Abstract:
Deoxidizing compositions and two-stage methods for deoxidizing molten steel produce purer steel products, at lower cost, than traditional aluminum and silicon-based deoxidation technologies. A first deoxidizing composition is free from elemental aluminum and reduces oxygen content primarily by the conversion of carbon to carbon monoxide, along with the conversion of silicon to silica. A subsequently-added second deoxidizing composition lowers the oxygen content to a highly desirable level, with the production of substantially less alumina impurities than the prior art technologies.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The present invention relates to compositions and methods for the deoxidation of molten ferrous metals such as steel, especially the deoxidation of steel produced by continuous casting (CC) methods.  
           [0003]    2. Description of the Background Art  
           [0004]    Steels are alloys of iron and carbon with other metals, and typically have a carbon content of less than 1%, with some alloys having a very low carbon content of less than 20 ppm. The physical properties of steel are chiefly influenced by the interaction between the chemical composition of the steel, the thermal treatment of the steel, and the cleanliness of the steel which is influenced by the method used to remove oxygen from the steel (referred to herein as “deoxidation”).  
           [0005]    A very important chemical reaction during steelmaking is the oxidation of carbon. Its gaseous product, carbon monoxide, goes into the off-gas, but, before it does that, it generates the carbon monoxide boil, a phenomenon common to all steelmaking processes and very important for mixing. Mixing enhances chemical reactions, purges hydrogen and nitrogen, and improves heat transfer. Adjusting the carbon content is important, but it is often oxidized below specified levels, so that carbon powder must be injected to raise the carbon again.  
           [0006]    As the carbon level is lowered in liquid steel, the level of dissolved oxygen theoretically increases according to the relationship % C×% O=(0.0020 at 2900° F.). This means that, for example, a steel with 0.1 percent carbon, at equilibrium, contains about 0.025 percent, or 250 parts per million, dissolved oxygen. The level of dissolved oxygen in liquid steel must be lowered because oxygen reacts with carbon during solidification and forms carbon monoxide and blowholes in the cast. This reaction can start earlier, too, resulting in a dangerous carbon monoxide boil in the ladle. In addition, a high oxygen level creates many oxide inclusions that are harmful for most steel products. Therefore, usually at the end of steelmaking during the tapping stage, liquid steel is deoxidized by adding aluminum or silicon. Both elements are strong oxide formers and react with dissolved oxygen to form alumina (Al 2 O 3 ) or silica. These float to the surface of the steel, where they are absorbed by the slag. The upward movement of these inclusions is often slow because they are small (e.g., 0.05 mm), and combinations of various deoxidizers are sometimes used to form larger inclusions, or liquid inclusions, that float more readily.  
           [0007]    In order for the CC process to be successful, the amount of dissolved oxygen in the steel has to be very low, normally lower than 10 parts per million (ppm). However, in the primary steel making process (melting and pre-refining), oxygen is captured in the steel either from the oxygen being injected to accelerate the process and gain in productivity, or simply it is taken absorbed from the ambient atmosphere according to the law of equilibrium.  
           [0008]    For low carbon steels, (0.02 to 0.05% tapping carbon) the quantity of oxygen in the steel varies from about 700 to 1200 ppm. Even for medium carbon steels which are tapped at 0.1% carbon the oxygen content of the steel is about 300 ppm. Thus, in order for the steel to be continuously cast, oxygen has to be reduced to less than about 10 ppm. Additionally, in the primary steel making process, a slag is formed due to the gangue content of some raw materials such as Direct Reduced Iron (DRI) or Hot Briquetted Iron (HBI), the oxidation of silicon (Si) or manganese (Mn) from Hot Metal (HM), or dirt from the scrap. The majority of these materials render an acid slag, i.e. composed mainly of silica (SiO 2 ), and alumina (Al 2 O 3 ). Oxygen also may be injected during the process for refining purposes (de-carburation and de-siliconization of HM), or to increase productivity (EAF). In any case, iron is also oxidized and goes to the slag as FeO, typically requiring the addition of lime (CaO) and magnesia (MgO) to provide the slag with the proper basicity, fluidity, and to render it less aggressive to the refractory. During tapping, even if measures are taken to prevent it, some slag is carried on to the secondary metallurgy step. Depending on the sulphur and cleanliness requirements of the steel, this slag has to be partially deoxidized (reduction of FeO and MnO; conventional casting) or almost totally deoxidized (thin slab casting).  
           [0009]    Oxygen is present in molten steel and is removed by one of two methods. In “rimmed” steels not produced by continuous casting, a portion of the oxygen leaves the steel in the form of carbon monoxide during the solidification process. This results in a lower concentration of carbon in the steel at the surface and thus a skin on the steel that is much more ductile than the bulk of the material. A more uniform product is obtained by the addition of an element such as aluminum or silicon to the molten steel and allowing it to react with the oxygen and form compounds that are separated from the molten steel. Steel that is produced in this manner is known as “killed” steel.  
           [0010]    It has been widely accepted that aluminum and silicon are the best deoxidizing agents due to their affinity with oxygen and the economies of their use in the process. Steels commonly are classified as aluminum killed steels (steels deoxidized by aluminum), and silicon killed steels (steels deoxidized by silicon). Aluminum is mostly used because it is an excellent deoxidizer and its price is lower than other very high oxygen affinity elements such as zirconium, calcium, magnesium and titanium. On the other hand, most of the applications for flat steel products (those to be formed, coated and those of high surface quality requirements) do not tolerate the presence of relatively high levels of silicon in the steel chemistry.  
           [0011]    Although the use of aluminum has been widely accepted as a deoxidizing additive in the steelmaking process, the following reactions are known to take place:  
           [0012]    In the steel: 2Al+3O→Al2O3  
           [0013]    In the slag: 2Al+3 FeO→Al2O3+3Fe  
           [0014]    In both cases alumina (Al2O3) is formed, which is solid at steel making temperatures. As noted above, alumina is not dissolved in the steel but remains in the steel in the form of small particles (5 to 200 microns). Some of the particles, specially the larger ones, are floated out to the slag by means of a gentle rinsing of the steel mass with an inert gas, normally argon, but many remain in the steel, rendering the final product “dirty” and affecting its forming properties, such as drawing, stretching, etc. Also, if the steel is dirty, it is prone to exhibit a large number of surface defects known as non-metallic inclusions (NMI), seams or slivers. These surface defects are a cause of rejection or downgrading of the product.  
           [0015]    Oxygen is a deleterious element in the steel chemistry and can be classified as active toward carbon, which means that it is dissolved in the melt and non-combined, and inactive toward carbon which means that it is combined, forming oxides with other elements and will not react with carbon. In the aluminum or silicon killed steels, aluminum or silicon, because of their very high affinity with oxygen, react immediately with it and change the state of the oxygen from being a dissolved gas active toward carbon, to a solid (or liquid) and non-dissolved oxide. Cleanliness in the steel is defined as the absence of oxygen in the steel, and it is measured as the inverse value of the total oxygen content; the lower the oxygen content, the cleaner the steel, understanding the total oxygen content as the sum of dissolved oxygen, plus combined oxygen. Thus, strong deoxidizers such as aluminum or silicon solve the problem of preventing the oxygen in the melt to react with carbon which in turn allows the steel to be continuous cast, but create solid compounds which can not be floated out completely and undesirably remain in the steel. During tapping, the normal oxygen content for a low carbon steel is 800 to 900 ppm, or 850 grams per ton, and by deoxidation with aluminum this amount of oxygen converts to 1700 grams of alumina (or “dirt”) per metric ton of steel.  
           [0016]    In many steel making facilities, the slab, bloom or billet is cooled down and surface conditioned to remove these defects, adding time and cost to the process. This conditioning does not affect the substantial portion of metal lying below the surface, however. In most modern processes such as the conventional-direct hot charge, thin slab casting or strip casting, there is no possibility so far to remove the defects, since these processes (aimed at saving energy and time), do not allow the cast product to cool down and instead charge it hot to the rolling mills, or simply produce the strip directly.  
           [0017]    Accordingly, a need remains for improvements to the deoxidation of steel—a need addressed by the present invention.  
         SUMMARY OF THE INVENTION  
         [0018]    In one aspect the present invention provides deoxidizing compositions, for addition to molten ferrous metal during steelmaking, which provide excellent deoxidation and, advantageously, generate a far lesser amount of particulate residues within or on the surface of the finished product.  
           [0019]    A first deoxidizing composition is comprised of, on a total weight basis, from about 38 to about 42% CaO, about 38 to 42% Al 2 O 3 , about 4 to about 6% MgO, about 14 to 17% C, and about 0 to about 5% Si.  
           [0020]    A second deoxidizing composition is comprised of, on a total weight basis, from about 68 to about 72% CaO, about 4 to about 6% MgO, and about 24 to 27% Al.  
           [0021]    The invention also relates to methods for deoxidizing molten ferrous metals, including steel. One aspect of the invention provides a method of deoxidizing molten steel, comprising a first deoxidation step comprising adding to molten steel an amount of carbon and silicon effective to substantially lower the reactive oxygen content in the absence of elemental aluminum, whereby said carbon is converted to carbon monoxide and said silicon is converted to silica, followed by a second deoxidation step comprising adding elemental aluminum to further lower the reactive oxygen content of the molten steel.  
           [0022]    Another aspect of the invention relates to methods for producing steel, comprising adding to the molten metal, sequentially during the tapping of the steelmaking furnace, the first deoxidizing composition and the second deoxidizing composition. This aspect of the invention includes a two-step deoxidation process, comprising a first step that is free from the use of elemental aluminum, thereby avoiding the formation of solid alumina residues that remain in the final product. This step relies primarily upon the oxidation of carbon to form carbon monoxide and the oxidation of silicon to form silica (silicon dioxide). The second deoxidizing step includes the addition of elemental aluminum. However, at that point, the reactive oxygen content of the metal has been lowered substantially, whereby the formation of alumina is minimized. The compositions may be added to molten metal within the ladle. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    The present invention provides advantages over the prior art deoxidation processes and additives, which are based primarily on aluminum and/or silicon chemistry. Unlike such prior art processes, the present invention utilizes carbon and silicon as deoxidizing compounds in a first deoxidation step of a two-stage deoxidation process for molten ferrous metals. Carbon is considered to be only partially removed by oxygen, and oxygen will react with carbon only if other elements of higher oxygen affinity are not present. However, if higher affinity elements such as aluminum are not present, oxygen will react with carbon to form carbon monoxide, a gas, according to the following expression:  
         C+O→CO  
         [0024]    Also:  
         Si+O→SiO 2    
         [0025]    Advantageously, the CO is in the form of a gas and the SiO 2  is in the form of a liquid when, in accordance with the present invention, aluminum content is low.  
         [0026]    While the prior art relies upon deoxidants that generate solid oxides in the steel and, later on, have to be at the best partially floated out to the slag, the present invention takes advantage of the affinity and Gibbs energy of the compounds to deoxidize the steel from the tapping oxygen content to as low as 100 ppm. Deoxidation is carried out with carbon and silicon that generate gas and liquid compounds, respectively, that leave the steel rapidly due to the partial pressures and the bath stirring by the stream of steel being poured to the ladle or by the argon stirring common to today&#39;s steel making practices. Deoxidation of the steel in a process that creates far fewer solids (or even liquids that solidify with it), provides an improved final product. Use of the deoxidation compositions and processes of the present invention results in the formation of only 20% or even less of the alumina and other particulate impurities, on a per-ton basis, that arise when aluminum is used as the primary deoxidation material.  
         [0027]    Advantageously, use of these compositions in the methods of the present invention provides a first deoxidation step wherein no elemental aluminum is added. This first step deoxidizes the steel down to approximately 100 to 200 ppm oxygen, via the conversion of carbon to carbon monoxide (gas) and silicon to silica (liquid). Deoxidation can then be further carried out, to bring the oxygen content down to approximately 10 ppm or less, via addition of the second deoxidation composition which contains a mix of aluminum and lime. The small amounts of alumina which form in this step are efficiently removed from the molten metal by being absorbed in the form of inclusion bodies which form from the liquid slag generated by the deoxidation compositions. This reduces the alumina generation to less than about 300 grams per metric ton. The process, in addition to producing a purer final product, reduces the processing costs because the carbonaceous materials provided in the first deoxidizing composition (i.e. silicon, supplied in the form of silicon carbide and carbon, supplied in the form of coke and silicon carbide) are lower in cost than aluminum. Other advantages include a better balance of acid compounds (SiO2 &amp; Al2O3 ) with basic ones in the final product, and the formation of a liquid slag with an increased alumina adsorption capacity (since less alumina is generated as deoxidation product, so that the slag is less saturated with alumina).  
         [0028]    It will be appreciated that the processes and compositions of the present invention are useful in a variety of steelmaking operations. Use of the present invention as part of a continuous casting operation, particularly a thin slab continuous casting operation, is particularly preferred.  
         [0029]    In one preferred embodiment, the first deoxidation composition is comprised of, on a total weight basis, from about 38 to about 42% CaO, about 38 to 42% Al2O3, about 4 to about 6% MgO, about 14 to 17% C, and about 0 to about 5% Si. The preferred particle size profile of this composition is such that the particles are larger than about ⅛ inch and not larger than about 1¼ inch. This first deoxidation composition may be added to the ladle in the form of a particulate blend at an addition rate of between about 7 and about 9 pounds per (short) ton of steel. A particularly preferred addition rate is 8 pounds per ton of steel in the ladle. The first deoxidation composition is essentially free of elemental aluminum.  
         [0030]    In a preferred embodiment, the second deoxidation composition is comprised of, on a total weight basis, from about 68 to about 72% CaO, about 4 to about 6% MgO, and about 24 to 27% Al. The MgO preferably is supplied in the form of magnesium carbonate from dolomitic lime. The preferred particle size profile of this composition is such that the particles are larger than about {fraction (1/16)} inch and not larger than about ¾ inch. This composition may be added to the ladle as a particulate blend at an addition rate of between about 9 and about 11 pounds per (short) ton of steel. A particularly preferred addition rate is 10 pounds per ton of steel in the ladle.  
         [0031]    Particularly preferred first and second deoxidation compositions are manufactured by dry blending the following ingredients in the proportions set forth below. All percentages are expressed as weight percent based upon the total weight of the composition:  
         [0032]    First Deoxidation Composition  
         [0033]    Calcium aluminate 70%  
         [0034]    Lime 12%  
         [0035]    Coke 14%  
         [0036]    Silicon carbide 4%  
         [0037]    Second Deoxidation Composition  
         [0038]    Lime 62%  
         [0039]    Dolomitic lime 12%  
         [0040]    Aluminum 26%  
         [0041]    Because steelmaking processes are well known, the methods by which the compositions of the present invention can be incorporated into molten ferrous metal will be apparent to persons skilled in this technical field. One preferred process according to the present invention comprises a first deoxidation step characterized by the addition of the first deoxidizing composition (which advantageously is free from elemental aluminum), at a rate of approximately 8 pounds per short ton of steel, after a “cushion” of approximately 10 to 15 tons of steel have been tapped to the ladle. The presence of such a cushion of steel in the ladle is important so as to avoid undesirably rapid chemical reactions. Additional steel is added to the ladle, and a second deoxidizing step is carried out by adding the second deoxidizing composition at a rate of approximately 10 pounds per short ton when only about 20% of the total capacity remains to be tapped into the ladle.  
         [0042]    Those skilled in this field will be familiar with the compositions of molten ferrous metals useful in the steelmaking process. If desired, ferroalloys such as ferromanganese, ferrosilicon, or any other that could react with the dissolved oxygen of the steel, should be added after the first deoxidizing composition. If desired, such alloys can be added simultaneously with the second deoxigenation composition. The addition of additional lime generally is required in at an addition rate of approximately 7 pounds per short ton of steel to reach a balance with the other compounds and rapidly desulphurize the steel. Such lime can be added after addition of the second deoxidizing composition. Also, trimming additions of aluminum are generally required, primarily to finish the deoxidation of the slag and to compensate for process variations.  
         [0043]    Although the present invention has been described in connection with certain preferred embodiments, it is not so limited. Variations within the scope of the invention will be apparent to persons skilled in this technical field.