Method for predicting delayed fracture time of press-formed products, apparatus for predicting delayed fracture time of press-formed products, and program for predicting delayed fracture time of press-formed products

Abstract

Predicting when delayed fracture will occur in press-formed steel plate products. [Solution] The method for predicting the delayed fracture time of a press-formed product according to the present invention predicts the time at which delayed fracture occurs in a press-formed product of a steel sheet, and includes the steps of: (S1) deriving a delayed fracture time prediction formula that is a mathematical expression using the stress and strain applied to a uniaxial tensile test piece 11 of a steel sheet, in which a delayed fracture occurs in a delayed fracture test in which various strains and stresses are applied to the uniaxial tensile test piece 11 and the test piece is held in a predetermined hydrogen ingress environment; (S3) calculating the strain distribution and stress distribution of the press-formed product; (S5) obtaining the strain and stress at the part of the press-formed product where delayed fracture is a concern based on the calculated strain distribution and stress distribution of the press-formed product; and (S7) substituting the strain and stress at the part of the press-formed product where delayed fracture is a concern into the delayed fracture time prediction formula to predict the delayed fracture time at which delayed fracture occurs in the part of the press-formed product where delayed fracture is a concern.

Description

[Technical Field]

[0001] The present invention relates to a method for predicting the delayed fracture time of a press-formed product, a device for predicting the delayed fracture time of a press-formed product, and a program for predicting the delayed fracture time of a press-formed product, which predict when delayed fracture will occur in a press-formed product made of steel sheet. [Background technology]

[0002] In response to stricter environmental regulations regarding carbon dioxide emissions and other factors, there is a growing demand for lighter vehicle bodies to improve fuel efficiency. At the same time, improved collision safety performance is also required for vehicle bodies. To meet these needs, the application of high-tensile steel sheets with a tensile strength of 1 GPa or higher to vehicle frame components is progressing. Automotive frame components are generally manufactured by press forming. However, in press-formed products using high-tensile steel sheets with a tensile strength exceeding 980 MPa, there are concerns about strain and residual stress generated during the press forming process and component assembly process, as well as delayed fracture caused by hydrogen entering the vehicle during manufacturing or use.

[0003] Therefore, several methods for evaluating and predicting the delayed fracture characteristics of press-formed high-tensile steel sheets (high-strength steel sheets) have been proposed. For example, Patent Document 1 discloses a method for evaluating delayed fracture characteristics by placing a test piece of deep-drawn high-tensile steel sheet in a hydrogen-ingress environment and observing the crack initiation conditions that occur in the flange portion of the test piece. Furthermore, Patent Document 2 discloses a method for introducing hydrogen into a test specimen of steel material subjected to plastic strain and evaluating hydrogen embrittlement characteristics (delayed fracture characteristics) based on the amount of plastic strain. Furthermore, Patent Document 3 discloses a technique for predicting which parts of a press-formed product will experience delayed fracture based on the strain and stress introduced during press forming. [Prior art documents] [Patent Documents]

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0005] [[ID=2"0]] Generally, for structures, equipment, vehicles, etc., a service life is required at the time of their design. And in order to accurately determine the service life, it is necessary to calculate the life for each cause that can cause deterioration or failure of the object. For the life of automotive parts, in addition to the fatigue life caused by fatigue fracture, there is also a painting life caused by corrosion, etc. However, in order to more accurately calculate the service life of automotive parts, it is necessary to predict the delayed fracture life due to delayed fracture at the time of design.

[0006] Regarding when delayed fracture occurs in a press-formed product obtained by press-forming a steel plate and assembling parts, it has been evaluated by conducting a delayed fracture test in which the press-formed product is held in a predetermined hydrogen intrusion environment, but it has required a great deal of cost. Therefore, a technique for predicting when delayed fracture occurs without conducting a delayed fracture test on the press-formed product has been desired. However, although the techniques of Patent Documents 1 to 3 can evaluate the delayed fracture characteristics of press-formed products and predict the parts at risk of delayed fracture, they cannot predict when delayed fracture will occur in press-formed products without conducting a delayed fracture test.

[0007] The present invention has been made to solve the above problems, and an object thereof is to provide a method for predicting the delayed fracture time of a press-formed product, a device for predicting the delayed fracture time of a press-formed product, and a program for predicting the delayed fracture time of a press-formed product, which can predict when delayed fracture will occur in a press-formed product of a steel plate without conducting a delayed fracture test.

Means for Solving the Problems

[0008] (1) The method for predicting the delayed fracture time of a press-formed product according to the present invention predicts the time at which delayed fracture occurs in a press-formed product of a steel sheet, A delayed fracture time prediction formula acquisition step is performed to obtain a delayed fracture time prediction formula that is mathematically formulated using the stress and strain applied to the test piece, in a delayed fracture test in which various strains and stresses are applied to the steel plate test piece and the test piece is held in a predetermined hydrogen ingress environment, and the delayed fracture time at which delayed fracture occurs in the test piece is obtained. A press-formed product strain distribution and stress distribution calculation process for calculating the strain distribution and stress distribution of the press-formed product, Based on the calculated strain distribution and stress distribution of the press-formed product, a delayed fracture risk area strain and stress acquisition step is performed to acquire the strain and stress at the delayed fracture risk area of ​​the press-formed product, The present invention is characterized by including a delayed fracture time prediction step, which involves substituting the strain and stress in the area of ​​concern for delayed fracture into the delayed fracture time prediction formula to predict the delayed fracture time at which delayed fracture will occur in the area of ​​concern for delayed fracture.

[0009] (2) In the items described in (1) above, The method for obtaining the delayed fracture time prediction formula is characterized by formulating the delayed fracture time prediction formula using the equivalent plastic strain and the maximum principal stress applied to the test specimen.

[0010] (3) In the items described in (2) above, The method for obtaining the delayed fracture time prediction formula is characterized by applying a uniform equivalent plastic strain and the maximum principal stress to the area in the test specimen where delayed fracture occurs.

[0011] (4) In any of the items described in (1) to (3) above, The present invention is characterized in that, in the process of acquiring the strain and stress of the delayed fracture area, the delayed fracture area is defined as the edge of the plate of the press-formed product.

[0012] (5) The delayed fracture time prediction device for press-formed products according to the present invention predicts the time at which delayed fracture occurs in a press-formed product of a steel sheet, In a delayed fracture test in which a steel plate test specimen is subjected to predetermined strain and stress and held in a predetermined hydrogen ingress environment, a delayed fracture time prediction formula acquisition unit acquires a delayed fracture time prediction formula that is a mathematical expression of the stress and strain, which determines the delayed fracture time at which delayed fracture occurs in the test specimen. A press-formed product strain distribution and stress distribution calculation unit calculates the strain distribution and stress distribution of the press-formed product, A unit for acquiring delayed fracture risk strain and stress in the calculated press-formed product, and for acquiring the strain and stress in the identified delayed fracture risk area, The invention is characterized by comprising: a delayed fracture time prediction unit that substitutes the strain and stress in the area of ​​concern for delayed fracture into the delayed fracture time prediction formula and predicts the time at which delayed fracture will occur in the area of ​​concern for delayed fracture.

[0013] (6) The delayed fracture time prediction program for press-formed products according to the present invention predicts the time at which delayed fracture occurs in a press-formed product of a steel sheet, Computers, A delayed fracture time prediction formula acquisition unit acquires a delayed fracture time prediction formula that is a mathematical expression of the stress and strain, which is obtained by a delayed fracture test in which a steel plate test piece is subjected to a predetermined strain and stress and held in a predetermined hydrogen ingress environment, and the delayed fracture time at which the test piece in which delayed fracture occurs is measured, A press-formed product strain distribution and stress distribution calculation unit calculates the strain distribution and stress distribution of the press-formed product, A unit for acquiring delayed fracture risk strain and stress in the calculated press-formed product, and for acquiring the strain and stress in the identified delayed fracture risk area, The system is characterized by having a delayed fracture time prediction unit that substitutes the strain and stress in the area of ​​concern for delayed fracture into the delayed fracture time prediction formula and predicts the time at which delayed fracture will occur in the area of ​​concern for delayed fracture, and has a function to perform this function. [Effects of the Invention]

[0014] According to the present invention, by predicting the delayed fracture time of a press-formed steel sheet, it is possible to predict the lifespan of the press-formed steel sheet without conducting a delayed fracture test. [Brief explanation of the drawing]

[0015] [Figure 1] This is a flowchart showing the processing flow in a method for predicting the delayed fracture time of a press-formed product according to an embodiment of the present invention. [Figure 2] This graph shows the relationship between strain and hydrogen concentration obtained from a hydrogen charge test using test specimens strained 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] This figure illustrates the uniaxial tensile test specimen used in the delayed fracture test in this embodiment ((a) before the delayed fracture test, (b) after the occurrence of delayed fracture). [Figure 4] This figure shows the background to the present invention and the examples, illustrating the panel-shaped press-formed product used for predicting delayed fracture time ((a) shape, (b) photograph). [Figure 5] This contour plot shows (a) the equivalent plastic strain distribution and (b) the maximum principal stress distribution of a press-formed product used to predict the delayed fracture time, which is the background to the present invention. [Figure 6] This figure shows the configuration of a delayed fracture time prediction device and a delayed fracture time prediction program for press-formed products according to an embodiment of the present invention. [Figure 7] This graph shows the relationship between the delayed fracture time and the equivalent plastic strain, as measured by a delayed fracture test using a uniaxial tensile specimen in the example. [Figure 8] This graph shows the relationship between the delayed fracture time and the maximum principal stress, as measured by a delayed fracture test using a uniaxial tensile specimen in the example. [Figure 9]The graph in the example shows the relationship between the delayed fracture time measured by a delayed fracture test of a panel-shaped press-formed product and the bending radius of the bent portion of the press-formed product. [Modes for carrying out the invention]

[0016] Before describing embodiments of the present invention, the background leading to the present invention will be explained. [Background leading to the present invention] Delayed fracture is a brittle fracture phenomenon caused by hydrogen infiltration into steel, and is thought to occur in areas where strain, stress, and hydrogen concentration reach certain conditions. Therefore, the inventors proposed that the delayed fracture time t is the time at which delayed fracture occurs in steel. f As shown in equation (1) below, the hydrogen concentration C at the site where delayed fracture occurs H I thought it might be possible to express this using a mathematically formulated function that utilizes strain ε and stress σ.

number

[0017] Next, the inventors conceived of a method to predict the delayed fracture time of a press-formed steel sheet: conducting a delayed fracture test on a test piece using the steel sheet as the test material to determine the strain, stress, and delayed fracture time at the location where delayed fracture occurs, and then formulating the delayed fracture time mathematically in relation to the strain and stress. Furthermore, we conceived the idea that if we could obtain the strain and stress of a part of a press-formed product where delayed fracture is a concern (referred to in this application as the "part of the press-formed product where delayed fracture is a concern"), we could use the delayed fracture time, which is mathematically formulated using strain and stress, to predict the delayed fracture time without conducting a delayed fracture test on the press-formed product. Therefore, based on this idea, the inventors conducted the following investigations (a) to (e) to determine whether it was possible to predict the delayed fracture time of press-formed steel plates.

[0018] [Study (a): Relationship between delayed fracture time and stress] In mechanics of materials, there is a theory that when the maximum principal stress reaches a certain value, a crack occurs at that point, and the crack propagates perpendicular to the maximum principal stress, leading to fracture (maximum principal stress theory). To verify this in delayed fracture, the inventors placed a press-formed product in a predetermined hydrogen-impregnated environment for a certain period of time and observed the occurrence of delayed fracture and the crack propagation behavior. As a result, it was confirmed that in the press-formed product, delayed fracture occurred in areas with high maximum principal stress, and the crack propagated perpendicular to the maximum principal stress.

[0019] From these verification results, the effect of stress on the delayed fracture time of press-formed products is the maximum principal stress σ at the area of ​​concern for delayed fracture. max Since it is thought to depend on the value of , the aforementioned equation (1) can be expressed in the following form.

number

[0020] [Study (b): Relationship between delayed fracture time, strain, and hydrogen concentration] From the study (1), it became clear that delayed fracture occurs as explained by the maximum principal stress theory, but the maximum principal stress σ is the limiting stress that leads to the occurrence of delayed fracture. max The value of is thought to be determined by the strain and hydrogen concentration at the site of delayed fracture. Therefore, the inventors investigated the relationship between hydrogen concentration and strain with respect to delayed fracture time.

[0021] Generally, press-formed steel sheets exhibit strains resulting from various deformation modes, including bending (plane strain), bulging (biaxial tension), uniaxial tension, and uniaxial compression. Therefore, the inventors prepared test pieces of steel sheets with strain applied in various deformation modes, conducted a hydrogen charging test in which the test pieces were placed in a hydrogen intrusion environment for a certain period of time and then the amount of hydrogen in the test pieces was measured, and investigated the relationship between strain and hydrogen concentration with different deformation modes. In this investigation, cold-rolled steel sheets with a thickness of 1.2 mm and a tensile strength of 1470 MPa grade were used as test materials, and various strains were applied in deformation modes of rolling (plane strain), uniaxial tension, uniaxial compression, or biaxial tension / compression.

[0022] Figure 2 is an example of the result of measuring the amount of hydrogen in the test pieces after immersing the test pieces with various strains applied in each deformation mode in ammonium thiocyanate and McIlvaine buffer solution with pH = 4.0 and a concentration of 0.1% for 30 hours and organizing the relationship between hydrogen concentration and strain. Figure 2(a) shows the relationship between the equivalent plastic strain applied to the test piece and the hydrogen concentration, and Figure 2(b) shows the relationship between the deformation mode and the hydrogen concentration when the equivalent plastic strain applied to the test piece is constant (= 0.01, 0.02). Regarding the correspondence between the ratio of the maximum principal strain and the minimum principal strain (hereinafter referred to as the "strain ratio") and the deformation mode in Figure 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.

[0023] As shown in Figure 2(b), the hydrogen concentration C H is found to depend on the equivalent plastic strain ε p eq regardless of the deformation mode. It is considered that as the equivalent plastic strain ε p eq increases, the hydrogen concentration C H increases because the defects in the test piece where hydrogen is trapped increase with the increase of the equivalent plastic strain ε p eq , so the amount of hydrogen invading the test piece increases. From this, the hydrogen concentration C H can be expressed by the following formula (3).

Equation

[0024] Substituting equation (3) into equation (2), we get the delayed breakdown time t. f It can be expressed by the following equation (4).

number

[0025] [Consideration (c): Test specimens to be used in delayed fracture testing] Next, the inventors considered test specimens that reproduce the stress and strain in areas of press-formed products where delayed fracture is a concern.

[0026] Although the shape of the test specimen used to reproduce stress and strain is not considered to be particularly limited, the inventors decided to use, as an example, a uniaxial tensile test specimen 11 described in the publicly known document, Japanese Patent Publication No. 7563424, as shown in Figure 3. This is because, in the uniaxial tensile test specimen 11, a uniaxial tensile stress corresponding to the positive maximum principal stress can be uniformly applied to the parallel section 13 where cracking occurs in a normal uniaxial tensile test. Furthermore, regarding the strain applied to the uniaxial tensile test specimen 11, a strain equivalent to the equivalent plastic strain of the area of ​​concern for delayed fracture was uniformly applied to the parallel section 13 by cutting out steel plates that had been uniformly strained by bending (plane strain), stretching (biaxial tension), uniaxial tension, uniaxial compression, etc.

[0027] [Consideration (d): Calculation of strain distribution and stress distribution of press-formed products] Next, the inventors investigated a method for determining strain and stress in press-formed products. Strain and stress in press-formed products can be determined by CAE analysis using the finite element method, such as press forming analysis, shear analysis of cutting, trimming, and piercing of sheet metal before press forming, and springback analysis. These CAE analyses are commonly used analytical methods in the design of automotive frame parts and the like. It is considered that the equivalent plastic strain and maximum principal stress values ​​of the CAE analysis element corresponding to the delayed fracture site of a press-formed product can be obtained by using the equivalent plastic strain and maximum principal stress values ​​of the CAE analysis element corresponding to that site.

[0028] [Consideration (e): Areas of concern regarding delayed fracture in press-formed products] Next, the inventors examined areas in press-formed products that were at risk of delayed fracture. Delayed fracture in press-formed products is said to occur at the edges of the sheet metal. The reason for this is that the edges of press-formed products used as structural components for automobiles, etc., are sheared edges created when the blanks for press forming are manufactured from steel sheets, and the strain and stress at these sheared edges are considered to be the cause. On the other hand, inside the sheet metal, because there is material surrounding it, even if the strain and stress are high, the material does not open up, and cracking is less likely to occur.

[0029] Therefore, in order to confirm that delayed fracture is likely to occur at the edges of the press-formed product, a recess 23 was formed extending in the thickness direction from the edge of the plate into the interior of the plate, as shown in Figure 4, and a delayed fracture test was conducted on the press-formed product 21 with higher strain and stress inside the plate than at the edge. Figure 4(a) is a diagram showing the shape of the press-formed product 21 in plan view, and Figure 4(b) is a photograph of the external appearance of the press-formed product 21. The bending radius of the bent portion 23a in the recess 23 was set to 10 mm.

[0030] Next, the inventors performed a CAE analysis of the press-formed product 21 process and calculated the strain distribution and stress distribution of the press-formed product 21. Figure 5(a) is a contour plot of the equivalent plastic strain distribution obtained by the CAE analysis, and Figure 5(b) is a contour plot of the maximum principal stress distribution.

[0031] In Figure 5, the equivalent plastic strain inside the plate, indicated by the arrow, is 0.0779 MPa, and the maximum principal stress is 1389 MPa. On the other hand, the equivalent plastic strain at the plate edge, indicated by the arrow, is 0.0023 MPa, and the maximum principal stress is 1081 MPa. Both the equivalent plastic strain and the maximum principal stress are smaller at the plate edge than inside the plate.

[0032] Furthermore, four press-formed products 21 were prepared from a cold-rolled steel sheet (1.2 mm thick) with a tensile strength of 1470 MPa. These were immersed in a 0.1% ammonium thiocyanate and McIlbain buffer solution with a pH of 4.0, and the locations where cracks occurred and the time until cracks occurred were measured. In all four press-formed parts 21 produced, cracks occurred at the edges of the plates, as indicated by the arrows in Figure 4, whereas no cracks occurred inside the plates. This indicates that delayed fracture of the press-formed parts 21 occurs at the edges of the plates.

[0033] Based on the results of the above studies (a) to (e), the inventors further investigated specific means for predicting the delayed fracture time in press-formed steel sheets. The present invention is based on these studies, and its specific configuration is described below.

[0034] [Embodiment] <Method for predicting delayed breakdown time> The delayed fracture time prediction method for press-formed products according to an embodiment of the present invention (hereinafter simply referred to as the "delayed fracture time prediction method") predicts the time at which delayed fracture occurs in a press-formed product made of steel sheet. As shown in Figure 1, the delayed fracture time prediction method includes a delayed fracture time prediction formula acquisition step S1, a press-formed product strain distribution and stress distribution calculation step S3, a delayed fracture concern strain and stress acquisition step S5, and a delayed fracture time prediction step S7. Each of these steps will be described below.

[0035] ≪Process for obtaining a delayed failure time prediction formula≫ Step S1, which involves obtaining a delayed fracture time prediction formula, is a process of deriving a delayed fracture time prediction formula that mathematically represents the delayed fracture time at which a steel plate test specimen undergoes delayed fracture in a predetermined hydrogen immersion environment while being subjected to various strains and stresses, using the stresses and strains applied to the test specimen.

[0036] In this embodiment, as described in the above consideration (c), a delayed fracture test is performed using the uniaxial tensile test specimen 11 shown in Figure 3. The uniaxial tensile test specimen 11 is made from a steel plate that has been subjected to various equivalent plastic strains by rolling or other means. The uniaxial tensile test specimen 11 is then held in a hydrogen-impregnated environment with a predetermined maximum principal stress applied to the parallel section 13, and the delayed fracture time until delayed fracture occurs in the uniaxial tensile test specimen is measured. The steel plate used in the delayed fracture test is made of the same material as the press-formed product used for predicting the delayed fracture time.

[0037] For the hydrogen intrusion environment in the delayed fracture test, for example, the uniaxial tensile test specimen 11 may be immersed in a 0.1% concentration ammonium thiocyanate and McIlbain buffer solution at pH=4.0. Furthermore, the delayed fracture time can be measured based on the change in load applied to the parallel section 13 using a jig with strain gauges embedded in the bolt, as shown in, for example, the aforementioned Japanese Patent Publication No. 7563424.

[0038] Then, in the delayed fracture time prediction formula acquisition process S1, the delayed fracture time measured by the delayed fracture test is expressed mathematically using the equivalent plastic strain and maximum principal stress applied to the uniaxial tensile test specimen 11, and the delayed fracture time prediction formula is derived (see formula (4) above). The specific delayed fracture time prediction formula will be explained in the example described later.

[0039] <<Process for calculating strain and stress distribution in press-formed products>> Step S3, which calculates the strain distribution and stress distribution of a press-formed product, is a step in which the strain distribution and stress distribution of the press-formed product are calculated.

[0040] In this embodiment, step S3, which calculates the strain distribution and stress distribution of a press-formed product, calculates the strain and stress distribution of the press-formed product by calculating the strain and stress for each CAE analysis element of the press-formed product.

[0041] CAE analysis methods commonly used in automobile design and other applications can be applied to calculate the strain and stress distribution of press-formed products. Examples of CAE analysis methods include press-forming CAE analysis, shear CAE analysis of cutting, trimming, and piercing of sheet metal before press-forming, springback CAE analysis of press-formed products, and CAE analysis of press-formed product assembly. By determining the stress and strain for each CAE element in the press-formed product using CAE analysis with the finite element method, the stress and strain distribution of the press-formed product can be calculated.

[0042] <<Process for obtaining strain and stress in areas of concern for delayed fracture>> Step S5, which involves obtaining the strain and stress of the areas of concern for delayed fracture, is a step in which the strain and stress of the press-formed product are obtained based on the calculated strain distribution and stress distribution of the press-formed product.

[0043] In this embodiment, in step S5, which involves acquiring the strain and stress of the delayed fracture area, the edge of the press-formed product is designated as the delayed fracture area, as described in the above study (e), and the equivalent plastic strain and maximum principal stress are acquired.

[0044] <<Delayed Failure Time Prediction Process>> The delayed fracture time prediction step S7 is a step in which the strain and stress at the area of ​​concern for delayed fracture are substituted into the delayed fracture time prediction formula to predict the delayed fracture time at which delayed fracture will occur at the area of ​​concern for delayed fracture.

[0045] In this embodiment, the equivalent plastic strain and maximum principal stress at the area of ​​concern for delayed fracture, obtained in step S5, are substituted into the delayed fracture time prediction formula derived in step S1, and the delayed fracture time is calculated.

[0046] As described above, in the delayed fracture time prediction method for press-formed products according to this embodiment, by predicting the delayed fracture time of a press-formed product made of steel sheet, it is possible to predict the lifespan of the press-formed product due to delayed fracture without conducting a delayed fracture test on the press-formed product made of steel sheet.

[0047] Furthermore, according to the delayed fracture time prediction method for press-formed products of this embodiment, the delayed fracture time of different types of press-formed products can be predicted using the same criteria without conducting delayed fracture tests. The same criteria refers to the same hydrogen ingress environment as used in delayed fracture tests using test specimens.

[0048] Furthermore, according to the present invention, it becomes possible to appropriately implement measures to counter delayed fracture, such as changing the shape of the press-formed product or the press-forming process, based on the predicted lifespan due to delayed fracture. Specific countermeasures will be considered in accordance with the shape and molding process of individual press-formed products, but generally, measures to reduce strain by promoting material flow during press forming, or measures to reduce stress on press-formed products during use by increasing the plate thickness, can be considered. In any case, according to the present invention, a quantitative guideline for reducing strain and stress can be obtained.

[0049] In the above explanation, the delayed fracture time prediction formula derived in the delayed fracture time prediction formula acquisition process was a mathematical expression of the delayed fracture time in terms of equivalent plastic strain and maximum principal stress. However, the present invention is not limited to equivalent plastic strain and maximum principal stress. Nevertheless, as described in the above considerations (a) and (b), it is preferable to mathematically express the delayed fracture time in terms of equivalent plastic strain and maximum principal stress.

[0050] Furthermore, this embodiment involved performing a delayed fracture test using the uniaxial tensile test specimen 11 shown in Figure 3. However, the present invention does not particularly limit the shape of the test specimen used in the delayed fracture test; any shape that can apply uniform strain (equivalent plastic strain) and stress (maximum principal stress) to the part of the test specimen where delayed fracture occurs is acceptable. For example, a test specimen that can be subjected to uniform tensile stress is the "plate-shaped test specimen" as defined in JIS Z2241. One method for applying uniform plastic strain to a steel plate is to apply uniaxial tensile strain to the parallel section of the "plate-shaped test specimen" as defined in JIS Z2241, and then cut out the uniaxial tensile test specimen 11 shown in Figure 3(a) from the parallel section.

[0051] The above explanation, as stated in consideration (e), assumed that the edges of the press-formed product were areas of concern for delayed fracture. However, if other areas prone to delayed fracture are known in advance, these areas may also be designated as areas of concern for delayed fracture, and the delayed fracture time may be calculated accordingly.

[0052] The delayed fracture time prediction method for press-formed products according to this embodiment involves measuring the delayed fracture time by conducting delayed fracture tests on uniaxial tensile test specimens subjected to various strains and stresses, and deriving a delayed fracture time prediction formula that mathematically expresses the delayed fracture time in terms of equivalent plastic strain and maximum principal stress. However, the delayed fracture time prediction method for press-formed products according to the present invention is not limited to conducting a delayed fracture test, but may also involve obtaining a known delayed fracture time prediction formula that is mathematically formulated using strain and stress for steel plates of the same material (steel type, tensile strength).

[0053] Furthermore, the present invention can be preferably applied to press-formed products of high-tensile steel sheets where delayed fracture is a concern, and is more preferably applied to press-formed products of high-tensile steel sheets with a tensile strength of 1 GPa or higher.

[0054] Furthermore, while the press-formed products targeted by this invention are not limited to specific parts, they can be preferably applied to automotive body frame components such as center pillars and A-pillar lowers, which are press-formed from high-tensile steel sheets.

[0055] <Prediction device for delayed fracture time of press-formed products> The delayed fracture time prediction device for press-formed products according to an embodiment of the present invention (hereinafter simply referred to as "delayed fracture time prediction device 1") predicts the time at which delayed fracture occurs in a press-formed product made of steel sheet. As shown in Figure 6, the delayed fracture time prediction device 1 comprises a delayed fracture time prediction formula acquisition unit 3, a press-formed product strain distribution and stress distribution calculation unit 5, a delayed fracture concern area strain and stress acquisition unit 7, and a delayed fracture time prediction unit 9. The delayed failure time prediction device 1 may be configured using the CPU (Central Processing Unit) of a computer (PC, etc.). In this case, each of the above components functions when the computer's CPU executes a predetermined program.

[0056] ≪Delayed Destruction Time Prediction Formula Acquisition Unit≫ The delayed fracture time prediction formula acquisition unit 3 derives a delayed fracture time prediction formula that mathematically represents the delayed fracture time at which delayed fracture occurs in a steel plate test specimen in a predetermined hydrogen immersion environment while various strains and stresses are applied to the specimen, using the stress and strain applied to the specimen.

[0057] In this embodiment, the delayed fracture time prediction formula acquisition unit 3 acquires the delayed fracture time measured by a delayed fracture test performed using the uniaxial tensile test specimen 11 shown in Figure 3, as well as the equivalent plastic strain and maximum principal stress applied to the parallel portion 13 of the uniaxial tensile test specimen 11. The delayed fracture time prediction formula acquisition unit 3 then formulas the acquired delayed fracture time using the equivalent plastic strain and maximum principal stress to derive a delayed fracture time prediction formula.

[0058] Furthermore, the steel plate used in deriving the delayed fracture test time prediction formula shall be a steel plate of the same material (tensile strength, plate thickness, etc.) as the press-formed product used for predicting the delayed fracture time.

[0059] <Press-formed product strain distribution and stress distribution calculation unit> The press-formed product strain distribution and stress distribution calculation unit 5 calculates the strain distribution and stress distribution of the press-formed product.

[0060] In this embodiment, the press-formed product strain distribution and stress distribution calculation unit 5 calculates the strain distribution and stress distribution of the press-formed product by calculating the strain and stress for each CAE analysis element of the press-formed product.

[0061] The calculation of the strain distribution and stress distribution of a press-formed product by the press-formed product strain distribution and stress distribution calculation unit 5 can be performed using CAE analysis methods commonly used in automobile design, similar to the press-formed product strain distribution and stress distribution calculation process S3 described above.

[0062] <<Strain and stress acquisition areas in areas of concern for delayed fracture>> The delayed fracture risk area strain and stress acquisition section 7 acquires the strain and stress at the delayed fracture risk area of ​​the press-formed product based on the calculated strain distribution and stress distribution of the press-formed product.

[0063] In this embodiment, the delayed fracture risk area strain and stress acquisition section 7, similar to the delayed fracture risk area strain and stress acquisition step S5 described above, uses the edge of the press-formed product as the delayed fracture risk area and acquires the equivalent plastic strain and maximum principal stress.

[0064] ≪Delayed Destruction Time Prediction Unit≫ The delayed fracture time prediction unit 9 substitutes the strain and stress at the area of ​​concern for delayed fracture into the delayed fracture time prediction formula and predicts the delayed fracture time at which delayed fracture will occur at the area of ​​concern for delayed fracture.

[0065] In this embodiment, the equivalent plastic strain and maximum principal stress at the delayed fracture site, obtained by the delayed fracture site strain and stress acquisition unit 7, are substituted into the delayed fracture time prediction formula derived by the delayed fracture time prediction formula acquisition unit 3 to calculate the delayed fracture time.

[0066] <Prediction program for delayed fracture time of press-formed parts> Embodiments of the present invention can be configured as a delayed fracture time prediction program for press-formed products (hereinafter simply referred to as the "delayed fracture time prediction program"). In other words, the delayed fracture time prediction program predicts the time at which delayed fracture will occur in a press-formed steel sheet. The delayed fracture time prediction program has the function of running a computer as follows, for example as shown in Figure 6: a delayed fracture time prediction formula acquisition unit 3, a press-formed product strain distribution and stress distribution calculation unit 5, a delayed fracture concern area strain and stress acquisition unit 7, and a delayed fracture time prediction unit 9.

[0067] As described above, the delayed fracture time prediction device 1 and delayed fracture time prediction program according to this embodiment can predict when delayed fracture will occur in a press-formed steel sheet, similar to the delayed fracture time prediction method according to this embodiment described above. This makes it possible to predict the lifespan of a press-formed steel sheet due to delayed fracture without conducting a delayed fracture test on the press-formed steel sheet.

[0068] Furthermore, according to the delayed fracture time prediction device 1 and delayed fracture time prediction program of this embodiment, the delayed fracture time can be predicted using the same criteria for different types of press-molded products without conducting delayed fracture tests. [Examples]

[0069] Experiments and analyses were conducted to confirm the effects and benefits of the present invention, and these are described below.

[0070] In this example, the delayed fracture time was predicted for the press-formed product 21 shown in Figure 4. The press-formed product 21 is made by press-forming a cold-rolled steel sheet with a thickness of 1.2 mm and a tensile strength of 1470 MPa. As shown in Figure 4, a recess 23 is formed in the thickness direction from the edge of the sheet to the interior of the sheet. The press-formed product 21 has a bending radius of 10 mm, 20 mm, or 25 mm at the bent portion 23a in the recess 23.

[0071] In order to predict the delayed fracture time of the press-formed product 21, a delayed fracture time prediction formula was derived for the cold-rolled steel sheet used for the press-formed product 21. First, the cold-rolled steel sheet used as the test material was rolled in the thickness direction to apply a uniform strain. The uniform strain applied to the cold-rolled steel sheet was set to an equivalent plastic strain of 0.05, 0.1, or 0.39.

[0072] Next, a uniaxial tensile test specimen 11, shown in Figure 3, was cut from a cold-rolled steel sheet that had been subjected to uniform strain. The uniaxial tensile test specimen 11 was prepared by cutting it in such a way that the equivalent plastic strain described above was uniformly applied to the parallel section 13. Furthermore, uniaxial tensile test specimens 11 were also cut from cold-rolled steel sheets that had not been rolled and therefore had not been subjected to equivalent plastic strain (equivalent plastic strain was 0).

[0073] Next, a delayed fracture test was conducted on a uniaxial tensile test specimen 11, which was held in a hydrogen-impregnated environment while a tensile load was applied to it so that the maximum principal stress in the parallel section 13 was 600, 800, 1000, or 1200 MPa. The time until cracking occurred (delayed fracture time) was measured. Here, the hydrogen-impregnated environment consisted of ammonium thiocyanate and McIlbain buffer solution with a pH of 4.0 and a concentration of 0.1%.

[0074] In the delayed fracture test, the crack in the uniaxial tensile specimen 11 occurred in the parallel section 13, as shown in Figure 3(b). From this result, in order to clarify the influence of strain and stress on the delayed fracture time of the uniaxial tensile specimen 11, the relationship between the measured delayed fracture time, the equivalent plastic strain in the parallel section 13 applied by rolling the steel plate, and the maximum principal stress applied to the parallel section 13 was confirmed.

[0075] Figure 7 shows the delayed fracture time t measured by the delayed fracture test. f And the equivalent plastic strain ε applied to the parallel section 13 of the uniaxial tensile test specimen 11 p eq This graph shows the relationship between and . Figure 8 also shows the delayed fracture time t measured by the delayed fracture test. fThe maximum principal stress σ applied to the parallel section 13 of the uniaxial tensile test specimen 11. max This graph shows the relationship between and . As shown in Figures 7 and 8, equivalent plastic strain ε p eq or maximum principal stress σ max It can be seen that the larger the value, the lower the delay and destruction time.

[0076] From the results shown in Figures 7 and 8, the delayed fracture time t measured by the delayed fracture test using the uniaxial tensile test specimen 11 is shown. f The equivalent plastic strain ε applied to the parallel section 13 p eq and maximum principal stress σ max This was then formulated mathematically, and the delayed failure time prediction formula shown in equation (5) below was derived.

number

[0077] Figures 7 and 8 show the delayed failure time t calculated by equation (5). f This is shown by a solid line. The plots representing the measured delayed fracture times shown in Figures 7 and 8 are located almost exactly on the solid line representing the predicted value of the delayed fracture time obtained by equation (5). From this, it can be seen that the delayed fracture time prediction formula expressed by equation (5) is able to accurately formulate the measured value of the delayed fracture time in the delayed fracture test of the uniaxial tensile test specimen 11.

[0078] Thus, various equivalent plastic strains ε p eq and the maximum principal stress σ max From the results of the delayed fracture test of the uniaxial tensile test specimen 11 to which the value was applied, the equivalent plastic strain was 0 ≤ ε p eq≤0.39, maximum principal stress is 600 MPa ≤ σ max Delayed failure time t in the range of ≤1200MPa f We were able to derive a predictable delayed failure time prediction formula.

[0079] Next, we verified whether the derived delayed fracture time prediction formula could predict the delayed fracture time of press-formed products. For the verification, a blank was prepared from a cold-rolled steel sheet with a thickness of 1.2 mm and a tensile strength of 1470 MPa, and then press-formed into the press-formed product 21 shown in Figure 4. In the press-formed product 21, the bending radius of the bent portion 23a in the recess 23 was set to 10 mm, 20 mm, or 25 mm. Then, the press-formed product 21 was immersed in ammonium thiocyanate and McIlbain buffer solution with a pH of 4.0 and a concentration of 0.1%, and the locations where cracks occurred in the press-formed product 21 and the time until cracks occurred were measured.

[0080] Next, a CAE analysis was performed on the press-formed product 21 to calculate the equivalent plastic strain distribution and the maximum principal stress distribution. Then, the plate edges where cracks occurred in the delayed fracture test of the press-formed product 21 were identified as areas of concern for delayed fracture, and the equivalent plastic strain and maximum principal stress of the CAE analysis elements corresponding to these areas of concern for delayed fracture were determined. Furthermore, the equivalent plastic strain and maximum principal stress values ​​of the areas of concern for delayed fracture were substituted into the aforementioned delayed fracture time prediction formula (Equation (5)) to calculate the predicted value of the delayed fracture time.

[0081] Table 1 shows the equivalent plastic strain and maximum principal stress at the delayed fracture concern area, and the predicted value of the delayed fracture time calculated using the delayed fracture time prediction formula, for press-formed products 21 with a bending radius of 10 mm, 20 mm, or 25 mm at the bent portion 23a. Figure 9 is a graph comparing the measured and predicted delayed fracture times for each press-formed product. [Table 1]

[0082] As shown in Figure 9, the difference between the measured and predicted delayed fracture times was within +8 hours. This result demonstrates that the delayed fracture time of press-formed parts can be predicted with high accuracy. [Explanation of Symbols]

[0083] 1. Delayed Destruction Time Prediction Device 3. Unit for obtaining the delayed breakdown time prediction formula 5. Press-formed product strain distribution and stress distribution calculation unit 7. Areas of concern regarding delayed fracture: Strain and stress acquisition areas 9. Delayed Destruction Time Prediction Unit 11 Uniaxial tensile test specimens 13 Parallel section 21 Press-formed products 23 Recess 23a Bent section

Claims

1. A method for predicting the delayed fracture time of a press-formed steel plate product, which predicts the time at which delayed fracture occurs in the press-formed product, A delayed fracture time prediction formula acquisition step is performed to obtain a delayed fracture time prediction formula that is mathematically formulated using the stress and strain applied to the test piece, in a delayed fracture test in which various strains and stresses are applied to the steel plate test piece and the test piece is held in a predetermined hydrogen ingress environment, and the delayed fracture time at which delayed fracture occurs in the test piece is obtained. A press-formed product strain distribution and stress distribution calculation process for calculating the strain distribution and stress distribution of the press-formed product, Based on the calculated strain distribution and stress distribution of the press-formed product, a delayed fracture risk area strain and stress acquisition step is performed to acquire the strain and stress at the delayed fracture risk area of ​​the press-formed product, A method for predicting the delayed fracture time of a press-formed product, characterized by including a delayed fracture time prediction step of substituting the strain and stress in the area of ​​concern for delayed fracture into the delayed fracture time prediction formula to predict the delayed fracture time at which delayed fracture will occur in the area of ​​concern for delayed fracture.

2. The method for predicting the delayed fracture time of a press-formed product according to claim 1, characterized in that, in the step of obtaining the delayed fracture time prediction formula, the delayed fracture time prediction formula is formulated mathematically using the equivalent plastic strain and the maximum principal stress applied to the test piece.

3. The method for predicting the delayed fracture time of a press-formed product according to claim 2, characterized in that, in the step of obtaining the delayed fracture time prediction formula, a uniform equivalent plastic strain and a maximum principal stress are applied to the part of the test piece where delayed fracture occurs.

4. A method for predicting the delayed fracture time of a press-formed product according to any one of claims 1 to 3, characterized in that, in the step of acquiring the strain and stress of the area of ​​concern for delayed fracture, the area of ​​concern for delayed fracture is the end of the plate of the press-formed product.

5. A device for predicting the delayed fracture time of a press-formed steel plate, which predicts the time at which delayed fracture occurs in the press-formed steel plate, In a delayed fracture test in which a steel plate test specimen is subjected to predetermined strain and stress and held in a predetermined hydrogen ingress environment, a delayed fracture time prediction formula acquisition unit acquires a delayed fracture time prediction formula that is a mathematical expression of the stress and strain, which determines the delayed fracture time at which delayed fracture occurs in the test specimen. A press-formed product strain distribution and stress distribution calculation unit calculates the strain distribution and stress distribution of the press-formed product, A unit for acquiring delayed fracture risk strain and stress in the calculated press-formed product, and for acquiring the strain and stress in the identified delayed fracture risk area, A delayed fracture time prediction device for a press-formed product, comprising: a delayed fracture time prediction unit that substitutes the strain and stress in the area of ​​concern for delayed fracture into the delayed fracture time prediction formula and predicts the time at which delayed fracture will occur in the area of ​​concern for delayed fracture.

6. A program for predicting the delayed fracture time of a press-formed steel plate, which predicts the time at which delayed fracture occurs in the press-formed steel plate, Computers, A delayed fracture time prediction formula acquisition unit acquires a delayed fracture time prediction formula that is a mathematical expression of the stress and strain, which is obtained by a delayed fracture test in which a steel plate test piece is subjected to a predetermined strain and stress and held in a predetermined hydrogen ingress environment, and the delayed fracture time at which the test piece in which delayed fracture occurs is measured, A press-formed product strain distribution and stress distribution calculation unit calculates the strain distribution and stress distribution of the press-formed product, A unit for acquiring delayed fracture risk strain and stress in the calculated press-formed product, and for acquiring the strain and stress in the identified delayed fracture risk area, A delayed fracture time prediction program for a press-formed product, characterized by comprising a delayed fracture time prediction unit that substitutes the strain and stress in the area of ​​concern for delayed fracture into the delayed fracture time prediction formula and predicts the time at which delayed fracture will occur in the area of ​​concern for delayed fracture, and a function to execute this unit.

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