US1932252A - Process of producing alloys - Google Patents

Description

Patented Got; 24 1933 UNITED STATES 1,932,252 rnocnss F PRODUCING ALLoYs William Bell. Arness, Baltimore, Md., assignor to Alloy Research Corporation, Baltimore, Md., a corporation of Delaware No Drawing. Application August 15, 1931 Serial No. 557,375

16 Claims.

This invention relates to the art of producing alloy iron and steel, more particularly rustless iron and steel, and more especially to the art of producing such alloy iron whereinthe alloy metal is directly reduced from reducible ores or oxides thereof.

One of the objects of my invention is the production in a simple, direct and economical manner of alloy iron and steel of desired physical characteristics such as workability, hardenability, corrosion resistance, heat resistance and the like made to a desired analysis from inexpensive raw materials.

Another object is the reliable and consistent production in a highly eincient manner, of alloy metal of the class indicated, to a desired specification of alloying elements or ingredients such as chromium, tungsten, titanium, vanadium, manganese, or the like, together with desired low values of other ingredients such as silicon and carbon.

Another object of my invention is the production of an alloy iron or steel of the class noted from readily available and inexpensive raw materials, with a minimum of such materials all at maximum efficiency and minimum manufacturing cost.

Other objects will be in part obvious and in part pointed out hereinafter.

The invention accordingly consists in the several steps and in the relation of each of the same to one or more of the others as described herein and the scope of the application of which is indicated in the following claims.

As conducive to a clearer understanding of certain features of my invention at this point it may be noted that in heretofore known and/or used processes for the manufacture of alloy iron and steel of high alloy content by means of direct reduction of a reducible oxide of an alloy metal by a reducing agent wherein an excess of reducible oxide is employed, more oxide or ore is used in proportion to reducing agent than is chemically equivalent to the reducing agent. Such a process necessarily entails low alloy yield.

It has been heretofore necessary to maintain such reducible oxide in the order of about 60% excess so that contamination of the bath by the reducing agent, which in practice is ordinarily 50% ferro-silicon, may not occur to an undue extent. Where the excess of reducible oxide in such a process is decreased to about 60% excess the silicon content of the bath at the end of the reducing period will necessarily be considerably more than 1% and probably about 2%. And

where the excess is further decreased to about 40% this silicon contamination may cause the metal bath to pick up as high as 5% or 6% silicon.

Furthermore, in processes for the manufacture of alloy steels and irons which containa high percentage of alloy that is easily reducible from an ore thereof and which involve a direct reduction of such alloy content from the ore there is an inherent danger of producing a metal which upon solidification fails to show a normal shrinkage, and which often swells or rises in the molds, thus resulting in objectionable blow-holes in the finished product which render the same unusable for most purposes.

Several explanations for this peculiar gaseous condition have been advanced (this is a gaseous condition which even occurs in what are ordinarily considered fully killed or fully deoxidized metals), such as over oxidation, over deoxldation, or pouring at an excessively high temperature. None of these explanations, however, adequately account for these undesirable conditions, as will appear more fully hereinafter.

Furthermore, in heretofore known and/or used processes of alloy iron and steel manufacture wherein the alloy content of the product is directly reduced from the ores and oxides there- 'of difficulty is encountered in maintaining a satisfactory electric arc in the presence of the large slag volumes which are normal to. such processes. In such processes when silicon is used, which is normal practice, it is desirable to promote a condition in the slag which would render it as conductive as possible to an electric current.

By maintaining a large excess, in the order of 60%, of reducible oxide or ore and using in addition to this ore or oxide only the reducing agent (customarily 50% ferro-silicon) and forming a slag chiefly from these ingredients alone, this slag is rendered fairly conductive to the electric current and aids in maintaining fair- 13! good are conditions within the furnace. Where other ingredients such as burnt lime or limestone, or other fluxes such as fluorspar, are used in large quantities in such a slag the electric conductivity of the slag is decreased, thus making it more difficult to maintain satisfactory are conditions. (By satisfactory are conditions is meant the ability of the operator to maintain at will a visible are above the slag.)

Wherea satisfactory arc is not maintained at all times during the reducing period, the electrodes dip into the slag thereby, at least partially, M0

converting the furnace into a resistance furnace. Under this operating condition the metal bath becomes highly superheated and the furnace above the slag remains at subnormal temperatures which results in undue dimculty in effecting complete fusion and fluidity of the slag. Of further objection, unless a visible arc is consistentlymaintained, there is a tendency for the carbon electrodes to contribute carbon rapidly to the slag and hence from the slag to the metal bath resulting in undesirable carbon contamination of the metal.

One of the outstanding objects of my invention is, in the production of alloy irons and steels wherein the alloy content of the iron or steel is supplied by a direct reduction of ores or oxides of the alloy metal, to greatly decrease the necessary quantities of such ores or oxides over heretofore known and/or used methods of production.

Another object is to produce, in the manufacture of ferrous alloys in accordance with the process indicated above, consistently sound and fully formed ingots of alloy metal of any desired high content of alloying element, such as chromium, and any desired low content of the elements carbon and silicon with small amounts of sulfur and phosphorous.

Another object is to achieve more reliable and more efiicient furnace operating conditions, thus permitting greater economy of production, than in heretofore known processes of the class described.

Referring now to the practice of my invention, a suitable furnace, such as a six ton Heroult electric furnace rated three phase, 120 volts, 25

cycles, 1500 KVA and having a magnesite bottom and silica sidewalls and roof, is charged with, illustratively, 7500 pounds of ordinary low carbon steel scrap together with about' 100 pounds of iron ore or mill scale and some'300 to 500 pounds of a basic slag making material such as burnt lime. This charge melts down giving a ferrous metal bath of low carbon content covered with a basic slag. A portion of this slag is preferably removed prior to the addition of the alloying ingredients specially prepared and in the relative proportions now added.

A reducible oxide of'the alloying metal described (for example the ores of chromium, tungsten, titanium, vanadium, manganese, or the like either singly or in combination when, respectively, a chromium, tungsten, titanium, va-

nadium, manganese or like ,alloy is desired), il-

oxide), is crushed to about one quarter inch.

size and carefully pre-dried, as more fully described hereinafter. This crushed -and dried ore is preferably thoroughly mixed with about a chemically equivalent amount of a similarly crushed and pre-dried reducing agent (for example calcium, silicon, aluminum, aluminumsilicon alloy, ferro-aluminum alloy, ferro-silicon alloy, or the like), illustratively 2,700 to 3,200 pounds of 50% ferro-silicon, and a carefully pre-dried basic fiuxing agent such as burnt lime in an amount equal to some three to five times the weight of the effective reducing element of the reducing agent employed; illustratively 5,500 to 7,500 pounds of burnt lime for the ingredients and proportions indicated above.

This mixture is shoveled intothe furnace as rapidly as the ingredients can be fused. In this manner a shovelful of the mixture added to the slag, is fused, and begins to "react almost at once. The next shovelful follows as rapidly as the maintenance of good fluid slag conditions permit. The ore or reducible oxide, illustratively chrome ore, and the reducing agent, ferrosilicon, react and the resulting metals, iron and chromium, gravitate into the metal bath while the resulting silicates are retained in the slag.

At one or more intervals during the reduction operation the accumulated slag blanket may be removed at the convenience of the operator. It may at this point be noted that due to the thorough intermixing of ingredients prior to the introduction of the same into the furnace a large excess of the basic slag forming agent is preserved at all times thus assuring a substantially constant rate of reaction between reducible oxide and reducing agent and the carrying out of this reaction under basic slag conditions, the advantage of which appears more fully hereinafter.

After the mixture has all been added to the furnace, has all fused and reacted properly, the accumulated slag is quite largely removed leaving only a thin blanket of about one-half inch to two inches in thickness. The metal is then refined in accordance with conventional steel making methods. During this latter stage it is desirable to maintain a fairly tight furnace to prevent unnecessary oxidation of the metal. By further additions of burnt lime and fine ferrosilicon any metal remaining in the slag as oxides is recovered.

After the refining stage of the process is completed in accordance with standard practice, the furnace is tapped and the refined metal poured into suitable ingot molds. The heat comprises about six tons of ingots analyzing about 17.5% chromium, from .3% to'.5% silicon, less than .10% carbon, with small amounts of sulfur and phosphorous and the balance principally iron. There is no evidence in the product of swelling or rising in the molds nor are any blow-holes found in the metal. The ingots are consistently sound, being particularly free from the effects of gas and are of a desired low carbon and low silicon content. v

To achieve a high recovery of the alloy metal from the reducible oxide, illustratively, chromium from chrome ore, the proportion of reducible oxide to reducing agent is approximately in direct accordance with the molecular weights of these ingredients; a maximum recovery of alloy metal is obtained, however, where an excess of from 10% to 20% of reducing agent is employed (a small percentage of the reducing agent being I lost in the furnace).

To prevent contamination of the ferrous metal by the reducing agent, illustratively, ferro-silicon, a strongly basic slag is maintained throughout the entire reduction period. A readily available and inexpensive basic slag forming material such as burnt lime is used in an amount of from three to five times by weight of the amount of the effective reducing element of the reducing agent employed.

The particular amount of lime required, however, in addition to that necessary to maintain basic conditions in spite of the large quantities of acid constituents formed as a result of the reaction between reducible oxides and reducing agent is dependent upon the amount of acid constituents in the raw materials as well as upon the nature and condition of the furnace lining. Thus, when chrome ore is reduced by a silicon containing reducing agent there is a resultant 150 silica content which may vary from two percent to ten percent. And where the refractory sidewalls and roof of the furnace are made of silica brick, in accordance with the usual practice, a variable quantity of silica may be directly absorbed into the slag from the furnace lining. The amount of basic slag forming material used is maintained sufficient to give a strongly basic slag even in the presence of the various acid agents a number of which are set forth above.

When the required amount of lime is used and a strongly basic condition maintained in the presence of chemical equivalents of reducible oxide to reducing agent, or even of a preferred excess of reducing agent, it is possible to effect a highly eflicient reduction of alloy and at thesame time avoid silicon contamination of the.

metal bath.

Lime itself, however, does not directly enter into this reaction as lime is not an active reagent with respect to either the bath, the alloy, or the reducing agent. Nevertheless, after the reducing agent has reacted with the reducible oxide to form the alloy metal (illustratively chromium), and the oxide of silicon (SiO2) this oxide of silicon, commonly referred to as silica, does react vigorously with the lime. As a result of this reaction there is formed a series of calcium silicates, which, dependent upon temperature and chemical conditions in the slag, may contain more or less calcium oxide in proportion to silica. These calcium silicates balance themselves, acid (silica) against base (lime), to form neutral compounds which are among the most stable components of the slag.

These compounds apparently govern the susceptibility of the metal bath to silicon'contamination. It is known, for instance, that in acid processes of steel manufacture there is a normal pick up of silicon from the slag by means of dissociation of silica. This is quite pronounced in the electric arc furnace but it not unknown in other types of furnaces. It occurs in the presence of normally high oxide content in the metal bath and is explainedas a normal function of an acid process, and more particularly of an acid slag.

Thus by avoiding the use of an acid slag in the production of a ferrous alloy where the alloy content is obtained by direct reduction of an oxide of the alloy, and using a strongly basic slag as indicated above, numerous practical advantages are achieved. For example silicon contamination of the metal is effectively prevented, a high recovery of the alloy metal from a desired ore or oxide is obtainable, a decreased amount of relatively expensive oxide or ore of the alloy is found wholly adequate, and a general decrease is realized in cost of materials as well as in cost of operation.

At this point it may be noted that under the operating conditions of the electric furnace extremely high temperatures are obtained in the electric arcs. These temperatures are sufllcient to dissociate moisture or water vapor contained in the furnace atmosphere into the component elements oxygen and hydrogenp The oxygen thus liberated readily reacts with the s1ag,and with the metal bath in the furnace thus leaving the hydrogen free and in a very active state The hydrogen gas dissolves in the slag' and metal (that dissolved in the slag later going into the metal) while the metal is in a molten condition. This effect is greatly aggravated where the hydrogen is maintained in contact with the metal as evidenced by a slight pressure being built up within the furnace, due to the production of hydrogen gases more rapidly than it may be dissipated into the atmosphere outside of the furnace, thus causing unusually large quantities of hydrogen to enter the slag and metal.

Such gas is retained by the metal until the metal is permitted to solidify. Thus when the heat of metal is tapped into the molds and there allowed to cool and solidify, the contained hydrogen in excess of that which will remain dissolved in the solid metal comes out of solution thus causing rising or swelling" of the metal in the molds as well as a deleterious blow-hole condition which renders the metal unsatisfactory for many applications and unfit for most.

Now the source of such hydrogen which gives rise to many objectionable effects in the tapped metal, a number of which are set forth above, is moisture introduced into the process through the raw materials such as the alloy ore or oxides, any iron ore or oxide employed and particularly the fluxing or slag forming agents, as well as the moisture in the air which gains access to the furnace chamber.

In order to minimize the introduction of moisture into the furnace and thus effectively reduce the quantity of hydrogen which is produced and enters the metal, thus causing the above-mentioned highly undesirable effects, all of the ingredients are carefully pre-dried prior to the charging of these ingredients into the furnace, all as more particularly described and claimed in my United States Patent 1,925,916 granted September 5, 1933 and entitled Process of producing alloys.

It is to be noted, however, that in pre-drying the reducible oxide, illustratively chrome ore, and the fluxing agent, illustratively burnt lime, these ingredients are maintained at a temperature of approximately 2500 degrees F. under suitable ventilation conditions and for a period of time sufficient to effect a removal of all appreciable moisture or other voltable content. The reducing agent, however, illustratively ferro-silicon, should not be pre-dried at a temperature above 1000 degrees F. since higher temperatures would cause premature oxidation of this agent and thus detract from the desired properties of reduction.

Certain economies in operation are achieved by mixing the ingredients immediately after the above-mentioned drying step, while still hot, and then charging this hot mixture intothe furnace. By this procedure there is an appreciable lessening of the cooling action of this mixture on the.

molten metal within the furnace and so permits rapid charging and minimum duration of the reduction period described above with a consequent decrease in the production cost and a general increase in operating efficiency.

To minimize the highly undesirable absorption by the molten metal of any hydrogen produced, as more particularly described above, the furnace is preferably well vented during the reducing stages of the process. This effectively prevents the gases that are given off during this stage of the process from remaining in intimate contact with the slag and metal, as evidenced by the building up of an increased atmospheric pressure within the furnace, and causing an excessive absorption of hydrogen by the metal." The furnace chamber may be freed from these generated may be opened during the reducing stage and closed during the succeeding refining stage to give a tight furnace as indicated above, or the desired result may be accomplished by merely opening the furnace charging door a suitable amount to permit free outward passage of the gases evolved.

Referring, now, back to the manner of preventing of silicon absorption of the metal bath as by maintaining a strongly basic slag on the metal one practical difliculty is encountered in furnace operation. The electrical resistance of this type of slag is appreciably higher than that of the slag ordinarily employed so that there is a tendency for the furnace electrodes to automatically dip into the slag and shorten the electrical path between electrodes in the normal automatic power regulation of the furnace. Any direct contact between electrodes and slag or metal is'accompanied by an undesirable pickup of carbon. The danger of operating the furnace with the electrodes dipping into the slag is avoided by operating the furnace at a sufficiently increased voltage so that an arc may be preserved at the end of each electrode. While the particular operating voltage is determined by the thickness of the slag blanket as well as the total amount of slag carried, good results are achieved where an over voltage of 15% to 65% is used (a total voltage of 140 to 180 for the 120 volt furnace illustratively described).

Resulting from the use of the strongly basic slag in the reduction stage of the process there are certain inherent improvements in elimination of sulfur. A large portion of this impurity (introduced with the raw materials is retained by the slag and does not enter the metal as in heretofore known processes where an acid slag is used or where a strongly basic slag is not employed. The sulfur then is taken off with the slag leaving the metal uncontaminated thereby.

Thus it will be seen that there has been provided in this invention an art of producing alloy iron and steel in which the various objects hereinbefore noted, together with many thoroughly practical advantages, are successfully achieved.

It will be seen that the art is simple, direct, economical and highly eificient; that the alloy metal so produced is sound, clean and singularly free from gas; and that the elements carbon and silicon in the alloy metal are subject to precise control.

While in the embodiment of my invention hereinbefore set forth the pre-dried ingredients reducible oxide, reducing agent and basic slag forming agent are preferably thoroughly mixed prior to the introduction of the same into the furnace it will be understood that these ingredients may be separately introduced in proper proportions where such procedure is found desirable.

As many possible embodiments may be made of my invention and many changes made in the embodiment hereinbefore set forth it will be understood that all matter described herein is to be interpreted as illustrative, and not in a limiting sense.

What I claim is:

1. In the production of alloy iron and steel in an electric arc furnace, the art which includes preparing a ferrous metal bath, and adding to said bath a reducible oxide of a desired metal, an amount of silicon containing reducing agent in excess of the chemically equivalent amount of said oxide and an amount of a basic slag-forming material sufiicient to maintain basic conditions during the reduction of said ore.

2. In the production of alloy iron and steel, the art which includes preparing a ferrous metal bath of low carbon content, and adding to said bath a mixture of reducible oxide of a desired metal, a silicon containing reducing agent chemically. in excess of said oxide by about 10 per cent to 20 per cent and a basic slag-forming material in an amount sufficiently in excess of said reducing agent to maintain basic conditions during the reduction of said ore.

3. In the production of alloy iron and steel, the art which includes preparing a ferrous metal bath, and adding to said bath a mixture of a reducible oxide together with a silicon containing reducing agent chemically in excess of the oxide by about 10 per cent and a basic slag-forming material of about four times the amount of the silicon content of said reducing agent, whereby said oxide is reduced, said reduction being carried out under basic slag conditions.

4. In the production of alloy iron and steel in an electric arc furnace, the art which includes preparing a ferrous metal bath, and adding to said bath a mixture of a pre-dried reducible oxide of a desired alloy metal, a silicon reducing agent in amount substantially chemically equivalent to the amount of reducible oxide and an amount of a basic slag-forming ingredient calculated to assure the reduction ofsaid oxide under basic slag conditions.

5. In the production of alloy iron and steel in an electric arc furnace, the art which includes, preparing a ferrous metal bath, and adding to said bath a hot mixture comprising reducible oxides of desired alloy metals from which substantially all moisture has been removed, a silicon containing reducing agent in excess of the chemically equivalent amounts of said oxides and a large excess of a pre-dried basic slag forming ingredient.

6. In the production of rustless iron and steel in an electric arc furnace, the art which includes, preparing a ferrous metal bath, and adding to said bath reducible oxides of desired metals including oxides of chromium together with ferrosilicon in an amount sufficiently in excess of the chemically equivalent amount of said oxides so as to reduce all of said reducible oxides, and burnt lime in an amount sufiiciently in excess of the silicon content of said ferro-silicon to maintain basic slag conditions, whereby said oxides are reduced, the desired metals entering the bath, and the resulting silicates combine with the burnt lime to-prevent silicon contamination of the metal bath.

7. In the production of rustless iron and steel in an electric arc furnace, the art which includes, preparing a ferrous metal bath of low carbon content, and adding to said bath a chromium containing ore together with a silicon reducing agent chemically in excess of said ore and a basic slag forming material in sufficient excess of said reducing agent to maintain basic slag conditions during the reduction of said ore.

8. In the production of a corrosion-resistant iron alloy, the art which includes preparing a ferrous metal bath of low carbon content, and adding to said bath a chromium containing ore together with ferro-silicon chemically in excess of said ore and burnt lime in amount of from three to five times that of the silicon content of said ferro-silicon, whereby said ore is reduced, said reduction being carried out under basic slag conditions.

9. In the production of alloy irons and steels in an electric arc furnace, the art which includes, reducing chrome ore with an amount of ferrosilicon chemically in excess of the ore in the presence of a. ferrous metal bath and a slag maintained strongly basic by an amount of burnt lime in excess of the ferrosilicon, whereby a high recovery of chromium from the ore is achieved with low silicon contamination of the finished metal.

10. In the production of rustless irons and steels in an electric arc furnace, the art which includes, reducing chrome ore with an amount of ferrosilicon chemically in excess of said ore by about 10 per cent in the presence of a ferrous metal bath and a slag maintained strongly basic, whereby a high recovery of chromium from the ore is achieved with low silicon contamination of the metal bath.

' 11. In the productionof alloy irons and steels in an electric arc furnace, the art which includes, reducing chrome ore from which substantially all normal moisture is removed with a silicon containing reducing agent chemically in excess of the amount of ore in the presence of a ferrous metal bath and a slag maintained strongly basic, whereby a high recovery of chromium from the ore is achieved with low silicon contamination of the metal bath.

12. A process for producing steels and irons having a high alloy content and a low silicon content which comprises initially melting a charge of steel scrap and iron ore together with a slagforming constituent in a furnace, to form a molten metal bath, removing the slag after the completion of the melt-down and adding an alloy metal by the direct reduction of a reducible ore of said alloy metal throughout which reduction operation I a gradual addition is made to the molten bath of an amount of a reducible ore of an alloy metal and ferro-silicon to reduce said ore, said ferrosilicon being in excess of the amount chemically equivalent to the reducible ore and commingling with said oxide and reducing agent a quantity of predried calcined lime in an amount approximating at least three times by weight of the amount of sicilon employed in the process to maintain a strongly basic I slag throughout said reduction, whereby the silicon is maintained-at the desired low content.

13. In a process of producing irons and steels having a high alloy content by means of the direct reduction of the ores of the alloy metals in the presence of a molten bath of iron or steel by a reducing agent containing silicon wherein an excess of reducible ore to reducing agent is normally used to prevent the absorption by said bath of an excess of silicon over the content desired, the improvement which comprises decreasing the amount of reducible ore to an amount less than a substantial chemical equivalency with the reducing agent whereby a greater alloy recovery is obtained, and providing a strongly basic slag formed by commingling with saidreducible ore and reducing agent an amount of calcined lime approximating at least three times by weight the amount of silicon employed in the process whereby an excessive absorption of silicon by the metal bath is prevented.

14. In a process of producing irons and steels having a high alloy content by means of the direct reduction of the ores of the alloy metals in the presence of a molten bath of iron or steel by a reducing agent containing silicon wherein an excess of reducible ore to reddcing agent is normally used to prevent the absorption by said bath of an excess of silicon over the content desired, the improvement which comprises decreasing the amount of reducible ore to substantial chemical equivalency with the reducing agent whereby a greater alloy recovery is obtained, and providing a strongly basic slag formed by commingling with said reducible ore and reducing agent an amount of calcined lime approximating at least three times by weight the amount of silicon employed in the process whereby an excessive absorption of silicon is prevented, and drying said calcined lime previous to its addition whereby the amount of hydrogen dissolved in the iron is maintained below that which will remain in solution in the metal after solidification. a

15. In a process of producingirons and steels having a high alloy content by means of the direct reduction of the ores of the alloy metals in the presence of a molten bath of iron or steel by a reducing agent containing silicon wherein an excess of reducible ore to reducing agent is normally used to prevent the absorption by said bath of an excess of silicon overthe content desired, the improvement which comprises decreasing the amount of reducible ore to less than chemical equivalency of the reducing agent whereby a greater alloy recovery is obtained, and providing a strongly basic slag formed by commingling with said reducible ore and reducing agent an amount of calcined lime suificient to produce a strongly basic slag throughout said reduction period, whereby an excessive absorption of silicon by the bath is prevented.

16. In a process of producing irons and steels having a high alloy content by means of the direct reduction in an electric furnace of the arc type of the-ores of the alloy metals in the presence of a molten bath oi iron or steel by a reducing agent containing silicon wherein an excess of reducible ore to reducing agent is normally used to prevent the absorption by said bath of an excess or silicon over the content desired, the improvement which comprises decreasing the amount of reducible ore to substantial chemical equivalency with the reducing agent whereby a greater alloy recovery is obtained, providing a strongly basic slag formed by commingling with said reducible ore and reducing'agent an amount of calcined lime approximating at least three times by weight, the amount of silicon employed in the process to maintain a strongly basic slag whereby an excessive absorption of silicon is prevented, and maintaining a visible are above the slag by increasing the voltage over that normally employed.

WILLIAM BELL ARNESS.