JP7709049B2 - Method for evaluating critical stress for liquid metal embrittlement crack initiation, method for evaluating critical displacement for liquid metal embrittlement crack initiation, and method for evaluating liquid metal embrittlement susceptibility - Google Patents

Description

This 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.

When welding metal plates, etc., cracks due to liquid metal embrittlement (LME) can be a problem during spot welding, etc. Liquid metal embrittlement is the embrittlement of solid metal that is inherently ductile when tensile stress is applied while the solid metal is in contact with liquid metal, and typically occurs between steel (solid metal) and zinc (liquid metal).

For example, Patent Document 1 discloses a coated steel sheet having a specific composition and structure, and describes, as an example, that the probability of the number of cracks in a resistance spot weld is less than 5 on average, resulting in a welded structure with low LME susceptibility.

Patent document 2 discloses a plated steel material in which the Zn-Al-Mg plating layer has a specific composition, and describes, as an example, how to obtain a bead-on-plate test piece by bead-on-plate welding to a steel plate using a stainless steel welding wire, and how to check for the presence or absence of cracks using a penetrant test and evaluate the LME based on the length of the cracks in the weld.

Patent Document 3 discloses a hot stamped body having a base material and a plating layer with a specific composition and structure, and describes, as an example, observing the occurrence of liquid metal embrittlement cracking by observing backscattered electron images using a scanning electron microscope (SEM) and a backscattered electron detector in a cross section in the thickness direction of the steel material at a hot V-bend portion.

Patent document 4 discloses an austenitic hot-dip aluminum-plated steel sheet in which a hot-dip aluminum-plated layer is formed on the surface of a base steel sheet having a specific composition, and describes, as an example, that 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, and the difference value from the lower limit to the current at which LME cracks do not occur is defined as the "current range where LME cracks do not occur" to evaluate the spot weldability of the steel sheet.

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. It describes that a tensile test piece cut from a zinc-plated steel sheet or a galvannealed hot-dip steel sheet is processed to simulate the temperature history of a hot stamping process, and the maximum crack depth is measured as an evaluation of the LME crack depth in the plastically deformed area.

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 and average penetration depth of the liquid metal, as well as the penetration length of the liquid metal at the fracture surface.
Non-Patent Document 2 shows that stress and strain are dominant factors that significantly promote the diffusion of liquid metal at grain boundaries and cause LME cracking. Furthermore, an LME model is described regarding the effect of stress and strain on the diffusion of liquid metal at grain boundaries.

Special table 2019-506530 publication International Publication No. 2018/139620 International Publication No. 2017/195269 Special table 2019-504205 publication Patent No. 6043272

Journal of the Japan Welding Society Vol. 52 No. 1 1983 pp.56-61 On the Role of Stress, Strain and Diffusion in Dissolution - Condensation Mechanism of Liquid Metal Embrittlement, Defect and Diffusion Forum, Vol. 264, pp141-149

Patent Documents 1 to 5 and Non-Patent Documents 1 and 2 describe the evaluation of LME cracking or LME susceptibility in welded parts and bent parts of plated steel sheets based on the presence or absence of cracks, the number of cracks, and the length of the cracks. However, they do not describe a method for evaluating the critical stress or critical displacement at which LME cracking occurs.
Therefore, an object of the present disclosure is to provide a method for evaluating the critical stress or critical displacement at which liquid metal embrittlement cracking occurs in a metallic material, and a method for evaluating liquid metal embrittlement susceptibility.

Means for solving the above problems include the following aspects.
<1> A step of preparing a test material including a first metal portion and a second metal portion that is disposed on at least a part of the first metal portion and has a lower melting start temperature than the first metal portion, and in which liquid metal embrittlement of the first metal portion occurs when the second metal portion is liquefied by heating;
A step of performing a high-temperature elongation test in which the test material is heated to a temperature range that is equal to or higher than the melting start temperature of the second metal portion and lower than the melting start temperature of the first metal portion, a load is applied to the test material to give it elongation, and the application of the load is stopped before the test material breaks;
A step of checking the presence or absence of cracks extending from the surface of the first metal part on which the second metal part is disposed toward the inside of the test specimen subjected to the high-temperature elongation test;
Including,
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 at the same test temperature and strain rate, and with different stresses for each of the test specimens;
A method for evaluating the critical stress for liquid metal embrittlement crack initiation, in which, in the process of confirming the presence or absence of cracks, the critical stress at which the cracks will occur in the test materials is evaluated by confirming the presence or absence of cracks for the multiple test materials that have been subjected to the high temperature elongation test.
<2> The method for evaluating the critical stress for liquid metal embrittlement crack initiation described in <1>, in which two or more types of the high-temperature elongation tests with different strain rates are performed on each of the multiple test materials, and the critical stress is evaluated for each strain rate.
<3> The method for evaluating the liquid metal embrittlement crack initiation critical stress described in <1> or <2>, in which two or more types of the high-temperature elongation tests are performed at different test temperatures for each of the plurality of test materials, and the critical stress is evaluated for each of the test temperatures.
<4> The method for evaluating the critical stress for liquid metal embrittlement crack initiation described in any one of <1> to <3>, wherein the first metal portion and the second metal portion of the plurality of test specimens are the same type as each other.
<5> The method for evaluating the critical stress for liquid metal embrittlement crack initiation according to any one of <1> to <3>, wherein the plurality of test specimens are different from each other in the types of at least one of the first metal portion and the second metal portion.
<6> The method for evaluating a critical stress for liquid metal embrittlement crack initiation according to any one of <1> to <5>, wherein the first metal portion is a steel material.
<7> The method for evaluating critical stress for liquid metal embrittlement crack initiation according to any one of <1> to <6>, wherein the first metal portion is a base steel plate, and the second metal portion is a plating layer.
<8> The test material has the plating layer disposed on both sides of the base steel sheet,
The method for evaluating the critical stress for liquid metal embrittlement crack initiation described in <7>, in which a thermocouple is attached to a position outside the region in which the crack occurs on either one surface of the base steel plate and the high temperature elongation test is performed.
<9> The test material has the plating layer disposed on one surface of the base steel sheet,
The method for evaluating the critical stress for liquid metal embrittlement crack initiation described in <7>, wherein a thermocouple is attached to the other surface of the base steel plate at a position corresponding to the area in which the crack occurs on the one surface, and the high temperature elongation test is performed.
<10> The method for evaluating the critical stress for liquid metal embrittlement crack initiation 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 so that the stress is different for each of the test specimens, the high-temperature elongation test is performed so that the displacement amount is different for each of the test specimens;
A method for evaluating the critical displacement amount for liquid metal embrittlement crack initiation, in the process of confirming the presence or absence of a crack, which evaluates the critical displacement amount at which the crack occurs in the test material instead of evaluating the critical stress at which the crack occurs in the test material.
<11> A method for evaluating liquid metal embrittlement susceptibility, comprising: a method for evaluating a liquid metal embrittlement crack initiation critical stress according to any one of <1> to <9>; and a method for evaluating a liquid metal embrittlement crack initiation critical displacement according to <10>; and evaluating the liquid metal embrittlement susceptibility of the test material based on at least one of the critical stress and the critical displacement.

The present disclosure provides a method for evaluating the critical stress or critical displacement at which liquid metal embrittlement cracking occurs in metallic materials, and a method for evaluating liquid metal embrittlement susceptibility.

FIG. 1 is a diagram showing an example of a displacement-stress curve when a high-temperature tensile test is performed using a plated steel sheet and a non-plated steel sheet. FIG. 2 is a diagram showing the shape of a test material used in the examples as an example of the shape of the test material. 1 shows SEM images of cross sections in the thickness direction and tensile direction of each test material after a high-temperature tensile test was conducted at a tensile speed of 50 mm/s (strain rate: 8.3/s). FIG. 1 is a diagram showing the relationship between displacement and stress for each test material subjected to a high-temperature tensile test at a tensile speed of 50 mm/s (strain rate: 8.3/s). 1 shows SEM images of cross sections in the thickness direction and tensile direction of each test material after a high-temperature tensile test was conducted at a tensile speed of 5 mm/s (strain rate: 0.83/s). FIG. 1 is a diagram showing the relationship between the displacement and the stress of each test material subjected to a high-temperature tensile test at a tensile speed of 5 mm/s (strain rate: 0.83/s). FIG. 1 is a diagram showing the stress at which an LME crack occurred (critical stress for LME crack initiation) for each steel type at each temperature in a tensile test. 1 is a SEM image showing an LME crack occurring at the thermocouple location. FIG. 1 is a diagram showing an example of a position (position marked with an x) at which a thermocouple is attached to a double-sided plated test piece. 1 is a SEM image showing an LME crack occurring in a one-sided plated product. 1 is a SEM image showing an LME crack occurring in a double-sided plated product. FIG. 13 is a diagram showing the relationship between the distance from the center position of the parallel portion of each plated material and the crack length. FIG. 1 is a diagram showing the critical stress for LME crack initiation when the strain rate is set to 0.1/s for three test materials of different steel types. FIG. 1 is a diagram showing the critical stress for LME crack initiation when the strain rate is set to 10/s for three test materials of different steel types. FIG. 1 is a diagram showing the critical displacement for LME crack initiation when the strain rate is set to 0.1/s for three test materials of different steel types.

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

The inventors have conducted extensive research into methods for evaluating the critical stress or critical displacement at which LME cracks occur in metallic materials, and have come to the following conclusions.

When high-temperature tensile tests are performed on plated and unplated steel sheets, as shown in Figure 1, a difference in stress (Δstress) occurs in the displacement-stress diagram depending on whether the steel sheet is plated or not. Therefore, the inventors have investigated a method for evaluating the critical stress at which LME cracks occur. Then, multiple test materials (test pieces) 10 with the shape shown in Figure 2 were taken from the same zinc-plated steel sheet, and high-temperature tensile tests were performed on each test material under the same test temperature and strain rate, with the strain and stress varied.

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 was set to 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 was set to 5 mm/s (strain rate: 0.83/s). Fig. 3B and Fig. 4B each show the relationship between the displacement and the stress. Note that all the test materials were taken from 980 MPa-class zinc-based plated steel sheets, and the length of the parallel portion 14 was 6 mm.
As shown in Figures 3A and 4A, at all strain rates, cracks formed before the specimens broke, and the number of LME cracks increased with increasing strain and stress.
In this way, in a high-temperature tensile test at a constant strain rate at the temperature at which LME cracking occurs, the stress differs for each test material, and by stopping the test before the test material breaks and observing the cross-section of the test material, it is possible to confirm the stress (critical stress) at which LME cracking occurs at that temperature and strain rate.
From these test results, it was found that the strain rate also affects the initiation of LME cracks, and that the LME crack initiation stress also changes.

Furthermore, the inventors changed the test temperature and steel type and measured the critical stress at which cracks occurred in each test material. Figure 5 shows the stress at which LME cracks occurred at each temperature for each steel type (critical stress for LME crack initiation). The test temperature, steel type, and critical stress for LME crack initiation are different, and the components of the test material also affect the critical stress for LME crack initiation. In the figure, the "LME initiation region" is the region where LME cracks occur in the GA LME-resistant steel plate used in this experiment.
From these results, for example, when spot welding, by setting each condition so that the critical stress for LME crack initiation is not reached, it is possible to prevent or suppress the occurrence of LME cracks during spot welding.

Furthermore, in the case of "plated", as shown in Figure 1, in the high temperature tensile test, the stress increases and the displacement gradually increases, and when an LME crack occurs, the stress drops sharply but the displacement increases. If the tensile test is continued, the test material will break, but the tensile test is performed on multiple test materials at the same test temperature and strain rate with different displacements, and the tensile test is stopped before breakage, and the presence or absence of an LME crack in each test material after the test can be confirmed, thereby evaluating the critical displacement.
Based on these experiments and studies, the evaluation method according to the present disclosure has been found. First, the evaluation method for liquid metal embrittlement crack initiation critical stress according to the present disclosure will be specifically described below.

[Method for evaluating the critical stress for liquid metal embrittlement crack initiation]
The method for evaluating the critical stress for liquid metal embrittlement crack initiation according to the present disclosure includes the steps of:
preparing a test material including a first metal portion and a second metal portion disposed on at least a portion of the first metal portion and having a lower melting start temperature than the first metal portion, the first metal portion being subject to liquid metal embrittlement when the second metal portion is liquefied by heating;
A step of performing a high-temperature elongation test in which the test material is heated to a temperature range that is equal to or higher than the melting start temperature of the second metal portion and lower than the melting start temperature of the first metal portion, a load is applied to the test material to give it elongation, and the application of the load is stopped before the test material breaks;
A step of checking the presence or absence of cracks extending from the surface of the first metal part on which the second metal part is disposed toward the inside of the test specimen subjected to the high-temperature elongation test;
Includes.
Then, in the process of preparing test materials, multiple test materials are prepared, and in the process of performing a high-temperature elongation test, the high-temperature elongation test is performed on each of the multiple test materials with the same test temperature and strain rate and with different stresses for each test material, and in the process of confirming the presence or absence of cracks, the presence or absence of cracks is confirmed for each of the multiple test materials that have been subjected to the high-temperature elongation test, thereby evaluating the critical stress at which cracks occur in the test materials.
In this way, the test temperature and strain rate are uniformed for multiple test materials that may cause LME, and high-temperature elongation tests are performed on each of the test materials so that the stress is different for each test material, and the load is stopped before the test material breaks. By checking the test materials for the presence or absence of cracks after the test, the critical stress at which cracks occur in the test materials can be evaluated.
Each step will now be described in detail.

<Preparation of test material>
A test material is prepared which includes a first metal portion and a second metal portion which is disposed on at least a portion of the first metal portion and has a lower melting initiation temperature than the first metal portion, and in which liquid metal embrittlement of the first metal portion occurs when the second metal portion is liquefied by heating.
Here, multiple test materials are prepared, and the test materials may be multiple test materials in which the first metal part and the second metal part are of the same type, or multiple test materials in which at least one of the first metal part and the second metal part is of a different type.

Below, we will explain the case where a test material is used in which the surface of a metal substrate is formed as the first metal part and a metal layer is formed as the second metal part, but 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 with each other by a method other than lamination.

The metal substrate is a support member that does not melt even in a high-temperature elongation test and remains a solid metal, and examples of such metal materials include plate-shaped, rod-shaped, wire-shaped, and tubular metal materials. The metal layer is a layer that covers at least a portion of the surface of the metal substrate, and at least a portion of which melts and becomes 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 or both sides of the metal substrate.

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

In addition, the test material is not particularly limited in terms of the lamination form of the metal substrate and the metal layer, as long as the combination is such that LME occurs. For example, plated materials such as plated sheets, plated wires, plated steel bars, and other test materials other than plated materials may be used. For example, bonded materials formed by bonding multiple sheets of different types such as steel or aluminum together in a stacked state and embedding them in the thickness direction with a rivet or the like at high speed may be used. In the case of such bonded materials, LME may occur due to heat and stress during bonding. In addition, clad materials made by bonding different metals together, or clad materials with a plated layer formed on the surface thereof may be used. In addition, test materials in which a metal layer is provided on the outer and/or inner surfaces of a cylindrical metal member may be used.

In addition, other layers such as metal pre-plating may be present between the metal substrate (first metal portion) and the metal layer (second metal portion) that causes LME in the metal substrate, and other layers such as an organic film may be present on the metal layer (second metal portion) (i.e., on the opposite side to the metal substrate).

Hereinafter, as an example of a test material, a plated steel sheet, particularly a zinc-plated steel sheet (hereinafter sometimes simply referred to as "plated steel sheet") in which a plating layer (metal layer) as a second metal part is formed on the surface of a base steel sheet (metal substrate) as a first metal part, will be mainly described.
In the case of plated steel sheets, the plated layer (metal layer) may be liquefied by spot welding, etc., resulting in LME of the steel sheet (metal substrate). For example, a hot-dip galvanized steel sheet or a non-plated cold-rolled steel sheet electroplated with zinc can be used as a test material.

The method of evaluating the critical stress for LME crack initiation according to the present disclosure may be performed by taking multiple test specimens from the same type of plated steel sheet as shown in Figures 3A and 4A, or by taking specimens from different types of plated steel sheets as shown in Figure 5.

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

(Different types of plated steel sheets)
The term "different types of plated steel sheets" means that the types of at least one of the base steel sheet and the plating layer are different from each other.
The different types of base steel sheets are steel sheets that are different from each other in at least one of chemical composition and metal structure. For example, even when cold-rolled steel sheets are manufactured from slabs with the same chemical composition, steel sheets with different metal structures may be manufactured due to differences in thermal history. For example, even if the chemical composition is the same, if the metal structures are different, the mechanical properties and chemical properties will be different, and the critical stress for LME crack initiation will also be different.

Different types of plating layers refer to plating layers that differ in at least one of the chemical composition, metal structure, and coating weight, and are typically plating layers formed under different conditions on the same or different types of base steel sheets.

Therefore, the multiple test specimens used in the method for evaluating the critical stress for LME crack initiation according to the present disclosure include test specimens taken from any of the following groups of plated steel sheets (A) to (D).
(A) A group of homogeneous plated steel sheets in which the same type of plating layer is formed on the same type of base steel sheet; (B) A group of heterogeneous plated steel sheets in which different types of plating layers are formed on the same type of base steel sheet; (C) A group of heterogeneous plated steel sheets in which the same type of plating layer is formed on different base steel sheets; (D) A group of heterogeneous plated steel sheets in which different types of plating layers are formed on different base steel sheets. For example, a plurality of test specimens may be taken from the plated steel sheet of (A) to evaluate the critical stress for LME crack initiation, or a test specimen may be taken from each of the plated steel sheets of the group of plated steel sheets corresponding to any of (B) to (D) to evaluate the critical stress for LME crack initiation.

(Base material steel plate)
The composition, structure, thickness, etc. of the base steel sheet are not particularly limited, and any base steel sheet may be used in which the critical stress for LME crack initiation is to be evaluated in combination with the coating layer.

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

On the other hand, when plating is performed under the same conditions and then alloyed by heating, some of the components of the base steel sheet may migrate to the plating layer, resulting in a slight difference in the composition of the plating layer. However, the difference in the composition of the plating layer caused by the difference in the composition of the base steel sheet is slight and has little effect on LME cracking, so if the plating layer is formed under the same conditions except for the type of base steel sheet, it may be considered to be the same type of plating layer. However, there may be a significant difference in the composition of the plating layer between the case where alloying treatment is not performed after plating under the same conditions and the case where it is performed. Therefore, when evaluating the critical stress for LME crack initiation for multiple test materials, for example, the above-mentioned (C) group of heterogeneous plated steel sheets in which the same type of plating layer is formed on heterogeneous base steel sheets, the presence or absence of alloying treatment after plating under the same conditions should be the same.

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

Furthermore, the thickness of the test material is not limited, but since comparative evaluation becomes difficult if cracks penetrate in the thickness direction during a tensile test, the thickness of the test material is preferably 0.5 mm or more.
In addition, when the test material is a wire material, it is difficult to compare it if a crack penetrates through it like a plate material, and cutting it for cross-sectional observation is difficult, so 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 preferable that the portion be 1.0 mm or more.

<High temperature elongation test>
A high-temperature elongation test is performed by heating a test material to a temperature range that is equal to or higher than the melting temperature of the second metal portion and lower than the melting temperature of the first metal portion, applying a load to the test material to give it elongation, and stopping the application of the load before the test material breaks. Here, the high-temperature elongation test is performed on each of a plurality of test materials, with the test temperature and strain rate being uniform and the stress being different for each test material.

The type of high-temperature elongation test is not particularly limited, and may be, for example, a uniaxial tensile test, a biaxial tensile test, a ball-head extension test, or the like. Below, the case of a uniaxial tensile test (sometimes referred to as a "high-temperature tensile test" or simply a "tensile test" in this disclosure) will be described.
High-temperature tensile tests are performed by heating the test material to a temperature range that is equal to or higher than the melting start temperature of the coating layer and lower than the melting start temperature of the base steel sheet, but it is preferable to perform the test at a temperature (test temperature) at which the coating layer does not volatilize.
Furthermore, as the temperature approaches the melting start temperature of the steel sheet, the test material becomes more susceptible to deformation, making the tensile test difficult. Therefore, the maximum temperature of the test material in the tensile test is preferably set to be 20°C or higher than the melting start temperature of the coating layer and 200°C or lower than the melting start temperature of the steel sheet. Note that the "maximum temperature" here refers to the temperature of the surface of the test material, i.e., the coating layer. When using a hot-dip galvanized steel sheet as the test material, the maximum temperature in the tensile test should be 500 to 1200°C.

The heating (temperature increase) of the test material in the tensile test can be performed by electrical heating or furnace heating. The heating rate is preferably 50°C/s or more (e.g., 50 to 1000°C/s) to prevent volatilization of zinc (plating layer).
The test material may be tensioned by applying a tensile load before heating, or may be tensioned at the maximum temperature (e.g., 500°C, 700°C, or 900°C) without applying a tensile load. Alternatively, the test material may be tensioned by first heating to the maximum temperature (e.g., 1000°C) and then cooling to the processing temperature (e.g., 800°C, 700°C, 600°C, or 500°C) at which tension is performed.

It is desirable to change these temperature histories in the tensile test depending on the temperature history and processing temperature 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 accompanying electrode release in spot welding, it is preferable to first heat (e.g., 1000°C), and then cool with nitrogen gas at a slower cooling rate than in actual spot welding, and perform tensile testing at the temperature at the time of electrode release (e.g., 800°C), although this depends on the location of stress generation. In this temperature range, there are _A1 point and _A3 point, although they depend on the steel composition. Therefore, the crystal structure of the steel may be a body-centered cubic lattice (BCC) or a face-centered cubic lattice (FCC), or may be two of these phases, depending on the temperature history. Since the sensitivity of each crystal structure may differ, it is desirable to determine the test temperature in the tensile test, i.e., the temperature history and processing temperature (tensile temperature), depending on the purpose of use of the plated steel sheet.

The strain rate is not particularly limited, but if the strain rate is too slow, the alloying of steel and zinc will progress, which may reduce the accuracy of evaluating the critical stress for liquid metal embrittlement crack initiation. Also, as shown in Figures 3A and 4A, the higher the strain rate, the easier it is to cause cracks and to check for their presence or absence. For this reason, it is desirable to increase the strain rate to a certain extent and perform the tensile test in a short period of time. For example, the parallel portion of the test piece is 1 to 10 mm, and tension is performed at a tensile speed of 1 to 100 mm/s (strain rate of 0.1 to 100/s).

In a tensile test, the test temperature and strain rate are consistent for each test material, a tensile load is applied so that the stress varies for each test material, and the test is stopped before the test material breaks (intermediate stop). For example, a tensile test is performed so that the stress increases with each test.

(Thermocouple position)
In the tensile test, it is preferable to measure the temperature of the test material by attaching a thermocouple to the test material, and in order to grasp and control the temperature of the test material more accurately, it is better to attach the thermocouple to the center of the parallel part, which is the position where LME cracks will occur. However, in the case of double-sided plating, LME cracks will 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 microspot welding, but stress is likely to concentrate at this welded part. Therefore, LME cracks are likely to occur at the thermocouple position, as shown in Figure 6.
Therefore, when a tensile test is performed using a double-sided plated specimen, it is preferable to attach a thermocouple away from the parallel portion of the specimen (the region where LME cracks occur in the tensile test). As shown in Fig. 7, the test temperature difference between the parallel portion and the position marked with an x that is offset from the parallel portion between the dotted lines is measured in advance, and the difference is estimated. In the actual test, a thermocouple is attached at the position marked with an x, and temperature control is performed based on the temperature measurement result, thereby eliminating the effect of stress concentration due to the thermocouple.

On the other hand, in the case of single-sided plating, LME cracks do not occur on the side without plating (non-plated side). Therefore, by plating one side of the base steel sheet and attaching a thermocouple to the non-plated side to measure the temperature, stress concentration due to the thermocouple can be avoided. However, if the sheet thickness is too thin, it is possible that the stress due to the thermocouple attached to the non-plated side will affect the occurrence of LME cracks on the plated side. Therefore, when performing LME evaluation using a test material with single-sided plating, it is preferable that the sheet thickness is thick. For example, when using a base steel sheet with a sheet thickness of 1.1 mm or more, it is preferable to plate one side and attach a thermocouple to the center of the parallel part of the non-plated side (the position corresponding to the area where LME cracks occur on the plated side) to measure the temperature and perform a tensile test.
Although contact temperature measurement using a thermocouple as described above is preferable, a non-contact temperature measurement method may also be used.

<Checking for cracks>
The test specimens that have been subjected to the high-temperature elongation test are checked for the presence or absence of cracks extending inward from the surface where the second metal part of the first metal part is disposed. By checking the presence or absence of cracks for each of the multiple test specimens that have been subjected to the high-temperature elongation test, the critical stress at which cracks occur in the test specimens can be evaluated.
In a high-temperature tensile test, a cross section perpendicular to the surface of the test material (galvanized steel sheet) (i.e., in the sheet thickness direction) and parallel to the tensile direction of the high-temperature tensile test (sometimes referred to as a "tensile direction cross section" in this disclosure) is observed, and the liquid metal embrittlement crack initiation critical stress of the base steel sheet is evaluated based on cracks that have occurred from the surface of the steel sheet (i.e., the interface between the steel sheet and the plating layer after the high-temperature tensile test) toward the inside.
The following describes the case of a uniaxial tensile test, but in the case of a biaxial tensile test or a ball-head extension test, etc., a cross section can be determined appropriately for observation. For example, the principal stress direction can be set as the tensile direction, and the cross section can be taken as the tensile direction cross section.

The cross section of the test material in the tensile direction after the high-temperature tensile test is observed, for example, by a scanning electron microscope (SEM) or an optical microscope. If an LME crack occurs, the crack advances from the surface of the steel sheet toward the inside of the steel sheet in the cross section, and the presence or absence of the crack can be confirmed by an SEM image, an optical microscope, or the like.
Then, among a plurality of test specimens subjected to high temperature tensile tests, the minimum stress at which cracks occur can be evaluated as the LME crack initiation critical stress of that test specimen or a stress close to that critical stress.
In addition, the critical stress for LME crack initiation may be estimated more accurately by performing further tensile tests at multiple stresses between the minimum stress at which cracks occur and the closest stress that is smaller than that stress.

As shown in Figures 3A and 4A, two or more types of high-temperature tensile tests with different strain rates may be performed on each of a number of test materials, and the critical stress may be evaluated for each strain rate. Different strain rates result in different crack initiation conditions even at the same stress, making it possible to evaluate the LME crack initiation stress, including the effects of strain rate.

As shown in Figure 5, two or more types of high-temperature elongation tests at different test temperatures may be performed on each of a number of test materials, and the critical stress may be evaluated for each test temperature. Different test temperatures result in different LME crack initiation stresses, making it possible to evaluate the LME crack initiation region while also taking into account the effects of temperature.

(Difference between single-sided plating and double-sided plating)
A 1470 MPa grade cold rolled base sheet (sheet thickness 2 mm) was electroplated with zinc on one side or both sides with a plating amount of 50 g/ ^m2 on each side, and the samples were stretched to a nominal stress of 130 MPa at 800°C, and the lengths of cracks that had developed were compared. Note that the thermocouple was installed in the center of the parallel part on the non-plated side for the one-sided plated material, and the installation position of the thermocouple was shifted to a position not overlapping the parallel part for the double-sided plated material. Figures 8 and 9 are cross-sectional photographs of each plated material after the tensile test.

It can be seen that cracks of similar length have occurred in the cross-sectional photographs obtained by either method. Figure 10 shows a graph in which the crack length is plotted against the distance from the center position of the parallel section. This graph also shows that there is little difference between single-sided plating and double-sided plating. Therefore, since the effect on crack length due to the difference between single-sided plating and double-sided plating is small, it is thought that there is no problem with either method of evaluation. With single-sided plating, even if a thermocouple is attached to the parallel section on the non-plated side, it has little effect on the occurrence of LME cracks, so it is possible to measure and control the temperature at the crack occurrence section with high precision, and it is an effective method for evaluating the critical stress for LME crack occurrence while avoiding stress concentration caused by thermocouples.

The above has described the method of evaluating the critical stress for LME crack initiation according to the present disclosure, in which a steel plate (sheet material) is used as the metal substrate (first metal part), a uniaxial tensile test (high-temperature tensile test) is performed as a 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 initiation based on the presence or absence of cracks. However, the metal substrate for evaluating the critical stress for LME crack initiation is not limited to a steel plate.
For example, when a steel bar or wire is used as the metal substrate, after a uniaxial tensile test as a high temperature elongation test, the LME crack initiation critical stress can be evaluated based on the cracks (length, number, etc.) by observing a cut surface passing through the central axis and parallel to the tensile direction. Also, when a thick plate is used as the metal substrate, the LME crack initiation critical stress can be evaluated using a specimen cut into a rod shape.
Furthermore, the plating components are not limited to zinc-based, and the critical stress for LME crack initiation can also be evaluated when a plating layer other than zinc-based, such as aluminum, is formed on a metal substrate such as a steel plate.
Furthermore, the first metal part is not limited to steel materials such as steel plate, steel bar, and steel wire, but may be a metal material other than steel that generates LME in combination with the second metal part.

The method for evaluating the critical stress for LME crack initiation according to the present disclosure is not particularly limited to certain applications, but since it is possible to evaluate the critical stress for LME crack initiation for a specific plating layer with high accuracy for multiple types of metal materials with different components, structures, etc., it can be suitably applied to the selection of materials and conditions that are less likely to cause LME cracks, such as combinations of steel sheets and plating, and heating temperatures during processing such as spot welding. It may also be applied to the selection of steel types to be laminated for manufacturing clad materials and joining members.

[Method for evaluating the critical displacement for liquid metal embrittlement crack initiation]
The above-mentioned method of evaluating the critical stress for liquid metal embrittlement crack initiation involves stopping the high-temperature elongation test midway so that the stress differs for each test material, and evaluating the critical stress from the relationship between the stress and the initiation of LME cracking. However, it is also possible to stop the high-temperature elongation test midway so that the displacement amount differs for each test material, and evaluate the critical displacement amount from the relationship between the displacement amount and the initiation of LME cracking.
In other words, the method for evaluating the critical displacement for liquid metal embrittlement cracking initiation according to the present disclosure is a method in which, among the steps included in the method for evaluating the critical stress for liquid metal embrittlement cracking initiation, in the step of performing a high temperature elongation test, instead of performing a high temperature elongation test so that the stress is different for each test material, a high temperature elongation test is performed so that the displacement amount is different for each test material, and in the step of checking for the presence or absence of cracks, instead of evaluating the critical stress at which LME cracking occurs in the test material, the critical displacement amount at which LME cracking occurs in the test material is evaluated.
Note that, except for the evaluation based on the amount of displacement instead of the stress, the method is the same as the evaluation method for the critical displacement amount for LME crack initiation described above, so a description thereof will be omitted here.

[Method for evaluating liquid metal embrittlement susceptibility]
The critical stress and critical displacement obtained by each of the above-mentioned evaluation methods are related to the liquid metal embrittlement susceptibility of the test material (metal material). The larger the critical stress, the lower the liquid metal embrittlement susceptibility, and the same is true for the relationship between the critical displacement and the liquid metal embrittlement susceptibility. Therefore, the liquid metal embrittlement susceptibility of a metal material can be evaluated by using the evaluation method for the critical stress for liquid metal embrittlement cracking or the evaluation method for the critical displacement for liquid metal embrittlement cracking according to the present disclosure. That is, the evaluation method for liquid metal embrittlement susceptibility according to the present disclosure is a method for evaluating the liquid metal embrittlement susceptibility of a test material based on at least one of the critical stress and the critical displacement using at least one of the above-mentioned evaluation methods for the critical stress for liquid metal embrittlement cracking and the evaluation method for the critical displacement for liquid metal embrittlement cracking.

By evaluating the critical stress, critical displacement, or liquid metal embrittlement susceptibility using the method disclosed herein, it is possible to assist in the selection of a combination of a first metal part and a second metal part that is less susceptible to LME cracking, and in the setting of welding conditions for spot welding, etc.

Below, as a representative example of the evaluation method according to the present disclosure, the evaluation method for the critical stress for liquid metal embrittlement cracking and the critical displacement will be described in more detail with examples. However, the following examples do not limit the evaluation method for the critical stress for liquid metal embrittlement cracking according to the present disclosure.

Example 1
(Preparation of test materials)
Cold-rolled steel sheets having different components and heat treatments and a thickness of 1.6 mm were used as test materials, and both sides of each cold-rolled steel sheet were electroplated with zinc to produce the following electroplated zinc-plated steel sheets A, B, and C.
Steel sheet A: 980 MPa-class electrogalvanized steel sheet (50 g/m ² )
Steel plate B: 1180 MPa-class electrogalvanized steel plate (50 g/m ² )
Steel sheet C: 1470 MPa-class hot stamp steel sheet electroplated with zinc (70 g/m ² )
Ten test pieces (samples) each having a shape with a parallel portion (6 mm) in the center of the necked portion as shown in FIG. 2 were taken from each steel plate.

(High temperature tensile test)
Thermocouples were attached to each sample taken from steel plate A at intervals of 0.5 mm from the boundary of the parallel part so as not to overlap the parallel part, and the samples were heated to the maximum temperature (900°C) and then cooled to 800°C at 50°C/s, and tension was performed. Heating was performed by electrical heating at 300°C/s, cooling was performed by nitrogen gas cooling, and the strain rate was 0.1/s, and a tensile test was performed without breaking the sample. The tensile test was performed so that the stress of each sample was different.

Tensile tests were also conducted on the samples taken from steel plates B and C in the same manner as the sample from steel plate A.

(Check for cracks)
After the tensile test, the cross section of each sample in the tensile direction was observed. The sample was cut along a cutting line that was perpendicular to the surface of the sample, parallel to the tensile direction, and passed through approximately the center of the sample in the width direction, and the cross section was observed under a SEM (magnification: 25 times).
The stress at which a crack was initiated from the surface toward the inside of each sample taken from the same steel plate was evaluated as the LME crack initiation critical stress of that steel plate.
The critical stress for LME crack initiation for each steel plate is shown in Figure 11A. Note that, for steel plate A, LME cracking did not occur at a stress of 170 MPa or less.

Example 2
Samples were taken from each of the steel plates A to C used in Example 1 in the same manner as in Example 1, and a tensile test was performed and the presence or absence of cracks was 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 initiation for each steel plate is shown in FIG. 11B.

(Evaluation Results)
11A and 11B , under all conditions, the critical stress for LME crack initiation was greatest for steel plates A, B, and C. From this, it can be evaluated that under the conditions of Example 1 or Example 2, steel plates A, B, and C have decreasing LME susceptibility in this order, and are less likely to cause LME in spot welds, etc.
Note that steel plate B with a strain rate of 0.1/s has a higher LME crack initiation critical stress than steel plate A with a strain rate of 10/s. Therefore, when applying the evaluation method for the critical stress for LME crack initiation according to the present disclosure to the evaluation of the LME susceptibility of each steel plate, it is important to compare the critical stress for LME crack initiation by performing high-temperature tensile tests with the same strain rate as well as the same test temperature.

Example 3
(Preparation of test materials)
Cold-rolled steel sheets with different compositions and heat treatments and a sheet thickness of 1.6 mm were used as test materials, and one side of each cold-rolled steel sheet was hot-dip galvanized. Then, each hot-dip galvanized steel sheet was immersed in a salt bath (500°C) to perform an alloying treatment, to produce the following hot-dip galvanized steel sheets E, F, and G. Furthermore, a test piece (sample) having a shape with a parallel portion (6 mm) in the center of the necked portion was taken from each steel sheet, as shown in Figure 2.
Steel plate E: 980 MPa-class galvannealed steel plate (50 g/m ² )
Steel plate F: 1180 MPa-class galvannealed steel plate (50 g/m ² )
Steel plate G: 1470 MPa-class galvannealed steel plate (70 g/m ² )

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

(Check for cracks)
After the tensile test, each sample was checked for the presence or absence of cracks in the same manner as in Example 1. The critical displacement for LME crack initiation of each steel plate is shown in FIG.
12, it can be seen that the critical displacement and LME susceptibility are largest for steel plates E, F, and G in that order. Note that no LME cracks occurred in steel plate E up to a displacement of 3 mm.

10: Test piece 12: Necked portion 14: Parallel portion

Claims (11)

preparing a test material including a first metal portion and a second metal portion disposed on at least a portion of the first metal portion and having a lower melting start temperature than the first metal portion, the first metal portion being subject to liquid metal embrittlement when the second metal portion is liquefied by heating;
A step of performing a high-temperature elongation test in which the test material is heated to a temperature range that is equal to or higher than the melting start temperature of the second metal portion and lower than the melting start temperature of the first metal portion, a load is applied to the test material to give it elongation, and the application of the load is stopped before the test material breaks;
A step of checking the presence or absence of cracks extending from the surface of the first metal part on which the second metal part is disposed toward the inside of the test specimen subjected to the high-temperature elongation test;
Including,
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 at the same test temperature and strain rate, and with different stresses for each of the test specimens;
A method for evaluating the critical stress for liquid metal embrittlement crack initiation, in which, in the process of confirming the presence or absence of cracks, the critical stress at which the cracks will occur in the test materials is evaluated by confirming the presence or absence of cracks for the multiple test materials that have been subjected to the high temperature elongation test. The method for evaluating the critical stress for liquid metal embrittlement crack initiation described in claim 1, in which two or more types of high-temperature elongation tests with different strain rates are performed for each of the multiple test materials, and the critical stress is evaluated for each strain rate. The method for evaluating the critical stress for liquid metal embrittlement crack initiation described in claim 1 or claim 2, in which two or more types of high-temperature elongation tests with different test temperatures are performed for each of the multiple test materials, and the critical stress is evaluated for each test temperature. The method for evaluating the critical stress for liquid metal embrittlement crack initiation described in any one of claims 1 to 3, wherein the first metal part and the second metal part of the multiple test specimens are the same type. The method for evaluating the critical stress for liquid metal embrittlement crack initiation according to any one of claims 1 to 3, wherein the first metal part and/or the second metal part of the multiple test specimens are different from each other. A method for evaluating the critical stress for liquid metal embrittlement crack initiation according to any one of claims 1 to 5, wherein the first metal part is a steel material. A method for evaluating the critical stress for liquid metal embrittlement crack initiation 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. The test material has the plating layer disposed on both sides of the base steel sheet,
The method for evaluating the critical stress for liquid metal embrittlement crack initiation according to claim 7, wherein the high temperature elongation test is performed by attaching a thermocouple to a position outside the region where the crack occurs on either one surface of the base steel plate. The test material has the plating layer disposed on one surface of the base steel sheet,
The method for evaluating the critical stress for liquid metal embrittlement crack initiation according to claim 7, wherein the high temperature elongation test is performed by attaching a thermocouple to the other surface of the base steel plate at a position corresponding to the area in which the crack occurs on the one surface. Among the steps included in the method for evaluating the critical stress for liquid metal embrittlement crack initiation 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 so that the stress is different for each of the test specimens, the high-temperature elongation test is performed so that the displacement amount is different for each of the test specimens;
A method for evaluating the critical displacement amount for liquid metal embrittlement crack initiation, in the process of confirming the presence or absence of a crack, which evaluates the critical displacement amount at which the crack occurs in the test material instead of evaluating the critical stress at which the crack occurs in the test material. A method for evaluating liquid metal embrittlement susceptibility, which uses at least one of the evaluation method for liquid metal embrittlement crack initiation critical stress described in any one of claims 1 to 9 and the evaluation method for liquid metal embrittlement crack initiation critical displacement described in claim 10, and evaluates the liquid metal embrittlement susceptibility of the test material based on at least one of the critical stress and the critical displacement.