Effects of Second Phase Morphology on RetainedAustenite Morphology and Tensile Properties in a TRIP-aided Dual-phase Steel Sheet.

Therelationship betweensecond phasemorphologyand retained austenite morphologyand the influences of these two kinds of morpho[ogy on tensile properties of a 0.1 7C-1 .41Si=2.00Mn (masso/o) TRIP-aided dual-phase steel have been investigated in a temperature range between 20 and 400"C.A Iarge amount of fine retained austenite was obtained when the second phase morphology was "a network structure" or "an isolated fine and acicular one."Theretained 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 significantlyditfered 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 enhancedeffectively the ductility, particularly at 50-1 OO'C, due to the moderate strain induced transformation.On the other hand, retained austenite films along bainite [ath boundaryscarcely influenced on tensile properties of the steel.Theseresults were discussed on the basis of a continuum theory.