US-12625043-B2 - Delayed fracture characteristic evaluation method and program

US12625043B2US 12625043 B2US12625043 B2US 12625043B2US-12625043-B2

Abstract

To further enhance the evaluation accuracy of a delayed fracture. Focusing on the fact that a calculated stress value serving as the reference for the occurrence of the delayed fracture changes depending on analysis conditions of a forming analysis, a value obtained by changing a stress value serving as the reference for the occurrence of the delayed fracture according to the analysis conditions for analyzing an intended formed article (article for practical use) is used as the reference for evaluating the delayed fracture. For example, analysis conditions of a forming analysis in an evaluation test of the delayed fracture are matched with analysis conditions of a forming analysis of an article for practical use represented by an actual automobile component.

Inventors

Yuichi Matsuki
Toyohisa Shinmiya
Kinya Nakagawa
Yuji Yamasaki

Assignees

JFE STEEL CORPORATION

Dates

Publication Date: 20260512
Application Date: 20210729
Priority Date: 20201203

Claims (17)

1 . A delayed fracture characteristic evaluation method comprising: a test step of applying a deformation to a test piece containing a metal sheet, and evaluating a generation status of a crack generated in the test piece by placing the deformed test piece in a hydrogen entry environment, a first step of calculating, for the deformation determined to have a delayed fracture based on the evaluation, a maximum residual stress generated in the test piece after the deformation by applying the deformation to the test piece by a forming analysis under first analysis conditions by a computer, and determining, from the calculated maximum residual stress, a reference stress for determining whether the delayed fracture occurs in a formed article of the metal sheet in a hydrogen environment; a second step of determining a residual stress generated in the formed article by forming the metal sheet into an intended formed article by a forming analysis under second analysis conditions by the computer; a third step of determining, from a correlation between a first stress calculated by the forming analysis under the first analysis conditions and a second stress calculated by the forming analysis under the second analysis conditions when press forming is performed under a same forming condition, a conversion factor for bringing the stress calculated by the forming analysis under the first analysis conditions closer to the stress calculated by the forming analysis under the second analysis conditions; and an evaluation step of evaluating a delayed fracture characteristic of the intended formed article from a comparison between the residual stress determined in the second step and a stress after converting the reference stress by the conversion factor.
2 . The delayed fracture characteristic evaluation method according to claim 1 , wherein the forming analysis is a forming analysis by a finite element method, and the first analysis conditions and the second analysis conditions include one or more conditions selected from an element type, an element size, a stress output location in the formed article, and an integration point position where the stress is output in a shell element.
3 . The delayed fracture characteristic evaluation method according to claim 2 , the first analysis conditions and the second analysis conditions including the element type, the method comprising: setting the conversion factor based on a difference between the element types of the first analysis conditions and the second analysis conditions.
4 . The delayed fracture characteristic evaluation method according to claim 3 , wherein a sheet thickness of the metal sheet before the forming is set to t [mm], the element type is set to a 2D solid element, a 3D solid element, or a shell element, the second analysis conditions are that the element type is the shell element and a mesh size of an element used for the forming analysis is m [mm], a mesh size of an element in the first analysis conditions is m [mm], and the conversion factor K for converting the stress calculated by the forming analysis under the first analysis conditions into the stress calculated by the forming analysis under the second analysis conditions is expressed by Equation (1) below, K =α[β( m/t )+1] (1), wherein a coefficient α is set to a constant selected from a range of 0.7 to 0.9 when the element type of the first analysis conditions is the 2D solid element, and is set to 1 when the element type of the first analysis conditions is the 3D solid element or the shell element, and a coefficient β is set to a constant selected from a range of 0.05 to 0.15.
5 . The delayed fracture characteristic evaluation method according to claim 4 , wherein the forming analysis is the forming analysis by the finite element method, and with respect to the stress output location in a shape after the forming or the integration point position where the stress is output in the case of the shell element in the forming analysis under the first analysis conditions and the second analysis conditions, a maximum stress in all of the elements or the integration points in the sheet thickness is output.
6 . The delayed fracture characteristic evaluation method according to claim 3 , wherein a sheet thickness of the metal sheet before the forming is set to t [mm], the element type is set to a 2D solid element, a 3D solid element, or a shell element, the first analysis conditions are that the element type is the shell element and a mesh size of an element used for the forming analysis is m [mm], a mesh size of an element in the second analysis conditions is m [mm], and the conversion factor K for converting the stress calculated by the forming analysis under the first analysis conditions into the stress calculated by the forming analysis under the second analysis conditions is expressed by Equation (2) below, K= 1/(α[β( m/t )+1]) (2), wherein a coefficient α is set to a constant selected from a range of 0.7 to 0.9 when the element type of the second analysis conditions is the 2D solid element, and is set to 1 when the element type of the second analysis conditions is the 3D solid element or the shell element, and a coefficient β is set to a constant selected from a range of 0.05 to 0.15.
7 . The delayed fracture characteristic evaluation method according to claim 3 , wherein the forming analysis is the forming analysis by the finite element method, and with respect to the stress output location in a shape after the forming or the integration point position where the stress is output in the case of the shell element in the forming analysis under the first analysis conditions and the second analysis conditions, a maximum stress in all of the elements or the integration points in the sheet thickness is output.
8 . The delayed fracture characteristic evaluation method according to claim 2 , wherein the forming analysis is the forming analysis by the finite element method, and with respect to the stress output location in a shape after the forming or the integration point position where the stress is output in the case of the shell element in the forming analysis under the first analysis conditions and the second analysis conditions, a maximum stress in all of the elements or the integration points in the sheet thickness is output.
9 . The delayed fracture characteristic evaluation method according to claim 1 , wherein the metal sheet is a high-tensile steel sheet having a tensile strength of 980 MPa or more.
10 . The delayed fracture characteristic evaluation method according to claim 1 , wherein the intended formed article is a constituent component of an automobile.
11 . The delayed fracture characteristic evaluation method according to claim 1 , wherein the forming analysis is the forming analysis by the finite element method, and with respect to the stress output location in a shape after the forming or the integration point position where the stress is output in the case of the shell element in the forming analysis under the first analysis conditions and the second analysis conditions, a maximum stress in all of the elements or the integration points in the sheet thickness is output.
12 . A non-transitory computer-readable medium storing a computer program thereon, the program configured to perform a delayed fracture characteristic evaluation method comprising: a test step of receiving test data obtained from applying a deformation to a test piece containing a metal sheet, and evaluating a generation status of a crack generated in the test piece by placing the deformed test piece in a hydrogen entry environment, a first step of inputting information about the deformation determined to have a delayed fracture by the evaluation, calculating a maximum residual stress generated in the test piece after the deformation by applying the deformation to the test piece by a forming analysis under first analysis conditions by a computer, and determining, from the calculated maximum residual stress, a reference stress for determining whether the delayed fracture occurs in a formed article of the metal sheet in a hydrogen environment; a second step of determining a residual stress generated in the formed article by forming the metal sheet into an intended formed article by a forming analysis under second analysis conditions by the computer; a third step of determining, from a correlation between a first stress calculated by the forming analysis under the first analysis conditions and a second stress calculated by the forming analysis under the second analysis conditions when press forming is performed under a same forming condition, a conversion factor for bringing the stress calculated by the forming analysis under the first analysis conditions closer to the stress calculated by the forming analysis under the second analysis conditions; and an evaluation step of evaluating the delayed fracture characteristics of the intended formed article from a comparison between the residual stress determined in the second step and a stress after converting the reference stress by the conversion factor, the program causing the computer to realize the first step and the third step.
13 . The non-transitory computer-readable medium storing the program according to claim 12 , wherein the forming analysis is a forming analysis by a finite element method, and the first analysis conditions and the second analysis conditions include one or more conditions selected from an element type, an element size, a stress output location in the formed article, and an integration point position where the stress is output in a shell element.
14 . The non-transitory computer-readable medium storing the program according to claim 13 , the first analysis conditions and the second analysis conditions including the element type, the program comprising: setting the conversion factor based on a difference between the element types of the first analysis conditions and the second analysis conditions.
15 . The non-transitory computer-readable medium storing the program according to claim 14 , wherein a sheet thickness of the metal sheet before the forming is set to t [mm], the element type is set to a 2D solid element, a 3D solid element, or a shell element, the second analysis conditions are that the element type is the shell element and a mesh size of an element used for the forming analysis is m [mm], a mesh size of an element in the first analysis conditions is m [mm], and the conversion factor K for converting the stress calculated by the forming analysis under the first analysis conditions into the stress calculated by the forming analysis under the second analysis conditions is expressed by Equation (1) below, K =α[β( m/t )+1] (1), wherein a coefficient α is set to a constant selected from a range of 0.7 to 0.9 when the element type of the first analysis conditions is the 2D solid element, and is set to 1 when the element type of the first analysis conditions is the 3D solid element or the shell element, and a coefficient β is set to a constant selected from a range of 0.05 to 0.15.
16 . The non-transitory computer-readable medium storing the program according to claim 14 , wherein a sheet thickness of the metal sheet before the forming is set to t [mm], the element type is set to a 2D solid element, a 3D solid element, or a shell element, the first analysis conditions are that the element type is the shell element and a mesh size of an element used for the forming analysis is m [mm], a mesh size of an element in the second analysis conditions is m [mm], and the conversion factor K for converting the stress calculated by the forming analysis under the first analysis conditions into the stress calculated by the forming analysis under the second analysis conditions is expressed by Equation (2) below, K= 1/(α[β( m/t )+1]) (2), wherein a coefficient α is set to a constant selected from a range of 0.7 to 0.9 when the element type of the second analysis conditions is the 2D solid element, and is set to 1 when the element type of the second analysis conditions is the 3D solid element or the shell element, and a coefficient β is set to a constant selected from a range of 0.05 to 0.15.
17 . The non-transitory computer-readable medium storing the program according to claim 12 , wherein the forming analysis is the forming analysis by the finite element method, and for the stress output location in a shape after the forming or the integration point position where the stress is output in the case of the shell element in the forming analysis under the first analysis conditions and the second analysis conditions, a maximum stress in all of the elements or the integration points in the sheet thickness is output.

Description

TECHNICAL FIELD The present invention is a technology related to a method for evaluating the characteristics of a delayed fracture occurring in a formed article after press forming of a metal sheet and a program used therefor. In particular, the present invention is a technology suitable for structural components of automobiles in which metal sheets contain high-tensile steel sheets. For example, the present invention is a technology suitable for evaluating the delayed fracture characteristics of flange end portions in press formed vehicle body structural members, such as center pillars and A-pillar lowers containing the high-tensile steel sheets. BACKGROUND ART At present, automobiles have been required to improve fuel consumption by a reduction in weight and collision safety. For the purpose of achieving both the reduction in weight of a vehicle body and the protection of passengers in the event of a collision, high-strength steel sheets are used for the vehicle body. Particularly in recent years, ultrahigh-strength steel sheets which are high-strength steel sheets having a tensile strength of 980 MPa or more have been applied to the vehicle body. As one of the problems when the ultrahigh-strength steel sheets are applied to the vehicle body, a delayed fracture is mentioned. The delayed fracture is a fracture phenomenon caused by a residual stress and a plastic strain after press forming and hydrogen entering from a hydrogen environment during use. Therefore, the application of high-tensile steel sheets to the vehicle body requires the evaluation of the delayed fracture characteristics according to press forming conditions and the prediction of the occurrence of the delayed fracture. Conventional evaluation methods of the high-tensile steel sheets for press forming for automobiles include, for example, methods described in PTLs 1 to 3. PTL 1 describes a method for evaluating the delayed fracture for a status where a test piece is V-bent, and then further a bending stress by fastening is applied. PTLs 2, 3 describe a method for evaluating the delayed fracture for a status where a tensile residual stress is applied after compressive deformation by deep drawing, stamping, or stamping drawing forming. In all of these conventional findings, the stress generated in the test piece placed in the hydrogen environment is calculated. For example, a stress generated by the deformation applied to the test piece above by the forming is calculated by a simulation analysis by a computer (forming analysis). CITATION LIST Patent Literatures PTL 1: JP 2017-142086 APTL 2: JP 2018-185184 APTL 3: JP 2018-185183 A SUMMARY OF INVENTION Technical Problem The present inventors have examined the delayed fracture characteristics of the high-tensile steel sheets after press forming, and thus have obtained a finding that a calculated stress value in a simulation by a computer in a delayed fracture evaluation test depends on analysis conditions. The simulation by a computer is a forming analysis by the CAE, for example. The analysis conditions as used herein include an element type, an element size, a stress output location in a formed article, and an integration point position where the stress is output in a shell element in the forming analysis by the finite element method. Therefore, even when the delayed fracture characteristics are evaluated by the evaluation methods described in PTLs above and a reference value of a stress serving as the limit of the occurrence of the delayed fracture (limit stress) is calculated, the following problems have arisen. More specifically, when the analysis conditions in calculating the reference value are different from the analysis conditions of the forming analysis for evaluating an actual formed article, there is a difference in the calculated values due to the difference in the analysis conditions. This has posed a problem that the accuracy of the delayed fracture evaluation may deteriorate corresponding to the difference. Herein, as compared with the shape of a press formed article of the test piece, articles for practical are components having a large size and a complicated shape in many cases. Therefore, with respect to the articles for practical use, a forming analysis under analysis conditions of a small number of elements and a small computational load is adopted. On the other hand, the test piece is small and has a simple shape also after deformation. Therefore, there is a tendency to relatively increase the number of elements and relatively increase the computational load in the analysis condition when the reference value is calculated using the test piece. More specifically, there is a tendency to calculate a stress serving as the reference with high accuracy. The reference value of the stress (reference stress) is a value determined by various delayed fracture evaluation methods, and is a limit value at which the delayed fracture occurs. Then, the reference value of the stress is applie