Weldable and non-weldable reinforcing steel bars are among the most important steel products applied in civil construction. The available strength level of Niobium (Nb)-bearing rebar has been increased with the 345, 390 and 490MPa family of grades in addition to positive developments in the 600MPa series. Traditionally, higher strength grades were produced with vanadium and/or various controlled cooling processes. However, recent mechanical property and microstructural heterogeneity issues have led to increased global production of Nb- containing reinforcement bar products of all diameters. The combination of clean steelmaking practices at the melt shop with selective furnace heating practices, as well as controlled cooling practices at the rolling mill, are major operational focus areas.
Production practices relating to lower carbon equivalent construction-type, earthquake and fire-resistant steel rebars from the melting stage through the crack-free continuous casting of the billets and extending through hot rolling and strategic cooling steps are critical. Specifically, the practices maximize the effectiveness of Nb when manufacturing high quality, high strength reinforcing bar grades.
The step-change trend in rebar production is the design of cost-effective lower carbon, lower residual contents (including sulfur and phosphorous), reduced nitrogen, lower manganese and more homogeneous microstructure replacing traditional mixed care/shell rebar. Adaptation of these product characteristics can be cross-applied into lower strength non-earthquake/fire-resistant rebar, thereby reducing operational cost per ton and improving product quality for the end user in addition to reducing scrap rates during construction.