Method for analyzing the thickness of a low solid solution Si concentration layer and method for evaluating the LME resistance of steel materials

Abstract

The present invention provides an analytical method for easily determining the thickness of a low solid-solution Si concentration layer present on the surface of a steel material. The analytical method for determining the thickness of a low solid-solution Si concentration layer present on the surface of a steel material comprises: a measurement step of measuring the fluorescence X-ray intensity of Fe from the surface of the steel material, wherein the low solid-solution Si concentration layer contains Si-based oxides and the solid-solution Si concentration is in a region below a predetermined reference value; and a calculation step of calculating the thickness of the low solid-solution Si concentration layer using the fluorescence X-ray intensity of Fe.

Description

Technical Field

[0001] The present invention relates to a method for analyzing the thickness of a low solid-solution Si concentration layer present on the surface layer of a steel material. Further, the present invention relates to a method for evaluating the LME resistance of a steel material.

Background Art

[0002] In recent years, due to the increasing awareness of environmental issues, in order to achieve both weight reduction of automotive materials for CO2 emission reduction and improvement of collision safety performance due to high strength of the vehicle body, the high strength of automotive steel sheets has been promoted. For example, as a skeletal material for an automotive cabin, the application of a high strength steel sheet with a tensile strength of 980 MPa or more is required.

[0003] In the assembly of automobiles using such high strength steel sheets, from the viewpoints of efficiency and cost, it is common to overlap press-formed members and join them by resistance spot welding. Therefore, in order to ensure collision safety, it is also required to improve the strength at the welded part.

[0004] However, when a plurality of steel sheets including a high strength steel sheet provided with a zinc-based plating layer are overlapped and resistance spot welded, it is known that cracks occur in the welded part. This crack is caused by liquid metal, and is called a liquid metal embrittlement (LME) crack (hereinafter, referred to as an "LME crack"). That is, usually, since the melting point of the zinc-based plating layer is lower than the melting point of the base steel sheet, the zinc-based plating layer melts during welding, and the molten zinc penetrates into the grain boundaries of the steel sheet. As a result, the grain boundary strength of the steel sheet decreases and cracks occur.

[0005] Also, even when a high strength steel sheet is not provided with a zinc-based plating, when the steel sheets are overlapped and welded and the mating material is a zinc-based plated steel sheet, it is known that an LME crack occurs due to the influence of the zinc in the molten zinc-based plating of the mating material.

[0006] Non-patent document 1 shows that the amount of Si in steel influences the occurrence of LME cracking. Specifically, the higher the Si concentration in the steel, the more likely LME cracking is to occur.

[0007] Generally, steel sheets used in applications such as automobiles tend to have a high Si content to improve strength and ductility. This trend is particularly pronounced in high-strength steel sheets with a tensile strength of 980 MPa or higher, resulting in a high risk of LME cracking.

[0008] To address the risk of LME cracking, it has been proposed to provide a layer with a low solid-solution Si concentration (low solid-solution Si concentration layer) on the surface of the steel sheet.

[0009] For example, Patent Document 1 proposes a method for improving LME resistance in the manufacture of high-strength hot-dip galvanized steel sheets by subjecting the steel sheet to oxidation and reduction treatments prior to hot-dip galvanizing. In the steel sheet obtained in this way, Si oxide is formed inside the steel sheet, and the amount of solid-solution Si around it decreases. As a result, a low solid-solution Si concentration layer containing Si oxide is formed on the surface of the steel sheet.

[0010] Furthermore, Patent Document 2 proposes a method for improving LME resistance by forming a region with a specific thickness on the surface of a steel plate where the solid solution Si concentration is low.

[0011] Thus, LME resistance can be improved by providing a low-solid-solution Si concentration layer on the surface of the steel sheet. In this case, the LME resistance of the final steel sheet is thought to depend on the thickness of the low-solid-solution Si concentration layer. Therefore, controlling the thickness of the low-solid-solution Si concentration layer is important to obtain the desired LME resistance.

[0012] Therefore, in developing steel materials with excellent LME resistance, it is necessary to analyze the thickness of the low solid solution Si concentration layer formed on the surface of the steel material.

[0013] For example, Patent Document 2 describes measuring the Si concentration distribution by performing line analysis from the surface of a steel plate in the thickness direction using a field emission electron probe microanalyzer (FE-EPMA).

[0014] Furthermore, as described in the examples below, the Si concentration distribution in the plate thickness direction can also be measured by energy dispersive X-ray spectroscopy (EDS). [Prior art documents] [Patent Documents]

[0015] [Patent Document 1] International Publication No. 2023 / 182524 [Patent Document 2] International Publication No. 2021 / 200579 [Patent Document 3] Japanese Patent Publication No. 2023-75027 [Non-patent literature]

[0016] [Non-Patent Document 1] D. Bhattacharya et al., Materials Science & Engineering A, 2021, Vol. 823, p.141569 [Overview of the Initiative] [Problems that the invention aims to solve]

[0017] However, the measurement methods using EPMA and EDS described above had the following problems.

[0018] That is, in these measurement methods, since it is necessary to observe the cross-section of the steel material, it is necessary to cut out a sample for observing the cross-section in the plate thickness direction in advance from the steel material. Further, in these measurement methods, the information obtained in one measurement is only the information on a specific observed cross-section. Therefore, in order to obtain average information over the entire steel material, it is necessary to perform measurements on a plurality of cross-sections.

[0019] Due to such circumstances, the above conventional measurement methods had a problem that a considerable amount of time and labor were required for sample preparation and measurement.

[0020] On the other hand, in Patent Document 3, a method for obtaining the thickness of the grain boundary oxide layer using fluorescent X-rays has been proposed. Specifically, first, the fluorescent X-ray intensity of the elements constituting the oxide in the grain boundary oxide layer is measured. Next, based on the correlation between the fluorescent X-ray intensity obtained in advance and the thickness of the grain boundary oxide layer, the thickness of the grain boundary oxide layer is calculated from the measured fluorescent X-ray intensity.

[0021] By using this method, it may be possible to measure the thickness of the low solid-solution Si concentration layer. That is, as described in Patent Document 1 and the like, the low solid-solution Si concentration layer is formed by oxidizing the solid-solution Si contained in the base steel plate. Therefore, if the thickness of the layer in which the Si oxide is formed can be measured, it is expected that the thickness of the low solid-solution Si concentration layer can be known.

[0022] However, as a result of the inventors' study, it was found that in the method described in Patent Document 3, the thickness of the layer containing Mn oxide can be measured with sufficient accuracy, while the thickness of the layer containing Si oxide cannot be measured with sufficient accuracy.

[0023] The present invention has been made in view of the above circumstances, and an object thereof is to provide an analysis method capable of easily obtaining the thickness of the low solid-solution Si concentration layer present on the surface layer of a steel material.

Means for Solving the Problems

[0024] First, we will explain the results of the studies conducted by the inventors to solve the above-mentioned problems.

[0025] First, the inventors investigated the X-ray fluorescence intensity of Si obtained by X-ray fluorescence analysis. As a result, it was found that there was no sufficient correlation between the X-ray fluorescence intensity of Si and the thickness of the low solid solution Si concentration layer. The reason for this is thought to be as follows.

[0026] When analyzing the silicon content on the surface of steel using X-ray fluorescence analysis, the surface of the steel is irradiated with X-rays, and the resulting fluorescent X-rays are detected. Then, the intensity of the fluorescent X-rays attributable to silicon is determined from the detected fluorescent X-rays.

[0027] However, as mentioned above, the low solid-solution Si concentration layer is formed by oxidizing the solid-solution Si contained in the steel material. Therefore, the surface layer of the steel material contains both Si-based oxides formed by the oxidation of Si in the base material and solid-solution Si that remains unoxidized. Consequently, the fluorescence X-ray intensity of Si measured by an X-ray fluorescence analyzer is the sum of the fluorescence X-ray intensity caused by the Si-based oxides present on the surface layer of the steel material and the fluorescence X-ray intensity caused by the solid-solution Si. As a result, a sufficient correlation cannot be obtained between the measured fluorescence X-ray intensity of Si and the thickness of the low solid-solution Si concentration layer, and therefore, it is difficult to determine the thickness of the low solid-solution Si concentration layer from the fluorescence X-ray intensity of Si.

[0028] Therefore, we also investigated determining the thickness of the low-solid-solution Si concentration layer using the fluorescence X-ray intensity of oxygen (O), which constitutes Si-based oxides. However, we could not obtain a sufficient correlation between the measured fluorescence X-ray intensity of O and the thickness of the low-solid-solution Si concentration layer. Consequently, it was difficult to determine the thickness of the low-solid-solution Si concentration layer from the fluorescence X-ray intensity of O. This is thought to be because the fluorescence X-rays of O are low energy. In other words, because the escape depth of the fluorescence X-rays of low-energy O is shallow, it is greatly affected by the native oxide film present on the very surface of the steel material. Therefore, it is difficult to measure the O intensity of Si-based oxides present inside the steel material with high accuracy.

[0029] Based on the above results, further investigation revealed, unexpectedly, a good correlation between the X-ray fluorescence intensity of Fe contained in the steel material and the thickness of the low solid-solution Si concentration layer. The reason for this is not entirely clear, but it is thought to be as follows.

[0030] In other words, the low-solid-solution Si concentration layer is formed by the formation of Si-based oxides due to oxygen supplied from the outside, which reduces the amount of solid-solution Si around them. Therefore, the concentration of elements such as Fe present in the low-solid-solution Si concentration layer decreases from the initial concentration (concentration in the matrix phase of the steel) by the amount of oxygen that has entered. As a result, a correlation is found between the fluorescence X-ray intensity of elements such as Fe contained in the steel and the thickness of the low-solid-solution Si concentration layer.

[0031] However, since the concentration of invading oxygen is significantly lower than the concentration of Fe, the main component of steel, the change in the fluorescence X-ray intensity of Fe due to oxygen intrusion should normally be extremely small. However, our investigations have revealed that there is a higher correlation between the fluorescence X-ray intensity of Fe and the thickness of the low solid solution Si concentration layer compared to other elements such as Mn.

[0032] This invention was completed based on the above-mentioned novel findings, and its gist is as follows.

[0033] 1. A method for analyzing the thickness of a low solid-solution Si concentration layer present on the surface of a steel material, The low solid-solution Si concentration layer is a region containing Si-based oxides and in which the solid-solution Si concentration is below a predetermined reference value. A measurement step of measuring the fluorescence X-ray intensity of Fe from the surface of the steel material, A calculation step of calculating the thickness of the low solid solution Si concentration layer using the fluorescence X-ray intensity of the Fe, A method for analyzing the thickness of a low solid-solution Si concentration layer, comprising the following features.

[0034] 2. A method for evaluating the LME resistance of a steel material, comprising an evaluation step of evaluating the LME resistance of the steel material based on the thickness calculated using the analysis method for the thickness of the low solid solution Si concentration layer described in 1 above. [Effects of the Invention]

[0035] According to the present invention, the thickness of the low solid-solution Si concentration layer present on the surface of a steel material can be easily determined. [Brief explanation of the drawing]

[0036] [Figure 1] This is a flowchart illustrating a method for analyzing the thickness of a low solid-solution Si concentration layer according to one embodiment of the present invention. [Figure 2] This is a flowchart illustrating a method for evaluating the LME resistance of steel materials according to one embodiment of the present invention. [Figure 3] This graph shows the correlation between the Si-K intensity of X-ray fluorescence and the thickness of the low solid-solution Si concentration layer. [Figure 4] This graph shows the correlation between the OK intensity of fluorescent X-rays and the thickness of the low solid-solution Si concentration layer. [Figure 5] This graph shows the correlation between the Fe-K intensity of X-ray fluorescence and the thickness of the low solid-solution Si concentration layer. [Modes for carrying out the invention]

[0037] The present invention will be described in detail below. The following description is an example of a preferred embodiment of the present invention, and the present invention is not limited to the embodiments described below. Furthermore, in this specification, "%" as a unit of elemental content refers to "mass%" unless otherwise specified.

[0038] [Analysis method for determining the thickness of the low solid solution Si concentration layer] Figure 1 is a flowchart showing a method for analyzing the thickness of a low solid-solution Si concentration layer according to one embodiment of the present invention. The analysis method in this embodiment is a method for analyzing the thickness of a low solid-solution Si concentration layer present on the surface of a steel material, and comprises a measurement step S1 and a calculation step S2.

[0039] (Steel) The steel material to be measured is not particularly limited, and any steel material can be used. However, considering the purpose of measuring the thickness of the low solid solution Si concentration layer, it is usually sufficient to use a steel material containing Si. The Si concentration in the steel material is not particularly limited, but the higher the Si concentration, the more likely LME cracking is to occur, and therefore the greater the need to provide a low solid solution Si concentration layer. For this reason, the Si concentration in the steel material to be measured is preferably 0.4% or higher, more preferably 0.5% or higher, and even more preferably 1.0% or higher. On the other hand, there is no particular upper limit to the Si concentration. However, if the Si concentration is excessively high, it will lead to a decrease in the toughness of the steel material and the wettability during hot-dip galvanizing, so industrially produced high-strength steel sheets usually have a Si content of 2.5% or less. For this reason, the Si concentration in the steel material to be measured may be 2.5% or less. In one embodiment of the present invention, the Si concentration in the steel material to be measured is preferably 0.4 to 2.5%, more preferably 0.5 to 2.5%, and even more preferably 1.0 to 2.5%.

[0040] In some cases, a plating layer may be provided on the surface of the steel material. Examples of such plating layers include hot-dip galvanized layers, alloyed hot-dip galvanized layers, and electroplated zinc-based layers. If the steel material to be measured has a plating layer on its surface, the plating layer should be removed prior to the measurement process. The method for removing the plating layer is not particularly limited, but for example, it can be dissolved by immersion in an acid such as hydrochloric acid. In this case, it is preferable to add an inhibitor to the acid used in order to suppress the effect on the steel material (base material).

[0041] Furthermore, since the analytical method of the present invention is applicable to steel materials of any strength, the tensile strength of the steel material is not particularly limited. However, since LME cracking is more likely to occur with higher steel materials, the analytical method of the present invention is particularly useful when the steel material being measured is a high-strength steel material. For this reason, the tensile strength of the steel material is preferably 980 MPa or higher. On the other hand, there is no particular upper limit to the tensile strength, but it may be, for example, 2.0 GPa or less.

[0042] (Low solid solution Si concentration layer) The low-solid-solution Si concentration layer is a layer that affects the LME resistance of steel. In other words, while Si is an element added to increase the strength of steel, it negatively affects LME resistance. Therefore, Si present from the surface to the interior of the steel is oxidized to form oxides (granular, dendrites, etc.). This creates a layer with a lower Si concentration than the matrix phase of the steel, thereby improving LME resistance.

[0043] In this invention, a region containing Si-based oxides and in which the solid-solution Si concentration is below a predetermined reference value is defined as a low-solid-solution Si concentration layer. In other words, the thickness of the low-solid-solution Si concentration layer is defined as the depth from the surface of the steel material to the position where the solid-solution Si concentration reaches a predetermined reference value.

[0044] The aforementioned reference value, that is, the specific depth from the outermost surface of the steel material to which the low solid-solution Si concentration layer is considered, can be determined as appropriate.

[0045] In one embodiment of the present invention, the reference value may be determined based on the absolute value of the solid-solution Si concentration. For example, if the reference value is set to 0.5%, the region where the solid-solution Si concentration is 0.5% or less is considered a low solid-solution Si concentration layer. Any value can be used as the reference value, but typically, it is preferable to select the reference value from between 0.1% and 1.0%, and more preferably from between 0.3% and 0.6%.

[0046] In other embodiments of the present invention, the reference value may be defined as a ratio to the Si concentration of the steel material (base material). For example, if the ratio is 1 / 2, the reference value will be 1 / 2 of the Si concentration of the steel material. If the Si concentration of the steel material is 1.0%, the reference value will be 0.5%. The ratio can be set arbitrarily, but it is preferable to select from the range of 1 / 10 to 9 / 10 (0.1 to 0.9), more preferably from the range of 1 / 5 to 4 / 5 (0.2 to 0.8), and even more preferably from the range of 1 / 4 to 3 / 4 (0.25 to 0.75).

[0047] When determining a reference value as a ratio to the Si concentration of steel, the average Si concentration at the 1 / 4 thickness position of the steel plate can be used as the Si concentration of the steel. The average Si concentration can be determined by the following method.

[0048] First, a sample is cut from the steel material so that the cross-section parallel to the rolling direction of the steel material serves as the observation surface. The observation surface of the cut sample is mirror-polished using diamond paste, and then finished polished using colloidal silica. Subsequently, point analysis is performed using a field emission electron probe microanalyzer (FE-EPMA) to measure the Si concentration at a position 1 / 4 of the plate thickness on the observation surface at 10 or more randomly selected points. The electron beam diameter at this time is set to 1 μm. The average value of the Si concentrations obtained in this way is defined as the average Si concentration.

[0049] Furthermore, from the viewpoint of more appropriately evaluating the LME resistance of steel materials, it is also preferable to determine the reference value such that the correlation between the thickness of the obtained low solid solution Si concentration layer and the actual LME resistance is higher. In other words, the analysis method in one embodiment of the present invention may further include a reference value determination step in which a reference value is determined prior to the measurement step.

[0050] The specific method for determining the reference value in the aforementioned reference value determination step is not particularly limited; the reference value should be determined in such a way that a higher correlation is obtained between the thickness of the obtained low solid-solution Si concentration layer and the actual LME resistance. For example, the solid-solution Si concentration profile (distribution in the thickness direction) of several steel materials with known LME resistance can be measured by a method other than the present invention, and the reference value can be determined based on the results. More specifically, the following method can be applied.

[0051] First, several steel materials are prepared, each having a low solid-solution Si concentration layer formed under different conditions. Then, the LME resistance of each of these steel materials is experimentally evaluated. The experimental evaluation method for LME resistance is not particularly limited, but for example, the method described later can be used.

[0052] Furthermore, the solid-solution Si concentration profile is measured for each of the aforementioned steel materials using a method other than that of the present invention. Examples of such methods include the EPMA method used in Patent Document 2 and the EDS method used in the examples of this specification. Next, the thickness of the low-solid-solution Si concentration layer is calculated from the obtained solid-solution Si concentration profile, using several different reference values. The thickness of the low-solid-solution Si concentration layer obtained in this way is compared with the evaluation results of LME resistance. The reference value used to determine the thickness of the low-solid-solution Si concentration layer that showed the best correlation with LME resistance is adopted as the reference value used in the analytical method of the present invention.

[0053] (Measurement process) The measurement step involves measuring the intensity of Fe X-ray fluorescence from the surface of the steel material. The peak intensity of the Fe emission line, such as the Fe-L line peak intensity or the Fe-K line peak intensity, can be used as the Fe X-ray fluorescence intensity. However, since it is necessary to obtain depth information for evaluating the thickness of the low solid-solution Si concentration layer, it is preferable to use the Fe emission line with high energy. Therefore, it is preferable to use the Fe-K line peak intensity. The Fe X-ray fluorescence intensity can be measured using a general-purpose X-ray fluorescence analyzer.

[0054] (calculation process) In the calculation step, the thickness of the low-solid-solution Si concentration layer is calculated from the X-ray fluorescence intensity of Fe measured in the measurement step described above. The specific calculation method is not particularly limited, but typically, a formula (prediction formula) that expresses the relationship between the X-ray fluorescence intensity and the thickness of the low-solid-solution Si concentration layer, which has been determined in advance, can be used. The prediction formula is a formula in which the X-ray fluorescence intensity of Fe is the explanatory variable and the thickness is the dependent variable, and the thickness of the low-solid-solution Si concentration layer can be calculated from the X-ray fluorescence intensity of Fe by inputting the X-ray fluorescence intensity of Fe into the prediction formula.

[0055] The aforementioned prediction formula can be obtained, for example, by following the procedure below.

[0056] First, prepare several steel materials with different thicknesses of low-solid-solution Si concentration layers. The method for forming the low-solid-solution Si concentration layer is not particularly limited; for example, known methods described in Patent Documents 1 and 2 can be used. To form low-solid-solution Si concentration layers of different thicknesses, for example, the annealing conditions can be changed. All other conditions are the same for all steel materials.

[0057] Then, the fluorescence X-ray intensity of Fe is measured for each of the above steel materials. The measurement conditions should be the same as those used when actually using the method of the present invention.

[0058] Furthermore, the thickness of the low solid-solution Si concentration layer in each of the aforementioned steel materials is measured. Methods other than those of the present invention, such as EPMA or EDS, are used for this measurement. As an example, the specific procedure for measurement using EPMA is described below.

[0059] First, a sample is cut from each of the steel materials so that the cross-section parallel to the rolling direction of the steel material serves as the observation surface. The observation surface of the cut sample is mirror-polished using diamond paste, and then finished polished using colloidal silica. After that, line analysis is performed using FE-EPMA to measure the Si concentration distribution in the thickness direction of the observation surface. The electron beam diameter at this time is set to 0.1 μm. The thickness of the low solid-solution Si concentration layer is determined from the obtained Si concentration distribution in the thickness direction. At this time, as described above, the region in which the solid-solution Si concentration is below a predetermined reference value is considered to be the low solid-solution Si concentration layer.

[0060] Note that the measurement results by FE-EPMA include not only solid-solution Si but also Si existing as an oxide. However, since the Si as an oxide is in granular form, its concentration is discontinuous from other parts and is therefore distinguishable. Accordingly, when determining the thickness of the low solid-solution Si concentration layer from the Si concentration distribution, Si detected as Si-based oxide (granular material) should not be considered.

[0061] Here, FE-EPMA was used as an example, but the thickness of the low-solid-solution Si concentration layer can also be determined by energy-dispersive X-ray spectroscopy (EDS) using a scanning transmission electron microscope (STEM) instead of FE-EPMA. In that case, the sample should be cut using the focused ion beam (FIB) method so that the cross section in the thickness direction from the surface of the steel becomes the observation surface, and EDS line profiles should be acquired at 0.01 μm intervals using STEM-EDS.

[0062] From the fluorescent X-ray intensity of Fe obtained in this way and the thickness of the low solid-solution Si concentration layer, a predictive equation representing the relationship between the two is determined. The predictive equation may be a linear equation, for example, as shown in the examples described later.

[0063] [Evaluation method for LME resistance] Next, a method for evaluating LME resistance in one embodiment of the present invention will be described. Figure 2 is a flowchart showing a method for evaluating the LME resistance of steel according to one embodiment of the present invention. The method for evaluating LME resistance of the present invention includes an evaluation step S3 in which the LME resistance of steel is evaluated based on the thickness calculated using the analysis method for the thickness of the low solid solution Si concentration layer described above.

[0064] As already explained, there is a correlation between the thickness of the low-solid-solution Si concentration layer and the LME resistance of the steel material. Therefore, the LME resistance of the steel material can be evaluated (estimated) from the thickness of the low-solid-solution Si concentration layer calculated using the analytical method described above.

[0065] The method for evaluating the LME resistance of steel materials based on the thickness of the low-solid-solution Si concentration layer is not particularly limited and can be performed using any method. Basically, it can be determined that the thicker the low-solid-solution Si concentration layer, the better the LME resistance.

[0066] For a more quantitative evaluation, it is preferable to first determine the correlation between the thickness of the low-solid-solution Si concentration layer and the actual LME resistance, and then evaluate the LME resistance of the steel material based on the thickness of the low-solid-solution Si concentration layer according to this correlation.

[0067] (LME resistance) There are no particular limitations on the method used to evaluate the actual LME resistance of steel materials; any method can be used. For example, the steel material to be evaluated for LME resistance can be spot-welded to a galvanized steel sheet, and the LME resistance can be evaluated based on the occurrence of cracks in the weld. The following describes an example of a specific method for evaluating LME resistance.

[0068] First, a steel plate coated with hot-dip Zn is placed on top of the steel plate to be evaluated, and spot welding is performed. For the spot welding, for example, a servo motor-driven resistance spot welding machine with a welding gun attached and operating on single-phase AC (50Hz) may be used. As the electrode tip, for example, a pair of chromium copper DR-type electrodes may be used. For the DR-type electrodes, for example, electrodes with a tip radius of curvature R of 40 mm and a tip diameter of 6 mm can be used.

[0069] The above spot welding should preferably be performed under conditions that are prone to LME cracking. For example, spot welding should be performed while satisfying at least one of the following conditions (1) to (5). Under any of the conditions (1) to (5), it is possible to locally increase the temperature and / or tensile stress of the weld when the electrode is released, which makes LME cracking likely to occur.

[0070] (1) The welding electrode and the two overlapping steel plates have a striking angle of 0.2 degrees or more. Here, "striking angle" is defined as the angle at which the electrode is inclined with respect to the steel plate, that is, "the angle between the direction of electrode pressure and the direction of steel plate thickness". (2) The condition that the misalignment of the pair of welding electrodes is 0.1 mm or more. Here, "misalignment" means that the central axes of the pair of welding electrodes are not aligned, and "amount of misalignment" is defined as the distance between the central axis of the upper electrode and the central axis of the lower electrode. (3) The condition that there is a gap of 0.5 mm or more between either electrode and the steel plate (gap between electrode and steel plate). Here, "gap between electrode and steel plate" is defined as the distance between either electrode and the steel plate when the steel plate and the pair of electrodes are placed before pressurization begins, and is the greater of the two values. (4) The condition that there is a gap of 0.5 mm or more between two overlapping steel plates (distance between steel plates). Here, "gap between steel plates" is defined as the maximum distance between adjacent steel plates in the vertical direction. (5) The condition that the shortest distance from the center of the weld point to the edge of the steel plate is 10 mm or less. Here, "distance from the center of the weld point to the edge of the steel plate" is defined as the shortest distance from the center of the weld point to the edge of either of the two overlapping steel plates.

[0071] Welding is performed under five different conditions that satisfy at least one of the above conditions (1) to (5) to create a welded joint. After that, the center of the weld of the obtained welded joint is cut with a microcutter, and the cross-section of the weld is observed to evaluate the presence and depth of cracks.

[0072] Specific evaluation criteria can be determined as needed, but for example, the following criteria can be used.

[0073] If a crack with a depth of 5 μm or more is found in the cross-section of the weld, it is determined that "cracking has occurred." Then, the LME crack resistance is evaluated according to the following criteria based on the number of samples with cracks out of the five samples (welded joints). Generally, a score of 3 or 4 according to the following criteria can be considered acceptable. Rating 4: Five welded joints without cracks. Rating 3: 4 welded joints without cracks, 1 welded joint with cracks. Rating 2: 3 welded joints without cracks, 2 welded joints with cracks. Score 1: Two or fewer welded joints without cracks, and three or more welded joints with cracks.

[0074] According to the present invention, once the correlation between the thickness of the low solid solution Si concentration layer and the actual LME resistance is determined, the LME resistance of the steel material can then be evaluated (estimated) by a simple method of measuring the fluorescent X-ray intensity of the surface layer of the steel plate by fluorescent X-ray analysis. Therefore, the present invention is extremely effective in developing high-strength steel materials with excellent LME resistance. [Examples]

[0075] Next, the present invention will be described in more detail based on examples, but the present invention is not limited to these examples.

[0076] First, the following experiment was conducted to understand the correlation between X-ray fluorescence intensity and the thickness of the low-solid-solution Si concentration layer.

[0077] Steel materials (high-strength steel plates) with the component composition and tensile strength shown in Table 1 were prepared as test materials. Fluorescent X-ray analysis was performed on each of the test materials, and the peak intensities of fluorescent X-rays attributable to Si, O, and Fe were measured. For each measurement, the Si-K line, OK line, and Fe-K line peaks were used. The measurement results are shown in Table 1.

[0078] Next, cross-sectional TEM samples were prepared from each test material using FIB, and the Si concentration distribution in the thickness direction from the steel plate surface was measured using STEM-EDS. This measurement result includes not only solid-solution Si but also Si existing as an oxide. However, since the Si as an oxide exists as granular material, its concentration is discontinuous from other parts and can be distinguished. Therefore, the distribution of solid-solution Si concentration was obtained by removing the Si intensity at the locations where Si-based oxides were detected.

[0079] Based on the obtained Si concentration distribution, the thickness of the region where the solid-solution Si concentration is 0.5% or less from the surface of the high-strength steel plate (low solid-solution Si concentration layer) was calculated. In other words, a reference value of 0.5% was used to determine the thickness of the low solid-solution Si concentration layer. This corresponds to 1 / 3 of the Si concentration of the steel material used, which is 1.5%. The obtained thicknesses of the low solid-solution Si concentration layer are shown in Table 1.

[0080] Next, the correlation between the obtained X-ray fluorescence intensity and the thickness of the low-solid-solution Si concentration layer was determined. Figures 3-5 are graphs showing the correlation between the peak intensities of the Si-K line, OK line, and Fe-K line, and the thickness of the low-solid-solution Si concentration layer, respectively. Figures 3-5 also show the equation obtained by approximating the thickness of the low-solid-solution Si concentration layer (y) as a linear function of the X-ray fluorescence intensity (x) using the least squares method, and the coefficient of determination R 2 The coefficient of determination R was shown. 2 A value closer to 1 indicates a higher correlation.

[0081] As can be seen from the results shown in Figures 3-5, the correlation between the thickness of the low solid-solution Si concentration layer and the peak intensity of the Fe-K line was the highest. The relationship between the peak intensity of the Fe-K line (kcps) and the thickness of the low solid-solution Si concentration layer (μm) shown in Figure 5 is approximated by the following equation (1). y = -1.754x + 666.31 …(1) Here, y is the thickness of the low solid-solution Si concentration layer (μm), and x is the intensity of the Fe-K line (kcps).

[0082] Next, the LME resistance of the high-strength steel plate described above was evaluated. The LME resistance was evaluated using the evaluation method described earlier. The results are shown in Table 1.

[0083] As shown in Table 1, when the thickness of the low solid-solution Si concentration layer was less than 2.00 μm, the score was 2 or less, indicating a failure in LME resistance. However, when the thickness was 4.80 μm or more, the score was 4, indicating the best LME resistance.

[0084] Next, based on the relationship between LME resistance and the thickness of the low solid solution Si concentration layer obtained in the above experiment, the LME resistance of steel materials with unknown LME resistance was evaluated (estimated).

[0085] Specifically, four types of high-strength steel plates with the component compositions and tensile strengths shown in Table 2 were prepared as test materials. Fluorescent X-ray analysis was performed on each of the test materials, and the peak intensity of the Fe-K line was measured. From the measured Fe-K line peak intensities, the thickness of the low-solid-solution Si concentration layer was calculated using equation (1) above. The results are shown in Table 2. For convenience, the calculated thickness of the low-solid-solution Si concentration layer is shown rounded to the third decimal place.

[0086] Table 2 also shows the thickness of the low-solid-solution Si concentration layer measured by the STEM-EDS method for each of the above high-strength steel plates. From these results, it can be seen that the thickness of the low-solid-solution Si concentration layer can be determined with sufficient accuracy according to the analytical method of the present invention.

[0087] Furthermore, LME resistance was predicted based on the correlation shown in Table 1, using the thickness of the low solid-solution Si concentration layer determined by the method of the present invention. Both the predicted results and the measured LME resistance for each high-strength steel plate are shown together in Table 2. The LME resistance predicted by the method of the present invention is consistent with the evaluation of the actual LME resistance, indicating that the LME resistance of steel materials can be appropriately evaluated using the present invention.

[0088] [Table 1]

[0089] Table 2

Claims

1. A method for analyzing the thickness of a low solid-solution Si concentration layer present on the surface of a steel material, The low solid-solution Si concentration layer is a region containing Si-based oxides and in which the solid-solution Si concentration is below a predetermined reference value. A measurement step of measuring the fluorescence X-ray intensity of Fe from the surface of the steel material, A calculation step of calculating the thickness of the low solid solution Si concentration layer using the fluorescence X-ray intensity of the Fe, A method for analyzing the thickness of a low solid-solution Si concentration layer, comprising the following features.

2. A method for evaluating the LME resistance of a steel material, comprising an evaluation step of evaluating the LME resistance of the steel material based on the thickness calculated using the method for analyzing the thickness of the low solid solution Si concentration layer described in claim 1.

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