WO2023132342A1 - Hot-rolled steel sheet and method for producing same - Google Patents

Hot-rolled steel sheet and method for producing same Download PDF

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Publication number
WO2023132342A1
WO2023132342A1 PCT/JP2023/000071 JP2023000071W WO2023132342A1 WO 2023132342 A1 WO2023132342 A1 WO 2023132342A1 JP 2023000071 W JP2023000071 W JP 2023000071W WO 2023132342 A1 WO2023132342 A1 WO 2023132342A1
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hot
steel sheet
rolled steel
crystal grains
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PCT/JP2023/000071
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French (fr)
Japanese (ja)
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隆 安富
栄作 桜田
玄紀 虻川
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日本製鉄株式会社
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Priority to JP2023572480A priority Critical patent/JPWO2023132342A1/ja
Publication of WO2023132342A1 publication Critical patent/WO2023132342A1/en

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    • 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Definitions

  • the present invention relates to a hot-rolled steel sheet and a method for manufacturing the same.
  • This application claims priority based on Japanese Patent Application No. 2022-001424 filed in Japan on January 7, 2022, the content of which is incorporated herein.
  • the metal structure is substantially a two-phase structure of ferrite and bainite, and carbides containing Ti and Mo are dispersed and precipitated in the ferrite phase.
  • a steel plate is disclosed.
  • Patent Document 1 does not consider ductility and hole expansibility.
  • the automobile suspension parts described above are manufactured by performing multiple forming processes on steel plates. Therefore, steel sheets that are applied to automobile chassis parts are required to have excellent formability even after being subjected to a certain degree of prestrain in the preceding process.
  • multi-step forming if a rapid change in strain path occurs in the process of changing the deformation path from the previous process to the subsequent process, deformation-induced transformation of retained austenite tends to progress rapidly, and the retained austenite fraction decreases. As a result, the work hardening rate in the latter stage of deformation in the subsequent forming process tends to decrease, and the inherent formability of the hot-rolled steel sheet may not be exhibited.
  • the present invention has been made in view of the above circumstances, and aims to provide a hot-rolled steel sheet having high strength, excellent ductility and hole expansibility, and excellent formability after prestraining, and a method for producing the same. aim.
  • the inventors of the present invention have obtained the following findings as a result of creative studies on the method for obtaining the above-mentioned hot-rolled steel sheet and its manufacturing method.
  • the uniform elongation does not decrease significantly even during deformation in which the deformation path changes rapidly.
  • the angle ⁇ between the longitudinal direction of the grains of retained austenite, fresh martensite, and tempered martensite and the rolling direction should be concentrated in a specific direction. It was found that it is important that the
  • the hot-rolled steel sheet according to one aspect of the present invention has a chemical composition, in mass%, C: 0.11 to 0.23%, Si: 0.70 to 1.80%, Mn: 1.95-3.10%, P: 0.060% or less, S: 0.005% or less, Al: 0.010 to 0.430%, N: 0.0070% or less, Ti: 0.006-0.055%, Nb: 0.006 to 0.040%, B: 0.0001 to 0.0030%, Cr: 0-0.470%, Mo: 0-0.120%, V: 0 to 0.10%, Cu: 0-0.40%, Ni: 0 to 0.30%, Ca: 0 to 0.0200%, Mg: 0-0.0200%, REM: 0 to 0.1000%, Bi: 0 to 0.020%, Total of Zr, Co, Zn and W: 0 to 1.00%, and Sn: 0 to 0.05%, The balance consists of Fe and impurities
  • the area ratio of the tempered martensite out of the total area ratio of the fresh martensite and the tempered martensite is 80.0% as a percentage. or more.
  • a method for manufacturing a hot-rolled steel sheet according to another aspect of the present invention is the method for manufacturing a hot-rolled steel sheet according to (1) above, A heating step of holding a slab having the chemical composition described in (1) above in a temperature range of 1220 to 1300° C. for 40 minutes or more; A rough rolling step in which rough rolling is performed so that each rolling reduction in the first to third passes is 10 to 30% and each rolling reduction in the fourth and subsequent passes is 15 to 50%; Perform the final pass rolling at a temperature range of 940 to 1020 ° C. and a reduction rate of more than 25%, or perform rolling one pass before the final pass at a temperature range of 940 to 1020 ° C.
  • CT in the above formula (1) is the cooling stop temperature.
  • the hot-rolled steel sheet according to the present embodiment has a chemical composition in mass% of C: 0.11 to 0.23%, Si: 0.70 to 1.80%, Mn: 1.95 to 3.10%. , P: 0.060% or less, S: 0.005% or less, Al: 0.010 to 0.430%, N: 0.0070% or less, Ti: 0.006 to 0.055%, Nb: 0 0.006-0.040%, B: 0.0001-0.0030%, and the balance: containing Fe and impurities.
  • C 0.11 to 0.23%
  • Si 0.70 to 1.80%
  • Mn 1.95 to 3.10%.
  • P 0.060% or less
  • S 0.005% or less
  • Al 0.010 to 0.430%
  • N 0.0070% or less
  • Nb 0 0.006-0.040%
  • B 0.0001-0.0030%
  • balance containing Fe and impurities.
  • C 0.11-0.23%
  • C is an element necessary for obtaining the desired tensile strength of the hot-rolled steel sheet. Desired tensile strength cannot be obtained as C content is less than 0.11%. Therefore, the C content is made 0.11% or more.
  • the C content is preferably 0.12% or more, 0.13% or more, or 0.15% or more.
  • the C content is made 0.23% or less.
  • the C content is preferably 0.21% or less, more preferably 0.19% or less.
  • Si 0.70-1.80% Si is an element that stabilizes retained austenite. If the Si content is less than 0.70%, the desired amount of retained austenite cannot be obtained, and the ductility of the hot-rolled steel sheet deteriorates. Therefore, the Si content is set to 0.70% or more.
  • the Si content is preferably 0.90% or more or 1.00% or more, more preferably 1.10% or more. On the other hand, when the Si content exceeds 1.80%, the amount of retained austenite becomes too large, and the hole expansibility of the hot-rolled steel sheet deteriorates. Therefore, the Si content is set to 1.80% or less.
  • the Si content is preferably 1.60% or less, more preferably 1.50% or less.
  • Mn 1.95-3.10%
  • Mn is an element necessary for improving the strength of the hot-rolled steel sheet. If the Mn content is less than 1.95%, the ferrite area ratio becomes too high and the desired tensile strength cannot be obtained. Therefore, the Mn content is set to 1.95% or more.
  • the Mn content is preferably 2.00% or more, more preferably 2.10% or more.
  • the Mn content exceeds 3.10%, the strength of the hot-rolled steel sheet becomes too high and the ductility of the hot-rolled steel sheet deteriorates. Therefore, the Mn content is set to 3.10% or less.
  • the Mn content is preferably 3.00% or less, 2.80% or less, and more preferably 2.60% or less.
  • P 0.060% or less
  • P is an element that segregates in the thickness central portion of the hot-rolled steel sheet.
  • P is also an element that embrittles the weld zone. If the P content exceeds 0.060%, slab cracking is likely to occur, making casting difficult. Therefore, the P content should be 0.060% or less.
  • the P content is preferably 0.020% or less, more preferably 0.010% or less. The lower the P content is, the more preferable it is, and 0% is preferable. Therefore, the P content may be 0.0005% or more.
  • S 0.005% or less
  • S is an element that embrittles the slab by existing as a sulfide.
  • S is also an element that deteriorates the formability of the hot-rolled steel sheet. If the S content exceeds 0.005%, the hole expansibility of the hot-rolled steel sheet deteriorates. Therefore, the S content is made 0.005% or less.
  • the S content is preferably 0.004% or less, more preferably 0.003% or less. The lower the S content, the better, preferably 0%. Therefore, the S content may be 0.0005% or more.
  • Al 0.010-0.430%
  • Al is an element that acts as a deoxidizing agent and improves the cleanliness of steel. If the Al content is less than 0.010%, a sufficient deoxidizing effect cannot be obtained, and a large amount of inclusions (oxides) are formed in the steel sheet. Such inclusions deteriorate the formability of the hot-rolled steel sheet. Therefore, the Al content is set to 0.010% or more.
  • the Al content is preferably 0.020% or more, more preferably 0.030% or more. On the other hand, if the Al content exceeds 0.430%, casting becomes difficult. Therefore, the Al content is set to 0.430% or less.
  • the Al content is preferably 0.400% or less, 0.300% or less, and more preferably 0.100% or less.
  • N 0.0070% or less
  • N is an element that forms coarse nitrides in steel and deteriorates the hole expansibility of the hot rolled steel sheet. If the N content exceeds 0.0070%, the hole expansibility of the hot-rolled steel sheet deteriorates. Moreover, when a large amount of N is contained, the risk of slab cracking increases. Therefore, the N content is set to 0.0070% or less.
  • the N content is preferably 0.0050% or less or 0.0040% or less, more preferably 0.0035% or less. The lower the N content is, the more preferable it is, preferably 0%. Therefore, the N content may be 0.0005% or more.
  • Ti 0.006-0.055%
  • Ti is an element that increases the strength of a hot-rolled steel sheet by forming fine nitrides in the steel. Desired tensile strength cannot be obtained as Ti content is less than 0.006%. Therefore, the Ti content is set to 0.006% or more.
  • the Ti content is preferably 0.010% or more, 0.020% or more, or 0.025% or more.
  • the Ti content exceeds 0.055%, the hole expansibility of the hot-rolled steel sheet deteriorates.
  • the Ti content should be 0.055% or less.
  • the Ti content is preferably 0.050% or less, more preferably 0.045% or less.
  • Nb 0.006-0.040%
  • Nb is an element that suppresses abnormal grain growth of austenite grains during hot rolling.
  • Nb is also an element that increases the strength of the hot-rolled steel sheet by forming fine carbides. If the Nb content is less than 0.006%, the prior austenite grains cannot be refined, and the hole expansibility of the hot rolled steel sheet deteriorates. Therefore, the Nb content is made 0.006% or more.
  • the Nb content is preferably 0.010% or more or 0.013% or more, more preferably 0.015% or more.
  • the Nb content exceeds 0.040%, the hole expansibility of the hot-rolled steel sheet deteriorates. Therefore, the Nb content is set to 0.040% or less.
  • the Nb content is preferably 0.035% or less, more preferably 0.030% or less.
  • B 0.0001 to 0.0030%
  • B is an element that suppresses the formation of ferrite in the cooling process and increases the strength of the hot-rolled steel sheet. If the B content is less than 0.0001%, the desired tensile strength cannot be obtained. Therefore, the B content is made 0.0001% or more.
  • the B content is preferably 0.0002% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if the B content exceeds 0.0030%, the hot deformation resistance increases, making hot rolling difficult. Therefore, the B content is set to 0.0030% or less.
  • the B content is preferably 0.0025% or less.
  • the remainder of the chemical composition of the steel sheet according to this embodiment may be Fe and impurities.
  • impurities refers to ores used as raw materials, scraps, or impurities that are mixed in from the manufacturing environment or the like, or impurities that are allowed within a range that does not adversely affect the steel sheet according to the present embodiment.
  • the steel sheet according to the present embodiment may contain the following arbitrary elements instead of part of Fe.
  • the lower limit of the content is 0% when the optional element is not included. Each arbitrary element will be described below.
  • Cr 0-0.470% Cr is an element that exhibits effects similar to those of Mn.
  • the Cr content is preferably 0.001% or more in order to reliably obtain the effect of increasing the strength of the hot-rolled steel sheet by containing Cr.
  • the Cr content is set to 0.470% or less.
  • Mo is an element that increases the strength of a hot-rolled steel sheet by forming fine carbides in the steel. In order to reliably obtain this effect, the Mo content is preferably 0.001% or more. On the other hand, if the Mo content exceeds 0.120%, the hole expansibility of the hot-rolled steel sheet deteriorates. Therefore, Mo content shall be 0.120% or less.
  • V 0-0.10%
  • V is an element that increases the strength of the hot-rolled steel sheet by forming fine carbides in the steel.
  • the V content is preferably 0.01% or more.
  • the V content is set to 0.10% or less.
  • Cu 0-0.40%
  • the Cu content is preferably 0.01% or more in order to more reliably obtain the effects of the above action.
  • the Cu content is set to 0.40% or less.
  • Ni 0-0.30%
  • Ni has the effect of increasing the hardenability of the steel sheet and increasing the strength of the hot-rolled steel sheet.
  • Ni has the effect of effectively suppressing intergranular cracking of the slab caused by Cu.
  • the Ni content is preferably 0.02% or more. Since Ni is an expensive element, it is economically unfavorable to contain a large amount of Ni. Therefore, the Ni content should be 0.30% or less.
  • REM refers to a total of 17 elements consisting of Sc, Y and lanthanides, and the content of REM above refers to the total content of these elements.
  • lanthanides they are industrially added in the form of mischmetals.
  • Bi 0-0.020%
  • Bi has the effect of increasing the formability of the hot-rolled steel sheet by refining the solidified structure.
  • the Bi content is preferably 0.0005% or more.
  • the Bi content is set to 0.020% or less.
  • the Bi content is preferably 0.010% or less.
  • the chemical composition of the hot-rolled steel sheet described above can be analyzed using a spark discharge emission spectrometer or the like.
  • C and S values identified by burning in an oxygen stream and measuring by an infrared absorption method using a gas component analyzer or the like are adopted.
  • N a value identified by melting a test piece taken from a hot-rolled steel sheet in a helium stream and measuring it by a thermal conductivity method is adopted.
  • the hot-rolled steel sheet according to the present embodiment has a metal structure in terms of area %, bainite: 25.0% or more, total of fresh martensite and tempered martensite: 70.0% or less, retained austenite: 4.0 to 12.0%, ferrite: 5.0% or less, pearlite: 3.0% or less, the average grain size of prior austenite grains is 25.0 ⁇ m or less, and the retained austenite, the fresh martensite and the
  • the angle formed by the major axis direction of the tempered martensite crystal grains and the rolling direction is ⁇
  • the ratio of the crystal grains where ⁇ is 0 to 15°
  • the crystal grains where ⁇ is more than 15° and 30° or less
  • the proportion of the crystal grains that are more than 30° and 45° or less the proportion of the crystal grains that are more than 45° and 60° or less
  • the proportion of the crystal grains that are more than 60° and 75° or less and the proportion of the
  • the metallographic structure is defined at the position of 1/4 of the thickness of the hot-rolled steel sheet (the region from 1/8 of the thickness from the surface to 3/8 of the thickness from the surface). The reason is that the metallographic structure at this position shows the typical metallographic structure of the steel plate.
  • Area ratio of bainite 25.0% or more Bainite is a structure that increases the strength, ductility, and expansibility of the hot-rolled steel sheet. If the area ratio of bainite is less than 25.0%, desired strength, ductility and/or hole expansibility cannot be obtained. Therefore, the area ratio of bainite is set to 25.0% or more. Preferably, it is 30.0% or more, 40.0% or more, 55.0% or more, 60.0% or more, or 65.0% or more. Although the upper limit of the area ratio of bainite is not particularly limited, it may be 96.0% or less in view of the relationship with the area ratio of retained austenite. The area ratio of bainite may be 90.0% or less or 85.0% or less.
  • Total area ratio of fresh martensite and tempered martensite 70.0% or less
  • Fresh martensite and tempered martensite are effective structures for increasing the strength of hot-rolled steel sheets. However, if the total area ratio of these structures exceeds 70.0%, the ductility and hole expansibility of the hot-rolled steel sheet deteriorate. Therefore, the total area ratio of fresh martensite and tempered martensite is set to 70.0% or less. Preferably, it is 40.0% or less, 20.0% or less, or 10.0% or less. In addition, it is not necessary to contain both fresh martensite and tempered martensite, and only one of them may be contained. In addition, the total area ratio of fresh martensite and tempered martensite may be 0%, 1.0% or more, or 3.0% or more.
  • the ratio of the area ratio of tempered martensite 80.0% or more in percentage
  • the ratio of the area ratio of the reverted martensite is preferably 80.0% or more in percentage.
  • Area ratio of retained austenite 4.0 to 12.0% Retained austenite is a structure that increases the ductility of hot-rolled steel sheets. If the area ratio of retained austenite is less than 4.0%, desired ductility may not be obtained, or excellent formability may not be obtained after prestraining. Therefore, the area ratio of retained austenite is set to 4.0% or more. Preferably, it is 5.0% or more or 6.0% or more. On the other hand, if the area ratio of retained austenite exceeds 12.0%, the hole expansibility of the hot-rolled steel sheet deteriorates. Therefore, the area ratio of retained austenite is set to 12.0% or less. Preferably, it is 11.0% or less, 10.0% or less, or 8.0% or less.
  • the area ratio of ferrite is set to 5.0% or less. It is preferably 3.0% or less or 2.0% or less, and may be 0.0%.
  • the area ratio of pearlite is set to 3.0% or less. It is preferably 2.0% or less or 1.0% or less, and may be 0.0%.
  • the method for measuring the area ratio of each tissue will be described below. From the hot-rolled steel sheet, in the cross section parallel to the rolling direction, the position of 1/4 of the plate thickness from the surface (1/8 depth of the plate thickness from the surface to 3/8 depth of the plate thickness from the surface) and width A test piece is taken so that the metal structure can be observed at the direction center position.
  • Crystallographic orientation information is obtained by electron backscatter diffractometry.
  • an EBSD apparatus composed of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used.
  • the degree of vacuum in the EBSD apparatus is 9.6 ⁇ 10 ⁇ 5 Pa or less
  • the acceleration voltage is 15 kV
  • the irradiation current level is 13
  • the electron beam irradiation level is 62.
  • the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device, a region with a crystal structure of fcc is identified, and the region of this region is identified. Calculate the area ratio. Thereby, the area ratio of retained austenite is obtained.
  • the maximum value of the "Grain Average IQ" of the ferrite region under the condition that the 15° grain boundary is defined as the grain boundary in the remaining region (the region where the "Grain Orientation Spread" exceeds 1°) is I ⁇ .
  • a region of more than I ⁇ /2 is extracted as bainite, and a region of I ⁇ /2 or less is extracted as “pearlite, fresh martensite and tempered martensite”.
  • the area ratio of the extracted bainite is obtained.
  • the GAM "Grain Average Misorientation" function is used for the same field of view, and the “Grain Average Misorientation” is set to 0.5 under the condition that the 5° grain boundary is regarded as the grain boundary.
  • a region of more than 50° and 0.75° or less is extracted as bainite, and a region of more than 0.75° is extracted as "pearlite, fresh martensite and tempered martensite”.
  • pearlite, fresh martensite and tempered martensite By calculating the area ratio of the extracted bainite, the area ratio of bainite is obtained.
  • the perlite, fresh martensite and tempered martensite are distinguished by the following method.
  • a method such as buffing using alumina particles with a particle size of 0.1 ⁇ m or less or Ar ion sputtering may be used.
  • Average grain size of prior austenite grains 25.0 ⁇ m or less
  • the average grain size of prior austenite grains is set to 25.0 ⁇ m or less. It is preferably 20.0 ⁇ m or less and 15.0 ⁇ m or less. The smaller the average grain size of the prior austenite grains, the better the hole expansibility of the hot-rolled steel sheet. Therefore, the average grain size of the prior austenite grains may be 7.0 ⁇ m or more.
  • Measurement method of the grain size of prior austenite grains It is a thickness cross section perpendicular to the rolling direction of the hot-rolled steel sheet, and the position of 1/4 of the plate thickness from the surface (1/8 depth of the plate thickness from the surface to 3/3 of the plate thickness The sample is taken so that the 8-deep region) can be observed.
  • the texture of the thickness cross-section is revealed by an etchant in which a sodium dodecylbenzenesulfonate etchant is added to a picric acid saturated aqueous solution.
  • the grain size of the prior austenite grains is measured from metallographic photographs taken at three locations within a range of 200 ⁇ m in the direction perpendicular to the rolling direction. An equivalent circle diameter is calculated for one of the prior austenite grains included in each observation field.
  • the old austenite grains where the entire old austenite grains are not included in the imaging field such as the edge of the imaging field, perform the above operation for all the old austenite grains contained in each observation field, and all the old austenite grains in each imaging field Obtain the circle-equivalent diameter of the austenite grains.
  • the average grain size of the prior austenite grains is obtained.
  • the ratio of the crystal grains where ⁇ is 0 to 15 °, more than 15 ° 30
  • the ratio of the crystal grains that are 30° to 45°, the ratio of the crystal grains that are 45° to 60°, and the crystal grains that are 60° to 75° and the proportion of the grains that are more than 75° and 90° or less 40% or less in terms of percentage respectively
  • the long axis directions of the grains of retained austenite, fresh martensite and tempered martensite are concentrated in a specific direction Then, due to deformation in a plurality of steps, deformation-induced transformation of retained austenite tends to progress, and the retained austenite fraction tends to decrease.
  • the long axis direction of the crystal grains is appropriately dispersed.
  • the lower limit is not particularly limited, it may be 0% or more, 10% or more, or 15%
  • the above crystal grain ratio is measured by the following method.
  • the 1/4 position of the plate thickness from the surface (1/8 depth of the plate thickness from the surface to 3/8 depth of the plate thickness from the surface) and the plate Retained austenite, fresh martensite and tempered martensite are identified in the metallographic structure at the center position in the width direction.
  • the grains of retained austenite, fresh martensite and tempered martensite are approximated by ellipsoids to determine the direction of their long axes.
  • An angle ⁇ (0 to 90°) formed by the longitudinal direction and the rolling direction of the hot-rolled steel sheet is calculated.
  • the resulting ⁇ was 0 to 15°, 15° to 30°, 30° to 45°, 45° to 60°, 60° to 75°, and 75° to 90° at intervals of 15°. Subdivide and create a histogram. By determining the existence ratio of each section as a percentage, the crystal grain ratio in each section is obtained.
  • a method of approximating the grains of retained austenite, fresh martensite, and tempered martensite with ellipsoids will be described.
  • FIG. 1 by approximating with an ellipsoid so that the sum of the area S out of the crystal grain region not included in the ellipsoid and the area S in of the region other than the crystal grain in the ellipsoid is minimized , x0, y0, a, b, ⁇ . If the major axis length a of the ellipsoid is 1 ⁇ m or less, the crystal grains are not included in the measurement. If the value (a/b) obtained by dividing the length a of the major axis of the ellipsoid by the length b of the minor axis is less than 1.1, the crystal grain is not included in the measurement.
  • the rolling direction of the hot-rolled steel sheet can be determined by the following method. First, a test piece is taken so that the thickness cross section of the hot-rolled steel sheet can be observed. After mirror-polishing the plate thickness cross-section of the sampled test piece, it is observed using an optical microscope. The observation range is the entire plate thickness, and areas with dark luminance are determined to be inclusions. Among the inclusions, the direction parallel to the direction in which the inclusions extend is determined as the rolling direction in the inclusions having a major axis length of 5 ⁇ m or more.
  • Tensile strength 1100 MPa or more
  • the hot-rolled steel sheet according to the present embodiment may have a tensile strength of 1100 MPa or more. By setting the tensile strength to 1100 MPa or more, it can be suitably applied to various automotive underbody parts.
  • the tensile strength may be 1200 MPa or higher, or 1300 MPa or higher. The higher the tensile strength, the better, but it may be 1400 MPa or less.
  • the hot-rolled steel sheet according to the present embodiment may have a uniform elongation of 5.0% or more.
  • the uniform elongation may be suitably applied to automotive underbody parts.
  • it is 5.5% or more, 6.0% or more, or 7.0% or more.
  • the upper limit is not particularly limited, it may be 20.0% or less.
  • Tensile strength and uniform elongation are measured by performing a tensile test according to JIS Z 2241:2011 using a No. 5 test piece of JIS Z 2241:2011.
  • the tensile test piece is taken at the central position in the sheet width direction, and the direction perpendicular to the rolling direction is taken as the longitudinal direction.
  • the uniform elongation means "total elongation at maximum test force" according to JIS Z 2241:2011.
  • Hole expansion rate 30% or more
  • the hot-rolled steel sheet according to the present embodiment may have a hole expansion rate of 30% or more.
  • the hole expansion ratio By setting the hole expansion ratio to 30% or more, it can be suitably applied to automotive underbody parts. Preferably, it is 35% or more, 40% or more, 45% or more or 50% or more.
  • the upper limit is not particularly defined, it may be 80% or less.
  • the hole expansion rate is measured by conducting a hole expansion test in accordance with JIS Z 2256:2020.
  • Amount of decrease in uniform elongation after prestraining 2.0% or less
  • the uniform elongation should not decrease significantly even in deformation in which the deformation path suddenly changes. is important.
  • the amount of decrease in uniform elongation when the deformation path suddenly changes is evaluated by the following method.
  • a No. 5 test piece of JIS Z 2241:2011 is taken from a hot-rolled steel sheet.
  • a tensile predeformation with a true strain of 0.03 is applied to this test piece in a direction parallel to the rolling direction. After that, small tensile test pieces are taken from the test piece so that the tensile direction is 0°, 45°, and 90° with respect to the tensile direction.
  • a tensile test is performed using these tensile test pieces to obtain uniform elongation when the tensile directions are 0° direction, 45° direction and 90° direction.
  • the amount of decrease in uniform elongation when the tensile direction is 45 ° and 90 ° relative to the uniform elongation when the tensile direction is 0 ° (uniform elongation in 0 ° direction - uniform elongation in 45 ° direction, (uniform elongation in 0° direction - uniform elongation in 90° direction) is calculated.
  • the amount of decrease in uniform elongation is 2.0% or less, it is judged that uniform elongation does not decrease significantly even in deformation in which the deformation path suddenly changes. In other words, it is judged that it has excellent moldability even after prestraining.
  • a tensile test is performed based on JISZ2241:2011.
  • the hot-rolled steel sheet according to this embodiment may be a surface-treated steel sheet by providing a plating layer on the surface for the purpose of improving corrosion resistance.
  • the plating layer may be an electroplating layer or a hot dipping layer.
  • the electroplating layer include electrogalvanizing and electroplating of Zn—Ni alloy.
  • hot-dip coating layers include hot-dip galvanizing, hot-dip galvannealing, hot-dip aluminum plating, hot-dip Zn--Al alloy plating, hot-dip Zn--Al--Mg alloy plating, and hot-dip Zn---Al--Mg--Si alloy plating. be.
  • the amount of plating deposited is not particularly limited, and may be the same as the conventional one. Further, it is possible to further improve the corrosion resistance by applying an appropriate chemical conversion treatment (for example, applying a silicate-based chromium-free chemical conversion treatment solution and drying) after plating.
  • the temperatures described below refer to the surface temperature of the slab or steel plate unless otherwise specified.
  • a preferred method for manufacturing the hot-rolled steel sheet according to the present embodiment is A heating step of holding the slab having the above chemical composition in a temperature range of 1220 to 1300° C. for 40 minutes or more; A rough rolling step in which rough rolling is performed so that each rolling reduction in the first to third passes is 10 to 30% and each rolling reduction in the fourth and subsequent passes is 15 to 50%; Perform the final pass rolling at a temperature range of 940 to 1020 ° C. and a reduction rate of more than 25%, or perform rolling one pass before the final pass at a temperature range of 940 to 1020 ° C.
  • 245 ⁇ 0.942 ⁇ CT+0.00092 ⁇ CT 2 ⁇ T ⁇ 686 ⁇ 2.38 ⁇ CT+0.0022 ⁇ CT 2 (1) is the cooling stop temperature.
  • the heating temperature is set to 1220° C. or higher. It is preferably 1240° C. or higher.
  • the heating temperature is set to 1300° C. or less. From the viewpoint of energy cost, the heating temperature is preferably 1280° C. or lower.
  • the holding time in the temperature range of 1220 to 1300° C. is set to 40 minutes or longer. It is preferably 60 minutes or more and 80 minutes or more. Although the upper limit of the retention time is not particularly limited, it may be 200 minutes or less.
  • the slab to be heated is not particularly limited except that it has the chemical composition described above.
  • An ingot casting method, a thin slab casting method, or the like may be employed instead of the continuous casting method.
  • the rolling reduction ratios of the first to third passes are set to 10% or more, and the rolling reduction ratios of the fourth and subsequent passes are set to 15% or more.
  • each rolling reduction in the first to third passes is 15% or more or 20% or more, and each rolling reduction in the fourth pass or later is 20% or more or 25% or more.
  • finish rolling is performed in a state where the prior austenite grains are non-uniform.
  • This facilitates the formation of prior austenite grains elongated in a specific direction after finish rolling.
  • Crystal grains tend to concentrate in a specific direction, and as a result, the ratio of crystal grains increases, resulting in deterioration of the hole expansibility of the hot-rolled steel sheet. Therefore, the rolling reduction ratios of the 1st to 3rd passes are set to 30% or less, and the rolling reduction ratios of the 4th and subsequent passes are set to 50% or less.
  • each rolling reduction in the first to third passes is 25% or less
  • each rolling reduction in the fourth and subsequent passes is 40% or less.
  • the rolling reduction of each pass can be expressed by ⁇ 1-(t1/t0) ⁇ 100(%), where t0 is the inlet plate thickness of each pass and t1 is the outlet plate thickness of each pass.
  • the temperature at which rough rolling is completed is not particularly limited, it is preferably 1070°C or higher from the viewpoint of hot deformation resistance. In addition, from the viewpoint of reducing flaws due to entrapment of scale, the temperature is preferably 1200° C. or lower.
  • Finish Rolling Step In the finish rolling step, if the rolling reduction and/or the rolling temperature in the following condition I or condition II are too low, recrystallization does not proceed sufficiently, and the average grain size of the prior austenite grains cannot be reduced. Also, if the rolling temperature in I or II below is too high, the prior austenite grains become coarse and the hole expandability of the hot-rolled steel sheet deteriorates. Therefore, in the finish rolling step, finish rolling is performed so as to satisfy either Condition I or Condition II.
  • Condition I Final pass rolling is performed at a temperature range of 940 to 1020° C. and a rolling reduction of more than 25%.
  • Condition II Rolling one pass before the final pass is performed at a temperature range of 940 to 1020° C. and a rolling reduction of more than 25%, and the final pass is rolled at a rolling reduction of less than 15%.
  • the rolling one pass before the final pass is preferably performed at 960°C or higher, preferably at 1020°C or lower, and preferably at a rolling reduction of 30% or higher.
  • the rolling reduction may be 60% or less.
  • the rolling one pass before the final pass is preferably performed at a rolling reduction of 30% or more, and the upper limit may be 60% or less.
  • the rolling in the final pass is preferably performed at a rolling reduction of less than 10%, and the lower limit may be 5% or more.
  • Cooling Step If the time from the completion of finish rolling to the start of cooling exceeds 2.0 seconds, grain growth of recrystallized austenite grains proceeds, and the hole expansibility of the hot-rolled steel sheet deteriorates. Therefore, cooling is started within 2.0 seconds after completion of finish rolling.
  • the time until the start of cooling means the time until the start of water cooling after rolling in the final pass of finish rolling.
  • cooling may be started immediately after the finish rolling is completed, it is necessary to inject cooling water directly below the finish rolling mill, which impairs productivity.
  • the time until the start of cooling is preferably 1.0 second or longer.
  • the cooling time from the completion of finish rolling to the temperature range of 300 to 480°C exceeds 17.0 seconds, the area ratio of ferrite increases and the ductility and hole expansibility of the hot rolled steel sheet deteriorate. Therefore, after the start of cooling, the steel sheet is cooled to a temperature range of 300 to 480° C. within 14.0 seconds after finishing rolling is completed. A shorter cooling time is more preferable, but in order to cool to a desired temperature range in a short time, it is necessary to increase the mass density, which increases the load on the cooling nozzle and impairs economic efficiency. Therefore, the cooling time to the temperature range of 300 to 480° C. is preferably 5.0 seconds or more or 7.0 seconds or more after completion of finish rolling.
  • the cooling stop temperature is set to 300° C. or higher. Preferably, it is 320°C or higher.
  • the cooling stop temperature is set to 480° C. or lower. It is preferably 460° C. or less.
  • Winding process Immediately after cooling is stopped, the material is wound into a coil. That is, the winding temperature and the cooling stop temperature are the same temperature.
  • the temperature here means the surface temperature of the steel sheet on the outermost circumference of the coil.
  • the heating amount ⁇ T is set to exceed the left side of the above equation (1).
  • the heating amount ⁇ T is set to be less than the right side of the above equation (1).
  • the time required for ⁇ T heating exceeds 30 minutes, a carbide reaction occurs near the bainite interface, which reduces the amount of retained austenite. Therefore, the time required for ⁇ T heating is preferably within 30 minutes.
  • Air cooling is preferably carried out to a temperature range of 100° C. or less in consideration of safety during coil transportation.
  • the hot-rolled steel sheet according to the present embodiment can be manufactured by the manufacturing method including the steps described above.
  • the method for manufacturing a hot-rolled steel sheet according to the present embodiment may further include the following steps.
  • the present invention is not limited to this one conditional example. Various conditions can be adopted in the present invention as long as the object of the present invention is achieved without departing from the gist of the present invention.
  • the area ratio of each structure, the average grain size of the prior austenite grains, the angle formed by the major axis direction of the crystal grains of retained austenite, fresh martensite and tempered martensite and the rolling direction are determined by the above-described method.
  • ⁇ is ⁇
  • the ratio of the crystal grains where ⁇ is 0 to 15°
  • the ratio of the crystal grains where ⁇ is more than 15° and 30° or less
  • the ratio of the crystal grains where ⁇ is more than 30° and 45° or less is 45°.
  • TM, P, etc. in Tables 5A to 5C respectively indicate the following.
  • TM tempered martensite
  • P pearlite
  • ⁇ ferrite B: bainite FM: fresh martensite
  • ⁇ r retained austenite
  • the ratio of the crystal grains where ⁇ is 0 to 15°
  • the ratio of the crystal grains where the ratio is more than 15° and 30° or less
  • the ratio of the crystal grains where ⁇ is more than 30° and 45° or less
  • the proportion of the crystal grains that are more than 45° and 60° or less the proportion of the crystal grains that are more than 60° and 75° or less
  • TM ratio The ratio of the area ratio of tempered martensite to the total area ratio of fresh martensite and tempered martensite
  • the hot-rolled steel sheet was judged to have excellent ductility and was judged to pass. On the other hand, when the uniform elongation was less than 5.0%, the hot-rolled steel sheet did not have excellent ductility and was determined to be unacceptable.
  • the hole expansion ratio was 30% or more, it was judged to be a hot-rolled steel sheet with excellent hole expansion properties and judged to be acceptable. On the other hand, when the hole expansion rate was less than 30%, the hot rolled steel sheet did not have excellent hole expandability and was determined to be unacceptable.
  • the hot rolled steel sheet When the amount of decrease in uniform elongation after prestraining was 2.0% or less, the hot rolled steel sheet was judged to have excellent formability after prestraining and was judged to pass. On the other hand, when the amount of decrease in uniform elongation after prestraining was more than 2.0%, the hot rolled steel sheet did not have excellent formability after prestraining and was judged to be unacceptable.
  • Tables 5A and 5B show that the steel sheets according to the invention examples have high strength, excellent ductility and hole expansibility, and excellent formability after prestraining.
  • the total area ratios of fresh martensite and tempered martensite among the total area ratios of fresh martensite and tempered martensite, hot-rolled steel sheets in which the area ratio of tempered martensite is 80.0% or more in terms of percentage are more excellent in hole expansion. It can be seen that the On the other hand, it can be seen that the steel sheets according to the comparative examples are inferior in at least one of the above properties.

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Abstract

A hot-rolled steel sheet according to the present invention has a specific chemical composition, while having a metal structure that contains, in area%, 25.0% or more of bainite, a total of 70.0% or less of fresh martensite and tempered martensite, 4.0% to 12.0% of residual austenite, 5.0% or less of ferrite and 3.0% or less of pearlite, with the average prior austenite grain size being 25.0 µm or less. With respect to the metal structure, if θ is the angle between the rolling direction and the respective major axis directions of the crystal grains of residual austenite, fresh martensite and tempered martensite, the proportion of the crystal grains having θ of 0° to 15°, the proportion of the crystal grains having θ of more than 15° but not more than 30°, the proportion of the crystal grains having θ of more than 30° but not more than 45°, the proportion of the crystal grains having θ of more than 45° but not more than 60°, the proportion of the crystal grains having θ of more than 60° but not more than 75° and the proportion of the crystal grains having θ of more than 75° but not more than 90° are 40% or less in percentage.

Description

熱延鋼板およびその製造方法Hot-rolled steel sheet and manufacturing method thereof
　　本発明は、熱延鋼板およびその製造方法に関する。
　本願は、２０２２年１月７日に、日本に出願された特願２０２２－００１４２４号に基づき優先権を主張し、その内容をここに援用する。 TECHNICAL FIELD The present invention relates to a hot-rolled steel sheet and a method for manufacturing the same.
This application claims priority based on Japanese Patent Application No. 2022-001424 filed in Japan on January 7, 2022, the content of which is incorporated herein.
　近年、自動車および機械部品の軽量化が進められている。部品形状を最適な形状に設計することで剛性を確保することにより、自動車および機械部品の軽量化が可能である。さらに、プレス成形部品等のブランク成形部品では、部品材料の板厚を減少させることで軽量化が可能となる。 In recent years, efforts have been made to reduce the weight of automobiles and machine parts. It is possible to reduce the weight of automobiles and machine parts by ensuring rigidity by designing the parts to have an optimum shape. Furthermore, in blank molded parts such as press molded parts, it is possible to reduce the weight by reducing the plate thickness of the part material.
　しかしながら、板厚を減少させながら静破壊強度および降伏強度などの部品の強度特性を確保しようとした場合、高強度材料を用いることが必要となる。特に、ロアアーム、トレールリンクおよびナックルなどの自動車足回り部品では、７８０ＭＰａ級超の鋼板の適用が検討され始めている。これらの自動車足回り部品は、鋼板にバーリング、伸びフランジおよび曲げ成形等を施すことで製造される。そのため、これらの自動車足回り部品に適用される鋼板は成形性、特に延性および穴広げ性に優れることが要求される。 However, when trying to secure the strength characteristics of parts such as static fracture strength and yield strength while reducing the plate thickness, it is necessary to use high-strength materials. In particular, the application of steel sheets of over 780 MPa class is beginning to be considered for automotive underbody parts such as lower arms, trail links and knuckles. These automotive underbody parts are manufactured by subjecting steel sheets to burring, stretch flanging, bending forming, and the like. Therefore, the steel sheets applied to these automobile chassis parts are required to have excellent formability, particularly excellent ductility and hole expansibility.
　例えば、特許文献１には、金属組織が実質的にフェライトとベイナイトの２相組織であり、フェライト相中にＴｉと、Ｍｏとを含む炭化物が分散析出していることを特徴とする、高強度鋼板が開示されている。 For example, in Patent Document 1, the metal structure is substantially a two-phase structure of ferrite and bainite, and carbides containing Ti and Mo are dispersed and precipitated in the ferrite phase. A steel plate is disclosed.
日本国特開２００３－３２１７２５号公報Japanese Patent Application Laid-Open No. 2003-321725
　しかしながら、特許文献１では、延性および穴広げ性について考慮されていない。 However, Patent Document 1 does not consider ductility and hole expansibility.
　上述のような自動車足回り部品は、鋼板に対して複数工程の成形を行うことで製造される。そのため、自動車足回り部品に適用される鋼板は、前工程である程度の予歪みを受けた後においても優れた成形性を有することが求められる。複数工程の成形において、前工程から後工程へ変形経路が変化する過程で、急速な歪経路の変化が生じた場合には、残留オーステナイトの加工誘起変態が急速に進展しやすく、残留オーステナイト分率が減少する。これにより、後工程の成形における変形後期の加工硬化率が低下しやすく、熱延鋼板が持つ本来の成形性を発揮することができない場合がある。 The automobile suspension parts described above are manufactured by performing multiple forming processes on steel plates. Therefore, steel sheets that are applied to automobile chassis parts are required to have excellent formability even after being subjected to a certain degree of prestrain in the preceding process. In multi-step forming, if a rapid change in strain path occurs in the process of changing the deformation path from the previous process to the subsequent process, deformation-induced transformation of retained austenite tends to progress rapidly, and the retained austenite fraction decreases. As a result, the work hardening rate in the latter stage of deformation in the subsequent forming process tends to decrease, and the inherent formability of the hot-rolled steel sheet may not be exhibited.
　本発明は上記実情に鑑みてなされたものであり、高い強度、優れた延性および穴広げ性、並びに、予歪み付与後において優れた成形性を有する熱延鋼板およびその製造方法を提供することを目的とする。 The present invention has been made in view of the above circumstances, and aims to provide a hot-rolled steel sheet having high strength, excellent ductility and hole expansibility, and excellent formability after prestraining, and a method for producing the same. aim.
　本発明者らは、上記熱延鋼板およびその製造方法を得るための方法について創意検討した結果、以下の知見を得た。 The inventors of the present invention have obtained the following findings as a result of creative studies on the method for obtaining the above-mentioned hot-rolled steel sheet and its manufacturing method.
　予歪み付与後の成形性を高めるためには、特に、変形経路が急激に変化する変形においても、一様伸びが大きく低下しないことが重要である。変形経路が変化した場合に一様伸びを低下させないためには、残留オーステナイト、フレッシュマルテンサイトおよび焼き戻しマルテンサイトの結晶粒の長軸方向と圧延方向とがなす角度θが特定の方向に集中しておらず、適度に分散していることが重要であることを知見した。 In order to improve formability after prestraining, it is especially important that the uniform elongation does not decrease significantly even during deformation in which the deformation path changes rapidly. In order not to reduce the uniform elongation when the deformation path changes, the angle θ between the longitudinal direction of the grains of retained austenite, fresh martensite, and tempered martensite and the rolling direction should be concentrated in a specific direction. It was found that it is important that the
　上述のような金属組織を得るためには、粗圧延条件および仕上げ圧延条件を厳格に制御することが有効である。 　In order to obtain the metallographic structure described above, it is effective to strictly control the conditions for rough rolling and finish rolling.
　上記知見に基づいてなされた本発明の要旨は以下の通りである。
（１）本発明の一態様に係る熱延鋼板は、化学組成が、質量％で、
Ｃ　：０．１１～０．２３％、
Ｓｉ：０．７０～１．８０％、
Ｍｎ：１．９５～３．１０％、
Ｐ　：０．０６０％以下、
Ｓ　：０．００５％以下、
Ａｌ：０．０１０～０．４３０％、
Ｎ　：０．００７０％以下、
Ｔｉ：０．００６～０．０５５％、
Ｎｂ：０．００６～０．０４０％、
Ｂ　：０．０００１～０．００３０％、
Ｃｒ：０～０．４７０％、
Ｍｏ：０～０．１２０％、
Ｖ　：０～０．１０％、
Ｃｕ：０～０．４０％、
Ｎｉ：０～０．３０％、
Ｃａ：０～０．０２００％、
Ｍｇ：０～０．０２００％、
ＲＥＭ：０～０．１０００％、
Ｂｉ：０～０．０２０％、
Ｚｒ、Ｃｏ、ＺｎおよびＷの合計：０～１．００％、並びに
Ｓｎ：０～０．０５％を含有し、
　残部がＦｅおよび不純物からなり、
　金属組織が、面積％で、
　ベイナイト：２５．０％以上、
　フレッシュマルテンサイトおよび焼き戻しマルテンサイトの合計：７０．０％以下、
　残留オーステナイト：４．０～１２．０％、
　フェライト：５．０％以下、並びに
　パーライト：３．０％以下であり、
　旧オーステナイト粒の平均粒径が２５．０μｍ以下であり、
　前記残留オーステナイト、前記フレッシュマルテンサイトおよび前記焼き戻しマルテンサイトの結晶粒の長軸方向と圧延方向とがなす角度をθとしたとき、θが、０～１５°である前記結晶粒の割合、１５°超３０°以下である前記結晶粒の割合、３０°超４５°以下である前記結晶粒の割合、４５°超６０°以下である前記結晶粒の割合、６０°超７５°以下である前記結晶粒の割合、および７５°超９０°以下である前記結晶粒の割合が百分率でそれぞれ４０％以下である。
（２）上記（１）に記載の熱延鋼板は、前記フレッシュマルテンサイトおよび前記焼き戻しマルテンサイトの面積率の合計のうち、前記焼き戻しマルテンサイトの面積率の割合が百分率で８０．０％以上であってもよい。
（３）上記（１）または（２）に記載の熱延鋼板は、前記化学組成が、質量％で、
Ｃｒ：０．００１～０．４７０％、
Ｍｏ：０．００１～０．１２０％、
Ｖ　：０．０１～０．１０％、
Ｃｕ：０．０１～０．４０％、
Ｎｉ：０．０２～０．３０％、
Ｃａ：０．０００５～０．０２００％、
Ｍｇ：０．０００５～０．０２００％、
ＲＥＭ：０．０００５～０．１０００％、および
Ｂｉ：０．０００５～０．０２０％、
からなる群のうち１種または２種以上を含有してもよい。
（４）本発明の別の態様に係る熱延鋼板の製造方法は、上記（１）に記載の熱延鋼板の製造方法であって、
　上記（１）に記載の化学組成を有するスラブを１２２０～１３００℃の温度域で４０分以上保持する加熱工程と、
　１～３パス目の各圧下率が１０～３０％、４パス目以降の各圧下率が１５～５０％となるように粗圧延を行う粗圧延工程と、
　最終パスの圧延を９４０～１０２０℃の温度域且つ２５％超の圧下率で行う、または、最終パスから１パス前の圧延を９４０～１０２０℃の温度域且つ２５％超の圧下率で行い、最終パスの圧延を１５％未満の圧下率で行う仕上げ圧延工程と、
　仕上げ圧延完了後、２．０秒以内に冷却を開始し、且つ、１７．０秒以内に３００～４８０℃の温度域に冷却する冷却工程と、
　前記温度域で巻取る巻取り工程と、
　前記巻取り工程後、３０分以内に、加熱量ΔＴが下記式（１）を満たすように加熱する再加熱工程と、
　２００℃以下の温度域まで空冷する空冷工程と、を順次行う。
　２４５－０．９４２×ＣＴ＋０．０００９２×ＣＴ２＜ΔＴ＜６８６－２．３８×ＣＴ＋０．００２２×ＣＴ２　…（１）
　ただし、上記式（１）中のＣＴは冷却停止温度である。 The gist of the present invention made based on the above knowledge is as follows.
(1) The hot-rolled steel sheet according to one aspect of the present invention has a chemical composition, in mass%,
C: 0.11 to 0.23%,
Si: 0.70 to 1.80%,
Mn: 1.95-3.10%,
P: 0.060% or less,
S: 0.005% or less,
Al: 0.010 to 0.430%,
N: 0.0070% or less,
Ti: 0.006-0.055%,
Nb: 0.006 to 0.040%,
B: 0.0001 to 0.0030%,
Cr: 0-0.470%,
Mo: 0-0.120%,
V: 0 to 0.10%,
Cu: 0-0.40%,
Ni: 0 to 0.30%,
Ca: 0 to 0.0200%,
Mg: 0-0.0200%,
REM: 0 to 0.1000%,
Bi: 0 to 0.020%,
Total of Zr, Co, Zn and W: 0 to 1.00%, and Sn: 0 to 0.05%,
The balance consists of Fe and impurities,
The metal structure, in area %,
Bainite: 25.0% or more,
Total of fresh martensite and tempered martensite: 70.0% or less,
Retained austenite: 4.0 to 12.0%,
ferrite: 5.0% or less, and pearlite: 3.0% or less,
The average grain size of the prior austenite grains is 25.0 μm or less,
When the angle formed by the long axis direction of the crystal grains of the retained austenite, the fresh martensite and the tempered martensite and the rolling direction is θ, the ratio of the crystal grains where θ is 0 to 15 °, 15 The ratio of the crystal grains that are more than 30° and 45° or less, the ratio of the crystal grains that are more than 45° and 60° or less, the ratio of the crystal grains that are more than 60° and 75° or less The percentage of crystal grains and the percentage of said crystal grains having an angle of more than 75° and 90° or less are respectively 40% or less.
(2) In the hot-rolled steel sheet according to (1) above, the area ratio of the tempered martensite out of the total area ratio of the fresh martensite and the tempered martensite is 80.0% as a percentage. or more.
(3) The hot-rolled steel sheet according to (1) or (2) above, wherein the chemical composition is, in mass%,
Cr: 0.001 to 0.470%,
Mo: 0.001 to 0.120%,
V: 0.01 to 0.10%,
Cu: 0.01 to 0.40%,
Ni: 0.02 to 0.30%,
Ca: 0.0005 to 0.0200%,
Mg: 0.0005-0.0200%,
REM: 0.0005-0.1000% and Bi: 0.0005-0.020%,
You may contain 1 type(s) or 2 or more types out of the group which consists of.
(4) A method for manufacturing a hot-rolled steel sheet according to another aspect of the present invention is the method for manufacturing a hot-rolled steel sheet according to (1) above,
A heating step of holding a slab having the chemical composition described in (1) above in a temperature range of 1220 to 1300° C. for 40 minutes or more;
A rough rolling step in which rough rolling is performed so that each rolling reduction in the first to third passes is 10 to 30% and each rolling reduction in the fourth and subsequent passes is 15 to 50%;
Perform the final pass rolling at a temperature range of 940 to 1020 ° C. and a reduction rate of more than 25%, or perform rolling one pass before the final pass at a temperature range of 940 to 1020 ° C. and a reduction rate of more than 25%, A finish rolling step in which the final pass is rolled at a rolling reduction of less than 15%;
A cooling step of starting cooling within 2.0 seconds after completion of finish rolling and cooling to a temperature range of 300 to 480 ° C. within 17.0 seconds;
A winding step of winding in the temperature range;
A reheating step of heating so that the heating amount ΔT satisfies the following formula (1) within 30 minutes after the winding step;
and an air-cooling step of air-cooling to a temperature range of 200° C. or less.
245−0.942×CT+0.00092×CT 2 <ΔT<686−2.38×CT+0.0022×CT 2 (1)
However, CT in the above formula (1) is the cooling stop temperature.
　本発明に係る上記態様によれば、高い強度、優れた延性および穴広げ性、並びに、予歪み付与後において優れた成形性を有する熱延鋼板およびその製造方法を提供することができる。 According to the above aspect of the present invention, it is possible to provide a hot-rolled steel sheet having high strength, excellent ductility and hole expansibility, and excellent formability after prestraining, and a method for producing the same.
結晶粒を楕円体で近似する方法を説明するための図である。It is a figure for demonstrating the method of approximating a crystal grain with an ellipsoid.
　以下、本実施形態に係る熱延鋼板について、詳細に説明する。ただし、本発明は本実施形態に開示の構成のみに制限されることなく、本発明の趣旨を逸脱しない範囲で種々の変更が可能である。 The hot-rolled steel sheet according to this embodiment will be described in detail below. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications can be made without departing from the gist of the present invention.
　以下に記載する「～」を挟んで記載される数値限定範囲には、下限値および上限値がその範囲に含まれる。「未満」、「超」と示す数値には、その値が数値範囲に含まれない。化学組成についての「％」は全て「質量％」のことを指す。 　The numerical limitation range described below with "~" in between includes the lower limit and the upper limit. Numerical values indicated as "less than" and "greater than" do not include the value within the numerical range. All "%" in chemical composition refer to "% by mass".
　本実施形態に係る熱延鋼板は、化学組成が、質量％で、Ｃ：０．１１～０．２３％、Ｓｉ：０．７０～１．８０％、Ｍｎ：１．９５～３．１０％、Ｐ：０．０６０％以下、Ｓ：０．００５％以下、Ａｌ：０．０１０～０．４３０％、Ｎ：０．００７０％以下、Ｔｉ：０．００６～０．０５５％、Ｎｂ：０．００６～０．０４０％、Ｂ：０．０００１～０．００３０％、並びに、残部：Ｆｅおよび不純物含む。以下、各元素について詳細に説明する。 The hot-rolled steel sheet according to the present embodiment has a chemical composition in mass% of C: 0.11 to 0.23%, Si: 0.70 to 1.80%, Mn: 1.95 to 3.10%. , P: 0.060% or less, S: 0.005% or less, Al: 0.010 to 0.430%, N: 0.0070% or less, Ti: 0.006 to 0.055%, Nb: 0 0.006-0.040%, B: 0.0001-0.0030%, and the balance: containing Fe and impurities. Each element will be described in detail below.
　Ｃ：０．１１～０．２３％
　Ｃは、熱延鋼板の所望の引張強さを得るために必要な元素である。Ｃ含有量が０．１１％未満であると、所望の引張強さを得ることができない。そのため、Ｃ含有量は０．１１％以上とする。Ｃ含有量は、好ましくは０．１２％以上、０．１３％以上または０．１５％以上である。
　一方、Ｃ含有量が０．２３％超では、マルテンサイトの面積率が過剰となり、熱延鋼板の延性および／または穴広げ性が劣化する。そのため、Ｃ含有量は０．２３％以下とする。Ｃ含有量は、好ましくは０．２１％以下であり、より好ましくは０．１９％以下である。 C: 0.11-0.23%
C is an element necessary for obtaining the desired tensile strength of the hot-rolled steel sheet. Desired tensile strength cannot be obtained as C content is less than 0.11%. Therefore, the C content is made 0.11% or more. The C content is preferably 0.12% or more, 0.13% or more, or 0.15% or more.
On the other hand, if the C content exceeds 0.23%, the martensite area ratio becomes excessive, and the ductility and/or hole expandability of the hot-rolled steel sheet deteriorates. Therefore, the C content is made 0.23% or less. The C content is preferably 0.21% or less, more preferably 0.19% or less.
　Ｓｉ：０．７０～１．８０％
　Ｓｉは、残留オーステナイトを安定化させる元素である。Ｓｉ含有量が０．７０％未満では、所望量の残留オーステナイトを得ることができず、熱延鋼板の延性が劣化する。そのため、Ｓｉ含有量は０．７０％以上とする。Ｓｉ含有量は、好ましくは０．９０％以上または１．００％以上であり、より好ましくは１．１０％以上である。
　一方、Ｓｉ含有量が１．８０％超であると、残留オーステナイト量が多くなりすぎて、熱延鋼板の穴広げ性が劣化する。そのため、Ｓｉ含有量は１．８０％以下とする。Ｓｉ含有量は、好ましくは１．６０％以下であり、より好ましくは１．５０％以下である。 Si: 0.70-1.80%
Si is an element that stabilizes retained austenite. If the Si content is less than 0.70%, the desired amount of retained austenite cannot be obtained, and the ductility of the hot-rolled steel sheet deteriorates. Therefore, the Si content is set to 0.70% or more. The Si content is preferably 0.90% or more or 1.00% or more, more preferably 1.10% or more.
On the other hand, when the Si content exceeds 1.80%, the amount of retained austenite becomes too large, and the hole expansibility of the hot-rolled steel sheet deteriorates. Therefore, the Si content is set to 1.80% or less. The Si content is preferably 1.60% or less, more preferably 1.50% or less.
　Ｍｎ：１．９５～３．１０％
　Ｍｎは、熱延鋼板の強度を向上させるために必要な元素である。Ｍｎ含有量が１．９５％未満であると、フェライトの面積率が高くなりすぎ、所望の引張強さを得ることができない。そのため、Ｍｎ含有量は１．９５％以上とする。Ｍｎ含有量は、好ましくは２．００％以上であり、より好ましくは２．１０％以上である。
　一方、Ｍｎ含有量が３．１０％超であると、熱延鋼板の強度が高くなりすぎて、熱延鋼板の延性が劣化する。そのため、Ｍｎ含有量は３．１０％以下とする。Ｍｎ含有量は、好ましくは３．００％以下、２．８０％以下であり、より好ましくは２．６０％以下である。 Mn: 1.95-3.10%
Mn is an element necessary for improving the strength of the hot-rolled steel sheet. If the Mn content is less than 1.95%, the ferrite area ratio becomes too high and the desired tensile strength cannot be obtained. Therefore, the Mn content is set to 1.95% or more. The Mn content is preferably 2.00% or more, more preferably 2.10% or more.
On the other hand, when the Mn content exceeds 3.10%, the strength of the hot-rolled steel sheet becomes too high and the ductility of the hot-rolled steel sheet deteriorates. Therefore, the Mn content is set to 3.10% or less. The Mn content is preferably 3.00% or less, 2.80% or less, and more preferably 2.60% or less.
　Ｐ：０．０６０％以下
　Ｐは、熱延鋼板の板厚中央部に偏析する元素である。またＰは、溶接部を脆化させる元素でもある。Ｐ含有量が０．０６０％超であると、スラブ割れが生じやすくなり、鋳造を行うことが困難となる。そのため、Ｐ含有量は０．０６０％以下とする。Ｐ含有量は、好ましくは０．０２０％以下であり、より好ましくは０．０１０％以下である。
　Ｐ含有量は低い程好ましく、０％であることが好ましいが、Ｐ含有量を過剰に低減すると脱Ｐコストが著しく増加する。そのため、Ｐ含有量は０．０００５％以上としてもよい。 P: 0.060% or less P is an element that segregates in the thickness central portion of the hot-rolled steel sheet. P is also an element that embrittles the weld zone. If the P content exceeds 0.060%, slab cracking is likely to occur, making casting difficult. Therefore, the P content should be 0.060% or less. The P content is preferably 0.020% or less, more preferably 0.010% or less.
The lower the P content is, the more preferable it is, and 0% is preferable. Therefore, the P content may be 0.0005% or more.
　Ｓ：０．００５％以下
　Ｓは、硫化物として存在することで、スラブを脆化させる元素である。またＳは、熱延鋼板の成形性を劣化させる元素でもある。Ｓ含有量が０．００５％超であると、熱延鋼板の穴広げ性が劣化する。そのため、Ｓ含有量は０．００５％以下とする。Ｓ含有量は、好ましくは０．００４％以下であり、より好ましくは０．００３％以下である。
　Ｓ含有量は低い程好ましく、０％であることが好ましいが、Ｓ含有量を過剰に低減すると脱Ｓコストが著しく増加する。そのため、Ｓ含有量は０．０００５％以上としてもよい。 S: 0.005% or less S is an element that embrittles the slab by existing as a sulfide. S is also an element that deteriorates the formability of the hot-rolled steel sheet. If the S content exceeds 0.005%, the hole expansibility of the hot-rolled steel sheet deteriorates. Therefore, the S content is made 0.005% or less. The S content is preferably 0.004% or less, more preferably 0.003% or less.
The lower the S content, the better, preferably 0%. Therefore, the S content may be 0.0005% or more.
　Ａｌ：０．０１０～０．４３０％
　Ａｌは、脱酸剤として作用し、鋼の清浄度を向上させる元素である。Ａｌ含有量が０．０１０％未満であると、十分な脱酸効果が得られず、鋼板中に多量の介在物（酸化物）が形成される。このような介在物は、熱延鋼板の成形性を劣化させる。そのため、Ａｌ含有量は０．０１０％以上とする。Ａｌ含有量は、好ましくは０．０２０％以上であり、より好ましくは０．０３０％以上である。
　一方、Ａｌ含有量が０．４３０％超では、鋳造が困難となる。そのため、Ａｌ含有量は、０．４３０％以下とする。Ａｌ含有量は、好ましくは０．４００％以下、０．３００％以下であり、より好ましくは０．１００％以下である。 Al: 0.010-0.430%
Al is an element that acts as a deoxidizing agent and improves the cleanliness of steel. If the Al content is less than 0.010%, a sufficient deoxidizing effect cannot be obtained, and a large amount of inclusions (oxides) are formed in the steel sheet. Such inclusions deteriorate the formability of the hot-rolled steel sheet. Therefore, the Al content is set to 0.010% or more. The Al content is preferably 0.020% or more, more preferably 0.030% or more.
On the other hand, if the Al content exceeds 0.430%, casting becomes difficult. Therefore, the Al content is set to 0.430% or less. The Al content is preferably 0.400% or less, 0.300% or less, and more preferably 0.100% or less.
　Ｎ：０．００７０％以下
　Ｎは、鋼中に粗大な窒化物を形成し、熱延鋼板の穴広げ性を劣化させる元素である。Ｎ含有量が０．００７０％超であると、熱延鋼板の穴広げ性が劣化する。また、Ｎが多量に含まれるとスラブ割れのリスクが高まる。そのため、Ｎ含有量は０．００７０％以下とする。Ｎ含有量は、好ましくは０．００５０％以下または０．００４０％以下であり、より好ましくは０．００３５％以下である。
　Ｎ含有量は低い程好ましく、０％であることが好ましいが、Ｎ含有量を過剰に低減すると脱Ｎコストが著しく増加する。そのため、Ｎ含有量は０．０００５％以上としてもよい。 N: 0.0070% or less N is an element that forms coarse nitrides in steel and deteriorates the hole expansibility of the hot rolled steel sheet. If the N content exceeds 0.0070%, the hole expansibility of the hot-rolled steel sheet deteriorates. Moreover, when a large amount of N is contained, the risk of slab cracking increases. Therefore, the N content is set to 0.0070% or less. The N content is preferably 0.0050% or less or 0.0040% or less, more preferably 0.0035% or less.
The lower the N content is, the more preferable it is, preferably 0%. Therefore, the N content may be 0.0005% or more.
　Ｔｉ：０．００６～０．０５５％
　Ｔｉは、鋼中に微細な窒化物を形成することで、熱延鋼板の強度を高める元素である。Ｔｉ含有量が０．００６％未満であると、所望の引張強さを得ることができない。そのため、Ｔｉ含有量は０．００６％以上とする。Ｔｉ含有量は、好ましくは０．０１０％以上、０．０２０％以上または０．０２５％以上である。
　一方、Ｔｉ含有量が０．０５５％超であると、熱延鋼板の穴広げ性が劣化する。また、Ｔｉが多量に含まれるとスラブ割れのリスクが高まる。そのため、Ｔｉ含有量は、０．０５５％以下とする。Ｔｉ含有量は、好ましくは０．０５０％以下であり、より好ましくは０．０４５％以下である。 Ti: 0.006-0.055%
Ti is an element that increases the strength of a hot-rolled steel sheet by forming fine nitrides in the steel. Desired tensile strength cannot be obtained as Ti content is less than 0.006%. Therefore, the Ti content is set to 0.006% or more. The Ti content is preferably 0.010% or more, 0.020% or more, or 0.025% or more.
On the other hand, if the Ti content exceeds 0.055%, the hole expansibility of the hot-rolled steel sheet deteriorates. Moreover, when a large amount of Ti is contained, the risk of slab cracking increases. Therefore, the Ti content should be 0.055% or less. The Ti content is preferably 0.050% or less, more preferably 0.045% or less.
　Ｎｂ：０．００６～０．０４０％
　Ｎｂは、熱間圧延でのオーステナイト粒の異常な粒成長を抑制する元素である。またＮｂは、微細な炭化物を形成することで熱延鋼板の強度を高める元素でもある。Ｎｂ含有量が０．００６％未満であると、旧オーステナイト粒を微細化することができず、熱延鋼板の穴広げ性が劣化する。そのため、Ｎｂ含有量は０．００６％以上とする。Ｎｂ含有量は、好ましくは０．０１０％以上または０．０１３％以上であり、より好ましくは０．０１５％以上である。
　一方、Ｎｂ含有量が０．０４０％超であると、熱延鋼板の穴広げ性が劣化する。そのため、Ｎｂ含有量は０．０４０％以下とする。Ｎｂ含有量は、好ましくは０．０３５％以下であり、より好ましくは０．０３０％以下である。 Nb: 0.006-0.040%
Nb is an element that suppresses abnormal grain growth of austenite grains during hot rolling. Nb is also an element that increases the strength of the hot-rolled steel sheet by forming fine carbides. If the Nb content is less than 0.006%, the prior austenite grains cannot be refined, and the hole expansibility of the hot rolled steel sheet deteriorates. Therefore, the Nb content is made 0.006% or more. The Nb content is preferably 0.010% or more or 0.013% or more, more preferably 0.015% or more.
On the other hand, if the Nb content exceeds 0.040%, the hole expansibility of the hot-rolled steel sheet deteriorates. Therefore, the Nb content is set to 0.040% or less. The Nb content is preferably 0.035% or less, more preferably 0.030% or less.
　Ｂ：０．０００１～０．００３０％
　Ｂは、冷却工程でのフェライトの生成を抑制し、熱延鋼板の強度を高める元素である。Ｂ含有量が０．０００１％未満であると、所望の引張強さを得ることができない。そのため、Ｂ含有量は０．０００１％以上とする。Ｂ含有量は、好ましくは、０．０００２％以上、０．０００５％以上または０．００１０％以上である。
　一方、Ｂ含有量が０．００３０％超であると、熱間変形抵抗が上昇し、熱間圧延を行うことが困難となる。そのため、Ｂ含有量は０．００３０％以下とする。Ｂ含有量は、好ましくは０．００２５％以下である。 B: 0.0001 to 0.0030%
B is an element that suppresses the formation of ferrite in the cooling process and increases the strength of the hot-rolled steel sheet. If the B content is less than 0.0001%, the desired tensile strength cannot be obtained. Therefore, the B content is made 0.0001% or more. The B content is preferably 0.0002% or more, 0.0005% or more, or 0.0010% or more.
On the other hand, if the B content exceeds 0.0030%, the hot deformation resistance increases, making hot rolling difficult. Therefore, the B content is set to 0.0030% or less. The B content is preferably 0.0025% or less.
　本実施形態に係る鋼板の化学組成の残部は、Ｆｅおよび不純物であってもよい。本実施形態において、不純物とは、原料としての鉱石、スクラップ、または製造環境等から混入されるもの、あるいは、本実施形態に係る鋼板に悪影響を与えない範囲で許容されるものを意味する。 The remainder of the chemical composition of the steel sheet according to this embodiment may be Fe and impurities. In the present embodiment, the term "impurities" refers to ores used as raw materials, scraps, or impurities that are mixed in from the manufacturing environment or the like, or impurities that are allowed within a range that does not adversely affect the steel sheet according to the present embodiment.
　本実施形態に係る鋼板は、Ｆｅの一部に代えて、以下の任意元素を含んでもよい。任意元素を含有させない場合の含有量の下限は０％である。以下、各任意元素について説明する。 The steel sheet according to the present embodiment may contain the following arbitrary elements instead of part of Fe. The lower limit of the content is 0% when the optional element is not included. Each arbitrary element will be described below.
　Ｃｒ：０～０．４７０％
　Ｃｒは、Ｍｎと類似した効果を発現する元素である。Ｃｒ含有による熱延鋼板の強度を高める効果を確実に得るためには、Ｃｒ含有量は０．００１％以上とすることが好ましい。
　一方、Ｃｒ含有量が０．４７０％超であると、マルテンサイト分率が増加し、熱延鋼板の延性が劣化する。そのため、Ｃｒ含有量は０．４７０％以下とする。 Cr: 0-0.470%
Cr is an element that exhibits effects similar to those of Mn. The Cr content is preferably 0.001% or more in order to reliably obtain the effect of increasing the strength of the hot-rolled steel sheet by containing Cr.
On the other hand, when the Cr content is more than 0.470%, the martensite fraction increases and the ductility of the hot-rolled steel sheet deteriorates. Therefore, the Cr content is set to 0.470% or less.
　Ｍｏ：０～０．１２０％
　Ｍｏは、鋼中に微細な炭化物を形成することで熱延鋼板の強度を高める元素である。この効果を確実に得るためには、Ｍｏ含有量は０．００１％以上とすることが好ましい。
　一方、Ｍｏ含有量が０．１２０％超であると、熱延鋼板の穴広げ性が劣化する。そのため、Ｍｏ含有量は０．１２０％以下とする。 Mo: 0-0.120%
Mo is an element that increases the strength of a hot-rolled steel sheet by forming fine carbides in the steel. In order to reliably obtain this effect, the Mo content is preferably 0.001% or more.
On the other hand, if the Mo content exceeds 0.120%, the hole expansibility of the hot-rolled steel sheet deteriorates. Therefore, Mo content shall be 0.120% or less.
　Ｖ：０～０．１０％
　Ｖは、鋼中に微細な炭化物を形成することで熱延鋼板の強度を高める元素である。この効果を確実に得るためには、Ｖ含有量は０．０１％以上とすることが好ましい。
　一方、Ｖ含有量が０．１０％超であると、熱延鋼板の穴広げ性が劣化する。そのため、Ｖ含有量は０．１０％以下とする。 V: 0-0.10%
V is an element that increases the strength of the hot-rolled steel sheet by forming fine carbides in the steel. In order to reliably obtain this effect, the V content is preferably 0.01% or more.
On the other hand, if the V content exceeds 0.10%, the hole expansibility of the hot-rolled steel sheet deteriorates. Therefore, the V content is set to 0.10% or less.
　Ｃｕ：０～０．４０％
　Ｃｕは、鋼板の焼入れ性を高める作用および低温で鋼中に炭化物として析出して熱延鋼板の強度を高める作用を有する。上記作用による効果をより確実に得るためには、Ｃｕ含有量は０．０１％以上とすることが好ましい。
　しかし、Ｃｕ含有量が０．４０％超では、スラブの粒界割れが生じる場合がある。したがって、Ｃｕ含有量は０．４０％以下とする。 Cu: 0-0.40%
Cu has the effect of increasing the hardenability of the steel sheet and the effect of increasing the strength of the hot-rolled steel sheet by being precipitated as carbides in the steel at low temperatures. The Cu content is preferably 0.01% or more in order to more reliably obtain the effects of the above action.
However, when the Cu content exceeds 0.40%, intergranular cracking of the slab may occur. Therefore, the Cu content is set to 0.40% or less.
　Ｎｉ：０～０．３０％
　Ｎｉは、鋼板の焼入性を高めて熱延鋼板の強度を高める作用を有する。またＮｉは、Ｃｕを含有させる場合においては、Ｃｕに起因するスラブの粒界割れを効果的に抑制する作用を有する。上記作用による効果をより確実に得るためには、Ｎｉ含有量を０．０２％以上とすることが好ましい。
　Ｎｉは、高価な元素であるため、多量に含有させることは経済的に好ましくない。したがって、Ｎｉ含有量は０．３０％以下とする。 Ni: 0-0.30%
Ni has the effect of increasing the hardenability of the steel sheet and increasing the strength of the hot-rolled steel sheet. In addition, when Cu is contained, Ni has the effect of effectively suppressing intergranular cracking of the slab caused by Cu. In order to more reliably obtain the effects of the above action, the Ni content is preferably 0.02% or more.
Since Ni is an expensive element, it is economically unfavorable to contain a large amount of Ni. Therefore, the Ni content should be 0.30% or less.
　Ｃａ：０～０．０２００％
　Ｍｇ：０～０．０２００％
　ＲＥＭ：０～０．１０００％
　Ｃａ、ＭｇおよびＲＥＭはいずれも、介在物の形状を好ましい形状に制御することにより、熱延鋼板の成形性を高める作用を有する。したがって、これらの元素の１種または２種以上を含有させてもよい。上記作用による効果をより確実に得るためには、Ｃａ、ＭｇおよびＲＥＭのいずれか１種以上を０．０００５％以上とすることが好ましい。しかし、Ｃａ含有量またはＭｇ含有量が０．０２００％を超えると、あるいはＲＥＭ含有量が０．１０００％を超えると、鋼中に介在物が過剰に生成され、熱延鋼板の延性を劣化させる場合がある。したがって、Ｃａ含有量およびＭｇ含有量をそれぞれ０．０２００％以下、ＲＥＭ含有量を０．１０００％以下とする。 Ca: 0-0.0200%
Mg: 0-0.0200%
REM: 0-0.1000%
All of Ca, Mg and REM have the effect of improving the formability of the hot-rolled steel sheet by controlling the shape of inclusions to a preferred shape. Therefore, one or more of these elements may be contained. In order to more reliably obtain the effects of the above action, it is preferable that at least one of Ca, Mg and REM is 0.0005% or more. However, when the Ca content or Mg content exceeds 0.0200%, or the REM content exceeds 0.1000%, inclusions are excessively formed in the steel, deteriorating the ductility of the hot rolled steel sheet. Sometimes. Therefore, the Ca content and Mg content are set to 0.0200% or less, respectively, and the REM content is set to 0.1000% or less.
　ここで、ＲＥＭは、Ｓｃ、Ｙおよびランタノイドからなる合計１７元素を指し、上記ＲＥＭの含有量は、これらの元素の合計含有量を指す。ランタノイドの場合、工業的にはミッシュメタルの形で添加される。 Here, REM refers to a total of 17 elements consisting of Sc, Y and lanthanides, and the content of REM above refers to the total content of these elements. In the case of lanthanides, they are industrially added in the form of mischmetals.
　Ｂｉ：０～０．０２０％
　Ｂｉは、凝固組織を微細化することにより、熱延鋼板の成形性を高める作用を有する。この作用による効果をより確実に得るためには、Ｂｉ含有量を０．０００５％以上とすることが好ましい。しかし、Ｂｉ含有量を０．０２０％超としても、上記作用による効果は飽和してしまい、経済的に好ましくない。したがって、Ｂｉ含有量を０．０２０％以下とする。Ｂｉ含有量は、好ましくは０．０１０％以下である。 Bi: 0-0.020%
Bi has the effect of increasing the formability of the hot-rolled steel sheet by refining the solidified structure. In order to more reliably obtain the effect of this action, the Bi content is preferably 0.0005% or more. However, even if the Bi content exceeds 0.020%, the above effect is saturated, which is economically unfavorable. Therefore, the Bi content is set to 0.020% or less. The Bi content is preferably 0.010% or less.
　Ｚｒ、Ｃｏ、ＺｎおよびＷの合計：０～１．００％
　Ｓｎ：０～０．０５％
　Ｚｒ、Ｃｏ、ＺｎおよびＷについて、本発明者らは、これらの元素を合計で１．００％以下含有させても、本実施形態に係る熱延鋼板の効果は損なわれないことを確認している。そのため、Ｚｒ、Ｃｏ、ＺｎおよびＷのうち１種または２種以上を合計で１．００％以下含有させてもよい。
　また、本発明者らは、Ｓｎを少量含有させても本実施形態に係る熱延鋼板の効果は損なわれないことを確認しているが、熱間圧延時に疵が発生する場合があるため、Ｓｎ含有量は０．０５％以下とする。 Sum of Zr, Co, Zn and W: 0-1.00%
Sn: 0-0.05%
Regarding Zr, Co, Zn and W, the present inventors have confirmed that even if these elements are contained in a total amount of 1.00% or less, the effect of the hot-rolled steel sheet according to the present embodiment is not impaired. there is Therefore, one or more of Zr, Co, Zn and W may be contained in a total amount of 1.00% or less.
In addition, the present inventors have confirmed that even if a small amount of Sn is contained, the effects of the hot-rolled steel sheet according to the present embodiment are not impaired. The Sn content should be 0.05% or less.
　上述した熱延鋼板の化学組成は、スパーク放電発光分光分析装置などを用いて、分析すればよい。なお、ＣおよびＳはガス成分分析装置などを用いて、酸素気流中で燃焼させ、赤外線吸収法によって測定することで同定された値を採用する。また、Ｎは、熱延鋼板から採取した試験片をヘリウム気流中で融解させ、熱伝導度法によって測定することで同定された値を採用する。 The chemical composition of the hot-rolled steel sheet described above can be analyzed using a spark discharge emission spectrometer or the like. For C and S, values identified by burning in an oxygen stream and measuring by an infrared absorption method using a gas component analyzer or the like are adopted. For N, a value identified by melting a test piece taken from a hot-rolled steel sheet in a helium stream and measuring it by a thermal conductivity method is adopted.
　次に、本実施形態に係る熱延鋼板の金属組織について説明する。
　本実施形態に係る熱延鋼板は、金属組織が、面積％で、ベイナイト：２５．０％以上、フレッシュマルテンサイトおよび焼き戻しマルテンサイトの合計：７０．０％以下、残留オーステナイト：４．０～１２．０％、フェライト：５．０％以下、並びに、パーライト：３．０％以下であり、旧オーステナイト粒の平均粒径が２５．０μｍ以下であり、前記残留オーステナイト、前記フレッシュマルテンサイトおよび前記焼き戻しマルテンサイトの結晶粒の長軸方向と圧延方向とがなす角度をθとしたとき、θが、０～１５°である前記結晶粒の割合、１５°超３０°以下である前記結晶粒の割合、３０°超４５°以下である前記結晶粒の割合、４５°超６０°以下である前記結晶粒の割合、６０°超７５°以下である前記結晶粒の割合、および７５°超９０°以下である前記結晶粒の割合が百分率でそれぞれ４０％以下である。 Next, the metal structure of the hot-rolled steel sheet according to this embodiment will be described.
The hot-rolled steel sheet according to the present embodiment has a metal structure in terms of area %, bainite: 25.0% or more, total of fresh martensite and tempered martensite: 70.0% or less, retained austenite: 4.0 to 12.0%, ferrite: 5.0% or less, pearlite: 3.0% or less, the average grain size of prior austenite grains is 25.0 μm or less, and the retained austenite, the fresh martensite and the When the angle formed by the major axis direction of the tempered martensite crystal grains and the rolling direction is θ, the ratio of the crystal grains where θ is 0 to 15°, the crystal grains where θ is more than 15° and 30° or less , the proportion of the crystal grains that are more than 30° and 45° or less, the proportion of the crystal grains that are more than 45° and 60° or less, the proportion of the crystal grains that are more than 60° and 75° or less, and the proportion of the crystal grains that are more than 75° and 90 ° or less is respectively 40% or less in percentage.
　本実施形態では、熱延鋼板の板厚１／４位置（表面から板厚の１／８深さ～表面から板厚の３／８深さの領域）における金属組織を規定する。その理由は、この位置における金属組織が鋼板の代表的な金属組織を示すためである。 In this embodiment, the metallographic structure is defined at the position of 1/4 of the thickness of the hot-rolled steel sheet (the region from 1/8 of the thickness from the surface to 3/8 of the thickness from the surface). The reason is that the metallographic structure at this position shows the typical metallographic structure of the steel plate.
　ベイナイトの面積率：２５．０％以上
　ベイナイトは熱延鋼板の強度、延性、穴広げ性を高める組織である。ベイナイトの面積率が２５．０％未満であると、所望の強度、延性および／または穴広げ性を得ることができない。そのため、ベイナイトの面積率は２５．０％以上とする。好ましくは、３０．０％以上、４０．０％以上、５５．０％以上、６０．０％以上または６５．０％以上である。
　ベイナイトの面積率の上限は特に限定しないが、残留オーステナイトの面積率との関係から、９６．０％以下としてもよい。ベイナイトの面積率は、９０．０％以下または８５．０％以下としてもよい。 Area ratio of bainite: 25.0% or more Bainite is a structure that increases the strength, ductility, and expansibility of the hot-rolled steel sheet. If the area ratio of bainite is less than 25.0%, desired strength, ductility and/or hole expansibility cannot be obtained. Therefore, the area ratio of bainite is set to 25.0% or more. Preferably, it is 30.0% or more, 40.0% or more, 55.0% or more, 60.0% or more, or 65.0% or more.
Although the upper limit of the area ratio of bainite is not particularly limited, it may be 96.0% or less in view of the relationship with the area ratio of retained austenite. The area ratio of bainite may be 90.0% or less or 85.0% or less.
　フレッシュマルテンサイトおよび焼き戻しマルテンサイトの面積率の合計：７０．０％以下
　フレッシュマルテンサイトおよび焼き戻しマルテンサイトは熱延鋼板の強度を高めるために有効な組織である。しかし、これらの組織の面積率の合計が７０．０％超であると、熱延鋼板の延性および穴広げ性が劣化する。そのため、フレッシュマルテンサイトおよび焼き戻しマルテンサイトの面積率の合計は７０．０％以下とする。好ましくは、４０．０％以下、２０．０％以下または１０．０％以下である。
　なお、フレッシュマルテンサイトおよび焼き戻しマルテンサイトの両方が含まれている必要はなく、いずれか一方のみを含んでいてもよい。また、フレッシュマルテンサイトおよび焼き戻しマルテンサイトの面積率の合計は０％であってもよく、１．０％以上または３．０％以上としてもよい。 Total area ratio of fresh martensite and tempered martensite: 70.0% or less Fresh martensite and tempered martensite are effective structures for increasing the strength of hot-rolled steel sheets. However, if the total area ratio of these structures exceeds 70.0%, the ductility and hole expansibility of the hot-rolled steel sheet deteriorate. Therefore, the total area ratio of fresh martensite and tempered martensite is set to 70.0% or less. Preferably, it is 40.0% or less, 20.0% or less, or 10.0% or less.
In addition, it is not necessary to contain both fresh martensite and tempered martensite, and only one of them may be contained. In addition, the total area ratio of fresh martensite and tempered martensite may be 0%, 1.0% or more, or 3.0% or more.
　フレッシュマルテンサイトおよび焼き戻しマルテンサイトの面積率の合計のうち、焼き戻しマルテンサイトの面積率の割合：百分率で８０．０％以上
　フレッシュマルテンサイトおよび焼き戻しマルテンサイトの面積率の合計のうち、焼き戻しマルテンサイトの面積率の割合は百分率で８０．０％以上とすることが好ましい。焼き戻しマルテンサイトの割合を百分率で８０．０％以上とすることで、残留オーステナイトに隣接したフレッシュマルテンサイトが、残留オーステナイトの安定性を低下させることを抑制できる。その結果、変形経路が変化した場合においても一様伸びの低下をより抑制することができる。
　上限は特に限定しないが１００％としてもよい。 Of the total area ratio of fresh martensite and tempered martensite, the ratio of the area ratio of tempered martensite: 80.0% or more in percentage The ratio of the area ratio of the reverted martensite is preferably 80.0% or more in percentage. By setting the percentage of tempered martensite to 80.0% or more, fresh martensite adjacent to retained austenite can be prevented from lowering the stability of retained austenite. As a result, even when the deformation path changes, it is possible to further suppress the deterioration of the uniform elongation.
Although the upper limit is not particularly limited, it may be 100%.
　残留オーステナイトの面積率：４．０～１２．０％
　残留オーステナイトは熱延鋼板の延性を高める組織である。残留オーステナイトの面積率が４．０％未満であると、所望の延性を得ることができない場合、または、予歪み付与後において優れた成形性を得ることができない場合がある。そのため、残留オーステナイトの面積率は４．０％以上とする。好ましくは、５．０％以上または６．０％以上である。
　一方、残留オーステナイトの面積率が１２．０％超であると、熱延鋼板の穴広げ性が劣化する。そのため、残留オーステナイトの面積率は、１２．０％以下とする。好ましくは、１１．０％以下、１０．０％以下または８．０％以下である。 Area ratio of retained austenite: 4.0 to 12.0%
Retained austenite is a structure that increases the ductility of hot-rolled steel sheets. If the area ratio of retained austenite is less than 4.0%, desired ductility may not be obtained, or excellent formability may not be obtained after prestraining. Therefore, the area ratio of retained austenite is set to 4.0% or more. Preferably, it is 5.0% or more or 6.0% or more.
On the other hand, if the area ratio of retained austenite exceeds 12.0%, the hole expansibility of the hot-rolled steel sheet deteriorates. Therefore, the area ratio of retained austenite is set to 12.0% or less. Preferably, it is 11.0% or less, 10.0% or less, or 8.0% or less.
　フェライトの面積率：５．０％以下
　フェライトの面積率が５．０％超であると、熱延鋼板の強度が劣化する。そのため、フェライトの面積率は５．０％以下とする。好ましくは３．０％以下または２．０％以下であり、０．０％であってもよい。 Area ratio of ferrite: 5.0% or less When the area ratio of ferrite exceeds 5.0%, the strength of the hot-rolled steel sheet deteriorates. Therefore, the area ratio of ferrite is set to 5.0% or less. It is preferably 3.0% or less or 2.0% or less, and may be 0.0%.
　パーライトの面積率：３．０％以下
　パーライトの面積率が３．０％超であると、熱延鋼板の強度、延性および／または穴広げ性が劣化する場合、または、予歪み付与後において優れた成形性を得ることができない場合がある。そのため、パーライトの面積率は３．０％以下とする。好ましくは２．０％以下または１．０％以下であり、０．０％であってもよい。 Area ratio of pearlite: 3.0% or less If the area ratio of pearlite exceeds 3.0%, the strength, ductility and/or hole expansibility of the hot-rolled steel sheet are deteriorated, or the hot-rolled steel sheet is excellent after prestraining. In some cases, it may not be possible to obtain moldability. Therefore, the area ratio of pearlite is set to 3.0% or less. It is preferably 2.0% or less or 1.0% or less, and may be 0.0%.
　以下に、各組織の面積率の測定方法を説明する。
　熱延鋼板から、圧延方向に平行な断面で、表面から板厚の１／４位置（表面から板厚の１／８深さ～表面から板厚の３／８深さの領域）且つ板幅方向中央位置における金属組織が観察できるように試験片を採取する。 The method for measuring the area ratio of each tissue will be described below.
From the hot-rolled steel sheet, in the cross section parallel to the rolling direction, the position of 1/4 of the plate thickness from the surface (1/8 depth of the plate thickness from the surface to 3/8 depth of the plate thickness from the surface) and width A test piece is taken so that the metal structure can be observed at the direction center position.
　上記試験片の断面を＃６００から＃１５００の炭化珪素ペーパーを使用して研磨した後、粒度１～６μｍのダイヤモンドパウダーをアルコール等の希釈液や純水に分散させた液体を使用して鏡面に仕上げる。次に、室温においてアルカリ性溶液を含まない粒径０．２５μｍのコロイダルシリカを用いて８分間研磨し、サンプルの表層に導入されたひずみを除去する。サンプル断面の長手方向の任意の位置において、サンプルの長手方向に５０μｍ、表面から板厚の１／８深さ～表面から板厚の３／８深さの領域を、０．１μｍの測定間隔で電子後方散乱回折法により測定して結晶方位情報を得る。 After polishing the cross section of the above test piece using silicon carbide paper of #600 to #1500, diamond powder with a particle size of 1 to 6 μm is applied to a mirror surface using a diluted solution such as alcohol or a liquid dispersed in pure water. Finish. Next, the sample is polished for 8 minutes with colloidal silica having a particle size of 0.25 μm that does not contain an alkaline solution at room temperature to remove the strain introduced to the surface layer of the sample. At any position in the longitudinal direction of the sample cross section, 50 μm in the longitudinal direction of the sample, the area from 1/8 of the plate thickness from the surface to 3/8 of the plate thickness from the surface, at a measurement interval of 0.1 μm Crystallographic orientation information is obtained by electron backscatter diffractometry.
　測定には、サーマル電界放射型走査電子顕微鏡（ＪＥＯＬ製ＪＳＭ－７００１Ｆ）とＥＢＳＤ検出器（ＴＳＬ製ＤＶＣ５型検出器）とで構成されたＥＢＳＤ装置を用いる。この際、ＥＢＳＤ装置内の真空度は９．６×１０－５Ｐａ以下、加速電圧は１５ｋＶ、照射電流レベルは１３、電子線の照射レベルは６２とする。得られた結晶方位情報から、ＥＢＳＤ解析装置に付属のソフトウェア「ＯＩＭ　Ａｎａｌｙｓｉｓ（登録商標）」に搭載された「Ｐｈａｓｅ　Ｍａｐ」機能を用いて、結晶構造がｆｃｃである領域を特定し、この領域の面積率を算出する。これにより、残留オーステナイトの面積率を得る。 For the measurement, an EBSD apparatus composed of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used. At this time, the degree of vacuum in the EBSD apparatus is 9.6×10 −5 Pa or less, the acceleration voltage is 15 kV, the irradiation current level is 13, and the electron beam irradiation level is 62. From the obtained crystal orientation information, using the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device, a region with a crystal structure of fcc is identified, and the region of this region is identified. Calculate the area ratio. Thereby, the area ratio of retained austenite is obtained.
　次に、結晶構造がｂｃｃであるものをベイナイト、フェライト、パーライト、フレッシュマルテンサイトおよび焼き戻しマルテンサイトと判断する。これらの領域について、ＥＢＳＤ解析装置に付属のソフトウェア「ＯＩＭ　Ａｎａｌｙｓｉｓ（登録商標）」に搭載された「Ｇｒａｉｎ　Ｏｒｉｅｎｔａｔｉｏｎ　Ｓｐｒｅａｄ」機能を用いて、１５°粒界を結晶粒界とみなす条件下で、「Ｇｒａｉｎ　Ｏｒｉｅｎｔａｔｉｏｎ　Ｓｐｒｅａｄ」が１°以下の領域をフェライトとして抽出する。抽出したフェライトの面積率を算出することで、フェライトの面積率を得る。 Next, those with a bcc crystal structure are judged to be bainite, ferrite, pearlite, fresh martensite, and tempered martensite. For these regions, using the "Grain Orientation Spread" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device, "Grain Orientation Spread” is extracted as ferrite in the region of 1° or less. By calculating the area ratio of the extracted ferrite, the area ratio of ferrite is obtained.
　続いて、残部領域（「Ｇｒａｉｎ　Ｏｒｉｅｎｔａｔｉｏｎ　Ｓｐｒｅａｄ」が１°超の領域）の内、１５°粒界を結晶粒界の定義とした条件下で、フェライト領域の「Ｇｒａｉｎ　Ａｖｅｒａｇｅ　ＩＱ」の最大値をＩαとしたとき、Ｉα／２超となる領域をベイナイト、Ｉα／２以下となる領域を「パーライト、フレッシュマルテンサイトおよび焼き戻しマルテンサイト」として抽出する。抽出したベイナイトの面積率を算出することで、ベイナイトの面積率を得る。
　観察視野においてフェライトが抽出されない場合には、同視野に対して、ＧＡＭ「Ｇｒａｉｎ　Ａｖｅｒａｇｅ　Ｍｉｓｏｒｉｅｎｔａｔｉｏｎ」機能を用いて、５°粒界を結晶粒界とみなす条件下で、「Ｇｒａｉｎ　Ａｖｅｒａｇｅ　Ｍｉｓｏｒｉｅｎｔａｔｉｏｎ」が０．５０°超、０．７５°以下となる領域をベイナイトとして抽出し、０．７５°超となる領域を「パーライト、フレッシュマルテンサイトおよび焼き戻しマルテンサイト」として抽出する。抽出したベイナイトの面積率を算出することで、ベイナイトの面積率を得る。 Subsequently, the maximum value of the "Grain Average IQ" of the ferrite region under the condition that the 15° grain boundary is defined as the grain boundary in the remaining region (the region where the "Grain Orientation Spread" exceeds 1°) is Iα. , a region of more than Iα/2 is extracted as bainite, and a region of Iα/2 or less is extracted as “pearlite, fresh martensite and tempered martensite”. By calculating the area ratio of the extracted bainite, the area ratio of bainite is obtained.
When ferrite is not extracted in the observation field, the GAM "Grain Average Misorientation" function is used for the same field of view, and the "Grain Average Misorientation" is set to 0.5 under the condition that the 5° grain boundary is regarded as the grain boundary. A region of more than 50° and 0.75° or less is extracted as bainite, and a region of more than 0.75° is extracted as "pearlite, fresh martensite and tempered martensite". By calculating the area ratio of the extracted bainite, the area ratio of bainite is obtained.
　抽出した「パーライト、フレッシュマルテンサイトおよび焼き戻しマルテンサイト」について、下記方法によってパーライト、フレッシュマルテンサイトおよび焼き戻しマルテンサイトを区別する。 Regarding the extracted "perlite, fresh martensite and tempered martensite", the perlite, fresh martensite and tempered martensite are distinguished by the following method.
　ＥＢＳＤ測定領域と同領域をＳＥＭで観察するために、観察位置近傍にビッカース圧痕を打刻する。その後、観察面の組織を残して、表層のコンタミを研磨除去し、ナイタールエッチングする。次に、ＥＢＳＤ観察面と同一視野をＳＥＭにより倍率３０００倍で観察する。ＥＢＳＤ測定において、「パーライト、フレッシュマルテンサイトおよび焼き戻しマルテンサイト」と判別された領域の内、粒内に下部組織を有し、かつ、セメンタイトが複数のバリアントを持って析出している領域を焼き戻しマルテンサイトと判断する。セメンタイトがラメラ状に析出している領域をパーライトと判断する。輝度が大きく、かつ下部組織がエッチングにより現出されていない領域をフレッシュマルテンサイトと判断する。それぞれの面積率を算出することで、焼き戻しマルテンサイト、パーライト、およびフレッシュマルテンサイトの面積率を得る。 In order to observe the same area as the EBSD measurement area with an SEM, a Vickers indentation is stamped near the observation position. After that, leaving the structure of the observation surface, contamination on the surface layer is removed by polishing, and nital etching is performed. Next, the same field of view as the EBSD observation surface is observed with a SEM at a magnification of 3000 times. In the EBSD measurement, among the regions discriminated as "pearlite, fresh martensite and tempered martensite", the region that has a substructure in the grain and where cementite is precipitated with multiple variants is tempered. Judged as reverted martensite. A region in which cementite is precipitated in a lamellar shape is determined to be pearlite. A region with high brightness and in which the substructure is not revealed by etching is judged to be fresh martensite. By calculating the respective area ratios, the area ratios of tempered martensite, pearlite, and fresh martensite are obtained.
　なお、観察面表層のコンタミ除去については、粒子径０．１μｍ以下のアルミナ粒子を用いたバフ研磨、あるいはＡｒイオンスパッタリング等の手法を用いればよい。 For removing contaminants from the surface layer of the observation surface, a method such as buffing using alumina particles with a particle size of 0.1 μm or less or Ar ion sputtering may be used.
　旧オーステナイト粒の平均粒径：２５．０μｍ以下
　旧オーステナイト粒の粒径が大きいと、局部伸びが劣化する結果、熱延鋼板の穴広げ性が劣化する。旧オーステナイト粒の平均粒径が２５．０μｍ超であると、熱延鋼板の穴広げ性の劣化が顕著となる。そのため、旧オーステナイト粒の平均粒径は２５．０μｍ以下とする。好ましくは２０．０μｍ以下、１５．０μｍ以下である。
　旧オーステナイト粒の平均粒径が小さければ小さい程熱延鋼板の穴広げ性は向上するが、７．０μｍ未満としてもその効果は飽和する。そのため、旧オーステナイト粒の平均粒径は７．０μｍ以上としてもよい。 Average grain size of prior austenite grains: 25.0 µm or less When the grain size of prior austenite grains is large, local elongation is degraded, resulting in poor hole expandability of the hot-rolled steel sheet. When the average grain size of the prior austenite grains exceeds 25.0 μm, the hole expansibility of the hot-rolled steel sheet significantly deteriorates. Therefore, the average grain size of prior austenite grains is set to 25.0 μm or less. It is preferably 20.0 μm or less and 15.0 μm or less.
The smaller the average grain size of the prior austenite grains, the better the hole expansibility of the hot-rolled steel sheet. Therefore, the average grain size of the prior austenite grains may be 7.0 μm or more.
　旧オーステナイト粒の粒径の測定方法
　熱延鋼板の圧延方向と直行する板厚断面であり、表面から板厚の１／４位置（表面から板厚の１／８深さ～板厚の３／８深さの領域）が観察できるようにサンプルを採取する。ピクリン酸飽和水溶液にドデシルベンゼンスルホン酸ナトリウム腐食液を加えた腐食液によって板厚断面の組織を現出させる。このサンプルの表面から板厚の１／４深さ位置（表面から板厚の１／８深さ～板厚の３／８深さの領域）について、５００倍の倍率で、板厚方向に２００μｍ、圧延方向と直行する方向に２００μｍの範囲の３か所について撮影した金属組織写真から、旧オーステナイト粒の粒径を測定する。各観察視野に含まれる旧オーステナイト粒の１つについて、円相当直径を算出する。撮影視野の端部等、旧オーステナイト粒の全体が撮影視野に含まれていない旧オーステナイト粒を除き、各観察視野に含まれる全ての旧オーステナイト粒について上記操作を行い、各撮影視野における全ての旧オーステナイト粒の円相当直径を求める。各撮影視野において得られた旧オーステナイト粒の円相当直径の平均値を算出することで、旧オーステナイト粒の平均粒径を得る。 Measurement method of the grain size of prior austenite grains It is a thickness cross section perpendicular to the rolling direction of the hot-rolled steel sheet, and the position of 1/4 of the plate thickness from the surface (1/8 depth of the plate thickness from the surface to 3/3 of the plate thickness The sample is taken so that the 8-deep region) can be observed. The texture of the thickness cross-section is revealed by an etchant in which a sodium dodecylbenzenesulfonate etchant is added to a picric acid saturated aqueous solution. 200 μm in the thickness direction at a magnification of 500 times for the 1/4 depth position of the plate thickness from the surface of this sample (region from 1/8 depth of plate thickness to 3/8 depth of plate thickness from surface) , the grain size of the prior austenite grains is measured from metallographic photographs taken at three locations within a range of 200 μm in the direction perpendicular to the rolling direction. An equivalent circle diameter is calculated for one of the prior austenite grains included in each observation field. Except for the old austenite grains where the entire old austenite grains are not included in the imaging field such as the edge of the imaging field, perform the above operation for all the old austenite grains contained in each observation field, and all the old austenite grains in each imaging field Obtain the circle-equivalent diameter of the austenite grains. By calculating the average value of the equivalent circle diameters of the prior austenite grains obtained in each imaging field, the average grain size of the prior austenite grains is obtained.
　残留オーステナイト、フレッシュマルテンサイトおよび焼き戻しマルテンサイトの結晶粒の長軸方向と圧延方向とがなす角度をθとしたとき、θが、０～１５°である前記結晶粒の割合、１５°超３０°以下である前記結晶粒の割合、３０°超４５°以下である前記結晶粒の割合、４５°超６０°以下である前記結晶粒の割合、６０°超７５°以下である前記結晶粒の割合、および７５°超９０°以下である前記結晶粒の割合：百分率でそれぞれ４０％以下
　残留オーステナイト、フレッシュマルテンサイトおよび焼き戻しマルテンサイトの結晶粒の長軸方向が特定の方向に集中していると、複数工程における変形により残留オーステナイトの加工誘起変態が進展しやすくなり、残留オーステナイト分率が減少しやすくなる。その結果、熱延鋼板の予歪み付与後の成形性が劣化する。よって、本実施形態では、前記結晶粒の長軸方向を適度に分散させる。本実施形態では、残留オーステナイト、フレッシュマルテンサイトおよび焼き戻しマルテンサイトの結晶粒の長軸方向と圧延方向とがなす角度をθとしたとき、θが、０～１５°である前記結晶粒の割合、１５°超３０°以下である前記結晶粒の割合、３０°超４５°以下である前記結晶粒の割合、４５°超６０°以下である前記結晶粒の割合、６０°超７５°以下である前記結晶粒の割合、および７５°超９０°以下である前記結晶粒の割合（以下、単に結晶粒割合と記載する場合がある）を百分率でそれぞれ４０％以下とする。好ましくは、３８％以下、３５％以下または３０％以下である。
　下限は特に限定しないが、０％以上、１０％以上または１５％以上としてもよい。また、２０％以上としてもよい。 When the angle between the major axis direction of the crystal grains of retained austenite, fresh martensite, and tempered martensite and the rolling direction is θ, the ratio of the crystal grains where θ is 0 to 15 °, more than 15 ° 30 The ratio of the crystal grains that are 30° to 45°, the ratio of the crystal grains that are 45° to 60°, and the crystal grains that are 60° to 75° and the proportion of the grains that are more than 75° and 90° or less: 40% or less in terms of percentage respectively The long axis directions of the grains of retained austenite, fresh martensite and tempered martensite are concentrated in a specific direction Then, due to deformation in a plurality of steps, deformation-induced transformation of retained austenite tends to progress, and the retained austenite fraction tends to decrease. As a result, the formability of the hot-rolled steel sheet after prestraining is degraded. Therefore, in the present embodiment, the long axis direction of the crystal grains is appropriately dispersed. In the present embodiment, when the angle formed by the long axis direction of the crystal grains of retained austenite, fresh martensite, and tempered martensite and the rolling direction is θ, the ratio of the crystal grains where θ is 0 to 15 ° , the ratio of the crystal grains that are 15° to 30°, the ratio of the crystal grains that are 30° to 45°, the ratio of the crystal grains that are 45° to 60°, and the crystal grains that are 60° to 75° The ratio of certain crystal grains and the ratio of crystal grains having an angle of more than 75° to 90° or less (hereinafter sometimes simply referred to as the crystal grain ratio) shall each be 40% or less in percentage. Preferably, it is 38% or less, 35% or less or 30% or less.
Although the lower limit is not particularly limited, it may be 0% or more, 10% or more, or 15% or more. Moreover, it is good also as 20% or more.
　上述の結晶粒割合は以下の方法により測定する。
　各組織の面積率を測定したときと同じ方法により、表面から板厚の１／４位置（表面から板厚の１／８深さ～表面から板厚の３／８深さの領域）且つ板幅方向中央位置における金属組織において、残留オーステナイト、フレッシュマルテンサイトおよび焼き戻しマルテンサイトを特定する。次に、残留オーステナイト、フレッシュマルテンサイトおよび焼き戻しマルテンサイトの結晶粒を楕円体で近似してその長軸の方向を定める。この長軸方向と、熱延鋼板の圧延方向とがなす角度θ（０～９０°）を算出する。得られたθを０～１５°、１５°超３０°以下、３０°超４５°以下、４５°超６０°以下、６０°超７５°以下、および７５°超９０°以下と１５°間隔で分割して、ヒストグラムを作成する。各区間の存在割合を百分率で求めることで、各区間における結晶粒割合を得る。 The above crystal grain ratio is measured by the following method.
By the same method as when measuring the area ratio of each tissue, the 1/4 position of the plate thickness from the surface (1/8 depth of the plate thickness from the surface to 3/8 depth of the plate thickness from the surface) and the plate Retained austenite, fresh martensite and tempered martensite are identified in the metallographic structure at the center position in the width direction. Next, the grains of retained austenite, fresh martensite and tempered martensite are approximated by ellipsoids to determine the direction of their long axes. An angle θ (0 to 90°) formed by the longitudinal direction and the rolling direction of the hot-rolled steel sheet is calculated. The resulting θ was 0 to 15°, 15° to 30°, 30° to 45°, 45° to 60°, 60° to 75°, and 75° to 90° at intervals of 15°. Subdivide and create a histogram. By determining the existence ratio of each section as a percentage, the crystal grain ratio in each section is obtained.
　残留オーステナイト、フレッシュマルテンサイトおよび焼き戻しマルテンサイトの結晶粒を楕円体で近似する方法について説明する。
　図１に示すように、楕円体に含まれない結晶粒の領域の面積Ｓｏｕｔと、楕円体内の結晶粒でない領域の面積Ｓｉｎとの総和が最小となるように楕円体で近似することで、ｘ０、ｙ０、ａ、ｂ、θを求める。楕円体の長径の長さａが１μｍ以下である場合は、その結晶粒は測定に含めない。また、楕円体の長径の長さａを短径の長さｂで除した値（ａ／ｂ）が１．１未満である場合は、その結晶粒は測定に含めない。 A method of approximating the grains of retained austenite, fresh martensite, and tempered martensite with ellipsoids will be described.
As shown in FIG. 1, by approximating with an ellipsoid so that the sum of the area S out of the crystal grain region not included in the ellipsoid and the area S in of the region other than the crystal grain in the ellipsoid is minimized , x0, y0, a, b, θ. If the major axis length a of the ellipsoid is 1 μm or less, the crystal grains are not included in the measurement. If the value (a/b) obtained by dividing the length a of the major axis of the ellipsoid by the length b of the minor axis is less than 1.1, the crystal grain is not included in the measurement.
　なお、熱延鋼板の圧延方向は以下の方法で判別することができる。
　まず、熱延鋼板の板厚断面が観察できるように試験片を採取する。採取した試験片の板厚断面を鏡面研磨で仕上げた後、光学顕微鏡を用いて観察する。観察範囲は板厚の全厚とし、輝度が暗い領域を介在物と判定する。介在物のうち長軸の長さが５μｍ以上である介在物において、介在物が伸展している方向と平行な方向を圧延方向と判別する。 The rolling direction of the hot-rolled steel sheet can be determined by the following method.
First, a test piece is taken so that the thickness cross section of the hot-rolled steel sheet can be observed. After mirror-polishing the plate thickness cross-section of the sampled test piece, it is observed using an optical microscope. The observation range is the entire plate thickness, and areas with dark luminance are determined to be inclusions. Among the inclusions, the direction parallel to the direction in which the inclusions extend is determined as the rolling direction in the inclusions having a major axis length of 5 μm or more.
　引張強さ：１１００ＭＰａ以上
　本実施形態に係る熱延鋼板は、引張強さが１１００ＭＰａ以上であってもよい。引張強さを１１００ＭＰａ以上とすることで、様々な自動車足回り部品に好適に適用することができる。引張強さは、１２００ＭＰａ以上、または１３００ＭＰａ以上としてもよい。
　引張強さは高い程好ましいが、１４００ＭＰａ以下としてもよい。 Tensile strength: 1100 MPa or more The hot-rolled steel sheet according to the present embodiment may have a tensile strength of 1100 MPa or more. By setting the tensile strength to 1100 MPa or more, it can be suitably applied to various automotive underbody parts. The tensile strength may be 1200 MPa or higher, or 1300 MPa or higher.
The higher the tensile strength, the better, but it may be 1400 MPa or less.
　一様伸び：５．０％以上
　本実施形態に係る熱延鋼板は、一様伸びが５．０％以上であってもよい。一様伸びを５．０％以上とすることで、自動車足回り部品に好適に適用することができる。好ましくは、５．５％以上、６．０％以上または７．０％以上である。
　上限は特に限定しないが、２０．０％以下としてもよい。 Uniform elongation: 5.0% or more The hot-rolled steel sheet according to the present embodiment may have a uniform elongation of 5.0% or more. By setting the uniform elongation to 5.0% or more, it can be suitably applied to automotive underbody parts. Preferably, it is 5.5% or more, 6.0% or more, or 7.0% or more.
Although the upper limit is not particularly limited, it may be 20.0% or less.
　引張強さおよび一様伸びは、ＪＩＳ　Ｚ　２２４１：２０１１の５号試験片を用いて、ＪＩＳ　Ｚ　２２４１：２０１１に準拠して引張試験を行うことで、測定する。引張試験片の採取位置は、板幅方向中央位置とし、圧延方向に垂直な方向を長手方向とする。
　なお、一様伸びは、ＪＩＳ　Ｚ　２２４１：２０１１でいう「最大試験力時全伸び」のことである。 Tensile strength and uniform elongation are measured by performing a tensile test according to JIS Z 2241:2011 using a No. 5 test piece of JIS Z 2241:2011. The tensile test piece is taken at the central position in the sheet width direction, and the direction perpendicular to the rolling direction is taken as the longitudinal direction.
The uniform elongation means "total elongation at maximum test force" according to JIS Z 2241:2011.
　穴広げ率：３０％以上
　本実施形態に係る熱延鋼板は、穴広げ率が３０％以上であってもよい。穴広げ率を３０％以上とすることで、自動車足回り部品に好適に適用することができる。好ましくは、３５％以上、４０％以上、４５％以上または５０％以上である。上限は特に規定しないが、８０％以下としてもよい。
　穴広げ率は、ＪＩＳ　Ｚ　２２５６：２０２０に準拠して穴広げ試験を行うことで測定する。 Hole expansion rate: 30% or more The hot-rolled steel sheet according to the present embodiment may have a hole expansion rate of 30% or more. By setting the hole expansion ratio to 30% or more, it can be suitably applied to automotive underbody parts. Preferably, it is 35% or more, 40% or more, 45% or more or 50% or more. Although the upper limit is not particularly defined, it may be 80% or less.
The hole expansion rate is measured by conducting a hole expansion test in accordance with JIS Z 2256:2020.
　予歪み付与後の一様伸びの低下量：２．０％以下
　予歪み付与後においても成形性を高めるためには、変形経路が急激に変化する変形においても、一様伸びが大きく低下しないことが重要である。本実施形態において、変形経路が急激に変化するときの一様伸びの低下量は、以下の方法により評価する。
　熱延鋼板からＪＩＳ　Ｚ　２２４１：２０１１の５号試験片を採取する。この試験片に対し、圧延方向に平行な方向に、真歪０．０３の引張予変形を与える。その後、試験片から、引張方向に対して引張方向が０°、４５°、９０°となるように小型の引張試験片を採取する。これらの引張試験片を用いて引張試験を行い、引張方向が０°方向、４５°方向および９０°方向である場合の一様伸びを得る。引張方向が０°方向の場合の一様伸びに対する、引張方向が４５°方向および９０°方向の場合の一様伸びの低下量（０°方向の一様伸び－４５°方向の一様伸び、０°方向の一様伸び－９０°方向の一様伸び）を算出する。この一様伸びの低下量がいずれも２．０％以下である場合、変形経路が急激に変化する変形においても、一様伸びが大きく低下しないと判断する。すなわち、予歪み付与後においても優れた成形性を有すると判断する。
　なお、引張試験はＪＩＳ　Ｚ　２２４１：２０１１に準拠して行う。 Amount of decrease in uniform elongation after prestraining: 2.0% or less In order to improve formability even after prestraining, the uniform elongation should not decrease significantly even in deformation in which the deformation path suddenly changes. is important. In this embodiment, the amount of decrease in uniform elongation when the deformation path suddenly changes is evaluated by the following method.
A No. 5 test piece of JIS Z 2241:2011 is taken from a hot-rolled steel sheet. A tensile predeformation with a true strain of 0.03 is applied to this test piece in a direction parallel to the rolling direction. After that, small tensile test pieces are taken from the test piece so that the tensile direction is 0°, 45°, and 90° with respect to the tensile direction. A tensile test is performed using these tensile test pieces to obtain uniform elongation when the tensile directions are 0° direction, 45° direction and 90° direction. The amount of decrease in uniform elongation when the tensile direction is 45 ° and 90 ° relative to the uniform elongation when the tensile direction is 0 ° (uniform elongation in 0 ° direction - uniform elongation in 45 ° direction, (uniform elongation in 0° direction - uniform elongation in 90° direction) is calculated. When the amount of decrease in uniform elongation is 2.0% or less, it is judged that uniform elongation does not decrease significantly even in deformation in which the deformation path suddenly changes. In other words, it is judged that it has excellent moldability even after prestraining.
In addition, a tensile test is performed based on JISZ2241:2011.
　本実施形態に係る熱延鋼板は、表面に耐食性の向上等を目的としてめっき層を備えさせて表面処理鋼板としてもよい。めっき層は電気めっき層であってもよく溶融めっき層であってもよい。電気めっき層としては、電気亜鉛めっき、電気Ｚｎ－Ｎｉ合金めっき等が例示される。溶融めっき層としては、溶融亜鉛めっき、合金化溶融亜鉛めっき、溶融アルミニウムめっき、溶融Ｚｎ－Ａｌ合金めっき、溶融Ｚｎ－Ａｌ－Ｍｇ合金めっき、溶融Ｚｎ－Ａｌ－Ｍｇ－Ｓｉ合金めっき等が例示される。めっき付着量は特に制限されず、従来と同様としてよい。また、めっき後に適当な化成処理（例えば、シリケート系のクロムフリー化成処理液の塗布と乾燥）を施して、耐食性をさらに高めることも可能である。 The hot-rolled steel sheet according to this embodiment may be a surface-treated steel sheet by providing a plating layer on the surface for the purpose of improving corrosion resistance. The plating layer may be an electroplating layer or a hot dipping layer. Examples of the electroplating layer include electrogalvanizing and electroplating of Zn—Ni alloy. Examples of hot-dip coating layers include hot-dip galvanizing, hot-dip galvannealing, hot-dip aluminum plating, hot-dip Zn--Al alloy plating, hot-dip Zn--Al--Mg alloy plating, and hot-dip Zn--Al--Mg--Si alloy plating. be. The amount of plating deposited is not particularly limited, and may be the same as the conventional one. Further, it is possible to further improve the corrosion resistance by applying an appropriate chemical conversion treatment (for example, applying a silicate-based chromium-free chemical conversion treatment solution and drying) after plating.
　次に、本実施形態に係る熱延鋼板の好ましい製造方法について説明する。
　なお、下記に説明する温度は特に指定のない限りスラブまたは鋼板の表面温度のことをいう。 Next, a preferred method for manufacturing the hot-rolled steel sheet according to this embodiment will be described.
The temperatures described below refer to the surface temperature of the slab or steel plate unless otherwise specified.
　本実施形態に係る熱延鋼板の好ましい製造方法は、
　上述の化学組成を有するスラブを１２２０～１３００℃の温度域で４０分以上保持する加熱工程と、
　１～３パス目の各圧下率が１０～３０％、４パス目以降の各圧下率が１５～５０％となるように粗圧延を行う粗圧延工程と、
　最終パスの圧延を９４０～１０２０℃の温度域且つ２５％超の圧下率で行う、または、最終パスから１パス前の圧延を９４０～１０２０℃の温度域且つ２５％超の圧下率で行い、最終パスの圧延を１５％未満の圧下率で行う仕上げ圧延工程と、
　仕上げ圧延完了後、２．０秒以内に冷却を開始し、且つ、１７．０秒以内に３００～４８０℃の温度域に冷却する冷却工程と、
　前記温度域で巻取る巻取り工程と、
　前記巻取り工程後、３０分以内に、加熱量ΔＴが下記式（１）を満たすように加熱する再加熱工程と、
　２００℃以下の温度域まで空冷する空冷工程と、を順次行う。
　２４５－０．９４２×ＣＴ＋０．０００９２×ＣＴ２＜ΔＴ＜６８６－２．３８×ＣＴ＋０．００２２×ＣＴ２　…（１）
　ただし、上記式（１）中のＣＴは冷却停止温度である。
　以下、各工程について説明する。 A preferred method for manufacturing the hot-rolled steel sheet according to the present embodiment is
A heating step of holding the slab having the above chemical composition in a temperature range of 1220 to 1300° C. for 40 minutes or more;
A rough rolling step in which rough rolling is performed so that each rolling reduction in the first to third passes is 10 to 30% and each rolling reduction in the fourth and subsequent passes is 15 to 50%;
Perform the final pass rolling at a temperature range of 940 to 1020 ° C. and a reduction rate of more than 25%, or perform rolling one pass before the final pass at a temperature range of 940 to 1020 ° C. and a reduction rate of more than 25%, A finish rolling step in which the final pass is rolled at a rolling reduction of less than 15%;
A cooling step of starting cooling within 2.0 seconds after completion of finish rolling and cooling to a temperature range of 300 to 480 ° C. within 17.0 seconds;
A winding step of winding in the temperature range;
A reheating step of heating so that the heating amount ΔT satisfies the following formula (1) within 30 minutes after the winding step;
and an air-cooling step of air-cooling to a temperature range of 200° C. or less.
245−0.942×CT+0.00092×CT 2 <ΔT<686−2.38×CT+0.0022×CT 2 (1)
However, CT in the above formula (1) is the cooling stop temperature.
Each step will be described below.
　加熱工程
　加熱温度が１２２０℃未満であると、溶体化が進まず、フェライト量が多くなり、熱延鋼板の強度が劣化する。そのため、加熱温度は１２２０℃以上とする。好ましくは１２４０℃以上である。
　一方、加熱温度が１３００℃超であると、加熱時にオーステナイト粒が粗大化し、結果として熱延鋼板の穴広げ性が劣化する。そのため、加熱温度は１３００℃以下とする。エネルギーコストの観点から、加熱温度は、１２８０℃以下であることが好ましい。 Heating Step If the heating temperature is lower than 1220° C., solution treatment does not proceed, the amount of ferrite increases, and the strength of the hot-rolled steel sheet deteriorates. Therefore, the heating temperature is set to 1220° C. or higher. It is preferably 1240° C. or higher.
On the other hand, if the heating temperature exceeds 1300° C., the austenite grains become coarse during heating, resulting in deterioration of the hole expansibility of the hot-rolled steel sheet. Therefore, the heating temperature is set to 1300° C. or less. From the viewpoint of energy cost, the heating temperature is preferably 1280° C. or lower.
　１２２０～１３００℃の温度域での保持時間が４０分未満であると、溶体化が進まず、フェライト量が多くなり、熱延鋼板の強度が劣化する。そのため、上記温度域での保持時間は４０分以上とする。好ましくは６０分以上、８０分以上である。
　保持時間の上限は特に限定しないが、２００分以下としてもよい。 If the holding time in the temperature range of 1220 to 1300° C. is less than 40 minutes, solution treatment does not progress, the amount of ferrite increases, and the strength of the hot-rolled steel sheet deteriorates. Therefore, the holding time in the above temperature range is set to 40 minutes or longer. It is preferably 60 minutes or more and 80 minutes or more.
Although the upper limit of the retention time is not particularly limited, it may be 200 minutes or less.
　なお、加熱するスラブについては、上述した化学組成を有する点以外については特に限定されない。例えば、転炉又は電気炉等を用いて上記化学組成の溶鋼を溶製し、連続鋳造法により製造したスラブを用いることができる。連続鋳造法に代えて、造塊法、薄スラブ鋳造法等を採用してもよい。 The slab to be heated is not particularly limited except that it has the chemical composition described above. For example, it is possible to use a slab produced by melting molten steel having the above-mentioned chemical composition using a converter or an electric furnace and producing it by a continuous casting method. An ingot casting method, a thin slab casting method, or the like may be employed instead of the continuous casting method.
　粗圧延工程
　１～３パス目において圧下率１０％未満の圧延を行うと、または４パス目以降において圧下率が１５％未満の圧延を行うと、旧オーステナイト粒が粗大化する。そのため、１～３パス目の各圧下率は１０％以上とし、４パス目以降の各圧下率は１５％以上とする。好ましくは、１～３パス目の各圧下率は１５％以上または２０％以上であり、４パス目以降の各圧下率は２０％以上または２５％以上である。
　また、１～３パス目において圧下率３０％超の圧延を行うと、または４パス目以降において圧下率が５０％超の圧延を行うと、旧オーステナイト粒が不均一な状態で仕上げ圧延が行われ、仕上げ圧延後に特定の方向に伸張した旧オーステナイト粒が形成されやすくなる。結晶粒が特定の方向に集中しやすくなり、結晶粒割合が高くなる結果、熱延鋼板の穴広げ性が劣化する。そのため、１～３パス目の各圧下率は３０％以下とし、４パス目以降の各圧下率は５０％以下とする。好ましくは、１～３パス目の各圧下率は２５％以下であり、４パス目以降の各圧下率は４０％以下である。 Rough Rolling Step When rolling is performed with a rolling reduction of less than 10% in the 1st to 3rd passes, or when rolling is performed with a rolling reduction of less than 15% in the 4th and subsequent passes, the prior austenite grains become coarse. Therefore, the rolling reduction ratios of the first to third passes are set to 10% or more, and the rolling reduction ratios of the fourth and subsequent passes are set to 15% or more. Preferably, each rolling reduction in the first to third passes is 15% or more or 20% or more, and each rolling reduction in the fourth pass or later is 20% or more or 25% or more.
Further, when rolling is performed with a rolling reduction of more than 30% in the 1st to 3rd passes, or when rolling is performed with a rolling reduction of more than 50% in the 4th and subsequent passes, finish rolling is performed in a state where the prior austenite grains are non-uniform. This facilitates the formation of prior austenite grains elongated in a specific direction after finish rolling. Crystal grains tend to concentrate in a specific direction, and as a result, the ratio of crystal grains increases, resulting in deterioration of the hole expansibility of the hot-rolled steel sheet. Therefore, the rolling reduction ratios of the 1st to 3rd passes are set to 30% or less, and the rolling reduction ratios of the 4th and subsequent passes are set to 50% or less. Preferably, each rolling reduction in the first to third passes is 25% or less, and each rolling reduction in the fourth and subsequent passes is 40% or less.
　なお、各パスの圧下率は、各パスの入口板厚ｔ０とし、各パスの出口板厚ｔ１としたとき、｛１－（ｔ１／ｔ０）｝×１００（％）で表すことができる。 The rolling reduction of each pass can be expressed by {1-(t1/t0)}×100(%), where t0 is the inlet plate thickness of each pass and t1 is the outlet plate thickness of each pass.
　粗圧延完了温度（粗圧延の最終パスの出側温度）は特に限定しないが、熱間変形抵抗の観点から１０７０℃以上が好ましい。また、スケール噛み込みによる疵を減らす観点からは１２００℃以下が好ましい。 Although the temperature at which rough rolling is completed (temperature on the delivery side of the final pass of rough rolling) is not particularly limited, it is preferably 1070°C or higher from the viewpoint of hot deformation resistance. In addition, from the viewpoint of reducing flaws due to entrapment of scale, the temperature is preferably 1200° C. or lower.
　仕上げ圧延工程
　仕上げ圧延工程において、以下の条件Ｉまたは条件ＩＩにおける圧下率および／または圧延温度が低すぎる場合、再結晶が十分に進まず、旧オーステナイ粒の平均粒径を小さくすることができない。また、以下のＩまたはＩＩにおける圧延温度が高すぎる場合、旧オーステナイト粒が粗大化し、熱延鋼板の穴広げ性が劣化する。そのため、仕上げ圧延工程では条件Ｉまたは条件ＩＩのいずれかを満たすように仕上げ圧延を行う。
　条件Ｉ：最終パスの圧延を９４０～１０２０℃の温度域且つ２５％超の圧下率で行う。
　条件ＩＩ：最終パスから１パス前の圧延を９４０～１０２０℃の温度域且つ２５％超の圧下率で行い、最終パスの圧延を１５％未満の圧下率で行う。 Finish Rolling Step In the finish rolling step, if the rolling reduction and/or the rolling temperature in the following condition I or condition II are too low, recrystallization does not proceed sufficiently, and the average grain size of the prior austenite grains cannot be reduced. Also, if the rolling temperature in I or II below is too high, the prior austenite grains become coarse and the hole expandability of the hot-rolled steel sheet deteriorates. Therefore, in the finish rolling step, finish rolling is performed so as to satisfy either Condition I or Condition II.
Condition I: Final pass rolling is performed at a temperature range of 940 to 1020° C. and a rolling reduction of more than 25%.
Condition II: Rolling one pass before the final pass is performed at a temperature range of 940 to 1020° C. and a rolling reduction of more than 25%, and the final pass is rolled at a rolling reduction of less than 15%.
　条件Ｉにおいて、最終パスから１パス前の圧延は、９６０℃以上で行うことが好ましく、１０２０℃以下で行うことが好ましく、３０％以上の圧下率で行うことが好ましい。圧下率は６０％以下としてもよい。条件ＩＩにおいて、最終パスから１パス前の圧延は３０％以上の圧下率で行うことが好ましく、上限は６０％以下としてもよい。最終パスの圧延は１０％未満の圧下率で行うことが好ましく、下限は５％以上としてもよい。 In condition I, the rolling one pass before the final pass is preferably performed at 960°C or higher, preferably at 1020°C or lower, and preferably at a rolling reduction of 30% or higher. The rolling reduction may be 60% or less. In Condition II, the rolling one pass before the final pass is preferably performed at a rolling reduction of 30% or more, and the upper limit may be 60% or less. The rolling in the final pass is preferably performed at a rolling reduction of less than 10%, and the lower limit may be 5% or more.
　冷却工程
　仕上げ圧延の完了から冷却開始までの時間が２．０秒超であると、オーステナイト再結晶粒の粒成長が進み、熱延鋼板の穴広げ性が劣化する。そのため、仕上げ圧延完了後、２．０秒以内に冷却を開始する。ここでいう冷却開始までの時間とは、仕上げ圧延の最終パスでの圧延後、水冷が開始されるまでの時間のことをいう。 Cooling Step If the time from the completion of finish rolling to the start of cooling exceeds 2.0 seconds, grain growth of recrystallized austenite grains proceeds, and the hole expansibility of the hot-rolled steel sheet deteriorates. Therefore, cooling is started within 2.0 seconds after completion of finish rolling. Here, the time until the start of cooling means the time until the start of water cooling after rolling in the final pass of finish rolling.
　仕上げ圧延の完了直後に冷却を開始してもよいが、仕上げ圧延機直下への冷却水を噴射することが必要となり、製造性を損ねる。冷却開始までの時間は１．０秒以上とすることが好ましい。 　Although cooling may be started immediately after the finish rolling is completed, it is necessary to inject cooling water directly below the finish rolling mill, which impairs productivity. The time until the start of cooling is preferably 1.0 second or longer.
　仕上げ圧延の完了から３００～４８０℃の温度域までの冷却時間が１７．０秒超であると、フェライトの面積率が高まり、熱延鋼板の延性および穴広げ性が劣化する。そのため、冷却開始後は、仕上げ圧延が完了してから１４．０秒以内に、３００～４８０℃の温度域まで冷却する。冷却時間は短い程好ましいが、短時間で所望の温度域まで冷却するためには量密度を高める必要があり、冷却ノズルへの負荷が増大するため経済性を損ねる。そのため、３００～４８０℃の温度域までの冷却時間は、仕上げ圧延の完了から５．０秒以上または７．０秒以上とすることが好ましい。 If the cooling time from the completion of finish rolling to the temperature range of 300 to 480°C exceeds 17.0 seconds, the area ratio of ferrite increases and the ductility and hole expansibility of the hot rolled steel sheet deteriorate. Therefore, after the start of cooling, the steel sheet is cooled to a temperature range of 300 to 480° C. within 14.0 seconds after finishing rolling is completed. A shorter cooling time is more preferable, but in order to cool to a desired temperature range in a short time, it is necessary to increase the mass density, which increases the load on the cooling nozzle and impairs economic efficiency. Therefore, the cooling time to the temperature range of 300 to 480° C. is preferably 5.0 seconds or more or 7.0 seconds or more after completion of finish rolling.
　３００～４８０℃の温度域まで冷却した後、冷却を停止する。冷却停止温度が３００℃未満であると、マルテンサイト量が増え、熱延鋼板の延性が劣化する。そのため、冷却停止温度は３００℃以上とする。好ましくは３２０℃以上である。
　一方、冷却停止温度が４８０℃超であると、パーライト量が増え、且つ残留オーステナイト量が減ることで、熱延鋼板の強度、延性および穴広げ性が劣化する。そのため、冷却停止温度は４８０℃以下とする。好ましくは４６０℃以下である。 After cooling to a temperature range of 300-480° C., cooling is stopped. If the cooling stop temperature is less than 300°C, the amount of martensite increases and the ductility of the hot-rolled steel sheet deteriorates. Therefore, the cooling stop temperature is set to 300° C. or higher. Preferably, it is 320°C or higher.
On the other hand, when the cooling stop temperature is higher than 480°C, the amount of pearlite increases and the amount of retained austenite decreases, thereby deteriorating the strength, ductility and expansibility of the hot-rolled steel sheet. Therefore, the cooling stop temperature is set to 480° C. or lower. It is preferably 460° C. or less.
　巻取り工程
　冷却停止後は直ちにコイル状に巻取る。つまり、巻取り温度と冷却停止温度とは同じ温度となる。 Winding process Immediately after cooling is stopped, the material is wound into a coil. That is, the winding temperature and the cooling stop temperature are the same temperature.
　再加熱工程
　巻取り後は、３０分以内に、単位℃での加熱量ΔＴが上記式（１）を満たすように加熱する。なお、ここでの温度は、コイルの最外周の鋼板表面温度のことである。 Reheating Step Within 30 minutes after winding, the film is heated so that the amount of heating ΔT in °C satisfies the above formula (1). The temperature here means the surface temperature of the steel sheet on the outermost circumference of the coil.
　加熱量ΔＴが上記式（１）の左辺以下であると、残留オーステナイトを安定化することができず、所望量の残留オーステナイトを得ることができない。そのため、加熱量ΔＴは上記式（１）の左辺超とする。
　一方、加熱量ΔＴが上記式（１）の右辺以上であると、パーライトが多量に生成されてしまう。そのため、加熱量ΔＴは上記式（１）の右辺未満とする。 If the heating amount ΔT is equal to or less than the left side of the above formula (1), the retained austenite cannot be stabilized and the desired amount of retained austenite cannot be obtained. Therefore, the heating amount ΔT is set to exceed the left side of the above equation (1).
On the other hand, if the heating amount ΔT is equal to or larger than the right side of the above formula (1), a large amount of pearlite will be generated. Therefore, the heating amount ΔT is set to be less than the right side of the above equation (1).
　ΔＴ加熱に要する時間が３０分を超えると、ベイナイト界面近傍で炭化物反応が生じることで、残留オーステナイト量が低減する。そのため、ΔＴ加熱に要する時間は３０分以内とすることが好ましい。 When the time required for ΔT heating exceeds 30 minutes, a carbide reaction occurs near the bainite interface, which reduces the amount of retained austenite. Therefore, the time required for ΔT heating is preferably within 30 minutes.
　加熱量ΔＴが上記式（１）を満たすように加熱した後は空冷すればよい。空冷は、コイル搬送時の安全性を考慮し、１００℃以下の温度域まで行うことが好ましい。 After heating so that the heating amount ΔT satisfies the above formula (1), air cooling may be performed. Air cooling is preferably carried out to a temperature range of 100° C. or less in consideration of safety during coil transportation.
　以上説明した工程を備える製造方法によって、本実施形態に係る熱延鋼板を製造することができる。本実施形態に係る熱延鋼板の製造方法では、更に以下の工程を備えてもよい。 The hot-rolled steel sheet according to the present embodiment can be manufactured by the manufacturing method including the steps described above. The method for manufacturing a hot-rolled steel sheet according to the present embodiment may further include the following steps.
　次に、実施例により本発明の一態様の効果を更に具体的に説明するが、実施例での条件は、本発明の実施可能性および効果を確認するために採用した一条件例であり、本発明はこの一条件例に限定されるものではない。本発明は、本発明の要旨を逸脱せず、本発明の目的を達成する限りにおいて、種々の条件を採用し得る。 Next, the effects of one aspect of the present invention will be described in more detail with reference to examples. The present invention is not limited to this one conditional example. Various conditions can be adopted in the present invention as long as the object of the present invention is achieved without departing from the gist of the present invention.
　表１および表２に示す化学組成を有するスラブを連続鋳造により製造した。得られたスラブを用いて、表３Ａ～表４Ｂに示す条件により、板厚３．０ｍｍの鋼板を製造した。なお、再加熱工程後は１００℃以下の温度域まで空冷した。また、試験Ｎｏ．５４は、熱間での変形抵抗が高かったため、仕上げ圧延以降の製造を中止した。
　表１および表２中の空欄は、当該元素を意図的に含有させていないことを示す。 Slabs having chemical compositions shown in Tables 1 and 2 were produced by continuous casting. Using the obtained slabs, steel sheets with a thickness of 3.0 mm were manufactured under the conditions shown in Tables 3A to 4B. After the reheating step, air cooling was performed to a temperature range of 100° C. or lower. Also, test no. No. 54 had high deformation resistance in hot working, so production after finish rolling was discontinued.
A blank in Tables 1 and 2 indicates that the element is not intentionally contained.
　得られた鋼板について、上述の方法により各組織の面積率、旧オーステナイト粒の平均粒径、残留オーステナイト、フレッシュマルテンサイトおよび焼き戻しマルテンサイトの結晶粒の長軸方向と圧延方向とがなす角度をθとしたとき、θが、０～１５°である前記結晶粒の割合、１５°超３０°以下である前記結晶粒の割合、３０°超４５°以下である前記結晶粒の割合、４５°超６０°以下である前記結晶粒の割合、６０°超７５°以下である前記結晶粒の割合、および７５°超９０°以下である前記結晶粒の割合、引張強さ、一様伸び、穴広げ率、並びに、予歪み付与後の一様伸びの低下量を求めた。
　得られた結果を表５Ａ～表５Ｃに示す。 For the obtained steel sheet, the area ratio of each structure, the average grain size of the prior austenite grains, the angle formed by the major axis direction of the crystal grains of retained austenite, fresh martensite and tempered martensite and the rolling direction are determined by the above-described method. When θ is θ, the ratio of the crystal grains where θ is 0 to 15°, the ratio of the crystal grains where θ is more than 15° and 30° or less, and the ratio of the crystal grains where θ is more than 30° and 45° or less is 45°. Percentage of the crystal grains that are more than 60° or less, percentage of the crystal grains that are more than 60° and less than or equal to 75°, and percentage of the crystal grains that are more than 75° and 90° or less, tensile strength, uniform elongation, hole The expansion ratio and the amount of decrease in uniform elongation after prestraining were determined.
The results obtained are shown in Tables 5A-5C.
　なお、表５Ａ～表５ＣにおけるＴＭ、Ｐ等はそれぞれ以下を示す。
　ＴＭ：焼き戻しマルテンサイト
　Ｐ：パーライト
　α：フェライト
　Ｂ：ベイナイト
　ＦＭ：フレッシュマルテンサイト
　γｒ：残留オーステナイト
　結晶粒割合の最大値：残留オーステナイト、フレッシュマルテンサイトおよび焼き戻しマルテンサイトの結晶粒の長軸方向と圧延方向とがなす角度をθとしたとき、θが、０～１５°である前記結晶粒の割合、１５°超３０°以下である前記結晶粒の割合、３０°超４５°以下である前記結晶粒の割合、４５°超６０°以下である前記結晶粒の割合、６０°超７５°以下である前記結晶粒の割合、および７５°超９０°以下である前記結晶粒の割合のうち、最大値
　ＴＭ割合：フレッシュマルテンサイトおよび焼き戻しマルテンサイトの面積率の合計のうち、焼き戻しマルテンサイトの面積率の割合 TM, P, etc. in Tables 5A to 5C respectively indicate the following.
TM: tempered martensite P: pearlite α: ferrite B: bainite FM: fresh martensite γr: retained austenite When the angle formed by the rolling direction is θ, the ratio of the crystal grains where θ is 0 to 15°, the ratio of the crystal grains where the ratio is more than 15° and 30° or less, the ratio of the crystal grains where θ is more than 30° and 45° or less Of the proportion of the crystal grains, the proportion of the crystal grains that are more than 45° and 60° or less, the proportion of the crystal grains that are more than 60° and 75° or less, and the proportion of the crystal grains that are more than 75° and 90° or less, Maximum value TM ratio: The ratio of the area ratio of tempered martensite to the total area ratio of fresh martensite and tempered martensite
　引張強さが１１００ＭＰａ以上であった場合、高い強度を有する熱延鋼板であるとして合格と判定した。一方、引張強さが１１００ＭＰａ未満であった場合、高い強度を有さない熱延鋼板であるとして不合格と判定した。 When the tensile strength was 1100 MPa or more, it was judged to be a hot-rolled steel sheet with high strength and passed. On the other hand, when the tensile strength was less than 1100 MPa, the hot-rolled steel sheet did not have high strength and was determined to be unacceptable.
　一様伸びが５．０％以上であった場合、優れた延性を有する熱延鋼板であるとして合格と判定した。一方、一様伸びが５．０％未満であった場合、優れた延性を有さない熱延鋼板であるとして不合格と判定した。 When the uniform elongation was 5.0% or more, the hot-rolled steel sheet was judged to have excellent ductility and was judged to pass. On the other hand, when the uniform elongation was less than 5.0%, the hot-rolled steel sheet did not have excellent ductility and was determined to be unacceptable.
　穴広げ率が３０％以上であった場合、優れた穴広げ性を有する熱延鋼板であるとして合格と判定した。一方、穴広げ率が３０％未満であった場合、優れた穴広げ性を有さない熱延鋼板であるとして不合格と判定した。 When the hole expansion ratio was 30% or more, it was judged to be a hot-rolled steel sheet with excellent hole expansion properties and judged to be acceptable. On the other hand, when the hole expansion rate was less than 30%, the hot rolled steel sheet did not have excellent hole expandability and was determined to be unacceptable.
　予歪み付与後の一様伸びの低下量が２．０％以下であった場合、予歪み付与後において優れた成形性を有する熱延鋼板であるとして合格と判定した。一方、予歪み付与後の一様伸びの低下量が２．０％超であった場合、予歪み付与後において優れた成形性を有さない熱延鋼板であるとして不合格と判定した。 When the amount of decrease in uniform elongation after prestraining was 2.0% or less, the hot rolled steel sheet was judged to have excellent formability after prestraining and was judged to pass. On the other hand, when the amount of decrease in uniform elongation after prestraining was more than 2.0%, the hot rolled steel sheet did not have excellent formability after prestraining and was judged to be unacceptable.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000005
Figure JPOXMLDOC01-appb-T000005
Figure JPOXMLDOC01-appb-T000006
Figure JPOXMLDOC01-appb-T000006
Figure JPOXMLDOC01-appb-T000007
Figure JPOXMLDOC01-appb-T000007
Figure JPOXMLDOC01-appb-T000008
Figure JPOXMLDOC01-appb-T000008
　表５Ａ～表５Ｂを見ると、本発明例に係る鋼板は、高い強度、優れた延性および穴広げ性、並びに、予歪み付与後において優れた成形性を有することが分かる。
　本発明例の中でも、フレッシュマルテンサイトおよび焼き戻しマルテンサイトの面積率の合計のうち、焼き戻しマルテンサイトの面積率の割合が百分率で８０．０％以上の熱延鋼板は、より優れた穴広げ性を有することが分かる。
　一方、比較例に係る鋼板は、上記特性のいずれか一つ以上が劣ることが分かる。 Tables 5A and 5B show that the steel sheets according to the invention examples have high strength, excellent ductility and hole expansibility, and excellent formability after prestraining.
Among the total area ratios of fresh martensite and tempered martensite, among the total area ratios of fresh martensite and tempered martensite, hot-rolled steel sheets in which the area ratio of tempered martensite is 80.0% or more in terms of percentage are more excellent in hole expansion. It can be seen that the
On the other hand, it can be seen that the steel sheets according to the comparative examples are inferior in at least one of the above properties.
　本発明に係る上記態様によれば、高い強度、優れた延性および穴広げ性、並びに、予歪み付与後において優れた成形性を有する鋼板およびその製造方法を提供することができる。 According to the above aspect of the present invention, it is possible to provide a steel sheet having high strength, excellent ductility and hole expansibility, and excellent formability after prestraining, and a method for producing the same.

Claims (4)

  1. 　化学組成が、質量％で、
    Ｃ　：０．１１～０．２３％、
    Ｓｉ：０．７０～１．８０％、
    Ｍｎ：１．９５～３．１０％、
    Ｐ　：０．０６０％以下、
    Ｓ　：０．００５％以下、
    Ａｌ：０．０１０～０．４３０％、
    Ｎ　：０．００７０％以下、
    Ｔｉ：０．００６～０．０５５％、
    Ｎｂ：０．００６～０．０４０％、
    Ｂ　：０．０００１～０．００３０％、
    Ｃｒ：０～０．４７０％、
    Ｍｏ：０～０．１２０％、
    Ｖ　：０～０．１０％、
    Ｃｕ：０～０．４０％、
    Ｎｉ：０～０．３０％、
    Ｃａ：０～０．０２００％、
    Ｍｇ：０～０．０２００％、
    ＲＥＭ：０～０．１０００％、
    Ｂｉ：０～０．０２０％、
    Ｚｒ、Ｃｏ、ＺｎおよびＷの合計：０～１．００％、並びに
    Ｓｎ：０～０．０５％を含有し、
    　残部がＦｅおよび不純物からなり、
    　金属組織が、面積％で、
    　ベイナイト：２５．０％以上、
    　フレッシュマルテンサイトおよび焼き戻しマルテンサイトの合計：７０．０％以下、
    　残留オーステナイト：４．０～１２．０％、
    　フェライト：５．０％以下、並びに
    　パーライト：３．０％以下であり、
    　旧オーステナイト粒の平均粒径が２５．０μｍ以下であり、
    　前記残留オーステナイト、前記フレッシュマルテンサイトおよび前記焼き戻しマルテンサイトの結晶粒の長軸方向と圧延方向とがなす角度をθとしたとき、θが、０～１５°である前記結晶粒の割合、１５°超３０°以下である前記結晶粒の割合、３０°超４５°以下である前記結晶粒の割合、４５°超６０°以下である前記結晶粒の割合、６０°超７５°以下である前記結晶粒の割合、および７５°超９０°以下である前記結晶粒の割合が百分率でそれぞれ４０％以下であることを特徴とする熱延鋼板。     The chemical composition, in mass %,
    C: 0.11 to 0.23%,
    Si: 0.70 to 1.80%,
    Mn: 1.95-3.10%,
    P: 0.060% or less,
    S: 0.005% or less,
    Al: 0.010 to 0.430%,
    N: 0.0070% or less,
    Ti: 0.006-0.055%,
    Nb: 0.006 to 0.040%,
    B: 0.0001 to 0.0030%,
    Cr: 0-0.470%,
    Mo: 0-0.120%,
    V: 0 to 0.10%,
    Cu: 0-0.40%,
    Ni: 0 to 0.30%,
    Ca: 0 to 0.0200%,
    Mg: 0-0.0200%,
    REM: 0 to 0.1000%,
    Bi: 0 to 0.020%,
    Total of Zr, Co, Zn and W: 0 to 1.00%, and Sn: 0 to 0.05%,
    The balance consists of Fe and impurities,
    The metal structure, in area %,
    Bainite: 25.0% or more,
    Total of fresh martensite and tempered martensite: 70.0% or less,
    Retained austenite: 4.0 to 12.0%,
    ferrite: 5.0% or less, and pearlite: 3.0% or less,
    The average grain size of the prior austenite grains is 25.0 μm or less,
    When the angle formed by the long axis direction of the crystal grains of the retained austenite, the fresh martensite and the tempered martensite and the rolling direction is θ, the ratio of the crystal grains where θ is 0 to 15 °, 15 The ratio of the crystal grains that are more than 30° and 45° or less, the ratio of the crystal grains that are more than 45° and 60° or less, the ratio of the crystal grains that are more than 60° and 75° or less A hot-rolled steel sheet, wherein the percentage of crystal grains and the percentage of said crystal grains having an angle of more than 75° and 90° or less are each 40% or less.
  2. 　前記フレッシュマルテンサイトおよび前記焼き戻しマルテンサイトの面積率の合計のうち、前記焼き戻しマルテンサイトの面積率の割合が百分率で８０．０％以上であることを特徴とする請求項１に記載の熱延鋼板。 The heat according to claim 1, wherein the area ratio of the tempered martensite is 80.0% or more in percentage of the total area ratio of the fresh martensite and the tempered martensite. Rolled steel plate.
  3. 　前記化学組成が、質量％で、
    Ｃｒ：０．００１～０．４７０％、
    Ｍｏ：０．００１～０．１２０％、
    Ｖ　：０．０１～０．１０％、
    Ｃｕ：０．０１～０．４０％、
    Ｎｉ：０．０２～０．３０％、
    Ｃａ：０．０００５～０．０２００％、
    Ｍｇ：０．０００５～０．０２００％、
    ＲＥＭ：０．０００５～０．１０００％、および
    Ｂｉ：０．０００５～０．０２０％、
    からなる群のうち１種または２種以上を含有することを特徴とする請求項１または２に記載の熱延鋼板。     The chemical composition, in mass %,
    Cr: 0.001 to 0.470%,
    Mo: 0.001 to 0.120%,
    V: 0.01 to 0.10%,
    Cu: 0.01 to 0.40%,
    Ni: 0.02 to 0.30%,
    Ca: 0.0005 to 0.0200%,
    Mg: 0.0005-0.0200%,
    REM: 0.0005-0.1000% and Bi: 0.0005-0.020%,
    The hot-rolled steel sheet according to claim 1 or 2, containing one or more of the group consisting of:
  4. 　請求項１に記載の熱延鋼板の製造方法であって、
    　請求項１に記載の化学組成を有するスラブを１２２０～１３００℃の温度域で４０分以上保持する加熱工程と、
    　１～３パス目の各圧下率が１０～３０％、４パス目以降の各圧下率が１５～５０％となるように粗圧延を行う粗圧延工程と、
    　最終パスの圧延を９４０～１０２０℃の温度域且つ２５％超の圧下率で行う、または、最終パスから１パス前の圧延を９４０～１０２０℃の温度域且つ２５％超の圧下率で行い、最終パスの圧延を１５％未満の圧下率で行う仕上げ圧延工程と、
    　仕上げ圧延完了後、２．０秒以内に冷却を開始し、且つ、１７．０秒以内に３００～４８０℃の温度域に冷却する冷却工程と、
    　前記温度域で巻取る巻取り工程と、
    　前記巻取り工程後、３０分以内に、加熱量ΔＴが下記式（１）を満たすように加熱する再加熱工程と、
    　２００℃以下の温度域まで空冷する空冷工程と、を順次行うことを特徴とする熱延鋼板の製造方法。
    　２４５－０．９４２×ＣＴ＋０．０００９２×ＣＴ２＜ΔＴ＜６８６－２．３８×ＣＴ＋０．００２２×ＣＴ２　…（１）
    　ただし、上記式（１）中のＣＴは冷却停止温度である。     A method for manufacturing a hot-rolled steel sheet according to claim 1,
    A heating step of holding a slab having the chemical composition according to claim 1 in a temperature range of 1220 to 1300 ° C. for 40 minutes or more;
    A rough rolling step in which rough rolling is performed so that each rolling reduction in the first to third passes is 10 to 30% and each rolling reduction in the fourth and subsequent passes is 15 to 50%;
    Perform the final pass rolling at a temperature range of 940 to 1020 ° C. and a reduction rate of more than 25%, or perform rolling one pass before the final pass at a temperature range of 940 to 1020 ° C. and a reduction rate of more than 25%, A finish rolling step in which the final pass is rolled at a rolling reduction of less than 15%;
    A cooling step of starting cooling within 2.0 seconds after completion of finish rolling and cooling to a temperature range of 300 to 480 ° C. within 17.0 seconds;
    A winding step of winding in the temperature range;
    A reheating step of heating so that the heating amount ΔT satisfies the following formula (1) within 30 minutes after the winding step;
    and an air-cooling step of air-cooling to a temperature range of 200° C. or less.
    245−0.942×CT+0.00092×CT 2 <ΔT<686−2.38×CT+0.0022×CT 2 (1)
    However, CT in the above formula (1) is the cooling stop temperature.
PCT/JP2023/000071 2022-01-07 2023-01-05 Hot-rolled steel sheet and method for producing same WO2023132342A1 (en)

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Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2009263715A (en) * 2008-04-24 2009-11-12 Nippon Steel Corp Hot-rolled steel plate superior in hole expandability and manufacturing method therefor
JP2014510838A (en) * 2011-02-18 2014-05-01 ティッセンクルップ スチール ヨーロッパ アクチェンゲゼルシャフト Hot rolled flat steel product manufactured from composite phase steel and method for manufacturing the same
WO2018151273A1 (en) * 2017-02-16 2018-08-23 新日鐵住金株式会社 Hot rolled steel sheet and method for manufacturing same
WO2020170681A1 (en) * 2019-02-18 2020-08-27 日本製鉄株式会社 Hot-rolled steel sheet and method for manufacturing same
WO2020203943A1 (en) * 2019-04-04 2020-10-08 日本製鉄株式会社 Galvanized steel sheet and method for producing same
WO2021187321A1 (en) * 2020-03-17 2021-09-23 Ｊｆｅスチール株式会社 High-strength steel sheet and method for manufacturing same

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2009263715A (en) * 2008-04-24 2009-11-12 Nippon Steel Corp Hot-rolled steel plate superior in hole expandability and manufacturing method therefor
JP2014510838A (en) * 2011-02-18 2014-05-01 ティッセンクルップ スチール ヨーロッパ アクチェンゲゼルシャフト Hot rolled flat steel product manufactured from composite phase steel and method for manufacturing the same
WO2018151273A1 (en) * 2017-02-16 2018-08-23 新日鐵住金株式会社 Hot rolled steel sheet and method for manufacturing same
WO2020170681A1 (en) * 2019-02-18 2020-08-27 日本製鉄株式会社 Hot-rolled steel sheet and method for manufacturing same
WO2020203943A1 (en) * 2019-04-04 2020-10-08 日本製鉄株式会社 Galvanized steel sheet and method for producing same
WO2021187321A1 (en) * 2020-03-17 2021-09-23 Ｊｆｅスチール株式会社 High-strength steel sheet and method for manufacturing same

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