DE2320165A1 - PROCEDURE FOR REFRESHING MOLTEN METAL - Google Patents

Description

PATENT Attorney DIPL.-ING. GERHARD SCHWAN

L-8741-G

JOSLYN MFG. AND SUPPLY CO. 2 O ZU IDO 155 North Wacker Drive, Chicago, Illinois 60606, V.St.A.

and

UNION CARBIDE CORPORATION 27O Park Avenue, New York, N.Y. 10017, V.St.A.

Process for refining molten metal

The invention is generally concerned with a method of freshening a molten ferrous metal that allows the nitrogen content of the refined metal to be controlled. In particular The invention relates to a refining process, in the course of which the molten metal is decarburized by sour. · ' substance and a monatomic gas blown below the bath surface will.

A method of refining molten metal by injection of oxygen and argon or an equivalent monatomic gas below the bath surface, in particular to lower the The carbon content of the melt is known (US Pat. Nos. 3,252,790 and 3,046,107).

This well-known argon-oxygen decarburization process acts it is a duplex process that is particularly suitable for refining rust-proof steels without significant loss of chromium

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suitable. After scrap and suitable alloys have melted down in a furnace, the molten metal is purged and transferred to a fresh vessel in which the melt is decarburized by adding the argon-oxygen mixture below the bath surface is blown in. The molten metal is then reduced, refined and tapped into a pouring ladle.

"Reduction" is understood here to mean that from the slag metallic substances, for example chromium or manganese, which have been oxidized during the decarburization stage, are recovered by a less valuable material, for example silicon or aluminum, is added, which has a greater affinity for oxygen than the desired substances, which causes the reduced metallic chromium or manganese to go back into the melt. However, the reduction is not limited to the implementation with solid substances. Due to the performance, with the solved Hydrogen is eliminated from the melt with the inert gas It is possible to remove the oxidized metallic substances in the slag by blowing in hydrogen-releasing gases to reduce, for example of hydrogen, ammonia (NH ^) or a gaseous hydrocarbon, d. H. Methane, propane or natural gas.

In the present context, “fine” is intended to mean any process or process Understood a sequence of operations that involve the reduction to prepare the molten metal for the

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This is followed by tapping and potting, i.e. purifying, desulphurising, Adjusting the final composition, adjusting the temperature and deoxidation.

A suitable fresh vessel that can be turned into a horizontal position for loading, picking up, taking samples and tapping is described in FR-PS 2,056,845. During the Freshly, the vessel is rotated to the vertical position; different Composite mixtures of argon and oxygen are injected through nozzles in the floor or in the side walls of the vessel are attached.

So far, steel manufacturers have been able to determine the final nitrogen content of the Steels only to a small extent mastered, because of the nitrogen content of the steel generally depended on the particular steelmaking process used. For example, were in carbon steels the residual nitrogen values of steels produced using the Siemens-Martin process the lowest, while progressively higher nitrogen values were obtained in steels, for their Production using the normal oxygen converter process, the electric steel process and the Bessemer method were used. The high residual nitrogen contents of Bessemer steels are used of these steels in certain main areas of application] they have consequently contributed to this process in the United States of America practically abandoned and its use has been significantly restricted in other parts of the world.

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Most of the world's production of rustproof steels takes place with the help of electric arc furnace. In this process, the residual nitrogen levels depend on a number of variables away. The most important of these variables include the melt rate, the type of batch, the composition the melt, the freshness and the type of fining. Because these variables are generally based on primary considerations such as the economic efficiency, the availability of raw materials, the Alloy compositions and other residues such as oxygen and sulfur, the steelmaker has little practical use Influence on the residual nitrogen content. It goes without saying possible to increase the final nitrogen content of a melt by adding nitrogen-containing electrolytic to the melt Manganese or ferro alloys are added. However, this approach is associated with various disadvantages. First and foremost, manganese and ferro-alloys are expensive. Then the nitrogen content of the melt is often influenced unpredictable. If the amount of the alloy addition is large, there will be a large amount of heat due to the melting and dissolving of the alloy being added absorbed so that the batch must remain in the furnace for an additional period of time. The addition made in this way nitrogen is therefore at the expense of productivity and economy.

Although US Pat. No. 3,046,107 indicates that nitrogen is equivalent to argon in the argon-oxygen process, this is the case Equivalence is only applicable insofar as it relates to the function during

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of the decarburization process goes, and even this only with those Steel grades in which large residues of nitrogen can be tolerated. This is due to the fact that the exchange from nitrogen against argon to a fresh melt that contains an amount of nitrogen that differs only slightly from differs in the amount that is in equilibrium with the nitrogen in a nitrogen-containing atmosphere, which is the melt at external pressure and the respective temperature and composition of the melt. This amount is commonly referred to as nitrogen f denotes equilibrium value; it can be calculated in a known manner on the basis of theoretical thermodynamic considerations will. In this context, reference is made, for example, to Chipman and Corrigan's "Prediction of the Solubility of Nitrogen in Molten Steel ", Trans. AIME, Vol. 233, July 1965 =

The invention is intended to provide a method for producing a metal to provide dissolved nitrogen at any desired level between about 10 ppm and about 90% of the nitrogen equilibrium level in the molten metal; This method should be easy to implement and economical to design be,

The invention is also intended to improve the argon-oxygen decarburization process which leads to greater economy, namely by during decarburization one or more predetermined proportions of argon by less expensive Nitrogen can be replaced without the nitrogen target

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- 6 value of the generated metal is exceeded.

The invention specifically relates to a method for refining molten ferrous metals or alloys, by means of which the nitrogen content of the refined molten metal can be adjusted to any desired value between 10 ppm and 90% of the value at which nitrogen under external pressure is in equilibrium with the in the melt is dissolved nitrogen; The method includes a decarburization step in which a gas mixture containing oxygen and a monatomic gas is blown into the molten bath surface. According to the invention, such a method is characterized in that, during an initial part of the decarburization, a gas mixture consisting essentially of oxygen and nitrogen is blown in, the nitrogen percentage being kept at a value at which the nitrogen partial pressure in the atmosphere in contact with the melt is greater than the partial pressure of nitrogen is in equilibrium with the desired nitrogen content to be achieved in the refined melt, so that the nitrogen content of the melt at the end of the initial part of the decarburization is greater than the desired nitrogen content; that, furthermore, a gas mixture consisting essentially of oxygen and a monatomic gas or of oxygen, a monatomic gas and nitrogen is blown in during the remainder of the decarburization and that the blowing process is continued with a monatomic gas and / or nitrogen until the nitrogen content of the melt is up the desired value has been lowered.

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For the monatomic used during the remainder of the decarburization Gas can either be supplied by using some of the nitrogen in that used during the initial part of the decarburization Gas mixture is replaced or that the monatomic gas is the one in question Gas mixture is added.

The invention can be used to advantage with a variety of ferrous metals and alloys. They include low-carbon iron, carbon steels, rust-proof steels including ferro alloys with 3O to 4O % chromium, and nickel-based alloys. They can contain tungsten, vanadium, zirconium, copper, aluminum, silicon, sulfur, titanium, manganese, molybdenum and other normally used alloy components. Refining can also be done with separate batches of molten metal or with a changing batch, such as in a continuous process =

The only figure shows a graphic representation of the change the nitrogen content of a melt during the refining process according to the invention, in which an oxygen-nitrogen mixture is blown during an initial part of the decarburization, whereupon a second part follows, within which either argon takes the place of nitrogen in the mixture (curve X) or argon takes the place of nitrogen Mixture is added (curve Y), while the decarburized melt is then only refined with argon.

From the figure it follows that during the initial part of the decarburization,

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while an oxygen-nitrogen mixture is used, the nitrogen content in the melt is raised up to the equilibrium value N ₁ for the respective temperature and composition of the melt and for the external nitrogen gas pressure in the fresh vessel. During the second part of the decarburization, the exchange of nitrogen for argon (curve X) causes a rapid decrease in the nitrogen content of the melt. This second part is continued until the desired carbon content and a predetermined nitrogen value Np are reached, the latter depending on the final nitrogen content N- desired during tapping. Since the continued blowing in of argon during the reduction and refining causes a further, albeit relatively small, decrease in the nitrogen content, the point N ₁ at which the switch is made from nitrogen to argon depends on the amount of argon used during the second part of the Decarburization is blown in, as well as of the final nitrogen content N. ,, which the tapped metal should have.

The curve Y shows the course of the nitrogen content in the melt when the tapped charge is to have a higher nitrogen content N ₄ . During the second part of the decarburization, a three-component gas mixture of argon, oxygen and nitrogen is used. Curve Z shows the course that the nitrogen content in the melt would take if the same final nitrogen value N _{2 were} strived for when using a three-component mixture of nitrogen, oxygen and argon during the second part of the decarburization. In this case it would be the first

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Part earlier, namely at time T. instead of time T _? ended as in the previous example because the rate at which the nitrogen content in the melt falls becomes slower if the blowing gas mixture contains nitrogen instead of, as in the case of curve X, no nitrogen.

In the practical application of the argon-oxygen decarburization process to stainless steel, in which argon is the only gas used in addition to oxygen, it has been observed that the final nitrogen values obtained in the melt after decarburization, reduction and refining have increased by 30 to 50 % below the levels normally found in conventional electric steel processes. On the other hand, as mentioned above, a completely different problem arose when nitrogen is used as the only gas in addition to oxygen during the freshening processes. In the latter case, the nitrogen dissolved in the melt approaches the equilibrium value at the end of the refining process. Although this is not surprising on the basis of theoretical considerations, practical experience so far has shown that a close approximation to the theoretically calculated equilibrium value is never achieved in practice. In this context, reference is made to Ward "The Physical Chemistry of Iron and Steelmaking" 1952, pages 182 and 183. Furthermore, the Bessemer process, in which the melt is blown with air (which contains approximately 79% nitrogen), is the observed nitrogen content of decarburized steel normally between O, O1 and O, O2 %, during the equilibrium value

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- ίο - -

is about 0.04 % . The measured final nitrogen content in such melts is generally between 25 and 50% of the value resulting from theoretical equilibrium considerations. The view that no nitrogen uptake takes place in the melt under oxidizing conditions, such as are given during decarburization, is also represented in US Pat. No. 2,537,103.

Contrary to these well-known opinions, it turned out that ^; When the argon-oxygen decarburization process is carried out in practice with nitrogen as the inert gas, significantly higher nitrogen contents remain in the decarburized melt than would be expected. For example, when using nitrogen and oxygen to refine a 17 t batch of stainless steel AISI 304 (Cr 18 to 20 %, Ni 8 to 10 %, Mn at most 2 %, Si at most 1.0%, C at most 0.08) %) the actual nitrogen content at 0.136%, while the equilibrium value is approximately 0.145 % . The melt thus reached almost 94% of the equilibrium value. The injection of nitrogen during the reduction, desulfurization and refining increased the content to 0.207 % compared to a calculated equilibrium value of 0.247 %, i.e. the nitrogen content was approximately 80% of the equilibrium value.

During the refining, during which the composition of the melt is adjusted so that the desired final composition is achieved, nitrogen can thereby be added in a simple manner.

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be given that nitrogen gas is blown into the melt for a period of time which depends on the desired final value. This process can be referred to as "nitrogen alloying".
Contrary to expectations, it was found that the speed of the
Nitrogen uptake is reproducible to a high degree and, for the reasons mentioned above, is surprisingly high. Using this method, residual nitrogen levels in the melt of up to about 50 % of the equilibrium Np value in one atmosphere can be obtained quickly and economically. Contents of more than 50 % of the equilibrium value can also be achieved ^ However, the speed of the Np uptake begins to decrease and is consequently less effective. Table 1 below shows the results of five tests in which the nitrogen content of commercially available rustproof steels was increased by blowing N _{2 for} only 20 to 69 seconds
became.

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Tabel 1 N ₂
(m3 / h) Time
(S) stole
variety At first-
% N ₂ end
% N ₂ 34O 20th 316 * O.O21 0.032 340 45 316 0.019 0.041 283 34 304 ** 0.044 0.061 283 52 3O4L *** O.O43 0.066 283 69 3Q4L O.O28 0.059

* Cr 16-18% Ni 1O-14%, Mn not more than 2.0 %, Si not more than 1.0%

C at most 0.10 %
** Cr 18-20 %, Ni 8-10%, Mn at most 2.0% Si at most 1.0%

C 0.08 % or less
*** Cr 18-20% Ni 8-10%, Mn not more than 2.0% Si not more than 1.0%

C 0.03% or less. ;

In numerous previous studies of absorption-desorption kinetics of nitrogen it has been found that degassing processes, which work with either vacuum or argon are very inefficient in terms of nitrogen removal are. For example, Pehlke and Elliott refer in "Solubility of Nitrogen in Liquid Iron Alloys - II Kinetics", Trans. of Met. Soc. AIME (1963) points out that this is especially the case when surface-active elements, such as oxygen or sulfur, available. Consequently, it was to be expected that it would be under

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oxidizing conditions, as they exist during argon-oxygen decarburization, would be difficult to drive out significant amounts of dissolved nitrogen from the melt. Surprisingly, however, it has been found that even under the oxidizing conditions of the decarburization process, considerable amounts of nitrogen can be removed. As a result, nitrogen can be used in place of argon during at least the early stages of the decarburization process, although a significant amount of nitrogen is absorbed by the melt. The amount of nitrogen that can be used instead of argon depends on the final value of the melt desired during tapping. For example, if the residual nitrogen content of rustproof steel 3O4 is below 0.05%, nitrogen can be used during decarburization until about 60 % of the oxygen that is mathematically necessary for decarburization has been blown in. In general, the point at which argon is substituted for nitrogen is after 50 to 70 percent of the oxygen calculated to be required for decarburization has been blown. This amount of oxygen is calculated according to conventional stoichiometric rules, taking into account not only the oxygen required for oxidizing des.als CO, but also the oxygen that is used to oxidize silicon and other metallic elements in the melt, which is usually used in the slag pass as oxides. At a point calculated in this way, the nitrogen is replaced by argon in order to ensure further decarburization and the dissolved nitrogen content of the melt to the desired value

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lower.

Two preferred embodiments of the present process are explained with the aid of the following examples.

example 1

This example shows the embodiment in which nitrogen is used as the only gas in addition to oxygen during the initial part of the decarburization, after which the nitrogen is replaced by argon during the second part. Before decarburization, the melt of rustproof steel contained 0.78 % C, 0.51 % Mn, 0.41% Si, 18.25 % Cr and 8.05% Ni. The batch size was 17 t. Table 2 below shows the changes in temperature, carbon content, gas flow rates and nitrogen content at the beginning, during the first and second part of the decarburization and after the reduction process.

Table 2

Temp. Time C. °? Ar N ₂ N ₂ ( ⁰ C) (min) (%) (m3 / h) (Wt.%) Beginning 1477 O 0.78 - _ O.O42 Decarburization
1 . part 1638 25th O, 31 453 - 226 O, O75 Decarburization
Part 2 1620 15th 0.06 198 396 O.O48 reduction 1549 4th 0.07 - 283 - O.O41

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It follows from Table 2 that the nitrogen content of the melt increased from 0.042% to 0.075 % during the initial part of the decarburization due to the blowing in of a gas mixture with a ratio of two parts of oxygen to one part of nitrogen. During the second part of the decarburization, however, the nitrogen content fell to O.O48 % because a gas mixture with one part of oxygen was blown to two parts of argon. The further blowing only with argon during the reduction process lowered the nitrogen content to 0.041 % .

Example 2

This example illustrates a further embodiment of the process described, in which nitrogen is used together with oxygen during the initial part of the decarburization, and then the nitrogen is replaced by argon during the second part of the decarburization. Before decarburization, the rustproof steel melt contained 0.35 % C, 0.34 % Mn, O, 36 % Si, 16.22% Cr and O, 14 % Ni. The batch weighed 17 t. Table 3 below shows the changes in temperature, carbon content, gas flow rates and nitrogen content for the beginning, after the first and second part of the decarburization and after the reduction process.

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Table 3

Temp. Time 1 C. - Ar N ₂ 396 N ₂ (° c) (min) O (%) 452 (m3 / h) 283 (Weight 56) Beginning 1538 O 0 , 35 198 - - O, O36 Decarburization
Part 1 1693 4O 0 , 21 * 283 226 O, O68 * Decarburization
Part 2 1693 8th , 05 O.O75 reduction 1621 4th , 05 O.O56

* Sample taken after 35 minutes of decarburization

Table 3 shows that the high nitrogen content of
0.075 % caused by the use of nitrogen during the initial part of the decarburization, which was pushed down to 0.056% by blowing with argon. The value of 0.036 % nitrogen given in the table does not represent an actual initial value (this was not determined), but is typical for the composition of the melt concerned. Hold on
is that the oxygen flow remained on during the reduction process. This was not done for reasons of decarburization, but to prevent the temperature of the batch from dropping excessively.

In the above examples, the oxygen: argon ratio of the second part of the decarburization became lower than the oxygen:

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Nitrogen ratio during the first part of the decarburization held in order to oxidize as little chromium as possible, but still to ensure that the carbon is oxidized. For the same If nitrogen is used in addition to argon during the second part, the ratio of oxygen becomes the basis to argon and nitrogen is preferably kept less than the oxygen: nitrogen ratio of the initial part of the decarburization.

In the examples above, the various embodiments of the procedure are treated as separate individual processes. It is understood, however, that the various embodiments can also be used in diverse combinations' in order to achieve the most favorable conditions in terms of gas consumption, reproducibility and safe control of the nitrogen content of the end product.

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Claims (4)

- 18 claims 1. A method for refining molten ferrous metals or ferro-alloys while adjusting the nitrogen content of the refined molten metal to any desired value between 10 ppm and 90 % of the value for nitrogen under external pressure in the. Equilibrium with the nitrogen dissolved in the melt, with a decarburization stage in which a gas mixture containing oxygen and a monatomic gas is blown in below the surface of the melt bath, characterized in that a gas mixture consisting essentially of oxygen and nitrogen is blown in during an initial part of the decarburization , wherein the nitrogen percentage is maintained at a value at which the nitrogen partial pressure in the atmosphere in contact with the melt is greater than the partial pressure of nitrogen in equilibrium with the desired nitrogen content to be achieved in the refined melt, so that the nitrogen content of the melt is at The end of the initial part of the decarburization is greater than the desired nitrogen content; that during the remainder of the decarburization a gas mixture consisting essentially of oxygen and a monatomic gas or of oxygen, a monatomic gas and nitrogen is blown in and that the blowing process is continued with a monatomic gas and / or nitrogen until the nitrogen content of the melt reaches the desired value is lowered. 309844/096 5 2. The method according to claim 1, characterized in that in the gas mixture used during the remaining period of decarburization the ratio of oxygen to monatomic gas or the ratio of oxygen to monatomic gas and nitrogen is lower than the ratio of oxygen to nitrogen is kept in the initial part of decarburization. 3. The method according to claim 1 or 2, characterized in that the initial part of the decarburization is completed after 50 to 7O % of the oxygen has been blown in, which is mathematically required for the decarburization. 4. The method according to any one of the preceding claims, characterized in that argon is used as the monatomic gas will. 309844/0965 ZO Blank