Microstructures and mechanical properties of austenitic light-weight steels, and prediction of property distribution in large-scale slab

The effects of the cooling rate, after solution heat treatment, on the microstructures and mechanical properties of Fe-22Mn-8Al-0.8C-0.02Nb (low carbon) and Fe-20Mn-8Al-1.1C-0.1Nb (high carbon) light-weight steels were systematically investigated. The cooling process was controlled to achieve six different cooling rates, ranging from -0.016 to -465.1 oC/s. Under the slowest cooling rate (furnace cooling), intra-granular and inter-granular precipitations of κ-carbides were observed throughout the austenite grains. The higher the C content was, the larger the size of the inter-granular κ-carbides was. The formation of κ-carbides resulted in an increase in yield strength, and a decrease in elongation and impact absorbed energy. In the Fe-20Mn-8Al- 1.1C-0.1Nb, the inter-granular precipitation of κ-carbides caused a drastic decrease in the impact absorbed energy and the inter-granular brittle fracture. To predict the distribution of yield strength and impact absorbed energy at production scale (a 10-ton scale slab), finite element analysis was conducted for water cooling and air cooling conditions. The average cooling rates at the center of the slab under water cooling and air cooling were predicted to be -0.126 and -0.031 oC/s, respectively. Based on predicted cooling rates, the distribution of mechanical properties was determined. The prediction suggested that a large-scale slab of the light-weight steel with low C content would have good toughness at the center of the slab regardless of cooling condition.