US2455074A - Production of stainless steel - Google Patents

Description

Patented Nov. 30, 1948 2,455,074 PRODUCTION OF STAINLESS STEEL Donald L. Loveless, Baltimore, Md., asslgnor to Armco Steel Corporation, a corporation of Ohio No Drawing. Application February 18, 1946,

Serial No. 648,564

Claims. I

The present invention relates to the production of chromium-nickel stainless steels, particularly those of low carbon contents, and more especially to a process for producing the same employing chrome ore as a substantial source of chromium.

One of the objects of my invention is the efiicient and reliable production of chromium-nickel stainless steel of desired alloy content using inexpensive and readily available raw materials.

A further object of my invention is the consistent production of chromium-nickel grade stainless steel of low carbon content in an electric arc furnace at minimum cost of raw materials, power, labor and the like and with minimum shut-down of equipment for replacement and repair.

'A still further object is the simple, practical and economical production of chromium-nickel stainless steel of permitted alloy content having a desired extremely low carbon analysis and desired physical characteristics, utilizing known and tried furnaclng and operating equipment.

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

The invention accordingly consists in the combination of ingredients and in the several operational steps and in th 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;

As conducive to a clearer understanding of certain features of my invention, it may be noted at this point that the chromium-nickel stainless steels are defined as steels which include about to 35% or more chromium, nickel from considerably less than 1% up to 30% or more, with or without supplementary amounts of such elemanganese, aluminum, or the purposes, and the remainder substantially all iron. While the carbon content of the steels referred to may be 0.5% or more, the extremely low carbon stainless steels have a carbon content which preferably does not exceed about 0.03%. Also the sulphur and phosphorus contents ordinarily must be low, these respectively being less than about 0.030%.

Among the heretofore known processes for the production of chromium-nickel stainless steel, there are those which depend upon the utilization of relatively cheap raw material source or sources of chromium to supply the element chromium in the melt. These processes, as distinguished from earlier processes wherein low-carbon ferrochromium, a highly expensive pre-alloy, was employed as the accepted sole source of chromium for the melt, have rapidly a n P ity. This is true in view of the materially lowered production costs and numerous other advantages made possible through their use, Some of the processes referred to embody the direct utilization of such cheap sources of chromium as chrome ore, and stainless steel scrap either of straight chromium or chromium-nickel grade. They involve furnace melting a charge including the chromium-bearing materials, and iron oxide, under conditions to form a stainless steel bath comprising chromium and nickel, thenickel usually also being recovered insubstantial amount from the charge as from chromium-nickel stainless steel scrap.

An outstanding advantage in melting practices which are based upon the consumption of chrome ore, is in the derivation of a valuable quantity of chromium directly from its original source rather than from some relatively expensive pre-alloy. In this. connection, however. the bulk of the ore on the one hand, and the limitation of furnace capacity on the other, impose a restriction upon the quantity of ore permissible for proper melting conditions. -Thereiore, the chromium from the ore frequently is supplemented by chromium from such materials as stainless steel scrap, either of straight chromium or chromium-nickel grade or both. By this means there has been a consequent alleviation of the problem of disposing of waste metal about the stainless steel melt shop and rolling mill, particularly as regards that in the form of stainless steel ingot butts, crop ends, and the like, all with ensuing savings in cost of Some further importance of the reclamation melting of scrap appears from the remelting of considerable quantities of the same obtained from the various customer plants where stainless steel sheet, strip, bars, and the like, are fabricated. In the customer processing of bar stock, for example, about 25% to 30% results in scrap. As the number of working and forming operations increase so does the quantity of scrap, as to some 40% to 50% in the production of sheet and strip and may even amount to to where the sheet or strip is further fabricated into various ultimate articles of manufacture, such as machine or burner parts, kitchenware, automobile trim, hardware, architectural applications, and the like. Those quantities of stainless steel scrap in the customer plants, being in demand ior raw material in remelting accordity. that is about 0.02 This is unfortunate because in many of the chromium-- nickel grades, carbon is present only as an impurity. I find that the difllculty of obtaining a low-carbon grade of chromium-nickel stainless steel enormously increases with the lowering, especially as the limit of solubility is approached, and thereafter. The heretofore known processes, I find, usually are fairly eflicient for the elimination of carbon down to about 0.10% but usually any further removal is not readily or reliably accomplished.

A further and outstanding object of my invention accordingly is the eflicient, reliable and economical production of chromium-nickel stainless steel of low carbon content, and of desired analysis, directly employing chrome ore as a substantial source of chromium and using for this purpose the conventional electric arc furnace having carbonaceous electrodes.

In producing chromium-nickel stainless steel in accordance with the practice of my invention, I employ an electric arc furnace. as for example the conventional carbonaceous-electrode Heroult steel making furnace, and provide a meltdown charge therein comprising chrome ore in such quantity as is consistent with furnace capacity and good melting contact in view of other ingredients of the charge, among these advantageously being stainless steel scrap and base carbon steel scrap. The stainless steel scrap, at least in part, conveniently is such as to provide a source of nickel as well as of chromium and iron. As a further, and highly important, meltdown ingredient I use nickel oxide, with or without iron oxide as for example roll sca e or iron ore. I find that under proper conditions the nickel oxide serves as a remarkably potent and highly effective carbon elimination agent. This is particularly important in the production of extremely low carbon chromium-nickel stainless steel, especially in the presence of carbonaceous materials in the furnace.

In the production of stainless steel of extremely low carbon content (on the order of 0.03% say) I give special attention to the manner of assimilating the chrome ore in the furnace, this to avoid detrimental carbon pick-up from the ore. In this connection, as more fu ly pointed out in my copending application, Serial No. 628,673 filed November 14, 1945, and entitled Production of stainless steel, the various commercial grades of chrome ore are most often found to contain substantial quantities of carbon, usually somewhere in the vicinity of 0.06% to 0.20% as represented by a dozen different batches of ore which analyzed, 067%, 065%, 064%, 089%, 057% 198%, .0'78%, 080%, 145%, 018%. 020% and 124% carbon. The carbon, moreover, is in such form that attempts toward elimination by pre-drying and especially with a 1 carbon content approaching the limit of solubilcomes into the process 174% nickel grade) of the order containing or pre-heating, even to a red heat, are not successful. A major portion apparently exists in a combined form, perhaps in the ganguc'as a carbonate of calcium or magnesium. Then too, the carbon may exist in free state as in slivers of wood, dirt and the like in the ore.

In particular, for the reliable evoitiance of residual amounts of carbon in the final chromium-nlckel stainless steel melt, I charge the chrome ore onto the furnace banks, this however only in such quantity as may be assimillated during the meltdown period. In of extremely low carbon content, it is important that substantially no chrome ore charged onto the furnace bottom and side-walls remain unassimilated after the meltdown, otherwise it at a time when it may not be eliminated. For a lfi-ton Heroult furnace suited to the production of an 11 or 12-ton heat. the chrome ore of the initial charge usually does not exceed 3,500 to 4,000 pounds. I add the remainder of the ore, in fact the major portion of the ore, at a subsequent stage of the operation as appears more fully hereinafter. Where an extremely low carbon content is not required, larger quantities of chrome ore preferably are included in the initial charge in order to take advantage of the heat available throughout the prolonged melting period.

As a further operation in charging the furnace. I spread a thin layer of base steel scrap over the furnace bottom to facilitate melting. Onto this scrap I pile a substantial amount of nickeloxide with or without iron oxide, this oxidizing material being located in the middle of the furnace and inwardly of the furnace electrodes, care being taken, however, that the piled oxide is not placed immediately beneath the individual electrodes otherwise electrode consumption would be excessive and efficiency of the oxide greatly sacrificed. I find that the nickel oxide gives a quicker and more effective decarburization during the meltdown than does iron oxide. It thus saves time in the elimination of carbon. On the other hand, the presence of nickel oxide up to largely prevailing amounts is a relatively cheap oxidizing material.

The form of nickel oxide which I prefer for use is of substantially pure grade (as for example 0.6% cobalt, 0.3% iron, 0.2% sulphur, 0.25% manganese, a trace of phosphorus, 1% to 2% magnesium, 0.5% acid insoluble SiO2, and not more than amounts which require spectrographic measurement each of copper and zinc. The substantial freedom from serious contamination by the nickel oxide allows more latitude in quality of other ingredients in the charge for achieving a desired analysis of the melt. Moreover, it permits the production of stainless steel which is virtually free of sulphur and phosphorus.

After charging the nickel oxide, I bank the heavy scrap, mostly stainless steel scrap either or both of straight chromium or chromium-nickel grade but also including base scrap where desired, over the chrome ore, and over the base steel scrap on the furnace bottom. I employ any convenient amount of stainless steel scrap in the charge, this advantageously ranging from approximately 20% to by weight of the metal tapped. This addition is disposed outwardly of the furnace electrodes. My relative placement of base scrap,

from the standpoint of assuring minimum pickpro'iucing the steel conditions of the electric a rmen up of carbon from the furnace electrodes but from the point of assuring such melting of the chrome ore as to permit elimination of the carbon contained therein as more particularly referred to hereinafter. J

The amounts of nickel oxide which'may be used in the furnace are controlled by such considerations as the percentage of nickel expected in the final melt, by the carbon specification, and by the nickel and carbon content of the scrap metal used in the charge. As pointed out above, the nickel oxide may be relied upon wholly as the oxidizing agent, or may be used in conjunction with other oxidizing agents such as iron ore con-v centrates or mill scale. It is difficult, therefore, to present a precise definition of the exact limits of the usable amounts of nickel oxide; these amounts are a matter of circumstance and usually depend upon each individual case. In general, however, the oxide material, whether it be nickel oxide alone, or nickel oxide and iron oxide together, usually amounts to at least 15% of the tapping weight of the metal.

Following the achievement of the charging operation, I melt down the charge and bring the resulting bath of metal to a relatively high tem- 6 tic'ularly being that carbon picked up in the subsequent periods undesirably persists in the finished chromium-nickel stainless steel as mentioned above. This, it will be realized, is of major importance in producing stainless steel of extremely low carbon content.

As a matter of further importance in the process, however, there are considerable quantities of chrominum, nickel and iron available in the slag in the form of oxides of these metals which have accumulated during the melting operation. I recover these metals substantially completely from the slag all with a minimum of carbon contamination. This I achieve in a reducing period as, for example, through the addition of ferrosilicon, or other suitable silicon-containing reducing I material with or without the ferrosilicon; the

perature. The oxidizing material, comprising the nickel oxide, actively fluxes the chrome ore in the .charge thus forming a supernatant slag on the metal. This fiuxing action, enhanced by the nickel oxide, I consider to be highly important for the efiective release and prompt oxidation of carbonaceous material from the chrome ore in producing my extremely low carbon chromiumnickel stainless steel. aid of iron oxide where desired, behaves as an energetic oxidizing agent. Its presence in the slag materially assists in preventing carbon from the furnace electrodes from passing into the metal. The slag oxidizes that carbon which comes from the scrap in the charge. Of further importance the oxidizing material in the charge also tends to eliminate any carbon which may be picked up from the electrodes as during the first stages of the melting operation before the oxidizing slag forms.

The melt resulting from the charge desirably comes to a natural boil under high temperature furnace. It is during this boil that supplemental amounts of chrome ore are charged into the furnace as a further source of chromium, all without hurtful pick-up of carbon from the ore. For steel of extremely low carbon content the major portion of the chrome ore concerned in the process is introduced at this time. For stainless steel of other grades, however, as indicated above, the major portion of the chrome is introduced amount then is added at this point. In both instances, I introduced the quantities of chrome ore in the boiling metal; it is fluxed at intense heat by the strongly oxidizing slag bearing the nickel oxide. With the aid of the nickel oxide present, more slag forms and a prompt and thoroughly effective oxidation of carbon ensues under the continued high temperature. During this period of assimilation of supplemental ore and elimination of carbon it is expeditious to rabble the bath.

The carbon content is at a very low level at the successful conclusion of the meltdown period, usually somewhere in the approximate range of 0.01%, or less, up to about 0.02%. From this point on, one of the outstanding problems is that of precluding substantial carbon contamination of the melt the problem further and more par- The nickel oxide, with the n tained from a different source,

amount added generally being considerably in excess chemically of the oxides of chromium, nickel and iron involved in the reduction.

I prefer to use at least some ferrosilicon during the reducing period for the material ordinarily is readily procurable, yet enables a highly effective yield of chromium, nickel and iron from the slag. A highly important consideration in the production of extremely low carbon stainless steel through the use of silicon-containing reducing agents, however, resides in the discovery that commercial grades of these materials, particularly ferrosilicon, usually contain such amounts of carbon as to contaminate the melt to a harmful extent. As mentioned in my copendingapplication for patent referred to above, it is the finer material in size, considering the various particle and lump sizes of crushed ferrosilicon and the like, which contain carbon in greatest quantity. For example in initially crushed ferrosilicon, lumps ranging up to about five-inch screen size or more, yet which remain on a screen having openings, of about one square inch in dimensions, that is, not less than one-inch screen size, usually contain far less carbon collectively than those which are smaller and pass through the screen. An analysis of crushed 75% ferrosilicon graded as above, in one typical instance showed the lump material to include 0.04% carbon as distinguished from 0.095% in the fines. Another batch of crushed 75% ferrosilicon ohand also graded as above, contained 0.013% and 0.043% in the fines. ferent 75% A sample of still differrosilicon contained 0.036% carbon in the lumps while carbon in the fin-es amounted initially; a minor As a preferred measure in producing steel of extremely low carbon content, therefore, I add to the slag blanket a reducing agent comprising effective amounts of silicon reducing agent, preferably ferrosilicon, which has been substantially relieved of fines by screening or the like, for example lump ferrosilicon of a screen size ranging from about one or two inches up to five inches or more. This graded quality of ferrosilicon, which incidentally may if desired be a product of a plurality of crushing-and screening operations wherein the fines have been removed before each crushing treatment, I find is especially useful in avoiding carbon contaminatozi of the melt during the reducing stage.

I usually add burnt lime to the slag a further ingredient during-the reducing stage. It has a negligible carbon content. The addition generally improves efl'ici'ency of the reducing action and minimizes silicon contamination of the bath. In this connection, I prefer to include carbon in the lumps reducing agent as it melts after addition. I

prefer to maintain a high temperature in the furnace for increased activity; also a long are between the furnace electrodes and my extremely low carbon bath to minimize carbon pick-up while the agent exerts its reducing effect upon the slag. The reduction results in the recovery of valuable quantities of metallic chromium, nickel and iron from the slag which gravitate to enrich the underlying bath. I then bring thereducing period to an end, that is, after all the reducing agent and burnt lime have been added, fused, and their reactions have continued to the substantially complete recovery of metallic values from the oxides in the slag without harmi, This, of course, is considerably higher than is ful contamination of the melt with carbon.

Immediately following the .reducing period I find it preferable to draw off the residual slag and thereafter to prepare a basic finishing slag of lime and fluorspar on the metal surface. At this point, I usually add such materials as lowcarbon lump ferrosilicon, again preferably of the screened or graded type described hereinbefore, low-carbon ferromanganese, ferronickel, electrolytic nickel or other alloying materials of sufiicient purity and content to bring the melt to a i I then tap the resulting exdesired analysis. tremely low carbon chromium-nickel stainless steel. The tapped stainless steel has a maximum carbon content of 0.07% and usually appreciably lower than this. Where a minimum of chrome ore is initially employed, with the major portion being added during or after meltdown, the carbon content of the tapped metal ordinarily does not exceed 0.03%..

A manner of producing 0.03% maximum carbon chromium-nickel grade stainless steel having a maximum chromium content of 18% and 11.5% to 12.5% nickel, will serve as a specific illustration of the practice of my invention. In F this instance, I employ a l6-ton Heroult electric arc furnace. The furnace is equipped with the usual carbon electrodes, as for example amorphous carbon or graphite electrodes. It preferably has a chromite brick bottom. The side wall lining also preferably is of chromite brick extending well above the slag line. The hearth lin-' ing is inevitably eroded by the slag and the metal bath, but the cost of refractories nevertheless is minimized by the recovery of chromium from the chromium oxide content of the eroded portion.-

As in ordinary practice the furnace roof, and side walls in upper portion, may be of silica brick.

I introduce in the furnace an initial charge consisting of about 4,000 pounds of chrome ore containing about 51% chromium oxide (CrzOa), this ore being laid down on the furnace banks, 5,175 pounds of plain low-carbon steel scrap spread over the furnace bottom, nickel oxide having available therein 1,750 pounds of metallic nickel, and 4,800 pounds of iron ore containing about 3,360 pounds of metallic iron, the nickel oxide and the iron oxide preferably being disposed on the scrap on the bottom of the furnew, and in the center thereof in such manner in the initial charge.

that the ore is not beneath individual electrodes. The placement of the materials, as described,

gives a more effective removal of carbon from the melt with the assistance of the nickel oxide and in addition assures decreased carbon pick-up from the electrodes of the furnace. i

With the turning on of furnace electrical power I initiate a high temperature meltdown of the charge. A slag-covered pool of boiling metal grows in contact with the nickel oxide by virtue of heat from the electrodes and with the continued application of power. I use about 360 pounds of flucrspar as a flux, preferably without lime, during the melting operation. As the melting progresses, I distribute about 6,000 pounds of chrome ore over the stainless steel scrap in the furnace to supplement the chrome ore added The supplemental ore comes into contact with the boiling bath, and the slag which is rich in nickel and iron oxides, serves effectively to flux and oxidize carbon from the ore as the melting further continues. I believe the temperature ofthe metal beneath the slag blanket to. be from about 3000 F. to 3300 F.

used in ordinary electric steel melting practice. The higher operating temperature combined with the nickel oxide effect and placement of materials renders the removal and exclusion of carbon from the melt more efiective. After the charge has been substantially melted, nickel oxide or iron oxide or both may be added to supplement the oxide material in the initial charge. On occasions, if desired, iron oxide may be employed exclusively of nickel oxide in the initial product, the reducing operation then may be started. Measured from the time of first applying the power, the melting of the illustrative chargetakes about 1 /2 to 2 hours for substantial completion. Following the period of time just mentioned, an analysis of the bath, for example shows the carbon content to be about 0.015%. I therefore initiate the reducing period. In so doing, I add to the bath about 11,750 pounds of hot dry burnt lime and approximately 3,740

pounds of lump low-carbon 75% ferrosilicon which has been relieved substantially of carbonaceous fines. As the reducing action progresses, I-add to the slag a further amount of the ferrosilicon, say about 850 extra pounds. A further reduction of metal values from the oxides in the slag ensues. At a still later time when the reducible oxide content has been lowered to a relatively small percentage, as for example to 2% or less, I draw oil the slag. This Ireplace with a basic finishing slag such as of lime and fluorspar, and then adjust the analysis of the bath with such materials as ferrochromium, ferromanganese, electrolytic manganese and nickel, all of minimum carbon grade. Such additions as those just noted as well as supplementary additions of copper, tungsten, vanadium, aluminum, titanium, columbium, molybdenum, zirconium, and the like, when used, may if desired be held over from the furnace to the ladle in accordance 0.62% silicon, 0.80% manganese, with sulphurand phosphorous eachlaround 0.015% and the remainder iron.

Thus it will be seen that in this invention there is provided an art of producing chromium-nickel stainless steel including the chromium-nickel steels of extremely low-carbon quality, in which the various objects hereinbefore noted, together with many thoroughly practical advantages are successfully achieved. It will be seen that the process lends itself to the eiflcient, economical and reliable manufacture 'of chromium-nickel stainless steel of desired low carbon content employing nickel oxide and numerous other raw materials, among which are chrome ore, all to maximum advantage in obtaining clean metal.

While in the illustrative example given above, chromium-nickel stainless steel scrap is used as a steel scrap source of chromium and nickel as well as of iron, it will be readily understood on this score, and with regard to other charge ingredients, that there is considerable permissible latitude as to the quality and quantity of ingredients which may be employed and as to replacement of the same with other materials, even in producing a given chromiummickel grade of my extremely low carbon steel. The chromiumnickcl stainless steel scrap, for example, may be replaced in Whole or in part by straight chromium stainless steel scrap, with dependence upon some other suitable source of nickel as where the quantity of nickel oxide desired for use does not afford an adequate supply. I r In the practice of my invention certain conveniences and advantages in production are enjoyable through the use. of chromium-nickel 10 2. In producing chromium-nickel stainless steel of low carbon content in an electric arc furnace having carbonaceous electrodes, the art which comprises melting down a charge consisting principally of a minor portion of chrome ore,

disposed along the furnace banks and bottom, base and stainless steel scrap thereon, and oxidizing material including substantial quantities of nickel oxide and iron oxide in the middle of the furnace; introducing a major amount of chrome ore during said meltdown, thereby forming a chromium-rich and iron-rich bath having a carbon content substantially less than 0.03%

and a supernatant slag thereon including chrostainless steel scrap as a furnace charge ingredient, particularly where the steels under production are to have substantial nickel contents, as for example those steels which are to contain say more than 6% nickel. Under certain conditions, therefore, I prefer the use of stainless steel scrap as a source of nickel in the furnace.

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

I claim:

1. In producing chromium-nickel stainless steel of'low carbon content in an-electric arc furnace having carbonaceous electrodes, the art which comprises melting down a charge consisting principally of chrome ore disposed along the banks of said furnace, base steel scrap over the furnace bottom, oxidizing material including substantial quantities of nickel oxide in the middle of the furnace and the scrap, and stainless steel scrap over the chrome ore; adding further amounts of chrome ore during said melting while maintaining the melted metal at a boil, thereby forming a chromium-rich and iron-rich bath having a low carbon content and a supernatant slag thereon including chromium and nickel oxing principally of chrome ore ides; and adding to said slag crushed ferrosilicon and nickel oxides; 00

mium and nickel oxides; and thereafter adding crushed ferrosilicon, thereby recovering the metallic chromium and nickel from their oxides in the slag.

3. In producing chromium-nickel stainless steel of 0.03% maximum carbon content in an electric arc furnace having carbonaceous electrodes, the art which comprises melting down a charge arranged for maximum carbon elimination and decreased carbon pick-up and consisting principally of chrome ore disposed along the furnace banks and bottom in such amount as to be de-carbonized in the melting operation, base and stainless steel scrap thereon, and oxidizing material including substantial quantities of nickel oxide on the scrap in the middle of the furnace but not immediately beneath individual electrodes, thereby forming a chromium-rich and iron-rich bath of molten metal having a carbon content on the order of 0.015% and a supernatant slag thereon including chromium and nickel oxides; and thereafter adding to said slag a suitable reducing agent of desired low carbon content for recovering the metallic chromium and nickel from their oxides.

4. In producing chromium-nickel stainless steel having a carbon 0.03% using chrome ore as a source of chromium in an electric arc furnace having carbon electrodes, the art which comprises melting down a charge arranged for maximum carbon elimina tion and decreased carbon pick-up and consist:

ing principally of a minor amount of chrome ore disposed in usable portion along the banks of said furnace, base scrap on the furnace bottom, stainless steel scrap over the ore, and oxidizing material including substantial amounts of nickel oxide of high purity on said base scrap and away from said electrodes; introducing a major portion of chrome ore during said meltdown while maintaining the melted metal at a boil, thereby forming an iron and chromium rich bath having a carbon content on the order of 0.015% and a supernatant slag thereon including chromium and adding to said slag a silicon-containing reducing agent of low carbon content for recovering the metallic chromium and nickel from their oxides.

5. In producing chromium-nickel stainless steel of low carbon content in an electric arc furnace having carbonaceous electrodes, the art which comprises melting down a charge consistdisposed along the banks of the furnace, base steel scrap on the bottom of the furnace, stainless steel scrap on the chrome ore, and oxidizing material including substantial quantities of nickel oxide on the base steel scrap, thereby forming a chromium-rich and iron-rich bath having a low carbon content and a supernatant slag thereon including chromium and nickel oxides; and adding to the slag content not exceeding a 1 l crushed ferrosilicon which has been substantially relieved of fines for recovering the metallic chromium and'nlckel from their oxides.

DONALD L LOVELESS.

REFERENCES CITED The tollowing references are of record in the file of this patent:

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