Designing quenching and partitioning thermal routes of medium-Mn steel intended for ultra-high-strength plates

The applicability of the quenching & partitioning (Q&P) process to medium-Mn steel intended for plates with increased cross section produced in integrated thermomechanical processing lines has been studied in this work. The modeling in JMatPro and experimental approaches using DIL805 dilatometer were used to investigate 0.17C–5Mn–0.88Si–0.76Al–0.22Mo–0.06Nb steel and optimize time–temperature parameters of the Q&P process. Standard lever rule and its novel variant were applied for a precise determination of target interrupted quenching temperatures. The Q&P variants were investigated for a wide parameters range to record the microstructural evolution tendencies: at quenching temperatures from 240 to 300 °C, for partitioning temperatures from 350 to 450 °C, and partitioning time from 90 to 1800s. The obtained samples were characterized by various phase compositions including fresh martensite, tempered martensite, and lath-type retained austenite. Obtained results showed that the steel can be air-cooled after hot working without a risk of formation of ferrite, pearlite, or bainite; however, martensitic transformation kinetics is highly dependent on the post-deformation cooling rate which is of great importance for an industrial production of this steel type. The cooling rate and quenching temperature are the most important factors determining a final phase composition of investigated medium-Mn steel intended for plates. Studies have shown that a quenching temperature of 240 °C or lower is required to stabilize retained austenite. At higher quenching temperatures, blocky martensite grains form in the microstructure. Partitioning temperature and time have a smaller impact. The most favorable phase composition was achieved by quenching at 240 °C and partitioning at 420 °C for 300 s, resulting in the highest retained austenite content (17.8 vol.%) with no fresh martensite formed during final quenching.