EFFECTS OF 2ND PHASE MORPHOLOGY ON RETAINED AUSTENITE MORPHOLOGY AND TENSILE PROPERTIES IN A TRIP-AIDED DUAL-PHASE STEEL SHEET

The relationship between second phase morphology and retained austenite morphology and the influences of these two kinds of morphology on tensile properties of a 0.17C-1.41Si-2.00Mn (mass%) TRIP-aided dual-phase steel have been investigated in a temperature range between 20 and 400-degrees-C. A large amount of fine retained austenite was obtained when the second phase morphology was ''a network structure'' or ''an isolated fine and acicular one.'' The retained austenite particles were nearly isolated in the ferrite matrix away from bainite islands and were moderately stable. On the other hand, ''an isolated coarse structure'' of second phase resulted in a small amount of more stable retained austenite film along bainite lath boundary. The influence of second phase morphology on the flow curve significantly differed from that of a conventional ferrite-martensite dual-phase steel. Isolated retained austenite particles lowered the flow stress, and resultantly reduced the effects of second phase morphology (i.e., network effect or fine grain size effect) on flow stress. However, the isolated retained austenite particles enhanced effectively the ductility, particularly at 50-100-degrees-C, due to the moderate strain induced transformation. On the other hand, retained austenite films along bainite lath boundary scarcely influenced on tensile properties of the steel. These results were discussed on the basis of a continuum theory.