Pressing method and material steel sheet

Abstract

This press method includes a raw material preparation step for preparing a raw material steel sheet or a pre-processed raw material steel sheet which are configured so that there is a portion, of an end surface of the raw material steel sheet or the pre-processed raw material steel sheet, in which the maximum principal strain ε1 which occurs at the end surface due to pressing and the hole expansion ratio λ of the raw material steel sheet or the pre-processed raw material steel sheet satisfy formula (1), and when such portion is considered to be a first region, in a section of or the entirety of that first region the following are true: the converted tensile strength obtained by converting from the hardness of the end surface is 1.50 times or less the tensile strength of the raw material steel sheet or the pre-processed raw material steel sheet; the arithmetic mean roughness Ra is 3.00 μｍ or less in relation to the line roughness along the extension direction of the end surface of the raw material steel sheet or the pre-processed raw material steel sheet; and the ratio (Rz / RSm) of the maximum height Rz in relation to the line roughness along the extension direction of the end surface to the average length RSm of a roughness curve element is 0.20 or less. The press method also includes a press step for pressing the raw material steel sheet or the pre-processed raw material steel sheet.

Description

Technical Field

[0001] The present invention relates to a pressing method and a material steel sheet. This application claims priority based on Japanese Patent Application No. 2023-200665 filed in Japan on November 28, 2023, and incorporates its content herein.

Background Art

[0002] When press-forming a shape having a shape in which the end of a material steel sheet is stretched by press-forming (hereinafter sometimes referred to as "stretch flange forming"), if the strength of the steel sheet is high, cracks due to elongation deformation of the end (hereinafter sometimes referred to as "stretch flange cracks") are likely to occur. In order to manufacture a good press-formed product, it is required to suppress stretch flange cracks.

[0003] Automobile steel sheets used for automobile body parts are being strengthened to improve fuel efficiency and collision safety by reducing the weight of the vehicle body. In particular, in recent years, the application of high-strength steel sheets with a tensile strength of 980 MPa or more has been increasing, and suppressing stretch flange cracks has become a major issue. Therefore, various countermeasure technologies for stretch flange cracks have been proposed.

[0004] For example, Patent Document 1 discloses a technique for improving the ductility of a steel sheet by heating a sheared end at 600°C or higher and lower than 800°C to further suppress stretch flange cracks.

[0005] Also, Patent Document 2 discloses a technique for reducing the residual strain of a sheared end face, which is a cause of stretch flange cracks, by arranging a heating coil in a non-contact state on the end face of the punching end of a punched hole and along the end face, passing an electric current through the heating coil to generate an induced electromotive force in the steel sheet, and heating it.

[0006] Patent Document 3 discloses a technique for improving the elongation resistance of the sheared end face by performing a double shearing process, forming a first region in the first shearing process where the cutting allowance for the second shearing process is 5 mm or less, and performing the second shearing process while constraining the movement of the end side of the first region, thereby reducing the tensile residual stress and work hardened layer due to cutting on the sheared end face. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2021-139012 [Patent Document 2] Japanese Patent Publication No. 2022-108601 [Patent Document 3] Japanese Patent No. 7176549 [Overview of the project] [Problems that the invention aims to solve]

[0008] However, the technology disclosed in Patent Document 1 has the problem that a heating device is required to heat the ends and productivity is significantly reduced. Similarly, the technology disclosed in Patent Document 2 also has the problem that equipment such as heating coils is required and productivity is significantly reduced compared to a normal press. In the technology of Patent Document 3, the stretch-resistant flange cracking characteristics decrease when the roughness of the end face is large, so even if two shearing processes are performed, a certain level of roughness remains on the sheared end face, which has the problem that improvement in stretch-resistant flange cracking characteristics is limited.

[0009] This invention was made in view of the above circumstances, and aims to provide a pressing method and steel sheet material that are excellent in productivity and have excellent resistance to elongation flange cracking. [Means for solving the problem]

[0010] To solve the aforementioned problems, the present invention proposes the following means. (1) A pressing method according to embodiment 1 of the present invention is a pressing method for obtaining a press-formed product by pressing a raw steel sheet or a pre-processed raw steel sheet, When the portion of the end face of the raw steel sheet or the pre-processed raw steel sheet that satisfies the following formula (1) is defined as the first region, In part or all of the first region, The converted tensile strength calculated from the hardness of the end face is 1.50 times or less the tensile strength of the base steel sheet or the pre-processed base steel sheet. The process includes: a material preparation step of preparing the material steel sheet or pre-processed material steel sheet, wherein the arithmetic mean roughness Ra along the extending direction of the end face of the material steel sheet or pre-processed material steel sheet is 3.00 μm or less, and the ratio Rz / RSm of the maximum height Rz related to the line roughness along the extending direction of the end face to the average length RSm of the roughness curve elements is 0.20 or less; and a pressing step of pressing the material steel sheet or pre-processed material steel sheet. 5×ln(1+λ / 100)≧ε1≧0.5×ln(1+λ / 100)···(1) (2) Embodiment 2 of the present invention is a material preparation step of the pressing method of Embodiment 1 in which a part or all of the end face of the first region of the base steel sheet or the pre-processed base steel sheet is cut to obtain a base steel sheet or the pre-processed base steel sheet. (3) Embodiment 3 of the present invention is a material preparation step of the pressing method of Embodiment 1 in which a part or all of the end face of the first region of the base steel sheet or pre-processed base steel sheet is laser processed to obtain the base steel sheet or the pre-processed base steel sheet. (4) Embodiment 4 of the present invention is a pressing method according to any one of embodiments 1 to 3, wherein the tensile strength of the raw steel sheet or the pre-processed raw steel sheet is 500 MPa or more. (5) The material steel sheet of embodiment 5 of the present invention is a material steel sheet that includes a first region in which the maximum principal strain ε1 and hole expansion ratio λ generated at the end face when pressed satisfy the following formula (2), wherein in part or all of the first region, The converted tensile strength calculated from the hardness of the end face is 1.50 times or less the tensile strength of the steel sheet material. With respect to the linear roughness of the end face of the aforementioned steel sheet material along the extending direction, the arithmetic mean roughness Ra is 3.00 μm or less, and the ratio Rz / RSm obtained by dividing the maximum height Rz of the linear roughness of the end face along the extending direction by the average length RSm of the roughness curve elements is 0.20 or less. 5×ln(1+λ / 100)≧ε1≧0.5×ln(1+λ / 100)···(2) (6) Embodiment 6 of the present invention is the material steel sheet of Embodiment 5, A portion or all of the end face of the first region of the material steel plate has machining marks. (7) Embodiment 7 of the present invention is a material steel sheet of Embodiment 5 wherein a part or all of the end face of the first region of the material steel sheet has laser processing marks. (8) Embodiment 8 of the present invention is a material steel sheet of any one of embodiments 5 to 7, The tensile strength of the aforementioned steel sheet material is 500 MPa or more. [Effects of the Invention]

[0011] According to the above-described aspect of the present invention, it is possible to provide a pressing method and a steel sheet material that are excellent in productivity and have excellent resistance to elongation flange cracking. [Brief explanation of the drawing]

[0012] [Figure 1] This is a flowchart of the pressing method according to the first embodiment of the present invention. [Figure 2] This is a perspective view of an example of a steel sheet material according to the first embodiment of the present invention. [Figure 3] This is a perspective view of an example of a press-formed product obtained by pressing a steel sheet material according to the first embodiment of the present invention. [Figure 4] This is a flowchart of a pressing method according to a second embodiment of the present invention. [Figure 5] This is a perspective view of an example of a steel sheet material according to a second embodiment of the present invention. [Figure 6]This is a diagram for explaining the configuration of the mold used in the examples. [Figure 7] This is a diagram for explaining the measurement location of the height of the press-formed product.

Embodiments for Carrying Out the Invention

[0013] The inventors conducted studies on various press-forming processes. As a result, in drawing, foam-forming, and bending, which are common press-forming methods, it was found that when the maximum principal strain ε1 generated on the end surface by press-working is less than 0.5×ln(1 + λ / 100), where λ is the hole expansion value of the material steel plate, no elongation flange cracking occurs. Here, the hole expansion rate λ is the percentage of the diameter expansion rate in a conical hole expansion test in which a circular hole provided in the material is expanded by pushing in a conical punch.

[0014] In addition, the material steel plate used for ordinary press-formed products is cut out from the base steel plate by shearing. However, the end surface after shearing (hereinafter sometimes referred to as the sheared end surface) has a roughness of a certain level or more. When the arithmetic mean roughness Ra with respect to the line roughness along the extending direction of the end surface of the material steel plate exceeds 3.00 μm, the elongation flange property deteriorates as compared with the case where the arithmetic mean roughness Ra is 3.00 μm or less. As a result of intensive studies by the inventors, although the end surface of the sheared surface is work-hardened and hardened due to plastic deformation accompanying shearing, when the converted tensile strength converted from the hardness of this end surface is 1.50 times or less the tensile strength of the material steel plate, even if the end surface is not completely smoothed, the arithmetic mean roughness Ra with respect to the line roughness along the extending direction of the end surface of the material steel plate is 3.00 μm or less, and if the ratio Rz / RSm of the maximum height Rz to the average length RSm of the roughness curve elements is 0.20 or less, it was found that the elongation flange property can be significantly improved. Here, the converted tensile strength converted from the hardness is used as an index. This is because it is technically difficult to measure the tensile strength of the local steel plate end surface by a tensile test or the like. Therefore, the surface hardness, which is easy to measure and highly correlated with the tensile strength, is measured, and the tensile strength estimated from the measured surface hardness (converted tensile strength converted from the hardness) is used instead of the original tensile strength.

[0015] The present invention is based on the above findings, and the pressing method according to this embodiment comprises a material preparation step of preparing a steel sheet, in which, when a first region is defined as a portion of the end face of a steel sheet or a pre-processed steel sheet in which the maximum principal strain ε1 generated on the end face by pressing and the hole expansion ratio λ of the steel sheet satisfy the following equation (1), the converted tensile strength calculated from the hardness of the end face is 1.50 times or less of the tensile strength of the steel sheet in part or all of the first region, the arithmetic mean roughness Ra along the extending direction of the end face of the steel sheet is 3.00 μm or less, and the ratio Rz / RSm of the maximum height Rz to the average length RSm of the roughness curve elements in the line roughness is 0.20 or less; and a pressing step of pressing the steel sheet.

[0016] 5×ln(1+λ / 100)≧ε1≧0.5×ln(1+λ / 100)···(1)

[0017] <First Embodiment> The pressing method and steel sheet material according to one embodiment of the present invention will be described below with reference to the drawings. Figure 1 is a flowchart of the pressing method according to one embodiment of the present invention. As shown in Figure 1, the pressing method of this disclosure comprises a material preparation step S1 and a pressing step S2 for pressing the steel sheet material. Each step will be described below.

[0018] (Material preparation process) In the material preparation step S1, a first region is defined as the part of the end face of a raw steel sheet or pre-processed raw steel sheet where the maximum principal strain ε1 generated on the end face by pressing and the hole expansion ratio λ of the raw steel sheet satisfy the above formula (1). In part or all of the first region, the converted tensile strength calculated from the hardness of the end face is 1.50 times or less the tensile strength of the raw steel sheet or pre-processed raw steel sheet, the arithmetic mean roughness Ra along the extending direction of the end face of the raw steel sheet or pre-processed raw steel sheet is 3.00 μm or less, and the ratio Rz / RSm of the maximum height Rz to the average length RSm of the roughness curve elements in the line roughness is 0.20 or less. Here, pre-processing refers to processing including the formation of a plating layer, pre-forming, pre-heating, formation of holes or notches, and joining of steel sheets of different strengths or different types of steel sheets. Pre-processing may include one or more combinations of processes such as forming a plating layer, pre-forming, pre-heating, forming holes or notches, and joining steel sheets of different strengths or different types of steel sheets. In the first embodiment, the base steel sheet 1 has machining marks on part or all of the end faces 30 of the first region S. In this specification, the term "end face of the base steel sheet" means the surface of the base steel sheet that is exposed in a direction perpendicular to the thickness direction. In the base preparation step S1, the base steel sheet 1 is obtained by machining.

[0019] "Material steel plate" Figure 2 is a perspective view of the base steel sheet 1 according to this embodiment. Figure 3 is a perspective view of the press-formed product 100 obtained by pressing the base steel sheet. The base steel sheet 1 is a steel sheet used to obtain the press-formed product 100. The base steel sheet 1 is, for example, a steel sheet with low ductility and a tensile strength of 500 MPa or more. A more preferable base steel sheet 1 has a tensile strength of 700 MPa or more. By having a tensile strength of 700 MPa or more, it is possible to further reduce the weight of vehicle body parts and the like. The base steel sheet 1 may be pre-formed by pre-bending, pre-cutting, drilling, and other preliminary processing so that it takes the shape of the press-formed product 100 after pressing. The base steel sheet 1 may also further include a plating layer formed by a plating treatment, a paint film formed by a painting treatment, and so on. In the following description, the base steel sheet 1 will be used, but "base steel sheet" may be replaced with "pre-processed base steel sheet".

[0020] "First area" The first region S is defined as the portion of the end face of the steel sheet 1 in which the maximum principal strain ε1 generated at the end face 30 by pressing and the hole expansion ratio λ of the steel sheet 1 satisfy the above equation (1). The steel sheet 1 includes the first region in which the maximum principal strain ε1 generated at the end face and the hole expansion ratio λ satisfy the above equation (1) when pressed. That is, the first region S includes the portion of the end face 120 of the press-formed product 100 obtained by pressing the steel sheet 1 in which the maximum principal strain ε1 generated and the hole expansion ratio λ of the steel sheet 1 satisfy the above equation (1).

[0021] Here, the maximum principal strain ε1 generated at the end face 30 by pressing can be determined by forming simulation using the finite element method. The hole expansion ratio λ can be obtained by performing a conical hole expansion test based on JIS Z 2256:2010 using a steel plate with a circular hole created by shearing as a test specimen.

[0022] "In part or all of the first region, the converted tensile strength calculated from the hardness of the end face is 1.50 times or less the tensile strength of the base steel sheet." It is preferable that in part or all of the first region S of the steel sheet 1, the converted tensile strength calculated from the hardness of the end face is 1.50 times or less the tensile strength of the steel sheet 1. By having a converted tensile strength of 1.50 times or less the tensile strength of the steel sheet 1 in part or all of the first region S of the steel sheet 1, stretch flange cracking due to pressing can be suppressed even if the end face of the first region S is not smooth. The converted tensile strength may be 1.0 times or more the tensile strength of the steel sheet 1. It is more preferable that the converted tensile strength in part or all of the first region S of the steel sheet 1 is 1.3 times or less the tensile strength of the steel sheet 1.

[0023] Here, the converted tensile strength of the flange end face (end face of the first region) of the material steel sheet 1 is measured at the end face corresponding to the first region by a micro-Vickers test based on JIS Z 2244:2009, and then the obtained Vickers hardness HV is converted based on the conversion table specified in the SEA standard (SAE J 417:1983). The load used when measuring the Vickers hardness is 0.025 kgf. The tensile strength of material steel sheet 1 is obtained by measuring it in accordance with JIS Z 2241:2011.

[0024] In this embodiment, in part or all of the first region S of the material steel sheet 1, the region in which the converted tensile strength calculated from the hardness of the end face 30 is 1.50 times or less the tensile strength of the material steel sheet is such that the arithmetic mean roughness Ra along the extending direction t1 of the end face 30 is 3.00 μm or less, and the ratio Rz / RSm of the maximum height Rz related to the line roughness along the extending direction of the end face to the average length RSm of the roughness curve elements is 0.20 or less.

[0025] A portion of the first region S is, for example, 20% or more of the total area of ​​the end face 30 of the first region S. In 40% or more of the first region S, the converted tensile strength calculated from the hardness of the end face may be 1.50 times or less the tensile strength of the base steel sheet. The wider the area where the converted tensile strength satisfies the predetermined conditions, the better the resistance to elongation flange cracking. In 80% or less of the total area of ​​the end face 30 of the first region S, the converted tensile strength calculated from the hardness of the end face may be 1.50 times or less the tensile strength of the base steel sheet.

[0026] "In part or all of the first region, the arithmetic mean roughness Ra along the extension direction of the end face is 3.00 μm or less." In part or all of the first region S, the arithmetic mean roughness Ra along the extending direction t1 of the end face 30 of the material steel sheet 1 is 3.00 μm or less. A more preferable arithmetic mean roughness Ra is 2.00 μm or less. The arithmetic mean roughness Ra may be 1.00 μm or more. By having an arithmetic mean roughness Ra of 3.00 μm or less along the extending direction t1 of the end face 30 of the material steel sheet 1 in part or all of the first region S, stretch flange cracking due to pressing can be suppressed.

[0027] A portion of the first region S is, for example, 20% or more of the total area of ​​the end face 30 of the first region S. In 40% or more of the total area of ​​the end face 30 of the first region S, the arithmetic mean roughness Ra along the end face 30 of the material steel sheet 1 may be 3.00 μm or less. In 80% or less of the total area of ​​the end face 30 of the first region S, the arithmetic mean roughness Ra along the extending direction t1 of the end face 30 of the material steel sheet 1 may be 3.00 μm or less.

[0028] "In part or all of the first region, the ratio Rz / RSm, which is the ratio of the maximum height Rz related to the line roughness along the extension direction of the end face to the average length RSm of the roughness curve elements, is 0.20 or less." In part or all of the first region S, the ratio Rz / RSm of the maximum height Rz along the extending direction t1 of the end face 30 to the average length RSm of the roughness curve elements is 0.20 or less. By having the ratio Rz / RSm of the maximum height Rz along the extending direction t1 of the end face 30 to the average length RSm of the roughness curve elements be 0.20 or less in part or all of the first region S, stretch flange cracking due to pressing can be suppressed. Preferably, the ratio Rz / RSm of the maximum height Rz along the extending direction t1 of the end face 30 to the average length RSm of the roughness curve elements is 0.10 or less.

[0029] A portion of the first region S is, for example, 20% or more of the total area of ​​the end face of the first region S. In 40% or more of the total area of ​​the end face of the first region S, the ratio Rz / RSm of the maximum height Rz along the extending direction t1 of the end face 30 to the average length RSm of the roughness curve elements may be 0.20 or less. In 80% or less of the total area of ​​the end face 30 of the first region S, the ratio Rz / RSm of the maximum height Rz along the extending direction t1 of the end face 30 to the average length RSm of the roughness curve elements may be 0.20 or less. In mass production of press parts, when considering changes in the properties of the steel sheet end face due to variations in manufacturing conditions and wear of the shear blade during shearing, even if there is an effect on stretch flange properties, the larger the proportion of a portion of the first region S where Rz / RSm is 0.20 or less relative to the total area of ​​the end face of the first region S, the more stably stretch flange cracking can be suppressed.

[0030] The arithmetic mean roughness Ra, maximum height Rz, and average length RSm of the roughness curve elements along the extension direction of the end face of the first region can be determined by measuring the line roughness along the extension direction of the end face of the first region using a shape analysis laser microscope with a magnification of 20x and a cutoff value λc of 0.11 or 0.19 mm, in accordance with JIS B 0601:2013.

[0031] The manufacturing method for the base steel sheet 1 is not particularly limited. For example, a first region S is identified from a base steel sheet cut to a predetermined outer shape by shearing, using a forming simulation by the finite element method. Machining is performed on 20% or more of the obtained first region S. Machining is performed in part or all of the first region S such that the converted tensile strength calculated from the hardness of the end face is 1.50 times or less the tensile strength of the base steel sheet, the arithmetic mean roughness Ra along the extending direction of the end face is 3.00 μm or less, and the ratio Rz / RSm of the maximum height Rz related to the line roughness along the extending direction of the end face to the average length RSm of the roughness curve elements is 0.20 or less. When the end face of the base steel sheet is machined, part or all of the end face of the first region S of the base steel sheet 1 will have machining marks. When machining marks are present on the end face, the condition of the end face becomes more uniform than when shearing is performed, thus improving the elongation flange crack resistance characteristics. The presence or absence of machining marks can be determined, for example, by checking for machining lines on the end face. The base steel plate may have undergone the pre-processing described above.

[0032] The cutting conditions are not particularly limited as long as they satisfy the above conditions. They may be determined by investigating the relationship between the converted tensile strength of the surface of the end face 30, the arithmetic mean roughness Ra along the extending direction of the end face, the maximum height Rz, and the average length RSm of the roughness curve elements and the cutting conditions, or they may be determined for each steel type by model testing of shearing. Cutting may be performed, for example, with an end mill. When cutting is performed with an end mill, the outer cutting edge of the end mill bites into the material during cutting, so machining marks are formed.

[0033] The area to be machined may be the entire first region S, or it may be limited to the region where stretch flange cracking is a particular concern based on press tests or experience. For end faces where the maximum principal strain ε1 and hole expansion ratio λ generated after press working do not satisfy the above equation (1), the shearing process may be left as is, or further machining may be performed.

[0034] (Pressing process S2) In the pressing process S2, the raw steel sheet 1 prepared in the raw material preparation process S1 or the pre-processed raw steel sheet is pressed (press-formed) to obtain a press-formed product 100. The press-formed product 100 can be obtained by pressing the raw steel sheet 1 using a die and a known method. Examples of press forming methods include deep drawing, form forming, and bending.

[0035] The pressing method and the steel sheet material according to the first embodiment have been described above. In the steel sheet material 1 or pre-processed steel sheet material used in the pressing method according to the first embodiment, when the area of ​​the end face 30 where the maximum principal strain ε1 generated on the end face 30 by pressing and the hole expansion ratio λ of the steel sheet material 1 satisfy the above formula (1) is defined as the first region S, in part or all of the first region S, the converted tensile strength calculated from the hardness of the end face 30 is 1.50 times or less the tensile strength of the steel sheet material 1, the arithmetic mean roughness Ra along the extension direction t1 of the end face 30 of the steel sheet material 1 is 3.00 μm or less, and the ratio Rz / RSm of the maximum height Rz to the average length RSm of the roughness curve elements is 0.20 or less. Therefore, even in areas where stretch flange cracking is likely to occur, stretch flange cracking is suppressed, thereby improving the degree of freedom in design.

[0036] <Second Embodiment> Figure 4 is a flowchart of a pressing method according to a second embodiment of the present invention. As shown in Figure 4, the pressing method of this disclosure comprises a material preparation step S1A and a pressing step S2 for pressing a material steel sheet 1 or a pre-processed material steel sheet. Each step will be described below.

[0037] (Material preparation process) In the material preparation step S1A, a first region SA is defined as the portion of the end face 30A of the material steel sheet 1A where the maximum principal strain ε1 generated at the end face 30A by pressing and the hole expansion ratio λ of the material steel sheet 1A satisfy the above formula (1). In part or all of the first region SA, the converted tensile strength calculated from the hardness of the end face 30A is 1.50 times or less the tensile strength of the material steel sheet 1A, and the arithmetic mean roughness Ra along the extension direction of the end face 30A of the material steel sheet 1A is 3.00 μm or less, and the ratio Rz / RSm of the maximum height Rz to the average length RSm of the roughness curve elements is 0.20 or less. In the material steel sheet 1A according to the second embodiment, part or all of the end faces 30A of the first region SA have laser processing marks. When laser processing marks are present on the end faces, the condition of the end faces becomes more uniform than when shear processing is performed, thus improving the resistance to elongation flange cracking. The presence or absence of laser processing marks can be determined, for example, by the presence of streaks in the thickness direction on the end faces.

[0038] "Material steel plate" Figure 5 is a perspective view of the steel sheet 1A according to this embodiment. The steel sheet 1A is a steel sheet used to obtain the press-formed product 100. The steel sheet 1A is, for example, a steel sheet with low ductility and a tensile strength of 500 MPa or more. A more preferable steel sheet 1A has a tensile strength of 700 MPa or more. By having a tensile strength of 700 MPa or more, it is possible to further reduce the weight of the vehicle body parts, etc. The steel sheet 1A may be pre-processed, such as by cutting, so that it will take the shape of the press-formed product 100 after pressing. The steel sheet 1A may also further include a plating layer formed by a plating treatment, a paint film formed by a painting treatment, etc.

[0039] "First area" The first region SA is defined as the portion of the end face 30A of the steel sheet 1A in which the maximum principal strain ε1 generated on the surface of the end face 30A by pressing and the hole expansion ratio λ of the steel sheet 1 satisfy the following equation (1). The steel sheet 1A according to this embodiment includes the first region SA.

[0040] The maximum principal strain generated on the surface of the end face 30A by pressing and the hole expansion ratio λ of the material steel sheet 1 can be measured in the same manner as in the first embodiment.

[0041] "In part or all of the first region, the converted tensile strength calculated from the hardness of the end face 30A is 1.50 times or less the tensile strength of the base steel sheet." In part or all of the first region SA of the steel sheet 1A, it is preferable that the converted tensile strength, calculated from the hardness of the end face 30A, is 1.50 times or less the tensile strength of the steel sheet. By having a converted tensile strength of 1.50 times or less the tensile strength of the steel sheet 1A in part or all of the first region S of the steel sheet 1, stretch flange cracking due to pressing can be suppressed even if the end face of the first region SA is not smooth. The converted tensile strength may be 1.00 times or more the tensile strength of the steel sheet 1A. It is more preferable that the converted tensile strength in part or all of the first region SA of the steel sheet 1 is 1.30 times or less the tensile strength of the steel sheet 1A.

[0042] A portion of the first region SA is, for example, 20% or more of the total area of ​​the end face 30 in the first region SA. In the first region SA, the converted tensile strength calculated from the hardness of the end face may be 1.50 times or less of the tensile strength of the base steel sheet in 40% or more of the total area of ​​the end face 30. The wider the region where the converted tensile strength satisfies the predetermined conditions, the better the elongation flange crack resistance characteristics. In the first region SA, the converted tensile strength calculated from the hardness of the end face may be 1.50 times or less of the tensile strength of the base steel sheet in 80% or less of the total area of ​​the end face 30.

[0043] "In part or all of the first region, the arithmetic mean roughness Ra along the extension direction of the end face is 3.00 μm or less." In part or all of the first region SA, the arithmetic mean roughness Ra along the extending direction t1A of the end face 30A of the material steel sheet 1A is 3.00 μm or less. A more preferable arithmetic mean roughness Ra is 2.00 μm or less. The arithmetic mean roughness Ra may be 1.00 μm or more. By having an arithmetic mean roughness Ra of 3.00 μm or less along the extending direction t1A of the end face 30A of the material steel sheet 1A in part or all of the first region SA, stretch flange cracking due to pressing can be suppressed.

[0044] A portion of the first region SA is, for example, 20% or more of the total area of ​​the end face 30A in the first region SA. In the first region SA, the arithmetic mean roughness Ra along the end face 30A of the material steel sheet 1A may be 3.00 μm or less in 40% or more of the total area of ​​the end face 30A. The wider the region where the arithmetic mean roughness Ra satisfies the predetermined conditions, the better the elongation flange crack resistance characteristics. In the first region SA, the arithmetic mean roughness Ra along the extending direction t1 of the end face 30A of the material steel sheet 1 may be 3.00 μm or less in 80% or less of the total area of ​​the end face 30A.

[0045] "In part or all of the first region, the ratio Rz / RSm, which is the ratio of the maximum height Rz related to the line roughness along the extension direction of the end face to the average length RSm of the roughness curve elements, is 0.20 or less." In part or all of the first region SA, the ratio Rz / RSm of the maximum height Rz along the extending direction t1A of the end face 30A to the average length RSm of the roughness curve elements is 0.20 or less. By having the ratio Rz / RSm of the maximum height Rz along the extending direction t1A of the end face 30A to the average length RSm of the roughness curve elements be 0.20 or less in part or all of the first region SA, stretch flange cracking due to pressing can be suppressed. Preferably, the ratio Rz / RSm of the maximum height Rz along the extending direction t1 of the end face 30A to the average length RSm of the roughness curve elements is 0.10 or less.

[0046] A portion of the first region SA is, for example, 20% or more of the total area of ​​the end face 30A in the first region SA. In the first region SA, in 40% or more of the total area of ​​the end face 30A, the ratio Rz / RSm of the maximum height Rz along the extending direction t1A of the end face 30A to the average length RSm of the roughness curve elements may be 0.20 or less. The wider the region where the ratio Rz / RSm satisfies the predetermined conditions, the better the elongation crack resistance of the flange. In the first region SA, in 80% or less of the total area of ​​the end face 30A, the ratio Rz / RSm of the maximum height Rz along the extending direction t1A of the end face 30A to the average length RSm of the roughness curve elements may be 0.20 or less.

[0047] The manufacturing method for the steel sheet 1A is not particularly limited. For example, a first region SA is identified using a finite element method forming simulation based on the shape formed by laser processing. A portion of the obtained first region SA that accounts for 20% or more of the region may be cut by laser processing. Laser cutting is performed such that in part or all of the first region SA, the converted tensile strength calculated from the hardness of the end face is 1.50 times or less the tensile strength of the steel sheet, in part or all of the first region SA, the arithmetic mean roughness Ra along the extending direction of the end face is 3.00 μm or less, and in part or all of the first region SA, the ratio Rz / RSm of the maximum height Rz related to the line roughness along the extending direction of the end face to the average length RSm of the roughness curve elements is 0.20 or less. When the end is cut by laser processing, part or all of the end face of the first region SA of the steel sheet 1A will have laser processing marks.

[0048] The laser processing conditions are not particularly limited as long as the above conditions are satisfied. The relationship between the converted tensile strength of the end face 30A, the arithmetic mean roughness Ra along the extension direction t1A of the end face 30A, the maximum height Rz, the average length RSm of the roughness curve elements, and the laser processing conditions may be investigated and determined. When the base steel plate is cut with a laser, laser processing marks are formed in the thickness direction of the plate.

[0049] The area of ​​the base steel plate to be cut may be the entirety of the first region SA. That is, all ends may be cut with a laser. Alternatively, after shearing, only the areas where stretch flange cracking is a particular concern, based on press tests or experience, may be laser-processed (cut). End faces where the maximum principal strain ε1 and hole expansion ratio λ generated after press working do not satisfy the above formula (1) may be left as sheared.

[0050] (Pressing process S2) In the pressing process S2, the steel sheet 1A prepared in the material preparation process S1A or the pre-processed steel sheet is pressed (press-formed) to obtain a press-formed product 100. The press-formed product 100 can be obtained by pressing the steel sheet 1 using a die and a known method. Examples of press forming methods include deep drawing, form forming, and bending.

[0051] The pressing method and the steel sheet material according to the second embodiment have been described above. In the steel sheet material 1A used in the pressing method according to the second embodiment, when the area of ​​the end face 30A where the maximum principal strain ε1 generated on the end face 30A by pressing and the hole expansion ratio λ of the steel sheet material 1 satisfy the above formula (1) is defined as the first region SA, in part or all of the first region SA, the converted tensile strength calculated from the hardness of the end face 30A is 1.50 times or less the tensile strength of the steel sheet material 1A, the arithmetic mean roughness Ra along the extension direction t1A of the end face 30A of the steel sheet material 1A is 3.00 μm or less, and the ratio Rz / RSm of the maximum height Rz to the average length RSm of the roughness curve elements is 0.20 or less. Furthermore, in the second embodiment, since cutting is done by laser processing, the hardness of the end face can be reduced, which increases the degree of design freedom.

[0052] It should be noted that the technical scope of the present invention is not limited to the embodiments described above, and various modifications can be made without departing from the spirit of the invention. Furthermore, it is possible to replace the components in the embodiments with well-known components as appropriate, without departing from the spirit of the invention, and the above-described modifications can be combined as appropriate. [Examples]

[0053] The following describes embodiments of the present invention. However, the conditions in the embodiments are merely examples of conditions adopted to confirm the feasibility and effectiveness of the present invention. The present invention is not limited to these examples of conditions. The present invention can adopt various conditions as long as it does not depart from the spirit of the invention and achieves the objectives of the present invention.

[0054] (Tensile strength TS, yield stress YS, total elongation EL) The tensile strength, yield stress YS, and total elongation EL of steel plates A and B (hereinafter referred to as base steel plates) before processing, as shown in Table 1, were measured in accordance with JIS Z 2241:2011. The obtained results are shown in Table 1.

[0055] (Hole expansion ratio λ) The hole expansion ratio λ of base steel plates A and B was determined by a conical hole expansion test based on JIS Z 2256:2010, using steel plates with circular holes created by shearing as test specimens. The results are shown in Table 1.

[0056] (Vickers hardness of the base steel plate) The Vickers hardness of base steel plates A and B was measured at the end face 30 of the first region S, in accordance with JIS Z 2244:2009. Specifically, the Vickers hardness was measured at the end face 30 of the first region S. The Vickers hardness was taken as the average of 10 randomly measured points. The load used when measuring Vickers hardness was 0.025 kgf.

[0057] (Press molding experiment) Next, we will explain the press forming experiment. In the press forming experiment, two types of steel sheets with the material properties shown in Table 1 were bent using the die shown in Figure 6, and the occurrence of stretch flange cracks was compared. Figure 6 is a diagram illustrating the configuration of the die. The die 51 has an alignment part 52. The raw steel sheet 1 is provided with alignment holes, and alignment is performed by inserting the alignment part 52 through the holes. After placing the raw steel sheet 1 in the die 51, a presser 53 was placed, and then press forming was performed with a punch 54 to obtain a press-formed product. Figure 7 is a diagram illustrating the height of the press-formed product after press forming. During press forming, a large strain ε1 occurs at the end face of the center of the flange. In the press forming experiment, the size of ε1 was changed by changing the flange length H after bending. When H is large, ε1 is large, and when H is small, ε1 is small. When ε1 is large, flange cracks occur at the end of the center of the flange (part A in Figure 7). Therefore, in the press forming experiment, ε1 was changed by changing H.

[0058] (0.5 × ln(1 + λ / 100)) The maximum principal strain ε1 generated at the end face during press working was estimated by forming simulation using the finite element method. The value 0.5 × ln(1 + λ / 100) was calculated from the hole expansion ratio λ and the maximum principal strain ε1 obtained from the simulation.

[0059] (Preparation of the steel sheet material) Furthermore, the maximum principal strain ε1 generated at the end face due to press working was estimated by forming simulations using the finite element method under each condition. The base steel sheet was processed under three conditions: shearing the end face of the flange (first region) where there was concern about elongation flange cracking, as determined by the simulation (condition 1); cutting after shearing (condition 2); and laser cutting (condition 3). Specifically, the base steel sheet was processed by changing the cutting conditions (amount of material removed, cutting speed) and laser processing conditions (laser power, cutting speed, etc.) so that the converted tensile strength / tensile strength, arithmetic mean roughness Ra, and maximum height Rz / average length RSm of the roughness curve elements were as shown in Table 2, thereby obtaining the base steel sheets for Experiments No. 1 to 17.

[0060] (Equivalent tensile strength) The converted tensile strength of the flange end face (end face of the first region) was measured on the end face corresponding to the first region by micro-Vickers testing based on JIS Z 2244:2009. Next, the converted tensile strength was obtained by converting the Vickers hardness HV of the flange end face using the conversion table specified in the SEA standard (SAE J 417:1983).

[0061] (Arithmetic mean roughness Ra, maximum height Rz, and average length RSm of roughness curve elements) The arithmetic mean roughness Ra, maximum height Rz, and average length RSm of the roughness curve elements along the extension direction of the end face of the first region were measured by the following method. In accordance with JIS B 0601:2013, the linear roughness along the extension direction of the end face of the first region was measured using a shape analysis laser microscope. The end face of the first region S was measured with a magnification of 20x and a cutoff value λc of 0.11 or 0.19 mm. The evaluation range was the area within the first region S that had been cut or laser-processed. The results are shown in Table 2.

[0062] (Stretch flange cracking) The stretch flange crack suppression effect of the present invention was evaluated by conducting the press forming experiment described above. A press-formed product was deemed acceptable if no cracks occurred, and unacceptable if cracks occurred. The results are shown in Table 2.

[0063] [Table 1]

[0064] [Table 2]

[0065] In Experiments No. 1, No. 2, and No. 3, material A with a hole expansion ratio λ of 32% was press-formed to a height of H9 mm. The processing conditions for the end faces of Experiments No. 1, No. 2, and No. 3 were shearing (condition 1), cutting after shearing (condition 2), and laser cutting (condition 3), respectively. In Experiment No. 1, Rz / RSm was greater than 0.20, and in Experiment No. 3, the converted tensile strength σm was greater than 1.50 times the tensile strength of the base steel sheet. However, in Experiments No. 1, No. 2, and No. 3, the maximum principal strain ε1 and hole expansion ratio λ generated at the end face did not have a first region that satisfies equation (1) above, so no stretch flange cracking occurred.

[0066] In Experiment No. 4, material A was press-formed to a thickness of 11 mm. The processing conditions for the end face of the steel sheet material in Experiment No. 4 were condition 1, and it had a first region. Since the arithmetic mean roughness Ra of the end face in the first region was greater than 3.00 μm, stretch flange cracking occurred.

[0067] In Experiment No. 5, material A was press-formed to a thickness of 11 mm. The processing conditions for the end faces of the steel sheet material in Experiment No. 5 were condition 3 (laser processing marks were present on the end faces), and it had a first region. At all end faces in the first region, the converted tensile strength was greater than 1.50 times the tensile strength of the steel sheet material, so elongation flange cracking occurred.

[0068] In Experiment No. 6, material A was press-formed to a height of 11 mm. The processing conditions for the end face of the steel sheet material in Experiment No. 6 were condition 1, and it had a first region. In the first region, the ratio Rz / RSm, which is the ratio of the maximum height Rz to the average length RSm of the roughness curve elements, was greater than 0.20, so elongation flange cracking occurred.

[0069] In Experiment No. 7, material A was press-formed to a height of 11 mm. The processing conditions for the end faces of the steel sheet material in Experiment No. 7 were condition 2 (machining marks were present on the end faces), and it had a first region. However, at all end faces in the first region, the converted tensile strength σm was 1.50 times or less the tensile strength of the steel sheet material, Ra was 3.00 μm or less, and the ratio of the maximum height Rz to the average length RSm of the roughness curve elements Rz / RSm was 0.20 or less, so no elongation flange cracking occurred.

[0070] In Experiment No. 8, material A was press-formed to a thickness of 11 mm. The processing conditions for the end face of the material steel sheet in Experiment No. 8 were condition 3 (laser processing marks were present on the end face), and it had a first region. However, in all end faces of the first region of the material steel sheet in Experiment No. 8, the converted tensile strength σm was 1.50 times or less the tensile strength of the material steel sheet, Ra was 3.00 μm or less, and the ratio of the maximum height Rz to the average length RSm of the roughness curve elements Rz / RSm was 0.20 or less, so no elongation flange cracking occurred.

[0071] In Experiment No. 9, material A was press-formed to a height of 13 mm. The processing conditions for the end face of the material steel sheet in Experiment No. 9 were condition 3 (laser processing marks were present on the end face), and it had a first region. At all end faces of the first region of the material steel sheet in Experiment No. 9, the converted tensile strength σm was 1.50 times or less the tensile strength of the material steel sheet, Ra was 3.00 μm or less, and the ratio of the maximum height Rz to the average length RSm of the roughness curve elements Rz / RSm was 0.20 or less, so no elongation flange cracking occurred.

[0072] In Experiment No. 10, material A was press-formed to a height of 15 mm. The processing conditions for the end face of the steel sheet material in Experiment No. 10 were condition 3 (laser processing marks were present on the end face), and it had a first region. At all end faces of the first region of the steel sheet material in Experiment No. 10, the converted tensile strength σm was 1.50 times or less the tensile strength of the steel sheet material, Ra was 3.00 μm or less, and the ratio of the maximum height Rz to the average length RSm of the roughness curve elements Rz / RSm was 0.20 or less, so no elongation flange cracking occurred.

[0073] In Experiment No. 11, material B with a hole expansion ratio λ of 54% was press-formed to a height of 13 mm. The processing conditions for the end face of the material steel plate in Experiment No. 11 were condition 1. The ratio Rz / RSm of the maximum height Rz to the average length RSm of the roughness curve elements was 0.20 or higher, but no first region satisfying equation (1) above was found, so no stretch flange cracking occurred.

[0074] Experiment No. 12 involved press-forming material B to a height of 15 mm. The processing conditions for the end face of the steel sheet material in Experiment No. 12 were condition 1, and it had a first region. In the first region, the ratio Rz / RSm, which is the ratio of the maximum height Rz to the average length RSm of the roughness curve elements, was 0.20 or higher, resulting in elongation flange cracking.

[0075] In Experiment No. 13, material B was press-formed to a height of 17 mm. The processing conditions for the end faces of the steel sheet material in Experiment No. 13 were condition 2 (machining marks were present on the end faces), and it had a first region. At all end faces in the first region, the converted tensile strength σm was 1.50 times or less the tensile strength of the steel sheet material, Ra was 3.00 μm or less, and the ratio of the maximum height Rz to the average length RSm of the roughness curve elements Rz / RSm was 0.20 or less, so no elongation flange cracking occurred.

[0076] In Experiment No. 14, material B was press-formed to a height of 17 mm. The processing conditions for the end face of the steel sheet material in Experiment No. 14 were condition 3 (laser processing marks were present on the end face), and it had a first region. In all end faces of the first region of the steel sheet material in Experiment No. 14, the converted tensile strength σm was 1.50 times or less the tensile strength of the steel sheet material, Ra was 3.00 μm or less, and the ratio of the maximum height Rz to the average length RSm of the roughness curve elements Rz / RSm was 0.20 or less, so no elongation flange cracking occurred.

[0077] In Experiment No. 15, material B was press-formed to a height of 20 mm. The processing conditions for the end face of the steel sheet material in Experiment No. 15 were condition 3 (laser processing marks were present on the end face), and it had a first region. In all end faces of the first region of the steel sheet material in Experiment No. 15, the converted tensile strength σm was 1.50 times or less the tensile strength of the steel sheet material, Ra was 3.00 μm or less, and the ratio of the maximum height Rz to the average length RSm of the roughness curve elements Rz / RSm was 0.20 or less, so no elongation flange cracking occurred.

[0078] In Experiment No. 16, material B was press-formed to a height of 22 mm. The processing conditions for the end face of the steel sheet material in Experiment No. 16 were condition 3 (laser processing marks were present on the end face), and it had a first region. In all end faces of the first region of the steel sheet material in Experiment No. 16, the converted tensile strength σm was 1.50 times or less the tensile strength of the steel sheet material, Ra was 3.00 μm or less, and the ratio of the maximum height Rz to the average length RSm of the roughness curve elements Rz / RSm was 0.20 or less, so no elongation flange cracking occurred.

[0079] In Experiment No. 17, material A was press-formed to a thickness of 11 mm. The end face of the steel sheet material in Experiment No. 17 was a laser-processed surface (with laser processing marks on the end face) and had a first region. In 30% of the end face of the first region of the steel sheet material in Experiment No. 17, the converted tensile strength σm was 1.50 times or less the tensile strength of the steel sheet material, Ra was 3.00 μm or less, and the ratio of the maximum height Rz to the average length RSm of the roughness curve elements Rz / RSm was 0.20 or less, so no elongation flange cracking occurred. [Industrial applicability]

[0080] The pressing method disclosed herein offers excellent productivity and superior resistance to elongation flange cracking, making it highly applicable to industrial use. [Explanation of symbols]

[0081] 1. Material: Steel sheet, 30. End face, 100. Press-formed product, S. First area

Claims

1. In a pressing method for obtaining a press-formed product by pressing a raw steel sheet or a pre-processed raw steel sheet, When the portion of the end face of the raw steel sheet or the pre-processed raw steel sheet that satisfies the following formula (1) is defined as the first region, In part or all of the first region, The converted tensile strength calculated from the hardness of the end face is 1.48 times or less the tensile strength of the base steel sheet or the pre-processed base steel sheet. A pressing method comprising: a material preparation step of preparing a material steel sheet or a pre-processed material steel sheet, wherein the arithmetic mean roughness Ra along the extending direction of the end face of the material steel sheet or the pre-processed material steel sheet is 1.47 μm or less, and the ratio Rz / RSm of the maximum height Rz of the line roughness along the extending direction of the end face to the average length RSm of the roughness curve elements is 0.11 or less; and a pressing step of pressing the material steel sheet or the pre-processed material steel sheet. 5×ln(1+λ / 100)≧ε1≧0.5×ln(1+λ / 100)...(1)

2. The pressing method according to claim 1, wherein in the material preparation step, a part or all of the end faces of the first region of the base steel sheet or the pre-processed base steel sheet are cut to obtain the base steel sheet or the pre-processed base steel sheet.

3. The pressing method according to claim 1, wherein in the material preparation step, a part or all of the end faces of the first region of the base steel sheet or the pre-processed base steel sheet are laser-processed to obtain the base steel sheet or the pre-processed base steel sheet.

4. The pressing method according to claim 1, wherein the tensile strength of the material steel sheet or the pre-processed material steel sheet is 500 MPa or more.

5. A steel sheet material that includes a first region where the maximum principal strain ε1 and hole expansion ratio λ generated at the end face when pressed satisfy the following equation (2), In part or all of the first region, The converted tensile strength calculated from the hardness of the end face is 1.48 times or less the tensile strength of the steel sheet material. The steel sheet material has a linear roughness along the extending direction of the end face, wherein the arithmetic mean roughness Ra is 1.47 μm or less, and the ratio Rz / RSm obtained by dividing the maximum height Rz of the linear roughness along the extending direction of the end face by the average length RSm of the roughness curve elements is 0.11 or less. 5×ln(1+λ / 100)≧ε1≧0.5×ln(1+λ / 100)...(2)

6. The steel sheet material according to claim 5, wherein a part or all of the end faces of the first region of the steel sheet material have machining marks.

7. The steel sheet material according to claim 5, wherein a part or all of the end faces of the first region of the steel sheet material have laser processing marks.

8. The steel sheet material according to claim 5, wherein the tensile strength of the steel sheet material is 500 MPa or more.

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