FI73740B - KONTROLL AV KVAEVEHALTEN I ROSTFRITT STAOL I SAMBAND MED STAOLFRAMSTAELLNINGEN. - Google Patents

Description

73740

Adjusting the nitrogen content of stainless steel during steelmaking

The control is based on the cross-section of the ground and the ground floor.

The invention relates to a process for adjusting the nitrogen content of stainless steel to a predetermined desired level during steelmaking between 10 ppm and 90% of the equilibrium nitrogen content of molten steel at ambient pressure, the method comprising the step of quenching molten steel by injecting a gas mixture containing oxygen and at least one gases argon and nitrogen.

A process for refining molten metal by injecting oxygen and argon or a similar monoatomic gas below the surface, especially to reduce the carbon content of the molten metal, is disclosed in U.S. Patent No. 3,252,790 and an improvement to programmed blowing is disclosed in U.S. Patent No. 3,046,107.

Such a known argon-oxygen annealing process (AOD process) is a two-step process and is particularly useful for refining stainless steels without high chromium loss. After the scrap and suitable alloys are melted in the furnace, the slag is removed from the metal, which is transferred to a refining vessel where it is quenched by blowing an argon-oxygen mixture under the surface. The molten metal is then reduced, finished and poured into a ladle.

2 73740 As used herein, the term "reduction" refers to the recovery of metals such as chromium and manganese oxidized in the slag by adding a less expensive material, such as silicon or aluminum, with a higher oxygen affinity than the desired materials, returning the reduced chromium or manganese to the melt. However, the reduction is not limited to use with solid materials. As a result of the efficiency with which dissolved hydrogen can be removed from the melt by an inert gas, it is possible to reduce oxidized metals from the slag by injecting hydrogen or a hydrogen-releasing gas such as ammonia (NH4) or a hydrocarbon gas such as methane, propane or natural gas.

The term "finishing" is used to refer to some or all of the conventional post-reduction steps that prepare molten metal for pouring and casting, such as slag removal, desulfurization, final composition control, temperature control, and oxygen removal.

A suitable refining vessel that can be rotated horizontally for loading, holding, sampling, and pouring is disclosed in FR Patent Publication 2,056,845. For refining, the vessel is rotated to a vertical position and varying amounts of argon and oxygen mixtures are injected from nozzles located at the bottom or walls of the vessel. Until now, steelmakers have had little control over the nitrogen content of the steels they produce, as the nitrogen content of steel has generally been specific to the particular process used to make it. For example, in carbon steels, the residual nitrogen contents have been the lowest in martin-produced steels and progressively higher nitrogen concentrations have been obtained in steels produced by the basic siemens-martin method, the electric furnace method, and the bessermer method. In fact, the high residual nitrogen contents of Bessermer steel have prevented the use of such a material for some major uses and have therefore contributed to the direct abandonment of this method in the United States and the sharp reduction in its use in other countries.

3,73740

Most of the world’s stainless steel is made in electric arc furnaces. In this method, the residual nitrogen concentrations depend on a number of variables, the most important of which include the melting rate, the nature of the batch, the melt composition, the refining time, and the finishing methods. Because these variables have generally been adjusted for determinants such as economy, available raw materials, alloy requirements, and other residual concentrations such as oxygen and sulfur, the steelmaker has had very little practical ability to adjust the residual nitrogen content. It is, of course, possible to increase the final nitrogen content of the melt by adding a nitrogen-containing electrolytic manganese or iron alloy, but this procedure has many disadvantages. First, manganese and iron alloys are expensive and second, the result of adjusting the molten nitrogen content is uncertain. If the amount of mixture added is large, its melting and dissolution will absorb a large amount of heat, requiring more furnace time to replenish the heat. The nitrogen addition carried out in this way has been at the expense of the yield and economy of the process.

Although the aforementioned U.S. Patent No. 3,046,107 discloses that nitrogen is equivalent to argon in an argon-oxygen process, this equality can only be applied to its function in rejuvenation, and even then only to steels that allow high residual nitrogen concentrations. This is because the exchange of argon for nitrogen results in the refined melt containing an amount of nitrogen close to that in equilibrium with the nitrogen of the nitrogen-containing atmosphere surrounding the melt at ambient pressure and melt temperature and composition. This amount is commonly referred to as equilibrium nitrogen content and can be calculated by theoretical thermodynamic examination using a technique familiar to the person skilled in the art; see, e.g., Chipman and Corrigan, "Prediction of the Solubility of Nitrogen in Molten Steel," Trans. ΑΙΜΕ, Vol. 233, July 1965.

Autere, E., Ingman, A., Tennilä, P., "Foundry Technology", Helsinki 1969, pp. 46 ... 47 generally states that nitrogen gas does not dissolve in cast iron. According to the publication, the removal of dissolved nitrogen from molten cast iron is in principle possible by gas purging, even using nitrogen gas. However, the publication does not disclose the use of nitrogen 4,73740 as an argon replacement gas in the argon-oxygen process, nor a model for how the nitrogen content in molten metal could be purposefully controlled.

It is an object of the present invention to provide a simple and economical process for the manufacture of stainless steel containing dissolved nitrogen in an amount ranging from 10 ppm to 90% of the equilibrium nitrogen content of the molten metal as required. Another object of the present invention is to provide an economic improvement in the argon-oxygen annealing process by replacing one or more portions of argon with cheaper nitrogen in the annealing, without exceeding the nitrogen requirements of the metal produced.

In particular, this invention relates to a process for the refining of molten steel as set out in the preamble, characterized in that a gas mixture of oxygen and nitrogen is injected through the initial stage of the quenching with a nitrogen content sufficient to keep the ambient nitrogen partial pressure higher than that. a partial pressure of nitrogen in equilibrium with the refined melt when the desired final nitrogen content of the melt is reached, thus causing the molten nitrogen content at the end of the initial quenching to be higher than the desired final nitrogen concentration and injecting a gas mixture containing oxygen and argon and possibly nitrogen, and that after the annealing step, argon and / or nitrogen are injected to finally adjust the molten composition until a predetermined desired nitrogen concentration is reached.

The argon used to complete the annealing can be added either by replacing part of the TV contained in the gas mixture used in the initial stage of the annealing, or by adding it directly to such a gas mixture.

The invention can be suitably applied to a wide range of stainless steels, of which chromium steels containing 3 to 40% chromium. These may include tungsten, vanadium, zirconium, copper, aluminum, silicon, sulfur, titanium, manganese, molybdenum, and other commonly used dopants. In addition, refining can be applied to either private molten metal batches or moving pulp, e.g., in a continuous process.

I! 73740 5

The accompanying drawing shows graphically the change in molten nitrogen content during refining according to the invention, in which the oxygen-nitrogen mixture is injected at the beginning of the quench, followed by a second step in which argon is either completely replaced by nitrogen (curve X) or added to the mixture (curve Y). The refined melt is then finished with argon alone.

It can be seen from this figure that when a mixture of oxygen and nitrogen is used in the initial stage of quenching, the nitrogen content in the melt rises close to its equilibrium Νχ under those specific conditions, melt temperature and composition, and nitrogen pressure surrounding the refinery. Replacement of nitrogen with argon in the second stage of the quench (curve X) causes a rapid decrease in the nitrogen content of the melt. This second step is continued until the desired carbon content and the specified nitrogen content N2 are reached, which depends on the desired nitrogen content for casting. Since continuous argon injection during reduction and finishing causes an increased, albeit small, decrease in nitrogen content, it seems obvious that the point Νχ at which nitrogen is changed to argon depends on both the amount of argon injected in the second stage of quenching and the desired final nitrogen concentration N3 in the poured metal.

Curve Y depicts the path followed by the molten nitrogen content if a higher nitrogen content N4 is desired in the poured metal, and in which a ternary argon-oxygen-nitrogen mixture is used in the second stage of the quench. Curve Z depicts the path that the nitrogen content of the melt would follow if the same final nitrogen content N2 were applied using a three-component nitrogen-oxygen-argon mixture in the second stage of the quench. In this case, the first stage would end earlier in time Τχ and not in time T2, as in the previous example because the nitrogen content of the melt decreases more slowly when there is nitrogen in the injected gas than if it does not contain nitrogen, as in curve X.

In the commercial process of argon-oxygen sintering of stainless steel, where argon has been the only gas added to oxygen, it has been found that the final nitrogen concentrations in the melt after annealing, reduction and finishing have been 30-50% lower than those normally obtained in a conventional arc furnace process. On the other hand, 6,73740, as noted above, a significantly different problem has been observed if nitrogen has been used as the sole oxygen-added gas in the refining process. In the latter case, the nitrogen content dissolved in the melt approaches the equilibrium concentration at the end of the refining step. While this is not surprising from a theoretical point of view, previous practical experiment has shown that in a practical system one never gets close to the theoretically calculated equilibrium concentration; see Ward, "The Physical Chemistry of Iron and Steelmaking," 1952 pp. 182 ... 183. Furthermore, in the Bessermer process, in which air (containing about 79% nitrogen) is blown into the melt, the nitrogen content observed in the melted steel is usually 0.01 to 0.02% when the equilibrium content is about 0.04%. Thus, the final nitrogen content measured in such melts is typically 25 to 50% of the value predicted in theoretical equilibrium studies. The invention that nitrogen absorption does not occur under oxidative conditions, such as in melting, is also disclosed in U.S. Patent No. 2,537,103.

Unlike in the past, it has now been found that when nitrogen is used as an inert gas in the argon-oxygen bubbling process, much higher nitrogen concentrations remain in the milled melt than would be expected. For example, when nitrogen and oxygen were used to refine a 17-ton charge of AISI 304-type stainless steel (Cr 18 ... 20%, Ni 8 ... 10%, Mn 2.0 max, Si 1.0 max, C 0.08 max.) The nitrogen content was 0.136%, while the equilibrium content is about 0.145%. Thus, the melt reached almost 94% of the equilibrium concentration. Nitrogen injection during reduction, desulfurization and finishing increased the concentration to 0.207%, compared to the calculated equilibrium value of 0.247%, i.e. about 80% of the equilibrium value.

The equilibrium partial pressure of the nitrogen in the molten metal can be calculated, for example, according to the method described in the above-mentioned Chipman and Corrigan publication. Thus, for example, when a 304 type stainless steel having a melt composition at the beginning of the annealing of 0.17% C, 0.96% Mn, 0.27% Si, 19.38% Cr and 8.54% Ni is annealed according to the invention, and the desired maximum nitrogen content of the refined steel is 0.08%, the partial pressure of nitrogen in equilibrium with the desired nitrogen content is calculated, according to Chipman and Corrigan, to be about 0.1 atm. In the initial stage of the quenching, nitrogen is incorporated into the quenching gas to such an extent that the nitrogen content of the gas in contact with the melt is 7,73740 or more at least 0.1 atm. Significant amounts of carbon monoxide are formed during the initial stage of mulching, which tends to lower the N2 partial pressure. Thus, according to the invention, at the initial stage of the quench, the partial pressure of nitrogen must be kept higher than said equilibrium partial pressure. In the second stage of the digestion, considerably less carbon monoxide is formed, and thus in the second stage the nitrogen in the digestion gas can be partially or completely replaced by argon.

During finishing, when the composition of the melt is adjusted to meet its final requirements, the addition of nitrogen can be accomplished simply by injecting nitrogen into the melt for a period of time that depends on the desired final concentration. This procedure can be called a "nitrogen mixture". Unexpectedly, it has been found that the rate of nitrogen absorption is quite reproducible and unexpectedly high for the reasons mentioned above. Using this technique, it is possible to achieve residual nitrogen concentrations in the melt as high as about 50% of the equilibrium concentration at 1 atm nitrogen pressure, quickly and economically. Concentrations above 50% of equilibrium can also be achieved, but the rate of nitrogen absorption still begins to decrease and thus becomes ineffective. Table 1 shows the results of five tests in which the nitrogen content of commercial stainless steels was increased by injecting nitrogen for only 20 to 69 seconds.

table 1

Steel N2 N2 N2 Time type% beginning% end m3 / hs 316+ 0.021 0.032 340 20 316 0.019 0.041 340 45 304 ++ 0.044 0.061 283 34 304L +++ 0.043 0.066 283 52 304L 0.028 0.059 283 69 + Cr 16 ... 18% , Ni 10 ... 14%, Mn 2.0 max., Si 1.0 max., C 0.08 max.

++ Cr 18 ... 20%, Ni 8 ... 10%, Mn 2.0 max, Si 1.0 max., C 0.08 max.

+++ Cr 18 ... 20%, Ni 8 ... 10%, Mn 2.0 max., Si 1.0 max., C 0.03 max.

and 73740

Many previous studies of nitrogen absorption-desorption kinetics have found that vacuum or argon degassing methods are very inefficient to remove nitrogen (cf., e.g., Leitner-Plöckinger, "Die Edelstahlerzeugung", Vienna 1965, II extended edition, p. 265}. Pehlke and Elliot, "Solubility of Nitrogen in Liquid Iron Alloys - II Kinetics," Trans. Of Met.Soc.ΑΙΜΕ (1963), suggest that this is especially the case when surfactants such as oxygen and sulfur are present. It was therefore expected that it would be difficult to remove large amounts of dissolved nitrogen from the melt under the oxidizing conditions of argon-oxygen melting, however, it has been unexpectedly found that significant amounts of nitrogen can be removed even under oxidizing conditions of melting. , although a considerable amount of nitrogen is absorbed in the melt.The amount of nitrogen replacing argon depends on the melt poured. the desired final concentration. For example, if the desired residual nitrogen content is less than 0.05% in stainless steel grade 304, nitrogen may be used in the annealing until about 60% of the calculated oxygen required for the annealing has been injected. In general, the time at which argon replaces nitrogen is at the point where about 50-70% of the calculated oxygen required for the quench is injected. This amount of oxygen is calculated in a conventional stoichiometric manner, taking into account the oxygen required not only to oxidize the carbon removed as carbon monoxide, but also to oxidize silicon and other molten metals that normally migrate to the slag as oxides. At such a calculated point, argon is used to replace the nitrogen as the quenching is continued and to reduce the concentration of dissolved nitrogen in the melt to the desired degree.

The following examples illustrate two preferred embodiments of the present invention.

Example 1 This example illustrates embodiments of the invention in which nitrogen is used as the sole addition in the first stage of oxygen quenching, followed by a second stage in which argon replaces nitrogen. Prior to melting, the stainless steel contained: 0.78% C, 0.51% Mn, 0.41% Si, 18.25% Cr and 8, C3% Ni. The amount of melt was 17 tons. Table 2 of the table below 9 73740 describes changes in temperature, carbon content and gas flows, as well as nitrogen content at the beginning, in the first and second stages of quenching and after reduction.

It can be seen from Table 2 that the nitrogen content of the melt increased in the first stage of the riot from 0.042% to 0.075% when injected with a gas with an oxygen to nitrogen ratio of 2: 1. However, in the second stage of the quenching, the nitrogen content decreased to 0.048% when injected with a gas with an oxygen to argon ratio of 1: 2. When the injection was continued with argon alone during the reduction, the nitrogen content dropped to 0.041%.

Table 2 Temperature Time C O? Is N? N2 ° C min% m-Vh ~~ p-%

Start 1477 0 0.78 - 0.042

1st stage of mulching 1638 25 0.31 453 - 226 0.075

2nd stage of mulching 1620 15 0.06 198 396 - 0.048

Reduction 1549 4 0.07 - 283 - 0.041

Example 2 This example illustrates another embodiment of the invention in which nitrogen is used with oxygen in a first stage of quenching, followed by a second stage in which argon is used to replace nitrogen. Prior to melting, the stainless steel melt contained: 1.35% C, 0.34% Mn, 0.36% Si, 16.22% Cr and 0.41% Ni. The amount of melt was 17 tons. Table 3 below depicts changes in temperature, carbon content, and gas flows, as well as nitrogen concentration at the beginning, in the first and second stages of quenching, and after reduction.

10 73740

Table 3 Temperature Time C o2_Ar_N2 N2 ° C min% m3 / h wt%

Start 1538 0 1.35 - - 0.036

1st stage of mulching 1693 40 0.21 + 452 - 226 0.068+

2nd stage of mulching 1693 8 0.05 198 396 - 0.075

Reduction 1621 4 0.05 283 283 - 0.056 + Sample taken after 35 minutes of agitation.

As can be seen from Table 3, the high nitrogen content, 0.075%, caused by the nitrogen used in the first stage of quenching was reduced to 0.056% when injected with argon. The nitrogen content shown in the table, 0.036%, was not an actual initial value (which was not measured), but is a value typical of the melt composition in question. It should be noted that oxygen flow was maintained during the reduction. This was not done for rioting, but to prevent the melt temperature from dropping too much.

In the above examples, the oxygen-argon ratio of the second stage of the quenching was kept lower than the oxygen-nitrogen ratio of the first stage to minimize the oxidation of chromium while the oxidation of carbon continued. For the same reason, when nitrogen is used in addition to argon in the second stage, the ratio of oxygen to argon and nitrogen should be kept lower than the ratio of oxygen to nitrogen in the first stage of quenching.

The above examples have treated various embodiments of the invention as separate, individual procedures. However, it will be apparent to those skilled in the art that different embodiments may be used in different combinations within the limits of the requirements in order to achieve the greatest possible advantages in gas economy, reproducibility, and nitrogen control of the final product.

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Claims (3)

11 73740 A method for adjusting the nitrogen content of stainless steel to a predetermined desired level in steelmaking between 10 ppm and 90% of the equilibrium nitrogen content of molten steel at ambient pressure, the method comprising the step of annealing molten steel by injecting a gas mixture containing oxygen and at least one gas below the molten surface. argon and nitrogen, characterized in that a mixture of oxygen and nitrogen is injected through the initial stage of annealing with a nitrogen content sufficient to keep the ambient nitrogen partial pressure in contact with the molten surface higher than the partial pressure of nitrogen equilibrated with the refined melt when the desired final nitrogen concentration nitrogen content of the melt at the end of the initial stage of the riot is higher than the desired final nitrogen concentration and that a gas mixture containing oxygen and argon and possibly prevented nitrogen, and that after the annealing step, argon and / or nitrogen are injected to finally adjust the molten composition until a predetermined desired nitrogen concentration is reached. A method according to claim J, characterized in that the ratio of oxygen to argon or the ratio of oxygen to nitrogen and argon together in said gas mixture used in said final stage of quenching is kept lower than the ratio of oxygen to nitrogen in the first stage. Method according to Claim 1 or 2, characterized in that the first stage of the annealing is stopped when 50 to 70% of the calculated acid required for the annealing has been sprayed.