US2005409A - Process of producing alloys - Google Patents

Description

Patented June 13, 1935 UNITED STATES PATENT OFFICE No Drawing. Application August 15, 1931, Serial No. 557,371

6 Claims. (Cl. 75-45) This invention relates to the production of alloy irons and steels of low silicon content, especially rustless irons and steels, and more particularly to such irons and steels wherein the alloy content thereof is derived by a direct reduction of a reducible oxide of the alloy using a silicon containing reducing agent.

One of the objects of my invention is the production in a simple, practical and economical manner, of alloy irons and steels, such as chromium, manganese, vanadium, tungsten, and like irons and steels (wherein the alloy content is high and the carbon, silicon and the like content is low) from readily available and inexpensive raw materials and/or alloy scrap, employing and familiar operating equipment.

I known Another object is the highly eflicient and thoroughly reliable elimination of silicon from fer- 0 rous alloys of the class indicated, of objectionably high silicon content and desirably high alloy content without undue loss of such readily oxidizable alloy content.

pointed out hereinafter.

The invention accordingly consists in the several steps and in the relation of each of the same 5 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 it may at this D point be noted that in heretofore known and/or used processes of producing alloy irons and steels, and especially rustless or high chromium irons and steels, wherein the alloy content is obtained by a direct reduction of an oxide or ore of 5 the alloy ingredient into the ferrous metal, a silicon containing reducing agent is generallyemployed due to the commercial availability, coinparative cheapness and general eificiency of such a reducing agent.

Silicon, however, is readily soluble in the molten ferrous metal in all propor-' tions. Special precautions, therefore, need be observed to control or limit the amount of silicon that may enter the metal in the successful prac-' tice of such a process.

In heretoforeknown processes,

one manner for 'ficiency and economy of production are measpi'eventing silicon contamination is to utilize a substantial excess of reducible oxide in relation to the silicon containing reducing agent. By this means, if the technique of the process is carefully governed, silicon contamination of the metal bath to an undesirable extent may be prevented. To effectively employ such a means, however, it is necessary to maintain an excess of reducible oxide of sixty per cent, or more, in order that the silicon content of the product may be reliably mainm tained below .50 per cent. This method, however, gives but a low alloy recovery from the reducible oxide.

In heretofore known processes where the excess of reducible oxide is decreased materially below sixty per cent, the silicon content of the product proportionately increases. For example, where the excess of reducible oxide is decreased to forty per cent the silicon content becomes of the order of two per cent or more. Such a high silicon content is objectionable for most purposes and heretofore known attempts of decreasing this silicon content any appreciable extent results in undue loss of the readily oxidizable and expensive alloy elements.

One of the outstanding Objects of this invention is the production of alloy irons and steels of low silicon content and a desired high alloy content by direct reduction of the alloy ingredient from a reducible oxide thereof wherein the quantities 'of raw materials employed are decreased over heretofore known and/or used processes of the sort indicated and wherein the efureably increased.

Referring now to the practice of my invention, in the production of a ferrous alloy of low silicon content from available and comparative-. ly inexpensive raw materials, a bath of ferrous metal is prepared in a suitable furnace. Conveniently, ordinary plain low carbon steel scrap is charged into' an electric furnace and melted down therein together with suitable amounts of aniron oxideand a basic slag forming agent in accordance with the usual practice so as to produce a bath of ferrous metal of low carbon content.- The slag formed during this period of initial melt down and carbon elimination is preferably removed prior to the. next step wherein the alloy element is added to the-bath. I

' To economically produce a ferrous alloy of the class indicated from inexpensive raw materials,

a reducible oxide of a desired alloy ingredient,

ordinarily an ore of the alloy, is charged'into the bath of ferrous metal. With this oxide.

there is charged an appropriate amount of reducing agent. A silicon containing reducing agent and preferably a fifty per cent ferro-silicon. due to the comparative cheapness, efficacy and availability of this material is ordinarily preferred.

In accordance with the provisions of my invention the proportion of reducible oxide in excess of the chemical equivalent of reducing agent by forty per cent or less. This decreased amount of excess oxide over heretofore known methods wherein an excess of reducible oxide is employed to avoid undue silicon contamination of the product permits a direct saving in raw materials, allows supplementary economies in handling the material and is conducive to improved furnace operation during the reduction period and increased rapidity of operation.

Incident to the reduction of the oxide of the alloy element, silicon from the reducing agent is directly absorbed by the bath of molten metal since, as indicatedabove, silicon is readily ,soluble in molten iron.

To free the metal from the objectionable quantity of silicon thus introduced, an oxidizing slag comprising a mixture of an iron oxide together with a quantity of a basic slag forming material such as calcined lime sufficient to assure fluidity of the slag, is then prepared on the surface of the ferrous alloy bath. A minimum amount of slag and a slag of maximum eificiency is assured by first freeing the surface of the bath of molten metal from the slag remaining after reduction period described above.

The concomitant objectionable oxidation of the added alloy ingredient is prevented by maintaining the bath of metal at a temperature favorable to the oxidation of the objectionable silicon but unfavorable or at least considerably less favorable to the attendant oxidation of the alloy metal. When the silicon content is brought down to the desired low value this stage of the process is complete. The oxidizing slag is then removed and the metal is refined and finished for pouring in the usual manner.

As illustrative of the practice of my invention in the production of high chromium or, rustless, iron and steel of about seventeen per cent chromium, 0.10 per cent carbon and about 0.50 per cent silicon, about 5,000 pounds of ordinary steel scrap is melted down in a six ton Heroult electric furnace. Chrome ore together with a. fifty per cent ferro-silicon is then charged into the furnace in such quantities that upon reduction of the ore the desired alloy content of the metal is achieved.

When a typical chrome ore analyzing fortyeight per cent CrzO: and fifteen per cent FeO is used about seventeen per cent by weight of silicon is theoretically required to reduce all the available chromium and iron in this ore. Thus, for example, one hundred pounds of silicon are chemically equivalent to about six hundred pounds of ore and two hundred pounds of a fifty per cent ferro-silicon are equivalent to six hundred pounds of chrome ore.

In order that excessively high silicon contamination may be prevented and so that a preponderant excess of ore may be avoided an excess of chrome ore of about forty per cent above that theoretically required is preferably employed. Thus eight-hundred and forty pounds of chrome ore are used in proportion to one hundred pounds of silicon or two hundred pounds of a fifty per cent ferro-silicon.

For a six ton heat of metal, about ten thousand pounds of chrome ore and two thousand five hundred pounds of fifty per cent ferro-sllicon are charged into the furnace after the melt down of the five thousand pounds of ordinary steel scrap as indicated above.

As a convenient outlet for the large quantity of chromium steel scrap which is readily available, about four thousand pounds of this chromium steel scrap is charged into the furnace along with the chrome ore and ferro-silicon. Although alloy scrap analyzing about the same in chromium and carbon as desired in the final product is preferably used where convenient, good results are achieved where the alloy scrap is high in alloy content and low in carbon. The alloy scrap readily melts down and enters the bath of ferrous metal, thus directly supplying chromium and iron with the introduction of but a minimum of impurities.

Under the action of the intense heat of the electric furnace and the direct contact with the hot bath of ferrous metal an action between the reducible oxide, illustratively chrome ore, and the reducing agent, illustratively fifty per cent ferrosilicon, immediately sets in and rapidly progresses. The chrome ore is readily reduced and the resultant metals, chromium and iron, directly enter the bath beneeath. Incident to the reduction'of the reducible oxide a thick slag forms on the bath of metal and from time to time this slag is conveniently removed.

After the action between the reducible oxide and the reducing agent is at an end the reduction period is complete. At this stage of the process the bath of molten metal has a chromium content of about 17.5 per cent, a carbon content under 0.10 per cent, and a silicon content of about 1.75 per cent.

While the chromium and carbon contents are about the values desired in the final product, the silicon content is objectionably high. In accordance with the further provisions of my invention all slag remaining on the bath of metal after the completion of the above described reduction period is removed from the surface of this bath, leaving the metal free of slag and exposed to the furnace atmosphere.

The electric power supplied the furnace is then cut off and the furnace electrodes are raised out of contact with the bath of metal. With the furnance chamber thus unobstructed there isadded to the surface of the bath of ferrous alloy a mixture of about two hundred pounds of magnetically concentrated iron ore and about two hundred pounds of calcined lime for the ingredients and proportions illustratively set forth above. This mixture of ingredients quickly spreads over the entire exposed surface of the bath of molten metal.

The furnace electrodes are now lowered and the electric power turned on so that arcs are established above the loose mixture on the surface of the bath. Under the heating action of the furnace the iron ore and lime readily fuse to form a highly active oxidizing slag which attacks the silicon content of the bath. The slag in addition to serving as an oxidizing agent for the silicon content of the bath also serves to protect the bath of metal from carbon contamination from the furnace electrodes during the further operations.

To increase the fluidity of this slag and render it strongly basic and accelerate the attack on silicon there is preferably added a further amount of about two hundred and fifty pounds of calcined lime as soon as the mixture commences to fuse.

The total quantity of active oxidizing agent such as iron ore and/or mill scale, together with dred pounds and three hundred-pounds and the burnt lime between one hundred pounds and six hundred pounds. Theparticular amount of lime employed isgoverned by the character of the slag formedii Just sufficient lime is used as is required to give a dry, limey, or basic slag, 'a slag which is especially conducive to the elimination of silicon from the bath of metal. Where insufilcient lime is used the slag is acid in characterand the silicon content of the bath is not readily oxidized.

While this oxidizing slag readily attacks the silicon absorbed by the bath of metal from the silicon containing reducing agent during the reduction stage of the process and thus decreases the total silicon content of the bath, it also attacks the alloy metal, illustratively chromium. The loss of chromium through oxidation is highly undesirable since there would be required the initial introduction of an excess of chromium to compensate for this later loss, all at a considerable increased cost.

To achieve efflcient oxidation of silicon contained in the bath and yet prevent undue loss of the chromium, the temperature of the bath is maintained within fairly narrow limits. Where the temperature of the bath is allowed to rise much above 3000 F. the attack on the chromium is accelerated, and, where the temperature is allowed to fall much below 2800 F. all action on both the contained chromium and silicon is very slow. Dm'ing this stage of the process wherein the silicon content of the bath is brought down to a desired low value the temperature of the bath of metal is thus preferably maintained between 2800" F. and 3000 F.

Under the action of the actively oxidizing slag the silicon content of the metalis lowered to about 0.5 per cent in about forty-five minutes, for the conditions illustratively set forth above, accompanied by a negligible loss of chromium.

After the elimination of silicon has sufilciently progressed and the silicon content of the bath is brought to a desired low value, the oxidizing slag is preferably completely removed from the surface of the bath and a refining slag made up of about four hundred fifty pounds of calcined lime, seventyfive pounds of fiuorspar and about seventy-five pounds of fine ferro-silicon is added in accordance with the usual practice. This refining slag promotes deoxidation of dissolved oxides of iron, chromium, and silicon that are dissipated throughout the metal. When sufficient time for refining has elapsed the metal is tapped into suitable molds. Upon analysis the heat of metal is found to contain about seventeen per cent chmmium. from 0.30 per cent to 0.60 per cent silicon, and less than 0.10 per cent carbon.

It will be seen that there has been provided in this invention an art in which the various objects hereinbefore noted, together with many thoroughly practical results, are successfully achieved.

Although as illustrative of my invention undesirable silicon is eliminated from a bath of alloy iron or steel wherein the alloy content of the bath is obtained by a direct reduction of a reducible oxide of the alloy by means of a silicon containing reducing agent, it will be understood that silicon may be eliminated from a bath of ferrous alloy metal of objectionally high silicon content however produced; as for example, by the melt down of ferrous alloy scrap of objectionable silicon content in, for example, an induction type of electric furnace. I

And, although in the illustrative embodiment of my invention, the production of a high chromium iron orsteel of low. silicon content is specifically described it will be understood that high manganese, vanadium, tungstenand like alloy irons and steels, or combinations of the same, of low silicon contents may be achieved by a direct reduction of one or more ores or oxides of the alloy metals with a silicon reducing agent, obtaining improved recovery over heretofore known methods, and thereafter selectively oxidizing the excess silicon content of the iron or steel by maintaining the metal under basic oxidizing slag conditions at a temperature favorable to the oxidation of silicon but unfavorable to the oxidation of the desired alloy constituent all as more particularly described above.

As many possible embodiments may be made of my invention and as many changes may be made in the embodiments hereinbefore set forth it is to 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 rustless irons and steels, the method of eliminating silicon from a bath of iron or steel having a high percentage of chromium and an objectionable amount of silicon which comprises, forming thereon a basic iron oxide slag substantially free from oxides of chromium, and maintaining the temperature of the bath between 2800 F. and 3000 F., whereby the silicon contained in said bath is selectively oxidized thereby achieving ironand steel high in chromium and'substantially free of silicon and sub stantially free from chromium oxide inclusions.

2. In the production of alloy irons and steels high in a desned oxidizable alloy and low in sili- .con, the art which includes, reducing an ore of the desired alloying ingredient with a silicon-containing reducing agent in the presence of a ferrous metal bath, the amount of said ore being chemically in excess of said reducing agent by less than 40 'per cent, thereby achieving a bath of ferrous metal high in the desired alloying ingredient and objectionably high in silicon covered by a slag, removing the slag overlying said bath of metal, forming on said bath a basic oxidizing slag of iron oxide and burnt lime, and maintaining said bath and overlying oxidizing slag at a temperature favorable to the oxidation of silicon from said bath but relatively unfavorable to the oxidation of the desired alloying ingredient contained therein, whereby iron or steel of a de-- high in chromium and objectionably high in silicon covered by a slag from which chromium is substantially completely recovered, withdrawing said slag from the surface of said bath and forming thereon a basic oxidizing slag of iron oxide and burnt lime greatly in excess of the silicon present, and maintaining said bath and secondmentioned slag at a temperature from about 2800 F. to 3000 F., thereby selectively oxidizing the silicon present in said bath with a minimum oxidation of the chromium present.

4. In the production of rustless iron of low silicon content, the art which comprises, preparing a ferrous metal bath; adding to said bath a mixture of a reducible oxide of chromium and ferrosilicon wherein said oxide is in excess of the chemical equivalent of said ferrosilicon by less than 40 per cent, whereby said oxide is reduced, the chromium metal entering the bath together with silicon in an amount greater than desired in the final product and leaving a slag on the surface of said bath from which the metallic valuesare largely recovered; removing said slag to expose the clean metal of the bath and adding thereto iron oxide and lime thus forming a basic oxidizing slag; and maintaining said bath at a temperature favorable to the oxidation of silicon and substantially unfavorable to the oxidation of chromium, thereby decreasing the silicon content of said bath.

'5. In the production of rustless iron of low silicon content, the art which comprises, preparing a ferrous metal bath; adding to said bath a mixture of a reducible oxide of chromium and a silicon-containing reducing agent therefor wherein said oxide is in excess of the chemical equivalent of said reducing agent by forty per cent or less whereby said oxide is reduced, the chromium metal entering the slag together with about two per cent of silicon leaving a slag on the surface of said bath; removing substantially all of said slag to obtain a clean surface of said bath and forming thereon a basic slag rich in iron oxide; and maintaining said bath and slag at a temperature of from 2800 F. to 3000 F. whereby the silicon content of the bath is selectively oxidized.

6. In the production of rustless iron of desired chromium content and low silicon content, the art which includes,- preparing a ferrous metal bath, charging chrome ore and a silicon-containing reducing agent onto the surface of said bath in proportions such that the ore is in chemical excess of the reducing agent by less than 40 per cent whereby the ore is reduced accompanied by a contamination of said bath with silicon, removing the slag overlying said bath from which the metallic values have been recovered and forming on said bath a basic oxidizing slag, maintaining said bath and slag at a temperature favorable to the selective oxidation of silicon in the presence of iron and chromium until a desired low silicon content of the bath is reached, and removing said desiliconizing slag and forming on the surface of said bath a refining slag of ferrosilicon and lime to free the metal of oxide inclusions and achieve clean rustless iron of a desired low silicon content.

WILLIAM BELL ARNESS.