Effect of rare earth and calcium treatments on the mechanical, physical, and electrochemical properties of a 16Mn steel

Two treatment methods, Ca--Si injection into the ladle and rare earth (RE) additions to the mold, were investigated for their effectiveness in controlling the microstructure, particularly sulfide inclusions, and hence the properties of a 16Mn plain-carbon steel. A combined Ca + RE treatment increased the critical crack opening displacement in the transverse direction of sheet material to that in the rolling direction. The combined treatment also improved the resistance to stress-corrosion cracking (SCC) in a boiling 20 wt. % NH sub 4 NO sub 3 solution. Too large a RE addition ultimately lowers SCC resistance. The RE additions retarded austenite grain growth, whereas the Ca-only addition accelerated grain growth. The changes in fracture behavior, SCC resistance, and austenite grain growth behavior are explained in terms of the segregation of the RE to the grain boundaries and the subsequent changes in the electrochemical and mechanical properties of the grain boundaries relative to the grain interiors. A combined treatment of injecting a 3.4 kg Ca--Si /ton of steel into the molten steel and adding 0.03 wt.% RE to the mold yielded optimum properties.