RETAINED AUSTENITE AND MECHANICAL-PROPERTIES IN BAINITE TRANSFORMED LOW-ALLOY STEELS

Uniform ductility and formability of low alloy steels can be improved by the transformation plasticity effect of metastable retained austeni te. In this work, intercritical annealing followed by bainite transfor mation resulted in the retention of austenite with sufficient stabilit y for transformation plasticity interactions. The effect of retained a ustenite on mechanical properties was studied in two low-alloy steels. Bainite transformation was carried out in the range of 400 to 500 deg rees C. The strength properties (yield strength and ultimate tensile s trength) were more sensitive to bainite isothermal transformation temp erature than holding time. Maximum strength properties were obtained f or the lower transformation temperatures. On the other hand, high unif orm and total elongation Values were obtained at lower transformation temperatures but were sensitive to bainite isothermal transformation t ime. Variations in uniform elongation with holding time were linked to variations in retained austenite stability. Maximum values of uniform elongation occurred at the same holding times as the maximum amount o f retained austenite. The same was true for total elongation and ultim ate tensile strength. The above results indicate a strong correlation between retained austenite stability and uniform ductility and suggest that further optimisation regarding chemical composition and processi ng with respect to austenite stabilisation may lead to a new class of triple-phase high-strength high-formability low-alloy steels.