Method for evaluating the delayed fracture properties of steel plates

Abstract

This invention provides a method for evaluating the delayed fracture properties of steel plates that offers excellent evaluation accuracy and easily reflects the state of plastic strain and residual stress expected to occur in actual parts in the test specimen. [Solution] The present invention provides a method for evaluating the delayed fracture properties of a steel sheet, comprising: a first step of applying plastic strain to a steel sheet; a second step of obtaining multiple types of test pieces from the evaluation target area of ​​the steel sheet to which plastic strain has been applied, each differing in at least one of the cutting direction and cutting position; a third step of applying elastic stress to each of the multiple types of test pieces; and a fourth step of evaluating the delayed fracture properties of each of the multiple types of test pieces to which elastic stress has been applied.

Description

Technical Field

[0001] The present invention relates to a method for evaluating the delayed fracture property of a steel sheet. Specifically, the present invention relates to a highly accurate method for evaluating the delayed fracture property of a steel sheet that takes into account the effects of plastic strain and residual stress.

Background Art

[0002] In recent years, fuel efficiency regulations for automobiles have been tightened worldwide, and the application of high-tensile steel sheets to vehicle body materials has been promoted to reduce the weight of automobiles. In addition, along with the trend toward carbon neutrality, electrification of automotive power sources is progressing, and further higher tensile strength of steel sheets is required from the perspective of improving battery protection performance, and the application of ultra-high-tensile steel sheets is also being considered.

[0003] Parts such as automotive parts formed from steel sheets may experience a phenomenon in which cracks occur after a certain period of time, the so-called delayed fracture. In particular, parts formed from ultra-high-tensile steel sheets may have large tensile residual stress generated at end faces produced by processing such as cutting and punching, which may trigger delayed fracture. For this reason, for the purpose of clarifying the risk of such delayed fracture occurring in parts, evaluation of delayed fracture property has been performed using test specimens to which plastic strain and residual stress that may occur in formed parts are applied. 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 is being considered, a method for evaluating delayed fracture property that is also applicable to ultra-high-tensile steel sheets is required.

[0004] As a conventional method for evaluating delayed fracture property, for example, Patent Document 1 discloses that a drawn formed product is produced by subjecting a circular blank made of a steel sheet to cylindrical deep drawing, a C-shaped steel sheet is produced by removing the cylindrical portion and a part of the flange portion from the drawn formed product, a load is further applied to the C-shaped steel sheet to elastically deform the C-shaped steel sheet in the in-plane direction, then the C-shaped steel sheet is restrained so that the elastic deformation is maintained and no out-of-plane deformation occurs in the C-shaped steel sheet, the load is removed in this state to produce a test specimen, and the test specimen is used to perform the delayed fracture property evaluation.

[0005] Furthermore, Patent Document 2 discloses a method for evaluating delayed fracture properties using a test specimen made of a high-tensile steel plate in the shape of a regular polygon, which is subjected to form forming or form drawing using a mold. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2023-44079 [Patent Document 2] Patent No. 6614197 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] Delayed fracture occurs due to plastic strain and residual stress introduced during the processing of steel sheets into parts. In particular, at end faces such as punched end faces, sheared end faces, or cut end faces formed by processing such as punching, shearing, or cutting with a die, it is necessary to consider residual stress and plastic strain generated by shearing in addition to normal press working. When ultra-high-strength steel sheets are applied to automotive parts, it is expected that residual stress will be further increased, and the influence of plastic strain on delayed fracture characteristics will be more pronounced. In order to smoothly apply ultra-high-strength steel sheets to automotive parts, it is necessary to accurately understand the relationship between the plastic strain and residual stress generated at the various end faces mentioned above and the delayed fracture characteristic when evaluating delayed fracture properties.

[0008] However, conventional delayed fracture evaluation methods cannot adequately assess the influence of plastic strain and residual stress on delayed fracture. Some proposed evaluation methods only induce uniform deformation in the steel plate, or do not allow for individual setting of the plastic strain and residual stress applied to the test specimen. Therefore, it is difficult to adjust and appropriately evaluate the plastic strain and residual stress generated at the various end faces mentioned above.

[0009] For example, the method disclosed in Patent Document 1 above involves applying uniform plastic strain to a circular blank by cylindrical drawing, and then generating residual stress by applying a load to a C-shaped steel plate. However, in the method disclosed in Patent Document 1, if the state of plastic strain and residual stress differs from the state of plastic strain and residual stress that occurs in actual automobile parts or other components, there is a risk that an appropriate delayed fracture evaluation may not be possible.

[0010] Furthermore, in the evaluation method disclosed in Patent Document 2 mentioned above, since the blank subjected to cylindrical deep drawing has a regular polygonal shape, not only plastic strain but also residual stress is applied to the test piece during deep drawing. In other words, in the evaluation method disclosed in Patent Document 2, it is not possible to individually set the plastic strain and residual stress applied to the test piece. For this reason, even in the evaluation method disclosed in Patent Document 2, if the state of plastic strain and residual stress differs from the state of plastic strain and residual stress that occurs in actual automobile parts or other components, there is a risk that an appropriate delayed fracture evaluation may not be possible.

[0011] This invention has been made in view of the above circumstances, and aims to provide a method for evaluating the delayed fracture properties of steel plates that has excellent evaluation accuracy and can easily reflect the state of plastic strain and residual stress that is expected to occur in actual parts in the test specimen. [Means for solving the problem]

[0012] The present invention includes the following embodiments.

[0013] (Aspect 1) A method for evaluating the delayed fracture properties of steel plates, The first step involves applying plastic strain to the steel plate, A second step involves obtaining multiple types of test pieces from the evaluation target area of ​​the steel plate to which the above plastic strain has been applied, wherein at least one of the cutting direction and cutting position is different. A third step involves applying elastic stress to each of the above-mentioned multiple types of test specimens, A fourth step of evaluating the delayed fracture property for each of the plurality of types of test pieces applied with the elastic stress, A method for evaluating delayed fracture properties, comprising:

[0014] (Aspect 2) The method for evaluating delayed fracture properties according to the above aspect 1, wherein the first step is a step of subjecting the steel sheet to bulging with a cylindrical punch.

[0015] (Aspect 3) The method for evaluating delayed fracture properties according to the above aspect 1, wherein the first step is a step of subjecting the steel sheet to biaxial tension.

[0016] (Aspect 4) The method for evaluating delayed fracture properties according to any one of the above aspects 1 to 3, wherein the second step is a step of obtaining the plurality of types of test pieces by punching using a die.

[0017] (Aspect 5) The method for evaluating delayed fracture properties according to any one of the above aspects 1 to 4, wherein the second step further includes a step of adjusting the thickness of each of the plurality of types of test pieces such that the thicknesses of the plurality of types of test pieces are equal to each other.

[0018] (Aspect 6) The method for evaluating delayed fracture properties according to any one of the above aspects 1 to 5, wherein the third step is a step of subjecting each of the plurality of types of test pieces to four-point bending. [Effects of the Invention]

[0019] According to the present invention, it is possible to provide a method for evaluating delayed fracture properties of a steel sheet that is excellent in evaluation accuracy, in which the states of plastic strain and residual stress assumed to occur in an actual component can be easily reflected in a test piece. [Brief Description of the Drawings]

[0020] [Figure 1] Figure 1 is a diagram schematically showing an example of the first step and the second step of the delayed fracture property evaluation method according to one embodiment of the present invention. (a) of Figure 1 is the first step, and (b) is the second step. [Figure 2] Figure 2 is a diagram schematically showing another example of the first step and the second step. (a) in Figure 2 is the first step, and (b) is the second step. [Figure 3] Figure 3 is a diagram schematically showing still another example of the first step and the second step. (a) in Figure 3 is the first step, and (b) is the second step. [Figure 4] Figure 4 is a diagram schematically showing an example of the third step. (a) in Figure 4 is four-point bending, and (b) is uniaxial tension. [Mode for Carrying Out the Invention]

[0021] To achieve the above object, the present inventors have conducted intensive investigations focusing on the relationship between the state of plastic strain and residual stress assumed to occur in actual components and the delayed fracture property. As a result, the present inventors have found a delayed fracture property evaluation method capable of simultaneously controlling pre-strain and stress, in which an arbitrary pre-strain state (a state where plastic strain is applied) is imparted to an evaluation target location of a steel sheet, a plurality of types of test pieces differing in pre-strain state are cut out from the evaluation target location, and then elastic stress is imparted to each of the plurality of types of test pieces. According to this delayed fracture property evaluation method, since the state of plastic strain and residual stress assumed to occur in actual components such as automobile components can be easily reflected in the test piece, the influence of the state of plastic strain and residual stress on delayed fracture can be evaluated with high accuracy.

[0022] The present invention has been completed based on the above findings, and includes the aspects of the embodiments described below.

[0023] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In the present specification and the drawings, elements having substantially the same functional configuration are denoted by the same reference numerals, thereby omitting redundant descriptions.

[0024] A method for evaluating the delayed fracture properties of a steel sheet according to one embodiment of the present invention is a method comprising the following first to fourth steps. That is, the delayed fracture evaluation method of this embodiment is a method comprising: a first step of applying plastic strain to a steel sheet; a second step of obtaining multiple types of test pieces from the evaluation target area of ​​the steel sheet to which plastic strain has been applied, each having at least one of the cutting direction and cutting position being different; a third step of applying elastic stress to each of the multiple types of test pieces; and a fourth step of evaluating the delayed fracture properties of each of the multiple types of test pieces to which elastic stress has been applied.

[0025] (Effects and Benefits) In this embodiment, the delayed fracture evaluation method involves two separate steps: a first step of applying plastic strain to a steel sheet and a third step of applying elastic stress to each of several types of test pieces. This allows for independent control of pre-strain and stress. Furthermore, this embodiment includes a second step of obtaining several types of test pieces from the evaluation target area of ​​the steel sheet to which plastic strain has been applied, each differing in at least one of the cutting direction and cutting position. This makes it easier to reflect the state of plastic strain and residual stress expected to occur in actual automobile parts and other components in the test pieces. As a result, this embodiment can accurately evaluate the influence of the state of plastic strain and residual stress on delayed fracture.

[0026] The following describes in detail each step of the delayed fracture evaluation method of this embodiment.

[0027] (1st step) In the delayed fracture evaluation method of this embodiment, the first step is to apply plastic strain to the steel sheet. This first step makes it possible to reflect the state of plastic strain that is expected to occur in actual automobile parts and other components in the test specimen. It is necessary to ensure that no residual stress is generated in the evaluation area of ​​the steel sheet during this first step.

[0028] In the first step, the means and amount of plastic strain applied to the steel sheet are not limited to specific ones, but should be selected according to the state of plastic strain expected to occur in actual automobile parts and other components. CAE analysis may be used in making these selections.

[0029] In this specification, the state of plastic strain expected to occur in an actual part refers not only to the state of plastic strain that actually occurs in the part, but also to the state of plastic strain that is assumed to have a certain correlation with the state of plastic strain that actually occurs. The same applies to the state of stress expected to occur in an actual part.

[0030] The means of applying plastic strain are not particularly limited as long as they can apply appropriate plastic strain to the steel plate, but examples include tension, shear, and bulging. Tension may be uniaxial tension or biaxial tension such as cruciform tension. Shear may be simple shear or pure shear. Furthermore, bulging may be bulging using a cylindrical punch (Marciniac method) or bulging using a punch of other shapes.

[0031] For example, if uniaxial tension is used as the means of applying plastic strain, uniaxial strain can be applied to the steel plate. If biaxial tension is used, uniaxial strain, plane strain, or biaxial strain can be applied to the steel plate. Furthermore, if simple shear is used as the means of applying plastic strain, no reduction in plate thickness occurs, making it possible to apply pre-strain up to the high strain range. In addition, if bulging using a cylindrical punch (hereinafter sometimes simply referred to as "cylindrical bulging") is used as the means of applying plastic strain, uniaxial strain, plane strain, or biaxial strain can be applied to the steel plate. Furthermore, the above-described means of applying plastic strain are designed so as not to generate residual stress in the evaluation area of ​​the steel plate.

[0032] In this embodiment, the first step is preferably a step of bulging the steel plate using a cylindrical punch. As described above, when bulging using a cylindrical punch is adopted as a means of imparting plastic strain, it is possible to impart uniaxial strain, plane strain, or biaxial strain to the steel plate, and it is possible to impart a state of plastic strain to the steel plate that is more in line with reality.

[0033] Here, Figure 1 is a schematic diagram showing an example of the first and second steps of the delayed fracture evaluation method according to this embodiment, and Figure 2 is a schematic diagram showing another example of the first and second steps. In Figures 1 and 2, (a) shows the first step and (b) shows the second step. In the example shown in Figure 1, the first step involves applying uniaxial strain to the steel plate 1 by bulging it with a cylindrical punch 3. On the other hand, in the example shown in Figure 2, except that the size of the steel plate 1 and the bulging conditions are changed, plane strain is applied to the steel plate 1 by bulging it with a cylindrical punch 3, similar to the example shown in Figure 1.

[0034] Thus, when cylindrical bulging is used as a means of imparting plastic strain, the state of plastic strain applied to the steel plate can be arbitrarily controlled by appropriately selecting the size, shape, and bulging conditions of the steel plate. Therefore, a state of plastic strain that is more in line with actual conditions can be applied to the steel plate.

[0035] Furthermore, in this embodiment, it is preferable that the first step is a step of applying biaxial tension to the steel plate. As described above, even when biaxial tension is used as a means of applying plastic strain, it is possible to apply uniaxial strain, plane strain, or biaxial strain to the steel plate, and it is possible to apply a state of plastic strain to the steel plate that is more in line with reality.

[0036] Here, Figure 3 schematically shows yet another example of the first and second steps. In Figure 3, (a) shows the first step and (b) shows the second step. In the example shown in Figure 3, the first step applies biaxial strains perpendicular to each other to the cross-shaped steel plate 1 by performing biaxial tension perpendicular to each other on the steel plate 1.

[0037] Furthermore, even when employing biaxial tension as a means of applying plastic strain, it is possible to arbitrarily control the state of plastic strain applied to the steel plate by appropriately selecting the size, shape, and tensile conditions of the steel plate. This allows for the application of a more realistic state of plastic strain to the steel plate.

[0038] Furthermore, in this embodiment, the first step may involve applying an arbitrary strain state to the steel plate and then cutting off the edges to alter the strain path. If the actual parts are subjected to such processing (cutting off the edges), it is possible to apply a more realistic plastic strain state to the steel plate.

[0039] (2nd process) In the delayed fracture evaluation method of this embodiment, the second step is to obtain multiple types of test pieces from the evaluation target area of ​​a steel sheet to which plastic strain has been applied, with at least one of the cutting direction and cutting position being different. This second step makes it easier to reflect the state of plastic strain and residual stress that is expected to occur in actual automobile parts and other components in the test pieces, thereby enabling accurate evaluation of the influence of the state of plastic strain and residual stress on delayed fracture.

[0040] In the second step, the size, shape, cutting direction, and cutting position of the test specimen are not limited to specific ones, but should be selected according to the state of plastic strain and residual stress expected to occur in actual automobile parts and other components. CAE analysis may be used in making these selections.

[0041] In this specification, the cutting direction of the test specimen means the direction along the longitudinal direction of the test specimen being cut.

[0042] In the examples shown in Figure 1(b) and Figure 2(b), the test piece 2 is cut along the longitudinal direction of the steel plate 1. However, in the second step, in addition to the test piece 2 shown in this way, one or more test pieces are cut out with at least one different cutting direction and cutting position. The number of test pieces cut out should be set to two or more, depending on the state of plastic strain and residual stress expected to occur in actual automobile parts and other components. Furthermore, the cutting direction and cutting position of the second and subsequent test pieces should also be set according to the state of plastic strain and residual stress expected to occur in actual components. CAE analysis may be used in setting these parameters.

[0043] (Method of cutting) The means used to cut out the test specimen are not particularly limited and include, for example, cutting methods such as laser cutting and punching methods such as molds.

[0044] In this embodiment, it is preferable that the second step is a step of obtaining the above-mentioned multiple types of test pieces by punching using a die. Cutting out multiple types of test pieces by such punching has the advantage that the second step can be carried out quickly and the cut end surface of the processed part expected in the actual part can be reproduced.

[0045] Furthermore, it is desirable to adjust the plate thickness between different samples before punching. As mentioned above, when multiple types of test specimens are cut out by punching, residual stress is generated in the specimens during the punching process. Since this residual stress is affected by the thickness of the steel plate, the influence of this residual stress can be minimized by adjusting the thickness of each test specimen so that the thicknesses of each of the multiple types of test specimens are equivalent.

[0046] Therefore, in this embodiment, the second step may further include a step of adjusting the thickness of each of the multiple types of test pieces so that the thickness of each of the multiple types of test pieces is equivalent.

[0047] The means for adjusting the plate thickness are not particularly limited, but one method is to reduce the plate thickness by grinding one surface of the steel plate (for example, the inner surface of the bend where compressive stress is applied) using any grinding means.

[0048] (3rd step) In the delayed fracture evaluation method of this embodiment, the third step is to apply elastic stress to each of the above-mentioned multiple types of test specimens. This third step makes it possible to reflect the state of residual stress that is expected to occur in actual automobile parts and other components in the test specimens. Furthermore, by performing the stress application in this third step within the range of elastic deformation by referring to the plastic strain applied in the first step, it is also possible to evaluate only the effect of the plastic strain applied in the first step.

[0049] In the third step, the means of applying elastic stress to the test specimen and the magnitude of the stress are not limited to specific ones, but should be selected according to the state of residual stress expected to occur in actual automobile parts and other components. CAE analysis may be used in making these selections.

[0050] The means of applying elastic stress are not particularly limited as long as they can apply appropriate residual stress to the test specimen, but examples include bending and tension. The bending may be three-point bending or four-point bending. The tension may be uniaxial tension, etc. Here, Figure 4 is a schematic diagram showing an example of the third step. Figure 4(a) is four-point bending, and (b) is uniaxial tension.

[0051] For example, if four-point bending is used as a means of applying elastic stress, since much of the stress applied to actual automobile parts and other components is bending stress, it is possible to apply a stress state to the test piece that is more in line with reality, and the starting point of delayed fracture cracks can be limited to the outside of the bend. This makes it possible to evaluate the influence of plastic strain and residual stress on delayed fracture properties with greater accuracy.

[0052] Therefore, in this embodiment, it is preferable that the third step is a step of performing four-point bending on each of the above-mentioned multiple types of test pieces.

[0053] The means of performing the four-point bending are not particularly limited and may be any means commonly used in this field. For example, the four-point bending may be performed using a four-point bending test apparatus such as the one shown in Figure 4(a). The apparatus shown in Figure 4(a) consists of a structure equipped with a load jig 4a and a support jig 4b as a holder 4, and a four-point bending test can be performed by bending the test piece 2 using the two support points of the load jig 4a and the two support points of the support jig 4b.

[0054] On the other hand, when tension is used as a means of applying elastic stress, no stress distribution occurs in the thickness direction of the test piece. Therefore, if actual parts are subjected to such processing (tensile processing), a stress state that more accurately reflects the actual situation can be applied to the steel plate. Tensile testing can be performed, for example, by a tensile test using a tensile test fixture 5 as shown in Figure 4(b).

[0055] (4th step) In the delayed fracture evaluation method of this embodiment, the fourth step is to evaluate the delayed fracture for each of the above-mentioned multiple types of test specimens to which elastic stress has been applied. In the delayed fracture evaluation method of this embodiment, since the test specimens are subjected to plastic strain and residual stress states that are in line with reality in the first to third steps described above, the delayed fracture can be evaluated with high accuracy in this fourth step.

[0056] The means for evaluating delayed fracture in the fourth step are not particularly limited, and any method for evaluating delayed fracture commonly used in this industry can be adopted. Such evaluation methods include, for example, introducing hydrogen into a test specimen and evaluating the state of crack initiation in the specimen.

[0057] There are no particular restrictions on the method of introducing hydrogen into the test specimen; any known method may be used. Examples of such methods include electrolytically charging the test specimen in an electrolyte, holding the test specimen under a high-pressure hydrogen gas atmosphere, and immersing the test specimen in an etching solution.

[0058] Furthermore, there are no particular restrictions on the method for evaluating the crack formation state. The method involves visually checking for crack formation on the surface of the specimen while introducing hydrogen into the specimen or after introducing hydrogen, observing the surface of the specimen using an optical microscope or electron microscope, or measuring internal cracks using ultrasonic testing. The evaluation should then determine whether cracks formed in the specimen due to hydrogen introduction, the extent of the cracks, and the time elapsed until crack formation.

[0059] (steel plate) The steel sheet to which the delayed fracture evaluation method of this embodiment can be applied may be any steel sheet capable of delayed fracture. For example, the steel sheet may be a so-called high-strength cold-rolled steel sheet, specifically a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more, or a steel sheet for hot stamping. Generally, as the strength of the steel sheet increases, it becomes more difficult to evaluate the delayed fracture under conditions closer to those of actual automobile parts and other components. However, in the delayed fracture evaluation method of this embodiment, a more appropriate evaluation of delayed fracture can be achieved by performing the first to third steps described above.

[0060] Therefore, the delayed fracture evaluation method of this embodiment can also be applied to steel plates having very high strength. Examples of such steel plates include those having tensile strengths of 980 MPa or more, 1180 MPa or more, 1470 MPa or more, 1600 MPa or more, 1800 MPa or more, or 2000 MPa or more. Furthermore, there is no particular upper limit to the tensile strength of the steel plate, but for example, the tensile strength of the steel plate may be 3150 MPa or less, 2850 MPa or less, or 2500 MPa or less.

[0061] The tensile strength of the steel plate is measured by preparing a No. 5 test specimen in accordance with JIS Z 2241:2022 and conducting a tensile test. If there are size constraints on the steel plate and it is not possible to obtain a JIS No. 5 test specimen, an ASTM 1 / 2 test specimen may be used in accordance with ASTM standard E8.

[0062] The method for evaluating the delayed fracture properties of steel plates according to the present invention is not limited to the embodiments described above, and can be appropriately combined, substituted, or modified without departing from the purpose and spirit of the present invention. [Explanation of symbols]

[0063] 1 steel plate 2 Test specimens 3. Cylindrical punch 4 holders 4a Load jig 4b Receiving jig 5. Fixture for tensile testing

Claims

1. A method for evaluating the delayed fracture properties of steel plates, A first step of applying plastic strain to the steel plate, A second step involves obtaining multiple types of test pieces from the evaluation target area of ​​the steel plate to which the plastic strain has been applied, each having at least one of the cutting direction and cutting position being different. A third step involves applying elastic stress to each of the aforementioned multiple types of test specimens, A fourth step involves evaluating the delayed fracture properties of each of the multiple types of test specimens to which the aforementioned elastic stress has been applied. A method for evaluating delayed destructiveness, including the method described above.

2. The delayed fracture evaluation method according to claim 1, wherein the first step is a step of extending the steel plate with a cylindrical punch.

3. The delayed fracture evaluation method according to claim 1, wherein the first step is a step of performing biaxial tension on the steel plate.

4. The delayed fracture evaluation method according to any one of claims 1 to 3, wherein the second step is a step of obtaining the plurality of types of test pieces by punching using a die.

5. The delayed fracture evaluation method according to any one of claims 1 to 3, wherein the second step further includes a step of adjusting the thickness of each of the multiple types of test pieces so that the thickness of each of the multiple types of test pieces is equivalent.

6. The delayed fracture evaluation method according to any one of claims 1 to 3, wherein the third step is a step of performing four-point bending on each of the plurality of types of test specimens.

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