US1812941A - Manufacture of stainless iron - Google Patents

Description

Patented July 7,1931

.UNITED STATES PATENT OFFICE ALExANnnR L. mum), on cANToN', 01110, ASSiGNCR, BY uns'NE .assreNurnNars,v TO nnrunmc s'rnm. conronarIoN, or YOUNGSTOWN, 01310, A CORPORATION on NEW MANUFACTURE OF STAINLESS IRON No Drawing. I Application filed December 18, 1926, Serial No..155,767. Renewed January 14, 1931. I

This invention relates to the manufacture of those-low-carbon, high-chromium steels,

characterized by resistance to corrosion, tarnish and heat, and which, because of these properties and their low carbon content, are variously designated as stainless, stable-surface or rustless irons.

It has hitherto been customary in the commercial manufacture of these irons to proceed by way of. the more or less obvious method of adding to a molten bath of low-carbon steel the necessary quantity of chromium as low-carbon ferro-chrome, the carbon content of the ferro-chrome being low enough to yield by carefully conducted melting operations a product of satisfactory analysis so far as carbon and chromium are concerned. In

the manufacture of a stainless iron containing approximately 17% chromium and less than 0.10% carbon, it has been necessary, for instance, to use a low-carbon ferrochrome containing not over 0.10% carbon and approximately 72% chromium.

For the manufacture of iron-chromium alloys. within the stainless iron range of analysis, low-carbon ferro-chrome is a relatively expensive raw material. High carbon ferro-chrome of the 4 to 6% carbon grade, which is used so extensively in the manufacture of ordinary chromium steels, may be had at a price, per pound of contained chromium, which is somewhere between one-third and one-half that of the low-carbon ferrochrome. The commercial advantage of employing high-carbon ferro-chrome instead of low-carbon ferro-chrome, if a suitable method of utilizing the former could be devised, has long been apparent; and it ha "been equally apparent that such a'process'gwould of'necessity involve decarbonization. A mixture of carbon-free iron with an amount of 4% carbon ferro-chrome sufiicient to yield a 12% chromium alloy would under ideal conditions of simple melting give rise to a product containing approximately 0.68% carbon. Under similar conditions, a 6% carbon ferro-chrome, admixed with pure iron togive an 18% chromium alloy, would lead to a carbon. content of approximately 1.53%. In both instances the high-carbon ferrochrome has been assumed to contain chromium. Accordingly, in the interests of lower manufacturing costs, methods based on decarbonization and on the employment of hilgh-carbon ferro-chrome have been propose I have found by observation and experiment that such methods as have been hitherto proposed are either inoperative, or are characterized by extremely grave operating difiiculties.

the low value required and the ineflicacy of the procedures followed to restrain oxidation of chromium. Heats conducted in accordance with such processes are unduly prolonged, furnace refractories are subjected to One of the chief difficulties isthe slow rate of decarbonization as the carbon content of the metal bath falls toward" severe wear and tear, slag volume tends to in- My inventionrelates to a method for the manufacture of low-carbon, high-chromium ferrous alloys of the type known as stainless irons from chromium alloys relatively high in carbon. It relates in particular to the manufacture of commercial stainless irons, containing 12 to 18% chromium and not more than- 0.12% carbon from high-carbon ferrochroine of the 4 to 6% carbon grade, i. e. the common co mercial product. This method embraces and is based upon certain new and novel features not hitherto described or employed in the art and constitutes a distinct advance in stainless iron metallurgy so far as manufacturing costs are concerned.

My method of manufacture is characterized by the following novel procedures: (A) Completing decarbonization of the entire charge high-carbon ferro-chrome with from 6 to 11.5% chromium in the metal bath, as against 12 to 18% chromium in the case of prior processes, thereby decreasing by approximately one-half the retarding effect of chromium on rate of final carbon removal from the metal bath.

(B) Conducting decarbonization by means of molten iron ore or roll scale, in absence of additions of lime or other basic material chemically inert so far as oxidation of carbon is concerned, as against decarbonization by means of a-limey or strongly basic slag in the case of prior processes, thereby bringing about rapid removal of carbon from the metal bath.

(C) Repressing chromium oxidation and its loss to the decarbonizing slag entirely by controlling within certain limits the proportion of iron ore or roll scale charged.

(D) Conducting decarbonization on approximately that amount of high-carbon ferro-chrome which is required on the basis of practically completerecovery of chromium in'the finished product, as against conducting decarbonization on a large excess of high-carbon ferro-chrome, thereby decreasing the time and expense of decarbonization.

(E) subjecting to a silicon reduction operation a non-basic slag, high in reducible oxides and formed by the action of iron ore or'roll scale on a metal bath in which highcarbon ferro-chrome has been incorporated, by means of additions of lime and ferrosilicon, thereby avoiding unnecessary dilution of the reducible oxides and promoting a rapid rate and high efliciency for the reduction operation.

(F) By means of the silicon reduction operation above-mentioned, returning a molten high-chromium iron-chromium alloy, substantially higher in chromium than the commercial stainless irons, to the molten low-chromium iron-chromium alloy obtained at the end of decarbonization and increasing the chromium in the metal bath-from an initial value of 6 to 11.5% to a final value of 12 to 18%, thereby producing the desired stainless iron product by the rapid admixing of two molten iron-chromium'alloys, neither of which is of commercial stainless iron type and each of which is produced under condi-- tions which have unique advantages in regard to production economy.

.(G Obtaining the desired stainless iron product under a non-decarbonizing, non-carbonizing slag and bringing the metal bath for the first time to a stainless iron composition with respect to carbon and chromium under a slag of the aforesaid type, thereby insuring an accuratecontrol of-the analysis of the final product and an almost complete recov-' ery'of chromium.v

-My process comprises two distinct and separate steps in operation. In carrying out the first step, high-carbon ferro-chrome is incorporated in the bath under conditions which result' (a) in elimination of carbon by oxidation and (b) in the distribution of the chromium" originally contained in the. ferro-chrome between the metal bath and the slag. The second-step of the process is.

carried out in the same furnace without removlng the slag and consists in reducmg the oxidized chromium from the slag by means of a metallic reducing agent and recovering it in the metal bath below. In addition to the initial charge of low-carbon steel which serves as a base, the materials employedin the first or oxidizing step are high-' carbon ferro-chrome and roll scale (or iron ore). In carrying out the second step, line and a metallic reducing agent, preferably pulverized per cent ferro-silicon, are the .raw materials.

porated in the bath, nor have I found it possible, regardless of chromium loss, to pro.- duce by the first step alone a product with a sufficiently high-chromium content (12% or more) and a .sufliciently low-carbon content (0.12% or less) to, fall within any of the commercially accepted analysis limits for stainless iron. The second step alone makes possible .the attainment of an acceptable stainless iron analysis and at' the same time reduces the chromium loss to such a relative- I 1y low figure that the commercial advantages of the process in competition with established processes are immensely enhanced.

In order that the principles and practice involved may be more clearly understood I shall now describe in some detail how the invention has been successfully applied in actual operation.

In carrying out ployed'a 3-ton I-Icroult furnace of standard design equipped with a Westinghouse automatic regulator and preferably operated the. process I have em-- with three 12-inch carbon electrodes on 3- phase current at -volts. The bottom lining may be of any suitable refractory. The roof is constructed of silica brick in accordance with regular practice.

steel scrap, preferably of low-carbon content, is charged directly on the furnace bottom and melted down in absence of nonmetallic or slag-forming additions of any sort. The resulting metal bath carries a light slag resulting from the oxidation of the charge but this slag, as a rule, is not sufiicient in amount to cover the surface of the metal.

When the bath has reached a rather high debon and 'ferro-chrome has gree of super heat (about 3250 iron ore, preferably of low silica content, or clean roll scale is charged into the furnace in relatively large amount. The amount of iron ore or scale used depends primarily upon the final chromium content of the stainless iron and is roughly proportional to the latter. In the manufacture of a product analyzing 16.5 to 18% chromium I have employed between 1400 and 1750 lbs. of roll scale per ton (2240 lbs.) of steel scrap charged or 700 to 875- lbs. per ton of stainless iron produced. The roll scale readily melts to form a fluid slag, and causes a rapid drop in the carbon content of the molten steel to a very low figure (approx. 0.02'0.04% carbon) in case the scrap initially contained appreciable carbon. Highcarbon ferro-chrome analyzing 4 to 6% carpreferably in lump form is next charged into the furnace.

is added gradually to avoid undue chilling of the bath and also to prevent the vigorous boiling? action from jecting the slag and metal through the doors. This boiling ac tion is similar both in appearance and in origin to the well-known carbon boil in the open-hearth furnace and is due to the reaction between the carbon originally contained in the ferro-chrome and the iron oxide in the bath. In making stainless iron of 17% chromium and 0.10% carbon content from highcarbon ferro-chrome analyzing 66% chromium and 5.9% carbon, the total ferro-chromc charged amounts to approximately 1180 lbs. per ton of steel scrap charged or to 590 lbs. per ton of stainless iron produced. The electrodes are raised during a violent boil in order to avoid undue electrode consumption and possible contamination of the bath With carbon from the electrodes.

The metal bath should be maintained at a sufficiently high temperature and theslag suificiently high in iron oxide to cause the carbon content of the metal bath to remain always at a comparatively low value. Toward the completion of this procedure of charging high-carbon ferro-chrome the rate of carbon oxidation will be retarded appreciably if there is any deficiency in the amount of roll scale or ore originally charged. In such an event, a supplementary quantity of roll scale may be charged in order to bring the carbon down to the desired percentage.

en the entire quantity of high-carbon been charged, the carbon content is brought down, by an ore addition if necessary, to the percentage desired in the finished product, and preferably 0.010.02% lower than the maximum specified. After practice has been established in the case of any particular furnace and grade of raw materlals, such a supplementary ore addition is unnecessary and, in fact, becomes undesirable, since it prolongs the decarbonizing peri d The ferro-alloy.

I oxidizing Since a low silica content is desired in the oxidizing slag, I prefer to employ a high-carbon ferro-chrome which does not exhibit an unduly high silicon content. For the same.

reason, I prefer clean roll scale to iron ore; the latter when used should be low in silica. To avoid excessive drippings from the silica roof which are possible at the high temperature at which the bath is worked during the period, I prefer not to prolong this period of operation beyond the minimum time required. The oxidizing slag at the conclusion of the first step will generally contain 5 to 20% SiO 0 to 15% CaO, 3 to 15% A1 0 1 to 3% MnO, 0 to 5% MgO, and from to 80% of the oxides of iron and chromium combined. There is a consistent increase in the total amount by weight of chromium oxide in the slag throughout this period of operation, but the actual percentage at any particular time depends largely on the manner in which the ore or scale is added.

I am of the opinion that, when high-carbon ferro-chrome is added to the bath, a large proportion of its contained carbon may be oxidized and escape as a gas, concurrently with the melting of the lumps of ferrochrome, without actually becoming thoroughly incorporated and dissolved in the metal bath. Oxidation of the chromium content of the ferro-chrome probably proceeds side by side with the oxidation of carbon, so that a part of the chromium may never become diffused through the metal bath. When the iron oxide content of the slag becomes relatively lowered due to its reaction with the metal' bath and entry of -metal in finally brought down to 0.10%, it

will be generally observed that approximately one-half by weight of the chromium added a stainless iron sively that such a reaction never proceeds and,

there are good theoretical grounds for concluding that it cannot procee under the operating conditions process. The total weight of chromium in the metal bath'during the oxidizing period never suffers an increase except what is due directly to an addition of high-carbon ferrochrome.

Analyses of samples taken from the metal bath at various times during the oxidizing period of representative heats, together with the chromium oxide and iron oxide content of several slag samples taken at the particular times in question are given below:

Metal bath Slag Carbon Chromium CnO F010;

Per cent Per-cent Per cent Per cent Thetabulated data above indicate clearly what importance attaches to each 0.01% of carbon within the stainless range. It is readily possible to manufacture by my oxidizing step'alone an alloy analyzing 17% chromium and 0.17 018% carbon and with a loss of added chromium amounting to only about In going to an alloy of 0.10% carbon content, the maximum chromium obtainable such an alloy by my oxidizing step alone is in the neighborhood of 10%, with a loss of added'chromium amounting-to approximately 50%. '1-

I now come to the second step of the process. This stage of operation is begun with a highly heated metal bath above which is a slag containing chromium oxide oxide as essential constituents. The metal bath should contain 6 "to 11.5% chromium and not more than 0.12% carbon. The chromium oxide content of the slag is derived, by oxidation during the first step of operation, from the CllIOIIlllllIl originally contained in the high-carbon ferro-chrome. The addition agents used during the second step of operaburnt lime (Ca()) and pulverized tion are ferro-sihcon. The exact manner in which oxidized and carbon and 17% which characterize this and iron be varied at these additions are made may However, I

will to a considerable degree. prefer to make an which is spread over the surface of the bath. A mixture of lime and pulverized ferro-silicon is then fed gradually into the bath, preferably around or beneath the electrodes. The pulverized ferro-silicon reduces the chromiuni and iron oxide contained in the slag and forms silica, which acidic oxide isin turn neutralized, or else combines with, the lime present. As a result, the slag which at the beginning of this operation was black or dark brown'in color due to its high percentage of iron andchromium oxides gradually verges towardsa basic character and acquires a lighter color. High recoveries of chromium may be obtained without actually converting the original black slag into a greenish or greyish-white basic slag of the disintegrating type. The ratio of lime to ferro-silicon (50%) which leads to the best results lies be-.

initial addition of lime slag at the end of the reducing period lies as a rule between 25 and Since the metalbath at the end of the oxidizing period'contains a substantially lower percentage 0t chromium than that required in the stainless iron product, it is a. requirement of my method of manufacture that the metal returned to metal bath from the slag by silicon reduction must be of suflicient total weight and sufliciently high in'its percentage of chromium to 'enrich the decarbonized metal in chromium to the desired degree. In order that this requirement be met, the relative proportions of iron or steel scrap, high carbon ferro-chrome, and iron ore or roll scale incorporated in the bath prior to silicon reduction must be carefully adjusted in the manner already described above. When a reducing agent containing iron, such as ordinary 50 per cent ferro-silicon, is employed, the metalreturned to the metal bath contains roughly 35% chromium.

The metal ath at the completion of the reducing period should be low in silicon, and as a rule does not exceed 0.040.05%. Reduction of chromium and iron oxides by the sili- .con contained in the pulverized ferro-silic'o'n occurs, to all practical purposes, entirely within the slag layer, the reduced metals findmg thelr way subsequently into the metal bath. The introduction of lump ferro-silicon into the metal bath for reduction purposes, while not beyond the scope of the in-.

vention, is not the'preferred practice. When the silicon content of the metal bath is increased by additions of lump ferro-silicon before reduction of chromium oxide from the slag is practically complete, the carbon content of the metal bath is inclined to riseand its chromium content to fall appreciably.

Whereas the second step of operation is designated as a reducing step, this term refers. only to the action of the silicon content of the ferro-silicon in reducing the oxides of iron and chromium from the slag. The slag itself is never of a reducing type at any stage of the second period. When reduction by silicon is complete, the slag corresponds to a neutral or basiccalcium silicate slag. Before reduction by silicon is completed or when this reduction is incomplete, the slag contains the oxides of iron and chromium and remains of an oxidizing type within the generally accepted meaning of the term.- Em ployinent of pulverized coke or calcium carbide to assist in reducing the oxides of chromium and iron from the slag and simultaneously to refine the metal is unnecessary and, according to my particular experience, 'almost invariably increases the carbon content of the metal bath.

When it is desired for any reason to finish the heat under a new'slag, thus continuing the operation beyond the com letion of the second period, it is preferred a) to remove all or the major portion of the calcium silicat-e slag, which is formed during the second period and which attains to a considerable volume before reduction is completed, (b) to add suflicient lump ferro-silicon to raise the silicon content of the metal bath up to 0.300.40%, and ('0) to build up a normal volume of basic calcium silicate slag by means of lime and powdered ferro-silicon (or sand) in the usual way. This procedure is wellknown in the art and regularly employed in electric furnace practice so that it need not be considered as a novel or essential feature of the present invention. It is employed to particular advantage during the period when the furnace is being held up prior to, tapping on a preliminaryanalysis for chromium in the metal bath. When the product is being made, to narrow specification limits as to chromium, it is frequently necessary to increase the chromium percentage of the metal bath by a per cent or a fraction thereof by means of a small addition of low-carbon ferro-chrome or to lower the chromium content in the same wa an addition of low-carbon steel or ingot iron. When the metal bath is adjusted to the desired analysis as to car- 'bon and chromium, temperature tests are cold stainless iron scrap may be employed to cool the bath rapidly to the desired temperature. If too cold, the power is again ap lied. As a rule, however, the. bath will be ound to be on the hot side, which circumstance provides an economical outlet for an appreciable quantityof stainless iron scrap.

As soon as the second step of operation is completed and the heat brought to the proper analysis and temperature, the metal may be tapped into a ladle, the specified content of silicon and manganese introduced by ladle additions of ferro-siliconand low-carbon ferro-manganese, respectivel and the ladle metal poured into ingot mp1 s or castings in the customary manner.

The product which is obtained by this process is sound and free from blow-holes when cast into ingots even when the silicon content is 0.04% or less. In this respect the product difl'ers markedly from all other stainless-irons regardless of the process by standing difliculties hitherto encountered in the manufacture of stainless iron has been that of degasifying the final product so that it solidifies into an ingot free from blowholes. This undesirable property of ordinary stainless iron'does not exist in the case of my product. Silicon above 0.04% in the finished product need be added only for the purpose of meeting chemical or physical specifications. This peculiar freed ,m from gas evolution during solidification characterizes this chromium-iron alloy throughout the process of manufacture. A sample taken from the furnace at the completion of the first or oxidizing period freezes quietly to a sound ingot or test-piece,.just as does the finished product, without any deoxidizing or other additions whatever being made.

I have described in detail the procedure which I now prefer to. follow in conducting my process for the manufacture of sta nless iron alloys, but I do not wish to limit the scope of the invention to the particular procedure outlined. I may, for instance, if I so desire, melt up all or a portion of the iron ore (or scale) and the high-carbon ferro-chrome with the initial steel scrap charge in a single melting-down operation. Again, the iron ore (or scale) and hi h-carbon ferro-chrome may be added to the ath alternately in lots of any convenient Weight. Another variation, which I have actually employed in practice, consists in operating with not s mly one oxidizing and one reducing period ut two or more such cycles. Instead of ferro-silicon as a reducing agent, I may, if desired employ any one of several other wellknown metallic reducing agents, such as silicon metal, aluminum, calcium-silicon, ferroaluminum, silicon-zirconium, etc., without departure from the invention. I prefer to conduct both the firs't am]. second periods of the process in absence of any added flux or similar slag-forming ingredients but realize that the process may be worked satisfactorily in the presence of moderate amounts of such fluxes as fluorspar, rutile, sand, soda, manganese oxide, etc.

The product is usually madeto a specified silicon and manganese analysis but may be modified in its composition by the addition of various alloying elements such as nickel, copper, molybdenum, vanadium, etc..

By means of the process of the present in-. vention I am able to produce a rustless or stainless iron within the usual limits of 0.07 to 0.12% carbon and 12.00 to 18.00% chro: mium by the use of a high carbon ferrochrome, with a chromium loss of 10% or less, and at a cost appreciably less than the cost of the same iron made with low carbon ferrochrome.

, I claim: I

1. A process for making stainless iron which comprises the steps of forming a highly heated bath of a ferrous base metal, 5 charging iron oxide, removing carbon from the metal by oxidation at relatively high temperature, chargin high carbon ferro-chrome, maintaining the %ath at or above said high I temperature to oxidize carbon from the bath, 56 charging a silicon reducing agentand lime,

and maintaining the bath at high temperature until the slag is, substantially free fircm iron and chromium. --2. A process for making stainless iron which comprises the steps of forming; a highly heated bath of a ferrous base metal,

charging iron oxide, removing carbon from the metal byoxidation, charging high carbon ferro-chrome, maintaining the bath-at a 40 temperature of approximately 3250 F. until a the metal contains not more than 0.12 per cent carbon, charging powderedferro-silicon and lime, maintaining the bath at a relatively high temperature until the slag is substan tially free fnom' i'ron 'and chromium, and cooling the metal to the desired tapping temperature.

In'testimony whereof I afiix my signature. ALEXANDER L. FEILD.