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Peculiarities of producing intermediate product for manufacturing especially low-carbon steel in electric arc furnace.

At present production of steels with especially low content of carbon and other impurities, characterized by high ductility properties (IF-steels), which are widely used in production of thin sheets for their subsequent deep (super-deep) drawing and application of coatings by hot method, is widely used in the world. At the same time steels, composition of which is close to that of IF-steels, due to low value of their tensile strength (less than 300-340 N/[mm.sup.2]) represent interest as super-ductile rolled wire for production of thin wires. Due to low content of carbon, alloying elements and impurities conversion of such rolled wire is possible by drawing without intermediate annealing, which allows significant reducing of hardware production cost.

All IF-steels are molten in converters with combined blowing by oxygen and argon [1]. Present trends in development of state-of-the-art steel making industry is bringing nearer possibilities of the main steel-melting units of ferrous metallurgy-oxygen converters and arc steel-melting furnaces (ASMF) and even development of a kind of the arc steel-melting converter [2]. In state-of-the-art ASMFs, which operate with blowing of significant amount of oxygen, in technological chain of the ladle-furnace and evacuator units steels of the whole assortment are molten, which were previously produced only in oxygen converters, including those with low content of carbon.

Assortment of profiled rolled stock produced by Company <<Moldavian Metallurgical Works>> (MMW) is extraordinary wide and includes both high-carbon grades of steel for metal cord and low-carbon ones for manufacturing wire and reinforcement bars. Wide technological possibilities and high culture of production of MMW allowed setting task to melt especially low-carbon steel for producing high-ductility rolled wire from electric furnace intermediate product.

Technological possibilities of MMW in producing low-carbon steels. Electric arc furnace of MMW DSP-2 has rated capacity 120 t and power 95 MV-A (in standby 80 + 12%). Technology of production envisages feeding into the furnace for heating of the metal rather high amount of carbon-containing materials (coke, natural gas) and blowing of oxygen for combustion and oxidizing of metal charge impurities by means of several kinds of burners and lances. So, on the DSP-2 furnace the following equipment is used:

* a system of wall gas-oxygen burners (4-6 single-type jet burners of 3.6 MW power with flow of oxygen 4800 [m.sup.3]/h), ensuring production of strong flame, which allows efficient heating and cutting of metal charge and burning up of CO and C[O.sub.2] in working space of the furnace);

* manipulator <<Palmur>> for blowing of oxygen and powder-like coke for the purpose of acceleration of the metal charge melting in cold zone of the furnace, mixing of molten metal due to high rate of oxygen injection (flow of oxygen is up to 3000 [m.sup.3]/h and of powder-like coke--up to 40 kg/min), decarbonization of molten steel pool by oxygen, burning up of CO by injection of gaseous oxygen, and formation and maintaining of foamed slag;

* MMW manipulator for acceleration of the melting process and improving metal quality due to melting of metal charge in the area of the ASMF working window by gas-oxygen burner and controlled decarbonization of the melt by powerful and compact jet of oxygen (flow of oxygen in oxygen lance is up to 3000 [m.sup.3]/h, in gas-oxygen burner of 7.3 or 10 MW power--up to 2200 m /h, flow of natural gas is up to 1100 [m.sup.3]/h) and saving of electric power due to inclusion of additional alternative power source and reduction of the furnace operation period under current;

* oxygen lance of the Stein Lange company for producing additional thermal energy by feeding into the furnace of natural gas and oxygen (the power is 3.2 MW, flow of natural gas is up to 350 [m.sup.3]/h, flow of oxygen per a lance is up to 2000 [m.sup.3]/h and per a burner--up to 700 [m.sup.3]/h) which allows increasing productivity of a steel making unit, reducing consumption of electric power and graphitized electrodes, and increasing general power;

* system of side oxygen lances for ensuring supply of gaseous oxygen in the pool through two lances in the area of the fifth and the seventh water-cooled panels of the furnace, which ensures intensification of the process of the metal charge melting in the zones that are not affected by direct action of electric arcs or gas-oxygen burners, mixing of the melt in the process of steel melting, oxidation of carbon, and burning up of CO in working space of the furnace.

If necessary, application of oxygen tube is allowed in technology of steel production.

[FIGURE 1 OMITTED]

Lining of the furnace working layer is made of high-resistant periclase-carbonaceous refractory materials (more than 700 melts).

Mentioned possibilities allow melting in the electric arc furnace of intermediate products with different content of carbon, including low one (less than 0.03%), which significantly simplifies in further out-of-furnace processing production of especially low-carbon steel.

Technological possibilities of production allow performing further out-of-furnace processing of an intermediate product in the evacuator and the ladle-furnace unit according to the schemes: ASMF-ladle-furnace-evacuator-CCM (a direct one) and ASMF-evacuator-ladle-furnace-CCM (a reverse one).

The goal of this investigation consisted in development of technology for producing a low-carbon intermediate product in electric arc furnace DSP-2 of MMW for manufacturing especially low-carbon steel for ductile rolled wire. Maximum allowable content of carbon in steel was up to 0.01%, silicon and manganese--up to 0.01 and 0.12%.

Statistical analysis of technology for production of low-carbon intermediate product in electric arc furnace. For the purpose of substantiating rational parameters of technology for producing especially low-carbon steels statistical analysis of parameters of more than 200 melts of current production was carried out.

For determining efficiency of carbon oxidation in the DSP-2 pool by different blowing devices dependences of its final content in the intermediate product upon specific consumption of oxygen in these devices were established (Table).

It was established that consumption of oxygen in fuel devices (<<Palmur>> and Stein Lange oxygen lance) insignificantly effected content of carbon in metal at the outlet; influence of oxygen consumption in <<non-fuel>> devices--its supply to bottom lances and oxygen tube that was used not in all melts (40.43 and 30.85% of general number of the sampling), was appreciable, which is a reserve for obtaining low content of carbon at the outlet.

[FIGURE 2 OMITTED]

Slag conditions of the pool heating during the last period of melting, which excludes transition of carbon into the metal from electrodes of electric arc furnace, may enable obtaining of low carbon content in the metal at the outlet.

Oxidation degree of the metal (measured by CELOX sensors) at the outlet was determined by weight share of carbon in the metal (Figure 1, a) and depended upon temperature of the metal (Figure 1, b).

Significant scatter of values of the metal oxidation degree, depending upon carbon content, should be noted, which was determined by chemical analysis, whereby in majority of cases actual values of oxidation degree exceeded those determined by measurements. Mentioned differences may be stipulated by the time of sampling and oxidation measurement.

As a whole, possibility of producing low content of carbon at the outlet was statistically authentically established. Weight share of carbon in the intermediate product varied, according to chemical analysis data, within 0.025-0.095%, and measured oxidation--within 0.025-0.038%. Oxidation degree and temperature of metal at the outlet (1100-2100 ppm, 1653-1741[degrees]C) significantly exceeded usual values in converter processes of steel production.

Content of carbon and active oxygen in initial metal before ladle-furnace significantly effected melting loss of deoxidizer-elements (directly proportionally and inversely proportionally). Dependences of silicon and manganese melting loss upon content of carbon at the furnace outlet (Figure 2, a) and changes of carbon content in the ladle-furnace (Figure 2, b) showed that the fuller was decarbonization of metal in the ladle-furnace, the lower was melting loss of deoxidizers.

Analysis of data on content of sulfur in intermediate product at the DSP-2 outlet showed that it was significantly higher than average in the metal charge, even if one assumes the content to be at the level of upper limit for common grades of steel (0.04% S). In the furnace increase of weight share of sulfur due to injection of carbon-containing materials was registered.

At the same time in steel melting processes sulfur was partially removed into gaseous phase, provided blowing by oxygen and high values of temperature and degree of metal oxidation were ensured. So, in [3] data are presented that during oxidation period of melting in electric arc furnace 36-50% S, carried in by all charge materials, are removed from the furnace with furnace gases. Connection between concentration of sulfur in the intermediate product at outlet and oxidation degree of the metal was also detected in considered melting processes (Figure 3).

Essential dependence between content of sulfur in the intermediate product and metal temperature at the outlet was not established that proves prevailing influence of oxygen injection into the furnace and degree of metal oxidation on sulfur removal into gas phase.

As far as content of sulfur at outlet from the furnace is significantly higher than it is required by the standards, it is evident that desulphurization of the metal is performed in the course of out-of-furnace processing. Naturally, lower content of sulfur in the metal at outlet enables better proceeding of further desulphurization process, reduction of consumption of slag-forming components and desulfurizers.

Substantiation and experimental check of technological peculiarities of low carbon intermediate product production. Main thermodynamic parameters of steel desulphurization reaction, presented in literature sources, were calculated for the temperature 1600[degrees]C. However, real temperature at outlet from ASMF is significantly higher. That's why it was interesting to estimate equilibrium values of oxygen content in the metal at 0.002-0.016% C for temperature range 1550-1750[degrees]C (Figure 4).

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Results of the calculation showed that as weight share of carbon in the metal reduced, equilibrium content of oxygen in iron increased, and at weight share below 0.004% C and temperature 1750[degrees]C exceeded oxygen solubility range in iron.

As temperature reduces to evacuation level and beginning of refining, in the ladle-furnace conditions are created for removal of carbon due to super-equilibrium (at these temperatures) amount of oxygen in the metal (presence of FeO in the form of a separate phase).

So, oxidation of the intermediate product when producing especially low-carbon steel should be sufficient for ensuring removal of carbon from initial intermediate product up to the assigned limit, carbon coming from ferroalloys (in steel deoxidation) and electrodes (in heating of steel in the ladle-furnace), and from periclase-carbonaceous lining of the steel-teeming ladle (content of carbon in the area of slag zone is 10-12%, and in lining of walls and bottom 6 %).

Minimum necessary amount of active oxygen for obtaining in steel 0.005% C, its initial content in the intermediate product being different, was determined by calculation (Figure 5).

Comparison of calculated and statistical data analysis shows that real oxidation of the intermediate product at outlet from DSP-2 fluctuates within 458-1997 ppm (mean value of 200 melts is 1015 ppm).

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

It was experimentally established that in case of out-of-furnace processing of the intermediate product (evacuation and refining in the ladle-furnace) final content of carbon in the metal below 0.01% was achieved, even when its initial content was 0.07-0.08 % and oxidation of the metal at outlet from the furnace was above 1100 ppm. Such oxidation of the metal at the outlet ensured as well oxidation of additional amount of carbon, which entered into the metal.

So, degree of oxidation at the outlet is sufficient (and frequently excessive) for removal of carbon in further out-of-furnace processing of the metal.

In this connection experimental melts were carried out, for which amount of oxygen necessary for oxidation of impurities was calculated, constituting difference between general consumption of oxygen in the furnace and that used for combustion of coke (content of carbon is assumed equal 95%) and methane.

Experimental data prove that there is close interconnection between consumption of oxygen above stoichiometry of fuel combustion reactions, oxidation, produced at the outlet, and temperature of the metal (Figure 6). It is evident that for getting high level of the metal oxidation at the outlet it is necessary to increase flow of oxygen injected into ASMF for oxidation of steel impurities (in addition to oxygen necessary for combustion of fuel).

High degree of the metal oxidation at outlet was registered in those cases, when flow of oxygen, injected into ASMF, exceeded the one necessary according to stoichiometry of fuel combustion reactions.

[FIGURE 7 OMITTED]

Due to the amount of oxygen, which exceeds stoichiometric level, oxidation of carbon and other impurities of the metal charge occurs. At the same time achievement of a very high oxidation is undesirable because of reduction of the efficient metal yield (melting loss of iron) and reduction of the lining resistance.

So, results of the trial confirm dependences, produced by statistical processing of parameters of current production melts.

Level of the metal overoxidation was estimated (Figure 7), which was determined as difference between measured oxidation degree of the intermediate product and minimum content of oxygen necessary for decarbonization.

It is rational to remove metal overoxidation by introduction of aluminium (it is envisaged by technical instructions) or carbon-containing materials, whereby level of addition of materials for removal of oxidation should be determined by degree of the metal overoxidation. So, in case of aluminium overconsumption, potential possibility of carbon removal in evacuation is reduced (additional introduction of oxygen in one or another form is necessary), while in case of lack of aluminium excessive melting loss of silicon manganese occurs.

Theoretical and experimental investigations showed that possibilities of electric arc furnace of MMW company allow producing intermediate product fit for melting of especially low-carbon steel. It is established that for producing steel of a targeted composition oxidation degree of the intermediate product should ensure completeness of the metal vacuum-carbon deoxidation and approach the minimum one. Possible entry of carbon into metal in ladle-furnace with ferroalloys and as a result its transition from graphitized electrodes and refractory lining of the steel-teeming ladle should be taken into account.

The results obtained were used by the authors in development of technology for further out-of-furnace processing (evacuation, deoxidation, and heating in the ladle-furnace) of the intermediate product for producing especially low-carbon steel.

CONCLUSIONS

1. It is shown that for obtaining especially low content of carbon in ready steel the intermediate product at the DSP-2 melt outlet should have high values of oxidation and temperatures.

2. It is established that for achieving low carbon content in the metal at outlet one may recommend slag conditions of the pool heating during the last period of melting and increased flow of oxygen in non-fuel ejection devices.

3. Results of experimental-commercial trial proved possibility of producing especially low-carbon steel (weight share of carbon in the metal is less than 0.01%) under conditions of the complex ASMF-evacuator-ladle-furnace without using oxygen in out-of-furnace processing of the intermediate product.

[1.] Petrunenkov, A.A. (1990) Metallurgy and application of IF-steels. Stal, 9, 102 -103.

[2.] Berry, B., Ritt, A., Grayssel, M. (1999) Retrospectives of twentieth century steel. News Steel, Nov., 131-133.

[3.] Medzhibozhsky, M.Ya. (1986) Principles of thermodynamics and kinetics of steelmaking processes. Kiev-Donetsk: Vysshaya Shkola.

A.N. SAVIUK (1), I.V. DEREVYANCHENKO (1), O.L. KUCHERENKO (1), Yu.S. PROJDAK (2), A.P. STOVPCHENKO (2), L.V. KAMKINA (2) and Yu.N. GRISHCHENKO (1)

(1) Company <<Moldavian Metallurgical Works>>, Rybnitsa, Moldova

(2) National Metallurgical Academy of Ukraine, Dnepropetrovsk, Ukraine
Main technological parameters of melting in DSP-2 furnace

Specific Flow of oxygen, [m.sup.3]/t
consumption
of electric Manipulator Side MMW
power, (kWxh)/t <<Palmur>> lances manipulator

430 13.35 1.4 5.9
344 1.90 0.7 2.7
639 23.10 4.2 12.0

Specific Flow of oxygen, [m.sup.3]/t
consumption
of electric Stein Lang Oxygen
power, (kWxh)/t oxygen lance tube

430 4.20 1.27
344 2.05 0.10
639 6.95 3.50

Note. Mean, minimum and maximum data of 200 melts are
presented.
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Title Annotation:ELECTROMETALLURGY OF STEEL AND FERROALLOYS
Author:Saviuk, A.N.; Derevyanchenko, I.V.; Kucherenko, O.L.; Projdak, Yu.S.; Stovpchenko, A.P.; Kamkina, L.
Publication:Advances in Electrometallurgy
Article Type:Report
Date:Jul 1, 2006
Words:2718
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