US2113880A - Production of rustless iron - Google Patents

Description

Patented Apr. 12, 1938 PRODUCTION or RUSTLIESS IRON Alexander I}. Feild, Baltimore, Md, assignor to Rustless Iron and Steel Corporation, Baltimore, Md., a corporation of Delaware No Drawing. Original application May 31, 1934,

Serial No. 728,415. Divided and this application September 11, 1936, Serial No. 100,381

3 Claims.

This application is a division of my United States Letters Patent 2,076,885 of April 13,1937 entitled Production of rustless iron, and the invention relates to the manufacture of chromiummanganese irons and steels, and especially to the manufacture of chromium-manganese irons of high chromium and manganese contents and low carbon contents.

Among the objects of my invention is the eflicient, economical and thoroughly practical production of chromium-manganese irons and steels .(irons and steels analyzing approximately, chromium 10% to 35%, manganese 6% to 20%, carbon .05% to .15% for the alloy iron and .15% to 1% for the alloy steel, together with desired supplementary additions of nicke copper, aluminum, silicon, molybdenum, tun ,.,ten and the like for special purposes and with the usual low percentages of sulphur and phosphorus and the balance substantially iron), and especially to the simple, direct and thoroughly reliable production of chromium-manganese irons, employing inexpensive and readily available raw materials and utilizing known furnacing and operating equipment.

The invention accordingly consists in the combination of elements, mixture of materials and composition of ingredients and in the several steps and 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 it may be noted at this point that in various marine, aviation,

automotive, industrial and architectural applications where a bright surface pleasing to the eye and resistant to the corrosive attack of various acid, alkali or salt atmospheres or solutions is required, or where metal which is strong, tough and durable and resistant to the corrosive attack of' various acids, alkalies and salts at high tem-.

peratures is required, the well known austenitic chromium-nickel irons and steels are in widespread use at the present time. The extent of the use of such alloy irons and steels is greatly handicapped, however, by the comparative great expense of these materials as compared with ordinary irons and steels which are less attractive to do the eye and which are less resistant to corrosive attacks of the atmosphere or various acid, alkali and salt-mediums.

While numerous substitutes have been proposed for the expensive austenitic chromium-nickel irons and steels the only alloy iron or steel which used methods of production the expensive sources of chromium and manganese, are added to a bath of low carbon iron maintained at a desired temperature. Such methods of production, while simple in procedure, require prolonged heating with a continuous consumption of power and yield a very expensive product, largely because of the great expense of the raw materials used in obtaining the alloying additions, chromium and manganese.

While known methods of producing chromium-' manganese irons and steels are ordinarily satisfactory in the production of the usual low chromium low manganese alloys these methods as applied to the production of high chromium high manganese austenitic irons and steels render the product prohibitively expensive. In fact, the cost of producing the austenitic chromium-manganese irons and steels in accordance with known and/or used methods is so great that the margin of savings over the production of the well-known austenitic chromium-nickel irons and steels is so small that there is little incentive to replace, for general purposes, these costly irons and steels with the chromium-manganese irons and steels.

One of the outstanding objects of my invention, therefore, is the production of high chromium high manganese irons and steels, and especially the production of austenitic chromium-manganese irons in a simple, direct and economical manner employing inexpensive and readily avail able raw materials in the most economical proportions depending upon the availability and fluctuations in current market prices, achieving high grade, sound/clean metal which may be manufactured and sold at a price considerably beneath that of the comparatively expensive chromium-nickel irons and steels and which may be introduced into a wider range of applications supplanting known inferior but less expensive metals.

In the practice of my invention a material high in manganese content, such as any one of the manganese ores, pyrolusite MI1O2, manganite MnOlOH), hausmannite 'MnsO i, braunite 3Mn2O3.MnSiO3, rhodocrosite MnCOx and/or high carbon ferromanganese (produced by smelting any one of the above ores with coke achieving a product analyzing approximately, iron 10% to 30%, manganese 80% to 60%, silicon 1% and carbon 5% to 8%) is melted down in a suitable furnace with chrome ore FeOCmOa and/or high carbon ferrochrome (produced by smelting this ore with coke and silica achieving a product analyzing approximately, 60% to 70% chromium, 4% to 7% carbon, 20% to 30% iron) and a desired quantity of available low carbon steel scrap, either with or without a readily available amount of high chromium high manganese iron or steel scrap or straight chromium rustless iron or steel scrap or high manganese iron or steel scrap, and a sufficient quantity of an oxidizing agent, such as roll scale or magnetic iron ore concentrate to exclude and/or remove carbon from the ingredients, thereby forming a bath of ferrous metal containing manganese and chromium with an overlying oxidizing slag rich in the oxides of chromium and manganese.

To assure the production of sound metal, free from gas-pockets, pits and the like, the large quantities of ore used are, preferably, thoroughly dried at a high temperature prior to charging into the furnace. The pre-drying of the ores is carried out in any suitable manner, as by a long heating in a rotary gas-fired kiln at such temperatures as to rid the ores of substantially all free and combined moisture normally present. The use of predried materials effectively minimizes the amount of moisture introduced into the furnace and consequently limits the amount of hydrogen available to contaminate the metal during melt-down, as a result of the decomposition of this moisture by the action of the electric furnace arcs, and to subsequently come out during solidification of the metal after teeming to cause gas-pockets and like defects, all as more particularly indicated in the Patent No. 1,925,916 granted to William Bell Arness' on September 5, 1933 and entitled Process of producing alloys.

In order to achieve temperatures sufficiently high to effectively melt the refractory ingredients comprising the charge, an electric arc furnace is preferably employed as a means of heating and furnacing the ingredients. Conveniently, a Heroult furnace, or other furnace of the direct arc type, employing carbon or graphite electrodes and lined with chromite brick to a height somewhat above -the slag line and having side-walls and roof of silica brick is used in this practice.

Prior to charging the ingredients into the electric arc furnace, the furnace is preheated in any suitable manner, as by arcing the furnace on electrode butts or by means of a gas torch. After the furnace bottom and walls have been adequately preheated, the preheating means are withdrawn and the raw materials, indicated above, comprising the initial charge of ingredients for a heat of high chromium high manganese iron or steel are charged into the furnace.

In the production of a heat of high chromium high manganese iron or steel to a desired specification of chromium, manganese and carbon contents (with or without one or more supplementary additions of nickel, copper, aluminum, silicon, molybdenum, tungsten, vanadium and the like in specified amounts), the relative amounts of chromium contributed by the chromium-containing iron or steel scrap, high carbon ferrochrome and chrome ore, and the relative amounts of manganese contributed by the manganese-containing iron or steel scrap, high carbon ferromanganese and manganese ore, all as used in proportions consistent with good furnace operating conditions, are largely determined by the availability of these various ingredients and the respective current market prices. Ordinarily the contained chromium is least expensive in the form of chrome ore, is more expensive in the form of chromium-containing iron or steel scrap and is most expensive as high carbon ferrochrome. Chromium in all of these forms, however, is much less expensive than chromium as the generally used low carbon ferrochrome. Similarly, manganese is least expensive in the form of manganese ore and progressively more expensive as manganese-containing scrap iron or steel, high carbon ferromanganese and the generally used low carbon ferromanganese.

To achieve clean, sound, high grade metal at a minimum of expense, a maximum of the less expensive chromium-containing and manganesecontaining ingredients as is consistent with good furnace operation conditions (greatly limited by the permissible volume of slag which may be handled) is therefore employed. Now, since the chromium-bearing ingredients are considerably more expensive than the manganese-containing materials, and since the cost differential between chrome ore and high carbon ferrochrome is appreciably greater than that between manganese ore and high carbon ferromanganese a maximum of the inexpensive chrome ore is preferably used in the practice of my invention.

Where the practice indicates that further volumes of slag may be adequately handled, as in the production of the high chromium high manganese irons and steels of the lower ranges of chromium and manganese contents, much of the addition of manganese may be made in the form of the inexpensive manganese ore. Practice has shown, however, that unsatisfactory furnacing conditions are encountered where the chromium and manganese additions are made by chrome ore and manganese ore respectively, as for example, in the production of the high chromium high manganese irons and steels of a medium range of analyses.

Highly satisfactory results are achieved in the production of high grade, inexpensive metal by employing a maximum of chrome ore for the chromium addition and the readily available high carbon ferromanganese as the source of manganese. In many instances, as in the production of the high chromium high manganese irons and steels of the upper analyses ranges of chromium and manganese, much of the chromium is added in the form of high carbon ferrochrome.

Where desired the additions of chromium and manganese may be largely made in the form of high carbon ferrochrome and high carbon ferromanganese respectively, or as high carbon ferrochrome-manganese, substituting in whole or in part for additions of chrome ore and manganese ore. Under present market conditions, however, the use of great quantities of high carbon ferrochrome and high carbon ferromanganese as sources .of chromium and manganese is found to'be appreciably more expensive than the use of substantial quantities of the ores of chromium andmanganese for making these additions.

In the practice of my process for producing high chromium high manganese irons and steels the amount of high chromium high manganese iron and steel scrap employed as a source of chromium and manganese (as contrasted with the high manganese steel scrap-10% manganese, 1% carbon and the balance ironwhich is well known and readily available) is generally determined by the availability of scrap metal in and around the melt shop and various customer plants. As more particularly indicated in the recently granted Patent No. 2,056,162 of William B. Amess, issued October 6, 1936, and entitled Production of rustless iron, the amount of scrap metal available in balanced manufacture in the form of ingot butts, crop ends and the like, is about 20% of the tapped metal. Where this metal is processed into bar stock the available scrap is then about 25% to 30% of the tapped metal. Where the metal is further processed into sheet and strip at various customer plants for example, the scrap available as ingot butts, crop ends, scrap sheet, punchings, clippings and the like amounts to from 40% to 50% of'the metal tapped. This figure may even amount to some 60% or 70% where the sheet or strip is fabricated into various ultimate articles of manufacture, such as machine or burner parts, kitchen ware, automobile trim, architectural applications and similar products.

While as an economical measure the amount of high chromium high manganese iron or steel scrap employed in the production of a heat of metal is proportional to that rendered available about the melt shop, this amount may be greatly increased where large quantities of this scrap are available at a favorable market price and similarly, the amount of scrap employed may be greatly lessened, or omitted entirely, where an acute shortage of scrap is encountered or where the market price of scrap becomes excessive.

The highly flexible nature of my. process of producing high chromium high manganese irons and steels assures the production of high grade metal at a minimum of expense by using a maximum of inexpensive raw materials, the relative proportions of which are varied as desired to meet fluctuating conditions of availability and market price.

As illustrative of the practice of my invention, in the production of a heat of high chromium highmanganese iron to a desired specification of chromium 17.0% to 19%, manganese 8% to 10%, usual silicon, sulphur and phosphorus and the balance iron, 4,000 pounds of low carbon steel scrap, 6,700 pounds of chromium manganese iron scrap (having an average analysis of chromium 17.5%, manganese 10.0%, carbon .12% and the balance substantially iron), 3,000 pounds of chrome ore (48% CrzOz), 800 pounds of high carbon ferrochrome (analyzing 4% to 6% carbon, 70% chromium and the balance iron), 2,500 pounds of ferromanganese (analyzing 6% to 8% carbon, 80% manganese and the balance iron), and 4,000 pounds of roll scale, are charged onto the bottom of a chromite lined 6-ton three-phase Heroult electric arc furnace rated 25 cycles, 120 to 180 volts, 2500 kva. preheated as indicated above.

. Alternating current electrical energy is supplied the furnace and the charge of ingredients begins to melt down forming individual pools of ferrous metal containing carbon, chromium and manganese immediately beneath the furnace electrodes. Under the continuing action of the intense heat of the electric furnace arcs the melting charge of ingredients soon forms a single bathof ferrous metal containing considerable quantities of chromium and manganese and apthe slag and metal and with a consequent 'minimization of the amount of carbon directly con tributed to the metal bath. This is a feature of considerable practical importance where the elimination of carbon requires the use of additional materials and the expenditure of much time and effort.

The amounts of chromium and manganese present in the bath of .metal at this stage of the process are largely dependent upon the amount of high chromium high manganese iron or steel scrap added to the charge (and the average chromium and manganese contents of this scrap) and the amount of high carbon ferrochrome and high carbon ferromanganese melted down. Likewise, the quantities of chromium oxide and manganese oxide present in the slag overlying the bath of metal are in a great measure dependent upon the amounts of chrome ore and manganese ore present in the initial charge of ingredients.

The quantities of chromium and manganese appearing in. the metal bath as the alloying elements, as compared with the amounts of chromium and manganese appearing in the slag as oxides of chromium and manganese, are further dependent upon the oxidizing character of the slag overlying the metal bath and the tendency for this slag to oxidize these elements from the bath under the operating conditions encountered in practice. The excessive loss of chromium and manganese into the slag as oxides of these elements is effectively prevented, as appears more fully hereinafter, by conducting the melting operation at a temperature of super-heat and by initially proportioning relative amounts of chromium and manganese going to form the metal bath and the overlying slag.

The strongly oxidizing character of the slag blanket overlying the bath of metal throughout the period of meltingdown of the charge of ingredients is effective in oxidizing the carbon supplied the bath of metal by the low carbon steel scrap, the chromium-containing or manganesecontaining iron or steel scrap and the high carbon ferrochrome and the high carbon ferromanganese. The practical difficulties in oxidizing carbon fromthe metal are greatly lessened where -metal to pick up carbon from the electrodes is reduced to a minimum since manganese does not have the thirst for carbon that is characteristic of chromium.

The strongly oxidizing character of this slag furthermore acts as an effective barrier between the bath of metal and the furnace carbon or graphite electrodes to prevent the pickup of carbon from these electrodes in spite of the great avidity of a chromium-containing ferrous metal bath for this element.

In order to minimize the oxidation of chromium and manganese from the metal bath incident to the removal and/or exclusion of carbon, the melting operation is preferably conducted at a high melt-down temperature. The use of these high operating temperatures, which for convenience I designate as temperatures of superheat, furthermore assures a very active oxidation of carbon from the metal bath and the realization of an extremely low carbon .product in a minimum of time and with the consumption of a minimum of power.

While no reliable method is known to me for precisely determining the temperatures of metal and slag during the melt-down period, it is estimated that these temperatures are from about 3100 F. to 3250 R, which is some 150 to 300 F. higher than those ordinarily encountered in ordinary electric steel making practices. The use of such extremely high operating temperatures is permitted, as indicated above, by the highly refractory nature of the chromite brick furnace lining employed.

The excessive oxidation of chromium and manganese from the metal bath, which is ordinarily incidental to the oxidation of carbon, is further lessened by the inhibiting effect of the large quantities of chromium and manganese present in the slag as oxides of chromium and manganese. The tendency toward the oxidation of these alloying elements from the molten metal is greatly lessened by the initially high balance between the molten chromium and manganese oxides in the slag overlying the metal and the oxides of chromium and manganese dissolved in the metal. In this manner certain further savings are realized in material and labor required to recover chromium and manganese from the slag, as more particularly described hereinafter, thereby achieving a highly efficient and economical process.

When the charge of ingredients is completely melted down and samples taken from the bath for purposes of analysis indicate that the carbon content is several points below the maximum value permissible the melt-down period is at an end. At this stage of the process, there are available in the slag great quantities of iron, chromium and manganese in the form of oxides of these metals, although much of the iron oxide in the slag has been lost during the melt-down oxidation period in removing and/or excluding carbon from the melting metal and in the oxidation of chromium and manganese incidental to the oxidation of carbon, all as more particularly described above. The large quantities of metal which are found in the slag as oxides at this stage of the process, are recovered in a reducing period, where preferably a non-carbonaceous reducing agent, such as ferrosilicon, chemically in excess of the oxides of iron, chromium and manganese contained in the slag, is charged onto the slag overlying the bath of metal. The precise amount of reducing agent required to-effect a high recovery of the metals from the slag is ordinarily determined empirically.

The contamination of the metal bath with the silicon of the reducing agent during this stage of the process is efiectively prevented in spite of the use of the excessive quantities of silicon, by conducting the reduction under strongly basic slag conditions. The desired basic conditions are preferably achieved by burnt lime in an amount of about three to five times the total silicon content of the ferrosilicon employed. Burnt lime, preferably predried to free the lime of-substantially all free and combined moisture normally present, is conveniently charged onto the slag along with the silicon-containing reducing agent.

For the illustrative charge of ingredients set forth above 2,500 pounds of crushed ferrosilicion of the 75% grade and 8,000 pounds of predried burnt lime are charged onto the slag overlying the bath of metal.

Prior to charging the silicon reducing agent and lime onto the slag, these ingredients are conveniently mixed on the floor of the melt shop. This mixture is then charged onto the slag overlying the bath of metal from time to time as furnace conditions permit. By charging a large proportion of burnt lime along with the reducing agent in this manner the maintenance of strongly 'basic slag conditions during the reducing stage of the process is assured.

By carrying out the-reduction of the oxides contained in the slag under strongly basic conditions silicon contamination of the metal is precluded. The acid silicates resulting from the reduction of the reducible oxides of the slag by the silicon reducing agent employed react with the basic lime added to the slag and form a series of calcium silicates, all as more particularly described in the Patent No. 1,932,252 of William B. Arness, granted October 24, 1933 and entitled Process of producing alloys. These calcium silicates are among the most stable components of the slag.

The large quantities of burnt lime charged onto the slag overlying the bath ofmetal during the reducing period, as more particularly described above, serve not only to render the slag strongly basic and prevent silicon contamination of the metal but also serve to give body to the slag (which is highly fluid and watery because of the large percentage of manganese oxide present in the slag) which in a manner not fully understood by me is effective in achieving a highly eflicient reduction of the reducible oxide content of the slag.

The continued heating of the metal and slag during the prolonged reducing period affords an opportunity to melt down further quantities of chrome ore and manganese ore. These materials are preferably charged along with 'the silicon reducing agent and burnt lime in corresponding successive batches. The addition of chrome ore and/or manganese ore at this stage of the process permits the use of a maximum quantity of these cheap sources of chromium and manganese (a feature particularly important in the produc-- tion of high chromium high manganese irons and steels of the higher chromium and manganese analyses) while maintaining a manageable volume of slag within the furnace throughout both the melt down oxidation period and the subsequent reduction period with a minimum submersion of the furnace electrodes in the furnace metal thereby limiting objectionable carbon pickup from the furnace electrodes throughout the complete process. Illustratively, 2,500 pounds of chrome ore and 200 pounds of pyrolusite (MnOz) are charged along with the ferrosilicon and lime.

After all of the reducing agent, burnt lime and chromium and manganese ores have been added and have fused and completed their reactions with the ingredients present in the slag and metal and a substantially complete recovery of the oxides of chromium and manganese contained in the slag is achieved, as evidenced by a (ill the final silicon content of the metal.

change in color of successive samples taken from the furnace from a black to a light green or gray, the reduction stage of the process is at an end. The slag overlying the bath of metal from which iron, chromium and manganese are completely recovered is then completely drawn off the metal.

A basic finishing slag of burnt lime and fine ferrosilicon is scattered over the exposed surface of the metal bath. (About 500 pounds of burnt lime and about pounds of the 75% grade ferrosilicon are employed for the example given.) The heating of the bath is continued at the reduced power input necessary to maintain the metal at a desired temperature until the refining of the metal is completed. The necessary duration of the refining period is greatly decreased in the production of high chromium high manganese steels because of the presence of a large percentage of manganese in the metal which is effective in refining and cleansing the metal of distributed oxides. During this period lump ferrosilicon is added as desired to adjust In the example given, about 50 pounds of 75% ferrosilicon is used to make the final adjustment of the silicon content of the metal.

After the desired refining of the metal is effected the application of power to the furnace is discontinued, the furnace electrodes are raised and the heat of metal is tapped into a ladle for teeming. There is produced about 18,200 pounds of high chromium high manganese iron ingots analyzing approximately, .07 carbon, 17.2% chromium, 9.8% manganese, .50% silicon with the usual percentages of sulphur and phosphorus and the balance substantially iron. The metal is clean, sound and comparatively free of objectionable oxide inclusions.

Where desired supplementary additions of nickel, copper, aluminum, silicon, molybdenum, tungsten, vanadium, zirconium and the like may be made in accordance with standard practice either in the furnace or in the ladle.

The production of high chromium high manganese irons and steels with a minimum repair and/or replacement of furnace linings and a minimum furnace shut-down and loss of operating time, is enjoyed by virtue of the chromite lining employed. This linin'g, as compared to heretofore known and/or used linings, is particularly resistant to destructive attack, at the high temperatures employed, by.the oxidizing melt-down slag followed by the reducing slag used in the second stage of the process, all as more particularly pointed out in my Patent No. 1,925,182,

granted September 5, 1933 and entitled Process the slag from which the reducible oxides are effectively recovered, supplying iron and chromium to the bath of metal, thus, in a measure, compensating for the cost of the erosion of the lining.

Thus, it will be seen that there has been pro- .vided in this invention an art of producing high chromium high manganese irons and steels and especially the produc'tionof high chromium high manganese irons (metal in which the carbon ploying a maximum of available and inexpensive raw materials consistent with the realization of good furnace operating conditions and the production of clean metal. Because of the presence of a high percentage of manganese in the metal, and the tendency for manganese to reduce chromium oxides, larger quantities of the inexpensive source of chromium, chrome ore, are effectively used without risking the production of dirty metal, objectionably contaminated with oxides.

It will be further seen that the process is particularly favorable to obtaining a desired economic balance between the raw materials employed as sources of the alloy metals, chromium and manganese, by relatively adjusting the proportions of ingredients (to such an extent as is consistent with the maintenance of good furnace operating conditions) chromium, manganese iron or steel scrap, chrome ore and high carbon ferromanganese, in accordance with variations in the availability of these materials and the fluctuations in their market prices.

While as illustrative of the practice of my invention substantial quantities of chrome ore and manganese ore are added both along with the initial charge of ingredients melted down and, with the reducing agent added after meltdown is complete, in order to take advantage of the comparatively long melt-down and reduction periods to effect a thorough heating of these refractory ingredients, it will be understood that chrome ore and/or manganese ore may be added while the melt-down period is in progress, especially toward the end of this period when samples of metal have been taken and the heat of metal is being held in the furnace awaiting a report 'of the carbon analysis, in order to realize the benefits of the available furnace heat during this period. Likewise, it will be understood that where furnacing conditions permit substantially all of the ore may be added during the melt-down period, thus in a measure simplifying the procedure.

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

I claim:

1. In the production of-high manganese rustless irons and steels in an electric arc furnace, the art which includes, melting down in said furnace iron scrap, high carbon ferromanganese and chrome ore thereby forming a bath of ferrous metal containing manganese and chromium covered by a slag containing the oxides of iron and chromium, and after melt-down is complete reducing the oxides contained in said slag thereby effecting an enrichment of the bath in chromium.

2. In the production of high manganese rustless irons and steels in an electric arc furnace,

- the art which includes, melting down in said furnace iron scrap, high carbon ferromanganese and chrome ore thereby forming a bath of ferrous metal containing manganese covered by a. slag containing the oxides of iron and chromium, bringing said bath of metal and the overlying blanket of slag up to a temperature of superheat thereby oxidizing and/or excluding carbon from said bath, and after melt-down is complete and the bath has reached a desired low carbon content reducing the oxides contained in said overlying blanket of slag thereby effecting an enrichment of the bath in manganese and chromium.

3. In the production of high manganese rust- 10 less irons and steels in an electric arc furnace,

the art which includes, melting down in said