Use of Direct Reduced Iron in the Electric Furnace P CaO + MgO... o. 20. Mn Cr... <o.o05

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1 Use of Direct Reduced Iron in the Electric Furnace DIRECTLY reduced iron is coming more and more into the headlines of the steelmaking industry and interest is widespread and increasing. Much work has been done and published on the development of old as well as new processes capable of producing sponge irons'ranging in quality from a lower degree of reduction with appreciable amounts of gangue contents to materials with the highest practicable degree of reduction and negligible gangue contents. It is estimated that in 1944 there were no less than 9m patents granted in the United States on the production of sponge iron, and it is likely that since then many others have been added, but few of them have any economic value. Very limited data have been collected in this country on the use of these direct reduced irons, but it is no doubt justifiable to say that increasing availability of sponge iron in coming years will have a marked influence on price and quality of scrap. This paper is a short report on a few problems and possibilities in the use of sponge iron in electric furnaces. As mentioned before, it will probably be years before a complete picture can be drawn. More specifically, this report will deal with reduced iron produced by the so-called Hoeganaes method, using a high-grade magnetite concentrate as a raw material, giving a sponge iron with a very high iron content and high degree of reduction. The approximate analysis of the product is: CONSTITUENT PCT Fe,total Fe, metallic.....:....: Degree of reduction o Oxygen as iron oxides..... I.o 2 c S P Si CaO + MgO..... o. 20 ~ 1 ~... 0 ~ Ti vzos Mn Cr... <o.o05 Ni Mo... <o.o1 CU Co Zn In dealing with a material of this quality, high degree of reduction, and low gangue content, no serious problems are involved in any type of steel furnace, although the amounts that can be used in open hearth furnaces, for a number of reasons, generally will have to be kept lower than in the electric arc furnaces and induction furnaces. The material will behave like a lowcarbon high-quality melting stock and for all practical purposes will have approximately the same influence on furnace linings, yield, power, or fuel consumption, and melting procedure in general. It can be used equally well in acid and in basic furnaces. It requires no more power to melt than a normal heat consisting of a mixture of pig iron and scrap. The electric-arc furnace, because of its versatility and good temperature control, is especially well suited for operation with sponge iron. Very large amounts can be used in the charge without apparent disadvantage and, more often than not, production can thus be increased in comparison with a standard

2 charge by permitting full power during the entire meltdown period and often, shorter oxidation and reduction periods. Often it can be advantageous to add recarb to increase the carbon content of the charge. Less publicized but highly successful use is being made of sponge iron in induction furnaces, large and small. High-grade Hoeganaes sponge iron in amounts from 10 to 60 pct is regularly used in Sweden in acid high-frequency furnaces of up to 12 tons rated capacity, primarily in the manufacture of casehardeningsteels, heavy forgings, tool steels, and to some extent stainless steel, and in basic crucibles, mostly for stainless steel. The workability, purity, quality and uniformity of the steel has thereby been greatly improved. Since no refining can be done in induction furnaces, this sponge iron with approximately 0.17 pct carbon has not until recently been used in higher amounts for stainless steel with extra low carbon specifications in this type of furnace. Sponge iron with the analysis reported (carbon 0.17 pct and oxygen 1.0 pct) in the form of briquettes 3 in. long and approximately ~)5-in. diameter is now regularly used also in this country in the manufacture of a series of steels with low carbon specifications, including stainless. A slight amount of carbon is considered as one of the great benefits of the sponge iron, since in all types of furnaces it will react with the oxygen in the material, providing for a slow boil that keeps the bath open and prevents gas contamination, particularly by hydrogen and nitrogen. The foregoing analysis on carbon and oxygen shows that oxygen is present in excess. Theoretically, therefore, the resulting carbon content should be extremely low, and experience shows that actually this is true. Oxygen content of only I pct is sufficient to provide for extremely low carbon contents even if the charge contains some pig iron or high-carbon scrap. Recarb or added pig iron would have MELTING; COS T REDUCTIONS =45 to be supplied in greater quantities for steels requiring a certain carbon content. The reaction obviously will start at approximately 7o0 C and in most cases continues after meltdown. It could be expected that other metals or alloying elements with a high affinity for carbon and/or oxygen would interfere with the reaction and cause carbon pickup of the melt and/or low yields of certain elements such as chromium in induction melting. Practical experience now shows that this does not always occur. The dry reduction takes place to a considerable degree also in charges containing chromium and other alloying elements with affinity for carbon and/or oxygen. This opens up the interesting possibility of combining sponge iron with less expensive, higher carbon ferroalloys than are normally used for low-carbon specification steels. In order to avoid excessive losses of chromium and carbon pickup, it is usually recommended to charge the induction furnaces in the following manner: Nickel, molybdenum, and similar alloying elements are placed on the furnace bottom. The furnace is then filled up with sponge iron briquettes. If required, additional sponge iron is added during meltdown, after which a slagoff is recommended. Scrap and alloying elements are then added. Since it may not always be practical to follow this procedure, especially if larger amounts of return scrap are used, alloying elements, sponge iron, and scrap may be charged together and melted down, followed by a slagoff. This method gives an excellent steel analysis but tends to somewhat lower chromium yield compared with the first method. A few more words should perhaps be said about the slagoff, since this is not always the practice in many shops using induction melting. All sponge iron materials contain some gangue and, in addition, tend to oxidize slightly during meltdown,

3 246 PROCEEDINGS OF ELECTRIC FURNACE CONEERENCE, 1958 because of their porous nature, forming a certain amount of slag. Since, in addition, the slag tends to be oxidizing, practical experience has shown that a slagoff prior to addition of alloying elements is to be preferred. Basic Electric Arc Furnace, 20 Tons Charge The Midvale-Heppenstall Co., Philadelphia, Pennsylvania, regularly uses sponge iron in the production of steel to the following specifications: CONSTITUENT PCT C max. Mn ~-0.25 P ,. 025 max. S " " Si... : Ni " Cr..... o. 10 " Mo " Cu " Al added The charge consists of 86 pct sponge iron and 14 pct low-phosphorus pig iron, with the pig iron on, the furnace bottom. In addition, and prior to meltdown, 50 Ib of limestone and 6 Ib of fluorspar per ton of metallic charge are added. Full power is applied from the beginning and the steel melts down hot, providing for a boil,' which, after meltdown, is aided by oxygen. C and Mn analysis is taken and oxygen blowing continued until 0.03 pct C is reached. The oxidizing slag is then removed and 30 lb of lime and 4 lb of spar are added per ton of charge. Later 0.15 pct Al is added in the furnace and 0.25 pct Mn, and the remainder of the A1 in the ladle. Calculated charged carbon in this case is about 0.50 to 0.70 pct. The charge usually melts down below 0.10 pct C and as a consequence it has been possible to shorten. the oxidation period. A normal charge previously melted down about 0.05 to 0.20 pct C. Between 300 and 500 lb of ore was added before or after meltdown. Time savings have been possible with sponge iron, particularly so since fewer tests are required and a purer steel, well within specifications, is obtained at a lower cost. Sulphur is usually about o.015 pct and phosphorus about pct. It is further reported that the sponge iron heats yield an easier forging steel than normal heats. Table I shows a comparison between three heats with sponge iron and three heats with the high-cost ingot iron previously used. TABLE I-Use of Sponge Irolt vs Ingot Iron 1- Analysis, Pct Con- uents Basic Electric Arc Furnace, I 10-Ton Charge The following log is one example of heats made with 60 pct sponge iron in the charge of a carbon steel. Time 0.00 Start charge Low residual scrap.. 19,900 Sponge iron ,000 Brokenmolds... 16,500 Carbon bricks 3000 Burnt lime :o7 Back charge Low residual scrap. 15,100 Sponge iron ,600 Broken molds.... 9,900 Limestone.... 3,000 Burnt lime.... 3,000 1:5o Back charge Sponge iron ,400 Low residual scrap. 27,000 Total charge ,400

4 MELTING; COST REDUCTIONS *47 2:37 Melt: C Mn P S Cu Ni When all is melted, they slag off and add 3 : the alloying elements O Records of two heats made by.411oy Cr M0 Sn Si Steel Castings Co. are shown in Table z. 2: : Ib spar working heat TABLE 2-Beals Made by Alloy Sleel 1255 lb burnt lime working heat Very active during the working period 3:s' 2815' 3: carbon 3:39 FeSi (50 pct) 700 Ib 3:47 FeMn 3000 Ib 3: Start tap 222,600 tap weight 215,300 ingot weight 96.8 pct ingot yield 480 kw-hr per ton The charge was dense and the electrodes penetrated slowly, but with no hanging. The heats melted without difficulty in about normal time. Bath action after back charges was the same as for scrap heats, and after meltdown and during the working period, it was reported moderate to active. A normal slag analysis with about 23 pct FeO was reported. The volume was slightly greater than normal, but it was run off easl+---- The ingot yield was considered unusually high for the grade tapped. Basic Ifidticlion Furnace, 330 Poulzds Charge Alloy Steel Castings Co., of Southampton, Pennsylvania, uses sponge iron briquettes in the way described in the second paragraph of page 246 for a number of stainless steels. Heats with sponge iron melted in basic crucibles show carbons at least as low as normal heats, or lower. The Company charges with very good results some sponge iron on the bottom of the furnace, then return scrap, sponge iron again, and then some additional scrap. No damage is apparent in the furnace bottom. Grade of Stalnless Steel ; Castilzgs Company ( Sponge Iron in Charge Pounds 1 Pct Steel Analysis 1 Per Cent Constituent L The Company reports that chromium yield and power consumption and production are equal to those of normal heats. It is possible, by a special refining process, to upgrade the Hoeganaes sponge iron to a. considerable degree. Such an upgraded material may find expanding use for special applications where high purity is required. It should be remembered again that the iron in sponge iron, regardless of the gangue content, is considerably less contaminated than is scrap or ingot iron. The following percentages illustrate the improvements that are possible in the regular Hoeganaes sponge iron, grade E: C, pct; Mn, 0.033; P, 0.007; S, 0.005; Si, 0.14; Cu, 0.012; Ni, 0.016; Cr, 0.001; V, 0.10; Ti, 0.063; Al, 0.051; A1203, 0.042; N, 0.002; Sn, This material has been satisfactorily used by the American Brake Shoe Corn-

5 pany's Research Laboratories in basic electric arc as well as basic induction furnaces in work on high-strength 4300 and other low-alloy steels. The Company reports high strength and, as an added feature, considerably higher ductilities than with previous materials. This sponge iron grade is reported to be fully equal in result to electrolytic melting stock, with no apparent disadvantage or difficulty in handling or melting. The United States Steel Corporation's Applied Research Laboratory has satisfactorily used this material in limited quantities as a partial replacement or substitute for electrolytic iron in the air induction melting of a wide range of alloyed steel. The following are the results of one heat: The material was melted in a 100-lb MgO crucible and the melting proceeded without difficulty. Tapping temperature was about 2950 F. The only addition made was the aluminum for deoxidation. Analyses of the finished ingot and of the slag are given in Table 3. 'TABLE 3-Analysesfro?n U. S. Steel Corpo-..ratio% Applied Research Laboratory... Steel Analysis, Pct Slag Analysis, Pct Sioz Mn PeO P FezOa S..... o.010 Fe Si ~ I Feint ::: Cu... o.ooi Alzos Ni CaO..... I. 11 Cr MgO Mo..... <0.005 MnO Al cr2oa v VtOs Ti..... <o. OOI TiOz..... z.31 ~n / Upgraded sponge iron is currently being supplied in chip form (similar to electro- lytic iron) or in briquette form of about ~$6-in. diameter by 2 in. W. C. KOLLMANN, CHAIRMAN-Thank you for an excellent presentation, Mr. Gummeson. We will defer discussion of this paper until after the next paper on the same subject has been presented. Our next speaker is Mr. R. W. Farley, Director of Process Development, Republic Steel Corporation, Chicago.