Delayed fracture prediction method, device and program of press molded product, and manufacturing method of press molded product

Abstract

To provide a delayed fracture prediction method, a device and a program of a press molded product, and a manufacturing method of a press molded product in which a portion where a delayed fracture occurs is predicted in a press molded product of a high strength steel plate.SOLUTION: A delayed fracture prediction method of a press molded product comprises the steps of: calculating stress distribution of the press molded product and distortion distribution thereof (S1); calculating stress gradient in a crack propagation direction due to delayed fracture at a delayed fracture risk portion of the press molded product (S3); configuring hydrogen concentration distribution corresponding to the distortion distribution of the press molded product (S5); performing hydrogen diffusion analysis of the press molded product based on the stress distribution and the hydrogen concentration distribution and calculating a hydrogen concentration distribution after hydrogen diffusion (S7); and predicting whether the delayed fracture occurs at the delayed fracture risk portion or not based on a delayed fracture determination condition on the basis of stress, distortion and hydrogen concentration at the stress gradient of the delayed fracture risk portion and stress, distortion and hydrogen concentration at the delayed fracture risk portion (S9).SELECTED DRAWING: Figure 1

Description

[Technical field]

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

[0002] In response to stricter environmental regulations on carbon dioxide emissions, etc., there is a demand for lighter car bodies to improve fuel efficiency. At the same time, there is also a demand for improved collision safety performance for car bodies. In response to these needs, the use of high-tensile steel plates with tensile strength of 1 GPa or more in car frame parts is progressing. Automotive structural components are generally manufactured by press forming. However, press-formed parts made of high-tensile steel plates with tensile strengths of over 980 MPa are susceptible to delayed fracture due to distortion and residual stress that occur during the press forming and part assembly processes, as well as hydrogen that enters the body during the manufacture and use of the vehicle.

[0003] Therefore, several methods have been proposed to evaluate the delayed fracture properties of press-formed high-tensile steel sheets (high strength later). For example, Patent Document 1 discloses a method for evaluating delayed fracture properties by placing a test piece of deep-drawn high-tensile steel plate in a hydrogen penetration environment and observing the state of crack generation in the flange portion of the test piece. Furthermore, Patent Document 2 discloses a method of introducing hydrogen into a test piece of a steel material to which plastic strain has been imparted, and evaluating the hydrogen embrittlement properties (delayed fracture properties) based on the amount of plastic strain. Furthermore, Patent Document 3 discloses a method for evaluating delayed fracture properties at a preselected evaluation portion of a high-strength steel plate product by determining the amount of hydrogen corresponding to the strain of crystal grains in the structure of the evaluation portion. This method determines the amount of hydrogen corresponding to the strain of crystal grains in the evaluation portion by using a relationship that correlates the amount of hydrogen contained in the steel material when delayed fracture occurs, the residual stress, and the strain of crystal grains in the structure. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2019-174124 A [Patent Document 2] JP 2020-41838 A [Patent Document 3] JP 2011-33600 A Summary of the Invention [Problem to be solved by the invention]

[0005] 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 plate, and does not take into account the effect of strain on the occurrence of delayed fracture. In addition, the method disclosed in Patent Document 2 evaluates the delayed fracture properties (hydrogen embrittlement properties) of steel materials based on the amount of plastic strain, and has the problem that it cannot take into account the effects of stress distribution and hydrogen concentration distribution on the occurrence of delayed fracture in press-formed products. Furthermore, the method disclosed in Patent Document 3 selects an evaluation portion for evaluating delayed fracture properties in a steel sheet formed product, measures the residual stress, grain distortion, and hydrogen content at the evaluation portion, and uses the grain distortion, residual stress, and hydrogen content as criteria for determining the occurrence of delayed fracture. However, this method has a problem in that it takes a huge amount of time and cost to measure the grain distortion and hydrogen content of the entire press-formed product in the case of a large part such as a press-formed product.

[0006] In both of the methods disclosed in Patent Document 1 and Patent Document 3, a delayed fracture test is performed on an actual press-formed steel sheet, and the delayed fracture property of a specific portion of the press-formed steel sheet is evaluated based on the results of the delayed fracture test. This requires a lot of time and cost, and it is difficult to predict which portion of a press-formed product after press forming or assembly of parts 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. In addition, it is known that the strain gradient (stress gradient) affects cracking in press-formed products (for example, Ishiwata Akinobu, Urabe Masaki, Inazumi Toru, "Press Forming Analysis Technology Contributing to Expanded Application of High Tensile Steel," JFE Technical Report, No. 30 (August 2012), pp. 19-24.), but none of Patent Documents 1 to 3 evaluates the effect of the strain gradient (or stress gradient) on delayed fracture.

[0007] The present invention has been made to solve the above-mentioned problems, and has an object to provide a method, an apparatus, and a program for predicting a location at which delayed fracture will occur in a press-formed product using a high-tensile steel plate. Furthermore, the present invention aims to provide a method for manufacturing a press-molded product, which determines press-molding conditions so that delayed fracture does not occur in a portion where delayed fracture is predicted to occur by the above-mentioned method, and manufactures a press-molded product under the determined press-molding conditions. [Means for solving the problem]

[0008] (1) A method for predicting delayed fracture of a press-formed product according to the present invention predicts a location where delayed fracture will occur in a press-formed product of a high-tensile steel plate, A stress distribution and strain distribution calculation step of calculating a stress distribution and a strain distribution of the press-molded product; a stress gradient calculation step of identifying a portion of the press-formed product having a high maximum principal stress as a portion of concern of delayed fracture where delayed fracture is likely to occur based on the calculated stress distribution of the press-formed product, and calculating a stress gradient in a direction perpendicular to the maximum principal stress in the identified portion of concern of delayed fracture as a stress gradient in a direction in which a crack propagates 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 equation between strain and hydrogen concentration previously obtained for the high-tensile steel plate; a hydrogen diffusion analysis step of performing a hydrogen diffusion analysis on the press-molded 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; and a delayed fracture occurrence prediction process for predicting whether or not delayed fracture will occur in the portion at which delayed fracture is suspected, based on the acquired delayed fracture judgment conditions, the stress, strain, and hydrogen concentration after hydrogen diffusion in the portion at which delayed fracture is suspected, which are obtained from the stress distribution and strain distribution calculated in the stress distribution and strain distribution calculation process and the hydrogen concentration distribution after hydrogen diffusion calculated in the hydrogen diffusion analysis process.

[0009] (2) In the above (1), The delayed fracture judgment condition obtained in the delayed fracture occurrence prediction step is characterized in that 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 in a state where 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 a relationship is obtained 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.

[0010] (3) In the above (1), The delayed fracture judgment condition obtained in the delayed fracture occurrence prediction process is characterized in that 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 in which the crack propagates, calculated for the portion at which delayed fracture is suspected, 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 in a state in which the stress gradient is applied, and a relationship is obtained 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.

[0011] (4) In any one of (1) to (3) above, In the stress distribution and strain distribution calculation step, an equivalent plastic strain distribution is calculated as the strain distribution, The hydrogen concentration distribution setting process is characterized in that 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.

[0012] (5) In any one of (1) to (4) above, The high tensile steel plate is characterized in that it has a tensile strength of 1 GPa or more.

[0013] (6) The delayed fracture prediction device for a press-formed product according to the present invention predicts a location where delayed fracture will occur in a press-formed product of a high-tensile steel plate, A stress distribution and strain distribution calculation unit that calculates a stress distribution and a strain distribution of the press-molded product; a stress gradient calculation unit that identifies a portion of the press-formed product having a high maximum principal stress as a portion of concern for delayed fracture where delayed fracture is a concern 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 portion of concern for delayed fracture as a stress gradient in a direction in which a crack propagates 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; a hydrogen diffusion analysis unit that performs a hydrogen diffusion analysis on the press-molded 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, calculated for the portion at which delayed fracture is suspected, and predicts whether or not delayed fracture will occur in the portion at which delayed fracture is suspected, based on the acquired delayed fracture judgment conditions, the stress, strain, and the hydrogen concentration after hydrogen diffusion in the portion at which delayed fracture is suspected 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.

[0014] (7) In the above (6), The stress distribution and strain distribution calculation unit calculates an equivalent plastic strain distribution as the strain distribution, The hydrogen concentration distribution setting unit is characterized in that it acquires a relational equation 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 equation between equivalent plastic strain distribution and hydrogen concentration.

[0015] (8) A program for predicting delayed fracture of a press-formed product according to the present invention predicts a location where delayed fracture will occur in a press-formed product of a high-tensile steel plate, Computer, A stress distribution and strain distribution calculation unit that calculates a stress distribution and a strain distribution of the press-molded product; a stress gradient calculation unit that identifies a portion of the press-formed product having a high maximum principal stress as a portion of concern for delayed fracture where delayed fracture is a concern 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 portion of concern for delayed fracture as a stress gradient in a direction in which a crack propagates 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; a hydrogen diffusion analysis unit that performs a hydrogen diffusion analysis on the press-molded 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; The delayed fracture occurrence prediction unit 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, which is calculated for the portion at which delayed fracture is suspected, and the occurrence of delayed fracture, and predicts whether or not delayed fracture will occur in the portion at which delayed fracture is suspected based on the acquired delayed fracture judgment conditions and the stress, strain, and hydrogen concentration after hydrogen diffusion in the portion at which delayed fracture is suspected 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.

[0016] (9) In the above (8), The stress distribution and strain distribution calculation unit calculates an equivalent plastic strain distribution as the strain distribution, The hydrogen concentration distribution setting unit is characterized in that it acquires a relational equation 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 equation between equivalent plastic strain distribution and hydrogen concentration.

[0017] (10) A method for manufacturing a press-formed product according to the present invention is for suppressing the occurrence of delayed fracture in a press-formed product using a high-tensile steel plate, and for manufacturing the press-formed product, A temporary press molding condition setting process for setting temporary press molding conditions for the press-molded product; a delayed fracture occurrence determination process for predicting whether or not delayed fracture will occur in the portion of the press-formed product identified in the press-formed product by a delayed fracture prediction method for a press-formed product according to any one of (1) to (5) above based on the provisional press-forming conditions, and determining whether or not delayed fracture will occur in the press-formed product based on the predicted result; a temporary press-molding condition changing step of changing the temporary press-molding conditions when it is determined that delayed fracture has occurred in the press-molded product in the delayed fracture occurrence determining step; a repeating step of repeatedly executing the temporary press-forming condition changing step and the delayed fracture occurrence determining step until it is determined that no delayed fracture has occurred in the press-formed product in the delayed fracture occurrence determining step; a press-molding condition determination step of determining, when it is determined that no delayed fracture has occurred in the press-molded product in the delayed fracture occurrence determination step, the provisional press-molding conditions in that case as press-molding 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. Effect of the Invention

[0018] According to the present invention, it is possible to identify a portion of the press-formed product having a high maximum principal stress as a portion at which delayed fracture is likely to occur, based on the stress distribution of the press-formed product, and to predict whether delayed fracture will occur during use of a press-formed product of a high-tensile steel plate, taking into consideration the stress gradient in the direction in which a crack propagates due to delayed fracture in the identified portion at which delayed fracture is likely to occur. 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 design stage of the part, 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.

[0019] Furthermore, in the present invention, the presence or absence of delayed fracture in a press-formed product of a high tensile steel plate is judged, and press-forming conditions that suppress the occurrence of delayed fracture are determined based on the judgment result. 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 plate. Furthermore, by press-forming the press-formed product under the press-forming conditions thus determined, it is possible to manufacture a press-formed product in which measures have been taken to suppress the occurrence of delayed fracture. [Brief description of the drawings]

[0020] [Figure 1] FIG. 1 is a flowchart showing a process flow of a delayed fracture prediction method for a press-formed product according to a first embodiment of the present invention. [Diagram 2] FIG. 1 shows the results of an investigation into the relationship between crack initiation sites and hydrogen concentration distribution in uniaxial tensile test pieces of high-tensile steel plate ((a) uniaxial tensile test piece, (b) crack initiation sites, (c) hydrogen concentration distribution). [Diagram 3] This graph shows the relationship between strain and hydrogen concentration obtained by a hydrogen charging test using test pieces to which strain was applied in various deformation modes, with an immersion time of 30 hours ((a) relationship between equivalent plastic strain and hydrogen concentration, (b) relationship between strain mode and hydrogen concentration). [Figure 4]1 is a graph showing the relationship between equivalent plastic strain and hydrostatic stress when a uniaxial tensile test piece was immersed for 30 hours in a constant load hydrogen charging test. [Diagram 5] This is a graph showing the relationship between the equivalent plastic strain and stress and the occurrence of delayed fracture when the immersion time of a uniaxial tensile test piece is set to 30 hours and 20 hours in a delayed fracture test ((a) immersion time 30 hours, (b) immersion time 20 hours). [Figure 6] 1 is a graph showing the relationship between the equivalent plastic strain and hydrogen concentration of a uniaxial tensile test piece measured after immersion times of 30 hours, 20 hours, and 10 hours in a delayed fracture test. [Figure 7] 1 is a graph showing delayed fracture judgment criteria under constant stress conditions (1100 MPa, 1000 MPa, 900 MPa) obtained from delayed fracture test results. [Figure 8] FIG. 2 is a diagram illustrating a uniaxial tensile test specimen having a notch, which was used in a delayed fracture test. [Figure 9] 1 is a graph showing the relationship between the notch R (punching radius R) of a notched portion and the stress gradient when a uniaxial tensile test piece having a notched portion is subjected to uniaxial tensile stress in a delayed fracture test. [Figure 10] 1 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 piece having a notch. [Figure 11] FIG. 1 is a diagram illustrating a delayed fracture test in which a four-point bending test piece, to which a stress gradient has been applied in the plate thickness direction by a four-point bending test, is immersed in a hydrogen penetration environment for a predetermined period of time. [Figure 12] 1 is an example of a graph showing the relationship between sheet thickness and stress gradient in a four-point bending test piece in which a stress gradient is applied in the sheet thickness direction by a four-point bending test. [Figure 13] 1 is a diagram showing a configuration of a delayed fracture prediction device and program for a press-formed product according to a first embodiment of the present invention. [Figure 14] FIG. 11 is a flowchart showing a process flow of a method for manufacturing a press-molded product according to a second embodiment of the present invention. [Figure 15]1A and 1B are diagrams for explaining press-formed products that are targets for prediction of delayed fracture and manufacture in Examples 1 and 2 ((a) is a perspective view, (b) is a cross-sectional view). [Figure 16] 2A and 2B are contour diagrams 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). [Figure 17] FIG. 1 is a contour diagram showing the hydrogen concentration distribution before hydrogen diffusion calculated for the press-molded 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)). [Figure 18] FIG. 1 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)). [Figure 19] 1A and 1B are graphs showing the results of predicting whether delayed fracture will occur in a region of a press-formed product where delayed fracture is a concern in Example 1 ((a) region of a press-formed product where delayed fracture is a concern, (b) graph showing the prediction of delayed fracture occurrence using hydrogen concentration corresponding to equivalent plastic strain, (c) graph showing the prediction of delayed fracture occurrence using hydrostatic stress and hydrogen concentration corresponding to equivalent plastic strain). [Figure 20] 13 is a graph showing the results of predicting whether delayed fracture occurs in a bent portion of a press-formed product in Example 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] <Background to the invention> The inventors, in examining a method for predicting a site where delayed fracture will occur in a press-formed product of a high-tensile steel plate, conducted an examination by a delayed fracture test of a high-tensile steel plate under uniaxial tensile stress.

[0022] First, a hydrogen charging test was carried out on a uniaxial tensile test piece 21, which was a test material made of high-tensile steel plate shown in Fig. 2(a), in order to measure the amount of hydrogen in the uniaxial tensile test piece 21 after placing it in a hydrogen intrusion environment for a certain period of time while a constant load of uniaxial tensile stress was applied. Furthermore, a uniaxial tensile test was carried out in which the uniaxial tensile test piece 21 was held in the hydrogen intrusion environment until cracks (delayed fracture) occurred (hereinafter referred to as a hydrogen charging test under constant load).

[0023] In a normal uniaxial tensile test in which a constant load is applied to the uniaxial tensile test piece 21 without placing it in a hydrogen penetration environment, a crack occurs near the center of the parallel portion 23 of the uniaxial tensile test piece 21. However, in the uniaxial tensile test piece 21 subjected to a hydrogen charging test under a constant load, a crack occurs 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 FIG.

[0024] The inventors believed that the hydrogen concentration distribution in the uniaxial tensile test piece 21 was related to this cause, and carried out a hydrogen diffusion analysis taking into account the hydrostatic pressure distribution, and investigated the correspondence between the crack occurrence site in the uniaxial tensile test piece 21 and the hydrogen concentration distribution after hydrogen diffusion.

[0025] FIG. 2(c) shows the hydrogen concentration distribution after hydrogen diffusion, which was obtained by hydrogen diffusion analysis. In the uniaxial tensile test piece 21, hydrogen is concentrated at the boundary between the R section 25 and the parallel section 23 due to the distribution of hydrostatic pressure, and it can be seen that the area with high hydrogen concentration corresponds to the crack initiation area (see Fig. 2(b)). From this result, it was concluded that it is important to consider the water concentration distribution after hydrogen diffusion in order to predict delayed fracture.

[0026] Next, in order to predict the occurrence of delayed fracture in press-formed products, the relationship between the amount of hydrogen that penetrates into the press-formed product and the strain of the press-formed product was also examined. When high-tensile steel sheets are press-formed, various deformation modes exist in the press-formed product, such as bending (plane strain), extension (biaxial tension), and uniaxial tension. Therefore, high-tensile steel sheets (for example, 1470 MPa cold-rolled steel sheets, sheet thickness 1.2 mm) were used as test materials, and test specimens were created in which strain was imparted in various deformation modes, such as rolling (plane strain), uniaxial tension, uniaxial compression, and biaxial tension-compression. The strained test specimens were then placed in a hydrogen intrusion environment for a certain period of time, after which a hydrogen charging test was conducted to measure the amount of hydrogen in the test specimens.

[0027] Figure 3 shows an example of the relationship between the hydrogen concentration and strain after immersing a test piece with strain applied in various deformation modes in a 0.1% ammonium thiocyanate and McIlvaine buffer solution with a pH of 4.0 for 30 hours to measure the amount of hydrogen in the test piece. Here, Figure 3(a) shows the equivalent plastic strain ε 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 at a constant strain (=0.01, 0.02) is shown in Fig. 3(b). Regarding the relationship between the ratio of the maximum principal strain to the minimum principal strain (hereafter referred to as the "strain ratio") and the deformation mode in Fig. 3(b), a strain ratio of 1 represents biaxial tension, a strain ratio of 0 represents plane strain, and a strain ratio of -2 represents uniaxial tension and uniaxial compression.

[0028] Based on the results shown in FIG. 3, the inventors concluded that the hydrogen concentration is proportional to the equivalent plastic strain ε p eq The reason why the hydrogen concentration increases with increasing equivalent plastic strain is believed to be because the number of defects in the test specimens that trap hydrogen increases with increasing equivalent plastic strain, and the amount of hydrogen that penetrates into the steel increases. Based on the results shown in FIG. 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 an equivalent plastic strain relationship equation, for example, as shown in equation (1).

[0029]

number

[0030] Additionally, in the constant load hydrogen charging test, the relationship between the amount of hydrogen that penetrates into the press-formed product and the stress distribution in the press-formed product was also investigated. In the uniaxial tensile test piece 21, hydrostatic stress occurs near the boundary between the R portion 25 where the crack occurred and the parallel portion 23. Here, hydrostatic stress refers to the average stress defined as the average of the normal stresses in three directions acting on the surface of the object. When hydrostatic stress occurs in the tensile direction, it is thought that the crystal lattice in the steel opens, the amount of hydrogen dissolved in the crystal lattice increases, and the amount of hydrogen penetrating into the steel increases.

[0031] Therefore, as a representative example of a hydrogen charging test under constant load, a high-tensile steel plate (as an 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, 300 MPa, which is converted to hydrostatic stress of 300 MPa, 200 MPa, 100 MPa) was applied in the longitudinal direction of the parallel portion 23 of the uniaxial tensile test piece 21. The parallel portion 23 was then immersed in a pH of 4.0, 0.1% ammonium thiocyanate and McIlvaine buffer solution for 30 hours, and the amount of hydrogen in the parallel portion 23 of the uniaxial tensile test piece 21 was measured.

[0032] 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 C that dissolves between the crystal lattices as shown in formula (2). p and the tensile hydrostatic stress p. The hydrogen concentration C p Increase in ΔC p is expressed by equation (3).

[0033]

number

[0034] Furthermore, as factors related to delayed fracture of press-formed products, excluding environmental factors such as temperature, it is considered that the residual stress, in addition to the amount of hydrogen and strain in the press-formed product, is 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.

[0035] Therefore, a test was conducted to evaluate the occurrence of cracks by changing the level of hydrogen that penetrated into the uniaxial tensile test specimen, which was made of high-tensile steel plate that had been subjected to rolling strain (equivalent plastic strain) and was subjected to a constant uniaxial tensile stress load (hereinafter referred to as a delayed fracture test).

[0036] Figure 5 shows the results of a delayed fracture test in which a uniaxial tensile test piece made of cold-rolled steel sheet (tensile strength 1470 MPa, sheet thickness 1.2 mm) with rolling strain was immersed in ammonium thiocyanate and McIlvaine buffer solution with a pH of 4.0 and a concentration of 0.1% for a specified period of time. As an example, Figure 5 is a graph showing the relationship between stress and equivalent plastic strain and the occurrence of delayed fracture when the immersion times are 30 and 20 hours. In Figure 5, the circles indicate the results of plotting equivalent plastic strain and stress of uniaxial tensile test pieces in which delayed fracture did not occur, and the crosses indicate the results of plotting equivalent plastic strain and stress of uniaxial tensile test pieces in which delayed fracture occurred. Based on the delayed fracture test results, the delayed fracture criterion was derived, which is given by the relationship between equivalent plastic strain and stress shown in Figure 5. In Figure 5, the region where equivalent plastic strain and stress are higher than the delayed fracture criterion (gray region in the figure) indicates the region where delayed fracture occurs in the uniaxial tensile test specimen. In the results shown in Fig. 5, cracks occurred at lower stress and lower equivalent plastic strain after 30 hours of immersion compared to 20 hours of immersion, indicating that the criteria for determining delayed fracture differ depending on the immersion time.

[0037] Figure 6 shows the relationship between hydrogen concentration and equivalent plastic strain when uniaxial tensile test pieces with various equivalent plastic strains were immersed in ammonium thiocyanate and McIlvaine buffer solution with a pH of 4.0 and a concentration of 0.1% for a set time (30 hours, 20 hours, 10 hours) and the amount of hydrogen was measured. As shown in Figure 6, it can be seen that the longer the immersion time under the same stress and strain conditions, the higher the hydrogen concentration. And, from 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 cracks (delayed fracture) occur.

[0038] Figure 7 shows the delayed fracture judgment conditions (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.

[0039] The above results (Figures 4 to 7) show that in order to accurately determine which location in a press-formed product using high-tensile steel plate will cause delayed fracture, it is effective to accurately predict not only the stress and strain distribution in the press-formed product, but also the hydrogen concentration distribution.

[0040] 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.

[0041] Therefore, a delayed fracture test was carried out using a uniaxial tensile test piece 27 having a notch 29 in the parallel portion 23 as shown in Fig. 8. Here, a high tensile steel plate (1470 MPa class cold rolled steel plate, 1.2 mm thick, as an example) 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 concentration of 0.1% and a pH of 4.0 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 was 1000 MPa.

[0042] 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 direction in which the crack propagates. Here, the stress gradient in the direction in which the crack propagates is 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 ), y is the stress gradient (MPa / mm) and x is the notch radius (mm).

[0043] As shown in FIG. 9, the smaller the notch R of the notch portion 29, the larger the stress gradient. The notch R=0 corresponds to the uniaxial tensile test piece 21 shown in FIG. 2(a) in which no notch is formed, and the stress gradient in the parallel portion 23 of the uniaxial tensile test piece 21 is 0. The uniaxial tensile test piece 21 in which the notch R approaches 0 is theoretically considered to correspond to the uniaxial tensile test piece 27 in which the notch portion 29 of notch R→∞ is formed. Therefore, it is considered that the stress gradient approaches 0 when the notch R of the notch portion 29 of the uniaxial tensile test piece 27 approaches 0 when the notch R of the notch portion 29 approaches ∞. From this, the relationship between the notch R of the notch portion 29 of the uniaxial tensile test piece 27 and the stress gradient in the direction in which the crack propagates can be expressed using an exponential function formula, for example, as shown in FIG. 9.

[0044] FIG. 10 shows the relationship between the stress gradient in the direction in which a crack propagates due to delayed fracture and the delayed fracture time, obtained by a delayed fracture test using a uniaxial tensile test piece 27 having a notch 29 formed therein. As shown in Fig. 10, the larger the stress gradient in the direction of crack propagation, the longer the delayed fracture time, which indicates that the occurrence of delayed fracture takes longer. This is believed to be because the surrounding areas with low stress suppress the deformation of the areas with high stress where the crack occurs, lengthening the time it takes for the crack to occur.

[0045] From the results shown in Figures 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 a crack propagates due to delayed fracture.

[0046] Furthermore, the inventors thought that when a portion where delayed fracture will occur is predicted based on the stress, strain, hydrogen concentration, and stress gradient in a press-formed product, the press-formed product should be manufactured so that the portion has stress and strain that are predicted not to cause delayed fracture. To this end, the inventors came to the conclusion that it is possible to determine press-formed conditions that result in stress and strain that are predicted not to cause delayed fracture by repeatedly changing the press-formed conditions and predicting the occurrence of delayed fracture in the press-formed product under the changed press-formed conditions. Then, the inventors found that it is possible to manufacture a press-formed product to which measures for suppressing the occurrence of delayed fracture have been taken by performing press-formation under the press-formed conditions determined in this way.

[0047] 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.

[0048] [Embodiment 1] <Prediction method for delayed fracture of press-formed products> The method for predicting delayed fracture of a press-formed product according to the first embodiment of the present invention predicts a location where delayed fracture will occur in a press-formed product of a high-tensile steel plate. The method for predicting delayed fracture of a press-formed product according to the first embodiment 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, as illustrated in Fig. 1. Each of the above steps will be described below.

[0049] <Stress distribution and strain distribution calculation process> 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.

[0050] 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.

[0051] CAE analysis methods commonly used in automobile design and the like can be applied to calculate the stress distribution and strain distribution of a press-formed product. Examples of CAE analysis methods include press forming CAE analysis, shear CAE analysis of cutting, trimming, and piercing of sheet material before press forming, springback CAE analysis of press-formed products, and CAE analysis of assembly of press-formed products. The stress distribution and strain distribution of the press-formed product 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.

[0052] <Stress gradient calculation process> The stress gradient calculation step S3 is a step of identifying a portion of the press-formed product where the maximum principal stress is high and delayed fracture is a concern as a portion of delayed fracture 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 of delayed fracture concern as the stress gradient in the direction in which a crack propagates due to delayed fracture.

[0053] 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.

[0054] In addition, 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 in the area where delayed fracture is suspected is calculated as the stress gradient in the direction in which the crack propagates due to delayed fracture.

[0055] <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.

[0056] 1, in the present embodiment 1, a relational equation between strain and hydrogen concentration in a high tensile steel sheet used in press forming of a press-formed product (the above-mentioned equation (1)) is derived by performing a relational equation deriving step S11 between strain and hydrogen concentration prior to a hydrogen concentration distribution setting step S5. Then, based on the derived equation (1), a 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.

[0057] In the hydrogen concentration distribution setting step S5, the hydrogen concentration distribution of the press-molded product may be set by the following procedure. First, the relational equation (the above-mentioned equation (3)) between the hydrostatic stress and the hydrogen concentration in a high-tensile steel sheet used in the press forming of a press-formed product is obtained in advance. Next, the hydrostatic stress distribution is calculated from the stress distribution of the press-molded product calculated in the stress distribution and strain distribution calculation process S1, and the hydrogen concentration distribution corresponding to the hydrostatic stress distribution is calculated based on the previously obtained relational equation between the hydrostatic stress and the hydrogen concentration (the above-mentioned equation (3)). Then, the hydrogen concentration distribution of the press-formed product is 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 based on the above-mentioned formula (1) in the stress distribution and strain distribution calculation process S1.

[0058] (Process of deriving the relation 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.

[0059] Regarding the relationship between hydrogen concentration and strain in high-tensile steel plate, the hydrogen concentration depends only on the equivalent plastic strain, regardless of the deformation mode, as shown in Fig. 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 for example, it is advisable to derive the relational equation between equivalent plastic strain and hydrogen concentration expressed by the above-mentioned equation (1).

[0060] The constants a and b in formula (1) can be determined by conducting a typical hydrogen charging test. Figure 3(a) shows an example of the results of the measurement of equivalent plastic strain and hydrogen concentration determined by the hydrogen charging test, and the relational equation (a=0.52, b=3.9) between equivalent plastic strain and hydrogen concentration derived from the measurement results.

[0061] <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.

[0062] Hydrogen in a press-formed product diffuses according to the hydrogen concentration gradient and hydrostatic stress gradient. Therefore, in order to accurately predict the hydrogen concentration distribution in a press-formed product, it is necessary to solve the diffusion equation shown in the following equation (4) to redistribute hydrogen in each CAE element of the press-formed product.

[0063]

number

[0064] When stress concentration exists in a press-formed product, the hydrogen concentration when hydrogen accumulates at the stress concentration site can be accurately predicted by solving equation (2) in hydrogen diffusion analysis step S7.

[0065] <Delayed fracture occurrence prediction process> The delayed fracture occurrence prediction step S9 is a step of predicting whether or not delayed fracture will occur in the delayed fracture suspected portion identified in the stress gradient calculation step S3.

[0066] In order to predict delayed fracture in the delayed fracture occurrence prediction step S9, first, a delayed fracture judgment condition is obtained that represents the relationship between 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 area at which delayed fracture is suspected, and the occurrence of delayed fracture. Then, based on the acquired delayed fracture judgment conditions and the stress, strain, and hydrogen concentration after hydrogen diffusion in the area where delayed fracture is suspected, which are obtained from the stress distribution, strain distribution, and hydrogen concentration distribution after hydrogen diffusion, it is predicted whether delayed fracture will occur in the area where delayed fracture is suspected.

[0067] The stress and strain in the region at risk of delayed fracture are obtained 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 obtained from the hydrogen concentration distribution after hydrogen diffusion in the press-formed product calculated in the hydrogen diffusion analysis step S7. Furthermore, the stress gradient in the portion at which delayed fracture is suspected is the stress gradient in a 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 propagates due to delayed fracture.

[0068] The delayed fracture judgment condition obtained in the delayed fracture occurrence prediction step S9 is obtained by performing a delayed fracture judgment condition derivation step S13 prior to the delayed fracture occurrence prediction step S9, as shown in FIG.

[0069] (Derivation of delayed fracture criteria) The delayed fracture determination condition deriving process S13 is a process in which a delayed fracture test is performed on high-tensile steel plate to be used in press forming, and 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 is derived as the delayed fracture determination condition.

[0070] In the delayed fracture judgment condition derivation step S13, delayed fracture test pieces 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 pieces with different levels of uniaxial tensile stress and hydrogen amount (changing the immersion time in the acidic solution).Then, the relationship between the occurrence of cracks (delayed fracture) in the delayed fracture test pieces and the stress, equivalent plastic strain, hydrogen concentration in the uniaxial tensile test piece (amount of hydrogen penetrating into the steel), and the stress gradient in the direction in which the cracks propagate due to delayed fracture is obtained, and the delayed fracture judgment conditions are derived.

[0071] In a 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 specimen used in the delayed fracture judgment condition derivation process S13 has a shape such that the stress gradient in the direction of crack propagation in the delayed fracture suspected area obtained in the stress gradient calculation process S3 coincides with the stress gradient in the direction of crack propagation in the delayed fracture test specimen.

[0072] For example, when a uniaxial tensile test piece 27 (Fig. 8) with a notch 29 formed in the parallel portion 23 is used as a delayed fracture test piece, the relationship between the stress gradient in the direction in which the crack propagates and the notch R is expressed by the formula shown in Fig. 9. Therefore, the notch R of the notch 29 can be determined so as to obtain a predetermined stress gradient according to the formula shown in Fig. 9.

[0073] In the above explanation, in the delayed fracture determination condition derivation 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.

[0074] However, in the delayed fracture determination condition derivation step S13, a delayed fracture test may be performed by a four-point bending test of a four-point bending test piece 31, as shown in FIG. 11, in which a high-tensile steel plate having various equivalent plastic strains imparted thereto by rolling or the like is used as a test material.

[0075] In the four-point bending test, supports 33a and 33b are placed above the four-point bending test piece 31, and supports 33c and 33d are placed below it. Supports 33a and 33b are lowered or supports 33c and 33d are raised to perform bending processing to form a bend apex 35 on the outer side of the bend of the four-point bending test piece 31. In the four-point bending test, the raising and lowering of the support is stopped when a predetermined amount of pressing (amount of lifting and lowering of the support) is reached. At that time, the bend apex 35 on the outer side of the bend of the four-point bending test piece 31 is the part with the maximum tensile stress, and the tensile stress of the bend apex 35 can be changed to various levels depending on the amount of pressing.

[0076] In the 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 to the inside of the bend of the four-point bending test piece 31. In the four-point bending test piece 31 bent at four points, a crack propagates in the thickness direction from the bend apex on the outside of the bend, and the relationship between the stress gradient in the crack propagation direction and the thickness t of the four-point bending test piece 31 is expressed by an equation as 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 by a four-point bending test, by fixing the indentation amount of the supports 33a to 33d into the four-point bending test piece 31 and changing the plate thickness of the four-point bending test piece 31, a desired stress gradient can be imparted to the bend apex 35 as shown in FIG. 12.

[0077] In such a delayed fracture test by a four-point bending test, for example, a four-point bending test jig 41 shown in FIG. 11(c) may be used (see JP 2017-179589 A).

[0078] The four-point bending test jig 41 has fulcrums 45a and 45b made of insulating glass that are provided on a sample stage 43 and arranged above the four-point bending test piece 31, and fulcrums 45c and 45d made of insulating glass that are arranged below the four-point bending test piece 31. Furthermore, the four-point bending test jig 41 has a screw 47 that is provided so as to be able to push the fulcrums 45c and 45d upward.

[0079] In four-point bending using the four-point bending test jig 41, first, the supports 45c and 45d are pressed into the four-point bending test piece 31 by the screw 47, and the screw 47 is fixed with a pressing amount that gives a desired tensile stress on the outside of the bend. This gives the four-point bending test piece 31 a tensile stress and a stress gradient in the plate thickness direction. Next, the four-point bending test piece 31 is immersed for a predetermined time in a hydrogen intrusion environment (e.g., pH=4.0, 0.1% ammonium thiocyanate and McIlvaine buffer solution) together with the four-point bending test jig 41 with the screw 47 fixed at a predetermined pressing amount. Furthermore, by changing the time of immersion in the hydrogen intrusion environment, the amount of hydrogen invading the four-point bending test piece 31 is changed to various levels, and the presence or absence of cracking in the four-point bending test piece 31 is evaluated.

[0080] Even in delayed fracture tests using such a four-point bending test jig, it is possible to derive the delayed fracture judgment criteria that express the relationship between the stress, strain, hydrogen concentration, and stress gradient in the direction in which the crack propagates, and the presence or absence of delayed fracture.

[0081] As described above, according to the method for predicting delayed fracture of a press-formed product of this embodiment 1, based on the stress distribution of the press-formed product, areas in the press-formed product where the maximum principal stress is high are identified as areas at risk of delayed fracture where delayed fracture is a concern, and by taking into account the stress gradient in the direction in which a crack propagates due to delayed fracture in the identified areas at risk of delayed fracture, it is possible to accurately predict whether delayed fracture will occur during use of a press-formed product made of high-tensile steel plate. 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 part in the press-formed product where delayed fracture will occur. 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, so as to prevent delayed fracture at the design stage of the press-formed product.

[0082] <Prediction device for delayed fracture of press-formed products> A delayed fracture prediction device 1 for a press-formed product according to a first embodiment of the present invention (hereinafter referred to as "delayed fracture prediction device 1") predicts a location where delayed fracture will occur in a press-formed product of a high-tensile steel plate. As shown in Fig. 13 as an example, the delayed fracture prediction device 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 occurrence prediction unit 11. The delayed fracture prediction device 1 may be configured by a CPU (Central Processing Unit) of a computer (such as a PC). In this case, the above-mentioned units function as a result of the CPU of the computer executing a predetermined program.

[0083] <Stress distribution and strain distribution calculation section> The stress distribution and strain distribution calculation unit 3 calculates the stress distribution and strain distribution of the press-formed product.

[0084] In the delayed fracture prediction device 1 of 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.

[0085] CAE analysis methods generally used in automobile design and the like can be applied to the calculation of the stress distribution and strain distribution of the 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 plate material before press forming, springback CAE analysis of the press-formed product, CAE analysis of assembly of the press-formed product, etc. Then, the stress distribution and strain distribution of the press-formed product can be calculated by obtaining the stress and strain for each CAE element in the press-formed product by CAE analysis using the finite element method.

[0086] <Stress gradient calculation section> 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, which is at risk of delayed fracture, 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.

[0087] The stress gradient calculation unit 5 calculates the maximum principal stress of each CAE analysis element by using, for example, 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 having a high calculated maximum principal stress can be identified as a portion of the press-formed product at risk of delayed fracture.

[0088] <Hydrogen concentration distribution setting section> 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.

[0089] In the present embodiment 1, 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 equation (the above-mentioned equation (1)) between the equivalent plastic strain and the hydrogen concentration previously obtained for the high-tensile steel plate. This makes it possible to set the hydrogen concentration distribution corresponding to the strain distribution of the press-formed product.

[0090] The hydrogen concentration distribution setting unit 7 may set the hydrogen concentration distribution of the press-molded product by the following procedure. First, the relational equation (the above-mentioned equation (3)) between the hydrostatic stress and the hydrogen concentration in a high-tensile steel sheet used in the press forming of a press-formed product is obtained in advance. Next, the hydrostatic stress distribution is calculated from the stress distribution of the press-molded 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 equation between hydrostatic stress and hydrogen concentration (the above-mentioned equation (3)). 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, to set the hydrogen concentration distribution of the press-formed product.

[0091] <Hydrogen Diffusion Analysis Section> 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.

[0092] As described above, hydrogen in the press-formed product diffuses according to the hydrogen concentration gradient and the hydrostatic stress gradient. Therefore, the hydrogen diffusion analysis unit 9 solves the diffusion equation shown in the above-mentioned 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, and calculates the hydrogen concentration for each CAE element of the press-formed product. 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.

[0093] <Delayed fracture prediction section> The delayed fracture occurrence predicting unit 11 predicts whether or not delayed fracture will occur in the delayed fracture suspected portion identified by the stress gradient calculating unit 5 .

[0094] In order to predict delayed fracture by the delayed fracture occurrence prediction unit 11, first, a delayed fracture judgment condition is obtained, which 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 in which the crack propagates, calculated for the portion at risk of delayed fracture. As the delayed fracture judgment condition, for example, the relationship between the occurrence of delayed fracture and the stress, strain, hydrogen concentration, and the stress gradient in the direction in which the crack propagates, derived by performing the delayed fracture judgment condition derivation step S13 of the method for judging delayed fracture of a press-formed product according to the first embodiment, may be obtained.

[0095] Then, based on the acquired delayed fracture judgment conditions and the stress, strain, and hydrogen concentration after hydrogen diffusion in the area where delayed fracture is suspected, which are obtained from the stress distribution, strain distribution, and hydrogen concentration distribution after hydrogen diffusion, it is predicted whether delayed fracture will occur in the area where delayed fracture is suspected.

[0096] The stress and strain in the region where delayed fracture is suspected are obtained from the stress distribution and strain distribution of the press-formed product calculated by the stress and strain distribution calculation unit 3. Moreover, the hydrogen concentration after hydrogen diffusion in the region where delayed fracture is suspected is obtained 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 portion at which delayed fracture is suspected is set to the stress gradient in a 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.

[0097] The stress, strain, and hydrogen concentration after hydrogen diffusion in the area at risk of delayed fracture may be the stress, strain, and hydrogen concentration after hydrogen diffusion calculated for the CAE analysis element with high maximum principal stress identified by the stress gradient calculation unit 5 as the area at risk of delayed fracture.

[0098] <Program for predicting delayed fracture in high-tensile steel plates> 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 delayed fracture prediction program for a press-formed product according to the first embodiment (hereinafter referred to as the "delayed fracture prediction program") predicts a portion where delayed fracture will occur in a press-formed product of a high tensile steel plate. The delayed fracture prediction program has a function of causing a computer to execute 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.

[0099] As described above, in the delayed fracture prediction device and program for a press-formed product according to the first embodiment, similarly to the delayed fracture prediction method for a press-formed product according to the first embodiment described above, 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 it is possible to predict whether delayed fracture will occur during use of a press-formed product of a high tensile steel plate by taking into consideration the stress gradient in the direction in which a crack propagates due to delayed fracture in the identified portion at risk of delayed fracture. 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 design stage of the part, 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.

[0100] In addition, the method for predicting delayed fracture of press-formed products according to the above-mentioned embodiment 1 involves carrying out step S11 of deriving a relational equation between strain and hydrogen concentration, and experimentally determining the relational equation between strain and hydrogen concentration in high-tensile steel plate used in press forming through a hydrogen charging test. However, the method for predicting delayed fracture of press-formed products according to the present invention is not limited to performing step S11 of deriving the relational equation between strain and hydrogen concentration, but may also be for obtaining a known relational equation between strain and hydrogen concentration for a high-tensile steel plate of the same material (steel type, tensile strength).

[0101] Further, in the method for predicting delayed fracture of a press-formed product according to the first embodiment, the delayed fracture judgment condition is experimentally derived by carrying out the delayed fracture judgment condition deriving step S13. However, the method for predicting delayed fracture of a press-formed product according to the present invention is not limited to performing the delayed fracture judgment condition derivation step S13, but may also obtain delayed fracture judgment conditions that have already been derived for a high-tensile steel plate of the same material (steel type, tensile strength).

[0102] [Embodiment 2] <Method of manufacturing press molded products> The manufacturing method of a press-formed product according to the second embodiment of the present invention manufactures a press-formed product by suppressing the occurrence of delayed fracture in the press-formed product using a high-tensile steel plate. The manufacturing method of a press-formed product according to the second embodiment includes, as an example, 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, as shown in FIG. 14. Furthermore, the manufacturing method of a press-formed product according to the second embodiment includes a press-forming condition determination step S29 and a press forming step S31, as shown in FIG. Each of the above steps will be described below.

[0103] <Temporary press molding condition setting process> The provisional press-molding condition setting step S21 is a step of setting provisional press-molding conditions for a press-molded product.

[0104] For example, when a press-formed product having a bent portion obtained by bending a steel plate is targeted, the provisional press-forming conditions set in the provisional press-forming condition setting step S21 include the bending R (punch shoulder radius) of the bent portion. Other factors include the clearance between the punch and the die, the thickness of the steel plate, etc. In the case of the draw-bend method (tensile bending / bending extension), further factors include the die shoulder radius, the clearance of the side wall, and the blank holding force.

[0105] <Process for determining whether delayed fracture occurs> The delayed fracture occurrence determination step S23 predicts whether or not delayed fracture will occur in the portion of the press-formed product that is identified as being at risk of delayed fracture, based on the provisional press-forming conditions set in the provisional press-forming condition setting step S21. The portion of the press-formed product that is at risk of delayed fracture occurrence and the prediction of delayed fracture are performed by the method for predicting delayed fracture of a press-formed product according to the present invention described above. The delayed fracture occurrence determination step S23 is a step for determining whether or not delayed fracture will occur in the press-formed product, based on the results of the prediction of delayed fracture in the portion of the delayed fracture occurrence.

[0106] In this embodiment 2, the process S23 for determining whether or not a delayed fracture has occurred sequentially performs a stress distribution and strain distribution calculation process S1, a stress gradient calculation process S3, a hydrogen concentration distribution setting process S5, a hydrogen diffusion analysis process S7, and a delayed fracture occurrence prediction process S9, as shown in FIG. 14.

[0107] The stress distribution and strain distribution calculation step S1, the stress gradient calculation step S3, the hydrogen concentration distribution setting step S5, the hydrogen diffusion analysis step S7, and the delayed fracture occurrence prediction step S9 are the same as those in the above-described embodiment 1. However, in the stress distribution and strain distribution calculation step S1, a 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 the press-formed product press-formed under the tentative press forming conditions are calculated. Then, a shear CAE analysis, a springback CAE analysis, a CAE analysis of the assembly of the press-formed product, and the like may be performed as necessary.

[0108] Furthermore, in the delayed fracture occurrence determination step S23, if delayed fracture is predicted to occur in the delayed fracture suspected region in the delayed fracture occurrence prediction step S9, it is determined that delayed fracture has occurred in the press-formed product (S23a). On the other hand, if delayed fracture is predicted not to occur in the delayed fracture suspected region in the delayed fracture occurrence prediction step S9, it is determined that delayed fracture has not occurred in the press-formed product (S23a).

[0109] <Temporary press molding condition change process> The provisional press-forming condition changing step S25 is a step of changing the provisional 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.

[0110] The tentative press forming conditions can be changed so as to reduce the stress and strain in the area at which delayed fracture is suspected to occur in the press-formed product in the delayed fracture occurrence prediction process S9; for example, the bending radius of the bent portion that has been bent can be increased.

[0111] <Repetitive process> In the repeating step S27, the delayed fracture occurrence determination step S23 and the temporary press-forming condition changing step S25 are repeatedly performed under the tentative press-forming conditions changed in the tentative press-forming condition changing 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, when it is not determined that delayed fracture has not occurred by changing the tentative press-forming conditions only once, the tentative press-forming condition changing step S25 is also repeated.

[0112] <Press molding condition determination process> The press-forming condition determination step S29 is a step of determining, when it is determined in the delayed fracture occurrence determination step S23 that the press-formed product has not experienced delayed fracture, tentative press-forming conditions in that case as the press-forming conditions.

[0113] <Press molding process> 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.

[0114] As described above, in the manufacturing method of a press-formed product according to the second embodiment, the occurrence or non-occurrence of delayed fracture in the press-formed product is judged 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 judgment result. 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 plate. Furthermore, by press-forming the press-formed product under the press-forming conditions thus determined, it is possible to manufacture a press-formed product in which measures have been taken to suppress the occurrence of delayed fracture.

[0115] The method for predicting the occurrence of delayed fracture of 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, in which the occurrence of delayed fracture is a concern.

[0116] 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 formed by press forming high tensile steel sheets. EXAMPLES

[0117] Experiments and analyses were carried out to confirm the effects of the present invention, which will be described below.

[0118] In this Example 1, a press-formed product 51 shown in FIG. 15 was used as a target, and a location where delayed fracture would occur was predicted. 15, the press-formed product 51 has a bent portion 53 and pieces 55 extending from both ends of the bent portion 53. In the press-formed product 51, the bent portion 53 has a curvature radius of 5 mm.

[0119] 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 bent portion 53.

[0120] Next, a portion where delayed fracture will occur in the press-formed product 51 was predicted by the method for predicting delayed fracture of a press-formed product according to the first embodiment described above.

[0121] First, a press forming CAE analysis was performed on the process of bending a 1,470 MPa class cold-rolled steel sheet with a thickness of 1.2 mm into a press-formed product 51, and the 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, as shown in Figure 16.

[0122] The maximum principal stress in the bent portion 53 is the stress in the longitudinal direction of the press-formed product 51, and it can be seen from the stress distribution shown in Fig. 16(a) that the maximum principal stress is high from the outer side 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 other parts. Therefore, parts A and B in the bent portion 53 were identified as parts at risk of delayed fracture.

[0123] Next, the stress gradient in the direction perpendicular to the maximum principal stress in the area where delayed fracture is suspected, i.e., the stress gradient in the plate thickness direction at the bending apex of bent part 53, was calculated as the stress gradient in the direction in which a crack propagates due to delayed fracture. In the stress distribution of press-formed product 51 shown in Fig. 16(a), the stress gradient in the direction perpendicular to the maximum principal bending stress in parts A and B was both 466 MPa / mm.

[0124] 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.

[0125] In Case 1, first, based on the relational equation 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 so as to correspond to the equivalent plastic strain of the press-formed product 51.

[0126] On the other hand, in Case 2, first, the hydrostatic stress distribution was calculated from the stress distribution of the press-molded product 51, and the hydrogen concentration corresponding to the hydrostatic stress was calculated for each CAE analysis element of the press-molded product 51 based on the relational equation between the hydrostatic stress and hydrogen concentration shown in the above-mentioned equation (3). 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, while Figure 17(c) shows the hydrogen concentration distribution calculated by adding the hydrogen concentration corresponding to the hydrostatic stress to the hydrogen concentration corresponding to the strain (equivalent plastic strain) for each CAE analysis element.

[0127] Next, a hydrogen diffusion analysis was performed based on the stress and hydrogen concentration calculated for each CAE analysis element of the press-molded product 51, and the hydrogen concentration distribution after hydrogen diffusion was obtained by calculating the hydrogen concentration after hydrogen diffusion for each CAE analysis element. Figures 18(a) and (b) show the hydrogen concentration distribution after hydrogen diffusion in the press-molded product 51 obtained for (Case 1) and (Case 2), respectively.

[0128] Next, in order to derive the conditions for determining delayed fracture at the portions (parts A and B) where delayed fracture is suspected, 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 described above.

[0129] Regarding the shape of the uniaxial tensile test piece 27, based on the above-mentioned FIG. 9, a notch R was determined that can impart to the notch portion 29 a stress gradient equivalent to the stress gradient in the direction in which the crack propagates in parts A and B, which are areas at risk of delayed fracture. As described above, the stress gradient in the direction in which the crack propagates was 466 MPa / mm in parts A and B. Therefore, it can be seen from Fig. 9 that the notch R for imparting this stress gradient to the notch portion 29 should be set to 2.9 mm.

[0130] Next, with the above-mentioned stress gradient applied to the notch 29, the uniaxial tensile test piece 27 was immersed and held in a hydrogen penetration environment (pH=4.0, 0.1% ammonium thiocyanate and McIlvaine buffer solution) for a predetermined time. 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 the delayed fracture judgment condition.

[0131] 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 or not delayed fracture would occur at the delayed fracture-prone areas.

[0132] In 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 FIG. 19(b) was used as the delayed fracture criterion in part A. On the other hand, in 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 FIG. 19(b) was used as the delayed fracture judgment condition in part B. The relationship between the equivalent plastic strain and the hydrogen concentration when σ = 1120 MPa and σ = 980 MPa shown in Figure 19(b) was derived from a delayed fracture test using a uniaxial tensile test piece 27 in which a stress gradient (= 466 MPa / mm) was applied to the notch 29 at the area (part A, part B) where delayed fracture is suspected.

[0133] Table 1 shows the hydrogen concentrations in parts A and B in Case 1 and Case 2. In Case 2, by taking into account the hydrogen concentration distribution corresponding to the hydrostatic stress distribution and strain (equivalent plastic strain) distribution, 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.

[0134] [Table 1]

[0135] 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 (cracks) 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.

[0136] 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.

[0137] 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.

[0138] 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.

[0139] From these judgment results, in both (Case 1) and (Case 2) of the press-formed product 51, it was predicted that 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 would occur in the delayed fracture test.

[0140] 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 a high-tensile steel plate. EXAMPLES

[0141] 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.

[0142] The press-formed product 51 was obtained by press-forming a 1470 MPa-class cold-rolled steel sheet (sheet thickness: 1.2 mm), which was a high-tensile steel sheet, in the same manner as in Example 1. Then, a tentative press-forming condition was set in which the bending radius of the bent portion 53, which was feared to cause delayed fracture in Example 1, was changed from 5 mm to 7 mm. The stress at part A (see FIG. 20(a)) of bent part 53 of press-formed product 51 press-formed under the set tentative press forming conditions decreased from 1120 MPa (bending R=5 mm) to 1020 MPa (bending R=7 mm). In addition, the equivalent plastic strain at part A of bent part 53 decreased from 0.023 (bending R=5 mm) to 0.010 (bending R=7 mm).

[0143] In addition, in the bent portion 53 with a bending radius of 5 mm, the stress gradient in the direction perpendicular to the maximum principal stress in parts A and B was 466 MPa / mm. Furthermore, in the bent portion 53 with a bending radius of 7 mm, the stress gradient in the direction perpendicular to the maximum principal stress in both parts A and B was 424 MPa / mm.

[0144] Furthermore, the hydrogen concentration distribution in the press-molded product 51 was set in the same manner as in (Case 1) and (Case 2) of the above-mentioned Example 1. Here, (Case 1) is a case where a hydrogen concentration corresponding to the equivalent plastic strain of each CAE analysis element is set for each CAE analysis element. Also, (Case 2) is a case where a hydrogen concentration obtained by adding a hydrogen concentration corresponding to a hydrostatic stress calculated from stress to a hydrogen concentration corresponding to the equivalent plastic strain of each CAE analysis element is 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-molded product 51, thereby calculating the hydrogen concentration distribution after hydrogen diffusion.

[0145] The hydrogen concentrations at part A of bent portion 53 were determined from the hydrogen concentration distributions calculated for (Case 1) and (Case 2), and were 2.5 ppm and 2.7 ppm, respectively, before hydrogen diffusion and 4.0 ppm and 4.3 ppm, respectively, after hydrogen diffusion.

[0146] Next, as in Example 1 described above (see FIG. 19), delayed fracture was judged using the stress, equivalent plastic strain, and hydrogen concentration after hydrogen diffusion calculated for part A of bent portion 53. As a result, FIG. 20 shows the delayed fracture judgment condition (σ=1020 MPa, black solid line) when bending R of bent portion 53 is 7 mm, and the hydrogen concentration (★ in FIG. 20) after hydrogen diffusion analysis for (Case 1) and (Case 2). Furthermore, for comparison, FIG. 20 also shows the delayed fracture judgment condition (σ=1120 MPa, gray dashed line) when bending R of bent portion 53 of Example 1 is 5 mm, and the hydrogen concentration (☆ in FIG. 20) after hydrogen diffusion analysis for (Case 1) and (Case 2).

[0147] The criteria for determining delayed fracture at a bend R=5 mm were derived from a delayed fracture test using a uniaxial tensile test piece 27 (FIG. 8, notch R=2.9 mm) in which a stress gradient (466 MPa / mm) equivalent to the stress gradient in the direction in which a crack propagates in the bend 53 with a bend R=5 mm was applied to the notch 29. Furthermore, the criteria for determining delayed fracture at a bend R = 7 mm were derived from a delayed fracture test using a uniaxial tensile test piece 27 (Figure 8, notch R = 3.2 mm) in which a stress gradient (424 MPa / mm) equivalent to the stress gradient in the direction in which the crack propagates in the bend 53 at a bend R = 5 mm was applied to the notch 29. In the delayed fracture test in Example 2, similarly to Example 1, the uniaxial tensile test piece 27 was immersed in a hydrogen penetration environment of pH=4.0 and a 0.1% concentration of ammonium thiocyanate and McIlvaine buffer solution for a predetermined period of time.

[0148] As shown in Figure 20, by changing the bending radius of bend 53 from 5 mm to 7 mm, the delayed fracture judgment criteria were relaxed, and the hydrogen concentration after hydrogen diffusion analysis also decreased. As a result, the hydrogen concentration after hydrogen diffusion analysis when bend 53 has a bending radius of 7 mm (★ in Figure 20) is lower than the hydrogen concentration for the delayed fracture judgment criteria (σ = 1020 MPa) for a bending radius of 7 mm in both (Case 1) and (Case 2). Therefore, it was predicted that delayed fracture would not occur in bend 53 with a bending radius of 7 mm. Therefore, the tentative press forming conditions in which the bending radius of the bent portion 53 was changed to 7 mm were determined as the press forming conditions. Furthermore, the press-formed product 51 was press-formed under the determined press forming conditions, and a delayed fracture test was carried out. As a result, delayed fracture did not occur in the bent portion 53 of the press-formed product 51.

[0149] As described above, it has been demonstrated that the manufacturing method of the press-formed product according to the present invention determines press-forming conditions under which delayed fracture does not occur in high-tensile steel plate, 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. [Explanation of symbols]

[0150] 1. Delayed fracture prediction device 3 Stress 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 Cutout 31 Four-point bending test piece 33a, 33b, 33c, 33d fulcrum 35 Bend apex 41 4-point bending test jig 43 Sample stage 45a, 45b, 45c, 45d fulcrum 47 Screw 51 Press-molded products 53 Bending section 55 One side

Claims

1. A method for predicting delayed fracture of a press-formed product, which predicts a location where delayed fracture will occur in a press-formed product of a high-tensile steel plate, A stress distribution and strain distribution calculation step of calculating a stress distribution and a strain distribution of the press-molded product; a stress gradient calculation step of identifying a portion of the press-formed product having a high maximum principal stress as a portion of concern of delayed fracture where delayed fracture is likely to occur based on the calculated stress distribution of the press-formed product, and calculating a stress gradient in a direction perpendicular to the maximum principal stress in the identified portion of concern of delayed fracture as a stress gradient in a direction in which a crack propagates 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 equation between strain and hydrogen concentration previously obtained for the high-tensile steel plate; a hydrogen diffusion analysis step of performing a hydrogen diffusion analysis on the press-molded 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 a stress gradient in a direction in which the crack propagates, the delayed fracture judgment conditions being calculated 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 in the stress distribution and strain distribution calculation step and the hydrogen content distribution after hydrogen diffusion calculated in the hydrogen diffusion analysis step.

2. 2. The method for predicting delayed fracture of a press-formed product according to claim 1, wherein the delayed fracture judgment condition obtained in the delayed fracture occurrence prediction step is 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 in a state in which a stress gradient corresponding to the stress gradient in the direction in which the crack propagates, which is calculated for the portion at risk of delayed fracture, 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 obtained.

3. 2. The method for predicting delayed fracture of a press-formed product according to claim 1, wherein the delayed fracture judgment condition obtained in the delayed fracture occurrence prediction step is 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 corresponding to the stress gradient in the direction in which the crack propagates, calculated for the portion at risk of delayed fracture, is applied to a bent portion of the four-point bending test piece, and the four-point bending test piece is held in a hydrogen penetration environment in that state, and a relationship is obtained 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.

4. In the stress distribution and strain distribution calculation step, an equivalent plastic strain distribution is calculated as the strain distribution, 2. The method for predicting delayed fracture of a press-formed product according to claim 1, characterized in that, in the hydrogen concentration distribution setting process, a relational equation between equivalent plastic strain and hydrogen concentration is acquired in advance, and a hydrogen concentration distribution corresponding to the equivalent plastic strain distribution is set based on the acquired relational equation between equivalent plastic strain distribution and hydrogen concentration.

5. 2. The method for predicting delayed fracture of a press-formed product according to claim 1, 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-molded product; a stress gradient calculation unit that identifies a portion of the press-formed product having a high maximum principal stress as a portion of concern for delayed fracture where delayed fracture is a concern 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 portion of concern for delayed fracture as a stress gradient in a direction in which a crack propagates 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; a hydrogen diffusion analysis unit that performs a hydrogen diffusion analysis on the press-molded 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, calculated for the portion at which delayed fracture is suspected, and a delayed fracture occurrence prediction unit that predicts whether or not delayed fracture will occur in the portion 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.

7. The stress distribution and strain distribution calculation unit calculates an equivalent plastic strain distribution as the strain distribution, The delayed fracture prediction device for press-formed products according to claim 6, characterized in that the hydrogen concentration distribution setting unit acquires a relational equation 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 equation between equivalent plastic strain distribution and hydrogen concentration.

8. A program for predicting delayed fracture of a press-formed product, which predicts a portion where delayed fracture will occur in a press-formed product of a high-tensile steel plate, Computer, A stress distribution and strain distribution calculation unit that calculates a stress distribution and a strain distribution of the press-molded product; a stress gradient calculation unit that identifies a portion of the press-formed product having a high maximum principal stress as a portion of concern for delayed fracture where delayed fracture is a concern 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 portion of concern for delayed fracture as a stress gradient in a direction in which a crack propagates 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; a hydrogen diffusion analysis unit that performs a hydrogen diffusion analysis on the press-molded 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; a delayed fracture occurrence prediction unit that predicts whether or not delayed fracture will occur in the region at risk of delayed fracture based on the acquired delayed fracture judgment condition, the stress, strain, and hydrogen concentration after hydrogen diffusion in 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.

9. The stress distribution and strain distribution calculation unit calculates an equivalent plastic strain distribution as the strain distribution, The program for predicting delayed fracture of a press-formed product according to claim 8, characterized in that the hydrogen concentration distribution setting unit acquires a relational equation 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 equation between equivalent plastic strain distribution and hydrogen concentration.

10. A method for manufacturing a press-formed product by suppressing the occurrence of delayed fracture in a press-formed product using a high-tensile steel plate, comprising: A temporary press molding condition setting process for setting temporary press molding conditions for the press-molded product; a delayed fracture occurrence determination process for predicting whether or not delayed fracture will occur in the portion of the press-formed product identified in 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 provisional press-forming conditions, and determining whether or not delayed fracture will occur in the press-formed product based on the predicted result; a temporary press-molding condition changing step of changing the temporary press-molding conditions when it is determined that delayed fracture has occurred in the press-molded product in the delayed fracture occurrence determining step; a repeating step of repeatedly executing the temporary press-forming condition changing step and the delayed fracture occurrence determining step until it is determined that no delayed fracture has occurred in the press-formed product in the delayed fracture occurrence determining step; a press-molding condition determination step of determining, when it is determined that no delayed fracture has occurred in the press-molded product in the delayed fracture occurrence determination step, the provisional press-molding conditions in that case as press-molding 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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