Press molding method and press molding device

Abstract

To provide a press molding method that can apply compressive stress to a work-piece to prevent spring back with a simple facility.SOLUTION: The press molding method comprises a preliminary molding step of molding a preliminary molded product 1 with a hat-shape cross section while leaving a molding margin (an excess thickness part 3) on a shoulder part 5a, and a main molding step of molding the product into a final shape. In the press molding method, a first movable mold 38 is pressed against a top plate 2 of a hat part 5 to mold the product, while making a second movable mold 39 that moves in a height direction of the hat part 5 constrain an opening-side end part of the preliminary molded product 1. Pressing force for pressing the second movable mold 39 is smaller than pressing force for pressing the first movable mold 38.SELECTED DRAWING: Figure 17

Description

[Technical field]

[0001] The present invention relates to a press-forming method and a press-forming apparatus for bending a steel sheet into a hat-shaped cross section. [Background technology]

[0002] In recent years, ultra-high tensile steel, which has higher strength than high tensile steel and a tensile strength exceeding 1000 MPa, has come to be used as the steel material for automobile bodies. This type of ultra-high tensile steel is cold bent into a desired shape. Ultra-high tensile steel has a larger amount of springback after bending compared to general high tensile steel. For this reason, in the past, the mold was re-machined to obtain the target dimensional accuracy in anticipation of springback.

[0003] Re-machining the die requires a lot of man-hours, and if re-machining is not enough, the die may have to be remade. This will increase the amount of iron used by remaking the die, which is not good for the environment. In order to reduce such die corrections as much as possible, a press forming method has been devised in which the die is formed multiple times, springback is corrected, and the die is formed into the final shape. A conventional press forming method for correcting springback is described in, for example, Patent Document 1. The press forming method described in Patent Document 1 is a method for forming a steel sheet into a hat-shaped cross section while suppressing the occurrence of springback. A hat-shaped cross section is a shape in which a flange portion is provided at the open end of a hat portion consisting of a top plate and vertical plates on both sides. The press forming method described in Patent Document 1 employs a configuration in which, when forming a steel sheet into a hat-shaped cross section, the formed product is formed into a final shape through two steps, a preforming step and a main forming step.

[0004] In the preforming process, a preform with a hat-shaped cross section is formed while allowing springback to occur. In this forming process, the vertical walls on both sides of the preform and the flanges at both ends are pressed from the side against a punch, which is a fixed mold, by a laterally moving die, while the hat portion is sandwiched between a pad and a punch to form the preform, and the preform is compressed and formed into the final shape. Compressing the preform suppresses the occurrence of springback. The laterally moving die is driven by converting the vertical movement of the upper mold including the pad into horizontal movement by a cam mechanism so as to work in conjunction with the pad that forms the hat portion. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2022-39327 A Summary of the Invention [Problem to be solved by the invention]

[0006] The molding method shown in Patent Document 1 requires a cam mechanism to drive a laterally moving die, which causes problems in that the metal mold becomes complicated and large in size.

[0007] An object of the present invention is to provide a press-forming method and a press-forming apparatus that are capable of applying compressive stress to a workpiece and preventing springback using simple equipment. [Means for solving the problem]

[0008] In order to achieve this object, the press molding method of the present invention includes a preforming step of molding a molded product having a hat-shaped cross section and a flange portion at an opening end of a hat portion consisting of a top plate and vertical plates on both sides, using a preforming die so that molding allowances remain on both shoulder portions of the hat portion, and a main molding step of compressing the preform formed in the preforming step using a main molding die to mold a molded product in a final shape, the main molding die including a wall against which a tip surface of the flange portion abuts, a convex portion that fits into the hat portion, and a concave portion formed between the wall and the convex portion, a fixed die having a molding surface for molding the inner surface of the molded product in the final shape, a first movable die configured to be movable in the height direction of the hat portion, and configured to sandwich the top plate side of the hat portion between the fixed die and the convex portion to mold the hat portion into the final shape, and and a second movable die configured to be movable in a vertical direction and to sandwich the preformed product including the open end side of the hat portion between the second movable die and the concave portion of the fixed die to mold it into a final shape, the main molding process comprising a first main molding process in which the preformed product is placed on the convex portion of the fixed die, a second main molding process in which the preformed product is sandwiched between the concave portion of the fixed die and the second movable die, and a third main molding process in which the preformed product is molded into a final shape by the convex portion of the fixed die and the first movable die in a state in which the preformed product is pressed against the concave portion of the fixed die by the second movable die, and a pressing force with which the second movable die presses the preformed product in the second main molding process is smaller than a pressing force with which the first movable die presses the preformed product in the third main molding process.

[0009] In the press molding method of the present invention, the portions of the fixed die and the second movable die that mold the connection between the vertical plate of the hat portion and the flange portion may be formed by rounded portions with curved cross-sectional shapes, and in the second main molding process, the rounded portion of the second movable die may push the vertical plate of the pre-formed product toward the fixed die as the second movable die moves to a molding end position.

[0010] In the press molding method of the present invention, a chamfered portion may be formed at the tip of the wall of the fixed die, and in the second main molding process, the tip portion of the flange portion may pass through a position close to the chamfered portion.

[0011] In the press molding method of the present invention, the wall of the fixed die may be formed by a die part separate from other parts of the fixed die and configured to be detachable, the boundary between the bottom surface of the recess and the wall may be a right angle, and in the third main molding process, the tip surface of the flange portion may abut against the wall without any gap.

[0012] The press molding apparatus according to the present invention comprises a preforming die for molding a hat-shaped cross-sectioned molded product having a flange portion at an opening end of a hat portion consisting of a top plate and both vertical plates such that molding allowances remain on both shoulder portions of the hat portion, and a main molding die for compressing the preform molded by the preforming die to mold a molded product of a final shape, the main molding die having a wall against which a tip surface of the flange portion abuts, a convex portion that fits into the hat portion, and a concave portion formed between the wall and the convex portion, the main molding die having a molding surface for molding an inner surface of the molded product of the final shape, and a fixed die having a molding surface that is movable in the height direction of the hat portion, the hat portion being movable in the height direction of the hat portion. The mold has a first movable mold which sandwiches the top plate side of the child portion between it and the convex portion of the fixed mold to mold it into a final shape, and a second movable mold which is configured to be movable in the height direction of the hat portion and sandwiches the preform including the opening end side of the hat portion between it and the concave portion of the fixed mold to mold it into a final shape, wherein the first movable mold presses the preform against the convex portion of the fixed mold to mold it into a final shape while the second movable mold is pressing the preform against the concave portion of the fixed mold, and the pressing force used by the second movable mold to press the preform is smaller than the pressing force used by the first movable mold to press the preform.

[0013] In the press molding apparatus of the present invention, the portions of the fixed die and the second movable die that form the connection between the vertical plate of the hat portion and the flange portion may be formed by rounded portions that have curved cross-sectional shapes.

[0014] In the press molding apparatus of the present invention, a chamfered portion may be formed at the tip of the wall of the fixed die, and the chamfered portion may be formed outside the movement trajectory of the flange portion that moves by the second movable die pressing the preform against the fixed die.

[0015] In the press molding apparatus of the present invention, the wall of the fixed mold may be formed by a mold part separate from other parts of the fixed mold and configured to be removable, the boundary between the bottom surface of the recess and the wall may be a right angle, and the tip surface of the flange portion of the final shaped molded product may abut against the wall without any gaps. Effect of the Invention

[0016] According to the present invention, it is possible to provide a press-forming method and a press-forming apparatus that can impart compressive stress to a workpiece and prevent springback using simple equipment. [Brief description of the drawings]

[0017] [Figure 1] FIG. 1 is a flowchart illustrating a press molding method according to the present invention. [Diagram 2] FIG. 2 is a cross-sectional view of a preform. [Diagram 3] FIG. 3 is a cross-sectional view of the molded product in its final shape. [Figure 4] FIG. 4 is a cross-sectional view showing a preforming mold. [Diagram 5] FIG. 5 is an enlarged cross-sectional view showing a part of the preforming die. [Figure 6] FIG. 6 is a cross-sectional view showing a mold for the main molding. [Figure 7] FIG. 7 is a cross-sectional view showing a punch of a die for full molding. [Figure 8] FIG. 8 is an enlarged cross-sectional view showing a part of the fixed die of the main molding die. [Figure 9] FIG. 9 is an enlarged cross-sectional view showing a part of the first movable mold. [Figure 10] FIG. 10 is an enlarged cross-sectional view showing one of the second movable dies. [Figure 11] FIG. 11 is a cross-sectional view of a die for main molding showing the first main molding step. [Figure 12] FIG. 12 is a cross-sectional view of the die for main molding showing an early stage of the second main molding step. [Figure 13] FIG. 13 is a cross-sectional view of the mold for main molding, showing a state in which the second movable mold is pressing the vertical plate in the second main molding step. [Figure 14] FIG. 14 is a cross-sectional view of the metal mold for main molding, showing the state after the second main molding step is completed. [Figure 15] FIG. 15 is an enlarged cross-sectional view showing a part of the mold for the main molding. [Figure 16] FIG. 16 is a cross-sectional view of a die for main molding showing an early stage of the third main molding step. [Figure 17] FIG. 17 is an enlarged cross-sectional view showing a part of the mold for the main molding. [Figure 18] FIG. 18 is a cross-sectional view of a part of a molded product, which diagrammatically illustrates tensile stress and compressive stress. [Figure 19] FIG. 19 is a cross-sectional view of the metal mold for main molding showing the state after the third main molding step is completed. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Hereinafter, one embodiment of a press-molding method and press-molding apparatus according to the present invention will be described in detail with reference to Figs. The press forming method according to the present invention is a method in which a preforming step S1 and a main forming step S2 are carried out in this order, as shown in the flow chart of Fig. 1. In the preforming step S1, a preform 1 as shown in Fig. 2 is formed from a flat plate (not shown) made of high tensile steel or ultra-high tensile steel. The preform 1 is formed to have a hat-shaped cross section, with a pair of vertical plates 4 connected to both sides of a top plate 2 via excess pad parts 3, and flange parts 6 connected to the open ends of the hat part 5. The excess pad parts 3 are formed in a shape such that shoulder parts 5a on both sides of the hat part 5 partially bulge outward from the hat part 5. This preforming step S1 is carried out by a preforming die 11 (see Fig. 4) described later.

[0019] The main forming step S2, which will be described in detail later, is a step in which the preliminary formed product 1 is formed into a formed product 12 having a final shape as shown in Fig. 3 by carrying out first to third main forming steps S2A to S2C. The formed product 12 having the final shape is formed by correcting deformation of the preliminary formed product 1 due to springback, and is formed into a hat-shaped cross section having a hat portion 15 consisting of a flat top plate 13 and vertical plates 14 extending linearly on both sides, and a pair of flange portions 16 connected to the open ends of the hat portion 15.

[0020] In the molded product 12 in its final shape, a shoulder portion 15a, which is a connection portion between the top plate 13 and the vertical plate 14 of the hat portion 15, is formed into an arc-shaped cross section by crushing the excess pad portion 3 provided in the preliminary molded product 1. In addition, a connection portion 15b between the vertical plate 14 and the flange portion 16 of the hat portion 15 is formed into a concave curved surface when viewed from the front side of the molded product. The main forming step S2 is performed by a main forming die 17 shown in Figures 6 and 7. The main forming die 17 and the above-mentioned preforming die 11 constitute a press forming apparatus 18 according to the present invention.

[0021] As shown in FIG. 4, the preforming die 11 includes a preforming punch 21 having a mountain-shaped cross section for forming the inner surface of the preformed product 1, and a pair of preforming dies 22, 23 and a preforming pad 24 for forming the outer surface of the preformed product 1. The preforming punch 21 is fixed to a preforming lower die 25. The preforming lower die 25 is fixed to a base member 26. As shown in Fig. 5, the tip of the preforming punch 21 is formed with a flat plate portion 21a for forming the top plate 2 of the preform 1 and a bulging portion 21b for forming the excess material portion 3.

[0022] The preforming dies 22, 23 are used to form both side portions including the outer half of the excess pad portion 3 of the preform 1, and the preforming pad 24 is used to form the inner half of the excess pad portion 3 of the preform 1 and the top plate 2. The preforming dies 22 and 23 and the preforming pad 24 are formed with recesses 22a, 23a, and 24a for sandwiching the preform 1 between them and the bulging portion 21b of the preforming punch 21.

[0023] The preforming dies 22, 23 and the preforming pad 24 are fixed to a preforming upper die 27. The preforming upper die 27 is configured to be movable in the height direction of the hat portion 5 of the preforming product 1 (vertical direction in FIG. 4, that is, the thickness direction of the top plate 2 and the flange portion 6) and is driven by a preforming pressure device (not shown). During preforming, the preforming upper die 27 is driven downward by the preforming pressure device, and the preforming dies 22, 23 and the preforming pad 24 press the steel plate (not shown) against the preforming punch 21.

[0024] In the preform 1 formed by this preform die 11, as shown in FIG. 2, a forming allowance consisting of excess pad portions 3 remains on both shoulder portions 5a of the hat portion 5, and spring back occurs so that the opening sides of the hat portion 5 expand outwardly to both sides.

[0025] As shown in FIG. 6, the mold 17 for this molding has a fixed mold 32 including a lower mold 31 for this molding which is depicted at the bottom in FIG. 6, and a movable mold 34 including an upper mold 33 for this molding which is depicted at the top in FIG. 6. The fixed die 32 has a final-forming punch 35 formed to have a mountain-shaped cross section, and a pair of restraining pieces 36 arranged on both sides of the final-forming punch 35. The final-forming punch 35 is a die that forms the inner surface 15c (see FIG. 3) of the cap portion 15 of the molded product 12 in its final shape and one surface 16a of the flange portion 16 connected to the inner surface 15c. The final-forming punch 35 and the restraining piece 36 are fixed to the final-forming lower die 31. That is, the fixed die 32 is composed of the final-forming lower die 31, the final-forming punch 35, and the restraining piece 36. The final-forming lower die 31 is configured so as not to be movable.

[0026] As shown in FIG. 7, the forming punch 35 has a first plane 35a which is depicted at the top in FIG. 7, convex curved surfaces 35b connected to both sides of the plane 35a, an outer surface 35c, a first rounded portion 35d, a second plane 35e, and the like. The first plane 35a is a molding surface that molds the inner surface of the top plate 13 of the cap portion 15 of the final shaped molded product 12. The convex curved surface 35b is a molding surface that molds the inner surfaces of both shoulder portions 15a of the cap portion 15. The outer surface 35c is a molding surface that molds the inner surface of the vertical plate 14. The first rounded portion 35d is a molding surface that molds the inner surface of the connection portion 15b between the vertical plate 14 of the cap portion 15 and the flange portion 16. The second plane 35e is a molding surface that molds the lower surface (one surface 16a) of the flange portion 16. In this embodiment, the portion of the main molding punch 35 from the first plane 35a to the outer surface 35c corresponds to the "protruding portion of the fixed die" as defined in the present invention.

[0027] The first rounding portion 35d is formed by a concave curved surface having a curved cross-sectional shape. In the main forming punch 35, the first flat surface 35a, the convex curved surface 35b, the outer surface 35c, the first rounding portion 35d, and the second flat surface 35e become forming surfaces that form the inner surface of the formed product 12 in its final shape. The restraining piece 36 becomes a wall against which the tip surface 16b (see FIG. 3) of the flange portion 16 abuts in the main forming step S2, and is formed as a die component separate from the other portions of the fixed die 32, and protrudes toward the main forming upper die 33 side from the second flat surface 35e of the main forming punch 35. The restraining piece 36 is detachably attached to the main forming lower die 31. At the tip end (projecting end) of this restraining piece 36 adjacent to the main forming punch 35, a chamfered portion 37 is formed as shown in FIG.

[0028] As shown in Fig. 6, the movable mold 34 is configured by assembling a first movable mold 38 and a pair of second movable molds 39 to an upper mold for main molding 33. The upper mold for main molding 33 is configured to be movable in the height direction of the hat portion 15 of the molded product 12 in its final shape (the up and down direction in Fig. 6), and is connected to a pressurizing device for main molding (not shown). The first movable die 38 is used to sandwich the top plate 13 side of the cap portion 15 between itself and the fixed die 32 (the punch for main forming 35) to form the cap portion 15 into a final shape, and is fixed to the upper die for main forming 33 and is movable together with the upper die for main forming 33 in the height direction of the cap portion 15. As shown in Fig. 9, the first movable die 38 is formed with a third flat surface 38a for sandwiching the molded product between itself and the first flat surface 35a of the punch for main forming 35, a concave curved surface 38b for sandwiching the molded product between itself and the convex curved surface 35b of the punch for main forming 35, and an inner surface 38c for sandwiching the molded product between itself and the outer surface 35c of the punch for main forming 35.

[0029] The second movable mold 39 is for molding the cap portion 15 into a final shape by sandwiching the opening end side and the flange portion 16 between the fixed mold 32 and the second movable mold 39. The second movable mold 39 according to this embodiment is suspended from the first movable mold 38 by a plurality of spool retainers 43 in a state in which it can move vertically relative to the first movable mold 38. A keeper plate (not shown) can be used instead of the spool retainer 43. The second movable mold 39 is connected to the main molding upper mold 33 via a gas spring 40. The gas spring 40 has a cylinder 40a fixed to the main molding upper mold 33 and a piston rod 40b protruding from the cylinder 40a toward the main molding lower mold 31. The piston rod 40b is urged toward the main molding lower mold 31 by the pressure of the compressed gas in the cylinder 40a. The second movable mold 39 is attached to the tip of the piston rod 40b. The gas spring 40 according to this embodiment is attached to the upper mold 33 for main molding in a state in which the cylinder 40a passes through the first movable mold 38. The spring force of the gas spring 40, i.e., the pressing force with which the second movable die 39 is pressed against the final molding punch 35, is smaller than the pressing force with which the final molding pressurizing device presses the final molding upper die 33 against the final molding punch 35 in the final molding step S2.

[0030] As shown in FIG. 10, the second movable die 39 has a convex portion 41 protruding toward the opposite side of the main forming upper die 33, and an inner surface 39a for sandwiching the vertical plate 14 of the molded product 12 in the final shape between the convex portion 41 and the outer surface 35c of the main forming punch 35. The convex portion 41 is formed in a shape that fits into a groove 42 (see FIG. 8) formed by the main forming punch 35 and the restraining piece 36. That is, the convex portion 41 is formed with a second rounding portion 41a that sandwiches the molded product between the first rounding portion 35d of the main forming punch 35, and a fourth flat surface 41b that sandwiches the molded product between the second flat surface 35e of the main forming punch 35. The second rounding portion 41a is formed by a convex curved surface that has a curved cross-sectional shape. In this embodiment, the groove 42 corresponds to the "recess" in the present invention. The fourth flat surface 41 b is formed so as to be smaller than the flange portion 6 of the preform 1 .

[0031] One side wall of the groove 42 is formed by a restraining piece 36 that is separate from the lower die 31 for main forming and the punch 35 for main forming. Therefore, a right angle can be formed at the boundary between the bottom surface of the groove 42 and one side wall. That is, the groove 42 can be formed so that no gap is generated between the restraining wall 36 and the flange portion 6 of the preformed product 1 when the final shaped molded product 12 is obtained from the preformed product 1, in other words, so that the tip surface of the flange portion 6 can properly abut against the restraining wall 36. In addition, since the lower die 31 for main forming is configured not to move, the tip surface of the flange portion 6 can properly abut against the restraining wall 36 more stably.

[0032] Next, a description will be given of the main molding step S2 for obtaining the final molded product 12 from the preform 1 using the main molding die 17. In molding the final molded product 12, first, a first main molding step S2A is carried out. In the first main forming step S2A, as shown in Fig. 11, the preform 1 is placed on a main forming punch 35. At this time, the main forming punch 35 is inserted into the cap portion 5 of the preform 1. The step of placing the preform 1 on the main forming punch 35 is the first main forming step S2A in the main forming step S2.

[0033] In the preform 1, due to the occurrence of spring back, the flange portion 6 becomes largely separated from the outer surface 35c of the main forming punch 35. After the preform 1 is placed on the punch 35 for main forming, the pressurizing device for main forming is operated to move the movable die 34 toward the fixed die 32, as shown in Fig. 12. When the movable die 34 moves in this manner, the second movable die 39 first comes into contact with the vertical plate 4 of the cap portion 5 of the preform 1. When the movable die 34 moves further, the second movable die 39 presses the vertical plate 4 of the preform 1 against the outer surface 35c of the punch 35 for main forming, as shown in Fig. 13. At this time, the second rounding portion 41a of the second movable die 39 comes into contact with the vertical plate 4 and slides against it, thereby pressing the vertical plate 4 toward the outer surface 35c of the punch 35 for main forming.

[0034] The vertical plate 4 is pressed against the final forming punch 35, so that the flange portion 6 approaches the final forming punch 35. Then, as shown in FIG. 14, the convex portion 41 of the second movable die 39 is inserted into the groove 42 between the final forming punch 35 and the restraining piece 36 and reaches the forming end position, so that the opening of the cap portion 5 and the flange portion 6 are accommodated in the groove 42. The movement trajectory of the tip of the flange portion 6 at this time is shown by a two-dot chain line L in FIG. 15. That is, the tip of the flange portion 6 passes near the chamfered portion 37 on the groove side of the restraining piece 36 and enters the groove 42 without coming into contact with the restraining piece 36. The tip surface (fourth plane 41b) of the convex portion 41 of the second movable die 39 is formed to be smaller than the flange portion 6. For this reason, when the convex portion 41 presses the flange portion 6, the tip side of the flange portion 6 is not supported by anything, and even if the flange portion 6 comes into contact with the chamfered portion 37 of the restraining piece 36, the tip of the flange portion 6 will slide on the chamfered portion 37 while bending.

[0035] The main molding upper die 33 continues to move toward the fixed die 32 even after the second movable die 39 enters the groove 42. At this time, the second movable die 39 presses the opening portion of the cap portion 5 and the flange portion 6 against the fixed die 32 by the spring force of the gas spring 40. In this manner, the opening portion of the hat portion 5 and the flange portion 6 are sandwiched between the second movable mold 39 and the fixed mold 32, whereby they are molded into a shape close to the final shape. The process in which the opening portion of the hat portion 5 and the flange portion 6 are sandwiched between the fixed mold 32 and the second movable mold 39 in this manner corresponds to the second main molding process S2B in the main molding process S2.

[0036] When the second movable die 39 presses the preform 1 against the fixed die 32 in the second main molding process S2B, a part of the vertical plate 4 escapes toward the excess pad portion 3. As a result, as shown in Fig. 14, the top plate 2 of the cap portion 5 is pressed from both sides, and the central portion of the top plate 2 is deformed and curved so as to be convex upward.

[0037] Thereafter, when the main forming upper die 33 moves further, the first movable die 38 comes into contact with the excess pad portion 3 of the preformed product 1 as shown in Fig. 16, and presses it toward the main forming punch 35. When the first movable die 38 comes into contact with the preformed product 1 in this manner, the third main forming step S2C of the main forming step S2 is started. The excess pad portion 3 is deformed to become flat by being pressed by the first movable die 38, and presses the vertical plate 4 toward the opening side of the cap portion 5 as shown by the arrow A in Fig. 17. By pressing the vertical plate 4 in this manner, the flange portion 6 connected to the vertical plate 4 is also pressed toward the tip side as shown by the arrow B.

[0038] Since the spring force of the gas spring 40 is smaller than the force with which the first movable die 38 presses the excess pad portion 3, the flange portion 6 can be displaced toward the restraining piece 36 while being pressed by the second movable die 39. When the flange portion 6 is displaced by being pressed from the vertical wall side, the tip surface 6a of the flange portion 6 abuts against the restraining piece 36, and further displacement of the flange portion 6 is restricted. In other words, the opening portion of the cap portion 5 and the flange portion 6 are restrained by the fixed die 32 and the second movable die 39. With the tip of the flange portion 6 in contact with the restraining piece 36 in this manner, the second movable die 39 clamps and presses the preform 1 between it and the fixed die 32. When the excess material portion 3 of the preform 1 is compressed by the first movable die 38 in this state, the material flows from the excess material portion 3 to the flange portion 6, and the preform 1 can be molded so that the open shape of the preform 1 becomes straight.

[0039] Just before the preform 1 is pressed by the first movable die 38, tensile stress P1 and compressive stress P2 remain inside the top plate 2 and excess pad 3 of the preform 1, as shown by the arrows in Fig. 18(A). That is, tensile stress P1 remains in the portion that has been stretched by molding, and compressive stress P2 remains in the portion that has been compressed by molding. When the first movable die 38 presses the preform 1 in this state toward the fixed die 32, a part of the excess pad 3 is brought toward the center of the top plate 2, and the center of the top plate 2 is curved even more significantly, so that the top plate 2 and excess pad 3 of the preform 1 are compressed while being deformed into a corrugated shape.

[0040] Thereafter, as shown in FIG. 19, the first movable die 38 further presses the preformed product 1, and while the preformed product 1 is pressed against the fixed die 32 by the second movable die 39, the entire molding surface (the third flat surface 38a, the concave curved surface 38b and the inner surface 38c) of the first movable die 38 comes into close contact with the preformed product 1 and presses it against the main molding punch 35. As a result of the first movable die 38 coming into close contact with the preform 1, the portions of the top plate 2 and the excess pad 3 of the preform 1 that were stretched to generate tensile stress P1 are compressed (compressive stress P2 acts on them) as shown in Fig. 18(B). Also, the portions that were compressed to generate compressive stress P2 are stretched (tensile stress P1 acts on them). In the process in which the top plate 2 and the excess pad 3 are compressed into their final shape while being deformed into a corrugated shape, that is, in the process of deformation and compression, the tensile stress P1 and the compressive stress P2 cancel each other out, and springback is suppressed.

[0041] In this manner, the first movable mold 38 is brought into close contact with the preform 1, whereby the hat portion 5 and the flange portion 6 of the preform 1 are molded into the final shapes of the hat portion 15 and the flange portion 6 while compressive stress is generated in the planar direction of the hat portion 5 and the flange portion 6. The "surface direction" here refers to a direction perpendicular to the thickness direction, which is the left-right direction in the top plate 2 and flange portion 6 of the hat portion 5, and the up-down direction in the vertical plate 4. The third main forming step S2C is completed when the first movable die 38 cannot move any further. After the third main forming step S2C is completed, the movable die 34 moves away from the fixed die 32, and the molded product 12 in the final shape is obtained. In the shoulder portion 15a of the molded product 12 in the final shape and the connection portion 15b between the vertical plate 14 of the hat portion 15 and the flange portion 16, the tensile stress generated on the outer periphery of the curved portion is reduced. Therefore, the molded product 12 in the final shape has a significantly reduced occurrence of springback.

[0042] Therefore, according to the press molding method and press molding apparatus 18 of this embodiment, the preform 1 can be compressed to mold the molded product 12 in its final shape by moving the first and second movable dies 38, 39 only in the height direction of the cap portion 15. The press molding apparatus 18 of this embodiment does not use a cam mechanism used in conventional press molding apparatuses, and therefore has a simple structure. Therefore, according to this embodiment, it is possible to provide a press-forming method and a press-forming apparatus that can impart compressive stress to a workpiece and prevent springback using simple equipment.

[0043] In the fixed die 32 and the second movable die 39 according to this embodiment, the portions for forming the connection portion 15b between the vertical plate 14 of the cap portion 15 and the flange portion 16 are formed by the first rounded portion 35d and the second rounded portion 41a, each of which has a curved cross-sectional shape. In the second main forming step S2B, the second rounded portion 41a of the second movable die 39 presses the vertical plate 4 of the preformed product 1 toward the fixed die 32 while the second movable die 39 is moving to the forming end position. Therefore, the preformed product 1 expanded by springback can be formed by squeezing it toward the flange portion 6 by the second movable die 39.

[0044] In this embodiment, a chamfered portion 37 is formed at the tip of the restraining piece 36. In the second main forming process S2B, the tip of the flange portion 6 passes close to the chamfered portion 37. Therefore, the restraining piece 36 can be positioned so that the flange portion 6 fits between the restraining piece 36 and the main forming punch 35 while avoiding interference between the flange portion 6 and the restraining piece 36. Therefore, the final shape of the formed product 12 is compressed reliably when the third main forming process S2C is performed, and the occurrence of springback can be further reduced.

[0045] The restraining piece 36 in this embodiment is formed as a mold part separate from the other parts of the fixed mold 32 and is configured to be detachable. The boundary between the bottom surface of the groove 42 and the restraining piece 36 forms a right angle. The tip surface of the flange portion 16 of the molded product 12 in its final shape abuts against the restraining wall 36 with no gap. If a protrusion equivalent to the restraining piece 36 were formed integrally with the lower die 31 for main forming, a rounded portion would be generated at the boundary between the bottom surface of the groove 42 and the protrusion. In this case, the tip surface of the flange portion 16 would not be able to abut against the wall in the third main forming process S2C, and the flange portion 16 of the molded product 12 in its final shape would not be able to be formed in the correct shape. However, by forming the restraining piece 36 separately from the other parts of the fixed die 32 as shown in this embodiment, a wall (workpiece abutment surface) against which the tip surface of the flange portion 16 abuts without any gaps can be easily formed by the right-angle forming portion. [Explanation of symbols]

[0046] 1...preliminary formed product, 2,13...top plate, 3...excess material portion, 4,14...vertical plate, 5,15...cap portion, 6,16...flange portion, 11...die for preliminary forming, 12...final shaped formed product, 15a...shoulder portion, 15c...inner surface of cap portion, 17...die for main forming, 18...press forming device, 32...fixed die, 35d,41a...rounding portion, 36...restraint piece (wall), 37...chamfered portion, 38...first movable die, 39...second movable die, S1...preliminary forming process, S2...main forming process, S2A...first main forming process, S2B...second main forming process, S2C...third main forming process.

Claims

1. a preforming step of forming a hat-shaped cross-section molded product having a flange at an open end of a hat portion composed of a top plate and both vertical plates using a preforming die such that molding allowances remain on both shoulder portions of the hat portion; a main molding step of compressing the preform formed in the preforming step by a mold for main molding to form a molded product in a final shape, The mold for this molding is a fixed mold having a wall against which a tip end surface of the flange portion comes into contact, a convex portion that fits into the hat portion, and a concave portion formed between the wall and the convex portion, the fixed mold having a molding surface for molding an inner surface of the molded product having a final shape; a first movable die configured to be movable in a height direction of the hat portion and configured to sandwich the top plate side of the hat portion between the first movable die and the convex portion of the fixed die to mold the hat portion into a final shape; a second movable mold configured to be movable in a height direction of the hat portion and configured to sandwich the preform including the opening end side of the hat portion between the second movable mold and the recess of the fixed mold to mold the preform into a final shape; The main molding step includes: a first main molding step of placing the preform on the protruding portion of the fixed mold; a second main molding step of sandwiching the preform between the recess of the fixed mold and the second movable mold; a third main molding step of molding the preform into a final shape by the convex portion of the fixed die and the first movable die in a state in which the preform is pressed against the concave portion of the fixed die by the second movable die, A pressing force when the second movable mold presses the preformed product in the second main molding step is smaller than a pressing force when the first movable mold presses the preformed product in the third main molding step. A press molding method comprising the steps of:

2. The press molding method according to claim 1, a portion of the fixed die and a portion of the second movable die that form a connection between the vertical plate of the hat portion and the flange portion are formed by a rounded portion having a curved cross-sectional shape, A press molding method characterized in that, in the second main molding process, the rounding portion of the second movable mold pushes the vertical plate of the preform toward the fixed mold as the second movable mold moves to a molding end position.

3. The press molding method according to claim 2, A chamfered portion is formed at the tip of the wall of the fixed mold, a tip portion of the flange portion passing a position adjacent to the chamfered portion in the second main forming step.

4. The press molding method according to claim 1, The wall of the stationary mold is formed by a mold part separate from other parts of the stationary mold and is configured to be detachable, A press molding method characterized in that the boundary between the bottom surface of the recess and the wall has a right angle shape, and in a third main molding process, the tip surface of the flange portion abuts against the wall without any gap.

5. a preforming die for forming a hat-shaped cross-section having a flange at an open end of a hat portion including a top plate and vertical plates on both sides, with molding allowances remaining on shoulders on both sides of the hat portion; and a mold for main molding which compresses the preform molded by the mold for preforming to mold a molded product of a final shape, The mold for this molding is The mold for this molding is a fixed mold having a wall against which a tip end surface of the flange portion comes into contact, a convex portion that fits into the hat portion, and a concave portion formed between the wall and the convex portion, the fixed mold having a molding surface for molding an inner surface of the molded product having a final shape; a first movable die configured to be movable in a height direction of the hat portion and configured to sandwich the top plate side of the hat portion between the first movable die and the convex portion of the fixed die to mold the hat portion into a final shape; a second movable mold configured to be movable in a height direction of the hat portion and configured to sandwich the preform including the opening end side of the hat portion between the second movable mold and the recess of the fixed mold to mold the preform into a final shape; the first movable die presses the preform against the convex portion of the fixed die while the second movable die presses the preform against the concave portion of the fixed die, thereby forming the preform into a final shape; A press molding apparatus, characterized in that a pressing force when said second movable die presses said preform is smaller than a pressing force when said first movable die presses said preform.

6. The press molding apparatus according to claim 5, A press molding apparatus characterized in that the portions of the fixed mold and the second movable mold that form the connection between the vertical plate of the hat portion and the flange portion are each formed by a rounded portion that has a curved cross-sectional shape.

7. The press forming apparatus according to claim 5, A chamfered portion is formed at the tip of the wall of the fixed mold, A press molding apparatus, characterized in that the chamfered portion is formed outside a movement trajectory of the flange portion that moves as the second movable die presses the preform against the fixed die.

8. The press forming apparatus according to claim 5, The wall of the stationary mold is formed by a mold part separate from other parts of the stationary mold and is configured to be detachable, A press molding apparatus characterized in that the boundary between the bottom surface of the recess and the wall forms a right angle, and the tip surface of the flange portion of the final molded product abuts against the wall without any gap.

Images