HIGH-STRENGTH COLD-ROLLED STEEL SHEET HAVING EXCELLENT FORMABILITY AND COLLISION CHARACTERISTICS AND HAVING TENSILE STRENGTH OF 980 MPa OR MORE, AND METHOD FOR PRODUCING SAME

Abstract

A high-strength cold-rolled steel sheet, in which the metal structure at a position of ¼ of the sheet thickness satisfies (1) to (4) below. (1) The area ratio of ferrite is 0% or more and 10% or less, with the balance being a hard phase including quenched martensite and retained austenite and including at least one selected from the group consisting of bainitic ferrite, bainite, and tempered martensite. (2) The volume ratio V γ of retained austenite is 5% to 30%. (3) The area ratio V MA of an MA structure in which quenched martensite and retained austenite are combined is 3% to 25%, and the average circle-equivalent diameter of the MA structure is 2.0 μm or less. (4) The ratio V MA /V γ of the area ratio V MA of the MA structure to the volume ratio V γ of the retained austenite is 0.50 to 1.50.

1 . A cold-rolled steel sheet having a tensile strength of 980 MPa or more, the cold-rolled steel sheet comprising, in mass %:
 C: 0.10% or more to 0.5% or less,   Si: 1.0% or more to 3% or less,   Mn: 1.5% or more to 7% or less,   P: more than 0% to 0.1% or less,   S: more than 0% to 0.05% or less,   Al: 0.005% or more to 1% or less,   N: more than 0% to 0.01% or less, and   O: more than 0% to 0.01% or less,   wherein   a metal structure at a position of ¼ of a sheet thickness satisfies (1) to (4) below:   (1) when the metal structure is observed with a scanning electron microscope, an area ratio of ferrite relative to a whole of the metal structure is 0% or more to 10% or less, with a balance being a hard phase including quenched martensite and retained austenite and including at least one selected from the group consisting of bainitic ferrite, bainite, and tempered martensite,   (2) when the metal structure is measured by X-ray diffractometry, a volume ratio V γ  of retained austenite relative to the whole of the metal structure is 5% or more to 30% or less,   (3) when the metal structure is observed with an optical microscope, an area ratio V MA  of an MA structure, in which quenched martensite and retained austenite are combined, relative to the whole of the metal structure is 3% or more to 25% or less, and an average circle-equivalent diameter of the MA structure is 2.0 μm or less, and   (4) a ratio V MA /V γ  of the area ratio V MA  of the MA structure to the volume ratio V γ  of the retained austenite satisfies a formula (i) below:
   0.50≦ V   MA   /V   γ ≦1.50   (i).
 
   
     
     
         2 . The cold-rolled steel sheet according to  claim 1 , further comprising, as other elements, one or more of any of (a) to (e) below, in mass %:
 (a) at least one selected from the group consisting of Cr: more than 0% to 1% or less and Mo: more than 0% to 1% or less,   (b) at least one selected from the group consisting of Ti: more than 0% to 0.15% or less, Nb: more than 0% to 0.15% or less, and V: more than 0% to 0.15% or less,   (c) at least one selected from the group consisting of Cu: more than 0% to 1% or less and Ni: more than 0% to 1% or less,   (d) B: more than 0% to 0.005% or less, and   (e) at least one selected from the group consisting of Ca: more than 0% to 0.01% or less, Mg: more than 0% to 0.01% or less, and REM: more than 0% to 0.01% or less.   
     
     
         3 . An electrogalvanized steel sheet having an electrogalvanized layer on a surface of the high-strength cold-rolled steel sheet according to  claim 1 . 
     
     
         4 . A hot-dip galvanized steel sheet having a hot-dip galvanized layer on a surface of the cold-rolled steel sheet according to  claim 1 . 
     
     
         5 . A hot-dip galvannealed steel sheet having a hot-dip galvannealed layer on a surface of the cold-rolled steel sheet according to  claim 1 . 
     
     
         6 . A method for producing a cold-rolled steel sheet according to  claim 1 , having a tensile strength of 980 MPa or more, the method comprising:
 hot rolling steel with a rolling rate at a final stand of finish rolling being 5 to 25% and with a finish rolling end temperature being an Ar 3  point or higher to 900° C. or lower, coiling the steel with a coiling temperature being 600° C. or lower, and cooling the steel to room temperature;   cold rolling the steel;   heating the steel, at an average heating rate of 10° C./second or more, to a temperature region of an Ac 3  point or higher, and soaking the steel while holding the steel in the temperature region for 50 seconds or more;   cooling the steel at an average cooling rate of 10° C./second or more, to an arbitrary cooling stop temperature T° C. that lies in a temperature range of 100° C. or higher and an Ms point or lower; and   heating and holding the steel in a temperature region of higher than the cooling stop temperature T° C. to 550° C. or lower for 50 seconds or more, and thereafter cooling the steel to room temperature.   
     
     
         7 . A method for producing a hot-dip steel sheet according to  claim 1 , having a tensile strength of 980 MPa or more and being excellent in formability and crashworthiness, the method comprising:
 hot rolling steel with a rolling rate at a final stand of finish rolling being 5 to 25% and with a finish rolling end temperature being an Ar 3  point or higher to 900° C. or lower, coiling the steel with a coiling temperature being 600° C. or lower, and cooling the steel to room temperature;   cold rolling the steel;   heating the steel, at an average heating rate of 10° C./second or more, to a temperature region of an Ac 3  point or higher, and soaking the steel while holding the steel in the temperature region for 50 seconds or more;   cooling the steel at an average cooling rate of 10° C./second or more, to an arbitrary cooling stop temperature T° C. that lies in a temperature range of 100° C. or higher and an Ms point or lower; and   heating and holding the steel in a temperature region of higher than the cooling stop temperature T° C. to 550° C. or lower for 50 seconds or more, and after performing hot-dip galvanizing within a holding time, cooling the steel to room temperature.   
     
     
         8 . A method for producing a hot-dip steel sheet according to  claim 1 , having a tensile strength of 980 MPa or more and being excellent in formability and crashworthiness, the method comprising:
 hot rolling steel with a rolling rate at a final stand of finish rolling being 5 to 25% and with a finish rolling end temperature being an Ar 3  point or higher to 900° C. or lower, coiling the steel with a coiling temperature being 600° C. or lower, and cooling the steel to room temperature   cold rolling the steel;   heating the steel, at an average heating rate of 10° C./second or more, to a temperature region of an Ac 3  point or higher, and soaking the steel while holding the steel in the temperature region for 50 seconds or more;   cooling the steel at an average cooling rate of 10° C./second or more, to an arbitrary cooling stop temperature T° C. that lies in a temperature range of 100° C. or higher and an Ms point or lower; and   heating and holding the steel in a temperature region of higher than the cooling stop temperature T° C. to 550° C. or lower for 50 seconds or more, and after performing hot-dip galvanizing within a holding time, further performing an alloying treatment and thereafter cooling the steel to room temperature.