EFFECTS OF SILICON AND MANGANESE ADDITION ON MECHANICAL-PROPERTIES OF HIGH-STRENGTH HOT-ROLLED SHEET STEEL CONTAINING RETAINED AUSTENITE

An 80 kgf/mm2 grade high-strength hot rolled sheet steel with a significantly high product of tensile strength and total elongation (TS x El = 3 000) has been developed by utilizing transformation-induced plasticity of retained austenite in 0.2% carbon sheet steels by optimizing the silicon and manganese content and hot rolling conditions. Finish rolling temperature and coiling temperature are important factors in terms of introducing a large amount of retained austenite. Silicon addition over 1.0% results in a significant increase in the volume fraction of retained austenite due to the change in second phase from bainite+pearlite to bainitic ferrite. The maximum volume fraction of retained austenite and the consequential optimum combination of tensile strength and ductility is obtained in a 0.2%C-2.0%Si-1.5%Mn steel. The effect of retained austenite on ductility becomes small with further addition of manganese over 1.5%, because the retained austenite transforms in the early stage of the straining process due to the presence of martensite.