Metal mold and method for manufacturing hot press molded article

Abstract

The present invention provides a metal mold capable of using hot press molding to manufacture a hot press molded article that has a region having high strength and a region having low strength. A metal mold (10) according to the present disclosure comprises an upper mold (11) and a lower mold (12). A first molding surface (110) of the upper mold (11) includes a first rapid cooling region (1A) and a first slow cooling region (1B). A second molding surface (120) of the lower mold (12) includes a second rapid cooling region (2A) and a second slow cooling region (2B). The contact area percentage of the first rapid cooling region (1A) is 95% or more. The contact area percentage of the second rapid cooling region (2A) is 95% or more. The contact area percentage of the first slow cooling region (1B) is 9 to 33%. The contact area percentage of the second slow cooling region (2B) is 9 to 33%.

Description

Mold and method for manufacturing hot press-formed product

[0001] The present disclosure relates to a mold, and more particularly to a mold for hot press forming.

[0002] In the automotive field, weight reduction of vehicle bodies is required to improve fuel efficiency. Furthermore, in the automotive field, improvement of collision safety is also required to protect occupants in the event of a collision. Reducing the weight of vehicle bodies is effective in reducing the thickness of structural members used in the body. In order to improve collision safety while reducing the thickness of structural members, high strength is required for materials used in structural members of the body. On the other hand, materials with high strength have low press formability. Therefore, hot press forming has been proposed as a method for forming structural members using such high-strength materials.

[0003] In hot press forming, a blank (steel plate) is heated to a temperature range where the microstructure becomes austenite single phase. The heated blank is then hot-pressed using a hot press device equipped with a die. In hot press forming, the blank is softened by heating. This makes it easier to press-form the blank into a desired shape. Furthermore, in hot press forming, the blank comes into contact with almost the entire forming surface of the die during hot press forming. At this time, the die in contact with the blank removes heat from the blank and quenches it. As described above, in hot press forming, the blank is quenched simultaneously as it is formed. Therefore, hot press forming can easily produce high-strength hot-press-formed products.

[0004]

[0003] Meanwhile, hot press-formed products used as structural members of automobiles are required to have high strength, but may also be required to have partially low strength. For example, when a hot press-formed product is used as a B-pillar, the lower part of the B-pillar is required to have low strength to enhance impact absorption. On the other hand, the remaining part other than the lower part is required to have high strength (high rigidity).

[0005] A hot press molding die for forming a hot press-formed product having such a high-strength region and a low-strength region is proposed in Patent Document 1 (JP 2014-79790 A). In the hot press molding disclosed in Patent Document 1, a heated material is hot press-formed using a die. During hot press molding, a refrigerant is flowed through a portion of the gap between the die and the material to partially quench the material. Specifically, the die of Patent Document 1 has a supply port on the die surface that supplies refrigerant from a refrigerant path inside the die to the outside of the die. The die of Patent Document 1 further has a convex portion that contacts the boundary region between the high-strength portion and the low-strength portion of the material. During hot press molding, the refrigerant is supplied from the supply port to the gap between the high-strength portion and the die. On the other hand, the refrigerant is not supplied to the gap between the low-strength portion and the die. The convex portion that contacts the boundary between the high-strength portion and the low-strength portion blocks the refrigerant. Therefore, the refrigerant filled in the gap between the high strength portion and the die does not flow into the gap between the low strength portion and the die, resulting in a hot press-formed product having a high strength portion and a low strength portion.

[0006] JP 2014-79790 A

[0007] The mold disclosed in Patent Document 1 can produce a hot press-formed product having a high strength region and a low strength region. However, a hot press-formed product having a high strength region and a low strength region may be produced by a technique different from the technique described in Patent Document 1.

[0008] An object of the present disclosure is to provide a mold capable of producing a hot press-formed product having a region with high strength and a region with low strength by hot press forming.

[0009] The die disclosed herein is a die for performing hot press forming on a blank to form a hot press-formed product. The die disclosed herein includes an upper die having a first forming surface and a lower die having a second forming surface. The lower die is disposed opposite the first forming surface across the blank during hot press forming, and hot press forms the blank together with the first forming surface. The first forming surface includes a first quenching region and a first slow cooling region. The first quenching region contacts the blank to cool it during hot press forming. The first slow cooling region contacts the blank to cool it slower than the first quenching region during hot press forming. The second forming surface includes a second quenching region and a second slow cooling region. The second quenching region is disposed opposite the first quenching region across the blank during hot press forming, and contacts the blank to cool it. The second slow cooling region is disposed opposite the first slow cooling region across the blank during hot press forming, and contacts the blank to cool it slower than the second quenching region. When the proportion per unit area of ​​the first quenching region that contacts the blank during hot press forming is defined as the contact area ratio of the first quenching region, the contact area ratio of the first quenching region is 95% or more. When the proportion per unit area of ​​the second quenching region that contacts the blank during hot press forming is defined as the contact area ratio of the second quenching region, the contact area ratio of the second quenching region is 95% or more. When the proportion per unit area of ​​the first slow cooling region that contacts the blank during hot press forming is defined as the contact area ratio of the first slow cooling region, the contact area ratio of the first slow cooling region is 9 to 33%. When the proportion per unit area of ​​the second slow cooling region that contacts the blank during hot press forming is defined as the contact area ratio of the second slow cooling region, the contact area ratio of the second slow cooling region is 9 to 33%.

[0010] The mold according to the present disclosure can produce a hot-press-formed product having regions with high strength and regions with low strength by hot-press forming.

[0011] FIG. 1 is a front view showing an example of a hot press apparatus 1 for hot press forming. FIG. 2 is a perspective view of the mold in FIG. 1. FIG. 3 is a perspective view of a hot press-formed product formed by hot press forming using the mold in FIG. 2. FIG. 4 is an example of a plan view of the first or second quenching region. FIG. 5A is an example of a plan view of the first or second slow cooling region. FIG. 5B is another example of a plan view of the first or second slow cooling region that is different from FIG. 5A. FIG. 5C is another example of a plan view of the first or second slow cooling region that is different from FIGS. 5A and 5B. FIG. 5D is another example of a plan view of the first or second slow cooling region that is different from FIGS. 5A to 5C. FIG. 5E is another example of a plan view of the first or second slow cooling region that is different from FIGS. 5A to 5D. FIG. 5F is another example of a plan view of the first or second slow cooling region that is different from FIGS. 5A to 5E. FIG. 5G is another example of a plan view of the first or second gentle cooling region, different from FIGS. 5A to 5F. FIG. 6 is a schematic diagram for explaining the positional relationship between the top surfaces of the convex portions in the first gentle cooling region and the top surfaces of the convex portions in the second gentle cooling region during hot press forming. FIG. 7 is a perspective view of a mold according to a second embodiment. FIG. 8 is a perspective view of a hot press-formed product formed by hot press forming using the mold of FIG. 7. FIG. 9 is a perspective view of a mold according to a third embodiment. FIG. 10 is a perspective view of a hot press-formed product formed by hot press forming using the mold of FIG. 9. FIG. 11 is a perspective view of a mold according to a fourth embodiment. FIG. 12 is a perspective view of a hot press-formed product formed by hot press forming using the mold of FIG. 11. FIG. 13 is an example of an enlarged plan view of the first or second gentle cooling region in a mold according to a fifth embodiment.

[0012] The die of this embodiment according to the first configuration is a die for performing hot press forming on a blank to form a hot press-formed product. This die includes an upper die having a first forming surface and a lower die having a second forming surface. The lower die is disposed opposite the first forming surface across the blank during hot press forming, and hot press forms the blank together with the first forming surface. The first forming surface includes a first quenching region and a first slow cooling region. The first quenching region contacts the blank to cool it during hot press forming. The first slow cooling region contacts the blank to cool it slower than the first quenching region during hot press forming. The second forming surface includes a second quenching region and a second slow cooling region. The second quenching region is disposed opposite the first quenching region across the blank during hot press forming, and contacts the blank to cool it. The second slow cooling region is disposed opposite the first slow cooling region across the blank during hot press forming, and contacts the blank to cool it slower than the second quenching region. When the proportion per unit area of ​​the first quenching region that contacts the blank during hot press forming is defined as the contact area ratio of the first quenching region, the contact area ratio of the first quenching region is 95% or more. When the proportion per unit area of ​​the second quenching region that contacts the blank during hot press forming is defined as the contact area ratio of the second quenching region, the contact area ratio of the second quenching region is 95% or more. When the proportion per unit area of ​​the first slow cooling region that contacts the blank during hot press forming is defined as the contact area ratio of the first slow cooling region, the contact area ratio of the first slow cooling region is 9 to 33%. When the proportion per unit area of ​​the second slow cooling region that contacts the blank during hot press forming is defined as the contact area ratio of the second slow cooling region, the contact area ratio of the second slow cooling region is 9 to 33%.

[0013] Here, "during hot press forming" means a state in which the first forming surface of the upper mold and the second forming surface of the lower mold are in contact with the blank and applying an external force to the blank, and means a state in which the mold is closed and the blank is cooled by the mold while the upper mold is held at the bottom dead center.

[0014] The mold according to the first configuration includes two types of regions (first and second quenching regions and first and second slow cooling regions) having different contact area ratios with the blank during hot press forming. The first and second slow cooling regions have a smaller contact area ratio with the blank during hot press forming than the first and second quenching regions. Therefore, the contact heat transfer coefficient of the first and second slow cooling regions can be lowered compared to the first and second quenching regions. Therefore, during hot press forming, the cooling rate of the blank region sandwiched between and in contact with the first and second slow cooling regions can be slower than the cooling rate of the blank region sandwiched between and in contact with the first and second quenching regions. Therefore, the blank region sandwiched between the first and second slow cooling regions can have a different structure from the blank region sandwiched between the first and second quenching regions. For example, the blank region sandwiched between the first and second slow cooling regions can be a low-strength region that is made of a structure containing hard structure (martensite and / or bainite) and retained austenite, thereby reducing the strength and making it prone to plastic deformation, while the blank region sandwiched between the first and second rapid cooling regions can be a high-strength region made of martensite and having a strength higher than that of the low-strength region.

[0015] Preferably, in the mold of this embodiment according to the first configuration, each of the first and second rapid cooling regions is smooth. Furthermore, each of the first and second slow cooling regions includes a plurality of protrusions. The plurality of protrusions are arranged in a plurality of directions. The protrusions have top surfaces that come into surface contact with the blank during hot press forming. The top surfaces of the plurality of protrusions are arranged with gaps between them (second configuration).

[0016] In the mold of the second configuration, the top surfaces of adjacent convex portions in each of the first and second slow cooling regions are arranged with a gap between them. Therefore, during hot press forming, the contact area ratio of the first and second slow cooling regions in contact with the blank is low. Furthermore, air is present in the gaps formed between the convex portions. During hot press forming, the air present in these gaps absorbs heat from the blank and retains the absorbed heat. Therefore, the contact heat transfer coefficient of the first and second slow cooling regions is reduced.

[0017] On the other hand, the first and second quenching regions are smooth. Therefore, during hot press forming, the contact area ratio of the first and second quenching regions in contact with the blank is significantly higher than the contact area ratio of the first and second slow cooling regions. Furthermore, because the first and second quenching regions are smooth, air is less likely to be trapped. As a result, the contact heat transfer coefficient of the first and second quenching regions is significantly higher than the contact heat transfer coefficient of the first and second slow cooling regions.

[0018] Preferably, in a mold of the second configuration, when the first and second slow cooling regions are in contact with the blank, sandwiching the blank, during hot press forming, when the first and second slow cooling regions are viewed from the normal direction of the first slow cooling region, the top surfaces of the convex portions of the first slow cooling region at least partially overlap with the top surfaces of the corresponding convex portions of the second slow cooling region (third configuration).

[0019] In the mold of the third configuration, when the first and second slow cooling regions are in contact with the blank, sandwiching the blank between them, the convex portions of the first slow cooling region are prevented from entering the gaps between the multiple convex portions of the second slow cooling region during hot press forming, thereby preventing the blank from being deformed into a wavy shape by hot press forming.

[0020] Preferably, in the mold of the second configuration, the first or second slow cooling region further includes a rib portion. The rib portion is formed surrounding a plurality of protrusions. The top surfaces of the rib portion are disposed with a gap provided between them and the top surfaces of the protrusions (fourth configuration). This configuration can be applied to the mold of the third configuration (fifth configuration).

[0021] In the fourth and fifth mold configurations, the rib portion is formed to surround the periphery of the multiple protrusions. In this case, air present in the gaps between the top surfaces of the multiple protrusions is less likely to flow out of the rib portion. Therefore, air that absorbs heat during hot press forming tends to remain inside the rib portion. This further reduces the contact heat transfer coefficient between the first or second slow cooling region and the blank.

[0022] Preferably, in the mold of the fourth configuration, the rib portion includes a plurality of protrusions arranged in a plurality of directions, and the gap between adjacent protrusions is smaller than the gap between adjacent convex portions (sixth configuration). Such a configuration can be applied to the mold of the fifth configuration (seventh configuration).

[0023] In the molds of the sixth and seventh configurations, adjacent protrusions in the rib portion are arranged with a gap smaller than the gap between adjacent convex portions inside the rib portion. In this case, the air interposed in the gaps between the protrusions is less likely to flow than the air interposed in the gaps between the convex portions. Therefore, during hot press forming, the air interposed in the gaps between the protrusions is more likely to absorb heat from the blank and accumulate inside the rib portion, while the air interposed in the gaps between the convex portions is more likely to absorb heat from the blank and accumulate inside the rib portion. This further reduces the contact heat transfer coefficient between the first or second slow cooling region and the blank.

[0024] Preferably, in the mold of any one of the first to seventh configurations, the hot press-formed product extends in the longitudinal direction. The first quenching region and the first slow cooling region are arranged in a direction corresponding to the longitudinal direction of the hot press-formed product sandwiched between the upper and lower dies during hot press forming. The second quenching region and the second slow cooling region are arranged in a direction corresponding to the longitudinal direction of the hot press-formed product sandwiched between the upper and lower dies during hot press forming (eighth configuration).

[0025] In the die of the eighth configuration, for example, the end portion (lower portion) of a hot press-formed product having a longitudinal direction, such as a B-pillar, can be made a low-strength region, and the remaining portion can be made a high-strength region, so that the end portion of the hot press-formed product can absorb impact, and the remaining portion can ensure strength.

[0026] Preferably, in a mold of any one of the first to seventh configurations, the first molding surface includes a plurality of first quenching regions and a plurality of first slow-cooling regions. The second molding surface includes a plurality of second quenching regions and a plurality of second slow-cooling regions. The hot-press-formed product extends in the longitudinal direction. In a direction corresponding to the longitudinal direction of the hot-press-formed product sandwiched between the upper and lower dies during hot press forming, each of the plurality of first quenching regions is alternately arranged with each of the plurality of first slow-cooling regions. In a direction corresponding to the longitudinal direction of the hot-press-formed product sandwiched between the upper and lower dies during hot press forming, each of the plurality of second quenching regions is alternately arranged with each of the plurality of second slow-cooling regions (ninth configuration).

[0027] When hot press forming is performed using the die of the ninth configuration, in a hot press formed product having a longitudinal direction, such as a front side member, low strength regions and high strength regions are arranged alternately in the longitudinal direction, so that deformation of the low strength regions is widely dispersed to the high strength regions, and the entire hot press formed product has a shock absorbing effect.

[0028] Preferably, in the mold of any one of the first to seventh configurations, the hot press-formed product includes a top plate, a vertical wall, and a ridge portion connecting the top plate and the vertical wall. The first and second slow cooling regions are arranged at positions corresponding to the ridge portion of the hot press-formed product sandwiched between the upper and lower dies during hot press forming. The first and second quenching regions are arranged at positions corresponding to the top plate or the vertical wall of the hot press-formed product sandwiched between the upper and lower dies during hot press forming (tenth configuration).

[0029] In the die of the tenth configuration, the portion corresponding to the ridge line of the hot press-formed product can be made into a low-strength region during hot press forming, thereby suppressing the occurrence of wrinkles and cracks at the ridge line of the hot press-formed product during hot press forming.

[0030] Preferably, in a mold of any one of the first to seventh configurations, the hot press-formed product includes a top plate, a pair of vertical walls connected to both sides of the top plate and arranged opposite each other, and a pair of flanges connected to the vertical walls opposite the top plate and protruding outward from the vertical walls. The first and second slow cooling regions are arranged at positions corresponding to the flanges of the hot press-formed product sandwiched between the upper and lower dies during hot press forming. The first and second quenching regions are arranged at positions corresponding to the top plate and vertical walls of the hot press-formed product sandwiched between the upper and lower dies during hot press forming (eleventh configuration).

[0031] In the die of the eleventh configuration, the flange of the hot press-formed product can be a low-strength region. Welding, typically spot welding, is often performed on the flange of the hot press-formed product. When welding is performed, a weld heat-affected zone (HAZ) is formed in the area adjacent to the weld. The HAZ becomes soft due to the influence of the welding heat. Therefore, if the flange has high strength, the difference in hardness between the HAZ and the portion of the flange not affected by the welding heat increases, making cracks more likely to occur. By making the flange a low-strength region, the difference in hardness between the HAZ and the portion not affected by the welding heat is reduced. Therefore, the occurrence of cracks is suppressed.

[0032] Preferably, in the mold, the first quenching zone is 500 mm ２ The contact area ratio per 500 mm is 95% or more. ２ In the first slow cooling region, the contact area ratio is 95% or more. ２ In the second slow cooling region, the contact area ratio is 9 to 33%. ２ The contact area ratio per contact is 9 to 33% (twelfth configuration).

[0033] A method for manufacturing a hot press-formed product according to a thirteenth aspect includes a step of preparing a blank, and a step of forming the prepared blank into a ｃ３The manufacturing method of the thirteenth aspect includes the steps of: heating a blank to a temperature equal to or higher than the temperature at which the blank is heated; hot-press-forming the heated blank using a die having any one of the first to twelfth configurations; and releasing the hot-press-formed blank from the die to produce a hot-press-formed product. The manufacturing method of the thirteenth aspect makes it possible to produce a hot-press-formed product having a region having high strength and a region having low strength.

[0034] Hereinafter, the mold according to the present embodiment will be described with reference to the drawings. Note that the same or corresponding components in the various drawings will be denoted by the same reference numerals, and the same description will not be repeated.

[0035] [First Embodiment] [Configuration of Hot Press Apparatus 1] Fig. 1 is a front view showing an example of a hot press apparatus 1 for hot press forming. Referring to Fig. 1, the hot press apparatus 1 has substantially the same configuration as a known hot press apparatus, except for a die 10 according to this embodiment. The hot press apparatus 1 includes a frame 2, a slide 3, a bolster 4, and dies 10 (upper die 11 and lower die 12). In the following description, the vertical (up-down) direction of the hot press apparatus 1 will be referred to as the V direction, the width direction of the hot press apparatus 1 as the W direction, and the direction perpendicular to the V direction and W direction as the L direction.

[0036] Referring to FIG. 1 , a frame 2 is disposed on top of a hot press apparatus 1. The frame 2 supports a slide 3 disposed below the frame 2 so that the slide 3 can be raised and lowered. The frame 2 is provided with a drive device (not shown) that raises and lowers the slide 3. The drive device may be a mechanical mechanism or a hydraulic mechanism. The slide 3 is attached to the frame 2 and can be raised and lowered in the vertical direction by the drive device provided in the frame 2. An upper mold 11 is attached to the lower surface of the slide 3. A bolster 4 is disposed below the slide 3. The upper surface of the bolster 4 faces the lower surface of the slide 3. A lower mold 12 is attached to the upper surface of the bolster 4. In this case, the lower mold 12 is disposed below the upper mold 11.

[0037] The mold 10 includes the above-mentioned upper mold 11 and the above-mentioned lower mold 12. In Fig. 1, the upper mold 11 and the lower mold 12 extend in the L direction, for example. However, the shapes of the upper mold 11 and the lower mold 12 are not particularly limited.

[0038] As described above, the upper die 11 is fixed to the lower surface of the slide 3, and the lower die 12 is fixed to the upper surface of the bolster 4. The lower die 12 is disposed below the upper die 11. When hot press forming is performed, a heated blank is first placed on the lower die 12. After the blank is placed, the upper die 11 slides in the V direction relative to the lower die 12, sandwiching and contacting the blank while applying an external force to the blank. In other words, the upper die 11 and the lower die 12 hot press form the blank. This forms the blank into a desired shape. Furthermore, during hot press forming, the forming surfaces of the upper die 11 and the lower die 12 come into contact with the blank, and the upper die 11 and the lower die 12 remove heat from the blank and quench it. As a result, a hot press-formed product having a desired shape and increased strength is produced.

[0039] The hot press apparatus 1 may include other components not shown in FIG. 1 . The hot press apparatus 1 may include, for example, a cooling device for cooling the mold 10. In this case, for example, a flow path through which a cooling medium passes is provided inside the mold 10. Furthermore, a pump for supplying the cooling medium to the mold 10 is disposed inside the mold 10. The hot press apparatus 1 may further include a transport mechanism for transporting a material to the hot press apparatus 1. The hot press apparatus 1 may include a component not shown in FIG. 1 that is included in a known hot press apparatus.

[0040] [Configuration of Mold 10] Fig. 2 is a perspective view of the mold 10 in Fig. 1. Referring to Fig. 2, the upper mold 11 has a first molding surface 110. The first molding surface 110 is disposed on the lower surface of the upper mold 11. In Fig. 2, the first molding surface 110 includes a valley-shaped surface 110a extending in the longitudinal direction (L direction) of the upper mold 11 at the center in the width direction of the upper mold 11.

[0041] The lower mold 12 has a second molding surface 120. The second molding surface 120 is disposed on the upper surface of the lower mold 12. In FIG. 2 , the second molding surface 120 includes a mountain-shaped surface 120a extending in the longitudinal direction (L direction) of the lower mold 12 at the center in the width direction of the lower mold 12. During hot press forming, the second molding surface 120 is disposed opposite the first molding surface 110, sandwiching the blank 60 between them. The second molding surface 120, together with the first molding surface 110, is in contact with the blank 60 and sandwiches the blank to perform hot press forming.

[0042] [Configuration of first molding surface 110 and second molding surface 120] The first molding surface 110 includes a first quenching region 1A and a first slow cooling region 1B indicated by a shaded area. In Fig. 2, the first slow cooling region 1B is formed in a portion of the first molding surface 110, and the remaining portion of the first molding surface 110 constitutes the first quenching region 1A. In Fig. 2, the first quenching region 1A and the first slow cooling region 1B are arranged in the longitudinal direction (L direction) of the mold 10 (upper mold 11).

[0043] Similarly, the second molding surface 120 includes a second quenching region 2A and a second slow cooling region 2B indicated by a shaded area. In Fig. 2, the second slow cooling region 2B is formed in a portion of the second molding surface 120, and the remaining portion of the second molding surface 120 is the second quenching region 2A. In Fig. 2, the second quenching region 2A and the second slow cooling region 2B are arranged in the longitudinal direction (L direction) of the mold 10 (lower mold 12).

[0044] During hot press forming, the second quenching region 2A is disposed opposite the first quenching region 1A across the blank. Also, during hot press forming, the second slow cooling region 2B is disposed opposite the second quenching region 2A across the blank.

[0045] Note that no injection holes for injecting a cooling medium such as water are opened on the first forming surface 110 and the second forming surface 120. During hot press forming, the first forming surface 110 and the second forming surface 120 come into contact with the blank 60, thereby removing heat from the blank 60, without directly supplying the cooling medium to the blank 60.

[0046] 1 and 2, during hot press forming, the blank 60 is sandwiched between the upper mold 11 and the lower mold 12. At this time, the valley-shaped surface 110a of the first forming surface 110 fits into the peak-shaped surface 120a of the second forming surface 120. As a result, the blank 60 is formed into a hat-shaped hot press-formed product 61 as shown in FIG.

[0047] The upper mold 11 and the lower mold 12 of the mold 10 come into contact with the blank 60 during hot press forming, thereby removing heat from the blank 60. The blank 60 is molded while sandwiched between the upper mold 11 and the lower mold 12, and is also cooled by the upper mold 11 and the lower mold 12.

[0048] In the blank 60, the blank region sandwiched between the first and second quenching regions 1A and 2A is cooled faster than the blank region sandwiched between the first and second slow cooling regions 1B and 2B. Therefore, in the hot press-formed product 61, the region 61A formed between the first and second quenching regions 1A and 2A becomes a high-strength region having high strength.

[0049] On the other hand, in the blank 60, the blank region sandwiched between the first and second slow cooling regions 1B, 2B is cooled more slowly than the blank region sandwiched between the first and second quenching regions 1A, 2A. Therefore, in the hot press-formed product 61, the region 61B formed by being sandwiched between the first and second slow cooling regions 1B, 2B forms a low-strength region 61B that is weaker than the high-strength region 61A and is prone to plastic deformation. Below, the first and second quenching regions 1A, 2A and the first and second slow cooling regions 1B, 2B of the mold 10 will be described.

[0050] [Regarding the first and second quenching regions, and the first and second slow cooling regions] The proportion per unit area of ​​the first quenching region that comes into contact with the blank 60 during hot press forming is defined as the contact area ratio (%) of the first quenching region. Similarly, the proportion per unit area of ​​the second quenching region that comes into contact with the blank 60 during hot press forming is defined as the contact area ratio (%) of the second quenching region.

[0051] The contact area ratio of the first quenching region 1A is 95% or more. The contact area ratio of the second quenching region 2A is also 95% or more. That is, during hot press forming, almost the entire surface of the first quenching region 1A and almost the entire surface of the second quenching region 2A come into contact with the blank 60. Therefore, the first and second quenching regions 1A and 2A cool the blank 60 during hot press forming by removing heat.

[0052] If the contact area ratio of each of the first and second quenching regions 1A, 2A is less than 95% during hot press forming, a sufficient contact heat transfer coefficient cannot be obtained with the blank 60. In this case, the cooling rate of the blank region sandwiched between the first and second quenching regions 1A, 2A during hot press forming is not sufficiently fast. Therefore, a high-strength martensitic structure cannot be sufficiently obtained in the region 61A of the hot press-formed product 61 sandwiched between the first and second quenching regions 1A, 2A during hot press forming.

[0053] Therefore, the contact area ratio between the first and second quenching regions 1A and 2A is set to 95% or more. In this case, the region 61A of the hot press-formed product 61 sandwiched between the first and second quenching regions 1A and 2A during hot press forming becomes a high-strength region in which a high-strength martensitic structure is sufficiently obtained.

[0054] The proportion per unit area of ​​the first slow cooling region 1B that comes into contact with the blank 60 during hot press forming is defined as the contact area ratio (%) of the first slow cooling region 1B. Similarly, the proportion per unit area of ​​the second slow cooling region 2B that comes into contact with the blank 60 during hot press forming is defined as the contact area ratio (%) of the second slow cooling region 2B.

[0055] The contact area ratio of the first slow cooling region 1B is 9 to 33%. The contact area ratio of the second slow cooling region 2B is 9 to 33%. That is, during hot press forming, less than half of the entire surface of the first slow cooling region 1B and less than half of the entire surface of the second slow cooling region come into contact with the blank 60. Therefore, the amount of heat removed from the first and second slow cooling regions 1B and 2B during hot press forming is significantly less than the amount of heat removed from the first and second rapid cooling regions 1A and 2A. As a result, the cooling rate of the blank region sandwiched between the first and second slow cooling regions 1B and 2B is slower than the cooling rate of the blank region sandwiched between the first and second rapid cooling regions 1A and 2A.

[0056] If the contact area ratio of each of the first and second slow cooling regions is less than 9% during hot press forming, the contact heat transfer coefficient with the blank 60 becomes excessively low. In this case, the cooling rate of the blank region sandwiched between the first and second slow cooling regions 1B and 2B becomes excessively slow during hot press forming. As a result, the strength of the region 61B of the hot press-formed product 61 sandwiched between the first and second slow cooling regions 1B and 2B during hot press forming becomes excessively low, and pro-eutectoid ferrite, which serves as a starting point for fracture, is generated.

[0057] On the other hand, if the contact area ratio of each of the first and second slow cooling regions exceeds 33% during hot press forming, the contact heat transfer coefficient with the blank 60 becomes excessively high. In this case, the cooling rate of the blank region sandwiched between the first and second slow cooling regions 1B and 2B becomes excessively fast during hot press forming. As a result, martensite is excessively generated in region 61B of the hot press-formed product 61 sandwiched between the first and second slow cooling regions 1B and 2B during hot press forming, resulting in excessively high strength.

[0058] During hot press forming, if the contact area ratio of each of the first and second slow cooling regions is 9 to 33%, the contact heat transfer coefficient with the blank 60 will be in an appropriate range. In this case, during hot press forming, the cooling rate of the blank region sandwiched between the first and second slow cooling regions 1B, 2B will be in an appropriate range. Therefore, during hot press forming, region 61B of the hot press-formed product 61 sandwiched between the first and second slow cooling regions 1B, 2B will have a structure in which a hard structure (bainite or martensite) and retained austenite are mixed in appropriate amounts, and will be a low-strength region that is lower in strength and more susceptible to plastic deformation than region 61A.

[0059] As described above, in the mold 10 of this embodiment, the contact area ratio of each of the first and second quenching regions 1A and 2A is 95% or more, and the contact area ratio of each of the first and second slow cooling regions is 9 to 33%. In this case, during hot press forming, the cooling rate of the blank region sandwiched between the first and second quenching regions 1A and 2A is faster, and the cooling rate of the blank region sandwiched between the first and second slow cooling regions 1B and 2B is slower. As a result, the region 61A of the hot press-formed product 61 formed between the first and second quenching regions 1A and 2A becomes a high-strength region, and the region 61B formed between the first and second slow cooling regions 1B and 2B becomes a low-strength region that has lower strength than the high-strength region and is more susceptible to plastic deformation. Therefore, a hot press-formed product 61 having both a high-strength region and a low-strength region can be produced.

[0060] [Shape of the First and Second Quenching Regions 1A, 2A] The shape of the first and second quenching regions 1A, 2A is not particularly limited as long as the contact area ratio of each of the first and second quenching regions 1A, 2A is 95% or more. Figure 4 is an example of a plan view of the first or second quenching region 1A, 2A. In this specification, a plan view refers to the appearance of a target surface or region when developed into a plane. Referring to Figure 4, the first or second quenching region 1A, 2A is smooth.

[0061] [Configuration of First and Second Slow Cooling Regions 1B, 2B] The configuration of the first and second slow cooling regions 1B, 2B is not particularly limited as long as the contact area ratio of each of the first and second slow cooling regions 1B, 2B is 9 to 33%. Figure 5A is an example of a plan view of the first or second slow cooling region 1B, 2B.

[0062] 5A , the first slow cooling region 1B includes a plurality of convex portions 30. The convex portions 30 protrude in the normal direction of the first forming surface 110. Similarly, the second slow cooling region 2B includes a plurality of convex portions 30. The convex portions 30 protrude in the normal direction of the second forming surface 120. Each convex portion 30 has a top surface 31. The top surface 31 is flat. The multiple top surfaces 31 come into surface contact with the blank 20 during hot press forming. The top surfaces 31 of the multiple convex portions 30 are arranged with gaps between them.

[0063] During hot press forming, gaps are formed between adjacent top surfaces 31, and air is present in the gaps. In other words, air is present in the gaps between adjacent protrusions 30. When the top surfaces 31 come into contact with the blank 60 during hot press forming, the air present in the gaps between the top surfaces 31 absorbs heat from the blank 60 and retains the absorbed heat to a certain extent. This reduces the contact heat transfer coefficient with the blank 60.

[0064] In Fig. 5A, the top surface 31 of the protrusion 30 is rectangular, and the protrusion 30 is substantially rectangular. However, the shape of the protrusion 30 is not limited to this. For example, as shown in Fig. 5B, the top surface 31 of the protrusion 30 may be circular, and the protrusion 30 may be cylindrical. Furthermore, as shown in Fig. 5C, the top surface 31 of the protrusion 30 may be rectangular and longer than the top surface 31 in Fig. 5A.

[0065] 5A to 5C, the plurality of protrusions 30 are arranged in multiple directions, rather than in one direction. By arranging the plurality of protrusions 30 in multiple directions, rather than in one direction only, temperature variations during cooling are suppressed in the blank regions in contact with the first and second slow cooling regions 1B and 2B.

[0066] The size of the protrusions 30 is not particularly limited. For example, the height of the protrusions 30 is 0.1 to 5.0 mm. The maximum length of the top surface 31 of the protrusions 30 is 1 to 8 mm. The ratio of the maximum length d1 of the protrusions 30 to the width d2 of the top surface 31 in the direction perpendicular to the maximum length (= d1 / d2) is defined as the aspect ratio. The aspect ratio of the top surface 31 is, for example, 5.0 or less. The area of ​​the top surface 31 of the protrusions 30 is, for example, 100 mm ２ The following is the result.

[0067] 5D , the first gentle cooling region 1B preferably includes a plurality of convex portions 30 and a rib portion 40. Similarly, the second gentle cooling region 2B includes a plurality of convex portions 30 and a rib portion 40. The rib portion 40 is arranged around the plurality of convex portions 30, surrounding the plurality of convex portions 30. The rib portion 40 has a top surface 41. The top surface 41 is flat. The top surface 41, together with the plurality of top surfaces 31, comes into surface contact with the blank 20 during hot press forming. The top surface 41 of the rib portion 40 is arranged with a gap provided between it and the top surfaces 31 of the plurality of convex portions 30.

[0068] As described above, the rib portion 40 is formed so as to surround the periphery of the plurality of protrusions 30 in a plan view. Therefore, the air present in the gaps between the plurality of top surfaces 31 is less likely to flow out of the rib portion 40. Therefore, the air that absorbs heat during hot press forming tends to remain inside the rib portion 40. This allows the heat absorbed by the air to be retained for a longer period of time. As a result, the contact heat transfer coefficient with the blank 60 can be further reduced.

[0069] As shown in FIG. 5E , the first or second slow cooling region 1B or 2B may include a plurality of rib portions 40. Also, in FIGS. 5D and 5E , the rib portion 40 is rectangular in plan view. However, the shape of the rib portion 40 is not limited thereto. For example, as shown in FIG. 5F , the rib portion 40 may be circular. Furthermore, the rib portion 40 does not have to completely surround the plurality of protrusions 30. For example, as shown in FIG. 5G , the rib portion 40 may include a plurality of divided rib portions 42 and 43, with gaps formed between adjacent divided rib portions 42 and 43. As long as the rib portion 40 is formed so as to substantially surround the plurality of protrusions 30, the rib portion 40 may be composed of a plurality of divided rib portions, and the shape of the rib portion 40 is not particularly limited.

[0070] As described above, the configuration of the first and second slow cooling regions 1B, 2B is not particularly limited as long as the contact area ratio of each of the first and second slow cooling regions 1B, 2B is 9 to 33%. The first and second slow cooling regions 1B, 2B include, for example, a plurality of protrusions 30, or a plurality of protrusions 30 and one or a plurality of rib portions 40, as shown in Figures 5A to 5G.

[0071] Preferably, during hot press forming, when the first and second slow cooling regions 1B, 2B sandwich the blank 60, as shown in Fig. 6, when the first and second slow cooling regions 1B, 2B are viewed from the normal direction of the first slow cooling region 1B, the top surfaces 31 of the convex portions 30 in the first slow cooling region 1B at least partially overlap with the top surfaces 31 of the convex portions 30 in the second slow cooling region 2B. Fig. 6 shows the first and second slow cooling regions 1B, 2B when developed in a plane, with the top surfaces 31 of the convex portions 30 in the first slow cooling region 1B indicated by solid lines and the top surfaces 31 of the convex portions 30 in the second slow cooling region 2B indicated by dashed lines. In this case, the convex portions 30 in the second slow cooling region 2B enter between adjacent convex portions 30 in the first slow cooling region 1B, thereby suppressing the occurrence of wavy deformation in the blank 60 due to the convex portions 30.

[0072] Preferably, in the first slow cooling region 1B which is in contact with the blank during hot press forming, the cooling time is 500 mm ２ The contact area ratio per unit area is 9 to 33%. In the second slow cooling region 2B, which is in contact with the blank during hot press forming, the contact area ratio per unit area is 500 mm ２ The contact area ratio is 9 to 33%.

[0073] In this case, in the first and second slow cooling regions 1B and 2B, ２ A plurality of minute protrusions 30 (or protrusions 30 and rib portions 40) are formed to such an extent that the contact area ratio per unit area falls within the above-mentioned range. Therefore, in the first and second slow cooling regions 1B, 2B, the contact heat transfer coefficient can be reduced, and temperature variations in the blank regions in contact with the first and second slow cooling regions 1B, 2B can be suppressed.

[0074] Preferably, the first quenching region 1A, which is in contact with the blank during hot press forming, is 500 mm２ The contact area ratio per second quenching zone 2A, which is in contact with the blank during hot press forming, is 95% or more. ２ The contact area ratio is 95% or more.

[0075] The contact area ratio of each region can be determined, for example, by the following method: A pressure-sensitive paper is placed between the upper and lower dies, and the upper die is lowered to the bottom dead center. The upper die is then raised, the pressure-sensitive paper is removed, and the removed pressure-sensitive paper is cut into multiple rectangular pieces. The area of ​​each rectangular piece is, for example, 500 mm. ２ The size of each rectangular piece is preferably, for example, √500 mm (22.36 mm) in length and √500 mm (22.36 mm) in width. Then, for each rectangular piece, the area of ​​the part where the color tone has changed is measured. The contact area ratio of each region can be determined from the area of ​​this color tone change part and the area of ​​the rectangular piece.

[0076] In the first embodiment, a hot press-formed product 61 shown in Fig. 3 is formed by using the mold 10 shown in Fig. 2. Referring to Fig. 3, the hot press-formed product 61 extends in the longitudinal direction. Referring to Figs. 2 and 3, the first quenching region 1A and the first slow cooling region 1B are arranged in the L direction, which corresponds to the longitudinal direction of the hot press-formed product 61 formed by being sandwiched between the upper mold 11 and the lower mold 12 during hot press forming. The second quenching region 2A and the second slow cooling region 2B are also arranged in the L direction.

[0077] The hot press-formed product 61 is, for example, an automobile part such as a B-pillar. When the die 10 of the first embodiment is used, the end portion of the hot press-formed product 61 having such a longitudinal direction (for example, the lower portion of the B-pillar) can be made into a low-strength region that is low in strength and easy to plastically process, and the remaining portion can be made into a high-strength region. Therefore, the end portion of the hot press-formed product 61 can absorb impact, and the remaining portion can ensure strength.

[0078] Second Embodiment In the second embodiment, the arrangement of the first and second slow cooling regions 1B and 2B in the mold is different from that in the first embodiment. The mold of the second embodiment will be described below.

[0079] Fig. 7 is a perspective view of a mold 70 according to the second embodiment. Referring to Fig. 7, the mold 70 includes an upper mold 71 and a lower mold 72. The upper mold 71 has a first molding surface 710. The lower mold 72 has a second molding surface 720. The first molding surface 710 includes a plurality of first quenching regions 1A and a plurality of first slow cooling regions 1B. The second molding surface 720 includes a plurality of second quenching regions 2A and a plurality of second slow cooling regions 2B.

[0080] Fig. 8 is a perspective view of a hot-press-formed product 62 formed by hot-press forming using the die 70 of Fig. 7. Referring to Fig. 8, the hot-press-formed product 62 extends in the longitudinal direction. In the hot-press-formed product 62, a plurality of high-strength regions 62A and a plurality of low-strength regions 62B are alternately arranged in the longitudinal direction.

[0081] 7 , in the L direction, which corresponds to the longitudinal direction of the hot press-formed product 62, the multiple first quenching regions 1A of the first forming surface 710 are alternately arranged with the multiple first slow cooling regions 1B of the first forming surface 710. Also, in the L direction, the multiple second quenching regions 2A of the second forming surface 720 are alternately arranged with the multiple second slow cooling regions 2B of the second forming surface 720. During hot press forming, each of the first quenching regions 1A is disposed opposite the corresponding second quenching region 2A across the blank 60. Also, during hot press forming, each of the first slow cooling regions 1B is disposed opposite the corresponding second slow cooling region 2B across the blank 60.

[0082] 8 is, for example, a front side member. When hot press forming is performed using the die 70 of the second embodiment, for example, a plurality of high strength regions are arranged alternately with a plurality of low strength regions in the longitudinal direction of the hot press formed product 62. Therefore, an excellent impact absorption effect is obtained in the entire hot press formed product 62.

[0083] [Third Embodiment] In the third embodiment, the arrangement of the first and second slow cooling regions 1B, 2B in the mold is different from that in the first and second embodiments. The mold of the third embodiment will be described below.

[0084] Fig. 9 is a perspective view of a mold 80 according to the third embodiment. Referring to Fig. 9, the mold 80 includes an upper mold 81 and a lower mold 82. The upper mold 81 has a first molding surface 810. The lower mold 82 has a second molding surface 820. The first molding surface 810 includes a first rapid cooling region 1A and a first slow cooling region 1B. The second molding surface 820 includes a second rapid cooling region 2A and a second slow cooling region 2B.

[0085] Fig. 10 is a perspective view of a hot-press-formed product 63 formed by hot press forming using the die 80 of Fig. 9. Referring to Fig. 10, the hot-press-formed product 63 includes a top plate 631, a pair of vertical walls 632, and a pair of ridge portions 633. The pair of vertical walls 632 are arranged on both sides of the top plate 631 and face each other. Each of the pair of ridge portions 633 connects the top plate 631 and the corresponding vertical wall 632.

[0086] The ridge line portion 633 connecting the top plate 631 and the vertical wall 632 is subjected to the most strain during hot press forming. Therefore, wrinkles and cracks are likely to occur in the ridge line portion 633. Therefore, in the hot press-formed product 63, the ridge line portion 633 is made into a low-strength region 63B. In this case, the ridge line portion 633 has a soft structure, which increases its plastic deformability. As a result, the occurrence of wrinkles or cracks due to hot press forming can be suppressed.

[0087] Referring to Figure 9, the first slow cooling region 1B and the second slow cooling region 2B are positioned on the corresponding first forming surface 810 and second forming surface 820 at positions corresponding to the ridge portion 633 of the hot press-formed hot press-formed product 63 so that the ridge portion 633 of the hot press-formed hot press-formed product 63 becomes a low-strength region.

[0088] With the above-described configuration, the ridge line portion 633 of the hot press-formed product 63 can be made into a low-strength region that has low strength and is prone to plastic deformation during hot press forming, thereby suppressing the occurrence of wrinkles and cracks at the ridge line portion 633 of the hot press-formed product 63 during hot press forming.

[0089] [Fourth embodiment] In the fourth embodiment, the arrangement of the first and second slow cooling regions 1B, 2B in the mold is different from that in the first to third embodiments. The mold of the fourth embodiment will be described below.

[0090] Fig. 11 is a perspective view of a mold 90 according to the fourth embodiment. Referring to Fig. 11, the mold 90 includes an upper mold 91 and a lower mold 92. The upper mold 91 has a first molding surface 910. The lower mold 92 has a second molding surface 920. The first molding surface 910 includes a first quenching region 1A and a first slow cooling region 1B. The second molding surface 920 includes a second quenching region 2A and a second slow cooling region 2B.

[0091] Fig. 12 is a perspective view of a hot-press-formed product 64 formed by hot-press forming using the die 90 of Fig. 11. Referring to Fig. 12, the hot-press-formed product 64 includes a top plate 631, a pair of vertical walls 632, and a pair of flanges 634. Each of the pair of flanges 634 is connected to the vertical wall 632 on the side opposite to the top plate 631, and protrudes outward from the vertical wall 632.

[0092] The hot press-formed product 64 is usually welded at the flange 634. The welding method is, for example, spot welding. When welding is performed at the flange 634, a HAZ is formed in the area adjacent to the weld. The strength of the HAZ is reduced due to the influence of welding heat. Therefore, if the strength of the flange 634 is high, the difference in strength between the HAZ formed after welding and the area not affected by the heat of welding becomes large. Therefore, cracks may occur due to the difference in strength.

[0093] Therefore, in the hot press-formed product 64, the flange 634 is made into a low-strength region 64B. In this case, the flange 634 becomes a soft structure with low strength. Therefore, it is possible to reduce the difference in strength between the HAZ formed after welding and the region not affected by the heat of welding. As a result, it is possible to suppress the occurrence of cracks due to the difference in strength.

[0094] Referring to Figure 11, the first slow cooling region 1B and the second slow cooling region 2B are positioned on the corresponding first forming surface 910 and second forming surface 920 at positions corresponding to the flange 634 of the hot press-formed hot press-formed product 64 so that the flange 634 of the hot press-formed product 64 becomes a low strength region 64B.

[0095] With the above-described configuration, the flange 634 of the hot-press-formed product 64 can be made into a low-strength region during hot press forming. Therefore, when the hot-press-formed product 64 is welded, the occurrence of cracks in the flange 634 can be suppressed.

[0096] Fifth Embodiment In the fifth embodiment, the configuration of the rib portion 40 formed to surround the plurality of protrusions 30 is different from that of the first to fourth embodiments. The mold of the fifth embodiment will be described below.

[0097] Fig. 13 is an example of an enlarged plan view of the first or second slow cooling region 1B, 2B in the mold of the fifth embodiment. Fig. 13 shows some of the multiple convex portions 30 and some of the rib portion 40. Referring to Fig. 13, the rib portion 40 includes multiple protrusions 44. In the rib portion 40 of the first slow cooling region 1B, the protrusions 44 protrude in the normal direction of the first molding surface 110. In the rib portion 40 of the second slow cooling region 2B, the protrusions 44 protrude in the normal direction of the second molding surface 120.

[0098] In the rib portion 40, the multiple protrusions 44 are arranged with minute gaps c44 between them. In other words, minute gaps c44 are formed between adjacent protrusions 44. The gaps c44 between the protrusions 44 are smaller than the gaps c30 between adjacent convex portions 30. The gaps c44 are the smallest gaps between the protrusions 44. The gaps c30 are the smallest gaps between the convex portions 30. The multiple protrusions 44 are not arranged in one direction, but are arranged in multiple directions.

[0099] Each protrusion 44 has a top surface 441. The top surfaces 441 are flat. The top surfaces 441 come into surface contact with the blank 20 during hot press forming.

[0100] In the mold of the fifth embodiment, adjacent protrusions 44 in the rib portion 40 are arranged with a gap c44 that is smaller than the gap c30 between adjacent convex portions 30 inside the rib portion 44. In this case, the air present in the gaps c44 between the protrusions 44 is less likely to flow than the air present in the gaps c30 between the convex portions 30. Therefore, during hot press forming, the air present in the gaps c44 between the protrusions 44 absorbs heat from the blank 60 and tends to accumulate inside the rib portion 40, while the air present in the gaps c30 between the convex portions 30 absorbs heat from the blank 60 and tends to accumulate inside the rib portion 40.

[0101] Furthermore, the surface area of ​​the rib portion 40 is increased by the multiple protrusions 44 compared to when the top surface 441 of the rib portion 40 is flat over the entire area of ​​the rib portion 40. Therefore, during hot press forming, heat conducted to the rib portion 40 (multiple protrusions 44) through contact with the blank 60 is easily transferred to the air present in the gaps c44 between the protrusions 44, and this air is likely to become hot. This makes it possible to further reduce the contact heat transfer coefficient between the first or second slow cooling region 1B, 2B and the blank 60.

[0102] The size of the gap c44 between the protrusions 44 is not particularly limited as long as it is smaller than the gap c30 between the convex portions 30. The gap c44 between the protrusions 44 is, for example, 2 to 10 mm. The gap c30 between the convex portions 30 is, for example, 16 to 80 mm.

[0103] In the example shown in Fig. 13, the top surface 441 of the protrusion 44 is rectangular, and the protrusion 44 is substantially rectangular. However, the shape of the protrusion 44 is not limited to this. For example, the top surface 441 of the protrusion 44 may be circular, and the protrusion 44 may be cylindrical. Furthermore, the top surface 441 of the protrusion 44 may be rectangular, longer than the top surface 441 in Fig. 13.

[0104] Furthermore, the area of ​​the top surface 441 of the protrusion 44 is smaller than the area of ​​the top surface 30 of the convex portion 30. However, the area of ​​the top surface 441 of the protrusion 44 may be the same as the area of ​​the top surface 30 of the convex portion 30, or may be larger than the area of ​​the top surface 30 of the convex portion 30, as long as the gap c44 between the protrusions 44 is smaller than the gap c30 between the convex portions 30.

[0105] [Method for manufacturing a hot press-formed product using the die of this embodiment] A method for manufacturing a hot press-formed product by hot press forming using the die (10, 70, 80 or 90) of this embodiment will be described. The method for manufacturing a hot press-formed product of this embodiment includes the following steps: (Step 1) Preparation step (Step 2) Heating step (Step 3) Hot press forming step (Step 4) Demolding step Each step will be described below.

[0106] [Preparation Step] In the preparation step, a blank 60 having a desired chemical composition is prepared. In the present embodiment, the blank 60 is not particularly limited. The blank 60 is, for example, a steel plate. When the blank 60 is a steel plate, the type of steel plate is not particularly limited. The blank 60 may be, for example, a steel plate that has been subjected to a surface treatment such as plating, or may be a steel plate that has not been subjected to a surface treatment such as plating (so-called bare material). When plating is performed, the plating may be a hot-dip galvanizing treatment, a galvannealed hot-dip plating treatment, or an aluminum plating treatment.

[0107] The thickness of the blank 60 is not particularly limited, but is selected depending on the characteristics of the hot press-formed product to be obtained. The thickness of the blank 60 is, for example, 0.6 to 3.2 mm. The mechanical properties of the blank 60 are also not particularly limited. The mechanical properties of the blank 60 are appropriately selected depending on the characteristics of the hot press-formed product to be obtained. The method for preparing the blank 60 is not particularly limited. For example, the blank 60 may be manufactured from molten steel having a desired chemical composition by a known manufacturing method. The blank 60 may also be prepared by purchasing a blank 60 manufactured by a third party.

[0108] [Heating Step] In the heating step, the prepared blank 60 is heated by heating the blank 60 in a ｃ３The blank 60 is heated to a temperature equal to or higher than the above temperature point. In the heating step, the method for heating the blank 60 is not particularly limited. For example, the blank 60 may be heated using a heating furnace such as an electric furnace, a gas furnace, a far-infrared furnace, or a near-infrared furnace. The blank 60 may also be heated using an electric heating device or a high-frequency induction heating device. In the heating step, the method for heating the blank 60 is not particularly limited, and any known heating method may be appropriately selected.

[0109] [Hot press forming process] In the hot press forming process, ｃ３ The blank 60, which has been heated to a temperature above that point, is hot press-formed using the above-mentioned mold (10, 70, 80, or 90). In the hot press-forming process, the blank 60, which has been heated in the heating process, is placed on the second forming surface of the lower mold 12. Then, the upper mold 11 is moved relatively close to the lower mold 12, and the mold 10 is closed. At this time, the blank 60 comes into contact with the first forming surface of the upper mold 11 and the second forming surface of the lower mold 12. In other words, the blank 60 is sandwiched between the first forming surface of the upper mold 11 and the second forming surface of the lower mold 12. The blank 60 is hot press-formed by the upper mold 11 and the lower mold 12.

[0110] In the hot press forming process according to this embodiment, a cooling medium is not directly supplied to the blank 60 during hot press forming. Instead, heat is removed from the blank 60 by the mold 10, which comes into contact with the blank 60 during hot press forming. When the mold 10 is closed, the first and second quenching regions 1A and 2A are arranged opposite each other across the blank 60, and the first and second quenching regions 1A and 2A come into contact with the blank 60. Furthermore, the first and second slow cooling regions 1B and 2B are arranged opposite each other across the blank 60, and the first and second slow cooling regions 1B and 2B come into contact with the blank 60. At this time, the temperatures of the upper mold 11 and the lower mold 12 are sufficiently lower than the temperature of the blank 60. Therefore, heat is removed from the blank 60 by the first and second quenching regions 1A and 2A and the first and second slow cooling regions 1B and 2B. At this time, the blank region sandwiched between the first and second rapid cooling regions 1A, 2A cools faster than the blank region sandwiched between the first and second slow cooling regions 1B, 2B.

[0111] [Molding Process] In the mold-releasing process, the hot-press-formed blank 60 is released from the mold 10 to produce a hot-press-formed product. In the mold-releasing process, the temperature of the blank when it is released from the mold 10 is defined as the cooling stop temperature. For example, if the cooling stop temperature of the blank region sandwiched between the first and second slow-cooling regions 1B and 2B is between the Mf point and the Ms point, or between above the Ms point and 500°C, the region of the hot-press-formed product sandwiched between the first and second slow-cooling regions 1B and 2B will have a soft structure in which a hard structure (martensite and bainite, or bainite) and retained austenite are mixed. In other words, the region of the hot-press-formed product sandwiched between the first and second slow-cooling regions 1B and 2B has low strength and becomes a low-strength region that is prone to plastic deformation. On the other hand, the region of the hot press-formed product sandwiched between the first and second quenching regions 1A and 2A has a faster cooling rate than the region of the hot press-formed product sandwiched between the first and second slow cooling regions 1B and 2B. Therefore, the region of the hot press-formed product sandwiched between the first and second quenching regions 1A and 2A becomes a high-strength region having a higher strength than the low-strength region.

[0112] The Ms point and Mf point of the blank 60 vary depending on the chemical composition of the blank 60. Therefore, when forming high-strength and low-strength regions in a hot-press-formed product using the blank 60 as a material, the preferred cooling stop temperature varies depending on the chemical composition of the blank 60. However, heat transfer simulations or experiments can be used to determine the cooling rate, temperature change over time, and temperature distribution in the blank region sandwiched between the first and second slow-cooling regions 1B and 2B of the blank 60. Therefore, these heat transfer simulations or experiments can determine the preferred cooling stop temperature or the time from the start of hot press forming to demolding. Therefore, the mold according to this embodiment can produce a hot-press-formed product including high-strength and low-strength regions by hot press forming, depending on the chemical composition of the blank 60.

[0113] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

[0114] REFERENCE SIGNS LIST 1 Hot press device 10, 70, 80, 90 Mold 11, 71, 81, 91 Upper mold 12, 72, 82, 92 Lower mold 30 Convex portion 31, 41, 441 Top surface 40, 42, 43 Rib portion 44 Protrusion 110, 710, 810, 910 First molding surface 120, 720, 820, 920 Second molding surface 1A First quenching region 2A Second quenching region 1B First slow cooling region 2B Second slow cooling region

Claims

1. A mold for performing hot press forming on a blank to form a hot press formed product, comprising: an upper mold having a first forming surface; a lower mold which, during hot press forming, is disposed opposite to the first forming surface with the blank interposed therebetween and has a second forming surface for hot press forming the blank together with the first forming surface; The first forming surface: a first rapid cooling region that contacts the blank and cools the blank during the hot press forming; a first slow cooling region that contacts the blank and cools the blank more slowly than the first rapid cooling region during the hot press forming; The second forming surface: a second rapid cooling region that is disposed opposite to the first rapid cooling region with the blank interposed therebetween and contacts the blank to cool the blank during the hot press forming; a second slow cooling region that is disposed opposite to the first slow cooling region with the blank interposed therebetween and contacts the blank to cool the blank more slowly than the second rapid cooling region during the hot press forming; When the ratio per unit area of the portion of the first rapid cooling region that contacts the blank during the hot press forming is defined as the contact area ratio of the first rapid cooling region, the contact area ratio of the first rapid cooling region is 95% or more; When the ratio per unit area of the portion of the second rapid cooling region that contacts the blank during the hot press forming is defined as the contact area ratio of the second rapid cooling region, the contact area ratio of the second rapid cooling region is 95% or more; When the ratio per unit area of the portion of the first slow cooling region that contacts the blank during the hot press forming is defined as the contact area ratio of the first slow cooling region, the contact area ratio of the first slow cooling region is 9 - 33%; When the ratio per unit area of the portion of the second slow cooling region that contacts the blank during the hot press forming is defined as the contact area ratio of the second slow cooling region, the contact area ratio of the second slow cooling region is 9 - 33%. A mold.

2. The mold according to Claim 1, wherein: each of the first and second rapid cooling regions is smooth; each of the first and second slow cooling regions includes a plurality of convex portions arranged in a plurality of directions; the convex portion has a top surface that is in surface contact with the blank during the hot press forming; the top surfaces of the plurality of convex portions are arranged with gaps therebetween. A mold.

3. The mold according to Claim 2, wherein: When the first and second slow cooling regions are in contact with the blank with the blank therebetween during the hot press forming, when viewing the first and second slow cooling regions from the normal direction of the first slow cooling region, at least a part of the top surface of the convex portion of the first slow cooling region overlaps with the top surface of the corresponding convex portion of the second slow cooling region. Die.

4. The die according to claim 2, wherein the first or second slow cooling region further includes a rib portion formed surrounding a plurality of the convex portions, and the top surface of the rib portion is disposed with a gap from the top surface of the convex portion. Die.

5. The die according to claim 3, wherein the first or second slow cooling region further includes a rib portion formed surrounding a plurality of the convex portions, and the top surface of the rib portion is disposed with a gap from the top surface of the convex portion. Die.

6. The die according to claim 4, wherein the rib portion includes a plurality of protrusions arranged in a plurality of directions, and a gap between adjacent ones of the protrusions is smaller than a gap between adjacent ones of the convex portions. Die.

7. The die according to claim 5, wherein the rib portion includes a plurality of protrusions arranged in a plurality of directions, and a gap between adjacent ones of the protrusions is smaller than a gap between adjacent ones of the convex portions. Die.

8. The die according to any one of claims 1 to 7, wherein the hot press formed product extends in a longitudinal direction, and the first rapid cooling region and the first slow cooling region are arranged in a direction corresponding to the longitudinal direction of the hot press formed product formed by being sandwiched between the upper die and the lower die during hot press forming, and the second rapid cooling region and the second slow cooling region are arranged in a direction corresponding to the longitudinal direction of the hot press formed product formed by being sandwiched between the upper die and the lower die during hot press forming. Die.

9. The die according to any one of claims 1 to 7, wherein the first forming surface includes a plurality of the first rapid cooling regions and a plurality of the first slow cooling regions, the second forming surface includes a plurality of the second rapid cooling regions and a plurality of the second slow cooling regions, the hot press formed product extends in a longitudinal direction, and in a direction corresponding to the longitudinal direction of the hot press formed product formed by being sandwiched between the upper die and the lower die during hot press forming, each of the plurality of the first rapid cooling regions is alternately arranged with each of the plurality of the first slow cooling regions. In a direction corresponding to the longitudinal direction of the hot press-formed product formed by being sandwiched between the upper die and the lower die during hot press forming, each of the plurality of the second rapid cooling regions is arranged alternately with each of the plurality of the second slow cooling regions. Die.

10. The die according to any one of Claims 1 to 7, wherein the hot press-formed product includes a top plate, a vertical wall, and a ridge line portion connecting the top plate and the vertical wall. The first slow cooling region and the second slow cooling region are arranged at positions corresponding to the ridge line portion of the hot press-formed product formed by being sandwiched between the upper die and the lower die during hot press forming. The first rapid cooling region and the second rapid cooling region are arranged at positions corresponding to the top plate or the vertical wall of the hot press-formed product formed by being sandwiched between the upper die and the lower die during hot press forming. Die.

11. The die according to any one of Claims 1 to 7, wherein the hot press-formed product includes a top plate, a pair of vertical walls each connected to both sides of the top plate and arranged opposite to each other, and a pair of flanges each connected to the side of the vertical wall opposite to the top plate and protruding outward from the vertical wall. The first slow cooling region and the second slow cooling region are arranged at positions corresponding to the flanges of the hot press-formed product formed by being sandwiched between the upper die and the lower die during hot press forming. The first rapid cooling region and the second rapid cooling region are arranged at positions corresponding to the top plate and the vertical walls of the hot press-formed product formed by being sandwiched between the upper die and the lower die during hot press forming. Die.

12. The die according to any one of Claims 1 to 7, In the first rapid cooling region, the contact area ratio per 500 mm ２ is 95% or more, In the second rapid cooling region, the contact area ratio per 500 mm ２ is 95% or more, In the first slow cooling region, the contact area ratio per 500 mm ２ is 9 to 33%, In the second slow cooling region, the contact area ratio per 500 mm ２ is 9 to 33%, Die.

13. A method for manufacturing a hot press-formed product, comprising: a step of preparing a blank; Heating the prepared blank to a temperature of A or more ｃ３ and a step of heating to a temperature of at least a step of performing hot press forming on the heated blank using the die according to any one of Claims 1 to 7; and a step of separating the hot press-formed blank from the die to manufacture a hot press-formed product. A method for manufacturing a hot press-formed product.