JP7534612B2 - Surface-treated steel sheet and manufacturing method for processed material - Google Patents

Description

This application discloses a method for manufacturing surface-treated steel sheets and processed materials.

Patent document 1 discloses a technology for shearing a steel plate using a punch and a die to obtain a processed material. In this technology, the fracture surface of the punched material is pressed against the fracture surface of the processed material, thereby reducing the tensile residual stress in the fracture surface of the processed material.

Patent Document 2 discloses a technique for cutting a zinc-based plated steel sheet having a specified zinc weight and thickness. In Patent Document 2, the zinc-based plated steel sheet is cut using a punch and die having a specified shape and a specified clearance, thereby improving the rust resistance of the cut end surface.

International Publication No. 2016/136909 JP 2009-287082 A

As disclosed in Patent Document 1, the tensile residual stress at the fracture surface of a processed material obtained by shearing a steel sheet may be large. This is also true for surface-treated steel sheets that are plated or the like. From this perspective, a new technology is needed that can reduce the tensile residual stress at the fracture surface of a processed material when a surface-treated steel sheet is sheared to obtain the processed material.

As one of the means for solving the above problems, the present application provides:
A surface-treated steel sheet having a steel sheet and a surface treatment layer,
At least one of the front surface and the back surface is formed by the surface treatment layer,
The coefficient of friction of the front surface is 1.4 times or more the coefficient of friction of the back surface,
The difference between the coefficient of friction of the front surface and the coefficient of friction of the back surface is 0.10 or more.
A surface-treated steel sheet is disclosed.

In the surface-treated steel sheet disclosed herein, the front surface and the back surface may be formed by the surface treatment layer, and the type of the surface treatment layer forming the front surface and the type of the surface treatment layer forming the back surface may be the same or different.

The surface-treated steel sheet of the present disclosure may have a sheared edge,
The shear end surface has a sag, a fracture surface, and a burr,
The sagging may be present on the front surface side, and the burr may be present on the back surface side.

In the surface-treated steel sheet disclosed herein, the thickness of the surface treatment layer may be 1 μm or more and 50 μm or less.

In the surface-treated steel sheet disclosed herein, the surface treatment layer may include plating.

In the surface-treated steel sheet disclosed herein, the surface treatment layer may include a coating film.

In the surface-treated steel sheet disclosed herein, the tensile strength of the steel sheet may be 980 MPa or more.

In the surface-treated steel sheet disclosed herein, the tensile strength of the steel sheet may be 1470 MPa or more.

As one of the means for solving the above problems, the present application provides:
Placing the surface-treated steel sheet of the present disclosure between a first blade and a second blade, wherein the front surface of the surface-treated steel sheet is placed on the first blade side and the back surface of the surface-treated steel sheet is placed on the second blade side.
shearing the surface-treated steel sheet by moving the first blade and the second blade relatively;
Including,
A method for producing a charge stock is disclosed.

When the surface-treated steel sheet disclosed herein is sheared, it is easy to obtain a processed material with reduced tensile residual stress on the fracture surface.

1 is a schematic diagram for explaining an example of a flow of shearing of a surface-treated steel sheet, in which (A) shows a state in which the surface-treated steel sheet is disposed between a first blade and a second blade, (B) shows a state in which the first blade and the second blade are moved relatively close to each other to bring the bottom surface of the first blade into contact with the front surface of the surface-treated steel sheet and the bottom surface of the second blade into contact with the back surface of the surface-treated steel sheet, (C) shows a state in which a part of the surface-treated steel sheet is punched out by the first blade, and (D) shows a state in which the first blade and the second blade are separated and returned to the position of (A). 1 is a schematic diagram for explaining an example of a mechanism for forming a sheared end surface when a surface-treated steel sheet 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 surface-treated steel sheet. (A) shows a state in which a sag is formed in the surface-treated steel sheet by pressing the first blade and the second blade against the surface-treated steel sheet, (B) shows a state in which a crack is generated in the surface-treated steel sheet by further pressing the first blade and the second blade against the surface-treated steel sheet after the sag is formed, and (C) shows a state in which a part of the surface-treated steel sheet is punched out by further pressing the first blade and the second blade against the surface-treated steel sheet after the crack is formed. Schematic diagrams for explaining new findings by the inventors. (A) shows a case where the crack propagates from the first blade, (B) shows a case where the crack propagates from both the first and second blades, and (C) shows a 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. 1A is a schematic cross-sectional view for explaining an example of the configuration of a surface-treated steel sheet, in which (A) has a surface treatment layer on the back surface (second surface), (B) has a surface treatment layer on the front surface (first surface), and (C) has surface treatment layers on both the front and back surfaces. FIG. 2 is a schematic cross-sectional view for explaining an example of the configuration of a surface-treated steel sheet (processed material) having a sheared end surface. 1 is a schematic diagram for explaining an example of a configuration of a sheared end surface, showing the sheared end surface as viewed from the front. 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 sagging side on the fracture surface and the direction (angle θ) of the hydrogen embrittlement cracking. FIG. 2 is a schematic diagram for explaining an example of a flow of a manufacturing method for a processed material. FIG. 4 is a schematic diagram for explaining a shear angle.

1. Issues and new findings The flow of shearing a surface-treated steel sheet to obtain a processed material is, for example, as follows. First, as shown in FIG. 1(A), the surface-treated steel sheet 5 is placed between the first blade 21 and the second blade 22. Here, the surface-treated steel sheet 5 has a surface (first surface) 10a and a back surface (second surface) 10b, the first blade 21 has a first bottom surface 21a, a first side surface 21b, and a first tip portion 21x (see FIG. 2), 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). As shown in FIG. 1(B), the first bottom surface 21a contacts the surface 10a of the surface-treated steel sheet 5, and the second bottom surface 22a contacts the back surface 10b of the surface-treated steel sheet 5. The first blade 21 may be a punch, and the second blade 22 may be a die. 1(B) and (C), the first blade 21 and the second blade 22 are moved relatively to shear the surface-treated steel sheet 5. As a result, as shown in Fig. 1(C) and (D), a part of the surface-treated steel sheet 5 is punched out as scrap 15 by the first blade 21, and the remaining part of the surface-treated steel sheet 5 can become a processed material 10 having a sheared end surface 1. The scrap 15 may be used for some kind of product.

1(A) to (D) show a form 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, in Figs. 1(A) to (D) show a form in which the intersection line between the first bottom surface 21a and the first side surface 21b of the first blade 21 (the tip of the first blade 21) extends linearly in the longitudinal direction of the first blade 21, but the tip of the first blade 21 may extend curvedly in the longitudinal direction. In other words, the shearing may be performed so that the shape of the sheared end surface 1 in a plan view is linear, or curved, or a combination of linear and curved shapes. In addition, although FIGS. 1(A) to (D) show a form in which the end of the surface-treated steel sheet 5 is sheared and removed with the first blade 21 and the second blade 22, it is also possible to form a hole or a slit in a part of the surface-treated steel sheet 5 by shearing the surface-treated steel sheet 5 with the first blade 21 and the second blade 22. In this case, too, it is possible to obtain a processed material 10 having a sheared end surface 1.

An example of the mechanism of forming the sheared end surface 1 will be described. As shown in Figures 2(A) to (C), a processed material 10 having a sheared end surface 1 is obtained by shearing a surface-treated steel sheet 5 with a first blade 21 and a second blade 22. As shown in Figure 2(A), a sag 1a is formed on the surface 10a side of the surface-treated steel sheet 5 by pressing a first bottom surface 21a of the first blade 21 against the surface 10a of the surface-treated steel sheet 5. The sag 1a is formed in the process until the first tip portion 21x of the first blade 21 bites into the steel sheet 5. After the sag 1a is formed, a sheared surface 1e (see Figure 5) may be formed in the process in which the first tip portion 21x bites into the surface-treated steel sheet 5. As shown in Figure 2(B), after the sag 1a and the sheared surface 1e are formed, a first crack 1dx occurs from the first blade 21 side toward the second blade 22 side. On the other hand, on the second blade 22 side, after the second tip 22x bites into the back surface 10b of the surface-treated steel sheet 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 progress and join with each other to form a fracture surface 1b. In addition, by further moving the first blade 21 and the second blade 22, the surface-treated steel sheet 5 is separated into the scrap 15 and the processed material 10, which is the target. At this time, as shown in FIG. 2C, a burr 1c may be formed at the corner of the sheared 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, or the shape of the sheared end surface 1 in plan view (straight line, curved line, or a combination of these, punched hole, slit, etc.), the sheared end surface 1 can be formed by the mechanism shown in Figures 2 (A) to (C).

In the sheared end surface 1 formed as described above, compressive residual stress or tensile residual stress may occur due to damage or distortion caused by shearing. If a large tensile residual stress exists in the sheared end surface 1, the hydrogen embrittlement resistance or fatigue strength of the sheared end surface 1 may decrease. In this regard, in order to obtain a processed material 10 with high performance, one issue may be how to reduce the tensile residual stress in the sheared end surface 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 end surface 1.

The inventors conducted numerous experiments and analyses on the relationship between the shear conditions of the surface-treated steel sheet 5 and the properties of the sheared end surface 1 caused by the shear, and as a result, obtained the following new findings.

As shown in Figures 3(A) to (C), a case will be described in which a part 11 of the surface-treated steel sheet 5 is punched out by the first blade 21 and another part 12 of the surface-treated steel sheet 5 is punched out by the second blade 22. In this case, as shown in Figure 3(A), if the crack propagates preferentially from the first blade 21 side, the tensile residual stress at the shear end surface of the part 11 becomes large, while the tensile residual stress at the shear end surface of the other part 12 becomes small. In other words, the part 11 can be used as scrap 15, while the other part 12 can be suitably used as the product (processed material 10). Also, as shown in Figure 3(B), if the 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 shear end surfaces of both the part 11 and the other part 12. In other words, the variation in the characteristics of the part 11 and the other part 12 is suppressed. In this respect, it can be said to be suitable when both the part 11 and the other part 12 are used as products. Furthermore, as shown in FIG. 3(C), if the crack propagates preferentially from the second blade 22 side, the tensile residual stress at the sheared end surface of the other part 12 becomes large, while the tensile residual stress at the sheared end surface of the part 11 becomes small. In other words, the other part 12 is scrapped 15, while the part 11 can be suitably used as the product (processed material 10).

Based on the above findings, the present inventors have found the following (1) to (4).
(1) The tensile residual stress generated in the fracture surface 1b of the sheared 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 propagating from the sag 1a 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.

The present inventors have conducted numerous experiments and analyses to control the propagation direction and length of the cracks 1dx and 1dy when the surface-treated steel sheet 5 is sheared, and as a result, have obtained the following new findings.
(4) By making the coefficient of friction of the front surface 10a of the surface-treated steel sheet 5 larger than the coefficient of friction of the back surface 10b by a certain amount or more, stress concentration is likely to occur when the first blade 21 contacts the front surface 10a during shearing of the surface-treated steel sheet 5, and the first crack 1dx is likely to preferentially occur and propagate from the front surface 10a side. As a result, the tensile residual stress of the sheared end surface 1 (particularly the fracture surface 1b) can be reduced.

The surface-treated steel sheet disclosed herein was developed based on the above findings.

4(A) to 4(C), the surface-treated steel sheet 5 of the present disclosure has a steel sheet 5x and a surface-treated layer 5y, and at least one of the front surface 10a and the back surface 10b is constituted by the surface-treated layer 5y. In the surface-treated steel sheet 5, the friction coefficient of the front surface 10a is 1.4 times or more the friction coefficient of the back surface 10b, and the difference between the friction coefficient of the front surface 10a and the friction coefficient of the back surface 10b is 0.10 or more.

The specific values of the friction coefficient of the front surface 10a and the back surface 10b of the surface-treated steel sheet 5 are not particularly limited as long as the above friction coefficient ratio is satisfied. For example, the friction coefficient of the front surface 10a and the back surface 10b may be in the range of 0.01 to 3.00.

As shown in Figures 4(A) and (B), the surface-treated steel sheet 5 of the present disclosure may have either the front surface 10a or the back surface 10b formed by a surface treatment layer 5y. Alternatively, as shown in Figure 4(C), the surface-treated steel sheet 5 of the present disclosure may have the front surface 10a and the back surface 10b formed by surface treatment layers 5y, 5y, in which case the type of surface treatment layer 5y forming the front surface 10a and the type of surface treatment layer 5y forming the back surface 10b may be the same or different.

2.1 Steel Plate The steel plate 5x may have a plate shape that can be sheared, and the thickness of the steel plate 5x may be determined according to the application. The thickness of the steel plate 5x may be, for example, 0.8 mm or more, 1.2 mm or more, 3.0 mm or less, or 1.8 mm or less. If the thickness of the steel plate 5x is thin, the effect of the technology of the present disclosure on hydrogen embrittlement resistance is small, but the effect on fatigue strength is considered to be exerted. On the other hand, if the thickness of the steel plate 5x is too thick, the effect of the technology of the present disclosure is exerted, but cracks may progress from an unintended side during shearing. The steel plate 5x may be formed into some shape by bending or the like.

The steel plate 5x may have a front surface 5xa and a back surface 5xb. The front surface 5xa and the back surface 5xb of the steel plate 5x may be parallel to each other. Note that "parallel" in this application is not limited to perfect parallelism, but may be substantially parallel. In other words, even if the front surface 5xa and the back surface 5xb are not perfect parallelism, 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 front surface 5xa and the back surface 5xb is 0°±1°, the front surface 5xa and the back surface 5xb are considered to be parallel.

The mechanical properties of the steel plate 5x are not particularly limited, and may be appropriately determined depending on the application of the steel plate 5x. However, problems such as a decrease in hydrogen embrittlement resistance due to tensile residual stress are particularly likely to occur in high-strength steel plates. In this regard, the tensile strength of the steel plate 5x 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 plate 5x 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 plate in this application is in accordance with ISO 6892-1:2009.

The chemical composition and metal structure of the steel plate 5x are not particularly limited and may be appropriately determined depending on the application of the surface-treated steel plate 5. According to the technology disclosed herein, the tensile residual stress at the fracture surface 1b can be reduced regardless of the chemical composition and metal structure of the steel plate 5x. As an example of the chemical composition, the steel plate 5x 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%, 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 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%.

2.2 Surface Treatment Layer The surface treatment layer 5y may be a layer provided on at least one of the front surface 5xa and the back surface 5xb of the steel sheet 5x, and the type is not particularly limited. For example, the surface treatment layer 5y may include plating. Examples of plating include metal plating and alloy plating containing at least one metal selected from the group consisting of zinc, tin, chromium, nickel, magnesium, aluminum, copper, and iron, and these may be used appropriately depending on the application. The plating can be provided on the surface of the steel sheet 5x by a known method. The plating may be hot-dip plating or electrolytic plating.

The surface treatment layer 5y may include a coating film. The surface treatment layer 5y may also include a film other than plating or coating film, such as diamond-like carbon (DLC). The coating film may be, for example, a resin film attached to the surface of the steel plate 5x, or a resin paint applied to the surface of the steel plate 5x. Examples of resins that can form the coating film include polyester resin, melamine resin, isocyanate resin, silicone acrylic resin, etc., and two or more of these may be combined.

The surface treatment layer 5y may be one layer or multiple layers. The surface treatment layer 5y may be formed on a surface that has been chemically treated. For example, when a coating film is formed on the surface of the steel sheet 5x, the surface of the steel sheet 5x may be chemically treated to improve adhesion between the steel sheet 5x and the coating film. The same applies when a coating film is formed on the surface of the plating after plating the steel sheet 5x, and the surface of the plating may be chemically treated to improve adhesion between the plating and the coating film. The chemical treatment may include, for example, a chemical treatment containing at least one chemical component selected from the group consisting of phosphoric acid, zircon, sulfide, and oxide, or a chemical treatment containing multiple components, and these may be used appropriately depending on the application. In addition, oil may be appropriately applied on the chemical treatment, taking advantage of the surface unevenness of the chemical treatment.

The thickness of the surface treatment layer 5y is not particularly limited and may be appropriately determined depending on the purpose. For example, the thickness of the surface treatment layer 5y (if it is composed of multiple layers, the total thickness of the multiple layers) may be 1 μm or more, 5 μm or more, 10 μm or more, 80 μm or less, 60 μm or less, or 40 μm or less.

The surface-treated steel sheet 5 may comprise the steel sheet 5x and the surface treatment layer 5y described above. Here, the surface-treated steel sheet 5 may be coated with a lubricant, metal soap, soap, or the like. When a lubricant or the like is coated, the presence of the lubricant or the like is considered to constitute the front and back surfaces of the steel sheet 5x and the surface treatment layer 5y. For example, when a lubricant is coated on the surface treatment layer 5y, the surface of the lubricant is the "surface of the surface-treated steel sheet 5".

2.3 Friction Coefficient As shown in Figures 4(A) to (C), the surface-treated steel sheet 5 has a front surface 10a and a back surface 10b. Here, in the surface-treated steel sheet 5, the friction coefficient of the front surface 10a is 1.4 times or more than the friction coefficient of the back surface 10b, and the difference between the friction coefficient of the front surface 10a and the friction coefficient of the back surface 10b is 0.10 or more. The friction coefficient ratio may be 1.6 times or more, 1.8 times or more, 2.0 times or more, 2.5 times or more, 3.0 times or more, 4.0 times or more, or 5.0 times or more. The friction coefficient difference may be 0.20 or more, or 0.40 or more. The upper limit of the friction coefficient ratio and the friction coefficient difference is not particularly limited. By increasing the dynamic friction coefficient of the surface 10a of the surface-treated steel sheet 5 (reducing sliding), the side of the blade bites into the surface 10a when deforming the surface-treated steel sheet 5 in the early stage of shearing, which makes it easier for stress concentration to occur on the surface 10a and thus makes it easier for cracks to occur in the surface-treated steel sheet 5. As a result, in the sheared end surface 1 after shearing, the area ratio of the portion originating from the first crack 1dx propagating from the sag 1a side tends to be larger than the area ratio of the portion originating from the second crack 1dy propagating from the burr 1c side, making it easier to reduce the tensile residual stress on the fracture surface 1b.

The friction coefficient of the front surface 10a and the back surface 10b of the surface-treated steel sheet 5 are measured by a pin-on-disk as follows. The material of the pin of the pin-on-disk is SKD11 (equivalent to HRC 58-60), and the pin diameter is φ0.71 cm (0.357×0.357×π=0.4 cm ² (0.9 MPa at 29.4 kN)). The load is 29.4 kN. The measurement conditions are linear motion (the pin is moved so as to always slide on the new surface) and scanning of the pin on the front surface or back surface of the surface-treated steel sheet at a speed of 30 mm/sec. The scanning time is 7 seconds (travel distance 30 mm/sec×7 seconds). The average value of the measured values excluding the first and last seconds is calculated, and this is taken as the kinetic friction coefficient. The dynamic friction coefficient is measured at three different points on each of the front and back surfaces of the surface-treated steel sheet, and the average values are regarded as the "front surface friction coefficient" and the "back surface friction coefficient" of the surface-treated steel sheet.

There are various methods for making the coefficient of friction of the front surface 10a of the surface-treated steel sheet 5 1.4 times or more that of the back surface 10b. For example, a surface treatment layer 5y with a relatively large coefficient of friction may be provided on the front surface 10a. Alternatively, a surface treatment layer 5y with a relatively small coefficient of friction may be provided on the back surface 10b. Alternatively, the type and amount of lubricant applied to the front surface 10a and back surface 10b may be different.

When a surface treatment layer 5y having a relatively large friction coefficient is provided on the surface 10a of the surface-treated steel sheet 5, specific examples of such a surface treatment layer 5y include at least one metal plating or alloy plating selected from aluminum, nickel, and zinc, or a resin coating as described above.

When a surface treatment layer 5y having a relatively small coefficient of friction is provided on the back surface 10b of the surface-treated steel sheet 5, specific examples of such a surface treatment layer 5y include a combination of phosphating and oil coating, MoS, _MoS2 , or a combination of these with oil coating, or a composite plating of Ni-P and PTFE. Alternatively, regardless of the type of surface treatment layer 5y, it is also possible to reduce the coefficient of friction by applying a lubricating oil or soap only to the back surface 10b side.

2.4 Example of Sheared End Surface The surface-treated steel sheet 5 may be sheared. For example, as shown in Figures 5 and 6, the surface-treated steel sheet 5 may have a sheared end surface 1, and the sheared end surface 1 may have a sag 1a, a fractured surface 1b, and a burr 1c, with the sag 1a present on the front surface 10a side and the burr 1c present on the back surface 10b side. In this way, in the sheared end surface 1 having the sag 1a on the front surface 10a side, the tensile residual stress of the fractured surface 1b is likely to be reduced as described above.

As shown in Figures 5 and 6, the sheared end surface 1 has a sag 1a, a fracture surface 1b, and a burr 1c. The sheared end surface 1 may also have a sheared surface 1e. Of the sheared end surface 1, the sag 1a, the burr 1c, and the sheared surface 1e may have any shape. The sag 1a, the burr 1c, and the sheared surface 1e may have the same shape as conventional ones.

As shown in Figures 5 and 6, the fracture surface 1b may include a first portion 1bx and a second portion 1by. The first portion 1bx may be formed by a first crack 1dx that has progressed from the sag 1a side to the burr 1c side, and the second portion 1by may be formed by a second crack 1dy that has progressed from the burr 1c side to the sag 1a side.

The propagation direction of the first crack 1dx may be from the sag 1a side to the burr 1c side. The propagation direction of the first crack 1dx may be along the thickness direction of the surface-treated steel plate 5 (perpendicular to the front surface 10a and the back 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. The propagation direction of the second crack 1dy may be along the thickness direction of the surface-treated steel plate 5 (perpendicular to the front surface 10a and the back surface 10b) or inclined to the thickness direction. For example, when the surface-treated steel plate 5 is sheared, if a clearance C (see FIG. 8) 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 to the thickness direction, and the larger the clearance, the greater the inclination.

The first crack 1dx needs only to start from the sag 1a side and progress toward the burr 1c side, and join with the second crack 1dy on the burr 1c side; it does not necessarily have to progress along the shortest path from the sag 1a side to the second crack 1dy on the burr 1c side. For example, the first crack 1dx may progress in the plate width direction or plate length direction (such as the back-to-front direction of the paper in FIG. 2(B)) on the way from the sag 1a side to the burr 1c side. The same applies to the second crack 1dy.

In the shear end surface 1, the area ratio of the first portion 1bx in the fracture surface 1b may be greater than the area ratio of the second portion 1by in the fracture surface 1b. In other words, in the shear end surface 1, the average length of the first crack 1dx that has propagated from the sag 1a side toward the burr 1c side may be greater than the average length of the second crack 1dy that has propagated from the burr 1c side toward the sag 1a side. As described above, when the area ratio of the portion of the fracture surface 1b originating from the crack 1dx that has propagated from the sag 1a side is greater than the area ratio of the portion of the fracture surface 1b originating from the crack 1dx that has propagated from the burr 1c side, the tensile residual stress in the fracture surface 1b is relatively easy to reduce.

In addition, when determining the area ratio of each of the first portion 1bx and the second portion 1by in the fracture surface 1b and the length of each 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. 6, when the shear end surface 1 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 fracture surface 1b is larger than the area ratio of the second portion 1by in the fracture surface 1b.

According to the inventor's findings, the greater the area ratio of the first portion 1bx in the fracture surface 1b, the more the tensile residual stress in the fracture surface 1b is reduced. For example, the area ratio of the first portion 1bx in 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, or 2.5 times or more of the area ratio of the second portion 1by in the fracture surface 1b.

The burrs 1c may be large enough that they cannot be seen with the naked eye. Which of the front surface 10a and back surface 10b of the surface-treated steel sheet 5 is the surface with the sag 1a and which is the surface with the burrs 1c can be easily determined by observing the shape of the surface-treated steel sheet 5, even if the burrs 1c cannot be seen.

In the sheared end surface 1, the sheared surface 1e and the fractured surface 1b have different properties. For example, the roughness (glossiness) of the sheared surface 1e and the fractured surface 1b are different. In this regard, the sheared surface 1e and the fractured surface 1b can be easily distinguished from each other just by observing their appearance.

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 shear end surface 1. 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 Figures 7(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.

3. Manufacturing method of the workpiece As shown in Fig. 8(A), in the manufacturing method of the workpiece 10, first, the surface-treated steel sheet 5 of the present disclosure is placed between the first blade 21 and the second blade 22. Here, the front surface 10a of the surface-treated steel sheet 5 is placed on the first blade 21 side, and the back surface 10b of the surface-treated steel sheet 5 is placed on the second blade 22 side. Next, as shown in Fig. 8(B), the first blade 21 and the second blade 22 are moved relatively to shear the surface-treated steel sheet 5.

3.1 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.

The shape of the first bottom surface 21a can be determined according to the shape of the desired shear end surface 1 of the processed material 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 surface 10a when the surface-treated steel sheet 5 is sheared.

The shape of the first side surface 21b can be determined according to the shape of the desired sheared end surface 1 of the processed material 10. The first side surface 21b may be a flat surface, a curved surface, or a combination of a flat surface and a curved surface.

The first tip 21x may extend linearly or curvedly in the longitudinal direction of the first blade 21 (the forward-backward direction in the plane of the paper in FIG. 8), and may be determined according to the shape of the intended sheared end surface 1 of the processed material 10. When a punching hole is provided in the surface-treated steel sheet 5, the shape of the first tip 21x may be annular along the edge of the punching hole.

In a standby state before the shearing operation, the first blade 21 may be positioned above the second blade 22. In this case, the first blade 21 may be a punch that punches out, from top to bottom, a portion of the surface-treated steel sheet 5 placed on the second bottom surface 22a of the second blade 22.

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

3.2 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 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 an 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 a line of intersection 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 line of intersection toward both the second bottom surface 22a side and the second side surface 22b side may be regarded as the second tip portion 22x.

The shape of the second bottom surface 22a can be determined according to the shape of the desired sheared end surface 1 of the processed material 10. The second bottom surface 22a may have a flat surface or a curved surface, and the flat surface or the curved surface can face the back surface 10b when the surface-treated steel sheet 5 is sheared.

The shape of the second side surface 22b can be determined according to the shape of the desired sheared end surface 1 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.

The second tip 22x may extend linearly or curvedly in the longitudinal direction of the second blade 22 (the forward-backward direction in the plane of the paper in FIG. 8), and can be determined according to the shape of the intended sheared end surface 1 of the processed material 10. When a punch hole is provided in the surface-treated steel sheet 5, the shape of the second tip 22x can be annular along the edge of the punch hole.

In a standby state before the shearing operation, the second blade 22 may be positioned below the first blade 21. In this case, the second blade 22 may be a die on which the surface-treated steel sheet 5 is placed.

The second blade 22 is made of a material that is generally used for blades in shearing processes. 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. The second blade 22 may also have a second coating on its surface.

3.3 Arrangement of the Surface-Treated Steel Sheet In the manufacturing method of the present disclosure, the surface-treated steel sheet 5 is arranged between the first blade 21 and the second blade 22 as described above. There is no particular restriction on the arrangement of the surface-treated steel sheet 5 between the first blade 21 and the second blade 22, and the surface-treated steel sheet 5 may be arranged so that the first bottom surface 21a is in contact with the front surface 10a and the second bottom surface 22a is in contact with the back surface 10b, allowing the surface-treated steel sheet 5 to be appropriately sheared. For example, as shown in FIG. 8(A), the surface-treated steel sheet 5 may be placed on the second bottom surface 22a of the second blade 22 while the first blade 21 is arranged above the surface-treated steel sheet 5. In addition, in the manufacturing method of the present disclosure, in order to make the shearing of the surface-treated steel sheet 5 easier, when the surface-treated steel sheet 5 is arranged between the first blade 21 and the second blade 22, the surface-treated steel sheet 5 may be pressed against the first bottom surface 21a or the second bottom surface 22a by a pressing member (holder) not shown. The shape of the pressing member is not particularly limited, and a general pressing member may be used.

3.4 Actions and Relationship Between the First Blade and the Second Blade During Shearing In the manufacturing method disclosed herein, the surface-treated steel sheet 5 is placed between the first blade 21 and the second blade 22, and then the first blade 21 and the second blade 22 are moved relatively to each other to shear the surface-treated steel sheet 5. 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.

8(A) and (B), a clearance C may be provided between the first blade 21 and the second blade 22 during shearing. The clearance C may be appropriately determined depending on the material, thickness, etc. of the surface-treated steel sheet 5. For example, the clearance C may be 5% or more of the sheet thickness of the surface-treated steel sheet 5, or may be 25% or less of the sheet thickness. Note that the "clearance" referred to in this application complies with ISO 16630:2009.

3.4.2 Shear angle As shown in Fig. 9, a shear angle α may be provided between the first blade 21 and the second blade 22 during shearing. The shear angle α may be appropriately determined depending on the material, shape, etc. of the surface-treated steel sheet 5. For example, the shear angle α may be 0° or more and 10° or less. Furthermore, according to the inventor's new findings, when the shear angle is 0° or more and 1° or less, the first crack 1dx is more likely to propagate in the surface-treated steel sheet 5 before the second crack 1dy.

4. Processed material According to the manufacturing method of the present disclosure, a processed material 10 in which the tensile residual stress is reduced, particularly in the fractured surface 1b of the sheared end surface 1, is easily obtained. The processed material 10 may be similar to the surface-treated steel plate 5 having the sheared end surface 1. That is, as shown in Figs. 5 and 6, the sheared end surface 1 of the processed material 10 includes a sag 1a, a fractured surface 1b, and a burr 1c. The fractured surface 1b may include 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 fractured surface 1b may be greater than the area ratio of the second portion 1by to the fractured surface 1b. The surface-treated steel sheet 5 having the sheared end surface 1 may be further sheared to obtain a processed material 10 having a plurality of sheared end surfaces 1 .

The processed material 10 may have a sheared end surface 1, and the configuration other than the sheared end surface is not particularly limited. The shape of the processed material 10 corresponds to the shape of the surface-treated steel plate 5 described above. That is, the processed material 10 may be in the form of a plate as described above. In addition, the processed material 10 may have a surface 10a and a back surface 10b opposite to the surface 10a as surfaces other than the sheared end surface 1, and the surface 10a and the back surface 10b may be connected via the sheared end surface 1. The surface 10a and the back surface 10b may be parallel to each other. The processed material 0 may be formed by bending or the like. The mechanical properties and chemical composition of the steel plate 5x constituting the processed material 10 are also as described above.

5. Actions and Effects As described above, by shearing the surface-treated steel sheet 5 according to the present disclosure, a processed material 10 having reduced tensile residual stress at the fracture surface 1b is easily obtained. By reducing the tensile residual stress at the fracture surface 1b, for example, hydrogen embrittlement resistance or fatigue strength at the sheared end surface 1 can be improved.

1. Steel Plates As the steel plates constituting the surface-treated steel plates, which are the processed materials, steel plate a (thickness: about 1.8 mm) with a tensile strength of 1470 MPa, steel plate b (thickness: about 1.8 mm) with a tensile strength of 1180 MPa, and steel plate c (thickness: about 1.8 mm) with a tensile strength of 980 MPa were prepared. The front and back surfaces of each steel plate were subjected to the surface treatment described below to obtain surface-treated steel plates A to C for evaluation.

2. Surface Treatment Layer 2.1 Reference Example 1
A Zn-Fe alloy layer (GA) having a thickness of 8 μm was provided on both the front and back surfaces of the steel sheet, and then the back surface was subjected to a chemical conversion treatment (thickness 1 μm) using manganese phosphate, and further, a lubricating oil was applied to a coating thickness of 1 μm to obtain a surface-treated steel sheet according to Reference Example 1.

2.2 Example 2
A Zn-Fe alloy layer (GA) having a thickness of 8 μm was provided on both the front and back surfaces of the steel sheet. Then, the back surface was subjected to a chemical conversion treatment (thickness 0.009 μm) using manganese phosphate, and further, metal soap (thickness 1 μm) and sodium stearate soap (thickness 1 μm) were applied to obtain a surface-treated steel sheet according to Example 2.

2.3 Reference Example 3
A Zn-Fe alloy layer (GA) having a thickness of 8 μm was provided on both the front and back surfaces of the steel sheet. Then, the back surface was subjected to a chemical conversion treatment (thickness 1 μm) using phosphate (iron), and further, metal soap (thickness 1 μm) and sodium stearate soap (thickness 2 μm) were applied to obtain a surface-treated steel sheet according to Reference Example 3.

2.4 Example 4
A Zn-Fe alloy layer (GA) having a thickness of 8 μm was provided on both the front and back surfaces of the steel sheet. Then, the back surface was subjected to a chemical conversion treatment (thickness 2 μm) using zinc phosphate, and further, a metal soap (thickness 1 μm) and a sodium stearate soap (thickness 2 μm) were applied to obtain a surface-treated steel sheet according to Example 4.

2.5 Example 5
A Zn-plated layer having a thickness of 12 μm was provided on the front surface of the steel sheet, and a Zn-plated layer having a predetermined thickness was provided on the back surface. Then, a coating film (40 μm thick) made of melamine resin and polyester resin was provided on the front surface, and MoS ₂ was sprayed on the back surface to provide a MoS ₂ layer, thereby obtaining a surface-treated steel sheet according to Example 5. Note that, on the back surface of the surface-treated steel sheet according to Example 5, the MoS ₂ layer and the Zn-plated layer were alloyed, and a MoS ₂ -Zn layer (4 μm thick) and a MoS ₂ layer (2 μm thick) were formed on the Zn-plated layer (10 μm thick).

2.6 Example 6
A Zn-plated layer having a thickness of 3 μm was provided on the front surface of the steel sheet, and a Zn-plated layer having a predetermined thickness was provided on the back surface. _MoS2 was then sprayed onto the back surface to provide a _MoS2 layer, and lubricating oil was further applied to a coating thickness of 1 μm to obtain a surface-treated steel sheet according to Example 6. Note that on the back surface of the surface-treated steel sheet according to Example 6, the _MoS2 layer and the Zn-plated layer were alloyed, and a _MoS2 -Zn layer (thickness 3 μm) and a _MoS2 layer (thickness 2 μm) were formed on the Zn-plated layer (thickness 5 μm).

2.7 Example 7
A 4 μm thick Ni-plated layer was provided on the front surface of the steel sheet, and a 8 μm thick Ni-plated layer was provided on the back surface. Then, _MoS2 was sprayed on the back surface to provide a _MoS2 layer (thickness 2 μm), thereby obtaining a surface-treated steel sheet according to Example 7.

2.8 Example 8
An Al-Si plating layer having a thickness of 30 μm was provided on the front surface of the steel sheet, and an Al-Si plating layer having a predetermined thickness was provided on the rear surface. MoS ₂ was then sprayed onto the rear surface to provide an MoS ₂ layer, thereby obtaining a surface-treated steel sheet according to Example 8. Note that on the rear surface of the surface-treated steel sheet according to Example 8, the MoS ₂ layer and the Al-Si plating layer were alloyed, and an MoS ₂ -Al-Si layer (thickness 3 μm) and an MoS ₂ layer (thickness 3 μm) were formed on the Al-Si plating layer (thickness 27 μm).

2.9 Example 9
A Zn-Al-Mg plating layer having a thickness of 40 μm was formed on both the front and back surfaces of the steel sheet. Thereafter, the back surface was subjected to a chemical conversion treatment (thickness 0.01 μm) using manganese phosphate, and further, metal soap (thickness 1 μm) and sodium stearate soap (thickness 1 μm) were applied to obtain a surface-treated steel sheet according to Example 9.

2.10 Example 10
A 3 μm thick Ni—W(P) plating layer was provided on the front surface of the steel sheet, and a 6 μm thick Ni—W(P) plating layer was provided on the back surface of the steel sheet. Then, a DLC film (thickness: 1 μm) was provided on the back surface to obtain a surface-treated steel sheet according to Example 10.

2.11 Example 11
A 1 μm-thick Cr plating layer was formed on the front surface of the steel sheet, and a 5 μm-thick Cr plating layer was formed on the back surface of the steel sheet. Then, a DLC film (thickness: 0.5 μm) was formed on the back surface of the steel sheet, thereby obtaining a surface-treated steel sheet according to Example 11.

2.12 Example 12
A 3 μm thick Ni plating layer was provided on the front surface of the steel sheet, and a 10 μm thick composite plating layer of Ni-P and PTFE was provided on the back surface. Then, a coating film (40 μm thick) made of isocyanate resin and polyester resin was provided on the front surface, and lubricating oil was applied to the back surface to a coating thickness of 1 μm to obtain a surface-treated steel sheet according to Example 12.

2.13 Reference Example 13
A 2 μm thick Ni-plated layer was provided on the front surface of the steel sheet, and a 5 μm thick Ni-plated layer was provided on the back surface of the steel sheet. Then, a PTFE layer (10 μm thick) was provided on the back surface of the steel sheet to obtain a surface-treated steel sheet according to Reference Example 13.

2.14 Example 14
A 10 μm thick Fe plating layer was provided on the front surface of the steel sheet, while the back surface was subjected to a chemical conversion treatment (thickness: 0.007 μm) using phosphate (iron), and further, metal soap (thickness: 0.5 μm) and sodium stearate soap (thickness: 1 μm) were applied to obtain a surface-treated steel sheet according to Example 14.

2.15 Example 15
A 10 μm-thick Sn-plated layer was provided on the front surface of the steel sheet, and a 8 μm-thick Ni-plated layer was provided on the back surface. Then, a polyester resin-containing coating (50 μm thick) was provided on the front surface, and a MoS ₂ layer (3 μm thick) was provided on the back surface by thermal spraying with MoS _2. Furthermore, a lubricating oil was applied to a coating thickness of 1 μm to obtain a surface-treated steel sheet according to Example 15.

2.16 Example 16
A Zn plating layer having a thickness of 20 μm was formed on both the front and back surfaces of the steel sheet, and then a chemical conversion treatment (thickness 0.008 μm) with zinc phosphate was performed on the front surface, and a coating film (thickness 30 μm) made of silicone acrylic resin was further formed, and a chemical conversion treatment (thickness 0.008 μm) with zinc phosphate was performed on the back surface, and further a lubricating oil was applied to a coating thickness of 1 μm to obtain a surface-treated steel sheet according to Example 16.

2.17 Reference Example 17
A 10 μm thick Fe-plated layer was provided on the front surface of the steel sheet, and a 5 μm thick Ni-plated layer was provided on the back surface. Then, a 10 μm thick PTFE layer was provided on the back surface to obtain a surface-treated steel sheet according to Reference Example 17.

2.18 Reference Example 18
The front surface of the steel sheet was subjected to a polishing treatment without providing a surface treatment layer to adjust the surface roughness Ra to 5 μm, and then a 2 μm thick Fe plating layer was provided, while the back surface was subjected to a polishing treatment to adjust the surface roughness Ra to 0.5 μm, and then a 5 μm thick Zn plating layer was provided, thereby obtaining a surface-treated steel sheet according to Reference Example 18.

2.19 Reference Example 19
A 5 μm-thick Ni plating layer was provided on the front surface of the steel sheet, and a 10 μm-thick PTFE layer was further provided on the rear surface, which was then subjected to a chemical conversion treatment (0.007 μm thick) using phosphate (iron), and further coated with metal soap (0.5 μm thick) and sodium stearate soap (1 μm thick) to obtain a surface-treated steel sheet according to Reference Example 19.

2.20 Comparative Example 1
Neither the front nor back surface of the steel sheet was subjected to a surface treatment.

2.21 Comparative Example 2
A Zn-Fe alloy layer (GA) having a thickness of 8 μm was provided on both the front and back surfaces of the steel sheet, and a chemical conversion treatment (thickness 1 μm) was performed using manganese phosphate. Furthermore, a lubricating oil was applied to a coating thickness of 1 μm to obtain a surface-treated steel sheet according to Comparative Example 2.

2.22 Comparative Example 3
A surface-treated steel sheet according to Comparative Example 3 was obtained by applying a 5 μm-thick Ni plating to the front surface of the steel sheet and a 5 μm-thick Zn plating to the back surface.

2.23 Comparative Example 4
A Zn-plated layer having a thickness of 20 μm was provided on the front surface of the steel sheet, and a Zn-plated layer having a predetermined thickness was provided on the back surface. Then, a chemical conversion treatment (thickness 0.008 μm) was performed on the front surface using zinc phosphate, and a lubricant was applied to a thickness of 1 μm. Meanwhile, MoS ₂ was sprayed on the back surface to provide a MoS ₂ layer, and further, a lubricant was applied to a thickness of 1 μm, to obtain a surface-treated steel sheet according to Comparative Example 4. Note that, on the back surface of the surface-treated steel sheet according to Comparative Example 4, the MoS ₂ layer and the Zn-plated layer were alloyed, and a MoS ₂ -Zn layer (thickness 3 μm) and a MoS ₂ layer (thickness 2 μm) were formed on the Zn-plated layer (thickness 5 μm).

2.24 Comparative Example 5
A surface-treated steel sheet according to Comparative Example 5 was obtained by providing a 3 μm-thick Zn-plated layer on the front surface of the steel sheet and a 8 μm-thick Zn—Fe alloy layer (GA) on the back surface.

Details of the surface treatment layer are summarized in Tables 1 and 2 below.

3. Evaluation conditions Each surface-treated steel sheet was placed between a punch and a die, and a portion of the sheet was punched out by moving the punch and the die relative to each other to obtain a processed material having a sheared end surface on the die. When the surface-treated steel sheet was placed, the front surface of the steel sheet faced the punch and the back surface faced the die. The friction coefficients of the front and back surfaces of the surface-treated steel sheet before shearing were measured as follows, and the properties of the sheared end surface of the processed material after shearing were evaluated as follows.

3.1 Measurement of friction coefficient For each of the surface-treated steel sheets, the friction coefficient of the punch side surface and the friction coefficient of the die side surface were measured using a pin-on-disk. When a lubricant was present, the friction coefficient was measured in the presence of the lubricant. The details of the measurement method have already been described.

3.2 Properties of sheared end surface The residual stress of the sheared end surface was measured for each processed material as follows. That is, residual stress was measured by X-ray with a spot diameter of φ500 μm at the center position in the plate thickness direction (three different locations in the plate width direction). The residual stress was measured in three directions: the plate thickness direction, the plate width direction, and the direction at 45 degrees from the plate thickness direction, and the sin ² ψ method was used to calculate the residual stress. The residual stress in the normal direction of the end surface was assumed to be zero, and the maximum principal stress was calculated from the residual stress in the three directions calculated. The maximum principal stress values calculated at the three locations were averaged.

In the case of surface-treated steel plate A1 (1470 MPa class), the residual stress value was evaluated as "A" when it was less than 600 MPa, "B" when it was 600 MPa or more and less than 1000 MPa, "C" when it was 1000 MPa or more and less than 1400 MPa, "D" when it was 1400 MPa or more and less than 1800 MPa, and "E" when it was 1800 MPa or more.
In the case of surface-treated steel plate B1 (1180 MPa class), the residual stress value was evaluated as "A" when it was less than 450 MPa, "B" when it was 450 MPa or more and less than 800 MPa, "C" when it was 800 MPa or more and less than 1150 MPa, "D" when it was 1150 MPa or more and less than 1450 MPa, and "E" when it was 1450 MPa or more.
In the case of surface-treated steel plate C1 (980 MPa class), the residual stress value was evaluated as "A" when it was less than 400 MPa, "B" when it was 400 MPa or more and less than 650 MPa, "C" when it was 650 MPa or more and less than 950 MPa, "D" when it was 950 MPa or more and less than 1200 MPa, and "E" when it was 1200 MPa or more.
The evaluation results A to D were judged as pass, and E was judged as fail.

4. Evaluation Results The evaluation results are shown in Table 3 below.

As shown in Table 3, by changing the type of surface treatment layer on the front and back of the surface-treated steel sheet, the friction coefficient of the front (punch side surface) of the surface-treated steel sheet is greater than the friction coefficient of the back (die side surface) by a predetermined amount and the friction coefficient difference is greater than a predetermined amount. In this case, when the surface-treated steel sheet is sheared, the tensile residual stress of the fracture surface can be significantly reduced at the sheared end surface of the processed material obtained after shearing. This is thought to be because the area ratio of the first portion originating from the first crack propagating from the punch side can be increased at the fracture surface of the processed material. From various experiments conducted by the inventors, the above effect is observed regardless of the thickness or strength of the steel sheet.

The surface-treated steel sheet material disclosed herein can be used, for example, as a component material for automobiles, home appliances, building structures, ships, bridges, construction machinery, various plants, penstocks, etc.

1 Shear end surface 1a Sagging 1b Fracture surface 1bx First portion 1by Second portion 1c Burr 1dx First crack 1dy Second crack 1e Shear surface 5x Steel plate 5y Surface treatment layer 5 Surface-treated steel plate 10 Workpiece 10a Surface 10b Back surface 11 Part of steel plate 12 Other part of steel plate 15 Scrap 21 First blade 21a First bottom surface 21b First side surface 21x First tip portion 22 Second blade 22a Second bottom surface 22b Second side surface 22x Second tip portion

Claims (7)

A surface-treated steel plate having a steel plate, a surface treatment layer, and a sheared end surface ,
At least one of the front surface and the back surface is formed by the surface treatment layer,
The coefficient of friction of the front surface is 4.0 times or more the coefficient of friction of the back surface,
The difference between the coefficient of friction of the front surface and the coefficient of friction of the back surface is 0.40 or more ;
The shear end surface has a sag, a fracture surface, and a burr,
The sagging is present on the front surface side and the burr is present on the back surface side,
The tensile strength of the steel plate is 980 MPa or more ;
Surface treated steel sheet. The front surface and the back surface are formed by the surface treatment layer,
The type of the surface treatment layer constituting the front surface and the type of the surface treatment layer constituting the back surface are the same or different;
The surface-treated steel sheet according to claim 1. The thickness of the surface treatment layer is 1 μm or more and 50 μm or less.
The surface-treated steel sheet according to claim 1 or 2 . The surface treatment layer includes plating.
The surface-treated steel sheet according to any one of claims 1 to 3 . The surface treatment layer includes a coating film.
The surface-treated steel sheet according to any one of claims 1 to 4 . The tensile strength of the steel plate is 1470 MPa or more;
The surface-treated steel sheet according to any one of claims 1 to 5 . a surface-treated steel plate is disposed between a first blade and a second blade, wherein the surface-treated steel plate has a steel plate and a surface treatment layer, at least one of the front surface and the back surface is constituted by the surface treatment layer, the friction coefficient of the front surface is 4.0 times or more the friction coefficient of the back surface, the difference between the friction coefficient of the front surface and the friction coefficient of the back surface is 0.40 or more, the tensile strength of the steel plate is 980 MPa or more, the front surface of the surface-treated steel plate is disposed on the first blade side, and the back surface of the surface-treated steel plate is disposed on the second blade side;
The first blade and the second blade are moved relative to each other to shear the surface-treated steel sheet to obtain a processed material having a sheared end surface , wherein the sheared end surface has a sag, a fracture surface, and a burr, and the sag is present on the front surface side and the burr is present on the back surface side.
Including,
Manufacturing method of processed materials.