Press method and raw 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

Pressing method and steel sheet material

[0001] The present invention relates to a pressing method and a base steel sheet. This application claims priority based on Japanese Patent Application No. 2023-200665, filed on November 28, 2023, the contents of which are incorporated herein by reference.

[0002] When a shape in which the edges of a material steel sheet are stretched by press forming (hereinafter, sometimes referred to as "stretch flange forming") is formed by press working, if the strength of the steel sheet is high, cracks due to elongation deformation at the edges (hereinafter, sometimes referred to as "stretch flange cracking") are likely to occur. In order to produce good press-formed products, it is necessary to suppress stretch flange cracking.

[0003] Automotive steel sheets used in automobile body parts are becoming increasingly stronger in order to improve fuel efficiency and collision safety by reducing the weight of the vehicle body. In particular, in recent years, the use of high-strength steel sheets with tensile strengths of 980 MPa or more has increased, and the prevention of stretch flange cracking has become a major issue. Therefore, various countermeasures against stretch flange cracking have been proposed.

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

[0005] Furthermore, Patent Document 2 discloses a technology in which a heating coil is arranged in a non-contact manner on the end surface of the punched hole so as to face the end surface, and an electric current is passed through the heating coil to generate an induced electromotive force in the steel sheet, thereby heating the steel sheet and reducing residual strain on the shear end surface, which is a cause of stretch flange cracking.

[0006] Patent Document 3 discloses a technology in which double shearing is performed, and the first cut in the double shearing forms a first region in which the cutting allowance for the second shearing is 5 mm or less, and the second cut in the double shearing is performed while restricting movement of the end of the first region, thereby reducing the tensile residual stress at the sheared end surface and the work-hardened layer caused by the cut, and improving the stretch flange crack resistance of the sheared end surface.

[0007] Japanese Patent Publication No. 2021-139012 Japanese Patent Publication No. 2022-108601 Japanese Patent Publication No. 7176549

[0008] However, the technology disclosed in Patent Document 1 has the problem that a heating device is required to heat the end portion, and productivity is significantly reduced. Furthermore, the technology disclosed in Patent Document 2 also has the problem that equipment such as a heating coil is required, and productivity is significantly reduced compared to normal presses. The technology disclosed in Patent Document 3 has the problem that stretch flange cracking resistance decreases with increasing roughness of the end surface, and therefore, even if double shearing is performed, a certain level of roughness remains on the sheared end surface, and therefore improvement in stretch flange cracking resistance is limited.

[0009] The present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide a pressing method and a base steel sheet that are excellent in productivity and stretch flange crack resistance.

[0010] In order to solve the above problems, the present invention proposes the following means. (1) A pressing method according to a first aspect of the present invention is a pressing method for pressing a stock steel sheet or a pre-processed stock steel sheet to obtain a press-formed product, wherein, when a portion of an end face of the stock steel sheet or the pre-processed stock steel sheet where a maximum principal strain ε1 generated at the end face by pressing and a hole expansion ratio λ of the stock steel sheet or the pre-processed stock steel sheet satisfy the following formula (1) is defined as a first region, in part or all of the first region, the converted tensile strength converted from the hardness of the end face is 1.50 times or less the tensile strength of the stock steel sheet or the pre-processed stock steel sheet, The method includes a material preparation step of preparing the material steel plate or the pre-processed material steel plate, in which the arithmetic mean roughness Ra of the material steel plate or the pre-processed material steel plate along the extending direction of the end face is 3.00 μm or less, and the ratio Rz / RSm of the maximum height Rz of the material steel plate along the extending direction of the end face to the average length RSm of the roughness curve element is 0.20 or less, and a pressing step of pressing the material steel plate or the pre-processed material steel plate. １≧ 0.5 × ln(1 + λ / 100) (1) (2) Aspect 2 of the present invention relates to the pressing method of Aspect 1, wherein in the material preparation step, a part or all of the end faces of the first region of a base steel plate or a pre-processed base steel plate are machined to obtain a base steel plate or the pre-processed base steel plate. (3) Aspect 3 of the present invention relates to the pressing method of Aspect 1, wherein in the material preparation step, a part or all of the end faces of the first region of a base steel plate or a pre-processed base steel plate are laser processed to obtain the base steel plate or the pre-processed base steel plate. (4) Aspect 4 of the present invention relates to the pressing method of any one of Aspects 1 to 3, wherein the base steel plate or the pre-processed base steel plate has a tensile strength of 500 MPa or more. (5) A material steel sheet according to a fifth aspect of the present invention is a material steel sheet including a first region in which the maximum principal strain ε1 generated at the end face when pressed and the hole expansion ratio λ satisfy the following formula (2), and in part or all of the first region, the converted tensile strength converted from the hardness of the end face is 1.50 times or less the tensile strength of the material steel sheet, the arithmetic mean roughness Ra of the line roughness along the extension direction of the end face of the material steel sheet is 3.00 μm or less, and the ratio Rz / RSm obtained by dividing the maximum height Rz of the line roughness along the extension direction of the end face by the average length RSm of the roughness curve elements is 0.20 or less. 5×ln(1+λ / 100)≧ε １ ≧ 0.5 × ln(1 + λ / 100) (2) (6) Aspect 6 of the present invention is the base steel plate of Aspect 5, wherein a part or all of the end faces of the first region of the base steel plate have cutting marks. (7) Aspect 7 of the present invention is the base steel plate of Aspect 5, wherein a part or all of the end faces of the first region of the base steel plate have laser processing marks. (8) Aspect 8 of the present invention is the base steel plate of any one of Aspects 5 to 7, wherein the base steel plate has a tensile strength of 500 MPa or more.

[0011] According to the above aspects of the present invention, it is possible to provide a pressing method and a base steel sheet that are excellent in productivity and stretch flange crack resistance.

[0012] FIG. 1 is a flowchart of a pressing method according to a first embodiment of the present invention. FIG. 2 is a perspective view of an example of a base steel plate according to the first embodiment of the present invention. FIG. 3 is a perspective view of an example of a press-formed product obtained by pressing the base steel plate according to the first embodiment of the present invention. FIG. 4 is a flowchart of a pressing method according to a second embodiment of the present invention. FIG. 5 is a perspective view of an example of a base steel plate according to the second embodiment of the present invention. FIG. 6 is a diagram illustrating an example of a configuration of a die used in examples. FIG. 7 is a diagram illustrating an example of a measurement point for the height of a press-formed product.

[0013] The inventors have investigated various types of press forming and found that in common press forming methods such as drawing, forming, and bending, stretch flange cracking does not occur if the maximum principal strain ε1 generated on the edge surface by press working is less than 0.5 × ln(1 + λ / 100), where λ is the hole expansion value of the base steel sheet. The hole expansion ratio λ is the percentage of the expansion ratio in a conical hole expansion test in which a circular hole made in a material is expanded by forcing a conical punch into it.

[0014] Furthermore, the base steel sheet used for ordinary press-formed products is cut from a base steel sheet by shearing, and the end surface after shearing (hereinafter sometimes referred to as the sheared end surface) has a certain degree of roughness or more, and when the arithmetic mean roughness Ra of the line roughness along the extending direction of the end surface of the base steel sheet exceeds 3.00 μm, the stretch flangeability is reduced compared to when the arithmetic mean roughness Ra is 3.00 μm or less. The inventors have diligently studied and found that, although the end surface of the sheared surface hardens due to plastic deformation caused by shearing, when the converted tensile strength calculated from the hardness of this end surface is 1.50 times or less than the tensile strength of the base steel sheet, stretch flangeability can be significantly improved even without completely smoothing the end surface, as long as the arithmetic mean roughness Ra of the line roughness along the extending direction of the end surface of the base 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 element is 0.20 or less. Here, the converted tensile strength converted from hardness is used as an index. However, since it is technically difficult to measure the tensile strength of a local steel sheet end surface by a tensile test or the like, the surface hardness, which is easy to measure and has a high correlation with tensile strength, is measured, and the tensile strength estimated from the measured surface hardness (converted tensile strength converted from hardness) is used instead of the actual tensile strength.

[0015] The present invention is an invention based on the above findings, and the pressing method according to this embodiment defines a first region as a portion of the end surface of a raw material steel sheet or a pre-processed raw material steel sheet where the maximum principal strain ε generated on the end surface by pressing and the hole expansion ratio λ of the raw material steel sheet satisfy the following formula (1): In part or all of the first region, the converted tensile strength converted from the hardness of the end surface is 1.50 times or less the tensile strength of the raw material steel sheet, and the line roughness along the extension direction of the end surface of the raw material steel sheet has an arithmetic mean roughness Ra of 3.00 μm or less, and the ratio Rz / RSm of the maximum height Rz to the average length RSm of the roughness curve element is 0.20 or less. The method includes: a raw material preparation step of preparing a raw material steel sheet; and a pressing step of pressing the raw material steel sheet.

[0016] 5×ln(1+λ / 100)≧ε １ ≧0.5×ln(1+λ / 100)...(1)

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

[0018] (Material Preparation Step) In material preparation step S1, a material steel sheet or pre-processed material steel sheet is prepared, in which, when a portion of the end face of the material steel sheet or pre-processed material steel sheet where the maximum principal strain ε generated at the end face by pressing and the hole expansion ratio λ of the material steel sheet satisfy the above formula (1) is defined as a first region, the converted tensile strength converted from the hardness of the end face is 1.50 times or less the tensile strength of the material steel sheet or pre-processed material steel sheet in part or all of the first region, the arithmetic mean roughness Ra of the line roughness 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 to the average length RSm of the roughness curve element is 0.20 or less. Here, pre-processing refers to processing including forming a plating layer, preforming, preheating, forming holes or notches, and joining steel sheets of different strengths or different types. The pre-processing may be one or a combination of two or more of the following: forming a coating layer, pre-forming, pre-heating, forming holes or notches, and joining steel sheets of different strengths or different types. In the base steel sheet 1 according to the first embodiment, some or all of the end faces 30 of the first region S have cutting marks. In this specification, the term "end face of the base steel sheet" refers to the surface of the base steel sheet that is exposed in a direction perpendicular to the thickness direction. In the base steel sheet preparation step S1, the base steel sheet 1 is obtained by cutting.

[0019] "Material Steel Plate" FIG. 2 is a perspective view of a material steel plate 1 according to this embodiment. FIG. 3 is a perspective view of a press-formed product 100 obtained by pressing the material steel plate. The material steel plate 1 is a steel plate used to obtain the press-formed product 100. The material steel plate 1 is, for example, a steel plate with low ductility and a tensile strength of 500 MPa or more. A more preferred material steel plate 1 has a tensile strength of 700 MPa or more. A tensile strength of 700 MPa or more can further reduce the weight of vehicle body parts and the like. The material steel plate 1 may be preformed, such as pre-bending, or pre-processed, such as cutting or drilling, so that it will have the shape of the press-formed product 100 after pressing. The material steel plate 1 may further include a plating layer formed by a plating process, a coating film formed by a painting process, or the like. In the following description, the material steel plate 1 will be used, but the term "material steel plate" may be replaced with "pre-processed material steel plate."

[0020] "First region" A portion of the end face of the material steel sheet 1 where the maximum principal strain ε1 generated at the end face 30 by pressing and the hole expansion ratio λ of the material steel sheet 1 satisfy the above formula (1) is defined as the first region S. The material steel sheet 1 includes a first region where the maximum principal strain ε1 generated at the end face when pressed and the hole expansion ratio λ of the material steel sheet 1 satisfy the above formula (1). In other words, the material steel sheet 1 includes a first region S where the maximum principal strain ε1 generated at the end face 120 of the press-formed product 100 obtained by pressing the material steel sheet 1 and the hole expansion ratio λ of the material steel sheet 1 satisfy the above formula (1).

[0021] Here, the maximum principal strain ε generated on the end surface 30 by pressing １ can be obtained 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 having a circular hole formed by shearing as a test piece.

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

[0023] Here, the converted tensile strength of the flange end face (end face of the first region) of the base steel plate 1 can be obtained by measuring the end face corresponding to the first region using a micro-Vickers test based on JIS Z 2244:2009, and then converting the obtained Vickers hardness HV based on the conversion table defined by the SAE standard (SAE J 417:1983). The load used to measure the Vickers hardness is 0.025 kgf. The tensile strength of the base steel plate 1 can be obtained by measurement in accordance with JIS Z 2241:2011.

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

[0025] The portion of the first region S may be, 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 region in which the converted tensile strength satisfies the specified conditions, the more improved the stretch flange cracking resistance. 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 extending 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. When 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 in part or all of the first region S, stretch flange cracking due to pressing can be suppressed.

[0027] The part of the first region S refers to, for example, 20% 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 plate 1 may be 3.00 μm or less over 40% or more 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 plate 1 may be 3.00 μm or less over 80% or less of the total area of ​​the end face 30 of the first region S.

[0028] "In part or all of the first region, 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 0.20 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. In part or all of the first region S, 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, stretch flange cracking due to pressing can be suppressed. 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 preferably 0.10 or less.

[0029] The portion of the first region S may be, 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 element 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 element may be 0.20 or less. In the mass production of pressed parts, when considering changes in the properties of the steel sheet end face due to changes in conditions such as variations in manufacturing conditions and wear of the shear blade during shearing, even if there is an impact on stretch flangeability, the greater the proportion of the 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 element regarding the line roughness along the extension direction of the end face of the first region can be determined in accordance with JIS B 0601:2013 by measuring the line roughness along the extension direction of the end face of the first region using a shape analysis laser microscope at a magnification of 20 times and a cutoff value λc of 0.11 or 0.19 mm.

[0031] The manufacturing method of the base steel sheet 1 is not particularly limited. For example, a base steel sheet cut to a predetermined outer shape by shearing is subjected to a forming simulation using the finite element method or the like to identify a first region S. At least 20% of the obtained first region S is then subjected to cutting. The cutting is performed so that, in part or all of the first region S, the converted tensile strength calculated from the hardness of the end face is 1.50 times or less of the tensile strength of the base steel sheet, the arithmetic mean roughness Ra along the extension direction of the end face is 3.00 μm or less, and the ratio Rz / RSm of the maximum height Rz of the line roughness along the extension direction of the end face to the average length RSm of the roughness curve element is 0.20 or less. When the end face of the base steel sheet is machined by cutting, part or all of the end face of the first region S of the base steel sheet 1 has cutting marks. When cutting marks are present on the end face, the condition of the end face becomes more uniform than when shearing is performed, thereby further improving stretch flange cracking resistance. The presence or absence of cutting marks can be determined by, for example, the presence or absence of cutting streaks on the end surface. Note that the base steel sheet may be subjected to the above-mentioned pre-processing.

[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 extension direction of the end face, the maximum height Rz, and the average length RSm of the roughness curve elements, and the cutting conditions. Alternatively, they may be determined for each steel type through a shearing model test. Cutting may be performed, for example, using an end mill. When cutting with an end mill, the peripheral cutting edge of the end mill bites into the material during cutting, leaving marks that become cutting marks.

[0033] The cutting area may be the entire first region S, or only the region where stretch flange cracking is a particular concern based on press tests and experience. End faces where the maximum principal strain ε1 and the hole expansion ratio λ generated after press working do not satisfy the above formula (1) may be left as they are after shearing, or may be further cut.

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

[0035] The above describes the pressing method and the base steel sheet according to the first embodiment. In the base steel sheet 1 or pre-processed base steel sheet used in the pressing method according to the first embodiment, when the portion of the end surface 30 where the maximum principal strain ε1 generated at the end surface 30 by pressing and the hole expansion ratio λ of the base steel sheet 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 converted from the hardness of the end surface 30 is 1.50 times or less the tensile strength of the base steel sheet 1, and in the line roughness along the extension direction t1 of the end surface 30 of the base steel sheet 1, the arithmetic mean roughness Ra 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 element is 0.20 or less. Therefore, stretch flange cracking is suppressed even in areas where stretch flange cracking is likely to occur, improving design freedom.

[0036] Second Embodiment Fig. 4 is a flowchart of a pressing method according to a second embodiment of the present invention. As shown in Fig. 4, the pressing method of the present disclosure includes a material preparation step S1A and a pressing step S2 of 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 process S1A, when a portion of the end surface 30A of the material steel sheet 1A where the maximum principal strain ε generated at the end surface 30A by pressing and the hole expansion ratio λ of the material steel sheet 1A satisfy the above formula (1) is defined as a first region SA, a material steel sheet 1A is prepared in which, in part or all of the first region SA, the converted tensile strength converted from the hardness of the end surface 30A is 1.50 times or less the tensile strength of the material steel sheet 1A, the line roughness along the extension direction of the end surface 30A of the material steel sheet 1A has an arithmetic mean roughness Ra of 3.00 μm or less, and the ratio Rz / RSm of the maximum height Rz to the average length RSm of the roughness curve element is 0.20 or less. The material steel sheet 1A may be pre-formed, such as pre-bending, or pre-processed, such as cutting or drilling, so that it has the shape of the press-formed product 100 after pressing. In the material preparation process S1A, a base steel sheet 1A is obtained by cutting using laser processing. In the base steel sheet 1A according to the second embodiment, some 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 state of the end faces becomes more uniform than when shear processing is performed, thereby further improving stretch flange cracking resistance. The presence or absence of laser processing marks can be determined, for example, by the presence of striations in the plate thickness direction on the end faces.

[0038] "Material Steel Plate" FIG. 5 is a perspective view of a material steel plate 1A according to this embodiment. The material steel plate 1A is a steel plate for obtaining the press-formed product 100. The material steel plate 1A is, for example, a steel plate with low ductility and a tensile strength of 500 MPa or more. A more preferred material steel plate 1A has a tensile strength of 700 MPa or more. By making the tensile strength 700 MPa or more, the weight of vehicle body parts and the like can be further reduced. The material steel plate 1A may be subjected to pre-processing such as cutting in advance so that it has the shape of the press-formed product 100 after pressing. Furthermore, the material steel plate 1A may further include a plating layer formed by a plating process, a coating film formed by a painting process, or the like.

[0039] "First region" A portion of the end face 30A of the material steel plate 1A where the maximum principal strain ε1 generated on the surface of the end face 30A by pressing and the hole expansion ratio λ of the material steel plate 1 satisfy the following formula (1) is defined as a first region SA. The material steel plate 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 by the same method as in the first embodiment.

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

[0042] A portion of the first region SA may be, 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 converted from the hardness of the end face may be 1.50 times or less 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 in which the converted tensile strength satisfies the predetermined conditions, the more improved the stretch flange cracking resistance. In the first region SA, the converted tensile strength converted from the hardness of the end face may be 1.50 times or less 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 extending 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 the arithmetic mean roughness Ra along the extending direction t1A of the end face 30A of the material steel sheet 1A be 3.00 μm or less in part or all of the first region SA, stretch flange cracking due to pressing can be suppressed.

[0044] The portion of the first region SA may be, 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 plate 1A may be 3.00 μm or less over 40% or more of the total area of ​​the end face 30A. The wider the region in which the arithmetic mean roughness Ra satisfies the predetermined conditions, the more improved the stretch flange cracking resistance. In the first region SA, the arithmetic mean roughness Ra along the extending direction t1 of the end face 30A of the material steel plate 1A may be 3.00 μm or less over 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 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 0.20 or less." In part or all of the first region SA, the ratio Rz / RSm of the maximum height Rz of the line roughness 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. In part or all of the first region SA, the ratio Rz / RSm of the maximum height Rz of the line roughness 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, thereby suppressing stretch flange cracking due to pressing. The ratio Rz / RSm of the maximum height Rz of the end face 30A along the extending direction t1 to the average length RSm of the roughness curve elements is preferably 0.10 or less.

[0046] The portion of the first region SA may be, 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 ratio Rz / RSm of the maximum height Rz of the end face 30A along the extending direction t1A to the average length RSm of the roughness curve elements may be 0.20 or less over 40% or more of the total area of ​​the end face 30A. The wider the region in which the ratio Rz / RSm satisfies the specified conditions, the more improved the stretch flange cracking resistance. In the first region SA, the ratio Rz / RSm of the maximum height Rz of the end face 30A along the extending direction t1A to the average length RSm of the roughness curve elements may be 0.20 or less over 80% or less of the total area of ​​the end face 30A.

[0047] The manufacturing method of the base steel sheet 1A is not particularly limited. For example, the first region SA is identified using a forming simulation using the finite element method based on the shape formed by laser processing. A portion of the resulting first region SA that accounts for 20% or more of the first region SA may be cut by laser processing. The cutting by laser processing is performed so 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 base 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, 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 element is 0.20 or less. When the end portion is cut by laser processing, part or all of the end face of the base steel sheet 1A in the first region SA has laser processing marks.

[0048] The laser processing 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 end face 30A, the arithmetic mean roughness Ra, the maximum height Rz, and the mean length RSm of the roughness curve elements along the extending direction t1A of the end face 30A, and the laser processing conditions. When cutting a base steel plate with a laser, laser processing marks are left in the plate thickness direction.

[0049] The portion of the base steel sheet to be cut may be the entire first region SA. That is, all edges may be cut with a laser. Also, after shearing, only the area where stretch flange cracking is particularly likely based on press tests and experience may be processed (cut) with a laser. 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 remain sheared.

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

[0051] The above describes the pressing method and the base steel sheet according to the second embodiment. In the base steel sheet 1A used in the pressing method according to the second embodiment, when the portion of the end surface 30A where the maximum principal strain ε1 generated at the end surface 30A by pressing and the hole expansion ratio λ of the base steel sheet 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 converted from the hardness of the end surface 30A is 1.50 times or less of the tensile strength of the base steel sheet 1A, and in the line roughness along the extension direction t1A of the end surface 30A of the base steel sheet 1A, the arithmetic mean roughness Ra 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 element is 0.20 or less. Therefore, stretch flange cracking is suppressed even in areas where stretch flange cracking is likely to occur, improving the degree of freedom in design. Furthermore, in the second embodiment, since cutting is performed by laser processing, the hardness of the end face can be reduced, which further improves the degree of freedom in design.

[0052] The technical scope of the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. In addition, the components in the above-described embodiments can be replaced with well-known components as appropriate, and the above-described modifications can be combined as appropriate, without departing from the spirit of the present invention.

[0053] Examples of the present invention will be described below. However, the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to these examples. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0054] (Tensile strength TS, yield stress YS, total elongation EL) The tensile strength, yield stress YS, and total elongation EL of the steel sheets A and B (hereinafter referred to as base steel sheets) before processing 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 the base steel sheets A and B was determined by a conical hole expansion test based on JIS Z 2256: 2010 using a steel sheet having a circular hole formed by shearing as a test piece. The obtained results are shown in Table 1.

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

[0057] (Press-forming experiment) Next, the press-forming experiment will be described. In the press-forming experiment, two types of steel sheets having the material properties shown in Table 1 were bent using the die shown in FIG. 6 and compared for the occurrence of stretch flange cracking. FIG. 6 is a diagram for explaining the configuration of the die. The die 51 has an alignment portion 52. The base steel sheet 1 is provided with an alignment hole, and alignment is performed by inserting the alignment portion 52 into the hole. After the base steel sheet 1 is placed in the die 51, a presser 53 is placed, and then press-forming is performed with a punch 54 to obtain a press-formed product. FIG. 7 is a diagram for explaining the height of the press-formed product after press-forming. During press-forming, a large strain ε １ In the press forming experiment, the flange length H after bending was changed to １ The magnitude of H was changed. When H is large, ε １ becomes large, and when H is small, ε １ becomes smaller. ε １ If the value is too large, flange cracking occurs at the center end of the flange (part A in Figure 7). １ was changed.

[0058] (0.5 x ln(1 + λ / 100)) Maximum principal strain ε generated on the end surface due to press working １ was estimated by forming simulation using the finite element method. The hole expansion ratio λ and the maximum principal strain ε obtained by the simulation １ The value of 0.5×ln(1+λ / 100) was calculated from the above.

[0059] (Preparation of steel plate) The maximum principal strain ε generated on the edge surface by press working １was estimated by forming simulation using the finite element method under each condition. The base steel sheet was processed under three conditions: when the end surface (first region) of the flange, which was at risk of stretch flange cracking, obtained by simulation was sheared (condition 1), when it was cut after shearing (condition 2), and when it was laser cut (condition 3). Specifically, the base steel sheet was processed by changing the cutting conditions (cutting amount, 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 element in Table 2 were obtained, and the base steel sheets of Experiments No. 1 to 17 were obtained.

[0060] (Converted 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 a micro Vickers test 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 a conversion table defined by the SAE 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 roughness curve elements related to the line roughness 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 line 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 at a magnification of 20x and a cutoff value λc of 0.11 or 0.19 mm. The evaluation range was the area in the first region S that was cut or laser processed. The results are shown in Table 2.

[0062] (Stretch flange cracking) The effect of suppressing stretch flange cracking according to the present invention was evaluated by the press molding experiment described above. When no cracking occurred in the press-molded product, it was judged as passed, and when cracking occurred in the press-molded product, it was judged as failed. The results are shown in Table 2.

[0063]

[0064]

[0065] In Experiments No. 1, 2, and 3, Material A with a hole expansion ratio λ of 32% was press-formed to a height H of 9 mm. The processing conditions for each end face in Experiments No. 1, 2, and 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, 2, and 3, the maximum principal strain ε1 generated at the end face and the hole expansion ratio λ did not have the first region satisfying the above formula (1), so stretch flange cracking did not occur.

[0066] In Experiment No. 4, Material A was press-formed to H11 mm. The processing conditions for the end surface of the base steel sheet in Experiment No. 4 were Condition 1, and the end surface had a first region. Since the arithmetic mean roughness Ra of the end surface of the first region was greater than 3.00 μm, stretch flange cracking occurred.

[0067] In Experiment No. 5, Material A was press-formed to H11 mm. The processing conditions for the end surface of the base steel sheet in Experiment No. 5 were Condition 3 (there were laser processing marks on the end surface), and the first region was included. At all end surfaces of the first region, the converted tensile strength was greater than 1.50 times the tensile strength of the base steel sheet, so stretch flange cracking occurred.

[0068] In Experiment No. 6, Material A was press-formed to H11 mm. The processing conditions for the end surface of the base steel sheet in Experiment No. 6 were Condition 1, which included a first region. In the first region, the ratio Rz / RSm of the maximum height Rz to the average length RSm of the roughness curve element was greater than 0.20, so stretch flange cracking occurred.

[0069] In Experiment No. 7, Material A was press-formed to H11 mm. The processing conditions for the end surface of the base steel plate in Experiment No. 7 were Condition 2 (there were cutting marks on the end surface), and the first region was included. However, in all end surfaces of the first region, the converted tensile strength σm was 1.50 times or less the tensile strength of the base steel plate, Ra was 3.00 μm or less, and the ratio Rz / RSm of the maximum height Rz to the average length RSm of the roughness curve element was 0.20 or less, so stretch flange cracking did not occur.

[0070] In Experiment No. 8, Material A was press-formed to H11 mm. The processing conditions for the end surface of the base steel sheet in Experiment No. 8 were Condition 3 (with laser processing marks on the end surface), and the first region was present. However, in all end surfaces of the first region of the base steel sheet in Experiment No. 8, the converted tensile strength σm was 1.50 times or less the tensile strength of the base steel sheet, Ra was 3.00 μm or less, and the ratio Rz / RSm of the maximum height Rz to the average length RSm of the roughness curve element was 0.20 or less, so stretch flange cracking did not occur.

[0071] In Experiment No. 9, Material A was press-formed to H13 mm. The processing conditions for the end surface of the base steel sheet in Experiment No. 9 were Condition 3 (with laser processing marks on the end surface), and the first region was included. In all end surfaces of the first region of the base steel sheet in Experiment No. 9, the converted tensile strength σm was 1.50 times or less the tensile strength of the base steel sheet, Ra was 3.00 μm or less, and the ratio Rz / RSm of the maximum height Rz to the average length RSm of the roughness curve element was 0.20 or less, so stretch flange cracking did not occur.

[0072] In Experiment No. 10, Material A was press-formed to H15 mm. The processing conditions for the end surface of the base steel sheet in Experiment No. 10 were Condition 3 (with laser processing marks on the end surface), and the first region was included. In all end surfaces of the first region of the base steel sheet in Experiment No. 10, the converted tensile strength σm was 1.50 times or less the tensile strength of the base steel sheet, Ra was 3.00 μm or less, and the ratio Rz / RSm of the maximum height Rz to the average length RSm of the roughness curve element was 0.20 or less, so stretch flange cracking did not occur.

[0073] In Experiment No. 11, Material B with a hole expansion ratio λ of 54% was press-formed to H13 mm. The processing conditions for the end surface of the base steel sheet in Experiment No. 11 were Condition 1. Although the ratio Rz / RSm of the maximum height Rz to the average length RSm of the roughness curve element was 0.20 or more, stretch flange cracking did not occur because there was no first region satisfying the above formula (1).

[0074] In Experiment No. 12, Material B was press-formed to H15 mm. The processing conditions for the end surface of the base steel sheet in Experiment No. 12 were Condition 1, which included a first region. In the first region, the ratio Rz / RSm of the maximum height Rz to the average length RSm of the roughness curve element was 0.20 or more, so stretch flange cracking occurred.

[0075] In Experiment No. 13, Material B was press-formed to H17 mm. The processing conditions for the end surface of the base steel sheet in Experiment No. 13 were Condition 2 (there were cutting marks on the end surface), and the first region was formed. In all end surfaces of the first region, the converted tensile strength σm was 1.50 times or less the tensile strength of the base steel sheet, Ra was 3.00 μm or less, and the ratio Rz / RSm of the maximum height Rz to the average length RSm of the roughness curve element was 0.20 or less, so stretch flange cracking did not occur.

[0076] In Experiment No. 14, Material B was press-formed to H17 mm. The processing conditions for the end surface of the base steel sheet in Experiment No. 14 were Condition 3 (with laser processing marks on the end surface), and the first region was included. In all end surfaces of the first region of the base steel sheet in Experiment No. 14, the converted tensile strength σm was 1.50 times or less the tensile strength of the base steel sheet, Ra was 3.00 μm or less, and the ratio Rz / RSm of the maximum height Rz to the average length RSm of the roughness curve element was 0.20 or less, so stretch flange cracking did not occur.

[0077] In Experiment No. 15, Material B was press-formed to H20 mm. The processing conditions for the end surface of the base steel sheet in Experiment No. 15 were Condition 3 (with laser processing marks on the end surface), and the first region was included. In all end surfaces of the first region of the base steel sheet in Experiment No. 15, the converted tensile strength σm was 1.50 times or less the tensile strength of the base steel sheet, Ra was 3.00 μm or less, and the ratio Rz / RSm of the maximum height Rz to the average length RSm of the roughness curve element was 0.20 or less, so stretch flange cracking did not occur.

[0078] In Experiment No. 16, Material B was press-formed to H22 mm. The processing conditions for the end surface of the base steel sheet in Experiment No. 16 were Condition 3 (with laser processing marks on the end surface), and the first region was included. In all end surfaces of the first region of the base steel sheet in Experiment No. 16, the converted tensile strength σm was 1.50 times or less the tensile strength of the base steel sheet, Ra was 3.00 μm or less, and the ratio Rz / RSm of the maximum height Rz to the average length RSm of the roughness curve element was 0.20 or less, so stretch flange cracking did not occur.

[0079] In Experiment No. 17, Material A was press-formed to H11 mm. The end surface of the base steel sheet in Experiment No. 17 was a laser-processed surface (with laser processing marks on the end surface) and had a first region. In Experiment No. 17, in 30% of the end surface of the first region of the base steel sheet, the converted tensile strength σm was 1.50 times or less the tensile strength of the base steel sheet, Ra was 3.00 μm or less, and the ratio Rz / RSm of the maximum height Rz to the average length RSm of the roughness curve element was 0.20 or less, so stretch flange cracking did not occur.

[0080] The pressing method of the present disclosure has excellent productivity and excellent stretch flange crack resistance, and therefore has high industrial applicability.

[0081] 1: Material steel plate, 30: End surface, 100: Press-molded product, S: First region

Claims

1. A pressing method for obtaining a press-formed product by pressing a stock steel plate or a pre-processed stock steel plate, comprising: a material preparation step of preparing the stock steel plate or the pre-processed stock steel plate in which, when a portion of an end face of the stock steel plate or the pre-processed stock steel plate where a maximum principal strain ε1 generated on the end face by pressing and a hole expansion ratio λ of the stock steel plate or the pre-processed stock steel plate satisfy the following formula (1) is defined as a first region, the converted tensile strength converted from the hardness of the end face is 1.50 times or less of the tensile strength of the stock steel plate or the pre-processed stock steel plate in a part or all of the first region, the arithmetic mean roughness Ra of the line roughness along the extension direction of the end face of the stock steel plate or the pre-processed stock steel plate is 3.00 μm or less, and the ratio Rz / RSm of the maximum height Rz of the line roughness along the extension direction of the end face to the average length RSm of the roughness curve element is 0.20 or less; and a pressing step of pressing the steel sheet or the pre-processed steel sheet. １ ≧0.5×ln(1+λ / 100)...(1) 2. A pressing method as described in claim 1, wherein in the material preparation process, a part or all of the end faces of the first region of the base steel plate or the pre-processed base steel plate are machined to obtain the base steel plate or the pre-processed base steel plate.

3. A pressing method as described in claim 1, wherein in the material preparation process, a part or all of the end faces of the first region of the base steel plate or the pre-processed base steel plate are laser processed to obtain the base steel plate or the pre-processed base steel plate.

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

5. A base steel sheet including a first region in which the maximum principal strain ε1 generated at an end face when pressed and the hole expansion ratio λ satisfy the following formula (2), wherein in a part or all of the first region, the converted tensile strength converted 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 of the line roughness along the extension direction of the end face of the base steel sheet is 3.00 μm or less, and the ratio Rz / RSm obtained by dividing the maximum height Rz of the line roughness along the extension direction of the end face by the average length RSm of the roughness curve element is 0.20 or less. 5×ln(1+λ / 100)≧ε １ ≧0.5×ln(1+λ / 100)...(2) 6. The base steel plate according to claim 5, wherein a part or all of the end faces of the first region of the base steel plate have cutting marks.

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

8. The base steel plate according to claim 5, wherein the base steel plate has a tensile strength of 500 MPa or more.

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