The influence of unary and multi-component microalloying on hot deformation and dynamic recrystallization in ultra-fine bainitic rail steel

Microalloying technology is an effective way to enhance the strength and toughness of bainitic rail steel. Hot deformation parameters strongly influence these properties by controlling recrystallization. In the present study, three microalloyed steels (V, V-B, and V-B-Nb) were designed, and their flow stress under different hot deformation conditions was examined using Gleeble-3800 compression tests. Based on the results, constitutive equations and dynamic recrystallization models were established. The results showed that adding B to the V-microalloyed steel lowered the V precipitation temperature, reducing flow stress. In contrast, the combined addition of V and Nb caused compound precipitation at all deformation temperatures, resulting in the highest flow stress in the V-B-Nb microalloyed steel. Constitutive equations were established for all three microalloyed steels, with the V-microalloyed steel showing the highest hot deformation activation energy of 389.9 kJ. In the V-B-Nb microalloyed steel, Nb offset the reduction in hot deformation activation energy caused by B addition. The dynamic recrystallization model showed that recrystallization was more difficult in the V-B-Nb steel than in the V-B microalloyed steel, consistent with the Grain Orientation Spread analysis, which indicated a higher recrystallization degree in the V-B microalloyed steel. Grain size results further showed that Nb still significantly refined austenite grains when added with B atoms. On the whole, multi-component microalloying significantly influences the hot processing window of ultrafine bainitic rail steel, with V-B-Nb microalloyed steel demonstrating optimal hot workability.