Classifications

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  • 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
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  • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
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  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
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  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/008—Heat treatment of ferrous alloys containing Si
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
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  • 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
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
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  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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  • C22C—ALLOYS
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  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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  • 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
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
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  • 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
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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  • 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
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • 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
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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  • 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
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • 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
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • 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
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys 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
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling

Definitions

• the present invention relates to a hot rolled steel sheet.
• the workability generally decreases when the strength of the steel sheet is increased.
• the part of the steel plate applicable to the part may be limited. be. Therefore, from the viewpoint of yield, it is preferable that the characteristics are uniform in the plate width direction of the steel sheet.
• Patent Document 1 in terms of mass%, C: 0.05% or more, 0.2% or less, Si: 0.01% or more, 0.6% or less, Mn: 0.5. % Or more, 2.5% or less, P: 0.001% or more, 0.1% or less, S: 0.0005% or more, 0.05% or less, Al: 0.01% or more, 0.2% or less , N: 0.0001% or more, 0.010% or less, Mo: 0.05% or more, 0.5% or less, Ti: 48N / 14 + 0.01% or more, 0.14% or less, B: 0.0003 % Or more and 0.005% or less, 70 ⁇ 300 ⁇ C (mass%) + 33 ⁇ Mn (mass%) + 22 ⁇ Cr (mass%) + 11 ⁇ Mo (mass%) + 11 ⁇ Si (mass%) + 17 ⁇ Ni (% by mass) It is contained in the range satisfying the formula of ⁇ 100, the balance has a steel composition consisting of iron and unavoidable impurities,
• a high-strength hot-rolled steel sheet having a high yield ratio and a high-strength hot-rolled steel sheet, characterized in that the variation in yield strength in the width direction is within 50 MPa is described. Further, according to Patent Document 1, it is possible to obtain a high-yield ratio high-strength steel plate having a small variation in strength in the plate width direction and an excellent yield strength of 960 MPa or more and a yield ratio of 0.83 or more by the above configuration. Have been described.
• Patent Document 2 a slab produced by continuous casting and containing C: 0.05 to 0.12%, N: 0.001 to 0.005% and Ti: 0.04 to 0.15% by weight% is used. It is charged into a heating furnace and heated, and is retained under the condition that the retention time t (hours) is T. A method for manufacturing a Ti-added high-strength hot-rolled steel sheet in which the variation in strength between steel sheets due to rolling is small is described. Further, in Patent Document 2, by quantifying the heating conditions for sufficiently dissolving the added Ti in a solid solution, it is possible to suppress the variation in the strength of the steel sheet due to the insufficient solution of the Ti, and the strength specifications are hardly deviated, and the steel sheet is demoted. It is stated that the steel plate will run out.
• Patent Document 3 in terms of mass%, C: 0.020 to 0.065%, Si: 0.1% or less, Mn: 0.40 to less than 0.80%, P: 0.030% or less, S: It contains 0.005% or less, Ti: 0.08 to 0.20%, Al: 0.005 to 0.1%, N: 0.005% or less, and the balance consists of Fe and unavoidable impurities.
• a high-strength hot- rolled steel sheet is described in which 80% or more of Ti is precipitated as a Ti carbide.
• Patent Document 3 a high-strength hot-rolled steel sheet having high strength, excellent ductility and stretch flangeability, and good material uniformity with little variation in strength in the steel sheet due to the above configuration, more specifically. It is described that a high-strength hot-rolled steel sheet having a variation in tensile strength (TS) of ⁇ TS of 15 MPa or less can be obtained.
• TS tensile strength
• Patent Document 4 C: 0.05 to 0.18 mass%, Si: 0.7 to 1.5 mass%, Mn: 0.6 to 1.8 mass%, P: 0.04 mass% or less, substantially S: 0.005 mass% or less, Al: 0.01 to 0.10 mass%, N: 0.005 mass% or less, Mo: 0.05 to 1.5 mass%, and the balance is composed of Fe.
• a high-workability, high-strength hot-rolled steel sheet with little variation in the material inside the coil is described. Further, Patent Document 4 describes that the material of the hot-rolled steel sheet is uniform over the entire length and width of the coil, and the material variation in the coil is appropriately suppressed.
• the solid solution V has a component composition of 0.05% or more and less than 0.15%, and the area ratio of the ferrite phase to the entire structure is Fine carbides containing Ti and V and having an average particle size of less than 10 nm are dispersed and precipitated in a matrix of 95% or more, and the volume ratio of the fine carbides to the entire structure is 0.0050 or more, and the particles containing Ti.
• a high-strength hot-rolled steel plate having a structure in which the ratio of the number of carbides having a diameter of 30 nm or more to the total number of carbides is less than 10% and having a tensile strength of 980 MPa or more is described.
• Patent Documents 1 to 3 there are relatively many studies on suppressing the strength variation in the hot-rolled steel sheet, but even if the strength variation is simply suppressed, various processing is involved. When manufacturing a part having a complicated shape, cracks may occur depending on the part of the steel plate applied to the part, and in such a case, the yield is lowered as a result.
• Patent Documents 4 and 5 also study the uniformity in the width direction in characteristics other than strength, for example, in Patent Document 4, the specific measurement position in the width direction is not always clear. Further, also in Patent Document 5, although the difference in characteristics between the central portion of the plate width and the 1/4 width position is shown, a region relatively close to the end portion in the plate width direction in which it is more difficult to control the characteristics is included. Homogeneity has not always been fully investigated. If the characteristics are not sufficiently uniform even in the region relatively close to the end in the plate width direction, cracks may occur depending on the part of the steel plate applied to more complicated parts, resulting in a decrease in yield. It will be invited.
• an object of the present invention is to provide a hot-rolled steel sheet capable of suppressing the occurrence of cracks and improving the yield even when manufacturing a part having a complicated shape.
• the present inventors have made a hole in a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more, rather than characteristics such as tensile strength and yield strength as proposed in the prior art. Focusing on the expandability, it has been found that by controlling this so as to satisfy a predetermined formula, even a part having a complicated shape can be manufactured with a high yield, and the present invention has been completed.
• ⁇ W1 and ⁇ W2 indicate the hole expansion ratio (%) at the 1/8 position of the plate width from one end in the plate width direction of the hot-rolled steel sheet orthogonal to the rolling direction and the other end on the opposite side.
• ⁇ C indicates the hole expansion ratio (%) at the center of the plate width, and ⁇ W1 , ⁇ W2 and ⁇ C are 40% or more, respectively.
• TS W1 and TS W2 indicate the tensile strength (MPa) at the position of 1/8 of the plate width from one end in the plate width direction of the hot-rolled steel sheet orthogonal to the rolling direction and the other end on the opposite side.
• TS C indicates the tensile strength (MPa) at the center of the plate width.
• TS C indicates the tensile strength (MPa) at the center of the plate width.
• ⁇ E1 and ⁇ E2 determine the hole expansion ratio (%) at a position of 75 mm from one end in the plate width direction of the hot-rolled steel sheet orthogonal to the rolling direction and the other end on the opposite side to the center of the plate width.
• ⁇ C indicates the hole expansion rate (%) at the center of the plate width.
• the chemical composition is mass%.
• the chemical composition is mass%.
• the hot-rolled steel sheet according to (6) above which comprises at least one selected from the group consisting of.
• (8) The hot-rolled steel sheet according to (6) or (7) above, wherein the
• thermoforming steel sheet that can be manufactured with a high yield even for parts having a complicated shape.
• Such hot-rolled steel sheets are particularly suitable for manufacturing parts having a more complicated shape and requiring high strength, such as undercarriage parts of automobiles, and thus have industrial applicability. Is also very expensive.
• the hot-rolled steel sheet according to the embodiment of the present invention has a tensile strength of 780 MPa or more, a plate thickness of 1.2 to 4.0 mm, and a plate width of 750 mm or more, and is characterized by satisfying the following formula 1. -15 ⁇ ( ⁇ W1 + ⁇ W2 ) / 2- ⁇ C ⁇ 15 ⁇ ⁇ ⁇ Equation 1
• ⁇ W1 and ⁇ W2 indicate the hole expansion ratio (%) at the 1/8 position of the plate width from one end in the plate width direction of the hot-rolled steel sheet orthogonal to the rolling direction and the other end on the opposite side.
• ⁇ C indicates the hole expansion ratio (%) at the center of the plate width
• ⁇ W1 , ⁇ W2 and ⁇ C are 40% or more, respectively.
• the characteristics are uniform in the plate width direction of the steel sheet from the viewpoint of yield.
• the application of high-strength steel sheets to automobile members has been actively studied for the purpose of improving the durability and collision safety of automobiles.
• the strength of a steel sheet is increased, the workability generally deteriorates, and the characteristics of the steel sheet are strongly influenced by the structure of the steel sheet. It may not be possible to make it sufficiently uniform in the plate width direction. As a result, the material properties of the steel sheet may differ significantly in the plate width direction.
• the position of the burring processed part and the stretch flange formed part during press forming differs for each part, so the strength variation, especially the tensile strength, Even if the variation in yield strength is suppressed in the plate width direction of the steel sheet, cracks may occur during press forming depending on the portion of the steel sheet applied to the part, resulting in a decrease in yield.
• the present inventors pay attention to the hole expansion ratio of the steel sheet rather than the characteristics such as the tensile strength, and are measured in the plate width direction. It has been found that by controlling the hole expansion ratio of the above formula 1 so as to satisfy the relationship of the above formula 1, it is possible to suppress the occurrence of cracks and the like even for parts having a complicated shape and to manufacture the parts with good yield. Therefore, according to the hot-rolled steel sheet according to the embodiment of the present invention, even in the manufacture of parts having a more complicated shape and high strength, such as undercarriage parts of automobiles.
• the part of the steel plate that can be applied to the part is not limited, not only the degree of design freedom can be increased, but also it is very advantageous from the viewpoint of yield.
• strength characteristics such as tensile strength are controlled in the plate width direction in order to improve desired characteristics of the steel plate, and the structure of the steel plate is used to suppress variations in strength such as tensile strength.
• the technical idea of controlling the hole expandability in the plate width direction more specifically, the high-strength hot material having a tensile strength of 780 MPa or more, particularly 850 MPa or more or 980 MPa or more.
• the technical idea of controlling the hole expandability in the plate width direction so as to satisfy a predetermined formula in order to manufacture a part having a complicated shape from a rolled steel plate with a high yield has never been achieved. It was first discovered by them.
• the hot-rolled steel sheet according to the embodiment of the present invention has a tensile strength of 780 MPa or more, and may be, for example, 850 MPa or more, 980 MPa or more, 990 MPa or more, or 1040 MPa or more.
• the hot-rolled steel sheet according to the embodiment of the present invention has such a high tensile strength, the hole expandability is sufficiently controlled in the plate width direction, so that the hot-rolled steel sheet is complicated by, for example, press forming. Even when manufacturing a part having a shape, it is possible to remarkably suppress the occurrence of cracks or the like in the portion of the steel plate used without particular limitation.
• the upper limit of the tensile strength is not particularly limited, but for example, the tensile strength of the hot-rolled steel sheet may be 2000 MPa or less, 1470 MPa or less, 1250 MPa or less, or 1180 MPa or less.
• Tensile strength is determined by collecting JIS Z2241: 2011 No. 5 tensile test pieces from the plate width 1/8 position of the hot-rolled steel sheet in the direction perpendicular to the rolling direction, and performing a tensile test twice in accordance with JIS Z2241: 2011. , Determined by averaging the values of tensile strength obtained. More specifically, the lower value of TS W1 and TS W2 , which will be described in detail later, is determined as the tensile strength of the hot-rolled steel sheet.
• the hot-rolled steel sheet may be any material satisfying the requirement that the tensile strength is 780 MPa or more. Therefore, the chemical composition of the hot-rolled steel sheet is not particularly limited, and may be appropriately determined within a range in which the tensile strength satisfies the requirement of 780 MPa or more. More specifically, as described above, it is an object of the present invention to provide a hot-rolled steel sheet capable of suppressing the occurrence of cracks and improving the yield even when a part having a complicated shape is manufactured.
• the plurality of hole expansion ratios measured in the plate width direction are controlled so as to satisfy the relationship of the above formula 1. It achieves the purpose. Therefore, it is clear that the chemical composition of the hot-rolled steel sheet is not an essential technical feature for achieving the object of the present invention.
• preferable chemical compositions for a hot-rolled steel sheet having a tensile strength of 780 MPa or more according to an embodiment of the present invention will be described in detail, but these descriptions describe the hot-rolled steel sheet having a tensile strength of 780 MPa or more.
• the present invention is intended merely as an example, and is not intended to limit the present invention to hot-rolled steel sheets having such a specific chemical composition.
• “%”, which is a unit of the content of each element, means “mass%” unless otherwise specified.
• "-" indicating a numerical range is used to mean that the numerical values described before and after the numerical range are included as the lower limit value and the upper limit value unless otherwise specified.
• C is an element effective for increasing the strength of the steel sheet.
• the C content is preferably 0.01% or more.
• the C content may be 0.03% or more, 0.05% or more, 0.08% or more, 0.10% or more, or 0.12% or more.
• the toughness may decrease. Therefore, the C content is preferably 0.50% or less.
• the C content may be 0.40% or less, 0.35% or less, 0.30% or less, 0.25% or less, 0.22% or less, or 0.19% or less.
• Si is an element effective for increasing the strength as a solid solution strengthening element.
• the Si content is preferably 0.01% or more.
• the Si content may be 0.05% or more, 0.10% or more, 0.20% or more, 0.30% or more, 0.50% or more, or 0.80% or more.
• the Si content is preferably 3.50% or less.
• the Si content may be 3.00% or less, 2.50% or less, 2.00% or less, 1.50% or less, 1.20% or less, or 1.00% or less.
• Mn is an element effective for increasing strength as a hardenable and solid solution strengthening element.
• the Mn content is preferably 0.20% or more.
• the Mn content may be 0.50% or more, 0.80% or more, or 1.00% or more.
• the Mn content is preferably 3.00% or less.
• the Mn content may be 2.70% or less, 2.50% or less, 2.00% or less, 1.60% or less, or 1.40% or less.
• the P content is preferably 0.100% or less.
• the P content may be 0.080% or less, 0.050% or less, 0.030% or less, or 0.025% or less.
• the lower limit of the P content is not particularly limited and may be 0%, but an excessive reduction causes an increase in cost. Therefore, the P content may be 0.0001% or more, 0.001% or more, or 0.005% or more.
• the Si content is preferably 0.0200% or less.
• the S content may be 0.0150% or less, 0.0100% or less, or 0.0050% or less.
• the lower limit of the S content is not particularly limited and may be 0%, but an excessive reduction causes an increase in cost. Therefore, the S content may be 0.0001% or more or 0.0005% or more.
• N 0.0100% or less
• the N content is preferably 0.0100% or less.
• the N content may be 0.0080% or less or 0.0050% or less.
• the lower limit of the N content is not particularly limited and may be 0%, but an excessive reduction causes an increase in cost. Therefore, the N content may be 0.0001% or more or 0.0005% or more.
• Al is an element that acts as a deoxidizing agent.
• the Al content is preferably 0.001% or more.
• the Al content may be 0.005% or more, 0.010% or more, or 0.015% or more.
• the Al content is preferably 1.000% or less.
• the Al content may be 0.500% or less, 0.300% or less, 0.200% or less, 0.100% or less, 0.050% or less, or 0.030% or less.
• the basic chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention is as described above. Further, the hot-rolled steel sheet may contain at least one of the following optional elements in place of a part of Fe in the balance, if necessary.
• Cu is an element that contributes to the improvement of strength and / or corrosion resistance.
• the Cu content may be 0%, but in order to obtain these effects, the Cu content is preferably 0.001% or more.
• the Cu content may be 0.01% or more, 0.05% or more, or 0.10% or more.
• the Cu content is preferably 1.00% or less.
• the Cu content may be 0.80% or less, 0.60% or less, 0.40% or less, 0.25% or less, or 0.15% or less.
• Ni is an element that enhances the hardenability of steel and contributes to the improvement of strength and / or heat resistance.
• the Ni content may be 0%, but in order to obtain these effects, the Ni content is preferably 0.001% or more.
• the Ni content may be 0.01% or more, 0.03% or more, or 0.05% or more.
• the Ni content is preferably 0.50% or less.
• the Ni content may be 0.40% or less, 0.30% or less, 0.20% or less, or 0.10% or less.
• Cr is an element that enhances the hardenability of steel and contributes to the improvement of strength and / or corrosion resistance.
• the Cr content may be 0%, but in order to obtain these effects, the Cr content is preferably 0.001% or more.
• the Cr content may be 0.01% or more, 0.03% or more, or 0.10% or more.
• the Cr content is preferably 2.00% or less.
• the Cr content may be 1.50% or less, 1.00% or less, 0.50% or less, 0.30% or less, or 0.15% or less.
• Mo is an element that enhances the hardenability of steel and contributes to the improvement of strength, and is also an element that contributes to the improvement of corrosion resistance.
• the Mo content may be 0%, but in order to obtain these effects, the Mo content is preferably 0.001% or more.
• the Mo content may be 0.005% or more, 0.01% or more, or 0.02% or more.
• the Mo content is preferably 3.00% or less.
• the Mo content may be 2.00% or less, 1.00% or less, or 0.50% or less.
• the material variation may be relatively high in the high-strength steel plate.
• material properties such as hole expandability in the plate width direction can be made uniform regardless of the Mo content. Therefore, the Mo content may be 0% as described above, for example, less than 0.05%, 0.04% or less, or 0.03% or less.
• W is an element that enhances the hardenability of steel and contributes to the improvement of strength.
• the W content may be 0%, but in order to obtain such an effect, the W content is preferably 0.001% or more.
• the W content may be 0.005% or more or 0.01% or more.
• the W content is preferably 0.10% or less.
• the W content may be 0.08% or less, 0.05% or less, or 0.03% or less.
• Nb is an element that contributes to the improvement of strength by strengthening precipitation.
• the Nb content may be 0%, but in order to obtain such an effect, the Nb content is preferably 0.001% or more.
• the Nb content may be 0.005% or more, 0.010% or more, or 0.020% or more.
• the Nb content is preferably 0.060% or less.
• the Nb content may be 0.050% or less or 0.030% or less.
• V is an element that contributes to the improvement of strength by strengthening precipitation and the like.
• the V content may be 0%, but in order to obtain such an effect, the V content is preferably 0.001% or more.
• the V content may be 0.01% or more, 0.03% or more, or 0.05% or more.
• the V content is preferably 1.00% or less.
• the V content may be 0.80% or less, 0.50% or less, 0.30% or less, 0.11% or less, or 0.07% or less.
• Ti is an element that contributes to the improvement of strength by strengthening precipitation.
• the Ti content may be 0%, but in order to obtain such an effect, the Ti content is preferably 0.001% or more.
• the Ti content may be 0.01% or more, 0.03% or more, or 0.05% or more.
• the Ti content is preferably 0.20% or less.
• the Ti content may be 0.15% or less, 0.12% or less, or 0.07% or less.
• B is an element that enhances the hardenability of steel and contributes to the improvement of strength.
• the B content may be 0%, but in order to obtain such an effect, the B content is preferably 0.0001% or more.
• the B content may be 0.0002% or more, 0.0003% or more, or 0.0005% or more.
• the B content is preferably 0.0040% or less.
• the B content may be 0.0030% or less, 0.0020% or less, or 0.0010% or less.
• O is an element mixed in the manufacturing process.
• the O content may be 0%.
• the O content may be 0.0001% or more, 0.0005% or more, or 0.001% or more.
• the O content is preferably 0.020% or less.
• the O content may be 0.015% or less, 0.010% or less, or 0.005% or less.
• Ta is an element effective in controlling the morphology of carbides and increasing their strength.
• the Ta content may be 0%, but in order to obtain these effects, the Ta content is preferably 0.001% or more.
• the Ta content may be 0.005% or more, 0.01% or more, or 0.02% or more.
• the Ta content is preferably 0.10% or less.
• the Ta content may be 0.08% or less, 0.06% or less, or 0.04% or less.
• Co is an element that contributes to the improvement of hardenability and / or heat resistance.
• the Co content may be 0%, but in order to obtain these effects, the Co content is preferably 0.001% or more.
• the Co content may be 0.01% or more, 0.02% or more, or 0.05% or more.
• the Co content is preferably 3.00% or less.
• the Co content may be 2.00% or less, 1.00% or less, 0.50% or less, 0.20% or less, or 0.10% or less.
• Sn is an element effective for improving corrosion resistance.
• the Sn content may be 0%, but in order to obtain such an effect, the Sn content is preferably 0.001% or more.
• the Sn content may be 0.005% or more, 0.01% or more, or 0.02% or more.
• the Sn content is preferably 1.00% or less.
• the Sn content may be 0.80% or less, 0.50% or less, 0.30% or less, 0.10% or less, or 0.05% or less.
• Sb is an element effective for improving corrosion resistance.
• the Sb content may be 0%, but in order to obtain such an effect, the Sb content is preferably 0.001% or more.
• the Sb content may be 0.005% or more or 0.01% or more.
• excessive content of Sb may lead to a decrease in toughness. Therefore, the Sb content is preferably 0.50% or less.
• the Sb content may be 0.30% or less, 0.10% or less, or 0.05% or less.
• the As content may be 0%, but in order to obtain such an effect, the As content is preferably 0.001% or more.
• the As content may be 0.005% or more or 0.010% or more.
• the As content is 0.050% or less.
• the As content may be 0.040% or less, 0.030% or less, or 0.020% or less.
• Mg is an element that can control the morphology of sulfides.
• the Mg content may be 0%, but in order to obtain such an effect, the Mg content is preferably 0.0001% or more.
• the Mg content may be 0.0005% or more, 0.001% or more, or 0.005% or more.
• the Mg content is preferably 0.050% or less.
• the Mg content may be 0.030% or less, 0.020% or less, or 0.015% or less.
• Zr is an element that can control the morphology of sulfides.
• the Zr content may be 0%, but in order to obtain such an effect, the Zr content is preferably 0.0001% or more.
• the Zr content may be 0.003% or more, 0.005% or more, or 0.01% or more.
• the Zr content is preferably 0.050% or less.
• the Zr content may be 0.040% or less, 0.030% or less, or 0.020% or less.
• Ca is an element whose morphology of sulfide can be controlled by adding a small amount.
• the Ca content may be 0%, but in order to obtain such an effect, the Ca content is preferably 0.0001% or more.
• the Ca content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more.
• the Ca content is preferably 0.0500% or less.
• the Ca content may be 0.0300% or less, 0.0200% or less, 0.0100% or less, 0.0070% or less, or 0.0040% or less.
• REM 0 to 0.0500%
• the REM content may be 0%, but in order to obtain such an effect, the REM content is preferably 0.0001% or more.
• the REM content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more.
• the REM content is preferably 0.0500% or less.
• the REM content may be 0.0300% or less, 0.0200% or less, 0.0100% or less, 0.0070% or less, or 0.0040% or less.
• the REMs are scandium (Sc) having an atomic number of 21, yttrium (Y) having an atomic number of 39, and lanthanum (La) having an atomic number of 57 to lutetium (Lu) having an atomic number of 71, which are lanthanoids. ) Is a general term for the 17 elements, and the REM content is the total content of these elements.
• the balance other than the above elements consists of Fe and impurities.
• Impurities are components that are mixed in by various factors in the manufacturing process, including raw materials such as ore and scrap, when hot-rolled steel sheets are industrially manufactured.
• the hot-rolled steel sheet according to the embodiment of the present invention has a plate thickness of 1.2 to 4.0 mm. By defining the plate thickness within an appropriate range, it is possible to ensure that the hole expansion ratio in the plate width direction satisfies Equation 1.
• the plate thickness may be 1.5 mm or more or 2.0 mm or more, and / or 3.5 mm or less or 3.0 mm or less.
• the plate thickness means the plate thickness at the center of the plate width.
• the hot-rolled steel sheet according to the embodiment of the present invention has a plate width of 750 mm or more.
• the plate width may be 800 mm or more, 900 mm or more, or 1000 mm or more.
• the upper limit of the plate width is not particularly limited, but from the viewpoint of ensuring that the hole expansion ratio in the plate width direction satisfies Equation 1, the plate width is preferably 2500 mm or less, and 2000 mm or less and 1800 mm or less. It may be 1600 mm or less, 1500 mm or less, 1400 mm or less, or 1300 mm or less.
• the hot-rolled steel sheet according to the embodiment of the present invention satisfies the following formula 1.
• ⁇ W1 and ⁇ W2 indicate the hole expansion ratio (%) at the 1/8 position of the plate width from one end in the plate width direction of the hot-rolled steel sheet orthogonal to the rolling direction and the other end on the opposite side.
• ⁇ C indicates the hole expansion rate (%) at the center of the plate width.
• one end of the hot-rolled steel sheet in the plate width direction and the other end on the opposite side thereof may be in a relationship opposite to each other, and are not limited to a specific side of the hot-rolled steel sheet. Therefore, one end may be the so-called work side (one side in the plate width direction of the steel plate operated by the operator) or the drive side (the other side in the plate width direction of the steel plate in which the drive system device is installed). Similarly, the other end may be the work side or the drive side.
• the hole expansion property in the plate width direction is made uniform.
• the burring workability and the stretch flange formability in the above can be made uniform.
• ( ⁇ W1 + ⁇ W2 ) / 2- ⁇ C is preferably -14 or more, more preferably -12 or more, even more preferably -10 or more, and most preferably -8 or more.
• ( ⁇ W1 + ⁇ W2 ) / 2- ⁇ C is preferably 14 or less, more preferably 12 or less, even more preferably 10 or less, and most preferably 8 or less.
• the hole expansion ratios ⁇ W1 , ⁇ W2 and ⁇ C are 40% or more, respectively.
• the drilling ratios ⁇ W1 , ⁇ W2 and ⁇ C may be 41% or more, 42% or more, 43% or more, 44% or more, 45% or more, 47% or more, 49% or more or 52% or more, respectively. ..
• the upper limit is not particularly limited, but the hole expansion rates ⁇ W1 , ⁇ W2 , and ⁇ C may be, for example, 90% or less, 85% or less, or 80% or less, respectively.
• the drilling ratios ⁇ W1 , ⁇ W2 and ⁇ C are determined as follows by performing a drilling test in accordance with JIS Z2256: 2020. First, 1/8 of the plate width on the same line in the direction perpendicular to the rolling direction from the end in the plate width direction of either the work side or the drive side of the hot-rolled steel plate toward the center of the plate width. , The test piece is collected from the center of the plate width and further from the 7/8 position of the plate width.
• the hole diameter d1 mm is measured, and the hole expansion ratio ⁇ (%) of each test piece is obtained by the following formula.
• the hot-rolled steel sheet further satisfies the following formula 2 in addition to the above formula 1.
• TS W1 and TS W2 indicate the tensile strength (MPa) at the position of 1/8 of the plate width from one end in the plate width direction of the hot-rolled steel sheet orthogonal to the rolling direction and the other end on the opposite side.
• TS C indicates the tensile strength (MPa) at the center of the plate width.
• (TS W1 + TS W2 ) / 2-TS C is preferably -60 or more, more preferably -40 or more, even more preferably -30 or more, and most preferably -25 or more.
• (TS W1 + TS W2 ) / 2-TS C is preferably 60 or less, more preferably 40 or less, even more preferably 30 or less, and most preferably 25 or less.
• the tensile strengths TS W1 , TS W2 and TS C are determined as follows. First, 1/8 of the plate width on the same line in the direction perpendicular to the rolling direction from the end in the plate width direction of either the work side or the drive side of the hot-rolled steel plate toward the center of the plate width. , JIS Z2241: 2011 No. 5 tensile test piece is collected from the central part of the plate width and further from the 7/8 position of the plate width in the direction perpendicular to the rolling direction. Next, a tensile test based on JIS Z2241: 2011 is performed using each of the collected test pieces to determine the tensile strength (MPa) of each test piece.
• MPa tensile strength
• the hot-rolled steel sheet further satisfies the following formula 3 in addition to the above formula 1 and / or the formula 2.
• ⁇ E1 and ⁇ E2 determine the hole expansion ratio (%) at a position of 75 mm from one end in the plate width direction of the hot-rolled steel sheet orthogonal to the rolling direction and the other end on the opposite side to the center of the plate width.
• ⁇ C indicates the hole expansion rate (%) at the center of the plate width as described in Equation 1 above.
• Equation 3 By satisfying the relationship of Equation 3 in the hole expanding ratios of both end portions and the central portion in the plate width direction, the hole expanding property is surely made uniform up to the region closer to the end portion in the plate width direction. Therefore, the burring workability and the stretch flange formability in the plate width direction of the hot-rolled steel sheet can be made more uniform as compared with the case where the formula 1 is simply satisfied, and the parts having a complicated shape by press forming or the like can be made more uniform. Can be manufactured with even better yield.
• ( ⁇ E1 + ⁇ E2 ) / 2- ⁇ C is preferably -14 or more, more preferably -12 or more, even more preferably -10 or more, and most preferably -8 or more.
• ( ⁇ E1 + ⁇ E2 ) / 2- ⁇ C is preferably 14 or less, more preferably 12 or less, even more preferably 10 or less, and most preferably 8 or less.
• the specific values of the hole expansion ratios ⁇ E1 and ⁇ E2 are not particularly limited as long as they satisfy the above equation 3, but are preferably 30% or more, respectively.
• the drilling ratios ⁇ E1 and ⁇ E2 may be 33% or more, 35% or more, 40% or more, 45% or more, 47% or more, 49% or more, or 52% or more, respectively.
• the upper limit is not particularly limited, but the hole expansion rates ⁇ E1 and ⁇ E2 may be, for example, 90% or less, 85% or less, or 80% or less, respectively.
• the hole expansion ratios ⁇ E1 and ⁇ E2 are changed from the 1/8 position and the 7/8 position of the plate width, from the position of 75 mm to the center of the plate width from one end in the plate width direction and the other end on the opposite side, respectively. Except for the fact that the test piece was taken, the hole expansion rate ⁇ W1 and ⁇ W2 are determined by performing a hole expansion test in accordance with JIS Z2256: 2020 in the same manner as described above.
• the microstructure of the hot-rolled steel sheet may be any microstructure that satisfies the requirement that the tensile strength is 780 MPa or more.
• the microstructure of the hot-rolled steel sheet may contain ferrite and bainite in a total amount of more than 50 area%, 55 area% or more, 60 area% or more, or 70 area% or more.
• the microstructure of the hot-rolled steel sheet may be composed of only ferrite and bainite, that is, may contain 100 area% of ferrite and bainite in total.
• the microstructure of the hot-rolled steel sheet may contain ferrite and bainite in a total amount of 95 area% or less, 90 area% or less, 85 area% or less, or 80 area% or less.
• the microstructure of the hot-rolled steel sheet may contain ferrite in an amount of 90 area% or less, 80 area% or less, 75 area% or less, or 70 area% or less.
• the baynite in the microstructure of the hot-rolled steel sheet may be 15 area% or more, 25 area% or more, 35 area% or more, 45 area% or more or 50 area% or more, 90 area% or less, 95 area% or more.
• it may be 85 area% or less, 75 area% or less, 65 area% or less, or 60 area% or less.
• the microstructure of the hot-rolled steel sheet does not have to contain martensite, but when it contains martensite, the martensite content is 20 area% or less, 15 area% or less, 10 area% or less, or 5 area% or less. Is preferable.
• the microstructure of the hot-rolled steel sheet can also contain structures other than ferrite, bainite and martensite, such as retained austenite and pearlite, and these residual structures are preferably 20 area% or less, 15 area% or less, 10 Area% or less or 5 area% or less.
• the microstructure is identified and the area ratio is calculated by the following method. First, a sample collected from a depth of 1/4 of the thickness of a hot-rolled steel sheet is polished and then etched with nital. Then, by performing image analysis on the histological photograph obtained in the field of view of 300 ⁇ m ⁇ 300 ⁇ m using an optical microscope, the area ratio of ferrite and pearlite, and the total area ratio of bainite and martensite are obtained. Then, using a sample corroded by the repera, retained austenite and martensite were subjected to image analysis on the microstructure photograph obtained in the field of view of 300 ⁇ m ⁇ 300 ⁇ m at the depth position of 1/4 of the plate thickness using an optical microscope. Calculate the total area ratio of.
• the volume fraction of retained austenite is determined by X-ray diffraction measurement using a sample collected from a depth of 1/4 of the plate thickness from the direction normal to the rolled surface. Since the volume fraction of retained austenite is equivalent to the area fraction, this is used as the area fraction of retained austenite.
• the area ratio of martensite can be calculated by subtracting the area ratio of retained austenite obtained by X-ray diffraction measurement from the total area ratio of retained austenite and martensite obtained by an optical microscope and image analysis.
• the area ratio of bainite can be calculated by subtracting the area ratio of this martensite from the total area ratio of bainite and martensite obtained by an optical microscope and image analysis. Therefore, by the above method, the area ratios of ferrite, bainite, martensite, retained austenite and pearlite can be obtained.
• a preferred method for producing a hot-rolled steel sheet according to an embodiment of the present invention includes a hot-rolling step of hot-rolling a slab having a predetermined chemical composition, and a cooling step of cooling and winding the obtained rolled material.
• the difference between the rolling load (ton) of the final rolling stand in the hot rolling process and the average cooling rate due to water cooling between the 1/8 position of the plate width and the center of the plate width from both ends in the plate width direction in the cooling process ( °C / s) is characterized by satisfying the following formula 4.
• ⁇ CR is the average cooling rate CR1 (° C./s) by water cooling at the center of the plate width in the cooling process and the average cooling rate CR2 (° C./s) by water cooling at 1/8 of the plate width from both ends in the plate width direction.
• the difference from (CR1-CR2) is shown, and CR1 is 20 ° C./s or more.
• Hot rolling process for example, a slab having the chemical composition described above for a hot-rolled steel sheet is subjected to hot-rolling.
• the slab to be used is preferably cast by a continuous casting method from the viewpoint of productivity, but may be manufactured by an ingot forming method or a thin slab casting method. Further, the cast slab may be roughly rolled before the finish rolling, optionally for the purpose of adjusting the plate thickness and the like. Such rough rolling is not particularly limited as long as the desired seat bar size can be secured.
• Hot rolling can be performed under any suitable conditions except for the requirements for controlling the rolling load, which will be described in detail later, and is not particularly limited, for example, such that the completion temperature of finish rolling is 750 ° C. or higher. It is done under the conditions.
• the upper limit is not particularly limited, but for example, the completion temperature of finish rolling is 1050 ° C. or lower.
• the reduction rate of the final stage may be appropriately determined in consideration of a desired plate thickness and the like, and is not particularly limited, but may be, for example, 10% or more or 20% or more.
• the rolled material after hot rolling is water-cooled on a runout table (ROT) under the cooling conditions described in detail later, and then wound up at a temperature of, for example, 600 ° C. or lower or 500 ° C. or lower.
• the average cooling rate by water cooling is 20 ° C./s or more at the central portion of the plate width (that is, CR1), and may be 30 ° C./s or more or 40 ° C./s or more in order to obtain a desired tensile strength.
• the upper limit of the average cooling rate by water cooling is not particularly limited, but for example, the average cooling rate by water cooling is 200 ° C./s or less, 150 ° C./s or less, 100 ° C./s or less, or 80 ° C./s or less at the center of the plate width. May be.
• the material properties of the steel sheet can be made uniform in the plate width direction relatively easily in the cold-rolling process and the subsequent annealing process, but in the case of a hot-rolled steel sheet, this is the case. Since there is no such process, it is generally very difficult to make the material properties such as hole expandability uniform in the plate width direction.
• the state of recrystallization in the plate width direction is determined by appropriately controlling the cooling rate while considering the strain distribution in the plate width direction. It is possible to achieve uniform hole-spreading property in the plate width direction so as to satisfy Equation 1 by controlling it well.
• the corrugated shape is mainly controlled by the deflection of the rolling roll to control the crown of the steel plate (a phenomenon in which the central part of the plate width becomes thicker than the end in the plate width direction) and the transformation shrinkage during cooling.
• the focus is on control, and sufficient control is not performed on the strain distribution and steel sheet characteristics in the plate width direction.
• the present inventors analyzed the temperature history of the steel sheet and the strain due to hot rolling using a model, actual temperature, etc., and as a result, as shown in Equation 4 below, according to the strain distribution during hot rolling. It has been found that the hole expanding property of the hot-rolled steel sheet in the plate width direction can be made uniform by appropriately controlling the cooling rate in the subsequent cooling step.
• ⁇ CR is the average cooling rate CR1 (° C./s) by water cooling at the center of the plate width in the cooling process and the average cooling rate CR2 (° C./s) by water cooling at 1/8 of the plate width from both ends in the plate width direction. The difference from (CR1-CR2) is shown, and CR1 is 20 ° C./s or more.
• the cooling by water cooling is a two-stage cooling including non-water cooling such as air cooling
• the average cooling rate CR2 differs on both sides in the plate width direction, the one with the smaller average cooling rate is defined as CR2.
• the hole-spreading property is improved by randomizing the aggregate structure and forming an isotropic structure as described above. Therefore, in addition to the control by Equation 4, for example, the crown is made smaller to make the strain distribution in the plate width direction as uniform as possible, and if necessary, other parameters related to hot rolling and subsequent cooling are appropriately set. By adjusting, it is possible to control the state of recrystallization in the plate width direction well, thereby achieving uniform hole expanding property in the plate width direction so as to satisfy Equation 1.
• the strain distribution in the plate width direction is caused by the crown of the steel sheet and the deflection of the rolling roll.
• the thickness and load of the steel plate are the main controlling factors for the deflection of the crown and the rolling roll.
• the change in plate thickness as a crown appears as a strain distribution at the final rolling stand in the hot rolling process, and affects the subsequent transformation behavior. Therefore, the strain distribution in the plate width direction can be known from the plate thickness t (mm) at the center of the plate width of the hot-rolled steel plate and the rolling load R (ton) of the final rolling stand.
• the distribution is defined as t ⁇ R 0.5 .
• the steel sheet immediately after finish rolling in the hot rolling process has a non-uniform temperature distribution in the plate width direction, and has a temperature distribution in which the central portion is high and the end portion is low. This is due to the fact that the plate thickness of the end portion is thinner than that of the central portion, and that the cooling water flows from the central portion to the end portion due to such a gradient of the plate thickness. Therefore, when hot rolling with a high load is performed, the amount of decrease in temperature increases toward the end in the plate width direction.
• the average cooling rate CR1 is located at the center of the plate width where the strain is relatively small. It is necessary to increase the value and decrease the average cooling rate CR2 at the end portion in the plate width direction in which the strain is relatively large, that is, it is necessary to increase the difference ⁇ CR of the average cooling rate represented by CR1-CR2.
• the method for achieving the desired ⁇ CR by changing the average cooling rate at the central portion of the plate width and the end portion in the plate width direction is not particularly limited, and any appropriate method known to those skilled in the art can be used.
• the desired ⁇ CR can be realized by stopping the injection of the cooling water to a specific location in the plate width direction or appropriately adjusting the injection amount thereof.
• the cooling shutdown temperature may be, for example, 600 ° C. or lower or 500 ° C. or lower.
• the rolling load may be 800 tons or more, and may be 850 tons or more or 900 tons or more.
• the rolling load may be 3000 ton or less, and may be 2500 ton or less or 2000 ton or less. According to the above manufacturing method, it is possible to reliably and stably manufacture a hot-rolled steel sheet having uniform material properties in the plate width direction.
• the value of t ⁇ R 0.5 / ⁇ CR is controlled so as to satisfy the following formula 5.
• 2.5 ⁇ t ⁇ R 0.5 / ⁇ CR ⁇ 7.5 ⁇ ⁇ ⁇ Equation 5 the material properties can be set to a region closer to the end portion in the plate width direction, specifically, from one end in the plate width direction and the other end on the opposite side to a position of 75 mm toward the center portion of the plate width. It becomes possible to make it uniform.
• the plate thickness t (mm) at the center of the plate width of the hot-rolled steel sheet, the rolling load R (ton) of the final rolling stand in the hot-rolling process, and the rolling load R (ton) in the hot-rolling process so as to satisfy the formula 5.
• the hot-rolled steel sheet of the present invention has uniform material properties in the plate width direction, and therefore, by using the hot-rolled steel sheet of the present invention, the yield is obtained even for parts having a complicated shape. Can be manufactured well. Further, since the hot-rolled steel sheet of the present invention has a high tensile strength of 780 MPa or more, it is required to have a more complicated shape and high strength such as an automobile undercarriage part such as a lower arm. Especially useful for use in parts.
• a slab having the chemical composition shown in Table 1 was produced by a continuous casting method.
• the rolling load R (ton) of the final rolling stand, especially in hot rolling and the average cooling rate CR1 by water cooling at the center of the plate width in subsequent cooling.
• the difference between (° C./s) and the average cooling rate CR2 (° C./s) by water cooling at 1/8 of the plate width from both ends in the plate width direction ⁇ CR (CR1-CR2) is changed as shown in Table 2.
• the change in the average cooling rate between the central portion of the plate width and the end portion in the plate width direction was performed by stopping the injection of the cooling water to a specific location in the plate width direction or by appropriately adjusting the injection amount thereof.
• the chemical composition obtained by analyzing the sample collected from the manufactured hot-rolled steel sheet was almost unchanged from the chemical composition of the slab shown in Table 1. Further, for the microstructure of the hot-rolled steel sheet, the area ratio (%) of ferrite ( ⁇ ), bainite (B), martensite (M) and other structures is analyzed by image analysis using an optical microscope as described above. Was decided by doing.
• the characteristics of the obtained rolled steel sheet were measured and evaluated by the following methods.
• the tensile strengths TS W1 , TS W2 and TS C in Table 2 were determined as follows. First, 1/8 of the plate width on the same line in the direction perpendicular to the rolling direction from the end in the plate width direction of either the work side or the drive side of the hot-rolled steel plate toward the center of the plate width. , JIS Z2241: 2011 No. 5 tensile test pieces were taken from the central part of the plate width and further from the 7/8 position of the plate width in the direction perpendicular to the rolling direction. Next, a tensile test based on JIS Z2241: 2011 was performed using each of the collected test pieces, and the tensile strength (MPa) of each test piece was determined.
• MPa tensile strength
• This tensile test was performed twice on different test pieces, and the tensile strength at the 1/8 position of the plate width and the central portion of the plate width from one end (drive side) in the plate width direction and the other end (work side) on the opposite side, respectively.
• Each average value of (MPa) was determined as TS W1 , TS W2 and TS C , respectively. Further, the lower value of TS W1 and TS W2 was determined as the tensile strength of the hot-rolled steel sheet.
• the hole expansion rates ⁇ W1 , ⁇ W2 and ⁇ C in Table 2 were determined as follows by performing a hole expansion test in accordance with JIS Z2256: 2020. First, 1/8 of the plate width on the same line in the direction perpendicular to the rolling direction from the end in the plate width direction of either the work side or the drive side of the hot-rolled steel plate toward the center of the plate width. , The test pieces were collected from the central part of the plate width and further from the 7/8 position of the plate width.
• the hole diameter d1 mm was measured, and the hole expansion ratio ⁇ (%) of each test piece was determined by the following formula.
• This hole expansion test was performed 5 times on different test pieces, and the holes at 1/8 of the plate width and the center of the plate width from one end (drive side) in the plate width direction and the other end (work side) on the opposite side, respectively.
• the mean values of the spread ratio (%) were determined as ⁇ W1 , ⁇ W2 , and ⁇ C , respectively.
• ⁇ 100 ⁇ (d1-d0) / d0
• the hole expansion ratios ⁇ E1 and ⁇ E2 are changed from the 1/8 position and the 7/8 position of the plate width, from the position of 75 mm to the center of the plate width from one end in the plate width direction and the other end on the opposite side, respectively.
• the hole expansion rate ⁇ W1 and ⁇ W2 were determined by performing a hole expansion test in accordance with JIS Z2256: 2020 in the same manner as described above.
• the hole expansion ratio measured in the plate width direction satisfies the relationship of Equation 1 while keeping the plate thickness and the plate width in appropriate ranges.
• Examples 3, 5, 7, 8, 10-12, 14 of the present invention manufactured by controlling the formula 4 in the range of 2.5 to 7.5 that is, manufactured so as to satisfy the formula 5).
• 16, 19, 22 and 24 hot-rolled steel sheets satisfy Equation 3, ie -15 ⁇ ( ⁇ E1 + ⁇ E2 ) / 2- ⁇ C ⁇ 15, and therefore holes to the region closer to the end in the plate width direction. It can be seen that the spreadability is uniform and it is very useful from the viewpoint of yield.

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