Method for evaluating liquid metal embrittlement susceptibility

Abstract

To provide a liquid metal embrittlement susceptibility evaluation method that can evaluate liquid metal embrittlement susceptibility of a metal material highly accurately.SOLUTION: A method is configured to: prepare a plurality of sample materials including a first metal part and a second metal part arranged in at least a part of the first metal part and being lower in a melting start temperature than the first metal part, in which at least one of a chemical composition of the first metal part and a metallographic structure thereof is different from each other, and a basis weight of the second metal part and chemical composition thereof are organized; apply a load to give an elongation so that a test temperature of a plurality of sample materials and a displacement magnitude or a stress value thereof are organized; perform a high-temperature elongation test for stopping the application of the load before the sample material fractures; and evaluate liquid metal embrittlement susceptibility of the first metal part on the basis of cracking occurring in the first metal part by the high-temperature elongation test.SELECTED DRAWING: Figure 17

Description

Technical Field

[0001] The present disclosure relates to 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) may be a problem in spot welding or the like. Liquid metal embrittlement refers to the phenomenon that 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. Typically, it 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 because the probability of the number of cracks in the resistance spot weld is less than an average of 5.

[0004] Patent Document 2 discloses a plated steel material in which a Zn - Al - Mg plating layer has a specific composition. As an example, a bead - on - plate test piece obtained by bead - on - plate welding a steel sheet 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 formed 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 - rolled V - bending, the presence or absence of liquid metal embrittlement cracking 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. The difference value from the lower limit to the current at which no LME crack occurs is defined as the "LME crack non-occurrence current range" to evaluate the spot weldability of the steel sheet.

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

[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 describes performing high-temperature tension on a test piece obtained by applying electro-galvanizing to various high-strength steel sheets with different microstructures at a thickness of about 10 μm per side until fracture, heating at 1000 °C / s for 0.5 s to simulate spot welding, and evaluating the Zn-LME susceptibility based on the crack length and the number of cracks generated.

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, No. 1, 1983, pp. 56-61 [Non-Patent Document 2 Materials Science & Engineering A 804 (2021) 140391 [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) or LME susceptibility is evaluated based on the presence or absence of cracks, the number of cracks, and the crack length in the welded or bent portions of plated steel sheets. However, a method capable of evaluating the LME susceptibility of metal materials with higher accuracy is desirable. Therefore, an object of the present disclosure is to provide a method for evaluating the liquid metal embrittlement susceptibility that can accurately evaluate the liquid metal embrittlement susceptibility of metal materials. [Means for Solving the Problems

[0012] The means for solving the above problems include the following aspects. <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 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 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, and performing a high-temperature elongation test in which a load is applied to the test specimen to impart elongation. Evaluating the liquid metal embrittlement susceptibility of the first metal part based on cracks generated from the surface where the second metal part of the first metal part is disposed toward the inside by the high-temperature elongation test; comprising: In the step of preparing the test piece, as the test piece, a plurality of test pieces are prepared in which at least one of the chemical composition and the metal structure of the first metal part is different from each other, and the coating weight and the chemical composition of the second metal part are uniform; In the step of performing the high-temperature elongation test, the load is applied to impart elongation so that the test temperature and the displacement amount or the stress value of the plurality of test pieces are uniform, and the application of the load is stopped before the test piece breaks. A method for evaluating liquid metal embrittlement susceptibility. <2> The method for evaluating liquid metal embrittlement susceptibility according to <1>, wherein the first metal part is a steel material. <3> The method for evaluating liquid metal embrittlement susceptibility according to <1> or <2>, wherein the first metal part is a base metal steel plate and the second metal part is a plating layer. <4> In the test piece, the plating layers are disposed on both surfaces of the base metal steel plate, The method for evaluating liquid metal embrittlement susceptibility according to <3>, wherein a thermocouple is attached at a position outside the region where the crack occurs on either surface of the base metal steel plate to perform the high-temperature elongation test. <5> In the test piece, the plating layer is disposed on one surface of the base metal steel plate, The method for evaluating liquid metal embrittlement susceptibility according to <3>, 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 metal steel plate to perform the high-temperature elongation test.

Advantages of the Invention

[0013] According to the present disclosure, there is provided a method for evaluating liquid metal embrittlement susceptibility that can accurately evaluate the liquid metal embrittlement susceptibility of a metal material.

Brief Description of the Drawings

[0014]

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MODE FOR CARRYING OUT THE INVENTION

[0015] An embodiment which is an example of the present disclosure will be described. In addition, 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 present inventors repeatedly studied a method capable of more accurately evaluating the LME susceptibility of metal materials and obtained the following findings.

[0017] (1) Influence by coating weight (coating amount per unit area) Regarding the influence of the coating amount on LME cracks, in order to quantitatively evaluate the coating amount, the coating weight was changed in a high-temperature tensile test and its influence was examined. Figure 1 shows the coating weights of 0 g / m 2 (no coating), 10 g / m 2 , 50 g / m 2 , 70 g / m 2 , or 100 g / m 2For each test specimen of electro-galvanized steel sheet (thickness 1.6 mm), after heating at 900 °C (heating rate 100 °C / s), a tensile test (tensile rate 50 mm / s) was carried out at 800 °C (cooling rate 50 °C / s), and the relationship between the plating adhesion amount of each test specimen and the LME crack length (maximum, average, minimum) measured by cross-sectional observation is shown. Tensile tests were also carried out with the temperature during the tensile test being 900 °C or 700 °C. In both cases, when the plating amount was 50 g / m 2 LME cracks occurred when the above conditions were met, and the crack length tended to increase as the plating amount increased. From these results, even when tensile tests were carried out under the same conditions using test specimens with different weights per unit area and the LME susceptibility was evaluated based on the crack length, since the plating amount (weight per unit area) affects the crack length, it is considered that the LME susceptibility cannot be accurately evaluated. In other words, it is considered that conducting LME evaluation with the same weight per unit area contributes to improving the accuracy of LME susceptibility evaluation.

[0018] (2) Influence by plating composition Hot-dip galvanized steel sheets (thickness 1.6 mm, plating thickness 10 μm) were immersed in a salt bath at 500 °C for 15 s, 30 s, 50 s, 100 s, or 300 s, and then annealed by cooling at 0.2 - 0.3 °C / s. When the Fe concentration in the Zn plating was measured by ICP (inductively coupled plasma), as shown in Fig. 2, the Fe concentration in the plating changed according to the salt bath treatment time, and became almost constant when it exceeded 100 s. Among these, for specimens A, B, and C, that is, specimens with Fe concentrations (mass%) in the plating of 2%, 8%, and 12%, tensile tests were carried out in the same manner as when investigating the influence by the above-mentioned weight per unit area, and the LME crack length was measured by observing the cross-section of each specimen. When examining the relationship between the Fe concentration during plating and the length of LME cracks, as shown in Fig. 3, at any tensile temperature, the higher the Fe concentration during plating, the smaller the crack length tended to be. From these results, even if tensile tests are conducted on test materials with the same basis weight under the same conditions and the LME susceptibility is evaluated based on the crack length, since the plating composition affects the crack length, it is considered that the LME susceptibility cannot be accurately evaluated. In other words, it is considered that conducting LME evaluation with the same plating composition also contributes to improving the accuracy of LME susceptibility evaluation.

[0019] Based on the above considerations, it was thought that if the deposition amount per unit area (basis weight) of the plating on the test material and the plating composition were made the same and tensile tests were conducted under the same conditions, the LME susceptibility could be evaluated with high precision. However, when the test material was fractured by a tensile test under the same conditions, it was considered that there was a possibility that factors other than the LME susceptibility affected the crack length because the crack length was large in the vicinity of the fracture part. Therefore, regarding a hot-dip galvanized steel sheet with a thickness of 1.6 mm (cold-rolled steel sheet of 1180 MPa grade), heating was carried out up to 900 °C at 100 °C / s by electric heating, and after reaching 900 °C, tensile tests were conducted at a crosshead speed of 50 mm / s at 900 °C to fracture the test material, and an examination was made on the difference between the cracks when the test material was fractured and the cracks when the tensile test was stopped (interrupted) before fracture. The interruption was stopped at a displacement of 1 mm and a nominal stress of 116 MPa.

[0020] (3) Influence by stress distribution in tensile test Fig. 4 shows the cross-section (cross-section in the tensile direction and the plate thickness direction) when the test material is fractured by a tensile test, and Fig. 5 shows the occurrence status of each crack in the cross-section when the tensile test is interrupted without fracturing the test material. Fig. 6 shows the relationship between the crack occurrence position and the crack length in each test material after the tensile test. In the samples stopped midway, the crack length is almost horizontal across the tensile direction, while in the fractured samples, it can be seen that the crack length near the fracture part is longer. It is considered that more deformation progressed near the fracture part, and both the stress and strain were higher than those in other parts. As a result, it is considered that the crack length became longer. Therefore, since the crack length is affected by stress or strain, it is considered necessary to eliminate this influence on the evaluation of the susceptibility of the steel plate based on the crack length.

[0021] (4) Influence of cracks other than LME cracks When deformed until fracture, depending on the plating adhesion amount, the susceptibility of the steel plate, the test temperature, the tensile speed, etc., not only LME cracks but also further plastic deformation progresses at the tip and the crack may progress. LME cracks generally exhibit brittle crack propagation. For example, at the tip of the crack near the fracture position shown in Fig. 7, it appears to be undergoing ductile fracture. In Fig. 8, which is an enlarged view of the vicinity of the fracture position shown in Fig. 7, when point analysis by SEM (scanning electron microscope) - EDS (energy dispersive X-ray spectroscopy) is performed on the locations indicated by 1, 2, and 3, the results shown in Figs. 9 to 11 are obtained respectively. No zinc peak was obtained at position 3, which is considered to have undergone ductile fracture. By this point analysis, when measuring the location marked with an 'x' in Fig. 8, it was separated into a region with 'Zn detected' and a region with 'Zn not detected' as divided by the dashed line. Therefore, it is considered that the crack at the position where Zn was not detected is not an LME crack. From these facts, when performing LME susceptibility evaluation by tensile test, if carried out until fracture, large deformation occurs and the cracks may be a combination of LME cracks and ductile fracture accompanying deformation, and it may not be appropriate as an evaluation of the LME susceptibility of the steel plate based on the crack length. Therefore, for example, it is considered possible to evaluate the LME susceptibility with high precision by performing susceptibility evaluation based on the crack length after stopping midway during uniform deformation or before that.

[0022] In addition, in this specification, the "high-temperature elongation test" refers to a high-temperature test that applies elongation to at least a part of the test specimen, and includes not only a general tensile test but also a cantilever test and the like.

[0023] Hereinafter, embodiments of the method for evaluating liquid metal embrittlement susceptibility according to the present disclosure will be specifically described.

[0024] The method for evaluating liquid metal embrittlement susceptibility according to the present disclosure includes 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 preparing a test specimen in which liquid metal embrittlement (LME) of the first metal part occurs when the second metal part is liquefied by heating. heating the test specimen in 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, and performing a high-temperature elongation test in which a load is applied to the test specimen to apply elongation. evaluating the liquid metal embrittlement susceptibility of the first metal part based on cracks generated from the surface of the first metal part where the second metal part is disposed toward the inside during the high-temperature elongation test. and In the step of preparing the test specimen, a plurality of test specimens are prepared as the test specimen, in which at least one of the chemical composition and the metal structure of the first metal part is different from each other, and the basis weight and the chemical composition of the second metal part are uniform. In addition, in the step of performing the high-temperature elongation test, the load is applied so that the test temperature and the displacement amount or the stress value of the plurality of test specimens are uniform to apply elongation, and the application of the load is stopped before the test specimen breaks. By thus making the basis weight and the chemical composition of the second metal part uniform, and performing the high-temperature elongation test while making the test temperature and the displacement amount or the stress value uniform without breaking the test specimen, the influence due to the difference in the basis weight and the chemical composition of the second metal part, and cracks other than cracks due to LME are eliminated or suppressed, and the LME susceptibility can be evaluated with high accuracy based on the length of the cracks generated in the first metal part. Hereinafter, each step will be specifically described.

[0025] <Preparation of Specimen <Prepare a specimen 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. Here, as the specimens, prepare a plurality of specimens in which at least one of the chemical composition and the metal structure of the first metal part is different from each other, and the areal weight and the chemical composition of the second metal part are uniform. <Hereinafter, the case of using a 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.

[0026] <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 in the form of plates, rods, wires, and cylinders. 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 in the form of a plate, the metal layer may be laminated on one side or both sides of the metal substrate.

[0027] <The material of the specimen is not particularly limited as long as the combination of the metal substrate and the metal layer 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.

[0028] In addition, as long as the test specimen is a combination that causes LME, 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 in a state where a plurality of different plates such as steel and aluminum are stacked and rotating them at high speed in the thickness direction may be used. In the case of such a joined material, LME may occur due to heat and stress during joining. Also, 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. Note that the "coating weight" in the present disclosure means the mass per unit area. When the second metal part is a plating layer, it is the plating adhesion amount per unit area, and when the second metal part is a plate material, it means the mass per unit area of the plate material.

[0029] In addition, another layer such as metal pre-plating may exist between the metal base material (the first metal part) and the metal layer (the 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 (the second metal part) (that is, on the side opposite to the metal base material).

[0030] 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 zinc-plated 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.

[0031] (Base steel sheet) A plurality of base material steel plates are used, with at least one of the chemical composition and the metal structure being different from each other. For example, even when producing cold-rolled steel plates from slabs with the same chemical composition, steel plates with different metal structures can be produced due to differences in the thermal history. For example, if the chemical compositions are the same but the metal structures are different, the mechanical properties and chemical properties will be different, and the LME susceptibility will also be different.

[0032] (Plating) Zinc plating is applied to each base material steel plate. Here, a zinc plating layer is formed on each base material steel plate so that the coating weight and the chemical composition are uniform. Note that the plating method is not limited, and it may be hot-dip galvanizing or electro-galvanizing. For example, by plating each base material steel plate under the same conditions, a plating layer with uniform coating weight and chemical composition can be formed on each base material steel plate. Note that the "plating layer with uniform coating weight and chemical composition" means that the coating weight and the chemical composition of the plating layer in each plated steel plate are the same. If plating is carried out under the same conditions, even if the composition or structure of the base material steel plate is different, basically, a plating layer with the same coating weight and the same composition will be formed.

[0033] On the other hand, when heating and alloying are carried out after plating under the same conditions, a part of the components of the base material steel plate may move to 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 steel plate is slight, and the influence on LME cracks is small, so it is acceptable. However, a significant difference in the composition in the plating layer can occur between the case where alloying treatment is not carried out and the case where it is carried out after plating under the same conditions. Therefore, the test specimens to be evaluated should also have the same presence or absence of alloying treatment after plating under the same conditions. That is, the treatment may be carried out under the same conditions from plating on the base material steel plate to the start of the high-temperature elongation test.

[0034] The shape of the plated steel sheet is not particularly limited. However, when performing a high-temperature elongation test such as a high-temperature tensile test, in order to limit the site where stress is concentrated and observed in the tensile test (i.e., the site where cracks occur), for example, as shown in FIG. 12, it is preferable to use a test specimen 10 having a constricted portion 12 and a parallel portion 14 where both edges are parallel to the tensile direction in the tensile test at the center of the constricted portion 12.

[0035] Also, the thickness of the test specimen is not limited, but if cracks penetrate in the thickness direction during the tensile test, it becomes difficult to make a comparative evaluation. Therefore, the thickness of the test specimen is preferably 0.5 mm or more. 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.

[0036] <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, and applying a load to the test specimen to impart elongation. The type of high-temperature elongation test is not particularly limited. For example, in addition to a uniaxial tensile test, a biaxial tensile test or a spherical bulge test 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 sheet. It is desirable to perform the test at a 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 prone to deformation, making the tensile test difficult. 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 temperature of 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 - 1200°C.

[0037] Heating (temperature increase) of the test specimen in the tensile test can be carried out by electric heating or furnace heating. The heating rate is preferably 50°C / s or higher (for example, 50 - 150°C / s) to prevent the volatilization of zinc (plating layer). Tension can be applied to the test specimen by applying a tensile load before heating and then heating, or by heating without applying a tensile load and then performing tension at the maximum temperature reached (for example, 500°C, 700°C, or 900°C). Alternatively, once heated to the maximum temperature reached (for example, 1000°C), cooled to the processing temperature (for example, 800°C, 700°C, 600°C, or 500°C) at which tension is to be performed, and then tension can be carried out.

[0038] 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 preferable to heat once (for example, 1000°C), then cool at a rate slower than that of actual spot welding but with nitrogen gas cooling, and perform tension at the temperature at the time of electrode release (for example, 800°C). Note that in such a temperature range, 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 in each crystal structure may be different, 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.

[0039] The tensile speed is not particularly limited. However, if the tensile speed is too slow, the alloying of steel and zinc will progress, making it difficult to evaluate the liquid metal embrittlement susceptibility. Therefore, it is desirable to perform the tensile test in a short time. For example, the tensile test is performed at a tensile speed of 1 to 100 mm / s.

[0040] In the tensile test, a tensile load is applied to each test specimen so that the test temperature, displacement amount, or stress value is the same (the same displacement amount or the same stress value), and the test is stopped (interrupted) before the test specimen breaks. When evaluating LME based on the crack length from the surface to the inside of the steel plate, it is necessary to open the crack, so it is preferable to introduce at least a strain of 0.01. By stopping the tensile test before the test specimen breaks, the deformation and expansion of the crack due to the influence of fracture are eliminated, the variation in strain due to the position in the tensile direction is small, the variation in crack length is also small, and it becomes easier to observe and evaluate the cross-section in the tensile direction.

[0041] (Position of thermocouple) In the tensile test, it is preferable to attach a thermocouple to the test specimen to measure the temperature of the test specimen. In order to more accurately grasp and control the temperature of the test specimen, it is advisable to attach a thermocouple at the center of the parallel part, which is the position where the LME crack occurs, for temperature measurement. 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 specimen (test piece) by welding such as micro-spot welding, but stress concentration is likely to occur at this welded part. Therefore, as shown in Fig. 13, LME cracks are likely to occur at the thermocouple position. Therefore, when performing a tensile test using a test specimen with double-sided plating, it is preferable to attach the thermocouple offset from the parallel part of the test specimen. As shown in Fig. 14, the test temperature difference between the position marked with a cross offset from the parallel part sandwiched by the dotted line and the parallel part is measured in advance, and based on this difference, in the actual test, the thermocouple is attached at the position marked with a cross, and temperature control is performed according to the temperature measurement result, so that the influence of stress concentration due to the thermocouple can be eliminated.

[0042] On the other 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 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. The above-described contact temperature measurement using a thermocouple is preferred, but a non-contact temperature measurement method may also be used.

[0043] <Evaluation of LME Sensitivity> By means of a high-temperature tensile test, a cross-section 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 (which may be referred to as the "tensile direction cross-section" in the present disclosure) is observed, and the liquid metal embrittlement sensitivity of the base steel plate is evaluated 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) toward the inside. Hereinafter, the case of a uniaxial tensile test will be described. However, in the cases of a biaxial tensile test, a spherical head bulge test, etc., a cross-section can be determined and observed as appropriate. For example, with the principal stress direction as the tensile direction, a tensile direction cross-section can be obtained.

[0044] The tensile direction cross-section of the specimen is observed, for example, by a scanning electron microscope (SEM) or an optical microscope. In this cross-section, cracks are propagating from the steel plate surface toward the inside of the steel plate, and the length and number of the cracks can be measured by means of a backscattered electron image of the SEM or the like. For example, using the SEM image, the length of the crack from the surface of the base steel plate toward the inside is measured to evaluate the liquid metal embrittlement sensitivity of the steel plate.

[0045] The length of the crack is an index directly representing the susceptibility to liquid metal embrittlement. When evaluating the crack length, for example, the maximum value of the crack length may be measured and evaluated in the SEM image. However, from the perspective of evaluation accuracy, as shown in Fig. 17, it is preferable to plot the change in crack length according to the distance from the center position of the parallel part of the test material. In this plot, the behavior is classified according to the steel type and surface state, and the susceptibility to liquid metal embrittlement of each test material can be accurately evaluated by the maximum value, average value, and / or integral value of the crack length, etc. In the evaluation of the crack length, a test material with a small maximum value, average value, and / or integral value can be evaluated as a preferable combination with low susceptibility to liquid metal embrittlement of the steel plate with respect to the plating layer formed on the surface. Note that the evaluation index of LME is not limited to the crack length, and the susceptibility to LME may be evaluated based on the number of cracks.

[0046] (Difference between single-sided plating and double-sided plating) Electro-galvanized plating was applied to a cold-rolled base plate of 1470 MPa grade (plate thickness: 2 mm) with single-sided plating or double-sided plating at a plating amount of 50 g / m 2 for each side, and the crack lengths in the samples subjected to tension 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 not covering the parallel part. Figs. 15 and 16 are cross-sectional photographs of each plated material after the tensile test.

[0047] Regardless of the cross-sectional photos obtained by any method, it can be seen that cracks with similar lengths have occurred. Fig. 17 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 influence on the crack length due to the difference between single-sided plating and double-sided plating is small, it is considered that there is no problem in evaluating with either. In the case of 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 accurately evaluate the LME sensitivity by avoiding stress concentration due to the thermocouple.

[0048] As described above, regarding the method for evaluating the liquid metal embrittlement sensitivity according to the present disclosure, a steel plate (plate 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 plate material and parallel to the tensile direction of the high-temperature tensile test is observed to evaluate the LME sensitivity based on the crack length or the penetration distance of the liquid metal. However, the metal base material for evaluating the LME sensitivity is not limited to the steel plate. For example, when using a round steel or wire as the metal base material, after the 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 LME sensitivity can be evaluated based on the crack (length, number, etc.). Also, when using a thick plate as the metal base material, the LME sensitivity may be evaluated using a test specimen cut out in a rod shape. Also, the plating component is not limited to the zinc-based system, and the LME sensitivity can also be evaluated when a plating layer other than zinc-based such as aluminum is formed on a metal base material such as a steel plate. 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 in which LME occurs in combination with the second metal part.

[0049] The scenarios where the method for evaluating liquid metal embrittlement susceptibility according to the present disclosure is applied are not particularly limited. However, since the LME susceptibility to a specific plating layer can be evaluated with high precision for a plurality of types of metal 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 steel plates and plating, and the heating temperature during processing such as spot welding. Further, it may be applied to the selection of steel types to be laminated for manufacturing clad materials and joining members.

Example

[0050] Hereinafter, the method for evaluating liquid metal embrittlement susceptibility of the present disclosure will be described in more detail with reference to examples. However, the following examples do not limit the method for evaluating liquid metal embrittlement susceptibility of the present disclosure.

[0051] <Example 1> (Preparation of test specimens) As test specimens, cold-rolled steel plates with different components and heat treatments and a plate thickness of 1.6 mm were used, and electro-galvanizing was performed on both sides of each cold-rolled steel plate with a plating amount of 50 g / m 2 of zinc to produce the following electro-galvanized steel plates A and B. Further, as shown in FIG. 12, test specimens (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 A: 980 MPa grade electro-galvanized steel plate (50 g / m 2 ) Steel plate B: 1180 MPa grade electro-galvanized steel plate (50 g / m 2 )

[0052] (High-temperature tensile test) Thermocouples were attached at intervals of 0.5 mm from the boundary of the parallel portion of each sample so as not to be applied to the parallel portion. After heating the sample to the maximum temperature (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, and the tensile speed was 50 mm / s. The tensile test was carried out until the stress value reached 120 MPa as the nominal stress without breaking the sample. Also, the tensile test was carried out under the same conditions so that the displacement amount became 0.8 mm.

[0053] (Measurement of crack length) For each sample after the tensile test, the cross-section in the tensile direction was observed. The sample was cut along a cutting line that was perpendicular to the surface of each sample, parallel to the tensile direction, and passed through the approximate center in the width direction of the sample, and SEM observation of the cross-section (magnification: 25 times) was performed. Regarding the cracks generated from the surface to the inside of each sample, the crack position (distance from the center position of the parallel part) and the crack length were measured and plotted. The results are shown in FIGS. 18 and 19. Also, the average crack length of the cracks generated in each sample with the stress value set to 120 MPa is as follows. Average crack length of steel plate A: 0.58 mm Average crack length of steel plate B: 0.36 mm Also, the average crack length of the cracks generated in each sample with the displacement amount set to 0.8 mm is as follows. Average crack length of steel plate A: 0.58 mm Average crack length of steel plate B: 0.37 mm

[0054] <Comparative Example 1> A tensile test was performed on the test pieces prepared in the same manner as in Example 1 until fracture. Regarding the cracks generated from the surface to the inside of each sample, the distance from the fracture position and the crack length were measured and plotted. Note that the fracture position is the tip position of the fractured part. The results are shown in FIG. 20. Also, the average crack length of the cracks generated in each sample is as follows. Average crack length of steel plate A: 0.60 mm Average crack length of steel plate B: 0.39 mm

[0055] (Evaluation of LME sensitivity) As is clear from FIGS. 18, 19, and 20, the variation in the crack length distribution of each steel plate in Example 1 was smaller than the variation in the crack length distribution of each steel plate in Comparative Example 1. From these results, it is considered that in Example 1, the LME sensitivity of each base metal steel plate could be evaluated with higher accuracy than in Comparative Example 1.

[0056] <Example 2> (Preparation of Test Specimens) As test specimens, cold-rolled steel sheets with different components and heat treatments and a thickness of 1.6 mm were used. Zinc was electroplated on one side of each cold-rolled steel sheet at a plating amount of 100 g / m 2 . During the electroplating process, the surface was protected by attaching a heat-resistant protective sheet to one side. Next, each electroplated zinc steel sheet was immersed in a salt bath (500 °C) for alloying treatment without removing the protective sheet, and the following alloyed electroplated zinc steel sheets C and D were produced. Furthermore, as shown in Fig. 12, test pieces (samples) having a shape with a parallel portion (6 mm) at the center of the constricted portion were taken from each steel sheet. Steel sheet C: 980 MPa grade alloyed electroplated zinc steel sheet (100 g / m 2 ) Steel sheet D: 1180 MPa grade alloyed electroplated zinc steel sheet (100 g / m 2 )

[0057] (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 on each sample under the same conditions as in Example 1 without breaking it.

[0058] (Measurement of Crack Length) For each sample after the tensile test, the cross-section in the tensile direction was observed in the same manner as in Example 1, and the crack position (distance from the center position of the parallel portion) and crack length were measured and plotted. The results are shown in Fig. 21. Also, the average length of the cracks generated in each sample is as follows. Average crack length of steel sheet C: 0.27 mm Average crack length of steel sheet D: 0.49 mm

[0059] <Comparative Example 2> Tensile tests were performed on the test pieces prepared in the same manner as in Example 2 until they broke. For the cracks generated from the surface to the inside of each sample, the distance from the fracture position and the crack length were measured and plotted. The fracture position is the tip position of the fracture part. The results are shown in Fig. 22. Also, the average length of cracks generated in each sample is as follows. Average crack length of steel plate C: 0.28 mm Average crack length of steel plate D: 0.51 mm

[0060] (Evaluation of LME susceptibility) As is apparent from FIGS. 21 and 22, the variation in the crack length distribution of each steel plate in Example 2 was smaller than the variation in the crack length distribution of each steel plate in Comparative Example 2. From these results, it is considered that in Example 2, the LME susceptibility of each base steel plate could be evaluated with higher accuracy than in Comparative Example 2.

Description of reference numerals

[0061] 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 second metal part having a lower melting start temperature than 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 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, and a load is applied to the test specimen to impart elongation; A step of evaluating the liquid metal embrittlement susceptibility of the first metal part based on cracks generated from the surface of the first metal part where the second metal part is disposed toward the inside by the high-temperature elongation test; comprising In the step of preparing the test specimen, as the test specimen, a plurality of test specimens are prepared in which at least one of the chemical composition and the metal structure of the first metal part is different from each other, and the basis weight and the chemical composition of the second metal part are the same; In the step of performing the high-temperature elongation test, the load is applied to impart elongation so that the test temperature and the displacement amount or the stress value of the plurality of test specimens are the same, and the application of the load is stopped before the test specimen breaks. A method for evaluating liquid metal embrittlement susceptibility.

2. The method for evaluating liquid metal embrittlement susceptibility according to claim 1, wherein the first metal part is a steel material.

3. The method for evaluating liquid metal embrittlement susceptibility according to claim 1 or claim 2, wherein the first metal part is a base metal steel plate and the second metal part is a plating layer.

4. In the test specimen, the plating layers are disposed on both surfaces of the base metal steel plate, The method for evaluating liquid metal embrittlement susceptibility according to claim 3, wherein a thermocouple is attached at a position outside the region where the crack occurs on either surface of the base metal steel plate to perform the high-temperature elongation test.

5. In the test specimen, the plating layer is disposed on one surface of the base metal steel plate, The method for evaluating liquid metal embrittlement susceptibility according to claim 3, 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 metal steel plate to perform the high-temperature elongation test.

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