Steel sheet and component including same

Abstract

The present disclosure provides a steel sheet having excellent LME resistance and plating properties by means of a novel configuration.　A steel sheet according to the present disclosure has a specific chemical composition, and the emission intensities of B, Nb, Mo, and O measured by GDS analysis satisfy relationships (1) to (3).　Relationship 1: Isi(B) / Iba(B) ≥ 0.5 Relationship 2: Isx(Nb + Mo) / Iba(Nb + Mo) ≥ 0.6 Relationship 3: Isa(O) / Iba(O) ≤ 20 Isi(B) is the minimum value of the light emission intensity of B in the steel sheet surface layer, Iba(B) is the average value of the light emission intensity of B in bulk state, Isx(Nb + Mo) is the maximum value of the total of the light emission intensity of Nb and the light emission intensity of Mo in the steel sheet surface layer, Iba(Nb + Mo) is the average value of the total of the light emission intensity of Mo and the light emission intensity of Nb in bulk state, Isa(O) is the average value of the light emission intensity of O in the steel sheet surface layer, and Iba(O) is the average value of the light emission intensity of O in bulk state.

Description

Steel plates and parts containing them

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

[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 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 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 overlapping 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 portion to a thickness of 5 μm or more from the surface in the thickness direction, and a B-enriched portion having a B-strength of at least twice the B-strength at a depth of 50 μm determined by TOF-SIMS measurement exists to a thickness of 1.0 μm or more from the surface.

[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.

[0009] International Publication No. 2019 / 116531, International Publication No. 2020 / 218575, International Publication No. 2025 / 032898, International Publication No. 2025 / 032899, International Publication No. 2025 / 032900

[0010] To meet the demands for further weight reduction and increased strength of automotive parts and other components, there is a need for steel sheets that are less prone to LME cracking, i.e., steel sheets with excellent LME resistance.

[0011] On the other hand, in the manufacture of high-strength steel sheets, heat treatment such as annealing is generally 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 heat treatment described above, forming an oxide-containing layer near the surface of the steel sheet. Such layers can take two forms: an external oxide layer in which oxides containing elements such as Si and Mn are formed as a film on the outside (surface) of the steel sheet, and an internal oxide layer in which oxides are formed inside (surface) the steel sheet.

[0012] Furthermore, in steel sheets with an external oxide layer formed on them, when forming a plating layer (e.g., a zinc-based plating layer) on the surface of the steel sheet, the presence of oxides as a film on the surface of the steel sheet inhibits the interdiffusion between the steel component (e.g., Fe) and the plating component (e.g., Zn), which can reduce the adhesion between the steel and the plating, resulting in insufficient plating (i.e., an increase in unplated areas).

[0013] This disclosure is made in view of the circumstances described above, and aims to provide a steel sheet with excellent LME resistance and plating properties, as well as parts containing the same, through a novel configuration.

[0014] This disclosure includes at least the following aspects:

[0015] (Aspect 1) A steel sheet, wherein the chemical composition of the steel sheet is, in mass%, C: 0.05 to 0.40%, Si: 0.70 to 3.00%, Mn: 1.00 to 5.00%, B: 0.0005 to 0.0100%, Ti: 0.010 to 0.150%, sol. Al: 0-3.00%, 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%, A steel sheet characterized by containing REM: 0 to 0.100%, with the remainder being Fe and impurities, and having a total Nb content and Mo content of 0.010% or more, and the emission intensities of B, Nb, Mo, and O measured by glow discharge emission spectrometry satisfying 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) 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 in the steel plate surface layer, Iba(B) is the average value of the luminescence intensity of B in the bulk, Isx(Nb+Mo) is the maximum value of the sum of the luminescence intensity of Nb and Mo in the steel plate surface layer, Iba(Nb+Mo) is the average value of the sum of the luminescence intensity of Nb and Mo in the bulk, Isa(O) is the average value of the luminescence intensity of O in the steel plate surface layer, Iba(O) is the average value of the luminescence intensity of bulk oxygen.

[0016] (Aspect 2) The chemical composition of the steel sheet is, in mass%, Nb: 0.0001 to 0.150%, Mo: 0.0001 to 1.000%, O: 0.001 to 0.01%, V: 0.001 to 0.150%, Cr: 0.001 to 2.00%, Ni: 0.001 to 2.00%, Cu: 0.001 to 2.00%, W: 0.001 to 1.00%, Ca: 0.0001 to 0.100%, Mg: 0.0001 to 0.100%, Zr: 0.001 to 0.500%, Hf: 0.0001 to 0.100%, Sn: 0.0001 to 0.100%, The steel sheet according to embodiment 1, characterized in that it contains one or more of the following: As: 0.0001 to 0.100%, and REM: 0.0001 to 0.100%.

[0017] (Aspect 3) The steel sheet according to aspect 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 the above aspects 1 to 3, characterized in that the emission intensity of B measured by the above glow discharge emission spectroscopy satisfies the following formula (4): Isi(B) / Iba(B)≧0.8 ...(4)

[0019] (Aspect 5) The steel sheet according to any one of the above aspects 1 to 4, characterized in that the emission intensity of B measured by the above glow discharge emission spectroscopy satisfies the following formula (5): Isi(B) / Iba(B) ≥ 1.0 ... (5)

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

[0021] (Aspect 7) A steel sheet according to any one of aspects 1 to 6, characterized in that the sum of the Nb content and Mo content is 0.040% or more, and the emission intensity of Nb and Mo measured by glow discharge emission spectroscopy satisfies the following formula (7): Isx(Nb+Mo) / Iba(Nb+Mo)≧1.0 ...(7)

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

[0023] (Aspect 9) The steel plate according to any one of the above aspects 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 aspects 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 aspects 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 aspects 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 described in any of the above aspects 1 to 12.

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

[0029] (Aspect 15) A component comprising the plated steel sheet described in aspect 13 above.

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

[0031] LME cracking is considered to occur as follows. First, during welding such as spot welding, the metal structure of the steel sheet is heated and transforms into austenite. Molten zinc (Zn) generated by melting of zinc plating intensively penetrates into austenite grain boundaries in the surface layer of the steel sheet, particularly grain boundaries where grain boundary energy is locally lowered, thereby embrittling the steel sheet. Furthermore, it is considered that LME cracking occurs when tensile stress is applied to the steel sheet during welding.

[0032] For suppressing such LME cracking, segregating B (boron) in the surface layer of the steel sheet is effective. When B is segregated in the surface layer of the steel sheet in a solid solution state, Zn becomes less likely to penetrate into grain boundaries, so LME cracking can be suppressed. In the present specification, the property of suppressing LME cracking in a steel sheet is referred to as "LME resistance". Conversely, the property of readily causing LME cracking in a steel sheet is referred to as "LME sensitivity".

[0033] On the other hand, as described above, the decrease in platability occurs because 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 composed of oxides of Si, Mn and the like on the exterior (surface) of the steel sheet.

[0034] Accordingly, the present inventors focused on the LME resistance provided by the aforementioned segregation of B, and conducted intensive studies on a new method for segregating more B in the surface layer of the steel sheet (i.e., improving LME resistance) while suppressing the formation of an external oxide layer (i.e., while securing platability).

[0035] As a result, the inventors found that when producing a steel sheet, by performing annealing under specific high dew point conditions in the annealing step, a trace amount of Si-Mn internal oxides is formed (i.e., the formation of an external oxide layer is suppressed) to secure platability, followed by annealing under specific low dew point conditions, which causes B to be incorporated into the aforementioned Si-Mn internal oxides, thereby suppressing boron removal from the surface layer of the steel sheet. According to this method, B and at least one of Nb (niobium) and Mo (molybdenum), which have high affinity with B, can coexist in the surface layer of the steel sheet while suppressing the formation of an external oxide layer.

[0036] The steel sheet obtained by the above method can exhibit excellent platability because the formation of an external oxide layer is suppressed. Furthermore, it is considered that LME cracking can be more effectively suppressed because B and at least one of Nb and Mo coexist in the surface layer of the steel sheet.

[0037] The present disclosure has been completed based on the above findings, and includes aspects of the following embodiments.

[0038] Hereinafter, preferred embodiments of the steel sheet of the present disclosure will be described in detail.

[0039] <Steel Sheet> A steel sheet according to one embodiment of the present disclosure contains, in mass%,C: 0.05 to 0.40%, Si: 0.70 to 3.00%, Mn: 1.00 to 5.00%, B: 0.0005 to 0.0100%, Ti: 0.010 to 0.150%, sol.Al: 0 to 3.00%, Nb: 0 to 0.150%, Mo: 0 to 1.000%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0200% or less, O: 0 to 0.01%, V: 0 to 0.150%, Cr: 0 to 2.00%, Ni: 0 to 2.00%, Cu: 0 to 2.00%, W: 0 to 1.00%, Ca: 0 to 0.100%, Mg: 0 to 0.100%, Zr: 0 to 0.500%, Hf: 0 to 0.100%, Sn: 0 to 0.100%, As: 0 to 0.100%, REM: 0 to 0.100%, and the balance: Fe and impurities, and has a specific chemical composition in which the total content of Nb and Mo is 0.010% or more.

[0040] Furthermore, the steel sheet of this embodiment has a characteristic 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 in the steel plate surface layer, Iba(B) is the average value of the luminescence intensity of B in the bulk, Isx(Nb+Mo) is the maximum value of the sum of the luminescence intensity of Nb and Mo in the steel plate surface layer, Iba(Nb+Mo) is the average value of the sum of the luminescence intensity of Nb and Mo in the bulk, Isa(O) is the average value of the luminescence intensity of O in the steel plate surface layer, Iba(O) is the average value of the luminescence intensity of bulk oxygen.

[0041] (Effects) 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). That is, in the steel sheet of this embodiment, a certain amount of B and Nb and / or Mo coexist in 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, these Nb and Mo attract B, causing more cosegregation of B in 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) of the steel sheet of this embodiment satisfies the following equation (1). That is, the steel sheet of this embodiment has at least 0.5 times the concentration of B in the bulk present on the surface of the steel sheet. Isi(B) / Iba(B)≧0.5 ...(1) Here, Isi(B) is the minimum emission intensity of B on the surface of the steel sheet. Iba(B) is the average value of the emission intensity of B in the bulk.

[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 layer 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 of the steel plate to a depth of 5.0 μm in the thickness direction of the steel plate. "Bulk" refers to the region of the steel plate other than the steel plate surface layer, and the method for calculating the luminescence intensity of bulk materials such as Iba(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 considered to be 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 emission intensity at a sputtering time of 1000 seconds is considered to be the bulk emission intensity (details will be described later, but it is the average of the emission intensities of 51 points in total: the center point + 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 Fe emission intensity in the GDS analysis reaches 0.7 times or more the Fe emission intensity of the bulk, 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 Isi(B) of B on the surface of the steel plate 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 of the emission intensity of B in the bulk Iba(B) is the average value of the emission 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 luminescence intensity of B measured by GDS analysis preferably satisfies the following equation (4), and more preferably satisfies the following equation (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 luminescence 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 of the steel sheet together with the above-mentioned B. Isx(Nb+Mo) / Iba(Nb+Mo)≧0.6 ...(2) Here, Isx(Nb+Mo) is the maximum value of the sum of the luminescence intensities of Nb and Mo on the surface of the steel sheet. Iba(Nb+Mo) is the average value of the sum of the luminescence intensities of Nb and Mo in the bulk.

[0054] Regarding the emission intensity of Nb and Mo measured by GDS analysis, the maximum value Isx(Nb+Mo), which is the sum of the emission intensity of Nb and Mo on the surface of the steel plate, is the maximum value of the sum of the emission intensity of Nb and Mo on the surface of the steel plate obtained when GDS analysis is performed in the thickness direction from the outermost surface of the steel plate, and represents the maximum total concentration of Nb and Mo on the surface of the steel plate. If the maximum concentration of Nb and Mo is low, it becomes the starting point for LME cracking, and the effect of preventing the occurrence of 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 intensity of Nb and Mo on the surface of the steel plate, 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 plate 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 luminescence intensities of Nb and Mo measured by GDS analysis preferably satisfy the following equation (6), and more preferably satisfy the following equation (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 luminescence intensity of Nb and Mo measured by GDS analysis satisfies the above formula (6). 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 luminescence 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 plate of this embodiment satisfies the following equation (3). That is, in the steel plate of this embodiment, the average concentration of O present on the surface of the steel plate 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 emission intensity of O on the surface of the steel plate. Iba(O) is the average value of the emission intensity of O in the bulk.

[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 Iba(O) of the emission intensity of bulk oxygen 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) (Isa(O) / Iba(O)) be 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 the area where the plating layer is not formed (hereinafter sometimes referred to as "unplated area") exceeds 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, the "GDS850A" glow discharge emission spectrometer 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 of B, Nb, and Mo in the depth direction 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: 4 mmφ RF (Radio Frequency) output: 30W Measurement time: 200 to 1500 seconds On the other hand, the measurement method for the luminescence intensity of bulk B, Nb, Mo, and O is basically the same as above. However, the calculation method differs from 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: center point + 25 points before and after) 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 method and calculation method for GDS analysis related to Fe are also the same as above. The luminescence intensity of bulk Fe is the luminescence intensity at a sputtering time of 1000 seconds (average of 51 points: center point + 25 points before and after). Excluding the Fe emission intensity of the bulk material, the Fe emission intensity at each depth position from the outermost surface of the steel plate is obtained by removing the background from the acquired data and then using a moving average of 11 points (center point + 5 points before and after it). 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, when manufacturing the steel sheet, the steel sheet should be annealed under specific high dew point conditions 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 layer. 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 surface layer of the steel sheet. Specific methods and conditions will be described later.

[0073] [Chemical Composition] Next, the chemical composition of the steel sheet of this embodiment will be described in detail. As described above, the steel sheet of this embodiment has the following composition in mass%, C: 0.05 to 0.40%, Si: 0.70 to 3.00%, Mn: 1.00 to 5.00%, B: 0.0005 to 0.0100%, Ti: 0.010 to 0.150%, sol. Al: 0-3.00%, 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%, It has a specific chemical composition in which REM is 0-0.100%, and the remainder consists of Fe and impurities, and 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 Si is present in excess, 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 forming a hard structure and improving the strength of steel. 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 to 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 plate 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, or 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, or 0.030% or less.

[0080] [sol. Al: 0-3.00%] Al is an element that promotes ferrite stabilization and decarburization by being dissolved in steel. Here, sol. Al means Al ２ O ３This refers to acid-soluble Al, which is not an oxide 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%. Excessive sol. Al content can lead to external oxidation during high-dew-point annealing, forming oxides (scale) on the surface of the steel sheet, which can 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 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 more co-segregation of 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 an impurity commonly found in steel. P is an element that segregates at grain boundaries and promotes embrittlement of steel. A lower P content is preferable, so the lower limit is 0%. However, excessive reduction of the 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 an impurity commonly found in steel. S is an element that generates nonmetallic inclusions such as MnS in steel, leading to a decrease in the ductility of steel parts. A lower S content is preferable, so the lower limit is 0%. However, excessive reduction of the S content can lead to a significant increase in costs. 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 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 an impurity commonly found in steel. N forms coarse nitrides in steel, reducing the workability and weldability of steel sheets. A lower N content is preferable, so the lower limit is 0%. However, excessive reduction of the 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 mixed in 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 the 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 should 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 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 the 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, which may impair 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 strength of steel. Ni 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 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 sheet of this disclosure; therefore, the lower limit of the 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 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, so 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 contributes to controlling the morphology of nonmetallic inclusions, particularly to the fine dispersion of nonmetallic inclusions, thereby enhancing toughness. 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 the 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, if Ca is included in excess, deterioration of surface properties may become apparent; 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 contributes to controlling the morphology of nonmetallic inclusions, particularly to the fine dispersion of nonmetallic inclusions, thereby enhancing toughness. 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; therefore, 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 contributes to controlling the morphology of nonmetallic inclusions, particularly to the fine dispersion of nonmetallic inclusions, thereby enhancing toughness. 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 Zr is included in excess, deterioration of surface properties may become apparent; therefore, 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 contributes to controlling the morphology of nonmetallic inclusions, particularly to the fine dispersion of nonmetallic inclusions, thereby enhancing toughness. 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; therefore, 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 effect 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 content of Sn and As 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, or 0.002% or less, respectively.

[0097] [REM: 0-0.100%] REM (rare earth elements) are elements that contribute to controlling the morphology of nonmetallic inclusions, particularly to the fine dispersion of nonmetallic inclusions, thereby enhancing toughness. REM 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 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 stated above, the chemical composition of the steel sheet of this disclosure requires that the total Nb and Mo content be 0.010% or more. When the total Nb and Mo content is 0.010% or more, Nb and / or Mo diffuse into the grain boundaries together 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. In order to obtain this effect more reliably, the total Nb and Mo content 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 as follows (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%. The material may contain one or more of the following: As: 0.0001 to 0.100%, and REM: 0.0001 to 0.100%. Furthermore, the chemical composition of the steel sheet 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 test piece with a width and length of 35 mm square (for example, a 35 mm square test piece with a sample thickness of 3 / 4 the thickness of the steel sheet) 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. However, if analysis values ​​from 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 to each of the above ranges by appropriately adjusting the chemical composition of the steel plate.

[0104] The Vickers hardness of steel plates is determined in accordance with JIS Z 2244-1:2024 as follows: First, a test piece 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 piece is polished using silicon carbide sandpaper of #600 to #1500 grit. Next, the thickness cross-section of the test piece 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, using a Vickers hardness tester, the Vickers hardness (HV1) of the test piece is measured at intervals of at least three times the diagonal length of the indentation with a test force of 9.807 N. Specifically, 20 measurements are taken at 1 mm intervals at a depth of 1 / 4 of the thickness of the test piece, 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 to be 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] When taking samples from automotive parts for various measurements and analyses, the following locations (i) to (iv) should be avoided: (i) Welded areas: within 20 mm of the toe of spot welds, and within 20 mm of the toe of arc / laser welds (ii) Machined areas: machined areas with a radius of curvature of less than 15 mm, and within 5 mm of the above machined areas (iii) Ends: ends within 5 mm of the cut end face of the part (iv) Red rust: within 5 mm of areas where red rust is visible

[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 for Manufacturing Steel Sheets> Next, a preferred method for manufacturing the steel sheet according to one embodiment of the present 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 method 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 in which a slab is hot-rolled to obtain a hot-rolled steel sheet. 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 in which hot-rolled steel sheets are cold-rolled to obtain cold-rolled steel sheets. The reduction ratio in cold rolling can be appropriately determined according to the desired metal structure and sheet thickness, and may be, for example, 20 to 80%. After the cold rolling process, the sheet may be cooled to room temperature by air cooling, for example.

[0125] [Annealing Process] The annealing process is a process of annealing cold-rolled steel sheets under specific dew point conditions. In the annealing process, first, the steel sheet is 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 sheet is 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 layer. 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 layer 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 luminescence 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, respectively.

[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, 2 volume percent hydrogen and nitrogen in balance).

[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 formulas (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 create 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 a 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 structure 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 molybdenum become more concentrated on 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 / m. ２ The projection rate is preferably 60 kg / m². ２ The above, and more preferably 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 projected in the shot blasting process is 60 kg / m ２ The above applies. (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. However, a steel sheet that satisfies such characteristics is manufactured by adopting all of the following conditions (i) to (iv): (i) The amount of abrasive material projected in the shot blasting process is 100 kg / m２ As described above. (ii) When heating from room temperature to 750°C in the annealing step, the dew point is set to -5°C to 10°C. (iii) When heating from 750°C to the holding temperature in the annealing step, the dew point is set to -40°C to -30°C. (iv) The holding temperature in the annealing step is set to 820°C to 900°C.

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

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

[0141] Furthermore, it is preferable that the luminescence intensity of O measured by GDS analysis of the steel sheet of this embodiment satisfies formula (8) above. A steel sheet that satisfies such characteristics is manufactured by adopting all of the following conditions (i) to (iii): (i) The amount of abrasive material projected in the shot blasting process is 100 kg / m ２ The above applies. (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 is a process in which brush grinding is performed on the surface of the hot-rolled steel sheet after pickling. By brush grinding the surface of the hot-rolled steel sheet, a specific strain is applied to the surface of the steel sheet, and the Nb and Mo-deficient layer on the surface of the steel sheet that was generated 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 intensity of B and the emission intensities of Nb and Mo, as 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 5 g / m². ２ It is preferable to perform brush grinding under the above conditions. The amount of grinding performed by brush grinding is preferably as high as possible, for example 8 g / m², in order to more reliably impart a specific strain to the surface of the steel plate and to more reliably remove the Nb and Mo-deficient layer on the surface of the steel plate. ２ It is more preferable that the amount be greater than or equal to 10 g / m ２ It is even more preferable that the above conditions are met. The upper limit of the amount of material removed by brush grinding is not particularly limited, but for example, 20 g / m ２ The following applies: 15 g / 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.

[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. Then, this slab was cooled, reheated to 1200°C, hot-rolled, and wound at a winding temperature of 520°C or higher. Hot rolling was carried out 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 subsequently by heating it from 750°C to a holding temperature of 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, the steel plate of Test No. 1, which is the example of this disclosure, was obtained.

[0154] (Manufacturing of steel sheet 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 for Test No. 1 to obtain the plated steel sheet for Test No. 2, which is the present disclosure example.

[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 following projection rate. In the plating process, the annealed steel plate was immersed in a 450°C molten zinc plating bath for 3 seconds, then pulled out at 100 mm / second, and N ２ The plating adhesion rate was reduced to 50 g / m² by 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 sheets 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 the plated steel sheets for tests No. 3-32, 38, and 40, which are examples of this disclosure.

[0157] The types of plating shown in Table 2 are as follows: a: Unplated (i.e., no plating treatment applied) b: Alloyed hot-dip galvanized (Zn-0.09%Al-10%Fe(GA)) c: Hot-dip galvanized (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]

[0161] For each of the steel sheets obtained as described above (Test Nos. 1-40), GDS analysis for B, Nb, Mo, and O was performed on the surface layer and bulk of the steel sheet, and various Vickers hardness measurements were carried out. Furthermore, the plating properties and LME resistance of each steel sheet (Test Nos. 1-40) were evaluated according to the following evaluation method. 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 1 mm x 1 mm area of ​​the surface of each plated steel 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 distinguished, 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 percentage were classified as "unplated steel sheets," and the plating properties of the steel sheets were determined according to the evaluation criteria below based on the number of unplated steel sheets that were generated.

[0164] (Evaluation Criteria) Evaluation AA: 0 out of 10 unplated steel sheets Evaluation A: 1 to 2 out of 10 unplated steel sheets Evaluation B: 3 or more out of 10 unplated steel sheets

[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 sample measuring 50 mm x 100 mm 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 plate assembly using a dome radius type welding electrode with a tip diameter of 8 mm under the conditions of a striking angle of 5°, a pressing force of 4.0 kN, an energizing time of 1.6 seconds, and an energizing current of 13 kA to create a welded joint.

[0167] The counter steel plates used are shown in Table 2. The types of "counter steel plates" in Table 2 are as follows: "Same type": Steel plate of the same type as the steel plate being evaluated. "GA": Steel plate of the same type as the steel plate being evaluated, but 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 described 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 LME resistance was determined based on the longest crack length directly outside the pressure-welded joint.

[0170] (Evaluation Criteria) AAA: 0 μm AA: Greater than 0 μm, less than 60 μm A: 60 μm or more, less than 120 μm B: 120 μm or more

[0171] The evaluation criteria are as follows: an evaluation of A or higher (i.e., A, AA, AAA) indicates excellent resistance to LME, while an evaluation of B indicates poor resistance to LME. An evaluation of AAA indicates the highest level of 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]

[0174] The steel sheets No. 1-32, 38, and 40 in the test are examples of the present disclosure, 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 present disclosure, are comparative examples, and their chemical composition and GDS analysis results of the steel sheet surface and bulk were outside the scope of the present disclosure, resulting in inferior plating properties and LME resistance in all of them.

[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 poor.

[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 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 deemed to be inferior.

[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 poor.

[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 plating properties and LME resistance were likely inferior.

[0180] In Test No. 39, the dew point was low when the temperature was raised from room temperature to 750°C during the annealing process. This resulted in the formation of a large amount of external oxide, making it difficult for internal oxidation to proceed and hindering the trapping of B, Nb, and Mo in the internal oxide layer. As a result, the LME resistance is thought to have been poor.

[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 specified herein. The results are shown in Tables 3 and 4 below. Note that underlines on various numerical values ​​in Tables 3 and 4 indicate that they are outside the scope of this disclosure.

[0182]

[0183]

[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. A steel plate, wherein 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: 0.010-0.150%, 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%, A steel sheet characterized by containing REM: 0 to 0.100%, with the remainder being Fe and impurities, and having a total Nb content and Mo content of 0.010% or more, and the emission intensities of B, Nb, Mo, and O measured by glow discharge emission spectrometry satisfying 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) 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 in the steel plate surface layer, Iba(B) is the average value of the luminescence intensity of B in the bulk, Isx(Nb+Mo) is the maximum value of the sum of the luminescence intensity of Nb and Mo in the steel plate surface layer, Iba(Nb+Mo) is the average value of the sum of the luminescence intensity of Nb and Mo in the bulk, Isa(O) is the average value of the luminescence intensity of O in the steel plate surface layer, Iba(O) is the average value of the luminescence intensity of bulk oxygen.

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

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 sheet according to any one of claims 1 to 3, 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 any one of claims 1 to 4, 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 steel sheet according to any one of claims 1 to 5, characterized in that the sum of the Nb content and Mo content is 0.020% or more, and the emission intensities of Nb and Mo measured by glow discharge emission spectroscopy satisfy the following formula (6): Isx(Nb+Mo) / Iba(Nb+Mo)≧0.8 ...(6) 7. The steel sheet according to any one of claims 1 to 6, characterized in that the sum of the Nb content and Mo content is 0.040% or more, and the emission intensities of Nb and Mo measured by glow discharge emission spectroscopy satisfy the following formula (7): Isx(Nb+Mo) / Iba(Nb+Mo)≧1.0 ...(7) 8. The steel sheet according to any one of claims 1 to 7, 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 any one of claims 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.

10. The steel plate according to any one of claims 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.

11. The steel plate according to any one of claims 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.

12. The steel plate according to any one of claims 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.

13. A plated steel sheet having a zinc-based plating layer on at least a portion of the steel sheet according to any one of claims 1 to 12.

14. A component comprising a steel plate as described in any one of claims 1 to 12.

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

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