The Role of Niobium during Heat Treatment of High-Vanadium High-Speed Steel Rolls for Improved Mechanical Properties

Abstract High-speed steel (HSS) provides superior wear resistance and high surface quality of rolled-steel products. In this study, the influence of niobium addition on the solidification behavior and microstructure of high-speed steel rolls and their impact on the roll’s hardness and wear resistance were investigated by a combination of thermodynamic simulations of solidification using ThermoCalc, optical and scanning microscopy and hardness testing. The study was conducted on two centrifugal-cast commercial high-vanadium high-speed steel sleeves whose compositions differed in niobium content. Thermodynamic simulations showed that niobium promotes the precipitation of proeutectic Nb-rich MC carbide during solidification. Microscopic observations showed that the proeutectic carbide promotes carbide dispersion in the microstructure. This was found to improve the microhardness of the martensitic matrix and counteract softening during repeated tempering treatment. The addition of less than 2 wt.% Nb in the HSS roll yielded better hot hardness as a result of the roll’s better microstructural integrity and response to HT. The conclusion was that Nb-containing HSS rolls can be produced for the hot-rolling stands that operate at higher rolling force and temperature if the cost-benefit analysis is favorable.