METHOD FOR MAKING STEEL IN AN ELECTRIC ARC FURNACE
The invention relates to the field of metallurgy, and specifically to a method for producing steel in an electric arc furnace. A known method for making steel in an electric arc furnace includes, at least, loading, into the working space of a furnace, a solid metal charge and, at least, solid carbon-containing materials, melting the charge using electric arcs, carburizing the metal using the solid carbon-containing materials during melting, and outputting metal and slag from the furnace. It is proposed to conduct the melting process with the addition, to the working space of the furnace, of a high-carbon carburizer in the form of a liquid phase of iron recovered from the arc combustion zone for the additional carburization of the metal, wherein the high-carbon carburizer is obtained from iron oxides and carbonaceous material. The iron oxides and carbonaceous material are fed into the arc combustion zone, the dimensions of which are limited by D=dp+6del, where dp is the electrode pitch circle diameter and del is the diameter of an electrode. The total carbon content, in free and dissolved forms in the liquid iron phase, does not exceed 30%. The total amount of carburizer used in melting does not exceed 20% of the mass of the metal charge. The carburizer is fed continuously or periodically into the working space of the furnace during the charge melting process. The feeding of carburizer begins as charge melting begins. The use of the invention results in increased liquid metal output by means of regulating carbon content in the course of melting, increasing the carburization level of metal from the very beginning of melting, and reducing the loss of iron to slag and smoke.
The present invention relates to the field of metallurgy, and more specifically, to a steelmaking method in an electric arc furnace (EAF).
A steelmaking method in an electric arc furnace known in the art comprises melting a charge in an electric arc furnace followed by conducting two periods: oxidation and reduction. The main objective of the first, oxidation, period is to remove most of the phosphorus into slag by using solid oxidizing agents, typically, iron ore. The obtained slag is completely removed from the oven, if possible, and treated with lime and chamotte. The metal is heated under said slag, and carbon is oxidized to the set limits.
Prior to the second, reduction, period, the slag in the furnace is completely removed and replaced with fresh non-ferrous slag by adding lime and fluorite; said slag is deoxidized with carbon, silicon, and aluminum to remove the maximum amount of sulfur from the metal. The steel is then alloyed and deoxidized, mainly in the furnace. Said technique limited the furnace capacity and steel quality. [Electrometallurgy of steel and ferroalloys. D. Ya. Povolotsky, V. Ye. Roschin, M. A. Ryss, A. I. Stroganov, M. Ya. Yartsev. Study manual. Moscow. “Metallurgy”. 1974, pp. 213-276].
A new and more effective steelmaking method was developed in the last few decades. It is based on the wide use of bulk oxygen and carbon-bearing materials to intensify the longest process of electric smelting, the melting period [A. N. Morozov, Modern steelmaking in arc furnaces. 2nd edition, Chelyabinsk, Metallurgy, 1987, 175 p.]. Another special feature of the new method is moving a significant part of the manufacturing process: desulphurization, alloying, deoxidation, and, in some cases, decarburization, from the furnace to the ladle in the secondary refining unit [A. N. Morozov, Modern steelmaking in arc furnaces. 2nd edition, Chelyabinsk, Metallurgy, 1987, p. 175; Innovative development of electric steelmaking. A. G. Shalimov, A. Ye. Syomin, M. P. Galkin, K. L. Kosyrev. Monograph. M. Metallurg. Publish., 2014, p. 308]. Today, this steelmaking technique is widely used and is the predominant method of steelmaking in EAFs [A. N. Morozov, Modern steelmaking in arc furnaces. 2nd edition, Chelyabinsk, Metallurgy, 1987, 175 p.].
One type of the modern steelmaking methods uses synthetic composite materials of various compositions, from cast iron- and iron-oxide-based composites (Synticome) to carbon oxide materials [Yu. A. Bondarev, G. N. Yelansky, V. I. Lemyakin, et al. Experiment conducting melting in electric furnaces with carbon oxide cakes. Proceedings from the fifth congress of steelworkers. M. Chermetinformation, 1999, 218 p.].
There is a known method for steelmaking in electric arc furnaces comprising: loading the furnace; charging (loading) with scrap metal, direct-reduced iron, iron- and carbon-bearing materials, and fluxes; loading a solid carbon-bearing material (CBM) in the first charge portion (first bucket or basket); electrode slipping and replacement; adding charge, if needed; supplying electric power, fuel, oxygen gas, fluxes, granulated CBM for foaming the slag; melting the charge; loading with additional CBM through the door in the furnace arch roof, if needed, after the start of melting of the charge; carburizing (carbonizing) the metal during the melting process; heating and decarburizing the metal bath; slag skimming; and metal (semifinished steel) tapping [G. A. Dorofeev, P. R. Yantovsky, Ya. M. Stepanov, et al. Energy Efficiency of Arc Steelmaking Furnaces and Prospective Use of Composite Process Materials. Proceedings from the thirteenth congress of steelworkers. Moscow-Polevsky. 2014. pp. 87-92].
A steelmaking method described in inventor's certificate #1435614 is known in the art (Specification of author's certificate #1435614, priority date Jul. 14, 1986, published Nov. 7, 1988). In said known steelmaking method, liquid resins or pitch, dispersed in the stream of the carrier gas, are introduced into the melt during melting. The carburizer rate varies in the 0.3-25 kg/m3 range of the carrier gas. During carburization, the metal is heated and its gas content goes down.
Carburization, however, can only take place if the furnace charge is completely melted, i.e. during the liquid period of melting, which is very brief in modern furnaces, about 10 min., which constitutes only 10-15% of the time of the total melting cycle. Thus, the balk of melting occurs with no carburization of the metal, which lowers the efficiency of said method. The carbon uptake from the liquid carburizer (resin or pitch) injected into the melt is lowered because the balk of said material is carried out to the bath surface in the return gas flow, thus, increasing the carburizer consumption.
The use of very viscous liquid resin or pitch as a carburizer requires heating thereof, which complicates the EAF construction and the melting process. Therefore, the steelmaking method involving carburization with resin or liquid pitch didn't come into wide practice.
The method of steelmaking in an electric arc furnace disclosed in patent RF 2539890 (Specifications to patent RF 2539890 from Dec. 30, 2013, IPC C21C 5/52, published Jan. 27, 2015) is the closest to the proposed invention in its technical essence and the achieved result. In said method, the charge comprising metal scraps and sintered carbon oxide materials is loaded into the furnace proper; electric power is applied; fuel, carburizer, flux, and oxygen gas are injected; and the charge is heated and melted with electric arcs while the metal bath is decarburized; the metal and slag are then tapped. Before the start of the melting, the central furnace area abutting the ignition area of the electric arcs and not exceeding in size D=(dp+3.5 del), where dp-electrode pitch circle diameter and del-electrode diameter, is loaded with the first portion of metal charge together with a portion of carbon oxide materials in the amount of 10-90% from their total amount used in melting, and the remaining carbon oxide materials are loaded into the molten charge in the course of melting with the specific loading speed of 0.5-10 kg/min per 1 MV-A transformer power of the electric arc furnace, wherein the size of the carbon oxide material pieces is selected in the 5-80 mm range. The walls of the main body of the furnace, along their perimeter, feature at least three apertures for the introduction of carbon oxide materials into the central furnace region, which are located 0.2-1 m below the upper mark level of the furnace body. Said invention allows for the lowering of the specific power consumption used to melt the metal charge, increased yield of iron from the carbon oxide materials, and an increase in the relative amount of said materials in the total charge.
The disadvantages of the known method of steelmaking in an EAF are:
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- 1. Carbon will dissolve in the metal formed in the furnace during the melting period when the solid charge is completely melted and a liquid metal bath has been already formed. Because of that, carburization of the metal is slowed down and deferred until the very end of the melting period. The unique and distinct nature of carbon-bearing materials (CBMs) and their chemical and physical properties, which impede their ability to dissolve in liquid iron, limit the possible carbon content in liquid iron.
Another negative factor is the lower level of carbon uptake (no more than 50%) owing to the partial CBM combustion in the furnace. As a result, the carbon content becomes even lower by the end of the melting process, often not exceeding 0.03-0.05%, especially in the latest generation EAFs, which operate with elevated oxygen gas consumption (35-45 m3/T). Said low carbon content in the metal prior to tapping significantly increases the melting loss and decreases the yield of the metal by 1-4%.
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- 2. Carbon is diffused from the CBM into the liquid iron, and the metal is actually carburized by the carbon dissolved in iron only when the initial metal charge has been completely melted, when the melt is heated to 1,520-1,570° C., only 6-10 min. before the end of the melting process and tapping. Thus, there is not much metal decarburization occurs during the longest and most energy-consuming smelting period, the melting of the charge, which takes the longest time during smelting (20-35 min.). Accordingly, boiling and mixing of the metal and slag, which increase the uptake of the bath energy of the electric arcs and the speed of heating the metal, also remain underdeveloped. The specific power consumption during steelmaking in an EAF, therefore, also increases.
Said disadvantages of the known method based on carburizing the metal with a solid CBM, in the end, adversely affect said parameters of steelmaking in an EAF and create a need to find a technical solution for improving the efficiency of carburizing the metal.
The technical result achieved by the present invention is demonstrated in the following:
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- increased yield of the liquid metal, attained by regulating the carbon content during smelting; increased carburization level from the very beginning of the smelting; and decreased iron loss into the slag and smoke;
- decreased specific electric power consumption for the steelmaking in an EAF because of the additional carburization with no increased power consumption to prepare a liquid carburizer (carbonizer) and continuous decarburizing of the molten metal bath throughout the entire smelting process, including the melting period, when the molten metal bath is in the solid-liquid state and is only moderately heated;
- decreased oxygen content in the metal, achieved by regulating the oxygen content during the smelting process, and increased metal carburization level from the very beginning of smelting, from the start of the melting period, with no increased consumption during the smelting of solid CBMs, loaded into the furnace together with the metal charge.
The metal in the proposed steelmaking method is carburized (carbonized) with a liquid solution of carbon in direct-reduced iron; said solution was formed during the melting of the initial metal charge right in the EAF from the loaded into the furnace materials containing iron oxide and the reducing agent (carbon), and not from the charge, as observed in the known method. The use of the liquid high carbon iron melt instead of the solid CBM, which takes a long time to dissolve in iron, considerably accelerates the carburization of the metal. The presence of carbon from the very beginning of melting in the metal melt was formed from the melting of the metal charge promoted an earlier and more intensive carbon oxidation. Carbon monoxide formed in the decarburization causes bubbling, boiling, and mixing of the metal and slag, thereby putting the smelting into a continuous boiling mode. Said factors increase the heat uptake and accelerate the heating of the metal, which reduces power consumption. Iron loss into the slag and smoke is concurrently reduced, which increases the metal yield. Said factor, therefore, is especially important because it puts the melting process into a continuous boiling bath mode, from the start of the metal charge melting to the end of smelting.
To solve said problem and to achieve the desired technical result in the known method of steelmaking in an electric arc furnace comprising: loading a solid metal charge and at least solid carbon-bearing materials into the furnace proper; melting the charge with electric arcs; carburizing the metal with solid carbon-bearing materials during the melting process; and tapping the metal and slag from the furnace, the melting process is conducted with a high-carbon carburizer as a liquid phase of the iron reduced in the arc ignition zone added into the furnace proper in order to further carburize the metal, wherein the high-carbon-content carburizer is obtained from iron oxides and a carbonaceous material.
To achieve the best technical result:
the total carbon content in free and dissolved forms in the liquid iron phase should not exceed 30%;
the total carburizer used for melting should not exceed 20% from the weight of the metal charge;
iron oxides and the carbonaceous material should be injected into the arc ignition zone with the diameter not exceeding D=(dp+6del), where dp electrode pitch circle diameter and del electrode diameter;
the carburizer should be injected into the furnace cavity continuously or periodically during the melting of the charge;
the start of the carburizer feed should coincide with the start of the charge melting;
iron oxides and the carbonaceous material should be injected into the arc ignition zone at the same time;
iron oxides and the carbonaceous material should be mixed together prior to being injected into the furnace, and should be loaded as an intimate mixture;
Iron oxides and the carbonaceous material should be sintered prior to being injected into the furnace, and should be loaded sintered;
the size of the iron oxide particles and the carbonaceous material before being loaded into the furnace should not exceed 25 mm;
the total carbon content in free and dissolved forms in the liquid iron phase should be at least 2%;
the total amount of the carburizer used in smelting should be at least 20% of the metal charge weight.
Additional carburization of the metal during the melting process is conducted in tandem and simultaneously with the existing carburization method with a solid CBM, comprising the following steps: injecting said CBM into the charge, loading it into the furnace, melting the charge, and dissolving the carbon in the metal. The combination of two carburization techniques guarantees an early carburization of the metal, thereby achieving the desired carbon concentration at the start of the melting, when the first portions of the molten metal appear. Carbon oxidation and monoxide bubbling take place while stirring the metal and slag over the entire melting period, from the beginning to the end of melting, which lowers the energy consumption and increases the yield of iron from the charge.
The option, contemplated by said solution, of injecting the raw materials in the course of melting into the central hottest zone and into the cavities formed in the charge (so called ‘wells”), which have the highest energy concentration and exceptionally high temperatures, also plays part. Said factors contribute to the fast formation of the direct-reduced iron; carburization thereof; production of molten high carbon iron, which serves as a “liquid” carburizer directly in the EAF during melting; and acceleration of the whole carburization process.
Using the liquid solution of carbon in iron, formed during the charge melting, rather than a solid CBM leads to an earlier carburization of the molten mass formed from the solid metal charge at the very beginning of smelting (melting period), moreover, conducted in a controlled mode. That makes the use of CBMs in electric smelting more efficient and also improves the EAF performance in steelmaking, including electrical energy and metal charge consumption.
The presence of carbon in two states, (i.e. in the atomic state (diluted) chemically bound to iron and in the free state (as ultrafine particles)) in the “liquid” carburizer can be explained as follows. On the one hand, that can greatly increase the carburizing ability of the material as compared to the potential carburizers: high carbon iron alloys such as cast iron, and thereby cut back or completely cut out their use in smelting. On the other hand, it guarantees that the solid CBM is preliminarily dispersed into the smallest particles, about 10−3-10−6 cm, and maintains its activity on the highest level, close to 1. That, in turn, guarantees that the carbon is quickly dissolved in the metal bath of the furnace after the influx of the “liquid” carburizer into the bath and that the iron-carbon melt has the highest weight uniformity. That protects the metal from retaining some carbon groups, which would adversely affect the steel quality.
Total carbon content concentrations in the solution of carbon in iron are selected in the 2-30% range. A low carbon content (less than 2%) is undesirable both because of the increased refractory quality of the carburizer, and because of the reduced concentration of the main element, carbon, which lowers the efficiency of the method. A high carbon content, more than 30%, impedes the feed of said carburizer from its formation zone into the metal bath because the molten carburizer becomes less fluid due to the presence of solid carbon particles therein.
The total consumption of the carburizer (liquid carbonator) during melting is set to be in the 1-20% range from the weight of the charge. A low consumption of the carburizer (less than 1%) impedes the carburization effect because of the insufficient amount of carbon entering the metal. A high consumption of the carburizer (more than 20%) isn't advisable, as the carbon content in the metal bath then reaches extremely high concentrations due to the formation of large amounts of the carburizer.
The steps of injecting iron- and carbon-bearing materials into the furnace throughout the melting of the metal charge, the following heating of the materials, iron reduction, carburization thereof, formation of the carbon solution in liquid iron (“liquid carburizer) all concur in time and space and occur in tandem and simultaneously in the present invention as opposed to the known method. The proposed method allows for a partial or, if needed, a full replacement of the existing consecutive carburization with a solid CBM, with simultaneous carburization, which is faster and more efficient.
All the stages of the carburization process in the proposed method of steelmaking in an EAF can be combined by injecting iron- and carbon-bearing materials into the hotter region of the furnace, wherein a large amount of heat is released due to the electric-to thermal-energy conversion. Because of that, the heat-producing region is combined with the operational area, in which the injected materials react with each other producing a liquid solution of carbon in iron (“liquid” carburizer). That occurrence greatly facilitates the carburization of the metal, and does that from the very beginning of melting. Accordingly, the decarburization of the bath also starts from the moment of melting and lasts until the end of smelting, ensuring that the metal and the slag boil and mix together throughout the smelting, including the melting period. As a result, energy consumption is reduced and the yield of iron from the charge is increased.
The area of the region wherein raw materials are loaded, should not exceed D=dp+6d3n. If its size is increased any further, the materials end up in the cooler regions, get stuck together, and form conglomerates on the walls of the furnace. Their melting would then require additional energy and time, which reduces the efficiency of the method.
The proposed method of steelmaking in an electric arc furnace comprises the following steps: charging the furnace; loading the furnace proper with a solid metal charge and, as a minimum, solid carbon-bearing materials for the carburization of the metal; melting the charge with electric arcs; carburizing the metal with carbon-bearing materials throughout the melting process; tapping the metal and slag from the furnace, wherein as the charge begins melting, the molten metal formed from the metal charge is injected with a carburizer as the liquid phase of the metal, reduced in the arc ignition region, with the total carbon content in free and atomic states in the liquid iron phase not exceeding 30%, thereby ensuring a supplementary carburization of the metal with another type of carburizer in the concurrency mode.
The carburizer as a liquid phase of the carburized metal reduced in the arc-burning region is obtained from iron oxides and a carbon-bearing material having particle size not exceeding 25 mm, which is injected into the arc-burning region and the adjacent regions thereto (into the central hottest zone of the furnace), with the size in the range not exceeding D=dp+6del, where dp electrode pitch circle diameter and del electrode diameter. The total consumption of the carburizer during smelting doesn't exceed 20% of the metal charge weight. Injection of the carburizer into the molten metal occurs throughout the melting process of the charge, wherein the start of the carburizer injection is timed with the start of the melting thereof; either that, or injection of the carburizer into the molten metal is conducted throughout the smelting. In order to achieve the best technical result, iron oxides and the carbon-bearing material are premixed together and injected as an intimate mixture. To speed up the carburizer formation, it is advisable to pre-sinter the iron oxides and the carbon-bearing material.
Metallic-iron-based solid materials are used as the metal charge. They include: scrap steel, pig iron, Synticome, scrap materials, nameplate charge, various metal waste, and direct-reduced iron in the form of metallized pellets, sponge iron, ball iron, and partially reduced iron ore.
Said carbon-bearing material loaded into the furnace proper throughout the melting process together with iron-bearing materials in order to obtain a carburizer include: coke; graphite; anthracite; thermal anthracite; coal; charcoal; and metallurgical, chemical, and other waste that contains carbon as the main ingredient, including low-mesh coke and crushed electrodes. Said material is the source of carbon, which simultaneously reduces iron from the oxides thereof and carburizes molten iron formed from the metal charge.
Said iron-bearing materials comprise solid oxidizers containing iron oxides Fe2O3, Fe3O4, and FEO, the typical examples of which are: iron ores, concentrates, superconcentrates, agglomerates, and the mixtures thereof as well as metallic iron particles formed in steelmaking and metalworking, i.e. turnings; both steel and cast iron; buckshot; metal scrap formed during metal cutting; etc.
After carbothermic reduction, iron oxides form liquid direct-reduced metallic iron, which is simultaneously carburized and then flows into the metal bath thereby carburizing it. To achieve that, the CBM content in the total mass of the charged iron- and carbon-bearing materials should exceed the stoichiometric value from the (Fe2O3)+3C=2[Fe]+3{COr} reduction reaction, which is 321 kg of carbon per 1 T of iron.
The proposed method was employed to conduct a series of 17G1S-U and 22G21-7 steel melts in a modern electric arc furnace, DSP-160 model, with the rated charge capacity of 175 T, operating with a 100% solid charge, following the technique with liquid metal and slag remains (“hot heel” practice). Parallel to the experimental melts according to the proposed method, comparative melts were also conducted, melting the same gauge steel. The comparative steel melts were conducted following the effective engineering process documentation using modern oxygen and injection techniques. The experimental melts were conducted with the same power engineering parameters as the comparative melts, i.e. no additional power engineering parameters were applied to prepare the carburizers.
In contrast to the current method, in the proposed method, in addition to the carburization of the metal by breeze coke added to the initial metal charge (first basket), the metal was simultaneously carburized with a liquid carburizer obtained directly in the furnace. For that, iron- and carbon-bearing materials in the form of dross and breeze coke were injected into the furnace during the melting period, with the obtained carburizer used for the melting totaling 20% from the weight of the charge.
Said materials were loaded through the door in the roof of the furnace into the arc ignition zone in the center of the hot region of the furnace, measuring 4.5 m. That conformed to the parameters of the present invention, according to which the diameter of said zone should not exceed d=dc-F6del. The content of the breeze coke in the total amount of the loaded materials was 4-48%.
In some of the melts, breeze coke was injected before the dross, and in others, they were loaded simultaneously. In addition, sometimes, breeze coke and dross were loaded as a mixture, and sometimes, they were loaded after sintering, as briquettes measuring 60×60×80 mm. When making briquettes, cement was used as a binder, in the amount of 8-12% from the weight of a briquette. Most of the breeze coke and dross were 0.5-1.0 mm in size.
Said materials in the amount of 1-20% were loaded into the furnace 1-3 min. after the power had been turned on and free voids started to form in the top layer of the metal charge due to the melting of the solid charge and liquefying thereof, thus freeing a part of the volume filled with the charge. When the raw materials, dross and breeze coke, entered the arc ignition zone, they were heated to high temperatures and reacted with each other. The products of said carbothermic reaction were liquid direct-reduced iron and carbon monoxide. Owing to a significant excess of carbon, the reduced iron carburized, forming a liquid carburizer. The latter consisted of a saturated solution of carbon in iron and a solid phase dispersed therein as ultrafine graphite particles, forming a colloidal dispersion system. The high carbon melt (up to 30%) flowed into the metal bath while carbonizing the metal from the start of the metal charge melting and throughout the entire melting process. Carbon monoxide released in the carbothermic reduction of iron oxides was partially burned to CO2 emitting heat. Said heat transferred to the materials melting in the furnace, additionally heating them, thereby reducing the energy consumption.
During the smelting process, EAF operating conditions, including total time of the melting cycle; time of the furnace operating under current; furnace downtime; consumption of charge materials, electric energy, natural gas, oxygen, carburizer, iron- and carbon-bearing components used in the smelting process; and lime, metal, and slag composition, etc. were recorded. The summary of the technical and economic performance indicators of the melts conducted according to the present invention and following the known method, using the charge with the same parameters, are shown in Table A.
As seen from the results of the experimental melts, the proposed method offers higher yields of the liquid metal, lower energy consumption, and reduced oxygen content in the metal during tapping owing to the higher carbon content throughout the smelting. That stems from the additional carburizing of the metal with a liquid carburizer obtained directly from the iron- and carbon-bearing components loaded into the furnace.
The lower oxidation level of the metal and slag due to the elevated carbon content in the metal throughout the melting process of the bath improves the steel quality as to the oxide non-metallic inclusions.
Comparing parameters of the proposed steelmaking method with the known method parameters confirms its efficiency.
The proposed method of steelmaking in an EAF is fundamentally different from the known method in the mechanism, kinetics, and thermodynamics of the process of carbon diffusion into the metal and in the total character of carburization (carbonization).
The additional carburization step introduced into the steelmaking process in an EAF, which is different from the known method in the specific nature, mechanism, and the input mode thereof, drastically modifies the electric smelting technique, assigning the paramount role to the new process. That can be attributed to the fact that carbon oxidation is the key reaction in steelmaking and the base of all modern steelmaking methods, including steelmaking in an EAF, and it defines the dominant parameters of steelmaking. Thus, said feature is quite significant, thereby characterizing the novelty and importance of our technical solution.
Owing to said technique, the carbon content during smelting can be regulated in the proposed steelmaking method throughout the melting process, which is undoable in the known steelmaking method employing the carburization with solid CBMs. In addition to regulating the carbon performance mode, the present invention can significantly increase the carburization level of the metal, and do that from the very beginning of smelting, from the starting moment of the melting period. That creates conditions for the continuous decarburization of the metal bath throughout the smelting, including the melting period when the bath is in a solid-liquid state and is poorly heated.
The metal in the proposed steelmaking method is not carburized with the carbon from the initial charge, as observed in the known method, but with the liquid carbon in direct-reduced iron solution, formed right in the EAF during melting of the initial metal charge from the raw materials injected into the furnace, which are 25 mm in size and contain iron oxides and a reducing agent (carbon) that were loaded into the furnace.
Using the liquid high carbon iron melt instead of a solid CBM, which takes a long time to dissolve in iron, considerably accelerates the carburization of the metal. The presence of carbon in the metal melt formed from the very beginning of melting, during the melting of the metal charge, resulted in earlier and more intensive oxidation of carbon. Carbon monoxide formed in decarburization causes bubbling, boiling, and mixing of the metal and the slag, thereby putting the smelting into a continuous boiling mode. Said factors increase the degree of heat uptake and accelerate the heating of the metal, which reduces power consumption as well as lowers the oxidation level of the metal and slag.
At the same time, the loss of iron into slag and smoke is also reduced, which increases the yield of the metal.
The liquid solution of carbon in direct-reduced iron, in addition to the common forms of carbon, at the same time also contain its other forms, which can be attributed to the higher consumption of the carbonaceous material. Under such conditions, the balk of carbon exists in the free form as separate ultrafine particles 10−3-10−7 cm in size, forming a separate solid phase dispersed in the iron-carbon melt similar to common cast iron. That can be attributed to the fact that in the arc ignition zone, carbon in a solid CBM, similar to cast iron, is transformed into graphite, which structure is naturally similar to graphite.
The rest of the carbon is in the solution of carbon in iron in the atomic (dissolved) form, forming a true iron-carbon solution. The latter has characteristic chemical bonds between said elements and thus, the carbon in such form is bound to iron, forming the Fe—C chemical bond. Due to said noted aspects, the proposed method offers a different type of a carburizer, used as a liquid. Carburization of the particles of said type, as a whole, is a colloidal dispersion system having boundary lines with liquid iron.
From the chemical and physical standpoint, said carburizer is a liquid system comprising a saturated solution of carbon in iron and a solid phase in the form of graphite particles, ranging in size from 10−3 to 10−7 cm, dispersed therein. In other words, the carburizer of this type is a colloidal dispersion system, wherein carbon particles exist both as individual atoms and in a free state as ultrafine graphite particles, which, unlike true solution, have iron-carbon phase boundaries with liquid iron. Characteristics of such systems are carefully examined and shown in the monograph [Properties of Iron Melts. A. A. Vertman, A. M. Samarin. Publisher “Science”, 1969, 1-280].
The total carbon content in the liquid carburizer, which is in the 2-30% range, is considerably higher than the carbon concentration in iron-carbon melts used in steelmaking; thereby considerably increasing the carbonizing ability thereof compared to cast iron, which is known for its highest carbon content. Thus, said feature is essential, as it creates the greatest degree of metal carburization as compared to other potential carburizers based on the iron-carbon system.
Iron- and carbon-bearing materials are injected into an EAF throughout the smelting in the amount of 1-20% from the weight of the metal charge, thus creating a “liquid” carburizer. Iron oxides and the carbonaceous material, when present in said range in the injected materials, create the necessary and sufficient conditions for the carbothermal reduction of iron to take place at high speed and to produce liquid reduced carburized iron. The molten iron with high carbon content of 2-30% is a special kind of a carburizer, differing from the known solid CBM by being liquid. Therefore, the obtained carbon solution, already during the formation thereof, flows into the metal bath as a “liquid” carburizer in iron, thereby carburizing said bath. Advantages of said carburizer over a solid carburizer are obvious.
One of the essential features of the present invention is the unique nature of the carburizing as a whole. There is a continuous steelmaking method known in the art using a solid CBM as the carburizer; said CBM is injected into the charge, said charge is melted, and the molten metal is carburized. Carburization is conducted differently in the proposed method: all steps and processes are integrated in time and space; they occur in tandem and simultaneously. Said processes include: carbothermal reduction of iron from its oxides, formation of a liquid solution of carbon in direct reduced iron, said solution flowing into the metal bath followed by carburization of the metal formed in the furnace from the solid charge upon melting thereof. The integrated carbonization processes shorten the carburization time and the time of the melting period as compared to the consecutive processes. That results in reduced energy consumption and increased yield of the iron from the charge. Thus, the change in the nature of the carburization process in the proposed smelting method is an essential feature.
Iron- and carbon-bearing materials are injected into the central arc-burning region and the adjacent regions thereto (the hottest zone of the furnace), with the size in the range not exceeding D=dp+6del. Carbothermal reduction of iron from its oxides is energy-consuming and endothermic. Increased heat concentration and increased temperatures, therefore, accelerate said reaction and the comprehensiveness of iron oxides transformation into the metallic state. The arc-burning region is known in the art of steelmaking for the highest energy concentration because of the high specific power rating, about 10 mVA/m3 or 1500 kVA/T of the metal.
According to various sources, the temperatures in the ignition zone of the electric arcs is in the about 4,000-15,000° K range, which is close to the temperature of low-temperature plasma. The temperature on the surface of the metal bath, located directly under the electrodes, is about 2,600° C. Being significantly higher than the melting points of iron and its alloys with carbon, the aforementioned temperatures facilitate the formation of direct reduced iron and highly concentrated carbon solution therefrom. Thus, the proposed injection of iron oxides and a reducing agent, which constitute the basis of the iron- and carbon-bearing materials, into the central EAF zone is an essential feature of the proposed technical solution.
Additional advantages of said technical solution are in the considerably increased heat uptake from the burning electric arcs, which increases the density of the charge due to the material being injected into the hottest region of the furnace, which has a high concentration of energy and temperatures. That, in turn, curbs the loss of heat and lowers the energy cost. With that in mind, injection of the materials into the central region of the furnace is a quite significant element of the new steelmaking method.
Injection of iron- and carbon-bearing materials into the hottest region of the furnace is especially important from the standpoint of improving the energy performance of AEFs. Here, metallurgical materials are injected into the arc-burning zone with thermal conditions close to those of low-temperature plasma. Because of that, the high temperature heat generation zone overlaps with the process area, in which the cold raw materials are heated to form a solution of carbon in liquid iron with high (30%) carbon concentration. Said overlap in time and space between the heat generation zone and the process area that uses the applied heat makes heating, melting, iron reduction, iron carbonizing, and, as a whole, obtaining a “liquid” carburizer conditions close to the ideal. This drastically improves the heating conditions of melting, accelerating the heat transfer from the arc to the injected materials, thereby reducing heat loss.
Claims
1. A method for steelmaking in an electric arc furnace comprising the following steps: at least loading a solid metal charge and at least solid carbon-bearing materials into the furnace proper; melting said charge with electric arcs; carburizing the metal with the solid carbon-bearing materials throughout the melting process; and tapping the metal and slag from the furnace, wherein during the melting process, a high carbon content carburizer is added into the furnace proper as a liquid phase of the iron reduced in the arc ignition zone, in order to additionally carburize the metal, wherein the high carbon content carburizer is obtained from iron oxides and a carbonaceous material.
2. The steelmaking method of claim 1, wherein the total carbon content in free and dissolved forms in the liquid iron phase does not exceed 30%.
3. The steelmaking method of claim 1, wherein the total carbon consumption for the smelting does not exceed 20% of the metal charge weight.
4. The steelmaking method of claim 1, wherein iron said oxides and said carbonaceous material are injected into the arc ignition zone not exceeding D=(dp+6del) in size, where dp-electrode pitch circle diameter and del-electrode diameter.
5. The steelmaking method of claim 1, wherein the carburizer is injected into the furnace body continuously or periodically throughout the melting process of the charge.
6. The steelmaking method of claim 1, wherein the start of the carburizer injection coincides with the start of the charge melting.
7. The steelmaking method of claim 1, wherein the iron oxides and the carbonaceous material are injected into the arc-burning zone at the same time.
8. The steelmaking method of claim 1, wherein prior to being injected into the furnace, the iron oxides and the carbonaceous material are premixed together and injected into the furnace as an intimate mixture.
9. The steelmaking method of claim 1, wherein prior to being injected into the furnace, the iron oxides and the carbonaceous material are presintered and injected into the furnace sintered.
10. The steelmaking method of claim 1, wherein prior to being injected into the furnace, the particles of the iron oxides and the carbonations material do not exceed 25 mm in size.
11. The steelmaking method of claim 1, wherein the overall carbon content in free in dissolved forms in the liquid iron phase is at least 2%.
12. The steelmaking method of claim 1, wherein the total amount of the carburizer used in the melting is at least 20% from the weight of the metal charge.
Type: Application
Filed: Jul 25, 2016
Publication Date: Oct 25, 2018
Inventors: Genrikh Alekseevich DOROFEEV (Tula), Pavel Rudolfovich YANTOVSKY (Khimki), Konstantin Gennadievich SMIRNOV (Khimki)
Application Number: 15/749,789
