Manufacturing method of processed material and processed material

Abstract

To reduce tensile residual stress of a fracture surface on a cutting end surface having a relatively large curvature radius when a steel material is processed so as to form a cutting end surface, having a relatively small curvature radius, and a cutting end surface, having a relatively large curvature radius, to produce a processed material.SOLUTION: A method of cutting a steel material to produce a processed material includes: performing cutting to the steel material to form a first cutting end surface having a relatively small curvature radius; and performing shearing to the steel material after the first cutting end surface is formed to form a second cutting end surface having a relatively large curvature radius. In the method, the second cutting end surface is continuous with the first cutting end surface.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present application discloses a method for producing a workpiece and the workpiece. [Background technology]

[0002] Patent Document 1 discloses a technique for obtaining a processed material by shearing a steel material using a punch and a die. In Patent Document 1, the fracture surface of the punched material is pressed against the fracture surface of the processed material to reduce the tensile residual stress of the fracture surface of the processed material.

[0003] Patent Document 2 discloses a technique for punching a steel plate using a punch and a die to form a punch hole. In Patent Document 2, in the curved portion with a small radius of curvature among the sheared end faces defining the punch hole, the length of the fracture surface in the plate thickness direction is increased and the proportion of the sheared surface is reduced, thereby suppressing stretch flange cracking when the curved portion is stretch flanged. On the other hand, in the portion with a large radius of curvature other than the curved portion among the sheared end faces defining the punch hole, the length of the fracture surface in the plate thickness direction is shortened and the proportion of the sheared surface is increased, thereby suppressing a decrease in fatigue strength. In Patent Document 2, when shearing the steel material, a sheared surface is formed in the other portion prior to the portion corresponding to the curved portion. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2016 / 136909 [Patent Document 2] International Publication No. 2016 / 092657 Summary of the Invention [Problem to be solved by the invention]

[0005] As disclosed in Patent Document 1, the tensile residual stress at the fracture surface may be large in a processed material obtained by shearing a steel material. When a processed material is obtained by shearing a steel material, a new technology capable of reducing the tensile residual stress at the fracture surface of the processed material is required.

[0006] In addition, when a first cut end surface 1x (see, for example, FIG. 1) having a relatively small radius of curvature and a second cut end surface 1y (see, for example, FIG. 1) having a relatively large radius of curvature are formed with respect to the steel material, the first cut end surface 1x is more likely to deform during subsequent forming processing, for example, than the second cut end surface 1y, and the tensile residual stress is more likely to be eliminated. In other words, even if a large tensile residual stress occurs in the first cut end surface 1x having a small radius of curvature, the performance of the processed material is less likely to be degraded. In this regard, when a first cut end surface 1x having a relatively small radius of curvature and a second cut end surface 1y having a relatively large radius of curvature are formed with respect to the steel material, it is preferable to reduce the tensile residual stress in the second cut end surface 1y. [Means for solving the problem]

[0007] As one of the means for solving the above problems, the present application provides: A method for cutting a steel material to produce a processed material, comprising the steps of: A cutting process is performed on the steel material to form a first cut end surface having a relatively small radius of curvature; and After forming the first cut end surface, the steel material is subjected to a shearing process to form a second cut end surface having a relatively large radius of curvature, wherein the second cut end surface is continuous with the first cut end surface; Including, Manufacturing method of processed materials Disclose.

[0008] In the manufacturing method of the present disclosure, a ratio R2 / R1 of a radius of curvature R2 (mm) of the second cut end surface to a radius of curvature R1 (mm) of the first cut end surface may be 2.0 or more.

[0009] In the manufacturing method of the present disclosure, the first cut end surface may be formed by shearing.

[0010] In the manufacturing method of the present disclosure, the steel material may be in a plate shape.

[0011] In the manufacturing method of the present disclosure, the steel material may have a tensile strength of 980 MPa or more.

[0012] In the manufacturing method of the present disclosure, the steel material may have a tensile strength of 1470 MPa or more.

[0013] As one of the means for solving the above problems, the present application provides: A workpiece made of a steel material having a cut end surface, The cut end surface includes a first cut end surface having a relatively small radius of curvature and a second cut end surface having a relatively large radius of curvature, The first cut end surface and the second cut end surface are continuous, the first cut end surface includes a first sag, a first fracture surface, and a first burr; The second cut end surface includes a second sag, a second fracture surface, and a second burr, the first fracture surface includes a first portion and a second portion; the second fracture surface includes a third portion and a fourth portion; The first portion is formed by a first crack that propagates from the first sag side toward the first burr side, The second portion is formed by a second crack that propagates from the first burr side toward the first sag side, The third portion is formed by a third crack that propagates from the second sag side toward the second burr side, The fourth portion is formed by a fourth crack that propagates from the second burr side toward the second sag side, The area ratio of the first portion to the first fracture surface is X1 (%), The area ratio of the second portion to the first fracture surface is X2 (%), The area ratio of the third portion to the second fracture surface is Y1 (%), When the area ratio of the fourth portion to the second fracture surface is Y2 (%), Y1 is 1.1 times or more than X1. processed material Disclose.

[0014] In the processed material disclosed herein, Y2 <Y1であってもよい。

[0015] In the processed material disclosed herein, X1 / X2 <Y1 / Y2であってもよい。

[0016] In the processed material disclosed herein, at the boundary between the first cut end surface and the second cut end surface, the boundary line between the first portion and the second portion at the first cut end surface and the boundary line between the third portion and the fourth portion at the second cut end surface may be discontinuous. Effect of the Invention

[0017] According to the manufacturing method disclosed herein, when a processed material is manufactured by forming a first cut end surface having a relatively small radius of curvature with respect to a steel material and a second cut end surface (shear end surface) having a relatively large radius of curvature, the tensile residual stress of the fracture surface at the second cut end surface having the relatively large radius of curvature can be reduced. [Brief description of the drawings]

[0018] [Figure 1] Schematic diagrams for explaining an example of the flow of shear processing, in which (A) shows a state in which the first blade and the second blade are separated, (B) shows a state in which a steel material is placed between the first blade and the second blade, (C) shows a state in which the first blade and the second blade are moved relatively and brought closer to each other to punch out a part of the steel material, and (D) shows a state in which the first blade and the second blade are separated and returned to the position of (A). [Diagram 2]1 is a schematic diagram for explaining an example of a mechanism for forming a cut end surface when a steel material is sheared. The cross section is along the relative movement direction of the first blade and the second blade, and shows the form of the cross section including the first blade, the second blade, and the steel material. (A) shows a state in which a sag is formed in the steel material by pressing the first blade and the second blade against the steel material, (B) shows a state in which a crack is generated in the steel material by further pressing the first blade and the second blade against the steel material after the sag is formed, and (C) shows a state in which a part of the steel material is punched out by further pressing the first blade and the second blade against the steel material after the crack is formed. [Diagram 3] Schematic diagrams for explaining new findings by the inventors. (A) shows the case where the crack propagates from the first blade, (B) shows the case where the crack propagates from both the first and second blades, and (C) shows the case where the crack propagates from the second blade. "◯" means that the tensile residual stress is small, "△" means that the tensile residual stress is medium, and "×" means that the tensile residual stress is large. [Figure 4] 1A and 1B are schematic cross-sectional views for explaining the relationship between the scrap restraint and the crack propagation direction, where (A) is the case where the scrap restraint is small, and (B) is the case where the scrap restraint is large. [Diagram 5] 1 is a schematic diagram for explaining an example of a flow of a manufacturing method for a processed material. In a plan view, the central part of a steel material is punched to provide a hole. (A) shows the cutting line for the steel material with a dotted line, (B) shows the state of the steel material after the first cutting stage, and (C) shows the state of the processed material obtained after the second shearing stage. [Figure 6] 1 is a schematic cross-sectional view for explaining another example of the flow of the manufacturing method of the processed material. In plan view, the entire circumference of the outer edge of the steel material is punched out, and the central part of the steel material is left as the processed material. (A) shows the cutting line for the steel material with a dotted line, (B) shows the state of the steel material after the first cutting stage, and (C) shows the state of the processed material obtained after the second shearing stage. [Figure 7]FIG. 1 is a schematic cross-sectional view for explaining another example of the flow of the manufacturing method of the processed material. In plan view, one end of the steel material is punched out in a curved shape to form an open cross section. (A) shows the cutting line for the steel material with a dotted line, (B) shows the state of the steel material after the first cutting stage, and (C) shows the state of the processed material obtained after the second shearing stage. [Figure 8] 1 is a schematic diagram for explaining an example of the configuration of a processed material manufactured by the manufacturing method of the present disclosure, which shows a cross-sectional shape including a second cut end surface (shear end surface) having a large radius of curvature of the processed material. [Figure 9] 1 is a schematic diagram for explaining an example of the configuration of a second cut end surface (sheared end surface) of a processed material according to the present disclosure, showing the second cut end surface as viewed from the front. [Figure 10] 1A is a schematic diagram for explaining a method for distinguishing between a first portion and a second portion on a fracture surface, in which (A) shows a schematic diagram of the direction of hydrogen embrittlement cracking occurring on the fracture surface, and (B) shows a schematic diagram of the relationship between an arbitrary position X between the burr side and the sag side on the fracture surface and the direction (angle θ) of the hydrogen embrittlement cracking. [Figure 11] 1 is a schematic diagram for explaining an example of properties of a processed material when both a first cut end surface and a second cut end surface are formed by "shearing". The diagram shows the state in which each of the first cut end surface and the second cut end surface is viewed from the front. [Figure 12] 1 is a schematic diagram for explaining the shape of a cutting line for a steel plate in a plan view in relation to Example 1 and Comparative Example 1. The cutting line is indicated by a dotted line. [Figure 13] FIG. 2 is a schematic diagram for explaining a method for measuring residual stress in a cut end surface in relation to Example 1 and Comparative Example 1. [Figure 14] 12A and 12B are diagrams showing the inclination (inclination angle) that occurs in the scrap when the second cut end surface is formed, in relation to Example 1 and Comparative Example 1. The diagrams show the state in the X-X' cross section of FIG. 12. (A) is Example 1, and (B) is Comparative Example 1. [Figure 15] 4 is a graph showing the tensile residual stress values ​​generated on the second cut end faces on the scrap side and the processed material side for Example 1 and Comparative Example 1. [Figure 16] 1 is a schematic diagram for explaining the shape of a cutting line for a steel plate in a plan view in relation to Example 2 and Comparative Example 2. The cutting line is indicated by a dotted line. [Figure 17] 10 is a graph showing the tensile residual stress values ​​generated on the second cut end faces on the scrap side and the processed material side for Example 2 and Comparative Example 2. [Figure 18] 1 is a schematic diagram for explaining the shape of a cutting line for a steel plate in a plan view in relation to Example 3 and Comparative Example 3. The cutting line is indicated by a dotted line. [Figure 19] 10 is a graph showing the tensile residual stress values ​​generated on the second cut end faces on the scrap side and the processed material side for Example 3 and Comparative Example 3. [Figure 20] 1 is a schematic diagram for explaining the shape of a cutting line for a steel plate in a plan view in relation to Example 4 and Comparative Example 4. The cutting lines are indicated by dotted lines. [Figure 21] 10 is a graph showing the tensile residual stress values ​​generated on the second cut end faces on the scrap side and the processed material side for Example 4 and Comparative Example 4. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] 1. Issues and new findings 1(A) to (D) show an example of a flow when a steel material is sheared to obtain a processed material. First, as shown in FIG. 1(A) and (B), a steel material 5 is placed between a first blade 21 and a second blade 22 in a state where the first blade 21 and the second blade 22 are separated from each other. Here, the steel material 5 has a first surface 10a and a second surface 10b opposite to the first surface 10a, the first blade 21 has a first bottom surface 21a, a first side surface 21b, and a first tip portion 21x (see FIG. 2(A)), and the second blade 22 has a second bottom surface 22a, a second side surface 22b, and a second tip portion 22x (see FIG. 2(A)). The first tip portion 21x and the second tip portion 22x each have a portion with a small radius of curvature and a portion with a large radius of curvature in a plan view. As shown in Figs. 1(B) and (C), the first bottom surface 21a contacts the first surface 10a of the steel material 5, and the second bottom surface 22a contacts the second surface 10b of the steel material 5. The first blade 21 may be a punch, and the second blade 22 may be a die. Next, as shown in Fig. 1(C), the first blade 21 and the second blade 22 are moved relatively to shear the steel material 5. As a result, as shown in Figs. 1(C) and (D), a part of the steel material 5 is punched out as scrap 15 (see Fig. 2(C)) by the first blade 21, and the remaining part of the steel material 5 can become a processed material 10 having a cut end surface 1. The scrap 15 may be used for some kind of product. As shown in FIG. 1(D), the cut end surface 1 of the processed material 10 has a first cut end surface 1x with a small radius of curvature and a second cut end surface 1y with a large radius of curvature, corresponding to the shapes of the above-mentioned first tip portion 21x and second tip portion 22x.

[0020] 1(A) to (D) show a form in which a punch hole is formed in a part of the steel material 5 by shearing the steel material 5 with the first blade 21 and the second blade 22, but the end of the steel material 5 may be sheared and removed with the first blade 21 and the second blade 22, or a slit or the like may be formed by shearing the steel material 5 with the first blade 21 and the second blade 22. The shape of the cut end surface 1 in plan view is also not limited to the form shown in the figures, and may be any shape as long as it is a combination of a first cut end surface 1x having a relatively small radius of curvature and a second cut end surface 1y having a relatively large radius of curvature.

[0021] An example of the mechanism for forming a cut end surface by shearing will be described. As shown in Figs. 2(A) to (C), a workpiece 10 having a cut end surface 1 is obtained by shearing a steel material 5 with a first blade 21 and a second blade 22. As shown in Fig. 2(A), a first bottom surface 21a of the first blade 21 is pressed against a first surface 10a of the steel material 5, so that a sag 1a is formed on the first surface 10a side of the steel material 5. The sag 1a is formed in the process until the first tip portion 21x of the first blade 21 bites into the steel material 5. After the sag 1a is formed, a shear surface 1e (see Fig. 8) may be formed in the process in which the first tip portion 21x bites into the steel material 5. After the sag 1a is formed, or after the sag 1a and the shear surface 1e are formed, a first crack 1dx occurs from the first blade 21 side toward the second blade 22 side, as shown in Fig. 2(B). On the other hand, on the second blade 22 side, after the second tip portion 22x bites into the second surface 10b of the steel material 5, a second crack 1dy is generated from the second blade 22 side toward the first blade 21 side. As shown in FIG. 2(C), the first crack 1dx and the second crack 1dy each progress and join together to form a fracture surface 1b. In addition, by further moving the first blade 21 and the second blade 22, the steel material 5 is separated into scrap 15 and the processed material 10, which is the target. At this time, as shown in FIG. 2(C), a burr 1c may be formed at the corner of the cut end surface 1 of the processed material 10 on the second blade 22 side. Regardless of the presence or absence of a shear angle between the first blade 21 and the second blade 22 and the form of the cut end surface 1 in a plan view (open cross section, closed cross section, etc.), the cut end surface 1 may be formed by shearing through the mechanism shown in FIG. 2(A) to (C).

[0022] In the cut end surface 1 formed as described above, compressive residual stress or tensile residual stress may occur due to damage or distortion caused by shear. If a large tensile residual stress exists in the cut end surface 1 caused by shear, for example, the hydrogen embrittlement resistance or fatigue strength of the cut end surface 1 is likely to decrease. In this regard, in order to obtain a processed material 10 with high performance, how to reduce the tensile residual stress in the cut end surface 1 can be an issue. In particular, as disclosed in Patent Document 1, it is desirable to reduce the tensile residual stress in the fracture surface 1b of the cut end surface 1.

[0023] 1, when the steel material 5 is cut to form a first cut end surface 1x having a relatively small radius of curvature and a second cut end surface 1y having a relatively large radius of curvature, the first cut end surface 1x is more likely to deform during subsequent forming and the like than the second cut end surface 1y, and the tensile residual stress is more likely to be eliminated. That is, even if a large tensile residual stress occurs in the first cut end surface 1x having a small radius of curvature, the performance of the processed material 10 is less likely to be degraded. In this regard, when the steel material 5 is cut to form a first cut end surface 1x having a relatively small radius of curvature and a second cut end surface 1y having a relatively large radius of curvature, it is preferable that the tensile residual stress can be reduced in the second cut end surface 1y more than in the first cut end surface 1x.

[0024] The present inventors have conducted numerous experiments and analyses on the relationship between the conditions of shearing the steel material 5 and the properties of the cut end surface 1 caused by the shearing, and as a result have obtained the following new findings.

[0025] As shown in Figs. 3(A) to (C), a case will be described in which a part 11 of a steel material 5 is punched out by the first blade 21 and another part 12 of the steel material 5 is punched out by the second blade 22. In this case, as shown in Fig. 3(A), if a crack propagates preferentially from the first blade 21 side, the tensile residual stress at the cut end surface of the part 11 becomes large, while the tensile residual stress at the cut end surface of the other part 12 becomes small. That is, the part 11 is used as scrap 15, while the other part 12 can be suitably used as a product (processed material 10). Also, as shown in Fig. 3(B), if a crack propagates equally from both the first blade 21 side and the second blade 22 side, the same tensile residual stress can be generated at the cut end surfaces of both the part 11 and the other part 12. That is, the variation in the properties of the part 11 and the other part 12 is suppressed. In this respect, it can be said to be suitable for using both the part 11 and the other part 12 as products. 3(C), when the crack propagates preferentially from the second blade 22 side, the tensile residual stress at the cut end surface of the other part 12 becomes large, while the tensile residual stress at the cut end surface of the part 11 becomes small. In other words, the other part 12 is treated as scrap 15, while the part 11 can be suitably used as the product (processed material 10).

[0026] From the above findings, the following can be concluded: (1) The tensile residual stress generated at the fracture surface 1b of the cut edge 1 varies depending on the propagation direction and length of the cracks 1dx and 1dy that form the fracture surface 1b. (2) At the fracture surface 1b, as the crack 1dx that propagates from the sag 1a side (the first blade 21 side) becomes longer, the tensile residual stress at the fracture surface 1b of the processed material 10 becomes smaller and the tensile residual stress at the fracture surface of the scrap 15 becomes larger. (3) In other words, when the area ratio of the portion at the fracture surface 1b of the processed material 10 originating from the first crack 1dx propagating from the sag 1a side is larger than the area ratio of the portion at the fracture surface 1b originating from the second crack 1dy propagating from the burr 1c side, the tensile residual stress at the fracture surface 1b can be relatively reduced compared to when the area ratio of the portion at the first crack 1dx propagating from the sag 1a side is smaller than the area ratio of the portion at the second crack 1dy propagating from the burr 1c side.

[0027] The present inventors have conducted numerous experiments and analyses to control the propagation direction and length of the cracks 1dx and 1dy when the steel material 5 is sheared, and as a result, have obtained the following new findings.

[0028] As shown in Figures 4(A) and (B), the propagation direction and length of the cracks 1dx and 1dy change depending on the state of restraint of the scrap 15 during shearing. Specifically, when the scrap 15 is little restrained, the inclination angle α is likely to occur toward the scrap 15 during shearing, and as shown by the white arrow in Figure 4(A), the crack 1dx is likely to propagate from the first blade 21, and as a result, the tensile residual stress at the fracture surface 1b of the processed material 10 is likely to be small. On the other hand, when the scrap 15 is little restrained, the inclination angle α is unlikely to occur toward the scrap 15 during shearing, and as shown by the white arrow in Figure 4(B), the cracks 1dx and 1dy are likely to propagate from both the first blade 21 and the second blade 22, and as a result, the tensile residual stress at the fracture surface 1b of the processed material 10 is likely to be large.

[0029] The restraint state of the scrap during shearing varies depending on the radius of curvature of the cut end surface, etc. For example, when a cut end surface with a large radius of curvature is formed, the scrap 15 is less restrained and the inclination angle α is more likely to occur toward the scrap 15 during shearing than when a cut end surface with a small radius of curvature is formed. Also, when an open cross section such as trimming is formed, the scrap 15 is less restrained and the inclination angle α is more likely to occur toward the scrap 15 during shearing than when a closed cross section such as a punched hole is formed.

[0030] From the above findings, the following can be concluded: (4) In the case where a part of the steel material 5 is sheared to form a first cut end surface 1x having a small radius of curvature and a second cut end surface 1y having a large radius of curvature, if an attempt is made to simultaneously form the first cut end surface 1x and the second cut end surface 1y by a single shear, the constraint on the scrap 15 becomes large, making it difficult to generate a tilt angle α toward the scrap 15, and tensile residual stress is likely to occur in both the first cut end surface 1x and the second cut end surface 1y. (5) When a part of the steel material 5 is sheared to form a first cut end surface 1x with a small radius of curvature, and then a second cut end surface 1y with a large radius of curvature is formed, the first cut end surface 1x is subjected to a large constraint by the scrap 15, so that tensile residual stress occurs in the first cut end surface 1x. However, when the second cut end surface 1y is subsequently formed, the scrap 15 is subjected to a small constraint, so that the tensile residual stress occurring in the second cut end surface 1y can be relatively reduced. Here, as described above, the tensile residual stress occurring in the first cut end surface 1x with a small radius of curvature is unlikely to degrade the performance of the processed material 10. (6) In the case where the first cut end surface 1x having a small curvature radius is formed on the steel material 5, and then the second cut end surface 1y having a large curvature radius is formed, the first cut end surface 1x may be formed by a method other than shear processing. That is, even in the case where the first cut end surface 1x having a small curvature radius is formed by a cutting processing not limited to shear processing, and then the second cut end surface 1y having a large curvature radius is formed by shear processing, the tensile residual stress can be reduced in the second cut end surface 1y.

[0031] The manufacturing method of the processed material disclosed herein has been completed based on the above findings.

[0032] 2. Manufacturing method of processed material (first embodiment) 5(A) to (C) show an example of a method for cutting a steel material 5 to produce a processed material 10. As shown in FIG. 5(A) to (C), the manufacturing method of the present disclosure includes the following steps: A cutting process is performed on the steel material 5 to form a first cut end surface 1x having a relatively small radius of curvature (FIGS. 5(A) and 5(B)); and After forming the first cut end surface 1x, the steel material 5 is subjected to a shearing process to form a second cut end surface 1y having a relatively large radius of curvature, where the second cut end surface 1y is continuous with the first cut end surface 1x (FIG. 5(C)). Includes.

[0033] 2.1 Steel materials The shape of the steel material 5 is not particularly limited as long as it can be sheared. The steel material 5 may be, for example, plate-shaped or rod-shaped. When the steel material 5 is plate-shaped, the plate thickness may be, for example, 0.8 mm or more and 3.0 mm or less. When the steel material 5 is rod-shaped, the cross-sectional shape is not particularly limited, and may be, for example, circular or polygonal, and the circle-equivalent diameter of the cross-section may be 10 mm or more and 150 mm or less. Furthermore, the steel material 5 may be formed into some shape by bending or the like.

[0034] As shown in FIG. 5(A), the steel material 5 may have a first surface 10a and a second surface 10b opposite to the first surface 10a. The first surface 10a and the second surface 10b may be parallel to each other. In this application, "parallel" is not limited to perfect parallel, but may be substantially parallel. In other words, even if the first surface 10a and the second surface 10b are not perfect parallel, they are considered to be parallel as long as they are within the range of tolerance allowed in industrial production. Specifically, when the angle between the first surface 10a and the second surface 10b is 0°±1°, the first surface 10a and the second surface 10b are considered to be parallel.

[0035] The steel material 5 may have a surface treatment layer. Examples of the surface treatment layer include a plating layer and a coating film. The steel material 5 may also include a plurality of layers of different steel types. For example, clad steel may be used as the steel material 5.

[0036] The mechanical properties of the steel material 5 are not particularly limited, and may be appropriately determined depending on the application of the processed material 10. However, the problem of reduced hydrogen embrittlement resistance due to tensile residual stress is particularly likely to occur in high-strength steel materials. In this regard, the tensile strength of the steel material 5 may be, for example, 980 MPa or more, 1180 MPa or more, or 1470 MPa or more. The upper limit of the tensile strength of the steel material 5 is not particularly limited, but may be, for example, 2500 MPa or less, 2200 MPa or less, or 2000 MPa or less. The "tensile strength" of the steel material in this application is in accordance with ISO 6892-1:2009.

[0037] The chemical composition and metal structure of the steel material 5 are not particularly limited, and may be appropriately determined depending on the application of the processed material 10. According to the technology of the present disclosure, it is possible to reduce the tensile residual stress at the fracture surface 1b regardless of the chemical composition and metal structure of the steel material 5. As an example of the chemical composition, the steel material 5 has, in mass %, C: 0.050 to 0.800%, Si: 0.01 to 3.00%, Mn: 0.01 to 10.00%, Al: 0.001 to 0.500%, P: 0.100% or less, S: 0.050% or less, N: 0.010% or less, Cr: 0 to 3.000%, Mo: 0 to 1.000%, B: 0 to 0.0100%, Ti: 0 to 0.500%, Nb: 0 to 0.500%, V: 0 to 0.500%, Cu: 0 to 0.50 %, Ni: 0-0.50%, O: 0-0.020%, W: 0-0.100%, Ta: 0-0.10%, Co: 0-0.50%, Sn: 0-0.050%, Sb: 0-0.050%, As: 0-0.050%, Mg: 0-0.050%, Ca: 0-0.050%, Y: 0-0.050%, Zr: 0-0.050%, La: 0-0.050%, Ce: 0-0.050%, and the balance: Fe and impurities. In addition, in the above chemical composition of steel material 5, the lower limit of the content of the optionally added elements Cr, Mo, B, Ti, Nb, V, Cu, Ni, O, W, Ta, Co, Sn, Sb, As, Mg, Ca, Y, Zr, La, and Ce may be 0.0001% or 0.001%.

[0038] 2.2 Formation of the first cut edge In the manufacturing method of the present disclosure, first, the steel material 5 as described above is cut to form a first cut end surface 1x having a relatively small radius of curvature. For example, as shown in Figs. 5(A) and (B), when a part of the steel material 5 is punched out to form a punched hole, the entire cutting line L defining the punched hole is not punched out at once, but the part of the cutting line L having a relatively small radius of curvature is cut first to form the first cut end surface 1x. Note that the part of the cutting line L having a relatively small radius of curvature and the part of the cutting line L having a large radius of curvature may be appropriately determined according to the part where the tensile residual stress is to be reduced. The manufacturing method of the present disclosure is characterized in that the cutting and shearing are performed while changing the restraint state of the scrap 15 (see Figs. 2(C), 4(A) and (B)) by cutting the entire cutting line L in one go, but by cutting it in two or more parts. It is not necessary to strictly specify the boundary between the part with the relatively small radius of curvature and the part with the relatively large radius of curvature. 5(B), for example, the first cut end surface 1x may include a curved portion and a small straight portion of the cutting line L. In addition, the portion where it is desired to reduce the tensile residual stress the most may be included in a second cut end surface 1y described later, and the other portion may be the first cut end surface 1x.

[0039] The radius of curvature of a portion of the cutting line L that has a relatively small radius of curvature may be, for example, 100 mm or less. On the other hand, the portion of the cutting line L that has the largest radius of curvature may be a straight line.

[0040] The first cut end surface 1x may have a relatively smaller radius of curvature than the second cut end surface 1y described later in plan view. For example, the ratio R2 / R1 of the radius of curvature R2 (mm) of the second cut end surface 1y to the radius of curvature R1 (mm) of the first cut end surface 1x may be 2.0 or more, or 5.0 or more. In the present application, the radius of curvature R1 of the first cut end surface 1x and the radius of curvature R2 of the second cut end surface 1y are specified as follows. That is, the portions of the cut end surfaces 1x and 1y with the smallest radius of curvature are specified and regarded as R1 and R2, respectively. Specifically, the radius of curvature is measured based on three adjacent points spaced 2 mm apart on the plane of the steel material 5, the portions where the measured radius of curvature is smallest are specified, and the radius of curvature R1 of the first cut end surface 1x and the radius of curvature R2 of the second cut end surface 1y are specified.

[0041] The length of the first cut end surface 1x in plan view is not particularly limited, but may be, for example, 10 mm or more or 500 mm or less. Alternatively, the ratio L1 / L2 of the length L1 of one first cut end surface 1x to the length L2 of one second cut end surface 1y may be 0.5 or more or 3.0 or less.

[0042] The formation of the first cut end surface 1x is not limited to shear processing. For example, the first cut end surface 1x may be formed by a general cutting process other than shear processing, such as laser processing or mechanical processing. However, from the viewpoint of standardizing the manufacturing equipment and simplifying the process, the first cut end surface 1x may be formed by shear processing. An example of the shear processing will be described later.

[0043] 2.3 Formation of the second cut edge In the manufacturing method of the present disclosure, after forming the first cut end surface 1x, the steel material 5 is subjected to shear processing to form a second cut end surface 1y having a relatively large radius of curvature. Here, the second cut end surface 1y is continuous with the first cut end surface 1x. That is, the first cut end surface 1x and the second cut end surface 1y can be connected to each other to form one cut end surface. The second cut end surface 1y may be formed by one shearing or may be formed by multiple shearing. In this way, in the manufacturing method of the present disclosure, the formation of the first cut end surface 1x and the formation of the second cut end surface 1y are separated, and the restraint state of the scrap 15 (see FIG. 2(C), FIG. 4(A) and (B)) is changed between the first cutting stage and the second shearing stage, so that the propagation direction of the crack when the second cut end surface 1y is formed can be appropriately controlled, and as a result, the tensile residual stress in the fracture surface 1b of the second cut end surface 1y can be reduced.

[0044] 2.4 An example of shear processing In the manufacturing method of the present disclosure, the first cut end surface 1x or the second cut end surface 1y may be formed by a shearing process as described below.

[0045] For example, as shown in Figs. 1 and 2, the steel material 5 is placed between the first blade 21 and the second blade 22. Here, the first blade 21 has a first bottom surface 21a, a first side surface 21b, and a first tip portion 21x, and the second blade 22 has a second bottom surface 22a, a second side surface 22b, and a second tip portion 22x. The first tip portion 21x and the second tip portion 22x have a shape corresponding to the first cut end surface 1x or the second cut end surface 1y. After the steel material 5 is placed between the first blade 21 and the second blade 22, the first blade 21 and the second blade 22 are moved relatively to shear the steel material 5, thereby forming the first cut end surface 1x or the second cut end surface 1y. Needless to say, when both the first cut end surface 1x and the second cut end surface 1y are formed by shearing using the first blade 21 and the second blade 22, the first blade 21 and the second blade 22 for forming the first cut end surface 1x have different blade shapes from the first blade 21 and the second blade 22 for forming the second cut end surface 1y.

[0046] 2.4.1 First blade The first blade 21 has a first bottom surface 21a, a first side surface 21b, and a first tip portion 21x. The first bottom surface 21a may have a surface that intersects with the relative movement direction of the first blade 21, or may have a surface that is perpendicular to the movement direction. The first side surface 21b may have a surface that is aligned with the relative movement direction of the first blade 21, or may have a surface that is inclined with respect to the movement direction. The first tip portion 21x refers to a portion near the intersection line between the first bottom surface 21a and the first side surface 21b. For example, it may be a portion that is within a range of 2 mm from the intersection line between the first bottom surface 21a and the first side surface 21b toward both the first bottom surface 21a side and the first side surface 21b side. In cases where the tip of the first blade 21 is machined to have an R or where the tip is chamfered, a line of intersection between a surface extending along the first bottom surface 21a and a surface extending along the first side surface 21b can be assumed, and the portion included within a range of R+2 mm from the line of intersection toward both the first bottom surface 21a side and the first side surface 21b side can be regarded as the first tip portion 21x.

[0047] The shape of the first bottom surface 21a can be determined according to the shape of the intended cut end surfaces 1x, 1y of the workpiece 10. The first bottom surface 21a may have a flat surface or a curved surface, and the flat surface or the curved surface can face the first surface 10a when the steel material 5 is sheared.

[0048] The shape of the first side surface 21b can be determined according to the shapes of the intended cut end surfaces 1x, 1y of the processed material 10. The first side surface 21b may have a flat surface or a curved surface.

[0049] The first tip portion 21x can be determined according to the shapes of the cut end faces 1x, 1y of the target workpiece 10. When forming a punch hole in the steel material 5 as shown in Fig. 5(A) to (C), the shape of the first tip portion 21x of the first blade 21 used when forming the second cut end face 1y may be annular along the edge of the punch hole.

[0050] In a standby state before the shearing operation, the first blade 21 may be disposed above the second blade 22. In this case, the first blade 21 may be a punch that punches out a part of the steel material 5 placed on the second bottom surface 22a of the second blade 22 from top to bottom.

[0051] The first blade 21 is made of a material that is generally used for a blade in a shearing process. For example, the first blade 21 may be made of SKD11. The first blade 21 may have a first coating on its surface.

[0052] 2.4.2 Second blade The second blade 22 has a second bottom surface 22a, a second side surface 22b, and a second tip portion 22x. The second bottom surface 22a may have a surface that intersects with the relative movement direction of the second blade 22, or may have a surface that is perpendicular to the movement direction. The second side surface 22b may have a surface that is along the relative movement direction of the second blade 22, or may have a surface that is inclined with respect to the movement direction. The second tip portion 22x refers to a portion near the intersection line between the second bottom surface 22a and the second side surface 22b. For example, it may be a portion that is within a range of 2 mm from the intersection line between the second bottom surface 22a and the second side surface 22b toward both the second bottom surface 22a side and the second side surface 22b side. When the tip of the second blade 22 is processed to have R or the tip is chamfered, the second tip portion 22x can be specified in the same manner as the first tip portion 21x described above. In other words, assuming an intersection line between a surface extending along the second bottom surface 22a and a surface extending along the second side surface 22b, the portion included within a range of R+2 mm from the intersection line toward both the second bottom surface 22a side and the second side surface 22b side may be regarded as the second tip portion 22x.

[0053] The shape of the second bottom surface 22a can be determined according to the shape of the intended cut end surfaces 1x, 1y of the workpiece 10. The second bottom surface 22a may have a flat surface or a curved surface.

[0054] The shape of the second side surface 22b can be determined according to the shape of the intended cut end surfaces 1x, 1y of the processed material 10. The second side surface 22b may be a flat surface, a curved surface, or a combination of a flat surface and a curved surface.

[0055] The second tip portion 22x can be determined according to the shape of the intended cut end surfaces 1x, 1y of the workpiece 10. When forming a punch hole in the steel material 5 as shown in Fig. 5(A) to (C), the shape of the second tip portion 22x of the second blade 22 used when forming the second cut end surface 1y may be annular along the edge of the punch hole.

[0056] In a standby state before the shearing operation, the second blade 22 may be disposed below the first blade 21. In this case, the second blade 22 may be a die on which the steel material 5 is placed.

[0057] The first blade 21 is made of a material that is generally used for blades in shearing processing. For example, the second blade 22 may be made of SKD11. The material of the second blade 22 may be the same as or different from the material of the first blade 21. In addition, the second blade 22 may have a second coating on its surface.

[0058] 2.4.3 Steel arrangement In the manufacturing method of the present disclosure, when forming the first cut end surface 1x or the second cut end surface 1y on the steel material 5, the steel material 5 may be disposed between the first blade 21 and the second blade 22 as described above. There is no particular limitation on the arrangement of the steel material 5 between the first blade 21 and the second blade 22, as long as the steel material 5 is appropriately sheared. For example, as shown in FIG. 1(B), the steel material 5 may be placed on the second bottom surface 22a of the second blade 22 while the first blade 21 is disposed above the steel material 5. In addition, in the manufacturing method of the present disclosure, in order to control the restraint state of the scrap 15 when the steel material 5 is sheared, when the steel material 5 is disposed between the first blade 21 and the second blade 22, the steel material 5 may be pressed against the first bottom surface 21a or the second bottom surface 22a by a pressing member (holder) (not shown). The form of the pressing member is not particularly limited, and a general pressing member may be adopted.

[0059] 2.4.4 Action and relationship of the first and second blades during shearing In the manufacturing method of the present disclosure, after the steel material 5 is placed between the first blade 21 and the second blade 22, the first blade 21 and the second blade 22 are moved relatively to each other, thereby shearing the steel material 5 to form the first cut end surface 1x and the second cut end surface 1y. The relative movement of the first blade 21 and the second blade 22 may be performed by a moving device (not shown). Alternatively, at least one of the first blade 21 and the second blade 22 may be moved manually.

[0060] During shearing, a clearance may be provided between the first blade 21 and the second blade 22. The clearance may be appropriately determined depending on the material and thickness of the steel material 5. For example, when the steel material 5 is plate-shaped, the clearance may be 5% or more or 6% or more of the thickness, and may be 25% or less or 18% or less. According to the inventor's new findings, when the clearance is 10% or more and 14% or less, it is easier to more appropriately control the propagation direction of the cracks 1dx and 1dy during shearing of the steel material 5. The "clearance" in this application is in accordance with ISO 16630:2009.

[0061] During shearing, a shear angle may be provided between the first blade 21 and the second blade 22. The shear angle may be 0° or more, or 10° or less. According to the inventor's new findings, when the shear angle is 0° or more and 1° or less, it is easy to appropriately control the propagation direction of the cracks 1dx and 1dy during shearing of the steel material 5.

[0062] 3. Manufacturing method of processed material (second embodiment) In the manufacturing method shown in Fig. 5(A)-(C), a form in which a punch hole is formed by punching a part of the steel material 5 is shown, but the manufacturing method of the present disclosure is not limited to this form. For example, as shown in Fig. 6(A)-(C), the manufacturing method of the present disclosure can also be adopted in a form in which the steel material 5 is trimmed. That is, as shown in Fig. 6(A) and (B), even when the steel material 5 is trimmed, the entire cutting line L is not punched out at once, but a part of the cutting line L with a relatively small radius of curvature is cut first to form a first cut end surface 1x. After forming the first cut end surface 1x, as shown in Fig. 6(C), the steel material 5 is subjected to a shearing process to form a second cut end surface 1y with a relatively large radius of curvature. This can reduce the tensile residual stress in the fracture surface 1b of the second cut end surface 1y.

[0063] 4. Manufacturing method of processed material (third embodiment) In the manufacturing method shown in Figs. 5(A)-(C) and 6(A)-(C), the cutting line L of the steel material 5 is annular, but the method of the present disclosure is not limited to this form. For example, as shown in Figs. 7(A)-(C), the manufacturing method of the present disclosure can also be adopted in a form in which an open cross section is formed on the steel material 5. That is, as shown in Figs. 7(A) and (B), even when an open cross section is formed on the steel material 5, the entire cutting line L is not punched out at once, but a portion of the cutting line L with a relatively small radius of curvature is first cut to form a first cut end surface 1x. After forming the first cut end surface 1x, as shown in Fig. 7(C), the steel material 5 is subjected to a shearing process to form a second cut end surface 1y with a relatively large radius of curvature. This can reduce the tensile residual stress in the fracture surface 1b of the second cut end surface 1y.

[0064] 5. Supplementary Information In the above, the first cut end surface 1x is formed, and the portion of the scrap 15 is supported by a "cantilever" through the cutting line L that becomes the second cut end surface 1y (the portion of the scrap 15 is held by the processed material 10 through only one cutting line L). However, the manufacturing method of the present disclosure is not limited to this form. Even if the second cut end surface 1y is supported at both ends or more after the first cut end surface 1x is formed, the first cut end surface 1x is formed before the second cut end surface 1y is formed. Compared with the case where the first cut end surface 1x and the second cut end surface 1y are sheared simultaneously, the constraint on the scrap 15 side is reduced, and the inclination angle α is easily generated on the scrap 15 side when the second cut end surface 1y is formed. Therefore, the tensile residual stress generated on the fracture surface 1b of the second cut end surface 1y can be relatively reduced. However, according to the inventor's new findings, after the first cut end surface 1x is formed, the portion on the scrap 15 side is supported in a cantilevered manner from the portion on the processed material 10 side via the cutting line L which becomes the second cut end surface 1y. This makes it easier to create a larger inclination angle α on the scrap 15 side when the second cut end surface 1y is then formed by shearing, and further reduces the tensile residual stress generated in the fracture surface 1b of the second cut end surface 1y.

[0065] 6. Examples of processed material properties In the processed material 10 obtained by the manufacturing method of the present disclosure, the tensile residual stress can be reduced, particularly in the fracture surface 1b, in the second cut end surface 1y having a relatively large radius of curvature. Hereinafter, an example of the second cut end surface 1y of the processed material 10 will be described, but the second cut end surface 1y is not limited to the following form. As shown in Figures 8 and 9, the second cut end surface 1y of the processed material 10 has a sag 1a, a fracture surface 1b, and a burr 1c. The fracture surface 1b includes a first portion 1bx and a second portion 1by. The first portion 1bx is formed by a first crack 1dx that has progressed from the sag 1a side to the burr 1c side, and the second portion 1by is formed by a second crack 1dy that has progressed from the burr 1c side to the sag 1a side. The area ratio of the first portion 1bx to the fracture surface 1b is greater than the area ratio of the second portion 1by to the fracture surface 1b.

[0066] 8 and 9, the second cut end surface 1y has a sag 1a, a fracture surface 1b, and a burr 1c. The second cut end surface 1y may also have a shear surface 1e. The sag 1a, the burr 1c, and the shear surface 1e of the second cut end surface 1y may have any shape depending on the shape of the processed material 10. The sag 1a, the burr 1c, and the shear surface 1e may have the same shape as in the past.

[0067] The processed material 10 has one characteristic in the configuration of the fracture surface 1b of the second cut end surface 1y. As shown in Figures 8 and 9, the fracture surface 1b includes a first portion 1bx and a second portion 1by. The first portion 1bx is formed by a first crack 1dx that progresses from the sag 1a side to the burr 1c side, and the second portion 1by is formed by a second crack 1dy that progresses from the burr 1c side to the sag 1a side.

[0068] The propagation direction of the first crack 1dx may be from the sag 1a side to the burr 1c side. When the processed material 10 is plate-shaped, the propagation direction of the first crack 1dx may be along the thickness direction of the processed material 10 (perpendicular to the first surface 10a and the second surface 10b) or inclined to the thickness direction. The propagation direction of the second crack 1dy may be from the burr 1c side to the sag 1a side. When the processed material 10 is plate-shaped, the propagation direction of the second crack 1dy may be along the thickness direction of the processed material 10 (perpendicular to the first surface 10a and the second surface 10b) or inclined to the thickness direction. For example, when steel material 5 is sheared, if a clearance is provided between the first blade 21 and the second blade 22, the propagation direction of the first crack 1dx and the second crack 1dy may be inclined relative to the plate thickness direction, and the larger the clearance, the greater the inclination.

[0069] The first crack 1dx may propagate from the sag 1a side to the burr 1c side and join with the second crack 1dy on the burr 1c side, but does not necessarily have to propagate from the sag 1a side to the second crack 1dy on the burr 1c side via the shortest path. For example, the first crack 1dx may propagate toward the front of the paper in FIG. 2(B) (for example, in the plate width direction when the processed material 10 is plate-shaped) on the way from the sag 1a side to the burr 1c side. The same applies to the second crack 1dy.

[0070] In the second cut end surface 1y, the area ratio of the first portion 1bx in the fractured surface 1b may be larger than the area ratio of the second portion 1by in the fractured surface 1b. In other words, in the second cut end surface 1y, the average length of the first crack 1dx propagating from the sag 1a side toward the burr 1c side may be longer than the average length of the second crack 1dy propagating from the burr 1c side toward the sag 1a side. As described above, when the area ratio of the portion of the fractured surface 1b originating from the crack 1dx propagating from the sag 1a side is larger than the area ratio of the portion of the fractured surface 1b originating from the crack 1dx propagating from the burr 1c side, the tensile residual stress of the fractured surface 1b can be relatively reduced.

[0071] In addition, when determining the area ratio of each of the first portion 1bx and the second portion 1by in the fractured surface 1b and the length of each of the first crack 1dx and the second crack 1dy, the unevenness of the surface of the fractured surface 1b is not taken into consideration. For example, as shown in Fig. 10(A), when the second cut end surface 1y is viewed from the front, if the position of the starting point of the first crack 1dx is P1, the position of the starting point of the second crack 1dy is P2, and the position where the first crack 1dx and the second crack 1dy join is P3, if the distance between P1 and P3 is larger than the distance between P2 and P3, it can be determined that the area ratio of the first portion 1bx in the fractured surface 1b is larger than the area ratio of the second portion 1by in the fractured surface 1b.

[0072] According to the findings of the present inventors, the larger the area ratio of the first portion 1bx in the fractured surface 1b, the more the tensile residual stress in the fractured surface 1b is reduced. For example, in the processed material 10, the area ratio of the first portion 1bx in the fractured surface 1b may be 1.2 times or more, 1.5 times or more, 1.7 times or more, 2.0 times or more, 2.2 times or more, or 2.5 times or more of the area ratio of the second portion 1by in the fractured surface 1b.

[0073] The burrs 1c may be of a size that cannot be visually confirmed. Which of the first surface 10a and the second surface 10b of the processed material 10 is the surface on the sag 1a side and which is the surface on the burr 1c side can be easily determined by observing the shape of the processed material 10, even if the burrs 1c cannot be confirmed.

[0074] In the second cut end surface 1y, the sheared surface 1e and the fractured surface 1b have different properties. For example, the sheared surface 1e and the fractured surface 1b have different roughness (glossiness). In this regard, the sheared surface 1e and the fractured surface 1b can be easily distinguished from each other just by observing the appearance.

[0075] In the fracture surface 1b, the boundary between the first portion 1bx and the second portion 1by (the position where the first crack 1dx and the second crack 1dy join) can be identified, for example, by introducing a large amount of hydrogen into the second cut end surface 1y. As described above, the stress generated during the propagation of the crack depends on the propagation direction of the crack. That is, as shown in FIG. 10(A) and (B), it can be said that the residual stress suddenly changes at the position where the first crack 1dx and the second crack 1dy join. Therefore, the direction of the hydrogen embrittlement crack caused by the penetration of hydrogen also suddenly changes at the position where the first crack 1dx and the second crack 1dy join. Considering this, the position where the direction of the hydrogen embrittlement crack suddenly changes can be regarded as the position where the first crack 1dx and the second crack 1dy join.

[0076] The processed material 10 may have a first cut end surface 1x and a second cut end surface 1y, and the configuration other than the cut end surfaces is not particularly limited. The processed material 10 may have an end surface other than the cut end surfaces. The shape of the processed material 10 corresponds to the shape of the steel material 5 described above. That is, the processed material 10 may be a plate-like or rod-like shape as described above. The processed material 10 may also have a first surface 10a and a second surface 10b opposite to the first surface 10a as surfaces other than the cut end surfaces, and the first surface 10a and the second surface 10b may be connected via the cut end surfaces. The first surface 10a and the second surface 10b may be parallel to each other. The processed material 10 may also have a surface treatment layer as described above. The processed material 10 may also include multiple layers of different steel types. The mechanical properties and chemical composition of the processed material 10 are also as described above.

[0077] 7. Other examples of processed material properties In the manufacturing method of the present disclosure, when the first cut end surface 1x and the second cut end surface 1y are both formed by "shearing", the first cut end surface 1x and the second cut end surface 1y of the processed material may have the following configurations. Figure 11 shows a schematic configuration of each of the first cut end surface 1x and the second cut end surface 1y of the processed material 10.

[0078] The workpiece 10 shown in Fig. 11 is made of a steel material having a cut end face 1. The cut end face 1 includes a first cut end face 1x with a relatively small radius of curvature and a second cut end face 1y with a relatively large radius of curvature. The first cut end face 1x and the second cut end face 1y are continuous. The first cut end face 1x includes a first sag 1xa, a first fracture surface 1xb, and a first burr 1xc. The first cut end face 1x may further have a first sheared surface 1xe. The second cut end face 1y includes a second sag 1ya, a second fracture surface 1yb, and a second burr 1yc. The second cut end face 1y may further have a second sheared surface 1ye. The first fracture surface 1xb includes a first portion 1xb1 and a second portion 1xb2. The second fracture surface 1yb includes a third portion 1yb1 and a fourth portion 1yb2. The first portion 1xb1 is formed by a first crack 1xd1 that progresses from the first sag 1xa side toward the first burr 1xc side. The second portion 1xb2 is formed by a second crack 1xd2 that progresses from the first burr 1xc side toward the first sag 1xa side. The third portion 1yb1 is formed by a third crack 1yd1 that progresses from the second sag 1ya side toward the second burr 1yc side. The fourth portion 1yb2 is formed by a fourth crack 1yd2 that progresses from the second burr 1yc side toward the second sag 1ya side. When the area ratio of the first portion 1xb1 in the first fracture surface 1xb is X1 (%), the area ratio of the second portion 1xb2 in the first fracture surface 1xb is X2 (%), the area ratio of the third portion 1yb1 in the second fracture surface 1yb is Y1 (%), and the area ratio of the fourth portion 1yb2 in the second fracture surface 1yb is Y2 (%), Y1 may be 1.1 times or more of X1.

[0079] Also, as shown in Fig. 11, in the workpiece 10, the above Y1 and Y2 may satisfy the relationship of Y2 < Y1.

[0080] Also, as shown in Fig. 11, in the workpiece 10, the above X1, X2, Y1, and Y2 may satisfy the relationship of X1 / X2 < Y1 / Y2.

[0081] Furthermore, as shown in FIG. 11, in the processed material 10, at the boundary between the first cut end surface 1x and the second cut end surface 1y, the boundary line between the first portion 1xb1 and the second portion 1xb2 at the first cut end surface 1x and the boundary line between the third portion 1yb1 and the fourth portion 1yb2 at the second cut end surface 1y may be discontinuous.

[0082] In addition, as for "X1", "X2", "Y1" and "Y2", as well as the "boundary line between the first portion 1bx1 and the second portion 1bx2 at the first cut end surface 1x" and the "boundary line between the third portion 1by1 and the fourth portion 1by2 at the second cut end surface 1y", as described above, they can be determined by introducing hydrogen into the fracture surface and identifying the position where the direction of the hydrogen embrittlement crack suddenly changes. EXAMPLES

[0083] The steel materials prepared for cutting were steel plate A with a tensile strength of 1470 MPa (plate thickness: 1.6 mm), steel plate B with a tensile strength of 1310 MPa (plate thickness: 1.6 mm), steel plate C with a tensile strength of 1180 MPa (plate thickness: 1.6 mm), and steel plate D with a tensile strength of 980 MPa (plate thickness: 1.6 mm).

[0084] 1. Cutting and evaluation of steel plate A For the above steel plate A, a cut end surface was formed consisting of a portion with a small radius of curvature (R = 30 mm) and a portion with a large radius of curvature (straight line) continuing from the small radius of curvature, as shown in Figure 12, and the tensile residual stress was measured in the portion with the large radius of curvature of the cut end surface.

[0085] The tensile residual stress was measured as follows. The method for measuring the tensile residual stress is shown in Fig. 13. As shown in Fig. 13, residual stress was measured by X-rays with a spot diameter of φ500μm at the center position in the thickness direction (three different positions in the width direction). The residual stress was measured in three directions: the thickness direction, the width direction, and a 45 degree direction from the thickness direction. The residual stress was calculated using sin 2 The ψ method was used. The residual stress in the normal direction of the end face was assumed to be zero, and the maximum principal stress was calculated from the residual stresses in the three directions. The maximum principal stress values ​​calculated at the three locations were averaged.

[0086] 1.1 Example 1 When forming the cut end surface of steel plate A, a portion with a small radius of curvature was cut to form a first cut end surface, and then a portion with a large radius of curvature was cut to form a second cut end surface (see Figs. 7(A) to (C)). Each cut was performed by shearing using a punch and a die. Fig. 14(A) shows the inclination of the scrap when the second cut end surface was formed. Fig. 15 shows the measurement results of the tensile residual stress at the second cut end surface for each of the scrap side and the product (processed material) side.

[0087] 1.2 Comparative Example 1 To form the cut end surface on steel sheet A, a punch and a die were used to simultaneously shear the portion with a small radius of curvature and the portion with a large radius of curvature. Figure 14(B) shows the inclination of the scrap during shearing. Figure 15 shows the measurement results of the tensile residual stress in the portion with a relatively large radius of curvature (corresponding to the second cut end surface) on both the scrap side and the product (processed material) side.

[0088] 1.3 Evaluation results As is clear from Figures 14(A) and (B), the inclination (inclination angle) of the scrap during shearing was larger in the Example than in the Comparative Example, and the crack was able to propagate preferentially from the punch side. As a result, as shown in Figure 15, in the Comparative Example, the tensile residual stress at the second cut end surface was almost equally large on the processed material (product) side and the scrap side, whereas in the Example, the tensile residual stress at the second cut end surface on the processed material (product) side was significantly reduced.

[0089] 2. Cutting and evaluation of steel plates B to D As with steel plate A, steel plates B to D were also cut in the same manner, and the tensile residual stress generated at the second cut end surface was compared between Example 1 and Comparative Example 1. The results for steel plates B to D are shown in Table 1 below, together with the results for steel plate A.

[0090] [Table 1]

[0091] As is clear from the results shown in Table 1, for steel plates B to D, when shearing was performed in two stages as in the examples, the tensile residual stress at the second cut end surface on the processed material (product) side was significantly reduced.

[0092] 3. Consideration of the radius of curvature The shape of the cutting line on the steel plate was changed to perform shear processing. During shear processing, a cut end surface was formed consisting of a part with a small radius of curvature (R = 30 mm) and a part with a large radius of curvature (straight line (R = ∞), R = 150 mm, R = 60 mm), and the tensile residual stress was measured in the part with the large radius of curvature of the cut end surface.

[0093] 3.1 Example 2 To form the cut end surface of steel plate A, as shown in Fig. 16, a portion with a small radius of curvature (R = 30 mm) was cut to form a first cut end surface, and then a portion with a large radius of curvature (R = ∞) was cut to form a second cut end surface. Each cut was performed by shearing using a punch and a die. Fig. 17 shows the measurement results of the tensile residual stress at the second cut end surface for both the scrap side and the product (processed material) side.

[0094] 3.2 Comparative Example 2 In forming the cut end surface as shown in Fig. 16, a punch and a die were used to simultaneously shear a portion with a small radius of curvature (R = 30 mm) and a portion with a large radius of curvature (R = ∞) on the steel plate A. Fig. 17 shows the measurement results of the tensile residual stress on the second cut end surface for both the scrap side and the product (processed material) side.

[0095] 3.3 Example 3 To form the cut end surface of steel plate A, as shown in Fig. 18, a portion with a small radius of curvature (R = 30 mm) was cut to form a first cut end surface, and then a portion with a large radius of curvature (R = 150 mm) was cut to form a second cut end surface. Each cut was performed by shearing using a punch and a die. Fig. 19 shows the measurement results of the tensile residual stress at the second cut end surface for both the scrap side and the product (processed material) side.

[0096] 3.4 Comparative Example 3 In forming the cut end surface as shown in Fig. 18 for steel plate A, a punch and a die were used to simultaneously shear a portion with a small radius of curvature (R = 30 mm) and a portion with a large radius of curvature (R = 150 mm). Fig. 19 shows the measurement results of the tensile residual stress at the second cut end surface for each of the scrap side and the product (processed material) side.

[0097] 3.5 Example 4 To form the cut end surface of steel plate A, as shown in Fig. 20, a portion with a small radius of curvature (R = 30 mm) was cut to form a first cut end surface, and then a portion with a large radius of curvature (R = 60 mm) was cut to form a second cut end surface. Each cut was performed by shearing using a punch and a die. Fig. 21 shows the measurement results of the tensile residual stress at the second cut end surface for both the scrap side and the product (processed material) side.

[0098] 3.6 Comparative Example 4 In forming the cut end surface as shown in Fig. 20 for steel plate A, a punch and a die were used to simultaneously shear a portion with a small radius of curvature (R = 30 mm) and a portion with a large radius of curvature (R = 60 mm). Fig. 21 shows the measurement results of the tensile residual stress at the second cut end surface for each of the scrap side and the product (processed material) side.

[0099] 3.7 Evaluation Results 17, 19 and 21, in Comparative Examples 2 to 4, the tensile residual stress at the second cut end surface was almost equally large on the processed material (product) side and the scrap side, whereas the tensile residual stress at the second cut end surface on the processed material (product) side could be significantly reduced in Examples 2 to 4. In particular, it can be seen that the larger the radius of curvature at the second cut end surface, as in Example 2, the greater the effect of reducing the tensile residual stress.

[0100] 4. Cutting and evaluation of steel plates B to D As with steel plate A, steel plates B to D were also cut in the same manner, and the tensile residual stresses generated at the second cut end surface were compared between Examples 2 to 4 and Comparative Examples 2 to 4. The results for steel plates B to D are shown in Tables 2 to 4 below, along with the results for steel plate A. Table 2 is for the case of R=∞ shown in FIG. 16, Table 3 is for the case of R=150 mm shown in FIG. 18, and Table 4 is for the case of R=60 mm shown in FIG. 20.

[0101] [Table 2]

[0102] [Table 3]

[0103] [Table 4]

[0104] As is clear from the results shown in Tables 3 to 5, the tensile residual stress at the second cut end surface on the processed material (product) side could be significantly reduced for steel plates B to D when shearing was performed in two stages as in Examples 2 to 4. In particular, it can be seen that the larger the radius of curvature at the second cut end surface is as in Example 2, the greater the effect of reducing the tensile residual stress.

[0105] 5. Check the properties of the sheared end surface Steel sheet A was subjected to shear processing under the conditions according to Examples 1 to 4 or Comparative Examples 1 to 4 to obtain a processed material. An electrolytic solution was prepared by adding 3 g / L of NH4SCN to a 3% NaCl solution. The processed material was immersed in the prepared electrolytic solution, and a current density of 10 mA / cm was applied to the processed material. 2 Electrolytic charging was performed for 3 hours under the condition of , and hydrogen was introduced at least to the sheared edge of the processed material. After electrolytic charging, the processed material was removed from the solution and the properties of the sheared edge were observed. Hydrogen embrittlement cracking was confirmed on the sheared edge due to the introduction of a large amount of hydrogen. As shown in Figure 10, the part where the direction of hydrogen embrittlement cracking suddenly changes on the fracture surface constituting the sheared edge was regarded as "the boundary between the first part caused by the crack propagating from the first burr side and the second part caused by the crack propagating from the first sag side" and "the boundary between the third part caused by the crack propagating from the second burr side and the fourth part caused by the crack propagating from the second sag side", and the area ratios X1 and X2 of the first fracture surface and the area ratios Y1 and Y2 of the second fracture surface were specified as shown in Figure 11. The results are shown in Table 5 below.

[0106] [Table 5]

[0107] As shown in Table 5, it can be seen that in the processed material according to the embodiment, Y1 is sufficiently large relative to X1. Specifically, Y1 is 1.1 times or more of X1. In contrast, in the processed material according to the comparative example, X1 and Y1 are equivalent. Also, in the processed material according to the embodiment, Y1 is sufficiently large relative to Y2, whereas in the processed material according to the comparative example, Y2 and Y1 are equivalent. Furthermore, in the processed material according to the embodiment, Y1 / Y2 is sufficiently large relative to X1 / X2, whereas in the processed material according to the comparative example, X1 / X2 and Y1 / Y2 are equivalent. [Industrial Applicability]

[0108] The processed materials produced by the method of the present disclosure can be used, for example, as constituent materials for automobiles, home appliances, building structures, ships, bridges, construction machinery, various plants, penstocks, etc. [Explanation of symbols]

[0109] 1x First cutting edge (small radius of curvature) 1y Second cut end surface (part with large radius of curvature) 1 Cut end surface 5 Steel material 10 Processed materials 10a 1st page 10b 2nd side 11 Part of steel 12 Other parts of steel 15 Scrap 21 1st blade 22 2nd blade

Claims

1. A workpiece made of a steel material having a cut end surface, The cut end surface includes a first cut end surface having a relatively small radius of curvature and a second cut end surface having a relatively large radius of curvature, The first cut end surface and the second cut end surface are continuous with each other, The first cut end surface includes a first sag, a first fracture surface, and a first burr, The second cut end surface includes a second sag, a second fracture surface, and a second burr, the first fracture surface includes a first portion and a second portion; the second fracture surface includes a third portion and a fourth portion; The first portion is formed by a first crack that propagates from the first sag side toward the first burr side, The second portion is formed by a second crack that propagates from the first burr side toward the first sag side, The third portion is formed by a third crack that propagates from the second sag side toward the second burr side, The fourth portion is formed by a fourth crack that propagates from the second burr side toward the second sag side, The area ratio of the first portion to the first fracture surface is X1 (%), The area ratio of the second portion to the first fracture surface is X2 (%), The area ratio of the third portion to the second fracture surface is Y1 (%), When the area ratio of the fourth portion to the second fracture surface is Y2 (%), Y1 is 1.1 times or more of X1; Processed material.

2. Y2<Y1; The processed material according to claim 1.

3. X1 / X2<Y1 / Y2; The processed material according to claim 1 or 2.

4. At a boundary between the first cut end surface and the second cut end surface, a boundary line between the first portion and the second portion at the first cut end surface and a boundary line between the third portion and the fourth portion at the second cut end surface are discontinuous. The processed material according to any one of claims 1 to 3.

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