Effect of Microstructure on Mechanical Properties of 1.0G TRIP Steels Produced by Quenching and Partitioning (Q&P) Process

The influence of microstructure on tensile properties and stretch-flangeability of TRIP steels with tensile strengths higher than 1.0 GPa has been investigated under various Quenching and Partitioning conditions. Lowering the quenching stop temperature (QT) from 300 degrees C to 190 degrees C after intercritical annealing increased the volume fraction of tempered martensite (TM), decreased that of bainite, and maintains that of ferrite and retained austenite constant in the final microstructure. 300 degrees C was Ms temperature, thus, QT 300 degrees C sample consisted of negligible TM and 36% bainite. So, as the QT was reduced to 190 degrees C, the TM phase was integrated into the bainite up to 13.8%. The yield strength (YS), tensile strength (TS), and hole expansion ratio (HER) of QT 300 degrees C steel were inferior compared to that of QT 190 degrees C steel; however, the total elongation was improved by 2.9% for QT 300 degrees C steel. The elevated YS and TS of the QT 190 degrees C steel were attributed to the increased micro hardness caused by the increased TM fraction. Furthermore, the high HER of the QT 190 degrees C steel was elucidated by the reduced micro void formation compared to the QT 300 degrees C steel under the same reduction ratio. That was because QT 300 degrees C steel, comprised mainly of bainite, has a higher fraction of blocky retained austenite that, when subjected to deformation, transforms into mechanically induced blocky martensite, acting as a significant source of micro voids. Finally, enhanced total elongation in QT 300 degrees C steel was attributed to more stable retained austenite during deformation compared to the QT 190 degrees C steel.