Method for evaluating critical stress for occurrence of liquid metal embrittlement crack, method for evaluating critical displacement amount for occurrence of liquid metal embrittlement crack, and method for evaluating liquid metal embrittlement susceptibility

Abstract

To provide a method that evaluates critical stress or a critical displacement magnitude in which liquid metal embrittlement cracking of a metal material occurs, and to provide a method that evaluates liquid metal embrittlement susceptibility.SOLUTION: A method is configured to: organize a test temperature and strain velocity, and perform a high-temperature elongation test with respect to a plurality of sample materials so that stress is different for each sample material; stop the test before the sample material fractures; and evaluate critical stress in which cracking occurs in the sample material by checking presence or absence of LME cracking for the plurality of sample materials undergoing the high-temperature elongation test. The high-temperature test is performed so that a displacement magnitude is different instead of making the stress different, and a critical displacement magnitude in which the cracking occurs in the sample material may be evaluated. Liquid metal embrittlement susceptibility may be evaluated based on at least one of the critical stress and critical displacement magnitude.SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] The present disclosure relates to a method for evaluating the critical stress for liquid metal embrittlement crack initiation, a method for evaluating the critical displacement for liquid metal embrittlement crack initiation, and a method for evaluating liquid metal embrittlement susceptibility.

Background Art

[0002] When welding a metal plate or the like, cracking due to liquid metal embrittlement (LME) in spot welding or the like may be a problem. Liquid metal embrittlement is the phenomenon in which a solid metal that originally exhibits ductility becomes brittle when a tensile stress is applied in a state where a liquid metal is in contact with the solid metal, and typically occurs between steel (solid metal) and zinc (liquid metal).

[0003] For example, Patent Document 1 discloses a coated steel sheet in which a steel sheet has a specific composition and structure. As an example, it is described that a welding structure having a low LME susceptibility can be obtained, where the probability of the number of cracks in the resistance spot weld is less than an average of five.

[0004] Patent Document 2 discloses a plated steel material in which a Zn - Al - Mg plating layer has a specific composition. As an example, for a steel sheet, a bead - on - plate test piece obtained by bead - on - plate welding with a stainless steel welding wire is obtained, and the presence or absence of cracks is confirmed by a penetration flaw detection test, and the LME is evaluated by the length of cracks in the welded part.

[0005] Patent Document 3 discloses a hot - stamp forming body including a base material and a plating layer having a specific component and structure. As an example, in the cross - section in the thickness direction of the steel material at the site of hot - formed V - bending, the presence or absence of liquid metal embrittlement cracks is observed by observing a reflected electron image using a scanning electron microscope (SEM) and a backscattered electron detector.

[0006] Patent Document 4 discloses an austenitic molten aluminum-plated steel sheet in which a molten aluminum-based plating layer is formed on the surface of a base steel sheet having a specific composition. As an example, when the thickness of the steel sheet is t, the welding current at the point when the nugget diameter becomes smaller than 4√t is determined as the lower limit, and the welding current at the point when the splashing phenomenon occurs is determined as the upper limit, and spot welding is performed. It is described that the spot weldability of the steel sheet is evaluated by taking the difference value from the lower limit to the current at which no LME crack occurs as the "LME crack non-occurring current range".

[0007] Patent Document 5 discloses a method for manufacturing a press-formed product in which a steel sheet is held at a predetermined temperature for a predetermined time and then forming is started within a predetermined temperature range. For a tensile test piece cut out from a galvanized steel sheet or an alloyed molten zinc-plated steel sheet, processing is performed to simulate the temperature history of the hot stamping process, and as an evaluation of the LME crack depth in the plastically deformed part, it is described that the maximum crack depth is measured.

[0008] Non-Patent Document 1 describes a method for evaluating LME by performing a tensile test on a steel test piece in a Cd-Zn liquid metal and measuring the maximum penetration length, average penetration depth, and penetration length of the liquid metal on the fracture surface. Non-Patent Document 2 shows that stress and strain significantly promote the diffusion of liquid metal at grain boundaries and are the dominant factors causing LME cracks. Furthermore, an LME model regarding the influence of stress and strain on the diffusion of liquid metal at grain boundaries is described.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

[0010] [Non-Patent Document 1] Journal of the Welding Society, Vol. 52, 1983, No. 1, pp. 56-61 [Non-Patent Document 2] On the Role of Stress, Strain and Diffusion in Dissolution - Condensation Mechanism of Liquid Metal Embrittlement, Defect and Diffusion Forum, Vol. 264, pp141-149 [Summary of the Invention] [Problems to be Solved by the Invention]

[0011] In Patent Documents 1 to 5 and Non-Patent Documents 1 and 2, it is described that the liquid metal embrittlement (LME) cracks or LME susceptibility are evaluated by the presence or absence of cracks, the number of cracks, and the length of cracks in the welded part or bent part of the plated steel sheet. However, a method for evaluating the critical stress or critical displacement amount at which LME cracks occur is not described. Therefore, an object of the present disclosure is to provide a method for evaluating the critical stress or critical displacement amount at which liquid metal embrittlement cracks occur in a metal material, and a method for evaluating liquid metal embrittlement susceptibility. [Means for Solving the Problems]

[0012] Means for solving the above problems include the following aspects. <1> A step of preparing a test piece including a first metal part and a second metal part disposed on at least a part of the first metal part, the second metal part having a lower melting start temperature than the first metal part, and causing liquid metal embrittlement of the first metal part when the second metal part is liquefied by heating. A step of heating the test specimen to a temperature range that is equal to or higher than the melting start temperature of the second metal part and lower than the melting start temperature of the first metal part, applying a load to the test specimen to impart elongation, and stopping applying the load before the test specimen breaks, to perform a high-temperature elongation test; A step of checking for the presence or absence of cracks from the surface of the first metal part where the second metal part is disposed toward the inside of the test specimen that has undergone the high-temperature elongation test; comprising: In the step of preparing the test specimen, a plurality of test specimens are prepared, In the step of performing the high-temperature elongation test, the high-temperature elongation test is performed on the plurality of test specimens with the test temperature and strain rate being made uniform, and with the stress being different for each test specimen; In the step of checking for the presence or absence of cracks, by checking for the presence or absence of cracks in the plurality of test specimens that have undergone the high-temperature elongation test, a critical stress at which cracks occur in the test specimen is evaluated. A method for evaluating the critical stress for liquid metal embrittlement crack generation. <2> The method for evaluating the critical stress for liquid metal embrittlement crack generation according to <1>, wherein for each of the plurality of test specimens, two or more types of the high-temperature elongation tests with different strain rates are performed, and the critical stress is evaluated for each strain rate. <3> The method for evaluating the critical stress for liquid metal embrittlement crack generation according to <1> or <2>, wherein for each of the plurality of test specimens, two or more types of the high-temperature elongation tests with different test temperatures are performed, and the critical stress is evaluated for each test temperature. <4> The method for evaluating the critical stress for liquid metal embrittlement crack generation according to any one of <1> to <3>, wherein the plurality of test specimens have the same types of the first metal part and the second metal part. <5> The method for evaluating the critical stress for liquid metal embrittlement crack generation according to any one of <1> to <3>, wherein at least one of the types of the first metal part and the second metal part of the plurality of test specimens is different from each other. <6> The method for evaluating the critical stress for liquid metal embrittlement crack generation according to any one of <1> to <5>, wherein the first metal part is a steel material. <7> The first metal part is a base steel plate, and the second metal part is a plating layer. The method for evaluating the critical stress for liquid metal embrittlement cracking according to any one of <1> to <6>. <8> In the test piece, the plating layers are disposed on both surfaces of the base steel plate, The high-temperature elongation test is performed by attaching a thermocouple at a position deviated from the region where the crack occurs on either surface of the base steel plate. The method for evaluating the critical stress for liquid metal embrittlement cracking according to <7>. <9> In the test piece, the plating layer is disposed on one surface of the base steel plate, The high-temperature elongation test is performed by attaching a thermocouple at a position corresponding to the region where the crack occurs on the other surface of the base steel plate with respect to the one surface. The method for evaluating the critical stress for liquid metal embrittlement cracking according to <7>. <10> Among the steps included in the method for evaluating the critical stress for liquid metal embrittlement cracking according to any one of <1> to <9>, In the step of performing the high-temperature elongation test, instead of performing the high-temperature elongation test such that the stress is different for each test piece, the high-temperature elongation test is performed such that the displacement amount is different for each test piece, In the step of checking the presence or absence of the crack, instead of evaluating the critical stress at which the crack occurs in the test piece, the critical displacement amount at which the crack occurs in the test piece is evaluated. A method for evaluating the critical displacement amount for liquid metal embrittlement cracking. <11> Using at least one of the evaluation methods of the method for evaluating the critical stress for liquid metal embrittlement cracking according to any one of <1> to <9> and the method for evaluating the critical displacement amount for liquid metal embrittlement cracking according to <10>, a method for evaluating the liquid metal embrittlement susceptibility of the test piece based on at least one of the critical stress and the critical displacement amount.

Advantages of the Invention

[0013] According to the present disclosure, a method for evaluating the critical stress or critical displacement amount at which liquid metal embrittlement cracking occurs in a metal material, and a method for evaluating liquid metal embrittlement susceptibility are provided.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Figure 3A

Figure 3B

Figure 4A

Figure 4B

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11A

Figure 11B

Figure 12

Mode for Carrying Out the Invention

[0015] An embodiment which is an example of the present disclosure will be described. In this specification, the term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.

[0016] The inventors have repeatedly studied a method for evaluating the critical stress or critical displacement amount at which LME cracks occur in metal materials and obtained the following findings.

[0017] When high-temperature tension is performed on plated steel sheets and non-plated steel sheets respectively, as shown in FIG. 1, a stress difference (Δstress) due to the presence or absence of plating occurs in the displacement-stress diagram. Therefore, the inventors have advanced the study of a method for evaluating the critical stress at which LME cracks occur. Then, a plurality of test materials (test pieces) 10 having the shape shown in FIG. 2 were collected from the same zinc-plated steel sheet, and for each test material, the test temperature and strain rate were the same conditions, and a high-temperature tension test was performed by changing the strain and stress.

[0018] FIG. 3A is an SEM image showing a cross section in the thickness direction and the tensile direction of each test material after the tensile speed is 50 mm / s (strain rate: 8.3 / s), and FIG. 4A is an SEM image showing a cross section in the thickness direction and the tensile direction of each test material after the tensile speed is 5 mm / s (strain rate: 0.83 / s). Also, FIGS. 3B and 4B show the relationship between displacement and stress respectively. The test materials were all collected from a 980 MPa grade zinc-based plated steel sheet, and the length of the parallel portion 14 is 6 mm. As shown in FIGS. 3A and 4A, at any strain rate, cracks occur before the test piece breaks, and the number of LME cracks increases as the strain and stress increase. Thus, in a high-temperature tensile test where the strain rate is constant at the temperature at which LME cracks occur, the stress varies for each specimen. By stopping the test before the specimen breaks and observing the cross-section of the specimen, the stress (critical stress) at which LME cracks occur at that temperature and strain rate can be confirmed. From such test results, it was found that the strain rate also affects the occurrence of LME cracks and the LME crack initiation stress also changes.

[0019] Furthermore, the inventors changed the test temperature and steel type and measured the critical stress at which cracks occurred in each specimen. FIG. 5 shows the stress (LME crack initiation critical stress) at which LME cracks occurred at each temperature for each steel type. The test temperature, steel type, and LME crack initiation critical stress are different, and the components of the specimen etc. also affect the LME crack initiation critical stress. In the figure, the "LME occurrence region" is the region where LME cracks occur in the GA LME-resistant steel sheet used in this experiment. From these results, for example, when spot welding, by setting each condition so as not to reach the LME crack initiation critical stress, the occurrence of LME cracks during spot welding can be prevented or suppressed.

[0020] Also, as shown in FIG. 1, in the case of "with plating", as the stress increases in the high-temperature tensile test, the displacement amount also gradually increases. When an LME crack occurs, the stress rapidly decreases, while the displacement amount increases. If the tensile test is continued, the specimen breaks. However, for a plurality of specimens, the tensile test is performed with the test temperature and strain rate being the same and the displacement amounts being different, and the tensile test is stopped before breakage. By checking the presence or absence of LME cracks in each specimen after the test, the critical displacement amount can be evaluated. Based on these experiments and considerations, the evaluation method according to the present disclosure has been found. Hereinafter, first, the evaluation method of the liquid metal embrittlement crack initiation critical stress according to the present disclosure will be specifically described.

[0021] [Evaluation Method of Liquid Metal Embrittlement Crack Initiation Critical Stress] The evaluation method of the liquid metal embrittlement crack initiation critical stress according to the present disclosure is A step of preparing a test specimen including a first metal part and a second metal part disposed on at least a part of the first metal part and having a lower melting start temperature than the first metal part, wherein liquid metal embrittlement of the first metal part occurs when the second metal part is liquefied by heating. A step of performing a high-temperature elongation test in which the test specimen is heated to a temperature range equal to or higher than the melting start temperature of the second metal part and lower than the melting start temperature of the first metal part, a load is applied to the test specimen to impart elongation, and the application of the load is stopped before the test specimen breaks. A step of checking for the presence or absence of cracks from the surface of the first metal part where the second metal part is disposed toward the inside of the test specimen that has undergone the high-temperature elongation test. It includes. In the step of preparing the test specimen, a plurality of test specimens are prepared. In the step of performing the high-temperature elongation test, the high-temperature elongation test is performed for each of the plurality of test specimens with the test temperature and strain rate being made uniform and the stress being different for each test specimen. In the step of checking for the presence or absence of cracks, the critical stress at which cracks occur in the test specimen is evaluated by checking for the presence or absence of cracks in each of the plurality of test specimens that have undergone the high-temperature elongation test. In this way, for a plurality of test specimens that can cause LME, the test temperature and strain rate are made uniform, and the high-temperature elongation test is performed for each test specimen with the stress being different for each test specimen, and the load is stopped before the test specimen breaks. By checking for the presence or absence of cracks in the test specimen after the test, the critical stress at which cracks occur in the test specimen can be evaluated. Hereinafter, each step will be specifically described.

[0022] <Preparation of Test Specimen> Prepare a test specimen including a first metal part and a second metal part disposed on at least a part of the first metal part and having a lower melting start temperature than the first metal part, wherein liquid metal embrittlement of the first metal part occurs when the second metal part is liquefied by heating. Here, a plurality of test specimens are prepared. The test specimens may be a plurality of test specimens in which the types of the first metal part and the second metal part are the same as each other, or may be a plurality of test specimens in which at least one of the types of the first metal part and the second metal part is different from each other.

[0023] Hereinafter, the case of using a test specimen in which a metal layer is formed as the second metal part on the surface of a metal substrate as the first metal part will be described. However, the shapes of the first metal part and the second metal part are not limited to this, and the first metal part and the second metal part may be in direct or indirect contact by a method other than lamination.

[0024] The metal substrate is a member that remains a solid metal without melting even in a high-temperature elongation test as a support, and examples thereof include metal materials such as plate-shaped, rod-shaped, wire-shaped, and cylindrical. The metal layer is a layer that covers at least a part of the surface of the metal substrate and at least a part of which melts into a liquid metal in a high-temperature elongation test. For example, when the metal substrate is plate-shaped, the metal layer may be laminated on one side of the metal substrate or may be laminated on both sides.

[0025] The material of the test specimen is not particularly limited as long as the metal substrate is a combination that causes LME due to the liquefaction of the metal layer. Combinations that cause LME include, for example, combinations of steel materials and metal layers such as Zn, Sn, Cu, Cd, In, Hg, Bi, Na, Cu-Pb, Cu-Sn, Zn-Sn, Cu-Pd, Cd-Zn, or Al-Sn-Cu.

[0026] In addition, as long as the test specimen is a combination in which LME occurs, the lamination form of the metal base material and the metal layer is not particularly limited. For example, plated materials such as plated sheets, plated wire rods, and plated bars, and further, as test specimens other than plated materials, for example, joined materials joined by embedding rivets or the like by rotating them at high speed in the thickness direction in a state where a plurality of different plates such as steel and aluminum are stacked may be used. In the case of such a joined material, there is a possibility that LME may occur due to heat and stress during joining. Further, a clad material in which different metals are bonded together, or a material in which a plating layer is further formed on the surface of the clad material may be used. Further, a test specimen in which a metal layer is provided on the outer peripheral surface and / or the inner peripheral surface of a cylindrical metal member may be used.

[0027] In addition, another layer such as electroless plating may exist between the metal base material (first metal part) and the metal layer (second metal part) that causes LME of the metal base material, or another layer such as an organic film may exist on the metal layer (second metal part) (that is, on the side opposite to the metal base material).

[0028] Hereinafter, as an example of the test specimen, the case of using a plated steel sheet in which a plating layer (metal layer) as the second metal part is formed on the surface of a base steel sheet (metal base material) as the first metal part, particularly a galvanized steel sheet (hereinafter, may be simply referred to as "plated steel sheet") will be mainly described. In the case of a plated steel sheet, the plating layer (metal layer) may be liquefied by spot welding or the like, and LME of the steel sheet (metal base material) may occur. As the test specimen, for example, a hot-dip galvanized steel sheet or a non-plated cold-rolled steel sheet electroplated with zinc can be used.

[0029] The method for evaluating the critical stress for LME crack generation according to the present disclosure may be evaluated by collecting from the same type of plated steel sheet as shown in FIGS. 3A and 4A as a plurality of test specimens, or may be evaluated by collecting from different types of plated steel sheets as shown in FIG. 5.

[0030] (Same type of plated steel sheet) The same type of plated steel sheet means that all types of the base steel sheet and the plating layer are the same. The same type of base metal steel plate means steel plates with the same chemical composition and metal structure, typically steel plates manufactured under the same conditions. The same type of plating layer means a plating layer with the same chemical composition, metal structure, and coating weight of the plating layer, typically a plating layer formed under the same conditions for the same type of base metal steel plate. For example, a single plated steel plate can be cut to obtain a plurality of test specimens.

[0031] (Dissimilar plated steel plates) Dissimilar plated steel plates mean that at least one of the types of the base metal steel plate and the plating layer is different from each other. Different types of base metal steel plates mean steel plates with at least one of the chemical composition and metal structure different from each other. For example, even when manufacturing cold-rolled steel plates from the same slab with the same chemical composition, steel plates with different metal structures can be produced due to differences in the heat history. For example, if the chemical compositions are the same but the metal structures are different, the mechanical properties and chemical properties are different, and the critical stress for LME crack generation is also different.

[0032] Different types of plating layers mean plating layers with at least one of the chemical composition, metal structure, and coating weight of the plating layer different from each other, typically plating layers formed under different conditions for the same or different types of base metal steel plates.

[0033] Therefore, as the plurality of test specimens used in the method for evaluating the critical stress for LME crack generation according to the present disclosure, test specimens taken from any of the following groups (A) to (D) of plated steel plates can be mentioned. (A) A group of the same type of plated steel plates in which the same type of plating layer is formed on the same type of base metal steel plate (B) A group of dissimilar plated steel plates in which different types of plating layers are formed on the same type of base metal steel plate (C) A group of dissimilar plated steel plates in which the same type of plating layer is formed on different types of base metal steel plates (D) A group of dissimilar plated steel plates in which different types of plating layers are formed on different types of base metal steel plates For example, a plurality of test specimens may be collected from the plated steel sheet of (A) to evaluate the critical stress for LME crack initiation, or test specimens may be collected from each plated steel sheet of the group of plated steel sheets corresponding to any of (B) to (D) to evaluate the critical stress for LME crack initiation.

[0034] (Base metal steel sheet) The components, structure, thickness, etc. of the base metal steel sheet are not particularly limited, and a base metal steel sheet for which it is desired to evaluate the critical stress for LME crack initiation in combination with the plating layer may be used.

[0035] (Plating) When evaluating the critical stress for LME crack initiation of a galvanized steel sheet, the base metal steel sheet is galvanized. The plating method is not limited, and it may be hot-dip galvanizing or electro-galvanizing. For example, by plating each base metal steel sheet under the same conditions, a plating layer with a uniform coating weight and chemical composition can be formed on each base metal steel sheet. Note that the "plating layer with a uniform coating weight and chemical composition" means that the coating weight and chemical composition of the plating layer in each plated steel sheet are the same. Even if the composition or structure of the base metal steel sheet is different, if plating is performed under the same conditions, basically, a plating layer with the same coating weight and the same composition will be formed.

[0036] On the other hand, when alloying is performed by heating after plating under the same conditions, a part of the components of the base metal steel sheet may move into the plating layer, and there may be a slight difference in the composition in the plating layer. However, the difference in the composition of the plating layer caused by the difference in the composition of the base metal steel sheet is slight and has little effect on LME cracks. Therefore, when forming a plating layer under the same conditions except for the difference in the type of base metal steel sheet, it may be regarded as the same type of plating layer. However, a significant difference may occur in the composition in the plating layer between the case where no alloying treatment is performed and the case where alloying treatment is performed after plating under the same conditions. Therefore, when evaluating the critical stress for LME crack initiation for a group of different plated steel sheets in which the same type of plating layer is formed on different base metal steel sheets of (C) described above, for example, as a plurality of test specimens, the presence or absence of alloying treatment after plating under the same conditions is also made uniform.

[0037] The shape of the plated steel sheet is not particularly limited. However, when performing a high-temperature elongation test, for example, a high-temperature tensile test, in order to limit the site where stress is concentrated and observed (i.e., the site where cracks occur) in the tensile test, for example, as shown in FIG. 2, it is preferable to use a test specimen 10 having a constricted portion 12 and a parallel portion 14 in the center of the constricted portion 12 where both edges are parallel to the tensile direction in the tensile test.

[0038] Also, although the thickness of the test specimen is not limited, if cracks penetrate in the thickness direction in the tensile test, it becomes difficult to conduct a comparative evaluation. Therefore, the thickness of the test specimen is preferably 0.5 mm or more. In addition, when the test specimen is a wire rod, if cracks penetrate as in the case of a plate material, it is difficult to make a comparison, and it is also difficult to cut for cross-sectional observation. Therefore, the diameter is preferably 1.0 mm or more, and more preferably 3.0 mm or more. If a parallel portion is provided, it is preferably 1.0 mm or more in that portion.

[0039] <High-temperature elongation test> A high-temperature elongation test is performed by heating the test specimen in a temperature range that is equal to or higher than the melting start temperature of the second metal part and lower than the melting start temperature of the first metal part, applying a load to the test specimen to impart elongation, and stopping applying the load before the test specimen breaks. Here, for a plurality of test specimens, the test temperature and strain rate are made uniform, and a high-temperature elongation test is performed for each test specimen such that the stress is different for each test specimen.

[0040] The type of high-temperature elongation test is not particularly limited. For example, in addition to a uniaxial tensile test, a biaxial tensile test, a ball-headed bulge test, etc. may be used. Hereinafter, the case of a uniaxial tensile test (which may be referred to as a "high-temperature tensile test" or simply a "tensile test" in the present disclosure) will be described. The high-temperature tensile test is performed by heating the test specimen in a temperature range that is equal to or higher than the melting start temperature of the plating layer and lower than the melting start temperature of the base steel plate. However, it is desirable to perform the test at a temperature (test temperature) at which the plating layer does not volatilize. Also, as the melting start temperature of the steel sheet is approached, the test specimen becomes more likely to deform, making it difficult to conduct a tensile test. Therefore, the maximum temperature reached by the test specimen in the tensile test is preferably set to be +20°C or higher than the melting start temperature of the plating layer and -200°C or lower than the melting start temperature of the steel sheet. Here, the "maximum temperature reached" means the temperature of the surface of the test specimen, that is, the plating layer. When using a hot-dip galvanized steel sheet as the test specimen, the maximum temperature reached in the tensile test is preferably 500 to 1200°C.

[0041] Heating (temperature increase) of the test specimen in the tensile test can be performed by electric heating or furnace heating. The heating rate is preferably 50°C / s or higher (for example, 50 to 1000°C / s) in order to prevent volatilization of zinc (plating layer). Tensile force can be applied to the test specimen before heating and then heated, or it can be heated without applying tensile force and then tensile tested at the maximum temperature reached (for example, 500°C, 700°C, or 900°C). Alternatively, it can be heated to the maximum temperature reached (for example, 1000°C) once, cooled to the processing temperature for tensile testing (for example, 800°C, 700°C, 600°C, or 500°C), and then tensile tested.

[0042] These temperature histories in the tensile test are preferably changed according to the temperature histories and processing temperatures at which liquid metal embrittlement becomes a problem in the use of the plated steel sheet. For example, when evaluating liquid metal embrittlement due to stress generation associated with electrode release in spot welding, depending on the location where stress is generated, it is preferably heated once (for example, 1000°C), and then cooled at a slower rate than actual spot welding but with nitrogen gas cooling, and tensile tested at the temperature at the time of electrode release (for example, 800°C). Note that in such temperature ranges, although depending on the steel material composition, there are A1 point and A3 point. Therefore, the crystal structure of the steel material may be body-centered cubic lattice (BCC), face-centered cubic lattice (FCC), or a two-phase state of these depending on the temperature history. Since the susceptibility may be different in each crystal structure, it is desirable to determine the test temperature in the tensile test, that is, the temperature history and processing temperature (tensile temperature) according to the purpose of use of the plated steel sheet.

[0043] The strain rate is not particularly limited. However, if the strain rate is too slow, the alloying of steel and zinc may progress, and the accuracy of evaluating the critical stress for liquid metal embrittlement crack generation may decrease. Also, as shown in FIGS. 3A and 4A, the higher the strain rate, the easier it is for cracks to occur and the easier it is to confirm the presence or absence of cracks. Therefore, it is desirable to increase the strain rate to a certain extent and perform the tensile test in a short time. For example, when the parallel portion of the test piece is 1 to 10 mm, the tensile test is performed at a tensile speed of 1 to 100 mm / s (a strain rate of 0.1 to 100 / s).

[0044] In the tensile test, for each test material, the test temperature and the strain rate are made the same, and a tensile load is applied so that the stress is different for each test material, and the test is stopped (interrupted) before the test material breaks. For example, the tensile test is performed so that the stress increases each time the tensile test is performed.

[0045] (Position of thermocouple) In the tensile test, it is preferable to attach a thermocouple to the test material to measure the temperature of the test material in order to measure the temperature of the test material. In order to more accurately grasp and control the temperature of the test material, it is preferable to attach a thermocouple at the center of the parallel portion, which is the position where LME cracks occur. However, in the case of double-sided plating, LME cracks also occur at the position where the thermocouple is attached. The thermocouple can be attached to the test material (test piece) by welding such as micro spot welding, but stress concentration is likely to occur at this welded portion. Therefore, as shown in FIG. 6, LME cracks are likely to occur at the thermocouple position. Therefore, when performing a tensile test using a test material with double-sided plating, it is preferable to attach the thermocouple offset from the parallel portion of the test material (the region where LME cracks occur in the tensile test). As shown in FIG. 7, the test temperature difference between the position of the × mark shifted from the parallel portion sandwiched by the dotted line and the parallel portion is actually measured in advance, and based on that difference, in the actual test, a thermocouple is attached at the position of the × mark and temperature control is performed based on the temperature measurement result, thereby eliminating the influence of stress concentration due to the thermocouple.

[0046] On one hand, in the case of single-sided plating, no LME cracks occur on the surface of the non-plated side. Therefore, by plating one side of the base steel plate and attaching a thermocouple to the non-plated side for temperature measurement, stress concentration caused by the thermocouple can be avoided. However, if the plate thickness is too thin, it is conceivable that the stress caused by the thermocouple attached to the non-plated side may affect the occurrence of LME cracks on the plated side. Therefore, when performing LME evaluation using a single-sided plated specimen, a thicker plate thickness is preferred. For example, when using a base steel plate with a plate thickness of 1.1 mm or more, it is preferable to perform single-sided plating, attach a thermocouple to the center of the parallel part on the non-plated side (the position corresponding to the area where LME cracks occur on the plated side) for temperature measurement, and perform a tensile test. In addition, although the contact type temperature measurement using the above thermocouple is preferred, a non-contact type temperature measurement method may also be used.

[0047] <Crack confirmation> For the specimen subjected to the high-temperature elongation test, check for the presence or absence of cracks from the surface where the second metal part of the first metal part is arranged towards the inside. And by checking the presence or absence of cracks for each of the plurality of specimens subjected to the high-temperature elongation test, the critical stress at which cracks occur in the specimen can be evaluated. By means of a high-temperature tensile test, observe a cross-section (which may be referred to as the "tensile direction cross-section" in the present disclosure) that is perpendicular to the surface of the specimen (zinc-plated steel plate), i.e., in the plate thickness direction, and parallel to the tensile direction of the high-temperature tensile test. Based on the cracks generated from the surface of the steel plate (i.e., the interface between the steel plate and the plating layer after the high-temperature tensile test) towards the inside, evaluate the critical stress for the occurrence of liquid metal embrittlement cracks in the base steel plate. Hereinafter, the case of a uniaxial tensile test will be described. Similarly, in the case of a biaxial tensile test, a spherical head bulge test, etc., an appropriate cross-section can be defined and observed. For example, with the principal stress direction as the tensile direction, a tensile direction cross-section can be used.

[0048] The tensile-direction cross-section of the test piece after the high-temperature tensile test is observed by, for example, a scanning electron microscope (SEM) or an optical microscope. When LME cracks have occurred, in this cross-section, the cracks are progressing from the steel plate surface toward the inside of the steel plate, and the presence or absence of cracks can be confirmed by SEM images, optical microscopes, etc. And among the plurality of test pieces subjected to the high-temperature tensile test, the minimum stress at which cracks have occurred can be evaluated as the LME crack initiation critical stress of that test piece or a stress close thereto. Note that the LME crack initiation critical stress may be more accurately estimated by further performing tensile tests by dividing into a plurality of stresses between the minimum stress at which cracks have occurred and the stress that is smaller than that stress and closest to it.

[0049] Also, as shown in FIGS. 3A and 4A, for each of the plurality of test pieces, two or more types of high-temperature tensile tests with different strain rates may be performed, and the critical stress may be evaluated for each strain rate. Since the crack generation situation is different at the same stress due to different strain rates, the LME crack initiation stress including the influence of the strain rate can be evaluated.

[0050] Also, as shown in FIG. 5, for each of the plurality of test pieces, two or more types of high-temperature elongation tests with different test temperatures may be performed, and the critical stress may be evaluated for each test temperature. Since the LME crack initiation stress is also different due to different test temperatures, the LME crack initiation region including the influence of temperature can be evaluated.

[0051] (Difference between single-sided plating and double-sided plating) Electro-galvanizing was performed on a cold-rolled base plate of 1470 MPa grade (plate thickness: 2 mm) with single-sided plating or double-sided plating with a plating amount of 50 g / m 2 for each surface, and the crack lengths in the samples that were pulled until a nominal stress of 130 MPa was applied at 800 °C were compared. For the single-sided plated material, the thermocouple was installed at the center of the parallel part on the non-plated side, and for the double-sided plated material, the installation position of the thermocouple was shifted to a position that does not cover the parallel part. FIGS. 8 and 9 are cross-sectional photographs of each plated material after the tensile test.

[0052] Looking at the cross-sectional photos obtained by any method, it can be seen that cracks with similar lengths have occurred. Fig. 10 shows a graph plotting the crack length with the distance from the center position of the parallel part on the horizontal axis. It can be seen from this graph that the difference between single-sided plating and double-sided plating is small. Therefore, since the difference between single-sided plating and double-sided plating has little effect on the crack length, it is considered that there is no problem in evaluating with either method. In single-sided plating, even if a thermocouple is attached to the parallel part on the non-plated side, the influence on the occurrence of LME cracks is small. Therefore, it is possible to measure and control the temperature at the crack generation part with high precision, and it is effective as a method to avoid stress concentration due to the thermocouple and evaluate the critical stress for LME crack generation.

[0053] As described above, regarding the method for evaluating the critical stress for LME crack generation according to the present disclosure, a steel plate (sheet material) is used as the metal base material (first metal part), a uniaxial tensile test (high-temperature tensile test) is performed as the high-temperature elongation test, and a cross-section perpendicular to the surface of the sheet material and parallel to the tensile direction of the high-temperature tensile test is observed to evaluate the critical stress for LME crack generation based on the presence or absence of cracks. However, the metal base material for evaluating the critical stress for LME crack generation is not limited to steel plates. For example, when using a round steel or wire as the metal base material, after a uniaxial tensile test as the high-temperature elongation test, a cut surface passing through the central axis and parallel to the tensile direction is observed, and the critical stress for LME crack generation can be evaluated based on the cracks (length, number, etc.). Also, when using a thick plate as the metal base material, the critical stress for LME crack generation may be evaluated using a test specimen cut out in a rod shape. Also, the plating component is not limited to zinc-based. Even when a plating layer other than zinc-based such as aluminum is formed on a metal base material such as a steel plate, the critical stress for LME crack generation can be evaluated. Furthermore, the first metal part is not limited to steel materials such as steel plates, round steels, and steel wire rods, and may be a metal material other than steel that causes LME in combination with the second metal part.

[0054] The scenarios where the method for evaluating the critical stress for LME crack generation according to the present disclosure is applied are not particularly limited. However, since it is possible to highly accurately evaluate the critical stress for LME crack generation with respect to a specific plating layer for a plurality of types of metallic materials having different components, structures, etc., for example, it can be suitably applied to the selection of materials and conditions where LME cracks are less likely to occur, such as the combination of a steel plate and plating, and the heating temperature during processing such as spot welding. Further, it may be applied to the selection of steel grades to be laminated for manufacturing clad materials and joining members.

[0055] [Method for Evaluating Critical Displacement Amount for Liquid Metal Embrittlement Crack Generation] The above-described method for evaluating the critical stress for LME crack generation evaluates the critical stress from the relationship between the stress and the occurrence of LME cracks by stopping the high-temperature elongation test midway so that the stress is different for each test specimen. However, it is also possible to evaluate the critical displacement amount from the relationship between the displacement amount and the occurrence of LME cracks by stopping the high-temperature elongation test midway so that the displacement amount is different for each test specimen. That is, the method for evaluating the critical displacement amount for LME crack generation according to the present disclosure is a method in which, among the steps included in the method for evaluating the critical stress for LME crack generation, in the step of performing the high-temperature elongation test, instead of performing the high-temperature elongation test so that the stress is different for each test specimen, the high-temperature elongation test is performed so that the displacement amount is different for each test specimen, and in the step of confirming the presence or absence of cracks, instead of evaluating the critical stress at which LME cracks occur in the test specimen, the critical displacement amount at which LME cracks occur in the test specimen is evaluated. Note that since it is the same as the above-described method for evaluating the critical displacement amount for LME crack generation except for evaluating based on the displacement amount instead of the stress, the description here is omitted.

[0056] [Method for Evaluating Liquid Metal Embrittlement Sensitivity] The critical stress and critical displacement obtained by each of the above-described evaluation methods are related to the liquid metal embrittlement susceptibility of the test material (metal material). The greater the critical stress, the lower the liquid metal embrittlement susceptibility, and the same relationship holds between the critical displacement and the liquid metal embrittlement susceptibility. Therefore, by using the evaluation method for the critical stress for the occurrence of liquid metal embrittlement cracks or the evaluation method for the critical displacement for the occurrence of liquid metal embrittlement cracks according to the present disclosure, the liquid metal embrittlement susceptibility of the metal material can be evaluated. That is, the evaluation method for liquid metal embrittlement susceptibility according to the present disclosure uses at least one of the above-described evaluation methods for the critical stress for the occurrence of liquid metal embrittlement cracks and the evaluation method for the critical displacement for the occurrence of liquid metal embrittlement cracks, and evaluates the liquid metal embrittlement susceptibility of the test material based on at least one of the critical stress and the critical displacement.

[0057] By evaluating the critical stress, the critical displacement, or the liquid metal embrittlement susceptibility according to the method of the present disclosure, it is possible to contribute to the selection of a combination of the first metal part and the second metal part in which LME cracks are less likely to occur, the setting of welding conditions such as spot welding, and the like.

Example

[0058] Hereinafter, as a representative example of the evaluation method according to the present disclosure, examples of the evaluation method for the critical stress for the occurrence of liquid metal embrittlement cracks and the critical displacement will be given for more specific explanation. However, the following examples do not limit the evaluation method for the critical stress for the occurrence of liquid metal embrittlement cracks of the present disclosure.

[0059] <Example 1> (Preparation of Test Material) As test materials, cold-rolled steel sheets with different components and heat treatments and a plate thickness of 1.6 mm were used, and electro-galvanizing was applied to both sides of each cold-rolled steel sheet to produce the following electro-galvanized steel sheets A, B, and C. Steel sheet A: 980 MPa grade electro-galvanized steel sheet (50 g / m 2 ) Steel sheet B: 1180 MPa grade electro-galvanized steel sheet (50 g / m 2 ) Steel sheet C: Zinc electroplated on a 1470 MPa grade hot stamping steel sheet (70 g / m 2 ) From each steel plate, as shown in Fig. 2, 10 test pieces (samples) having a shape with a parallel portion (6 mm) at the center of the constricted portion were collected respectively.

[0060] (High-temperature tensile test) For each sample collected from steel plate A, thermocouples were attached at intervals of 0.5 mm from the boundary of the parallel portion of the sample so as not to cover the parallel portion, and after heating the sample to the maximum temperature reached (900 °C), it was cooled to 800 °C at 50 °C / s and then tension was applied. The heating was by electric heating at 300 °C / s, the cooling was by nitrogen gas cooling, the strain rate was 0.1 / s, and the tensile test was carried out without breaking the sample. The tensile test was carried out so that the stress of each sample was different.

[0061] Tensile tests were also carried out on each sample collected from steel plates B and C in the same manner as the samples of steel plate A.

[0062] (Confirmation of crack occurrence) For each sample after the tensile test, observation of the cross-section in the tensile direction was carried out. The sample was cut along a cutting line perpendicular to the surface of each sample, parallel to the tensile direction, and passing through the approximate center in the width direction of the sample, and SEM observation of the cross-section (magnification: 25 times) was performed. The stress at which cracks occurred from the surface to the inside of each sample collected from the same steel plate was evaluated as the critical stress for LME crack occurrence of that steel plate. The critical stress for LME crack occurrence of each steel plate is shown in Fig. 11A. Note that for steel plate A, no LME cracks occurred at stresses of 170 MPa or less.

[0063] (Example 2) Samples were collected from steel plates A to C used in Example 1 in the same manner as in Example 1, and tensile tests and the presence or absence of cracks were confirmed in the same manner as in Example 1, except that the strain rate in the high-temperature tensile test was changed to 10 / s. The critical stress for LME crack occurrence of each steel plate is shown in Fig. 11B.

[0064] (Evaluation results) As shown in FIGS. 11A and 11B, the critical stress for LME crack generation was higher for steel plates A, B, and C in that order under any conditions. From this, it can be evaluated that under the conditions of Example 1 or Example 2, in the order of steel plates A, B, and C, the LME sensitivity is low and LME is less likely to occur in spot welding or the like. Note that the steel plate B with a strain rate of 0.1 / s has a higher critical stress for LME crack generation than the steel plate A with a strain rate of 10 / s. Therefore, when applying it to the evaluation of the LME sensitivity of each steel plate by the method for evaluating the critical stress for LME crack generation according to the present disclosure, in addition to the test temperature, it is important to perform a high-temperature tensile test with the same strain rate to compare the critical stress for LME crack generation.

[0065] <Example 3> (Preparation of test materials) As test materials, cold-rolled steel plates with different components and heat treatments and a plate thickness of 1.6 mm were used, and hot-dip galvanizing was applied to one side of each cold-rolled steel plate. Next, each hot-dip galvanized steel plate was immersed in a salt bath (500 °C) for an alloying treatment to produce the following alloyed hot-dip galvanized steel plates E, F, and G. Further, as shown in FIG. 2, test pieces (samples) having a shape with a parallel portion (6 mm) at the center of the constricted portion were taken from each steel plate. Steel plate E: 980 MPa grade alloyed hot-dip galvanized steel plate (50 g / m 2 ) Steel plate F: 1180 MPa grade alloyed hot-dip galvanized steel plate (50 g / m 2 ) Steel plate G: 1470 MPa grade alloyed hot-dip galvanized steel plate (70 g / m 2 )

[0066] (High-temperature tensile test) A thermocouple was attached to the center position of the parallel portion on the non-plated surface of each sample, and a tensile test was performed under the same conditions as in Example 1 so that the displacements of the samples were different without breaking each sample. The maximum displacement was set to 3 mm.

[0067] (Confirmation of crack generation) For each sample after the tensile test, the presence or absence of cracks was confirmed in the same manner as in Example 1. The critical displacement at which LME cracks occur in each steel plate is shown in Fig. 12. From Fig. 12, it can be seen that the critical displacement is large and the LME sensitivity is low in the order of steel plates E, F, and G. Note that no LME cracks occurred in steel plate E up to a displacement of 3 mm.

Explanation of symbols

[0068] 10 Specimen 12 Necking part 14 Parallel part

Claims

1. A step of preparing a test specimen including a first metal part and a second metal part disposed on at least a part of the first metal part, the melting start temperature of the second metal part being lower than that of the first metal part, and when the second metal part is liquefied by heating, liquid metal embrittlement of the first metal part occurs; A step of heating the test specimen in a temperature range that is equal to or higher than the melting start temperature of the second metal part and lower than the melting start temperature of the first metal part, applying a load to the test specimen to impart elongation, and performing a high-temperature elongation test of stopping the application of the load before the test specimen breaks; A step of checking for the presence or absence of cracks from the surface of the first metal part where the second metal part is disposed toward the inside of the test specimen that has undergone the high-temperature elongation test; comprising; In the step of preparing the test specimen, a plurality of test specimens are prepared; In the step of performing the high-temperature elongation test, the high-temperature elongation test is performed on the plurality of test specimens with the test temperature and strain rate being made uniform and the stress being different for each test specimen; A method for evaluating the critical stress for crack generation due to liquid metal embrittlement, wherein in the step of checking for the presence or absence of cracks, the critical stress for crack generation in the test specimen is evaluated by checking for the presence or absence of cracks in the plurality of test specimens that have undergone the high-temperature elongation test.

2. The method for evaluating the critical stress for crack generation due to liquid metal embrittlement according to claim 1, wherein for each of the plurality of test specimens, two or more types of the high-temperature elongation tests with different strain rates are performed, and the critical stress is evaluated for each strain rate.

3. The method for evaluating the critical stress for crack generation due to liquid metal embrittlement according to claim 1 or claim 2, wherein for each of the plurality of test specimens, two or more types of the high-temperature elongation tests with different test temperatures are performed, and the critical stress is evaluated for each test temperature.

4. The method for evaluating the critical stress for crack generation due to liquid metal embrittlement according to any one of claims 1 to 3, wherein the various types of the first metal part and the second metal part of the plurality of test specimens are the same as each other.

5. The method for evaluating the critical stress for crack generation due to liquid metal embrittlement according to any one of claims 1 to 3, wherein at least one type of the first metal part and the second metal part of the plurality of test specimens is different from each other.

6. The method for evaluating the critical stress for crack generation due to liquid metal embrittlement according to any one of claims 1 to 5, wherein the first metal part is a steel material.

7. The evaluation method for the critical stress of liquid metal embrittlement crack generation according to any one of claims 1 to 6, wherein the first metal part is a base steel plate and the second metal part is a plating layer.

8. In the test specimen, the plating layers are arranged on both surfaces of the base steel plate, The evaluation method for the critical stress of liquid metal embrittlement crack generation according to claim 7, wherein a thermocouple is attached at a position outside the region where the crack occurs on either one of the surfaces of the base steel plate, and the high-temperature elongation test is performed.

9. In the test specimen, the plating layer is arranged on one surface of the base steel plate, The evaluation method for the critical stress of liquid metal embrittlement crack generation according to claim 7, wherein a thermocouple is attached at a position corresponding to the region where the crack occurs on the one surface on the other surface of the base steel plate, and the high-temperature elongation test is performed.

10. Among the steps included in the evaluation method for the critical stress of liquid metal embrittlement crack generation according to any one of claims 1 to 9, In the step of performing the high-temperature elongation test, instead of performing the high-temperature elongation test such that the stress is different for each test specimen, the high-temperature elongation test is performed such that the displacement amount is different for each test specimen, In the step of confirming the presence or absence of the crack, instead of evaluating the critical stress at which the crack occurs in the test specimen, the critical displacement amount at which the crack occurs in the test specimen is evaluated. An evaluation method for the critical displacement amount of liquid metal embrittlement crack generation.

11. Using at least one of the evaluation methods for the critical stress of liquid metal embrittlement crack generation according to any one of claims 1 to 9 and the evaluation method for the critical displacement amount of liquid metal embrittlement crack generation according to claim 10, an evaluation method for the liquid metal embrittlement susceptibility of the test specimen based on at least one of the critical stress and the critical displacement amount.

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