EFFECTS OF VOLUME FRACTION AND STABILITY OF RETAINED AUSTENITE ON DUCTILITY IN TRIP-AIDED DUAL-PHASE STEEL SHEETS

The effects of silicon and manganese contents on volume fraction and stability of retained austenite in 0.2C-(1.0-2.5) Si-(1.0-2.5) Mn (mass%) TRIP-aided dual-phase steels were investigated. And, relationship between the above retained austenite parameters and ductility at room and moderate temperatures was discussed through studies on strain-induced transformation behavior of retained austenite. Increasing silicon and manganese contents except for 2.5 mass% manganese steel, initial volume fraction of retained austenite increased with reducing carbon concentration in retained austenite. It was found that ductilities of these steels became maximum at a given temperature between 23-degrees-C and 175-degrees-C, i.e., peak temperature. The peak temperature of ductility was concluded to differ among the steels and agree well with the temperature at which strain-induced martensite transformation of retained austenite was suppressed moderately. Moreover, the peak temperature T(P)(degrees-C) was related to estimated martensite-start temperature M(S)(degrees-C) of the retained austenite as T(P) = 3.04.M(S) + 187. Strength-ductility balance at the peak temperature linearly increased with an increase in initial volume fraction of the retained austenite.