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

Abstract

The present invention provides a die for performing drawing by hot pressing and a method for manufacturing a hot-pressed drawn article, both of which make it possible to more reliably suppress the occurrence of wrinkles. A die 24 comprises: a first die 26 that comes into contact with a first surface 30F of a blank 30; a second die 28 that comes into contact with a second surface 30R of the blank 30; and a holder 58 that is disposed outside the second die 28 so as to face the first die 26 and that comes into contact with the second surface 30R of an outer edge part 30P of the blank 30. The holder 58 has a plurality of first holder parts 60 and a plurality of second holder parts 62. The outer edge part 30P is disposed between each of the plurality of first holder parts 60 and the first die 26. Each of the plurality of second holder parts 62 is disposed at a position farther from the first die 26 than the plurality of first holder parts 60, and comes into contact with a part of the second surface 30R of the deformed outer edge part 30P.

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 a die.

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

[0004] When performing drawing forming by hot press, it is required to more surely 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 surely 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 die for performing deep drawing by hot pressing on a heated blank, comprising: a first die in contact with a first surface of the blank; a second die positioned opposite to the first die and in contact with a second surface of the blank opposite to the first surface; and a holder located outside the second die and opposite to the first die, in contact with the second surface of the outer edge portion including the outer edge of the blank, wherein the holder has a plurality of first holder portions and a plurality of second holder portions positioned between the plurality of first holder portions, the outer edge portion is positioned between each of the plurality of first holder portions and the first die, and each of the plurality of second holder portions is positioned further away from the first die than the plurality of first holder portions and in contact with a part of the second surface of the deformed outer edge portion, a die for hot pressing. [Aspect 2] The first mold has an inner projection and a recessed outer peripheral recess relative to the second mold, the outer peripheral recess is provided so as to surround the inner projection, and the second mold has an inner recess and a projection relative to the first mold, the inner recess is provided at a position opposite to the inner projection, and the outer peripheral projection is provided at a position opposite to the outer peripheral recess so as to surround the inner recess, the mold for hot press forming according to Aspect 1. [Aspect 3] The inner projection is columnar, and the outer peripheral recess and the outer peripheral projection are annular, the mold for hot press forming according to Aspect 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. [Aspect 5] 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, after the blank has started to deform, suppressing the deformation of the blank in the thickness direction in a plurality of deformation suppression regions in the outer edge portion including the outer edge of the blank more than the deformation in the thickness direction in the easily deformable regions between the plurality of deformation suppression regions, and supporting a part of the outer edge portion of the easily deformable regions in the thickness direction while proceeding with deep drawing press forming.[Aspect 6] A method for manufacturing a hot press-formed product according to aspect 5, further comprising a third step of releasing the outer edge in the deformation suppression region after the second step to complete the deep drawing press forming. [Aspect 7] A method for manufacturing a hot press-formed product according to aspect 5 or 6, wherein the deep drawing press forming includes deep drawing in which the central part of the blank inside 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 part protrudes in the opposite direction to the one direction. [Aspect 8] A method for manufacturing a hot press-formed product according to any one of aspects 5 to 7, 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 of drawing a blank in the die according to this embodiment, and shows the state in which the blank has started to deform. This is a schematic cross-sectional view showing the outer edge of the blank as seen from the outside of the die in the state of Figure 7. This is a schematic cross-sectional view showing the state of drawing a blank in the die according to this embodiment, and shows the state in which the outer edge of the blank is positioned between the first die and the holder. This is a schematic cross-sectional view showing the outer edge of the blank as seen from the outside of the die in the state of Figure 9. This is a schematic cross-sectional view showing the state in which the die according to this embodiment has been operated to the bottom dead center. 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] [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 (not shown) 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 18A (Figure 4) perpendicular to the central axis Z, and a bottom surface 18S surrounded by the outer edge 18A.

[0017] 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.

[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 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.

[0019] 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.

[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 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.

[0022] 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.

[0023] [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 apparatus, although it is not shown in the figures. 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.

[0024] 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.

[0025] (First mold) The first mold 26 is equipped with a refrigerant flow path (not shown). The first mold 26 is sized according to the size of the wheel disc 10. The first mold 26 has a first molding surface 26S and a pressing surface 44 located outside the first molding surface 26S on the side facing the second mold 28 across the blank 30.

[0026] 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 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.

[0027] 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).

[0028] 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).

[0029] 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.

[0030] 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.

[0031] The pressing surface 44 is positioned outside the first wall surface 42 so as to surround the first wall surface 42. The pressing 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 pressing surface 44 is an annular shape centered on the central axis Z and is a plane perpendicular to the central axis Z.

[0032] (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.

[0033] 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.

[0034] 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).

[0035] 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).

[0036] 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 (Figure 6) positioned downward from the upper edge 56A parallel to the central axis Z. The second wall surface 56 is cylindrical in shape.

[0037] (Holder) The mold 24 is further equipped with a holder 58 on the outside of the second mold 28. The holder 58 is positioned opposite the pressing surface 44 of the first mold 26, with the blank 30 in between. The holder 58 is provided concentrically with the second mold 28 and is annular when viewed from the central axis Z. The holder 58 is provided to be independently movable in the vertical direction relative to the first mold 26 and the second mold 28.

[0038] The holder 58 has a plurality of first holder portions 60 arranged at predetermined intervals along the outer circumference of the second mold 28, and a plurality of second holder portions 62 arranged between the plurality of first holder portions 60. The number of first holder portions 60 and second holder portions 62 may be the same or different, and is not particularly limited, but for example, there may be two to ten of each.

[0039] The outer edge 30P of the blank 30 is positioned between the first holder portion 60 and the first mold 26. The first holder portion 60 has a first holder surface 60S positioned opposite the pressing surface 44. The minimum length H3 in the direction of the central axis Z between the pressing surface 44 and the first holder surface 60S is greater than the thickness of the blank 30. The minimum length H3 also varies depending on the mechanical properties and thickness of the blank 30, and can be, for example, 6 mm or more. The first holder surface 60S may contact the second surface 30R of the blank 30 during hot press forming. That is, the blank 30 may contact the pressing surface 44 or the first holder surface 60S during hot press forming. The first holder surface 60S is a plane perpendicular to the central axis Z and has a circular sector shape when viewed from the central axis Z.

[0040] The second holder portion 62 is positioned further downward from the first mold 26 than the first holder portion 60. The second holder portion 62 has a second holder surface 62S that is positioned opposite the pressing surface 44. The second holder surface 62S is further downward from the pressing surface 44 than the first holder surface 60S. In hot press forming, the second holder surface 62S may contact the second surface 30R of the blank 30, where the first surface 30F is separated from the pressing surface 44. In hot press forming, the second holder surface 62S may make point contact or line contact with the second surface 30R of the outer edge portion 30P of the blank 30 that has been deformed in the thickness direction.

[0041] The length from the pressing surface 44 to the second holder surface 62S in the direction of the central axis Z can be appropriately selected depending on the thickness of the blank 30, and may be longer than the length from the pressing surface 44 to the first holder surface 60S in the direction of the central axis Z, for example, 10 mm to 30 mm. The second holder surface 62S is a plane perpendicular to the central axis Z and has a circular sector shape when viewed from the central axis Z.

[0042] As shown in Figure 5, the first holder surface 60S and the second holder surface 62S of the holder 58 have the same circumferential length, and a step is provided between the first holder surface 60S and the second holder surface 62S. The holder 58 is provided so as to be movable up and down along the central axis Z by an actuator 64 (Figure 6). The actuator 64 is controlled by a control device (not shown). In hot press forming, the control device moves the holder 58 up and down in accordance with the movement of the first die 26. The control device moves the holder 58 downward faster than the first die 26 so that the first holder surface 60S separates from the second surface 30R just before the first die 26 reaches its bottom dead center.

[0043] (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 and the first holder surface 60S and second holder surface 62S of the holder 58 appropriately support 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.

[0044] [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.

[0045] 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.

[0046] 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. Further, the chemical composition may contain one or more selected from the group consisting of Ti, Nb, V, Cr, Mo, Cu, and Ni instead of a part of Fe.

[0047] 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 stably ensure the strength after quenching.

[0048] [Manufacturing Method] Next, a method for manufacturing the wheel disk 10 by hot press will be described. The manufacturing method of the wheel disk 10 includes a heating step and a quenching step.

[0049] (Heating process) In the heating process, the blank 30 is heated to a temperature above the Ac3 transformation point of the blank 30 to austenite it.

[0050] The Ac3 transformation point, which indicates the austenitization transformation temperature, is the temperature at which the blank 30 made of the aforementioned steel material is completely austenitized, and is shown as follows as an example.

[0051] 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 is carbon, Si is silicon, Mn is manganese, Cr is chromium, S is sulfur, Al is aluminum, and Ti is titanium.

[0052] (Quenching Process) The blank 30, which has been heated to above the Ac3 transformation point in the heating process, is hot press-formed using the mold 24 installed in the press machine of the hot press device. After hot press forming, the wheel disc 10 is rapidly cooled by removing heat from the press-formed product using the first mold 26 and the second mold 28 while the molds are clamped, thereby causing the wheel disc 10 to undergo martensitic transformation. In other words, the quenching process includes the steps of setting, deep drawing, and cooling.

[0053] - The blank 30, which has been heated to above the Ac3 transformation point in the set heating process, is set between the first mold 26 and the second mold 28 as shown in Figure 6. At this time, the blank 30 is positioned in the mold 24 by positioning pins (not shown).

[0054] - Deep drawing Deep drawing has a first step, a second step, and a third step. First, in the first step, as shown in Figure 7, the first mold 26 is moved downward toward the second mold 28. As a result, the central part 30C of the blank 30 is pushed downward by the inner protrusion 32 of the first mold and deep-drawn, and at the same time, the outer peripheral part 30M around it is pushed upward by the outer peripheral protrusion 48 of the second mold 28, and the blank 30 is deep-drawn in the reverse direction (hereinafter sometimes referred to as "reverse deep drawing"). In this way, when deep drawing is started on a blank 30 that is at a temperature above the Ac3 transformation point, the blank 30 begins to deform.

[0055] Although the outer edge 30P of the blank 30 is being pressed downward by the pressing surface 44 of the first mold 26, the holder 58 is not yet held by the pressing surface 44 of the first mold 26 and the first holder surface 60S of the first holder portion 60 because the holder 58 is sufficiently far from the first mold 26.

[0056] 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 may deform into a wave shape that oscillates in the vertical direction.

[0057] Next, in the second step, as shown in Figure 9, the first mold 26 is moved downward until the length in the direction of the central axis Z between the pressing surface 44 and the first holder surface 60S becomes the minimum length H3. By making the length in the direction of the central axis Z between the pressing surface 44 and the first holder surface 60S the minimum length H3, deformation in the thickness direction of the outer edge portion 30P is suppressed.

[0058] As shown in Figure 10, the outer edge portion 30P is positioned between the pressing surface 44 of the first mold 26 and the first holder surface 60S of the first holder portion 60. The region of the outer edge portion 30P positioned between the pressing surface 44 of the first mold 26 and the first holder surface 60S of the first holder portion 60 is defined as the deformation suppression region 30PH. The deformation suppression region 30PH is a region extending in the circumferential direction with respect to the central axis Z, and is an annular sector shape when viewed from the central axis Z. In Figure 10, the outer edge portion 30P is in contact with the pressing surface 44 but not with the first holder surface 60S, but this is not the only case. That is, the outer edge portion 30P is in contact with the first holder surface 60S but not with the pressing surface 44, including the case where the outer edge portion 30P is in contact with the first holder surface 60S but not with the pressing surface 44. More specifically, the outer edge portion 30P may come into contact with the pressing surface 44 or the first holder surface 60S in multiple deformation suppression regions 30PH. Furthermore, in the deformation suppression region 30PH, a portion of the outer edge 30P may be in contact with the pressing surface 44 or the first holder surface 60S, while the rest of the outer edge 30P may be in contact with the first holder surface 60S or the pressing surface 44.

[0059] The second holder portion 62, positioned between the first holder portions 60 in the circumferential direction, is positioned below the first holder portions 60, allowing the outer edge portion 30P to be easily deformed in a direction parallel to the central axis Z. The region of the outer edge portion 30P corresponding to the second holder surface 62S is defined as the easily deformable region 30PN. The easily deformable region 30PN is positioned between the deformation-restricting regions 30PH, extends in the circumferential direction centered on the central axis Z, and has a circular fan shape when viewed from the central axis Z. The easily deformable region 30PN is more easily deformed in the thickness direction than the deformation-restricting regions 30PH, and a portion of it is in contact with at least one of the pressing surface 44 and the second holder surface 62S. That is, the outer edge portion 30P can make point contact or line contact with the pressing surface 44 and the second holder surface 62S in the multiple easily deformable regions 30PN between the multiple deformation-restricting regions 30PH. In the easily deformable region 30PN, a part of the outer edge 30P may be in contact with the pressing surface 44 or the second holder surface 62S, while the other part of the outer edge 30P may be in contact with the second holder surface 62S or the pressing surface 44.

[0060] Furthermore, in the third step, as shown in Figure 11, 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 second and third steps, the outer edge portion 30P moves toward the central axis Z from between the pressing surface 44 and the first holder surface 60S and the second holder surface 62S. Furthermore, the holder 58 moves downward faster than the first mold 26, releasing the outer edge portion 30P in the deformation suppression region 30PH in a direction parallel to the central axis Z. As a result, the outer edge portion 30P is pulled between the first wall surface 42 and the second wall surface 56, becoming the peripheral edge fixing portion 16 of the wheel disc 10.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] [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.

[0065] In the first step of deep drawing, the outer edge 30P of the blank 30 is not held by the first mold 26 and the first holder 60, and the deep drawing process is carried out. As a result, the blank 30 is smoothly pulled into the central part 30C in the initial stage of deep drawing, thereby suppressing the reduction in plate thickness in the central part 30C.

[0066] In the second step, before significant shrinkage flange deformation occurs at the outer edge 30P of the blank 30 as the deep drawing process progresses, the outer edge 30P of the blank 30 is positioned between the pressing surface 44 of the first mold 26 and the first holder surface 60S of the first holder portion 60. This prevents the corrugated outer edges 30P from overlapping and being sandwiched between the first mold 26 and the first holder portion 60, thereby suppressing an increase in the thickness of that portion.

[0067] In the second step, the deep drawing press forming is carried out while suppressing the deformation in the thickness direction of the deformation-suppressing region 30PH of the outer edge portion 30P more than the deformation of the easily deformable region 30PN. That is, the first holder portion 60 suppresses the deformation of the outer edge portion 30P in the deformation-suppressing region 30PH and prevents the material of the outer edge portion 30P in the easily deformable region 30PN from flowing into the deformation-suppressing region 30PH. More specifically, the material of the outer edge portion 30P has its circumferential movement suppressed between the deformation-suppressing region 30PH and the easily deformable region 30PN, and moves inward toward the central axis Z by the deep drawing press forming. The outer edge portion 30P of the easily deformable region 30PN is more easily deformed in the direction of the central axis Z because the length in the direction parallel to the central axis Z between the pressing surface 44 and the second holder surface 62S is larger, but it is in contact with the second holder surface 62S. In other words, the second holder surface 62S supports the second surface 30R of the deformed outer edge portion 30P, thereby suppressing the deformation of the outer edge portion 30P in the direction of the central axis Z in the easily deformable region 30PN to a certain level or less. Therefore, it is possible to suppress large deformation of the outer edge portion 30P in the easily deformable region 30PN and suppress the occurrence of wrinkles in the peripheral fixing portion 16. Furthermore, in the second step, by suppressing the deformation of the outer edge portion 30P in the easily deformable region 30PN to a certain level or less, the occurrence of wrinkles in the peripheral fixing portion 16 can be suppressed more reliably.

[0068] While the first mold 26 is moving to its bottom dead center, the holder 58 moves downward faster than the first mold 26, releasing the outer edge 30P in the deformation suppression region 30PH. This prevents a localized increase in the thickness of the lower edge 16B of the peripheral fixing portion 16. As a result, the rate of change in the thickness of the peripheral fixing portion 16 relative to the thickness of the blank 30 can be kept below a certain level. Therefore, in this embodiment, the occurrence of wrinkles can be suppressed more reliably.

[0069] 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.

[0070] [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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] In the above embodiment, a case was described in which a step is provided between the first holder surface and the second holder surface, but the present invention is not limited to this. For example, the first holder surface and the second holder surface may be smoothly connected via an inclined surface or a curved surface.

[0075] In the above embodiment, the first and second holder surfaces were described as being planes perpendicular to the central axis, but the present invention is not limited to this, and may also be curved surfaces that oscillate in the vertical direction, for example.

[0076] In the above embodiment, the first holder surface and the second holder surface were described as being arranged with a predetermined distance between them. However, the present invention is not limited to this, and for example, the circumferential lengths of the first holder surface and the second holder surface may be different.

[0077] 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.

[0078] Furthermore, for example, the hot-pressed product may be the cover 68 of the torque converter 66, as shown in Figure 12. The cover 68 of the torque converter 66 shown in Figure 12 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 12, 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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 Mold 26 First mold 26S First molding surface 28 Second mold 28S Second molding surface 30 Blank 30F First surface 30R Second surface 30C Center 30M Outer periphery 30P Outer edge 30E Outer edge 30PH Deformation suppression region 30PN Deformable region 32 Inner convex portion 34 Outer concave portion 36 Tip surface 36A Outer edge 38 First inner inclined surface 38A Lower edge 38B Upper edge 40 First outer inclined surface 40A Inner edge 40B Outer edge 42 First wall surface 42A Upper edge 42B Lower edge 44 Pressing surface 44A Inner edge 44B Outer edge 46 Inner concave portion 48 Outer convex portion 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 Second wall surface 56A Upper edge 56B Lower edge 58 Holder 60 First holder portion 60S First holder surface 62 Second holder portion 62S Second holder surface 64 Actuator 66 Torque converter 68 Cover 70 Front cover 72 Rear cover 74 Recess 76 Annular inclined portion 78 Vertical wall portion H1 Height (hub mounting portion) H2 Height (annular inclined portion) H3 Minimum length between the retaining surface and the first holder surface Z Central axis

Claims

1. A die for performing deep drawing by hot pressing on a heated blank, comprising: a first die in contact with a first surface of the blank; a second die positioned opposite to the first die and in contact with a second surface of the blank opposite to the first surface; and a holder located outside the second die, opposite to the first die, and in contact with the second surface of the outer edge portion including the outer edge of the blank, wherein the holder has a plurality of first holder portions and a plurality of second holder portions positioned between the plurality of first holder portions, the outer edge portion being positioned between each of the plurality of first holder portions and the first die, and each of the plurality of second holder portions being positioned further away from the first die than the plurality of first holder portions and in contact with a part of the second surface of the deformed outer edge portion.

2. The first mold has 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 mold has 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 so as to surround the inner recess, the mold for hot press forming according to claim 1.

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

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

5. 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, after the blank has started to deform, suppressing the deformation of the blank in the thickness direction in a plurality of deformation suppression regions in the outer edge portion including the outer edge of the blank more than the deformation in the thickness direction in the easily deformable regions between the plurality of deformation suppression regions, and proceeding with deep drawing press forming while supporting a part of the outer edge portion of the easily deformable regions in the thickness direction.

6. The method for manufacturing a hot press-formed product according to claim 5, further comprising a third step of releasing the outer edge in the deformation-suppressing region after the second step to complete the deep drawing press forming.

7. The method for manufacturing a hot press-formed product according to claim 5 or 6, 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.

8. The method for manufacturing a hot press-formed product according to claim 5 or 6, wherein the hot press-formed product is a wheel disc.

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