US2893861A - Method of refining crude iron - Google Patents

Description

July 7, 195 R. F. RINESCH I 2,893,861 I METHOD OF REFINING CRUDE IRON Filed July 12, 1957 2 Sheets-Sheet 1 RUDOZF FRANZ R/A/EJM INVENTOR.

July 7, 1959 R. F. RINESCH 2,893,861

METHOD OF REFINING CRUDE IRON Filed July 12, 1957 2- SheetS-Sheat 2 B01701 FRA/VZ lP/NESU/ IN VEN TOR.

METHOD OF REFININ G CRUDE IRON Rudolf Franz Rinesch, Froschberg, Linz, Austria, as-

signor to Bot Brassert Oxygen Technik A.G., Zurich, Switzerland, 2: company of Switzerland Application July 12, 1957, Serial No. 671,459

Claims priority, application Austria May 10, 1957 6 Claims. (Cl. 75-52) This invention relates to a method of refining crude iron containing more than 0.5% phosphorus by blowing pure oxygen or an oxygen-enriched gas onto the surface of molten crude iron and producing a limited reaction area of high temperature at the surface of the bath.

Processes of this kind, which have been developed in recent times, have been described as so-called surface blowing converter processes. They aim at utilizing the economical and technological advantages of converter processes without involving the disadvantages thereof. The blowing of pure oxygen or gases with a high content of oxygen onto the surface of the bath of molten crude iron produces a high-grade steel which is free of nitrogen and which has properties comparable or even superior to those of steels produced in open-hearths. A typical surface blowing converter process is the so-called LD Process developed by the assignor of this invention and described more fully in Stahl und Eisen, 1952, pp. 992 et seq., and in the book Drei Jahre LD-Stahl published by the assignor of this invention in 1956.

In the surface blowing converter processes the dephosphorization of a phosphorus-containing crude iron is effected with the aid of a lime-ferrite slag, which is capable of absorbing phosphorus. Suflicient amounts of iron oxide and lime are required for producing a reactive lime-ferrite slag, as well as a relatively high temperature-this should be attained at the beginning of. the blowing process, if possible-in order to achieve a slag of high fluidity, which is essential for. the absorption of phosphorus by the slag at a sufiiciently high rate.

Two known methods have previously been available for the production of a lime-ferrite slag: After the necessary amount of lime or at least the major portion of said amount has been charged at the beginning of the process the necessary amount of iron oxides may be continually added, e.g., in one-minute intervals, during the refining process or the process may be carried out without adding iron oxide, the amount of iron oxide required for forming the lime-ferrite slag being produced from the crude iron charge itself by oxidizing a portion of the iron in the charge; this can be achieved by a slight blowing of the oxygen under lower pressure or by increasing the reaction area owing to a greater distance of the blowing nozzle from the surface of the bath or by other variations of the blowing technique. Whereas this second method is relatively expensive and causes appreciablelosses of iron it is sometimes preferred to the first method involving an addition of iron oxide because the second method has the advantage that the reactive slag is formed more quickly and in a more highly fluid state because the oxidation of iron in the bath is an exothermic process. When it is desired to make steel containing less than 0.030% P from crude iron having more than 0.5% P one or several deslagging operations are required in both of the known methods.

A consideration of the metallurgical reactions taking place during the refining of crude iron with oxygen under ite tates Patent 2,893,861 Patented July 7, 1959 2 an iron-oxide containing slag reveals the following relations:

From the theoretical relations in the system ironcarbon-oxygen it is known that an equilibrium relation exists between the carbon content and the oxygen content of the melt. This relation may be represented in a simplified manner by curves coordinated to difierent temperatures. Such a representation is given in Fig. l, which shows three curves indicating the carbon content of the bath in percent by weight as a function of the oxygen content of the bath in percent by weight at temperatures of 1500 deg. C., 1600 deg. C., and 1650 deg. 0., respectively, in the state of equilibrium.

It is also known that the equilibrium relation which is characteristic for the decarburization of crude iron is influenced by the composition of the slag inasmuch as the FeO content of the slag tends also to establish an equilibrium with the oxygen content of the slag. The higherthe FeO content of the slag the higher is the oxygen content of the bath where other conditions are equal. In other words, in an equilibrium condition a steel having a given carbon content and refined under a slag rich in iron oxide will contain more oxygen than a steel having the same carbon content and refined under a slag having a lower content of iron oxide.

Whereas this equilibrium can never be actually achieved in commercial refining operations, of course, because it would be established only after an infinitely long time, these relations involve an important requirement: At the end of the refining process, when the desired carbon content has been reached, the oxygen content should not be substantially above the equilibrium curve because a higher oxygen content will impart brittle properties to the steel produced and will preclude its use for special applications. In the LD process this requirement is fulfilled in an almost ideal manner. The steels produced by the LD process are known to have a minimum oxygen content of 0.020 to 0.040% in conjunction with a carbon content of 0.20 to 0.05%. A comparison of the carbonoxygen relation achieved during the LD process with the equilibrium curve, e.g., for the temperature of 1600 deg. C., shows that the values obtained in practice do not greatly difier from the curve. Fig. 2 shows the results achieved in practical experiments with LD charges in the form of the solid line A in the carbon-oxygen chart. the line G1 in Fig. 2 is the equilibrium curve for 1600 deg. C.

If the problem involved in the dephosphorization is reconsidered with all the general metallurgical aspects set forth hereinbefore being taken into account, it must be stated first that the known methods of dephosphorization mentioned hereinbefore do not fulfill the important requirement that the carbon and oxygen values at the end of the refining process should remain close to the equilibrium curve. It is true, however, that in this connection the metallurgist faces a dilemma which cannot be readily solved because on the one hand there must be an ample amount of iron oxide in the slag to ensure a reactive lime-ferrite slag for an effective dephosphorization, on the other hand the steel should have only a minimum oxygen content. This problem has not been solved before. Entering the carbon-oxygen values obtained by the known dephosphorizing method mentioned in the first place hereinbefore, in which iron oxide is added in oneminute intervals, into the chart of Fig. 2 will give a curve which corresponds to the dash-and-dot line C. The carbon-oxygen values obtained when iron oxide is formed by an oxidation of the charge, according to the second method, correspond also approximately to the dash-anddot line of the chart, which is also designated with D. In both cases it is apparent that the oxygen contents achievable when refining according to line C, D are considerably abo e or spaced from the equilibrium curve G1 and no steel can be produced which contains less than 0.050% O, which is inadmissibly high for high-grade steels.

It is an object ot the invention to make a steel containing less than 0.030% phosphorus and less than "0.050% oxygen from crude ironcontaining more than ).5% phospl'forus. According to the invention this object is achieved by adding anexcessive amount of iron oxide as a single hatch to the bath when a carbon content between 2.0% and 0.8%, preferably between 1.5% and 1.0% has been reached, which batch forms together with the other con- :stituents of the slag a reactive slag, which absorbs the phosphorus and is then skimmed off, whereafter the process is continued in the same manner as an usual LD process and completed in known manner. If the carbonoxygen values obtained with the process according to the invention are entered in a similar manner as those obtained with the known methods into the chart of Fig. 2 the solid line B will be obtained, which coincides with thelin'eA of the LD process. Thus, the process according to the invention gives at the end of the refining process the same favorable oxygen values as the known LD process and the steel thus produced is suitable for the highest requirements.

The results achieved according to the invention may be explained with the fact that the establishment of the equilibrium between the FeO content of the slag and the oxygen content of the bath proceeds more slowly than the reaction between the C210 and FeO in the slag and the phosphorus content of the bath, by which reaction the phosphorus is extracted from the bath. Assuming that phosphorus is present as phosphide, the decisive reactionis the following.

' Thereaction is dependent onternperature; the equilibrium will be displaced toward the right if a relatively 'cold slagcovers a hot bath, and toward the left if a hot slag covers a relatively colder bath. It has been found that avery short time of contact is suflicient for a considerable dephosphorization and for a formation of considerable amounts of Ca (PO During this short time of contact theequilibrium of distribution between the FeO in the slag and the bath willnot be established and the bath will not absorb oxygen. Any surplus FeO in the slag 'will react with the carbonof the bath andthe Values approach gradually the equilibrium curve.

1 The characteristic features of the process according to the invention compared to the known methods are apparent from Fig. 3, which shows the relations between the carbon content of the bath and the FeO content of the slag. Line G1 corresponds to the equilibrium curve; line A represents the known progress of an LD process, using a slag of 6 to 8% by weight FeO and continuing the process until "a carbon content of 0.3 to 0.2% Chas been reached. Line C shows the known method of adding iron oxide in one-minute intervals and line D rep resents the known method of surface-blowing oxygen at low pressure to cause a formation of iron oxide'from the crude iron charge. In both of the known methods the iron oxide content is above 20% FeO throughout the refinin process; this involves high oxygen contents or the charges, as is apparent from the lines C and D of Fig. 3 corresponding to line C, D of Fig. 2.

As contrasted therewith the line B in Fig. 3 illustrates anoexample of the process according to the invention. In'this example the LD process is exactly followed until acarbon content of 1.1% has been reached, whereafter a'single batch of iron oxide is added to the'bath. As is shownby curve B of Fig.3 there is a sudden strong increase in the iron oxide content of the slag e.g., to values above 25% or even above 30%; having a high content of iron oxide this slag reacts suddenlywithdhe phosphorus in the "bath. This is follow'ed by deslagging,

whereafter the process is continued according to the known LD refining curve (line A). During the short time of contact between the slag having a high content of iron oxide and the bath the equilibrium of distribution between the FeO content of the slag and the oxygen content of the bath cannot be established. Line B of Fig. 2 shows that the oxygen content of the bath does not or does not substantially increase during the dephosphoringphase. Because accurate oxygen values are not available for this phase owing to the violent reaction, line B has been shown only as a broken line in that region in Fig. 2. In any case the oxygen values after deslagging and continuation of the blowing are not higher than before dephosphorizing.

The addition of a single batch of iron oxide according to the invention should preferably be effected when a carbon content between 1.5% and 1.0% has been reached, as has been mentioned hereinbefore. Whereas the addition may also be made at a somewhat lower carbon content the time available for finishing the charge is then relatively short and there may be shortage of time if the iron oxide is added when the carbon content is less than 0.8%.

The amount of iron oxide added should be suflicient to 'eifec't 'a strong increase in the FeO content of the "slag, at least of 50% and preferably of 200%. After approximately half a minute the iron oxide has completely dissolved and "the dephosphorizing reaction begins; 'the high-phosphorus slag may be skimmed after about "one minute.

Additional 'CaO and, if desired, some FeO maybe added after deslag'gin'g. However, the slag added after dephos'phoriz'in'g should not contain more iron oxide than an ordinary LD charge, i.'e. less than approximately 15% P60.

As has been mentioned hereinbefore the dephosphor- 'izing reactionis dependent on temperature and its progmm is optimum if the sla'g'is colder than the bath. From this aspect too the addition or a larger amount of iron oxide as'a single batch, as taught by'the invention, is favorable because it has a ccooling 'e'tfe'ct. For this reason the 'dephos'phoriz'ation 'takes place within a very short time in the "process according 'to the invention.

The slagskitninedotf'after the addition of iron oxide may contain 15% and more H0 It is very suitable as a'fertilizer.

The process according to the invention will be explainedmo're fully by the 'followingexamplesz Example 1 The following charge is introduced into a tiltable converter provided with a refractory lining: 30,210 'kg. molten crude iron containing 3.96% C, 0.72% Si, 1.74% Mn, 1.14% P, 0.042% S; 4,330 kg. scrap. g 2,700 kg. 'CaO, 150 kg. scarfing scale and 400 kg. CaE are added as slagforming agents. Then an oxygen blowing tubehaving a nozzle 35 mm. in diameter is adjusted at a distance of cm.'above the bath surface and oxygen isblown vertically onto the surface at a pressure of 10kg./sq. om. gauge. 7

After ablowing time of 11.30 minutes the nozzle is removed and 200kg. of scarfing scale and 200 kg. ore (containing 76% iron) are added. A violent reaction begins in'the timebetween 11.45 minutes and 12.25 minutesanerrhebeginning of the'blowing time. The converter'is "slowly lowered; the 'slag is skimmed off after samples "of steel 'and s'la'g have been taken. The steel sample has thefollowin'g analysis: 1.5% C, 0.91% Mn, 0.22% 11 02014 S. The temperature measured with a Pt/Pt-Rh iinrnersion pyro'met'er is 1565 deg. C.

The slag has thefollowing analysis: 11% Fe, 5.83% Mn, 12.26% SiO 3.28% A1 0 50% CaO, 4.79% MgO, 4.99%1.

After raising the converter, 1800 kg. CaO, .100 kg. scarting scale, -300 kg. bauxite, 300 kg. manganese ore and350 kg. quartz sand are added 21.80minutes after the beginning of the blowing time. 22.80 minutes after the beginning of the blowing time the oxygen nozzle is again adjusted to a distance of 80 cm. from the bath surface and the blowing of oxygen is continued at a pressure of 8 kg./sq. cm. gauge. 30 minutes after the beginning of the heat the nozzle is removed, the converter is lowered and samples of steel and slag are again taken. The steel sample has the following analysis: 0.07% C, 0.25% Mn, 0.020% P, 0.01% S. The temperature is 1615 deg. C.

The slag sample has the following analysis: 15.95% Fe, 5.92% Mn, 8.34% Si0 4.19% A1 0 51% CaO, 4.19% MgO, 1.73% P.

The steel is poured into the ladle and 70 kg. ferromanganese (77% Mn) are added. The final analysis of the steel is: 0.07% C, 0.0% Si, 0.31% Mn, 0.015% P, 0.01% S.

Example 2 The following charge is introduced into a converter similar to that of Example 1: 29,300 kg. molten crude iron containing 3.80% C, 0.72% Si, 1.7% Mn, 2.03% P, 0.043% S; 3,500 kg. scrap.

2000 kg. CaO, 250 kg. bauxite, 150 kg. CaF and 460 kg. molten LD slag from a previous charge are added to the charge. The LD slag has the following composition: 15.4% Fe, 7.50% Mn, 10.74% SiO 1.24% A1 0 51.8% CaO, 4.22% MgO, 0.63% P.

The blowing tube equipped with a 35-min. nozzle is adjusted to a distance of 1 meter from the bath surface and oxygen under a pressure of kg./sq. om. gauge is blown vertically on the surface for a period of 11.20 minutes. 700 kg. CaCO are added in increments in the fifth to tenth blowing minutes. After a blowing time of 11.20 minutes a batch of 500 kg. scarfing scale is added to the bath. The dephosphorizing reaction begins, which continues until the end of the twelfth minute. The slag is skimmed oif after samples of steel and slag have been taken. In this stage the steel has the following analysis: 0.87% C, 0.34% Mn, 0.34% P, 0.032% S. The temperature measured with the Pt/Pt-Rh immersion pyrometer is 1505 deg. C.

The slag has the following composition: 15.53% Fe, 7.1% Mn, 16% SiO 1.06% A1 0 40% CaO, 6.14% MgO, 7.85% P.

10 minutes are required for skimming ofi the slag while the converter is tilted so that 22 minutes have elapsed after the beginning of the blowing time.

After the converter has been raised, 1053 kg. CaO, 100 kg. scarfing scale, 100 kg. bauxite, 300 kg. manganese ore and 250 kg, quartz sand are added. After the 23rd minute the oxygen blowing tube is again adjusted to a distance of 1 meter above the bath surface and the blowing is continued for 7.7 minutes so that 30.7 minutes have elapsed after the beginning of the heat. Then the nozzle is removed, the converter is lowered and samples of steel and slag are taken.

The steel sample has the following anaylsis: 0.03% C, 0.27% Mn, 0.033% P, 0.024% S. The temperature is 1605 deg. C.

The slag sample has the following analysis: 17.30% Fe, 5.22% Mn, 7.76% SiO 4.25% A1 0 44.8% CaO, 4% MgO, 3.56% P.

The steel is carburized while being poured into the ladle and 50 kg. ferromanganese are added. The final analysis is: 0.11% C, 0.0% Si, 0.45% Mn, 0.02% P, 0.018% S.

What I claim is:

1. A method of refining molten crude iron containing more than 0.5% phosphorus in a refractory-lined converter, which comprises blowing oxygen onto the surface of the molten crude iron to produce a reaction zone in which the oxygen reacts with the carbon and other impurities accompanying the iron in the presence of a basic slag, adding an iron oxide carrier to the bath when a carbon content between 2.0% and 0.8% in the molten charge has been reached to increase the iron oxide content of the slag at least 50% and form thereby a reactive slag and allowing said slag to react with the bath for a period not substantially in excess of one minute to extract at least the major portion of the phosphorus, skimming oif the slag and continuing the blowing until a steel having the desired content of carbon and a content of less than 0.030% phosphorus is produced.

2. A method of refining molten crude iron containing carbon and other oxidizable impurities and more than 0.5% phosphorus in a refractory-lined converter which comprises blowing oxygen against the surface of the molten crude iron to produce a reaction area in which the oxygen reacts with the carbon and other impurities accompanying the iron in the presence of a basic slag containing iron oxide in an amount insufficient to extract the phosphorus from the bath, adding a batch of an iron oxide carrier to the bath when a carbon content between 2.0% and 0.8% in the molten charge has been reached in an amount to increase the iron oxide content of the slag by between about 50% and 200% to form thereby with the other constituents of the slag a highly reactive slag, allowing said slag to react with the bath for a period not substantially in excess of one minute, skimming otf the reacted slag containing the major portion of the phosphorus of the bath and continuing the blowing until a steel having the desired content of carbon and a content of less than 0.030% phosphorus is produced.

3. A method as set forth in claim 2, in which said excessive amount of an iron oxide carrier is added, when a carbon content between 1.5 and 1.0% has been reached.

4. A method of refining molten crude iron containing more than 0.5 phosphorus in a refractory-lined converter, which comprises blowing oxygen against the surface of the molten crude iron in a sufiicient amount to produce a high temperature reaction zone in which a rapid reaction between the oxygen and the carbon and other impurities accompanying the iron takes place in the presence of a basic slag containing burnt lime and less than 15% by weight iron oxide, adding an iron oxide carrier to the bath as a single batch to form a reactive basic slag having an iron content of more than 25% by 7 weight, when a carbon content between 2.0% and 0.8% in the molten charge has been reached, allowing said slag enriched with iron oxide to react with the bath for a period not substantially in excess of one minute, skimming oIf the reacted slag containing at least the major portion of the phosphorus contained in the crude iron and continuing the blowing under a basic slag having an iron oxide content of less than 15% by weight until a steel having the desired final content of carbon and a content of less than 0.030% phosphorus is produced.

5. A method as set forth in claim 4, in which the blowing after deslagging is continued under conditions causing a combustion of the carbon still contained in the bath at a lower rate than before the batch-wise addition of iron oxide.

6. A method as set forth in claim 5, in which the blowing after deslagging is continued at a lower pressure of oxygen than before the batch-wise addition of iron oxide.

References Cited in the file of this patent UNITED STATES PATENTS 2,050,803 Perrin Aug. 11, 1936 2,668,759 Tenenbaum Feb. 9, 1954 2,793,110 Kosmider et a1 May 21, 1957 FOREIGN PATENTS 492,740 Great Britain Sept. 21, 1938 717,975 Great Britain Nov. 3, 1954 OTHER REFERENCES Ser. No. 288,900, Bertscherer et a1. (A.P.C.), pub lished May 4, 1943, now abandoned.

Claims (1)

1. A METHOD OF REFINING MOLTEN CRUDE IRON CONTAINING MORE THAN 0.5% PHOSPHORUS IN A REFRACTORY-LINED CONVERTER, WHICH COMPRISES BLOWING OXYGEN ONTO THE SURFACE OF THE MOLTEN CRUDE IRON TO PRODUCE A REACTION ZONE IN WHICH THE OXYGEN REACTS WITH THE CARBON AND OTHER IMPURITIES ACCOMPANYING THE IRON IN THE PRESENCE OF A BASIC SLAG, ADDING AN IRON OXIDE CARRIER TO THE BATH WHEN A CARBON CONTENT BETWEEN 2.0% AND 0.8% IN THE MOLTEN CHARGE HAS BEEN REACHED TO INCREASE THE IRON OXIDE CONTENT OF THE SLAG AT LEAST 50% AND FORM THEREBY A REACTIVE SLAG AND ALLOWING SAID SLAG TO REACT WITH THE BATH FOR A PERIOD NOT SUBSTANTIALLY IN EXCESS OF ONE MINUTED TO EXTRACT AT LEAST THE MAJOR PORTION OF THE PHOSPHOROUS, SKIMMING OFF THE SLAG AND CONTINUING THE BLOWING UNTIL A OF LESS THAN 0.030% PHOSPHORUS IS PRODUCED.

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