Delayed fracture property evaluation method, delayed fracture prediction method, program, and method for manufacturing press-molded product

Abstract

The present invention provides a technique with which it is possible to evaluate the influence of a strain amount prior to trim molding of a rear trim part on a delayed fracture characteristic of a shear end surface. Provided is a delayed fracture characteristic evaluation method for evaluating a delayed fracture characteristic of a shear end surface of a metal plate (3) made of a high-strength steel sheet, wherein: the metal plate (3) to at least a part of which plastic strain has been applied is sheared in a position including the part to which the plastic strain has been applied, so that a test piece (1) of the metal plate having the shear end surface is prepared; and the delayed fracture characteristic of the shear end surface is evaluated by setting the test piece (1) in a hydrogen intrusion environment in a state in which a load stress is applied to the shear end surface of the test piece (1).

Description

[Technical Field]

[0001] The present invention relates to a technique suitable for evaluating delayed fracture at a sheared edge of a press-formed product manufactured from a metal sheet made of high-strength steel sheet, and a technique utilizing the evaluation technique. In this specification, high-strength steel sheet refers to a steel sheet with a tensile strength of 980 MPa or more. [Background technology]

[0002] Currently, automobiles are required to improve fuel efficiency and crashworthiness through weight reduction. To achieve both weight reduction and occupant protection in the event of a collision, there is a trend toward using high-strength steel sheets for the components that make up the body. In recent years, ultra-high-strength steel sheets with tensile strengths of 1470 MPa or more have also been used in car bodies. One of the issues when using high-strength steel sheets in car bodies is delayed fracture. Delayed fracture, which occurs at the edge after shearing, is a particularly important issue for high-strength steel sheets with tensile strengths of 980 MPa or more. The edge after shearing is also referred to as the sheared edge. It is known that large tensile stress remains at this sheared edge. For this reason, there is concern about the occurrence of delayed fracture at the sheared edge.

[0003] A method for evaluating delayed fracture at a sheared edge is, for example, the method described in Patent Document 1. The method described in Patent Document 1 describes evaluating hydrogen embrittlement of a thin steel plate by injecting hydrogen into a stressed test specimen. Furthermore, Patent Document 2 describes a method for evaluating delayed fracture properties by partially cutting out the compressed edge from a test piece produced by deep drawing, and applying stress to the test piece. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5196926 [Patent Document 2] Japanese Patent Publication No. 2022-14784 Summary of the Invention [Problem to be solved by the invention]

[0005] Here, there are two types of sheared end faces of automotive parts: the front trim portion and the rear trim portion. The front trim portion is the portion that is sheared before press forming. The rear trim portion is the portion that is sheared after press forming. The latter type of rear trimming occurs, for example, when a plate holder is required during draw forming. In this case, after press forming, the excess flange for draw forming is sheared (rear trimmed) along the product outline (see reference numeral 8 in Figure 9). In the case of such post-trim, a sheared end surface is formed by shearing a portion in which plastic strain has already been introduced by forming.

[0006] The inventors have found that such a rear trim portion may have different properties at the sheared end surface from a front trim portion obtained by shearing a metal plate to which no plastic strain has been introduced prior to press forming. However, conventional methods such as those described in Patent Document 1 do not particularly focus on evaluating the shear end surface of such a rear trim portion.

[0007] Furthermore, Patent Document 2 aims to evaluate delayed fracture by introducing compressive strain into an already formed sheared edge. In contrast, the aim of the present invention is to evaluate a newly formed sheared edge after introducing strain, and the implementation of the present invention is different from that of Patent Document 2.

[0008] Considering the production process of actual press-formed products, particularly mass-produced automobile parts, the evaluation of delayed fracture associated with such rear trim portions is extremely important. The present invention has been made with this in mind, and aims to provide a technology that can evaluate the effect of the amount of strain before trim forming on the delayed fracture properties of the shear end surface of the rear trim portion. [Means for solving the problem]

[0009] One aspect of the present invention is to evaluate delayed fracture occurring at the sheared edge of a formed product made of high-strength steel plate. This aspect of the present invention provides a technology for evaluating delayed fracture taking into account the amount of plastic strain in the rear trim portion before shear processing, which was not taken into account in conventional techniques. As a result, this aspect of the present invention aims to provide a press-formed product with excellent delayed fracture resistance in the manufacture of a formed product having a rear trim portion.

[0010] In order to achieve this object, one aspect of the present invention is a method for evaluating delayed fracture properties of a sheared edge of a metal plate made of a high-strength steel plate, in which a metal plate having at least a portion thereof imparted with plastic strain is sheared at a position including the portion to which the plastic strain is imparted, thereby preparing a test piece of the metal plate having the sheared edge, and placing the test piece in a hydrogen penetration environment while applying a load stress to the sheared edge of the test piece, thereby evaluating the delayed fracture properties of the sheared edge with respect to the plastic strain. [Effects of the Invention]

[0011] According to the present invention, it is possible to evaluate the delayed fracture properties of the sheared edge according to the plastic strain applied before the shearing process to form the sheared edge. Therefore, according to the present invention, it is possible to evaluate the influence of the strain amount before trimming of the rear trim portion on the delayed fracture properties of the sheared edge. As a result, according to the aspects of the present invention, it is possible to further improve delayed fracture resistance when high-strength steel sheets are applied to various parts such as panel parts, structural and skeletal parts of automobiles. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 2 is a diagram illustrating a processing flow for evaluating delayed fracture properties according to an embodiment of the present invention. [Figure 2] FIG. 10 is a diagram illustrating an example in which plastic strain is imparted by tensile deformation in the minor axis. [Figure 3] FIG. 10 is a diagram illustrating an example in which plastic strain is imparted by compressive deformation in the minor axis. [Figure 4] FIG. 10 is a diagram illustrating an example of processing of a program according to an embodiment of the present invention. [Figure 5] FIG. 10 is a diagram illustrating an example in which plastic strain is imparted by tensile deformation in the examples. [Figure 6] FIG. 10 is a diagram illustrating an example in which plastic strain is imparted by compressive deformation in an embodiment. [Figure 7] FIG. 1 is a diagram showing an example of a delayed fracture limit line with respect to tensile strain in an example. [Figure 8] FIG. 1 is a diagram showing an example of a delayed fracture limit line with respect to compressive strain in an example. [Figure 9] FIG. 2 is a top view showing a press-formed product in the example. [Figure 10] Figure 7 shows a diagram reflecting the information in Table 3. [Figure 11] Figure 8 shows a diagram reflecting the information in Table 3. [Figure 12] Figure 7 shows the information in Table 4. [Figure 13] Figure 8 shows the information in Table 4. DETAILED DESCRIPTION OF THE INVENTION

[0013] Next, an embodiment of the present invention will be described with reference to the drawings. (overview) First, an outline of this embodiment will be described. In this embodiment, focusing on the amount of strain in the trimmed portion before trimming, plastic strain is introduced into the portion of the test specimen where the shear edge is to be formed. The plastic strain is introduced, for example, by uniaxial tension or compression in the direction of the planned shear edge extension. The plastically strained test specimen is then sheared to form the shear edge. The plastically strained area is then sheared and stress is applied to the sheared end surface formed.The stressed test piece is then placed in a hydrogen penetration environment to determine the limit of the applied stress at which delayed fracture does not occur (hereinafter referred to as the delayed fracture limit stress).

[0014] From the above test, it is possible to obtain a combination of the amount of tensile or compressive strain (amount of plastic strain) before shearing and the limit value of the applied stress after shearing at which delayed fracture does not occur. Then, from the information on the combination, information on the correlation between the amount of plastic strain before shearing and the limit value of the applied stress is obtained. The information on the correlation may be in the form of a known format, such as a function or a table.

[0015] Furthermore, by referencing the correlation information and judging the CAE analysis information of the press-formed product, it is predicted whether or not delayed fracture will occur in the current press-formed product. Based on this prediction, the product is designed to prevent delayed fracture. This enables the production of press-formed products with excellent delayed fracture resistance. This process may be configured as a program. Alternatively, the limit value of the allowable load stress may be calculated from the amount of plastic strain and the above correlation information. The amount of plastic strain is the amount of plastic strain imparted to the end surface of the target press-formed product up to the shearing process. In this case, the press-formed product may be determined by CAE analysis so that the load stress (forming stress) during press forming is equal to or less than the calculated limit value of the load stress. In this specification, the forming stress refers to the load stress imparted to the sheared end surface after shearing. The forming stress refers to the stress remaining in the target press-formed product during its use.

[0016] Next, an example of this embodiment will be described with reference to the drawings. (Delayed fracture property evaluation method) The present embodiment is a delayed fracture property evaluation method for evaluating the delayed fracture property of a sheared end surface of a metal plate. The present invention is particularly effective when the metal plate is a high-strength steel plate. In this embodiment, the process of evaluating the delayed fracture properties of the metal plate to be evaluated includes a test step including actual experiments and an evaluation step. Specifically, as shown in Fig. 1, the delayed fracture property evaluation method of this embodiment includes a first step 10A to a sixth step 10F. The first step 10A to the fifth step 10E correspond to testing steps. The sixth step 10F corresponds to an evaluation step.

[0017] <First step 10A> A metal plate 3 is prepared under the same conditions as the metal plate to be evaluated. The first step 10A is a step of applying plastic strain to at least a partial region of the metal plate 3. The metal plate 3 used is preferably a metal plate made of the same material and having the same thickness as the metal plate to be evaluated. The plastic strain is preferably imparted to the entire region that will become the shear end face of the test piece 1 described later. The plastic strain is preferably imparted by uniaxial deformation in tension or compression.

[0018] The first step 10A is, for example, a step of introducing plastic strain into the metal plate 3 at a location where the sheared edge of the test piece 1 is to be formed. Specifically, the first step 10A is a step of introducing uniaxial tensile or compressive plastic strain in the direction of the planned sheared edge extension. That is, the tensile or compressive direction in which the plastic strain is introduced is the direction along the extension direction of the sheared edge. This makes it possible to introduce nearly uniform plastic strain into the sheared edge of the test piece 1 along the extension direction of the sheared edge. The extension direction of the sheared edge is, for example, linear.

[0019] One example of such a method is to impart plastic strain through tensile deformation, as follows: As shown in Figure 2, both ends of a rectangular metal plate (test piece 1) are held with chucks 2, and the metal plate is subjected to tensile deformation. The following method can be used to impart plastic strain through compressive deformation. Specifically, as shown in Figure 3, a metal plate 3 is deep-drawn to form a drawn part 3A. During the forming process, compressive deformation is introduced into the flange portion of the drawn part 3A. The flange portion of the produced drawn part 3A then becomes the region that can be used as the test piece 1.

[0020] During the application of this plastic deformation, the increase or decrease in plate thickness according to the amount of tensile or compressive plastic strain of the uniaxial deformation and the change in width of the marking line previously drawn on the metal plate 3 are measured. Alternatively, the measured values ​​of a strain meter or strain gauge previously attached to the metal plate 3 are measured. It is desirable to adjust the application of plastic deformation while referring to the measurements and measuring the amount of strain introduced in the region that will become the shear end face of the test piece 1. The amount of plastic strain to be applied is, for example, 0.001 or more.

[0021] Here, it is conceivable that the first step 10A involves applying plastic strain by bending. However, bending can be considered a combination of tension outside the bend and compression inside the bend. Therefore, from the perspective of ease of evaluation of the applied plastic strain, applying plastic strain by uniaxial tensile or compressive deformation is more convenient and preferable. However, unlike uniaxial deformation, bending causes the metal plate to bend, which may cause problems in the subsequent shearing process.

[0022] <Second step 10B> The second step 10B is a step of preparing a test piece 1 from the plastically strained metal plate 3. That is, in the second step 10B, the metal plate 3 is sheared to form a sheared end surface of the test piece 1 so as to include the region of the metal plate 3 to which plastic strain has been applied. Here, if the width of the punched-out side during shearing is too short, there is a concern that the plastic deformation region at the sheared end face will be reduced, resulting in an improvement in delayed fracture resistance. Hereinafter, the width of the punched-out side during shearing will also be referred to as the punching allowance. Therefore, it is desirable to set the punching allowance to at least twice the thickness of the metal plate 3 that will become the test piece 1.

[0023] For example, in the case of a metal plate (test piece 1 (see Figure 2)) to which plastic strain has been imparted by tensile deformation, the metal plate is sheared at position 7 by laser cutting or the like to obtain strip-shaped test piece 1. Furthermore, a metal plate 3 (see FIG. 3(a)) in which compressive deformation has been imparted to the flange portion by drawing is cut out as follows to obtain a test piece 1. That is, in the region of the flange portion (shaded area) of the drawn part 3A, the compressively deformed portion is cut out into strips at the position indicated by the reference numeral 6 by laser cutting or the like to obtain the test piece 1.

[0024] <Third step 10C> In the test piece 1 formed by shearing the plastically strained portion to be evaluated, the portion of the end face other than the sheared end face is called the non-sheared end face portion. In the third step 10C, it is confirmed whether the non-sheared end face portion is an end face that is relatively less damaged than a sheared end face such as a ground end face or a laser end face and has excellent delayed fracture properties. This is to avoid delayed fracture at the end surface other than the evaluation surface, which would make evaluation impossible in the delayed fracture test in the later process. If there is a problem, additional processing such as cutting the test piece 1 is carried out as necessary, or the test piece 1 is remade.

[0025] <Fourth step 10D> The fourth step 10D is a step of applying a predetermined external load stress to the sheared end surface formed by shearing the plastically strained portion and restraining the sheared end surface under this load. The stress application method is, for example, tensile stress application or bending stress application. In this case, a bending stress application method using a jig is particularly desirable from the viewpoint of simplicity. Any known method may be used for this loading method. For example, one method of loading is to apply stress to a test piece with a sheared edge by four-point bending. Stress adjustment is preferably performed using a strain gauge attached to a metal plate in advance. Alternatively, axial tension can be used. Here, it is possible to prepare multiple test pieces by varying the applied stress in sufficiently small increments, such as 100 MPa.

[0026] <5th step 10E> In the fifth step 10E, the test piece 1 is subjected to an external load stress in the fourth step 10D, and the test piece 1 is restrained in this state and placed in a predetermined hydrogen penetration environment for a predetermined time. Then, in the fifth step 10E, the test piece 1 in this state is evaluated for crack occurrence. In this case, it is preferable to set the hydrogen penetration environment and installation time based on conditions that will result in an amount of hydrogen penetration equivalent to the amount of hydrogen that is estimated to penetrate under the environment in which the material to be evaluated will actually be used.

[0027] The test specimen 1 is placed in a hydrogen penetration environment by immersing the test specimen 1 in a bath containing an acid solution such as hydrochloric acid or an aqueous solution of NHSCN. The concentration of the acid solution and the immersion time are set so that the amount of hydrogen that is preset as the allowable upper limit is penetrated into the test specimen 1. Then, the third step 10C to the fifth step 10E are repeatedly performed on each test piece 1 prepared in the first step 10A and the second step 10B. At this time, the conditions of the applied plastic strain and the applied load stress are changed and the steps are repeatedly performed. In this way, by selecting multiple load levels of the external load stress, it is possible to determine the limit value of the load stress at which delayed fracture does not occur for each amount of plastic strain. Hereinafter, this limit value of the load stress will also be referred to as the delayed fracture limit stress.

[0028] <6th step 10F> In the sixth step 10F, multiple combinations of the plastic strain applied in the first step 10A and the critical load stress at which delayed fracture does not occur, as determined in the fifth step 10E, are obtained. The multiple combinations of data are used to determine the correlation between the plastic strain before shearing and the corresponding critical load stress. The plastic strain is distinguished between the strain due to compressive deformation and the strain due to tensile deformation. The correlation information may be calculated using a known calculation method, such as a multiple regression model or a model formula based on machine learning. A plurality of combination data may also be used as the correlation information.

[0029] In this way, by obtaining correlation information in advance, it is possible to predict whether delayed fracture will occur in an actual press-formed product or in a shape of a press-formed product to be formed. Specifically, in the sixth step 10F, correlation information is referenced to predict whether delayed fracture will occur based on the strain amount before trimming and the load stress after trimming. The load stress is, for example, the residual stress when the metal plate 3 is press-formed. In this case, the residual stress becomes the forming stress. The load stress may also be the load stress after trimming, which is the residual stress (forming stress) when the metal plate 3 is press-formed, plus the assembly stress of the press-formed parts thereafter. In this case, the load stress after trimming becomes the forming stress. Conversely, if the upper limit of the load stress after forming the sheared end face (after post-trim) is determined, it is also possible to evaluate the limit of the pre-trim strain at which post-trim can be applied.

[0030] As described above, it is advisable to design the part shape and post-trim process so as to prevent delayed fracture by determining in advance the risk of delayed fracture in the post-trim portion. In this case, according to the present invention, it is possible to manufacture a press-formed product having a sheared edge with excellent delayed fracture resistance. The amount of tensile and compressive plastic strain at each location before trimming in a press-formed product, as well as residual stress from forming, can be calculated using standard CAE forming analysis. If the amount of plastic strain before trimming differs at multiple locations on the sheared edge, the evaluation can be performed as follows: That is, the occurrence of delayed fracture can be evaluated for each of the multiple locations, or for a representative location, using the amount of plastic strain at that location as a variable.

[0031] (Method of manufacturing press-molded products) When producing a press-formed product having a sheared end surface obtained by shearing at a position including a portion to which plastic strain has been applied, it is preferable to produce the press-formed product as follows. That is, the method for evaluating delayed fracture properties of this embodiment determines the load stress conditions after shearing under which the press-formed product is evaluated as not suffering from delayed fracture, and then produces the press-formed product under forming conditions that satisfy the determined load stress conditions. Alternatively, the method for evaluating delayed fracture properties of this embodiment is used to determine the limit of the amount of plastic strain before shearing that is allowable when manufacturing a target press-formed product, and the plastic strain imparted to the metal plate 3 before shearing is adjusted so that it is equal to or less than the determined limit of the amount of plastic strain.

[0032] In this embodiment, when shearing (post-trimming) is performed after press forming, the following adjustment is made, for example. That is, the amount of strain imparted to the region that will become the sheared end face in press forming is adjusted to be equal to or less than the limit value of the calculated plastic strain amount. In this case, the external load stress is, for example, residual stress caused by further press forming after shearing. In addition, the external load stress is, for example, assembly stress generated when the press-formed part is assembled to another structure.

[0033] (program) An example of a program used in the above-described delayed fracture property evaluation method will be shown below. The program of this example stores information 40A relating to the correlation between the amount of plastic strain before shearing and the limit value of the external load stress that can be applied to the sheared end surface (threshold stress for delayed fracture) in a storage unit 40. The correlation information 40A can be obtained by processing the delayed fracture property evaluation method described above. The program of this example is a program for executing a process that refers to the correlation information 40A and determines the limit value of the load stress that can be applied after shearing, which corresponds to the input amount of plastic strain before shearing.

[0034] Another example of the program is a program for causing a computer to execute a first processing step and a second processing step. The first processing step executes a process of referencing correlation information 40A and determining a limit value of the load stress corresponding to the input amount of plastic strain before shearing. The second processing step executes a process of determining whether or not delayed fracture has occurred based on the forming stress, which is the load stress when press-forming the metal sheet 3 into the shape of the press-formed product, and the limit value of the load stress determined in the first processing step.

[0035] Next, an example of processing by a program of the evaluation method described above will be explained with reference to Fig. 4. If evaluation is performed using the processing shown in Fig. 4, delayed fracture can be evaluated more efficiently. 4 includes a correlation information calculation unit 20, an evaluation main unit 30, and a storage unit 40. The programs for performing the processes of the correlation information calculation unit 20 and the evaluation main unit 30 are stored in the storage unit 40, such as a RAM or ROM, of a computer, and are executed by the computer.

[0036] <Storage section 40> The storage unit 40 is made up of a recording medium such as a database. The tests of the first step 10A to the fifth step 10E are repeated while variously changing the material conditions of the metal plate 3, the hydrogen environment conditions, the shearing conditions, and the amount of forming strain. Through this process, data on the load stress d calculated against the plastic strain is calculated using the test conditions as variables for each of the material conditions of the metal plate 3, the hydrogen environment conditions, and the amount of plastic strain before shearing. The calculated data is stored in the memory unit 40. The memory unit 40 also stores information 40A on the calculated correlation. The information 40A on the correlation may be stored in the same medium as the program.

[0037] <Correlation information calculation unit 20> In step S10, the correlation information calculation unit 20 first prompts the operator to input the basic conditions for evaluation, such as the type of material (steel type and thickness) and the hydrogen environment conditions (acidity and installation time), which are the conditions for delayed fracture. Thereafter, the correlation information calculation unit 20 acquires the above inputs through the operator's input operation. Next, the correlation information calculation unit 20 prompts the input of the plastic strain application conditions before shearing in step S20, and acquires the above input through the operator's input operation. Next, in step S30, a data group of load stresses for each plastic strain amount that matches the conditions input in steps S10 and step 20 is acquired from the storage unit 40. The data group is a collection of data of (plastic strain amount before shearing, load stress). Alternatively, it prompts the input of a data group of load stresses for each plastic strain amount obtained by the test, and acquires the above input information through the operator's input operation. The acquired data is stored in the storage unit 40.

[0038] Next, in step S40, referring to the data group of load stresses for the plastic strain amount acquired in step S30, a correlation detection process is performed based on a known processing method. Specifically, in step S40, an arithmetic process for obtaining the load stress d as a function f(x) with the plastic strain amount x as a variable is executed. Next, in step S50, the function of the load stress d obtained in step S40 is changed to an expression considering the safety factor s (: 0 < s ≤ 1) as shown in the following formula. d = s·f(x) Then, the information of the obtained function of the load stress d (correlation information 40A) is stored in the storage unit 40 with the test conditions as the key.

[0039] <Evaluation main body unit 30> In the evaluation main body unit 30, first, in step S100, it prompts the input of the conditions of the material type (steel type and thickness) of the evaluation target and the conditions of the hydrogen environment (acidity and installation time), which are the conditions of delayed fracture. Then, through the operator's input operation, the evaluation main body unit 30 acquires the above input. Next, in step S110, it prompts the input of the plastic strain amount x to be applied to the region that will become the shearing end face before shearing and the load stress g applied to the metal plate 3 after the formation of the shearing end face. Then, through the operator's input operation, the evaluation main body unit 30 acquires the above input. These pieces of information may be acquired from the analysis information of the CAE analysis.

[0040] In step S120, correlation information 40A consisting of the function "s·f(x)" of the applied stress d that matches the conditions input in step S100 is obtained from the storage unit 40. Then, the limit value d of the applied stress corresponding to the input amount of plastic strain x is calculated. A process of outputting the limit value d of the applied stress may be executed. In step S130, the limit value d (= s·f(x)) of the load stress calculated in step S120 is compared with the load stress g applied to the metal sheet 3 after the sheared edge is formed. Then, based on this comparison, it is determined whether there is a risk of delayed fracture. The load stress g is, for example, a forming stress. Here, in step S130 in Fig. 4, it is determined whether or not there is a risk of delayed fracture, but this is not limitative. The margin of load stress (= dg) up to delayed fracture may also be output.

[0041] In the above description, the correlation information calculation unit 20 calculates correlation information 40A as d = f(x) or d = s · f(x) using the plastic strain amount x as a variable. The correlation information 40A may also calculate the critical plastic strain amount x = h(d) using the applied stress d as a variable. In this case, in step S120, the correlation information 40A is referenced to calculate the critical plastic strain amount x corresponding to the applied stress d after the planned sheared edge formation. Then, in step S130, the plastic strain amount imparted to the region that will become the sheared edge before shearing is compared with the calculated critical plastic strain amount x. The comparison is then used to determine whether there is a risk of delayed fracture.

[0042] (effect) In this embodiment, it is possible to set the critical stress for delayed fracture or to determine the occurrence of delayed fracture by taking into account the amount of plastic strain in the rear trim portion of a press-formed product. As a result, in this embodiment, it is possible to improve delayed fracture resistance when applying high-strength steel sheets to various parts such as panel parts, structural and frame parts of automobiles.

[0043] (others) The present disclosure may also have the following configuration. (1) Disclosure 1 is a delayed fracture property evaluation method for evaluating the delayed fracture property of a sheared end surface of a metal plate made of a high-strength steel plate, A metal plate having at least a portion thereof subjected to plastic strain is subjected to shearing at a position including the portion to which the plastic strain is applied, thereby producing a metal plate test piece having the sheared end surface; Thereafter, the test piece is placed in a hydrogen penetration environment while a load stress is applied to the sheared end surface of the test piece. Evaluating the delayed fracture characteristics of the sheared end surface with respect to the plastic strain. Delayed fracture property evaluation method.

[0044] (2) Disclosure 2 describes preparing a plurality of test pieces with different plastic strains applied before shearing, The delayed fracture properties of the metal plate are evaluated for each of the plastic strains.

[0045] (3) Disclosure 3 determines the limit value of the load stress at the sheared end surface after shearing that does not cause delayed fracture in a hydrogen penetration environment relative to the amount of plastic strain before the shearing process, Based on the limit value of the load stress thus determined, the delayed fracture properties of the sheared end surface are evaluated.

[0046] (4) Disclosure 4 is based on the delayed fracture property evaluation method of the present disclosure, and determines in advance the correlation between the amount of plastic strain before the shearing process and the limit value of the load stress, Based on this correlation, the allowable load stress for the plastic strain applied to the sheared end surface before the shearing process is evaluated.

[0047] (5) Disclosure 5 is a prediction method for predicting whether or not a delayed fracture will occur in a press-formed product manufactured by shearing a press part having at least a portion to which plastic strain has been applied at a position including the portion to which plastic strain has been applied, the method comprising: For the metal plate, a correlation between the amount of plastic strain before the shearing process and the limit value of the load stress is previously determined based on the delayed fracture property evaluation method of the present disclosure; Based on this correlation, it is possible to predict whether or not delayed fracture will occur in the press-formed product.

[0048] (6) Disclosure 6 is a method for evaluating delayed fracture properties, wherein information on the correlation between the amount of plastic strain before the shearing process and the limit value of the load stress, which is obtained based on the disclosed delayed fracture property evaluation method, is stored in a storage unit; A program for causing a computer to execute a process of referencing the stored correlation information and determining the limit value of the load stress that can be applied after shearing corresponding to the input amount of plastic strain before shearing.

[0049] (7) Disclosure 7 is a program for predicting whether or not a delayed fracture will occur in a press-formed product manufactured by shearing a press part having at least a portion to which plastic strain has been applied at a position including the portion to which the plastic strain has been applied, information on the correlation between the amount of plastic strain before the shearing process and the limit value of the load stress, which is obtained based on the method for evaluating delayed fracture properties disclosed herein, is stored in a storage unit; On the computer, a first processing step of determining a limit value of the load stress corresponding to the input amount of plastic strain before shearing by referring to the stored correlation information; a second processing step of determining whether or not delayed fracture has occurred based on a forming stress, which is a load stress applied to a sheared end surface formed by the shearing of the press part after the shearing, and the limit value of the load stress obtained in the first processing step; A program to execute.

[0050] (8) Disclosure 8 is a method for manufacturing a press-molded product having a sheared end surface obtained by shearing at a position including a portion to which plastic strain is applied, Determine the load stress conditions under which the press-formed product is evaluated not to undergo delayed fracture by the delayed fracture property evaluation method of the present disclosure; manufacturing the press-molded product under molding conditions that satisfy the determined load stress conditions; Manufacturing method for press-molded products. [Example]

[0051] Next, an example based on this embodiment will be described. In this example, the metal plate to be evaluated is a test material A made of a 1.4 mm thick steel with a strength of 1470 MPa. However, the present invention is not limited to such a metal plate. The present invention can be suitably applied to metal materials, including high-tensile steels with a tensile strength of 980 MPa or more, which are likely to experience delayed fracture at the sheared edge.

[0052] (strain applied) First, plastic strain was introduced into the location of the specimen A where the shear end face was to be formed by uniaxial tension or compression in the direction of the planned shear end face extension. As a method for introducing plastic strain, tensile deformation was performed by gripping a rectangular specimen 1 (specimen A) with a chuck 2 and subjecting it to tensile deformation, as shown in Figure 5. A strain gauge 4 was attached to specimen 1 (specimen A) to measure the amount of tensile deformation. The amount of plastic strain was then adjusted by subtracting the amount of elastic deformation from the amount of deformation. However, the outer peripheral edge of specimen 1 (specimen A) was previously cut into a cutting edge.

[0053] To impart compressive deformation, a metal plate 3 (specimen A) was first drawn to produce a drawn part 3A, as shown in Figure 6. The drawn part 3A was adjusted so that a portion of the flange was subjected to compressive deformation. Furthermore, as shown in Figure 6(a), a scribe line 5 was marked in advance in the compressively deformed portion. The amount of plastic strain was estimated from the change in the scribe line width before and after compressive deformation. Here, a 50 mm diameter ball-head punch can be used as the punch for deep drawing. During deep drawing, it is desirable to position the sheet holder so that wrinkles do not appear around the punch.

[0054] (shearing) The flange portion (compression-deformed portion) of the compressively deformed metal plate 3 (sample A) was cut out by cutting. The compressively deformed portion was then cut out into strips by cutting. Reference numeral 6 in Figure 6 indicates the cutting position. Furthermore, shearing was performed on the compressively deformed test piece 1 (test material A) shown in Fig. 5. Specifically, the long side of the test piece 1 (test material A) was trimmed using a flat shearing die to obtain a strip-shaped test piece 1. Reference numeral 7 in Fig. 5 indicates the shearing position. The clearance during shearing was 12% of the initial plate thickness before deformation. Shearing was performed so that the part that underwent tensile or compressive deformation was at the center of the cutting line. The shearing allowance was 10 mm, more than twice the plate thickness. Furthermore, the tensile plastic strain was changed in the range of 0 or more and 0.03 or less, and the compressive plastic strain was changed in the range of 0 or more and 0.30 or less, and a plurality of test pieces 1 were prepared.

[0055] (Application of load stress) Thereafter, a tensile stress was applied to the sheared end surface of each test piece 1 by four-point bending, with the shear fracture surface side facing outward. The applied stress was adjusted as follows: Before four-point bending, a strain gauge was attached to the surface of the test piece 1, and the applied stress was calculated and adjusted by multiplying the strain of the strain gauge by 205 GPa, which is the Young's modulus. In this example, a plurality of samples were prepared in which the applied stress was changed from 100 MPa to 1400 MPa in increments of 100 MPa for each plastic strain.

[0056] (Installation in an environment where hydrogen is invaded) Thereafter, each test piece 1 was immersed in hydrochloric acid of pH 2.5 for 96 hours. Then, the presence or absence of cracks on the sheared edge after the immersion test was investigated to determine whether or not delayed fracture had occurred.

[0057] (result) The test results are shown in Tables 1 and 2. In Tables 1 and 2, the amount of plastic strain introduced into test piece 1 before shearing is listed in the left column. For each amount of plastic strain, the limiting value of the applied stress at which delayed fracture did not occur is listed in the right column. Table 1 shows the case where the plastic strain is tensile, and Table 2 shows the case where the plastic strain is compressive.

[0058] [Table 1]

[0059] [Table 2]

[0060] The test results clarified the relationship between the amount of plastic strain before shearing and the critical stress for delayed fracture at the sheared end surface after post-trimming (after shearing).

[0061] FIG. 7 is a graph showing correlation information 40A when the plastic strain is tensile plastic strain based on Table 1. FIG. 8 is a graph showing correlation information 40A when the plastic strain is compressive plastic strain based on Table 2. In FIGS. 7 and 8, the horizontal axis represents the amount of plastic strain before shearing, and the vertical axis represents the delayed fracture critical stress of the sheared end surface after post-trimming, plotting the limit line. It is predicted that delayed fracture will not occur in the region below this limit line. In FIGS. 7 and 8, examples of the limit line are shown with dashed lines.

[0062] Next, the obtained limit line was used to predict delayed fracture in the rear trim of the press-formed product, and countermeasures were implemented. In this example, the shape of the metal sheet 3 before shearing was a press-formed product as shown in Fig. 9. In Fig. 9, reference numeral 8 denotes a trim line (the position that becomes the sheared end surface). Ten locations A to J (not shown) were set as representative locations above this trim line 8 that contain forming strain before trimming. Then, the amount of strain before post-trim and the forming residual stress after trimming at each trimmed location were calculated by CAE analysis using FEM. In this example, the post-trim residual stress (molding stress) was the assembly stress that occurs when the press-molded product after shearing is assembled to another structure. The assembly stress was calculated, and the calculated value was used as the post-trim residual stress (molding stress). The results are shown in Table 3.

[0063] [Table 3]

[0064] The results of Table 3 are plotted on Figures 7 and 8, along with the results of delayed fracture tests carried out on the same parts as described above, in Figures 10 and 11. However, the compressive strain has been converted to a positive value in order to align the horizontal axis. As can be seen from FIGS. 10 and 11, it has been found that, based on the present invention, it is possible to accurately predict delayed fracture of the rear trim portion of a press-formed product. Furthermore, referring to the predictions in Figs. 10 and 11, parts were separately prepared in which the strain-stress ratio was changed in the area at risk of delayed fracture. 10 and 11, representative locations C, F, I, and J are locations where delayed fracture characteristics are predicted to occur. Table 4 shows an example of a part where the strain-stress relationship at representative locations C, F, I, and J was changed. That is, Table 4 shows the results of calculating the strain amount before post-trim and the molding residual stress after trimming for representative locations A to J of the modified part, using FEM CAE analysis.

[0065] [Table 4]

[0066] The results of Table 4 are superimposed on Figures 7 and 8, and plotted together with the results of delayed fracture tests conducted on the same parts as in the previous period in Figures 12 and 13. Figures 12 and 13 show that delayed fracture in the rear trim of press-formed parts can be predicted with high accuracy. Therefore, by using this as a reference, it is possible to manufacture press-formed parts with excellent delayed fracture resistance.

[0067] In actual press working, linear regions frequently appear as outlines of blanks. In contrast, the present disclosure enables the fabrication of test pieces having linear regions including linearly extending sheared edges, and the evaluation of these linear regions. Furthermore, the linear region is different from, for example, a region where a sheared end surface is formed by punching a cylindrical region. That is, when shearing a linear region to produce a linear sheared end surface, the influence of the shear angle and clearance during shearing becomes important in practice. In the present disclosure, it is possible to evaluate the shearing process taking into account the influence of the shear angle and clearance during shearing. For example, consider the case of post-trimming a section with a compressive plastic strain of 0.20. In this case, it is possible to measure the effects of the shear angle and clearance, as shown in Table 5.

[0068] [Table 5]

[0069] Table 5 shows the shear angle and clearance during shearing set as shear conditions, and the critical stress for each shear angle and clearance is calculated. The unit of critical stress is MPa. In this way, in a linear region, the critical stress differs depending on the shear conditions that form the sheared end face.

[0070] The entire contents of Japanese Patent Application No. 2023-072153 (filed April 26, 2023), from which this application claims priority, are incorporated herein by reference. While the present invention has been described with reference to a limited number of embodiments, the scope of the invention is not limited thereto, and modifications of each embodiment based on the above disclosure would be obvious to those skilled in the art. [Explanation of symbols]

[0071] 1 test piece 3 metal plate 3A Aperture parts 10A First Step 10B Second step 10C Third step 10D Fourth step 10E 5th step 10F 6th step 20 Correlation information calculation unit 30 Evaluation Body 40 Storage section 40A Correlation Information

Claims

1. A delayed fracture property evaluation method for evaluating the delayed fracture property of a sheared end surface of a metal plate made of a high-strength steel plate, comprising: A metal plate having at least a portion thereof imparted with plastic strain is subjected to shearing at a position including the portion to which the plastic strain is imparted, thereby forming a sheared end surface to be evaluated, and preparing a metal plate test piece having the sheared end surface; Thereafter, the test piece is placed in a hydrogen penetration environment while a load stress is applied to the sheared end surface of the test piece. Evaluating the delayed fracture characteristics of the sheared end surface with respect to the plastic strain. Delayed fracture property evaluation method.

2. As the test piece, a plurality of test pieces with different plastic strains applied before shearing were prepared, evaluating the delayed fracture properties of the metal plate for each of the plastic strains; 2. The delayed fracture property evaluation method according to claim 1.

3. The limit value of the applied stress at the sheared end surface after shearing that does not cause delayed fracture in a hydrogen penetration environment is determined for the amount of plastic strain before the shearing process. Based on the obtained limit value of the load stress, the delayed fracture characteristics of the shear end surface are evaluated.

3. A delayed fracture property evaluation method according to claim 1 or 2.

4. a correlation between the amount of plastic strain before the shearing process and the limit value of the applied stress is previously determined based on the delayed fracture property evaluation method according to claim 3; Using this correlation, the allowable load stress for the plastic strain applied to the sheared end surface before the shearing process is evaluated. Delayed fracture property evaluation method.

5. A method for predicting whether or not a delayed fracture will occur in a press-formed product manufactured by shearing a press part having at least a portion to which plastic strain has been applied at a position including the portion to which plastic strain has been applied, comprising: determining in advance a correlation between the amount of plastic strain before the shearing process and the limit value of the applied stress for the metal plate based on the delayed fracture property evaluation method of claim 3; The correlation is used to predict whether or not delayed fracture will occur in the press-formed product. Delayed fracture prediction method.

6. information on the correlation between the amount of plastic strain before the shearing process and the limit value of the applied stress, which is obtained based on the delayed fracture property evaluation method according to claim 3, is stored in a storage unit; A program for causing a computer to execute a process of referencing the stored correlation information and determining the limit value of the load stress that can be applied after shearing corresponding to the input amount of plastic strain before shearing.

7. A program for predicting whether or not a delayed fracture will occur in a press-formed product manufactured by shearing a press part having at least a portion to which plastic strain has been applied at a position including the portion to which plastic strain has been applied, information on the correlation between the amount of plastic strain before the shearing process and the limit value of the applied stress, which is obtained based on the delayed fracture property evaluation method according to claim 3, is stored in a storage unit; On the computer, a first processing step of determining a limit value of the load stress corresponding to the input amount of plastic strain before shearing by referring to the stored correlation information; a second processing step of determining whether or not delayed fracture has occurred based on a forming stress, which is a load stress applied to a sheared end surface formed by the shearing of the press part after the shearing, and the limit value of the load stress obtained in the first processing step; A program to execute.

8. A method for manufacturing a press-molded product having a sheared end surface obtained by shearing at a position including a portion to which plastic strain is applied, determining a load stress condition under which the press-formed product is evaluated not to undergo delayed fracture by the delayed fracture property evaluation method according to claim 4; manufacturing the press-molded product under molding conditions that satisfy the determined load stress conditions; Manufacturing method for press-molded products.

Images