Tensile Properties and Stretch-Flangeability of TRIP Steels Produced by Quenching and Partitioning (Q&P) Process with Different Fractions of Constituent Phases

The influence of microstructure on tensile properties and stretch-flangeability of TRIP steels with tensile strengths higher than 1.2 GPa has been investigated under various Quenching and Partitioning (Q&P) conditions. As lowering the quenching stop temperature, QT, below Ms temperature in the range of 340°C to 280°C, volume fractions of tempered martensite and retained austenite increased and volume fractions of bainite and fresh martensite decreased in the final microstructure. The higher the QT temperature in the range of 280°C to 330°C, the more the relative proportion of untransformed austenite at the end of the partitioning step was transformed into fresh martensite. The microstructural characteristics of fresh martensite and retained austenite under different QT conditions were analyzed by EBSD. The fresh martensite phase was identified by a new method applying the threshold values of both Image Quality (IQ) and Kernel Average Misorientation (KAM). It is suggested that the decrease in the HER (Hole Expansion Ratio) value with increasing QT temperature is due to the increase in the size and the volume fraction of fresh martensite particle.The mechanical properties of Q&P steels were evaluated before and after tempering at 200°C for 1 hour. Under conditions where the initial volume fraction of fresh martensite before tempering was higher, tensile elongation and HER values were improved by tempering. Tensile elongation was increased with the volume fraction of retained austenite. Lower HER values were obtained with higher volume fractions of fresh martensite, regardless of tempering.