Compositional Optimization of a Commercially Produced Ferritic Steel Using Microstructural Heterogeneity and Mechanical Properties

Abstract

Titanium- and niobium-added medium-strength ferritic steels find widespread application in automotive industries. Carbon and nitrogen play an important role in controlling microstructure and mechanical properties of these alloys. In this study, five alloys were selected from commercial heats of the same grade. Carbon and nitrogen contents were different for them. The microstructure of steel consisted of polygonal ferrite with a small quantity of cementite. Two types of precipitates were identified. One group was coherent/semicoherent niobium carbide (NbC) of size <20 nm. Other group was incoherent titanium nitride (TiN)-NbC with size ≥20 nm. The quantity of NbC precipitates primarily contributed in strengthening the mechanism and the rate of strain hardening. The quantity of complex TiN-NbC was controlled by bulk nitrogen content of steel. These incoherent carbides were responsible for the deterioration of the mechanical properties of the alloy.