steel

Abstract

To provide a steel material which has reduced tensile residual stress particularly in a fracture surface of a shear end surface.SOLUTION: There is provided a steel material comprising a shear end surface. The shear end surface comprises sags, a fracture surface, and burrs. The fracture surface includes a first part and a second part. The first part is formed by a first crack that has developed from the sags side to the burrs side, the second part is formed by a second crack that has developed from the burrs side to the sags side. An area ratio of the first part occupying the fracture surface is larger than an area ratio of the second part occupying the fracture surface.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present application discloses a steel product having a sheared edge. [Background technology]

[0002] Patent Document 1 discloses a technique for shearing steel material using a punch and a die. In Patent Document 1, the fracture surface of the material punched out by the punch is pressed against the fracture surface of the steel material, thereby reducing the tensile residual stress at the fracture surface of the steel material. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2016 / 136909 Summary of the Invention [Problem to be solved by the invention]

[0004] As disclosed in Patent Document 1, tensile residual stress may be large at the fracture surface among the sheared end surfaces of steel material. A new steel material with reduced tensile residual stress, particularly at the fracture surface among the sheared end surfaces, is needed. [Means for solving the problem]

[0005] As one of the means for solving the above problems, the present application provides: A steel material having a shear end surface, The sheared end surface has a sag, a fracture surface, and a burr, the fracture surface includes a first portion and a second portion; the first portion is formed by a first crack that propagates from the sag side to the burr side, the second portion is formed by a second crack that propagates from the burr side to the sag side, an area ratio of the first portion to the fracture surface is greater than an area ratio of the second portion to the fracture surface; steel material Disclose.

[0006] In the steel material of the present disclosure, the area ratio of the first portion to the fracture surface may be 1.5 times or more the area ratio of the second portion to the fracture surface.

[0007] In the steel material of the present disclosure, the area ratio of the first portion to the fracture surface may be 2.0 times or more the area ratio of the second portion to the fracture surface.

[0008] The steel material of the present disclosure may be in the form of a plate.

[0009] The steel material of the present disclosure may have a tensile strength of 980 MPa or more.

[0010] The steel material of the present disclosure may have a tensile strength of 1470 MPa or more. [Effects of the Invention]

[0011] In the steel material of the present disclosure, the tensile residual stress at the fracture surface of the sheared end surface can be reduced. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a schematic diagram illustrating an example of the flow of shearing, in which (A) shows a state in which a workpiece is placed between the first blade and the second blade, (B) shows a state in which the first blade and the second blade are moved closer to each other so that the bottom surface of the first blade is in contact with the first surface of the workpiece and the bottom surface of the second blade is in contact with the second surface of the workpiece, (C) shows a state in which a part of the workpiece has been punched out by the first blade, and (D) shows a state in which the first blade and the second blade have been separated and returned to the position of (A). [Figure 2]1A and 1B are schematic diagrams for explaining an example of the mechanism for forming a sheared edge when a workpiece is sheared. The diagrams are cross sections along the relative movement direction of the first blade and the second blade, and show the cross-sectional form including the first blade, the second blade, and the workpiece. (A) shows a state in which a sag is formed in the workpiece by pressing the first blade and the second blade against the workpiece, (B) shows a state in which a crack is generated in the workpiece by further pressing the first blade and the second blade against the workpiece after the sag is formed, and (C) shows a state in which a portion of the workpiece is punched out by further pressing the first blade and the second blade against the workpiece after the crack is formed. [Figure 3] These are 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] 1 is a schematic diagram for explaining an example of the configuration of a steel material according to the present disclosure, showing a cross section of the steel material. [Figure 5] 1 is a schematic diagram for explaining an example of the configuration of a sheared end surface of a steel material according to the present disclosure, showing the sheared end surface as viewed from the front. [Figure 6] 1A and 1B are schematic diagrams for explaining a method for distinguishing between the first and second portions on a fracture surface, in which (A) schematically shows the orientation of hydrogen embrittlement cracks occurring on the fracture surface, and (B) schematically shows the relationship between an arbitrary position X between the sag side and the burr side on the fracture surface and the orientation (angle θ) of the hydrogen embrittlement cracks. DETAILED DESCRIPTION OF THE INVENTION

[0013] 1. Challenges and new findings A steel material having a sheared edge can be obtained, for example, as follows. First, as shown in FIG. 1(A), a steel workpiece 5 is placed between a first blade 21 and a second blade 22. Here, the workpiece 5 has a first surface 10a and a second surface 10b opposite the first surface 10a. The first blade 21 has a bottom surface 21a that contacts the first surface 10a, and the second blade 22 has a bottom surface 22a that contacts the second surface 10b. The first blade 21 may be a punch, and the second blade 22 may be a die. Next, as shown in FIGS. 1(B) and 1(C), the first blade 21 and the second blade 22 are moved relative to each other to shear the workpiece 5. As a result, as shown in FIGS. 1(C) and 1(D), a portion of the workpiece 5 is punched out as scrap 15 by the first blade 21, and the remaining portion of the workpiece 5 becomes a steel material 10 having a sheared edge 1. The scrap 15 may be used for some product.

[0014] 1(A) to 1(D) show a configuration in which no shear angle is provided between the first blade 21 and the second blade 22, but a shear angle may be provided between the first blade 21 and the second blade 22. Also, while FIGS. 1(A) to 1(D) show a configuration in which the intersection line between the bottom surface 21a and the side surface 21b of the first blade 21 (the cutting edge (tip) of the first blade 21) extends linearly in the longitudinal direction of the first blade 21, the cutting edge of the first blade 21 may extend curvedly in the longitudinal direction. That is, the cutting edge 1 may be sheared so that its shape in plan view is linear, curved, or a combination of linear and curved shapes. 1(A) to 1(D) show a form in which the end of the workpiece 5 is sheared and removed with the first blade 21 and the second blade 22, but it is also possible to form a hole, a slit, or the like in part of the workpiece 5 by shearing the workpiece 5 with the first blade 21 and the second blade 22. In this case, too, a steel material 10 having a sheared end surface 1 can be obtained.

[0015] An example of the mechanism by which the sheared edge 1 is formed will be described. As shown in FIGS. 2A to 2C, consider the case where a steel material 10 having a sheared edge 1 is obtained by shearing a workpiece 5 with a first blade 21 and a second blade 22. As shown in FIG. 2A, the bottom surface 21a of the first blade 21 is pressed against the first surface 10a of the workpiece 5, forming a sag 1a on the first surface 10a side of the workpiece 5. The sag 1a is formed during the process in which the cutting edge (tip) of the first blade 21 penetrates into the workpiece 5. After the sag 1a is formed, a sheared surface 1e (see FIG. 4) may also be formed during the process in which the cutting edge of the first blade 21 penetrates into the workpiece 5. As shown in FIG. 2B, after the sag 1a and sheared surface 1e are formed, a first crack 1dx is generated from the first blade 21 side toward the second blade 22 side. Similarly, on the second blade 22 side, after the cutting edge of the second blade 22 penetrates into the second surface 10b of the workpiece 5, a second crack 1dy is generated from the second blade 22 side toward the first blade 21 side. As shown in FIG. 2C, the first crack 1dx and the second crack 1dy each propagate and merge with each other, forming a fracture surface 1b. Further movement of the first blade 21 and the second blade 22 separates the workpiece 5 into scrap 15 and the target steel material 10. At this time, as shown in FIG. 2C, a burr 1c may be formed at the corner of the sheared edge 1 of the steel material 10 on the second blade 22 side. The sheared edge 1 can be formed by the mechanism shown in FIGS. 2A to 2C, regardless of whether a shear angle is present between the first blade 21 and the second blade 22 or the shape of the sheared edge 1 in a planar view (linear, curved, or a combination thereof).

[0016] In the sheared edge 1 formed as described above, compressive residual stress or tensile residual stress may occur due to damage or distortion caused by shearing. If large tensile residual stress exists in the sheared edge 1, for example, the hydrogen embrittlement resistance or fatigue strength of the sheared edge 1 is likely to decrease. In this regard, one issue in order to obtain a high-performance steel material 10 is how to reduce the tensile residual stress in the sheared edge 1. In particular, as disclosed in Patent Document 1, it is desirable to be able to reduce the tensile residual stress in the fracture surface 1b of the sheared edge 1.

[0017] The inventors have conducted numerous experiments and analyses on the relationship between the shearing conditions of the workpiece 5 and the properties of the sheared end surface 1 produced by the shearing, and as a result have obtained the following new findings.

[0018] As shown in FIGS. 3A to 3C, a case will be described in which a first blade 21 punches out a portion 11 of a workpiece 5, and a second blade 22 punches out another portion 12 of the workpiece 5. In this case, if a crack propagates preferentially from the first blade 21 side as shown in FIG. 3A, the tensile residual stress at the sheared edge of the portion 11 increases, while the tensile residual stress at the sheared edge of the other portion 12 decreases. In other words, the portion 11 can be used as scrap 15, while the other portion 12 can be used as a product (steel material 10). Furthermore, if a crack propagates equally from both the first blade 21 side and the second blade 22 side as shown in FIG. 3B, the same tensile residual stress can be generated at the sheared edge of both the portion 11 and the other portion 12. In other words, variation in the properties of the portion 11 and the other portion 12 is reduced. In this respect, this is suitable for using both the portion 11 and the other portion 12 as a product. 3(C), when the crack propagates preferentially from the second blade 22 side, the tensile residual stress at the sheared end surface of the other portion 12 increases, while the tensile residual stress at the sheared end surface of the part 11 decreases. In other words, the other portion 12 is used as scrap 15, while the part 11 can be suitably used as a product (steel material 10).

[0019] From the above, the following (1) to (3) can be stated. (1) The tensile residual stress occurring at the fracture surface 1b of the shear edge 1 varies depending on the propagation direction and length of the cracks 1dx and 1dy that form the fracture surface 1b. (2) On the fracture surface 1b, as the crack 1dx that propagates from the sag 1a side becomes longer, the tensile residual stress on the fracture surface 1b of the steel material 10 becomes smaller and the tensile residual stress on 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 steel 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.

[0020] The steel material 10 of the present disclosure has been completed based on the above findings. The configuration of the steel material 10 of the present disclosure will be described below.

[0021] 2. Steel material As shown in Figures 4 and 5, the steel material 10 of the present disclosure has a sheared edge 1. The sheared edge 1 includes a sag 1a, a fractured surface 1b, and a burr 1c. The fractured surface 1b includes a first portion 1bx and a second portion 1by. The first portion 1bx is formed by a first crack 1dx that propagates from the sag 1a side to the burr 1c side, and the second portion 1by is formed by a second crack 1dy that propagates from the burr 1c side to the sag 1a side. The area ratio of the first portion 1bx to the fractured surface 1b is greater than the area ratio of the second portion 1by to the fractured surface 1b.

[0022] 2.1 Sheared edge As shown in Figures 4 and 5, the sheared edge 1 has a sag 1a, a fractured surface 1b, and a burr 1c. The sheared edge 1 may also have a sheared surface 1e. Of the sheared edge 1, the sag 1a, the burr 1c, and the sheared surface 1e may take any form depending on the form of the steel material 10. The sag 1a, the burr 1c, and the sheared surface 1e may have the same forms as conventional forms. However, as will be described later, the technology of the present disclosure is more effective when the area ratio of the sheared surface 1e is small (i.e., when the area ratio of the fractured surface 1b is relatively large) than when it is large.

[0023] The steel material 10 of the present disclosure has one feature in the configuration of the fracture surface 1b of the shear end surface 1. As shown in Figures 4 and 5, 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 propagates from the sag 1a side to the burr 1c side, and the second portion 1by is formed by a second crack 1dy that propagates from the burr 1c side to the sag 1a side.

[0024] The propagation direction of the first crack 1dx may be a direction from the sag 1a side to the burr 1c side. When the steel material 10 is plate-shaped, the propagation direction of the first crack 1dx may be a direction along the plate thickness direction of the steel material 10 (a direction perpendicular to the first surface 10a and the second surface 10b) or a direction inclined relative to the plate thickness direction. Furthermore, the propagation direction of the second crack 1dy may be a direction from the burr 1c side to the sag 1a side. When the steel material 10 is plate-shaped, the propagation direction of the second crack 1dy may be a direction along the plate thickness direction of the steel material 10 (a direction perpendicular to the first surface 10a and the second surface 10b) or a direction inclined relative to the plate thickness direction. For example, when shearing the workpiece 5, 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.

[0025] The first crack 1dx does not necessarily have to propagate along the shortest path from the sag 1a side to the second crack 1dy on the burr 1c side as long as it starts from the sag 1a side and propagates toward the burr 1c side, merging with the second crack 1dy on the burr 1c side. For example, the first crack 1dx may propagate toward the front of the plane of the paper in FIG. 2(B) (for example, in the plate width direction if the steel material 10 is plate-shaped) as it propagates from the sag 1a side to the burr 1c side. The same applies to the second crack 1dy.

[0026] At the sheared edge 1, the area ratio of the first portion 1bx to the fractured surface 1b is larger than the area ratio of the second portion 1by to the fractured surface 1b. In other words, at the sheared edge 1, the average length of the first cracks 1dx that propagate from the sag 1a side toward the burr 1c side is longer than the average length of the second cracks 1dy that propagate 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 cracks 1dx that propagated from the sag 1a side is larger than the area ratio of the portion originating from the cracks 1dx that propagated from the burr 1c side, the tensile residual stress at the fractured surface 1b can be relatively reduced.

[0027] Note that, when specifying the area ratios of the first portion 1bx and the second portion 1by on the fracture surface 1b and the lengths of the first crack 1dx and the second crack 1dy, the unevenness of the surface of the fracture surface 1b is not taken into consideration. For example, as shown in Fig. 5, when the shear edge 1 is viewed from the front, if the position where the first crack 1dx starts is P1, the position where the second crack 1dy starts 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 on the fracture surface 1b is larger than the area ratio of the second portion 1by on the fracture surface 1b.

[0028] According to the findings of the present inventors, the larger the area ratio of the first portion 1bx to the fracture surface 1b, the more the tensile residual stress at the fracture surface 1b is reduced. For example, in the steel material 10, the area ratio of the first portion 1bx to the fracture 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, 2.5 times or more, 3.0 times or more, 3.5 times or more, 4.0 times or more, 4.5 times or more, or 5.0 times or more of the area ratio of the second portion 1by to the fracture surface 1b.

[0029] 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 steel 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 steel material 10, even if the burrs 1c cannot be confirmed.

[0030] In the sheared edge 1, 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 simply by observing their appearance. The larger the proportion of the fractured surface 1b in the sheared edge 1, the larger the area where tensile residual stress needs to be reduced, and the more excellent the effects of the technology of the present disclosure can be expected. In other words, the larger the proportion of the fractured surface 1b in the edge of the steel material 10, the better. For example, the product of the cut length and the cut thickness (plate thickness) of the steel material 10 (cut length × plate thickness) is taken as the reference (100%), and the area ratio of the fractured surface 1b is preferably 50% or more, more preferably 55% or more, even more preferably 60% or more, and particularly preferably 65% ​​or more. The upper limit of the area ratio of the fractured surface 1b is not particularly limited, and may be 100%, 95% or less, or 90% or less.

[0031] On 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 meet) can be identified, for example, by introducing a large amount of hydrogen into the shear edge 1. As described above, the stress generated during crack propagation depends on the direction of crack propagation. That is, as shown in Figures 6(A) and 6(B), the residual stress suddenly changes at the position where the first crack 1dx and the second crack 1dy meet. Therefore, the direction of hydrogen embrittlement cracking caused by hydrogen penetration also suddenly changes at the position where the first crack 1dx and the second crack 1dy meet. Considering this, the position where the direction of hydrogen embrittlement cracking suddenly changes can be considered to be the position where the first crack 1dx and the second crack 1dy meet. The conditions for introducing a large amount of hydrogen into the shear edge 1 will be described in detail in the following examples.

[0032] 2.2 Structure other than the sheared end face The steel material 10 may have a sheared end surface 1, and the configuration other than the sheared end surface is not particularly limited. The steel material 10 may be, for example, plate-shaped or rod-shaped. When the steel material 10 is plate-shaped, its thickness may be, for example, 0.8 mm or more, 5.0 mm or less, or 3.0 mm or less. When the steel material 10 is rod-shaped, its cross-sectional shape is not particularly limited, and it may be, for example, circular or polygonal, and the circle-equivalent diameter of the cross section may be 5 mm or more and 100 mm or less. Furthermore, the steel material 10 may be formed into some shape by bending or the like while leaving the sheared end surface 1.

[0033] As shown in FIG. 4 , the steel material 10 may have a first surface 10a and a second surface 10b opposite the first surface 10a as surfaces other than the sheared end surface 1, and the first surface 10a and the second surface 10b may be connected via the sheared end surface 1. The first surface 10a and the second surface 10b may be parallel to each other. Note that the term “parallel” used herein does not necessarily mean perfectly parallel, but may mean substantially parallel. In other words, even if the first surface 10a and the second surface 10b are not perfectly parallel, they are considered to be parallel as long as the difference is within the 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.

[0034] The steel material 10 may have a surface treatment layer. Examples of the surface treatment layer include a plating layer and a coating film. The steel material 10 may also include multiple layers of different steel types. For example, clad steel may be used as the steel material 10.

[0035] 2.3 Mechanical properties The mechanical properties of the steel material 10 are not particularly limited and can be determined appropriately depending on the application of the steel material 10. The steel material 10 having the above-described sheared edge 1 can have reduced tensile residual stress at the fracture surface 1b regardless of its mechanical properties. However, problems such as reduced hydrogen embrittlement resistance due to tensile residual stress are particularly likely to occur in high-strength steel materials. In this regard, the tensile strength of the steel material 10 is preferably 980 MPa or more, more preferably 1180 MPa or more, and particularly preferably 1470 MPa or more. The upper limit of the tensile strength of the steel material 10 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 referred to in this application conforms to ISO 6892-1:2009. Furthermore, the higher the strength of the steel material 10, the greater the proportion of the fracture surface 1b in the sheared end surface 1, i.e., the larger the area where the tensile residual stress needs to be reduced, and the more excellent the effects of the technology of the present disclosure can be expected. The preferable area ratio of the fracture surface 1b is as described above.

[0036] 2.4 Chemical composition The chemical composition and metal structure of the steel material 10 are not particularly limited and can be determined appropriately depending on the application of the steel material 10. The steel material 10 having the above-described sheared edge 1 can reduce tensile residual stress at the fracture surface 1b regardless of its chemical composition or metal structure. As an example of the chemical composition, the steel material 10 of the present disclosure has, in mass %, C: 0.050-0.800%, Si: 0.01-3.00%, Mn: 0.01-10.00%, Al: 0.001-0.500%, P: 0.100% or less, S: 0.050% or less, N: 0.010% or less, Cr: 0-3.000%, Mo: 0-1.000%, B: 0-0.0100%, Ti: 0-0.500%, Nb: 0-0.500%, V: 0-0.500%, and Cu: 0-0. 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 the steel material 10, 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%.

[0037] 3. Steel manufacturing method As described above, the steel material 10 having the sheared edge 1 can be produced by controlling the propagation direction and length of the cracks 1dx and 1dy during shearing. For example, as shown in FIGS. 1(A)-(D) and 2(A)-(C), consider a case in which a portion of the workpiece 5 is sheared using the first blade 21 and the second blade 22 and removed as scrap 15, and the remaining portion of the workpiece 5 is used as the target steel material 10. In this case, the first crack 1dx propagates preferentially from the first blade 21 side, thereby obtaining the steel material 10 having the predetermined sheared edge 1. To facilitate shearing of the workpiece 5, when the workpiece 5 is positioned between the first blade 21 and the second blade 22, a press member (holder) (not shown) may be used to press the workpiece 5 against the first bottom surface 21a or the second bottom surface 22a. The shape of the press member is not particularly limited; a general press member may be used.

[0038] Various methods are conceivable for preferentially propagating the first crack 1dx from the first blade 21 side. For example, the coefficient of friction of the bottom surface 21a or the side surface 21b of the first blade 21 may be increased, or the coefficient of friction of the first surface 10a of the workpiece 5 may be increased. Furthermore, when performing R-machining on the tip of at least one of the first blade 21 and the second blade 22, the R of the tip of the first blade 21 may be made smaller than the R of the tip of the second blade 22. Another possible method is to use a material that is more likely to cause cracks on the first surface 10a side of the workpiece 5. Specifically, to make cracks more likely to occur on the first surface 10a side than on the second surface 10b side, the hardness of the first surface 10a side of the workpiece 5 may be increased compared to the hardness of the second surface 10b side. Alternatively, strain or the like may be imparted to the first surface 10a side of the workpiece 5 before shearing to make the first surface side more susceptible to cracking. An example of the manufacturing conditions for the steel material 10 will be described in detail in the following examples.

[0039] 4. Actions and Effects As described above, the steel material 10 of the present disclosure can reduce the tensile residual stress, particularly at the fracture surface 1b, of the sheared edge 1. By reducing the tensile residual stress at the fracture surface 1b, for example, the hydrogen embrittlement resistance or fatigue strength at the sheared edge 1 can be improved. [Example]

[0040] 1.Work material The workpieces prepared were steel plate A (thickness 1.6 mm) with a tensile strength of 590 MPa, steel plate B (thickness 1.6 mm) with a tensile strength of 980 MPa, and steel plate C (thickness 1.6 mm) with a tensile strength of 1470 MPa.

[0041] 2. Shear conditions for steel plates A steel sheet was placed between a punch and a die, and the punch and die were moved relative to each other to punch out a portion of the steel sheet, thereby obtaining a steel product having a sheared end surface. At the sheared end surface, a sag was formed on the punch side and a burr was formed on the die side. Each of steel sheets A to C was sheared under the conditions according to the following Examples 1 to 4 and Comparative Examples 1 to 4, and the properties of each sheared end surface were confirmed.

[0042] 2.1 Example 1 A punch and die were used to form holes in the steel plate (φ10 mm pierce punching). The clearance between the punch and die was 10% of the steel plate thickness. The shape of the punch cutting edge (tip) was R = 0.1 mm, and the shape of the die cutting edge (tip) was R = 0.4 mm. The die was previously coated with TiCN, while the punch was not. Furthermore, when shearing the steel plate, a lubricant (rust-preventive oil) was applied only to the surface of the steel plate facing the die (burr side).

[0043] 2.2 Example 2 The steel sheets were sheared in the same manner as in Example 1, except that no lubricant was applied to either the punch-side surface or the die-side surface of the steel sheets when they were sheared.

[0044] 2.3 Example 3 The steel plate was sheared in the same manner as in Example 1, except that neither the punch nor the die was coated.

[0045] 2.4 Example 4 The steel sheet was sheared in the same manner as in Example 1, except that neither the punch nor the die was coated, and no lubricant was applied to either the punch-side surface or the die-side surface of the steel sheet.

[0046] 2.5 Comparative Example 1 The steel plate was sheared in the same manner as in Example 1, except that the shape of the cutting edge (tip) of the punch was R = 0.1 mm, the shape of the cutting edge (tip) of the die was R = 0.1 mm, neither the punch nor the die was coated, and no lubricant was applied to either the punch side surface or the die side surface of the steel plate when shearing the steel plate.

[0047] 2.6 Comparative Example 2 The steel plate was sheared in the same manner as in Example 1, except that the shape of the cutting edge (tip) of the punch was R = 0.4 mm, the shape of the cutting edge (tip) of the die was R = 0.1 mm, neither the punch nor the die was coated, and when shearing the steel plate, a lubricant (rust-preventive oil) was applied only to the surface of the steel plate on the punch side (sagging side).

[0048] 2.7 Comparative Example 3 The steel plate was sheared in the same manner as in Example 1, except that the shape of the cutting edge (tip) of the punch was R = 0.4 mm, the shape of the cutting edge (tip) of the die was R = 0.1 mm, the punch was previously coated with TiCN while the die was not particularly coated, and no lubricant was applied to either the punch side surface or the die side surface of the steel plate when shearing the steel plate.

[0049] 2.8 Comparative Example 4 The steel plate was sheared in the same manner as in Example 1, except that the shape of the cutting edge (tip) of the punch was R = 0.4 mm, the shape of the cutting edge (tip) of the die was R = 0.1 mm, the punch was previously coated with TiCN, but the die was not particularly coated, and when shearing the steel plate, a lubricant was applied only to the surface of the steel plate on the punch side (sagging side).

[0050] 3. Evaluation conditions Each of the steel plates A to C was sheared under the conditions of Examples 1 to 4 and Comparative Examples 1 to 4, and the properties of the sheared end surface of each steel material were evaluated as follows.

[0051] 3.1 Identifying the area ratio of the first part on the fracture surface An electrolyte was prepared by adding 3 g / L of NHSCN to a 3% NaCl solution. After immersing the steel material in the electrolyte, a current density of 10 mA / cm was applied to the steel material. 2 Electrolytic charging was carried out for three hours under these conditions, and hydrogen was introduced into at least the sheared edge of the steel. After electrolytic charging, the steel 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 6, the part of the fracture surface that makes up the sheared edge where the direction of the hydrogen embrittlement cracking suddenly changes was considered to be the "boundary between the first part caused by the crack propagating from the sag side and the second part caused by the crack propagating from the burr side," and the area ratio of the first part to the fracture surface was determined.

[0052] 3.2 Measurement of tensile residual stress on fracture surface Residual stress was measured by irradiating X-rays with a spot diameter of φ500 μm, centered on three boundary positions that divide the fracture surface into four equal parts in the thickness direction. The residual stress was measured in three directions: the thickness direction, the width direction, and a 45-degree angle from the thickness direction. The residual stress was calculated using sin 2 The ψ method was used. The residual stress in the normal direction to the end face was assumed to be zero, and the maximum principal stress was calculated from the residual stresses calculated in the three directions. The maximum principal stress values ​​calculated at the three locations were averaged.

[0053] 4. Evaluation Results The evaluation results are shown in Table 1 below.

[0054] [Table 1]

[0055] As shown in Table 1, it can be seen that the tensile residual stress of the fracture surface can be reduced when the area ratio of the first portion (the portion originating from the first crack propagating from the punch side) to the fracture surface is larger than the area ratio of the second portion (the portion originating from the second crack propagating from the die side) to the fracture surface. This effect is observed regardless of the steel type. It can be seen that the higher the strength of the steel sheet, the greater the effect. When the area ratio of the fracture surface was measured on the sheared edge of Steel Sheet B in Example 1, the area ratio of the fracture surface was 65%, with the product of the cut length and the sheet thickness of Steel Sheet B (cut length × sheet thickness) being taken as the reference (100%). Furthermore, the area ratio of the fracture surface on the sheared edge of Steel Sheet B in Examples 2 to 4 and the sheared edge of Steel Sheet C in Examples 1 to 4 was equal to or larger than that of Steel Sheet B in Example 1. Thus, it is believed that when the ratio of the fracture surface to the sheared edge is large, reducing the tensile residual stress can provide a greater effect.

[0056] In the above examples, a steel plate is used as the workpiece, but the technology of the present disclosure is not limited to this. As described above, the technology of the present disclosure is characterized in that the crack propagation direction on the fracture surface is controlled, and the same effect can be achieved even with shapes other than a plate. For example, a rod-shaped steel material may be used. [Industrial Applicability]

[0057] The steel material of the present disclosure can be used as a constituent material for, for example, automobiles, home appliances, building structures, ships, bridges, construction machinery, various plants, penstocks, etc. [Explanation of symbols]

[0058] 1 Sheared end face 1a Dare 1b Fracture surface 1bx 1st part 1by 2nd part 1c Bali 1dx First crack 1dy 2nd crack 1e Shear surface 5 Work material 10 Steel material 11 Part of the workpiece 12 Other parts of the workpiece 15 Scrap 21 1st blade 21a Bottom surface of first blade 21b Side of the first blade 22 2nd blade 22a Bottom surface of second blade 22b Side of second blade

Claims

1. A steel material having a shear end surface, The sheared end surface has a sag, a fracture surface, and a burr, the fracture surface includes a first portion and a second portion; the first portion is formed by a first crack that propagates from the sag side to the burr side, the second portion is formed by a second crack that propagates from the burr side to the sag side, an area ratio of the first portion to the fractured surface is greater than an area ratio of the second portion to the fractured surface; Steel material.

2. an area ratio of the first portion to the fracture surface is 1.5 times or more of an area ratio of the second portion to the fracture surface; The steel material according to claim 1.

3. an area ratio of the first portion to the fracture surface is 2.0 times or more of an area ratio of the second portion to the fracture surface; The steel material according to claim 1.

4. It is plate-shaped, The steel material according to any one of claims 1 to 3.

5. The tensile strength is 980 MPa or more. The steel material according to any one of claims 1 to 4.

6. The tensile strength is 1470 MPa or more. The steel material according to claim 5.

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