EP4438747A1 - Hot-rolled steel sheet - Google Patents

Hot-rolled steel sheet Download PDF

Info

Publication number
EP4438747A1
EP4438747A1 EP22898648.5A EP22898648A EP4438747A1 EP 4438747 A1 EP4438747 A1 EP 4438747A1 EP 22898648 A EP22898648 A EP 22898648A EP 4438747 A1 EP4438747 A1 EP 4438747A1
Authority
EP
European Patent Office
Prior art keywords
present
hot
less
steel sheet
rolled steel
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP22898648.5A
Other languages
German (de)
French (fr)
Other versions
EP4438747A4 (en
Inventor
Shohei Yabu
Takuya Kuwayama
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Nippon Steel Corp
Original Assignee
Nippon Steel Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Nippon Steel Corp filed Critical Nippon Steel Corp
Publication of EP4438747A1 publication Critical patent/EP4438747A1/en
Publication of EP4438747A4 publication Critical patent/EP4438747A4/en
Pending legal-status Critical Current

Links

Classifications

    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • 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
    • C21D8/0221—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
    • 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
    • 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
    • C21D8/04—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 to produce plates or strips for drawing, e.g. for deep-drawing
    • C21D8/0421—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 to produce plates or strips for drawing, e.g. for deep-drawing characterised by the working steps
    • C21D8/0426—Hot rolling
    • 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
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • 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
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • 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
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • 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
    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
    • 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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • 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
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • 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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • 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
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • 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
    • C22C38/10—Ferrous alloys, e.g. steel alloys containing cobalt
    • 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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing 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
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
    • 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • 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

Definitions

  • the present invention relates to a hot-rolled steel sheet.
  • Patent Document 1 discloses a low yield ratio type high-burring high-strength hot-rolled steel sheet in which the amount of insoluble Ti measured by an extraction residue method in a steel sheet is 30% or more and 70% or less of a total Ti content.
  • Patent Document 1 Japanese Unexamined Patent Application, First Publication No. 2009-263774
  • Vehicle members are formed by press forming, and blank sheets for the press forming are often manufactured by punching.
  • punching for a high strength steel sheet of 980 MPa or more, it is desired to control an end surface after punching with high accuracy, that is, to have excellent punching property.
  • Patent Document 1 As a result of an investigation by the present inventors, it was found that in Patent Document 1, it is necessary to further increase the strength, and there is room for improvement in yield ratio and punching property in a case where the strength is increased.
  • An object of the present invention is to provide a hot-rolled steel sheet having high strength and yield ratio and excellent punching property.
  • the gist of the present invention is as follows.
  • the hot-rolled steel sheet according to the present embodiment includes, by mass%, C: 0.045% to 0.150%, Si: 0.10% to 2.50%, Mn: 1.50% to 3.50%, sol. Al: 0.010% to 1.000%, Ti: 0.050% to 0.200%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, and a remainder: Fe and impurities.
  • C 0.045% to 0.150%
  • Si 0.10% to 2.50%
  • Mn 1.50% to 3.50%
  • sol. 0.010% to 1.000%
  • Ti 0.050% to 0.200%
  • P 0.100% or less
  • S 0.100% or less
  • S 0.100% or less
  • N 0.1000% or less
  • O 0.0100% or less
  • a remainder Fe and impurities.
  • C is an element necessary to obtain a desired strength.
  • a C content is less than 0.045%, a desired strength cannot be obtained. Therefore, the C content is set to 0.045% or more.
  • the C content is preferably 0.050% or more, 0.055% or more, or 0.060% or more.
  • the C content is set to 0.150% or less.
  • the C content is preferably 0.120% or less, 0.100% or less, or 0.080% or less.
  • Si has an action of improving ductility of the hot-rolled steel sheet by promoting the generation of ferrite and has an action of increasing the strength of the hot-rolled steel sheet by solid solution strengthening of ferrite.
  • Si has an action of achieving soundness of steel by deoxidation (suppressing the occurrence of a defect such as a blowhole in steel).
  • the Si content is set to 0.10% or more.
  • the Si content is preferably 0.50% or more or 0.70% or more.
  • the Si content is set to 2.50% or less.
  • the Si content is preferably 2.00% or less, 1.80% or less, or 1.50% or less.
  • Mn is an element that improves hardenability and increases the strength of the hot-rolled steel sheet.
  • the Mn content is set to 1.50% or more.
  • the Mn content is preferably 1.80% or more, 2.00% or more, or 2.30% or more.
  • the Mn content is set to 3.50% or less.
  • the Mn content is preferably 3.30% or less, 3.00% or less, or 2.80% or less.
  • Al has an action of achieving soundness of steel by deoxidation and also has an action of controlling ferritic transformation.
  • a sol. Al content is set to 0.010% or more.
  • the sol. Al content is preferably 0.030% or more, 0.050% or more, 0.080% or more, or 0.100% or more.
  • the sol. Al content is set to 1.000% or less.
  • the sol. Al content is preferably 0.800% or less, 0.600% or less, 0.400% or less, or 0.200% or less.
  • sol. Al means acid-soluble Al, and indicates solute Al present in the steel in a solid solution state.
  • Ti is precipitated in steel as a carbide or a nitride and has an action of refining the microstructure by an austenite pinning effect and increasing the strength and yield ratio of the hot-rolled steel sheet by precipitation hardening.
  • a Ti content is less than 0.050%, an effect by the above action cannot be obtained. Therefore, the Ti content is set to 0.050% or more.
  • the Ti content is preferably 0.080% or more, 0.100% or more, or 0.130% or more.
  • the Ti content is set to 0.200% or less.
  • the Ti content is preferably 0.180% or less or 0.150% or less.
  • P is an element that is generally contained in steel as an impurity, and the lower a P content is, the more preferable it is.
  • the P content is set to 0.100% or less.
  • the P content is preferably 0.080% or less, 0.060% or less, or 0.040% or less.
  • the P content is preferably 0%, but may be set to 0.001% or more from the viewpoint of a refining cost.
  • S is an element that is generally contained in steel as an impurity, and the lower a S content is, the more preferable it is.
  • the S content is set to 0.0300% or less.
  • the S content is preferably 0.0200% or less or 0.0100% or less.
  • the S content is preferably 0%, but may be set to 0.0001% or more from the viewpoint of a refining cost.
  • N is an element that is generally contained in steel as an impurity, and the lower a N content is, the more preferable it is.
  • the N content is set to 0.1000% or less.
  • the N content is preferably 0.0800% or less, 0.0600% or less, or 0.0400% or less.
  • the N content is preferably 0%, but may be set to 0.0010% or more from the viewpoint of a refining cost.
  • O is an element that, when contained in steel in a large amount, forms a coarse oxide that becomes an origin of fracture, causing brittle fractures and hydrogen-induced cracks.
  • the O content is set to 0.0100% or less.
  • the O content is preferably 0.0080% or less, 0.0060% or less, or 0.0040% or less.
  • the O content may be set to 0.0005% or more or 0.0010% or more to disperse a large number of fine oxides when molten steel is deoxidized.
  • the remainder of the chemical composition of the hot-rolled steel sheet according to the present embodiment may be Fe and impurities.
  • the impurities mean substances that are incorporated from ore as a raw material, scrap, a manufacturing environment, or the like and/or substances that are permitted to an extent that the hot-rolled steel sheet according to the present embodiment is not adversely affected.
  • the hot-rolled steel sheet according to the present embodiment may contain the following elements as optional elements instead of a portion of Fe.
  • optional elements instead of a portion of Fe.
  • a lower limit of amounts thereof is 0%.
  • Nb 0.001% to 0.050%
  • Nb is an element that is finely precipitated in steel as a carbide and a nitride and improves the strength of steel by precipitation hardening.
  • a Nb content is preferably set to 0.001% or more.
  • the Nb content is set to 0.050% or less.
  • V 0.001% to 0.050%
  • V is, similar to Nb, an element that is finely precipitated in steel as a carbide and a nitride and improves the strength of steel by precipitation hardening.
  • a V content is preferably set to 0.001% or more.
  • the V content is set to 0.050% or less.
  • a B content is preferably set to 0.0001% or more.
  • the B content is set to 0.0100% or less.
  • Cu has an action of enhancing the hardenability of the hot-rolled steel sheet and an action of increasing the strength of the hot-rolled steel sheet by being precipitated as a carbide in steel at a low temperature.
  • a Cu content is preferably set to 0.01% or more.
  • the Cu content is more than 2.00%, there are cases where grain boundary cracking occurs in a slab. Therefore, the Cu content is set to 2.00% or less.
  • a Cr content is preferably set to 0.01 % or more.
  • the Cr content is set to 2.00% or less.
  • Mo has an action of enhancing the hardenability of the hot-rolled steel sheet and an action of increasing the strength of the hot-rolled steel sheet by being precipitated as a carbide in steel.
  • a Mo content is preferably set to 0.001% or more.
  • the Mo content is set to more than 1.000%, the effect by the actions is saturated, which is not economically preferable. Therefore, the Mo content is set to 1.000% or less.
  • Ni has an action of enhancing the hardenability of the hot-rolled steel sheet.
  • a Ni content is preferably set to 0.01% or more.
  • Ni is an expensive element, it is not economically preferable to contain a large amount of Ni. Therefore, the Ni content is set to 2.00% or less.
  • Ca has an action of enhancing the yield ratio of the hot-rolled steel sheet by adjusting a shape of an inclusion in steel to a preferable shape.
  • a Ca content is preferably set to 0.0005% or more.
  • the Ca content is set to 0.0200% or less.
  • Mg has an action of enhancing the yield ratio of the hot-rolled steel sheet by adjusting a shape of an inclusion in steel to a preferable shape.
  • a Mg content is preferably set to 0.0005% or more.
  • the Mg content is set to 0.0200% or less.
  • REM has an action of enhancing the yield ratio of the hot-rolled steel sheet by adjusting a shape of an inclusion in steel to a preferable shape.
  • a REM content is preferably set to 0.0005% or more.
  • the REM content is set to 0.1000% or less.
  • REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoids, and the REM content refers to the total amount of these elements.
  • Lanthanoids are added in the form of mischmetal in industry.
  • Bi has an action of enhancing the yield ratio of the hot-rolled steel sheet by refining a solidification structure.
  • a Bi content is preferably 0.0005% or more.
  • the Bi content is set to 0.0200% or less.
  • the present inventors have confirmed that, even when these elements are contained in a total amount of 1.0000% or less, the effects of the hot-rolled steel sheet according to the present embodiment are not impaired. Therefore, one or two or more of Zr, Co, Zn, and W may be contained in a total amount of 1.0000% or less.
  • a Sn content is set to 0.050% or less.
  • the chemical composition of the above-described hot-rolled steel sheet may be measured by a general analytical method.
  • the chemical composition may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES).
  • ICP-AES inductively coupled plasma-atomic emission spectrometry
  • sol. Al may be measured by ICP-AES using a filtrate that is obtained after a sample is heated and decomposed with an acid.
  • C and S may be measured using a combustion-infrared absorption method
  • N may be measured using an inert gas fusion-thermal conductivity method
  • O may be measured using an inert gas fusion-non-dispersive infrared absorption method.
  • the chemical composition is analyzed after front and back surfaces of the steel sheet including the plating layer on a surface are mechanically ground by 150 ⁇ m or more.
  • the microstructure at a 1/4 position from the surface includes,
  • the 1/4 position from the surface indicates a region from a 1/8 thickness depth from the surface to a 3/8 thickness depth from the surface of the hot-rolled steel sheet.
  • the reason for specifying the microstructure at this position is that the microstructure at this position indicates a typical microstructure of the hot-rolled steel sheet.
  • Ferrite is a structure generated when fcc transforms into bcc at a relatively high temperature.
  • an area ratio of ferrite is less than 2.0%, a desired yield ratio cannot be obtained. Therefore, the area ratio of ferrite is set to 2.0% or more.
  • the area ratio of ferrite is preferably 5.0% or more, 8.0% or more, or 10.0% or more.
  • the area ratio of ferrite is set to 30.0% or less.
  • the area ratio of ferrite is preferably 27.0% or less, 25.0% or less, or 20.0% or less.
  • Bainite 60.0% to 93.0%
  • Bainite is a structure including fine grains and a carbide.
  • an area ratio of bainite is set to 60.0% or more.
  • the area ratio of bainite is preferably 65.0% or more, 70.0% or more, 75.0% or more, or 80.0% or more.
  • the area ratio of bainite is set to 93.0% or less.
  • the area ratio of bainite is preferably 90.0% or less, 87.0% or less, or 80.0% or less.
  • Martensite is a structure that increases the strength of the hot-rolled steel sheet.
  • an area ratio of martensite is set to 5.0% or more.
  • the area ratio of martensite is preferably 8.0% or more or 10.0% or more.
  • the area ratio of martensite is set to 20.0% or less.
  • the area ratio of martensite is preferably 18.0% or less or 15.0% or less.
  • the hot-rolled steel sheet according to the present embodiment may contain retained austenite and pearlite as a remainder in the microstructure in a total amount of less than 5.0%.
  • the area ratio of each structure is measured by the following method.
  • a test piece is collected from the hot-rolled steel sheet in a sheet thickness cross section parallel to a rolling direction so that the microstructure at the 1/4 position from the surface (the region from the 1/8 depth from the surface to the 3/8 depth from the surface) and at a center position in a sheet width direction can be observed.
  • the cross section of the test piece is polished using #600 to #1500 silicon carbide paper and is thereafter mirror-finished using a liquid obtained by dispersing a diamond powder having a particle size of 1 to 6 ⁇ m in a diluted solution such as alcohol or in pure water.
  • the cross section of the test piece is polished at room temperature using colloidal silica containing no alkaline solution to remove strain introduced into a surface layer of the sample.
  • a region having a length of 50 ⁇ m from the 1/8 thickness depth from the surface to the 3/8 thickness depth from the surface is measured by electron backscatter diffraction at a measurement interval of 0.1 ⁇ m to obtain crystal orientation information.
  • an EBSD analyzer including a thermal field-emission scanning electron microscope (JSM-7001F manufactured by JEOL Ltd) and an EBSD detector (DVC5 type detector manufactured by TSL solutions) is used.
  • a degree of vacuum in the EBSD analyzer is set to 9.6 ⁇ 10 -5 Pa or less
  • an accelerating voltage is set to 15 kV
  • an irradiation current level is set to 13
  • an irradiation level of an electron beam is set to 62.
  • a Grain Average Image Quality map (GAIQ map) is obtained using the "Grain Average Image Quality" function.
  • GAIQ map a region surrounded by grain boundaries having a crystal orientation difference of 15° or more is defined as a grain.
  • a maximum value of a "Grain Average Image Quality Value (GAIQ Value)" of the region extracted as ferrite is defined as I ⁇
  • a region having a relative GAIQ value of more than I ⁇ /2 is extracted as bainite
  • a region having a relative GAIQ value of I ⁇ /2 or less is extracted as martensite.
  • the area ratio of each of bainite and martensite is obtained by calculating an area ratio of the region of the extracted bainite and an area ratio of the region of the extracted martensite.
  • the area ratio of the remainder in the microstructure is obtained by subtracting the area ratios of the structures from 100%.
  • a method such as buffing using alumina particles having a particle size of 0.1 ⁇ m or less or Ar ion sputtering may be used.
  • the martensite can be rephrased as martensite that satisfies the following conditions (I) to (III).
  • the area ratio of the martensite that satisfies the above conditions (1) to (III) is less than 5.0%, the punching property of the hot-rolled steel sheet deteriorates. Therefore, the area ratio of the martensite is set to 5.0% or more.
  • the area ratio of martensite is preferably 8.0% or more or 10.0% or more.
  • the area ratio of the martensite may be set to 20.0% or less or 15.0% or less.
  • the GAIQ value is a parameter that can reflect a dislocation density of a grain.
  • the martensite having a relative GAIQ value of I ⁇ /3 or less is a harder type of martensite among other types of martensite.
  • the area ratio of the hard martensite that is in contact with a 30° grain boundary is controlled.
  • the martensite having a grain size of less than 2.0 ⁇ m does not affect the punching property of the hot-rolled steel sheet and therefore does not need to be particularly controlled.
  • the area ratio of the martensite is measured by the following method.
  • a test piece is collected and treated by the same method as in the measurement of the area ratios of the structures.
  • a measurement position is set to a 1/4 position from a surface (a region from a 1/8 depth from the surface to a 3/8 depth from the surface) and a center position in a sheet width direction.
  • the 30° grain boundary is specified using the "Grain Orientation Spread" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer.
  • a GAIQ map is obtained by the same method as in the measurement of the area ratios of the structures, and ferrite and martensite are extracted.
  • the maximum value I ⁇ of the GAIQ value of the ferrite is obtained, and the martensite having a relative GAIQ value of I ⁇ /3 or less is specified. Accordingly, the martensite that is in contact with a 30° grain boundary and has a relative GAIQ value of I ⁇ /3 or less is specified (condition (I) and condition (II)).
  • the martensite that is in contact with a 30° grain boundary also includes martensite that is present on a 30° grain boundary.
  • the grain size of the martensite is obtained by calculating an equivalent circle diameter of the martensite that satisfies condition (I) and condition (II). Accordingly, martensite having a grain size of 2.0 ⁇ m or more is specified (Condition (III)).
  • the area ratio of the martensite that satisfies the conditions (I) to (III) by calculating the area ratio of the martensite that satisfies the conditions (I) to (III), the area ratio of the martensite that is in contact with a 30° grain boundary, has a relative GAIQ value of I ⁇ /3 or less, and has a grain size of 2.0 ⁇ m or more is obtained.
  • the hot-rolled steel sheet according to the present embodiment has a tensile strength of 980 MPa or more.
  • a tensile strength of 980 MPa or more can contribute to a further reduction in the weight of vehicle bodies.
  • An upper limit of the tensile strength is not particularly specified, but may be set to 1,400 MPa or less.
  • the tensile strength is measured by a tensile test according to JIS Z 2241: 2011.
  • a test piece a No. 5 test piece of JIS Z 2241: 2011 is used.
  • a position from which the tensile test piece is collected is set to a 1/4 portion from an end portion in a sheet width direction, and a longitudinal direction thereof may be a direction perpendicular to a rolling direction.
  • the hot-rolled steel sheet according to the present embodiment has a yield ratio of 0.75 or more.
  • the yield ratio is obtained by dividing a yield stress by a tensile strength (yield stress / tensile strength).
  • the yield stress is obtained by conducting a tensile test in the above-described method.
  • an upper yield point is regarded as the yield stress
  • a 0.2% proof stress is regarded as the yield stress.
  • the hot-rolled steel sheet according to the present embodiment has excellent punched end surface properties when subjected to a punching test.
  • the punched end surface properties are evaluated by the following method.
  • a test piece is collected from the hot-rolled steel sheet.
  • a punched hole is prepared from the test piece with a hole diameter of 20 mm, a shear angle of 5°, a clearance of 15.0%, and a punching speed of 80 mm/s.
  • a test piece is collected so that four cross sections can be checked at intervals of about 90°, the test piece is embedded in a resin, and a punched end surface is photographed with a scanning electron microscope. The obtained picture is observed, and in a case where no large cracks of more than 100 ⁇ m are observed in a direction perpendicular to the sheet thickness direction, the hot-rolled steel sheet can be determined to have excellent punching property. In a case where no cracks of 50 ⁇ m or more are observed in the direction perpendicular to the sheet thickness direction, the hot-rolled steel sheet can be determined to have particularly excellent punching property.
  • a sheet thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited and may be set to 0.6 to 8.0 mm.
  • an excessive rolling force can be suppressed, thereby facilitating hot rolling.
  • the sheet thickness can be easily obtained.
  • the hot-rolled steel sheet may be a surface-treated steel sheet provided with a plating layer on the surface for the purpose of improving corrosion resistance or the like.
  • the plating layer may be an electroplating layer or a hot-dip plating layer.
  • the electroplating layer include electrogalvanizing, and electro Zn-Ni alloy plating.
  • the hot-dip plating layer include hot-dip galvanizing, hot-dip galvannealing, hot-dip aluminum plating, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, and hot-dip Zn-Al-Mg-Si alloy plating.
  • a plating adhesion amount is not particularly limited and may be the same as in the related art.
  • it is also possible to further enhance the corrosion resistance by performing an appropriate chemical conversion treatment (for example, application and drying of a silicate-based chromium-free chemical conversion liquid) after plating.
  • a temperature of a slab and a temperature of a steel sheet in the present embodiment refer to a surface temperature of the slab and a surface temperature of the steel sheet.
  • the hot-rolled steel sheet according to the present embodiment can be stably manufactured by a manufacturing method in which the above steps are closely and inseparably controlled.
  • the slab to be subjected to the hot rolling is heated after casting and before being cooled to a temperature range of 500°C or lower and retained in a temperature range of 1,220°C or higher for 30 minutes or longer.
  • the temperature of the steel sheet may be changed or may be kept constant.
  • the slab stage the slab is not cooled to a temperature range of 500°C or lower, whereby the precipitation of coarse Ti is suppressed, and Ti can be sufficiently solutionized. Accordingly, a precipitation hardening ability of ferrite and bainite can be enhanced, and the strength and yield ratio of the hot-rolled steel sheet can be increased.
  • a precipitation hardening ability of ferrite and bainite can be enhanced, and the strength and yield ratio of the hot-rolled steel sheet can be increased.
  • sufficient solutionizing can be achieved, and prior austenite grains are coarsened, whereby an effect of the slab width reduction in the subsequent step can be maximized.
  • the area ratio of the martensite that is in contact with a 30° grain boundary when the maximum value of the GAIQ value of ferrite is indicated as I ⁇ , which has a relative GAIQ value of I ⁇ /3 or less, and which has a grain size of 2.0 ⁇ m or more (hereinafter, sometimes referred to as martensite area ratio at a 30° grain boundary) can be increased.
  • the slab width reduction is performed in a temperature range of 1,200°C or higher with a rolling reduction of 10% or larger.
  • the rolling reduction of the slab width reduction can be expressed as, when a length of the slab in a width direction before the reduction is indicated as w0 and the length of the slab in the width direction after the reduction is indicated as w1, (1 - w1/w0) ⁇ 100 (%).
  • a method of performing the slab width reduction for example, there is a method of rolling a slab using rolls installed so that a rotation axis is perpendicular to a sheet surface of the slab.
  • the rough rolling finishing temperature is set to a temperature range of 1,100°C or higher, the cumulative rolling reduction of all the stands is set to 70% or larger, the rolling reduction of each of the final three stands is set to smaller than 20%, and the rolling reduction of each of all the stands is smaller than 40%.
  • the rough rolling finishing temperature is set to the temperature range of 1,100°C or higher, setting the cumulative rolling reduction of all the stands to 70% or larger, and setting the rolling reduction of each of the final three stands to smaller than 20%, prior austenite grains elongated in the sheet thickness direction can be made equiaxed, and uniform prior austenite grains can be obtained by promoting recrystallization in an intragranular deformation band. As a result, the martensite area ratio at a 30° grain boundary can be increased.
  • reverse rolling is not desirable. This is because when reverse rolling is performed during the rough rolling, a shape of the prior austenite grains cannot be preferably controlled, and as a result, the martensite area ratio at a 30° grain boundary cannot be preferably controlled.
  • the temperature of the steel sheet may be kept constant or may be changed in a temperature range of 1,000°C or higher.
  • the finishing temperature FT is set to a temperature range of T1 (°C) - 100°C or higher, the cumulative rolling reduction of the finish rolling is set to 75% or larger, the cumulative rolling reduction of final two stands is set to smaller than 30%.
  • the average cooling rate in the temperature range of the finishing temperature FT to 700°C is preferably set to 30 °C/s or faster.
  • the average cooling rate refers to a value obtained by dividing a temperature drop width of the steel sheet from the start of the cooling to the completion of the cooling by a time required from the start of the cooling to the completion of the cooling.
  • Coiling is preferably performed in a temperature range of 450°C to 650°C.
  • a coiling temperature By setting a coiling temperature to a temperature range of 450°C or higher, desired amounts of ferrite and martensite can be obtained.
  • desired amounts of ferrite and bainite can be obtained.
  • cooling to room temperature may be performed. Thereafter, pickling and cold rolling may be performed by an ordinary method as necessary. In the cold rolling, a cumulative rolling reduction may be 50% or larger. Furthermore, as necessary, temper rolling may be performed in order to flatten the hot-rolled steel sheet and adjust a surface roughness.
  • the obtained hot-rolled steel sheets were subjected to microstructure observation, a tensile test, and a punching test by the above-described methods.
  • the obtained measurement results are shown in Tables 4A and 4B.
  • the hot-rolled steel sheet was considered to be a hot-rolled steel sheet having high strength and determined to be acceptable.
  • the hot-rolled steel sheet was not regarded as a hot-rolled steel sheet having high strength and determined to be unacceptable.
  • the hot-rolled steel sheet was considered to be a hot-rolled steel sheet having a high yield ratio and determined to be acceptable.
  • the hot-rolled steel sheet was not considered to be a hot-rolled steel sheet having a high yield ratio and determined to be unacceptable.
  • the punching property was considered to be particularly good and indicated as "E" (Excellent) in the tables.
  • E Excellent
  • the punching property was considered to be good and indicated as "G” (Good) in the tables.
  • the punching property was considered to be inferior and indicated as "B" (Bad) in the tables.
  • the hot-rolled steel sheets according to the present invention examples have high strength and yield ratio and excellent punching property.
  • the hot-rolled steel sheets according to the comparative examples do not have any one or more of the above properties.

Landscapes

  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)

Abstract

In this hot-rolled steel sheet, a microstructure at a 1/4 position from a surface includes, by area%, ferrite: 2.0% to 30.0%, bainite: 60.0% to 93.0%, and martensite: 5.0% to 20.0%, an area ratio of martensite, which is in contact with a 30° grain boundary, when a maximum value of a GAIQ value of the ferrite is indicated as Iα, which has a relative GAIQ value of Iα/3 or less, and which has a grain size of 2.0 µm or more, is 5.0% or more, and a tensile strength is 980 MPa or more.

Description

    [Technical Field of the Invention]
  • The present invention relates to a hot-rolled steel sheet.
  • Priority is claimed on Japanese Patent Application No. 2021-191745, filed November 26, 2021 , the content of which is incorporated herein by reference.
    • [Background Art]
  • In recent years, from the viewpoint of protecting the global environment, efforts have been made to reduce the amount of carbon dioxide gas emitted in many fields. Vehicle manufacturers are also actively developing techniques for reducing the weight of vehicle bodies for the purpose of reducing fuel consumption. However, it is not easy to reduce the weight of vehicle bodies since the emphasis is placed on improvement in collision resistance to secure safety of occupants.
  • In order to achieve both the reduction in the weight of vehicle bodies and collision resistance, thinning a member using a high strength steel sheet has been examined. Therefore, a steel sheet having both high strength and excellent workability is strongly desired. Several techniques have been hitherto proposed to meet these demands.
  • For example, Patent Document 1 discloses a low yield ratio type high-burring high-strength hot-rolled steel sheet in which the amount of insoluble Ti measured by an extraction residue method in a steel sheet is 30% or more and 70% or less of a total Ti content.
    • [Prior Art Document] [Patent Document]
  • [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2009-263774
    • [Disclosure of the Invention] [Problems to be Solved by the Invention]
  • Vehicle members are formed by press forming, and blank sheets for the press forming are often manufactured by punching. In particular, for a high strength steel sheet of 980 MPa or more, it is desired to control an end surface after punching with high accuracy, that is, to have excellent punching property.
  • As a result of an investigation by the present inventors, it was found that in Patent Document 1, it is necessary to further increase the strength, and there is room for improvement in yield ratio and punching property in a case where the strength is increased.
  • The present invention has been made in view of the above circumstances. An object of the present invention is to provide a hot-rolled steel sheet having high strength and yield ratio and excellent punching property.
    • [Means for Solving the Problem]
  • The gist of the present invention is as follows.
    1. (1) A hot-rolled steel sheet according to an aspect of the present invention includes, as a chemical composition, by mass%:
      • C: 0.045% to 0.150%;
      • Si: 0.10% to 2.50%;
      • Mn: 1.50% to 3.50%;
      • sol. Al: 0.010% to 1.000%;
      • Ti: 0.050% to 0.200%;
      • P: 0.100% or less;
      • S: 0.0300% or less;
      • N: 0.1000% or less;
      • O: 0.0100% or less;
      • Nb: 0% to 0.050%;
      • V: 0% to 0.050%;
      • B: 0% to 0.0100%;
      • Cu: 0% to 2.00%;
      • Cr: 0% to 2.00%;
      • Mo: 0% to 1.000%;
      • Ni: 0% to 2.00%;
      • Ca: 0% to 0.0200%;
      • Mg: 0% to 0.0200%;
      • REM: 0% to 0.1000%;
      • Bi: 0% to 0.0200%;
      • one or two or more of Zr, Co, Zn, and W: 0% to 1.0000% in total;
      • Sn: 0% to 0.050%; and
      • a remainder comprising Fe and impurities,
      • in which a microstructure at a 1/4 position from a surface includes,
      • by area%,
        • ferrite: 2.0% to 30.0%,
        • bainite: 60.0% to 93.0%, and
        • martensite: 5.0% to 20.0%,
      • an area ratio of martensite,
        • which is in contact with a 30° grain boundary,
        • which has a relative GAIQ value of Iα/3 or less, where Iα is a maximum value of a GAIQ value of the ferrite, and
        • which has a grain size of 2.0 µm or more, is 5.0% or more, and a tensile strength of the hot-rolled steel sheet is 980 MPa or more.
    2. (2) In the hot-rolled steel sheet according to (1), the chemical composition may contain, by mass%, one or two or more of the group consisting of
      • Nb: 0.001% to 0.050%,
      • V: 0.001% to 0.050%,
      • B: 0.0001% to 0.0100%,
      • Cu: 0.01% to 2.00%,
      • Cr: 0.01% to 2.00%,
      • Mo: 0.001% to 1.000%,
      • Ni: 0.01% to 2.00%,
      • Ca: 0.0005% to 0.0200%,
      • Mg: 0.0005% to 0.0200%,
      • REM: 0.0005% to 0.1000%, and
      • Bi: 0.0005% to 0.0200%.
    [Effects of the Invention]
  • According to the aspect of the present invention, it is possible to provide a hot-rolled steel sheet having high strength and yield ratio, and excellent punching property.
    • [Embodiments of the Invention]
  • Hereinafter, a chemical composition and a microstructure of a hot-rolled steel sheet according to the present embodiment will be more specifically described. However, the present invention is not limited to configurations disclosed in the present embodiment, and various modifications can be made without departing from the gist of the present invention.
  • The numerical limit range described below with "to" includes the lower limit and the upper limit. Numerical values indicated as "less than" or "more than" do not fall within the numerical range. In the following description, % regarding the chemical composition of the hot-rolled steel sheet is mass% unless otherwise specified.
    • Chemical Composition
  • The hot-rolled steel sheet according to the present embodiment includes, by mass%, C: 0.045% to 0.150%, Si: 0.10% to 2.50%, Mn: 1.50% to 3.50%, sol. Al: 0.010% to 1.000%, Ti: 0.050% to 0.200%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, and a remainder: Fe and impurities. Each element will be described in detail below.
    • C: 0.045% to 0.150%
  • C is an element necessary to obtain a desired strength. When a C content is less than 0.045%, a desired strength cannot be obtained. Therefore, the C content is set to 0.045% or more. The C content is preferably 0.050% or more, 0.055% or more, or 0.060% or more.
  • On the other hand, when the C content is more than 0.150%, weldability of the hot-rolled steel sheet decreases. Therefore, the C content is set to 0.150% or less. The C content is preferably 0.120% or less, 0.100% or less, or 0.080% or less.
    • Si: 0.10% to 2.50%
  • Si has an action of improving ductility of the hot-rolled steel sheet by promoting the generation of ferrite and has an action of increasing the strength of the hot-rolled steel sheet by solid solution strengthening of ferrite. In addition, Si has an action of achieving soundness of steel by deoxidation (suppressing the occurrence of a defect such as a blowhole in steel). When a Si content is less than 0.10%, an effect by the above action cannot be obtained. Therefore, the Si content is set to 0.10% or more. The Si content is preferably 0.50% or more or 0.70% or more.
  • On the other hand, when the Si content is more than 2.50 %, the weldability of the hot-rolled steel sheet decreases. Therefore, the Si content is set to 2.50% or less. The Si content is preferably 2.00% or less, 1.80% or less, or 1.50% or less.
    • Mn: 1.50% to 3.50%
  • Mn is an element that improves hardenability and increases the strength of the hot-rolled steel sheet. When the Mn content is less than 1.50%, a desired strength cannot be obtained. Therefore, the Mn content is set to 1.50% or more. The Mn content is preferably 1.80% or more, 2.00% or more, or 2.30% or more.
  • On the other hand, when the Mn content is more than 3.50%, the hardenability becomes excessive and a yield ratio of the hot-rolled steel sheet decreases. Therefore, the Mn content is set to 3.50% or less. The Mn content is preferably 3.30% or less, 3.00% or less, or 2.80% or less.
    • sol. Al: 0.010% to 1.000%
  • Al has an action of achieving soundness of steel by deoxidation and also has an action of controlling ferritic transformation. When a sol. Al content is less than 0.010%, an effect by the above action cannot be obtained. Therefore, the sol. Al content is set to 0.010% or more. The sol. Al content is preferably 0.030% or more, 0.050% or more, 0.080% or more, or 0.100% or more.
  • On the other hand, when the sol. Al content is more than 1.000%, alumina precipitated in the form of a cluster is generated and the yield ratio of the hot-rolled steel sheet decreases. Therefore, the sol. Al content is set to 1.000% or less. The sol. Al content is preferably 0.800% or less, 0.600% or less, 0.400% or less, or 0.200% or less.
  • In addition, sol. Al means acid-soluble Al, and indicates solute Al present in the steel in a solid solution state.
    • Ti: 0.050% to 0.200%
  • Ti is precipitated in steel as a carbide or a nitride and has an action of refining the microstructure by an austenite pinning effect and increasing the strength and yield ratio of the hot-rolled steel sheet by precipitation hardening. When a Ti content is less than 0.050%, an effect by the above action cannot be obtained. Therefore, the Ti content is set to 0.050% or more. The Ti content is preferably 0.080% or more, 0.100% or more, or 0.130% or more.
  • On the other hand, when the Ti content is more than 0.200%, punching property of the hot-rolled steel sheet deteriorates due to excessive precipitation of TiC. Therefore, the Ti content is set to 0.200% or less. The Ti content is preferably 0.180% or less or 0.150% or less.
    • P: 0.100% or Less
  • P is an element that is generally contained in steel as an impurity, and the lower a P content is, the more preferable it is. In particular, when the P content is more than 0.100%, deterioration of workability and weldability of the hot-rolled steel sheet becomes significant, and the punching property also deteriorates. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.080% or less, 0.060% or less, or 0.040% or less.
  • The P content is preferably 0%, but may be set to 0.001% or more from the viewpoint of a refining cost.
    • S: 0.0300% or Less
  • S is an element that is generally contained in steel as an impurity, and the lower a S content is, the more preferable it is. When the S content is more than 0.0300%, the yield ratio of the hot-rolled steel sheet significantly decreases. Therefore, the S content is set to 0.0300% or less. The S content is preferably 0.0200% or less or 0.0100% or less.
  • The S content is preferably 0%, but may be set to 0.0001% or more from the viewpoint of a refining cost.
    • N: 0.1000% or Less
  • N is an element that is generally contained in steel as an impurity, and the lower a N content is, the more preferable it is. When the N content is more than 0.1000%, the yield ratio of the hot-rolled steel sheet significantly decreases. Therefore, the N content is set to 0.1000% or less. The N content is preferably 0.0800% or less, 0.0600% or less, or 0.0400% or less.
  • The N content is preferably 0%, but may be set to 0.0010% or more from the viewpoint of a refining cost.
    • O: 0.0100% or Less
  • O is an element that, when contained in steel in a large amount, forms a coarse oxide that becomes an origin of fracture, causing brittle fractures and hydrogen-induced cracks. When an O content is more than 0.0100%, brittle fractures and hydrogen-induced cracks are likely to be initiated. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less, 0.0060% or less, or 0.0040% or less.
  • The O content may be set to 0.0005% or more or 0.0010% or more to disperse a large number of fine oxides when molten steel is deoxidized.
  • The remainder of the chemical composition of the hot-rolled steel sheet according to the present embodiment may be Fe and impurities. In the present embodiment, the impurities mean substances that are incorporated from ore as a raw material, scrap, a manufacturing environment, or the like and/or substances that are permitted to an extent that the hot-rolled steel sheet according to the present embodiment is not adversely affected.
  • The hot-rolled steel sheet according to the present embodiment may contain the following elements as optional elements instead of a portion of Fe. In a case where such optional elements are not contained, a lower limit of amounts thereof is 0%. Hereinafter, the optional elements will be described in detail.
    • Nb: 0.001% to 0.050%
  • Nb is an element that is finely precipitated in steel as a carbide and a nitride and improves the strength of steel by precipitation hardening. In order to reliably obtain this effect, a Nb content is preferably set to 0.001% or more.
  • However, when the Nb content is more than 0.050%, the yield ratio of the hot-rolled steel sheet deteriorates. Therefore, the Nb content is set to 0.050% or less.
    • V: 0.001% to 0.050%
  • V is, similar to Nb, an element that is finely precipitated in steel as a carbide and a nitride and improves the strength of steel by precipitation hardening. In order to reliably obtain the effect, a V content is preferably set to 0.001% or more.
  • However, when the V content is more than 0.050%, the yield ratio of the hot-rolled steel sheet deteriorates. Therefore, the V content is set to 0.050% or less.
    • B: 0.0001% to 0.0100%
  • B has an action of enhancing the hardenability of the hot-rolled steel sheet. In order to reliably obtain this effect, a B content is preferably set to 0.0001% or more.
  • However, when the B content is more than 0.0100%, the yield ratio of the hot-rolled steel sheet significantly decreases. Therefore, the B content is set to 0.0100% or less.
    • Cu: 0.01% to 2.00%
  • Cu has an action of enhancing the hardenability of the hot-rolled steel sheet and an action of increasing the strength of the hot-rolled steel sheet by being precipitated as a carbide in steel at a low temperature. In order to reliably obtain these effects, a Cu content is preferably set to 0.01% or more.
  • However, when the Cu content is more than 2.00%, there are cases where grain boundary cracking occurs in a slab. Therefore, the Cu content is set to 2.00% or less.
    • Cr: 0.01% to 2.00%
  • Cr has an action of enhancing the hardenability of the hot-rolled steel sheet. In order to reliably obtain this effect, a Cr content is preferably set to 0.01 % or more.
  • However, when the Cr content is more than 2.00%, chemical convertibility of the hot-rolled steel sheet significantly decreases. Therefore, the Cr content is set to 2.00% or less.
    • Mo: 0.001% to 1.000%
  • Mo has an action of enhancing the hardenability of the hot-rolled steel sheet and an action of increasing the strength of the hot-rolled steel sheet by being precipitated as a carbide in steel. In order to reliably obtain these effects, a Mo content is preferably set to 0.001% or more.
  • However, even when the Mo content is set to more than 1.000%, the effect by the actions is saturated, which is not economically preferable. Therefore, the Mo content is set to 1.000% or less.
    • Ni: 0.01% to 2.00%
  • Ni has an action of enhancing the hardenability of the hot-rolled steel sheet. In order to reliably obtain this effect, a Ni content is preferably set to 0.01% or more.
  • However, since Ni is an expensive element, it is not economically preferable to contain a large amount of Ni. Therefore, the Ni content is set to 2.00% or less.
    • Ca: 0.0005% to 0.0200%
  • Ca has an action of enhancing the yield ratio of the hot-rolled steel sheet by adjusting a shape of an inclusion in steel to a preferable shape. In order to reliably obtain this effect, a Ca content is preferably set to 0.0005% or more.
  • However, when the Ca content is more than 0.0200%, an excessive amount of the inclusion is generated in steel, and the yield ratio of the hot-rolled steel sheet decreases. Therefore, the Ca content is set to 0.0200% or less.
    • Mg: 0.0005% to 0.0200%
  • Mg has an action of enhancing the yield ratio of the hot-rolled steel sheet by adjusting a shape of an inclusion in steel to a preferable shape. In order to reliably obtain this effect, a Mg content is preferably set to 0.0005% or more.
  • However, when the Mg content is more than 0.0200%, an excessive amount of the inclusion is generated in steel, and the yield ratio of the hot-rolled steel sheet decreases. Therefore, the Mg content is set to 0.0200% or less.
    • REM: 0.0005% to 0.1000%
  • REM has an action of enhancing the yield ratio of the hot-rolled steel sheet by adjusting a shape of an inclusion in steel to a preferable shape. In order to reliably obtain this effect, a REM content is preferably set to 0.0005% or more.
  • However, when the REM content is more than 0.1000%, an excessive amount of the inclusion is generated in steel, and the yield ratio of the hot-rolled steel sheet decreases. Therefore, the REM content is set to 0.1000% or less.
  • Here, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoids, and the REM content refers to the total amount of these elements. Lanthanoids are added in the form of mischmetal in industry.
    • Bi: 0.0005% to 0.0200%
  • In addition, Bi has an action of enhancing the yield ratio of the hot-rolled steel sheet by refining a solidification structure. In order to more reliably obtain an effect by this action, a Bi content is preferably 0.0005% or more.
  • However, when the Bi content is more than 0.0200%, the effect by the action is saturated, which is not economically preferable. Therefore, the Bi content is set to 0.0200% or less.
    • One or Two or More of Zr, Co, Zn, and W: 0% to 1.0000% in Total Sn: 0% to 0.050%
  • Regarding Zr, Co, Zn, and W, the present inventors have confirmed that, even when these elements are contained in a total amount of 1.0000% or less, the effects of the hot-rolled steel sheet according to the present embodiment are not impaired. Therefore, one or two or more of Zr, Co, Zn, and W may be contained in a total amount of 1.0000% or less.
  • In addition, the present inventors have confirmed that, even when a small amount of Sn is contained, the effects of the hot-rolled steel sheet according to the present embodiment are not impaired. However, when a large amount of Sn is contained, there are cases where a defect occurs during hot rolling. Therefore, a Sn content is set to 0.050% or less.
  • The chemical composition of the above-described hot-rolled steel sheet may be measured by a general analytical method. For example, the chemical composition may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). In addition, sol. Al may be measured by ICP-AES using a filtrate that is obtained after a sample is heated and decomposed with an acid. C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method.
  • In a case where the hot-rolled steel sheet is a surface-treated steel sheet having a plating layer, the chemical composition is analyzed after front and back surfaces of the steel sheet including the plating layer on a surface are mechanically ground by 150 µm or more.
    • Microstructure of Hot-Rolled Steel Sheet
  • Next, the microstructure of the hot-rolled steel sheet according to the present embodiment will be described.
  • In the hot-rolled steel sheet according to the present embodiment, the microstructure at a 1/4 position from the surface includes,
    • by area%,
      • ferrite: 2.0% to 30.0%,
      • bainite: 60.0% to 93.0%, and
      • martensite: 5.0% to 20.0%, and
    • an area ratio of martensite,
      • which is in contact with a 30° grain boundary,
      • which has a relative GAIQ value of Iα/3 or less, where Iα is a maximum value of a GAIQ value of the ferrite, and
      • which has a grain size of 2.0 µm or more, is 5.0% or more.
  • In the present embodiment, the 1/4 position from the surface indicates a region from a 1/8 thickness depth from the surface to a 3/8 thickness depth from the surface of the hot-rolled steel sheet. The reason for specifying the microstructure at this position is that the microstructure at this position indicates a typical microstructure of the hot-rolled steel sheet.
    • Hereinafter, each specification will be described. Area Ratio of Ferrite: 2.0% to 30.0%
  • Ferrite is a structure generated when fcc transforms into bcc at a relatively high temperature. When an area ratio of ferrite is less than 2.0%, a desired yield ratio cannot be obtained. Therefore, the area ratio of ferrite is set to 2.0% or more. The area ratio of ferrite is preferably 5.0% or more, 8.0% or more, or 10.0% or more.
  • On the other hand, when the area ratio of ferrite is more than 30.0%, a desired strength cannot be obtained. Therefore, the area ratio of ferrite is set to 30.0% or less. The area ratio of ferrite is preferably 27.0% or less, 25.0% or less, or 20.0% or less.
    • Bainite: 60.0% to 93.0%
  • Bainite is a structure including fine grains and a carbide. When an area ratio of bainite is less than 60.0%, a desired strength and yield ratio cannot be obtained. Therefore, the area ratio of bainite is set to 60.0% or more. The area ratio of bainite is preferably 65.0% or more, 70.0% or more, 75.0% or more, or 80.0% or more.
  • On the other hand, when the area ratio of bainite is more than 93.0%, a desired yield ratio cannot be obtained. Therefore, the area ratio of bainite is set to 93.0% or less. The area ratio of bainite is preferably 90.0% or less, 87.0% or less, or 80.0% or less.
    • Martensite: 5.0% to 20.0%
  • Martensite is a structure that increases the strength of the hot-rolled steel sheet. When an area ratio of martensite is less than 5.0%, a desired strength cannot be obtained. Therefore, the area ratio of martensite is set to 5.0% or more. The area ratio of martensite is preferably 8.0% or more or 10.0% or more.
  • On the other hand, when the area ratio of martensite is more than 20.0%, a desired yield ratio cannot be obtained. Therefore, the area ratio of martensite is set to 20.0% or less. The area ratio of martensite is preferably 18.0% or less or 15.0% or less.
  • The hot-rolled steel sheet according to the present embodiment may contain retained austenite and pearlite as a remainder in the microstructure in a total amount of less than 5.0%.
  • The area ratio of each structure is measured by the following method.
  • First, a test piece is collected from the hot-rolled steel sheet in a sheet thickness cross section parallel to a rolling direction so that the microstructure at the 1/4 position from the surface (the region from the 1/8 depth from the surface to the 3/8 depth from the surface) and at a center position in a sheet width direction can be observed.
  • The cross section of the test piece is polished using #600 to #1500 silicon carbide paper and is thereafter mirror-finished using a liquid obtained by dispersing a diamond powder having a particle size of 1 to 6 µm in a diluted solution such as alcohol or in pure water. Next, the cross section of the test piece is polished at room temperature using colloidal silica containing no alkaline solution to remove strain introduced into a surface layer of the sample. At a random position of the sample cross section in a longitudinal direction, a region having a length of 50 µm from the 1/8 thickness depth from the surface to the 3/8 thickness depth from the surface is measured by electron backscatter diffraction at a measurement interval of 0.1 µm to obtain crystal orientation information.
  • For the measurement, an EBSD analyzer including a thermal field-emission scanning electron microscope (JSM-7001F manufactured by JEOL Ltd) and an EBSD detector (DVC5 type detector manufactured by TSL solutions) is used. In this case, a degree of vacuum in the EBSD analyzer is set to 9.6 × 10-5 Pa or less, an accelerating voltage is set to 15 kV, an irradiation current level is set to 13, and an irradiation level of an electron beam is set to 62.
  • From the obtained crystal orientation information, using the "Grain Orientation Spread" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer, a region having a "Grain Orientation Spread" of 1° or less is extracted as ferrite under a condition in which a boundary with a crystal orientation difference of 15° or more is regarded as a grain boundary. By calculating an area ratio of the extracted ferrite, the area ratio of ferrite is obtained.
  • Subsequently, a Grain Average Image Quality map (GAIQ map) is obtained using the "Grain Average Image Quality" function. In the obtained GAIQ map, a region surrounded by grain boundaries having a crystal orientation difference of 15° or more is defined as a grain. When a maximum value of a "Grain Average Image Quality Value (GAIQ Value)" of the region extracted as ferrite is defined as Iα, a region having a relative GAIQ value of more than Iα/2 is extracted as bainite, and a region having a relative GAIQ value of Iα/2 or less is extracted as martensite. The area ratio of each of bainite and martensite is obtained by calculating an area ratio of the region of the extracted bainite and an area ratio of the region of the extracted martensite.
  • The area ratio of the remainder in the microstructure is obtained by subtracting the area ratios of the structures from 100%.
  • For the removal of contamination on a surface layer of an observed section, a method such as buffing using alumina particles having a particle size of 0.1 µm or less or Ar ion sputtering may be used.
  • Area Ratio of Martensite, Which Is in Contact with 30° Grain Boundary, When Maximum Value of GAIQ Value of Ferrite Is Indicated as Iα, Relative GAIQ Value of Iα/3 or Less, and Which Has Grain Size of 2.0 µm or More: 5.0% or More
  • The martensite can be rephrased as martensite that satisfies the following conditions (I) to (III).
    1. (I) Is in contact with a 30° grain boundary.
    2. (II) When the maximum value of the GAIQ value of ferrite is indicated by Iα, the relative GAIQ value is Iα/3 or less.
    3. (III) The grain size is 2.0 µm or more.
  • When the area ratio of the martensite that satisfies the above conditions (1) to (III) is less than 5.0%, the punching property of the hot-rolled steel sheet deteriorates. Therefore, the area ratio of the martensite is set to 5.0% or more. The area ratio of martensite is preferably 8.0% or more or 10.0% or more.
  • Although an upper limit thereof is not particularly specified, the area ratio of the martensite may be set to 20.0% or less or 15.0% or less.
  • A higher GAIQ value indicates a lower dislocation density, while a lower GAIQ value indicates a higher dislocation density. Therefore, the GAIQ value is a parameter that can reflect a dislocation density of a grain.
  • When the maximum value of the GAIQ value of ferrite is indicated as Iα, the martensite having a relative GAIQ value of Iα/3 or less is a harder type of martensite among other types of martensite. In the present embodiment, in order to improve the punching property of the hot-rolled steel sheet, the area ratio of the hard martensite that is in contact with a 30° grain boundary is controlled. The martensite having a grain size of less than 2.0 µm does not affect the punching property of the hot-rolled steel sheet and therefore does not need to be particularly controlled.
  • The area ratio of the martensite is measured by the following method.
  • First, a test piece is collected and treated by the same method as in the measurement of the area ratios of the structures. A measurement position is set to a 1/4 position from a surface (a region from a 1/8 depth from the surface to a 3/8 depth from the surface) and a center position in a sheet width direction. Next, the 30° grain boundary is specified using the "Grain Orientation Spread" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. Next, a GAIQ map is obtained by the same method as in the measurement of the area ratios of the structures, and ferrite and martensite are extracted. The maximum value Iα of the GAIQ value of the ferrite is obtained, and the martensite having a relative GAIQ value of Iα/3 or less is specified. Accordingly, the martensite that is in contact with a 30° grain boundary and has a relative GAIQ value of Iα/3 or less is specified (condition (I) and condition (II)). The martensite that is in contact with a 30° grain boundary also includes martensite that is present on a 30° grain boundary.
  • In the GAIQ map, the grain size of the martensite is obtained by calculating an equivalent circle diameter of the martensite that satisfies condition (I) and condition (II). Accordingly, martensite having a grain size of 2.0 µm or more is specified (Condition (III)).
  • In the above GAIQ map, by calculating the area ratio of the martensite that satisfies the conditions (I) to (III), the area ratio of the martensite that is in contact with a 30° grain boundary, has a relative GAIQ value of Iα/3 or less, and has a grain size of 2.0 µm or more is obtained.
    • Strength
  • The hot-rolled steel sheet according to the present embodiment has a tensile strength of 980 MPa or more. A tensile strength of 980 MPa or more can contribute to a further reduction in the weight of vehicle bodies. An upper limit of the tensile strength is not particularly specified, but may be set to 1,400 MPa or less.
  • The tensile strength is measured by a tensile test according to JIS Z 2241: 2011. As a test piece, a No. 5 test piece of JIS Z 2241: 2011 is used. A position from which the tensile test piece is collected is set to a 1/4 portion from an end portion in a sheet width direction, and a longitudinal direction thereof may be a direction perpendicular to a rolling direction.
    • Yield Ratio
  • The hot-rolled steel sheet according to the present embodiment has a yield ratio of 0.75 or more. The yield ratio is obtained by dividing a yield stress by a tensile strength (yield stress / tensile strength).
  • The yield stress is obtained by conducting a tensile test in the above-described method. In a case where the hot-rolled steel sheet yields discontinuously, an upper yield point is regarded as the yield stress, and in a case where the hot-rolled steel sheet yields continuously, a 0.2% proof stress is regarded as the yield stress.
    • Punching Property
  • It is preferable that the hot-rolled steel sheet according to the present embodiment has excellent punched end surface properties when subjected to a punching test. The punched end surface properties are evaluated by the following method.
  • A test piece is collected from the hot-rolled steel sheet. A punched hole is prepared from the test piece with a hole diameter of 20 mm, a shear angle of 5°, a clearance of 15.0%, and a punching speed of 80 mm/s. For the punched hole, a test piece is collected so that four cross sections can be checked at intervals of about 90°, the test piece is embedded in a resin, and a punched end surface is photographed with a scanning electron microscope. The obtained picture is observed, and in a case where no large cracks of more than 100 µm are observed in a direction perpendicular to the sheet thickness direction, the hot-rolled steel sheet can be determined to have excellent punching property. In a case where no cracks of 50 µm or more are observed in the direction perpendicular to the sheet thickness direction, the hot-rolled steel sheet can be determined to have particularly excellent punching property.
    • Sheet Thickness
  • A sheet thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited and may be set to 0.6 to 8.0 mm. By setting the sheet thickness of the hot-rolled steel sheet to 0.6 mm or more, an excessive rolling force can be suppressed, thereby facilitating hot rolling. In addition, by setting the sheet thickness to 8.0 mm or less, the above-described microstructure can be easily obtained.
    • Plating Layer
  • The hot-rolled steel sheet may be a surface-treated steel sheet provided with a plating layer on the surface for the purpose of improving corrosion resistance or the like. The plating layer may be an electroplating layer or a hot-dip plating layer. Examples of the electroplating layer include electrogalvanizing, and electro Zn-Ni alloy plating. Examples of the hot-dip plating layer include hot-dip galvanizing, hot-dip galvannealing, hot-dip aluminum plating, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, and hot-dip Zn-Al-Mg-Si alloy plating. A plating adhesion amount is not particularly limited and may be the same as in the related art. In addition, it is also possible to further enhance the corrosion resistance by performing an appropriate chemical conversion treatment (for example, application and drying of a silicate-based chromium-free chemical conversion liquid) after plating.
    • Manufacturing Conditions
  • In a suitable manufacturing method for the hot-rolled steel sheet according to the present embodiment, the following steps (1) to (7) are sequentially performed. A temperature of a slab and a temperature of a steel sheet in the present embodiment refer to a surface temperature of the slab and a surface temperature of the steel sheet.
    1. (1) After casting a slab having the above-described chemical composition, the slab is heated before being cooled to a temperature range of 500°C or lower and retained in a temperature range of 1,220°C or higher for 30 minutes or longer.
    2. (2) Slab width reduction is performed at a rolling reduction of 10% or larger in a temperature range of 1,200°C or higher.
    3. (3) In rough rolling, a rough rolling finishing temperature is set to a temperature range of 1,100°C or higher, a cumulative rolling reduction of all stands is set to 70% or larger, a rolling reduction of each of final three stands is set to smaller than 20%, and a rolling reduction of each of all the stands is set to smaller than 40%.
    4. (4) Retention is performed in a temperature range of 1,000°C or higher for 30 seconds or longer after the completion of the rough rolling and before the start of finish rolling.
    5. (5) A finishing temperature FT is set to a temperature range of T1 (°C) - 100°C or higher, a cumulative rolling reduction of the finish rolling is set to 75% or larger, a cumulative rolling reduction of final two stands is set to smaller than 30%.
      Here, T1 (°C) can be obtained by Formula (A) below. An element symbol in the following formula represents the amount of each element by mass%, and is substituted with 0 in a case where the corresponding element is not contained. T1 = 937 + 168 × Ti + 3545 × Nb + 4500 × B
      Figure imgb0001
    6. (6) An average cooling rate in a temperature range of the finishing temperature FT to 700°C is set to 30 °C/s or faster.
    7. (7) Coiling is performed in a temperature range of 450°C to 650°C.
  • The hot-rolled steel sheet according to the present embodiment can be stably manufactured by a manufacturing method in which the above steps are closely and inseparably controlled.
  • Hereinafter, each step will be described.
    • (1) Slab Heating
  • It is preferable that the slab to be subjected to the hot rolling is heated after casting and before being cooled to a temperature range of 500°C or lower and retained in a temperature range of 1,220°C or higher for 30 minutes or longer. In addition, in the retention in a temperature range of 1,220°C or higher, the temperature of the steel sheet may be changed or may be kept constant.
  • In the slab stage, the slab is not cooled to a temperature range of 500°C or lower, whereby the precipitation of coarse Ti is suppressed, and Ti can be sufficiently solutionized. Accordingly, a precipitation hardening ability of ferrite and bainite can be enhanced, and the strength and yield ratio of the hot-rolled steel sheet can be increased. In addition, by performing retention in a temperature range of 1,220°C or higher for 30 minutes or longer, sufficient solutionizing can be achieved, and prior austenite grains are coarsened, whereby an effect of the slab width reduction in the subsequent step can be maximized. As a result, the area ratio of the martensite that is in contact with a 30° grain boundary, when the maximum value of the GAIQ value of ferrite is indicated as Iα, which has a relative GAIQ value of Iα/3 or less, and which has a grain size of 2.0 µm or more (hereinafter, sometimes referred to as martensite area ratio at a 30° grain boundary) can be increased.
  • Other manufacturing steps preceding the hot rolling are not particularly limited. Subsequent to melting by a blast furnace, an electric furnace, or the like, various secondary smelting processes may be performed, and then the slab may be cast by a method such as ordinary continuous casting. Scrap may be used as a raw material.
    • (2) Slab Width Reduction
  • After the above heating and retention, it is preferable that the slab width reduction is performed in a temperature range of 1,200°C or higher with a rolling reduction of 10% or larger. By performing the slab width reduction under this condition, the coarsened prior austenite grains can be elongated in the sheet thickness direction. As a result, the martensite area ratio at a 30° grain boundary can be increased.
  • In addition, the rolling reduction of the slab width reduction can be expressed as, when a length of the slab in a width direction before the reduction is indicated as w0 and the length of the slab in the width direction after the reduction is indicated as w1, (1 - w1/w0) × 100 (%). As a method of performing the slab width reduction, for example, there is a method of rolling a slab using rolls installed so that a rotation axis is perpendicular to a sheet surface of the slab.
    • (3) Rough Rolling
  • In the rough rolling, it is preferable that the rough rolling finishing temperature is set to a temperature range of 1,100°C or higher, the cumulative rolling reduction of all the stands is set to 70% or larger, the rolling reduction of each of the final three stands is set to smaller than 20%, and the rolling reduction of each of all the stands is smaller than 40%. By setting the rough rolling finishing temperature to the temperature range of 1,100°C or higher, setting the cumulative rolling reduction of all the stands to 70% or larger, and setting the rolling reduction of each of the final three stands to smaller than 20%, prior austenite grains elongated in the sheet thickness direction can be made equiaxed, and uniform prior austenite grains can be obtained by promoting recrystallization in an intragranular deformation band. As a result, the martensite area ratio at a 30° grain boundary can be increased.
  • In addition, by setting the rolling reduction of each of all the stands to smaller than 40%, elongation of the prior austenite grains in the rolling direction can be suppressed. As a result, the martensite area ratio at a 30° grain boundary can be increased.
  • In the rough rolling, reverse rolling is not desirable. This is because when reverse rolling is performed during the rough rolling, a shape of the prior austenite grains cannot be preferably controlled, and as a result, the martensite area ratio at a 30° grain boundary cannot be preferably controlled.
    • (4) Retention After Completion of Rough Rolling and Before Start of Finish Rolling
  • It is preferable to perform retention in a temperature range of 1,000°C or higher for 30 seconds or longer after the completion of the rough rolling and before the start of the finish rolling. By performing retention under this condition, recrystallization in the intragranular deformation band is promoted, and uniform prior austenite grains can be obtained. As a result, the martensite area ratio at a 30° grain boundary can be increased.
  • As a method of performing retention in the above temperature range, for example, there is a method of performing heating in a heating furnace after the completion of rough rolling or a method of using a heat-retaining cover. In addition, in the retention, the temperature of the steel sheet may be kept constant or may be changed in a temperature range of 1,000°C or higher.
    • (5) Finish Rolling
  • It is preferable that the finishing temperature FT is set to a temperature range of T1 (°C) - 100°C or higher, the cumulative rolling reduction of the finish rolling is set to 75% or larger, the cumulative rolling reduction of final two stands is set to smaller than 30%. By performing the finish rolling under this condition, it is possible to control the prior austenite grains to be equiaxed while promoting recrystallization. As a result, the martensite area ratio at a 30° grain boundary can be increased.
    • (6) Cooling After Completion of Finish Rolling
  • After the completion of the finish rolling, the average cooling rate in the temperature range of the finishing temperature FT to 700°C is preferably set to 30 °C/s or faster. By performing cooling under this condition, desired amounts of ferrite and bainite can be obtained.
  • In the present embodiment, the average cooling rate refers to a value obtained by dividing a temperature drop width of the steel sheet from the start of the cooling to the completion of the cooling by a time required from the start of the cooling to the completion of the cooling.
    • (7) Coiling
  • Coiling is preferably performed in a temperature range of 450°C to 650°C. By setting a coiling temperature to a temperature range of 450°C or higher, desired amounts of ferrite and martensite can be obtained. In addition, by setting the coiling temperature to a temperature range of 650°C or lower, desired amounts of ferrite and bainite can be obtained.
  • After the coiling, cooling to room temperature may be performed. Thereafter, pickling and cold rolling may be performed by an ordinary method as necessary. In the cold rolling, a cumulative rolling reduction may be 50% or larger. Furthermore, as necessary, temper rolling may be performed in order to flatten the hot-rolled steel sheet and adjust a surface roughness.
    • [Examples]
  • Next, effects of one aspect of the present invention will be more specifically described using examples, but conditions in the examples are simply examples of the conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples of the conditions. The present invention may adopt various conditions to achieve the object of the present invention without departing from the scope of the present invention.
  • Steels having the chemical composition shown in Table 1 were melted and continuously cast to manufacture slabs having a thickness of 240 to 300 mm. The obtained slabs were used to obtain hot-rolled steel sheets shown in Tables 4A and 4B under the manufacturing conditions shown in Tables 2A to 3B.
  • The obtained hot-rolled steel sheets were subjected to microstructure observation, a tensile test, and a punching test by the above-described methods. The obtained measurement results are shown in Tables 4A and 4B.
  • In a case where the tensile strength was 980 MPa or more, the hot-rolled steel sheet was considered to be a hot-rolled steel sheet having high strength and determined to be acceptable. On the other hand, in a case where the tensile strength was less than 980 MPa, the hot-rolled steel sheet was not regarded as a hot-rolled steel sheet having high strength and determined to be unacceptable.
  • In a case where the yield ratio (yield stress / tensile strength) was 0.75 or more, the hot-rolled steel sheet was considered to be a hot-rolled steel sheet having a high yield ratio and determined to be acceptable. On the other hand, in a case where the yield ratio was less than 0.75, the hot-rolled steel sheet was not considered to be a hot-rolled steel sheet having a high yield ratio and determined to be unacceptable.
  • At the punching test, in a case where no cracks of 50 µm or more in the direction perpendicular to the sheet thickness direction were observed in the obtained structure photograph, the punching property was considered to be particularly good and indicated as "E" (Excellent) in the tables. In addition, in a case where a crack of more than 50 µm and 100 µm or less in the direction perpendicular to the sheet thickness direction was observed, the punching property was considered to be good and indicated as "G" (Good) in the tables. In addition, in a case where a crack of more than 100 µm in the direction perpendicular to the sheet thickness direction was observed, the punching property was considered to be inferior and indicated as "B" (Bad) in the tables.
  • In a case of being evaluated as "E" and "G", the hot-rolled steel sheet was considered to be a hot-rolled steel sheet having excellent punching property and determined to be acceptable. On the other hand, in a case of being evaluated as "B", the hot-rolled steel sheet was not considered to be a hot-rolled steel sheet having excellent punching property and determined to be unacceptable. [Table 1]
    Kind of steel Chemical composition (mass%), remainder: Fc and impurities T1 (°C) Note
    C Si Mn sol. Al Ti P S N O Others
    A 0.090 0.70 1.90 0.100 0.130 0.008 0.0010 0.0020 0.0020 Nb: 0.020 1030 Present Invention Steel
    B 0.065 1.48 1.95 0.305 0.120 0.010 0.0020 0.0040 0.0010 957 Present Invention Steel
    C 0.051 1.28 2.30 0.030 0.110 0.011 0.001 0 0.0020 0.0010 955 Present Invention Steel
    D 0.121 0.80 1.80 0.020 0.140 0.010 0.0010 0.0020 0.0020 961 Present Invention Steel
    E 0.080 0.24 2.30 0.030 0.120 0.009 0.0010 0.0020 0.0010 957 Present Invention Steel
    F 0.058 2.20 2.20 0.020 0.130 0.010 0.0010 0.0010 0.0030 959 Present Invention Steel
    G 0.081 0.90 1.61 0.030 0.060 0.009 0.0020 0.0020 0.0020 947 Present Invention Steel
    H 0.078 1.10 3.10 0.020 0.080 0.011 0.0010 0.0020 0.0010 950 Present Invention Steel
    I 0.100 1.20 2.50 0.030 0.101 0.010 0.0020 0.0020 0.0020 954 Present Invention Steel
    J 0.131 1.10 1.70 0.020 0.160 0.010 0.0020 0.0030 0.0010 964 Present Invention Steel
    K 0.075 0.70 2.10 0.030 0.091 0.010 0.0010 0.0020 0.0010 Nb: 0.041 1098 Present Invention Steel
    L 0.065 0.40 2.10 0.030 0.098 0.013 0.0020 0.0010 0.0010 V: 0.040 953 Present Invention Steel
    M 0.068 0.50 1.87 0.030 0.112 0.010 0.0020 0.0020 0.0010 Cu: 0.25 956 Present Invention Steel
    N 0.058 0.38 1.94 0.020 0.110 0.011 0.0010 0.0010 0.0020 Cr: 0.44 955 Present Invention Steel
    O 0.084 0.83 2.10 0.030 0.056 0.013 0.0010 0.0010 0.0020 Mo: 0.100 946 Present Invention Steel
    P 0.059 1.20 1.75 0.030 0.100 0.010 0.0010 0.0020 0.0010 Ni: 0.80 954 Present Invention Steel
    Q 0.110 1.12 2.21 0.030 0.089 0.010 0.0010 0.0020 0.0010 Ca: 0.0150 952 Present Invention Steel
    R 0.065 1.32 1.89 0.030 0.078 0.013 0.0010 0.0010 0.0010 Mg: 0.0120 950 Present Invention Steel
    S 0.049 1.38 2.71 0.030 0.084 0.010 0.0010 0.0010 0.0020 REM: 0.0700 951 Present Invention Steel
    T 0.094 1.75 1.86 0.030 0.110 0.011 0.0020 0.0020 0.0010 Bi: 0.0180 955 Present Invention Steel
    U 0.081 1.83 2.42 0.020 0.124 0.013 0.0010 0.0010 0.0010 Zr: 0.0025 958 Present Invention Steel
    V 0.076 2.11 2.15 0.020 0.142 0.011 0.0020 0.0010 0.0010 Co: 0.0040 961 Present Invention Steel
    W 0.058 2.31 1.51 0.040 0.095 0.013 0.0010 0.0010 0.0030 Zn: 0.0210 953 Present Invention Steel
    X 0.140 1.81 1.95 0.030 0.055 0.010 0.0020 0.0020 0.0010 W: 0.0400 946 Present Invention Steel
    Y 0.071 1.25 2.41 0.050 0.112 0.010 0.0010 0.0010 0.0010 Sn: 0.050 956 Present Invention Steel
    Z 0.070 1.22 2.20 0.030 0.130 0.010 0.0010 0.0030 0.0010 Nb: 0.018, B: 0.0020 1032 Present Invention Steel
    AA 0.085 0.35 1.89 0.730 0.110 0.010 0.0010 0.0020 0.0010 Nb: 0.010 991 Present Invention Steel
    AB 0.034 1.45 1.98 0.020 0.095 0.012 0.0020 0.0020 0.0020 953 Comparative Steel
    AC 0.065 0.05 2.45 0.020 0.098 0.012 0.0020 0.0010 0.0010 953 Comparative Steel
    AD 0.098 1.30 4.80 0.030 0.123 0.013 0.0010 0.0010 0.0020 958 Comparative Steel
    AE 0.100 1.05 1.02 0.020 0.109 0.015 0.0010 0.0010 0.0010 955 Comparative Steel
    AF 0.089 1.05 2.30 0.030 0.250 0.012 0.0020 0.0020 0.0010 979 Comparative Steel
    AG 0.067 1.53 2.21 0.030 0.032 0.011 0.0010 0.0010 0.0010 942 Comparative Steel
    The underlined indicates outside of the range of the present invention. [Table 2A]
    Sample No. Kind of steel Slab heating Slab width reduction Rough rolling Retention Note
    Heating start temperature °C Retention time in temperature range of 1,220°C or higher min Rolling reduction % Rolling temperature °C Rough rolling finishing temperature °C Cumulative rolling reduction of all stands % Maximum rolling reduction % Maximum rolling reduction of final three stands % Retention time in temperature range of 1000°C or higher after completion of rough rolling and before start of finish rolling s
    1 A 550 60 15 1230 1150 84 34 18 78 Present Invention Example
    2 A 650 75 18 1165 1102 78 38 18 58 Comparative Example
    3 B 620 55 21 1215 1120 79 38 17 81 Present Invention Example
    4 B 590 68 16 1208 1056 76 39 19 64 Comparative Example
    5 C 520 54 13 1240 1185 75 35 18 90 Present Invention Example
    6 C 610 89 6 1235 1176 78 36 16 85 Comparative Example
    7 C 580 70 15 1225 1145 78 37 16 77 Comparative Example
    8 D 580 63 13 1220 1118 77 31 16 68 Present Invention Example
    9 D 530 75 17 1218 1146 65 29 14 90 Comparative Example
    10 D 550 80 21 1231 1155 80 35 18 76 Comparative Example
    11 E 600 75 15 1230 1115 86 34 17 75 Present Invention Example
    12 E 620 70 16 1211 1127 76 43 17 87 Comparative Example
    13 F 650 59 14 1210 1117 76 36 18 70 Present Invention Example
    14 F 570 78 18 1221 1148 79 33 17 80 Comparative Example
    15 F 580 65 17 1235 1121 78 38 17 85 Comparative Example
    16 G 580 120 19 1220 1135 81 35 16 69 Present Invention Example
    17 G 520 10 15 1205 1113 78 36 19 75 Comparative Example
    18 G 550 85 15 1230 1140 79 33 17 8 Comparative Example
    19 H 860 89 24 1230 1141 88 34 17 70 Present Invention Example
    20 H 560 65 16 1224 1135 83 35 16 85 Comparative Example
    21 I 730 130 14 1230 1108 84 31 15 68 Present Invention Example
    22 I 30 45 12 1220 1136 90 29 16 75 Comparative Example
    23 J 550 69 19 1240 1121 85 29 19 73 Present Invention Example
    24 J 600 74 18 1229 1135 91 39 19 80 Comparative Example
    25 K 530 40 24 1210 1110 83 38 17 77 Present Invention Example
    26 L 610 77 21 1220 1105 76 33 17 80 Present Invention Example
    27 M 570 250 11 1230 1115 77 35 18 75 Present Invention Example
    The underlined indicates that the manufacturing conditions arc not preferable. [Table 2B]
    Sample No. Kind of steel Slab heating Slab width reduction Rough rolling Retention Note
    Heating start temperature °C Retention time in temperature range of 1,220°C or higher min Rolling reduction % Rolling temperature °C Rough rolling finishing temperature °C Cumulative rolling reduction of all stands % Maximum rolling reduction % Maximum rolling reduction of final three stands % Retention time in temperature range of 1,000°C or higher after completion of rough rolling and before start of finish rolling s
    28 N 600 180 14 1210 1109 82 39 16 76 Present Invention Example
    29 O 650 100 12 1220 1142 74 31 17 80 Present Invention Example
    30 P 610 110 19 1215 1130 85 33 18 75 Present Invention Example
    31 Q 510 120 15 1230 1115 81 29 19 77 Present Invention Example
    32 R 550 47 11 1210 1125 83 38 16 65 Present Invention Example
    33 S 630 95 13 1220 1155 82 31 17 60 Present Invention Example
    34 T 580 66 23 1240 1135 84 29 16 65 Present Invention Example
    35 U 620 43 19 1225 1140 85 38 17 63 Present Invention Example
    36 V 630 38 27 1230 1135 73 35 17 61 Present Invention Example
    37 W 630 65 17 1215 1150 78 34 19 83 Present Invention Example
    38 X 610 80 21 1210 1145 75 38 18 82 Present Invention Example
    39 Y 580 100 14 1230 1150 74 31 15 71 Present Invention Example
    40 Z 540 35 15 1210 1145 85 39 16 68 Present Invention Example
    41 Z 520 60 12 1245 1151 80 37 33 73 Comparative Example
    42 AA 590 80 23 1220 1130 76 33 18 73 Present Invention Example
    43 AB 540 55 19 1230 1140 82 36 16 95 Comparative Example
    44 AC 630 80 19 1230 1140 79 32 19 78 Comparative Example
    45 AD 520 99 15 1220 1130 83 37 17 75 Comparative Example
    46 AE 560 75 21 1220 1125 77 39 16 68 Comparative Example
    47 AF 530 81 15 1210 1150 78 34 17 63 Comparative Example
    48 AG 550 75 12 1215 1130 83 39 is 71 Comparative Example
    49 A 510 62 16 1210 1118 91 39 18 81 Present Invention Example
    50 B 600 81 12 1220 1126 78 31 19 71 Present Invention Example
    51 E 580 54 21 1225 1139 88 33 16 66 Present Invention Example
    52 H 520 91 11 1240 1154 82 39 17 82 Present Invention Example
    53 H 550 73 16 1230 1141 78 35 18 85 Present Invention Example
    The underlined indicates that the manufacturing conditions arc not preferable. [Table 3A]
    Sample No. Kind of steel Finish rolling Coiling Note
    Cumulative rolling reduction % Cumulative rolling reduction of final two stands % Finishing temperature FT °C T1 - 100 °C Average cooling rate in temperature range of FT to 700°C °C/s Coiling temperature °C
    1 A 89 29 980 930 35 500 Present Invention Example
    2 A 82 28 955 930 50 575 Comparative Example
    3 B 83 26 900 857 80 550 Present Invention Example
    4 B 76 28 890 857 71 580 Comparative Example
    5 C 81 27 930 855 75 480 Present Invention Example
    6 C 79 27 900 855 80 510 Comparative Example
    7 C 79 26 900 855 76 685 Comparative Example
    8 D 79 28 940 861 40 530 Present Invention Example
    9 D 76 28 911 861 80 550 Comparative Example
    10 D 79 38 910 861 56 608 Comparative Example
    11 E 80 27 910 857 55 560 Present Invention Example
    12 E 78 26 888 857 70 590 Comparative Example
    13 F 77 29 890 859 79 550 Present Invention Example
    14 F 81 27 825 859 68 550 Comparative Example
    15 F 79 29 906 859 15 620 Comparative Example
    16 G 83 27 940 847 32 580 Present Invention Example
    17 G 78 29 900 847 80 530 Comparative Example
    is G 77 25 905 847 55 610 Comparative Example
    19 H 87 25 950 850 80 635 Present Invention Example
    20 II 81 27 925 850 75 398 Comparative Example
    21 I 90 28 940 854 77 575 Present Invention Example
    22 I 86 27 920 854 85 590 Comparative Example
    23 J 85 27 955 864 85 546 Present Invention Example
    24 J 71 27 928 864 75 585 Comparative Example
    25 K 89 27 999 998 57 550 Present Invention Example
    26 L 85 29 940 853 45 580 Present Invention Example
    27 M 90 26 890 856 69 520 Present Invention Example
    The underlined indicates that the manufacturing conditions are not preferable. [Table 3B]
    Sample No. Kind of steel Finish rolling Coiling Note
    Cumulative rolling reduction % Cumulative rolling reduction of final two stands % Finishing temperature FT °C T1 - 100 °C Average cooling rate in temperature range of FT to 700°C °C/s Coiling temperature °C
    28 N 89 27 900 855 81 495 Present Invention Example
    29 O 82 28 920 846 44 520 Present Invention Example
    30 P 78 29 910 854 39 500 Present Invention Example
    31 Q 80 27 890 852 75 495 Present Invention Example
    32 R 78 27 906 850 65 560 Present Invention Example
    33 S 79 28 889 851 77 565 Present Invention Example
    34 T 80 28 890 855 80 580 Present Invention Example
    35 U 78 27 905 858 49 540 Present Invention Example
    36 V 81 26 915 861 58 500 Present Invention Example
    37 W 79 29 910 853 69 485 Present Invention Example
    38 X 81 28 900 846 70 600 Present Invention Example
    39 Y 77 27 896 856 84 550 Present Invention Example
    40 Z 78 28 983 932 75 580 Present Invention Example
    41 Z 77 29 942 932 80 500 Comparative Example
    42 AA 80 26 910 891 84 570 Present Invention Example
    43 AB 78 28 890 853 85 550 Comparative Example
    44 AC 80 27 896 853 74 610 Comparative Example
    45 AD 79 28 899 858 59 490 Comparative Example
    46 AE 82 27 895 855 63 500 Comparative Example
    47 AF 81 28 911 879 75 630 Comparative Example
    48 AG 79 28 895 842 66 500 Comparative Example
    49 A 88 26 962 930 61 500 Present Invention Example
    50 B 79 27 944 857 73 460 Present Invention Example
    51 E 81 25 912 857 32 590 Present Invention Example
    52 H 86 25 937 850 44 510 Present Invention Example
    53 II 84 26 926 854 39 460 Present Invention Example
    The underlined indicates that the manufacturing conditions are not preferable. [Table 4A]
    Sample No. Kind of steel Microstructure Mechanical properties Note
    Ferrite area% Bainite area% Martensite area% Remainder in microstructure area% Martensite at 30° grain boundary area% Tensile strength TS MPa Yield ratio- Punching property
    1 A 15.0 72.2 12.0 0.8 6.0 1045 0.78 G Present Invention Example
    2 A 100 76.2 13.0 0.8 4.1 989 0.92 B Comparative Example
    3 B 14.0 74.8 10.0 1.2 9.0 983 0.89 E Present Invention Example
    4 B 15.0 67.7 16.0 1.3 4.1 982 0.87 B Comparative Example
    5 C 12.0 78.5 8.0 1.5 8.0 986 0.9 G Present Invention Example
    6 C 15.0 71.4 12.0 1.6 2.8 1008 0.87 B Comparative Example
    7 c 34.0 45.9 18.0 2.1 9.0 789 0.92 G Comparative Example
    8 D 16.0 66.9 16.0 1.1 12.0 1221 0.78 E Present Invention Example
    9 D 9.0 78.9 11.0 1.1 47 1001 0.91 B Comparative Example
    10 D 14.0 73.9 11.0 1.1 48 1276 0.76 B Comparative Example
    11 E 14.0 75.1 10.0 0.9 8.0 1005 0.91 G Present Invention Example
    12 E 11.0 75.9 11.0 2.1 3.9 1164 0.82 B Comparative Example
    13 F 100 81.8 7.0 1.2 6.0 991 0.87 G Present Invention Example
    14 F 18.0 72.6 8.0 1.4 3.1 990 0.89 B Comparative Example
    15 F 35.0 56.9 7.0 1.1 5.0 894 0.88 G Comparative Example
    16 G 17.0 70.2 12.0 0.8 9.0 984 0.91 E Present Invention Example
    17 G 15.0 70.9 12.0 2.1 3.5 895 0.74 B Comparative Example
    18 G 13.0 77.5 8.0 1.5 4.3 989 0.78 B Comparative Example
    19 II 12.0 72.9 14.0 1.1 11.0 1016 0.89 E Present Invention Example
    20 H 1.1 94.7 3.1 1.1 2.8 908 0.72 E Comparative Example
    21 I 15.0 71.2 12.0 1.8 9.0 1085 0.91 E Present Invention Example
    22 I 12.0 71.6 15.0 1.4 6.0 974 0.72 G Comparative Example
    23 J 12.0 70.1 17.0 0.9 13.0 1205 0.76 E Present Invention Example
    24 J 21.0 71.2 7.0 0.8 4.6 991 0.85 B Comparative Example
    25 K 11.0 76.2 11.0 1.8 6.0 994 0.92 G Present Invention Example
    26 L 16.0 74.5 8.0 1.5 12.0 987 0.88 E Present Invention Example
    27 M 12.0 75.9 10.0 2.1 13.0 981 0.87 E Present Invention Example
    The underlined indicates outside of the range of the present invention, and that the properties are not preferable. [Table 4B]
    Sample No. Kind of steel Microstructure Mechanical properties Note
    Ferrite area% Bainite area% Martensite area% Remainder in microstructure area% Martensite at 30° grain boundary area% Tensile strength TS MPa Yield ratio- Punching property
    28 N 6.0 86.4 6.0 1.6 11.0 987 0.91 E Present Invention Example
    29 O 13.0 73.7 12.0 1.3 10.0 1032 0.91 E Present Invention Example
    30 P 15.0 74.1 9.0 1.9 6.0 1001 0.9 G Present Invention Example
    31 Q 13.0 69.9 15.0 2.1 11.0 1176 0.88 E Present Invention Example
    32 R 12.0 75.6 11.0 1.4 9.0 998 0.92 E Present Invention Example
    33 S 14.0 75.2 9.0 1.8 7.0 1012 0.91 G Present Invention Example
    34 T 17.0 65.7 15.0 23 11.0 1115 0.88 E Present Invention Example
    35 U 15.0 69.1 14.0 1.9 12.0 1186 0.89 E Present Invention Example
    36 V 14.0 73.7 11.0 1.3 7.0 1085 0.76 G Present Invention Example
    37 W 18.0 71.1 9.0 1.9 6.0 1006 0.91 G Present Invention Example
    38 X 16.0 630 19.0 2.0 14.0 1209 0.86 E Present Invention Example
    39 Y 13.0 74.2 11.0 1.8 7.0 1002 0.91 G Present Invention Example
    40 Z 16.0 68.9 14.0 1.1 12.0 986 0.85 E Present Invention Example
    41 Z 17.0 74.9 7.0 1.1 4.4 991 0.81 B Comparative Example
    42 AA 14.0 72.3 13.0 0.7 8.0 999 0.86 G Present Invention Example
    43 AB 13.0 78.5 7.0 1.5 5.0 809 0.86 G Comparative Example
    44 AC 14.0 74.4 11.0 0.6 7.0 860 0.89 G Comparative Example
    45 AD 1.0 72.6 250 1.4 12.0 1211 0.72 G Comparative Example
    46 AE 19.0 75.5 4.0 1.5 2.0 805 0.87 B Comparative Example
    47 AF 13.0 69.9 15.0 2.1 7.0 1015 0.92 B Comparative Example
    48 AG 12.0 76.4 10.0 1.6 7.0 870 0.71 G Comparative Example
    49 A 14.0 71.8 11.6 26 5.3 1033 0.81 G Present Invention Example
    50 B 2.1 78.1 17.1 27 9.5 1026 0.78 G Present Invention Example
    51 E 29.0 63.8 6.1 1.1 5.7 992 0.93 E Present Invention Example
    52 II 2.9 91.1 5.8 0.2 5.2 997 0.85 G Present Invention Example
    53 H 77 71.2 19.1 2.0 18.8 1005 0.78 G Present Invention Example
    The underlined indicates outside of the range of the present invention, and that the properties are not preferable.
  • From Table 4A and Table 4B, it can be seen that the hot-rolled steel sheets according to the present invention examples have high strength and yield ratio and excellent punching property.
  • 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 the above properties.
    • [Industrial Applicability]
  • According to the aspect of the present invention, it is possible to provide a hot-rolled steel sheet having high strength and yield ratio, and excellent punching property.

Claims (2)

  1. A hot-rolled steel sheet comprising, as a chemical composition, by mass%:
    C: 0.045% to 0.150%;
    Si: 0.10% to 2.50%;
    Mn: 1.50% to 3.50%;
    sol. Al: 0.010% to 1.000%;
    Ti: 0.050% to 0.200%;
    P: 0.100% or less;
    S: 0.0300% or less;
    N: 0.1000% or less;
    O: 0.0100% or less;
    Nb: 0% to 0.050%;
    V: 0% to 0.050%;
    B: 0% to 0.0100%;
    Cu: 0% to 2.00%;
    Cr: 0% to 2.00%;
    Mo: 0% to 1.000%;
    Ni: 0% to 2.00%;
    Ca: 0% to 0.0200%;
    Mg: 0% to 0.0200%;
    REM: 0% to 0.1000%;
    Bi: 0% to 0.0200%;
    one or two or more of Zr, Co, Zn, and W: 0% to 1.0000% in total;
    Sn: 0% to 0.050%; and
    a remainder comprising Fe and impurities,
    wherein a microstructure at a 1/4 position from a surface includes, by area%,
    ferrite: 2.0% to 30.0%,
    bainite: 60.0% to 93.0%, and
    martensite: 5.0% to 20.0%,
    an area ratio of martensite,
    which is in contact with a 30° grain boundary,
    which has a relative GAIQ value of Iα/3 or less, where Iα is a maximum value of a GAIQ value of the ferrite, and
    which has a grain size of 2.0 µm or more,
    is 5.0% or more, and
    a tensile strength of the hot-rolled steel sheet is 980 MPa or more.
  2. The hot-rolled steel sheet according to claim 1, wherein the chemical composition contains, by mass%, one or two or more of the group consisting of
    Nb: 0.001% to 0.050%,
    V: 0.001% to 0.050%,
    B: 0.0001% to 0.0100%,
    Cu: 0.01% to 2.00%,
    Cr: 0.01% to 2.00%,
    Mo: 0.001% to 1.000%,
    Ni: 0.01% to 2.00%,
    Ca: 0.0005% to 0.0200%,
    Mg: 0.0005% to 0.0200%,
    REM: 0.0005% to 0.1000%, and
    Bi: 0.0005% to 0.0200%.
EP22898648.5A 2021-11-26 2022-11-25 HOT-ROLLED STEEL SHEET Pending EP4438747A4 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
JP2021191745 2021-11-26
PCT/JP2022/043513 WO2023095866A1 (en) 2021-11-26 2022-11-25 Hot-rolled steel sheet

Publications (2)

Publication Number Publication Date
EP4438747A1 true EP4438747A1 (en) 2024-10-02
EP4438747A4 EP4438747A4 (en) 2025-02-26

Family

ID=86539590

Family Applications (1)

Application Number Title Priority Date Filing Date
EP22898648.5A Pending EP4438747A4 (en) 2021-11-26 2022-11-25 HOT-ROLLED STEEL SHEET

Country Status (7)

Country Link
US (1) US20250171868A1 (en)
EP (1) EP4438747A4 (en)
JP (1) JP7817596B2 (en)
KR (1) KR20240097885A (en)
CN (1) CN118284714A (en)
MX (1) MX2024005967A (en)
WO (1) WO2023095866A1 (en)

Family Cites Families (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP5326709B2 (en) 2008-04-03 2013-10-30 新日鐵住金株式会社 Low yield ratio type high burring high strength hot rolled steel sheet and method for producing the same
JP6264507B2 (en) * 2016-02-10 2018-01-24 Ｊｆｅスチール株式会社 High strength galvanized steel sheet and manufacturing method thereof
EP4056723A4 (en) * 2019-11-06 2023-10-18 Nippon Steel Corporation HOT ROLLED STEEL SHEET AND PROCESS FOR PRODUCTION THEREOF
JP7777399B2 (en) 2020-06-05 2025-11-28 住友化学株式会社 Salt, acid generator, resist composition and method for producing resist pattern

Also Published As

Publication number Publication date
WO2023095866A1 (en) 2023-06-01
CN118284714A (en) 2024-07-02
KR20240097885A (en) 2024-06-27
JPWO2023095866A1 (en) 2023-06-01
US20250171868A1 (en) 2025-05-29
MX2024005967A (en) 2024-06-11
JP7817596B2 (en) 2026-02-19
EP4438747A4 (en) 2025-02-26

Similar Documents

Publication Publication Date Title
EP4039842B1 (en) Hot-rolled steel sheet
EP3904552B1 (en) High-strength hot-dip galvanized steel sheet and method for manufacturing same
EP4098762B1 (en) Hot-rolled steel sheet
EP3106528B1 (en) High-strength hot-dip galvanized steel sheet, and method for manufacturing high-strength alloyed hot-dip galvanized steel sheet
EP3715493A1 (en) High strength steel sheet and method for producing same
EP3936628A1 (en) Hot rolled steel sheet
CN116096514B (en) Hot rolled steel plate
EP4279617A1 (en) Hot-rolled steel sheet
CN116113716B (en) Hot rolled steel plate
CN115943224B (en) Hot rolled steel plate
EP4234742A1 (en) Hot-rolled steel sheet
EP3936630A1 (en) Hot-rolled steel sheet
EP3875616A1 (en) Steel sheet, member, and manufacturing method of these
EP4206344A1 (en) Hot-rolled steel sheet
EP4644578A1 (en) Hot-rolled steel sheet
EP4119690A1 (en) Hot-rolled steel sheet
EP4098761A1 (en) Hot-rolled steel sheet
CN115244203B (en) Hot rolled steel sheet
EP4613899A1 (en) Galvanized steel sheet and member, and method for producing same
EP4438756A1 (en) Zinc-plated steel sheet
EP4379083A1 (en) Steel sheet and method for producing same
EP4368736A1 (en) Hot-rolled steel sheet
EP4438747A1 (en) Hot-rolled steel sheet
EP4239093A1 (en) Hot rolled steel sheet
EP4660342A1 (en) Hot-rolled steel sheet

Legal Events

Date Code Title Description
STAA Information on the status of an ep patent application or granted ep patent

Free format text: STATUS: THE INTERNATIONAL PUBLICATION HAS BEEN MADE

PUAI Public reference made under article 153(3) epc to a published international application that has entered the european phase

Free format text: ORIGINAL CODE: 0009012

STAA Information on the status of an ep patent application or granted ep patent

Free format text: STATUS: REQUEST FOR EXAMINATION WAS MADE

17P Request for examination filed

Effective date: 20240529

AK Designated contracting states

Kind code of ref document: A1

Designated state(s): AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC ME MK MT NL NO PL PT RO RS SE SI SK SM TR

REG Reference to a national code

Ref country code: DE

Ref legal event code: R079

Free format text: PREVIOUS MAIN CLASS: C21D0009460000

Ipc: C21D0008020000

A4 Supplementary search report drawn up and despatched

Effective date: 20250128

RIC1 Information provided on ipc code assigned before grant

Ipc: C22C 38/04 20060101ALI20250122BHEP

Ipc: C22C 38/10 20060101ALI20250122BHEP

Ipc: C22C 38/08 20060101ALI20250122BHEP

Ipc: C22C 38/06 20060101ALI20250122BHEP

Ipc: C22C 38/12 20060101ALI20250122BHEP

Ipc: C22C 38/02 20060101ALI20250122BHEP

Ipc: C22C 38/16 20060101ALI20250122BHEP

Ipc: C22C 38/38 20060101ALI20250122BHEP

Ipc: C22C 38/14 20060101ALI20250122BHEP

Ipc: C22C 38/34 20060101ALI20250122BHEP

Ipc: C22C 38/00 20060101ALI20250122BHEP

Ipc: C21D 9/46 20060101ALI20250122BHEP

Ipc: C21D 8/04 20060101ALI20250122BHEP

Ipc: C21D 8/02 20060101AFI20250122BHEP

DAV Request for validation of the european patent (deleted)
DAX Request for extension of the european patent (deleted)
STAA Information on the status of an ep patent application or granted ep patent

Free format text: STATUS: EXAMINATION IS IN PROGRESS

17Q First examination report despatched

Effective date: 20251002

GRAP Despatch of communication of intention to grant a patent

Free format text: ORIGINAL CODE: EPIDOSNIGR1

STAA Information on the status of an ep patent application or granted ep patent

Free format text: STATUS: GRANT OF PATENT IS INTENDED

RIC1 Information provided on ipc code assigned before grant

Ipc: C21D 8/02 20060101AFI20260225BHEP

Ipc: C21D 8/04 20060101ALI20260225BHEP

Ipc: C21D 9/46 20060101ALI20260225BHEP

Ipc: C22C 38/00 20060101ALI20260225BHEP

Ipc: C22C 38/34 20060101ALI20260225BHEP

Ipc: C22C 38/14 20060101ALI20260225BHEP

Ipc: C22C 38/38 20060101ALI20260225BHEP

Ipc: C22C 38/16 20060101ALI20260225BHEP

Ipc: C22C 38/02 20060101ALI20260225BHEP

Ipc: C22C 38/12 20060101ALI20260225BHEP

Ipc: C22C 38/06 20060101ALI20260225BHEP

Ipc: C22C 38/08 20060101ALI20260225BHEP

Ipc: C22C 38/10 20060101ALI20260225BHEP

Ipc: C22C 38/04 20060101ALI20260225BHEP

INTG Intention to grant announced

Effective date: 20260330