US2206711A - Method for producing alloys containing iron and chromium - Google Patents

Description

Patented July 2, 1940 UNITED STATES METHOD FOR PRODUCING ALLOYS CON TAINING IRON AND CHROMIUM Marvin J. Udy, Niagara Falls, N. x.

No Drawing.

11 Claims.

This invention relates to the production of alloys containing iron and chromium and has for an object the provision of an improved process for producing such alloys. More particularly, the

g invention contemplates the provision of certain improvements in processes for producing chromium-bearing iron and steel products of low carbon contents.

According to some heretofore customary practices, low-carbon chromium-bearing iron andsteel products are formed by adding low-carbon ferrochromium to suitably prepared molten iron or steel baths in suitable furnaces. The low-carbon ferrochromium employed may be produced,

iii for example, by reducing chromite ore with carbon to produce high-carbon ferrochromium, smelting the high-carbon ferrochromium with silica and coke to produce low-carbon, highsilicon ferrochrome silicon and subsequently to eliminating the silcon from the ferrochrome silicon by oxidation. Such a procedure for producing low-oarbon ferrochromium is expensive. Moreover, the ferrochromium produced is obtained and marketed in the form of a solid relam tively high-melting point alloy which can be melted and incorporated in molten iron or steel readily only in furnaces, such as electric furnaces, where large supplies of heat are available.

The addition of solid ferrochromium to molten metal baths causes chilling of the baths. The amount of ferrochromium which can be added efllciently to a molten metal bath in any furnace depends upon the supply of heat available to melt the added ferrochromium and maintain the metal of the bath at the proper temperature. In electric furnaces, ample heat is available for melting commercial ferrochromium and for maintaining alloys of the usual commercial compositions at proper temperatures, so, practically,

there is no limit to the amount or proportion of ferrochromium which may be added to molten metal baths in such furnaces. In combustion furnaces such, for example, as open hearth furnaces; on the other hand, the quantities of heat available and the temperatures attainable are limited, and, therefore, only limited quantities of solid ferrochromium may be added efi'ectively to moltenjmetal baths in such furnaces. The chillo' iing effects are limited customarily by employing only high-chromium ferrochromium and thus avoiding the introduction of large quantities of diluentfj. iron, which requires heat for melting, into the molten metal baths with the required *amount of chromium. The production of high- Application October 16, 1939. Serial No. 299,897

chromium ferrochromium by direct reduction requires the use of high-grade chromite ores which contain suitably high ratios of chromium to iron. ,Low-grade ores, containing relatively low ratios of chromium to iron require preliminary 5 beneficiation for iron removal prior to reduction for the production of ferrochromium. Because of the necessity for employing high-grade ores or beneficiated ores for the production of ferrochromium according to heretofore cus- 10 tomary'practices, the production of chromium alloys, such as chromium iron and steel products,

is relatively expensive.

It has been proposed also to introduce chromium into low-carbon chromium alloys by re- 1 ducing chromite ore in contact with molten metal to which the chromium is to be added by means of non-carbonaceous reducing agents. Such proposed procedures have not been highly satisfactory because a large proportion of the chromium W is chemically combined in the form of dimcultly reducible compounds with magnesia and alumina.

The present invention providesan effective and relativelyinexpensive method of producing lowcarbon chromium-bearing alloys. According to as the invention, chromium is introduced into the furnace in which the alloy is to be made in the form of oxidized ferrochromium. A suitable noncarbonaceous reducing agent capable of reducing to the metallic state the chromium oi. the so oxidized ferrochromium is placed in contact with the oxidized ferrochromium in the furnace. The non-carbonaceous reducing agent, aided by the heat available in the furnace, functions to re duce the iron and chromium of the oxidized ierrog5 chromium, delivering molten -metal containing metallic iron and metallic chromium in the proportions in which those elements are present in the oxidized ferrochromium.

The oxidized ferrochromium employed is pro- 40 duced, preferably, by roasting finely divided highcarbon ferrochromium with access of air' at an elevated temperature below its fusing temperature and with or without the aid of one or more oxidation promotors. Roasting of the finely divided ferrochromium effects carbon removal by oxidation and also causes oxidation of iron and chromiumpresent in the ferrochromium. The oxidized ferrochromium contains iron and chromium in the proportions in which those elements are present in the ferrochromium subjected to oxidation and in oxidized forms which may be reduced readily by non-carbonaceous reducing agents such, for example, as silicon, ferrosilicon, 5

ferrochrome silicon and aluminum, and it may be free or largely free of carbon.

Since the process of the invention is capable of delivering molten metal containing iron and chromium in the proportions in which those elements are present in ferro-chromium produced by direct reduction of chromite ore, the invention provides an effective method of adding the chromium of ferrochromium'to molten metal baths without chilling the baths. Furthermore, since the process is capable of delivering the metal in the molten state and without chilling molten metal, the necessity for employing highchromium ferrochromium to avoid the chilling effect. of large quantities of iron is eliminated.

Thus, it is possible to employ ferrochroniium carrying out the method or process of the invention may be produced, for example, in accordance with any of the procedures outlined in my aforementioned copending application Serial No. 228,860, filed September '1, 1938, which has matured into Patent No. 2,176,688, of which this application is a continuation-in-part.

The ferrochromium may be subjected to the oxidizing treatment alone or in the presence of one or more addition agents such as lime or soda ash which serve as oxidation promoters and which react with the oxides resulting from oxidation of the iron and chromium. When the ferrochromium is subjected to the oxidizing treatment alone in the first instance the product of such oxidizing treatment may be subjected to a further oxidizing treatment in the presence of one or more agents of the aforementioned type to accomplish further oxidation, to bind the oxides chemically, or to accomplish both-of these objectives.'

Oxidation is readily controlled to form products containing suiilcient oxygen to support combustion of reducing agents which. may be substantially complete, if desired. The iron of the ferrochromium is converted readily to ferric oxide (Fe-20:), and the chromium is converted readily to chromic oxide (ClzOa). Oxidation may be further controlled to convert a large proportion of the chromium to'chromic anhydride (CrOa) if desired. Depending upon the conditions under which the oxidized ferrochromium products are to be reduced, they may contain one or more additional oxygen-containing substances such, for example, as sodium nitrate, sodium chlorate and manganese dioxide to provide additional heat for melting the metal and slag produced. When the oxidized ferrochromium is to be reduced in an electric furnace where ample heat is available, it

is unnecessary to oxidize the chromium to a state ried out in the presence of one or more of such agents, additional amounts may be added to the product of that stage of roasting before subjecting it to the'next stage of roasting. Thus, for example, when chromate formation is desired, I prefer to first roast finely divided ferrochromiurn at a relatively high temperature above 1000i C. inthe presence of'suflicient lime to form calcium chromite with all of the chromium in the ferrochromium and subsequently roast the product obtained by such roasting treatment at a temperature below 1000" C. in the presence of sumcient additional lime and soda ash to provide a total amount of calcium oxide and sodium oxide to combine with all of the chromic oxide remaining unchanged to form chromate and chromite of calcium and sodium.

Roasting operations may be facilitated by selection of the ferrochromium to be treated. High-carbon ferrochromium products grind more readily and can be converted to desirably small i particles more easily than low-carbon ferrochromium. Thus, for example, ferrochromium containing 8 to 10 per cent or more carbon can be reduced to the form of a powder comprising very small particles quite easily; ferrochromium containing 6 to 8 percent carbon is more diflicultly reducible to particles of desirably small sizes; and ferrochromium containing less than about 6 percent carbon can be reduced to particles desirably small in size only with considerable difliculty relatively to the diiliculties encountered in finely dividing ferrochromium products containing more than 6 percent carbon.

The grinding ofrelatively low-carbon ferrochromium is facilitated if the ferrochromium also contains silicon. Thus, forexample, ferrochromium containing about 4 to 6 percent carbon and amounts of silicon up to about 3 percent can be ground quite readily.

In producing oxidized ferrochromium I prefer to employ i'errochromium as nearly saturated as possible with carbon, or, alternatively, ferrochromium containing smaller amounts of carbon, but containing, also, suiilcient silicon to compensate, in its influence upon grinding characteristics, for the carbon-deficiency. When I produce the ferrochromium for roastingv by reduction treatments of chromium-bearing materials, I prefer to employ suificient carbon to incorporate in the resulting ferrochromium as much carbon as possible, and, if'the conditions of operation are not such as to permit the production of products of the higher carbon content, I operate the reduction process under conditions such as to reduce silicon from silica contained in the charge and form a product containing, preferably, about 1 to 3 percent of silicon. Larger quantities of silicon improve the grinding characteristics of the ferro- .chromium produced, but they are objectionable because they increase the bulk of the slag produced in theultimate reduction of the oxidized products of the invention;

I have discovered that oxidation of the iron, chromium and carbon proceeds rapidly and eflectively when alloys containing these elements are heated, in a flne state of division, to temperatures substantially higher than 1200" C., but below the melting points of the alloys. At temperatures below 1000 C., oxidation proceeds more slowly,

,and oxidized products containing less than about 0.30 percent of carbon (by weight) are difficult to obtain. Even at temperatures as high as 1200 0., products containing less than 0.20 percent by weight of carbon are diiiicult or impossi- '75 oxidizing conditions. At temperatures substantially higher than 1200 0., products containing not more than about 0.10 percent by weight of carbon and containing even as little as a trace of carbon can be produced by roasting under oxidizing conditions for periods of time shorter than two hours. In roasting to effect a high degree of carbon elimination, I prefer to employ temperatures in excess of 1250 C. or even in excess of tain less than about 0.05 percent of carbon. 1 may produce, also, oxidized products containing.

carbon in amounts up to 0.10, 0.20, 0.30 percent or more.

According to the preferred method of the invention, the oxidized ferrochromium is added to a molten metal bath in a suitable furnace to form a layer on the surface of the bath in contact with the upper surface of the slag layer if the metal is covered by slag or in contact with the upper surface of the metalif the metal is free or partly free of slag. .ii'he non-carbonaceous reducing agent is then placed on top of the layer of oxidized ferrochromium and it functions to reduce the iron and chromium of the oxidized ferrochromium, producing molten metallic iron and molten metallic chromium which enter the molten metal of the bath.

The molten metal bath may be produced and" refined or otherwise treated according to customary practices or in any other suitable manner prior to addition of the oxidized ferrochromium. Slag produced in the melting operation may be removed prior to addition of the oxidized ferrochromium, or it may be retained in whole or in part and the oxidized ferrochromium may be added to the upper surface of the slag layer. If the slag produced in the melting operation is removed in whole or in part, a new, clean slag may be provided prior to the addition of the oxidized ferrochromium.

In a preferred method or process of theinvention for producing chromium-bearing iron or steel products, I prepare a molten bath of ferrous metal by melting scrap iron or steel or pig iron or other suitable metal products or combinations of metal products in a combustion furnace, elec tric furnace or other suitable furnace. The melting charge may include suitable slag-forming materials such as lime and silica, and, if desired, the molten bath may be treated with iron oxide to remove carbon from the metal. The slag may then be treated with ferrosilicon to reduce oxides contained therein. All or a portion of the slag may then be removed or the slag may be retained on the surface of the metal. The oxidized ferrochromium may then be placed in the form ofa layer on the surafce of the molten bath in contact with the slag layer, or in contact with the metal if no slag layer is present, and the reducing agent may be added on top of the layer of oxidized ferrochromium.

The oxidized ferrochromium and the non-carbonaceous reducing agent may be employed in the ble to produce with many hours of roasting under form of particles of any suitable sizes. Particles of oxidized ferrochromium about one-half inch in size and smaller and partlclesof nonoarbonaceous reducing agent about one-quarter inch 0A,") in size and smaller may be employed 5 satisfactorily. Preferably both the oxidized ferrochromium and the non-carbonaceous. reducing agent are employed in the form of particlesminus -mesh in size. a 1

I prefer to employ silicon-containing materials 10 such as silicon, ferrosilicon, and ferrochrome silicon as the reducing agents. In employing. silicon-containing reducing agents, better results are obtained with materials containing more than about fifty (50%) percent of silicon bylweight and optimum results are obtained with j,lmate-" rials containing not less than about seventy-five (75%) percent of silicon by weight. Materials containing higher percentages of silicon have lower specific gravities and the particles of} such ,20 materials tend to remain within or onfthfesurface of the layer of oxidized ferroch inium. Silicon contamination of the metal whichyhiight result from the use of particles of material of high specific gravities is thus avoided. JGenerally, it is advisable to determine the particle size of the silicon-containing material to be employed in accordance with the silicon content} infusing materials of low silicon content, smaller (pa icles should be used, and in using materials silicon content, larger particles may be,

Lime (CaO) should be provided in s quantity to flux the silica (S102) or qt substance formed by oxidation of the A agent resulting from reduction of the i f chromium. Lime may be provided in] sufiicicnt to produce a calcium silicate sla suitable composition, but I prefer .to' lime in an amount sufficient to provide one molcule ofcalcium oxide for each m'filecule of silica produced. The amount of lim added with the oxidized ferrochromium nd the reducing agent will depend upon the am unt of calcium oxide (CaO) present in the oxidized ferrochromium, that is the lime introducedifn carrying out the roasting operation. If arifl addition of lime is required at the time of adding the oxidized ferrochromium and the reducing' lagent, I prefer to add the lime in admixture with the reducing agent. In using ferrosilicompiflor 6x ample, I may grind the ferrosilicon with the required amount of lime and add the tw {materias as van intimate mixture to a p' viously added layer of oxidized ferrochromium. 11

The reducing agent may be employedflin any suitable amount relatively to the amountsgof iron and chromium to be reduced from the oxidized ferrochromium. In employing a siliconcontaining material, I prefer to employ an amount in excess (up to about ten (10%) percent excess or more) of the amount required theoretically to reduce the iron and chromium of thefloxidized ferrochromium. The silicon may be added at one time or at intervals.

The following example illustrates the; use of 65 the process of my invention in making high chromium steel: In order to make one ton of steel, I melted a charge comprising 1555 pounds of scrap and pig iron in an electric furnace. I then oxidized the 70 carbon with iron oxide, reducing the carbon content to about 0.06 percent. The slag was then highly oxidizing and I added sufficient ferrosilicon containing seventy-five (75%) percent silicon "to reduce the iron oxide in the slag and suflicient 75 lime to flux the silica formed in reducing the iron oxide. I then formed a layer of oxidized ferrochromium on the surface of the bath by-adiding.

526 pounds of oxidized ferrochromium analyzing Per cent 01- 0; 174.0 F 0 23.81 CaO 1.47 510, 0.73

On top of the layer of oxidized ferrochromium,

I placed a mixture comprising 217.5 unds oi ferrosilicon containing seventy-five 5%) percent of silicon and 450 pounds of lime in the-form of particles one-quarter inch (8 and smaller in size. The silicon of the ferrochromium reacted with the oxidized iron and chromium of the oxidized ferrochromium. producing molten .metailic iron and molten metallic chromium which entered the steel bath. After the reaction was complete, the steel'was tapped and finished in the usual-manner. The recovery of chromium 'was about ninety (90%) percent.

I claim:

'on the surface of the molten bath, and placing a finely divided silicon-containing reducing agent Icontaining' not less than about 50 percent of silicon by weight on the layer of oxidized ferrochromium.

3. The method of producing an alloy containing iron and chromium which comprises forming a molten bath of metallic iron-bearing material in a suitable receptacle, forming a layer of oxidized ferrochromium in finely divided form on the surface of the molten bath, and placing a finely divided silicon-containing reducing agent containing not less than about 75 percent of silicon by weight on the layer of oxidized ferrochromium.

4. The method of producing chromium-bearing steel which comprises heating a charge comprising steel scrap and slag-forming materials'and forming a'slag-covered molten metal bath, treating the molten bath with iron oxide to remove carbon from the metal, treating the resulting slag with a reducing agent to remove excessiron oxide, subsequently forming a layer of oxidized ferrochromium on the surface of the molten bath, and placing a finely divided silicon-containing reducing agenton the layer of oxidized ferrochromium. b

5. The method ofproducing an alloy contain- 1. The method offproducing an alloycontaining iron and chromium which comprises forming a-molten bath of metallic iron-bearing material in a suitable" receptacle, forming a layer of oxidized ferrochromium in finely divided form on the surface or the molten bath, and placing finely divided ferrosilicon on the layer of oxidized ferrochromium. I

6. The method of producing an alloy containing iron and chromium which comprises forming a molten bath of metallic iron-bearing material in a suitable receptacle, forming a layerof oxidized ferrochromium in finely divided form on the surface of the molten bath, and placing finely divided ferrochrome silicon on the layer of oxidized ferrochromium.

'1. The method of producing an alloy containingiron and chromium which comprises forming a molten bath of metallic iron-bearing material in a suitable receptacle, forming a layer of oxidized-ferrochromium in solid, finely divided form on the surface of the molten bath, and placing lime and a silicon-containing reducing agent on the layer of oxidized ferrochromium. 8. The method of producing an alloy' containing iron and chromium which comprises forming a molten bath of metallic iron-bearing material in a suitable receptacle, forming a layer of oxidized ferrochromium in solid, finely divided form on the surface of the bath, and placing an intimate mixture of lime and a silicon-containing reducing agent on the layer of oxidized ferrochromium.

9. The method of producing an alloy containing iron and chromium which comprises forming a molten bath of metallic iron-bearing material in a suitable receptacle, forming a layer of 0x1- dized ferrochromium in finely divided form on the surface of the molten bath, and placing a finely divided non-carbonaceous reducing agent on the layer of oxidized ferrochromium. 10. The method of producing an alloy containing iron and chromium which comprises forming a molten bath of metallic iron-bearing material in a suitable receptacle, forming a layer of oxidized ferrochromium' in finely divided form on the surface of the'molten bath, and placing a finely divided silicon-containing reducing agent on the layer of oxidized ferrochromium. the major portions of said oxidized ferrochromium and said reducing agent consisting of particles minus 100- 'mesh in size. j

' 11. The method of producing an alloy containing iron and chromium which comprises forming a molten bath of metallic iron-bearing material in a suitable receptacle, forming a layer of oxidized ferrochromium in finely divided form on agent consisting of particles smaller than onequarter inch in size.

- MARVIN J. UDY. j