US2741554A - Method of refining iron - Google Patents

Description

Filed Aug. 26, 1955 1 1/ I I I /2 INVENTOR FRANZ RUDOLF RINESCH IS ATTORNEYS United States Patent METHOD 0? REFINTNG IRGN Rudolf Franz Rinesch, Linz, Austria Application August 26, 1955, Serial No. 530,7ii9

19 Claims. (Cl. 75-52) This invention relates to improvements in methods of refining metals, and it relates particularly to methods of refining pig iron to produce steels of a quality at least as good as open-hearth steels.

This application is a continuation-in-part of copending applications Serial Nos. 485,571, filed February 1, 1955, and 493,024, filed March 8, 1955.

The manufacture of steel in this country is largely conducted by the open-hearth method. Some steel, although a minor proportion thereof, is made by the acid Bessemer process involving blowing in a Bessemer converter containing an acid lining. An even smaller amount is made in Bessemer converters having a basic lining.

Steels made by the open-hearth method are generally high in quality and may be used for many purposes. The principal disadvantages in their manufacture are the cost of the open-hearth installation, long residence time in the hearth, and the cost of labor involved in the operation of the open-hearth.

Bessemer processes are not as satisfactory in operation as the open-hearth method largely due to the fact that the phosphorous and nitrogen content of the resulting steel is relatively high, thereby imparting properties to the steel which render them unsuitable for many purposes and otherwise less satisfactory than the open-hearth steel. Moreover, the thermal eficiency of the Bessemer processes is low and loss of iron in the slag and dust is quite high.

The present invention relates to a method of refining pig iron or pig iron and scrap economically with inexpensive equipment as compared with open-hearth equipment to produce steel similar to and in some instances superior to open-hearth steel.

More particularly, the method embodying the present invention enables pig irons of varying composition and including substantial proportions of phosphorous to be refined by blowing substantially pure oxygen against the surface of the molten metal in a converter and by regulating the slag composition and temperature to enable the removal of phosphorous and other impurities without interrupting the refining operation so that it can be completed in a relatively short time as compared with prior practice.

The process is practiced with converters of relatively small size, that is, having a capacity of 2 to 50 metric tons of charge which are, as compared with an open-hearth converter, relatively inexpensive and are easily built and maintained. The principal expense in an installation of the type embodying the present invention is the cost of oxygen generating apparatus for supplying substantially pure oxygen (i. e., about 99% oxygen or higher with the remainder principally of inert gases) for the refining or converting operation. The provision of two converters of the type mentioned enables almost continuous production of steel because each converter is capable of being operated through two hundred or more blows before relining is required. In the normal course of operation, this amounts to seven to ten days of almost shut down.

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continuous operation, thereby enabling one converter to be laid up for repair while the other continues in service. As compared with an open-hearth converter, only a small maintenance and operating force is required and the controls for operation of the system are relatively few and inexpensive thereby enabling a highly productive plant to be installed at relatively small initial, maintenance and operating cost.

For a better understanding of the present invention, reference may be had to the accompanying drawing in which the single figure is a view in vertical section of a typical converter for practicing the present invention.

As shown in the drawing, the converter 10 includes an outer shell 11 formed of steel plate or the like and having a permanent lining of magnesite brick 12. The lining 12 is extended around the bottom of the converter and may include two or three layers of magnesite brick. The inner surface of the lining 12 is coated with a dolomite-tar mix which is rammed into position to form a ram coat or lining 13 and the bottom lining 14 of the converter. An inner lining of dolomite-tar bricks 15 is also disposed Within the lining 13 to form the wearresistant lining of the converter. The layer of bricks 15 and the dolomite mix lining 13 is basic or alkaline inasmuch as the magnesitic dolomites have about the following composition:

Percent MgO to 80 CaO 10 to 25 SiO2 2 to 5 A1203 l FezOa 4 t0 6 Ignition loss 0.5 to 2 The bricks and the ram lining are formed by mixing dolomite With a suitable tar. The bricks are pressed from this mixture. The lining described above has the advantage that it shows the wear on the lining and also serves to indicate when the converter should be taken out of service for relining. For example, when the inner lining 15 is burned through, the ram lining 13 is exposed indicating that the time for repair is approaching and when the lining 13 wears or erodes through, the permanent lining 12 is exposed indicating that the converter should be This avoids danger of rupture of or serious damage to the converter. While a basic lining is used in the present operation, it will be understood that acid or other linings can be used when the converter is used for refining other pig irons. Magnesite brick can be used also for the inner lining 15.

Experience shows that the lining of the converter lasts for about seven to ten days of practically continuous temperatures promote the removal of phosphorous sothat the slag temperature should be maintained below the temperature of the molten metal. By adding limestone, lime or cold slag to cool the slag at about the time a the slag attains the temperature of the molten metal, the

removal of phosphorous is assured without the need for stopping the blowing operation for deslagging. lnthis way, the retiring operationcan be made continuous and completed in about 15 to 30 minutes under most conditions. Limestone is the most satisfactory cooling agent because it removes heat at almost twice the rate of burnt lime, for example.

The oxygen for refining the-metal is introduced through a lance 20 or pipe, preferably having a copper tip or nozzle and having a water-cooled jacket thereon to prevent damage of the nozzle 'by the intense heat to which it is subjected. The lance is suspended from a crane or hoist 22 which allows it to be adjusted up and down in the converter to space it a proper distance from the surface or the molten metal in the converter. The diameter of the nozzle is between about 1.1 and 1.4 inches for a 30 ton converter.

()xygen is supplied to the lance ;at apressure between about and 12 atmospheres and 4000 to 8000 cubic meters of oxygen per hour are supplied to the converter. When using a nozzle or tip 1.35 inches in diameter, the oxygen is supplied at atmospheres pressure.

For a typical charge of approximately 34 metric tons containing about 83% pig iron and the remainder scrap, the blowing time is about eighteen minutes thereby requiring approximately 2000 cubic meters of oxygen for 34 tons of charge. The theoretical oxygen consumption to convert the carbon in the pig iron to carbon monoxide, amounts to about 57 cubic meters per metric ton of ingots and is only slightly exceeded in the present operation.

A typical operation in the production'of a suitable steel may involve the use of a pig iron having almost any desired composition. Pi'giron produced froin'o'r'e'co'ntaining as much as 2.5% phosphorous hasb'een refined successfully by this method. Pig irons having about the following cornposition are typical:

3.8 to 4.2 percent carbon 0.2 to 1.5 percent silicon 0.6 to 2.8 percent manganese 0.10 to 2.00 percent phosphorous Maximum of 0.08 percent sulfur Pig iron of this general composition is charged into the converter in a molten condition and scrap in proportion of about 16 to 18 percent is added. The amount of scrap is based on the silicon and phosphorous content of the pig iron. Usually, the scrap is charged by means of chutes into the tilted converter in about the first three minutes of the operation. Liquid pig iron is-introduced into the converter to produce an approximately'30'metric ton charge in the converter. After the pig-ironischarged, the converter is'swung to an upright position and suitable stag-forming ingredients are introduced by'chutes on each side of the converter. Burnt lime having an average chemical composition about as follows may be used:

CaO, 86%

H2O as Ca (OH)2, 2%

CO2(CO)3, 3%

r M m 4% For slag development, lime and small-lumpy limestone may be used. The percentage of CaO *may vary between about 3% and 12% by weightofthe 'molten'ine'tal depending on the silicon, manganese and phosphorous conresist/amen in the steel.

The blowing of the converter is then started with the nozzle adjusted to a spacing of about 36 inches above the surface of the molten metal. Oxygen is supplied to the nozzle at about 10 atmospheres pressure and at a volume of about 5000 to 5500 cubic meters per hour when measured at atmospheric pressure. This corresponds to about 160 cubic meters per ton per hour. Within about threetenths of a minute, the carbon flame produced by oxidation of the carbon in the charge and burning of the carbon monoxide, extends to 3 to 6 feet over the converter mouth and at the end of the first minute of blowing, the flame grows to about ZO-feet in length.

During about the first three minutes of blowing, enough oxygen is supplied to make possible the simultaneous oxidation of silicon and carbon thereby generating heat and raising the temperature to a high degree. For'example, about 10% excess of oxygen is supplied above the theoretical requirements for equilibrium between the carbon and oxygen of the bath at a temperature of 1650 C. In this way, the temperature of the converter is raised quickly.

During the next four minutes of blowing, the oxygen feed is increased to between about 6000 and 7000 cubic meters'per hour (180 cubic meters per ton per hour).

At about the end of the eighth minute of blowing, it is necessary to begin the addition of sla -cooling components to assure removal of phosphorous to a degree cornparabie to the phosphorous content of open-hearth steels. Cooling is accomplished by the addition of such slagformingingredients as lime, cold slag but most desirably by limestone in small granular or particle size which, in being decomposed by heat, reduces the temperature of the slag. Thelimestone is added at two minute intervals until about the fourteenth minute of blowing. With very high phosphorous metals, the addition of limestone can be continued until about two minutes before the end of the blow. The principal object of the addition of the slag-cooling ingredients is to keep the temperature of the slag below the temperature of the metal and, for best results, on the order of 20 C. below that of the molten metal. Evidently, the temperature of the slag approaches the temperature of the metal at about the eighth minute of blowing and the ability of the slag to retain the phosphorous products is decreased with an increase in slag temperature. Slag-forming ingredients can be added Within the range of about 4 percent to 20 percent of the molten metal charged.

The total slag additions for a typical heat will be between about 15% and 17% and this slag will have about the following composition:

The limestone should be added at such a rate that the ratio of silica to lime is maintained between about 1 to 2 and 1 to 3. The'additionsat the end of each two minute interval maintain the slag at the relatively lower temperature until the steel reaches the desired temperature of about 1620 C.

As blowing proceeds, the flame becomes brighter until the flame temperature attains its peak between about the twelfth and seventeenth blowing'minute. At the end of about eighteen minutes of blowing, the flame becomes shorter and less brilliant'and begins to flutter indicating the end of the reaction period. The blowing nozzle '20 is th'enretracted'and'theoxygen supplied thereto is shutoff. Immediately'thereafter, or at the e'nd of about 18.3 min utes of blowing, "slag and steel samples are taken'a'nd deslagging of the melt and stiifening thereof with burnt lime or crushed cold slag is accomplished. The steel is then poured into a ladle of suitable capacity to be turned into molds and immediately following this operation, the converter liner can be repaired, where necessary, With tar magnesite-dolomite. The total time required to charge, blow and empty the converter in the above-described example is about forty minutes, but can vary from 30 to 60 minutes. The converter can be reused repeatedly so that as many as 45 charges can be refined during a period of twenty-four hours. The total output, therefore, of two 30 ton converters operating alternately (one in operation and the other undergoing repair) is on the order of 36,000 metric tons of steel per month.

It is a distinctive advantage of the herein described top blowing steel process that the oxygen pressure may be varied within relatively wide limits. This provides a means for exercising precise control over the carbon end point and the consequent results of obtaining a finished steel with a minimum oxygen content, which in most cases does not require the use of subsequent deoxidizing practices.

In accordance with the foregoing, a preferred embodiment of the invention provides for a reduction in the oxygen pressure near the end of the blowing time in order to reduce the rate of flow of the oxygen so that the rate of carbon oxidation in the critical range near the end of the carbon end point, for example, 0.15% to 0.10% is reduced. This prevents a sudden collapse of the carbon flame and provides a longer period of time for observance of the color to flame length change and flame drop indicative of the carbon end point, for example, 0.05%.

In actual practice, the foregoing precise control over the carbon end point is exercised by reducing the oxygen pressure (as measured'in the line feeding at the lance nozzle) from the selected operating pressure existing during the major portion of the blowing time, for example, from 8 to 12 atmospheres to from about 4 to 8 atmospheres. The time at which such reduction in oxygen pressure is efiected depends upon the total blowing time which is predetermined for each blow, depending upon such factors as pig iron analysis, scrap addition, or other factors. In other words, the rate at which carbon oxidation is made to occur, as well as the burning out of the other impurities, such as silicon and phosphorous, presents the basis for selecting the total blowing time. The time at which the reduction in pressure is effected varies inversely with the total blowing time, that is to say, for a total blowing time of 18 or 19 minutes, the oxygen pressure is reduced about 4 minutes before the end of the blow, whereas for a total blowing time of from 21 to 23 minutes, the oxygen pressure is reduced about 2 minutes before the end of the blow. In any event, it is the preferred practice to reduce the oxygen pressure within a range of from about 2 to 6 minutes prior to the end of the predetermined blowing time.

A typical example of the method with a pig iron produced from a high phosphorous content ore is as follows:

A 30 ton converter of the type described above is charged with 5 tons of scrap and 26 tons of pig iron having the following composition:

3.8% carbon 1.5% silicon 1.9% manganese 0.25% phosphorous 0.05% sulfur 2 tons of burnt lime are added as slag forming components.

A blowing nozzle having a diameter of 1.3 inches is introduced into the converter and oxygen is blown through the nozzle at the rate of 160 normal cubic meters per ton per hour with a nozzle spacing of 36 inches from the surface of the molten metal. At the end of the first 3 minutes of blowing, the rate of oxygen feed is raised to 180 normal cubic feet per ton per hour.

At the eighth minute of blowing, the temperature of the slag has approached the temperature of the molten metal and the addition of slag cooling limestone is begun. Additions of limestone are made at the rate of five hundred pounds every two minutes beginning at the eighth minute and ending at the fourteenth minute. At the end of the fifteenth minute of blowing, the rate of oxygen supply is reduced to normal cubic feet per ton per hour and this blowing rate is maintained until the flame becomes shorter and begins to flutter and die indicating the end of the refining operation. In the example given, the flame began to die at the eighteenth minute of blowing and the lance was retracted and the oxygen shut off at eighteen and one-half minutes after the start of the blow.

The steel resulting from the operation had the following composition:

0.06% carbon 0.020% phosphorous 0.025% sulfur 0.37% manganese The steel resulting from the refining operation described above will be controlled, of course, by the composition of the charge. Thus, the manganese content of the resulting steel will depend upon the original manganese content of the pig iron, the quantity and kind of slag and the temperature of the melt. For example, a pig iron containing 1.4 to 1.8% manganese can be converted by the present process to a steel containing 0.30% to 0.35% manganese while a pig iron containing 2% to 2.20% manganese can be converted to a steel containing 0.40% to 0.45% manganese. If a high manganese content is required in the final product, it can be increased by the addition of ferromanganese in the ladle.

The phosphorous content of a steel can be reduced to about 0.005% to 0.04% with most of the steel containing about .020% phosphorous by operating in the manner described above even with a pig iron containing as high as 0.175% phosphorous or even higher, as indicated above. The control of slag temperature by addition of slag-cooling agents acts to reduce the phosphorous content of the steel Without requiring interruption of the blowing for deslagging.

The sulfur content of the melt is also reduced to about fifty percent of its initial amount. Steels containing about between .028% and .012% sulfur can be produced from pig iron containing 0.045% sulfur.

The nitrogen content of the steel likewise is very low because the substantially pure oxygen introduced has a very low nitrogen loading. The nitrogen content of the resulting steel is between a-bott 0.002% and 0.006% and usually about .003

The silicon content of the pig iron controls the addition of scrap to the converter. Ordinarily, pig irons of the type indicated require between about 5% and 30% scrap. The amount of scrap added to the charge averages between about 16% and 20%.

The carbon content of the steel can also be regulated as required and various types of steel, for example, rimming steels containing 0.03% to 0.20% carbon, semikilled steels up to 0.24% carbon, killed steels up to 0.60% carbon and more, and structural steels containing up to 1.5% manganese Cr A. S. O. can be produced by adding FeMn or FeCr A. S. O. and carbon in the converter or ladle in accordance with the present invention.

The method and installation embodying the present invention provide considerable flexibility in the production of steels of high quality and, in fact, are capable of producing steel comparable and in many instances superior to open-hearth steel without the installation costs which characterize plants for the manufacture of openhearth steel. The method is particularly suitable for ing phosphorous content, which comprises blowing sub st'antially pure oxygen onto the surface of said molten metal in a basic reaction Zone, maintaining a basic slag on said molten metal, controlling the temperature of said slag such that it is maintainedbelow that of the molten metal by addition of a slag cooling agent to the slag during blowing to reduce the phosphorous content of the metal at the carbon end-point to that comparable to openhearth steel, and supplying oxygen to the molten metal at a rate such that refining is completed in from about to about 30 minutes, while limiting the total oxygen supplied to an amount not substantially in excess of that stoichiometrically required for refining the molten metal to the refined steel analysis based on oxidation of the carbon to carbon monoxide.

2. A process according to claim 1 in which the oxygen issupplied during blowing at a rate of from about 160 to about 180 cubic 'meters per hour per ton of metallic charge.

3. A process according to claim 3 in which limestone is added periodically during blowing to the existing slag to cool the same and to maintain the silica to lime ratio of the'basic slag between about 1 to 2 and about 1 to 3.

4. A process according. to claim 3 in which the basic slag is formed by addition of lime to maintain the silica to limeratio-of the slag between about 1 to 2 and about 1 to 3, and cold basic slag is added periodically during blowing to the existing slag as the slag cooling agent.

5. A process for producing steel having the characteristics of open-hearth steel from pig irons of varying phosphorous content, which comprises introducing a charge of steel scrap and a predominant proportion of molten pig iron into a basic reaction zone, introducing lime into the reaction zone to form a basic slag, blowing the molten metal by directing a jet of substantially pure oxygen downwardly into contact with the surface thereof, maintaining the temperature of the slag below that of the molten metal by addition of a basic slag-forming agent during blowing to cool the slag and to reduce the phosphorous content of the metal at the carbon end-point to that comparable to open-hearth steel in a single slagging operation, and supplying oxygen at a rate such that refining is completed in from 15 to 30 minutes, While limiting the total oxygen supplied to an amount not substantially in excess of that stoichiometrically required for refining the molten metal to the refined steel analysis based on oxidation of carbon to carbon monoxide.

6. A process according to claim 5 in which the phos-:

a 7. A process according to claim 5 in which oxygen is supplied during blowing at a rate of from about 1-60 to about cubic meters per hour per ton of metallic charge. a

8. A process according to claim 5 in which the slag forming and cooling agent is limestone, said limestone being added to the existing slag atabout two minute intervals from about the eighth minute of blowing and at a rate to maintain a silica to lime ratio between about 1 to 2 and 1 to 3.

9. A process of refining predominantly pig iron-containing metal to produce steel having characteristics at least equal to that of open hearth steel with respect to phosphorous, nitrogen and sulfur content, which comprises introducing a charge of said molten metal containing carbon, silicon, sulfur, manganese, and phosphorous into a basic reaction zone, adding a lime-containing slag'forming agent to form a basic slag, blowing the molten metal with substantially pure oxygen directed downwardly into contact with the surface thereof, cooling the .slag during blowing by periodic additions of limestone to the slag to reduce the phosphorous content of the metal at the carbon end point to that comparable to open-hearth steel, supplying oxygen at arate such that refining is completed in from 15 to 30 minutes, and reducing the rate of'flow of oxygen to the molten metal in the latter portion of the blow before the carbon end-point is reached.

10. A process according to claim ,9 in which the rate of flow of oxygen is reduced from about 180 cubic meters per hour per ton of metallic charge to about'160 cubic meters per hour per ton from about 2 to about 6 minutes prior to the end of the blow.

References Cited in the file of this patent UNITED STATES PATENTS 1,032,653 Brassert July 16, 19.12 1,338,655 McCaifery Apr. 27, 1920 FOREIGN PATENTS 496,545 Great Britain Nov. 28,1938 546,020 Great Britain June 24, 1942 615,611 Great Britain Jan. 10, .1949 685,326 Great Britain Dec. 31, 1952 700,255 Great Britain Nov. 25, 1953

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