US2455073A - Production of stainless steel - Google Patents

Description

Patented Nov. 30, 1948 PRODUCTION STAINLESS STEEL Donald L. Loveless, Baltimore, Md., assignor, by

mesne assignments, to Armco Steel tion, a corporation of Ohio Corpora- No Drawing. Application November 14, 1945, Serial No. 628,673

7 Claims. 1

The present invention relates to the manufacture of high-chromium steels of extremely low carbon content, more particularly to a process for producing the same using chrome ore as a substantial source of chromium. While the composition of the low-carbon steels referred to may vary considerably, the invention is importantly directed to the production of stainless steels with chromium ranging from about 10% up to 85% or more and with a carbon content not exceeding Among the objects of my invention is the emcient and reliable production of stainless steel of extremely low carbon content and of required analysis of chromium, with or without supplementary additions of nickel, copper, molybdenum, titanium, tungsten, silicon, manganese, vanadium, aluminum, sulphur, or the like, utilizing in such production readily available and inexpensive raw materials including large quantities of chrome ore and employing known and reliable furnacing and operating equipment.

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

As conducive to a clearer understanding of certain features of my invention it may be noted at this point that of the heretofore known processes for the production of 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 expansive pre-alloy, was the accepted sole source of chromium for the melt, have rapidly gained popularity. This is particularly 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. They involve melting a charge including the chromium-bearing materials and iron oxide in a furnace under conditions to form a stainless steel bath which is rich in iron and chromium. Any chromium oxide present is subsequently reduced with ferrosilicon or like reducing agent to effect a recovery of the chromium.

In melting practices based on the consumption of chrome ore the chromium content is derived at minimum cost because it comes directly from its original source rather than from some more expensive pre-alloy or the like. chrome ore which may be employed, however, usually is limited by the furnace capacity. The quantity of ore handled must not exceed that permissible for proper melting conditions. Therefore, the chromium from the ore frequently issupplemented by chromium from such materials as stainless steel scrap.

With the adoption' of melting practices based on the consumption of stainless steel scrap 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 ingot butts, crop ends, and the like. These are successfully reclaimed by the remelting. Some further importance of the reclamation melting practices is represented by the statistical information that of the stainless steel used for processing into bar stock about 25% to 30% results in scrap. The total amount of scrap metal increases with the number of working or forming operations and amounts to some 40% to 50% in theproduction of sheet and strip and may even amount to some to where the sheet or strip is further fabricated into various ultimate articles of manufacture, such as machine or burner parts, kitchen ware, automobile trim, architectural applications and the like.

The processes wherein chrome ore serves as a useful furnace charge ingredient for the production of stainless steel, both of straight chromium grades and the chromium-nickel varieties, in many instances have proven to be suited for the electric arc furnace production of moderate and medium carbon quality stainless steel. Great difilculty, however, is encountered in adapting such processes to the commercial manufacture of stainless steels with a carbon content approaching the limit of solubility, that is about 0.02% to 0.025%. This is unfortunate because certain low-carbon steels, namely the austenitic chromium-nickel steels containing less than about 0.030% carbon, may be substituted satisfactorily for the more expensive stabilized steels such as 188 chromium-nickel steels of the columbium and titanium varieties. I find that the difficulty of obtaining a low-carbon grade of stainless steel enormously increases with the lowering, especiallyas the limit of carbon solubility is approached, and thereafter. In fact, I-find that although the carbon is fairly readily removed down to ap- The amount of proximately 0.05%. the processes usually do not function to effect a'fiirther removal.

An outstanding object of my invention accordingly is the eiiicient, reliable and economical production of stainless steel of extremely low carbon content, and of desired analysis directly employing chrome ore as asubstantial source of cliromium and using .for 'this purpose the conventional electric arc furnace having carbonaceous electrodes.

Referring now more particularly to the practice of my invention, I provide in an electric furnace, such as the conventional carbonaceouselectrode Heroult steel making furnace, a meltdown charge including chrome ore, stainless steel scrap, base steel scrap and a material high in iron oxide as for example roll scale or iron ore. The chrome ore advantageously is employed in such quantity as will avoid crowding the furnace in view of other ingredients in the charge and enable good melting contact.

In connection with the commercial grades of chrome ore employed, I have made the important discovery that such ore contains a material quantity of carbon,usually somewhere in the vicinity of 0.06% to 0.20%. A dozen difierent batches of chrome ore analyzed 067%, 065%, 064%, .089 92, 057%, .198%, 078%, 080%, 145%, 018%,

020% and .124% carbon. Moreover, this carbon is in such form that it is not eliminated by predrying or pre-heating even to a red heat. Apparently a major portion appears in a combined form, perhaps in the gangue as a carbonate of calcium or magnesium. Also, it may appear as free carbon existing as dirt, slivers of wood and i the like. I

I find that the ore requires special attention to enable its successful use in the production of an extremely low carbon stainless steel melt. In particular, for the reliable avoidance of excessive amounts of residual carbon in the melt, I charge the chrome ore onto the furnace banks, this however only in such quantity as may be assimi lated in the melting period. This generally will not exceed 3,500 to 4,000 pounds for a 12-ton Heroult furnace. The remainder of the ore, in fact the major portion of the ore. is added at a subsequent stage of the operation as appears more fully hereinafter. Where larger quantities of chrome ore are added in the initial charge, it

is my observation that this excess of ore is assimilated, not in the melting period but in the prolonged reducing period which follows. During that period the carbon coming into the bath is retained by the bath for the reason that carbon is eliminated only under oxidizing conditions. It is not eliminated under reducing conditions. Accordingly, in the finished metal there is had an undesirably high carbon content.

Continuing with the charging of the furnace, a thin layer of base steel scrap is spread over the furnace bottom to facilitate melting. Next, iron oxide is charged in a pile onto this scrap, this pile being located in the middle of the furnace and inwardly of the furnace electrodes, care being taken, however, that the iron oxide is not placed immediately beneath the individual electrodes otherwise electrode consumption would be excessive and the oxidizing efilciency of the iron oxide greatly sacrificed. Following the charging of iron oxide the heavy scrap, mostly stainless steel scrap but also including base scrap where desired. is banked over the chrome ore and steel scrap on the furnace bottom. This addition is placed outwardly of the furnace el ctrodes. The relative placement of base scrap. iron oxide, chrome ore and stainless steel scrap is important, not only from the standpoint of assuring a minimum pick-up of carbon from the furnace electrodes, but also from the point of assuring such melting of the chrome to permit elimination of the carbon contained therein as more particularly referred to above.

As for the stainless steel scrap in the charge, I employ any convenient amount, this advantageously ranging from approximately 20% to of the tapping weight of the metal. The iron oxide-containing material such as the iron ore or the roll scale, ordinarily amounts to at least 20%, this also based on the tapping weight of the metal.

Upon completion of the charging operation, I melt down the charge and bring the resulting bath to a relatively high temperature. The roll scale and the chrome ore form a supernatant slag on the metal, the chrome ore in the charge being fluxed by the iron oxide. This I consider to be important because thorough fluxing of the chrome ore appears necessary to its release of carbonaceous material and consequent oxidation of the same. The iron oxide in the roll scale or iron ore employed behaves as an energetic oxidizing agent. Its presence in the slag serves as a carbon oxidizin barrier to prevent the carbon from the electrodes from passing into the metal. Moreover, the iron oxide in the charge also tends to oxidize any carbon which may get into the charge from the electrodes during the initial stages of the melting operation before the formation of the oxide-containing meltdown slag. Of further importance, as pointed out above, the slag oxidizes the carbon coming from the chrome ore, also whatever carbon comes from the scrap material in the charge.

The bath desirably comes to a natural boil under the high temperature imparted by the electric furnace. It is during this boil in the production of my low-carbon stainless steel, I find, that supplemental amounts of chrome ore are useful as a source of chromium without excessive carbon pick-up from the ore. In fact, the major portion of the chrome ore employed preferably is added during this period or, in any event, subsequent to the initial meltdown. In this connection. I introduce the supplemental quantities of chrome ore in the boiling metal; it is fluxed by the intense heat and the high iron oxide content of the slag. More slag forms, and the oxidation of the carbon content of the ore takes place in a thoroughly efiective manner under the continued high temperature. The bath is continuously rabbled durin this period.

At the successful conclusion of the melting period the carbon content of the bath is at a very low level, usually somewhere in the approximate range of 0.01%, or less. up to about 0.02%. One of the outstanding problems confronted at this point is that of maintaining the low carbon level or precluding appreciable carbon contamination during subsequent operations; the problem further and more particularly being that carbon picked up during the subsequent periods undesirably persists in the finished stainless steel as mentioned above. In this connection there are available in the meltdown slag considerable quantities of chromium and iron 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 minimum carbon contamination. This is achieved in a reducing period, preferably through the addition of ferrosilicon, or other suitable silicon-containing reducing material, either with or without the ferrosilicon, the amount added generally bein considerably in excess chemically of the oxides of iron and Chromium in the slag.

In the reducing operation I prefer to use at least some ferrosilicon for the material ordinarily is readily available and enables a highly satisfactory yield of chromium and iron from the slag. Of great importance, howeverfl have discovered that the commercial grades of silicon-containing reducing agents, particularly ferrosilicon, contain enough carbon to contaminate the melt to a harmful extent in the production of my low-carbon stainless steel. More especially, I find that of the various particle and lump sizes of crushed ferrosilicon it is the finer material in size which contain the most carbon.

In initially crushed ferrosilicon, I find, for example, that lumps ranging from one-inch up to about five-inch screen size or more and which remain on a screen having openings of about one square inch in dimensions, that is, a one-inch screen, usually contain far less carbon collectively than those particles which pass through the screen.- An analysis of crushed 75% silicon, graded as above, in one typical instance showed the lump material to contain 0.04% carbon as distinguished from 0.095% in the fines. Another batch of crushed 75% ferrosilicon obtained from a different source, and also graded as above, contained 0.013% carbon in the lumps and 0.043% carbon in the fines. A sample of still different 75% ferrosilicon contained 0.036% carbon in the lumps and 0.208% carbon in the fines.

In the reducing operation, therefore I preferably add to the slag overlying the molten metal, a reducing agent comprising effective amounts of crushed, or otherwise divided, ferrosilicon for example lump ferrosilicon of a screen size ranging from about one or two inches up to five inches or more. The ferrosilicon in any event is one which has been relieved of substantially all fines as by screening. 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 note is especially useful in avoiding carbon contamination of the melt during the reducing stage.

Along with the silicon-containing reducing agent, I usually add burnt lime. Its carbon content, I find, is negligible. This addition generally improves the efiiciency of the reducing action and minimizes silicon contamination of the bath; I

prefer to add the lime in amounts from about 3 to 5 times the silicon content of the siliconcontaining reducing agent added. It is convenient to mix the lime and reducing agent together on the melt shop fioor prior to charging, and then to charge the mixture onto the slag from time to time as furnace conditions permit. As a further preferred measure I carry out the reducing operation, while maintaining a high furnace voltage and a long are to minimize carbon pick-up from the furnace electrodes.

As the added materials melt down, the chromium and iron oxides are acted upon by the reducing agent. and consequently valuable quantities of metallic chromium and metallic iron are recovered and gravitate from the slag to enrich the low-carbon metal bath. At the successful been added, fused, and their reactions with the slag ingredientshave continued to the substantially complete recovery of chromium from the oxides in the slag without harmful contamination of the melt with carbon, I bring the reducing period to an end.

With the conclusion of the reducing period I find it preferable to Withdraw the residual slag from the low-carbon bath and thereafter prepare on the metal surface a basic finishing slag of lime and fluorspar. At this point, I usually add such materials as low-carbon lump ferrosilicon, again preferably of the screened or graded type described hereinbefore, low-carbon ferromanganese, or other alloying materials need to bring the melt to desired analysis. I then tap the resulting low-carbon stainless steel. The tapped metal is a stainless steel with a carbon content not exceeding 0.03%, and usually appreciably lower than this.

As a specific example of the practice of my invention, production of the following heat will serve. For this purpose, there is employed a 16- ton Heroult electric arc furnace. The furnace has the usual carbon electrodes. as for example amorphous carbon or graphite electrodes. It preferably has a chromite brickbottom. The side wall lining also preferably is of chromite brick extending to well above the slag line. The hearth lining 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. The furnace roof and upper side walls may be of silica brick, in accordance with ordinary practice.

In the p"oduction of a 0.03% maximum carbon chromium-nickel grade of stainless steel having a, minimum chromium content of about 18% and 11.5% to 12.5% nickel, I introduce in. the furnace an initial charge consisting of about 4,000 pounds of chrome ore containing about 51% chromium oxide (CrzOa), the 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 iron oxide preferably being disposed on the bottom of the furnace in the center thereof in such manner that the ore is not beneath individual electrodes, and finally 7,200

pounds of 18-8 chromium-nickel stainless steel scrap over the chrome ore and base scrap and adjacent to the furnace electrodes. The place ment of the materials, as described, gives a more effective removal of carbon from the melt and in addition assures decreased carbon pick-up from the furnace electrodes.

I apply the furnace power and thus begin a high temperature meltdown .of the charge. A slagcovered pool of boiling metal grows beneath the electrodes with the continued application of power. I use about 360 pounds of fluorspar 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 in the initial charge. The added ore comes in contact with the boiling bath, which serves effectively to oxidize the carbon in the ore as the melting further continues. The temperature of the metal beneath the slag I believe to be in the order of 3100 F. or higher. This is considerably higher than is used in ordinary electric steel melting practices. The high temperature renders the removal and exclusion of carbon from the melt by the slag effective. After the charge has been substantially melted, additional iron oxide-bearing material such as mill scale or iron ore may be added to the slag ifrequired. Measured from the time of first applying the power, the melting of the illustrative charge takes about 1 /2 to 2 hours for substantial completion.

When analysis of samples taken from the oath shows that the carbon content of the bath is sufficiently low, usually somewhat below maximum content desired in the stainless steel product as finished, the reducing operation then may be started. In the present instance, analysis shows the carbon content of the bath to be about 0.015%. I therefore 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 continues I add to the slag about 850 extra pounds of lump low-carbon 75% ferrosilicon, preferably substantially relieved of fines. A further reduction of metal from the oxides in the slag ensues with the melting of the ferrosilicon. When the reducible oxide content has been lowered to a relatively small percentage, 2% or less of iron and chromium oxides, I draw off the slag. A basic finishing slag such as'of lime and fluorspar then is formed on the bath. Thereafter I adjust the analysis of the bath with such materials as ferrochromium, ferromanganese, electrolytic manganese and nickel, all of minimum carbon grade. The heat advantageously is finished as rapidly as possible to prevent pick-up of carbon from the furnace electrodes. I then tap the heat. In the case of the illustrative example, there are had stainless steel ingots weighing 21.480 pounds and analyzing about 0.029% carbon. 18.02% chromium, 11.68% nickel, 0.62% silicon, 0.80% manganese, with sulphur and phosphorus each around 0.015% and the remainder lIOn.

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

Thus it will be seen that in this invention there is provided an art of producing stainless steel of extremely low-carbon quality in which the various objects hereinbefore noted, together with oany thoroughly practical advantages are successfully achieved. It will further be seen that the process lends itself to the efficient, economical and reliable manufacture of stainless steel employing a maximum of available and inexpensive raw materials, including large quantities of chrome ore, consistent with the production of clean metal.

I claim:

1. In producing stainless steel of 0.03% maximum carbon con-tent 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 banks of said furnace, and in such amount as to be assimilated in the melting operation, base steel scrap and stainless steel scrap, and a material high in iron oxide content, thereby forming an iron and chromium rich bath having a low carbon content and a supernatant slag thereon -including iron and chromium oxides; and adding to the slag crushed ferrosilicon of low-carbon content relieved substantially of fines of less than about one-inch screen size for recovering the metallic iron and chromium from their oxides.

2. In producing stainless steel of 0.03% maximum carbon content using chrome ore as a principal source of chromium 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 banks of said furnace, and in such amount as to be decarbonized during the melting operation, base steel scrap over the furnace bottom, a material high in iron oxide in the middle of the furnace and 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 an iron and chromium rich bath having a low carbon content and a supernatant slag thereon including iron and chromium oxides; and adding crushed ferrosilicon, relieved substantially of relatively high-carbon fines of less than approximately one-inch screen size, for recovering the metallic iron and chromium from their oxides in the slag.

3. In producing stainless steel of 0.03% maximum carbon content in an electric arc furnace having carbon electrodes, the art which comprises melting down a charge arranged for maximum carbon elimination and decreased carbon pick-up and consisting principally of a minor portion of chrome ore disposed in usable quantity along the banks of said furnace, base and stainless steel scrap, and a material high in iron oxide in the middle of the furnace; introducing a major amount of chrome ore during said meltdown, thereby forming an iron and chromium rich bath having a carbon content substantially 1ess than 0.03% and a supernatant slag thereon including iron and chromium oxides; and thereafter adding crushed f errosilicon, relieved substantially of relatively high-carbon fines of less than approximately one-inch screen size, thereby recovering the metallic iron and chromium from their oxides in the slag.

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

5. In producing stainless steel having a carbon content not exceeding 0.03% using chrome ore as a principal source of chromium in an electric arc furnace having carbon electrodes, the art which comprises melting down a charge arranged for maximum carbon elimination and decreased carbon pick-up and consisting essentially of a minor amount of chrome ore disposed in usable quantity along the banks of said furnace, base scrap on the furnace bottom, stainless steel scrap over the ore, and a material high in iron oxide on said base scrap and away from said electrodes; introducing a major amount 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 iron and chromium oxides; and adding to said slag a silicon-containing reducing agent in lump form and of low carbon content for recovering the metallic iron and chromium from theiroxides.

6. In producing stainless steel having a carbon content not exceeding 0.03% in an electric arc furnace having carbon electrodes, the art which includes forming in said furnace an iron and chromium rich bath of molten metal having a low carbon content and a supernatant slag thereon including iron and chromium oxides, and adding to said bath and slag a crushed silicon-containing reducing agent relieved substantially of fines of less than about one-inch screen size for recovering the metallic iron and chromium from their oxides without contaminating the bath with appreciable carbon content.

'7, In producing stainless steel having a carbon content not exceeding 0.03% in an electric arc furnace having carbon electrodes, the art which includes, forming in said furnace an iron and chroinium rich bath of molten metal having a sufficlently low carbon content and a supernatant DONALD L. L08.

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

UNITED STATES PATENTS Number Name Date 2,276,074 Vignos Mar. 10, 1942 FOREIGN PATENTS Number Country Date 437,186 Great Britain Oct. 23, 1935