PRECIPITATION BEHAVIOR AND PRECIPITATION STRENGTHENING OF NANOSCALE CEMENTITE IN CARBON STEELS DURING ULTRA FAST COOLING

In recent years, the precipitation strengthening by cementite, which is a common and economical second phase constituent in steels, has drawn renewed attention in the context of precipitation strengthening, because if cementites can be effectively refined to the scale of a few nanometers, they can induce significant precipitation strengthening effect. Therefore, nanoscale cementite is viewed as a viable option to replace precipitates of microalloying elements for reducing alloy costs in steel products. Given that cementites are usually to form lamellar pearlite structure in a traditional cooling process and generally tend to coarsen at relatively high temperatures, the thermodynamic feasibility for the formation of nanoscale cementite precipitates during cooling has been determined in the previous study, and the non-equilibrium precipitation of nanoscale cementite can be realized by increasing the cooling rate after hot rolling. Thus, the ultra fast cooling (UFC) technology was applied after the hot strip rolling for the research of precipitation behavior and precipitation strengthening of nanoscale cementite in carbon steels. The experimental results demonstrated that the UFC technology shows the unique effects on strengthening in carbon steels and a large number of dispersed nanoscale cementite precipitates with the size of 10 similar to 100 nm have been formed in 0.17%C and 0.33%C steels. The nanoscale precipitation of cementite was realized in the microstructure by UFC without the microalloy elements addition. Both the yield strength and tensile strength of the steels increased gradually with the stop temperature of UFC decreasing, and the yield strength increments of 0.17%C and 0.33%C steel were more than 100 MPa, when the stop temperature of UFC decreased from 890 degrees C to 600 degrees C. Besides, thermomechanical treatment (TMT) process was introduced after UFC to explore uniform nucleation of cementite in hot-rolled carbon steels, and it is a feasible way to realize the uniform precipitation of nanoscale cementite in the entire miscrostructure for the further strengthening improvement. This was accomplished by subjecting the UFC cooled steel to a small degree of plastic deformation, with the aim to increase the dislocation density evidently. By combining UFC and TMT processing, the yield strength of 0.17%C steel is greater than 600 MPa, leading to a superior strengthening effect.