STEEL SHEET SUITABLE FOR IMPACT ABSORBING MEMBER AND METHOD FOR ITS MANUFACTURE

摘要

A steel sheet suitable as a starting material for a vehicle impact absorbing member with high absorption of impact energy and resistance to cracking contains, by mass %, C: 0.08-0.30%, Mn: 1.5-3.5%; Si+Al: 0.50-3.0%, P: 0.10% or less, S: at most 0.010%, and N: at most 0.010%, and optionally, one or more types selected from Cr: at most 0.5%, Mo: at most 0.5%, B: at most 0.010%, Ti: less than 0.04%, Nb: less than 0.030%, V: less than 0.5%, Ca: at most 0.010%, Mg: at most 0.010%, REM: at most 0.050%, and Bi: at most 0.050%. The microstructure contains, by area %, bainite: more than 50%, martensite: 3-30%, and retained austenite: 3-15%, the remainder comprising ferrite having an average grain diameter of less than 5 mm. The product of uniform elongation and hole expansion ratio is at least 300%2 and 5% effective flow stress is at least 900 MPa.

主权项

1. A steel sheet comprising:
a chemical composition containing, by mass %, C: at least 0.08% and at most 0.30%, Mn: at least 1.5% and at most 3.5%, Si+Al: at least 0.50% and at most 3.0%, P: at most 0.10%, S: at most 0.010%, N: at most 0.010%, Cr: 0 to at most 0.5%, Mo: 0 to at most 0.5%, B: 0 to at most 0.01%, Ti: 0 to less than 0.04%, Nb: 0 to less than 0.030%, V: 0 to less than 0.5%, Ca: 0 to at most 0.010%, Mg: 0 to at most 0.010%, REM: 0 to at most 0.050%, and Bi: 0 to at most 0.050%, the remainder being Fe and impurities; a microstructure containing, by area %, bainite: more than 50%, martensite: at least 3% and at most 30%, and retained austenite: at least 3% and at most 15%, the remainder consisting of ferrite having an average grain diameter of less than 5 μm; and mechanical properties in which the product of uniform elongation and hole expansion ratio is at least 300%2, and an effective flow stress when 5% true strain is applied is at least 900 MPa.