Delay breakdown test method

Abstract

To provide a delay breakdown test method capable of determining delay breakdown characteristics in a delay breakdown concern part of a press-molded product.SOLUTION: A delay breakdown test method according to the present invention includes: acquiring maximum main stress, equivalent plastic strain and stress gradient perpendicular to the maximum main stress in a delay breakdown concern part of a press-molded product (S1); producing an uniaxial tensile test piece 111 that can be applied with a stress gradient corresponding to the stress gradient in the delay breakdown concern part and is previously applied with strain corresponding to the equivalent plastic strain in the delay breakdown concern part (S3A); and loading the tensile stress corresponding to the maximum main stress in the delay breakdown concern part on a notch R part 117 of the uniaxial tensile test piece 111, to conduct a delay breakdown test while applying the stress gradient corresponding to the stress gradient in the delay breakdown concern part, to acquire a crack occurrence time until the occurrence of a crack, and to determine delay breakdown characteristics in the delay breakdown concern part of the press-molded product (S7A).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a delayed fracture testing method for determining delayed fracture properties of a portion of a press-formed product where delayed fracture is a concern. [Background technology]

[0002] In response to stricter regulations on reducing carbon dioxide emissions, there is a demand for lighter automobile bodies to improve fuel efficiency. At the same time, automobile bodies are also required to have improved collision safety performance. In response to these needs, high-tensile steel plates with tensile strengths of 1 GPa or higher are increasingly being used in the structural components of automobile bodies.

[0003] Automobile body frame components are generally manufactured by press forming. However, there are concerns about delayed fracture in frame components (press-formed products) that are press-formed using high-tensile steel plates with a tensile strength exceeding 980 MPa. Delayed fracture is thought to be caused by strain and stress (residual stress) that occurs from the press forming process to the frame component assembly process, as well as hydrogen that penetrates into the frame component during the manufacturing and use of the automobile.

[0004] Therefore, several methods and devices have been proposed to evaluate the delayed fracture properties of press-formed products made using high-tensile steel sheets according to the manufacturing conditions (press-forming conditions). 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 cracks that occur 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 by applying stress to the bent portion of a thin steel plate that has been bent into a V shape and then holding the plate in a hydrogen penetration environment, based on the occurrence of cracks in the bent portion, and also discloses a stress application jig for applying stress to the bent portion. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2019-174124 [Patent Document 2] Japanese Patent Publication No. 2020-41838 [Patent Document 3] Japanese Patent Application Laid-Open No. 2010-107297 Summary of the Invention [Problem to be solved by the invention]

[0006] The method of 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. Furthermore, the method of Patent Document 2 evaluates the delayed fracture resistance (hydrogen embrittlement resistance) of steel materials based on the amount of plastic strain, and has the problem of not being able to take into account the effect of stress distribution on the occurrence of delayed fracture in press-formed products. Furthermore, although the technology of Patent Document 3 can apply a stable stress to the top of the bent portion where strain has been introduced, it is limited to evaluating the delayed fracture properties of a thin steel plate that has been subjected to V-bending. Therefore, there is a problem in that it is not possible to evaluate the delayed fracture properties of a portion where delayed fracture is a concern (a portion where delayed fracture is a concern) in a press-formed product manufactured by press working a steel plate.

[0007] It is also known that strain gradients affect cracking in press-formed products (for example, Akinobu Ishiwata, Masaki Urabe, and Toru Inazumi, "Press Forming Analysis Technology Contributing to Expanded Applications of High-Tens Steel," JFE Technical Report, No. 30 (August 2012), pp. 19-24.). Therefore, it is thought that delayed fracture in press-formed products is also affected by not only strain and stress in areas where delayed fracture is a concern, but also strain gradients, i.e., stress gradients. However, none of Patent Documents 1 to 3 evaluated the effect of stress gradients on delayed fracture.

[0008] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a delayed fracture testing method for determining delayed fracture properties in a press-formed product, taking into account the effects of strain, stress, and stress gradient at a site at which delayed fracture is likely to occur. [Means for solving the problem]

[0009] (1) The delayed fracture testing method according to the present invention is for determining the delayed fracture characteristics of a portion of a press-formed product where delayed fracture is a concern, and includes the steps of: a delayed fracture suspected portion identifying step of calculating a stress distribution and a strain distribution of the press-formed product, identifying a portion having a high maximum principal stress as the delayed fracture suspected portion, and acquiring the maximum principal stress, the equivalent plastic strain, and the stress gradient in a direction perpendicular to the maximum principal stress at the identified delayed fracture suspected portion; a test piece manufacturing process for manufacturing a test piece capable of imparting a stress gradient corresponding to the stress gradient in the region at risk of delayed fracture and having a strain corresponding to the equivalent plastic strain in the region at risk of delayed fracture imparted in advance; a delayed fracture testing process in which a tensile stress corresponding to the maximum principal stress at the portion at which delayed fracture is suspected is applied to the manufactured test piece, and the test piece is held in a predetermined hydrogen penetration environment while a stress gradient corresponding to the stress gradient at the portion at which delayed fracture is suspected is applied, and a crack initiation time until a crack is generated in the test piece is obtained; and a delayed fracture property acquisition process for determining the delayed fracture property of the press-formed product at the portion at risk of delayed fracture based on the acquired crack initiation time, the strain previously applied to the test piece, the tensile stress applied to the test piece, and the stress gradient applied to the test piece by the applied tensile stress.

[0010] (2) In the above (1), The test piece is a uniaxial tensile test piece having a notched R portion formed in the parallel portion, In the test piece manufacturing step, the strain is applied to the parallel portion in advance, and a notch radius of the notch R portion is determined based on a stress gradient at the portion at risk of delayed fracture; In the delayed fracture testing process, a uniaxial tensile stress equivalent to the maximum principal stress of the portion at risk of delayed fracture is applied to the notch R portion, and a stress gradient equivalent to the stress gradient of the portion at risk of delayed fracture is imparted.

[0011] (3) In the above (1), The test piece is a four-point bending test piece in which a bent portion is formed by four-point bending, In the test piece manufacturing step, a strain is applied to the bent portion of the four-point bending test piece in advance, and a plate thickness of the four-point bending test piece is determined based on a stress gradient at the portion at which delayed fracture is suspected; In the delayed fracture testing process, the four-point bending test piece is bent at four points, so that a bending stress equivalent to the maximum principal stress of the portion at risk of delayed fracture is applied to the outer side of the bent portion, thereby imparting a stress gradient equivalent to the stress gradient of the portion at risk of delayed fracture.

[0012] (4) The delayed fracture testing method according to the present invention is the one described in (2) or (3) above, If the stress gradient at the portion at which delayed fracture is suspected is equal to or less than a predetermined value, the delayed fracture test method described in (2) above is carried out, When the stress gradient at the portion at which delayed fracture is suspected exceeds a predetermined value, the delayed fracture testing method described in (3) above is carried out. [Effects of the Invention]

[0013] According to the present invention, by reproducing the stress, strain, and stress gradient at the portion of the press-formed product where delayed fracture is suspected in a test piece and conducting a delayed fracture test, it is possible to determine the delayed fracture properties while taking into account the effects of the stress, strain, and stress gradient at the portion of the press-formed product where delayed fracture is suspected. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a flowchart showing the processing flow of a delayed fracture testing method according to the first embodiment. [Figure 2] This graph shows an example of the relationship between strain and hydrogen concentration obtained by a hydrogen charging test using test specimens 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 3] 1A and 1B are diagrams illustrating uniaxial tensile test specimens used in delayed fracture tests ((a) a general uniaxial tensile test specimen, (b) a uniaxial tensile support specimen with a notched R portion in the parallel portion). [Figure 4] 1 is a graph showing an example of the relationship between the notch radius (notch R) of a notch R portion and the stress gradient when a uniaxial tensile stress is applied to a uniaxial tensile test piece provided with a notch R portion in a delayed fracture test. [Figure 5] 1 is a graph showing an example of the relationship between the delayed fracture time and the stress gradient in the crack propagation direction, obtained by a delayed fracture test. [Figure 6] 1A and 1B are diagrams showing an example of a delayed fracture testing apparatus used in a delayed fracture testing method according to the first embodiment ((a) delayed fracture testing apparatus, (b) detailed view of a tension jig that applies tensile stress to a uniaxial tensile test piece). [Figure 7]10A and 10B are diagrams illustrating a delayed fracture test using a four-point bending test piece in a delayed fracture test method according to the second embodiment ((b) four-point bending test piece before bending stress is applied, (c) four-point bending test piece after bending stress is applied, (c) an example of a bending jig for bending a four-point bending test piece at four points). [Figure 8] This shows the relationship between the stress gradient in the thickness direction and the plate thickness when a maximum principal stress of 1000 MPa is applied to the outside of the bend of a four-point bending test piece. [Figure 9] FIG. 1 is a diagram illustrating strain and stress caused by bending deformation in the elastic region of a four-point bending test piece. [Figure 10] 1A and 1B are diagrams showing a press-formed product from which delayed fracture properties are obtained in the examples ((a) perspective view, (b) cross-sectional view). [Figure 11] 1A and 1B are contour diagrams showing the longitudinal maximum principal stress distribution and the equivalent plastic strain distribution calculated for a press-formed product in the examples ((a) longitudinal maximum principal stress distribution, (b) equivalent plastic strain distribution). DETAILED DESCRIPTION OF THE INVENTION

[0015] [Background to the invention] As described above, in press-formed products of high-tensile steel sheets, there is a concern that delayed fracture may occur due to strain and stress that occur in the press-formed product during the press-forming process and hydrogen that penetrates into the press-formed product. Therefore, one possible measure to suppress the occurrence of delayed fracture in press-formed products is to manufacture press-formed products in which delayed fracture does not occur in regions where delayed fracture is a concern.

[0016] In order to find the manufacturing conditions (press forming conditions) for manufacturing such press-formed products, it is important to understand the delayed fracture properties of the portion at risk of delayed fracture. Therefore, the inventors have investigated a method of determining the stress and strain of the portion at risk of delayed fracture, simulating the determined stress and strain in a test piece to perform a delayed fracture test, and determining the results obtained from the delayed fracture test as the delayed fracture properties of the portion at risk of delayed fracture.

[0017] First, the inventors considered that the stress and strain in areas of a press-formed product where delayed fracture is a concern could be determined by CAE analysis using the finite element method, such as press forming analysis, shear analysis of cutting, trimming, piercing, etc. of the plate material before press forming, and springback analysis.

[0018] Next, in order to perform a delayed fracture test by simulating the stress and strain found for the portion at risk of delayed fracture in a test piece, the inventors studied the stress and strain to be reproduced in the test piece and the shape of the test piece.

[0019] In fracture mechanics, it is believed that the stress that initiates and propagates cracks is not compressive stress but tensile stress. Based on this idea, the inventors believed that in press-formed products, areas where tensile stress occurs are at risk of delayed fracture. To verify this, the inventors placed bent press-formed products in a specified hydrogen penetration environment for a certain period of time and investigated areas where delayed fracture would occur. As a result, it was confirmed that cracks (delayed fracture) occurred in the center of the plate thickness near the bend apex, where the maximum principal stress, equivalent to the stress in the tensile direction, is greatest in the press-formed products.

[0020] From the above investigations, the inventors have concluded that in press-formed products, delayed fracture is a concern in areas where the value of the maximum principal stress, which corresponds to the stress in the tensile direction, is high. Therefore, the inventors have considered that, in order to reproduce the stress in areas where delayed fracture is a concern in a delayed fracture test using a test specimen, it is preferable to apply a stress in the tensile direction, i.e., a positive maximum principal stress, to the test specimen.

[0021] In addition, delayed fracture is caused by hydrogen that penetrates into press-formed products, and the more hydrogen that penetrates, the more likely it is to occur. The amount of hydrogen that penetrates into press-formed products is thought to be related to the plastic strain that occurs in the press-formed products. In other words, as plastic strain increases, the number of defects that trap hydrogen increases, and the amount of hydrogen that penetrates into press-formed products is also thought to increase.

[0022] On the other hand, press-formed products made from high-tensile steel sheets have strains caused by various deformation modes, such as bending (plane strain), extension (biaxial tension), uniaxial tension, uniaxial compression, etc. Therefore, the inventors prepared test pieces of high-tensile steel sheets to which strains were applied in various deformation modes, and after placing the test pieces in a hydrogen penetration environment for a certain period of time, conducted a hydrogen charging test to measure the amount of hydrogen in the test pieces, and investigated the relationship between strains caused by different deformation modes and hydrogen concentration. In this study, cold-rolled steel sheets with a thickness of 1.2 mm and a strength of 1470 MPa were used as test materials, and strain was applied in various deformation modes, such as rolling (plane strain), uniaxial tension, uniaxial compression, or biaxial tension-compression.

[0023] Figure 2 shows an example of the results of measuring the hydrogen concentration in the test specimens after immersing them in a 0.1% ammonium thiocyanate and McIlvaine buffer solution with pH 4.0 for 30 hours, to which strain was applied in various deformation modes. The relationship between the hydrogen concentration and strain was then summarized. Figure 2(a) shows the equivalent plastic strain ε p eq and hydrogen concentration, and Fig. 2(b) shows the relationship between the equivalent plastic strain ε p eq The relationship between the deformation mode and hydrogen concentration is shown for a constant strain (=0.01, 0.02). 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. 2(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.

[0024] As shown in Figure 2, the inventors found that the hydrogen concentration (amount of hydrogen) in the test piece increased with the equivalent plastic strain ε p eq Furthermore, the hydrogen concentration increases with increasing equivalent plastic strain. This is thought to be because the number of defects in the specimen that trap hydrogen increases with increasing equivalent plastic strain, and the amount of hydrogen that penetrates into the specimen increases.

[0025] From the above results, it was found that in order to perform a delayed fracture test by reproducing the strain in the area of ​​a press-formed product where delayed fracture is a concern in a test specimen, it is preferable to impart a strain equivalent to the equivalent plastic strain in the area where delayed fracture is a concern to the test specimen.

[0026] For example, in the case of a 1.2 mm thick, 1470 MPa grade cold-rolled steel sheet, if the equivalent plastic strain is 0.03 or less, a uniform uniaxial or biaxial strain equivalent to the desired equivalent plastic strain can be applied by uniaxial tension, uniaxial compression, or biaxial tension.

[0027] However, when the equivalent plastic strain exceeds 0.03, necking, a localized deformation, occurs in uniaxial tension and biaxial tension, and buckling, a localized out-of-plane deformation, occurs in uniaxial compression, making it difficult to impart uniform strain. Therefore, when the equivalent plastic strain exceeds 0.03, it was decided to impart uniform plane strain by rolling.

[0028] Next, the inventors investigated the shape of a test piece that would reproduce the stress and strain in a portion of a press-formed product where delayed fracture is a concern. Although the shape of the test specimen to which strain corresponding to the desired equivalent plastic strain is applied is not considered to be particularly limited, the inventors decided to utilize a uniaxial tensile test specimen 101 used in a normal uniaxial tensile test, as shown in Fig. 3(a). This is because, in the uniaxial tensile test specimen 101, a uniaxial tensile stress corresponding to the maximum positive principal stress can be uniformly applied to the parallel portion 103 where cracks occur in a normal uniaxial tensile test.

[0029] Therefore, we decided to produce a uniaxial tensile test piece 101 in which a strain equivalent to the equivalent plastic strain in the area at risk of delayed fracture was uniformly imparted to the parallel portion 103 by cutting out a steel plate to which strain had been uniformly imparted by bending (plane strain), extension (biaxial tension), uniaxial tension, uniaxial compression, etc.

[0030] Next, the inventors conducted extensive research into the relationship between the stress gradient in the direction in which a crack propagates due to delayed fracture (hereinafter also referred to as the "crack propagation direction") and the delayed fracture time, in order to reproduce the stress gradient in a test specimen at a location in a press-formed product where delayed fracture is a concern.

[0031] In this study, a high-tensile steel plate (e.g., a 1470 MPa cold-rolled steel plate with a thickness of 1.2 mm) was used as a test material, and a delayed fracture test was performed using a uniaxial tensile test piece 111 having a notch R portion 117 formed in a parallel portion 113, as shown in FIG. 3(b). In the delayed fracture test, the uniaxial tensile test piece 111 was immersed in a solution of ammonium thiocyanate and McIlvaine buffer solution with a pH of 4.0 and a concentration of 0.1%, while a constant uniaxial tensile load was applied to the uniaxial tensile test piece 111 so that the tensile stress applied to the notch R portion 117 would be 1000 MPa. The inventors then investigated the relationship between the notch radius (notch R) of the notch R portion 117 and the stress gradient, and the relationship between the time until cracks (delayed fracture) occurred in the uniaxial tensile test piece 111 and the stress gradient.

[0032] FIG. 4 is an example of a graph showing the relationship between the notch radius (notch R) of the notch R portion 117 and the stress gradient when a tensile stress of 1000 MPa is applied to the notch R portion 117 with various notch radii. Here, the stress gradient is the gradient of the maximum principal stress in the crack propagation direction (direction perpendicular to the maximum principal stress). Also, FIG. 4 shows the equation y=1082.5e which shows the relationship between the notch R (=x) and the stress gradient (=y). -0.293x Shows.

[0033] As shown in Fig. 4, the smaller the notch R of the notch R portion 117, the larger the stress gradient. Also, notch R = ∞ corresponds to the uniaxial tensile test piece 101 in which the notch R portion 117 is not formed, as shown in Fig. 3(a), and the stress gradient in the parallel portion 103 of the uniaxial tensile test piece 101 is 0. Therefore, the relationship between the notch R of the notch R portion 117 and the stress gradient in the crack propagation direction can be expressed using an exponential function, for example, as shown in Fig. 4.

[0034] FIG. 5 is a graph showing an example of the relationship between the stress gradient in the crack propagation direction in delayed fracture and the delayed fracture time, obtained by a delayed fracture test using a uniaxial tensile test piece 111. As shown in Figure 5, the larger the stress gradient in the notch R portion 117, the longer the delayed fracture time and the longer it takes for the delayed fracture to occur. This is thought to be because the surrounding low-stress regions suppress deformation in the high-stress region where the crack occurs, prolonging the time it takes for the crack to occur.

[0035] In this way, the inventors have found that in order to determine the delayed fracture characteristics of a press-formed product, it is necessary to conduct a delayed fracture test by reproducing not only the stress, strain, and hydrogen concentration at the site at which delayed fracture is suspected, but also the stress gradient in the crack propagation direction on a test piece.

[0036] The present invention has been made based on such findings, and first and second embodiments of the present invention will be described below.

[0037] [Embodiment 1] The delayed fracture testing method according to the first embodiment of the present invention is for determining the delayed fracture properties of a portion at risk of delayed fracture in a press-formed product, where delayed fracture is a concern. As shown in Fig. 1, the delayed fracture testing method according to the first embodiment includes a delayed fracture suspected portion identifying step S1, a test piece manufacturing step S3A, a delayed fracture testing step S5A, and a delayed fracture property obtaining step S7A. Each step of the delayed fracture testing method according to the first embodiment will be described below.

[0038] <Process for identifying areas at risk of delayed fracture> The delayed fracture risk portion identifying step S1 is a step of calculating the stress distribution and strain distribution of the press-formed product and identifying a portion with a high maximum principal stress as a delayed fracture risk portion. Furthermore, the delayed fracture risk portion identifying step S1 is a step of acquiring the maximum principal stress, equivalent plastic strain, and stress gradient in a direction perpendicular to the maximum principal stress in the identified delayed fracture risk portion.

[0039] CAE analysis methods commonly used in automobile design and the like can be applied to calculate the stress and strain distributions of press-formed products. Examples of CAE analysis methods include press forming analysis, shear analysis of sheet metal cutting, trimming, piercing, etc. before press forming, springback analysis of press-formed products, and assembly analysis of press-formed products. Then, by using CAE analysis using the finite element method to find the stress and strain for each CAE analysis element in the press-formed product, the stress and strain distributions of the press-formed product can be calculated.

[0040] In the delayed fracture suspected portion identifying step S1, 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. Then, the CAE analysis element with the highest calculated maximum principal stress can be identified as the delayed fracture suspected portion in the press-formed product. In the delayed fracture suspected portion identifying step S1, the equivalent plastic strain in the delayed fracture suspected portion is calculated based on the following formula.

[0041]

number

[0042] Furthermore, the direction in which a crack propagates (crack propagation direction) when delayed fracture occurs is perpendicular to the maximum principal stress. Therefore, in the delayed fracture suspected part identification step S1, the stress gradient in the direction perpendicular to the maximum principal stress in the delayed fracture suspected part is calculated as the stress gradient in the crack propagation direction in delayed fracture.

[0043] <Test piece manufacturing process> The test piece production process S3A is a process for producing a test piece that can impart a stress gradient equivalent to the stress gradient in the region at risk of delayed fracture identified in the region at risk of delayed fracture identification process S1, and that has been given a strain equivalent to the equivalent plastic strain in the region at risk of delayed fracture in advance.

[0044] In the present first embodiment, the test piece manufacturing step S3A manufactures a uniaxial tensile test piece 111 having a notched R portion 117 formed in a parallel portion 113 as shown in FIG. 3(b). Specifically, a strain equivalent to the equivalent plastic strain of the area at risk of delayed fracture is imparted to the steel plate, and the steel plate is cut out so that the area to which the strain is imparted becomes the parallel portion 113, and a notch R portion 117 is formed in the parallel portion 113.

[0045] The strain may be imparted to the steel sheet, for example, as follows. When the equivalent plastic strain in a portion of a press-formed product where delayed fracture is a concern is relatively small, a uniform uniaxial strain or biaxial strain equivalent to the desired equivalent plastic strain can be imparted to the steel plate by uniaxial tension, uniaxial compression, or biaxial tension. On the other hand, when the equivalent plastic strain in the portion at risk of delayed fracture is relatively large, a uniform plane strain can be imparted to the steel sheet by rolling.

[0046] For example, in a 1.2 mm thick, 1470 MPa grade cold-rolled steel sheet, if the equivalent plastic strain is relatively small, 0.03 or less, strain can be imparted by uniaxial tension, uniaxial compression, or biaxial tension. On the other hand, if the equivalent plastic strain is relatively large, exceeding 0.03, strain can be imparted by rolling.

[0047] In the test piece production process S3A, a notch R portion 117 is provided in the parallel portion 113 so as to impart a stress gradient equivalent to the stress gradient in the portion at risk of delayed fracture. The notch radius of the notch R portion 117 is determined as follows.

[0048] First, an FEM analysis or the like is performed in which various tensile stresses are applied to a uniaxial tensile test piece 111 provided with a notch R portion 117. Then, the FEM analysis is performed on uniaxial tensile test pieces provided with notch R portions 117 of various notch radii, and the relationship between the notch radius of the notch R portion 117 and the stress gradient in the crack propagation direction at the notch R portion 117 is obtained.

[0049] 4 is an example of a graph showing the relationship between the notch radius (notch R) of the notch R portion 117 and the stress gradient in the crack propagation direction in the notch R portion 117 when a tensile stress of 1000 MPa is applied to the notch R portion 117. Here, the crack propagation direction in the notch R portion 117 is set to be perpendicular to the tangent to the shear end face at the notch bottom of the notch R portion 117.

[0050] Then, the notch radius of the notch R portion 117 corresponding to the stress gradient in the direction perpendicular to the maximum principal stress in the portion at risk of delayed fracture can be determined from the relationship between the notch R and the stress gradient shown in FIG.

[0051] In addition, since it is difficult to guarantee dimensional accuracy from the viewpoint of processing accuracy when the notch radius is 1 mm or less, it is desirable that the notch radius be 1 mm or more in order to perform a stable delayed fracture test. Therefore, based on the relationship between the notch radius (notch R) and the stress gradient shown in Figure 4, a stable delayed fracture test can be performed using the uniaxial tensile test piece 111 formed with the notch R portion 117 when the stress gradient is 800 MPa / mm or less.

[0052] <Delayed fracture testing process> The delayed fracture testing step S5A is a step of applying a tensile stress corresponding to the maximum principal stress at the portion at which delayed fracture is suspected to occur to the test specimen fabricated in the test specimen fabrication step, and imparting a stress gradient corresponding to the stress gradient at the portion at which delayed fracture is suspected to occur. The delayed fracture testing step S5A is a step of performing a delayed fracture test in which the test specimen is held in a predetermined hydrogen penetration environment while being subjected to a tensile stress and with a stress gradient imparted thereto, and obtaining the crack initiation time until a crack is generated in the test specimen.

[0053] In the present embodiment 1, the delayed fracture test step S5A performs a delayed fracture test using a delayed fracture test apparatus 1 shown in FIG. 6(a).

[0054] <Delayed fracture testing equipment> The delayed fracture testing apparatus 1 includes a tensile jig 3, a strain gauge 5, and a strain time history acquisition unit 7. Furthermore, the delayed fracture testing apparatus 1 has a function of acquiring the time history of strain occurring in the tensile jig 3 so as to be able to detect cracks occurring in the notch R portion 117 when a uniaxial tensile test piece 111, to which uniaxial tensile stress has been applied using the tensile jig 3, is held in a hydrogen penetration environment 21.

[0055] (Tension jig) The tensile jig 3 applies uniaxial tensile stress to the notch R portion 117 of the uniaxial tensile test piece 111, and generates strain in conjunction with the notch R portion 117. The tensile jig 3 includes a pair of gripping portions 9 and a stress applying portion 11, as shown in FIG. 6(b).

[0056] The pair of gripping portions 9 grip both ends 115 of the uniaxial tensile test piece 111 . The stress loading section 11 applies a predetermined tensile stress to the notch R section 117 of the uniaxial tensile test piece 111 by applying a load in a pulling direction to a pair of gripping sections 9 that are holding both ends 115 of the uniaxial tensile test piece 111. Furthermore, the stress application section 11 has a pair of bolts 13 on the left and right sides of the uniaxial tensile test piece 111, and two lock nuts 15, one above the other, screwed onto each bolt 13.

[0057] The bolt 13 is a rod-shaped body that is distorted in conjunction with the parallel portion 113 to which tensile stress is applied together with the notched R portion 117, and is screwed into both the left and right ends of the upper and lower gripping portions 9, respectively. The lock nut 15 is threaded onto the bolt 13 between the upper and lower gripping portions 9 .

[0058] The stress loading section 11 can apply a load (tensile load) to the uniaxial tensile test piece 111 in a direction that separates the pair of gripping sections 9 by tightening the lock nut 15 threaded onto the bolt 13 to generate a pressing force against each of the upper and lower gripping sections 9. When a tensile load is applied to the uniaxial tensile test piece 111 by the stress loading section 11, the upper and lower lock nuts 15 receive a reaction force from the gripping section 9 in a direction that brings them closer to each other, causing compressive strain to occur in the shank 13a of the bolt 13. As a result, the strain gauge 5 attached to the shank 13a outputs a strain that is opposite in sign to the strain occurring in the parallel section 113 of the uniaxial tensile test piece 111.

[0059] In this way, uniaxial tensile stress is applied to notch R portion 117 by bolt 13 and lock nut 15, which causes strain (elastic strain) in notch R portion 117 and, in conjunction with this, causes strain in shank 13a of bolt 13. Then, by adjusting the tightening torque of the lock nut 15, a predetermined uniaxial tensile stress can be applied to the notched R portion 117 of the uniaxial tensile test piece 111.

[0060] The left and right lock nuts 15 are tightened so that their torque values ​​are the same, thereby preventing a rotational moment from being applied to the uniaxial tensile test piece 111. Furthermore, by using a lock nut 15 in the stress application portion 11, the tightening force on the grip portion 9 side does not loosen, and it is possible to prevent a decrease in the uniaxial tensile stress applied to the notch R portion 117 of the uniaxial tensile test piece 111, which is preferable.

[0061] (strain gauge) The strain gauge 5 is attached to the shaft portion 13a of the bolt 13, which is a rod-shaped body of the tension jig 3, and detects the strain occurring in the shaft portion 13a. The strain gauge 5 is embedded in the shaft portion 13a of the bolt 13, as shown in FIG. 6(a).

[0062] (Strain time history acquisition section) The strain time history acquisition unit 7 acquires the strain time history of the shank 13 a of the bolt 13 detected by the strain gauge 5 . The strain-time history acquisition unit 7 includes a strain measuring instrument that measures strain based on signals output from the strain gauges 5, and a strain recorder that records the time history of the strain measured by the strain measuring instrument. In the strain-time history acquisition unit 7, the strain measuring instrument is connected to the strain gauges 5 via lead wires 17.

[0063] <Delayed fracture testing using a delayed fracture testing device> In the delayed fracture test step S5A, a delayed fracture test is performed using the delayed fracture test device 1 as follows.

[0064] First, using the tension jig 3, a uniaxial tensile stress equivalent to the maximum principal stress of the region at risk of delayed fracture calculated in the delayed fracture suspected region identifying step S1 is applied to the notch R portion 117 of the uniaxial tensile test piece 111. The notch radius of the notch R portion 117 of the uniaxial tensile test piece 111 is determined based on the above-mentioned FIG. 4 so that a stress gradient equivalent to the stress gradient of the region at risk of delayed fracture is imparted when a tensile stress equivalent to the maximum principal stress is applied. Therefore, a stress gradient equivalent to the stress gradient of the region at risk of delayed fracture is imparted to the notch R portion 117 of the uniaxial tensile test piece 111.

[0065] Next, the uniaxial tensile test piece 111 is placed in a hydrogen entry environment 21 and maintained until cracks appear in the notch R portion 117 of the uniaxial tensile test piece 111. The hydrogen entry environment 21 is, for example, a solution of ammonium thiocyanate with a pH of 4.0 and a concentration of 0.1% and McIlvaine buffer solution. The time (test time) maintained in the hydrogen entry environment 21 is not particularly limited, but since the amount of hydrogen entry becomes saturated after a certain immersion time, the immersion time (test time) is preferably at most about 100 hours.

[0066] Next, the time history of the strain occurring in the tensioning jig 3 is acquired. In the present embodiment 1, as shown in Fig. 6(a), a strain time history acquisition unit 7 is used to acquire the time history of the strain detected by the strain gauge 5 attached to the shank 13a of the bolt 13, which is a rod-shaped body of the tensioning jig 3.

[0067] Then, based on the acquired time history of strain, cracks occurring in the notch R portion 117 of the uniaxial tensile test piece 111 are detected, and the time from the start of the delayed fracture test until cracks occur is acquired as the crack occurrence time.

[0068] <Delayed fracture property acquisition process> The delayed fracture property acquisition process S7A is a process for determining the delayed fracture property of a portion of a press-formed product at which delayed fracture is suspected, based on the crack occurrence time acquired in the delayed fracture testing process S5A, the strain previously applied to the test piece, the tensile stress applied to the test piece, and the stress gradient applied to the test piece.

[0069] In the present embodiment 1, the delayed fracture property acquisition step S7A acquires the crack initiation time acquired in the delayed fracture testing step S5A and the strain previously applied to the uniaxial tensile test piece 111 in the test piece production step S3A. Furthermore, the delayed fracture property acquisition step S7A acquires the tensile stress applied to the notch R portion 117 in the delayed fracture testing step S5A and the stress gradient applied to the notch R portion 117 by the tensile stress applied to the notch R portion 117. Then, the acquired crack initiation time, strain, tensile stress, and stress gradient are found as the delayed fracture property of the portion of the press-formed product at which delayed fracture is suspected.

[0070] The stress gradient acquired in the delayed fracture property acquisition step S7A may be a stress gradient found from the relationship between the notch R of the notch R portion 117 and the stress gradient, as shown in Fig. 4. Alternatively, an FEM analysis may be performed on the process of applying a stress equivalent to the tensile stress at the site at risk of delayed fracture to the notch R portion 117, and the stress gradient in the direction perpendicular to the tangent to the shear end face of the notch bottom 117a of the notch R portion 117 may be found.

[0071] <Action and effect> According to the delayed fracture testing method of this embodiment 1, the delayed fracture characteristics of the portion at risk of delayed fracture can be determined by a delayed fracture test in which the stress, strain, and stress gradient at the portion at risk of delayed fracture of the press-formed product are reproduced in the notch R portion 117 of the uniaxial tensile test piece 111.

[0072] [Embodiment 2] The delayed fracture testing method according to the second embodiment of the present invention uses a four-point bending test piece 121 in which a bent portion 123 is formed by four-point bending as a test piece for the delayed fracture test, instead of the uniaxial tensile test piece 111 (FIG. 3(b)) of the first embodiment described above. The delayed fracture testing method according to the second embodiment includes a delayed fracture suspected region identifying step S1, a test piece manufacturing step S3B, a delayed fracture testing step S5B, and a delayed fracture property acquiring step S7B, as shown in FIG. 1, similar to the first embodiment. Here, the delayed fracture suspected area identification process S1 in embodiment 2 is common to embodiment 1, so below we will explain the test piece production process S3B, the delayed fracture testing process S5B, and the delayed fracture property acquisition process S7B in embodiment 2.

[0073] <Test piece manufacturing process> The test piece production process S3B is a process for producing a four-point bending test piece 121 that can impart a stress gradient to the area at risk of delayed fracture identified in the delayed fracture risk area identification process S1 and that has been given a strain equivalent to the equivalent plastic strain in the area at risk of delayed fracture in advance.

[0074] The four-point bending test piece 121 is strained in the same manner as in the test piece production process S3A of the first embodiment described above, by applying a strain to the steel plate that is equivalent to the equivalent plastic strain of the area at risk of delayed fracture, and then cutting out the steel plate so that the area to which the strain is applied is the area where the tensile stress is maximum.

[0075] In addition, in the test piece manufacturing process S3B, the thickness of the four-point bending test piece 121 is determined as follows so that a stress gradient equivalent to the stress gradient in the area at risk of delayed fracture can be imparted to the bending portion 123 of the four-point bending test piece 121.

[0076] First, the four-point bending test piece 121 is bent at four points by FEM analysis or the like, and a bending stress (tensile stress) equivalent to the maximum principal stress in the portion at risk of delayed fracture is applied to the outside of the bent portion 123 to determine the stress gradient in the thickness direction on the outside of the bent portion 123. Then, by performing FEM analysis or the like on four-point bending test pieces 121 of various thicknesses, the relationship between the thickness of the four-point bending test piece 121 and the stress gradient in the thickness direction on the outside of the bent portion 123 is determined in advance by FEM analysis or the like.

[0077] FIG. 8 is an example of a graph showing the relationship between the thickness of the four-point bending test piece 121 and the stress gradient in the thickness direction on the outside of the bend of the bent portion 123 when a bending stress of 1000 MPa, which corresponds to the maximum principal stress in the area at risk of delayed fracture, is applied to the bent portion 123. Then, the thickness of the four-point bending test piece 121 corresponding to the maximum principal stress in the portion at risk of delayed fracture can be determined from the relationship between the thickness and the stress gradient as shown in FIG.

[0078] The thickness of the four-point bending test piece 121 may be calculated by dividing the maximum principal stress (MPa) obtained in the process S1 for identifying areas at risk of delayed fracture by the stress gradient (MPa / mm) obtained in the process S1 for identifying areas at risk of delayed fracture.

[0079] The reason why the predetermined stress gradient applied to the four-point bending test piece 121 can be adjusted by the plate thickness is as follows.

[0080] Figure 9 shows a bent portion 123 obtained by bending and deforming a four-point bending test piece 121 in the elastic region (R: bending radius, θ: bending angle). In Figure 9, R is the bending radius, θ is the bending angle, t is the plate thickness, and x is the plate thickness direction. The position of the neutral axis in the plate thickness direction is defined as x = 0, and the plate thickness direction from the neutral axis to the outside of the bend is defined as the +x direction, and the plate thickness direction from the neutral axis to the inside of the bend is defined as the -x direction.

[0081] The line length of the neutral axis located at the center of the plate thickness (x=0) is Rθ, and its value does not change before and after bending deformation. The line length at position x is Rθ, the same as the line length of the neutral axis, before bending deformation, and is (R+x)θ after bending deformation, so the change in line length due to bending deformation is xθ.

[0082] The strain of the bending portion 123 in the elastic region is the value obtained by dividing the change in line length by the line length before bending deformation, so the strain at position x is x / R, which is the change in line length xθ divided by the line length Rθ before bending deformation.

[0083] The bending stress in the elastic region is the Young's modulus E of the material (e.g., steel plate) used in the four-point bending test piece 121 multiplied by the strain, so the bending stress σ at position x is Ex / R, and the position x and the bending stress σ are in a linear relationship. The bending stress on the outside of the bend (x=t / 2) is Et / 2R, the bending stress on the inside of the bend (x=-t / 2) is -Et / 2R, and the bending stress on the inside of the bend is the bending stress on the outside of the bend multiplied by -1. That is, when the four-point bending test piece 121 is bent at four points, a stress gradient can be generated in the bent portion 123 from the outside of the bend (the apex of the bend) to the inside of the bend.

[0084] Based on the above relationship, a case will be considered in which the same bending stress σ1 is applied to the outer side of the bend of the bent portion 123 to the four-point bending test pieces 121 having the thickness t1 and the thickness t2. In this case, the bending stress on the inside of the bend is -σ1, and the difference between the bending stress on the outside and inside of the bend is 2σ1. Furthermore, as described above, the bending stress θ1 and the position x in the thickness direction have a linear relationship in the elastic region, so the stress gradient in the thickness direction of the four-point bending test piece 121 with a thickness t1 is 2σ1 / t1, which is the difference between the stress on the outside of the bend and the stress on the inside of the bend, 2σ1, divided by the thickness t1. Similarly, the stress gradient in the thickness direction of the four-point bending test piece 121 with a thickness t2 is 2σ1 / t2, which is the difference between the stress on the outside of the bend and the stress on the inside of the bend, 2σ1, divided by the thickness t2. Therefore, when the same bending stress is applied to the outside of the bend to four-point bending test pieces 121 with different thicknesses, the stress gradient in the thickness direction is inversely proportional to the thickness of the four-point bending test piece 121.

[0085] Furthermore, when the amount of pressing when bending the four-point bending test piece 121 at four points is constant, the bending stress applied to the outer side of the bent portion 123 is constant. Therefore, as shown by the relationship between the plate thickness and the stress gradient described above, the stress gradient applied to the bent portion 123 can be changed by changing the plate thickness of the four-point bending test piece 121. Therefore, in order to apply a predetermined stress gradient to the four-point bending test piece 121, it is sufficient to change the plate thickness t of the four-point bending test piece 121.

[0086] The range of stress gradient that can be applied to the four-point bending test piece 121 is determined by the range of thicknesses of steel plates that can be manufactured, and therefore varies depending on the type of steel. For example, the upper limit of the thickness range (t) of a 1470 MPa-class cold-rolled steel sheet that satisfies the required mechanical properties (yield stress, tensile strength, elongation, r-value) for the product is 2.5 mm. Therefore, if the bending stress (tensile stress) on the outside of the bend is σ, the stress gradient in the thickness direction is Δσ = 2σ / t, so the bending stress and stress gradient can be expressed as 0.8σ < Δσ.

[0087] For example, the relationship between the stress gradient and the material thickness of the four-point bending test piece 121 shown in FIG. 8 is expressed as y = 2000 / x. Here, y is the stress gradient, and x is the thickness. The relationship between the stress gradient and the thickness shown in FIG. 8 is obtained when a bending stress of 1000 MPa is applied to the outer side of the bent portion 123 of the four-point bending test piece 121. Therefore, the lower limit of the stress gradient that can be applied to the bent portion 123 is the difference between the bending stress on the outer side and the inner side of the bend (1000 MPa - (-1000 MPa)) divided by the upper limit of the thickness (= 2.5 mm), i.e., 2000 MPa / 2.5 mm = 800 MPa / mm. The lower limit of the stress gradient calculated in this way satisfies the above-mentioned relationship of 0.8σ < Δσ (bending stress σ = 1000 MPa, stress gradient Δ = 800 MPa).

[0088] <Delayed fracture testing process> The delayed fracture test step S5B is a step of performing a delayed fracture test using the four-point bending test piece 121 and obtaining the crack occurrence time until a crack occurs in the four-point bending test piece 121. In the delayed fracture testing step S5B, first, the four-point bending test piece 121 is bent at four points, so that a bending stress equivalent to the maximum principal stress in the portion at risk of delayed fracture is applied to the outer side of the bent portion 123, and a stress gradient equivalent to the stress gradient in the portion at risk of delayed fracture is applied. Then, the four-point bending test piece 121 is held in a predetermined hydrogen penetration environment in a state in which a bending stress is applied to the outer side of the bent portion 123 and a stress gradient is applied, and the crack initiation time until a crack occurs in the four-point bending test piece 121 is obtained.

[0089] Four-point bending of the four-point bending test piece 121 can be performed, for example, as shown in Figures 7(a) and (b). First, as shown in Figure 7(a), supports 31a and 31b are placed above the four-point bending test piece 121, and supports 31c and 31d are placed below the four-point bending test piece 121. Then, as shown in Figure 7(b), supports 31a and 31b or supports 31c and 31d are raised and lowered.

[0090] In such four-point bending, the movement of the fulcrums 31a to 31d stops when a predetermined amount of compression (amount of movement of the fulcrums) is reached. At this time, the bending stress (tensile stress) on the outer side of the bend of the bent portion 123 formed by four-point bending becomes maximum, and the bending stress on the outer side of the bend can be adjusted to various levels by changing the amount of compression. In addition, when bending the four-point bending test piece 121 at four points, the bending stress applied to the four-point bending test piece 121 may be adjusted by attaching a strain gauge to the outside of the bend of the bending portion 123 to measure the strain, and calculating the tensile stress by multiplying the measured strain value by Young's modulus.

[0091] FIG. 7(c) is an example of a bending jig 41 for bending the four-point bending test piece 121 at four points, and is a diagram schematically showing a state in which the four-point bending test piece 121 is attached to the bending jig 41. The four-point bending test piece 121 is fixed to the sample stage 43 at four fulcrums 45a to 45d, and the bending stress applied to the bent portion 123 of the four-point bending test piece 121 is adjusted by the amount of tightening of the screw 47 (= the amount of pressing into the four-point bending test piece 121). Furthermore, by making the supports 45a to 45d out of insulating glass, the four-point bending test piece 121 is insulated from the specimen stage 43. Then, the four-point bending test piece 121 is set on the specimen stage 43 and immersed in a hydrogen penetration environment (in a test liquid) to perform a delayed fracture test. The hydrogen penetration environment is, for example, a solution of ammonium thiocyanate with a pH of 4.0 and a concentration of 0.1% and McIlvaine buffer solution.

[0092] In this way, the four-point bending test piece 121 is bent at four points, a bending stress equivalent to the maximum principal stress in the area at risk of delayed fracture is applied to the outside of the bend of the bent portion 123, and a delayed fracture test can be performed in this state while applying a stress gradient equivalent to the stress gradient in the area at risk of delayed fracture.

[0093] In the delayed fracture testing process S5B, the immersion time (test time) of the four-point bending test piece 121 in the test liquid is not particularly limited, but since the amount of hydrogen penetration becomes saturated after a certain immersion time, it is preferable that the immersion time (test time) be at most about 100 hours.

[0094] In addition, the time to crack occurrence can be obtained, for example, by attaching a strain gauge to the sample stage 43 or the shank of the screw 47 in Figure 7(c) and based on changes in strain occurring in the sample stage 43 or the shank of the screw 47.

[0095] <Delayed fracture property acquisition process> The delayed fracture property acquisition process S7B is a process for determining the delayed fracture property at the portion of the press-formed product at which delayed fracture is suspected, based on the crack occurrence time acquired in the delayed fracture testing process S5B, the strain previously applied to the four-point bending test piece 121, the tensile stress applied to the four-point bending test piece 121, and the stress gradient applied to the four-point bending test piece 121.

[0096] <Action and effect> According to the delayed fracture testing method of this embodiment 2, the delayed fracture characteristics of the area at risk of delayed fracture can be determined by a delayed fracture test in which the stress, strain, and stress gradient at the area at risk of delayed fracture of a press-formed product are reproduced on the outside of the bend in the bending portion 123 of the four-point bending test piece 121.

[0097] As described above, in the delayed fracture testing method according to the first embodiment, the stress gradient that can be stably reproduced in the notch R portion 117 of the uniaxial tensile test piece 111 is in the range of 800 MPa / mm or less. In contrast to this, in the delayed fracture testing method according to the second embodiment, the stress gradient that can be stably reproduced in the bent portion 123 of the four-point bending test piece 121 is in the range exceeding 800 MPa / mm.

[0098] Therefore, the delayed fracture testing method of the present invention can be configured to switch between the delayed fracture testing method of embodiment 1 and the delayed fracture testing method of embodiment 2 based on the stress gradient at the portion of the press-formed product at risk of delayed fracture.

[0099] That is, when the stress gradient in the direction perpendicular to the maximum principal stress in the area at risk of delayed fracture is below a predetermined value (for example, below 800 MPa / mm), the delayed fracture test method according to embodiment 1 is carried out using a uniaxial tensile test piece 111 having a notch R portion 117 in the parallel portion 113. On the other hand, when the stress gradient in the portion at risk of delayed fracture exceeds a predetermined value (for example, exceeds 800 MPa / mm), a delayed fracture testing method according to the second embodiment using a four-point bending test piece 121 is carried out.

[0100] In this way, by switching between the delayed fracture testing method according to embodiment 1 and the delayed fracture testing method according to embodiment 2 depending on the magnitude of the stress gradient in the area of ​​the press-formed product where delayed fracture is a concern, it is possible to determine delayed fracture properties over a wide range of stress gradients. [Example]

[0101] An experiment was conducted to confirm the effects of the delayed fracture testing method according to the present invention, and the results will be described below.

[0102] In the experiment, the press-formed product 201 shown in FIG. 10 was used as the subject, and the delayed fracture properties of the press-formed product 201 at a portion where delayed fracture is suspected were determined by the delayed fracture testing method according to the first embodiment described above.

[0103] The press-formed product 201 was obtained by press-forming a 1470 MPa-class cold-rolled steel sheet having a thickness of 1.2 mm as a test material through bending, and as shown in Fig. 10, the press-formed product 201 has a bent portion 203 and pieces 205 extending from both ends of the bent portion 203. Here, the bending radius of the bent portion 203 is R = 4 mm.

[0104] Next, a CAE analysis was performed on the process of press-forming the press-formed product 201, and the stress distribution and strain distribution of the press-formed product 201 were calculated. The maximum principal stress was the stress in the longitudinal direction of the press-formed product 201. Figure 11 shows the maximum principal stress distribution and equivalent plastic strain distribution of the press-formed product 201 calculated by CAE analysis. From the maximum principal stress distribution shown in Figure 11, the center of the plate thickness of the bent part 203, where the maximum principal stress is high, was identified as a part at risk of delayed fracture.

[0105] Then, from the maximum principal stress distribution and equivalent plastic strain distribution shown in Figure 11, the equivalent plastic strain, maximum principal stress, and stress gradient in the direction perpendicular to the maximum principal stress (i.e., the thickness direction) were calculated for the portion of the press-formed product 201 where delayed fracture is suspected. Here, the equivalent plastic strain was 0.09, the maximum principal stress was 1260 MPa, and the stress gradient in the thickness direction was 466 MPa / mm. Since the stress gradient calculated for the portion where delayed fracture is suspected was 800 MPa / mm or less, it was decided to use the uniaxial tensile test piece 111 for the delayed fracture test.

[0106] Next, a uniaxial tensile test piece 111 to be used in a delayed fracture test was produced (see FIG. 3). First, a 1470 MPa grade cold rolled steel sheet (thickness: 1.2 mm) was rolled to a thickness of 1.11 mm at a thickness reduction rate of 7.5% in order to impart a uniform plane strain equivalent to an equivalent plastic strain of 0.09. Next, the notch radius of the notch R portion 117 was determined so that a stress gradient equivalent to the stress gradient (466 MPa / mm) in the direction perpendicular to the maximum principal stress at the location where delayed fracture is suspected can be imparted to the tensile test piece. In the experiment, the notch R was determined to be 2.9 mm based on the relationship between the notch radius of the notch R portion 117 of the uniaxial tensile test piece 111 shown in Figure 3 and the stress gradient in the crack propagation direction. Then, the steel plate to which the above-mentioned equivalent plastic strain had been imparted was cut out, and a uniaxial tensile test piece 111 was produced in which a strain equivalent to an equivalent plastic strain of 0.09 had been imparted to the parallel portion 113 and a notch R portion 117 with a notch radius of 2.9 mm had been provided.

[0107] Next, using the tension jig 3 shown in Fig. 6(b), a uniaxial tensile stress equivalent to the maximum principal stress (=1260 MPa) in the region where delayed fracture is suspected in the press-formed product 201 was applied to the notch R portion 117 of the uniaxial tensile test piece 111. This applied a stress gradient equivalent to the stress gradient (=466 MPa / mm) in the region where delayed fracture is suspected.

[0108] Then, the uniaxial tensile test piece 111 with the above-mentioned uniaxial tensile stress and stress gradient applied was immersed together with the tensile jig 3 in a solution of ammonium thiocyanate and McIlvaine buffer solution with a pH of 4.0 and a concentration of 0.1%, and a delayed fracture test was performed. In the delayed fracture test, the time history of the strain output from the strain gauge 5 was acquired by a strain time history acquisition unit 7 connected to the strain gauge 5 embedded in the tensile jig 3 .

[0109] Next, based on the acquired time history of strain, cracks were detected in the parallel part 113 of the uniaxial tensile test piece 111. Since the strain significantly decreased 29 hours after the start of the delayed fracture test, it was detected that cracks had occurred in the parallel part 113 of the uniaxial tensile test piece 111 at that time. From these results, delayed fracture characteristics were obtained in the bent portion 203 of the press-formed product 201, in which delayed fracture occurred 29 hours after the start of the delayed fracture test in a hydrogen penetration environment using ammonium thiocyanate and McIlvaine buffer solution with a pH of 4.0 and a concentration of 0.1%.

[0110] Furthermore, the press-formed product 201 was immersed in a solution of ammonium thiocyanate and McIlvaine buffer solution with a pH of 4.0 and a concentration of 0.1% for 30 hours, and the presence or absence of delayed fracture was observed in the press-formed product 201. As a result, it was confirmed that delayed fracture had occurred in the center of the plate thickness near the apex of the bent portion 203 of the press-formed product 201.

[0111] From the above, it has been demonstrated that the delayed fracture testing method according to the first embodiment can determine the delayed fracture properties of areas where delayed fracture is a concern by performing a delayed fracture test in which the strain, stress, and stress gradient at areas where delayed fracture is a concern in a press-formed product are reproduced in a uniaxial tensile test piece. [Explanation of symbols]

[0112] 1. Delayed fracture testing equipment 3 Tensile jig 5 Strain gauges 7 Strain time history acquisition unit 9 Grip 11 Stress loading section 13 volts 13a Shaft 15 Lock nut 17 Lead wire 21 Hydrogen intrusion environment 31a, 31b, 31c, 31d fulcrum 41 Bending jig 43 Sample stage 45a, 45b, 45c, 45d fulcrum 47 screws 101 Uniaxial tensile test specimen 103 Parallel section 111 Uniaxial tensile test specimen 113 Parallel section 115 End 117 Notch R part 117a Notched bottom 121 Four-point bending test piece 123 Bent section 201 Press-molded products 203 Bending section 205 One side

Claims

1. A delayed fracture testing method for determining delayed fracture properties of a portion of a press-formed product where delayed fracture is a concern, comprising: a delayed fracture suspected portion identifying step of calculating a stress distribution and a strain distribution of the press-formed product, identifying a portion having a high maximum principal stress as the delayed fracture suspected portion, and acquiring the maximum principal stress, the equivalent plastic strain, and the stress gradient in a direction perpendicular to the maximum principal stress at the identified delayed fracture suspected portion; a test piece manufacturing process for manufacturing a test piece capable of imparting a stress gradient corresponding to the stress gradient in the region at risk of delayed fracture and having a strain corresponding to the equivalent plastic strain in the region at risk of delayed fracture imparted in advance; a delayed fracture testing process in which a tensile stress corresponding to the maximum principal stress at the portion at which delayed fracture is suspected is applied to the manufactured test piece, and the test piece is held in a predetermined hydrogen penetration environment while a stress gradient corresponding to the stress gradient at the portion at which delayed fracture is suspected is applied, and a crack initiation time until a crack is generated in the test piece is obtained; and a delayed fracture property acquisition step of determining the delayed fracture property of the press-formed product at the portion at which delayed fracture is suspected based on the acquired crack initiation time, the strain previously applied to the test piece, the tensile stress applied to the test piece, and the stress gradient applied to the test piece by the applied tensile stress.

2. The test piece is a uniaxial tensile test piece having a notched R portion formed in the parallel portion, In the test piece manufacturing step, the strain is applied to the parallel portion in advance, and a notch radius of the notch radius portion is determined based on a stress gradient at the portion at risk of delayed fracture; 2. The delayed fracture testing method according to claim 1, wherein in the delayed fracture testing step, a uniaxial tensile stress equivalent to a maximum principal stress in the portion at risk of delayed fracture is applied to the notch R portion, and a stress gradient equivalent to a stress gradient in the portion at risk of delayed fracture is applied.

3. The test piece is a four-point bending test piece in which a bent portion is formed by four-point bending, In the test piece manufacturing step, a strain is applied to the bent portion of the four-point bending test piece in advance, and a plate thickness of the four-point bending test piece is determined based on a stress gradient at the portion at which delayed fracture is suspected; 2. The delayed fracture testing method according to claim 1, wherein in the delayed fracture testing step, the four-point bending test piece is bent at four points, so that a bending stress equivalent to a maximum principal stress in the portion at risk of delayed fracture is applied to the bending outer side of the bent portion, thereby imparting a stress gradient equivalent to a stress gradient in the portion at risk of delayed fracture.

4. The delayed fracture test method according to claim 2 or 3, When the stress gradient at the portion at which delayed fracture is suspected is equal to or less than a predetermined value, the delayed fracture testing method according to claim 2 is carried out, 4. A delayed fracture testing method, comprising the steps of: performing the delayed fracture testing method according to claim 3 when the stress gradient at the portion at risk of delayed fracture exceeds a predetermined value.

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