Ferrosilicochromium with a low admixture content, method of its production and its application in production of low carbon ferrochrome

Abstract

Ferrosilicochromium with a low admixture content comprising as a percentage by mass of the total weight of the alloy: not less than 33 chromium, not less than 47 silicon, not more than 0.025 carbon, not more than 0.02 phosphorus, not more than 0.015 nitrogen, not more than 0.03 hydrogen, not more than 0.4 oxygen, not more than 0.005 sulphur, the remainder is iron and traces of impurities, with a chromium to iron ratio of not less than 2.0. The invention also includes a method for producing ferrosilicochromium with a low admixture content and its use in the production of low carbon ferrochrome.

Description

[0001] Ferrosilicochromium with a low admixture content, method of its production and its application in production of low carbon ferrochrome

[0002] The invention relates to ferrosilicochromium with a low admixture content not more than 0.025 wt. % carbon, 0.02 wt. % phosphorus, 0.03 wt. % hydrogen, 0.4 wt. % oxygen, 0.005 wt. % sulphur and 0.015 wt. % nitrogen, with a chromium to iron ratio of not less than 2.0. The invention also relates to a method for producing ferro-silico-chromium with a low admixture content and to the use of ferro-silico-chromium in the production of low carbon ferrochrome (FeCr LC). The invention comprises a solution in the field of metallurgy.

[0003] Ferrosilicochromium is produced in electric submerged arc furnaces using a carbothermic reduction reaction of silica and iron oxide in the presence of high carbon ferrochrome, and then subjected to ladle refining to reduce the carbon content, as well as casting and cooling in a way that limits the entry of gaseous inclusions into the alloy.

[0004] Various types of iron alloys with silicon and chromium are known in the state of the art, including those described in the international standard ISO 5449:1980 (Ferrosilicochromium - Specification and conditions of delivery), which describes an iron alloy with chromium and silicon obtained by reduction or fusion containing chromium in an amount of 20.0 to 65.0 wt.% and silicon in an amount of 10.0 to 60.0 wt.%.

[0005] The international patent application No. WO2019240589 A1 discloses a silicon- based alloy containing 45-95 wt.% silicon, not more than 0.05 wt.% carbon, 0.4-30 wt.% chromium, 0.01-10 wt.% aluminium, 0.01-0.3 wt.% calcium, not more than 0.10 wt.% titanium, not more than 25 wt.% manganese, 0.005-0.07 wt.% phosphorus, 0.001-0.02% wt. of sulphur, and the remainder is iron and incidental impurities in the usual amount. The alloy described said international patent application differs from that described in the present patent application in terms of its calcium content.

[0006] The European patent application No. EP3075869 A1 discloses a method for producing FeSiAl alloys in which carbonaceous rock is mixed with quartzite, iron-bearing material and wood chips, high-volatility carbon if required, in specific amounts, and the homogenised charge mix is loaded into a melting furnace to produce FeSiAl alloy. The furnace charge contains 1.5-4.5% Fe2Os, 55-65% SiC>2, 25-35% AI2O3, 32-34% CaO, 0.3-3% MgO 0.3-2% TiC>2 and traces of phosphorus and sulphur. The alloy obtained in described process contains (wt. %): 40-85% silicon, 1-40% aluminium, 0.001-1.0% carbon, up to 2% titanium, up to 1.0% calcium and traces of phosphorus and sulphur, the remainder is iron.

[0007] From the European patent No. EP2295614 B1 , on the other hand, the composition of the alloy for deoxidation and introduction of alloying additives is known, containing 45.0-63.0 wt.% silicon, 10.0-25.0 wt.% aluminium, 1.0-10.0 wt.% calcium, 1.0- 10.0 wt.% barium, 0.3-0.5 wt.% vanadium, 1.0-10.0 wt.% titanium, 0.1-1.0 wt.% carbon, the remainder is iron.

[0008] Ferrosilicochromium with a low admixture content, particularly of carbon and nitrogen, is currently an especially desirable product. This type of alloy is used in the production of low-carbon ferrochrome (FeCr LC), which in turn is used in the steel industry for the production of special alloy steels and materials, including corrosionresistant, heat-resistant, acid-resistant and high-strength steels for demanding industries, including aeronautics, defence, chemical and energy. However, achieving low levels of admixtures, particularly carbon and nitrogen in ferrosilicochromium is a real technological challenge.

[0009] The purpose of the invention was to obtain ferrosilicochromium with a low admixture content, in particular carbon and nitrogen, and to develop a method for its manufacture and use in the production of low carbon ferrochrome.

[0010] The object of the invention is ferrosilicochromium with a low content of admixtures characterised in that it comprises, as a weight percentage of the total weight of the alloy: not less than 33 of chromium, not less than 47 of silicon, not more than 0.025 of carbon, not more than 0.02 of phosphorus, not more than 0.015 of nitrogen, not more than 0.03 of hydrogen, not more than 0.4 of oxygen, not more than 0.005 of sulphur, the remainder is iron and trace impurities, the chromium to iron ratio is not less than 2.0. The proportion of chromium of not less than 33% and silicon of not less than 47% ensures an adequate reduction potential of ferrosilicochromium. The lower chromium content reduces the efficiency of the FeCr LC production process and its usefulness in the production of alloy steels. The lower silicon content results in a decrease in the reduction potential of the alloy. Preferably, wherein ferrosilicochromium with a low admixture content characterised in that it comprises, as a weight percentage relative to the total weight of the alloy: from 33 to 36 chromium, from 47 to 50 silicon, from 0.020 to 0.025 carbon, from 0.01 to 0.02 phosphorus, from 0.01 to 0.015 nitrogen, from 0.01 to 0.03 hydrogen, from 0.2 to 0.4 oxygen, from 0.001 to 0.005 sulphur, the remainder is iron and trace impurities.

[0011] Preferably, wherein ferrosilicochromium with a low admixture content is characterised in that it comprises as a percentage by weight of the total weight of the alloy: 34.02 chromium, 49.12 silicon, 0.019 carbon, 0.013 phosphorus, 0.011 nitrogen, 0.026 hydrogen, 0.24 oxygen, 0.0015 sulphur, the remainder is iron and trace impurities. This chemical composition guarantees a high reduction potential relative to the chromite ore and ensures minimal non-metallic inclusions in the finished alloy steel.

[0012] Preferably, ferrosilicochromium with a low admixture content is characterised by a chromium to iron ratio of 2.0. A lower value adversely affects the chemical composition of FeCr LC and its usefulness in the production of alloy steels.

[0013] In another aspect, the invention relates to a method for obtaining ferrosilicon- chromium with a low admixture content characterised in that the oxides of silicon (SiC>2) and iron (FeO) are reduced in a carbothermic process in the presence of chromium and iron derived from high carbon ferrochrome in a submerged arc furnace, followed by ladle decarburisation refining using silica fume and subsequent cooling of the melt, the cooling being carried out using a water jet. Controlling the carbon content of alloy steels is extremely important because of the variation in mechanical properties of the finished products depending on the iron to carbon ratio and the separation of M23C6 carbides, which are the main cause of intergranular corrosion. It turned out that the intuitive use of an argon atmosphere shield and argon cooling, which was aimed at cutting off the access of air during the cooling process, did not have the expected effect of obtaining satisfactorily low nitrogen contents in the ferrosilicochromium (Table 6). Such a method also did not ensure sufficiently rapid cooling and did not sufficiently limit the absorption of gases by the alloy. In addition, the use of argon shielding proved to be a complex process to implement on a technical scale. Unexpectedly, it turned out that the use of water jet cooling (pouring water onto the metal) excellently reduced the access of air to the alloy (the protective layer of water vapour formed during the contact of water with the hot metal limited the contact time between the hot metal and the air) and, moreover, the cooling occurred very quickly, leading to the expected ferrosilicochromium meeting expectations in terms of sufficiently low nitrogen content. Preferably, the method for obtaining ferrosilicon-chromium with low admixture content is characterised in that the ladle decarburisation refining using silica fume is carried out up to a temperature of 1400°C. Refining carried out at higher or lower temperatures does not yield favourable results and is inefficient.

[0014] In another aspect, the invention relates to a use of ferrosilicochromium with low admixture content in the production of low carbon ferrochrome.

[0015] The main advantage of the invention is an appropriate chromium to iron mass ratio of no less than 2.0, a chromium content of no less than 33% and a silicon content of no less than 47% to ensure adequate reduction potential of the ferrosilicochromium. An additional advantage of the invention is the reduction of the gaseous inclusions in the alloy, in particular nitrogen, oxygen and hydrogen, below the thresholds required by manufacturers of low carbon ferrochrome as well as high-grade stainless, heat-resistant, acid-resistant and high-strength alloy steels. Ferrosilicochromium with reduced gaseous inclusions is a precursor for the production of low carbon ferrochrome (FeCr LC) and is used as a reductant in its manufacture, so the requirements for these alloys are becoming more stringent as the steel market and metallurgical sciences develop. Nitrogen is an austenite-forming element and, as such, can significantly affect the structure of steel, which in alloy steels, is the main factor affecting their properties. Hydrogen is an element that reduces the mechanical properties of steel through hydrogen embrittlement and the potential for diffusion through the crystal lattice. Oxygen is directly responsible for the corrosion of steel and its increased content can lead to the depletion of steel grains in chromium by increasing the proportion of the oxidised layer. Another advantage of the invention is the low carbon, phosphorus and sulphur content. Controlling the carbon content of alloy steels is extremely important because of the variation in mechanical properties of the finished products depending on the iron to carbon ratio and the separation of M23C6 carbides, which are the main cause of intergranular corrosion. Phosphorus is an element that deteriorates the mechanical properties of steels through what is known as cold embrittlement, due to its dissolution in ferrite and its effect on the formation of band structures in steels. Sulphur has a harmful effect on the mechanical properties of steel by binding iron into sulphides, which affect so-called hot embrittlement. In yet another aspect, a fundamental advantage of the invention is the use of water jet cooling (pouring water over the metal), which excellently reduced the access of air and, moreover, the cooling occurred very quickly leading to the expected ferrosilicochromium meeting expectations in terms of sufficiently low nitrogen content. The object of the invention is illustrated in an embodiment, which does not limit its scope.

[0016] The iron-silicon-chromium alloy presented in the embodiment was obtained by a process in which a raw materials mixture for the production of the alloy is introduced periodically in defined proportions into the working space of the low-shaft, semi-closed submerged arc furnace, using the method of simultaneous high carbon ferrochrome transition into liquid with the parameters listed in Table 1 , reduction of SiC>2 contained in quartzite with the parameters listed in Table 2, the reduction of FeO contained in the mill scale (scale) with the parameters listed in Table 3 by means of the elemental carbon contained in the hard coal with the parameters listed in Table 4 and the chromium carbides contained in the high carbon ferrochrome, followed by dissolution of the resulting Si - Fe alloy in liquid ferrochrome, the melting products are tapped from the submerged arc furnace bottom into a taping ladle, the alloy is transferred into a refining ladle and decarburisation is carried out, followed by casting of the alloy into a set of cast iron ingot moulds and intensive cooling of the alloy using a waterjet.

[0017] Table 1. Chemical composition of high carbon ferrochrome

[0018] Table 2. Chemical composition of quartzite

[0019] Table 3. Chemical composition of mill scale (scale) in terms of pure elements Table 4. Parameters and chemical composition of hard coal

[0020] The furnace used to produce alloy of iron, silicon and chromium according to the embodiment is equipped with a 12 MVA three-phase transformer, a round bath, three self-baking electrodes with a diameter of 900 mm, a low hood with five windows intended for the introduction of the batch mixture using buckets of loading machines and one tapping hole intended for periodic tapping of the smelting products, operating in continuous mode with an active power of 7.8 MW and an electrode current in the range of 32-35 kA.

[0021] The smelting of ferrosilicon-chromium with a low admixture content, particularly carbon and nitrogen, was carried out using raw materials for which the consumption rates are shown in Table 5. The addition of mill scale is required to achieve a certain chromium to iron ratio of at least. 2:1. The chemical composition of FeSiCr is shown in Table 6. Refining of the finished product was based on coating the metal surface with a layer of silica fume at a rate of 50kg per 8,000kg of refined alloy, which binds the silicon carbide formed during lowering the temperature from 1650 - 1550 °C to 1400°C. The tapping was intended to be done through a hole in the side of the ladle in an argon atmosphere and cooling was also done in an argon shield with an argon flow rate of 50 dm3 / min. Such a method did not ensure rapid cooling and did not sufficiently limit the absorption of gases by the alloy, so the cooling method was changed so that a specific chemical composition could be achieved. To this end, a stream of metal was poured directly from the ladle into the mould, bringing the two elements as close as possible to each other (stream length max. 50 cm) at a speed of 1 ,600 kg per minute, and then water was poured over the metal with such intensity that the entire surface of the metal was covered by a layer of water of a minimum thickness of 1-3 mm, thus increasing the cooling rate and creating a protective vapour layer and reducing the contact time between the hot liquid metal and the air.

[0022] Table 5. Raw material consumption rates for the production of the alloy according to the invention

[0023] Table 6 Chemical composition of the alloys with comparison to the composition assumed in the periods performed

[0024] * Individual samples: 1 - FeSiCr unrefined; 2 - FeSiCr after decarburisation refining; 3 - FeSiCr after decarburisation refining and argon cooling; 4 - FeSiCr after decarburisation refining and water cooling; 5-8 - FeSiCr reference samples after decarburisation refining and water cooling.

[0025] Table 6 shows the chemical composition [%] of the alloy obtained depending on the treatment applied. The unrefined alloy comprises significant amounts of carbon (0.170%) and nitrogen (0.023%) disqualifying it as a material for use in the further production of low carbon ferrochrome. Decarburisation improves the parameters of the alloy in terms of carbon content and the values achieved (0.020%) predestine such an alloy for further use in the production of low carbon ferrochrome; however, the nitrogen content (0.025%) is still too high. The intuitive use of argon cooling to restrict air access to the melt and reduce the content of unwanted gaseous impurities in the melt still did not achieve the expected nitrogen content (a nitrogen content of 0.017% was obtained). In addition, the argon cooling process is difficult to implement on a technical scale. Only the use of water cooling unexpectedly allowed obtaining a product with the expected nitrogen content (0.011%).

[0026] The production of low carbon ferrochrome is based on the reduction of chromite ore with ferrosilicochromium. Thanks to the reduction reaction of silicon with O2O3 , there is no need to supplement the reaction with more carbon and consequently maintain a low carbon content in the finished alloy. Ferrochrome prepared in this way can be successfully used in the production of austenitic stainless steels, in which the amount of carbon must be strictly controlled to ensure adequate mechanical properties of the finished product.

Claims

Claims1. Ferrosilicochromium with low admixture content characterised in that it comprises as a mass percentage of the total mass of the alloy: not less than 33 chromium, not less than 47 silicon, not more than 0.025 carbon, not more than 0.02 phosphorus, not more than 0.015 nitrogen, not more than 0.03 hydrogen, not more than 0.4 oxygen, not more than 0.005 sulphur, the remainder is iron and traces impurities, the chromium / iron ratio is not less than 2.0.

2. Ferrosilicochromium with a low admixture content according to claim. 1 characterised in that it comprises, as a mass percentage relative to the total weight of the alloy:33 to 36 chromium,47 to 50 silicon,0.020 to 0.025 coal,0.01 to 0.02 phosphorus,0.01 to 0.015 nitrogen,0.01 to 0.03 hydrogen,0.2 to 0.4 oxygen,0.001 to 0.005 sulphur, the remainer is iron and trace impurities.

3. Ferrosilicochromium with a low admixture content according to claim 1 , characterised in that it comprises as a mass percentage of the total weight of the alloy: 34.02 chromium, 49.12 silicon, 0.019 carbon, 0.013 phosphorus, 0.011 nitrogen, 0.026 hydrogen, 0.24 oxygen, 0.0015 sulphur, the remainder is iron and trace impurities.

4. Ferrosilicochromium according to any of the claims 1 to 3, characterised in that the chromium to iron ratio is 2.

05. A method for obtaining ferrosilicochromium with a low admixture content according to any of the claims 1 to 4, characterised in that the SiC>2 and FeO oxides are reduced in a carbothermic process in the presence of chromium and iron derived from high carbon ferrochrome in a submerged arc furnace, followed by ladle decarburisation refining using silica fume and subsequent cooling of the melt, the cooling being carried out using a waterjet.

6. The method for obtaining ferrosilicochromium with a low admixture content according to claim 5, characterised in that ladle decarburisation refining using silica fume is carried out up to a temperature of 1400°C.

7. Use of the ferrosilicochromium with a low admixture content according to any of claims 1 to 4, characterised in that it is used in the production of low carbon ferrochrome.

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