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Título

TMCP COMBUSTION METALLURGY

Autoria

Jansto, Steven G.

DOI

10.5151/9785-9785-32214

Downloads

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Resumo

 

The technological integration of the process and physical metallurgical advancements of value-added niobium (Nb) microalloyed thermo-mechanical controlled process (TMCP) steels continue to be developed for more demanding end user requirements and cost-effective steel production. The market demand for reduced fuel consumption and CO2 emissions in the automotive and construction sectors have further increased the demand for these new and advanced higher quality microalloyed grades produced via TMCP. 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 consistent 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. However, the transition from laboratory melted and TMCP hot rolled heats to the production scale is challenging. Aberrations in the reheating process step at the industrial furnace are often the root cause of mechanical property quality issues when the prescribed mechanical metallurgy TMCP practices are met. These combustion process metallurgy parameters connect directly to the resultant TMCP product quality and is defined as Combustion Metallurgy® (CM®). The quality and efficiency of the reheating process has a profound effect on the prior austenite grain size and uniformity of grain size along the entire length of the slab or billet. The resultant ferrite grain size in the final hot rolled product is significantly governed by the initial prior austenite grain size (PAGS). This reheating step in the steelmaking process often receives low priority in the evaluation of product quality and mechanical property performance. In laboratory studies, the furnace heating step is typically quite uniform resulting in a homogeneous and fine PAGS. Unfortunately, in industrial operations, it is much more difficult to control the uniformity of heating along the entire length and through the thickness of the work piece. Such inhomogeneity and efficiency of heating is highly influenced by several combustion process variables such as the air-to-gas ratio, furnace burner condition, furnace pressure and refractory condition. The optimum air-to-gas ratio of 1.10 yields the highest adiabatic flame temperature. Often in actual operations, cracked burner orifice plates, poor burner tuning, and inefficient combustion fan performance contribute to variations in the air-to-gas ratio. These situations have a huge effect on the optimal adiabatic flame temperature performance which translates into inhomogeneous austenite grain size and variations in mechanical properties in the hot rolled product. Mechanical property variations may result in quality issues involving the toughness, ductile-to-brittle transition temperature (DBTT), bendability, formability, yield-to-tensile ratio and drop weight tear tests (DWTT). CM® corrective actions and operational practice recommendations are presented to minimize inhomogeneous heating effects on product quality. 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.

Palavras-chave

Adiabatic Flame Temperature; air-to-gas ratio; austenite grain size; combustion; quality