Title:
Composition for reducing steelmaking slag
Kind Code:
A1


Abstract:
A slag composition containing steelmaking slag and from about 0.5 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 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, 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 5 to about 80 weight percent of calcium carbide and from about 10 to about 70 weight percent of an admixture containing silicon or titanium or silicon carbide or combinations thereof.



Inventors:
Jones, Jeremy A. T. (Lees Summit, MO, US)
Application Number:
11/251708
Publication Date:
04/19/2007
Filing Date:
10/17/2005
Primary Class:
Other Classes:
75/327, 75/323
International Classes:
C21B3/02
View Patent Images:



Primary Examiner:
YANG, JIE
Attorney, Agent or Firm:
JOHN M. HAMMOND (Lakeville, NY, US)
Claims:
I claim:

1. A slag composition comprised of steelmaking slag and from about 0.5 to about 15 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 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, 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 5 to about 80 weight percent of calcium carbide and from about 10 to about 70 weight percent of a material selected from the group consisting of silicon, titanium, silicon carbide, and mixtures thereof.

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 material selected from the group consisting of silicon, titanium, and silicon carbide, and mixtures thereof have similar particle size distributions.

4. The slag composition as recited in claim 1, wherein the source of said titanium is titanium oxide.

5. The slag composition as recited in claim 1, wherein the source of said titanium is ferrotitanium.

6. The slag composition as recited in claim 1, wherein the source of said silicon is calcium silicon.

7. A slag composition comprised of steelmaking slag and from about 0.5 to about 15 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 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, 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 5 to about 80 weight percent of calcium carbide and from about 10 to about 70 weight percent of a first material selected from the group consisting of silicon, titanium, silicon carbide, and mixtures thereof; and from about 5 to about 80 weight percent of a second material including 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.

8. The slag composition as recited in claim 7, 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.

9. The slag composition as recited in claim 7, wherein the source of said titanium is titanium oxide.

10. The slag composition as recited in claim 7, wherein the source of said titanium is ferrotitanium.

11. The slag composition as recited in claim 7, wherein the source of said silicon is calcium silicon.

12. The slag composition as recited in claim 7, wherein the source of said calcium aluminate is recycled ladle slag.

13. The slag composition as recited in claim 7, wherein the source of said calcium silicate is recycled ladle slag.

14. An additive used in steel making for making a ladle slag composition, said additive comprising about 5 to about 60 weight percent of a first reducing agent, and about 10 to about 70 weight percent of a second reducing agent, wherein: a. said first reducing agent consists essentially of calcium carbide; and b. said second reducing agent is selected from the group consisting of silicon, titanium, silicon carbide, and mixtures thereof.

15. The additive as recited in claim 14, 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.

16. The additive as recited in claim 15, wherein said calcium carbide and said second reducing agent selected from the group consisting of silicon, titanium, and silicon carbide, and mixtures thereof have similar particle size distributions.

17. The additive as recited in claim 14, wherein the source of said titanium is titanium oxide.

18. The additive as recited in claim 14, wherein the source of said titanium is ferrotitanium.

19. The additive as recited in claim 14, wherein the source of said silicon is calcium silicon.

20. The additive as recited in claim 14, further comprising a fluxing material selected from the group consisting of lime, calcium aluminate, calcium silicate, glass, calcium fluoride, by-product oxides, steelmaking slag, recycled slag, or combinations thereof.

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 enables the production of a desired slag within a specified time.

BACKGROUND OF THE INVENTION

1. Field of the Invention

An additive used in steel making for making a ladle slag composition, which comprises calcium carbide, a second reducing agent (chosen from silicon metal, ferro-silicon, calcium silicon, silicon carbide, titanium metal, ferro-titanium, titanium carbide) and optionally at least one fluxing agent 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 1A, IIA, and IIIA of the Periodic Table, silicon carbide, and mixtures thereof, and from about 0 to about 20 weight percent of a metal carbonate.

2. Description of Related Art

Heretofore, a number of patents and publications have disclosed compositions for synthesizing and/or optimizing and/or treating ladle slags in steel making.

U.S. Pat. No. 5,279,639, of Kemeny et al., discloses a composition for synthesizing and treating ladle slags 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.

The composition of Kemeny, et al is effective and has achieved a substantial amount of commercial success. However, since the commercial introduction of this composition, pressures have increased upon the domestic steel making industry to substantially increase productivity, especially with the increased sales of foreign steel dumped into the United States. 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.

U.S. Pat. No. 6,267,798, of Kemeny et al., discloses a composition for treating steelmaking slags 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.

The composition of the U.S. Pat. No. 6,267,798 patent is also effective and has achieved a substantial amount of commercial success. This patent provides a method to more readily facilitate the production of the desired ladle slag than prior art compositions but only for those grades of steel for which the process route allows for the use of aluminum as a reducing agent. However, 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 fall into two categories known as silicon deoxidized and titanium deoxidized steels. For these grades of steel, it is desired to provide a method to more rapidly facilitate the production of the desired ladle slag than prior art compositions. To the best of the applicant's knowledge, such a method, and a composition used in such a method, is not provided in the prior art.

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 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 5 to about 20 weight percent of magnesium oxide, from about 0 to about 10 weight percent of aluminum oxide, from about 0 to about 15 weight percent of calcium fluoride, and from about 0.5 to about 12 weight percent of a mixture containing ferrous oxide and manganese oxide.

The desired refining slag preferably serves 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 silicon oxide or titanium oxide), to be either non-oxidizing or reducing with respect to 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.

Accordingly, embodiments of the present invention are provided that meet at least one or more of the following objects of the present invention.

It is an object of this invention to provide an additive for treating ladle slag specifically for steels 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 said ladle slag which achieves the desired results at a lower concentration than prior art compositions.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided an additive for making a ladle slag composition which comprises calcium carbide, a second reducing agent, and optionally at least one fluxing agent selected from the group consisting of calcium aluminate, glass, oxides of elements of Groups IA, IIA, IIIA, IVA of the Periodic table, fluorides of elements of Groups 1A, IIA, and IIIA of the Periodic table, silicon carbide, and mixtures thereof.

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 5 to about 60 weight percent of a first reducing agent, and about 10 to about 70 weight percent of a second reducing agent, wherein the first reducing agent consists essentially of calcium carbide; and the second reducing agent is selected from the group consisting of silicon, titanium, silicon carbide, and mixtures thereof. The additive may further comprise a fluxing material selected from the group consisting of lime, calcium aluminate, calcium silicate, glass, calcium fluoride, by-product oxides, steelmaking slag, 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 0.5 to about 15 percent of reducing agent by total weight of steelmaking slag and reducing agent, wherein the steelmaking slag is comprised of from about 20 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, 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 the reducing agent is comprised of from about 5 to about 80 weight percent of calcium carbide and from about 10 to about 70 weight percent of a material selected from the group consisting of silicon, titanium, silicon carbide, and mixtures thereof.

In accordance with the present invention, there is further provided a slag composition comprised of steelmaking slag and from about 0.5 to about 15 percent of reducing agent by total weight of steelmaking slag and reducing agent, wherein the steelmaking slag is comprised of from about 20 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, 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 the reducing agent is comprised of from about 5 to about 80 weight percent of calcium carbide and from about 10 to about 70 weight percent of a first material selected from the group consisting of silicon, titanium, silicon carbide, and mixtures thereof; and from about 5 to about 80 weight percent of a second material including a fluxing material selected from the group consisting of lime, calcium aluminate, calcium silicate, glass, calcium fluoride, by-product oxides, steelmaking slag, recycled slag, or combinations thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described by reference to the drawing of FIG. 1, in which like numerals refer to like elements. FIG. 1 is a flow chart of a preferred process for producing the desired slag.

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.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

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 inorganic metal oxide solids within an ionic liquid usually floating upon the surface of molten steel.

As used herein, a “fluxing agent” is a composition which enhances the dissolution rate of solid components of the slag and thereby typically increases the rate of reduction reaction by one or more reducing agents that are present in the additives of the present invention.

As used herein, the term “primary steelmaking” refers to a process involving the production of raw liquid steel using oxygen.

As used herein, the term “secondary steelmaking” refers to a process involving removal of dissolved oxygen and adjustment of alloy composition in the molten steel.

The preferred additive of this invention for making a ladle slag composition is comprised of a first reducing agent consisting of calcium carbide, a second reducing agent, and at least one fluxing agent 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 1A, IIA, and IIIA of the Periodic Table, silicon carbide, and mixtures thereof, and from about 0 to about 10 weight percent of a metal carbonate.

The preferred additive is comprised of from about 5 to about 60 weight percent, by total weight of additive, of calcium carbide. It is preferred that the calcium carbide be in particulate form and that at least about 90 weight percent of its particles have a particle size in the range between from about 0.1 to about 0.7 inches. In an even more preferred embodiment, at least about 90 weight percent of the calcium carbide particles have a particle size in the range of from about 0.25 to about 0.4 inches. As used herein, particle size is meant to indicate a diameter of an approximately spherical particle, or the maximum dimension of an oblong particle.

Without wishing to be bound to any particular theory, applicants believe that the use of calcium carbide with the desired particle size distribution contributes to the desired but unexpected efficacy of applicants' composition.

In one preferred embodiment, from about 30 to about 50 weight percent of the calcium carbide is used in the composition of the invention.

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 85 weight percent of calcium carbide. Such miner's grade calcium carbide is sold, e.g., by Carbide International, LLC. of Louisville, Ky.

The preferred additive of this invention also comprises from about 10 to about 70 weight percent of a second reducing agent. The second reducing agent may be comprised of silicon, titanium, or silicon carbide. In one embodiment, the additive is comprised of from about 20 to about 50 weight percent of silicon or titanium or silicon carbide.

The silicon used to form the additive of this invention may be pure elemental silicon. Alternatively, or additionally, it may be elemental silicon in combination with other materials (ferro-silicon, calcium silicon) or silicon carbide.

The titanium used to form the additive of this invention may be pure elemental titanium. Alternatively, or additionally, it may be elemental titanium in combination with other materials (ferro-titanium) or titanium carbide.

However, with regard to the manner in which the second source of reducing agent used in the process of the invention is constituted, whether by itself or in admixture with other material(s), it is preferred that the second reducing agent material have a particle size distribution such that at least about 90 weight percent of its particles have a particle size in the range between from about 0.1 to about 0.7inches. In an even more preferred embodiment, at least about 90 weight percent of the second reducing agent has a particle size in the range of from about 0.25 to about 0.4 inches.

Without wishing to be bound to any particular theory, applicants believe that the particle size distribution of the second. reducing agent material must be substantially the same as the particle size distribution of the calcium carbide material. If the particle size distributions of these materials differ substantially, segregation of the particle compacts within the mixture occurs during shipment of the material, and a non-homogeneous additive is produced.

Regardless of how much calcium carbide and second reducing agent individually are present in the additive composition, it is preferred that, in one embodiment, in combination, the calcium carbide and the second reducing agent comprise no more than about 70 weight percent of the desired additive.

Applicant believes that the use of the second reducing agent in the additive of this invention is unexpectedly advantageous in that it results in slag that is more fluid and thereby increases the kinetic rates of reaction of slag reduction. When the second reducing agent is not present in the additive, the reaction rate is substantially slower and all of the advantages of the additive are not achieved. This is not obvious. The additive of this invention, when added to the slag to be treated, undergoes an exothermic reaction which facilitates the production of the desired slag.

The slag to be treated may vary in composition. One typical such slag contains 45 weight percent of calcium oxide, 28 weight percent of ferrous oxide (FeO), 12 weight percent of magnesium oxide, 10 weight percent of silicon 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 20 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 15 minutes. It is noteworthy that the composition of the aforementioned U.S. Pat. No. 5,279,639 is not capable of achieving this result within the specified time.

In one embodiment, the additive of this invention contains from about 25 to about 30 weight percent of the second reducing agent, and from about 70 to about 75 weight percent of the aforementioned calcium carbide, by total weight of silicon, titanium and calcium carbide. In this embodiment, one may add additional flux materials to the additive, but such materials are not required. It will be appreciated that, when concentrations of silicon and/or titanium and/or calcium carbide are specified herein, these are to be considered as the being the concentrations of the pure materials.

In another embodiment, a flux material is added to the second reducing agent and the calcium carbide. In this embodiment, no more than about 70 weight percent of the additive is comprised of second reducing agent and calcium carbide.

It is preferred that the fluxing agent used 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 1A, IIA, and IIIA of the Periodic Table, silicon carbide, and mixtures thereof, and from about 0 to about 20 weight percent of a metal carbonate. The additive of this invention is preferably made by dry blending the calcium carbide, the material containing the silicon and/or titanium and/or silicon carbide, and, optionally, one or more of the fluxes in the desired stoichiometry. In embodiments where silicon carbide is used as a fluxing agent as well as a reducing agent, some amount of the silicon carbide forms a silicon oxide reaction product, which functions as the actual fluxing agent. If glass is used as a fluxing agent, such glass may include any complex silicate glass, such as e.g., crushed bottles or window glass.

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 charged to a paddle mixer and dry blended. Typically, the materials are dry blended for from about 5 to about 15 minutes, depending upon the batch size. The blended materials are then discharged into shipping containers.

Description of a Preferred Process for Using the Additive of this Invention

The present invention provides for the manufacture in situ of a ladle slag by the addition of a mixture of materials including silicon, calcium silicon and/or titanium and/or silicon carbide, calcium carbide, and, optionally, one or more fluxing materials 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 applicant believes that when one or more fluxes are used, they dissolve the CaO and SiO2 or TiO2 reaction products that form on the surfaces of the reducing agents during the reduction reaction, thereby presenting further reducing agent surfaces that may contact the slag to continue the reduction reactions.

The glass and calcium fluoride, when used, act as fluxes, i.e., they solubilize, other ingredients. Alumina, silica, calcium fluoride, and lime are slag formers. These raw materials provide the components necessary to achieve a desired composition with appropriate physical and chemical properties for steel refining in the ladle. It should be noted that each component may serve multiple functions.

It should be appreciated that, although the invention is discussed in the context of secondary steelmaking, i.e., secondary ladle slags, the invention has applicability to a wide range of refining procedures. 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.

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 second reducing agent required, based on the amount of iron and manganese oxide to be reduced and the desired composition of the refining slag. The amount of gas generation from this amount of calcium carbide is calculated, and compared with what is thought to be the maximum acceptable amount of gas generation. If the gas generation or gas generation rate are predicted to be excessive, an amount of calcium carbide is substituted by an amount of second reducing agent to provide equal reducing power but to decrease the gas generation or gas generation rate to an acceptable level (the gas generation level that is acceptable will vary by location and process). is Likewise, based on the final desired composition of the refining slag, the second reducing agent may be substituted for calcium carbide to achieve the desired chemistry.

The amount of carbon that is included in the steel when using calcium carbide slag reducing agents is preferably determined depending on process parameters and desired final FeO level. This amount of carbon pick-up will add to the existing carbon level in the steel. In some cases, the incremental carbon will create an unacceptably high carbon content of the steel. In that case, the maximum amount, or tolerance for calcium carbide slag reducing agent is calculated. The second reducing agent will then be substituted for the calcium carbide component to achieve the desired slag deoxidation.

In the above methodologies, calcium carbide tolerance is preferably first calculated, since calcium carbide slag reducing agents are less costly than slag reducing agents based on the second reducing agent. There are some instances where the tolerance for calcium carbide is very low, for example in tire cord carbon steels. In those cases, the primary slag reducing agent is based on the second reducing agent by necessity. The effectiveness of the silicon and/or titanium based reducing agents can be greatly improved by the addition of calcium carbide. The calcium carbide causes agitation within the slag, thereby mixing the silicon and/or titanium based conditioner into the slag for more efficient reaction. The calcium carbide's reaction products also help to dissolve the silicon and/or titanium oxide reaction products, and helps to fluidize the slag. It also reduces the lime requirement in the flux additions, thereby reducing its cost. It should be noted that similar agitation benefits can be obtained through the use of silicon carbide but will produce a slag higher in silica content. In the case of ultra low carbon steels, a blend of second reducing agent, calcium carbide, and fluxes is added in the required amount to reduce the iron and manganese oxides. This blend will typically contain just enough calcium carbide to create the desired synergistic effect, with silicon and/or titanium being the primary reducing agent; the synergistic effect being to provide reaction products from the reactants that combine to form a liquid at steel refining temperatures and help to flux the slag and thereby increase the reaction rate. In this case, the ratio of calcium carbide to second reducing agent may be kept constant for each heat of steel that is treated.

The solubility of magnesia in the final desired slag composition is determined, and that amount may be added in the form of magnesia containing materials in the slag additive. The required fluxing agents are added to ensure that the lime, silica and titanium oxide reaction products generated by the reduction reactions are dissolved and that the reducing agent surface is continually available for contact with the iron and manganese oxides that are present in the slag. It will be appreciated 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., silica, calcium fluoride, sodium oxide, alumina, carbon, 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.

The slag additives of the present invention are 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 steel after the tap.

FIG. 1 is a flow diagram illustrating a preferred process of this invention.

Referring to FIG. 1, it will be seen that a ladle 10 receives via line 12 molten steel and molten steel making slag. The molten steelmaking slag generally contains from about 25 to about 55 weight percent of calcium oxide, from about 10 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 via line 12 such that these two components comprise at least about 90 weight percent of molten steel, and more preferably, at least about 95 weight percent of molten steel.

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, generally via line 23. In the preferred embodiment depicted in FIG. 1, the reducing agent is a combination of reagents “A” and “B”, to be described in detail later in this specification. However, it should be understood that the process depicted in FIG. 1 may be used with only one of reducing agents A or B, with both reducing agents A and B, with three reducing agents (not shown), etc.

Thus, as is known to those skilled in the art, one may supply ladle 10 via line 23 with one or more reducing agent(s) such as calcium carbide, silicon carbide, ferrosilicon, titanium, titanium carbide, ferrotitanium, calcium silicon and combinations thereof. The process of this invention may be used with any or all of these reducing agents and/or other reducing agents. However, in some embodiments, the use of aluminum as a reducing agent is restricted due to downstream steel processing considerations.

FIG. 1 illustrates a preferred process, in which reducing agent A is calcium carbide, and reducing agent B is ferro-silicon. These reducing agents may be added in the desired proportion either by premixed combination or by separate proportional addition. Thus, e.g., the calcium carbide is preferably used by charging miner's grade calcium carbide from hopper 22, and the ferro-silicon is preferably used by charging from hopper 24.

In one embodiment of the process depicted in FIG. 1, only the calcium carbide and silicon reagents are added. 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 calcium carbide material. Alternatively, one or more of the additional fluxing agent(s) may be present in hopper 24 admixed with the silicon material. 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, and in the preferred embodiment depicted therein, it will be seen that material dispensed from hoppers 22 and 24 are forced through 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 reagent A and how much of reagent B to charge to ladle 10. The controller 29 evaluates historical data and real-time data in determining how much of each of reagents A and B to charge to the 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 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 made immediately prior to the current batch, (4) the weight of the batch of steel made during the session immediately prior to the current batch, and (5) the average final slag composition for the grade of steel being made in the current batch.

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 necessarily 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 planned for the batch in question by the steel mill additions programs, which often involve the addition of materials such as alloys, calcium oxide, calcium aluminate, etc., (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 camera 40 continuously monitors 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 the data charged via lines 36 and 38, 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 containing material to be charged from hopper 22. In making such a calculation, it is desirable to note that the stoichiometric amount of calcium carbide required (3.0 moles of iron oxide per moles calcium carbide) is generally insufficient for full reduction of the iron oxide, for some of the calcium carbide is not effective.

Thus, controller 29 utilizes an “efficiency factor” to appropriately increase the amount of calcium carbide above the stoichiometric amount., Once the controller 29 calculates how much calcium carbide should be charged to the ladle 10, it then determines whether the reaction mixture within ladle 10 can accept the presence of such an amount of calcium carbide. As is known to those skilled in the art, in some systems the addition of calcium carbide to molten slag causes excessive foaminess of the slag, which often is undesirable. Additionally, since calcium carbide contains carbon, the controller must determine how much carbon the reaction mixture in the ladle 10 can accept without compromising the quality of the steel; if too much carbon is present, it may adversely affect the properties of the steel made. The controller thus calculates how much less than the required amount of calcium carbide should be charged based upon the foaminess and carbon uptake considerations.

The shortfall of calcium carbide thus calculated is then made up with the addition of the second reducing agent, i.e. silicon, titanium and/or silicon carbide material, which is caused to be discharged from hopper 24 in the required amount by the controller 29.

If the difference between the theoretically ideal amount of calcium carbide and the amount which can be tolerated by the system is less than about ten percent, then it is preferred to charge 90 percent of calcium carbide (by weight of carbide and second reducing agent material) from hopper 22, and 10 percent of second reducing agent material from hopper 24.

In any event, based upon all of the data fed into controller 29, the controller can determine factors such as the concentrations of the calcium carbide and the second reducing agent materials added, and the concentration(s), if any, of the flux material(s) added. In either event, the calcium carbide material and the second reducing agent material are added at or about the same time to the molten reaction mixture so that they act in concert with each other, cause an exothermic oxidation-reduction reaction to occur, and facilitate the formation of the desired refining slag.

The reagents added are generally present only within the slag layer 16. In general, the amount of calcium carbide in the slag layer 16, when added to the amount of second reducing agent in the slag 16, is of sufficient weight that, when such amounts are combined, they are equivalent to from about 0.5 to about 10 weight percent of the total mixture of the weights of slag on the ladle after tap, the calcium carbide added, and the second reducing agent added. The presence of such reagents in the slag on the ladle 10 after tap facilitates reactions which generally produce the desired refining slag in less than about 15 minutes. In one embodiment, from about 2 to about 5 weight percent of the total mixture of slag on the ladle 10 after tap is calcium carbide and second reducing agent. Considering only the calcium carbide and the second reducing agent reagents together, whatever form they may be in, the calcium carbide represents from about 5 to about 60 weight percent of the total weight of these two materials, and the second reducing agent represents from about 10 to about 50 weight percent of such total weight.

Prior to the introduction of the calcium carbide and the second reducing agent into ladle 10, molten steel in ladle 10 is generally maintained at a temperature of from about 2,600 to about 3,000 degrees Fahrenheit. After the introduction of the calcium carbide and the second reducing agent into ladle 10, it is preferred to continue maintaining the steel at such temperature of from about 2,600 to about 3,000 degrees Fahrenheit. Under such conditions, the desired refining slag is generally produced in less than about 15 minutes.

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, second reducing agent, and one or more fluxing agents).

When one or more fluxing agent(s) is used, it is preferred that less than ten weight percent of the total amount of fluxing agent used is a metal carbonate. It is even more preferred that less than 5 weight percent of the fluxing agent is a metal carbonate. If a metal carbonate is used in the configuration depicted in FIG. 1, it is preferred to charge it from hopper 24, with the second reducing agent.

When one or more fluxing agents is used, it is preferred to also use particulate glass with a particle size substantially identical to the calcium carbide. The amount of glass used is generally from about 0 to about 50 weight percent of the total amount of flux used, and preferably is from about 10 to about 30 weight percent. Preferred glass compositions may include complex oxides primarily comprised of silicate, with minor amounts of calcia, alumina, magnesia, and other metal oxides; as well as calcium fluoride. Examples of suitable sources of glass materials include crushed bottles or auto glass, complex oxide industrial byproducts and by-product oxides, recycled steelmaking slag, and/or minerals such as volcanic glass.

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.

When one or more fluxing agents is used, it is preferred to also use calcium oxide. The amount of calcium oxide used is generally from about 0 to about 50 weight percent of the total amount of flux used, and preferably is from about 5 to about 20 weight percent.

When one or more fluxing agents is used, it is preferred to also use silica. The amount of silica used is generally from about 0 to about 50 weight percent of the total amount of flux used, and preferably is from about 5 to about 20 weight percent.

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.