Ultra-high strength medium-Mn lightweight steel by dislocation slip band refinement and suppressed intergranular κ-carbide with Cr addition

Abstract

Ultra-high strength lightweight steel has great potential in the automotive industry. An ultra-high strength Fe-12Mn-9Al-1.4C-3Cr austenite-based lightweight steel has been developed in the study. The alloy is designed based on the precipitation strengthening of κ-carbides while preventing the formation of intergranular κ-carbides by sub-rapid solidification and Cr addition. The results show that the addition of Cr element reduces C segregation at grain boundaries by expanding the solid solution ability of austenite matrix. It can effectively inhibit the precipitation of intergranular κ-carbides at the grain boundaries. Therefore, the ductility of as-cast steel is greatly improved from 14.0% to 47.8% by Cr addition. Besides, the strain hardening ability of the alloy is also enhanced with refined slip bands. The yield strength and tensile strength of the cold rolled steel reach approximately 1.5 GPa, with an elongation up to 15%. The ultra-high strength comes from the high-density dislocation hardening and precipitation strengthening of κ-carbides. This work provides potential alloy design and processing route for automotive applications requiring ultra-high strength and lightweight. • The precipitation of coarse intergranular κ-carbides is greatly reduced. • Cr expands the solid solution limit of carbon in austenite. • Ultra-high strength of 1.5 GPa with a 15% elongation is achieved. • The dislocation slip bands spacing is refined in Cr-alloyed steel.