High-strength steel sheet and production method for same, and production method for high-strength galvanized steel sheet

Abstract

Disclosed is a high-strength steel sheet having a predetermined chemical composition and a steel microstructure that contains, by area, 25-80% of ferrite and bainitic ferrite in total, and 3-20% of martensite, and that contains, by volume, 10% or more of retained austenite, in which the retained austenite has a mean grain size of 2 μm or less, a mean Mn content in the retained austenite in mass % is at least 1.2 times the Mn content in the steel sheet in mass %, an area ratio of retained austenite having a mean C content in mass % at least 2.1 times the C content in the steel sheet in mass % is 60% or more of an area ratio of the entire retained austenite.

1 . A high-strength steel sheet comprising:
 a chemical composition containing, in mass %, C: 0.08% or more and 0.35% or less, Si: 0.50% or more and 2.50% or less, Mn: 1.60% or more and 3.00% or less, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, and N: 0.0005% or more and 0.0100% or less, and optionally at least one element selected from the group consisting of Al: 0.01% or more and 1.00% or less, Ti: 0.005% or more and 0.100% or less, Nb: 0.005% or more and 0.100% or less, Cr: 0.05% or more and 1.00% or less, Cu: 0.05% or more and 1.00% or less, Sb: 0.0020% or more and 0.2000% or less, Sn: 0.0020% or more and 0.2000% or less, Ta: 0.0010% or more and 0.1000% or less, Ca: 0.0003% or more and 0.0050% or less, Mg: 0.0003% or more and 0.0050% or less, and REM: 0.0003% or more and 0.0050% or less, and the balance consisting of Fe and incidental impurities;   a steel microstructure that contains, by area, 25% or more and 80% or less of ferrite and bainitic ferrite in total, and 3% or more and 20% or less of martensite, and that contains, by volume, 10% or more of retained austenite, wherein   the retained austenite has a mean grain size of 2 μm or less,   a mean Mn content in the retained austenite in mass % is at least 1.2 times the Mn content in the steel sheet in mass %, and   an area ratio of retained austenite having a mean C content in mass % at least 2.1 times the C content in the steel sheet in mass % is 60% or more of an area ratio of the entire retained austenite.   
     
     
         2 . (canceled) 
     
     
         3 . A production method for a high-strength steel sheet, the method comprising:
 heating a steel slab having the chemical composition as recited in  claim 1  to 1100° C. or higher and 1300° C. or lower;   hot rolling the steel slab with a finisher delivery temperature of 800° C. or higher and 1000° C. or lower to obtain a steel sheet;   coiling the steel sheet at a mean coiling temperature of 450° C. or higher and 700° C. or lower;   subjecting the steel sheet to pickling treatment;   optionally, retaining the steel sheet at a temperature of 450° C. or higher and Ac 1  transformation temperature or lower for 900 s or more and 36000 s or less;   cold rolling the steel sheet at a rolling reduction of 30% or more;   subjecting the steel sheet to first annealing treatment whereby the steel sheet is heated to a temperature of 820° C. or higher and 950° C. or lower;   cooling the steel sheet to a first cooling stop temperature at or below Ms at a mean cooling rate to 500° C. of 15° C./s or higher;   subjecting the steel sheet to second annealing treatment whereby the steel sheet is reheated to a temperature of 740° C. or higher and 840° C. or lower;   cooling the steel sheet to a temperature in a second cooling stop temperature range of 300° C. to 550° C. at a mean cooling rate of 10° C./s or higher and 50° C./s or lower; and   retaining the steel sheet at the second cooling stop temperature range for 10 s or more, to produce the high-strength steel sheet as recited in  claim 1 .   
     
     
         4 . The production method for a high-strength steel sheet according to  claim 3 , the method further comprising after the retaining at the second cooling stop temperature range, subjecting the steel sheet to third annealing treatment whereby the steel sheet is heated to a temperature of 100° C. or higher and 300° C. or lower. 
     
     
         5 . A production method for a high-strength galvanized steel sheet, the method comprising subjecting the high-strength steel sheet as recited in  claim 1  to galvanizing treatment.