The Effects of Heat-treatment Conditions on Stretch-flangeability and Bendability of High-strength Low Alloy TRIP-aided Sheet Steels with Annealed Martensitic Matrix

Abstract

The effects of heat-treatment conditions on stretch-flangeability and bendability of 800 MPa grade 0.2C-1.5Si-1.5Mn low alloy TRIP-aided sheet steels with annealed martensitic matrix or "TRIP-aided annealed martensite steels" were investigated for automotive applications. Good stretch-flangeability and excellent bendability, as well as large ductility, were achieved in the steels intercritically annealed at 780-820°C, followed by austempering at 375-400°C for 100-1000 s. In this case, initial microstructure prior to intercritical annealing was martenite or fine bainite structure transformed at temperatures below 400°C. These good formabilities were considered to be caused by fine annealed lath matrix with low dislocation density, interlath stable retained austenites and relatively low difference in strength between second phase and matrix.