EP1029089A1

EP1029089A1 - Method for continuous smelting of solid metal products

Info

Publication number: EP1029089A1
Application number: EP98956840A
Authority: EP
Inventors: Wilhelm Burgmann; Jean Monai; Jean-Luc Roth; Henri Radoux
Original assignee: Paul Wurth SA
Current assignee: Paul Wurth SA
Priority date: 1997-10-17
Filing date: 1998-09-24
Publication date: 2000-08-23
Anticipated expiration: 2018-09-24
Also published as: EP1029089B1; ZA988966B; AU1334599A; BR9812926A; LU90154B1; MA24658A1; US6314123B1; DE69806796D1; DE69806796T2; AR013667A1; WO1999020802A1; TW400389B; ATE221134T1; ES2178285T3

Abstract

The invention concerns a method for continuous smelting of solid metal products in a reactor comprising a smelting zone and a metallurgical processing zone, which consists in continuously heating the solid metal products in the smelting zone, gradually transferring said smelted products into the metallurgical process zone, refining the smelted products in the metallurgical process zone in an oxidising slag medium, separating the metallurgical process zone slag from the smelting zone slag, transforming the oxidising slag in the metallurgical process zone into a reducing slag, desulphurizing the smelted products in the metallurgical processing zone in a reducing slag medium and casting the smelted metal.

Description

Process for the continuous melting of solid metallic products

The present invention relates to a continuous melting process for solid metal products.

The process relates more particularly to the melting of metallic solids, such as solid iron, solid cast iron, scrap iron or cast iron, pre-reductions, etc., which are used with the optional addition of liquid cast iron, for example. for steel production. The process can take place inside a reactor comprising for example an electric furnace, in which the energy necessary for melting is produced by an electric arc and / or a furnace with gas, fuel oil or charcoal and / or plasma torch oven. The continuous melting of solid products generally takes place in a reactor which comprises 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 then melted under the effect of a considerable energy supply. The metal thus melted is transferred progressively to the second zone and undergoes a metallurgical treatment there in order to adjust its chemical composition. This metallurgical treatment generally involves refining the liquid metal during which refining gases, such as oxygen, are injected into the metal bath using blowing lances in order to reduce the carbon content and made of silicon from the steel produced. Such a process is described, for example, in French patent application FR-A-1, 482,929.

However, in addition to carbon and silicon, the molten metal contains other impurities which have negative effects on the physical and mechanical properties of the steel produced. Among these impurities, mention is made in particular of sulfur which reduces, among other things, the resilience of steel, its resistance to fatigue, its resistance to corrosion and its weldability. Unfortunately, sulfur cannot be removed at the same time as carbon since desulphurization requires very different operating conditions from those of decarburization.

Indeed, during decarburization, we are in the presence of a slag which is oxidizing due to the insufflation of large quantities of oxygen. Desulphurization is all the more effective the slag is reducing.

In the refining zone of the ovens as described in the French patent application FR-A-1, 482,929, the conditions are such that it is barely able to remove 30% of the sulfur initially contained in the metallic charge charged born. This means that a secondary metallurgical treatment must always be carried out in a container placed downstream of the fusion reactor, eg a pocket furnace.

The object of the present invention is therefore to provide a process for the continuous melting of solid metal products which allows, in addition to a reduction in the carbon content, also a reduction in the sulfur content of the molten metal.

According to the invention, this objective is achieved by a process of continuous melting of solid metal products in a reactor with two distinct zones, a melting zone and a metallurgical treatment zone, which comprises the steps consisting in a) heating in continuous solid metal products in the melting zone until the solid products merge, b) gradually transfer the molten products in the metallurgical treatment zone, c) refine the molten products in the metallurgical treatment zone in an oxidizing slag medium, d) separating the slag from the metallurgical treatment zone from the slag from the melting zone, e) transforming the oxidizing slag in the metallurgical treatment zone into a reducing slag, f) desulfurizing the molten products in the metallurgical treatment zone in a reducing slag medium, g) pouring the molten metal.

The solid metal is continuously melted in the melting zone. In this zone, we are in the presence of an oxidizing slag which allows the elimination of a large part of the phosphorus contained in the liquid metal. Indeed, phosphorus, which among other things reduces the ductility and weldability of steel, is transferred to the oxidizing slag by an exchange reaction with this slag. As it melts, the liquid metal is transferred to the second zone in which the actual metallurgical treatment takes place.

Arrived in the second zone, the metallurgical treatment of the molten metal is done in two phases. In the first phase, a reduction in the carbon and silicon contents of the metal bath is mainly carried out under oxidizing conditions. This refining is carried out by injecting oxygen into the metal bath and by adding, for example, CaO to form slag. The carbon and silicon contents of the metal bath can thus be reduced to predetermined values, which are preferably between 0.05% and 0.1% for carbon.

After this oxidative refining, the conditions in the treatment zone are modified to pass from an oxidizing medium to a reducing medium. This transformation of the conditions is carried out by adding aluminum Al or / and silicon Si and / or carbon C in the slag. The slag is thus calmed and passes from a more oxidizing slag to a more reducing slag. It should be noted that silicon and / or carbon are added so as not to increase their contents again in the metal bath, which would reduce the effect of the previous refining, but so as only to reduce the FeO in the slag and lower the oxygen content in the metal.

Under the reducing conditions thus created, the second phase of the metallurgical treatment is then carried out, namely the desulfurization of the metal bath. During this desulphurization, the metal bath is preferably stirred by bubbling inert gas, eg argon, in order to facilitate the exchange between the metal bath and the slag. The slag being reducing, the sulfur passes largely in the slag.

Under these conditions, desulphurization of more than 80% becomes possible and, depending on the sulfur content of the solid products and depending on the quantity of slag used, we arrive at final contents of 0.010% of sulfur in the steel. sunk from the second reactor area.

The proposed process thus allows the production of steel with low carbon and sulfur content in a two-zone reactor and therefore makes it possible to avoid pocket furnace treatment in the production of mass steels such as concrete reinforcing bars. , for which a content of 0.020 to 0.030% of sulfur is targeted on the final product.

For new electric steelworks producing high-end steels with scrap substitutes which make it possible to achieve low copper contents (cast iron, pre-reduced, iron carbide), low sulfur steels (less than 0.010% sulfur on final product ) are difficult to produce in an electric steel plant: in fact, the oxidizing conditions of the electric furnace do not allow desulfurization by more than 30%, i.e. that we remove at best 30% of the sulfur in the oven. However, with the exception of desulfurized cast iron, scrap substitutes contain much more sulfur than the pure scrap they replace: 0.020% S to 0.100% S for pre-reduced (DRI) depending on the origin and 0.050% at 0.100% S for non-desulfurized cast iron. By massively using a substitute containing more than 0.030% S, one cannot reach the objective of less than 0.020% S with the casting of the conventional electric furnace. The present process which makes it possible to carry out a more effective desulfurization is therefore an important advantage for the massive use of pre-reduced or non-desulfurized cast iron as a substitute for scrap. This process therefore widens the range of raw materials which can be used for the production in the electric oven of high purity products, eg containing less than 0.010% sulfur, which today require only the use of scrap or pre-reduced very pure, or desulfurized cast iron. In the current high productivity processes, the oxidative refining and the reducing refining take place in different places. The present process makes it possible to make the two mutually exclusive reactions in the same place and successively.

At the same time, it makes it possible to produce steels with a much lower nitrogen content than steels produced according to conventional processes. In fact, in the metallurgical treatment zone, the two main sources of nitrogen pollution that are the electric arc and the scrap melting late are avoided and the residual nitrogen content is reduced by bubbling a neutral gas in particular argon in the metallurgical treatment area. In addition, denitriding is facilitated by the low sulfur content of the metal bath.

For a load of 100% scrap for example, a steel will be produced in a conventional electric furnace at 70 to 80 ppm N, while with the present process it is possible to produce steels having a nitrogen content of about 40 ppm or even less. According to a first preferred embodiment, the reducing slag is evacuated from the metallurgical treatment zone before, during or after step g), ie. pouring the liquid metal. It is in fact preferable to eliminate the sulfur-rich reducing slag before refining the new charge of liquid metal, this in order to prevent the sulfur contained in the slag from re-entering the bath during refining. of liquid metal.

Under certain conditions, e.g. when there is too much slag in the melting zone or when more slag is needed in the metallurgical treatment zone, it may be advantageous to transfer oxidizing slag contained in the melting zone towards the metallurgical treatment zone before, during or after the refining of the molten products in the metallurgical treatment zone.

The reverse, i.e. the transfer of oxidizing slag from the metallurgical treatment zone to the melting zone during or after the refining of the molten products in the metallurgical treatment zone can be advantageous. In fact, in the metallurgical treatment zone, the slag is foaming and contains a lot of iron oxides and drops of metallic iron. During the transit of the slag to through the melting zone, the slag is deoxidized on contact with the liquid metal with a higher carbon content and the metal drops are decanted there. A countercurrent mass exchange is thus carried out, which makes it possible to minimize the loss of iron. In addition, in an arc furnace, the foaming slag formed which is transferred to the melting zone has the effect of stabilizing the electric arc and increasing its efficiency.

At certain points in the process, - p. ex. during the casting of the metal and / or during the desulfurization of the metal - it may be advantageous to completely interrupt or to substantially reduce the transfer of the molten products to the metallurgical treatment zone.

Depending on the composition of the raw materials, it may be advantageous to pre-ripen the molten products in the melting zone.

In a preferred embodiment, the gases released during the refining of the molten products are transferred to the melting zone in order to heat the solid products present in this zone. In fact, the refining of the steel bath is accompanied by the formation of abundant amounts of CO (almost half of the CO released by the process is produced during refining). The energy contained in this CO gas can be used to heat the solid products in the melting zone as well as the solid products in an optional solid product preheater either in counter-current or in partial co-current. It is thus possible to recover the energy contained in the hot gases to increase the energy efficiency of the reactor.

In another preferred embodiment, the melting zone is continuously charged with solid products. Since the loading of the melting zone into solid products is continuous, the melting zone permanently contains solid products and the energy efficiency of the melting zone can be maximized.

The solid products are advantageously preheated before loading with hot gases from the reactor. In this way, the gases released during the melting and refining can be recovered to increase the temperature of the solid products before they are loaded into the furnace. Solid products therefore reach their melting temperature more quickly and the melting time is considerably shortened. This leads to an increase in the overall thermal efficiency of the reactor, and possibly in its productivity. Preheating is carried out for example in a preheater which can be executed in the form of a vertical or inclined hopper extending the melting zone or in the form of an inclined rotary drum.

It should be noted that the heating and / or the melting of the solid products can (can) be carried out either using an electric arc or using gas, oil or gas burners. coal either using a combination of these different means.

In addition to the advantages described above, the process of the present invention has other advantages over conventional fusion processes. In fact, since the melting zone can operate continuously and the batch casting is carried out from the metallurgical treatment zone, the downtime caused by loading and casting in conventional furnaces is eliminated and the reduction in the power usable in the final period known as refining and overheating is no longer necessary. Other features and characteristics of the invention will emerge from the detailed description of some advantageous embodiments presented below, by way of illustration, with reference to the accompanying drawings. These show:

Fig. 1: a longitudinal section of an electric furnace for continuously melting solid products during the melting / refining phase - overheating,

Fig. 2: a longitudinal section of an electric furnace for continuously melting solid products during the melting / slag reduction and desulfurization of steel phase, Fig. 3: a longitudinal section of an electric melting furnace continuous solid products during the casting and cleaning phase of the sulfur-rich slag, Fig. 1 shows a section through a continuous melting reactor 10 of solid products, such as solid iron, solid cast iron, scrap iron or pre-reduced iron (DRI), etc., which are used p .ex. for steel production. The reactor 10 is produced as an electric furnace, in which the energy necessary for melting is produced by an electric arc and by burners 12 mounted in the lower lateral part of the furnace 10.

The electric oven 10 comprises a hearth 14 made of a refractory material, surmounted by a tank 16 and a vault 18. At least one electrode 20 mounted on a mast (not shown) via an arm (not shown) ) is introduced into the oven 10 through an opening 22 made in the roof 18. The arm can slide on the mast so as to be able to raise and lower the electrode 20.

The electric oven 10 is subdivided into two separate zones. The first zone, called the fusion zone 24, is loaded, preferably continuously, with scrap metal 25 using a vertical hopper 26 arranged above the fusion zone 24. In this fusion zone 24 , the scrap metal 25 is melted using the electrodes 20 passing through the roof 18 of the furnace 10. To increase the rate of melting of the scrap metal 25, an additional supply of energy is made using the burners 12 in the wall side of the furnace 10. In the melting zone, advantageously, even when the main energy source is an electric arc, an additional 10 to 20 kg of carbon per tonne of steel produced. This carbon is supplied either in the form of coke or of anthracite, or in the form of carburetted metal, eg cast iron. About half of this carbon is removed by injecting oxygen using lances or submerged nozzles so that the molten metal transferred to the metallurgical treatment zone has an intermediate carbon content of the order of 0.5 to 1%. . Due to the injection of oxygen, we are in the presence of an oxidizing slag, which allows the elimination of a large part of the phosphorus in the melting zone. As the scrap 25 melts, liquid metal accumulates in the bottom 14 of the melting zone 24 and when this has reached a certain level, it pours over a weir 27 in the second zone of the furnace called metallurgical treatment zone 28.

In the first phase of the process, the liquid metal in the treatment zone is subjected to conventional refining operations by injection of gases such as oxygen by means of a lance 32 in order to adjust the chemical composition of the metal liquid. During such refining operations, the carbon content of the steel can be reduced from approximately 1% by weight to approximately 0.1%.

The hot gases released during the refining of the molten products are transferred to the melting zone 24 and are then sucked by the hopper 26 supplying the furnace 10 with scrap metal. A large part of the energy contained in these gases can be used to heat the scrap metal 25 in the melting zone 24 as well as the solid products contained in the preheater hopper 26. Here we are approaching a counter-current heat exchanger which has optimal thermal efficiency. Lime (CaO) is added to the melting zone and to the metallurgical treatment zone in order to form a slag there. Various additives such as fluxes can also be added.

In the metallurgical treatment zone 28, the slag is foaming 34 and contains a lot of iron oxides and drops of metallic iron during the refining phase.

The slag contained in the two zones is separated by a slag barrier 36, possibly removable, installed between the two zones at the level of the weir 27. This barrier prevents the slag from passing from the melting zone 24 into the metallurgical treatment zone 28 The fact of separating the slag from the two zones is especially important in the second phase of the process, the desulphurization phase. During the refining and overheating phase, the slag contained in the melting zone 24 and that contained in the metallurgical treatment zone 28 have similar chemical properties, namely that the slags of the two zones are oxidizing slags. For this reason, it is not necessary to separate them during this first phase. The dam 36 can therefore be removed entirely or partially to allow a slag exchange of these two zones. On the other hand, during the desulfurization phase, the chemical properties of the slag contained in the melting and metallurgical treatment zones are different and incompatible.

While the chemical composition of the slag from the melting zone 24 remains unaffected, aluminum and / or silicon and possibly carbon are added to the metallurgical treatment zone 28 in order to transform the oxidizing slag into a reducing slag. Of course, the oxygen lance 32 is stopped during this desulfurization phase.

To guarantee good homogenization of the liquid metal bath, a neutral gas (argon) is injected into the liquid steel bath through one or more porous block (s) 38 through the bottom of the metallurgical treatment zone 28 The eddies created by this injection of gas improve the contact between the liquid steel to be treated and the reducing slag so that the desulfurization takes place under the best conditions.

During this desulfurization phase, it is possible to reduce or even completely shut off the supply of liquid metal coming from the melting zone 24 in order to limit the quantity of unrefined metal in the metallurgical treatment zone 28 during the desulfurization .

Fig. 3 shows the last phase of the process, the pouring of the liquid metal. After refining and desulfurization, the sulfur-rich reducing slag is discharged through the scouring door 40 and the liquid metal is poured from the metallurgical treatment zone 28 through a tap hole 30 while preserving a metal bottom liquid in this area. This bath foot is used to reduce the wear of the refractory lining. It should be noted that the taphole 30 can be arranged in the side wall of the metallurgical treatment zone 28 as well as in the bottom of this zone.

It is advantageous to remove most of the reducing slag at the end of the desulphurization phase and in any case before the next refining to avoid the sulfur contained in the reducing slag does not pass back into the liquid steel when the next charge is refined.

Although in the reactor described above the metallurgical treatment zone 28 operates in discontinuous mode, it should be noted that the melting zone 24 operates continuously. The non-power times caused by the loading and pouring procedures in conventional ovens are therefore eliminated and the reduction in the usable power in the final period known as refining and overheating is no longer necessary.

The material flows and the energy flows from the furnace can be summarized as follows: scrap metal 25 is introduced into the furnace by the hopper 26, it crosses the melting zone 24 and is then drawn off by the treatment zone metallurgical 28.

The gas flow passes through the furnace in the opposite direction. In fact, the gases are injected or formed in the metallurgical treatment zone 28 and the melting zone 24 to be sucked in by the hopper 26.

The slag contained or formed in the metallurgical treatment zone 28 is evacuated by the scouring door 40 located in this zone while the slag contained in the melting zone 24 can be evacuated by a scouring door 42 located in the zone fusion 24. The effectiveness of the process is subsequently illustrated with the aid of two examples.

In the first example, the efficiency of removing sulfur and phosphorus for a charge of ordinary scrap in a conventional electric oven and in a two-zone oven implementing the method according to the present invention is compared. In both cases, 100 kg of slag per tonne of steel is considered.

In the conventional oven, the slag is very oxidizing and contains <0.1% by weight of carbon and approximately 25% by weight of FeO. The partition coefficient for sulfur i.e. the sulfur content ratio in the slag / sulfur content of the metal is less than 5 and that for phosphorus is approximately 50. It therefore succeeds in removing 70% to 80% of the phosphorus initially contained in the metal and about 25% to 30% of the sulfur. The conventional furnace therefore makes it possible to obtain steels from scrap with a very reduced concentration of phosphorus but with a non-negligible concentration of sulfur.

In the process according to the present invention, in the melting zone, a medium oxidizing slag is formed which contains less than 10% by weight of FeO and which has a basicity of approximately 2.5. The partition coefficient under such conditions is 5 to 10 for sulfur and about 25 for phosphorus. Assuming that 32 kg of CaO is used to form 80 kg of slag / 1 of steel in the melting zone, it is possible to eliminate inside the melting zone between 30% and 40% of the sulfur. and between 60% and 70% of phosphorus.

The metal with these reduced sulfur and phosphorus contents is then transferred to the metallurgical treatment zone. In this zone, 20 kg of slag are formed per t of steel by adding 12 kg of CaO per tonne of steel and possibly fluxing agents. After the oxidative refining, the slag is made reducing by adding either aluminum or silicon and / or carbon. The FeO content in the slag is reduced to 0 and the basicity of the resulting slag is approximately 3. The metallurgical treatment zone is subjected to strong stirring with argon. When using aluminum, the partition coefficient for sulfur is around 500 while it is only around 100 when the slag is deoxidized by silicon. Under the aforementioned conditions and using aluminum to calm the slag, it is possible to remove between 80 and 90% of the sulfur contained in the steel supplying the metallurgical treatment zone. When silicon is used to deoxidize the slag, between 60% and 70% of the sulfur contained in the steel fed to the metallurgical treatment zone is removed.

The overall reduction in the sulfur content per 100 kg of slag / 1 of steel is therefore 86% by weight when aluminum is used and 72% in the case of silicon. The overall reduction in phosphorus content is 60% by weight. The present process therefore makes it possible to obtain much lower sulfur contents than in the conventional processes while having comparable performances with regard to phosphorus.

The same analysis can be made for a metal oven comprising a large proportion of pre-reduced or non-desulphurized cast iron; such an oven is used to produce quality end products requiring low copper and sulfur contents.

The table below summarizes the metallurgical performance of a conventional melting process to that of a melting process according to the present invention. In the case of the production of concrete rods from scrap, it can be seen that the method according to the present invention makes it possible to avoid an additional step of desulfurization in the ladle.

When producing wire for wire drawing, the present process makes it possible to use cheaper raw materials or else when the same raw materials are used, it makes it possible to dispense with a second desulfurization step.

Table 1. Comparison of the performances of a conventional fusion process with those of the process according to the present invention.

Claims

claims

1. A method of continuously melting solid metal products in a reactor with two separate zones, the reactor comprising a melting zone and a metallurgical treatment zone, said method comprising the steps of a) continuously heating the solid metal products in the melting zone until the solid products melt, b) gradually transfer the molten products to the metallurgical treatment zone, c) refine the molten products to the metallurgical treatment zone in an oxidizing slag medium, d) separating the slag from the metallurgical treatment zone from the slag from the melting zone, e) transforming the oxidizing slag in the metallurgical treatment zone into a reducing slag, f) desulfurizing the molten products in the metallurgical treatment zone reducing slag medium, g) pouring the molten metal.

2. Method according to claim 1, characterized by the evacuation of the reducing slag from the metallurgical treatment zone before, during or after step g).

3. Method according to one of claims 1 or 2, characterized by the transfer of oxidizing slag from the melting zone to the metallurgical treatment zone before, during or after step c).

4. Method according to one of claims 1 or 2, characterized by the transfer of oxidizing slag from the metallurgical treatment zone to the melting zone before, during or after step c).

5. Method according to one of claims 1 to 4, characterized by the interruption of the transfer of the molten products to the metallurgical treatment zone during step f).

6. Method according to one of the preceding claims, characterized by a pre-refining of the molten products in the melting zone.

7. Method according to one of the preceding claims, characterized in that the gases released in the metallurgical treatment zone during the refining of the molten products are transferred to the melting zone in order to heat the solid products found in this zone .

8. Method according to one of the preceding claims, characterized in that the melting zone is continuously charged with soda products.

9. Method according to one of the preceding claims, characterized by charging liquid iron into the reactor.

10. Method according to one of the preceding claims, characterized in that before their loading, the solid products are preheated using hot gases from the reactor.

11. Method according to one of claims 1 to 10, characterized in that the heating of the solid products is carried out using an electric arc and / or burners and / or plasma torches.

12. Method according to one of the preceding claims, characterized by an energy supply in the metallurgical treatment zone during the initial phase of the process.