Quantifying the Evolution of Strengthening Mechanisms Along Coil Length for Hot-Rolled Nb-Ti HSLA Steel Sheet

Niobium (Nb) additions to high-strength, low-alloy (HSLA) steel enables strength uniformity along the coil length of commercially produced hot-rolled sheet to avoid downgrading of product that fails strength specifications. In addition to contributing to precipitation strengthening through the growth of niobium-titanium carbides ((Nb,Ti)-C), niobium deters recrystallization and refines ferrite grain size. The potency of these strengthening mechanisms relies heavily on the austenite to ferrite transformation kinetics of the hot-rolling process. While niobium’s effect on precipitation strengthening, Hall-Petch strengthening, dislocation strengthening, and solute strengthening have all been studied in literature independently, the interactions of these mechanisms with each other and coiling temperature has not been optimized. The strengthening behavior of niobium microalloyed HSLA steels compared to a non-niobium HSLA steel is quantified and correlated using industry samples and a design of experiments with factors of niobium composition, coil temperature, and location on coil length. Hall-Petch grain size strengthening and geometrically necessary dislocation strengthening is determined from electron backscatter diffraction mapping. Orowan precipitation strength is quantified from image analysis of (Nb,Ti)-C precipitates captured on carbon extraction replica films using a scanning transmission electron microscope. Solubility of alloying elements in the ferrite matrix, and therefore solute strengthening, is calculated with Thermo-Calc. With niobium’s improvements to HSLA steel strength uniformity quantified, there is justification for use of niobium in commercial production of HSLA steel sheet.