Steel sheet and component including same

Abstract

The present disclosure provides: a high-strength steel sheet which has excellent LME resistance by means of a novel configuration; and a component including the same.　A steel sheet and a component including the same according to the present disclosure are characterized by having a specific chemical composition and a microstructure, and are also characterized in that, with respect to the signal intensity of BO2 - as determined by time-of-flight secondary ion mass spectrometry, pixels exhibiting a signal intensity that is not less than 7.5 times the average value of the signal intensities of the BO2 - are in contact with each other, and if a region in which 5-16 pixels are gathered is defined as one specific B precipitate, the number density of the specific B precipitate is 10,000 / mm2 or less, and the tensile strength of the steel sheet is 1,760 MPa or more.

Description

Steel plates and parts containing them

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

[0002] Galvanized steel sheets are widely used in various fields, such as automobiles, to improve the corrosion resistance of structural members. However, when spot welding is performed on plate assemblies including galvanized steel sheets for the assembly of vehicle bodies, molten zinc can penetrate into the steel of the galvanized steel sheets and other steel sheets within the assembly, causing cracks to occur inside the galvanized steel sheets or other steel sheets (non-galvanized steel sheets). This phenomenon is called liquid metal embrittlement cracking (so-called LME cracking), and in recent years, various proposals have been made to suppress the occurrence of such LME cracking in galvanized steel sheets and other steel sheets (non-galvanized steel sheets).

[0003] For example, Patent Document 1 discloses a zinc alloy plated steel material having a zinc alloy plating layer on the surface of the steel material, wherein the steel material has a predetermined chemical composition, and the sensitivity index E value of liquid metal embrittlement, shown by the formula: E value = [%C] + [%Si] / 17 + [%Mn] / 7.5 + [%Ni] / 17 + [%Nb] / 2 + [%V] / 1.5 + [%Zr] / 2, exceeds 0.24, and the B content satisfies 3 ppm or more and -102 × E + 61 ppm or less, and is a zinc alloy plated steel material with excellent weldability.

[0004] Furthermore, Patent Document 2 describes a material having a predetermined chemical composition and metal structure, in a surface region from the surface to a depth of 100 μm in the thickness direction of the plate, with (Fe, Mn) having a circular equivalent diameter of 50 to 300 nm. ２ B is present in 1 unit / 500 μm ２ Steel plates existing at the above number densities are disclosed.

[0005] Furthermore, Patent Document 3 describes a steel structure having a predetermined component composition, satisfying the requirement that the effective Ti mole fraction (xTi, eff) obtained from xTi, eff = xTi - xN - xS (where xTi, xN, and xS are the mole fractions of each element in the steel) is 0.001 or higher, and that at the 1 / 4 position of the plate thickness, the area ratio of martensite is 60% or more and 99% or less, and the total area ratio of ferrite and / or the volume ratio of retained austenite is more than 0% and 40% or less, and the boron atom concentration at prior austenite grain boundaries with an orientation difference of 15 degrees or more is 0.3 at% or more and 6.0 at% or less, and the number density of precipitates with a size of 2 μm or more is 150 particles / mm ２ The following high-strength steel plates are disclosed.

[0006] Japanese Patent Publication No. 2006-249521, Japanese Patent No. 6750759, International Publication No. 2024 / 252889

[0007] Patent Document 1 discloses a zinc-based alloy plated steel material that, when welding zinc-based alloy plated steel materials used as welded structural members for automobiles and the like using various methods, can suppress liquid metal embrittlement cracking in the heat-affected zone during welding, making it possible to provide welded structures made of zinc-based alloy plated steel materials with excellent weld quality. Furthermore, the steel plate disclosed in Patent Document 2 is said to have improved LME resistance, particularly improved LME resistance in welding methods such as spot welding, as well as high strength and excellent ductility. Moreover, the high-strength steel plate disclosed in Patent Document 3 was also insufficient as an LME countermeasure.

[0008] Therefore, the present disclosure aims to provide a high-strength steel plate and a component containing the same, which have excellent LME resistance through a novel configuration.

[0009] This disclosure includes the following aspects:

[0010] (Aspect 1) The chemical composition is in mass %, C: 0.27 to 0.40%, Si: 0.01 to 2.50%, Mn: 1.00 to 4.00%, Al: 0.001 to 1.500%, Ti: 0.001 to 0.100%, B: 0.0005 to 0.0100%, P: 0.050% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0100% or less, Cr: 0 to 1.00%, Cu: 0 to 1.00%, Mo: 0 to 1.00%, Ni: 0 to 1.00%, Co: 0 to 3.00%, W: 0 to 1.00%, Sn: 0 to 1.00%, Sb: 0 to 0.50%, Nb: 0 to 0.200%, V: 0 to 1.00%, As: 0 to 0.10%, Zn: 0 to 1.00%, Mg: 0 to 0.0500%, Ca: 0 to 0.0500%, Zr: 0 to 0.5000%, Ce: 0 to 0.0300%, La: 0 to 0.0150%, Hf: 0 to 0.0100%, Bi: 0 to 0.0100%, REM other than Ce and La: 0 to 0.1000%, and the balance: Fe and impurities, and the contents of C and B satisfy the following formula (1), the microstructure at a depth of 1 / 4 of the plate thickness from the steel plate surface is in area %, total of ferrite and bainite: 5% or less, retained austenite: 10% or less, martensite: 85% or more, and the balance: 5% or less, Regarding the signal intensity of BO ２ － measured by time-of-flight secondary ion mass spectrometry, when pixels showing a signal intensity 7.5 times or more the average value of the signal intensity of the above BO ２ － are in contact with each other and a region where 5 to 16 pixels are gathered is regarded as one specific B precipitate, the number density of the above specific B precipitate is 10000 pieces / mm ２ or less, The steel plate is characterized in that the tensile strength of the above steel plate is 1760 MPa or more.

[0011] T ｐ < 1000...(1) However, T ｐ = 555 + 179×[C] + 8083×√[B] [C]: Content of C (mass %) [B]: Content of B (mass %)

[0012] (Aspect 2) The above chemical composition is as follows, in mass%, Cr: 0.001 to 1.00%, Cu: 0.001 to 1.00%, Mo: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Co: 0.01 to 3.00%, W: 0.001 to 1.00%, Sn: 0.001 to 1.00%, Sb: 0.001 to 0.50%, Nb: 0.001 to 0.200%, V: 0.001 to 1.00%, As: 0.001 to 0.10%, Zn: 0.001 to 1.00%, Mg: 0.0001 to 0.0500%, Ca: 0.0001 to 0.0500%, The steel sheet according to embodiment 1, characterized by containing one or more of the following: Zr: 0.0001 to 0.5000%, Ce: 0.0001 to 0.0300%, La: 0.0001 to 0.0150%, Hf: 0.0001 to 0.0100%, Bi: 0.0001 to 0.0100%, and REM other than Ce and La: 0.0001 to 0.1000%.

[0013] (Aspect 3) The steel sheet according to aspect 1 or 2, characterized in that the emission intensity of C measured by glow discharge emission spectroscopy in the thickness direction of the steel sheet satisfies the following formula (2).

[0014]

[0015] C ３０ : The luminescence intensity of C at a depth of 30 μm from the surface of the steel plate. １ｔ／４ : Luminous intensity of C at a depth of 1 / 4 of the thickness of the steel plate mentioned above

[0016] (Aspect 4) The steel sheet according to aspect 1, characterized in that the above chemical composition contains, by mass%, N: 0.0100% or less, Co: 0 to 1.00%, B: 0.0005 to 0.0030%, Mg: 0 to 0.0100%, Ca: 0 to 0.0100%, Zr: 0 to 0.0100%, Ce: 0 to 0.0150%, and REM other than Ce and La: 0 to 0.0100%.

[0017] (Aspect 5) A component characterized by including the steel plate described in any of the above aspects 1 to 4.

[0018] According to this disclosure, it is possible to provide high-strength steel plates having excellent LME resistance and parts including the same.

[0019] Figure 1 is a schematic diagram illustrating the binning process.

[0020] This disclosure may define characteristics of specific locations in the thickness direction of a steel plate. Such characteristics of specific locations in the thickness direction of a steel plate are characteristics that use the position of the steel plate in the thickness direction with respect to the surface of the steel plate. Therefore, we will first explain such specific locations in the thickness direction of a steel plate.

[0021] First, since the "thickness direction" and "depth direction" of a steel plate are synonymous, in this specification, the position of the steel plate in the thickness direction relative to the surface of the steel plate may be referred to as the "depth position."

[0022] In this specification, "position at x / y depth of plate thickness (where x and y are natural numbers satisfying x < y)" means a position in the thickness direction of the steel plate, that is, a position moved from the surface of the steel plate toward the center of the steel plate by a distance (depth) of x / y of the plate thickness in the thickness direction. For example, if the thickness of the steel plate is t mm, "position at 1 / 8 depth of plate thickness" means a position that is 1 × t / 8 mm deep from the surface of the steel plate toward the thickness direction.

[0023] In particular, with respect to the "steel plate surface," which serves as the reference for the position in the thickness direction of the steel plate, i.e., the depth position of the steel plate, in the glow discharge emission spectroscopy analysis (sometimes referred to as "GDS analysis") described later, in order to eliminate the influence of contamination on the steel plate surface (even if there is no coating on the surface), when the Fe emission intensity is measured from the surface side toward the interior of the steel plate, the depth position where the Fe emission intensity reaches 0.7 times the internal Fe emission intensity is defined as the 0 μm position, and this 0 μm position is considered the steel plate surface. The internal Fe emission intensity is the Fe emission intensity in a sufficiently deep region of the base steel plate. This region is a region where there is almost no change in Fe concentration in the depth direction, and is a region that is judged to be "steel" according to common technical knowledge. The internal Fe emission intensity can be, for example, the Fe emission intensity at a sputtering time of 3000 seconds.

[0024] Furthermore, 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 "steel sheet surface" that serves as the reference for the depth position of the steel sheet is the steel sheet surface of the base steel sheet. For example, in GDS analysis, the depth position where the Fe emission intensity reaches 0.7 times the internal Fe emission intensity is defined as the surface of the base steel sheet, i.e., the 0 μm position. In the time-of-flight secondary ion mass spectrometry method described later, Fe ＋ The shallowest position (i.e., the position closest to the steel plate surface) where the ion signal intensity is 0.7 times that at a depth of 1 / 4 of the plate thickness is defined as the surface of the base steel plate, i.e., the 0 μm position.

[0025] Similarly, expressions such as "a depth of 30 μm from the surface of the steel plate" also mean a position 30 μm away from the surface of the steel plate, in the thickness direction, towards the center of the steel plate.

[0026] Preferred embodiments of the steel sheet, the component containing the same, and the method for manufacturing the steel sheet described herein will be explained in detail below. In this specification, unless otherwise specified, the "~" indicating a numerical range includes the numbers before and after it as the lower and upper limits. However, if "greater than" or "less than" is appended to a number, that number is not included.

[0027] When welding plated steel sheets, molten metals such as zinc in the plating layer, melted by the heat of welding, can penetrate the grain boundaries of the weld structure, potentially causing the aforementioned LME cracks inside the steel sheet. Furthermore, LME cracking in spot welding is a phenomenon in which molten zinc penetrates the grain boundaries of the steel due to the tensile stress generated by the thermal contraction of the heat-affected zone during the subsequent cooling process, as well as the stress caused by the steel sheet being pressed by the electrodes during spot welding, resulting in macroscopic cracks.

[0028] To suppress LME cracking, it is important to prevent molten zinc from penetrating the grain boundaries. One possible method to prevent molten zinc from penetrating the grain boundaries is to segregate solid-solution B at the grain boundaries. This method utilizes the property that B easily segregates at grain boundaries, causing the solid-solution B in the steel plate to segregate at the grain boundaries during welding heating, and this segregated solid-solution B at the grain boundaries suppresses the penetration of molten zinc into the interior of the steel plate.

[0029] However, if there are many precipitates of B in the bulk material, it may not be possible to adequately supply the solid-solution B to the grain boundaries and cause segregation. In such cases, tensile stress generated during the subsequent cooling process may trigger molten zinc to penetrate into grain boundaries where the solid-solution B has not segregated, potentially leading to LME cracking.

[0030] Therefore, the Disclosers focused on and diligently investigated methods for efficiently segregating solid-solution B at grain boundaries during welding. As a result, the Disclosers discovered that BO can be measured by time-of-flight secondary ion mass spectrometry. ２ － Regarding the signal strength of the BO ２ － When pixels exhibiting a signal intensity 7.5 times or more than the average signal intensity are in contact with each other, and a region consisting of 5 to 16 such pixels is defined as one specific B precipitate, it was found that by using a steel plate with fewer of these specific B precipitates, solid solution B can be sufficiently segregated at the grain boundaries even in spot welding, resulting in a significant improvement in the LME resistance of the steel plate.

[0031] Although the mechanism by which reducing the number density of specific B precipitates improves LME resistance is not clear, B-deficient regions may form near the interface between specific B precipitates and the bulk material. Such B-deficient regions act as a barrier to B diffusion during high-temperature holding, thereby hindering the supply of solid-solution B to the grain boundaries. In particular, in welding methods with low heat input, such as spot welding, the short holding time at high temperatures makes it difficult to completely eliminate B-deficient regions. Therefore, it is presumed that reducing the number density of specific B precipitates reduces the volume proportion of B-deficient regions in the entire material, thereby efficiently supplying B to the grain boundaries and improving LME resistance.

[0032] Furthermore, the Disclosers have found that, in addition to the above configuration, by appropriately controlling the chemical composition and microstructure of the steel sheet, excellent LME resistance can be obtained even with high-strength steel sheets that are prone to LME cracking, more specifically, high-strength steel sheets with a tensile strength (TS) of 1760 MPa or higher.

[0033] This disclosure is completed based on these findings and includes aspects of each of the following embodiments.

[0034] First, preferred embodiments of the steel sheet of this disclosure will be described in detail.

[0035] <Steel Plate> A steel plate according to one embodiment of the present disclosure has the following unique characteristic configuration.

[0036] First, the chemical composition of the steel sheet in this embodiment is as follows (in mass%): C: 0.27-0.40%, Si: 0.01-2.50%, Mn: 1.00-4.00%, Al: 0.001-1.500%, Ti: 0.001-0.100%, B: 0.0005-0.0100%, P: 0.050% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0100% or less, Cr: 0-1.00%, Cu: 0-1.00%, Mo: 0-1.00%, Ni: 0-1.00%, Co: 0-3.00%, W: 0-1.00%, Sn: 0-1.00%. Sb: 0-0.50%, Nb: 0-0.200%, V: 0-1.00%, As: 0-0.10%, Zn: 0-1.00%, Mg: 0-0.0500%, Ca: 0-0.0500%, Zr: 0-0.5000%, Ce: 0-0.0300%, La: 0-0.0150%, Hf: 0-0.0100%, Bi: 0-0.0100%, REM other than Ce and La: 0-0.1000%, and the remainder: Fe and impurities, and the content (mass%) of C and B satisfies the following formula (1).

[0037] T ｐ <1000 ... (1) However, T ｐ = 555 + 179 × [C] + 8083 × √[B] [C]: Content of C (mass%) [B]: Content of B (mass%)

[0038] Furthermore, in this embodiment, the microstructure of the steel sheet at a depth of 1 / 4 of the sheet thickness from the surface of the steel sheet is as follows in area percentage: total of ferrite and bainite: 5% or less, retained austenite: 10% or less, martensite: 85% or more, and the remainder: 5% or less.

[0039] Furthermore, the steel plate of this embodiment is measured by time-of-flight secondary ion mass spectrometry. ２ － Regarding the signal strength, the above BO ２ －When pixels exhibiting a signal intensity 7.5 times or more than the average signal intensity are in contact with each other, and a region consisting of 5 to 16 such pixels is defined as one specific B precipitate, the number density of this specific B precipitate is 10,000 pixels / mm³. ２ The following applies:

[0040] Furthermore, the steel plate of this embodiment has a tensile strength of 1760 MPa or more.

[0041] As described above, the steel sheet of this embodiment contains the above-mentioned specific B precipitate, i.e., BO measured by time-of-flight secondary ion mass spectrometry. ２ － Regarding the signal strength, the above BO ２ － When pixels exhibiting a signal intensity 7.5 times or more than the average signal intensity are in contact with each other, and a region consisting of 5 to 16 such pixels is defined as one specific B precipitate, the number of specific B precipitates is 10,000 per mm. ２ The number density is reduced to the following range. If specific B precipitates exceeding the above range are present, B will not be sufficiently supplied to the grain boundaries during heating in spot welding, and the effect of suppressing LME cracking described above may not be fully obtained. However, the steel plate of this embodiment has the above-mentioned specific B precipitates reduced to 10,000 pieces / mm ２ By reducing the number density to the following levels, the B-deficient regions that inhibit B diffusion are reduced, allowing solid-solution B to be efficiently supplied along grain boundaries and segregate. As a result, the steel sheet of this embodiment can exhibit excellent LME resistance.

[0042] Furthermore, in addition to having a low concentration of such specific B precipitates, the steel sheet of this embodiment has its chemical composition and microstructure controlled within the appropriate ranges described above. Therefore, even high-strength steel sheets, more specifically those with a strength of 1760 MPa or higher, which are prone to LME cracking, can fully exhibit excellent LME resistance.

[0043] The following describes the characteristic configurations of the steel plate in this embodiment.

[0044] First, the chemical composition of the steel sheet in this embodiment will be described. In the following description, the unit "%" for the content of each element means "mass%" unless otherwise specified.

[0045] [C: 0.27-0.40%] Carbon (C) is an element necessary for improving the strength of steel plates. Sufficient strength cannot be obtained if the C content is less than 0.27%, so the C content should be 0.27% or more. The C content may also be 0.30% or more, 0.32% or more, or 0.34% or more. On the other hand, if the C content exceeds 0.40%, the strength of the welded joint may increase and the LME resistance may decrease, so the C content should be 0.40% or less. The C content may also be 0.38% or less or 0.36% or less.

[0046] [Si: 0.01-2.50%] Silicon (Si) is an element that suppresses the precipitation of iron-based carbides and contributes to improved strength and moldability. In addition, Si is Fe ２３ (C, B) ６ It is an element that contributes to suppressing coarsening. However, if the Si content is less than 0.01%, the above effect may not be sufficiently obtained. Therefore, the Si content should be 0.01% or more. The Si content may be 0.10% or more, 0.50% or more, or 1.00% or more. On the other hand, if the Si content is excessive, the penetration of molten zinc into the steel sheet is promoted, which may reduce the LME resistance of the steel sheet. Therefore, the Si content should be 2.50% or less. The Si content may be 1.80% or less, 1.60% or less, or 1.50% or less.

[0047] [Mn: 1.00–4.00%] Manganese (Mn) is an element that enhances hardenability and contributes to improving the strength of steel sheets. However, if the Mn content is less than 1.00%, these effects may not be fully obtained. Therefore, the Mn content should be 1.00% or more. The Mn content may also be 1.20% or more, 1.50% or more, or 1.70% or more. On the other hand, if the Mn content is excessive, the austenite phase in the steel sheet may be stabilized, which may reduce the LME resistance of the steel sheet. Therefore, the Mn content should be 4.00% or less. The Mn content may also be 3.60% or less, 3.40% or less, 3.20% or less, 3.00% or less, 2.50% or less, or 2.00% or less.

[0048] [Al: 0.001 to 1.500%] Aluminum (Al) is a deoxidizing element and also contributes to improving strength by suppressing the formation of iron-based carbides. However, if the Al content is less than 0.001%, the deoxidizing effect may not be sufficiently obtained. Therefore, the Al content should be 0.001% or more. The Al content may also be 0.005% or more, 0.008% or more, or 0.010% or more. On the other hand, if the Al content exceeds 1.500%, the ferrite transformation in the steel sheet may be promoted, and sufficient strength may not be obtained. Therefore, the Al content should be 1.500% or less. The Al content may also be 1.000% or less, 0.800% or less, 0.500% or less, 0.300% or less, 0.100% or less, or 0.050% or less.

[0049] [Ti: 0.001 to 0.100%] Titanium (Ti) is an element that contributes to improving the strength of steel sheets through precipitation strengthening, fine grain strengthening by suppressing grain growth, and dislocation strengthening through suppression of recrystallization. Furthermore, when manufacturing the steel sheet of this embodiment, Ti also has the effect of suppressing the formation of boron nitride (BN) by combining with solid-solution nitrogen, by forming titanium nitride and consuming the solid-solution nitrogen in the steel. However, if the Ti content is less than 0.001%, these effects may not be sufficiently obtained. Therefore, the Ti content should be 0.001% or more. The Ti content may also be 0.005% or more, 0.008% or more, or 0.010% or more. On the other hand, if the Ti content exceeds 0.100%, bainite transformation in the steel sheet may be suppressed, and sufficient ductility may not be obtained. Therefore, the Ti content should be 0.100% or less. The Ti content may be 0.080% or less, 0.060% or less, or 0.050% or less.

[0050] [B: 0.0005 to 0.0100%] Boron (B) is an element that dissolves during welding and segregates at grain boundaries, contributing to improved LME resistance. However, if the B content is less than 0.0005%, B cannot be sufficiently segregated at grain boundaries, and the effect of improving LME resistance may not be fully obtained. Therefore, the B content should be 0.0005% or more. The B content may also be 0.0008% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if the B content exceeds 0.0100%, coarse carbides and / or nitrides are more likely to be formed, making it difficult to fully obtain the above effect, and the steel plate may become more prone to cracking and its ductility may decrease. Therefore, the B content should be 0.0100% or less. The B content may be 0.0080% or less, 0.0060% or less, 0.0040% or less, 0.0030% or less, 0.0028% or less, 0.0025% or less, or 0.0020% or less.

[0051] [P: 0.050% or less] Phosphorus (P) is an element that inhibits weldability. The P content may be 0%, but reducing the P content to less than 0.0001% requires more time for refining, which may lead to a decrease in productivity. Therefore, the P content may be 0.0001% or more. The P content may be 0.0005% or more, 0.001% or more, or 0.002% or more. On the other hand, if the P content exceeds 0.050%, the steel sheet may become brittle due to P segregation at the grain boundaries, which may reduce LME resistance. Therefore, the P content should be 0.050% or less. The P content may be 0.045% or less, 0.040% or less, or 0.035% or less.

[0052] [S: 0.0100% or less] Sulfur (S) is an element that causes hot embrittlement and inhibits weldability and corrosion resistance. The S content may be 0%, but reducing the S content to less than 0.0001% requires more time for refining, which may lead to a decrease in productivity. Therefore, the S content may be 0.0001% or more. The S content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more. On the other hand, if the S content exceeds 0.0100%, it may lead to embrittlement of the steel plate and a decrease in LME resistance. Therefore, the S content should be 0.0100% or less. The S content may be 0.0080% or less, 0.0060% or less, or 0.0050% or less.

[0053] [N: 0.0200% or less] Nitrogen (N) is an element that forms nitrides, inhibiting ductility and flange properties, and also causes blowholes during welding. The N content may be 0%, but reducing the N content to less than 0.0001% requires time for refining, which may lead to a decrease in productivity. Therefore, the N content may be 0.0001% or more. The N content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more. On the other hand, if the N content exceeds 0.0200%, ductility and flange properties may decrease significantly, and blowholes may easily occur during welding. In addition, if the N content exceeds 0.0200%, boron nitride (BN) may be generated, which may result in a decrease in LME resistance. Therefore, the N content should be 0.0200% or less. The N content may be 0.0180% or less, 0.0160% or less, 0.0140% or less, 0.0120% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0050% or less.

[0054] [O: 0.0100% or less] Oxygen (O) is an element that forms oxides, inhibiting ductility and flangeability, and is also a cause of blowhole formation during welding. The O content may be 0%, but reducing the O content to less than 0.0001% requires time for refining, which may lead to a decrease in productivity. Therefore, the O content may be 0.0001% or more. The O content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if the O content exceeds 0.0100%, ductility and flangeability may be significantly reduced, and blowhole formation during welding becomes more likely. Therefore, the O content should be 0.0100% or less. The O content may be 0.0080% or less, 0.0060% or less, or 0.0050% or less.

[0055] The basic component composition of the steel sheet of this embodiment is as described above. Furthermore, the steel sheet of this embodiment may contain one or more of the following optional elements in place of a portion of the remaining Fe, if necessary.

[0056] [Cr: 0-1.00%] Chromium (Cr) is an element that enhances the hardenability of steel and contributes to improving the strength of steel sheets. The Cr content may be 0%, but to obtain these effects fully, it is preferable that the Cr content be 0.001% or more. The Cr content may be 0.005% or more, 0.010% or more, or 0.020% or more. On the other hand, if the Cr content exceeds 1.00%, pickling properties, weldability, and hot workability may decrease. Therefore, the Cr content should be 1.00% or less. The Cr content may be 0.80% or less, 0.60% or less, 0.50% or less, 0.40% or less, or 0.20% or less.

[0057] [Cu: 0-1.00%] Copper (Cu) is an element that contributes to improving the strength of steel sheets. The Cu content may be 0%, but to obtain this effect fully, it is preferable that the Cu content be 0.001% or more. The Cu content may be 0.005% or more, 0.010% or more, or 0.015% or more. On the other hand, if the Cu content exceeds 1.00%, it can lead to embrittlement of the steel sheet, and cracks may occur during hot rolling. Therefore, the Cu content should be 1.00% or less. The Cu content may be 0.80% or less, 0.60% or less, or 0.50% or less.

[0058] [Mo: 0-1.00%] Molybdenum (Mo) is an element that enhances the hardenability of steel, contributes to improving the strength of steel sheets, and also contributes to improving LME resistance. In addition, it has the effect of suppressing the precipitation of B. The Mo content may be 0%, but in order to fully obtain these effects, it is preferable that the Mo content be 0.01% or more. The Mo content may be 0.02% or more, 0.03% or more, or 0.04% or more. On the other hand, if the Mo content exceeds 1.00%, the strength of the hot-rolled sheet may increase and the cold-rollability may decrease. Therefore, the Mo content should be 1.00% or less. The Mo content may be 0.90% or less, 0.80% or less, 0.70% or less, 0.50% or less, 0.30% or less, or 0.15% or less.

[0059] [Ni: 0-1.00%] Nickel (Ni) is an element that contributes to improving strength. The Ni content may be 0%, but to obtain this effect fully, it is preferable that the Ni content be 0.01% or more. The Ni content may be 0.02% or more, 0.03% or more, or 0.04% or more. On the other hand, if the Ni content exceeds 1.00%, the strength of the hot-rolled sheet may increase and the cold-rollability may decrease. Therefore, the Ni content should be 1.00% or less. The Ni content may be 0.90% or less, 0.80% or less, 0.70% or less, or 0.50% or less.

[0060] [Co: 0-3.00%] Cobalt (Co) is an element that contributes to improving strength. The Co content may be 0%, but to obtain these effects fully, it is preferable that the Co content be 0.01% or more. The Co content may be 0.02% or more, 0.03% or more, or 0.04% or more. On the other hand, if the Co content exceeds 3.00%, ferrite transformation and / or pearlite transformation in the steel sheet may be promoted, and sufficient strength may not be obtained. Therefore, the Co content should be 3.00% or less. The Co content may be 2.50% or less, 2.00% or less, 1.50% or less, 1.00% or less, 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, or 0.10% or less.

[0061] [W: 0-1.00%] Tungsten (W) is an element that is effective in increasing strength and also contributes to improving LME resistance. The W content may be 0%, but in order to obtain these effects fully, it is preferable that the W content be 0.001% or more. The W content may be 0.005% or more, 0.007% or more, or 0.010% or more. On the other hand, if the W content exceeds 1.00%, the strength of the hot-rolled sheet may increase and the cold-rollability may decrease. Therefore, the W content should be 1.00% or less. The W content may be 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, 0.10% or less, or 0.05% or less.

[0062] [Sn: 0-1.00%] Tin (Sn) is an element contained in steel when scrap is used as a raw material, and a lower amount is preferable. The Sn content may be 0%, but reducing the Sn content to less than 0.001% requires more time for refining, which may lead to a decrease in productivity. Therefore, the Sn content may be 0.001% or more. The Sn content may be 0.002% or more, 0.003% or more, or 0.005% or more. On the other hand, if the Sn content exceeds 1.00%, it can lead to embrittlement of the steel sheet, and cracks may occur during hot rolling. Therefore, the Sn content should be 1.00% or less. The Sn content may be 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, 0.10% or less, or 0.05% or less.

[0063] [Sb: 0-0.50%] Antimony (Sb), like Sn, is an element present when scrap is used as a raw material for steel, and a lower amount is preferable. The Sb content may be 0%, but reducing the Sb content to less than 0.001% requires more time for refining, which may lead to a decrease in productivity. Therefore, the Sb content may be 0.001% or more. The Sb content may be 0.002% or more, 0.003% or more, or 0.005% or more. On the other hand, if the Sb content exceeds 0.50%, it may lead to embrittlement of the steel sheet and a decrease in ductility. Therefore, the Sb content should be 0.50% or less. The Sb content may be 0.45% or less, 0.40% or less, 0.35% or less, 0.25% or less, 0.10% or less, or 0.05% or less.

[0064] [Nb: 0-0.200%] Niobium (Nb) is an element that contributes to improving the strength of steel sheets through precipitation strengthening, fine grain strengthening by suppressing grain growth, and dislocation strengthening through suppression of recrystallization. In addition, it has the effect of suppressing the precipitation of boron. The Nb content may be 0%, but in order to obtain these effects sufficiently, it is preferable that the Nb content be 0.001% or more. The Nb content may be 0.005% or more, 0.008% or more, or 0.010% or more. On the other hand, if the Nb content exceeds 0.200%, coarse carbides may precipitate and become the starting point for cracks, which may cause a decrease in LME resistance. Therefore, the Nb content should be 0.200% or less. The Nb content may be 0.150% or less, 0.120% or less, 0.100% or less, 0.080% or less, 0.060% or less, or 0.040% or less.

[0065] [V: 0-1.00%] Vanadium (V) is an element that contributes to improving the strength of steel sheets through precipitation strengthening, fine grain strengthening by suppressing grain growth, and dislocation strengthening by suppressing recrystallization. The V content may be 0%, but in order to obtain these effects fully, it is preferable that the V content be 0.001% or more. The V content may be 0.005% or more, 0.008% or more, or 0.010% or more. On the other hand, if the V content exceeds 1.00%, coarse carbides may precipitate and become the starting point for cracks, which may cause a decrease in LME resistance. Therefore, the V content should be 1.00% or less. The V content may be 0.80% or less, 0.60% or less, 0.50% or less, 0.40% or less, or 0.30% or less.

[0066] [As: 0-0.10%] Arsenic (As), like Sn and Sb, is an element present when scrap is used as a raw material for steel, and a lower amount is preferable. The As content may be 0%, but reducing the As content to less than 0.001% requires more time for refining, which may lead to a decrease in productivity. Therefore, the As content may be 0.001% or more. The As content may be 0.002% or more, 0.003% or more, or 0.005% or more. On the other hand, if the As content exceeds 0.10%, it may lead to embrittlement of the steel sheet and a decrease in ductility. Therefore, the As content should be 0.10% or less. The As content may be 0.08% or less, 0.06% or less, or 0.05% or less.

[0067] [Zn: 0-1.00%] Zinc (Zn) is an effective element for controlling the morphology of sulfides and improving local ductility and elongation flange properties. The Zn content may be 0%, but to obtain these effects fully, it is preferable that the Zn content be 0.001% or more. The Zn content may be 0.002% or more, 0.003% or more, or 0.005% or more. On the other hand, if the Zn content exceeds 1.00%, the number of inclusions increases, which may cause defects on the surface and inside the steel sheet. Therefore, the Zn content should be 1.00% or less. The Zn content may be 0.80% or less, 0.60% or less, 0.40% or less, 0.25% or less, or 0.10% or less.

[0068] [Mg: 0-0.0500%] Magnesium (Mg) is an element that can control the form of sulfides with trace additions and also contributes to improving LME resistance. The Mg content may be 0%, but to fully obtain these effects, it is preferable that the Mg content be 0.0001% or more. The Mg content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if the Mg content exceeds 0.0500%, it may lead to embrittlement of the steel sheet and a decrease in ductility. Therefore, the Mg content should be 0.0500% or less. The Mg content may be 0.0450% or less, 0.0400% or less, 0.0350% or less, 0.0300% or less, 0.0250% or less, 0.0200% or less, 0.0150% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0050% or less.

[0069] [Ca: 0-0.0500%, Zr: 0-0.5000%, Ce: 0-0.0300%, La: 0-0.0150%, Hf: 0-0.0100%, Bi: 0-0.0100%, REM other than Ce and La: 0-0.1000%] Calcium (Ca), zirconium (Zr), cerium (Ce), lanthanum (La), hafnium (Hf), and REM (rare earth elements) other than Ce and La are all elements that can control the form of sulfides and contribute to improving LME resistance. In addition, Bi (bismuth) is an element that reduces the microsegregation of substitutional alloy elements such as Mn and Si in steel. For this reason, one or more of these elements may be included as needed. The content of each of these elements may be 0%, but in order to fully obtain the above effects, it is preferable that the content of Ca, Zr, Ce, La, Hf, Bi, and REM other than Ce and La be 0.0001% or more. The content of Ca, Zr, Ce, La, Hf, Bi, and REM other than Ce and La may be 0.0002% or more, 0.0003% or more, 0.0005% or more, or 0.0010% or more, respectively.

[0070] On the other hand, if the Ca content exceeds 0.0500%, it can lead to embrittlement of the steel sheet and a decrease in ductility. Therefore, the Ca content should be 0.0500% or less.

[0071] Similarly, if the Ce content exceeds 0.0300%, it can lead to embrittlement of the steel sheet and a decrease in ductility. Therefore, the Ce content should be 0.0300% or less.

[0072] Similarly, if the Zr content exceeds 0.5000%, it can lead to embrittlement of the steel sheet and a decrease in ductility. Therefore, the Zr content should be 0.5000% or less.

[0073] Similarly, if the content of Hf and Bi exceeds 0.0100%, or if the content of La exceeds 0.0150%, it may lead to embrittlement of the steel sheet and a decrease in ductility. Therefore, the content of Hf and Bi should be 0.0100% or less, and the content of La should be 0.0150% or less.

[0074] Furthermore, if the content of REMs other than Ce and La exceeds 0.1000%, it may lead to embrittlement of the steel sheet and a decrease in ductility. Therefore, the content of REMs other than Ce and La should be 0.1000% or less.

[0075] The Ca content may be 0.0400% or less, 0.0300% or less, 0.0200% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0050% or less.

[0076] Furthermore, the Ce content may be 0.0250% or less, 0.0200% or less, 0.0150% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0050% or less.

[0077] Furthermore, the Zr content may be 0.4000% or less, 0.3000% or less, 0.2000% or less, 0.1000% or less, 0.0750% or less, 0.0500% or less, 0.0300% or less, or 0.0100% or less.

[0078] Furthermore, the content of Hf and Bi may be 0.0080% or less, 0.0060% or less, or 0.0050% or less, respectively.

[0079] Furthermore, the La content may be 0.0120% or less, 0.0110% or less, or 0.0100% or less.

[0080] Furthermore, the content of REM other than Ce and La may be 0.0800% or less, 0.0600% or less, 0.0400% or less, 0.0200% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0050% or less.

[0081] In this specification, REM refers to the collective term for 17 elements, including scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content is the total content of these elements.

[0082] In this embodiment, the chemical composition of the steel sheet is as follows, in mass%, for the above optional elements: Cr: 0.001 to 1.00%, Cu: 0.001 to 1.00%, Mo: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Co: 0.01 to 3.00%, W: 0.001 to 1.00%, Sn: 0.001 to 1.00%, Sb: 0.001 to 0.50%, Nb: 0.001 to 0.200%, V: 0.001 to 1.00%, As: 0.001 to 0.10%, Zn: 0.001 to 1.00%, Mg: 0.0001 to 0.0500%. It may also contain one or more of the following: Ca: 0.0001 to 0.0500%, Zr: 0.0001 to 0.5000%, Ce: 0.0001 to 0.0300%, La: 0.0001 to 0.0150%, Hf: 0.0001 to 0.0100%, Bi: 0.0001 to 0.0100%, and REM other than Ce and La: 0.0001 to 0.1000%.

[0083] Furthermore, the above chemical composition may contain, in mass percent, N: 0.0100% or less, Co: 0 to 1.00%, B: 0.0005 to 0.0030%, Mg: 0 to 0.0100%, Ca: 0 to 0.0100%, Zr: 0 to 0.0100%, Ce: 0 to 0.0150%, and REM other than Ce and La: 0 to 0.0100%.

[0084] In the steel sheet of this embodiment, the remainder of the elements other than those mentioned above consists of Fe and impurities. Here, impurities are components that are mixed in during the industrial production of steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap. It should be noted that impurities include components that were not intentionally added to the steel sheet of this embodiment (so-called unavoidable impurities). Furthermore, impurities also include elements other than those mentioned above that are present in the steel sheet to such an extent that their specific effects do not affect the properties of the steel sheet of this embodiment.

[0085] [Content of C and B: Formula (1)] In the steel sheet of this embodiment, the content (mass%) of C and B satisfies the following formula (1). T calculated by the following formula (1) ｐ This refers to the precipitation start temperature (°C) of B, and T ｐ The temperature should be 1000°C or lower.

[0086] T ｐ <1000 ... (1) However, T ｐ = 555 + 179 × [C] + 8083 × √[B] [C]: Content of C (mass%) [B]: Content of B (mass%)

[0087] In the above formula (1), T ｐ B is Fe ２３ (C, B) ６ This refers to the precipitation initiation temperature when precipitation occurs, and in this specification, it is conveniently referred to as the "precipitation initiation temperature of B". Fe ２３ (C, B) ６ If precipitation occurs, B may not be able to segregate sufficiently at the grain boundaries, and the effect of improving LME resistance may not be fully obtained. Therefore, T ｐ The temperature should be less than 1000°C. ｐ The temperature may be less than 950°C, less than 900°C, less than 850°C, or less than 800°C.

[0088] Note that the above equation (1) is an approximate formula derived from calculation values ​​obtained using the integrated thermodynamic calculation software "Thermo-Calc" (registered trademark). ｐ In the expression representing T ｐThe unit is "℃", but the unit on the right side also includes the term √B. However, if we calculate [C] and [B] in the formula using "mass%", the unit of the calculation result will be "℃".

[0089] 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 inductively coupled plasma atomic emission spectroscopy (ICP-AES). Specifically, the front and back surfaces of the steel sheet are ground to a depth of 200 μm from each surface to take test pieces, and the chemical composition of the steel sheet can be determined by measuring it using a measuring device such as the Shimadzu ICPS-8100 under conditions based on a pre-established calibration curve. C and S, which cannot be measured by ICP-AES, can be measured using combustion-infrared absorption spectroscopy, N can be measured using inert gas fusion-thermal conductivity spectroscopy, and O can be measured using inert gas fusion-nondispersive infrared absorption spectroscopy.

[0090] Furthermore, if analysis values ​​for molten steel, slabs, or other steel sheets manufactured from the same molten steel are available, the analysis of test pieces taken from the steel sheet may be omitted, and those analysis values ​​may be considered as the chemical composition of the steel sheet.

[0091] Next, the microstructure of the steel sheet in this embodiment will be described.

[0092] As described above, in this embodiment, the microstructure of the steel sheet at a depth of 1 / 4 of the sheet thickness from the surface of the steel sheet is as follows in area percentage: total of ferrite and bainite: 5% or less, retained austenite: 10% or less, martensite: 85% or more, remainder: 5% or less.

[0093] The following describes each of these organizations.

[0094] [Total content of ferrite and bainite: 5% or less] Ferrite is a soft structure with excellent ductility. On the other hand, bainite is a structure that is effective in ensuring the ductility and strength of steel sheets. These structures are included in order to improve the elongation of the steel sheet, depending on the required strength or ductility. From the viewpoint of ensuring the strength of the steel sheet, the total content of ferrite and bainite shall be 5% or less by area percentage. The total content of ferrite and bainite may be 4% or less, 3% or less, or 2% or less by area percentage. The total content of ferrite and bainite may be 0% by area percentage, but from the viewpoint of improving the ductility of the steel sheet, it may be 1% or more or 2% or more.

[0095] [Retained Austenite: 10% or less] Retained austenite is a structure that improves the ductility of steel sheets through the TRIP effect, which is the transformation into martensite during deformation of the steel sheet (i.e., work-induced transformation). The retained austenite content may be 0% by area percentage, but from the viewpoint of improving the ductility of the steel sheet, the retained austenite content may be 1% or more, 3% or more, or 5% or more. On the other hand, if retained austenite is present in excess, crack propagation may be promoted, which may reduce the LME resistance of the steel sheet. Therefore, the retained austenite content should be 10% or less by area percentage, and may be 8% or less, 6% or less, 4% or less, or 2% or less.

[0096] [Martensite: 85% or more] Martensite is a high-strength structure that increases the tensile strength of steel sheets. Therefore, it is an important structure for ensuring the desired steel sheet strength. The martensite referred to here includes fresh martensite and tempered martensite. Fine cementite present within tempered martensite is considered part of the tempered martensite. To ensure sufficient strength, the martensite content shall be 85% or more by area percentage. The martensite content may be 90% or more, 92% or more, or 95% or more. Alternatively, the martensite content may be 100% or less, 99% or less, or 98% or less.

[0097] [Remainder: 5% or less] Considering the chemical composition of the steel sheet of this embodiment and the preferred manufacturing conditions, cementite and other precipitates are measured as the area percentage of the microstructure in which they exist. These microstructures are common in high-strength steel sheets used in fields such as automobiles, home appliances, and building materials, which are the subject of this application. In this disclosure, the effect on strength and LME resistance is largely due to martensite, which makes up most of the microstructure, and the ferrite, bainite, and retained austenite present with it, so even if cementite and alloy precipitates of at most about 5% are formed, there is no need to consider the effect on strength and LME resistance. Furthermore, if the manufacturing method conforms to the preferred manufacturing conditions of this disclosure, it is easy to keep the area percentage of the remaining region to 5% or less. The area percentage of the remaining region is more preferably 2% or less, and even more preferably 1% or less or 0%. The effect of the remaining region on strength and LME resistance is negligible.

[0098] The remaining region may contain perlite. However, since there is very little perlite, the remaining region does not necessarily need to contain perlite.

[0099] (Identification of Microstructure and Calculation of Area Ratio) For the above-mentioned microstructure, the area ratio of retained austenite can be calculated by X-ray diffraction. In addition, the total of ferrite and bainite, as well as martensite, can be identified and their area ratios calculated by observing and measuring the cross-section of the steel sheet using a scanning electron microscope.

[0100] Specifically, the area ratio of retained austenite is calculated by X-ray diffraction. First, the area from the surface of the steel plate to a depth of 1 / 4 of the plate thickness is removed by mechanical and chemical polishing. Next, the surface of the polished sample is examined using MoKα rays as characteristic X-rays to obtain the diffraction peaks of the bcc phase (200) and (211), and the fcc phase (200), (220), and (311). At this time, the total irradiated area is 1 mm². ２ This concludes the explanation. The integral intensity ratio of each diffraction peak is used to calculate the structural fraction of retained austenite, and this is taken as the area fraction of retained austenite.

[0101] The total area ratio of ferrite and bainite is measured by the following method. First, a sample is taken from the thickness cross section of the steel plate parallel to the rolling direction, and the observation surface is mechanically polished to a mirror finish, followed by electropolishing. Next, in one or more observation fields centered in the thickness direction at a depth of 1 / 4 of the steel plate thickness on the observation surface, crystal structure analysis and orientation analysis are performed for Iron-Alpha and Iron-Gamma by electron backscatter diffraction using a scanning electron microscope. At this time, each observation field has a length of 200 μm in the thickness direction and a length of 200 μm in the horizontal direction of the plate surface, and the distance between evaluation points (step) is 0.20 μm. For the analysis of the data obtained by electron backscatter diffraction, "OIM Analysis® 6.0" manufactured by TSL Corporation is used. From the measurement data, the average grain orientation difference (GAM) for Iron-Alpha is calculated. Then, the area fraction of the region with a GAM value of less than 0.1° is multiplied by the area fraction of the bcc phase to obtain the total area fraction of ferrite and bainite.

[0102] Here, "the value obtained by multiplying the area fraction of the region with a GAM value of less than 0.1° by the area fraction of the bcc phase" is "the value of (area of ​​the region with a GAM value of less than 0.1° / area of ​​Iron-Alpha) × (1 - area fraction of retained austenite)". Furthermore, the average orientation difference within a crystal grain is the value obtained by calculating the orientation difference between adjacent measurement points in a region surrounded by grain boundaries where the crystal orientation difference is 5° or more, and averaging this value over all measurement points within the crystal grain.

[0103] The total area percentage of martensite is measured by the following method. First, nital etching is performed on the same observation surface of the sample that was observed using the electron backscatter diffraction method described above. Then, secondary electron imaging is performed using a scanning electron microscope on the same field of view as observed using electron backscatter diffraction on the etched observation surface. This observation should be performed at a magnification of 1,000 to 50,000. To observe the same field of view as observed using electron backscatter diffraction, it is advisable to mark the surface beforehand with Vickers indentations or other markers. In the field of view of the obtained secondary electron image, the area percentage of tissue that is relatively unetched, has a lath-like understructure, and contains fine cementite or retained austenite less than 1 μm in size is measured by point counting, and the area percentage of martensite is calculated by subtracting the area percentage of the retained austenite. The finer the lattice spacing when performing point counting, the more accurate the value will be. For example, a lattice spacing of 2 μm is appropriate.

[0104] If the total area ratio of each tissue obtained by the above method is less than 100%, the remaining area is considered to be other small amounts of tissue. If the total area ratio of each tissue obtained by the above evaluation method exceeds 100%, the area ratio of each tissue is determined by multiplying the area ratio of each tissue by 100 / (total area ratio of each tissue).

[0105] [BO ２ － When pixels exhibiting a signal intensity 7.5 times or more than the average signal intensity are adjacent to each other, and a region consisting of 5 to 16 such pixels is defined as one specific B precipitate, the number density of specific B precipitates is 10,000 pixels / mm³. ２ [The following] In this embodiment, when the steel plate was measured using Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS) in the range from a depth of 30 μm to a depth of 130 μm from the surface of the steel plate, BO ２ － When pixels exhibiting a signal intensity 7.5 times or more than the average signal intensity are in contact with each other, and a region consisting of 5 to 16 such pixels is defined as one specific B precipitate, the number density of such specific B precipitates is 10,000 pixels / mm² per area.２ The steel sheet of this embodiment has the following characteristic configuration (hereinafter sometimes referred to as the "characteristic configuration relating to specific B precipitates"). When such specific B precipitates are present, B may not be sufficiently supplied to the grain boundaries, and the effect of suppressing LME cracking described above may not be fully obtained. However, since the specific B precipitates are reduced in the steel sheet of this embodiment, solid-solution B can be efficiently supplied toward the grain boundaries and segregate. As a result, the steel sheet of this embodiment can exhibit excellent LME resistance.

[0106] Furthermore, the lower the number density of specific B precipitates, the greater the effect of suppressing LME cracking, with a number density of specific B precipitates of 8000 particles / mm³. ２ Below, 6500 pieces / mm ２ Below, 5000 pieces / mm ２ The following or 3500 pieces / mm ２ The following is preferable: The number density of specific precipitate B is 2000 particles / mm³. ２ The following or 1000 pieces / mm ２ The following is also acceptable. Furthermore, in order to improve strength, the number density of specific B precipitates should be 1000 particles / mm³. ２ The above is preferable.

[0107] The number density of specific precipitate B is measured by the following method.

[0108] A sample is taken from the cross-section of the steel plate parallel to the rolling direction, and the observation surface is polished to a mirror finish with diamond paste. Then, in one or more observation fields at depths of 30 μm to 130 μm from the surface of the steel plate on the observation surface, TOF-SIMS analysis is performed according to the following procedure. ２ ― Measure the signal strength.

[0109] For TOF-SIMS analysis, commercially available analytical instruments can be used. In this embodiment, an ION-TOF TOF-SIMS5 is used, and the measuring ion gun is Bi １ ＋ (30kV) will be used. The measurement pitch will be 0.05μm, with 2008 points in the thickness direction and 2008 points horizontally to the thickness, for a total of 4,032,064 points. ２ －The signal strength (hereafter referred to as "BO") ２ － The signal strength of the signal is sometimes simply referred to as "signal strength." The signal strength of the signal is measured. The obtained data is binned to convert it into 251-point x 251-point data. Here, Figure 1 is a schematic diagram to explain the binning process. The binning process is an operation that adds the signal strengths of adjacent 8-point x 8-point data to convert it into a single point. Coordinates are assigned to the measurement points before conversion as shown in Figure 1(a), and the signal strength at that position is calculated as I ｎ （ｉ，ｊ） In this case, the signal intensity I at the transformed coordinate (i,j) shown in Figure 1(b) （ｉ，ｊ） It can be calculated using the following formula.

[0110]

[0111] By performing binning on 4,032,064 measurement data points, the number of data points is reduced to 1 / 64th.

[0112] In Figure 1, one measurement point is displayed as a 0.05 μm square rectangular area. In binning, the signal strengths of adjacent 8x8 points are added together to convert it into a single data point. Binning is commonly used in image processing or data processing, and it can eliminate "noise" such as errors contained in the data, interpolate appropriate values, and furthermore, enable high-speed processing of large amounts of data. Through the above measurement and binning process, in a rectangular area ranging from a depth of 30 μm to 130 μm from the surface of the steel plate in the thickness direction, and in a range of 100 μm in the horizontal direction of the plate thickness (i.e., perpendicular to the thickness direction), BO ２ ― The signal strength measurement is obtained.

[0113] Next, in the 251 x 251 data obtained by binning, BO ２ － Pixels showing a signal intensity 7.5 times or more than the average signal intensity are in contact with each other, and a region where 5 to 16 pixels are clustered together is identified as a single specific B precipitate. Here, the "clustered region" refers to BO ２ －When focusing on any single pixel (measurement point) that shows a signal intensity 7.5 times or more than the average signal intensity, BO occurs in that pixel and in any of the eight pixels adjacent to it vertically, horizontally, or diagonally. ２ － This refers to a region formed by pixels whose signal intensity is 7.5 times or more the average value. Furthermore, the eight pixels adjacent to the said pixel vertically, horizontally, and diagonally are considered to be BO. ２ － Determining that any single pixel exhibiting a signal strength 7.5 times or more than the average signal strength of the entire spectrum constitutes a contact between that pixel and any other single pixel means that even if the pixels are only touching at their vertices (i.e., only diagonally), they are still considered adjacent.

[0114] Therefore, "BO ２ － "A region where pixels showing a signal intensity 7.5 times or more than the average signal intensity are adjacent to each other, and where 5 to 16 pixels are clustered together" is, in other words, BO ２ － This refers to a region where 5 to 16 pixels that exhibit a signal strength 7.5 times or more than the average signal strength are adjacent to each other.

[0115] After identifying the specific B precipitates using the method described above, count the number of specific B precipitates present within the observation field. At this time, specific B precipitates touching the top, bottom, left, and right edges of the 251x251 area are treated as 0.5. The number of specific B precipitates obtained is divided by the area of ​​the 251x251 area to obtain the number density of specific B precipitates (precipitates / mm²). ２ It is calculated as follows:

[0116] In the steel sheet of this embodiment, the characteristic configuration relating to the specific B precipitate described above may be provided on only one surface of the steel sheet, or on both surfaces of the steel sheet. In the latter case, the state of the specific B precipitate can be made uniform on both the front and back surfaces of the steel sheet, and the effects of this disclosure can be more reliably achieved.

[0117] Note BO ２ －Pixels showing a signal strength of 7.5 times or more the average value of the signal strength are in contact with each other, and in the precipitates in a region where less than 5 pixels are gathered, they can be completely dissolved during welding, and since B can be supplied to grain boundaries without special control, the influence is considered small. On the other hand, BO ２ － In the precipitates in a region where pixels showing a signal strength of 7.5 times or more the average value of the signal strength are in contact with each other and more than 16 pixels are gathered, they can hardly be dissolved during welding, and since the contribution to the supply of B to grain boundaries is relatively small, the influence is considered small.

[0118] Therefore, it is important to suppress the generation of precipitates (i.e., specific B precipitates) in a region where pixels showing a signal strength of 7.5 times or more the average value of the signal strength of BO ２ － are in contact with each other and 5 to 16 pixels are gathered.

[0119] The means for realizing the characteristic configuration regarding the specific B precipitate as described above, that is, the means for reducing the specific B precipitate, will be described in the manufacturing method of the steel sheet described later.

[0120] [Tensile strength: 1760 MPa or more] As described above, in addition to having the characteristic configuration regarding the specific B precipitate, the steel sheet of the present embodiment has its chemical composition and microstructure appropriately controlled. Therefore, even at a high strength where LME cracking is likely to occur, more specifically, at a high strength with a tensile strength (TS) of 1760 MPa or more, excellent LME resistance can be sufficiently exhibited. That is, the steel sheet of the present embodiment can achieve a high strength with a tensile strength of 1760 MPa or more while having excellent LME resistance. Note that the tensile strength of the steel sheet may be 1800 MPa or more or 2000 MPa or more. The upper limit of the tensile strength is not particularly limited, but from the viewpoint of workability and the like, for example, it is 3000 MPa or less, and may be 2800 MPa or less.

[0121] Incidentally, the tensile strength (TS) of the steel sheet can be measured by taking a No. 5 tensile test piece of JIS Z2241:2022 with the longitudinal direction being the direction orthogonal to the rolling direction and the sheet thickness direction from the steel sheet and conducting a tensile test in accordance with JIS Z2241:2022. When the No. 5 tensile test piece cannot be taken from the steel sheet of the measurement sample (for example, when the size of the measurement sample is small), a tensile test piece of any size with the longitudinal direction being the direction orthogonal to the rolling direction and the sheet thickness direction may be adopted instead of the No. 5 tensile test piece.

[0122] In the above electron backscatter diffraction analysis, TOF-SIMS measurement, and tensile test, when the rolling direction cannot be specified, a sample with an arbitrary direction orthogonal to the sheet thickness direction as the observation surface or a tensile test piece with the longitudinal direction may be adopted.

[0123] (Decarburization) It is preferable that the steel sheet of this embodiment satisfies the following formula (2) for the emission intensity of C measured by glow discharge optical emission spectroscopy (GDS analysis) in the sheet thickness direction.

[0124]

[0125] C ３０ : Emission intensity of C at a depth position of 30 μm from the steel sheet surface C １ｔ／４ : Emission intensity of C at a depth position of 1 / 4 of the sheet thickness of the steel sheet

[0126] In this specification, the region in the sheet thickness direction from the steel sheet surface to a depth position of 30 μm is the surface layer portion of the steel sheet. [[ID=二十]]

[0127] The above formula (2) means that the carbon concentration at a depth position of 30 μm from the steel sheet surface is less than 0.50 times the carbon concentration at a depth position of 1 / 4 of the sheet thickness. By reducing the carbon concentration in the surface layer portion of the steel sheet (hereinafter sometimes simply referred to as "decarburization") so as to satisfy this formula (2), the resistance to LME cracking can be further improved.

[0128] In the above formula (2), C ３０ / C １ｔ／４ may be 0.45 or less, 0.40 or less, or 0.35 or less. Also, C ３０ / C １ｔ／４It may be 0, but it may also be 0.10 or greater, 0.15 or greater, or 0.20 or greater.

[0129] The degree of decarburization can be controlled by adjusting the atmosphere from 650°C to the maximum heating temperature during the heat treatment process, which is described later in step (D) of the steel sheet manufacturing method.

[0130] Furthermore, GDS analysis measurements will be performed at five arbitrary positions in accordance with JIS K0144:2018 "Surface Chemical Analysis - General Rules for Glow Discharge Emission Spectroscopy." ３０ The average value of the luminescence intensity of C at a depth of 30 μm from the surface of the steel plate at five arbitrary positions is adopted. Specifically, C ３０ Each is measured using a glow discharge emission spectrometer. For GDS analysis, the surface of the steel plate to be measured is placed in an Ar atmosphere, and a voltage is applied to generate a glow plasma. The surface of the steel plate is then sputtered and analyzed in the depth direction. The element-specific emission spectral wavelengths emitted when atoms are excited in the glow plasma are used to identify the element C contained in the steel plate, and the emission intensity of the identified element C is estimated. １ｔ／４ Similarly, measurements can be taken for C, and the average value of the luminescence intensity of C at a depth of 1 / 4 of the steel plate thickness can be adopted.

[0131] 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. The sputtering time should be set so that the sputtering depth is at least 30 μm or more.

[0132] For GDS analysis, commercially available analytical instruments can be used. In this embodiment, a high-frequency glow discharge emission spectrometer GD-Profiler2 (registered trademark) manufactured by Horiba, Ltd. is used. The detection pitch is set to 0.1 seconds. After removing the background from the obtained data, filtering is performed. Filtering is performed using the moving average method. Specifically, a moving average of 51 points, consisting of the center point plus 25 points before and after it, is calculated. The time value corresponding to a depth of 30 μm is C30. Other measurement conditions are as follows.

[0133] Ar gas pressure: 600 Pa, Anode diameter: 4 mmφ, RF output: 35 W

[0134] In this specification, as described above, the 0 μm position is defined as the depth position where the Fe emission intensity determined by GDS analysis reaches 0.7 times the internal Fe emission intensity. In this definition, the internal Fe emission intensity may be, for example, the Fe emission intensity at a sputtering time of 3000 seconds.

[0135] In the steel sheet of this embodiment, the characteristic configuration for decarburization described above may be provided on only one surface of the steel sheet, or on both surfaces of the steel sheet. In the latter case, the decarburization state can be made uniform on both the front and back surfaces of the steel sheet, and the effects of this disclosure can be more reliably achieved.

[0136] (Plating layer) The steel sheet of this embodiment may also have a zinc-containing plating layer (hereinafter sometimes referred to as the "zinc-containing plating layer") on at least one surface. The plating layer may be a zinc-containing plating layer having any known composition, and may also contain additive elements other than zinc, such as aluminum or magnesium. Furthermore, this zinc-containing plating layer may or may not be subjected to alloying treatment. In addition, the amount of zinc-containing plating layer that adheres is not particularly limited and may be a general amount.

[0137] If the steel sheet has a zinc-containing plating layer, molten zinc from the zinc-containing plating layer can naturally penetrate into the base steel sheet during welding, causing LME cracks inside the steel sheet. Furthermore, even with unplated steel sheets, for example, when welding to a galvanized steel sheet, molten zinc from the galvanized sheet can penetrate into the unplated steel sheet, causing LME cracks. However, as described above, the steel sheet of this embodiment has a reduced number density of specific B precipitates, allowing solid-solution B to be efficiently supplied along the grain boundaries and segregate during spot welding heating, resulting in excellent LME resistance.

[0138] (Application Examples) As described above, the steel sheet of this embodiment is a high-strength steel sheet with excellent LME resistance, and can therefore be applied to various structural components such as automobiles and other transportation machinery, industrial machinery, and buildings where excellent strength and durability are required. In particular, this disclosure can more reliably prevent LME cracking in spot welding using galvanized steel sheets, and can therefore be used especially suitably in the manufacture of automobile bodies and parts.

[0139] <Parts> In one preferred embodiment of the present disclosure, an automotive part is provided which includes a steel sheet according to the embodiment of the present disclosure described above. Examples of automotive parts include frame parts, bumpers, and other structural and reinforcing parts that require strength. Further examples of automotive parts include exterior parts such as roofs, hoods, fenders, and doors that require high aesthetic appeal. These parts only need to include a steel sheet according to the embodiment of the present disclosure in at least a portion of them. Therefore, these parts satisfy the characteristics of the steel sheet of the embodiment described above in at least a portion of them. In forming processes such as press forming, the characteristics of the steel sheet do not particularly change before and after forming in areas of the steel sheet that do not come into direct contact with the mold or, even if they come into direct contact with the mold, are processed to a relatively low degree. For example, in a part including a steel sheet according to the embodiment of the present disclosure, the portion of the sample collection site described later (i.e., the portion avoiding the locations described in (i) to (iv) below) can be recognized as an unprocessed portion, and this portion retains the characteristics of the steel sheet of the embodiment described above before and after forming as a part.

[0140] The steel sheet according to the embodiments of this disclosure can be used as various automotive parts as described above, for example, after a chemical conversion coating or paint film is optionally formed on its surface. Whether or not an automotive part having a paint film or chemical conversion coating includes the steel sheet according to the embodiments of this disclosure can be determined (without removing the paint film or chemical conversion coating) by performing GDS analysis or TOF-SIMS analysis, etc., on a sample taken from the automotive part using the method described above. However, if there are problems such as the need for a long time for sputtering of the paint film, chemical conversion coating, or plating in the GDS analysis, etc., the plating, chemical conversion coating, or paint film may be removed as necessary. The sampling locations are as follows.

[0141] 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 said 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.

[0142] Methods for removing coatings, chemical conversion coatings, or plating include known chemical methods (for example, a method for removing coatings using a coating remover (specifically, Neoriver® #160, manufactured by Sansai Chemical Co., Ltd.), a method for removing chemical conversion coatings in accordance with JIS K 3151:1996, or a method for dissolving and removing the plating layer with an acidic aqueous solution containing an inhibitor (for example, an acidic aqueous solution at room temperature with 10% hydrochloric acid and 0.04% by mass of Ibit 710K (manufactured by Asahi Chemical Industry Co., Ltd.)), known mechanical methods (removal using a grinder or sandpaper, etc.), or methods combining these. In particular, in GDS analysis, it is preferable to limit the removal to a portion and leave some of these portions intact before subjecting the material to analysis tests using the methods described above. If these materials are completely removed, it is preferable to confirm by some means that a portion of the base steel sheet has not been removed, or that the analysis position is at the plate thickness position specified in each analysis method (for example, 1 / 4 of the plate thickness from the surface of the steel sheet).

[0143] <Method for Manufacturing Steel Sheets> Next, a method for manufacturing steel sheets according to one embodiment of the present disclosure will be described. The following description is intended to illustrate characteristic methods for manufacturing steel sheets according to the embodiment of the present disclosure, and is not intended to limit the steel sheets to those manufactured by the manufacturing method described below.

[0144] A method for manufacturing a steel sheet according to one embodiment of this disclosure is a manufacturing method having the following steps (A) to (D). These steps will be described below.

[0145] [Process (A)] In this embodiment, process (A) is a process of hot rolling a slab having the same chemical composition as the specific chemical composition described above with respect to the steel sheet. Process (A) is a process in which the slab is finish-rolled, the hot-rolled steel sheet after the finish-rolling is completed is cooled and wound up to form a hot-rolled coil, and in the finish-rolling process T ｐThe cumulative reduction ratio at the following temperatures is 30% or less, the finish rolling completion temperature is 800°C or higher, and the temperature of the hot-rolled steel sheet during the cooling process after the finish rolling is T ｐ The thermal history from reaching the temperature to the start of winding satisfies equation (3) below, and the thermal history from the start of winding to cooling to 100°C satisfies equation (4) below. x1 calculated by equation (3) below is when the temperature of the hot-rolled steel sheet is T ｐ x1 represents the cumulative average diffusion distance (m) of B from the time the temperature reaches 100°C until the start of winding. To suppress nucleation of B between finish rolling and winding, x1 is set to 0.0060 m or less. x2, calculated by the following formula (4), represents the cumulative average diffusion distance (m) of B from the start of winding until the steel sheet temperature reaches 100°C. To suppress the growth and coarsening of precipitates after winding, x2 is set to 3.7 m or less.

[0146] Here, the cooling process refers to the process in which heat is removed from the steel sheet via cooling water and air after finish rolling. In cases where there is a large amount of heat generated due to phase transformation, etc., it is conceivable that the steel sheet temperature may rise, but the section in which the steel sheet temperature rises due to the heat generated by the steel sheet itself is also included in the cooling process. Note that if the steel sheet temperature rises during the cooling process due to the heat generated as described above, ｐ If the temperature exceeds T ｐ The point at which the temperature of the hot-rolled steel sheet reaches the T temperature during the cooling process is defined as "the temperature of the hot-rolled steel sheet during the cooling process is T ｐ It is considered that the temperature has reached that point.

[0147]

[0148] t: The temperature of the hot-rolled steel sheet is T ｐ The elapsed time (in seconds) from the point when the temperature reached t ＣＴ : The temperature of the hot-rolled steel sheet is T ｐTime (seconds) from reaching the specified temperature until winding begins. T(t): Temperature of the steel plate at time t (°C). However, the integration step is 1 second. If the measurement interval is not 1 second, the temperature at each second is calculated by linear interpolation of the temperatures between measurement points. For example, if a certain measurement point is the time origin (t=0) and the measurement interval is every 3 seconds, data points t=0, 3, 6, ... are accumulated. In this case, when determining the temperatures at t=1 and t=2, the temperature gradient (dT / dt) between t=0 and t=3 is calculated, and T(1) = T(0) + dT / dt and T(2) = T(0) + 2 × dT / dt are calculated.

[0149] Furthermore, in equation (3), the unit of x1 is "m", but the unit of the right-hand side is t, where T(t) in the equation is in "℃" from t=0 seconds. ＣＴ When calculations are performed down to the second, the unit of the calculation result becomes "m".

[0150]

[0151] t: Elapsed time (seconds) since winding began １００℃ : Time elapsed (seconds) from the start of winding until the temperature of the hot-rolled steel sheet reaches 100°C T(t): Temperature of the steel sheet at time t (°C) However, the integration step is 1 second. If the measurement interval is not 1 second, the temperature every second is calculated by linearly interpolating the temperature between measurement points in the same way as in equation (3) above.

[0152] Furthermore, in equation (4), the unit of x² is "m", but the unit of the right-hand side is t, where T(t) in the equation is in "℃" from t=0 seconds. １００℃ When calculations are performed down to the second, the unit of the calculation result becomes "m".

[0153] In process (A), T ｐ By keeping the cumulative reduction ratio at the following temperatures below 30% and the finish rolling completion temperature above 800°C, Fe after the completion of finish rolling ２３ (C, B) ６ By suppressing nucleation, the formation of B precipitates after winding can be suppressed, and the number density of specific B precipitates in the final product can be reduced.

[0154] In process (A), during the cooling process after the completion of finish rolling, the temperature of the hot-rolled steel sheet is T ｐ By ensuring that the thermal history from the time the temperature is reached until winding begins satisfies equation (3) above, Fe ２３ (C, B) ６ By suppressing nucleation, the formation of B precipitates after winding can be suppressed, and the number density of specific B precipitates in the final product can be reduced.

[0155] In equation (3) above, x1 is less than 0.0060m, but may be 0.0050m or less, or 0.0030m or less. The lower limit of x1 is not particularly limited, but x1 may be, for example, 0.0001m or more, 0.0003m or more, or 0.0006m or more.

[0156] Furthermore, by ensuring that the thermal history in process (A), from the start of winding to cooling to 100°C, satisfies the above equation (4), the formation of B precipitates after winding can be suppressed, and the number density of specific B precipitates in the final product can be reduced.

[0157] In equation (4) above, x² is less than 3.7m, but may be 3.6m or less, 3.5m or less, or 3.4m or less. The lower limit of x² is not particularly limited, but x² may be, for example, 1.0m or more, 1.5m or more, 2.0m or more, or 2.5m or more.

[0158] In process (A), the slab is finish-rolled, the hot-rolled steel sheet after finish-rolling is cooled, and then wound up to form a hot-rolled coil. ｐ The cumulative reduction ratio in the following cases is 30% or less, the finish rolling completion temperature is 800°C or higher, and the temperature of the hot-rolled steel sheet during the cooling process after the finish rolling is T ｐOther conditions are not particularly limited, as long as the thermal history from reaching the specified temperature until the start of winding satisfies equation (3) above, and the thermal history from the start of winding until cooling to 100°C satisfies equation (4) above. For example, the slab may be hot-rolled after casting, either directly or after cooling once and then reheated. When reheating, the heating temperature of the slab is, for example, 1100°C or higher, and although there is no particular upper limit, it may be, for example, 1250°C or lower.

[0159] T in finish rolling ｐ The cumulative reduction ratio at the following temperatures is 30% or less, but may be 29% or less, 28% or less, 25% or less, or 20% or less. ｐ The lower limit of the cumulative reduction rate at the following temperatures is not particularly limited and may be 0%. ｐ The cumulative reduction rate at the following temperatures may be 1% or more, 5% or more, or 10% or more.

[0160] Furthermore, in process (A), for example, rough rolling may be optionally performed on the cast slab before finish rolling to adjust the plate thickness. Such rough rolling is only required to ensure the desired sheet bar dimensions are achieved, and the conditions are not particularly limited.

[0161] Slabs that have undergone rough rolling or slabs that have not undergone rough rolling are subjected to finish rolling, T ｐ Other conditions are not particularly limited, as long as the cumulative reduction ratio at the following temperatures is 30% or less and the finish rolling completion temperature is 800°C or higher. For example, the finish rolling completion temperature may be controlled to be in the range of 800 to 950°C. If the finish rolling completion temperature is 950°C or lower, the ferrite content in the hot-rolled steel sheet increases, making it less prone to embrittlement, and thus less likely to crack during cold rolling. The finish rolling completion temperature may be 930°C or lower or 910°C or lower. On the other hand, if the finish rolling completion temperature is 850°C or higher, the ferrite content in the hot-rolled steel sheet decreases, increasing the yield stress of the hot-rolled steel sheet, making it easier to obtain sufficient friction on the steel sheet surface during cold rolling, as described later. The finish rolling completion temperature may be 870°C or higher or 890°C or higher.

[0162] In addition, in process (A), finish rolling is performed on the slab that has undergone rough rolling or the slab that has not undergone rough rolling, the hot-rolled steel sheet after the finish rolling is cooled and wound up to form a hot-rolled coil, at which point the temperature of the hot-rolled steel sheet reaches T ｐ The thermal history from reaching the specified temperature until the start of winding must satisfy the above equation (3). Other conditions are not particularly limited as described above, but for example, during the cooling process from the completion of finish rolling to the start of winding, the hot-rolled steel sheet may be air-cooled, water-cooled, or intermediate air-cooling may be performed in the range of 600 to 750°C during water cooling. If the intermediate air-cooling temperature is 750°C or higher, Ti and Nb carbides will precipitate inside the ferrite contained in the hot-rolled steel sheet, increasing the yield stress of the hot-rolled steel sheet, making it easier to obtain sufficient friction on the steel sheet surface during cold rolling as described later. The intermediate air-cooling temperature may be 730°C or lower or 700°C or lower. On the other hand, if the intermediate air-cooling temperature is 600°C or higher, the ferrite content in the hot-rolled steel sheet will increase, making it less prone to embrittlement, and thus less likely to crack during cold rolling. The intermediate air-cooling temperature may be 620°C or higher or 650°C or higher.

[0163] Furthermore, in process (A), it is necessary to ensure that the thermal history from the start of winding the hot-rolled steel sheet until it cools to 100°C satisfies the above formula (4). Other conditions are not particularly limited as described above, but for example, the winding temperature when winding the hot-rolled steel sheet may be 400 to 700°C. If the winding temperature is 700°C or lower, the ferrite grain size in the hot-rolled steel sheet does not become too large, and sufficient strength is easily obtained after annealing of the cold-rolled steel sheet. The winding temperature may be 680°C or lower or 650°C or lower. On the other hand, if the winding temperature is 400°C or higher, the hot-rolled steel sheet does not become too hard and is less prone to embrittlement, so cracks are less likely to occur on the surface of the steel sheet in subsequent processes. The winding temperature may be 420°C or higher or 450°C or higher.

[0164] [Process (B)] In this embodiment, process (B) is a process in which the hot-rolled steel sheet after process (A) described above is unwound and pickled. By performing such pickling, the oxide scale on the surface of the hot-rolled steel sheet can be removed, thereby improving the chemical conversion treatment properties and plating properties of the cold-rolled steel sheet. The pickling conditions are not particularly limited, and general pickling conditions can be used. In addition, pickling may be performed in one step or in multiple steps.

[0165] [Process (C)] In this embodiment, process (C) is a process of cold rolling a hot-rolled steel sheet after pickling at a reduction ratio of 30% to 75%, and is a process of cold rolling under conditions that satisfy the following formula (5). The value of f calculated by the following formula (5) is a parameter relating to the friction of the steel sheet surface and is greater than 0.11. Note that the unit of the calculated f is considered to be dimensionless when V, r and Y are substituted with the values ​​in the units shown below, and f is greater than 0.11.

[0166]

[0167] V: Inlet speed of the sheet during cold rolling (m / min) r: Total reduction ratio during cold rolling (%) Y: Yield stress of the hot-rolled steel sheet (MPa)

[0168] In process (C), the hot-rolled steel sheet after pickling is cold-rolled at a reduction ratio of 30% to 75%. By setting the cold-rolling reduction ratio to 30% or more, the shape of the cold-rolled steel sheet can be kept flat, and a decrease in ductility in the final product can be suppressed. The cold-rolling reduction ratio may be 35% or more, or 40% or more. On the other hand, by setting the cold-rolling reduction ratio to 75% or less, it is possible to prevent the rolling load from becoming excessive and making rolling difficult. The cold-rolling reduction ratio may be 70% or less, or 65% or less. The number of rolling passes and the reduction ratio for each pass are not particularly limited and should be set appropriately so that the overall cold-rolling reduction ratio falls within the above range.

[0169] Furthermore, in process (C), by cold rolling under conditions that satisfy the above formula (5), the frictional force between the cold rolling roll and the steel sheet is increased, a large strain is applied to the surface of the steel sheet, and recrystallization on the surface of the steel sheet is promoted during the heat treatment (annealing) described later, thereby refining the microstructure of the steel sheet surface, promoting the dissolution of B precipitates, and reducing the number density of specific B precipitates in the final product.

[0170] In this process (C), as long as the cold rolling is performed under the specified reduction ratio and conditions that satisfy formula (5) above, other conditions are not particularly limited, and any known cold rolling conditions may be adopted.

[0171] The tensile strength (TS) of hot-rolled steel sheets can be measured by taking a No. 5 tensile test specimen from the steel sheet, with its longitudinal direction perpendicular to the rolling direction and thickness direction, and performing a tensile test in accordance with JIS Z2241:2011. If a No. 5 tensile test specimen cannot be taken from the steel sheet being measured (for example, if the size of the sample is small), a tensile test specimen of any size with its longitudinal direction perpendicular to the rolling direction and thickness direction may be used instead of the No. 5 tensile test specimen. In addition, if the rolling direction cannot be determined in the tensile test, a specimen with its longitudinal direction perpendicular to the thickness direction may be used.

[0172] [Process (D)] In this embodiment, process (D) is a process of heat-treating the cold-rolled steel sheet obtained in process (C) described above, and is a process of heat-treating under the following conditions (D1) to (D5).

[0173] (D1): In the process of heating the cold-rolled steel sheet to a maximum heating temperature of Ac3 + 20°C to 960°C, the sheet is heated at an average heating rate of 0.5°C / sec to 500°C / sec from 650°C until it reaches the maximum heating temperature. However, if Ac3 is less than 800°C, the sheet is heated from 800°C to 960°C.

[0174] (D2): Hold the temperature at Ac3 + 20°C to 960°C for 1 second to 1000 seconds. However, if Ac3 is less than 800°C, hold the temperature at 800°C to 960°C for 1 second to 1000 seconds.

[0175] (D3): The steel plate held at Ac3 + 20°C to 960°C is cooled to a temperature of Ms - 100°C or lower, with an average cooling rate of 10°C / second or more from 700°C to 500°C. However, if Ac3 is less than 800°C, the steel plate held at 800°C to 960°C is cooled to a temperature of Ms - 100°C or lower, with an average cooling rate of 10°C / second or more from 700°C to 500°C.

[0176] (D4): After cooling, the steel plate is held at 200°C to 350°C for 1 to 600 seconds.

[0177] (D5): In (D3) above, the thermal history from when the steel plate temperature reaches 800°C until it first reaches 550°C satisfies the following equation (6). x3 calculated by the following equation (3) represents the cumulative average diffusion distance (m) of B from when the steel plate temperature reaches 800°C until it reaches 550°C, and x3 shall be less than 0.020 m.

[0178]

[0179] t: Time elapsed (seconds) since the steel plate reached a temperature of 800°C. ５５０℃ : The time (seconds) from when the steel plate temperature reaches 800°C until it first reaches 550°C. T(t): The temperature of the steel plate at time t (°C). However, the integration step is 1 second. If the measurement interval is not 1 second, the temperature every second is calculated by linearly interpolating the temperature between measurement points in the same way as in equation (3) above.

[0180] Note that in equation (6), the unit of x3 is "m", but the unit of the right-hand side is t, where T(t) in the equation is in "℃" from t=0 seconds. ５５０℃ When calculations are performed down to the second, the unit of the calculation result becomes "m".

[0181] Furthermore, in process (D), the Ac3 point (°C) is determined according to the following formula.

[0182] Ac3 = 912 - 230.5 [C] - 20.4 [Mn] + 31.6 [Si] - 14.8 [Cr] - 18.1 [Ni] + 16.8 [Mo] + 100 [Al] - 39.8 [Cu] In the above formula, [C], [Mn], [Si], [Cr], [Ni], [Mo], [Al], and [Cu] represent the content (mass %) of each element.

[0183] Furthermore, in (D3), the Ms point (°C) is determined according to the following formula.

[0184] Ms = 561 - 474 [C] - 33 [Mn] - 7.5 [Si] - 17 [Cr] - 17 [Ni] - 21 [Mo] + 10 [Co] In the above formula, [C], [Mn], [Si], [Cr], [Ni], [Mo], and [Co] represent the content (mass %) of each element.

[0185] In process (D), by heat-treating the cold-rolled steel sheet under the conditions of processes (D1) to (D5) described above, it is possible to suppress the increase in the number density of specific B precipitates during heat treatment (annealing).

[0186] In process (D1), the average heating rate to the maximum heating temperature is set to 0.5 to 500°C / second from the viewpoint of promoting ferrite recrystallization and suppressing austenite coarsening. The average heating rate to the maximum heating temperature may be 1.0°C / second or more, or 2.0°C / second or more. Alternatively, the average heating rate may be 400°C / second or less, or 300°C / second or less. Here, "average heating rate" refers to the value obtained by dividing the difference between 650°C and the maximum heating temperature by the elapsed time from 650°C to the maximum heating temperature.

[0187] In processes (D1) to (D2), the maximum heating temperature shall be between Ac3 + 20°C and 960°C, from the viewpoint of promoting austenitization and suppressing the coarsening of the austenite diameter. The holding time at the maximum heating temperature shall be between 1 and 1000 seconds, from the viewpoint of promoting austenitization and productivity. During holding, the steel sheet does not necessarily need to be held at a constant temperature and may fluctuate within the above range of Ac3 + 20°C to 960°C.

[0188] In process (D3), in order to obtain the desired microstructure, the steel plate is held at Ac3 + 20°C to 960°C, and then cooled to Ms point - 100°C or below, with an average cooling rate of 10°C / second or more from 700°C to 500°C. However, if Ac3 is less than 800°C, the steel plate held at 800°C to 960°C is cooled to a temperature of Ms - 100°C or below, with an average cooling rate of 10°C / second or more from 700°C to 500°C. The average cooling rate from 700°C to 500°C may be 20°C / second or more, 30°C / second or more, or 50°C / second or more.

[0189] In addition, when process (D) and the hot-dip galvanizing process described later are carried out as separate processes, it is conceivable that the "cooling process from 700°C to 500°C" may be repeated multiple times. In this case, the "average cooling rate from 700°C to 500°C" explained as (D3) is calculated for the steel plate held at Ac3 + 20°C to Ac3 + 75°C, or, if Ac3 is less than 780°C, for the steel plate held at 800°C to 855°C, from the point when the steel plate temperature first reaches 700°C to the point when it first reaches 500°C.

[0190] In process (D4), in order to obtain the desired structure, the steel sheet is cooled to below the Ms point -100°C and then held at 200 to 350°C for 1 to 600 seconds. The holding temperature of the steel sheet after cooling may be 300°C or below, or 250°C or below. The holding time of the steel sheet after cooling may be 500 seconds or below, or 400 seconds or below. The holding time of the steel sheet after cooling may be 50 seconds or more, 100 seconds or more, or 150 seconds or more. During holding, the steel sheet does not necessarily need to be held at a constant temperature and may fluctuate within the range of 200 to 350°C.

[0191] The holding time in the 200-350°C temperature range is calculated by summing the residence times in the 200-350°C temperature range for the thermal history after the steel plate temperature reaches Ms point - 100°C due to the cooling in (D3) above. The steel plate may also be reheated after the temperature falls below 200°C and held in the 200-350°C temperature range. In addition to the case of reheating, it is also conceivable that the "holding process in the 200-350°C range" may be repeated multiple times, such as when process (D) and the hot-dip galvanizing process described later are carried out as separate processes. In this case as well, the "holding time in the 200-350°C range" explained as (D4) is calculated by summing the residence times in the 200-350°C temperature range for the thermal history after the steel plate temperature reaches Ms point - 100°C due to the cooling in (D3) above.

[0192] In process (D5), the thermal history from when the steel plate temperature in (D3) reaches 800°C until it first reaches 550°C satisfies the above equation (6). By cooling under these conditions, the formation of specific B precipitates can be suppressed. The cumulative average diffusion distance x3 of B is preferably as small as possible, and may be 0.015 or less, 0.010 or less, or 0.005 or less.

[0193] In addition, when process (D) and the hot-dip galvanizing process described later are carried out as separate processes, it is conceivable that the "thermal history from 800°C to reaching 550°C for the first time" may be repeated multiple times. In this case, the "thermal history from 800°C to reaching 550°C for the first time," as explained as (D5), is calculated for the period from when the steel plate temperature first reaches 800°C to when it first reaches 500°C during the cooling process of a steel plate held at Ac3 + 20°C to Ac3 + 75°C, or, if Ac3 is less than 780°C, when the steel plate temperature is held at 800°C to 855°C.

[0194] In this step (D), as long as the cold-rolled steel sheet is heat-treated under the conditions of steps (D1) to (D5) described above, the other conditions are not particularly limited, and any known heat treatment (annealing) conditions may be adopted.

[0195] Furthermore, in this embodiment, it is preferable that the conditions of step (D1) in step (D1) are such that the atmosphere surrounding the cold-rolled steel sheet satisfies the following formula (7) while the cold-rolled steel sheet is heated from 650°C to the maximum heating temperature.

[0196]

[0197] pH ２ O: Partial pressure of water vapor pH ２ : Hydrogen partial pressure

[0198] In process (D), by creating an atmosphere around the cold-rolled steel sheet that satisfies formula (7) while heating the cold-rolled steel sheet from 650°C to the maximum heating temperature, the surface layer of the steel sheet can be decarburized to satisfy formula (2), thereby further improving the LME crack resistance of the resulting steel sheet.

[0199] (Plating Treatment) In this embodiment, a zinc-containing plating layer may be formed on at least one surface of the steel sheet. The means for forming such a plating layer is not particularly limited, and any known method may be used. For example, during or after process (D3), the sheet may be heated or cooled to (zinc plating bath temperature - 40)°C to (zinc plating bath temperature + 50)°C as needed, and hot-dip galvanizing may be performed. The hot-dip galvanizing process forms a hot-dip galvanized layer on at least one surface of the cold-rolled steel sheet. When such a hot-dip galvanized layer is formed, the corrosion resistance of the cold-rolled steel sheet can be improved.

[0200] (Post-treatment) In addition, the surface of the hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet may be subjected to a top-layer plating treatment or other various treatments, such as chromate treatment, phosphate treatment, lubricity improvement treatment, and weldability improvement treatment, for the purpose of improving paintability, weldability, etc.

[0201] (Skin Pass Rolling) Furthermore, skin pass rolling may be applied to the steel sheet for the purpose of correcting the shape of the steel sheet and introducing movable dislocations to improve ductility. The reduction ratio of skin pass rolling after heat treatment may be in the range of 0.1 to 1.5%. If the reduction ratio is 0.1% or more, the desired effect can be sufficiently obtained and it is easy to control. Also, if the reduction ratio is 1.5% or less, productivity can be easily ensured.

[0202] The steel plates, parts containing them, and methods for manufacturing steel plates described herein are not limited to the embodiments described above or the examples described later, and can be appropriately combined, substituted, or modified without departing from the purpose and spirit of this disclosure.

[0203] The present disclosure will be described in more detail below with reference to examples, but the present disclosure is not limited in any way to these examples. The present disclosure may adopt various conditions, insofar as they do not depart from the gist of the present disclosure and achieve the objectives of the present disclosure.

[0204] (Preparation of steel plates) First, slabs were produced by casting steel with various chemical compositions. These slabs were then hot-rolled to produce hot-rolled steel plates. Furthermore, these hot-rolled steel plates were sequentially subjected to grinding, cold-rolling, and heat treatment to produce cold-rolled steel plates.

[0205] A portion of the obtained cold-rolled steel sheets was subjected to plating.

[0206] Table 1 below shows the molten steel analysis values ​​for the various steel sheets obtained as described above. The remainder other than the components shown in Table 1 consists of Fe and impurities. Underlines next to the chemical compositions in Table 1 indicate that they are outside the scope of this disclosure.

[0207]

[0208] The hot rolling conditions described above were those shown in Table 2. In Table 2, x1 to x3 represent the cumulative average diffusion distance of B.

[0209]

[0210]

[0211] Subsequently, the various steel sheets obtained as described above were pickled. Furthermore, the pickled steel sheets were cold-rolled at the reduction ratios shown in Table 2. In Table 2, V is the sheet feeding speed during cold rolling (m / min), r is the reduction ratio (%), and Y is the yield stress (MPa) of the hot-rolled steel sheet.

[0212] Subsequently, the cold-rolled steel sheets were subjected to heat treatment. The heat treatment involved heating to the maximum heating temperature, holding, and then cooling. Furthermore, after cooling to a temperature below the Ms point - 100°C, the sheets were held at 200-350°C. Steel sheet No. 15, whose cooling completion temperature to below the Ms point - 100°C was higher than the holding temperature at 200-350°C, was held during the subsequent cooling process after the completion of cooling to below the Ms point - 100°C. Steel sheets other than No. 15 were reheated to a predetermined temperature after the completion of cooling to below the Ms point - 100°C and then held. These conditions and the log(pH) from 650°C to the maximum heating temperature were measured. ２ 0 / pH ２ The values ​​of ) are shown in Table 3. Here, pH ２ O is the partial pressure of water vapor, and pH ２ is the partial pressure of hydrogen. Also, in Table 3, Ms is the martensitic transformation point (°C) of the steel used.

[0213] Subsequently, some of the steel sheets underwent continuous hot-dip galvanizing, and some of those underwent alloying treatment. The plating conditions were not special, but rather known and general conditions. In Table 2, GA refers to alloyed hot-dip galvanized steel sheets. GI refers to hot-dip galvanized steel sheets that have not undergone alloying treatment. CR refers to cold-rolled steel sheets that have not been plated.

[0214] The various steel sheets obtained as described above underwent microstructural identification, surface analysis by time-of-flight secondary ion mass spectrometry and glow discharge emission spectrometry, and tensile strength measurement according to the various measurement methods described above. The results of these measurements are shown in Table 4. Hot-dip galvanized steel sheets were used in the tests without removing the hot-dip galvanizing.

[0215] In Table 4, "α + B" represents ferrite and bainite, "γ" represents retained austenite, and "M" represents martensite.

[0216] In addition, the underlines next to various numerical values ​​in Tables 2, 3, and 4 indicate that they are outside the scope of this disclosure, that the manufacturing conditions are such that the steel sheet described herein cannot be obtained, or that the various properties of the steel sheet are undesirable.

[0217] (Evaluation of LME resistance) To evaluate the resistance of spot welds to liquid metal embrittlement (LME) cracking, test specimens measuring 150 mm wide x 50 mm long were taken from the steel plates manufactured as described above, and a two-piece spot welding test was performed. The plate assembly consisted of two plates, with the steel plate shown in Table 3 on top and the mating material on the bottom, and welding was performed with a striking angle of 3°. For the mating material, GI steel plates with a tensile strength difference of ±20 MPa from the steel plate shown in Table 3 were used. A servo motor-driven stationary spot welding test machine was used as the testing machine. The power supply was single-phase AC 50 Hz, the pressing force was 400 kgf, the energizing time was 20 cycles, and the holding time was 5 cycles. The welding current values ​​were set to current values ​​that resulted in a molten nugget diameter of 4.0 times, 5.0 times, and 5.5 times √t (t: plate thickness / mm). For the electrodes, we used chromium copper electrodes with a tip diameter of φ6 mm and a tip radius of curvature of R40 mm.

[0218] Cross-sectional observation of the nugget portion was performed on the welded samples. Specifically, the sample was cut by a plane passing through the center of the nugget and perpendicular to the steel plate surface. After the cut surface was polished to a mirror finish by mechanical polishing, the cross-section was observed with an optical microscope, and the length of the largest crack was measured.

[0219] Based on cross-sectional observation, samples were graded as follows: those with cracks of 0.20 mm or larger at any current value were graded 0 (failure); those with cracks between 0.10 and less than 0.20 mm at any current value were graded 1 (pass); those with cracks between 0.05 and less than 0.10 mm at any current value were graded 2 (pass); and those with no cracks of 0.05 mm or larger at any current value were graded 3 (pass). The evaluation results for LME resistance are shown in Table 4.

[0220]

[0221] As shown in Table 4, the specific chemical composition and microstructure described above, and the number density of specific B precipitates, are 10,000 particles / mm³. ２ The steel plates No. 1-15, 20, and 22-28, which are embodiments of this disclosure, were found to all have excellent LME resistance and tensile strength.

[0222] On the other hand, the comparative steel sheets No. 16-19, 21, and 35 were found to have poor LME resistance due to a high number density of specific B precipitates. Furthermore, the comparative steel sheets No. 29, 31, 32, and 34 were found to have poor LME resistance because their chemical composition or microstructure was not within the specified range.

[0223] Note that steel plate No. 34 has a number density of specific B precipitates of 0 particles / mm². ２ In other words, steel plate No. 34 had a very low content of specific B precipitate, and no specific B precipitate was observed within the measurement range.

[0224] Furthermore, while the comparative steel plates No. 30 and 33 exhibited excellent LME resistance, it was found that their tensile strength was low and their overall strength was inferior because their chemical composition or microstructure was not within the specified range mentioned above.

[0225] In particular, the reasons why steel plates No. 5, 9, 13, 27, and 28, which received a rating of "3" for LME resistance, exhibited such exceptionally high LME resistance are thought to be as follows.

[0226] No. 5: x1 was particularly small, and x2 and x3 were also relatively small, resulting in a number density of specific B precipitates of 5000 particles / mm³. ２ The following results suggest that particularly excellent LME resistance was achieved.

[0227] No. 9: x2 was particularly small, and x1 and x3 were also relatively small, resulting in a number density of specific B precipitates of 5000 particles / mm³. ２ The following results suggest that particularly excellent LME resistance was achieved.

[0228] No. 13: x3 was particularly small, and x1 and x2 were also relatively small, resulting in a number density of specific B precipitates of 5000 particles / mm³. ２ The following results suggest that particularly excellent LME resistance was achieved.

[0229] No. 27: The addition of Mo suppresses the precipitation of B, and the number density of specific B precipitates is 5000 particles / mm³. ２ The following results suggest that particularly excellent LME resistance was achieved.

[0230] No. 28: The addition of Nb suppresses the precipitation of B, and the number density of specific B precipitates is 5000 particles / mm³. ２ The following results suggest that particularly excellent LME resistance was achieved.

Claims

1. The chemical composition, in mass%, is as follows: C: 0.27-0.40%, Si: 0.01-2.50%, Mn: 1.00-4.00%, Al: 0.001-1.500%, Ti: 0.001-0.100%, B: 0.0005-0.0100%, P: 0.050% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0100% or less, Cr: 0-1.00%, Cu: 0-1.00%, Mo: 0-1.00%, Ni: 0-1.00%, Co: 0-3.00%, W: 0-1.00%, Sn: 0-1.00%. Sb: 0-0.50%, Nb: 0-0.200%, V: 0-1.00%, As: 0-0.10%, Zn: 0-1.00%, Mg: 0-0.0500%, Ca: 0-0.0500%, Zr: 0-0.5000%, Ce: 0-0.0300%, La: 0-0.0150%, Hf: 0-0.0100%, Bi: 0-0.0100%, REM other than Ce and La: 0-0.1000%, and the remainder: Fe and impurities, and the content of C and B satisfies the following formula (1), and the microstructure at a depth of 1 / 4 of the plate thickness from the surface of the steel plate is, in area %, total of ferrite and bainite: 5% or less, The composition is: retained austenite: 10% or less, martensite: 85% or more, and remainder: 5% or less, and measured by time-of-flight secondary ion mass spectrometry. ２ － Regarding the signal strength of the BO ２ － When pixels exhibiting a signal intensity 7.5 times or more than the average signal intensity are in contact with each other, and a region consisting of 5 to 16 such pixels is defined as one specific B precipitate, the number density of the specific B precipitate is 10,000 pixels / mm³. ２ A steel plate characterized in that the tensile strength of the steel plate is 1760 MPa or more. ｐ <1000 ... (1) However, T ｐ = 555 + 179 × [C] + 8083 × √[B] [C]: Content of C (mass%) [B]: Content of B (mass%) 2. The chemical composition is as follows, in mass%, Cr: 0.001-1.00%, Cu: 0.001-1.00%, Mo: 0.01-1.00%, Ni: 0.01-1.00%, Co: 0.01-3.00%, W: 0.001-1.00%, Sn: 0.001-1.00%, Sb: 0.001-0.50%, Nb: 0.001-0.200%, V: 0.001-1.00%, As: 0.001-0.10%, Zn: 0.001-1.00%, Mg: 0.0001-0.0500%, Ca: 0.0001-0.0500%, The steel sheet according to claim 1, characterized by containing one or more of the following: Zr: 0.0001 to 0.5000%, Ce: 0.0001 to 0.0300%, La: 0.0001 to 0.0150%, Hf: 0.0001 to 0.0100%, Bi: 0.0001 to 0.0100%, and REM other than Ce and La: 0.0001 to 0.1000%.

3. The steel sheet according to claim 1 or 2, characterized in that the emission intensity of C measured by glow discharge optical emission spectrometry in the thickness direction of the steel sheet satisfies the following formula (2). C ３０ : Emission intensity of C at a depth of 30 μm from the surface of the steel sheet C １ｔ／４ : Emission intensity of C at a depth of 1 / 4 of the thickness of the steel sheet 4. The steel sheet according to claim 1, characterized in that the chemical composition contains, by mass%, N: 0.0100% or less, Co: 0 to 1.00%, B: 0.0005 to 0.0030%, Mg: 0 to 0.0100%, Ca: 0 to 0.0100%, Zr: 0 to 0.0100%, Ce: 0 to 0.0150%, and REM other than Ce and La: 0 to 0.0100%.

5. A component characterized by comprising a steel plate as described in any one of claims 1 to 4.

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