1067°C) , so has the effect of promoting the incorporation of the liquid phase Cu into the oxide scale. Therefore, it becomes possible to prevent cracking flaws caused by Cu. S exhibits its effects when the concentration is 0.010 mass% or more, but when the S concentration becomes high, the S concentrated at the interface forms a sulfide with the Fe, and the melting point thereof is low, i.e., 940°C, therefore induces grain boundary embrittlement. When the S concentration exceeds 0.050 mass%, the embrittlement by the FeS becomes remarkable, therefore, the S concentration is made 0.010 mass% to 0.050 mass%. Note that when S is contained in this way, by incorporating the Mn in a concentration so as to satisfy Mn/S>7 in weight ratio, the embrittlement due to S can be eased. Namely, this is because when Mn is contained in the steel, S is fixed as MnS, therefore, the embrittlement due to S can be eased. Here, as the Mn concentration in the steel, Mn/S>7 in weight ratio is sufficient. Note that, the upper limit of the Mn concentration is not particularly defined and may be appropriately set in accordance with the object, purpose etc., but usually is 2. 5 mass% or less from the viewpoint of the material in many cases. S, in addition to the above action, is precipitated in the steel as MnS, so also has the action of suppressing the growth of austenite grains and refining the particle size. In addition, when a nitride such as TiN is precipitated, the Mns precipitates using the previously precipitated nitride as nuclei, therefore further refinement of the particle size of the austenite becomes possible. As a result, the hot shortness caused by Cu can be advantageously prevented. Next, the invention disclosed in claim 6 will be explained. In the invention disclosed in claim 6, the steel material adjusted to the ingredients of the invention disclosed in claim 5 contains a precipitate which is one type or two or more types of carbides, nitrides, or carbonitrides of Ti, Nb, and V, has a particle size of 10 nm to 1 µm, and has a particle density of precipitate of 105/mm2 or more. For the steel of the present invention, the particulate density of the fine precipitate of 10 nm to 1 \xm was examined. The method of examination here was observation of multiple fields of vision under a high power by a transmission electron microscope (for example, observation of 100 fields by 100,000X). As a result, it was discovered that, when the particulate density of fine precipitate of 10 nm to 1 µm becomes larger, i.e., lo5 .mm2 or more, the grain growth could be obstructed, and the austenite particle size could be held at the minute state. Here, the particle size of the precipitate means a diameter corresponding to a circle. Further, for various types of samples, the relationships between the particulate density of the precipitate contained in the steel and cracking were examined by the previously mentioned method. As a result, as shown in FIG. 3, it was seen that cracking was suppressed when the particulate density of the precipitate of 10 nm to 1 i-un became 105/mm2 or more. Namely, when the particulate density of the precipitate is less than 105/mm2, cracking cannot be suppressed, therefore it is important to make the particulate density to 105/mm2 or more. By making the particle size of the precipitate at that time 10 nm to 1 µm, the desired particulate density can be achieved. As described above, for the steel of the present invention, by making the particulate density of the fine precipitate having a particle size of 10 nm to 1 µm to 105/mm2, the hot shortness can be advantageously suppressed. It could be confirmed together that these precipitates were comprised of carbides, nitrides, and carbonitride of Ti, Nb, and v when examinined by analysis of the EDS (Energy Dispersive Spectrometry) and electron-beam diffraction pattern by a transmission electron microscope. Next, an explanation will be given of the invention disclosed in claim 7 relating to the first method of production for producing a steel material excellent in surface properties by avoiding hot shortness. As mentioned before, the inventors heated steel materials containing Cu and discovered that the Cu was volatilized from the surface layer of the oxide scale other than the concentration of Cu at the oxide scale/metal interface in the case of the low oxygen concentration atmosphere condition when oxide scale made of wustite was produced. This first method of production suppresses the hot shortness caused by Cu by utilizing this phenomenon. First, an explanation will be given of the low oxygen concentration atmosphere condition where oxide scale made of wustite is produced when the steel material is oxidized and the high oxygen concentration atmosphere condition where oxide scale made of three layers of hematite, magnetite and wustite is produced. In general, it is known that oxide scale made of three layers of hematite, magnetite, and wustite is produced when the iron is oxidized at a high temperature. In this case, the amount of oxidation proceeds by a parabolic rate advanced proportional to the square root of the time. The oxidation speed at this time is expressed as in the following equations. (Equation Removed) where, w: amount of increase of oxidation (g.cm-2), t: Time (s), kp: Parabolic rate constant (kpo =0. 60g2-cm-2-s-1) E: Activation energy (E=140kJ-mol-1-K-1) R: Gas constant T; Temperature (K) . The growth of oxidation by such a parabolic rate occurs in the case where the diffusion of iron ions in the oxide scale becomes regular in speed and the oxide scale grows. The prerequisite is that oxygen sufficient for the reaction exists in the atmospheric gas. This case will be referred to as the "high oxygen concentration atmosphere condition". However, when oxygen gas sufficient for maintaining the parabolic rate cannot be supplied from the gas phase to the oxide scale surface, the supply of the oxygen gas from the gas phase becomes regular in speed. In that case, the oxygen speed is proportional to the oxygen concentration and becomes the linear rule expressed by the following equation. At this time, the hematite layer and the magnetite layer do not exist, so an oxide scale made of a wustite layer is formed. This case will be referred to as the "low oxygen concentration atmosphere condition". (Equation Removed) where, Ki: Linear rate constant (0.6 x 10-6 .g.cm-2 .s-1 .%-1) Po2- Oxygen concentration When the steel material is actually heated by the combustion gas, the slower of the oxidation speeds by the parabolic rate and the linear rate regulates the oxidation. Accordingly, the oxidation speed of iron is expressed as in Equation (11): (Equation Removed) From the above, the boundary between the high oxygen concentration atmosphere condition under which an oxide scale made of three layers of hematite, magnetite, and wustite is formed and oxidation is advanced under a parabolic rate and the low oxygen concentration atmosphere condition under which an oxide seal© made of only wustite is formed and the scale is formed with the linear rate is found from Equation (12) giving equal oxidation speeds under both conditions. Further, the relationship between the amount of increase of oxidation and the oxide scale thickness is found by Equation (13), therefore Equation (4) and Equation (14) give the oxygen concentration which becomes the border between the high oxygen concentration atmosphere condition and the low oxygen concentration atmosphere condition. As apparent from Equation (4) and Equation (14) including the parabolic rate constant kp depending upon the temperature and the oxide scale thickness x or the increased amount of oxidation w, the oxygen concentration forming the border of both conditions is not determined by only the oxidation concentration, and the oxidation concentration forming the border changes according to the oxide scale thickness and the temperature at that time: (Equation Removed) where, x: Oxide scale thickness µm (Equation Removed) Next, the behavior of Cu where the steel containing the Cu is oxidized will be mentioned. Where the steel containing Cu is oxidized, the Cu which is the element more precious than the iron is concentrated at the oxide scale/metal interface, and the Cu of the liquid phase appears. when the temperature is l080°C or more as the melting point of the Cu, it appears as the liquid phase. The wustite and the liquid phase Cu have extremely high coatabilities. Further, fine pores exist at the three main points of the grain boundary of the oxide scale. These pores are distributed while being connected in a mesh state. From the fact that this high coatability exists and connected pores exist in the oxide scale, the Cu of the liquid phase can easily move in the oxide scale by capillary action. Namely, the Cu of the liquid phase appearing in the lower portion of the oxide scale of the wustite can easily move to the surface layer of the oxide scale. When the oxide scale made of wustite is formed under a low oxygen concentration atmosphere condition, it is volatilized from the oxide scale surface layer as Cu or CuO having a relatively high vapor pressure. For the volatilization of the Cu, the low oxygen concentration atmosphere condition under which the oxide scale made of wustite is formed is indispensable. This is because, under a high oxygen concentration atmosphere condition wherein an oxide scale made of three layers of hematite, magnetite, and wustite is formed, Cu is dissolved into the magnetite layer. When an oxide .scale made of three layers of hematite, magnetite, and wustite is formed in the high oxygen concentration atmosphere, the phenomenon of the Cu dissolving into the magnetite layer can be explained as follows. When considering the behavior of a minute amount of metal in steel at the time of the formation of the oxide scale, it is important to consider the solubility of the minute amount of metal into the oxide scale. Almost no Cu can be dissolved into the wustite, but a lot of Cu can be dissolved into magnetite having a spinel structure. This is because the composition can be changed while maintaining the spinel structure from Fe2FeO4 as the magnetite to Fe2Cu04. Further, when considering the balanced oxygen potential of Cu and the oxide of Cu, it is seen that the Cu can exist as the oxide above the magnetite in the oxide scale on the steel material. Accordingly, the Cu is not only concentrated at the oxide scale/metal interface, but is also dissolved into above the magnetite layer produced near the surface layer of the oxide scale". Namely, under a high oxygen, concentration atmosphere condition wherein an oxide scale made of three layers of hematite, magnetite, and wustite is formed, the Cu of the liquid phase appearing while being concentrated at the oxide scale/metal interface permeates through the holes at the grain boundary of the oxide scale by a capillary phenomenon and moves to above the oxide scale, but when the magnetite layer is near the surface layer, the Cu will be dissolved into the magnetite layer. The first method of production for preventing the hot shortness of Cu is to heat the iron under a low oxygen concentration atmosphere condition when heating the steel material before the hot rolling and volatilize the Cu into the atmosphere. Due to this, the amount of Cu concentrated at the oxide scale/metal interface can be reduced, so the hot shortness can be suppressed. Further, the oxidation speed under the low oxygen concentration atmosphere condition is proportional to the oxygen concentration, therefore there is also the action of reducing the oxygen concentration so as to reduce the amount of formation of the oxide scale and reducing the amount of Cu itself appearing concentrated at the oxide scale/metal interface. In the case of the usual oxidation under a high oxygen concentration atmosphere condition where the steel material is made of three layers of hematite, magnetite, and wustite, part of the Cu removed from the steel material by the oxidation is dissolved into the magnetite layer, but a constant ratio of Cu is concentrated at the oxide scale/metal interface. In this case, it is seen that the amount of Cu concentration Ecu (µg.cm-2) per unit surface area of the Cu-containing steel material concentrated near the -oxide scale/metal interface becomes about 18.6Ccu X d where the effective thickness of the steel material obtained by dividing the sectional area £ of the cross-section of the steel material vertical to the rolling direction after the end of the hot rolling by its circumferential length 1 is d (mm). Accordingly, when the amount of Cu concentrated at the oxide scale/metal interface can be reduced in the present invention, the amount of Cu concentration Ecu(µg.cm-2) per unit surface area concentrated near the oxide scale/metal interface can be made less than 18.5Ccu x d, and the hot shortness can be avoided as mentioned before. This phenomenon that the Cu is volatilized is continued so far as the oxide scale holds a structure made of a wustite layer. Accordingly, the amount of Cu concentrated at the oxide scale/metal interface can be reduced. In this case, it is necessary for Cu to move in the oxide scale of wustite as the liquid phase, therefore, in the present invention, it is the prerequisite that the temperature be the 1080°C or more of the melting point of the Cu where the Cu of the liquid phase is produced. Further, the atmosphere at the time of the heating must be the low oxygen concentration atmosphere condition wherein it becomes not more than the oxygen concentration expressed in Equation (4) or Equation (14) . An oxide scale made of the wustite is produced under the low oxygen concentration atmosphere condition, while an oxide scale made of three layers of hematite, magnetite, and wustite is produced under the high oxygen concentration atmosphere condition. When the low oxygen concentration atmosphere condition is changed to the high oxygen concentration atmosphere condition and the atmosphere condition is inversely changed with respect to the former in the state where the oxide scale is produced, the oxide scale structure also changes in accordance with the atmosphere condition. For example, even if the oxide scale produced under the high oxygen concentration atmosphere condition exists at first, by changing the atmosphere condition to the low oxygen concentration atmosphere condition from the middle, the structure changes to the oxide scale structure made of wustite. At that time, the Cu dissolved in the magnetite layer under the first high oxygen concentration atmosphere condition cannot be dissolved into the- wustite when the atmosphere condition shifts to the low oxygen concentration atmosphere condition and the oxide scale made of the wustite layer is formed, and the Cu is volatilized from the oxide scale surface layer and diffused into the atmospheric gas. Accordingly, it is not necessary to achieve the low oxygen concentration atmosphere condition in the entire region in the heating furnace where the surface of the steel material becomes 1080°C or more temperature. Even if part of the region in the heating furnace is set to the low oxygen concentration atmosphere condition, the volatilization phenomenon of Cu appears in that region, therefore it is possible to suppress the hot shortness. Under the low oxygen concentration atmosphere condition, the Cu moves in the grain boundary of the oxide scale from the oxide scale/metal interface and is volatilized from the oxide scale surface. The inventors engaged in in-depth studies and consequently found a fact that the oxide scale was formed at the time of the formation of the oxide scale, and the time (s) during which Cu moves in the scale and starts to volatilize was expressed as in Equation (15) in the relationship with the temperature T(K), Therefore, preferably the oxidation under the low oxygen concentration atmosphere condition is carried out for a time of t. seconds or more expressed by the following equation corresponding to the surface of the steel material temperature: Logio(t/60)=-0.00301 x T + 4.83 (15) Usually, when a combustion gas using coke furnace gas, LNG, or the like as fuel is used, the atmosphere of the high oxygen concentration atmosphere condition and the atmosphere of the low oxygen concentration atmosphere condition can be obtained by suppressing the air ratio at the time of the combustion. When the air ratio is increased, the oxygen concentration in the combustion gas atmosphere increases, while when the air ratio is reduced, the oxygen concentration in the combustion gas atmosphere is reduced. The oxygen concentration in the heating furnace can be measured by an oxygen concentration meter. The low oxygen concentration atmosphere condition can be obtained also by mixing an inert gas such as nitrogen gas, argon gas, or helium gas into the combustion gas, or raising the temperature of the steel material'. Note that the present method can also be worked in the case where a heating furnace not using combustion gas, for example, an induction heating furnace or "a high frequency heating furnace, is used. In this case, the non-oxidizing gas such as nitrogen gas, argon gas, or helium gas can be used as the atmospheric gas. When part of the region in the heating furnace is set to the low oxygen concentration atmosphere condition, and rest of the region is set to the high oxygen concentration atmosphere condition, preferably a partition is provided at the position forming the border between the regions. By providing the partition, the low oxygen concentration atmosphere condition and the high oxygen concentration atmosphere condition can be clearly partitioned. Further, use can be also made of a combustion method using a heat storage type combustion burner (regeneration burner) as the heating method when part of the region in the heating furnace is set to the low oxygen concentration atmosphere condition, and the rest of the region is set to the high oxygen concentration atmosphere condition. In this combustion method, the combustion gas released from the burner enters into heat storage chambers of the burner facing to each other, therefore, the amount of the combustion gas flowing out into the other region is small, and the change of the atmosphere condition of part of the region is easy. In Equation (4), the oxide scale thickness must be considered. However, it is impossible to measure the oxide scale thickness at the time of the actual production of steel plate in real time. Accordingly, by integrating the oxide scale found by Equation (11) and converting it to the oxide scale thickness in Equation (13), the oxide scale thickness in generation can be found by computation. The temperature of the surface of the steel material required at this time can be easily measured by a radiation thermometer. Further, it is also possible to find the temperature distribution of the steel material from the temperature distribution of the atmosphere by computation of the heat conduction. The low oxygen concentration atmosphere condition wherein the Cu is volatilized from the oxide scale surface is expressed by the oxygen concentration or less expressed in Equation (4} or Equation (14) and changes according to the thickness and temperature of the oxide scale, so cannot be correctly described as a specific oxygen concentration or less. However, the thickness of the oxide scale formed in the heating furnace is about 500 µm to 3000 µm. Under the heating temperature condition of about 1080°C to 1250°C under this thickness condition, the low oxygen concentration atmosphere condition is exhibited so far as the oxygen concentration is 0.5 vol% or less from Equation (4), the Cu can be vaporized into the atmosphere, and the hot shortness can be suppressed. FIG. 4 schematically shows an example of a general embodiment of the preferred heating furnace for working the first method of production of the present invention and a situation of formation of oxide scale corresponding to the oxygen concentration condition. This example applies the present invention to a case where a steel material (slab) 1 containing 0.05 to 3 mass% of Cu is inserted into a heating furnace 2 at room temperature, heated in a temperature atmosphere of ilOO to 1300°C, and removed at the temperature of 1100 to 1300°C, then the heating furnace oxide scale is removed by a descaling device (high pressure water) 3 and hot rolled by a hot rolling mill 4. The characteristic feature thereof resides in a point that part of the regioQ of the heating furnace is set to the low oxygen concentration atmosphere condition. Further, a partition 5 is provided in the heating furnace for partitioning the region of the low oxygen concentration atmosphere condition and the region of the high oxygen atmosphere condition. In this embodiment, in order to heat the steel material in the low oxygen concentration atmosphere condition, the oxide scale to be formed is constituted by wustite. When the oxide scale is formed and the steel material is heated up to 1100 to 1300°C or above the melting point 1300°C or more of Cu, as shown in FIG. 4, the Cu is concentrated at the oxide scale/metal interface and appears as the liquid phase. The Cu of the liquid phase permeates through the grain boundaries of the oxide scale and reaches the surface of the oxide scale, where the Cu is vaporized or oxidized and vaporized and scattered as the CuO vapor. These vaporization and scattering of Cu continue during the period under the low oxygen concentration atmosphere condition, therefore the amount of Cu concentration at the oxide scale/metal interface of the surface of the steel material can be reduced. In this way, in the present invention, the amount of concentration of Cu at the oxide scale/metal interface can be much suppressed, and the hot shortness caused by Cu at the time of the hot rolling can be advantageously prevented. In the same figure, the reduced amount of the Cu concentration layer is indicated by the broken line. Next, an explanation will be given of the invention disclosed in claim 8 concerning the second method of production for suppressing hot shortness. In the present invention, all of the following three phenomena newly discovered by the inventors are utilized, (a) The Cu appearing as the liquid phase at the oxide scale/metal interface easily moves in the grain boundaries of the oxide scale, (b) In the case of oxide scale not forming magnetite, that is, in the case of oxide scale made of a wustite layer, the Cu of the liquid phase from th.e oxide scale/metal interface moves in the oxide scale (grain boundary) and reaches the surface of the oxide scale where it is vaporized and scattered as the vapor of the Cu or CuO. (c) When an-oxide scale made of three layers of hematite, magnetite, and wustite is produced, the Cu of the liquid phase moves from the oxide scale/metal interface through the oxide scale (grain boundary), and the Cu is dissolved into the magnetite layer. These natures include a nature that the liquid phase Cu _ concentrated at the oxide scale/metal interface is sucked through the grain boundaries of the oxide scale by the capillary phenomenon, and the Cu is moved to a location away from the interface, that is the, oxide scale has a sponge-like nature absorbing the liquid phase Cu. The steel is heated and hot rolled after the oxide scale formed on the surface of the steel material is removed. The concentration of the Cu at the oxide scale/metal interface of the oxide scale formed in the heating stage becomes the problem of hot shortness. If the amount of Cu concentrated by the heating can be reduced before the start of the hot rolling, the hot shortness can be suppressed. Usually, a single oxide scale removal treatment is applied before the start of the hot rolling. In the present invention, this oxide scale removal treatment is carried out twice or more. Usually the steel material is rolled in the ambient air atmosphere, therefore the oxide scale is formed on the surface during each oxide scale removal treatment. The oxide scale formed during the period of this oxide scale removal treatment acts to absorb the Cu of the liquid phase into the oxide scale as mentioned before. Accordingly, as the oxide scale removal treatment is executed more than the usual one time, the amount of concentrated Cu at the oxide scale/metal interface is reduced, and the hot shortness can be suppressed. As apparent also from the above actions, two or more oxide scale removal treatments must be applied after the heating of the steel material and before the first hot rolling. Further, re-oxidation forming the oxide .scale is necessary, the atmosphere must be the oxidizing atmosphere, and the ambient air atmosphere in which the ordinary rolling is carried out can be easily and conveniently utilized.,Even when the scale at the time of the re-oxidation is an oxide scale made of wustite, and even in the case where an oxide scale made of three layers of hematite, magnetite, and wustite is produced, the oxide scale has the action of absorbing the liquid phase Cu in any case, so there is the action of reducing the Cu concentrated at the oxide scale/metal interface. Accordingly, the atmosphere to which the steel material is exposed during the period of twice or more oxide scale removal treatments may be any of the low oxygen concentration atmosphere condition or high oxygen concentration atmosphere condition. Further, in order to utilize the phenomenon of the liquid phase Cu being absorbed into the oxide scale, the temperature of the surface of the steel material must be 1080°C ox more of the melting point of the Cu. Note that, the temperature of the surface of the steel material can be easily measured by a radiation thermometer. If a steel material made of three layers of hematite, magnetite, and wustite obtained by heating the steel material under a high oxygen concentration atmosphere condition, descaling it once, then hot rolling it, it is seen that the amount of Cu concentration Ecu (µg . cm-2) per unit surface area of the Cu-containing steel material concentrated near the oxide scale/metal interface becomes about 18.6Ccu x d where the effective thickness of the steel material obtained by dividing the sectional area s_ of the cross-section of the steel material vertical to the rolling direction after the end of the hot rolling by ,its circumferential length 1 is d (mm). Accordingly, if the amount of Cu concentrated at the oxide scale/metal interface can be reduced in the present invention, the amount of Cu concentration Ecu (µg.cm-2) per unit surface area concentrated near the oxide scale/metal interface can be made less than 18.6Ccu X d, and the hot shortness can be avoided as mentioned before. For the re-oxidation of the steel material during twice or more oxide scale removal treatments, heat treatment for re-oxidation may be applied. As the means of heating and/or soaking in the re-oxidation treatment, preferably use is made of a means of heating or soaking by electrical energy - which is excellent in energy efficiency, response, and controllability, for example, radiation heating or soaking in an electric furnace, or induction heating or current heating. So far as the steel material temperature is high and the surface of the steel material can be held at 1080°C or more during the re-oxidation treatment, the usage of the soaking means for holding the steel material in the state covered by a soaking material is a preferred embodiment in terms of the energy efficiency. By any method, the object of the present invention can be attained so far as the surface of the steel material is 1080°C or more and the atmosphere is an oxidation atmosphere, therefore, a method having a good energy efficiency may be appropriately selected. As this oxidation atmosphere, preferably use is made of atmospheric air which can be easily utilized. The method of removal of the oxide scale includes conventionally well known methods, for example, the method of ejecting high pressure water onto the surface of the steel material, a method of rolling the surface of the steel material forming the product surface, and a method of pressing the side surface of the steel material forming the product surface downward in the width direction and can be appropriately selected or combined. FIG. 5 schematically shows a general view of a preferred facility for working the second method of production of the present invention and the situation of production of the oxide scale in this facility. Here, the steel material 1 is heated in a combustion gas atmosphere (high oxygen concentration atmosphere condition) in the heating furnace 2. At the time of this heating, the oxide scale is produced on the surface of the steel material. The Cu of the liquid phase appears at the oxide scale/metal interface accompanied with this. Part of that melted (liquid phase) Cu moves in the grain boundaries of the oxide scale, reaches the magnetite layer, and dissolves into the magnetite layer. The steel material heated to the predetermined temperature for the predetermined time is removed from the heating furnace, and the heating furnace oxide scale is removed in the descaling device (high pressure water) 3. Due to this, the Cu dissolved in the magnetite layer is removed together with the oxide scale. Thereafter, when the steel material moves in the ambient atmosphere, the re-oxidized scale is produced on the surface of the steel material by the oxygen in the atmospheric air. This region is a xe-oxidation treatment zone 6. Due to this, part of the Cu of the liquid phase appearing in the heating furnace and remaining on the surface of the iron even after the descaling is absorbed into the scale of the re-oxidized scale, moves to the magnetite layer, and is dissolve or volatilizes into the atmosphere, so the concentrated Cu amount at the oxide scale/metal interface will be reduced. The Cu concentration layer reduced in amount is indicated by the broken line in the figure. Thereafter, it is descaled in the descaling device (high pressure water) 3 before the hot rolling and rolled by the hot rolling mill 4, but the concentrated Cu amount at the oxide scale/metal interface has become small, so the hot shortness (surface cracking) at the time of the hot rolling can be advantageously prevented. Finally, an explanation will be given of the invention disclosed in claim 9. This simultaneously uses the first method of production and the second method of production suppressing the red shortness. The first method of production is the method of heating the steel material, and the second method is the method of removing the oxide scale of the steel material during the period from the heating to the first rolling. They can be simultaneously carried out. By simultaneously performing them, the effect of suppression of the hot shortness can be further raised. Examples (Example 1) An experiment was performed casting steel materials containing Cu and Sn and producing steel plates by hot rolling. The heating preceding the hot rolling was carried out while changing the air ratio at the time of the combustion by heating by combustion to various ratios. The steel materials were heated to a temperature of 1100 to 1250°C, then hot rolled to produce steel plates having various steel material thicknesses (effective thicknesses of the steel materials). The Cu concentration of the base material, the Sn concentration of the base material, and the Ni concentration of the base material are shown in Table 1. Further, the effectiveness thickness of the obtained steel material, the Cu amount (amount of Cu concentration) per unit surface area concentrated near the oxide scale/metal interface, the Sn amount (amount of Sn concentration) per unit surface area concentrated near the oxide scale/metal interface, the Sb amount (amount of Sb concentration) per unit surface area concentrated near the oxide scale/metal interface, and the As amount (amount of As concentration) per unit surface area concentrated near the oxide scale/metal interface, and the situation of occurrence of the cracking due to the hot shortness of the surface are shown in Table 1 together. The amount of Cu concentration and the amount of Sn concentration were found by GDS analysis. The degree of occurrence of cracking due to the hot shortness of the obtained steel material was indicated by the marks such as ©: no occurrence of cracking, O: occurrence of minute cracking but no problem in quality and outer appearance, X: cracking which becomes a problem in quality and outer appearance. In steel plates having small amounts of concentration of Cu, Sn, 3b, and As as the hot shortness inducing elements at the oxide scale/metal interface and satisfying Equation (1) and Equation (2) (No. 1 to 9), no occurrence of cracking due to hot shortness which became a problem in quality and outer appearance was seen, while cracking due to hot shortness which becomes a problem in quality and outer appearance occurred in steel plates not satisfying Equation (1) or Equation (2) {No. 10 to 13). Also, in steel plates obtained by adding Ni satisfying Equation (3) (No. 8, 9), no hot shortness occurred and an excellent surface quality could be obtained. Due to this, in the examples according to the present invention, it is seen that no hot shortness occurred at the time of hot rolling even if the steel material contains hot shortness inducing elements of Cu and Sn. (Table Removed) (Example 2) An experiment was performed casting steel materials containing Cu and Sn and producing wire materials by hot rolling. The heating preceding the hot rolling was carried out while changing the air ratio at the time of the combustion by the heating by combustion to various ratios. The steel materials were heated to a temperature of 1100 to 1250°C, then hot rolled to produce wire materials having various diameters (steel material thicknesses). The Cu concentration of the base material, the Sn concentration of the base material, and the Ni concentration of the base material are shown in Table 1. Further, the effective-thicknesses of the obtained steel materials (radii of wire materials), the Cu amount (amount of Cu concentration) per unit surface area concentrated near the oxide scale/metal interface, the Sn amount (amount of Sn concentration) per unit surface area concentrated near the oxide scale/metal interface,^ and the situation of occurrence of the cracking due to the hot shortness of the surface are shown in Table 2 together. The amount of Cu concentration and the amount of Sn concentration were found from the results by surface analysis of the cross-section of the oxide scale by EPMA. The degree of occurrence of cracking due to the hot shortness of the obtained steel materials was indicated by marks such as @: no occurrence of cracking, O: occurrence of minute cracking but no problem in quality and outer appearance, X: cracking which becomes a problem in quality and outer appearance. In wire materials having small amounts of concentration of Cu and Sn as hot shortness inducing elements at the oxide scale/metal interface and satisfying Equation (1) and Equation (2) (No. 14 to 21), no occurrence of cracking due to hot shortness which becomes a problem in quality and outer appearance was seen. Cracking due to hot shortness which becomes a problem in quality and outer appearance occurred in wire materials not satisfying Equation (1) or Equation (2) (No- 22 to 24). Also, in the wire material obtained by adding Ni satisfying Equation (3) (No. 21), hot shortness did not occur, and an excellent surface quality could be obtained. Due to this, in the examples according to the present invention, it is seen that the hot shortness does not occur at the time of hot rolling even if the steel material contains the hot shortness inducing elements of Cu and Sn. [Table 2] (Table Removed) (Example 3) An experiment was performed casting steel materials containing Cu and Sn and producing H-shaped steels by hot rolling. The heating preceding the hot rolling was carried out while changing the air ratio at the time of the combustion by heating by combustion to various ratios. The steel materials were heated to a temperature of 1150 to 1300°C, then hot rolled to produce H-shaped steels having various thicknesses (effective thicknesses of the steel materials). The Cu concentration of the base material, the Sn concentration of the base material, and the Ni concentration of the base material are shown in Table 3. Further, the effective thicknesses of the obtained steel materials, the Cu amount (amount of Cu concentration) per unit surface area concentrated near the oxide scale/metal interface, the Sn amount (amount of Sn concentration) per unit surface area concentrated near the oxide scale/metal interface, and the situation of occurrence of cracking due to the hot shortness of the surface are shown in Table 3 together. The amount of Cu concentration and the amount of Sn concentration were found by GDS analysis for 3 points of one surface of a web portion, and an inner surface and an outer surface of a flange portion, and the mean value of them is shown. The degree of occurrence of the cracking due to the hot shortness of the obtained steel materials was indicated by the marks such as @: no occurrence of cracking, O: occurrence of minute cracking but no problem in quality and outer appearance, x: cracking which becomes a problem in quality and outer appearance. In the steel materials having small amounts of concentration of Cu and Sn as the hot shortness inducing elements at the oxide scale/metal interface and satisfying Equation (1) and Equation (2) (No. 25 to 32), no occurrence of cracking due to hot shortness which becomes a problem in quality and outer appearance was seen, while cracking due to hot shortness which becomes a problem in quality and outer appearance occurred in steel materials not satisfying Equati'on (1) or Equation (2) (No. 33 to 35). Also, in the steel materials obtained by adding Ni and satisfying Equation (3) (No. 30. 31), no hot shortness occurred and an excellent surface quality could be obtained. Due to this, in the examples according to the present invention, it is seen that no hot shortness occurs at the time of hot rolling even if the steel material contains the hot shortness inducing elements of Cu and Sn. (Table Removed) (Example 4) An experiment was performed casting steel materials containing Cu and Sn and producing seamless steel pipes by hot rolling. The heating preceding the hot rolling was carried out while changing the air ratio at the time of the combustion by the heating by combustion to various ratios. The steel materials were heated to a temperature of 1100 to 1250°C, then hot rolled to produce seamless steel pipes having various thicknesses (effective thicknesses of the steel material). The Cu concentration of the base material, the Sn concentration of the,base material, and the Ni concentration of the base material are shown in Table 4. Further, the effective thicknesses of the obtained steel materials, the Cu amount (amount of Cu concentration) per unit surface area concentrated near the oxide scale/metal interface, the Sn amount (amount of Sn concentration) per unit surface area concentrated near the oxide scale/metal interface, and the situation of occurrence of the cracking due to the hot shortness of the surface are shown in Table 4 together. The amount of Cu concentration and the amount of Sn concentration were found from the results of surface analysis of the cross-section of the oxide scale on the outer surface and the inner surface of the steel material by EPMA, and the mean value thereof is shown. The degree of occurrence of the cracking due to the hot shortness of the obtained steel materials was indicated by the marks such as (Q): no occurrence of cracking, O: occurrence of minute cracking but no problem in quality and outer appearance, X: cracking which becomes a problem in quality and outer appearance. In steel materials having small amounts of concentration of Cu and Sn as the hot shortness inducing elements at the oxide scale/metal and satisfying Equation (1) and Equation (2) (No. 36 to 41), no occurrence of cracking due to hot shortness which becomes a problem in quality and outer appearance was seen, while cracking due to hot shortness which becomes a problem in quality and outer appearance occurred in steel materials not satisfying Equation (1) or Equation (2) (No. 42 to 44) . Also, in the steel material obtained by adding Ni and satisfying Equation (3) (No. 41), no hot shortness occurred, and an excellent surface quality could be obtained. Due to this, in the examples according to the present invention, it is seen that no hot shortness occurs at the time of hot rolling even if the steel material contains the hot shortness inducing elements of Cu and Sn. (Table Removed) (Example 5) An experiment was performed casting steel materials obtained by adding one type or two or more types of elements of Ti, V, Nb, S, P, and an REM into steel materials having ingredients including Cu, Sn, Sb, and As so as to adjust the ingredients and producing steel materials by hot rolling. The heating preceding the hot rolling was carried out while changing the air ratio at the time of the combustion by heating by combustion to various ratios. The steel materials were heated to a temperature of 1100 to 1250°C, then hot rolled to produce 3.2 mm thick steel plates. The ingredients of the base material are shown in Table 5. Further, the effective thicknesses of the obtained steel materials, the Cu amount (amount of Cu concentration) per unit surface area concentrated near the oxide scale/metal interface, the Sn amount (amount of Sn concentration) per unit surface area concentrated near the oxide scale/metal interface, the Sb amount (amount of Sb concentration) per unit surface area concentrated near the oxide scale/metal interface, the As amount (amount of As concentration) per unit surface area concentrated near the oxide scale/metal interface, and the situation of occurrence of the cracking due to the hot shortness of the surface are shown in Table 5 together. The amount of Cu concentration, the amount of Sn concentration, the amount of Sb concentration, and the amount of As concentration were found from the CDS analysis results. The degree of occurrence of cracking due to hot shortness of the obtained steel materials was indicated by the marks such as @: no occurrence of cracking, O: occurrence of minute cracking but no problem in quality and outer appearance, X: cracking which becomes a problem in quality and outer appearance. In the steel plates containing Ti, V, Nb, and an REM within the range of present invention, having small amounts of concentration of Cu and Sn as the hot shortness inducing elements at the oxide scale/metal interface, and satisfying Equation (1) and Equation (2) (No. 45 to 53), no occurrence of cracking due to hot shortness which becomes a problem in quality and outer appearance was seen, while cracking due to hot shortness which becomes a problem in quality and outer appearance occurred in steel plates not containing Ti, V, Nb, or an REM and not satisfying Equation (1) or Equation (2) (No, 54 to 56) . Due to this, in the examples according to the present invention, it is seen that hot shortness does not occur at the time of hot rolling even if the steel material contains hot shortness inducing elements such as Cu, Sn, Sb, and As. (Table Removed) (Example 6) A steel material containing, as chemical ingredients, 0.05 mass% of C, 0.01 mass% of Si, 0.25 mass% of Mn, 0.012 mass% of P, 0.006 mass% of S, 1.64 mass% of Cu, 0.01 mass% of Ni, and 0.02 mass% of Cr was heated in a heating furnace by using a combustion gas using LNG as fuel. The steel material, having an initial oxide scale thickness of 300 µm, was heated up to 123G°C for 90 minutes by making the oxygen concentration in the entire heating furnace (heating zone and soaking zone) 0.5 vol% and then holding the temperature at 1230°C for 40 minutes. The oxygen concentration condition in this case was a low oxygen concentration atmosphere condition in the entire heating furnace. Thereafter, the steel material was taken out from the heating furnace, descaled by high pressure water, then hot rolled. No occurrence of red shortness was confirmed on the surface of the steel material after the hot rolling. On the other hand, when the entire heating furnace was heated under a high oxygen concentration atmosphere condition with an oxygen concentration of 5 vol%, cracking on the surface of the steel material due to hot shortness occurred. (Example 7) A steel material containing, as chemical ingredients, 0.04 mass% of C, 0.01 mass% of Si, 0.33 mass% of Mn, 0.010 mass% of P, 0.011 mass% of S, 0.74 mass% of Cu, 0.04 mass% of Ni, and 0.07 mass% of Cr was heated in a heating furnace by using a combustion gas using coke furnace gas as fuel. The oxide scale thickness before placing the steel material in the heating furnace was 500 µm. In this heating furnace heating, first, the steel material was heated up to 1200°C for 80 minutes under a high oxygen concentration atmosphere condition (oxygen concentration of 5 vol%), the steel material was held at 1200°C for 20 minutes in the atmosphere as it was, then was held at 1200°C for 30 minutes under a low oxygen concentration atmosphere condition (oxygen concentration of 0.4 vol%) partitioned to front and rear by a partition. The steel material was held again at 1200°C for 30 minutes under a high oxygen concentration atmosphere condition (oxygen concentration of 5 vol%), then removed from the heating furnace. Thereafter, the heating furnace oxide scale of the surface of the steel material was removed by high pressure water, then the material was hot rolled. No occurrence of surface cracking of the steel material due to hot shortness was confirmed at the surface of the steel material after hot rolling. On the other hand, the oxide scale thickness before the steel material was heated under a conventional high oxygen concentration atmosphere condition not providing a heating zone exhibiting a low oxygen concentration atmosphere condition in the heating zone, that is, before being placed in the heating furnace, was 500 µm. The steel material was heated up to 1200°C for 80 minutes under a high oxygen concentration atmosphere condition (oxygen concentration of 5 vol%), held in the atmosphere and at the temperature as it was for 80 minutes, then removed. In the case of these steel material, when descaling by high pressure water, then hot rolling, cracking of the surface of the steel material due to hot shortness occurred at the surface of the steel material after hot rolling. (Example 8) A steel material containing, as chemical ingredients, 0.05 mass! of C, 0.01 mass% of Si, 0.25 mass% of Mn, 0.011 mass% of P, 0.006 mass% of S, 1.60 mass% of Cu, 0.01 mass% of Ni, and 0,02 mass% of Cr was heated in a heating furnace by using a combustion gas using LNG as fuel. The oxide scale thickness before placing the steel material in the heating furnace was 500 µm. In this heating, the entire interior of the heating furnace was made an oxygen concentration of 5 vol%. First, the steel material was heated up to 1200°C for 80 minutes and held at 1200°C for 20 minutes in the -atmosphere as it was. The heating during this period corresponds to the high oxygen concentration atmosphere condition. Thereafter, the steel material was heated up to 1300°C and held there for 30 minutes. Up to 1300°C, 10 minutes after heating corresponds to the low oxygen concentration atmosphere condition. Thereafter, as the oxide scale becomes thicker, the condition shifts to the higher oxygen concentration atmosphere condition. Thereafter, the oxide scale on the surface of the steel material was removed by high pressure water, and hot rolling was performed. No occurrence of cracking of the surface of the steel material due to hot shortness was confirmed on the surface of the steel material after hot rolling. On the other hand, when heating without raising the steel material temperature in the middle of the heating zone, that is, when the oxide scale thickness before placing the steel material in the heating furnace was 500 µm, heating all of the steel material under a high oxygen concentration atmosphere condition (oxygen concentration of 5 vol%) up to 1200°C for 80 minutes, holding the steel material in the atmosphere as it was for 50 minutes, then removing it, if descaling by high pressure water, then hot rolling, cracking of the surface of the steel material due to hot shortness occurred at the surface of steel material after the hot rolling. (Example 9) A steel material containing, as chemical ingredients, 0.002 mass% of C, 0.02 mass% of Si, 0T12 mass% of Mn, 0.010 mass% of P, 0.007 mass% of S, 1.02 mass% of Cu, 0.02 mass% of Ni, and 0.03 mass% of Cr was heated by heating using coke furnace gas as fuel up to 1150°C while changing the oxygen concentration at l080°C or more to 0.5 vol% as the low oxygen concentration atmosphere condition and to 2 vol% as the high oxygen concentration atmosphere condition and holding at that temperature for 1 minute. Immediately after the extraction of the steel material from the heating' furnace, the oxide scale was removed by high pressure water. Thereafter, the steel material was moved into the atmosphere, and the oxide scale was removed again by high pressure water immediately before the first hot rolling. No cracking due to hot shortness occurred at a steel material having a thickness of 2.5 mm hot rolled by the present method. On the other hand, the steel material was heated under similar heating conditions. Rather than descaling by high pressure water immediately after removal from the heating furnace, the scale was removed by high pressure water only before the start of the first hot rolling. The steel material was rolled to form a steel plate having the same thickness of 2.5 mm. The steel plate was heated by an oxygen concentration of 0.5 vol% as the low oxygen concentration atmosphere condition (present invention). Consequently, no cracking due to hot shortness occurred on the surface of the steel plate, but cracking due to hot shortness occurred on the surface of the steel plate heated in an atmosphere of an oxygen concentration of 2 vol% as the high oxygen concentration atmosphere condition (comparative example). (Example 10) A steel material containing, as chemical ingredients, 0,05 mass% of C, 0.01 mass% of Si, 0.25 mass% of Mn, 0.012 mass% of P, 0.006 raass% of S, 1.61 mass% of Cu, 0.01 mass% of Ni, and 0,02 mass% of Cr was heated in a heating furnace using coke furnace gas as fuel up to 1230°C and held at that temperature for'90 minutes. The oxygen concentration of the atmosphere at this time was made 3 vol% as the high oxygen concentration atmosphere condition. Rolling with a reduction ratio of 31 was applied to the steel material removed from the heating furnace in the width direction, and the oxide scale of the surface of the steel material was removed. Thereafter, the steel material was placed in a soaking cover covered by a heat insulation material and held in the state where the lowest temperature of_the surface of the steel material was llOO°C or more for 5 minutes. The atmosphere at the time of the soaking was made the atmospheric air. After soaking, the oxide scale was removed again by high pressure water and hot rolling was carried out. Further, as a comparative example, after heating, a steel material obtained by applying hot rolling immediately after removing the oxide scale in the heating furnace by high pressure water was prepared. As a result, cracking did not occur on the surface of the steel material held in the soaking cover in the example of the present invention, but cracking occurred on the surface of the steel material not soaked in a comparative example. INDUSTRIAL APPLICABILITY As explained above, according to the present invention, by enabling advantageous suppression of the occurrence of hot shortness of a steel material due to Cu when hot rolling a steel material containing 0.05 to 3 mass% of Cu without a change of the steel ingredients such as addition of Ni and Si, it is possible to provide a Cu-containing steel material excellent in surface properties and a method of production of the same. We Claim: 1. A method of production of a Cu- containing steel material by heating a steel material containing Cu by mass%, of 0.05% to 3% of a base material in a heating furnace, then starting hot rolling, said method of production of a Cu-containing steel material excellent in surface properties characterized by, forming an atmosphere of an oxygen concentration P02 (vol%) shown in the equation as herein described (low oxygen concentration atmosphere condition) when heating by the heating furnace, in the entire region or partial region in the heating furnace where the temperature of the surface of the steel material becomes at least 1080°C so as to cause the formation of an oxide scale made of wustite, thereby to make the amount of Cu concentration Ecu (ug.cm-2) per unit surface area of the Cu-containing steel material concentrated near the oxide scale/metal interface as herein described, wherein, when the effective thickness of the steel material after hot rolling is obtained by dividing a sectional area s of a cross-section of the steel material vertical to the rolling direction by its circumferential length 1 is d (mm). 2. The method as claimed in claim 1, optionally comprising performing at least twice treatment for removing oxide scale formed on the surface of the steel material after taking out of the steel material from the heating furnance and before the hot rolling and forming scale by an atmosphere giving an oxygen concentration of or more than P02 (vol%) as herein described (high oxygen concentration atmosphere condition) during the at least twice oxide spale removal treatments in the steel material after hot rolling. 3. A Cu-containing steel material excellent in surface properties containing Cu by mass % of 0.5% to 3% of a base material, produced by the method of production as claimed in claim 1 and 2, wherein, total concentrations of the base materials of elements inducing hot shortness which are more noble than iron for oxidation in a temperature range of 1000°C to 1300°C and having a melting point of 1300°C or less, that is, a total concentration of base materials of hot shortness inducing elements is Ci as herein described. 4. The Cu-containing steel material excellent in surface properties as claimed in claim 1 to 3, wherein, one type of hot shortness inducing element is Cu, and the rest is one type or two or more types of Sn, Sb, and As. 5. The Cu-containing steel material excellent in surface properties as claimed in claim 1 to 4, wherein, Ni concentration CNi (mass%) in base material and a Cu concentration Ccu (mass%) in base material is related as herein described. 6. The Cu-containing steel material excellent in surface properties as claimed in claim 1 to 5, wherein, the Cu- containing steel material contains, by mass%, at least one type or two or more types of 0.01 to 0.15% of Ti, 0.01 to 0.15% of Nb, and 0.01 to 0.15% of V and further contains one type or two or more types of 0.010 to 0.100% of P, 0.010 to 0.050% of S, and 0.002 to 0.150% of an REM. 7. The Cu-containing steel material excellent in surface properties as claimed in claim 1 to 6, wherein, the Cu-containing steel material contains a precipitate comprised of a carbide, nitride, or carbonitride of one type or two or more types of Ti, Nb, and V and having a particle size of 10 nm to 1 um and particle distribution density of at least l05/mm2.

Documents:

1199-delnp-2005-abstract.pdf

1199-delnp-2005-claims.pdf

1199-delnp-2005-complete specification(as files).pdf

1199-delnp-2005-complete specification(granted).pdf

1199-DELNP-2005-Correspondence-Others-(12-10-2009).pdf

1199-delnp-2005-correspondence-others.pdf

1199-delnp-2005-correspondence-po.pdf

1199-delnp-2005-description (compelete).pdf

1199-delnp-2005-drawings.pdf

1199-delnp-2005-form-1.pdf

1199-delnp-2005-form-18.pdf

1199-delnp-2005-form-2.pdf

1199-DELNP-2005-Form-3-(12-10-2009).pdf

1199-delnp-2005-form-3.pdf

1199-delnp-2005-form-5.pdf

1199-delnp-2005-gpa.pdf

1199-delnp-2005-pct-210.pdf

1199-delnp-2005-pct-304.pdf

1199-delnp-2005-pct-308.pdf

1199-delnp-2005-pct-332.pdf

1199-delnp-2005-pct-409.pdf

1199-delnp-2005-petition-137.pdf

1199-delnp-2005-petition-138.pdf