US20130019715A1

US20130019715A1 - Composition for reducing the oxygen potential of slag

Info

Publication number: US20130019715A1
Application number: US13/185,357
Authority: US
Inventors: Frank L. Kemeny
Original assignee: Nupro Co
Current assignee: Nupro Co; Nuflux LLC
Priority date: 2011-07-18
Filing date: 2011-07-18
Publication date: 2013-01-24

Abstract

A slag composition containing steelmaking slag and from about 0.3 to about 10 weight percent of reducing agent. The steelmaking slag contains from about 20 to about 55 weight percent of calcium oxide, from about 8 to about 50 weight percent of ferrous oxide, from about 4 to about 20 weight percent of magnesium oxide, from about 8 to about 30 weight percent of silicon oxide, from 0.5 to about 10 weight per cent aluminum oxide, and from about 0.5 to about 10 weight percent of manganese oxide. The reducing agent contains from about 15 to about 70 weight percent of calcium carbide and from about 10 to about 50 weight percent silicon carbide, wherein the ratio of calcium carbide to silicon carbide is between 0.7 and 7.

Description

This invention relates in one embodiment to an additive for making a ladle slag composition used in steel making, and more particularly to an additive that that reduces the oxygen potential of slag.

BACKGROUND

1. Field of the Invention
Additives used in steel making for improving the composition of slag disposed on molten steel during the steel making process.
2. Description of Related Art
Slag is a mixture of metal oxides and other non-metallic components, which is typically found in partially molten liquid form floating upon the surface of molten steel during a steel making process. During steel making, the molten steel and slag may be contained in an open vessel known as a ladle. The composition of the slag during steel making is critical in achieving the desired composition of the finished steel.
Heretofore, a number of patents and publications have disclosed compositions for synthesizing and/or optimizing and/or treating ladle slag in steel making.
U.S. Pat. No. 5,279,639, of Kemeny et al., discloses a composition for synthesizing and treating ladle slag comprised of from about 5 to about 50 weight percent of calcium carbide, from about 10 to about 20 weight percent of magnesium carbonate, from about 40 to about 55 weight percent of calcium carbonate, from about 5 to about 20 weight percent of alumina, and from about 2 to about 5 weight percent of coke. The entire disclosure of this United States patent is hereby incorporated by reference into this specification.
One problem with the Kemeny et al composition is that is takes a relatively long period of time for it to treat the slag in order to produce the desired slag composition. The reaction of the Kemeny et al composition is endothermic and therefore the progress of the reaction is often limited by heat transfer to the reactants. It is common that the rate limiting step in steel production is the treatment of slag in the ladle. Therefore, it would be of great benefit if the process of reducing the oxygen potential of slag on the steel in the ladle and creating a desired composition of slag could be hastened.
U.S. Pat. No. 6,267,798, of Kemeny et al., discloses a composition for treating steelmaking slag wherein a slag composition is formulated containing steelmaking slag and from about 0.5 to about 10 weight percent of reducing agent. The steelmaking slag contains from about 25 to about 55 weight percent of calcium oxide, from about 10 to about 50 weight percent of ferrous oxide, from about 5 to about 20 weight percent of magnesium oxide, from about 5 to about 20 weight percent of silicon oxide, and from about 0.5 to about 8 weight percent of manganese oxide. The reducing agent contains both calcium carbide and elemental aluminum. From about 5 to about 80 weight percent of the reducing agent is comprised of calcium carbide, and from about 10 to about 50 weight percent of such reducing agent is comprised of elemental aluminum. The entire disclosure of this United States patent is hereby incorporated by reference into this specification.
In accordance with the disclosure of this patent, the addition of elemental aluminum to the reagent, as well as the removal of carbonates, results in an exothermic reagent that allows the reaction to progress more quickly, thereby shortening the treatment time of a rate limiting process. However, this reagent mixture can be used only for those grades of steel for which the process route allows for the use of aluminum as a reducing agent. Many grades of steel are restricted from the use of aluminum as a reducing agent because this may cause processing problems with the steel in subsequent downstream operations such as casting. Generally these grades of steel are deoxidized with silicon. For these grades of steel to which the compositions of U.S. Pat. No. 6,267,798 are not applicable, there is a need for a method to more rapidly facilitate the production of the desired ladle slag than prior art compositions.
The desired refining slag composition disposed on top of the steel in the ladle will vary according to steel grade and other parameters. In general, however, this desired refining slag composition for silicon deoxidized steel products is comprised of from about 40 to about 65 weight percent of calcium oxide, from about 10 to about 40 weight percent of silicon oxide, from about 4 to about 15 weight percent of magnesium oxide, from about 0 to about 15 weight percent of aluminum oxide, from about 0 to about 10 weight percent of calcium fluoride, less than about 1 weight percent of manganese oxide and less than about 4 weight percent of iron oxide.
It is known that there may be various levels of restriction on the weight percent of iron oxide and manganese oxide in the desired refining slag. For example, for steel batches that require extensive removal of sulfur during the ladle refining stage, it may be desirable to have less than about 0.3 weight percent of manganese oxide and less than about 2 weight percent of iron oxide in the desired refining slag. Alternatively, for steel batches that require no significant chemistry modification and that will be applied to products of lower quality requirements, it may be acceptable to have up to about 2 weight percent manganese oxide and up to about 8 weight percent iron oxide in the desired refining slag. It will be apparent that, for a given quantity of ladle slag, the greater the restriction on the allowable weight percent of oxides of manganese and iron, the greater is the quantity required of slag reducing additive of a given composition.
The desired refining slag preferably serves a range of purposes simultaneously. The slag is needed to provide a continuous partially molten oxide phase on the surface of the steel being treated, to capture and retain inclusive non-metallic material present in the steel (such as oxides of calcium, magnesium, silicon and aluminum), to be either non-oxidizing or reducing with respect to the oxygen potential of the steel, to control the sulfur content of the steel, to provide a non-corrosive environment for the refractory ladle linings, to promote stable arcing during electric arc reheating in the ladle, to protect the steel from contact with the atmosphere, and to provide thermal insulation on the top surface of the molten steel.
The desired refining slag has both solid and liquid components, with there being tradeoffs in the proportioning of the two phases. The reaction kinetics of FeO and MnO reduction by the slag reducing agents is favored by a higher fraction of liquid slag. The capture and retention of non-metallic material present in steel is also favored by a higher fraction of liquid slag. However, a higher fraction of solid matter in the slag improves the thermal insulation properties and also provides better protection against corrosion and erosion of refractory materials that comprise the ladle lining. Therefore, there is an optimum range of solid and liquid fractions of slag disposed on the steel in the ladle. Many of the slag reducing agents used in the prior art have significant impact on the percentage of solid in the ladle slag. For example, the use of calcium carbide to reduce iron and manganese oxides in the slag may significantly increase the fraction of solid in the slag, thereby requiring the addition of a fluidizer such as calcium fluoride to reduce the fraction of solid in the slag. Conversely, the addition of aluminum to the slag as a reducing agent may significantly decrease the fraction of solid in the slag, thereby requiring the addition of a slag thickening agent, such as lime, to counter the fluidizing effect.
In steel making, there is often a “see-saw” effect, in which reducing agent, is used, causing a high viscosity problem which halts the reducing reaction; then excessive fluidizer is used to lower the viscosity, which causes refractory erosion and heat loss. Reducing agent or slag thickener is then added again, raising the viscosity, again halting useful chemical reactions and requiring the addition of more fluidizer, and the cycle is repeated. Slag requiring a large amount of deoxidation often leads to breaks in the process flow due to the excessive time used up by this see-saw effect. Extended cycle time and yield loss result from this effect. Thus when formulating an additive to improve the performance of slag, there is generally a need that the additive have a relatively neutral effect on the slag viscosity, thereby facilitating the maintenance of the optimal ratio of solid to liquid in the slag.

SUMMARY

It is therefore an object of this invention to provide an additive for treating ladle slag specifically for steel grades that are restricted from the use of aluminum as a reducing agent, which more readily facilitates the production of the desired ladle slag than prior art compositions.
It is another object of this invention to provide an additive for treating ladle slag, which achieves the desired results at a lower relative proportion than prior art compositions and without significant impact on the desired slag viscosity, thereby improving process efficiency such that the usage of additive for a given slag reducing requirement is closer to the stoichiometric amount.
It is yet another object of this invention to provide an additive for treating ladle slag to reduce the FeO and MnO content in the slag to desirable levels at a faster rate than is provided for with prior art methods, such that the step of ladle slag treatment is not the rate limiting step in the process of steel making.
To accomplish at least one or more of these objects of the invention, there is provided an additive used in steel making for reducing the oxygen potential of slag disposed on steel in a ladle, which comprises the reducing agents calcium carbide and silicon carbide in a ratio such that the reaction products produced result in a liquid calcium silicate at steel making temperatures. Optionally, additional fluxing additives are included to increase the liquid fraction of the slag and to promote the quick dissolution of the reaction products and reactant slag components, thereby decreasing processing time required. The instant slag additive facilitates the fast and efficient removal of FeO and MnO from slag disposed on top of steel in a ladle, while having a relatively neutral effect on the slag viscosity, thereby facilitating the maintenance of the optimal ratio of solid to liquid in the slag. It has been discovered that the ratio of the two main components in the additive, calcium carbide and silicon carbide, must be maintained within a specific range in order for the additive to be effective, and that providing the additive with this range of ratios results in maintaining the slag in partially liquid form and at the correct viscosity during the FeO and MnO reduction reactions. This facilitates performing the reactions at lower relative proportions of the additive at higher reaction rates, thereby lowering operating costs and increasing throughput. In addition, the slag viscosity is maintained at sufficiently high level to minimize heat loss and dissolution of refractory lining material.
More specifically, in accordance with the present invention, there is provided an additive for making a ladle slag composition which comprises calcium carbide and silicon carbide in a ratio wherein the reaction products produce a calcium silicate compound within the slag that is substantially liquid at steel making temperatures.
The ratio of calcium carbide to silicon carbide is in the range 0.7 to 7. More preferably, the ratio may be in the range 1.5 to 4.5, with the optimal ratio in the range 2.3 to 3.5.
In accordance with the present invention, there is further provided an additive used in steel making for making a ladle slag composition, the additive comprising about 15 to about 70 weight percent of calcium carbide, and about 10 to about 50 weight percent of silicon carbide, wherein the ratio of calcium carbide to silicon carbide is in the range 0.7 to 7. The additive may further comprise a fluxing material selected from the group consisting of calcium oxide, calcium aluminate, calcium silicate, glass, calcium fluoride, magnesium oxide, by-product oxides, recycled slag, or combinations thereof. The reducing agents of the additive are preferably provided in the form of particles, and at least about 90 weight percent of the particles are within a size range of from about 0.1 to about 0.7 inches.
In accordance with the present invention, there is further provided a slag composition comprised of steelmaking slag and from about 0.3 to about 10 percent of reducing agent by total weight of steelmaking slag and reducing agent. The steelmaking slag is comprised of from about 20 to about 55 weight percent of calcium oxide, from about 8 to about 50 weight percent of ferrous oxide, from about 4 to about 20 weight percent of magnesium oxide, from about 8 to about 30 weight percent of silicon oxide, from 0.5 to about 10 weight per cent aluminum oxide, and from about 0.5 to about 10 weight percent of manganese oxide. The reducing agent may be comprised of from about 15 to about 70 weight percent of calcium carbide and from about 10 to about 50 weight percent of silicon carbide. The reducing agent may be further comprised of from about 0 to about 80 weight percent of a fluxing material selected from the group consisting of calcium oxide, calcium aluminate, calcium silicate, glass, calcium fluoride, magnesium oxide, by-product oxides, recycled slag, or combinations thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described by reference to the drawings, in which like numerals refer to like elements, and in which:

FIG. 1 is a schematic illustration of an apparatus and process for producing the desired slag; and

FIG. 2 is a phase diagram depicting various phases that may be present in slag comprised of oxides of silicon, calcium, and magnesium.

DETAILED DESCRIPTION

The present invention will be described in connection with a preferred embodiment, however, it will be understood that there is no intent to limit the invention to the embodiment described. On the contrary, the intent is to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.
In describing the present invention, a variety of terms are used in the description. Standard terminology is widely used in steel making art.
As used herein, the term “slag” is meant to indicate a mixture of metal oxides and other non-metallic components, which is typically found in partially molten liquid form floating upon the surface of molten steel.
As used herein, a “fluxing agent” is a composition which helps to fluidize the components of the slag and thereby increases the rate of reduction reaction by one or more reducing agents that are present in the additives of the present invention. Fluxing agents are also referred to herein as “fluxes.”
One preferred additive of this disclosure for making a ladle slag composition is comprised of a first reducing agent comprising calcium carbide, a second reducing agent comprising silicon carbide, and optionally one or more fluxing agents selected from the group consisting of calcium aluminate, glass, oxides of elements of Groups IA, IIA, IIIA, IVA of the Periodic Table of the Elements, fluorides of elements of Groups IA, IIA, and IIIA of the Periodic Table, and mixtures thereof.
The preferred additive is comprised of from about 15 to about 70 weight percent, by total weight of additive, of calcium carbide and about 10 to 50 weight percent, by total weight of additive, of silicon carbide. The calcium carbide and the silicon carbide may be in particulate form, with at least about 90 weight percent of its particles having a particle size in the range between from about 0.1 to about 0.7 inches. More preferably, at least about 90 weight percent of the calcium carbide particles may have a particle size in the range of from about 0.25 to about 0.4 inches. The calcium carbide and the silicon carbide may have similar particle size distributions. As used herein, particle size is meant to indicate a diameter of an approximately spherical particle, or the maximum dimension of an oblong or irregularly shaped particle.
Without wishing to be bound to any particular theory, the Applicants believe that the use of calcium carbide and silicon carbide with the desired particle size distribution contributes to the desired efficacy of the Applicants' composition. If the particles are not similar in size and shape, then there may be segregation of the mixture during transport, causing the additive to have variable composition as batches are withdrawn from the transport container. If the particle size of a batch is too small, the reaction proceeds too quickly, releasing CO gas at a fast rate. This causes slag to be ejected from the ladle, which is undesirable. If the particle size of a batch is too big, the reaction proceeds too slowly, which is counter to the purpose of the invention.
In one preferred embodiment, a ratio of between 2.3 and 3.5 of the calcium carbide to silicon carbide is used in the additive composition. Using this ratio, the reaction product calcium silicate is substantially liquid at common steel making temperatures of around 2,900 degrees Fahrenheit, thereby facilitating the continuous release of nascent reducing agent to contact the FeO and MnO reactants within the slag.
One may use any of the commercially available calcium carbide compositions to produce the additive of this invention. Thus, e.g., one may use commercial grade calcium carbide such as, e.g., miner's grade calcium carbide which may contain about 80 weight percent of calcium carbide. Such miner's grade calcium carbide is sold, e.g., by Carbide Industries, LLC. of Louisville, Ky. Similarly, one may use any of the commercially available metallurgical grade silicon carbide compositions to produce the additive of this invention. Thus, e.g., one may use CARBOLON MA which may contain about 94% silicon carbide, sold by Washington Mills Hennepin, Inc. of Hennepin, Ill.
The slag to be treated may vary in composition. One typical such slag contains 35 weight percent of calcium oxide, 30 weight percent of ferrous oxide (FeO), 13 weight percent of magnesium oxide, 10 weight percent of silicon oxide, 5 weight percent of aluminum oxide, and 5 weight percent of manganese oxide. Typically, during the steel making process, such a slag is at a temperature of about 2,900 degrees Fahrenheit.
When 10 weight percent of the additive of the invention (by total weight of slag to be treated and additive) is added to the slag, and the slag is maintained at a temperature of 2,900 degrees Fahrenheit by constant heat input, the desired refining slag is produced in no more than about 10 minutes, and often in as little as 5 minutes. It is noteworthy that the composition of the aforementioned U.S. Pat. No. 5,279,639 of Kemeny et al. is not capable of achieving this result within the specified time. It is not uncommon for steel made by prior art methods to be treated for 20 minutes to improve slag chemistry, and sometimes even longer. In some circumstances, the connection to a sequence cast is missed due to high sulfur content in the steel, because the slag could not be treated in a sufficiently short time.
In one embodiment, the additive of this invention contains from about 40 to about 60 weight percent calcium carbide and from about 15 to about 25 weight percent of silicon carbide, where the ratio of calcium carbide to silicon carbide is between about 2.3 and about 3.5. In this embodiment, one may add additional flux materials to the additive, but such materials are not required. It is to be understood that, when concentrations of silicon carbide and/or calcium carbide are specified herein, these are to be considered as the being the concentrations of the pure materials.
In another embodiment of the additive composition, a fluxing agent is added to the reducing agents. The fluxing agent is added to further reduce the melting point of the reaction products and to adjust the slag disposed on the steel in the ladle to the desired chemical composition. In this way, both oxygen potential reduction and chemistry adjustment of the slag can be accomplished with one addition, thereby further decreasing the processing time. The fluxing agent may contain ingredients that reduce the reactivity of the reaction products with the ladle lining. For example, magnesium oxide may be added to prevent the dissolution of ladle lining material comprised primarily of magnesium oxide.
It is preferred that the fluxing agent be selected from the group consisting of calcium aluminate, glass, oxides of elements of Groups IA, IIA, IIIA, IVA of the Periodic Table of the Elements, fluorides of elements of Groups IA, IIA, and IIIA of the Periodic Table, and mixtures thereof. If glass is used as a fluxing agent, such glass may include crushed auto glass or bottle glass obtained from glass recycling operations. Alternatively, low melting point oxide mixtures or glasses produced as by-products of industrial processes, such as the manufacture of vanadium metal, may be used as the fluxing agent.
The additive of this invention may be made by dry blending the calcium carbide, the silicon carbide, and, optionally, one or more of the fluxing agents in the desired stoichiometry. The raw materials preferably contain less than about 0.2 weight percent of moisture and, if needed, are dried until the moisture content is reduced to or below this level. The dried material can then be mixed, e.g. charged to a paddle mixer and dry blended from about 5 to about 15 minutes, depending upon the batch size. The blended materials are then discharged into suitable shipping containers or other suitable batch packaging as required. Any such packaging should be moisture resistant since the calcium carbide is highly reactive with water.
Description of a Preferred Process for Using the Additive
The Applicants' slag additive provides for the manufacture in situ of a ladle slag by the addition of a mixture of materials including calcium carbide, silicon carbide, and, optionally, one or more fluxing agents such as glass or other complex oxides of low melting point, alkali metal salts, alkali earth metal salts, and slag raw materials containing components such as silica, calcium fluoride, alumina, lime, magnesia, and calcium aluminate which are required to achieve the desired slag composition. Depending upon specific conditions at each ladle refining installation, it may be appropriate to manufacture in situ a ladle slag in more than one step through more than one addition of mixtures of the above materials.
Without wishing to be bound to any particular theory, the Applicants believe that when one or more fluxing agents are used, they dissolve the solid fraction of the slag surrounding the FeO and MnO reactants and the calcium carbide and silicon carbide slag additive, thereby releasing trapped FeO and MnO and presenting these components for reduction by the slag additive. In the absence of any fluxing agents, the FeO and MnO reactants may not come into contact with the calcium carbide and silicon carbide slag additive if they are “frozen” in solid slag, or rendered immobile by highly viscous slag. Overall, the addition of one or more fluxes may thus enhance the speed of the slag reduction reactions, especially in slag which contains a large fraction of solid matter. The addition of fluxes to the slag additive may not be beneficial when the solid fraction of the slag is insignificant.
More specifically, The reducing agent compounds react with the FeO and MnO in the slag at a temperature of around 1500 to 1600° C. The FeO reactions are as follows:
CaC₂+3FeO=3Fe+CaO+2CO
SiC+3FeO=3Fe+SiO₂+CO
Analogous reactions occur with Mn and its oxide. For both the Fe and Mn reactions, the CO reaction product escapes the slag as gas bubbles and further oxidizes to CO₂when it contacts the oxygen in the air.
The SiO₂and the CaO are optimally soluble to form an ionic liquid in the slag. In order for that to occur, the ratio of those components should be such that the slag is in the liquid range of the phase diagram as shown in FIG. 2. It will be apparent that a slag is more complex and contains more than just the three compounds shown at the apices in the phase diagram of FIG. 2. Each additional compound may change the range of effective ratios for CaO:SiO₂to remain in the liquid region 50. By empirical and theoretical means, the Applicant has discovered that for all practical slag compositions, that ratio is between 0.7 and 7. For most effective additive compositions that ratio can be narrowed to between 1.4 and 4.5. If no flux is used, but only the commercially available materials containing the CaC₂and SiC, that ratio range is between 2.3 and 3.5. Accordingly, through this discovery, it has been determined that the required ratio of calcium carbide to silicon carbide in the instant slag additive is between about 0.7 and 7, and preferably between about 1.4 and 4.5, and more preferably between about 2.3 and 3.5.
It should be appreciated that, although aspects of the instant slag additives are discussed in the context of treatment of slag in the ladle of steel after it has been poured from the primary steel making vessel or furnace, the additives have applicability to a wide range of refining procedures. For example, the treatment of slag is often done within an electric arc melting furnace in steel foundry facilities, since reheating of steel in the ladle after pouring from the furnace is not facilitated. Similarly it should be understood that the sequence of steps and addition of composition components in accordance with the invention may be varied substantially depending upon the requirements of a particular application, provided that the ratio of calcium carbide to silicon carbide is maintained as specified herein.
In one embodiment, the final slag properties and characteristics required or desired for a given ladle refining system are calculated or otherwise determined. The minimum amount of required ladle slag depth is established, usually around 2 inches if no arc reheating is applied, or 4-6 inches if arc reheating will be applied.
The primary furnace slag chemistry is measured or approximated using historical and real time data. The amount of this slag that is carried into the ladle is determined in two steps. First, the tendency for slag carry over is estimated based on historical data for the particular steel grade and steel making conditions, especially with regard to steel oxygen potential, duration of tapping, use of slag retention devices, condition of tap hole, etc. One may then determine the amount of ferrous oxide (FeO) and manganese oxide (MnO) in the slag to be treated which must be reduced.
The tapping process is preferably observed, and the actual slag carry over quantity is measured using one or more of commercially available slag detection devices, or using visual means. The excess quantities of iron and manganese oxides in the ladle slag can now be calculated, and this amount will determine the required slag reducing agent.
One may then calculate the amount of calcium carbide and silicon carbide required, based on the amount of iron and manganese oxide to be reduced and the desired composition of the refining slag. In addition, one may estimate the solid fraction of the slag and, if substantial, calculate the amount of fluxes that must be included in the additive, if any.
The solubility of magnesia in the final desired slag composition is determined, and, if the slag contains less than that amount, the deficiency may be added in the form of magnesia containing materials in the slag additive. The required fluxing agents are added to ensure that the slag to be treated is mostly liquid to facilitate reaction of the FeO and MnO in the slag with the reagents in the additive. It is to be understood that, when a particular step of the process calls for adding one or more fluxing agents, this step may be omitted when the desired composition does not contain such fluxing agent.
Other fluxing agents may be added such as, e.g., materials that contain silica, calcium fluoride, sodium oxide, alumina, lime, magnesia, and calcium aluminate; the addition of these reagents, in some embodiments, will fulfill the requirements necessary to achieve the desired slag composition and properties. Depending upon specific conditions at each ladle refining installation, it may be appropriate to manufacture in situ a secondary slag in more than one step through more than one addition of mixtures of the above materials. However, where possible, it is beneficial to add one reagent mixture to the slag to achieve the required slag chemistry and thereby save processing time.
The slag additives may be added to the ladle during the tapping of steel, preferably approximately one-half to two-thirds through the tap, or alternatively, to the top of the contents of the ladle after the tap.
FIG. 1 is a schematic illustration of an apparatus and process for producing the desired slag. Referring to FIG. 1, it will be seen that a ladle 10 receives molten steel and molten steel making slag, as indicated by line 12. The molten steelmaking slag generally contains from about 25 to about 55 weight percent of calcium oxide, from about 8 to about 50 weight percent of ferrous oxide (FeO), from about 5 to about 20 weight percent of magnesium oxide, from about 5 to about 20 weight percent of silicon oxide, from about 2 to about 10 weight percent alumina and from about 0.5 to about 10 weight percent of manganese oxide.
The molten steel generally contains at least about 70 weight percent of molten, elemental iron and less than about 1 percent of elemental carbon dissolved therein.
The molten steel and the molten slag are charged in such concentrations as indicated by line 12 such that these two components comprise at least about 90 weight percent of the material added to the ladle, and more preferably, at least about 95 weight percent of the material added to the ladle.
Referring again to FIG. 1, which is not drawn to scale, it will be seen that the slag 16 is disposed on top of the molten steel 18, shielding it from contact with the ambient atmosphere indicated by arrow 20. It will be appreciated that, although this is generally the situation in steel making, there are instances in which the slag layer 16 does not fully cover the molten steel 18. To the ladle 10 containing the molten steel 18 and the slag 16 is added a reducing agent, as indicated generally by line 23. In the preferred embodiment depicted in FIG. 1, the reducing agent is a combination of calcium carbide and silicon carbide in a ratio of between 0.7 and 7.
In one embodiment of the process depicted in FIG. 1, only the calcium carbide and silicon carbide reagents, apportioned in the ratio range specified, are added via a hopper 22. In another embodiment, one or more fluxing agents are also added. In this latter embodiment, the additional fluxing agent(s) may be charged from hopper 22, where it is present in admixture with the reducing agent material.
Alternatively, one or more of the additional fluxing agent(s) may be present in hopper 24. Alternatively, all of the reagents may be present in an admixture in hopper 22, hopper 24, or another hopper (not shown).
Referring again to FIG. 1, it will be seen that material dispensed from hoppers 22 and 24 may be forced through a pipe 26, preferably by means of dry gas transport. In the embodiment depicted, a valve 28 allows nitrogen to flow through pipe 26 and to entrain the materials discharged from hoppers 22 and 24, thus carrying such materials through such pipe into ladle 10.
The amount of inert gas flowing through pipe 26, and the amount and type of reagent(s) flowing from hoppers 22 and 24 are preferably controlled via controller 29 which communicates with hoppers 22 and 24 via control lines 30, 32, and 34 and controls valves 36 and 38. Controller 29 also communicates with and controls valve 28 via line 35. It is to be understood that lines 30, 32, 34, 36, 38, and 35 are typical process control communication lines, and may comprise numerous electrical wires and/or optical fibers as is known in the process control arts.
The controller 29 is comprised of a computer (not shown), with a microprocessor, input and output devices, and communication modules; controller 29 also includes a software program (not shown) which evaluates a multiplicity of factors in determining how much of each reagent to charge to ladle 10.
Referring again to FIG. 1, it will be seen that historical data is input to controller 29 via line 36. This historical data may include (but is not limited to) (1) the average and standard deviation of the weight of slag carried over from the furnace (not shown) to the ladle 10 during previous manufacturing sessions, (2) the average weight percent of ferrous oxide and manganese oxide present in the steel making slag during previous manufacturing sessions, (3) the time required to tap the batch of steel immediately made prior to the batch in question, and (4) the average final slag composition disposed on the steel in the ladle after treatment is complete.
Referring again to FIG. 1, real time data is input to controller 29 via line 38. This real time data may include (but is not limited to) one or more of such factors as (1) the concentration, in parts per million, of oxygen dissolved in the steel before tapping, (2) the scheduled additions of alloys and other materials planned for the batch in question by the steel mill additions programs, (3) the chemical analyses of the steel prior to tapping, (4) the chemical analyses of the slag prior to tapping, (5) whether a slag retention device (such as a refractory shape) is being used in the process, (6) the amount of slag which actually went into ladle 10, and (7) the temperature of the steel. Referring again to FIG. 1, it will be seen that a camera 40 may continuously monitor the tapping stream fed via line 12 for the duration of the tap and, thus, can assist the controller 29 in determining the flow rate of the steel.
Armed with all of the above data, controller 29 is programmed to calculate the amount of ferrous oxide and manganese oxide which needs to be reduced within ladle 10 in order to obtain the optimum refining slag. Based upon this calculation, the controller 29 then is capable of determining the amount of calcium carbide and silicon carbide containing material to be charged from hopper 22.
In one embodiment, from about 2 to about 5 weight percent of the total mixture of slag on the ladle 10 after tap is reducing agent containing calcium carbide and silicon carbide, wherein the ratio of calcium carbide to silicon carbide is between about 2.3 and about 3.5.
When one or more fluxing agents is used and is charged from hopper 22 and/or 24, the fluxing agent(s) is also used in an amount such that from about 0.5 to 10 weight percent (and preferably from about 2 to 5 weight percent) of such fluxing agent is present by weight of the total weight of the slag layer 16 (which will comprise slag, calcium carbide containing material, silicon carbide containing material, and one or more fluxing agents).
When one or more fluxing agents is used, it is preferred to also use calcium fluoride. The amount of calcium fluoride used is generally from about 0 to about 30 weight percent of the total amount of flux used, and preferably is from about 10 to about 20 weight percent. However, some steel production facilities limit or prohibit the use of calcium fluoride as an additive. In these cases only, it is preferred to not use calcium fluoride as a flux material.
When one or more fluxing agents is used, and calcium fluoride use is limited or prohibited, then it is preferred to use a low melting point oxide selected from the group glass, by-product oxide, recycled slag, or mixtures thereof. The amount of low melting point oxide used is generally from about 0 to about 50 weight percent of the total amount of flux used, and preferably is from about 20 to about 30 weight percent.
The following examples are used to illustrate the claimed invention but are not to be deemed limitative thereof. Unless otherwise stated, all parts are by weight, and all temperatures are in degrees Fahrenheit.

EXAMPLE 1

Steel was made in an electric arc furnace and tapped into a ladle. The steel making slag within the furnace had an approximate composition of 23% CaO, 30% FeO, 17% MgO, 15% SiO₂, 6% MnO, and 7% Al₂O₃. It was estimated that 122 tons of steel and 2,500 pounds of this slag flowed from the furnace into the ladle during the tapping process. When the ladle was approximately half full, 400 pounds of slag additive reagent was added, with an approximate composition of 47% CaC₂, 14% SiC, 23% CaF₂, 10% CaO, 3% SiO₂, and 2% Al₂O₃. The reagent had a particle size range of approximately 4 to 15 millimeters. In addition to the reagent, 2,300 pounds of SiMn and 500 pounds of FeSi alloys were added, and 800 pounds of CaO was also added to the ladle. All additions were complete prior to three quarters of the way through the tapping process. During and after tapping, the contents of the ladle were stirred by injecting argon gas through a porous element in the bottom of the ladle. Approximately 8 minutes after tap, the slag formed on the surface of the ladle was sampled and analysis thereof indicated an approximate composition of 53% CaO, 26% SiO₂, 14% MgO, 4% Al₂O₃, 3% CaF₂, 0.9% FeO and 0.3% MnO. This slag was very effective for refining the steel and reducing impurities. The sulfur content of the steel dropped from 0.055% to 0.028%, indicating the effectivity of the refining slag produced by this method.

EXAMPLE 2

In the following hypothetical example, steel is made in an electric arc furnace and then tapped into a ladle. During the tapping process, alloys and lime are added to create the desired steel and slag chemistry. After tapping, the ladle is moved to a refining station where it is heated and further refined. Upon analysis, the slag on the ladle may be found to contain 6% FeO and 2% MnO. These levels of the reducible oxides are too high for effective refining of the steel and it is desired to lower them to a maximum of 2% FeO and 0.5% MnO. A reducing agent comprised of a mixture of 75% commercial grade calcium carbide and 25% commercial grade silicon carbide with average particle size of ⅜ inch is used to adjust the slag. The reagent has approximately 60% CaC₂and 23.5% SiC, with a ratio of 2.55 CaC₂to SiC. It is calculated that about 450 pounds of FeO and MnO will need to be reduced to create a desirable slag chemistry. Based on stoichiometry and reagent efficiency, it is further calculated that approximately 180 pounds of reagent is required to adjust the slag to the desired chemistry. The ladle of steel is heated using electric arc reheating and stirred by argon injected through a porous element in the bottom of the ladle. The 180 pounds of reagent is added to the surface of the slag by dispensing the reagent from a hopper and conveying it through a pipe using nitrogen gas. Load cells on the hopper are used to determine when the desired amount of reagent has been dispensed. Stirring and heating are continued for 5 minutes after the reagent is added. The resulting slag chemistry is desirable for refining and production of clean steel with the desired chemistry. The FeO and MnO contents are reduced to 1% and 0.5% respectively by the reagent.
It is, therefore, apparent that there has been provided, in accordance with the present invention, an additive used in steel making for making a ladle slag composition, and a method and apparatus for using such additive in steelmaking. While this invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.

Claims

1. A slag composition comprised of steelmaking slag and from about 0.3 to about 10 percent of reducing agent by total weight of steelmaking slag and reducing agent, wherein:

a) said steelmaking slag is comprised of from about 20 to about 55 weight percent of calcium oxide, from about 8 to about 50 weight percent of ferrous oxide, from about 4 to about 20 weight percent of magnesium oxide, from about 8 to about 30 weight percent of silicon oxide, from 0.5 to about 10 weight per cent aluminum oxide, and from about 0.5 to about 10 weight percent of manganese oxide; and

b) said reducing agent is comprised of from about 15 to about 70 weight percent of calcium carbide and from about 10 to about 50 weight percent of silicon carbide, wherein the ratio of calcium carbide to silicon carbide is between 0.7 and 7.

2. The slag composition as recited in claim 1, wherein said reducing agent is provided in the form of particles, and at least about 90 weight percent of said particles are within a size range of from about 0.1 to about 0.7 inches.

3. The slag composition as recited in claim 2, wherein said calcium carbide and said silicon carbide have similar particle size distributions.

4. A slag composition comprised of steelmaking slag and from about 0.3 to about 10 percent of reducing agent by total weight of steelmaking slag and reducing agent, wherein:

b) said reducing agent is comprised of from about 15 to about 70 weight percent of calcium carbide and from about 10 to about 50 weight percent of silicon carbide, wherein the ratio of calcium carbide to silicon carbide is between 0.7 and 7; and from about 5 to about 80 weight percent of a fluxing material selected from the group consisting of lime, calcium aluminate, calcium silicate, glass, calcium fluoride, by-product oxides, recycled slag, and combinations thereof.

5. The slag composition as recited in claim 4, wherein said reducing agent is provided in the form of particles, and at least about 90 weight percent of said particles are within a size range of from about 0.1 to about 0.7 inches.

6. An additive used in steel making comprised of reducing agents for adjusting ladle slag composition, said additive comprising about 15 to about 70 weight percent of calcium carbide and about 10 to about 50 weight percent of silicon carbide, wherein the ratio of calcium carbide to silicon carbide is between about 0.7 and about 7.

7. The additive as recited in claim 6 wherein the ratio of calcium carbide to silicon carbide is in the range 1.5 to 4.5.

8. The additive as recited in claim 7 wherein the ratio of calcium carbide to silicon carbide is in the range 2.3 to 3.5.

9. The additive as recited in claim 6, wherein said reducing agents are provided in the form of particles, and at least about 90 weight percent of said particles are within a size range of from about 0.1 to about 0.7 inches.

10. The additive as recited in claim 9, further comprising a fluxing material selected from the group consisting of lime, calcium aluminate, calcium silicate, glass, calcium fluoride, by-product oxides, recycled slag, or combinations thereof.