Effect of heating rate on microstructure and mechanical properties of TRIP-aided multiphase steel

• Fast-heating annealing process was applied to TRIP-aided Multiphase steels.
• Effect of heating rates on austenite formation was demonstrated.
• Various microstructures were obtained by different heating rates.
• Improved mechanical properties were obtained in fast-heating process.

Optimization of high strength and high formability of multiphase cold rolled TRIP-aided steel sheet is based on the continuous annealing conditions. The influence of heating rate on the microstructure and mechanical properties of TRIP steel was investigated with Gleeble-3500 and a thermal system using resistance heating method. Experiments with different heating rates (5 and 500 K/s) and annealing temperatures from 883 K to 1223 K were applied to study effects on the progress of austenitization and recrystallization kinetics. Different initial heating rates were applied to examine the influence of heating rate on the mechanical properties of TRIP steel. It appeared that the final tensile strength of the TRIP steel increased with increasing heating rate held at 1073 K. The phase-transformation kinetics during the intercritical annealing was investigated by means of dilatometry. The results indicated that samples with higher heating rates have a much higher austenite volume fraction for a given annealing time of 80 s held at 1073 K. In addition, the higher heating rate results in a larger volume fraction of retained austenite (RA) and lower retained austenite carbon content in comparison to the lower heating rate after held at the same intercritical temperature. The morphology of RA under different annealing conditions was also tentatively discussed.