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

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.