Hot-rolled steel sheet and hot-dip galvanized steel sheet

Abstract

Provided is a hot-rolled steel sheet by which excellent LME resistance can be achieved. A hot-rolled steel sheet according to the present disclosure contains, in terms of mass%, 0.030-0.110% C, 0.01-0.50% Si, 0.50-1.50% Mn, 0.025% or less P, 0.010% or less S, 0.010-0.070% Al, 0.0070% or less N, 0.055-0.200% Ti, 0.030-0.150% Nb, 0.0010-0.0050% B, 0-0.20% V, and 0-1.00% Cr, with the remainder consisting of Fe and impurities. The area ratio of bainitic ferrite is 85% or greater, and in a surface layer region from the surface of the hot-rolled steel sheet to a depth position of 0.2 mm in the sheet thickness direction, the total length of the prior austenite grain boundary in which BO2 - is detected by time-of-flight secondary ion mass spectrometry is 1.20-5.00 mm per 100 μm × 100 μm, and the B segregation degree at the prior austenite grain boundary is 40 or greater.

Description

Hot-rolled steel sheets and hot-dip galvanized steel sheets

[0001] The present disclosure relates to a hot-rolled steel sheet and a hot-dip galvanized steel sheet having a hot-dip galvanized layer formed on the surface of the hot-rolled steel sheet.

[0002] Hot-rolled steel sheets are widely used in automobiles, electrical machinery, building materials, construction machinery, etc. In some cases, a hot-dip galvanized layer is formed on the surface of the hot-rolled steel sheet used for these applications to improve corrosion resistance. Hereinafter, the hot-rolled steel sheet on which the hot-dip galvanized layer is formed is also referred to as a hot-dip galvanized steel sheet.

[0003] A hot-dip galvanized steel sheet may be welded to another steel member. During welding, a portion of the hot-dip galvanized coating layer melts. At this time, the molten metal (zinc) may penetrate into grain boundaries in the surface layer of the hot-rolled steel sheet, causing cracks. Such cracks are called liquid metal embrittlement cracking (LME: Liquid Metal Embrittlement).

[0004] Hot-rolled steel sheets are required to have a property capable of suppressing the occurrence of LME when a hot-dip galvanized layer is formed on the surface of the hot-rolled steel sheet (hereinafter, this property is referred to as LME resistance).

[0005] Japanese Patent Application Laid-Open No. 2018-145500 (Patent Document 1) proposes a hot-dip Zn-Al-Mg-plated steel sheet with excellent LME resistance.

[0006] The hot-dip Zn-Al-Mg-plated steel sheet of Patent Document 1 includes a base steel sheet and a hot-dip Zn-Al-Mg-plated alloy layer. The base steel sheet has a chemical composition containing, by mass%, C: 0.01 to 0.08%, Si: 0.8% or less, Mn: 0.5 to 1.8%, P: 0.05% or less, S: 0.005% or less, N: 0.001 to 0.005%, Ti: 0.02 to 0.2%, B: 0.0005 to 0.010%, and Al: 0.005 to 0.1%, with the balance being Fe and unavoidable impurities. In the above chemical composition, the Ti / C equivalent ratio (= (Ti / 48) / (C / 12)) is 0.4 to 1.5. The base steel sheet further has a dislocation density of 1.8 × 10 １４ / m ２ ~5.7 x 10 １４ / m ２The base steel sheet has a main phase consisting of either a bainitic ferrite phase or a ferrite phase, or a phase containing both the bainitic ferrite phase and the ferrite phase, and the area ratio of the hard second phase and cementite is 3% or less. The base steel sheet further has dispersed precipitates of carbides containing Ti and having an average particle size of 20 nm or less.

[0007] Patent Document 1 describes that the hot-dip Zn—Al—Mg-plated steel sheet of Patent Document 1 has the above-mentioned chemical composition and microstructure, thereby achieving excellent LME resistance.

[0008] JP 2018-145500 A

[0009] The hot-rolled steel sheet disclosed in Patent Document 1 has excellent LME resistance. However, excellent LME resistance may be obtained by a means different from that of the hot-rolled steel sheet disclosed in Patent Document 1.

[0010] An object of the present disclosure is to provide a hot-rolled steel sheet and a hot-dip galvanized steel sheet that can provide excellent LME resistance.

[0011] The hot-rolled steel sheet of the present disclosure contains, by mass%, C: 0.030 to 0.110%, Si: 0.01 to 0.50%, Mn: 0.50 to 1.50%, P: 0.025% or less, S: 0.010% or less, Al: 0.010 to 0.070%, N: 0.0070% or less, Ti: 0.055 to 0.200%, Nb: 0.030 to 0.150%, B: 0.0010 to 0.0050%, V: 0 to 0.20%, and Cr: 0 to 1.00%, with the balance consisting of Fe and impurities. In the microstructure, the area ratio of bainitic ferrite is 85% or more. In a surface region of the hot-rolled steel sheet from the surface to a depth of 0.2 mm in the sheet thickness direction, BO is determined by time-of-flight secondary ion mass spectrometry. ２ － The total length of the prior austenite grain boundaries where B is detected is 1.20 to 5.00 mm per 100 μm×100 μm, and the degree of B segregation at the prior austenite grain boundaries is 40 or more.

[0012] The hot-dip galvanized steel sheet of the present disclosure includes the above-mentioned hot-rolled steel sheet and a hot-dip galvanized layer formed on the surface of the above-mentioned hot-rolled steel sheet and containing 65.00% or more of Zn by mass%.

[0013] The hot-rolled steel sheet and hot-dip galvanized steel sheet according to the present disclosure can achieve excellent LME resistance.

[0014] Fig. 1 is a schematic diagram for explaining line analysis by time-of-flight secondary ion mass spectrometry. Fig. 2 is a schematic diagram of the B detection intensity distribution obtained by line analysis. Fig. 3 is a schematic diagram of an LME resistance evaluation test in an example. Fig. 4 is a cross-sectional view of the LME resistance evaluation test of Fig. 3 as seen from the side.

[0015] A certain level of strength is required for hot-rolled steel sheets suitable for applications such as automobiles, electrical machinery, building materials, and construction machinery. Therefore, the present inventors conducted research into hot-rolled steel sheets suitable for these applications from the perspective of chemical composition. As a result, they concluded that sufficient strength could be obtained if the hot-rolled steel sheet had a chemical composition containing, in mass%, C: 0.030 to 0.110%, Si: 0.01 to 0.50%, Mn: 0.50 to 1.50%, P: 0.025% or less, S: 0.010% or less, Al: 0.010 to 0.070%, N: 0.0070% or less, Ti: 0.055 to 0.200%, V: 0 to 0.20%, and Cr: 0 to 1.00%, with the balance being Fe and impurities, and further, if the area fraction of bainitic ferrite is 85% or more.

[0016] Therefore, the present inventors further investigated means for improving the LME resistance of a hot-rolled steel sheet having the above-mentioned chemical composition and microstructure. As described above, LME occurs when molten metal (zinc) penetrates into the grain boundaries in the surface layer of the steel sheet. Therefore, the present inventors considered adding 0.0010 to 0.0050% B to the above-mentioned chemical composition. B segregates at the grain boundaries and strengthens them. This may improve LME resistance.

[0017] However, there have been cases where sufficient LME resistance cannot be obtained simply by adding B to the above-mentioned chemical composition. Therefore, the present inventors conducted further studies. Here, the present inventors focused on the length of prior austenite grain boundaries in the surface layer of a hot-rolled steel sheet, where B is segregated. LME tends to progress along prior austenite grain boundaries. Therefore, the longer the length of the prior austenite grain boundaries where B is segregated, the more easily it becomes possible to suppress the progression of LME along the grain boundaries.

[0018] Based on the above findings, the present inventors further investigated means for increasing the length of prior austenite grain boundaries where B is segregated. As a result, the present inventors found that the total length of prior austenite grain boundaries where the degree of B segregation is high can be increased by further adding 0.030 to 0.150% of Nb to the above chemical composition.

[0019] The hot-rolled steel sheet of the present embodiment, which has been completed based on the above findings, and the hot-dip galvanized steel sheet using the hot-rolled steel sheet, have the following configurations.

[0020] The hot-rolled steel sheet of this embodiment contains, by mass%, C: 0.030 to 0.110%, Si: 0.01 to 0.50%, Mn: 0.50 to 1.50%, P: 0.025% or less, S: 0.010% or less, Al: 0.010 to 0.070%, N: 0.0070% or less, Ti: 0.055 to 0.200%, Nb: 0.030 to 0.150%, B: 0.0010 to 0.0050%, V: 0 to 0.20%, and Cr: 0 to 1.00%, with the balance consisting of Fe and impurities. In the microstructure, the area ratio of bainitic ferrite is 85% or more. In a surface region of the hot-rolled steel sheet from the surface to a depth of 0.2 mm in the sheet thickness direction, BO was determined by time-of-flight secondary ion mass spectrometry. ２ － The total length of the prior austenite grain boundaries where B is detected is 1.20 to 5.00 mm per 100 μm×100 μm, and the degree of B segregation at the prior austenite grain boundaries is 40 or more.

[0021] The hot-rolled steel sheet of the present embodiment may further contain, in mass %, one or more elements selected from the group consisting of V: ​​0.01 to 0.20% and Cr: 0.01 to 1.00%.

[0022] The hot-dip galvanized steel sheet of this embodiment includes the hot-rolled steel sheet of this embodiment and a hot-dip galvanized layer formed on the surface of the hot-rolled steel sheet and containing 65.00% or more of Zn by mass %.

[0023] The hot-rolled steel sheet and the hot-dip galvanized steel sheet according to the present embodiment will be described in detail below. Unless otherwise specified, "%" regarding elements means mass %.

[0024] [Hot-rolled steel sheet] The hot-rolled steel sheet of this embodiment satisfies the following features 1 to 4. (Feature 1) The chemical composition contains, in mass%, C: 0.030 to 0.110%, Si: 0.01 to 0.50%, Mn: 0.50 to 1.50%, P: 0.025% or less, S: 0.010% or less, Al: 0.010 to 0.070%, N: 0.0070% or less, Ti: 0.055 to 0.200%, Nb: 0.030 to 0.150%, B: 0.0010 to 0.0050%, V: 0 to 0.20%, and Cr: 0 to 1.00%, with the balance consisting of Fe and impurities. (Feature 2) In the microstructure, the area fraction of bainitic ferrite is 85% or more. (Feature 3) In the surface layer region from the surface of the hot-rolled steel sheet to a depth of 0.2 mm in the sheet thickness direction, BO was detected by time-of-flight secondary ion mass spectrometry. ２ － The total length of the prior austenite grain boundaries where BO is detected is 1.20 to 5.00 mm per 100 μm × 100 μm. (Feature 4) Time-of-flight secondary ion mass spectrometry ２ － The degree of B segregation at the prior austenite grain boundary where B is detected is 40 or more. Features 1 to 4 will be described below.

[0025] [(Feature 1) Chemical Composition] The chemical composition of the hot-rolled steel sheet according to this embodiment contains the following elements.

[0026] C: 0.030 to 0.110% Carbon (C) combines with Ti to form Ti carbide. Ti carbide increases the strength of the hot-rolled steel sheet through precipitation strengthening. C also facilitates the formation of bainitic ferrite when the Ti content in the chemical composition is 0.055 to 0.200%. If the C content is less than 0.030%, the above effect cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content exceeds 0.110%, even if the contents of other elements are within the ranges of this embodiment, excessive bainite is formed in the microstructure, and the area ratio of bainitic ferrite is excessively reduced. This reduces the workability of the hot-rolled steel sheet. Therefore, the C content is 0.030 to 0.110%. The preferred lower limit of the C content is 0.032%, more preferably 0.034%, even more preferably 0.036%, and even more preferably 0.040%. The upper limit of the C content is preferably 0.105%, more preferably 0.100%, even more preferably 0.095%, even more preferably 0.090%, and still more preferably 0.085%.

[0027] Si: 0.01 to 0.50% Silicon (Si) deoxidizes steel. Si also increases the strength of hot-rolled steel sheets through solid-solution strengthening. If the Si content is less than 0.01%, the above effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content exceeds 0.50%, polygonal ferrite is likely to form in the hot-rolled steel sheet even if the contents of other elements are within the ranges of this embodiment. As a result, the area ratio of bainitic ferrite in the hot-rolled steel sheet decreases. In this case, the strength of the hot-rolled steel sheet decreases. Therefore, the Si content is 0.01 to 0.50%. The preferred lower limit of the Si content is 0.02%, more preferably 0.03%, and even more preferably 0.04%. The preferred upper limit of the Si content is 0.45%, more preferably 0.40%, and even more preferably 0.35%.

[0028] Mn: 0.50 to 1.50% Manganese (Mn) increases the strength of hot-rolled steel sheets through solid-solution strengthening. If the Mn content is less than 0.50%, the above effect cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content exceeds 1.50%, Mn segregation is likely to occur in the hot-rolled steel sheet, even if the contents of other elements are within the ranges of this embodiment. If the Mn content exceeds 1.50%, bainite is more likely to form in the hot-rolled steel sheet. As a result, the area ratio of bainitic ferrite in the hot-rolled steel sheet decreases. In this case, the cold workability of the hot-rolled steel sheet decreases. Therefore, the Mn content is 0.50 to 1.50%. The preferred lower limit of the Mn content is 0.55%, more preferably 0.60%, even more preferably 0.65%, and even more preferably 0.70%. The upper limit of the Mn content is preferably 1.40%, more preferably 1.35%, even more preferably 1.30%, even more preferably 1.25%, and still more preferably 1.20%.

[0029] P: 0.025% or less Phosphorus (P) segregates at grain boundaries and reduces the cold workability of hot-rolled steel sheets. P also reduces the weldability of hot-rolled steel sheets. If the P content exceeds 0.025%, the cold workability and weldability of hot-rolled steel sheets are significantly reduced, even if the contents of other elements are within the ranges of this embodiment. Therefore, the P content is 0.025% or less. The lower the P content, the better. However, excessive reduction in the P content reduces productivity and increases manufacturing costs. Therefore, considering industrial production, the lower limit of the P content is preferably greater than 0%, more preferably 0.001%, even more preferably 0.002%, even more preferably 0.003%, and even more preferably 0.005%. The upper limit of the P content is preferably 0.023%, even more preferably 0.022%, and even more preferably 0.020%.

[0030] S: 0.010% or less Sulfur (S) segregates at grain boundaries and reduces the cold workability of hot-rolled steel sheets. If the S content exceeds 0.010%, the cold workability of the hot-rolled steel sheets will be significantly reduced, even if the contents of other elements are within the ranges of this embodiment. Therefore, the S content is 0.010% or less. The S content is preferably as low as possible. However, excessive reduction in the S content reduces productivity and increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the S content is more than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the S content is 0.009%, even more preferably 0.008%, and even more preferably 0.007%.

[0031] Al: 0.010 to 0.070% Aluminum (Al) deoxidizes steel. Furthermore, Al combines with N to form Al nitrides, which inhibit B from combining with N. If the Al content is less than 0.010%, the above effect cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Al content exceeds 0.070%, excessive coarse Al nitrides are formed, even if the contents of other elements are within the ranges of this embodiment. This results in a decrease in the cold workability of the hot-rolled steel sheet. Therefore, the Al content is 0.010 to 0.070%. The preferred lower limit of the Al content is 0.012%, more preferably 0.014%, even more preferably 0.016%, even more preferably 0.018%, and even more preferably 0.020%. The preferred upper limit of the Al content is 0.065%, even more preferably 0.060%, and even more preferably 0.055%.

[0032] N: 0.0070% or less Nitrogen (N) combines with B to form BN, reducing the amount of solute B in the hot-rolled steel sheet. N also combines with Ti to form TiN, inhibiting the formation of Ti carbides. If the N content exceeds 0.0070%, even if the contents of other elements are within the ranges of this embodiment, excessive BN and TiN are formed. As a result, the LME resistance of the hot-rolled steel sheet is reduced. Furthermore, the strength of the hot-rolled steel sheet is also reduced. Therefore, the N content is 0.0070% or less. The N content is preferably as low as possible. However, excessive reduction of the N content reduces productivity and increases manufacturing costs. Therefore, considering industrial production, the lower limit of the N content is preferably greater than 0%, more preferably 0.0001%, even more preferably 0.0005%, even more preferably 0.0010%, even more preferably 0.0015%, and even more preferably 0.0020%. The upper limit of the N content is preferably 0.0065%, more preferably 0.0060%, even more preferably 0.0055%, even more preferably 0.0050%, and still more preferably 0.0040%.

[0033] Ti: 0.055 to 0.200% Titanium (Ti) combines with C to form Ti carbide. Ti carbide increases the strength of hot-rolled steel sheets through precipitation strengthening. Furthermore, the formation of Ti carbide adjusts the amount of dissolved C in the steel sheet. Therefore, bainitic ferrite is more likely to form in the hot-rolled steel sheet. If the Ti content is less than 0.055%, polygonal ferrite is more likely to form, and the area ratio of bainitic ferrite is lowered, even if the contents of other elements are within the ranges specified in this embodiment. In this case, the strength of the hot-rolled steel sheet is reduced. On the other hand, if the Ti content exceeds 0.200%, excessive coarse TiC is formed, even if the contents of other elements are within the ranges specified in this embodiment. As a result, the cold workability of the hot-rolled steel sheet is reduced. Therefore, the Ti content is 0.055 to 0.200%. The lower limit of the Ti content is preferably 0.060%, more preferably 0.065%, even more preferably 0.070%, even more preferably 0.075%, even more preferably 0.080%, and even more preferably 0.085%. The upper limit of the Ti content is preferably 0.190%, more preferably 0.180%, and even more preferably 0.170%.

[0034] Nb: 0.030 to 0.150% Niobium (Nb) forms Nb precipitates (Nb carbides or Nb carbonitrides) and forms Nb precipitates within 1 mm of the hot-rolled steel sheet. ２Nb increases the total length of prior austenite grain boundaries per unit area. Nb also promotes the segregation of B to prior austenite grain boundaries. This increases the degree of B segregation at prior austenite grain boundaries. As a result, the LME resistance of the hot-rolled steel sheet is improved. Nb precipitates also increase the strength of the hot-rolled steel sheet through precipitation strengthening. If the Nb content is less than 0.030%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Nb content exceeds 0.150%, excessive Nb precipitates are formed. In this case, the cold workability of the hot-rolled steel sheet is reduced even if the contents of other elements are within the ranges of this embodiment. Therefore, the Nb content is 0.030 to 0.150%. The preferred lower limit of the Nb content is 0.035%, more preferably 0.040%, and even more preferably 0.045%. The upper limit of the Nb content is preferably 0.145%, more preferably 0.140%, and even more preferably 0.135%.

[0035] B: 0.0010 to 0.0050% Boron (B) dissolves in the hot-rolled steel sheet and segregates at the prior austenite grain boundaries. The segregated B increases the grain boundary strength. Therefore, B improves the LME resistance of the hot-rolled steel sheet. B also improves the hardenability of the steel. If the B content is less than 0.0010%, the above effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. If the B content is less than 0.0010%, the austenite-to-ferrite transformation start temperature also increases. In this case, the Ti carbide precipitation start temperature also increases. Therefore, the Ti carbide becomes coarse. As a result, the strength of the hot-rolled steel sheet decreases. On the other hand, if the B content exceeds 0.0050%, the hardenability becomes excessively high even if the contents of other elements are within the ranges of this embodiment. In this case, bainite is more likely to form, and the area ratio of bainitic ferrite decreases. As a result, the workability of the steel sheet decreases. If the B content exceeds 0.0050%, Fe is further introduced into the prior austenite grain boundaries. ２３ CB ６precipitates. In this case, the grain boundary strength of the prior austenite grains decreases. As a result, LME resistance decreases. Therefore, the B content is 0.0010 to 0.0050%. A preferred lower limit of the B content is 0.0012%, more preferably 0.0015%, even more preferably 0.0020%, and even more preferably 0.0025%. A preferred upper limit of the B content is 0.0045%, more preferably 0.0043%, even more preferably 0.0040%, and even more preferably 0.0035%.

[0036] The balance of the chemical composition of the hot-rolled steel sheet of this embodiment contains Fe and impurities. Here, impurities refer to substances that are mixed in from raw materials such as ore or scrap or the manufacturing environment during industrial production of the hot-rolled steel sheet, and are acceptable within a range that does not adversely affect the hot-rolled steel sheet of this embodiment. Preferably, the balance of the chemical composition of the hot-rolled steel sheet of this embodiment consists of Fe and impurities. Here, the impurities are, for example, one or more selected from the group consisting of O: 0-0.01%, Mo: 0-0.10%, Cu: 0-0.50%, Ni: 0-0.30%, Co: 0-0.10%, Zr: 0-0.05%, Sn: 0-0.10%, Sb: 0-0.10%, Hf: 0-0.02%, Sc: 0-0.02%, Te: 0-0.01%, Sr: 0-0.02%, Bi: 0-0.10%, Ta: 0-0.02%, Zn: 0-0.02%, and rare earth elements (REM): 0-0.01%. The total content of these impurities is, for example, 0-1.00%.

[0037] [Optional Elements] The chemical composition of the heat-rolled steel sheet of this embodiment may further contain one or more elements selected from the group consisting of V: ​​0 to 0.20% and Cr: 0 to 1.00%, instead of a portion of Fe. All of these elements are optional elements.

[0038] V: 0 to 0.20% Vanadium (V) is an optional element and does not necessarily need to be contained. That is, the V content may be 0%. When vanadium is contained, that is, when the V content exceeds 0%, V combines with C to form V carbide. V carbide increases the strength of the hot-rolled steel sheet through precipitation strengthening. Even if even a small amount of V is contained, the above effect can be obtained to some extent. However, if the V content exceeds 0.20%, excessive V carbide will be formed even if the contents of other elements are within the ranges of this embodiment. In this case, the workability of the hot-rolled steel sheet will decrease. Therefore, the V content is 0 to 0.20%, and when contained, the V content is 0.20% or less. The preferred lower limit of the V content is 0.01%, more preferably 0.05%, and even more preferably 0.08%. The preferred upper limit of the V content is 0.18%, more preferably 0.16%, even more preferably 0.14%, and even more preferably 0.12%.

[0039] Cr: 0 to 1.00% Chromium (Cr) is an optional element and does not necessarily need to be contained. That is, the Cr content may be 0%. When chromium is contained, that is, when the Cr content exceeds 0%, Cr segregates at prior austenite grain boundaries, improving the LME resistance of the hot-rolled steel sheet. Even if even a small amount of Cr is contained, the above effect can be obtained to some extent. However, if the Cr content exceeds 1.00%, the cold workability of the hot-rolled steel sheet will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cr content is 0 to 1.00%, and when contained, the Cr content is 1.00% or less. The preferred lower limit of the Cr content is 0.01%, more preferably 0.05%, even more preferably 0.10%, even more preferably 0.20%, and even more preferably 0.30%. The upper limit of the Cr content is preferably 0.90%, more preferably 0.80%, even more preferably 0.70%, even more preferably 0.60%, and still more preferably 0.55%.

[0040] [Method for measuring the chemical composition of a hot-rolled steel sheet] The chemical composition of the hot-rolled steel sheet of this embodiment can be measured by a known elemental analysis method in accordance with JIS G0321:2017. Specifically, chips are collected from the hot-rolled steel sheet using a cutting tool such as a drill. The collected chips are dissolved in acid to obtain a solution. ICP-MAS (Inductively Coupled Plasma Mass Spectrometry) is performed on the solution to perform elemental analysis of the chemical composition. The C content and S content are determined by a known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using a known inert gas fusion-thermal conductivity method.

[0041] The content of each element is determined by rounding off the measured value to the nearest significant digit based on the significant digits specified in this embodiment, to obtain the value to the least significant digit of the content of each element specified in this embodiment. For example, the C content of the steel material in this embodiment is specified as a value to three decimal places. Therefore, the C content is determined as a value to three decimal places obtained by rounding off the fourth decimal place of the measured value. Similarly, the content of other elements other than the C content of the steel material in this embodiment is determined as the value obtained by rounding off the measured value to the least significant digit specified in this embodiment. Note that rounding means rounding down if the fraction is less than 5, and rounding up if the fraction is 5 or more.

[0042] [(Feature 2) Area Ratio of Bainitic Ferrite] In the microstructure of the heat-rolled steel sheet of this embodiment, the area ratio of bainitic ferrite is 85% or more. The microstructure of the heat-rolled steel sheet of this embodiment may be a single phase of bainitic ferrite. When the microstructure of the heat-rolled steel sheet of this embodiment is composed of bainitic ferrite and another structure, the other structure is, for example, one or more types selected from the group consisting of polygonal ferrite, bainite, and cementite.

[0043] Bainitic ferrite can be distinguished from polygonal ferrite and bainite in the following respects.

[0044] [Distinguishing between bainitic ferrite and polygonal ferrite] Bainitic ferrite is an aggregate of grains with slightly different crystal orientations. Therefore, differences in contrast are observed within the crystal grains. On the other hand, polygonal ferrite has a structure with almost no crystal orientation difference within the grains. Therefore, the crystal grains are observed with uniform contrast. Therefore, bainitic ferrite can be distinguished from polygonal ferrite based on the contrast caused by the crystal orientation difference.

[0045] [Distinguishing Between Bainitic Ferrite and Bainite] The crystal structure of bainitic ferrite is a bcc structure, similar to the crystal structure of bainite. Therefore, it is difficult to distinguish bainitic ferrite from bainite based on the crystal structure. Furthermore, it is difficult to distinguish bainitic ferrite from bainite based on the crystal orientation. However, bainitic ferrite can be distinguished from bainite based on the presence or absence of Fe carbides within the grains and at the grain boundaries. Here, Fe carbides refer to carbides containing Fe, such as cementite.

[0046] Specifically, in bainitic ferrite, iron carbides are not present within the grains or at the grain boundaries, whereas in bainite, iron carbides are present within the laths and / or at the lath boundaries. Therefore, bainitic ferrite can be distinguished from bainite based on the presence or absence of iron carbides within the grains and at the grain boundaries.

[0047] [Effect of Area Fraction of Bainitic Ferrite] In the microstructure of a hot-rolled steel sheet in which the content of each element in the chemical composition falls within the range of this embodiment, if the area fraction of bainitic ferrite is 85% or more, high strength can be obtained.

[0048] The lower limit of the area ratio of bainitic ferrite is preferably 88%, more preferably 90%, still more preferably 92%, still more preferably 94%, and still more preferably 96%. The area ratio of bainitic ferrite may be 100%.

[0049] [Method for Measuring Area Ratio of Bainitic Ferrite] The area ratio of bainitic ferrite can be determined by the following method. Microstructure observation is performed using a field emission scanning electron microscope (FE-SEM). Microstructure observation is performed by electron channeling contrast image (ECCI). Observation conditions are an acceleration voltage of 20 kV, tilt (T) = 0°, and backscattered electron mode. Electron backscatter diffraction (EBSD) is used to measure the crystal orientation.

[0050] The test specimen is taken from the center of the plate width of the hot-rolled steel plate. The measurement position is a depth of 1 / 4 of the plate thickness from the surface of the test specimen in the plate thickness direction of the hot-rolled steel plate, and the measurement range is 100 μm × 100 μm. The measurement range is a longitudinal cross section including the L direction (longitudinal direction of the hot-rolled steel plate) and the T direction (thickness direction of the hot-rolled steel plate). The measurement range is divided into regular hexagonal pixel units. The center-to-center distance between adjacent pixels (EBSD measurement interval) is 0.1 μm. The measurement point is the center position of a pixel that includes the entire regular hexagonal pixel within the measurement range. In other words, pixels that are partially outside the observation field of view are excluded from the measurement target. The reliability index (CI value) is 0.1 or more.

[0051] The measurement data is analyzed using EBSD analysis software according to the following procedure to identify and quantify polygonal ferrite and bainitic ferrite. (Step 1) A region surrounded by grain boundaries with a crystal orientation misorientation of 15° or more is defined as one crystal grain. Note that if the circle-equivalent diameter of a region surrounded by grain boundaries with a crystal orientation misorientation of 15° or more is 1.0 μm or less, the region is determined to be measurement noise and is not recognized as a crystal grain. In other words, regions determined to be measurement noise are excluded. (Step 2) The average value of the crystal orientation misorientation within each crystal grain (Grain Average Misorientation: hereinafter referred to as GAM value) is calculated. Crystal grains with a GAM value of 0.5° or less are defined as polygonal ferrite. Crystal grains with a GAM value of more than 0.5° are defined as bainitic ferrite.

[0052] In the above-described microstructure observation, structures different from bainitic ferrite and polygonal ferrite (bainite and cementite) can be easily distinguished by contrast.

[0053] The identified bainitic ferrite is quantified. Then, the area ratio (%) of bainitic ferrite is calculated based on the area of ​​the quantified bainitic ferrite and the total area of ​​the measurement range (100 μm × 100 μm). Note that areas determined to be measurement noise are excluded from the total area of ​​the measurement range.

[0054] The EBSD analysis program for determining the GAM value may be a well-known program, such as OIM Data Collection / Analysis 6.2.0 manufactured by TSL Solutions Co., Ltd.

[0055] [(Feature 3) Grain boundary length GL of prior austenite grains in the surface layer] In the hot-rolled steel sheet of this embodiment, further, in the surface layer region, BO ２ － The total length of the prior austenite grain boundaries where BO is detected is 1.20 to 5.00 mm per 100 μm×100 μm. ２ － The total length of the prior austenite grain boundaries where the above-mentioned grain boundary length is detected is referred to as the "grain boundary length GL" (mm).

[0056] If the grain boundary length GL is too short, the grain boundary region where B segregates is small. In this case, B segregation cannot sufficiently contribute to suppressing LME cracking. As a result, the LME resistance of the hot-rolled steel sheet is reduced. If the grain boundary length GL is 1.20 mm or more, the grain boundary length GL is sufficiently long. In this case, the grain boundary region where B segregation occurs becomes sufficiently large in the surface layer region of the hot-rolled steel sheet. As a result, the LME resistance of the hot-rolled steel sheet is improved.

[0057] The lower limit of the grain boundary length GL is preferably 1.25 mm, more preferably 1.40 mm, even more preferably 1.60 mm, even more preferably 1.80 mm, even more preferably 1.90 mm, even more preferably 2.00 mm, and even more preferably 2.10 mm. The upper limit of the grain boundary length GL is not particularly limited. However, if the grain boundary length GL is made excessively long, the manufacturing cost will increase significantly. Therefore, taking industrial production into consideration, the upper limit of the grain boundary length GL is 5.00 mm, more preferably 4.95 mm, even more preferably 4.80 mm, even more preferably 4.60 mm, and even more preferably 4.40 mm.

[0058] [Method for measuring grain boundary length GL] The grain boundary length GL of the surface layer region is determined by the following method. One test piece is taken, with the cross section of the hot-rolled steel sheet parallel to the rolling direction and thickness direction as the observation surface. The size of the observation surface is, for example, 10 mm in the rolling direction × sheet thickness. The observation surface of each test piece is mirror-polished with diamond paste. After mirror polishing, the observation surface is chemically polished with colloidal silica liquid to remove any process-affected layer introduced during mirror polishing.

[0059] On the observation surface, one rectangular observation field of 100 μm × 100 μm is selected from the surface region of the hot-rolled steel sheet extending from the surface to a depth of 0.2 mm in the sheet thickness direction. Bainitic ferrite grain boundaries, polygonal ferrite grain boundaries, and bainite grain boundaries are identified in the observation field based on the method described in the above-mentioned [Method for measuring the area fraction of bainitic ferrite].

[0060] In the same observation field, the B segregation at the prior austenite grain boundary is measured by time-of-flight secondary ion mass spectrometry (TOF-SIMS). First, as a pretreatment, the observation field is sputtered with a Cs (cesium) ion beam to remove dirt and impurities such as oxides from the observation field. After the pretreatment, the observation field is irradiated with primary ions from the primary ion source of the TOF-SIMS device to sputter the observation field. In the measurement, Bi is used as the primary ion. １＋ The primary ion beam diameter is set to 0.1 μm, the acceleration voltage is set to 30 keV, and the current value is set to 2 pA.

[0061] Among the secondary ions generated by sputtering with primary ions, BO is a secondary ion containing B. ２ － (mass-to-charge ratio m / z = 43) ２ － By measuring the time of flight of the B ions, a map of the B distribution within the observation field is created. ２ － 1 is a map showing the locations where the

[0062] Bainitic ferrite grain boundaries, polygonal ferrite grain boundaries, and bainite grain boundaries are identified in the created B distribution map. In the B distribution map, grain boundaries other than bainitic ferrite grain boundaries, polygonal ferrite grain boundaries, and bainite grain boundaries are defined as prior austenite grain boundaries. Among the prior austenite grain boundaries, BO boundaries with an intensity of 40 counts or more are defined as prior austenite grain boundaries. ２ － The total length of the grain boundary portions where the grain boundary regions are detected is determined by well-known image processing. The total length of the grain boundaries obtained is defined as the grain boundary length GL (mm). The grain boundary length GL is the value obtained by rounding the obtained value to one decimal place.

[0063] The TOF-SIMS device used is, for example, a TOF-SIMS.5 manufactured by IONTOF.

[0064] [(Feature 4) B segregation at prior austenite grain boundaries] In the hot-rolled steel sheet of this embodiment, furthermore, the degree of B segregation at prior austenite grain boundaries was measured by time-of-flight secondary ion mass spectrometry. ２ －The degree of B segregation at the prior austenite grain boundary where this is detected is 40 or more. If the degree of B segregation at the prior austenite grain boundary is high, the prior austenite grain boundary is strengthened. Therefore, it is possible to suppress the propagation of LME cracking along the grain boundary. If the degree of B segregation at the prior austenite grain boundary is 40 or more, a grain boundary length GL with a high degree of B segregation is formed. Therefore, it is possible to significantly suppress the propagation of LME cracking along the prior austenite grain boundary. As a result, the LME resistance of the hot-rolled steel sheet is significantly improved.

[0065] The preferred lower limit of the degree of B segregation is 42, more preferably 44, even more preferably 46, even more preferably 50, even more preferably 52, even more preferably 55, even more preferably 58, and even more preferably 60. The upper limit of the degree of B segregation is not particularly limited. However, in the case of a hot-rolled steel sheet satisfying Features 1 to 3, the upper limit of the degree of B segregation at the prior austenite grain boundary is, for example, 160, 150, 140, 130, or 120.

[0066] [Method for measuring the degree of B segregation at prior austenite grain boundaries] The degree of B segregation at prior austenite grain boundaries is determined by the following method. ２ － The prior austenite grain boundaries where the above-mentioned grain boundary is detected are identified. Line analysis across the grain boundaries is carried out at any five points on the identified grain boundaries as follows.

[0067] FIG. 1 is a schematic diagram for explaining line analysis. Referring to FIG. 1, BO in the observation field of view ２ － The primary ion beam is scanned in a direction approximately perpendicular to the prior austenite grain boundary AG where B is detected by TOF-SIMS to detect the B intensity. In the line analysis of TOF-SIMS, Bi is used as the primary ion. １ ＋The beam diameter of the primary ions is set to 0.1 μm. At this time, the length L1 of the line segment SE to be analyzed is set to 15 μm, and the measurement pitch is set to 0.2 μm. The line segment SE is set to intersect only one grain boundary AG, and not to intersect multiple grain boundaries AG. Furthermore, the line analysis is performed so that the grain boundary AG is located at approximately the center of the length L1 of the line segment SE. The line analysis provides a B detection intensity distribution on the line segment SE.

[0068] 2 is a schematic diagram of the B detection intensity distribution on the line segment SE. The vertical axis in FIG. 2 represents the B detection intensity, and the horizontal axis represents the distance on the line segment SE. Referring to FIG. 2, the maximum value of the detection intensity in the obtained B detection intensity distribution BC is designated as BO. ２ － B detection intensity B at the prior austenite grain boundary AG ｐ In the B detection intensity distribution BC, the end point E of the line segment SE Ｌ All B detection intensities measured at 0.2 μm pitches within a range L10 of 0.15×L1 from the end point E of the line segment SE Ｒ The arithmetic mean value of all the B detection intensities measured at 0.2 μm pitches within the range R10 of 0.15×L1 is calculated as BO ２ － The average B detection intensity B within the prior austenite grains surrounded by the prior austenite grain boundary AG where B is detected ａｖｅ Let's say.

[0069] The detected intensity B at the prior austenite grain boundary AG obtained ｐ , and the average B detection intensity B within the prior austenite grains ａｖｅ The degree of B segregation is calculated based on the formula (A) using the formula: ｐ / B ａｖｅ × B content (ppm) in the chemical composition of the hot-rolled steel sheet (A)

[0070] The arithmetic average value of the degree of B segregation obtained by the line analysis at five locations is defined as the degree of B segregation at the prior austenite grain boundary in the surface layer region. The obtained degree of B segregation is an integer value obtained by rounding off the value to one decimal place.

[0071] [Effects of the Hot-Rolled Steel Sheet of the Present Embodiment] The hot-rolled steel sheet of the present embodiment satisfies Features 1 to 4. Therefore, the hot-rolled steel sheet of the present embodiment has excellent LME resistance.

[0072] [Strength of the Hot-Rolled Steel Sheet of the Present Embodiment] The hot-rolled steel sheet of the present embodiment satisfies Features 1 to 4. Therefore, the tensile strength of the hot-rolled steel sheet of the present embodiment is 590 MPa or more. Therefore, the hot-rolled steel sheet of the present embodiment can achieve both high strength and excellent LME resistance.

[0073] The lower limit of the tensile strength of the hot-rolled steel sheet is preferably 600 MPa, more preferably 610 MPa, even more preferably 620 MPa, even more preferably 630 MPa, even more preferably 640 MPa, and even more preferably 650 MPa. The upper limit of the tensile strength of the hot-rolled steel sheet is not particularly limited, but is, for example, 930 MPa.

[0074] [Method for Measuring Tensile Strength] The tensile strength of a hot-rolled steel sheet can be determined by a tensile test in accordance with JIS Z2241:2022.

[0075] Specifically, a plate-shaped tensile test specimen corresponding to the JIS No. 5 test specimen specified in JIS Z2241:2022 is taken from the hot-rolled steel sheet. The longitudinal direction of the test specimen is perpendicular to the rolling direction of the hot-rolled steel sheet. A tensile test is carried out in air at room temperature in accordance with JIS Z2241:2022 to determine the tensile strength TS (MPa).

[0076] [Regarding a balance between strength and burring ability] The hot-rolled steel sheet of this embodiment preferably further has excellent hole expandability (burring ability) in addition to the above-mentioned excellent strength. When the hot-rolled steel sheet is used for applications such as automobiles, electrical machinery, building materials, and construction machinery, it may be subjected to various processes such as press working and stretch flange working. When such processes are performed, it is preferable that the hot-rolled steel sheet has high burring ability.

[0077] Preferably, the hot-rolled steel sheet of this embodiment has an excellent balance of strength and burring properties. Specifically, the strength-burring properties balance SB, defined by the following formula using the tensile strength TS (MPa) and the hole expansion ratio λ (%), is preferably 40,000 (MPa·%) or more. In this case, when the hot-rolled steel sheet is used for the above-mentioned applications, it is possible to reduce the weight of the member. SB = TS × λ

[0078] [Method for measuring the strength-burring ability balance SB] The strength-burring ability balance SB can be determined by the following method. A test piece measuring 70 mm x 70 mm x thickness is taken from a hot-rolled steel sheet. A punched hole is formed in the center of the test piece using a first punch and a die. The diameter D0 of the first punch is 10.0 mm. A die with a clearance of 12% of the thickness of the test piece is used. The punched hole is circular, and is formed so that the center position of the punched hole is the center position of the surface of the 70 mm x 70 mm of the test piece.

[0079] A second punch with an apex angle of 60° is pressed into the punched hole from the side opposite the burr formed on the edge of the punched hole, enlarging the punched hole. The moving speed of the second punch is 10 mm / min. The second punch is stopped when the punched hole expands and a crack penetrates through the plate thickness direction. After stopping the second punch, the inner diameter Db (mm) of the punched hole is measured. The hole expansion ratio λ (%) is calculated using the diameter DO (mm) of the first punch and the inner diameter Db (mm) of the punched hole according to the following formula: λ = (Db - DO) / DO x 100

[0080] Using the tensile strength TS (MPa) of the hot-rolled steel sheet obtained by the above-mentioned [Method for measuring tensile strength] and the hole expansion ratio λ (%), the strength-burring ability balance SB (MPa·%) is calculated by the following formula: SB = TS × λ

[0081] [Regarding the thickness of the hot-rolled steel sheet of this embodiment] The thickness of the hot-rolled steel sheet of this embodiment is not particularly limited. The thickness of the hot-rolled steel sheet of this embodiment is, for example, 1.6 to 8.0 mm. The preferred lower limit of the thickness of the hot-rolled steel sheet of this embodiment is 1.8 mm, and more preferably 2.0 mm. The preferred upper limit of the thickness of the hot-rolled steel sheet of this embodiment is 7.8 mm, and more preferably 7.6 mm.

[0082] [Regarding the hot-dip galvanized steel sheet using the hot-rolled steel sheet of the present embodiment] The hot-dip galvanized steel sheet of the present embodiment includes the hot-rolled steel sheet of the present embodiment described above and a hot-dip galvanized coating layer mainly containing Zn. The hot-dip galvanized coating layer is formed on the surface of the hot-rolled steel sheet. The hot-dip galvanized coating layer has a well-known configuration. The hot-dip galvanized coating layer will be described below.

[0083] [Regarding the hot-dip galvanized layer] As described above, the hot-dip galvanized layer mainly contains Zn. Specifically, the hot-dip galvanized layer contains 65.00% or more Zn by mass. The hot-dip galvanized layer may be a layer made of so-called hot-dip galvanized (GI). Hot-dip galvanized coating contains 1.00% or less by mass of elements other than Zn, with the remainder being Zn. The hot-dip galvanized layer may have a chemical composition other than that of hot-dip galvanized (GI). When the Zn content of the hot-dip galvanized layer is 65.00% or more by mass, sufficient corrosion resistance can be obtained. The lower limit of the Zn content of the hot-dip galvanized layer is preferably 70.00%, and more preferably 73.00%.

[0084] As described above, the hot-dip galvanized layer may have a chemical composition other than GI. The chemical composition of the hot-dip galvanized layer may be within a known range. The chemical composition of the hot-dip galvanized layer may contain, for example, 65.00% or more of Zn and 0.05 to 35.00% of Al, in mass %. The hot-dip galvanized layer may contain impurities. Here, impurities refer to substances mixed in from raw materials when hot-dip galvanizing is performed.

[0085] The lower limit of the Al content is preferably 0.08%, more preferably 0.10%, and even more preferably 0.15%. The upper limit of the Al content is preferably 33.00%, more preferably 30.00%, more preferably 28.00%, more preferably 25.00%, more preferably 23.00%, and even more preferably 21.00%.

[0086] The chemical composition of the hot-dip galvanized layer according to this embodiment may further contain one or more elements selected from the following Groups 1 to 7. It is well known that these optional elements can be contained in a hot-dip galvanized layer. The "%" in the content of each optional element below means % by mass. [Group 1] Mg: 0 to 30.0% [Group 2 (Sn group)] One or more selected from the group consisting of Sn: 0 to 2.00%, Bi: 0 to 2.00%, and In: 0 to 2.00% [Group 3 (Ca group)] One or more selected from the group consisting of Ca: 0 to 3.00%, Y: 0 to 3.00%, La: 0 to 3.00%, and Ce: 0 to 3.00% [Group 4] Si: 0 to 2.50% [Group 5 (Cr group)] One or more selected from the group consisting of Cr: 0 to 0.5%, Ti: 0 to 0.5%, Ni: 0 to 0.5%, Co: 0 to 0.5%, V: 0 to 0.5%, Nb: 0 to 0.5%, Cu: 0 to 0.5%, and Mn: 0 to 0.5% [Group 6] Fe: 0 to 5.0% [Group 7 (Sr group)] One or more selected from the group consisting of Sr: 0 to 0.5%, Sb: 0 to 0.5%, Pb: 0 to 0.5%, and B: 0 to 0.5%.

[0087] Mg is an easily oxidizable element and enhances the corrosion resistance of the hot-dip galvanized layer through sacrificial corrosion protection. Tin (Sn), bismuth (Bi), and indium (In) form intermetallic compounds with Mg when the hot-dip galvanized layer contains Mg. As a result, the corrosion resistance of the hot-dip galvanized steel sheet is enhanced. Calcium (Ca), yttrium (Y), lanthanum (La), and selenium (Ce) form intermetallic compounds with Al and Zn in the hot-dip galvanized layer. As a result, the corrosion resistance of the hot-dip galvanized steel sheet is enhanced. Silicon (Si) enhances the corrosion resistance of the hot-dip galvanized steel sheet. Chromium (Cr), titanium (Ti), nickel (Ni), cobalt (Co), vanadium (V), niobium (Nb), copper (Cu), and manganese (Mn) improve the appearance quality of the hot-dip galvanized steel sheet. Iron (Fe) increases the hardness of the hot-dip galvanized layer and improves the workability of the hot-dip galvanized steel sheet. Strontium (Sr), antimony (Sb), lead (Pb) and boron (B) enhance the metallic luster of the hot-dip galvanized layer and improve the appearance quality of the hot-dip galvanized steel sheet.

[0088] [Method for measuring the chemical composition of the hot-dip galvanized layer] The chemical composition of the hot-dip galvanized layer can be determined by the following method. The hot-dip galvanized layer is dissolved using hydrochloric acid containing an inhibitor. The inhibitor can be, for example, IBIT, a product name of Asahi Chemical Industry Co., Ltd. The solution of the hot-dip galvanized layer is subjected to elemental analysis in the same manner as in the method described above in [Method for measuring the chemical composition of a hot-rolled steel sheet]. The chemical composition of the hot-dip galvanized layer can be determined by the above method.

[0089] The above-described hot-rolled steel sheet and hot-dip galvanized steel sheet having a hot-dip galvanized coating layer can provide excellent LME resistance.

[0090] [Method for manufacturing hot-rolled steel sheet] An example of a method for manufacturing a hot-rolled steel sheet according to this embodiment will be described below. The method for manufacturing a hot-rolled steel sheet described below is an example for manufacturing a hot-rolled steel sheet according to this embodiment.

[0091] An example of a method for manufacturing a hot-rolled steel sheet according to this embodiment includes the following steps: (Step 1) Material preparation step (Step 2) Rough rolling step (Step 3) Finish rolling step (Step 4) Coiling step The above manufacturing method is carried out using a manufacturing line facility. The manufacturing line facility includes, from upstream to downstream, a heating furnace, a roughing mill (rougher), a finishing mill (finisher), a water cooling device, and a down coiler. A plurality of transport rolls are arranged between each piece of equipment. Each step will be described below.

[0092] [(Step 1) Material Preparation Step] In the material preparation step, a material having a chemical composition that satisfies Feature 1 is prepared. A slab is produced, for example, by the following method. Molten steel having a chemical composition that satisfies Feature 1 is produced. The molten steel is used to produce a material (slab or ingot) by a casting method. For example, a slab is produced by a well-known continuous casting method using the molten steel. Alternatively, an ingot is produced by a well-known ingot-making method using the molten steel.

[0093] [(Step 2) Rough Rolling Step] In the rough rolling step, a prepared material (slab or ingot) is hot rolled (rough rolling) using a reverse rough rolling mill to produce an intermediate steel plate (rough bar). The heating temperature of the material in the rough rolling step is, for example, 1250 to 1300°C. The storage time of the material in the heating furnace is 30 minutes or more, preferably 60 minutes or more. The upper limit of the storage time is not particularly limited, but is, for example, 240 minutes.

[0094] [(Step 3) Finish Rolling Step] In the finish rolling step, the intermediate steel plate is further hot rolled (finish rolled) using a finishing rolling mill to produce a steel plate. The finishing rolling mill is a tandem rolling mill having a plurality of rolling stands ST arranged in a row. １ ~ST Ｋ (K is a natural number). Each rolling stand ST is equipped with a pair of work rolls. Among the multiple rolling stands of the finishing mill, the rolling stand ST that rolls down the intermediate steel plate last is Ｌ The surface temperature of the steel sheet at the delivery side of the rolling stand ST (where L is a natural number equal to or less than K) is defined as the finish rolling temperature FT (°C). Ｌis the rolling stand ST located at the most downstream of the finishing mill. Ｋ or the rolling stand ST Ｋ The rolling stand ST may be arranged upstream of the rolling stand ST.

[0095] The finish rolling process satisfies the following conditions: (Condition 1) The finish rolling temperature FT (°C) is set to 850 to 950°C. (Condition 2) The rolling stand ST that rolls down the intermediate steel plate last in the finish rolling mill is set to 850 to 950°C. Ｌ and a rolling stand ST Ｌ Rolling stand ST is located next to Ｌ－１ Reduction rate R Ｌ－１ (Condition 3) Rolling stand ST Ｌ Reduction rate R Ｌ is the reduction rate R Ｌ－１ In other words, R Ｌ / R Ｌ－１ is 0.90 or more. (Condition 4) Rolling stand ST Ｌ The water cooling device installed on the exit side of the rolling stand ST Ｌ (Condition 5) Cooling (water cooling) of the steel sheet is started within 0.30 seconds after the completion of rolling at rolling stand ST. Ｌ－１ Cooling (water cooling) of the steel sheet in the water cooling device is started within 2.00 seconds after completion of rolling at 1000°C. (Condition 6) The cooling rate CR1 from the finish rolling temperature FT to the finish rolling temperature FT-40°C is set to 150°C / sec or more and 250°C / sec or less. (Condition 7) The cooling rate CR2 from the finish rolling temperature FT-40°C to the coiling temperature CT is set to 25°C / sec or more and 75°C / sec or less. Conditions 1 to 7 will be explained below.

[0096] [(Condition 1) Regarding the finish rolling temperature FT] If the finish rolling temperature FT exceeds 950°C, the grain boundary length GL becomes excessively short. Also, the distance that B in the prior austenite grains diffuses to the prior austenite grain boundaries increases, and the degree of B segregation at the prior austenite grain boundaries decreases. On the other hand, if the finish rolling temperature FT is less than 850°C, an excessive load is applied to the rolling stand ST. Therefore, the finish rolling temperature FT is set to 850 to 950°C.

[0097] [(Condition 2) Reduction rate R Ｌ－１ About the rolling stand ST, which is the last rolling stand in the finishing mill to reduce the intermediate steel plate. Ｌ and a rolling stand ST Ｌ Rolling stand ST is located next to Ｌ－１ Reduction rate R Ｌ－１ If the reduction ratio R is too low, the grain boundary length GL becomes excessively short. In addition, the distance that B in the prior austenite grains travels to diffuse to the prior austenite grain boundary increases, and the degree of B segregation at the prior austenite grain boundary decreases. Therefore, Ｌ－１ to 25% or more.

[0098] [(Condition 3) Reduction rate R Ｌ About Rolling Stand ST Ｌ Reduction rate R Ｌ If too low, the rolling stand ST Ｌ－１ The strain given to the steel plate (rough bar) at rolling stand ST Ｌ Therefore, the strain accumulated in the steel sheet immediately after the completion of finish rolling becomes insufficient. In this case, the grain boundary length GL becomes excessively short. In addition, the distance that B in the prior austenite grains travels to diffuse to the prior austenite grain boundary increases, and the degree of B segregation at the prior austenite grain boundary decreases. Therefore, the reduction ratio R Ｌ is the reduction rate R Ｌ－１ The value shall be 0.90 times or more.

[0099] [(Condition 4) Cooling start time t Ｌ About Rolling Stand ST Ｌ After the rolling of the steel plate is completed at the rolling stand ST Ｌ The time until the water cooling device arranged on the outlet side starts cooling the steel plate is defined as the cooling start time t Ｌ The cooling start time t Ｌ If the cooling start time t is too long, the grain boundary length GL becomes excessively short. In addition, the distance that B in the prior austenite grains travels to diffuse to the prior austenite grain boundary becomes large, and the degree of B segregation at the prior austenite grain boundary decreases. Therefore, the cooling start time t Ｌ is set to 0.30 seconds or less.

[0100] Cooling start time t Ｌcan be obtained by the following method: Ｌ The moving speed (m / sec) of the steel plate at the exit side of the rolling stand ST Ｌ The exit side is the rolling stand ST Ｌ The point FP immediately after the steel plate in contact with the pair of rolls separates from the pair of work rolls Ｌ This means that the point FP Ｌ From the rolling stand ST Ｌ The cooling start time t is calculated based on the distance to the water cooling start position where the water cooling device arranged on the outlet side of the steel sheet starts water cooling the steel sheet and the moving speed of the steel sheet. Ｌ The obtained value is rounded to two decimal places and used as the cooling start time t Ｌ (seconds).

[0101] [(Condition 5) Water cooling start time t Ｌ－１ About Rolling Stand ST Ｌ－１ After the rolling of the steel plate is completed at the rolling stand ST Ｌ The time until the water cooling device arranged on the outlet side starts cooling the steel plate is defined as the cooling start time t Ｌ－１ The cooling start time t Ｌ－１ If it is too long, the rolling stand ST Ｌ－１ After the reduction is completed at the rolling stand ST Ｌ In this case, the time until the reduction starts becomes excessively long. Ｌ－１ The strain imparted to the steel plate by rolling in the rolling stand ST Ｌ In this case, the grain boundary length GL becomes excessively short. Also, the distance that B in the prior austenite grains travels to diffuse to the prior austenite grain boundary increases, and the degree of B segregation at the prior austenite grain boundary decreases. Therefore, the cooling start time t Ｌ－１ is set to 2.00 seconds or less.

[0102] Cooling start time t Ｌ－１ can be obtained by the following method: Ｌ－１ The moving speed (m / sec) of the steel plate at the exit side of the rolling stand ST Ｌ－１ The exit side is the rolling stand ST Ｌ－１The point FP immediately after the steel plate in contact with the pair of rolls separates from the pair of work rolls Ｌ－１ This means that the point FP Ｌ－１ From rolling stand ST Ｌ The exit point FP Ｌ Based on the distance to the point FP and the moving speed of the steel plate, Ｌ－１ From rolling stand ST Ｌ The exit point FP Ｌ The transportation time (seconds) of the steel plate up to the cooling start time t Ｌ－１ The sum of the transport time and the cooling start time tL is defined as the cooling start time t Ｌ－１ (seconds).

[0103] [(Condition 6) Cooling Rate CR1] The average cooling rate in the temperature range between the finish rolling temperature FT and the finish rolling temperature FT-40°C is defined as the cooling rate CR1 (°C / sec). If the cooling rate CR1 in this temperature range is too slow, the grain boundary length GL becomes excessively short. Furthermore, the distance that B in the prior austenite grains diffuses to the prior austenite grain boundaries increases, and the degree of B segregation at the prior austenite grain boundaries decreases. On the other hand, if the cooling rate CR1 is too fast, the area ratio of bainitic ferrite in the produced hot-rolled steel sheet becomes excessively small. Therefore, the cooling rate CR1 is set to 150°C / sec or more and 250°C / sec or less.

[0104] The cooling rate CR1 can be determined based on the time it takes for the surface temperature of the steel sheet to drop from the finish rolling temperature FT to the finish rolling temperature FT-40°C. The cooling rate CR1 is an integer value obtained by rounding off the obtained value to one decimal place. Note that the steel sheet is continuously cooled in the temperature range from the finish rolling temperature FT to FT-40°C. In other words, the temperature of the steel sheet decreases continuously due to cooling.

[0105] [(Condition 7) Cooling Rate CR2] The average cooling rate in the temperature range from the finish rolling temperature FT-40°C to the coiling temperature CT (°C) is defined as the cooling rate CR2. If the cooling rate CR2 is too slow, the segregation of B to the prior austenite grain boundaries decreases. Furthermore, polygonal ferrite is generated in excess, and a sufficient area ratio of bainitic ferrite cannot be obtained. Furthermore, the grain boundary length GL becomes excessively short, and the degree of B segregation becomes low. On the other hand, if the cooling rate CR2 is too fast, the area ratio of bainitic ferrite in the produced hot-rolled steel sheet becomes excessively small. Therefore, the cooling rate CR2 is set to 25°C / s or more and 75°C / s or less.

[0106] The cooling rate CR2 can be determined based on the time it takes for the surface temperature of the steel sheet to reach the coiling temperature CT (°C) from the finish rolling temperature FT-40°C. The cooling rate CR2 is an integer value obtained by rounding the obtained value to one decimal place. Note that in the temperature range from the finish rolling temperature FT-40°C to the coiling temperature CT2, the steel sheet is continuously cooled. In other words, the temperature of the steel sheet decreases continuously due to cooling.

[0107] After the steel sheet is cooled from the finish rolling temperature FT to the finish rolling temperature FT-40°C at a cooling rate CR1, the steel sheet is quickly cooled at a cooling rate CR2 without providing a residence time. In other words, cooling at the cooling rate CR1 and cooling at the cooling rate CR2 are carried out continuously. If a residence time of 0.2 seconds or more is provided after cooling at the cooling rate CR1, B segregated at the prior austenite grain boundaries will diffuse, and Feature 3 or Feature 4 will not be satisfied.

[0108] [(Step 4) Coiling Step] In the coiling step, the steel sheet that has passed through the cooling device is wound into a coil by a coiling device. In the coiling step, Ti carbides are generated in the steel sheet. Here, the surface temperature of the steel sheet at the start of coiling is defined as the coiling temperature CT (°C). The coiling temperature CT also affects the microstructure of the hot-rolled steel sheet (the proportions of bainitic ferrite, polygonal ferrite, and bainite). Therefore, the coiling temperature CT is set to 500 to 630°C.

[0109] If the coiling temperature CT is higher than 630°C, coiling starts before the transformation from austenite to bainitic ferrite is completed in the microstructure of the steel sheet. Therefore, part of the austenite transforms into polygonal ferrite. As a result, the area ratio of bainitic ferrite in the hot-rolled steel sheet decreases.

[0110] On the other hand, if the coiling temperature CT is less than 500°C, bainite is formed in the hot-rolled steel sheet, and therefore the area ratio of bainitic ferrite in the hot-rolled steel sheet decreases.

[0111] The hot-rolled steel sheet according to this embodiment is manufactured by the above-described manufacturing steps. As described above, the hot-rolled steel sheet according to this embodiment may be manufactured by a manufacturing method other than the above-described manufacturing method.

[0112] [Other Steps in the Method for Producing Hot-Rolled Steel Sheet] The method for producing a hot-rolled steel sheet according to this embodiment may include steps other than those described above. For example, a temper rolling step may be performed after the finish rolling step and before the coiling step, or after the coiling step. In the temper rolling step, temper rolling is performed on the hot-rolled steel sheet. The temper rolling step adjusts the shape of the hot-rolled steel sheet, adjusts the surface roughness, and adjusts the yield strength. To effectively obtain the above effects, the thickness reduction rate in the temper rolling step is, for example, 0.1% or more. A preferred upper limit of the thickness reduction rate in the temper rolling step is 3.0%. In this case, the introduction of excessive strain into the hot-rolled steel sheet is suppressed, and good ductility, bendability, and flangeability can be maintained.

[0113] [Method for manufacturing hot-dip galvanized steel sheet including the hot-rolled steel sheet of this embodiment] The hot-dip galvanized steel sheet including the hot-rolled steel sheet of this embodiment can be manufactured by carrying out the following well-known hot-dip galvanizing treatment step.

[0114] [Hot-dip galvanizing process] In the hot-dip galvanizing process, a hot-dip galvanized layer having the above-mentioned chemical composition is formed on the surface of a hot-rolled steel sheet. Specifically, a galvanizing bath is prepared. The composition of the galvanizing bath is adjusted depending on the composition of the hot-dip galvanized layer to be formed. After immersing the hot-rolled steel sheet in the galvanizing bath for a certain period of time, the hot-rolled steel sheet is pulled out of the galvanizing bath by a well-known method. For example, a sink roll is disposed in the galvanizing bath. The hot-rolled steel sheet immersed in the galvanizing bath has its traveling direction changed upward by the sink roll.

[0115] A hot-dip galvanized coating is applied to the surface of a hot-rolled steel sheet pulled out of a coating bath. A known gas wiping device is used to adjust the amount of the hot-dip galvanized coating applied to the hot-rolled steel sheet. The hot-dip galvanized coating applied to the hot-rolled steel sheet pulled out of the coating bath solidifies to form a hot-dip galvanized coating layer. A hot-dip galvanized steel sheet is manufactured through the above steps.

[0116] [Other Steps in the Manufacturing Method of Hot-Dip Galvanized Steel Sheet] The manufacturing method of the hot-dip galvanized steel sheet of this embodiment may include manufacturing steps other than the hot-dip galvanizing treatment step. For example, the manufacturing method of the hot-dip galvanized steel sheet of this embodiment may include a Ni pre-plating step before the hot-dip galvanizing treatment step. In the Ni pre-plating step, the above-mentioned hot-rolled steel sheet is Ni-plated to form a Ni-plated layer on the surface of the hot-rolled steel sheet. The hot-dip galvanizing treatment step is then performed on the hot-rolled steel sheet on which the Ni-plated layer has been formed. In this case, the adhesion of the hot-dip galvanized coating layer to the hot-rolled steel sheet is improved.

[0117] The method for producing a hot-dip galvanized steel sheet according to this embodiment may further include a chemical conversion treatment step after the hot-dip galvanizing treatment step. In the chemical conversion treatment step, a chemical conversion treatment is performed on the produced hot-dip galvanized steel sheet to form a chemical conversion coating on the hot-dip galvanized layer. When the chemical conversion treatment step is performed, the method of chemical conversion treatment is not particularly limited, and a well-known method may be used. For example, a well-known chromium chemical conversion coating may be formed as the chemical conversion coating.

[0118] The method for producing a hot-dip galvanized steel sheet according to this embodiment may further include a temper rolling step after the hot-dip galvanizing treatment step. In the temper rolling step, the produced hot-dip galvanized steel sheet is subjected to temper rolling. When the chemical conversion treatment step described above is carried out, the adhesion of the chemical conversion coating can be improved by carrying out the temper rolling step before the chemical conversion treatment step. The sheet thickness reduction rate in the temper rolling step is not particularly limited. A preferred sheet thickness reduction rate in the temper rolling step is, for example, 0.1 to 3.0%.

[0119] The method for producing a hot-dip galvanized steel sheet may further include other production steps. The above-described production method is an example of a production method for obtaining the hot-dip galvanized steel sheet of the present embodiment. Therefore, the production method for a hot-dip galvanized steel sheet of the present embodiment is not limited to the above-described production method.

[0120] The effects of one aspect of the hot-rolled steel sheet and hot-dip galvanized steel sheet of this embodiment will be described more specifically using examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the hot-rolled steel sheet and hot-dip galvanized steel sheet of this embodiment. Therefore, the hot-rolled steel sheet and hot-dip galvanized steel sheet of this embodiment are not limited to this one example of conditions.

[0121] Hot-rolled steel sheets having the chemical compositions shown in Table 1 were produced.

[0122]

[0123] In Table 1, "-" means that the corresponding element was not contained. The chemical composition of the steel sheet having each test number was determined based on the method described in the above-mentioned [Method for measuring the chemical composition of a hot-rolled steel sheet].

[0124] Molten steel was continuously cast to produce a slab. The slab was subjected to a rough rolling process and a finish rolling process. Specifically, the slab was heated at 1250 to 1300°C for 60 minutes. The heated slab was rolled in a rough rolling mill to produce an intermediate steel plate. Furthermore, the intermediate steel plate was hot rolled using a finish rolling mill to produce a steel plate. In the finish rolling process, (Condition 1) the finish rolling temperature FT (°C) and (Condition 2) the rolling reduction R Ｌ－１ (%), (Condition 3) Reduction rate R Ｌ / Reduction rate R Ｌ－１, (Condition 4) Cooling start time t Ｌ (seconds), (condition 5) water cooling start time t Ｌ－１ , (Condition 6) cooling rate CR1 (°C / sec), and (Condition 7) cooling rate CR2 (°C / sec) were as shown in Table 2. After the steel sheet was cooled from the finish rolling temperature FT to the finish rolling temperature FT-40°C at the cooling rate CR1, the steel sheet was quickly cooled at the cooling rate CR2 without providing a residence time. In other words, cooling at the cooling rate CR1 and cooling at the cooling rate CR2 were carried out continuously. Furthermore, in the temperature range from the finish rolling temperature FT to FT-40°C, the temperature of the steel sheet was continuously lowered by cooling. Similarly, in the temperature range from the finish rolling temperature FT-40°C to the coiling temperature CT2, the temperature of the steel sheet was continuously lowered by cooling.

[0125]

[0126] The steel sheet after finish rolling was wound into a coil by a winding device at a coiling temperature CT. The coiling temperature CT for each test number was 500 to 630°C. The coiled steel sheet was allowed to cool to room temperature, and a hot-rolled steel sheet with each test number shown in Table 1 was manufactured. The thickness of the hot-rolled steel sheet was 2.3 mm.

[0127] [Evaluation Tests] The following evaluation tests were carried out on the hot-rolled steel sheets with each test number: (Test 1) Measurement test of the area ratio of bainitic ferrite (Test 2) Measurement test of the grain boundary length GL of prior austenite grains in the surface layer (Test 3) Measurement test of the degree of B segregation at prior austenite grain boundaries (Test 4) Measurement test of tensile strength (Test 5) Measurement test of the balance between strength and burring properties (Test 6) Evaluation test of LME resistance of hot-dip galvanized steel sheets Tests 1 to 6 will be described below.

[0128] [(Test 1) Measurement test of area ratio of bainitic ferrite] The area ratio (%) of bainitic ferrite in the hot-rolled steel sheet of each test number was determined based on the method described in [Method for measuring area ratio of bainitic ferrite] above. The obtained area ratios of bainitic ferrite are shown in the "BF area ratio (%)" column in Table 3.

[0129]

[0130] [(Test 2) Measurement test of grain boundary length GL of prior austenite grains in surface layer] Based on the method described in [Method for measuring grain boundary length GL] above, the grain boundary length GL (mm) in the surface layer region of the heat-rolled steel sheet of each test number was determined. The obtained grain boundary lengths GL are shown in the "Grain boundary length GL (mm)" column in Table 3.

[0131] [(Test 3) Measurement test of B segregation at prior austenite grain boundaries] The B segregation in the surface layer region of the hot-rolled steel sheet of each test number was determined based on the method described in [Method for measuring B segregation at prior austenite grain boundaries] above. The obtained B segregation is shown in the "B segregation" column in Table 3.

[0132] [(Test 4) Tensile Strength Measurement Test] The tensile strength TS (MPa) of the hot-rolled steel sheet of each test number was determined based on the method described in the above-mentioned [Method for measuring tensile strength]. The obtained tensile strengths TS are shown in the "TS (MPa)" column in Table 3.

[0133] [(Test 5) Strength-burring ability balance measurement test] Based on the method described in the above [Method for measuring strength-burring ability balance SB], the hole expandability λ (%) of the hot-rolled steel sheet of each test number was determined. Furthermore, the strength-burring ability balance SB was determined using the tensile strength TS (MPa) and hole expandability (%) obtained in Test 4. The obtained hole expandability λ (%) is shown in the "λ (%)" column in Table 3. The obtained strength-burring ability balance SB is shown in the "SB (MPa %)" column in Table 3.

[0134] [(Test 6) Evaluation Test for LME Resistance of Hot-Dip Galvanized Steel Sheets] [Manufacturing of Hot-Dip Galvanized Steel Sheets] In order to evaluate the LME resistance of the hot-dip galvanized steel sheets, first, hot-dip galvanized steel sheets were manufactured using the hot-rolled steel sheets of each test number. Specifically, a well-known hot-dip galvanizing process was performed on the hot-rolled steel sheets of each test number to form a hot-dip galvanized coating layer having a chemical composition shown in Table 4 on the surface of the hot-rolled steel sheet. The "Coating Number" column of "Coated Steel Sheet" in Table 3 shows the coating numbers of the hot-dip galvanized coating layer formed on the hot-rolled steel sheet of each test number. The coating numbers shown in the "Coating Number" column of "Coated Steel Sheet" in Table 3 correspond to the coating numbers in Table 4. Hot-dip galvanized steel sheets were manufactured by the above manufacturing process.

[0135]

[0136] In Table 4, the element symbols written to the left of the values ​​indicate the contained elements. For example, plating number P2 indicates that 0.11% by mass of Sn is contained as an element of the Sn group. The chemical composition of the plating layer of each plating number was determined by the method described in the above-mentioned "Method for measuring the chemical composition of a hot-dip galvanized coating layer."

[0137] The LME resistance of the manufactured hot-dip galvanized steel sheets of each test number was evaluated by the following method. Sample steel sheets measuring 100 mm x 75 mm x thickness were taken from the hot-dip galvanized steel sheets of each test number. Arc welding as shown in Figure 3 was performed using the sample steel sheets. Specifically, a cylindrical boss member 1 with a diameter of 20 mm and a length of 25 mm was prepared. The boss member 1 was made of a steel material equivalent to SS400 specified in JIS G3101:2015.

[0138] As shown in FIG. 3 , the boss member 1 was positioned at the center of the sample steel plate 2 so that the axial direction of the boss member 1 was normal to the surface of the sample steel plate 2. The positioned boss member 1 was welded to the sample steel plate 2 by arc welding. The welding wire used was YM-70CS as specified in JIS Z3312:2009 G69A2UCN1M2T. In the arc welding, the weld bead 3 made one full clockwise revolution around the boss member 1 from the welding start point in a plan view, and the arc welding continued even after it passed the welding start point until an overlap region 4 of the weld bead was formed. The width of the overlap region 4 was approximately 15 mm.

[0139] The current value during arc welding was 190 A and the voltage value was 23 V. The welding speed was 0.3 m / min. A 20% by volume CO shielding gas was used during arc welding. ２ The gas mixture was 80% by volume of argon gas and 100% by volume of argon gas. The flow rate of the shielding gas during arc welding was 20 L / min.

[0140] Before performing the arc welding shown in Fig. 3 , sample steel plate 2 was joined in advance to restraint plate 5 as shown in Fig. 4 . Constraint plate 5 measured 120 mm × 95 mm × 4 mm thick, and was a steel plate equivalent to SS400 specified in JIS G3101:2015. Sample steel plate 2 was placed on the surface of restraint plate 5. The entire periphery of the placed sample steel plate 2 was welded to restraint plate 5. The welding wire and welding conditions were the same as those used when welding boss member 1 to sample steel plate 2.

[0141] Before welding the boss member shown in Fig. 3 , sample steel plate 2 circumferentially welded to restraint plate 5 was placed on stand 6 as shown in Fig. 4 , and sample steel plate 2 and restraint plate 5 were fixed to stand 6 with clamps (not shown). After sample steel plate 2 was fixed to stand 6 with the clamps, boss member 1 was welded to sample steel plate 2 by arc welding as shown in Fig. 3 .

[0142] After arc welding the boss member 1 to the sample steel plate 2, as shown in FIG. 3 , the boss member 1, the sample steel plate 2, and the restraint plate 5 were cut at a cut surface 7 passing through the central axis of the boss member 1 and the overlap region 4 of the weld bead 3. The cut surface 7 was then observed at 100x magnification using an optical microscope. The presence or absence of cracks (liquid metal embrittlement cracking) in the sample steel plate 2 was visually confirmed. If cracks were observed, their lengths were measured. Specifically, the crack lengths (mm) from the surface of the hot-rolled steel plate (the interface between the coating layer and the hot-rolled steel plate) in the depth direction (sheet thickness direction) were determined. The maximum crack length among the measured crack lengths was identified. If the maximum crack length was 1.0 mm or less, the specimen was judged to have excellent LME resistance (indicated as "E" (Excellent) in the "LME Resistance Evaluation" column in Table 3). On the other hand, when the maximum crack length exceeded 1.0 mm, the LME resistance was determined to be low (shown as "B" (Bad) in the "LME resistance evaluation" column in Table 3).

[0143] [Evaluation Results] Referring to Tables 1 to 4, the hot-rolled steel sheets of test numbers 1 to 18 satisfied features 1 to 4. Therefore, excellent LME resistance was obtained. Note that the hot-rolled steel sheets of these test numbers had a tensile strength TS of 590 MPa or more, and high strength was obtained.

[0144] On the other hand, in test number 19, the Nb content was too low, which resulted in a short grain boundary length GL and a low degree of B segregation, and as a result, sufficient LME resistance was not obtained.

[0145] In test number 20, the B content was too low, and therefore the degree of B segregation was low, resulting in insufficient LME resistance.

[0146] In test numbers 21 and 22, the finish rolling temperature FT was too high, which resulted in a short grain boundary length GL and a low degree of B segregation, making it impossible to obtain sufficient LME resistance.

[0147] In test numbers 23 and 24, the rolling reduction rate R Ｌ－１ was too low. Therefore, the grain boundary length GL was short and the degree of B segregation was low. As a result, sufficient LME resistance was not obtained.

[0148] In test numbers 25 and 26, the rolling reduction rate R Ｌ Reduction rate R Ｌ－１ Ratio to (= R Ｌ / R Ｌ－１ ) was too low. Therefore, the grain boundary length GL was short and the degree of B segregation was low. As a result, sufficient LME resistance was not obtained.

[0149] In Test Nos. 27 and 28, the cooling start time t Ｌ was too long. Therefore, the grain boundary length GL was short and the degree of B segregation was low. As a result, sufficient LME resistance was not obtained.

[0150] In Test Nos. 29 and 30, the water cooling start time t Ｌ－１ was too long. Therefore, the grain boundary length GL was short and the degree of B segregation was low. As a result, sufficient LME resistance was not obtained.

[0151] In test numbers 31 and 32, the cooling rate CR1 was too slow, which resulted in a short grain boundary length GL and a low degree of B segregation, making it impossible to obtain sufficient LME resistance.

[0152] In test number 33, the cooling rate CR1 was too fast, which resulted in a low area ratio of bainitic ferrite. As a result, the strength-burring ability balance SB was low.

[0153] In test numbers 34 to 36, the cooling rate CR2 was too slow. Therefore, in test numbers 34 and 35, the area ratio of bainitic ferrite was low, and further, the degree of B segregation was low. In test number 36, the degree of B segregation was also low. As a result, in test numbers 34 and 35, sufficient LME resistance was not obtained, and the tensile strength was less than 590 MPa. Furthermore, the strength-burring property balance SB was low. In addition, in test number 36, sufficient LME resistance was not obtained.

[0154] In test number 37, the cooling rate CR2 was too fast, which resulted in a low area ratio of bainitic ferrite. As a result, the strength-burring ability balance SB was low.

[0155] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. A hot-rolled steel sheet containing, by mass%, C: 0.030 to 0.110%, Si: 0.01 to 0.50%, Mn: 0.50 to 1.50%, P: 0.025% or less, S: 0.010% or less, Al: 0.010 to 0.070%, N: 0.0070% or less, Ti: 0.055 to 0.200%, Nb: 0.030 to 0.150%, B: 0.0010 to 0.0050%, V: 0 to 0.20%, and Cr: 0 to 1.00%, with the remainder consisting of Fe and impurities, and in the microstructure, the area ratio of bainitic ferrite is 85% or more, In the surface region of the hot-rolled steel sheet from the surface to a depth of 0.2 mm in the sheet thickness direction, BO was detected by time-of-flight secondary ion mass spectrometry. ２ － a total length of prior austenite grain boundaries in which B is detected is 1.20 to 5.00 mm per 100 μm×100 μm, and a degree of B segregation at the prior austenite grain boundaries is 40 or more.

2. The hot-rolled steel sheet according to claim 1, containing, by mass%, one or more elements selected from the group consisting of V: ​​0.01 to 0.20%, and Cr: 0.01 to 1.00%.

3. A hot-dip galvanized steel sheet comprising: the hot-rolled steel sheet according to claim 1 or 2; and a hot-dip galvanized layer formed on the surface of the hot-rolled steel sheet and containing 65.00% or more Zn by mass.

Images