Fundamental Study on Non-Oxidizing Direct Heating Technology

Non-oxidizing furnace (NOF) sections have become involved into continuous annealing lines (CAL) and continuous galvanizing lines (CGL). In fact, a number of large scale lines are successfully operated also in Japanese steel mills. Such a vertical type direct heating method has the following advantages; (1) The current requirements of decreasing the furnace dimensions and layout space limit are fulfilled. (2) Reduction heating which results in better strip surface quality is permitted. (3) Due to rapid heating, improvement of metallurgical properties is expected. However, it is not yet well understood why and how the reducing (or non-oxidizing) heating of steel strip is achieved by impinging a flame at right angle, even when the air-fuel ratio is set at over 0.90 which is believed to be of oxidizing range in the theory of chemical equilibrium. This paper describes; (1) a new type nozzle mixing burner superior to the conventional ones in the capability of non-oxidizing heating, (2) experience of actual CGL with non-oxidizing direct heating zone (NOF) placed before RT burner zones, (3) experimental results obtained in a bench scale experimental furnace, aiming at clarifying the possible mechanisms in direct heating. It has been found through the experiments that the residual oxygen concentration in the flame must be lower than 0.1% by volume, and the volume ratios of CO/CO2 and H2/H2O in burnt gas should be over 0.4 and 0.09, respectively, to realize this concept.