Delayed fracture susceptibility evaluation method

Abstract

To perform the evaluation precision by performing the evaluation test using a plurality of test pieces and easily reflecting a state of the plastic strain and residual stress expected to occur in a component of a product shape on the test piece when performing the delay fracture evaluation at the end surface.SOLUTION: A delayed fracture evaluation method includes the steps of: performing the cylindrical drawing on a circular blank made of steel plate to manufacture a C-shaped steel plate by removing a cylindrical part and a part of a flange part from a drawn molding; applying a load to the C-shaped steel plate to elastically deform the C-shaped steel plate in the in-plane direction; maintaining the elastic deformation of the C-shaped steel plate and restraining the C-shaped steel plate so as not to cause the out-of-plane deformation of the C-shaped steel plate when the load on the C-shaped steel plate is removed; manufacturing a test piece 1 by removing the load in a state where the C-shaped steel plate is restrained; and performing the delay fracture evaluation using the test piece 1.SELECTED DRAWING: Figure 9

Description

[Technical field]

[0001] The present invention relates to a method for evaluating delayed fracture susceptibility of a steel sheet. [Background technology]

[0002] In recent years, fuel efficiency regulations for automobiles have become stricter around the world, and the use of high-tensile steel sheets as body materials is being promoted to reduce the weight of automobiles. In addition, as the trend toward carbon neutrality leads to the advancement of electric power sources for automobiles, there is a demand for even higher tensile strength steel sheets from the perspective of improving battery protection performance, and the use of ultra-high tensile steel sheets is also being considered.

[0003] Automobile parts and other components formed from steel sheets may suffer from a phenomenon in which cracks appear after a certain time has passed, known as delayed fracture. For this reason, in order to prevent delayed fracture in components, it is common to evaluate delayed fracture susceptibility using test pieces that have been given the plastic strain and residual stress that occur in the product shape of the component. Since delayed fracture is more likely to occur as the tensile strength of the steel sheet increases, in recent years, when the application of ultra-high tensile steel sheets has been considered, evaluation methods for delayed fracture susceptibility that can also be used for ultra-high tensile steel sheets have been considered.

[0004] As a conventional method for evaluating delayed fracture resistance, Patent Document 1 discloses a method in which a test piece is prepared by deep cylindrical drawing of a blank made of a regular polygonal high-tensile steel plate, and the delayed fracture resistance is evaluated using the test piece. Patent Document 2 discloses an evaluation method in which a test piece for evaluating hydrogen embrittlement of thin steel plate is attached to a jig in an electrolytic cell, and tensile stress is applied to the test piece while the test piece and the jig are immersed in an electrolytic solution. Patent Document 3 discloses a method in which a test piece of high-strength steel plate is U-bent or V-bent, and compressive stress is applied to both sides of the bent test piece to evaluate delayed fracture resistance. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6614197 [Patent Document 2] Patent No. 4901662 [Patent Document 3] Patent No. 4646134 Summary of the Invention [Problem to be solved by the invention]

[0006] Delayed fracture occurs due to the influence of plastic strain and residual stress that occur during the forming process of steel plate into parts. In particular, punched end faces, sheared end faces, cut end faces, and other end faces formed by processing such as punching, shearing, or cutting of steel plate are prone to plastic strain and residual stress during the forming process into parts of the product shape. For this reason, when evaluating delayed fracture susceptibility, it is necessary to fully understand the relationship between the plastic strain or residual stress that occurs on end faces such as punched end faces, sheared end faces, and cut end faces and delayed fracture susceptibility.

[0007] However, the magnitude of the plastic strain and the magnitude of the residual stress occurring at the end face vary depending on the shape of the end face and the shape of the periphery of the end face. Therefore, in order to accurately evaluate the delayed fracture susceptibility at the end face, it is preferable to be able to individually impart the magnitude of the plastic strain and the magnitude of the residual stress to the test piece used in the evaluation test.

[0008] From this viewpoint, in the evaluation method described in Patent Document 1, since the blank to be subjected to cylindrical deep drawing has a regular polygonal shape, not only plastic strain but also residual stress is imparted to the test piece during deep drawing. That is, in the evaluation method described in Patent Document 1, the magnitude of plastic strain and the magnitude of residual stress imparted to the test piece cannot be set separately. For this reason, depending on the shape of the part to be evaluated for delayed fracture susceptibility, the state of plastic strain and residual stress assumed to occur in the part cannot be reflected in the test piece, and there is room for improvement in terms of the evaluation accuracy of delayed fracture susceptibility.

[0009] In addition, in the evaluation method described in Patent Document 1, plastic strain and residual stress are simultaneously applied to the test specimen, so it is not possible to prepare a test specimen to which only plastic strain or only residual stress is applied. Therefore, depending on the shape of the part to be evaluated for delayed fracture susceptibility, the state of plastic strain or residual stress expected to occur in the part cannot be reflected in the test specimen, and there is room for improvement in terms of the evaluation accuracy of delayed fracture susceptibility.

[0010] In the evaluation method described in Patent Document 2, it is possible to freely set the magnitude of the residual stress applied to the test piece. However, this method requires a tensile testing machine for pulling the test piece in the electrolytic solution, and the test device used for the evaluation test of delayed fracture resistance is a large-scale device. For this reason, it is difficult to perform the evaluation test using multiple test pieces, and when there are multiple test pieces to be evaluated, it takes a long time to complete the evaluation of all the test pieces.

[0011] The evaluation method described in Patent Document 3 uses test pieces obtained by bending a rectangular blank in the out-of-plane direction, and therefore cannot evaluate delayed fracture susceptibility at end faces such as punched end faces, sheared end faces, or cut end faces.

[0012] The present invention has been made in view of the above circumstances, and has an object to enable evaluation tests to be performed using a plurality of test specimens when evaluating delayed fracture susceptibility at an end surface, such as a punched end surface, a sheared end surface, or a cut end surface, and to improve evaluation accuracy by making it easier to reflect the states of plastic strain and residual stress assumed to occur in a part having a product shape in the test specimens. [Means for solving the problem]

[0013] The present invention, which solves the above-mentioned problems, is a method for evaluating delayed fracture tendency, which comprises the steps of: preparing a drawn product by performing cylindrical drawing on a circular blank made of a steel plate; preparing a C-shaped steel plate by removing a cylindrical portion and a part of a flange portion from the drawn product; applying a load to the C-shaped steel plate in a direction in which both ends of the C-shaped steel plate approach or move away from each other, thereby elastically deforming the C-shaped steel plate in an in-plane direction; restraining the C-shaped steel plate such that the elastic deformation of the C-shaped steel plate is maintained and no out-of-plane deformation occurs in the C-shaped steel plate when the load on the C-shaped steel plate is removed; preparing a test piece by removing the load in a state in which the C-shaped steel plate is restrained; and using the test piece to evaluate delayed fracture tendency.

[0014] According to another aspect of the present invention, there is provided a method for evaluating delayed fracture property, comprising the steps of: preparing a C-shaped steel plate by punching, shearing or cutting a blank made of a steel plate; applying a load to the C-shaped steel plate in a direction in which both ends of the C-shaped steel plate approach or move away from each other to elastically deform the C-shaped steel plate in an in-plane direction; restraining the C-shaped steel plate such that the elastic deformation of the C-shaped steel plate is maintained and out-of-plane deformation of the C-shaped steel plate is not caused when the load on the C-shaped steel plate is removed; and removing the load in a state in which the C-shaped steel plate is restrained to prepare a test piece; The test piece is used to evaluate delayed fracture properties. Effect of the Invention

[0017] According to the present invention, when evaluating the delayed fracture susceptibility of an end surface such as a punched end surface, a sheared end surface, or a cut end surface, an evaluation test can be performed using a plurality of test specimens, and the state of plastic strain and residual stress expected to occur in a part having a product shape can be easily reflected in the test specimens, thereby improving the evaluation accuracy. [Brief description of the drawings]

[0018] [Figure 1] FIG. 1 is a diagram showing a test piece used for evaluating delayed fracture properties according to one embodiment of the present invention. [Diagram 2] FIG. [Diagram 3] FIG. 3 is a perspective view showing a drawn product obtained by subjecting the circular blank of FIG. 2 to cylindrical drawing. [Figure 4] 1 is a perspective view showing an annular steel plate from which a cylindrical portion of a drawn product has been removed. FIG. [Diagram 5] FIG. 2 is a diagram showing a C-shaped steel plate. [Figure 6] FIG. 13 is a diagram showing a state in which a load is applied to a C-shaped steel plate. [Figure 7] FIG. 13 is a view showing a state in which a C-shaped steel plate is restrained by a restraining member. [Figure 8] 8 is a view of FIG. 7 as seen in the direction of arrow Z. [Figure 9] FIG. 2 is a diagram showing a state in which a test piece is immersed in an immersion liquid. [Figure 10] 1A to 1C are diagrams showing examples of the shape of a C-shaped steel plate having a notch. [Figure 11] 1A to 1C are diagrams showing examples of shapes of a drawn product having a through hole provided in a flange portion. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In this specification and the drawings, elements having substantially the same functional configuration are designated by the same reference numerals, and redundant description will be omitted.

[0020] The delayed fracture susceptibility evaluation method according to this embodiment is a method in which a test piece as shown in FIG. 1 to which plastic strain and residual stress are imparted is prepared, and the delayed fracture susceptibility is evaluated using the test piece. The tensile strength of the steel plate used for the test piece is not particularly limited, but delayed fracture is more likely to occur as the tensile strength of the steel plate increases. For this reason, the delayed fracture susceptibility evaluation method according to this embodiment is particularly useful when performing an evaluation test using a steel plate having a tensile strength of 780 MPa or more.

[0021] An example of a method for preparing a test piece will be described below.

[0022] First, a steel plate is punched to prepare a circular blank 2 as shown in Fig. 2. Note that the "circular blank" in this specification does not include an elliptical blank. Also, even if the outer shape of a blank is circular, if a through hole or a notch is formed in the blank, the blank is not included in the "circular blank" in this specification.

[0023] Next, as shown in FIG. 3, the circular blank 2 is subjected to cylindrical drawing. As a result, a drawn product 3 having a cylindrical portion 3a and a flange portion 3b is obtained. In this drawn product 3, plastic strain occurs in the cylindrical portion 3a and the flange portion 3b, and the magnitude of the plastic strain that occurs here changes depending on the forming height H (drawing depth) of the cylindrical portion 3a. In other words, by adjusting the forming height H when performing cylindrical drawing, it is possible to impart a desired plastic strain to the cylindrical portion 3a and the flange portion 3b. That is, by determining the forming height H according to the magnitude of the plastic strain assumed to occur in the part in the product shape, it is possible to set a plastic strain equivalent to the plastic strain that occurs in the part in the product shape for the test piece 1 used for delayed fracture property evaluation.

[0024] In addition, when cylindrical drawing is performed on a polygonal blank, residual stress occurs in the flange portion, but when cylindrical drawing is performed from a circular blank 2 as in this embodiment, residual stress is unlikely to occur in the flange portion 3b. That is, in the drawn product 3 obtained by cylindrical drawing of the circular blank 2, only the desired plastic strain can be set without causing residual stress. In addition, in the drawn product of the polygonal blank, the magnitude of the plastic strain occurring in the flange portion changes along the circumferential direction of the flange portion, but in the drawn product 3 of the circular blank 2, the magnitude of the plastic strain tends to be uniform. In the test piece 1 in which such uniform plastic strain occurs, the plastic strain is not localized, so that the generated strain can be easily evaluated accurately. Therefore, it becomes easier to evaluate the effect of plastic strain on delayed fracture resistance, and delayed fracture resistance can be evaluated more accurately.

[0025] Next, as shown in FIG. 4, the cylindrical portion 3a is removed from the drawn product 3 to leave only the flange portion 3b, and an annular steel plate 4 is produced. The method for removing the cylindrical portion 3a from the drawn product 3 is not particularly limited, but for example, the cylindrical portion 3a is cut off by cutting such as laser cutting. As described above, when the cylindrical drawing process is performed on the circular blank 2, no residual stress is generated in the flange portion 3b, so even if the cylindrical portion 3a is removed from the drawn product 3, the magnitude of the plastic strain generated in the flange portion 3b is unlikely to change. That is, in the annular steel plate 4, the plastic strain of the flange portion 3b after the cylindrical drawing process shown in FIG. 3 remains.

[0026] Next, a part of the circumferential region of the annular steel plate 4 is removed to produce the C-shaped steel plate 5 shown in Fig. 5. The method for removing the part of the annular steel plate 4 is not particularly limited, but for example, the part of the region is cut off by cutting such as laser cutting or shearing. The interval of the opening 6 of the C-shaped steel plate 5, i.e., the interval between one end 5a and the other end 5b of the C-shaped steel plate 5, may be such that both ends 5a, 5b of the C-shaped steel plate 5 do not come into contact with each other when a load is applied to the C-shaped steel plate 5 as described later.

[0027] The C-shaped steel plate 5 in this embodiment has two flat portions 7. Each flat portion 7 is formed by removing two parts of the outer edge of the C-shaped steel plate 5 by cutting such as laser cutting or shearing. Each flat portion 7 is parallel to the symmetry axis A of the C-shaped steel plate 5 having an axisymmetric shape, and the distances from each flat portion 7 to the inner diameter center O of the C-shaped steel plate 5 are equal.

[0028] The procedure for producing the C-shaped steel plate 5 shown in Fig. 5 from the drawn product 3 shown in Fig. 3 is not limited to the procedure described in this embodiment. For example, the C-shaped steel plate 5 may be produced by forming the opening 6 and the two flat portions 7 in the flange portion 3b of the drawn product 3 shown in Fig. 3, and then removing the cylindrical portion 3a.

[0029] Next, as shown in Fig. 6, a load is applied to the C-shaped steel plate 5 in a direction in which both ends 5a, 5b of the C-shaped steel plate 5 approach each other, and the C-shaped steel plate 5 is elastically deformed in an in-plane direction (parallel to the paper surface of Fig. 6). In other words, the C-shaped steel plate 5 is bent and deformed in a direction in which both ends 5a, 5b of the C-shaped steel plate 5 approach each other so as not to plastically deform the C-shaped steel plate 5.

[0030] In this embodiment, the two flat surfaces 7 of the C-shaped steel plate 5 are clamped by a vice 8, and a load is applied toward the symmetric axis A side of the C-shaped steel plate 5. When a load is applied to the C-shaped steel plate 5, stress is generated in the C-shaped steel plate 5, and in this embodiment, the stress generated here becomes the residual stress of the test piece 1 described below. That is, by adjusting the load applied to the C-shaped steel plate 5, the stress of the C-shaped steel plate 5 can be adjusted, and the desired residual stress can be applied to the test piece 1. Therefore, by determining the load applied to the C-shaped steel plate 5 according to the magnitude of the residual stress assumed to be generated in the product-shaped part, the residual stress generated in the product-shaped part can be set for the test piece 1.

[0031] The means for applying a load to the C-shaped steel plate 5 is not particularly limited as long as it is capable of generating a desired stress. For example, when applying a load to the C-shaped steel plate 5, the load may be applied while restraining the C-shaped steel plate 5 with a weak restraining force that suppresses out-of-plane deformation of the C-shaped steel plate 5 and does not inhibit elastic deformation in the in-plane direction, using two restraining members 9 described later.

[0032] In addition, in this embodiment, the two flat surfaces 7 are provided to clamp the C-shaped steel plate 5 with the vice 8 and apply a load, but if a desired stress can be applied to the C-shaped steel plate 5 by other load application means, it is not necessary to provide the two flat surfaces 7. In addition, the load input position for the C-shaped steel plate 5 is not limited to the position described in this embodiment.

[0033] 7 and 8, while a load is being applied to the C-shaped steel plate 5, the C-shaped steel plate 5 is clamped between two restraining members 9 from the out-of-plane direction of the C-shaped steel plate 5 (the direction perpendicular to the paper surface of FIG. 7). After that, the two restraining members 9 are fixed to each other, for example, by bolt fastening. By restraining the C-shaped steel plate 5 in this manner, the out-of-plane deformation of the C-shaped steel plate 5 is suppressed.

[0034] The magnitude of the restraining force of the restraining member 9 on the C-shaped steel plate 5 is set so that the stress generated when a load is applied to the C-shaped steel plate 5 remains as a residual stress when the load applied to the C-shaped steel plate 5 is removed as shown in Fig. 1. In other words, the C-shaped steel plate 5 is restrained with a restraining force such that the in-plane elastic deformation generated when a load is applied to the C-shaped steel plate 5 is maintained as it is even when the load is removed. As a result, the stress applied for the delayed fracture evaluation in the load application process shown in Fig. 6 remains in the C-shaped steel plate 5 as a residual stress.

[0035] Although the restraining member 9 in this embodiment is a rectangular plate, the shape, thickness, and number of the restraining members 9 are not particularly limited as long as they are capable of restraining the C-shaped steel plate 5. The method of fixing the two restraining members 9 is not limited to bolt fastening. The material of the two restraining members 9 and the material of the fasteners that fix the restraining members 9 to each other may be different from the material of the C-shaped steel plate 5.

[0036] The test piece 1 used for the delayed fracture susceptibility evaluation in this embodiment is prepared by the above procedure. Then, as shown in Fig. 9, the test piece 1 is immersed in an immersion liquid such as hydrochloric acid or ammonium thiocyanate to perform the delayed fracture susceptibility evaluation. Note that a known test method can be applied to the evaluation test of delayed fracture susceptibility performed after the preparation of the test piece 1.

[0037] Since the outer and inner surfaces of the C-shaped steel plate 5 correspond to end faces such as punched end faces, sheared end faces or cut end faces of a part in the product shape, by performing a delayed fracture susceptibility evaluation using the above-mentioned test piece 1, it is possible to evaluate the relationship between the plastic strain or residual stress at the end face and the delayed fracture susceptibility.

[0038] According to the method for producing the test piece 1 in this embodiment, the plastic strain and the residual stress to be applied to the test piece 1 can be set individually. That is, the magnitude of the plastic strain can be set freely, and the magnitude of the residual stress can also be set freely. Therefore, the plastic strain and the residual stress assumed to be generated in a part having a product shape can be applied to the test piece 1, and by using this test piece 1, the delayed fracture tendency can be evaluated more accurately than before.

[0039] Furthermore, according to the method for evaluating delayed fracture susceptibility in this embodiment, an evaluation test can be performed simply by immersing the test piece 1 to which plastic strain and residual stress have been imparted in the immersion liquid, so that it is possible to immerse a plurality of test pieces 1 in the immersion liquid. In other words, it is possible to evaluate the delayed fracture susceptibility of a plurality of test pieces 1 in a single evaluation test.

[0040] The C-shaped steel plate 5 may have a notch or a through hole formed by punching, shearing, cutting, or the like. In the example shown in Fig. 10, a notch 10 formed by punching is provided at the position where the outer peripheral surface of the C-shaped steel plate 5 intersects with the symmetry axis A. By using the C-shaped steel plate 5 with such a notch 10 formed therein and imparting residual stress by the method shown in Figs. 6 to 8, it is possible to perform a delayed fracture susceptibility evaluation assuming the punched end face of a part in which residual stress occurs on the outside of the bend. The position of the notch or through hole is appropriately determined depending on the shape of the part to be evaluated for delayed fracture susceptibility.

[0041] In the above embodiment, the test piece 1 was prepared so that both plastic strain and residual stress were imparted, but in the delayed fracture evaluation, evaluation may be performed on a part in which plastic strain does not occur and only residual stress occurs. When evaluating delayed fracture assuming such a part, for example, the process of preparing the annular steel plate 4 from the circular blank 2 shown in Figs. 2 to 4 may be omitted. Specifically, the C-shaped steel plate 5 shown in Fig. 5 may be prepared by punching, shearing, or cutting the blank of the steel plate, and then the test piece 1 may be prepared by the method shown in Figs. 6 to 8. This makes it possible to prepare the test piece 1 in which only residual stress occurs without plastic strain. By using this test piece 1 to evaluate delayed fracture, it is possible to evaluate delayed fracture assuming a part in which only residual stress occurs.

[0042] In the above embodiment, in order to set a residual stress in the C-shaped steel plate 5, a load is applied in a direction in which both ends 5a, 5b of the C-shaped steel plate 5 approach each other, but a load may be applied in a direction in which both ends 5a, 5b of the C-shaped steel plate 5 move away from each other (moving away). For example, the upper and lower parts of the C-shaped steel plate 5 divided by the symmetrical axis A shown in Fig. 5 may be elastically deformed so as to be pulled in opposite directions, and the C-shaped steel plate 5 may be restrained in this state by the restraining member 9 to prepare the test piece 1.

[0043] Furthermore, in the evaluation of delayed fracture susceptibility, there are cases where evaluation is performed on a component in which no residual stress occurs and only plastic strain occurs. When evaluating delayed fracture susceptibility assuming such a component, for example, an annular steel plate 4 produced through the steps shown in Figs. 2 to 4 may be used as a test piece 1 to perform the delayed fracture susceptibility evaluation. This test piece 1 is in a state in which a desired plastic strain has been imparted by cylindrical drawing as shown in Fig. 3, and no residual stress has been imparted. By using this test piece 1 to evaluate delayed fracture susceptibility, it is possible to evaluate delayed fracture susceptibility assuming a component in which only plastic strain occurs.

[0044] Furthermore, for example, in order to evaluate the delayed fracture susceptibility of a part end surface to which only plastic strain has been imparted, a test piece 1 may be prepared by cylindrically drawing a circular blank 2 as shown in FIG. 3, and then providing a notch or a through hole in a flange portion 3b of a drawn product 3. In the example shown in FIG. 11, a through hole 11 is formed in the flange portion 3b by punching. By using this drawn product 3 with the through hole 11 formed therein as the test piece 1, it is possible to perform a delayed fracture susceptibility evaluation assuming a part in which only plastic strain has occurred around the through hole 11.

[0045] Although the embodiment of the present invention has been described above, the present invention is not limited to such examples. It is clear that a person skilled in the art can come up with various modified or altered examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally belong to the technical scope of the present invention. [Industrial Applicability]

[0046] The present invention can be applied to evaluation of delayed fracture susceptibility of steel sheets. [Explanation of symbols]

[0047] 1 Test piece 2 Circular Blanks 3. Squeeze-molded products 3a Cylindrical part 3b Flange part 4. Circular steel plate 5 C-shaped steel plate 5a end 5b End 6 Openings 7 Plane part 8. Vice 9 Restraining member 10 Notch 11 Through hole A axis of symmetry H Molding height O Center of inner diameter

Claims

1. A cylindrical drawing process is performed on a circular blank made of a steel plate to produce a drawn product. A C-shaped steel plate is produced by removing a cylindrical portion and a part of a flange portion from the drawn product, A load is applied to the C-shaped steel plate in a direction in which both ends of the C-shaped steel plate approach or move away from each other, thereby elastically deforming the C-shaped steel plate in an in-plane direction; Restraining the C-shaped steel plate so that the elastic deformation of the C-shaped steel plate is maintained and out-of-plane deformation of the C-shaped steel plate is not generated when the load on the C-shaped steel plate is removed; A test piece is prepared by removing the load while the C-shaped steel plate is restrained, A method for evaluating delayed fracture susceptibility, comprising: evaluating delayed fracture susceptibility using the test piece.

2. A C-shaped steel plate is produced by punching, shearing or cutting a blank made of a steel plate; A load is applied to the C-shaped steel plate in a direction in which both ends of the C-shaped steel plate approach or move away from each other, thereby elastically deforming the C-shaped steel plate in an in-plane direction; Restraining the C-shaped steel plate so that the elastic deformation of the C-shaped steel plate is maintained and out-of-plane deformation of the C-shaped steel plate is not generated when the load on the C-shaped steel plate is removed; A test piece is prepared by removing the load while the C-shaped steel plate is restrained, A method for evaluating delayed fracture susceptibility, comprising: evaluating delayed fracture susceptibility using the test piece.

3. 3. The method for evaluating delayed fracture property according to claim 1 or 2, characterized in that, when restraining the C-shaped steel plate, the C-shaped steel plate is clamped between two restraining members from the out-of-plane direction of the C-shaped steel plate, and the C-shaped steel plate is restrained by fixing the two restraining members to each other.

4. The method for evaluating delayed fracture susceptibility according to any one of claims 1 to 3, characterized in that the C-shaped steel plate has a notch or a through hole formed therein.

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