Title of Invention	"A METAL PLATE MATERIAL HOT MOLDING APPARATUS AND METHOD FOR PRESS MOLDING A HEATED METAL PLATE MATERIAL"
Abstract	A metal plate material hot molding apparatus wherein, in a metal plate material hot molding apparatus for press molding a heated metal plate material (1), supply piping (6) for a cooling medium is provided in a mold (2, 3), ejection holes (4) for the cooling medium are provided in a molding surface of the mold, the supply piping (6) and the ejection holes communicate with each other, characterized in that plural projections (13) having an area ratio between 1% and 90%, a diameter or circumcircle diameter between 10 µm and 5 mm, and a height between 5 µm and 1 mm are provided on at least part of the molding surface of the mold.

Title of Invention

"A METAL PLATE MATERIAL HOT MOLDING APPARATUS AND METHOD FOR PRESS MOLDING A HEATED METAL PLATE MATERIAL"

Abstract

A metal plate material hot molding apparatus wherein, in a metal plate material hot molding apparatus for press molding a heated metal plate material (1), supply piping (6) for a cooling medium is provided in a mold (2, 3), ejection holes (4) for the cooling medium are provided in a molding surface of the mold, the supply piping (6) and the ejection holes communicate with each other, characterized in that plural projections (13) having an area ratio between 1% and 90%, a diameter or circumcircle diameter between 10 µm and 5 mm, and a height between 5 µm and 1 mm are provided on at least part of the molding surface of the mold.

Full Text	DESCRIPTION METAL PLATE MATERIAL HOT PRESS MOLDING APPARATUS AND HOT PRESS MOLDING METHOD Technical Field The present invention relates to a metal plate material hot press molding apparatus and hot press molding method for heating a metal plate material and rapidly and uniformly cooling the molded material during and/or after hot press molding. Background Art Press molding of a metal plate material is the most common working method which is widely known in manufacturing of automobiles, machines, electric equipment, transport equipment, and so on because of its high productivity and high-precision working ability. In recent years, increase in the strength of steel plate, for example, as a material for automobile parts has been advanced in terms of reduction in the weight of parts, and in press molding of a high-tensile steel plate, a problem that springback, wrinkling, and so on occur, which tends to cause defective shapes becomes manifest. Furthermore, increase in the strength of the metal plate material causes increase in the pressure of a contact surface with a mold at the time of press molding, which raises a problem that a frictional force between the mold and the metal plate material exceeds the withstand load of a lubricant oil to thereby cause a defective surface due to die galling or the like and damage the mold, and consequently productivity reduces. Concerning these problems, to prevent the occurrence of molding defects such as crack, wrinkling, and galling of the metal plate material after press molding, a method of forming plural recesses in part or all of the surface of the mold and confining the lubricant oil between the surface of the mold and the metal plate material to thereby improve a sliding property is proposed (for example, Patent Document 1). However, this method has a problem that if the friction force increases because of the increase in the strength of the metal plate material, a sufficient lubricating effect cannot be obtained. Moreover, it is conventionally known that when a metal plate material with low press moldability is molded, a hot press molding method of heating the metal plate material and pressing it at a high temperature is effective. In this hot press molding, importance is put on cooling of the metal plate material after molding in terms of productivity, and a method of cooling with a refrigerant after press molding at a high temperature is proposed (for example, Patent Documents 2 and 3). However, the method proposed in Patent Document is designed to supply air from an air output provided at a peripheral portion of a punch of a warm press mold and perform cooling with the air with low heat capacity and heat conductivity as a medium, and has difficulty in changing the air with air existing in a gap between the mold and the metal plate material, whereby it has a problem that the cooling efficiency is low. Furthermore, the method proposed in Patent Document 3 is designed to define a clearance between the mold and the metal plate material, provide refrigerant introducing grooves in a molding surface of the mold which touches the metal plate material, and increase the cooling rate using the refrigerant. However, when the refrigerant flows into the refrigerant introducing grooves, the temperature at the outlet side becomes higher than that at the inlet side, and the refrigerant becomes difficult to flow along the grooves due to deformation of the metal plate material at the time of molding, which makes uniform cooling difficult. Additionally, there is a problem that a continuous groove shape tends to be transferred to the molded metal plate material. (Patent Document 1) Japanese Patent Application Laid-open No. Hei 6-210370 (Patent Document 2) Japanese Patent Application Laid-open No. Hei 7-47431 (Patent Document 3) Japanese Patent Application Laid-open No. 2002-282951 Summary of the Invention An object of the present invention is to provide a metal plate material hot press molding apparatus and hot press molding method which makes it possible to, in a hot press molding apparatus for heating and molding a metal plate material, accelerate cooling of a mold and a molded piece to obtain a pressed product excellent in strength and dimensional accuracy in a short period of time and further suppress heat storage into the mold to improve productivity of the pressed product. The present invention has been made based on findings obtained by elucidating the sliding property and heat transfer phenomenon between the metal plate material and the mold in hot press molding and examining the cooling behavior of the metal plate material by a cooling medium in detail, and its summary is as follows. (1) A metal plate material hot molding apparatus, wherein in a metal plate material hot molding apparatus for press molding a heated metal plate material, supply piping for a cooling medium is provided in a mold, ejection holes for the cooling medium are provided in a molding surface of the mold, and the supply piping and the ejection holes communicate with each other. (2) The metal plate material hot molding apparatus of (1), wherein the ejection holes for the cooling medium have a diameter between 100 ^m and 10 mm and a pitch between 100 \|^m and 1000 mm. (3) The metal plate material hot molding apparatus of (1) or (2), wherein discharge piping for the cooling medium is provided in the mold, discharge holes for the cooling medium are provided in the molding surface of the mold, and the discharge piping and the discharge holes communicate with each other. (4) The metal plate material hot molding apparatus of (3), wherein the discharge holes for the cooling medium have a diameter between 100 j^m and 10 mm and a pitch between 100 jam and 1000 mm. (5) The metal plate material hot molding apparatus of any of (1) to (4), wherein at least part of the mold is formed of porous metal having plural holes. (6) The metal plate material hot molding apparatus of any of (1) to (5), wherein cooling piping is provided in the mold. (7) The metal plate material hot molding apparatus of any of (I) to (6), wherein a valve mechanism is provided in the ejection hole. (8) The metal plate material hot molding apparatus of any of (1) to (7), wherein a sealing mechanism which prevents the cooling medium from flowing out is provided at a periphery of the mold. (9) The metal plate material hot molding apparatus of any of (1) to (8), wherein plural projections having an area ratio between 1% and 90%, a diameter or circumcircle diameter between 10 jj,m and 5 mm, and a height between 5 (am and 1 mm are provided on at least part of the molding surface of the mold. (10) The metal plate material hot molding apparatus of (9), wherein the projection is a NiW- plated layer or chrome-plated layer with a thickness between 10 (xm and 80 (im. (11) The metal plate material hot molding apparatus of any of (1) to (10), wherein the ejection hole for the cooling medium is provided only in a portion where a heat transfer coefficient between the metal plate material and the mold is 2000 W/m2K or less . (12) A metal plate material hot molding method, wherein in a metal plate hot molding method of press molding a heated metal plate material using the metal plate material hot molding apparatus of any of (1) to (11), molding is performed while a cooling medium is ejected to a gap between the metal plate material and a mold from ejection holes. (13) The metal plate material hot molding method of (12), wherein the cooling medium ejected to the gap between the metal plate material and the mold is discharged from the ejection holes and/or discharge holes. (14) The metal plate material hot molding method of (12) or (13), wherein the cooling medium is ejected only to a portion where a heat transfer coefficient calculated by measuring temperatures of the metal plate material and the mold is 2000 W/m2K or less . (15) The metal plate material hot molding method of any of (12) to (14), wherein the cooling medium is one kind or two kinds or more of water, a polyhydric alcohol, a polyhydric alcohol solution, polyglycol, a mineral oil with a flash point of 120°C or higher, synthetic ester, a silicon oil, a fluorine oil, grease with a dropping point of 120°C or higher, and a water emulsion obtained by mixing a surfactant into a mineral oil or synthetic ester. (16) The metal plate material hot molding method of any of (12) to (15), wherein the cooling medium is ejected during holding at a press bottom dead center. Brief Description of the Drawings Fig. 1A is a sectional view showing an example of a mold of the present invention provided with ejection holes and supply piping for a cooling medium; Fig. IB is a perspective view of the example of the mold in Fig. 1A; Fig. 2A is a sectional view showing an example of a mold of the present invention provided with ejection holes, supply piping, discharge holes, and discharge piping for a cooling medium; Fig. 2B is a perspective view of the example of the mold in Fig. 2A; Fig. 3A is a sectional view showing an example of a mold provided with ejection holes, supply piping, and cooling piping for a cooling medium; Fig. 3B is a perspective view of the example of the mold in Fig. 3A; Fig. 4 is a view schematically showing part of the surface of a mold provided with ejection holes, discharge holes, and projections; Fig.5A is a view schematically showing part of a section of an example of the mold provided with the ejection holes, the discharge holes, and the projections; and Fig. 5B is a view schematically showing another example of the mold in Fig. 5A. Detailed Description of the Preferred Embodiments The present invention is designed such that in a metal plate material hot press molding method of heating a metal plate material to a predetermined temperature (for example, between 700°C and 1000°C) by an electric heating furnace or a heating device by induction heating, electric current heating, or the like, setting the high-temperature metal plate material in a mold of a press molding apparatus, pressing the metal plate material by molding surfaces of the mold, that is, contact surfaces of opposed punch and die, and holding the mold at a bottom dead center, a cooling medium is ejected from the mold during and/or after molding to forcibly cool a molded piece and the mold. Examples of molds of the present invention shown in Fig. 1 to Fig. 3 will be described in detail below Figs. 1A and IB schematically show an aspect in which ejection holes 4 and supply piping 6 for the cooling medium of the present invention are provided in a die 2 being a lower mold, and the supply piping 6 for the cooling medium provided in the die 2 and a die holder 2' are connected by bolts via 0-rings 11. In Fig. 1A, a rubber 0-ring as a sealing mechanism 12 which prevents the cooling medium from flowing out is provided at a periphery of the die 2. Figs. 1A and IB show the example in which the ejection holes 4 for the cooling medium are provided in a vertical wall portion of the die, but they may be provided in a bottom portion or may be provided in both the vertical wall portion and the bottom portion. Figs. 2A and 2B schematically show an example in which the ejection holes 4 and discharge holes 5 for the cooling medium are provided in a punch 3 being an upper mold, the supply piping 6 for the cooling medium is provided in a punch holder 3', and discharge piping 7 for the cooling medium is provided in a core 3" and the punch holder 3'. In Figs. 2A and 2B, the supply piping 6 for the cooling medium is formed by the core 3" provided inside the punch 3. The discharge piping 7 provided in the punch holder 3' and the core 3", and the supply piping 6 for the cooling medium in the punch holder 3' and the punch 3 are respectively connected by bolts via the 0-rings 11. As in Fig. 1, the rubber 0-ring as the sealing mechanism 12 for the cooling medium is provided at the periphery of the lower die 2. An ejection valve 9 with a spring mechanism is provided in the ejection hole 4 in Figs. 2A and 2B, and closes an outlet of the supply piping 6 for the cooling medium, for example, when the punch reaches the bottom dead center at the time of pressing, and when the internal pressure of the cooling medium is increased, the ejection valve 9 opens and the cooling medium is ejected from the ejection hole 4 to the surface of the mold. The ejected cooling medium is discharged from the discharge piping 7 through an intermediate barrel 10 which crosses the supply piping 6 from a discharge hole 5. Incidentally, Figs 2A and 2B show the example in which the ejection holes 4 and discharge holes 5 for the cooling medium are provided in a vertical wall portion of the punch, but they may be provided in a bottom portion or may be provided in both the vertical wall portion and the bottom portion. Fig. 3 shows an example in which cooling piping 8 is further provided in the die 2 provided with the ejection holes 4 and supply piping 6 for the cooing medium shown in Fig. 1. The mold is cooled by the supply piping 6 for the cooling medium, but by further providing the cooling piping 8, the cooling of the mold is accelerated. The cooling piping 8 is also effective in accelerating the cooling of the mold provided with the supply piping 6 and discharge piping 7 for the cooling medium shown in Fig. 2. Moreover, by providing the cooling piping 8, for example, it is possible to suppress an increase in the temperature of the mold when press molding is performed until the bottom dead center is reached without the cooling medium being supplied to the supply piping 6. Figs. 1 to 3 each show the example in which the ejection holes 4, supply piping 6, discharge holes 5, discharge piping 7, and cooing piping 8 for the cooling medium are provided in either of the punch 3 and the die 2, but they may be provided in both of the punch 3 and the die 2. Moreover, it is necessary to provide at least the ejection holes 4 and supply piping 6 for the cooling medium. In this case, it is possible to continuously eject the cooling medium from the ejection holes while continuing to supply the cooling medium to the supply piping 6, and it is also possible to discharge the cooling medium if the supply of the cooling medium to the supply piping 6 is stopped to bring the internal pressure to a negative pressure. Accordingly, depending on the size and shape of the mold, it can be selected appropriately whether the ejection holes 4 and the supply piping 6 are used for discharging the cooling medium or the independent discharge holes 5 and discharge piping 7 are further provided. When the shapes of the ejection hole 4 and the discharge hole 5 are circular, a sufficient supply of liquid cannot be obtained due to pressure loss if their diameter is less than 100 jim, whereby it is desirable that the lower limit of the diameter be 100 jam or more. On the other hand, if the diameter of the ejection hole 4 and the discharge hole 5 is more than 10 mm, the shapes thereof are transferred to the metal plate material, whereby it is desirable that the upper limit of the diameter be 10 mm or less. Incidentally, when the shapes of the ejection hole 4 and the discharge hole 5 are rectangular or elliptical and when the ejection hole 4 and the discharge hole 5 have indeterminate forms such as holes of porous metal, the area of a flow path needs to be equal to that of a circle with a diameter between 100 jam and 10 mm. When the pitch of the ejection holes 4 and the discharge holes 5, that is, the distance between the adjacent ejection holes 4 when only the ejection holes 4 are provided or the distance between the adjacent ejection holes 4 or discharge holes 5 when both the ejection holes 4 and the discharge holes 5 are provided is less than 100 j^m, the number of holes increases, resulting in an increase in the cost of the mold. On the other hand, the pitch of the ejection holes 4 and the discharge holes 5 is more than 1000 mm, cooling capacity becomes sometimes insufficient. Accordingly, it is desirable that the pitch of the ejection holes 4 and the discharge holes 5 be between 100 jam and 1000 mm. It is desirable that die steel for hot working be used as a material for the mold in terms of hot strength. When the cooling piping is provided in both the punch and the die, die steel for cold working which ha high heat conductivity and is resistant to heat storage may be used. The ejection holes, the discharge holes, and the cooling piping can be provided by mechanical drilling by a drill or by drilling by electric discharge machining. Furthermore, instead of drilling the ejection holes and discharge holes for the cooling medium in the mold, the supply piping for the cooling medium may be connected to porous metal having pores which penetrate from within the mold to the outer surface. In this case, it is desirable to use porous metal having plural holes with a diameter between 100 p.m and 1 mm and a pitch between 100 jam and 10 mm which penetrate in a thickness direction. For example, if in a punch having a structure such as shown in Fig. 2, die steel is used for the core 3" and porous metal is used for the punch 3, the punch 3 having the fine ejection holes 4 and discharge holes 5 with a small pitch can be manufactured. Such porous metal can be manufactured by sintering powder after molding or by unidirectional solidification for making the direction of a solidification structure fixed by temperature control after melting metal. Incidentally, it is also possible to manufacture the entire punch 3 by the porous metal, or it is also possible to provide holes in portions corresponding to the ejection holes 4 and discharge holes 5 for the cooling medium in Figs. 2A and 2B by machining and join the porous metal into the holes by shrink fitting or the like. Moreover, by providing projections 13 on the molding surface of the mold, the area of contact between the mold and the metal plate material can be reduced, and hence the occurrence of die galling can be suppressed. Furthermore, since the area of contact between the mold, that is, the die 2 or the punch 3 and the metal plate material 1 is reduce by these projections 13, excessive cooling of the metal plate material 1 due to the movement of heat to the mold during press molding can be suppressed. When the cooling medium is ejected at the bottom dead center, it becomes easy to circulate the cooling medium through gaps between the projections 13 and the metal plate material 1, which makes it possible to increase cooling efficiencies of the mold and the metal plate material 1. A schematic view and sectional views of the surface of part of the mold provided with the projections 13 on its molding surface are shown in Figs. 4 and 5, respectively. The projections 13 shown in Figs. 4 and 5 as an example are circular cylinders which are provided at predetermined intervals on the molding surface of the mold, but it is desirable that the shape of their horizontal sections be any of a circular shape, a polygonal shape, and a star-shape, and that the shape of their vertical section be rectangular or trapezoidal. They also may be hemispherical. Incidentally, it is desirable that plural projections 3 of the mold be provided on the molding surface, and the projections 13 may be provided on part of the molding surface or may be provided on the entire surface. Furthermore, they may be provided on either or both of the punch and the die. Incidentally, as shown in Fig. 5A, the projections 13 of the mold may be provided as they are on the surface of the molding surface, but depending on molding conditions, marks of the projections 13 are sometimes transferred to the molded piece. To prevent this, it is recommended to remove only peripheries of the projections 13 as shown in Fig. 5B. Furthermore, it is also possible to remove portions where the projections 13 are provided to a depth equal to the height of the projection 13 and provide the projections 13. It is desirable that the height of the projections 13 on the molding surface of the mold be between 5 \|im and 1 mm. This is because if the height of the projections 13 is lower than 5 \|j,m, the gap between the mold and the metal plate material 1 is too small, so that it is difficult to circulate liquid between the mold and the metal plate material 1, and if the height is higher than 1 mm, the gap is too large, so that the cooling rate by heat conductivity of the liquid lowers. It is desirable that the area ratio of the projections 13 on the molding surface of the mold be between 1% and 90%. This is because if the area ratio of the projections 13 is less than 1%, projection shapes on the surface of the mold tend to be transferred to the metal plate material, and if it is more than 90%, the gap between the projections is narrow, whereby pressure loss becomes larger and the liquid can neither be filled nor flow, which causes a slight reduction in cooling efficiency. It is desirable that the diameter of the projection when the shape of the horizontal section of the projection on the molding surface of the mold is circular or the diameter of a circumcircle of the projection when the shape thereof is polygonal or star-shaped be between 10 ^m and 5 mm. This is because if the diameter of the projection or the diameter of the circumcircle is less than 10 f^m, the projection wears badly, and cannot produce an effect over a long period, and if the diameter thereof is more than 5 mm, uniform cooling cannot be performed. The projections on the molding surface of the mold can be formed by electrochemical machining, chemical etching, electric discharge machining, or a plating method. The chemical etching can be performed in the following manner. First, after a visible light curing photosensitive resin is applied on the surface of the mold and dried, visible light is irradiated to cure an irradiated portion while the surface is covered with a mask for cutting off the visible light. Then, the resin except that on the cured portion is removed by an organic solvent. For example, it is recommended to perform etching by immersing the surface of the mold in an etching solution such as a sodium chloride solution for one minute to thirty minutes. The diameter or pitch of the projections may be selected appropriately depending on the shape of the mask for cutting off the visible light, and the height of the projections may be adjusted appropriately depending on the etching time. Electro discharge texturing is a processing method in which a copper electrode having recesses each with an inverted shape of the targeted projection as a surface pattern is placed opposite the mold and a pulse direct current is passed while its current peak value and pulse width are changed. The desirable current value is between 2 A and 100 A and pulse width is between 2 psec and 1000 f^sec, and they need to be adjusted appropriately according to the material of the mold and the desired shape of the projections. In the case of the plating method, in order that the diameter of the hemispherical projection is set to 10 f^m or more, it is desirable that the thickness of plating be 10 f^m or more, and that the upper limit thereof be 80 ^m or less to prevent exfoliation. After alkaline degreasing and electrolytic etching of electrolyzing the mold as an anode in a plating solution, a plating layer can be formed at a predetermined bath temperature and current density. Incidentally, a plating layer with a thickness between 10 jam and 80 {jm can be provided under conditions of a current density approximately between 1 A/dm2 and 200 A/dm2 and a bath temperature approximately between 30°C and 60°C in a chrome plating solution in the case of chrome plating, and under conditions of a current density approximately between 1 A/dm2 and 100 A/dm2 and a bath temperature approximately between 30°C and 60°C in a NiW plating solution in the case of NiW plating. Incidentally, in order to form a plating layer having a hemispherical projection shape, for example, it is required to perform plating at a fixed current density after the current density is increased stepwise. Furthermore, it is desirable that the ejection holes 4, the discharge holes 5, and the projections 13 be each provided at a portion where the heat transfer coefficient between the mold and the metal plate material is 2000 W/m2K or less. For example, by performing hot press molding while measuring the temperatures of the mold and the metal plate material using a thermocouple, a radiation thermometer, or the like before the ejection holes 4, the discharge holes 5, and the projections 13 are each provided, the portion where the heat transfer coefficient between the mold and the metal plate material is 2000 W/m2K or less can be worked out from the temperature changes of the mold and the metal plate material. It is also possible to calculate the deformation behavior and gap amount between the mold and the metal plate material by FEM and determine the portion where the heat transfer coefficient is 2000 W/m2K or less. Consequently, it becomes possible to eject the cooling medium to a portion which requires acceleration of cooling and enhance cooling, which enables uniform cooling and reductions in the manufacturing cost and cooling cost of the mold. A hot press molding method of the present invention is designed to enhance cooling by ejecting the cooling medium to the gap between the mold and the metal plate material during and/or after press molding. For example, when the metal plate material 1 is press-molded using the hot press molding apparatus shown in Figs. 1 and 3, the cooling medium is supplied from the supply piping 6 and ejected to the gap between the mold and the metal plate material 1 from the ejection holes 4 while the punch 3 is lowered to and held at the bottom dead center. In this case, if the internal pressure in the supply piping 6 is brought to a negative pressure, the cooling medium can be discharged from the ejection holes 4, and hence, if the ejection and discharge of the cooling medium are repeated intermittently, the cooling effect increases. Similarly, also in the case of the hot press molding apparatus provided with the discharge holes 5 and the discharge piping 7 shown in Fig. 2, the cooling medium can be discharged from the ejection holes 4. Incidentally, when the nucleate boiling of the cooling medium is predicted from a calculation based on the boiling point of the cooling medium, heat conductivity, the heat capacity of the metal plate material, and so on, it is desirable to constantly eject the cooling medium from the ejection holes to let it flow to the discharge holes. When the nucleate boiling of the cooling medium is not predicted, the gap between the mold and the metal plate material may remain filled with the cooling medium. The cooling medium may be any of water, a polyhydric alcohol, a polyhydric alcohol solution, polyglycol, a mineral oil with a flash point of 120°C or higher, synthetic ester, a silicon oil, a fluorine oil, grease with a dropping point of 120°C or higher, and a water emulsion obtained by mixing a surfactant into a mineral oil or synthetic ester, or a mixture of these may be used in terms of flame retandancy and corrosiveness. Furthermore, the cooling medium may be liquid or vapor. Hot-press molding according to the present invention is also applicable to any of metal plate materials such as an Al-plated steel plate, a Zn-plated steel plate, ordinary steel, copper, and aluminum. Incidentally, when the material of the metal plate material is steel, it is desirable that the temperature of the entire steel plate be maintained at not higher than a martensitic transformation point of the steel at the bottom dead center. -Examples- The present invention will be more specifically described below by examples. A hat-shaped product is manufactured by way of trial by manufacturing the mold which is schematically shown in Fig. 2 by machining, and further drawing Al-plated steel using the hot press molding apparatus provided with the projections 13 which is schematically shown in Figs. 4 and 5. The length of a specimen is 300 mm, width is 100 mm, thickness is 1.2 mm, and surface roughness is 1.0 (j,m. The material of the die and the punch is S45C, shoulder width is 5 mm, die width is 70 mm, and die molding depth is 60 mm. Porous metal is fabricated by unidirectional solidification of fixing a rod with a diameter of 10 mm which is made of stainless steel composed of a SUSS 04L-based component in a high-pressure container, moving a portion to be heated while partially melting the rod by high-frequency induction heating, and thereby continuously melting and solidifying the rod. Ejection holes, discharge holes, and projections of the mold are those shown in Table 1, and the surface roughness is 1.0 µm. Incidentally, before processing of providing the ejection holes, the discharge holes, and the projections, hot-press molding is performed while the temperature is measured by a thermocouple to specify portions where the heat transfer coefficient is 2000 W/m2K or less, and more specifically, the ejection holes, the discharge holes, and the projections are provided in sidewall surfaces of the die and the punch. The Al-plated steel plate is heated to approximately 950°C in an atmosphere furnace, and the heated steel plate is set at a molding position between the punch and the die, subjected to hot press molding, held for two seconds at the bottom dead center, and cooled by ejecting the cooling medium. In comparative example 12, it is held for ten seconds at the bottom dead center. Thereafter, the mold is released, and the product is taken out. This molding is performed continuously 100 times. Furthermore, using the specimen and the mold under the same conditions, a comparative product is manufactured by heating the specimen to approximately 950°C, hot press molding it, and then immediately cooling it by immersing it in a tank without holding it. The hardness, shape, surface damage, and mold surface temperature regarding each of the obtained products are evaluated, and results thereof are shown in Table 1. The hardness of the product is measured at a pitch of 10 mm in a longitudinal direction. If the hardnesses at all positions of all the products are higher than the hardness of the comparative product, the hardness is regarded as good and shown by "©". The shape of the product is evaluated by comparing the shape of the product measured by a laser displacement meter with a designed shape, and if the error between the shape of the product and the designed shape is within 10%, the shape is regarded as good and shown by "©". The evaluation of surface damage is performed by visually examining a sidewall portion of the product, and if no galling is observed in all the products, the evaluation of surface damage is regarded as good and shown by "©". If the percent defective of hardness, shape, and surface damage is 1% or less, the comprehensive evaluation is regarded as good and shown by "O", and if it is more than 1%, the comprehensive evaluation is regarded as bad and shown by "x". Furthermore, after molding, the mold surface temperature is measured by a contact-type surface thermometer, and if the mold surface temperature is 80°C or lower, it is regarded as good and shown by "O", and if it is higher than 80°C, it is regarded as bad and shown by "x". As shown in Table 1, the products manufactured within the scope of the hot press molding method of the present invention using the hot press molding apparatus of the present invention have good hardnesses and shapes, have no surface damage, cause a small increase in mold temperature, and receive good comprehensive evaluations. On the other hand, in comparative examples 11 and 12, a conventional molding apparatus provided with no ejection hole for the cooling medium is used, and the comparative example 12 which has a longer holding time than the comparative example 11 has good hardness and shape, but receives a bad comprehensive evaluation. [TABLE 1] (Table Removed) Industrial Applicability The present invention makes an extremely remarkable industrial contribution such that when a pressed product excellent in strength and dimensional accuracy is manufactured using a high-strength metal plate material with low press moldability as a material by hot press molding, it is possible to increase productivity and further suppress heat storage into a mold to lengthen the life of the mold, thereby reducing a manufacturing cost. We Claim: 1. A metal plate material hot molding apparatus wherein, in a metal plate material hot molding apparatus for press molding a heated metal plate material (1), supply piping (6) for a cooling medium is provided in a mold (2, 3), ejection holes (4) for the cooling medium are provided in a molding surface of the mold, the supply piping (6) and the ejection holes communicate with each other, characterized in that plural projections (13) having an area ratio between 1% and 90%, a diameter or circumcircle diameter between 10 um and 5 mm, and a height between 5 µm and 1 mm are provided on at least part of the molding surface of the mold. 2. The metal plate material hot molding apparatus as claimed in claim 1, wherein a valve mechanism is provided in the ejection hole (4). 3. The metal plate material hot molding apparatus as claimed in claim 1, wherein a sealing mechanism (12) which prevents the cooling medium from flowing out is provided at a periphery of the mold (2, 3). 4. The metal plate material hot molding apparatus as claimed in claim 1, wherein at least part of the molding surface of the mold is formed of porous metal having plural holes penetrating, and the supply piping (6) and the plural pores penetrating of the porous metal communicate with each other. 5. The metal plate material hot molding apparatus as claimed in claim 4, wherein the projection is a NiW-plated layer or chrome-plated layer with a thickness between 10 µm and 80 urn. 6. The metal plate material hot molding apparatus as claimed in any one of claims 1 to 5, wherein the ejection hole (4) for the cooling medium is provided only in a portion where a heat transfer coefficient between the metal plate material and the mold is 2000 W/m2 K or less. 7. The metal plate material hot molding apparatus as claimed in any one of claims 1 to 4, wherein discharge piping (7) for a cooling medium is provided in the mold, discharge holes (5) for the cooling medium are provided in a molding surface of the mold, and the discharge piping and the discharge holes communicate with each other. 8. The metal plate material hot molding apparatus as claimed in any one of claims 1 to 4, wherein cooling piping (8) is provided in the mold. 9. A metal plate material hot molding method, wherein in a metal plate hot molding method of press molding a heated metal plate material (1) using the metal plate material hot molding apparatus as claimed in any one of claims 1 to 8, wherein molding is performed while a cooling medium is ejected to a gap between the metal plate material and a mold from ejection holes (4). 10. The metal plate material hot molding method as claimed in claim 9, wherein the cooling medium ejected to the gap between the metal plate material and the mold is discharged from the ejection holes (4) and/or discharge holes (5). 11. The metal plate material hot molding method as claimed in claim 9, wherein the cooling medium is ejected only to a portion where a heat transfer coefficient calculated by measuring temperatures of the metal plate material and the mold is 2000 W/m2 K or less. 12. The metal plate material hot molding method as claimed in claim 9, wherein the cooling medium is one kind or two kinds or more of water, a polyhydric alcohol, a polyhydric alcohol solution, polyglycol, a mineral oil with a flash point of 120°C or higher, synthetic ester, a silicon oil, a fluorine oil, grease with a dropping point of 120°C or higher, and a water emulsion obtained by mixing a surfactant into a mineral oil or synthetic ester. 13. The metal plate material hot molding method as claimed in claim 9, wherein the cooling medium is ejected during holding at a press bottom dead center.

Full Text

DESCRIPTION
METAL PLATE MATERIAL HOT PRESS MOLDING APPARATUS AND
HOT PRESS MOLDING METHOD
Technical Field
The present invention relates to a metal plate material hot press molding apparatus and hot press molding method for heating a metal plate material and rapidly and uniformly cooling the molded material during and/or after hot press molding.
Background Art
Press molding of a metal plate material is the most common working method which is widely known in manufacturing of automobiles, machines, electric equipment, transport equipment, and so on because of its high productivity and high-precision working ability. In recent years, increase in the strength of steel plate, for example, as a material for automobile parts has been advanced in terms of reduction in the weight of parts, and in press molding of a high-tensile steel plate, a problem that springback, wrinkling, and so on occur, which tends to cause defective shapes becomes manifest. Furthermore, increase in the strength of the metal plate material causes increase in the pressure of a contact surface with a mold at the time of press molding, which raises a problem that a frictional
force between the mold and the metal plate material exceeds the withstand load of a lubricant oil to thereby cause a defective surface due to die galling or the like and damage the mold, and consequently productivity reduces.
Concerning these problems, to prevent the occurrence of molding defects such as crack, wrinkling, and galling of the metal plate material after press molding, a method of forming plural recesses in part or all of the surface of the mold and confining the lubricant oil between the surface of the mold and the metal plate material to thereby improve a sliding property is proposed (for example, Patent Document 1). However, this method has a problem that if the friction force increases because of the increase in the strength of the metal plate material, a sufficient lubricating effect cannot be obtained.
Moreover, it is conventionally known that when a metal plate material with low press moldability is molded, a hot press molding method of heating the metal plate material and pressing it at a high temperature is effective. In this hot press molding, importance is put on cooling of the metal plate material after molding in terms of productivity, and a method of cooling with a refrigerant after press molding at a high temperature is proposed (for example, Patent Documents 2 and 3).
However, the method proposed in Patent Document
is designed to supply air from an air output provided at a peripheral portion of a punch of a warm press mold and perform cooling with the air with low heat capacity and heat conductivity as a medium, and has difficulty in changing the air with air existing in a gap between the mold and the metal plate material, whereby it has a problem that the cooling efficiency is low. Furthermore, the method proposed in Patent Document 3 is designed to define a clearance between the mold and the metal plate material, provide refrigerant introducing grooves in a molding surface of the mold which touches the metal plate material, and increase the cooling rate using the refrigerant. However, when the refrigerant flows into the refrigerant introducing grooves, the temperature at the outlet side becomes higher than that at the inlet side, and the refrigerant becomes difficult to flow along the grooves due to deformation of the metal plate material at the time of molding, which makes uniform cooling difficult. Additionally, there is a problem that a continuous groove shape tends to be transferred to the molded metal plate material.
(Patent Document 1)
Japanese Patent Application Laid-open No. Hei 6-210370
(Patent Document 2)
Japanese Patent Application Laid-open No. Hei 7-47431
(Patent Document 3)
Japanese Patent Application Laid-open No. 2002-282951
Summary of the Invention
An object of the present invention is to provide a metal plate material hot press molding apparatus and hot press molding method which makes it possible to, in a hot press molding apparatus for heating and molding a metal plate material, accelerate cooling of a mold and a molded piece to obtain a pressed product excellent in strength and dimensional accuracy in a short period of time and further suppress heat storage into the mold to improve productivity of the pressed product.
The present invention has been made based on findings obtained by elucidating the sliding property and heat transfer phenomenon between the metal plate material and the mold in hot press molding and examining the cooling behavior of the metal plate material by a cooling medium in detail, and its summary is as follows.
(1) A metal plate material hot molding apparatus, wherein in a metal plate material hot molding apparatus for press molding a heated metal plate material, supply piping for a cooling medium is provided in a mold, ejection holes for the cooling medium are provided in a molding surface of the mold, and the supply piping and the ejection holes communicate with each other.
(2) The metal plate material hot molding
apparatus of (1), wherein the ejection holes for the
cooling medium have a diameter between 100 ^m and 10
mm and a pitch between 100 |^m and 1000 mm.
(3) The metal plate material hot molding
apparatus of (1) or (2), wherein discharge piping for
the cooling medium is provided in the mold, discharge
holes for the cooling medium are provided in the
molding surface of the mold, and the discharge piping
and the discharge holes communicate with each other.
(4) The metal plate material hot molding
apparatus of (3), wherein the discharge holes for the
cooling medium have a diameter between 100 j^m and 10
mm and a pitch between 100 jam and 1000 mm.
(5) The metal plate material hot molding
apparatus of any of (1) to (4), wherein at least part
of the mold is formed of porous metal having plural
holes.
(6) The metal plate material hot molding
apparatus of any of (1) to (5), wherein cooling
piping is provided in the mold.
(7) The metal plate material hot molding
apparatus of any of (I) to (6), wherein a valve
mechanism is provided in the ejection hole.
(8) The metal plate material hot molding
apparatus of any of (1) to (7), wherein a sealing
mechanism which prevents the cooling medium from
flowing out is provided at a periphery of the mold.
(9) The metal plate material hot molding
apparatus of any of (1) to (8), wherein plural projections having an area ratio between 1% and 90%, a diameter or circumcircle diameter between 10 jj,m and 5 mm, and a height between 5 (am and 1 mm are provided on at least part of the molding surface of the mold.
(10) The metal plate material hot molding
apparatus of (9), wherein the projection is a NiW-
plated layer or chrome-plated layer with a thickness
between 10 (xm and 80 (im.
(11) The metal plate material hot molding
apparatus of any of (1) to (10), wherein the ejection
hole for the cooling medium is provided only in a
portion where a heat transfer coefficient between the
metal plate material and the mold is 2000 W/m2K or
less .
(12) A metal plate material hot molding method,
wherein in a metal plate hot molding method of press
molding a heated metal plate material using the metal
plate material hot molding apparatus of any of (1) to
(11), molding is performed while a cooling medium is ejected to a gap between the metal plate material and a mold from ejection holes.
(13) The metal plate material hot molding method
of (12), wherein the cooling medium ejected to the
gap between the metal plate material and the mold is
discharged from the ejection holes and/or discharge
holes.
(14) The metal plate material hot molding method
of (12) or (13), wherein the cooling medium is
ejected only to a portion where a heat transfer coefficient calculated by measuring temperatures of the metal plate material and the mold is 2000 W/m2K or less .
(15) The metal plate material hot molding method
of any of (12) to (14), wherein the cooling medium is
one kind or two kinds or more of water, a polyhydric
alcohol, a polyhydric alcohol solution, polyglycol, a
mineral oil with a flash point of 120°C or higher,
synthetic ester, a silicon oil, a fluorine oil,
grease with a dropping point of 120°C or higher, and a
water emulsion obtained by mixing a surfactant into a
mineral oil or synthetic ester.
(16) The metal plate material hot molding method
of any of (12) to (15), wherein the cooling medium is
ejected during holding at a press bottom dead center.
Brief Description of the Drawings
Fig. 1A is a sectional view showing an example of a mold of the present invention provided with ejection holes and supply piping for a cooling medium;
Fig. IB is a perspective view of the example of the mold in Fig. 1A;
Fig. 2A is a sectional view showing an example of a mold of the present invention provided with ejection holes, supply piping, discharge holes, and discharge piping for a cooling medium;
Fig. 2B is a perspective view of the example of
the mold in Fig. 2A;
Fig. 3A is a sectional view showing an example of a mold provided with ejection holes, supply piping, and cooling piping for a cooling medium;
Fig. 3B is a perspective view of the example of the mold in Fig. 3A;
Fig. 4 is a view schematically showing part of the surface of a mold provided with ejection holes, discharge holes, and projections;
Fig.5A is a view schematically showing part of a section of an example of the mold provided with the ejection holes, the discharge holes, and the projections; and
Fig. 5B is a view schematically showing another example of the mold in Fig. 5A.
Detailed Description of the Preferred Embodiments
The present invention is designed such that in a metal plate material hot press molding method of heating a metal plate material to a predetermined temperature (for example, between 700°C and 1000°C) by an electric heating furnace or a heating device by induction heating, electric current heating, or the like, setting the high-temperature metal plate material in a mold of a press molding apparatus, pressing the metal plate material by molding surfaces of the mold, that is, contact surfaces of opposed punch and die, and holding the mold at a bottom dead center, a cooling medium is ejected from the mold
during and/or after molding to forcibly cool a molded piece and the mold.
Examples of molds of the present invention shown in Fig. 1 to Fig. 3 will be described in detail below
Figs. 1A and IB schematically show an aspect in which ejection holes 4 and supply piping 6 for the cooling medium of the present invention are provided in a die 2 being a lower mold, and the supply piping 6 for the cooling medium provided in the die 2 and a die holder 2' are connected by bolts via 0-rings 11. In Fig. 1A, a rubber 0-ring as a sealing mechanism 12 which prevents the cooling medium from flowing out is provided at a periphery of the die 2. Figs. 1A and IB show the example in which the ejection holes 4 for the cooling medium are provided in a vertical wall portion of the die, but they may be provided in a bottom portion or may be provided in both the vertical wall portion and the bottom portion.
Figs. 2A and 2B schematically show an example in which the ejection holes 4 and discharge holes 5 for the cooling medium are provided in a punch 3 being an upper mold, the supply piping 6 for the cooling medium is provided in a punch holder 3', and discharge piping 7 for the cooling medium is provided in a core 3" and the punch holder 3'. In Figs. 2A and 2B, the supply piping 6 for the cooling medium is formed by the core 3" provided inside the punch 3. The discharge piping 7 provided in the punch holder 3' and the core 3", and the supply piping 6 for the
cooling medium in the punch holder 3' and the punch 3 are respectively connected by bolts via the 0-rings 11. As in Fig. 1, the rubber 0-ring as the sealing mechanism 12 for the cooling medium is provided at the periphery of the lower die 2.
An ejection valve 9 with a spring mechanism is provided in the ejection hole 4 in Figs. 2A and 2B, and closes an outlet of the supply piping 6 for the cooling medium, for example, when the punch reaches the bottom dead center at the time of pressing, and when the internal pressure of the cooling medium is increased, the ejection valve 9 opens and the cooling medium is ejected from the ejection hole 4 to the surface of the mold. The ejected cooling medium is discharged from the discharge piping 7 through an intermediate barrel 10 which crosses the supply piping 6 from a discharge hole 5. Incidentally, Figs 2A and 2B show the example in which the ejection holes 4 and discharge holes 5 for the cooling medium are provided in a vertical wall portion of the punch, but they may be provided in a bottom portion or may be provided in both the vertical wall portion and the bottom portion.
Fig. 3 shows an example in which cooling piping 8 is further provided in the die 2 provided with the ejection holes 4 and supply piping 6 for the cooing medium shown in Fig. 1. The mold is cooled by the supply piping 6 for the cooling medium, but by further providing the cooling piping 8, the cooling
of the mold is accelerated. The cooling piping 8 is also effective in accelerating the cooling of the mold provided with the supply piping 6 and discharge piping 7 for the cooling medium shown in Fig. 2. Moreover, by providing the cooling piping 8, for example, it is possible to suppress an increase in the temperature of the mold when press molding is performed until the bottom dead center is reached without the cooling medium being supplied to the supply piping 6.
Figs. 1 to 3 each show the example in which the ejection holes 4, supply piping 6, discharge holes 5, discharge piping 7, and cooing piping 8 for the cooling medium are provided in either of the punch 3 and the die 2, but they may be provided in both of the punch 3 and the die 2. Moreover, it is necessary to provide at least the ejection holes 4 and supply piping 6 for the cooling medium. In this case, it is possible to continuously eject the cooling medium from the ejection holes while continuing to supply the cooling medium to the supply piping 6, and it is also possible to discharge the cooling medium if the supply of the cooling medium to the supply piping 6 is stopped to bring the internal pressure to a negative pressure. Accordingly, depending on the size and shape of the mold, it can be selected appropriately whether the ejection holes 4 and the supply piping 6 are used for discharging the cooling medium or the independent discharge holes 5 and
discharge piping 7 are further provided.
When the shapes of the ejection hole 4 and the discharge hole 5 are circular, a sufficient supply of liquid cannot be obtained due to pressure loss if their diameter is less than 100 jim, whereby it is desirable that the lower limit of the diameter be 100
jam or more. On the other hand, if the diameter of the ejection hole 4 and the discharge hole 5 is more than 10 mm, the shapes thereof are transferred to the metal plate material, whereby it is desirable that the upper limit of the diameter be 10 mm or less. Incidentally, when the shapes of the ejection hole 4 and the discharge hole 5 are rectangular or elliptical and when the ejection hole 4 and the discharge hole 5 have indeterminate forms such as holes of porous metal, the area of a flow path needs to be equal to that of a circle with a diameter between 100 jam and 10 mm. When the pitch of the ejection holes 4 and the discharge holes 5, that is, the distance between the adjacent ejection holes 4 when only the ejection holes 4 are provided or the distance between the adjacent ejection holes 4 or discharge holes 5 when both the ejection holes 4 and the discharge holes 5 are provided is less than 100 j^m, the number of holes increases, resulting in an increase in the cost of the mold. On the other hand, the pitch of the ejection holes 4 and the discharge holes 5 is more than 1000 mm, cooling capacity becomes sometimes insufficient. Accordingly, it is
desirable that the pitch of the ejection holes 4 and the discharge holes 5 be between 100 jam and 1000 mm.
It is desirable that die steel for hot working be used as a material for the mold in terms of hot strength. When the cooling piping is provided in both the punch and the die, die steel for cold working which ha high heat conductivity and is resistant to heat storage may be used. The ejection holes, the discharge holes, and the cooling piping can be provided by mechanical drilling by a drill or by drilling by electric discharge machining.
Furthermore, instead of drilling the ejection holes and discharge holes for the cooling medium in the mold, the supply piping for the cooling medium may be connected to porous metal having pores which penetrate from within the mold to the outer surface. In this case, it is desirable to use porous metal having plural holes with a diameter between 100 p.m and 1 mm and a pitch between 100 jam and 10 mm which penetrate in a thickness direction. For example, if in a punch having a structure such as shown in Fig. 2, die steel is used for the core 3" and porous metal is used for the punch 3, the punch 3 having the fine ejection holes 4 and discharge holes 5 with a small pitch can be manufactured. Such porous metal can be manufactured by sintering powder after molding or by unidirectional solidification for making the direction of a solidification structure fixed by temperature control after melting metal.
Incidentally, it is also possible to manufacture the entire punch 3 by the porous metal, or it is also possible to provide holes in portions corresponding to the ejection holes 4 and discharge holes 5 for the cooling medium in Figs. 2A and 2B by machining and join the porous metal into the holes by shrink fitting or the like.
Moreover, by providing projections 13 on the molding surface of the mold, the area of contact between the mold and the metal plate material can be reduced, and hence the occurrence of die galling can be suppressed. Furthermore, since the area of contact between the mold, that is, the die 2 or the punch 3 and the metal plate material 1 is reduce by these projections 13, excessive cooling of the metal plate material 1 due to the movement of heat to the mold during press molding can be suppressed. When the cooling medium is ejected at the bottom dead center, it becomes easy to circulate the cooling medium through gaps between the projections 13 and the metal plate material 1, which makes it possible to increase cooling efficiencies of the mold and the metal plate material 1.
A schematic view and sectional views of the surface of part of the mold provided with the projections 13 on its molding surface are shown in Figs. 4 and 5, respectively. The projections 13 shown in Figs. 4 and 5 as an example are circular cylinders which are provided at predetermined
intervals on the molding surface of the mold, but it is desirable that the shape of their horizontal sections be any of a circular shape, a polygonal shape, and a star-shape, and that the shape of their vertical section be rectangular or trapezoidal. They also may be hemispherical. Incidentally, it is desirable that plural projections 3 of the mold be provided on the molding surface, and the projections 13 may be provided on part of the molding surface or may be provided on the entire surface. Furthermore, they may be provided on either or both of the punch and the die.
Incidentally, as shown in Fig. 5A, the projections 13 of the mold may be provided as they are on the surface of the molding surface, but depending on molding conditions, marks of the projections 13 are sometimes transferred to the molded piece. To prevent this, it is recommended to remove only peripheries of the projections 13 as shown in Fig. 5B. Furthermore, it is also possible to remove portions where the projections 13 are provided to a depth equal to the height of the projection 13 and provide the projections 13.
It is desirable that the height of the
projections 13 on the molding surface of the mold be between 5 |im and 1 mm. This is because if the height of the projections 13 is lower than 5 |j,m, the gap between the mold and the metal plate material 1 is too small, so that it is difficult to circulate
liquid between the mold and the metal plate material 1, and if the height is higher than 1 mm, the gap is too large, so that the cooling rate by heat conductivity of the liquid lowers.
It is desirable that the area ratio of the projections 13 on the molding surface of the mold be between 1% and 90%. This is because if the area ratio of the projections 13 is less than 1%, projection shapes on the surface of the mold tend to be transferred to the metal plate material, and if it is more than 90%, the gap between the projections is narrow, whereby pressure loss becomes larger and the liquid can neither be filled nor flow, which causes a slight reduction in cooling efficiency.
It is desirable that the diameter of the projection when the shape of the horizontal section of the projection on the molding surface of the mold is circular or the diameter of a circumcircle of the projection when the shape thereof is polygonal or star-shaped be between 10 ^m and 5 mm. This is because if the diameter of the projection or the diameter of the circumcircle is less than 10 f^m, the projection wears badly, and cannot produce an effect over a long period, and if the diameter thereof is more than 5 mm, uniform cooling cannot be performed.
The projections on the molding surface of the mold can be formed by electrochemical machining, chemical etching, electric discharge machining, or a plating method. The chemical etching can be
performed in the following manner. First, after a visible light curing photosensitive resin is applied on the surface of the mold and dried, visible light is irradiated to cure an irradiated portion while the surface is covered with a mask for cutting off the visible light. Then, the resin except that on the cured portion is removed by an organic solvent. For example, it is recommended to perform etching by immersing the surface of the mold in an etching solution such as a sodium chloride solution for one minute to thirty minutes. The diameter or pitch of the projections may be selected appropriately depending on the shape of the mask for cutting off the visible light, and the height of the projections may be adjusted appropriately depending on the etching time.
Electro discharge texturing is a processing method in which a copper electrode having recesses each with an inverted shape of the targeted projection as a surface pattern is placed opposite the mold and a pulse direct current is passed while its current peak value and pulse width are changed. The desirable current value is between 2 A and 100 A and pulse width is between 2 psec and 1000 f^sec, and they need to be adjusted appropriately according to the material of the mold and the desired shape of the projections.
In the case of the plating method, in order that the diameter of the hemispherical projection is set
to 10 f^m or more, it is desirable that the thickness of plating be 10 f^m or more, and that the upper limit thereof be 80 ^m or less to prevent exfoliation. After alkaline degreasing and electrolytic etching of electrolyzing the mold as an anode in a plating solution, a plating layer can be formed at a predetermined bath temperature and current density. Incidentally, a plating layer with a thickness between 10 jam and 80 {jm can be provided under conditions of a current density approximately between 1 A/dm2 and 200 A/dm2 and a bath temperature
approximately between 30°C and 60°C in a chrome plating solution in the case of chrome plating, and under conditions of a current density approximately between 1 A/dm2 and 100 A/dm2 and a bath temperature approximately between 30°C and 60°C in a NiW plating solution in the case of NiW plating. Incidentally, in order to form a plating layer having a hemispherical projection shape, for example, it is required to perform plating at a fixed current density after the current density is increased stepwise.
Furthermore, it is desirable that the ejection holes 4, the discharge holes 5, and the projections 13 be each provided at a portion where the heat transfer coefficient between the mold and the metal plate material is 2000 W/m2K or less. For example, by performing hot press molding while measuring the temperatures of the mold and the metal plate material
using a thermocouple, a radiation thermometer, or the like before the ejection holes 4, the discharge holes 5, and the projections 13 are each provided, the portion where the heat transfer coefficient between the mold and the metal plate material is 2000 W/m2K or less can be worked out from the temperature changes of the mold and the metal plate material. It is also possible to calculate the deformation behavior and gap amount between the mold and the metal plate material by FEM and determine the portion where the heat transfer coefficient is 2000 W/m2K or less. Consequently, it becomes possible to eject the cooling medium to a portion which requires acceleration of cooling and enhance cooling, which enables uniform cooling and reductions in the manufacturing cost and cooling cost of the mold.
A hot press molding method of the present invention is designed to enhance cooling by ejecting the cooling medium to the gap between the mold and the metal plate material during and/or after press molding. For example, when the metal plate material 1 is press-molded using the hot press molding apparatus shown in Figs. 1 and 3, the cooling medium is supplied from the supply piping 6 and ejected to the gap between the mold and the metal plate material 1 from the ejection holes 4 while the punch 3 is lowered to and held at the bottom dead center. In this case, if the internal pressure in the supply piping 6 is brought to a negative pressure, the
cooling medium can be discharged from the ejection holes 4, and hence, if the ejection and discharge of the cooling medium are repeated intermittently, the cooling effect increases. Similarly, also in the case of the hot press molding apparatus provided with the discharge holes 5 and the discharge piping 7 shown in Fig. 2, the cooling medium can be discharged from the ejection holes 4.
Incidentally, when the nucleate boiling of the cooling medium is predicted from a calculation based on the boiling point of the cooling medium, heat conductivity, the heat capacity of the metal plate material, and so on, it is desirable to constantly eject the cooling medium from the ejection holes to let it flow to the discharge holes. When the nucleate boiling of the cooling medium is not predicted, the gap between the mold and the metal plate material may remain filled with the cooling medium.
The cooling medium may be any of water, a polyhydric alcohol, a polyhydric alcohol solution,
polyglycol, a mineral oil with a flash point of 120°C or higher, synthetic ester, a silicon oil, a fluorine
oil, grease with a dropping point of 120°C or higher, and a water emulsion obtained by mixing a surfactant into a mineral oil or synthetic ester, or a mixture of these may be used in terms of flame retandancy and corrosiveness. Furthermore, the cooling medium may be liquid or vapor.
Hot-press molding according to the present invention is also applicable to any of metal plate materials such as an Al-plated steel plate, a Zn-plated steel plate, ordinary steel, copper, and aluminum. Incidentally, when the material of the metal plate material is steel, it is desirable that the temperature of the entire steel plate be maintained at not higher than a martensitic transformation point of the steel at the bottom dead center.
-Examples-
The present invention will be more specifically described below by examples.
A hat-shaped product is manufactured by way of trial by manufacturing the mold which is schematically shown in Fig. 2 by machining, and further drawing Al-plated steel using the hot press molding apparatus provided with the projections 13 which is schematically shown in Figs. 4 and 5. The length of a specimen is 300 mm, width is 100 mm, thickness is 1.2 mm, and surface roughness is 1.0 (j,m. The material of the die and the punch is S45C, shoulder width is 5 mm, die width is 70 mm, and die molding depth is 60 mm.
Porous metal is fabricated by unidirectional solidification of fixing a rod with a diameter of 10 mm which is made of stainless steel composed of a SUSS 04L-based component in a high-pressure container, moving a portion to be heated while partially melting
the rod by high-frequency induction heating, and thereby continuously melting and solidifying the rod.
Ejection holes, discharge holes, and projections of the mold are those shown in Table 1, and the surface roughness is 1.0 µm. Incidentally, before processing of providing the ejection holes, the discharge holes, and the projections, hot-press molding is performed while the temperature is measured by a thermocouple to specify portions where the heat transfer coefficient is 2000 W/m2K or less, and more specifically, the ejection holes, the discharge holes, and the projections are provided in sidewall surfaces of the die and the punch.
The Al-plated steel plate is heated to
approximately 950°C in an atmosphere furnace, and the heated steel plate is set at a molding position between the punch and the die, subjected to hot press molding, held for two seconds at the bottom dead center, and cooled by ejecting the cooling medium. In comparative example 12, it is held for ten seconds at the bottom dead center. Thereafter, the mold is released, and the product is taken out. This molding is performed continuously 100 times. Furthermore, using the specimen and the mold under the same conditions, a comparative product is manufactured by
heating the specimen to approximately 950°C, hot press molding it, and then immediately cooling it by immersing it in a tank without holding it.
The hardness, shape, surface damage, and mold
surface temperature regarding each of the obtained products are evaluated, and results thereof are shown in Table 1. The hardness of the product is measured at a pitch of 10 mm in a longitudinal direction. If the hardnesses at all positions of all the products are higher than the hardness of the comparative product, the hardness is regarded as good and shown
by "©".
The shape of the product is evaluated by comparing the shape of the product measured by a laser displacement meter with a designed shape, and if the error between the shape of the product and the designed shape is within 10%, the shape is regarded as good and shown by "©". The evaluation of surface damage is performed by visually examining a sidewall portion of the product, and if no galling is observed in all the products, the evaluation of surface damage is regarded as good and shown by "©".
If the percent defective of hardness, shape, and surface damage is 1% or less, the comprehensive evaluation is regarded as good and shown by "O", and if it is more than 1%, the comprehensive evaluation
is regarded as bad and shown by "x". Furthermore, after molding, the mold surface temperature is measured by a contact-type surface thermometer, and if the mold surface temperature is 80°C or lower, it is regarded as good and shown by "O", and if it is higher than 80°C, it is regarded as bad and shown by "x".
As shown in Table 1, the products manufactured within the scope of the hot press molding method of the present invention using the hot press molding apparatus of the present invention have good hardnesses and shapes, have no surface damage, cause a small increase in mold temperature, and receive good comprehensive evaluations. On the other hand, in comparative examples 11 and 12, a conventional molding apparatus provided with no ejection hole for the cooling medium is used, and the comparative example 12 which has a longer holding time than the comparative example 11 has good hardness and shape, but receives a bad comprehensive evaluation. [TABLE 1]

(Table Removed)
Industrial Applicability
The present invention makes an extremely remarkable industrial contribution such that when a pressed product excellent in strength and dimensional accuracy is manufactured using a high-strength metal plate material with low press moldability as a material by hot press molding, it is possible to increase productivity and further suppress heat storage into a mold to lengthen the life of the mold, thereby reducing a manufacturing cost.

We Claim:
1. A metal plate material hot molding apparatus wherein, in a metal plate material hot
molding apparatus for press molding a heated metal plate material (1), supply piping (6)
for a cooling medium is provided in a mold (2, 3), ejection holes (4) for the cooling
medium are provided in a molding surface of the mold, the supply piping (6) and the
ejection holes communicate with each other, characterized in that
plural projections (13) having an area ratio between 1% and 90%, a diameter or circumcircle diameter between 10 um and 5 mm, and a height between 5 µm and 1 mm are provided on at least part of the molding surface of the mold.
2. The metal plate material hot molding apparatus as claimed in claim 1, wherein a valve mechanism is provided in the ejection hole (4).
3. The metal plate material hot molding apparatus as claimed in claim 1, wherein a sealing mechanism (12) which prevents the cooling medium from flowing out is provided at a periphery of the mold (2, 3).
4. The metal plate material hot molding apparatus as claimed in claim 1, wherein at least part of the molding surface of the mold is formed of porous metal having plural holes penetrating, and the supply piping (6) and the plural pores penetrating of the porous metal communicate with each other.
5. The metal plate material hot molding apparatus as claimed in claim 4, wherein the projection is a NiW-plated layer or chrome-plated layer with a thickness between 10 µm and 80 urn.
6. The metal plate material hot molding apparatus as claimed in any one of claims 1 to 5, wherein the ejection hole (4) for the cooling medium is provided only in a portion where a heat transfer coefficient between the metal plate material and the mold is 2000 W/m2 K or less.
7. The metal plate material hot molding apparatus as claimed in any one of claims 1 to 4, wherein discharge piping (7) for a cooling medium is provided in the mold, discharge holes (5) for the cooling medium are provided in a molding surface of the mold, and the discharge piping and the discharge holes communicate with each other.
8. The metal plate material hot molding apparatus as claimed in any one of claims 1 to 4, wherein cooling piping (8) is provided in the mold.
9. A metal plate material hot molding method, wherein in a metal plate hot molding method of press molding a heated metal plate material (1) using the metal plate material hot molding apparatus as claimed in any one of claims 1 to 8, wherein molding is performed

while a cooling medium is ejected to a gap between the metal plate material and a mold from ejection holes (4).
10. The metal plate material hot molding method as claimed in claim 9, wherein the cooling medium ejected to the gap between the metal plate material and the mold is discharged from the ejection holes (4) and/or discharge holes (5).
11. The metal plate material hot molding method as claimed in claim 9, wherein the cooling medium is ejected only to a portion where a heat transfer coefficient calculated by measuring temperatures of the metal plate material and the mold is 2000 W/m2 K or less.
12. The metal plate material hot molding method as claimed in claim 9, wherein the cooling medium is one kind or two kinds or more of water, a polyhydric alcohol, a polyhydric alcohol solution, polyglycol, a mineral oil with a flash point of 120°C or higher, synthetic ester, a silicon oil, a fluorine oil, grease with a dropping point of 120°C or higher, and a water emulsion obtained by mixing a surfactant into a mineral oil or synthetic ester.
13. The metal plate material hot molding method as claimed in claim 9, wherein the cooling medium is ejected during holding at a press bottom dead center.

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Patent Number

269557

Indian Patent Application Number

2343/DELNP/2006

PG Journal Number

44/2015

Publication Date

30-Oct-2015

Grant Date

27-Oct-2015

Date of Filing

27-Apr-2006

Name of Patentee

NIPPON STEEL & SUMITOMO METAL CORPORATION

Applicant Address

6-1, Marunouchi 2-chome, Chiyoda-ku, TOKYO 100-8071, JAPAN

Inventors:

#	Inventor's Name	Inventor's Address
1	YASUSHI KURISU	C/O NIPPON STEEL CORPORATION, TECHNICAL DEVELOPMENT BUREAU, 20-1, SHINTOMI, FUTTSU - SHI, CHIBA 293-8511, JAPAN.
2	KAZUTO YAMAMURA	C/O NIPPON STEEL CORPORATION, TECHNICAL DEVELOPMENT BUREAU, 20-1, SHINTOMI, FUTTSU - SHI, CHIBA 293-8511, JAPAN.
3	YUUICHI ISHIMORI	C/O NIPPON STEEL CORPORATION, TECHNICAL DEVELOPMENT BUREAU, 20-1, SHINTOMI, FUTTSU - SHI, CHIBA 293-8511, JAPAN.
4	HIROYUKI MITAKE	C/O NIPPON STEEL CORPORATION, TECHNICAL DEVELOPMENT BUREAU, 20-1, SHINTOMI, FUTTSU - SHI, CHIBA 293-8511, JAPAN.
5	HIROSHI FUKUCHI	C/O NIPPON STEEL CORPORATION, TECHNICAL DEVELOPMENT BUREAU, 20-1, SHINTOMI, FUTTSU - SHI, CHIBA 293-8511, JAPAN.
6	YOSHIAKI SHIA	C/O NIPPON STEEL CORPORATION, TECHNICAL DEVELOPMENT BUREAU, 20-1, SHINTOMI, FUTTSU - SHI, CHIBA 293-8511, JAPAN.
7	TETSUO SHIMA	C/O NIPPON STEEL CORPORATION, TECHNICAL DEVELOPMENT BUREAU, 20-1, SHINTOMI, FUTTSU - SHI, CHIBA 293-8511, JAPAN.
8	NORIMASA YAMASAKI	C/O NIPPON STEEL CORPORATION, TECHNICAL DEVELOPMENT BUREAU, 20-1, SHINTOMI, FUTTSU - SHI, CHIBA 293-8511, JAPAN.

PCT International Classification Number

B21D 22/20

PCT International Application Number

PCT/JP2004/014174

PCT International Filing date

2004-09-28

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	2003-344309	2003-10-02	Japan