WO1999053105A1 - Method for making stainless steel and steel with high alloy addition content in an electric furnace comprising two separate zones - Google Patents

Abstract

The invention concerns a method for making stainless steel or steel with high alloy addition content from solid metal products in an electric furnace with two separate zones, namely a melting zone and a metallurgical processing zone, comprising the following steps: a) continuously heating the solid metal products in the melting zone until the solid products are melted; b) adding at least part of the alloy products in the melting zone; c) gradually transferring the melted products into the metallurgical processing zone; d) decarburizing the melted products in the metallurgical processing zone until the final carbon content is not more than 0.6 wt.%; e) casting the melted metal.

Description

MANUFACTURE OF STAINLESS STEELS AND STEELS WITH HIGH CONTENT OF ALLOY ELEMENTS IN AN ELECTRIC OVEN COMPRISING TWO SEPARATE AREAS

Introduction

The present invention relates to a process for producing stainless steels or steels with high contents of alloying elements from solid metallic products.

Traditionally, stainless steels are produced using an electric furnace in which mild steel scrap, stainless steel scrap and alloying elements are melted together. Once the steels have melted, the furnace is shut down and the steels are poured and decarburized in vacuum degassers and / or in converters. Such a sector is described in the review "Steel Times International" of November 1991 pp. 12-16. The productivity of this sector is limited for several reasons:

- the alloying elements added have high melting points and consequently take much longer to melt than scrap, which considerably lengthens the time between two castings,

- the electrical power must be interrupted with each coating, - the cast steels still contain a very high carbon content of between 1.5% and 2%, so subsequent treatments to decarburize are difficult and require a lot of time.

Subject of the invention

The object of the present invention is to provide a process for the production of stainless steels or steels with high contents of alloying elements from solid metal products in an electric furnace having a higher yield. General description of the invention

In accordance with the invention, this objective is achieved by a process for the production of stainless steels or steels with high contents of alloying elements from solid metallic products in an electric furnace with two distinct zones, namely a zone a melting zone and a metallurgical treatment zone, which comprises the steps consisting in: a) continuously heating the solid metal products in the melting zone until the solid products melt, b) adding at least part of the products alloys in the melting zone, c) gradually transfer the molten products in the metallurgical treatment zone, d) decarburize the molten products in the metallurgical treatment zone until a final carbon content less than or equal to 1 % and preferably less than or equal to 0.6% by weight, e) pouring the molten metal. The cast metal is then transferred to a converter and / or a vacuum degasser (VOD) and treated according to the traditional method.

This process increases the productivity of a production line for stainless steels or steels with high alloy contents. In fact, in conventional processes, the cast metal still contains approximately 1.5% to 2% by weight of carbon, since it is not possible to decarburize the metal without having significant losses in terms of alloying elements such as chromium only when temperatures exceed 1600 ° C. However, since chromium is an expensive alloying element, the metal is not decarburized in the electric furnace. If one waited until the liquid metal had reached an adequate temperature for decarburization, the time between two flows would be considerably extended and the efficiency of the electric furnace would decrease.

One of the advantages of this process is that it makes it possible to obtain a steel with a relatively low carbon content, the treatment times in the converter or the vacuum degasser are also shortened since the amount of carbon to be removed is less. The increased productivity of the furnace affects the entire production line for stainless steels or steels with high contents of alloying elements. The method according to the present invention makes it possible to refine the molten metal to a final content of less than or equal to 1% and preferably less than or equal to 0.6% in C because in the metallurgical treatment zone where decarburization is carried out, the temperatures are permanently at around 1700 ° C. Losses of alloying elements and in particular of chromium are not to be feared under these conditions. As the solid metal products are continuously melted and transferred as soon as they are melted in the metallurgical treatment zone, decarburization can be done in masked time, without lengthening the time between two castings.

As the carbon contents are lower at the start, the equipment ensuring the subsequent treatment can be used in a more rational way and their productivity can be increased accordingly.

In addition, as scrap is always present around electric arcs, the scrap is fused with better thermal efficiency.

Due to continuous melting, a certain amount of liquid metal is always kept in the melting zone and the incorporation of the charge of alloying elements is greatly accelerated insofar as this is charged directly into a bath. of liquid metal. The times between two successive flows can be reduced to between 40 and 50 minutes while for conventional methods the times between two flows are of the order of 80 minutes.

In addition to the electric arc, the heating and melting of the solid products can be carried out using burners, plasma torches or using a combination of these means.

Advantageously, an energy supply in the metallurgical treatment zone during the initial phase of the process can be carried out using burners incorporated in the walls of this zone. According to an advantageous embodiment, the content of alloying elements is checked and possibly adjusted before the casting of the molten metal from the metallurgical treatment zone.

The oxygen lance used for decarburization must be located at a predetermined distance from the surface of the bath in order to obtain an optimal yield from decarburization. For this reason, it is preferable to stop decarburization, i.e. oxygen insufflation during the pouring of the liquid metal.

In order to prevent the decarburized charge from being contaminated by molten metal with a higher carbon content, the transfer of molten products from the melting zone to the metallurgical treatment zone can be interrupted during casting. This can be achieved by a removable barrier which is introduced between the two zones at the time of casting.

A certain quantity of molten metal is permanently maintained in the melting zone and in the metallurgical treatment zone. This foot of bath serves to reduce the wear of the refractory lining of the reactor.

If necessary, a pre-refining of the molten products can be carried out in the melting zone by means of moderate oxygen flow rates respectively of a gas containing oxygen. During such pre-refining operations, the carbon content of the steel can be reduced up to approximately 1.6%. The thermal efficiency of the furnace can be increased if the gases released in the melting zone and 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 in this zone. .

Before loading, the solid products are preheated using hot reactor gases which are sucked through the reactor feed hopper.

Description using figures

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, Fig.2: a top view of the oven in Fig. 1.

In the figures, the same references designate identical or similar elements.

Fig. 1 shows a section through a continuous fusion reactor 10 for the production of stainless steels or steels with a high content of alloying elements from solid metal products such as a mixture of scrap from mild steels, of stainless steel and various ferro-alloys. Typically a charge for producing conventional stainless steel with 18% chromium (Cr) and 8% nickel (Ni) consists of about 30% mild steel scrap, about 25% steel scrap stainless, approximately 25% FeCrC (approximate composition Cr 54%, Fe 35%, Si 4%), approximately 20% FeNi (approximate composition Fe 66%, Cr 1%, Ni 30%, Si 1, 5%, C 1, 5%) and about 0.5% in FeSi (approximate composition Fe 25%, Si 75%). Of course, the constituents and their proportions can change depending on availability and the steel that is to be obtained.

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 raise and lower the electrode 20.

The electric oven 10 comprises 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 25 is melted using the electrodes 20 passing through the roof 18 of the furnace 10. To increase the melting speed of the scrap 25, an additional supply of energy is made using the burners 12 in the side wall of the furnace 10.

As the scrap 25 melts, liquid metal accumulates in the bottom 14 of the furnace 10 and when this reaches a certain level, it pours over a weir 27 in the second zone of the oven called metallurgical treatment zone 28.

The metallurgical treatment zone 28 can be produced so that it can be removed from the oven 10 and its content transferred to a converter or to a vacuum degasser. The second zone 28 would therefore not only play a role in metallurgy but would also serve as a transport pocket.

The roof 18 of the oven 10 includes an opening or airlock 29 through which the alloys are introduced into the oven. These alloys are generally heavy ferro-alloys with slow melting and can constitute from 30 to 50% of the total load of the furnace 10. These products fall directly into the foot of bath containing molten metal and start to melt as of their introduction in the oven 10.

As the melting zone 24 operates continuously, the speed of melting of the blocks of alloying elements is no longer a limiting factor of the process. In fact, if the blocks of alloying elements melt more slowly than scrap, it suffices to add a certain excess in the melting zone. As it can be assumed that the melting rate of the blocks of alloying elements is constant, at least when they are submerged in liquid steel, it suffices to add a certain excess in order to obtain a predetermined concentration d alloying elements in the metallurgical treatment zone 28.

Alternatively, part of the total quantity of alloying elements required can be loaded into the melting zone 24, the remainder can be added to the metallurgical treatment zone 28 or even in the converter or in the vacuum degasser.

As the melting zone operates continuously, the melting speed is no longer a limiting factor of the process. In the first phase of the process, the liquid metal in the melting zone 24 is subjected to pre-refining operations by injection of moderate flow rates of approximately 5 m ³ / t of gas such as oxygen by means of a lance (not shown) in order to adjust the chemical composition of the liquid metal. During this pre-refining operation, the carbon content of the steel can be reduced to approximately 1.6%.

The hot gases released during the pre-refining of the molten products are 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 preheating hopper 26. We are approaching here a counter-current heat exchanger which has optimal thermal efficiency.

Lime (CaO) is added mainly to the melting zone intermittently up to approximately 6% of the metal charge in order to form a slag there. Various additives such as fluxes can also be added.

The slag contained in the metallurgical treatment zone 28 is held there by a slag barrier (not shown), possibly removable, installed between the two zones at the level of the weir 27. This barrier prevents the passage of slag from the melting zone 24 in the metallurgical treatment zone 28. The excess slag is discharged through the scouring door 38 fitted in the melting zone 24.

In the second phase of the process, the liquid metal in the metallurgical treatment zone 28 is subjected to further decarburization by injection of high flow rates (25m ³ / min) of gases such as oxygen by means of a lance 32 to decrease the carbon content of the steel to around 0.6%.

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) 40 through the bottom of the metallurgical treatment zone 28 in order to homogenize the bath and to ensure that decarburization takes place in the best conditions. The losses of alloying elements are thus minimized. 8

During this decarburization phase, it is possible to reduce or even completely shut off the arrival 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 decarburization. After decarburization, the liquid metal is analyzed with regard to its temperature and its content of alloying elements and it is possible, if necessary, to adjust the contents of these elements by adding fine-grained ferroalloys in the zone of metallurgical treatment 28.

The liquid metal is poured from the metallurgical treatment zone 28 through a tap hole 30 while retaining a bottom of liquid metal in this zone. This bath foot serves to reduce the wear of the refractory lining and thanks to it the oxygen blowing for decarburization can be resumed immediately after casting.

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.

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 and it passes through 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 present process therefore makes it possible to obtain carbon contents which are much lower than those obtained by conventional methods and this while having comparable performance in terms of chromium and much better results in productivity and energy consumption.

Claims

10 Claims 1. Method for producing stainless steels or steels with high contents of alloying elements from solid metallic products in an electric furnace with two distinct zones, namely a melting zone and a metallurgical treatment zone, which includes the steps of: a) continuously heating the solid metal products in the melting zone until the solid products melt, b) adding at least part of the alloy products in the melting zone, c) transferring as the products melted in the metallurgical treatment zone progressively, d) decarburize the molten products in the metallurgical treatment zone up to a final carbon content of less than or equal to 1% and preferably less than or equal to 0.6% in weight, e) pour the molten metal. 2. Method according to claim 1, characterized in that the heating and the melting of the solid products is carried out incidentally using burners and / or plasma torches. 3. Method according to claim 1 or 2, characterized by an energy supply in the metallurgical treatment zone during the initial phase of the process. 4. Method according to any one of the preceding claims, characterized in that the remainder of the alloying elements is added to the metallurgical treatment zone. 5. Method according to any one of the preceding claims, characterized in that the content of alloying elements is controlled and possibly adjusted before the casting of the molten metal from the metallurgical treatment zone. 1 1 6. Method according to any one of the preceding claims, characterized in that the decarburization is stopped during the pouring of the liquid metal. 7. Method according to claim 6, characterized in that a slag is formed in the melting zone and in that the slag is kept there. 8. Method according to any one of the preceding claims, characterized in that the transfer of molten products from the melting zone to the metallurgical treatment zone is interrupted during casting. 9. Method according to one of the preceding claims, characterized by a pre-refining of the molten products in the melting zone. 10. 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 into the melting zone in order to heat the solid products present in this zone . 11. Method according to one of the preceding claims, characterized in that the melting zone is continuously charged with solid products. 12. Method according to one of the preceding claims, characterized by charging liquid iron into the reactor.