RU2268310C2 - Duplex process for stainless steel melting - Google Patents

Abstract

FIELD: non-iron metallurgy, method for high-chromium (more than 9 % of chromium content) intermediate melting in electric arc furnace followed by refining in bottom-blowing converter.

SUBSTANCE: claimed method includes melting of steel waste and alloy additives, deoxidizer feeding, and slag foaming in electric furnace. Than chromium containing in slag is reduced; oxygen is fed into bath, intermediate and slag discharged into overflow ladle, slag is skimmed, metal is charged into bottom-blowing converter, melt is decarbonized, and desired chemical composition is adjusted. For finished and fuller chromium reducing slag-forming additives and deoxidizers are fed in process of intermediate and slag discharging into overflow ladle, when said ladle is filled on 2/3. Method of present invention makes in possible to increase chromium yield from slag at least to 70 %.

EFFECT: increased chromium yield, decreased ferrous alloys consumption, accelerated melt process and reduced energy consumption.

2 cl, 1 ex

Description

The invention relates to ferrous metallurgy, and more particularly to methods for the production of stainless steel by the duplex process, and can be used in the smelting of high-chromium (more than 9% chromium) intermediate in an electric arc furnace with its subsequent refinement in converters with bottom blasting (AOD, CLU, KCB- processes S et al.).

A known method of reducing the chromium content in slag during the smelting of stainless steel in an electric arc furnace, including the melting of steel scrap and alloying additives, the release of molten steel and slag into a ladle, the separation of slag from metal, removal of furnace slag from the ladle, pouring molten steel from the ladle into a converter for refining and regulating its composition by adding alloying elements. In this method, to reduce the chromium content in the slag during the period between the melting of the charge and the release of the melt, finely ground granular ferrosilicon with a fraction of 0.1-5.0 mm, preferably 0.3-3.0 mm, is added to the slag and / or slag (one). This method is the closest in technical essence to the invention and adopted as a prototype.

The disadvantages of this method include the insufficiently high efficiency of mixing furnace slag with seated deoxidizers. Another disadvantage of this method is the use for the recovery of chromium from slag of more expensive deoxidizing agents requiring additional production costs (crushing of ferrosilicon, sieving, granulation, etc.). The degree of extraction of chromium from slag by this method is at least 50%.

The main task to which the invention is directed is to reduce the loss of chromium during the smelting of high-chromium intermediate in an electric arc furnace, as well as reducing the consumption of expensive ferroalloys for alloying steel.

Obtaining a sufficiently low content of chromium oxides in the furnace slag (not more than 3.0%) is ensured by the use of relatively inexpensive and sufficiently effective deoxidizers, as well as by intensive mixing of the sitting deoxidizers with slag.

This object is achieved by the fact that the proposed method of smelting stainless steel by the duplex process, including the melting of steel waste and alloying additives, the deoxidation and foaming of slag in an electric furnace, the reduction of chromium in it, the introduction of oxygen into the bath, the release of the intermediate with slag into the overflow bucket, the downloading of slag , pouring metal into the converter with bottom blasting, subsequent decarburization of the melt and bringing the melting according to chemical analysis to the required contents, while for a final and more complete recovery chrome-plating produces an additive of slag-forming additives and deoxidizers in the process of releasing the intermediate and slag in the overflow bucket, which is carried out in the bucket at the time of its filling by 2/3.

As deoxidants, crushed ferrosilicon, coke, and aluminum waste are used.

The production of stainless steel by the duplex process in comparison with the traditional method of smelting in electric furnaces has several advantages:

- use in the smelting of cheaper ferroalloys with a high carbon content;

- high through assimilation of the main alloying elements (more than 95%);

- a high degree of purity of the metal in terms of gas saturation (oxygen, nitrogen, hydrogen) and non-metallic inclusions;

- high decarburization rates of the melt during steelmaking in the converter;

- high productivity of processes, etc.

In the production of stainless steel by the duplex process (electric furnace-converter) in an electric arc furnace, steel waste (both alloyed, containing chromium and nickel, and carbonaceous) and alloying additives (chromium alloys, nickel and its alloys, etc.) are melted. To reduce chromium fumes during smelting of a high-chromium intermediate in particleboard, as well as taking into account the need for intensive subsequent processing in a converter provided by carbon oxidation and metal heating during the process, melting of the intermediate in an electric furnace is completed at a carbon content of 0.6-1.5 % and silicon content <0.40%.

One of the main elements of the proposed technology is the fast and economical melting of the charge, which ensures maximum productivity with the least wear of the refractory lining and minimal oxidation of alloying elements, in particular chromium. This is achieved by shielding arcs. Moreover, if at the beginning and in the middle of the melting period the electric arcs are shielded by a metal charge, then by the end of the melting the arcs open, causing intense thermal irradiation of the walls and the arch of the electric furnace. In this regard, the main condition for using an intense electric mode during operation at the higher stages of the secondary voltage of the transformer is to shield the electric arc with a melt (foamed slag).

The main conditions for the production of foamed slag during the smelting of a high-chromium intermediate are the reduction of the content of chromium oxides in it by reducing them from the slag to contents not exceeding 15-20%, diluting the slag with lime (increasing and maintaining the basicity of the slag within 1.2-1.7) , as well as the introduction of gas bubbles into it (2).

In order to prevent the oxidation of chromium during the melting of the charge and to reduce the content of chromium oxides in the slag at the end of melting, thereby contributing to the formation of active, liquid-moving slag, as well as heating the melt to the required temperature, up to 5.0 kg / t of crushed ferrosilicon is planted with a basement. An additive of more than 5.0 kg / t of ferrosilicon will require the addition of lime to obtain the required basicity of slag for its effective foaming.

To guide and maintain active, foamed slag during the melting of the charge and at the end of the oxidative purge, the following foaming additives are added as foaming additives: lime - in portions of 200-300 kg (to dilute the slag and maintain basicity within 1.2-1.7), coke with a fraction of up to 8 mm - 50-100 kg each (with the aim of forming CO bubbles, which, when released, will actively mix slag with a deoxidizing agent, thereby contributing to more efficient recovery of chromium from slag) and crushed ferrosilicon or fluorspar, with a fraction of up to 40 mm - 100-200 kg each ( To obtain high activity flowable slag and lowering the content of Cr ₂ O ₃ content in the slag to less than 15-20%, at which slag foaming ability is maximum).

At the end of the melting period, in order to intensify the process, average and obtain the necessary temperature and chemical analysis of the intermediate, the bath is purged with oxygen with a total flow rate of up to 20 m ³ / t at an intensity of 50-100 m ³ / min. Oxygen consumption of more than 20 m ³ / t will lead to excessive carbon oxidation, in which, under the conditions of an electric furnace, intense chromium oxidation will occur. The oxygen purge rate of less than 50 m ³ / min will lead to a delay in the melting time, more than 100 m ³ / min to intensive oxidation of chromium and to obtain a dense heterogeneous slag, which is not very suitable for foaming.

In order to reduce the loss of chromium with slag during the melting of the charge, oxidative blowing of the melt and active foaming, the slag is released through the working window during the entire smelting partially or not at all; obtaining the required deoxidation, reactivity and basicity of slag is achieved by additives deoxidizing agents (ferrosilicon, coke, etc.) and slag-forming materials (lime, fluorspar).

At the end of the oxidative purge of the metal, after obtaining the necessary temperature and the required chemical analysis of the intermediate, the metal together with slag is discharged into the overflow bucket.

The discharge of metal and slag from the furnace is carried out in such a way that the bulk of the slag is released before filling 2/3 of the bucket; at this moment, slag-forming materials (fluorspar and (or) lime) and deoxidizers (crushed ferrosilicon and (or) coke, aluminum production wastes, etc.) are deposited in the bucket for the final, deeper slag deoxidation in quantities ensuring active liquid-moving slag, the complete extraction of chromium from the slag and obtaining the required contents of carbon (0.6-1.5%) and silicon (not more than 0.40%) in the intermediate. In addition to chromium, iron and manganese are almost completely reduced from slag.

The release of metal with slag and the simultaneous addition of deoxidizing agents to the overflow bucket facilitates a more complete (high-quality) extraction of chromium from slag due to rather intensive mixing of slag with deoxidizing agents and metal, which provides a significant increase in the contact surface of pieces of deoxidizing agent with slag.

After the smelting is released, the main part of the slag from the overflow ladle is removed, the intermediate product, together with the remains of the furnace slag, is poured into the converter with bottom blast to further obtain the finished steel by refining (decarburization) of the melt.

According to the proposed technology, more than 100 smelting intermediates of various grades of stainless steel were produced, the extraction of chromium from slag formed during the smelting of the intermediate was not less than 70%, while ensuring the required chemical analysis of the intermediate.

Example. For the smelting of semi-products of 08Kh18N10T grade steel, alloyed (B-26, B-28) and carbon low-phosphorous wastes (AF-15), carbon ferrochrome, and lime - 2 tons were dumped into a heavy-duty arc furnace. The total weight of the charge is 105 tons, the estimated chromium content is 19 , 0%, nickel - 10.5%, carbon - 1.5%. After melting the filling, a basement was made consisting of carbon wastes and nickel, before the introduction of which crushed ferrosilicon in the amount of 500 kg was planted in the furnace. During the period of melting and oxidative purging of the melt, in order to form highly active foamy slag to protect the lining of the walls and maximize the use of the power of the transformer, freshly calcined lime was planted in portions in the furnace to deoxidate and maintain the slag foam - 200-300 kg (based on the calculation of the basicity of slag 1.2-1.7), coke with a fraction of 3-8 mm - 50-100 kg and crushed ferrosilicon - 100-200 kg. After melting the bulk of the charge, after consuming 80% of the electric power, the melt was purged with oxygen through wall tuyeres-burners with an oxygen supply rate of 70 m ³ / min in an amount of 1520 m ³ . At the end of the oxidative purge, a temperature was measured and a sample of the metal and slag was taken. The temperature of the intermediate before release was 1653 ° C; the carbon content in the intermediate was 1.12%, silicon - 0.01%, manganese - 0.53%, sulfur - 0.021%, phosphorus - 0.029%, chromium - 18.32%, nickel - 9.53%. Slag did not go down during the melting and oxidative purge period. The chemical analysis of the slag at the end of the oxidative purge was as follows: CaO - 44.8%, SiO ₂ - 27.2%, Cr ₂ O ₃ - 13.6%, MnO - 2.2%, FeO - 1.23%.

When a semi-finished product with slag was produced, 2/3 of the bucket was seated in an overflow bucket before filling: fluorspar - 250 kg, lime - 730 kg, coke (with a fraction of 3-8 mm) - 130 kg, crushed ferrosilicon - 265 kg, aluminum production waste (with aluminum content 38%) - 210 kg.

After melting, the slag from the overflow ladle (about 80% of the total mass) was poured into an argon-oxygen refining unit and turned into a slag bowl, then an intermediate with residues of furnace slag was poured into the converter for subsequent decarburization of the melt and obtaining finished steel. The temperature of the intermediate after pouring was 1580 ° C. Before starting the process, 1 ton of lime was planted in the converter to increase the basicity of the slag and the decarburization process was started.

In the intermediate before the start of refining, the carbon content was 1.15%, silicon - 0.15%, manganese - 0.55%, sulfur - 0.015%, phosphorus - 0.029%, chromium - 18.96%, nickel - 9.45% . The chemical analysis of the slag before its removal from the converter was as follows: CaO - 46.7%, SiO ₂ - 32.2%, Cr ₂ O ₃ - 1.43%, MnO - 1.14%, FeO - 0.51%.

Using the proposed method for smelting stainless steel by the duplex process allows: to increase the extraction of chromium from slag by at least 70%, to reduce the consumption of ferroalloys for melting, and also, by transferring the operation of slag deoxidation from the furnace to the ladle, to reduce the melting time and consumption electricity.

Sources of information taken into account:

1. European patent EP 1144697 A1, 7C C 21 C 5/52, etc. 20.01.99, the prototype.

2. A.N. Volkodaev, O.K. Tokova and others, “Foaming of chromium slag in a high-power arc furnace,” Steel magazine, No. 6, 1997

Claims (2)

1. The method of smelting stainless steel by the duplex process, including the melting of steel waste and alloying additives, the addition of deoxidizing agents and foaming of slag in an electric furnace, the reduction of chromium in it, the introduction of oxygen into the bath, the release of the intermediate with slag into the overflow bucket, the loading of slag, and the pouring of metal into a bottom blast converter, subsequent decarburization of the melt and bringing the smelting according to chemical analysis to the required contents, characterized in that for the final and more complete reduction of chromium, the additive of slag-forming additives and deoxidizers in the process of releasing the intermediate and slag in the overflow bucket, which is carried out in the bucket at the time of its filling by 2/3. 2. The method according to claim 1, characterized in that crushed ferrosilicon, coke, aluminum waste are used as a deoxidant.