Method, device, and program for predicting delayed fracture in press-molded article, and method for manufacturing press-molded article

Abstract

A method for predicting delayed fracture in a press-molded article according to the present invention comprises: calculating the stress distribution and the strain distribution in a press-molded article (S1); calculating the stress gradient in the direction of crack propagation due to delayed fracture in sites in the press-molded article that are at risk for delayed fracture (S3); setting the hydrogen concentration distribution corresponding to the strain distribution in the press-molded article (S5); performing hydrogen diffusion analysis on the press-molded article on the basis of the stress distribution and the hydrogen concentration distribution, and calculating the hydrogen concentration distribution after hydrogen diffusion (S7); and predicting whether or not delayed fracture occurs in a delayed fracture risk site on the basis of a delay fracture determination condition that is based on stress, strain, and hydrogen concentration at the stress gradient of the delayed fracture risk site, and on the basis of stress, strain, and hydrogen concentration at the delayed fracture risk site (S9).

Description

Method, device and program for predicting delayed fracture of press-formed product, and method for manufacturing press-formed product

[0001] The present invention relates to a method, an apparatus, and a program for predicting delayed fracture in a press-formed part using a high-tensile steel sheet. The present invention also relates to a method for manufacturing a press-formed part, in which press-forming conditions are determined so as to suppress delayed fracture based on the results of predicting the occurrence of delayed fracture.

[0002] In response to increasingly strict environmental regulations on carbon dioxide emissions and other factors, there is a demand for lighter automobile bodies to improve fuel efficiency. At the same time, automobile bodies are also required to have improved collision safety. To meet these needs, the use of high-tensile steel plates with tensile strengths of 1 GPa or higher is increasingly being applied to automobile body frame parts. Automotive body frame parts are generally manufactured by press forming. However, press-formed parts made of high-tensile steel plates with tensile strengths exceeding 980 MPa are susceptible to delayed fracture due to strain and residual stress generated during the press forming and parts assembly processes, as well as hydrogen intrusion during automobile manufacturing and use.

[0003] Therefore, several methods for evaluating delayed fracture characteristics of press-formed high-tensile steel sheets (high-strength steel sheets) have been proposed. For example, Patent Document 1 discloses a method for evaluating delayed fracture characteristics by placing a deep-drawn high-tensile steel sheet test piece in a hydrogen penetration environment and examining the occurrence of cracks in the flange portion of the test piece. Patent Document 2 discloses a method for introducing hydrogen into a steel test piece to which plastic strain has been applied, and evaluating hydrogen embrittlement characteristics (delayed fracture characteristics) based on the amount of plastic strain. Furthermore, Patent Document 3 discloses a method for evaluating delayed fracture characteristics at a preselected evaluation portion of a high-strength steel sheet product by determining the amount of hydrogen corresponding to the strain of crystal grains in the metal structure of the evaluation portion. This method is said to determine the amount of hydrogen corresponding to the strain of crystal grains in the evaluation area using the relationship that correlates the amount of hydrogen contained in the steel when delayed fracture occurs, residual stress, and strain of crystal grains in the structure.

[0004] JP 2019-174124 A JP 2020-41838 A JP 2011-33600 A JP 2017-179589 A

[0005] Ishiwata, A., Urabe, M., and Inazumi, T., "Press Forming Analysis Technology Contributing to Expanded Applications of High-Tens Steel," JFE Technical Report, No. 30 (August 2012), pp. 19-24.

[0006] The method disclosed in Patent Document 1 evaluates the maximum residual stress at which delayed fracture occurs in the flange portion of a test piece of deep-drawn high-tensile steel sheet, but does not consider the effect of strain on the occurrence of delayed fracture. Furthermore, the method disclosed in Patent Document 2 evaluates the delayed fracture resistance (hydrogen embrittlement resistance) of a steel material based on the amount of plastic strain, but does not consider the effects of stress distribution and hydrogen concentration distribution on the occurrence of delayed fracture in a press-formed product. Furthermore, the method disclosed in Patent Document 3 selects an evaluation region for evaluating delayed fracture resistance in a steel sheet formed product, measures the residual stress, grain strain, and hydrogen content at the evaluation region, and uses the grain strain, residual stress, and hydrogen content as criteria for determining the occurrence of delayed fracture. However, this method requires a significant amount of time and cost to measure the grain strain and hydrogen content of the entire press-formed product for large parts such as press-formed products.

[0007] The methods disclosed in Patent Documents 1 and 3 each involve performing a delayed fracture test on an actual press-formed steel sheet and evaluating the delayed fracture properties of a specific portion of the press-formed steel sheet based on the results of the delayed fracture test. This requires a significant amount of time and cost, and it is difficult to predict which portion of a press-formed product, after press forming or assembly, is at risk of delayed fracture. Therefore, it is difficult to manufacture a press-formed product under press forming conditions that can suppress the occurrence of delayed fracture in the press-formed product. Furthermore, although it is known that strain gradient (stress gradient) affects cracking in press-formed products (see, for example, Non-Patent Document 1), none of the methods disclosed in Patent Documents 1 to 3 evaluates the effect of strain gradient (or stress gradient) on delayed fracture.

[0008] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a method, an apparatus, and a program for predicting a portion where delayed fracture will occur in a press-formed product made of a high-tensile steel sheet. Another object of the present invention is to provide a method for manufacturing a press-formed product, which determines press forming conditions so that delayed fracture will not occur in the portion where delayed fracture is predicted to occur by the above-mentioned method, and manufactures the press-formed product under the determined press forming conditions.

[0009] A method for predicting delayed fracture of a press-formed product according to the present invention predicts a site where delayed fracture will occur in a press-formed product of a high-tensile steel plate, and includes: a stress distribution and strain distribution calculation step of calculating a stress distribution and a strain distribution of the press-formed product; a stress gradient calculation step of identifying a site where a high maximum principal stress is high in the press-formed product from the calculated stress distribution of the press-formed product as a site where delayed fracture is likely to occur, and calculating a stress gradient in a direction perpendicular to the maximum principal stress in the identified site where delayed fracture is likely to occur as a stress gradient in a direction in which a crack will propagate due to delayed fracture; a hydrogen concentration distribution setting step of setting a hydrogen concentration distribution corresponding to the strain distribution of the press-formed product calculated in the stress distribution and strain distribution calculation step, based on a relational expression between strain and hydrogen concentration previously obtained for the high-tensile steel plate; and a hydrogen diffusion coefficient setting step of setting a hydrogen concentration distribution for the press-formed product based on the stress distribution calculated in the stress distribution and strain distribution calculation step and the hydrogen concentration distribution set in the hydrogen concentration distribution setting step. and a delayed fracture occurrence prediction step of acquiring delayed fracture judgment conditions that indicate the relationship between the occurrence of delayed fracture and the stress, strain, and hydrogen concentration at a stress gradient corresponding to the stress gradient in the direction in which the crack propagates, calculated for the region at risk of delayed fracture, and predicting whether or not delayed fracture will occur in the region at risk of delayed fracture based on the acquired delayed fracture judgment conditions, the stress distribution and strain distribution calculated in the stress distribution and strain distribution calculation step, and the hydrogen concentration distribution after hydrogen diffusion calculated in the hydrogen diffusion analysis step.

[0010] The delayed fracture judgment condition obtained in the delayed fracture occurrence prediction step may be one obtained by carrying out a delayed fracture test in which a predetermined uniaxial tensile load is applied to a uniaxial tensile test piece having a notch formed in a parallel portion, and the uniaxial tensile test piece is held in a hydrogen penetration environment while a stress gradient corresponding to the stress gradient in the direction of crack propagation calculated for the delayed fracture suspected portion is applied to the notch, and the relationship between the stress, strain, and hydrogen concentration at the notch to which the stress gradient is applied and the presence or absence of delayed fracture at the notch is determined.

[0011] The delayed fracture judgment condition obtained in the delayed fracture occurrence prediction step may be a delayed fracture test in which a predetermined bending load is applied to a four-point bending test piece, and a stress gradient corresponding to the stress gradient in the direction of crack propagation calculated for the delayed fracture suspected portion is applied to the bent portion of the four-point bending test piece, and the four-point bending test piece is held in a hydrogen penetration environment while the stress gradient is applied to the bent portion, and the relationship between the stress, strain, and hydrogen concentration in the bent portion to which the stress gradient is applied and the presence or absence of delayed fracture in the bent portion is determined.

[0012] In the stress distribution and strain distribution calculation step, an equivalent plastic strain distribution is calculated as the strain distribution, and in the hydrogen concentration distribution setting step, a relational expression between equivalent plastic strain and hydrogen concentration is obtained in advance, and a hydrogen concentration distribution corresponding to the equivalent plastic strain distribution is set based on the obtained relational expression between equivalent plastic strain distribution and hydrogen concentration.

[0013] The high-tensile steel plate preferably has a tensile strength of 1 GPa or more.

[0014] The delayed fracture prediction device for press-formed products according to the present invention predicts a site where delayed fracture will occur in a press-formed product of a high-tensile steel plate, and includes a stress distribution and strain distribution calculation unit that calculates the stress distribution and strain distribution of the press-formed product, a stress gradient calculation unit that identifies a site in the press-formed product where a high maximum principal stress is high as a site at risk of delayed fracture based on the calculated stress distribution of the press-formed product, and calculates a stress gradient in a direction perpendicular to the maximum principal stress in the identified site at risk of delayed fracture as a stress gradient in a direction in which a crack will propagate due to delayed fracture, a hydrogen concentration distribution setting unit that sets a hydrogen concentration distribution corresponding to the strain distribution of the press-formed product calculated by the stress distribution and strain distribution calculation unit based on a relational expression between strain and hydrogen concentration previously obtained for the high-tensile steel plate, and a hydrogen concentration setting unit that sets a hydrogen concentration distribution corresponding to the strain distribution of the press-formed product calculated by the stress distribution and strain distribution calculation unit. and a delayed fracture occurrence prediction unit that acquires delayed fracture judgment conditions that indicate the relationship between the occurrence of delayed fracture and the stress, strain, and water content at a stress gradient corresponding to the stress gradient in the direction in which the crack propagates, calculated for the region at risk of delayed fracture, and the stress, strain, and hydrogen content after hydrogen diffusion at the region at risk of delayed fracture that are obtained from the stress distribution and strain distribution calculated by the stress distribution and strain distribution calculation unit and the hydrogen content distribution after hydrogen diffusion calculated by the hydrogen diffusion analysis unit.

[0015] The stress distribution and strain distribution calculation unit calculates an equivalent plastic strain distribution as the strain distribution, and the hydrogen concentration distribution setting unit acquires a relational expression between equivalent plastic strain and hydrogen concentration in advance, and sets a hydrogen concentration distribution corresponding to the equivalent plastic strain distribution based on the acquired relational expression between equivalent plastic strain distribution and hydrogen concentration.

[0016] A program for predicting delayed fracture of a press-formed product according to the present invention predicts a site where delayed fracture will occur in a press-formed product of a high-tensile steel plate, and includes a computer including: a stress distribution and strain distribution calculation unit that calculates a stress distribution and a strain distribution of the press-formed product; a stress gradient calculation unit that identifies a site in the press-formed product where a high maximum principal stress is high in the stress distribution of the press-formed product from the calculated stress distribution of the press-formed product as a site where delayed fracture is likely to occur, and calculates a stress gradient in a direction perpendicular to the maximum principal stress in the identified site where delayed fracture is likely to occur as a stress gradient in a direction in which a crack will propagate due to delayed fracture; a hydrogen concentration distribution setting unit that sets a hydrogen concentration distribution corresponding to the strain distribution of the press-formed product calculated by the stress distribution and strain distribution calculation unit based on a relational expression between strain and hydrogen concentration previously obtained for the high-tensile steel plate; and a hydrogen concentration setting unit that sets a hydrogen concentration distribution corresponding to the strain distribution of the press-formed product calculated by the stress distribution and strain distribution calculation unit based on a relational expression between strain and hydrogen concentration previously obtained for the high-tensile steel plate. and a delayed fracture occurrence prediction unit that acquires delayed fracture judgment conditions that indicate the relationship between the occurrence of delayed fracture and the stress, strain, and hydrogen concentration at a stress gradient corresponding to the stress gradient in the direction in which the crack propagates, calculated for the region at risk of delayed fracture, and the stress, strain, and hydrogen concentration after hydrogen diffusion at the region at risk of delayed fracture that are obtained from the stress distribution and strain distribution calculated by the stress distribution and strain distribution calculation unit and the hydrogen concentration distribution after hydrogen diffusion calculated by the hydrogen diffusion analysis unit.

[0017] The stress distribution and strain distribution calculation unit calculates an equivalent plastic strain distribution as the strain distribution, and the hydrogen concentration distribution setting unit acquires a relational expression between equivalent plastic strain and hydrogen concentration in advance, and sets a hydrogen concentration distribution corresponding to the equivalent plastic strain distribution based on the acquired relational expression between equivalent plastic strain distribution and hydrogen concentration.

[0018] The method for manufacturing a press-formed product according to the present invention is for manufacturing a press-formed product using a high-tensile steel plate while suppressing the occurrence of delayed fracture in the press-formed product, and includes a temporary press-forming condition setting step for setting temporary press-forming conditions for the press-formed product, a delayed fracture occurrence determination step for predicting whether or not delayed fracture will occur in the delayed fracture-suspected portion identified for the press-formed product based on the temporary press-forming conditions using a delayed fracture prediction method for a press-formed product according to the present invention, and determining whether or not delayed fracture will occur in the press-formed product based on the predicted result, and a delayed fracture occurrence determination step for determining whether or not delayed fracture will occur in the press-formed product in the delayed fracture occurrence determination step. a tentative press-forming condition changing step of changing the tentative press-forming conditions when it is determined that delayed fracture has occurred; a repeating step of repeatedly executing the tentative press-forming condition changing step and the delayed-fracture occurrence determining step until it is determined that delayed fracture has not occurred in the press-formed product in the delayed-fracture occurrence determining step; a press-forming condition determining step of determining the tentative press-forming conditions as press-forming conditions when it is determined that delayed fracture has not occurred in the press-formed product in the delayed-fracture occurrence determining step; and a press-forming step of press-forming the high-tensile steel sheet into the press-formed product under the determined press-forming conditions.

[0019] According to the present invention, based on the stress distribution of a press-formed product, a portion in the press-formed product where the maximum principal stress is high is identified as a portion at risk of delayed fracture, and by taking into account the stress gradient in the direction of propagation of a crack due to delayed fracture in the identified portion at risk of delayed fracture, it is possible to predict whether delayed fracture will occur during use of a press-formed product made of a high-tensile steel sheet. As a result, it is possible to predict not only whether delayed fracture will occur in a press-formed part but also the portion at which delayed fracture will occur, at the part design stage, and it becomes possible to take measures against delayed fracture, such as changing the part shape or press-forming process, so as to prevent delayed fracture.

[0020] Furthermore, in the present invention, the presence or absence of delayed fracture in a press-formed product of a high-tensile steel sheet is determined, and press-forming conditions that suppress the occurrence of delayed fracture are determined based on the determination results. This makes it possible to shorten the time required to determine press-forming conditions that can suppress the occurrence of delayed fracture in a press-formed product using a high-tensile steel sheet. Furthermore, by press-forming the press-formed product under the press-forming conditions determined in this manner, it is possible to manufacture a press-formed product in which measures have been taken to suppress the occurrence of delayed fracture.

[0021] Fig. 1 is a flow chart showing the processing flow of the method for predicting delayed fracture of press-formed products according to the first embodiment of the present invention. Fig. 2 shows the results of an investigation into the relationship between crack initiation locations and hydrogen concentration distribution in uniaxial tensile test specimens of high-tensile steel sheets ((a) uniaxial tensile test specimen, (b) crack initiation locations, (c) hydrogen concentration distribution). Fig. 3 is a graph showing the relationship between strain and hydrogen concentration obtained in a hydrogen charging test using test specimens strained in various deformation modes after a 30-hour dipping time ((a) relationship between equivalent plastic strain and hydrogen concentration, (b) relationship between strain mode and hydrogen concentration). Fig. 4 is a graph showing the relationship between equivalent plastic strain and hydrostatic stress after a 30-hour dipping time for a uniaxial tensile test specimen in a constant load hydrogen charging test. Figure 5 is a graph showing the relationship between equivalent plastic strain and stress and delayed fracture initiation when a uniaxial tensile test specimen is immersed for 30 hours and 20 hours in a delayed fracture test ((a) immersion time 30 hours, (b) immersion time 20 hours). Figure 6 is a graph showing the relationship between equivalent plastic strain and hydrogen concentration of a uniaxial tensile test specimen measured after immersion for 30 hours, 20 hours, and 10 hours in a delayed fracture test. Figure 7 is a graph showing the delayed fracture judgment criteria under constant stress conditions (1100 MPa, 1000 MPa, 900 MPa) obtained from the delayed fracture test results. Figure 8 is a diagram illustrating a notched uniaxial tensile test specimen used in the delayed fracture test. Figure 9 is a graph showing the relationship between the notch radius (punching radius R) of the notch and the stress gradient when a uniaxial tensile stress is applied to a notched uniaxial tensile test specimen in a delayed fracture test. Fig. 10 is a graph showing the relationship between the stress gradient and the delayed fracture time obtained by a delayed fracture test in which a tensile load was applied to a uniaxial tensile test specimen with a notch. Fig. 11 is a diagram explaining a delayed fracture test in which a four-point bending test specimen, to which a stress gradient was applied in the thickness direction by a four-point bending test, is immersed in a hydrogen penetration environment for a predetermined time. Fig. 12 is an example of a graph showing the relationship between the sheet thickness and the stress gradient in a four-point bending test specimen, to which a stress gradient was applied in the thickness direction by a four-point bending test.Fig. 13 is a diagram showing the configuration of a delayed fracture prediction device and program for a press-formed product according to embodiment 1 of the present invention. Fig. 14 is a flow chart showing the process flow of a manufacturing method for a press-formed product according to embodiment 2 of the present invention. Fig. 15 is a diagram explaining the press-formed product that was the target of delayed fracture prediction and manufacturing in Examples 1 and 2 ((a) perspective view, (b) cross-sectional view). Fig. 16 is a contour diagram showing the stress distribution and equivalent plastic strain distribution calculated for the press-formed product in Example 1 ((a) longitudinal stress distribution, (b) equivalent plastic strain distribution). Fig. 17 is a contour diagram showing the hydrogen concentration distribution before hydrogen diffusion calculated for the press-formed product in Example 1 ((a) hydrogen concentration distribution corresponding to equivalent plastic strain (Case 1), (b) hydrogen concentration distribution corresponding to hydrostatic stress, (c) hydrogen concentration distribution corresponding to hydrostatic stress and equivalent plastic strain (Case 2)). Fig. 18 is a contour diagram showing the hydrogen concentration distribution after hydrogen diffusion calculated for the press-formed product in Example 1 ((a) hydrogen concentration distribution corresponding to equivalent plastic strain (Case 1), (b) hydrogen concentration distribution corresponding to hydrostatic stress and equivalent plastic strain (Case 2)). Fig. 19 is a graph showing the results of predicting whether delayed fracture will occur in a region of the press-formed product where delayed fracture is suspected in Example 1 ((a) region of the press-formed product where delayed fracture is suspected, (b) graph showing the predicted occurrence of delayed fracture using the hydrogen concentration corresponding to equivalent plastic strain, (c) graph showing the predicted occurrence of delayed fracture using the hydrogen concentration corresponding to hydrostatic stress and equivalent plastic strain). Fig. 20 is a graph showing the predicted result of whether delayed fracture will occur in a bent portion of the press-formed product in Example 2.

[0022] <Background to the Invention> When investigating a method for predicting the location where delayed fracture will occur in a press-formed product of high-tensile steel sheet, the inventors conducted an investigation using a delayed fracture test of high-tensile steel sheet under uniaxial tensile stress.

[0023] First, a hydrogen charging test was conducted on a uniaxial tensile test piece 21, which was a high-tensile steel plate shown in Fig. 2(a) and used as a test material, in which the test piece was placed in a hydrogen entry environment for a certain period of time while being subjected to a constant uniaxial tensile stress load, and then the amount of hydrogen in the uniaxial tensile test piece 21 was measured. Furthermore, a uniaxial tensile test was conducted in which the uniaxial tensile test piece 21 was held in the hydrogen entry environment until cracks (delayed fracture) occurred (hereinafter referred to as a hydrogen charging test under constant load).

[0024] In a normal uniaxial tensile test in which a constant load is applied to the uniaxial tensile test specimen 21 without placing it in a hydrogen penetration environment, cracks occur near the center of the parallel portion 23 of the uniaxial tensile test specimen 21. However, in the uniaxial tensile test specimen 21 subjected to a hydrogen charging test under a constant load, cracks occurred not near the center of the parallel portion 23 but at the boundary between the parallel portion 23 and the R portion 25, as shown in Figure 2(b).

[0025] The inventors believed that the hydrogen concentration distribution in the uniaxial tensile test piece 21 was related to this factor, and carried out a hydrogen diffusion analysis taking into account the hydrostatic pressure distribution, and investigated the correspondence between the crack occurrence location in the uniaxial tensile test piece 21 and the hydrogen concentration distribution after hydrogen diffusion.

[0026] Figure 2(c) shows the hydrogen concentration distribution after hydrogen diffusion obtained by hydrogen diffusion analysis. In the uniaxial tensile test piece 21, hydrogen is concentrated at the boundary between the R portion 25 and the parallel portion 23 due to the distribution of hydrostatic pressure, and it can be seen that the area with high hydrogen concentration corresponds to the area where cracks occur (see Figure 2(b)). From these results, we have concluded that it is important to consider the water concentration distribution after hydrogen diffusion when predicting delayed fracture.

[0027] Next, to predict delayed fracture in press-formed products, we investigated the relationship between the amount of hydrogen that penetrates into the press-formed product and the strain of the press-formed product. When high-tensile steel sheets are press-formed, various deformation modes occur, including bending (plane strain), bulge forming (biaxial tension), and uniaxial tension. Therefore, we used high-tensile steel sheets (e.g., 1470 MPa cold-rolled steel sheets, 1.2 mm thick) as test materials and prepared specimens strained in various deformation modes, including rolling (plane strain), uniaxial tension, uniaxial compression, and biaxial tension / compression. We then conducted hydrogen charging tests to measure the amount of hydrogen in the specimens after placing the strained specimens in a hydrogen intrusion environment for a certain period of time.

[0028] Figure 3 shows a typical example of the relationship between the hydrogen concentration and strain in a specimen that has been subjected to strain in various deformation modes and then immersed in a 0.1% ammonium thiocyanate solution with a pH of 4.0 and a McIlvaine buffer solution for 30 hours. p eq and hydrogen concentration, and Fig. 3(b) shows the relationship between the equivalent plastic strain ε p eq The relationship between the deformation mode and hydrogen concentration is shown in Fig. 3(b) when the ratio of the maximum principal strain to the minimum principal strain (hereafter referred to as the "strain ratio") corresponds to the deformation mode: a strain ratio of 1 indicates biaxial tension, a strain ratio of 0 indicates plane strain, and a strain ratio of -2 indicates uniaxial tension and uniaxial compression.

[0029] Based on the results shown in FIG. 3, the inventors concluded that the hydrogen concentration is related to the equivalent plastic strain ε p eqIt was found that the hydrogen concentration increases only with increasing equivalent plastic strain. The reason why the hydrogen concentration increases with increasing equivalent plastic strain is thought to be because the number of defects in the test specimen that trap hydrogen increases with increasing equivalent plastic strain, and the amount of hydrogen that penetrates into the steel increases. From the results shown in Figure 3(a), the relationship between the amount of hydrogen that penetrates into the press-formed product and the strain of the press-formed product can be expressed as a relational equation of equivalent plastic strain, for example, as shown in Equation (1).

[0030] C ε is the hydrogen concentration (ppm) when strain is applied, ε p eq is the equivalent plastic strain, and a and b are constants.

[0031] Furthermore, the relationship between the amount of hydrogen that penetrates into a press-formed product and the stress distribution in the press-formed product was also investigated in a constant-load hydrogen charging test. Hydrostatic stress occurs near the boundary between the R portion 25 and the parallel portion 23, where a crack occurred, in the uniaxial tensile test piece 21. Hydrostatic stress refers to the average stress defined as the average of the normal stresses acting on the surface of an object in three directions. When hydrostatic stress occurs in the tensile direction, the crystal lattice in the steel opens, increasing the amount of hydrogen that solid-solves in the crystal lattice, and it is thought that this increases the amount of hydrogen that penetrates into the steel.

[0032] Therefore, as a representative example of a hydrogen charging test under constant load, a high-tensile steel plate (for example, a 1470 MPa-class cold-rolled steel plate with a plate thickness of 1.2 mm) was used as the material, and a constant uniaxial tensile stress (900 MPa, 600 MPa, or 300 MPa, which is equivalent to a hydrostatic stress of 300 MPa, 200 MPa, or 100 MPa) was applied in the longitudinal direction of the parallel portion 23 of the uniaxial tensile test piece 21. The uniaxial tensile test piece 21 was then immersed in a solution of ammonium thiocyanate and McIlvaine buffer solution with a pH of 4.0 and a concentration of 0.1% for 30 hours, and the amount of hydrogen in the parallel portion 23 of the uniaxial tensile test piece 21 was measured.

[0033] 4 shows the results of summarizing the relationship between the hydrogen concentration and hydrostatic stress in the uniaxial tensile test piece 21. From this result, the relationship between the hydrostatic stress and the amount of hydrogen (hydrogen concentration) penetrating into the steel can be expressed, for example, as the hydrogen concentration Cp and the tensile hydrostatic stress p. The hydrogen concentration C p The increase in ΔC p is expressed by equation (3).

[0034] C p is the hydrogen concentration (ppm) when stress is applied, C0 is the hydrogen concentration (=0.55 ppm) when no stress is applied, ΔC p is the hydrogen concentration C when stress is applied p is the increase in (ppm), p is the hydrostatic stress (MPa). H is the hydrogen molar volume change in steel (=2.0 cm 3 ), R is the gas constant (=8.31 ​​N m K mol), and T is the temperature (=300 K).

[0035] Furthermore, as factors related to delayed fracture of press-formed products, excluding environmental factors such as temperature, residual stress as well as the amount of hydrogen and strain in the press-formed product are considered to be important, as described above. Based on this, the inventors considered that delayed fracture occurs when the amount of hydrogen, strain, and stress satisfy certain conditions.

[0036] Therefore, a test was conducted to evaluate whether or not cracks occurred by varying the level of hydrogen penetration into the uniaxial tensile test specimens, which were made of high-tensile steel plates that had been subjected to rolling strain (equivalent plastic strain). The test specimens were subjected to a constant uniaxial tensile stress load, and the test results were referred to as delayed fracture tests.

[0037] Figure 5 shows the results of delayed fracture tests in which uniaxial tensile test specimens, using cold-rolled steel sheets (tensile strength 1470 MPa, thickness 1.2 mm) with rolling strain applied as test materials, were immersed in a 0.1% ammonium thiocyanate solution with a pH of 4.0 and a McIlvaine buffer solution for a predetermined period of time. Figure 5 also shows the relationship between stress and equivalent plastic strain and delayed fracture initiation after immersion times of 30 and 20 hours, respectively. In Figure 5, circles represent plots of equivalent plastic strain versus stress for uniaxial tensile test specimens in which delayed fracture did not occur, while crosses represent plots of equivalent plastic strain versus stress for uniaxial tensile test specimens in which delayed fracture did occur. Based on the delayed fracture test results, the delayed fracture threshold given by the relationship between equivalent plastic strain and stress shown in Figure 5 was derived. In Figure 5, the region where equivalent plastic strain and stress are higher than the delayed fracture threshold (gray region) indicates the region in which delayed fracture occurs in the uniaxial tensile test specimen. The results shown in Figure 5 indicate that cracks occurred at lower stress and lower equivalent plastic strain after 30 hours of immersion compared to 20 hours, indicating that the criteria for determining delayed fracture differ depending on the immersion time.

[0038] Figure 6 shows the relationship between hydrogen concentration and equivalent plastic strain when uniaxial tensile test specimens with various equivalent plastic strains were immersed in a 0.1% ammonium thiocyanate solution (pH 4.0) and a McIlvaine buffer solution for a set period of time (30 hours, 20 hours, and 10 hours) and the amount of hydrogen measured. As shown in Figure 6, the longer the immersion time under the same stress and strain conditions, the higher the hydrogen concentration. Furthermore, based on the results shown in Figure 5, it is inferred that the longer the immersion time, i.e., the higher the hydrogen concentration, the lower the stress and equivalent plastic strain at which cracking (delayed fracture) occurs.

[0039] Figure 7 shows the delayed fracture judgment criteria (critical values ​​of equivalent plastic strain and hydrogen concentration at which delayed fracture occurs) under constant stress conditions (1100 MPa, 1000 MPa, 900 MPa) obtained from the delayed fracture test results. It can be seen that the higher the hydrogen concentration, the lower the stress and strain at which cracks occur.

[0040] The above results (Figs. 4 to 7) show that in order to accurately determine the location where delayed fracture will occur in press-formed products made from high-tensile steel sheet, it is effective to accurately predict not only the stress and strain distribution in the press-formed product but also the hydrogen concentration distribution.

[0041] Furthermore, in predicting delayed fracture in press-formed products, the inventors also investigated the relationship between the stress gradient in the direction in which a crack propagates (advances) due to delayed fracture and delayed fracture properties.

[0042] Therefore, a delayed fracture test was carried out using a uniaxial tensile test piece 27 having a notch 29 formed in the parallel portion 23 as shown in Fig. 8. A high-tensile steel plate (e.g., a 1470 MPa-class cold-rolled steel plate with a thickness of 1.2 mm) was used as the test material for the uniaxial tensile test piece 27. In the delayed fracture test, the test piece was immersed in a solution of ammonium thiocyanate and McIlvaine buffer solution with a pH of 4.0 and a concentration of 0.1% until cracks (delayed fracture) occurred, while a constant uniaxial tensile load was applied so that the tensile stress at the apex (bottom) of the notch 29 became 1000 MPa.

[0043] FIG. 9 shows an example of the relationship between the notch R (punching radius R of the notch 29) of the notch 29 in the uniaxial tensile test piece 27 and the stress gradient in the crack propagation direction. Here, the stress gradient in the crack propagation direction is set to a direction perpendicular to the maximum principal stress. FIG. 9 also shows an equation that expresses the relationship between the notch R and the stress gradient. The equation shown in FIG. 9 (y=1082.5e -0.293x ) where y is the stress gradient (MPa / mm) and x is the notch radius (mm).

[0044] As shown in Figure 9, the smaller the notch radius of the notch 29, the larger the stress gradient. Notch radius R = 0 corresponds to the uniaxial tensile test specimen 21 shown in Figure 2(a) without a notch, and the stress gradient in the parallel portion 23 of the uniaxial tensile test specimen 21 is zero. Theoretically, a uniaxial tensile test specimen 21 with a notch radius R approaching zero is considered to be equivalent to a uniaxial tensile test specimen 27 with a notch radius 29 where notch radius R → infinity. Therefore, the stress gradient is considered to approach zero as the notch radius R of the notch 29 changes from infinity to infinity. Therefore, the relationship between the notch radius R of the notch 29 in the uniaxial tensile test specimen 27 and the stress gradient in the crack propagation direction can be expressed using an exponential function, as shown in Figure 9, for example.

[0045] Figure 10 shows the relationship between the stress gradient in the direction of crack propagation due to delayed fracture and the delayed fracture time, obtained from a delayed fracture test using a uniaxial tensile test piece 27 with a notch 29. As shown in Figure 10, the larger the stress gradient in the direction of crack propagation, the longer the delayed fracture time, which indicates that the delayed fracture occurs for a longer period of time. This is thought to be because the deformation of the high-stress area where the crack occurs is suppressed by the surrounding low-stress areas, which prolongs the time it takes for the crack to occur.

[0046] From the results shown in FIGS. 8 to 10, the inventors have concluded that in order to accurately predict delayed fracture of press-formed products, it is necessary to consider not only the stress, strain, and hydrogen concentration, but also the stress gradient in the direction in which cracks propagate due to delayed fracture.

[0047] Furthermore, the inventors considered that, when a region where delayed fracture will occur is predicted based on the stress, strain, hydrogen concentration, and stress gradient in a press-formed product, it would be possible to manufacture the press-formed product so that the region has stress and strain that are predicted to prevent delayed fracture. To this end, they concluded that by repeatedly changing the press-forming conditions and predicting the occurrence of delayed fracture in the press-formed product under the changed press-forming conditions, it would be possible to determine press-forming conditions that result in stress and strain that are predicted to prevent delayed fracture. They then discovered that by press-forming under the press-forming conditions determined in this way, it is possible to manufacture a press-formed product that has been designed to prevent delayed fracture.

[0048] The present invention has been made based on the results of such investigations, and first and second embodiments of the present invention will be described below.

[0049] [Embodiment 1] <Method for predicting delayed fracture of press-formed product> A method for predicting delayed fracture of a press-formed product according to embodiment 1 of the present invention predicts a location where delayed fracture will occur in a press-formed product made of a high-tensile steel plate. As illustrated in Fig. 1 , the method for predicting delayed fracture of a press-formed product according to embodiment 1 includes a stress distribution and strain distribution calculation step S1, a stress gradient calculation step S3, a hydrogen concentration distribution setting step S5, a hydrogen diffusion analysis step S7, and a delayed fracture occurrence prediction step S9. Each of these steps will be described below.

[0050] <Stress distribution and strain distribution calculation step> The stress distribution and strain distribution calculation step S1 is a step of calculating the stress distribution and strain distribution of the press-formed product.

[0051] In the first embodiment, the stress distribution and strain distribution calculation step S1 calculates the stress and strain for each CAE analysis element of the press-formed product, thereby calculating the stress distribution and strain distribution of the press-formed product.

[0052] CAE analysis methods commonly used in automobile design and the like can be applied to calculate the stress and strain distributions of press-formed products. Examples of CAE analysis methods include press-forming CAE analysis, shearing CAE analysis of sheet metal cutting, trimming, piercing, etc. before press-forming, springback CAE analysis of press-formed products, and CAE analysis of press-formed product assembly. The stress and strain distributions of press-formed products can be calculated by determining the stress and strain for each CAE element in the press-formed product through CAE analysis using the finite element method.

[0053] <Stress gradient calculation step> The stress gradient calculation step S3 is a step of identifying, as a portion at risk of delayed fracture, a portion in the press-formed product where the maximum principal stress is high and delayed fracture is a concern, based on the stress distribution of the press-formed product calculated in the stress distribution and strain distribution calculation step S1. Furthermore, the stress gradient calculation step S3 is a step of calculating the stress gradient in a direction perpendicular to the maximum principal stress in the identified portion at risk of delayed fracture as the stress gradient in the direction in which a crack propagates due to delayed fracture.

[0054] In the stress gradient calculation step S3, for example, the maximum principal stress of each CAE analysis element is calculated using the stress calculated for each CAE analysis element of the press-formed product in the stress distribution and strain distribution calculation step S1. Then, the CAE analysis element having a high calculated maximum principal stress can be identified as a portion of the press-formed product at risk of delayed fracture.

[0055] Furthermore, since the direction in which a crack propagates when delayed fracture occurs is perpendicular to the maximum principal stress, the stress gradient in the direction perpendicular to the maximum principal stress at the site at which delayed fracture is suspected is calculated as the stress gradient in the direction in which a crack propagates due to delayed fracture.

[0056] <Hydrogen concentration distribution setting process> The hydrogen concentration distribution setting process S5 is a process of setting a hydrogen concentration distribution corresponding to the strain distribution of the press-formed product calculated in the stress distribution and strain distribution calculation process S1, based on the relational equation between strain and hydrogen concentration previously obtained for the high-tensile steel plate.

[0057] 1 , in the first embodiment, a relational equation between strain and hydrogen concentration (the above-mentioned equation (1)) is derived by performing a step S11 of deriving a relational equation between strain and hydrogen concentration in a high-tensile steel sheet used in press-forming of a press-formed product prior to the hydrogen concentration distribution setting step S5. Then, based on the derived equation (1), the hydrogen concentration corresponding to the strain (equivalent plastic strain) calculated for each CAE element of the press-formed product in the stress distribution and strain distribution calculation step S1 is obtained for each CAE element, thereby setting the hydrogen concentration distribution of the press-formed product.

[0058] Furthermore, in the hydrogen concentration distribution setting step S5, the hydrogen concentration distribution of the press-formed product may be set by the following procedure. First, the relational expression (the aforementioned formula (3)) between hydrostatic stress and hydrogen concentration in a high-tensile steel sheet used in press forming of the press-formed product is obtained in advance. Next, the hydrostatic stress distribution is calculated from the stress distribution of the press-formed product calculated in the stress distribution and strain distribution calculation step S1, and the hydrogen concentration distribution corresponding to the hydrostatic stress distribution is calculated based on the previously obtained relational expression (the aforementioned formula (3)) between hydrostatic stress and hydrogen concentration. The hydrogen concentration distribution of the press-formed product is then set by adding the hydrogen concentration distribution corresponding to the hydrostatic stress distribution to the hydrogen concentration distribution corresponding to the strain (equivalent plastic strain) distribution calculated in the stress distribution and strain distribution calculation step S1 based on the aforementioned formula (1).

[0059] (Step of deriving a relational equation between strain and hydrogen concentration) The step S11 of deriving a relational equation between strain and hydrogen concentration is a step of experimentally determining the relational equation between strain and hydrogen concentration in a high-tensile steel sheet used in press forming by a hydrogen charging test.

[0060] Regarding the relationship between hydrogen concentration and strain in high-tensile steel sheets, the hydrogen concentration depends only on the equivalent plastic strain, regardless of the deformation mode, as shown in Figure 3. Therefore, in the hydrogen concentration distribution setting step S5, it is preferable to use equivalent plastic strain as the strain in the relational equation between strain and hydrogen concentration, and it is advisable to derive, for example, the relational equation between equivalent plastic strain and hydrogen concentration expressed by the above-mentioned equation (1).

[0061] The constants a and b in equation (1) can be determined by conducting a typical hydrogen charging test. Figure 3(a) shows an example of the measurement results of equivalent plastic strain and hydrogen concentration obtained by the hydrogen charging test, and the relationship between equivalent plastic strain and hydrogen concentration (a = 0.52, b = 3.9) derived from the measurement results.

[0062] <Hydrogen Diffusion Analysis Process> The hydrogen diffusion analysis process S7 is a process of performing hydrogen diffusion analysis on the press-molded product based on the stress distribution calculated in the stress distribution and strain distribution calculation process S1 and the hydrogen concentration distribution set in the hydrogen concentration distribution setting process S5, and calculating the hydrogen concentration distribution after hydrogen diffusion.

[0063] Hydrogen in a press-formed product diffuses according to the hydrogen concentration gradient and hydrostatic stress gradient. Therefore, to accurately predict the hydrogen concentration distribution in a press-formed product, it is necessary to solve the diffusion equation shown in Equation (4) below to redistribute hydrogen in each CAE element of the press-formed product.

[0064] x i is the in-plane direction of the press-molded product, J is the concentration flux, D is the diffusion coefficient, s is the solubility, and κ p is a coefficient representing stress dependency, c is the hydrogen concentration, and p is the hydrostatic stress. The first term on the right side represents the hydrogen concentration gradient, and the second term represents the dependence of the hydrostatic stress gradient on hydrogen diffusion.

[0065] If stress concentration exists in the press-formed product, the hydrogen concentration when hydrogen accumulates at the stress concentration site can be accurately predicted by solving equation (2) in the hydrogen diffusion analysis step S7.

[0066] <Delayed Fracture Prediction Step> The delayed fracture prediction step S9 is a step of predicting whether or not delayed fracture will occur in the delayed fracture-prone region identified in the stress gradient calculation step S3.

[0067] To predict delayed fracture in the delayed fracture occurrence prediction step S9, first, a delayed fracture judgment condition is acquired which indicates the relationship between the occurrence of delayed fracture and the stress, strain, and water content at a stress gradient corresponding to the stress gradient in the direction of crack propagation calculated for the region at risk of delayed fracture. Then, whether or not delayed fracture will occur in the region at risk of delayed fracture is predicted based on the acquired delayed fracture judgment condition and the stress, strain, and hydrogen content after hydrogen diffusion at the region at risk of delayed fracture which are obtained from the stress distribution, strain distribution, and hydrogen content distribution after hydrogen diffusion.

[0068] The stress and strain in the region at risk of delayed fracture are determined from the stress distribution and strain distribution of the press-formed product calculated in the stress and strain distribution calculation step S1. The hydrogen concentration after hydrogen diffusion in the region at risk of delayed fracture is determined from the hydrogen concentration distribution after hydrogen diffusion in the press-formed product calculated in the hydrogen diffusion analysis step S7. The stress gradient in the region at risk of delayed fracture is defined as the stress gradient in the direction perpendicular to the maximum principal stress calculated in the stress gradient calculation step S3 as the stress gradient in the direction in which a crack due to delayed fracture propagates.

[0069] The delayed fracture determination conditions obtained in the delayed fracture occurrence prediction step S9 are obtained by performing the delayed fracture determination condition derivation step S13 prior to the delayed fracture occurrence prediction step S9, as shown in FIG.

[0070] (Delayed fracture determination condition deriving step S13) The delayed fracture determination condition deriving step S13 is a step of conducting a delayed fracture test on a high-tensile steel plate used in press forming, and deriving the relationship between the stress, strain, water content at the location where delayed fracture occurs, and the stress gradient in the direction in which a crack propagates due to delayed fracture, as the delayed fracture determination condition.

[0071] In the delayed fracture judgment condition deriving step S13, delayed fracture test specimens are prepared from high-tensile steel plates to which various equivalent plastic strains have been imparted by rolling or the like, and delayed fracture tests are carried out on the delayed fracture test specimens with different levels of uniaxial tensile stress and hydrogen content (by varying the immersion time in an acidic solution).Then, the relationship between the occurrence of cracks (delayed fracture) in the delayed fracture test specimens and the stress, equivalent plastic strain, hydrogen concentration in the uniaxial tensile test specimen (amount of hydrogen penetrating into the steel), and the stress gradient in the direction of crack propagation due to delayed fracture is determined, and the delayed fracture judgment conditions are derived.

[0072] In the delayed fracture test for deriving the delayed fracture judgment conditions, the tensile stress in the notch 29 can be changed to various levels by the tensile load applied to the uniaxial tensile test piece 27. In addition, the delayed fracture test piece used in the delayed fracture judgment condition deriving step S13 has a shape such that the stress gradient in the direction of crack propagation in the delayed fracture suspected portion calculated in the stress gradient calculation step S3 coincides with the stress gradient in the direction of crack propagation in the delayed fracture test piece.

[0073] For example, when a uniaxial tensile test specimen 27 (FIG. 8) having a notch 29 formed in the parallel portion 23 is used as a delayed fracture test specimen, the relationship between the stress gradient in the crack propagation direction and the notch radius is expressed by the equation shown in FIG. 9. Therefore, the notch radius of the notch 29 can be determined in accordance with the equation shown in FIG. 9 so as to obtain a predetermined stress gradient.

[0074] In the above description, in the delayed fracture determination condition deriving step S13, a constant uniaxial tensile load is applied to a uniaxial tensile test piece 27 having a notch 29 in the parallel portion 23 in order to impart a stress gradient in the direction of crack propagation.

[0075] However, in the step S13 of deriving the delayed fracture judgment conditions, as shown in FIG. 11, a delayed fracture test may be performed by a four-point bending test on a four-point bending test piece 31, which is a high-tensile steel plate to which various equivalent plastic strains have been imparted by rolling or the like, as a test material.

[0076] In a four-point bending test, supports 33a and 33b are placed above a four-point bending test piece 31 and supports 33c and 33d are placed below the four-point bending test piece 31. Supports 33a and 33b are lowered or supports 33c and 33d are raised to perform bending to form a bend apex 35 on the outside of the bend of the four-point bending test piece 31. In the four-point bending test, the raising and lowering of the supports is stopped when a predetermined amount of compression (amount of lifting and lowering of the supports) is reached. At this time, the bend apex 35 on the outside of the bend of the four-point bending test piece 31 is the part with the greatest tensile stress, and the tensile stress of the bend apex 35 can be changed to various levels depending on the amount of compression.

[0077] Furthermore, in a four-point bending test, a stress gradient can be generated in the thickness direction from the bend apex 35 on the outside of the bend of the four-point bending test specimen 31 toward the inside of the bend. In the four-point bending test specimen 31, a crack propagates in the thickness direction from the bend apex on the outside of the bend. The relationship between the stress gradient in the crack propagation direction and the thickness t of the four-point bending test specimen 31 is expressed by the equation shown in FIG. 12. In the equation shown in FIG. 12, x is the thickness and y is the stress gradient. Therefore, in delayed fracture in a four-point bending test, a desired stress gradient can be imparted to the bend apex 35 by fixing the indentation amount of the supports 33a to 33d on the four-point bending test specimen 31 and changing the thickness of the four-point bending test specimen 31, as shown in FIG. 12.

[0078] Such a delayed fracture test using a four-point bending test may be carried out using, for example, a four-point bending test jig 41 shown in FIG. 11(c) (see Patent Document 4).

[0079] The four-point bending test jig 41 has fulcrums 45a and 45b made of insulating glass that are provided on the sample stage 43 and positioned above the four-point bending test piece 31, and fulcrums 45c and 45d made of insulating glass that are positioned below the four-point bending test piece 31. Furthermore, the four-point bending test jig 41 has a screw 47 that is provided so that the fulcrums 45c and 45d can be pushed upward.

[0080] In four-point bending using a four-point bending test jig 41, first, the supports 45c and 45d are pressed into the four-point bending test specimen 31 with a screw 47, and the screw 47 is fixed with a pressing force that produces a desired tensile stress on the outside of the bend. This applies a tensile stress and a stress gradient in the thickness direction to the four-point bending test specimen 31. Next, the four-point bending test specimen 31, together with the four-point bending test jig 41 with the screw 47 fixed with a predetermined pressing force, is immersed in a hydrogen intrusion environment (e.g., pH 4.0, 0.1% ammonium thiocyanate and McIlvaine buffer solution) for a predetermined time. Furthermore, by changing the immersion time in the hydrogen intrusion environment, the amount of hydrogen that penetrates into the four-point bending test specimen 31 is varied, and the presence or absence of cracking in the four-point bending test specimen 31 is evaluated.

[0081] Even in delayed fracture tests using such a four-point bending test jig, it is possible to derive delayed fracture judgment criteria that represent the relationship between stress, strain, hydrogen concentration, and the stress gradient in the direction of crack propagation, and the presence or absence of delayed fracture.

[0082] As described above, according to the method for predicting delayed fracture of a press-formed product according to the first embodiment, a portion of the press-formed product having a high maximum principal stress is identified as a portion at risk of delayed fracture based on the stress distribution of the press-formed product, and the stress gradient in the direction of propagation of a crack due to delayed fracture in the identified portion at risk of delayed fracture is taken into consideration, thereby making it possible to accurately predict whether delayed fracture will occur during use of a press-formed product made of a high-tensile steel sheet. Furthermore, according to the method for predicting delayed fracture of a press-formed product according to the first embodiment, it is possible to predict not only whether delayed fracture will occur in the press-formed product but also the portion at which delayed fracture will occur in the press-formed product. This makes it possible to take measures against delayed fracture, such as changing the shape of the press-formed product or the press-forming process, at the design stage of the press-formed product to prevent delayed fracture.

[0083] <Device for Predicting Delayed Fracture of Press-Formed Product> A device for predicting delayed fracture of a press-formed product 1 (hereinafter referred to as "delayed fracture prediction device 1") according to a first embodiment of the present invention predicts a location where delayed fracture will occur in a press-formed product made of a high-tensile steel plate. As shown in FIG. 13 as an example, the device for predicting delayed fracture 1 includes a stress distribution and strain distribution calculation unit 3, a stress gradient calculation unit 5, a hydrogen concentration distribution setting unit 7, a hydrogen diffusion analysis unit 9, and a delayed fracture prediction unit 11. The device for predicting delayed fracture 1 may be configured by a central processing unit (CPU) of a computer (such as a PC). In this case, the above-described units function when the CPU of the computer executes a predetermined program.

[0084] <Stress Distribution and Strain Distribution Calculation Unit> The stress distribution and strain distribution calculation unit 3 calculates the stress distribution and strain distribution of the press-formed product.

[0085] In the delayed fracture prediction device 1 according to this embodiment 1, the stress distribution and strain distribution calculation unit 3 calculates the stress distribution and strain distribution of the press-formed product by calculating the stress and strain for each CAE analysis element of the press-formed product.

[0086] CAE analysis methods commonly used in automobile design and the like can be applied to the calculation of the stress distribution and strain distribution of a press-formed product by the stress distribution and strain distribution calculation unit 3. Examples of CAE analysis methods include press forming CAE analysis, shear CAE analysis of cutting, trimming, piercing, etc. of sheet material before press forming, springback CAE analysis of press-formed products, and CAE analysis of assembly of press-formed products. Then, by determining the stress and strain for each CAE element in the press-formed product through CAE analysis using the finite element method, the stress distribution and strain distribution of the press-formed product can be calculated.

[0087] <Stress gradient calculation unit> The stress gradient calculation unit 5 identifies a portion of the press-formed product where the maximum principal stress is high as a portion at risk of delayed fracture, where delayed fracture is a concern, based on the stress distribution of the press-formed product calculated by the stress distribution and strain distribution calculation unit 3. Furthermore, the stress gradient calculation unit 5 calculates the stress gradient in a direction perpendicular to the maximum principal stress in the identified portion at risk of delayed fracture as the stress gradient in the direction in which a crack propagates due to delayed fracture.

[0088] The stress gradient calculation unit 5 calculates the maximum principal stress of each CAE analysis element, for example, by using the stress calculated for each CAE analysis element of the press-formed product by the stress distribution and strain distribution calculation unit 3. Then, the CAE analysis element with a high calculated maximum principal stress can be identified as a portion of the press-formed product at risk of delayed fracture.

[0089] <Hydrogen concentration distribution setting unit> The hydrogen concentration distribution setting unit 7 sets a hydrogen concentration distribution corresponding to the strain distribution of the press-formed product calculated by the stress distribution and strain distribution calculation unit 3, based on the relational equation between strain and hydrogen concentration previously obtained for the high-tensile steel plate.

[0090] In the first embodiment, the hydrogen concentration distribution setting unit 7 determines the hydrogen concentration corresponding to the strain (equivalent plastic strain) for each CAE analysis element of the press-formed product based on the relational expression between equivalent plastic strain and hydrogen concentration (the above-mentioned expression (1)) previously obtained for the high-tensile steel sheet. This makes it possible to set a hydrogen concentration distribution corresponding to the strain distribution of the press-formed product.

[0091] The hydrogen concentration distribution setting unit 7 may also set the hydrogen concentration distribution of the press-formed product by the following procedure. First, the relational expression (the aforementioned equation (3)) between hydrostatic stress and hydrogen concentration in a high-tensile steel sheet used in press-forming of the press-formed product is obtained in advance. Next, the hydrostatic stress distribution is calculated from the stress distribution of the press-formed product calculated by the stress distribution and strain distribution calculation unit 3, and the hydrogen concentration distribution corresponding to the hydrostatic stress distribution is calculated based on the relational expression (the aforementioned equation (3)) between hydrostatic stress and hydrogen concentration. Then, the hydrogen concentration distribution corresponding to the hydrostatic stress distribution is added to the hydrogen concentration distribution corresponding to the strain distribution calculated by the stress distribution and strain distribution calculation unit 3, thereby setting the hydrogen concentration distribution of the press-formed product.

[0092] <Hydrogen diffusion analysis unit> The hydrogen diffusion analysis unit 9 performs hydrogen diffusion analysis on the press-molded product based on the stress distribution calculated by the stress distribution and strain distribution calculation unit 3 and the hydrogen concentration distribution set by the hydrogen concentration distribution setting unit 7, and calculates the hydrogen concentration distribution after hydrogen diffusion.

[0093] As described above, hydrogen in a press-formed product diffuses in accordance with the hydrogen concentration gradient and the hydrostatic stress gradient. Therefore, the hydrogen diffusion analysis unit 9 calculates the hydrogen concentration for each CAE element of the press-formed product by solving the diffusion equation shown in the above-described formula (4) based on the stress distribution calculated by the stress distribution and strain distribution calculation unit 3 and the hydrogen concentration distribution set by the hydrogen concentration distribution setting unit 7. In this way, the hydrogen diffusion analysis unit 9 can calculate the hydrogen concentration distribution after hydrogen diffusion, taking into account the hydrogen concentration gradient and the hydrostatic stress gradient.

[0094] <Delayed Fracture Prediction Unit> The delayed fracture prediction unit 11 predicts whether or not delayed fracture will occur in the delayed fracture-prone region identified by the stress gradient calculation unit 5 .

[0095] In order for the delayed fracture occurrence prediction unit 11 to predict delayed fracture, first, a delayed fracture judgment condition is obtained that indicates the relationship between the occurrence of delayed fracture and the stress, strain, and hydrogen concentration at a stress gradient corresponding to the stress gradient in the direction of crack propagation calculated for the delayed fracture suspected portion. As the delayed fracture judgment condition, for example, the relationship between the occurrence of delayed fracture and the stress, strain, hydrogen concentration, and stress gradient in the direction of crack propagation derived by performing the delayed fracture judgment condition derivation step S13 of the delayed fracture judgment method for a press-formed product according to the first embodiment may be obtained.

[0096] Then, based on the acquired delayed fracture judgment conditions and the stress, strain, and hydrogen concentration after hydrogen diffusion at the delayed fracture suspected area obtained from the stress distribution, strain distribution, and hydrogen concentration distribution after hydrogen diffusion, it is predicted whether delayed fracture will occur at the delayed fracture suspected area.

[0097] The stress and strain in the region where delayed fracture is suspected are determined from the stress distribution and strain distribution of the press-formed product calculated by the stress distribution and strain distribution calculation unit 3. The hydrogen concentration after hydrogen diffusion in the region where delayed fracture is suspected is determined from the hydrogen concentration distribution after hydrogen diffusion in the press-formed product calculated by the hydrogen diffusion analysis unit 9. Furthermore, the stress gradient in the region where delayed fracture is suspected is the stress gradient in the direction perpendicular to the maximum principal stress calculated by the stress gradient calculation unit 5 as the stress gradient in the direction in which a crack propagates due to delayed fracture.

[0098] The stress, strain, and hydrogen concentration after hydrogen diffusion in the region at risk of delayed fracture may be the stress, strain, and hydrogen concentration after hydrogen diffusion calculated for the CAE analysis element with a high maximum principal stress identified by the stress gradient calculation unit 5 as the region at risk of delayed fracture.

[0099] <Program for Predicting Delayed Fracture of High-Tensile Steel Plate> The first embodiment of the present invention can be configured as a program for predicting delayed fracture of a press-formed product. That is, the program for predicting delayed fracture of a press-formed product (hereinafter referred to as the "delayed fracture prediction program") according to the first embodiment predicts a location where delayed fracture will occur in a press-formed product of a high-tensile steel plate. The delayed fracture prediction program causes a computer to function as a stress distribution and strain distribution calculation unit 3, a stress gradient calculation unit 5, a hydrogen concentration distribution setting unit 7, a hydrogen diffusion analysis unit 9, and a delayed fracture occurrence prediction unit 11, as shown in FIG. 13 as an example.

[0100] As described above, in the delayed fracture prediction device and program for a press-formed product according to the first embodiment, similarly to the method for predicting delayed fracture for a press-formed product according to the first embodiment, based on the stress distribution of the press-formed product, a portion of the press-formed product having a high maximum principal stress is identified as a portion at risk of delayed fracture, where delayed fracture is a concern, and the stress gradient in the direction of propagation of a crack due to delayed fracture in the identified portion at risk of delayed fracture is taken into consideration, thereby making it possible to predict whether delayed fracture will occur during use of a press-formed product made of a high-tensile steel plate. As a result, it is possible to predict not only whether delayed fracture will occur in a press-formed part but also the portion at which delayed fracture will occur, at the part design stage, and to take measures against delayed fracture, such as changing the part shape or the press-forming process, so as to prevent delayed fracture.

[0101] The method for predicting delayed fracture of a press-formed product according to the first embodiment described above involves experimentally determining the relational expression between strain and hydrogen concentration in a high-tensile steel sheet used in press forming through a hydrogen charging test by carrying out step S11 of deriving the relational expression between strain and hydrogen concentration. However, the method for predicting delayed fracture of a press-formed product according to the present invention is not limited to carrying out step S11 of deriving the relational expression between strain and hydrogen concentration, and may involve obtaining a known relational expression between strain and hydrogen concentration for a high-tensile steel sheet of the same material (steel type, tensile strength).

[0102] Furthermore, the method for predicting delayed fracture of a press-formed product according to the first embodiment experimentally derives the delayed fracture judgment condition by carrying out the delayed fracture judgment condition derivation step S13. However, the method for predicting delayed fracture of a press-formed product according to the present invention is not limited to carrying out the delayed fracture judgment condition derivation step S13, and may also be a method for obtaining delayed fracture judgment conditions that have already been derived for a high-tensile steel plate of the same material (steel type, tensile strength).

[0103] [Embodiment 2] <Method for manufacturing press-formed product> A method for manufacturing a press-formed product according to embodiment 2 of the present invention manufactures a press-formed product while suppressing the occurrence of delayed fracture in the press-formed product using a high-tensile steel sheet. As an example, as shown in FIG. 14 , the method for manufacturing a press-formed product according to embodiment 2 includes a provisional press-forming condition setting step S21, a delayed fracture occurrence determination step S23, a provisional press-forming condition changing step S25, and a repeating step S27. Furthermore, as shown in FIG. 14 , the method for manufacturing a press-formed product according to embodiment 2 includes a press-forming condition determination step S29 and a press-forming step S31. Each of the above steps will be described below.

[0104] <<Temporary Press-Molding Condition Setting Step>> The temporary press-molding condition setting step S21 is a step of setting temporary press-molding conditions for the press-molded product.

[0105] Examples of the provisional press-forming conditions set in the provisional press-forming condition setting step S21 include, for example, the bending R (punch shoulder radius) of the bent portion when the target is a press-formed product having a bent portion obtained by bending a steel plate. Other examples include the clearance between the punch and the die (die / die for press) and the thickness of the steel plate. In addition, in the case of a draw-bending method (stretch bending / bending and stretching), further examples include the radius of the die shoulder part, the clearance of the side wall, and blank holder pressure.

[0106] <Delayed fracture occurrence determination step S23> The delayed fracture occurrence determination step S23 predicts whether delayed fracture will occur in the delayed fracture susceptible region identified in the press-formed product, based on the tentative press-forming conditions set in the tentative press-forming condition setting step S21. The delayed fracture susceptible region in the press-formed product is identified and the delayed fracture is predicted by the delayed fracture prediction method for a press-formed product according to the present invention described above. The delayed fracture occurrence determination step S23 is a step of determining whether delayed fracture will occur in the press-formed product, based on the results of the prediction of delayed fracture in the delayed fracture susceptible region.

[0107] In the second embodiment, the delayed fracture occurrence determination step S23 sequentially performs a stress distribution and strain distribution calculation step S1, a stress gradient calculation step S3, a hydrogen concentration distribution setting step S5, a hydrogen diffusion analysis step S7, and a delayed fracture occurrence prediction step S9, as shown in FIG. 14 .

[0108] The stress distribution and strain distribution calculation step S1, stress gradient calculation step S3, hydrogen concentration distribution setting step S5, hydrogen diffusion analysis step S7, and delayed fracture occurrence prediction step S9 are the same as those in the first embodiment. However, in the stress distribution and strain distribution calculation step S1, press-forming CAE analysis is performed using the tentative press-forming conditions set in the tentative press-forming condition setting step S21, and the stress distribution and strain distribution of a press-formed product press-formed under the tentative press-forming conditions are calculated. Then, shear CAE analysis, springback CAE analysis, CAE analysis of press-formed product assembly, etc. may be performed as needed.

[0109] Furthermore, in the delayed fracture occurrence determination step S23, if it is predicted that delayed fracture will occur in the delayed fracture suspected region in the delayed fracture occurrence prediction step S9, it is determined that delayed fracture will occur in the press-formed product (S23a). On the other hand, if it is predicted that delayed fracture will not occur in the delayed fracture suspected region in the delayed fracture occurrence prediction step S9, it is determined that delayed fracture will not occur in the press-formed product (S23a).

[0110] <<Temporary Press-Forming Condition Changing Step>> The temporary press-forming condition changing step S25 is a step of changing the temporary press-forming conditions when it is determined in the delayed fracture occurrence determining step S23 that delayed fracture has occurred in the press-formed product.

[0111] The tentative press forming conditions can be changed so as to reduce the stress and strain in the area where delayed fracture is suspected to occur in the press-formed product in the delayed fracture occurrence prediction step S9, for example, by increasing the bending radius of the bent portion.

[0112] <<Repetition Step>> In the repetition step S27, the delayed fracture occurrence determination step S23 and the temporary press-forming condition change step S25 are repeatedly performed under the temporary press-forming conditions changed in the temporary press-forming condition change step S25. This repetition is performed until it is determined that delayed fracture has not occurred in the delayed fracture occurrence determination step S23. Therefore, if it is not determined that delayed fracture has not occurred after changing the temporary press-forming conditions only once, the temporary press-forming condition change step S25 is also repeated.

[0113] <Press-forming condition determination process> The press-forming condition determination process S29 is a process for determining, if it is determined in the delayed fracture occurrence determination process S23 that delayed fracture has not occurred in the press-formed product, tentative press-forming conditions in that case as press-forming conditions.

[0114] <Press-forming step> The press-forming step S31 is a step of press-forming a high-tensile steel sheet into a press-formed product under the press-forming conditions determined in the press-forming condition determination step S29.

[0115] As described above, in the method for manufacturing a press-formed product according to the second embodiment, the occurrence of delayed fracture in the press-formed product is determined based on the method for predicting the occurrence of delayed fracture in a press-formed product according to the present invention, and press-forming conditions that suppress the occurrence of delayed fracture are determined based on the determination results. This makes it possible to shorten the time required to determine press-forming conditions that can suppress the occurrence of delayed fracture in a press-formed product made of a high-tensile steel sheet. Furthermore, by press-forming the press-formed product under the press-forming conditions determined in this manner, it is possible to manufacture a press-formed product in which measures have been taken to suppress the occurrence of delayed fracture.

[0116] The method for predicting the occurrence of delayed fracture in a press-formed product and the method for manufacturing a press-formed product according to the present invention are intended for high-tensile steel plates, and can be particularly preferably applied to high-tensile steel plates having a tensile strength of 1 GPa or more, for which the occurrence of delayed fracture is a concern.

[0117] Furthermore, the press-formed product targeted by the present invention is not limited to a specific part, but can be preferably applied to automobile body frame parts such as center pillars and A-pillar lowers that are press-formed from high-tensile steel sheets.

[0118] Experiments and analyses were carried out to confirm the effects of the present invention, and these will be described below.

[0119] In this Example 1, a location where delayed fracture occurs was predicted for a press-formed product 51 shown in Fig. 15. The press-formed product 51 was obtained by press-forming a 1470 MPa-class cold-rolled steel sheet (sheet thickness: 1.2 mm), which is a high-tensile steel sheet, and as shown in Fig. 15, had a bent portion 53 and pieces 55 extending from both ends of the bent portion 53. In the press-formed product 51, the radius of curvature of the bent portion 53 was set to 5 mm.

[0120] First, a delayed fracture test was carried out on the press-formed product 51, and a crack (delayed fracture) was generated in the center of the plate thickness near the apex of the bend 53.

[0121] Next, the location where delayed fracture will occur in the press-formed product 51 was predicted using the method for predicting delayed fracture of a press-formed product according to the first embodiment described above.

[0122] First, a press forming CAE analysis was performed on the process of bending a 1470 MPa class cold-rolled steel sheet with a thickness of 1.2 mm into a press-formed product 51, and as shown in Figure 16, stress and equivalent plastic strain were calculated for each CAE analysis element as the stress distribution and strain distribution of the press-formed product 51.

[0123] The maximum principal stress in the bent portion 53 is the stress in the longitudinal direction of the press-formed product 51, and the stress distribution shown in Figure 16(a) shows that the maximum principal stress is high from the outside of the bend near the bend apex of the bent portion 53 to the center of the plate thickness. In particular, the maximum principal stress in the center of the plate thickness (part A) and the surface (part B) at the bend apex of the bent portion 53 were 1120 MPa and 980 MPa, respectively, which were higher values ​​than in other parts. Therefore, parts A and B in the bent portion 53 were identified as parts at risk of delayed fracture.

[0124] Next, the stress gradient in the direction perpendicular to the maximum principal stress at the site where delayed fracture is suspected, i.e., the stress gradient in the plate thickness direction at the bending apex of the bent portion 53, was calculated as the stress gradient in the direction in which a crack propagates due to delayed fracture. In the stress distribution of the press-formed product 51 shown in Figure 16(a), the stress gradient in the direction perpendicular to the maximum principal bending stress at parts A and B was both 466 MPa / mm.

[0125] Next, a hydrogen concentration distribution was set in the press-formed product 51. In Example 1, a hydrogen concentration distribution was set in the press-formed product 51 for both Case 1 and Case 2 described below.

[0126] In Case 1, first, based on the relational expression between equivalent plastic strain and hydrogen concentration shown in the above-mentioned equation (1), the hydrogen concentration of the press-formed product 51 corresponding to the strain (equivalent plastic strain) for each CAE analysis element of the press-formed product 51 was calculated, and the hydrogen concentration distribution of the press-formed product 51 was set. Fig. 17(a) shows the hydrogen concentration distribution set to correspond to the equivalent plastic strain of the press-formed product 51.

[0127] On the other hand, in Case 2, the hydrostatic stress distribution was first calculated from the stress distribution of the press-formed product 51. Then, the hydrogen concentration corresponding to the hydrostatic stress was calculated for each CAE analysis element of the press-formed product 51 based on the relationship between hydrostatic stress and hydrogen concentration shown in Equation (3) above. Next, for each CAE analysis element, the hydrogen concentration corresponding to the hydrostatic stress was added to the hydrogen concentration corresponding to the strain (equivalent plastic strain). The hydrogen concentration calculated for each CAE analysis element of the press-formed product 51 in this manner was set as the hydrogen concentration distribution corresponding to the hydrostatic stress distribution and strain (equivalent plastic strain) distribution of the press-formed product 51. Figure 17(b) shows the hydrogen concentration distribution calculated by calculating the hydrogen concentration corresponding to the hydrostatic stress for each CAE analysis element. Furthermore, Figure 17(c) shows the hydrogen concentration distribution calculated for each CAE analysis element by adding the hydrogen concentration corresponding to the hydrostatic stress to the hydrogen concentration corresponding to the strain (equivalent plastic strain).

[0128] Next, a hydrogen diffusion analysis was performed based on the stress and hydrogen concentration calculated for each CAE analysis element of the press-formed product 51, and the hydrogen concentration after hydrogen diffusion was calculated for each CAE analysis element to determine the hydrogen concentration distribution after hydrogen diffusion. Figures 18(a) and 18(b) show the hydrogen concentration distributions after hydrogen diffusion in the press-formed product 51 obtained for (Case 1) and (Case 2), respectively.

[0129] Next, in order to derive the conditions for determining delayed fracture at the portions (A and B) where delayed fracture is suspected, a delayed fracture test was carried out using the uniaxial tensile test piece 27 having the notch 29 formed in the parallel portion 23 as shown in FIG. 8 described above.

[0130] Regarding the shape of the uniaxial tensile test piece 27, a notch R was determined based on the aforementioned Figure 9 so that a stress gradient equivalent to the stress gradient in the direction of crack propagation in parts A and B, which are regions at risk of delayed fracture, could be imparted to the notch 29. As mentioned above, the stress gradient in the direction of crack propagation in parts A and B was 466 MPa / mm. Therefore, it can be seen from Figure 9 that the notch R required to impart this stress gradient to the notch 29 should be 2.9 mm.

[0131] Next, with the above-mentioned stress gradient applied to the notch 29, the uniaxial tensile test piece 27 was immersed and held for a predetermined time in a hydrogen penetration environment (pH = 4.0, 0.1% ammonium thiocyanate and McIlvaine buffer solution). Then, the stress, strain, and hydrogen concentration in the notch 29 were changed to various levels, and the relationship between the stress, strain, and hydrogen concentration in the notch 29 and the occurrence of delayed fracture in the notch 29 was derived as a delayed fracture judgment condition.

[0132] Furthermore, based on the delayed fracture judgment conditions derived from a delayed fracture test using a uniaxial tensile test piece 27 and the stress, equivalent plastic strain, and hydrogen concentration at the delayed fracture-prone areas (parts A and B) of the press-formed product 51, it was predicted whether delayed fracture would occur at the delayed fracture-prone areas.

[0133] At part A of bent portion 53, the stress was 1120 MPa, so the relationship between equivalent plastic strain and hydrogen concentration at a stress of 1120 MPa shown in Figure 19(b) was used as the criterion for determining delayed fracture at part A. On the other hand, at part B of bent portion 53, the stress was 980 MPa, so the relationship between equivalent plastic strain and hydrogen concentration at a stress of 980 MPa shown in Figure 19(b) was used as the criterion for determining delayed fracture at part B. The relationships between equivalent plastic strain and hydrogen concentration at σ = 1120 MPa and σ = 980 MPa shown in Figure 19(b) were both derived from delayed fracture tests using uniaxial tensile test specimens 27 in which a stress gradient (= 466 MPa / mm) was applied to notches 29 at the parts susceptible to delayed fracture (parts A and B).

[0134] Table 1 shows the hydrogen concentrations in parts A and B for (Case 1) and (Case 2). By taking into account the hydrogen concentration distribution corresponding to the hydrostatic stress distribution and strain (equivalent plastic strain) distribution in (Case 2), the hydrogen concentration increased by a maximum of +13.6% (after hydrogen diffusion in part B) and an average of +7.7% compared to (Case 1).

[0135]

[0136] First, when the hydrogen concentration before hydrogen diffusion analysis was used to judge delayed fracture in part A of (Case 1), as shown in Figure 19(b-1), the hydrogen concentration before hydrogen diffusion analysis (☆A in the figure) was lower than the hydrogen concentration when the equivalent plastic strain was equal under the crack judgment condition of σ = 1120 MPa, so it was judged that delayed fracture (crack) would not occur. However, the hydrogen concentration after hydrogen diffusion analysis (★A in the figure) was higher than the hydrogen concentration under the crack judgment condition, so it was judged that delayed fracture would occur.

[0137] Next, when the hydrogen concentration before hydrogen diffusion analysis was used to judge delayed fracture in part B of (Case 1), as shown in Figure 19(b-1), the hydrogen concentration before hydrogen diffusion analysis (★B in the figure) was higher than the hydrogen concentration when the equivalent plastic strain was equal under the crack judgment condition of σ = 980 MPa, so it was judged that delayed fracture would occur. However, the hydrogen concentration after hydrogen diffusion analysis (☆B in the figure) was lower than the hydrogen concentration under the crack judgment condition, so it was judged that delayed fracture would not occur.

[0138] When the hydrogen concentration before hydrogen diffusion analysis was used to judge delayed fracture in part A of (Case 2), as shown in Figure 19(b-2), the hydrogen concentration before hydrogen diffusion analysis (☆A in the figure) was lower than the hydrogen concentration when the equivalent plastic strain was equal under the crack judgment condition of σ = 1120 MPa, so it was judged that delayed fracture would not occur. However, the hydrogen concentration after hydrogen diffusion analysis (★A in the figure) was higher than the hydrogen concentration under the crack judgment condition, so it was judged that delayed fracture would occur.

[0139] When the hydrogen concentration before hydrogen diffusion analysis was used to judge delayed fracture in part B of (Case 2), as shown in Figure 19(b-2), the hydrogen concentration before hydrogen diffusion analysis (★B in the figure) was higher than the hydrogen concentration when the equivalent plastic strain was equal under the crack judgment condition of σ = 980 MPa, so it was judged that delayed fracture would occur. However, the hydrogen concentration after hydrogen diffusion analysis (☆B in the figure) was lower than the hydrogen concentration under the crack judgment condition, so it was judged that delayed fracture would not occur.

[0140] From these judgment results, it was predicted that in the press-formed product 51, in both (Case 1) and (Case 2), delayed fracture would not occur on the surface (part B) of the bent part 53, but would occur in the center part (part A) of the plate thickness of the bent part 53. This predicted result was in good agreement with the position where delayed fracture occurred in the delayed fracture test.

[0141] As described above, it has been demonstrated that the present invention makes it possible to accurately predict the location where delayed fracture will occur in a press-formed product of high-tensile steel sheet.

[0142] In Example 2, for the press-formed product 51 shown in Figure 15, the tentative press-forming conditions were changed so that delayed fracture would not occur, and the occurrence of delayed fracture was determined, and the press-formed product 51 was press-formed under the press-forming conditions under which it was determined that delayed fracture would not occur.

[0143] The press-formed product 51 was obtained by press-forming a 1470 MPa-class cold-rolled steel sheet (thickness: 1.2 mm), which is a high-tensile steel sheet, as in Example 1. A tentative press-forming condition was set in which the bending radius of the bent portion 53, which was a concern in Example 1 and which was likely to cause delayed fracture, was changed from 5 mm to 7 mm. The stress at part A of the bent portion 53 of the press-formed product 51 press-formed under the set tentative press-forming condition decreased from 1120 MPa (bending radius = 5 mm) to 1020 MPa (bending radius = 7 mm). Furthermore, the equivalent plastic strain at part A of the bent portion 53 decreased from 0.023 (bending radius = 5 mm) to 0.010 (bending radius = 7 mm).

[0144] In the bent portion 53 with a bending radius of 5 mm, the stress gradient in the direction perpendicular to the maximum principal stress at parts A and B was 466 MPa / mm. In the bent portion 53 with a bending radius of 7 mm, the stress gradient in the direction perpendicular to the maximum principal stress at parts A and B was 424 MPa / mm.

[0145] Furthermore, the hydrogen concentration distribution in the press-formed product 51 was set in the same manner as in (Case 1) and (Case 2) of Example 1 described above. Here, (Case 1) is a case where a hydrogen concentration corresponding to the equivalent plastic strain of each CAE analysis element was set for each CAE analysis element. Also, (Case 2) is a case where a hydrogen concentration obtained by adding a hydrogen concentration corresponding to the hydrostatic stress calculated from the stress to the hydrogen concentration corresponding to the equivalent plastic strain of each CAE analysis element was set for each CAE analysis element. Then, for each of (Case 1) and (Case 2), a hydrogen diffusion analysis was performed based on the stress and hydrogen concentration calculated for each CAE analysis element of the press-formed product 51, and the hydrogen concentration distribution after hydrogen diffusion was calculated.

[0146] The hydrogen concentration at part A of the bent portion 53 was determined from the hydrogen concentration distribution calculated for each of (Case 1) and (Case 2), and was 2.5 ppm and 2.7 ppm, respectively, before hydrogen diffusion, and 4.0 ppm and 4.3 ppm, respectively, after hydrogen diffusion.

[0147] Next, as in Example 1 (see FIG. 19 ), delayed fracture was assessed using the calculated stress, equivalent plastic strain, and hydrogen concentration after hydrogen diffusion for part A of bent portion 53. As a result, FIG. 20 shows the delayed fracture assessment criteria (σ = 1020 MPa, black solid line) for a bent portion 53 with a bending radius of 7 mm and the hydrogen concentrations (★ in FIG. 20 ) for Case 1 and Case 2 after hydrogen diffusion analysis. Furthermore, for comparison, FIG. 20 also shows the delayed fracture assessment criteria (σ = 1120 MPa, gray dashed line) for a bent portion 53 of Example 1 with a bending radius of 5 mm and the hydrogen concentrations (☆ in FIG. 20 ) for Case 1 and Case 2 after hydrogen diffusion analysis.

[0148] The criteria for determining delayed fracture at a bending radius of 5 mm were determined by a delayed fracture test using a uniaxial tensile test piece 27 (Fig. 8, notch radius = 2.9 mm) with a notch 29 provided with a stress gradient (466 MPa / mm) corresponding to the stress gradient in the direction of crack propagation in a bent portion 53 with a bending radius of 5 mm. Furthermore, the criteria for determining delayed fracture at a bending radius of 7 mm were determined by a delayed fracture test using a uniaxial tensile test piece 27 (Fig. 8, notch radius = 3.2 mm) with a notch 29 provided with a stress gradient (424 MPa / mm) corresponding to the stress gradient in the direction of crack propagation in a bent portion 53 with a bending radius of 5 mm. In the delayed fracture test in Example 2, as in Example 1, the uniaxial tensile test piece 27 was immersed for a predetermined time in a hydrogen penetration environment containing 0.1% ammonium thiocyanate and McIlvaine buffer solution at pH 4.0.

[0149] As shown in Figure 20, changing the bending radius of the bent portion 53 from 5 mm to 7 mm relaxed the delayed fracture criteria and also reduced the hydrogen concentration after hydrogen diffusion analysis. As a result, the hydrogen concentration after hydrogen diffusion analysis for the bent portion 53 with a bending radius of 7 mm (marked with a star in Figure 20) was lower than the hydrogen concentration for the delayed fracture criteria (σ = 1020 MPa) for a bent radius of 7 mm in both Case 1 and Case 2. Therefore, delayed fracture was predicted not to occur in the bent portion 53 with a bending radius of 7 mm. Therefore, provisional press-forming conditions were determined as press-forming conditions, changing the bending radius of the bent portion 53 to 7 mm. Furthermore, a press-formed product 51 was press-formed under the determined press-forming conditions, and a delayed fracture test was conducted. As a result, delayed fracture did not occur in the bent portion 53 of the press-formed product 51.

[0150] As described above, it has been demonstrated that the method for manufacturing a press-formed product according to the present invention determines press-forming conditions under which delayed fracture does not occur in high-tensile steel sheets, and by performing press-forming under the determined press-forming conditions, it is possible to manufacture a press-formed product in which measures have been taken to suppress the occurrence of delayed fracture.

[0151] According to the present invention, it is possible to provide a method, an apparatus, and a program for predicting a portion where delayed fracture will occur in a press-formed product made of a high-tensile steel sheet. Furthermore, according to the present invention, it is possible to provide a method for manufacturing a press-formed product, which determines press-forming conditions so that delayed fracture will not occur in the portion where delayed fracture is predicted to occur by the above-mentioned method, and manufactures the press-formed product under the determined press-forming conditions.

[0152] DESCRIPTION OF SYMBOLS 1 Delayed fracture prediction device 3 Stress distribution and strain distribution calculation section 5 Stress gradient calculation section 7 Hydrogen concentration distribution setting section 9 Hydrogen diffusion analysis section 11 Delayed fracture occurrence prediction section 21 Uniaxial tensile test piece 23 Parallel section 25 R section 27 Uniaxial tensile test piece 29 Notch section 31 Four-point bending test piece 33a, 33b, 33c, 33d Support point 35 Bending apex section 41 Four-point bending test jig 43 Sample stage 45a, 45b, 45c, 45d Support point 47 Screw 51 Press-formed product 53 Bent section 55 Piece section

Claims

1. A method for predicting a portion where delayed fracture will occur in a press-formed product of a high tensile steel plate, comprising: a stress distribution and strain distribution calculation step for calculating a stress distribution and a strain distribution of the press-formed product; a stress gradient calculation step for identifying a portion of the press-formed product having a high maximum principal stress from the calculated stress distribution of the press-formed product as a portion where delayed fracture is likely to occur, and calculating a stress gradient in a direction perpendicular to the maximum principal stress in the identified portion where delayed fracture is likely to occur, as a stress gradient in a direction in which a crack will propagate due to delayed fracture; a hydrogen concentration distribution setting step for setting a hydrogen concentration distribution corresponding to the strain distribution of the press-formed product calculated in the stress distribution and strain distribution calculation step, based on a relational equation between strain and hydrogen concentration previously obtained for the high tensile steel plate; and a hydrogen diffusion analysis step for performing a hydrogen diffusion analysis of the press-formed product based on the stress distribution calculated in the stress distribution and strain distribution calculation step and the hydrogen concentration distribution set in the hydrogen concentration distribution setting step, and calculating a hydrogen concentration distribution after hydrogen diffusion. a delayed fracture occurrence prediction step of acquiring delayed fracture judgment conditions that indicate a relationship between the occurrence of delayed fracture and the stress, strain, and water content at a stress gradient corresponding to the stress gradient in the direction in which the crack propagates, the delayed fracture judgment conditions being acquired for the portion at which delayed fracture is suspected, the stress distribution and strain distribution being calculated in the stress distribution and strain distribution calculation step, and the hydrogen content distribution after hydrogen diffusion being calculated in the hydrogen diffusion analysis step, and predicting whether or not delayed fracture will occur in the portion at which delayed fracture is suspected.

2. The method for predicting delayed fracture of a press-formed product as described in claim 1, wherein the delayed fracture judgment conditions obtained in the delayed fracture occurrence prediction step are obtained by carrying out a delayed fracture test in which a predetermined uniaxial tensile load is applied to a uniaxial tensile test piece having a notch formed in a parallel portion, and the uniaxial tensile test piece is held in a hydrogen penetration environment while a stress gradient equivalent to the stress gradient in the direction in which the crack propagates calculated for the delayed fracture suspected portion is applied to the notch, and the relationship between the stress, strain and hydrogen concentration in the notch to which the stress gradient is applied and the presence or absence of delayed fracture in the notch is obtained.

3. The method for predicting delayed fracture of a press-molded product as described in claim 1, wherein the delayed fracture judgment conditions obtained in the delayed fracture occurrence prediction process are obtained by carrying out a delayed fracture test in which a predetermined bending load is applied to a four-point bending test piece, and a stress gradient equivalent to the stress gradient in the direction in which the crack propagates calculated for the delayed fracture suspected area is applied to the bent portion of the four-point bending test piece, and the four-point bending test piece is held in a hydrogen penetration environment while the bent portion is subjected to the stress gradient, the stress gradient being equivalent to the stress gradient in the direction in which the crack propagates calculated for the delayed fracture suspected area.

4. A method for predicting delayed fracture of a press-formed product as described in any one of claims 1 to 3, wherein in the stress distribution and strain distribution calculation process, an equivalent plastic strain distribution is calculated as the strain distribution, and in the hydrogen concentration distribution setting process, a relational equation between equivalent plastic strain and hydrogen concentration is obtained in advance, and a hydrogen concentration distribution corresponding to the equivalent plastic strain distribution is set based on the obtained relational equation between equivalent plastic strain distribution and hydrogen concentration.

5. A method for predicting delayed fracture of a press-formed product according to any one of claims 1 to 4, wherein the high-tensile steel plate has a tensile strength of 1 GPa or more.

6. A delayed fracture prediction device for a press-formed product that predicts a portion where delayed fracture will occur in a press-formed product of a high tensile steel plate, comprising: a stress distribution and strain distribution calculation unit that calculates a stress distribution and a strain distribution of the press-formed product; a stress gradient calculation unit that identifies a portion of the press-formed product with a high maximum principal stress from the calculated stress distribution of the press-formed product as a portion at risk of delayed fracture where delayed fracture is a concern, and calculates a stress gradient in a direction perpendicular to the maximum principal stress in the identified portion at risk of delayed fracture as a stress gradient in a direction in which a crack will propagate due to delayed fracture; a hydrogen concentration distribution setting unit that sets a hydrogen concentration distribution corresponding to the strain distribution of the press-formed product calculated by the stress distribution and strain distribution calculation unit, based on a relational equation between strain and hydrogen concentration previously obtained for the high tensile steel plate; and a hydrogen diffusion analysis unit that performs a hydrogen diffusion analysis on the press-formed product based on the stress distribution calculated by the stress distribution and strain distribution calculation unit and the hydrogen concentration distribution set by the hydrogen concentration distribution setting unit, and calculates a hydrogen concentration distribution after hydrogen diffusion. and a delayed fracture occurrence prediction unit that acquires delayed fracture judgment conditions that indicate the relationship between the occurrence of delayed fracture and the stress, strain, and water content at a stress gradient corresponding to the stress gradient in the direction in which the crack propagates, the delayed fracture judgment conditions being acquired for the portion at which delayed fracture is suspected, and the stress, strain, and hydrogen content after hydrogen diffusion at the portion at which delayed fracture is suspected being obtained from the stress distribution and strain distribution calculated by the stress distribution and strain distribution calculation unit and the hydrogen content distribution after hydrogen diffusion calculated by the hydrogen diffusion analysis unit.

7. A delayed fracture prediction device for press-formed products as described in claim 6, wherein the stress distribution and strain distribution calculation unit calculates an equivalent plastic strain distribution as the strain distribution, and the hydrogen concentration distribution setting unit acquires in advance a relational equation between equivalent plastic strain and hydrogen concentration, and sets a hydrogen concentration distribution corresponding to the equivalent plastic strain distribution based on the acquired relational equation between equivalent plastic strain distribution and hydrogen concentration.

8. A program for predicting delayed fracture of a press-formed product that predicts a portion where delayed fracture will occur in a press-formed product of a high-tensile steel plate, comprising: a computer; a stress distribution and strain distribution calculation unit that calculates a stress distribution and a strain distribution of the press-formed product; a stress gradient calculation unit that identifies a portion of the press-formed product with a high maximum principal stress from the calculated stress distribution of the press-formed product as a portion at risk of delayed fracture where delayed fracture is a concern, and calculates a stress gradient in a direction perpendicular to the maximum principal stress in the identified portion at risk of delayed fracture as a stress gradient in a direction in which a crack will propagate due to delayed fracture; a hydrogen concentration distribution setting unit that sets a hydrogen concentration distribution corresponding to the strain distribution of the press-formed product calculated by the stress distribution and strain distribution calculation unit, based on a relational equation between strain and hydrogen concentration previously obtained for the high-tensile steel plate; and a hydrogen diffusion analysis unit that performs a hydrogen diffusion analysis of the press-formed product based on the stress distribution calculated by the stress distribution and strain distribution calculation unit and the hydrogen concentration distribution set by the hydrogen concentration distribution setting unit, and calculates a hydrogen concentration distribution after hydrogen diffusion. and a delayed fracture occurrence prediction unit that acquires delayed fracture judgment conditions that indicate the relationship between the occurrence of delayed fracture and the stress, strain, and water content at a stress gradient corresponding to the stress gradient in the direction in which the crack propagates, the delayed fracture judgment conditions being calculated for the region at which delayed fracture is suspected, and predicts whether or not delayed fracture will occur in the region at which delayed fracture is suspected, based on the acquired delayed fracture judgment conditions, the stress distribution and strain distribution calculated by the stress distribution and strain distribution calculation unit, and the hydrogen concentration distribution after hydrogen diffusion calculated by the hydrogen diffusion analysis unit.

9. A program for predicting delayed fracture of a press-formed product as described in claim 8, wherein the stress distribution and strain distribution calculation unit calculates an equivalent plastic strain distribution as the strain distribution, and the hydrogen concentration distribution setting unit acquires in advance a relational equation between equivalent plastic strain and hydrogen concentration, and sets a hydrogen concentration distribution corresponding to the equivalent plastic strain distribution based on the acquired relational equation between equivalent plastic strain distribution and hydrogen concentration.

10. A manufacturing method for a press-formed product which manufactures a press-formed product using a high tensile steel plate while suppressing the occurrence of delayed fracture, comprising: a tentative press-forming condition setting step for setting tentative press-forming conditions for the press-formed product; a delayed fracture occurrence determination step for predicting whether or not delayed fracture will occur in the portion of the press-formed product identified for the press-formed product by a delayed fracture prediction method for a press-formed product according to any one of claims 1 to 5 based on the tentative press-forming conditions, and determining whether or not delayed fracture will occur in the press-formed product based on the predicted result; a tentative press-forming condition changing step for changing the tentative press-forming conditions when it is determined that delayed fracture has occurred in the press-formed product in the delayed fracture occurrence determination step; and a repeating step for repeatedly executing the tentative press-forming condition changing step and the delayed fracture occurrence determination step until it is determined that delayed fracture has not occurred in the press-formed product in the delayed fracture occurrence determination step, a press-forming condition determination step of, when it is determined in the delayed fracture occurrence determination step that delayed fracture has not occurred in the press-formed product, determining the tentative press-forming conditions in that case as press-forming conditions; and a press-forming step of press-forming the high-tensile steel sheet into the press-formed product under the determined press-forming conditions.

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