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

Abstract

Provided is a hot-rolled steel sheet that exhibits excellent LME resistance. The hot-rolled steel sheet according to the present disclosure contains, in mass%, 0.030-0.110% of C, 0.01-0.50% of Si, 0.50-1.50% of Mn, 0.025% or less of P, 0.010% or less of S, 0.010-0.070% of Al, 0.0070% or less of N, 0.055-0.200% of Ti, 0.020-0.100% of W, 0.0010-0.0050% of B, and the balance being Fe with impurities. In the microstructure, the area ratio of bainitic ferrite is 85% or more. In a surface layer region from the surface of the hot-rolled steel sheet to a depth of 0.2 mm in the sheet thickness direction, the total length of the old austenite grain boundary in which BO2- is detected by the time-of-flight secondary ion mass spectrometry is 0.80-5.00 mm per 100 μm x 100 μm. The B segregation degree at the old austenite grain boundary is 80 or more.

Description

TITLE OF INVENTION: HOT-ROLLED STEEL SHEET AND HOT-DIP GALVANIZED STEEL SHEET TECHNICAL FIELD

[0001] The present disclosure relates to a hot-rolled steel sheet, and to a hot-dip galvanized steel sheet obtained by forming a hot dip galvanized layer on the surface of the hot-rolled steel sheet. BACKGROUND ART

[0002] Hot-rolled steel sheets are widely utilized for automobiles, electrical machinery, building materials, construction equipment and the like. In hot-rolled steel sheets to be used for these applications, a hot dip galvanized layer may be formed in some cases on the surface of the hot-rolled steel sheets to increase corrosion resistance. Hereinafter, a hot-rolled steel sheet on which a hot dip galvanized layer has been formed is also referred to as a "hot-dip galvanized steel sheet".

[0003] In some cases, a hot-dip galvanized steel sheet may be welded to another steel member. During welding, a part of the hot dip galvanized layer melts. At such time, molten metal (zinc) may penetrate into grain boundaries at the outer layer of the hot-rolled steel sheet, leading to the occurrence of cracks. Such cracks are referred to as liquid metal embrittlement (LME).

[0004] Hot-rolled steel sheets are required to have a characteristic such that the occurrence of LME can be suppressed in a case where a hot dip galvanized layer is formed on the surface of the hot-rolled steel sheet (hereinafter, this characteristic is referred to as "LME resistance").

[0005] Japanese Patent Application Publication No. 2018-145500 (Patent Literature 1) proposes a molten Zn-Al-Mg-based plated steel sheet that is excellent in LME resistance.

[0006] The molten Zn-Al-Mg-based plated steel sheet proposed in Patent Literature 1 includes a base steel sheet and a molten Zn-Al-Mg-based alloy coating layer. The base steel sheet has a chemical composition containing, in 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, a Ti / C equivalence ratio (= (Ti / 48) / (C / 12)) is 0.4 to 1.5. In the base steel sheet, in addition, the dislocation density is 1.8 x 1014 / m2 to 5.7 x 1014 / m2. In the base steel sheet, either one of a bainitic ferrite phase and a ferrite phase is a single phase, or a phase containing a bainitic ferrite phase and a ferritic phase is a principal phase, and the area fraction of a hard second phase and cementite is 3% or less. In addition, carbides containing Ti that have a mean particle diameter of 20 nm or less are dispersed and precipitated in the base steel sheet.

[0007] Patent Literature 1 describes that by having the aforementioned chemical composition and microstructure, excellent LME resistance is obtained in the molten Zn-Al-Mg-based plated steel sheet of Patent Literature 1. CITATION LIST PATENT LITERATURE

[0008] Patent Literature 1: Japanese Patent Application Publication No. 2018-145500 SUMMARY OF INVENTION TECHNICAL PROBLEM

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

[0010] An objective of the present disclosure is to provide a hot-rolled steel sheet and a hot-dip galvanized steel sheet in which excellent LME resistance is obtained. SOLUTION TO PROBLEM

[0011] A hot-rolled steel sheet of the present disclosure 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%, W: 0.020 to 0.100%, B: 0.0010 to 0.0050%, Nb: 0 to 0.150%, V: 0 to 0.20%, and Cr: 0 to 1.00%, with the balance being Fe and impurities. In the microstructure, an area fraction of bainitic ferrite is 85% or more. In an outer layer region from a surface of the hot-rolled steel sheet to a position at a depth of 0.2 mm in a sheet thickness direction, a total length of prior-austenite grain boundaries where BO2- is detected by time-of-flight secondary ion mass spectrometry is 0.80 to 5.00 mm per 100 pm x 100 pm, and a degree of B segregation at the prior-austenite grain boundaries is 80 or more.

[0012] A hot-dip galvanized steel sheet of the present disclosure includes the hot-rolled steel sheet described above, and a hot dip galvanized layer which is formed on a surface of the hot-rolled steel sheet and which contains Zn in an amount of 65.00% by mass or more. ADVANTAGEOUS EFFECTS OF INVENTION

[0013] Excellent LME resistance is obtained in the hot-rolled steel sheet and the hot-dip galvanized steel sheet of the present disclosure. The method for producing a hot-rolled steel sheet according to the present disclosure can produce the aforementioned hot-rolled steel sheet. BRIEF DESCRIPTION OF DRAWINGS

[0014] [FIG. 1] FIG. 1 is a schematic diagram for describing line analysis by time-of-flight secondary ion mass spectrometry. [FIG. 2] FIG. 2 is a schematic diagram of a B detection intensity distribution obtained by line analysis. [FIG. 3] FIG. 3 is a schematic diagram illustrating an LME resistance evaluation test carried out in the Examples. [FIG. 4] FIG. 4 is a cross-sectional view of the LME resistance evaluation test of FIG. 3 as viewed from the side. DESCRIPTION OF EMBODIMENTS

[0015] Hot-rolled steel sheets suitable for applications such as automobiles, electrical machinery, building materials, and construction equipment are required to have a certain level of strength. Therefore, the present inventors conducted studies from the viewpoint of the chemical composition with respect to a hot-rolled steel sheet that is suitable for these applications. As a result, the present inventors have considered that if 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 to 0.150%, V: 0 to 0.20%, and Cr: 0 to 1.00%, with the balance being Fe and impurities, and in addition, the area fraction of bainitic ferrite is 85% or more, there is a possibility that sufficient strength can be obtained.

[0016] Therefore, the present inventors also conducted studies regarding means for increasing the LME resistance of a hot-rolled steel sheet having the chemical composition and microstructure described above. As mentioned above, LME occurs due to molten metal (zinc) penetrating into grain boundaries in the outer layer of the steel sheet. Therefore, the present inventors have considered including B in an amount of 0.0010 to 0.0050% in the aforementioned chemical composition. B segregates to grain boundaries and strengthens the grain boundaries. Hence, there is a possibility that the LME resistance will increase.

[0017] However, in some cases, sufficient LME resistance could not be obtained merely by including B in the aforementioned chemical composition. Therefore, the present inventors conducted further studies. Here, among the prior-austenite grain boundaries in the outer layer of the hot-rolled steel sheet, the present inventors focused on the length of the grain boundaries where B is segregated. LME tends to propagate along prior-austenite grain boundaries. Therefore, the longer the length of prior-austenite grain boundaries where B is segregated is, the easier it will be to suppress the propagation of LME along the grain boundaries.

[0018] Based on the above finding, the present inventors conducted further investigations regarding means for increasing the length of prior-austenite grain boundaries where B is segregated. As a result, it has been discovered that by further including W in an amount of 0.020 to 0.100% in the chemical composition described above, the total length of prior-austenite grain boundaries where there is a high degree of B segregation can be increased.

[0019] The reason why B segregation at grain boundaries is markedly increased by containing W in the chemical composition is not certain. However, it is considered that the reason may be as follows. When B segregates to prior-austenite grain boundaries, Fe23CB6 carbides form at the grain boundaries. Formation of Fe23CB6 reduces segregation of B at the prior-austenite grain boundaries. W suppresses the formation of Fe23CB6, and consequently the amount of dissolved B at the prioraustenite grain boundaries can be secured. Therefore, it is considered that by containing W, B segregation at the prior-austenite grain boundaries can be increased.

[0020] The reason described above is an assumption, and there is also a possibility that the reason why W increases B segregation at prior-austenite grain boundaries is different from the reason described above. However, the fact that W increases B segregation at prior-austenite grain boundaries and, as a result, increases the LME resistance of the hot-rolled steel sheet is demonstrated by Examples that are described later.

[0021] A hot-rolled steel sheet, a hot-dip galvanized steel sheet that uses the hot-rolled steel sheet, and a method for producing a hot-rolled steel sheet of the present embodiment that have been completed based on the above findings are as follows.

[0022] A hot-rolled steel sheet of the present embodiment 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%, W: 0.020 to 0.100%, B: 0.0010 to 0.0050%, Nb: 0 to 0.150%, V: 0 to 0.20%, and Cr: 0 to 1.00%, with the balance being Fe and impurities. In the microstructure, an area fraction of bainitic ferrite is 85% or more. In an outer layer region from a surface of the hot-rolled steel sheet to a position at a depth of 0.2 mm in a sheet thickness direction, a total length of prior-austenite grain boundaries where BO2- is detected by time-of-flight secondary ion mass spectrometry is 0.80 to 5.00 mm per 100 gm x 100 gm, and a degree of B segregation at the prior-austenite grain boundaries is 80 or more.

[0023] The hot-rolled steel sheet of the present embodiment may further contain one or more types of element selected from a group consisting of, in mass%, Nb: 0.001 to 0.150%, V: 0.01 to 0.20% and Cr: 0.01 to 1.00%.

[0024] A hot-dip galvanized steel sheet of the present embodiment includes the hot-rolled steel sheet of the present embodiment, and a hot dip galvanized layer which is formed on a surface of the hot-rolled steel sheet and which contains Zn in an amount of 65.00% by mass or more.

[0025] Hereinafter, the hot-rolled steel sheet and the hot-dip galvanized steel sheet according to the present embodiment are described in detail. The symbol "%" in relation to an element means mass percent unless otherwise stated.

[0026] [Hot-rolled steel sheet] The hot-rolled steel sheet of the present embodiment satisfies the following feature 1 to feature 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%, W: 0.020 to 0.100%, B: 0.0010 to 0.0050%, Nb: 0 to 0.150%, V: 0 to 0.20%, and Cr: 0 to 1.00%, with the balance being Fe and impurities. (Feature 2) In the microstructure, an area fraction of bainitic ferrite is 85% or more. (Feature 3) In an outer layer region from a surface of the hot-rolled steel sheet to a position at a depth of 0.2 mm in the sheet thickness direction, a total length of prioraustenite grain boundaries where BO2- is detected by time-of-flight secondary ion mass spectrometry is 0.80 to 5.00 mm per 100 ^m x 100 um. (Feature 4) The degree of B segregation at the prior-austenite grain boundaries where BO2- is detected by time-of-flight secondary ion mass spectrometry is 80 or more.

[0027] [(Feature 1) Regarding chemical composition] The chemical composition of the hot-rolled steel sheet according to the present embodiment contains the following elements.

[0028] C: 0.030 to 0.110% Carbon (C) combines with Ti to form Ti carbides. The Ti carbides increase the strength of the hot-rolled steel sheet by precipitation strengthening. In addition, in a case where the content of Ti in the chemical composition is 0.055 to 0.200%, C facilitates the formation of bainitic ferrite. If the content of C is less than 0.030%, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of C is more than 0.110%, even if the contents of other elements are within the range of the present embodiment, bainite will excessively form in the microstructure and the area fraction of bainitic ferrite will excessively decrease. Consequently, workability of the hot-rolled steel sheet will decrease. Therefore, the content of C is 0.030 to 0.110%. A preferable lower limit of the content of C is 0.032%, more preferably is 0.034%, further preferably is 0.035%, and further preferably is 0.036%. A preferable upper limit of the content of C is 0.105%, more preferably is 0.100%, further preferably is 0.095%, and further preferably is 0.090%.

[0029] Si: 0.01 to 0.50% Silicon (Si) deoxidizes the steel. Si also increases the strength of the hot-rolled steel sheet by solid-solution strengthening. If the content of Si is less than 0.01%, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Si is more than 0.50%, polygonal ferrite will easily form in the hot-rolled steel sheet even if the contents of other elements are within the range of the present embodiment. Consequently, the area fraction of bainitic ferrite in the hot-rolled steel sheet will decrease. In such case, the strength of the hot-rolled steel sheet will decrease. Therefore, the content of Si is 0.01 to 0.50%. A preferable lower limit of the content of Si is 0.02%, more preferably is 0.03%, and further preferably is 0.04%. A preferable upper limit of the content of Si is 0.45%, more preferably is 0.40%, further preferably is 0.35%, and further preferably is 0.33%.

[0030] Mn: 0.50 to 1.50% Manganese (Mn) increases the strength of the hot-rolled steel sheet by solidsolution strengthening. If the content of Mn is less than 0.50%, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Mn is more than 1.50%, even if the contents of other elements are within the range of the present embodiment, Mn segregation will easily occur in the hot-rolled steel sheet. In addition, if the content of Mn is more than 1.50%, bainite will easily form in the hot-rolled steel sheet. As a result, the area fraction of bainitic ferrite in the hot-rolled steel sheet will decrease. In such case, cold formability of the hot-rolled steel sheet will decrease. Therefore, the content of Mn is 0.50 to 1.50%. A preferable lower limit of the content of Mn is 0.55%, more preferably is 0.60%, further preferably is 0.65%, further preferably is 0.70%, further preferably is 0.75%, and further preferably is 0.80%. A preferable upper limit of the content of Mn is 1.45%, more preferably is 1.40%, further preferably is 1.35%, further preferably is 1.30%, and further preferably is 1.20%.

[0031] P: 0.025% or less Phosphorus (P) segregates to grain boundaries and decreases the cold formability of the hot-rolled steel sheet. P also decreases the weldability of the hot-rolled steel sheet. If the content of P is more than 0.025%, the cold formability and weldability of the hot-rolled steel sheet will markedly decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of P is 0.025% or less. The content of P is preferably as low as possible. However, excessively reducing the content of P will lower productivity and increase the production cost. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of P is more than 0%, more preferably is 0.001%, further preferably is 0.002%, further preferably is 0.003%, further preferably is 0.005% further preferably is 0.010%, and further preferably is 0.015%. A preferable upper limit of the content of P is 0.023%, more preferably is 0.022%, and further preferably is 0.020%.

[0032] S: 0.010% or less Sulfur (S) segregates to grain boundaries and decreases the cold formability of the hot-rolled steel sheet. If the content of S is more than 0.010%, the cold formability of the hot-rolled steel sheet will markedly decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of S is 0.010% or less. The content of S is preferably as low as possible. However, excessively reducing the content of S will lower productivity and increase the production cost. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of S is more than 0%, more preferably is 0.001%, further preferably is 0.002%, and further preferably is 0.003%. A preferable upper limit of the content of S is 0.009%, more preferably is 0.008%, and further preferably is 0.007%.

[0033] Al: 0.010 to 0.070% Aluminum (Al) deoxidizes the steel. Al also combines with N to form Al nitrides. By this means, Al suppresses the combining of B with N. If the content of Al is less than 0.010%, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Al is more than 0.070%, coarse Al nitrides will excessively form even if the contents of other elements are within the range of the present embodiment. Consequently, the cold formability of the hot-rolled steel sheet will decrease. Therefore, the content of Al is 0.010 to 0.070%. A preferable lower limit of the content of Al is 0.012%, more preferably is 0.014%, further preferably is 0.016%, further preferably is 0.020%, and further preferably is 0.025%. A preferable upper limit of the content of Al is 0.065%, more preferably is 0.060%, and further preferably is 0.055%.

[0034] N: 0.0070% or less Nitrogen (N) combines with B to form BN, and thereby reduces the amount of B dissolved in the hot-rolled steel sheet. N also combines with Ti to form TiN, and thereby inhibits the formation of Ti carbides. If the content of N is more than 0.0070%, excessive amounts of BN and TiN will form even if the contents of other elements are within the range of the present embodiment. As a result, the LME resistance of the hot-rolled steel sheet will decrease. In addition, the strength of the hot-rolled steel sheet will decrease. Therefore, the content of N is 0.0070% or less. The content of N is preferably as low as possible. However, excessively reducing the content of N will lower productivity and increase the production cost. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of N is more than 0%, more preferably is 0.0001%, further preferably is 0.0005%, further preferably is 0.0010%, further preferably is 0.0015%, and further preferably is 0.0020%. A preferable upper limit of the content of N is 0.0060%, more preferably is 0.0050%, and further preferably is 0.0040%.

[0035] Ti: 0.055 to 0.200% Titanium (Ti) combines with C to form Ti carbides. The Ti carbides increase the strength of the hot-rolled steel sheet by precipitation strengthening. In addition, the amount of dissolved C in the steel sheet is adjusted by formation of the Ti carbides. Therefore, formation of bainitic ferrite in the hot-rolled steel sheet is facilitated. If the content of Ti is less than 0.055%, even if the contents of other elements are within the range of the present embodiment, polygonal ferrite will easily form. Therefore, the area fraction of bainitic ferrite in the hot-rolled steel sheet decreases. In such case, the strength of the hot-rolled steel sheet will decrease. On the other hand, if the content of Ti is more than 0.200%, coarse TiC will excessively form even if the contents of other elements are within the range of the present embodiment. As a result, the cold formability of the hot-rolled steel sheet will decrease. Therefore, the content of Ti is 0.055 to 0.200%. A preferable lower limit of the content of Ti is 0.060%, more preferably is 0.065%, further preferably is 0.070%, further preferably is 0.075%, further preferably is 0.080%, and further preferably is 0.085%. A preferable upper limit of the content of Ti is 0.190%, more preferably is 0.180%, and further preferably is 0.170%.

[0036] W: 0.020 to 0.100% Tungsten (W) promotes segregation of B to grain boundaries. Consequently, the degree of B segregation at prior-austenite grain boundaries increases, and consequently the LME resistance of the hot-rolled steel sheet increases. If the content of W is less than 0.020%, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of W is more than 0.100%, even if the contents of other elements are within the range of the present embodiment, cold formability may decrease and weldability may decrease. Furthermore, if the content of W is more than 0.100%, the production cost will increase. Therefore, the content of W is 0.020 to 0.100%. A preferable lower limit of the content of W is 0.025%, more preferably is 0.030%, further preferably is 0.035%, further preferably is 0.040%, and further preferably is 0.045%. A preferable upper limit of the content of W is 0.095%, more preferably is 0.090%, and further preferably is 0.085%.

[0037] B: 0.0010 to 0.0050% Boron (B) dissolves in the hot-rolled steel sheet, and segregates to prioraustenite grain boundaries. The segregated B increases the grain boundary strength. Therefore, B increases the LME resistance of the hot-rolled steel sheet. B also increases the hardenability of the steel. If the content of B is less than 0.0010%, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. In addition, if the content of B is less than 0.0010%, the temperature at which transformation from austenite to ferrite starts will increase. In such case, the temperature at which precipitation of Ti carbides starts will also increase. Consequently, the Ti carbides will be coarse. As a result, the strength of the hot-rolled steel sheet will decrease. On the other hand, if the content of B is more than 0.0050%, the hardenability will be excessively high even if the contents of other elements are within the range of the present embodiment. In such case, bainite will tend to form and the area fraction of bainitic ferrite will decrease. As a result, workability of the steel sheet will decrease. In addition, if the content of B is more than 0.0050%, Fe23CB6 will precipitate at prior-austenite grain boundaries. In such case, the grain boundary strength of the prior-austenite grains will decrease. Consequently, the LME resistance will decrease. Therefore, the content of B is 0.0010 to 0.0050%. A preferable lower limit of the content of B is 0.0012%, more preferably is 0.0015%, further preferably is 0.0020%, and further preferably is 0.0025%. A preferable upper limit of the content of B is 0.0045%, more preferably is 0.0043%, further preferably is 0.0041%, further preferably is 0.0040%, and further preferably is 0.0035%.

[0038] The balance of the chemical composition of the hot-rolled steel sheet of the present embodiment contains Fe and impurities. Here, the term "impurities" refers to substances which are mixed in from ore or scrap as the raw material or from the production environment or the like when industrially producing the hot-rolled steel sheet, and which are allowed within a range that does not adversely affect the hot-rolled steel sheet of the present embodiment. Preferably, the balance of the chemical composition of the hot-rolled steel sheet of the present embodiment is Fe and impurities. Here, the impurities are, for example, one or more types of element selected from a group consisting of O: 0 to 0.01%, Mo: 0 to 0.10%, Cu: 0 to 0.50%, Ni: 0 to 0.30%, Co: 0 to 0.10%, Zr: 0 to 0.05%, Sn: 0 to 0.10%, Sb: 0 to 0.10%, Hf: 0 to 0.02%, Sc: 0 to 0.02%, Te: 0 to 0.01%, Sr: 0 to 0.02%, Bi: 0 to 0.10%, Ta: 0 to 0.02%, Zn: 0 to 0.02%, and rare earth metal (REM): 0 to 0.01%. The total content of these impurities is, for example, 0 to 1.00%.

[0039] [Optional elements] The chemical composition of the hot-rolled steel sheet of the present embodiment may further contain one or more types of element selected from a group consisting of Nb: 0 to 0.150%, V: 0 to 0.20% and Cr: 0 to 1.00%, in lieu of a part of Fe. Each of these elements is an optional element.

[0040] Nb: 0 to 0.150% Niobium (Nb) is an optional element, and does not have to be contained. That is, the content of Nb may be 0%. When contained, that is, when the content of Nb is more than 0%, Nb forms Nb precipitates (Nb carbides or Nb carbo-nitrides) and increases the grain boundary length of the prior-austenite grains. Nb also promotes segregation of B to the prioraustenite grain boundaries. By this means, the degree of B segregation at the prioraustenite grain boundaries is increased. As a result, the LME resistance of the hot-rolled steel sheet increases. The Nb precipitates also increase the strength of the hot-rolled steel sheet by precipitation strengthening. If even a small amount of Nb is contained, the aforementioned advantageous effects will be obtained to a certain extent. However, if the content of Nb is more than 0.150%, Nb precipitates will excessively form. In such case, the cold formability of the hot-rolled steel sheet will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of Nb is 0 to 0.150%. A preferable lower limit of the content of Nb is 0.001%, more preferably is 0.010%, further preferably is 0.030%, further preferably is 0.035%, further preferably is 0.040%, and further preferably is 0.045%. A preferable upper limit of the content of Nb is 0.145%, more preferably is 0.140%, further preferably is 0.135%, further preferably is 0.130%, further preferably is 0.125%, and further preferably is 0.120%.

[0041] V: 0 to 0.20% Vanadium (V) is an optional element, and does not have to be contained. That is, the content of V may be 0%. When contained, that is, when the content of V is more than 0%, V combines with C to form V carbides. The V carbides increase the strength of the hot-rolled steel sheet by precipitation strengthening. If even a small amount of V is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of V is more than 0.20%, V carbides will excessively form. In such case, the cold formability of the hot-rolled steel sheet will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of V is 0 to 0.20%, and when contained, the content of V is 0.20% or less. A preferable lower limit of the content of V is 0.01%, more preferably is 0.05%, and further preferably is 0.08%. A preferable upper limit of the content of V is 0.18%, more preferably is 0.16%, further preferably is 0.14%, and further preferably is 0.12%.

[0042] Cr: 0 to 1.00% Chromium (Cr) is an optional element, and does not have to be contained. That is, the content of Cr may be 0%. When contained, that is, when the content of Cr is more than 0%, Cr segregates to prior-austenite grain boundaries, and thereby increases the LME resistance of the hot-rolled steel sheet. If even a small amount of Cr is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Cr is more than 1.00%, the cold formability of the hot-rolled steel sheet will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of Cr is 0 to 1.00%, and when contained, the content of Cr is 1.00% or less. A preferable lower limit of the content of Cr is 0.01%, more preferably is 0.05%, further preferably is 0.10%, further preferably is 0.20%, and further preferably is 0.30%. A preferable upper limit of the content of Cr is 0.90%, more preferably is 0.80%, further preferably is 0.70%, further preferably is 0.60%, further preferably is 0.55%, further preferably is 0.50%, further preferably is 0.45%, and further preferably is 0.40%. [Method for measuring chemical composition of hot-rolled steel sheet] The chemical composition of the hot-rolled steel sheet of the present embodiment can be measured by a well-known composition analysis method in accordance with JIS G0321: 2017. Specifically, a machined chip is collected from the hot-rolled steel sheet using a cutting tool such as a drill. The collected machined chip is dissolved in acid to obtain a liquid solution. The liquid solution is subjected to ICP-MAS (Inductively Coupled Plasma Mass Spectrometry) to perform elemental analysis of the chemical composition. The content of C and the content of S are determined by a well-known high-frequency combustion method (combustion-infrared absorption method). The content of N is determined using a well-known inert gas fusion-thermal conductivity method.

[0044] Note that, a numerical value up to the least significant digit of the content of each element defined in the present embodiment obtained by rounding off a fraction of the measured numerical value based on the significant figures defined in the present embodiment is adopted for the content of each element. For example, the content of C in the steel material of the present embodiment is defined as a numerical value to the third decimal place. Therefore, the content of C is taken as a numerical value to the third decimal place obtained by rounding off the fourth decimal place of the measured numerical value. Similarly, for the content of each element other than the content of C in the steel material of the present embodiment, a value obtained by rounding off a fraction of the measured numerical value to the least significant digit defined in the present embodiment is taken as the content of the relevant element. Note that, the term "rounding off" means rounding down if the fraction is less than 5, and rounding up if the fraction is 5 or more.

[0045] [(Feature 2) Regarding area fraction of bainitic ferrite] In the microstructure of the hot-rolled steel sheet of the present embodiment, the area fraction of bainitic ferrite is 85% or more. The microstructure of the hot-rolled steel sheet of the present embodiment may be a bainitic-ferrite single phase microstructure. In a case where the microstructure of the hot-rolled steel sheet of the present embodiment is composed of bainitic ferrite and one or more other phases, the one or more other phases are, for example, one or more types selected from a group consisting of polygonal ferrite, bainite, and cementite.

[0046] Bainitic ferrite can be distinguished from polygonal ferrite and bainite as follows.

[0047] [Distinguishing between bainitic ferrite and polygonal ferrite] Bainitic ferrite is an aggregation of grains having slightly different crystal orientations to each other. Therefore, a difference in contrast within the grains can be recognized. On the other hand, polygonal ferrite is a structure in which there is almost no crystal misorientation within the grains. Therefore, the interiors of the grains are observed with a uniform contrast. Therefore, bainitic ferrite can be distinguished from polygonal ferrite based on the contrast that is attributable to crystal misorientation.

[0048] [Distinguishing between bainitic ferrite and bainite] The crystal structure of bainitic ferrite is a bcc structure, similarly to the crystal structure of bainite. Hence, it is difficult to distinguish bainitic ferrite from bainite based on crystal structure. In addition, it is difficult to distinguish bainitic ferrite from bainite based on crystal misorientation. However, bainitic ferrite can be distinguished from bainite based on whether or not Fe carbides are present within grains and at crystal grain boundaries. Here, the term "Fe carbides" refers to carbides that contain Fe, such as cementite.

[0049] Specifically, in bainitic ferrite, Fe carbides are not present within grains and at crystal grain boundaries. On the other hand, in bainite, Fe carbides are present within laths and / or at lath boundaries. Therefore, bainitic ferrite can be distinguished from bainite based on whether or not Fe carbides are present within grains and at crystal grain boundaries.

[0050] [Advantageous effect produced by 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 is within the range of the present embodiment, if the area fraction of bainitic ferrite is 85% or more, high strength will be obtained.

[0051] A preferable lower limit of the area fraction of bainitic ferrite is 88%, more preferably is 90%, further preferably is 92%, further preferably is 94%, and further preferably is 96%. The area fraction of bainitic ferrite may be 100%.

[0052] [Method for measuring area fraction of bainitic ferrite] The area fraction of bainitic ferrite can be determined by the following method. Microstructure observation is performed using a field emission scanning electron microscope (FE-SEM). The microstructure observation is performed using an electron channeling contrast image (ECCI). The observation conditions are to be set as follows: acceleration voltage of 20 kV, tilt (T) = 0°, and backscattered electron mode. Electron back scatter diffraction (EBSD) is used as the method for measuring the crystal orientation.

[0053] A test specimen is taken from a central position of the width of the hot-rolled steel sheet. The measurement position is to be at a depth of 1 / 4 of the thickness in the sheet thickness direction of the hot-rolled steel sheet from the surface of the test specimen, and the measurement range is to be set to 100 um x 100 urn. The measurement range is to be a longitudinal section that includes an L direction (longitudinal direction of the hot-rolled steel sheet) and a T direction (thickness direction of the hot-rolled steel sheet). The measurement range is divided into regular hexagonal pixel units. The distance between the centers of pixels that are adjacent to each other (EBSD step size) is set to 0.1 um. The center position of each regular hexagonal pixel that is a pixel which is entirely included within the measurement range is taken as a measurement point. That is, any pixel in which a part of the pixel is outside the observation field is excluded from the targets for measurement. Further, the confidence index (CI value) is to be 0.1 or more.

[0054] The measurement data is analyzed by the following procedures using EBSD analysis software to identify and quantify polygonal ferrite and bainitic ferrite. (Procedure 1) A region surrounded by grain boundaries where the crystal misorientation is 15° or more is defined as a single grain. Note that, in a case where the equivalent circular diameter of a region surrounded by grain boundaries where the crystal misorientation is 15° or more is 1.0 pm or less, it is determined that the region is measurement noise, and the region is not recognized as a grain. In other words, regions which are determined as being measurement noise are excluded from the analysis. (Procedure 2) An average value of the crystal misorientation within each grain (grain average misorientation: hereinafter, referred to as "GAM value") is calculated. Grains in which the GAM value is 0.5° or less are defined as polygonal ferrite. Grains in which the GAM value is more than 0.5° are defined as bainitic ferrite.

[0055] Note that, in the microstructure observation described above, phases which are different from bainitic ferrite and polygonal ferrite (that is, bainite and cementite) can be easily distinguished by contrast.

[0056] The identified bainitic ferrite is quantified. The area fraction (%) of bainitic ferrite is then determined based on the area of the quantified bainitic ferrite and the total area of the measurement range (100 pm x 100 pm). Note that, regions which were determined to be measurement noise are excluded from the total area of the measurement range.

[0057] Note that, it suffices to use a well-known program as an EBSD analysis program for determining the GAM values. For example, OIM Data Collection / Analysis 6.2.0 manufactured by TSL Solutions KK may be used.

[0058] [(Feature 3) Regarding grain boundary length GL of prior-austenite grains in outer layer] In the hot-rolled steel sheet of the present embodiment, furthermore, in the outer layer region, the total length of prior-austenite grain boundaries where BO2- is detected by time-of-flight secondary ion mass spectrometry is 0.80 to 5.00 mm per 100 pm x 100 pm. Here, the total length, per 100 pm x 100 pm, of prior-austenite grain boundaries where BO2- is detected by time-of-flight secondary ion mass spectrometry is referred to as "grain boundary length GL" (mm).

[0059] If the grain boundary length GL is too short, there are fewer grain boundary regions where B segregation occurs. In such case, B segregation cannot sufficiently contribute to suppressing LME cracks. Consequently, the LME resistance of the hot-rolled steel sheet decreases. If the grain boundary length GL is 0.80 mm or more, the grain boundary length is sufficiently long. In such case, there are a sufficiently large number of grain boundary regions where B segregation occurs within the outer layer region of the hot-rolled steel sheet. As a result, the LME resistance of the hot-rolled steel sheet increases.

[0060] A preferable lower limit of the grain boundary length GL is 0.90 mm, more preferably is 0.95 mm, further preferably is 1.00 mm, further preferably is 1.01 mm, and further preferably is 1.05 mm. The upper limit of the grain boundary length GL is not particularly limited. However, the production cost will significantly increase if it is attempted to make the grain boundary length GL excessively long. Therefore, when taking industrial production into consideration, a preferable upper limit of the grain boundary length GL is 5.00 mm, more preferably is 4.95 mm, further preferably is 4.90 mm, further preferably is 4.50 mm, further preferably is 4.00 mm, further preferably is 3.50 mm, and further preferably is 3.00 mm.

[0061] [Regarding method for measuring grain boundary length GL] The grain boundary length GL of prior-austenite grain boundaries per pm2 in the outer layer is determined by the following method.

[0062] Five test specimens in which a cross section parallel to the rolling elongation direction and the sheet thickness direction of the hot-rolled steel sheet is defined as an observation surface are taken from the hot-rolled steel sheet. The size of the observation surface is set to, for example, 10 mm in the rolling elongation direction x the sheet thickness. The observation surface of each test specimen is mirror-polished with diamond paste. After mirror polishing, the observation surface is chemically polished with colloidal silica solution to remove the mechanically affected layer introduced during mirror polishing.

[0063] On the observation surface, one rectangular observation field measuring 100 gm x 100 gm is selected in an outer layer region from the surface of the hot-rolled steel sheet to a position at a depth of 0.2 mm in the sheet thickness direction. The bainitic ferrite grain boundaries, polygonal ferrite grain boundaries, and bainite grain boundaries in the observation field are identified based on the method described above in the section [Method for measuring area fraction of bainitic ferrite].

[0064] In the same observation field, B segregation to prior-austenite grain boundaries is measured by time-of-flight secondary ion mass spectrometry (TOF-SIMS). First, as a preliminary treatment, 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 preliminary treatment, primary ions are irradiated onto the observation field from a primary ion source of a TOF-SIMS apparatus to sputter the observation field with the primary ions. For the measurement, Bi1+ is used as the primary ion. The beam diameter of the primary ions is set to 0.1 gm, the acceleration voltage is set to 30 keV, and the current value is set to 2 pA.

[0065] Among the secondary ions generated by sputtering with primary ions, BO2-(mass-to-charge ratio m / z = 43) is detected as a secondary ion containing B. By measuring the time of flight of BO2-, a map of the B distribution within the observation field is created. The B distribution map is a map that shows the positions where BO2- was detected. The bainitic ferrite grain boundaries, polygonal ferrite grain boundaries, and bainite grain boundaries in the created B distribution map are identified. In the B distribution map, the grain boundaries other than bainitic ferrite grain boundaries, polygonal ferrite grain boundaries, and bainite grain boundaries are defined as prioraustenite grain boundaries. Among the prior-austenite grain boundaries, the total length of grain boundary portions where BO2- with an intensity of 40 counts or more is detected is determined by well-known image processing. The determined total length of the grain boundaries is defined as the grain boundary length GL (mm). The grain boundary length GL is to be made a value to the second decimal place by rounding off to the second decimal place of the determined value.

[0067] Note that, for example, an apparatus with the product name "TOF-SIMS.5" manufactured by IONTOF GmbH is used as the TOF-SIMS apparatus.

[0068] [(Feature 4) Regarding degree of B segregation at prior-austenite grain boundaries] In the hot-rolled steel sheet of the present embodiment, furthermore, in the outer layer region, the degree of B segregation at prior-austenite grain boundaries where BO2- is detected by time-of-flight secondary ion mass spectrometry is 80 or more. When the degree of B segregation at prior-austenite grain boundaries is high, the prior-austenite grain boundaries are strengthened. Therefore, the propagation of LME cracks along the grain boundaries can be suppressed. If the degree of B segregation at the prior-austenite grain boundaries is 80 or more, a grain boundary length GL where the degree of B segregation is high is formed. Therefore, the propagation of LME cracks along the prior-austenite grain boundaries can be markedly suppressed. As a result, the LME resistance of the hot-rolled steel sheet will markedly increase.

[0069] A preferable lower limit of the degree of B segregation is 82, more preferably is 84, further preferably is 85, further preferably is 90, and further preferably is 92. The upper limit of the degree of B segregation is not particularly limited. However, in the case of a hot-rolled steel sheet satisfying feature 1 to feature 3, the upper limit of the degree of B segregation at the prior-austenite grain boundaries is, for example, 160, 150, 140, 130, or 120.

[0070] [Regarding method for measuring degree of B segregation at prior-austenite grain boundaries] The degree of B segregation at the prior-austenite grain boundaries is determined by the following method. Prior-austenite grain boundaries where BO2- is detected within the observation field are identified by the method described above in the section [Regarding method for measuring grain boundary length GL]. At five arbitrary locations along the identified grain boundaries, line analysis across the grain boundary is performed as follows.

[0071] FIG. 1 is a schematic diagram for describing the line analysis. Referring to FIG. 1, at a prior-austenite grain boundary AG where BO2- is detected within the observation field, TOF-SIMS is used to scan a primary ion beam in an approximately perpendicular direction to detect the B intensity. In the line analysis by TOF-SIMS, Bi1+ is used as the primary ion. The diameter of the primary ion beam is set to 0.1 pm. At such time, a length L1 of a line segment SE to be subjected to line analysis is set to 15 pm, and the measurement interval is set to 0.2 pm. Note that, the line segment SE is to be set so as to intersect with only one grain boundary AG and to ensure that the line segment SE does not intersect with a plurality of grain boundaries AG. In addition, the line analysis is performed so that the grain boundary AG is positioned at approximately the center position of the length L1 of the line segment SE. The B detection intensity distribution on the line segment SE is obtained by the line analysis.

[0072] FIG. 2 is a schematic diagram illustrating the B detection intensity distribution on the line segment SE. In FIG. 2, the ordinate represents the B detection intensity, and the abscissa represents the distance on the line segment SE. Referring to FIG. 2, in the obtained B detection intensity distribution BC, the maximum value of the detection intensity is defined as a B detection intensity Bp at the prior-austenite grain boundary AG where BO2- was detected. In the B detection intensity distribution BC, the arithmetic average value of all B detection intensities measured at intervals of 0.2 gm within a range L10 of 0.15 x L1 from an endpoint El of the line segment SE and all B detection intensities measured at intervals of 0.2 gm within a range R10 of 0.15 x L1 from an endpoint Er of the line segment SE is defined as an average B detection intensity Bave within the prior-austenite grain surrounded by the grain boundary AG where BO2- is detected.

[0073] The obtained B detection intensity Bp on the grain boundary AG and the average B detection intensity Bave within the prior-austenite grain are used to calculate the degree of B segregation based on Formula (A). Degree of B segregation = Bp / Bave x content of B (ppm) in chemical composition of hot-rolled steel sheet (A)

[0074] The arithmetic average value of the degrees of B segregation obtained by line analysis at the five locations is defined as the degree of B segregation at the prioraustenite grain boundaries in the outer layer region. The obtained degree of B segregation is to be made an integer obtained by rounding off the value at the first decimal place.

[0075] [Advantageous effect of hot-rolled steel sheet of present embodiment] The hot-rolled steel sheet of the present embodiment satisfies feature 1 to feature 4. Therefore, excellent LME resistance is obtained in the hot-rolled steel sheet of the present embodiment.

[0076] [Regarding strength of hot-rolled steel sheet of present embodiment] The hot-rolled steel sheet of the present embodiment satisfies feature 1 to feature 4. Therefore, the tensile strength of the hot-rolled steel sheet of the present embodiment is 590 MPa or more. Thus, high strength and excellent LME resistance can both be achieved in the hot-rolled steel sheet of the present embodiment. A preferable lower limit of the tensile strength of the hot-rolled steel sheet is 595 MPa, more preferably is 600 MPa, further preferably is 605 MPa, further preferably is 610 MPa, further preferably is 620 MPa, and further preferably is 650 MPa. Although not particularly limited, the upper limit of the tensile strength of the hot-rolled steel sheet is, for example, 930 MPa.

[0078] [Method for measuring tensile strength] The tensile strength, total elongation, and yield ratio of the hot-rolled steel sheet can be determined by a tensile test in accordance with JIS Z2241: 2022.

[0079] Specifically, a sheet-shaped tensile test specimen corresponding to a JIS No. 5 test specimen specified in JIS Z 2241: 2022 is taken from the hot-rolled steel sheet. The longitudinal direction of the test specimen is to be orthogonal to the rolling elongation direction of the hot-rolled steel sheet. In accordance with JIS Z 2241: 2022, a tensile test is conducted at normal temperature in air, and a tensile strength TS (MPa) is determined.

[0080] [Regarding achieving balance between strength and burring properties] Preferably, while having excellent strength as described above, the hot-rolled steel sheet of the present embodiment also has excellent hole expandability (burring properties). When hot-rolled steel sheets are used for applications such as automobiles, electrical machinery, building materials, and construction equipment, the hot-rolled steel sheets may be subjected to various processing such as press working and stretch flanging. In a case where such processing is performed, it is preferable for the hot-rolled steel sheet to have high burring properties.

[0081] Preferably, the hot-rolled steel sheet of the present embodiment is excellent in the balance between excellent strength and burring properties. Specifically, preferably a balance between strength and burring properties SB defined by the following formula using the tensile strength TS (MPa) and a hole expansion ratio X (%) is 40000 (MPa-%) or more. In this case, when the hot-rolled steel sheet is used for the aforementioned applications, it is possible to reduce the weight of the member. SB = TS x X

[0082] [Method for measuring balance between strength and burring properties SB] The balance between strength and burring properties SB can be determined by the following method. A test specimen with dimensions of 70 mm x 70 mm x the sheet thickness is taken from the hot-rolled steel sheet. A first punch and a die are used to form a punched hole at the center part of the test specimen. A diameter D0 of the first punch is to be 10.0 mm. A die that allows a clearance that is 12% of the sheet thickness of the test specimen to be obtained is used. The punched hole is formed in a manner so that the punched hole is a circular shape and the center position of the punched hole is at the center position of the 70 mm x 70 mm surface of the test specimen.

[0083] From the opposite side to the burr formed at the edge of the punched hole as it is in a punched state, a second punch with an apex angle of 60° is pushed into the punched hole to enlarge the punched hole. At such time, the movement speed of the second punch is set to 10 mm / min. The second punch is stopped at a time point at which the punched hole has been enlarged and a crack has penetrated through the test specimen in the sheet thickness direction. After the second punch is stopped, the inner diameter Db (mm) of the punched hole is measured. The hole expansion ratio X (%) is calculated by the following formula using the diameter DO (mm) of the first punch and the inner diameter Db (mm) of the punched hole. X = (Db-D0) / D0 x 100

[0084] The balance between strength and burring properties SB (MPa-%) is calculated by the following formula using the tensile strength TS (MPa) of the hot-rolled steel sheet obtained by the above [Method for measuring tensile strength], and the hole expansion ratio X (%). SB = TS x X

[0085] [Regarding thickness of hot-rolled steel sheet of present embodiment] The thickness of the hot-rolled steel sheet of the present embodiment is not particularly limited. The thickness of the hot-rolled steel sheet of the present embodiment is, for example, 1.6 to 8.0 mm. A preferable lower limit of the thickness of the hot-rolled steel sheet of the present embodiment is 1.8 mm, and more preferably is 2.0 mm. A preferable upper limit of the thickness of the hot-rolled steel sheet of the present embodiment is 7.8 mm, and more preferably is 7.6 mm.

[0086] [Regarding hot-dip galvanized steel sheet which uses hot-rolled steel sheet of 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 layer that principally contains Zn. The hot dip galvanized layer is formed on the surface of the hot-rolled steel sheet. The hot dip galvanized layer has a well-known composition. Hereinafter, the hot dip galvanized layer is described.

[0087] [Regarding hot dip galvanized layer] As mentioned above, the hot dip galvanized layer principally contains Zn. Specifically, the hot dip galvanized layer contains Zn in an amount of 65.00% by mass or more. The hot dip galvanized layer may be a layer composed of a so-called "hot-dip galvanized coating (GI)". A hot-dip galvanized coating contains elements other than Zn in an amount of 1.00% by mass or less, with the balance being Zn. The hot dip galvanized layer may have a chemical composition other than the chemical composition of a hot-dip galvanized coating (GI). As long as the content of Zn in the hot dip galvanized layer is 65.00% by mass or more, sufficient corrosion resistance will be obtained. A preferable lower limit of the content of Zn in the hot dip galvanized layer is 70.00%, and more preferably is 73.00%.

[0088] [Regarding chemical composition of hot dip galvanized layer] As mentioned above, the hot dip galvanized layer may have a chemical composition other than GI. It suffices that the chemical composition of the hot dip galvanized layer is within a well-known range. For example, the chemical composition of the hot dip galvanized layer may contain, in mass%, Zn in an amount of 65.00% or more, and Al: 0.05 to 35.00%. The hot dip galvanized layer may contain impurities. Here, the term "impurities" refers to substances which are mixed in from the raw material when performing a hot-dip galvanizing treatment.

[0089] A preferable lower limit of the content of Al is 0.08%, more preferably is 0.10%, and further preferably is 0.15%. A preferable upper limit of the content of Al is 33.00%, more preferably is 30.00%, further preferably is 28.00%, further preferably is 25.00%, further preferably is 23.00%, and further preferably is 21.00%.

[0090] The chemical composition of the hot dip galvanized layer according to the present embodiment may further contain one or more elements selected from the following first group to seventh group in lieu of a part of Zn. It is well-known that these optional elements are optional elements that can be contained in a hot dip galvanized layer. The symbol "%" in relation to the content of each optional element hereinafter means "mass percent". [First group] Mg: 0 to 30.0% [Second group (Sn group)] One or more types of element selected from a group consisting of Sn: 0 to 2.00%, Bi: 0 to 2.00%, and In: 0 to 2.00%. [Third group (Ca group)] One or more types of element selected from a group consisting of Ca: 0 to 3.00%, Y: 0 to 3.00%, La: 0 to 3.00%, and Ce: 0 to 3.00%. [Fourth group] Si: 0 to 2.50% [Fifth group (Cr group)] One or more types of element selected from a 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%. [Sixth group] Fe: 0 to 5.0% [Seventh group (Sr group)] One or more types of element selected from a group consisting of Sr: 0 to 0.5%, Sb: 0 to 0.5%, Pb: 0 to 0.5%, and B: 0 to 0.5%.

[0091] Mg is an easily oxidized element, and increases the corrosion resistance of the hot dip galvanized layer by sacrificial protection. Tin (Sn), bismuth (Bi), and indium (In) are elements which, when the hot dip galvanized layer contains Mg, form intermetallic compounds with Mg. As a result, the corrosion resistance of the hot-dip galvanized steel sheet increases. 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 increases. Silicon (Si) increases 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) enhance the external appearance quality of the hot-dip galvanized steel sheet. Iron (Fe) increases the hardness of the hot dip galvanized layer, and increases the workability of the hot-dip galvanized steel sheet. Strontium (Sr), antimony (Sb), lead (Pb), and boron (B) increase the metallic luster of the hot dip galvanized layer, and thereby enhance the external appearance quality of the hot-dip galvanized steel sheet.

[0092] [Method for measuring chemical composition of 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. For example, a product with the trade name "IBIT" manufactured by Asahi Chemical Co., Ltd. can be used as the inhibitor. The solution of the hot dip galvanized layer is subjected to elemental analysis in the same manner as in the method described above in the section [Method for measuring chemical composition of hot-rolled steel sheet]. The chemical composition of the hot dip galvanized layer can be determined by the above method.

[0093] In a hot-dip galvanized steel sheet that includes the hot-rolled steel sheet and the hot dip galvanized layer described above, excellent LME resistance is obtained.

[0094] [Method for producing hot-rolled steel sheet] One example of a method for producing the hot-rolled steel sheet according to the present embodiment will now be described. The method for producing a hot-rolled steel sheet described below is one example for producing the hot-rolled steel sheet according to the present embodiment.

[0095] One example of a method for producing the hot-rolled steel sheet of the present embodiment includes the following processes. (Process 1) Starting material preparation process (Process 2) Rough rolling process (Process 3) Finish rolling process (Process 4) Coiling process Note that, the production method described above is carried out using production line equipment. The production line equipment includes, in order from the upstream side toward the downstream side, a heating furnace, a rougher, a finisher, a water cooling apparatus, and a down coiler. A plurality of transfer rolls are arranged between each piece of equipment. Each process is described below.

[0096] [(Process 1) Starting material preparation process] In the starting material preparation process, a starting material having a chemical composition that satisfies feature 1 is prepared. For example, a slab is produced by the following method. Molten steel having a chemical composition that satisfies feature 1 is produced. The molten steel is used to produce a starting material (a slab or an ingot) by a casting process. For example, the molten steel is used to produce a slab by a well-known continuous casting process. Alternatively, the molten steel is used to produce an ingot by a well-known ingot-making process.

[0097] [(Process 2) Rough rolling process] In the rough rolling process, a reverse-type rougher is used to perform hot rolling (rough rolling) on the prepared starting material (slab or ingot) to produce an intermediate steel sheet (sheet bar). The heating temperature of the starting material in the rough rolling process is, for example, 1250 to 1300°C. The in-furnace time of the starting material in the heating furnace is 30 mins or more, and preferably is 60 mins or more. Although not particularly limited, the upper limit of the in-furnace time is, for example, 240 minutes.

[0098] [(Process 3) Finish rolling process] In the finish rolling process, the intermediate steel sheet is subjected to further hot rolling (finish rolling) using a finisher to produce a steel sheet. The finisher is a tandem rolling mill, and includes a plurality of roll stands ST1 to STk (K represents a natural number) arranged in a row. Each roll stand ST includes a pair of work rolls. The surface temperature of the steel sheet on the exit side of a roll stand STl (L represents a natural number equal to or less than K) that, among the plurality of roll stands of the finisher, is the last stand to perform rolling of the intermediate steel sheet is defined as a rolling finishing temperature FT (°C). The roll stand STl may be the roll stand STk that is located furthest downstream in the finisher, or may be a roll stand ST that is located upstream of the roll stand STk.

[0099] In the finish rolling process, the following conditions are to be satisfied. (Condition 1) The rolling finishing temperature FT (°C) is to be 850 to 950°C. (Condition 2) A rolling reduction Rl-1 at a roll stand STl-1 which is upstream of the roll stand STl that performs the last rolling of the intermediate steel sheet in the finisher and which is located next to the roll stand STl is to be 25% or more. (Condition 3) A rolling reduction Rl at the roll stand STl is to be 0.90 times or more the rolling reduction Rl-1. In other words, Rl / Rl-1 is to be 0.90 or more. (Condition 4) Cooling (water cooling) of the steel sheet by a water cooling apparatus located on the exit side of the roll stand STl is to be started within 0.30 seconds after completion of rolling at the roll stand STl. (Condition 5) Cooling (water cooling) of the steel sheet by the water cooling apparatus is to be started within 2.00 seconds after completion of rolling at the roll stand STl-1. (Condition 6) A cooling rate CR1 from the rolling finishing temperature FT to a temperature corresponding to the rolling finishing temperature FT - 40°C is to be within the range of 150°C / sec or more to 250°C / sec or less. (Condition 7) A cooling rate CR2 from the temperature corresponding to the rolling finishing temperature FT - 40°C to a coiling temperature CT is to be within the range of 25°C / sec or more to 75°C / sec or less. Hereinafter, condition 1 to condition 7 are described.

[0100] [(Condition 1) Regarding rolling finishing temperature FT] If the rolling finishing temperature FT is more than 950°C, the grain boundary length GL will be excessively short. Further, the moved distance for diffusion of B within the prior-austenite grains to the prior-austenite grain boundaries will increase, and the degree of B segregation at the prior-austenite grain boundaries will decrease. On the other hand, if the rolling finishing temperature FT is less than 850°C, an excessive load will be applied to the roll stand ST. Therefore, the rolling finishing temperature FT is to be within the range of 850 to 950°C.

[0101] [(Condition 2) Regarding rolling reduction Rl-1] If the rolling reduction Rl-1 at the roll stand STl-1 which is upstream of the roll stand STl that performs the last rolling of the intermediate steel sheet in the finisher and which is located next to the roll stand STl is too low, the grain boundary length GL will be excessively short. Further, the moved distance for diffusion of B within the prior-austenite grains to the prior-austenite grain boundaries will increase, and the degree of B segregation at the prior-austenite grain boundaries will decrease. Therefore, the rolling reduction Rl-1 is to be 25% or more.

[0102] [(Condition 3) Regarding rolling reduction Rl] If the rolling reduction Rl at the roll stand STl is too low, strain imparted to the steel sheet (sheet bar) at the roll stand STl-1 will decrease during rolling at the roll stand STl, and hence the strain accumulated in the steel sheet immediately after completion of finish rolling will be insufficient. In such case, the grain boundary length GL will be excessively short. Further, the moved distance for diffusion of B within the prior-austenite grains to the prior-austenite grain boundaries will increase, and the degree of B segregation at the prior-austenite grain boundaries will decrease. Therefore, the rolling reduction Rl is to be 0.90 times or more the rolling reduction Rl-1.

[0103] [(Condition 4) Regarding cooling start time tl] The time period from when rolling of the steel sheet at the roll stand STl is completed until cooling of the steel sheet by a water cooling apparatus located on the exit side of the roll stand STl is started is defined as a "cooling start time tl". If the cooling start time tl is too long, the grain boundary length GL will be excessively short. Further, the moved distance for diffusion of B within the prior-austenite grains to the prior-austenite grain boundaries will increase, and the degree of B segregation at the prior-austenite grain boundaries will decrease. Therefore, the cooling start time tl is to be 0.30 seconds or less.

[0104] The cooling start time tl can be determined by the following method. The movement speed (m / sec) of the steel sheet at the exit side of the roll stand STl is determined. Here, the phrase "exit side of the roll stand STl" means a point FPl immediately after the steel sheet that is in contact with the pair of rolls of the roll stand STl separates from the work rolls. The cooling start time tl (secs) is calculated based on the distance from the point FPl to a water cooling start position where the water cooling apparatus located on the exit side of the roll stand STl starts water cooling of the steel sheet, and the aforementioned movement speed of the steel sheet. A value to the second decimal place obtained by rounding off to the second decimal place of the calculated numerical value is defined as the cooling start time tl (secs).

[0105] [(Condition 5) Regarding water cooling start time tl-1] The time period from when rolling of the steel sheet at the roll stand STl-1 is completed until cooling of the steel sheet by the water cooling apparatus located on the exit side of the roll stand STl is started is defined as a cooling start time tl-1. If the cooling start time tl-1 is too long, the time period from when rolling at the roll stand STl-1 is completed until rolling is started at the roll stand STl will be excessively long. In such case, strain that was imparted to the steel sheet by the rolling at the roll stand STl-1 will decrease during rolling at the roll stand STl. In such case, the grain boundary length GL will be excessively short. Further, the moved distance for diffusion of B within the prior-austenite grains to the prioraustenite grain boundaries will increase, and the degree of B segregation at the prioraustenite grain boundaries will decrease. Therefore, the cooling start time tl-1 is to be 2.00 seconds or less.

[0106] The cooling start time tl-1 can be determined by the following method. The movement speed (m / sec) of the steel sheet at the exit side of the roll stand STl-1 is determined. Here, the phrase "exit side of the roll stand STl-1" means a point FPl-1 immediately after the steel sheet that is in contact with the pair of rolls of the roll stand STl-1 separates from the work rolls. The conveying time (secs) of the steel sheet from the point FPl-1 to the point FPl on the exit side of the roll stand STl is calculated based on the distance from the point FPl-1 to the point FPl on the exit side of the roll stand STl and the movement speed of the steel sheet. A value to the second decimal place obtained by rounding off to the second decimal place of the calculated numerical value is defined as the cooling start time tl-1 (secs). A time obtained by adding the conveying time and the cooling start time tl is defined as the cooling start time tl-1 (secs).

[0107] [(Condition 6) Regarding cooling rate CR1] An average cooling rate in a temperature range from the rolling finishing temperature FT to the rolling finishing temperature FT - 40°C is defined as a cooling rate CR1 (°C / sec). If the cooling rate CR1 in this temperature range is too slow, the grain boundary length GL will be excessively short. Further, the moved distance for diffusion of B within the prior-austenite grains to the prior-austenite grain boundaries will increase, and the degree of B segregation at the prior-austenite grain boundaries will decrease. On the other hand, if the cooling rate CR1 is too fast, the area fraction of bainitic ferrite in the produced hot-rolled steel sheet will be excessively small. Therefore, the cooling rate CR1 is to be 150°C / sec or more and 250°C / sec or less.

[0108] The cooling rate CR1 can be determined based on the time it takes for the surface temperature of the steel sheet to decrease from the rolling finishing temperature FT to the rolling finishing temperature FT - 40°C. The cooling rate CR1 is to be made an integer obtained by rounding off decimals of the determined numerical value. Note that, in the temperature range from the rolling finishing temperature FT to FT - 40°C, the steel sheet is continuously cooled. That is, the temperature of the steel sheet continuously decreases due to cooling.

[0109] [(Condition 7) Regarding cooling rate CR2] The average cooling rate in a temperature range from the rolling finishing temperature FT - 40°C to the coiling temperature CT is defined as a cooling rate CR2. If the cooling rate CR2 is too slow, segregation of B to prior-austenite grain boundaries will decrease. In addition, polygonal ferrite will excessively form and an area fraction of bainitic ferrite will not be sufficiently obtained. Furthermore, the grain boundary length GL may become excessively short, or the degree of B segregation may become low. On the other hand, if the cooling rate CR2 is too fast, the area fraction of bainitic ferrite in the produced hot-rolled steel sheet will be excessively small. Therefore, the cooling rate CR2 is to be 25°C / sec or more and 75°C / sec or less.

[0110] The cooling rate CR2 can be determined based on the time it takes for the surface temperature of the steel sheet to decrease from the rolling finishing temperature FT - 40°C to the coiling temperature CT (°C). The cooling rate CR2 is to be made an integer obtained by rounding off decimals of the determined numerical value. Note that, in the temperature range from the rolling finishing temperature FT - 40°C to the coiling temperature CT2, the steel sheet is continuously cooled. That is, the temperature of the steel sheet decreases continuously due to cooling.

[0111] After the steel sheet has been cooled at the cooling rate CR1 from the rolling finishing temperature FT to the rolling finishing temperature FT - 40°C, the steel sheet is then immediately cooled at the cooling rate CR2 without any retention time being provided. In other words, the cooling at the cooling rate CR1 and the cooling at the cooling rate CR2 are performed in a continuous manner. If a retention time of 0.2 seconds or more were provided after the cooling at the cooling rate CR1, the B that segregated to the prior-austenite grain boundaries would diffuse, and feature 3 and feature 4 would no longer be satisfied.

[0112] [(Process 4) Coiling process] In the coiling process, the steel sheet that passed through the cooling apparatus is coiled into a coil shape by a down coiler. In the coiling process, Ti carbides form in the steel sheet. Here, the surface temperature of the steel sheet when coiling starts is defined as "coiling temperature CT (°C)". The coiling temperature CT also influences the microstructure of the hot-rolled steel sheet (the proportions of bainitic ferrite, polygonal ferrite, and bainite). Therefore, the coiling temperature CT is to be 500 to 630°C.

[0113] If the coiling temperature CT is higher than 630°C, coiling will be started before transformation from austenite to bainitic ferrite in the microstructure of the steel sheet is completed. Therefore, a part of the austenite will transform to polygonal ferrite. As a result, the area fraction of bainitic ferrite in the hot-rolled steel sheet will be low.

[0114] On the other hand, if the coiling temperature CT is less than 500°C, bainite will form in the hot-rolled steel sheet. Consequently, the area fraction of bainitic ferrite in the hot-rolled steel sheet will be low.

[0115] The hot-rolled steel sheet according to the present embodiment is produced by the production processes described above. As mentioned above, the hot-rolled steel sheet of the present embodiment may also be produced by a production method other than the production method described above.

[0116] [Other processes in method for producing hot-rolled steel sheet] The method for producing the hot-rolled steel sheet of the present embodiment may also include other processes apart from the processes described above. For example, a temper rolling process may be performed at a stage that is after the finish rolling process and is before the coiling process, or may be performed after the coiling process. In the temper rolling process, the hot-rolled steel sheet is subjected to temper rolling. The temper rolling process adjusts the shape of the hot-rolled steel sheet, adjusts the surface roughness, and adjusts the yield strength. A sheet thickness reduction ratio in the temper rolling process for effectively obtaining the aforementioned advantageous effects is, for example, 0.1% or more. A preferable upper limit of the sheet thickness reduction ratio in the temper rolling process is 3.0%. In this case, introduction of excessive strain into the hot-rolled steel sheet is suppressed, and good ductility, bendability, and flangeability can be maintained.

[0117] [Method for producing hot-dip galvanized steel sheet including hot-rolled steel sheet of present embodiment] A hot-dip galvanized steel sheet that includes the hot-rolled steel sheet of the present embodiment can be produced by performing the following well-known hot-dip galvanizing process.

[0118] [Hot-dip galvanizing process] In the hot-dip galvanizing process, a hot dip galvanized layer having the chemical composition described above is formed on the surface of the hot-rolled steel sheet. Specifically, a plating bath is prepared. The composition of the plating bath is adjusted according to the composition of the hot dip galvanized layer to be formed. After the hot-rolled steel sheet has been dipped in the plating bath for a certain time period, the hot-rolled steel sheet is lifted up from the plating bath by a well-known method. For example, a sink roll is arranged in the plating bath. The travelling direction of the hot-rolled steel sheet that is dipped in the plating bath is changed to the upward direction by the sink roll.

[0119] A hot-dip Zn-based coating is adhered to the surface of the hot-rolled steel sheet that is lifted up from the plating bath. The amount of the hot-dip Zn-based coating adhering to the hot-rolled steel sheet is adjusted using a well-known gas wiping apparatus. The hot-dip Zn-based coating adhering to the hot-rolled steel sheet that was lifted up from the plating bath solidifies to form a hot dip galvanized layer. The hot-dip galvanized steel sheet is produced by the above process.

[0120] [Other processes in method for producing hot-dip galvanized steel sheet] The method for producing the hot-dip galvanized steel sheet of the present embodiment may also include other production processes apart from the hot-dip galvanizing process. For example, the method for producing the hot-dip galvanized steel sheet of the present embodiment may include an Ni pre-plating process before the hot-dip galvanizing process. In the Ni pre-plating process, the hot-rolled steel sheet described above is subjected to Ni plating to form an Ni plating layer on the surface of the hot-rolled steel sheet. The hot-rolled steel sheet on which the Ni plating layer has been formed is subjected to the hot-dip galvanizing process. In this case, the adhesion of the hot dip galvanized layer to the hot-rolled steel sheet increases.

[0121] The method for producing the hot-dip galvanized steel sheet of the present embodiment may also include a chemical treatment process after the hot-dip galvanizing process. In the chemical treatment process, the produced hot-dip galvanized steel sheet is subjected to a chemical treatment to form a chemical conversion coating on the hot dip galvanized layer. When performing a chemical treatment process, the chemical treatment method is not particularly limited, and a well-known method can be used. For example, a well-known chromium conversion coating may be formed as a chemical conversion coating.

[0122] In the method for producing the hot-dip galvanized steel sheet of the present embodiment, in addition, a temper rolling process may be performed after the hot-dip galvanizing process. In the temper rolling process, the produced hot-dip galvanized steel sheet is subjected to temper rolling. In a case where the aforementioned chemical treatment process is performed, the adhesion of the chemical conversion coating can be increased by performing a temper rolling process before the chemical treatment process. The sheet thickness reduction ratio in the temper rolling process is not particularly limited. A preferable sheet thickness reduction ratio in the temper rolling process is, for example, 0.1 to 3.0%.

[0123] The method for producing the hot-dip galvanized steel sheet may also include other production processes. The production method described above is one example of a production method for obtaining the hot-dip galvanized steel sheet of the present embodiment. Therefore, a method for producing the hot-dip galvanized steel sheet of the present embodiment is not limited to the production method described above. EXAMPLES

[0124] Hereinafter, advantageous effects of one aspect of the hot-rolled steel sheet and the hot-dip galvanized steel sheet of the present embodiment are described more specifically by way of examples. The conditions adopted in the following examples are one example of conditions employed for confirming the feasibility and advantageous effects of the hot-rolled steel sheet and the hot-dip galvanized steel sheet of the present embodiment. Accordingly, the hot-rolled steel sheet and the hot-dip galvanized steel sheet of the present embodiment are not limited to this one example of the conditions.

[0125] Hot-rolled steel sheets having the chemical compositions shown in Table 1 were produced.

[0126] [Table 1] TABLE Test Number Chemical Composition (in mass%; balance is Fe and impurities) C Si Mn P S Al N Ti W B Nb V Cr 1 0.068 0.15 0.95 0.022 0.005 0.030 0.0035 0.060 0.045 0.0041 - - - 2 0.039 0.04 0.81 0.020 0.003 0.030 0.0029 0.072 0.055 0.0032 - - - 3 0.082 0.05 1.11 0.018 0.004 0.035 0.0035 0.119 0.073 0.0025 - - - 4 0.080 0.02 1.35 0.022 0.005 0.035 0.0028 0.091 0.065 0.0021 0.040 - - 5 0.050 0.10 1.22 0.019 0.003 0.035 0.0028 0.090 0.055 0.0017 - 0.11 - 6 0.040 0.09 0.85 0.019 0.004 0.035 0.0031 0.060 0.089 0.0033 - - 0.40 7 0.098 0.30 0.95 0.022 0.003 0.030 0.0033 0.080 0.060 0.0029 0.053 0.05 8 0.035 0.01 0.90 0.022 0.003 0.030 0.0033 0.105 0.060 0.0029 - 0.08 0.35 9 0.080 0.33 0.95 0.020 0.003 0.055 0.0026 0.080 0.055 0.0031 0.120 - - 10 0.039 0.04 0.81 0.020 0.003 0.030 0.0029 0.072 0.055 0.0032 - - - 11 0.039 0.04 0.81 0.020 0.003 0.030 0.0029 0.072 0.055 0.0032 - - - 12 0.039 0.04 0.81 0.020 0.003 0.030 0.0029 0.072 0.055 0.0032 - - - 13 0.082 0.05 1.11 0.018 0.004 0.035 0.0035 0.119 0.073 0.0025 - - - 14 0.082 0.05 1.11 0.018 0.004 0.035 0.0035 0.119 0.073 0.0025 - - - 15 0.082 0.05 1.11 0.018 0.004 0.035 0.0035 0.119 0.073 0.0025 - - - 16 0.082 0.05 1.11 0.018 0.004 0.035 0.0035 0.119 0.073 0.0025 - - - 17 0.104 0.45 1.30 0.023 0.003 0.031 0.0040 0.088 0.066 0.0019 - - - 18 0.080 0.03 1.31 0.007 0.009 0.065 0.0022 0.065 0.042 0.0029 - - - 19 0.099 0.21 0.98 0.009 0.003 0.016 0.0063 0.193 0.060 0.0042 - - - 20 0.082 0.05 1.11 0.018 0.004 0.035 0.0035 0.119 0.014 0.0015 - - - 21 0.072 0.05 0.99 0.018 0.004 0.033 0.0035 0.111 0.066 0.0005 - - - 22 0.082 0.05 1.11 0.018 0.004 0.035 0.0035 0.119 0.073 0.0025 - - - 23 0.080 0.02 1.35 0.022 0.005 0.035 0.0028 0.091 0.065 0.0021 0.040 - - 24 0.082 0.05 1.11 0.018 0.004 0.035 0.0035 0.119 0.073 0.0025 - - - 25 0.080 0.02 1.35 0.022 0.005 0.035 0.0028 0.091 0.065 0.0021 0.040 - - 26 0.082 0.05 1.11 0.018 0.004 0.035 0.0035 0.119 0.073 0.0025 - - - 27 0.080 0.02 1.35 0.022 0.005 0.035 0.0028 0.091 0.065 0.0021 0.040 - - 28 0.082 0.05 1.11 0.018 0.004 0.035 0.0035 0.119 0.073 0.0025 - - - 29 0.080 0.02 1.35 0.022 0.005 0.035 0.0028 0.091 0.065 0.0021 0.040 - - 30 0.082 0.05 1.11 0.018 0.004 0.035 0.0035 0.119 0.073 0.0025 - - - 31 0.080 0.02 1.35 0.022 0.005 0.035 0.0028 0.091 0.065 0.0021 0.040 - - 32 0.082 0.05 1.11 0.018 0.004 0.035 0.0035 0.119 0.073 0.0025 - - - 33 0.080 0.02 1.35 0.022 0.005 0.035 0.0028 0.091 0.065 0.0021 0.040 - - 34 0.082 0.05 1.11 0.018 0.004 0.035 0.0035 0.119 0.073 0.0025 - - - 35 0.082 0.05 1.11 0.018 0.004 0.035 0.0035 0.119 0.073 0.0025 - - - 36 0.080 0.02 1.35 0.022 0.005 0.035 0.0028 0.091 0.065 0.0021 0.040 - - 37 0.080 0.02 1.35 0.022 0.005 0.035 0.0028 0.091 0.065 0.0021 0.040 - - 38 0.082 0.05 1.11 0.018 0.004 0.035 0.0035 0.119 0.073 0.0025 - - -

[0127] The symbol "-" in Table 1 means that the chemical compositions did not contain the corresponding element. Note that, the chemical composition of the steel sheet of each test number was determined based on the method described above in the section [Method for measuring chemical composition of hot-rolled steel sheet].

[0128] Molten steel was subjected to continuous casting to produce a slab. The slab was subjected to a hot rolling process (a rough rolling process and a finish rolling process). The slab was heated for 60 mins at a temperature of 1250 to 1300°C. After heating, the slab was subjected to rolling with a rougher to produce an intermediate steel sheet. Further, the intermediate steel sheet was subjected to rolling using a finisher to produce a steel sheet. In the finish rolling process, (Condition 1) the rolling finishing temperature FT (°C), (Condition 2) the rolling reduction Rl-1 (%), (Condition 3) the rolling reduction Rl / rolling reduction Rl-1, (Condition 4) the cooling start time tl (sec), (Condition 5) the water cooling start time tl-1 (sec), (Condition 6) the cooling rate CR1 (°C / sec), and (Condition 7) the cooling rate CR2 (°C / sec) were as shown in Table 2. Note that, after the steel sheet had been cooled at the cooling rate CR1 from the rolling finishing temperature FT to the rolling finishing temperature FT - 40°C, the steel sheet was immediately cooled at the cooling rate CR2 without any retention time being provided. In other words, the cooling at the cooling rate CR1 and the cooling at the cooling rate CR2 were performed in a continuous manner. Further, in the temperature range from the rolling finishing temperature FT to FT - 40°C, the temperature of the steel sheet was reduced continuously by cooling. Similarly, in the temperature range from the rolling finishing temperature FT - 40°C to the coiling temperature CT2, the temperature of the steel sheet was reduced continuously by cooling.

[0129] [Table 2] TABLE 2 Test Number Condition 1 Condition 2 Condition 3 Condition 4 Condition 5 Condition 6 Condition 7 FT (°C) Rl-i(%) Rl / Rl-i tL (secs) tL-1 (secs) CR1 (°C / secs) CR2 (°C / secs) 1 910 28 0.95 0.22 1.85 180 35 2 895 29 0.99 0.25 1.66 187 40 3 903 27 1.01 0.19 1.55 193 35 4 870 31 0.95 0.23 1.35 201 28 5 932 26 0.98 0.23 1.76 166 32 6 888 32 0.97 0.28 1.88 189 38 7 894 28 0.97 0.15 1.65 160 28 8 870 32 1.03 0.25 1.52 179 32 9 888 29 0.97 0.26 1.33 205 42 10 865 28 0.95 0.23 1.51 180 29 11 935 27 1.01 0.25 1.48 190 27 12 910 28 0.97 0.23 1.59 193 32 13 880 29 0.99 0.25 1.56 181 32 14 870 28 0.98 0.26 1.51 170 29 15 895 27 0.99 0.27 1.48 182 29 16 888 27 0.99 0.25 1.59 185 29 17 880 28 0.95 0.10 1.02 244 70 18 902 28 0.99 0.25 1.31 205 43 19 878 31 0.97 0.23 1.55 190 40 20 865 29 0.98 0.21 1.56 166 27 21 910 29 0.98 0.25 1.62 171 29 22 970 28 0.97 0.18 1.65 170 30 23 965 28 0.97 0.22 1.51 175 35 24 888 23 0.97 0.26 1.48 170 29 25 895 22 0.99 0.25 1.59 176 29 26 900 29 0.85 0.25 1.56 165 29 27 875 29 0.83 0.25 1.48 179 27 28 905 31 0.95 0.36 1.92 170 32 29 888 26 0.95 0.35 1.89 180 32 30 876 32 0.97 0.29 2.08 175 32 31 860 29 0.97 0.29 2.10 180 27 32 911 28 0.98 0.22 1.51 142 29 33 899 29 0.99 0.21 1.48 139 29 34 901 28 0.93 0.25 1.78 259 40 35 885 29 0.99 0.25 1.59 179 22 36 890 28 0.97 0.18 1.51 170 21 37 865 29 0.98 0.21 1.53 181 24 38 888 30 1.01 0.27 1.88 199 80

[0130] The steel sheet after the finish rolling was coiled into a coil shape at the coiling temperature CT by a down coiler. The coiling temperature CT of each test number was in the range of 500 to 630°C. The coil-shaped steel sheet after coiling was allowed to cool to normal temperature, to thereby produce the hot-rolled steel sheet of each test number shown in Table 1. The sheet thickness of each hot-rolled steel sheet was 2.3 mm.

[0131] [Evaluation tests] The hot-rolled steel sheet of each test number was subjected to the following evaluation tests. (Test 1) Test to measure area fraction of bainitic ferrite (Test 2) Test to measure grain boundary length GL of prior-austenite grains in outer layer (Test 3) Test to measure degree of B segregation at prior-austenite grain boundaries (Test 4) Test to measure tensile strength (Test 5) Test to measure balance between strength and burring properties (Test 6) Test to evaluate LME resistance of hot-dip galvanized steel sheet Test 1 to test 6 are described below.

[0132] [(Test 1) Test to measure area fraction of bainitic ferrite] The area fraction (%) of bainitic ferrite in the hot-rolled steel sheet of each test number was determined based on the method described above in the section [Method for measuring area fraction of bainitic ferrite]. The determined area fraction of bainitic ferrite is shown in the column "BF Area Fraction (%)" in Table 3.

[0133] [Table 3] TABLE 3 Test Number BF Area Fraction (%) Grain Boundary Length GL (mm) Degree of B Segregation TS (MPa) X (%) SB (MPa%) Coated Steel Sheet Coating Number LME Resistance Evaluation Remarks 1 92 1.05 98 750 55 41250 P1 E Inventive Example 2 87 1.11 103 630 75 47250 P2 E Inventive Example 3 89 1.01 95 832 60 49920 P3 E Inventive Example 4 91 1.85 90 860 65 55900 P4 E Inventive Example 5 87 1.15 85 695 70 48650 P5 E Inventive Example 6 87 1.00 120 665 65 43225 P6 E Inventive Example 7 91 2.55 115 888 47 41736 P7 E Inventive Example 8 86 1.75 113 722 88 63536 P8 E Inventive Example 9 90 4.89 100 875 47 41125 P8 E Inventive Example 10 88 1.11 105 640 68 43520 P9 E Inventive Example 11 88 1.15 110 599 70 41930 P10 E Inventive Example 12 88 1.09 108 605 72 43560 P11 E Inventive Example 13 89 1.08 103 833 51 42483 P12 E Inventive Example 14 90 1.11 105 827 52 43004 P13 E Inventive Example 15 89 1.12 102 830 51 42330 P14 E Inventive Example 16 90 1.10 110 820 50 41000 P15 E Inventive Example 17 99 4.77 108 855 48 41040 P1 E Inventive Example 18 91 2.01 104 801 53 42453 P1 E Inventive Example 19 87 1.95 110 888 47 41736 P1 E Inventive Example 20 92 1.11 68 781 53 41393 P8 B Comparative Example 21 93 0.95 20 765 60 45900 P9 B Comparative Example 22 89 0.71 70 811 55 44605 P10 B Comparative Example 23 92 0.74 75 840 63 52920 P11 B Comparative Example 24 90 0.71 71 808 55 44440 P12 B Comparative Example 25 88 0.75 75 831 60 49860 P13 B Comparative Example 26 95 0.72 72 815 55 44825 P14 B Comparative Example 27 93 0.74 75 832 66 54912 P15 B Comparative Example 28 87 0.73 71 822 54 44388 P1 B Comparative Example 29 88 0.75 69 833 66 54978 P1 B Comparative Example 30 88 0.73 75 799 55 43945 P1 B Comparative Example 31 89 0.75 78 803 68 54604 P1 B Comparative Example 32 86 0.69 73 788 54 42552 P1 B Comparative Example 33 86 0.75 75 811 69 55959 P1 B Comparative Example 34 79 1.29 88 844 44 37136 P1 E Comparative Example 35 81 0.73 75 571 68 38828 P1 B Comparative Example 36 80 0.75 73 581 67 38927 P1 B Comparative Example 37 86 0.85 75 865 62 53630 P1 B Comparative Example 38 75 1.91 95 865 43 37195 P1 E Comparative Example

[0134] [(Test 2) Test to measure grain boundary length GL of prior-austenite grains in outer layer] The grain boundary length GL (mm) in the outer layer region of the hot-rolled steel sheet of each test number was determined based on the method described above in the section [Regarding method for measuring grain boundary length GL]. The determined grain boundary length GL is shown in the column "Grain Boundary Length GL (mm)" in Table 3.

[0135] [(Test 3) Test to measure degree of B segregation at prior-austenite grain boundaries] The degree of B segregation in the outer layer region of the hot-rolled steel sheet of each test number was determined based on the method described above in the section [Method for measuring degree of B segregation at prior-austenite grain boundaries]. The determined degree of B segregation is shown in the column "Degree of B Segregation" in Table 3.

[0136] [(Test 4) Test to measure tensile strength] The tensile strength TS (MPa) of the hot-rolled steel sheet of each test number was determined based on the method described above in the section [Method for measuring tensile strength]. The determined tensile strength TS is shown in the column "TS (MPa)" in Table 3.

[0137] [(Test 5) Test to measure balance between strength and burring properties] The hole expandability k (%) of the hot-rolled steel sheet of each test number was determined based on the method described above in the section [Method for measuring balance between strength and burring properties SB]. In addition, the balance between strength and burring properties SB was determined using the tensile strength TS (MPa) determined in test 4, and the hole expandability (%). The determined hole expansion ratio X (%) is shown in the column "X (%)" in Table 3. The determined balance between strength and burring properties SB is shown in the column "SB (MPa-%)" in Table 3.

[0138] [(Test 6) Test to evaluate LME resistance of hot-dip galvanized steel sheet] [Production of hot-dip galvanized steel sheet] In order to evaluate the LME resistance of the hot-dip galvanized steel sheets, first, hot-dip galvanized steel sheets were produced using the hot-rolled steel sheet of each test number. Specifically, the hot-rolled steel sheet of each test number was subjected to a well-known hot-dip galvanizing process to form a hot dip galvanized layer having a chemical composition shown in Table 4 on the surface of the hot-rolled steel sheet. The coating number of the hot dip galvanized layer formed on the hot-rolled steel sheet of each test number is shown in the column "Coating Number " of the column "Coated Steel Sheet" in Table 3. The coating numbers shown in the column "Coating Number " of the column "Coated Steel Sheet" in Table 3 correspond to the coating numbers in Table 4. Hot-dip galvanized steel sheets were produced by the above production process.

[0139] [Table 4] TABLE 4 Coating Number Coating Composition (in mass%; balance is Fe and impurities) Zn Al Mg Sn Group (Sn, Bi, In) Ca Group (Ca, Y, La, Ce) Si Cr Group (Cr, Ti, Ni, Co, V, Nb, Cu, Mn) Fe Sr Group (Sr, Sb, Pb, B) P1 73.70 19.50 6.8 - - - - - - P2 74.39 19.50 6.0 Sn:0.11 - - - - - P3 74.91 19.20 5.8 Bi:0.09 - - - - - P4 78.92 16.70 4.3 In:0.08 - - - - - P5 74.58 18.90 6.4 - Ca:0.12 - - - - P6 76.78 17.70 5.4 - Y:0.12 - - - - P7 74.95 20.50 4.5 - La:0.05 - - - - P8 77.93 17.20 4.8 - Ce:0.07 - - - - P9 75.55 18.50 5.7 - - 0.25 - - - P10 73.70 19.80 6.4 - - - Ni:0.1 - - P11 76.20 18.10 5.2 - - - - 0.5 - P12 75.00 18.80 6.1 - - - - - Sr:0.1 P13 84.27 12.00 3.5 - - 0.23 - - - P14 90.75 6.50 2.7 - - 0.05 - - - P15 99.89 0.11 - - - - - - -

[0140] Note that, the symbol of an element written on the left side of a numerical value in Table 4 means the contained element. For example, for coating number P2, it means that, as an element of the Sn group, Sn is contained in an amount of 0.11% by mass. Note that, the chemical composition of the coating layer of each coating number was determined by the method described above in the section [Method for measuring chemical composition of hot dip galvanized layer].

[0141] The LME resistance of the produced hot-dip galvanized steel sheet of each test number was evaluated by the following method. A sample steel sheet having dimensions of 100 mm x 75 mm x the sheet thickness was taken from the hot-dip galvanized steel sheet of each test number. Arc welding illustrated in FIG. 3 was performed using the sample steel sheet. Specifically, a cylindrical boss member 1 having 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 defined in JIS G3101: 2015.

[0142] As illustrated in FIG. 3, the boss member 1 was arranged at the center position of the sample steel sheet 2 in a manner so that the axial direction of the boss member 1 was the normal direction of the surface of the sample steel sheet 2. The arranged boss member 1 was welded to the sample steel sheet 2 by arc welding. The welding wire used was YM-70CS welding wire defined in JIS Z 3312: 2009 G69A2UCN1M2T. In the arc welding, a weld bead 3 was run around the circumference of the boss member 1 one time in the clockwise direction from the welding starting point in plan view, and after passing the welding starting point, the arc welding was further continued so as to continue the welding until an overlapping region 4 of the weld bead was formed. The width of the overlapping region 4 was approximately 15 mm.

[0143] The current value during the arc welding was set to 190 A, and the voltage value was set to 23 V. The welding speed was set to 0.3 m / min. A gaseous mixture composed of 20% by volume of CO2 gas and 80% by volume of argon gas was used as a shielding gas during the arc welding. The shielding gas flow rate during the arc welding was set to 20 L / min.

[0144] Before performing the arc welding illustrated in FIG. 3, the sample steel sheet 2 was joined in advance to a restraint plate 5 as illustrated in FIG. 4. A steel sheet equivalent to SS400 defined in JIS G3101: 2015 having dimensions of 120 mm x 95 mm x 4 mm in thickness was used as the restraint plate 5. The sample steel sheet 2 was placed on the surface of the restraint plate 5. The entire circumference of the placed sample steel sheet 2 was welded to the restraint plate 5. The welding wire and welding conditions were the same as the welding wire and welding conditions used when welding the boss member 1 to the sample steel sheet 2.

[0145] Before performing the welding of the boss member illustrated in FIG. 3, as illustrated in FIG. 4, the sample steel sheet 2 which was peripherally welded to the restraint plate 5 was placed on a base 6, and the sample steel sheet 2 and the restraint plate 5 were fixed to the base 6 by an unshown clamp. After the sample steel sheet 2 had been fixed to the base 6 by the clamp, the boss member 1 was welded to the sample steel sheet 2 by the arc welding as illustrated in FIG. 3.

[0146] After the boss member 1 was arc-welded to the sample steel sheet 2, as illustrated in FIG. 3, the boss member 1, the sample steel sheet 2, and the restraint plate 5 were cut along a cutting plane 7 that passed through the central axis of the boss member 1 and passed through the overlapping region 4 of the weld bead 3. The resulting cross section at the cutting plane 7 was then observed at a magnification of 100x using an optical microscope. In the observation, whether or not cracks (liquid metal embrittlement) were present in the sample steel sheet was confirmed by visual observation. If a crack was observed, the crack length was measured. Specifically, the length (mm) of each crack in the depth direction (sheet thickness direction) from the surface of the hot-rolled steel sheet (interface between the coating layer and the hot-rolled steel sheet) was determined. The largest crack length among the measured crack lengths was identified. If the largest crack length was 1.0 mm or less, it was determined that the sample steel sheet was excellent in LME resistance (indicated by "E" (Excellent) in the column "LME Resistance Evaluation" in Table 3). On the other hand, if the largest crack length was more than 1.0 mm, it was determined that the LME resistance was low (indicated by "B" (Bad) in the column "LME Resistance Evaluation" in Table 3).

[0147] [Evaluation results] Referring to Table 1 to Table 4, the hot-rolled steel sheets of Test Nos. 1 to 19 satisfied feature 1 to feature 4. Therefore, excellent LME resistance was obtained. Note that, in the hot-rolled steel sheets of theses test numbers, the tensile strength TS was 590 MPa or more and thus high strength was obtained.

[0148] On the other hand, in Test No. 20, the content of W was too low. Consequently, the degree of B segregation was low. As a result, sufficient LME resistance was not obtained.

[0149] In Test No. 21, the content of B was too low. Consequently, the degree of B segregation was low. As a result, sufficient LME resistance was not obtained.

[0150] In Test Nos. 22 and 23, the rolling finishing temperature FT was too high. Consequently, 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 Nos. 24 and 25, the rolling reduction Rl-1 was too low. Consequently, the grain boundary length GL was short and the degree of B segregation was low. As a result, sufficient LME resistance was not obtained.

[0152] In Test Nos. 26 and 27, the ratio (= Rl / Rl-1) of the rolling reduction Rl to the rolling reduction Rl-1 was too low. Consequently, the grain boundary length GL was short and the degree of B segregation was low. As a result, sufficient LME resistance was not obtained.

[0153] In Test Nos. 28 and 29, the cooling start time tl was too long. Consequently, the grain boundary length GL was short and the degree of B segregation was low. As a result, sufficient LME resistance was not obtained.

[0154] In Test Nos. 30 and 31, the water cooling start time tl-1 was too long. Consequently, the grain boundary length GL was short and the degree of B segregation was low. As a result, sufficient LME resistance was not obtained.

[0155] In Test Nos. 32 and 33, the cooling rate CR1 was too slow. Consequently, the grain boundary length GL was short and the degree of B segregation was low. As a result, sufficient LME resistance was not obtained.

[0156] In Test No. 34, the cooling rate CR1 was too fast. Consequently, the area fraction of bainitic ferrite was low. As a result, the balance between strength and burring properties SB was low.

[0157] In Test Nos. 35 to 37, the cooling rate CR2 was too slow. Consequently, in Test Nos. 35 and 36, the area fraction of bainitic ferrite was low, the grain boundary length GL was short, and the degree of B segregation was low. Further, in Test No. 37, the degree of B segregation was low. As a result, in Test Nos. 35 and 36, sufficient LME resistance was not obtained, and the tensile strength was less than 590 MPa. In addition, the balance between strength and burring properties SB was low. Furthermore, in Test No. 37, sufficient LME resistance was not obtained.

[0158] In Test No. 38, the cooling rate CR2 was too fast. Consequently, the area fraction of bainitic ferrite was low. As a result, the balance between strength and burring properties SB was low.

[0159] An embodiment of the present disclosure has been described above. However, the embodiment described above is merely an example for carrying out the present disclosure. Therefore, the present disclosure is not limited to the abovedescribed embodiment, and can be implemented by appropriately modifying the above embodiment within a range that does not depart from the gist of the present disclosure.

Claims

1. A hot-rolled steel sheet comprising, 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%,W: 0.020 to 0.100%,B: 0.0010 to 0.0050%,Nb: 0 to 0.150%,V: 0 to 0.20%, andCr: 0 to 1.00%,with the balance being Fe and impurities,wherein:in the microstructure, an area fraction of bainitic ferrite is 85% or more,in an outer layer region from a surface of the hot-rolled steel sheet to a position at a depth of 0.2 mm in a sheet thickness direction, a total length of prioraustenite grain boundaries where BO2- is detected by time-of-flight secondary ion mass spectrometry is 0.80 to 5.00 mm per 100 pm x 100 pm, anda degree of B segregation at the prior-austenite grain boundaries is 80 or more.

2. The hot-rolled steel sheet according to claim 1, containing one or more types of element selected from a group consisting of, in mass%,Nb: 0.001 to 0.150%,V: 0.01 to 0.20%, andCr: 0.01 to 1.00%.

3. A hot-dip galvanized steel sheet, comprising:the hot-rolled steel sheet according to claim 1 or claim 2, anda hot dip galvanized layer which is formed on the surface of the hot-rolled steel sheet and which contains Zn in an amount of 65.00% by mass or more.

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