MORPHOLOGY OF 2ND PHASE AND TENSILE PROPERTIES IN A TRIP-AIDED SUPER HIGH-STRENGTH DUAL-PHASE STEEL SHEET

A relationship between retained austenite and second phase morphologies and the influences of these morphologies on tensile properties have been investigated using a 0.17%C-1.41%Si-2.00%Mn TRIP-aided dual-phase steel containing retained austenite. A large amount (above 10 vol%) of retained austenite lay isolated in the ferrite matrix and away from the hard phase particle if austenite morphology in α + γ region during annealing is a network structure or a fine grained island. In the other case, the retained austenite film (below 5 vol%) remained inside the hard phases and along these lath boundaries. This type of retained austenite was stabilized. The retained austenite increased the flow stress and elongation. The influence of second phase morphology on flow stress appeared to be small because of the retained austenite. Elongation depended on the morphology. The network structure of the second phase enhanced internal stress and strain hardening rate after most retained austenite had transformed to marteniste. The steel with fine-grained isolated particles exhibited increased elongation since void formation at hard phase/matrix interface was suppressed compared to the other type of steel.