Continuous annealing furnaces are devoted to steel recrystallisation following cold rolling. Recrystallisation is carried out at high temperatures under reducing atmospheres (H_2/N_2 mixtures). However, this heat treatment has effects not only on the bulk steel properties, but also on the surface quality of steel; even though, in most cases, iron is not oxidized, internal oxidation of the alloying elements (Mn, Al, P, Si,) generally occurs, which may influence the further behavior of the steel surface, especially its wettability during hot dip galvanizing. Increasing demands on product quality have already led to improvements of the process control in continuous annealing furnaces. However, little is known about the flow field in annealing furnaces and its influence on the temperature and gas conditions. Many benefits of an improved control of gas flow within annealing furnaces are expected: 1. control of gas leaks, which would create oxidizing atmospheres greater than required, leading to excessive oxidation of the steel surface before dipping into the zinc bath; a poor wettability of the steel surface would then lead to a non uniform zinc coverage and most likely to zinc bare spots; 2. control of the hydrogen content in the gas; high hydrogen contents in the soaking and especially cooling zones may lead to hydrogen pick up by steel and, later on, to blistering; 3. control of the oxidizing and reducing functions of each zone (preheating, soaking, cooling) which is generally achieved by creating a counter current between the steel strip and the gas would result from the control of the local gas composition; 4. control of the furnace efficiency by using exactly the required amount of hydrogen which would generally result in gas savings, and the highest productivity level by clearly separating the cooling and heating functions; 5. design of localized gas treatments in reactive zones of the annealing line, which would give new surface and bulk properties to our steel products. These thermochemical treatments, based on high temperature gas-metal reactions require the isolation of the reactive section from the other parts of the furnace. This paper presents ways of assessing the flow pattern of gases within continuous annealing furnaces: from gas mass balances to tracer injection and their validation through CFD (Computational Fluid Dynamics) models and industrial measurements. It also shows some solutions to control, decrease and eliminate gas recirculation.
展开▼
