Heat treatment effects on microstructure and properties of advanced high-strength steels

The effects of the austempering time on the microstructure and properties of two third-generation high-strength steels are investigated for austempering (TBF), and for quenching and partitioning (QP). The process parameters are chosen according to the capacity of an existing continuous annealing or galvanising line. More than 10% retained austenite can be stabilised although carbides start to form at short overageing times with the simulated cycles. The product of the tensile strength and total elongation for TBF is higher than for QP, and it peaks with increasing austempering time for TBF, but decreases monotonously for QP. The hole expansion capacity for QP is higher than for TBF and it peaks with austempering time for both TBF and QP.