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 austenit e morphology and the influences of these two kinds of morphology on te nsile properties of a 0.17C-1.41Si-2.00Mn (mass%) TRIP-aided dual-phas e steel have been investigated in a temperature range between 20 and 4 00-degrees-C. A large amount of fine retained austenite was obtained w hen the second phase morphology was ''a network structure'' or ''an is olated fine and acicular one.'' The retained austenite particles were nearly isolated in the ferrite matrix away from bainite islands and we re moderately stable. On the other hand, ''an isolated coarse structur e'' of second phase resulted in a small amount of more stable retained austenite film along bainite lath boundary. The influence of second p hase 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 t he effects of second phase morphology (i.e., network effect or fine gr ain size effect) on flow stress. However, the isolated retained austen ite particles enhanced effectively the ductility, particularly at 50-1 00-degrees-C, due to the moderate strain induced transformation. On th e other hand, retained austenite films along bainite lath boundary sca rcely influenced on tensile properties of the steel. These results wer e discussed on the basis of a continuum theory.