Influence of niobium microalloying on the thermal stability of medium-carbon carbide-free bainitic steel

This paper investigates the influence of niobium on the thermal stability of ultra-fine bainite microstructures and their mechanical properties by means of an isothermal transformation at 340 degrees C, followed by tempering in the range of 200-700 degrees C for two medium-carbon, carbide-free bainitic steels with and without 0.018 wt. % Nb. The results reveal that ultra-fine bainite microstructures, especially film-like retained austenite, is refined, and the average carbon content of retained austenite is also improved as a result of the addition of Nb element. Both of the bainitic steels in this paper exhibit good thermal stability at low temperatures (<= 400 degrees C) and precipitation hardening arising from carbide precipitation following tempering at 500 degrees C. The precipitation strengthening compensates for the decrease in strength induced by the decrement in the dislocation density and coarsening of the bainitic ferrite plates during tempering process. M23C6- and MC-type carbides are precipitated in Nb-free and Nb steels, respectively, when the tempering temperature exceeds 400 degrees C. The further coarsening of the carbides and bainitic ferrite plates, and the decrement in the dislocation density in the latter, relate to the decrease in the mechanical properties for both of the tested bainitic steels following tempering at 500 and 600 degrees C. Compared to those in the bainitic steel without Nb, the larger size and superior thermal stability of MC-type carbides results in greater strength and elongation of the bainitic steel containing 0.018 wt. % of the Nb element, but does harm to its impact toughness after tempering at 700 degrees C. (C) 2021 The Author(s). Published by Elsevier B.V.

Automated summary

The addition of niobium improves the thermal stability and mechanical properties of bainitic steels, especially at low temperatures. After tempering, MC-type carbides contribute positively to the strength and elongation of the bainitic steel, but may detrimentally affect its impact toughness.