The Integration of Process and Product Metallurgy in Niobium Bearing Steels

A review of the technological integration of both the process and physical metallurgical advancements of value-added niobium (Nb) microalloyed thermo-mechanical controlled process (TMCP) steels have evolved into the development of higher quality steels for more demanding end user requirements. The connection of process and physical metallurgy is evolving through the integration of research that is aimed at improving product quality. However, often the connection of the process metallurgical parameters is not reported, especially with industrial data. The importance of this innovative metallurgical connection is validated by the market demand for reduced fuel consumption, improved quality, and CO2 emissions in both the automotive and construction sectors. This situation has further increased the demand for new higher quality Nb-bearing steel grades. This integrative process/physical metallurgical (IP/PM) approach applies to both low and high strength steel grades in numerous applications. Often, the transition from laboratory melted and TMCP to the production scale is challenging. The methodology, process control, and key production steps that are required during the melting, ladle metallurgy, continuous casting, thermal, and hot rolling production conditions often vary significantly from the laboratory conditions. Understanding the reasons and corrective action for these variations is a critical product development success factor. These process metallurgy parameters for the industrial melting, casting, reheating, and hot rolling of Nb grades are connected and correlated to the resultant microstructures, physical metallurgy, and mechanical properties. These advanced high strength steels are microalloyed with Nb, V, Ti and/or other elements, which affect the austenite-ferrite transformation. Niobium enables the achievement of substantial grain refinement when the plate or sheet is rolled with the proper reheat, hot reduction, and thermal schedule. A recently developed key metallurgical transition is in progress applying this integrative approach with the use of MicroNiobium. A reduction of Mn and C levels with the complementary refinement of the microstructural grain size through MicroNiobium additions improves the robustness of the steel to better accommodate some process metallurgy variations. Applications are evolving in lower strength steels with Nb to achieve complementary grain refinement