Industrial electric heating furnaces – Arc furnace device – Charging or discharging
Reexamination Certificate
2000-09-25
2001-11-06
Hoang, Tu Ba (Department: 3742)
Industrial electric heating furnaces
Arc furnace device
Charging or discharging
C373S043000
Reexamination Certificate
active
06314123
ABSTRACT:
The present invention relates to a method for the continuous melting of solid metallic products.
The method relates more particularly to the melting of metallic solids such as solid iron, solid pig iron, scrap iron or pig iron, pre-reduced products etc., which are used, possibly with the addition of liquid pig iron for example, for the production of steel. The method can be implemented inside a reactor comprising for example an electric furnace, in which the energy required for the melting is produced by an electric arc and/or by a gas-fired, oil-fired or coal-fired furnace and/or by a plasma torch furnace.
The continuous melting of solid products is generally carried out in a reactor comprising two adjacent zones, namely a melting zone and a metallurgical treatment zone. The solid products are loaded into the melting zone of the reactor and are then melted under the effect of a considerable input of energy. The metal thus melted is progressively transferred into the second zone and undergoes a metallurgical treatment there with the aim of adjusting its chemical composition. This metallurgical treatment generally comprises a refining of the liquid metal during which a refining gas, such as oxygen for example, is injected into the metal bath using a lance in order to reduce the carbon and silicon content of the steel produced. Such a method is described, for example, in the French patent application FR-A-1,482,929.
However, apart from carbon and silicon, the molten metal contains other impurities which have deleterious effects on the physical and mechanical properties of the steel produced. Among these impurities particular mention may be made of sulphur which, inter alia, reduces the impact strength of the steel, its fatigue strength, its corrosion resistance and its weldability.
Unfortunately, the sulphur cannot be eliminated at the same time as the carbon since desulphurisation requires very different operating conditions from decarburisation.
In fact, during decarburisation, a slag is present which is oxidising because of the insufflation of large quantities of oxygen. Desulphurisation, however, becomes more efficient when the slag is a reducing slag.
In the refining zone of furnaces such as those described in the French patent application FR-A-1,482,929, the conditions are such that barely 30% of the sulphur contained initially in the metal charge loaded into the furnace is eliminated. This means that it is always necessary to carry out a secondary metallurgical treatment in a receptacle placed downstream from the fusion reactor, for example a ladle furnace.
The object of the present invention is therefore to propose a method for the continuous melting of solid metallic products which permits not only a reduction in carbon content but also a reduction in the sulphur content of the molten metal.
In conformity with the invention, this objective is attained by a method for the continuous melting of solid metallic products in a reactor with two separate zones, a melting zone and a metallurgical treatment zone, which comprises steps consisting in
a) continuously heating the solid metallic products in the melting zone until the solid products melt,
b) progressively transferring the melted products into the metallurgical treatment zone,
c) refining the melted products in the metallurgical treatment zone in an environment comprising an oxidising slag,
d) separating the slag of the metallurgical treatment zone from the slag of the melting zone,
e) converting the oxidising slag in the metallurgical treatment zone into a reducing slag,
f) desulphurising the melted products in the metallurgical treatment zone in an environment comprising a reducing slag,
g) pouring the molten metal.
The solid metal is melted continuously in the melting zone. An oxidising slag is present in this zone, which enables a large proportion of the phosphorus contained in the molten metal to be eliminated. In fact, the phosphorus, which, inter alia, reduces the ductility and the weldability of the steel, is transferred to the oxidising slag by an exchange reaction with this slag. As its melting progresses, the molten metal is transferred to the second zone, in which the metallurgical treatment proper occurs.
After arrival in the second zone, the metallurgical treatment of the molten metal takes place in two stages. In the first stage, the main process is a reduction of the carbon and silicon contents of the metallic bath under oxidising conditions. This refining is achieved by oxygen injection into the metallic bath and by the addition of, for example, CaO in order to form the slag. The carbon and silicon contents of the metallic bath may be reduced in this way to predetermined values, preferably lying between 0.05% and 0.1% for carbon.
After this oxidising refining, the conditions in the treatment zone are modified in order to change from an oxidising environment to a reducing environment. Such a transformation of conditions is achieved by an addition of aluminium Al and/or of silicon Si and/or of carbon C to the slag. The slag is thus killed and changes from a more oxidising slag into a more reducing slag. It should be noted that the silicon and/or the carbon are added in such a way as not once again to increase their concentrations in the metallic bath, which would diminish the effect of the earlier refining, but in such a way as to reduce only the FeO in the slag and to lower the oxygen content in the metal.
Under the reducing conditions created in this way, the second stage of the metallurgical treatment is then carried out, i.e. the desulphurisation of the metallic bath. During this desulphurisation, the metallic bath is preferably stirred by bubbling an inert gas, argon for example, through it in order to facilitate the exchange between the metallic bath and the slag. Since the slag is a reducing agent, a large proportion of the sulphur passes into the slag.
Under these conditions, a desulphurisation of more than 80% becomes possible and, depending on the sulphur content of the solid products and the quantity of slag involved, final sulphur contents of 0.010% are achieved in the steel poured from the second zone of the reactor.
The proposed method thus makes possible the production of steel with low carbon and sulphur contents in a two-zone reactor and hence makes it possible to avoid ladle furnace treatment in the mass production of steels, such as that of rods for reinforced concrete, for which a sulphur content of between 0.020% and 0.030% is aimed at in the finished product.
For new electric steel plants producing high quality steels with substitutes for scrap iron which enable low copper contents to be achieved (pig iron, pre-reduced products, iron carbide), low-sulphur steels (less than 0.010% of sulphur in the finished product) are difficult to produce in an electric steel plant: in fact, the oxidising conditions in an electric furnace do not allow desulphurisation by more than 30%, i.e. a maximum of 30% of the sulphur loaded into the furnace is eliminated. However, apart from desulphurised pig iron, substitutes for scrap iron contain much more sulphur than the pure scrap iron that they replace: 0.020% S to 0.100% S for pre-reduced products (DRI) depending on their source and 0.050% to 0.100% S for the non-desulphurised pig iron. With massive use of a substitute containing more than 0.030% S, it is impossible to attain the objective of less than 0.020% S in the pouring from a conventional electric furnace. The present method, which enables a more efficient desulphurisation to be achieved, is therefore an important benefit when envisaging the massive use of pre-reduced products or non-desulphurised pig iron as a substitute for the scrap iron. This method thus widens the range of raw materials usable for the production in the electric furnace of high-purity products, for example containing less than 0.010% sulphur, which at present requires that use may be made only of very pure scrap iron, very pure pre-reduced products, or desulphurised pig iron.
In current high-productivity methods, the oxidisi
Burgmann Wilhelm
Monai Jean
Radoux Henri
Roth Jean-Luc
Hoang Tu Ba
Paul Wurth S.A.
Smith , Gambrell & Russell, LLP
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