1. Field of the Invention
The present invention relates to an improvement in a scrap melting method, and provides a method capable of melting scrap with reduced consumption of energy.
The above term "scrap" refers not only to iron scrap but also to all other materials containing iron that are used as materials for manufacturing steel together with iron scrap. Therefore, the term also includes such iron-containing materials as solid pig iron, pellet, sponge iron and reduced iron.
1. State of the Art
When manufacturing special steel using an electric arc furnace, scrap is usually melted beforehand in the arc furnace with electricity. It is not advantageous, in terms of cost reduction, to consume expensive electric power as the energy source throughout the procedure of scrap melting. Therefore, it is desired that the energy required to heat the scrap for melting should be supplied by fuel, a cheaper energy source, as a substitute for electric power.
Tests have been carried out heating the scrap with both an electric arc and, for instance, a powdered coal burner simultaneously. By thus substituting part of the scrap melting procedure, and part of the energy supply required therefor, the efficiency with respect to energy consumption can be improved. However, since the energy load (i.e. the maximum quantity of energy which can be introduced into unit volume of the furnace, and per unit time) has a limit, the consumption of electric power cannot be reduced to the desired extent. Further, CO.sub.2 produced by the burning reacts with the graphite electrodes to cause carbon solution, which results in an inferior unit consumption of electrodes.
Since the flame temperature of the powdered coal burner is about 2000.degree. C., while that of the graphite arc heater is about 3000.degree. C., it is considered rational to use the burner and the arc heater in a switching manner wherein the former is used in the lower temperature region while the latter is used in the higher temperature region. In order to increase the proportion of fuel substitution for electric power within the total energy supply required for scrap melting, it is desirable to use the burner to as high a temperature region as possible. However, if the burner is used on the scrap in a temperature region exceeding 1000.degree. C., rapid oxidation of iron takes place, which decreases the yield with respect to the used material.
The present inventors have long been studying methods to substitute powdered coal burner heating for electric arc heating in the first half of the scrap melting procedure, and have already proposed a scrap melting method, as disclosed in Japanese Patent Disclosure No. 59-215427, wherein such substitution can be achieved with high heating efficiency and with a controlled amount of oxidized iron. The proposed method comprises burning the powdered coal mixed with air while the scrap temperature is low, and burning the powdered coal mixed with oxygen or oxygen-enriched air instead of air when the scrap temperature increases above a certain temperature, more specifically, above about 500.degree. C., thereby increasing the flame temperature of the burner, and thus increasing the proportion of the energy substitution.
According to a preferred embodiment of the above proposed method, a pair of furnaces are employed, to alternately heat the scrap with a burner, and melt the scrap.