High-strength cold rolled steel sheet and method of producing same

Abstract

A high-strength cold rolled steel sheet has both high strength, i.e. a tensile strength of 900 MPa or more, and high workability, i.e. a total elongation of 20% or more, and also has a high yield ratio of 60% or more. The high-strength cold rolled steel sheet comprises: a predetermined chemical composition; and a steel structure having, in area ratio, ferrite: 30% to 75%, martensite: 15% to 40%, and retained austenite: 10% to 30%. In the case where a distribution of crystal grain circularity of the retained austenite is represented by a histogram with a class range of more than 0.1×(n−1) and 0.1×n or less and a class value of 0.1×n (n is an integer from 1 to 10), a mode of the crystal grain circularity of the retained austenite is 0.6 or less.

1 . A high-strength cold rolled steel sheet comprising:
 a chemical composition containing, in mass %,   C: 0.15% to 0.35%,   Si: 1.0% to 2.0%,   Mn: 1.8% to 3.5%,   P: 0.020% or less,   S: 0.0040% or less,   Al: 0.01% to 0.1%, and   N: 0.01% or less,   with a balance consisting of Fe and inevitable impurities; and   a steel structure having, in area ratio,   ferrite: 30% to 75%,   martensite: 15% to 40%, and   retained austenite: 10% to 30%,   wherein in the case where a distribution of crystal grain circularity of the retained austenite is represented by a histogram with a class range of more than 0.1×(n−1) and 0.1×n or less and a class value of 0.1×n where n is an integer from 1 to 10, a mode of the crystal grain circularity of the retained austenite is 0.6 or less, and   circularity of each crystal grain of the retained austenite is calculated according to the following formula:
   circularity=4π S/L   2  
 
   where S is an area of the crystal grain of the retained austenite, and L is a perimeter of the crystal grain of the retained austenite.   
     
     
         2 . The high-strength cold rolled steel sheet according to  claim 1 , comprising
 a galvanized layer on a surface thereof.   
     
     
         3 . A method of producing a high-strength cold rolled steel sheet, the method comprising:
 heating a steel slab to 1100° C. or more and 1200° C. or less, the steel slab containing, in mass %,   C: 0.15% to 0.35%,   Si: 1.0% to 2.0%,   Mn: 1.8% to 3.5%,   P: 0.020% or less,   S: 0.0040% or less,   Al: 0.01% to 0.1%, and   N: 0.01% or less,   with a balance consisting of Fe and inevitable impurities;   thereafter hot rolling the steel slab at a finisher delivery temperature of 850° C. or more and 950° C. or less, to obtain a hot rolled sheet;   cooling the hot rolled sheet, with a cooling rate in a temperature range from the finisher delivery temperature to 700° C. of 50° C./s or more;   coiling the hot rolled sheet at 300° C. or more and less than 550° C.;   thereafter cooling the hot rolled sheet to 100° C. or less by water cooling, after the coiling;   pickling the hot rolled sheet;   thereafter cold rolling the hot rolled sheet to obtain a cold rolled sheet;   annealing the cold rolled sheet at 730° C. or more and 820° C. or less;   thereafter cooling the cold rolled sheet to a temperature range of 300° C. or more and 500° C. or less;   
       and
 performing an overaging treatment of holding the cold rolled sheet in the temperature range of 300° C. or more and 500° C. or less for 100 s or more and 1000 s or less. 
 
     
     
         4 . The method of producing a high-strength cold rolled steel sheet according to  claim 3 , comprising
 performing a galvanizing treatment after the overaging treatment.