Die for hot press molding, hot press device, and method for manufacturing hot press molded article

Abstract

Provided are a die for hot press molding, a hot press device, and a method for manufacturing a hot press molded article with which it is possible to more reliably suppress the occurrence of breakage when drawing is performed by hot pressing. The die 24 is for drawing a heated blank 30 by hot pressing. The die 24 comprises: a first die 26 that comes into contact with a first surface 30F of the blank 30; and a second die 28 that is disposed facing the first die 26 and that comes into contact with a second surface 30R of the blank 30. The first die 26 has an inner die 33 and an outer die 35. The inner die 33 protrudes with respect to the second die 28. The outer die 35 is recessed with respect to the second die 28 and is provided so as to surround the inner die 33. The second die 28 has an inner recess 46, which is recessed with respect to the first die 26, and a outer circumferential protrusion 48, which protrudes with respect to the first die 26. The inner recess 46 is provided at a position facing the inner die 33, and the outer circumferential protrusion 48 is provided at a position facing the outer die 35 so as to surround the inner recess 46.

Description

Die for hot press forming, hot press apparatus, and method for manufacturing a hot press formed product

[0001] The present invention relates to a die for hot press forming, a hot press apparatus, and a method for manufacturing a hot press formed product.

[0002] In hot press forming, various manufacturing methods for suppressing the occurrence of breakage have been proposed. For example, Patent Document 1 discloses a hot press deep drawing forming method using a holder that holds the outer edge portion of a blank between the holder and a die.

[0003] Japanese Patent Application Laid-Open No. 2015-182081

[0004] When performing drawing forming by hot press, it is required to more reliably suppress the occurrence of breakage.

[0005] An object of the present invention is to provide a die for hot press forming, a hot press apparatus, and a method for manufacturing a hot press formed product that can more reliably suppress the occurrence of breakage when performing drawing forming by hot press.

[0006] As one means for solving the above problems, the present application discloses the following plurality of aspects.

[0007] [Aspect 1] A die for performing deep drawing by hot pressing on a heated blank, wherein the die comprises, in hot pressing, a first die that contacts a first surface of the blank, and a second die positioned opposite to the first die and contacting a second surface of the blank opposite to the first surface, the first die having an inner die and an outer die positioned outside the inner die, the inner die protruding from the second die, the outer die recessed from the second die and surrounding the inner die, the second die having a recessed inner recess and a protruding outer circumferential protrusion relative to the first die, the inner recess being positioned opposite the inner die and the outer circumferential protrusion being positioned opposite the outer die and surrounding the inner recess, a die for hot pressing. [Aspect 2] The die for hot pressing according to aspect 1, wherein the inner die is columnar and the outer die is annular. [Aspect 3] The outer mold comprises an intermediate mold and an outer edge mold positioned outside the intermediate mold, and the mold for hot press forming is performed on the blank in the order of the intermediate mold and the outer edge mold, as described in Aspect 1 or 2. [Aspect 4] The mold for hot press forming according to any one of Aspects 1 to 3, for forming a wheel disc by hot press forming. [Aspect 5] A method for manufacturing a hot press-formed product, comprising: a first step of performing deep drawing on the central part of a heated blank; and a second step of performing deep drawing on the outer peripheral part surrounding the outside of the central part, wherein the first step is completed before the second step. [Aspect 6] The method for manufacturing a hot press-formed product according to Aspect 5, wherein the first step is deep drawing to make the central part protrude in one direction, and the second step is reverse deep drawing to make the outer peripheral part protrude in the opposite direction to the one direction. [Aspect 7] The method for manufacturing a hot press-formed product according to Aspect 5 or 6, wherein the hot press-formed product is a wheel disc.[Aspect 8] A hot press apparatus for forming a wheel disc, comprising a die and a press machine to which the die is attached, wherein in hot press forming, the die comprises a first die that contacts a first surface of the blank and a second die that is positioned opposite to the first die and contacts a second surface of the blank opposite to the first surface, the first die comprises an inner die and an outer die positioned outside the inner die, and the hot press apparatus for forming a wheel disc is performed on the blank in the order of the inner die and the outer die. [Aspect 9] The hot press apparatus according to aspect 8, wherein the inner die protrudes relative to the second die, the outer die is recessed relative to the second die and is provided to surround the inner die, the second die has a recessed inner recess and a protruding outer circumferential protrusion relative to the first die, the inner recess is provided at a position opposite to the inner die and the outer circumferential protrusion is provided at a position opposite to the outer die and surrounds the inner recess. [Aspect 10] The hot press apparatus according to aspect 8 or 9, wherein the inner mold is columnar and the outer mold is annular. [Aspect 11] The hot press apparatus according to any one of aspects 8 to 10, wherein the outer mold comprises an intermediate mold and an outer edge mold positioned outside the intermediate mold, and deep drawing is performed on the blank in the order of the intermediate mold and the outer edge mold. [Aspect 12] The hot press apparatus according to any one of aspects 8 to 11, wherein a wheel disc is formed by the hot press.

[0008] According to the present invention, when deep drawing is performed by hot pressing, the occurrence of fracture can be more reliably suppressed.

[0009] This is a front view of a wheel equipped with a wheel disc according to the first embodiment. This is a longitudinal cross-sectional view of a wheel equipped with a wheel disc according to the first embodiment. This is a schematic perspective view of a wheel disc according to the first embodiment. This is a schematic central longitudinal cross-sectional view of a wheel disc according to the first embodiment. This is a schematic cross-sectional view of a hot press apparatus according to the first embodiment, showing a blank set in the die. This is a schematic perspective view of a die for performing hot press forming according to the first embodiment. This is a schematic cross-sectional view showing the state of drawing a blank with the die according to the first embodiment, showing the inner die performing press forming. This is a schematic cross-sectional view showing the state of drawing a blank with the die according to the first embodiment, showing the inner die and intermediate die having completed press forming. This is a schematic cross-sectional view showing the state of drawing a blank with the die according to the first embodiment, showing the outer die having completed press forming. This is a schematic cross-sectional view of a hot press apparatus for performing hot press forming according to the second embodiment. This is a perspective view showing a hot press-formed product according to a modified example.

[0010] Embodiments of the present invention will be described below with reference to the drawings.

[0011] 1. First Embodiment [Wheel] A two-piece type automobile wheel 1 comprising a wheel disc 10 as a hot-pressed product will be described with reference to Figures 1 and 2. The wheel 1 comprises a rim 2 and a wheel disc 10. The rim 2 is substantially cylindrical with a central axis Z and has a drop portion 3, bead seat portions 4 provided on both sides of the drop portion 3 in the direction of the central axis Z, and flange portions 5 provided on the outer edge of each bead seat portion 4 in the direction of the central axis Z. A tire is mounted on the outer circumference of the rim 2.

[0012] The wheel disc 10 is fitted onto the inner circumference of the rim 2. The wheel disc 10 is substantially disc-shaped with a central axis Z at its center, and has a hub mounting portion 12 as a recess in the center, and, extending radially outward from the hub mounting portion 12, an annular inclined portion 14 and a peripheral edge fixing portion 16 as a vertical wall. The hub mounting portion 12 has a hub hole and a plurality of bolt holes, although these are not shown in the figure.

[0013] The peripheral fixing portion 16 of the wheel disc 10 is welded to the drop portion 3 of the rim 2 while fitted into the inner circumference of the drop portion 3 of the rim 2. The wheel disc 10 is positioned on one side of the rim 2 in the direction of the central axis Z. The wheel 1 has a space 6 formed by the rim 2 and the wheel disc 10 on the other side of the rim 2 in the direction of the central axis Z.

[0014] [Wheel Disc] The wheel disc 10 will be described with reference to Figures 3 and 4. The central axis Z passes through the center of the wheel disc 10 and is parallel to the thickness direction of the blank before press forming. In the following description, one side and one direction parallel to the central axis Z will be referred to as the upper side and the upward direction in Figure 3, and the other side and other direction of the central axis Z will be referred to as the lower side and the downward direction in Figure 3. In the direction perpendicular to the central axis Z, the side and direction approaching the central axis Z will be referred to as the inward side and the inward direction, and the side and direction moving away from the central axis Z will be referred to as the outward side and the outward direction. The direction of rotation about the central axis Z will be referred to as the circumferential direction. "Orthogonal" and "parallel" include the cases of "approximately orthogonal" and "approximately parallel," respectively.

[0015] Figure 3 is a schematic perspective view of the wheel disc 10 according to this embodiment, as seen from the outer surface. The wheel disc 10 is roughly disc-shaped. In Figure 3, the surface visible on the outside is the outer surface 10A, and the opposite back surface is the inner surface 10B.

[0016] The hub mounting portion 12 is located in the center of the wheel disc 10 and is recessed in a mortar shape. The hub mounting portion 12 has a bottom portion 18 and an inclined cylindrical portion 20. The bottom portion 18 includes the central axis Z and has a circular outer edge 18E (Figure 4) perpendicular to the central axis Z, and a bottom surface 18S surrounded by the outer edge 18E.

[0017] The inclined cylindrical portion 20 is shaped like an inverted truncated cone, having a circular lower edge 20E as viewed from the central axis Z, an upper edge 20L having a larger diameter than the lower edge 20E and positioned above the lower edge 20E, and an inclined cylindrical surface 20S connecting the lower edge 20E and the upper edge 20L. The lower edge 20E is connected to the outer edge 18E of the base portion 18. The inclined cylindrical portion 20 is inclined outward from the lower edge 20E toward the upper edge 20L. The connection portion between the outer edge 18E of the base portion 18 and the lower edge 20E of the inclined cylindrical portion 20 may have a cross-sectional radius (R). The diameter of the lower edge 20E of the inclined cylindrical portion 20 is, for example, 40 mm or more and 200 mm or less. The diameter of the upper edge 20L of the inclined cylindrical portion 20 is, for example, 60 mm or more and 250 mm or less.

[0018] The annular inclined portion 14 is provided on the outside of the hub mounting portion 12 so as to surround the hub mounting portion 12, and is positioned above the bottom portion 18. The annular inclined portion 14 is provided concentrically with the hub mounting portion 12 and is annular when viewed from the central axis Z. The annular inclined portion 14 has an inner edge 14E, an outer edge 14L positioned outside and below the inner edge 14E, and an inclined surface 14S connecting the inner edge 14E and the outer edge 14L.

[0019] The inner edge 14E of the annular inclined portion 14 is connected to the upper edge 20L of the inclined cylindrical portion 20. The outer edge 14L of the annular inclined portion 14 is also the outer edge of the wheel disc 10. The inclined surface 14S is a gently curved surface that slopes downward from the inner edge 14E to the outer edge 14L and is convex upward. The outer edge 14L of the annular inclined portion 14 is, for example, 250 mm or more and 500 mm or less.

[0020] The annular inclined portion 14 may have a plurality of through holes 22 in the inclined surface 14S. In the inclined surface 14S shown in Figure 3, four through holes 22 are formed evenly in the circumferential direction. The through holes 22 are in the shape of an annular sector when viewed from the central axis Z.

[0021] The peripheral fixing portion 16 is provided on the outside of the annular inclined portion 14 so as to surround the annular inclined portion 14. The peripheral fixing portion 16 is provided concentrically with the annular inclined portion 14 and is cylindrical in shape, having a circular upper edge 16E when viewed from the central axis Z, a lower edge 16L which is located below the upper edge 16E and has the same diameter as the upper edge 16E, and a cylindrical surface 16S which connects the upper edge 16E and the lower edge 16L. The upper edge 16E of the peripheral fixing portion 16 is connected to the outer edge 14L of the annular inclined portion 14. The lower edge 16L of the peripheral fixing portion 16 is open.

[0022] The height H1 of the hub mounting portion 12, that is, the vertical distance between the bottom surface 18S and the upper edge 20L of the inclined cylindrical portion 20, is, for example, 15 mm or more and 80 mm or less. The height H2 of the annular inclined portion 14, that is, the vertical distance between the inner edge 14E and the outer edge 14L of the annular inclined portion 14, is, for example, 5 mm or more and 70 mm or less.

[0023] [Hot Pressing Apparatus] As shown in Figure 5, the hot pressing apparatus 21 comprises a press machine 23 and a die 24 attached to the press machine 23. In this specification, the central axis Z of the press machine 23, the upper side and upward direction, the lower side and downward direction, the inner side and inward direction, and the outer side and outward direction are assumed to be the same as those of the wheel disc 10, and will be described below accordingly.

[0024] The press machine 23 has a slide 25 and a bolster 27. The slide 25 is located on the upper side of the die 24. The slide 25 has a surface 25S perpendicular to the central axis Z. The slide 25 has an inner slide 25C, an intermediate slide 25M located outside the inner slide 25C, and an outer edge slide 25E located outside the intermediate slide 25M. The intermediate slide 25M and the outer edge slide 25E are annular when viewed with respect to the central axis Z. Each slide is connected to a drive device (not shown) and can be raised and lowered independently. During hot press forming, the slide 25 descends toward the die 24 and applies a load to the die 24. The bolster 27 is located on the lower side of the die 24. The bolster 27 has a surface 27S perpendicular to the central axis Z.

[0025] [Mold] A mold 24 for hot press forming used in the manufacturing method of the wheel disc 10 according to this embodiment will be described with reference to Figures 5 and 6. The mold 24 is used to perform hot press forming on a blank when manufacturing a wheel disc 10 having a mortar-shaped hub mounting portion 12, an annular inclined portion 14 provided so as to surround the hub mounting portion 12, and a cylindrical peripheral edge fixing portion 16 provided so as to surround the annular inclined portion 14.

[0026] The mold 24 shown in Figure 5 comprises a first mold 26 and a second mold 28. Figures 5 and 6 show a state in which a blank 30 is placed between the upper mold, which is the first mold 26, and the lower mold, which is the second mold 28.

[0027] (First mold) The first mold 26 is equipped with a refrigerant flow path (not shown). The first mold 26 is sized to match the size of the wheel disc 10. The first mold 26 has a first molding surface 26S on the side facing the second mold 28 across the blank 30, a pressing surface 44 positioned outside the first molding surface 26S, and a first mounting surface 26FS provided on the side opposite to the first molding surface 26S. The first molding surface 26S is in contact with the first surface 30F of the blank 30. The first molding surface 26S is provided with an inner convex portion 32 and an outer peripheral concave portion 34. The first mounting surface 26FS is a plane perpendicular to the central axis Z. The first mold 26 is fixed to the slide 25 with the first mounting surface 26FS in contact with the surface 25S of the slide 25.

[0028] The first mold 26 has an inner mold 33 and an outer mold 35 positioned outside the inner mold 33. The inner mold 33 is columnar, located in the center of the first mold 26, and protrudes downward relative to the second mold 28. The inner mold 33 has a tip surface 36 and a first inner inclined surface 38. The tip surface 36 includes the central axis Z, is circular when viewed from the central axis Z, and is positioned to protrude downward. The tip surface 36 is a plane perpendicular to the central axis Z. The outer edge 36E of the tip surface 36 is connected to the first inner inclined surface 38.

[0029] The first inner inclined surface 38 has an inner edge 38E connected to the outer edge 36E of the tip surface 36, and an outer edge 38L having a larger diameter than the inner edge 38E and positioned above the tip surface 36. The first inner inclined surface 38 is inclined outward from the inner edge 38E toward the outer edge 38L. The connection portion between the outer edge 36E of the tip surface 36 and the inner edge 38E of the first inner inclined surface 38 may have a cross-sectional radius (R).

[0030] The outer mold 35 has an intermediate mold 37 and an outer edge mold 39 positioned outside the intermediate mold 37. The intermediate mold 37 is positioned outside the inner mold 33 so as to surround the inner mold 33 and is recessed upward relative to the second mold 28. The intermediate mold 37 is annular when viewed from the central axis Z. The intermediate mold 37 has a first outer inclined surface 40. The first outer inclined surface 40 has an inner edge 40E connected to the outer edge 38L of the first inner inclined surface 38, and an outer edge 40L having a larger diameter than the inner edge 40E and positioned below the inner edge 40E. The first outer inclined surface 40 is a curved surface that slopes downward from the inner edge 40E to the outer edge 40L and is gently recessed upward. The connection portion between the outer edge 38L of the first inner inclined surface 38 and the inner edge 40E of the first outer inclined surface 40 may have a cross-sectional radius (R).

[0031] The outer edge mold 39 is positioned outside the intermediate mold 37 so as to surround the intermediate mold 37 and protrudes downward relative to the second mold 28. The outer edge mold 39 is annular when viewed from the central axis Z. The outer edge mold 39 has a first wall surface 42 and a pressing surface 44. The first wall surface 42 has an upper edge 42E and a lower edge 42L that is positioned downward from the upper edge 42E parallel to the central axis Z. The outer edge 40L of the first outer inclined surface 40 is in contact with the first wall surface 42 between the upper edge 42E and the lower edge 42L of the first wall surface 42. The first wall surface 42 has the shape of the inner surface of a cylinder.

[0032] The pressing surface 44 is positioned outside the first wall surface 42 and protruding downward, so as to surround the first wall surface 42. The pressing surface 44 has an inner edge 44E connected to the lower edge 42L of the first wall surface 42, and an outer edge 44L positioned outward from the inner edge 44E perpendicular to the central axis Z. The pressing surface 44 is an annular shape centered on the central axis Z and is a plane perpendicular to the central axis Z.

[0033] As described above, the tip surface 36 and the first inner inclined surface 38 form an inner protrusion 32 that protrudes relative to the second mold 28. The first inner inclined surface 38 and the first outer inclined surface 40 form an outer peripheral recess 34 that is recessed relative to the second mold 28. The first mold 26 forms the bottom surface 18S and the inclined cylindrical surface 20S of the hub mounting portion 12 by the tip surface 36 and the first inner inclined surface 38, the inclined surface 14S of the annular inclined portion 14 by the first outer inclined surface 40, and the cylindrical surface 16S of the peripheral fixing portion 16 by the first wall surface 42.

[0034] The inner type 33 is fixed to the inner slide 25C, the intermediate type 37 to the intermediate slide 25M, and the outer edge type 39 to the outer edge slide 25E. The inner type 33, intermediate type 37, and outer edge type 39 can be raised and lowered independently by their respective slides.

[0035] (Second mold) The second mold 28 is equipped with a refrigerant flow path (not shown). The second mold 28 is sized to match the size of the wheel disc 10. The second mold 28 has a second molding surface 28S on the side facing the first mold 26 across the blank 30, which corresponds to the first molding surface 26S. The second molding surface 28S is in contact with the second surface 30R of the blank 30.

[0036] The second molding surface 28S is provided with an inner recess 46 and an outer peripheral protrusion 48 (Figure 6). The inner recess 46 is located in the center of the second molding surface 28S and is recessed downward relative to the first mold 26. The inner recess 46 is provided complementary to the inner protrusion 32 of the first mold 26. The outer peripheral protrusion 48 is positioned outside the inner recess 46 so as to surround it and protrudes upward relative to the first mold 26. The outer peripheral protrusion 48 is provided concentrically with the inner recess 46 and is annular when viewed from the central axis Z. The outer peripheral protrusion 48 is provided complementary to the outer peripheral recess 34 of the first mold 26.

[0037] The second molding surface 28S includes a central axis Z and has a concave bottom surface 50 that is circular when viewed from the central axis Z. The concave bottom surface 50 is a plane perpendicular to the central axis Z. The outer edge 50E of the concave bottom surface 50 is connected to the second inner inclined surface 52. The second inner inclined surface 52 has an inner edge 52E connected to the outer edge 50E of the concave bottom surface 50, and an outer edge 52L having a larger diameter than the inner edge 52E and positioned above the concave bottom surface 50. The second inner inclined surface 52 is inclined outward from the inner edge 52E toward the outer edge 52L. The connection portion between the outer edge 50E of the concave bottom surface 50 and the inner edge 52E of the second inner inclined surface 52 may have a cross-sectional radius (R).

[0038] The outer edge 52L of the second inner inclined surface 52 is connected to the second outer inclined surface 54. The second outer inclined surface 54 has an inner edge 54E connected to the outer edge 52L of the second inner inclined surface 52, and an outer edge 54L having a larger diameter than the inner edge 54E and positioned below the inner edge 54E. The second outer inclined surface 54 is a curved surface that slopes downward from the inner edge 54E toward the outer edge 54L and gently protrudes upward. The connection portion between the outer edge 52L of the second inner inclined surface 52 and the inner edge 54E of the second outer inclined surface 54 may have a cross-sectional radius (R).

[0039] The second outer inclined surface 54 is connected to the second wall surface 56. The second wall surface 56 has an upper edge 56E connected to the outer edge 54L of the second outer inclined surface 54, and a lower edge 56L (Figure 6) positioned downward from the upper edge 56E parallel to the central axis Z. The second wall surface 56 is cylindrical in shape.

[0040] (Relationship between mold and wheel disc) The front surface 36 of the first mold 26 and the concave bottom surface 50 of the second mold 28 form the bottom portion 18 of the wheel disc 10. The first inner inclined surface 38 of the first mold 26 and the second inner inclined surface 52 of the second mold 28 form the inclined cylindrical portion 20 of the wheel disc 10. The first outer inclined surface 40 of the first mold 26 and the second outer inclined surface 54 of the second mold 28 form the annular inclined portion 14 of the wheel disc 10. The first wall surface 42 of the first mold 26 and the second wall surface 56 of the second mold 28 form the peripheral fixing portion 16 of the wheel disc 10. The pressing surface 44 of the first mold 26 contributes to the formation of the peripheral fixing portion 16.

[0041] [Blank] The blank 30 is, for example, a steel plate with a thickness of 1 mm or more and 5 mm or less. After hot press forming, the first surface 30F of the blank 30 becomes the outer surface 10A, and the second surface 30R becomes the inner surface 10B. The blank 30 preferably has a tensile strength of 1000 MPa or more after quenching. The blank 30 has a circular shape when viewed in the thickness direction. The blank 30 has a plurality of through-holes 22P. The through-holes 22P shown in FIG. 5 are in the shape of a sector of an annulus and are evenly arranged in four positions in the circumferential direction of the blank 30. The through-holes 22P become the through-holes 22 in the wheel disk 10 after hot press forming. In the blank 30, a circular range centered on the central axis Z from the center in the radial direction to the inner edge 22E of the through-hole 22P is defined as the central portion 30C, an annular range centered on the central axis Z from the inner edge 22E to the outer edge 22L of the through-hole 22P is defined as the outer peripheral portion 30M, and an annular range centered on the central axis Z from the outer edge 22L of the through-hole 22P to the outer edge of the blank 30 is defined as the outer edge portion 30P.

[0042] The steel material used as this blank 30 contains, for example, in mass%, C: 0.1% or more and 0.8% or less, Si: 0.001% or more and 2.0% or less, Mn: 0.5% or more and 3.0% or less, P: 0.15% or less, and S: 0.01% or less as its chemical composition.

[0043] Further, the steel material preferably contains, in mass%, sol.Al: 0.001% or more and 1.0% or less, N: 0.01% or less, and B: 0.01% or less as its chemical composition, and the balance consists of Fe and impurities. Also, the chemical composition may contain one or more selected from the group consisting of Ti, Nb, V, Cr, Mo, Cu, and Ni in place of a part of Fe.

[0044] As other additive elements, Ti, Nb, V, Cr, Mo, Cu, and Ni may be added as necessary to improve the hardenability of the steel and ensure the strength after quenching stably.

[0045] [Manufacturing Method] Next, a manufacturing method of the wheel disk 10 by hot pressing will be described. The manufacturing method of the wheel disk 10 includes a heating step and a quenching step.

[0046] (Heating Process) In the heating process, the blank 30 is heated to a temperature above the Ac3 transformation point of the blank 30 to be austenitized.

[0047] The Ac3 transformation point indicating the austenitization transformation point temperature is the temperature at which the blank 30 made of the above-described steel material is completely austenitized, and is shown by the following formula as an example.

[0048] Ac3 (°C) = 910 - 203 × √C (mass%) + 44.7 × Si (mass%) - 30 × Mn (mass%) - 11 × Cr (mass%) + 700 × S (mass%) + 400 × Al (mass%) + 50 × Ti (mass%) Here, C represents carbon, Si represents silicon, Mn represents manganese, Cr represents chromium, S represents sulfur, Al represents aluminum, and Ti represents titanium.

[0049] (Quenching Process) The blank 30 heated above the Ac3 transformation point in the heating process is hot press formed using the above die 24 installed in the press of the hot press device. After hot press forming, the heat of the press formed product is rapidly removed and cooled by the first die 26 and the second die 28 while the dies are clamped, thereby martensite transforming the wheel disk 10. That is, the quenching process includes the steps of setting, drawing, and cooling.

[0050] - In the set heating process, the blank 30 heated above the Ac3 transformation point is set between the first die 26 and the second die 28 as shown in FIG. 6. At this time, the blank 30 is positioned in the die 24 by a positioning pin (not shown).

[0051] - Drawing The drawing has a first step, a second step, and a third step. The first step is completed before the completion of the second step. First, in the first step, as shown in FIG. 7, the inner die 33 is moved downward toward the second die 28. Then, the central portion 30C of the blank 30 is pushed downward by the inner die 33 of the first die 26, and at the same time, the outer peripheral portion 30M around it is pushed upward by the outer peripheral convex portion 48 of the second die 28, and the blank 30 is drawn. Thus, when drawing is started on the blank 30 at a temperature above the Ac3 transformation point, the blank 30 starts to deform.

[0052] As the deep drawing process begins, a shrinkage flange deformation occurs at the outer edge 30P of the blank 30, in which the blank 30 is compressed in the circumferential direction and pulled towards the central part 30C.

[0053] Next, in the second step, as shown in Figure 8, the intermediate mold 37 is moved downward toward the second mold 28. As a result, the outer circumference 30M of the blank 30 is pushed downward by the intermediate mold 37, causing the blank 30 to be drawn in the reverse direction (hereinafter sometimes referred to as "reverse drawing"). Note that before the second step is completed, that is, before the intermediate mold 37 reaches its bottom dead center, the first step by the inner mold 33 is completed, that is, the inner mold 33 reaches its bottom dead center.

[0054] Furthermore, in the third step, as shown in Figure 9, the outer edge mold 39 is moved downward toward the second mold 28. As a result, the outer edge portion 30P of the blank 30 is pushed downward by the outer edge mold 39, and the outer edge portion 30P is drawn between the first wall surface 42 and the second wall surface 56, becoming the peripheral fixing portion 16 of the wheel disc 10. Cooling When the first mold 26 reaches its bottom dead center, the molded blank 30 is in close contact with the first mold 26 and the second mold 28. This state is called mold clamping. In the mold-clamped state, the heat of the blank 30 is rapidly removed by the first mold 26 and the second mold 28. As a result, the blank 30 undergoes martensitic transformation. That is, the blank 30 is hardened, and the blank 30 becomes the wheel disc 10.

[0055] One method for removing heat from the blank 30 is to indirectly cool it using a refrigerant flowing inside the first mold 26 and the second mold 28. Another method for removing heat is to directly cool the blank 30 by injecting a refrigerant from the first mold 26 and the second mold 28.

[0056] The time from the heating process to the start of cooling shall be within 15 seconds. That is, the time from when the blank 30 heated in the heating process leaves the heating furnace until the first mold 26 reaches the bottom dead center shall be within 15 seconds. [Operation and Effects] In the first step of deep drawing, the deep drawing process is carried out by the inner mold 33 without holding the outer peripheral portion 30M and the outer edge portion 30P of the blank 30. In the initial stage of deep drawing, the blank 30 is smoothly pulled into the central portion 30C, thereby suppressing the reduction in plate thickness in the central portion 30C. Therefore, in this embodiment, fracture in the central portion 30C of the blank 30 can be suppressed.

[0057] Furthermore, in the second step, the blank 30 is not held in place by the outer edge 30P, and the reverse drawing process is carried out using the intermediate mold 37. The blank 30 is smoothly pulled in from the outer edge 30P toward the central axis Z, thereby suppressing the reduction in plate thickness at the outer circumference 30M. Therefore, in this embodiment, fracture at the outer circumference 30M of the blank 30 can be suppressed.

[0058] Finally, the hot press device 21 forms the peripheral fixing portion 16 of the wheel disc 10 by drawing the outer edge portion 30P of the blank 30 with the outer edge mold 39.

[0059] Thus, in this embodiment, by performing deep drawing in the order of inner mold 33, intermediate mold 37, and outer edge mold 39, the material is smoothly pulled from the outer edge 30E of the blank 30 toward the central axis Z. That is, when deep drawing is performed on the central portion 30C of the blank 30 by the inner mold 33, the material outside the central portion 30C can flow toward the central axis Z. As a result, the hot press device 21 can suppress the reduction in plate thickness of the bottom portion 18 and the inclined cylindrical portion 20 of the wheel disc 10.

[0060] Next, when the intermediate die 37 is used to draw the outer periphery 30M of the blank 30, the central portion 30C is held by the inner die 33, but the outer edge portion 30P is not held. That is, in conjunction with the drawing by the intermediate die 37, the material of the outer edge portion 30P can flow toward the central axis Z. As a result, the hot press device 21 can suppress the reduction in the thickness of the annular inclined portion 14 of the wheel disc 10.

[0061] In this embodiment, by performing unidirectional drawing and reverse drawing in the opposite direction in a single press operation, localized temperature drops in the blank 30 can be suppressed. Therefore, the strength of the wheel disc 10 after quenching can be ensured.

[0062] 2. Second Embodiment [Hot Pressing Apparatus] A mold according to the second embodiment of the present invention will be described with reference to Figure 10, which has the same configuration as in Figure 5 and is denoted by the same reference numerals. Note that the same configuration as in the first embodiment described above is denoted by the same reference numerals and its description is omitted.

[0063] The hot press apparatus 21A shown in Figure 10 comprises a press machine 23A and a die 24A attached to the press machine 23A. The press machine 23A has a slide 25A and a bolster 27. The slide 25A has an inner slide 25C and an outer slide 25P provided on the outside of the inner slide 25C. The outer slide 25P is annular when viewed from the central axis Z.

[0064] [Mold] Mold 24A comprises a first mold 26A and a second mold 28A. The first mold 26A comprises an inner mold 33A and an outer mold 35A. The inner mold 33A is substantially columnar, located in the center of the first mold 26A, and recessed upward relative to the second mold 28A. The inner mold 33A has a concave bottom surface 50A and a first inner inclined surface 38A. The concave bottom surface 50A includes the central axis Z and is circular when viewed from the central axis Z. The outer edge 50EA of the concave bottom surface 50A is connected to the first inner inclined surface 38A.

[0065] The first inner inclined surface 38A has an inner edge 38EA connected to the outer edge 50EA of the concave bottom surface 50A, and an outer edge 38LA having a larger diameter than the inner edge 38EA and positioned below the concave bottom surface 50A. The first inner inclined surface 38A is inclined outward from the inner edge 38EA toward the outer edge 38LA. The connection portion between the outer edge 50EA of the concave bottom surface 50A and the inner edge 38EA of the first inner inclined surface 38A may have a cross-sectional radius (R).

[0066] The outer mold 35A is positioned outside the inner mold 33A so as to surround it, and protrudes downward relative to the second mold 28A. The outer mold 35A is annular when viewed from the central axis Z. The outer mold 35A has a first outer inclined surface 40A. The first outer inclined surface 40A has an inner edge 40EA connected to the outer edge 38LA of the first inner inclined surface 38A, and an outer edge 40LA having a larger diameter than the inner edge 40EA and positioned above the inner edge 40EA. The first outer inclined surface 40A is a curved surface that slopes upward from the inner edge 40EA toward the outer edge 40LA and protrudes gently downward. The connection portion between the outer edge 38LA of the first inner inclined surface 38A and the inner edge 40EA of the first outer inclined surface 40A may have a cross-sectional radius (R).

[0067] The first outer inclined surface 40A is connected to the first wall surface 42A. The first wall surface 42A has a lower edge 42LA connected to the outer edge 40LA of the first outer inclined surface 40A, and an upper edge 42EA positioned upward from the lower edge 42LA parallel to the central axis Z. The first wall surface 42A is cylindrical in shape.

[0068] The inner type 33A is fixed to the inner slide 25C, and the outer type 35A is fixed to the outer slide 25P. The inner type 33A and the outer type 35A can be raised and lowered independently by their respective slides.

[0069] The second mold 28A has an inner protrusion 32A and an outer peripheral recess 34A on its second molding surface 28SA. The inner protrusion 32A is columnar, located in the center of the second molding surface 28SA, and protrudes upward relative to the first mold 26A. The outer peripheral recess 34A is positioned outside the inner protrusion 32A so as to surround it, and is recessed downward relative to the first mold 26A. The outer peripheral recess 34A is provided concentrically with the inner protrusion 32A and is annular when viewed from the central axis Z.

[0070] The second molding surface 28SA includes the central axis Z and has a tip surface 36A that is circular when viewed from the central axis Z. The outer edge 36EA of the tip surface 36A is connected to the second inner inclined surface 52A. The second inner inclined surface 52A has an inner edge 52EA connected to the outer edge 36EA of the tip surface 36A, and an outer edge 52LA having a larger diameter than the inner edge 52EA and located below the tip surface 36A. The second inner inclined surface 52A is inclined outward from the inner edge 52EA toward the outer edge 52LA. The connection portion between the outer edge 36EA of the tip surface 36A and the inner edge 52EA of the second inner inclined surface 52A may have a cross-sectional radius (R).

[0071] The outer edge 52LA of the second inner inclined surface 52A is connected to the second outer inclined surface 54A. The second outer inclined surface 54A has an inner edge 54EA connected to the outer edge 52LA of the second inner inclined surface 52A, and an outer edge 54LA having a larger diameter than the inner edge 54EA and positioned above the inner edge 54EA. The second outer inclined surface 54A is a curved surface that slopes upward from the inner edge 54EA toward the outer edge 54LA and is gently concave downward. The connection portion between the outer edge 52LA of the second inner inclined surface 52A and the inner edge 54EA of the second outer inclined surface 54A may have a cross-sectional radius (R).

[0072] The second outer inclined surface 54A is connected to the second wall surface 56A. The second wall surface 56A has a lower edge 56LA connected to the outer edge 54LA of the second outer inclined surface 54, and an upper edge 56EA positioned upward from the lower edge 56LA parallel to the central axis Z. The second wall surface 56A has the shape of the inner surface of a cylinder.

[0073] The pressing surface 44A is positioned outside the second wall surface 56A so as to surround the second wall surface 56A. The pressing surface 44A has an inner edge 44EA connected to the upper edge 56EA of the second wall surface 56A, and an outer edge 44LA positioned outward from the inner edge 44EA perpendicular to the central axis Z. The pressing surface 44A is an annular shape centered on the central axis Z and is a plane perpendicular to the central axis Z.

[0074] (Relationship between mold and wheel disc) The concave bottom surface 50A of the first mold 26A and the front end surface 36A of the second mold 28A form the bottom portion 18 of the wheel disc 10. The first inner inclined surface 38A of the first mold 26A and the second inner inclined surface 52A of the second mold 28 form the inclined cylindrical portion 20 of the wheel disc 10. The first outer inclined surface 40A of the first mold 26A and the second outer inclined surface 54A of the second mold 28A form the annular inclined portion 14 of the wheel disc 10. The first wall surface 42A of the first mold 26A and the second wall surface 56A of the second mold 28A form the peripheral fixing portion 16 of the wheel disc 10. The pressing surface 44A of the first mold 26A contributes to the formation of the peripheral fixing portion 16.

[0075] (Operation and Effects) In the second embodiment, drawing is performed on the blank 30 in the order of the central portion 30C, the outer peripheral portion 30M, and the outer edge portion 30P. That is, the hot press device 21A performs drawing on the blank 30 in the order of the inner mold 33A and the outer mold 35A. As a result, the hot press device and the method for manufacturing a wheel disc according to the second embodiment can obtain the same effects as the first embodiment.

[0076] 3. Modifications The present invention is not limited to the embodiments described above and can be modified as appropriate within the scope of the spirit of the invention. The examples described below can be implemented individually or in combination with the above embodiments as appropriate.

[0077] In the above embodiment, the wheel disc was described as having through holes in the annular inclined portion that are in the shape of a circular fan. However, the present invention is not limited to this, and the through holes may be circular or square. Furthermore, although the above embodiment described as having four through holes formed in the annular inclined portion, the present invention is not limited to this, and may have three or fewer, or five or more. Moreover, the wheel disc does not need to have through holes in the annular inclined portion.

[0078] In the above embodiment, the first mold and the second mold were described as being circular when viewed from the central axis Z, but the present invention is not limited to this, and the first mold and the second mold may be rectangular when viewed from the central axis.

[0079] In the above embodiment, the outer mold was described as having an intermediate mold and an outer edge mold, but the present invention is not limited to this. For example, the outer mold may be one or three or more.

[0080] In the above embodiment, the case where the hot-press-formed product is a disc for an automobile wheel was described, but the present invention is not limited to this, and may be, for example, a part formed by deep drawing and having large irregularities in a direction parallel to the central axis.

[0081] Furthermore, for example, the hot-pressed product may be the cover 68 of the torque converter 66, as shown in Figure 11. The cover 68 of the torque converter 66 shown in Figure 11 is filled with fluid inside. The cover 68 rotates in response to engine input from the front. The cover 68 has a front cover 70 positioned at the front and a rear cover 72 connected to the rear end of the front cover 70. Regarding the front cover 70, although the surface shape is not shown in Figure 11, both the front cover 70 and the rear cover 72 are substantially disc-shaped with a central axis Z, and are provided with a recess 74, an annular inclined portion 76, and a vertical wall portion 78 in order from the central axis Z outward. The cover 68 of the torque converter 66 according to this modified example can be manufactured using the same manufacturing method, mold, and manufacturing apparatus as in the above embodiment, and the same effects as in the above embodiment can be obtained.

[0082] The above embodiments describe the case of manufacturing a substantially disc-shaped hot press-formed product, but the present invention is not limited thereto. That is, the present invention is suitable for deep drawing, and the external shape in plan view is not particularly limited; for example, the external shape in plan view may be elliptical, rectangular, polygonal, etc. Similarly, the blank is not limited to a circular shape when viewed from the thickness direction, but may be elliptical, rectangular, polygonal, etc., to match the hot press-formed product.

[0083] In the above embodiment, the hot-pressed product was described as having a recess and an annular inclined portion formed to surround the recess in a plan view, but the present invention is not limited to this. For example, the present invention may be applied to a hot-pressed product having one or more recesses or one or more protrusions.

[0084] Furthermore, the present invention may be applied to a hot press-formed product having a convex portion and an annular recess formed to surround the convex portion. In this case, the first mold is provided with a recessed inner recess and a protruding outer peripheral convex portion relative to the second mold, and the outer peripheral convex portion may be provided to surround the inner recess. Alternatively, the second mold is provided with a protruding inner convex portion and a recessed outer peripheral recess relative to the first mold, and the inner convex portion may be provided at a position opposite to the inner recess, while the outer peripheral recess may be provided at a position opposite to the outer peripheral convex portion and to surround the inner convex portion.

[0085] 1 Wheel 2 Rim 3 Drop section 4 Bead seat section 5 Flange section 6 Space 10 Wheel disc (hot press molded product) 10A Outer surface 10B Inner surface 12 Hub mounting section (recess) 14 Annular inclined section 14E Inner edge 14L Outer edge 14S Inclined surface 16 Peripheral fixing section (vertical wall section) 16E Upper edge 16L Lower edge 16S Cylindrical surface 18 Bottom section 18E Outer edge 18S Bottom surface 20 Inclined cylindrical section 20E Lower edge 20L Upper edge 20S Inclined cylindrical surface 21, 21A Hot press device 22 Through hole (wheel disc) 22P Through hole (blank) 22E Inner edge 22L Outer edge 23 Press machine 24, 24A Mold 25 Slide 25S Surface 25C Inner slide 25M Intermediate slide 25E Outer edge slide 26, 26A First mold 26S First molding surface 26FS First mounting surface 27 Bolster 27S Surface 28, 28A Second mold 28S, 28SA Second molding surface 30 Blank 30F First surface 30R Second surface 30C Center 30M Outer periphery 30P Outer edge 30E Outer edge 30PH Holding area 30PN Non-holding area 32, 32A Inner convex part 33, 33A Inner mold 34, 34A Outer recess 35, 35A Outer mold 36, 36A Tip surface 36E, 36EA Outer edge 37 Intermediate mold 38, 38A First inner inclined surface 38E, 38EA Inner edge 38L, 38LA Outer edge 39 Outer edge type 40, 40A First outer inclined surface 40E, 40EA Inner edge 40L, 40LA Outer edge 42, 42A First wall surface 42E, 42EA Upper edge 42L, 42LA Lower edge 44, 44A Pressing surface 44E, 44EA Inner edge 44L, 44LA Outer edge 46 Inner recess 48 Outer peripheral protrusion 50, 50A Recessed bottom surface 50E, 50EA Outer edge 52, 52A Second inner inclined surface 52E, 52EA Inner edge 52L, 52LA Outer edge 54, 54A Second outer inclined surface 54E, 54EA Inner edge 54L, 54LA Outer edge 56, 56A Second wall surface 56E, 56EA Upper edge 56L, 56LA Lower edge 66 Torque converter 68 Cover 70 Front cover 72 Rear cover 74 Recess 76 Annular inclined section 78 Vertical wall section H1 Height (hub mounting section) H2 Height (Annular inclined section)H3: Length in the central axis direction from the first holder surface to the second holder surface. Z: Central axis.

Claims

1. A die for performing deep drawing by hot pressing on a heated blank, wherein the die comprises, in hot press forming, a first die that contacts a first surface of the blank, and a second die positioned opposite to the first die and contacting a second surface of the blank opposite to the first surface, the first die having an inner die and an outer die positioned outside the inner die, the inner die protruding relative to the second die, the outer die recessed relative to the second die and provided to surround the inner die, the second die having a recessed inner recess and a protruding outer circumferential protrusion relative to the first die, the inner recess being positioned opposite the inner die and the outer circumferential protrusion being positioned opposite the outer die and provided to surround the inner recess.

2. The mold for hot press forming according to claim 1, wherein the inner mold is columnar and the outer mold is annular.

3. The mold for hot press forming according to claim 1 or 2, wherein the outer mold comprises an intermediate mold and an outer edge mold positioned outside the intermediate mold, and drawing is performed on the blank in the order of the intermediate mold and the outer edge mold.

4. A mold for hot press molding according to claim 1 or 2, wherein a wheel disc is formed by the hot press.

5. A hot press apparatus comprising a die for hot press forming according to claim 1 or 2, and a press machine to which the die is attached, wherein deep drawing is performed on the blank in the order of the inner die and the outer die.

6. A method for manufacturing a hot press-formed product, comprising: a first step of performing deep drawing on the central part of a heated blank; and a second step of performing deep drawing on the outer peripheral part surrounding the outside of the central part, wherein the first step is completed before the second step.

7. The method for manufacturing a hot press-formed product according to claim 6, wherein the first step is a drawing process in which the central portion is made to protrude in one direction, and the second step is a reverse drawing process in which the outer peripheral portion is made to protrude in the opposite direction to the one direction.

8. The method for manufacturing a hot press-formed product according to claim 6 or 7, wherein the hot press-formed product is a wheel disc.

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