Thermodynamic Behaviors of Nb and P in Dephosphorization of Niobium-bearing Hot Metal with BaO-based Slag

The Bayan Obo iron ore contains valuable metallic elements such as niobium and rare earth elements. However, developing appropriate metallurgical processes to achieve effective dephosphorization of Nb-bearing hot metal meanwhile retaining Nb in the steel products remains a challenge. We carried out a thermodynamic study on dephosphorization of Nbbearing hot metal and investigated the effects of the initial [C] content and the dephosphorization temperature on oxidation behaviors of [Nb] and [P]. In particular, we focused on thermodynamic analysis of dephosphorization of Nb-bearing hot metal with highly basic BaO-based slag. The results showed that the highly basic BaO-based slag system is more suitable for dephosphorization under oxidative conditions, and increasing the [C] content favors dephosphorization of Nb-bearing hot metal without oxidizing [Nb]. Moreover, the values of equilibrium oxygen activity for oxidation reactions of [Nb] and [P] are increased as the dephosphorization temperature increases. Considering the hot metal containing 0.02% of [Nb] in Baotou Steel, thermodynamic calculations indicated that the content of [P] can be reduced to 0.02%–0.05% and [Nb] remains unoxidized when dephosphorization occurs at the cconditions of T=1573–1673 K, [C]=4.0%, [Nb]=0.02%, aP2O5=10-24, aNb2O5=10-10.

21^(st) Century Niobium-Bearing Structural Steels

Value-added applications of niobium (Nb) microalloyed steels continue to be developed for commercial implementation to meet increased material demands and improved properties for 21st century structural applications. These applications demand Nb-bearing steels that deliver improved toughness, fracture and fire resistance and weldability. Such applications include medium and jumbo beam, boiler, bridge, container, heavy equipment, long product, pressure vessel, ship, storage tank and windtower applications. Steel producers are challenged to develop microalloyed steel grades that cost effectively meet end user demands for higher strength at thinner cross sections, better low temperature toughness to resist brittle fracture in building, pressure vessel and ship structures, sustain higher loads per unit area in earthquake and hurricane zone product applications, demonstrate improved fire-resistance in buildings, bridges and tunnels and provide overall improved weldability. Niobium is often a key element to achieve these results. This paper will discuss Nb market opportunities and key operational practices required to successfully melt, cast and roll these high strength steel grades. Niobium process metallurgy is important to leverage the ability of niobium to obtain ultra-fine grain, homogeneous structural steel microstructures with superior mechanical properties. The process metallurgy, physical metallurgy and resultant properties are significantly determined by mill capabilities, mill practices, operational understanding and the culture of the steel mill. The optimal combination and implementation aspects that are unique to each mill we call metallurgical operational integration (MOI) . MOI is the bridge that links the product requirements to mill capability and process implementation.