US3472649A - Electric-arc steelmaking - Google Patents

Description

Oct. 14, 1969 J. G. SIBAKIN ET AL 3,472,649

ELECTRIC-ARC STEELMAKING Filed July 10, 1967 s Sheets-Sheet 1 lfw'VE/VTORS J. saunas S/BAK/N, canoe/v A. Rot-05m y PAUL H H. HOOK/NGS 4m 5w 8 Mulfiollaul ATTOR NEYS Oct. 14, 1969 5|BAK|N ET AL ELECTRIC-ARC STEELMAKING 6 Sheets-Sheet 2 Filed July 10, 1967 FIG. 3

m m m MEML INVENTORS J. GEORGE SIBAKIN, GORDON A. ROEDER 8 PAUL H. H. HOOKINGS ATTORNEYS Oct. 14 1969 J, S|BAK|N ET AL 3,472,649

ELECTRIC-ARC STEELMAKING Filed July 10, 1967 6 Sheets-Sheet .5

5 s3 SLAG LAYER I STEEL x BATH O; A,

M F G- 4 s2 SCRAP METAL CHARGE {PETROLEUM COKE FURNACE SPONGE IRON I SPONGE IRON PELLETS l 1 T' PELLETS .l L FURNACE I FURNACE 1 B l 8 I L .T J J uouro STEEL r0 comnvuous CASTING INVENTORS MOULDS J. GEORGE S/BAK/N,

GORDON A. ROEDER a y PAUL H. H. HOOK/N65 ATTORNEYS v Oct. 14, 1969 5.5 ET AL 3,472,649

ELECTRIC-ARC STEELMAKING Filed July 10, 19s? 6 Sheets-Sheet 4 000.. g E s 3 aaooo g g m y o q m 3. u. g g

3 2aoo0-- 5 YD Iv 4 v q; 2, a Q t, l" Q J g Y 4 I ..Q 5 r V g m MEL! DOWN 4 REFINING I, PERIOD PERIOD 3 50 ab a do 150 160 POWER INPUT- MEGAWATTS o u a a a \a o 0 150 1&0 15o HEAT TIME MINUTES F l 6 INVENTORS J. GEORGE suaAKm PRIOR ART BY eonoou M20506! a PAUL H. H. HOOKINGS 40 Shaman/14M ATTORNEYS Oct. 14, 1969 J, G, S|BAK|N ETAL 3,472,649

' ELECTRIC-ARC STEELMAKING Filed July 10, 1967 6 Sheets-Sheet s 3 no "1' 3 {3E -1 tu mow- 3 En: m O (L, -l 3 qt k u E 9 2?; 3211000 I G g I I q I a 1 HOLDING 5 3 I PERIOD 2 E I; m MELT oow/v u. PERIOD l I A l l 0 3b so 100 POWER INPUT. MEGAWATTS F 3b 6b 9'0 do 150 HEAT TIME MINUTES ATTORNEYS United States Patent 3,472,649 ELECTRIC-ARC STEELMAKING Jaroslaw George Sibakin, Ancaster, Ontario, Gordon A. Roeder, Burlington, Ontario, and Paul H. H. Hookings, Edmonton, Alberta, Canada, assignors to The Steel Company of Canada Limited, Hamilton, Ontario, Canada, a company of Canada, Metallgesellschaft A.G., Frankfurt am Main, Germany, a German company, and Pickands Mather & Co., Cleveland, Ohio, a company of Delaware Continuation-impart of application Ser. No. 571,837, Aug. 11, 1966. This application July 10, 1967, Ser. No. 652,143 Claims priority, application Canada, Sept. 3, 1965, 939,972; July 16, 1966, 965,617 Int. Cl. C21c 5/52 US. CI. 75-10 15 Claims ABSTRACT OF THE DISCLOSURE A method of steelmaking in a direct arc furnace utilizing continuous charging of discrete iron-bearing material at a controlled rate such that the particles fall through the slag to the molten metal bath without forming clusters of unmelted particles. Slag forming and alloy perfecting additions are made such that the desired tap temperature and carbon content are reached substantially concurrently with the end of the continuous charging. Agitation of the bath continues throughout the heat.

CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of co-pending US. application Ser. No. 571,837, filed Aug. 11, 1966.

BACKGROUND OF THE INVENTION This invention relates to series electric-arc steelmaking and is particularly directed to a method of continuously charging iron-bearing material into an electric-arc steelmaking furnace.

Conventional electric arc steelmaking furnaces, Heroult direct-arc type, utilize scrap metal as melting stock which is top-charged or door-charged into the furnace. The charging of scrap materials into the furnace necessitates, in addition to the initial furnace charge, from two to as many as six scrap recharges in order to obtain the desired final metal charge weight in the furnace, the number of recharges required depending upon the size and shape of the scrap used. Prior to each recharging of the furnace with scrap, the power is shut off, the hot electrodes withdrawn and, in a modern top charged furnace, the roof lifted and swung to one side. The scrap charge is then placed in the furnace by means of a drop-bottom bucket or the like which is brought to the open furnace and held in position over it by an overhead crane. This method of charging scrap to the furnace heretofore has been considered to be the quickest and most efficient technique available. The time taken to complete a recharge of scrap varies, a charging time of from 4 to 7 minutes being normal for a modern steelmaking furnace. It is evident that elimination of these interruptions for recharging from the steelmaking sequence would result not only in shorter heat times but also in reduced energy consumption per ton of steel produced since the considerable heat lost from the furnace chamber by radiation when it is opened to receive a scrap charge must be subsequently recovered.

In electric-arc steelmaking furnaces, the electric current passes through one electrode, across the are created between the foot of the electrode and the scrap or bath, then through the scrap or bath and up across another are to an adjacent electrode, completing the circuit through 3,472,649 Patented Oct. 14, 1969 this second electrode. The arcs constitute a variable resistance in the circuit which can be altered by raising or lowering the electrodes to change the arc length, the electrodes being moved up and down by automatic means which seek a predetermined position and therefore correct resistance to maintain the electric current and applied voltage at the value chosen by the furnace operator. Because the scrap tends to fall against the electrodes as the electrodes bore through the scrap charge, the arcs are frequently short-circuited, resulting during the meltdown period, in effect, in a series of short circuits, i.e. one or more of the three phases of the furnace transformer secondary circuit is short-circuited. Arc energy interruptions resulting from these short circuits normally are brief in duration being in the order of a few seconds. However, interruptions as long as 30 seconds sometimes occur if the electrode is short-circuited by scrap falling against it at a point some distance above the foot of the electrode necessitating the electrode to pull out of the bore cavity to a point above the touching scrap before elfective power can be resumed. It is clear that elimination of these frequent power interruptions by stabilizing the arc during the meltdown period of steelmaking would result in employment of a higher average electrical energy input and realization of a shorter heat time.

Conventional electric-arc furnaces furnish most of the heat for melting scrap by means of a direct are formed between each electrode and scrap material.

The electric-arc provides a very intense source of high temperature heat (6300 F. for the carbon arc). The heat is radiated from along the length of the arc and generated at the interfaces of arc/metal or slag, and arc/ electrode. These locations of high temperature referred to herein as the arc flare zones normally occupy positions at the foot of each electrode flaring away from the edge opposite to the furnace center toward the wall of the furnace and arcing downwardly to the bath. The rate at which a given scrap charge will absorb the heat from the three are flare zones is largely dependent upon the area of cold metal exposed to the radiation from these zones, the rate of heat transfer diminishing continuously as the average temperature of the scrap charge rises. It has always been a problem in the prior art that radiation from the are, particularly radiation from these zones, is extremely damaging to the furnace; so much so in fact that in usual practice, after meltdown, the energy dissipated by the arc Zone must be decreased in order to protect the refractory sidewalls and roof in line of sight of the arcs from overheating. It is clear that the presence of means or a method of protecting the furnace refractories would permit full power utilization at all times during steelmaking with resulting shortened heat time, i.e. the time to make one bath (heat) of steel.

The steelmaking cycle consists of five operations; the meltdown period when the scrap is melted; refining period when the impurities of the molten steel bath are removed and alloying and deoxidizing additions are made; tapping period when all of the molten charge is removed from the furnace chamber fettling period when the furnace bottom and banks are repaired in preparation for the next heat; and charging period when the scrap metal is placed in the furnace.

Of these operations, the refining period can be the most variable, the length of the refining period depending upon the composition of the metal bath on completion of the meltdown of the scrap. Because scrap is heterogeneous material of variable and often unknown chemical composition, having been collected from a multiplicity of sources, the composition of the metal bath at meltdown usually cannot be predicted with a reasonable degree of accuracy. For example, it is often found that the sulphur or phosphorus content of the metal at meltdown exceeds the amount specified for the finished steel. In such cases, a lengthy steel-making procedure known as the two-slag practice may be necessary to lower the content of these elements. This practice consists of shutting off the power, raising the electrodes, back-tilting the furnace slightly, and then raking the slag off the metal pool through the charging door using a rabble. A second slag is then made by charging for example lime, powdered coke, fluorspar and sand to the furnace. This procedure can take from 20 to 60 minutes.

It is also usually found that the carbon content of the bath after meltdown is either too high or too low for the grade of steel specified. The carbon content is decreased in the refining period by making additions of iron ore or mill scale or by lancing the bath with gaseous oxygen. The carbon content is increased by making additions of coke, coal or graphite to the bath. Dipping the graphite electrodes in the bath is sometimes used although this is an expensive method of recarburizing the bath.

The temperature of the steel prior to tapping must fall within a narrow specified temeprature range somewhat above the liquids temperature of the steel, particularly if the molten steel is to be continuously cast. It is evident that elimination or shortening of this prolonged period of bath composition and temperature adjustment would constitute a very significant improvement in electric-arc steelmaking.

Discrete iron-bearing material called sponge iron has been used in the past for charging with scrap metal to an electric-arc steelmaking furnace. In most cases, these attempts provided poor results in that longer heat times and higher power consumptions were required. The reason for this is believed to be that the sponge iron particles in the charge tend to pack together very tightly, thus producing a relatively impermeable layer or layers which trap liquid iron formed in the region of contact between the scrap and the furnace electrodes. Iron thus trapped cools and solidifies in situ, welding the sponge iron particles together. The welded layer or mass formed prevents gravitation of the liquid iron to the furnace hearth. This barrier to the descent of molten metal and slag creates an adverse melting condition; i.e. melting of the charge from the top to bottom. In this situation, the arcs are exposed to the roof and to the upper courses of the side wall bricks with resultant damage to the alfected refractories. Also, under this situation, the fused mass, which is diflicult to melt, necessitates a prolonged meltdown period with attendant higher than normal energy consumption. It is evident that a method for eliminating the formation of impermeable sponge iron layers or masses would be important in permitting effective utilization of sponge iron in the electric-arc furnace.

In many direct reduction processing plants, a considerable portion of the sponge iron produced is finer than inch and, in some of these plants, the entire output -is very much finer than this size. The very fine size tends to aggravate the formation of fused masses or clusters of sponge iron particles. To overcome this disadvantage, it has been conventional practice to cold or hot press the metatllic fines into dense briquettes having a suitable size and shape. Briquettes give no problems in conventional melting practice. Although briquetting represents a solution to the problem of employing fines in steelmaking, it also represents additional cost both of capital to purchase a briquetting press and of operation to power, maintain and to man the press. A steelmaking procedure whereby metallic fines could be used directly in the furnace without a decrease in productivity and an increase in energy consumption is desirable in obviating these capital costs.

Many techniques have been devised and attempted over the years to improve the operation of electric-arc steelmaking furnaces including the charging of sponge iron into the furnace through holes provided in the furnace roof. Heretofore such techniques have not lead to significant improvement in the steelmaking operation and, therefore, such methods have not met with ready acceptance and commercial use. For example, German Patent No. 954,699 issued Dec. 20, 1956, describes an apparatus for charging materials to an electric-arc steelmaking furnace, but does not give suflicient operating instructions to enable a steelmaker to carry out a process of concurrently charging, melting and refining the iron-bearing feed. Nor does the patent teach the optimum location of feeding iron-bearing material. For another example, United States Patent No. 3,153,588, issued Oct. 20, 1964 teaches a technique for the feeding of sponge iron into an electric-arc steelmaking furnace. According to this patent, an essential part of the operation is that the arc should be covered by sponge iron and, therefore, the arc submerged in the sponge iron. As mentioned previously, this practice leads to the formation of clusters which are difiicult to melt and which result in a condition of continuous electrical shorting of the electrode. Accordingly, the teachings of this patents are contrary to the process of the present invention which is found economically operative as will be described and claimed hereinbelow.

SUMMARY OF INVENTION We have found that these problems can be substantially overcome by forming a slag covered bath of molten metal in a direct are electric furnace and continuously feeding a discrete free-flowing, iron-bearing material having a composition within the range of from about 76 percent to 99.5 percent by weight total iron into said slag cover at a controlled rate relative to the power input to the furnace such that the molten metal has substantially reached the desired tap temperaure and carbon content concurrent with completion of the feeding of said iron-bearing material and any other additives to the furnace; thus providing a steelmaking cycle essentially free of interruptions due to recharges, optimum utilization of electrical energy, diminished refractory damage due to radiation, and decreased charge to tap time due to the almost complete elimination of a separate refining period.

The discrete iron-bearing material used in the process of the invention has a composition within the range of from 76 percent to 99.5 percent by weight total iron and residual oxygen content of 0.1 percent to 1.75 percent, and up to 5.0 percent, by weight. A residual oxygen content of from 0.1 percent to 1.75 percent by weight is preferred with the use of high percentages of sponge iron, e.g. 65 percent sponge iron content. For low percentages of sponge iron, e.g. 35 percent or less, the oxygen content becomes less critical and can be as high as about 5 percent. Throughout the application, figures relating to oxygen refer to residual oxygen combined with iron.

The initial charge of the electric furnace may include in addition to the metal and iron-bearing material some of the necessary fluxing, carbon and alloying additives. It is not necessary or even preferable that the discrete particles be present in the initial charge and, accordingly, the charge may initially be made up with only scrap metal.

After some of the scrap is melted and forms a slag covered pool, the discrete particles of iron-bearing material are continuously fed into the slag layer to the pool. When continuous feeding starts, the particles are believed to substantially melt as they fall through this hot slag layer and they are fed at a rate which is slow enough to prevent formation of clusters of unmelted particles on the slag surface. The rate is adjusted relative to power input such that the molten metal temperature and carbon content reach the desired tap temperature and carbon content substantially concurrently wih completion of the feeding to the furnace of said discrete iron-bearing material and any other additives, the molten metal temperature normally rising and the carbon content normally falling to the desired levels as will become evident as the description proceeds. The feed rate of the discrete ironbearing particles is controlled at a rate which will achieve or hold the desired temperature of the metal bath while still sufficiently slow to prevent the formation of clusters of particles. During this time bath temperature canbe determined and bath samples can be taken. A decarburizer such as gaseous oxygen or particulated oxide can be injected, or, if needed, a carburizer such as carbon can be injected.

At the same time, of course, slag-forming ingredients can be added to maintain an average basicity ratio, i.e., ratio of bases" to acids which, for example, can be a ratio of calcium oxide and magnesium oxide to silicon dioxide and aluminum oxide, in the range of between 1 and 2; and more particularly in about the range of 1.0 to 1.5. The effect of this is to maintain the slag with rather high electrical resistivity. The slag volume should be controlled such that it is great enough to permit submergence of the arc, the volume being controlled for example by restricting the slag flushed during the heat. This can also be done by forming a foamy slag. By foaming the slag, the effective resistance of the slag between the electrode and the metallic bath can be effectively increased, thus allowing increased basicity while still maintaining the arc submerged. It is believed that during submergence of the arcs, the arcs are struck between the electrodes and molten metal bath.

If no phosphorus elimination or manganese correction, for example, is required at the end of the heat, the temperature at the end of the addition of the discrete ironbearing material should approximate tap temperature. On the other hand, if carbon or manganese is to be added to the steel, then the feed of discrete iron-bearing particles may be completed at a lower temperature and the carbon or manganese added to the bath while maintaining the full power input. Again, this is possible because the arc is substantially submerged in slag. submergence of the arc in the slag also enhances heat transfer from the arc to the slag cover for increased efficiency of heat exchange.

Agitation of the bath also is believed important for efficiency of heat exchange between arc and bath. This agitation can result from the chemical reaction between carbon and oxygen. A movement of bath and slag also results from impact of the arc and associated magnetomotive forces in the bath.

The term steelmaking where used herein is intended to include making an iron carbon alloy, where the carbon is in the approximate range of from 0.02 percent to 1.8 percent by weight and all of the other constituents are in a refined state.

It is, therefore, an important object of the present invention to overcome the aforementioned problems and dis advantages inherent in prior art processes and, in particular, to shorten the steelmaking time and lower the total energy consumption by eliminating the need for periodic scrap recharges.

It is another object of the invention to eliminate frequent power interruption by stabilizing the arc during the meltdown period of steelmaking and thus provide a high average electrical energy input and shortened steelmaking time. Y

Another important object of the present invention is to protect the refractory materials in the sidewalls and roof of the furnace after the scrap is melted, while still maintaining full power input to the furnace.

Still another important object of the present invention is the provision of a simultaneous melting and refining by oxidation method of steelmaking to provide a steel within the desired ranges of metallurgical composition and tap temperature which effectively eliminates the need for the conventional refining period in the steelmaking cycle and which substantially shortens the total steelmaking time and decreases the energy consumption.

And another important object of the invention is to continuously charge discrete iron-bearing material at a rate which prevents the formation of particle clusters and permits ready assimilation of the said iron-bearing material in the molten metal bath.

And another important object of the invention is to enable the use of fine metallic materials in an electricarc steel-making furnace, having a particle size, for eX- ample, of less than W inch and in amounts of from about 15 percent up to about percent or more of the charge, with shorter heat times and lower energy consumption than required in conventional practice.

It is a further important object of the present invention to utilize discrete particles of iron-bearing material such as sponge iron having a known composition within the approximate range of 76 percent to 99.5 percent by weight total iron having a residual oxygen content of from 0.1 percent to 5.0 percent together with scrap iron and/or scrap steel, if desired, for the production of steel of predetermined composition.

And a still further important object of the present invention is to provide an agitation action to the molten metal bath for an accelerated refining rate.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects of the invention, and the mannet in which they can be attained, will become apparent from the following detailed description of the method and apparatus with reference to the accompanying drawings, in which:

FIGURE 1 is a perspective view, partly cut away, of a three-phase electric-arc furnace of the type employed in the present invention;

FIGURE 2 is a plan section taken along the line 22 of FIGURE 1 showing the formation of bore cavities consolidated into a trefoil-shaped pool within the furnace;

FIGURE 3 is an enlarged vertical section taken along the line 3--3 of FIGURE 2 showing the introduction of sponge iron in proximity to an electrode;

FIGURE 4 is a schematic illustration, in part, of the furnace illustrated in FIGURE 1 showing the charging system for continuous feeding of sponge iron to an open bath in the furnace;

FIGURE 5 is a flowsheet of a steelmaking system using the method of the present invention;

FIGURE 6 is a graphical illustration of charge weight and power input relative to heat time for conventional electric steelmaking practice utilizing scarp material showing meltdown and refining periods;

FIGURE 7 is a graphical illustration of charge weight and power input relative to heat time for the method of the invention utilizing controlled continuous charging of sponge iron pellets showing meltdown and holding periods; and

FIGURE 8 is a graphical illustration of the effect of residual oxygen on furnace productivity.

Like reference characters refer to like parts throughout the description of the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With particular reference to FIGURES l and 4, an embodiment of the structure of the present invention will now be described wherein furnace 10 has cylindrical side wall 12 with removable roof 14 secured to arms 16 extending from mast 18. Arms 16 are adapted to be raised and lowered above wall 12 by a hydraulic or mechanical system, not shown, formed in mast 18. Three electrodes 24, 26 and 28 are each mounted on masts 29, 30 and 31 by means of arms 32, 34 and 36 for independently raising and lowering each of said electrodes through openings formed in roof 14 equispaced about the roof center. Masts 18 and 29-31, in turn, are supported by platform 20 having rollers 22 for pivoting said platform about its axis, thereby moving roof 14 and electrodes 24, 26 and 28 to one side of furnace 10 for loading with scrap material and the like charge materials.

Three conduits 40, 42 and 44 having telescopic extensions 41, 43 and 45 retractable from and extensible into sleeves 46 formed in stationary ducts 48, 50 and 52 are in communication with openings formed in the furnace roof 14 at points between electrodes 24, 26 and 28 respectively and furnace side wall 12. Said conduits, sloping at an angle greater than the angle of repose of the material charged, in this embodiment sloping at an angle of about 40 to the horizontal, are in communication at their upper extremities with a conventional splitter box 56 having gating means for independently regulating the flow of material to each conduit. Mechanical, or pneumatic or hydraulic piston-cylinder units 58 permit the extension and retraction of conduit extensions 41, 43 and 45 to and from their respective sleeve 46 engaging positions. Bucket elevator 49 which receives sponge iron at a controlled rate from weigh feeder 51 in communication with hopper 53 continuously feeds splitter box 56.

In operation, roof 14 and electrodes 24, 26 and 28 are lifted and swung away from the furnace for charging to the furnace of scrap iron and/or scrap steel together with fluxing, alloying and carburizing additives, if desired, and said roof and electrodes then returned to their respective operative positions. It may also be desirable to add a portion of the sponge iron to the furnace with the initial charge by the procedure described in co-pending United States application, Ser. No. 571,837, filed Aug. 11, 1966.

The electrodes 24, 26 and 28 are lowered through their roof openings into proximity to the scrap contained in the furnace and a predetermined voltage applied to each electrode creating an are which bridges the space between the electrodes and scrap thereby heating and melting the scrap in proximity to the are. As the scrap melts and falls away from the electrodes, an electrical control 85' a power input of 7.4 megawatts to a 25 ton furnace, a sponge iron pellet feed rate of 700 pounds per minute, resulted in a 6 F. per minute fall in bath temperature, and a feed rate of 500 pounds per minute, resulted in a 7.5 F. per minute rise in bath temperature.

Typically, the slag analysis could be represented by the following range and example of a heat with slag within the range:

Typical slag example at Range meltdown (percent by (percent by weight) weight) O ther 4. 9 l

The slag basicity ratio of typical heats is in the range of 1.0 to 1.5. The basicity of the typical heat example given above was 1.08 i.e. the weight ratio of Ca0 MgO SiO +Al O was 1.08.

Surprisingly effective results have been attained by the methods of the invention as will be evident from the following general description and examples given hereinbelow carried out in two, three-phase electric arc steelmaking furnaces with nominal rated capacities of 15 and 25 tons, identified as furnace A and furnace B respectively and powered by 8000 kva transformers.

In general, according to the method of the invention, the furnace may be initially charged with sponge iron and steel scrap. Typical chemical compositions, by weight, of sponge iron pellets of the type disclosed are as follows:

Chemical analyses of sponge iron pellets by percent weight A B C D E F G Total Fe 92. s0 s7. 70 78. 70 90. 85 97.09 94. 59 89. 3 Metallic Fe 89. 94 82. 54 72. 73 90. 20 94.11 91. 35 83. 2 Iron 0x1de 3.68 5.17 7. as 3.83 3. 24 7. 86 yg In 11011 oxide 0. 82 1. 15 1. 71 0. l0 0. 85 0. 72 1. 75 Carbon 0. 098 0. 409 0. 125 0. 15 0. 194 0. 42 0- l9 Gangue:

Sulphur 0. 014 0. 008 0. 029 0. 010 0. 042

Phosphorus. 0. 046 Titania 10. 09

Lime 0. 7

Magnesia.-. 0. 7 0. 58

Silica. 1. 2 2.04 0.50 3. 88

Alumina. 5. 4 0. 96 0. 56 0. 17 Other g ngue o. o. 5 4. 60 Percent metalllzat 96. 9 94. 1 92. 4 99. 28 97. 0 96. 6 93. 2

system causes the electrodes to be repositioned vertically an optimum distance from the scrap for maintenance or re-ignition of the arc, as necessary, and for maximum production of heat. The electrodes thus independently bore through the scrap melting the scrap in proximity thereto creating a cavity '60 having a molten metal pool 61 with a slag cover 70 thereinbelow each electrode, the pools coalescing to form an initial generally trefoil-shaped pool centrally disposed within the furnace with a peripheral shell 71 of iron-bearing material forming a protective lining for the furnace wall 12 and hearth 62, as shown in FIGURES 2 and 3.

The sponge iron charge 73 can be initiated at this stage, preferably into an area of the bath in proximity to and surrounding the arc flare zones by means of conduits 40, 42 and 44, or later into the slag covered open bath 63 of molten metal shown in FIGURE 4.

The rate of continuous charging of the sponge iron to the furnace through the slag can be balanced with the power input and heat produced thereby to prevent clusters of unmelted particles from forming on the slag surface and to attain the desired tap temperature with concurrent achievement of the specified carbon and other alloying element content. For example, we have found that at Percent metallic Fe over percent total iron times equals percent metallization.

The scrap used consists of about 65 percent mixed, unprepared material and about 35 percent heavy melt material.

Carbon usually in the form of petroleum coke (due to its availability and low cost) is also added to the initial charge to provide carbon for the boil throughout the heat and to ensure sufficient carbon in the bath at the end of the heat.

This initial charge is partially melted using a circuit power in the furnace system of 7 to about 8 megawatts. When partial meltdown has been reached, controlled continuous pellet feeding is started. The initial feed rate in the various heats ranges from 300 to 500 pounds per minute. As the heat progresses, the feed rate is varied, as required, in order that the pellets will substantially melt as soon as they make contact with, and penetrate the slag. If mounds of pellets, i.e. sponge iron covers, start to accumulate, the feed rate is slightly reduced in order to clear the mounds and regain the bath temperature.

The pellet feed is completed at this level of power input with variations in the feed rate being dependent upon the bath temperature, which is checked periodically with an immersion thermocouple, to reach a bath temperature near the tapping range for continuous casting of about 2950 to 3000 F. at the same time as pellet feeding is completed.

During the heat, metal bath samples are taken in order to follow the carbon drop of the melt, it being desirable to have the bath carbon within tapping range at the same time that the bath temperature approaches the tapping range. A final bath sample is taken after the pellet feed is completed.

No conventional refining period is required and the heats are thus able to be tapped as soon as the chemistry of the final sample is known. Because no sulphur or phosphorus problems generally are encountered with the ironbearing materials used, it is possible to make the heats with little or no lime or limestone additions to the meltdown slag and without a second slag.

Steels were made to the following specifications, with the intermediate 32 and 28 reinforcing bar grades predominating:

sponge iron charge of 45,000 pounds or 82 percent of the metallic charge. Table II illustrates the time required for the steps of charging, melting and refining the scrap, coke and sponge iron.

Table II Operating step: Time elapsedminutes Power on 0 Initiation of controlled chip charging 32 All chips charged 99 Tap 138 Energy consumed was approximately 560 kwh./ton of steel tapped. The production rate was 10.6 tons of steel produced per hour. Comparable energy consumption and productivity for melting and refining an all-scrap charge on the same B furn'ace according to conventional practice was 580 kwh./ton tof steel tapped and 8.3 tons of steel produced per hour respectively. The productivity of the furnace was increased by about 28 percent, through the use of controlled, continuous feeding of sponge iron Range in percentages by weight of steel Grade of steel 0 Si Mn S (max.) P (max.)

Intermediate 32 .30/.34 .15/.30 .45/. 65 .050 .050 Intermediate 28---- .2s/. 32 .15/. .45/. 66 050 050 A 30 (ASTM grade) 12/. 17 15/. 30 .50/. 75 .045 .040

Example 1 chips. Productivity is defined as the production per unit An initial charge consisting of 6000 pounds of scrap and 12,400 pounds of sponge iron pellets of the composition described above as type A together with 400 pounds of petroleum coke were charged to the 15 ton A furnace. The electrodes was lowered and power applied for 24 minutes which resulted in the formation of a pool of molten metal therein. Sponge iron pellets, essentially spherical in shape and having a size range of minus to plus V inch, were then introduced through three conduits at locations near the arc flare zones at an average rate, when charging, of 400 pounds per minute until a total of 14,000 pounds of pellets from continuous charging was in the furnace to make a total pellet charge of 26,400 pounds or 81.5 percent of the metallic charge. Table I illustrates the time required for the steps of continuous charging, melting and refining the scrap, coke and sponge iron pellets.

Table I Operating step: Time elapsedminutes Power on 0 Initiation of controlled pellet charging 24 All pellets charged 69 Tap 84 Example 2 This example illustrates the operation of the invention wherein sponge iron chips, i.e. minus inch material having the composition described above as type B, were fed continuously into the 25 ton B electric-arc furnace. An initial charge consisting of 10,000 pounds of steel scrap and 20,400 pounds of sponge iron chips and 430 pounds of petroleum coke was placed in the furnace. The electrodes were lowered and power applied 32 minutes for the formation of a pool of molten metal before sponge iron chips were introduced with the continuous charging apparatus at an average rate of 500 pounds per minute until a total of 25,000 pounds of sponge iron from continuous charging was in the furnace to make a total of total furnace time.

Example 3 This sample illustrates the operation 'of the method of the invention wherein sponge iron to type C with high Ti0 content and scrap of the type stated in Example 1 were fed to an open bath in the furnace. An initial 13,700 pound charge of scrap, 4,900 pound charge of the pellets and 400 pound charge of coke was placed in the furnace. The electrodes were lowered and power applied for 26 minutes until the formation of the trefoil-shaped bath before sponge iron pellets were introduced through three conduits at locations near the arc flare zones at an average rate of 420 pounds per minute until a total of 19,000 pounds of pellets from continuous charging were in the furnace to make a total pellet charge of 23,900 pounds equivalent to 63.5 percent of the metallic charge. Table III illustrates the time required for the steps of charging, melting and refining the scrap, pig iron and sponge iron pellets.

Table III Openating step: Time-minutes Power to electrodes 0 Initiation of pellet charge 26 All pellets charged 71 Tap Energy consumed was approximately 712 kwh./ton of steel tapped. This is a very reasonable power consumption considering that this material contained a high gangue content; ie 19.5 percent, and a combined oxygen content of 1.71 percent.

A comparable heat time for melting and refining a similar type of scrap, as used in Example 3, according to conventional practice, was 171 minutes with an energy consumption of 652 kwh./ton of product. There exists a widespread opinion that titania-bearing materials are not amenable to steelmaking processing. However, We found that satisfactory slags could be produced according to the method of the present invention with optimum melting and refining of the sponge iron fed thereto at a reasonable energy consumption rate.

In the three foregoing examples a vigorous, steady boiling action took place in the bath throughout pellet feeding. This boiling action significantly improved heat transfer and permited the sponge iron pellets to be readily engulfed by slag, thereby increasing their rate of acceptance by the bath without the undesirable formation of floating mounds or clusters of unmelted material.

Example 4 This example illustrates the operation of the method of the present invention at the lower residual oxygen content level of 0.1 percent wherein sponge iron having the analysis of type D in the above table was fed into the B furnace. The initial charge consisted of 8,000 pounds of scrap, 200 pounds of coke and 12,935 pounds of pellets.

The electrodes were lowered and power applied for 27 minutes before starting to feed pellets into the arc flare zones. After the pellets were fed for a few minutes at a rate expected from previous tests to give the desired results, i.e. 450 pounds per minute, it was observed that the pellets tended to accumulate in mounds and the rate of pellet addition had to be decreased to 330 pounds per minute to balance the rate of feeding with the rate of melting of the pellets. With these highly metallized pellets no boil occurred in the bath and the melting of the pellets in the slag was retarded. The no boil condition of the bath and pellet build-ups near the banks caused irregular eddy currents to be created in the bath. These, in turn, produced waves on the surface of the bath, causing the arcs to alternately extinguish and re-ignite frequently. The overall energy input was, therefore lower than normal, making it necessary to further lower the rate of pellet feed to obtain the correct temperature rise. The heat time, 177 minutes, and power consumption, 635 kwh./ton of steel tapped, were comparable with that obtained in conventional practice.

Agitation of a bath into which highly metallized pellets are fed can be increased by feeding iron oxide, such as millscale or fine iron ore, simultaneously with the pellets, to make up the deficiency in combined oxygen in the highly metallized sponge iron pellets.

Example 5 This example illustrates the operation of the present invention wherein sponge iron having the analysis of type B from the above table was fed into the B furnace. This sponge was in a granular form with 97 percent passing through a mesh screen, i.e. having a particle size of minus inch.

The initial charge consisted of 5,800 pounds of scrap, 8,500 pounds of pig iron and 11,100 pounds of sponge. Power was applied for 47 minutes and continuous feeding was initiated. Granular sponge was fed to the furnace at an average rate of 438 pounds per minute. During the feed periods, the slag tended to foam and rise to envelop completely the arcs. The foaming condition, probably due to iron oxide reduction within the slag layer, produced the desirable result of effectively insulating the arcs and thereby eliminating refractory burning.

Thus we have solved the prior art problems of damage to the lining from radiation and heat loss through radiation by merely producing a foamy slag which, as previously described, can be effectively obtained by chemically reducing the iron oxide in the slag layer. The foam then permits higher power application and, accordingly, it is possible to run on high power throughout almost the entire heat.

Table V illustrates the time required for the steps of charging, melting and refining the scrap, coke and sponge lI'Ol'l.

Table V Operating step: Time elapsed-minutes Power on 0 Initiation of controlled sponge charging 47 All sponge charged 111 Tap 139 The energy consumed was approximately 540 kwh./ton of steel tapped, which was considerably below the average energy requirement of 570 kwh./ton for all-scrap beats made in the same 25 ton furnace. The lower energy consumption in the heat made with the granular sponge was due to the foamy slag which led to decreased heat losses from the metal bath. Steel productivity in the heat was 11.4 tons/hour or about 38 percent greater than the productivity of the average all-scrap heat of 8.3 tons/hour made in the same furnace.

A foamy slag about one foot thick was generated in each location of the continuous sponge feed effectively burying the electric arc to prevent refractory burning and improve heat transfer to the system. The melting rates with foamy slags induced by the continuous feed of fine sized sponge iron was about equal to the melting rates obtained by the feed of pellets but the energy consumption was enhanced. A series of trial runs indicated an energy consumption of 539 kwh./ton tapped, corrected to 525 kwh./ton, can be obtained with the controlled continuous feed of fine sized sponge iron.

Example 6 An initial charge of 13,500 pounds of scrap, 4700 pounds of pig iron and 13,800 pounds of sponge iron pellets of the composition described above as type F was charged to B furnace. The electrodes were lowered and power applied for 53 minutes for formation of a bath of molten metal before a mixture of sponge iron pellets and chips of type F was introduced in the manner described above at an average rate of 383 pounds per minute until a total of 23,000 pounds of pellets and chips from continuous charging was introduced to make a total sponge iron charge of 36,800 pounds or 69 percent of the metallic yield of 52,000 pounds. The sponge iron constituted 70 percent pellets within the size range of minus /s inch plus inch and 30 percent chips having a particle size smaller than inch. Table VI illustrates the time required for the steps of charging, melting and refining the scrap, pig iron and sponge iron pellets and chips.

Table VI Operating step: Time-minutes Power to electrodes 0 Initiation of pellet charge 53 All pellets charged 108 Tap 150 Energy consumption was approximately 555 kwh./ton of steel tapped at a power input of 7.1 megawatts; productivity being 10.4 tons/hour for an improvement in productivity of about 25.4 percent relative to a comparable all-scrap heat of 8.3 tons/hour.

-If an inexpensive and plentiful supply of suitable scrap material is readily available, it may be preferred to use a minimum of sponge iron and as much scrap as practicable by the normally undesirable batch charging of the scrap; the continuously fed sponge iron being introduced to obtain the important advantage of minimizing the refining period as described in the foregoing disclosure. Therefore, although the process of the present invention has proceeded with reference to the introduction of discrete particles of iron-bearing materials in the form of sponge iron to electric-arc furnaces in amounts of, for example, 63.5 percent, 82 percent and as much as about percent by weight of the total charge. It will be understood that the process can be carried out with a total continuously charged sponge iron content as low as about 15 percent of the total charge which in practice is suflicient to substantially eliminate the conventional refining period. The following example illustrates the operation of the process of the invention with continuous charging of sponge iron in the amount of about 20 percent after batch charging of scrap material.

Example 7 In a test heat in B furnace, four buckets of scrap were charged to the furnace, as if a normal all-scrap heat were being made. The scrap charge weighed 43,900 pounds, of

which 3500 pounds were pig iron. One thousand pounds of limestone was also charged. No sponge iron was included in these charges.

Power was applied for 86 minutes, including the time taken for the recharging, after which a molten bath with a temperature of 2870 F. was produced. Pellets having the analysis of type A were fed to the furnace at an average rate of 490 pounds per minute. The total weight of pellets continuously charged was 11,000 pounds. After several minutes, during which the temperature was corrected, the heat was tapped.

Table VII illustrates the time required for the charging, melting and refining of the heat.

Table VII Operating step: Time elapsedminutes The energy consumed in the heat was approximately 495 k-wh./ton of steel tapped, well below the average energy requirement of 570 kwh./ton for all-scrap heats made in the same furnace. The energy consumed in the heat was somewhat less than in the other examples cited because less gangue was melted due to the low percentage of pellets charged.

In heats made with low percentages of sponge iron, or of sponge iron having low residual oxygen contents, carbon correction and/or bath agitation can be achieved through the use of oxygen lancing. Some uncertainty with respect to carbon levels is inherent in the low percentage sponge practice because only a small portion of the sponge, with its known chemical composition, is used, and a reduction of carbon content may be necessary. Also, sponge iron having low residual oxygen contents provide a fiat bath and bath agitation, if desirable, may be effected by adding oxygen for reaction with carbon. The oxygen lancing preferably is carried out while the sponge iron is being charged so that the additional heat generated by the oxygen reacting with the bath carbon can be compensated for by an increase in the sponge iron feed rate. The use of oxygen, therefore, leads to shortened heat times and to further reduced electrical energy requirements. Likewise, carbon additions can be made at intervals or continuously during the continuous charging of ironbearing material to correct the carbon content of the bath if the carbon content should be too low.

The use of about 15 percent by weight, of the charge, of discrete particles of iron-bearing material, while not overcoming some problems incurred in the bath charging of scrap material as described hereinbefore, provides the important advantage of substantially minimizing the refining period which normally takes place after the meltdown of the charge. Although, the addition of as little as 15 percent discrete material such as sponge iron necessitates close control of carbon content, due to the uncertainty of composition of the scrap material, carbon control maintained by oxygen lancing provides exothermic heat which permits an increase in the rate of sponge iron addition and acceleration of the substantially concurrent melting and refining operations.

The examples described hereinabove showing the reduced energy consumptions attainable with the present invention were conducted in 15 and 25 ton furnaces, A and B, respectively. It will be evident that improved energy consumption as low as 350 kwh./ton of product for cold charges and 250 kwh./ton of product with the use of hot metal or the preheating of the charge constituents may be attained with furnaces of increased capacity as compared with 400 kwh./ton for all scrap charges. For example, iron-bearing materials such as sponge iron in the form of pellets introduced into the furnace can be heated in a neutral atmosphere to a temperature below the melting point of the pellets prior to charging to accelerate melting of the charge within the furnace.

FIGURE 5 exemplifies a steelmaking system utilizing the method and apparatus of the present invention wherein melt furnace A is arranged in series with one or more refining furnaces B such that furnace A which receives scrap metal and a carbon-containing material such as petroleum coke produces a molten metal which can be transferred to furnaces B by ladles or the like conveying means. The hot metal forms an open bath in each of furnaces B and, after a suitable slag blanket or layer has been formed, the iron-bearing material such as sponge iron pellets is continuously fed to the slag blanket, preferably in proximity to the arc flare zone and partly surrounding it, for melting and refining as described hereinabove. The refined steel can be poured into molds or into continuous casting units.

In geographic areas where the quality of scrap is poor, or is not readily available for use in electric-arc steelmaking, it may be preferable to retain a portion of a preceding heat in the furnace to provide a molten bath into which the continuously fed sponge iron can be charged. The molten metal comprising the heat can be recarburized by the addition of a carbon-containing material such as petroleum coke to provide a hot metal into which the sponge iron is continuously charged.

The present invention provides a number of important improvements in electric-arc steelmaking. By charging the furnace only once in the conventional manner, and by feeding the remainder of the metallic charge as sponge iron pellets through the roof into the area of the arc flare zones, a significant reduction in heat time is realized such that productivity of the furnace can be increased by more than 25 percent with, in some tests, increases of up to 60 percent being achieved.

The increase in productivity made possible by the controlled continuous charging method of the present invention, as illustrated in FIGURE 8 wherein productivities of the several iron-bearing materials treated in furnace B, namely types B, D, E, F and G are illustrated relative to their respective residual oxygen contents and compared with conventional treatment of scrap, may be understood with reference to the schematic graphs shown in FIGURES 6 and 7 from which it will be evident that the use of the present invention for feeding sponge iron at a controlled, continuous rate during meltdown eliminates the need for making recharges of scrap to attain the desired final weight of metal. By this practice the loss in operating time which accrues during each recharge and also the loss of heat energy from the furnace when the roof is removed are avoided. The elimination of recharges is an important improvement of electric-arc steelmaking, the rewards being shorter processing time with lower electrical energy consumption.

An advantage of this improvement in electric-arc steelmaking is that with a higher and more uniform power input to the furnace, as indicated by a comparison of FIGURES 6 and 7, the time required to melt the charge is shortened and the amount of circuit breaker maintenance is decreased. A further advantage of smaller surges in power may be realized in those furnace locations where the electrical power companies object to the severe swings in demand and insist upon additional reactance in the primary circuit to dampen these swings. In such localities the use of controlled, continuous sponge iron pellet feeding obviates such limitations to furnace power input. A further significant advantage from having submerged stabilized arcs is the quieter operation which is of considerable importance to the melting shop personnel.

The life of the furnace refractories is enhanced. Thus, as long as the arc is submerged in the slag and foam cover, maximum power setting may be employed without fear of seriously damaging the furnace refractories. The controlled generation of foamy slag which envelops the arc provides for improved heat transfer and reduced energy consumptions at high power inputs while protecting the refractory lining from radiant heat damage.

Another important improvement in electric-arc steelmaking realized in using the present invention is the virtual elimination of the separate refining period. As seen in the graph in FIGURE 7 and evident from the foregoing tables, a holding period from the completion of the feeding of the sponge pellets to the tapping of the refined steel has been indicated. This period reflects the time necessary for chemical analysis of the bath to determine its readiness for tap and with improved analysis apparatus and techniques has been shortened to as little as two minutes. Productivity figures include total time with inclusion of the holding period and would be revised upward with reduced holding time. The high purity of the sponge pellets allows, as soon as all of the charge is melted, the production of steel having a sulphur and phosphorus content below the specified percentages for commercial steel grades. The controlled, continuous feeding of sponge iron pellets containing from 0.1 to 1.75 percent and up to 5.0 percent, by weight, residual oxygen combined with iron, also allows the formation of a steady, active boil which removes carbon from the bath at a predictable and controllable rate so that at meltdown the bath of steel contains the desired or substantially desired carbon content.

In many areas Where electric-arc furnace steelmaking shops are located, the quality of the scrap that is available is very poor; i.e. the scrap has an unknown and highly variable composition. In these areas, it is particularly desirable to have, in addition to the scrap as melting stock, sponge iron having a known composition with a low tramp element content. The use of the present invention has proven to be an eifective way of utilizing sponge iron in amounts of up to 100 percent of the charge Without encountering the problems of cluster formation normally associated with sponge iron. This is an important advantage of the present invention.

It will be understood that although the description of the methods of the present invention has proceeded with reference to sponge iron, this term is intended to encompass discrete free-fiowing iron-bearing materials in general, briquettes, granules, punchings, borings and fragmentized scrap which have a composition Within the range of from 76 percent to 99.5 percent by Weight total iron and residual oxygen with iron content of 0.1 percent to 1.75 percent, and as high as 5.0 percent, by weight.

Alloying and fluxing materials such as ferromanganese, ferrosilicon, lime and the like addition to that used in the initial charge can be added to the molten metal for refining of the metal within the furnace. The furnace can be adapted to discharge refined molten metal continuously to a casting unit wherein the quantity of metal tapped would be brought into phase with the quantity of iron-bearing material charged to the furnace.

The apparatus of the present invention can be arranged in conjunction with a rotary kiln for receiving hot sponge iron, carburized, if desired, directly from said kiln.

The present invention may also be used in the manufacture of gray cast iron and the like by recarburizing the bath after meltdown.

It will be understood, of course that modifications can be made in the preferred embodiments of the invention described and illustrated herein without departing from the scope and purview of the appended claims.

We claim:

1. In the process of making steel having carbon in the approximate range of from 0.02 percent to 1.8 percent by weight and the other constituents in a refined state in a direct arc electric furnace having a slag covered bath of molten iron-bearing metal, the improvement comprising the step of continuously charging through said slag to said bath a discrete iron-bearing material having a composition within the range of from about 76 percent to about 99.5 percent by weight total iron while agitating said slag and molten metal, said charging being performed at a rate at which the particles fall through the slag toward the molten metal bath to avoid forming clusters of unmelted particles on the slag surface and at a rate relative to power input, such that the molten metal temperature and carbon content reach substantially the desired tap temperature and carbon content substantially concurrently With completion of the feeding to the furnace of said discrete iron-bearing material and any other additives.

2. A process of steelmaking according to claim 1 in which from about 15 percent to percent of the ironbearing material of the charge is continuously fed into the bath.

3. The process of steelmaking according to claim 1 in which the step of agitating is achieved by employing as the discrete iron-bearing material sponge iron particles having residual oxygen contents of between 0.1 and about 5 .0 percent.

4. The process of steelmaking according to claim 1 in which the step of agitating is achieved by employing as the discrete iron-bearing material sponge iron particles having residual oxygen contents of between 0.1 and about 1.75 percent.

5. A process of steelmaking according to claim 1 in which the step of agitating is achieved by blowing oxygen into the molten bath.

6. A process of steelmaking according to claim 1 which includes the step of charging iron oxide along with the discrete iron-bearing material in order to create agitation of the bath.

7. A process of steelmaking according to claim 1 in which the step of agitating the bath is achieved by magnetomotive forces.

8. The process of steelmaking of claim 1 in which the step of continuously charging discrete iron-bearing particles includes the step of feeding the material into the furnace to the three are flare regions.

9. The process of steelmaking of claim 8 in which the step of continuously charging discrete iron-bearing particles includes individually adjusting the rate of feed to each arc flare region.

10. The process of steelmaking of claim 1 which includes the step of tapping the refined steel from the furnace while the total energy consumption from a cold charge is within the range of from abut 250 to about 700 kwh./ ton of iron-bearing material, said steel having a carbon content by weight in the range from about 0.02 percent to about 1.8 percent.

11. The process of steelmaking of claim 1 in which ingredients of the slag are fed into the furnace to maintain the slag of a composition having a basicity ratio within the average range of from 1.0 to 1.5.

12. The process of steelmaking of claim 1 which includes the step of controlling the volume and composition of slag for maintaining the arcs from the electrodes substantially submerged to prevent substantial radiation to the furnace lining.

13. The process of steelmaking of claim 1 which includes the step of maintaining the arcs from the electrodes so that the arcs extend through the slag to the molten metal bath and are submerged to prevent substantial radiation to the furnace lining.

14. The process of steelmaking of claim 1 which includes the step of foaming the slag during continuous charging of the discrete iron-bearing material for enveloping the arcs by charging an iron-bearing material such as granular sponge iron to the slag.

15. The process of steelmaking according to claim 1 in which the step of charging is performed at a rate such that the molten metal temperature rises and its carbon content falls until the metal reaches substantially the desired tap temperature and carbon content.

(References on following page) 17 18 References Cited Iron, in Iron and Steel Engineer, XL(VIII): pp. 69-77, UNITED STATES PATENTS August1963- Trentini, B., Powder-Blowing Techniques for Electric 1,535,311 4/1925 Holloway Steelmaking, in Journal of Metals, 16(11): pp. 885-890, 1,902,638 3/1933 Gustafsson 75-12 19 4 2,502,259 3/1950 Hulmc 75-12X 5 3,188,197 6/ 1965 Where X L. DEWAYNE RUTLEDGE, Primary Examiner 2,704,248 3/1955 Madanas 7543 X 2,805,930 9/1957 Udy J. E. LEGRU, Assistant Examiner 3,153,588 10/1964 Madaras 7543 X 10 U.S. Cl. X.R.

OTHER REFERENCES Ayala, Jose, Electric Furnace Steelmaking with Sponge

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