The development and characterization of ultra high GIGA-strength ferritic hot band steels

The correlations existing among the hot mill processing, as-coiled microstructure and mechanical properties of a Mo–Ti–V microalloyed hot rolled high strength steel were investigated in this current study. Discrete processing parameters, i.e., finish rolling temperatures (FRT) and coiling temperatures (CT), were applied and the corresponding microstructures and mechanical properties were analyzed. It was found that the FRT had only a very minor influence on either microstructures or mechanical properties. However, the CTs strongly affected both the microstructures and mechanical properties. The microstructure in the matrix was observed to change from mainly polygonal ferrite to quasi-polygonal ferrite to granular bainite and upper bainite with falling CTs, accompanied by the formation of martensite/austenite (M/A) constituents at the lower CTs. The strength appeared to be increased by the dislocations originating by the shear component of the displacive phase transformation, and by fine (Ti, Mo)C precipitates, both formed during the coiling process. Strengths reaching values as high as 1166 MPa in yield strength and 1225 MPa in tensile strength were observed in specimens after coiling at 610 °C, and the steels still had a reasonable total elongation of around 20%. However, the hole expansion ratios of these steel conditions were rather low in this study, especially for those with higher strength. Several factors appeared to contribute to the poor hole expansion ratios found in these steel conditions: the presence of coarse TiN inclusions, a large amount of strengthening precipitates and a high dislocation density as well as the presence of M/A constituents.