US3004847A - Refining of crude iron - Google Patents

Description

1961 L. J. R LAMBERT ETAL 3,004,847

REFINING OF CRUDE IRON Filed Dec. 22, 1958 2 Sheets-Sheet 1 INVENTORS LOUIS JEAN RENE LAMBERT WILHELM SEDLACEK GEORGES MARIE MESSIN TH EIR ATTORNEYS Oct. 17, 1961 L. J; R. LAMBERT ETAL 3,004,847

REFINING 0F CRUDE IRON Filed Dec. 22, 1958 2 Sheets-Sheet 2 Min. /o Fe!) msmgtstwgag INVENTORS LOUIS JEAN RENE, LAMBERT WILHELM SEDLACEK GEORGES MARIE MESSIN THEIR ATTORNEYS United States Patent Ofiice 3,004,847 Patented Oct. 17, 1961 3.00434! REFINING F CRUDE IRON Louis lean Rene Lambert, Wilhelm Sedlacek, and Georges Marie Messin, Pompey, France, assignors to HGT Brassert Gxygen Tee AG, Zurich, Switzerland, :1

company of Switzerland Filed Dec. 22, 1958, Ser. No. 782,118 Claims priority, application Austria Dec. 23, 1957 1 Claim. (Cl. 75--52) The invention relates to a process of refining crude iron having a phosphorus content of more than 1%, particularly basic Bessemer crude iron containing 1.7 to 2.20% phosphorus, by blowing oxygen or an oxygen enriched refining gas ontothe surface of the molten metal in the presence of a basic slag.

Processes of this kind, which have been developed in recent times are referred to as so-called surface blowing processes. They have the object of utilizing the eco nomic and technological advantages of converter processes without involving the disadvantages thereof. T he blowing of pure oxygen or gases with a high content of oxygen, if desired, in conjunction with solids, onto the surface of the bath of molten crude iron, produces a highgrade nitrogen-free steel, which is comparable with or even superior to open-hearth steels. A typical surface blowing process is the so-called LD-process, which is decribed more fully in Stahl und Eisen, 1952, pp. 992 et seq.

Whereas surface blowing processes have previously been used primarily for steelmaking iron and other types of crude iron having a relatively low phosphorus content, attempts have also been made to extend the field of application of the surface blowing processes to highphosphorus crude iron such as basic Bessemer crude iron having a phosphorus content up to 2.20%. In accord ance with a known proposal the dephos-phorization is effected by the formation of a lime-ferrite slag, which is capable of absorbing the phosphorus from the bath. In order to produce a reactive lime-ferrite slag, adequate amounts of iron oxide and lime are, of course, required, as Well as a relatively high temperature, which should preferably be attained at the beginning of the process to ensure a slag of high fluidity, which is essential for the absorption of the phosphorus at a sufficiently high rate. In order to meet this requirement, a known technique comprises the continuous addition of portions of iron oxide or of ore to the bath, e.g., at one-minute intervals. In this procedure, care had to be taken that adequate heat is available during the process for dissolving the slagforming materials and the ore and for keeping them in a fluid condition.

This was the mm'n difiiculty of the known technique because-in view of the temperature required for the formation of slag the combustion of carbon, which is an exothermic reaction supplying the corresponding quantitles of heat, must be performed at a higher rate than would have been desirable for the desired early elimination of the phosphorus. The known processes for refining high-phospholus crude iron had other disadvantcges also. as will be apparent from the following.

A consideration of the metallurgical reactions taking place during the refining of crude iron with oxygen under an iron-oxide containing slag reveals the following interrelations:

Frc-m the theoretical relationships in the system ironcarbon-oxygen it is known that an equilibrium relation exists between the carbon and oxygen content of the melt. This relation may be represented in a simplified manner by curves coordinated to different temperatures. Such a representation is given in FIG. 1, 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 a temperature of 1500 deg. C., 1600 deg. 'C., and 1650 deg. C., respectively, in an equilibrium condition.

It is also known that the equilibrium relation which is characteristic for the decarburization 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 bath. The higher the FeO-content of the slag, the higher is the oxygen content of the bath, other conditions being equal. In other words, 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 because it would be established only after an infinitely long time, these relations involve an important requirement:' It is highly desirable to have at the end of the refining process, when the desired carbon content has been reached, an oxygen content which is not much above the equilibrium curve because an oxygen content higher will render the resulting steel brittle and susceptible to fracture and will preclude its use for certain applications.

The above requirement has been almost ideally fulfilled in the LD process when applied tosteel-making iron or other low-phosphorus crude iron. The steels obtained by the LD process have a minimum oxygen content of 0.020 to 0.040% when a carbon content of 0.20 to 0.05% is reached. If the carbon-oxygen functionobtained during the LD process is compared with the equilibrium curve at about 1600 C., it will be recognized that the values which are practically obtained are not greatly spaced from the curve. In FIG. 2 the result obtained in practical experiments with LD charges have been diagrammatically entered into the carbon oxygen chart in the form of the solid line LD. The equilibrium curve for 1600 C. is represented by the line Gl of FIG. 2.

If the problem of the dephosphorization is reconsidered with all the general metallurgical aspects set forth hereinbefore being taken in account, it must be stated first that the known method of dephosphorization mentioned hereinbefore does not fulfill the important requirement that the refining process should be so controlled that the carbon-oxygen values remain close to the equilibrium curve at the end of the refining process. Entering the carboo-oxygen values obtained when dephosphorizing with additions of iron oxide at one-minute intervals into the diagram of FIG. 2, a curve will be obtained which corresponds to the dash-and-dot line A. The oxygen values achievable when refining in accordance with line A are much above or spaced from the equilibrium curve Gland no steel can be produced which contains less than 0.050% O, which is already inad-missibly high for high-grade steel.

It is an object of the invention to make a steel containing less than 0.030% phosphorus and less than 0.050% carbon from a crude iron having a phosphorus content of more than 1.0%.

According to the invention this object is achieved by carrying out the refining process in three phases, in the first of which the molten charge is reacted with a liquid end slag from a preceding heat and the charge is blown with low blast energy until the carbon content is 3.5 to 2.5%, whereafter the bath is at least partially deslagged and the process is continued in the second phase, after an addition of further slag-forming materials, with low blast energy until the carbon content is 2.0 to 1.0%, whereafter the bath is completely deslagged, new slag forming materials are added and the process is completed in'the third phase, with increased blast energy until the desired carbon content has been reached. The blast energy may be simply controlled by varying the nozzle distance. Where a Laval nozzle or a standard nozzle is used having an orifice diameter of, e.g., 30 mm. and operated at a pressure of 7 to 10 kg./ sq. cm. gauge a distance of the nozzle from the bath of more than 1 meter may be used in the first two phases and a distance ,of less than 1 meter may be used in the third phase, in order to control the blast energy according to the invention. Another or additional method of varying the blast energy consists in the use of nozzles having different orifice diameters.

In order to avoid heat losses, the steel obtained at the end of the refining process may be removed through a tapping hole in the crucible vessel whereas the end slag remains in the vessel. The crude iron charge for the next heat is poured onto this end slag, whereupon the first blowing phase of the next charge is started.

If the carbon-oxygen values obtained with the process according to the invention are plotted into the diagram of FIG. 2 in the same manner as those obtained with the known process, the solid line B will be obtained, which coincides with the line LD of the LD process toward the end of the refining process. This'means that in the process according to the invention the same, favorable oxygen values as in the known LD process are obtained toward the end of the refining operation and the resulting steel is suitable for the highest requirements.

Thus, one main feature of the process according to the invention resides in that the heat-consuming new formation of a reactive slag for each heat is eliminated by utilizing the end slag from a preceding heat, having a low P O -content, e.g., below 6%, and an FeO-content of 15 to 25%. To obtain from this slag a reactive dephosphorizing slag it is sufiicient to add a corresponding quantity of CaO whereby a continuous addition of iron oxide or a formation of iron oxide from the charge by oxidation of iron is not necessary.

The dephosphorization begins immediately by the reaction with the slag. In the first phase of the process the existing phosphorus content is reduced to about 1.0 to 0.7% while the FeO-content of the slag drops at the same time to 10% or less. In the first phase of the process the slag is maintained at a high basicity, e.g., at a Cacontent of more than 50%, to ensure that the SiO finds sulficient free CaO in liquid form and to prevent the formation of acid slags inducing foaming. Oxygen is supplied with a low blast energy during the first phase by adjusting the nozzle tube to a distance of more than 1 meter from the bath surface and maintaining the pressure of the oxygen below 10 kg./sq. cm. gauge, e,g., at 7 kg./sq. cm. gauge. In this case the refining conditions can be roughly compared to the relations in an open-hearth furnace, in which the refining action under an oxidizing atmosphere is also efiected primarily by the slag while the combustion of carbon proceeds at a relatively slow rate. The bath is at least partially deslagged when a carbon content of 3.5 to 2.5% has been reached. It is preferred to permit to run off only one third to one half of the slag rather than the entire slag at the end of the first phase. The slag contains about 50% CaO, up to 20% P 0 and 8 to 10% FeO and may be used as a high-phosphorus fertilizer.

In the following, second phase of the process CaO is added first to increase again the basicity of the slag so that the content of free liquid 6210 will prevent the formation of foams which would disturb the process. The addition'of oxygen with a low blast energy is continued (elevated nozzle tube spaced more than 1 meter from the bath surface) and the refining is continued until the carbon content has been reduced to 2.0 to 1.0%. The phosphorus content toward the end of the second phase may be 0.6 to 0.2%. bath is very carefully deslagged; the slag contains about 50 to 55% CaO, to P 0 and 6 to 8% FeO. This slag has also a composition which enables it to be used as a fertilizer.

At the end of the second phase the In the subsequent, third phase of the process, new slag-forming materials, particularly CaO, sand, fluxes, such as fiuorspar, bauxite and, if desired, some iron oxide, are added to form a new slag and the refining process is completed with increased blast energy (nozzle distance less than 1 meter, e.g., 40 cm., pressure 10 to 15 lag/sq. cm. gauge). Because the phosphorus content had already been reduced to approximately 0.6 to 0.2% at the beginning of the third phase the charge may be finished like an ordinary LD charge without a disturbing effect of the phosphorus content. When a carbon content of 0.2 to 0.1% has been reached, the phosphorus content is 0.020 to 0.040% and the ox gen content is less than 0.050%. For this reason reblowing'is not required.

An important and preferred modification of the process according to the invention resides in that an excessive amount of iron oxide is added to the oath, as a single batch, in the last part of the second phase of the process so that there is a considerable rise in the FeO-content of the slag, by at least 50%, for a short time. The slag may contain more than FeO, e.g., or even Fe@ at this stage, so that it is extremely reactive. In this modification the oxygen supply is interrupted after the sudden addition of iron oxide and the slag having a high content of iron oxide is allowed to act on the bath for a short time e.g., one to two minutes. The slag reacts violently with the phosphorus present in the bath and reduces the phosphorus content down to less than 0.2% phosphorus during the short contact time. This is followed by immediate deslagging, whereafter the third phase is continued as described hereinbefore. It has been found that this batch-wise addition of iron oxide at the end of the second phase causes a great increase in the FeO-content of the slag and provides the conditions for a sudden reduction of the'phosphorus content whereas the oxygen content of the steel is not appreciably increased at the same time. This important effect may be explained by 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 slag constituents CaO and FeO and the phosphorus of the bath, by which reaction the phosphorus is removed from the bath. An other reason resides in that the dephosphorizing reaction (the slagging of the phosphorus) is dependent on temperature and is performed best when a relatively cold slag covers a hot bath. he sudden addition of a large amount of iron oxidecools the slag and thus promotes the reception of the phosphorus.

The characteristic features of the process according to the invention are more fully apparent from FiGS. 3 and 4, which diagrammatically illustrate the refining process. FIG. 3 shows the dependence of the carbon content of the bath on the FeO content of the slag. The line 61 correspondents again to the equilibrium curve. The dash-and-dot line A shows the known method of adding iron oxide in intervals of approximately one minute; the Pet) content lies above 20% FeO throughout the refining process and in accordance therewith the oxygen content of the steel from the heat is also high, as is apparent from the corresponding line A of FIG. 2.

As contrasted therewith, line B of FIG. 3 shows an example of the process according to the invention. The process is started with an end slag from a preceding heat. This slag contains approximately 25 FeQ. The refining process is performed as described, without addition of iron oxides, until the carbon content of 2.5%

has been reached. At the same time the FeQcontent of the slag drops to 10% and the oxygen content of the bath does not amount to more than 0.020%. After partial deslagging the second phase is performed as described and the single large amount of iron oxide is added to the bath at the end of the second phase. As is shown by curve B of FIG. 3 the iron oxide content of the slag rises suddenly to more than 30% FeO. This 5 slag is allowed to act on the bath for a short time. Without establishing the equilibrium of distribution between the FeO of the slag and tlze oxygen of the metal bath the slag is skimmed off at the end of the second phase. It is apparent from line B of FIG. 2 that the oxygen content does not rise substantially or does not rise at all. Because exact oxygen values are not available owing to the violent reaction in this phase, line B is shown only as a dash line in this region in FIG. 2. In any case, the oxygen: values, after the bath has been deslagged and the blowing is continued, are not higher than before the sudden addition of the large amount of iron oxide.

At the end of the third phase the FeO content of the slag and the oxygen content of the bath have reached the same levels as in the ordinary LD process. When a carbon content of 0.2 to 0.1% has been reached the FeO content of the slag is 15 to 25% and the oxygen content of the steel is 0.030 to 0.050%.

FIG. 3 shows also a time curve Z. In accordance therewith the first phase takes about 10 minutes, followed by the first deslagging in three minutes. This is followed by the second blowing period of eight minutes. The following addition of iron oxide and the deslagging after the second phase take 12 to 15 minutes. The blowing period of the third phase is 10 minutes. This is followed by a preliminary sampling of slag and steel, which takes about 3 minutes. Thus, the total duration of one heat is 45 to 50 minutes.

FIG. 4 shows the phosphorus curve (P) of the charge and the phosphorus pentoxide curve (P of the slag. It is apparent that the starting and end slags contain about 4 to 5% P 0 whereas the slags skimmed oif at the end of the first and second phases contain 15 to 20% P 0 The phosphorus curve is of particular interest at the second deslagging, when it suddenly drops to less than 0.2%. The phosphorus content has been reduced to about 0.030% when a carbon content of 1.0 to 0.5% has been reached. For this reason the heats may be intercepted by finishing the refining process when the desired carbon level has been reached without need of taking the phosphorus content into account. The temperature variation is shown by curve T in FIG. 4.

The iron oxide added at the end of the second phase may consist of scarfing scale, ore, or mixtures thereof.

The process according to the invention is explained in more detail in the following examples:

Example 1 The following charge is introduced into a cruciblelike tiltable vessel having a refractory lining: 30 metric tons of liquid crude iron containing 3.6% C, 1.8% P, 0.6% Si, 0.8% Mn, 0.04% S.

3750 kgs. slag from a preceding charge, having a composition of 55% CaO, 4% SiO 6% P 0 20% FeO, 4% MgO, 1.5% A1 0 are added to this charge, as well as 750 kgs. C210 and, if desired, CaF as a flux, to impart an adequate basicity to the slag. A vertically slidable oxygen blowing tube is adjusted to a distance of 150 cm. above the bath and oxygen under a pressure of 7 kg./ sq. cm. gauge is blown onto the surface of the bath. About one third of the amount of slag is allowed to run off after minutes.

A steel sample at this stage has the following analysis: 2.76% C., 0.70% P; the temperature measured with the immersion pyrometer is 1475 C.

A slag sample showed the following contents: 56.1% CaO, 7.1% SiO 8.1% FeO, 18.5% P 0 After an addition of further 600 kgs. CaO to increase the basicity and of further quantities of sand, bauxite and other slag-forming materials to replace the skimmed slag as far as possible, the blowing is continued with an elevated nozzle for eight minutes, whereafter the bath is carefully deslagged. The deslagging takes 8 minutes. The steel has now the following carbon and phosphorus levels: 1.86% C, 0.36% P; the temperature measured 6 with the immersion pyrometer is 1540 C. The slag shows contents of 57% CaO, 18.5% P 0 5.5% SiO 7.5% FeO. The total amount of slag skimmed after phase 1 and 2 is 7000 Rgs. which corresponds to about 250 kgs. per metric ton of steel. After deslagging, new sl'agforming' materials, consisting of CaO, quartz sand, manganese ore, bauxite and, if desired, fluorsp-ar, totalling 3000 kgs, are added and the oxygen nozzle is set to a distance of cm. 0xygen under a pressure of about 8 kg./sq. cm. gauge is blown for 8 minutes. The oxygen nozzle is then approached to the bath to a distance of 40 cm. and the blowing is continued for three additional minutes. Then the steel istapped. 3750 kgs. slag having approximately the following composition remain in the crucible: 55% CaO, 6 % P 05, 25% FeO, 4% SiO The temperature is 1600 C; The remaining slag is used for the following heat.

The steel bath has the following composition: 0.20% C, 0.024% P. i

70 kgs. ferromanganese are added after the steel has been pouredint'othe ladle.

Example 2 The following charge is introduced into a crucible like that of Example 1: 30 metric tons of crude iron containing 3.8% C, 2.00% P, 0.72% Si, 0.9% Mn, 0.03% S.

4000 kgs. liquid slag from a preceding heat, having a composition of about 54% CaO, 5% SiO-,,, 6.2% P 0 20% FeO, 5% MgO and 1.8% A1 0 are added to the slag, as well as 825 kgs. CaO serving to increase the basicity of the slag. After the oxygen blowing tube has been adjusted to a distance of cm., pure oxygen under a pressure of 6 kg./sq. cm. gauge is blown onto the bath for 8 minutes. The oxygen supply is then interrupted and in the following three minutes approximately one third of the slag runs off, having the following composition: 53% CaO, 9% SiO 20% P 0 9.4% FeO. At this stage the steel contains 2.67% C and 0.680% P. After the skimmed slag has been replaced by quartz sand, bauxite and additional 525 kgs. CaO, the blowing with elevated nozzle tube is continued for 10 minutes. The blowing is then interrupted and 675 kgs. scarfing scale (iron oxide) are added to the bath at once, whereby the iron oxide content of the slag is suddenly increased to more than 25 The slag is allowed to act on the bath for two minutes, in which a violent reaction may be observed. The bath is then carefully deslagged and steel and slag samples are taken. The steel has the following carbon and phosphorus contents: 1.5% C, 0.14% P; and the slag contains 56% CaO, 17% P 0 12% FeO as well as 6% SiO The addition of iron oxide, the action of the iron oxide slag on the bath and the deslagging take 15 minutes. so that the third phase of the process begins in the 37th minute. As described in Example 1, a new slag is built up and the blowing is continued for 7 minutes and completed with a nozzle distance of 80 cm. and a pressure of 10 kg./ sq. cm. gauge.

The finished steel contains 0.10% C and 0.018% P. The composition of the slag is similar to that of Example 1 but the P 0 content is lower and amounts to about 3.8 to 4%.

What we claim is:

A process of refining molten crude iron having a phosphorus content of more than 1% in a refractory-lined converter comprising introducing a charge of molten crude iron into a converter containing a molten end slag from a preceding heat, said slag containing more than 15% FeO, less than 6% P 0 and basic slag-forming material corresponding to more than 50% CaO, blowing oxygen against the surface of said charge with low blast energy substantially equal to the blast energy from a nozzle having a 30 mm. orifice spaced one to one and one half meters from the crude iron and at an oxygen pressure of about 7 to 10 kg. per square centimeter gauge 7 until the'carbon content of the molten metal is reduced to 3;5% and 2.5% and the FeO eon-tent of the slag is reduced to not substantially in excess of 10% removing between about one-third andt one-half of the slag from siad converter, adding CaO and other slag-forming material to the converter in an amount to replace the removed slag and maintain the CaO content of the slag about constant, reduce the phosphorus content of the slag and maintain the FeO content at not substantially in excess of 10%, blowing oxygen against the surface of said charge with said low blast energy until the carbon content is about 2.0 to 1.0%, completely removing the slag from said bath, adding slag-forming material composed principally of CaO and other 'basic slag-forming components to the converter and blowing with increased blast energy substantially equal to the blast energy from a nozzle having a 30 mm. orifice spaced about 40 to 80 cm. from said bath and at an, oxygen pressure of 'between about 10 and 15 kg. per square cm. gauge until the desired carbon content has been attained, and maintaining the FeO content of the slag sufliciently low during the blowing operations to produce an oxygen content in the blown iron of less than .050%.

References Cited in the file of this patent UNITED STATES PATENTS 1,137,681 Vogler Apr. 27, 1915 2,440,564 Allard Apr. 27, 1948 2,668,759 Tenenbaum Feb. 9, 1954 2,804,385 Graef Aug. 27, 1957 2,815,275 Richter Dec. 3, 1957 2,853,377 Kalling Sept. 23, 1958 2,893,861 Rinesch July 7, 1959 FOREIGN PATENTS 492,740 Great Britain Sept. 21, 1938 493,610 Great Britain Oct. 10, 1938 OTHER REFERENCES Harrison et al.: Oxygen in Iron and Steel Making, Butterworths Scientific Publications, London, 1956, page 70 relied on.

UNITED STATES PATENT. OFFICE CERTIFICATE OF CORRECTION Patent No, 3 004 847 October 17 1961 Louis Jean Rene Lambert e1; ale

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 7, line 2, before "305%" insert between This certificate superseades Certificate of Correction issued April 3 19620,

Signed and sealed thid 5th day of June 1962o (SEAL) Attest:

DAVID L. LADD ERNEST W. SWIDER Commissioner of Patents Attesting Officer

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