US1744374A - Manufacture of stainless iron - Google Patents

Description

Patented Jan. 21, 1930 WALTER M. IARNSWORTH, QF CANTON, OHIO, ASSIGNDR TO CENTRAL ALLOY STEEL CORPORATION, OF MAFSSILLON, 01-110, A GORPORATION OF NEW YORK MANUFACTURE OF STAINLESS IRON No Drawing. Application filed June 29,

This invention relates to the manufacture of those low-carbon, high-chromiumsteels, characterized by resistance to corrosion, tarnish and heat, and which, because'of these properties and their low carbon content, are variously designated as stainless, stablesurface or rustless irons.

'lhe usual'commercial limits for stainless iron are 0.07 to 0.12% carbon and 12.00 to 18.00% chromium, and one of the chief diffic ulties with which manufacturers of this product are confronted is the difficulty of holding the carbon to these low figures. Because of this difficulty it has been practically 1927. Serial No. 202,469.

the chromium, only about half of which oxidizes and passes into the slag, the other half remaining as an alloying element in the bath. At the conclusion of this first or oxidation stage of the process the carbon content of the bath is at or below the permissible maximum while the chromium content is much below the permissible minimum. Then follows the second or reducing stage of the process during which the chromium oxide in the slag is reduced, and the chromium, except for a small less, returned to the bath.

lln carrying out the process, an electric arc furnace of the Heroult type, or its equivalent,

impossible to make stainless iron by remeltis preferred. I have successfully used a 2-- mg stainless iron scrap, for unless extreme precautions are taken the scrap, during the re melt, invariably picks up carbon from the electrodes, the furnace atmosphere and elsewhere,-sometimes up to 0.10%, And it has been impossible to rid the bath of this carbon without, at the same time, experiencing an excessive chromium loss. llt has also been practically impossible to make stainless iron by adding the relatively inexpensive high carbon ferro-chrome (containing say 4: to 6% carbon) to a bath of molten low-carbon steel because in oxidizingthe excess carbon to bring the carbon content of the bath down to the permissible maximum the chromium is also oxidized. Consequently it is common practice to use the much more expensive low carbon ferro-chrome containing not over 0.10% carbon. I

his the object of the present invention to make stainless iron within the prescribed low carbon limits without experiencing excessive chromium losses, and in particular to make stainless iron by remelting stainless iron scrap.

In attaining this object I proceed as follows: The bath containing carbon and chro miuin is superheated and subjected to strongly oxidizing conditions. As the temperaturerises the afiinity of carbon for oxygen increases faster than that of chromium so that under the intensely oxidizing conditions obtaining in the bath at this stage of the process a very large part of the carbon is completely eliminated without-a complete oxidation of ton Heroult furnace of standard design equipped with a Westinghouse automatic regulator and operated with three 8-inch carbon electrodes on 3-phase current at 110 volts. Abasic refractory lining such as is ordinarily used in furnaces for the manufacture of alloy steels, may be used, but a bottom put in by burning in successive layers of ma'gnesite or dolomite will not stand up under continuous operation at the high temperatures necessary to maintain the necessary highly oxidizing conditions in the metal bath. A bottom built up of chrome brick fused in with a layer of finely crushed chrome ore stands up well during the oxidizing period, but when the reducing materials are added, the fluid slag which is formed, cuts into and damages the banks and causes considerable difficulty in obtaining uniform results in the chromium content of the bath. 1 have found that these difficulties are entirely overcome by ramming in a bottom consisting of granular basic refractory material, averaging 94 to 95% MgO, mixed with a binder of sodium silicate, and fusing it into place. The roof is constructed of silica brick in accordance with regular practice.

The scrap is preferably charged into the furnace with a part of the oxidizing material, which material should preferably be the purest form of iron oxide commercially available, such as clean roll scale. Special iron ore of low silica content may be used but when iron ore of the ordinary grade is used, con siderable difficulty is encountered in prop erly balancing the reducing materials, owing to the variables present therein. During the melting down period and at the ordinary refining .temperature, there is no oxidation of carbon, the chromium being oxidized more readily thanthe carbon, but when the temperature is increased, the speed of reaction between the oxygen and carbon increases more rapidly than the speed of reaction between the oxygen and chromium, thus making oxidation of the carbon possible. The ordinary refining temperature is about 1500 to 1600 C. The temperature necessary to effect a reaction between the oxygen and the carbon in the presence of the chromium is usually from to 250 higher. At the time the temperature is-raised the remainder of the iron oxide is added, the total charge amounting to about 20% of the weight of the scrap about 450 pounds per ton (2240 pounds). This higher temperature must be maintained until the carbon is reduced to the desired point, and when this is accomplished, the current input to the furnace is reduced as low as possible and the temperature permitted to drop. Meanwhile a substantial part of the chromium has been oxidized and forms with the iron oxide a heavy layer of slag usually carrying from to Cr O I now come to the second step of the process. This stage of operation is begun I with a highly heated metal bath above which is a slag containing chromium oxide and iron 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 chromium originally contained in the scrap. The addition agents used during the second step of operation are burnt lime (CaO) and pulverized ferro-silicon. The exact manner in which these additions are made may be varied at will to a considerable degree. However, I prefer to make an initial addition of lime which is spread over the surface of the bath. A mixture of lime and pulverized ferro-silicon is then fed gradually into the bath, preferabl around or beneath the electrodes. The pu verized ferro-"s'ilicon reduces the chromium and iron oxide contained in the slag and forms silica, which acidic oxide is in turn neutralized, combining with the lime present. As a result, the slag which at the beginning of this operation was black or dark brown incolor due to its high percentage of iron and chromium oxides gradually verges towards a 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 grayishwhite slag of the disintegrating type. The ratio of lime to ferrosilicon (50%) which leads to the best results lies between 1.5 and 3.0. At the completion of the second step, or reducing period, of operation the slag should not contain more than 2% oxide of chromium and 2% oxide of iron.

If the reduction process is not efiiciently con-' ducted, however, these percentages will be higher. The amount of residual chromium oxide in the slag corresponds to the net loss of chromium for the operation as a whole. The silica (SiO content of the slag at the end of the reducing period lies as a rule between 25 and 35%.

Alloying ingredients, necessary to bring the metal up to the desired specifications, are then added to the furnace, or if desired in the ladle prior to tapping. If the bath is too hot additional stainless iron scrap may be added to cool it. I

I shall now give a specific example of an actual heat. 3000 pounds of steel scrap having an approximate analysis of .08% carbon; 35% manganese;.020% sulphur;.020% phosphorus; 16.5% chromium and .80% silicon, were charged into a two-ton furnace with 300 pounds of mill roll scale. As soon as the charge was melted, a sample was taken for analysis, which ran .17% carbon, and 14% chromiuma carbon pick up of 0.09%. 'No reaction occurred in the bath until the excessively high temperature of 1700 C. was attained, whereupon another 300 pounds of roll scale was added to the bath: After the re action caused by this addition had subsided, a sample was taken which analyzed .07% carbon and 8.85% chromium. A slag sample taken at the same time had the following analysis: CI' O 60.3% .Fe O 22.9% MnO, 1.0%; SiO .4l%; CaO, nil. The remainder of the slag was probably MgO. The particular order for which this heat wasmade required the following analysis: .08/.12% carbon; .35/.45% manganese; 12/ 14% chromium and .35/.50% silicon, and the subsequentadditions were ma dc to meet this analysis.

The object of the procedure from this point was to reduce the chromium back into the metal bath, and this was accomplished by an addition of 300 pounds of burnt lime mixed with 150 pounds. of crushed ferro-silicon (containing 50% silicon"). After about twenty minutes the slag formed by this addition of burnt lime and ferro-silicon appeared light green in color and very fluid indicating completion of the reduction stage. An addition of 20 pounds of ferro-manganese fontaining 80% manganese and .75% car on) was then made to the bath and five minutes after this addition was made, the heat was tapped. The final analysis of the metal was as follows: 09% carbon; 40% manganese; 31% silicon; and 13.5% chromium. The analysis of the final slag was as follows: 01 30 F6203, M110 Slog, 32.8%; A1 0 7.4%; CaO, 52.8% and MgO, 4.0%. The duration of the heat from current on until tap was three hours and ten minutes.

(about 1650 C.) 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 1 400 and1' 50 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, the first small addition or so of the scale being sutficient to drop the carbon content of the molten steel to a very low figure (approx. ODE-0.04% carbon) in case the scrap initially contained appreciable 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% chromiumand 0.10% carbon content from high-carbon ferro-chrome analyzing 66% chromium and 5.9% carbon, the total ferro-chrome charged amounts to approximately 1180 lbs. per ton of steelscrap charged or to 590 lbs. per ton of stainless iron produced- This quantity of higlrcarbon ferrochrome has, as a rule, been added'intermittently in eight equal additions. Each of the eight additions may itself be gradually charged into the furnace, and at times this becomes necessary to avoid a too violent boiling action. The electrodes are raised when ferro-chrome is added and during a violent boil in order to avoid undue electrode con: sumption and possible contamination of the bath with carbon from, the electrodes.

The metalbath should be maintained at a sufliciently high temperature and the slag sufficiently high in iron oxide to cause the carbon content of the metal bath to remain always at a comparatively low value. ll prefer to operate under such conditions that a metal sample taken from the bath ten minutes after a ferro-chrome addition does not show by analysis more than 0.100.25% carbon. 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, supplementary additions of roll scale may be made in order to bring the carbon down to the desired percentage. l/Vhen the last addition of ferro-chrome has been made, the carbon content is brought down, by an ore addition if necessary, to the percentage desired in the finished product, and preferably Mil-0.02% lower than the maximum specified. Occasionally, the carbon content will apparently drop 0.01% or so during the second or reducing step, which will be later described, but this cannot be relied upon and, when it occasionally happens, may be due in part to unavoidable error in sampling and analysis.

The oxidizing slag at the conclusion of the first step will generally contain 5 to 20% SiO2, 0 to 15% CaO, 3 to 15% A1 0 1 to 3% M110, 0 to 5% Mg(), and from 45 to 80% of the oxides of iron and chromium combined. There is a consistent increase in the total amount of chromium oxide in the slag throughout this period of operation, but the actual percentage depends largely on the manner in which the additions of ore or scaleare made. 7

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 1n question are givenbelow:

The tabulated 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 QTY-0.18% carbon and With a loss of added chromium amounting to only about 30%. In going to an alloy of 0.10% carbon conready described in connection with the re-- melting of stainless iron scrap because the conditions obtaining in the bath and slag at the end of the oxidation period are substantially the same'no matter whether arrived at through remelting of stainless iron scrap or the addition of high carbon ferro-chromc to chromium free scrap.

By the process of the present invention I am able'to produce stainless iron within the usual commercial analysis limits and with low chromium losses by oxidizing the excess carbon and part of the chromium and subseuently reducing the chromium oxide thus ormed to return the chromium to the bath. Although capable of other applications, the process is of particular importance when applied to the remelting of stainless iron scrap. The production of large quantities of sore is unavoidable in the manufacture of stain ess iron and up to the time of the present invention there has been no satisfactory method gojor utilizing it. As a result large tonnages of this very valuable material have accumulated and present a serious problem to the stainless iron maker. By means of my process this Waste material can now be made up into a marketable product and a saving of considerable magnitude efiected.

I claim: a

1. The process of making stainless iron which comprises maintaining a bath of ferrous metal containing carbon and chromium under an oxidizingslag and at a temperature of superheat, thereby oxidizing a substantial part of the carbon and chromium, the chromium oxide thus formed passing into the slag, and then, without removing the slag, reducing the chromium'oxide to return a substantial amount of the chromium to the bath.

2. The process of making stainless iron which comprises adding sufficient iron oxide to a superheated bath of ferrous metal containing carbon and chromium to make it highly oxidizing, thereby oxidizing a substantial part of both the carbon and chromium, the chromium oxide passing into the slag, and later, without removing the slag, reducing the chromium oxide in the slag, thus returning a substantial amount of the chromium to the bath.

3. The process of making stainless iron which comprises adding suflicient iron oxide to a superheated bath of ferrous metal containing carbon and chromium to make it highly oxidizin thereby oxidizing a substantial part of both the carbon and chromi- .um, the chromium oxide passing into the slag, and then adding ferro-silicon and lime to the slag to reduce the chromium oxide and return the chromium to the bath.

4. The process of making stainless iron which comprises forming an oxidizing slag on a superheated bath of iron containing chromium and carbon in a furnace linedwith magnesia, thereby oxidizing part of the chromium, which passes into the slag as chromium oxide, and the carbon to the extent of holding it at or below the permissible maximum, and then adding ferro-silicon to the slag to reduce the chromium oxide in the slag and return a substantial amount of the chromium to the bath. I

5. The process of making stainless iron which comprises forming an oxidizing slag on a superheated bath of iron containing chromium and more than the permissible carbon, thereby lowering the carbon content to or below the permissible maximum and oxidizing a substantial part of the chromium, which passes into the slag, and then, without removing the slag, reducing a substantial amount of the chromium oxide thus formed to return the chromium to the bath.

6. The process of making stainless iron containing not more than 0.12% carbon and not less than 12% chromium which comprises superheating an iron bath containing higher carbon and substantially the required chromium, adding sufficient iron oxide to lower the carbon content to or below 0.12%, thereby oxidizing a substantial quantity of the chromium which passes into the slag and then adding ferro-silicon to reduce the chromium oxide and return the chromium to the bath, and adding lime to control the acidity of the slag;

7. The process of making stainless iron from stainless iron scrap which comprises melting down the scrap, superheating the bath under strongly oxidizing conditions whereby the carbon content of the bath which has increased during melting down is brought back to or below the permissible maximum, and a substantial part of the chromium oxidized and passed into the slag, and them without removing the slag, reducing the chromium oxide to return a substantial amount of the chromium to the bath.

8. The process of making stainless iron from stainless iron scrap which comprises melting down the scrap, superheating the bath under an iron oxide slag whereby the carbon content of the bath which has in-' creased during melting down is decreased to or below the original amount, and a sub stantial part of the chromium oxidized, the chromium oxide thus formed assing into the slag, and then adding a re ucing agent to the slag to reduce the chromium oxide and returna substantial amount of the chromium to the bath.

9. The process of making stainless iron from stainless-iron scrap which comprises melting down the scrap, adding 1ron made and superheating the bath about 50 to 250 melting down the scrap in the presence of iron oxide, adding additional iron oxide, superheating the bath to about 1700 C. whereby the carbon which has been picked up during melting down is oxidized, and the carbon content of the bath lowered to or below the original percentage, and part of the chromium'oxidized and passed into the slag, and (then adding ferro-silicon to reduce the chromium oxide and return the chromium to the bath, and adding lime to control the acidity of the slag.

11. The process of remelting low carbon, high chromium iron scrap in an electric furnace without a substantial increase in the carbon or decrease in the chromium content which comprises melting down the scrap under superheated and strongly oxidizing conditions, whereby any carbon picked up during the remelt is oxidized and the carbon content lowered to or below the original figure, while a substantial part of the chromium is oxidized and passed into the slag, and then reducin the chromium oxide thus formed, where y substantially all of the chromium in the slag is returned to the bath.

In testimony whereof I aflix my signature WALTER M. FARNSWORTH.