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 O intensity profile in the thickness direction from the surface of the steel material using glow discharge emission spectrometry; an integration step of obtaining the integral value of the O concentration in the surface portion of the steel material from the O intensity profile; and a calculation step of calculating the thickness of the low solid-solution Si concentration layer using the integral value of the O concentration.

Description

[Technical Field]

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

[0002] In recent years, with growing awareness of environmental issues, efforts have been made to increase the strength of automotive steel sheets, aiming to achieve both the reduction of CO2 emissions through the lightweighting of automotive materials and the improvement of collision safety performance through the increased strength of the vehicle body. For example, the use of high-strength steel sheets with a tensile strength of 980 MPa or higher is required for the structural materials of automobile cabins.

[0003] In the assembly of automobiles using such high-strength steel plates, it is common practice to overlap press-formed components and join them using resistance spot welding, from the perspective of efficiency and cost. Therefore, in order to ensure crash safety, it is also necessary to improve the strength of the welded joints.

[0004] However, it is known that cracks can occur in the weld when multiple steel plates, including high-strength steel plates with a zinc-based plating layer, are stacked and resistance spot welded together. These cracks are caused by the metal becoming liquefied and are called liquid metal embrittlement (LME) cracks (hereinafter referred to as "LME cracks"). In other words, since the melting point of the zinc-based plating layer is usually lower than that of the underlying steel plate, the zinc-based plating layer melts during welding, and the liquefied zinc penetrates into the grain boundaries of the steel plate. As a result, the grain boundary strength of the steel plate decreases, and cracks occur.

[0005] Furthermore, even in the case of high-strength steel sheets without a zinc-based plating layer, it is known that LME cracking occurs when these high-strength steel sheets are welded together with zinc-based plated steel sheets. This is because the zinc contained in the plating layer of the zinc-based plated steel sheet melts during welding and penetrates into the grain boundaries of the high-strength steel sheet.

[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, in Patent Document 2, line analysis is performed from the steel sheet surface in the sheet thickness direction using a field emission-electron probe micro analyzer (FE-EPMA), and the concentration distribution of Si is measured.

[0014] Further, as described in the Examples mentioned later, the Si concentration distribution in the sheet thickness direction can also be measured by energy dispersive X-ray spectroscopy (EDS).

Prior Art Literature

Patent Documents

[0015]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0016]

Non-Patent Document 1

Summary of the Invention

Problem to be Solved by the Invention

[0017] However, the measurement methods using EPMA or EDS as described above have the following problems.

[0018] That is, these measurement methods require observation of a cross-section of the steel material. Further, in these measurement methods, information obtained by one measurement is only information on a specific observed cross-section, so it is necessary to perform measurements on a plurality of cross-sections to obtain average information over the entire steel material.

[0019] Due to these circumstances, the conventional measurement method described above had the problem of requiring considerable time and effort for sample preparation and measurement.

[0020] The present invention has been made in view of the above circumstances, and aims to provide an analytical method that can easily determine the thickness of the low solid-solution Si concentration layer present on the surface of a steel material. [Means for solving the problem]

[0021] First, we will explain the results of the studies conducted by the inventors to solve the above-mentioned problems.

[0022] First, the inventors investigated the relationship between the distance in the depth direction (plate thickness direction) from the surface of the steel material obtained by glow discharge emission spectrometry (GDS analysis) and the Si concentration (Si concentration profile), specifically the relationship between the distance at which the Si concentration exceeds a certain value and the thickness of the low-solid-solution Si concentration layer. As a result, it was found that there was insufficient correlation between the above-mentioned distance obtained from the Si concentration profile and the thickness of the low-solid-solution Si concentration layer. The reason for this is thought to be as follows.

[0023] When analyzing the Si layer on the surface of steel using GDS analysis, the surface of the steel is sputtered, the emitted atoms are excited with plasma, and the resulting light is analyzed spectroscopically to obtain a Si concentration profile in the depth direction.

[0024] 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 matrix phase and solid-solution Si that remains unoxidized. Consequently, the Si concentration obtained by GDS analysis is the sum of the Si concentration due to Si-based oxides present on the surface layer of the steel material and the Si concentration due to solid-solution Si. As a result, a sufficient correlation cannot be obtained between the distance obtained from the acquired Si concentration profile and the thickness of the low solid-solution Si concentration layer. Therefore, it is difficult to determine the thickness of the low solid-solution Si concentration layer using the distance obtained from the Si concentration profile.

[0025] Therefore, instead of directly measuring Si, the inventors investigated indirectly determining the thickness of the low-solid-solution Si concentration layer by measuring oxygen (O), another element constituting Si-based oxides. In the following investigation, the measured value of the low-solid-solution Si concentration layer was obtained by energy-dispersive X-ray spectroscopy (EDS) using a scanning transmission electron microscope (STEM) (hereinafter referred to as STEM-EDS). Furthermore, as the steel material to be analyzed, a steel plate in which a low-solid-solution Si concentration layer was formed on the surface using a general method that utilizes oxidation was used.

[0026] (1) Integral value of O intensity First, we investigated a method for integrating the O intensity obtained by GDS analysis in the thickness direction of the plate.

[0027] Figure 1 is a graph showing the correlation between the thickness of the low solid solution Si concentration layer (measured value) and the integral value of the O strength from the steel surface measured by GDS. Although a slight correlation is observed between the two, the correlation coefficient R 2 The value was extremely low at 0.038. From this result, it can be seen that the thickness of the low solid-solution Si concentration layer cannot be determined with sufficient accuracy from the integrated value of the O intensity.

[0028] (2) Integrated value of O intensity (normalized by Fe intensity) As mentioned above, we considered the possibility that the O intensity values ​​measured by GDS analysis varied from sample to sample, which may be the reason for the lower accuracy. Specifically, to counteract this variation, we attempted a method of normalizing the measured O intensity value by the Fe intensity value, also measured by GDS analysis, and then integrating that value.

[0029] Figure 2 is a graph showing the correlation between the thickness of the low solid solution Si concentration layer and the integral value of the O intensity normalized by the Fe intensity. The R obtained by this method 2 The value was 0.0763, which was slightly higher than in the case shown in Figure 1. However, the tendency for a thinner low-solid-solution Si concentration layer to correspond to a higher integrated O intensity was unexpected. Thus, even using the integrated O intensity normalized by the Fe intensity, sufficient accuracy could not be obtained.

[0030] (3) Integrated value of O intensity (excluding the surface layer) Next, we investigated the possibility that the accuracy was reduced due to the influence of oxygen contained in the native oxide film present on the surface of the steel material. Specifically, when calculating the integral value of the O intensity using the same method as in Figure 1, we excluded the top 2 μm of the steel plate (the region between the outermost surface and a depth of 2 μm).

[0031] Figure 3 shows a graph plotting the integrated O intensity value, excluding the obtained surface layer of 2 μm, against the thickness of the low solid-solution Si concentration layer (measured value). Correlation coefficient R 2 The result was 0.0802, which was higher than the methods described in (1) and (2) above, but still not accurate enough.

[0032] (4) Integrated value of O intensity (excluding the surface layer and normalized by Fe intensity) Therefore, we attempted to use methods (2) and (3) above in combination. Specifically, when calculating the integral value of the O strength, we used the strength normalized by the Fe strength and excluded the top 2 μm of the steel plate. In other words, we performed integration in the depth direction from a position 2 μm below the surface of the steel plate.

[0033] Figure 4 shows a graph plotting the integrated O intensity (normalized by Fe intensity), excluding the obtained surface layer of 2 μm, against the thickness of the low solid-solution Si concentration layer (measured value). Correlation coefficient R 2 The value was 0.0539, and no improvement was observed compared to method (3) above.

[0034] (5) Integral value of O intensity Therefore, as an alternative approach, we tried using the integral value of oxygen concentration instead of the integral value of oxygen intensity. Specifically, after measuring the oxygen intensity profile obtained by GDS analysis, we converted the oxygen intensity to oxygen concentration to obtain an oxygen concentration profile, and then integrated that oxygen concentration in the thickness direction of the steel plate to obtain the integral value of oxygen concentration.

[0035] Figure 5 shows a graph plotting the obtained integral value of the O concentration against the thickness of the low solid-solution Si concentration layer (measured value). The resulting correlation coefficient R 2 The result was 0.7667, showing a significant improvement in accuracy compared to methods (1) to (4) above.

[0036] The reason for this remarkable improvement is unclear, but given this level of accuracy, it can be said that this method is perfectly usable in practice as a simpler measurement method, replacing conventional methods (such as EPMA and EDS) that require time and effort to measure.

[0037] (6) Integral value of O concentration (excluding the surface layer) In addition, to further improve accuracy, we also investigated how to exclude the effect of the native oxide film. Specifically, when calculating the integral value of the O concentration, we excluded the top 2 μm of the steel plate (the region between the outermost surface and a depth of 2 μm).

[0038] Figure 6 shows a graph plotting the integral value of the O concentration, excluding the obtained surface layer of 2 μm, against the thickness of the low solid-solution Si concentration layer (measured value). Correlation coefficient R 2The result was 0.9048, which was a significant improvement in accuracy compared to the case in (5) above. As can be seen from this result, when determining the integral value of the O concentration in the surface layer of the steel material, the thickness of the low solid solution Si concentration layer can be determined with even greater accuracy by excluding the range from the outermost surface of the steel material to a predetermined depth during integration.

[0039] This invention was completed based on the above-mentioned novel findings, and its gist is as follows.

[0040] 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 O strength profile in the thickness direction from the surface of the steel material using glow discharge emission analysis, An integration step is performed to obtain the integral value of the O concentration in the surface layer of the steel material from the O strength profile, A calculation step of calculating the thickness of the low solid-solution Si concentration layer using the integral value of the O concentration, A method for analyzing the thickness of a low solid-solution Si concentration layer, comprising the following features.

[0041] 2. The method for analyzing the thickness of a low solid-solution Si concentration layer according to 1 above, wherein in the integration step, the integral value of the O concentration is determined by excluding the range from the outermost surface of the steel material to a predetermined depth.

[0042] 3. The steel material is a zinc-plated steel material comprising a base steel material and a zinc-plated layer provided on the surface of the base steel material. The method for analyzing the thickness of a low solid solution Si concentration layer according to 1 or 2 above, wherein the calculation step involves calculating the thickness of a low solid solution Si concentration layer present on the surface of the underlying steel material.

[0043] 4. The steel material is an alloyed zinc-plated steel material comprising a base steel material, a zinc-plated layer provided on the base steel material, and an alloy layer formed between the base steel material and the zinc-plated layer. The method for analyzing the thickness of a low solid solution Si concentration layer according to 1 or 2 above, wherein the calculation step involves calculating the thickness of a low solid solution Si concentration layer present on the surface of the underlying steel material.

[0044] 5. In the measurement process, further, the C intensity profile in the thickness direction from the surface of the steel material is measured using glow discharge emission analysis. In the calculation step described above, the thickness of the low-C concentration layer, which is the region where the C concentration is below a predetermined reference value, is calculated based on the C intensity profile. Method for analyzing the thickness of the low solid solution Si concentration layer as described in any one of the above 1 to 4.

[0045] 6. 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 of the low solid solution Si concentration layer calculated using the analysis method for the thickness of the low solid solution Si concentration layer described in any one of 1 to 4 above.

[0046] 7. 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 of the low solid solution Si concentration layer and the thickness of the low C concentration layer calculated using the analytical method for the thickness of the low solid solution Si concentration layer described in item 5 above. [Effects of the Invention]

[0047] 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 determined accurately and easily. [Brief explanation of the drawing]

[0048] [Figure 1] This graph shows the correlation between the thickness of the low solid-solution Si concentration layer measured by STEM-EDS and the integrated value of the O strength from the steel surface measured by GDS. [Figure 2] This graph shows the correlation between the thickness of the low-solid-solution Si concentration layer measured by STEM-EDS and the integrated value of the O strength from the steel surface measured by GDS (normalized by Fe strength). [Figure 3]This graph shows the correlation between the thickness of the low-solid-solution Si concentration layer measured by STEM-EDS and the integrated value of the O intensity excluding the top 2 μm layer, measured by GDS. [Figure 4] This graph shows the correlation between the thickness of the low-solid-solution Si concentration layer measured by STEM-EDS and the integrated value of the O intensity (normalized by Fe intensity) excluding the top 2 μm layer, measured by GDS. [Figure 5] This graph shows the correlation between the thickness of the low solid-solution Si concentration layer measured by STEM-EDS and the integrated value of the O concentration from the steel surface measured by GDS. [Figure 6] This graph shows the correlation between the thickness of the low-solid-solution Si concentration layer measured by STEM-EDS and the integral value of the O concentration excluding the top 2 μm layer, measured by GDS. [Figure 7] 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 8] This is a flowchart illustrating a method for evaluating the LME resistance of steel materials according to another embodiment of the present invention. [Figure 9] This graph shows the correlation between the integral value of the oxygen concentration obtained by GDS analysis and the thickness of the low solid-solution silicon concentration layer measured by STEM-EDS. [Modes for carrying out the invention]

[0049] 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.

[0050] (First embodiment) An analytical method according to the first embodiment of the present invention 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, an integration step, and a calculation step.

[0051] (Steel) The steel material to be measured is not particularly limited, and any steel material can be used. 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 materials usually have a Si concentration of 2.5% or less. For this reason, the Si concentration in the steel material to be measured may be 2.5% or less. The Si concentration is preferably 0.4 to 2.5%, more preferably 0.5 to 2.5%, and even more preferably 1.0 to 2.5%.

[0052] Furthermore, while steel contains carbon (C), the C concentration of the steel to be measured in this invention is not particularly limited. However, if the C concentration is excessively high, the toughness and weldability of the steel will decrease, so the C concentration of industrially produced high-strength steel is usually 0.7% or less. Therefore, the C concentration in the steel to be measured is typically 0.7% or less, 0.6% or less, or 0.5% or less. On the other hand, the lower limit of the C concentration is also not particularly limited, but for example, it may be 0.01% or more, 0.1% or more, or 0.2% or more. The C concentration may be between 0.01% and 0.7%, between 0.1% and 0.6%, or between 0.2% and 0.5%.

[0053] The aforementioned steel material may be a zinc-plated steel material having a zinc-based plating layer on its surface. Here, "zinc-based plating layer" is a general term for zinc plating layers and zinc-based alloy plating layers. Examples of the aforementioned zinc-based plating layer include hot-dip zinc plating layers such as GI, alloyed hot-dip zinc plating layers such as GA, and electro-zinc plating layers such as EG.

[0054] For example, the steel material may be a zinc-plated steel material comprising a base steel material and a zinc-based plating layer provided on the surface of the base steel material. Alternatively, the steel material may be an alloyed zinc-plated steel material comprising a base steel material, a zinc-based plating layer provided on the base steel material, and an alloy layer formed between the base steel material and the zinc-based plating layer.

[0055] Furthermore, even when zinc-plated steel is the material to be measured, as described later, the O intensity profile can be measured from the surface of the steel material (i.e., the surface of the zinc-plated layer). Therefore, in this invention, the thickness of the low solid-solution Si concentration layer can be easily determined without performing any pretreatment such as stripping the plating layer.

[0056] 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 strength 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, the upper limit of the tensile strength is not particularly limited, but may be, for example, 2.0 GPa or less, or 1.5 GPa or less. The tensile strength may be between 980 MPa and 2.0 GPa, or between 980 MPa and 1.5 GPa. The tensile strength of the steel material can be measured by a tensile test in accordance with the provisions of JIS Z 2241.

[0057] (Low solid solution Si concentration layer) As already mentioned, the low solid-solution Si concentration layer is a layer added to improve the LME resistance of steel. That is, Si is an element included to increase the strength of steel, but it has an adverse effect on LME resistance. Therefore, the Si present from the surface to the interior of the steel (solid-solution Si) is oxidized to form oxides (granular, dendrite, etc.). As a result, a layer with a lower solid-solution Si concentration than the matrix phase of the steel is formed on the surface of the steel, and as a result, the LME resistance can be improved.

[0058] In this invention, a region containing Si-based oxides and having a solid-solution Si concentration below a predetermined standard value is defined as a low solid-solution Si concentration layer. For example, if the steel material does not have a plating layer on its surface, the depth from the surface of the steel material to the position where the solid-solution Si concentration reaches a predetermined standard value can be considered as the thickness of the low solid-solution Si concentration layer. On the other hand, if the steel material is a zinc-plated steel material or an alloyed zinc-plated steel material, the depth from the surface of the base steel material to the position where the solid-solution Si concentration reaches a predetermined standard value can be considered as the thickness of the low solid-solution Si concentration layer. The surface of the base steel material can also be described as the interface between the base steel material and the plating layer.

[0059] 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.

[0060] 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%.

[0061] 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 steel 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).

[0062] 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.

[0063] 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 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.

[0064] 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 analytical method in one embodiment of the present invention may further include a first reference value determination step in which a reference value of Si concentration is determined prior to the measurement step.

[0065] The specific method for determining the reference value of Si concentration in the first reference value determination step is not particularly limited; the reference value of Si concentration should be determined in such a way that a higher correlation is obtained between the thickness of the low solid-solution Si concentration layer obtained and the actual LME resistance. For example, the solid-solution Si concentration profile (distribution in the plate thickness direction) of several steel materials with known LME resistance can be measured by a method other than the GDS analysis used in the present invention, and the reference value of Si concentration can be determined based on the results. More specifically, the following method can be applied.

[0066] 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.

[0067] Furthermore, for each of the aforementioned steel materials, the solid solution Si concentration profile is measured 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 Si concentration 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 Si concentration 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 Si concentration reference value used in the analytical method of the present invention.

[0068] Next, the analysis procedure in this embodiment will be described in detail. Figure 7 is a flowchart showing an analysis method for determining the thickness of a low solid-solution Si concentration layer according to the first embodiment of the present invention. The analysis method in this embodiment comprises a measurement step S1, an integration step S2, and a calculation step S3.

[0069] (Measurement process) In the measurement process, the O strength profile in the thickness direction from the surface of the steel material is measured using GDS analysis. The GDS analysis can be performed in the thickness direction from the surface of the steel material regardless of whether or not there is a zinc-based plating layer.

[0070] The GDS analysis described above can be performed using any GDS analyzer without any particular limitations. Examples of commercially available GDS analyzers include the GD-profiler2 from Horiba, Ltd. When measuring the O intensity profile using the said apparatus, the conditions can be, for example, a measuring diameter of 4 mmφ, an Ar gas pressure of 300 Pa, and a power of 35 W.

[0071] (integral process) In the integration step, an integrated value of O concentration in the surface layer of the steel material is obtained from the O intensity profile obtained in the above measurement step. Specifically, the O intensity profile obtained in the measurement step may be converted into an O concentration profile, and then the O concentration may be integrated. The conversion from the O intensity profile to the O concentration profile is not particularly limited and may be performed by any method. Typically, O intensity may be converted into O concentration using a calibration curve prepared in advance using standard samples having different O concentrations.

[0072] In the integration step, the integrated value of O concentration in the surface layer of the steel material is obtained, and the depth-direction range over which O concentration is integrated may be determined in consideration of the O concentration distribution in the surface layer of the steel material or the like.

[0073] Hereinafter, preferred examples of the integration range will be described. In the following description, the integration start position when obtaining the integrated value of O concentration is defined as X1, and the integration end position is defined as X2. Both X1 and X2 are positions in the sheet thickness direction (depth direction) of the steel material, and are represented by the depth from the steel material surface (distance in the sheet thickness direction). Therefore, the steel material surface is zero, and X1 < X2. Further, when the steel material to be measured is a zinc-based plated steel material or an alloyed zinc-based plated steel material, the surface of the zinc-based plating layer is regarded as the surface of the steel material. Therefore, X1 and X2 are represented by depths from the steel material surface (that is, the surface of the zinc-based plating layer). In other words, the surface of the zinc-based plating layer is set to zero.

[0074] • Integration start position X1 The integration start position X1 may be zero, that is, the surface of the steel material. By using the value obtained by integrating O concentration from the surface of the steel material, the thickness of the low solid-solution Si concentration layer can be obtained with good accuracy as shown in Fig. 5.

[0075] Even when the steel material to be measured is a zinc-based plated steel material, integration may be started from the surface of the steel material (that is, the surface of the zinc-based plating layer). The reason therefor is as follows.

[0076] For example, if one attempts to start integration from the surface of the underlying steel material, it is necessary to pinpoint the location of the underlying steel material's surface. However, accurately pinpointing the location of the underlying steel material's surface in the O concentration profile is difficult. In particular, if the steel material is an alloyed zinc-plated steel, an alloy layer is formed between the underlying steel material and the zinc-plated layer, and the composition of this alloy layer changes continuously. Therefore, accurately pinpointing the location of the underlying steel material's surface is especially difficult. In contrast, if integration is started from the surface of the steel material (i.e., the surface of the zinc-plated layer), it is not necessary to pinpoint the location of the underlying steel material's surface.

[0077] Furthermore, the inventors confirmed that even in zinc-plated steel materials, the majority of the oxygen in the steel material exists on the surface of the underlying steel material as Si-based oxides involved in the formation of the low solid-solution Si concentration layer, and only a small amount of oxygen is contained in the plating layer. Therefore, it was found that even if integration is started from the surface of the zinc-plated layer, the amount of Si-based oxides involved in the formation of the low solid-solution Si concentration layer can be evaluated with sufficient accuracy.

[0078] Therefore, even when the steel material being measured is zinc-plated steel or alloyed zinc-plated steel, integration can be started from the surface of the steel material (i.e., the surface of the zinc-plated layer).

[0079] In one embodiment of the present invention, it is preferable to exclude the range from the outermost surface of the steel material to a predetermined depth in the integration step when determining the integral value of the O concentration. The reasons for this will be explained below.

[0080] A native oxide film is formed on the outermost surface of the steel material. Therefore, if X1 is the surface of the steel material, the resulting integral value will include not only the oxygen that constitutes the Si-based oxide involved in the formation of the low solid-solution Si concentration layer, but also the oxygen that constitutes the native oxide film. Therefore, as shown in Figure 6, by setting X1 in a way that minimizes the influence of the oxygen that constitutes the native oxide film, the thickness of the low solid-solution Si concentration layer can be determined more accurately.

[0081] Similarly, if the steel material is zinc-plated steel or alloyed zinc-plated steel, a native oxide film exists on the surface of the zinc-plated layer. Therefore, by setting X1 in a way that minimizes the influence of O, which constitutes the native oxide film, the thickness of the low solid-solution Si concentration layer can be determined more accurately.

[0082] In this case, there are no particular limitations on how X1 is determined; it can be determined by considering factors such as the thickness of the native oxide film and the distribution of oxides that contribute to the formation of the low solid solution Si concentration layer. If the steel material is zinc-plated steel or alloyed zinc-plated steel, X1 can be determined based on the surface of the zinc-plated layer.

[0083] For example, if the approximate thickness of the native oxide film can be determined from the thickness-direction profiles of each element obtained by GDS analysis, X1 can be determined according to that thickness. X1 may be a fixed value or it may be defined as a relative value. For example, in the O concentration profile of steel, if there is an O concentration peak near the surface, X1 can be defined as the position where the O concentration has decreased to a predetermined ratio (e.g., 1 / 2) of the O concentration at the peak top. Alternatively, X1 may be set so that a high correlation is obtained between the integral value of the O concentration and the actual thickness of the low solid solution Si concentration layer.

[0084] Depending on the steel material being tested, from the viewpoint of sufficiently eliminating the influence of the native oxide film, it is typically preferable that X1 be 0.1 μm or more, more preferably 0.2 μm or more, and even more preferably 0.5 μm or more. On the other hand, if X1 is excessively large, it may not be possible to obtain an integral value that accurately reflects the amount of O involved in the formation of the low solid solution Si concentration layer. Therefore, depending on the steel material being tested, it is typically preferable that X1 be 3 μm or less, and even more preferably 2 μm or less. X1 is preferably 0.1 to 3 μm, more preferably 0.2 to 3 μm, and even more preferably 0.5 to 2 μm.

[0085] ·Integration end position X2 On the other hand, the integration termination position X2 should be determined in a way that allows for the evaluation of the amount of O that constitutes the Si-based oxide involved in the formation of the low solid-solution Si concentration layer. Normally, oxygen that penetrates the surface of the steel material by the oxidation treatment to form the low solid-solution Si concentration layer diffuses to a certain depth, but is almost nonexistent at depths greater than a certain amount. Therefore, for example, X2 can be set to the position where the O concentration becomes zero in the O concentration profile obtained by GDS analysis. However, in actual measurements, the measured value may not be exactly zero. In such cases, for example, X2 may be set to the position below a predetermined reference value, or to a position where measurement points below the reference value are consecutive within a predetermined range.

[0086] However, as mentioned above, oxygen is almost nonexistent at depths greater than a certain point, so integrating to greater depths has little effect on the actual integral value. Therefore, the upper limit of X2 is not particularly limited and may be, for example, half the thickness (t) of the steel plate.

[0087] On the other hand, the lower limit of X2 is not particularly limited and can be any value as long as it is greater than X1 (at a deeper position than X1). However, if X1 and X2 are too close together, there is a risk that the range in which O exists cannot be covered. For this reason, X2 is preferably X1 + 10 μm or more, more preferably X1 + 13 μm or more, and even more preferably X1 + 15 μm or more. X2 is preferably (X1 + 10 μm) ~ t / 2, more preferably (X1 + 13 μm) ~ t / 2, and even more preferably (X1 + 15 μm) ~ t / 2.

[0088] In the measurement process, it is sufficient to perform the measurement in a way that covers the range over which the O concentration is integrated in this integration process. In other words, it is sufficient to measure at least the range from the surface of the steel material to a depth of X2. However, as will be described later, when calculating the thickness of the low C concentration layer, it is preferable to perform the measurement to a deeper position, for example, it is preferable to measure to a depth of X1 + 50 μm or more from the surface of the steel material. This is because, generally, the thickness of the low C concentration layer is greater than the thickness of the low solid solution Si concentration layer.

[0089] (calculation process) Next, in the calculation step, the thickness of the low-solid-solution Si concentration layer is calculated using the integral value of the O concentration obtained in the integration step. The specific calculation method is not particularly limited, but typically, an equation (prediction equation) that expresses the relationship between the integral value of the O concentration and the thickness of the low-solid-solution Si concentration layer, which has been determined in advance, can be used. The prediction equation is an equation in which the integral value of the O concentration is the explanatory variable and the thickness of the low-solid-solution Si concentration layer is the objective variable, and by inputting the integral value of the O concentration into the prediction equation, the thickness of the low-solid-solution Si concentration layer can be calculated from the integral value of the O concentration.

[0090] The aforementioned prediction formula can be obtained, for example, by following the procedure below.

[0091] First, several steel materials with different thicknesses of the low-solid-solution Si concentration layer are prepared. The method for forming the low-solid-solution Si concentration layer is not particularly limited, and known methods described in, for example, Patent Documents 1 and 2 can be used. The steel materials are the same except for the thickness of the low-solid-solution Si concentration layer. In particular, in the case of alloyed zinc-plated steel materials, the thickness of the low-solid-solution Si concentration layer tends to decrease during the alloying reaction, so when evaluating the thickness of the low-solid-solution Si concentration layer, it is preferable to use alloyed zinc-plated steel materials when calculating the prediction formula.

[0092] Then, for each of the above steel materials, an O strength profile is obtained by GDS analysis, and the integral value of the O concentration is calculated. The measurement conditions for the GDS analysis should be the same as those used when actually using the method of the present invention.

[0093] 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.

[0094] First, if the steel material has a zinc-based plating layer, it is preferable to remove most of the zinc-based plating layer, as this allows for the preparation of the sample described later by using the steel material after the zinc-based plating layer has been removed. The method for removing the zinc-based 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 underlying steel material.

[0095] From each of the aforementioned steel materials, a sample is cut out 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, for example, 0.1 μm. From the obtained Si concentration distribution in the thickness direction, the thickness of the low solid-solution Si concentration layer is determined. In this case, 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.

[0096] Note that the measurement results by FE-EPMA include not only solid-solution Si but also Si existing as an oxide. However, since Si as an oxide exists as granular material, 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.

[0097] Here, FE-EPMA was used as an example, but instead of FE-EPMA, energy dispersive X-ray spectroscopy (EDS) using a scanning transmission electron microscope (STEM) may also be used. 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 plane, and EDS line profiles should be acquired at 0.01 μm intervals using STEM-EDS.

[0098] From the integral values ​​of the thickness of the low-solid-solution Si concentration layer and the O concentration obtained in this way, a predictive equation representing the relationship between the two is obtained. The predictive equation may be a linear equation, for example, as shown in the example described later.

[0099] Using the prediction formula created in this way, the thickness of the low solid-solution Si concentration layer can be calculated from the integral value of the O concentration obtained in the integration step.

[0100] As mentioned above, if the steel material is zinc-plated steel, it is preferable to measure the thickness of the low-solid-solution Si concentration layer using a sample from which the zinc-plated layer has been removed, and to create a prediction formula using that value. The thickness of the low-solid-solution Si concentration layer calculated by the method of the present invention using the prediction formula thus created can be considered to be substantially the thickness (depth) from the surface of the underlying steel material. In other words, the thickness of the zinc-plated layer is not included in the thickness of the low-solid-solution Si concentration layer calculated by the method of the present invention using the prediction formula.

[0101] Furthermore, if the steel material is an alloyed zinc-plated steel, the alloy layer will also be removed when the zinc-plated layer is peeled off. Therefore, the thickness of the low solid-solution Si concentration layer calculated by the method of the present invention using the prediction formula does not include the thickness of the zinc-plated layer and the alloy layer. In the method of the present invention, the O concentration in the alloy layer is usually integrated as well, but since the influence of the alloy layer is relatively small, the thickness of the low solid-solution Si concentration layer can be determined without any problems.

[0102] However, from the standpoint of improving accuracy, it is preferable to create a prediction formula using the same type of steel material as the object being measured. For example, when analyzing the thickness of the low solid solution Si concentration layer in zinc-plated steel, it is preferable to use a prediction formula that has been prepared in advance using zinc-plated steel, and when analyzing the thickness of the low solid solution Si concentration layer in alloyed zinc-plated steel, it is preferable to use a prediction formula that has been prepared in advance using alloyed zinc-plated steel.

[0103] (Second Embodiment) Next, an analysis method according to a second embodiment of the present invention will be described. In the analysis method of this embodiment, in the measurement step, the carbon intensity profile in the plate thickness direction from the surface of the steel material is further measured using glow discharge emission analysis. Then, in the calculation step, the thickness of the low carbon concentration layer, which is the region in which the carbon concentration is below a predetermined reference value, is calculated based on the carbon intensity profile. A detailed explanation follows below.

[0104] (Low C concentration layer) The low-carbon concentration layer is a layer present on the surface of steel that affects its resistance to LME (Laser Metal Emission Control). In other words, carbon is an element included to increase the strength of steel, but increasing the strength of steel reduces its resistance to LME. Therefore, by decarburizing the surface of the steel and forming a layer with a lower carbon concentration than the base material (low-carbon concentration layer) on the surface of the steel, the surface of the steel is softened, and its resistance to LME can be improved.

[0105] When a low-carbon concentration layer is formed on the surface of a steel material in this manner, even if the thickness of the low-solid-solution Si concentration layer is the same, the LME resistance may differ depending on the thickness of the low-carbon concentration layer. Therefore, if the presence or absence of the low-carbon concentration layer and its thickness can be taken into consideration, the accuracy of evaluating the LME resistance of the steel material can be further improved. Accordingly, in this embodiment, the thickness of the low-carbon concentration layer is calculated.

[0106] In the present invention, as described above, the region in which the C concentration is below a predetermined standard value is defined as the low C concentration layer. For example, if the steel material does not have a zinc-based plating layer on its surface, the thickness of the low C concentration layer is defined as the depth from the surface of the steel material to the position where the C concentration reaches a predetermined standard value. On the other hand, if the steel material is a zinc-plated steel material or an alloyed zinc-plated steel material, the thickness of the low C concentration layer can be defined as the depth from the surface of the base steel material to the position where the C concentration reaches a predetermined standard value.

[0107] Furthermore, the reference value for the C concentration, that is, the specific depth range from the outermost surface of the steel material to be considered a low-C concentration layer, can be determined as appropriate.

[0108] In one embodiment of the present invention, the reference value for the C concentration may be determined based on the absolute value of the C concentration. For example, if the reference value for the C concentration is set to 0.03%, then the region where the C concentration is 0.03% or less is considered a low C concentration layer. Any value can be used as the reference value for the C concentration, but typically, it is preferable to select the reference value for the C concentration from between 0.01% and 0.04%.

[0109] Furthermore, from the viewpoint of more appropriately evaluating the LME resistance of steel materials, it is also preferable to determine the reference value of the C concentration such that the correlation between the thickness of the obtained low C 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 second reference value determination step in which a reference value of the C concentration is determined prior to the measurement step.

[0110] In the second reference value determination step, the specific method for determining the reference value of C concentration is not particularly limited. The reference value of C concentration should be determined in such a way that a higher correlation is obtained between the thickness of the low-C concentration layer obtained and the actual LME resistance. For example, the C strength profile (distribution in the plate thickness direction) of several steel materials with known LME resistance can be measured by GDS analysis, and the reference value of C concentration can be determined based on the results. More specifically, the following method can be applied.

[0111] First, several steel materials are prepared, each having a low carbon 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.

[0112] Furthermore, the carbon intensity profile is measured for each of the aforementioned steel materials using GDS analysis. Next, a calibration curve is created from the obtained carbon intensity profiles using standard samples with different carbon concentrations, and the carbon concentration profile is determined from the obtained calibration curve. Then, the thickness of the low carbon concentration layer is calculated from the obtained carbon concentration profile, using several different carbon concentration reference values. The thickness of the low carbon concentration layer obtained in this way is compared with the evaluation results of LME resistance. As a result, the carbon concentration reference value used to determine the thickness of the low carbon concentration layer that showed the best correlation with LME resistance is adopted as the carbon concentration reference value used in the analytical method of the present invention.

[0113] Furthermore, if the steel material is zinc-plated steel or alloyed zinc-plated steel, the position of the surface of the base steel material can be determined based on the concentration profile obtained by GDS analysis. For example, by measuring the Zn concentration profile by GDS analysis, the position where the Zn concentration is half of the maximum value can be considered the surface of the base steel material.

[0114] [Measurement process] To determine the thickness of the low-C concentration layer, the C intensity profile can be measured simultaneously with the O intensity profile using glow discharge emission spectrometry during the measurement process.

[0115] [Calculation process] Next, in the calculation step, the thickness of the low-C concentration layer, which is the region where the C concentration is below a predetermined reference value, is calculated based on the C intensity profile. The reference value for C concentration can be determined by the method described above.

[0116] Furthermore, when calculating the thickness of the low-C concentration layer, the measured C intensity profile can be converted into a C concentration profile. The conversion from the C intensity profile to the C concentration profile can be performed by any method without any particular limitations. Typically, a calibration curve prepared in advance using standard samples with different C concentrations can be used to convert the C intensity to C concentration.

[0117] [Evaluation method for LME resistance] Next, a method for evaluating LME resistance in one embodiment of the present invention will be described. Figure 8 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 S4 in which the LME resistance of steel is evaluated based on the thickness of the low solid solution Si concentration layer calculated using the analysis method for the thickness of the low solid solution Si concentration layer described above.

[0118] 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.

[0119] 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.

[0120] 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. For example, the value of the thickness of the low-solid-solution Si concentration layer can be divided into several ranges, and an index (score) of the predicted LME resistance for each range can be predetermined.

[0121] LME resistance can also be affected by the thickness of the low-C concentration layer. Therefore, it is even more preferable to evaluate LME resistance from the thickness of the low-Si concentration layer and the low-C concentration layer. For example, a two-dimensional plane with the thickness of the low-solid-solution Si concentration layer on the horizontal axis and the thickness of the low-C concentration layer on the vertical axis can be divided into multiple regions, and an index (score) of predicted LME resistance can be predetermined for each region.

[0122] (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 plate to be evaluated for LME resistance can be spot-welded to a galvanized steel plate, 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.

[0123] First, a steel plate coated with hot-dip galvanizing 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.

[0124] 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.

[0125] (1) The welding electrode and the assembled plate have a striking angle of 0.2 degrees or more. Here, "striking angle" is defined as the angle at which the electrode is tilted relative to the steel plate, that is, "the angle between the direction of electrode pressure and the direction of steel plate thickness".

[0126] (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.

[0127] (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.

[0128] (4) The condition that there is a gap of 0.5 mm or more between at least one pair of steel plates among the two or more overlapping steel plates (distance between steel plates). Here, "gap between steel plates" is defined as the maximum distance between one pair of steel plates that are adjacent in the vertical direction.

[0129] (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 any of the two or more overlapping steel plates.

[0130] 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.

[0131] Specific evaluation criteria can be determined as needed, but for example, the following criteria can be used.

[0132] A crack is considered present if a crack with a depth of 5 μm or more is found in the cross-section of the weld. Then, the LME resistance is evaluated according to the following criteria based on the number of samples with cracks out of 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.

[0133] 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 can then be evaluated (estimated) by a simple method of measuring the O strength profile from the steel surface using GDS analysis and calculating the integral value of the O concentration. Furthermore, the thickness of the low C concentration layer can also be measured simultaneously with GDS analysis, and by considering this, it becomes possible to evaluate the LME resistance with even higher accuracy. Therefore, the present invention is extremely effective in developing high-strength steel with excellent LME resistance. [Examples]

[0134] Next, the present invention will be described in more detail based on examples, but the present invention is not limited to these examples.

[0135] The steel material used for measurement was alloyed hot-dip galvanized steel sheet (GA), which has an alloyed hot-dip galvanized layer on the surface of a high-strength steel sheet (base steel sheet). The component composition and tensile strength of the high-strength steel sheet are shown in Table 1. The amount of plating adhesion was 45 g / m² per side. 2 The plate thickness was 1.4 mm.

[0136] GDS analysis was performed on each of the aforementioned steel materials to obtain profiles of oxygen (O) and carbon (C) intensity in the thickness direction from the surface of the steel material. Next, the O and C intensity profiles were converted into O concentration and C concentration profiles, respectively, using a pre-prepared calibration curve.

[0137] The thickness of the low-C concentration layer was determined from the obtained C concentration profile. Here, a reference value of 0.01% C concentration was used to determine the thickness of the low-C concentration layer, and the distance from the surface of the base steel plate until the C concentration reached 0.01% was calculated as the thickness of the low-C concentration layer. In addition, the Zn concentration profile was measured by GDS analysis, and the position where the Zn concentration was half of the maximum value was defined as the surface of the base steel plate.

[0138] Next, the integral value of the oxygen concentration was determined from the obtained oxygen concentration profile. In calculating the integral value, the integration start position X1 was set to a depth of 2 μm from the surface of the steel material, and the integration end position X2 was set to a depth of 20 μm from the surface of the steel material. The integral value was obtained by trapezoidal approximation of the oxygen concentration profile. That is, the calculation (yi+1+yi)×(xi+1-xi) / 2 was performed for the i-th to i+1-th plots from a depth of 2 μm to 20 μm, and the integral value was obtained by summing all the values.

[0139] Table 1 shows the thickness of the low-C concentration layer and the integral value of the O concentration obtained in this way.

[0140] Next, in order to understand the correlation between the integral value of the oxygen concentration and the thickness of the low solid-solution Si concentration layer, the thickness of the low solid-solution Si concentration layer of each steel material was measured using STEM-EDS. Specifically, first, the plating layer formed on the surface of each steel material was removed. This removal was performed by immersing the steel material in hydrochloric acid with an inhibitor added.

[0141] Next, cross-sectional TEM samples were prepared from the steel material using FIB, and the Si concentration distribution in the thickness direction from the surface of the underlying steel plate was measured using STEM-EDS. The Si concentration distribution obtained from this measurement 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 concentration at the locations where Si-based oxides were detected.

[0142] Based on the obtained distribution of solid-solution Si concentration, the thickness of the region from the surface of the base steel plate to the point where the solid-solution Si concentration is 0.5% or less (low solid-solution Si concentration layer) was calculated. In other words, a reference value of Si concentration of 0.5% was set 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, which is 1.5%. The obtained thicknesses of the low solid-solution Si concentration layer are shown in Table 1.

[0143] Figure 9 is a graph showing the correlation between the integral value of the O concentration obtained by GDS analysis and the thickness of the low solid-solution Si concentration layer measured by STEM-EDS. When the relationship between the two was approximated by a linear equation using the least squares method, the following equation (1) was obtained. y = 2.4554x - 0.2383 …(1) Here, y is the thickness of the low solid solution Si concentration layer (μm), and x is the integral value of the O concentration. And the correlation coefficient R 2 The value is high at 0.9263, indicating an extremely good correlation between the integral value of the O concentration obtained by GDS analysis and the thickness of the low solid-solution Si concentration layer measured by STEM-EDS.

[0144] Next, the LME resistance of each steel material was evaluated. The LME resistance was evaluated using the welded joint method described earlier. The results are shown in Table 1.

[0145] 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).

[0146] Specifically, first, eight types of galvannealed steel sheets having the component compositions and tensile strengths of the base steel sheets shown in Table 2 were prepared as test samples. GDS analysis was performed on each of the test samples to obtain profiles of C intensity and O intensity. After converting the obtained intensity profiles to concentration profiles by the method described above, the thickness of the low C concentration layer was obtained from the C concentration profile, and the integrated value of O concentration was obtained from the O concentration profile, respectively. Then, the thickness of the low solid-solution Si concentration layer was calculated from the obtained integrated value of O concentration using the above formula (1). The results are shown in Table 2. Here, for convenience, values obtained by rounding the calculated thickness of the low solid-solution Si concentration layer to the third decimal place are shown.

[0147] For each of the above galvannealed steel sheets, the thickness of the low solid-solution Si concentration layer actually measured by STEM-EDS after peeling off the plating layer is also shown in Table 2. From these results, it can be seen that according to the analysis method of the present invention, the thickness of the low solid-solution Si concentration layer can be obtained with accuracy sufficient to evaluate LME resistance.

[0148] Further, the thickness d of the low solid-solution Si concentration layer obtained by the method of the present invention Si was used to predict the score of LME resistance. In the prediction, the following criteria determined based on the correlation in Table 1 were used. · Score 4: d Si ≧5 μm · Score 3: 5 μm > d Si ≧2 μm · Score 2: 2 μm > d Si ≧1 μm · Score 1: 1 μm > d Si

[0149] Also, the thickness d of the low solid-solution Si concentration layer obtained by the method of the present invention Si and the thickness d of the low C concentration layer c was used to predict the score of LME resistance. In the prediction, the following criteria determined based on the correlation in Table 1 were used. · Score 4: d Si ≧5 μm · Score 3: 5 μm > d Si ≧2 μm or 2 μm > dSi and d c ≥41.5μm • Rating 2: 2μm > d Si ≥1μm and 41.5μm >d c or 1μm>d Si Furthermore, 41.5 μm > d c ≥20μm • Score 1: 1μm > d Si and 20 μm > d c

[0150] Thickness d of the low solid solution Si concentration layer Si The result predicted from only this is given a "score of A", and the thickness of the low solid solution Si concentration layer is d Si and the thickness d of the low C concentration layer c The predicted results are shown in Table 2 as "Score B". The measured LME resistance scores for each alloyed hot-dip galvanized steel sheet are also listed in Table 2.

[0151] As can be seen from the results shown in Table 2, the LME resistance predicted by the method of the present invention was in good agreement with the actual LME resistance. Considering that the measurement can be performed very simply and quickly according to the present invention, it can be said that the present invention can predict LME resistance with sufficiently practical accuracy. Furthermore, it can be seen that the LME resistance can be predicted with even higher accuracy when the thickness of the low-C concentration layer is used in addition to the integral value of the O concentration.

[0152] [Table 1]

[0153] [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 O strength profile in the thickness direction from the surface of the steel material using glow discharge emission analysis, An integration step is performed to obtain the integral value of the O concentration in the surface layer of the steel material from the O strength profile, A calculation step of calculating the thickness of the low solid solution Si concentration layer using the integral value of the O concentration, A method for analyzing the thickness of a low solid-solution Si concentration layer, comprising the following features.

2. The method for analyzing the thickness of a low solid-solution Si concentration layer according to claim 1, wherein in the integration step, the integral value of the O concentration is determined by excluding the range from the outermost surface of the steel material to a predetermined depth.

3. The steel material is a zinc-plated steel material comprising a base steel material and a zinc-plated layer provided on the surface of the base steel material. The method for analyzing the thickness of a low solid solution Si concentration layer according to claim 1 or 2, wherein the calculation step involves calculating the thickness of a low solid solution Si concentration layer present on the surface of the underlying steel material.

4. The steel material is an alloyed zinc-plated steel material comprising a base steel material, a zinc-based plating layer provided on the base steel material, and an alloy layer formed between the base steel material and the zinc-based plating layer. The method for analyzing the thickness of a low solid solution Si concentration layer according to claim 1 or 2, wherein the calculation step involves calculating the thickness of a low solid solution Si concentration layer present on the surface of the underlying steel material.

5. In the above measurement step, further, the C intensity profile in the thickness direction from the surface of the steel material is measured using glow discharge emission analysis. In the calculation step described above, the thickness of the low-C concentration layer, which is the region where the C concentration is below a predetermined reference value, is calculated based on the C intensity profile. A method for analyzing the thickness of a low solid-solution Si concentration layer according to claim 1 or 2.

6. In the measurement step, further, the C intensity profile in the thickness direction from the surface of the steel material is measured using glow discharge emission analysis, In the calculation step described above, the thickness of the low-C concentration layer, which is the region where the C concentration is below a predetermined reference value, is calculated based on the C intensity profile. The method for analyzing the thickness of a low solid-solution Si concentration layer according to claim 3.

7. In the measurement step, further, the C intensity profile in the thickness direction from the surface of the steel material is measured using glow discharge emission analysis, In the calculation step described above, the thickness of the low-C concentration layer, which is the region where the C concentration is below a predetermined reference value, is calculated based on the C intensity profile. A method for analyzing the thickness of a low solid-solution Si concentration layer according to claim 4.

8. 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 of the low solid solution Si concentration layer calculated using the analysis method for the thickness of the low solid solution Si concentration layer described in claim 1 or 2.

9. 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 of the low solid solution Si concentration layer calculated using the analysis method for the thickness of the low solid solution Si concentration layer described in Claim 3.

10. 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 of the low solid solution Si concentration layer calculated using the analysis method for the thickness of the low solid solution Si concentration layer described in Claim 4.

11. 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 of the low solid solution Si concentration layer and the thickness of the low C concentration layer calculated using the analytical method for the thickness of the low solid solution Si concentration layer described in claim 5.

12. 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 of the low solid solution Si concentration layer and the thickness of the low C concentration layer calculated using the analytical method for the thickness of the low solid solution Si concentration layer described in Claim 6.

13. 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 of the low solid solution Si concentration layer and the thickness of the low C concentration layer calculated using the analytical method for the thickness of the low solid solution Si concentration layer described in Claim 7.

Images