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

Abstract

The present invention provides a die for hot-press drawing that can more reliably suppress the occurrence of wrinkles, and a method for manufacturing a hot-press drawn product. The die (24) includes a first die (26) that contacts a first surface (30F) of a blank (30), a second die (28) that contacts a second surface (30R) of the blank (30), and a holder (58) that is disposed outside the second die (28) and faces the first die (26) and contacts the second surface (30R) of an outer edge portion (30P) of the blank (30). The holder (58) has a plurality of first holder portions (60) and a plurality of second holder portions (62). The outer edge portion (30P) is disposed between each of the plurality of first holder portions (60) and the first die (26). Each of the plurality of second holder portions (62) is disposed at a position farther from the first die (26) than the plurality of first holder portions (60), and contacts a portion of the second surface (30R) of the deformed outer edge portion (30P).

Description

[Technical Field]

[0001] The present invention relates to a mold for hot press forming and a method for manufacturing a hot press-formed product. [Background technology]

[0002] Various manufacturing methods have been proposed to prevent wrinkles from occurring in hot press forming. For example, Patent Document 1 discloses a hot press deep drawing method using a holder that holds the outer edge of a blank by clamping it between a die and the holder. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-182081 Summary of the Invention [Problem to be solved by the invention]

[0004] When drawing is performed by hot pressing, it is required to more reliably suppress the occurrence of wrinkles.

[0005] An object of the present invention is to provide a mold for hot press forming that can more reliably suppress the occurrence of wrinkles when drawing by hot press, and a method for manufacturing a hot press-formed product. [Means for solving the problem]

[0006] The present application discloses the following aspects as one of means for solving the above problems.

[0007] [Aspect 1] A die for performing drawing on a heated blank by hot pressing, In hot press forming, a first die that contacts a first surface of the blank; a second die disposed opposite the first die and in contact with a second surface of the blank opposite the first surface; a holder disposed outside the second mold and facing the first mold, and in contact with the second surface of an outer edge portion including an outer edge of the blank; the holder has a plurality of first holder portions and a plurality of second holder portions arranged between the plurality of first holder portions, the outer edge portion is disposed between each of the plurality of first holder portions and the first mold; Each of the plurality of second holder portions is disposed at a position farther from the first mold than the plurality of first holder portions and contacts a part of the second surface of the deformed outer edge portion. Dies for hot press forming. [Aspect 2] the first mold has a protruding inner convex portion and a recessed outer peripheral concave portion relative to the second mold, and the outer peripheral concave portion is provided so as to surround the inner convex portion; The second mold has a recessed inner recess and a protruding outer peripheral protrusion, the inner recess being provided at a position facing the inner protrusion, and the outer peripheral protrusion being provided at a position facing the outer peripheral recess and surrounding the inner recess. 2. A mold for hot press forming according to claim 1. [Aspect 3] 3. The die for hot press-forming according to claim 2, wherein the inner convex portion is columnar, and the outer peripheral concave portion and the outer peripheral convex portion are annular. [Aspect 4] A mold for hot press forming according to any one of aspects 1 to 3, wherein a wheel disc is formed by the hot press. [Aspect 5] A method for manufacturing a hot press-formed product, A first step of starting drawing press forming on a heated blank; and a second step of suppressing deformation in the thickness direction of the blank in a plurality of deformation suppression regions at an outer edge portion including an outer edge of the blank, compared to deformation in the thickness direction of an easy-to-deform region between the plurality of deformation suppression regions, after the blank starts to deform, and proceeding with drawing press forming while supporting a portion of the outer edge portion of the easy-to-deform region in the thickness direction. A manufacturing method for hot press formed products. [Aspect 6] A method for manufacturing a hot press-formed product according to aspect 5, further comprising a third step, after the second step, of releasing the outer edge portion in the deformation suppression region to complete the drawing press forming. [Aspect 7] A method for manufacturing a hot press-formed product according to aspect 5 or 6, wherein the drawing press forming includes drawing forming in which a central portion of the blank inside the outer edge portion is protruded in one direction relative to the heated blank, and reverse drawing forming in which an outer periphery portion between the outer edge portion and the central portion is protruded in a direction opposite to the one direction. [Aspect 8] A method for producing a hot-press-formed product according to any one of aspects 5 to 7, wherein the hot-press-formed product is a wheel disc. [Effects of the Invention]

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

[0009] [Figure 1] FIG. 2 is a front view of a wheel including a wheel disc according to the present embodiment. [Figure 2] 1 is a longitudinal cross-sectional view of a wheel including a wheel disc according to an embodiment of the present invention. [Figure 3] FIG. 2 is a perspective view schematically showing a wheel disc according to the embodiment. [Figure 4] 1 is a central vertical cross-sectional view schematically showing a wheel disc according to an embodiment of the present invention. [Figure 5]FIG. 2 is a perspective view schematically showing a mold for molding the wheel disc according to the embodiment. [Figure 6] FIG. 2 is a cross-sectional view schematically showing a state in which a blank is set in the mold according to the present embodiment. [Figure 7] FIG. 2 is a cross-sectional view schematically showing a state in which a blank is being draw-formed using a die according to the present embodiment, showing a state in which the blank has started to deform. [Figure 8] 8 is a cross-sectional view schematically showing the outer edge of the blank as seen from outside the mold in the state shown in FIG. 7. [Figure 9] FIG. 2 is a cross-sectional view schematically showing a state in which a blank is being draw-formed using a die according to this embodiment, showing a state in which the outer edge of the blank is positioned between a first die and a holder. [Figure 10] 10 is a cross-sectional view schematically showing the outer edge of the blank as seen from outside the mold in the state shown in FIG. 9. FIG. [Figure 11] FIG. 10 is a cross-sectional view schematically showing a state in which the mold according to the present embodiment has been operated to the bottom dead center. [Figure 12] FIG. 10 is a perspective view showing a hot press-formed product according to a modified example. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0011] [wheel] 1 and 2, a two-piece type automobile wheel 1 including a wheel disc 10 as a hot press-formed product will be described. The wheel 1 includes a rim 2 and a wheel disc 10. The rim 2 is substantially cylindrical and centered on 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 periphery of the rim 2.

[0012] The wheel disc 10 is fitted onto the inner periphery of the rim 2. The wheel disc 10 is generally disk-shaped and centered on the central axis Z, and has a hub mounting portion 12 as a recess in the center, and, moving radially outward from the hub mounting portion 12, in that order, an annular inclined portion 14 and a peripheral fixing portion 16 as a vertical wall portion. Although not shown, the hub mounting portion 12 has a hub hole and multiple bolt holes formed therein.

[0013] The peripheral fixing portion 16 of the wheel disc 10 is fitted into the inner periphery of the drop portion 3 of the rim 2 and welded to the drop portion 3. The wheel disc 10 is disposed 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 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 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 inner side and inner direction, and the side and direction away from the central axis Z will be referred to as the outer side and outer direction. The direction of rotation about the central axis Z will be referred to as the circumferential direction. The terms "perpendicular" and "parallel" include the cases of "approximately perpendicular" and "approximately parallel," respectively.

[0015] 3 is a perspective view of the wheel disc 10 according to this embodiment, as viewed from the outer surface. The wheel disc 10 is generally disk-shaped. The surface shown on the outside in FIG. 3 is the outer surface 10A, and the opposite back surface is the inner surface 10B.

[0016] The hub mounting portion 12 is provided 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 has a circular outer edge 18A (FIG. 4) that includes the central axis Z and is perpendicular to the central axis Z, and a bottom surface 18S that is surrounded by the outer edge 18A.

[0017] The inclined cylindrical portion 20 has an inverted truncated cone-like cylindrical shape, with a circular lower edge 20A when viewed from the central axis Z, an upper edge 20B having a larger diameter than the lower edge 20A and positioned higher than 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 bottom portion 18. The inclined cylindrical portion 20 is inclined outward from the lower edge 20A toward the upper edge 20B. The connecting portion between the outer edge 18A of the bottom portion 18 and the lower edge 20A of the inclined cylindrical portion 20 may have an R-shaped cross section. 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 below and outward from 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 inclined downward from the inner edge 14A to the outer edge 14B and is a gently curved surface that 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. Four through holes 22 are formed in the inclined surface 14S shown in Fig. 3, evenly spaced in the circumferential direction. The through holes 22 have a circular sector shape 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 when viewed from the central axis Z. The peripheral fixing portion 16 has a circular upper edge 16A, a lower edge 16B that is positioned below the upper edge 16A and has the same diameter as the upper edge 16A, and a cylindrical surface 16S that connects 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, i.e., the vertical distance between the bottom surface 18S and the upper edge 20B of the inclined tube portion 20, is, for example, 15 mm or more and 80 mm or less. The height H2 of the annular inclined portion 14, i.e., 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] 5 and 6, a description will be given of a die 24 used in the manufacturing method of the wheel disc 10 according to this embodiment. Although not shown, the die 24 is installed in a press of a hot press device. The die 24 is used to perform hot press forming on a blank when manufacturing the wheel disc 10 having the cone-shaped hub mounting portion 12, the annular inclined portion 14 provided so as to surround the hub mounting portion 12, and the cylindrical peripheral fixing portion 16 provided so as to surround the annular inclined portion 14.

[0024] The mold 24 shown in Fig. 5 includes a first mold 26 and a second mold 28. Figs. 5 and 6 show a state in which a blank 30 is disposed between an upper mold serving as the first mold 26 and a lower mold serving as the second mold 28. In the following description, the central axis Z, upper and upward directions, lower and downward directions, inside and inward directions, and outside and outward directions of the mold 24 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 from the central axis Z.

[0025] (First mold) The first mold 26 has a refrigerant flow path (not shown). The first mold 26 has a size corresponding to the size of the wheel disc 10. The first mold 26 has a first molding surface 26S on the side facing the second mold 28 with the blank 30 sandwiched therebetween, and a pressing surface 44 disposed outside the first molding surface 26S.

[0026] The first molding surface 26S contacts the first surface 30F of the blank 30. The first molding surface 26S is provided with an inner convex portion 32 and an outer peripheral concave portion 34. The inner convex portion 32 is columnar, is provided in the center of the first molding surface 26S, and protrudes downward relative to the second mold 28. The outer peripheral concave portion 34 is disposed outside the inner convex portion 32 so as to surround the inner convex portion 32, and is recessed upward relative to the second mold 28. The outer peripheral concave portion 34 is provided concentrically with the inner convex portion 32, and is annular when viewed from the central axis Z.

[0027] The first molding surface 26S has a tip surface 36 that includes the central axis Z and is circular when viewed from the central axis Z. An 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 that has a larger diameter than the lower edge 38A and is 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 connecting 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 an R-shaped cross section.

[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 connecting 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 an R-shaped cross section.

[0029] The first outer inclined surface 40 is connected to a 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 disposed downward from the upper edge 42A and 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 convex portion 32 is formed by the tip surface 36 and the first inner inclined surface 38 into a shape that protrudes relative to the second mold 28. The outer peripheral recess 34 is formed by the first inner inclined surface 38 and the first outer inclined surface 40 into a shape that is recessed relative to the second mold 28. The first mold 26 forms the bottom surface 18S and the inclined cylindrical surface 20S of the hub mounting portion 12 by the tip surface 36 and the first inner inclined surface 38, forms the inclined surface 14S of the annular inclined portion 14 by the first outer inclined surface 40, and forms the cylindrical surface 16S of the peripheral fixing portion 16 by the first wall surface 42.

[0031] The pressing surface 44 is disposed on the outside of 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 disposed outward from the inner edge 44A and perpendicular to the central axis Z. The pressing surface 44 is annular about the central axis Z and is a plane perpendicular to the central axis Z.

[0032] (Second mold) The second mold 28 is provided with a refrigerant flow path (not shown). The second mold 28 has a size corresponding to the size of the wheel disc 10. The second mold 28 has a second molding surface 28S corresponding to the first molding surface 26S on the side facing the first mold 26 across the blank 30. The second molding surface 28S contacts 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 provided 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 to be complementary to the inner protrusion 32 of the first mold 26. The outer peripheral protrusion 48 is disposed outside the inner recess 46 so as to surround the inner recess 46 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 to be complementary to the outer peripheral recess 34 of the first mold 26.

[0034] The second molding surface 28S has a concave bottom surface 50 that includes the central axis Z and is circular when viewed from the central axis Z. An outer edge 50A of the concave bottom surface 50 is connected to a 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 that has a larger diameter than the lower edge 52A and is 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 connecting 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 an R-shaped cross section.

[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 protrudes gently upward. The connecting 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 an R-shaped cross section.

[0036] The second outer inclined surface 54 is connected to a 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 (FIG. 6) disposed downward from the upper edge 56A and parallel to the central axis Z. The second wall surface 56 has the shape of the outer surface of a cylinder.

[0037] (holder) The mold 24 further includes a holder 58 on the outside of the second mold 28. The holder 58 is disposed opposite the pressing surface 44 of the first mold 26 with the blank 30 sandwiched between them. The holder 58 is disposed concentrically with the second mold 28 and has an annular shape when viewed from the central axis Z. The holder 58 is disposed so as to be movable in the vertical direction independently of the first mold 26 and the second mold 28.

[0038] The holder 58 has a plurality of first holder parts 60 arranged at predetermined intervals along the outer periphery of the second mold 28, and a plurality of second holder parts 62 arranged between the plurality of first holder parts 60. The number of first holder parts 60 and second holder parts 62 may be the same or different and is not particularly limited, but may be, for example, 2 to 10 of each.

[0039] The outer edge portion 30P of the blank 30 is positioned between the first holder part 60 and the first mold 26. The first holder part 60 has a first holder surface 60S arranged opposite the pressing surface 44. The minimum length H3 between the pressing surface 44 and the first holder surface 60S in the direction of the central axis Z is greater than the thickness of the blank 30. The minimum length H3 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 can contact the second surface 30R of the blank 30 during hot press forming. That is, the blank 30 can 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 part 62 is disposed at a position further downward from the first mold 26 than the first holder part 60. The second holder part 62 has a second holder surface 62S disposed opposite the pressing surface 44. The second holder surface 62S is further downward from the pressing surface 44 than the first holder surface 60S. During hot press forming, the second holder surface 62S can come into contact with the second surface 30R of the blank 30, whose first surface 30F is away from the pressing surface 44. During hot press forming, the second holder surface 62S can come into point contact or line contact with the second surface 30R of the outer edge portion 30P of the blank 30 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] The first holder surface 60S and the second holder surface 62S of the holder 58 shown in FIG. 5 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 (FIG. 6). The actuator 64 is controlled by a control device (not shown). During hot press forming, the control device moves the holder 58 up and down in accordance with the movement of the first die 26. Before the first die 26 reaches the bottom dead center, the control device moves the holder 58 downward faster than the first die 26 so that the first holder surface 60S moves away from the second surface 30R.

[0043] (Relationship between the mold and the wheel disc) The tip surface 36 of the first die 26 and the concave bottom surface 50 of the second die 28 form the bottom 18 of the wheel disc 10. The first inner inclined surface 38 of the first die 26 and the second inner inclined surface 52 of the second die 28 form the inclined cylindrical portion 20 of the wheel disc 10. The first outer inclined surface 40 of the first die 26 and the second outer inclined surface 54 of the second die 28 form the annular inclined portion 14 of the wheel disc 10. The first wall surface 42 of the first die 26 and the second wall surface 56 of the second die 28 form the peripheral fixing portion 16 of the wheel disc 10. The pressing surface 44 of the first die 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 having 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. It is preferable that the blank 30 has a tensile strength of 1000 MPa or more after quenching. The blank 30 has a circular shape when viewed in the thickness direction. The blank 30 has a plurality of through holes 22P. The through holes 22P shown in FIG. 5 are in the shape of a circular fan, and four of them are arranged evenly in the circumferential direction of the blank 30. The through holes 22P become the through holes 22 in the wheel disc 10 after hot press forming. In the blank 30, the circular range centered on the central axis Z from the radial center to the inner edge 22A of the through hole 22P is called the central portion 30C, the annular range centered on the central axis Z from the inner edge 22A to the outer edge 22B of the through hole 22P is called the outer peripheral portion 30M, and the 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 called the outer edge portion 30P.

[0045] The steel material used for this blank 30 has a chemical composition, for example, in mass %, of C: 0.1% to 0.8%, Si: 0.001% to 2.0%, Mn: 0.5% to 3.0%, P: 0.15% or less, and S: 0.01% or less.

[0046] The steel preferably has a chemical composition of, by mass%, sol.Al: 0.001% to 1.0%, N: 0.01% or less, B: 0.01% or less, with the balance being Fe and impurities. The chemical composition may also contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ti, Nb, V, Cr, Mo, Cu, and Ni.

[0047] Other additive elements such as Ti, Nb, V, Cr, Mo, Cu, and Ni may be added as necessary to improve the hardenability of the steel and ensure stable strength after hardening.

[0048] [Manufacturing method] Next, a description will be given of a method for manufacturing the wheel disc 10 by hot pressing. The method for manufacturing the wheel disc 10 includes a heating step and a quenching step.

[0049] (Heating process) In the heating step, the blank 30 is heated to a temperature equal to or higher than the Ac3 transformation point of the blank 30 to be austenitized.

[0050] The Ac3 transformation point, which indicates the austenitizing transformation temperature, is the temperature at which the blank 30 made of the above-mentioned steel material is completely austenitized, and is expressed by the following formula, for example.

[0051] Ac3(℃)=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.

[0052] (Quenching process) The blank 30 heated to the Ac3 transformation point or higher in the heating process is hot press-formed using the above-mentioned die 24 installed in the press of a hot press device. After hot press-forming, the wheel disc 10 is transformed into martensitic by rapidly removing heat from the press-formed product using the first die 26 and second die 28 while the dies are still clamped, thereby cooling it. In other words, the quenching process includes the steps of setting, drawing, and cooling.

[0053] ·set The blank 30 heated to the Ac3 transformation point or higher in the heating step 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 in the mold 24 by a positioning pin (not shown).

[0054] ·Squeeze molding The draw forming includes a first step, a second step, and a third step. First, in the first step, as shown in Fig. 7, the first die 26 is moved downward toward the second die 28. As a result, the central portion 30C of the blank 30 is pressed downward by the inner convex portion 32 of the first die and is drawn, and at the same time, the peripheral portion 30M around it is pressed upward by the outer peripheral convex portion 48 of the second die 28, and the blank 30 is drawn in the reverse direction (hereinafter sometimes referred to as "reverse drawing"). In this way, drawing is started on the blank 30 whose temperature is equal to or higher than the Ac3 transformation point, and the blank 30 begins to deform.

[0055] Although the outer edge portion 30P of the blank 30 is pressed downward by the pressing surface 44 of the first mold 26, since the holder 58 is sufficiently far away from the first mold 26, it 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.

[0056] As the drawing process begins, shrinkage flange deformation occurs at the outer edge portion 30P of the blank 30, in which the blank 30 is compressed in the circumferential direction and pulled into the central portion 30C. Specifically, as shown in Fig. 8, the outer edge portion 30P of the blank 30 may deform into a wave-like shape that oscillates in the up-down direction.

[0057] 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. When 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, deformation in the thickness direction of the outer edge portion 30P is suppressed.

[0058] As shown in FIG. 10 , the outer edge portion 30P is disposed 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 disposed between the pressing surface 44 of the first mold 26 and the first holder surface 60S of the first holder portion 60 is referred to as a deformation suppression region 30PH. The deformation suppression region 30PH extends in the circumferential direction centered on the central axis Z and has a circular sector shape when viewed from the central axis Z. In FIG. 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 limited to this. That is, the outer edge portion 30P may be in contact with the first holder surface 60S but not with the pressing surface 44. More specifically, the outer edge portion 30P may be in contact with the pressing surface 44 or the first holder surface 60S in multiple deformation suppression regions 30PH. In the deformation suppression region 30PH, a portion of the outer edge portion 30P may contact the pressing surface 44 or the first holder surface 60S, and another portion of the outer edge portion 30P may contact the first holder surface 60S or the pressing surface 44.

[0059] The second holder portion 62, which is disposed between the first holder portions 60 in the circumferential direction, is positioned lower than the first holder portions 60, allowing the outer edge portion 30P to more easily deform 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 referred to as the easy-to-deform region 30PN. The easy-to-deform region 30PN is disposed between the deformation-suppressed regions 30PH, extends circumferentially around the central axis Z, and has a circular sector shape when viewed from the central axis Z. The easy-to-deform region 30PN is more easily deformed in the thickness direction than the deformation-suppressed regions 30PH, and a portion of the easy-to-deform region 30PN 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 be in point contact or line contact with the pressing surface 44 and the second holder surface 62S in the multiple easy-to-deform regions 30PN between the multiple deformation-suppressed regions 30PH. In the easily deformable region 30PN, a part of the outer edge portion 30P may contact the pressing surface 44 or the second holder surface 62S, and another part of the outer edge portion 30P may contact the second holder surface 62S or the pressing surface 44.

[0060] 11 , the first die 26 is moved downward to the bottom dead center, and the drawing of the blank 30 is completed while the blank 30 is sandwiched between the first die 26 and the second die 28. From the second step to the third step, the outer edge portion 30P advances toward the central axis Z from between the pressing surface 44 and the first and second holder surfaces 60S, 62S. Furthermore, the holder 58 moves downward faster than the first die 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 fixing portion 16 of the wheel disc 10.

[0061] ·cooling When the first die 26 reaches the bottom dead center, the formed blank 30 is in close contact with the first die 26 and the second die 28. This state is called die clamping. In the die clamped state, heat from the blank 30 is rapidly removed by the first die 26 and the second die 28. As a result, the blank 30 undergoes martensitic transformation. In other words, the blank 30 becomes the wheel disc 10 by being quenched.

[0062] One method of removing heat from the blank 30 is to indirectly cool it using a refrigerant that flows inside the first mold 26 and the second mold 28. Another method of removing heat is to directly cool the blank 30 by spraying a refrigerant onto it from the first mold 26 and the second mold 28.

[0063] The time from the heating step to the start of cooling is set to within 15 seconds. In other words, the time from when the blank 30 heated in the heating step leaves the heating furnace to when the first die 26 reaches the bottom dead center is set to within 15 seconds.

[0064] [Action and effect] The functions and effects of the die 24 for hot press forming and the method for manufacturing the wheel disc 10 according to the present embodiment will be described.

[0065] In the first step of the drawing, the drawing is carried out without holding the outer edge portion 30P of the blank 30 by the first die 26 and the first holder portion 60. Therefore, in the initial stage of the drawing, the blank 30 is smoothly pulled toward the central portion 30C, thereby suppressing a reduction in the plate thickness at the central portion 30C.

[0066] In the second step, before the drawing advances and significant shrinkage flange deformation occurs in the outer edge portion 30P of the blank 30, the outer edge portion 30P of the blank 30 is positioned between the pressing surface 44 of the first die 26 and the first holder surface 60S of the first holder portion 60. This prevents the outer edge portions 30P, which have been deformed into a wave shape, from being sandwiched between the first die 26 and the first holder portion 60 in an overlapping state, thereby preventing an increase in the plate thickness of that portion.

[0067] In the second step, the drawing press forming is performed while suppressing deformation in the thickness direction of the deformation-suppressed region 30PH of the outer periphery 30P relative to deformation of the easy-to-deform region 30PN. That is, the first holder part 60 suppresses deformation in the deformation-suppressed region 30PH of the outer periphery 30P and prevents material of the outer periphery 30P from flowing into the deformation-suppressed region 30PN. More specifically, circumferential movement of the material of the outer periphery 30P is suppressed between the deformation-suppressed region 30PH and the easy-to-deform region 30PN, and the material moves inward toward the central axis Z during the drawing press forming. The outer periphery 30P of the easy-to-deform region 30PN is more likely to deform in the direction of the central axis Z due to the longer length between the pressing surface 44 and the second holder surface 62S in the direction parallel to the central axis Z, but it contacts the second holder surface 62S. That is, by the second holder surface 62S supporting the second surface 30R of the deformed outer edge portion 30P, deformation of the outer edge portion 30P in the easy-to-deform region 30PN in the direction of the central axis Z is suppressed to a certain level or less. Therefore, it is possible to suppress large deformation of the outer edge portion 30P in the easy-to-deform region 30PN and suppress the occurrence of wrinkles in the peripheral fixing portion 16. Furthermore, by suppressing the deformation of the outer edge portion 30P in the easy-to-deform region 30PN to a certain level or less in the second step, it is possible to more reliably suppress the occurrence of wrinkles in the peripheral fixing portion 16.

[0068] While the first die 26 is moving to the bottom dead center, the holder 58 moves downward faster than the first die 26, releasing the outer edge portion 30P in the deformation suppression region 30PH. This makes it possible to suppress a local 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 more reliably suppressed.

[0069] In this embodiment, by performing one-way drawing and reverse drawing in the opposite direction in a single press operation, it is possible to suppress a local temperature drop in the blank 30. Therefore, it is possible to ensure the strength of the wheel disc 10 after quenching.

[0070] [Variations] The present invention is not limited to the above-described embodiments and can be modified as appropriate within the scope of the present invention. The examples exemplified below can be implemented alone or in combination with the above-described embodiments.

[0071] In the above embodiment, the wheel disc has been described as having a through hole formed in the annular inclined portion that is a circular sector shape, but the present invention is not limited to this, and the through hole may be circular or rectangular. Furthermore, while the above embodiment has been described as having four through holes formed in the annular inclined portion, the present invention is not limited to this, and the number of through holes may be three or less, or five or more. Furthermore, the wheel disc does not need to have a through hole in the annular inclined portion.

[0072] In the above embodiment, the first mold is an upper mold and the second mold is a lower mold, but the present invention is not limited to this, and the first mold may be a lower mold and the second mold may be an upper mold.

[0073] In the above embodiment, the first mold and the second mold are 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 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 holder surface and the second holder surface are flat surfaces perpendicular to the central axis, but the present invention is not limited to this, and may 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 are described as being arranged at a predetermined distance from each other, but 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 hot press-formed product is described as a disk for an automobile wheel, but the present invention is not limited to this. For example, the hot press-formed product may be a part formed using drawing forming and having large irregularities in the direction of the central axis.

[0078] Furthermore, for example, the hot press-formed product may be a cover 68 of a torque converter 66 as shown in FIG. 12 . The cover 68 of the torque converter 66 shown in FIG. 12 is filled with a fluid. The cover 68 rotates upon receiving engine input from the front. The cover 68 includes a front cover 70 disposed at the front and a rear cover 72 connected to the rear end of the front cover 70. Although the surface shape of the front cover 70 is not shown in FIG. 12 , the front cover 70 and the rear cover 72 are both substantially disk-shaped and centered on the central axis Z. The cover 68 of the torque converter 66 according to this modification can be manufactured using the same manufacturing method, mold, and manufacturing apparatus as those of the above embodiment, and can achieve the same effects as those of the above embodiment.

[0079] Although the above embodiment has been described with reference to a case where a substantially disk-shaped hot press-formed product is produced, the present invention is not limited thereto. That is, the present invention is suitable for a case where drawing is performed, and the outer shape in plan view is not particularly limited. For example, the outer shape in plan view may be an ellipse, a rectangle, a polygon, or the like.

[0080] In the above embodiment, the hot press-formed product has 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 press-formed product having one or more recesses or one or more protrusions.

[0081] The present invention may also be applied to a hot press-formed product having a convex portion and an annular concave portion formed to surround the convex portion. In this case, the first mold may be provided with a recessed inner concave portion 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 concave portion. The second mold may be provided with an inner convex portion protruding relative to the first mold and a recessed outer peripheral concave portion, the inner convex portion being provided at a position facing the inner concave portion, and the outer peripheral concave portion being provided at a position facing the outer peripheral convex portion to surround the inner convex portion. [Explanation of symbols]

[0082] 1 wheel 2 rims 3 Drop section 4 Bead seat 5 Flange 6 Space 10 Wheel disc (hot press formed product) 10A outer surface 10B inner surface 12 Hub mounting part (recess) 14 Annular ramp 14A Inner edge 14B outer edge 14S Slope 16 Peripheral fixing part (vertical wall part) 16A Upper edge 16B Lower edge 16S Cylindrical surface 18 Bottom 18A outer edge 18S bottom 20 Inclined cylinder part 20A Lower edge 20B Upper edge 20S inclined cylinder surface 22 through hole (wheel disc) 22P through hole (blank) 22A Inner edge 22B outer edge 24 Mold 26 First mold 26S 1st molding surface 28 Second mold 28S 2nd molding surface 30 Blank 30F 1st page 30R 2nd side 30C central part 30M outer circumference 30P outer edge 30E outer edge 30PH Deformation suppression area 30PN Easy deformation area 32 Inner convex part 34 Outer periphery recess 36 Tip surface 36A outer edge 38 1st inner slope 38A lower edge 38B Upper edge 40 1st outer slope 40A Inner edge 40B outer edge 42 First Wall 42A Upper edge 42B Lower edge 44 Presser surface 44A Inner Margin 44B outer edge 46 Inner recess 48 Outer periphery convex part 50 concave bottom 50A outer edge 52 2nd inner slope 52A Lower edge 52B upper edge 54 Second outer slope 54A Inner Margin 54B outer edge 56 Second wall 56A Upper edge 56B Lower edge 58 Holder 60 First holder part 60S 1st holder surface 62 Second holder part 62S Second holder surface 64 Actuator 66 Torque converter 68 Cover 70 Front cover 72 Rear cover 74 recess 76 Circular ramp 78 Vertical wall section H1 Height (hub mounting part) H2 Height (annular slope) H3 Minimum length between the holding surface and the first holder surface Z center axis

Claims

1. A die for performing drawing on a heated blank by hot pressing, In hot press forming, a first die that contacts a first surface of the blank; a second die disposed opposite the first die and in contact with a second surface of the blank opposite to the first surface; a holder disposed outside the second mold and facing the first mold, the holder contacting the second surface of an outer edge portion including an outer edge of the blank; the holder has a plurality of first holder portions and a plurality of second holder portions arranged between the plurality of first holder portions, the outer edge portion is disposed between each of the plurality of first holder portions and the first mold; Each of the plurality of second holder portions is disposed at a position farther from the first mold than the plurality of first holder portions and is in contact with a part of the second surface of the deformed outer edge portion. Dies for hot press forming.

2. the first mold has a protruding inner convex portion and a recessed outer peripheral concave portion relative to the second mold, the outer peripheral concave portion being provided so as to surround the inner convex portion; the second mold has a recessed inner recess and a protruding outer peripheral protrusion, the inner recess being provided at a position facing the inner protrusion, and the outer peripheral protrusion being provided at a position facing the outer peripheral recess and surrounding the inner recess; The mold for hot press forming according to claim 1.

3. The die for hot press forming according to claim 2 , wherein the inner convex portion is columnar, and the outer peripheral concave portion and the outer peripheral convex portion are annular.

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

5. A method for manufacturing a hot press-formed product, A first step of starting drawing press forming on a heated blank; After the blank starts to deform, deformation in the thickness direction of the blank in a plurality of deformation suppression regions at an outer edge portion including the outer edge of the blank is suppressed to be smaller than deformation in the thickness direction of an easy-to-deform region between the plurality of deformation suppression regions, and drawing press forming is carried out while supporting a portion of the outer edge portion of the easy-to-deform region in the thickness direction. A manufacturing method for hot press formed products.

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

7. 7. The method for manufacturing a hot press-formed product according to claim 5 or 6, wherein the drawing press forming includes drawing forming in which a central portion of the blank inside the outer edge portion is protruded in one direction relative to the heated blank, and reverse drawing forming in which an outer periphery between the outer edge portion and the central portion is protruded in a direction opposite 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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