MicroNiobium Alloy Approach in Medium and High Carbon Steel Bar, Plate and Sheet Products

Abstract

The recently developed application of the MicroNiobium Alloy Approach® in medium- and high-carbon steel long products, sheets, and plate steels enhances both the metallurgical properties and processability, as well as reducing the operational cost per tonne of production. The process and product metallurgy improvements relate to the Nb-pinning effect of the austenite grain boundaries. The metallurgical mechanism of the MicroNiobium Alloy Approach is related to the retardation of austenite grain coarsening during reheat furnace soaking of the billets, slabs, or shapes before rolling. Variable grain size is induced by temperature fluctuations and inhomogeneity during the heating of the slabs in the reheat furnace. Such fluctuations can occur because of variations in the air- to gas-ratio, directly affecting the adiabatic flame temperature and heat input into the slabs. This approach contributes to the achievement of an ultrafine grain, homogeneous higher carbon microstructures that exhibit superior toughness, high strength, less mechanical property variation in the final hot-rolled product, and reduced cost of quality. The reduced cost of quality far exceeds the additional alloy cost for the Nb addition.