Heterogeneous quenching and partitioning from manganese-partitioned pearlite: retained austenite modification and formability improvement

Increasing quenching temperature in conventional quenching and partitioning (Q&P) process usually increases the fraction of retained austenite (RA) being dominant by blocky morphology, which ensures a good global formability but deteriorates the local formability. Instead of homogeneous austenite in conventional Q&P process, we propose a Q&P process purposely based on Mn-heterogeneous austenite inherited from Mn-partitioned pearlite. Therein, Mn-depleted austenite areas originated from ferrite lamellae are readily transformed into lath martensite during quenching; meanwhile, Mn-enriched austenite areas originated from cementite lamellae are readily retained as film RA at room temperature. Consequently, this heterogeneous Q&P process unusually achieves a large RA fraction being dominant by film morphology. The large RA fraction ensures a large total elongation and the dominant film morphology ensures a large post-uniform elongation, indicating good global and local formability, respectively. Additionally, the microstructure is refined through the above divided transformation of heterogeneous austenite, predominantly contributing to the increased yield strength by 200 ∼ 300 MPa. Introducing heterogeneous austenite into Q&P process provides a feasible way to achieve a large RA fraction being dominant by film morphology, leading to the successful production of high-strength Q&P steels simultaneously with good global and local formability.