М/A-Constituent in Bainitic Low-Carbon High-Strength Steel Structure. Part 2

The microstructure and mechanical properties of steel K65 (X80) with a significant proportion of martensite and residual austenite (M/A-constituent) within the microstructure, prepared in a laboratory rolling mill by controlled rolling using stepwise accelerated cooling, are studied. During accelerated cooling (AC) the temperature for the end of the first stage and the duration of pauses between AC stages was varied. A dependence is revealed for mechanical properties on temperature and time parameters of the AC process and microstructure. It is shown that the microstructure formed during two-stage AC, consisting of a matrix of low-carbon bainitic ferrite and secondary high-carbon phases in the form M/A-constituent “islands”, makes it possible to obtain rolled sheet with high strength and increased ductility, with a low σy/σf ratio, and good cold resistance. Formation of this microstructure with stepwise AC compared with single-stage cooling makes it possible to increase ultimate strength on average by 40 MPa, relative elongation on average by 5% (abs.), and uniform elongation by 3%. The optimum properties are achieved for steel sheet with a microstructure consisting of a matrix of bainitic ferrite and strong phase in the form of M/A-constituent “islands” with predominance of residual austenite.