A single-step Direct-Cast Hardening (DiCH) method for making property-gradient low-carbon steel at a lower cost by saving energy and manpower. An ultra-hard layer is introduced at the surface with free oxygen content before solidification and a specific cooling rate during solidification. Samples subjected to hardness tests after processing showed ultra-high levels (4.2GPa Vickers hardness and 6.3GPa nanoindentation hardness). Ultra-high hardness is achieved by a proper mix of acicular ferrite and ultra-fine lath-structured grains (LSG) and a specific cooling rate, which produces refined microstructure and high surface hardness.
