Highly formable high-strength steel sheet, warm working method, and warm-worked automobile part

Abstract

A high-strength steel sheet has a chemical composition including 0.05% to 0.3% of C, 1% to 3% of Si, 0.5% to 3% of Mn, 0% to 0.1% of P, 0% to 0.01% of S, 0.001% to 0.1% of Al, and 0.002% to 0.03% of N in mass percent; further includes iron and impurities; and has a structure including 50% to 90% of bainitic ferrite, 3% or more of retained austenite (γ R ), a total of 10% to 50% of martensite and the retained austenite, and 0% to 40% of polygonal ferrite in area percent based on the entire structure. The retained austenite has a carbon content (Cγ R ) of 0.5% to 1.2% by mass. The steel sheet includes 0.3% or more of the retained austenite that is surrounded by martensite. The high-strength steel sheet has a strength of 980 MPa or more and exhibits better ductility.

1 . A highly formable high-strength steel sheet having a chemical composition comprising:
 C in a content of from 0.05% to 0.3%;   Si in a content of from 1% to 3%;   Mn in a content of from 0.5% to 3%;   P in a content of from 0% to 0.1%;   S in a content of from 0% to 0.01%;   Al in a content of from 0.001% to 0.1%; and   N in a content of from 0.002% to 0.03%,   in mass percent,   wherein the steel sheet further comprises iron and impurities;   the steel sheet has a structure comprising bainitic ferrite in a content of from 50% to 90%, retained austenite in a content of from 3% or more, a total of martensite and retained austenite in a content of from 10% to 50%, and polygonal ferrite in a content of from 0% to 40%, in area percent, based on an entire structure area;   the retained austenite has a carbon content (Cγ R ) of from 0.5% to 1.2% in mass percent; and   the steel sheet comprises 0.3% or more, in area percent based on the entire structure, of retained austenite that is substantially surrounded by martensite.   
     
     
         2 . The steel sheet of  claim 1 , further comprising, as a chemical composition:
 Cr in a content of from 0.01% to 3%;   Mo in a content of from 0.01% to 1%;   Cu in a content of from 0.01% to 2%;   Ni in a content of from 0.01% to 2%;   B in a content of from 0.00001% to 0.01%;   or any combination thereof.   
     
     
         3 . The highly formable high-strength steel sheet of  claim 1 , further comprising, as a chemical composition:
 Ca in a content of from 0.0005% to 0.01%;   Mg in a content of from 0.0005% to 0.01%;   a rare-earth element (REM) in a content of 0.0001% to 0.01%;   or any combination thereof.   
     
     
         4 . A method of warm-working a high-strength steel sheet, the method comprising:
 heating the high-strength steel sheet of  claim 1  up to a temperature of from 200° C. to 400° C.; and   working the high-strength steel sheet within 3600 seconds after the heating.   
     
     
         5 . An automobile part, comprising:
 a mixture of at least one region receiving a true strain of 0.05 or more and at least one region receiving a true strain of less than 0.05 upon working,   wherein a difference in yield strength between a region receiving a maximum true strain and a region receiving a minimum true strain upon working is 200 MPa or less, and   wherein the automobile part is obtained by a process comprising the method of  claim 4 .   
     
     
         6 . The sheet of  claim 2 , further comprising, as a chemical composition:
 Ca in a content of from 0.0005% to 0.01%;   Mg in a content of from 0.0005% to 0.01%;   a rare-earth element in a content of from 0.0001% to 0.01%;   or any combination thereof