US20180023162A1 - Hot-rolled steel sheet - Google Patents

Hot-rolled steel sheet Download PDF

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US20180023162A1
US20180023162A1 US15/549,837 US201515549837A US2018023162A1 US 20180023162 A1 US20180023162 A1 US 20180023162A1 US 201515549837 A US201515549837 A US 201515549837A US 2018023162 A1 US2018023162 A1 US 2018023162A1
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range
steel sheet
hot
equal
rolled steel
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Natsuko Sugiura
Mitsuru Yoshida
Hiroshi Shuto
Tatsuo Yokoi
Masayuki Wakita
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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Assigned to NIPPON STEEL & SUMITOMO METAL CORPORATION reassignment NIPPON STEEL & SUMITOMO METAL CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: SHUTO, HIROSHI, SUGIURA, NATSUKO, WAKITA, MASAYUKI, YOKOI, TATSUO, YOSHIDA, MITSURU
Publication of US20180023162A1 publication Critical patent/US20180023162A1/en
Assigned to NIPPON STEEL CORPORATION reassignment NIPPON STEEL CORPORATION CHANGE OF NAME (SEE DOCUMENT FOR DETAILS). Assignors: NIPPON STEEL & SUMITOMO METAL CORPORATION
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/001—Heat treatment of ferrous alloys containing Ni
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties 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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—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
    • C22C38/02—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
    • C22C38/14—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
    • 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 excellent in workability and particularly relates to a hot-rolled steel sheet having a composite structure and excellent in stretch flangeability.
  • the steel sheets used for the structural member, the suspension member, and the like which account for about 20% of the vehicle body weight are press-formed mainly based on stretch flange processing and burring processing after performing blanking and drilling by shearing or punching. For this reason, excellent stretch flangeability is required for such steel sheets.
  • Patent Document 1 discloses a hot-rolled steel sheet in which the fraction and the size of the martensite, the number density, and the average gap between martensite is specified, and is excellent in elongation and hole expansibility.
  • Patent Document 2 discloses a hot-rolled steel sheet in which average particle diameters of ferrite and a second phase and a carbon concentration of the second phase are limited, and is excellent in burring workability.
  • Patent Document 3 discloses a hot-rolled steel sheet which is obtained by coiling the steel sheet at a low temperature after being kept at a temperature in a range of 750° C. to 600° C. for 2 to 15 seconds, and is excellent in workability, surface texture, and plate flatness.
  • Patent Document 1 since a primary cooling rate should be set to be equal to or higher than 50° C./s after completing the hot rolling, the load applied on an apparatus becomes higher. In addition, in a case of setting the primary cooling rate to be equal to or higher than 50° C./s, there is a problem in that unevenness in material properties is caused by unevenness in the cooling rate.
  • breaking occurs without the strains in the circumferential direction are hardly distributed; however, in the actual process of components, strain distribution is present, and thus a gradient of the strain and the stress in the vicinity of the broken portion affects a breaking limit. Accordingly, regarding the high-strength steel sheet, even if the stretch flangeability is sufficient in the hole expansion test, in a case of performing cold pressing, the breaking may occur due to the strain distribution.
  • Patent Documents 1 to 3 disclose that in all of the inventions, the hole expansibility is improved by specifying only the structures observed using an optical microscope. However, it is not clear whether or not sufficient stretch flangeability can be secured even in consideration of the strain distribution.
  • Patent Document 1 Japanese Unexamined Patent Application, First Publication No. 2013-19048
  • Patent Document 2 Japanese Unexamined Patent Application, First Publication No. 2001-303186
  • Patent Document 3 Japanese Unexamined Patent Application, First Publication No. 2005-213566
  • the present invention has been made in consideration of the above-described circumstance.
  • An object of the present invention is to provide a high-strength hot-rolled steel sheet which is excellent in the stretch flangeability and can be applied to a member which requires high strength and the strict stretch flangeability.
  • the stretch flangeability means a value evaluated by a product of limit forming height H (mm) and tensile strength TS (MPa) of the flange obtained as a result of the test by the saddle type stretch flange test method, which is an index of the stretch flangeability in consideration of the strain distribution.
  • the excellent stretch flangeability means that the product of the limit forming height H (mm) and the tensile strength TS (MPa) is equal to or greater than 19500 (mm ⁇ MPa).
  • the high strength means that the tensile strength is equal to or greater than 590 MPa. There is no need to particularly set the upper limit of the strength; however, in the range of the structure defined in the present invention, it is difficult to secure a strength of greater than 1470 MPa.
  • the improvement of the stretch flangeability has been performed by inclusion control, homogenization of structure, unification of structure, and/or reduction in hardness difference between structure, as disclosed in Patent Documents 1 to 3.
  • hole expansibility, workability, or the like have been improved by controlling the structure which can be observed using an optical microscope.
  • the present inventors made an intensive study by focusing an intragranular orientation difference in grains in consideration that the stretch flangeability under the presence of the strain distribution cannot be improved even by controlling only the structure observed using an optical microscope. As a result, it was found that it is possible to greatly improve the stretch flangeability by controlling the ratio of the grains in which the intragranular orientation difference is in a range of 5° to 14° with respect to the entire grains to be within a certain range.
  • the present invention is configured on the basis of the above findings, and the gists thereof are as follows.
  • a hot-rolled steel sheet includes, as a chemical composition, by mass %, C: 0.04% to 0.18%, Si: 0.10% to 1.70%, Mn: 0.50% to 3.00%, Al: 0.010% to 1.00%, B: 0% to 0.005%, Cr: 0% to 1.0%, Mo: 0% to 1.0%, Cu: 0% to 2.0%, Ni: 0% to 2.0%, Mg: 0% to 0.05%, REM: 0% to 0.05%, Ca: 0% to 0.05%, Zr: 0% to 0.05%, P: limited to equal to or less than 0.050%, S: limited to equal to or less than 0.010%, and N: limited to equal to or less than 0.0060%, with the remainder including of Fe and impurities; and a structure which includes, by area ratio, a ferrite and a bainite in a range of 75% to 95% in total, and a martensite in a range of 5% to 20%, in which in
  • a tensile strength may be equal to or greater than 590 MPa, and a product of the tensile strength and a limit forming height in a saddle type stretch flange test may be equal to or greater than 19500 mm ⁇ MPa.
  • the chemical composition may contain, by mass %, one or more selected from the group consisting of: B: 0.0001% to 0.005%, Cr: 0.01% to 1.0%, Mo: 0.01% to 1.0%, Cu: 0.01% to 2.0%, and Ni: 0.01% to 2.0%.
  • the chemical composition may contain, by mass %, one or more selected from the group consisting of: Mg: 0.0001% to 0.05%, REM: 0.0001% to 0.05%, Ca: 0.0001% to 0.05%, and Zr: 0.0001% to 0.05%.
  • FIG. 1 is an analysis result obtained by EBSD at 1 ⁇ 4t portion (a 1 ⁇ 4 thickness position from the surface in the sheet thickness direction) of a hot-rolled steel sheet according to the present embodiment.
  • FIG. 2 is a diagram showing a shape of a saddle-shaped formed product which is used in a saddle type stretch flange test method.
  • a hot-rolled steel sheet (hereinafter, referred to as a hot-rolled steel sheet according to the present embodiment in some case) of the embodiment of the present invention will be described in detail.
  • the hot-rolled steel sheet according to the present embodiment includes, as a chemical composition, by mass %, C: 0.04% to 0.18%, Si: 0.10% to 1.70%, Mn: 0.50% to 3.00%, Al: 0.010% to 1.00%, and optionally B: 0.005% or less, Cr: 1.0% or less, Mo: 1.0% or less, Cu: 2.0% or less, Ni: 2.0% or less, Mg: 0.05% or less, REM: 0.05% or less, Ca: 0.05% or less, Zr: 0.05% or less, P: limited to equal to or less than 0.050%, S: limited to equal to or less than 0.010%, and N: limited to equal to or less than 0.0060%, with the remainder including Fe and impurities.
  • a structure in the hot-rolled steel sheet according to the present embodiment, includes, by area ratio, ferrite and bainite in a range of 75% to 95% in total, and martensite in a range of 5% to 20%.
  • a boundary having an orientation difference of equal to or greater than 15° is defined as a grain boundary
  • an area which is surrounded by the grain boundary and has an equivalent circle diameter of equal to or greater than 0.3 ⁇ m is defined as a grain
  • the ratio of the grains having an intragranular orientation difference in a range of 5° to 14° is, by area ratio, in a range of 10% to 60%.
  • the amount (%) of the respective elements is based on mass %.
  • the C is an element which contributes to improvement of the strength of steel.
  • the lower limit of the C content is set to 0.04%.
  • the ratio of the grains having an intragranular orientation difference in a range of 5° to 14° is decreased. From this point, the lower limit of the C content is set to 0.04%.
  • the lower limit of the C content is preferably 0.045%, and the lower limit of the C content is further preferably 0.05%.
  • the C content is greater than 0.18%, the stretch flangeability and the weldability are deteriorated.
  • P is an impurity.
  • P causes the toughness, the workability, and the weldability to be deteriorated, and thus the less the content, the better.
  • the P content is greater than 0.050%, the stretch flangeability is remarkably deteriorated, and thus the P content is limited to be equal to or less than 0.050%.
  • the P content is further preferably equal to or less than 0.040%.
  • N is an element which forms AlN during the cooling after hot rolling, and deteriorates the formability of the steel sheet.
  • the upper limit of the N content is limited to be equal to or less than 0.0060%.
  • the upper limit of the N content is preferably 0.0040%.
  • Ni is an element which enhances the strength and improves the toughness of the steel sheet.
  • the Ni content is preferably equal to or greater than 0.01%.
  • the upper limit of the Ni content is preferably set to 2.0%.
  • each of the fraction (the area ratio) of the ferrite and the bainite is not necessarily limited, when the fraction of the ferrite is greater than 90%, sufficient strength cannot be obtained in some cases, and thus the fraction of the ferrite is preferably equal to less than 90%, and is further preferably less than 70%.
  • the fraction of the bainite is greater than 60%, the ductility may be deteriorated, and thus the fraction of the bainite is preferably less than 60%, and is further preferably less than 50%.
  • the structures of the remainders other than the ferrite, bainite, and martensite are not particularly limited, and for example, it may be residual austenite, pearlite, or the like.
  • the ratio of the structures of the remainders is preferably equal to or less than 5%, further preferably equal to or less than 3%, and still further preferably 0%, by area ratio.
  • the volume fraction of the residual austenite is obtained through X-ray diffraction measurement.
  • the volume fraction of the residual austenite is equivalent to the area ratio, and thus is set as the area ratio of the residual austenite.
  • the grains having such intragranular orientation difference are effective to obtain the steel sheet which has the strength and the workability in the excellent balance, and thus when the ratio is controlled, it is possible to greatly improve the stretch flangeability while maintaining a desired steel sheet strength.
  • the ratio of the grains having an intragranular orientation difference in a range of 5° to 14° is less than 10% by area ratio, the stretch flangeability is deteriorated.
  • the ratio of the grains having an intragranular orientation difference in a range of 5° to 14° is greater than 60% by area ratio, the ductility is deteriorated.
  • the “intragranular orientation difference” of the present invention means “Grain Orientation Spread (GOS)” which is an orientation dispersion in the grains, and the value thereof is obtained as an average value of reference crystal orientations and misorientations of all of the measurement points within the same grain as disclosed in “Misorientation Analysis of Plastic Deformation of Stainless Steel by EBSD and X-Ray Diffraction Methods”, KIMURA Hidehiko, journal of the Japan Society of Mechanical Engineers (Series A) Vol. 71, No. 712, 2005, p. 1722 to 1728.
  • the reference crystal orientation is an orientation obtained by averaging all of the measurement points in the same grain, a value of GOS can be calculated using “OIM Analysis (trademark) Version 7.0.1” which is software attached to the EBSD analyzer.
  • the hot-rolled steel sheet according to the present embodiment can be obtained using a manufacturing method including a hot rolling process and a cooling process as follows.
  • the cumulative strain ( ⁇ eff.) at the latter three stages in the finish rolling in the present embodiment can be obtained from the following Equation (1).
  • the rolling finishing temperature of the hot rolling is preferably in a range of Ar3° C. to Ar3+60° C.
  • the rolling finishing temperature is higher than Ar3+60° C.
  • the grain size of the hot-rolled sheet becomes larger, thus the workability is deteriorated, and the ratio of the grains having an intragranular orientation difference in a range of 5° to 14° is decreased, which is not preferable.
  • the rolling finishing temperature is lower than Ar3, the hot rolling is performed in the two phase region, thus the ferrite phase is deformed, the ductility and the hole expansibility of the hot-rolled steel sheet are deteriorated, and the ratio of the grains having an intragranular orientation difference in a range of 5° to 140 is decreased, which is not preferable.
  • the range of the dislocation density of austenite before the transformation can be limited, and as a result, it is possible to obtain a desired ratio of the grains having an intragranular orientation difference in a range of 5° to 14°.
  • Ar3 can be calculated by the following Expression (2) in consideration of the influence on the transformation point by rolling reduction.
  • [C], [Si], [P], [Al], [Mn], [Mo], [Cu], [Cr], and [Ni] each represent, by mass %, the amount of each of C, Si, P, Al, Mn, Mo, Cu, Cr, and Ni.
  • the elements which are not contained are calculated as 0%.
  • the ratio of the grains having an intragranular orientation difference in a range of 5° to 14° is less than 10%, which is not preferable.
  • a cooling stopping temperature in the first cooling is lower than 650° C.
  • the ratio of the grains having an intragranular orientation difference in a range of 5° to 14° is less than 10%, which is not preferable.
  • the cooling stopping temperature in the first cooling is higher than 750° C.
  • the martensite fraction is excessively low, the strength is decreased, and the ratio of the grains having an intragranular orientation difference in a range of 5° to 14° is greater than 60%, which is not preferable.
  • the retention time is shorter than three seconds at a temperature range of 650° C. to 750° C.
  • the martensite fraction is excessively high, the ductility is deteriorated, and the ratio of the grains having an intragranular orientation difference in a range of 5° to 14° is less than 10%, which is not preferable.
  • the martensite fraction is decreased, the strength is deteriorated, and the ratio of the grains having an intragranular orientation difference in a range of 5° to 14° is less than 10%, which is not preferable.
  • the cooling rate of the second cooling is lower than 30° C./s, the martensite fraction is decreased, the strength is deteriorated, and the ratio of the grains having an intragranular orientation difference in a range of 5° to 14° is greater than 60%, which is not preferable.
  • the cooling stopping temperature of the second cooling is higher than 100° C., the ratio of the grains having an intragranular orientation difference in a range of 5° to 14° is greater than 60%, which is not preferable.
  • the cooling rate in the first cooling and the second cooling may be set to be equal to or lower than 200° C./s in consideration of the equipment capacity of the cooling facility.
  • the above-described manufacturing method it is possible to obtain a structure which has, by area ratio, ferrite and bainite in a range of 75% to 95% in total, and martensite in a range of 5% to 20%, in which a boundary having an orientation difference of equal to or greater than 15° is set as a grain boundary, and in a case where an area which is surrounded by the grain boundary, and has an equivalent circle diameter of equal to or greater than 0.3 ⁇ m is defined as a grain, the ratio of the grains having an intragranular orientation difference in a range of 5° to 14° is, by area ratio, in a range of 10% to 60%.
  • the present invention will be described more specifically with reference to examples of the hot-rolled steel sheet of the present invention.
  • the present invention is not limited to Example described below, and can be implemented by being properly modified the extent that it can satisfy the object before and after description, which are all included in the technical range of the present invention.
  • the steel having the chemical composition indicated in the following Table 1 was melted, and continuous cast so as to produce a slab. Then, the slab was heated at a temperature indicated in Table 2, and was subjected to rough rolling. After the rough rolling, the finish rolling was performed under the conditions indicated in Table 2 so as to obtain a hot-rolled steel sheet having the sheet thickness in a range of 2.2 to 3.4 mm.
  • Ar3 (° C.) indicated in Table 2 was obtained from the chemical composition indicated in Table 1 using the following Expression (2).
  • [C], [Si], [P], [Al], [Mn], [Mo], [Cu], [Cr], and [Ni] each represent, by mass %, the amount of each of C, Si, P, Al, Mn, Mo, Cu, Cr, and Ni, by mass %, and in a case of not containing the elements, 0 is substituted.
  • ⁇ i0 represents a logarithmic strain at the time of rolling reduction
  • t represents a cumulative time immediately before the cooling in the pass
  • T represents a rolling temperature in the pass.
  • the blank column in Table 1 means that the analysis value was less than the detection limit.
  • each structure fraction (the area ratio), and the ratio of the grains having an intragranular orientation difference in a range of 5° to 14° were obtained.
  • the structure fraction (the area ratio) was obtained using the following method. First, a sample collected from the hot-rolled steel sheet was etched using nital. After etching, a structure photograph obtained at a 1 ⁇ 4 thickness position in a visual field of 300 ⁇ m ⁇ 300 ⁇ m using an optical microscope was subjected to image analysis, and thereby the area ratio of ferrite and pearlite, and the total area ratio bainite and martensite were obtained.
  • the ratio of the grains having an intragranular orientation difference in a range of 5° to 14° was measured using the following method. First, at a position of depth of 1 ⁇ 4 (1 ⁇ 4t portion) thickness t from surface of the steel sheet in a cross section vertical to a rolling direction, an area of 200 ⁇ m in the rolling direction, and 100 ⁇ m in the normal direction to the rolled surface was subjected to EBSD analysis at a measurement pitch of 0.2 ⁇ m so as to obtain crystal orientation information.
  • the EBSD analysis was performed using an apparatus which is configured to include a thermal field emission scanning electron microscope (JSM-7001F, manufactured by JEOL) and an EBSD detector (HIKARI detector manufactured by TSL), at an analysis speed in a range of 200 to 300 points per second. Then, with respect to the obtained crystal orientation information, an area having the orientation difference of equal to or greater than 15° and an equivalent circle diameter of equal to or greater than 0.3 ⁇ m was defined as a grain, the average intragranular orientation difference of the grains was calculated, and the ratio of the grains having an intragranular orientation difference in a range of 5° to 14° was obtained.
  • the grain defined as described above and the average intragranular orientation difference can be calculated using software “OIM Analysis (trademark)” attached to an EBSD analyzer.
  • the yield strength and the tensile strength were obtained in the tensile test, and the limit forming height was obtained by the saddle type stretch flange test.
  • a product of tensile strength (MPa) and limit forming height (mm) was evaluated as an index of the stretch flangeability, and in a case where the product thereof is equal to or greater than 19500 mm ⁇ MPa, it was determined that the steel sheet was excellent in the stretch flangeability.
  • the tensile test was performed based on JIS Z 2241 using tensile test pieces No. 5 of JIS which were collected in the longitudinal direction which is orthogonal to the rolling direction.
  • the saddle type stretch flange test was conducted by setting a clearance at the time of punching a corner portion to 11% using a saddle-type formed product in which a radius of curvature R of a corner was set to 60 mm, and an opening angle ⁇ was set to 120°.
  • the existence of the cracks having a length of 1 ⁇ 3 of the sheet thickness were visually observed after forming, and then a forming height of the limit in which the cracks were not present was determined as the limit forming height.
  • the chemical composition was outside the range of the present invention, and thus any one or both of the structure observed using the optical microscope and the ratio of the grains having an intragranular orientation difference in a range of 5° to 14° did not satisfy the range of the present invention. As a result, the stretch flangeability did not satisfy the target value. In addition, in some examples, the tensile strength is also decreased.
  • Nos. 24 to 36 are examples in which the manufacturing method was outside the preferable range, and thus any one or both of the structure observed using the optical microscope and the ratio of the grains having an intragranular orientation difference in a range of 5° to 14° did not satisfy the range of the present invention.
  • the stretch flangeability did not satisfy the target value.
  • the tensile strength was also decreased.
  • an high-strength hot-rolled steel sheet which is excellent in the stretch flangeability and can be applied to a member which requires high strength and the strict stretch flangeability.
  • the steel sheet contributes to improving fuel economy of vehicles, and thus has high industrial applicability.

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US11236412B2 (en) 2016-08-05 2022-02-01 Nippon Steel Corporation Steel sheet and plated steel sheet
US11884989B2 (en) 2019-05-31 2024-01-30 Nippon Steel Corporation Hot-stamping formed body
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US11904573B2 (en) 2019-05-31 2024-02-20 Nippon Steel Corporation Hot-stamp-molded article
US12054800B2 (en) 2019-05-31 2024-08-06 Nippon Steel Corporation Steel sheet for hot stamping

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KR101981876B1 (ko) 2019-05-23
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WO2016132545A1 (ja) 2016-08-25
CN107250404A (zh) 2017-10-13

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