Effect of alloying elements on the microstructure and properties of a hot-rolled low-carbon low-alloy bainitic steel

A two-level full factorial statistical experiment consisting of eight alloys was conducted to determine the effect of 2 pct cobalt, 1 pct nickel and 1 pct chromium on the hot-rolled microstructure and properties of a bainitic steel containing 0.2 pct C, 2 pct Mn, 1 pct Si, 0.75 pct Mo and 0.003 pct B. The results indicate that chromium induced the formation of the acicular bainitic structure while cobalt favored massive ferrite formation and resulted in islands of martensite and/or austenite. Nickel, when added singly, did not appear to influence the microstructure but in combination with chromium, enhanced the formation of the lower bainitic structure. The mechanical properties were statistically analyzed and statistical equations were obtained to predict optimized compositions. These equations indicate that chromium increased the toughness of these steels more than nickel. However, it was shown that with similar bainitic structures, nickel enhanced the toughness more than chromium. The results illustrate the short-coming of a pure statistical approach to the design of alloys.