Hot straining and quenching and partitioning of a trip-assisted steel: Microstructural characterization and mechanical properties

Advanced high strength steels (AHSS), with yield strengths and tensile strengths above 400 and 700 MPa, respectively, are becoming more noticeable in vehicle manufacturing. A novel processing route of a TRIP-assisted steel was developed. Characterization and modelling techniques were used to establish correlations between processing, microstructure and mechanical properties. TRIP-assisted steel was heat-treated by quenching and partitioning (Q&P) and a novel process of hot straining (HS) and Q&P (HSQ&P) treatments on a Gleeble ®3S50 thermo-mechanical simulator. The samples were intercritical annealed at 800 ºC, quenched at 318 ºC (optimal quenching temperature), and partitioned at 400 ºC for 100 s. In the HSQ&P process, isothermal straining at 800 ºC was applied. The influence of isothermal straining at high temperatures on the strain-induced transformation to ferrite (SIT) effect, austenite carbon enrichment, and carbide precipitation were investigated. Carbon, silicon, and manganese distribution in the martensite/austenite interfaces and carbide formation were analyzed by means of atom probe tomography (APT). The carbon enrichment in austenite was confirmed in all samples. The carbon enrichment in Q&P samples was slightly inferior than in HSQ&P, suggesting the contribution of the additional carbon partitioning to austenite from ferrite formed by the SIT-effect. The carbon accumulation at the interface of martensite/austenite was clearly observed by APT. The newly developed combined thermomechanical process (HSQ&P) is promising as the transformation induced plasticity can favor the energy absorption and formability, contributing to fill the gap of the third generation of high-strength steels.