Steel plates and parts containing them

Abstract

The present disclosure provides a steel sheet excellent in LME resistance and a component including the same with a novel configuration. The steel sheet of the present disclosure has a specific chemical composition in which the total content of Nb and Mo is 0.010% or more. In GDS analysis, a B-enriched portion having a B concentration 10.0 times or more that of the bulk B concentration exists in the steel sheet surface layer in the region from the outermost surface of the steel sheet to a depth position of 5.0 μm in the plate thickness direction. The thickness t of the B-enriched portion B satisfies the following formula (1). The emission intensities of Nb and Mo measured by GDS analysis satisfy the following formula (2). t B ≧2.0 μm ···(1) Isa(Nb + Mo) / Iba(Nb + Mo)≧0.5 ···(2) Here, Isa(Nb + Mo) is the average value of the sum of the emission intensities of Nb and Mo in the steel sheet surface layer. Iba(Nb + Mo) is the average value of the sum of the emission intensities of Nb and Mo in the bulk.

Description

[Technical Field]

[0001] This disclosure relates to steel plates and parts containing them. [Background technology]

[0002] In recent years, there has been a trend towards increasing the strength of steel sheets used in various fields such as automobiles, home appliances, and building materials. For example, in the automotive sector, the use of thin, high-strength steel sheets is increasing in order to lighten vehicle bodies and components and improve fuel efficiency.

[0003] Furthermore, in the case of high-strength steel sheets used in automobile bodies and parts, high-strength steel sheets with a surface plated with metals such as zinc, particularly zinc-plated steel sheets, are used from the viewpoint of rust prevention.

[0004] On the other hand, the assembly of automobile bodies and the attachment of parts are often carried out by welding. However, in welding using zinc-plated steel sheets, weldability may be reduced due to cracks caused by liquid metal embrittlement (LME), such as those disclosed in Patent Document 1 (hereinafter sometimes referred to as "LME cracks").

[0005] As a steel sheet with improved weldability by suppressing such LME cracking, for example, Patent Document 2 describes a steel sheet with 3,000 to 6,000 Si oxide particles with a particle size of 20 nm or larger per mm in the surface layer. 2 A steel sheet having a tensile strength of 600 MPa or more is disclosed, which exists at a specific number density and with a specific particle size distribution.

[0006] Furthermore, Patent Document 3 discloses a steel sheet having a tensile strength of 780 MPa or more, having a predetermined chemical composition, wherein in GDS measurement in the thickness direction of the steel sheet, the depth at which the luminescence intensity Bx at a depth of x (μm) and the luminescence intensity B150 at a depth of 150 μm satisfy Bx / B150 ≥ 5.0 is 0.5 μm or more from the surface of the steel sheet, the thickness of the oxide formed on the surface of the steel sheet is 0.5 μm or less, and an internal oxide layer of 1.0 μm or more in thickness exists in the thickness direction of the steel sheet from the surface of the steel sheet.

[0007] Patent Document 4 discloses a welded joint in which, among a plurality of stacked steel plates, at least one of the outermost steel plates is a high-strength steel plate having a Vickers hardness of 240 Hv or more at the center of the plate thickness, the high-strength steel plate has a predetermined chemical composition, a high-ferrite layer with a ferrite phase area ratio of 90% or more exists at a position 50 μm outward from the edge of the pressure-welded joint to a thickness of 5 μm or more from the surface in the thickness direction, and a B-enriched region having a B-strength of 1.0 μm or more from the surface having a B-strength of 2 times or more than the B-strength at a depth of 50 μm determined by TOF-SIMS measurement.

[0008] Furthermore, Patent Document 5 discloses a plated steel sheet having a tensile strength of 780 MPa or more, wherein the steel sheet has a predetermined chemical composition and the plating layer contains Zn, and in GDS measurement in the thickness direction of the plated steel sheet, the depth at which the luminescence intensity Bx at depth X (μm) and the luminescence intensity B150 at depth 150 μm satisfy Bx / B150 ≥ 5.0 is 0.5 μm or more from the interface between the steel sheet and the plating layer, the thickness of the oxide formed on the surface of the plating layer is 0.5 μm or less, and an internal oxide layer with a thickness of 1.0 μm or more exists in the thickness direction of the steel sheet from the interface. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] International Publication No. 2019 / 116531 [Patent Document 2] International Publication No. 2020 / 218575 [Patent Document 3] International Publication No. 2025 / 032898 [Patent Document 4] International Publication No. 2025 / 032899 [Patent Document 5] International Publication No. 2025 / 032900 [Overview of the Initiative]

Problems to be Solved by the Invention

[0010] In order to cope with further weight reduction and high strength of automotive parts and the like, there is a demand for a steel sheet in which such LME cracks are unlikely to occur, that is, a steel sheet excellent in LME resistance.

[0011] Therefore, an object of the present disclosure is to provide a steel sheet excellent in LME resistance and a component including the same with a novel configuration.

Means for Solving the Problems

[0012] The present disclosure includes at least the following aspects.

[0013] (Aspect 1) A steel sheet, wherein the chemical composition of the steel sheet is, in mass%, C: 0.05 to 0.40%, Si: 0.70 to 3.00%, Mn: 1.00 to 5.00%, B: 0.0005 to 0.0100%, Ti: 0.010 to 0.150%, sol.Al: 0 to 3.00%, Nb: 0 to 0.150%, Mo: 0 to 1.000%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0200% or less, O: 0 to 0.01%, V: 0 to 0.150%, Cr: 0 to 2.00%, Ni: 0 to 2.00%, Cu: 0 to 2.00%, W: 0 to 1.00%, Ca: 0 to 0.100%, Mg: 0 to 0.100%, Zr: 0 to 0.500%, Hf: 0 to 0.100%, Sn: 0 to 0.100%, As: 0~0.100%, REM: 0~0.100%, and The remainder: contains Fe and impurities, and The sum of the Nb content and Mo content is 0.010% or more. In glow discharge emission spectroscopy analysis, B-enriched regions, where the B concentration is 10.0 times or more compared to the bulk B concentration, exist in the surface layer of the steel plate, extending from the outermost surface to a depth of 5.0 μm in the thickness direction. The thickness t of the B-enriched area mentioned above. B However, if the following equation (1) is satisfied, A steel sheet characterized in that the emission intensities of Nb and Mo, measured by the above glow discharge emission spectroscopy, satisfy the following equation (2). t B ≥2.0μm ···(1) Isa(Nb+Mo) / Iba(Nb+Mo)≧0.5 ···(2) Here, Isa(Nb+Mo) is the average value of the sum of the luminescence intensities of Nb and Mo on the surface of the steel plate. Iba(Nb+Mo) is the average value of the sum of the emission intensities of bulk Nb and Mo.

[0014] (Aspect 2) The chemical composition of the above steel plate is, in mass%, Nb: 0.0001~0.150%, Mo: 0.0001~1.000%, O: 0.001~0.01%, V: 0.001~0.150%, Cr: 0.001~2.00%, Ni: 0.001~2.00%, Cu: 0.001~2.00%, W: 0.001~1.00%, Ca: 0.0001~0.100%, Mg: 0.0001~0.100%, Zr: 0.001~0.500%, Hf: 0.0001~0.100%, Sn: 0.0001~0.100%, As: 0.0001~0.100%, and REM: 0.0001~0.100%, The steel plate according to embodiment 1 above, characterized in that it contains one or more of the following.

[0015] (Aspect 3) The steel sheet according to embodiment 1 or 2, characterized in that the chemical composition of the steel sheet contains B: 0.0010 to 0.0100% by mass.

[0016] (Aspect 4) The sum of the above Nb content and Mo content is 0.020% or more. The thickness t of the B-enriched area mentioned above. B However, if the following equation (3) is satisfied, The steel sheet according to any one of embodiments 1 to 3, characterized in that the emission intensities of Nb and Mo measured by the glow discharge emission spectroscopy analysis described above satisfy the following equation (4). t B ≥5.0μm ···(3) Isa(Nb+Mo) / Iba(Nb+Mo)≧0.8 ···(4)

[0017] (Appendix 5) The sum of the above Nb content and Mo content is 0.040% or more. The steel sheet according to any one of embodiments 1 to 4, characterized in that the emission intensities of Nb and Mo measured by the glow discharge emission spectroscopy described above satisfy the following equation (5). Isa(Nb+Mo) / Iba(Nb+Mo)≧1.0 ···(5)

[0018] (Aspect 6) A steel plate according to any one of the above embodiments 1 to 5, characterized in that the Vickers hardness at a depth of 1 / 4 of the plate thickness of the steel plate is 300 Hv or more.

[0019] (Aspect 7) The steel plate according to any one of the above embodiments 1 to 6, characterized in that the Vickers hardness at a depth of 1 / 4 of the plate thickness of the steel plate is 360 Hv or more.

[0020] (Pattern 8) The steel plate according to any one of the above embodiments 1 to 7, characterized in that the Vickers hardness at a depth of 1 / 4 of the plate thickness of the steel plate is 460 Hv or more.

[0021] (Aspect 9) The steel plate according to any one of the above embodiments 1 to 8, characterized in that the Vickers hardness at a depth of 1 / 4 of the plate thickness of the steel plate is 550 Hv or more.

[0022] (Aspect 10) A component comprising a steel plate as described in any of the above embodiments 1 to 9. [Effects of the Invention]

[0023] According to this disclosure, it is possible to provide steel plates with excellent resistance to LME and parts containing the same. [Modes for carrying out the invention]

[0024] LME cracking is thought to occur as follows: First, during welding, such as spot welding, the metal structure of the steel sheet is heated and transformed into austenite. Molten zinc (Zn) produced by the melting of zinc plating concentrates and penetrates the grain boundaries of the austenite on the surface of the steel sheet, especially at grain boundaries where the grain boundary energy is locally low. This causes the steel sheet to become brittle, and further tensile stress is applied to the steel sheet during welding, which is thought to cause LME cracking.

[0025] To suppress LME cracking, segregating boron (B) on the surface of the steel sheet is effective. When boron is segregated on the surface of the steel sheet in a solid solution state, Zn has difficulty penetrating the grain boundaries, thus suppressing LME cracking. In this specification, the property of steel sheets in which LME cracking is suppressed is referred to as "LME resistance." Conversely, the property of steel sheets in which LME cracking is likely to occur is referred to as "LME susceptibility."

[0026] The inventors focused on the LME resistance due to segregation of B and diligently investigated methods to more effectively suppress LME cracking. As a result, the inventors discovered a new method in which, when manufacturing a steel sheet, a specific strain is applied to the surface layer of the steel sheet, and then the steel sheet is annealed under high dew point conditions above a controlled temperature, thereby enriching the surface layer of B and at least one of Nb (niobium) and Mo (molybdenum), which have a high affinity for B. It is believed that when a steel sheet with a specific strain applied to its surface layer is annealed under specific conditions, Si-Mn internal oxides are rapidly formed, and B is incorporated into these internal oxides, thereby suppressing the deboronization of the steel sheet surface and enriching the surface layer of B and at least one of Nb and Mo.

[0027] The steel sheet obtained by the above method is thought to have LME cracking more effectively suppressed because B and at least one of Nb and Mo coexist on the surface of the steel sheet, causing more cosegregation of B (i.e., making it even less likely for Zn to penetrate into the grain boundaries).

[0028] This disclosure is completed based on the findings described above and includes aspects of the following embodiments.

[0029] Preferred embodiments of the steel sheet of this disclosure will be described in detail below.

[0030] <Steel plate> A steel sheet according to one embodiment of this disclosure is, by mass %, C: 0.05~0.40%, Si: 0.70~3.00%, Mn: 1.00~5.00%, B: 0.0005~0.0100%, Ti: 0.010~0.150%, sol.Al: 0~3.00%, Nb: 0~0.150%, Mo: 0~1.000%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0200% or less, O: 0~0.01%, V: 0~0.150%, Cr: 0~2.00%, Ni: 0~2.00%, Cu: 0~2.00%, W: 0~1.00%, Ca: 0~0.100%, Mg: 0~0.100%, Zr: 0~0.500%, Hf: 0~0.100%, Sn: 0~0.100%, As: 0~0.100%, REM: 0~0.100%, and The remainder: contains Fe and impurities, and It has a specific chemical composition in which the sum of the Nb and Mo content is 0.010% or more.

[0031] Furthermore, in this embodiment, the steel sheet has a B-enriched region in the surface layer of the steel sheet, which is a region from the outermost surface of the steel sheet to a depth of 5.0 μm in the thickness direction, where the B concentration is 10.0 times or more compared to the bulk B concentration, as determined by glow discharge emission spectroscopy (GDS analysis). The thickness t of this B-enriched region B The following equation (1) is satisfied. t B ≥2.0μm ···(1)

[0032] Furthermore, the steel sheet of this embodiment has a characteristic configuration in which the luminescence intensity of Nb and Mo, as measured by GDS analysis, satisfies the following equation (2). Isa(Nb+Mo) / Iba(Nb+Mo)≧0.5 ···(2) Here, Isa(Nb+Mo) is the average value of the sum of the luminescence intensities of Nb and Mo on the surface of the steel plate. Iba(Nb+Mo) is the average value of the sum of the emission intensities of bulk Nb and Mo.

[0033] (Effects and Benefits) As described above, the steel sheet of the present embodiment has a specific chemical composition, includes a B-enriched portion satisfying formula (1) in the surface layer of the steel sheet, and the emission intensities of Nb and Mo measured by GDS analysis satisfy the above formula (2). That is, in the surface layer of the steel sheet of the present embodiment, a sufficient amount of B for suppressing the intrusion of Zn into the grain boundaries is concentrated, and a certain amount of Nb and / or Mo coexists in the surface layer of the steel sheet. Thereby, when B diffuses into the grain boundaries and segregates at the grain boundaries after the grain boundaries are formed during the cooling process after welding in the steel sheet of the present embodiment, Nb and Mo with high affinity for B also diffuse into the grain boundaries and segregate at the grain boundaries. Furthermore, by this Nb and Mo attracting B, more co-segregation of B can be caused. As a result, the steel sheet of the present embodiment can be made even less likely to cause the intrusion of Zn into the grain boundaries and can more effectively suppress LME cracking.

[0034] Hereinafter, each component of the steel sheet of the present embodiment will be described in detail.

[0035] [Emission intensity of B measured by glow discharge optical emission spectroscopy: B-enriched portion] [Thickness t of B-enriched portion B ≧2.0 μm] As described above, the steel sheet of the present embodiment has a thickness t of a B-enriched portion that becomes a B concentration 10.0 times or more the bulk B concentration in glow discharge optical emission spectroscopy (GDS analysis). B That is, in GDS analysis, there is a region (i.e., a B-enriched portion) in the surface layer of the steel sheet that exhibits a B emission intensity 10.0 times or more the bulk emission intensity of B, and the thickness t of that region B is 2.0 μm or more. Here, the "B-enriched portion" means a region in the surface layer of the steel sheet where B is concentrated and has a B concentration 10.0 times or more the B concentration in the bulk in GDS analysis. That is, in the steel sheet of the present embodiment, B having a concentration 10.0 times or more the concentration of B in the bulk exists in the surface layer of the steel sheet with a thickness of 2.0 μm or more as the B-enriched portion. t B ≧2.0 μm ···(1)

[0036] In this specification, "steel plate surface layer" refers to the region near the outermost surface of the steel plate, specifically, the region from the outermost surface of the steel plate to a depth of 5.0 μm in the thickness direction of the steel plate. "Bulk" refers to the region of the steel plate other than the steel plate surface layer, and the method for calculating the luminescence intensity of bulk materials such as Iba(Nb+Mo) will be described later. Therefore, in this embodiment, the steel sheet has a B-enriched region on its surface where the B concentration is 10.0 times or more than the bulk B concentration in GDS analysis. In other words, the B-enriched region is located on the outermost surface of the steel sheet. Therefore, the thickness t of the B-enriched region B If the thickness is 5.0 μm or less, all B-enriched areas are located in the "steel plate surface layer", and the thickness of the B-enriched area is t B If it exceeds 5.0 μm, the entire "steel plate surface" becomes a B-enriched area, and a portion of the "bulk" steel plate surface side (specifically, "t B The B-enriched area is a thickness of -5.0 μm. In addition, in the steel plate of this embodiment, the B-enriched area with a thickness of 2.0 to 5.0 μm exists in the "steel plate surface layer," that is, in the region from the outermost surface of the steel plate to a depth of 5.0 μm from the outermost surface of the steel plate.

[0037] Regarding the outermost surface of the steel plate, which serves as the reference for the depth position of the steel plate, this specification defines the 0 μm position as the depth position where the Fe emission intensity reaches 0.7 times or more the bulk Fe emission intensity in GDS analysis, and considers this 0 μm position as the outermost surface of the steel plate. The bulk Fe emission intensity is the Fe emission intensity in a sufficiently deep region of the steel plate. This region is a region where there is almost no change in Fe concentration in the depth direction, and is a region that is judged to be "steel" according to common technical knowledge. The bulk Fe emission intensity can be, for example, the Fe emission intensity at a sputtering time of 1000 seconds, under the measurement conditions of the GDS analysis described later. In this specification, the luminescence intensity at a sputtering time of 1000 seconds (details will be described later, but this is the average of the luminescence intensities of 51 points in total: the center point plus 25 points before and after it) is considered to be the luminescence intensity of the bulk material.

[0038] It should be noted that the "steel sheet" covered by this disclosure may be the "base steel sheet" of a steel sheet having some kind of coating on its surface, such as a plated steel sheet. In such cases, the outermost surface of the steel sheet that serves as the reference for the depth position of the steel sheet is the outermost surface of the base steel sheet (for example, the interface between the steel sheet and the plating layer). In such cases as well, the depth position where the emission intensity of Fe in the GDS analysis reaches 0.7 times or more the emission intensity of bulk Fe, i.e., the 0 μm position, is considered to be the outermost surface.

[0039] Equation (1) above represents the thickness of the region where the concentration of B on the surface of the steel plate is 10.0 times or more than the concentration of B in the bulk, i.e., the thickness t of the B-enriched region. B However, this is equivalent to being 2.0 μm or thicker, meaning that B at a concentration 10.0 times or more than the B concentration in the bulk exists on the surface of the steel plate as a B-enriched area with a thickness of 2.0 μm or more.

[0040] Thus, when a sufficient amount of B is concentrated in the surface layer of the steel plate, penetration of Zn into the grain boundaries is less likely to occur, and LME cracking can be suppressed.

[0041] In this embodiment, the thickness t of the B-enriched portion. B It is preferable that the following equation (3) is satisfied in order to more reliably suppress LME cracking. t B ≥5.0μm ···(3)

[0042] [Emission intensity of Nb and Mo measured by glow discharge emission spectroscopy] [Isa(Nb+Mo) / Iba(Nb+Mo)≧0.5] Furthermore, as described above, the emission intensities of Nb and Mo measured by GDS analysis in the steel sheet of this embodiment satisfy the following equation (2). That is, in the steel sheet of this embodiment, Nb and / or Mo are present on the surface layer of the steel sheet together with the above-mentioned B at a concentration of 0.5 times or more compared to the sum of the concentrations of Nb and Mo in the bulk. Isa(Nb+Mo) / Iba(Nb+Mo)≧0.5 ···(2) Here, Isa(Nb+Mo) is the average value of the sum of the luminescence intensities of Nb and Mo on the surface of the steel plate. Iba(Nb+Mo) is the average value of the sum of the emission intensities of bulk Nb and Mo.

[0043] Regarding the emission intensity of Nb and Mo measured by GDS analysis, the emission intensity Isa(Nb+Mo) of Nb and Mo on the surface of the steel sheet is the average value of the sum of the emission intensity of Nb and Mo on the surface of the steel sheet, obtained when GDS analysis is performed from the outermost surface of the steel sheet in the thickness direction. The emission intensity Iba(Nb+Mo) of Nb and Mo in the bulk is the average value of the sum of the emission intensity of Nb and Mo in the bulk, obtained when GDS analysis is performed from the outermost surface of the steel sheet in the thickness direction. These represent the average values ​​of the sum of the concentrations of Nb and Mo in the respective regions of the steel sheet surface and bulk.

[0044] Therefore, equation (2) above is equivalent to saying that the average concentration of Nb and Mo on the surface of the steel plate is 0.5 times or more the average sum of the concentrations of Nb and Mo in the bulk. In other words, it means that Nb and / or Mo are present on the surface of the steel plate together with B at a concentration of 0.5 times or more the average sum of the concentrations of Nb and Mo in the bulk. Note that it is sufficient for at least one of Nb and Mo to be present in the steel plate; for example, if only one of Nb or Mo is present in the steel plate, the average sum of the concentrations of Nb and Mo will be the average sum of the concentrations of either Nb or Mo alone. Furthermore, in the process of investigating the relationship between LME resistance and the elemental concentration distribution on the surface of steel sheets, the inventors found that in ordinary steel sheets, the average sum of the concentrations of Nb and Mo on the surface of the steel sheet is less than 0.5 times the average sum of the concentrations of Nb and Mo in the bulk.

[0045] In this embodiment, the emission intensities of Nb and Mo measured by GDS analysis preferably satisfy the following formula (4), and more preferably satisfy the following formula (5), in order to more reliably suppress LME cracking. Isa(Nb+Mo) / Iba(Nb+Mo)≧0.8 ···(4) Isa(Nb+Mo) / Iba(Nb+Mo)≧1.0 ···(5)

[0046] Furthermore, in this embodiment, in order to more reliably suppress LME cracking, the chemical composition of the steel sheet is such that the sum of the Nb content and Mo content is 0.020% or more, and the thickness of the B-enriched portion is t B It is particularly preferable that the above formula (3) is satisfied, and that the emission intensities of Nb and Mo measured by GDS analysis are satisfied by formula (4). A method for manufacturing steel sheets that satisfy these characteristics will be described later.

[0047] Similarly, in this embodiment, in order to more reliably suppress LME cracking, it is particularly preferable that the chemical composition of the steel sheet has a total Nb content and Mo content of 0.040% or more, and that the emission intensities of Nb and Mo measured by GDS analysis satisfy formula (5) above. A method for manufacturing a steel sheet that satisfies these characteristics will be described later.

[0048] The GDS analysis of each element in the surface layer and bulk of the steel plate described above should be carried out as follows.

[0049] (GDS analysis) Glow discharge optical emission spectrometry (GD-OES) of each element in the surface and bulk of steel sheets is performed according to the method specified in JIS K0144:2018 "Surface Chemical Analysis - General Rules for Glow Discharge Optical Emission Spectroscopy." Specifically, using a glow discharge optical emission spectrometer, the surface of the steel sheet to be measured is placed in an Ar atmosphere, and a voltage is applied to generate a glow plasma. The steel sheet surface is then sputtered and analyzed in the depth direction. The elements contained in the steel sheet are identified from the emission spectral wavelengths of the elements emitted when atoms are excited in the glow plasma, and the emission intensity of the identified elements is estimated.

[0050] The depth data can be estimated from the sputtering time. Specifically, by determining the relationship between sputtering time and sputtering depth in advance using a standard sample, the sputtering time can be converted to sputtering depth. Therefore, the sputtering depth converted from the sputtering time can be defined as the depth from the steel plate surface.

[0051] The GDS analysis is performed at five measurement points on the steel plate surface that are at least 5 mm apart from each other, and the arithmetic mean is used.

[0052] A commercially available glow discharge emission spectrometer can be used for GDS analysis. In this embodiment, a glow discharge emission spectrometer "GDS850A" manufactured by LECO Japan LLC is used. The measurement conditions are as follows: The detection pitch is 0.1 seconds. Background is removed from the obtained data. Then, the arithmetic mean of the sum of the emission intensities of Nb and Mo on the surface of the steel sheet, Isa(Nb+Mo), is adopted as the emission intensity Isa(Nb+Mo). Anode diameter: 4mmφ RF (Radio Frequency) output: 30W Measurement time: 200-1500 seconds

[0053] On the other hand, the measurement method for the average value Iba(Nb+Mo), which is the sum of the luminescence intensities of bulk Nb and Mo, is basically the same as described above. However, the calculation method differs from that of Isa(Nb+Mo), etc., and for bulk luminescence intensities such as Iba(Nb+Mo), the luminescence intensity at a sputtering time of 1000 seconds (average of 51 points: the center point + 25 points before and after it) is used. The sputtering speed under the above measurement conditions is often around 100 nm / s, which is a typical sputtering speed described in JIS K0144:2018 5.1. Thickness t of the B-enriched area BThe measurement and calculation methods for GDS analysis related to Fe are the same as described above. The emission intensity of B or Fe in bulk is the emission intensity at a sputtering time of 1000 seconds (average of 51 points: center point + 25 points before and after). Excluding this emission intensity of B or Fe in bulk, the emission intensity of B or Fe at each depth position from the outermost surface of the steel plate is obtained by removing the background from the obtained data and then using a moving average of 11 points: center point + 5 points before and after. In other words, the thickness t of the B-enriched area B The position of the outermost surface of the steel plate (0 μm position) is determined from the moving average of these 11 points.

[0054] Furthermore, in order for the emission intensity of B measured by GDS analysis to satisfy the relationships in equations (1) and (2) above, respectively, B, Nb, and / or Mo should be concentrated on the surface of the steel sheet by applying high dew point annealing above a controlled temperature to a steel sheet that has been subjected to a specific strain by brush grinding during the manufacturing process. Specific methods and conditions will be described later.

[0055] [Chemical composition] Next, the chemical composition of the steel sheet in this embodiment will be described in detail. As described above, the steel plate of this embodiment is, by mass %, C: 0.05~0.40%, Si: 0.70~3.00%, Mn: 1.00~5.00%, B: 0.0005~0.0100%, Ti: 0.010~0.150%, sol.Al: 0~3.00%, Nb: 0~0.150%, Mo: 0~1.000%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0200% or less, O: 0~0.01%, V: 0~0.150%, Cr: 0~2.00%, Ni: 0~2.00%, Cu: 0~2.00%, W: 0~1.00%, Ca: 0~0.100%, Mg: 0~0.100%, Zr: 0~0.500%, Hf: 0~0.100%, Sn: 0~0.100%, As: 0~0.100%, REM: 0~0.100%, and The remainder: contains Fe and impurities, and It has a specific chemical composition in which the sum of the Nb and Mo content is 0.010% or more.

[0056] The following provides a more detailed explanation of each of these elements.

[0057] [C : 0.05~0.40%] Carbon (C) is an important element for controlling the strength of steel. To ensure the strength of the steel, the C content should be 0.05% or more. To prevent the C concentration on the surface of the steel plate from becoming too high and for reasons of weldability, the C content should be 0.40% or less. The C content may be 0.08% or more, 0.10% or more, or 0.15% or more. The C content may be 0.37% or less, 0.35% or less, or 0.30% or less.

[0058] [Si: 0.70~3.00%] Si is an element that promotes ferrite stabilization and decarburization. When Si is present, decarburization proceeds easily on the surface of the steel sheet, and the stabilization of the ferrite on the surface of the steel sheet improves LME resistance. To obtain this effect fully, the Si content should be 0.70% or more. If the Si content is excessive, external oxidation will proceed during high dew point annealing, and oxides (scale) will form on the surface of the steel sheet, which may suppress decarburization at the outermost surface and reduce the effect of improving LME resistance. For this reason, the Si content should be 3.00% or less. The Si content may be 0.80% or more, 0.90% or more, or 1.00% or more. The Si content may be 2.50% or less, 2.00% or less, or 1.50% or less.

[0059] [Mn: 1.00~5.00%] Mn is an effective element for improving the strength of steel by forming a hard structure. Considering the strength of the steel, the Mn content should be 1.00% or more. Also, considering the decrease in workability due to Mn segregation, the Mn content should be 5.00% or less. The Mn content may be 1.50% or more, 2.00% or more, or 2.20% or more. The Mn content may be 4.50% or less, 4.00% or less, 3.50% or less, 3.00% or less, or 2.60% or less.

[0060] [B : 0.0005~0.0100%] B is an element that enhances hardenability and contributes to improved strength, as well as strengthening grain boundaries by segregating at them and improving toughness. By segregating on the surface of the steel sheet in a solid solution state, B can suppress the penetration of Zn into the grain boundaries and improve LME resistance. To fully obtain this effect, the B content should be 0.0005% or more. Also, from the viewpoint of ensuring sufficient toughness, the B content should be 0.0100% or less. The B content may be 0.0008% or more, 0.0010% or more, 0.0013% or more, or 0.0017% or more. The B content may be 0.0080% or less, 0.0060% or less, 0.0040% or less, 0.0030% or less, or 0.0025% or less.

[0061] [Ti: 0.010~0.150%] Ti is an element that precipitates as TiC during the cooling of steel and contributes to improving its strength. To fully obtain this effect, the Ti content should be 0.010% or more. On the other hand, if the Ti content is excessive, coarse TiN may be formed, which may impair toughness, so the Ti content should be 0.150% or less. The Ti content may be 0.015% or more, 0.020% or more. The Ti content may be 0.120% or less, 0.100% or less, 0.080% or less, 0.060% or less, 0.040% or less, 0.030% or less.

[0062] [sol.Al:0~3.00%] Al is an element that promotes ferrite stabilization and decarburization by solid dissolving in steel. Here, sol.Al refers to acid-soluble Al, which is not an oxide such as Al2O3 and is soluble in acid. Sol.Al is determined as Al measured after removing the insoluble residue on the filter paper that is generated during the Al analysis process. The above effect of including sol.Al can also be obtained by including Si, so sol.Al is not an essential element in the steel sheet of this disclosure. Therefore, the lower limit of the sol.Al content is 0%. If sol.Al is included in excess, external oxidation will progress during high dew point annealing, and oxides (scale) will be formed on the surface of the steel sheet, which may suppress decarburization at the outermost surface and reduce the effect of improving LME resistance. For this reason, the sol.Al content should be 3.0% or less. The sol.Al content may be 0.001% or more, 0.005% or more, 0.01% or more, or 0.05% or more. The sol.Al content may be 2.00% or less, 1.50% or less, 1.00% or less, 0.75% or less, 0.50% or less, 0.20% or less, or 0.10% or less.

[0063] [Nb: 0~0.150%] Nb is an element that enhances the hardenability of steel and contributes to improving its strength. Furthermore, Nb has a high affinity for B. During the cooling process after welding, Nb diffuses into the grain boundaries along with B after the grain boundaries are formed, causing grain boundary segregation, and further attracts B to the grain boundaries, thereby causing co-segregation of more B. Since this effect can also be obtained with Mo, Nb is not an essential element in the steel sheet of this disclosure. Therefore, the lower limit of the Nb content is 0%. However, in order to obtain the above effect, the total content of Nb and Mo should be 0.010% or more. Also, from the viewpoint of ensuring sufficient toughness, the Nb content should be 0.150% or less. The Nb content may be 0.0001% or more, 0.0005% or more, 0.001% or more, or 0.010% or more in order to fully obtain the above effect. On the other hand, the Nb content may be 0.120% or less, 0.100% or less, 0.080% or less, 0.055% or less, or 0.030% or less.

[0064] [Mo: 0~1.000%] Mo is an element that enhances the hardenability of steel and contributes to improving its strength. Furthermore, Mo has a high affinity for B. During the cooling process after welding, Mo diffuses into the grain boundaries along with B after the grain boundaries are formed, causing grain boundary segregation, and further attracts B to the grain boundaries, thereby causing more co-segregation of B. Since this effect can also be obtained with Nb, Mo is not an essential element in the steel sheet of this disclosure. Therefore, the lower limit of the Mo content is 0%. However, in order to obtain the above effect, the total content of Nb and Mo should be 0.010% or more. Also, from the viewpoint of ensuring sufficient toughness, the Mo content should be 1.000% or less. The Mo content may be 0.0001% or more, 0.0005% or more, or 0.001% or more in order to fully obtain the above effect. On the other hand, the Mo content may be 0.800% or less, 0.600% or less, 0.500% or less, 0.300% or less, 0.200% or less, 0.100% or less, or 0.050% or less.

[0065] [P:0.0300% or less] P is a common impurity found in steel. P is an element that segregates at grain boundaries, promoting embrittlement of steel. A lower P content is preferable, so the lower limit is 0%. However, excessive reduction of P content can lead to a significant increase in cost. For this reason, the P content may be 0.0001% or more, or 0.001% or more, or 0.005% or more. On the other hand, excessive P content can lead to embrittlement of steel due to grain boundary segregation, as described above. Therefore, the P content should be 0.0300% or less. The P content may also be 0.0250% or less, 0.0200% or less, 0.0150% or less, or 0.0100% or less.

[0066] [S:0.0300% or less] S is a common impurity found in steel. S forms nonmetallic inclusions such as MnS in steel, leading to a decrease in the ductility of steel components. A lower S content is preferable, and therefore the lower limit is 0%. However, excessive reduction of S content can lead to a significant increase in cost. For this reason, the S content may be 0.0001% or more, 0.0005% or more, 0.0010% or more, or 0.0020% or more. On the other hand, excessive S content can lead to a decrease in weldability and a decrease in workability such as bendability due to an increase in MnS precipitation. Therefore, the S content should be 0.0300% or less. The S content may also be 0.0250% or less, 0.0200% or less, 0.0150% or less, or 0.0100% or less.

[0067] [N: 0.0200% or less] N is a common impurity found in steel. N forms coarse nitrides in steel, reducing the workability and weldability of steel sheets. A lower N content is preferable, and therefore the lower limit is 0%. However, excessive reduction of N content can lead to a significant increase in manufacturing costs. For this reason, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive N content can form coarse nitrides, reducing the workability and weldability of steel sheets. Therefore, the N content should be 0.0200% or less. The N content may also be 0.0150% or less, 0.0100% or less, or 0.0080% or less.

[0068] [O : 0~0.01%] O is an element that is introduced during the manufacturing process and may be included as needed. Since O is not an essential element in the steel sheet of this disclosure, the lower limit of O content is 0%. The O content may be 0.001% or more, 0.003% or more, or 0.005% or more. On the other hand, from the viewpoint of suppressing the formation of coarse oxides and ensuring the ductility and formability of the steel sheet, the O content should be 0.01% or less. The O content may be 0.009% or less, 0.008% or less, or 0.007% or less.

[0069] [V : 0~0.150%] V is an element that enhances hardenability and contributes to improved strength. V is an element that may be included as needed and is not an essential element in the steel sheet of this disclosure; therefore, the lower limit of the V content is 0%. The V content may be 0.001% or more, 0.003% or more, 0.005% or more, or 0.008% or more. On the other hand, from the viewpoint of ensuring sufficient toughness, the V content shall be 0.150% or less. The V content may be 0.120% or less, 0.100% or less, 0.080% or less, 0.050% or less, or 0.030% or less.

[0070] [Cr: 0~2.00%] Cr is an effective element for increasing the hardenability and thus the strength of steel. Cr is an element that may be included as needed, and is not an essential element in the steel sheets of this disclosure; therefore, the lower limit of Cr content is 0%. The Cr content may be 0.001% or more, 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, if Cr is included in excess, a large amount of Cr carbides may be formed, impairing the hardenability; therefore, the Cr content should be 2.00% or less. The Cr content may be 1.80% or less, 1.50% or less, 1.20% or less, 1.00% or less, 0.70% or less, 0.50% or less, or 0.25% or less.

[0071] [Ni: 0~2.00%] Ni is an effective element for increasing the hardenability and thus the strength of steel. Ni is an element that may be included as needed and is not an essential element in the steel sheets of this disclosure; therefore, the lower limit of the Ni content is 0%. The Ni content may be 0.001% or more, 0.01% or more, 0.02% or more, or 0.05% or more. On the other hand, excessive addition of Ni increases costs, so the Ni content should be 2.00% or less. The Ni content may be 1.80% or less, 1.50% or less, 1.20% or less, 1.00% or less, 0.70% or less, 0.50% or less, or 0.25% or less.

[0072] [Cu: 0~2.00%] Cu is an effective element for increasing the hardenability and strength of steel. Cu is an element that may be included as needed and is not an essential element in the steel sheets of this disclosure; therefore, the lower limit of Cu content is 0%. The Cu content may be 0.001% or more, or 0.01% or more. On the other hand, in order to suppress the reduction of toughness and cracking of the slab after casting, the Cu content should be 2.00% or less. The Cu content may be 1.80% or less, 1.50% or less, 1.20% or less, 1.00% or less, 0.70% or less, 0.50% or less, or 0.25% or less.

[0073] [W : 0~1.00%] W is an element that is effective in increasing the hardenability and thus the strength of steel. W is an element that may be included as needed and is not an essential element in the steel sheet of this disclosure; therefore, the lower limit of the W content is 0%. The W content may be 0.001% or more, 0.005% or more, or 0.01% or more. On the other hand, in order to suppress the decrease in toughness, the W content should be 1.00% or less. The W content may be 0.80% or less, 0.50% or less, 0.30% or less, 0.10% or less, 0.05% or less, 0.02% or less, or 0.01% or less.

[0074] [Ca: 0~0.100%] Ca is an element that enhances toughness by controlling the morphology of nonmetallic inclusions, particularly by finely dispersing nonmetallic inclusions. Ca is an element that may be included as needed and is not an essential element in the steel sheet of this disclosure; therefore, the lower limit of Ca content is 0%. The Ca content may be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, excessive Ca content may lead to deterioration of surface properties; therefore, the Ca content should be 0.100% or less. The Ca content may be 0.080% or less, 0.050% or less, 0.030% or less, 0.010% or less, 0.006% or less, or 0.003% or less.

[0075] [Mg: 0~0.100%] Mg is an element that enhances toughness by contributing to the control of the morphology of nonmetallic inclusions, particularly the fine dispersion of nonmetallic inclusions. Mg is an element that may be included as needed and is not an essential element in the steel sheet of this disclosure; therefore, the lower limit of the Mg content is 0%. The Mg content may be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, if Mg is included in excess, deterioration of surface properties may become apparent, so the Mg content should be 0.100% or less. The Mg content may be 0.080% or less, 0.050% or less, 0.030% or less, 0.010% or less, 0.006% or less, or 0.003% or less.

[0076] [Zr:0~0.500%] Zr is an element that enhances toughness by contributing to the control of the morphology of nonmetallic inclusions, particularly the fine dispersion of nonmetallic inclusions. Zr is an element that may be included as needed and is not an essential element in the steel sheet of this disclosure; therefore, the lower limit of the Zr content is 0%. The Zr content may be 0.001% or more, 0.003% or more, or 0.005% or more. On the other hand, if the Zr content is excessive, deterioration of the surface properties may become apparent, so the Zr content should be 0.500% or less. The Zr content may be 0.450% or less, 0.400% or less, 0.350% or less, 0.300% or less, 0.200% or less, 0.100% or less, 0.050% or less, or 0.006% or less.

[0077] [Hf: 0~0.100%] Hf is an element that enhances toughness by contributing to the control of the morphology of nonmetallic inclusions, particularly the fine dispersion of nonmetallic inclusions. Hf is an element that may be included as needed and is not an essential element in the steel sheet of this disclosure; therefore, the lower limit of the Hf content is 0%. The Hf content may be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, if Hf is included in excess, deterioration of surface properties may become apparent, so the Hf content should be 0.100% or less. The Hf content may be 0.080% or less, 0.050% or less, 0.030% or less, 0.010% or less, 0.005% or less, or 0.002% or less.

[0078] [Sn: 0~0.100%] [As: 0~0.100%] Sn and As are elements that are effective in improving corrosion resistance. Sn and As are elements that may be included as needed and are not essential elements in the steel sheet of this disclosure; therefore, the lower limit of the Sn and As content is 0% each. The Sn and As content may be 0.0001% or more, 0.0005% or more, and 0.001% or more, respectively. On the other hand, if Sn and As are included in excess, the above effects will saturate, and including them in the steel sheet more than necessary will lead to an increase in manufacturing costs. Therefore, the Sn and As content should be 0.100% or less each. The Sn and As content may be 0.090% or less, 0.080% or less, 0.070% or less, 0.060% or less, 0.030% or less, 0.010% or less, and 0.002% or less, respectively.

[0079] [REM: 0~0.100%] Rare earth elements (REMs) are elements that contribute to controlling the morphology of nonmetallic inclusions, particularly to the fine dispersion of nonmetallic inclusions, thereby enhancing toughness. REMs are elements that may be included as needed and are not essential elements in the steel sheets of this disclosure; therefore, the lower limit of REM content is 0%. The REM content may be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, excessive REM content may lead to deterioration of surface properties; therefore, the REM content should be 0.100% or less. The REM content may be 0.080% or less, 0.050% or less, 0.030% or less, 0.010% or less, 0.005% or less, or 0.002% or less. REM stands for Rare Earth Metal, and refers to a total of 17 elements, consisting of scandium (Sc) and yttrium (Y), plus 15 other elements from lanthanum (La) to lutetium (Lu).

[0080] In the steel sheet of this disclosure, the remainder of the elements other than those mentioned above consists of Fe and impurities. Here, impurities are components that are inevitably mixed in during the industrial manufacture of steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap.

[0081] [Total Nb and Mo content: 0.010% or more] Furthermore, as described above, the chemical composition of the steel sheet in this disclosure must have a total content of 0.010% or more of Nb and Mo. When the total content of Nb and Mo is 0.010% or more, Nb and / or Mo diffuse into the grain boundaries along with B after the grain boundaries are formed during the cooling process after welding, causing grain boundary segregation, and further attracting B to the grain boundaries, thereby causing more cosegregation of B. To obtain this effect more reliably, the total content of Nb and Mo may be 0.020% or more, 0.040% or more, 0.060% or more, 0.080% or more, or 0.100% or more.

[0082] Furthermore, regarding optional components, the chemical composition of the steel sheet is expressed in mass percent. Nb: 0.0001~0.150%, Mo: 0.0001~1.000%, O: 0.001~0.01%, V: 0.001~0.150%, Cr: 0.001~2.00%, Ni: 0.001~2.00%, Cu: 0.001~2.00%, W: 0.001~1.00%, Ca: 0.0001~0.100%, Mg: 0.0001~0.100%, Zr: 0.001~0.500%, Hf: 0.0001~0.100%, Sn: 0.0001~0.100%, As: 0.0001~0.100%, and REM: 0.0001~0.100%, It may contain one or more of the following. Furthermore, the chemical composition of the steel plate may contain B: 0.0010 to 0.0100% by mass.

[0083] The chemical composition of steel sheets can be measured using general analytical methods. For example, the chemical composition of steel sheets can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) in accordance with JIS G 1201:2022. Specifically, a 35mm square specimen (for example, a 35mm square specimen with a sample thickness of 3 / 4 of the steel sheet thickness) is obtained, machined so that the analysis surface is located 1 / 4 of the sheet thickness away from the surface of the steel sheet and parallel to the surface of the steel sheet. The chemical composition can then be determined by measuring it using a Shimadzu ICPS-8100 or similar (measuring device) under conditions based on a pre-established calibration curve. C and S, which cannot be measured by ICP-AES, can be measured using the combustion-infrared absorption method, and N can be measured using the inert gas fusion-thermal conductivity method. Sol.Al can be measured by atomic absorption spectrometry in accordance with JIS G1257-10-2:2013. Furthermore, if analysis values ​​for molten steel, slabs, or other steel sheets manufactured from the same molten steel are available, the analysis of test pieces taken from the steel sheet may be omitted, and those analysis values ​​may be considered as the chemical composition of the steel sheet.

[0084] The thickness of the steel plate is not particularly limited, but generally it is between 0.2 and 8.0 mm. For example, the thickness may be 0.3 mm or more, 0.6 mm or more, 1.0 mm or more, 1.6 mm or more, or 2.0 mm or more. Similarly, the thickness of the steel plate may be 7.0 mm or less, 6.0 mm or less, 5.0 mm or less, or 4.0 mm or less.

[0085] [Vickers hardness] In the steel plate of this embodiment, the strength of the steel plate is not particularly limited, but for example, the Vickers hardness of the steel plate at a depth of 1 / 4 of the plate thickness may be 300 Hv or more. The Vickers hardness of the steel plate at a depth of 1 / 4 of the plate thickness may be 360 ​​Hv or more, 460 Hv or more, or 550 Hv or more. The upper limit of the Vickers hardness is not particularly limited, but for example, the Vickers hardness of the steel plate at a depth of 1 / 4 of the plate thickness may be 950 Hv or less, 900 Hv or less, 850 Hv or less, or 800 Hv or less. The Vickers hardness can be controlled within the above ranges by appropriately adjusting the chemical composition of the steel plate.

[0086] The Vickers hardness of the steel plate is determined in accordance with JIS Z 2244-1:2024 as follows. First, a test specimen is cut from any position on the steel plate, excluding the edges, so that a cross-section perpendicular to the surface (a cross-section along the thickness direction) can be observed. The thickness cross-section of the cut test specimen is polished using #600 to #1500 silicon carbide sandpaper. Next, the thickness cross-section of the test specimen is polished to a mirror finish using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water, and this thickness cross-section is used as the measurement surface. Then, the Vickers hardness (HV1) of the test specimen is measured using a Vickers hardness tester with a test force of 9.807 N at intervals of at least three times the diagonal length of the indentation. Specifically, 20 measurements are taken at 1 mm intervals at a depth of 1 / 4 of the thickness of the test specimen, and the arithmetic mean of these measurements is adopted as the Vickers hardness of the steel plate.

[0087] (Plating layer) Furthermore, the steel sheet of this embodiment may or may not have a plating layer on its surface for reasons described later. In other words, the steel sheet of this embodiment may be a plated steel sheet or an unplated steel sheet.

[0088] If the steel sheet in this embodiment is a plated steel sheet, the plating layer may be formed on only one side of the steel sheet surface, or on both sides. It may also be formed on only a portion of the sheet surface. The plating layer may also be an alloyed layer.

[0089] The chemical composition of the plating layer is not particularly limited, but examples include zinc-based plating layers. Examples of zinc-based plating layers include Zn-0.2%Al(GI), Zn-(0.3~1.5)%Al, Zn-4.5%Al, Zn-0.09%Al-10%Fe(GA), Zn-1.5%Al-1.5%Mg, Zn-11%Al-3%Mg-0.2%Si, Zn-11%Ni, Zn-15%Mg, as well as electro-zinc plating (EG).

[0090] The chemical composition of the plating layer can be determined by dissolving the plating layer in an acidic solution to which an inhibitor that suppresses corrosion of the steel sheet has been added, and then measuring the resulting solution by ICP (inductively coupled plasma) emission spectroscopy. As an acidic solution to which an inhibitor has been added to dissolve the plating layer, for example, a 10% hydrochloric acid solution to which 0.06% by mass of inhibitor (manufactured by Asahi Chemical Industries, Ltd., Ibit 710K) has been added can be used.

[0091] The thickness of the plating layer is not particularly limited, but for example, the thickness of the plating layer on one side may be 3 to 50 μm. Also, the amount of plating layer attached is not particularly limited, but for example, 10 to 170 g / m² per side. 2 This may be the case. The amount of plating layer can be determined by dissolving the plating layer in an acid solution containing an inhibitor that suppresses corrosion of the steel plate, and measuring the change in mass before and after dissolution of the plating layer.

[0092] Furthermore, the steel sheet of this embodiment can exhibit the effect of improved LME resistance even if it does not have a zinc-based plating layer. Generally, when spot welding two steel sheets that are not zinc-based, LME cracking will not occur unless contact with molten zinc occurs near the spot weld. However, when spot welding a zinc-plated steel sheet to an unplated steel sheet or a steel sheet with a plating other than zinc, molten zinc is generated on the overlapping surface of the steel sheets during welding, and this molten zinc may come into contact with the surface of the unplated steel sheet or the steel sheet with a plating other than zinc, potentially causing LME cracking. Therefore, the steel sheet of this embodiment can be suitably applied not only to zinc-plated steel sheets but also to unplated steel sheets and steel sheets with plating other than zinc that are welded to zinc-plated steel sheets.

[0093] <Parts> As described above, the steel sheet of this embodiment is a steel sheet with excellent resistance to LME (Luminous Meteorological Emissions). Therefore, the steel sheet of this embodiment is useful as a raw material for parts that require excellent resistance to LME. In particular, the steel sheet of this embodiment is useful as a raw material for parts in the automotive sector.

[0094] One embodiment of this disclosure is a part containing the steel plate of the above embodiment. Furthermore, examples of parts include automobile parts. Specific examples of automobile parts include the frame parts and bumpers of automobiles, as well as other structural and reinforcing parts that require strength. Further specific examples of automobile parts include exterior parts such as roofs, hoods, fenders, and doors, which require high aesthetic appeal. These parts only need to contain the steel plate of the above embodiment in at least a portion of the part. Therefore, these parts have the characteristics of the steel plate of the above embodiment in at least a portion of the part. It should be noted that in parts of the steel plate that do not directly come into contact with the mold during forming such as press forming, or that, even if they come into direct contact with the mold, undergo relatively little processing, the characteristics of the steel plate do not change particularly before and after forming.

[0095] Furthermore, when taking samples from automotive parts for various measurements and analyses, the following locations (i) to (iv) should be avoided. (i) Welds: within 20 mm from the toe of spot welds, and within 20 mm from the toe of arc / laser welds. (ii) Machining area: Machining area with a radius of curvature of less than 15 mm, and areas within 5 mm of the above-mentioned machining area. (iii) Ends: Ends within 5 mm of the cut end face of the part (iv) Red rust: Within 5 mm of the area where red rust is visible to the naked eye.

[0096] When taking samples from a coil for various measurements and analyses, the outermost part may have a changed surface condition. Therefore, samples should be taken from the third turn onwards from the outside of the coil, avoiding the end 100 mm away.

[0097] <Method of manufacturing steel plates> Next, a preferred method for manufacturing the steel sheet according to one embodiment of this disclosure will be described. The following description is intended to illustrate characteristic methods for manufacturing the steel sheet of this embodiment and is not intended to limit the steel sheet to those manufactured by the manufacturing methods described below.

[0098] The steel sheet of this embodiment can be manufactured by a manufacturing method that includes a casting step of casting molten steel with an adjusted chemical composition to form a slab, a hot rolling step of hot rolling the slab to obtain a hot-rolled steel sheet, a pickling step of pickling the hot-rolled steel sheet, a brush grinding step of brush grinding the surface of the hot-rolled steel sheet after pickling, a cold rolling step of cold rolling the hot-rolled steel sheet after brush grinding to obtain a cold-rolled steel sheet, and an annealing step of annealing the cold-rolled steel sheet under specific conditions. Optionally, a plating step of applying a plating treatment to the surface of the steel sheet after the annealing step may also be performed.

[0099] The following will explain in detail the preferred conditions for these processes.

[0100] [Casting Process] In the steel plate manufacturing method of this embodiment, the casting process is a process of forming a slab by casting molten steel with an adjusted chemical composition. The conditions of the casting process are not particularly limited. For example, the casting process may involve melting in a blast furnace or electric furnace, followed by various secondary smelting processes, and then casting using methods such as conventional continuous casting or ingot casting.

[0101] [Hot rolling process] The hot rolling process is a process of obtaining hot-rolled steel sheets by hot-rolling a slab. The hot rolling process is carried out by hot-rolling the cast slab either directly or after it has been cooled and then reheated. When reheating is performed, the heating temperature of the slab may be, for example, 1100 to 1250°C. In the hot rolling process, rough rolling and finish rolling are usually performed. The temperature and reduction ratio of each rolling can be appropriately determined according to the desired metal structure and sheet thickness. For example, the finishing temperature of the finish rolling may be 900 to 1050°C, and the reduction ratio of the finish rolling may be 10 to 50%.

[0102] The hot-rolled steel sheet, after finish rolling, is wound at a predetermined winding temperature and subjected to the next pickling process. The winding of the hot-rolled steel sheet is carried out at a winding temperature of 500°C or higher. The winding temperature may be 520°C or higher or 550°C or higher. The winding temperature may be 600°C or lower or 580°C or lower.

[0103] [Pickling process] The pickling process is a process of pickling hot-rolled steel sheets after the hot-rolling process. In the pickling process, the hot-rolled steel sheets are pickled to remove surface oxides and other contaminants. The conditions for the pickling process are not particularly limited, and it is sufficient to carry it out under conditions appropriate for removing surface oxides and other contaminants using a commonly used pickling solution, such as a hydrochloric acid solution of a predetermined concentration containing an inhibitor that suppresses corrosion of the steel sheet. Pickling may be carried out in one step, or it may be carried out in multiple steps to ensure that surface oxides and other contaminants are completely removed.

[0104] [Brush grinding process] The brush grinding process involves brush grinding the surface of the hot-rolled steel sheet after pickling. By brush grinding the surface of the hot-rolled steel sheet, specific strains can be applied to the surface of the steel sheet, and the Nb and Mo-deficient layer on the surface of the steel sheet that occurred during the winding of the hot-rolled steel sheet in the hot-rolling process can be removed.

[0105] In the brush grinding process, the hot-rolled steel sheet, after hot-rolling and pickling, is ground down at a rate of 5g / m². 2 Brush grinding is performed under the above conditions. The amount of material removed during brush grinding is preferably as high as possible, for example, 8 g / m², in order to more reliably impart specific strain to the steel plate surface and to more reliably remove the Nb and Mo deficient layer on the surface of the steel plate. 2 It is more preferable that the amount be greater than or equal to 10 g / m 2 It is even more preferable that the above conditions are met. The upper limit of the amount of material removed by brush grinding is not particularly limited, but for example, 20 g / m 2 The following is true: 15g / m 2 The following is also acceptable.

[0106] Furthermore, the amount of material removed during brush grinding can be adjusted by any suitable method known to those skilled in the art. For example, the amount of material removed during brush grinding can be adjusted by appropriately selecting the type of brush (e.g., Hotani H115), wire material, bristle length, rotational speed, density, brush reduction amount, and the coating liquid used.

[0107] [Cold rolling process] The cold rolling process is a process in which hot-rolled steel sheets, after brush grinding, are cold-rolled to obtain cold-rolled steel sheets. The reduction ratio in cold rolling can be appropriately determined according to the desired metal structure and sheet thickness, and may be, for example, 20 to 80%. After the cold rolling process, the sheet may be cooled to room temperature by air cooling, for example.

[0108] [Annealing process] The annealing process is a process of annealing cold-rolled steel sheets under specific conditions. In the annealing process, by applying annealing under specific conditions to cold-rolled steel sheets that have been subjected to specific strains by the aforementioned brush grinding, B, Nb, and / or Mo can be concentrated in the surface layer of the steel sheet.

[0109] The specific conditions for the annealing process are as follows: First, the steel sheet is heated from room temperature to a control temperature of 350 to 550°C in an atmosphere with a dew point of -50°C to -30°C. Next, the steel sheet is heated from the above control temperature to a holding temperature of 720 to 900°C in an atmosphere with a dew point of greater than -20°C to 20°C, and held at this holding temperature for 0 to 360 seconds.

[0110] When a steel sheet with a specific strain applied to its surface is annealed under such specific conditions, internal Si-Mn oxides are rapidly formed, and B is incorporated into these internal oxides. This suppresses the deboronization of the steel sheet surface, allowing for the concentration of B, Nb, and / or Mo on the steel sheet surface. Furthermore, by undergoing such a specific annealing process, the emission intensity of B and the emission intensities of Nb and Mo, as measured by GDS analysis, satisfy the relationships given by equations (1) and (2) above, respectively.

[0111] In the annealing process, the control temperature is preferably 500 to 550°C. Furthermore, the dew point when raising the temperature from the control temperature to the holding temperature is preferably -10°C to 20°C, and more preferably -5°C to 20°C. In addition, the holding time is preferably 40 to 160 seconds.

[0112] As described above, the steel sheet of this embodiment has a total Nb content and Mo content of 0.020% or more, and the thickness of the B-enriched portion is t B It is particularly preferable that the above formula (3) is satisfied and the emission intensities of Nb and Mo measured by GDS analysis are satisfied by formula (4). Steel sheets that satisfy these characteristics are manufactured by adopting all of the following conditions (i) to (iii). (i) The sum of the Nb and Mo content in the chemical composition shall be 0.020% or more. (ii) The amount of material removed during brush grinding is 10 g / m 2 That concludes this section. (iii) The dew point when raising the temperature from the control temperature to the holding temperature in the annealing process shall be -10°C to 20°C.

[0113] Similarly, it is particularly preferable that the steel sheet of this embodiment has a total Nb content and Mo content of 0.040% or more, and that the luminescence intensity of Nb and Mo measured by GDS analysis satisfies formula (5) above. Steel sheets that satisfy these characteristics are manufactured by adopting all of the following conditions (i) to (iv). (i) The sum of the Nb and Mo content in the chemical composition shall be 0.040% or more. (ii) The amount of material removed during brush grinding is 10 g / m 2 That concludes this section. (iii) The dew point when raising the temperature from the control temperature to the holding temperature in the annealing process shall be -5°C to 20°C. (iv) The holding time in the annealing process shall be 40 seconds to 160 seconds.

[0114] The atmosphere during the annealing process is preferably a reducing atmosphere. More specifically, a reducing atmosphere containing nitrogen and hydrogen is preferred. Examples of a reducing atmosphere include a reducing atmosphere with 1 to 10% hydrogen by volume (for example, 2% hydrogen by volume and the remainder being nitrogen).

[0115] The basic steps for manufacturing the steel sheet of this embodiment are as described above. The steel sheet of this embodiment, manufactured by the manufacturing method including the above steps, has a specific chemical composition, contains a B-enriched portion satisfying formula (1) in the surface layer of the steel sheet, and the emission intensity of Nb and Mo measured by GDS analysis satisfies formula (2) above, thus possessing a characteristic configuration. In other words, the steel sheet of this embodiment has a sufficient amount of B enriched in the surface layer of the steel sheet to suppress the penetration of Zn into the grain boundaries, and a certain amount of Nb and / or Mo coexist in the surface layer of the steel sheet. As a result, the steel sheet of this embodiment can exhibit excellent LME resistance.

[0116] Furthermore, the steel sheet manufacturing method of this embodiment may include any additional processing steps that are commonly performed in steel sheet manufacturing, in addition to the steps described above. For example, the steel sheet manufacturing method of this embodiment may include a plating step in which a plating treatment is applied to the surface of the annealed steel sheet.

[0117] [Plating process] The plating process involves applying a plating treatment to the surface of the annealed steel sheet. The plating treatment may be carried out according to methods known to those skilled in the art, for example, by hot-dip plating or by electroplating. Preferably, the plating treatment is carried out by hot-dip plating. The conditions for the plating treatment should be set appropriately considering the desired chemical composition, thickness, and amount of adhesion of the plating layer. After the plating treatment, a known alloying treatment may be performed to produce alloyed plating. The type and chemical composition of the plating layer are as described above.

[0118] As described above, the steel sheet of this disclosure has high resistance to LME (Long-Lasting Mechanism) cracking and can therefore be suitably used in a wide range of fields such as automobiles, home appliances, and building materials. It can be suitably used in the automotive field in particular. Steel sheets used in automobiles are often spot-welded, which makes them prone to LME cracking. For this reason, the steel sheet of this disclosure can be particularly suitably used in automobile parts.

[0119] This disclosure is not limited to the embodiments described above or the following examples, and can be combined, substituted, or modified as appropriate without departing from the purpose and spirit of this disclosure.

[0120] The present disclosure will be described in more detail below with reference to examples, but these examples are merely examples of the present disclosure and the present disclosure is not limited in any way to these examples. [Examples]

[0121] In the following embodiments, steel sheets according to the embodiments of the present disclosure and steel sheets that serve as comparative examples of the present disclosure were manufactured under various conditions, and the properties of the obtained steel sheets were investigated.

[0122] (Manufacturing of steel plate for Test No. 1) First, molten steel was cast using a continuous casting method to form a slab having the chemical composition shown in Test No. 1 in Table 1 below. After cooling this slab, it was reheated to 1200°C and hot-rolled, then wound up at a winding temperature of 520°C or higher. Hot rolling was performed by rough rolling and finish rolling, with the finishing rolling completion temperature being 900-1050°C and the reduction ratio of the finish rolling being 30%.

[0123] After pickling the obtained hot-rolled steel sheet, the surface of the hot-rolled steel sheet is polished using a grinding brush (H115, manufactured by Hotani Co., Ltd.) at a rate of 5g / m². 2 Brush grinding was performed with the specified grinding amount.

[0124] Next, the hot-rolled steel sheet after brush grinding was cold-rolled at a reduction ratio of 50% to obtain a cold-rolled steel sheet with a thickness of 1.6 mm.

[0125] The obtained cold-rolled steel sheet was then heated from room temperature to a control temperature of 350°C in an atmosphere with a dew point of -30°C. Next, the cold-rolled steel sheet was annealed by heating from the control temperature to a holding temperature of 800°C in an atmosphere with a dew point of -10°C. In this annealing process, the sheet was not held at the holding temperature, so the holding time was 0 seconds.

[0126] In this way, we obtained the steel plate for Test No. 1, which is the example of this disclosure.

[0127] (Manufacturing of steel plates for Test No. 2) The chemical composition was changed to that shown in Test No. 2 in Table 1 below. The brush grinding and annealing conditions were changed to those shown in Test No. 2 in Table 2 below. Furthermore, plating treatment was performed. Other conditions were the same as for the steel sheet in Test No. 1 to obtain the plated steel sheet of Test No. 2, which is the present disclosure example.

[0128] In the plating process for steel plates in Test No. 2, the annealed steel plates were immersed in a 450°C molten zinc plating bath for 3 seconds, then withdrawn at 100 mm / second, and the plating amount was reduced to 50 g / m² using N2 wiping gas. 2 The material was then adjusted to the specified temperature. Subsequently, an alloying treatment was performed at 520°C for 30 seconds to obtain alloyed hot-dip galvanized steel sheet (Zn-0.09%Al-10%Fe(GA)).

[0129] (Manufacturing of steel plates for tests No. 3-32, 40, and 42) The chemical composition was changed to that shown in Table 1 below. The brush grinding amount, annealing conditions, and plating type were changed to those shown in Table 2 below. Other conditions were the same as in Test No. 2 to obtain plated steel sheets for Tests No. 3-32, 40, and 42, which are examples of this disclosure.

[0130] The plating types in Table 2 are as follows: a: Unplated (i.e., not plated) b: Alloyed hot-dip galvanizing (Zn-0.09%Al-10%Fe(GA)) c: Hot-dip galvanizing (Zn-0.2%Al(GI))

[0131] (Manufacturing of steel plates for tests No. 33-39 and 41) The chemical composition was changed to that shown in Table 1 below. The brush grinding amount and annealing conditions were changed to those shown in Table 2 below. Other conditions were the same as in Test No. 1 to obtain comparative steel plates for Tests No. 33-39 and 41.

[0132] The chemical compositions of each steel sheet from Test No. 1 to 42 shown in Table 1 are based on molten steel analysis values.

[0133] [Table 1]

[0134] For each of the steel sheets obtained in Tests No. 1 to 42 as described above, GDS analysis of B, Nb, and Mo in the surface layer and bulk of the steel sheet, as well as various Vickers hardness measurements, were performed. Furthermore, the LME resistance of each steel sheet in Tests No. 1 to 42 was evaluated according to the evaluation method described below. The measurement results and evaluation results are shown in Table 2 below. Here, regarding Table 2, the thickness t of the B-enriched region B If the thickness is less than 5.0 μm, the B-enriched areas are all located on the surface of the steel plate, and the thickness of the B-enriched areas is t B If the thickness was 5.0 μm or greater, the entire surface layer of the steel plate was a B-enriched area.

[0135] The underlines next to the various values ​​in Tables 1 and 2 below indicate that they are outside the scope of this disclosure or that the manufacturing conditions are such that the steel sheets described herein cannot be obtained.

[0136] <Evaluation of LME resistance> A 50mm x 100mm sample was taken from each steel plate to be evaluated by cutting. Separately, a mating steel plate of the same size as this sample was prepared. The evaluation sample and the mating steel plate were overlapped to form a plate assembly, and spot welding was performed on this assembly using a dome radius type welding electrode with a tip diameter of 8mm under the conditions of a striking angle of 5°, a pressing force of 4.0kN, an energizing time of 1.6 seconds, and an energizing current of 13kA to create a welded joint.

[0137] The mating steel plates used are as shown in Table 2. The types of "matting steel plates" in Table 2 are as follows: "Same type": Steel plates of the same type as the steel plate being evaluated. "GA": A steel plate of the same type as the steel plate being evaluated, which has been treated with alloyed zinc plating.

[0138] Next, the welded joint prepared as described above was cut along the thickness direction, passing through the center of the nugget. The cut surface was then observed, and the length of the crack (LME crack) that occurred directly outside the pressure-welded area (corona bond) of the welded joint was measured. Based on the length of this crack, the LME resistance of each steel plate was determined according to the evaluation criteria below.

[0139] In determining LME resistance, the above-mentioned process from fabricating the welded joint to measuring the crack length was performed three times. Of these three measurements, the longest crack length directly outside the pressure-welded joint was used as the basis for determining LME resistance.

[0140] (Evaluation Criteria) Rating AAA: 0μm Rating AA: Greater than 0 μm, less than 60 μm Rating A: 60 μm or larger, less than 120 μm Rating B: 120 μm or larger

[0141] The evaluation criteria are as follows: a rating of A or higher (i.e., ratings A, AA, and AAA) indicates excellent LME resistance, while a rating of B indicates poor LME resistance. A rating of AAA indicates the best LME resistance.

[0142] Here, "directly outside the pressure-welded portion of the weld" refers to the area outside the pressure-welded portion, which is the part that is pressure-welded by spot welding on the overlapping surface of two steel plates, and specifically the area extending 1 mm outward from the edge of the pressure-welded portion.

[0143] [Table 2]

[0144] The steel sheets No. 1-32, 40, and 42 of the tests are examples of the disclosed material, and as shown in Table 2, all of them exhibited high LME resistance. On the other hand, the steel sheets No. 33-39 and 41, obtained under manufacturing conditions that did not yield the steel sheets of the disclosed material, are comparative examples. Their chemical composition and the results of GDS analysis of the steel sheet surface and bulk are outside the scope of the disclosed material, and all of them exhibited inferior LME resistance.

[0145] In Test No. 33, the combined content of Nb and Mo was low, which likely resulted in insufficient suppression of boron removal on the steel plate surface, as well as lower concentrations of Nb and Mo. Consequently, the LME resistance was likely inferior.

[0146] In Test No. 34, the amount of material removed by brush grinding was insufficient, which likely resulted in insufficient suppression of boron removal from the steel plate surface, as well as lower concentrations of Nb and Mo. Consequently, the LME resistance was likely inferior. The same applies to test No. 41.

[0147] In Test No. 35, the low control temperature during annealing likely led to the consumption of Si in the steel due to the progression of external oxidation at low temperatures, hindering the progression of fine internal oxidation at high temperatures and making it difficult to trap B, Nb, and Mo in the internal oxide layer. As a result, the LME resistance was likely poor.

[0148] In Test No. 36, the high control temperature during annealing likely resulted in the formation of a thick external oxide film on the surface of the steel sheet, hindering internal oxidation and making it difficult to trap B, Nb, and Mo in the internal oxide layer. Consequently, the LME resistance was likely reduced.

[0149] In Test No. 37, the dew point was high when raising the temperature from room temperature to the control temperature during the annealing process. As a result, internal oxidation and boron removal proceeded simultaneously during the heating process from room temperature to the control temperature, and it is thought that boron removal could not be sufficiently suppressed on the surface of the steel sheet. Consequently, the LME resistance was poor.

[0150] In Test No. 38, the dew point was low when the temperature rose from the control temperature to the holding temperature during the annealing process. This resulted in the formation of a thick external oxide film on the surface of the steel sheet, making it difficult for internal oxidation to proceed and hindering the trapping of B, Nb, and Mo in the internal oxide layer. Consequently, the LME resistance was deemed to be poor.

[0151] In Test No. 39, it is believed that the low holding temperature during annealing prevented sufficient concentration of B in the surface layer of the steel sheet. As a result, the LME resistance was likely inferior.

[0152] Here, as a reference example, GDS analysis was performed on some steel plates from the examples described in the prior art documents Patent Document 3 (International Publication No. 2025 / 032898) and Patent Document 5 (International Publication No. 2025 / 032900) (specifically, the steel plates of Examples No. 17, No. 20, and No. 22 in Patent Document 3, and the steel plates of Examples No. 3, No. 13, and No. 18 in Patent Document 5) using the method described herein. The results are shown in Tables 3 and 4 below. Underlines on various numerical values ​​in Tables 3 and 4 indicate that they are outside the scope of this disclosure.

[0153] [Table 3]

[0154] [Table 4]

[0155] As shown in Tables 3 and 4, it was found that the steel sheets described in the above prior art documents do not satisfy the constituent elements of this disclosure, which are formulas (1) and (2) above.

Claims

1. It is a steel plate, The chemical composition of the steel plate is, in mass%, C: 0.05-0.40%, Si: 0.70-3.00%, Mn: 1.00-5.00%, B: 0.0005-0.0100%, Ti: 0.010 to 0.150%, Sol. Al: 0-3.00%, Nb: 0 to 0.150%, Mo: 0-1.000%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0200% or less, O: 0 to 0.01%, V: 0 to 0.150%, Cr: 0-2.00%, Ni: 0-2.00%, Cu: 0-2.00%, W: 0-1.00%, Ca: 0-0.100%, Mg: 0-0.100%, Zr: 0 to 0.500%, Hf: 0-0.100%, Sn: 0-0.100%, As: 0 to 0.100%, REM: 0-0.100%, and Remainder: Contains Fe and impurities, and The combined content of Nb and Mo is 0.010% or more. In glow discharge emission spectroscopy, B-enriched regions, where the B concentration is 10.0 times or more compared to the bulk B concentration, exist in the surface layer of the steel plate, which is the region from the outermost surface of the steel plate to a depth of 5.0 μm in the thickness direction. The thickness t of the B-enriched portion Ｂ However, if the following equation (1) is satisfied, A steel sheet characterized in that the emission intensities of Nb and Mo measured by the glow discharge emission spectroscopy described above satisfy the following equation (2). t Ｂ ≧2.0μm ・・・(1) Isa(Nb+Mo) / Iba(Nb+Mo)≧0.5...(2) Here, Isa(Nb+Mo) is the average value of the sum of the luminescence intensities of Nb and Mo on the surface of the steel plate. Iba(Nb+Mo) is the average value of the sum of the luminescence intensities of bulk Nb and Mo.

2. The chemical composition of the steel plate is, in mass%, Nb: 0.0001 to 0.150%, Mo: 0.0001 to 1.000%, O: 0.001-0.01%, V: 0.001 to 0.150%, Cr: 0.001-2.00%, Ni: 0.001 to 2.00%, Cu: 0.001 to 2.00%, W: 0.001-1.00%, Ca: 0.0001-0.100%, Mg: 0.0001-0.100%, Zr: 0.001 to 0.500%, Hf: 0.0001-0.100%, Sn: 0.0001 to 0.100%, As: 0.0001 to 0.100%, and REM: 0.0001-0.100%, The steel plate according to claim 1, characterized in that it contains one or more of the following.

3. The steel sheet according to claim 1 or 2, characterized in that the chemical composition of the steel sheet contains B: 0.0010 to 0.0100% by mass.

4. The sum of the Nb content and Mo content is 0.020% or more. The thickness t of the B-enriched portion Ｂ However, if the following equation (3) is satisfied, The steel sheet according to claim 1 or 2, characterized in that the emission intensities of Nb and Mo measured by the glow discharge emission spectroscopy analysis satisfy the following formula (4). t Ｂ ≧5.0μm ・・・(3) Isa(Nb+Mo) / Iba(Nb+Mo)≧0.8...(4)

5. The sum of the Nb content and Mo content is 0.040% or more. The steel sheet according to claim 1 or 2, characterized in that the emission intensities of Nb and Mo measured by the glow discharge emission spectroscopy satisfy the following formula (5). Isa(Nb+Mo) / Iba(Nb+Mo)≧1.0...(5)

6. The steel plate according to claim 1 or 2, characterized in that the Vickers hardness at a depth of 1 / 4 of the plate thickness of the steel plate is 300 Hv or more.

7. The steel plate according to claim 1 or 2, characterized in that the Vickers hardness at a depth of 1 / 4 of the plate thickness of the steel plate is 360 Hv or more.

8. The steel plate according to claim 1 or 2, characterized in that the Vickers hardness at a depth of 1 / 4 of the plate thickness of the steel plate is 460 Hv or more.

9. The steel plate according to claim 1 or 2, characterized in that the Vickers hardness at a depth of 1 / 4 of the plate thickness of the steel plate is 550 Hv or more.

10. A component comprising the steel plate described in claim 1 or 2.

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