Evaluation method for delayed fracture properties of welded joints of metallic materials

Abstract

The purpose of the present invention is to provide a method for evaluating delayed fracture characteristics of a weld zone of a metal material, the method being capable of accurately and quantitatively evaluating delayed fracture characteristics of the weld zone of the metal material.　The method for evaluating delayed fracture characteristics of a weld zone of a metal material comprises: a hydrogen introduction step for introducing hydrogen into a test material composed of two or more metal material pieces having a weld zone; a tensile load application step for applying a tensile load to the hydrogen-introduced test material and acquiring a displacement-load curve; and a first evaluation step for evaluating an initial cracking load on the basis of the displacement-load curve.

Description

[Technical Field]

[0001] The present invention relates to a method for evaluating delayed fracture properties of welded portions of metallic materials. [Background technology]

[0002] In recent years, from the viewpoint of preventing global warming, there has been a demand for improving energy efficiency, for example, improving gasoline fuel efficiency in automobiles, by reducing the weight of mobile objects such as automobiles, ships, and railway vehicles. Steel, one of the materials used to construct mobile objects, requires high strength to ensure the same level of safety as before, even when the plate thickness is reduced to reduce weight. Meanwhile, automotive steel is plastically formed into a desired shape by press forming and then assembled to the vehicle body by welding, for example, spot welding. Therefore, automotive steel is required to have not only press formability but also the ability to be properly welded in various sheet combinations.

[0003] However, increasing the strength of steel materials increases the likelihood of a phenomenon known as delayed fracture. Delayed fracture is a phenomenon in which, after a certain period of time has passed while steel materials are subjected to a static load, sudden brittle fracture occurs without any apparent plastic deformation. Here, it refers to hydrogen embrittlement-type fracture caused by hydrogen entering the steel material. Known factors that influence delayed fracture include the hydrogen embrittlement susceptibility of the steel material, the applied load (e.g., residual stress), and the amount of hydrogen in the steel material. For example, as the strength of steel materials increases, the hydrogen embrittlement susceptibility increases significantly, leading to fracture (cracks) of the steel material.

[0004] In particular, hydrogen embrittlement susceptibility increases significantly in steel materials with a tensile strength of 1180 MPa or more (Non-Patent Document 1). Therefore, even if the strength required of steel materials increases, the steel materials are required to be free from delayed fracture, that is, to have excellent delayed fracture resistance. In particular, the welded parts of steel materials have higher strength than the base steel material, and therefore are prone to increase in hydrogen embrittlement susceptibility. Therefore, there is a need for a method to accurately and quantitatively evaluate the delayed fracture properties of welded parts of steel materials. In particular, it is considered important to accurately quantify the effects of the amount of hydrogen in steel materials and the applied load on delayed fracture properties.

[0005] Previously, as disclosed in Non-Patent Document 2, resistance spot-welded test specimens with gaps were prepared with varying nugget diameters, and hydrogen was introduced into the test specimens by immersing them in hydrochloric acid to evaluate delayed fracture properties (known as the gap test method). The resistance spot-welded test specimens with gaps used in this method were prepared by sandwiching a spacer steel plate between the ends of two steel sheets to create a gap, and then resistance spot welding the central gap. The reaction force that returns the gap acts on the nugget portion of the steel sheets. Therefore, by reducing the nugget diameter, the nugget area that receives the reaction force can be reduced, thereby increasing the stress (residual stress) imparted to the weld. Therefore, the relationship between the type of steel sheet and delayed fracture properties can be evaluated by preparing test specimens with varying nugget diameters, introducing hydrogen, and determining the minimum nugget diameter at which delayed fracture (cracks) does not occur in the test specimens.

[0006] Furthermore, Non-Patent Document 3 describes a method for quantitatively evaluating the amount of diffusible hydrogen and tensile shear strength, which are factors that affect delayed fracture properties, by preparing test pieces by spot welding high-tensile steel plates together and conducting tensile shear tests with diffusible hydrogen introduced into the test pieces. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] Shinsaku Matsuyama: Delayed Fracture, Nikkan Kogyo Shimbun, Tokyo, (1989) [Non-patent document 2] Transactions of the Japan Welding Society, 37, 3, 125-132 (2019) [Non-patent document 3] Journal of the Japan Institute of Metals, 85, 2, 75-83 (2021) Summary of the Invention [Problem to be solved by the invention]

[0008] The gap test method described in Non-Patent Document 2 has the following problems. The spot weld of the test specimen is composed of a nugget and a heat-affected zone, which have a structure different from that of the base material. Changing the nugget diameter also changes the residual stress and structure introduced by thermal effects during welding. Therefore, the gap test method evaluates delayed fracture properties using test specimens with different hydrogen embrittlement susceptibility and applied load (residual stress), which are factors that affect delayed fracture. In other words, while the gap test method can evaluate the tendency of the delayed fracture properties of welds between different materials to some extent, the evaluation also includes the influence of the applied load. Therefore, there was a problem in that it was not possible to quantitatively evaluate the delayed fracture properties of welds caused by hydrogen.

[0009] Furthermore, the method for evaluating delayed fracture properties described in Non-Patent Document 3 had the following problem. When test pieces were prepared by spot welding L-shaped steel plates together and the tensile shear strength was evaluated, it was found that the tensile shear strength was sometimes greater when a large amount of hydrogen was introduced into the test piece than when a small amount of hydrogen was introduced. In other words, it was found that the tensile shear strength obtained by the method described in Non-Patent Document 3 may not be able to accurately evaluate the delayed fracture properties of spot welds.

[0010] The present invention has been made in view of the above points, and an object of the present invention is to provide a method for evaluating the delayed fracture properties of welded joints of metallic materials, which is capable of quantitatively evaluating the delayed fracture properties of welded joints of metallic materials caused by hydrogen. [Means for solving the problem]

[0011] As a result of extensive research, the present inventors have found that the above object can be achieved by employing the following configuration, and have completed the present invention. [1] A hydrogen introduction step of introducing hydrogen into a test material consisting of two or more metal materials with a weld; a tensile load application step of applying a tensile load to the test material into which hydrogen has been introduced and obtaining a displacement-load curve; a first evaluation step of evaluating an initial crack initiation load based on the displacement-load curve; A method for evaluating delayed fracture properties of a welded portion of a metallic material, comprising: [2] The method for evaluating delayed fracture properties of a welded portion of a metallic material according to [1], wherein the welded portion is a spot welded portion and the nugget diameter of the spot welded portion is 1.7√t or more, where t is the plate thickness of the metallic material. [3] The tensile speed in the tensile load application step is 3.0 × 10 -4 The method for evaluating the delayed fracture properties of a welded portion of a metallic material according to [1] or [2], wherein the delayed fracture properties of the welded portion of the metallic material are measured at a speed of 1 / 200 rpm or less. [4] A method for evaluating the delayed fracture properties of welds of metallic materials according to any one of [1] to [3], further comprising a hydrogen amount measurement step of measuring the amount of hydrogen in the test material into which hydrogen has been introduced in the hydrogen introduction step. [5] A method for evaluating the delayed fracture properties of welds of metallic materials described in [4], further comprising a second evaluation step of evaluating the initial crack initiation critical load at which the initial crack initiation load is constant regardless of the hydrogen content, based on the relationship between the hydrogen content of the test material obtained in the hydrogen content measurement step and the initial crack initiation load obtained in the first evaluation step. [Effects of the Invention]

[0012] According to the present invention, the delayed fracture properties of welded parts of metallic materials can be evaluated quantitatively with high accuracy. [Brief explanation of the drawings]

[0013] [Figure 1] Figure 1 shows a schematic diagram (a) of the tensile test of the L-shaped joint-shaped test material and the tensile test results (b). [Figure 2] FIG. 2 shows the results of a tensile test on the L-shaped joint specimen after hydrogen was introduced. [Figure 3(a)] FIG. 3(a) is a schematic cross-sectional view of the welded portion. [Figure 3(b)] FIG. 3(b) shows the cross-sectional observation results of the L-shaped joint test material after hydrogen was introduced. [Figure 4] FIG. 4 is a graph showing the relationship between the hydrogen content at fracture and the initial crack initiation load for test materials with an L-shaped joint. [Figure 5] FIG. 5 shows the effect of tension speed on the initial crack initiation load of the L-jointed test material. [Figure 6] FIG. 6 is a schematic diagram showing the hydrogen introduction and tensile test of the L-shaped joint-shaped test material. DETAILED DESCRIPTION OF THE INVENTION

[0014] First, the inventors prepared a test material (L-shaped joint shape) by spot welding two L-shaped steel plates together, and performed a tensile shear test with and without introducing hydrogen into the test material, and compared in detail the behavior of the displacement-load curves obtained in the tensile shear test.

[0015] Figure 1(a) shows a schematic diagram of a tensile test of an L-jointed specimen without hydrogen injection, and (b) shows the displacement-load curve. The tensile test was conducted on an L-jointed specimen (30) consisting of two L-shaped steel plates (metallic material 1) welded together. A load was applied in the tensile direction (3). Figure 1(b) shows that initially, the load increases linearly with increasing displacement, and then decreases slightly at a certain displacement. As described below, at displacement 4, where the load decreases slightly, corona bond delamination occurs. Subsequently, the load increases linearly again with increasing displacement, and at a certain displacement, a sudden, large drop in load occurs. This sudden, large drop in load is believed to be caused by the corona bond delamination reaching the nugget and initiating a crack. Therefore, in the case of the displacement-load curve 5 when hydrogen is not introduced into the test material 30, the load at which the load suddenly drops significantly with an increase in displacement (the load at which the load drop occurs) can be used as the fracture strength of the weld to evaluate the material characteristics.

[0016] On the other hand, Figure 2 shows the results of a tensile test on a test piece 30 in the shape of an L-joint after hydrogen was introduced, and it can be seen that the displacement-load curve 6 when hydrogen was introduced into the test piece 30 does not show the phenomenon of a sudden large drop in load as shown in Figure 1. Therefore, the inventors considered it impossible to evaluate the superiority or inferiority of metallic materials using the index of fracture strength of the welded joint, and considered it important to use a new index for evaluation.

[0017] Therefore, the inventors interrupted the tensile shear test at various points on the displacement-load curve 6 in Figure 2 and performed detailed observations of the cross section of the weld 2 of the test material 30. Figure 3(a) shows a schematic cross-sectional view of the weld 2, and Figure 3(b) shows the cross-sectional observation results of the weld 2 at four points (1), (3), (4), and (5) on the displacement-load curve 6. The weld 2 has a nugget 21, which is the weld metal, and a heat-affected zone 23. The major axis of the nugget 21 is called the nugget diameter 22. Figure 3(b) (1) shows the cross-sectional observation result at a displacement slightly higher than the displacement at which the load slightly decreased ((0) in Figure 3(b)). This shows that corona bond delamination 24 occurred in the heat-affected zone 23. From this, it is inferred that the displacement at which the load slightly decreased ((0) in Figure 3(b)) is the displacement at which corona bond delamination 24 occurred. The displacement where the load decreases slightly ((0) in Figure 3(b)) is called the first gradient change point on the displacement-load curve 6, which moves from the initial position (displacement 0) to the fracture displacement (fracture displacement). The gradient change point refers to the point where the slope of the displacement and load changes. (3) in Figure 3(b) shows the cross-sectional observation results at a displacement slightly higher than (2) in Figure 3(b), which is the gradient change point on the displacement-load curve 6 between the corona bond delamination 24 (displacement at the first gradient change point) and the fracture displacement (fracture displacement). As shown in (3) in Figure 3(b), a crack 25 propagates inside the nugget 21. As the displacement increases, the crack 25 propagates further inside the nugget 21 ((4) in Figure 3(b)). As shown in (5) in Figure 3(b), the crack 25 leads to cracking (fracture) and fracture (fracture).

[0018] Here, the displacement of the gradient change point on the displacement-load curve 6 between the displacement at which corona bond delamination 24 occurs (the displacement at the first gradient change point) and the displacement at which fracture occurs (the fracture displacement) is defined as the displacement of the second gradient change point on the displacement-load curve 6, which moves from the initial stage (displacement 0) to the displacement at which fracture occurs (the fracture displacement). Compared to the gradient of the displacement-load curve at (1), where only corona bond delamination 24 has occurred, the gradient of the displacement-load curve at (3), where a crack 25 has occurred in the nugget 21, is smaller. This is likely due to the difference in the mechanisms between the two. The inventors therefore concluded that the displacement at the second gradient change point, where this gradient changes, is the displacement at which the above-mentioned mechanism changes, i.e., the displacement at which a crack 25 begins to occur in the nugget 21. In other words, based on this behavior, in the test specimen into which hydrogen was introduced, it is believed that the corona bond delamination 24 progressed from the displacement at which the corona bond delamination 24 occurred (the displacement at the first gradient change point) to the displacement at the second gradient change point, that the corona bond delamination 24 reached the nugget 21 near the displacement at the second gradient change point, and that a crack 25 occurred inside the nugget 21 near the displacement at the second gradient change point and progressed.

[0019] From the above, it was found that it is essential to evaluate the load at the second gradient change point of the displacement-load curve 6 from the initial stage to the fracture displacement as the fracture strength of the weld 2 due to hydrogen-induced delayed fracture (defined as the initial crack initiation load), and that the use of the initial crack initiation load makes it possible to quantitatively evaluate the characteristics of hydrogen-induced delayed fracture.

[0020] Furthermore, based on the finding that the hydrogen-induced delayed fracture resistance of the weld 2 can be quantitatively evaluated using the above-mentioned initial crack initiation load, the inventors varied the amount of hydrogen introduced into the test material and evaluated the relationship between the initial crack initiation load and the amount of hydrogen at the time of cracking (fracture). The results are shown in Figure 4. Figure 4 reveals that when hydrogen is introduced into the test specimen, the initial crack initiation load decreases and then remains constant regardless of the amount of hydrogen. It was also found that this constant value differs depending on the steel type. Based on this, the inventors believed that by using this constant value (defined as the initial crack initiation critical load 10) as an indicator of the hydrogen-induced delayed fracture resistance of the weld 2, it would be possible to quantitatively determine the relative merits of the hydrogen-induced delayed fracture resistance of a weld between different types of metallic material 1, for example, between different steel types in the case of steel.

[0021] A method for evaluating the delayed fracture properties of a welded portion of a metallic material according to one embodiment of the present invention will be described below. Note that the present invention is not limited to the following embodiment. Furthermore, the components in the following embodiment include those that are easily replaceable by a person skilled in the art, or those that are substantially identical.

[0022] A method for evaluating the delayed fracture properties of welds of metallic materials according to one embodiment of the present invention comprises a hydrogen introduction step of introducing hydrogen into a test piece 30 consisting of two or more metallic materials 1 each having a weld 2, a tensile load application step of applying a tensile load at a constant rate to the test piece into which hydrogen has been introduced, and a first evaluation step of evaluating the initial crack initiation load from the displacement-load curve obtained in the tensile load application step.

[0023] (Hydrogen introduction step) In the hydrogen introduction step, hydrogen is introduced into a test material composed of two or more metal materials 1 having a welded portion 2. The metallic material 1 is not particularly limited as long as it is weldable and exhibits hydrogen-induced delayed fracture characteristics. However, as mentioned above, in the case of steel, hydrogen embrittlement susceptibility increases significantly when the tensile strength of the steel is 1180 MPa or more. Therefore, high-strength steel, particularly steel with a tensile strength of 1180 MPa or more, is preferred. The tensile strength is more preferably 1320 MPa or more. The upper limit of the tensile strength is not particularly limited, but steel of 2000 MPa or less is preferred. The metallic material 1 may also be a plated steel sheet having the high-strength steel as a substrate. The composition of the steel material is not particularly limited, but examples thereof include, in mass %, C: 0.1 to 0.4%, Si: 0 to 3.0%, Mn: 1 to 10%, P: 0 to 0.05%, S: 0 to 0.005%, with the balance being Fe and unavoidable impurities, and those to which one or more of Cu, Ti, V, Al, Cr, Ni, etc. are added.

[0024] Examples of commercially available steel materials having the above tensile strength include JFE-CA1180, JFE-CA1370, JFE-CA1470, JFE-CA1180SF, JFE-CA1180Y1, and JFE-CA1180Y2 (all manufactured by JFE Steel Corporation).

[0025] In the present invention, the thickness of the steel material (base steel plate) that serves as the substrate is not particularly limited, but is preferably about 0.8 to 2.5 mm, and more preferably about 1.2 to 2.0 mm.

[0026] The welding method for forming the weld 2 is not particularly limited as long as it can weld the metallic material 1. Here, the weld 2 is composed of a weld metal and a heat-affected zone 23. When the welding method is spot welding, the conditions are not particularly limited as long as a nugget 21, which is a weld metal, is formed in the metallic material 1. The nugget diameter 22 is preferably 1.7√t or greater when the plate thickness of the metallic material 1 is t mm. If the nugget diameter 22 is 1.7√t or greater, the reproducibility of the displacement-load curve 6 during tensile shear is high, and the initial crack initiation load can be accurately evaluated. The nugget diameter 22 is more preferably 2.0√t or greater, even more preferably 3.0√t or greater, and most preferably 3.5√t or greater. On the other hand, if the nugget diameter 22 is too large, the initial crack initiation load becomes too high, necessitating an increase in the load of the tensile testing machine, which increases the size and cost of the equipment. Therefore, the nugget diameter 22 is preferably 10.0√t or less. The nugget diameter 22 is more preferably 7.0√t or less, and further preferably 5.0√t or less.

[0027] Furthermore, if the hardness of the nugget 21 region of the weld 2 is less than 200 HV on the Vickers hardness scale, the likelihood of delayed fracture is low. Therefore, the hardness of the nugget 21 region of the weld 2 is preferably 200 HV or more on the Vickers hardness scale. A Vickers hardness of 250 HV or more is more preferable. A Vickers hardness of 300 HV or more is even more preferable. A Vickers hardness of 350 HV or more is most preferable. There is no particular upper limit to the hardness of the nugget 21 region of the weld 2, but because a high hardness can cause initial cracking and make it impossible to evaluate cracking due to delayed fracture, the hardness of the nugget 21 region of the weld 2 is preferably 550 HV or less on the Vickers hardness scale. A Vickers hardness of 500 HV or less is more preferable. Note that hardness measurements are performed as described in the examples.

[0028] The shape of the test material is not particularly limited, and examples include test material in which two strip-shaped metal materials 1 are shifted in the long-side direction to form an overlapping portion and the overlapping portion is spot-welded, test material in which two strip-shaped metal materials 1 are overlapped in a cross shape and the overlapping portion is spot-welded (cross joint shape), test material in which the short sides of two L-shaped metal materials 1 are overlapped and spot-welded as shown in Figure 1(a) (L-joint shape), and test material in which two U-shaped metal materials 1 are overlapped and the bottom of the U is spot-welded (U shape).

[0029] In the present invention, in the tensile load application step described below, it is necessary to grip the test piece with a jig to apply a tensile load to the test material. The shape of the gripping portion of the test material is not particularly limited, as long as the jig can be attached to an area of ​​the test material other than the spot welds and a tensile load can be applied in the desired direction. For example, the gripping portion may be the end of the test piece itself, or may be formed at the end. Specifically, in a cross joint shape, the gripping portion may be formed at the end with a bolt or the like; in an L-joint shape, the long portion may be used as the gripping portion; and in a U-shaped joint, the two long portions may be used as the gripping portion, or holes may be drilled in the two long portions and a pin inserted through them to form the gripping portion.

[0030] The hydrogen introduction method is not particularly limited as long as it can introduce hydrogen into the test material. Figure 6 shows a schematic diagram of hydrogen introduction and tensile testing of a test material 30 welded into an L-shaped joint. Examples of hydrogen introduction methods include acid immersion, cathodic hydrogen charging in an electrolyte solution, high-pressure hydrogen gas exposure, and exposure to a corrosive environment. When evaluating the delayed fracture properties of steel, a method that sufficiently introduces hydrogen into various hydrogen trapping sites in the steel is preferred. From the perspective of controlling the amount of introduced hydrogen, cathodic hydrogen charging in an electrolyte solution 15 is preferred. Specifically, in the electrolyte solution 15, the test material 30 serves as the cathode and a platinum (Pt) electrode 17 or the like serves as the anode. A current is passed between the cathode and anode to electrolyze the solution, and the generated hydrogen is introduced into the test material. In the cathodic hydrogen charging method, the amount of hydrogen introduced into the test material 30 can be controlled by, for example, changing the current or potential. The hydrogen introduction step may be performed before the next step, the tensile load application step, or may be performed continuously during the tensile load application step. The direction in which the tensile load is applied is indicated by reference numeral 20. Examples of the electrolyte 15 include 0.1N NaOH, 1N NaOH+0.3 g / L NH4SCN, and 3 wt% NaCl+3 g / L NH4SCN.

[0031] (Tensile load application step) Next, a tensile load is applied to the test piece into which hydrogen has been introduced. A general tensile testing machine can be used to apply the tensile load. Figure 5 shows the relationship between the amount of hydrogen and the initial crack initiation load obtained when the tension speed is changed when applying a tensile load to steel material spot-welded in an L-shaped joint shape as an example. In the evaluation in Figure 5, cold-rolled steel sheets with a tensile strength of 1470 MPa were used, and welding was performed under conditions where the weld nugget diameter was set to 4√t (t is the thickness of the steel sheet) and the actual measured value was 4.73 mm. The hardness of the nugget 21 region of the resulting weld was 488 HV. The tension speed was 2.0 × 10 ―4 mm / s and 8.3 x 10 -6 At a tension speed of 1.0×10 mm / s, the curves of hydrogen content and initial crack initiation load are almost the same, whereas at a tension speed of 1.0×10 ―3At a tension speed of 3.0 x 10 mm / s, the initial crack initiation load is relatively high. This is presumably because delayed fracture of steel is caused by hydrogen penetrating and diffusing into the steel and accumulating at defects and stress concentration sources. The time it takes for hydrogen to diffuse through steel is often slow, and it is said to be even slower in high-strength steel, which is a problem in delayed fracture. Therefore, in order to properly evaluate the delayed fracture characteristics of hydrogen-induced welds, it is preferable to slow down the tension speed and ensure the time required for hydrogen to penetrate and diffuse into the steel and accumulate at defects and stress concentration sources. Therefore, the tension speed is set to 3.0 x 10 -4 The tensile speed is preferably 2.0 × 10 mm / s or less. -4 mm / s or less is more preferable, and 1.0 × 10 -4 It is more preferable that the speed is 1.0×10 mm / s or less. -5 On the other hand, the lower limit of the tensile speed is not particularly limited, but the tensile speed is preferably 6.0 × 10 mm / s or less because the test time becomes excessively long. -7 It is preferable to set the speed to 1.0×10 mm / s or more. -6 It is more preferable that the speed is 3.0×10 mm / s or more. -6 mm / s or more. This step is carried out until the displacement-load curve is obtained.

[0032] (First evaluation step) Next, the initial crack initiation load is evaluated from the displacement-load curve 6 obtained in the tensile load application step. The initial crack initiation load is the load at which cracks begin to form in the weld metal, and in the case of spot welding, it is the load at which cracks begin to form in the nugget, which is the weld metal. Here, as shown in Figure 3(b), the first gradient change point on the displacement-load curve 6, where the curve moves from the initial stage (displacement 0) to the displacement at which fracture occurs (fracture displacement), is defined as the first gradient change point. The displacement at the first gradient change point is the displacement at which the load slightly decreases, and is the displacement at which corona bond delamination occurs. In addition, the gradient change point of the displacement-load curve 6 from the displacement at which the corona bond peeling occurs to the fracture displacement was defined as the second gradient change point ((2) in Figure 3(b)).

[0033] In the present invention, the initial crack initiation load can be evaluated using the second gradient change point on the displacement-load curve 6, and can be evaluated using a load within ±1% of the load at the second gradient change point. The load at which cracks begin to occur in the weld metal (or the nugget in the case of spot welding) is synonymous with the load within ±1% of the load at the second gradient change point, which is the gradient change point on the displacement-load curve 6 from the displacement at which corona bond separation occurs to the fracture displacement shown in Figure 3(b).

[0034] In another embodiment of the present invention, a hydrogen amount measurement step may be provided to measure the hydrogen amount in order to evaluate the relationship between the hydrogen amount in the test piece 30 and the initial crack initiation load.

[0035] (Hydrogen amount measurement step) In the hydrogen content measurement step, the hydrogen content of the test material 30 into which hydrogen was introduced in the hydrogen introduction step is measured. The test material 30 used in this step may be cut from the test material 30 fractured in the tensile load application step to include the weld 2, to prepare a test piece 16 for hydrogen content measurement, and the hydrogen content may be measured. Alternatively, a separate test material 30 (prepared under the same conditions as the test material for the tensile test) may be prepared. Hydrogen is introduced into the test material 30 under the same conditions as the test material for the tensile test in the hydrogen introduction step (the test material before the tensile load application step). The test piece 16 for hydrogen content measurement may be cut to include the weld 2, and the hydrogen content may be measured. However, the former is preferred. In this case, the test piece 16 for hydrogen content measurement is preferably stored immersed in liquid nitrogen to prevent hydrogen desorption. When measuring the hydrogen content, it is preferable to remove it from the liquid nitrogen and immediately measure the hydrogen content. For example, thermal desorption analysis can be used to measure the hydrogen content. The hydrogen content measurement step may be performed any time after the hydrogen introduction step, in which hydrogen is introduced into the test material.

[0036] In another embodiment of the present invention, a second evaluation step may be provided in which the initial crack initiation critical load 10 is evaluated based on the relationship between the amount of hydrogen obtained in the hydrogen amount measurement step and the initial crack initiation load obtained in the first evaluation step, so that the relative merits of hydrogen-induced delayed fracture resistance between metallic materials can be quantitatively determined.

[0037] (Second evaluation step) The second evaluation step evaluates the initial crack initiation load 10 at which the initial crack initiation load remains constant regardless of the hydrogen content, based on the relationship between the hydrogen content measured in the hydrogen content measurement step and the initial crack initiation load measured in the first evaluation step. In this evaluation, it is preferable to maintain the nugget diameter 22 constant. Here, a constant nugget diameter 22 means that the fluctuation range (mm) of the nugget diameter 22 is preferably ±0.3√t or less, where t is the thickness of the metal material. This is because changes in the nugget diameter 22 can cause residual stresses and structural changes due to thermal effects, etc., which can affect the delayed fracture properties due to factors other than hydrogen. More preferably, the fluctuation range (mm) is ±0.2√t or less. While the lower limit of the fluctuation range (mm) of the nugget diameter 22 is not particularly limited, it is preferably 0.05√t or more. As mentioned above, the present inventors have discovered that when the hydrogen content and the initial crack initiation load are plotted, the initial crack initiation load exhibits a substantially constant value from a certain hydrogen content. In the present invention, the constant value at which the initial crack initiation load is constant regardless of the amount of hydrogen is defined as the initial crack initiation critical load (reference numeral 10 in FIG. 4). Furthermore, it has been found that the initial crack initiation critical load 10 exhibits different values ​​depending on the type of steel in the case of a metallic material 1, for example, a steel material, and that this makes it possible to quantitatively determine the superiority or inferiority of the hydrogen-induced delayed fracture resistance of a weld 2 of the metallic material 1. Therefore, the initial crack initiation critical load 10 can quantitatively determine the superiority or inferiority of the hydrogen-induced delayed fracture resistance of a weld 2 of a metallic material 1. [Example]

[0038] The present invention will be specifically described below with reference to examples, but the present invention is not limited to the examples described below.

[0039] (Hydrogen introduction step) First, steel plates having the chemical composition, thickness, and tensile strength of the representative components shown in Table 1 were cut into strips of 150 mm C × 30 mm L (C is the width direction of the steel plate, and L is the longitudinal direction of the steel plate), and then L-shaped steel plates were fabricated by bending the long sides vertically to form an L shape as shown in Figure 6.

[0040] The short sides of the two L-shaped steel plates were then placed back to back and spot welded under the conditions described below to produce test material 30 (L-shaped joint shape, see Figure 6). Test material 30 was produced for each of steel plates A to C. The spot welding conditions were as follows: Spot welding was performed at room temperature, and the electrodes were always water-cooled. Both the lower and upper electrodes used for welding were DR-type electrodes made of chromium copper with a tip diameter (tip diameter) of 6 mm and a curvature radius of 40 mm. Spot welding was performed using a welding current value in the range of 3.0 to 4.5 kA, under conditions such that the nugget diameters were the set values ​​shown in Tables 2 and 3. The actual nugget diameters obtained are also shown.

[0041] Next, the test material 30 was connected to a potentiostat 19, with the Pt electrode 17 as the cathode, the silver-silver chloride (Ag / AgCl) electrode 18 as the reference electrode, and hydrogen was introduced into the test material 30 by controlling the potential. The amount of hydrogen was varied by controlling the type of electrolyte 15 and the potential. The electrolyte 15 was either 1N NaOH + 0.3 g / L NH4CN or 0.1 N NaOH, and the set potential was set in the range of -1000 to -1500 mV vs. SHE. The next tensile load application step was performed while the cathodic hydrogen charging method for hydrogen introduction was still applied.

[0042] (Tensile load application step) A general tensile tester was used to apply the tensile load. The tensile speed when applying the tensile load was set to the conditions shown in Table 2.

[0043] (First evaluation step) The initial crack initiation load was evaluated from the displacement-load curve obtained in the tensile load application step. The load corresponding to the second gradient change point in the displacement-load curve from the initial (displacement 0) to the fracture displacement was defined as the initial crack initiation load. An example of the obtained results is shown in Table 2.

[0044] Table 2 shows that by using the index of initial crack initiation load, it is possible to quantitatively evaluate the hydrogen-induced delayed fracture characteristics under certain conditions of the hydrogen introduction step.

[0045] (Hydrogen amount measurement step) A 10 × 30 mm hydrogen content measurement specimen 16, including the weld 2, was cut from the test specimen 30 fractured in the tensile load application step. The hydrogen content measurement specimen 16 was then rapidly cooled in liquid nitrogen to prevent the hydrogen introduced into the test specimen 30 from desorbing. The hydrogen content of the rapidly cooled hydrogen content measurement specimen 16 was then measured using thermal desorption analysis. A low-temperature temperature-programmed hydrogen analyzer was used for the thermal desorption analysis. The thermal desorption analysis was performed at a temperature range of -50°C to 800°C at a heating rate of 200°C / h. The diffusible hydrogen content was calculated as the integrated value of the hydrogen content measured from -50°C to 200°C. The hydrogen content measurement step can be performed any time after the hydrogen introduction step, in which hydrogen is introduced into the test material.

[0046] (Second evaluation step) Furthermore, a second evaluation step was carried out to quantitatively evaluate the relative merits of the metallic materials. First, for test materials 30 having the same nugget diameter, the amount of hydrogen introduced was varied, and the initial crack initiation load and hydrogen amount were determined using the above procedure, and the relationship was plotted. An example is shown in Figure 4. As shown in Figure 4, the load at which the initial crack initiation load becomes constant with respect to the amount of hydrogen was determined as the initial crack initiation critical load. The steel plate, nugget diameter, and Vickers hardness in Figure 4 were the same as those shown in Figure 5, and the tension speed was 2.0 x 10 -4 mm / s.

[0047] Next, the magnitude of the critical load for initial crack initiation was evaluated for steel plates A, B, and C. The results are shown in Table 3. If the ranking of hydrogen-induced delayed fracture properties between metallic materials obtained by the method of the present invention for evaluating the delayed fracture properties of welds of metallic materials matches the ranking of delayed fracture properties of welds between metallic materials obtained by the conventional gap test method, it is determined that the delayed fracture properties of the welds have been appropriately evaluated. Here, the ranking of delayed fracture resistance properties obtained for steel plates A, B, and C using the conventional gap test method was (superior) steel plate A > steel plate B > steel plate C (poor).

[0048] Table 3 shows that by using the method of the present invention for evaluating the delayed fracture properties of welds of metallic materials, it is possible to evaluate the delayed fracture properties of welds using test pieces with the same nugget diameter, thereby making it possible to eliminate load stress as much as possible, quantitatively evaluate the delayed fracture properties of welds caused by hydrogen, and appropriately evaluate the superiority or inferiority of materials quantitatively.

[0049] (Vickers hardness evaluation) The Vickers hardness of weld 2 was calculated by pressing a square pyramidal diamond indenter into the surface of the weld with a load of 300 g, and using the following formula to calculate the Vickers hardness from the load and the average length of the diagonal line of the indentation. Using the above method, measurements were taken at 20 points at 0.2 mm intervals from the edge of the nugget to the center of the weld, and the average value was used as the Vickers hardness. The values ​​are shown in Tables 2 and 3. Vickers hardness = constant × (test force / surface area of ​​indentation)

[0050] [Table 1]

[0051] [Table 2]

[0052] [Table 3] [Explanation of symbols]

[0053] 1 Metal materials 2 Welded parts 3 Tensile direction 4. Displacement 5. Displacement-load curve when hydrogen is not introduced into the test material 6. Displacement-load curve when hydrogen is introduced into the test material 10. Critical load for initial crack initiation 15 Electrolyte 16. Test piece for measuring hydrogen content 17 Pt (platinum) electrode 18 Silver-silver chloride (Ag / AgCl) electrode 19 Potentiostat 20 Direction of tensile load application 21 Nuggets 22 nugget diameter 23 Heat-affected zone 24 Corona bond peeling 25 Crack 30 Test materials, test pieces

Claims

1. a hydrogen introduction step of introducing hydrogen into a test material composed of two or more metal materials having a weld; a tensile load application step of applying a tensile load to the test material into which hydrogen has been introduced and obtaining a displacement-load curve; a first evaluation step of determining, based on the displacement-load curve, a displacement at which the load is slightly reduced from the initial state (displacement 0) toward the displacement at which fracture occurs (fracture displacement), as a displacement at a first gradient change point, and determining, as a displacement at a gradient change point on the displacement-load curve between the displacement at the first gradient change point and the displacement at which fracture occurs, as a displacement at a second gradient change point, and evaluating an initial crack initiation load, which is the load at the second gradient change point; a hydrogen amount measurement step of measuring the amount of hydrogen in the test material into which hydrogen has been introduced in the hydrogen introduction step; a second evaluation step of evaluating the initial crack initiation critical load at which the initial crack initiation load is constant regardless of the hydrogen content, based on the relationship between the hydrogen content of the test material obtained in the hydrogen content measurement step and the initial crack initiation load obtained in the first evaluation step.

2. 2. The method for evaluating delayed fracture properties of a welded portion of a metallic material according to claim 1, wherein the welded portion is a spot welded portion and the nugget diameter of the spot welded portion is 1.7√t or more, where t is the plate thickness of the metallic material.

3. The tensile speed in the tensile load application step is 3.0 × 10 －４ 2. The method for evaluating delayed fracture properties of a welded portion of a metallic material according to claim 1, wherein the delayed fracture properties of the welded portion of the metallic material are evaluated by the following equation:

4. The tensile speed in the tensile load application step is 3.0 × 10 －４ 3. The method for evaluating delayed fracture properties of a welded portion of a metallic material according to claim 2, wherein the elongation rate is 1 / 2 mm / s or less.

Images