The methods to produce grain boundary and interphase boundary characters suitable for fine-grain superplasticity in three kinds of steels with phases including ferrite, austenite and cementite were investigated. It was observed that recrystallization of matrix was needed for superplasticity of microduplex structure whereas phase transformation had the most important role in ultra high carbon steel (UHCS). The volume fraction of second phase was also important to increase the fraction of high angle boundaries and for the refining of grain size. The microduplex stuctures with high angle boundaries exibited higher superplasticity than that formed by heavy warm rolling or cold rolling.
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