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
• • 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
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/008—Heat treatment of ferrous alloys containing Si
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—Cold 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
  • 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
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—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
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
• • C—CHEMISTRY; METALLURGY
  • 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
  • C21D2211/001—Austenite
• • 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
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present invention relates to a high-strength cold-rolled steel sheet having a high elongation (EL), a high hole expansion ratio ( ⁇ ), and a low yield ratio (YR) and a method for manufacturing the steel sheet, and in particular, to a high-strength cold-rolled steel sheet which can preferably be used for structural parts of, for example, an automobile.
• EL high elongation
• ⁇ high hole expansion ratio
• YR low yield ratio
• examples of known high-strength cold-rolled steel sheet having satisfactory formability and a high strength at the same time include a dual-phase steel sheet (DP steel sheet), which has a multi-phase microstructure composed of ferrite and martensite.
• DP steel sheet has a high elongation (EL)
• a DP steel sheet has a disadvantage in that, since a crack tends to occur due to stress concentration occurring at the interface between ferrite and martensite, there is a deterioration in bendability and hole expansion capability.
• Patent Literature 1 discloses a DP steel sheet in which the crystal grain diameter, volume fraction, and nanoindentation hardness of ferrite are controlled, and it is possible to achieve a high elongation (EL) and excellent bendability with this DP steel sheet.
• examples of a steel sheet having a high strength and a high elongation (EL) at the same time include TRIP steel sheet. Since this TRIP steel sheet has a steel sheet microstructure including retained austenite, a large elongation (EL) is achieved in the case where the steel sheet is deformed by work performed at a temperature equal to or higher than a temperature at which martensite transformation begins, because retained austenite transforms into martensite through transformation induced by stress.
• Patent Literature 2 discloses a TRIP steel sheet which includes bainitic ferrite in order to improve hole expansion capability.
• an object of the present invention is, by solving the problems described above, to provide a high-strength cold-rolled steel sheet excellent in terms of elongation (EL) and hole expansion ratio ( ⁇ ) having a low yield ratio (YR) and a method for manufacturing the steel sheet.
• the present inventors diligently conducted investigations, and, as a result, found that it is possible to achieve a high elongation (EL) and a high hole expansion ratio ( ⁇ ) while maintaining a low yield ratio (YR) by controlling the crystal grain diameters and volume fractions of steel sheet microstructures, that is, ferrite, retained austenite, and tempered martensite.
• EL elongation
• ⁇ hole expansion ratio
• YR low yield ratio
• DP steel has a low yield ratio (YR).
• YR yield ratio
• voids are formed at its interface, in particular, its interface with soft ferrite when punching work is performed in a hole expansion process, the voids then combine with each other when the punched hole is expanded, and a crack occurs as the combination of the voids progresses. Therefore, there is a decrease in the hole expansion ratio ( ⁇ ) of DP steel.
• ⁇ hole expansion ratio
• retained austenite significantly increases elongation (EL)
• retained austenite since retained austenite, as is the case with hard martensite, causes the formation of voids when punching work is performed in a hole expansion process, there is a decrease in hole expansion ratio ( ⁇ ).
• EL elongation
• ⁇ hole expansion ratio
• the present inventors diligently conducted investigations, and, as a result, found the tempering conditions used for forming tempered martensite in order to increase hole expansion ratio ( ⁇ ) while achieving low yield ratio (YR). Moreover, it was found that it is possible to inhibit the combination of voids in a hole expansion process by decreasing the average crystal grain diameter of retained austenite and tempered martensite in order to form a steel sheet microstructure in which retained austenite and tempered martensite are finely dispersed, which results in an increase in elongation (EL) and hole expansion ratio ( ⁇ ).
• EL elongation
• ⁇ hole expansion ratio
• fine martensite and retained austenite are formed by forming a microstructure composed of bainite and martensite in a first annealing process after cold rolling has been performed, by forming fine austenite through reverse transformation in a second annealing process, by then allowing bainite transformation to occur through cooling, and by then performing rapid cooling.
• EL elongation
• ⁇ hole expansion ratio
• a high-strength cold-rolled steel sheet refers to a cold-rolled steel sheet having a tensile strength (TS) of 980 MPa or more.
• an average cooling rate refers to a value derived by dividing a value derived by subtracting a cooling stop temperature from a cooling start temperature by a cooling time.
• an average heating rate refers to a value derived by dividing a value derived by subtracting a heating start temperature from a heating stop temperature by a heating time.
• TS tensile strength
• YR low yield ratio
• EL elongation
• ⁇ hole expansion ratio
• the high-strength cold-rolled steel sheet according to the present invention has a chemical composition containing, by mass%, C: 0.15% to 0.25%, Si: 1.0% to 2.0%, Mn: 1.8% to 2.5%, P: 0.10% or less, S: 0.010% or less, Al: 0.10% or less, N: 0.010% or less, and the balance being Fe and inevitable impurities, and a multi-phase microstructure including ferrite having an average crystal grain diameter of 5 ⁇ m or less in an amount of 30% to 55% in terms of volume fraction, retained austenite having an average crystal grain diameter of 2 ⁇ m or less in an amount of 5% to 15% in terms of volume fraction, and tempered martensite having an average crystal grain diameter of 2 ⁇ m or less in an amount of 30% to 60% in terms of volume fraction, in which the number of grains of the retained austenite existing in an area of 1000 ⁇ m 2 is 10 or more.
• C is a chemical element which is effective for increasing the strength of a steel sheet and which also contributes to the formation of second phases in the present invention, that is, tempered martensite and retained austenite.
• the C content is set to be 0.15% or more, or preferably 0.18% or more.
• the C content is set to be 0.25% or less, or preferably 0.23% or less.
• a main phase refers to a ferrite phase
• second phases refers to a tempered martensite phase and a retained austenite phase
• the microstructure of the high-strength cold-rolled steel sheet according to the present invention may include tempered bainite and pearlite.
• Si is a chemical element which is necessary for contributing to the formation of retained austenite by inhibiting the formation of carbides when bainite transformation occurs in the first and second annealing processes.
• the Si content is set to be 1.0% or more, or preferably 1.3% or more.
• the Si content is set to be 2.0% or less, or preferably 1.8% or less.
• Mn is a chemical element which contributes to an increase in strength through solid solution strengthening and by facilitating the formation of the second phases and which stabilizes austenite.
• the Mn content is set to be 1.8% or more.
• the Mn content is set to be 2.5% or less.
• the P content is set to be 0.10% or less, or preferably 0.05% or less.
• the S content is set to be 0.010% or less, or preferably 0.005% or less.
• the S content be 0.0005% or more.
• the Al content is set to be 0.10% or less, or preferably 0.08% or less.
• the Al content be 0.01% or more.
• the N content is set to be 0.010% or less, or preferably 0.006% or less.
• the remainder which is different from the constituent chemical elements described above is Fe and inevitable impurities.
• the inevitable impurities include Sb, Sn, Zn, and Co, and the acceptable ranges of the contents of these chemical elements are respectively Sb: 0.01% or less, Sn: 0.10% or less, Zn: 0.01% or less, and Co: 0. 10% or less.
• Ta, Mg, and Zr are added in amounts within the ranges which are common among ordinary steel chemical compositions, there is no decrease in the effects of the present invention.
• one, two, or more of the following chemical elements may be added in addition to the constituent chemical elements described above.
• V 0.10% or less
• V contributes to an increase in strength by forming fine carbonitrides
• V may be added as needed.
• the V content be 0.01% or more.
• the V content be 0.10% or less.
• Nb since V, contributes to an increase in strength by forming fine carbonitrides, Nb may be added as needed. In order to realize such an effect, it is preferable that the Nb content be 0.005% or more. On the other hand, there is a significant deterioration in elongation (EL) in the case where the Nb content is more than 0.10%. Therefore, it is preferable that the Nb content be 0.10% or less.
• Ti like V, contributes to an increase in strength by forming fine carbonitrides
• Ti may be added as needed. In order to realize such an effect, it is preferable that the Ti content be 0.005% or more. On the other hand, in the case where the Ti content is more than 0.10%, there is a significant deterioration in elongation (EL). Therefore, it is preferable that the Ti content be 0.10% or less.
• B is a chemical element which contributes to an increase in strength by increasing hardenability and by facilitating the formation of the second phases and which achieves hardenability without significantly increasing the hardness of tempered martensite
• B may be added as needed.
• the B content be 0.0003% or more.
• the B content be 0.010% or less.
• Cr is a chemical element which contributes to an increase in strength by facilitating the formation of the second phases
• Cr may be added as needed.
• the Cr content be 0.10% or more.
• the Cr content in the case where the Cr content is more than 0.50%, an excessive amount of tempered martensite is formed. Therefore, in the case where Cr is added, it is preferable that the Cr content be 0.50% or less.
• Mo is a chemical element which contributes to an increase in strength by facilitating the formation of the second phases and by partially forming carbides
• Mo may be added as needed. In order to realize such an effect, it is preferable that the Mo content be 0.05% or more. On the other hand, in the case where the Mo content is more than 0.50%, such an effect becomes saturated. Therefore, in the case where Mo is added, it is preferable that the Mo content be 0.50% or less.
• Cu is a chemical element which contributes to an increase in strength through solid solution strengthening and by facilitating the formation of the second phases
• Cu may be added as needed.
• the Cu content be 0.05% or more.
• the Cu content in the case where the Cu content is more than 0.50%, such an effect becomes saturated, and surface defects caused by Cu tend to occur. Therefore, in the case where Cu is added, it is preferable that the Cu content be 0.50% or less.
• Ni is, like Cu, a chemical element which contributes to an increase in strength through solid solution strengthening and by facilitating the formation of the second phases
• Ni may be added as needed.
• the Ni content be 0.05% or more.
• the Ni content is more than 0.50%, such effects become saturated. Therefore, in the case where Ni is added, it is preferable that the Ni content be 0.50% or less.
• Ca contributes to inhibiting a decrease in the hole expansion ratio ( ⁇ ) due to sulfides by spheroidizing the shape of sulfides
• Ca may be added as needed.
• the Ca content be 0.0005% or more.
• the Ca content in the case where the Ca content is more than 0.0050%, such an effect becomes saturated. Therefore, in the case where Ca is added, it is preferable that the Ca content be 0.0050% or less.
• REM since REM, like Ca, contributes to inhibiting a decrease in the hole expansion ratio ( ⁇ ) due to sulfides by spheroidizing the shape of sulfides, REM may be added as needed. In order to realize such an effect, it is preferable that the REM content be 0.0005% or more. On the other hand, in the case where the REM content is more than 0.0050%, such an effect becomes saturated. Therefore, in the case where REM is added, it is preferable that the REM content be 0.0050% or less.
• the high-strength cold-rolled steel sheet according to the present invention includes ferrite, retained austenite, and tempered martensite.
• the high-strength cold-rolled steel sheet according to the present invention may include tempered bainite as the remainder of the microstructure.
• the ferrite has an average crystal grain diameter of 5 ⁇ m or less and a volume fraction of 30% to 55%.
• the retained austenite has an average crystal grain diameter of 2 ⁇ m or less and a volume fraction of 5% to 15%.
• the tempered martensite has an average crystal grain diameter of 2 ⁇ m or less and a volume fraction of 30% to 60%.
• the number of grains of retained austenite having an average crystal grain diameter of 2 ⁇ m or less existing in an area of 1000 ⁇ m 2 is 10 or more.
• the volume fraction of ferrite described above is less than 30%, since there is an insufficient amount of soft ferrite, there is a decrease in elongation (EL). Therefore, the volume fraction of ferrite is set to be 30% or more, or preferably 35% or more. On the other hand, in the case where the volume fraction of ferrite is more than 55%, it is difficult to achieve a tensile strength (TS) of 980 MPa or more. Therefore, the volume fraction of ferrite is set to be 55% or less, or preferably 50% or less.
• the average crystal grain diameter of ferrite is set to be 5 ⁇ m or less.
• the volume fraction of retained austenite In order to achieve a high elongation (EL), it is necessary that the volume fraction of retained austenite be 5% to 15%. In the case where the volume fraction of retained austenite is less than 5%, it is not possible to achieve the desired elongation (EL). Therefore, the volume fraction of retained austenite is set to be 5% or more, or preferably 6% or more. On the other hand, in the case where the volume fraction of retained austenite is more than 15%, it is not possible to achieve the desired hole expansion ratio ( ⁇ ). Therefore, the volume fraction of retained austenite is set to be 15% or less, or preferably 12% or less. In addition, in order to achieve a high hole expansion ratio ( ⁇ ), the average crystal grain diameter of retained austenite is set to be 2 ⁇ m or less.
• the average crystal grain diameter of retained austenite is set to be 2 ⁇ m or less.
• the volume fraction of tempered martensite is set to be 30% to 60%. In the case where the volume fraction of tempered martensite is less than 30%, it is not possible to achieve a tensile strength of 980 MPa or more. On the other hand, in the case where the volume fraction of tempered martensite is more than 60%, it is difficult to achieve the desired elongation (EL). In addition, in order to achieve a high hole expansion ratio ( ⁇ ), the average crystal grain diameter of tempered martensite is set to be 2 ⁇ m or less.
• the upper limit of the average crystal grain diameter of tempered martensite is set to be 2 ⁇ m.
• tempered bainite may be partially formed in order to form retained austenite by allowing bainite transformation to occur in an annealing process.
• the volume fraction of this tempered bainite it is preferable that the volume fraction of tempered bainite be 30% or less in order to achieve a high elongation (EL).
• the number of grains of the above-described retained austenite having an average crystal grain diameter of 2 ⁇ m or less existing in an area of 1000 ⁇ m 2 be 10 or more. In the case where the number of grains of retained austenite existing in an area of 1000 ⁇ m 2 is less than 10, it is not possible to achieve the desired elongation (EL).
• the upper limit of the number of grains of retained austenite existing in an area of 1000 ⁇ m 2 in the case where the number of grains of retained austenite existing in an area of 1000 ⁇ m 2 is more than 50, voids which have been formed at the grain boundaries with ferrite tend to combine with each other. Therefore, it is preferable that the number of grains of retained austenite existing in an area of 1000 ⁇ m 2 be 50 or less.
• tempered bainite and pearlite are formed in addition to ferrite, retained austenite, and tempered martensite
• the volume fraction of pearlite be 5% or less.
• the volume fraction of tempered bainite be 30% or less.
• a SEM scanning electron microscope
• nital an alcohol solution containing nitric acid
• image analysis software Image-Pro ver. 7 produced by Media Cybernetics, Inc.
• the method for manufacturing the high-strength cold-rolled steel sheet according to the present invention includes, after having performed hot rolling and cold rolling on a steel slab having the chemical composition (constituent chemical elements) described above, performing continuous annealing on the cold-rolled steel sheet, in which heating is performed to a temperature of 850°C or higher, in which holding is performed at a first soaking temperature of 850°C or higher for 30 seconds or more, in which cooling is then performed from the first soaking temperature to a second soaking temperature of 320°C to 500°C at a first average cooling rate of 3°C/s or more, in which holding is performed at the second soaking temperature of 320°C to 500°C for 30 seconds or more, in which cooling is then performed to a temperature of 100°C or lower (for example, room temperature), in which heating is thereafter performed to a temperature of 750°C or higher at an average heating rate of 3°C/s to 30°C/s, in which holding is performed at a third soaking temperature of 750°C or
• the hot rolling process by performing rough rolling and finish rolling on a steel slab having the chemical composition described above after heating has been performed, a hot-rolled steel sheet is obtained.
• the steel slab used be manufactured by using a continuous casting method in order to prevent the macro segregation of the constituent chemical elements
• the slab may also be manufactured by using an ingot-making method or a thin-slab-casting method.
• the cast slab may not be reheated or may be reheated to a temperature of 1100°C or higher.
• an energy-saving process such as a hot direct rolling or a direct rolling, that is, a method in which a slab in the hot state is charged into a heating furnace without the slab having been cooled, a method in which a slab is rolled immediately after heat retention has been performed, or a method in which a slab in the cast state is rolled may be used without causing any problem.
• a slab heating temperature By controlling a slab heating temperature to be 1100°C or higher, it is possible to decrease rolling load and to improve productivity. On the other hand, by controlling the slab heating temperature to be 1300°C or lower, it is possible to decrease heating costs. Therefore, it is preferable that the slab heating temperature be 1100°C to 1300°C.
• a finishing delivery temperature 830°C or higher, since it is possible to finish hot rolling within an austenite single phase region, it is possible to inhibit a decrease in elongation (EL) and hole expansion ratio ( ⁇ ) due to an increase in the inhomogeneity of a microstructure in a steel sheet and the anisotropy of material properties after annealing.
• the finishing delivery temperature it is possible to inhibit deterioration in properties after annealing due to coarsening of a hot-rolled microstructure. Therefore, it is preferable that the finishing delivery temperature be 830°C to 950°C.
• a coiling temperature there is no particular limitation on the method used for cooling the hot-rolled steel sheet after hot rolling. Also, there is no particular limitation on a coiling temperature. However, by controlling a coiling temperature to be 700°C or lower, since it is possible to inhibit the formation of coarse pearlite, it is possible to prevent a decrease in elongation (EL) and hole expansion ratio ( ⁇ ) after annealing has been performed. Therefore, it is preferable that the coiling temperature be 700°C or lower, or more preferably 650°C or lower. On the other hand, although there is no particular limitation on the lower limit of the coiling temperature, by controlling the coiling temperature to be 400°C or higher, since it is possible to inhibit the formation of excessive amounts of hard bainite and martensite, it is possible to decrease cold rolling load. Therefore, it is preferable that the coiling temperature be 400°C or higher.
• pickling may be performed on the hot-rolled steel sheet after the hot rolling process described above. It is preferable that scale on the surface of the hot-rolled steel sheet be removed by performing pickling.
• pickling There is no particular limitation on the method used for pickling, and pickling may be performed by using a commonly used method.
• cold rolling in which rolling is performed in order to obtain a cold-rolled steel sheet having a specified thickness, is performed.
• cold rolling may be performed by using a commonly used method.
• intermediate annealing may be performed before the cold rolling process. By performing intermediate annealing, it is possible to decrease cold rolling load.
• annealing be performed at a temperature of 450°C to 800°C for 10 minutes to 50 hours.
• annealing is performed on the cold-rolled steel sheet.
• recrystallization is progressed, and retained austenite and tempered martensite are formed in a steel sheet microstructure in order to increase strength.
• ⁇ high hole expansion ratio
• a steel sheet microstructure including bainite, martensite, and retained austenite which are homogenized to some extent in the first annealing process it is possible to allow more homogeneous fine dispersion to occur in the second annealing process.
• tempering is performed after cooling is first performed to an excessive degree. With this, it is possible to achieve a high hole expansion ratio ( ⁇ ) while inhibiting a decrease in elongation (EL).
• heating is performed to a temperature of 850°C or higher, holding is performed at a first soaking temperature of 850°C or higher for 30 seconds or more, cooling is then performed from the first soaking temperature to a second soaking temperature of 320°C to 500°C at a first average cooling rate of 3°C/s or more, holding is performed at the second soaking temperature of 320°C to 500°C for 30 seconds or more, and cooling is then performed to a temperature of 100°C or lower (for example, room temperature).
• heating is performed to a temperature of 750°C or higher at an average heating rate of 3°C/s to 30°C/s, holding is performed at a third soaking temperature of 750°C or higher for 30 seconds or more, cooling is then performed from the third soaking temperature to a temperature of 350°C to 500°C at a second average cooling rate of 3°C/s or more, cooling is performed to a temperature of 100°C or lower at a third average cooling rate of 100°C/s to 1000°C/s, heating is performed to a temperature of 200°C to 350°C, and holding is then performed at a fourth soaking temperature of 200°C to 350°C for 120 seconds to 1200 seconds.
• the first annealing process heating is firstly performed to the first soaking temperature.
• This first soaking temperature is set to be a temperature in a temperature range in which an austenite single phase is formed.
• the first soaking temperature is lower than 850°C, since there is a decrease in the amount of bainite after the first annealing process, there is an increase in the crystal grain diameter of tempered martensite and retained austenite which are formed in the second annealing process, which results in a decrease in hole expansion ratio ( ⁇ ). Therefore, the lower limit of the first soaking temperature is set to be 850°C, or preferably 870°C or higher.
• the first soaking temperature be 1000°C or lower in order to prevent the crystal grain diameter of austenite from increasing.
• the holding time (soaking time) at the first soaking temperature is set to be 30 seconds or more. Although there is no particular limitation on the upper limit of this holding time, it is preferable that this holding time be 600 seconds or less in order to prevent coarse carbides from being formed in a steel sheet.
• the first annealing process in order to form a steel sheet microstructure including a large amount of bainite, cooling is performed to a second soaking temperature of 320°C to 500°C at a first average cooling rate of 3°C/s or more.
• the first average cooling rate is less than 3°C/s, since excessive amounts of ferrite, pearlite and spherical cementite are formed in a steel sheet microstructure, the lower limit of the first average cooling rate is set to be 3°C/s.
• the cooling stop temperature (hereinafter, also referred to as "second soaking temperature”) is lower than 320°C, since an excessive amount of massive martensite is formed in the cooling process, it is difficult to form a fine homogeneous steel sheet microstructure in the second annealing process, which makes it impossible to achieve the desired hole expansion ratio ( ⁇ ).
• the cooling stop temperature (second soaking temperature) is higher than 500°C, since there is an excessive increase in the amount of pearlite, it is difficult to form a fine homogeneous steel sheet microstructure in the second annealing process, which makes it impossible to achieve the desired hole expansion ratio ( ⁇ ). Therefore, the second soaking temperature is set to be 320°C to 500°C, or preferably 350°C to 450°C.
• the holding time at the second soaking temperature is set to be 30 seconds or more.
• Heating to third soaking temperature (750°C or higher) at an average heating rate of 3°C/s to 30°C/s)
• the third soaking temperature is lower than 750°C, since there is an excessively small amount of austenite formed, it is not possible to achieve the desired volume fractions of martensite and retained austenite formed. Therefore, the third soaking temperature is set to be 750°C or higher.
• the third soaking temperature be 900°C or lower in order to remove the influence of the steel sheet microstructure which has been formed in the first annealing process by performing annealing in an austenite single phase region is formed.
• the average heating rate to the third soaking temperature (750°C or higher) is more than 30°C/s, recrystallization is less likely to progress. Therefore, the average heating rate is set to be 30°C/s or less.
• the average heating rate to the third soaking temperature (750°C or higher) is less than 3°C/s, since ferrite grains are coarsened, it is not possible to achieve the specified average crystal grain diameter. Therefore, the average heating rate is set to be 3°C/s or more.
• the holding time at the third soaking temperature is set to be 30 seconds or more.
• cooling is performed to a temperature of 350°C to 500°C at a second average cooling rate of 3°C/s or more.
• the second average cooling rate is less than 3°C/s, excessive amounts of pearlite and spherical cementite are formed in a steel sheet microstructure. Therefore, the lower limit of the second average cooling rate is set to be 3°C/s.
• cooling at the second average cooling rate is performed to a temperature of lower than 350°C, since an excessive amount of martensite is formed in the cooling process, and the amounts of bainite transformation and retained austenite are decreased due to a decrease in the amount of untransformed austenite, it is impossible to achieve the desired elongation (EL). Therefore, cooling at the second average cooling rate should be performed to a temperature of 350°C or higher.
• the cooling at the second average cooling rate should be performed to a temperature of 500°C or lower, or preferably 370°C to 450°C.
• cooling is performed to a temperature of 100°C or lower at a third average cooling rate of 100°C/s to 1000°C/s.
• the third average cooling rate is set to be 100°C/s or more.
• the third average cooling rate is set to be 1000°C/s or less.
• this cooling it is preferable that water quenching be performed.
• a tempering treatment is performed.
• This tempering is performed in order to improve workability by softening martensite. That is, after cooling is performed as described above, in order to temper martensite, after heating has been performed to a temperature of 200°C to 350°C, holding is performed at a tempering temperature of 200°C to 350°C (hereinafter, also referred to as "fourth soaking temperature") for 120 seconds to 1200 seconds.
• the tempering temperature (fourth soaking temperature) is lower than 200°C, since the softening of martensite is insufficient, there is a decrease in hole expansion capability. Therefore, the fourth soaking temperature is set to be 200°C or higher.
• the tempering temperature (fourth soaking temperature) is higher than 350°C, there is an increase in yield ratio (YR). Therefore, the fourth soaking temperature is set to be 350°C or lower, or preferably 300°C or lower.
• the holding time at the fourth soaking temperature is set to be 120 seconds or more.
• the holding time at the fourth soaking temperature is set to be 1200 seconds or less.
• cooling method or cooling rate there is no limitation on cooling method or cooling rate.
• skin pass rolling may be performed after the annealing process. It is preferable that skin pass rolling be performed with an elongation ratio of 0.1% to 2.0%.
• a galvanized steel sheet may be manufactured by performing a galvanizing treatment in the annealing process, and a galvannealed steel sheet may be manufactured by performing an alloying treatment after a galvanizing treatment has been performed. Moreover, by performing an electroplating treatment on the cold-rolled steel sheet according to the present invention, an electroplated steel sheet may be manufactured.
• heating was performed to the first soaking temperatures given in Table 2, annealing was performed at the first soaking temperatures for the first soaking time (first holding time), cooling was the performed to the second soaking temperatures at the first average cooling rates (CR1) given in Table 2, holding was performed for the second soaking time (second holding time), and then cooling was performed to room temperature (25°C).
• cooling was then performed to the quenching start temperatures (cooling start temperatures of cooling performed at the third average cooling rates, that is, cooling stop temperatures: Tq) at the second average cooling rates (CR2) given in Table 2, cooling was then performed to room temperature (25°C) at the third average cooling rates (CR3), and then, in the tempering process, heating was performed to the fourth soaking temperatures given in Table 2, holding was performed for the fourth soaking times (fourth holding times given in Table 2), and then cooling was performed to room temperature (25°C).
• a steel sheet having a tensile strength (TS) of 980 MPa or more was judged as a high-strength steel sheet, a steel sheet having an elongation (EL) of 19% or more was judged as a steel sheet having a good elongation (EL), and a steel sheet having a yield ratio (YR) of 66% or less was judged as a steel sheet having the desired low yield ratio (YR).
• hole expansion capability in accordance with The Japan Iron and Steel Federation Standard (JFS T 1001 (1996), by punching a hole having a diameter of 10 mm ⁇ in a sample with a clearance of 12.5%, by setting the sample on a testing machine so that the burr was on the die side, and by forming the sample with a conical punch having a point angle of 60°, hole expansion ratio ( ⁇ ) was determined. A steel sheet having a ⁇ (%) of 30% or more was judged as a steel sheet having good hole expansion capability.
• steel sheet microstructure by using a SEM (scanning electron microscope), a TEM (transmission electron microscope), and an FE-SEM (field-emission-type scanning electron microscope), steel sheet microstructure was observed in order to identify ferrite, retained austenite, tempered martensite, and other kinds of steel microstructures.
• SEM scanning electron microscope
• TEM transmission electron microscope
• FE-SEM field-emission-type scanning electron microscope
• the volume fractions of ferrite and tempered martensite of the steel sheet were determined by polishing a cross section in the thickness direction parallel to the rolling direction of the steel sheet, by then etching the polished cross section through the use of a 3%-nital solution, by observing the etched cross section through the use of a SEM (scanning electron microscope) at magnifications of 2000 times and 5000 times, by determining the area fraction of each of the phases through the use of a point-counting method (in accordance with ASTM E562-83 (1988)), and by defining the area fraction as the volume fraction.
• SEM scanning electron microscope
• the average crystal grain diameters of ferrite, retained austenite, and tempered martensite since it was possible to calculate each area of the phases by inputting the steel sheet microstructure photographs, in which the crystal grains of ferrite, retained austenite, and tempered martensite had been identified in advance, into Image-Pro produced by Media Cybernetics, Inc., by calculating circle-equivalent diameters from the calculated areas, the average crystal grain diameter of each of the phases was defined as the average of the calculated circle-equivalent diameters.
• the volume fraction of retained austenite was determined by polishing the steel sheet in order to expose a surface located at 1/4 of the thickness of the steel sheet and by determining the X-ray diffraction intensities of the surface.
• determining the integrated intensities of X-ray diffraction of the (200) plane, (211) plane, and (220) plane of the ferrite of iron and the (200) plane, (220) plane, and (311) plane of the austenite of iron through the use of the K ⁇ ray of Mo as a radiation source with an acceleration voltage of 50 keV in X-ray diffractometry (apparatus: RINT-2200 produced by Rigaku Corporation), and by using the calculating formula described in " X-ray Diffraction Handbook" published by Rigaku Corporation (2000), pp. 26 and 62-64 , the volume fraction of retained austenite was determined.
• the number of grains of retained austenite were determined by counting the number in the observation of a steel sheet photograph taken through the use of a SEM.
• the hole expansion ratio ( ⁇ ) was less than 30%.
• the yield ratio (YR) was more than 66%, and the hole expansion ratio ( ⁇ ) was less than 30%.
• the hole expansion ratio ( ⁇ ) was less than 30%.
• the tensile strength (TS) was less than 980 MPa.
• the volume fraction of ferrite was more than 55%, where the average crystal grain diameter of ferrite was more than 5 ⁇ m, where the average crystal grain diameter of retained austenite was more than 2 ⁇ m, and where the volume fraction of tempered martensite was less than 30%
• the tensile strength (TS) was less than 980 MPa
• the yield ratio (YR) was more than 66%
• the hole expansion ratio ( ⁇ ) was less than 30%.
• the volume fraction of retained austenite was less than 5% and where the number of grains of retained austenite existing in an area of 1000 ⁇ m 2 was less than 10
• the elongation (EL) was less than 19%
• the yield ratio (YR) was more than 66%.
• the Si content was less than 1.0 mass%, where the volume fraction of retained austenite was less than 5%, and where the number of grains of retained austenite existing in an area of 1000 ⁇ m 2 was less than 10, the elongation (EL) was less than 19%.
• the Mn content was more than 2.5 mass%, where the average crystal grain diameter of retained austenite was more than 2 ⁇ m, where the volume fraction of tempered martensite was more than 60%, and where the average crystal grain diameter of tempered martensite was more than 2 ⁇ m, the elongation (EL) was less than 19%, the yield ratio (YR) was more than 66%, and the hole expansion ratio ( ⁇ ) was less than 30%.

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