Speaker
Description
Multiphase steels, such as medium-Mn and QP steels, are of key interest for automotive industrial applications. The primary characteristic of these steels is strain-induced martensitic transformation (SIMT) of retained austenite (RA) during deformation, enabling substantial energy absorption. This study investigates the effect of thermomechanical processing on the multiphase steels that contain simulated residual elements like Mo and Cu, subjected to Q&P treatment. Three steels were examined: a base composition of 0.17C-4Mn-0.8Al-0.5Si (wt.%), a base with 0.3 wt.% Mo, and a base with a combined 0.3 wt.% Mo and 1.0 wt.% Cu additions. The phase transformation behavior of deformed austenite was characterized by using a high-resolution Bahr DIL805 dilatometer, and the deformation continuous cooling transformation (DCCT) and deformation-temperature-time-transformation (DTTT) diagrams were developed. The evolution of prior austenite grain size (PAGS) was analyzed using different methods (EBSD, chemical etching and thermal etching). As a result, the effect of plastic deformation delays the formation of bainite in all analyzed steels. The Q&P treatments were then designed based on the DCCT and DTTT to identify optimal processing, quenching temperature, and partitioning parameters. Subsequently, selected variants were reproduced using salt bath processing to validate the dilatometric RA stability findings under near-industrial conditions. Mechanical behavior was assessed based on hardness and static tensile tests. The addition of Mo was found to enhance hardenability and delay bainitic transformations. The combined Mo + Cu alloy enhances this effect and optimizes phase fractions and RA stability. The results demonstrate that controlled thermomechanical Q&P processing combined with tailored alloying strategies enable effective tuning of microstructure and mechanical performance in advanced multiphase QP steels.
Acknowledgement
The authors would like to acknowledge Pro-quality program for support in starting scientific activities in new research topics as part of the Excellence Initiative – Research University program, grant no. 32/014/SDU/10-22-001.