Classifications

• • 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
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
  • B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
• • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
• • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/84—Controlled slow cooling
• • 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
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and 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
  • C21D6/005—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
  • C21D7/00—Modifying the physical properties of iron or steel by deformation
  • C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working
• • 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
  • C21D8/0247—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
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
  • C21D8/0426—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
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
  • C21D8/0463—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing 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
  • 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
  • C22C38/02—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
  • C22C38/12—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
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
  • B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
  • B21B2001/225—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length by 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
  • 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
  • C21D2211/003—Cementite
• • 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
  • C22C38/40—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

Definitions

• the present invention relates to a high strength hot rolled steel sheet having a tensile strength of 980 MPa or more, which is suitable for a material for structural parts and frameworks of automobiles, frames of trucks, steel pipes, and the like.
• the high strength hot rolled steel sheet having tensile strength: 980 MPa or more is highly expected to serve as a material capable of improving fuel efficiency of automobile by leaps and bounds or a material capable of reducing the construction cost of pipeline to a large extent.
• Patent Literature 1 proposes a hot rolled steel sheet with sheet thickness: 4.0 mm or more and 12 mm or less, having a composition containing, on a percent by mass basis, C: 0.04% to 0.12%, Si: 0.5% to 1.2%, Mn: 1.0% to 1.8%, P: 0.03% or less, S: 0.0030% or less, Al: 0.005% to 0.20%, N: 0.005% or less, Ti: 0.03% to 0.13%, and the balance being Fe and incidental impurities and a microstructure in which the area fraction of bainite phase is more than 95% and the average grain size of the bainite phase is 3 ⁇ m or less, wherein a difference between the Vickers hardness at the position at 50 ⁇ m from the surface layer and the Vickers hardness at the position one-quarter of the sheet thickness is specified to be 50 or less, and a difference between the Vickers hardness at the position one-quarter of the sheet thickness and the Vickers hardness at the position at one-half of the sheet thickness is specified
• Patent Literature 1 a high strength hot rolled steel sheet exhibiting excellent toughness and having tensile stress: 780 MPa or more is obtained by specifying the principal phase to be fine bainite and reducing the hardness distribution in the sheet thickness direction.
• Patent Literature 2 proposes a method for manufacturing a steel sheet, including the steps of heating a steel material satisfying, on a percent by mass basis, C: 0.05% to 0.18%, Si: 0.10% to 0.60%, Mn: 0.90% to 2.0%, P: 0.025% or less (excluding 0%), S: 0.015% or less (excluding 0%), Al: 0.001% to 0.1%, and N: 0.002% to 0.01%, and the balance being Fe and incidental impurities, to 950°C or higher and 1,250°C or lower, starting rolling, completing the rolling at 820°C or higher, performing cooling to 600°C to 700°C at a cooling rate of 20°C/s or more, performing holding at that temperature range for 10 to 200 seconds or performing slow cooling and, thereafter, performing cooling to 300°C ar lower at a cooling rate of 5°C/s or more, wherein the metal microstructure is specified to be ferrite: 70% to 90%, martensite or a mixed phase of martensite and austenite: 3% to
• Patent Literature 2 a high toughness steel sheet which has a tensile strength of 490 N/mm 2 or more and which exhibits a low yield ratio, where the yield ratio is 70% or less, is obtained by specifying the metal microstructure to be a microstructure including ferrite having fine crystal grains, martensite or a mixed phase of martensite and austenite, and the like.
• Patent Literature 3 proposes a method for manufacturing a thick high strength hot rolled steel sheet, including the steps of subjecting a steel material containing, on a percent by mass basis, C: 0.02% to 0.25%, Si: 1.0% or less, Mn: 0.3% to 2.3%, P: 0.03% or less, S: 0.03% or less, Al: 0.1% or less, Nb: 0.03% to 0.25%, and Ti: 0.001% to 0.10%, where (Ti + Nb/2)/C ⁇ 4 is satisfied, to hot rolling, applying first cooling after finish rolling of the hot rolling is completed, where accelerated cooling is performed at an average cooling rate of hot-rolled sheet surface of 20°C/s or more and less than martensite formation critical cooling rate until the surface temperature reaches the Ar 3 transformation temperature or lower and the Ms temperature or lower, applying second cooling, where quenching is performed until the sheet thickness center temperature reaches 350°C or higher and lower than 600°C, performing coiling into the shape of a coil at a coiling temperature of 350°C
• Patent Literature 3 a material for X65 grade or higher of high strength electric resistance welded steel pipe exhibiting excellent low-temperature toughness is obtained by specifying the microstructure of the hot rolled steel sheet to be a bainite phase or bainitic ferrite phase and, furthermore, adjusting the amount of grain boundary cementite to a specific value or less.
• Patent Literature 4 describes a method for manufacturing a high strength hot rolled steel sheet, including the steps of heating a steel having a composition containing, on a percent by mass basis, C: 0.05% to 0.15%, Si: 0.2% to 1.2%, Mn: 1.0% to 2.0%, P: 0.04% or less, S: 0.005% or less, Ti: 0.05% to 0.15%, Al: 0.005% to 0.10%, N: 0.007% or less, and the balance being Fe and incidental impurities to 1,150C to 1,350°C, and preferably higher than 1,200°C and 1,350°C or lower, applying hot rolling which is completed at a finishing temperature of 850°C to 950°C, and preferably higher than 900°C and 950°C or lower, applying cooling after the hot rolling is completed, where cooling to 530°C is performed at an average cooling rate of 30°C/s or more, applying cooling to coiling temperature: 300°C to 500°C at an average cooling rate of 100°C/s or more, and
• the stretch flangeability and the fatigue resistance are considerably improved while high strength of TS: 780 MPa or more is maintained by allowing the microstructure to become composed of a bainite single phase having an average grain size of 5 ⁇ m or less, and preferably more than 3.0 ⁇ m and 5.0 ⁇ m or less and allowing 0.02% or more of solid solution Ti to remain.
• the microstructure may be composed of 90% or more on an area fraction basis of bainite phase and a secondary phase other than the bainite phase, where the average grain size of the secondary phase is 3 ⁇ m or less, instead of the microstructure composed of the bainite single phase.
• Patent Literature 5 describes a method for manufacturing a high strength hot rolled steel sheet, including the steps of subjecting a slab containing, on a percent by mass basis, C: 0.01% to 0.08%, Si: 0.30% to 1.50%, Mn: 0.50% to 2.50%, P: 0.03% or less, S: 0.005% or less, one or two of Ti: 0.01% to 0.20% and Nb: 0.01% to 0.04%, and the balance being Fe and incidental impurities to hot rolling, where the finish rolling temperature is specified to be the Ar 3 transformation temperature to 950°C, performing cooling to 650°C to 800°C at a cooling rate of 20°C/s or more, performing air cooling for 2 to 15 s, performing further cooling to 350°C to 600°C at a cooling rate of 20°C/s or more, and performing coiling.
• the finish rolling temperature is specified to be the Ar 3 transformation temperature to 950°C
• Patent Literature 6 describes a high strength steel sheet exhibiting excellent hole expansion property and ductility.
• the high strength steel sheet described in Patent Literature 6 is a steel sheet containing, on a percent by mass basis, C: 0.01% to 0.20%, Si: 1.50% or less, Al: 1.5% or less, Mn: 0.5% to 3.5%, P: 0.2% or less, S: 0.0005% to 0.009%, N: 0.009% or less, Mg: 0.0006% to 0.01%, O: 0.005% or less, one or two of Ti: 0.01% to 0.20% and Nb: 0.01% to 0.10%, and the balance being Fe and incidental impurities, wherein all three formulae below Mg % ⁇ O % / 16 ⁇ 0.8 ⁇ 24 S % ⁇ Mg % / 24 - O % / 16 ⁇ 0.8 + 0.00012 ⁇ 32 S % ⁇ 0.0075 / Mn % are satisfied and the microstructure includes a bainite phase as a primary phase
• Patent Literature 1 the high strength hot rolled steel sheet having tensile strength: 980 MPa or more is obtained.
• the control of the bainite microstructure is insufficient and, thereby, there is a problem that excellent low-temperature toughness cannot be obtained stably.
• the metal microstructure of the steel is specified to be the structure including a ferrite phase as a primary phase, although in the case where the tensile strength is in the 980 MPa class, the toughness of the ferrite phase may be degraded significantly.
• the present invention solves the above-described problems included in the technologies of the related art advantageously, and it is an object to provide a high strength hot rolled steel sheet having high strength of tensile strength: 980 MPa or more, further exhibiting good toughness and, in particular, having a sheet thickness of 4 mm or more and 15 mm or less and a method for manufacturing the same.
• the aimed strength is tensile strength TS: 780 MPa or more, and when the C content is increased, high strength of tensile strength TS: 980 MPa or more can be obtained.
• the C content is increased to further enhance the strength, control of the amount of precipitation of Ti carbides becomes difficult, and there is a problem that 0.02% or more of solid solution Ti required for improving hole expansion property cannot be left easily stably.
• the steel sheet microstructure is specified to be the mixed microstructure of ferrite in which the proportion of ferrite having a grain size of 2 ⁇ m or more is 80% or more + bainite. Therefore, there are problems that the resulting steel sheet strength is about 976 MPa at the maximum, further higher strength of tensile strength TS: 980 MPa or more cannot be achieved easily, and even if the high strength of tensile strength TS: 980 MPa or more is obtained, the toughness of the ferrite phase is degraded significantly and excellent hole expansion property cannot be obtained.
• the present invention solves such problems included in the technologies of the related art, and it is an object to provide a high strength hot rolled steel sheet exhibiting excellent hole expansion workability while the high strength of tensile strength: 980 MPa or more has and a method for manufacturing the same.
• the high strength hot rolled steel sheet aimed in the present invention is a steel sheet having a sheet thickness of 2 to 4 mm.
• the present inventors conducted intensive research to improve the toughness of a hot rolled steel sheet while the high strength of tensile strength TS: 980 MPa or more had.
• the bainite phase was noted, where it is known that the bainite phase has good strength-toughness balance in general, and various factors affecting the strength and the toughness of the hot rolled steel sheet, in which the primary phase of the microstructure was bainite, were studied. As a result, it was found that allowing laths of the bainite phase to become fine was very effective in enhancing strength and improving toughness of the hot rolled steel sheet. Then, further studies were conducted.
• the present invention has been completed on the basis of the above-described findings and additional studies. That is, the gist configuration of the present invention is as described below.
• the present inventors conducted intensive research on various factors affecting the hole expansion workability while the high strength of tensile strength TS: 980 MPa or more has.
• the primary phase in the microstructure was specified to be the bainite phase and high strength of tensile strength TS: 980 MPa or more had, cementite functioned as a starting point of void formation during hole expansion working or local deformation, and as the amount of cementite increased, voids were connected to each other easily, the local ductility was degraded, and the hole expansion workability was degraded.
• the grain size of cementite increased, coarse voids were formed in the punched surface by punching, which was a pretreatment of hole expansion working, and the hole expansion property was degraded.
• the present inventors conducted further research and found that in order to improve the hole expansion property and, furthermore, the local ductility while the high strength of tensile strength TS: 980 MPa or more had, adjustment of the balance between the contents of C, Si, Ti, and V, further adjustment of cementite to 0.8% or less on a percent by mass basis and the average grain size of cementite to 150 nm or less by optimizing the production condition, and an increase in distance between cementite grains were important.
• the present invention has been completed on the basis of the above-described findings and additional studies. That is, the gist of the present invention is as described below.
• a high strength hot rolled steel sheet having a tensile strength of 980 MPa or more and exhibiting excellent toughness is obtained. Therefore, the car body weight can be reduced while the safety of the automobile is ensured and an environmental load can be reduced by applying the present invention to structural parts and frameworks of automobiles, frames of trucks, and the like.
• a welded steel pipe produced from the hot rolled steel sheet according to the present invention serving as a material instead of the UOE pipe produced from a steel plate serving as a material is applied to a transport pipe, the productivity is improved and the cost can be further reduced.
• the present invention can stably produce a hot rolled steel sheet exhibiting improved toughness while high strength of tensile strength: 980 MPa or more has and, therefore, is very useful for the industry.
• a hot rolled steel sheet exhibiting considerably improved hole expansion workability can be produced while high strength of tensile strength: 980 MPa or more has, so that an industrially remarkable effect is exerted. Also, effects that the car body weight can be reduced while the safety of the automobile is ensured and an environmental load can be reduced are exerted by applying the hot rolled steel sheet according to the present invention to materials for chassis parts, structural parts and frameworks of automobiles, frames of trucks, and the like.
• the C content is specified to be 0.05% or more and 0.18% or less, preferably 0.08% or more and 0.17% or less, and more preferably more than 0.10% and 0.16% or less.
• the amount of Mn is 2.5% or more and 3.5% or less
• the amount of C is preferably 0.06% or more and 0.15% or less.
• Si is an element which suppresses coarse oxides and cementite to impair the toughness and which contributes to solute strengthening. If the content is more than 1.0%, the surface quality of the hot rolled steel sheet is degraded significantly and degradation in the chemical conversion treatability and the corrosion resistance is caused. Therefore, the Si content is specified to be 1.0% or less, and preferably 0.4% or more and 0.8% or less.
• Mn 1.0% or more and 3.5% or less
• Mn is an element which contributes to enhancement of strength of the steel through solid solution and which facilitates formation of bainite through improvement of the hardenability. In order to obtain such effects, it is necessary that the Mn content be 1.0% or more. On the other hand, if the Mn content is more than 3.5%, center segregation becomes considerable, and the toughness of the hot rolled steel sheet is degraded. Therefore, the Mn content is specified to be 1.0% or more and 3.5% or less. In this regard, 1.5% or more and 3.0% or less is preferable and 1.8% or more and 2.5% or less is more preferable.
• P is an element which contributes to enhancement of strength of the steel through solid solution but is an element which segregates at grain boundaries, in particular prior-austenite grain boundaries, to cause degradation in low-temperature toughness and workability. Consequently, it is preferable that the P content be minimized, although the content up to 0.04% is allowable. Therefore, the P content is specified to be 0.04% or less. However, when the P content is excessively reduced, an effect corresponding to an increase in the smelting cost is not obtained, so that the P content is specified to be preferably 0.003% or more and 0.03% or less, and more preferably 0.005% or more and 0.02% or less.
• the S content is specified to be 0.006% or less.
• the S content is specified to be preferably 0.0003% or more and 0.004% or less, and more preferably 0.0005% or more and 0.002% or less.
• Al is an element which functions as a deoxidizing agent and which is effective in improving cleanliness of the steel.
• excessive addition of Al causes increases in oxide inclusions, degrades the toughness of the hot rolled steel sheet and, in addition, causes an occurrence of flaw. Therefore, the Al content is specified to be 0.10% or less, preferably 0.005% or more and 0.08% or less, and further preferably 0.01% or more and 0.05% or less.
• N bonds to Ti at a high temperature to form coarse nitrides easily and degrades the toughness of the hot rolled steel sheet. Consequently, the N content is specified to be 0.008% or less, preferably 0.001% or more and 0.006% or less, and more preferably 0.002% or more and 0.005% or less.
• Ti is one of the most important elements in the present invention. Ti contributes to enhancement of strength of the steel through formation of carbonitrides to make crystal grains fine and through precipitation strengthening. Also, Ti forms many fine (Ti,V)C clusters at low temperatures of 300°C or higher and 450°C or lower, reduces the amount of cementite in the steel, and improve the toughness of the hot rolled steel sheet. In order to exert such effects, it is necessary that the Ti content be 0.05% or more. On the other hand, if the Ti content is excessive and is more than 0.20%, the above-described effects are saturated, an increase in coarse precipitates is caused, and degradation in the toughness of the hot rolled steel sheet is caused. Therefore, the Ti content is limited to within the range of 0.05% or more and 0.20% or less, and preferably 0.08% or more and 0.007% or less.
• V more than 0.1% and 0.3% or less
• V is one of the most important elements in the present invention.
• V contributes to enhancement of strength of the steel through formation of carbonitrides to make crystal grains fine and through precipitation strengthening. Also, V improves the hardenability and contributes to formation and making fine of bainite phase.
• V forms many fine (Ti,V)C clusters at low temperatures of 300°C or higher and 450°C or lower, reduces the amount of cementite in the steel, and improves the toughness of the hot rolled steel sheet. In order to exert such effects, it is necessary that the V content be more than 0.1%. On the other hand, if the V content is excessive and is more than 0.3%, the above-described effects are saturated, so that the cost increases. Therefore, the V content is limited to within the range of more than 0.1% and 0.3% or less, and preferably 0.15% or more and 0.25% or less.
• the basic components of the hot rolled steel sheet according to the present invention are as described above.
• the hot rolled steel sheet according to the present invention may further contain, as necessary, at least one selected from Nb: 0.005% or more and 0.4% or less, B: 0.0002% or more and 0.0020% or less, Cu: 0.005% or more and 0.2% or less, Ni: 0.005% or more and 0.2% or less, Cr: 0.005% or more and 0.4% or less, and Mo: 0.005% or more and 0.4% or less for the purpose of, for example, improvement of toughness and enhancement of strength.
• Nb 0.005% or more and 0.4% or less
• Nb is an element which contributes to enhancement of strength of the steel through formation of carbonitrides. In order to exert such an effect, it is preferable that the Nb content be 0.005% or more. On the other hand, if the Nb content is more than 0.4%, deformation resistance increases, so that a rolling force of hot rolling increases in production of the hot rolled steel sheet, a load to a rolling mill becomes too large, and rolling operation in itself may become difficult. Meanwhile, if the Nb content is more than 0.4%, coarse precipitates are formed and the toughness of the hot rolled steel sheet tends to be degraded. Therefore, the Nb content is preferably specified to be 0.005% or more and 0.4% or less. In this regard, 0.01% or more and 0.3% or less is more preferable and 0.02% or more and 0.2% or less is further preferable.
• B is an element which segregates at austenite grain boundaries and which suppresses formation and growth of ferrite. Also, B is an element which improves the hardenability and which contributes to formation and making fine of bainite phase. In order to exert these effects, it is preferable that the B content be 0.007% or more. However, if the B content is more than 0.0020%, formation of martensite phase is facilitated, so that the toughness of the hot rolled steel sheet may be degraded significantly. Therefore, in the case where B is contained, the content thereof is specified to be preferably 0.0002% or more and 0.007% or less. In this regard, 0.0004% or more and 0.007% or less is more preferable.
• Cu is an element which contributes to enhancement of strength of the steel through solid solution. Also, Cu is an element which has a function of improving hardenability, which lowers, in particular, the bainite transformation temperature, and which contributes to making bainite phase fine. In order to obtain these effects, it is preferable that the Cu content be 0.005% or more, although if the content thereof is more than 0.2%, degradation in the surface quality of the hot rolled steel sheet is caused. Therefore, the Cu content is specified to be preferably 0.007% or more and 0.007% or less. In this regard, 0.007% or more and 0.15% or less is more preferable.
• Ni 0.005% or more and 0.2% or less
• Ni is an element which contributes to enhancement of strength of the steel through solid solution. Also, Ni has a function of improving hardenability and facilitates formation of bainite phase. In order to obtain these effects, it is preferable that the Ni content be 0.005% or more. However, if the Ni content is more than 0.2%, a martensite phase is generated easily, and the toughness of the hot rolled steel sheet may be degraded significantly. Therefore, the Ni content is specified to be preferably 0.005% or more and 0.2% or less, and more preferably 0.01% or more and 0.15% or less.
• the Cr forms carbides and contributes to enhancement of strength of the hot rolled steel sheet.
• the Cr content be 0.005% or more.
• the Cr content is specified to be preferably 0.005% or more and 0.4% or less, and more preferably 0.01% or more and 0.2% or less.
• Mo facilitates formation of bainite phase through improvement of the hardenability and contributes to improvement of the toughness and enhancement of strength of the hot rolled steel sheet.
• the Mo content be 0.007% or more.
• the Mo content is specified to be preferably 0.005% or more and 0.4% or less, and more preferably 0.01% or more and 0.2% or less.
• the hot rolled steel sheet according to the present invention may contain, as necessary, one or two selected from Ca: 0.0002% or more and 0.01% or less and REM: 0.0002% or more and 0.01% or less.
• the Ca is effective in controlling the shape of sulfide inclusions and improving bending workability and the toughness of the hot rolled steel sheet.
• the Ca content be 0.0002% or more.
• the Ca content is specified to be preferably 0.0002% or more and 0.01% or less. In this regard, 0.0004% or more and 0.005% or less is more preferable.
• REM controls the shape of sulfide inclusions and improves adverse influences of sulfide inclusions on the bending workability and the toughness of the hot rolled steel sheet.
• the REM content be 0.0002% or more.
• the content thereof is specified to be preferably 0.0002% or more and 0.01% or less. In this regard, 0.0004% or more and 0.005% or less is more preferable.
• the remainder other than those described above is composed of Fe and incidental impurities.
• incidental impurities include Sb, Sn, and Zn.
• Sb 0.01% or less
• Sn 0.1% or less
• Zn 0.01% or less are allowable.
• the hot rolled steel sheet according to the present invention has a microstructure in which a primary phase is more than 85% on an area fraction basis of bainite phase, a secondary phase is at least one of ferrite phase, martensite phase, and retained austenite phase, 0% or more and less than 15% in total on an area fraction basis of secondary phase is contained, the average lath interval of laths of the above-described bainite phase is 400 nm or less, and the average long axis length of the above-described laths is 5.0 ⁇ m or less.
• the primary phase of the hot rolled steel sheet according to the present invention is a bainite phase having excellent strength-toughness balance. If the fraction of the bainite phase is 85% or less on an area fraction basis, a hot rolled steel sheet provided with predetermined strength and toughness is not obtained. Therefore, the fraction of the bainite phase is specified to be more than 85% on an area fraction basis, preferably 87% or more, and more preferably 90% or more. It is still more preferable that the fraction of the bainite phase be 100% on an area fraction basis and the microstructure be a bainite single phase microstructure.
• the hot rolled steel sheet according to the present invention may include a secondary phase, which is composed of at least one of ferrite phase, martensite phase, and retained austenite phase, as a microstructure other than the bainite phase serving as the primary phase.
• the microstructure is specified to be preferably a bainite single phase microstructure to impart predetermined strength and toughness to the hot rolled steel sheet.
• the fraction of the above-described secondary phase in total is specified to be 0% or more and less than 15% on an area fraction basis, preferably 13% or less, and more preferably 11% or less.
• Average lath interval of laths of bainite phase 400 nm or less
• Average long axis length of laths of bainite phase 5.0 ⁇ m or less
• the average lath interval of laths of the bainite phase is specified to be 400 nm or less, and preferably 350 nm or less.
• the average long axis length of laths of the bainite phase is specified to be 5.0 ⁇ m or less, and preferably 4.0 ⁇ m or less.
• lower limits of the average lath interval of laths of the bainite and the average long axis length of laths of the bainite phase are not particularly specified.
• the lath interval and the long axis length are determined on the basis of the bainite transformation temperature and, therefore, usually the average lath interval of laths of the bainite phase is 100 nm or more and the average long axis length of laths of the bainite phase is 1.0 ⁇ m or more.
• a high strength hot rolled steel sheet having a tensile strength of 980 MPa or more and having toughness required of a material for structural parts of automobiles and a material for steel pipes, e.g., line pipes, is obtained by specifying the composition and the microstructure, as described above.
• the sheet thickness of the hot rolled steel sheet according to the present invention is not specifically limited, although the sheet thickness is specified to be preferably about 4 mm or more and 15 mm or less.
• the present invention is characterized by heating a steel having the above-described composition to 1,200°C or higher, applying hot rolling composed of rough rolling and finish rolling in which the accumulated rolling reduction is 50% or more in a temperature range of 1,000°C or lower and the finishing temperature is 820°C or higher and 930°C or lower, starting cooling within 4.0 s of the hot rolling, performing cooling at an average cooling rate of 20°C/s or more, and performing coiling at a coiling temperature of 300°C or higher and 450°C or lower.
• the method for manufacturing a steel is not necessarily particularly limited, and any common method can be applied, wherein a molten steel having the above-described composition is refined in a converter or the like, and a steel, e.g., a slab, is produced by a casting method, e.g., a continuous casting method.
• a casting method e.g., a continuous casting method.
• an ingot-making and blooming method may be used.
• electro-magnetic stirrer EMS
• IBSR intentional bulging soft reduction casting
• Equiaxial crystals are formed in the sheet thickness center portion by applying an electro-magnetic stirrer treatment, so that segregation can be reduced.
• segregation in the sheet thickness center portion can be reduced by preventing flowing of the molten steel in an unsolidified portion of the continuous casting slab.
• the toughness described below can be brought to a more excellent level by applying at least one of these segregation reduction treatments.
• Heating temperature of steel 1,200°C or higher
• the heating temperature of the steel is specified to be preferably 1,350°C or lower, and more preferably 1,220°C or higher and 1,300°C or lower.
• the steel material is heated to the heating temperature of 1,200°C or higher and is held for a predetermined time. If the holding time is more than 4,800 seconds, the amount of generation of scale increases and, as a result, scale biting and the like occurs easily in the following hot rolling step, and the surface quality of the hot rolled steel sheet tends to be degraded. Therefore, the holding time of the steel material in the temperature range of 1,200°C or higher is specified to be preferably 4,800 seconds or less, and more preferably 4,000 seconds or less.
• the steel material is subjected to hot rolling having rough rolling and finish rolling.
• the condition of the rough rolling is not specifically limited insofar as predetermined sheet bar dimensions are ensured.
• the finish rolling is applied. In this regard, preferably, descaling is performed before the finish rolling or between stands during rolling.
• the accumulated rolling reduction is specified to be 50% or more in a temperature range of 1, 000°C or lower and the finishing temperature is specified to be 820°C or higher and 930°C or lower.
• the rolling reduction in a relatively low temperature range be increased and crystal grains after rolling be allowed to become crystal grains elongated in the rolling direction (crystal grains having a high elongation rate). If the accumulated rolling reduction at 1,000°C or lower is less than 50%, it becomes difficult to make bainite having a predetermined lath structure (average lath interval: 400 nm or less, average long axis length: 5.0 ⁇ m or less), and the toughness of the hot rolled steel sheet is degraded. Therefore, the accumulated rolling reduction at 1,000°C or lower is specified to be 50% or more, and preferably 60% or more.
• the accumulated rolling reduction in a temperature range of 1,000°C or lower is excessively high, crystal grains are excessively elongated in the rolling direction and ferrite is generated easily, so that it may also be difficult to make bainite having a predetermined lath structure. Consequently, the accumulated rolling reduction in a temperature range of 1, 000°C or lower is specified to be preferably 80% or less.
• Finishing temperature 820°C or higher and 930°C or lower
• the finishing temperature of the finishing rolling is lower than 820°C, rolling is performed at a temperature of two-phase region of ferrite + austenite, so that a deformation microstructure remains after rolling and the toughness of the hot rolled steel sheet is degraded.
• the finishing temperature is higher than 930°C, austenite grains grow, and a bainite phase of the hot rolled steel sheet obtained after cooling is coarsened. As a result, it becomes difficult to make a predetermined microstructure, and the toughness of the hot rolled steel sheet is degraded. Therefore, the finishing temperature is specified to be 820°C or higher and 930°C or lower, and preferably 840°C or higher and 920°C or lower.
• the finishing temperature refers to the surface temperature of a sheet.
• Forced cooling is started within 4.0 s of, preferably just after, completion of the finish rolling, cooling is stopped at the coiling temperature, and coiling into the shape of a coil is performed. If the time from completion of the finish rolling to start of the forced cooling is more than 4.0 s and is long, austenite grains become coarse, and a bainite phase is coarsened. Also, austenite grains become coarse, so that the hardenability of the steel sheet increases and a martensite phase is generated easily. In the case where the bainite phase is coarsened and the martensite phase is generated easily, predetermined excellent toughness cannot be obtained. Therefore, the forced cooling start time is limited to within 4.0 s of completion of the finish rolling.
• Average cooling rate 20°C/s or more
• the above-described average cooling rate is specified to be 20°C/s or more, and preferably 30°C/s or more.
• the upper limit of the average cooling rate is not particularly specified.
• the average cooling rate is specified to be preferably 60°C/s or less.
• the above-described average cooling rate is specified to be an average cooling rate of the steel sheet surface.
• Coiling temperature 300°C or higher and 450°C or lower
• the coiling temperature is specified to be within the range of 300°C or higher and 450°C or lower, and preferably 330°C or higher and 430°C or lower.
• the hot rolled steel sheet may be subjected to temper rolling following the common method or be subjected to pickling to remove scale formed on the surface.
• a galvanization process e.g., hot dip galvanizing or electrogalvanizing, and a chemical conversion treatment may further be applied.
• a molten steel having the composition shown in Table 1 was refined in a converter, and a slab (steel) was produced by a continuous casting method.
• those other than Hot rolled steel sheet No. 1' of Steel Al in Tables 1 to 3 described below were subjected to electro-magnetic stirrer (EMS) for the purpose of segregation reduction treatment of the components.
• EMS electro-magnetic stirrer
• these steel materials were heated under the conditions shown in Table 2, and were subjected to hot rolling having rough rolling and finish rolling under the conditions shown in Table 2.
• cooling was performed under the conditions shown in Table 2,' and coiling was performed at coiling temperatures shown in Table 2, so that hot rolled steel sheets having sheet thicknesses shown in Table 2 were produced.
• Test pieces were taken from the resulting hot rolled steel sheets, and microstructure observation, a tensile test, and a Charpy impact test were performed.
• the microstructure observation method and various testing methods were as described below.
• a test piece for a scanning electron microscope (SEM) was taken from the hot rolled steel sheet, a sheet thickness cross-section parallel to the rolling direction was polished and, thereafter, the microstructure was allowed to appear with a corrosive liquid (3% nital solution). Photographs were taken in three fields of view of each of the position at one-quarter of the sheet thickness and the position at one-half of the sheet thickness (center position of the sheet thickness) with a scanning electron microscope (SEM) at the magnification of 3,000 times, and the area fraction of each phase was quantified on the basis of an image treatment.
• SEM scanning electron microscope
• a test piece having size: 10 mm ⁇ 15 mm was taken from the hot rolled steel sheet, thin film samples for transmission electron microscope (TEM) observation of the position at one-quarter of the sheet thickness and the position at one-half of the sheet thickness (center position of the sheet thickness) were produced, and photographs were taken in ten fields of view of each position with TEM at the magnification of 30,000 times.
• Five straight lines at intervals of 10 mm were drawn at right angles to long axes of at least three laths which were shown in each photograph having a size of 120 mm ⁇ 80 mm and which were successively arranged side by side.
• the length of each line segment between the intersection points of the straight line and the lath boundary was measured and the average value of the resulting lengths of the segments was specified to be the average lath interval.
• a test piece for a scanning electron microscope (SEM) was taken from the hot rolled steel sheet, a sheet thickness cross-section parallel to the rolling direction was polished and, thereafter, the microstructure was allowed to appear with a corrosive liquid (3% nital solution). Photographs were taken in five fields of view of each of the position at one-quarter of the sheet thickness and the position at one-half of the sheet thickness (center position of the sheet thickness) with a scanning electron microscope (SEM) at the magnification of 10,000 times.
• SEM scanning electron microscope
• JIS No. 5 test pieces (GL: 50 mm) were taken from the hot rolled steel sheet in such a way that the tensile direction and the rolling direction form a right angle.
• a tensile test was performed in conformity with JIS Z 2241 (2011) and the yield strength (yield point) YP, the tensile strength (TS), and the total elongation El were determined.
• a subsize test piece (V-notch) having a thickness of 5 mm was taken from the hot rolled steel sheet in such a way that the longitudinal direction of the test piece and the rolling direction form a right angle.
• a Charpy impact test was performed in conformity with JIS Z 2242, the Charpy impact value (vE -50 ) at a temperature of -50°C was measured, and the toughness was evaluated.
• the hot rolled steel sheet having a sheet thickness of more than 5 mm was subjected to double-side polishing to produce a test piece having a sheet thickness of 5 mm.
• a test piece having the original sheet thickness was produced. Then, the test pieces were subjected to the charpy impact test. In the case where the measured vE- 50 value was 40 J or more, the toughness was evaluated as good.
• the hot rolled steel sheets of Invention examples are hot rolled steel sheets having predetermined strength (TS: 980 MPa or more) and excellent toughness (vE- 50 value: 40 J or more) in combination. Also, the hot rolled steel sheets of Invention examples have predetermined strength and excellent toughness at each of the position at 1/4 of sheet thickness and the position at 1/2 of sheet thickness (sheet thickness center position) and, therefore, are hot rolled steel sheets having good characteristics in the entire region in the sheet thickness direction. On the other hand, the hot rolled steel sheets of Comparative examples out of the scope of the present invention are unable to obtained predetermined strength or are unable to obtained sufficient toughness.
• C is one of important elements in the present invention having a function of facilitating formation of bainite and enhancing strength of the steel. In order to obtain such effects, it is necessary that the C content be more than 0.1%.
• C bonds to Fe to form cementite so that if the C content is excessive, the number of cementite grains is increased, the distances between the cementite grains serving as starting points of voids are reduced, the local ductility is degraded, and the hole expansion workability is degraded.
• the C content is excessive and is more than 0.2%, the weldability is degraded. Consequently, C is limited to within the range of more than 0.1% and 0.2% or less. In this regard, 0.12% to 0.17% is preferable.
• Si is an element which contributes to enhancement of strength of the steel through solid solution and which has a function of suppressing generation of coarse cementite and, therefore, is one of important elements in the present invention.
• Si increases the intervals between cementite grains serving as starting points of voids through the function of suppressing generation of coarse cementite and, thereby, contributes to improvement of the local ductility and the hole expansion workability.
• the content is desirably 0.1% or more.
• the content is more than 1.0%, the surface quality of the steel sheet is degraded significantly, and degradation in the chemical conversion treatability and the corrosion resistance is caused. Therefore, Si is limited to 1.0% or less. In this regard, 0.5% to 0.9% is preferable.
• Mn is an element which contributes to enhancement of strength of the steel through solid solution and, in addition, which facilitates formation of a bainite phase through improvement of the hardenability.
• the Mn content be 1.5% or more.
• the amount of Mn is specified to be within the range of 1.5% to 2.5%. In this regard, the range of 1.7% to 2.2% is preferable.
• P contributes to enhancement of strength of the steel through solid solution but segregates at grain boundaries, in particular prior-austenite grain boundaries, to cause degradation in low-temperature toughness and workability. Consequently, it is preferable that P be minimized, although the content up to 0.05% is allowable. Therefore, P is specified to be 0.05% or less. In this regard, 0.03% or less is preferable, and 0.02% or less is further preferable.
• S forms coarse sulfides by bonding to Ti and Mn and degrades the workability. Consequently, it is preferable that S be minimized, although the content up to 0.005% is allowable. Therefore, S is limited to 0.005% or less. In this regard, 0.003% or less is preferable, and 0.001% or less is further preferable.
• Al is an element which functions as a deoxidizing agent and which is effective in improving cleanliness of the steel.
• the content is desirably 0.005% or more.
• Al is limited to 0.10% or less. In this regard, 0.01% to 0.05% is preferable.
• N precipitates as nitrides by bonding to nitride-forming elements and contributes to making crystal grains fine.
• Ti contributes to enhancement of strength of the steel through formation of carbonitrides to make crystal grains fine and through precipitation strengthening. Also, Ti forms many fine (Ti,V)C clusters at a temperature range of about 300°C to 500°C (coiling temperature), has a function of reducing the amount of cementite in the steel, and is one of important elements in the present invention. In order to exert such effects, it is necessary that the content be 0.07% or more. On the other hand, if the content is excessive and is more than 0.2%, the above-described effects are saturated, increases in coarse precipitates are caused, and degradation in the hole expansion workability is caused.
• Ti facilitates formation of a ferrite phase, so that a predetermined microstructure cannot be obtained and the hole expansion workability is degraded. Therefore, Ti is limited to within the range of 0.07% to 0.2%. In this regard, 0.1% to 0.15% is preferable.
• V more than 0.1% and 0.3% or less
• V is an element which contributes to enhancement of strength of the steel through formation of carbonitrides to make crystal grains fine and through precipitation strengthening and which also contributes to formation and making fine of bainite phase through an improvement of the hardenability.
• V forms many fine (Ti,V)C clusters in a temperature range of about 300°C to 500°C (coiling temperature), has a function of reducing the amount of cementite in the steel, and is one of important elements in the present invention. In order to exert such effects, it is necessary that the content be more than 0.1%. On the other hand, if the content is excessive and is more than 0.3%, the ductility is degraded and, in addition, an increase in the cost is caused. Therefore, V is limited to within the range of more than 0.1% and 0.3% or less. In this regard, 0.13% to 0.27% is preferable and 0.15% to 0.25% is further preferable.
• the above-described components are the basic components.
• at least one selected from Nb: 0.005% to 0.1%, B: 0.0002% to 0.002%, Cu: 0.005% to 0.3%, Ni: 0.005% to 0.3%, Cr: 0.005% to 0.3%, and Mo: 0.005% to 0.3% and/or one or two selected from Ca: 0.0003% to 0.01% and REM: 0.0003% to 0.1% may be further contained as selective elements.
• Each of Nb, B, Cu, Ni, Cr, and Mo is an element which contributes to enhancement of strength of the steel and at least one may be selected and contained, as necessary.
• Nb is an element which contributes to enhancement of strength of the steel through formation of carbonitrides. In order to exert such an effect, it is preferable that the content be 0.005% or more. On the other hand, if the content is more than 0.1%, deformation resistance increases, a rolling force of hot rolling increases, a load to a rolling mill becomes too large, rolling operation in itself becomes difficult and, in addition, coarse precipitates are formed, so that degradation in the workability is caused. Consequently, in the case where Nb is contained, Nb is limited to within the range of preferably 0.005% to 0.1%. In this regard, 0.01% to 0.05% is more preferable and 0.02% to 0.04% is further preferable.
• B is an element having functions of segregating at austenite grain boundaries, suppressing formation and growth of ferrite, improving hardenability, contributing to formation and making fine of bainite phase, and enhancing strength of the steel.
• the content be 0.0002% or more. However, if the content is more than 0.002%, the workability is degraded significantly. Therefore, in the case where B is contained, B is limited to within the range of preferably 0.0002% to 0.002%. In this regard, 0.0005% to 0.0015% is more preferable.
• Cu is an element having functions of enhancing strength of the steel through solid solution and improving hardenability.
• Cu lowers the bainite transformation temperature and contributes to making bainite phase fine.
• the content be 0.005% or more, although if the content is more than 0.3%, degradation in the surface quality is caused. Therefore, in the case where Cu is contained, Cu is limited to within the range of preferably 0.005% to 0.3%. In this regard, 0.01% to 0.2% is more preferable.
• Ni is an element having functions of enhancing strength of the steel through solid solution, improving hardenability, and facilitating formation of bainite phase. In order to obtain such effects, it is preferable that the content be 0.005% or more. However, if the content is more than 0.3%, a martensite phase is generated easily, and the hole expansion workability is degraded significantly. Therefore, in the case where Ni is contained, Ni is limited to within the range of preferably 0.005% to 0.3%. In this regard, 0.01% to 0.2% is more preferable.
• Cr is an element which forms carbides and contributes to enhancement of strength of the steel. In order to exert such effects, it is preferable that the content be 0.005% or more. On the other hand, if the content is excessive and is more than 0.3%, the corrosion resistance of the steel is degraded. Therefore, in the case where Cr is contained, Cr is limited to within the range of preferably 0.005% to 0.3%. In this regard, 0.01% to 0.2% is more preferable.
• Mo is an element having functions of improving hardenability, facilitating formation of bainite phase, and enhancing strength of the steel. In order to obtain such effects, it is preferable that the content be 0.005% or more. However, if the content is more than 0.3%, a martensite phase is generated easily, and the hole expansion workability is degraded significantly. Therefore, in the case where Mo is contained, Mo is limited to within the range of preferably 0.005% to 0.3%. In this regard, 0.01% to 0.2% is more preferable.
• Each of Ca and REM is an element which contributes to improvement of the hole expansion workability through shape control of inclusions and one or two may be selected and contained, as necessary.
• Ca is an element which controls the shape of inclusions and which contributes to improvement of the hole expansion workability effectively. In order to exert such effects, it is necessary that the content be 0.0003% or more. On the other hand, if the content is excessive and is more than 0.01%, the amount of inclusions increases and many surface defects are caused. Therefore, in the case where Ca is contained, Ca is limited to within the range of preferably 0.0003% to 0.01%.
• REM is an element which controls the shape of sulfide inclusions to improve adverse influences of sulfide inclusions on the hole expansion workability and, thereby, contributes to improvement of the hole expansion workability.
• the content In order to exert such effects, it is necessary that the content be 0.0003% or more.
• the content is excessive and is more than 0.01%, the amount of inclusions increases, the cleanliness of the steel is degraded, and the hole expansion workability is degraded. Therefore, in the case where REM is contained, REM is limited to within the range of preferably 0.0003% to 0.01%.
• the balance other than those described above is composed of Fe and incidental impurities.
• O oxygen
• W 0.1% or less
• Ta 0.1% or less
• Co 0.1% or less
• Sb 0.1% or less
• Sn 0.1% or less
• Zr 0.1% or less
• the primary phase is specified to be a bainite phase.
• the term "primary phase” refers to a phase having an area fraction of 90% or more. If a phase other than the bainite phase is specified to be the primary phase, predetermined high strength and good hole expansion workability cannot be obtained stably. Consequently, the primary phase is specified to be bainite phase having an area fraction of 90% or more. In this regard, 92% or more is preferable, and 95% or more is more preferable.
• the remainder other than the bainite phase serving as the primary phase is at least one selected from martensite phase, austenite phase (retained austenite phase), and ferrite phase.
• the phases of the reminder other than the primary phase are specified to be 10% or less in total (including 0%) on an area fraction basis. If the phases of the reminder other than the bainite phase are more than 10%, predetermined high strength and good hole expansion workability cannot be obtained stably. In particular, if the martensite phase increases, predetermined good hole expansion workability cannot be obtained stably.
• the hot rolled steel sheet according to the present invention has the above-described microstructure, where the microstructure shows that cementite is dispersed in the microstructure. Cementite is present while being dispersed mainly in the bainite phase, although may be present in the phases other than bainite or at the phase boundaries.
• cementite dispersed in the microstructure is specified to be 0.8% or less on a percent by mass basis and the average grain size is specified to be 150 nm or less.
• cementite is limited to 0.8% or less on a percent by mass basis. In this regard, 0.6% or less is preferable, and 0.5% or less is more preferable.
• the average grain size of cementite is limited to 150 nm or less.
• 130 nm or less is preferable, and 110 nm or less is further preferable.
• a hot rolled steel sheet is produced through the steps of heating a steel, applying hot rolling having rough rolling and finish rolling, performing cooling composed of two stages of first stage cooling and second stage cooling, and performing coiling.
• the method for manufacturing a steel serving as a starting material is not necessarily particularly limited, and any common manufacturing method can be applied, wherein a molten steel having the above-described composition is refined by a common refining method, e.g., a converter, and a steel, e.g., a slab, is produced by a common casting method, e.g., a continuous casting method.
• a common refining method e.g., a converter
• a steel e.g., a slab
• a common casting method e.g., a continuous casting method.
• an ingot-making and blooming method may be employed without problem.
• electro-magnetic stirrer EMS
• IBSR intentional bulging soft reduction casting
• Equiaxial crystals are formed in the sheet thickness center portion by applying an electro-magnetic stirrer treatment, so that segregation can be reduced.
• segregation in the sheet thickness center portion can be reduced by preventing flowing of the molten steel in an unsolidified portion of the continuous casting slab.
• the elongation and the hole expansion workability in tensile characteristics described below can be brought to a more excellent level by applying at least one of these segregation reduction treatments.
• the resulting steel is heated to heating temperature: 1,200°C or higher.
• Heating temperature 1,200°C or higher
• Carbonitride-forming elements e.g., Ti are contained in the steel employed in the present invention. Most of these carbonitride-forming elements are present as coarse carbonitrides (precipitates). In this regard, the presence of coarse carbonitride-forming elements, e.g., Ti, which remain coarse precipitates, causes reduction in the amount of fine precipitates, which contribute to solute strengthening. Consequently, the steel sheet strength is reduced. In order to allow these coarse precipitates to form solid solutions before hot rolling, the heating temperature is limited to 1,200°C or higher. In this regard, 1,220°C to 1,350°C is preferable.
• the heated steel is subjected to hot rolling composed of rough rolling and finish rolling.
• the condition of the rough rolling is not specifically limited insofar as predetermined sheet bar dimensions are ensured.
• the finish rolling with finishing temperature: 850°C to 950°C is applied.
• descaling is performed before the finish rolling or between finish rolling stands during rolling.
• Finishing temperature 850°C to 950°C
• finishing temperature is lower than 850°C, finish rolling is rolling in two-phase region of ferrite + austenite, so that a deformation microstructure remains after rolling and the hole expansion workability is degraded.
• the finishing temperature is high and is higher than 950°C, austenite grains grow, and a bainite phase of the hot rolled sheet obtained after cooling is coarsened. Consequently, the hole expansion workability is degraded. Therefore, the finishing temperature is limited to within the range of 850°C to 950°C. In this regard, 870°C to 930°C is preferable.
• finishing temperature refers to the surface temperature.
• cooling composed of two stages of first stage cooling and second stage cooling is applied.
• cooling is started within 1.5 s of, preferably just after, completion of the finish rolling, and cooling to a first stage cooling stop temperature of 500°C to 600°C is performed at an average cooling rate of 20°C/s to 80°C/s.
• the cooling start time of the first stage cooling is limited to within 1.5 s of completion of the finish rolling.
• the average cooling rate of the first stage cooling is less than 20°C/s and, therefore, cooling becomes slow, formation of ferrite or coarse bainite is facilitated, and predetermined high strength or hole expansion workability cannot be obtained.
• quenching is performed at more than 80°C/s, martensite is generated easily to become hard, and the hole expansion workability is degraded. Consequently, the average cooling rate of the first stage cooling is limited to within the range of 20°C/s to 80°C/s. In this regard, 25°C/s to 60°C/s is preferable.
• the first stage cooling stop temperature is lower than 500°C, a transition boiling region is reached, variations in steel sheet temperature increase, the microstructure becomes heterogeneous, and predetermined excellent hole expansion workability cannot be obtained.
• the first stage cooling stop temperature is a high temperature higher than 600°C, ferrite transformation is facilitated, and predetermined high strength cannot be obtained. Consequently, the first stage cooling stop temperature is limited to 500°C to 600°C. In this regard, 520°C to 580°C is preferable.
• the second stage cooling is started just after or within 3 s of, preferably just after, completion of the first stage cooling, and cooling to a second stage cooling stop temperature of 330°C to 470°C is performed at an average cooling rate of 90°C/s or more.
• the cooling start time of the second stage cooling is limited to within 3 s of completion of the first stage cooling.
• the average cooling rate of the second stage cooling is limited to 90°C/s or more.
• the upper limit of the average cooling rate of the second stage cooling is not specifically limited, although the upper limit is about 250°C/s in association with the sheet thickness of a sheet to be cooled and the capability of cooling equipment. In this regard, 100°C/s to 200°C/s is preferable.
• the second stage cooling stop temperature is lower than 330°C, hard martensite phase and retained austenite phase are formed in the steel sheet microstructure, a predetermined microstructure cannot be obtained, and the hole expansion workability is degraded.
• the second stage cooling stop temperature is a high temperature higher than 470°C, a ferrite phase and a martensite phase increase in the steel sheet microstructure, predetermined microstructure cannot be obtained, and the hole expansion workability is degraded significantly. Consequently, the second stage cooling stop temperature is limited to 330°C to 470°C. In this regard, 350°C to 450°C is preferable.
• hot rolled steel sheet (steel strip in coil) is produced by performing coiling into the shape of a coil, where a coiling temperature is specified to be the second stage cooling stop temperature.
• the above-described temperature refers to a steel sheet surface temperature.
• the hot rolled steel sheet may further be subjected to temper rolling following the common method.
• the resulting hot rolled steel sheet may be subjected to pickling to remove scale formed on the surface.
• a galvanization process e.g., hot dip galvanizing or electrogalvanizing, and a chemical conversion treatment may further be applied.
• a molten steel having the composition shown in Table 5 was refined in a converter, and a slab (steel) was produced by a continuous casting method.
• those other than Hot rolled steel sheet No. 1' of Steel A2 in Tables 5 to 7B described later were subjected to electro-magnetic stirrer (EMS) for the purpose of segregation reduction treatment of the components.
• EMS electro-magnetic stirrer
• these steels were heated under the conditions shown in Tables 6A and 6B, and were subjected to hot rolling composed of rough rolling and finish rolling under the conditions shown in Tables 6A and 6B.
• cooling was performed under the conditions shown in Tables 6A and 6B, and coiling was performed at coiling temperatures shown in Table 2, so that hot rolled steel sheets having sheet thicknesses shown in Tables 6A and 6B were produced. Cooling of some hot rolled steel sheets were specified to be single stage cooling.
• Test pieces were taken from the resulting hot rolled steel sheets, and microstructure observation, a tensile test, and a hole expanding test were performed. The testing methods were as described below.
• a test piece for a microstructure observation was taken from the resulting hot rolled steel sheet, a sheet thickness cross-section parallel to the rolling direction was polished, and the microstructure was allowed to appear with a corrosive liquid (3% nital solution).
• the microstructure of the position at one-quarter of the sheet thickness was observed with a scanning electron microscope (SEM), and photographs of the microstructure were taken in three fields of view (magnification: 3,000 times) .
• the microstructure fraction (area fraction) of each phase was calculated on the basis of identification of the microstructure and image analysis.
• a test piece (size: 10 mm ⁇ 15 mm) for replica was taken from the position at one-quarter of the sheet thickness of the resulting hot rolled steel sheet, a replica film was produced by a two-stage replica method, and cementite was taken.
• the resulting cementite was observed with a transmission electron microscope (TEM), and photographs were taken in five fields of view (magnification: 50,000 times).
• the grain size of each cementite was determined and the average grain size of cementite of the steel sheet concerned was determined by averaging.
• the average value of the long axis length and the short axis length was specified to be the grain size of the cementite concerned.
• test piece (size: t ⁇ 50 ⁇ 100 mm) for electrolytic residue extraction was taken from the resulting hot rolled steel sheet.
• the total thickness of the test piece was subjected to constant-current electrolysis in a 10 vol% AA electrolyte (10 vol% acetylacetone-1 mass% tetramethylammonium chloride ⁇ methanol) at current density: 20 mA/cm 2 .
• the resulting electrolyte was filtrated and the electrolytic residue remaining on the filter paper was analyzed with an inductively-coupled plasma spectrophotometric analyzer to measure the amount of Fe in the electrolytic residue. It was assumed that quantified Fe was entirely Fe 3 C, and the amount of precipitated cementite was calculated on the basis of the following formula.
• Fe 3 C percent by mass 1.0716 ⁇ quantified Fe g / electrolyzed weight g ⁇ 100
• the atomic weight of Fe was specified to be 55.85 (g/mol) and the atomic weight of C was specified to be 12.01 (g/mol).
• the electrolyzed weight was determined by cleaning the test piece for electrolysis after the electrolysis, measuring the weight, and subtracting the resulting weight from the test piece weight before electrolysis.
• JIS No. 5 test pieces (GL: 50 mm) were taken from the resulting hot rolled steel sheet in such a way that the tensile direction and the rolling direction form a right angle.
• a tensile test was performed in conformity with JIS Z 2241 and the yield strength (yield point) YP, the tensile strength TS, and the elongation El were determined.
• a test piece (size: t ⁇ 100 ⁇ 100 mm) for hole expanding test was taken from the resulting hot rolled steel sheet.
• a punched hole was punched in the center of the test piece with a 10 mm ⁇ punch, where clearance: 12.5% of sheet thickness.
• a test piece (size: t ⁇ 100 ⁇ 100 mm) for hole expanding test was taken from the resulting hot rolled steel sheet.
• a punched hole was punched in the center of the test piece with a 10 mm ⁇ punch, where clearance: 25.0% of sheet thickness.
• a 60° cone punch was inserted into the punched hole along the punching direction in such a way as to be pushed upward, and a hole diameter d mm at the point in time when a crack penetrated the sheet thickness was determined, and the hole expanding ratio ⁇ (%) was calculated by the above-described formula.
• the clearance refers to the proportion (%) relative to the sheet thickness.

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• 2014
  • 2014-03-17 CN CN201480020707.7A patent/CN105102662A/zh active Pending
  • 2014-03-17 WO PCT/JP2014/001509 patent/WO2014171063A1/fr active Application Filing
  • 2014-03-17 US US14/784,455 patent/US20160076124A1/en not_active Abandoned
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• 2015
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