High-strength steel sheet and production method therefor

Abstract

A high-strength steel sheet having a TS of 780 MPa or more, not only excellent ductility but also excellent stretch flangeability, and excellent in-plane anisotropy of TS is provided. A high-strength steel sheet comprises: a predetermined chemical composition; a steel microstructure including, in area fraction, ferrite: 20% or more and 50% or less, upper bainite: 5% or more and 45% or less, and martensite: 1% or more and 20% or less, and including, in volume fraction, retained austenite: 5% or more, the retained austenite having an average grain size of 2 μm or less; and a texture having an inverse intensity ratio of γ-fiber to α-fiber of 3.0 or less.

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: 1.50% 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, with the balance being Fe and inevitable impurities; 
   a steel microstructure including, in area fraction,
 ferrite: 20% or more and 50% or less, 
 upper bainite: 5% or more and 45% or less, and 
 martensite: 1% or more and 20% or less, and 
   including, in volume fraction,
 retained austenite: 5% or more, the retained austenite having an average grain size of 2 μm or less; and 
   a texture having an inverse intensity ratio of γ-fiber to α-fiber of 3.0 or less.   
     
     
         2 . The high-strength steel sheet according to  claim 1 ,
 wherein the chemical composition further contains, in mass %, 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, 
 V: 0.005% or more and 0.100% or less, 
 B: 0.0001% or more and 0.0050% 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. 
   
     
     
         3 . A production method for the high-strength steel sheet according to  claim 1 , the production method comprising:
 heating a steel slab having the chemical composition according to  claim 1  to 1100° C. or more and 1300° C. or less;   hot rolling the steel slab at a finisher delivery temperature of 800° C. or more and 1000° C. or less, to obtain a hot-rolled sheet;   coiling the hot-rolled sheet at a coiling temperature of 300° C. or more and 700° C. or less;   subjecting the hot-rolled sheet to pickling treatment;   thereafter optionally holding the hot-rolled sheet in a temperature range of 450° C. or more and 800° C. or less for a time of 900 s or more and 36000 s or less;   thereafter cold rolling the hot-rolled sheet with a rolling reduction of 30% or more, to obtain a cold-rolled sheet;   thereafter subjecting the obtained cold-rolled sheet to first annealing treatment of T 1  temperature or more and 950° C. or less;   thereafter cooling the cold-rolled sheet at an average cooling rate of 5° C./s or more at least to (T 2  temperature+100° C.);   thereafter cooling the cold-rolled sheet to room temperature;   thereafter reheating the cold-rolled sheet to a temperature range of 740° C. or more and the T 1  temperature or less to perform second annealing treatment;   thereafter cooling the cold-rolled sheet to the T 2  temperature or more and (the T 2  temperature+150° C.) or less at a first average cooling rate of 8° C./s or more as primary cooling;   thereafter cooling the cold-rolled sheet to a cooling end temperature at a second average cooling rate of 3° C./s or more as secondary cooling, the cooling end temperature being 150° C. or more and (the T 2  temperature−10° C.) or less, wherein an average cooling rate after the second annealing treatment is higher in the primary cooling than in the secondary cooling;   thereafter reheating the cold-rolled sheet to a reheating temperature range that is (the T 2  temperature−50° C.) or more and (the T 2  temperature+50° C.) or less and is (the cooling end temperature+5° C.) or more; and   holding the cold-rolled sheet in the reheating temperature range for a time of 10 s or more,   wherein the T 1  temperature in ° C.=946−203×[% C] 1/2 +45×[% Si]−30×[% Mn]+150×[% Al]−20×[% Cu]+11×[% Cr]+400×[% Ti], and   the T 2  temperature in ° C.=740−490×[% C]−100×[% Mn]−70×[% Cr],   where [% X] denotes a content of an element X in the steel sheet in mass %, and is 0 for any element not contained in the steel sheet.   
     
     
         4 . A high-strength galvanized steel sheet comprising:
 the high-strength steel sheet according to  claim 1 ; and   a galvanized layer on a surface of the high-strength steel sheet.   
     
     
         5 . A production method for the high-strength steel sheet according to  claim 2 , the production method comprising:
 heating a steel slab having the chemical composition according to  claim 2  to 1100° C. or more and 1300° C. or less;   hot rolling the steel slab at a finisher delivery temperature of 800° C. or more and 1000° C. or less, to obtain a hot-rolled sheet;   coiling the hot-rolled sheet at a coiling temperature of 300° C. or more and 700° C. or less;   subjecting the hot-rolled sheet to pickling treatment;   thereafter optionally holding the hot-rolled sheet in a temperature range of 450° C. or more and 800° C. or less for a time of 900 s or more and 36000 s or less;   thereafter cold rolling the hot-rolled sheet with a rolling reduction of 30% or more, to obtain a cold-rolled sheet;   thereafter subjecting the obtained cold-rolled sheet to first annealing treatment of T 1  temperature or more and 950° C. or less;   thereafter cooling the cold-rolled sheet at an average cooling rate of 5° C./s or more at least to (T 2  temperature+100° C.);   thereafter cooling the cold-rolled sheet to room temperature;   thereafter reheating the cold-rolled sheet to a temperature range of 740° C. or more and the T 1  temperature or less to perform second annealing treatment;   thereafter cooling the cold-rolled sheet to the T 2  temperature or more and (the T 2  temperature+150° C.) or less at a first average cooling rate of 8° C./s or more as primary cooling;   thereafter cooling the cold-rolled sheet to a cooling end temperature at a second average cooling rate of 3° C./s or more as secondary cooling, the cooling end temperature being 150° C. or more and (the T 2  temperature−10° C.) or less, wherein an average cooling rate after the second annealing treatment is higher in the primary cooling than in the secondary cooling;   thereafter reheating the cold-rolled sheet to a reheating temperature range that is (the T 2  temperature−50° C.) or more and (the T 2  temperature+50° C.) or less and is (the cooling end temperature+5° C.) or more; and   holding the cold-rolled sheet in the reheating temperature range for a time of 10 s or more,   wherein the T 1  temperature in ° C.=946−203×[% C] 1/2 +45×[% Si]−30×[% Mn]+150×[% Al]−20×[% Cu]+11×[% Cr]+400×[% Ti], and   the T 2  temperature in ° C.=740−490×[% C]−100×[% Mn]−70×[% Cr],   where [% X] denotes a content of an element X in the steel sheet in mass %, and is 0 for any element not contained in the steel sheet.   
     
     
         6 . A high-strength galvanized steel sheet comprising:
 the high-strength steel sheet according to  claim 2 ; and   a galvanized layer on a surface of the high-strength steel sheet.