Method for bending metal plate material, die for bending metal plate material

Abstract

To provide bending method for manufacturing a bent article which has high dimensional accuracy and is suppressed in spring back, and in which there is no possibility of poor strength at a bent portion.SOLUTION: A metal plate bending method is provided for bending a metal plate as a raw material 3 using a punch 1, the punch 1 comprises a pair of punch die surfaces 1b, and a projecting portion 1c provided at a punch tip. An apical surface of the projecting portion 1c is a convex surface having a curvature radius equivalent to a length of 30% or more and 50% or less of a plate thickness of the raw material 3, and a length of the projecting portion 1c is equivalent to a length of 30% or more and 60% or less of the plate thickness of the raw material 3. A die 2 is equipped with V-shaped groove-like die recess portion for the pair of punch die surfaces 1b, and a bottom of the die recess portion is equipped with a concave surface 2b having a curvature radius equivalent to a length of 60% or less of the plate thickness of the raw material 3. The punch 1 and the die 2 are caused to approach each other, and the punch 1 and the die 2 are each caused to be closer to the raw material 3, and at least a part of the projection 1c of the punch 1 is press-fitted into the raw material 3.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a method for bending a metal plate material, This relates to a die for bending metal plates. [Background technology]

[0002] Exterior and interior building materials include components used in highly visible locations such as window frames and curtain walls. These components require precise dimensions without distortion, small diameters at bent parts, and sharp bend ridges for aesthetic appeal and aesthetic appeal. To ensure the strength required for structural components, there has been a recent trend toward using metal materials with high yield strength of 300 MPa or more for these components.

[0003] When high-strength metal sheets are bent, the dimensional accuracy after bending can be reduced. If the processing conditions are made stricter in order to improve dimensional accuracy, cracks may occur in the bent portion. Furthermore, strict processing conditions can also result in transfer flaws and scoring flaws, which can impair the design. This is particularly problematic when the sheet is used for building materials where aesthetics are important.

[0004] Patent Document 1 describes a grooved austenitic stainless steel sheet that can provide a grooved steel sheet material that can form sharp ridgelines with excellent design properties through bending. However, Patent Document 1 requires the formation of V-grooves on the surface of the steel sheet, which is disadvantageous in terms of cost. Furthermore, the formation of V-grooves on the surface of the steel sheet reduces the thickness of the steel sheet, which reduces the thickness of the bent part of the bent product after bending, which may result in insufficient strength.

[0005] Patent Document 2 describes a press-forming method in which, when bending a metal plate using a bending device consisting of a punch and a die, a recess having a depth of 20% or less of the metal plate thickness is formed on at least a portion of the convex side of the bent metal plate in the final step of the forming stroke. However, since the recess is formed on the convex side of the processed product by the press-forming method described in Patent Document 2, the aesthetic appearance may be impaired, making it difficult to use as a raw material for exterior or interior building materials. Furthermore, Patent Document 2 assumes that the raw material is a soft aluminum alloy plate, and therefore may not be applicable to bending metal plate materials with high yield strength.

[0006] Patent Document 3 describes a method of bending a workpiece having a thickness of t at a desired angle using a bending die comprising a die and punch, in which the vicinity of the portion of the die and punch where the workpiece is to be bent is made higher than the other die surfaces by a height of at most 10% of the thickness t, and the apex corner of the die is rounded with a radius of approximately t. However, with the bending method described in Patent Document 3, it is difficult to suppress springback after bending. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-169423 [Patent Document 2] Patent No. 3633012 [Patent Document 3] Special Publication No. 47-25266 Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention has been made in view of the above circumstances, and provides a bending method for metal plate material, which makes it possible to stably obtain bent products with high dimensional accuracy, suppress springback, and eliminate the risk of insufficient strength in the bent portion; The object of the present invention is to provide a die for bending metal plate materials. [Means for solving the problem]

[0009] In order to solve the above problems, the present invention employs the following configuration. [1] A placement step of placing a material made of a metal plate between a punch and a die; a forming step of bending the blank by bringing the punch and the die closer to each other, The material is a metal plate material that satisfies the following requirements: 0.2% proof stress is 300 MPa or more, yield ratio is 0.9 or less, and plate thickness t is 6 mm or less; the punch comprises a pair of punch die surfaces approaching each other toward the tip of the punch, and a protrusion provided at the tip of the punch, the tip surface of the protrusion being a convex curved surface with a radius of curvature corresponding to a length of 30% to 50% of the thickness of the material, and the length of the protrusion is a length corresponding to 30% to 60% of the thickness of the material, the die is provided with V-groove-shaped die recesses corresponding to the pair of punch die surfaces, and a concave curved surface having a curvature radius corresponding to a length of 20% to 60% of a plate thickness of the material is provided at a bottom of the die recess; The forming process is a process of bringing the punch and the die close to each other so that the punch and the die are in close contact with the material, and pressing at least a portion of the protrusion of the punch into the material. [2] The bending method for a metal plate material according to [1], wherein the protrusion has the convex curved surface and a side wall surface connected to the convex curved surface and defining the width of the protrusion. [3] A placement step of placing a material made of a metal plate between the punch and the die; a forming step of bending the blank by bringing the punch and the die closer to each other, The material is a metal plate material that satisfies the following requirements: 0.2% proof stress of 300 MPa or more, a yield ratio of 0.9.0 or less, and a plate thickness t of 6.0 mm or less; the punch comprises a pair of punch die surfaces which approach each other as they approach the tip of the punch, and a protrusion provided at the tip of the punch, the tip surface of the protrusion is a convex curved surface with a radius of curvature corresponding to a length of 30% to 50% of the plate thickness of the material, the length of the protrusion is a length corresponding to 30% to 60% of the plate thickness of the material, and the width of the protrusion is a length corresponding to 50% to 150% of the plate thickness of the material, and a concave curved surface is provided at the boundary between the pair of punch die surfaces and the protrusion, and the radius of curvature of the concave curved surface is a radius of curvature corresponding to a length of 150% or less of the plate thickness of the material, the die is provided with V-groove-shaped die recesses corresponding to the pair of punch die surfaces, and a concave curved surface having a curvature radius corresponding to a length of 20% to 60% of a plate thickness of the material is provided at a bottom of the die recess; The forming process is a process of bringing the punch and the die close to each other so that the punch and the die are in close contact with the material, and pressing at least a portion of the protrusion of the punch into the material. [4] The method for bending a metal plate material described in [3], wherein the protrusion has the convex curved surface, a side wall surface that forms the width of the protrusion, and an inclined surface between the convex curved surface and the side wall surface. [5] A die for bending a material made of a metal plate material that satisfies the following requirements: 0.2% proof stress of 300 MPa or more, a yield ratio of 0.9 or less, and a plate thickness t of 6 mm or less. a punch having a pair of punch die faces approaching each other toward a tip of the punch and a protrusion provided at the tip of the punch; a die including a die block provided with V-groove-shaped die recesses corresponding to the pair of punch die surfaces; the tip surface of the protrusion is a convex curved surface having a radius of curvature corresponding to a length of 30% to 50% of the plate thickness of the material, and the length of the protrusion is a length corresponding to 30% to 60% of the plate thickness of the material, A die for bending metal plate material, wherein the bottom of the V-groove-shaped recess is provided with a concave curved surface having a radius of curvature corresponding to a length of 20% to 60% of the plate thickness of the material. [6] A die for bending metal plate material as described in [5], wherein the protrusion has the convex curved surface and a side wall surface connected to the convex curved surface and defining the width of the protrusion. [7] A die for bending a material made of a metal plate material that satisfies the following requirements: 0.2% proof stress of 300 MPa or more, a yield ratio of 0.90 or less, and a plate thickness t of 6.0 mm or less. a punch having a pair of punch die faces approaching each other toward a tip of the punch and a protrusion provided at the tip of the punch; a die including a die block provided with V-groove-shaped die recesses corresponding to the pair of punch die surfaces; a tip surface of the protrusion is a convex curved surface having a radius of curvature corresponding to a length of 30% to 50% of the plate thickness of the material, the length of the protrusion is a length corresponding to 30% to 60% of the plate thickness of the material, and the width of the protrusion is a length corresponding to 50% to 150% of the plate thickness of the material, and a concave curved surface is provided at the boundary between the pair of punch die surfaces and the protrusion, and the radius of curvature of the concave curved surface is a radius of curvature corresponding to a length of 150% or less of the plate thickness of the material, A die for bending metal plate material, wherein the bottom of the V-groove-shaped recess is provided with a concave curved surface having a radius of curvature corresponding to a length of 20% to 60% of the plate thickness of the material. [8] A die for bending metal sheet material as described in [7], wherein the protrusion has the convex curved surface, a side wall surface that forms the width of the protrusion, and an inclined surface between the convex curved surface and the side wall surface. [9] A plate part made of a metal plate material satisfying a 0.2% proof stress of 300 MPa or more, a yield ratio of 0.90 or less, and a plate thickness of 6.0 mm or less, and a bending part provided in the plate part, The radius of curvature of the convex curved surface on the outer side of the bent portion is a radius of a length corresponding to the plate thickness of the plate portion, A bent product made of a metal plate, wherein the minimum thickness of the bent portion is 0.75 times or more the plate thickness t of the plate portion.

[10] A bent product made of a metal plate material according to [9], in which a recess is provided along the longitudinal direction of the bent portion on the inner side of the bent portion, and a concave curved surface is provided on the inner surface of the recess.

[11] Vickers hardness HV in the plate part m Vickers hardness HV at the bent part r Ratio (HV r / HV m ) is in the range of 1.50 or more and 1.90 or less. [Effects of the Invention]

[0010] According to the bending method for metal sheet material of the present invention, when bending a workpiece using a punch and a die, a punch having a protrusion at its tip and a die having a V-groove-shaped die recess with a concavely curved bottom are used. By pressing at least a portion of the protrusion of the punch into the workpiece, the workpiece is pressed into the bottom of the die recess, resulting in material flow to the bottom of the die recess. Furthermore, since the radius of curvature of the concavely curved bottom of the die recess is set to a length equivalent to 60% or less of the workpiece thickness, the radius of curvature of the convexly curved surface on the outer side of the bent portion is reduced. Furthermore, by pressing the protrusion into the workpiece, the residual stress distribution in the bent portion is different from that in the bent portion when a punch without a protrusion is used. In conventional die bending, springback is thought to occur due to tensile residual stress on the outer side of the bend and compressive residual stress on the inner side of the bend. However, in the present invention, the residual stress distribution in the bent portion is changed compared to conventional methods, thereby reducing springback after die bending. Therefore, according to the present invention, it is possible to stably manufacture bent products with high dimensional accuracy using metal plate material that satisfies the requirements of a 0.2% proof stress of 300 MPa or more, a yield ratio of 0.90 or less, and a plate thickness of 6.0 mm or less. That is, according to the processing method of the present invention, it is possible to manufacture bent products in which the convex curved surface on the outside of the bent portion has a small radius of curvature and a sharp shape, and which have little springback after die bending.

[0011] Furthermore, according to the method for bending metal sheet material of the present invention, the width of the protrusion is set to 50 to 150% of the thickness of the material, and a concave curved surface with a predetermined radius of curvature is provided at the boundary between the punch die surface and the protrusion, thereby reducing the frequency of breakage of the protrusion when processing is repeated.

[0012] The die for bending metal sheet material of the present invention includes a punch with a protrusion at the tip and a die with a V-groove-shaped die recess with a concave curved bottom. By using this die as a die for bending a material made of metal sheet material that satisfies the following requirements: 0.2% proof stress of 300 MPa or more, yield ratio of 0.90 or less, and plate thickness of 6.0 mm or less, it is possible to stably produce bent products with high dimensional accuracy.

[0013] Furthermore, according to the die for bending metal sheet material of the present invention, the width of the protrusion is set to 50 to 150% of the thickness of the material, and a concave curved surface with a predetermined radius of curvature is provided at the boundary between the punch die surface and the protrusion, thereby reducing the frequency of breakage of the protrusion when repeatedly processed.

[0014] In the bent product made of the metal sheet material of the present invention, the radius of curvature of the convex curved surface on the outside of the bent portion is a radius of length equivalent to or less than the thickness of the sheet material, so that the radius of curvature of the convex curved surface on the outside of the bent portion is small and a sharp shape can be obtained. Furthermore, since the minimum thickness of the bent portion is 0.75 times or more the thickness of the sheet material, the strength of the bent portion can be made sufficient.

[0015] Furthermore, a bent product made of the metal plate material of the present invention has a recess formed along the longitudinal direction of the bent portion, and a concave curved surface is formed inside this recess. This recess is an indentation formed by the protrusion of the punch during die bending. By providing such an indentation, the bent portion of the bent product is in a state in which compressive residual stress is imparted in the thickness direction of the bent portion. Therefore, according to the present invention, springback resulting from the relationship between conventional compressive residual stress and tensile residual stress is reduced, and springback can be reduced. Furthermore, since the inner surface of the recess is a concave curved surface, cracks originating from the recess can also be prevented.

[0016] Furthermore, according to the bent product made of the metal plate material of the present invention, the Vickers hardness HV m Vickers hardness HV at the bent part r Ratio (HV r / HV m ) is in the range of 1.50 to 1.90, and the Vickers hardness of the bent portion is high. Therefore, even though the thickness of the bent portion is smaller than the plate thickness of the plate portion by providing a recess in the bent portion, sufficient strength can be imparted to the bent portion. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a schematic view showing a metal plate bending die according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram illustrating a method for bending a metal plate material according to a first embodiment of the present invention. [Figure 3] FIG. 3 is a schematic diagram illustrating a method for bending a metal plate material according to a first embodiment of the present invention. [Figure 4] FIG. 4 is a schematic diagram illustrating a method for bending a metal plate material according to a first embodiment of the present invention. [Figure 5] FIG. 5 is a cross-sectional view showing a bent product made of a metal plate according to the first embodiment of the present invention. [Figure 6] FIG. 6 is a schematic diagram illustrating a conventional method for bending a metal plate material. [Figure 7] FIG. 7 is a schematic diagram illustrating the length of the protrusions in the mold according to the first embodiment of the present invention. [Figure 8] FIG. 8 is a schematic view showing a metal plate bending die according to a second embodiment of the present invention. [Figure 9] FIG. 9 is a partially enlarged schematic view showing a main part of the punch of the die of FIG. [Figure 10] FIG. 10 is a partially enlarged view of FIG. 9, and is a schematic diagram illustrating the relationship between the range of the tip surface (angle θr) and the angle θq of the inclined surface. [Figure 11] FIG. 11 is a schematic diagram illustrating the length of the protrusions in the mold according to the second embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0018] (First embodiment) Below, we will explain a first embodiment of the present invention, which is a method for bending metal plate material (hereinafter sometimes referred to as a bending method), a mold for bending metal plate material (hereinafter sometimes referred to as a mold), and a bent product made of metal plate material (hereinafter sometimes referred to as a bent product).

[0019] The outline of this embodiment is as follows. The bending method of this embodiment includes a placement step of placing a blank 3 made of a metal plate between a punch 1 and a die 2, as shown in FIG. 2, and a forming step of bending the blank 3 by bringing the punch 1 and the die 2 relatively close to each other, as shown in FIGS. 3 and 4. The blank 3 is a metal plate satisfying the following requirements: a 0.2% proof stress of 300 MPa or more, a yield ratio of 0.90 or less, and a thickness t of 6.0 mm or less. As shown in FIG. 1, the punch 1 includes a pair of punch die faces 1b and a protrusion 1c provided on the punch tip 1a. The tip surface 1e of the protrusion 1c is a convex curved surface, and the length H of the protrusion 1c is a predetermined length. The die 2 is provided with a V-groove-shaped die recess 2a corresponding to the pair of punch die faces 1b. A concave curved surface 2b with a predetermined radius of curvature is provided on a bottom 2d of the die recess 2a. In the forming step, the punch 1 and the die 2 are brought close to each other so that the punch 1 and the die 2 are brought into close contact with the material 3, and at least a part of the protrusion 1c of the punch 1 is pressed into the material 3. As a result, the radius of curvature Rout of the convex curved surface 4c on the outside of the bent portion becomes small, as shown in Fig. 5, and a bent product 4 having a sharply shaped bent portion 4b is obtained. The present embodiment will be described below in order.

[0020] (material) The material 3 according to this embodiment is a metal plate material that satisfies the following requirements: a 0.2% proof stress of 300 MPa or more, a yield ratio of 0.90 or less, and a plate thickness t of 6.0 mm or less. The reasons for limiting the metal plate material suitable for the raw material 3 will be described below.

[0021] <0.2% yield strength> The 0.2% yield strength of the metal sheet material according to this embodiment is set to 300 MPa or more. This ensures sufficient strength when the metal sheet material according to this embodiment is used as a material for exterior and interior building materials. If the 0.2% yield strength is less than 300 MPa, the strength will be insufficient.

[0022] <Yield ratio> The yield ratio of the metal sheet material according to this embodiment is set to 0.90 or less. This makes it possible to suppress cracks in the bent portion, particularly on the outer surface of the bent portion, during bending. If the yield ratio exceeds 0.90, cracks will occur on the outer surface of the bent portion during bending, making it impossible to provide a bent product with excellent appearance. The ratio of yield strength (YS) to tensile strength (TS) is called the yield ratio (YS / TS), and here, 0.2% proof stress is defined as yield strength (YS).

[0023] <Thickness> The thickness t of the metal sheet material according to this embodiment is 6.0 mm or less, preferably 4.0 mm or less. The lower limit of the thickness t may be 0.5 mm or more, or may be 1 mm or more. By setting the thickness t to 6.0 mm or less, it is possible to reduce the springback of the bent product after bending.

[0024] (Mold) Next, a metal plate bending die used to manufacture the bent product of this embodiment will be described with reference to FIG. The die K according to this embodiment is a die for so-called die bending. That is, the die K according to this embodiment is used as a die for a processing method in which both ends 3a of the material 3 are used as free ends, and the die K (punch 1 and die 2) are pressed against the material 3 from above and below to bend it. The die K according to this embodiment will be described below.

[0025] The mold K according to this embodiment is composed of a punch 1 and a die 2.

[0026] As shown in Fig. 1, the punch 1 has a pair of punch die surfaces 1b that are inclined so as to approach each other toward the punch tip 1a, and a protrusion 1c provided on the punch tip 1a. The pair of punch die surfaces 1b have a relative angle θp that is, for example, in the range of 80 to 100°, preferably 85 to 95°, more preferably 88 to 92°, and even more preferably 90°. The pair of punch die surfaces 1b form the outer shape of the punch 1, and are inclined so as to approach each other toward the punch tip 1a. The punch tip 1a has a protrusion 1c.

[0027] The protrusion 1c protrudes from the punch tip 1a toward the die 2. As shown in Fig. 1, the protrusion 1c has a side wall surface 1d in contact with the punch die surface 1b and a tip surface 1e in contact with the side wall surface 1d. The side wall surface 1d is flat, and the tip surface 1e is a convex curved surface.

[0028] The tip surface 1e of the protrusion 1c is a convex curved surface with a curvature radius Rp. The curvature radius Rp of the tip surface 1e is a radius equivalent to a length of 30% to 50% of the thickness t of the raw material 3, i.e., a length of 0.30t to 0.50t. By setting the curvature radius Rp to a radius equivalent to a length of 30% or more (0.30t) of the thickness t of the raw material 3, sufficient compressive stress can be applied to the bent portion 4b, as described below, thereby reducing springback of the bent product 4. Furthermore, by setting the curvature radius Rp to a radius equivalent to a length of 50% or less (0.50t) of the thickness t of the raw material 3, the curvature radius Rout of the convex curved surface 4c on the outer side of the bent portion 4b after bending can be reduced. Furthermore, by setting the tip surface 1e to a convex curved surface, cracking of the bent portion 4b of the bent product 4 can be prevented.

[0029] The length H of the protrusion 1c is set to a length equivalent to 30% to 60% of the thickness t of the blank 3, i.e., a length equivalent to 0.30t to 0.60t. As shown in FIG. 1, the length H of the protrusion 1c is the length in the protruding direction of the protrusion 1c, which is the length from the boundary between the side wall surface 1d and the punch die surface 1b to the tip surface 1e. By setting the length H of the protrusion 1c to a length equivalent to 30% or more of the thickness t of the blank 3 (0.30t), material flow toward the bottom of the die recess 2a is promoted, thereby reducing the radius of curvature Rout of the convex curved surface 4c on the outer side of the bent portion 4b after bending. Furthermore, sufficient compressive stress can be applied to the bent portion 4b, thereby suppressing springback of the bent product. Furthermore, by setting the length H of the protrusion 1c to a length equivalent to 60% or less (0.60t) of the thickness t of the blank 3, when the punch 1 reaches the bottom dead center, the blank 3 can be tightly held between the punch 1 and the die 2 by being pressed against the punch die surface 1b of the punch 1 and the die recess 2a, thereby ensuring reliable bending. Furthermore, the protrusion 1c does not penetrate excessively into the blank 3, ensuring a sufficient thickness at the bent portion 4b of the bent product and suppressing spring go.

[0030] The length H of the protrusion 1c is defined as the length from the boundary between the side wall surface 1d and the punch die surface 1b to the tip surface 1e, but if the position of the boundary between the side wall surface 1d and the punch die surface 1b is unclear, the length H of the protrusion 1c may be determined as follows: That is, as shown in Fig. 7, the position of the intersection X between the extension of the contour of the punch die surface 1b and the extension of the contour of the side wall surface 1d is identified, and an imaginary horizontal line M is drawn from the intersection X in the direction of the width W of the protrusion 1c. Then, the distance between the horizontal line M and the tip of the protrusion 1c (the apex of the convex curved surface) is defined as the length H of the protrusion 1c.

[0031] The distance between the side wall surfaces 1d of the protrusions 1c, ie, the width W of the protrusions 1c, is preferably set to a length equivalent to 50% to 150% of the thickness t of the material 3 (0.50t to 1.50t).

[0032] The radius of curvature Rj of the concave curved surface 1g provided at the boundary between the pair of punch die surfaces 1b and the protrusion 1c is preferably set to a radius of curvature corresponding to a length of 150% or less of the thickness t of the material 3 (1.50t).

[0033] Next, the die 2 will be described. As shown in FIG. 1, the die 2 comprises a die block 2A having a die recess 2a formed therein. The die recess 2a is formed on the upper surface 2m of the die block 2A. The cross-sectional shape of the die recess 2a is a V-groove relative to the punch die surfaces 1b of the punch 1. That is, the die recess 2a is defined by a pair of die die surfaces 2c corresponding to the pair of punch die surfaces 1b and a concave curved surface 2b formed on the bottom 2d of the die recess 2a. The pair of die die surfaces 2c are inclined so as to approach each other toward the bottom 2d of the die recess 2a. The concave curved surface 2b is located between the pair of die die surfaces 2c. The pair of die die surfaces 2c and the concave curved surface 2b are in contact with each other.

[0034] The radius of curvature Rd of the concave curved surface 2b of the die recess 2a is set to a length that is 20% to 60% of the thickness t of the blank 3, i.e., a radius equivalent to a length of 0.20t to 0.60t. When bending is performed using a punch 1 with a protrusion, stress concentrates on the concave curved surface 2b of the die recess 2a. This stress concentration increases as the radius of curvature Rd of the concave curved surface 2b becomes smaller. When the radius of curvature Rd is less than 20% of the thickness t of the blank 3, the concentrated stress may exceed the fracture limit of the die 2, causing plastic deformation or cracking of the die 2. For this reason, the radius of curvature Rd of the concave curved surface 2b of the die recess 2a is set to be 20% or more of the thickness t of the blank 3 (0.20t). Furthermore, by setting the radius of curvature Rd to a radius of 60% (0.60t) or less of the plate thickness t of the raw material 3, a space in which the material can flow is secured at the bottom 2d of the die recess 2a, making it possible to reduce the radius of curvature Rout of the convex curved surface 4c on the outside of the bend in the bent portion 4b after bending processing, and also making it possible to apply an appropriate amount of compressive stress to the bent portion 4b so that spring back does not occur, thereby suppressing springback.

[0035] Next, the bending method of this embodiment will be described with reference to Figs. 2 to 4. In the bending method of this embodiment, a bent portion 4b is formed in the blank 3 by performing a die bending process on the blank 3 using a punch 1 and a die 2. The punch 1 and die 2 used in the bending process are as shown in Fig. 1. The bending method of this embodiment includes a placement step and a forming step.

[0036] (Placement process) In the placement step, as shown in FIG. 2, the blank 3 is placed on the upper surface 2m of the die block 2A of the die 2, and the punch 1 is placed above it. The blank 3 is placed on the die 2 so that the upper surface 3b faces the punch 1 and the lower surface 3c contacts the upper surface 2m of the die block 2A. In this way, the blank 3 is placed between the punch 1 and the die 2. Both ends 3a of the blank 3 are not constrained but are free ends. The punch 1 and die 2 are attached to a press molding machine (not shown).

[0037] (molding process) Next, in the forming step, the press molding machine is operated to bring the punch 1 and the die 2 relatively close to each other, and the blank 3 is subjected to a bending process.

[0038] In the forming process, as shown in Fig. 3, the punch 1 and the die 2 are brought relatively close to each other, and the protrusion 1c of the punch 1 is pressed against the upper surface 3b of the material 3. The material 3 against which the protrusion 1c is pressed is further pressed into the die recess 2a by the punch 1. As a result, bending deformation (plastic deformation) of the material 3 begins at the point where the protrusion 1c is in contact.

[0039] As the punch 1 and die 2 are brought closer together, the material 3 undergoes bending deformation (plastic deformation) and the lower surface 3c of the material comes into close contact with the die surface 2c of the die 2. Meanwhile, the lower surface 3c of the material 3 separates from the upper surface 2m of the die 2. At this stage, the lower surface 3c of the material does not come into contact with the concave curved surface 2b of the bottom 2d of the die 2, leaving a gap S1. Meanwhile, the upper surface 3b of the material 3 is in contact only with the protrusion 1c, and the upper surface 3b of the material 3 does not come into contact with the punch surface 1b. This creates a space S2 between the upper surface 3b of the material 3 and the punch surface 1b. There is still room to press the punch 1 further down until this space S2 is eliminated.

[0040] When the punch 1 and the die 2 are moved relatively close to each other until the punch surface 1b of the punch 1 contacts the top surface of the blank 3, the punch surface 1b and the die surface 2c contact the top surface 3b and bottom surface 3c of the blank 3, respectively, and the protrusions 1c are pressed into the inner surface of the bent portion 4b, as shown in FIG. 4 . The protrusions 1c apply compressive stress in the thickness direction to the bent portion 4b, resulting in a process similar to forging. This causes material flow in the bent portion 4b, filling part or all of the gap S1 that existed between the bottom surface 3c of the blank 3 and the concave surface 2b of the die 2. This reduces the radius of curvature Rout of the convex surface 4c on the outer side of the bent portion 4b. As a result of the above, a bent product 4 having the bent portion 4b, as shown in FIG. 5, is produced.

[0041] For comparison, Figure 6 shows a cross-sectional schematic diagram of a case where a punch 111 without protrusions is used. When a blank 3 is bent using a punch 111 without protrusions, the punch 111 and die 2 are moved relatively close to each other until the punch surface 111b and die surface 2c are in close contact with the upper surface 3b and lower surface 3c of the blank 3, respectively, as shown in Figure 6. However, because the punch 111 does not have protrusions, material flow does not occur in the bent portion 14b. Consequently, the bending process ends with a gap S1 remaining between the lower surface 3c of the blank 3 and the concave curved surface 2b of the die 2. Therefore, a bent product manufactured using a punch 111 without protrusions cannot reduce the radius of curvature Rout of the convex curved surface 4c on the outer side of the bent portion 14b. Furthermore, because compressive residual stress is not imparted in the thickness direction of the bent portion 14b, it is difficult to suppress springback.

[0042] Next, the bent product 4 according to this embodiment will be described. Fig. 5 shows a schematic cross-sectional view of the bent product according to this embodiment.

[0043] The bent product 4 according to this embodiment includes a plate portion 4a made of a metal plate having a 0.2% proof stress of 300 MPa or more, a yield ratio of 0.90 or less, and a plate thickness t of 6.0 mm or less, and a bent portion 4b provided in the plate portion 4a. The bent portion 4b has a convex curved surface 4c on the outside of the bend and a concave portion 4d on the inside of the bend.

[0044] The bending angle θ of the bent portion 4b is set in the range of 85° to 95°, more preferably in the range of 88 to 92°, and even more preferably at 90°. This means that when the target bending angle θ is 90°, the bending angle θ of the bent portion 4b is within a range of ±5° or ±2°. By setting the bending angle θ within the above range, the variation in the bending angle θ is reduced, and a sharp ridgeline is formed along the entire length of the outer side of the bend of the bent portion 4b.

[0045] The radius of curvature Rout of the convex curved surface 4c on the outside of the bend is set to a radius of a length equal to or less than the plate thickness of the plate portion 4a. By setting the radius of curvature Rout of the convex curved surface 4d on the outside of the bend to be equal to or less than the plate thickness of the plate portion 4a, the appearance of the outside of the bend of the bent portion 4b becomes angular and sharp, making it suitable for use as an exterior building material or an interior building material.

[0046] A recess 4d is provided on the inner side of the bent portion 4b. This recess 4d is an indentation formed by pressing in the protrusion 1c of the punch 1 in the final stage of the forming process. The recess 4d has a concave curved surface 4e formed by the tip surface 1e of the protrusion 1c. The inside of the recess 4d is the concave curved surface 4e, which prevents cracks from occurring starting from the recess 4d.

[0047] Furthermore, by providing the recess 4d on the inner side of the bend of the bent portion 4b, the thickness of the bent portion 4b becomes smaller than the plate thickness t of the plate portion 4a, but the minimum thickness of the bent portion 4b needs to be 0.75 times (0.75t) or more the plate thickness t of the plate portion 4a, thereby ensuring sufficient strength of the bent portion 4b.

[0048] In addition, the Vickers hardness HV m Vickers hardness HV at bent part 4b r Ratio (HV r / HV m ) is preferably in the range of 1.50 or more and 1.90 or less. In the bent portion 4b according to this embodiment, in addition to work hardening caused by plastic deformation of the raw material 3, work hardening is also caused by residual stress imparted in the thickness direction by the protrusion 1c during the forming process. Therefore, the hardness of the bent portion 4b of the bent part 4 according to this embodiment is higher than the hardness of the plate portion 4a, and the strength of the bent portion 4b increases. On the other hand, if the hardness of the bent portion 4b becomes too high, cracks are more likely to occur in the bent portion 4b. Therefore, the Vickers hardness HV of the plate portion 4a m Vickers hardness HV at bent part 4b r Ratio (HV r / HV m) is preferably in the range of 1.50 to 1.90.

[0049] The bent product 4 of this embodiment as described above can be suitably used as a material for exterior and interior building materials.

[0050] As described above, according to the bending method for metal sheet material of this embodiment, when performing die bending on the raw material 3 using the punch 1 and die 2, the punch 1 has a protrusion 1c on the punch tip 1a, and the die 2 has a V-groove-shaped die recess 2a with the bottom 2d having a concave curved surface 2b, and by using this, at least a portion of the protrusion 1c of the punch 1 is pressed into the raw material 4, so that the raw material 3 is pressed into the bottom 2d of the die recess 2a, and material flows into the bottom 2d of the die recess 2a.

[0051] In addition, since the radius of curvature of the concave curved surface 2b of the bottom 2d of the die recess 2a is set to a length equivalent to 20% or more and 60% or less of the plate thickness of the material 3, the radius of curvature Rout of the convex curved surface 4c on the outside of the bend of the bent portion 4b becomes small.

[0052] Furthermore, by press-fitting the protrusions 1c into the material 3, the residual stress distribution in the bent portion 4b can be made different from the residual stress distribution in the bent portion when die bending is performed using a punch without the protrusions 1c. That is, in conventional die bending, springback is thought to occur due to tensile residual stress on the outside of the bend and compressive residual stress on the inside of the bend, but in this embodiment, the residual stress distribution in the bent portion 4b changes compared to conventional methods, thereby reducing springback after die bending.

[0053] Therefore, according to this embodiment, a bent product 4 with high dimensional accuracy can be stably manufactured using a metal plate material 3 that satisfies the requirements of a 0.2% proof stress of 300 MPa or more, a yield ratio of 0.90 or less, and a plate thickness t of 6.0 mm or less. That is, according to the processing method according to this embodiment, the radius of curvature Rout of the convex curved surface 4c on the outside of the bent portion is small, resulting in a sharp shape, and a bent product 4 with little springback after die bending can be manufactured.

[0054] The die K for bending metal sheet material of this embodiment includes a punch 1 having a protrusion 1c at the punch tip 1a and a die 2 having a V-groove-shaped die recess 2a with a bottom 2d having a concave curved surface 2b. By using this die K as the die K when bending a raw material 3 made of a metal sheet material that satisfies the following requirements: a 0.2% proof stress of 300 MPa or more, a yield ratio of 0.90 or less, and a plate thickness t of 6.0 mm or less, it is possible to stably manufacture a bent product 4 with high dimensional accuracy.

[0055] According to the bent product 4 of this embodiment, the radius of curvature Rout of the convexly curved surface 4c on the outside of the bent portion 4b is a radius of a length equivalent to or less than the thickness t of the plate portion 4a, so that the radius of curvature Rout of the convexly curved surface 4c on the outside of the bent portion can be small and have a sharp shape. In addition, since the minimum thickness of the bent portion 4b is 0.75 times or more the thickness t of the plate portion 4a, the strength of the bent portion 4b can be made sufficient.

[0056] Furthermore, the bent product 4 of this embodiment has a recess 4d formed along the longitudinal direction of the bent portion 4b, and a concave curved surface 4e is formed inside this recess 4d. This recess 4d is an indentation formed by the protrusion 1c of the punch 1 during die bending. Due to the presence of this indentation, the bent portion 4b of the bent product 4 is in a state in which compressive residual stress is imparted in the thickness direction of the bent portion 4b. Therefore, according to this embodiment, springback resulting from the relationship between conventional compressive residual stress and tensile residual stress is alleviated, and springback can be reduced. Furthermore, because the inner surface of the recess 4d is the concave curved surface 4e, cracks originating from the recess 4d can also be prevented.

[0057] Furthermore, according to the bent product 4 of this embodiment, the Vickers hardness HV m Vickers hardness HV at bent part 4b r Ratio (HV r / HV m) is in the range of 1.50 to 1.90, and the Vickers hardness of the bent portion 4b is high. Therefore, even though the thickness of the bent portion 4b is smaller than the plate thickness t of the plate portion 4a by providing the recess 4d in the bent portion 4b, sufficient strength can be imparted to the bent portion 4b.

[0058] (Second embodiment) Next, a second embodiment of the present invention will be described. The second embodiment is different from the first embodiment in that the shape of the tip of the punch is changed. Specifically, by widening the width of the protrusion, it is possible to prevent damage to the tip of the punch that can occur when die bending is performed repeatedly. The die, bending method using the die, and bent product of this embodiment will be described below.

[0059] (Mold) As shown in Fig. 8, the die L according to this embodiment is a die for bending, similar to Fig. 1. The die L is composed of a punch 11 and a die 2. The shape of the die 2 is the same as in the first embodiment.

[0060] The punch 11 is provided with a pair of punch die surfaces 11b that are inclined so as to approach each other toward the punch tip 11a, and a protrusion 11c provided on the punch tip 11a. As in the first embodiment, the relative angle θp between the pair of punch die surfaces 11b is, for example, 80 to 100°, preferably 85 to 95°, more preferably 88 to 92°, and even more preferably 90°.

[0061] As shown in FIG. 9, the protrusion 11c has a pair of side wall surfaces 11d that define the width W of the protrusion 11c, a tip surface 11e (a convex curved surface) of the protrusion 11c, and an inclined surface 11f between the side wall surfaces 11d and the tip surface 11e. The side wall surfaces 11d are connected to the punch die surface 11b. A concave curved surface 11g with a curvature radius Rj is present between the side wall surfaces 11d and the punch die surface 11b. The side wall surfaces 11d are parallel to each other. The inclined surface 11f is connected to the side wall surfaces 11d, and the tip surface 11e is connected to the inclined surface 11f. The inclined surface 11f is inclined at an inclination angle θq with respect to the side wall surfaces 11d. Thus, the protrusion 11c is composed of the side wall surface 11d, the tip surface 11e (convex curved surface), and the inclined surface 11f, and there is a concave curved surface 11g with a curvature radius Rj between the protrusion 11c and the punch die surface 11b.

[0062] The tip surface 11e of the protrusion 11c is a convex curved surface with a curvature radius Rp. As in the first embodiment, the curvature radius Rp of the tip surface 11e is a radius corresponding to a length that is 30% to 50% (0.30t to 0.50t) of the thickness t of the raw material 3. By setting the curvature radius Rp to 30% (0.30t) or more of the thickness t of the raw material 3, sufficient compressive stress is applied to the bent portion 4b of the bent product 4, thereby reducing springback of the bent product 4. Furthermore, by setting the curvature radius Rp to 50% (0.50t) or less of the thickness t of the raw material 3, the curvature radius Rout of the convex curved surface 4c on the outer side of the bent portion 4b after bending can be reduced. Furthermore, by making the tip surface 11e a convex curved surface, cracks in the bent portion 4b of the bent product 4 can be prevented.

[0063] Furthermore, the tip surface 11e of this embodiment has an arc-shaped contour when viewed in cross section as shown in Fig. 9. When the range of tip surface 11e consisting of this arc-shaped contour line is defined by the central angle at the center O of the arc, the range of tip surface 11e preferably has an angle θr of 25° or more and 65° or less with respect to the normal direction L passing through the center of the width W direction of protrusion 11c. By defining tip surface 11e in such a range, the durability of punch 11 can be improved when repeated die bending is performed.

[0064] The inclined surface 11f is located on an extension of both ends of the tip surface 11e. The side wall surface 11d in contact with the inclined surface 11f extends parallel to the normal direction L. Therefore, by specifying the preferred range of the tip surface 11e, the angle θq of the inclined surface 11f with respect to the side wall surface 11d is set to a range of 25° to 65°. As shown in FIG. 10, since the relationship θr + θq = 90° holds, once the value of θr is determined, θq is also uniquely determined. By setting the angle θq of the inclined surface 11f within this range, the durability of the punch 11 can be improved when repeatedly bending.

[0065] The distance between the side wall surfaces 11d of the protrusions 11c, i.e., the width W of the protrusions 11c, is set to a length equivalent to 50% to 150% (0.50t to 1.50t) of the thickness t of the raw material 3. By setting the width W of the protrusions 11c to 50% (0.50t) or more of the thickness t of the raw material 3, stress concentration on the protrusions 11c during die bending is alleviated, preventing damage to the protrusions 11c when repeated die bending is performed. For the above reasons, the lower limit of the width W of the protrusions 11c is preferably 60% (0.60t) of the thickness t of the raw material 3, more preferably 80% (0.80t), and even more preferably 100% (1.00t). Furthermore, by setting the width W of the protrusions 11c to 150% (1.50t) or less of the thickness t of the raw material 3, the radius of curvature Rout of the convex curved surface 4c on the outer side of the bent portion 4b after bending can be made small and sharp. Furthermore, the protrusions 11c can be easily pressed into the material 3 during the forming process, and sufficient compressive stress can be applied to the bent portions 4b, thereby suppressing springback of the bent product 4.

[0066] As in the first embodiment, the length H of the protrusion 11c is set to a length (0.30t to 0.60t) equivalent to 30% to 60% of the thickness t of the blank 3. By setting the length H of the protrusion 11c to 30% (0.30t) or more of the thickness t of the blank 3, the flow of material toward the bottom of the die recess 2a is promoted, thereby reducing the radius of curvature of the bent portion 4b on the outer side of the bend after bending. Furthermore, sufficient compressive stress can be applied to the bent portion 4b, thereby suppressing springback of the bent product 4. Furthermore, by setting the length H of the protrusion 11c to 60% (0.60t) or less of the thickness t of the blank 3, the blank 3 can be tightly attached to the punch die surface 11b of the punch 11 and the die recess 2a when the punch 11 reaches the bottom dead center, and the blank 3 can be confined between the punch 11 and the die 2, thereby ensuring reliable bending. Furthermore, the protrusion 11c does not penetrate excessively into the material 3, ensuring a sufficient thickness at the bent portion 4b of the bent product and suppressing spring go.

[0067] The length H of the protrusion 11c is the length of the protrusion 11c in the protruding direction, as shown in Fig. 11, and is determined as follows: That is, as shown in Fig. 11, the position of the intersection X between the extension of the contour line of the punch die surface 11b and the extension of the contour line of the side wall surface 11d is identified, and an imaginary horizontal line M is drawn from the intersection X in the width W direction of the protrusion 11c. Then, the distance between the horizontal line L and the tip of the protrusion 11c (the apex of the convex curved surface) is defined as the length H of the protrusion 11c.

[0068] The curvature radius Rj of the concave curved surface 11g located at the boundary between the pair of punch die surfaces 11b and the protrusions 11c is set to a curvature radius corresponding to a length of 150% or less (1.50t) of the sheet thickness t of the blank 3. By setting the curvature radius Rj of the concave curved surface 11g to 150% or less (1.50t), sufficient compressive stress can be applied to the bent portion 4b during the forming process, thereby suppressing springback of the bent product 4. Furthermore, when die bending is performed using the punch 11 of this embodiment, the concave curved surface 11g located between the protrusions 11c and the punch die surfaces 11b is subject to the greatest stress concentration, making it a likely starting point for breakage. To mitigate stress concentration, the curvature radius Rj of the concave curved surface 11g is preferably set to 40% or more (0.40t) of the sheet thickness t of the blank 3. Setting the radius of curvature Rj to be 40% (0.40t) or more of the thickness t of the raw material 3 prevents stress concentration on the concave curved surface 11g during die bending, thereby suppressing damage to the punch 11. For the reasons described above, the lower limit of the radius of curvature Rj of the concave curved surface 11g is preferably 50% (0.50t) of the thickness t of the raw material 3, more preferably 60% (0.60t), and even more preferably 70% (0.70t).

[0069] The bending method of this embodiment is similar to that of the first embodiment except for the use of a die L, and forms a bent portion in the blank 3 by performing die bending on the blank 3 using a punch 11 and a die 2. The punch 11 and die 2 used in the bending are as shown in Figures 8 and 9. As with the first embodiment, the bending method of this embodiment sequentially performs a placement step and a forming step.

[0070] In the placement step, the raw material 3 is placed on the upper surface 2m of the die 2, as in the case of Fig. 2, and the punch 11 shown in Fig. 8 and Fig. 9 is placed above it. Both ends of the raw material 3 are not constrained but are left free ends.

[0071] Next, in the forming process, as in the case of FIG. 3, the punch 11 and the die 2 are brought relatively close to each other, and the blank 3 is subjected to a die bending process. The blank 3, against which the protrusions 11c are pressed, is pressed into the die recesses 2a by the punch 11. As a result, bending deformation (plastic deformation) of the blank 3 begins at the points where the protrusions 11c are in contact. Then, as in the case of FIG. 4, the punch 11 and the die 2 are brought relatively close to each other until the punch die surface 11b of the punch 11 comes into close contact with the upper surface 3b of the blank 3. As a result, the protrusions 11c are pressed into the inner side of the bent portion of the blank 3, and the protrusions 11c apply compressive stress in the thickness direction to the bent portion, resulting in a process similar to forging. Material flow occurs in the bent portion, and the radius of curvature of the convex curved surface on the outer side of the bent portion becomes smaller. In this way, a bent product is produced.

[0072] According to this embodiment, a bent product equivalent to the bent product 4 obtained in the first embodiment can be obtained. Furthermore, by using the die L of this embodiment, damage to the punch 11 can be prevented even if die bending is performed repeatedly.

[0073] According to the mold L and the method for bending metal sheet material of this embodiment, the width W of the protrusion 11c of the tip 11a of the punch 11 is set to a length equivalent to 50% or more and 150% or less of the plate thickness t of the material 3, so that stress concentration on the protrusion 11c during die bending is alleviated, and damage to the protrusion 11c when die bending is performed repeatedly can be prevented.

[0074] Furthermore, according to the die L and the method for bending a metal plate material of this embodiment, a concave curved surface 11g is provided at the boundary between the pair of punch die surfaces 11b and the protrusion portion 11c, and the radius of curvature Rj of the concave curved surface 11g is set to a radius of curvature corresponding to a length that is 150% or less of the plate thickness t of the material 3. This prevents stress concentration on the concave curved surface 11g during die bending, and suppresses damage to the punch 11 when die bending is performed repeatedly.

[0075] Furthermore, according to the die L and the method for bending a metal plate of this embodiment, the protrusion 11c has a tip surface (convex curved surface) 11e, a side wall surface 11d, and an inclined surface 11f. Because the inclined surface 11f is provided between the tip surface 11e and the side wall surface 11d of the protrusion 11c, the width W of the protrusion 11c can be ensured even if the radius of curvature Rp of the tip surface 11e is reduced. This improves the durability of the punch when repeatedly bending, and also enables the radius of curvature Rout of the convex curved surface 4c on the outside of the bent portion 4b after bending to be small, resulting in a sharp shape.

[0076] The technical scope of the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. As the metal material for the bending method and bent product of the above embodiment, for example, a ferritic-austenitic duplex stainless steel sheet having a 0.2% proof stress of 300 MPa or more, a yield ratio of 0.90 or less, and a thickness t of 6.0 mm or less is preferably used. Duplex stainless steel sheets have sufficient strength, are less expensive than austenitic stainless steel sheets, and have better corrosion resistance than ferritic stainless steel sheets. However, because they have higher proof stress than austenitic stainless steels and ferritic stainless steels, bending them can result in reduced dimensional accuracy. If the bending conditions are made stricter to improve dimensional accuracy, cracks may occur in the bent portion. Furthermore, stricter bending conditions can result in transfer flaws and scoring flaws, which can impair the design and aesthetic appeal. For this reason, it has been difficult to apply duplex stainless steel sheets to exterior and interior building materials that require bending in highly visible locations. By applying the present invention, bending can be performed without causing such problems, and a bent product with excellent design and aesthetic appeal can be provided.

[0077] Such stainless steel sheet may be, for example, a ferritic-austenitic duplex stainless steel sheet having a chemical composition, in mass %, of C: 0.050% or less, Si: 2.00% or less, Mn: 1.00 to 8.00%, P: 0.050% or less, S: 0.0500% or less, Ni: 0.10 to 6.00%, Cr: 17.00 to 25.00%, Mo: 0.10 to 2.50%, Cu: 0.10 to 3.00%, N: 0.080 to 0.300%, with the remainder being Fe and impurities. Furthermore, the above stainless steel sheet may further contain, in mass %, one or more of Sn: 0.01 to 1.00%, W: 0.01 to 1.00%, and V: 0.01 to 1.00%, in place of a portion of Fe. Furthermore, the above stainless steel sheet may contain, in mass %, one or more of the following instead of part of the Fe: Ca: 0.0050% or less, Mg: 0.0050% or less, Al: 0.50% or less, and rare earth elements: 0.50% or less. Furthermore, the above stainless steel sheet may further contain, in mass %, 0.100% or less Nb in place of a portion of Fe.

[0078] To ensure the corrosion resistance of the steel sheet, the C content may be set to 0.050% or less, preferably 0.040% or less. On the other hand, since C is an element that forms austenite, which constitutes a dual-phase structure, the C content may be set to preferably 0.005% or more, more preferably 0.010% or more.

[0079] For deoxidation, Si is preferably contained in an amount of 0.01% or more, more preferably 0.10% or more, and even more preferably 0.30% or more. Furthermore, if Si is contained in an amount exceeding 2.00%, the precipitation of the σ phase is promoted, so the Si amount is preferably 2.00% or less, more preferably 0.60% or less.

[0080] Mn is a deoxidizing agent and an austenite stabilizing element for forming a dual-phase structure, and should be contained in an amount of 1.00% or more, preferably 1.50% or more, and more preferably 2.00% or more. On the other hand, since a Mn content exceeding 8.00% deteriorates corrosion resistance, the Mn content should be 8.00% or less, preferably 5.00% or less, and more preferably 4.00% or less.

[0081] Since P deteriorates hot workability and toughness, the P content is limited to 0.050% or less, preferably 0.035% or less. On the other hand, since an excessive reduction in the P content increases refining costs, it may be set to 0.005% or more.

[0082] Since S deteriorates hot workability, toughness, and corrosion resistance, the S content is limited to 0.0500% or less, preferably 0.0100% or less, and more preferably 0.0010% or less. On the other hand, since an excessive reduction in the S content increases raw material costs and refining costs, it may be set to 0.0003% or more.

[0083] When Ni is contained in a coating of a steel sheet, it has the effect of suppressing the occurrence of pitting corrosion when the Fe concentration of the coating is high, and the effect of suppressing the progression of corrosion once corrosion has occurred. On the other hand, if the Ni content is excessive, the Cr concentration of the coating decreases too much, making it impossible to obtain sufficient corrosion resistance. Therefore, the Ni content is preferably set in the range of 0.10 to 6.00%. The lower limit of the Ni content may preferably be 0.30% or more, 0.50% or more, or 1.00% or more. The upper limit of the Ni content may preferably be 5.00% or less, 4.00% or less, or 3.00% or less.

[0084] In order to impart sufficient corrosion resistance to the steel, the Cr content is preferably in the range of 17.00 to 25.00%. The lower limit of the Cr content is preferably 18.00% or more, more preferably 20.00% or more, and even more preferably 21.00% or more. The upper limit of the Cr content is preferably 24.00% or less, more preferably 22.00% or less.

[0085] Mo is an element that improves corrosion resistance, and its effect is exhibited when it is contained in an amount of 0.10% or more. Mo may be contained up to 2.50%, but if the Mo content exceeds 2.50%, the σ phase is likely to precipitate during hot working. Therefore, the lower limit of the Mo content is 0.10% or more, preferably 0.20% or more, and more preferably 0.50% or more. The upper limit of the Mo content is 2.50% or less, preferably 2.00% or less, and more preferably 1.50% or less.

[0086] When Cu is contained in an amount of 0.10% or more, the effect of suppressing the progression of corrosion when corrosion occurs is obtained. Cu may be contained in an amount of 3.00% or less. Furthermore, if the Cu content exceeds 3.00%, cracks may be more likely to occur during casting. Therefore, the lower limit of the Cu content is 0.10% or more, preferably 0.30% or more, and more preferably 0.50% or more. The upper limit of the Cu content is 3.00% or less, preferably 2.50% or less, and more preferably 2.00% or less.

[0087] N is an element that significantly improves corrosion resistance and increases the amount of austenite phase, and as an austenite stabilizing element, it is preferable that the content be 0.080% or more. The lower limit of the N content is preferably 0.150% or more. On the other hand, if the N content is excessive, nitrides are formed in the steel, reducing corrosion resistance and toughness, so the upper limit of the N content should be 0.300% or less. The upper limit of the N content is preferably 0.250% or less.

[0088] Furthermore, the inclusion of a small amount of Sn improves corrosion resistance. Therefore, Sn is a useful element for improving corrosion resistance, and may be included within a range that does not impair cost. If the Sn content is less than 0.001%, the effect of improving corrosion resistance is not achieved, and if the Sn content exceeds 1.00%, cost increases become apparent and workability also deteriorates. Therefore, the appropriate range of Sn content is set to 0.001 to 1.00%. The lower limit of the Sn content is preferably 0.01% or more, and the upper limit of the Sn content is preferably 0.50% or less. Note that Sn is an optional element and need not be included. In other words, the Sn content may be 0%.

[0089] V and W may be contained as needed to improve corrosion resistance, particularly crevice corrosion resistance. However, excessive V or W content reduces workability and saturates the effect of improving corrosion resistance. Therefore, the lower limit of each of the V and W contents is set to 0.01% or more, and the upper limit of each of the V and W contents is set to 1.00% or less. The lower limit of each of the V and W contents is preferably 0.04% or more, and the upper limit of each of the V and W contents is preferably 0.50% or less. Note that V and W are optional elements and may not be contained. In other words, the V content and the W content may each be 0%.

[0090] Ca, Mg, and REM are elements that improve hot workability, and for that purpose, one or more of Ca, Mg, and REM may be contained. Since the effects of Ca and Mg are manifested at a content of 0.0002% or more, the lower limit of each of the Ca and Mg contents is set to 0.0002% or more. In the case of REM, the lower limit is set to 0.001% or more. Note that Ca, Mg, and REM are optional elements and do not necessarily need to be contained. In other words, the Ca content, Mg content, and REM content may each be 0%.

[0091] However, since excessive content of either element adversely reduces hot workability, it is preferable to set the upper and lower limits of their contents as follows: the amounts of Ca and Mg are each 0.0002 to 0.0050%, and the amount of REM is 0.001 to 0.50%.

[0092] The lower limit of each of the amounts of Ca and Mg is preferably 0.0005% or more. The upper limit of each of the amounts of Ca and Mg is preferably 0.0015% or less. The lower limit of the REM amount is preferably 0.005% or more, and the upper limit of the REM amount is preferably 0.30% or less.

[0093] Here, rare earth elements (REM) are generally defined as two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). They may be contained alone or as a mixture. The amount of REM is the total amount of these elements.

[0094] Al is useful as a deoxidizing element, but should not be contained in large amounts because it deteriorates workability. The upper limit of the Al content should be limited to 0.50% or less. The preferred range of the Al content is 0.30% or less. The lower limit of the Al content is 0.01% or more. Note that Al is an optional element and does not necessarily need to be contained. In other words, the Al content may be 0%.

[0095] Nb has a strong affinity with N and forms nitrides preferentially with chromium, thereby suppressing the decrease in the chromium content in the material and improving corrosion resistance. Therefore, Nb may be added as needed. However, excessive Nb content reduces toughness, so the Nb content is set to 0.100% or less, and may be 0.060% or less. On the other hand, to obtain the above effects, the Nb content is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.020% or more. Note that Nb is an optional element and does not necessarily need to be added. In other words, the Nb content may be 0%.

[0096] The balance other than the above-mentioned elements is Fe and impurities, but elements other than those described above may also be contained within a range that does not impair the effects of this embodiment.

[0097] By using stainless steel having the above chemical composition as the material for the bent product 4, the bent product 4 of this embodiment has excellent strength and corrosion resistance and can be suitably used as an exterior building material or an interior building material.

[0098] Furthermore, for example, the metal material may be a metal plate material that further satisfies the requirements of a tensile strength of 750 MPa or more and / or a total elongation of 20% or more. [Example]

[0099] Example 1 A ferritic-austenitic duplex stainless steel sheet was prepared with the chemical composition shown in Table 1 and the thickness t shown in Table 2. The mechanical properties of the ferritic-austenitic duplex stainless steel sheet were as shown in Table 1. The metal plate material made of the ferritic-austenitic duplex stainless steel sheet was cut into a size of 100 mm in length and 30 mm in width to prepare the raw material. In Table 2, YS means yield strength, TS means tensile strength, and T.El means total elongation, and 0.2 proof stress is shown as yield strength (YS).

[0100] A mold equipped with a punch and a die as shown in Figure 1 was also prepared. The punch used had a protrusion having a convex curved surface and a side wall surface connected to the convex curved surface. The relative angle θp of the punch die surface of the punch was 90°. The punch had a width of 80 mm and a length (length in the depth direction of Figure 1) of 60 mm. The width of the die recess was 80 mm and the length (length in the depth direction of Figure 1) of the die recess was 60 mm.

[0101] The punch and die were then attached to a press molding machine, and the material was placed between the punch and die as shown in Figure 2. The material was placed so that the length direction of the material coincided with the depth direction in Figure 2. The center of the material in the width direction was also aligned with the center of the mold. The press molding machine was then operated to move the punch and die closer to each other until they were in close contact with the material, thereby V-bending the material and forming a bent portion. In this way, a bent product having a bent portion was produced.

[0102] The bending angle θ of the bent portion of the obtained bent product was measured. Four measurement points were set at 1 cm intervals along the longitudinal direction of the bent portion, and the bending angle θ on the inner side of the bent portion was measured at each measurement point. The average bending angle θ at the four measurement points was then calculated. Δθ (= θ - θp) was calculated from the average bending angle θ and the relative angle θp (= 90°) of the punch die face. Δθ is the amount of angular change of the bent product relative to the bending punch. If Δθ > 0, springback occurred, and if Δθ < 0, spring-go occurred. From the perspective of shape fixability, a small absolute value of Δθ is desirable. Therefore, a range of Δθ between -1.5 / t and 1.5 / t was considered acceptable. More specifically, if Δθ was between 0 and 1.5 / t, springback was evaluated as good, and if Δθ was less than 0 and greater than -1.5 / t, spring-go was evaluated as good. In addition, since the likelihood of springback and spring-back and the absolute value of the angle change Δθ of the bent product relative to the bending punch vary depending on the plate thickness t, the plate thickness t was taken into consideration and the pass range for Δθ was set to be between -1.5 / t and 1.5 / t. The results are shown in Table 2.

[0103] As shown in Table 2, all of Inventive Examples A1 to A11 were subjected to die bending under conditions that satisfied the range of the present invention. As a result, the obtained bent products had a radius of curvature of the convex curved surface on the outside of the bent portion (outside diameter Rout in Table 2) that was less than or equal to the thickness t of the sheet material portion, a minimum thickness of the bent portion (tm in Table 2) that was more than 0.75 times the thickness t of the sheet material portion, and an angle change of the bent product with respect to the bending punch (Δθ in Table 2) that was greater than or equal to -1.5 / t and less than or equal to 1.5 / t. Therefore, the bent products of Inventive Examples A1 to A11 were suppressed in terms of springback and spring-go, there was no risk of insufficient strength in the bent portion, and the shape of the outside of the bent portion was also sharp.

[0104] In addition, in all of the invention samples A1 to A11, the Vickers hardness HV m Vickers hardness HV at the bent part r Ratio (HV r / HVm ) is now in the range of 1.50 to 1.90.

[0105] On the other hand, as shown in Table 2, in Comparative Example B1, the radius of curvature of the tip surface of the protrusion of the punch was less than 30% of the plate thickness of the material, so the compressive force of the bent portion was small and the springback was large.

[0106] In Comparative Example B2, the radius of curvature of the tip surface of the protrusion of the punch was more than 50% of the thickness of the material, so the radius of curvature on the outside of the bent portion exceeded the thickness of the material, and a sharp shape could not be obtained.

[0107] In Comparative Example B3, the length of the punch protrusion was less than 30% of the sheet thickness of the material, so the radius of curvature of the outer bent portion exceeded the sheet thickness, making it impossible to obtain a sharp shape. In addition, springback was large.

[0108] In Comparative Example B4, the length of the punch protrusion exceeded 60% of the blank thickness, creating a gap between the punch die surface and the blank, preventing the blank from being sufficiently constrained by the punch, resulting in a large spring go. In addition, the minimum wall thickness was reduced, resulting in a decrease in the strength and rigidity of the bent portion.

[0109] In Comparative Example B5, the radius of curvature of the bottom surface of the die recess of the die exceeded 60% of the thickness of the material, and therefore, there was no space for material flow to occur, so the radius of curvature on the outside of the bent part exceeded the thickness of the material, and a sharp shape could not be obtained. In addition, the compressive force on the bent part was too strong, resulting in a large spring back.

[0110] In Comparative Example B6, the radius of curvature of the bottom surface of the die recess of the die was less than 20% of the plate thickness of the material, and cracks occurred in the bottom surface of the die recess during bending, making it impossible to continue bending.

[0111] In each test example, the width W of the protrusion was a length equivalent to 50% or more and 150% or less of the thickness of the material, and the radius of curvature Rj of the concave curved surface at the boundary between the punch die surface and the protrusion was a radius of curvature equivalent to a length equivalent to 150% or less of the thickness of the material.

[0112] [Table 1]

[0113] [Table 2]

[0114] Example 2 As in Example 1, a ferritic-austenitic duplex stainless steel sheet was prepared having the chemical composition shown in Table 1 and the thickness t shown in Table 3A. The mechanical properties of the ferritic-austenitic duplex stainless steel sheet were as shown in Table 1. The metal plate material made of the ferritic-austenitic duplex stainless steel sheet was cut into a size of 100 mm in length and 30 mm in width to prepare a raw material.

[0115] A mold equipped with a punch and a die was also prepared, as shown in Figures 8 and 9. The punch used had a protrusion 11c with a tip surface 11e (convex curved surface), a side wall surface 11d, and an inclined surface 11f. The relative angle θp of the punch die surface of the punch was 90°. The width of the punch was 80 mm, and the length (length in the depth direction in Figures 8 and 9) was 60 mm. The width of the die recess was 80 mm, and the length (length in the depth direction in Figure 8) of the die recess was 60 mm.

[0116] The punch and die were then attached to a press molding machine, and the material was placed between the punch and die. The material was placed so that its length coincided with the depth direction in Figures 8 and 9. The widthwise center of the material was aligned with the center of the mold. The press molding machine was then operated to bring the punch and die closer to each other until they were in close contact with the material, thereby V-bending the material and forming a bent portion. In this way, a bent product having a bent portion was produced.

[0117] The bending angle θ of the bent portion of the obtained bent product was measured, and spring back and spring back were evaluated in the same manner as in Example 1. In addition, the Vickers hardness HV of the plate portion was m Vickers hardness HV at the bent part r Ratio (HV r / HV m ) was measured. The results are shown in Table 3B.

[0118] In addition, 1,000 metal blanks were prepared and V-bent using one die. The presence or absence of punch deformation was evaluated. Evaluation was performed according to the following evaluation criteria. The results are shown in Table 3B.

[0119] A: The punch does not deform even after 1000 times. B: Punch deformation occurs after 800 to 1000 times. C: Punch deformation occurs after 600 or more times but less than 800 times. D: Punch deformation occurs after 100 or more times but less than 600 times.

[0120] In addition, an overall evaluation was made based on the following criteria: If the bent product failed any of the radius of curvature of the convex curved surface on the outside of the bent portion (outside diameter Rout in Table 3B), the minimum wall thickness of the bent portion (tm in Table 3B), or the amount of change in angle of the bent product relative to the bending punch (Δθ in Table 3B), the mold deformation was not evaluated.

[0121] Pass (○): Mold deformation was rated A. Pass (△): Mold deformation was rated B to D. Fail (×): No evaluation of mold deformation was performed (for bent products, the radius of curvature of the convex curved surface on the outside of the bend was large, the minimum thickness of the bent part was small, or the angle change of the bent product relative to the bending punch was large).

[0122] As shown in Tables 3A and 3B, all of Inventive Examples A21 to A37 were subjected to die bending under conditions that satisfied the ranges of the present invention. As a result, the bent products obtained had a radius of curvature of the convex curved surface on the outside of the bent portion (outside diameter Rout in Table 3B) that was less than or equal to the thickness t of the sheet material portion, a minimum thickness of the bent portion (tm in Table 3B) that was at least 0.75 times the thickness t of the sheet material portion, and an angle change of the bent product with respect to the bending punch (Δθ in Table 3B) that was greater than or equal to -1.5 / t and less than or equal to 1.5 / t. Therefore, the bent products of Inventive Examples A21 to A37 were suppressed in terms of springback and spring-go, there was no risk of insufficient strength in the bent portion, and the shape of the outside of the bent portion was also sharp.

[0123] In addition, in all of the invention samples A21 to A37, the Vickers hardness HV m Vickers hardness HV at the bent part r Ratio (HV r / HV m ) is now in the range of 1.50 to 1.90.

[0124] Furthermore, it was confirmed that the punches and dies of the molds of invention samples A21 to A37 did not deform even after 1000 cycles of processing, or even if they did deform, they had acceptable durability.

[0125] On the other hand, as shown in Tables 3A and 3B, in Comparative Example B21, the radius of curvature Rp of the tip surface of the protrusion of the punch was less than 30% of the thickness of the material, so the compression force of the bent portion was small and the springback was large.

[0126] In Comparative Example B22, the radius of curvature Rp of the tip surface of the protrusion of the punch was more than 50% of the thickness of the material, so the radius of curvature on the outside of the bent portion exceeded the thickness, and a sharp shape could not be obtained.

[0127] In Comparative Example B23, the length H of the protruding portion of the punch was less than 30% of the thickness of the blank, so the radius of curvature of the outer bent portion exceeded the thickness of the blank, making it impossible to obtain a sharp shape. In addition, springback was large.

[0128] In Comparative Example B24, the length H of the punch protrusion exceeded 60% of the blank thickness, creating a gap between the punch die surface and the blank, preventing the blank from being sufficiently constrained by the punch, resulting in a large spring go. In addition, the minimum wall thickness was reduced, resulting in a decrease in the strength and rigidity of the bent portion.

[0129] In Comparative Examples B25 and B27, the width W of the protruding part of the punch was more than 150% of the thickness of the blank, so the radius of curvature on the outer side of the bent part exceeded the thickness of the blank, making it impossible to obtain a sharp shape. In addition, the protruding part of the punch was not pressed into the blank, which resulted in insufficient compression force being applied to the bent part, resulting in large springback.

[0130] In Comparative Examples B26 and B28, the radius of curvature Rj of the concave curved surface of the punch was more than 150% of the plate thickness of the material, and therefore the springback was large.

[0131] In Comparative Example B29, the radius of curvature of the bottom surface of the die recess of the die exceeded 60% of the thickness of the material, and therefore, there was no space for material flow to occur, and the radius of curvature of the outer side of the bent part exceeded the thickness of the material, making it impossible to obtain a sharp shape. In addition, the compressive force on the bent part was too strong, resulting in a large spring back.

[0132] In Comparative Example B30, the radius of curvature of the bottom surface of the die recess of the die was less than 20% of the plate thickness of the material, and cracks occurred at the bottom surface of the die recess during bending, making it impossible to continue bending.

[0133] [Table 3A]

[0134] [Table 3B] [Explanation of symbols]

[0135] 1...punch, 1a...punch tip, 1b...punch die surface, 1c...protrusion, 1e...tip surface of protrusion, 2...die, 2A...die block, 2a...die recess, 2c...concave curved surface of die, 2d...bottom, 3...material, 4...bent product (bent product made of metal plate), 4a...plate portion, 4b...bent portion, 4c...convex curved surface on the outside of the bent portion, 4d...recess in bent portion, 4e...concave curved surface of recess.

Claims

1. a placement step of placing a material made of a metal plate between the punch and the die; a forming step of bending the blank by bringing the punch and the die closer to each other, The material is a metal plate material that satisfies the following requirements: a 0.2% proof stress of 300 MPa or more, a yield ratio of 0.9 or less, and a plate thickness of 6 mm or less; the punch comprises a pair of punch die surfaces approaching each other toward the punch tip, and a protrusion provided at the punch tip, the tip surface of the protrusion being a convex curved surface with a radius of curvature corresponding to a length of 30% to 50% of the plate thickness of the material, and the length of the protrusion, which is the length from the boundary between the side wall surface of the protrusion and the punch die surface to the tip surface of the protrusion, is a length corresponding to 30% to 60% of the plate thickness of the material, the die is provided with V-groove-shaped die recesses corresponding to the pair of punch die surfaces, and a concave curved surface having a curvature radius corresponding to a length of 20% to 60% of a plate thickness of the material is provided at a bottom of the die recess; The forming process is a process of bringing the punch and the die close to each other so that the punch and the die are in close contact with the material, and pressing at least a portion of the protrusion of the punch into the material.

2. The method for bending a metal plate according to claim 1 , wherein the protrusion has the convex curved surface and a side wall surface connected to the convex curved surface and defining a width of the protrusion.

3. a placement step of placing a material made of a metal plate between the punch and the die; a forming step of bending the blank by bringing the punch and the die closer to each other, The material is a metal plate material that satisfies the following requirements: a 0.2% proof stress of 300 MPa or more, a yield ratio of 0.90 or less, and a plate thickness t of 6.0 mm or less; the punch comprises a pair of punch die surfaces approaching each other toward the punch tip, and a protrusion provided at the punch tip, the tip surface of the protrusion being a convex curved surface with a radius of curvature corresponding to a length of 30% to 50% of the plate thickness of the material, the length of the protrusion, which is the length from the boundary between the side wall surface of the protrusion and the punch die surface to the tip surface of the protrusion, being a length corresponding to 30% to 60% of the plate thickness of the material, and the width of the protrusion being a length corresponding to 50% to 150% of the plate thickness of the material, and a concave curved surface is provided at the boundary between the pair of punch die surfaces and the protrusion, and the radius of curvature of the concave curved surface is a radius of curvature corresponding to a length of 150% or less of the plate thickness of the material, the die is provided with V-groove-shaped die recesses corresponding to the pair of punch die surfaces, and a concave curved surface having a curvature radius corresponding to a length of 20% to 60% of a plate thickness of the material is provided at a bottom of the die recess; The forming process is a process of bringing the punch and the die close to each other so that the punch and the die are in close contact with the material, and pressing at least a portion of the protrusion of the punch into the material.

4. 4. The bending method for a metal plate according to claim 3, wherein the protrusion has the convex curved surface, a side wall surface forming the width of the protrusion, and an inclined surface between the convex curved surface and the side wall surface.

5. A die for bending a material made of a metal plate material that satisfies the following requirements: 0.2% proof stress of 300 MPa or more, a yield ratio of 0.9 or less, and a plate thickness of 6 mm or less. a punch having a pair of punch die faces approaching each other toward a tip of the punch and a protrusion provided at the tip of the punch; a die including a die block provided with V-groove-shaped die recesses corresponding to the pair of punch die surfaces; a tip end surface of the protrusion is a convex curved surface having a radius of curvature corresponding to a length that is 30% or more and 50% or less of a thickness of the blank, and a length of the protrusion, which is a length from a boundary position between a side wall surface of the protrusion and the punch die surface to the tip end surface of the protrusion, corresponds to a length that is 30% or more and 60% or less of a thickness of the blank, A die for bending metal plate material, wherein the bottom of the V-groove-shaped recess is provided with a concave curved surface having a radius of curvature corresponding to a length of 20% to 60% of the plate thickness of the material.

6. The die for bending a metal plate according to claim 5 , wherein the protrusion has the convex curved surface and a side wall surface connected to the convex curved surface and defining a width of the protrusion.

7. A die for bending a material made of a metal plate material that satisfies the following requirements: 0.2% proof stress of 300 MPa or more, a yield ratio of 0.90 or less, and a plate thickness of 6.0 mm or less. a punch having a pair of punch die faces approaching each other toward a tip of the punch and a protrusion provided at the tip of the punch; a die including a die block provided with V-groove-shaped die recesses corresponding to the pair of punch die surfaces; a tip surface of the protrusion is a convex curved surface having a radius of curvature corresponding to a length of 30% to 50% of the thickness of the material, a length of the protrusion, which is the length from the boundary between the side wall surface of the protrusion and the punch die surface to the tip surface of the protrusion, is a length corresponding to 30% to 60% of the thickness of the material, and a width of the protrusion is a length corresponding to 50% to 150% of the thickness of the material, and a concave curved surface is provided at the boundary between the pair of punch die surfaces and the protrusion, and a radius of curvature of the concave curved surface is a radius of curvature corresponding to a length of 150% or less of the thickness of the material, A die for bending metal plate material, wherein the bottom of the V-groove-shaped recess is provided with a concave curved surface having a radius of curvature corresponding to a length of 20% to 60% of the plate thickness of the material.

8. 8. The die for bending metal plate material according to claim 7, wherein the protrusion has the convex curved surface, a side wall surface that defines the width of the protrusion, and an inclined surface between the convex curved surface and the side wall surface.

9. A plate portion made of a metal plate material satisfying the following requirements: 0.2% proof stress of 300 MPa or more, a yield ratio of 0.90 or less, and a plate thickness of 6.0 mm or less; and a bending portion provided in the plate portion, The radius of curvature of the convex curved surface on the outer side of the bent portion is a radius of a length corresponding to the plate thickness of the plate portion, The method for bending a metal plate according to any one of claims 1 to 4, wherein a bent product made of a metal plate is manufactured, wherein the minimum thickness of the bent portion is 0.75 times or more the plate thickness t of the plate portion.

10. 10. The method for bending a metal plate according to claim 9, wherein a bent product is manufactured from the metal plate, the bent portion having a recess formed on the inner side thereof along the longitudinal direction of the bent portion, and the recess having a concave curved surface on the inner surface thereof.

11. Vickers hardness HV of the plate material ｍ Vickers hardness HV at the bent portion ｒ Ratio of (HV ｒ / HV ｍ 11. The method for bending a metal plate according to claim 9 or 10, wherein a bent product is produced in which the bending strength is in the range of 1.50 or more and 1.90 or less.