Mechanism of Microstructural Control and Mechanical Properties in Hot Rolled Plain C-Mn Steel during Controlled Cooling

In this study, plain C-Mn steels with different ferrite grain size were obtained through various thermomechanical processing. Fast cooling suppressed ferrite transformation at high temperature, but promoted ferrite nucleation at low temperature and restricted ferrite grain growth. The microstructure and mechanical properties were governed by the combined effects of cooling rate and cooling temperature. A new thermo-mechanical controlled processing (TMCP) based on ultra fast cooling (UFC) technology was adopted to increase the strength of plain C-Mn steel strips. The new TMCP approach involving UFC enabled significantly finer grain size to be obtained in relation to conventional TMCP. The grain size of Fe-0.17C-0.33Mn steel strips obtained by the new TMCP was refined to similar to 3-4 mu m, and both yield strength and tensile strength were increased by 60-100 MPa over the conventional TMCP.