Title of Invention	DIE STEEL
Abstract	A die steel which consists essentially of, by mass, 0.10 to 0.25% carbon, not more than 1.00% Si, not more than 2.00% Mn, 0.60 to 1.50% Ni, from more than 1.00 to not more than 2.50% Cr, at least one of Mo and W in the amount as defined by the equation of (Mo + (W/2))< 1.00%, 0.03 to 0.15% V (vanadium), 0.50 to 2.00% Cu, not more than 0.05% S (sulphur), not more than 0.10% Al, not more than 0.06% N (nitrogen), not more than 0.005% O (oxygen), and the balance of Fe and unavoidable impurities, which has a metal structure having a primary phase of lower bainite, and which has a hardness of HRC 34 to 45.

Title of Invention

DIE STEEL

Abstract

A die steel which consists essentially of, by mass, 0.10 to 0.25% carbon, not more than 1.00% Si, not more than 2.00% Mn, 0.60 to 1.50% Ni, from more than 1.00 to not more than 2.50% Cr, at least one of Mo and W in the amount as defined by the equation of (Mo + (W/2))< 1.00%, 0.03 to 0.15% V (vanadium), 0.50 to 2.00% Cu, not more than 0.05% S (sulphur), not more than 0.10% Al, not more than 0.06% N (nitrogen), not more than 0.005% O (oxygen), and the balance of Fe and unavoidable impurities, which has a metal structure having a primary phase of lower bainite, and which has a hardness of HRC 34 to 45.

Full Text	Technical Background The present invention relates to a die steel which is excellent in toughness, machinability, polishability and wear resistance, which has high hardness, and which is used mainly for production of a plastic molding die. The following characteristics are required for a die steel used for production of a plastic molding die: (1) a good mirror finish workability and a small tendency to occur with pinholes and very fine pits; (2) a good satinizing workability; (3) good corrosion and rust resistances; (4) good strength, wear resistance and toughness; and (5) a good machinability. A medium C-Mn-Cr-Mo-Fe system steel such as JIS SCM440 has been conventionally used as a die steel for plastic molding, but the above mentioned requirements have been increased, and among others, in recent years, a reduction in manufacture delivery time has been arisen strongly as an important customer's requirement. For the purpose of reducing the number of processing steps to meet such a requirement, various steels are commonly used, for example, such as a low carbon Mn-Cr-Mo-S-Fe system steel (see JP-A-48-093518), a modified steel of this (i.e. JP-A-48-093518) into which Ni is added in order to improve quenching ability (see JP-A-52-065557), and a sulfur-free and Cu containing steel improved in machinability (see JP-A-58-067850 or JP-A-60-204869). There has been also known a low carbon, low sulfur and low Al Mn-Cr-Mo system steel which is improved in the properties of mirror finish workability, the machinability and the weldability (see JP-A-03-115523). Further, there have been proposed a low carbon Mn-Cr-Mo (W)-V-Cu-Fe alloy and a low carbon Mn-Ni-Cr-Mo (W)-V-Cu-Fe alloy (see JP-A-07-062491) which have a suitable combination of a matrix structure and precipitates, and are subjected to a heat treatment to have an adjusted hardness of about HRC 30 so that they can have excellent machinability without a much amount of additive free cutting elements, and they have excellent properties of anti-rust, wear resistance and polishability. In recent years, there has been broadly used an engineering plastic material added with a glass fiber, a carbon fiber or the like in order to improve strength, wear resistance property and heat resistance property. With this, a die for molding a plastic material is strictly required to have desirable properties of wear resistance, toughness and machinability. Thus, a low carbon Mn-Ni-Cr-Mo-Cu-Al system steel (see JP-A-02-182860) has been proposed, in which Cu is finely precipitated so as to have a hardness level of HRC 40 while having excellent properties of toughness and machinability. Further, there has been proposed a low carbon Mn-Ni-(Mo, W)-Cu-Al system prehardened steel for plastic molding (see JP-A-07-278737), which is adjusted to have a lower bainite having a higher toughness in order to improve the die life. Brief Summary of the Invention The above mentioned steels for plastic molding, which are the low carbon Mn-Cr-Mo-S-Fe system steel and the low carbon Mn-Ni-Cr-Mo-S-Fe system steel, have a problem that a satisfactory die life can not be obtained, since when a large-sized die having a maximum length of about 2 meter is formed from such a steel, the properties of polishability, wear resistance and toughness are deteriorated due to segregation of sulfides and so on. The low carbon and free sulfur Mn-Cr-Cu-Fe system steel of JP-A-58-067850 has low softening resistance property to quenching and tempering treatment, so that the steel is deteriorated in hardness when it is subjected to a nitriding treatment at around 550°C. The low carbon Mn-Ni-Cr-Mo (W/2)-Cu-Fe system steel of JP-A-48-093518 is not necessarily satisfactory, because an enough precipitation hardening effect can not be obtained due to a low carbon content. In the low carbon, low sulfur, low Al and low oxygen Mn-Cr-Mo- system steel of JP-A-03-115523, since the machinability is improved by lowering the hardness of the base material to about HRC 30, the steel has a low hardness, and unsatisfactory properties of polishability and wear resistance while the mirror finish workability is improved by restraining occurrence of non-metallic inclusions. The low carbon Mn-Cr-Mo (W)-V-Cu-Fe steel or the low carbon Mn-Ni-Cr-Mo (W)-V-Cu-Fe steel of JP-A-07-062491 ensures that even when a large-sized die is formed from such a steel, the above requirements for the die for plastic molding are satisfied, and the steels have high strength and excellent machinability by enough precipitation hardening by virtue of Cr, Mo (W/2), Cu and V. However, the properties of polishability and wear resistance have become to be not necessarily satisfactory as the engineering plastic material is broadly used. Further, the low carbon Mn-Ni-Cr-Mo-Cu-Al system steel of JP-A-02-182860 is provided with a hardness level of HRC 40 by virtue of a precipitated intermetallic compound of Al and Ni, and has an adjusted uniform upper bainite by making the carbon content low to have excellent machinability, while the steel has a problem of insufficient toughness. The low carbon Mn-Ni-(Mo, W)-Cu-Al system prehardened steel for plastic molding, which is disclosed in JP-A-07-278737, and which has both of the machinability and toughness provided by the preparation thereof into the lower bainite rather than the upper bainite which has been considered to be a structure indispensable to enhance the machinability. Recently, however, a reduction in service cost of a die is extremely highly required, and thus it is necessary to prolong a die life, while maintaining the performance of the conventional die, so that the toughness of the die has not been necessarily satisfactory. With the above-described requirements in view, it is an object of the present invention to provide a die steel used mainly for plastic molding, which has excellent properties of toughness and machinability whereby being capable of achieving a prolongation of the life of a tool such as a die, while maintaining the performance of the conventional die, and which has both of excellent properties of polishability and wear resistance. The present inventors have made a detailed examination of the relationship among the chemical composition and the metal structure, and the properties of machinability, toughness and polishability, and consequently found a die steel optimal for use mainly in plastic molding, which has excellent properties of toughness, machinability and high hardness and also has both of excellent properties of polishability and wear resistance by selecting a suitable combination of components, and in particularly, by (1) making the steel to have a lower bainite rather than an upper bainite which has been considered conventionally to be indispensable in order to improve the machinability by mutually adjusting the Ni and Cu contents with relation to each other; and (2) employing the precipitation hardening mechanism obtainable by an optimal content adjustment of Cr, Mo (W/2), Cu and V, rather than employing the hardening mechanism attributable to the precipitation of the intermetallic compound of Al and Ni and the Cu- precipitation hardening, both of which have been considered to be a hardening mechanism indispensable to provide both of a toughness and a machinability at a hardness level of HRC 40. Thus, according to the present invention, there is provided a die steel which consists essentially of, by mass, 0.10 to 0.25% carbon, not more than 1.00% Si, not more than 2.00% Mn, 0.60 to 1.50% Ni, from more than 1.00 to not more than 2.50% Cr, at least one of Mo and W in the amount as defined by the equation of (Mo + (W/2)) bainite, and which has a hardness of HRC 34 to 45. Preferably, the die steel has a chemical composition (by mass%), which meets Equation 1: (%Ni) + 1.2(%Cu) = 1.30 to 2.70, and Equation 2: 60(%C) + 1.5(%Si) + 6(%Cr) + 2(%Mo and/or 1/2(%W)) + 20(%V) + 0.2(%Cu) = 21.00 to 28.70. Further, preferably, the content of at least one of Mo and W is defined by Mo + (W/2) = 0.10 to 1.00%. Furthermore, the sulfur content is in a range of 0.003 to 0.05% by mass. The invention die steel preferably meets one or more of the followings: not more than 0.05% by mass of Al; from more than 0.001 to not more than 0.005% by mass of oxygen; 0.60 to 1.20% by mass of Ni; 0.60 to 1.50% by mass of Cu; 0.13 to 0.20% by mass of carbon; and 1.40 to 2.20% by mass of Cr. According to the present invention, excellent properties of toughness, machinability and high hardness as well as both of excellent wear resistance can be obtained even if any free cutting element such as S (sulfur) is not added in a much amount, by the suitable combination of the components, particularly, by employing the precipitation hardening mechanism attributable to the optimization of the structure provided by mutually adjusting the Ni and Cu contents with relation to each other and the optimization of the contents of Cr, Mo (W/2), Cu and V. In addition, the segregation which is a problem in the case of production of a large-sized die can be decreased remarkably by optimization of the S (sulfur) content (the uniform dispersion of sulfides). Since the invention die steel is highly resistant to softening by the tempering treatment, even if the working surface of the die is subjected to a nitriding treatment, the hardness is less decreased. Additionally, since the die steel has the sufficient strength and wear resistance, it is highly effective when it is applied to a large-sized plastic molding die. Brief Description of the Several Views of the Drawings Fig. 1 is one example of a microscopic photograph of a typical metal structure of an invention die steel; Fig. 2 is one example of a microscopic photograph of a typical metal structure of a conventional die steel; Fig. 3 is one example of a microscopic photograph of a typical metal structure of a conventional die steel; and Fig. 4 is one example of a microscopic photograph of a typical metal structure of a conventional die steel. Detailed Description of the Invention One of key aspects of the invention is that a low carbon Mn-Ni-Cr-Mo (W)-V-Cu-Fe system alloy is used for the invention steel, which has a metal structure of a lower bainite phase obtained by optimally adjusting the relationship among the contents of the component elements in the steel, particularly, the relationship between the Ni and Cu contents. In addition, by virtue of employment of the precipitation hardening mechanism attributable to the optimal adjustment of contents of Cr, Mo (W/2), Cu and V, the steel has excellent properties of toughness, machinability and high hardness and also has both of excellent properties of polishability and wear resistance, even if any free cutting element such as sulfur is not added in a much amount. As mentioned above, the conventional low carbon Mn-Cr-Mo (W)-V-Cu-Fe system alloy steel, carbon Mn-Ni-Cr-Mo (W)-V-Cu-Fe system alloy steel, and low carbon Mn-Ni-Cr-Mo-Cu-Al system alloy steel have been adjusted to have the upper bainite in order to ensure the machinability. Although the upper bainite is excellent in machinability, it has a low in toughness. In order to hold the toughness, it has been necessary to control the hardness to about HRC 30. Therefore, the present inventors has employed a manner for adjusting the metal structure to have a bainite phase by a suitable combination of components, particularly, by mutually adjusting the Ni and Cu contents with relation to each other. In the case of the conventional die steel, while the metal structure thereof has been adjusted to have the upper bainite phase, and a strict management has been required at a heat treatment step in the manufacture in order to obtain the intended metal structure. Namely, there is a disadvantage that a fine control of cooling rate is essential, and an increased number of heat treatment steps is required. In the invention steel, however, since the chemical composition is appropriately adjusted, in the heat treatment step for achieving the intended lower bainite, the difficulty of management thereof is improved remarkably. More specifically, even if the invention steel after hot working is subjected to direct quenching of which cooling rate is higher than that of air cooling, the lower bainite is obtainable. In general, the bainite of the steel structure is one of transformation products obtainable when austenite is cooled, and is of a phase produced in 25 an intermediate temperature range between a pearlite forming temperature and a martensite forming temperature. As viewed microscopically, a phase produced at near the pearlite forming temperature has a feather-like form, and another phase produced at near the martensite forming temperature has an acicular form. The former is called an upper bainite, and the latter is called a lower bainite. The lower bainite defined in the present invention is, for example, specifically, a structure shown in Fig. 1. For comparison, a martensite (Fig. 2; a low carbon Mn-Ni-Cr-Mo (W)-Fe system alloy), an upper bainite (Fig. 3; a low carbon Mn-Ni-Cr-Mo (W)-V-Cu-Fe system alloy) and an upper bainite (Fig. 4; a low carbon Mn-Ni-Cr-Mo-Cu-Al-Fe system alloy) of conventional steels are shown. The term of "primary phase" of the lower bainite in the present invention is used taking account of occurrence of a nonuniform structure in a heat treatment (i.e. cooling) process due to a somewhat nonuniform composition and a somewhat nonuniform temperature. With regard to some quantities of the upper bainite and the martensitic which may be included in the lower bainite, also taking other inclusion factors into consideration, in the case of the present invention, if the lower bainite is ensured in a quantity of not less than 70% (by area) in a microscopic field of view (600 magnifications) in accordance with Fig. 1 (a microscopic photograph), there is no problem. Preferably, the quantity of the lower bainite is not less than 75% (by area), and more preferably not less than 80% (by area). Fig. 1 shows a steel having substantially the lower bainite of which quantity is controlled to be 80% (by area). Further, the present invention is aimed at the promotion of the uniform dispersion of sulfides by the proper suppression of the sulfur content and the addition of Cu. In the invention steel, the uniform lower bainite is produced by quenching. In addition, the steel is subjected to tempering treatment at a high temperature of not lower than 550°C to form a solid solution of Fe-Cu and precipitate carbides of Cr, Mo (W) and V so as to obtain a hardness of HRC 34 to 45. Further, by the tempering treatment, the precipitates are caused to agglomerate so as to provide the steel with a high strength while moderately embrittling the steel whereby providing an extremely good machinability to the steel matrix itself. Therefore, even if the content of S (sulfur) usually added in a large amount as a means for providing a free cutting property to a steel is limited to a small content level, excellent machinability is obtainable. In the invention steel, even if sulfur is added, the sulfur content can be limited to not more than 0.05%. Thus various problems such as the generation of pinholes upon welding and roughing of a discharge-processed surface due to the segregation of sulfides and further the deterioration of the polishability, the wear resistance and the roughing can be avoided, and excellent corrosion and rust resistances can be obtained in cooperation with the incorporation of Cr, Mo, W, Cu or Ni. As discussed above, the invention steel also has a feature in respect of that the sulfur content is reduced, from the viewpoint of the provision of the good machinability to the matrix itself. Mo and tungsten (W) may be incorporated alone or in combination in the invention steel, and in this case, the Mo content has an equivalent effect in combination of the content of W/2. Mo and tungsten (W) in the invention steel are important elements, since they increase the resistance to softening when quenching and tempering, and further dissolve into an Fe-Cr oxide film or a Cr oxide film on a surface of a die to form a solid solution whereby strengthening the oxide film to improve corrosion resistance property of the die. The invention steel is supplied in a prehardened state having a hardness of HRC 34 to 45 (in general, a tempered state by tempering at not lower than 550°C after quenching), and directly subjected to die forming working followed by finish-polishing thereafter provided to use. Namely, in the tempered state, the steel has a good machinability and an excellent polishability, and hence no heat treatment is required after the die forming working. In the invention steel, a reduced amount of free cutting elements such as sulfur is added, and thus it is unnecessary to concern about occurrence of significant segregation due to an increased size of a die made of steel. Therefore, even when the invention steel is applied to the production of not only a die for producing a smaller article but also a particularly large-sized die, for example, a die having a maximum length of one side on the order of 2000 mm, the invention steel exhibits a large effect and thus, a longer life can be provided without need for fear of wearing. The reason why the components in the invention steel are limited will be described below. Carbon (C) is a basic additive element required to maintain the quenched structure at the lower bainite having a good machinability and to bring about the hardening attributable to the precipitation of carbides of Cr, Mo (W) and V in the tempering treatment. If the carbon content is too large, the matrix is transformed into a martensite, resulting in a reduction in machinability, and excess carbides are formed, resulting in a reduction in machinability. Therefore, the carbon content is defined to be equal to or lower than 0.25%. On the other hand, if the carbon content is too small, the precipitation of ferrite is brought about and hence, the carbon content is defined to be equal to higher than 0.10%. Preferably, the carbon content is defined to be in a range of 0.13% to 0.20%. Si is an element for enhancing the corrosion resistance for an atmosphere provided at the time of service of the die. If the content of Si is too large, the production of ferrite is brought about and hence, the Si content is defined to be equal to or lower than 1.00%. If the Si content is reduced, the anisotropy of the mechanical properties is alleviated, and the stripe segregation is reduced to provide an excellent mirror finish workability. Therefore, the Si content is defined to be equal to or lower than 0.60%. To provide the corrosion resistance as described above, it is preferable to add silicon (Si) in an amount equal to or higher than 0.10%, more preferably in an amount equal to or higher than 0.20%. Mn is an element suitable to enhance the quenchability of the lower bainite of the invention steel, to inhibit the production of ferrite and to provide a moderate quenched and tempered hardness. However, if the content Mn is too large, the control of the heat treatment for maintaining the lower bainite is severe; the transformation into the martensite is promoted; and the toughness of the matrix is increased to reduce the machinability. Therefore, the Mn content is defined to be equal to or lower than 2.00%. To provide the quenchability, it is preferable to add Mn in an amount equal to or higher than 1.00% and more preferably in an amount equal to or higher than 1.20%. Cr is added to precipitate and agglomerate fine carbides in the tempering treatment to provide the invention steel with a high strength. Cr improves the corrosion resistance property of the invention steel and restrains occurrence of rust during the polishing treatment or during storage of the die. Further, when a nitriding treatment is carried out, chromium has an effect of increasing the hardness of a nitrided layer. However, if the Cr content is too large, the transformation into martensite is promoted from an action of finely dividing the lower bainite, and the toughness of the matrix is increased to reduce the machinability. If the Cr content is too small, the effect of the addition of Cr is not obtained. Therefore, the Cr content is defined to be in a range of from more than 1.00% to not more than 2.50%. Preferably, the Cr content is in a range of 1.40 to 2.20%, more preferably, in a range of 1.60 to 2.00%. To enhance the strength of the die, as described above, Cr may be added in a larger amount, but there is a limit for the addition of Cr, because the larger the Cr content, the more the machinability is reduced. Therefore, it is necessary to enhance the strength of the die by a technique which is not relied only on the addition of Cr. If it is considered that the die is subjected to a nitriding treatment and then put into use, it is necessary to guarantee the resistance to the softening in the tempering treatment at 550°C or more. In this point, only the addition of Cr does not suffice. Therefore, in the invention steel, the incorporation of Mo and W is important to solve the above problems. Mo and tungsten (W) are incorporated alone or in combination, from the viewpoint that they serve to precipitate and agglomerate fine carbides during the tempering treatment to enhance the strength of the invention steel and increase the resistance to the softening in the quenching and tempering treatment. Further, Mo and tungsten (W) also have an advantage of improving the corrosion resistance to a corrosive gas generated, for example, from a plastic during service of the die, because they are partially dissolved into an oxide film on the die surface to form a solid solution. In the case of this application, the steel need not to contain Mo and W in large contents. If the contents of Mo and W are too large, a reduction in machinability is brought about and hence, the contents of Mo and W in terms of Mo + 1/2(W) are defined to be not more than 1.00%. Preferably, they are in a range of 0.10 to 1.00%, more preferably, in a range of 0.10 to 0.70%. Vanadium (V) serves to increase the resistance to the softening in the tempering treatment and to inhibit the coalescing of crystal grains to contribute to an enhancement in toughness. Vanadium also has an effect of finely forming hard carbides to enhance the wear resistance. For this purpose, a vanadium content equal to or lager than 0.03% is required, but if the vanadium content is too large, a reduction in machinability is brought about. Therefore, the vanadium content is defined to be equal to or lower than 0.15%, and preferably, the vanadium content is in a range of 0.05 to 0.12%. Cu is suitable to precipitate and agglomerate 5 an Fe-Cu solid solution in the tempering treatment of the invention steel. What is to be specially mentioned is that the structure is controlled to the lower bainite by appropriate regulating the amounts of Cu and Ni (which will be described hereinafter) added. An excellent machinability is provided to the invention steel by cooperation of the precipitation and agglomeration of the solid solution and the control of the structure to the lower bainite with each other. Cu also has an effect of providing an excellent corrosion resistance, and it is important for the Cu content to be defined to be equal to or higher than 0.50%. If the Cu content is too large, the hot workability is reduced, and Cu acts to transform the structure into martensite, resulting in a more reduction in machinability. Therefore, the Cu content is defined to be equal to or lower than 2.00%, and preferably, the Cu content is in a range of 0.60 to 1.50%. Ni is an element for enhancing the quenchability of the lower bainite of the invention steel and for inhibiting the production of ferrite. As described above, Cu is also an element important to control the structure into the lower bainite by the appropriate adjustment of Ni and Cu, and in order to provide an excellent machinability to the invention steel, the content of Ni is defined to be equal to or higher than 0.60%. If the content of Ni is too large, the lower bainite is excessively finely divided; the transformation into martensite is promoted, and the toughness of the matrix is reduced to degrade the machinability. Therefore, the content of Ni is defined to be equal to or lower than 1.50%, and preferably, equal to or lower than 1.20%. Sulfur (S) has a sugnificant effect of enhancing the machinability by the presence of S in the form of non-metallic inclusion MnS. However, the presence of a large content of MnS causes not only effects during die-working such as the generation of pinholes during welding, the generation of pinholes during polishing and the roughing of the discharge-processed surface, but also provides a starting point for rusting, thereby causing the degradation of the nature of the die itself, such as the promotion of the anisotropy of the mechanical properties. Especially, in a large-sized die, the above adverse effect due to the segregation of MnS is significant. Therefore, to obtain the above effects, the preferable content of S is not less than 0.003%, but to inhibit these problems, it is necessary to limit the content of S to at most 0.05% or less. Al is usually used as a deoxidizer element during melting, but in the state of the invention steel, Al2Ci3 present in the steel degrades the mirror finish workability and hence, it is necessary to limit the content of Al to not more than 0.10%. Preferably, the content of Al is not more than 0.05%, more preferably, not more than 0.01%, further preferably, not more than 0.002%. Oxygen (0) is an element which forms oxides in the steel, thereby causing the cold plastic workability and the polishability to be deteriorated remarkably. Especially, in the present invention, an upper limit for the content of Al is defined as 0.005%, preferably as not more than 0.003%, from the viewpoint that it is important to inhibit the formation of the above-described A^Oa. For an enhancement in polishability, it is a desirable condition to limit and control the content of Al to a further low range, for example, to not more than 0.00.1%. In the present invention in which a reduction in content of Al2C>3 is aimed, however, in addition to Al already controlled to a lower content, even the control of the content of 0 itself to a low range is not desired particularly severely. Therefore, it is acceptable for the content of Al to exceed 0.001%. Nitrogen (N) is an element which forms nitrides in the steel. If the nitrides are formed in an excessive amount, the toughness of the die, the machinability and the polishability are considerably deteriorated. Therefore, it is preferable to limit the content of nitrogen (N) in the steel to a low level, and in the present invention, the content of nitrogen (N) is limited to not more than 0.06%. Desirably, the content of nitrogen (N) is limited to not more than 0.02%, more desirably, to not more than 0.005%. In order to ensure that the structure essentially comprising the lower bainite phase aimed by the present invention is realized in the invention steel and that the invention steel has both of the machinability and the toughness at high levels, effective further narrow ranges of components exist for this purpose, and the present invention is also characterized in that the above component ranges are defined clearly. More specifically, a further examination was made within the above mentioned ranges of basic components in the present invention, and consequently the inventors found that there are the narrow component ranges which meet the following equations, Equation 1: (%Ni) + 1.2 (%Cu) = 1.30 to 2.70, and Equation 2: 60 (%C) + 1.5 (%Si) + (%Ni) + 6 (%Cr) + 2 (%Mo + 1/2(%W) (alone or in combination)) + 20 (%V) + 0.2 (%Cu) = 21.00 to 28.70. More specifically, it is an aim for the invention steel to have the structure essentially comprising the lower bainite phase. The reason is that if the value given by Equation 1: (%Ni) +1.2 (%Cu) is smaller than 1.30, ferrite and upper bainite are liable to be produced, and if this value is larger than 2.70, the lower bainite finely divided and martensite are liable to be produced. Further, the reason is that even if the value given by the equation 1 satisfies 1.3 to 2.70, if the value given by the equation 2: 60 (%C) + 1.5 (%Si) + (%Ni) + 6 (%Cr) + 2 (%Mo + 1/2(%W) (alone or in combination)) + 20 (%V) + 0.2 (%Cu) is smaller than 21.00, the desired hardness is difficult to achieve, or the toughness is liable to be reduced, and if this value is larger than 28.70, the machinability is poor, or the toughness is liable to be reduced. In the present invention, any further toughness-improving element(s) and any machinability-improving element(s) can be added in a range which does not detract the above-described functional effects. For example, one or more of 0.5% or less (preferably 0.01 to 0.1%) of Nb, 0.15% or less of Ti, 0.15% or less of Zr and 0.15% or less of Ta can be added as the toughness-improving element(s). One or more of 0.003 to 0.2% of Zr, 0.0005 to 0.01% of Ca, 0.03 to 0.2% of Pb, 0.03 to 0.2% of Se, 0.01 to 0.15% of Te, 0.01 to 0.2% of Bi, 0.005 to 0.5% of In and 0.01 to 0.1% of Ce can be added as the machinability-improving element(s). Further, Y, La, Nd, Sm and other REM elements can be also incorporated in a total content of 0.0005 to 0.3%. First, various steels shown in Table 1 and having compositions containing the balance of iron and unavoidable impurities were melted with a high frequency vacuum melting furnace of 30 kg (values represented by the equations 1 and 2 of the present invention are shown in Table 2). Then, each of these steels was forged to a square bar having a size of 50 mm x 100 mm, which was subjected to a heat treatment and then put into the following evaluations. The heat treatment was carried out in the following manner so that a predetermined hardness was obtained: Each of the steels was heated to an austenite region of 900°C and kept at this temperature for 1 hour and then cooled at a cooling rate suitable for a practical ingot supposed. Thereafter, the resulting steel was tempered 1 hour at an appropriate temperature between 520°C and 590°C and air-cooled. Table I (mass%' Specimen No. C Si Mn P s Ni Cr W Mo V Cu hi N 0 Remarks 1 0.11 0.29 1.05 0.011 0.030 0.65 2.47 - 0.25 0.14 1.68 0.0800 0.0030 0.0021 Invention steel 2 0.15 0.47 1.87 0.011 0.015 0.82 1.85 - 0.35 0.07 1.23 0.0050 0.0020 0.0012 3 0.16 0.25 1.45 0.010 0.007 0.99 1.82 - 0.51 0.07 0.76 0.0010 0.0050 0.0018 4 0.20 0.26 1.42 0.007 0.015 1.21 1.83 - 0.31 0.08 1.05 0.0020 0.0060 0.0019 5 0.17 0.67 1.63 0.008 0.030 1.47 1.05 - 0.67 0.04 0.58 0.0400 0.0180 0.0021 6 0.14 0.42 1.32 0.007 0.022 1.04 1.81 - 0.37 0.09 0.70 0.0050 0.0078 0.0018 7 0.16 0.51 1.56 0.010 0.019 1.03 2.20 0.82 0.27 0.05 0.52 0.0910 0.0280 0.0024 8 0.22 0.32 0.85 0.012 0.025 1.05 1.84 - 0.41 0.07 0.52 0.0450 0.0025 0.0011 9 0.11 0.44 0.49 0.008 0.003 0.78 1.79 - 0.55 0.09 0.73 0.0088 0.0145 0.0025 10 0.16 0.30 0.99 0.007 0.012 0.76 2.15 - 0.62 0.04 0.51 0.0018 0.0087 0.0014 11 0.24 0.38 1.53 0.008 0.019 0.74 1.86 0.91 - 0.04 0.54 0.0945 0.0045 0.0011 12 0.16 0.25 1.95 0.009 0.035 1.08 1.88 0.22 0.36 0.12 0.77 0.0070 0.0121 0.0011 13 0.06 0.42 0.99 0.009 0.04 0.65 1.12 - 0.33 0.05 0.52 0.0018 0.0015 0.0012 Comparative steel 14 0.21 1.49 1.53 0.012 0.03 0.78 1.89 - 0.81 0.03 0.67 0.0840 0.0158 0.0015 15 0.23 0.47 1.08 0.006 0.08 1.20 1.78 - 0.62 0.09 1.20 0.0044 0.0046 0.0024 16 0.14 0.30 1.78 0.008 0.045 0.24 1.02 - 0.55 0.04 0.64 0.0068 0.0260 0.0018 17 0.18 0.42 1.85 0.006 0.022 1.84 1.54 - 0.27 0.05 0.95 0.0640 0.0119 0.0012 18 0.20 0.33 1.53 0.009 0.019 0.78 0.37 0.72 0.27 0.13 0.66 0.0040 0.0086 0.0015 19 0.12 0.41 1.41 0.012 0.04 0.98 3.44 - 0.36 0.07 0.87 0.0870 0.0715 0.0024 20 0.15 0.47 0.99 0.011 0.05 1.23 2.39 - 1.78 0.04 1.08 0.0791 0.0187 0.0072 21 0.11 0.31 1.34 0.008 0.035 0.84 1.86 - 0.48 0.01 0.91 0.0624 0.0056 0.0016 22 0.24 0.37 1.32 0.007 0.038 1.34 1.23 - 0.62 0.29 1.38 0.0018 0.0078 0.0020 23 0.15 0.32 0.85 0.009 0.015 0.69 1.31 - 0.41 0.04 2.42 0.0120 0.0103 0.0026 24 0.12 0.49 1.005 0.008 0.031 1.04 1.78 0.24 0.37 0.07 0.87 0.2200 0.0162 0.0013 25 0.38 0.33 1.3 0.006 0.012 0.91 1.70 - 0.16 - - 0.040 0.0089 0.0058 Conventional steel 26 0.25 0.12 1.1 0.016 0.018 0.92 1.93 - 0.64 0.08 0.29 .0.062 0.0105 0.0007 27 0.16 0.15 1.8 0.012 0.011 - 1.78 - 0.24 0.08 0.65 0.038 0.0094 0.0032 28 0.16 0.16 1.6 0.006 0.027 0.11 1.82 - 0.16 0.08 0.26 0.001 0.0098 0.0035 29 0.13 0.28 1.4 0.011 0.004 3.15 0.16 - 0.25 0.01 1.01 1.100 0.0080 0.0052 "Note: The hyphen (-) means "less than 0.01%' The evaluation of the machinability was made by carrying out a drilling test. More specifically, 50 holes were drilled in the specimen steel by a drill of (j)2mm made of a high-speed steel under conditions: a cutting rate of 15 m/min, a feed rate of 120 mm/min and a drilling depth of 20 mm, and thereafter, a maximum wear width of an edge of an outer peripheral surface of the tool was measured. The evaluation of the toughness was made by carrying out a Charpy impact test using a U-notch test piece (a test piece according to JIS No. 3) of 2 mm, wherein a Charpy impact value at room temperature was measured. The evaluation of the polishability was made in the following manner: Specimens each having an evaluation surface of 50 mm squares were taken and then subjected to a quenching and tempering treatment under the same conditions as the above-described conditions for the heat treatment, whereby the hardness was adjusted, and thereafter, each of the specimens was subjected to a mirror finishing by a system of grinder—»paper-»diamond compound. Then, the number of fine pits generated was counted using a magnifier (of 10 magnifications) and in this case, the specimens having less than 6 pits generated therein were indicated by A; the specimens having 6 to 10 pits generated therein were indicated by B; the specimens having 11 to 20 pits generated therein were indicated by C; and the specimens having more than 20 pits generated therein were indicated by D. These results are given in Table 2. Each of the structures of the specimens shown in Table 2, excluding those of Specimen Nos. 16 to 18, was a substantially single-phase structure, whose phase shown in Table 2 occupied 90% or more by area. Table 2 Equation 1: Equation 2: Specimen No. Hardness (HRC) Structure Wear width (mm) Toughness (J/cm2) Polishing property Value of Equation 1 Value of Equation 2 Whether Equations 1 and 2 are satisfied Remarks 1 40.6 lower bainite 0.15 64 B 2.67 26.14 Yes Invention steel 2 40.7 ditto 0.14 64 B 2.30 23.97 Yes 3 40.0 ditto 0.14 76 B 1.90 24.46 Yes 4 38.7 ditto 0.16 66 B 2.47 27.01 Yes 5 39.4 ditto 0.15 69 B 2.17 21.23 Yes 6 40.2 ditto 0.15 67 B 1.88 23.61 Yes 7 41.2 ditto 0.14 62 B 1.65 27.06 Yes 8 37.9 ditto 0.12 78 B 1.67 28.09 Yes 9 38.6 ditto 0.13 77 B 1.66 21.83 Yes 10 40.6 ditto 0.15 64 B 1.37 25.85 Yes 11 39.9 ditto 0.14 73 B 1.39 28.70 Yes 12 40.3 ditto 0.16 67 B 2.00 25. 61 Yes 13 32.8 upper bainite 0.17 78 C 1.27 13.36 No Comparative steel 14 39.8 f errite 0.19 43 C 1.58 29.31 No 15 40.1 lower bainite 0.12 36 D 2.64 29.67 No 16 39.1 upper bainite + ferrite 0.15 30 C 1.00 17.24 No 17 38.8 lower bainite + martensite 0.22 52 B 2.98 24.24 No 18 40.6 upper bainite + ferrite 0.18 32 C 1.52 19.49 No 19 41.2 martensite 0.31 54 D 2.02 31.73 No 20 41.3 lower bainite 0.23 44 D 2.53 29.85 No 21 32.8 ditto 0.18 70 C 1.93 20.41 No 22 38.4 ditto 0.22 50 B 3.00 30.99 No 23 39.5 ditto 0.19 34 B 3.59 20.13 No 24 39.7 ditto 0.17 31 C 2.08 22.21 Yes 25 40.1 martensite 0.33 42 B 0.91 34.73 No Conventional steel 26 39.8 upper bainite 0.45 40 B 1.27 30.62 No 27 40.3 ditto 0.28 41 B 0.78 22.72 No 28 32.2 ditto 0.18 82 C 0.42 22.84 No 29 40.6 ditto 0.12 16 A 4.36 13.23 No +(%Ni)+6(%Cr)+2(%Mo+l/2(%W)(at least one of Mo and W))+20(%V)+0.2(%Cu) Invention steel Each of Specimen Nos. 1 to 12 satisfying the chemical composition of the present invention is a die steel essentially comprising a substantially single-phase of lower bainite and satisfying the hardness of the present invention, because the values represented by the equations 1 and 2 also satisfy a preferred defined range of the present invention. The machinability of each of these specimens shows a best result in which the maximum wear width of the edge of the outer peripheral surface of the tool is equal to or smaller than 0.16 mm. Further, the toughness shows a very good result of not less than 60 J/cm2, and the polishability is good. Comparative steel Specimen No. 13 containing carbon (C) smaller than the constituent range of the present invention has an upper bainite, because the value given by the equation I is less than 1.30, and this specimen has a good machinability and a good toughness because of a lower hardness, but on the other hand, its polishability is not necessarily sufficient. Specimen No. 14 containing a larger content of Si, Specimen No. 16 containing a smaller content of Ni and Specimen No. 18 containing a smaller content of Cr satisfy the hardness of the present invention, because the values represented by the equations 1 and 2 satisfy the preferred defined range of the present invention, but their machinability and polishability are slightly poor, because the structure is a ferrite, or an upper bainite having less than 5% by area of ferrite mixed therein. Specimen No. 15 containing sulfur in a content exceeding 0.05% has a lower bainite having a satisfactory hardness defined in the present invention, and has a best machinability, because sulfide-based non-metallic inclusions are contained in a large content in this specimen. On the other hand, however, the toughness is poor, because the value given by the equation 2 is larger than 28.70. The sulfides are very soft as compared with the matrix, and during polishing, pits are liable to be generated from the sulfides, resulting in a poor polishability. Specimen No. 17 containing Ni in a large content has a structure in which martensite (about 60% by area) is mixed in a lower bainite excessively finely divided, because the value given by the equation 1 is larger than 2.70. Thus, this specimen is good in polishability and also good in toughness to a certain extent, but is slightly poor in machinability. Specimen No. 19 containing Cr in a content smaller than the constituent range of the present invention and nitrogen (N) in a larger content has an excessively fine martensite. Specimen No. 20 containing Mo in a large content and oxygen (0) in a large content has a single-phase substantially lower bainite. However, both of the specimens are slightly poor in machinability, because the value given by the equation 2 is larger than 28.70 and the structure contains a large content of carbides. Further, these specimens are considerably poor in polishability, because Specimen No. 19 contains an excess content of nitrides, and Specimen No. 20 contains an excess content of oxides. Each of Specimen Nos. 21 to 24 has a single-phase substantially lower bainite. However, Specimen No. 21 containing vanadium in a content smaller than the constituent range of the present invention has a slightly low hardness, because the value given by the equation 2 is less than 21.00; and for this reason, this specimen is good in machinability, but slightly poor in polishability. On the other hand, Specimen No. 22 containing vanadium (V) in a large content is slightly poor in machinability, because the values given by the equations 1 and 2 do not satisfy the preferred defined ranges of the present invention and because this specimen contains carbides in a large content. In Specimen No. 23 containing Cu in a large content, the value given by the equation 1 is larger than 2.70, and the micro-structure keeps the lower bainite excessively finely divided, but the machinability is slightly poor. In Specimen No. 24 containing Al in a large content, the value given by the equation 2 satisfies the preferred defined range of the present invention, but the hardening due to the precipitation of the metallic compound of Al and Ni, resulting in a reduction in toughness. In addition, there is a tendency for nitrides to be contained in a large content, and the polishability is slightly poor. Conventional steels No one of the conventional steel specimen Nos.25 and 29 satisfies the preferred constituent ranges represented by the equations 1 and 2 of the present invention, but in Specimen No. 25 containing carbon (C) in a larger content and vanadium (V) and Cu in smaller contents than the constituent ranges constituting the basic concept of the present invention, the hardness defined in the present invention is satisfied, and because this specimen has the martensitic micro-structure, the toughness and the polishability are good, but the machinability is poor. Each of Specimen No. 26 containing Cu in a small content and Specimen No. 27 containing Ni in a small content satisfies the hardness defined in the present invention and has a good polishability, but is slightly poor in toughness and poor in machinability, because it has an upper bainite. Specimen No. 28 containing Ni and Cu in contents smaller than the constituent range of the present invention has an upper bainite, and is good in machinability and toughness because of its low hardness. On the other hand, however, the polishability of this specimen is not necessarily sufficient. Specimen No. 29 containing Ni and Al in larger contents and Cr in a smaller content has an upper bainite, whose hardness defined in the present invention is satisfied. In addition, this specimen has best machinability and polishability in virtue of the employment of both of the hardening attributable to the precipitation of an intermetallic compound of Al and Ni and the hardening attributable to the precipitation of Cu, but on the other hand, the toughness of this specimen is considerably poor. Industrial Applicability The invention steel having an excellent toughness, which is not found in the conventional plastic-molding prehardened steel, is particularly suitable for production of a die for carrying out a more precise die-working, because the cracking is difficult to occur due to a thermal stress caused with the working or processing in a die. Claim: 1. A die steel which consists essentially of, by mass, 0.10 to 0.25% carbon, not more than 1.00% Si, not more than 2.00% Mn, 0.60 to 1.50% Ni, from more than 1.00 to not more than 2.50% Cr, at least one of Mo and W in the amount as defined by the equation of (Mo + (W/2)) not more than 0.05% S (sulphur), not more than 0.10% Al, not more than 0.06% N (nitrogen), not more than 0.005% O (oxygen), and the balance of Fe and unavoidable impurities, which has a metal structure having a primary phase of lower bainite, and which has a hardness of HRC 34 to 45. 2. A die steel as claimed in claim 1, which chemical composition meets the following equations (by mass%): (1) %Ni + 1.2 (%Cu) = 1.30 to 2.70, and (2) 60 (%C) + 1.5 (%Si) + %Ni + 6(%Cr) + 2(%Mo+l/2(%W)) + 20(%V) +0.2(%Cu) = 21.00 to 28.70 3. A die steel as claimed in claim 1, wherein the total amount of at least one of Mo and W is defined by Mo + (W/2) = 0.10 to 1.00% by mass%. 4. A die steel as claimed in claim 1, wherein the sulphur amount is 0.003 to 0.05% by mass. 5. A die steel as claimed in claim 1, wherein the Al amount is not more than 0.05% by mass and the oxygen amount is from more than 0.001 to not more than 0.005% by mass. 6. A die steel as claimed in claim 1, wherein the Ni amount is 0.60 to 1.20% by mass and the Cu amount is 0.60 to 1.50%) by mass. 7. A die steel as claimed in claim 1, wherein the carbon amount is 0.13 to 0.20% by mass and the Cr amount is 1.40 to 2.20%) by mass.

Full Text

Technical Background
The present invention relates to a die steel which is excellent in toughness, machinability, polishability and wear resistance, which has high hardness, and which is used mainly for production of a plastic molding die.
The following characteristics are required for a die steel used for production of a plastic molding die:
(1) a good mirror finish workability and a small
tendency to occur with pinholes and very fine pits;
(2) a good satinizing workability;
(3) good corrosion and rust resistances;
(4) good strength, wear resistance and toughness;
and
(5) a good machinability.
A medium C-Mn-Cr-Mo-Fe system steel such as JIS SCM440 has been conventionally used as a die steel for plastic molding, but the above mentioned requirements have been increased, and among others, in recent years, a reduction in manufacture delivery time has been arisen strongly as an important customer's requirement. For the purpose of reducing the number of processing steps to meet such a requirement, various steels are commonly used, for example, such as a low

carbon Mn-Cr-Mo-S-Fe system steel (see JP-A-48-093518), a modified steel of this (i.e. JP-A-48-093518) into which Ni is added in order to improve quenching ability (see JP-A-52-065557), and a sulfur-free and Cu containing steel improved in machinability (see JP-A-58-067850 or JP-A-60-204869).
There has been also known a low carbon, low sulfur and low Al Mn-Cr-Mo system steel which is improved in the properties of mirror finish workability, the machinability and the weldability (see JP-A-03-115523). Further, there have been proposed a low carbon Mn-Cr-Mo (W)-V-Cu-Fe alloy and a low carbon Mn-Ni-Cr-Mo (W)-V-Cu-Fe alloy (see JP-A-07-062491) which have a suitable combination of a matrix structure and precipitates, and are subjected to a heat treatment to have an adjusted hardness of about HRC 30 so that they can have excellent machinability without a much amount of additive free cutting elements, and they have excellent properties of anti-rust, wear resistance and polishability.
In recent years, there has been broadly used an engineering plastic material added with a glass fiber, a carbon fiber or the like in order to improve strength, wear resistance property and heat resistance property. With this, a die for molding a plastic material is strictly required to have desirable properties of wear resistance, toughness and machinability. Thus, a low carbon Mn-Ni-Cr-Mo-Cu-Al

system steel (see JP-A-02-182860) has been proposed, in which Cu is finely precipitated so as to have a hardness level of HRC 40 while having excellent properties of toughness and machinability. Further, there has been proposed a low carbon Mn-Ni-(Mo, W)-Cu-Al system prehardened steel for plastic molding (see JP-A-07-278737), which is adjusted to have a lower bainite having a higher toughness in order to improve the die life.
Brief Summary of the Invention
The above mentioned steels for plastic
molding, which are the low carbon Mn-Cr-Mo-S-Fe system steel and the low carbon Mn-Ni-Cr-Mo-S-Fe system steel, have a problem that a satisfactory die life can not be obtained, since when a large-sized die having a maximum length of about 2 meter is formed from such a steel, the properties of polishability, wear resistance and toughness are deteriorated due to segregation of sulfides and so on. The low carbon and free sulfur Mn-Cr-Cu-Fe system steel of JP-A-58-067850 has low softening resistance property to quenching and tempering treatment, so that the steel is deteriorated in hardness when it is subjected to a nitriding treatment at around 550°C. The low carbon Mn-Ni-Cr-Mo (W/2)-Cu-Fe system steel of JP-A-48-093518 is not necessarily satisfactory, because an enough precipitation hardening effect can not be obtained due

to a low carbon content.
In the low carbon, low sulfur, low Al and low oxygen Mn-Cr-Mo- system steel of JP-A-03-115523, since the machinability is improved by lowering the hardness of the base material to about HRC 30, the steel has a low hardness, and unsatisfactory properties of polishability and wear resistance while the mirror finish workability is improved by restraining occurrence of non-metallic inclusions. The low carbon Mn-Cr-Mo (W)-V-Cu-Fe steel or the low carbon Mn-Ni-Cr-Mo (W)-V-Cu-Fe steel of JP-A-07-062491 ensures that even when a large-sized die is formed from such a steel, the above requirements for the die for plastic molding are satisfied, and the steels have high strength and excellent machinability by enough precipitation hardening by virtue of Cr, Mo (W/2), Cu and V. However, the properties of polishability and wear resistance have become to be not necessarily satisfactory as the engineering plastic material is broadly used.
Further, the low carbon Mn-Ni-Cr-Mo-Cu-Al system steel of JP-A-02-182860 is provided with a hardness level of HRC 40 by virtue of a precipitated intermetallic compound of Al and Ni, and has an adjusted uniform upper bainite by making the carbon content low to have excellent machinability, while the steel has a problem of insufficient toughness.
The low carbon Mn-Ni-(Mo, W)-Cu-Al system

prehardened steel for plastic molding, which is disclosed in JP-A-07-278737, and which has both of the machinability and toughness provided by the preparation thereof into the lower bainite rather than the upper bainite which has been considered to be a structure indispensable to enhance the machinability. Recently, however, a reduction in service cost of a die is extremely highly required, and thus it is necessary to prolong a die life, while maintaining the performance of the conventional die, so that the toughness of the die has not been necessarily satisfactory.
With the above-described requirements in view, it is an object of the present invention to provide a die steel used mainly for plastic molding, which has excellent properties of toughness and machinability whereby being capable of achieving a prolongation of the life of a tool such as a die, while maintaining the performance of the conventional die, and which has both of excellent properties of polishability and wear resistance.
The present inventors have made a detailed examination of the relationship among the chemical composition and the metal structure, and the properties of machinability, toughness and polishability, and consequently found a die steel optimal for use mainly in plastic molding, which has excellent properties of toughness, machinability and high hardness and also has both of excellent properties of polishability and wear

resistance by selecting a suitable combination of components, and in particularly, by
(1) making the steel to have a lower bainite
rather than an upper bainite which has been considered
conventionally to be indispensable in order to improve
the machinability by mutually adjusting the Ni and Cu
contents with relation to each other; and
(2) employing the precipitation hardening
mechanism obtainable by an optimal content adjustment
of Cr, Mo (W/2), Cu and V, rather than employing the
hardening mechanism attributable to the precipitation
of the intermetallic compound of Al and Ni and the Cu-
precipitation hardening, both of which have been
considered to be a hardening mechanism indispensable to
provide both of a toughness and a machinability at a
hardness level of HRC 40.
Thus, according to the present invention, there is provided a die steel which consists essentially of, by mass, 0.10 to 0.25% carbon, not more than 1.00% Si, not more than 2.00% Mn, 0.60 to 1.50% Ni, from more than 1.00 to not more than 2.50% Cr, at least one of Mo and W in the amount as defined by the equation of (Mo + (W/2))
bainite, and which has a hardness of HRC 34 to 45.
Preferably, the die steel has a chemical composition (by mass%), which meets
Equation 1: (%Ni) + 1.2(%Cu) = 1.30 to 2.70, and
Equation 2: 60(%C) + 1.5(%Si) + 6(%Cr) + 2(%Mo and/or 1/2(%W)) + 20(%V) + 0.2(%Cu) = 21.00 to 28.70.
Further, preferably, the content of at least one of Mo and W is defined by Mo + (W/2) = 0.10 to 1.00%.
Furthermore, the sulfur content is in a range of 0.003 to 0.05% by mass.
The invention die steel preferably meets one or more of the followings:
not more than 0.05% by mass of Al;
from more than 0.001 to not more than 0.005% by mass of oxygen;
0.60 to 1.20% by mass of Ni;
0.60 to 1.50% by mass of Cu;
0.13 to 0.20% by mass of carbon; and
1.40 to 2.20% by mass of Cr.
According to the present invention, excellent properties of toughness, machinability and high hardness as well as both of excellent wear resistance can be obtained even if any free cutting element such as S (sulfur) is not added in a much amount, by the suitable combination of the components, particularly,

by employing the precipitation hardening mechanism attributable to the optimization of the structure provided by mutually adjusting the Ni and Cu contents with relation to each other and the optimization of the contents of Cr, Mo (W/2), Cu and V. In addition, the segregation which is a problem in the case of production of a large-sized die can be decreased remarkably by optimization of the S (sulfur) content (the uniform dispersion of sulfides). Since the invention die steel is highly resistant to softening by the tempering treatment, even if the working surface of the die is subjected to a nitriding treatment, the hardness is less decreased. Additionally, since the die steel has the sufficient strength and wear resistance, it is highly effective when it is applied to a large-sized plastic molding die.
Brief Description of the Several Views of the Drawings
Fig. 1 is one example of a microscopic
photograph of a typical metal structure of an invention die steel;
Fig. 2 is one example of a microscopic photograph of a typical metal structure of a conventional die steel;
Fig. 3 is one example of a microscopic photograph of a typical metal structure of a conventional die steel; and
Fig. 4 is one example of a microscopic

photograph of a typical metal structure of a conventional die steel.
Detailed Description of the Invention
One of key aspects of the invention is that a low carbon Mn-Ni-Cr-Mo (W)-V-Cu-Fe system alloy is used for the invention steel, which has a metal structure of a lower bainite phase obtained by optimally adjusting the relationship among the contents of the component elements in the steel, particularly, the relationship between the Ni and Cu contents. In addition, by virtue of employment of the precipitation hardening mechanism attributable to the optimal adjustment of contents of Cr, Mo (W/2), Cu and V, the steel has excellent properties of toughness, machinability and high hardness and also has both of excellent properties of polishability and wear resistance, even if any free cutting element such as sulfur is not added in a much amount.
As mentioned above, the conventional low carbon Mn-Cr-Mo (W)-V-Cu-Fe system alloy steel, carbon Mn-Ni-Cr-Mo (W)-V-Cu-Fe system alloy steel, and low carbon Mn-Ni-Cr-Mo-Cu-Al system alloy steel have been adjusted to have the upper bainite in order to ensure the machinability. Although the upper bainite is excellent in machinability, it has a low in toughness. In order to hold the toughness, it has been necessary to control the hardness to about HRC 30. Therefore,

the present inventors has employed a manner for adjusting the metal structure to have a bainite phase by a suitable combination of components, particularly, by mutually adjusting the Ni and Cu contents with relation to each other.
In the case of the conventional die steel, while the metal structure thereof has been adjusted to have the upper bainite phase, and a strict management has been required at a heat treatment step in the manufacture in order to obtain the intended metal structure. Namely, there is a disadvantage that a fine control of cooling rate is essential, and an increased number of heat treatment steps is required. In the invention steel, however, since the chemical composition is appropriately adjusted, in the heat treatment step for achieving the intended lower bainite, the difficulty of management thereof is improved remarkably. More specifically, even if the invention steel after hot working is subjected to direct quenching of which cooling rate is higher than that of air cooling, the lower bainite is obtainable.
In general, the bainite of the steel
structure is one of transformation products obtainable when austenite is cooled, and is of a phase produced in 25 an intermediate temperature range between a pearlite forming temperature and a martensite forming temperature. As viewed microscopically, a phase produced at near the pearlite forming temperature has a

feather-like form, and another phase produced at near the martensite forming temperature has an acicular form. The former is called an upper bainite, and the latter is called a lower bainite. The lower bainite defined in the present invention is, for example, specifically, a structure shown in Fig. 1. For comparison, a martensite (Fig. 2; a low carbon Mn-Ni-Cr-Mo (W)-Fe system alloy), an upper bainite (Fig. 3; a low carbon Mn-Ni-Cr-Mo (W)-V-Cu-Fe system alloy) and an upper bainite (Fig. 4; a low carbon Mn-Ni-Cr-Mo-Cu-Al-Fe system alloy) of conventional steels are shown. The term of "primary phase" of the lower bainite in the present invention is used taking account of occurrence of a nonuniform structure in a heat treatment (i.e. cooling) process due to a somewhat nonuniform composition and a somewhat nonuniform temperature. With regard to some quantities of the upper bainite and the martensitic which may be included in the lower bainite, also taking other inclusion factors into consideration, in the case of the present invention, if the lower bainite is ensured in a quantity of not less than 70% (by area) in a microscopic field of view (600 magnifications) in accordance with Fig. 1 (a microscopic photograph), there is no problem. Preferably, the quantity of the lower bainite is not less than 75% (by area), and more preferably not less than 80% (by area). Fig. 1 shows a steel having substantially the lower bainite of which

quantity is controlled to be 80% (by area).
Further, the present invention is aimed at the promotion of the uniform dispersion of sulfides by the proper suppression of the sulfur content and the addition of Cu. In the invention steel, the uniform lower bainite is produced by quenching. In addition, the steel is subjected to tempering treatment at a high temperature of not lower than 550°C to form a solid solution of Fe-Cu and precipitate carbides of Cr, Mo (W) and V so as to obtain a hardness of HRC 34 to 45. Further, by the tempering treatment, the precipitates are caused to agglomerate so as to provide the steel with a high strength while moderately embrittling the steel whereby providing an extremely good machinability to the steel matrix itself. Therefore, even if the content of S (sulfur) usually added in a large amount as a means for providing a free cutting property to a steel is limited to a small content level, excellent machinability is obtainable. In the invention steel, even if sulfur is added, the sulfur content can be limited to not more than 0.05%. Thus various problems such as the generation of pinholes upon welding and roughing of a discharge-processed surface due to the segregation of sulfides and further the deterioration of the polishability, the wear resistance and the roughing can be avoided, and excellent corrosion and rust resistances can be obtained in cooperation with the incorporation of Cr, Mo, W, Cu or Ni.

As discussed above, the invention steel also has a feature in respect of that the sulfur content is reduced, from the viewpoint of the provision of the good machinability to the matrix itself. Mo and tungsten (W) may be incorporated alone or in combination in the invention steel, and in this case, the Mo content has an equivalent effect in combination of the content of W/2. Mo and tungsten (W) in the invention steel are important elements, since they increase the resistance to softening when quenching and tempering, and further dissolve into an Fe-Cr oxide film or a Cr oxide film on a surface of a die to form a solid solution whereby strengthening the oxide film to improve corrosion resistance property of the die.
The invention steel is supplied in a
prehardened state having a hardness of HRC 34 to 45 (in general, a tempered state by tempering at not lower than 550°C after quenching), and directly subjected to die forming working followed by finish-polishing thereafter provided to use. Namely, in the tempered state, the steel has a good machinability and an excellent polishability, and hence no heat treatment is required after the die forming working. In the invention steel, a reduced amount of free cutting elements such as sulfur is added, and thus it is unnecessary to concern about occurrence of significant segregation due to an increased size of a die made of steel. Therefore, even when the invention steel is

applied to the production of not only a die for producing a smaller article but also a particularly large-sized die, for example, a die having a maximum length of one side on the order of 2000 mm, the invention steel exhibits a large effect and thus, a longer life can be provided without need for fear of wearing.
The reason why the components in the invention steel are limited will be described below.
Carbon (C) is a basic additive element required to maintain the quenched structure at the lower bainite having a good machinability and to bring about the hardening attributable to the precipitation of carbides of Cr, Mo (W) and V in the tempering treatment. If the carbon content is too large, the matrix is transformed into a martensite, resulting in a reduction in machinability, and excess carbides are formed, resulting in a reduction in machinability. Therefore, the carbon content is defined to be equal to or lower than 0.25%. On the other hand, if the carbon content is too small, the precipitation of ferrite is brought about and hence, the carbon content is defined to be equal to higher than 0.10%. Preferably, the carbon content is defined to be in a range of 0.13% to 0.20%.
Si is an element for enhancing the corrosion resistance for an atmosphere provided at the time of service of the die. If the content of Si is too large,

the production of ferrite is brought about and hence, the Si content is defined to be equal to or lower than 1.00%. If the Si content is reduced, the anisotropy of the mechanical properties is alleviated, and the stripe segregation is reduced to provide an excellent mirror finish workability. Therefore, the Si content is defined to be equal to or lower than 0.60%. To provide the corrosion resistance as described above, it is preferable to add silicon (Si) in an amount equal to or higher than 0.10%, more preferably in an amount equal to or higher than 0.20%.
Mn is an element suitable to enhance the quenchability of the lower bainite of the invention steel, to inhibit the production of ferrite and to provide a moderate quenched and tempered hardness. However, if the content Mn is too large, the control of the heat treatment for maintaining the lower bainite is severe; the transformation into the martensite is promoted; and the toughness of the matrix is increased to reduce the machinability. Therefore, the Mn content is defined to be equal to or lower than 2.00%. To provide the quenchability, it is preferable to add Mn in an amount equal to or higher than 1.00% and more preferably in an amount equal to or higher than 1.20%.
Cr is added to precipitate and agglomerate fine carbides in the tempering treatment to provide the invention steel with a high strength. Cr improves the corrosion resistance property of the invention steel

and restrains occurrence of rust during the polishing treatment or during storage of the die. Further, when a nitriding treatment is carried out, chromium has an effect of increasing the hardness of a nitrided layer. However, if the Cr content is too large, the transformation into martensite is promoted from an action of finely dividing the lower bainite, and the toughness of the matrix is increased to reduce the machinability. If the Cr content is too small, the effect of the addition of Cr is not obtained. Therefore, the Cr content is defined to be in a range of from more than 1.00% to not more than 2.50%. Preferably, the Cr content is in a range of 1.40 to 2.20%, more preferably, in a range of 1.60 to 2.00%.
To enhance the strength of the die, as described above, Cr may be added in a larger amount, but there is a limit for the addition of Cr, because the larger the Cr content, the more the machinability is reduced. Therefore, it is necessary to enhance the strength of the die by a technique which is not relied only on the addition of Cr. If it is considered that the die is subjected to a nitriding treatment and then put into use, it is necessary to guarantee the resistance to the softening in the tempering treatment at 550°C or more. In this point, only the addition of Cr does not suffice. Therefore, in the invention steel, the incorporation of Mo and W is important to solve the above problems.

Mo and tungsten (W) are incorporated alone or in combination, from the viewpoint that they serve to precipitate and agglomerate fine carbides during the tempering treatment to enhance the strength of the invention steel and increase the resistance to the softening in the quenching and tempering treatment. Further, Mo and tungsten (W) also have an advantage of improving the corrosion resistance to a corrosive gas generated, for example, from a plastic during service of the die, because they are partially dissolved into an oxide film on the die surface to form a solid solution. In the case of this application, the steel need not to contain Mo and W in large contents. If the contents of Mo and W are too large, a reduction in machinability is brought about and hence, the contents of Mo and W in terms of Mo + 1/2(W) are defined to be not more than 1.00%. Preferably, they are in a range of 0.10 to 1.00%, more preferably, in a range of 0.10 to 0.70%.
Vanadium (V) serves to increase the
resistance to the softening in the tempering treatment and to inhibit the coalescing of crystal grains to contribute to an enhancement in toughness. Vanadium also has an effect of finely forming hard carbides to enhance the wear resistance. For this purpose, a vanadium content equal to or lager than 0.03% is required, but if the vanadium content is too large, a reduction in machinability is brought about.

Therefore, the vanadium content is defined to be equal to or lower than 0.15%, and preferably, the vanadium content is in a range of 0.05 to 0.12%.
Cu is suitable to precipitate and agglomerate 5 an Fe-Cu solid solution in the tempering treatment of the invention steel. What is to be specially mentioned is that the structure is controlled to the lower bainite by appropriate regulating the amounts of Cu and Ni (which will be described hereinafter) added. An excellent machinability is provided to the invention steel by cooperation of the precipitation and agglomeration of the solid solution and the control of the structure to the lower bainite with each other. Cu also has an effect of providing an excellent corrosion resistance, and it is important for the Cu content to be defined to be equal to or higher than 0.50%. If the Cu content is too large, the hot workability is reduced, and Cu acts to transform the structure into martensite, resulting in a more reduction in machinability. Therefore, the Cu content is defined to be equal to or lower than 2.00%, and preferably, the Cu content is in a range of 0.60 to 1.50%.
Ni is an element for enhancing the
quenchability of the lower bainite of the invention steel and for inhibiting the production of ferrite. As described above, Cu is also an element important to control the structure into the lower bainite by the appropriate adjustment of Ni and Cu, and in order to

provide an excellent machinability to the invention steel, the content of Ni is defined to be equal to or higher than 0.60%. If the content of Ni is too large, the lower bainite is excessively finely divided; the transformation into martensite is promoted, and the toughness of the matrix is reduced to degrade the machinability. Therefore, the content of Ni is defined to be equal to or lower than 1.50%, and preferably, equal to or lower than 1.20%.
Sulfur (S) has a sugnificant effect of
enhancing the machinability by the presence of S in the form of non-metallic inclusion MnS. However, the presence of a large content of MnS causes not only effects during die-working such as the generation of pinholes during welding, the generation of pinholes during polishing and the roughing of the discharge-processed surface, but also provides a starting point for rusting, thereby causing the degradation of the nature of the die itself, such as the promotion of the anisotropy of the mechanical properties. Especially, in a large-sized die, the above adverse effect due to the segregation of MnS is significant. Therefore, to obtain the above effects, the preferable content of S is not less than 0.003%, but to inhibit these problems, it is necessary to limit the content of S to at most 0.05% or less.
Al is usually used as a deoxidizer element during melting, but in the state of the invention

steel, Al2Ci3 present in the steel degrades the mirror finish workability and hence, it is necessary to limit the content of Al to not more than 0.10%. Preferably, the content of Al is not more than 0.05%, more preferably, not more than 0.01%, further preferably, not more than 0.002%.
Oxygen (0) is an element which forms oxides in the steel, thereby causing the cold plastic workability and the polishability to be deteriorated remarkably. Especially, in the present invention, an upper limit for the content of Al is defined as 0.005%, preferably as not more than 0.003%, from the viewpoint that it is important to inhibit the formation of the above-described A^Oa. For an enhancement in polishability, it is a desirable condition to limit and control the content of Al to a further low range, for example, to not more than 0.00.1%. In the present invention in which a reduction in content of Al2C>3 is aimed, however, in addition to Al already controlled to a lower content, even the control of the content of 0 itself to a low range is not desired particularly severely. Therefore, it is acceptable for the content of Al to exceed 0.001%.
Nitrogen (N) is an element which forms nitrides in the steel. If the nitrides are formed in an excessive amount, the toughness of the die, the machinability and the polishability are considerably deteriorated. Therefore, it is preferable to limit the

content of nitrogen (N) in the steel to a low level, and in the present invention, the content of nitrogen (N) is limited to not more than 0.06%. Desirably, the content of nitrogen (N) is limited to not more than 0.02%, more desirably, to not more than 0.005%.
In order to ensure that the structure
essentially comprising the lower bainite phase aimed by the present invention is realized in the invention steel and that the invention steel has both of the machinability and the toughness at high levels, effective further narrow ranges of components exist for this purpose, and the present invention is also characterized in that the above component ranges are defined clearly.
More specifically, a further examination was made within the above mentioned ranges of basic components in the present invention, and consequently the inventors found that there are the narrow component ranges which meet the following equations,
Equation 1: (%Ni) + 1.2 (%Cu) = 1.30 to 2.70, and
Equation 2: 60 (%C) + 1.5 (%Si) + (%Ni) + 6 (%Cr) + 2 (%Mo + 1/2(%W) (alone or in combination)) + 20 (%V) + 0.2 (%Cu) = 21.00 to 28.70.
More specifically, it is an aim for the invention steel to have the structure essentially comprising the lower bainite phase. The reason is that if the value given by Equation 1: (%Ni) +1.2 (%Cu) is

smaller than 1.30, ferrite and upper bainite are liable to be produced, and if this value is larger than 2.70, the lower bainite finely divided and martensite are liable to be produced. Further, the reason is that even if the value given by the equation 1 satisfies 1.3 to 2.70, if the value given by the equation 2: 60 (%C) + 1.5 (%Si) + (%Ni) + 6 (%Cr) + 2 (%Mo + 1/2(%W) (alone or in combination)) + 20 (%V) + 0.2 (%Cu) is smaller than 21.00, the desired hardness is difficult to achieve, or the toughness is liable to be reduced, and if this value is larger than 28.70, the machinability is poor, or the toughness is liable to be reduced.
In the present invention, any further
toughness-improving element(s) and any machinability-improving element(s) can be added in a range which does not detract the above-described functional effects. For example, one or more of 0.5% or less (preferably 0.01 to 0.1%) of Nb, 0.15% or less of Ti, 0.15% or less of Zr and 0.15% or less of Ta can be added as the toughness-improving element(s). One or more of 0.003 to 0.2% of Zr, 0.0005 to 0.01% of Ca, 0.03 to 0.2% of Pb, 0.03 to 0.2% of Se, 0.01 to 0.15% of Te, 0.01 to 0.2% of Bi, 0.005 to 0.5% of In and 0.01 to 0.1% of Ce can be added as the machinability-improving element(s). Further, Y, La, Nd, Sm and other REM elements can be also incorporated in a total content of 0.0005 to 0.3%.
First, various steels shown in Table 1 and having compositions containing the balance of iron and

unavoidable impurities were melted with a high frequency vacuum melting furnace of 30 kg (values represented by the equations 1 and 2 of the present invention are shown in Table 2). Then, each of these steels was forged to a square bar having a size of 50 mm x 100 mm, which was subjected to a heat treatment and then put into the following evaluations. The heat treatment was carried out in the following manner so that a predetermined hardness was obtained: Each of the steels was heated to an austenite region of 900°C and kept at this temperature for 1 hour and then cooled at a cooling rate suitable for a practical ingot supposed. Thereafter, the resulting steel was tempered 1 hour at an appropriate temperature between 520°C and 590°C and air-cooled.

Table I
(mass%'
Specimen No. C Si Mn P s Ni Cr W Mo V Cu hi N 0 Remarks
1 0.11 0.29 1.05 0.011 0.030 0.65 2.47 - 0.25 0.14 1.68 0.0800 0.0030 0.0021 Invention steel
2 0.15 0.47 1.87 0.011 0.015 0.82 1.85 - 0.35 0.07 1.23 0.0050 0.0020 0.0012

3 0.16 0.25 1.45 0.010 0.007 0.99 1.82 - 0.51 0.07 0.76 0.0010 0.0050 0.0018

4 0.20 0.26 1.42 0.007 0.015 1.21 1.83 - 0.31 0.08 1.05 0.0020 0.0060 0.0019

5 0.17 0.67 1.63 0.008 0.030 1.47 1.05 - 0.67 0.04 0.58 0.0400 0.0180 0.0021

6 0.14 0.42 1.32 0.007 0.022 1.04 1.81 - 0.37 0.09 0.70 0.0050 0.0078 0.0018

7 0.16 0.51 1.56 0.010 0.019 1.03 2.20 0.82 0.27 0.05 0.52 0.0910 0.0280 0.0024

8 0.22 0.32 0.85 0.012 0.025 1.05 1.84 - 0.41 0.07 0.52 0.0450 0.0025 0.0011

9 0.11 0.44 0.49 0.008 0.003 0.78 1.79 - 0.55 0.09 0.73 0.0088 0.0145 0.0025

10 0.16 0.30 0.99 0.007 0.012 0.76 2.15 - 0.62 0.04 0.51 0.0018 0.0087 0.0014

11 0.24 0.38 1.53 0.008 0.019 0.74 1.86 0.91 - 0.04 0.54 0.0945 0.0045 0.0011

12 0.16 0.25 1.95 0.009 0.035 1.08 1.88 0.22 0.36 0.12 0.77 0.0070 0.0121 0.0011

13 0.06 0.42 0.99 0.009 0.04 0.65 1.12 - 0.33 0.05 0.52 0.0018 0.0015 0.0012 Comparative steel
14 0.21 1.49 1.53 0.012 0.03 0.78 1.89 - 0.81 0.03 0.67 0.0840 0.0158 0.0015

15 0.23 0.47 1.08 0.006 0.08 1.20 1.78 - 0.62 0.09 1.20 0.0044 0.0046 0.0024

16 0.14 0.30 1.78 0.008 0.045 0.24 1.02 - 0.55 0.04 0.64 0.0068 0.0260 0.0018

17 0.18 0.42 1.85 0.006 0.022 1.84 1.54 - 0.27 0.05 0.95 0.0640 0.0119 0.0012

18 0.20 0.33 1.53 0.009 0.019 0.78 0.37 0.72 0.27 0.13 0.66 0.0040 0.0086 0.0015

19 0.12 0.41 1.41 0.012 0.04 0.98 3.44 - 0.36 0.07 0.87 0.0870 0.0715 0.0024

20 0.15 0.47 0.99 0.011 0.05 1.23 2.39 - 1.78 0.04 1.08 0.0791 0.0187 0.0072

21 0.11 0.31 1.34 0.008 0.035 0.84 1.86 - 0.48 0.01 0.91 0.0624 0.0056 0.0016

22 0.24 0.37 1.32 0.007 0.038 1.34 1.23 - 0.62 0.29 1.38 0.0018 0.0078 0.0020

23 0.15 0.32 0.85 0.009 0.015 0.69 1.31 - 0.41 0.04 2.42 0.0120 0.0103 0.0026

24 0.12 0.49 1.005 0.008 0.031 1.04 1.78 0.24 0.37 0.07 0.87 0.2200 0.0162 0.0013

25 0.38 0.33 1.3 0.006 0.012 0.91 1.70 - 0.16 - - 0.040 0.0089 0.0058 Conventional steel
26 0.25 0.12 1.1 0.016 0.018 0.92 1.93 - 0.64 0.08 0.29 .0.062 0.0105 0.0007

27 0.16 0.15 1.8 0.012 0.011 - 1.78 - 0.24 0.08 0.65 0.038 0.0094 0.0032

28 0.16 0.16 1.6 0.006 0.027 0.11 1.82 - 0.16 0.08 0.26 0.001 0.0098 0.0035

29 0.13 0.28 1.4 0.011 0.004 3.15 0.16 - 0.25 0.01 1.01 1.100 0.0080 0.0052

"Note: The hyphen (-) means "less than 0.01%'

The evaluation of the machinability was made by carrying out a drilling test. More specifically, 50 holes were drilled in the specimen steel by a drill of (j)2mm made of a high-speed steel under conditions: a cutting rate of 15 m/min, a feed rate of 120 mm/min and a drilling depth of 20 mm, and thereafter, a maximum wear width of an edge of an outer peripheral surface of the tool was measured.
The evaluation of the toughness was made by carrying out a Charpy impact test using a U-notch test piece (a test piece according to JIS No. 3) of 2 mm, wherein a Charpy impact value at room temperature was measured.
The evaluation of the polishability was made in the following manner: Specimens each having an evaluation surface of 50 mm squares were taken and then subjected to a quenching and tempering treatment under the same conditions as the above-described conditions for the heat treatment, whereby the hardness was adjusted, and thereafter, each of the specimens was subjected to a mirror finishing by a system of grinder—»paper-»diamond compound. Then, the number of fine pits generated was counted using a magnifier (of 10 magnifications) and in this case, the specimens having less than 6 pits generated therein were indicated by A; the specimens having 6 to 10 pits generated therein were indicated by B; the specimens having 11 to 20 pits generated therein were indicated

by C; and the specimens having more than 20 pits generated therein were indicated by D. These results are given in Table 2. Each of the structures of the specimens shown in Table 2, excluding those of Specimen Nos. 16 to 18, was a substantially single-phase structure, whose phase shown in Table 2 occupied 90% or more by area.

Table 2

*Equation 1: *Equation 2:

Specimen No. Hardness (HRC) Structure Wear width (mm) Toughness (J/cm2) Polishing property Value of Equation 1 Value of Equation 2 Whether Equations 1 and 2 are satisfied Remarks
1 40.6 lower bainite 0.15 64 B 2.67 26.14 Yes Invention steel
2 40.7 ditto 0.14 64 B 2.30 23.97 Yes

3 40.0 ditto 0.14 76 B 1.90 24.46 Yes

4 38.7 ditto 0.16 66 B 2.47 27.01 Yes

5 39.4 ditto 0.15 69 B 2.17 21.23 Yes

6 40.2 ditto 0.15 67 B 1.88 23.61 Yes

7 41.2 ditto 0.14 62 B 1.65 27.06 Yes

8 37.9 ditto 0.12 78 B 1.67 28.09 Yes

9 38.6 ditto 0.13 77 B 1.66 21.83 Yes

10 40.6 ditto 0.15 64 B 1.37 25.85 Yes

11 39.9 ditto 0.14 73 B 1.39 28.70 Yes

12 40.3 ditto 0.16 67 B 2.00 25. 61 Yes

13 32.8 upper bainite 0.17 78 C 1.27 13.36 No Comparative steel
14 39.8 f errite 0.19 43 C 1.58 29.31 No

15 40.1 lower bainite 0.12 36 D 2.64 29.67 No

16 39.1 upper bainite + ferrite 0.15 30 C 1.00 17.24 No

17 38.8 lower bainite + martensite 0.22 52 B 2.98 24.24 No

18 40.6 upper bainite + ferrite 0.18 32 C 1.52 19.49 No

19 41.2 martensite 0.31 54 D 2.02 31.73 No

20 41.3 lower bainite 0.23 44 D 2.53 29.85 No

21 32.8 ditto 0.18 70 C 1.93 20.41 No

22 38.4 ditto 0.22 50 B 3.00 30.99 No

23 39.5 ditto 0.19 34 B 3.59 20.13 No

24 39.7 ditto 0.17 31 C 2.08 22.21 Yes

25 40.1 martensite 0.33 42 B 0.91 34.73 No Conventional steel
26 39.8 upper bainite 0.45 40 B 1.27 30.62 No

27 40.3 ditto 0.28 41 B 0.78 22.72 No

28 32.2 ditto 0.18 82 C 0.42 22.84 No

29 40.6 ditto 0.12 16 A 4.36 13.23 No

+(%Ni)+6(%Cr)+2(%Mo+l/2(%W)(at least one of Mo and W))+20(%V)+0.2(%Cu)

Invention steel
Each of Specimen Nos. 1 to 12 satisfying the chemical composition of the present invention is a die steel essentially comprising a substantially single-phase of lower bainite and satisfying the hardness of the present invention, because the values represented by the equations 1 and 2 also satisfy a preferred defined range of the present invention. The machinability of each of these specimens shows a best result in which the maximum wear width of the edge of the outer peripheral surface of the tool is equal to or smaller than 0.16 mm. Further, the toughness shows a very good result of not less than 60 J/cm2, and the polishability is good. Comparative steel
Specimen No. 13 containing carbon (C) smaller than the constituent range of the present invention has an upper bainite, because the value given by the equation I is less than 1.30, and this specimen has a good machinability and a good toughness because of a lower hardness, but on the other hand, its polishability is not necessarily sufficient. Specimen No. 14 containing a larger content of Si, Specimen No. 16 containing a smaller content of Ni and Specimen No. 18 containing a smaller content of Cr satisfy the hardness of the present invention, because the values represented by the equations 1 and 2 satisfy the preferred defined range of the present invention, but

their machinability and polishability are slightly poor, because the structure is a ferrite, or an upper bainite having less than 5% by area of ferrite mixed therein.
Specimen No. 15 containing sulfur in a content exceeding 0.05% has a lower bainite having a satisfactory hardness defined in the present invention, and has a best machinability, because sulfide-based non-metallic inclusions are contained in a large content in this specimen. On the other hand, however, the toughness is poor, because the value given by the equation 2 is larger than 28.70. The sulfides are very soft as compared with the matrix, and during polishing, pits are liable to be generated from the sulfides, resulting in a poor polishability. Specimen No. 17 containing Ni in a large content has a structure in which martensite (about 60% by area) is mixed in a lower bainite excessively finely divided, because the value given by the equation 1 is larger than 2.70. Thus, this specimen is good in polishability and also good in toughness to a certain extent, but is slightly poor in machinability.
Specimen No. 19 containing Cr in a content smaller than the constituent range of the present invention and nitrogen (N) in a larger content has an excessively fine martensite. Specimen No. 20 containing Mo in a large content and oxygen (0) in a large content has a single-phase substantially lower

bainite. However, both of the specimens are slightly poor in machinability, because the value given by the equation 2 is larger than 28.70 and the structure contains a large content of carbides. Further, these specimens are considerably poor in polishability, because Specimen No. 19 contains an excess content of nitrides, and Specimen No. 20 contains an excess content of oxides.
Each of Specimen Nos. 21 to 24 has a single-phase substantially lower bainite. However, Specimen No. 21 containing vanadium in a content smaller than the constituent range of the present invention has a slightly low hardness, because the value given by the equation 2 is less than 21.00; and for this reason, this specimen is good in machinability, but slightly poor in polishability. On the other hand, Specimen No. 22 containing vanadium (V) in a large content is slightly poor in machinability, because the values given by the equations 1 and 2 do not satisfy the preferred defined ranges of the present invention and because this specimen contains carbides in a large content. In Specimen No. 23 containing Cu in a large content, the value given by the equation 1 is larger than 2.70, and the micro-structure keeps the lower bainite excessively finely divided, but the machinability is slightly poor. In Specimen No. 24 containing Al in a large content, the value given by the equation 2 satisfies the preferred defined range of

the present invention, but the hardening due to the precipitation of the metallic compound of Al and Ni, resulting in a reduction in toughness. In addition, there is a tendency for nitrides to be contained in a large content, and the polishability is slightly poor. Conventional steels
No one of the conventional steel specimen Nos.25 and 29 satisfies the preferred constituent ranges represented by the equations 1 and 2 of the present invention, but in Specimen No. 25 containing carbon (C) in a larger content and vanadium (V) and Cu in smaller contents than the constituent ranges constituting the basic concept of the present invention, the hardness defined in the present invention is satisfied, and because this specimen has the martensitic micro-structure, the toughness and the polishability are good, but the machinability is poor. Each of Specimen No. 26 containing Cu in a small content and Specimen No. 27 containing Ni in a small content satisfies the hardness defined in the present invention and has a good polishability, but is slightly poor in toughness and poor in machinability, because it has an upper bainite.
Specimen No. 28 containing Ni and Cu in contents smaller than the constituent range of the present invention has an upper bainite, and is good in machinability and toughness because of its low hardness. On the other hand, however, the

polishability of this specimen is not necessarily sufficient. Specimen No. 29 containing Ni and Al in larger contents and Cr in a smaller content has an upper bainite, whose hardness defined in the present invention is satisfied. In addition, this specimen has best machinability and polishability in virtue of the employment of both of the hardening attributable to the precipitation of an intermetallic compound of Al and Ni and the hardening attributable to the precipitation of Cu, but on the other hand, the toughness of this specimen is considerably poor.
Industrial Applicability
The invention steel having an excellent toughness, which is not found in the conventional plastic-molding prehardened steel, is particularly suitable for production of a die for carrying out a more precise die-working, because the cracking is difficult to occur due to a thermal stress caused with the working or processing in a die.

Claim:
1. A die steel which consists essentially of, by mass, 0.10 to 0.25% carbon, not more
than 1.00% Si, not more than 2.00% Mn, 0.60 to 1.50% Ni, from more than 1.00 to
not more than 2.50% Cr, at least one of Mo and W in the amount as defined by the
equation of (Mo + (W/2)) not more than 0.05% S (sulphur), not more than 0.10% Al, not more than 0.06% N
(nitrogen), not more than 0.005% O (oxygen), and the balance of Fe and unavoidable
impurities,
which has a metal structure having a primary phase of lower bainite, and
which has a hardness of HRC 34 to 45.
2. A die steel as claimed in claim 1, which chemical composition meets the following
equations (by mass%):
(1) %Ni + 1.2 (%Cu) = 1.30 to 2.70, and
(2) 60 (%C) + 1.5 (%Si) + %Ni + 6(%Cr) + 2(%Mo+l/2(%W)) + 20(%V) +0.2(%Cu) = 21.00 to 28.70

3. A die steel as claimed in claim 1, wherein the total amount of at least one of Mo and W is defined by Mo + (W/2) = 0.10 to 1.00% by mass%.
4. A die steel as claimed in claim 1, wherein the sulphur amount is 0.003 to 0.05% by mass.
5. A die steel as claimed in claim 1, wherein the Al amount is not more than 0.05% by mass and the oxygen amount is from more than 0.001 to not more than 0.005% by mass.
6. A die steel as claimed in claim 1, wherein the Ni amount is 0.60 to 1.20% by mass and the Cu amount is 0.60 to 1.50%) by mass.
7. A die steel as claimed in claim 1, wherein the carbon amount is 0.13 to 0.20% by mass and the Cr amount is 1.40 to 2.20%) by mass.

Documents:

2265-DEL-2006-Abstract-(11-08-2009).pdf

2265-del-2006-abstract.pdf

2265-DEL-2006-Claims-(11-08-2009).pdf

2265-del-2006-claims.pdf

2265-DEL-2006-Correspondence-Others-(11-08-2009).pdf

2265-DEL-2006-Correspondence-Others-(24-07-2009).pdf

2265-del-2006-correspondence-others.pdf

2265-del-2006-description (complete).pdf

2265-del-2006-drawings.pdf

2265-DEL-2006-Form-1-(11-08-2009).pdf

2265-del-2006-form-1.pdf

2265-del-2006-form-18.pdf

2265-DEL-2006-Form-2-(11-08-2009).pdf

2265-del-2006-form-2.pdf

2265-DEL-2006-Form-3-(24-07-2009).pdf

2265-del-2006-form-3.pdf

2265-del-2006-form-5.pdf

2265-del-2006-gpa.pdf

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Patent Number

243289

Indian Patent Application Number

2265/DEL/2006

PG Journal Number

41/2010

Publication Date

08-Oct-2010

Grant Date

04-Oct-2010

Date of Filing

16-Oct-2006

Name of Patentee

HITACHI METALS LTD

Applicant Address

2-1,Shibaura-1-chome,Minato-ku, Tokyo, Japan

Inventors:

#	Inventor's Name	Inventor's Address
1	HIDESHI NAKASTU	1240-5,HASHIMACYO,YASUGI-SHI,SHIMANE,JAPAN
2	YOSHIYUKI INOUE	9-1-1-E303,NISHIFUKUBARA, YONAGO-SHI,TOTTORI, JAPAN
3	FUMIO TOHYAMA	714-68,KAMIHIGASHIKAWATSUCHO, MATSUE-SHI,SHIMANE, JAPAN
4	YASUSHI TAMURA	2739-13,ARASHIMACYO,YASUGI-SHI,SHIMANE,JAPAN
5	YASUHIRO HOSODA	579,UNAMI,HIROSECYO,YASUGI-SHI,SHIMANE,JAPAN
6	MITSUHIRO ANDOH	262-20,HASHIMACYO,YASUGI-SHI,SHIMANE,JAPAN

PCT International Classification Number

C22C38/00; B29C33/38; C22C38/18

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	2005-313181	2005-10-27	Japan