Quenching and partitioning in Si/Al-free steels: effect of Mn-heterogeneous distribution in high-temperature austenite

Conventionally, quenching and partitioning (Q&P) process involves homogeneous Mn distribution in high-temperature austenite. In addition, it is necessary to add Si and/or Al in order to inhibit competitive carbide precipitation and promote carbon partitioning from martensite to austenite. In the present study, through introducing heterogeneous Mn distribution in high-temperature austenite, Q&P-het steel realizes the carbon partitioning from martensite to austenite in the Si/Al-free steels. In comparison with Q&P-hom steel having Mn-homogeneous high-temperature austenite, Q&P-het steel has doubled fraction of retained austenite (RA). This is due to the absence of carbide precipitation in the Mn-depleted martensite, which assists the carbon partitioning into the neighboring Mn-enriched austenite. Q&P-het steel exhibits similar yield strength (∼1200 MPa) and ultimate tensile strength (∼1930 MPa) to Q&P-hom steel. However, the Q&P-het steel has larger both uniform elongation (7.7 ± 0.5 % vs. 6.2 ± 0.4 %) and total elongation (14.4 ± 1.5 % vs. 10.3 ± 0.5 %), which is ascribed to continuous RA-to-martensite transformation during tension. This study proves the feasibility to apply Q&P process to the Si/Al-free steels through heterogeneous Mn distribution in high-temperature austenite.