Effect of fast cooling rate on the microstructure and mechanical properties of low-carbon high-strength steel annealed in the intercritical region

The effect of fast cooling rate on the microstructure and mechanical properties of low-carbon high-strength steel annealed in the intercritical region was investigated using a Gleeble 1500 thermomechanical simulator and a continuous annealing thermomechanical simulator. The results showed that the microstructure consisted of ferrite and bainite as the main phases with a small amount of retained austenite and martensite islands at cooling rate of 5 and 50 °/s, respectively. Fast cooling after continuous annealing affected all constituents of the microstructure. The mechanical properties were improved considerably. Ultimate tensile strength (UTS) increased and total elongation (TEL) decreased with increasing cooling rate in all specimens. The specimen 1 at a cooling rate of 5 °/s exhibited the maximum TEL and UTS×TEL (20% and 27 200 MPa%, respectively) because of the competition between weakening by presence of the retained austenite plus the carbon indigence by carbide precipitation, and strengthening by martensitic islands and precipitation. The maximum UTS and YS (1 450 and 951 MPa, respectively) were obtained for specimen 2 at a cooling rate of 50 °/s. This is attributed to the effect of dispersion strengthening of finer martensite islands and the effect of precipitation strengthening of carbide precipitates.