Effect of step quenching heat treatments on the kinetics of ferrite formation and quenching & partitioning modeling for a commercial C-Mn-Si steel.

Abstract

Aiming to meet current demands from the automotive industry, a third generation of advanced high strength steel (AHSS) has been developed as an alternative to previous generations. Different alloys and innovative processes have been deeply studied as, for example, quenching and partitioning (Q&P). The published papers highlight that the best heat treatment parameters for a satisfactory Q&P execution are strongly dependent on the austenite conditioning and they can be optimized if thermodynamic and kinetics calculations are performed. In this context, this work evaluated the effect of step quenching (SQ) heat treatments on the kinetics of ferrite formation and Q&P modeling for a commercial C-Mn-Si steel, predicting the microstructural evolution and the final phase fractions as a function of the heat treatment parameters. The JMAK model was optimally fitted to the ferrite fraction, as well as to the microhardness data. The kinetics characterization and the thermodynamic modeling showed that the combination of SQ and Q&P can provide a high retained austenite fraction in a multiphase microstructure. As a conclusion, it is possible to state that a well-planned SQ heat treatment followed by an optimized Q&P cycle has the potential to generate an advanced steel with a final microstructure assisted by the TRIP effect.