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
  • 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
  • C21D8/0247—Modifying the physical properties 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 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 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
  • 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
  • C21D8/0278—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface treatment
  • C21D8/0284—Application of a separating or insulating coating
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • 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
  • C22C38/007—Ferrous alloys, e.g. steel alloys containing silver
• • 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
  • C22C38/10—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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
  • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
  • C23C2/0224—Two or more thermal pretreatments
• • 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
  • C21D2211/004—Dispersions; Precipitations
• • 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
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/12—All metal or with adjacent metals
  • Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
  • Y10T428/12771—Transition metal-base component
  • Y10T428/12785—Group IIB metal-base component
  • Y10T428/12792—Zn-base component
  • Y10T428/12799—Next to Fe-base component [e.g., galvanized]

Definitions

• the present invention relates to a high-strength hot-rolled steel sheet that has both high strength, or a tensile strength (TS) of 980 MPa or more, and excellent workability (particularly, bending workability), and is usefully applied in automobile members, and a method for producing the same.
• TS tensile strength
• ductility and workability are reduced as steel materials have higher strength. This may cause difficulties in forming a steel sheet with enhanced strength of an increased tensile strength of 980 MPa or more into the desired shape of components. For example, in the case where a steel sheet having a tensile strength of 980 MPa or more is subjected to press forming, it is difficult to form the steel sheet into the shape of components due to significant cracking, necking, and so on occurring at bending-processed portions.
• JP 2007-262429 A proposes a technique where a steel sheet is arranged to have a chemical composition containing, in mass%, C: 0.05 % to 0.20 % and Nb: 0.1 % to 1.0 %, and that the content of solute C is 0.03 % or less.
• a chemical system containing Nb and C it is said that by limiting the content of solute C by a chemical system containing Nb and C, such an abrasion-resistant steel sheet is obtained that has microstructures in which the matrix is ferrite phase which is soft in nature, and NbC is dispersed in the matrix as a hard secondary phase, and that have excellent bending workability.
• JP 2008-189978 A proposes a technique whereby a steel sheet is arranged to have a chemical composition containing, in mass%, C: 0.02 % to 0.2 %, Si: 0.01 % to 1.0 %, Mn: 0.1 % to 2.0 %, P: 0.2 % or less, sol.
• Al 0.001 % to 0.5 %, Ti: 0.1 % or less, Nb: 0.1 % or less, V: 0.5 % or less, Mo: 0.5 % or less, and Ti + Nb: 0.1 % or less, and have microstructures with ferrite as the main phase, where an average grain size of ferrite within a region from a surface of the steel sheet to a 1/4 depth of the thickness of the steel sheet and an increasing rate of the average grain size at 700 °C are defined. It is stated in the technique proposed by PTL 2 that a steel sheet having excellent workability is obtained.
• the technique proposed by PTL 1 involves substantially enhancing the strength of a steel sheet by dispersing NbC, and it is difficult to obtain a steel sheet having a tensile strength of 980 MPa or more by using this technique utilizing NbC. This is because while the degree of precipitation strengthening achieved by dispersing precipitates increases with increasing carbide volume fraction, it is not possible to increase the carbide volume fraction due to a small solubility product in steel and a large atomic density of NbC.
• Ti and V are added to steel as precipitation-strengthening elements, but Ti and V for forming carbides are contained in the steel in a small amount, or added to the steel in an inappropriate manner, in which case, again, the tensile strength of the steel sheet does not reach 980 MPa.
• the present invention has been made in view of these situations, and an object of the present invention is to provide a high-strength hot-rolled steel sheet that has a tensile strength of 980 MPa or more, and still exhibits excellent bending workability.
• the inventors of the present invention have focused on a technique for enhancing the strength of a ferrite single-phase steel sheet having good workability by achieving fine particle distribution of carbides therein, and made intensive studies on various factors that have an effect on enhancement of the strength of the steel sheet and workability, particularly bending workability, of the steel sheet.
• titanium (Ti) was identified as the most suitable element for this purpose.
• V vanadium
• the inventors of the present invention have searched for ways to impart excellent bending workability to a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more, to which Ti and V have been added in combination as described above, while maintaining the strength of the steel sheet.
• a hot-rolled steel sheet that has a tensile strength of 980 MPa or more and exhibits excellent bending workability may be obtained by arranging the steel sheet to have a component composition with optimized contents of C, Mn, Ti and V, while reducing Si content as much as possible.
• the present invention has been completed based on the aforementioned discoveries and the primary features thereof are as follows.
• a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more and excellent bending workability that is suitably applicable to automobile structural members or the like, that is highly advantageous in, for example, being capable of reducing the weight of automobile members, forming automobile members or the like, and that enables the even wider application of high-strength hot-rolled steel sheets, thereby causing a significantly advantageous effect in industrial terms.
• the hot-rolled steel sheet of the present invention has microstructures such that an area ratio of ferrite phase is 95 % or more, an average grain size of the ferrite phase is 8 ⁇ m or less, and carbides in grains of the ferrite phase have an average particle size of less than 10 nm.
• the metal structure of the matrix of the hot-rolled steel sheet is preferably ferrite single-phase structure having excellent workability.
• a secondary phase such as bainite phase, martensite phase, cementite or pearlite
• voids are generated at interfaces between the ferrite phase and the secondary phase having different hardness from each other, which deteriorates the bending workability of the steel sheet.
• the present invention allows carbides, such as Ti and/or V carbides, to precipitate in the steel sheet in order to ensure the desired strength of the steel sheet.
• carbides such as Ti and/or V carbides
• Most of these carbides are such carbides that undergo austenite to ferrite transformation and interphase precipitation at the same time during a cooling step after completion of the finish rolling in the process of producing a hot-rolled steel sheet.
• it is necessary to facilitate ferrite transformation to obtain more carbides for the desired strength (tensile strength: 980 MPa or more) of the steel sheet; if the area ratio of the ferrite phase is below 95 %, it is difficult to ensure a tensile strength of 980 MPa or more.
• the metal structure of the hot-rolled steel sheet is ferrite single-phase structure.
• the metal structure is not exact ferrite single phase, it is still possible to obtain the desired strength (tensile strength: 980 MPa or more) as long as the ferrite area ratio is 95 % or more. Therefore, the area ratio of the ferrite phase is to be 95 % or more, preferably 98 % or more.
• typical phases other than the ferrite phase that may be contained in the steel sheet include cementite, pearlite, bainite, martensite, and so on. If such phases are present in the steel sheet in a large amount, the properties (such as bending workability) of the steel sheet deteriorate. It is thus preferable to reduce such phases as much as possible, although a total area ratio of these phases is acceptable up to 5 %, and is preferably 2 % or less, relative to the entire metal structure of the steel sheet.
• Average grain size of ferrite phase 8 ⁇ m or less
• the upper limit of the average grain size of the ferrite phase is to be 8 ⁇ m.
• the average grain size of the ferrite phase is preferably 6 ⁇ m or less, more preferably 4.5 ⁇ m or less.
• the hot-rolled steel sheet of the present invention must allow fine precipitation of carbides in the grains of the ferrite phase.
• the carbides to be finely precipitated in the grains of the ferrite phase include Ti carbides, V carbides, composite carbides of Ti and V and carbides further containing Nb, W, Mo, Hf and Zr. Most of these carbides are such carbides that undergo austenite to ferrite transformation and interphase precipitation at the same time during a cooling step after completion of the finish rolling in the process of producing a hot-rolled steel sheet.
• Average particle size of carbides in the ferrite grains less than 10 nm
• the above-described fine dispersion of carbides mainly of composite carbides of Ti and V, is used to enhance the strength of the steel sheet, where finer carbides provide more particles interfering with dislocation movement, and hence result in a higher degree of enhancement of the strength achieved by the dispersion of the carbides.
• the average particle size of the carbides to be dispersed in the ferrite grains is to be less than 10 nm, preferably less than 7 nm, more preferably 5 nm or less.
• Carbon (C) is bonded to Ti, V or further to Nb to form carbides, which present with fine particle distribution in the steel sheet. That is, C is an element that forms fine carbides to significantly strengthen the ferrite phase and is essential for enhancing the strength of a hot-rolled steel sheet.
• C content in steel needs to be at least 0.06 % or more.
• C content in steel exceeding 0.1 % causes precipitation of a large amount of cementite, which deteriorates the bending workability of the steel sheet.
• the C content is to be 0.06 % or more and 0.1 % or less, preferably 0.07 % or more and 0.09 % or less.
• Si has been intentionally contained in conventional high-strength steel sheets as an element that effectively improves the strength of the steel sheets without deteriorating the ductility (elongation).
• Si tends to be concentrated on surfaces of the steel sheets and form fayalite (Fe 2 SiO 4 ) thereon. Since the fayalite is formed in wedge shape on a surface of the steel sheet, it would serve as the origin of cracking when the steel sheet is being subjected to bending working. Accordingly, it is desirable in the present invention to reduce Si content in steel as much as possible; however, up to 0.09 % is acceptable and the upper limit of the Si content is to be 0.09 %.
• the Si content is preferably 0.06 % or less. The Si content may be reduced to impurity level.
• Manganese (Mn) is an element that serves to refine carbides to be precipitated in grains of the ferrite phase of a hot-rolled steel sheet, and thus effectively enhances the strength of the steel sheet.
• Mn Manganese
• most of these carbides precipitated in the grains of the ferrite phase will undergo austenite to ferrite transformation and interphase precipitation at the same time during a cooling step after completion of the finish rolling in the process of producing a hot-rolled steel sheet.
• carbides would be precipitated at a high temperature range and experience coarsening during the cooling step arriving at coiling.
• the transformation temperature may be lowered to a coiling temperature range described later by containing a predetermined amount of Mn, and carbides may be precipitated concurrently with coiling of the steel sheet. Then, such carbides precipitated concurrently with the coiling without being exposed at a high temperature range for a long period of time would be kept in fine grain condition.
• Mn content in steel needs to be at least 0.7 % or more.
• the Mn content in steel exceeding 1.3 % leads to a significant deterioration in the workability of the steel sheet due to solute Mn, which makes it impossible to obtain the desired bending workability. Accordingly, the Mn content is to be 0.7 % or more and 1.3 % or less, preferably 0.8 % or more and 1.2 % or less.
• Phosphorus (P) is a harmful element that serves as the origin of intergranular cracking during working when segregated at grain boundaries and thereby deteriorates the bending workability of the steel sheet. It is thus preferable to reduce P content in steel as much as possible. Accordingly, to avoid this problem, the P content is to be 0.03 % or less, preferably 0.02 % or less in the present invention. The P content may be reduced to impurity level.
• S Sulfur
• inclusions such as MnS. Since such inclusions are hard in nature, interfaces between the matrix and the inclusions serve as the origins of voids when the steel sheet is being subjected to bending working, which results in a deterioration in the bending workability of the steel sheet. Accordingly, it is preferable in the present invention to reduce S content as much as possible.
• the S content is to be 0.01 % or less, preferably 0.008 % or less.
• the S content may be zero, in which case there is no problem.
• Aluminum (Al) is an element that functions as a deoxidizer. To obtain this effect, Al is desirably contained in steel in an amount of 0.02 % or more. However, if Al content exceeds 0.1 %, the adverse effect on the bending workability caused by inclusions, such as alumina, appears. Accordingly, the Al content is to be 0.1 % or less, preferably 0.08 % or less.
• N Nitrogen
• Ti which is a carbide-forming element
• N Nitrogen
• the N content is to be 0.01 % or less, preferably 0.008 % or less.
• the N content may be zero, in which case there is no problem.
• Titanium (Ti) is an element that is bonded to C to form carbides and thereby contributes to enhancing the strength of the steel sheet.
• Ti content needs to be 0.14 % or more.
• coarse Ti carbides may not be dissolved by heating the steel material (slab) prior to hot rolling, which results in coarse Ti carbides remaining in the finally obtained (coiled) hot-rolled steel sheet.
• the Ti content is to be 0.14 % or more and 0.20 % or less, preferably 0.15 % or more and 0.19 % or less.
• Vanadium (V) is an element that is bonded to C to form carbides and thereby contributes to enhancing the strength of the steel sheet, as is the case with Ti. V is bonded to Ti to form fine composite carbides, and thus is effective for enhancing the strength of the steel sheet. To ensure the desired strength (tensile strength: 980 MPa or more) of the hot-rolled steel sheet, V content needs to be 0.07 % or more. On the other hand, if V content is larger than the Ti content, it is difficult to allow V to precipitate, in which case more V would remain in the steel sheet in solute state.
• the V content needs to be equal to or smaller than the Ti content, i.e., 0.14 % or less. Accordingly, the V content is to be 0.07 % or more and 0.14 % or less, preferably 0.08 % or more and 0.13 % or less.
• composition of the steel sheet of the present invention may further contain Nb: 0.01 % or more and 0.05 % or less.
• Niobium is an element that acts to inhibit recrystallization of austenite grains before transforming from austenite to ferrite and thereby provide non-recrystallized structure in the hot rolling step in producing a hot-rolled steel sheet having substantially ferrite single-phase structure.
• Nb may increase the number of nucleation sites for ferrite phase in the hot rolling step, which enables refinement of grains of the ferrite phase.
• Nb content is preferably 0.01 % or more.
• Nb content is preferably 0.05 % or less, more preferably 0.02 % or more and 0.04 % or less.
• the composition of the steel sheet of the present invention may further contain at least one of molybdenum (Mo), tungsten (W), zirconium (Zr) and hafnium (Hf), in which case the content of each element is preferably controlled so that Mo: 0.05 % or less, W: 0.05 % or less, Zr: 0.05 % or less, and Hf: 0.05 % or less.
• Mo, W, Zr and Hf are elements that form carbides contributing to enhancing the strength of the steel sheet; however, they remain in the steel sheet in solute state in a large amount. These solute elements deteriorate the workability of the matrix and adversely affect the bending workability of the steel sheet.
• Mo, W, Zr and Hf precipitate at a low rate relative to their contents and remain in the steel sheet as solute elements in a large amount. Accordingly, it is desirable to reduce the contents of these elements as much as possible; however the content of each of these elements is acceptable up to 0.05 %, and thus the upper limit of each element is to be 0.05 %. Preferably, the content of each element is 0.03 % or less. Also, the contents of Mo, W, Zr and Hf may be zero.
• the composition of the steel sheet of the present invention may further contain at least one of O (oxygen), Se, Te, Po, As, Bi, Ge, Pb, Ga, In, Tl, Zn, Cd, Hg, Ag, Au, Pd, Pt, Co, Rh, Ir, Ru, Os, Tc, Re, Ta, Be, Sr, REM, B, Ni, Cr, Sb, Cu, Sn, Mg, and Ca, in a total amount of 0.2 % or less. From the viewpoint of the bending workability of the steel sheet, acceptable contents of these elements are up to 0.2 % in total, preferably not more than 0.09 % in total.
• the balance, or components other than those described above, of the composition of the steel sheet is Fe and incidental impurities.
• a plating layer may also be formed on a surface of the hot-rolled steel sheet of the present invention. Formation of such a plating layer on the surfaces improves the corrosion resistance property of the hot-rolled steel sheet, and thereby makes the steel sheet applicable to components such as automobile components that are used in a severe corrosion environment.
• the present invention is not limited to a particular type of plating layer, and so both an electroplated layer and an electroless-plated layer are applicable as the plating layer.
• the alloy components of the plating layer there is no particular limitation on the alloy components of the plating layer, and preferred examples of the alloy components include a hot-dip galvanized layer, a hot-dip galvannealed layer, and so on.
• the present invention is not limited to the disclosed components, and so any conventionally known components are applicable.
• the method of the present invention comprises: heating a steel material (slab) having the above-described composition; subjecting the steel material to hot rolling including rough rolling and finish rolling; and after completion of the finish rolling, cooling and coiling the steel material to gain a hot-rolled steel sheet.
• the steel material is heated at a temperature of 1100 °C or higher and 1350 °C or lower, the finish rolling is operated at a finish rolling temperature of 850 °C or higher, the cooling is initiated within 3 seconds after completion of the finish rolling, the cooling is operated at an average cooling rate of 20 °C/s or higher, and the coiling is operated at a coiling temperature of 550 °C or higher and 700 °C or lower.
• the present invention is not limited to a particular steelmaking method, and so any known steelmaking method may be adopted, such as using converter, electric furnace, and so on.
• secondary refinement may also be operated in a vacuum degassing furnace.
• any known casting method may also be used to cast a slab, such as ingot casting-blooming or thin slab continuous casting.
• Heating temperature of steel material 1100 °C to 1350 °C
• the steel material (steel slab) thus obtained is subjected to rough rolling and finish rolling. And, in the present invention, the steel material needs to be heated prior to the rough rolling so as to establish substantially uniform austenite phase and dissolve coarse carbides. If the steel material is heated at a temperature below 1100 °C, coarse carbides are not dissolved, and therefore, less carbides are subjected to fine particle distribution at the cooling and coiling step after completion of the hot rolling. This results in a significant deterioration in the strength of the finally obtained hot-rolled steel sheet. Alternatively, if the heating temperature is above 1350 °C, scale defects occur, degrading the surface appearance quality of the steel sheet.
• the steel material is to be heated at a temperature of 1100 °C or higher and 1350 °C or lower, preferably 1150 °C or higher and 1320 °C or lower.
• the steel material when the steel material is subjected to hot rolling, and if the steel material after casting is at a temperature range of 1100 °C or higher to 1350 °C or lower, or if the carbides in the steel material have been dissolved, then the steel material may be subjected to hot direct rolling without being heated.
• the present invention is not limited to a particular rough rolling condition.
• Finish rolling temperature 850 °C or higher
• the finish rolling temperature is to be 850 °C or higher, preferably 870 °C or higher. While the upper limit of the finish rolling temperature is not particularly specified herein, the finish rolling temperature is determined by the slab reheating temperature, sheet passage rate and steel sheet thickness. Thus, the upper limit of the finish rolling temperature is substantially 990 °C or lower.
• the present invention requires forced cooling to be initiated promptly after completion of the hot rolling for the purpose of suppressing strain-induced precipitation, and therefore, cooling is initiated within 3 seconds at the latest, preferably within 2 seconds, after completion of the finish rolling in the present invention.
• Average cooling rate 20 °C/s or higher
• the present invention suppresses austenite to ferrite transformation by means of a predetermined amount of Mn contained in the steel sheet, ferrite transformation would begin at high temperature if the cooling rate is low, in which case carbides are more susceptible to coarsening.
• rapid cooling is required after the finish rolling, and the steel sheet needs to be cooled at an average cooling rate of 20 °C/s or higher to avoid the above-described problems.
• the average cooling rate is preferably 40 °C/s or higher.
• the average cooling rate is preferably not higher than 150 °C/s.
• Coiling temperature 550 °C to 700 °C
• the coiling temperature is to be 550 °C or higher and 700 °C or lower, preferably 580 °C or higher and 680 °C or lower.
• the hot-rolled steel sheet having been subjected to hot rolling and coiling has such properties that will not change whether in a state where scales are attached to the surfaces or in a state where scales have been removed by pickling. In both of these states, the hot-rolled steel sheet exhibits excellent properties as described above.
• the hot-rolled steel sheet after coiling may also be subjected to plating treatment so that a plating layer is provided on a surface of the hot-rolled steel sheet.
• the hot-rolled steel sheet of the present invention shows a small variability of material properties even when subjected to heating treatment up to 740 °C for a short period of time.
• the steel sheet may be subjected to plating treatment to provide a plating layer on a surface thereof. Since the hot-rolled steel sheet of the present invention can be produced when heated at a temperature of 740 °C or lower during plating treatment, the hot-rolled steel sheet may be subjected to plating treatment without loss of the above-described effects of the present invention.
• the present invention is not limited to a particular type of plating layer, and so both an electroplated layer and an electroless-plated layer are applicable as the plating layer.
• alloy components of the plating layer there is no particular limitation on the alloy components of the plating layer, and preferred examples of the alloy components include a hot-dip galvanized layer, a hot-dip galvannealed layer, and so on.
• the present invention is not limited to the disclosed components, and so any conventionally known components are applicable.
• the present invention is not limited to a particular plating treatment method, and so any conventionally known methods are applicable. Exemplary methods include passing a steel sheet through a continuous galvanizing /galvannealing line with an annealing temperature of 740 °C or lower, followed by immersing the steel sheet in a molten bath and then lifting it from the molten bath. After the plating treatment, the steel sheet may also be subjected to alloying treatment by heating the surfaces of the steel sheet in a furnace, such as a gas furnace.
• a furnace such as a gas furnace.
• the present invention may provide such a hot-rolled steel sheet, by optimizing the composition and producing conditions thereof, that has microstructures such that an area ratio of ferrite phase is 95 % or more, an average grain size of the ferrite phase is 8 ⁇ m or less, and carbides in grains of the ferrite phase have an average particle size of less than 10 nm.
• the present invention involves enhancing the strength of the steel sheet, while reducing solute elements and coarse inclusions present in the steel sheet for the purpose of improving the bending workability of the steel sheet.
• the high-strength hot-rolled steel sheet according to the present invention may have excellent bending workability.
• the present invention specifies the producing conditions of the hot-rolled steel sheet, while optimizing the contents of carbide-forming elements (Ti and V, and furthermore, Nb, W, Mo, Hf and Zr) contained in the steel sheet.
• carbide-forming elements Ti and V, and furthermore, Nb, W, Mo, Hf and Zr
• This allows the above-described carbides having an average particle size of less than 10 nm to be precipitated in the ferrite grains sufficiently, and the tensile strength of the hot-rolled steel sheet to be increased to 980 MPa or more, while maintaining excellent bending workability of the steel sheet.
• the present invention is preferably applied to a hot-rolled steel sheet having a tensile strength of 1100 MPa or less, more preferably 1052 MPa or less.
• Steel materials (steel slabs) of 250 mm thick having the compositions shown in Table 1 were subjected to hot rolling under the hot rolling conditions shown in Table 2 to gain hot-rolled steel sheets having a sheet thickness of 1.4 mm to 3.2 mm, respectively.
• the cooling rate shown in Table 2 indicates the average cooling rate from the finish rolling temperature to the coiling temperature.
• the resulting hot-rolled steel sheets were passed through a hot-dip galvanizing line with an annealing temperature of 720 °C, and then immersed in a molten bath at 460 °C (plating composition: Zn-0.13 mass% Al), whereby hot-dip galvanized materials (GI materials) were obtained. Further, subsequent to the sheet passage through the hot-dip galvanizing line and the following immersion in the molten bath, some of the hot-dip galvanized materials (GI materials) were subjected to alloying treatment at 520 °C, whereby galvannealed materials (GA materials) were obtained. For both GI and GA materials, the coating weight was 45 g/m 2 to 55 g/m 2 per surface.
• Test specimens were taken from the hot-rolled steel sheets thus obtained (hot-rolled steel sheets, GI materials and GA materials) and subjected to the microstructure observation, tensile test and bend test to determine the following: area ratio of ferrite phase; types and area ratios of phases other than ferrite phase; average grain size of ferrite phase; average particle size of carbides; yield strength; tensile strength; elongation; and limit bending radius.
• the test method was as follows.
• the area ratio of ferrite phase was evaluated in the following procedure. At the central portion of the sheet thickness in a cross-section parallel to the rolling direction, 10 fields of microstructures on which corrosion emerged with 5 % nital were photographed under a scanning optical microscope at 400x magnification. Ferrite phase is such a phase with no corrosion traces or no cementite observed in the grains thereof. In addition, assuming ferrite includes polygonal ferrite, bainitic ferrite, acicular ferrite and granular ferrite, the following parameters were derived: area ratio of ferrite phase; average grain size of ferrite phase; and average particle size of carbides in grains of the ferrite phase.
• the area ratio of ferrite phase was determined by image analysis measuring an area ratio of ferrite phase to the observed field, while separating the ferrite phase from other phases such as bainite or martensite phases. In this case, grain boundaries appeared in linear form were construed as part of ferrite phase. The obtained results on the area ratio of ferrite phase are shown in Table 3.
• the average grain size of ferrite phase was determined by using an intersection method under ASTM E 112-10, where three horizontal lines and three vertical lines were respectively drawn in representative three images, among those taken at 400x magnification as described earlier, to calculate an average among the three images, which was considered as the final average grain size.
• the obtained results on the average grain size are shown in Table 3.
• the average particle size of carbides in grains of the ferrite phase was determined by using a microfilm method to fabricate samples from the central portion of the sheet thickness of each hot-rolled steel sheet obtained, which samples were then observed under a transmission electronic microscope (at 135,000x magnification) to calculate an average of the precipitate particle size measurements at 100 points or more.
• coarse cementite and nitrides having a grain size of 1.0 ⁇ m or more were excluded from the calculation.
• the obtained results on the average particle size of carbides are shown in Table 3.
• JIS No. 5 tensile test specimens were fabricated from the resulting hot-rolled steel sheets in a direction perpendicular to the rolling direction, and then subjected to tensile tests five times pursuant to JIS Z 2241 (2011) standard to determine the average values of yield strength (YS), tensile strength (TS) and total elongation (El). Besides, the tensile tests were conducted with crosshead speed of 10 mm/min.
• Strip test specimens (100 W mm x 35 L mm) were taken from the resulting hot-rolled steel sheets by shearing work so that their longitudinal direction is vertical to the rolling direction. In this case, a sheared surface and a fractured surface were directed in the same direction at an edge face of each test specimen.
• Each test specimen thus obtained was subjected to bend tests three times using the V-block bend test pursuant to JIS Z 2248, and after the tests, the external appearance of the curved portions of the samples were visually observed, where those samples were considered as having passed the tests if no defects, such as cracks or scars, were observed on the curved portion thereof.
• a smaller limit bending radius means a better result.
• Circle represents a good result where the limit bending radius is not more than 2.0, while cross indicates a poor result where the limit bending radius is more than 2.0.

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