High-strength thin steel sheet and method of producing the same

Abstract

This disclosure provides a high-strength thin steel sheet excellent in both tensile strength and elongation with small elongation anisotropy. The high-strength thin steel sheet has a specific chemical composition and a microstructure where a total area ratio of ferrite, tempered bainitic ferrite and bainitic ferrite is 40% or more and 70% or less, an area ratio of martensite is 5% or more and 30% or less, an area ratio of retained austenite is 10% or more and 35% or less, an average equivalent circular diameter of martensite and retained austenite (secondary phase) grains is 2.0 μm or less, an area ratio of secondary phase grains having an equivalent circular diameter of 2.0 μm or more is 10% or less, and an average minor axis length of secondary phase grains is 0.40 μm or less.

1 - 7 . (canceled) 
     
     
         8 . A high-strength thin steel sheet comprising:
 a chemical composition containing, by mass %,
 C: 0.10% or more and 0.35% or less, 
 Si: 0.5% or more and 2.0% or less, 
 Mn: 1.5% or more and 3.0% or less, 
 P: 0.001% or more and 0.050% or less, 
 S: 0.0001% or more and 0.0100% or less, 
 Al: 0.001% or more and 1.00% or less, and 
 N: 0.0005% or more and 0.0200% or less, 
 the balance being Fe and inevitable impurities, and 
 a ratio of C content in mass % to Mn content in mass %, expressed as C/Mn, being 0.08 or more and 0.20 or less; and 
   a microstructure where
 a total area ratio of ferrite, tempered bainitic ferrite and bainitic ferrite is 40% or more and 70% or less, 
 an area ratio of martensite is 5% or more and 30% or less, and 
 an area ratio of retained austenite is 10% or more and 35% or less, to a total area of the microstructure of the high-strength thin steel sheet, 
 an average equivalent circular diameter of martensite and retained austenite grains is 2.0 μm or less, an area ratio of martensite and retained austenite grains having an equivalent circular diameter of 2.0 μm or more to a total area of martensite and retained austenite is 10% or less, and 
 an average minor axis length of martensite and retained austenite grains is 0.40 μm or less. 
   
     
     
         9 . The high-strength thin steel sheet according to  claim 8 , wherein
 the chemical composition further contains one or more selected from among group (i) to group (iii) below:   group (i): at least one selected from the group consisting of
 Ti: 0.005% or more and 0.100% or less, 
 Nb: 0.005% or more and 0.100% or less, and 
 V: 0.005% or more and 0.100% or less, in mass %; 
   group (ii): at least one selected from the group consisting of
 Cr: 0.05% or more and 1.0% or less, 
 Ni: 0.05% or more and 0.50% or less, 
 Mo: 0.05% or more and 1.0% or less, 
 Cu: 0.005% or more and 0.500% or less, and 
 B: 0.0001% or more and 0.0100% or less, in mass %; and 
   group (iii): at least one of
 Ca: 0.0001% or more and 0.0050% or less, and 
 REM: 0.0005% or more and 0.0050% or less, in mass %. 
   
     
     
         10 . The high-strength thin steel sheet according to  claim 8 , further comprising a zinc or zinc alloy coated layer on surface. 
     
     
         11 . The high-strength thin steel sheet according to  claim 9 , further comprising a zinc or zinc alloy coated layer on surface. 
     
     
         12 . A method of producing a high-strength thin steel sheet, comprising
 preparing a steel sheet having the chemical composition according to  claim 8  and a microstructure where a total area ratio of bainitic ferrite and retained austenite to a total area of the microstructure of the steel sheet is 70% or more,   heating the steel sheet to 700° C. at an average heating rate of 15° C./s or more,   heating the steel sheet from 700° C. to a soaking temperature of 740° C. or higher and 860° C. or lower at an average heating rate of 15° C./s or less,   holding the heated steel sheet at the soaking temperature for 60 seconds or more and 600 seconds or less,   cooling the steel sheet to a cooling end temperature of 350° C. or higher and 550° C. or lower at an average cooling rate of 50° C./s or less, and   holding the cooled steel sheet at the cooling end temperature for 30 seconds or more and 1200 seconds or less.   
     
     
         13 . A method of producing a high-strength thin steel sheet, comprising
 preparing a steel sheet having the chemical composition according to  claim 9  and a microstructure where a total area ratio of bainitic ferrite and retained austenite to a total area of the microstructure of the steel sheet is 70% or more,   heating the steel sheet to 700° C. at an average heating rate of 15° C./s or more,   heating the steel sheet from 700° C. to a soaking temperature of 740° C. or higher and 860° C. or lower at an average heating rate of 15° C./s or less,   holding the heated steel sheet at the soaking temperature for 60 seconds or more and 600 seconds or less,   cooling the steel sheet to a cooling end temperature of 350° C. or higher and 550° C. or lower at an average cooling rate of 50° C./s or less, and   holding the cooled steel sheet at the cooling end temperature for 30 seconds or more and 1200 seconds or less.   
     
     
         14 . The method of producing a high-strength thin steel sheet according to  claim 12 , further comprising subjecting the steel sheet after being held at the cooling end temperature to zinc or zinc alloy coating treatment. 
     
     
         15 . The method of producing a high-strength thin steel sheet according to  claim 13 , further comprising subjecting the steel sheet after being held at the cooling end temperature to zinc or zinc alloy coating treatment.