Warm Formability of Ultra High-Strength Low Alloy TRIP-aided Sheet Steels with Bainitic Ferrite Matrix

Abstract

Forming temperature dependence of formability in 0.20C-1.51Si-1.51Mn ultra high-strength TRIP-aided sheet steels with bainitic ferrite matrix (TBF steel) which were austempered at 375 or 450°C was investigated for automotive applications. Warm formings at 20-50°C and 100°C respectively enhanced the stretch-formability and the deep drawability of the TBF steel through the TRIP effect due to the strain-induced martensite transformation of retained austenite. The stretch-flangeability became maximum by warm forming at 20-50°C and 100-200°C in the TBF steels austempered at 375°C and 450°C, respectively. Further good stretch-flangeability was completed by warm punching at 100-200°C and subsequent cold expanding at 20°C in the TBF steel austempered at 450°C. The mechanisms improving these formabilities by warm forming were proposed in terms of retained austenite characteristics and microstructural properties.