Steel plates and parts containing them

Abstract

The present disclosure provides a steel sheet with excellent LME resistance and plating properties by virtue of a novel configuration. The steel sheet of the present disclosure has a specific chemical composition, and the emission intensities of B, Nb, Mo, and O measured by GDS analysis satisfy the following formulas (1) to (3). Isi(B) / Iba(B)≧0.5 ···(1) Isx(Nb + Mo) / Iba(Nb + Mo)≧0.6 ···(2) Isa(O) / Iba(O)≦20 ···(3) Isi(B) is the minimum value of the emission intensity of B on the surface layer of the steel sheet, Iba(B) is the average value of the emission intensity of B in the bulk, Isx(Nb + Mo) is the maximum value of the sum of the emission intensities of Nb and Mo on the surface layer of the steel sheet, Iba(Nb + Mo) is the average value of the sum of the emission intensities of Nb and Mo in the bulk, Isa(O) is the average value of the emission intensity of O on the surface layer of the steel sheet, Iba(O) is the average value of the emission intensity of O 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 project]

Problems to be Solved by the Invention

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

[0011] On the other hand, in the production of high-strength steel sheets, generally, heat treatment such as annealing is performed after rolling. Among the elements typically contained in high-strength steel sheets, easily oxidizable elements such as Si and Mn may combine with oxygen in the atmosphere during the above heat treatment to form a layer containing oxides near the surface of the steel sheet. As forms of such a layer, there are a form in which oxides containing elements such as Si and Mn are formed as a film on the outside (surface) of the steel sheet (external oxide layer), and a form in which oxides are formed inside the steel sheet (surface layer) (internal oxide layer).

[0012] And in the steel sheet in which the external oxide layer is formed, when forming a plating layer (for example, a zinc-based plating layer, etc.) on the surface of the steel sheet, due to the presence of oxides as a film on the surface of the steel sheet, the mutual diffusion between the steel component (for example, Fe) and the plating component (for example, Zn) is inhibited, the adhesion between the steel and the plating decreases, and the plating property may become insufficient (that is, the non-plated part increases).

[0013] The present disclosure has been made in view of the above circumstances, and an object thereof is to provide a steel sheet excellent in LME resistance and plating property, and a component including the same, with a novel configuration.

Means for Solving the Problems

[0014] The present disclosure includes at least the following aspects.

[0015] (Aspect 1) A steel sheet, The chemical composition of the above steel sheet is in mass %, C: 0.05 to 0.40%, Si: 0.70 to 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%, [[ID=​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​Isx(Nb+Mo) is the maximum value of the sum of the luminescence intensity 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. Isa(O) is the average value of the luminescence intensity of O on the surface of the steel plate. Iba(O) is the average value of the luminescence intensity of bulk oxygen.

[0016] (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.

[0017] (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.

[0018] (Aspect 4) The steel sheet according to any one of embodiments 1 to 3, characterized in that the emission intensity of B measured by the glow discharge emission spectroscopy analysis described above satisfies the following equation (4). Isi(B) / Iba(B)≧0.8 ···(4)

[0019] (Appendix 5) The steel sheet according to any one of embodiments 1 to 4, characterized in that the emission intensity of B measured by the glow discharge emission spectroscopy analysis described above satisfies the following equation (5). Isi(B) / Iba(B)≧1.0 ···(5)

[0020] (Aspect 6) The sum of the above Nb content and Mo content is 0.020% or more. The steel sheet according to any one of embodiments 1 to 5, characterized in that the emission intensities of Nb and Mo measured by the above glow discharge emission spectroscopy satisfy the following formula (6). Isx(Nb+Mo) / Iba(Nb+Mo)≧0.8 ···(6)

[0021] (Aspect 7) 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 6, characterized in that the emission intensities of Nb and Mo measured by the glow discharge emission spectroscopy described above satisfy the following formula (7). Isx(Nb+Mo) / Iba(Nb+Mo)≧1.0 ···(7)

[0022] (Pattern 8) The steel sheet according to any one of embodiments 1 to 7, characterized in that the emission intensity of O measured by the glow discharge emission spectroscopy described above satisfies the following formula (8). Isa(O) / Iba(O)≦5 ···(8)

[0023] (Aspect 9) A 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 300 Hv or more.

[0024] (Aspect 10) The steel plate according to any one of the above embodiments 1 to 9, 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.

[0025] (Aspect 11) The steel plate according to any one of the above embodiments 1 to 10, 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.

[0026] (Aspect 12) The steel plate according to any one of the above embodiments 1 to 11, 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.

[0027] (Aspect 13) A plated steel sheet having a zinc-based plating layer on at least a portion of the steel sheet according to any of the above embodiments 1 to 12.

[0028] (Aspect 14) A component comprising a steel plate as described in any of the above embodiments 1 to 12.

[0029] (Aspect 15) A component comprising the plated steel sheet described in embodiment 13 above. [Effects of the Invention]

[0030] According to this disclosure, it is possible to provide steel sheets with excellent resistance to LME and plating, as well as parts containing the same. [Modes for carrying out the invention]

[0031] 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.

[0032] 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."

[0033] On the other hand, the decrease in plating properties occurs, as described above, when easily oxidizable elements such as Si and Mn contained in high-strength steel sheets combine with oxygen in the atmosphere during heat treatment such as annealing, forming an external oxide layer consisting of oxides of Si and Mn on the outside (surface) of the steel sheet.

[0034] Therefore, the inventors focused on the LME resistance due to segregation of B as described above, and diligently investigated a new method to segregate more B on the surface of the steel sheet (i.e., improve LME resistance) while suppressing the formation of an external oxide layer (i.e., ensuring plating properties).

[0035] As a result, the inventors have discovered a novel method for manufacturing steel sheets, in which, by annealing under specific high dew point conditions during the annealing process, a small amount of Si-Mn internal oxide is formed (i.e., the formation of an external oxide layer is suppressed) to ensure plating properties, and then by annealing under specific low dew point conditions, B is incorporated into the Si-Mn internal oxide, thereby suppressing the deboronization of the steel sheet surface layer. According to this method, B and at least one of Nb (niobium) and Mo (molybdenum), which have a high affinity for B, can coexist on the steel sheet surface layer while suppressing the formation of an external oxide layer.

[0036] The steel sheets obtained by the above method exhibit excellent plating properties because the formation of an external oxide layer is suppressed. Furthermore, the presence of B and at least one of Nb and Mo on the surface of the steel sheet is thought to more effectively suppress LME cracking.

[0037] This disclosure is completed based on the findings described above and includes the following embodiments.

[0038] Preferred embodiments of the steel sheet of this disclosure will be described in detail below.

[0039] <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 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.

[0040] Furthermore, the steel sheet of this embodiment has a distinctive configuration in which the emission intensities of B, Nb, Mo, and O, as measured by glow discharge emission spectroscopy, satisfy the following equations (1) to (3). Isi(B) / Iba(B)≧0.5 (1) Isx(Nb+Mo) / Iba(Nb+Mo)≧0.6 ···(2) Isa(O) / Iba(O)≦20 ··············(3) Here, the steel plate surface layer is the region from the outermost surface of the steel plate to a depth of 5.0 μm in the thickness direction. Isi(B) is the minimum value of the luminescence intensity of B on the surface of the steel plate. Iba(B) is the average value of the luminescence intensity of bulk B. Isx(Nb+Mo) is the maximum value of the sum of the luminescence intensity 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. Isa(O) is the average value of the luminescence intensity of O on the surface of the steel plate. Iba(O) is the average value of the luminescence intensity of bulk oxygen.

[0041] (Effects and Benefits) As described above, the steel sheet of this embodiment has a specific chemical composition, the emission intensity of B measured by GDS analysis satisfies formula (1), and the emission intensities of Nb and Mo measured by GDS analysis satisfy formula (2). In other words, in the steel sheet of this embodiment, a certain amount of B and Nb and / or Mo coexist on the surface layer of the steel sheet. As a result, in the steel sheet of this embodiment, when B diffuses into the grain boundaries and segregates at the grain boundaries after grain boundaries are formed during the cooling process after welding, Nb and Mo, which have a high affinity for B, also diffuse into the grain boundaries and segregate at the grain boundaries. Furthermore, this Nb and Mo attract B, causing more cosegregation of B on the surface layer of the steel sheet. As a result, the steel sheet of this embodiment makes it even less likely for Zn to penetrate into the grain boundaries, and can more effectively suppress LME cracking.

[0042] Furthermore, the emission intensity of oxygen (O) measured by GDS analysis of the steel sheet of this embodiment satisfies formula (3) above. In other words, the formation of an external oxide layer is suppressed to a certain level or lower in the steel sheet of this embodiment. As a result, the interdiffusion between the steel component (e.g., Fe) and the plating component (e.g., Zn) is not easily inhibited in the steel sheet of this embodiment, so high adhesion between the steel and the plating can be ensured, and excellent plating properties can be exhibited.

[0043] Based on the above, the steel sheet of this embodiment can exhibit excellent LME resistance and plating properties.

[0044] The following describes in detail each component of the steel plate in this embodiment.

[0045] [Isi(B) / Iba(B)≧0.5] As described above, the emission intensity of B measured by glow discharge emission spectroscopy (GDS analysis) in the steel sheet of this embodiment satisfies the following equation (1). That is, in the steel sheet of this embodiment, a concentration of B at least 0.5 times the concentration of B in the bulk is present on the surface of the steel sheet. Isi(B) / Iba(B)≧0.5 ···(1) Here, Isi(B) is the minimum value of the luminescence intensity of B on the surface of the steel plate. Iba(B) is the average value of the luminescence intensity of bulk B.

[0046] Furthermore, in steel plates, once a crack caused by LME occurs, it becomes difficult to stop the crack from propagating due to stress concentration at the crack tip. Therefore, the minimum value Isi(B) of the luminescence intensity of B on the surface of the steel plate (i.e., the weakest point), which governs the occurrence of the crack, is important.

[0047] 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 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, such as Iba(B), will be described later.

[0048] 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 is considered to be the bulk luminescence intensity (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).

[0049] 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.

[0050] Regarding the emission intensity of B measured by GDS analysis, the minimum emission intensity of B on the surface of the steel plate, Isi(B), is the minimum emission intensity of B on the surface of the steel plate obtained when GDS analysis is performed from the outermost surface of the steel plate in the thickness direction, and represents the minimum concentration of B on the surface of the steel plate. If the minimum concentration of B is low, it becomes the starting point for LME cracking, and the effect of preventing LME cracking becomes insufficient. On the other hand, the average value Iba(B) of the luminescence intensity of bulk B is the average value of the luminescence intensity of B in the bulk region obtained when GDS analysis is performed from the outermost surface of the steel plate in the thickness direction, and represents the concentration of B in the bulk region.

[0051] Therefore, equation (1) above is equivalent to saying that the minimum concentration of B on the surface of the steel plate is 0.5 times or more the concentration of B in the bulk, meaning that at least 0.5 times the concentration of B in the bulk is present on the surface of the steel plate.

[0052] In this embodiment, the emission intensity of B measured by GDS analysis preferably satisfies the following formula (4), and more preferably satisfies the following formula (5), in order to more reliably suppress LME cracking. Isi(B) / Iba(B)≧0.8 ···(4) Isi(B) / Iba(B)≧1.0 ···(5)

[0053] [Isx(Nb+Mo) / Iba(Nb+Mo)≧0.6] 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, the maximum concentration of Nb and / or Mo, which is 0.6 times or more the sum of the concentrations of Nb and Mo in the bulk, is present on the surface layer of the steel sheet together with the aforementioned B. Isx(Nb+Mo) / Iba(Nb+Mo)≧0.6 ···(2) Here, Isx(Nb+Mo) is the maximum value of the sum of the luminescence intensity 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.

[0054] Regarding the emission intensities of Nb and Mo measured by GDS analysis, the maximum value Isx(Nb+Mo), which is the sum of the emission intensities of Nb and Mo on the surface of the steel sheet, is the maximum value of the sum of the emission intensities of Nb and Mo on the surface of the steel sheet obtained when GDS analysis is performed in the thickness direction from the outermost surface of the steel sheet, and represents the maximum total concentration of Nb and Mo on the surface of the steel sheet. If the maximum concentrations of Nb and Mo are low, it becomes the starting point for LME cracking, and the effect of preventing LME cracking becomes insufficient. Furthermore, since the intrusion of Zn into the γ grain boundary leading to the occurrence of LME cracking stops at the position where the amount of grain boundary segregation of B is high and where Nb and Mo are most concentrated, the maximum value Isx(Nb+Mo), which is the sum of the emission intensities of Nb and Mo on the surface of the steel sheet, is important.

[0055] On the other hand, the average value Iba(Nb+Mo), which is the sum of the emission intensities of bulk Nb and Mo, is the average value of the sum of the emission intensities of Nb and Mo in the bulk region obtained when GDS analysis is performed from the outermost surface of the steel sheet in the thickness direction, and represents the total concentration of Nb and Mo in the bulk region.

[0056] Therefore, equation (2) above is equivalent to saying that the maximum sum of the concentrations of Nb and Mo on the surface of the steel plate is 0.6 times or more the average sum of the concentrations of Nb and Mo in the bulk. In other words, it means that the maximum concentration of Nb and / or Mo that is 0.6 times or more the average sum of the concentrations of Nb and Mo in the bulk is present on the surface of the steel plate. 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 sum of the concentrations of Nb and Mo will be the concentration of only one of Nb or Mo.

[0057] 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 maximum sum of the concentrations of Nb and Mo on the surface of the steel sheet is less than 0.6 times the average sum of the concentrations of Nb and Mo in the bulk.

[0058] In this embodiment, the emission intensities of Nb and Mo measured by GDS analysis preferably satisfy the following formula (6), and more preferably satisfy the following formula (7), in order to more reliably suppress LME cracking. Isx(Nb+Mo) / Iba(Nb+Mo)≧0.8 ···(6) Isx(Nb+Mo) / Iba(Nb+Mo)≧1.0 ···(7)

[0059] Furthermore, in this embodiment, in order to more reliably suppress LME cracking, it is particularly preferable that the total content of Nb and Mo is 0.020% or more, and that the emission intensity of Nb and Mo measured by GDS analysis satisfies formula (6) above. A method for manufacturing steel sheets that satisfy these characteristics will be described later.

[0060] Similarly, in this embodiment, in order to more reliably suppress LME cracking, it is particularly preferable that the total content of Nb and Mo is 0.040% or more, and that the emission intensity of Nb and Mo measured by GDS analysis satisfies formula (7) above. A method for manufacturing steel sheets that satisfy these characteristics will be described later.

[0061] [Isa(O) / Iba(O)≦20] As described above, the emission intensity of O measured by GDS analysis of the steel sheet of this embodiment satisfies the following equation (3). That is, the average concentration of O present on the surface of the steel sheet of this embodiment is 20 times or less than the concentration of O in the bulk. Isa(O) / Iba(O)≦20 ···(3) Here, Isa(O) is the average value of the luminescence intensity of O on the surface of the steel plate. Iba(O) is the average value of the luminescence intensity of bulk oxygen.

[0062] Regarding the emission intensity of oxygen measured by GDS analysis, the average value Isa(O) of the emission intensity of oxygen on the surface of the steel plate is the average value of the emission intensity of oxygen on the surface of the steel plate obtained when GDS analysis is performed from the outermost surface of the steel plate in the thickness direction, and represents the average concentration of oxygen on the surface of the steel plate. On the other hand, the average value of the emission intensity of bulk oxygen, Iba(O), is the average value of the emission intensity of oxygen in the bulk region obtained when GDS analysis is performed from the outermost surface of the steel plate in the thickness direction, and represents the concentration of oxygen in the bulk region.

[0063] In other words, the average value Isa(O) of the luminescence intensity of oxygen on the surface of the steel sheet is an indicator of the degree of external oxide layer formation, and the luminescence intensity of oxygen in the bulk is an indicator of the degree of internal oxide layer formation. Therefore, equation (3) above is equivalent to saying that the average concentration of oxygen present on the surface of the steel sheet is suppressed to 20 times or less compared to the concentration of oxygen in the bulk, meaning that the internal oxide layer is favorably formed and the formation of the external oxide layer is suppressed to a certain level or less.

[0064] Thus, when the formation of the external oxide layer is suppressed to a certain level or lower, the mutual diffusion between the steel component and the plating component is less likely to be inhibited during the plating process of the steel sheet. This ensures high adhesion between the steel and the plating, resulting in excellent plating performance. Since this effect can be obtained more reliably, it is preferable that the left side of equation (3) above (Isa(O) / Iba(O)) is as small as possible.

[0065] In this specification, "excellent plating properties" means that when a plating treatment is applied to the surface of a steel sheet, it is difficult to form a steel sheet in which areas where a plating layer is not formed (hereinafter sometimes referred to as "unplated areas") exceed 5.0 area percent (hereinafter, such a steel sheet may be referred to as "unplated steel sheet"). The specific evaluation method for plating properties will be described later.

[0066] In this embodiment, the emission intensity of O measured by GDS analysis preferably satisfies the following formula (8) in order to more reliably obtain excellent plating properties. Isa(O) / Iba(O)≦5 ···(8)

[0067] The GDS analysis of each element in the surface layer and bulk of the steel plate described above should be carried out as follows.

[0068] (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.

[0069] 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.

[0070] 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.

[0071] 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 emission intensity of O on the surface of the steel plate is adopted as the average value Isa(O). On the other hand, for the emission intensity in the depth direction of B, Nb, and Mo on the surface of the steel plate, a moving average of 11 points (center point + 5 points before and after) for each depth is adopted. Then, Isi(B) is determined from the minimum value of the moving average, and Isx(Nb+Mo) is determined from the maximum value of the moving average. Anode diameter: 4mmφ RF (Radio Frequency) output: 30W Measurement time: 200-1500 seconds On the other hand, the method for measuring the luminescence intensity of bulk B, Nb, Mo, and O is basically the same as described above. However, the calculation method differs from that of Isi(B) or Isx(Nb+Mo), and for the bulk luminescence intensity of these elements, 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 for the above measurement conditions is often around 100 nm / s, which is a typical sputtering speed described in JIS K0144:2018, 5.1. The measurement and calculation methods for GDS analysis related to Fe are the same as described above. The emission intensity of bulk Fe is the emission intensity at a sputtering time of 1000 seconds (average of 51 points: center point + 25 points before and after). Excluding this bulk Fe emission intensity, the Fe emission intensity 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 position of the outermost surface of the steel plate (0 μm position) is determined from this moving average of 11 points.

[0072] Furthermore, in order for the emission intensities of B, Nb, Mo, and O measured by GDS analysis to satisfy the relationships in equations (1), (2), and (3) above, respectively, the steel sheet should be annealed under specific high dew point conditions during manufacturing to form a small amount of Si-Mn internal oxide (i.e., suppress the formation of an external oxide layer), and then annealed under specific low dew point conditions to incorporate B into the Si-Mn internal oxide, thereby suppressing the deboronization of the steel sheet surface. With this method, the formation of an external oxide layer can be suppressed while B and at least one of Nb and Mo, which have a high affinity for B, can coexist on the steel sheet surface. Specific methods and conditions will be described later.

[0073] [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 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.

[0074] The following provides a more detailed explanation of each of these elements.

[0075] [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.

[0076] [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.

[0077] [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.

[0078] [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.

[0079] [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.

[0080] [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 generated during the Al analysis process. Since the above effects of including sol.Al can also be obtained by including Si, 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 form on the surface of the steel sheet, which may suppress decarburization at the outermost surface and reduce the effect of improving LME resistance. Therefore, the sol.Al content should be 3.00% 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.

[0081] [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.

[0082] [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.

[0083] [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.

[0084] [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.

[0085] [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.

[0086] [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.

[0087] [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.

[0088] [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.

[0089] [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.

[0090] [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.

[0091] [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.

[0092] [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.

[0093] [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.

[0094] [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, 0.005% or more, or 0.010% 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.

[0095] [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.

[0096] [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, or 0.060% or less, 0.030% or less, 0.010% or less, and 0.002% or less, respectively.

[0097] [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).

[0098] 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.

[0099] [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.

[0100] 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.

[0101] 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.

[0102] 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.

[0103] [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.

[0104] 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.

[0105] (Plating layer) Furthermore, the steel sheet of this embodiment may or may not have a plating layer for reasons described later. In other words, the steel sheet of this embodiment may be a plated steel sheet or an unplated steel sheet.

[0106] If the steel sheet in this embodiment is a plated steel sheet, the plating layer may be formed on only one side of the sheet surface, or on both sides. It may also be formed on only a portion of the sheet surface. The plating layer may be an alloyed layer.

[0107] The chemical composition of the plating layer is not particularly limited, but a zinc-based plating layer is one example. That is, the steel sheet of this embodiment may be a plated steel sheet having a zinc-based plating layer in at least a portion of it. Since a plated steel sheet having a zinc-based plating layer has a plating layer containing Zn, which is a cause of LME cracking, this disclosure is particularly advantageous when applied to such a plated steel sheet having a zinc-based plating layer.

[0108] The zinc-based plating layer is not particularly limited as long as it is a plating layer containing zinc (Zn), but examples 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).

[0109] 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 the acidic solution to which the 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.

[0110] 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.

[0111] Furthermore, even if the steel sheet of this embodiment does not have a plating layer, it can still exhibit the effect of improved LME resistance. Generally, when spot welding two steel sheets that are not zinc-plated, 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. However, since the steel sheet of this embodiment has excellent plating properties as well as excellent LME resistance, it is particularly advantageous when applied to plated steel sheets.

[0112] <Parts> As described above, the steel sheet of this embodiment is a steel sheet with excellent LME resistance and plating properties. Therefore, the steel sheet of this embodiment is useful as a raw material for parts that require excellent LME resistance and plating properties. In particular, the steel sheet of this embodiment is useful as a raw material for parts in the automotive sector.

[0113] One embodiment of this disclosure is a part containing the steel sheet or plated steel sheet of the above embodiment. Furthermore, examples of parts include automobile parts. Specific examples of automobile parts include the frame of an automobile, bumpers, and other structural and reinforcing parts that require strength. Further specific examples of automobile parts include exterior parts such as roofs, hoods, fenders, and doors that require high aesthetic appeal. These parts only need to contain the steel sheet of the above embodiment in at least a portion of the part. Therefore, these parts have the characteristics of the steel sheet of the above embodiment in at least a portion of the part. Note that in parts of the steel sheet that do not directly come into contact with the mold during forming such as press forming, or that are in direct contact with the mold but undergo relatively little processing, the characteristics of the steel sheet do not change particularly before and after forming.

[0114] 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.

[0115] 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.

[0116] <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.

[0117] 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 after the hot rolling step, a cold rolling step of cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet, and an annealing step of annealing the cold-rolled steel sheet under specific dew point conditions. Since the steel sheet obtained by this manufacturing method has excellent plateability, if the steel sheet is to be plated, a plating step should be performed after the annealing step described above.

[0118] In addition, optional processes such as shot blasting, in which the surface of the hot-rolled steel sheet after pickling is shot-blasted, or brush grinding, in which the surface of the hot-rolled steel sheet after pickling is brush-ground, may be performed.

[0119] The following will explain in detail the preferred conditions for these processes.

[0120] [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.

[0121] [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%.

[0122] The hot-rolled steel sheet, after finish rolling, is wound at a predetermined winding temperature. 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.

[0123] [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.

[0124] [Cold rolling process] The cold rolling process is a process of obtaining cold-rolled steel sheets by cold rolling hot-rolled steel sheets. The reduction ratio of the 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.

[0125] [Annealing process] The annealing process involves annealing cold-rolled steel sheets under specific dew point conditions. In the annealing process, first, the steel sheets are annealed under specific high dew point conditions during heating to form a small amount of Si-Mn internal oxide (i.e., suppress the formation of an external oxide layer), thereby ensuring plating properties. Subsequently, the steel sheets are annealed under specific low dew point conditions to incorporate B into the Si-Mn internal oxide, suppressing the deboronization of the steel sheet surface. This allows B and at least one of Nb and Mo, which have a high affinity for B, to coexist on the steel sheet surface while suppressing the formation of an external oxide layer.

[0126] The specific conditions for the annealing process are as follows: First, the steel plate is heated from room temperature to 750°C in an atmosphere with a dew point of -10°C to 20°C. Subsequently, the steel plate is heated to a holding temperature of 750°C to 900°C in an atmosphere with a dew point of -60°C to -20°C, and held at this holding temperature for 60 to 180 seconds.

[0127] Furthermore, by annealing cold-rolled steel sheets under these specific dew point conditions, the emission intensity of B, Nb and Mo, and O, as measured by GDS analysis, will satisfy the relationships given by equations (1), (2), and (3) above.

[0128] In the annealing process, the dew point when raising the temperature from room temperature to 750°C is preferably -5°C to 10°C. When raising the temperature from 750°C to the holding temperature, the dew point is preferably -40°C to -30°C. Furthermore, the holding temperature is preferably 800°C to 900°C.

[0129] 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 volume percent hydrogen (for example, a balance of 2 volume percent hydrogen and nitrogen).

[0130] The steel sheet obtained through the above-described process has a specific chemical composition, as described above, and is characterized in that the luminescence intensity of B, Nb and Mo, and O, as measured by GDS analysis, satisfy the relationships of equations (1), (2), and (3), respectively. In other words, in the steel sheet of this embodiment, a certain amount of B and Nb and / or Mo coexist on the surface layer of the steel sheet, and the formation of the external oxide layer is suppressed to a certain level or less. As a result, the steel sheet of this embodiment can more effectively suppress LME cracking and exhibit excellent plating properties.

[0131] As described above, the steel sheets obtained by this manufacturing method have excellent plating properties; therefore, if the steel sheets are to be plated, the plating process should be carried out after the annealing process described above.

[0132] [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.

[0133] As described above, the steel sheet of this embodiment can be made into a plated steel sheet.

[0134] Furthermore, the steel sheet manufacturing method of this embodiment may include any additional processing steps in addition to the steps described above. For example, the steel sheet manufacturing method of this embodiment may include a shot blasting step in which shot blasting is performed on the surface of the hot-rolled steel sheet after pickling, or a brush grinding step in which brush grinding is performed on the surface of the hot-rolled steel sheet after pickling.

[0135] [Shot blasting process] The shot blasting process involves applying shot blasting to the surface of a hot-rolled steel sheet after pickling. Shot blasting is a process in which spherical projectiles are used to project onto the surface of the hot-rolled steel sheet. By applying such shot blasting to the surface of the steel sheet, a specific strain can be applied to the surface of the steel sheet. As a result, the microstructure of the steel sheet surface becomes finer, and boron is trapped at the grain boundaries, which further suppresses boron removal. Consequently, boron, nb, and / or mo are more easily concentrated in the surface of the steel sheet. The surface roughness Ra of the steel sheet after shot blasting is, for example, 2.5 μm or more.

[0136] The abrasive material used in shot blasting is not particularly limited, but for example, steel balls (shots) with a central particle size of 40 to 450 μm can be used. Examples of such steel balls include WINOA IKK JAPAN's TSH30. The amount of abrasive material projected is, for example, 5 to 400 kg / m². 2 The projection rate is preferably 60 kg / m². 2 The above is a comfortable 100 kg / m 2 That's all.

[0137] As mentioned above, it is preferable that the luminescence intensity of B measured by GDS analysis of the steel sheet in this embodiment satisfies formula (4) above. A steel sheet that satisfies such characteristics is manufactured by adopting at least one of the following conditions (i) to (iii). (i) The amount of abrasive material to be projected in the shot blasting process is 60 kg / m 2 That concludes this section. (ii) The dew point when raising the temperature from room temperature to 750°C during the annealing process shall be -5°C to 10°C. (iii) The dew point when raising the temperature from 750°C to the holding temperature during the annealing process shall be -40°C to -30°C.

[0138] Similarly, it is even more preferable that the luminescence intensity of B measured by GDS analysis of the steel sheet of this embodiment satisfies formula (5) above. A steel sheet that satisfies such characteristics can be manufactured by adopting all of the following conditions (i) to (iv). (i) The amount of abrasive material to be projected in the shot blasting process is 100 kg / m 2 That concludes this section. (ii) The dew point when raising the temperature from room temperature to 750°C during the annealing process shall be -5°C to 10°C. (iii) The dew point when raising the temperature from 750°C to the holding temperature during the annealing process shall be -40°C to -30°C. (iv) The holding temperature during the annealing process shall be 820°C to 900°C.

[0139] Furthermore, it is particularly preferable that the total content of Nb and Mo in the steel sheet of this embodiment is 0.020% or more, and the emission intensities of Nb and Mo measured by GDS analysis satisfy the above formula (6). However, the steel sheet satisfying these characteristics is manufactured by adopting all of the following conditions (i) to (v). (i) The projection amount of the projection material in the shot blasting treatment is 100 kg / m 2 or more. (ii) The dew point when heating from room temperature to 750°C in the annealing process is -5°C to 10°C. (iii) The dew point when heating from 750°C to the holding temperature in the annealing process is -40°C to -30°C. (iv) The holding temperature in the annealing process is 820°C to 900°C. (v) The total content of Nb and Mo in the chemical composition is 0.020% or more.

[0140] Similarly, it is particularly preferable that the total content of Nb and Mo in the steel sheet of this embodiment is 0.040% or more, and the emission intensities of Nb and Mo measured by GDS analysis satisfy the above formula (7). However, the steel sheet satisfying these characteristics is manufactured by adopting all of the following conditions (i) to (v). (i) The projection amount of the projection material in the shot blasting treatment is 100 kg / m 2 or more. (ii) The dew point when heating from room temperature to 750°C in the annealing process is -5°C to 10°C. (iii) The dew point when heating from 750°C to the holding temperature in the annealing process is -40°C to -30°C. (iv) The holding temperature in the annealing process is 820°C to 900°C. (v) The total content of Nb and Mo in the chemical composition is 0.040% or more.

[0141] Also, it is preferable that the emission intensity of O measured by GDS analysis in the steel sheet of this embodiment satisfies the above formula (8). However, the steel sheet satisfying such characteristics is manufactured by adopting all of the following conditions (i) to (iii). (i) The amount of abrasive material to be projected in the shot blasting process is 100 kg / m 2 That concludes this section. (ii) The dew point when raising the temperature from room temperature to 750°C during the annealing process shall be -5°C to 10°C. (iii) The dew point when raising the temperature from 750°C to the holding temperature during the annealing process shall be -40°C to -30°C.

[0142] [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.

[0143] When a steel sheet, which has been subjected to specific strain on its surface by brush grinding, is annealed under the specific dew point conditions described above, internal Si-Mn oxides are rapidly formed, and B is incorporated into these internal oxides. As a result, the deboronization of the steel sheet surface is further suppressed, and B, Nb, and / or Mo become more concentrated on the steel sheet surface. Consequently, the emission intensities of B and Nb and Mo measured by GDS analysis are more likely to satisfy the relationships in equations (1) and (2) above, respectively.

[0144] 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 It is preferable to perform brush grinding under the above conditions. The amount of material removed by brush grinding is preferably as high as possible, for example, 8 g / m², from the standpoint of more reliably imparting a specific strain to the steel plate surface and more reliably removing 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.

[0145] 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.

[0146] As described above, the steel sheet of this disclosure has excellent LME resistance and plating properties, 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.

[0147] 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.

[0148] 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]

[0149] 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.

[0150] (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%.

[0151] Next, the obtained hot-rolled steel sheet was pickled, and then cold-rolled at a reduction ratio of 50% to obtain a cold-rolled steel sheet with a thickness of 1.6 mm.

[0152] The cold-rolled steel sheet was then annealed by heating it from room temperature to 750°C in an atmosphere with a dew point of -10°C, and then heating it to a holding temperature of 750°C to 800°C in an atmosphere with a dew point of -20°C, and holding it at this temperature for 60 seconds.

[0153] In this way, we obtained the steel plate for Test No. 1, which is the example of this disclosure.

[0154] (Manufacturing of steel plates for Test No. 2) The chemical composition was changed to that shown in Test No. 2 in Table 1 below. After pickling, shot blasting was performed, and the annealing conditions were changed to those shown in Test No. 2 in Table 2 below. Furthermore, plating was performed. Other conditions were the same as for the steel sheet in Test No. 1, and the plated steel sheet of Test No. 2, which is the present disclosure example, was obtained.

[0155] In addition, for the shot blasting treatment of the steel plate in Test No. 2, the abrasive material (WINOA IKK JAPAN, TSH30) was used at a rate of 60 kg / m². 2 The projection material was projected onto the steel plate surface at the specified projection rate. In the plating process, the annealed steel plate was 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)).

[0156] (Manufacturing of steel plates for tests No. 3-32, 38, and 40) The chemical composition was changed to that shown in Table 1 below. The amount of shot blasting, 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, 38, and 40, which are examples of this disclosure.

[0157] 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))

[0158] (Manufacturing of steel plates for tests No. 33-37 and 39) The chemical composition was changed to that shown in Table 1 below. After pickling, the irradiation rate was 50 kg / m². 2 Shot blasting was performed. Furthermore, the 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. 33-37 and 39.

[0159] The chemical compositions of each steel sheet from Test No. 1 to 40 shown in Table 1 are based on molten steel analysis values.

[0160] [Table 1]

[0161] For each of the steel sheets obtained in Test No. 1 to 40 as described above, GDS analysis of B, Nb, Mo, and O was performed on the surface layer and bulk of the steel sheet, and various Vickers hardness measurements were also carried out. Furthermore, the plating properties and LME resistance of each steel sheet in Test No. 1 to 40 were evaluated according to the evaluation method described below. Note that the plating properties were evaluated only for plated steel sheets. The measurement results and evaluation results are shown in Table 2 below.

[0162] 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.

[0163] <Evaluation of plating properties> Ten identical plated steel sheets were prepared, and a 1mm x 1mm area of ​​the surface of each sheet was observed using an optical microscope. From the observed images, the areas where the plating layer was formed (plated areas) and areas where the plating layer was not formed (unplated areas) were identified, and the area ratio of the unplated areas (area of ​​unplated areas / area of ​​the observed image) was calculated for each plated steel sheet. Plated steel sheets with an unplated area exceeding 5.0 area were classified as "unplated steel sheets," and the plating quality of the steel sheets was determined according to the evaluation criteria below based on the number of unplated steel sheets that were found.

[0164] (Evaluation Criteria) Rating AA: 0 out of 10 sheets were unplated steel. Rating A: 1-2 out of 10 sheets are unplated steel. Rating B: 3 or more out of 10 sheets are unplated steel.

[0165] The evaluation criteria are as follows: an A rating or higher (i.e., ratings A and AA) indicates excellent plating performance, while a B rating indicates poor plating performance. A rating of AA indicates extremely excellent plating performance.

[0166] <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.

[0167] 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.

[0168] 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.

[0169] 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.

[0170] (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

[0171] 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.

[0172] 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.

[0173] [Table 2]

[0174] The steel sheets in Tests No. 1-32, 38, and 40 are examples of the disclosed material, and as shown in Table 2, all of them exhibited excellent plating properties and LME resistance. On the other hand, the steel sheets No. 33-37 and 39, obtained under manufacturing conditions that did not yield the steel sheets of the disclosed material, are comparative examples. Their chemical composition and GDS analysis results for the steel sheet surface and bulk were outside the scope of the disclosed material, and all of them exhibited inferior plating properties and LME resistance.

[0175] 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.

[0176] In Test No. 34, it is believed that a large amount of external oxide was formed because the dew point was low when the temperature was raised from room temperature to 750°C during the annealing process. As a result, the plating performance was likely poor.

[0177] In Test No. 35, the dew point was high when the temperature was raised from 750°C to the holding temperature during the annealing process. This resulted in the formation of a thick external oxide layer on the steel sheet surface, making internal oxidation difficult and hindering the trapping of B, Nb, and Mo in the internal oxide layer. Consequently, the plating properties and LME resistance were likely to be poor.

[0178] In Test No. 36, it is believed that the holding time during annealing was too short, preventing sufficient concentration of B, Nb, and Mo in the surface layer of the steel sheet. As a result, the LME resistance was likely inferior.

[0179] In Test No. 37, the long holding time during annealing likely led to the formation of an external oxide layer on the steel sheet surface, hindering internal oxidation and making it difficult to trap Nb and Mo in the internal oxide layer. As a result, the plateability and LME resistance were likely inferior.

[0180] In Test No. 39, the low dew point during the annealing process, when the temperature was raised from room temperature to 750°C, likely resulted in the formation of a large amount of external oxide, 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.

[0181] 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.

[0182] [Table 3]

[0183] [Table 4]

[0184] 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 element of formula (2) of this disclosure.

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. The emission intensities of B, Nb, Mo, and O measured by glow discharge emission spectroscopy satisfy the following equations (1) to (3): A steel plate characterized by the following. Isi(B) / Iba(B)≧0.5 ・・・・・・・・・・・・(1) Isx(Nb+Mo) / Iba(Nb+Mo)≧0.6...(2) Isa(O) / Iba(O)≦20 ・・・・・・・・・・・・(3) Here, the steel plate surface layer is the region from the outermost surface of the steel plate to a depth of 5.0 μm in the thickness direction. Isi(B) is the minimum value of the luminescence intensity of B on the surface layer of the steel plate. Iba(B) is the average value of the luminescence intensity of bulk B. Isx(Nb+Mo) is the maximum value of the sum of the luminescence intensity 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. Isa(O) is the average value of the luminescence intensity of O on the surface layer of the steel plate. Iba(O) is the average value of the luminescence intensity of bulk oxygen.

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 steel plate according to claim 1 or 2, characterized in that the emission intensity of B measured by the glow discharge emission spectroscopy satisfies the following formula (4). Isi(B) / Iba(B)≧0.8...(4)

5. The steel sheet according to claim 1 or 2, characterized in that the emission intensity of B measured by the glow discharge emission spectroscopy satisfies the following formula (5). Isi(B) / Iba(B)≧1.0 (5)

6. The sum of the Nb content and Mo content is 0.020% 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 (6). Isx(Nb+Mo) / Iba(Nb+Mo)≧0.8...(6)

7. 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 (7). Isx(Nb+Mo) / Iba(Nb+Mo)≧1.0...(7)

8. The steel sheet according to claim 1 or 2, characterized in that the emission intensity of O measured by the glow discharge emission spectroscopy satisfies the following formula (8). Isa(O) / Iba(O)≦5...(8)

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 300 Hv or more.

10. 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.

11. 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.

12. 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.

13. A plated steel sheet having a zinc-based plating layer on at least a portion of the steel sheet according to claim 1 or 2.

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

15. A component comprising the plated steel sheet described in claim 13.

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