SU954440A1 - Method for vacuum treatment of molten steel - Google Patents

Description

The invention relates to ferrous metallurgy, in particular, to methods for controlling the process of vacuum treatment of liquid steel. In particular, the method relates to the technological methods implemented in the ferrous metallurgy using the vacuum treatment of liquid steel or by the ladle method.

A known method for the production of mild steel, according to which during the period of vacuum decarburization, the melt is blown with an inert gas through the bottom porous elements of the casting ladle. At the same time, the melt begins to be flushed with an inert gas from the moment of the beginning of the creation of a vacuum in the chamber and in fact continues until the end of the evacuation, changing only the consumption norms of the inert gas 1J.

The disadvantage of this technique of the known method is the fact that in most cases (and for boards, weighing 20-30 tons in 100% of cases) in the initial period of evacuation, even at the usual rate of reduction of residual pressure in the chamber, the boiling point of the melt in the ladle is so intense the nature that, in order to avoid overflow of metal over the edge of the bucket, it is necessary to repeatedly block the main valve through which the vacuum chamber communicates with the steam-jet pump. Under these conditions, purging the melt with an inert gas, carried out for the purpose of further mixing the metal with the slag and intensifying the reaction between oxygen and carbon, is useless and associated with unreasonable material costs. Moreover, this technique is associated with the danger of a breakthrough. evacuated metal through the porous element of the scab, since in this case the period of purging is significantly increased, and the durability of the porous element is limited (especially when blowing large-capacity melts).

Closest to the proposed

20 is a method for evacuating a liquid steel, including continuous measurement of the oxidation potential of a gas phase using an electrochemical cell, determining the completion of the degassing process to achieve an extreme value of the EMF value 2.

The disadvantages of the known method are that the use of the method for large-capacity melting, where the use of melt purging with an inert gas through bottom porous elements is required to achieve the efficiency of evacuation, is associated with a decrease in the accuracy of determining the end of the process of terminating the vacuum decarburization and degassing reaction. liquid steel. The decrease in accuracy is associated with the dilution of waste gases with an inert gas, as a result of which the maximum value of the emf reaches at the time when the release of CO is not finished yet. Moreover, the method does not contain any criteria that allow determining technologically justified moments of onset and termination of metal purging with an inert gas when evacuating. The use of a vacuum of a metal evacuated in the ladle with an inert gas through the bottom porous elements is a necessary technological onerration, especially for heats weighing more than 20 tons, since it is only in this case that the negative effect of the ferrostatic column of metal in the ladle can be compensated. Under these conditions, when vacuuming untreated steel grades, it is possible to most effectively use the deoxidizing ability of carbon in vacuum and, depending on the initial ratio between the oxidation and reduction potential of the metal / slag system, to obtain a melt deoxidized only with carbon to 0.00150, 008 % oxygen, or melt, in which the carbon content does not exceed 0.010%. In addition, with such vacuuming of both calm and unoxidized steel grades, stable removal of hydrogen from the melt to values of 1.5-2.0 g occurs, depending on its initial content. Determination of the technologically determined start and end of purging of the melt evacuated in the ladle with inert gas through the bottom porous elements optimizes the whole evacuation process in general, in particular, shortens the duration of degassing periods (for quiet steel) or deep decarburization or carbon deoxidation ( for non-oxidized steel) which in modern practice of vacuuming are in most cases unreasonably overestimated; reduce the heat loss of the metal during the evacuation, reduce the overheating of the steel in the steel-smelting units, improving the durability of their lining; Add a doping or deoxidizing agent to the melt at the most optimal moment; save inert gas by improving the service conditions of porous blowing elements. The purpose of the invention is to optimize the process of vacuuming, reducing the cost of steel production. The goal is achieved by the fact that according to the method of vacuuming liquid steel, which includes continuous measurement of the oxidation potential of the gas phase using an electrochemical cell, determining the completion of the degassing process to achieve an extreme value of EMF, upon reaching the first maximum on the EMF value curve, liquid steel begins to blow with an inert gas, and the production is completed with a linear decrease in the EMF value after reaching the second maximum on the same curve. When evacuating non-oxidized low carbon steel, carried out for the purpose of deep decarburization of the melt and obtaining less than 0.010% carbon in it, the achievement of the first maximum on the change curve: the EMF value, which determines the oxidative potential of the exhaust gases, means that with a given specific power of the exhaust system the reaction decarburization of the vacuum melt ceased. Usually this happens as a result of a decrease in the rate of arrival — Lenin to the place of the reaction of carbon or-; oxygen from oxidizing slag. At the same time, the carbon content in the melt is 0,012-0,014%, although the oxidation potential of the metal (Co f O, 05%) and slag (up to 17-20% FeO) remain quite high. Such a position is typical for vacuuming large masses of metal in a ladle when the ferrostatic pressure of the metal column above the reaction site plays a significant role. A similar picture is observed when evacuating unoxidized medium and high carbon steels, which is carried out in order to deoxidize steel to carbon. In this case, the reduction of the first maximum on the curve of the change in the EMF value also means that the reaction between the carbon and the oxygen of the melt has stopped, although the oxygen content in the melt remains high (0.010-0.015%). The termination of the reaction here is also due to the decrease in the rate of supply of the reacting components to the reaction site as a result of the negative influence of the ferrostatic pressure. For evacuating still steels, which are mainly carried out to remove hydrogen from the melt and refining it from non-metallic inclusions (to a lesser extent), the presence of the first maximum in the red line of the EMF changes is due to a decrease in the release rate

hydrogen from the melt, which ultimately occurs as a result of the negative effect of the ferrostatic pressure of the metal column

Further development of the reaction between carbon and oxygen of the melt is possible by increasing the rate of carbon supply to the reaction site and oxygen, for example, from slag due to decomposition of readily reducing iron oxides, which is achieved by using a melt blown with an inert gas through bottom porous ladle elements. Purging the melt with an inert gas in vacuum also contributes to an increase in the rate of hydrogen removal from the metal, since in this case the rate of melt circulation in the ladle increases and the specific surface area of the metal being evacuated which is in direct contact with the gas phase with a low hydrogen partial pressure increases .

Thus, the start of purging the melt with an inert gas at the time of reaching the first maximum in the EMF change curve, which determines the oxidative potential of the exhaust gases using an electrochemical cell, is optimal, since, starting from this moment, the reaction between oxygen and carbon of the melt stops. (for untreated steel) or predominantly hydrogen evolution from a melt (for deoxidized steel). An earlier injecting of a melt with an inert gas does not have a technological meaning, since with sufficient specific power of the pumping system, the boiling intensity of the metal, in order to avoid overflowing it over the edge of the ladle, must be artificially limited by disconnecting the reaction space from the vacuum pump. The late start of the purge entails unreasonable loss of time, which is associated with the loss of metal temperature. Attempts to associate the start of melt blowing with the intensity of its boiling in most cases, especially for evacuating medium to high carbon unoxidized steels, are inconsequential, since the boiling intensity can remain almost constant for up to 10 minutes due to oxygen entering the melt from the lining of the ladle, and during the last 5-7 minutes the oxygen content in the melt is kept constant and at a minimum level.

Purging the melt with an inert gas at the time of reaching the first maximum in the EMF change curve, which determines the oxidative potential of flue gases, leads to an increase in the rate of oxygen and carbon entering the reaction site, an increase in the rate of oxygen supply to the metal due to decomposition of readily soluble iron oxides of slag and the resumption of the reaction of vacuum carburization of the melt. In addition, the rate of hydrogen evolution from the melt increases, especially when it is subjected to vacuum treatment in a deoxidized state. Secondary

0 an increase in the rate of reaction between carbon and metal oxygen, as well as the evolution of hydrogen, entails an increase in the content of both the reaction products carbon monoxide, carbon dioxide (to a lesser extent), and hydrogen in the exhaust gases, which is reflected in the oxidative potential of these gases. gases. The maximum content of the considered components in the exhaust gases (co, H) corresponds to

0 to the maximum difference in the oxygen pressure in the standard electrode of the electrochemical cell and in the controlled gas phase. Reducing the EMF value after reaching the second maximum on the curve of its change means lowering the emission rate of the gases in question from the metal JIa. A linear decrease in the value of the EMF value after reaching the second maximum means

This is a practical cessation of the emission of CO, H2. from the melt, and is caused by diluting the concentration of these components in the exhaust gases with an inert gas, which is being blown at a constant rate. Thus, melt blowdown in a vacuum, having played its positive role, from this point on is useless and can only be resumed for other purposes,

0 For example for mixing the melted ferroalloys. .

FIG. Figure 1 shows the change in the EMF value, which determines the 9 acidic potential of the outgoing gas phase,

5 during evacuation and inert gas purging of unoxidized low carbon steel; n is the beginning of the melt boiling in vacuum, f. is the evacuation time, min. The arrows indicate the start of the purge.

0 melt with argon through bottom elements and the introduction of alloying and deoxidizing agents into the melt. Q is 1.75-10 nm / min-t specific argon flow rate for additional decarburization of the melt;

5 Q - O, 5 nm VMHH-t is the specific flow rate of argon for mixing in a melt of a deoxidizing or doping type; in fig. 2 - change of carbon content in the melt

0 in the process of its evacuation and purging with an inert gas.

Claims (1)

Example. In 100 tons of the main electric furnace, 100 tons of metal are smelted and filtered from sulfur, phosphorus, silicon and, partially, from manganese, after which the melt in an unoxidized state is drained into a ladle together with part of the oxidizing slag, the amount of which is 2, 0 t. The chemical composition of the metal before evacuation,%: C 0.050, Mp 0.08 Si 0.02; S 0.015; p o, o1o; o o, oO The content of readily repairable iron oxide in the slag before evacuation was 24% (in terms of FeO). After installing the metal bucket into the vacuum chamber and connecting the system of porous elements to the argon station, finely crushed coke in the amount of 0.20 kg / ton of steel is introduced into the bucket, which provides the initial weight ratio of oxygen in the metal and slag in iron oxide to carbon in a metal and deposited on the slag equal to 2.1 of their stoichiometric ratio in the oxidation of carbon. This level of this ratio provides a proven opportunity to be obtained in the melt after it has been flushed with argon under vacuum in less than 0.010% carbon. During the vacuum process, the oxidizing potential of the exhaust gases is continuously measured using an electrochemical cell. Immediately after reaching the first maximum on the EMF change curve (i.e., 3.5 min), the emitter begins to be flushed with argon through the bottom porous element of the cup, and the argon flow rate is kept constant at 1.75 x X 10 t (Fig. 1) . After the formation of the second maximum on the EMF curve and the subsequent reduction of the EMF value at a constant rate, i.e. for 7.5 minutes of evacuation, the flow rate of argon is reduced to 0.510 nmVmint and PeMn, FeSi, A1 are introduced into the melt in an amount that provides the specified composition of the finished steel. Purging the melt with an inert gas with a constant flow rate of -1.75 x 10 nm / minT for 4 min (Fig. 2) reduces the carbon content in the metal from 0.014 to 0.006%. The moment of reaching this carbon concentration is recorded directly during the vacuum treatment. Reducing the carbon content in the melt from 0.014 to 0.006% is essential, since the processing of steel with 0.014% carbon requires decarburization annealing of intermediate Rolling Profiles, which cost 15-20 rubles / ton of steel. Without this annealing in such steel, it is not possible to achieve the required level of electromagnetic properties (coercive force), while for a steel with 0.006% carbon, a decarburization annealing is not needed. Thus, the use of the methods of the method makes it possible to optimize the process of evacuation as a whole and to achieve a reduction in the cost of steel treated in vacuum. Optimization of the process is achieved by reducing the length of the period of vacuum decarburization or degassing of the melt, which. in modern practice, vacuuming is unreasonably overstated. In addition, the beginning and the end of the obligatory technological operation - the purging of the melt with an inert gas, carried out in order to achieve completeness of the process -. Neither the decarburization and vacuum deacidification of the metal, as well as its degassing (hydrogen removal), are determined using the techniques of the proposed method, based on objective parameters. Reductions in the cost of steel treated in vacuum are achieved by reducing the duration of vacuuming, reducing heat losses during vacuuming, reducing steel overheating in smelting units, increasing the productivity of smelting units; increase the durability of the lining ladle. The calculation of the economic efficiency of the application of the proposed method is made on the example of especially low-carbon steel (relay) steel, the production of which is carried out using bucket vacuum processing. Compared with the known method, with the steel-making redistribution of relay steel by the proposed method, the production of evacuated melts with a high carbon content, for example, with 0.014% carbon, which require decarburization annealing of intermediate rolled profiles, is excluded. The cost of such annealing is 15-20 rubles / ton of rolled steel. Considering that, according to the basic method, each 3-5 melts are characterized by a carbon content of 0.014% and require decarburization annealing in between. the exact profile of the car, the weight of which is 90 tons out of 100 tons of heat, poured, for example, into a slab mold, is eliminated by these costs. specific saving 90t X 20 rubles / t 3.6 rubles / t 5 X 100 tons The total economic efficiency, kt from the application of the method with an annual need for relay steel in the amount of 40,000 tons / year will be 3 .6 rubles / t X 40000 tons / year 144,000 rubles / year This economic effect is underestimated, since it can be obtained by producing only one grade of steel. If we consider that the industry is experiencing an annual need for an amount of 200,000 tons / year in electrical dynamo steel with a carbon content of less than 0.010%, and the specific economic effect from the application of the proposed method remains unchanged and for dynamical steel grades, the foregoing becomes apparent. The calculation of the economic effect, which can be obtained from the application of the method, is also carried out in comparison with the basic technology, which uses the cheapest and high-quality relay stalk, which has a high margin of electromagnetic properties (coercive force) over GOST standards in both cold rolled and in hot rolled pieces of rolled x profiles. . The essence of the basic technology is reduced to the implementation of the following technological operations. 100 t of electric arc furnaces produce semi-finished product with a carbon content of 0.03-0.10%, which, in an undiluted state, is released into the ladle together with the final furnace slag, the amount of which is 2.1-5.0% by weight of the metal. Immediately before vacuuming, oxidizing additives (ore, scale) or reducing (coke, electrode fights) types are introduced into the ladle on the basis of the ratio between the oxidizing and reducing potentials of the metal-slag system within 1.43-3.33. The metal is then subjected to vacuuming, which is divided into two stages. At the first stage, the metal is subjected to vacuum decarburization to less than 0.010% carbon, and the melt is flushed with an inert gas through the bottom porous element of the ladle for almost the entire period. The duration of the purge for this purpose reaches 10-12 minutes. On . In the second stage, the metal and slag are deoxidized by FeSi, SiCa, Al, and the deoxidation operation is also accompanied by flushing the melt with an inert gas. The economic effect that can be obtained by implementing the proposed method is calculated on the basis of the following savings. Reducing the duration of the decarburization stage, combined with melt blowing with an inert gas, is associated with a decrease in heat losses and vacuuming of the melt in the furnace. With reference to 100 tons of arc furnaces, a decrease in metal heating can not be expressed as a reduction in electricity consumption by 30 KWh / t or by 0.6 rubles / t of steel. Reducing the duration of smelting in 100 tons of arc furnace for 5 minutes, due to the reduction of time spent on overheating of the metal, provides an increase in furnace productivity by 3%, which saves 0.3 rub / t of steel. 3. Increasing the durability of the lining of the steel teeming ladle. The durability of the lining of the ladle with the evacuation of 100 tons of melts of unoxidized steel with an initial carbon content in the metal of 0.03-0.05% and an iron oxide content in the slag of 2535% is 2 melts. The bucket is equipped with one porous plug, with a resistance of 2 melts. The application of the method will practically halve the unreasonably high period of decarburization of the melt, which is characterized by increased wear of the lining of the ladle as a result of intensive mixing and contact of the over-oxidized slag with the lining. The intensity of mixing is caused by blowing the melt with an inert gas. At the present time, the cost of lining and equipping the ladle with a refractory porous element is 1478 rubles at the factory. steel using the proposed method is 3-5 heats. The durability of the purge porous element also increases to 3-4 heats. If you take the durability of the lining and porous blowing element of 3 melts,. then the economic effect for this. the article will amount to 1478 - 7.39 -4.93 7.39 rubles / t 100 х з 2.46 rubles / ton The total specific economy will be equal to 0.6 +0.3 + 2.46 3.36 rubles / ton General economic the effect of the application of the proposed method with an annual demand for relay steel in the amount of 40,000 tons / year, as compared with the base technology, will amount to 3.36 rubles / ton X 40,000 tons / year 134,000 rubles / year Invention Method Method for evacuating liquid steel, including continuous measurement of gas oxidation potential