Effect of Cooling Rate and Coiling Temperature on Microstructure and Precipitation Behavior of a 700 MPa Weathering Steel

The microstructures, mechanical properties and precipitation behavior of a high-Ti low-C weathering steel obtained by thermo-mechanical controlled process were investigated by means of electron backscatter diffraction and transmission electron microscope. The cooling rates have a significant effect on the continuous cooling transformation structure of deformed austenite in the tested steel, and the higher cooling rate is conducive to the formation of bainite. The grain orientation and precipitates of the steel at different cooling rates were discussed in detail. There were some large size precipitates which may lead to negative effect on the mechanical properties at the grain boundaries and within the grains at cooling rate of 0.5 °C/s. However, the higher cooling rate of 30 °C/s was favorable to obtain dispersive nano-scale precipitates. There is a greater degree of plastic deformation and dislocation density in the microstructures at 30 °C/s than 0.5 °C/s. The roles of precipitates, grain boundaries and sub-grain boundaries on the hindrance of dislocations movement were discussed. The precipitation strengthening effect of the precipitates can be brought into full play when the coiling temperature is about 610 than 560 °C. The yield strength of the high-Ti low-C weathering steel plate is 777 MPa at 610 °C.