WO2023084926A1 - 熱延鋼板、溶融めっき鋼板、及び、熱延鋼板の製造方法 - Google Patents
熱延鋼板、溶融めっき鋼板、及び、熱延鋼板の製造方法 Download PDFInfo
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Images
Classifications
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
Definitions
- the present disclosure relates to a hot-rolled steel sheet, a hot-dipped steel sheet having a hot-dip galvanized coating layer formed on the surface of the hot-rolled steel sheet, and a method for manufacturing the hot-rolled steel sheet.
- Hot-rolled steel sheets are widely used in automobiles, electrical machinery, building materials, and construction machinery. Hot-rolled steel sheets used for these applications are required to have high strength. On the other hand, hot-rolled steel sheets are processed into various shapes for use in the above applications. Therefore, hot-rolled steel sheets are required to have not only high strength but also excellent workability.
- Patent Document 1 JP-A-2018-003062
- Patent Document 2 JP-A-2017-179539
- the hot-rolled steel sheet disclosed in Patent Document 1 contains, in mass%, C: 0.04 to 0.18%, Si: 0.2 to 2.0%, Mn: 1.0 to 3.0%, P : 0.03% or less, S: 0.005% or less, Al: 0.01 to 0.100%, N: 0.010% or less, Ti: 0.03 to 0.15%, Cr: 0.10 ⁇ 0.50%, B: 0.0005-0.0050%, and the balance being Fe and unavoidable impurities.
- the area ratio of bainite phase is 85% or more
- the area ratio of austenite phase is 1 to 8%
- the area ratio of martensite phase is 3% or less.
- crystal grains with a diameter of 0.8 ⁇ m or less account for 70% or more of the entire austenite phase.
- Patent Document 1 the microstructure of the hot-rolled steel sheet is mainly composed of the bainite phase, and the fine austenite phase is dispersed in the bainite phase. Patent Literature 1 describes that this provides high strength and excellent workability.
- the hot-rolled steel sheet disclosed in Patent Document 2 contains, by mass%, C: 0.03 to 0.08%, Si: 0.01 to 1.50%, Mn: 0.1 to 1.5%, Ti : 0.05-0.15%, B: 0.0002-0.0030%, P: 0.1% or less, S: 0.005% or less, Al: 0.5% or less, N: 0.009 % or less, the total of Nb, Mo and V: 0 to 0.02%, and the total of Ca and REM: 0 to 0.01%, with the balance being Fe and impurities. Furthermore, the mass ratio of Ti content to C content (Ti/C) in the chemical composition is 0.625 to 3.000.
- This hot-rolled steel sheet further has a dislocation density of 1 ⁇ 10 14 to 1 ⁇ 10 16 m ⁇ 2 .
- the average diameter of TiC precipitates in the crystal grains is 2.0 nm or less, and the average number density of the TiC precipitates in the crystal grains is 1 ⁇ 10 17 to 5 ⁇ 10 18 pieces/cm 3 .
- the content of Ti existing as TiC precipitates in the matrix phase, not on dislocations, in crystal grains is 30% by mass or more of the total Ti content of the steel sheet.
- Patent Document 2 a high tensile strength of 780 MPa or more is obtained by increasing the dislocation density and forming TiC precipitates in the matrix phase not on the dislocations. Furthermore, Patent Document 2 describes that the workability of the hot-rolled steel sheet can be improved by suppressing the content of alloying elements.
- a hot-dip galvanized layer may be formed on the surface of the hot-rolled steel sheets in order to further improve corrosion resistance.
- a hot-rolled steel sheet on which a hot-dip galvanized layer is formed is also referred to as a hot-dip plated steel sheet.
- a hot-rolled steel sheet (hot-dip galvanized steel sheet) with a hot-dip galvanized layer may be welded to other steel members. During welding, part of the hot-dip galvanized layer melts. Then, the molten metal (zinc) may enter the grain boundaries of the hot-rolled steel sheet and cause cracks. Such cracks are called liquid metal embrittlement cracks (LME: Liquid Metal Embrittlement).
- Hot-rolled steel sheets not only have high strength and excellent workability, but also have the property of suppressing the occurrence of LME when a hot-dip galvanized layer is formed on the surface of the hot-rolled steel sheet (hereinafter, this property is referred to as LME resistance). ) is also required.
- Patent Document 3 proposes a hot-dip Zn-Al-Mg plated steel sheet that has high strength and excellent workability, as well as excellent LME resistance.
- the hot-dip Zn-Al-Mg-based plated steel sheet of Patent Document 3 includes a material steel plate and a hot-dip Zn-Al-Mg-based alloy plating layer.
- the material steel sheet in terms of % by mass, contains C: 0.01 to 0.08%, Si: 0.8% or less, Mn: 0.5 to 1.8%, P: 0.05% or less, S: 0.05%. 005% or less, N: 0.001 to 0.005%, Ti: 0.02 to 0.2%, B: 0.0005 to 0.010%, Al: 0.005 to 0.1% , with the balance being Fe and unavoidable impurities.
- the material steel sheet has a dislocation density of 1.8 ⁇ 10 14 /m 2 to 5.7 ⁇ 10 14 /m 2 .
- the main phase is either a single bainitic ferrite phase or a ferrite phase, or a phase containing a bainitic ferrite phase and a ferrite phase, and the area ratio of the hard second phase and cementite is 3% or less.
- carbides containing Ti having an average particle size of 20 nm or less are also dispersed and precipitated.
- the hot-dip Zn-Al-Mg-coated steel sheet of Patent Document 3 has the above-described chemical composition and microstructure, thereby obtaining high strength, excellent workability, and excellent LME resistance.
- hot-rolled steel sheets are sometimes required not only to have high strength, excellent workability, and excellent LME resistance when a hot-dip galvanized layer is formed, but also to have high rigidity.
- Patent Documents 1 to 3 high strength, excellent workability, and excellent LME resistance when a hot-dip zinc-based plating layer is formed are studied. A technique for obtaining high rigidity has not been studied.
- An object of the present disclosure is to provide a hot-rolled steel sheet, a hot-dip coated steel sheet, and a method for producing a hot-rolled steel sheet having high strength, excellent workability, and excellent LME resistance, as well as excellent stiffness. .
- the hot-rolled steel sheet, hot-dip plated steel sheet, and hot-rolled steel sheet manufacturing method according to the present disclosure have the following configurations.
- the hot-rolled steel sheet according to the present disclosure is in % by mass, C: 0.040 to 0.120%, Si: 0.01 to 0.60%, Mn: 0.50-1.50%, 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%, and B: 0.0010 to 0.0050%, the balance consists of Fe and impurities,
- the area ratio of bainitic ferrite is 85% or more
- dislocation density is 8.0 ⁇ 10 13 to 100.0 ⁇ 10 13 /m 2
- the average circle equivalent diameter of Ti carbides in the hot-rolled steel sheet is 10 nm or less
- the average circle equivalent diameter of the crystal grains of the bainitic ferrite is 15 ⁇ m or less.
- the hot-rolled steel sheet according to the present disclosure is in % by mass, C: 0.040 to 0.120%, Si: 0.01 to 0.60%, Mn: 0.50-1.50%, 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%, and B: 0.0010 to 0.0050%, Furthermore, it contains one or more selected from the group consisting of the first group and the second group, and the balance consists of Fe and impurities, In the microstructure, the area ratio of bainitic ferrite is 85% or more, dislocation density is 8.0 ⁇ 10 13 to 100.0 ⁇ 10 13 /m 2 , The average circle equivalent diameter of Ti carbides in the hot-rolled steel sheet is 10 nm or less, The average circle equivalent diameter of the crystal grains of the bainitic ferrite is 15 ⁇ m or less. [First group] Nb: 0.20% or less, and V: 0.20% or less, one or more selected
- the hot-dip plated steel sheet according to the present disclosure is The hot-rolled steel sheet described above, A hot-dip zinc-based coating layer formed on the surface of the hot-rolled steel sheet and containing 65.00% by mass or more of Zn.
- a method for manufacturing a hot-rolled steel sheet according to the present disclosure includes: A rough rolling step of rough rolling the raw material using a rough rolling mill to produce a rough bar; A finish rolling step of finish rolling the rough bar using a finish rolling mill to manufacture a steel plate and setting the finish rolling temperature FT to 850 to 950 ° C.; A cooling step of cooling the steel plate after completion of finish rolling; A winding step of winding the steel plate after the cooling step at a winding temperature of 470 to 620 ° C., In the cooling step, Within 3 seconds after the completion of the finish rolling, start cooling the steel sheet using cooling equipment, The period from when the cooling equipment starts cooling until the temperature of the steel sheet reaches the switching temperature ST is defined as a pre-cooling period, and the period from the switching temperature ST until the temperature of the steel sheet reaches the coiling temperature.
- the pre-cooling rate CR1 which is the cooling rate in the pre-cooling period, is less than 25°C/sec
- the switching temperature ST is set to 730 to 830 ° C.
- a post-stage cooling rate CR2 which is the cooling rate in the post-stage cooling period, is set to 25° C./second or more.
- the hot-rolled and hot-dip steel sheets according to the present disclosure have high strength, excellent workability, and excellent LME resistance, as well as excellent rigidity.
- the hot-rolled steel sheet manufacturing method according to the present disclosure can manufacture the hot-rolled steel sheet described above.
- FIG. 1 is a schematic diagram of an LME resistance evaluation test in Examples.
- FIG. 2 is a cross-sectional view of the LME resistance evaluation test of FIG. 1 viewed from the side.
- the present inventors examined hot-rolled steel sheets with high strength, excellent workability, and excellent LME resistance from the viewpoint of chemical composition.
- the chemical composition of the hot-rolled steel sheet is C: 0.040 to 0.120%, Si: 0.01 to 0.60%, Mn: 0.50 to 1 in mass%.
- Feature 1 The area ratio of bainitic ferrite in the microstructure is 85% or more.
- Feature 2 Dislocation density is 8.0 ⁇ 10 13 to 100.0 ⁇ 10 13 /m 2 .
- Feature 3 The average circle equivalent diameter of Ti carbides in the hot-rolled steel sheet is 10 nm or less.
- the hot-rolled steel sheet of the present embodiment, the hot-dipped steel sheet using the hot-rolled steel sheet, and the method for manufacturing the hot-rolled steel sheet have been completed based on the above-described technical concept, and have the following configurations.
- a hot-rolled steel sheet in % by mass, C: 0.040 to 0.120%, Si: 0.01 to 0.60%, Mn: 0.50-1.50%, 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%, and B: 0.0010 to 0.0050%, the balance consists of Fe and impurities,
- the area ratio of bainitic ferrite is 85% or more
- dislocation density is 8.0 ⁇ 10 13 to 100.0 ⁇ 10 13 /m 2
- the average circle equivalent diameter of Ti carbides in the hot-rolled steel sheet is 10 nm or less
- the average circle equivalent diameter of the crystal grains of the bainitic ferrite is 15 ⁇ m or less
- Hot-rolled steel plate in % by mass, C: 0.040 to 0.120%, Si: 0.01 to 0.60%, Mn: 0.50-1.50%, P: 0.025% or less, S: 0.010% or less,
- a hot-rolled steel sheet in % by mass, C: 0.040 to 0.120%, Si: 0.01 to 0.60%, Mn: 0.50-1.50%, 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%, and B: 0.0010 to 0.0050%, Furthermore, it contains one or more selected from the group consisting of the first group and the second group, and the balance consists of Fe and impurities, In the microstructure, the area ratio of bainitic ferrite is 85% or more, dislocation density is 8.0 ⁇ 10 13 to 100.0 ⁇ 10 13 /m 2 , The average circle equivalent diameter of Ti carbides in the hot-rolled steel sheet is 10 nm or less, The average circle equivalent diameter of the crystal grains of the bainitic ferrite is 15 ⁇ m or less, Hot-rolled steel plate. [First group] Nb: 0.20% or less, and V: 0.20% or less, one
- the pre-cooling rate CR1 which is the cooling rate in the pre-cooling period, is less than 25°C/sec
- the switching temperature ST is set to 730 to 830 ° C.
- the post-cooling rate CR2 which is the cooling rate in the post-cooling period, is set to 25 ° C./sec or more
- Carbon (C) combines with Ti to form Ti carbide.
- Ti carbide increases the strength of the hot-rolled steel sheet by precipitation strengthening, and improves the workability.
- C further facilitates formation of bainitic ferrite when the Ti content in the chemical composition is 0.055-0.200%. If the C content is less than 0.040%, high strength cannot be obtained even if the content of other elements is within the range of the present embodiment.
- the tensile strength TS of the hot-rolled steel sheet is less likely to be 780 MPa or more.
- the dislocation density becomes excessively high, and the workability of the hot-rolled steel sheet deteriorates.
- the C content exceeds 0.120%, polygonal ferrite tends to form in the microstructure even if the content of other elements is within the range of the present embodiment. Therefore, the area ratio of bainitic ferrite in the hot-rolled steel sheet is reduced. Furthermore, the dislocation density of the hot-rolled steel sheet is lowered. Furthermore, the average equivalent circle diameter of the crystal grains of the bainitic ferrite also becomes coarse. As a result, the rigidity of the hot-rolled steel sheet is lowered. Therefore, the C content is 0.040-0.120%. A preferable lower limit of the C content is 0.042%, more preferably 0.044%, and still more preferably 0.046%. A preferable upper limit of the C content is 0.115%, more preferably 0.110%, and still more preferably 0.105%.
- Si 0.01-0.60% Silicon (Si) deoxidizes steel. Si further enhances the strength of the hot-rolled steel sheet by solid-solution strengthening. If the Si content is less than 0.01%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Si content exceeds 0.60%, polygonal ferrite tends to form in the hot-rolled steel sheet even if the content of other elements is within the range of the present embodiment. Therefore, the area ratio of bainitic ferrite in the hot-rolled steel sheet is reduced. Furthermore, the dislocation density of the hot-rolled steel sheet is reduced. Furthermore, the average equivalent circle diameter of the crystal grains of the bainitic ferrite also becomes coarse.
- the Si content is 0.01-0.60%.
- a preferable lower limit of the Si content is 0.02%, more preferably 0.03%, and still more preferably 0.04%.
- a preferable upper limit of the Si content is 0.55%, more preferably 0.50%, and still more preferably 0.45%.
- Mn 0.50-1.50%
- Mn Manganese
- the Mn content is 0.50-1.50%.
- a preferable lower limit of the Mn content is 0.55%, more preferably 0.60%, and still more preferably 0.65%.
- a preferable upper limit of the Mn content is 1.40%, more preferably 1.30%, and still more preferably 1.20%.
- Phosphorus (P) is an impurity. P segregates at grain boundaries and lowers the workability of the hot-rolled steel sheet. P further reduces the weldability of the hot-rolled steel sheet. If the P content exceeds 0.025%, the workability and weldability of the hot-rolled steel sheet are remarkably lowered even if the content of other elements is within the range of the present embodiment. Therefore, the P content is 0.025% or less. The lower the P content is, the better. However, excessive reduction of the P content reduces productivity and increases manufacturing costs. Therefore, when considering normal industrial production, the preferable lower limit of the P content is more than 0%, more preferably 0.001%, more preferably 0.002%, and still more preferably 0.003%. %. A preferable upper limit of the P content is 0.023%, more preferably 0.020%, and still more preferably 0.015%.
- S 0.010% or less Sulfur (S) is an impurity. S segregates at grain boundaries and lowers the workability of the hot-rolled steel sheet. If the S content exceeds 0.010%, the workability of the hot-rolled steel sheet is remarkably lowered even if the content of other elements is within the range of the present embodiment. Therefore, the S content is 0.010% or less. It is preferable that the S content is as low as possible. However, excessive reduction of the S content reduces productivity and increases manufacturing costs. Therefore, when considering normal industrial production, the preferred lower limit of the S content is more than 0%, more preferably 0.001%, more preferably 0.002%, still more preferably 0.003 %. A preferable upper limit of the S content is 0.009%, more preferably 0.008%.
- Al 0.010-0.070%
- a preferable lower limit of the Al content is 0.012%, more preferably 0.014%, and still more preferably 0.016%.
- a preferable upper limit of the Al content is 0.065%, more preferably 0.060%, and still more preferably 0.055%.
- N 0.0070% or less Nitrogen (N) is an impurity. N combines with B to form BN and reduces the amount of solid solution B in the hot-rolled steel sheet. N further combines with Ti to form TiN, which inhibits the formation of Ti carbides. If the N content exceeds 0.0070%, BN and TiN are excessively produced even if the content of other elements is within the range of the present embodiment. As a result, the LME resistance of the hot-rolled steel sheet is lowered. Furthermore, the strength of the hot-rolled steel sheet is also reduced. Therefore, the N content is 0.0070% or less. N content is preferably as low as possible. However, excessive reduction of the N content reduces productivity and increases manufacturing costs.
- the preferable lower limit of the N content is more than 0%, more preferably 0.0001%, still more preferably 0.0005%, still more preferably 0.0010 %.
- a preferable upper limit of the N content is 0.0060%, more preferably 0.0050%, and still more preferably 0.0040%.
- Titanium (Ti) combines with C to form Ti carbides.
- Ti carbide increases the strength of the hot-rolled steel sheet by precipitation strengthening.
- bainitic ferrite tends to form in the hot-rolled steel sheet if the Ti content is appropriate.
- the Ti content is less than 0.055%, polygonal ferrite is likely to form even if the content of other elements is within the range of the present embodiment. Therefore, the area ratio of bainitic ferrite in the hot-rolled steel sheet is reduced, and the dislocation density of the hot-rolled steel sheet is also reduced. Furthermore, the average equivalent circle diameter of the crystal grains of the bainitic ferrite becomes coarse.
- the Ti content is 0.055-0.200%.
- the lower limit of the Ti content is preferably 0.060%, more preferably 0.065%, still more preferably 0.070%, still more preferably 0.075%, still more preferably 0.080 %, more preferably 0.085%.
- a preferable upper limit of the Ti content is 0.190%, more preferably 0.180%, and still more preferably 0.170%.
- B 0.0010 to 0.0050% Boron (B) dissolves in the hot-rolled steel sheet and segregates at prior austenite grain boundaries. Segregated B increases grain boundary strength. Therefore, B enhances the LME resistance of the hot-rolled steel sheet. B further enhances the hardenability of steel. If the B content is less than 0.0010%, sufficient LME resistance of the hot-rolled steel sheet cannot be obtained even if the content of other elements is within the range of the present embodiment. Furthermore, since the hardenability is insufficient, the dislocation density is lowered. Furthermore, the area ratio of bainitic ferrite is reduced. Furthermore, the transformation start temperature from austenite to ferrite increases. In this case, the precipitation start temperature of Ti carbide also increases. Therefore, the Ti carbide becomes coarse.
- the strength of the hot-rolled steel sheet is lowered, and the rigidity is also lowered.
- the B content exceeds 0.0050%, the hardenability becomes excessively high even if the contents of other elements are within the range of the present embodiment. In this case, the dislocation density of the hot-rolled steel sheet becomes excessively high. Furthermore, the area ratio of bainitic ferrite is reduced. As a result, the workability of the steel sheet deteriorates. If the B content exceeds 0.0050%, the LME resistance is further lowered. Therefore, the B content is 0.0010-0.0050%.
- a preferable lower limit of the B content is 0.0015%, more preferably 0.0020%, and still more preferably 0.0025%.
- a preferable upper limit of the B content is 0.0045%, more preferably 0.0040%, and still more preferably 0.0035%.
- the rest of the chemical composition of the hot-rolled steel sheet of this embodiment consists of Fe and impurities.
- the impurities are those that are mixed from ore, scrap, or the manufacturing environment as raw materials when industrially producing hot-rolled steel sheets, and are not intentionally contained. It means a range that does not adversely affect the hot-rolled steel sheet of the present embodiment.
- the chemical composition of the hot-rolled steel sheet of the present embodiment may further contain one or more selected from the group consisting of the first group and the second group instead of part of Fe.
- the first group and the second group will be described below.
- the hot-rolled steel sheet of the present embodiment may contain the first group instead of part of Fe. All of these elements combine with C to form carbides and increase the strength of the hot-rolled steel sheet. Each element will be described below.
- Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When present, that is, when the Nb content is greater than 0%, Nb combines with C to form Nb carbides. Nb carbide increases the strength of the hot-rolled steel sheet by precipitation strengthening. If even a small amount of Nb is contained, the above effect can be obtained to some extent. However, if the Nb content exceeds 0.20%, Nb carbides are excessively formed even if the content of other elements is within the range of the present embodiment. In this case, the workability of the hot-rolled steel sheet deteriorates.
- the Nb content is 0-0.20%, and if included, the Nb content is 0.20% or less.
- a preferable lower limit of the Nb content is 0.01%, more preferably 0.05%, and still more preferably 0.08%.
- a preferable upper limit of the Nb content is 0.18%, more preferably 0.16%, and still more preferably 0.14%.
- V 0.20% or less Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When present, that is, when the V content is greater than 0%, V combines with C to form V carbides. V carbide increases the strength of hot-rolled steel sheets by precipitation strengthening. If even a small amount of V is contained, the above effect can be obtained to some extent. However, if the V content exceeds 0.20%, excessive V carbide is formed even if the content of other elements is within the range of the present embodiment. In this case, the workability of the hot-rolled steel sheet deteriorates. Therefore, the V content is 0 to 0.20%, and when included, the V content is 0.20% or less. A preferable lower limit of the V content is 0.01%, more preferably 0.05%, and still more preferably 0.08%. A preferable upper limit of the V content is 0.18%, more preferably 0.16%, and still more preferably 0.14%.
- the hot-rolled steel sheet of the present embodiment may contain the second group instead of part of Fe. All of these elements enhance the LME resistance of the hot rolled steel sheet. Each element will be described below.
- Chromium (Cr) is an optional element and may not be contained. That is, the Cr content may be 0%. When contained, that is, when the Cr content is more than 0%, Cr segregates at the prior austenite grain boundaries to enhance the LME resistance of the hot-rolled steel sheet. If even a little Cr is contained, the above effect can be obtained to some extent. However, if the Cr content exceeds 1.0%, the workability of the hot-rolled steel sheet deteriorates even if the contents of other elements are within the ranges of the present embodiment. Therefore, the Cr content is 0-1.0%, and if included, the Cr content is 1.0% or less. A preferable lower limit of the Cr content is 0.1%, more preferably 0.2%, and still more preferably 0.3%. A preferable upper limit of the Cr content is 0.9%, more preferably 0.8%, and still more preferably 0.7%.
- Mo Molybdenum
- Mo is an optional element and may not be contained. That is, the Mo content may be 0%. When contained, that is, when the Mo content is more than 0%, Mo segregates at the prior austenite grain boundaries to enhance the LME resistance of the hot rolled steel sheet. If even a little Mo is contained, the above effect can be obtained to some extent. However, if the Mo content exceeds 1.0%, the workability of the hot-rolled steel sheet deteriorates even if the content of other elements is within the range of the present embodiment. Therefore, the Mo content is 0-1.0%, and if included, the Mo content is 1.0% or less.
- the lower limit of the Mo content is preferably 0.1%, more preferably 0.2%, still more preferably 0.3%.
- a preferable upper limit of the Mo content is 0.9%, more preferably 0.8%, and still more preferably 0.7%.
- the chemical composition of the hot-rolled steel sheet of this embodiment can be measured by a well-known component analysis method conforming to JIS G0321:2017. Specifically, chips are collected from the hot-rolled steel sheet using a cutting tool such as a drill. The collected chips are dissolved in acid to obtain a solution. ICP-MAS (Inductively Coupled Plasma Mass Spectrometry) is performed on the solution to perform elemental analysis of the chemical composition. The C content and S content are obtained by a well-known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using the well-known inert gas fusion-thermal conductivity method.
- ICP-MAS Inductively Coupled Plasma Mass Spectrometry
- each element content based on the significant digits defined in this embodiment, rounded off the measured numerical value, the numerical value up to the minimum digit of each element content defined in this embodiment do.
- the C content of the steel material of this embodiment is specified by a numerical value up to the third decimal place. Therefore, the C content is a numerical value to the third decimal place obtained by rounding off the measured numerical value to the fourth decimal place.
- the content of elements other than the C content of the steel material of this embodiment is a value obtained by rounding off the numerical value to the minimum digit specified in this embodiment with respect to the measured value. is the content of the element. Rounding off means rounding down if the fraction is less than 5, and rounding up if the fraction is 5 or more.
- the hot-rolled steel sheet of the present embodiment has a chemical composition in which the content of each element is within the range of the present embodiment, and satisfies the following features 1 to 4.
- Feature 1 The area ratio of bainitic ferrite in the microstructure is 85% or more.
- Feature 2 Dislocation density is 8.0 ⁇ 10 13 to 100.0 ⁇ 10 13 /m 2 .
- Feature 3 The average circle equivalent diameter of Ti carbides in the hot-rolled steel sheet is 10 nm or less.
- Feature 4 The average equivalent circle diameter of the bainitic ferrite crystal grains is 15 ⁇ m or less.
- the area ratio 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.
- the other phase is, for example, one selected from the group consisting of polygonal ferrite, pearlite, bainite, and cementite. That's it.
- Bainitic ferrite can be distinguished from polygonal ferrite and bainite in the following points.
- Bainitic ferrite is an aggregate of grains with slightly different crystal orientations. Therefore, a difference in contrast is recognized within the crystal grains.
- polygonal ferrite has a structure in which there is almost no crystal orientation difference within grains. Therefore, the inside of the crystal grain is observed with a uniform contrast. Therefore, bainitic ferrite can be distinguished from polygonal ferrite based on the contrast caused by the difference in crystal orientation.
- bainitic ferrite is a bcc structure, similar to the crystal structure of bainite. Therefore, it is difficult to distinguish bainitic ferrite from bainite based on its crystal structure. Moreover, it is difficult to distinguish bainitic ferrite from bainite based on the crystallographic misorientation. However, bainitic ferrite is distinguishable from bainite based on the presence or absence of Fe carbides within grains and at grain boundaries.
- the Fe carbide is a carbide containing Fe, such as cementite.
- bainitic ferrite Fe carbide does not exist in grains and grain boundaries.
- bainite Fe carbide exists within the laths and/or at the lath boundaries. Therefore, bainitic ferrite can be distinguished from bainite based on the presence or absence of Fe carbides in grains and grain boundaries.
- the preferable lower limit of the area ratio of bainitic ferrite is 88%, more preferably 90%, still more preferably 92%, still more preferably 94%, still more preferably 96%.
- the area ratio of bainitic ferrite can be obtained by the following method. Microstructural observation is performed using a field emission scanning electron microscope (FE-SEM). Microstructure observation is performed by an electron channeling contrast image (ECCI: Electron Channeling Contrast Image). Observation conditions are an acceleration voltage of 20 kV, a tilt (T) of 0°, and a backscattered electron mode. Electron Back Scatter Diffraction (EBSD) is used to measure the crystal orientation.
- FE-SEM field emission scanning electron microscope
- ECCI Electron Channeling Contrast Image
- Observation conditions are an acceleration voltage of 20 kV, a tilt (T) of 0°, and a backscattered electron mode.
- Electron Back Scatter Diffraction (EBSD) is used to measure the crystal orientation.
- the test piece is taken from the width center position of the hot-rolled steel sheet.
- the measurement position is the thickness/4 depth position in the thickness direction of the hot-rolled steel sheet from the surface of the test piece, the measurement range is 100 ⁇ m ⁇ 100 ⁇ m, and the measurement interval is 0.1 ⁇ m.
- the measurement range is a longitudinal section including the L direction (longitudinal direction of the hot-rolled steel sheet) and the T direction (thickness direction of the hot-rolled steel sheet).
- the measurement data is analyzed by the following procedure using analysis software to identify and quantify polygonal ferrite and bainitic ferrite.
- Step 1 A region surrounded by grain boundaries of 15° or more is defined as one crystal grain. If the equivalent circle diameter of a region surrounded by grain boundaries of 15° or more is 1.0 ⁇ m or less, the region is judged to be measurement noise and is not recognized as a crystal grain. In other words, areas determined to be measurement noise are excluded.
- GAM value An average value of crystal orientation differences in each crystal grain (Grain Average Misorientation: hereinafter referred to as GAM value) is calculated. Crystal grains with a GAM value of 0.5° or less are defined as polygonal ferrite. A crystal grain with a GAM value exceeding 0.5° is defined as bainitic ferrite.
- phases perlite, bainite, cementite
- bainitic ferrite and polygonal ferrite phases (perlite, bainite, cementite) different from bainitic ferrite and polygonal ferrite can be easily distinguished by contrast.
- a well-known program may be used as the EBSD analysis program for obtaining the GAM value.
- OIM Data Collection/Analysis 6.2.0 manufactured by TSL Solutions Co., Ltd. may be used.
- the hot-rolled steel sheet of this embodiment has a dislocation density of 8.0 ⁇ 10 13 to 100.0 ⁇ 10 13 /m 2 .
- the higher the dislocation density the higher the rigidity of the hot-rolled steel sheet.
- the strain of bainitic ferrite is higher than that of polygonal ferrite. Therefore, the dislocation density of bainitic ferrite is higher than that of polygonal ferrite. Therefore, if the area ratio of bainitic ferrite is 85% or more in the microstructure of the hot-rolled steel sheet, the dislocation density is high and the strength of the hot-rolled steel sheet is high.
- the hot-rolled steel sheet whose chemical composition contains each element within the range of the present embodiment satisfies the features 1, 3, and 4, if the dislocation density is too low, the stiffness will not be sufficiently high. .
- the content of each element in the chemical composition is within the range of the present embodiment and the dislocation density of the hot-rolled steel sheet is 8.0 ⁇ 10 13 to 100.0 ⁇ 10 13 /m 2 , feature 1, feature 3, and feature On the premise that 4 is satisfied, excellent workability and excellent LME resistance are obtained, and high strength and high rigidity are obtained.
- a preferable lower limit of the dislocation density is 10.0 ⁇ 10 13 /m 2 , more preferably 15.0 ⁇ 10 13 /m 2 , still more preferably 20.0 ⁇ 10 13 /m 2 .
- a preferable upper limit of the dislocation density is 90.0 ⁇ 10 13 /m 2 , more preferably 80.0 ⁇ 10 13 /m 2 , still more preferably 70.0 ⁇ 10 13 /m 2 .
- the dislocation density of the hot-rolled steel sheet of this embodiment can be determined by the following method.
- test piece for dislocation density measurement is taken from the width center position of the hot-rolled steel sheet.
- the dimensions of the test piece are width 20 mm x length 20 mm x plate thickness.
- the surface of the test piece is mirror-finished by polishing with #80 to #1500 sandpaper and buffing from the surface to the plate thickness/4 depth position. Furthermore, the test piece after mirror polishing is subjected to electrolytic polishing of 50 ⁇ m or more in the plate thickness direction using 10% by volume of perchloric acid (acetic acid solvent) to remove strain on the surface layer of the test piece. .
- the (110), (211), and (220) planes of the body-centered cubic structure (bcc structure) were analyzed by X-ray diffraction (XRD) on the surface (observation surface) of the electrolytically polished test piece. Find the half width ⁇ K of the peak.
- CoK ⁇ rays are used as a radiation source, the tube voltage is 30 kV, and the tube current is 100 mA, and the half width ⁇ K is measured. Furthermore, LaB 6 (lanthanum hexaboride) powder is used to measure the half-value width derived from the X-ray diffractometer.
- the non-uniform strain ⁇ of the test piece is obtained from the half width ⁇ K obtained by the above method and the Williamson-Hall equation (equation (I)).
- ⁇ K ⁇ cos ⁇ / ⁇ 0.9/D+2 ⁇ sin ⁇ / ⁇ (I)
- ⁇ means diffraction angle (°)
- ⁇ means X-ray wavelength (nm)
- D means crystallite size (nm).
- the average circle equivalent diameter of Ti carbides in the hot-rolled steel sheet is 10 nm or less.
- the equivalent circle diameter means the diameter of a circle having the same area as the Ti carbide.
- Ti carbide increases the strength of hot-rolled steel sheets by precipitation strengthening.
- the content of each element in the chemical composition is within the range of the present embodiment, if the average circle equivalent diameter of Ti carbides exceeds 10 nm, the Ti carbides in the hot-rolled steel sheet are coarse. If the Ti carbide is coarse, sufficient precipitation strengthening cannot be obtained. As a result, the strength of the hot-rolled steel sheet is not sufficiently high.
- the 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 average circle equivalent diameter of Ti carbide is 10 nm or less, the other characteristics 1, 2 and 4 are satisfied. , high strength and high stiffness are obtained while maintaining excellent workability and excellent LME resistance.
- a preferable upper limit of the average equivalent circle diameter of Ti carbide is 9 nm, more preferably 8 nm.
- the lower limit of the average equivalent circle diameter of Ti carbides is not particularly limited.
- the lower limit of the average equivalent circle diameter of Ti carbide is preferably 2 nm, more preferably 3 nm, still more preferably 4 nm, still more preferably 5 nm.
- the average equivalent circle diameter of Ti carbides can be obtained by the following method.
- a thickness sample of the hot-rolled steel sheet is taken from the central position of the width of the hot-rolled steel sheet. Using emery paper, both sides of the sample are ground and polished to prepare a sample having a thickness of 50 ⁇ m centered at a position of thickness/4 depth from the surface. After that, a disc-shaped sample with a diameter of 3 mm is taken.
- a disc-shaped sample is immersed in a 10% perchloric acid-glacial acetic acid solution and subjected to electrolytic polishing to prepare a thin film sample having a thickness of 100 nm.
- TEM transmission electron microscope
- the precipitates are identified based on the contrast.
- the identified precipitates are subjected to component analysis by EDS.
- Precipitates in which Ti and C are detected as a result of EDS analysis are identified as Ti carbides.
- the equivalent circle diameter of each identified Ti carbide is obtained.
- the arithmetic average value of the equivalent circle diameters of all the Ti carbides confirmed in the five fields of view is defined as the average equivalent circle diameter (nm) of the Ti carbides.
- the average equivalent circle diameter of the bainitic ferrite crystal grains is 15 ⁇ m or less.
- the equivalent circle diameter means the diameter of a circle having the same area as the crystal grain.
- the grain size of bainitic ferrite strongly affects its rigidity.
- a hot-rolled steel sheet having a chemical composition in which the content of each element is within the range of the present embodiment even if the characteristics 1 to 3 are satisfied, if the equivalent circle diameter of the bainitic ferrite crystal grains exceeds 15 ⁇ m, Although high strength, excellent workability and excellent LME resistance can be obtained, sufficient rigidity cannot be obtained.
- the upper limit of the average equivalent circle diameter of the bainitic ferrite crystal grains is preferably 14 ⁇ m, more preferably 13 ⁇ m, and still more preferably 12 ⁇ m.
- the lower limit of the average equivalent circle diameter of crystal grains of bainitic ferrite is not particularly limited.
- the lower limit of the average equivalent circle diameter of the bainitic ferrite crystal grains is preferably 1 ⁇ m, more preferably 2 ⁇ m, still more preferably 3 ⁇ m, and still more preferably 5 ⁇ m.
- the equivalent circle diameter of the bainitic ferrite crystal grains of the hot-rolled steel sheet can be obtained by the following method.
- the circle equivalent diameter of each crystal grain of bainitic ferrite identified by microstructure observation is determined by the method described in the above [Method for measuring area ratio of bainitic ferrite].
- the arithmetic mean value of the obtained equivalent circle diameters is defined as the average equivalent circle diameter ( ⁇ m) of the crystal grains of the bainitic ferrite. If the equivalent circle diameter of a region surrounded by grain boundaries of 15° or more is 1.0 ⁇ m or less, the region is judged to be measurement noise and is not recognized as a crystal grain. That is, if the equivalent circle diameter of a region surrounded by grain boundaries of 15° or more is 1.0 ⁇ m or less, that region is excluded from the analysis.
- the hot-rolled steel sheet of the present embodiment has the content of each element in the chemical composition within the above range, and further satisfies the features 1 to 4. Therefore, it has high strength, excellent workability, and when a hot-dip galvanized layer is formed on the surface of the hot-rolled steel sheet, it has excellent LME resistance and high rigidity.
- the tensile strength which is an indicator of strength
- the total elongation which is an index of workability
- the yield ratio which is an index of rigidity
- a preferable lower limit of the tensile strength of the hot-rolled steel sheet is 785 MPa, more preferably 790 MPa, still more preferably 795 MPa, still more preferably 800 MPa.
- the upper limit of the tensile strength of the hot-rolled steel sheet is not particularly limited, it is, for example, 950 MPa.
- the preferred lower limit of the total elongation of the hot-rolled steel sheet is 14.5%, more preferably 15.0%, still more preferably 15.5%.
- the upper limit of the total elongation of the hot-rolled steel sheet is not particularly limited, it is, for example, 20.0%.
- a preferable lower limit of the yield ratio of the hot-rolled steel sheet is 86%, more preferably 87%, still more preferably 88%, still more preferably 89%.
- the tensile strength, total elongation, and yield ratio of the hot-rolled steel sheet can be obtained by a tensile test based on JIS Z2241:2011.
- a plate-shaped tensile test piece corresponding to a JIS No. 5 test piece specified in JIS Z2241:2011 is taken from the width center position of the hot-rolled steel sheet.
- the longitudinal direction of the test piece is perpendicular to the rolling direction of the hot-rolled steel sheet.
- a tensile test was performed at room temperature in the air, yield strength YS, tensile strength TS, total elongation T. Find EL. At this time, 0.2% yield strength is defined as yield strength YS (MPa).
- yield strength YS MPa
- the hot-dip plated steel sheet of the present embodiment includes the hot-rolled steel sheet of the present embodiment described above and a hot-dip galvanized layer containing mainly Zn.
- a hot-dip galvanized layer is formed on the surface of the hot-rolled steel sheet.
- the hot-dip galvanized layer has a known structure. The hot-dip galvanized layer will be described below.
- the hot-dip galvanized layer mainly contains Zn. Specifically, the hot-dip galvanized layer contains 65.00% or more of Zn in mass %.
- the hot-dip galvanized layer may be a so-called hot-dip galvanized (GI) layer. Hot-dip galvanizing contains 1.00% or less by mass of elements other than Zn, and the balance is Zn. If the Zn content in the hot-dip galvanized layer is 65.00% by mass or more, sufficient corrosion resistance can be obtained.
- a preferable lower limit of the Zn content of the hot dip galvanized layer is 70.00%, more preferably 73.00%.
- the hot-dip galvanized layer may have a chemical composition other than GI.
- the chemical composition of the hot-dip galvanized layer may be within a well-known range.
- the chemical composition of the hot-dip galvanized layer contains, for example, the following elements.
- Al 0.05-35.00%
- Aluminum (Al) is an easily oxidizable element and enhances the corrosion resistance of the hot-dip galvanized layer by sacrificial corrosion protection. If the Al content is 0.05 to 35.00%, the above effect can be sufficiently obtained.
- a preferable lower limit of the Al content is 0.08%, more preferably 0.10%, and still more preferably 0.15%.
- the preferred upper limit of the Al content is 33.00%, more preferably 30.00%, still more preferably 28.00%, still more preferably 25.00%, still more preferably 23.00 %, more preferably 21.00%.
- the rest of the chemical composition of the hot-dip galvanized layer according to this embodiment consists of Zn and impurities.
- impurities refers to those that are mixed from raw materials during the hot-dip galvanizing treatment and are not intentionally included.
- the chemical composition of the hot-dip zinc-based plating layer according to the present embodiment may further contain one or more elements selected from the following 1st to 7th groups instead of part of Zn.
- the first to seventh groups will be described below.
- Si 2.50% or less
- Mg Group 1 (Mg) Mg: 30.0% or less
- Mg Magnesium
- the Mg content may be 0%.
- Mg is an easily oxidizable element and enhances the corrosion resistance of the hot-dip galvanized layer by sacrificial corrosion protection. As long as the Mg content is even small, the above effect can be obtained to some extent. However, if the Mg content is too high, oxidation dross will increase even if the content of other elements is within the range of the present embodiment. In this case, the appearance quality of the hot-dip plated steel sheet is deteriorated. Therefore, the Mg content is 0-30.0%, and if included, the Mg content is 30.0% or less.
- a preferable lower limit of the Mg content is more than 0%, more preferably 0.1%, more preferably 0.5%, more preferably 1.0%, more preferably 2.0% is.
- the preferred upper limit of the Mg content is 25.0%, more preferably 20.0%, still more preferably 15.0%, still more preferably 10.0%, still more preferably 8.0 %, more preferably 7.0%.
- Tin (Sn), bismuth (Bi), and indium (In) are optional elements and may not be contained. That is, the Sn content, Bi content, and In content may each be 0%. These elements form an intermetallic compound with Mg when the hot-dip zinc-based plating layer contains Mg. As a result, the corrosion resistance of the hot dip plated steel sheet is enhanced. If at least one of Sn, Bi and In is contained even in a small amount, the above effect can be obtained to some extent. However, if the contents of these elements are too high, the viscosity of the hot-dip galvanizing bath will increase even if the contents of other elements are within the range of the present embodiment. In this case, the appearance quality of the hot-dip plated steel sheet is deteriorated.
- the Sn content is 0-2.00%
- the Bi content is 0-2.00%
- the In content is 0-2.00%.
- the Sn content is 2.00% or less
- the Bi content is 2.00% or less
- the In content is 2.00% or less.
- a preferable lower limit of the content of each element is more than 0%, more preferably 0.01%, and still more preferably 0.05%.
- the upper limit of the content of each element is preferably 1.90%, more preferably 1.80%, still more preferably 1.70%.
- Calcium (Ca), yttrium (Y), lanthanum (La), and selenium (Ce) are all optional elements and may not be contained. That is, the content of these elements may be 0%. These elements form intermetallic compounds with Al and Zn in the hot dip galvanized layer. As a result, the corrosion resistance of the hot-dip plated steel sheet is enhanced. If these elements are contained even in small amounts, the above effect can be obtained to some extent. However, if the contents of these elements are too high, oxidation dross will increase even if the contents of other elements are within the range of the present embodiment. In this case, the appearance quality of the hot-dip plated steel sheet is deteriorated.
- the Ca content is 0-3.00%, the Y content is 0-3.00%, the La content is 0-3.00%, and the Ce content is 0-3.00. %.
- the Ca content is 3.00% or less, the Y content is 3.00% or less, the La content is 3.00% or less, and the Ce content is 3.00% or less is.
- the lower limit of the content of each element is preferably over 0%, more preferably 0.01%, still more preferably 0.05%, still more preferably 0.10%.
- the upper limit of the content of each element is preferably 2.80%, more preferably 2.50%, still more preferably 2.00%.
- Si is an optional element and may not be contained. That is, the Si content may be 0%. Si enhances the corrosion resistance of the hot-dip plated steel sheet. If even a small amount of Si is contained, the above effect can be obtained to some extent. However, if the Si content is too high, the viscosity of the hot-dip galvanizing bath increases even if the contents of other elements are within the range of the present embodiment. In this case, the appearance quality of the hot-dip plated steel sheet is deteriorated. Therefore, the Si content is 0-2.50%, and if included, the Si content is 2.50% or less.
- the lower limit of the Si content is preferably over 0%, more preferably 0.01%, still more preferably 0.05%, still more preferably 0.10%.
- a preferred upper limit of the Si content is 2.00%, more preferably 1.50%, still more preferably 1.00%, and still more preferably 0.50%.
- Chromium (Cr), titanium (Ti), nickel (Ni), cobalt (Co), vanadium (V), niobium (Nb), copper (Cu) and manganese (Mn) are all optional elements and are not contained. may That is, the content of these elements may be 0%. These elements improve the appearance quality of the hot-dip plated steel sheet. These elements further form intermetallic compounds with Al in the hot-dip galvanized layer. As a result, the corrosion resistance of the hot dip plated steel sheet is enhanced. If these elements are contained even in small amounts, the above effect can be obtained to some extent.
- the Cr content is 0-0.5%
- the Ti content is 0-0.5%
- the Ni content is 0-0.5%
- the Co content is 0-0.5%.
- V content is 0-0.5%
- Nb content is 0-0.5%
- Cu content is 0-0.5%
- Mn content is 0-0 0.5%.
- the Cr content is 0.5% or less
- the Ti content is 0.5% or less
- the Ni content is 0.5% or less
- the Co content is 0.5% or less
- the V content is 0.5% or less
- the Nb content is 0.5% or less
- the Cu content is 0.5% or less
- the Mn content is 0.5% or less .
- a preferable lower limit of the content of each element is more than 0%, more preferably 0.1%.
- a preferable upper limit of the content of each element is less than 0.5%, more preferably 0.4%.
- [Sixth group (Fe)] Fe 5.0% or less Iron (Fe) is an optional element and may not be contained. That is, the Fe content may be 0%. Fe increases the hardness of the hot-dip galvanized layer and enhances the workability of the hot-dip plated steel sheet. If even a little Fe is contained, the above effects can be obtained to some extent. However, if the Fe content is too high, the hardness of the hot-dip galvanized layer will be too high even if the contents of other elements are within the range of the present embodiment. In this case, the workability of the hot-dip plated steel sheet rather deteriorates. Therefore, the Fe content is 0-5.0%, and if included, the Fe content is 5.0% or less. A preferable lower limit of the Fe content is more than 0%, more preferably 0.1%, and still more preferably 0.5%. A preferable upper limit of the Fe content is 4.5%, more preferably 4.0%, and still more preferably 3.5%.
- Strontium (Sr), antimony (Sb), lead (Pb) and boron (B) are all optional elements and may not be contained. That is, the content of these elements may be 0%. These elements enhance the metallic luster of the hot-dip galvanized layer and improve the appearance quality of the hot-dip plated steel sheet. If these elements are contained even in small amounts, the above effect can be obtained to some extent. However, if the contents of these elements are too high, oxidation dross will increase even if the contents of other elements are within the range of the present embodiment. In this case, the appearance quality of the hot-dip plated steel sheet is deteriorated.
- the Sr content is 0-0.5%
- the Sb content is 0-0.5%
- the Pb content is 0-0.5%
- the B content is 0-0.5%. %.
- the Sr content is 0.5% or less
- the Sb content is 0.5% or less
- the Pb content is 0.5% or less
- the B content is 0.5% or less is.
- a preferable lower limit of the content of each element is more than 0%, more preferably 0.1%.
- a preferable upper limit of the content of each element is less than 0.5%, more preferably 0.4%.
- the chemical composition of the hot-dip galvanized layer can be obtained by the following method.
- the hot-dip zinc-based plating layer is dissolved using inhibitor-containing hydrochloric acid.
- As the inhibitor for example, IBIT (trade name) manufactured by Asahi Kagaku Kogyo Co., Ltd. can be used.
- the solution is subjected to elemental analysis in the same manner as the chemical composition analysis of the hot-rolled steel sheet described above.
- the chemical composition of the hot-dip galvanized layer can be obtained.
- the above hot-rolled steel sheet and hot-dip galvanized steel sheet having a hot-dip galvanized layer have not only high strength, high rigidity and excellent workability, but also excellent LME resistance.
- the hot-rolled steel sheet manufacturing method described below is an example for manufacturing the hot-rolled steel sheet according to the present embodiment. Therefore, the hot-rolled steel sheet having the configuration described above may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of the method for manufacturing a hot-rolled steel sheet according to this embodiment.
- An example of the method for manufacturing the hot-rolled steel sheet of the present embodiment includes the following steps.
- the above manufacturing method is carried out using production line equipment.
- the production line equipment includes, in order from upstream to downstream, a heating furnace, a rough rolling mill (Rougher), a finishing rolling mill (Finisher), a cooling device (Run-out table cooling equipment), and a winding device (Down Coiler). Prepare. A plurality of transport rolls are arranged between each facility.
- Step 2 Hot rolling step Finish rolling temperature FT: 850 to 950 ° C.
- Step 3 Cooling step Pre-cooling rate CR1: less than 25 ° C./sec Switching temperature ST from pre-cooling rate to post-cooling rate: 730 to 830 ° C.
- Post-stage cooling rate CR2 25 ° C./sec or more
- Step 4 Winding step ⁇ Winding temperature CT: 470 to 620 ° C.
- Step 1 Material preparation step
- a material is prepared in which the content of each element in the chemical composition is within the range of the present embodiment.
- the material is manufactured, for example, by the following method.
- Molten steel is produced in which the content of each element in the chemical composition is within the range of the present embodiment.
- a raw material (slab or ingot) is produced by casting using the molten steel.
- a slab is manufactured by a well-known continuous casting method using the molten steel.
- the molten steel is used to produce an ingot by a well-known ingot casting method.
- Step 2 A steel plate is produced by hot rolling the prepared material (slab or ingot).
- the hot rolling process includes a rough rolling process in which a raw material is roughly rolled to produce a rough bar (intermediate steel sheet), and a finish rolling process in which the rough bar is finish rolled to produce a steel sheet.
- the material (slab or ingot) is heated in a heating furnace.
- the heated material is rolled using a roughing mill to produce a rough bar.
- the heating temperature of the material in the rough rolling process is, for example, 1250 to 1300.degree.
- the time in which the material is kept in the heating furnace is 30 minutes or more, preferably 60 minutes or more. Although the upper limit of the time in the furnace is not particularly limited, it is, for example, 240 minutes.
- the rough bar is further rolled (finish rolling) using a finish rolling mill to produce a steel plate.
- a finishing mill includes a plurality of stands arranged in a row. Each stand has a pair of work rolls.
- the surface temperature of the steel sheet on the delivery side of the stand that finally rolls down the steel sheet among the plurality of stands of the finish rolling mill is defined as the finish rolling temperature FT (°C).
- the finish rolling temperature FT is set as follows. ⁇ Finish rolling temperature FT: 850 to 950 ° C
- Step 3 the steel sheet that has undergone finish rolling is rapidly cooled using a cooling device.
- a cooling device for the steel plate after finish rolling, from the viewpoint of productivity, for example, cooling in a cooling facility is started within 3 seconds after completion of finish rolling.
- a cooling medium is used to cool the steel plate.
- the cooling medium is for example water.
- the cooling rate of the steel sheet differs between the upstream side and the downstream side of the cooling equipment.
- the manufacturing conditions in the cooling process using cooling equipment are as follows. ⁇ Pre-stage cooling rate CR1: less than 25°C/sec ⁇ Switching temperature ST from pre-stage cooling rate to post-stage cooling rate: 730 to 830°C ⁇ Post-stage cooling rate CR2: 25 ° C./sec or more
- pre-cooling period the period from when the cooling equipment starts cooling until the steel plate temperature reaches the switching temperature ST
- post-cooling period the period from the switching temperature ST until the steel plate temperature reaches the coiling temperature CT.
- Pre-stage cooling rate CR1 less than 25°C/sec
- the steel sheet after completion of finish rolling is cooled at the pre-stage cooling rate CR1. That is, in the pre-cooling period, the steel plate is cooled at the pre-cooling rate CR1.
- the pre-stage cooling rate CR1 is 25° C./second or more, a non-recrystallized austenite region remains in the steel sheet when the steel sheet temperature reaches the switching temperature ST (° C.).
- the unrecrystallized austenite region tends to become polygonal ferrite during the post-cooling period. Therefore, the area ratio of bainitic ferrite becomes low in the hot-rolled steel sheet after production. In this case, the dislocation density is also low.
- the pre-stage cooling rate CR1 is less than 25°C/sec, recrystallization of austenite can be promoted. Therefore, the non-recrystallized austenite region in the steel sheet can be reduced. As a result, in the microstructure of the hot-rolled steel sheet, the area ratio of bainitic ferrite can be increased, and the dislocation density can be within an appropriate range.
- a preferable upper limit of the pre-stage cooling rate CR1 is 24° C./second, more preferably 23° C./second.
- the lower limit of the pre-stage cooling rate CR1 is not particularly limited. However, if the pre-stage cooling rate CR1 is too slow, the production efficiency will drop significantly. Therefore, the preferred lower limit of the pre-stage cooling rate CR1 is 5°C/sec.
- the steel plate temperature at which the cooling rate is switched from the pre-stage cooling rate CR1 to the post-stage cooling rate CR2 is defined as a switching temperature ST (°C).
- the switching temperature ST is lower than 730°C
- recrystallization of austenite is not completed in the pre-cooling period, and an unrecrystallized austenite region remains in the post-cooling period.
- polygonal ferrite is generated in the hot-rolled steel sheet during the post-cooling period.
- the area ratio of bainitic ferrite in the hot-rolled steel sheet becomes low.
- the dislocation density is also low.
- the pre-cooling period is appropriate. Therefore, after the recrystallization of austenite in the steel sheet is sufficiently promoted and the austenite unrecrystallized region is sufficiently reduced, cooling at the post-stage cooling rate CR2 can be started.
- a preferable upper limit of the switching temperature ST is 820°C, more preferably 810°C.
- a preferable lower limit of the switching temperature ST is 740°C, more preferably 750°C.
- the post-stage cooling rate CR2 is 25°C/sec or more, the cooling rate during the post-stage cooling period is sufficiently high. Therefore, on the premise that other manufacturing conditions are satisfied, the area ratio of bainitic ferrite in the hot-rolled steel sheet is 85% or more, and the average circle equivalent diameter of the bainitic ferrite crystal grains is 15 ⁇ m or less. .
- the post-stage cooling rate CR2 is the switching temperature ST (° C.), the coiling temperature CT (° C.), the rolling speed V (m/sec) of the steel sheet on the delivery side of the stand that finally rolls down the steel sheet, and the switching temperature ST and the distance L2 (m) between the thermometer for the winding temperature CT and the thermometer for the winding temperature CT.
- CR2 (ST-CT)/(L2/V)
- a preferable lower limit of the post-stage cooling rate CR2 is 30° C./sec.
- the upper limit of the post-stage cooling rate CR2 is not particularly limited. Considering the facility capacity, the preferable upper limit of the post-stage cooling rate CR2 is 70°C/sec.
- Step 4 Winding step
- the steel sheet that has passed through the cooling device is wound into a coil by a winding device.
- Ti carbide is generated in the steel sheet.
- the surface temperature of the steel sheet at the start of winding is defined as winding temperature CT (°C).
- the coiling temperature CT affects the average equivalent circle diameter of Ti carbide.
- the coiling temperature CT also affects the microstructure of the hot-rolled steel sheet (percentage of bainitic ferrite, polygonal ferrite and bainite). Therefore, the winding temperature CT is adjusted within the following range.
- ⁇ Winding temperature CT 470 to 620°C
- Winding temperature CT 470 to 620 ° C. If the winding temperature CT is higher than 620° C., the end temperature of the post-cooling period in the cooling step is too high. In this case, the coiling is started before the transformation from austenite to bainitic ferrite is completed in the microstructure of the steel sheet. Therefore, part of the austenite transforms into polygonal ferrite. As a result, the area ratio of bainitic ferrite in the hot-rolled steel sheet becomes low. Furthermore, the dislocation density is also lowered. If the coiling temperature CT is higher than 620°C, Ti carbides in the hot-rolled steel sheet are further coarsened.
- the coiling temperature CT is 470 to 620°C
- the area ratio of bainitic ferrite is 85% or more in the microstructure of the hot-rolled steel sheet, provided that other manufacturing conditions are satisfied.
- the average equivalent circle diameter of Ti carbide is 10 nm or less.
- the hot-rolled steel sheet according to the present embodiment is manufactured through the manufacturing process described above.
- the hot-rolled steel sheet of the present embodiment may be manufactured by a manufacturing method other than the manufacturing method described above.
- the production method of the hot-rolled steel sheet of the present embodiment is not particularly limited as long as the content of each element in the chemical composition is within the range of the present embodiment and the features 1 to 4 are provided.
- the method for manufacturing a hot-rolled steel sheet according to the present embodiment may include other steps than the steps described above.
- the temper rolling step may be performed after the cooling step and before the winding step, or after the winding step.
- 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.
- the sheet thickness reduction rate in the temper rolling step for effectively obtaining the above effects is, for example, 0.1% or more.
- a preferable upper limit of the strip thickness reduction rate in the skin pass rolling step 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.
- a hot-dip plated steel sheet, including the hot-rolled steel sheet of the present embodiment can be produced by performing the following well-known hot-dip plating process.
- a hot-dip galvanized layer having the chemical composition described above is formed on the surface of the hot-rolled steel sheet.
- a plating bath is prepared.
- the composition of the plating bath is adjusted according to the composition of the hot-dip zinc-based plating layer to be formed.
- the hot-rolled steel sheet is pulled out of the plating bath by a known method. For example, a sink roll is placed in the plating bath. The hot-rolled steel sheet immersed in the plating bath is turned upward by a sink roll.
- Hot-dip galvanized plating adheres to the surface of the hot-rolled steel sheet pulled up from the plating bath.
- a well-known gas wiping device is used to adjust the amount of hot-dip galvanized coating that adheres to the hot-rolled steel sheet.
- the hot-dip zinc-based coating adhered to the hot-rolled steel sheet pulled up from the coating bath solidifies to form a hot-dip zinc-based coating layer.
- a hot-dip plated steel sheet is manufactured through the above steps.
- the method for manufacturing the hot-dip plated steel sheet of the present embodiment may include manufacturing steps other than the hot-dip plating treatment step.
- the method for manufacturing the hot-dip plated steel sheet of the present embodiment may include a Ni pre-plating step before the hot-dip plating treatment step.
- the Ni pre-plating step the hot-rolled steel sheet described above is plated with Ni to form a Ni-plated layer on the surface of the hot-rolled steel sheet.
- a hot-dip plating process is performed on the hot-rolled steel sheet on which the Ni plating layer is formed. In this case, the adhesion of the hot-dip galvanized layer to the hot-rolled steel sheet increases.
- the method for manufacturing the hot-dip plated steel sheet of the present embodiment may further include a chemical conversion treatment step after the hot-dip plating step.
- the chemical conversion treatment step the manufactured hot-dip plated steel sheet is subjected to chemical conversion treatment to form a chemical conversion coating on the hot-dip zinc-based coating layer.
- the method of chemical conversion treatment is not particularly limited, and a well-known method may be used.
- a chromium conversion coating known as a conversion coating, may be applied.
- a temper rolling step may be further performed after the hot-dip plating treatment step.
- the manufactured hot-dip plated steel sheet is subjected to temper rolling.
- the adhesion of the chemical conversion coating can be enhanced by carrying out the temper rolling process before the chemical conversion treatment process.
- a preferable plate thickness reduction rate in the temper rolling step is 0.1 to 3.0%.
- the manufacturing method of the hot-dip plated steel sheet may further include other manufacturing processes.
- the manufacturing method described above is an example of a manufacturing method for obtaining the hot-dip plated steel sheet of the present embodiment. Therefore, the manufacturing method of the hot-dip plated steel sheet of the present embodiment is not limited to the manufacturing method described above.
- the effect of one aspect of the hot-rolled steel sheet and the hot-dip plated steel sheet of the present embodiment will be described more specifically by way of examples.
- the conditions in the following examples are examples of conditions adopted for confirming the feasibility and effects of the hot-rolled steel sheet and the hot-dip plated steel sheet of the present embodiment. Therefore, the hot-rolled steel sheet and the hot-dip plated steel sheet of the present embodiment are not limited to this one condition example.
- a hot-rolled steel sheet having the chemical composition shown in Table 1 was manufactured.
- the slab was manufactured by continuously casting molten steel.
- the slab was subjected to hot working steps (rough rolling step and finish rolling step).
- the slab was heated at 1250-1300°C for 60 minutes.
- the slab after heating was rolled by a roughing mill to produce a rough bar. Further, the rough bar was rolled using a finishing rolling mill to produce a steel plate.
- the finish rolling temperature FT (°C) of each test number was as shown in Table 2, "FT (°C)" column.
- a cooling process was performed on the steel sheet after finish rolling. Specifically, in any test number, cooling using a cooling device was started within 2 seconds after completion of finish rolling.
- the front-stage cooling rate CR1 (°C/sec), switching temperature ST (°C), and post-stage cooling rate CR2 (°C/sec) of each test number in the cooling process are shown in Table 2 as “CR1 (°C/sec)” and “ ST (°C)” and “CR2 (°C/sec)”.
- the steel plate that passed through the cooling equipment was wound into a coil by a winding device.
- the winding temperature CT (°C) for each test number was as shown in Table 2.
- the coiled steel sheet after winding was allowed to cool to room temperature, and hot-rolled steel sheets with test numbers shown in Table 1 were manufactured.
- the plate thickness of the hot-rolled steel plate was 2.3 mm.
- Test 5 LME resistance evaluation test of hot-dip plated steel sheet Tests 1 to 5 will be described below.
- a plate-shaped tensile test piece corresponding to a JIS No. 5 test piece specified in JIS Z2241:2011 was sampled from the width center position of the hot-rolled steel sheet of each test number.
- the longitudinal direction of the test piece was perpendicular to the rolling direction of the hot-rolled steel sheet.
- a tensile test was performed at room temperature in the air, yield strength YS, tensile strength TS, total elongation T. I asked for EL. 0.2% yield strength was defined as yield strength YS (MPa).
- yield strength YS MPa
- Yield ratio YR YS/TS Yield strength YS (MPa), tensile strength TS (MPa), yield ratio YR (%), total elongation T.O. EL (%) is shown in the columns of "YS (MPa)", “TS (MPa)", “YR (%)” and “T.EL (%)” in Table 3.
- plating number P2 means that 0.30% by mass of Sn is contained as an element of the Sn group.
- the LME resistance of the manufactured hot-dip plated steel sheets of each test number was evaluated by the following method.
- a sample steel plate of 100 mm ⁇ 75 mm ⁇ plate thickness was taken from the hot-dip plated steel plate of each test number.
- Arc welding shown in FIG. 1 was performed using the sample steel plates.
- 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 steel corresponding to SS400 specified in JIS G3101:2015.
- the boss member 1 was placed at the center of the sample steel plate 2 so that the axial direction of the boss member 1 was normal to the surface of the sample steel plate 2 .
- the arranged boss member 1 was welded to the sample steel plate 2 by arc welding.
- the welding wire was YGW12 specified in JIS Z3312:2009.
- the weld bead 3 circles around the boss member 1 clockwise from the welding start point, and after passing the welding start point, the arc welding is continued to form an overlap region 4 of the weld bead. Welding was continued until a The width of the overlapping region 4 was approximately 15 mm.
- a current value of 190 A and a voltage value of 23 V were used during arc welding.
- the welding speed was 0.3 m/min.
- a mixed gas of 20% by volume CO 2 gas and 80% by volume argon gas was used as a shielding gas during arc welding.
- the shield gas flow rate during arc welding was set to 20 L/min.
- the sample steel plate 2 was previously joined to the restraining plate 5 as shown in Fig. 2 .
- the constraining plate 5 is 120 mm x 95 mm x 4 mm thick, and is a steel plate corresponding to SS400 specified in JIS G3101:2015.
- a sample steel plate 2 was placed on the surface of the restraining plate 5 .
- the entire circumference of the arranged sample steel plate 2 was welded to the restraint plate 5 .
- the welding wire and welding conditions were the same as those for welding the boss member 1 to the sample steel plate 2 .
- the boss member 1 is arc-welded to the sample steel plate 2
- the boss member 1 and the sample steel plate 2 are welded together at a cutting plane 7 that passes through the central axis of the boss member 1 and the overlapping region 4 of the weld bead 3, as shown in FIG. , and the constraining plate 5 were cut.
- the cut surface 7 was observed with an optical microscope at a magnification of 100 times. In the observation, the presence or absence of cracks (molten metal embrittlement cracks) in the sample steel plate was visually confirmed. If cracks were observed, the crack length was measured. Among the measured crack lengths, the maximum crack length was identified.
- the content of each element in the chemical composition of the hot-rolled steel sheets of test numbers 1 to 29 was appropriate. Furthermore, the area ratio of bainitic ferrite in the hot-rolled steel sheets of test numbers 1 to 29 was 85% or more, and the average equivalent circle diameter of the bainitic ferrite crystal grains was 15 ⁇ m or less. Furthermore, the average circle equivalent diameter of Ti carbide in the hot-rolled steel sheets of test numbers 1 to 29 was 10 nm or less, and the dislocation density was 8.0 to 100.0 ⁇ 10 13 /m 2 . Therefore, the hot-rolled steel sheets of test numbers 1 to 29 had a tensile strength TS of 780 MPa or more. Furthermore, the yield ratio YR was 85% or more, indicating excellent rigidity. Furthermore, total elongation T.E. EL was 14.0% or more, indicating excellent workability (ductility).
- test number 30 the C content was too high.
- polygonal ferrite was generated in the microstructure of the hot-rolled steel sheet, and the area ratio of bainitic ferrite was less than 85%.
- the average circle equivalent diameter of the bainitic ferrite exceeded 15 ⁇ m.
- the dislocation density of the hot-rolled steel sheet was less than 8.0 ⁇ 10 13 /m 2 . Therefore, the yield ratio YR was less than 85%, and sufficient rigidity was not obtained.
- test number 31 the C content was too low. Therefore, the dislocation density of the hot-rolled steel sheet exceeded 100.0 ⁇ 10 13 /m 2 . Therefore, the total elongation T.D. EL was less than 14.0%, and sufficient workability was not obtained. Furthermore, the tensile strength TS of the hot-rolled steel sheet was less than 780 MPa, and sufficient strength was not obtained.
- the Si content was too high.
- polygonal ferrite was generated in the microstructure of the hot-rolled steel sheet, and the area ratio of bainitic ferrite was less than 85%.
- the average circle equivalent diameter of the bainitic ferrite exceeded 15 ⁇ m.
- the dislocation density of the hot-rolled steel sheet was less than 8.0 ⁇ 10 13 /m 2 . Therefore, the yield ratio YR was less than 85%, and sufficient rigidity was not obtained.
- test number 33 the Mn content was too high. Therefore, bainite was generated in the microstructure of the hot-rolled steel sheet, and the area ratio of bainitic ferrite was less than 85%. Furthermore, the dislocation density of the hot-rolled steel sheet exceeded 100.0 ⁇ 10 13 /m 2 . Therefore, the total elongation T.D. EL was less than 14.0%, and sufficient workability was not obtained.
- test number 34 the Ti content was too low.
- polygonal ferrite was generated in the microstructure of the hot-rolled steel sheet, and the area ratio of bainitic ferrite was less than 85%.
- the average circle equivalent diameter of the bainitic ferrite exceeded 15 ⁇ m.
- the dislocation density of the hot-rolled steel sheet was less than 8.0 ⁇ 10 13 /m 2 . Therefore, YR was less than 85%, and sufficient rigidity was not obtained.
- test number 35 the Ti content was too high. Therefore, the dislocation density of the hot-rolled steel sheet exceeded 100.0 ⁇ 10 13 /m 2 . Therefore, the total elongation T.D. EL was less than 14.0%, and sufficient workability was not obtained.
- the B content was too low. Therefore, in the microstructure of the hot-rolled steel sheet, the area ratio of bainitic ferrite was less than 85%. Furthermore, the average circle equivalent diameter of Ti carbides in the hot-rolled steel sheet exceeded 10 nm. Furthermore, the dislocation density was less than 8.0 ⁇ 10 13 /m 2 . Therefore, the tensile strength TS was less than 780 MPa, and sufficient strength was not obtained. Furthermore, the yield ratio YR was less than 85%, and sufficient rigidity was not obtained. Furthermore, sufficient LME resistance was not obtained.
- test numbers 38 and 39 the B content was too high. Therefore, the dislocation density of the hot-rolled steel sheet exceeded 100.0 ⁇ 10 13 /m 2 . Therefore, the total elongation T.D. EL was less than 14.0%, and sufficient workability was not obtained. Furthermore, sufficient LME resistance was not obtained.
- test numbers 40 and 41 the content of each element in the chemical composition of the hot-rolled steel sheet was appropriate.
- the finish rolling temperature FT during the manufacturing process was too high. Therefore, the average circle equivalent diameter of the crystal grains of the hot-rolled steel sheet bainitic ferrite exceeded 15 ⁇ m.
- the yield ratio YR was less than 85%, and sufficient rigidity was not obtained.
- test numbers 42 and 43 the content of each element in the chemical composition of the hot-rolled steel sheet was appropriate.
- the pre-stage cooling rate CR1 during the cooling step of the manufacturing process was too fast.
- polygonal ferrite was generated in the microstructure of the hot-rolled steel sheet, and the area ratio of bainitic ferrite was less than 85%.
- the dislocation density of the hot-rolled steel sheet was less than 8.0 ⁇ 10 13 /m 2 .
- the yield ratio YR was less than 85%, and sufficient rigidity was not obtained.
- test numbers 44 and 45 the content of each element in the chemical composition of the hot-rolled steel sheet was appropriate.
- the switching temperature ST in the cooling process of the manufacturing process was too high.
- the austenite grains became coarse, and the average circle equivalent diameter of bainitic ferrite in the hot-rolled steel sheet exceeded 15 ⁇ m.
- the yield ratio YR was less than 85%, and sufficient rigidity was not obtained.
- test numbers 46 and 47 the content of each element in the chemical composition of the hot-rolled steel sheet was appropriate.
- the switching temperature ST in the cooling process of the manufacturing process was too low.
- polygonal ferrite was generated in the microstructure of the hot-rolled steel sheet, and the area ratio of bainitic ferrite was less than 85%.
- the dislocation density of the hot-rolled steel sheet was less than 8.0 ⁇ 10 13 /m 2 .
- the yield ratio YR was less than 85%, and sufficient rigidity was not obtained.
- test numbers 48 and 49 the content of each element in the chemical composition of the hot-rolled steel sheet was appropriate.
- the post-stage cooling rate CR2 in the cooling step of the manufacturing process was too slow.
- polygonal ferrite was generated in the microstructure of the hot-rolled steel sheet, and the area ratio of bainitic ferrite was less than 85%.
- the dislocation density of the hot-rolled steel sheet was less than 8.0 ⁇ 10 13 /m 2 .
- the average equivalent circle diameter of Ti carbide exceeded 10 nm. Therefore, the tensile strength TS was less than 780 MPa, and sufficient strength was not obtained.
- the yield ratio YR was less than 85%, and sufficient rigidity was not obtained.
- test numbers 50 and 51 the content of each element in the chemical composition of the hot-rolled steel sheet was appropriate.
- the winding temperature CT in the winding process was too high.
- polygonal ferrite was generated in the microstructure of the hot-rolled steel sheet, and the area ratio of bainitic ferrite was less than 85%.
- the average equivalent circle diameter of Ti carbide exceeded 10 nm.
- the dislocation density of the hot-rolled steel sheet was less than 8.0 ⁇ 10 13 /m 2 . Therefore, the tensile strength TS was less than 780 MPa, and sufficient strength was not obtained.
- the yield ratio YR was less than 85%, and sufficient rigidity was not obtained.
- test numbers 52 to 54 the content of each element in the chemical composition of the hot-rolled steel sheet was appropriate.
- the winding temperature CT in the winding process was too low. Therefore, bainite was generated in the microstructure of the hot-rolled steel sheet. Therefore, the area ratio of bainitic ferrite was less than 85%, and the dislocation density of the hot-rolled steel sheet exceeded 100.0 ⁇ 10 13 /m 2 . Therefore, the total elongation T.D. EL was less than 14.0%, and sufficient workability was not obtained.
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Abstract
Description
質量%で、
C:0.040~0.120%、
Si:0.01~0.60%、
Mn:0.50~1.50%、
P:0.025%以下、
S:0.010%以下、
Al:0.010~0.070%、
N:0.0070%以下、
Ti:0.055~0.200%、及び、
B:0.0010~0.0050%、を含有し、
残部はFe及び不純物からなり、
ミクロ組織において、ベイニティックフェライトの面積率は85%以上であり、
転位密度は8.0×1013~100.0×1013/m2であり、
前記熱延鋼板中のTi炭化物の平均円相当径は10nm以下であり、
前記ベイニティックフェライトの結晶粒の平均円相当径は15μm以下である。
質量%で、
C:0.040~0.120%、
Si:0.01~0.60%、
Mn:0.50~1.50%、
P:0.025%以下、
S:0.010%以下、
Al:0.010~0.070%、
N:0.0070%以下、
Ti:0.055~0.200%、及び、
B:0.0010~0.0050%、を含有し、
さらに、第1群及び第2群からなる群から選択される1種以上を含有し、残部はFe及び不純物からなり、
ミクロ組織において、ベイニティックフェライトの面積率は85%以上であり、
転位密度は8.0×1013~100.0×1013/m2であり、
前記熱延鋼板中のTi炭化物の平均円相当径は10nm以下であり、
前記ベイニティックフェライトの結晶粒の平均円相当径は15μm以下である。
[第1群]
Nb:0.20%以下、及び、
V:0.20%以下、からなる群から選択される1種以上
[第2群]
Cr:1.0%以下、及び、
Mo:1.0%以下、からなる群から選択される1種以上
上述の熱延鋼板と、
前記熱延鋼板の表面上に形成されており、質量%でZnを65.00%以上含有する溶融亜鉛系めっき層と、を備える。
粗圧延機を用いて素材を粗圧延して粗バーを製造する粗圧延工程と、
仕上げ圧延機を用いて前記粗バーを仕上げ圧延して鋼板を製造し、仕上げ圧延温度FTを850~950℃とする仕上げ圧延工程と、
仕上げ圧延完了後の前記鋼板を冷却する冷却工程と、
冷却工程後の前記鋼板を470~620℃の巻取温度で巻き取る巻取工程とを備え、
前記冷却工程では、
前記仕上げ圧延が完了してから3秒以内に、冷却設備を用いた前記鋼板の冷却を開始し、
前記冷却設備で冷却を開始してから前記鋼板の温度が切替温度STに到達するまでの期間を前段冷却期間と定義し、前記切替温度STから前記鋼板の温度が巻取温度に到達するまでの期間を後段冷却期間と定義したとき、
前記前段冷却期間での冷却速度である前段冷却速度CR1を25℃/秒未満とし、
前記切替温度STを730~830℃とし、
前記後段冷却期間での冷却速度である後段冷却速度CR2を25℃/秒以上とする。
特徴1:ミクロ組織中のベイニティックフェライトの面積率が85%以上である。
特徴2:転位密度が8.0×1013~100.0×1013/m2である。
特徴3:熱延鋼板中のTi炭化物の平均円相当径が10nm以下である。
特徴4:ベイニティックフェライトの結晶粒の平均円相当径が15μm以下である。
熱延鋼板であって、
質量%で、
C:0.040~0.120%、
Si:0.01~0.60%、
Mn:0.50~1.50%、
P:0.025%以下、
S:0.010%以下、
Al:0.010~0.070%、
N:0.0070%以下、
Ti:0.055~0.200%、及び、
B:0.0010~0.0050%、を含有し、
残部はFe及び不純物からなり、
ミクロ組織において、ベイニティックフェライトの面積率は85%以上であり、
転位密度は8.0×1013~100.0×1013/m2であり、
前記熱延鋼板中のTi炭化物の平均円相当径は10nm以下であり、
前記ベイニティックフェライトの結晶粒の平均円相当径は15μm以下である、
熱延鋼板。
熱延鋼板であって、
質量%で、
C:0.040~0.120%、
Si:0.01~0.60%、
Mn:0.50~1.50%、
P:0.025%以下、
S:0.010%以下、
Al:0.010~0.070%、
N:0.0070%以下、
Ti:0.055~0.200%、及び、
B:0.0010~0.0050%、を含有し、
さらに、第1群及び第2群からなる群から選択される1種以上を含有し、残部はFe及び不純物からなり、
ミクロ組織において、ベイニティックフェライトの面積率は85%以上であり、
転位密度は8.0×1013~100.0×1013/m2であり、
前記熱延鋼板中のTi炭化物の平均円相当径は10nm以下であり、
前記ベイニティックフェライトの結晶粒の平均円相当径は15μm以下である、
熱延鋼板。
[第1群]
Nb:0.20%以下、及び、
V:0.20%以下、からなる群から選択される1種以上
[第2群]
Cr:1.0%以下、及び、
Mo:1.0%以下、からなる群から選択される1種以上
[2]に記載の熱延鋼板であって、
前記第1群を含有する、
熱延鋼板。
[2]又は[3]に記載の熱延鋼板であって、
前記第2群を含有する、
熱延鋼板。
[1]~[4]のいずれか1項に記載の熱延鋼板と、
前記熱延鋼板の表面上に形成されており、質量%でZnを65.00%以上含有する溶融亜鉛系めっき層と、を備える、
溶融めっき鋼板。
[1]~[4]のいずれか1項に記載の熱延鋼板の製造方法であって、
粗圧延機を用いて素材を粗圧延して粗バーを製造する粗圧延工程と、
仕上げ圧延機を用いて前記粗バーを仕上げ圧延して鋼板を製造し、仕上げ圧延温度FTを850~950℃とする仕上げ圧延工程と、
仕上げ圧延完了後の前記鋼板を冷却する冷却工程と、
冷却工程後の前記鋼板を470~620℃の巻取温度で巻き取る巻取工程とを備え、
前記冷却工程では、
前記仕上げ圧延が完了してから3秒以内に、冷却設備を用いた前記鋼板の冷却を開始し、
前記冷却設備で冷却を開始してから前記鋼板の温度が切替温度STに到達するまでの期間を前段冷却期間と定義し、前記切替温度STから前記鋼板の温度が巻取温度に到達するまでの期間を後段冷却期間と定義したとき、
前記前段冷却期間での冷却速度である前段冷却速度CR1を25℃/秒未満とし、
前記切替温度STを730~830℃とし、
前記後段冷却期間での冷却速度である後段冷却速度CR2を25℃/秒以上とする、
熱延鋼板の製造方法。
元素に関する「%」は、特に断りがない限り、質量%を意味する。
[化学組成]
本実施形態による熱延鋼板の化学組成は、次の元素を含有する。
炭素(C)は、Tiと結合してTi炭化物を生成する。Ti炭化物は析出強化により熱延鋼板の強度を高め、かつ、加工性を高める。Cはさらに、化学組成中のTi含有量が0.055~0.200%である場合に、ベイニティックフェライトを生成しやすくする。C含有量が0.040%未満であれば、他の元素含有量が本実施形態の範囲内であっても、高い強度が得られない。具体的には、熱延鋼板の引張強度TSが780MPa以上になりにくい。さらに、転位密度が過剰に高くなり、熱延鋼板の加工性が低下する。
一方、C含有量が0.120%を超えれば、他の元素含有量が本実施形態の範囲内であっても、ミクロ組織においてポリゴナルフェライトが生成しやすくなる。そのため、熱延鋼板のベイニティックフェライトの面積率が低下する。さらに、熱延鋼板の転位密度が低下する。さらに、ベイニティックフェライトの結晶粒の平均円相当径も粗大となる。その結果、熱延鋼板の剛性が低下する。
したがって、C含有量は0.040~0.120%である。
C含有量の好ましい下限は0.042%であり、さらに好ましくは0.044%であり、さらに好ましくは0.046%である。
C含有量の好ましい上限は0.115%であり、さらに好ましくは0.110%であり、さらに好ましくは0.105%である。
シリコン(Si)は鋼を脱酸する。Siはさらに、固溶強化により熱延鋼板の強度を高める。Si含有量が0.01%未満であれば、他の元素含有量が本実施形態の範囲内であっても、上記効果が十分に得られない。
一方、Si含有量が0.60%を超えれば、他の元素含有量が本実施形態の範囲内であっても、熱延鋼板においてポリゴナルフェライトが生成しやすくなる。そのため、熱延鋼板のベイニティックフェライトの面積率が低下する。さらに、熱延鋼板の転位密度が低下する。さらに、ベイニティックフェライトの結晶粒の平均円相当径も粗大となる。その結果、熱延鋼板の剛性が低下する。
したがって、Si含有量は0.01~0.60%である。
Si含有量の好ましい下限は0.02%であり、さらに好ましくは0.03%であり、さらに好ましくは0.04%である。
Si含有量の好ましい上限は0.55%であり、さらに好ましくは0.50%であり、さらに好ましくは0.45%である。
マンガン(Mn)は、固溶強化により熱延鋼板の強度を高める。Mn含有量が0.50%未満であれば、他の元素含有量が本実施形態の範囲内であっても、上記効果が十分に得られない。
一方、Mn含有量が1.50%を超えれば、他の元素含有量が本実施形態の範囲内であっても、熱延鋼板中にMn偏析が生じやすくなる。Mn含有量が1.50%を超えればさらに、熱延鋼板において、ベイナイトが生成しやすくなる。そのため、熱延鋼板のベイニティックフェライトの面積率が低下し、転位密度が過剰に高くなる。そのため、熱延鋼板の加工性が低下する。
したがって、Mn含有量は0.50~1.50%である。
Mn含有量の好ましい下限は0.55%であり、さらに好ましくは0.60%であり、さらに好ましくは0.65%である。
Mn含有量の好ましい上限は1.40%であり、さらに好ましくは1.30%であり、さらに好ましくは1.20%である。
リン(P)は不純物である。Pは粒界に偏析して熱延鋼板の加工性を低下する。Pはさらに、熱延鋼板の溶接性を低下する。P含有量が0.025%を超えれば、他の元素含有量が本実施形態の範囲内であっても、熱延鋼板の加工性及び溶接性が顕著に低下する。
したがって、P含有量は0.025%以下である。
P含有量はなるべく低い方が好ましい。しかしながら、P含有量の過剰な低減は、生産性を低下し、製造コストを高める。したがって、通常の工業生産を考慮した場合、P含有量の好ましい下限は0%超であり、さらに好ましくは0.001%であり、さらに好ましくは0.002%であり、さらに好ましくは0.003%である。
P含有量の好ましい上限は0.023%であり、さらに好ましくは0.020%であり、さらに好ましくは0.015%である。
硫黄(S)は不純物である。Sは結晶粒界に偏析して熱延鋼板の加工性を低下する。S含有量が0.010%を超えれば、他の元素含有量が本実施形態の範囲内であっても、熱延鋼板の加工性が顕著に低下する。
したがって、S含有量は0.010%以下である。
S含有量はなるべく低い方が好ましい。しかしながら、S含有量の過剰な低減は、生産性を低下し、製造コストを高める。したがって、通常の工業生産を考慮した場合、S含有量の好ましい下限は0%超であり、さらに好ましくは0.001%であり、さらに好ましくは0.002%であり、さらに好ましくは0.003%である。
S含有量の好ましい上限は0.009%であり、さらに好ましくは0.008%である。
アルミニウム(Al)は、鋼を脱酸する。Alはさらに、Nと結合してAl窒化物を形成する。これにより、BがNと結合するのを抑制する。Al含有量が0.010%未満であれば、他の元素含有量が本実施形態の範囲内であっても、上記効果が十分に得られない。
一方、Al含有量が0.070%を超えれば、他の元素含有量が本実施形態の範囲内であっても、粗大なAl窒化物が過剰に生成する。そのため、熱延鋼板の加工性が低下する。
したがって、Al含有量は0.010~0.070%である。
Al含有量の好ましい下限は0.012%であり、さらに好ましくは0.014%であり、さらに好ましくは0.016%である。
Al含有量の好ましい上限は0.065%であり、さらに好ましくは0.060%であり、さらに好ましくは0.055%である。
窒素(N)は不純物である。NはBと結合してBNを形成し、熱延鋼板中の固溶B量を低減する。Nはさらに、Tiと結合してTiNを形成し、Ti炭化物の形成を阻害する。N含有量が0.0070%を超えれば、他の元素含有量が本実施形態の範囲内であっても、BN及びTiNが過剰に生成する。その結果、熱延鋼板の耐LME性が低下する。さらに、熱延鋼板の強度も低下する。
したがって、N含有量は0.0070%以下である。
N含有量はなるべく低い方が好ましい。しかしながら、N含有量の過剰な低減は、生産性を低下し、製造コストを高める。したがって、通常の工業生産を考慮した場合、N含有量の好ましい下限は0%超であり、さらに好ましくは0.0001%であり、さらに好ましくは0.0005%であり、さらに好ましくは0.0010%である。
N含有量の好ましい上限は0.0060%であり、さらに好ましくは0.0050%であり、さらに好ましくは0.0040%である。
チタン(Ti)はCと結合してTi炭化物を形成する。Ti炭化物は、析出強化により、熱延鋼板の強度を高める。さらに、C含有量が0.040~0.120%である場合、Tiが適切な含有量であれば、熱延鋼板中にベイニティックフェライトが生成しやすくなる。Ti含有量が0.055%未満であれば、他の元素含有量が本実施形態の範囲内であっても、ポリゴナルフェライトが生成しやすくなる。そのため、熱延鋼板のベイニティックフェライトの面積率が低下し、さらに、熱延鋼板の転位密度も低下する。さらに、ベイニティックフェライトの結晶粒の平均円相当径が粗大となる。その結果、熱延鋼板の剛性が低下する。
一方、Ti含有量が0.200%を超えれば、他の元素含有量が本実施形態の範囲内であっても、熱延鋼板中の転位密度が過剰に高くなる。その結果、熱延鋼板の加工性が低下する。
したがって、Ti含有量は0.055~0.200%である。
Ti含有量の好ましい下限は0.060%であり、さらに好ましくは0.065%であり、さらに好ましくは0.070%であり、さらに好ましくは0.075%であり、さらに好ましくは0.080%であり、さらに好ましくは0.085%である。
Ti含有量の好ましい上限は0.190%であり、さらに好ましくは0.180%であり、さらに好ましくは0.170%である。
ボロン(B)は熱延鋼板に固溶して、旧オーステナイト粒界に偏析する。偏析したBは、粒界強度を高める。そのため、Bは、熱延鋼板の耐LME性を高める。Bはさらに、鋼の焼入れ性を高める。B含有量が0.0010%未満であれば、他の元素含有量が本実施形態の範囲内であっても、熱延鋼板の耐LME性が十分に得られない。さらに、焼入れ性が不足するため、転位密度が低下する。さらに、ベイニティックフェライトの面積率が低下する。さらに、オーステナイトからフェライトへの変態開始温度が高まる。この場合、Ti炭化物の析出開始温度も高まる。そのため、Ti炭化物が粗大になる。その結果、熱延鋼板の強度が低下し、剛性も低下する。
一方、B含有量が0.0050%を超えれば、他の元素含有量が本実施形態の範囲内であっても、焼入れ性が過剰に高くなる。この場合、熱延鋼板の転位密度が過剰に高くなる。さらに、ベイニティックフェライトの面積率が低下する。その結果、鋼板の加工性が低下する。B含有量が0.0050%を超えればさらに、耐LME性が低下する。
したがって、B含有量は0.0010~0.0050%である。
B含有量の好ましい下限は0.0015%であり、さらに好ましくは0.0020%であり、さらに好ましくは0.0025%である。
B含有量の好ましい上限は0.0045%であり、さらに好ましくは0.0040%であり、さらに好ましくは0.0035%である。
本実施形態の熱延鋼板の化学組成はさらに、Feの一部に代えて、第1群及び第2群からなる群から選択される1種以上を含有してもよい。
[第1群]
Nb:0.20%以下、及び、
V:0.20%以下、からなる群から選択される1種以上
[第2群]
Cr:1.0%以下、及び、
Mo:1.0%以下、からなる群から選択される1種以上
これらはいずれも任意元素である。以下、第1群及び第2群について説明する。
本実施形態の熱延鋼板は、Feの一部に代えて、第1群を含有してもよい。これらの元素はいずれも、Cと結合して炭化物を形成し、熱延鋼板の強度を高める。以下、各元素について説明する。
ニオブ(Nb)は任意元素であり、含有されなくてもよい。つまり、Nb含有量は0%であってもよい。
含有される場合、つまり、Nb含有量が0%超である場合、NbはCと結合してNb炭化物を形成する。Nb炭化物は、析出強化により熱延鋼板の強度を高める。Nbが少しでも含有されれば、上記効果がある程度得られる。
しかしながら、Nb含有量が0.20%を超えれば、他の元素含有量が本実施形態の範囲内であっても、Nb炭化物が過剰に生成する。この場合、熱延鋼板の加工性が低下する。
したがって、Nb含有量は0~0.20%であり、含有される場合、Nb含有量は0.20%以下である。
Nb含有量の好ましい下限は0.01%であり、さらに好ましくは0.05%であり、さらに好ましくは0.08%である。
Nb含有量の好ましい上限は0.18%であり、さらに好ましくは0.16%であり、さらに好ましくは0.14%である。
バナジウム(V)は任意元素であり、含有されなくてもよい。つまり、V含有量は0%であってもよい。
含有される場合、つまり、V含有量が0%超である場合、VはCと結合してV炭化物を形成する。V炭化物は、析出強化により熱延鋼板の強度を高める。Vが少しでも含有されれば、上記効果がある程度得られる。
しかしながら、V含有量が0.20%を超えれば、他の元素含有量が本実施形態の範囲内であっても、V炭化物が過剰に生成する。この場合、熱延鋼板の加工性が低下する。
したがって、V含有量は0~0.20%であり、含有される場合、V含有量は0.20%以下である。
V含有量の好ましい下限は0.01%であり、さらに好ましくは0.05%であり、さらに好ましくは0.08%である。
V含有量の好ましい上限は0.18%であり、さらに好ましくは0.16%であり、さらに好ましくは0.14%である。
本実施形態の熱延鋼板は、Feの一部に代えて、第2群を含有してもよい。これらの元素はいずれも、熱延鋼板の耐LME性を高める。以下、各元素について説明する。
クロム(Cr)は任意元素であり、含有されなくてもよい。つまり、Cr含有量は0%であってもよい。
含有される場合、つまり、Cr含有量が0%超である場合、Crは旧オーステナイト粒界に偏析して、熱延鋼板の耐LME性を高める。Crが少しでも含有されれば、上記効果がある程度得られる。
しかしながら、Cr含有量が1.0%を超えれば、他の元素含有量が本実施形態の範囲内であっても、熱延鋼板の加工性が低下する。
したがって、Cr含有量は0~1.0%であり、含有される場合、Cr含有量は1.0%以下である。
Cr含有量の好ましい下限は0.1%であり、さらに好ましくは0.2%であり、さらに好ましくは0.3%である。
Cr含有量の好ましい上限は0.9%であり、さらに好ましくは0.8%であり、さらに好ましくは0.7%である。
モリブデン(Mo)は任意元素であり、含有されなくてもよい。つまり、Mo含有量は0%であってもよい。含有される場合、つまり、Mo含有量が0%超である場合、Moは旧オーステナイト粒界に偏析して、熱延鋼板の耐LME性を高める。Moが少しでも含有されれば、上記効果がある程度得られる。
しかしながら、Mo含有量が1.0%を超えれば、他の元素含有量が本実施形態の範囲内であっても、熱延鋼板の加工性が低下する。
したがって、Mo含有量は0~1.0%であり、含有される場合、Mo含有量は1.0%以下である。
Mo含有量の好ましい下限は0.1%であり、さらに好ましくは0.2%であり、さらに好ましくは0.3%である。
Mo含有量の好ましい上限は0.9%であり、さらに好ましくは0.8%であり、さらに好ましくは0.7%である。
本実施形態の熱延鋼板の化学組成は、JIS G0321:2017に準拠した周知の成分分析法で測定できる。具体的には、ドリル等の切削工具を用いて、熱延鋼板から切粉を採取する。採取された切粉を酸に溶解させて溶液を得る。溶液に対して、ICP-MAS(Inductively Coupled Plasma Mass Spectrometry)を実施して、化学組成の元素分析を実施する。C含有量及びS含有量については、周知の高周波燃焼法(燃焼-赤外線吸収法)により求める。N含有量については、周知の不活性ガス溶融-熱伝導度法を用いて求める。
本実施形態の鋼材のC含有量以外の他の元素含有量も同様に、測定された値に対して、本実施形態で規定された最小桁までの数値の端数を四捨五入して得られた値を、当該元素含有量とする。
なお、四捨五入とは、端数が5未満であれば切り捨て、端数が5以上であれば切り上げることを意味する。
本実施形態の熱延鋼板は、化学組成中の各元素含有量が本実施形態の範囲内であり、かつ、次の特徴1~特徴4を満たす。
特徴1:ミクロ組織中のベイニティックフェライトの面積率が85%以上である。
特徴2:転位密度は8.0×1013~100.0×1013/m2である。
特徴3:熱延鋼板中のTi炭化物の平均円相当径が10nm以下である。
特徴4:ベイニティックフェライトの結晶粒の平均円相当径が15μm以下である。
以下、各特徴について説明する。
本実施形態の熱延鋼板のミクロ組織では、ベイニティックフェライトの面積率が85%以上である。本実施形態の熱延鋼板のミクロ組織は、ベイニティックフェライト単相であってもよい。本実施形態の熱延鋼板のミクロ組織が、ベイニティックフェライトと他の相とからなる場合、他の相は例えば、ポリゴナルフェライト、パーライト、ベイナイト、及びセメンタイトからなる群から選択される1種以上である。
ベイニティックフェライトは結晶方位がわずかに異なる粒の集合体である。そのため、結晶粒内にコントラストの差が認められる。一方、ポリゴナルフェライトは粒内の結晶方位差がほとんどない組織である。そのため、結晶粒内は均一なコントラストで観察される。したがってベイニティックフェライトは、結晶方位差に起因したコントラストに基づいて、ポリゴナルフェライトと区別可能である。
ベイニティックフェライトの結晶構造は、ベイナイトの結晶構造と同様に、bcc構造である。したがって、結晶構造に基づいて、ベイニティックフェライトをベイナイトと区別することは困難である。さらに、結晶方位差に基づいて、ベイニティックフェライトをベイナイトと区別することは困難である。しかしながら、ベイニティックフェライトは、結晶粒内及び結晶粒界でのFe炭化物の有無に基づいて、ベイナイトと区別可能である。ここで、Fe炭化物とは、Feを含有する炭化物であり、例えば、セメンタイトである。
化学組成中の各元素含有量が本実施形態の範囲内である熱延鋼板のミクロ組織において、ベイニティックフェライトの面積率が85%以上であれば、他の特徴2~4を満たすことを前提として、高い強度及び高い剛性が得られる。
ベイニティックフェライトの面積率は、次の方法で求めることができる。
電界放出型走査電子顕微鏡(FE-SEM)を用いてミクロ組織観察を行う。ミクロ組織観察は、電子チャンネリングコントラスト像(ECCI:Electron Channeling Contrast Image)により行う。観察条件は、加速電圧20kV、傾斜(T)=0°、反射電子モード、とする。結晶方位の測定には電子線後方散乱回折法(EBSD:Electron Back Scatter Diffraction)を用いる。
(手順1)
15°以上の粒界に囲まれた領域を、一つの結晶粒として定義する。なお、15°以上の粒界に囲まれた領域の円相当径が1.0μm以下である場合、その領域は測定ノイズであると判断して、結晶粒とは認定しない。つまり、測定ノイズと判定された領域は対象外とする。
(手順2)
各結晶粒内の結晶方位差の平均値(Grain Average Misorientation:以下、GAM値という)を算出する。GAM値が0.5°以下の結晶粒をポリゴナルフェライトと定義する。GAM値が0.5°を超える結晶粒をベイニティックフェライトと定義する。
本実施形態の熱延鋼板ではさらに、転位密度が8.0×1013~100.0×1013/m2である。
転位密度の好ましい上限は90.0×1013/m2であり、さらに好ましくは80.0×1013/m2であり、さらに好ましくは70.0×1013/m2である。
本実施形態の熱延鋼板の転位密度は、次の方法で求めることができる。
ΔK×cosθ/λ=0.9/D+2ε×sinθ/λ (I)
ここで、式(I)中において、θ:回折角度(°)、λ:X線の波長(nm)、D:結晶子サイズ(nm)、を意味する。
ρ=14.4×ε2/b2 (II)
ここで、式(II)中において、bは体心立方構造(鉄)のバーガースベクトル(b=0.248(nm))である。
本実施形態の熱延鋼板ではさらに、熱延鋼板中のTi炭化物の平均円相当径が10nm以下である。ここで、円相当径とは、Ti炭化物の面積と同じ面積の円の直径を意味する。
Ti炭化物の平均円相当径の下限は特に限定されない。Ti炭化物の平均円相当径の好ましい下限は2nmであり、さらに好ましくは3nmであり、さらに好ましくは4nmであり、さらに好ましくは5nmである。
Ti炭化物の平均円相当径は次の方法で求めることができる。熱延鋼板の板幅中央位置から熱延鋼板の板厚のサンプルを採取する。エメリー紙を用いてサンプルの両側から研削研磨を行い、表面から板厚/4深さの位置を中心として厚さ50μmのサンプルを作成する。その後、直径3mmの円盤状サンプルを採取する。円盤状サンプルを10%過塩素酸-氷酢酸溶液中に浸漬して電解研磨を実施し、厚さ100nmの薄膜試料を作成する。
本実施形態の熱延鋼板のミクロ組織ではさらに、ベイニティックフェライトの結晶粒の平均円相当径が15μm以下である。ここで、円相当径とは、結晶粒の面積と同じ面積の円の直径を意味する。
ベイニティックフェライトの結晶粒の平均円相当径の下限は特に限定されない。ベイニティックフェライトの結晶粒の平均円相当径の好ましい下限は1μmであり、さらに好ましくは2μmであり、さらに好ましくは3μmであり、さらに好ましくは5μmである。
熱延鋼板のベイニティックフェライトの結晶粒の円相当径は、次の方法で求めることができる。上述の[ベイニティックフェライトの面積率の測定方法]に記載の方法により、ミクロ組織観察で識別されたベイニティックフェライトの各結晶粒の円相当径を求める。得られた円相当径の算術平均値を、ベイニティックフェライトの結晶粒の平均円相当径(μm)と定義する。なお、15°以上の粒界に囲まれた領域の円相当径が1.0μm以下である場合、その領域は測定ノイズであると判断して、結晶粒とは認定しない。つまり、15°以上の粒界に囲まれた領域の円相当径が1.0μm以下である場合、その領域は対象外とする。
熱延鋼板の引張強度、全伸び、及び、降伏比は、JIS Z2241:2011に準拠した引張試験により求めることができる。
降伏比YR=YS/TS
本実施形態の溶融めっき鋼板は、上述の本実施形態の熱延鋼板と、主としてZnを含有する溶融亜鉛系めっき層とを備える。溶融亜鉛系めっき層は、熱延鋼板の表面上に形成される。溶融亜鉛系めっき層は公知の構成を有する。以下、溶融亜鉛系めっき層について説明する。
上述のとおり、溶融亜鉛系めっき層は、主としてZnを含有する。具体的には、溶融亜鉛系めっき層は、質量%で65.00%以上のZnを含有する。溶融亜鉛系めっき層は、いわゆる、溶融亜鉛めっき(GI)からなる層であってもよい。溶融亜鉛めっきは、質量%で1.00%以下のZn以外の元素を含有し、残部がZnからなる。溶融亜鉛系めっき層のZn含有量が質量%で65.00%以上であれば、十分な耐食性が得られる。溶融亜鉛系めっき層のZn含有量の好ましい下限は70.00%であり、さらに好ましくは73.00%である。
溶融亜鉛系めっき層は、GI以外の化学組成を有してもよい。溶融亜鉛系めっき層の化学組成は周知の範囲でよい。溶融亜鉛系めっき層の化学組成は例えば、次の元素を含有する。
Al:0.05~35.00%
アルミニウム(Al)は易酸化元素であり、犠牲防食によって溶融亜鉛系めっき層の耐食性を高める。Al含有量が0.05~35.00%であれば、上記効果が十分に得られる。
Al含有量の好ましい下限は0.08%であり、さらに好ましくは0.10%であり、さらに好ましくは0.15%である。Al含有量の好ましい上限は33.00%であり、さらに好ましくは30.00%であり、さらに好ましくは28.00%であり、さらに好ましくは25.00%であり、さらに好ましくは23.00%であり、さらに好ましくは21.00%である。
本実施形態による溶融亜鉛系めっき層の化学組成はさらに、Znの一部に代えて、次の第1群~第7群から選択される1元素以上を含有してもよい。以下、第1群~第7群について説明する。
[第1群]Mg:30.0%以下
[第2群(Sn群)]Sn:2.00%以下、Bi:2.00%以下、及び、In:2.00%以下、からなる群から選択される1種以上
[第3群(Ca群)]Ca:3.00%以下、Y:3.00%以下、La:3.00%以下、及び、Ce:3.00%以下、からなる群から選択される1種以上
[第4群]Si:2.50%以下
[第5群(Cr群)]Cr:0.5%以下、Ti:0.5%以下、Ni:0.5%以下、Co:0.5%以下、V:0.5%以下、Nb:0.5%以下、Cu:0.5%以下、及び、Mn:0.5%以下、からなる群から選択される1種以上
[第6群]Fe:5.0%以下
[第7群(Sr群)]Sr:0.5%以下、Sb:0.5%以下、Pb:0.5%以下、及び、B:0.5%以下、からなる群から選択される1種以上
Mg:30.0%以下
マグネシウム(Mg)は任意元素であり、含有されなくてもよい。すなわち、Mg含有量は0%であってもよい。
Mgは易酸化元素であり、犠牲防食によって溶融亜鉛系めっき層の耐食性を高める。Mg含有量が少しでも含有されれば、上記効果はある程度得られる。
しかしながら、Mg含有量が高すぎれば、他の元素含有量が本実施形態の範囲内であっても、酸化ドロスが増加する。この場合、溶融めっき鋼板の外観品質が低下する。
したがって、Mg含有量は0~30.0%であり、含有される場合、Mg含有量は30.0%以下である。
Mg含有量の好ましい下限は0%超であり、さらに好ましくは0.1%であり、さらに好ましくは0.5%であり、さらに好ましくは1.0%であり、さらに好ましくは2.0%である。
Mg含有量の好ましい上限は25.0%であり、さらに好ましくは20.0%であり、さらに好ましくは15.0%であり、さらに好ましくは10.0%であり、さらに好ましくは8.0%であり、さらに好ましくは7.0%である。
Sn:2.00%以下、Bi:2.00%以下、及び、In:2.00%以下、からなる群から選択される1種以上
これらの元素は、溶融亜鉛系めっき層がMgを含有している場合、Mgと金属間化合物を形成する。その結果、溶融めっき鋼板の耐食性が高まる。Sn、Bi及びInのいずれか1種以上が少しでも含有されれば、上記効果はある程度得られる。
しかしながら、これらの元素含有量が高すぎれば、他の元素含有量が本実施形態の範囲内であっても、溶融亜鉛めっき浴の粘度が高まる。この場合、溶融めっき鋼板の外観品質が低下する。
したがって、Sn含有量は0~2.00%であり、Bi含有量は0~2.00%であり、In含有量は0~2.00%である。含有される場合、Sn含有量は2.00%以下であり、Bi含有量は2.00%以下であり、In含有量は2.00%以下である。
各元素含有量の好ましい下限は0%超であり、さらに好ましくは0.01%であり、さらに好ましくは0.05%である。
各元素含有量の好ましい上限は1.90%であり、さらに好ましくは1.80%であり、さらに好ましくは1.70%である。
Ca:3.00%以下、Y:3.00%以下、La:3.00%以下、及び、Ce:3.00%以下、からなる群から選択される1種以上
これらの元素は、溶融亜鉛系めっき層中でAl及びZnと金属間化合物を形成する。その結果、溶融めっき鋼板の耐食性を高める。これらの元素が少しでも含有されれば、上記効果はある程度得られる。
しかしながら、これらの元素含有量が高すぎれば、他の元素含有量が本実施形態の範囲内であっても、酸化ドロスが増加する。この場合、溶融めっき鋼板の外観品質が低下する。
したがって、Ca含有量は0~3.00%であり、Y含有量は0~3.00%であり、La含有量は0~3.00%であり、Ce含有量は0~3.00%である。含有される場合、Ca含有量は3.00%以下であり、Y含有量は3.00%以下であり、La含有量は3.00%以下であり、Ce含有量は3.00%以下である。
各元素含有量の好ましい下限は0%超であり、さらに好ましくは0.01%であり、さらに好ましくは0.05%であり、さらに好ましくは0.10%である。
各元素含有量の好ましい上限は2.80%であり、さらに好ましくは2.50%であり、さらに好ましくは2.00%である。
Si:2.50%以下
ケイ素(Si)は任意元素であり、含有されなくてもよい。すなわち、Si含有量は0%であってもよい。
Siは溶融めっき鋼板の耐食性を高める。Siが少しでも含有されれば、上記効果はある程度得られる。
しかしながら、Si含有量が高すぎれば、他の元素含有量が本実施形態の範囲内であっても、溶融亜鉛めっき浴の粘度が高まる。この場合、溶融めっき鋼板の外観品質が低下する。
したがって、Si含有量は0~2.50%であり、含有される場合、Si含有量は2.50%以下である。
Si含有量の好ましい下限は0%超であり、さらに好ましくは0.01%であり、さらに好ましくは0.05%であり、さらに好ましくは0.10%である。
Si含有量の好ましい上限は2.00%であり、さらに好ましくは1.50%であり、さらに好ましくは1.00%であり、さらに好ましくは0.50%である。
Cr:0.5%以下、Ti:0.5%以下、Ni:0.5%以下、Co:0.5%以下、V:0.5%以下、Nb:0.5%以下、Cu:0.5%以下、及び、Mn:0.5%以下、からなる群から選択される1種以上
これらの元素は溶融めっき鋼板の外観品質を高める。これらの元素はさらに、溶融亜鉛系めっき層中でAlと金属間化合物を形成する。その結果、溶融めっき鋼板の耐食性が高まる。これらの元素が少しでも含有されれば、上記効果はある程度得られる。
しかしながら、これらの元素含有量が高すぎれば、他の元素含有量が本実施形態の範囲内であっても、溶融亜鉛めっき浴の粘度が高まる。この場合、溶融めっき鋼板の外観品質が低下する。
したがって、Cr含有量は0~0.5%であり、Ti含有量は0~0.5%であり、Ni含有量は0~0.5%であり、Co含有量は0~0.5%であり、V含有量は0~0.5%であり、Nb含有量は0~0.5%であり、Cu含有量は0~0.5%であり、Mn含有量は0~0.5%である。含有される場合、Cr含有量は0.5%以下であり、Ti含有量は0.5%以下であり、Ni含有量は0.5%以下であり、Co含有量は0.5%以下であり、V含有量は0.5%以下であり、Nb含有量は0.5%以下であり、Cu含有量は0.5%以下であり、Mn含有量は0.5%以下である。
各元素含有量の好ましい下限は0%超であり、さらに好ましくは0.1%である。
各元素含有量の好ましい上限は0.5%未満であり、さらに好ましくは0.4%である。
Fe:5.0%以下
鉄(Fe)は任意元素であり、含有されなくてもよい。すなわち、Fe含有量は0%であってもよい。
Feは溶融亜鉛系めっき層の硬さを高め、溶融めっき鋼板の加工性を高める。Feが少しでも含有されれば、上記効果がある程度得られる。
しかしながら、Fe含有量が高すぎれば、他の元素含有量が本実施形態の範囲内であっても、溶融亜鉛系めっき層の硬さが高くなりすぎる。この場合、溶融めっき鋼板の加工性がかえって低下する。
したがって、Fe含有量は0~5.0%であり、含有される場合、Fe含有量は5.0%以下である。
Fe含有量の好ましい下限は0%超であり、さらに好ましくは0.1%であり、さらに好ましくは0.5%である。
Fe含有量の好ましい上限は4.5%であり、さらに好ましくは4.0%であり、さらに好ましくは3.5%である。
Sr:0.5%以下、Sb:0.5%以下、Pb:0.5%以下、及び、B:0.5%以下、からなる群から選択される1種以上
これらの元素は溶融亜鉛系めっき層の金属光沢を高め、溶融めっき鋼板の外観品質を高める。これらの元素が少しでも含有されれば、上記効果はある程度得られる。
しかしながら、これらの元素含有量が高すぎれば、他の元素含有量が本実施形態の範囲内であっても、酸化ドロスが増加する。この場合、溶融めっき鋼板の外観品質が低下する。
したがって、Sr含有量は0~0.5%であり、Sb含有量は0~0.5%であり、Pb含有量は0~0.5%であり、B含有量は0~0.5%である。含有される場合、Sr含有量は0.5%以下であり、Sb含有量は0.5%以下であり、Pb含有量は0.5%以下であり、B含有量は0.5%以下である。
各元素含有量の好ましい下限は0%超であり、さらに好ましくは0.1%である。
各元素含有量の好ましい上限は0.5%未満であり、さらに好ましくは0.4%である。
溶融亜鉛系めっき層の化学組成は、次の方法で求めることができる。インヒビター入りの塩酸を用いて、溶融亜鉛系めっき層を溶解させる。インヒビターは、例えば、朝日化学工業株式会社製の商品名イビットを用いることができる。上述の熱延鋼板の化学組成分析と同様に、溶解液に対して元素分析を実施する。以上の方法により、溶融亜鉛系めっき層の化学組成を求めることができる。
本実施形態による熱延鋼板の製造方法の一例を説明する。以降に説明する熱延鋼板の製造方法は、本実施形態による熱延鋼板を製造するための一例である。したがって、上述の構成を有する熱延鋼板は、以降に説明する製造方法以外の他の製造方法により製造されてもよい。しかしながら、以降に説明する製造方法は、本実施形態による熱延鋼板の製造方法の好ましい一例である。
(工程1)素材準備工程
(工程2)熱間圧延工程
(工程3)冷却工程
(工程4)巻取工程
なお、上記製造方法は、製造ライン設備を用いて実施される。製造ライン設備は、上流から下流に向かって順に、加熱炉、粗圧延機(Rougher)、仕上げ圧延機(Finisher)、冷却装置(Run-out table cooling equipment)、及び、巻取装置(Down Coiler)を備える。各設備間には、複数の搬送ロールが配列されている。
(工程2)熱間圧延工程
・仕上げ圧延温度FT:850~950℃
(工程3)冷却工程
・前段冷却速度CR1:25℃/秒未満
・前段冷却速度から後段冷却速度への切替温度ST:730~830℃
・後段冷却速度CR2:25℃/秒以上
(工程4)巻取工程
・巻取温度CT:470~620℃
以下、各工程について説明する。
素材準備工程では、化学組成中の各元素含有量が本実施形態の範囲内である素材を準備する。素材は例えば、次の方法により製造される。化学組成中の各元素含有量が本実施形態の範囲内である溶鋼を製造する。上記溶鋼を用いて、鋳造法により素材(スラブ又はインゴット)を製造する。例えば、上記溶鋼を用いて周知の連続鋳造法によりスラブを製造する。又は、上記溶鋼を用いて周知の造塊法によりインゴットを製造する。
準備された素材(スラブ又はインゴット)に対して熱間圧延を実施して、鋼板を製造する。熱間圧延工程は、素材を粗圧延して粗バー(中間鋼板)を製造する粗圧延工程と、粗バーを仕上げ圧延して鋼板を製造する仕上げ圧延工程とを含む。
・仕上げ圧延温度FT:850~950℃
仕上げ圧延温度FTが950℃を超えれば、仕上げ圧延中に鋼板中のオーステナイト粒が過剰に粗大になる。そのため、製造された熱延鋼板のベイニティックフェライトの結晶粒が粗大になってしまう。一方、仕上げ圧延温度FTが850℃未満となれば、スタンドに過剰な負荷が掛かってしまう。
仕上げ圧延温度FTが850~950℃であれば、他の製造条件を満たすことを前提として、製造された熱延鋼板中のベイニティックフェライトの平均円相当径が15μm以下となる。
冷却工程では、仕上げ圧延が完了した鋼板を、冷却装置を用いて速やかに冷却する。具体的には、仕上げ圧延後の鋼板では、生産性の観点から、例えば、仕上げ圧延が完了してから3秒以内に、冷却設備での冷却が開始される。冷却設備では、冷却媒体を用いて、鋼板を冷却する。冷却媒体は例えば水である。
・前段冷却速度CR1:25℃/秒未満
・前段冷却速度から後段冷却速度への切替温度ST:730~830℃
・後段冷却速度CR2:25℃/秒以上
冷却工程では初めに、仕上げ圧延完了後の鋼板に対して、前段冷却速度CR1で冷却する。つまり、前段冷却期間では、鋼板を前段冷却速度CR1で冷却する。前段冷却速度CR1が25℃/秒以上であれば、鋼板温度が切替温度ST(℃)となった時点で、鋼板中にオーステナイト未再結晶領域が残存している。オーステナイト未再結晶領域は、後段冷却期間において、ポリゴナルフェライトになりやすい。そのため、製造後の熱延鋼板において、ベイニティックフェライトの面積率が低くなってしまう。この場合さらに、転位密度も低くなる。
CR1=(FT-ST)/(L1/V)
前段冷却速度CR1の下限は特に限定されない。しかしながら、前段冷却速度CR1が遅すぎれば、生産効率が顕著に低下する。したがって、前段冷却速度CR1の好ましい下限は5℃/秒である。
冷却装置において、冷却速度を前段冷却速度CR1から後段冷却速度CR2に切り替えるときの鋼板温度を、切替温度ST(℃)と定義する。
切替温度STの好ましい下限は740℃であり、さらに好ましくは750℃である。
前段冷却期間で鋼板温度が低下して切替温度STとなった後、後段冷却速度CR2での冷却(後段冷却期間)が開始される。後段冷却速度CR2が25℃/秒未満であれば、後段冷却期間での冷却速度が遅すぎる。この場合、後段冷却期間中に熱延鋼板にポリゴナルフェライトが生成してしまう。その結果、熱延鋼板中のベイニティックフェライトの面積率が低くなる。さらに、転位密度が低くなる。さらに、Ti炭化物が粗大になる。
CR2=(ST-CT)/(L2/V)
後段冷却速度CR2の上限は特に限定されない。設備能力を考慮した場合、後段冷却速度CR2の好ましい上限は70℃/秒である。
巻取工程では、冷却装置を通過した鋼板を巻取装置によりコイル状に巻き取る。巻取工程では、鋼板中にTi炭化物が生成する。ここで、巻き取り開始時の鋼板の表面温度を、巻取温度CT(℃)と定義する。巻取温度CTは、Ti炭化物の平均円相当径に影響を与える。巻取温度CTはさらに、熱延鋼板のミクロ組織(ベイニティックフェライト、ポリゴナルフェライト、及び、ベイナイトの割合)にも影響を与える。そこで、巻取温度CTを次の範囲に調整する。
・巻取温度CT:470~620℃
巻取温度CTが620℃よりも高い場合、冷却工程での後段冷却期間の終了温度が高すぎる。この場合、鋼板中のミクロ組織において、オーステナイトからベイニティックフェライトへの変態が完了する前に、巻取が開始される。そのため、オーステナイトの一部がポリゴナルフェライトに変態してしまう。その結果、熱延鋼板中のベイニティックフェライトの面積率が低くなる。さらに、転位密度も低くなる。巻取温度CTが620℃よりも高ければさらに、熱延鋼板中のTi炭化物が粗大化する。
本実施形態の熱延鋼板の製造方法は、上述の工程以外の他の工程を含んでもよい。例えば、冷却工程後であって巻取工程前に、又は、巻取工程後に、調質圧延工程を実施してもよい。調質圧延工程では、熱延鋼板に対して調質圧延を実施する。調質圧延工程により、熱延鋼板の形状を調整したり、表面粗さを調整したり、降伏強度を調整したりする。上記効果を有効に得るための調質圧延工程での板厚減少率は例えば、0.1%以上である。調質圧延工程での板厚減少率の好ましい上限は3.0%である。この場合、熱延鋼板に過度な歪が導入されるのが抑制され、良好な延性、曲げ性及びフランジ性を維持できる。
本実施形態の熱延鋼板を含む、溶融めっき鋼板は、次の周知の溶融めっき処理工程を実施することにより製造できる。
溶融めっき処理工程では、熱延鋼板の表面に上述の化学組成を有する溶融亜鉛系めっき層を形成する。具体的には、めっき浴を準備する。形成される溶融亜鉛系めっき層の組成に応じて、めっき浴の組成を調整する。めっき浴に熱延鋼板を一定時間浸漬した後、熱延鋼板をめっき浴から周知の方法で引き上げる。例えば、めっき浴中にはシンクロールが配置されている。めっき浴に浸漬された熱延鋼板は、その進行方向を、シンクロールにより上方に転換される。
本実施形態の溶融めっき鋼板の製造方法は、溶融めっき処理工程以外の他の製造工程を含んでもよい。例えば、本実施形態の溶融めっき鋼板の製造方法は、溶融めっき処理工程の前に、Niプレめっき工程を含んでもよい。Niプレめっき工程では、上述の熱延鋼板に対してNiめっきを実施して、熱延鋼板の表面にNiめっき層を形成する。Niめっき層が形成された熱延鋼板に対して、溶融めっき処理工程を実施する。この場合、溶融亜鉛系めっき層の熱延鋼板に対する密着性が高まる。
例えば、試験番号1のNb含有量は、小数第三位を四捨五入したときに0%であったことを意味する。試験番号1のCr含有量は、小数第二位を四捨五入したときに0%であったことを意味する。
各試験番号の熱延鋼板に対して、次の評価試験を実施した。
(試験1)ベイニティックフェライトの面積率、及び、ベイニティックフェライトの結晶粒の平均円相当径の測定試験
(試験2)Ti炭化物の平均円相当径測定試験
(試験3)転位密度測定試験
(試験4)機械特性評価試験
(試験5)溶融めっき鋼板の耐LME性評価試験
以下、試験1~試験5について説明する。
各試験番号の熱延鋼板に対して、上述の[ベイニティックフェライトの面積率の測定方法]及び[ベイニティックフェライトの結晶粒の円相当径の測定方法]に記載の方法により、ベイニティックフェライトの面積率(%)、及び、ベイニティックフェライトの結晶粒の平均円相当径(μm)を求めた。得られたベイニティックフェライトの面積率を表3中の「BF面積率(%)」欄に示す。また、得られたベイニティックフェライトの結晶粒の平均円相当径を表3中の「BF粒径(μm)」欄に示す。
各試験番号の熱延鋼板のTi炭化物の平均円相当径を、上述の[Ti炭化物の平均円相当径の測定方法]に記載の方法により求めた。得られたTi炭化物の平均円相当径を表3中の「TiC粒径(nm)」欄に示す。
各試験番号の熱延鋼板の転位密度を、上述の[転位密度の測定方法]に記載の方法により求めた。得られた転位密度を表3中の「転位密度(×1013/m2)」欄に示す。
各試験番号の熱延鋼板の引張強度TS、降伏比YR、全伸びT.ELをJIS Z2241:2011に準拠した引張試験により求めた。
降伏比YR=YS/TS
得られた降伏強度YS(MPa)、引張強度TS(MPa)、降伏比YR(%)、全伸びT.EL(%)を、表3の「YS(MPa)」、「TS(MPa)」、「YR(%)」、「T.EL(%)」欄に示す。
[溶融めっき鋼板の製造]
溶融めっき鋼板の耐LME性を評価するために、初めに、各試験番号の熱延鋼板を用いて、溶融めっき鋼板を製造した。具体的には、各試験番号の熱延鋼板に対して、周知の溶融めっき処理を実施して、表4に示す化学組成の溶融亜鉛系めっき層を熱延鋼板の表面に形成した。表5の「めっき鋼板」の「めっき番号」欄に、各試験番号の熱延鋼板上に形成された溶融亜鉛系めっき層のめっき番号を示す。表5の「めっき鋼板」の「めっき番号」欄に示すめっき番号は、表4のめっき番号に相当する。以上の製造工程により、溶融めっき鋼板を製造した。
各試験番号の溶融めっき鋼板から100mm×75mm×板厚のサンプル鋼板を採取した。サンプル鋼板を用いて図1に示すアーク溶接を実施した。具体的には、直径20mm、長さ25mmの円柱形状のボス部材1を準備した。ボス部材1はJIS G3101:2015に規定のSS400に相当する鋼材とした。
表1~表5を参照して、試験番号1~29の熱延鋼板の化学組成中の各元素含有量は適切であった。さらに、試験番号1~29の熱延鋼板のベイニティックフェライトの面積率は85%以上であり、ベイニティックフェライトの結晶粒の平均円相当径は15μm以下であった。さらに、試験番号1~29の熱延鋼板のTi炭化物の平均円相当径は10nm以下であり、転位密度は8.0~100.0×1013/m2であった。そのため、試験番号1~29の熱延鋼板では、引張強度TSが780MPa以上であった。さらに、降伏比YRが85%以上であり、優れた剛性を示した。さらに、全伸びT.ELは14.0%以上であり、優れた加工性(延性)を示した。
Claims (6)
- 熱延鋼板であって、
質量%で、
C:0.040~0.120%、
Si:0.01~0.60%、
Mn:0.50~1.50%、
P:0.025%以下、
S:0.010%以下、
Al:0.010~0.070%、
N:0.0070%以下、
Ti:0.055~0.200%、及び、
B:0.0010~0.0050%、を含有し、
残部はFe及び不純物からなり、
ミクロ組織において、ベイニティックフェライトの面積率は85%以上であり、
転位密度は8.0×1013~100.0×1013/m2であり、
前記熱延鋼板中のTi炭化物の平均円相当径は10nm以下であり、
前記ベイニティックフェライトの結晶粒の平均円相当径は15μm以下である、
熱延鋼板。 - 熱延鋼板であって、
質量%で、
C:0.040~0.120%、
Si:0.01~0.60%、
Mn:0.50~1.50%、
P:0.025%以下、
S:0.010%以下、
Al:0.010~0.070%、
N:0.0070%以下、
Ti:0.055~0.200%、及び、
B:0.0010~0.0050%、を含有し、
さらに、第1群及び第2群からなる群から選択される1種以上を含有し、残部はFe及び不純物からなり、
ミクロ組織において、ベイニティックフェライトの面積率は85%以上であり、
転位密度は8.0×1013~100.0×1013/m2であり、
前記熱延鋼板中のTi炭化物の平均円相当径は10nm以下であり、
前記ベイニティックフェライトの結晶粒の平均円相当径は15μm以下である、
熱延鋼板。
[第1群]
Nb:0.20%以下、及び、
V:0.20%以下、からなる群から選択される1種以上
[第2群]
Cr:1.0%以下、及び、
Mo:1.0%以下、からなる群から選択される1種以上 - 請求項2に記載の熱延鋼板であって、
前記第1群を含有する、
熱延鋼板。 - 請求項2に記載の熱延鋼板であって、
前記第2群を含有する、
熱延鋼板。 - 請求項1~請求項4のいずれか1項に記載の熱延鋼板と、
前記熱延鋼板の表面上に形成されており、質量%でZnを65.00%以上含有する溶融亜鉛系めっき層と、を備える、
溶融めっき鋼板。 - 請求項1~請求項4のいずれか1項に記載の熱延鋼板の製造方法であって、
粗圧延機を用いて素材を粗圧延して粗バーを製造する粗圧延工程と、
仕上げ圧延機を用いて前記粗バーを仕上げ圧延して鋼板を製造し、仕上げ圧延温度FTを850~950℃とする仕上げ圧延工程と、
仕上げ圧延完了後の前記鋼板を冷却する冷却工程と、
冷却工程後の前記鋼板を470~620℃の巻取温度で巻き取る巻取工程とを備え、
前記冷却工程では、
前記仕上げ圧延が完了してから3秒以内に、冷却設備を用いた前記鋼板の冷却を開始し、
前記冷却設備で冷却を開始してから前記鋼板の温度が切替温度STに到達するまでの期間を前段冷却期間と定義し、前記切替温度STから前記鋼板の温度が巻取温度に到達するまでの期間を後段冷却期間と定義したとき、
前記前段冷却期間での冷却速度である前段冷却速度CR1を25℃/秒未満とし、
前記切替温度STを730~830℃とし、
前記後段冷却期間での冷却速度である後段冷却速度CR2を25℃/秒以上とする、
熱延鋼板の製造方法。
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