Quenching and partitioning in Si/Al-free steels: effect of quenching temperature

Carbon partitioning is a feasible way to stabilize austenite against martensite transformation at room temperature in quenching and partitioning (Q&P) steels. It is necessary to add Si/Al in order to inhibit competitive reactions like carbide precipitation. However, Si is harmful to Zn coating while Al is related to nozzle clogging. Here, carbon partitioning from martensite to austenite, for the first time, is realized in a Si/Al-free Q&P steel. In the quenching step, high-temperature austenite is Mn-patterned through fast heating from Mn-enriched cementite and Mn-depleted ferrite, leading to the formation of Mn-enriched austenite and Mn-depleted martensite during quenching. Following partitioning, carbide precipitation is fully prevented in Mn-depleted martensite, which facilitates the carbon partitioning from Mn-depleted martensite to the adjacent Mn-enriched austenite. This phenomenon is striking and directly evidenced by electron microscopy characterization; it is probably ascribed to the Mn enriched in austenite, which has a strong attraction with carbon atoms. Increasing quenching temperature from 60 to 110 °C increases the width of retained austenite from 43.8 ± 11.8 to 47.7 ± 9.7 nm and its fraction from 18.7 % to 23.2 %. All the fractions and widths of retained austenite in Q&P process are larger than those (33.6 ± 6.4 nm; 11.2 %) after direct quenching from austenitization to room temperature. Additionally, the tensile properties are comparable with Si/Al-added Q&P steels having similar carbon content of 4 wt %, showing 2000 MPa tensile strength and 15 % total elongation. It is believed that this work offers a novel avenue to produce Q&P steels without Si/Al addition through Mn patterning between austenite and martensite.