Reheat Furnace Thermodynamic, Kinetic and Combustion Considerations for TMCP Processing

Abstract

The reheat furnace process step has a profound effect on the TMCP performance, final hot rolled steel quality and mechanical property consistency during the production of hot rolled steels. The uniformity of heating applied across the entire width and length of the slab or billet is critical in the achievement of customer properties regardless of the chemistry. The resultant ferrite grain size in the final hot rolled product is significantly governed by the initial prior austenite grain size. Numerous reheat furnace process metallurgy and combustion parameters in actual operation affect mill productivity, microstructure, austenite grain size, scrap rate and diverts. This reheating step in the steelmaking process often receives low priority in the evaluation of product quality and mechanical property performance, especially the toughness through the plate thickness. Heat transfer conditions of radiation, convection and conduction affect furnace heating efficiency. In laboratory studies, the furnace heating step is typically quite uniform resulting in a homogeneous and fine prior austenite grain size. During production, it is much more difficult to control the uniformity of heating and heat transfer consistency along the entire length and through the thickness of the work piece. The furnace conditions are correlated to product quality via furnace process variables such as the air to gas ratio, furnace burner condition, furnace pressure, energy efficiency, adiabatic flame temperature (AFT) and furnace refractory condition. Operational practice recommendations are presented to minimize inhomogeneous heating which results in inferior product quality, hot rolling model anomalies and toughness variations in the through-thickness-direction.