US2430117A - Production of stainless steel - Google Patents

Description

Nov. 4, '41947.

A. L. FElLD PRODUCTION oF STAINLEss STEEL original Filed Deo. 5o, 1940 Jigsaw/:M006

/Y/a irre/anw Patented Nov. 4, 1947 anni PRODUCTION F STAINLESS STEEL Alexander L. Feld, Baltimore, Md., assignor, by mesne assignments, to The American Rolling Mill Company, Middletown, Ohio, a corporation of Ohio continuation of application serial No. 372,401, December 30, 1940. This application December 27, 1944, Serial No. 570,022

Claims. 1

This application is a continuation of my copending application Serial No. 372,401, filed December 30, 1940, and entitled Production of stainless steel. The present invention relates to a method for the efcient and economical production of stainless steel.

One of the objects of my invention is the production of stainless steel in a simple, effective and thoroughly practical manner, employing available and inexpensive raw materials and using known and available melting equipment.

Another object is to adapt in advantageous manner the use of low-grade chrome ores such as are available in this country to the production of stainless steel in regions where such ores are available and electric power is comparatively inexpensive, carrying out the production oi high quality stainless steels entirely by the use of lowgrade, low-cost materials, in highly eicient, economical and practical manner.

A still further object of my invention is to produce stainless steel in such manner as to maintain a high degree of fluidity of the melt, a maximum amount of metal for a given furnace capacity, a minimum cost of material and energy, all with a minimum amount of labor, and expenditure of time.

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

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

In the accompanying drawings, there is given a flow sheet illustrating certain features of my process for producing stainless steel.

As conductive to a clearer understanding of certain features of my invention, it may be noted at this point that the stainless steels, in general, are alloy steels containing chromium from about up to about 35%. Other alloylng ingredients may be present, as desired, to achieve wellknown special properties. The carbon content may be maintained low, as from 0.03% to 0.10%, giving rise to a ierritic or martensitic iron. If the toughness and ductility of an austenitic metal is desired, nickel, and/or manganese may be added in proper quantities as from about '7% to 20%. Where desired, it will be understood that high carbon and nitrogen contents may be employed. Ordinarily, however, the carbon content is maintained at a low value, preferably lower than .2%, or even .07%.

A stainless steel distinguishes sharply from the pre-alloy known as ferrochrome, in which the chromium content is at least and ranges up to and in which the metal loses most of the characteristics of steel and adopts many of those of chromium. The distinction resides not only in the physical properties of the solid products but in the molten products as well, such as in melting points, fluidity and the like, as appears more fully hereinafter.

In the production of stainless steel in accordance with previously proposed methods, the chromium content of the steel is had through low carbon ferrochrome, stainless steel scrap, chrome ore, or high carbon ferrochrome. In the former instance, lump low-carbon ferrochrome is added to a bath of low carbon steel. In the second, highcarbo-n ferrochrome is added to a bath of steel and the excess carbon content is eliminated through oxidation with roll scale 0r another oxide of iron. Preferably the chromium, incidentally lost through oxidation, is subsequently recovered with a ferro-silicon reducing agent, all as more particularly described in my prior Patent 1,812,- 941 of July '7, 1931, entitled Manufacture of stainless iron. Stainless steel scrap and high carbon ferrochrome are used together in the production of stainless steel, as more particularly described and claimed in my Patent 1,925,182 of September 5, 1933, entitled Manufacture of ferro-alloys, particularly ferrochromium alloys. scrap and chrome ore afford the principal sources of chromium in the process forming the subject matter of the Patent 1,954,400, issued to William B. Arness on April 10, 1934, entitled Process of making rustless iron.

In connection with prior processes wherein ferrochrorne is iirst produced as a source of chromium, using a carbonaceous reducing agent, it is particularly noted that a high carbon content is essential in the product obtained from direct reduction of chrome ore. This high carbon content is required to maintain uidity of the melt, in the presence of the high chromium content. This represents a necessary disadvantage which it has been. impossible to avoid prior to my invention.

The procedure according to prior processes has been attended by many disadvantages, among which are complexity of process, high cost of furnacing and melting equipment and the expenditure of time in carrying the processes to completion. For example, in many instances a highcarbon ferrochrome was employed as the principal source of chromium. To produce a stainless steel of desired composition, it was then nec- Stainless steel essary in the rening stage to lower the relatively large carbon content to an extremely low value by the use of large quantities of suitable oxidizing agents, such as iron ore, mill scale or hammer scale. This results in a lowering of the chromium content, not only through loss by oxidation, as noted above, but through dilution by the iron obtained. Charging the furnace with these additive ingredients during the refining stage necessarily results in the consumption of considerable quantities of power required to melt down these additions, while the process is slowed to the extent of the time required for such melting to take place. Moreover, the ferrochrome is obtained in lump form and in utilizing this source of chromium, considerable expenditures of energy and loss of time are encountered in remelt.-y ing the alloy as a part of the steel making process.

To like effect, when using an iron-chromiumsilicon pre-alloy as a source of chromium in the production of stainless steel there is the problem of eliminating carbon. `In addition to this, and of even more serious consequence, there is the problem of eliminating silicon. I nud that silicon is even more of a problem than carbon because it requires large quantities of iron oxide together with a proportionately large quantity of lime to achieve slag fluidity. A substantial increase in slag value, and a corresponding decrease in furnace capacity, is inevitable.

Moreover, in producing the iron-chromiumsilicon alloy subsequently used in the production of stainless steel many problems are met. To prevent an excessive carbon content resulting from coke used as a reducing agent and carbon employed in furnace bed or lining great quantities of silica are employed. The reduction of this silica requires large amounts of power. It is found extremely wasteful to, put silicon into the metal, as an attempt to keep carbon out, this during the production of the iron-chromium-silicon alloy, and then remove this silicon subsequently in the production of stainless steel.

In direct ore processes of producing stainless steel, the practice is to carry out all steps in a single furnace. In such processes, chrome ore is charged onto a bath of steel, together with a reducing agent, the resulting chromium metal gravitating into the steel bath. Although an attempt is then made to rene. the metal, it is found that the metal is so dirty as to have a limited field of commercial use, Moreover, the process `is wasteful of chrome ore, since much of the metallic values are not recovered from the ore employed. Moreover it is difcult to eliminate carbon from the metal. In addition, such processes have the inherent; disadvantage that the discontinuous batch process resulted in a yield of metal per unit of time which was comparatively small.

I have found high quality stainless steel is achieved directly from the ore by accurately correlating the quantities of low-grade chrome ore and iron ore, and/or steel scrap, on the basis of their respective available chromium and iron contents, as appears more fully hereinafter, and

charging them, together with suitable car? bon-aceous reducing agent in an electric furnace in a continuous manner. Such steel is obtained with a melt which is fluid and thoroughly and readily workable in the smelting furnace, despite the presence of chromium in the approximate amount desired in the nal product. The fluid character of the melt is somewhat surprising since it is well-known that chromium tends to increase the freezing point of a metal bath and that the melt is sticky even at fairly high ternperatures. A low carbon product is had without recourse to large quantities of silicon, the presence of this ingredient being neither necessary nor desired.

I have found that a highly advantageous startllg material for my process is a low-grade chromium ore which can be obtained inexpensively in certain parts of this country. Iron ores of low silica content, or other sources of iron, can satisfactorily be used to supply the additional iron required. If they be available at low cost, steel scrap, mill scale or hammer scale can be substituted in part or entirely for the iron ore. I find that with a high grade source of iron there may be used chrome ores high in gangue content, this because the total amount of the chrome ore is substantially less than the amount of iron required.

By charging a batch of the metal, after completion of the first or smelt-.ing step, and preferably while still in molten condition, into another electric furnace, I have found that it is possible to refine the metal by eliminating the small remaining quantities of carbon and other impurities by the use of relatively simple and inexpensive procedural steps. Any chromium lost in this second or renning step may be recovered in a third step using a suitable ferrosilicon or other non-carbonaceous reducing agent. Most of the chromium remaining as chromium oxide in the smelting slag also may be recovered if desired. This is achieved by charging the metal slag into the second furnace during the second or reningstep, or even during the third step.

Accordingly, one object of my invention is the manufacture of stainless steel by means of a novel, economical and practical process which produces a large quantity of stainless steel of uniform good quality and at the same time requires a minimum of complexity in the process and a minimum of expenditure of time and energy, and which enables the advantageous use Qf low-grade ores especially in regions where electrical power costs are low.

Referring now more particularly to the pracw tice of my invention, as a first step a suitable lowgrade chrome ore, a source of additional iron, such as iron ore or scrap steel in small pieces, and a carbonaceous reducing agent, such as coke, are smelted in a suitable ferro-alloy furnace. Conveniently the ingredients are intimately mixed in proper proportions, as discussed hereinafter, on

- the oor of the melt shop. The materials are charged into a continuously operated three-phase ferro-alloy furnace, preferably having a magnesite lining. I und that with such a lining, but little of the furnace lining is taken up by the metal and slag. This action is due in part to the high stability of its principal component magnesia in the presence of the smelting slag. Moreover, such a lining, as distinguished from the conventional carbon bottom or lining is not taken up by slag and metal, particularly it does not dissolve in the metal as does carbon, subsequently presenting a difficult problem of removal.

Along with the charged ingredients may be included some lime, if desired, to serve as a fluxing agent, and to promote the general fluidity and workability of the slag. For purposes of power economy, it will be understood that the charging ingredients should be kept as dry as possible.

A very advantageous feature of my invention is that a low-grade chromium ore may be employed. This, of course, is quite distinct from the production of ferrochrome, where a chromium-rich, comparatively costly, ore of the irnported type is essential in a practical process. As pointed out hereinbefore, a low-grade chrome ore tends to limit the enrichment of the metal in chromium to approximately the chromium content desired. By low-grade chrome ore I mean an ore having a low ratio of chromium oxide to iron oxide, also an ore in which the ratio of chromium to irreducible oxides, such as alumina and magnesia, is low.

This chromium ore, such as is found in California and Oregon, where electric power is quite inexpensive, is usually found to range from about 15% to 45% chromium oxide (CrzOs), 10% to 35% iron oxide (FeO), 3% to 25% alumina (A1203), and 6.5% to 35% magnesia (MgO), together with silica and small quantities of other ingredients.

My new process is of particular utility in locales where iron and steel scrap are at a pre mium, but where low-grade chrome ore and inexpensive sources of iron are available. It reaches its principal utility in such regions, where the cost of electrical power is extremely low, thereby facilitating, from an economical standpoint, the use of low-grade ores, hitherto of but little commercial value, in electric furnace practice. In the far west, for example, the cost of such electric power is approximately only one-fifth of what it is on the Atlantic seaboard.

Where the chrome ore is rich in chromium, then the quantity of iron used is correspondingly increased. In choosing the initial ingredients, it is essential to preserve the balance between the chromium content of the chromium-containing ingredient and the iron content of the ironcontaining ingredient. This balance is always maintained at a value such that the resulting metal has a melting point sufficiently low to ensure fluidity under the furnacing conditions which obtain. It is my discovery that this ratio between available chromium and available iron must be on the order of .60 or less, this however going no lower than .125. A higher ratio than .60 invariably results in excess slag values, bulkiness of slag and loss of eiciency. This ratio I find to be critical.

As a further feature the chromium content of the metal in the smelting furnace is maintained at a low value, as contrasted to the chromium value of ferrochrome alloys. This is of great importance, not only in tending to simplify subsequent refining steps, but in making it possible to realize a comparatively low carbon content While still maintaining the metal in a fluid condition. A high chromium content tends to give a high melting point for the metaly as pointed out hereinbefore, a tendency which in ferro-alloy practice is counteracted, at least in part, by maintaining a high content of carbon. Thus with the chromium content maintained high, then, if the carbon content is not correspondingly high, the metal begins to solidify. The metal becomes sticky and begins to freeze on the bottom of the furnace, since this region is most remote from the furnace arcs. How-ever, by keeping the chromium content of my metal bath down to approximately the Value which is desired in the refined stainless steel, I find that the bath, even with its low carbon content, is even more fluid than in the case of the production of the known ferrochrcme alloys, where the high carbon content present promotes fluidity.

tively loW carbon content of 4%, a diierence of.

144 F. My further investigation has disclosed that with the chromium content kept at 20% Iv can lower the carbon content to 0.10% and still the freezing point will not rise beyond 2696 F., the significance of which will be appreciated when it is considered that this value is only slightly (54 F.) higher than the freezing point (2642,o F.) of commercial ferrochrome, containing 60% chromium and 4% carbon.

A low-melting point, and the accompanying uidity of the metal, permits impurities occluded or entrapped therein to be eliminated with comparative ease, resulting in a metal of greater purity and general uniformity.

By determining in advance the proportions of chrome ore to iron ore and/or iron scrap in order to fix the ratio of the chromium contient to the iron content in the raw materials or mix at a gure not exceeding .60, it is ensured that the metal in this smelting step has a reasonably low melting point, and good fluidity is preserved. The melting 'point is at least sufficiently low and the fluidity adequate to permit the realization of an intermediate product of hitherto unknown low carbon content, that is, a carbon content of less than 2% and customarily below 1%. Any silicon coming from the ore employed is insignificant, being less than 2%.

During the smelting stage wherein the metal is reduced from the ore, the carbonaceous material, under the heating action of the arc discharge, reacts with the oxides 0f chromium and iron in the charge. This occurs in the regions of most intense heating, that is, in the region of the interface between the slag and the molten metal. This reaction yields gases which, rising, percolate through the solid charge to form a gas blanket of high temperature. YThe scouring action of these hot gases serves effectively to vaporize and drive off all moisture contained in the charge and serves effectively to prevent the absorption by the metal of any hydrogen which might result from the dissociation of the moisture content of' the initial charge. Any consequent contamination of the metal is thus iavoided. Accordingingly, the moisture content of the initial charge passes olf in large part before the chargebecomes incorporated in the melt, so that no preliminary drying is required of the smelted ingredients.

It will be noted that the smelting step preferably is a continuous process, the slag being drawn olf from time to time, to prevent the quantity thereof in the furnace at any one time from becoming excessive, and to provide room for the additional charge. The charge preferably is added slowly and in a substantially continuous manner. The metal produced, now an impure stainless steel, is maintained in the furnace in a molten condition until it is ready to be tapped off at the time when it is actually needed, that is, when the refining furnaces are available. In tapping a portion of the melt always is retained to facilitate further operation of the furnace and prevent carbon contamination by precluding direct contact between the furnace electrodes and the steel scrap added.

The metal tapped at the completion of the smelting stage is an impure stainless steel. It

analyzes about 10% to 35% chromium, less than 2%/ carbon, with the balance consisting principally'of iron. This stainless steel has approximately the chromium content desired in the refined stainless steel. There remains then, in the rening stage, simply to eliminate the comparatively small remaining quantity of carbon and impurities to the desired degree. No silicon problernV is encountered.

The extent to which the carbon absorption can be minimized during the smelting process is dependent in large measure upon the required extent of the recovery of chromium from the ore. As the smelting process is continued, especially in the presence of an appreciable excess of carbonaceous4 reducing material, the chromium recovery is increased. However, the carbon absorportion in the metal is likewise increased, and this carbon absorption determines a limiting condition beyond which chromium recovery in this directsmelting process cannot be carried. I have found, however, that 90% or more of the chromium-can be recovered from the ore without increasing the carbon absorptionv beyond 1%. Nevertheless, it is probable that successful smelting could not be carried on with a carbon content of less than 0.25%, because the chromium recovery from the ore then would not be suiciently highV in view of the decreased amount of carbonaceous material used, and the low-carbon content Would prevent the metal bath from beingV sufficiently fluid -to be readily workable under the heavy slag encountered.

Although I prefer in most instances to employ iron ore along With chrome ore as the source of whatever iron is required, I am not bound by this practice. For example, where the chrome ore available is of high iron content as compared tothe chromium content, I find that little r no additional iron is either necessary or desired at this point, suiiicient iron being present in the iron oxide of the chrome ore. On the other hand, Where. an addition of iron is desired, this conveniently may be made with steel scrap which is substituted in part orY entirely for the iron ore in the smelting step. The choice of iron ore or steel scrap is one of economics. It depends upon the relative cost and availability of the two iron sources. A factor to be considered and weighed in this connection is that steel scrap requires a lower power consumption, and less reducing charge, than does the ore. Where steel scrap is employed, I find it advisable to use steel turnings, borings, clippings, and the like, the size of each particular piece of steel being sufliciently small to avoid the possibility of bridging across the electrodes and occasioning operating difficulties.

As contrasted with the production of the conventional high-carbon ferrochrome alloy obtained in previous ore reduction processes, the production of my new impure stainless steel displays many advantages. It has already been pointed out herein that there is a, material saving in power. As a matter of fact, the power required per pound of my new impure stainless steel is materially less than that required per pound of ferrochrome alloy of 60% or '70% chromium, this decrease in power requirement probably being due to the ease of reduction of iron oxide as compared to the chromium oxide.

As to the product itself, the impure stainless steel has a high malleability and approximately correct chromium content, as contrasted with the brittleness of high-carbon ferrochrome containing 60%to 75% chromium, a value much too high for useful steels.

As has been suggested, this metal is retained until ready for use in the smelting furnace, and from time to time a portion of the melt is drawn olf and carried over in molten, super-heated condition, to a refining furnace, now to be described. A considerable saving in power results from the use of charging materials which are already in molten form, for my experience has shown that by far the major portion of energy consumed in known refining processes wherein cold alloy is charged into the furnace is required in bringing the charge into molten condition. In my present process, the principal energy consumption is in the nrst, or continuous stage.

After the metal has been sufficiently reduced from the ore, the resulting impure stainless steel, of aforementioned composition, is either maintained in iuid state in the furnace until needed, or is tapped immediately and transferred to a suitable electric furnace Where subsequent reiining is carried out. This rening operation is the second step in my process of producing stainless steel. For this purpose a S-phase Heroult type furnace of 6 to 20 ton capacity, is satisfactory. The furnace preferably has a chromite lining extending to a point just above the slag line.

Experience has shown that the time that the metal is held in the smelting furnace is not critical, so that it can be retained there over a long interval, without detrimental effect. It is, of course, entirely feasible to employ a number of refining furnaces, all supplied by a single smelting furnace. The capacities of the several relining furnaces, of course, are properly weighted to take into account the time factor and are so correlated to the output of the smelting furnace as to ensure that a maximum efli-ciency of operation is obtained. Due recognition is given the fact that the smelting furnace is operated continuously, While the refining furnaces are operated intermittently, that is, in batches.

The molten impure stainless steel is charged into the refining furnace along with a sufficient quantity of a suitable oxidizing agent to ensure a lowering of the carbon content to the desired low value of say 0.03% to 0.20%. As the oxidizing agent, I find it convenient to use simply the iron ore hematite (Fe-203), in order to maintain the quantity of slag at a minimum, and thus increase the volume of metal which can be handled per heat. As I have suggested previously, in this and the possible subsequent reducing step, there is an addition of a certain amount of iron. The amount of iron ore employed is considerably in excess of the quantity theoretically required to substantially eliminate the carbon present. Because of the comparative thinness of the layer of slag, I can, for a given voltage, remove the carbon electrodes a greater distance from the surface of the melt than has been possible in earlier processes, thereby effectively decreasing carbon contamination of the metal by the furnace electrodes.

Although as stated, I find it desirable to maintain Athe oxidizing slag at a minimum in volume, for the reasons aforesaid, nevertheless I have found that frequently it is advantageous to add a certain quantity of chrome ore at this stage of the process in order to take advantage of the furnace heat available, and the length of time required for carbon elimination. As the carbon is oxidized from the metal, there is a tendency for some of the chromium of the metal to oxidize and pass into the slag. The presence in the slag of chromium oxide from the chrome ore addition tends to retard the oxidation of chromium from the metal.

Where desired, the chromium oxide addition made during the rening step may come from the slag resulting from the smelting stage. rIhis is particularly advantageous where the smelting is conducted to yield a very low carbon product at a sacrice to the extent of the recovery from the chrome ore. It is possible to use this slag as the entire source of additional chromium in this batch oxidizing process, if so desired. Additions of further sources of chromium may be made as desired.

I have found that the use of an impure staim less steel of approximately the desired ultimate chromium content, is of considerable advantage in simplifying, shortening the production of finished stainless steel. The advantages are many. For one thing, and as has been pointed out above, there is no necessity of charging steel scrap into the furnace, Thus there is a saving in power consumption, and a maintenance of power demand at a fairly uniform figure. Experience has shown, for example, that where high-carbon ferrochrome is charged into the refining furnace along with steel scrap, iron scale or other source of iron, the greatest quantity of power consumed in this step is in bringing the ingredients to a molten condition. Furthermore, irregularities in the contour and density of the charge result in irregular action of the furnace electrodes. By simply eliminating the necessity of using a source of additional iron, at this stage of the process, l am enabled to realize considerable savings in power.

Furthermore, substantial reductions in the quantity of electrical energy required result from the transfer of metal from the smelting furnace directly into the refining furnace while in molten condition. Preferably the metal is charged into the refining furnace in a condition of super-heat, that is, at a temperature which is substantially raised above the melting point of the metal. Thus, the refining step can be carried to completion quickly and advantageously, with the expenditure of only a small quantity of power. Since the initial or smelting step is a continuous one, with but little Variation in load demand once the process has been started, and since there is but little variation in power requirements of the plurality of simultaneously operating batch refining processes, I am enabled to achieve a uniform and high power load factor, thus further decreasing the cost of energy supply.

The described stable operation of the refining furnace is facilitated by charging the impure stainless steel in molten form. The surface of the charge is uid and level, rather than irregular, as when a solid charge is used. This, of course, permits more uniform operation of the electrode, without reciprocation and dipping of the carbon electrodes into the metal, another important advantage in rapidly obtaining a low-carbon product. Y Y

I have found that in this step, the iron ore, since it is-charged directly into the furnace with the metal, has little opportunity for initial drying.

' Furthermore, there is no gas blanket protecting the metal, as in the smelting step, so that there is possibility of unsoundness in the resultant metal due to gas effects should appreciable moisture be contained in the ore. To avoid this possibility,

10 nd it highly advantageous to predry carefuly the iron ore before it is charged into the furnace, thereby effectively inhibiting hydrogen contamination of the metal.

While the carbon content is being lowered to the desired value through oxidation by the iron oxide content of the slag, some of the chromium values of the metal likewise oxidize and pass over into the slag. It is desirable to recover these values, as Well as those introduced directly as chrome ore or the slag from the smelting operation. To this end as a third step in my process, a reducing agent is added to the melt. For example, for this purpose, a ferro-silicon is used, preferably in an amount slightly in excess of that theoretically required, say by 5% to 20%. Along with the ferro-silicon reducing agent there is added a quantity of lime sufcient to assure slag fluidity.

The silicon added rst reduces the chromium and iron oxides in the slag, so that the chromium and iron thus liberated pass back into the metal.

I have found that ferrosilicon of the low-grade type say containing 40% or less of silicon is used to advantage by charging it into the furnace in a molten condition. Certain economies in using this cheap reducing agent are possible. The advantage of this option and variation, however, is not so great in the case of the richer ferrosilicons, those containing say 50% to 75% silicon, because the amount of power and time saved byy using these ferrosilicons in molten form is not particularly large. As a matter of fact, there are certain advantages in charging ferrosilicon in the pulverized or granular form, directly onto the slag and outsof contact with the underlying bath of metal in order to confine the reducing reaction largely to the slag layer rather than the interface between slag and metal.

I fin-d that it is feasible to substitute ferrochrome silicon for ferrosilicon as the reducing agent. However, since it is difficult to employ ferrochrome silicon as a reducing agent without at the same time using a certain quantity of ordin nary ferrosilicon as a supplementary addition, ferrochrome silicon ordinarily may not entirely displace ferrosilicon. In a, self-contained plant, therefore, the use of ferrochrome silicon would 1call for an additional ferro-alloy smelting operaion.

In connection with the third stage of my process, I have found that, if desired, the chromium values of the smelting charge can be recovered by charging a portion of this slag into the furnace during this step. Charging the smelting slag at this point is not .so desirable, however, because it tends to give dirty metal. Of course, such slag,l addition should be made only if the procedure proves to be an economical one. The burnt lime used in the reducing step, since 1t is a material which readily absorbs moisture from the surrounding atmosphere, should be carefully predried before charging it into the furnace. This precaution prevents unsoundness of the metal which might result from a decomposition of the moisture under the action of the furnace arcs and a consequent absorption of hydrogen by the metal, Accordingly, it is desirable that the lime be dried at the plant just prior to use, or else be obtained and stored in air-tight containers.

When the metallic values have been recovered from the slag, this third or reducing step is complete. The metal is then finished in accordance with standard practice and the heat tapped. The finished metal is poured into ingot molds and al- I lowed to solidify and cool. It is found to have 1l a chromium content of about to,35%, asdid the impure stainless steel starting charge. The carbon content amounts to about 0.03% to 0.20%. Where special additions are desired, such as those noted above, rthese are usually added to the heat of metal during the finishing operation.

Thek production of stainless steel in accordance with my present process, in which an impure stainless steel is achieved inthe rst or smelting step, is of considerable advantage over known processes wherein high-carbon Yferrochrome is first produced. Firstly, the direct production of an impure stainless steel is realized without encountering objectionableconditions of slag and metal. Secondly, the use of impure stainless steel greatly simplifies the subsequent or rening steps. Thirdly, the quantity of oxidizing substance which must be added is small, due to the already low value of the carbon content in the impure steel. Flowing from this, there is a fourth advantage in that the amount of the expensive ingredient ferro-silicon necessary to recover the oxides of chromium and iron in the slag, some of which came from the oxides added during the second stage, is brought to a minimum. Moreover, as a nfth advantage, there is enjoyed in my process smoother furnace operation, a more even power demand and, in fact, a substantial decrease in the amo-unt of power ordinarily required in iirst making high-carbon ferrochrome and then using this either directly or indirectly in the production of nished stainless steel in accordance with known methods.

My new process not only makes possible a substantial saving in yelectric power, but it puts a premium on the high-iron grades of chrome ore,

such as are available in this country. Of like importance, a maximum output is enjoyed for a particular investment in plant and melting equipment. The use of a continuous smelting furnace, in combination with a plurality of simultaneously operating relining furnaces, enables a uniformity of power demand to be realized, with a substantially constant power load factor, superior to anything known in the priorlart, all as I have pointed out previously, in this disclosure. For a smelting furnace of a given electrical rating, the daily output is considerably higher in the case of chromium stainless steels than in the case of the ferrochromes. kIt will be observed that my new process makes it possible to produce in the same plant, simultaneously if desired, an impure stainless steel and ingots or rreiined stainless steel. Where desired, it will be understood that the impure stainless steel may be cast into suitable form and stored for subsequent use.

My invention is of particular advantagewhere low electric rates are available, and where there is available a chrome ore oflow chromium content. It has additional advantage where iron ore, even of an impoverished variety, is present at low cost, along with low cost of electrical power, although it is possible, as pointed out hereinbefore, to substitute all or part of the iron ore by scrap iron, mill or hammer scale, where these ingredients are available. Use of these ymaterials is attended by consequent lowered powerdemand, due to the comparative facility with'which they are melted. Y 4

The metal produced by my new process hasa high degree of purity, resulting among other causes from the highly iluid'condition` of the melt, throughout the smelting process. Fluidity at thatstage is important in korder that slag occlusions, oxides and entrapped `gases can pass readily to the surface of the metal. Purity of the metal also comes from the fact that little, if any, of the furnace lining is taken into the metal.

Thus it will be seen that there is provided in this invention, a process of producing stainless steel in which the many objects herein'before noted, together with many thoroughly practical advantages, is successfully achieved. It will be seen that the process is simple, direct and eiicient, that it makes use of the least expensive raw materials to achieve a desired chromium content, and that it permits a maximum tapping weight of metal, consistent with the use of inexpensive raw materials, for a given investment of furnacing equipment.

As many possible embodiments may be made of my invention, and as many changes may be made in the embodiment hereinbeiore 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:

l. In the production of stainless steel having a chromium content of 10% to 30% and a carbon content not exceeding about 0.2%, the art which comprises, smelting in a ferro-alloy furnace having a non-carbonaceous lining a charge consisting of the essential ingredients chrome ore, a carbonaceous reducing agent and one or more of the class consisting of -iron ore, roll scale and steel scrap, the charge ingredients being in the relative proportions that the available chromium content bears a relation to the'available iron content of .60 or less whereby an impure stainless steel is had with a chromium content not exceeding about 35%, a carbon content not exceeding about 2% and a Silicon contentnot exceeding about 2%; transferring said impure stainless steel to a steel-'making electric `arc furnace having carbon electrodes and there-'decarburizing the same with a thinslag of iron oxide, thus precluding contamination by said electrodes, some of the chromium of the steel incidentally vbeing oxidized with the decarburization, the oxides migrating into the slag; and `thereafter with minimum ccntamination of the bath with the furnace electrodes recovering the'metallic values of the oxides present with a silicon 'reducing agent.

2. In the production of stainless steel having a chromium content of 10% to 30% and a carbon content not exceeding about 0.2%,` the fart of carrying out simultaneously-in the same plant a smelting operation resulting. in the production of an impure 'stainless' steel of approximately the desired ultimate iron andV chromium contents, and a decarburizing operation, and employing to that end in economical and advantageous manner, a low grade Achrome ore, which includes, smelting the chromeore in a, magnesite-lined smelting furnace along with a -carbonaceous reducing material and a quantity of iron-containing material 'of the group `consisting of iron ore, roll scalegand steel scrap, the relative proportions of theingredients being such that the ratio of the available chromium and iron' contents does not exceed about .60 whereby impure stainless-steel is tapped from time to time, a substantial portion at all times being retained in the furnace, however, the low chromium content with resultant fluidity Vand the continuousfmaintenance of at least a portion of the melt in the vfurnace Vassuring tapped metal of acarboncontent not-exceeding 2% witha silicon contentr not exceeding about 2% and decarburizing the tapped metal in a steel-making furnace having carbonelectrodes by forming on the metal a thin slag of one or 'more of the group consisting of iron ore and roll scale and thereafter, with minimum contamination of the melt with the furnace electrodes because of the thin slag present, reducing the metal oxides present in the slag to achieve stainless steel having a low carbon content togetherwith a low silicon content.

3. In the production of stainless steel having a chromium content of to 30% and a carbon content not exceeding about 0.2%, the art which includes charging into a smelting furnace having a non-carbonaceous lining, the ingredients, chrome ore, a carbonaceous reducing agent, and a quantity of iron-containing material of the class consisting of iron ore, roll scale and steel scrap sufficient to give a ratio of available chrominum to iron not exceeding .60 and not less than .125 and yielding upon smelting an impure stainless steel having a chromium content not exceeding about 35%, the low chromium content of the charge and resultant fluid characteristics of the melt and the continued presence of at least a portion of the melt assuring a carbon content not exceeding 2% With a silicon content not exceeding about 2%; while retaining a portion of the melt in the smelting furnace to facilitate the smelting operation and assure a low carbon product, tapping oi a part of the metal and charging it into a steel-making furnace having carbon electrodes; decarburizing the metal with a slag made up of one or both of the group consisting of predried iron ore and roll scale thus oxidizing the carbon, a substantial quantity of predried chrome ore being added to preclude excessive oxidation of chromium during the oxidation of the carbon present in the melt; and thereafter adding a silicon reducing agent to the slag thereby reducing the oxides of iron and chromium present in slag and metal to achieve refined stainless steel.

4. In the production of stainless Steel, the art of carrying out simultaneously in the same plant, a smelting step resulting in the production of an impure stainless steel of approximately the desired ultimate iron and chromium content, and a refining step, and employing to that end in economical and advantageous manner a low-grade chrome ore, which includes, charging into a smelting furnace the chrome ore, a carbonaceous reducing agent and a quantity of iron-containing material s uicient to yield a melt containing from 10% to 35% chromium; reducing the metals from the charge into the melt, the low chromium content permitting low carbon content up to 2% with fluid characteristics of the melt; tapping off part of the melt; and charging it into a second furnace, along with some of the chromium-containing slag resulting from the smelting step; and there refining the metal by oxidizing the carbon and thereafter reducing the metal oxides present in the meltI and recapturing into the metal a part of the available chromium content of the slag thus charged in, until a rened metal is obtained.

5. In the production of an impure stainless steel having a chromium content not exceeding 35%, a carbon content not exceeding 2% and a silicon content not exceeding about 2%, and to that end employing a low grade chrome ore as a source of chromium, the art which comprises smelting in a ferro-alloy furnace having a noncarbonaceous lining a charge essentially consisting of said low-grade chrome ore, a carbonaceous reducing agent and one or more of the class consisting of iron ore, roll scale and steel scrap, the charge ingredients being in the relative proportions that the available chromium content bears a relation to the available iron content of .60 or less, and from time to time withdrawing a portion only of the melt, the retention of a portion of the melt and relationship between the ingredients of the charge assuring a product of 10W carbon and yet of low silicon content.

ALEXANDER L. FEILD.

REFERENCES CITED The following references are of record in the iile of this patent:

UNITED STATES PATENTS Number Name Date 1,365,091 Clement Jan. 1, 1921 1,686,206 Flodin et al Oct. 2, 1928 1,901,367 Gustafsson Mar. 14, 1933 1,925,916 Arness Sept. 5, 1933 2,021,979 Arness Nov, 26, 1935 2,176,689 Udy oct. 17, 1939

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