JP7477809B2 - Method for manufacturing mold and hot press molded product - Google Patents

Description

The present disclosure relates to a die and a method for manufacturing hot press-formed products using the die.

In the automotive field, weight reduction of the vehicle body 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. To reduce the weight of the vehicle body, it is effective to make the structural members used in the vehicle body thinner. In order to improve collision safety while making the structural members thinner, high strength is required for the materials used in the structural members of the vehicle body. On the other hand, materials with high strength have low press formability. When a material with high strength is cold press molded, cracks may occur during press molding, or the material may elastically deform (springback) due to the stress generated by press molding. Therefore, hot press molding has been proposed as a method of forming structural members using such high-strength materials.

In hot press forming, the material is heated to a temperature range where the microstructure becomes austenite single phase. The heated material is then hot press formed using a hot press device equipped with a die. In hot press forming, the material is softened by heating. Therefore, the press formability of the material in hot press forming is high. Furthermore, in hot press forming, the material during hot press forming comes into contact with almost the entire forming surface of the die. At this time, the material is heat removed and quenched by the die that comes into contact with the material. Thus, in hot press forming, the material is quenched at the same time as it is hot press formed. Therefore, hot press forming can easily produce high-strength hot press formed products.

Incidentally, excellent shock absorbing capabilities are required for shock absorbing components, which are a type of structural component for automobiles. Therefore, Patent Document 1 (JP 2014-79790 A) proposes a hot press molding die for molding shock absorbing components.

In the hot press molding disclosed in Patent Document 1, a heated material is hot press molded using a die. During hot press molding, a refrigerant is further flowed into a part 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 the refrigerant in the 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. As a result, a hot press molded product (shock absorbing member) with a low strength portion that has excellent shock absorbing ability is manufactured.

JP 2014-79790 A

The die disclosed in Patent Document 1 can produce a hot press-formed product having a low strength portion with excellent shock absorbing ability. However, a hot press-formed product with excellent shock absorbing ability may be produced by a technique different from the technique described in Patent Document 1.

The object of the present disclosure is to provide a mold capable of producing hot press-molded products having excellent impact absorption capabilities by hot press molding, and a method for producing hot press-molded products using the mold.

The die according to the present disclosure is a die for performing hot press forming on a material.
The die includes an upper die and a lower die. The upper die has a first molding surface. The lower die has a second molding surface. The second molding surface is disposed opposite the first molding surface during hot press forming, and hot press forms the material together with the first molding surface.
The first molding surface includes a first slow cooling region. The first slow cooling region has a plurality of first rib portions and a plurality of first groove portions. The plurality of first rib portions are arranged in a width direction of the first rib portion. The plurality of first groove portions are arranged in a width direction of the first groove portion. The first rib portion is formed between adjacent first groove portions. The width of the first rib portion is narrower than the width of the first groove portion.
The second molding surface includes a second slow cooling region. The second slow cooling region has a plurality of second rib portions and a plurality of second groove portions. The plurality of second rib portions are arranged in the width direction of the second rib portion. The plurality of second groove portions are arranged in the width direction of the second groove portion. The second rib portions are formed between adjacent second groove portions. The width of the second rib portions is narrower than the width of the second groove portions.
During hot press forming, when the first and second slow cooling regions are viewed from a normal direction of the first slow cooling region, the first rib portion and the second rib portion at least partially overlap each other.

The manufacturing method of a hot press-formed product according to the present disclosure includes a step of preparing a material, a step of heating the prepared material to a temperature of A _c3 point or higher, a step of performing hot press forming on the heated material using the above-mentioned die, and a step of releasing the hot press-formed material from the die to manufacture a hot press-formed product.

The mold according to the present disclosure can produce a hot press-molded product having excellent shock absorption capacity by hot press molding. The manufacturing method of a hot press-molded product according to the present disclosure can produce a hot press-molded product having excellent shock absorption capacity.

FIG. 1 is a front view showing an example of a hot press apparatus for hot press forming. FIG. 2 is a perspective view showing an example of a mold according to the present embodiment. FIG. 3 is a cross-sectional view showing the state of the die and the material during hot press forming in the slow cooling region of an example of the die according to this embodiment. FIG. 4 is a cross-sectional view showing the state of the die and the material during hot press forming in the quenching region of an example of the die according to this embodiment. FIG. 5A is an enlarged schematic diagram of region 100 in FIG. FIG. 5B is a schematic diagram showing the hot press forming process from FIG. 5A with material B removed. FIG. 5C is a cross-sectional view perpendicular to the extending direction of a first rib portion or a second rib portion having a different shape from those in FIGS. 5A and 5B. FIG. 5D is a cross-sectional view perpendicular to the extending direction of a first rib portion or a second rib portion having a different shape from those in FIGS. 5A to 5C. FIG. 6 is a schematic diagram of the region 100 in FIG. 3 as viewed from above (in the normal direction of the first and second slow cooling regions). FIG. 7 is an enlarged schematic diagram of the region 200 in FIG. FIG. 8 is a diagram showing the relationship between Fn1 and the temperature variation ΔT (° C.) in the first and second slow cooling regions. FIG. 9 is a schematic diagram of a heat conduction model used in the two-dimensional heat transfer simulation for creating FIG. FIG. 10 is a diagram showing the relationship between Fn2 and the cooling rate V (° C./sec) of the material B during hot press forming. FIG. 11 is a perspective view showing another example of the mold according to the present embodiment, which is different from the example shown in FIG. FIG. 12 is a perspective view showing another example of the mold according to the present embodiment, which is different from the example shown in FIG. 2 and FIG. FIG. 13 is a perspective view showing another example of the mold according to the present embodiment, which is different from the examples shown in FIGS. FIG. 14 is an example of an enlarged schematic diagram of the region 100 in FIG. 3, different from FIG. 5A. FIG. 15 is a cross-sectional view taken along line XV-XV in FIG. FIG. 16 is an example of an enlarged schematic diagram of the region 100 in FIG. 3 that is different from FIG. 5A and FIG. FIG. 17 is a schematic diagram showing only the rib portion when the region 100 in FIG. 16 is viewed from the normal direction of the first slow cooling region. FIG. 18 is an example of an enlarged schematic view of the region 100 in FIG. 3 that is different from FIGS. 5A, 14, and 16. In FIG. FIG. 19 is a schematic diagram showing only the rib portion when the region 100 in FIG. 18 is viewed from the normal direction of the first slow cooling region.

As mentioned above, among the structural components of an automobile, shock absorbing components are required to have improved shock absorption capabilities, i.e., the ability to absorb the impact energy during a collision. Impact energy is absorbed by the structural components undergoing plastic deformation. Therefore, in order to improve shock absorption capabilities, it is effective for the structural components to be able to exhibit excellent plastic deformability even when subjected to high stress.

The inventors of the present invention considered that if the cooling rate of the material during hot press forming could be reduced, the material would be able to exhibit excellent plastic deformability even when subjected to high stress. Therefore, the inventors of the present invention investigated the surface shape of the molding surface of the die. As a result, the inventors of the present invention considered that if a slow cooling region including multiple ribs and multiple grooves is formed on the molding surface, the cooling rate of the material during hot press forming could be reduced.

On the molding surface, the rib portion extends in a predetermined direction, and a cross section perpendicular to the extension direction has a convex shape. On the molding surface, the groove portion extends along the extension direction of the rib portion, and a cross section perpendicular to the extension direction has a concave shape. Multiple rib portions and multiple groove portions are formed alternately.

During hot press forming, in the slow cooling region, the rib portion comes into direct contact with the material and removes heat from the material. On the other hand, the groove portion does not come into direct contact with the material. Therefore, if the forming surface includes multiple rib portions and multiple groove portions, the area of contact between the die and the material during hot press forming can be reduced. As a result, the cooling rate of the material during quenching can be reduced.

Based on the above findings, the inventors conducted a more detailed study, focusing on the width of the rib portions and the width of the groove portions among the multiple rib portions and multiple groove portions in the slow cooling region of the molding surface of the mold. As a result, the inventors discovered that if the width of the rib portions is made narrower than the width of the groove portions, the heat removal from the material during hot press molding can be effectively reduced, and the cooling rate can be reduced. If the cooling rate of the material during hot press molding can be slowed down, the increase in strength of the material can be suppressed, and the impact absorption capacity can be improved.

By making the width of the rib portion in the slow cooling region narrower than the width of the groove portion, it is possible to form an impact buffer region with reduced strength in at least a part of the hot press-formed product. However, when the width of the rib portion is made narrower than the width of the groove portion, wavy deformation occurs on the surface of the hot press-formed product, and the dimensional accuracy of the hot press-formed product may decrease. The inventors therefore investigated the cause of this. As a result, the inventors have come to the following findings.

During hot press forming, a rib portion formed in the slow cooling area of the forming surface of one of a pair of dies (upper and lower dies) may get into a groove portion formed in the slow cooling area of the forming surface of the other die. In this case, the above-mentioned wavy deformation occurs on the surface of the hot press formed product.

Based on the above findings, the inventors conceived of making the rib portion formed in the slow cooling region of one die overlap the rib portion formed in the slow cooling region of the other die when viewed from the normal direction of the slow cooling region during hot press forming. If at least a portion of the rib portion formed in the slow cooling region of one die overlaps with the rib portion formed in the slow cooling region of the other die, it is possible to prevent the rib portion from entering the groove portion.

In the mold according to this embodiment, when the slow cooling regions are viewed from the normal direction of the slow cooling regions during hot press forming, the rib portion formed in one slow cooling region and the rib portion formed in the other slow cooling region at least partially overlap. As a result, it is possible to suppress the above-mentioned wavy deformation caused by the width of the rib portion being narrower than the width of the groove portion.

The die of this embodiment, which was completed based on the above findings, and the manufacturing method for hot press-formed products using the die of this embodiment, have the following configuration.

[1]
A die for performing hot press forming on a material,
an upper mold having a first molding surface;
and a lower mold having a second molding surface that is disposed opposite the first molding surface during hot press molding and hot press molds the material together with the first molding surface,
The first molding surface is
a first slow cooling region having a plurality of first rib portions and a plurality of first groove portions;
The first rib portions are arranged in a width direction of the first rib portion,
The first groove portions are arranged in a width direction of the first groove portions,
The first rib portion is formed between adjacent first groove portions,
The width of the first rib portion is narrower than the width of the first groove portion,
The second molding surface is
a second slow cooling region having a plurality of second rib portions and a plurality of second groove portions;
The second rib portions are arranged in a width direction of the second rib portion,
The second groove portions are arranged in a width direction of the second groove portions,
The second rib portion is formed between adjacent second groove portions,
The width of the second rib portion is narrower than the width of the second groove portion,
During hot press forming, when the first slow cooling region and the second slow cooling region are viewed from a normal direction of the first slow cooling region,
The first rib portion and the second rib portion at least partially overlap each other.
Mold.

The mold [1] can produce hot press molded products with excellent impact absorption capabilities through hot press molding.

[2]
The mold according to [1],
During hot press forming, when the first slow cooling region and the second slow cooling region are viewed from a normal direction of the first slow cooling region,
The first rib portion and the second rib portion extend in the same direction,
The first groove portion and the second groove portion extend in the same direction.
Mold.

The mold [2] can improve the accuracy of simulation of heat transfer in materials during hot press forming.

[3]
The mold according to [1] or [2],
The width of the first rib portion is 10 to 50% of the width of the first groove portion,
The width of the second rib portion is 10 to 50% of the width of the second groove portion.
Mold.

In the case of the die [3], the cooling rate can be slowed down during hot press forming. If the cooling rate can be slowed down, it becomes easier to stop cooling at the desired material temperature. For example, the amount of martensite and retained austenite produced in the microstructure of the material may be adjusted by stopping cooling between the Mf point and the Ms point during hot press forming and then carrying out the heating and holding process described later. Also, for example, the microstructure of the material may be made mainly of bainite by stopping cooling at a temperature between the Ms point and 500°C during hot press forming and then carrying out the heating and holding process described later.

[4]
A mold according to any one of [1] to [3],
In at least a portion of the first slow cooling region,
The width of the first rib portion is 1.0 to 8.0 mm,
The height of the first rib portion is 0.2 to 5.0 mm,
In at least a portion of the second slow cooling region,
The width of the second rib portion is 1.0 to 8.0 mm,
The height of the second rib portion is 0.2 to 5.0 mm.
Mold.

The mold [4] can suppress temperature variations in the material during hot press forming.

[5]
The mold according to [2],
The width of the first rib portion is 10 to 50% of the width of the first groove portion,
The width of the second rib portion is 10 to 50% of the width of the second groove portion,
The width of each of the first rib portion and the second rib portion is 1.0 to 8.0 mm,
The height of each of the first rib portion and the second rib portion is 0.2 to 5.0 mm,
Fn1 defined by formula (1) is 14 or less;
Mold.
Fn1 = Wr ^0.9 / P0 ^0.8 + 0.05Hr (1)
Here, in formula (1), Wr is the width (mm) of the first rib portion and the second rib portion, P0 = Wr/Ws, Ws is the width (mm) of the first groove portion and the second groove portion, and Hr is the height (mm) of the first rib portion and the second rib portion.

The die [5] can further suppress temperature variations in the material during hot press forming.

[6]
The mold according to [2] or [5],
The width of the first rib portion is 10 to 50% of the width of the first groove portion,
The width of the second rib portion is 10 to 50% of the width of the second groove portion,
The width of each of the first rib portion and the second rib portion is 1.0 to 8.0 mm,
The height of each of the first rib portion and the second rib portion is 0.2 to 5.0 mm,
Fn2 defined by formula (2) is 30 or more;
Mold.
Fn2 = ^Ws2 x ^Hr0.4 / Wr (2)
Here, Ws in formula (2) is the width (mm) of the first groove portion and the second groove portion, Wr is the width (mm) of the first rib portion and the second rib portion, and Hr is the height (mm) of the first rib portion and the second rib portion.

In the die [6], the cooling rate of the material can be slowed down. If the cooling rate can be slowed down, it becomes easier to stop cooling at the desired material temperature. For example, the amount of martensite and retained austenite produced in the microstructure of the material may be adjusted by stopping cooling between the Mf point and the Ms point during hot press forming and then carrying out the heating and holding process described later. Also, for example, the microstructure of the material may be made mainly of bainite by stopping cooling at a temperature between the Ms point and 500°C during hot press forming and then carrying out the heating and holding process described later.

[7]
The mold according to any one of [4] to [6],
In at least a portion of the first slow cooling region,
The width of the first rib portion is 1.0 to 4.0 mm and is 10 to 30% of the width of the first groove portion,
In at least a portion of the second slow cooling region,
The width of the second rib portion is 1.0 to 4.0 mm and is 10 to 30% of the width of the second groove portion.
Mold.

The die [7] can further suppress temperature variations in the material during hot press forming.

[8]
A method for producing a hot press-formed product, comprising:
A process of preparing materials;
Heating the prepared material to a temperature equal to or higher than A _c3 ;
A step of performing hot press forming on the heated material using a mold according to any one of [1] to [7];
and releasing the hot press molded material from the die to produce a hot press molded product.
A manufacturing method for hot press formed products.

The manufacturing method of hot press-formed products [8] can produce hot press-formed products having excellent impact absorption capabilities.

[9]
The method for producing a hot press-formed product according to [8], further comprising:
and holding the hot press molded product produced by releasing it from the mold at 100 to 500 ° C.
A manufacturing method for hot press formed products.

Hereinafter, the die according to this embodiment and the method for manufacturing a hot press molded product using the die according to this embodiment will be described with reference to the drawings. Note that the same or corresponding configurations in each drawing will be given the same reference numerals, and the same description will not be repeated.

[Configuration of hot press device 1]
Fig. 1 is a front view showing an example of a hot press apparatus 1 for hot press molding. 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 a die 10 (upper die 11 and lower die 12). In the following description, the vertical (up-down) direction of the hot press apparatus 1 is also referred to as the V direction, the width direction of the hot press apparatus 1 is also referred to as the W direction, and the direction perpendicular to the V direction and the W direction is also referred to as the L direction.

Referring to FIG. 1, the frame 2 is arranged on the upper part of the hot press apparatus 1. The frame 2 supports the slide 3 arranged below the frame 2 so that it can be raised and lowered. The frame 2 is equipped 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. The bolster 4 is arranged 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. At this time, the lower mold 12 is arranged below the upper mold 11.

The die 10 includes the above-mentioned upper die 11 and the above-mentioned lower die 12. The upper die 11 and the lower die 12 extend in the L direction in Fig. 1. Hereinafter, with respect to the hot press apparatus 1 and the die 10, the direction in which the upper die 11 and the lower die 12 extend is also referred to as the "longitudinal direction (L direction) of the die 10." With respect to the die 10, the direction perpendicular to the L direction and the V direction in Fig. 1 is also referred to as the "width direction (W direction) of the die 10."

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 material (blank) is first placed on the lower die 12. After the material is placed, the upper die 11 slides in the V direction relative to the lower die 12, and applies an external force to the material while contacting the material. In other words, the upper die 11 and the lower die 12 hot press form the material. This forms the material 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 material, and the upper die 11 and the lower die 12 remove heat from the material and quench it. Therefore, a hot press formed product having a desired shape and increased strength is manufactured.

The hot press apparatus 1 may include a configuration not shown in FIG. 1. The hot press apparatus 1 may include, for example, a cooling device for cooling the die 10. In this case, for example, a flow path through which a cooling medium passes is provided inside the die 10. Furthermore, a pump for supplying the cooling medium inside the die 10 is disposed. 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 configuration not shown in FIG. 1 that is included in a known hot press apparatus.

[Configuration of mold 10]
The configuration of the mold 10 in FIG. 1 will now be described in more detail.
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 recess extending in the L direction at the center in the W direction.

The lower die 12 has a second molding surface 120. The second molding surface 120 is disposed on the upper surface of the lower die 12. In FIG. 2, the second molding surface 120 includes a convex portion extending in the L direction at the center in the W direction. During hot press forming, the second molding surface 120 is disposed opposite the first molding surface 110. Then, the second molding surface 120, together with the first molding surface 110, comes into contact with the material (blank) and hot press forms the material.

[Configuration of first molding surface 110 and second molding surface 120]
The first molding surface 110 includes a first slow cooling region 113 indicated by a shaded area. In Fig. 2, the first slow cooling region 113 is formed in a part of the first molding surface 110. In Fig. 2, the first molding surface 110 includes the first slow cooling region 113 and a first rapid cooling region 114.
Similarly, the second molding surface 120 includes a second slow cooling region 123 indicated by a shaded area. In Fig. 2, the second slow cooling region 123 is formed in a portion of the second molding surface 120. In Fig. 2, the second molding surface 120 includes the second slow cooling region 123 and a second rapid cooling region 124.
During hot press forming, the first quenching region 114 faces the second quenching region 124. The first quenching region 114 and the second quenching region 124 form a high-strength region having high strength in the material after hot press forming (hot press formed product).
During hot press forming, the first slow cooling region 113 faces the second slow cooling region 123. The first slow cooling region 113 and the second slow cooling region 123 form an impact buffer region in the material after hot press forming (hot press formed product) that has a lower strength than the high strength region and is easily plastically deformed.

Figure 3 is a cross-sectional view including the V direction and the W direction, showing the state of the die 10 and the blank B in the first slow cooling region 113 and the second slow cooling region 123 during hot press forming. Figure 4 is a cross-sectional view including the V direction and the W direction, showing the state of the die 10 and the blank B in the first quenching region 114 and the second quenching region 124 during hot press forming. Figures 3 and 4 show the state in which the upper die 11 has reached the bottom dead center and the die 10 is closed. In other words, Figures 3 and 4 show the state in which substantially the entire first molding surface 110 of the upper die 11 and substantially the entire second molding surface 120 of the lower die 12 are in contact with the blank B and apply an external force to the blank B.

With reference to Figures 2 to 4, during hot press forming, material B is sandwiched between upper die 11 and lower die 12. At this time, the concave portion of first forming surface 110 fits into the convex portion of second forming surface 120. As a result, material B is formed into a hat-shaped hot press formed product.

2 further includes a slow cooling region (113 and 123) and a rapid cooling region (114 and 124). In the first molding surface 110 of the upper mold 11, the surface structure of the first slow cooling region 113 forming the impact buffer region of the hot press molded product is different from the surface structure of the first rapid cooling region 114 forming the high strength region of the hot press molded product. Similarly, in the second molding surface 120 of the lower mold 12, the surface structure of the second slow cooling region 123 forming the impact buffer region of the hot press molded product is different from the surface structure of the second rapid cooling region 124 forming the high strength region of the hot press molded product. The surface structures of the first molding surface 110 and the second molding surface 120 are further described below.

[Configuration of first molding surface 110]
[Configuration of the first slow cooling region 113]
5A is an enlarged view of the region 100 in the first slow cooling region 113 of the first molding surface 110 in FIG. 3. Referring to FIG. 5A, the first slow cooling region 113 of the first molding surface 110 of the upper mold 11 has a plurality of first rib portions 111 and a plurality of first groove portions 112. FIG. 6 is a schematic diagram of the region 100 in FIG. 3, viewed from above (the normal direction of the first slow cooling region 113 and the second slow cooling region 123). Referring to FIG. 5A and FIG. 6, the plurality of first rib portions 111 extend in a predetermined direction. In FIG. 5A and FIG. 6, as an example, the plurality of first rib portions 111 extend in the L direction. However, the extension direction of the plurality of first rib portions 111 is not limited to the L direction.

The multiple first rib portions 111 are arranged in the width direction of the first rib portion 111. The width direction of the first rib portion 111 is a direction perpendicular to the extension direction of the first rib portion 111. In FIG. 5A, the width direction of the first rib portion 111 is the W direction. However, the width direction of the first rib portion 111 is not limited to the W direction. In this embodiment, each first rib portion 111 extends in the L direction and is arranged in the W direction.

Referring to FIG. 5A, the multiple first groove portions 112 are arranged in the width direction of the first groove portions 112, and the first rib portions 111 are formed between adjacent first groove portions 112. In FIG. 5A, the first groove portions 112 extend in the L direction and are arranged in the W direction, similar to the first rib portions 111. Furthermore, the first rib portions 111 are formed between two adjacent first groove portions 112. In FIG. 5A, the multiple first groove portions 112 and the multiple first rib portions 111 both extend in the L direction, and the first groove portions 112 and the first rib portions 111 are arranged alternately in the W direction.

With reference to FIG. 5A, the width of the first rib portion 111 is narrower than the width of the first groove portion 112. As described above, the width of the first rib portion 111 means the width of the first rib portion 111 in a cross section perpendicular to the extension direction of the first rib portion 111. In FIG. 5A, the width of the first rib portion 111 means the width of the first rib portion 111 in the W direction. Similarly, the width of the first groove portion 112 means the width of the first groove portion 112 in a cross section perpendicular to the extension direction of the first groove portion 112. In FIG. 5A, the width of the first groove portion 112 means the width of the first groove portion 112 in the W direction. The width of the first rib portion 111 and the width of the first groove portion 112 can be easily determined, for example, using a vernier caliper or the like.

With reference to FIG. 5A, in this embodiment, the width of the first rib portion 111 is narrower than the width of the first groove portion 112. When the first slow cooling region 113 comes into contact with the material B during hot press forming, the amount of heat dissipated in the first groove portion 112 is less than the amount of heat dissipated in the first rib portion 111. Since the width of the first rib portion 111 is narrower than the width of the first groove portion 112, the amount of heat dissipated in the first rib portion 111 can be reduced. Therefore, in the first slow cooling region 113, the cooling rate of the material B can be slower than that in the first rapid cooling region 114. Therefore, when hot press forming is performed using the mold 10, the degree of quenching of the region of the material B that comes into contact with the first slow cooling region 113 can be reduced. As a result, a shock absorbing region with reduced strength can be formed in the material B (hot press formed product) after hot press forming.

[Configuration of the first quenching region 114]
FIG. 7 is an enlarged view of a region 200 in the first quenching region 114 of the first molding surface 110 in FIG. 4. Referring to FIG. 7, the first rib portion 111 and the first groove portion 112 are not formed in the first quenching region 114 of the first molding surface 110 of the upper mold 11. In other words, the first quenching region 114 of the first molding surface 110 is a smooth surface. Therefore, during hot press forming, substantially the entire first quenching region 114 comes into contact with the material B. Therefore, in the first quenching region 114, the cooling rate of the material B can be made faster than that in the first slow cooling region 113. Therefore, when hot press forming is performed using the mold 10, the degree of quenching of the region of the material B that comes into contact with the first quenching region 114 can be increased. As a result, a high-strength region having a higher strength than the impact buffer region can be formed in the material B (hot press-formed product) after hot press forming.

[Configuration of second molding surface 120]
[Configuration of the second slow cooling region 123]
5A, the second slow cooling region 123 of the second molding surface 120 of the lower mold 12 has a plurality of second rib portions 121 and a plurality of second groove portions 122. With reference to Fig. 5A and Fig. 6, the plurality of second rib portions 121 extend in a predetermined direction. In Fig. 5A and Fig. 6, the plurality of second rib portions 121 extend in the L direction. However, the extension direction of the plurality of second rib portions 121 is not limited to the L direction.

The multiple second rib portions 121 are arranged in the width direction of the second rib portion 121. The width direction of the second rib portion 121 is a direction perpendicular to the extension direction of the second rib portion 121. In FIG. 5A, the width direction of the second rib portion 121 is the W direction. However, the width direction of the second rib portion 121 is not limited to the W direction. In this embodiment, each second rib portion 121 extends in the L direction and is arranged in the W direction.

Referring to FIG. 5A, the multiple second groove portions 122 are arranged in the width direction of the second groove portions 122, and the second rib portions 121 are formed between adjacent second groove portions 122. In FIG. 5A, the second groove portions 122 extend in the L direction and are arranged in the W direction, similar to the second rib portions 121. Furthermore, the second rib portions 121 are formed between two adjacent second groove portions 122. In FIG. 5A, both the multiple second groove portions 122 and the multiple second rib portions 121 extend in the L direction, and the second groove portions 122 and the second rib portions 121 are arranged alternately in the W direction.

Referring to Figure 5A, furthermore, the width of the second rib portion 121 is narrower than the width of the second groove portion 122. As described above, the width direction of the second rib portion 121 and the second groove portion 122 corresponds to the width direction (W direction) of the mold 10. Therefore, in the cross section shown in Figure 5A, the width of the second rib portion 121 means the width of the second rib portion 121 in the W direction, and the width of the second groove portion 122 means the width of the second groove portion 122 in the W direction. The width of the second rib portion 121 and the width of the second groove portion 122 can be found, for example, using a vernier caliper.

Referring to FIG. 5A, the width of the second rib portion 121 is narrower than the width of the second groove portion 122. As described above, the width of the second rib portion 121 means the width of the second rib portion 121 in a cross section perpendicular to the extension direction of the second rib portion 121. Therefore, the same effect as the relationship between the width of the first rib portion 111 and the width of the first groove portion 112 described above can be obtained. Specifically, when the second slow cooling region 123 contacts the material B during hot press molding, the amount of heat dissipated in the second groove portion 122 is less than the amount of heat dissipated in the second rib portion 121. Since the width of the second rib portion 121 is narrower than the width of the second groove portion 122, the amount of heat dissipated in the second rib portion 121 can be suppressed to be smaller. Therefore, in the second slow cooling region 123, the cooling rate of the material B can be slower than that of the second rapid cooling region 124 described later. Therefore, when hot press molding is performed using the mold 10, the degree of quenching of the region of the material B that contacts the second slow cooling region 123 can be reduced. As a result, a shock absorbing region with reduced strength can be formed in the material B (hot press molded product) after hot press molding.

[Configuration of the second quenching region 124]
Referring to FIG. 7, the second rib portion 121 and the second groove portion 122 are not formed in the second quenching region 124 of the second molding surface 120 of the lower mold 12. That is, the second quenching region 124 of the second molding surface 120 is a smooth surface. Therefore, during hot press forming, substantially the entire second quenching region 124 comes into contact with the material B. Therefore, in the second quenching region 124, the cooling rate of the material B can be made faster than that in the second slow cooling region 123. Therefore, when hot press forming is performed using the mold 10, the degree of quenching of the region of the material B that comes into contact with the second quenching region 124 can be increased. As a result, a high-strength region having a strength higher than that of the impact buffer region can be formed in the material B (hot press-formed product) after hot press forming.

[Relationship between first rib portion 111 and second rib portion 121]
Furthermore, in the die 10, during hot press molding, when the first slow cooling region 113 of the first molding surface 110 and the second slow cooling region 123 of the second molding surface 120 are viewed from the normal direction of the first slow cooling region 113, the first rib portion 111 and the second rib portion 121 at least partially overlap. Here, "during hot press molding" refers to a state in which substantially the entire molding surface 110 of the upper die 11 and substantially the entire molding surface 120 of the lower die 12 are in contact with the material B and apply an external force to the material B, and refers to a state in which the die 10 is closed and the material B is cooled by the die 10 while the upper die 11 is held at the bottom dead center.

In this case, as shown in Figure 5A, during hot press forming, the first rib portion 111 does not enter the second groove portion 122, and the second rib portion 121 does not enter the first groove portion 112. Therefore, during hot press forming, the first rib portion 111 enters the second groove portion 122, and the second rib portion 121 enters the first groove portion 112, thereby preventing material B from being deformed into a wavy shape.

[Features of Mold 10]
As described above, in the mold 10 of this embodiment, the width of the first rib portion 111 in the first slow cooling region 113 is narrower than the width of the first groove portion 112. Furthermore, the width of the second rib portion 121 in the second slow cooling region 123 is narrower than the width of the second groove portion 122. Therefore, when hot press molding is performed using the mold 10, the cooling rate is suppressed in the region of the material B sandwiched between the first slow cooling region 113 and the second slow cooling region 123. Therefore, it is possible to form an impact buffer region with reduced strength and a high strength region with higher strength than the impact buffer region in the hot press molded product. Furthermore, when the first slow cooling region 113 and the second slow cooling region 123 are viewed from the normal direction of the first slow cooling region 113 during hot press molding, the first rib portion 111 and the second rib portion 121 at least partially overlap. Therefore, it is possible to suppress the material B from deforming into a wavy shape in the impact buffer region.

[Preferable form of mold 10]
In the first slow cooling region 113 and the second slow cooling region 123, the first rib portion 111 and the second rib portion 121 preferably extend in the same direction. Furthermore, the first groove portion 112 and the second groove portion 122 extend in the same direction. In Fig. 5A and Fig. 6, the first rib portion 111 and the second rib portion 121 extend in the L direction, and the first groove portion 112 and the second groove portion 122 extend in the L direction.

As described above, when the first rib portion 111, the first groove portion 112, the second rib portion 121, and the second groove portion 122 all extend in the same direction, it is sufficient to perform a two-dimensional simulation in the simulation of heat transfer of material B. In other words, if a two-dimensional simulation is performed on a cross section perpendicular to the extension direction of the first rib portion 111, the first groove portion 112, the second rib portion 121, and the second groove portion 122 (i.e., the cross section shown in FIG. 5A), the result will be substantially the same as the result obtained by a three-dimensional simulation. Therefore, it is not necessary to perform a three-dimensional simulation. Therefore, it is possible to more easily and accurately simulate the heat transfer of material B. In other words, it is possible to more easily and accurately control the cooling rate. In this case, it is possible to control the desired microstructure from the chemical composition of material B by using a simulation.

For example, when material B is a steel material, if the result of hot press forming material B is a microstructure containing not only a hard phase but also retained austenite, the impact absorption capacity of the hot press formed product is further improved. Here, the hard phase consists of martensite and/or bainite.

[Preferable Relationship Between the First Rib Portion 111 and the First Groove Portion 112, and Preferred Relationship Between the Second Rib Portion 121 and the Second Groove Portion 122]
Preferably, the width of the first rib portion 111 formed in the slow cooling region 113 is 10 to 50% of the width of the first groove portion 112, and the width of the second rib portion 121 formed in the slow cooling region 123 is 10 to 50% of the width of the second groove portion 122.

If the width of the first rib portion 111 is 10% or more of the width of the first groove portion 112, the first rib portion 111 appropriately removes heat from material B. In this case, the cooling rate of material B can be prevented from becoming excessively slow. This prevents ferrite and pearlite from forming in the microstructure of material B, and promotes the formation of a hard phase, or a hard phase and retained austenite. As a result, the impact absorption capacity of the hot press molded product is improved.

On the other hand, if the width of the first rib portion 111 is 50% or less of the width of the first groove portion 112, the cooling rate can be slowed down during hot press forming. If the cooling rate can be slowed down, it becomes easier to stop cooling at the desired material temperature. For example, cooling may be stopped between the Mf point and the Ms point to adjust the amount of martensite and retained austenite produced. For example, cooling may be stopped at a temperature higher than the Ms point to make the microstructure of the material mainly bainite.

Therefore, the width of the first rib portion 111 is preferably 10 to 50% of the width of the first groove portion 112 .
A more preferable upper limit of the ratio of the width of the first rib portion 111 to the width of the first groove portion 112 is 45%, more preferably 40%, and even more preferably 35%.
A more preferable lower limit of the ratio of the width of the first rib portion 111 to the width of the first groove portion 112 is 12%, and a more preferable lower limit is 14%.

FIG. 5B is a schematic diagram showing the hot press forming of FIG. 5A with the material B removed. Here, the width of the first rib portion 111 is defined as follows. Among the surfaces of the first rib portion 111, the surface facing the second molding surface 120 is defined as the top surface of the first rib portion 111. That is, among the surfaces of the first rib portion 111, the surface that contacts the material B during hot press forming is defined as the "top surface". As shown in FIG. 5B, in a cross section perpendicular to the extension direction of the first rib portion 111, the width W _111P of the top surface 111P is defined as the width of the first rib portion 111. That is, the width W _111P of the top surface 111P corresponds to the length of the top surface 111P in the direction perpendicular to the extension direction of the first rib portion 111 and the normal direction of the top surface 111P (W direction in FIG. 5B).

The height of the first rib portion 111 is defined as follows: In Fig. 5B, the height _H111P from the top surface 111P of the first rib portion 111 to the groove bottom 112P of the first groove portion 112 is defined as the height of the first rib portion 111. In other words, the length from the top surface 111P of the first rib portion 111 to the groove bottom 112P of the first groove portion 112 in the normal direction of the top surface 111P (V direction in Fig. 5B) is defined as the height _H111P of the first rib portion 111.

The width of the first groove portion 112 is defined as follows: As shown in Fig. 5B, in a cross section perpendicular to the extension direction of the first rib portion 111, the width _W112P of the gap between an end point of one top surface 111P of adjacent first rib portions 111 and an end point of the other top surface 111P is defined as the width of the first groove portion 112.

The width of the second rib portion 121 is defined as follows. Of the surfaces of the second rib portion 121, the surface facing the first molding surface 110 is defined as the top surface of the second rib portion 121. That is, of the surfaces of the second rib portion 121, the surface that comes into contact with the material B during hot press forming is defined as the "top surface". As shown in FIG. 5B, in a cross section perpendicular to the extension direction of the second rib portion 121, the width W _121P of the top surface 121P is defined as the width of the second rib portion 121. That is, the width W _121P of the top surface 121P corresponds to the length of the top surface 121P in the direction perpendicular to the extension direction of the second rib portion 121 and the normal direction of the top surface 121P (W direction in FIG. 5B).

The height of the second rib portion 121 is defined as follows: In Fig. 5B, the height _H121P from the top surface 121P of the second rib portion 121 to the groove bottom 122P of the second groove portion 122 is defined as the height of the second rib portion 121. In other words, the length from the top surface 121P of the second rib portion 121 to the groove bottom 122P of the second groove portion 122 in the normal direction of the top surface 121P (V direction in Fig. 5B) is defined as the height of the second rib portion 121.

The width of the second groove portion 122 is defined as follows: As shown in Fig. 5B, in a cross section perpendicular to the extension direction of the second rib portion 121, the width _W122P of the gap between an end point of one top surface 121P of adjacent second rib portions 121 and an end point of the other top surface 121P is defined as the width of the second groove portion 122.

The shape of the first rib portion 111 or the second rib portion 121 in a cross section perpendicular to the extension direction may be rectangular as shown in Figures 5A and 5B, or may be trapezoidal with a narrowing width toward the top surface 111P or 121P as shown in Figure 5C. In addition, in the first rib portion 111 or the second rib portion 121, the corners of the top surface 111P or 121P may be chamfered as shown in Figure 5D, or the base of the first rib portion 111 or the second rib portion 121 may be chamfered. In addition, the corners of the top surface 111P or 121P may be rounded (i.e., filleted), or the base of the first rib portion 111 or the second rib portion 121 may be rounded (i.e., filleted).

In addition, in the case where the top surface (111P or 121P) is chamfered or filleted, the width of the top surface (111P or 121P) shall be the width of the portion of the top surface (111P and 121P) that is not chamfered or filleted.

As described above, the die 10 of this embodiment is a die for hot press molding. During hot press molding, the temperature of the material B is A _c3 point or higher, which is higher than that during warm press molding. Therefore, the material B during hot press molding has lower hardness and better workability than the material during warm press molding. Therefore, the surface pressure applied to the material B during hot press molding is lower than that during warm press molding. Therefore, even if the width of the first rib portion 111 is 50% or less of the width of the first groove portion 112, the surface of the material B is less likely to be scratched.

Similar to the relationship between the first rib portion 111 and the first groove portion 112, if the width of the second rib portion 121 is 10% or more of the width of the second groove portion 122, an excessive decrease in the cooling rate of material B can be suppressed. This improves the impact absorption capacity of the hot press molded product.

On the other hand, if the width of the second rib portion 121 is 50% or less of the width of the second groove portion 122, the cooling rate of material B can be appropriately slowed. This makes it easier to adjust the cooling stop temperature (i.e., the temperature of material B when the die 10 is separated from material B) during hot press forming. For example, it becomes easier to adjust the cooling stop temperature to a temperature above the Ms point, to adjust the cooling stop temperature to between the Mf point and the Ms point, or to between above the Ms point and 500°C.

Therefore, the width of the second rib portion 121 is 10 to 50% of the width of the second groove portion 122 .
A preferred upper limit of the ratio of the width of the second rib portion 121 to the width of the second groove portion 122 is 45%, more preferably 40%, and even more preferably 35%.
A more preferable lower limit of the ratio of the width of the second rib portion 121 to the width of the second groove portion 122 is 12%, and a more preferable lower limit is 14%.

Similar to the relationship between the width of the first rib portion 111 and the width of the first groove portion 112 described above, even if the width of the second rib portion 121 is 50% or less of the width of the second groove portion 122, the surface of material B is less likely to be scratched.

[Preferable width and height of first rib portion 111, and preferred width and preferred height of second rib portion 121]
Preferably, in at least a portion of the first slow cooling region 113, the width of the first rib portion 111 is 1.0 to 8.0 mm and the height of the first rib portion 111 is 0.2 to 5.0 mm, and in at least a portion of the second slow cooling region 123, the width of the second rib portion 121 is 1.0 to 8.0 mm and the height of the second rib portion 121 is 0.2 to 5.0 mm. The height of the first rib portion 111 corresponds to the depth of the first groove portion 112. Similarly, the height of the second rib portion 121 corresponds to the depth of the second groove portion 122.

By setting the width of the first rib portion 111 or the width of the second rib portion 121 to 8.0 mm or less, it is possible to reduce the temperature variation in the width direction of the first rib portion 111 or the width direction of the second rib portion 121 in the material B. Therefore, it is possible to reduce the variation in the cooling rate caused by the temperature variation of the material B during hot press forming.

On the other hand, if the width of the first rib portion 111 or the width of the second rib portion 121 is 1.0 mm or more, the first rib portion 111 or the second rib portion 121 will be less likely to break when hot press forming is performed, improving productivity. Therefore, the preferred width of the first rib portion 111 is 1.0 to 8.0 mm. The preferred width of the second rib portion 121 is 1.0 to 8.0 mm.

A more preferred lower limit for the width of the first rib portion 111 is 1.8 mm, and more preferably 2.0 mm. A more preferred lower limit for the width of the second rib portion 121 is 1.8 mm, and more preferably 2.0 mm. In this case, the first rib portion 111 and the second rib portion 121 become even less likely to break. In addition, the dimensional accuracy of the first rib portion 111 and the second rib portion 121 is relaxed, making them easier to process.

If the height of the first rib portion 111 or the height of the second rib portion 121 is set to 0.2 mm or more, heat dissipation from material B in the first groove portion 112 or the second groove portion 122 can be suppressed. On the other hand, if the height of the first rib portion 111 or the height of the second rib portion 121 is set to 5.0 mm or less, the first rib portion 111 or the second rib portion 121 becomes less likely to break when hot press forming is performed, improving productivity. Therefore, the preferred height of the first rib portion 111 is 0.2 to 5.0 mm. The preferred height of the second rib portion 122 is 0.2 to 5.0 mm.

In addition, if the width or height of the first rib portion 111 or the second rib portion 121 is adjusted as described above in at least a portion of the first slow cooling region 113 and the second slow cooling region 123, the above-mentioned preferable effect can be obtained in at least a portion of the above-mentioned region. Therefore, in this embodiment, the width or height of the first rib portion 111 or the second rib portion 121 may be adjusted as described above in at least a portion of the first slow cooling region 113 and the second slow cooling region 123. Preferably, the width of the first rib portion 111 is 1.0 to 8.0 mm and the height is 0.2 to 5.0 mm throughout the first slow cooling region 113, and the width of the second rib portion 121 is 1.0 to 8.0 mm and the height is 0.2 to 5.0 mm throughout the second slow cooling region 123.

More preferably, in at least a portion of the first slow cooling region 113, the width of the first rib portion 111 is 1.0 to 4.0 mm and is 10 to 30% of the width of the first groove portion 112, and further, in at least a portion of the second slow cooling region 123, the width of the second rib portion 121 is 1.0 to 4.0 mm and is 10 to 30% of the width of the second groove portion 122. In this case, the cooling rate of material B can be further slowed. Therefore, it is easy to adjust the cooling stop temperature to the desired temperature. Furthermore, the temperature variation of material B during hot press forming can be reduced.

[About Fn1]
In the mold 10, it is assumed that the first rib portion 111 and the second rib portion 121 extend in the same direction, the first groove portion 112 and the second groove portion 122 extend in the same direction, the width of the first rib portion 111 is 10 to 50% of the width of the first groove portion 112, the width of the second rib portion 121 is 10 to 50% of the width of the second groove portion 122, the widths of the first rib portion 111 and the second rib portion 121 are 1.0 to 8.0 mm, and the heights of the first rib portion 111 and the second rib portion 121 are 0.2 to 5.0 mm.
In this case, Fn1 defined in formula (1) is preferably 14 or less.
Fn1 = Wr ^0.9 / P0 ^0.8 + 0.05Hr (1)
Here, Wr is the width (mm) of the first rib portion 111 and the second rib portion 121. P0=Wr/Ws, where Ws is the width (mm) of the first groove portion 112 and the second groove portion 122. Hr is the height (mm) of the first rib portion 111 and the second rib portion 121. Fn1 will be described below.

Figure 8 shows the relationship between Fn1 and the temperature variation ΔT (°C) in the first slow cooling region 113 and the second slow cooling region 123. Figure 8 shows the results obtained by the two-dimensional heat transfer simulation described below. Specifically, the temperature distribution of material B during hot press forming and the change in the temperature distribution over time were simulated using a finite difference method assuming the heat conduction model shown in Figure 9.

9 is a cross-sectional view in the width direction of the first rib portion 111 and the second rib portion 121 in the first slow cooling region 113 and the second slow cooling region 123. The material B was partitioned into a plurality of elements E having an element width D0 = 1 mm, a thickness D1 = the plate thickness of material B (assumed to be 1.4 mm), and a unit length in the L direction. The heat balance Q (W/ ^m2 ) per unit time (1 second) due to heat conduction and heat transfer in the i-th element E (i is a natural number) is expressed by the following formula.

Q = (Q2i) - (Q2i + 1) - 2Q1
Each term in this equation is defined as follows:
(When element E is in contact with the first rib portion 111 and the second rib portion 121)
Q1: Heat transfer to the die per unit time of element E due to contact heat transfer between material B and the rib portion (111 or 121) (W/m ² )
Q1=-h(Tr-Tbi)
Heat transfer coefficient h = 2000 W/ ^m2 K
Mold temperature Tr = 100 ° C.
Tbi: temperature of the i-th element E in the material B (when the element E is not in contact with the first rib portion 111 and the second rib portion 121)
Q1: The amount of heat transferred per unit time from element E to the mold by heat conduction through the air in the groove portion (112 or 122) (W/m ² )
Q1 = -λa (Tr - Tbi) / Hr
λa: Thermal conductivity of air = 0.04 W/m K
Q2i: The amount of heat transferred from the (i-1)th element E to the adjacent i-th element E per unit time due to thermal conduction within material B (W/m ² )
Q2i = -λb (Tbi - Tbi-1) / Δx
λb: Thermal conductivity of material B = 50 W/m K
Tbi: Temperature of the i-th element E (material B) Tbi-1: Temperature of the i-1-th element E adjacent to the i-th element E Δx: Distance between adjacent elements E = 1 mm

Meanwhile, the temperature change ΔTb (° C.) of element E per unit time associated with the heat balance Q is expressed by the following formula.
ΔTb=Q/(c·ρ·D0·D1)
c: Specific heat of material B = 0.435 J/(g K)
ρ: density of material B = 7.8 × 10^3 (g/m ³ )

In the above-mentioned heat conduction model, the first rib portion 111 and the second rib portion 121 extend in the same direction, and the first groove portion 112 and the second groove portion 122 extend in the same direction. Furthermore, the width of the first rib portion 111 is the same as the width of the second rib portion 121, and the height of the first rib portion 111 is the same as the height of the second rib portion 121. Furthermore, the width of the first groove portion 112 is the same as the width of the second groove portion 122. Furthermore, as shown in FIG. 9, during hot press forming, the entire top surface of the first rib portion 111 overlaps with the entire top surface of the second rib portion 121. Note that the cross-sectional shape perpendicular to the extension direction of the ribs (111 and 121) was rectangular.

As an initial condition, the temperature Tbi of element E at time 0 (seconds) was set to 622°C. Furthermore, in the sections 500 to 1000 mm to the right and left of Fig. 9, element E was in contact with the first rib portion 111 and the second rib portion 121 (i.e., the above sections were set to the slow cooling regions 113 and 123), and as a boundary condition, heat conduction within material B in element E at points 1000 mm to the right and left of Fig. 9 was set to be adiabatic. Then, by performing sequential calculations to calculate the temperature change ΔTb of element E at time intervals of 0.01 seconds, the temperature distribution of material B and its change over time were simulated.

The width and height of the rib portions (111 and 121) and the width of the groove portions (112 and 122) were changed, and the difference Tbimax-Tbimin between the maximum value Tbimax and the minimum value Tbimin of Tbi in the sections within 250 mm to the right and left of Figure 9 was defined as the temperature variation ΔT (°C) for material B. Figure 8 was created using the obtained ΔT.

With reference to Figure 8, when Fn1 is higher than 14, ΔT decreases rapidly as Fn1 decreases. Then, when Fn1 becomes 14 or less, the rate of decrease in ΔT associated with the decrease in Fn1 becomes more gradual. When Fn1 becomes 10 or less, the rate of decrease in ΔT associated with the decrease in Fn1 becomes even more gradual. Therefore, in the graph of Figure 8, there are inflection points near Fn1 = 14 and near Fn1 = 10.

Therefore, the preferred upper limit of Fn1 is 14, and more preferably 10. If Fn1 is 14 or less, for example, the temperature variation ΔT of material B will be 110°C or less. Also, if Fn1 is 10 or less, for example, the temperature variation ΔT of material B will be 40°C or less.

[About Fn2]
In the mold 10, it is assumed that the first rib portion 111 and the second rib portion 121 extend in the same direction, the first groove portion 112 and the second groove portion 122 extend in the same direction, the width of the first rib portion 111 is 10 to 50% of the width of the first groove portion 112, the width of the second rib portion 121 is 10 to 50% of the width of the second groove portion 122, the widths of the first rib portion 111 and the second rib portion 121 are 1.0 to 8.0 mm, and the heights of the first rib portion 111 and the second rib portion 121 are 0.2 to 5.0 mm.
In this case, Fn2 defined in formula (2) is preferably 30 or more.
Fn2 = ^Ws2 x ^Hr0.4 / Wr (2)
Here, Ws is the width (mm) of the first groove portion 112 and the second groove portion 122. Wr is the width (mm) of the first rib portion 111 and the second rib portion 121. Hr is the height (mm) of the first rib portion 111 and the second rib portion 121.

Figure 10 shows the relationship between Fn2 and the cooling rate V (°C/sec) of material B during hot press forming. Figure 10 was obtained by the above-mentioned two-dimensional heat transfer simulation using the heat conduction model shown in Figure 9. In the heat transfer simulation, the cooling rate V was calculated by taking the time average of the temperature change of element E per unit time from time 0 to the time when the temperature of element E reaches 400°C. Here, the element E used to calculate the cooling rate was the element whose temperature Tbi indicates Tbimax.

With reference to FIG. 10, as Fn2 increases, the cooling rate V of material B decreases rapidly, and then the degree of decrease in the cooling rate V becomes gradual. Therefore, the preferred lower limit of Fn2 is 30, more preferably 45, and even more preferably 90. If Fn2 is 30 or more, for example, the cooling rate V of material B becomes 80°C/sec or less. Therefore, during hot press molding, it becomes easier to adjust the cooling stop temperature (i.e., the temperature of material B when the die 10 is separated from material B) to the desired temperature. If Fn2 is 45 or more, for example, the cooling rate of material B becomes 70°C/sec or less. Therefore, it becomes even easier to adjust the cooling stop temperature to the desired temperature. If Fn2 is 90 or more, for example, the cooling rate V of material B becomes 50°C/sec or less. Therefore, it becomes even easier to adjust the cooling stop temperature to the desired temperature.

In the mold 10, it is more preferable that Fn1 is 14 or less and Fn2 is 30 or more. In this case, the temperature variation ΔT of the material B can be further suppressed, and the cooling stop temperature can be easily adjusted to the desired temperature.

More preferably, in the mold 10, the first rib portion 111 and the second rib portion 121 extend in the same direction, the first groove portion 112 and the second groove portion 122 extend in the same direction, the width of the first rib portion 111 is 1.0 to 4.0 mm and is 10 to 30% of the width of the first groove portion 112, and the width of the second rib portion 121 is 1.0 to 4.0 mm and is 10 to 30% of the width of the second groove portion 122, and Fn1 is 14 or less and/or Fn2 is 30 or more. In this case, the cooling rate of the material B can be further slowed. Therefore, it is easy to adjust the cooling stop temperature to the desired temperature. Furthermore, the temperature variation of the material B during hot press forming can be reduced.

[Other forms of the mold 10 of this embodiment]
[Regarding the shapes of the first molding surface 110 and the second molding surface 120 of the mold 10]
The mold 10 according to the present embodiment is not limited to the above configuration. For example, the mold 10 is not limited to the shape shown in FIG. 2. The first molding surface 110 and the second molding surface 120 of the mold 10 may be curved in the longitudinal direction. In addition, the cross-sectional shape perpendicular to the longitudinal direction (L direction) of the first molding surface 110 of the upper mold 11 of the mold 10 is not limited to a concave shape. The cross-sectional shape perpendicular to the longitudinal direction (L direction) of the second molding surface 120 of the lower mold 12 is not limited to a convex shape. As long as the first molding surface 110 of the upper mold 11 is fitted to the second molding surface 120 of the lower mold 12, the shapes of the first molding surface 110 and the second molding surface 120 are not particularly limited.

[Regarding the arrangement of the first gentle cooling region 113 of the first molding surface 110 and the second gentle cooling region 123 of the second molding surface 120]
Further, the arrangement of the first slow cooling region 113 of the first molding surface 110 is not particularly limited. Similarly, the arrangement of the second slow cooling region 123 of the second molding surface 120 is not particularly limited. It is sufficient that the first molding surface 110 includes the first slow cooling region 113 and the second molding surface 120 includes the second slow cooling region 123.

For example, the first slow cooling region 113 and the second slow cooling region 123 are not limited to the form shown in Fig. 2. Fig. 11 is a perspective view showing another example of the mold 10 according to this embodiment, different from Fig. 2. As shown in Fig. 11, the first slow cooling region 113 may be arranged on the entire bottom surface of the groove of the recess of the first molding surface 110 of the upper mold 11 of the mold 10. The second slow cooling region 123 may be arranged on the entire upper surface of the convex portion of the second molding surface 120 of the lower mold 12 of the mold 10.

Figure 12 is a perspective view showing another example of the mold 10 according to this embodiment, different from Figures 2 and 11. As shown in Figure 12, a first slow cooling area 113 may be arranged on a part of the side of the recess of the first molding surface 110 of the upper mold 11 of the mold 10. A second slow cooling area 123 may be arranged on a part of the side of the protrusion of the second molding surface 120 of the lower mold 12 of the mold 10.

Figure 13 is a perspective view showing another example of the mold 10 according to this embodiment, which is different from Figures 2, 11, and 12. In Figures 2, 11, and 12, the first molding surface 110 includes a first slow cooling region 113 and a first rapid cooling region 114. The second molding surface 120 includes a second slow cooling region 123 and a second rapid cooling region 124. In contrast, as shown in Figure 13, the entire first molding surface 110 may be the first slow cooling region 113. Also, the entire second molding surface 120 may be the second slow cooling region 123.

As described above, if the first molding surface 110 includes the first slow cooling region 113 and the second molding surface 120 includes the second slow cooling region 123, the position of the first slow cooling region 113 in the first molding surface 110 and the position of the second slow cooling region 123 in the second molding surface 120 are not particularly limited. However, when the upper mold 11 and the lower mold 12 are in contact with the material B during hot press molding and pressing the material B, at least a portion of the first slow cooling region 113 and at least a portion of the second slow cooling region 123 are arranged opposite each other.

In this embodiment, the first slow cooling region 113 can also be defined as a portion of the region in which the first rib portion 111 and the first groove portion 112 are formed. Similarly, the second slow cooling region 123 can also be defined as a portion of the region in which the second rib portion 121 and the second groove portion 122 are formed. In short, in this embodiment, in at least a portion of the first molding surface 110, the width of the first rib portion 111 is narrower than the width of the first groove portion 112, and in at least a portion of the second molding surface 120, the width of the second rib portion 121 is narrower than the width of the second groove portion 122.

[Extending direction of first rib portion 111 and second rib portion 121]
In this embodiment, the extending direction of the first rib portion 111 and the extending direction of the second rib portion 121 are not particularly limited. For example, the first rib portion 111 and the second rib portion 121 may not extend in the L direction shown in FIG. 5A and FIG. 6. FIG. 14 is another example, different from FIG. 5A, of the schematic diagrams in which the region 100 in FIG. 3 is enlarged. FIG. 15 is a cross-sectional view taken along line segment XV-XV in FIG. 14. Referring to FIG. 14 and FIG. 15, in this embodiment, the first rib portion 111, the first groove portion 112, the second rib portion 121, and the second groove portion 122 all extend in the W direction and are aligned in the L direction. In this case, the width of the first rib portion 111, the width of the first groove portion 112, the width of the second rib portion 121, and the width of the second groove portion 122 all refer to the width of the mold 10 in the L direction.

In hot press forming, the upper die 11 moves in the V direction together with the slide 3. As a result, the material B is hot press formed by the first forming surface 110 of the upper die 11 and the second forming surface 120 of the lower die 12. When the first forming surface 110 and the second forming surface 120 have the shape shown in Figure 2, the metal flow of the material B advances in the W direction by hot press forming. That is, in the region 100, the material B slides in the W direction relative to the first forming surface 110 and the second forming surface 120.

14 and 15, when the first rib portion 111, the first groove portion 112, the second rib portion 121, and the second groove portion 122 all extend in the W direction, material B is less susceptible to frictional resistance from the first rib portion 111 and/or the second rib portion 121, and material B is more likely to slide against the first rib portion 111 and/or the second rib portion 121. This makes it possible to suppress the formation of scratches on the surface of material B.

In this way, when the first rib portion 111, the first groove portion 112, the second rib portion 121, and the second groove portion 122 extend in the sliding direction of material B during the hot press forming process, the formation of defects on the surface of material B during the hot press forming process can be suppressed.

[Regarding the arrangement of the first rib portion 111 and the second rib portion 121]
In this embodiment, the arrangement of the first rib portion 111 and the second rib portion 121 may not completely overlap when the mold 10 is closed, as shown in Fig. 5A and Fig. 6. Fig. 16 is another example of a schematic diagram of an enlarged region 100 in Fig. 3, different from Fig. 5A and Fig. 14. Fig. 17 is a schematic diagram showing only the rib portion when the region 100 in Fig. 16 is viewed from the normal direction of the first slow cooling region 113. For example, with reference to Figs. 16 and 17, when the mold 10 is closed, the first rib portion 111 and the second rib portion 121 may be arranged in parallel and overlap with each other in a shifted state. In this way, the first rib portion 111 and the second rib portion 121 may at least partially overlap.

In this embodiment, the arrangement of the first rib portion 111 and the second rib portion 121 may not extend in the same direction as shown in FIG. 5A and FIG. 16. FIG. 18 is another example of a schematic diagram of an enlarged region 100 in FIG. 3, which is different from FIG. 5A, FIG. 14, and FIG. 16. FIG. 19 is a schematic diagram showing only the rib portion when the region 100 in FIG. 18 is viewed from the normal direction of the first slow cooling region. For example, as shown in FIG. 18 and FIG. 19, the multiple first groove portions 112 and the multiple first rib portions 111 may all extend in the L direction, and the multiple second groove portions 122 and the multiple second rib portions 121 may all extend in the W direction. Even in this case, referring to FIG. 19, when the mold 10 is closed, the first rib portion 111 and the second rib portion 121 at least partially overlap each other.

[Regarding the first gentle cooling region 113 and the second gentle cooling region 123]
In this embodiment, the first slow cooling region 113 and the second slow cooling region 123 preferably do not have a supply port for supplying a cooling medium to the surfaces of the first slow cooling region 113 and the second slow cooling region 123. If the first slow cooling region 113 and the second slow cooling region 123 had a supply port, the material B would be likely to lose excessive heat in the portions of the first slow cooling region 113 and the second slow cooling region 123 that have the supply port due to air remaining in the piping inside the mold 10 (upper mold 11, lower mold 12) that communicates with the supply port. This may promote temperature variation of the material B.

On the other hand, the upper die 11 and the lower die 12 may have cooling passages through which a cooling medium passes. In this case, the temperature of the upper die 11 and the lower die 12 during hot press forming can be kept sufficiently low.

[Method of manufacturing hot press molded product using die 10]
A method for manufacturing a hot press-formed product by hot press forming using the die 10 will be described. The method for manufacturing a hot press-formed product according to this embodiment includes the following steps.
- Preparation process - Heating process - Hot press molding process - Demolding process Each process will be explained below.

[Preparation process]
In the preparation step, a material B having a desired chemical composition is prepared. In the present embodiment, the material B is not particularly limited. The material B is, for example, a steel plate. When the material B is a steel plate, the type of the steel plate is not particularly limited. The material B 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 hot-dip galvannealing treatment, or an aluminum plating treatment.

As described above, the chemical composition of the base steel plate of material B is not particularly limited. The base steel plate of material B may have a chemical composition, for example, in mass%, of C: 0.10 to 0.60%, Si: 0 to 5.0%, Mn: 0 to 5.0%, P: 0.100% or less, S: 0.100% or less, N: 0.100% or less, O: 0.100% or less, Al: 0 to 1.0%, Cr: 0 to 3.0%, Mo: 0 to 5.0%, V: 0 to 2.0%, Nb: 0 to 1.0%, Ti: 0 to 1.0%, B: 0 to 1.0%, Ca: 0 to 1.0%, Mg: 0 to 1.0%, Zr: 0 to 1.0%, rare earth elements: 0 to 1.0%, Co: 0 to 5.0%, W: 0 to 5.0%, Ni: 0 to 3.0%, Cu: 0 to 3.0%, and the balance being Fe and impurities. Here, impurities refer to elements that are mixed in from raw materials such as ore, scrap, or the manufacturing environment when steel is industrially produced, and are acceptable within a range that does not adversely affect the hot press-formed product of this embodiment.

The thickness of material B is not particularly limited, but is selected according to the characteristics of the hot press molded product to be obtained. The thickness of material B is, for example, 0.6 to 3.2 mm. The mechanical properties of material B are also not particularly limited. The mechanical properties of material B are appropriately selected according to the characteristics of the hot press molded product to be obtained. The tensile strength of material B may be, for example, 400 MPa or more.

The method for preparing material B is not particularly limited. For example, material B may be manufactured from molten steel having the above-mentioned chemical composition by a known manufacturing method. Material B may also be prepared by purchasing material B manufactured by a third party.

[Heating process]
In the heating process, the prepared material B is heated to a temperature of A _c3 point or higher. If the heating temperature is lower than A _c3 point, the material B does not become austenite single phase. In this case, when the material B is cooled in the hot press forming process, the formation of hard phases (martensite and/or bainite) is insufficient in the region of the material B sandwiched between the first quenching region 114 and the second quenching region 124. Therefore, sufficient strength may not be obtained. If the heating temperature is A _c3 point or higher, the material B before the hot press forming process becomes austenite single phase. Therefore, when the material B is cooled in the hot press forming process, the hard phase is sufficiently formed in the region of the material B sandwiched between the first quenching region 114 and the second quenching region 124. As a result, the strength of the region can be increased.

Preferably, the heating temperature is less than 950°C. In this case, the heating time of material B can be shortened, and productivity can be increased. Furthermore, the fuel and electricity required for heating can be reduced, and therefore manufacturing costs can be kept down.

In the heating process, the method for heating material B is not particularly limited. For example, material B may be heated using a heating furnace such as an electric furnace, a gas furnace, a far-infrared furnace, or a near-infrared furnace. Material B may also be heated using an electric heating device or a high-frequency induction heating device. In the heating process, the method for heating material B is not limited, and a publicly known heating method may be appropriately selected.

[Hot press forming process]
In the hot press forming process, the material B heated to the A _c3 point or higher is hot press formed using the above-mentioned die 10. In the hot press forming process, the material B heated in the heating process is placed on the second forming surface 120 of the lower die 12. Then, as shown in FIG. 3, the upper die 11 is brought relatively close to the lower die 12, and the die 10 is closed. At this time, the material B comes into contact with the first forming surface 110 of the upper die 11 and the second forming surface 120 of the lower die 12. In other words, the material B is sandwiched between the first forming surface 110 of the upper die 11 and the second forming surface 120 of the lower die 12. The material B is hot press formed by the upper die 11 and the lower die 12.

In the hot press forming process according to this embodiment, the material B is not cooled using a cooling medium during hot press forming. Instead, the material B is removed by the die 10 that comes into contact with the material B during hot press forming. When the die 10 is closed, in the first slow cooling region 113, the first rib portion 111 of the first molding surface 110 of the upper die 11 comes into contact with the material B. Furthermore, in the second slow cooling region 123, the second rib portion 121 of the second molding surface 120 of the lower die 12 comes into contact with the material B. At this time, the upper die 11 and the lower die 12 have a temperature sufficiently lower than that of the material B. Therefore, the material B is removed by the first rib portion 111 and the second rib portion 121. The temperature of the upper die 11 and the lower die 12 is, for example, room temperature (20±15°C) to 200°C.

[Mold release process]
In the demolding step, the hot press molded material B is demolded from the die 10 to produce a hot press molded product. Here, in the demolding step, the temperature of the material B (hot press molded product) when it is demolded from the die 10 is defined as the cooling stop temperature. For example, if the cooling stop temperature is between the Mf point and the Ms point of the hot press molded product, or between the Ms point and 500 ° C., a structure consisting of a hard phase, or a structure consisting of a hard phase and residual austenite, is obtained as the microstructure of the hot press molded product. In this case, the obtained hot press molded product has excellent impact absorption ability. Therefore, preferably, in the demolding step according to this embodiment, the material B is demolded from the die 10 when the temperature of the region of the material B sandwiched between the first slow cooling region 113 and the second slow cooling region 123 is between the Mf point and the Ms point determined from the chemical composition of the material B, or between the Ms point and 500 ° C.

The Ms and Mf points of the material B differ depending on the chemical composition of the material B. Therefore, when it is desired to obtain a structure consisting of a hard phase or a structure consisting of a hard phase and residual austenite in the material B, the preferred cooling stop temperature differs depending on the chemical composition of the material B. However, according to the manufacturing method of the hot press molded product using the mold 10, the cooling rate, temperature change over time, and temperature distribution after a predetermined time has elapsed after hot press molding in the region of the material B sandwiched between the first slow cooling region 113 and the second slow cooling region 123 can be obtained by heat transfer simulation based on the width and height of the first rib portion 111, the first groove portion 112, the second rib portion 121, and the second groove portion 122 formed in the first slow cooling region 113 and the second slow cooling region 123, and the chemical composition of the material B. Therefore, the preferred cooling stop temperature or the time from the start of hot press molding to demolding can be obtained by these heat transfer simulations. Therefore, according to the die 10 of this embodiment, a hot press-formed product having a structure consisting of a hard phase, or a structure consisting of a hard phase and retained austenite, depending on the chemical composition of the material B, can be produced by hot press forming.

[Other steps]
The method for producing a hot press-formed product according to the present embodiment may further include other manufacturing steps other than those described above. For example, the method for producing a hot press-formed product according to the present embodiment may include a heating and holding step in a temperature range of 500° C. or less after the demolding step.

In the heating and holding process, the hot press molded product after the demolding process is heated and held at a temperature range of 500°C or less. Specifically, the hot press molded product produced by demolding from the mold 10 is held at a heating temperature of 100 to 500°C. In this case, the heating and holding can distribute carbon from the hard phase in the microstructure of the hot press molded product to the retained austenite. Since carbon in the retained austenite is concentrated, the generation of the retained austenite is promoted. As a result, the proportion of the retained austenite in the hot press molded product increases. In this case, the retained austenite is transformed into martensite during collision deformation, improving the ductility of the hot press molded product (the so-called TRIP (Transformation Induced Plasticity) effect). As a result, the impact absorption ability of the hot press molded product is further improved.

The preferred upper limit of the heating holding temperature is 400°C. The heating holding temperature is preferably Ms point -209°C or higher. In this case, the impact absorption capacity of the hot press molded product is stabilized and further improved. Therefore, when Ms point -209°C exceeds 100°C, the preferred lower limit of the heating holding temperature is Ms point -209°C. The heating holding time is not particularly limited. The holding time at the heating holding temperature is preferably, for example, 5 seconds to 30 minutes (1800 seconds).

The above describes the embodiments of the present disclosure. 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 modifying the above-described embodiments as appropriate within the scope of the spirit of the present disclosure.

Reference Signs List 1: Hot press device 10: Die 11: Upper die 110: First molding surface 111: First rib portion 112: First groove portion 113: First gentle cooling area 12: Lower die 120: Second molding surface 121: Second rib portion 122: Second groove portion 123: Second gentle cooling area

Claims (9)

A die for performing hot press forming on a material,
an upper mold having a first molding surface;
and a lower mold having a second molding surface that is disposed opposite the first molding surface during hot press molding and hot press molds the material together with the first molding surface,
The first molding surface is
a first slow cooling region having a plurality of first rib portions and a plurality of first groove portions;
The first rib portions are arranged in a width direction of the first rib portion,
The first groove portions are arranged in a width direction of the first groove portions,
The first rib portion is formed between adjacent first groove portions,
The width of the first rib portion is narrower than the width of the first groove portion,
The second molding surface is
a second slow cooling region having a plurality of second rib portions and a plurality of second groove portions;
The second rib portions are arranged in a width direction of the second rib portion,
The second groove portions are arranged in a width direction of the second groove portions,
The second rib portion is formed between adjacent second groove portions,
The width of the second rib portion is narrower than the width of the second groove portion,
During hot press forming, when the first slow cooling region and the second slow cooling region are viewed from a normal direction of the first slow cooling region,
The first rib portion and the second rib portion at least partially overlap each other.
Mold. The mold according to claim 1,
During hot press forming, when the first slow cooling region and the second slow cooling region are viewed from a normal direction of the first slow cooling region,
The first rib portion and the second rib portion extend in the same direction,
The first groove portion and the second groove portion extend in the same direction.
Mold. The mold according to claim 1 or 2,
The width of the first rib portion is 10 to 50% of the width of the first groove portion,
The width of the second rib portion is 10 to 50% of the width of the second groove portion.
Mold. The mold according to any one of claims 1 to 3,
In at least a portion of the first slow cooling region,
The width of the first rib portion is 1.0 to 8.0 mm,
The height of the first rib portion is 0.2 to 5.0 mm,
In at least a portion of the second slow cooling region,
The width of the second rib portion is 1.0 to 8.0 mm,
The height of the second rib portion is 0.2 to 5.0 mm.
Mold. The mold according to claim 2,
The width of the first rib portion is 10 to 50% of the width of the first groove portion,
The width of the second rib portion is 10 to 50% of the width of the second groove portion,
The width of each of the first rib portion and the second rib portion is 1.0 to 8.0 mm,
The height of each of the first rib portion and the second rib portion is 0.2 to 5.0 mm,
Fn1 defined by formula (1) is 14 or less;
Mold.
Fn1 = Wr ^0.9 / P0 ^0.8 + 0.05Hr (1)
Here, in formula (1), Wr is the width (mm) of the first rib portion and the second rib portion, P0 = Wr/Ws, Ws is the width (mm) of the first groove portion and the second groove portion, and Hr is the height (mm) of the first rib portion and the second rib portion. The mold according to claim 2 or claim 5,
The width of the first rib portion is 10 to 50% of the width of the first groove portion,
The width of the second rib portion is 10 to 50% of the width of the second groove portion,
The width of each of the first rib portion and the second rib portion is 1.0 to 8.0 mm,
The height of each of the first rib portion and the second rib portion is 0.2 to 5.0 mm,
Fn2 defined by formula (2) is 30 or more;
Mold.
Fn2 = ^Ws2 x ^Hr0.4 / Wr (2)
Here, Ws in formula (2) is the width (mm) of the first groove portion and the second groove portion, Wr is the width (mm) of the first rib portion and the second rib portion, and Hr is the height (mm) of the first rib portion and the second rib portion. The mold according to any one of claims 4 to 6,
In at least a portion of the first slow cooling region,
The width of the first rib portion is 1.0 to 4.0 mm and is 10 to 30% of the width of the first groove portion,
In at least a portion of the second slow cooling region,
The width of the second rib portion is 1.0 to 4.0 mm and is 10 to 30% of the width of the second groove portion.
Mold. A method for producing a hot press-formed product, comprising:
A process of preparing materials;
Heating the prepared material to a temperature equal to or higher than A _c3 ;
A step of performing hot press forming on the heated material using a die according to any one of claims 1 to 7;
and releasing the hot press molded material from the die to produce a hot press molded product.
A manufacturing method for hot press formed products. The method for producing a hot press-formed product according to claim 8, further comprising:
and holding the hot press molded product produced by releasing it from the mold at 100 to 500 ° C.
A manufacturing method for hot press formed products.

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