High-strength steel sheet and method for manufacturing same

Abstract

A high-strength steel sheet comprising, as component composition, by mass: C: 0.15 to 0.35%; total of Si and Al: 0.5 to 2.5%; Mn: 1.0 to 4.0%; P: more than 0% and 0.05% or less; and S: more than 0% and 0.01% or less, with the balance being Fe and inevitable impurities, wherein a steel structure satisfies, in ratio with respect to the whole structure: ferrite: more than 5 area % and 50 area % or less; total of tempered martensite and bainite: 30 area % or more; and retained austenite: 10 volume % or more, the steel structure further includes MA, and the steel structure satisfies: an average circle equivalent diameter of the MA: 1.0 μm or less; an average circle equivalent diameter of the retained austenite: 1.0 μm or less; and a volume ratio of retained austenite with a circle equivalent diameter of 1.5 μm or more to the whole retained austenite: 5% or more.

1 . A high-strength steel sheet comprising a component composition comprising Fe and, by mass:
 C: 0.15 to 0.35%;   a total of Si and Al: 0.5 to 2.5%;   Mn: 1.0 to 4.0%;   P: more than 0% and 0.05% or less; and   S: more than 0% and 0.01% or less,   wherein the component composition forms a steel structure comprising:   ferrite: more than 5 area % and 50 area % or less;   a total of tempered martensite and bainite: 30 area % or more; and   a total retained austenite: 10 volume % or more,   wherein the steel structure further comprises a martensite-austenite composite, and   wherein the steel structure satisfies:   an average circle equivalent diameter of the martensite-austenite composite: 1.0 μm or less;   an average circle equivalent diameter of the retained austenite: 1.0 μm or less; and   a volume ratio of a retained austenite with a circle equivalent diameter of 1.5 μm or more to the total retained austenite: 5% or more.   
     
     
         2 . The high-strength steel sheet of  claim 1 , wherein the amount of C in the component composition is 0.30% or less by mass. 
     
     
         3 . The high-strength steel sheet of  claim 1 , wherein the amount of Al in the component composition is less than 0.10% by mass. 
     
     
         4 . The high-strength steel sheet of  claim 1 , further comprising at least one of, by mass:
 (a) at least one selected from the group consisting of Cu, Ni, Mo, Cr and B, in a total amount of more than 0% and 1.0% or less;   (b) at least one selected from the group consisting of V, Nb, Ti, Zr and Hf, in a total amount of more than 0% and 0.2% or less; and   (c) at least one selected from the group consisting of Ca, Mg and REM, in a total amount of more than 0% and 0.01% or less.   
     
     
         5 . A method for manufacturing the high-strength steel sheet of  claim 1 , the method comprising (i)-(v) in order:
 (i) heating an original sheet comprising the component composition of  claim 1 , wherein the original sheet has an Ac 1  point and an Ac 3  point, to a temperature in a range of T1 or higher to lower than the Ac 3  point, wherein T1 satisfies:
   T1=the Ac 1  point×0.8+the Ac 3  point×0.2;
 
   (ii) rapidly cooling from a temperature T2 of 650° C. or more to a temperature T3a of 300 to 500° C. at an average cooling rate of 30° C./s or more and less than 200° C./s;   (iii) gradually cooling from T3a to a temperature T3b not less than 300° C. at an average cooling rate of 0° C./s or more and 10° C./s or less for 10 seconds or more and less than 300 seconds;   (iv) cooling from a temperature T3b to a temperature T4 of 100 to 300° C. at an average cooling rate of more than 10° C./s; and   (v) reheating to a temperature T5 of 300 to 500° C.   
     
     
         6 . The method of  claim 5 , wherein (iii) further comprises retaining a constant temperature of 300 to 500° C. 
     
     
         7 . A method for manufacturing the high-strength steel sheet of  claim 2 , the method comprising (i)-(v) in order:
 (i) heating an original sheet comprising the component composition of  claim 2 , wherein the original sheet has an Ac 1  point and an Ac 3  point, to a temperature in a range of T1 or higher to lower than the Ac 3  point, wherein T1 satisfies:
   T1=the Ac 1  point×0.8+the Ac 3  point×0.2;
 
   (ii) rapidly cooling from a temperature T2 of 650° C. or more to a temperature T3a \of 300 to 500° C. at an average cooling rate of 30° C./s or more and less than 200° C./s;   (iii) gradually cooling from T3a to a temperature T3b not less than 300° C. at an average cooling rate of 0° C./s or more and 10° C./s or less for 10 seconds or more and less than 300 seconds;   (iv) cooling from a temperature T3b to a temperature T4 of 100 to 300° C. at an average cooling rate of more than 10° C./s; and   (v) reheating to a temperature T5 of 300 to 500° C.   
     
     
         8 . A method for manufacturing the high-strength steel sheet of  claim 3 , the method comprising (i)-(v) in order:
 (i) heating an original sheet comprising the component composition of  claim 3 , wherein the original sheet has an Ac 1  point and an Ac 3  point, to a temperature in a range of T1 or higher to lower than the Ac 3  point, wherein T1 satisfies:
   T1=the Ac 1  point×0.8+the Ac 3  point×0.2;
 
   (ii) rapidly cooling from a temperature T2 of 650° C. or more to a temperature T3a of 300 to 500° C. at an average cooling rate of 30° C./s or more and less than 200° C./s;   (iii) gradually cooling from T3a to a temperature T3b not less than 300° C. at an average cooling rate of 0° C./s or more and 10° C./s or less for 10 seconds or more and less than 300 seconds;   (iv) cooling from a temperature T3b to a temperature T4 of 100 to 300° C. at an average cooling rate of more than 10° C./s; and   (v) reheating to a temperature T5 of 300 to 500° C.   
     
     
         9 . A method for manufacturing the high-strength steel sheet of  claim 4 , the method comprising (i)-(v) in order:
 (i) heating an original sheet comprising the component composition of  claim 4 , wherein the original sheet has an Ac 1  point and an Ac 3  point, to a temperature in a range of T1 or higher to lower than the Ac 3  point, wherein T1 satisfies:
   T1=the Ac 1  point×0.8+the Ac 3  point×0.2;
 
   (ii) rapidly cooling from a temperature T2 of 650° C. or more to a temperature T3a of 300 to 500° C. at an average cooling rate of 30° C./s or more and less than 200° C./s;   (iii) gradually cooling from T3a to a temperature T3b not less than 300° C. at an average cooling rate of 0° C./s or more and 10° C./s or less for 10 seconds or more and less than 300 seconds;   (iv) cooling from a temperature T3b to a temperature T4 of 100 to 300° C. at an average cooling rate of more than 10° C./s; and   (v) reheating to a temperature T5 of 300 to 500° C.   
     
     
         10 . The method of  claim 7 , wherein (iii) further comprises retaining a constant temperature of 300 to 500° C. 
     
     
         11 . The method of  claim 8 , wherein (iii) further comprises retaining a constant temperature of 300 to 500° C. 
     
     
         12 . The method of  claim 9 , wherein (iii) further comprises retaining a constant temperature of 300 to 500° C.