REVISITING THE ROLE OF Nb MICROALLOYING IN MEDIUM-HIGH CARBON STEELS

摘要

The metallurgical effects that niobium additions have in medium carbon (0.2-0.4 wt% C) grades have been far less analyzed than in low carbon steels. Besides the effect of carbon on Nb solubility in the reheating furnace and the interaction between recrystallization and strain induced precipitation, the combination of short interpass times at high strain rates during the finishing rolling stands of bar and wire mills could alter Nb impact on microstructural evolution. This paper analyzes the effect that Nb additions have in the microstructural evolution during hot working of medium-high carbon steel for different bar diameters. In the case of larger bars, Nb microalloying addition could exert an effect similar to the case of low carbon steels, that is, retard softening processes due to strain-induced precipitation leading to pancaked austenite and refined room temperature microstructures. However, for smaller diameters, the activation of dynamic recrystallization leads to the necessity of reconsidering the role of Nb. The comprehension of these differences during hot rolling will allow optimizing Nb additions in these medium-high carbon grades and the enhancement of final properties in hot rolled bars.