Delayed fracture characteristic evaluation method, delayed fracture prediction method, program, and press-molded article production method

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

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

[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 plate made of high-strength steel plate, and a technique utilizing the evaluation technique. In this specification, high-strength steel plate refers to a steel plate having a tensile strength of 980 MPa or more.

[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 vehicle body. In recent years, ultra-high-strength steel sheets with a tensile strength of 1,470 MPa or more have also been applied to vehicle bodies. One of the issues when using high-strength steel sheets in vehicle bodies is delayed fracture. In particular, for high-strength steel sheets with a tensile strength of 980 MPa or more, delayed fracture that occurs at the edge after shearing is a significant issue. The edge after shearing is also referred to as the sheared edge. It is known that large tensile stress remains at this sheared edge. Therefore, 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 described, for example, in Patent Document 1. The method in Patent Document 1 describes evaluating hydrogen embrittlement of a thin steel plate by injecting hydrogen into a stressed test piece. Patent Document 2 also describes a method for evaluating delayed fracture properties by partially cutting out a compressed edge from a test piece produced by deep drawing and applying stress to the test piece.

[0004] Japanese Patent No. 5196926 Japanese Patent Application Laid-Open No. 2022-14784

[0005] Here, there are two types of sheared end surfaces of automotive parts: front trim and rear trim. The front trim is a portion sheared before press forming. The rear trim is a portion sheared after press forming. The latter rear trim occurs, for example, when a sheet holding portion is required during draw forming. In this case, for example, after press forming, an excess flange portion for draw forming is sheared along the product outline (rear trim) (see reference numeral 8 in Figure 9). In the case of such rear trim, the sheared end surface is formed by shearing the portion that has already been plastically strained by forming.

[0006] The inventors have found that the properties of the sheared end surface of such a rear trim portion may be different from those of a front trim portion obtained by shearing a metal plate that has not been subjected to plastic strain prior to press forming. However, conventional methods such as those disclosed in Patent Document 1 do not particularly focus on evaluating the sheared end surface of such a rear trim portion.

[0007] In addition, 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 the strain has been introduced, and the implementation of the present invention is also 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.

[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.

[0011] According to an aspect of the present invention, it is possible to evaluate the delayed fracture properties of a sheared edge according to the plastic strain imparted before shearing to form the sheared edge. Therefore, according to an aspect of the present invention, it is possible to evaluate the influence of the amount of strain before trim forming of a rear trim portion on the delayed fracture properties of the sheared edge. As a result, according to an aspect of the present invention, it is possible to further improve the delayed fracture properties when applying high-strength steel sheets to various parts such as automotive panel parts, structural / framework parts, etc.

[0012] FIG. 1 is a diagram illustrating a processing flow for evaluating delayed fracture properties according to an embodiment of the present invention. FIG. 2 is a diagram illustrating an example of imparting plastic strain by tensile deformation in the minor axis. FIG. 3 is a diagram illustrating an example of imparting plastic strain by compressive deformation in the minor axis. FIG. 4 is a diagram illustrating an example of processing of a program according to an embodiment of the present invention. FIG. 5 is a diagram illustrating an example of imparting plastic strain by tensile deformation in an example. FIG. 6 is a diagram illustrating an example of imparting plastic strain by compressive deformation in an example. FIG. 7 is a diagram reflecting the information in Table 3. FIG. 8 is a diagram reflecting the information in Table 3. FIG. 7 is a diagram reflecting the information in Table 4. FIG. 8 is a diagram reflecting the information in Table 4.

[0013] Next, an embodiment of the present invention will be described with reference to the drawings. (Summary) First, an overview of this embodiment will be described. In this embodiment, focusing on the amount of strain in the trim 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. Stress is then applied to the shear edge formed by shearing the plastically strained portion. The stressed test specimen is then placed in a hydrogen penetration environment to determine the limit of 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 determining 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 makes it possible to manufacture a press-formed product 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 when the target press-formed product is in use.

[0016] Next, an example of this embodiment will be described with reference to the drawings. (Delayed fracture property evaluation method) This 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 delayed fracture property evaluation process for the metal plate to be evaluated includes a testing step involving 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 the testing step. The sixth step 10F corresponds to the 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 imparting plastic strain to at least a partial region of the metal plate 3. The metal plate 3 used is preferably made of the same material and thickness as the metal plate to be evaluated. The plastic strain is preferably imparted to the entire region that will become the shear end surface of the test piece 1 described below. Furthermore, the plastic strain is preferably imparted by uniaxial tension or compression.

[0018] The first step 10A is, for example, a step of introducing plastic strain into a location of the metal plate 3 where the sheared edge of the test piece 1 is to be formed. Specifically, the first step 10A is a step of introducing tensile or compressive plastic strain of uniaxial deformation in the planned sheared edge extension direction. 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 by tensile deformation, as follows. That is, as shown in FIG. 2, a method can be exemplified in which both ends of a rectangular metal plate (test piece 1) are gripped with chucks 2 and the metal plate is subjected to tensile deformation. Another example of a method to impart plastic strain by compressive deformation is as follows. That is, as shown in FIG. 3, a metal plate 3 is subjected to deep drawing to form a drawn part 3A. During this forming, 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 a 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 measurement and measuring the amount of strain introduced into 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 also conceivable that the first step 10A involves applying plastic strain by bending deformation. However, bending deformation can be considered as a combination of tension outside the bending and compression inside the bending. Therefore, from the viewpoint 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 deformation bends the metal plate, which may cause problems in the subsequent shearing process.

[0022] <Second Step 10B> The second step 10B is a step of preparing a test specimen 1 from the metal plate 3 to which plastic strain has been applied. That is, in the second step 10B, the metal plate 3 is sheared to form the sheared end surface of the test specimen 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 of the sheared end surface will be reduced, and the delayed fracture properties will also be improved. 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 be at least twice the plate thickness of the metal plate 3 to be the test specimen 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 the position indicated by the reference numeral 7 by laser cutting or the like to obtain the strip-shaped test piece 1. In addition, in the case of a metal plate 3 (see Figure 3(a)) to which compressive deformation has been imparted to the flange portion by drawing, the test piece 1 is obtained by cutting as follows. That is, in the region of the flange portion (shaded area) of the drawn part 3A, the compressively deformed portion is cut out into a strip shape 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> The end surface portion of the test piece 1 formed by shearing the plastically strained portion to be evaluated, other than the sheared end surface, is called the non-sheared end surface portion. In the third step 10C, it is confirmed whether the non-sheared end surface portion is relatively less damaged than a sheared end surface such as a ground end surface or a laser end surface, and whether it is an end surface with excellent delayed fracture properties. This is to avoid delayed fracture of the end surface other than the evaluation end surface in the delayed fracture test in the later step, making it impossible to evaluate. If there is a problem, additional processing is performed, such as by cutting the test piece 1, as necessary. Alternatively, 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 specimen in that loaded state. 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. This loading method may be any known method. For example, a method of applying stress to a test specimen having a sheared end surface by four-point bending is one example of the loading method. Stress adjustment is preferably performed using a strain gauge attached to a metal plate in advance. Alternatively, axial tension loading or the like can be used. Here, the applied stress can be varied in sufficiently small increments, such as 100 MPa, to prepare multiple test specimens.

[0026] <Fifth step 10E> In the fifth step 10E, the test specimen 1, which has been subjected to an external load stress in the fourth step 10D and restrained in that state, is placed in a predetermined hydrogen entry environment for a predetermined time. Then, in the fifth step 10E, the test specimen 1 in that state is evaluated for crack occurrence. In this case, the hydrogen entry environment and the exposure time are preferably set based on conditions that result in an amount of hydrogen entry equivalent to the amount of hydrogen that is estimated to enter the material to be evaluated in an environment in which it is actually used.

[0027] The test piece 1 is placed in a hydrogen penetration environment, for example, in a solution of hydrochloric acid or NH ４ This is done by immersing the test specimen 1 in a bath containing an acid solution such as an SCN aqueous solution. 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 penetrates the test specimen 1. Then, the third step 10C to the fifth step 10E are repeatedly performed on each test specimen 1 prepared in the first step 10A and the second step 10B. This is done by changing the conditions of the applied plastic strain and the applied load stress. 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] <Sixth Step 10F> In the sixth step 10F, a plurality of combination data of the amount of plastic strain applied in the first step 10A and the limit value of the load stress at which delayed fracture does not occur, determined in the fifth step 10E, is obtained. By referring to the plurality of combination data, correlation information between the amount of plastic strain before shearing and the corresponding limit value of the load stress is obtained. The amount of plastic strain is distinguished between the amount of strain due to compressive deformation and the amount of strain due to tensile deformation. The correlation information may be obtained using a known calculation method. For example, the correlation information may be a multiple regression model or a model formula based on machine learning. A plurality of combination data may 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 a press-formed product shape to be formed. Specifically, in the sixth step 10F, the correlation information is referenced to predict whether delayed fracture will occur based on the amount of strain before trimming and the load stress after trimming. The load stress is, for example, the residual stress resulting from press-forming the metal sheet 3. In this case, the residual stress serves as the forming stress. Alternatively, the load stress may be the load stress after trimming, which is the residual stress (forming stress) resulting from press-forming the metal sheet 3 plus the assembly stress of the press-formed parts. In this case, the load stress after trimming serves as the forming stress. Conversely, if an upper limit value for the load stress after sheared edge formation (post-trim) is determined, it is also possible to evaluate the limit of the amount of strain before trimming 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 the risk of delayed fracture in the post-trim portion in advance. In this case, according to an embodiment of 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 stresses from forming, can be calculated by conventional CAE forming analysis. When the amount of plastic strain before trimming differs at multiple locations on the sheared edge, the following evaluation can be performed. 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 for manufacturing press-formed product) When manufacturing a press-formed product having a sheared end surface formed by shearing at a position including a portion where plastic strain has been imparted, it is preferable to manufacture the press-formed product as follows. That is, by the delayed fracture property evaluation method of this embodiment, the load stress conditions after shearing are determined under which the press-formed product is evaluated as not suffering from delayed fracture. Then, the press-formed product is manufactured under forming conditions that satisfy the determined load stress conditions. Alternatively, by the delayed fracture property evaluation method of this embodiment, the limit value of the amount of plastic strain before shearing that is allowable when manufacturing a target press-formed product is determined. Then, the plastic strain imparted to the metal plate 3 before shearing is adjusted to be equal to or less than the determined limit value 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 described below. The program of this example stores, in a storage unit 40, 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 face (delayed fracture limit stress). 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 determining a limit value of the load stress corresponding to the input amount of plastic strain before shearing by referring to the correlation information 40A. 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 applied when the metal sheet 3 is press-formed 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 described with reference to Fig. 4. If evaluation is performed using the processing shown in Fig. 4, delayed fracture can be evaluated more efficiently. The example shown in Fig. 4 includes a correlation information calculation unit 20, an evaluation main unit 30, and a storage unit 40. The programs that perform the processing of the correlation information calculation unit 20 and the evaluation main unit 30 are stored in the storage unit 40, such as RAM or ROM, of a computer and executed by the computer.

[0036] <Storage Unit 40> The storage unit 40 is composed 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 with respect to 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 storage unit 40. The storage unit 40 also stores information 40A on the calculated correlation. The correlation information 40A may be stored in the same medium as the program storage location.

[0037] <Correlation Information Calculation Unit 20> First, in step S10, the correlation information calculation unit 20 prompts the operator to input the basic evaluation conditions, such as the type of material (steel type and thickness) and the hydrogen environment conditions (acidity and installation time), which are conditions for delayed fracture. The correlation information calculation unit 20 then acquires the input through the operator's input operation. Next, in step S20, the correlation information calculation unit 20 prompts the operator to input the conditions for applying plastic strain before shearing, and acquires the input through the operator's input operation. Next, in step S30, the correlation information calculation unit 20 acquires from the storage unit 40 a data set of applied stresses for each amount of plastic strain that matches the conditions entered in steps S10 and S20. The data set is a collection of data (amount of plastic strain before shearing, applied stress). Alternatively, the correlation information calculation unit 20 prompts the operator to input a data set of applied stresses for each amount of plastic strain determined by testing, and acquires the input information through the operator's input operation. The acquired data is stored in the storage unit 40.

[0038] Next, in step S40, the data group of applied stress versus plastic strain acquired in step S30 is referenced, and correlation detection processing is performed based on a known processing method. Specifically, in step S40, a calculation process is performed to determine the applied stress d as a function f(x) with the amount of plastic strain x as a variable. Next, in step S50, the function of applied stress d determined in step S40 is changed to the following equation, which takes into account a safety factor s (: 0<s≦1): d = s f(x). Then, information on the determined function of applied stress d (correlation information 40A) is stored in memory unit 40 using the test conditions as a key.

[0039] <Evaluation Main Unit 30> First, in step S100, the evaluation main unit 30 prompts the operator to input the conditions for the type of material (steel type and thickness) to be evaluated and the conditions for the hydrogen environment (acidity and installation time), which are the conditions for delayed fracture. The evaluation main unit 30 then acquires the above inputs through an input operation by the operator. Next, in step S110, the operator is prompted to input the amount of plastic strain x to be imparted to the region that will become the sheared edge before shearing, and the load stress g to be applied to the metal plate 3 after the sheared edge is formed. The evaluation main unit 30 then acquires the above inputs through an input operation by the operator. These pieces of information may be acquired from analysis information from a CAE analysis.

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

[0041] In the above description, the correlation information calculation unit 20 calculates the 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 load 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 load 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 / frame parts, etc. of automobiles.

[0043] (Other) The present disclosure may also have the following configurations: (1) Disclosure 1 is a delayed fracture property evaluation method for evaluating the delayed fracture property of a sheared edge of a metal plate made of a high-strength steel plate, comprising: preparing a metal plate test piece having the sheared edge by shearing a metal plate, at least a portion of which has been imparted with plastic strain, at a position including the portion to which the plastic strain has been imparted, and then placing the test piece in a hydrogen penetration environment while a load stress is applied to the sheared edge of the test piece, thereby evaluating the delayed fracture property of the sheared edge with respect to the plastic strain.

[0044] (2) In Disclosure 2, a plurality of test pieces are prepared with different plastic strains applied before shearing, and 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 which delayed fracture does not occur in a hydrogen penetration environment at the sheared end surface after shearing, relative to the amount of plastic strain before the shearing process, and evaluates the delayed fracture properties of the sheared end surface based on the determined limit value of the load stress.

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

[0047] (5) Disclosure 5 is a prediction method for predicting whether or not 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, the method comprising: determining in advance, for the metal plate, based on the delayed fracture property evaluation method disclosed herein, a correlation between the amount of plastic strain before the shearing and the limit value of the load stress; and predicting whether or not delayed fracture will occur in the press-formed product based on the correlation.

[0048] (6) Disclosure 6 provides a program for causing a computer to execute a process in which information on the correlation between the amount of plastic strain before shearing and the limit value of the load stress, which information is obtained based on the delayed fracture property evaluation method of the present disclosure, is stored in a storage unit, and the computer refers to the stored information on the correlation 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.

[0049] (7) Disclosure 7 is a program for predicting whether or not 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 imparted at a position including the portion to which the plastic strain has been imparted, the program storing information on the correlation between the amount of plastic strain before the shearing process and the limit value of the load stress, which is determined based on the delayed fracture property evaluation method of the present disclosure, in a memory unit, and causing a computer to execute: a first processing step of determining the limit value of the load stress corresponding to the input amount of plastic strain before the shearing process by referring to the stored correlation information; and a second processing step of determining whether or not delayed fracture will occur based on the forming stress, which is the load stress applied to the sheared end face formed by the shearing process of the press part after the shearing process, and the limit value of the load stress determined in the first processing step.

[0050] (8) Disclosure 8 is a manufacturing method for 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, the manufacturing method for a press-formed product comprising: determining load stress conditions under which the press-formed product is evaluated not to undergo delayed fracture by the delayed fracture property evaluation method disclosed herein; and manufacturing the press-formed product under molding conditions that satisfy the determined load stress conditions.

[0051] Next, an example based on this embodiment will be described. In this example, test material A made of 1470 MPa-class steel with a plate thickness of 1.4 mm will be described as the metal plate to be evaluated. 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 application) First, plastic strain was introduced into the area of ​​the test piece A where the shear end face was to be formed by uniaxial tension or compression in the planned shear end face extension direction. Regarding the method of introducing plastic strain, for tensile deformation, as shown in Figure 5, a rectangular test piece 1 (test piece A) was gripped with a chuck 2 and subjected to tensile deformation. At this time, a strain gauge 4 was attached to the test piece 1 (test piece 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 end face of the test piece 1 (test piece A) was previously cut into a cut end face.

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

[0054] (Shearing) The flange portion (compression-deformed portion) of the compressively deformed metal plate 3 (test material A) was cut out by cutting. Then, the compression-deformed portion was cut out into a strip by cutting. Reference numeral 6 in FIG. 6 indicates the cutting position. Furthermore, the compressively deformed test piece 1 (test material A) shown in FIG. 5 was subjected to shearing. Specifically, the long side of the test piece 1 (test material A) was trimmed using a flat-cut shearing die to obtain the strip-shaped test piece 1. Reference numeral 7 in FIG. 5 indicates the shearing position. Here, the clearance during shearing was 12% of the initial plate thickness before deformation. Shearing was performed so that the tensilely or compressively deformed portion was located at the center of the cutting line. Furthermore, the punching allowance during shearing was 10 mm, which was 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) Then, a tensile stress was applied to the shear end surface of each test piece 1 by four-point bending, with the shear fracture surface side facing outward. The load stress was adjusted as follows: That is, before four-point bending, a strain gauge was attached to the surface of the test piece 1, and the load stress was calculated and adjusted by multiplying the strain amount 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 load stress was changed from 100 MPa to 1400 MPa in 100 MPa increments for each plastic strain.

[0056] (Placement in a hydrogen penetration environment) Then, 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 end surface after the immersion test was examined to determine whether or not delayed fracture had occurred.

[0057] (Results) 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. In addition, for each amount of plastic strain, the limiting value of the load stress at which delayed fracture did not occur is listed in the right column. Table 1 is for the case where the plastic strain is tensile plastic strain. Table 2 is for the case where the plastic strain is compressive plastic strain.

[0058]

[0059]

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

[0062] Next, using the obtained limit line, delayed fracture in the rear trim portion of the press-formed product was predicted and countermeasures were implemented. In this example, the shape of the metal sheet 3 before shearing was as shown in Figure 9. In Figure 9, reference numeral 8 denotes the trim line (the position that becomes the shear end face). Ten locations A to J (not shown) above this trim line 8 were set as representative locations containing pre-trim forming strain. Then, CAE analysis using FEM was performed to calculate the amount of strain before rear trim and the post-trim forming residual stress at each trim location. In this example, the post-trim forming residual stress (forming stress) was defined as the assembly stress generated when the press-formed product after shearing was assembled to another structure. The assembly stress was then calculated, and the calculated value was used as the post-trim forming residual stress (forming stress). The results are shown in Table 3.

[0063]

[0064] The results of Table 3 are plotted on Figures 7 and 8 , along with the results of delayed fracture tests performed on the same parts as described above. However, the compressive strain has been converted to a positive value to align the horizontal axis. As can be seen from Figures 10 and 11 , the present invention makes it possible to accurately predict delayed fracture in the rear trim of a press-formed part. Furthermore, based on the predictions in Figures 10 and 11 , parts were separately prepared in which the strain-stress profiles of the risky areas for delayed fracture were varied. From Figures 10 and 11 , representative locations C, F, I, and J were predicted to exhibit delayed fracture characteristics. Table 4 shows an example of a part in which the strain-stress profiles of representative locations C, F, I, and J were varied. That is, Table 4 shows the results of CAE analysis using FEM to calculate the strain before rear trimming and the molding residual stress after trimming for representative locations A to J of the modified part.

[0065]

[0066] The results of Table 4 are superimposed on Figures 7 and 8, and plotted together with the results of delayed fracture tests performed 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 portion of a press-formed product can be predicted with high accuracy. Therefore, by using this as a reference, it is possible to manufacture press-formed products with excellent delayed fracture resistance.

[0067] In actual press working, linear regions frequently appear as the outline of a blank. In contrast, the present disclosure allows for the preparation of a test specimen having a linear region including a linearly extending sheared edge and the evaluation of the linear region. Furthermore, linear regions are different from regions where a sheared edge is formed, for example, by punching a cylindrical region. In other words, when shearing a linear region to produce a linear sheared edge, the effects of the shear angle and clearance during shearing are important in practice. The present disclosure allows for evaluation taking into account the effects of the shear angle and clearance during shearing. For example, consider the case of post-trimming a portion with a compressive plastic strain of 0.20. In this case, the effects of the shear angle and clearance can be measured, as shown in Table 5.

[0068]

[0069] Table 5 shows the critical stress for the shear angle and clearance set as shear conditions. The critical stress is expressed in MPa. In this way, in the linear region, the critical stress varies 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.

[0071] REFERENCE SIGNS LIST 1 Test piece 3 Metal plate 3A Drawn part 10A First step 10B Second step 10C Third step 10D Fourth step 10E Fifth step 10F Sixth step 20 Correlation information calculation unit 30 Evaluation main body 40 Storage unit 40A Correlation information

Claims

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

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

3. The limit value of the load stress at which delayed fracture does not occur in a hydrogen penetration environment at the sheared end surface after shearing 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 property of the shear end surface is evaluated.

3. A method for evaluating delayed fracture properties according to claim 1 or 2.

4. A method for evaluating delayed fracture properties according to claim 3, comprising: determining in advance a correlation between the amount of plastic strain before the shear processing and a limit value of the load stress; 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 the plastic strain has been applied, comprising: a correlation between the amount of plastic strain before the shearing process and the limit value of the applied stress is previously obtained for the metal plate based on the delayed fracture property evaluation method according to claim 3; By using the correlation, it is possible to predict whether or not delayed fracture will occur in the press-formed product. A method for predicting delayed fracture.

6. information on the correlation between the amount of plastic strain before the shear processing and the limit value of the load 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 referring to the stored correlation information and determining the limit value of the load stress that can be applied after shear processing corresponding to the input amount of plastic strain before shear processing.

7. A program for predicting the occurrence of delayed fracture in a press-formed product produced 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, comprising: information on the correlation between the amount of plastic strain before the shear processing and the limit value of the load 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 a load stress corresponding to an input amount of plastic strain before shear processing by referring to the stored correlation information; a second processing step of determining whether or not delayed fracture occurs based on a forming stress, which is a load stress applied to a sheared end surface formed by the shearing process of the press part after the shearing process, and the limit value of the load stress obtained in the first processing step; A program for executing the above.

8. A method for producing a press-molded product having a sheared end surface obtained by shearing at a position including a portion to which plastic strain is applied, comprising: determining a condition of a load stress 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; A manufacturing method for press-molded products.