Effects of Nb Addition on Transformation Kinetics and Microstructure Properties in Low-Carbon Bainitic Steels

Abstract

To investigate the effects of Nb addition on transformation kinetics and microstructure properties in low-carbon bainitic steels, two C–Si–Mn bainitic steels were designed, one of which was added with 0.025 (wt%) Nb and the other was C–Si–Mn steel. Heat treatment experiments were carried out on thermal simulator. The microstructures were observed by scanning electron microscope. The results show that lath-like bainite is obtained in both two steels. Film-like retained austenite distributes between bainite laths. In addition, Nb retards bainite transformation owing to smaller parent austenite grains in steel with Nb addition. However, Nb improves the strength of the low-carbon bainitic steel by grain refinement. Moreover, transformation kinetics equations for two tested steels are established based on the experimental data. The experimental results are useful to clarify the function of Nb in low-carbon bainitic steels and provide the theoretical reference for the composition design of low-carbon bainitic steels.