Continuous Cooling Transformation Microstructure and Strength-Toughness of 700 MPa Low Carbon Microalloyed Steel

Gleeble-3500 thermo-simulation machine was employed to determine the continuous cooling transformation (CCT) curves of 700 MPa low carbon microalloyed steel. The effects of cooling rate on phase transition behavior and microstructure were investigated. The results indicate that the CCT curves were in flat shape and the low carbon bainite structure was obtained in a large cooling rate range. The cooling rates had great effects on the form, proportion, distribution and micro-hardness of phases. The microstructure in experimental steel were changed from polygonal ferrite, acicular ferrite, granular bainite to lathe bainite with micro-hardness increasing when cooling rate became faster. When the cooling rate was between 10 ℃·s-1 and 30 ℃·s-1, the microstructure of the experimental steel was mainly compound of lathe bainite, and the M/A constituents dispersed on the grain boundaries. Futhermore, the size of lathe bainite became smaller with the cooling rate increasing. Therefore controlling the intermediate temperature transformation products using a proper cooling process could improve comprehensive mechanical property of experimental steel on the basis of analyzing of its phase transition characteristics.