High-strength steel sheet with excellent deep drawability at room temperature and warm temperature, and method for warm working same
This high-strength steel sheet has a component composition containing, in mass %, 0.02 to 0.3% C, 1 to 3% Si, 1.8 to 3% Mn, 0.1% or less P, 0.01% or less S, 0.001 to 0.1% Al, and 0.002 to 0.03% N, the remainder being iron and impurities. The high-strength steel sheet has a structure containing, in terms of area ratio relative to the entire structure, each of the following phases: 50 to 85% bainitic ferrite; 3% or more retained austenite (γ); 10 to 45% martensite and the aforementioned retained austenite (γ); and 5 to 40% ferrite. The ratio between the Mn concentration (Mn γR ) in the retained austenite (γ) and the average Mn concentration (Mn av ) in the entire structure is 1.2 or more (Mn γR / Mn av ) based on the Mn concentration distribution obtained by means of EPMA line analysis. As a consequence, the high-strength steel sheet exhibits strength of 980 MPa or more and exerts excellent deep drawability.
1. A steel sheet comprising, in mass percent:
C: 0.02-0.3%
Si: 1.0-3.0%;
Mn: 1.8-3.0%;
P: 0.1% or less (including 0%);
S: 0.01% or less (including 0%);
Al: 0.001-0.1%; and
N: 0.002-0.03%;
wherein a remainder is iron and impurities,
a microstructure comprises, in terms of area ratio relative to the entire structure, each of the following phases:
bainitic ferrite: 50-85%;
retained austenite: 3% or more;
martensite+the retained austenite: 10-45%; and
ferrite: 5-40%,
a C content (C γR ) in the retained austenite is 0.6-1.2mass%, and
a ratio Mn γR /Mn av of Mn content Mn γR in the retained austenite and average Mn content Mn av in the entire structure is 1.2 or more based on a Mn content distribution obtained by EPMA line analysis.
2. The steel sheet of claim 1 , further comprising:
Cr: 0.01-3.0%;
Mo: 0.01-1.0%;
Cu: 0.01-2.0%;
Ni: 0.01-2.0%; and
B: 0.00001-0.01%.
3. The steel sheet of claim 1 further comprising
Ca: 0.0005-0.01%;
Mg: 0.0005-0.01%; and
REM: 0.0001-0.01%.
4. A method for warm working a steel sheet, comprising
heating the steel sheet of claim 1 to 100-400° C.; and
working the steel sheet thereafter within 3,600 s.
5. The steel sheet of claim 2 , further comprising:
Ca: 0.0005-0.01%;
Mg: 0.0005-0.01%; and
REM: 0.0001-0.01%.
6. The method according to claim 4 ,
wherein the steel sheet further comprises:
Cr: 0.01-3.0%;
Mo: 0.01-1.0%;
Cu: 0.01-2.0%;
Ni: 0.01-2.0%; and
B: 0.00001-0.01%.
7. The method according to claim 4 ,
wherein the steel sheet further comprises:
Ca: 0.0005-0.01%;
Mg: 0.0005-0.01%; and
REM: 0.0001-0.01%.