JP7709050B2 - Evaluation method for liquid metal embrittlement susceptibility - Google Patents

Description

This disclosure relates to 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 describes that test pieces made of various high-strength steel sheets with different structures that were electrolytically plated with zinc to a thickness of about 10 μm per side were heated at 1000° C./s for 0.5 seconds to simulate spot welding, and subjected to high-temperature tensile testing until fracture, and Zn-LME susceptibility was evaluated based on the length and number of cracks that occurred.

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 Materials Science & Engineering A 804 (2021) 140391

Patent Documents 1 to 5 and Non-Patent Documents 1 and 2 describe evaluating LME cracking or LME susceptibility in welded or bent parts of plated steel sheets based on the presence or absence of cracks, the number of cracks, and their lengths. 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 liquid metal embrittlement susceptibility that can evaluate the liquid metal embrittlement susceptibility of a metallic material with high accuracy.

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, and a load is applied to the test material to elongate it;
a step of evaluating liquid metal embrittlement susceptibility of the first metal part based on cracks generated from a surface of the first metal part, on which the second metal part is disposed, toward the inside of the first metal part by the high temperature elongation test;
Including,
In the step of preparing the test material, a plurality of test materials are prepared as the test materials, in which at least one of the chemical composition and the metal structure of the first metal portion is different from each other and the basis weight and the chemical composition of the second metal portion are uniform;
In the step of performing the high-temperature elongation test, the load is applied to impart elongation so that the test temperatures and displacements or stress values of the multiple test specimens are uniform, and the application of the load is stopped before the test specimens break.
A method for evaluating liquid metal embrittlement susceptibility.
<2> The method for evaluating liquid metal embrittlement susceptibility according to <1>, wherein the first metal portion is a steel material.
<3> The method for evaluating liquid metal embrittlement susceptibility according to <1> or <2>, wherein the first metal portion is a base steel plate, and the second metal portion is a plating layer.
<4> The test material has the plating layer disposed on both sides of the base steel sheet,
The method for evaluating liquid metal embrittlement susceptibility described in <3>, in which a thermocouple is attached to a position outside the region where the crack occurs on either one surface of the base steel plate and the high-temperature elongation test is performed.
<5> The test material has the plating layer disposed on one surface of the base steel sheet,
The method for evaluating liquid metal embrittlement susceptibility described in <3>, in which a thermocouple is attached to the other surface of the base steel plate at a position corresponding to the area where the crack occurs on the one surface, and the high-temperature elongation test is performed.

The present disclosure provides a method for evaluating liquid metal embrittlement susceptibility that can evaluate the liquid metal embrittlement susceptibility of metal materials with high accuracy.

FIG. 1 is a diagram showing the relationship between plating weight and LME crack length (maximum length). FIG. 1 is a graph showing the relationship between the salt bath treatment time of a hot-dip galvanized steel sheet and the Fe concentration in the coating. FIG. 1 is a diagram showing the relationship between the Fe concentration in the plating and the LME crack length. 1 is a SEM image showing a cross section of a test material broken by a tensile test. 1 is a SEM image showing a cross section of a test material when the tensile test was stopped midway without breaking the material. FIG. 2 is a diagram showing the relationship between the crack initiation position and the crack length in each test material after a tensile test. 1 is a SEM image showing a crack caused by a factor other than LME that occurred near the fracture position. 8 is an enlarged SEM image of the vicinity of the fracture position in FIG. 7. FIG. 9 is a diagram showing the results of a point analysis by SEM-EDS for the portion 1 in FIG. 8. FIG. 9 is a diagram showing the results of a point analysis by SEM-EDS for the portion 2 in FIG. 8. FIG. 9 is a diagram showing the results of a point analysis by SEM-EDS for the portion 3 in FIG. 8. 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 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. 1 is a diagram showing the relationship between the distance from the center position of a parallel portion of a one-sided plated material and the crack length. 1 is a diagram showing the relationship between the crack position (distance from the center position of the parallel portion) and the crack length when the test material in Example 1 is stopped midway without fracturing at a constant stress value. 1 is a diagram showing the relationship between the crack position (distance from the center position of the parallel portion) and the crack length when the displacement amount is stopped halfway without fracturing the test material in Example 1 at a constant value. 1 is a diagram showing the relationship between the distance from the fracture position and the crack length when a test material is fractured in Comparative Example 1. FIG. FIG. 13 is a diagram showing the relationship between crack position and crack length when the test material in Example 2 is stopped midway without breaking. 1 is a diagram showing the relationship between crack position and crack length when a test material is fractured in Comparative Example 2. FIG.

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 that can evaluate the LME susceptibility of metal materials with greater precision, and have come to the following conclusions.

(1) Effect of coating weight (amount of plating per unit area) Regarding the effect of coating weight on LME cracks, in order to quantitatively evaluate the coating weight, the effect was examined by changing the coating weight in a high-temperature tensile test.
Fig. 1 shows the relationship between the coating weight of each test material and the ^{LME crack} length (maximum, average, minimum) measured by cross-sectional observation, when tensile tests (tensile speed 50 mm/ ^{s )} were performed on electrogalvanized steel sheets (1.6 mm thick) with coating weights of 0 g/ ^m2 (no coating), 10 g/ ^m2 , 50 g/m2, 70 g/m2, and 100 g/m2 for three types of base steel sheets with different structures, after heating to 900°C (heating rate 100°C/s) and 800°C (cooling rate 50°C/s). Tensile tests were also performed at temperatures of 900°C and 700°C, and it was found that LME cracks occurred when the coating weight was 50 g/ ^m2 or more at all tensile temperatures, and the crack length tended to increase with increasing coating weight. From these results, it is considered that even if tensile tests are performed under the same conditions using test materials with different coating weights and LME susceptibility is evaluated based on the crack length, the coating weight (coating weight) affects the crack length, and therefore LME susceptibility cannot be evaluated accurately. In other words, it is considered that performing LME evaluation with the same coating weight contributes to improving the accuracy of LME susceptibility evaluation.

(2) Effect of plating components Hot-dip galvanized steel sheets (sheet thickness 1.6 mm, plating thickness 10 μm) were immersed in a 500°C salt bath for 15, 30, 50, 100, or 300 seconds, and then annealed by cooling at 0.2 to 0.3°C/s. When the Fe concentration in the Zn plating was measured by ICP (inductively coupled plasma), the Fe concentration in the plating changed depending on the salt bath treatment time, as shown in Figure 2, and became almost constant when the treatment time exceeded 100 seconds.
Of these, test pieces A, B, and C, i.e., test pieces with Fe concentrations (mass%) in the plating of 2%, 8%, and 12%, were subjected to tensile tests in the same manner as in the case of investigating the effect of the coating weight, and the LME crack length was measured by observing the cross section of each test piece.
When the relationship between the Fe concentration in the coating and the LME crack length was investigated, as shown in Figure 3, the crack length tended to be shorter as the Fe concentration in the coating increased at any tensile temperature. From this result, it is considered that even if tensile tests are performed under the same conditions on test materials with the same basis weight and LME susceptibility is evaluated based on the crack length, the plating components affect the crack length, so it is considered that the LME susceptibility cannot be evaluated accurately. In other words, it is considered that performing LME evaluation with the same plating components contributes to improving the accuracy of the LME susceptibility evaluation.

Based on the above investigations, it was considered that LME susceptibility could be evaluated with high accuracy by uniforming the coating weight per unit area (base weight) and plating components of the test materials and conducting tensile tests under the same conditions.
However, when the test material was broken in a tensile test under the same conditions, the crack length was large near the fracture site, suggesting that factors other than LME susceptibility may be influencing the crack length.
Therefore, a hot-dip galvanized steel sheet (1180 MPa class cold rolled steel sheet) with a thickness of 1.6 mm was heated to 900°C by electrical heating at 100°C/s, and after reaching 900°C, tension was performed at 900°C with a crosshead speed of 50 mm/s to study the difference between cracks when the test material was broken and cracks when the tension was stopped (halted midway) before breaking. Note that the midway stopping was at a displacement of 1 mm and a nominal stress of 116 MPa.

(3) Influence of stress distribution in tensile test Figure 4 shows the cross section (cross section in the tensile direction and plate thickness direction) of the test material when it was broken by the tensile test, and Figure 5 shows the state of crack generation in each cross section when the test material was stopped midway without breaking by the tensile test. Figure 6 shows the relationship between the crack generation position and the crack length in each test material after the tensile test.
It can be seen that in the samples where the strain was stopped midway, the crack length remained almost flat in the tensile direction, whereas in the samples where the fracture occurred, the crack length was longer near the fractured area. It is believed that the deformation had progressed further near the fractured area, and both the stress and strain were higher than in other areas. As a result, the crack length was longer. Therefore, since the crack length is affected by the stress or strain, it is considered necessary to eliminate the effect of this on the sensitivity evaluation of the steel plate based on the crack length.

(4) Influence of cracks not caused by LME When deformation was performed until fracture, depending on the coating weight, the sensitivity of the steel sheet, the test temperature, the tensile speed, etc., not only the LME crack but also the tip of the crack may progress plastically and the crack may progress. LME cracks generally progress in a brittle manner, but for example, the tip of the crack near the fracture position shown in Figure 7 appears to have ductile fracture. In Figure 8, which shows an enlarged view of the fracture position shown in Figure 7, points indicated by 1, 2, and 3 were analyzed by SEM (scanning electron microscope)-EDS (energy dispersive X-ray spectroscopy), and the results shown in Figures 9 to 11 were obtained. No zinc peak was obtained at the point 3 where ductile fracture was thought to have occurred. When the point indicated by the x in Figure 8 was measured by this point analysis, the area was separated into a "Zn detected" area and a "Zn not detected" area as divided by the dashed line. Therefore, it is considered that the cracks at the positions where Zn was not detected were not caused by LME.
For these reasons, when an LME susceptibility evaluation is performed by a tensile test, if the test is performed until fracture, a large deformation occurs, and the cracks may include both LME cracks and ductile fractures due to deformation, so that it may not be appropriate to evaluate the LME susceptibility of a steel plate based on the crack length. Therefore, it is considered that the LME susceptibility can be evaluated with high accuracy by performing the susceptibility evaluation based on the crack length, for example, during uniform deformation or by stopping the test before that.

In this specification, "high-temperature elongation test" refers to a high-temperature test that applies elongation to at least a portion of the test material, and includes not only general tensile tests but also extension tests, etc.

The following is a detailed description of an embodiment of the liquid metal embrittlement susceptibility evaluation method according to the present disclosure.

The liquid metal embrittlement susceptibility evaluation method according to the present disclosure includes:
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 test material being such that liquid metal embrittlement (LME) 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, and a load is applied to the test material to elongate it;
a step of evaluating liquid metal embrittlement susceptibility of the first metal part based on cracks generated from a surface of the first metal part, on which the second metal part is disposed, toward the inside of the first metal part by the high temperature elongation test;
Includes.
Then, in the process of preparing the test material, a plurality of test materials are prepared in which at least one of the chemical composition and metal structure of the first metal portion is different from each other, and the basis weight and chemical composition of the second metal portion are consistent.
In addition, in the process of conducting the high-temperature elongation test, the load is applied to impart elongation so that the test temperatures and displacement amounts or stress values of the multiple test materials are consistent, and the application of the load is stopped before the test materials break.
In this way, by regulating the basis weight and chemical composition of the second metal part and performing a high-temperature elongation test at the same test temperature and displacement or stress value without fracturing the test material, the effects of differences in the basis weight and chemical composition of the second metal part, as well as cracks other than those caused by LME, are eliminated or suppressed, and the LME susceptibility can be evaluated with high accuracy based on the length of the crack generated in the first metal part, etc.
Each step will now be described in detail.

<Preparation of test material>
A test material is prepared, the test material including a first metal portion and a second metal portion disposed on at least a part of the first metal portion and having a lower melting start temperature than the first metal portion, the first metal portion suffering from liquid metal embrittlement when the second metal portion is liquefied by heating. Here, a plurality of test materials are prepared, the first metal portions being different from each other in at least one of the chemical composition and the metal structure, and the second metal portions being uniform in basis weight and chemical composition.
Below, we will explain the case where a test material is used in which a metal base material is formed on the surface of 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, a joint material formed by joining a plurality of different sheets of steel or aluminum, etc., by rotating a rivet at high speed and embedding them in the thickness direction may be used. In the case of such a joint material, LME may occur due to heat and stress during joining. In addition, a clad material made by bonding different metals together, or a clad material having a plated layer formed on its surface may be used. In addition, a test material having a metal layer provided on the outer and/or inner surface of a cylindrical metal member may be used. In addition, the "weight" in this disclosure means mass per unit area, and when the second metal part is a plated layer, it means the plating adhesion amount per unit area, and when the second metal part is a plated material, it means the mass per unit area of the plated material.

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.

(Base material steel plate)
A plurality of base steel sheets having different chemical compositions and/or metal structures are used. For example, even when cold-rolled steel sheets are produced from slabs having the same chemical composition, steel sheets having different metal structures may be produced 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 LME susceptibility will also be different.

(Plating)
Each base steel sheet is subjected to zinc plating. Here, a zinc plating layer is formed on each base steel sheet so that the coating weight and chemical composition are uniform. The plating method is not limited, and may be hot-dip galvanizing or electrolytic galvanizing. For example, by plating each base steel sheet under the same conditions, a plating layer with a uniform coating weight and chemical composition can be formed on each base steel sheet. The term "plating layer with a uniform coating weight and chemical composition" means that the coating weight and chemical composition of the plating layer on each plated steel sheet are all the same.
If plating is performed under the same conditions, a plating layer of the same coating weight and composition will basically be formed even if the composition or structure of the base steel sheet is different.

On the other hand, if the steel plate is plated under the same conditions and then heated for alloying, some of the components of the base steel sheet may migrate to the plated layer, resulting in a slight difference in the composition of the plated layer. However, the difference in the composition of the plated layer caused by the difference in the composition of the steel sheet is slight and has little effect on LME cracks, so it is acceptable. However, there may be a significant difference in the composition of the plated layer between the case where the alloying treatment is not performed after plating under the same conditions and the case where it is performed. Therefore, the test materials to be evaluated are the same in terms of whether or not they are alloyed after plating under the same conditions.
In other words, the treatment may be carried out under the same conditions from plating on the base steel sheet to the start of the high-temperature elongation test.

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 FIG. 12, 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 in which the test material is heated to a temperature range equal to or higher than the melting temperature of the second metal portion and lower than the melting temperature of the first metal portion, and a load is applied to the test material to elongate it.
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.
The high-temperature tensile test is performed by heating the test material to a temperature range that is equal to or higher than the melting point of the coating layer and lower than the melting point of the base steel sheet, but it is preferable to perform the test at a 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 150°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.

There are no particular limitations on the tensile speed, but if the tensile speed is too slow, alloying of the steel and zinc will progress, making it difficult to evaluate the liquid metal embrittlement susceptibility, so it is desirable to tensile for a short period of time. For example, tensile speed is 1 to 100 mm/s.

In the tensile test, a tensile load is applied to each specimen so that the test temperature and displacement or stress value are consistent (the same displacement or stress value), and the test is stopped before the specimen breaks (intermediate stop). When evaluating the 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 a strain of at least 0.01. By stopping the tensile test before the specimen breaks, the deformation and expansion of the crack due to the influence of the break is eliminated, the variation in strain due to the position in the tensile direction is small, and the variation in crack length is also small, making it easier to observe and evaluate the cross section in the tensile direction.

(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 13.
Therefore, when conducting a tensile test using a double-sided plated specimen, it is preferable to attach a thermocouple away from the parallel part of the specimen. As shown in Fig. 14, the test temperature difference between the parallel part and the position marked with an x that is offset from the parallel part 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.

Assessment of LME susceptibility
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 susceptibility of the base steel sheet is evaluated based on cracks that have developed 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 is observed, for example, by a scanning electron microscope (SEM) or an optical microscope. In the cross section, cracks propagate from the surface of the steel sheet toward the inside of the steel sheet, and the length and number of the cracks can be measured by the backscattered electron image of the SEM.
For example, the SEM image is used to measure the length of a crack extending from the surface of the base steel plate toward the inside, and the liquid metal embrittlement susceptibility of the steel plate is evaluated.

The length of the crack is an index that directly represents the liquid metal embrittlement susceptibility. When evaluating the crack length, for example, the maximum crack length may be measured in an SEM image, but from the viewpoint of evaluation accuracy, it is preferable to plot the transition of the crack length according to the distance from the center position of the parallel part of the test material as shown in FIG. 17. In the plot, the behavior is stratified according to the steel type and the surface layer state, and the liquid metal embrittlement susceptibility of each test material can be accurately evaluated by the maximum value, average value, and/or integral value of the crack length. 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 in which the liquid metal embrittlement susceptibility of the steel sheet is low with respect to the plating layer formed on the surface.
The evaluation index for LME is not limited to the crack length, and LME susceptibility may be evaluated based on the number of cracks.

(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 at 800°C to a nominal stress of 130 MPa, 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 15 and 16 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 17 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 accurately evaluating LME susceptibility by avoiding stress concentration caused by the thermocouple.

The above has described the liquid metal embrittlement susceptibility evaluation method according to the present disclosure in which a steel plate (plate 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 plate material and parallel to the tensile direction of the high-temperature tensile test is observed to evaluate LME susceptibility based on the crack length or the penetration distance of liquid metal, but the metal substrate for evaluating LME susceptibility 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 susceptibility 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 susceptibility can be evaluated using a specimen cut into a rod shape.
Furthermore, the plating components are not limited to zinc-based, and LME susceptibility 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 liquid metal embrittlement susceptibility evaluation method according to the present disclosure can be applied to any situation, but since it can evaluate the LME susceptibility of 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 the combination of steel sheet and plating, and the heating temperature during processing such as spot welding. It may also be applied to the selection of steel types to be layered to manufacture clad materials and joining members.

The liquid metal embrittlement susceptibility evaluation method of the present disclosure will be explained in more detail below with reference to examples. However, the following examples do not limit the liquid metal embrittlement susceptibility evaluation method of the present disclosure.

Example 1
(Preparation of test materials)
Cold-rolled steel sheets with different components and heat treatments and a sheet thickness of 1.6 mm were used as test materials, and both sides of each cold-rolled steel sheet were electroplated with zinc in an amount of 50 g/ ^m2 to produce the following electro-galvanized steel sheets A and B. Furthermore, test pieces (samples) having a shape with a parallel portion (6 mm) in the center of the necked portion were taken from each steel sheet, as shown in Figure 12.
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 )

(High temperature tensile test)
Thermocouples were attached to each sample at intervals of 0.5 mm from the boundary of the parallel part so as not to overlap the parallel part, and the sample was heated to the maximum temperature (900°C), then cooled to 800°C at 50°C/s, and tension was performed. The heating was performed by electrical heating at 300°C/s, the cooling was performed by nitrogen gas cooling, and the tension speed was 50 mm/s, and the tensile test was performed until the stress value reached 120 MPa as a nominal stress without breaking the sample. In addition, a tensile test was performed under the same conditions so that the displacement was 0.8 mm.

(Measurement of crack length)
After each tensile test, the cross section of the 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 an SEM (magnification: 25 times).
For cracks that had developed from the surface toward the inside of each sample, the crack positions (distance from the center position of the parallel part) and crack lengths were measured and plotted. The results are shown in Figures 18 and 19.
The average length of cracks generated in each sample when the stress value was 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
The average length of the cracks generated in each sample when the displacement was 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

<Comparative Example 1>
A tensile test was performed on the test pieces prepared in the same manner as in Example 1 until they broke, and the distance from the break position and the length of the cracks that had developed from the surface to the inside of each sample were measured and plotted. The break position was the tip position of the broken part. The results are shown in FIG.
The average length of the cracks that occurred in each sample was as follows:
Average crack length of steel plate A: 0.60 mm
Average crack length of steel plate B: 0.39 mm

(Assessment of LME susceptibility)
As is clear from Figures 18, 19 and 20, the variation in crack length distribution of each steel plate in Example 1 was smaller than the variation in crack length distribution of each steel plate in Comparative Example 1.
From these results, it is considered that in Example 1, the LME susceptibility of each base steel plate was evaluated with higher accuracy than in Comparative Example 1.

Example 2
(Preparation of test materials)
Cold-rolled steel sheets with different components and heat treatments and a thickness of 1.6 mm were used as test materials, and one side of each cold-rolled steel sheet was hot-dip galvanized with zinc at a plating amount of 100 g/ ^m2 . When hot-dip galvanization was performed, a heat-resistant protective sheet was attached to one side to protect the surface. Next, each hot-dip galvanized steel sheet was immersed in a salt bath (500°C) without removing the protective sheet to perform an alloying treatment, thereby producing the following hot-dip galvanized steel sheets C and D. Furthermore, test pieces (samples) having a shape with a parallel portion (6 mm) in the center of the necked portion were taken from each steel sheet, as shown in Figure 12.
Steel sheet C: 980 MPa-class galvannealed steel sheet (100 g/m ² )
Steel plate D: 1180 MPa-class galvannealed steel plate (100 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 without breaking each sample.

(Measurement of crack length)
For each sample after the tensile test, the cross section in the tensile direction was observed, and the crack position (distance from the center position of the parallel part) and crack length were measured and plotted in the same manner as in Example 1. The results are shown in FIG.
The average length of the cracks that occurred in each sample was as follows:
Average crack length of steel plate C: 0.27 mm
Average crack length of steel plate D: 0.49 mm

<Comparative Example 2>
A tensile test was performed on the test pieces prepared in the same manner as in Example 2 until they broke. For the cracks that had developed from the surface toward the inside of each sample, the distance from the break position and the crack length were measured and plotted. The break position is the tip position of the broken part. The results are shown in FIG.
The average length of the cracks that occurred in each sample was as follows:
Average crack length of steel plate C: 0.28 mm
Average crack length of steel plate D: 0.51 mm

(Assessment of LME susceptibility)
As is clear from Figures 21 and 22, the variation in crack length distribution of each steel plate in Example 2 was smaller than the variation in 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 was evaluated with higher accuracy than in Comparative Example 2.

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

Claims (5)

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, and a load is applied to the test material to elongate it;
a step of evaluating liquid metal embrittlement susceptibility of the first metal part based on cracks generated from a surface of the first metal part, on which the second metal part is disposed, toward the inside of the first metal part by the high temperature elongation test;
Including,
In the step of preparing the test material, a plurality of test materials are prepared as the test materials, in which at least one of the chemical composition and the metal structure of the first metal portion is different from each other and the basis weight and the chemical composition of the second metal portion are uniform;
In the step of performing the high-temperature elongation test, the load is applied to impart elongation so that the test temperatures and displacements or stress values of the multiple test specimens are uniform, and the application of the load is stopped before the test specimens break.
A method for evaluating liquid metal embrittlement susceptibility. The method for evaluating liquid metal embrittlement susceptibility according to claim 1, wherein the first metal part is a steel material. The method for evaluating liquid metal embrittlement susceptibility according to claim 1 or claim 2, 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 liquid metal embrittlement susceptibility according to claim 3, 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 liquid metal embrittlement susceptibility according to claim 3, 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 region where the crack occurs on the one surface.