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

Abstract

Provided are a die for hot press molding and a method for manufacturing a hot press molded article, by which it is possible to more reliably suppress the generation of wrinkles if press forming by hot pressing. A die 24 is for carrying out press forming of a heated blank 30 by hot pressing, and comprises: a first die 26 having a first forming surface 26S and an outer circumferential surface 44 disposed outside the first forming surface 26S; and a second die 28 disposed facing the first die 26 and having a second forming surface 28S corresponding to the first forming surface 26. A guide section 45 is provided at the outer circumferential surface 44. The guide section 45 protrudes with respect to the second die 28 and has an apex 47 extending in the direction from the outer edge of the first die 26 toward the inside of the first die 26. The guide section 45 has a pair of guide surfaces 45S inclined from the apex 47 toward the sides of the outer circumferential surface 44 intersecting the extension direction of the apex 47.

Description

Die for hot press forming and method for manufacturing hot press formed product

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

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

[0005] An object of the present invention is to provide a die for hot press forming and a method for manufacturing a hot press formed product that can more reliably suppress the generation of wrinkles 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 mold for performing deep drawing by hot pressing on a heated blank, comprising: a first mold having a first molding surface and an outer peripheral surface disposed outside the first molding surface; and a second mold disposed opposite to the first mold and having a second molding surface corresponding to the first molding surface, wherein a guide portion is provided on the outer peripheral surface, the guide portion protrudes from the second mold and has a top portion extending in a direction toward the inside of the first mold from the outer edge of the first mold, and the guide portion has a pair of guide surfaces inclined from the top portion toward the outer peripheral surface on the side intersecting the extending direction of the top portion, a mold for hot press forming. [Aspect 2] The mold for hot press forming according to aspect 1, wherein the top portion is inclined toward the side away from the second mold toward the inside of the first mold from the outer edge of the first mold. [Aspect 3] The first molding surface of the first mold is provided with an inner projection and a recessed outer peripheral recess relative to the second mold, the outer peripheral recess being provided so as to surround the inner projection, and the second molding surface of the second mold is provided with an inner recess and a projection relative to the first mold, the inner recess being provided at a position opposite to the inner projection, and the outer peripheral projection being provided at a position opposite to the outer peripheral recess and surrounding the inner recess, as described in Aspect 1 or 2. [Aspect 4] The inner projection is columnar, and the outer peripheral recess and the outer peripheral projection are annular, as described in Aspect 3. [Aspect 5] The first mold is provided with a first bead portion at the connection portion between the pair of guide surfaces and the outer peripheral surface, and the second mold is provided with a second bead portion that forms a bead on the blank between itself and the first bead portion, as described in any one of Aspects 1 to 3. [Aspect 6] A mold for hot press molding according to any one of aspects 1 to 5, for forming a wheel disc by the hot press.[Aspect 7] A method for manufacturing a hot press-formed product, comprising: a first step of starting deep drawing press forming on a heated blank; and a second step of proceeding with deep drawing press forming after the blank has started to deform, while restricting the circumferential movement of the blank with respect to at least a part of the outer edge, including the outer edge of the blank. [Aspect 8] The method for manufacturing a hot press-formed product according to aspect 7, wherein the deep drawing press forming includes deep drawing to cause the central part of the blank, which is inside the outer edge, to protrude in one direction relative to the heated blank, and reverse deep drawing to cause the outer circumference between the outer edge and the central part to protrude in the opposite direction to the one direction. [Aspect 9] The method for manufacturing a hot press-formed product according to aspect 7 or 8, wherein the hot press-formed product is a wheel disc.

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

[0009] This is a front view of a wheel equipped with a wheel disc according to this embodiment. This is a longitudinal cross-sectional view of a wheel equipped with a wheel disc according to this embodiment. This is a schematic perspective view showing the wheel disc according to this embodiment. This is a schematic central longitudinal cross-sectional view showing the wheel disc according to this embodiment. This is a schematic perspective view showing a die for forming the wheel disc according to this embodiment. This is a schematic cross-sectional view showing a blank set in the die according to this embodiment. This is a schematic cross-sectional view showing the state in which the blank is being drawn by the die according to this embodiment, and the blank has started to deform. This is a schematic view of the outer edge of the blank seen from the outside of the die in the state shown in Figure 5. This is a schematic cross-sectional view showing the die according to this embodiment in operation to the bottom dead center. This is a schematic perspective view showing a die for forming a wheel disc according to a modified example. This is a perspective view showing a hot press-formed product according to another modified example.

[0010] Embodiments of the present invention relate to the following aspects.

[0011] [Wheel] Referring to Figures 1 and 2, a two-piece type automobile wheel 1 comprising a wheel disc 10 as a hot-pressed product will be described. 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 around the central axis Z will be the circumferential direction. "Orthogonal" and "parallel" include the cases of "approximately orthogonal" and "approximately parallel," respectively. Also, angles will be considered inferior angles.

[0015] Figure 3 is a schematic perspective view of the wheel disc 10 according to this embodiment, as seen from the outer surface.

[0016] The wheel disc 10 is roughly disc-shaped. In Figure 3, the outer surface shown is the outer surface 10A, and the opposite back surface is the inner surface 10B.

[0017] 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 18A (Figure 5) perpendicular to the central axis Z, and a bottom surface 18S surrounded by the outer edge 18A.

[0018] The inclined cylindrical portion 20 is shaped like an inverted truncated cone, having a circular lower edge 20A as viewed from the central axis Z, an upper edge 20B having a larger diameter than the lower edge 20A and positioned above the lower edge 20A, and an inclined cylindrical surface 20S connecting the lower edge 20A and the upper edge 20B. The lower edge 20A is connected to the outer edge 18A of the base portion 18. The inclined cylindrical portion 20 is inclined outward from the lower edge 20A toward the upper edge 20B. The connection portion between the outer edge 18A of the base portion 18 and the lower edge 20A of the inclined cylindrical portion 20 may have a cross-sectional radius (R). The diameter of the lower edge 20A of the inclined cylindrical portion 20 is, for example, 40 mm or more and 200 mm or less. The diameter of the upper edge 20B of the inclined cylindrical portion 20 is, for example, 60 mm or more and 250 mm or less.

[0019] 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 14A, an outer edge 14B positioned outside and below the inner edge 14A, and an inclined surface 14S connecting the inner edge 14A and the outer edge 14B.

[0020] The inner edge 14A of the annular inclined portion 14 is connected to the upper edge 20B of the inclined cylindrical portion 20. The outer edge 14B 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 14A to the outer edge 14B and is convex upward. The outer edge 14B of the annular inclined portion 14 is, for example, 250 mm or more and 500 mm or less.

[0021] 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.

[0022] 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 16A when viewed from the central axis Z, a lower edge 16B located below the upper edge 16A and having the same diameter as the upper edge 16A, and a cylindrical surface 16S connecting the upper edge 16A and the lower edge 16B. The upper edge 16A of the peripheral fixing portion 16 is connected to the outer edge 14B of the annular inclined portion 14. The lower edge 16B of the peripheral fixing portion 16 is open.

[0023] The height H1 of the hub mounting portion 12, that is, the vertical distance between the bottom surface 18S and the upper edge 20B 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 14A and the outer edge 14B of the annular inclined portion 14, is, for example, 5 mm or more and 70 mm or less.

[0024] [Mold] The mold 24 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 installed in the press machine of a hot press device, although it is not shown in the figure. 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.

[0025] The mold 24 shown in Figure 5 comprises a first mold 26 and a second mold 28. Figures 5 and 6 show a blank 30 positioned between the upper mold, which is the first mold 26, and the lower mold, which is the second mold 28. In this specification, the central axis Z of the mold 24, 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. The first mold 26 and the second mold 28 are circular when viewed with respect to the central axis Z.

[0026] (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 and an outer peripheral surface 44 located outside the first molding surface 26S on the side facing the second mold 28 across the blank 30. The first molding surface 26S is in contact with the first surface 30F of the blank 30.

[0027] The first molding surface 26S is provided with an inner protrusion 32 and an outer peripheral recess 34. The inner protrusion 32 is columnar, located in the center of the first molding surface 26S, and protrudes downward relative to the second mold 28. The outer peripheral recess 34 is positioned outside the inner protrusion 32 so as to surround it, and is recessed upward relative to the second mold 28. The outer peripheral recess 34 is provided concentrically with the inner protrusion 32 and is annular when viewed from the central axis Z.

[0028] The first molding surface 26S includes a central axis Z and has a tip surface 36 that is circular when viewed from the central axis Z. The outer edge 36A of the tip surface 36 is connected to a first inner inclined surface 38. The first inner inclined surface 38 has a lower edge 38A connected to the outer edge 36A of the tip surface 36, and an upper edge 38B having a larger diameter than the lower edge 38A and positioned above the tip surface 36. The first inner inclined surface 38 is inclined outward from the lower edge 38A toward the upper edge 38B. The connection portion between the outer edge 36A of the tip surface 36 and the lower edge 38A of the first inner inclined surface 38 may have a cross-sectional radius (R).

[0029] The upper edge 38B of the first inner inclined surface 38 is connected to the first outer inclined surface 40. The first outer inclined surface 40 has an inner edge 40A connected to the upper edge 38B of the first inner inclined surface 38, and an outer edge 40B having a larger diameter than the inner edge 40A and positioned below the inner edge 40A. The first outer inclined surface 40 is a curved surface that slopes downward from the inner edge 40A to the outer edge 40B and is gently concave upward. The connection portion between the upper edge 38B of the first inner inclined surface 38 and the inner edge 40A of the first outer inclined surface 40 may have a cross-sectional radius (R).

[0030] The first outer inclined surface 40 is connected to the first wall surface 42. The first wall surface 42 has an upper edge 42A connected to the outer edge 40B of the first outer inclined surface 40, and a lower edge 42B positioned downward from the upper edge 42A parallel to the central axis Z. The first wall surface 42 has the shape of the inner surface of a cylinder.

[0031] The outer circumferential surface 44 is positioned outside the first wall surface 42 so as to surround the first wall surface 42. The outer circumferential surface 44 has an inner edge 44A connected to the lower edge 42B of the first wall surface 42, and an outer edge 44B positioned outward from the inner edge 44A perpendicular to the central axis Z. The outer circumferential surface 44 is annular with respect to the central axis Z and has a surface perpendicular to the central axis Z.

[0032] A guide portion 45 is provided on the outer circumferential surface 44. The guide portion 45 has an inverted V-shape when viewed from the outside of the first mold 26. The guide portion 45 has a apex portion 47 that protrudes relative to the second mold 28. The apex portion 47 extends in a direction toward the inside of the first mold 26 from the outer edge of the first mold 26. The apex portion 47 may be inclined toward the side away from the second mold 28 toward the inside of the first mold 26 from the outer edge of the first mold 26 (Figure 6). In the cross-section shown in Figure 6, the angle α between the extending direction of the apex portion 47 and the direction perpendicular to the central axis Z is preferably 0° or more and 15° or less, and more preferably 5° or more and 10° or less. The apex portion 47 is curved and protrudes relative to the second mold 28 when viewed from the outside of the first mold 26.

[0033] The guide portion 45 has a pair of guide surfaces 45S. The guide surfaces 45S are provided on both sides in the circumferential direction, flanking the top portion 47. Each guide surface 45S is inclined upward from the top portion 47 toward the outer peripheral surfaces 44 on both sides intersecting the direction of extension of the top portion 47, i.e., on both sides in the circumferential direction relative to the top portion 47. When viewing the guide portion 45 from the outside in a direction perpendicular to the central axis Z, the angle β between the extension of the tangent line touching the guide surface 45 and the outer peripheral surface 44 is preferably 10° or more and 60° or less, and more preferably 10° or more and 45° or less.

[0034] As described above, the inner protrusion 32 is formed to protrude from the second mold 28 by the tip surface 36 and the first inner inclined surface 38. The outer peripheral recess 34 is formed to be recessed from the second mold 28 by the first inner inclined surface 38 and the first outer inclined surface 40. 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.

[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. 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 outer edge 50A of the concave bottom surface 50 is connected to the second inner inclined surface 52. The second inner inclined surface 52 has a lower edge 52A connected to the outer edge 50A of the concave bottom surface 50, and an upper edge 52B having a larger diameter than the lower edge 52A and positioned above the concave bottom surface 50. The second inner inclined surface 52 is inclined outward from the lower edge 52A toward the upper edge 52B. The connection portion between the outer edge 50A of the concave bottom surface 50 and the lower edge 52A of the second inner inclined surface 52 may have a cross-sectional radius (R).

[0038] The upper edge 52B 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 54A connected to the upper edge 52B of the second inner inclined surface 52, and an outer edge 54B having a larger diameter than the inner edge 54A and positioned below the inner edge 54A. The second outer inclined surface 54 is a curved surface that slopes downward from the inner edge 54A to the outer edge 54B and gently protrudes upward. The connection portion between the upper edge 52B of the second inner inclined surface 52 and the inner edge 54A 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 56A connected to the outer edge 54B of the second outer inclined surface 54, and a lower edge 56B positioned downward from the upper edge 56A 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 guide surface 45S of the first mold 26 appropriately relieves the circumferential compressive force of the outer edge portion 30P of the blank 30 during hot press forming, thereby more reliably suppressing the occurrence of wrinkles in the peripheral fixing portion 16 and the annular inclined portion 14 of the wheel disc 10.

[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 from 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 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 22A of the through hole 22P is the central portion 30C, an annular range centered on the central axis Z from the inner edge 22A to the outer edge 22B of the through hole 22P is the outer peripheral portion 30M, and an annular range centered on the central axis Z from the outer edge 22B of the through hole 22P to the outer edge of the blank 30 is referred to 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, 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, B: 0.01% or less as its chemical composition, and the balance consists of Fe and impurities. Further, the chemical composition may contain one or more selected from the group consisting of Ti, Nb, V, Cr, Mo, Cu, Ni in place of a part of Fe.

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

[0045] [Manufacturing Method] Next, a method for manufacturing 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 steel material described above is completely austenitized, and is expressed 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-mentioned mold 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 mold 26 and the second mold 28 while the mold is clamped, so that the wheel disk 10 is martensite transformed. That is, the quenching process includes the processes 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 mold 26 and the second mold 28 as shown in FIG. 6. At this time, the blank 30 is positioned on the mold 24 by a positioning pin (not shown).

[0051] - Drawing The drawing has a first step and a second step. First, in the first step, as shown in FIG. 7, the first mold 26 is moved downward toward the second mold 28. Then, the central portion 30C of the blank 30 is pushed downward by the inner convex portion 32 of the first mold and drawn, 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 mold 28, and the blank 30 is drawn in the reverse direction (hereinafter, may be referred to as "reverse drawing"). 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 portion 30C. Specifically, as shown in Figure 8, the outer edge 30P of the blank 30 can deform into a wavy shape that oscillates in the vertical direction. The wavy deformed outer edge 30P moves inward toward the central axis Z while a portion of it contacts the guide surface 45S.

[0053] Then, in the second step, as shown in Figure 9, the first mold 26 is moved downward to its bottom dead center, and the drawing of the blank 30 is completed while it is sandwiched between the first mold 26 and the second mold 28. Between the first and second steps, the outer edge 30P of the blank 30 continues to move inward along the outer peripheral surface 44 and the guide surface 45S toward the central axis Z, and 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.

[0054] 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 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 thus becomes a 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 its bottom dead center shall be within 15 seconds.

[0057] [Operation and Effects] The operation and effects of the mold 24 for hot press forming and the method for manufacturing the wheel disc 10 according to the above embodiment will be explained.

[0058] In the first step of deep drawing, the blank 30 is subjected to deep drawing. At this time, the outer edge 30P of the blank 30 is not held. Therefore, in the initial stage of deep drawing, the blank 30 is smoothly pulled into the central part 30C, thereby suppressing the reduction in plate thickness in the central part 30C.

[0059] As the drawing process progresses and shrinkage flange deformation occurs at the outer edge 30P of the blank 30, a portion of the corrugated outer edge 30P comes into contact with the guide surface 45S. In this way, a portion of the outer edge 30P moves inward toward the central axis Z along the guide surface 45S, thereby partially relieving the circumferential compressive force on the outer edge 30. This restricts the circumferential movement of the outer edge 30P and prevents the outer edge 30P from overlapping and increasing the plate thickness.

[0060] The guide surface 45S is inclined from the top portion 47 toward the outer peripheral surface 44 on the side intersecting the extending direction of the top portion 47, making it easy for the corrugated outer edge portion 30P to come into surface contact with it. As a result, the outer edge portion 30P moves easily along the guide surface 45S, so the circumferential compressive force on the outer edge portion 30P can be more reliably relieved. In this way, in this embodiment, the occurrence of wrinkles in the peripheral fixing portion 16 can be more reliably suppressed.

[0061] The top portion 47 is inclined toward the inside of the first mold 26 and away from the second mold 28 from the outer edge portion 30P of the first mold 26, thereby enabling more reliable surface contact with the outer edge portion 30P, which has deformed into a wavy shape as the deep drawing process progresses.

[0062] 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.

[0063] [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.

[0064] For example, the mold according to this embodiment may include a bead portion for forming a bead on the wheel disc. A modified mold will be described with reference to Figure 10, which uses the same reference numerals for a configuration similar to that in Figure 4. Note that for a configuration similar to that of the above embodiment, the same reference numerals are used and detailed explanations are omitted. The mold 24A shown in Figure 10 includes a first mold 26A and a second mold 28A.

[0065] The first mold 26A has a first molding surface 26SA and an outer peripheral surface 44SA located outside the first molding surface 26SA on the side facing the second mold 28A. The first molding surface 26SA has a tip surface 36, a first inner inclined surface 38, a first outer inclined surface 40SA, and a first wall surface 42SA. The outer peripheral surface 44SA is located outside the first wall surface 42SA so as to surround it. A guide portion 45 is provided on the outer peripheral surface 44SA. The guide portion 45 has guide surfaces 45S on both sides in the circumferential direction, with the top portion 47 in between.

[0066] The first mold 26A has a plurality of first bead portions 58 between the first wall surface 42SA and the outer peripheral surface 44SA. One first bead portion 58 is provided on each side in the circumferential direction of the guide portion 45 provided on the outer peripheral surface 44SA. The first bead portions 58 have a recessed first bead surface 58S relative to the second mold 28A.

[0067] The second mold 28A has a second molding surface 28SA on the side facing the first mold 26A, which corresponds to the first molding surface 26SA. The second molding surface 28SA has a concave bottom surface 50, a second inner inclined surface 52, a second outer inclined surface 54, and a second wall surface 56SA.

[0068] The second mold 28A has a plurality of second bead portions 60 on the second wall surface 56SA. The second bead portions 60 are located in positions corresponding to the first bead portions 58 and are provided complementary to the first bead portions 58. The second bead portions 60 have a second bead surface 60S that protrudes relative to the first mold 26.

[0069] The first bead portion 58 and the second bead portion 60, although not shown in the figure, form a plurality of bead portions that protrude outward from the wheel disc.

[0070] The mold 24A according to this modified example can obtain the same effects as the above embodiment because the guide surface 45S is provided on the guide portion 45.

[0071] In hot press forming, as deep drawing progresses and shrinkage flange deformation occurs at the outer edge 30P of the blank 30, a portion of the corrugated outer edge 30P moves toward the central axis Z along the guide surface 45S. At a position corresponding to the inner edge 44A of the outer peripheral surface 44SA, a large compressive force acts on the blank 30 in the circumferential direction, making it easy for the plate thickness to increase. The mold 24A in this modified example suppresses localized increases in plate thickness by allowing the material of the blank 30 to escape into the first bead portion 58 and the second bead portion 60. As a result, the mold 24A in this modified example and the method for manufacturing a wheel disc using the mold 24A can more reliably suppress the occurrence of wrinkles.

[0072] 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 polygonal. 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 there may be three or fewer, or five or more. Moreover, the wheel disc does not need to have through holes in the annular inclined portion.

[0073] In the above embodiment, the case where the first mold is the upper mold and the second mold is the lower mold has been described, but the present invention is not limited to this, and the first mold may be the lower mold and the second mold may be the upper mold.

[0074] 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.

[0075] In the above embodiment, the top of the guide portion is described as being curved and protruding from the second mold when viewed from the outside of the first mold, but the present invention is not limited to this. The top of the guide portion may be the intersection of guide surfaces provided on both sides of the top in the circumferential direction. In this case, the guide portion is in the shape of an inverted triangle when viewed from the outside of the first mold. The top may also have a surface perpendicular to the central axis Z. In this case, the guide portion is in the shape of an inverted trapezoid when viewed from the outside of the first mold.

[0076] 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 the direction of the central axis.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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 14A Inner edge 14B Outer edge 14S Inclined surface 16 Peripheral fixing section (vertical wall section) 16A Upper edge 16B Lower edge 16S Cylindrical surface 18 Bottom section 18A Outer edge 18S Bottom surface 20 Inclined cylindrical section 20A Lower edge 20B Upper edge 20S Inclined cylindrical surface 22 Through hole (wheel disc) 22P Through hole (blank) 22A Inner edge 22B Outer edge 24, 24A Mold 26, 26A First mold 26S, 26SA First molding 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 Inner convex part 34 Outer recess 36 Tip surface 36A Outer edge 38 First inner inclined surface 38A Lower edge 38B Upper edge 40, 40SA First outer inclined surface 40A Inner edge 40B Outer edge 42, 42SA First wall surface 42A Upper edge 42B Lower edge 44, 44SA Outer periphery surface 44A Inner edge 44B Outer edge 45 Guide part 45S Guide surface 46 Inner recess 47 Top part 48 Outer convex part 50 Concave bottom surface 50A Outer edge 52 Second inner inclined surface 52A Lower edge 52B Upper edge 54 Second outer inclined surface 54A Inner edge 54B Outer edge 56, 56SA Second wall surface 56A Upper edge 56B Lower edge 58 First bead section 58S First bead surface 60 Second bead section 60S Second bead surface 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) Z Center axis α, β Angle

Claims

1. A die for performing deep drawing by hot pressing on a heated blank, comprising: a first die having a first forming surface and an outer peripheral surface disposed outside the first forming surface; and a second die disposed opposite to the first die and having a second forming surface corresponding to the first forming surface, wherein a guide portion is provided on the outer peripheral surface, the guide portion protrudes from the second die and has a top portion extending in a direction toward the inside of the first die from the outer edge of the first die, and the guide portion has a pair of guide surfaces inclined from the top portion toward the outer peripheral surface on the side intersecting the extending direction of the top portion, for hot press forming.

2. The top portion is inclined toward the side away from the second mold toward the inside of the first mold from the outer edge of the first mold, according to claim 1.

3. The mold for hot press forming according to claim 1 or 2, wherein the first molding surface of the first mold is provided with an inner projection and a recessed outer peripheral recess relative to the second mold, the outer peripheral recess being provided so as to surround the inner projection, and the second molding surface of the second mold is provided with an inner recess and a projection relative to the first mold, the inner recess being provided at a position opposite to the inner projection, and the outer peripheral projection being provided at a position opposite to the outer peripheral recess and so as to surround the inner recess.

4. The mold for hot press forming according to claim 3, wherein the inner protrusion is columnar, and the outer recess and the outer protrusion are annular.

5. The mold for hot press forming according to claim 1 or 2, wherein the first mold is provided with a first bead portion at the connection portion between the pair of guide surfaces and the outer peripheral surface, and the second mold is provided with a second bead portion that forms a bead on the blank between itself and the first bead portion.

6. The mold according to claim 1 or 2, wherein a wheel disc is formed by the hot press described above.

7. A method for manufacturing a hot press-formed product, comprising: a first step of starting deep drawing press forming on a heated blank; and a second step of continuing deep drawing press forming after the blank has started to deform, while restricting the circumferential movement of the blank with respect to at least a portion of the outer edge, including the outer edge of the blank.

8. The method for manufacturing a hot press-formed product according to claim 7, wherein the deep drawing press forming includes deep drawing in which the central portion of the blank, which is inward from the outer edge, protrudes in one direction relative to the heated blank, and reverse deep drawing in which the outer periphery between the outer edge and the central portion protrudes in the opposite direction to the one direction.

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

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