WO2023038084A1 - 熱延鋼板 - Google Patents

熱延鋼板 Download PDF

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WO2023038084A1
WO2023038084A1 PCT/JP2022/033730 JP2022033730W WO2023038084A1 WO 2023038084 A1 WO2023038084 A1 WO 2023038084A1 JP 2022033730 W JP2022033730 W JP 2022033730W WO 2023038084 A1 WO2023038084 A1 WO 2023038084A1
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hot
steel sheet
content
rolled steel
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PCT/JP2022/033730
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English (en)
French (fr)
Japanese (ja)
Inventor
充 吉田
和夫 匹田
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Nippon Steel Corp
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Nippon Steel Corp
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Priority to JP2023546984A priority Critical patent/JP7678370B2/ja
Priority to EP22867405.7A priority patent/EP4400621A4/en
Priority to US18/578,652 priority patent/US20240301537A1/en
Priority to CN202280057939.4A priority patent/CN117858971A/zh
Priority to MX2024002247A priority patent/MX2024002247A/es
Priority to KR1020247005884A priority patent/KR20240038998A/ko
Publication of WO2023038084A1 publication Critical patent/WO2023038084A1/ja
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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    • 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
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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    • 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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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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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
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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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    • 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
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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    • 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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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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    • 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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    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
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    • C21D2211/00—Microstructure comprising significant phases
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/009—Pearlite

Definitions

  • the present invention relates to hot-rolled steel sheets. Specifically, the present invention relates to a hot-rolled steel sheet that is used by being formed into various shapes by press working or the like, and particularly to a hot-rolled steel sheet that has high strength and excellent shear workability.
  • This application claims priority based on Japanese Patent Application No. 2021-146231 filed in Japan on September 08, 2021, the contents of which are incorporated herein.
  • Patent Document 1 the burr height after punching is controlled by controlling the ratio ds/db of the ferrite grain size ds in the surface layer to the ferrite crystal grain db in the inner layer to 0.95 or less. A technique for doing so is disclosed.
  • Patent Literature 2 discloses a technique for improving peeling and peeling on the plate end surface by reducing the P content.
  • Patent Document 1 targets IF steel, and it may be difficult to apply it to high-strength members of 980 MPa or more. Even in Patent Document 2, a strength of 980 MPa or more is not obtained, and the fracture surface roughness at the sheared end surface after shearing is not examined.
  • the present invention has been made in view of the above problems of the prior art, and an object of the present invention is to provide a hot-rolled steel sheet having high strength and excellent shear workability.
  • having excellent shearing workability means that the surface roughness Rz of the fractured surface of the sheared end surface after shearing is 30.0 ⁇ m or less.
  • having high strength indicates that the tensile strength is 980 MPa or more.
  • a hot rolling step in a high-temperature range is important to allow a certain amount or more of unmatched and coarse Ti-based carbides to exist. For example, it is effective to perform hot rolling in a temperature range of 1100° C. to SRT (° C.) with a total rolling reduction of 70% or more.
  • 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, in mass %, C: 0.050 to 0.200%, Si: 0.005 to 2.000%, Mn: 0.50 to 4.00 %, P: 0.100% or less, S: 0.0100% or less, sol.
  • Al 0.001-1.00%, Ti: 0.150-0.400%, N: 0.0010-0.0200%, Nb: 0-0.200%, V: 0-1.000% , Mo: 0-1.000%, Cu: 0-1.00%, Ni: 0-1.00%, Cr: 0-2.00%, W: 0-1.00%, B: 0- 0.0040%, Ca: 0-0.0100%, Mg: 0-0.0100%, REM: 0-0.0100%, Bi: 0-0.0200%, balance: chemical composition consisting of Fe and impurities
  • the metal structure at the 1/4 depth position is, in terms of area fraction, Retained austenite: less than 3.0%, ferrite: less than 30.0%, pearlite: less than 5.0%, and the average number density of Ti-based carbides having a major axis of 15 nm or more at the 1 ⁇ 4 depth position It has a grain size of 1.0 ⁇ 10 4
  • the hot-rolled steel sheet according to [1] has an average crystal grain size ds of the surface layer portion and the 1/4 depth ds/dq, which is a ratio to the average crystal grain size dq at the flat position, may be 0.95 or less.
  • the chemical composition is, in mass%, Nb: 0.001 to 0.200%, V: 0.005 to 1.000%, Mo : 0.001-1.000%, Cu: 0.02-1.00%, Ni: 0.02-1.00%, Cr: 0.02-2.00%, W: 0.020-1 .00%, B: 0.0001-0.0040%, Ca: 0.0002-0.0100%, Mg: 0.0002-0.0100%, REM: 0.0002-0.0100%, Bi: 0.0002 to 0.0200%, may contain one or more selected from the group consisting of.
  • the hot-rolled steel sheet according to the above aspect of the present invention is suitable as an industrial material used for automobile members, mechanical structural members, and building members.
  • a hot-rolled steel sheet according to one embodiment of the present invention has a predetermined chemical composition, and a region of 1/8 to 3/8 of the thickness in the thickness direction from the surface.
  • the metal structure at the 1/4 depth position has an area fraction of retained austenite: less than 3.0%, ferrite: less than 30.0%, and pearlite: 5.0%.
  • the average number density of Ti-based carbides having a major axis of 15 nm or more is 1 ⁇ 10 4 /mm 2 or more, and the average crystal grain size dq is 15.0 ⁇ m or less and the tensile strength of the hot-rolled steel sheet is 980 MPa or more.
  • steel sheet (hereinafter sometimes simply referred to as steel sheet) according to the present embodiment will be described more specifically below.
  • 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.
  • the numerical limits described below with “-” in between include the lower limit and the upper limit. Any numerical value indicated as “less than” or “greater than” excludes that value from the numerical range.
  • C is an element that increases the fraction of the hard phase and increases the strength of ferrite by combining with precipitation-strengthening elements such as Ti, Nb, and V. If the C content is less than 0.050%, it becomes difficult to obtain the desired strength. Therefore, the C content should be 0.050% or more.
  • the C content is preferably 0.060% or more, more preferably 0.070% or more, and even more preferably 0.080% or more.
  • the C content should be 0.200% or less.
  • the C content is preferably 0.150% or less.
  • Si is an element that has the effect of increasing the strength of the hot-rolled steel sheet by solid-solution strengthening.
  • Si has the effect of making steel sound by deoxidizing (suppressing the occurrence of defects such as blowholes in steel). If the Si content is less than 0.005%, the above effects cannot be obtained. Therefore, the Si content should be 0.005% or more.
  • the Si content is preferably 0.010% or more.
  • Si is an element that has the effect of deteriorating the surface properties and chemical conversion treatability of the hot-rolled steel sheet and promoting the formation of retained austenite by suppressing the precipitation of cementite from austenite.
  • the Si content should be 2.000% or less.
  • the Si content is preferably 1.500% or less, more preferably 1.300% or less.
  • Mn is an element that has the effect of suppressing ferrite transformation and increasing the strength of the hot-rolled steel sheet. If the Mn content is less than 0.50%, a tensile strength of 980 MPa or more cannot be obtained. Therefore, the Mn content should be 0.50% or more.
  • the Mn content is preferably 0.80% or more, more preferably 1.00% or more. Further, when the area fraction of ferrite is reduced, the Mn content is more preferably 1.40% or more, more preferably 1.50% or more.
  • the Mn content should be 4.00% or less.
  • the Mn content is preferably 3.50% or less, more preferably 3.00% or less.
  • P 0.100% or less
  • P is an element generally contained as an impurity.
  • P is an element that easily segregates, and if the P content exceeds 0.100%, the bending workability decreases due to grain boundary segregation. Therefore, the P content should be 0.100% or less.
  • the P content is preferably 0.030% or less.
  • the lower limit of the P content does not have to be specified, it is preferable to set the P content to 0.001% or more from the viewpoint of refining cost.
  • P is also an element that has the effect of increasing the strength of the hot-rolled steel sheet by solid-solution strengthening. Therefore, P may be positively contained. In that case, the P content may be 0.002% or more.
  • S is an element contained as an impurity, and is an element that forms sulfide-based inclusions in steel to reduce the bending workability of the hot-rolled steel sheet. If the S content exceeds 0.0100%, the bendability of the hot-rolled steel sheet is remarkably lowered. Therefore, the S content should be 0.0100% or less.
  • the S content is preferably 0.0050% or less.
  • the lower limit of the S content does not have to be specified, but from the viewpoint of refining cost, the S content is preferably 0.0001% or more.
  • sol. Al 0.001 to 1.00%
  • Al is an element that has the effect of deoxidizing steel and making the steel sound. sol. If the Al content is less than 0.001%, the above effects cannot be obtained. Therefore, sol. Al content shall be 0.001% or more. sol. The Al content is preferably 0.01% or more. On the other hand, sol. If the Al content exceeds 1.00%, the above effect is saturated and it is economically unfavorable. Therefore, sol. Al content is 1.00% or less. sol. The Al content is preferably 0.80% or less, more preferably 0.60% or less. sol. Al means acid-soluble Al, and indicates Al present in steel in a solid solution state.
  • Ti 0.150 to 0.400%
  • Ti is an element that reduces the surface roughness of the fracture surface at the sheared end surface by precipitating as coarse Ti-based carbides in the high-temperature region of hot rolling.
  • it is an element that suppresses the recovery/recrystallization and grain growth of the austenite structure and refines the metal structure after transformation.
  • Ti is an element that precipitates as fine Ti-based carbides even during cooling after hot rolling (after completion of finish rolling) and improves the strength of steel by precipitation strengthening. If the Ti content is less than 0.150%, the driving force for precipitation of Ti-based carbides in the high temperature range of hot rolling is small, and the desired number density of Ti-based carbides cannot be obtained.
  • the Ti content is set to 0.150% or more.
  • the Ti content is preferably 0.170% or more, more preferably 0.190% or more, and even more preferably 0.210% or more.
  • the Ti content is set to 0.400% or less.
  • the Ti content is preferably 0.350% or less, more preferably 0.300% or less.
  • Ti-based carbides refer to carbides containing Ti and having a NaCl-type crystal structure. If such carbides contain Ti, other carbide-generating alloying elements such as Mo, Nb, V, Cr, and W are included in small amounts within the chemical composition range specified in the present embodiment. be Carbonitrides in which part of carbon is replaced with nitrogen are also included.
  • N is an element that forms nitrides and carbonitrides with Ti, Nb, V, etc., and suppresses coarsening of austenite during slab heating, thereby refining the metal structure. If the N content is less than 0.0010%, it becomes difficult to exhibit the above effects. Therefore, the N content should be 0.0010% or more. The N content is preferably 0.0015% or more. On the other hand, if the N content exceeds 0.0200%, coarse Ti nitrides are formed and the bending workability is lowered. Therefore, the N content is made 0.0200% or less. The N content is preferably 0.0150% or less, more preferably 0.0100% or less, and even more preferably 0.0060% or less.
  • the rest of the chemical composition of the hot-rolled steel sheet according to the present embodiment may be Fe and impurities.
  • impurities refers to ores used as raw materials, scraps, or impurities that are mixed from the manufacturing environment, etc., and/or those that are allowed within a range that does not adversely affect the hot-rolled steel sheet according to the present embodiment. do.
  • the hot-rolled steel sheet according to the present embodiment includes Nb, V, Mo, Cu, Ni, Cr, W, B, Ca, Mg, REM, and Bi as optional elements instead of part of Fe, You may contain 1 type(s) or 2 or more types. Since it is not essential to contain the above optional elements, the lower limit of the content is 0%. The optional elements will be described in detail below.
  • Nb is an optional element.
  • Nb is an element that precipitates in steel as a carbide, nitride, carbonitride, or the like, and has the effect of increasing the tensile strength of the steel sheet.
  • the Nb content is preferably 0.001% or more.
  • the Nb content is more preferably 0.005% or more.
  • the Nb content should be 0.200% or less.
  • the Nb content is preferably 0.170% or less, more preferably 0.140% or less, even more preferably 0.110% or less.
  • V (V: 0 to 1.000%) V is an arbitrary element.
  • V is an element that precipitates in steel as a carbide, nitride, carbonitride, or the like, and has the effect of improving the tensile strength of the steel sheet.
  • the V content is preferably 0.005% or more.
  • the V content is more preferably 0.010% or more.
  • the V content is set to 1.000% or less.
  • the V content is more preferably 0.800% or less, still more preferably 0.600% or less.
  • Mo is an optional element. Mo is an element that has the effect of increasing the hardenability of steel and forming carbides and carbonitrides to increase the strength of the steel sheet. To obtain these effects, the Mo content is preferably 0.001% or more. Mo content is more preferably 0.005% or more. On the other hand, if the Mo content exceeds 1.000%, the cracking sensitivity of the slab may increase. Therefore, when Mo is contained, the content of Mo should be 1.000% or less. The Mo content is more preferably 0.800% or less, still more preferably 0.600% or less.
  • Cu is an optional element.
  • Cu is an element that has the effect of improving the toughness of steel and the effect of increasing the tensile strength. To obtain these effects, the Cu content is preferably 0.02% or more. Cu content is more preferably 0.08% or more.
  • the Cu content is set to 1.00% or less. The Cu content is more preferably 0.50% or less, still more preferably 0.30% or less.
  • Ni is an optional element.
  • Ni is an element that has the effect of improving the toughness of steel and the effect of increasing the tensile strength. To obtain these effects, the Ni content is preferably 0.02% or more. The Ni content is more preferably 0.10% or more.
  • the Ni content is set to 1.00% or less. The Ni content is more preferably 0.50% or less, still more preferably 0.30% or less.
  • Cr is an optional element. Cr is an element that has the effect of increasing the hardenability of steel and forming carbides and carbonitrides to increase the strength of the steel sheet. To obtain this effect, the Cr content is preferably 0.02% or more. The Cr content is more preferably 0.05% or more. On the other hand, if Cr is contained excessively, the chemical convertibility deteriorates. Therefore, when Cr is contained, the Cr content is set to 2.00% or less. The Cr content is more preferably 1.50% or less, still more preferably 1.00% or less, and particularly preferably 0.50% or less.
  • W is an arbitrary element.
  • W is an element having the effect of forming carbides and carbonitrides to increase tensile strength.
  • the W content is preferably 0.02% or more.
  • W content is set to 1.00% or less.
  • the W content is preferably 0.80% or less.
  • B is an arbitrary element.
  • B is an element that has the effect of increasing the tensile strength of the steel sheet by intergranular strengthening and solid-solution strengthening. To obtain this effect, the B content is preferably 0.0001% or more.
  • the B content is more preferably 0.0002% or more.
  • the B content should be 0.0040% or less.
  • the B content is more preferably 0.0030% or less, still more preferably 0.0020% or less.
  • Ca is an optional element.
  • Ca is an element that has the effect of dispersing a large number of fine oxides in molten steel and refining the metal structure of the steel sheet.
  • Ca is an element that has the effect of improving the stretch flangeability of a steel sheet by fixing S in molten steel as spherical CaS and suppressing the formation of stretched inclusions such as MnS.
  • the Ca content is preferably 0.0002% or more.
  • Ca content is more preferably 0.0005% or more.
  • the Ca content when the Ca content exceeds 0.0100%, the amount of CaO in the steel increases, which may adversely affect the toughness of the steel sheet. Therefore, when Ca is contained, the Ca content shall be 0.0100% or less.
  • the Ca content is more preferably 0.0050% or less, still more preferably 0.0030% or less.
  • Mg is an optional element. Like Ca, Mg is an element that forms oxides and sulfides in molten steel, suppresses the formation of coarse MnS, disperses many fine oxides, and has the effect of refining the metal structure of the steel sheet. be. To obtain these effects, the Mg content is preferably 0.0002% or more. The Mg content is more preferably 0.0005% or more. On the other hand, when the Mg content exceeds 0.0100%, oxides in the steel increase and adversely affect the toughness of the steel sheet. Therefore, when Mg is contained, the Mg content shall be 0.0100% or less. The Mg content is more preferably 0.0050% or less, still more preferably 0.0030% or less.
  • REM 0 to 0.0100%
  • the REM content is preferably 0.0002% or more.
  • the REM content is more preferably 0.0005% or more.
  • the REM content is preferably 0.0100% or less.
  • the REM content is more preferably 0.0050% or less, even more preferably 0.0030% or less.
  • REM rare earth elements
  • the REM content refers to the total content of these elements.
  • Bi is an arbitrary element.
  • B is an element that has the effect of refining the solidification structure and improving the formability of the steel sheet.
  • the Bi content is preferably 0.0001% or more.
  • the Bi content is more preferably 0.0005% or more.
  • the Bi content should be 0.0200% or less.
  • the Bi content is more preferably 0.0100% or less, and even more preferably 0.0070% or less.
  • the chemical composition of the hot-rolled steel sheet mentioned above can be measured by a general analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry).
  • sol. Al can be measured by ICP-AES using the filtrate obtained by thermally decomposing the sample with acid.
  • C and S may be measured using the combustion-infrared absorption method, and N may be measured using the inert gas fusion-thermal conductivity method.
  • the metal structure of the hot-rolled steel sheet according to this embodiment will be described.
  • the metal structure at the 1/4 depth position has an area fraction with less than 30.0% ferrite, less than 3.0% retained austenite, and less than 5.0% perlite.
  • the average grain size is 15.0 ⁇ m or less in the metal structure at the 1/4 depth position, and the average number density of Ti-based carbides having a major axis of 15 nm or more is 1.
  • Ferrite is a structure formed when fcc transforms to bcc at a relatively high temperature. Since ferrite has a high work hardening ability, if the area fraction of ferrite is too large, the amount of deformation of the fractured surface at the sheared end face increases, resulting in an increase in the roughness of the fractured surface. Therefore, the area fraction of ferrite is set to less than 30.0%.
  • the area fraction of ferrite is preferably 20.0% or less, more preferably 10.0% or less, and even more preferably 8.0% or less. The smaller the area fraction of ferrite, the more preferable it is, and it may be 0%. good too.
  • the area fraction of retained austenite is less than 3.0%.
  • the area fraction of retained austenite is preferably less than 1.5%, more preferably less than 1.0%.
  • the area fraction of retained austenite may be 0% because the smaller the retained austenite, the better.
  • Pearlite is a lamellar metal structure in which cementite is deposited in layers between ferrite particles. Pearlite is also a soft metal structure compared to bainite and martensite.
  • the area fraction of pearlite is 5.0% or more, carbon is consumed by cementite contained in pearlite, and the strength of martensite, tempered martensite, and bainite, which are the remaining structures, is lowered, and the tensile strength is 980 MPa or more. can't get Therefore, the area fraction of pearlite is set to less than 5.0%.
  • the perlite area fraction is preferably 3.0% or less, more preferably 2.0%, even more preferably 1.0% or less, or may be 0%.
  • the hot-rolled steel sheet according to the present embodiment has a residual structure other than retained austenite, ferrite, and pearlite, which is a hard structure, bainite, martensite, and tempered martensite. It is preferably composed of two or more kinds.
  • the area fraction of one or more of bainite, martensite and tempered martensite is preferably 70.0% or more, more preferably 80.0% or more, and even more preferably 90.0% or more.
  • the area fraction of each structure constituting the metal structure is measured by the following method.
  • a plate thickness cross-section parallel to the rolling direction is mirror-finished and polished with colloidal silica containing no alkaline solution at room temperature for 8 minutes to remove the strain introduced to the surface layer of the sample.
  • electron backscattering at a measurement interval of 0.1 ⁇ m in a region of 50 ⁇ m in length, 1/8 of the plate thickness from the surface to 3/8 of the plate thickness from the surface Crystal orientation information is obtained by measurement using a diffraction method.
  • 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.
  • a backscattered electron image is taken in the same field of view.
  • crystal grains in which ferrite and cementite are deposited in layers are specified from a backscattered electron image, and the area fraction of the crystal grains is calculated to obtain the area fraction of pearlite.
  • the crystal orientation information obtained for the crystal grains excluding the crystal grains determined to be pearlite is used with the "Grain Average Misorientation" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device.
  • a region with a grain average misorientation value of 1.0° or less is determined to be ferrite.
  • the area fraction of ferrite is obtained by calculating the area fraction of the region determined to be ferrite.
  • the maximum value of the "Grain Average IQ" of the ferrite region under the condition that the boundary of 5° or more in the residual region (the region where the grain average misorientation value exceeds 1.0°) is defined as the grain boundary is I ⁇
  • a region exceeding I ⁇ /2 is extracted as bainite
  • a region below I ⁇ /2 is extracted as “pearlite, martensite and tempered martensite”.
  • the area of martensite and tempered martensite Get the sum of the fractions.
  • Methods for measuring the area fraction of retained austenite include X-ray diffraction, EBSD (Electron Back Scattering Diffraction Pattern) analysis, magnetic measurement, and the like, and the measured value may vary depending on the measurement method. .
  • the area fraction of retained austenite is measured by X-ray diffraction.
  • the average number density of Ti-based carbides having a major axis of 15 nm or more is 1.0 ⁇ 10 4 /mm 2 or more
  • coarse Ti-based carbides having a major axis of 15 nm or more are precipitated. Due to the presence of the coarse Ti-based carbides and the refinement of the average crystal grain size, which will be described later, the voids at the time of fracture in the shearing work are dispersed, and the surface roughness of the fractured surface at the sheared end surface is reduced.
  • the average number density of Ti-based carbides having a major axis of 15 nm or more must be 1.0 ⁇ 10 4 (10000) pieces/mm 2 or more.
  • the average number density of Ti-based carbides having a major axis of 15 nm or more is preferably 2.0 ⁇ 10 4 pieces/mm 2 or more, more preferably 4.0 ⁇ 10 4 pieces/mm 2 or more.
  • the Ti-based carbide refers to a Ti-containing carbide having a NaCl-type crystal structure. If such carbides contain Ti, they may contain minor amounts of other carbide-forming alloying elements.
  • Ti-based carbides may contain other carbide-forming alloying elements such as Mo, Nb, V, Cr, and W within the chemical composition range specified in the present embodiment. Furthermore, Ti-based carbides may be carbonitrides in which part of the carbon is substituted with nitrogen.
  • the average number density of Ti-based carbides was obtained by photographing 1/4 depth position with a TEM at a magnification of 50,000 times, with 2.0 ⁇ m ⁇ 2.0 ⁇ m area defined as one field, and 20 fields of view, and observed in the field of view.
  • the precipitates are analyzed by energy dispersive X-ray spectroscopy (EDS), and the precipitates in which Ti and C are detected are determined to be Ti-based carbides, and the major axis (Ti-based carbides) of each precipitate (Ti-based carbide) Measure the longest diameter). Then, the number of Ti-based carbides having a major axis of 15 nm or more per 1 mm 2 is examined to determine the number density.
  • the average crystal grain size (dq) at the 1/4 depth position is set to 15.0 ⁇ m or less.
  • the average grain size is preferably 12.0 ⁇ m or less, more preferably 10.0 ⁇ m or less. Since the smaller the average crystal grain size, the better, the lower limit is not particularly limited. However, in ordinary hot rolling, it is technically difficult to refine grains to an average crystal grain size of less than 1.0 ⁇ m. Therefore, the average crystal grain size may be 1.0 ⁇ m or more, or 4.0 ⁇ m or more.
  • ds/dq which is the ratio of the average crystal grain size ds of the surface layer portion to the average crystal grain size dq of the quarter depth position: 0.95 or less
  • the inside of the bend is uniformly deformed as the work progresses, but as the amount of work increases, uniform deformation alone cannot support the deformation, and deformation progresses as the strain concentrates locally (the occurrence of shear deformation bands). As this shear deformation band grows further, a crack occurs and grows along the shear band from the inner surface of the bend. The reason why inner bending cracks are more likely to occur as the strength increases is that the deformation progresses unevenly due to the decrease in work hardening ability that accompanies the increase in strength. is presumed to be due to
  • the inventors investigated a method for suppressing bending inner cracks in high-strength steel sheets. As a result, it was found that the finer the crystal grain size in the surface layer of the hot-rolled steel sheet, the more the local strain concentration is suppressed and the cracks in bending are less likely to occur.
  • ds/dq is the ratio of the crystal grain size ds to the average crystal grain size dq at the quarter depth position. Therefore, when obtaining a hot-rolled steel sheet having excellent bendability (suppressing bending cracks during bending) in addition to high strength and excellent shear workability, ds/dq should be 0.95 or less. is preferred. ds/dq is more preferably 0.90 or less, and even more preferably 0.85 or less. Although the lower limit of ds/dq is not specified, it may be 0.50 or more.
  • the average grain size of each of the surface layer and the 1/4 depth position is the surface layer of the hot-rolled steel sheet in the thickness cross section parallel to the rolling direction of the hot-rolled steel sheet (area from the surface to the depth of 50 ⁇ m from the surface) and 1/4 depth position (area from 1/8 depth of plate thickness to 3/8 depth of plate thickness from surface) at 1200 times magnification, 1 field of view of 40 ⁇ m ⁇ 30 ⁇ m area
  • analysis is performed and measured using EBSD in at least 5 fields of view.
  • a grain boundary is defined as a region where the angle difference between adjacent measurement points is 15° or more and the equivalent circle diameter is 0.3 ⁇ m or more, and the area-average grain size is calculated.
  • the area-average crystal grain size obtained at each measurement position is defined as the average crystal grain size of the surface layer portion and the average crystal grain size of the 1/4 depth position.
  • the hot-rolled steel sheet according to this embodiment has a tensile (maximum) strength of 980 MPa or more. If the tensile strength is less than 980 MPa, the applicable parts are limited and the contribution to vehicle weight reduction is small.
  • the tensile strength is preferably 1000 MPa or higher, more preferably 1080 MPa or higher, and even more preferably 1180 MPa or higher. Although the upper limit is not particularly limited, the tensile strength may be 1780 MPa or less from the viewpoint of mold wear suppression.
  • the tensile strength of hot-rolled steel sheets is evaluated in accordance with JIS Z 2241:2011.
  • the test piece is a JIS Z 2241:2011 No. 5 test piece, and the test direction is perpendicular to the rolling direction.
  • the thickness of the hot-rolled steel sheet according to this embodiment is not particularly limited, but may be 1.2 to 10.0 mm. If the thickness of the hot-rolled steel sheet is less than 1.2 mm, it may become difficult to ensure the rolling completion temperature and the rolling load may become excessive, making hot rolling difficult. Therefore, the thickness of the hot-rolled steel sheet according to this embodiment may be 1.2 mm or more. More preferably, it is 1.4 mm or more. On the other hand, if the plate thickness exceeds 10.0 mm, it becomes difficult to refine the metal structure, and it may be difficult to obtain the metal structure described above. Therefore, the plate thickness may be 10.0 mm or less. More preferably, it is 8.0 mm or less. Even more preferably, it is 6.0 mm or less.
  • the method for manufacturing the hot-rolled steel sheet according to the present embodiment is not particularly limited, but the hot-rolled steel sheet can be obtained by a manufacturing method including the following steps.
  • a heating step of heating a slab or billet having a predetermined chemical composition (I) A hot rolling step of subjecting the slab or billet after the heating step to multipass hot rolling using a plurality of rolling stands to obtain a hot rolled steel sheet.
  • a winding step of winding the hot-rolled steel sheet are described below.
  • Heating temperature over 1300°C and SRT (°C) or higher
  • the heating temperature of the slab or billet to be hot-rolled shall be above 1300° C. and above the temperature SRT (° C.) represented by the following formula (1).
  • SRT ° C.
  • the temperature of the slab or billet is higher than 1300 ° C. and SRT (° C.) or more
  • the temperature of the slab or billet is higher than the higher temperature of 1300 ° C. and SRT (° C.) , or if the SRT (° C.) is greater than 1300° C., it means that the SRT and the temperature of the slab or billet are the same.
  • the heating temperature exceeds 1400° C., thick scales may be formed, resulting in a decrease in yield or significant damage to the heating furnace. Therefore, the heating temperature is preferably 1400° C. or less.
  • Hot rolling process In the hot rolling process, a heated slab or billet is subjected to multi-pass hot rolling using multiple rolling stands to produce a hot rolled steel sheet. Multi-pass hot rolling can be performed using a reverse mill or a tandem mill, but from the viewpoint of industrial productivity, it is preferable to use a tandem mill for at least the last few stages.
  • the method for producing a hot-rolled steel sheet according to the present embodiment by increasing the total rolling reduction of hot rolling in the temperature range of 1100 ° C. or more and SRT (° C.) or less, recrystallized austenite is refined and the major axis is 15 nm.
  • the coarse Ti-based carbides described above are precipitated by work-induced precipitation in a short period of time during rolling. When the total rolling reduction in this temperature range is low, it becomes difficult to obtain a fine structure and desired Ti-based carbides.
  • the total rolling reduction in the temperature range from 1100° C. to SRT (° C.) is set to 70% or more. If the total rolling reduction in the above temperature range is less than 70%, desired coarse Ti-based carbides cannot be obtained.
  • the total rolling reduction is preferably 75% or more, more preferably 80% or more. The higher the total rolling reduction in the temperature range of 1100° C. or more and SRT (° C.) or less, the better.
  • Total rolling reduction in the temperature range above the hot rolling completion temperature FT (°C) of less than 1100°C: 80% or more In the method for manufacturing a hot-rolled steel sheet according to the present embodiment, after controlling the rolling reduction in the temperature range of 1100 ° C. or higher as described above, the total rolling reduction in the temperature range of FT (° C.) or higher below 1100 ° C. is increased, and further By performing cooling after hot rolling under the conditions described later, the average crystal grain size is refined. If the total rolling reduction in the temperature range below 1100° C. FT (° C.) or above is less than 80%, the average crystal grain size after transformation becomes coarse.
  • the total rolling reduction in the temperature range of FT (°C) or higher below 1100°C is set to 80% or higher.
  • the total rolling reduction is preferably 85% or more, more preferably 90% or more. It is preferable that the total rolling reduction in the temperature range below 1100° C. FT (° C.) is as high as possible.
  • the total reduction in each temperature range is the total reduction in this temperature range based on the inlet plate thickness before the first pass in the specified temperature range ( difference between the inlet strip thickness before the first pass and the outlet strip thickness after the final pass in rolling in this temperature range).
  • FT Hot rolling completion temperature FT (°C): Ar3 (°C) or more determined by the following formula (2)
  • Ar3 melting point
  • FT is set to Ar3 (° C.) or more.
  • FT is preferably 1030°C or lower, more preferably 1010°C or lower.
  • the temperature during hot rolling refers to the surface temperature of the steel material, and can be measured with a radiation thermometer or the like.
  • Ar3 (° C.) 901 ⁇ 325 ⁇ [C]+33 ⁇ [Si] ⁇ 92 ⁇ [Mn]+287 ⁇ [P]+40 ⁇ [sol. Al] (2)
  • the [element symbol] in the above formula (2) indicates the content in mass % of each element, and 0 is substituted when it is not contained.
  • Average cooling rate to 600°C or less after completion of hot rolling 50°C/sec or more
  • accelerated cooling is performed to a temperature range of 600° C. or less at an average cooling rate of 50° C./sec or more in order to suppress the formation of ferrite and pearlite.
  • the average cooling rate here means the temperature drop range of the steel plate from the start of accelerated cooling (when the steel plate is introduced into the cooling equipment) to the completion of accelerated cooling (when the steel plate is taken out of the cooling equipment). It is the value obtained by dividing by the required time from to the completion of accelerated cooling.
  • the upper limit of the average cooling rate is not particularly defined, increasing the cooling rate requires a large-scale cooling facility and increases the facility cost. For this reason, considering the equipment cost, 300° C./second or less is preferable.
  • cooling after hot rolling in order to suppress the growth of austenite grains refined by hot rolling, it is cooled by 50 ° C. or more within 1.0 second after the completion of hot rolling (the temperature drop margin is 50° C. or higher) is more preferable.
  • cooling at a high average cooling rate is performed immediately after the completion of hot rolling. For example, cooling water may be sprayed onto the surface of the steel plate.
  • the grain size of the surface layer can be refined and the resistance to internal bending cracks can be improved (inside bending during bending It is possible to suppress the occurrence of cracks).
  • the residence time in the temperature range of 600 to 750°C. which is the ferrite transformation temperature range, is 5.0 seconds or less. is preferred. More preferably, the residence time in the temperature range of 600 to 750° C. is 2.0 seconds or less.
  • Winding process (Winding temperature: less than 600°C)
  • the hot-rolled steel sheet after cooling under the above conditions is wound up.
  • the coiling temperature (approximately equal to the cooling stop temperature) is set to a temperature range of less than 600°C.
  • a bainite or martensite structure can be obtained.
  • a fine structure with high strength can be obtained, and as a result, excellent shear workability can be obtained.
  • the structure and carbide precipitation state are controlled in the steps up to the winding step. Therefore, after the winding process, it is preferable not to perform any process that affects the structure or the state of the carbide.
  • 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.
  • a slab with a thickness of 240 to 300 mm was produced by melting steel having the chemical composition shown in Table 1 and continuously casting it. Using the obtained slabs, hot-rolled steel sheets shown in Tables 3A and 3B were obtained under the manufacturing conditions shown in Tables 2A and 2B. In both rolling in the temperature range of 1100°C to SRT (°C) and rolling in the temperature range of less than 1100°C to FT°C or more, reduction was performed by two or more passes.
  • the obtained measurement results are shown in Tables 3A and 3B.
  • the obtained hot-rolled steel sheets were evaluated for tensile strength TS, shear workability, and bending inner crack resistance in the following manner.
  • the tensile strength of the hot-rolled steel sheet was evaluated according to JIS Z 2241:2011.
  • the test piece was a JIS Z 2241:2011 No. 5 test piece, and the test direction was perpendicular to the rolling direction.
  • the tensile strength TS was 980 MPa or more, the hot-rolled steel sheet was judged to have high strength and was judged to be acceptable.
  • the tensile strength TS was less than 980 MPa, the hot-rolled steel sheet was judged to be inferior in strength and was judged to be unacceptable.
  • the shear workability of the hot-rolled steel sheet was evaluated by determining the surface roughness Rz ( ⁇ m) of the fracture surface at the end face after punching by a punching test. A punched hole with a clearance of 20% was produced at a hole diameter of 10 mm and a punching speed of 3 m/s. Next, using a laser microscope, the surface roughness Rz ( ⁇ m) of the fractured surface of the punched hole was measured at a total of four points in the rolling direction and the direction perpendicular to the rolling direction, and the maximum value among them was evaluated. When Rz was 30.0 ⁇ m or less, it was determined that the hot-rolled steel sheet had excellent shear workability. As shown in FIG. 1, the fracture surface is a punched end surface separated by a crack generated near the cutting edge after shear deformation.
  • a value obtained by dividing the average value of the minimum bending radii of the L-axis and the C-axis by the plate thickness was defined as the limit bending R/t and was used as an index value of bending inner crack resistance.
  • R/t was 2.5 or less, it was determined that the hot-rolled steel sheet was excellent in resistance to internal bending cracks.
  • the cross section of the test piece after the V-block 90° bending test is cut in a plane parallel to the bending direction and perpendicular to the plate surface.
  • the crack length observed inside the bending exceeded 30 ⁇ m, it was determined that there was a crack.
  • Tables 3A and 3B The obtained results are shown in Tables 3A and 3B.
  • the hot-rolled steel sheets according to the invention examples have excellent strength and shear workability. Further, among the examples of the present invention, it is found that the hot-rolled steel sheets with a ds/dq of 0.95 or less have not only the above properties but also excellent bending internal crack resistance. On the other hand, it can be seen that the hot-rolled steel sheets according to the comparative examples do not have any one or more of excellent strength and shear workability.
  • Test No. 6 which is a comparative example, had a low slab heating temperature. Therefore, Ti-based carbides were not sufficiently dissolved during heating, and the average number density of Ti-based carbides having a major axis of 15 nm or more decreased. As a result, the roughness of the fracture surface became rough (the shear workability was low).
  • Test No. 7 which is a comparative example, had a low total rolling reduction in the temperature range of 1100° C. or higher and SRT (° C.) or lower. Therefore, the average number density of Ti-based carbides having a major axis of 15 nm or more decreased. As a result, the roughness of the fracture surface became rough. Test No.
  • Test No. 8 which is a comparative example, had a low total rolling reduction in the temperature range below 1100°C and above FT (°C). Therefore, the average crystal grain size increased. As a result, the roughness of the fracture surface became rough.
  • Test No. 9 which is a comparative example, had a low average cooling rate to 600° C. or less after completion of hot rolling, and a long residence time at 600 to 750° C. Therefore, the area fraction of ferrite increased and the average crystal grain size increased. As a result, the tensile strength was low and the roughness of the fracture surface was rough. In addition, the ds/dq was high and the bending inner crack resistance was low.
  • Test No. 10 which is a comparative example, had a high winding temperature.
  • the area fraction of ferrite was high. As a result, the tensile strength was low and the roughness of the fracture surface was rough.
  • Sample No. 28, which is a comparative example had a low C content. As a result, the tensile strength was low.
  • Test No. 29, which is a comparative example had a high Si content. As a result, the area of retained austenite increased and the roughness of the fracture surface became rough.
  • Test No. 30, which is a comparative example had a low Ti content. Therefore, the average number density of Ti-based carbides having a major axis of 15 nm or more decreased. As a result, the roughness of the fracture surface became rough.
  • Test No. 31, which is a comparative example had a low Mn content. As a result, the tensile strength was low.
  • a hot-rolled steel sheet having high strength and excellent shear workability can be obtained.
  • INDUSTRIAL APPLICABILITY The hot-rolled steel sheet of the present invention is suitable as an industrial material used for automobile members, mechanical structural members, and building members, and has high industrial applicability.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
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  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Heat Treatment Of Sheet Steel (AREA)
  • Heat Treatment Of Steel (AREA)
PCT/JP2022/033730 2021-09-08 2022-09-08 熱延鋼板 Ceased WO2023038084A1 (ja)

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JP2023546984A JP7678370B2 (ja) 2021-09-08 2022-09-08 熱延鋼板
EP22867405.7A EP4400621A4 (en) 2021-09-08 2022-09-08 HOT-ROLLED STEEL SHEET
US18/578,652 US20240301537A1 (en) 2021-09-08 2022-09-08 Hot-rolled steel sheet
CN202280057939.4A CN117858971A (zh) 2021-09-08 2022-09-08 热轧钢板
MX2024002247A MX2024002247A (es) 2021-09-08 2022-09-08 Lamina de acero laminada en caliente.
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Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH10168544A (ja) 1996-12-10 1998-06-23 Nkk Corp 打ち抜き性に優れる冷延鋼板及びその製造方法
JP2005298924A (ja) 2004-04-13 2005-10-27 Nippon Steel Corp 打ち抜き加工性に優れた高強度熱延鋼板及びその製造方法
WO2019009410A1 (ja) * 2017-07-07 2019-01-10 新日鐵住金株式会社 熱延鋼板及びその製造方法
WO2021124864A1 (ja) * 2019-12-19 2021-06-24 日本製鉄株式会社 鋼板及びめっき鋼板
JP2021146231A (ja) 2020-03-16 2021-09-27 ウシオ電機株式会社 ガス供給装置

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US11332803B2 (en) 2017-04-21 2022-05-17 Nippon Steel Corporation High strength hot-dip galvanized steel sheet and production method therefor
JP7260824B2 (ja) * 2020-01-27 2023-04-19 日本製鉄株式会社 熱延鋼板
US12435382B2 (en) * 2020-01-27 2025-10-07 Nippon Steel Corporation Hot-rolled steel sheet

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH10168544A (ja) 1996-12-10 1998-06-23 Nkk Corp 打ち抜き性に優れる冷延鋼板及びその製造方法
JP2005298924A (ja) 2004-04-13 2005-10-27 Nippon Steel Corp 打ち抜き加工性に優れた高強度熱延鋼板及びその製造方法
WO2019009410A1 (ja) * 2017-07-07 2019-01-10 新日鐵住金株式会社 熱延鋼板及びその製造方法
WO2021124864A1 (ja) * 2019-12-19 2021-06-24 日本製鉄株式会社 鋼板及びめっき鋼板
JP2021146231A (ja) 2020-03-16 2021-09-27 ウシオ電機株式会社 ガス供給装置

Non-Patent Citations (1)

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Title
See also references of EP4400621A4

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EP4400621A4 (en) 2025-06-11
EP4400621A1 (en) 2024-07-17
US20240301537A1 (en) 2024-09-12
CN117858971A (zh) 2024-04-09
MX2024002247A (es) 2024-03-05

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