High-strength steel sheet and method for manufacturing the same

Abstract

The high-strength steel sheet has a predetermined chemical composition and has a microstructure containing a specific microstructure in a surface layer region and in an inner region. The surface layer region has an average grain size of 6 μm or less. A difference between a hardness of the surface layer region extending from the surface of the steel sheet to the position of one-tenth of the thickness of the steel sheet and a hardness of the inner region is 5% or more and 15% or less of [0.3×tensile strength (MPa)], and the steel sheet has a tensile strength of 980 MPa or more, a uniform elongation of 6% or more, and a ratio of a critical bending radius to a thickness of 1.5 or less.

1 . A high-strength steel sheet comprising:
 a chemical composition containing, in mass %:   C: 0.05% to 0.20%,   Si: 0.5% to 1.2%,   Mn: 1.5% to 4.0%,   P: 0.10% or less,   S: 0.03% or less,   Al: 0.001% to 2.0%,   N: 0.01% or less,   O: 0.01% or less,   B: 0.0005% to 0.010%, and the remainder being Fe and incidental impurities;   a microstructure in a surface layer region extending from a surface of the steel sheet to a position of one-tenth of a thickness of the steel sheet containing 80% by area or more of upper bainite and 2% by area or more of fresh martensite and/or retained austenite in total; and   a microstructure in an inner region extending from the position of one-tenth to a position of three-tenths of the thickness of the steel sheet containing 70% by area or more of upper bainite and 3% by area or more of fresh martensite and/or retained austenite in total,   wherein:   the surface layer region extending from the surface of the steel sheet to the position of one-tenth of the thickness of the steel sheet has an average grain size of 6 μm or less;   the steel sheet has a tensile strength of 980 MPa or more, a uniform elongation of 6% or more, and a ratio R/t of a critical bending radius R to a thickness t of 1.5 or less; and   a difference (HV2−HV1) between a hardness (HV1) of the surface layer region extending from the surface of the steel sheet to the position of one-tenth of the thickness of the steel sheet and a hardness (HV2) of the inner region extending from the position of one-tenth to the position of three-tenths of the thickness of the steel sheet is 5% or more and 15% or less of [0.3×tensile strength (MPa)].   
     
     
         2 . The high-strength steel sheet according to  claim 1 , wherein
 the area of fresh martensite and/or retained austenite in total in the surface layer region is smaller than the area of fresh martensite and/or retained austenite in total in the inner region.   
     
     
         3 . The high-strength steel sheet according to  claim 1 , wherein the chemical composition further contains at least one selected from following groups A to D consisting of:
 Group A: in mass %, at least one of
 Cr: 1:0% or less and 
 Mo: 1.0% or less; 
   Group B: in mass %, at least one of   Cu: 2.0% or less,   Ni: 2.0% or less,   Ti: 0.3% or less,   Nb: 0.3% or less, and   V: 0.3% or less;   Group C: in mass %,
 Sb: 0.005% to 0.020%; and 
   Group D: in mass %, at least one of
 Ca: 0.01% or less, 
 Mg: 0.01% or less, and 
 REM: 0.01% or less. 
   
     
     
         4 . The high-strength steel sheet according to  claim 2 , wherein the chemical composition further contains at least one selected from following groups A to D consisting of:
 Group A: in mass %, at least one of
 Cr: 1.0% or less and 
 Mo: 1.0% or less; 
   Group B: in mass %, at least one of
 Cu: 2.0% or less, 
 Ni: 2.0% or less, 
 Ti: 0.3% or less, 
 Nb: 0.3% or less, and 
 V: 0.3% or less; 
   Group C: in mass %,
 Sb: 0.005% to 0.020%; and 
   Group D: in mass %, at least one of
 Ca: 0.01% or less, 
 Mg: 0.01% or less, and 
 REM: 0.01% or less. 
   
     
     
         5 . (canceled) 
     
     
         6 . A method for manufacturing the high-strength steel sheet according to  claim 1 , comprising:
 heating a steel material having the chemical composition to a heating temperature of 1150° C. or more;   then hot-rolling including rough rolling and finish rolling the steel material into a hot-rolled steel sheet, the finish rolling being performed under conditions of a finishing temperature: (RC2−50° C.) or more and (RC2+120° C.) or less and a total rolling reduction of 25% or more and 80% or less at a temperature of RC1 or less;   cooling the hot-rolled steel sheet under conditions of a time from completion of the hot rolling to start of cooling: 2.0 seconds or less, an average cooling rate at the position of three-tenths of the thickness of the steel sheet: 15° C./s or more, and a cooling stop temperature: Trs or more and (Trs+250° C.) or less;   coiling the hot-rolled steel sheet after the cooling at a coiling temperature: Trs or more and (Trs+250° C.) or less; and   cooling the hot-rolled steel sheet to 100° C. or less at an average cooling rate of 20° C./s or less,   wherein RC1, RC2, and Trs are represented by the following formulae (1), (2), and (3), respectively,
     RC 1(° C.)=900+100× C+ 100× N+ 10× Mn+ 700× Ti+ 5000× B+ 10× Cr+ 50× Mo+ 2000× Nb+ 150× V   (1)
 
     RC 2(° C.)=750+100× C+ 100× N+ 10× Mn+ 350× Ti+ 5000× B+ 10× Cr+ 50× Mo+ 1000× Nb+ 150× V   (2)
 
     Trs (° C.)=500−450× C— 35× Mn− 15× Cr— 10× Ni− 20× Mo   (3)
 
   where each element symbol in the formulae (1), (2), and (3) denotes a corresponding element content (% by mass) and is 0 in the absence of the element.   
     
     
         7 . The method for manufacturing the high-strength steel sheet according to  claim 6 , wherein in the cooling after the hot rolling, an average cooling rate of the surface layer and the average cooling rate at the position of three-tenths of the thickness of the steel sheet satisfy the formula (4):
   Average cooling rate of surface layer−average cooling rate at position of three-tenths of thickness of steel sheet≥10° C./s  (4).
   
     
     
         8 . A method for manufacturing the high-strength steel sheet according to  claim 2 , comprising:
 heating a steel material having the chemical composition to a heating temperature of 1150° C. or more;   then hot rolling including rough rolling and finish rolling the steel material into a hot-rolled steel sheet, the finish rolling being performed under conditions of a finishing temperature: (RC2−50° C.) or more and (RC2+120° C.) or less and a total rolling reduction of 25% or more and 80% or less at a temperature of RC1 or less;   cooling the hot-rolled steel sheet under conditions of a time from completion of the hot rolling to start of cooling: 2.0 seconds or less, an average cooling rate at the position of three-tenths of the thickness of the steel sheet: 15° C./s or more, and a cooling stop temperature: Trs or more and (Trs+250° C.) or less;   coiling the hot-rolled steel sheet after the cooling at a coiling temperature: Trs or more and (Trs+250° C.) or less; and   cooling the hot-rolled steel sheet to 100° C. or less at an average cooling rate of 20° C./s or less,   wherein RC1, RC2, and Trs are represented by the following formulae (1), (2), and (3), respectively,
     RC 1(° C.)=900+100× C+ 100× N+ 10× Mn+ 700× Ti+ 5000× B+ 10× Cr+ 50× Mo+ 2000× Nb+ 150× V   (1)
 
     RC 2(° C.)=750+100× C+ 100× N+ 10× Mn+ 350× Ti+ 5000× B+ 10× Cr+ 50× Mo+ 1000× Nb+ 150× V   (2)
 
     Trs (° C.)=500−450× C— 35× Mn− 15× Cr— 10× Ni− 20× Mo   (3)
 
   where each element symbol in the formulae (1), (2), and (3) denotes a corresponding element content (% by mass) and is 0 in the absence of the element.   
     
     
         9 . The method for manufacturing the high-strength steel sheet according to  claim 8 , wherein in the cooling after the hot rolling, an average cooling rate of the surface layer and the average cooling rate at the position of three-tenths of the thickness of the steel sheet satisfy the formula (4):
   Average cooling rate of surface layer−average cooling rate at position of three-tenths of thickness of steel sheet≥10° C./s  (4).
   
     
     
         10 . A method for manufacturing the high-strength steel sheet according to  claim 3 , comprising:
 heating a steel material having the chemical composition to a heating temperature of 1150° C. or more;   then hot rolling including rough rolling and finish rolling the steel material into a hot-rolled steel sheet, the finish rolling being performed under conditions of a finishing temperature: (RC2−50° C.) or more and (RC2+120° C.) or less and a total rolling reduction of 25% or more and 80% or less at a temperature of RC1 or less;   cooling the hot-rolled steel sheet under conditions of a time from completion of the hot rolling to start of cooling: 2.0 seconds or less, an average cooling rate at the position of three-tenths of the thickness of the steel sheet: 15° C./s or more, and a cooling stop temperature: Trs or more and (Trs+250° C.) or less;   coiling the hot-rolled steel sheet after the cooling at a coiling temperature: Trs or more and (Trs+250° C.) or less; and   cooling the hot-rolled steel sheet to 100° C. or less at an average cooling rate of 20° C./s or less,   wherein RC1, RC2, and Trs are represented by the following formulae (1), (2), and (3), respectively,
     RC 1(° C.)=900+100× C+ 100× N+ 10× Mn+ 700× Ti+ 5000× B+ 10× Cr+ 50× Mo+ 2000× Nb+ 150× V   (1)
 
     RC 2(° C.)=750+100× C+ 100× N+ 10× Mn+ 350× Ti+ 5000× B+ 10× Cr+ 50× Mo+ 1000× Nb+ 150× V   (2)
 
     Trs (° C.)=500−450× C— 35× Mn− 15× Cr— 10× Ni− 20× Mo   (3)
 
   where each element symbol in the formulae (1), (2), and (3) denotes a corresponding element content (% by mass) and is 0 in the absence of the element.   
     
     
         11 . The method for manufacturing the high-strength steel sheet according to  claim 10 , wherein in the cooling after the hot rolling, an average cooling rate of the surface layer and the average cooling rate at the position of three-tenths of the thickness of the steel sheet satisfy the formula (4):
   Average cooling rate of surface layer−average cooling rate at position of three-tenths of thickness of steel sheet≥10° C./s  (4).
   
     
     
         12 . A method for manufacturing the high-strength steel sheet according to  claim 4 , comprising:
 heating a steel material having the chemical composition to a heating temperature of 1150° C. or more;   then hot rolling including rough rolling and finish rolling the steel material into a hot-rolled steel sheet, the finish rolling being performed under conditions of a finishing temperature: (RC2−50° C.) or more and (RC2+120° C.) or less and a total rolling reduction of 25% or more and 80% or less at a temperature of RC1 or less;   cooling the hot-rolled steel sheet under conditions of a time from completion of the hot rolling to start of cooling: 2.0 seconds or less, an average cooling rate at the position of three-tenths of the thickness of the steel sheet: 15° C./s or more, and a cooling stop temperature: Trs or more and (Trs+250° C.) or less;   coiling the hot-rolled steel sheet after the cooling at a coiling temperature: Trs or more and (Trs+250° C.) or less; and   cooling the hot-rolled steel sheet to 100° C. or less at an average cooling rate of 20° C./s or less,   wherein RC1, RC2, and Trs are represented by the following formulae (1), (2), and (3), respectively,
     RC 1(° C.)=900+100× C+ 100× N+ 10× Mn+ 700× Ti+ 5000× B+ 10× Cr+ 50× Mo+ 2000× Nb+ 150× V   (1)
 
     RC 2(° C.)=750+100× C+ 100× N+ 10× Mn+ 350× Ti+ 5000× B+ 10× Cr+ 50× Mo+ 1000× Nb+ 150× V   (2)
 
     Trs (° C.)=500−450× C— 35× Mn− 15× Cr— 10× Ni− 20× Mo   (3)
 
   where each element symbol in the formulae (1), (2), and (3) denotes a corresponding element content (% by mass) and is 0 in the absence of the element.   
     
     
         13 . The method for manufacturing the high-strength steel sheet according to  claim 12 , wherein in the cooling after the hot rolling, an average cooling rate of the surface layer and the average cooling rate at the position of three-tenths of the thickness of the steel sheet satisfy the formula (4):
   Average cooling rate of surface layer−average cooling rate at position of three-tenths of thickness of steel sheet≥10° C./s  (4).