Continuous cooling transformation behavior of titanium and niobium-titanium microalloyed steel for pallet container

The research on the continuous cooling transformation behavior of Ti and Nb-Ti microalloyed steel for pallet container is of great significance for promoting the industrial application of this type of steel and improving the quality of pallet container equipment. The microstructure phase transformation behavior of Ti and Nb-Ti microalloyed steel for pallet container at different cooling rates was determined by dilatometric method combined with metallographic-microhardness method using thermal simulation testing machine. The dynamic continuous cooling transformation (CCT) curves of the two test steels were drawn, and the continuous phase transformation rules were analyzed. The results showed that the microstructure of both test steels transformed to bainite and the hardness increased with the increase of cooling rate. Due to the precipitation of particles such as Nb(C,N) and Ti(C,N), Nb-Ti test steel pinched the grain boundaries and refined the grains, delaying the bainite transformation and resulting in the appearance of martensite at higher cooling rates. Combined with the analysis of microstructure and precipitates, it indicated that when the cooling rate was 0.2-1 ℃/s, grain refinement dominated the hardness; when the cooling rate was 2-10 ℃/s, the hardness difference between the two test steels was small; when the cooling rate was 20-50 ℃/s, the hardness of Nb-Ti test steel reached 330HV 5 due to grain refinement, martensite transformation and precipitation strengthening, which was significantly higher than that of Ti test steel (290HV 5). In addition, the CCT curve of Nb-Ti test steel had a new martensite transformation zone, and the phase transformation temperature and critical cooling rate were better, which proved that Nb could effectively improve the microstructure density and hardness of steel.