A Strategy for Simultaneous Increasement in the Strength-Ductility Balance of Directly-Quenched Ultra-High Strength Low Alloy Steel

This study investigated a strategy for simultaneous improving in the strength and ductility of the directly quenched ultra-high strength low alloy steel via low-temperature tempering. To stabilize the microstructure, sub-zero treatment was also employed at -73 °C for 30 min, and then tempering was performed for two types of temperature at 200 °C and 630 °C for 30 min. The microstructure of as-quenched steel consists of lath martensite and meta-stable retained austenite. After low temperature tempering (200 °C), some stable reverted austenite was formed at the austenite/martensite interface. When tempering was performed at high temperature (630 °C), the entire austenite remained as reverted austenite. In addition, Ti-rich carbides were observed at the martensite lath boundaries during tempering treatment. Tensile strength increased up to ∼ 1.8 GPa after low-temperature tempering, and ductility was also higher than that of as-quenched steel. In contrast, the steel which was tempered at 630 °C shows low mechanical properties compared to the as-quenched steel. Correlations between microstructure evolution (meta-stable to stable austenite transformation), mechanical properties and deformation behavior were also discussed and identified.