Journal
ISIJ INTERNATIONAL
Volume 50, Issue 1, Pages 162-168Publisher
IRON STEEL INST JAPAN KEIDANREN KAIKAN
DOI: 10.2355/isijinternational.50.162
Keywords
strech-flangeability; ultra high-strength steel; TRIP-aided steel; retained austenite; martensite; bainitic ferrite; microalloying
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Funding
- Iron and Steel Institute of Japan
- Amada Foundation for Metal Work Technology [AF-2003016]
- Ministry of Education, Science, Sports and Culture, Japan [2004-15560624]
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Microstructure, tensile properties and stretch-flangeability of 980-1 470 MPa grade Al or Al-Nb bearing TRIP-aided cold-rolled sheet steels with bainitic ferrite and/or martensite matrix microstructure (TBF steels) were investigated for automotive applications such as impact member reinforcements, sheet flames and so on. In addition, these properties were related with the microstructure and the retained austenite characteristics. Complex additions of 0.5% Al and 0.05% Nb into a base steel with chemical composition of 0.2% C, 1.5% Si and 1.5% Mn (in mass%) significantly enhanced the total elongation and stretch-flangeability, especially when austempered at temperatures below martensite-start temperature. The excellent stretch-flangeability was primarily associated with (i) refined prior austenitic grain by NbC precipitates and (ii) uniform fine mixed matrix microstructure of bainitic ferrite and martensite, as well as (iii) TRIP effect of metastable retained austenite.
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