Hardening Low-Carbon Steels by Engineering the Size and Distribution of Inclusions

Ultra-hard low-carbon steels usually need many processing steps after casting. This paper introduces a single-step direct-cast hardening (DiCH) method for making ultra-hard, low-carbon steels by manipulating two variables: free oxygen content before solidification and cooling rate during solidification. Without any post-casting steps required to enhance hardness, DiCH produced property-gradient steel with high surface hardness (4.2 GPa Vickers) directly from liquid metal. The optimum size, number, and distribution of oxide inclusions were achieved in condition of intermediate oxygen content (25 to 45 ppm) and high cooling rate (≥ 550K/s). Ultra-high hardness was achieved at the surface of DiCH samples with a mixture of refined acicular ferrite (AF) and martensite-like ferrite (MF). Two factors contributed to refinement of microstructure and enhancement of hardness: a high cooling rate during the solidification process, and a high density of submicron oxide inclusions in the cast steel. At cooling rates higher than 2500 K/s, refined AF and MF was obtained, accompanied by high densities (up to 600/mm²) of multiple-component, Ti-containing oxides of sizes between 0.5 and 0.7 μm.