JPH02179815A - Method and apparatus for refining stainless steel - Google Patents

Abstract

PURPOSE:To shorten the required decarburizing time and also to reduce wear of refractory by injecting gas toward almost perpendicular direction through plural tuyeres arranged at bottom part of a refining furnace in the subject method with AOD type. CONSTITUTION:In a refining apparatus for stainless composed of a furnace body 1 arranging refractory lining 12 on steel-made outer shell 11, the tuyeres 2 for blowing Ar gas and O2 gas into molten steel are arranged in plural pieces at the furnace bottom part so that gas injects toward almost perpendicular direction. Further, sliding nozzle type steel tapping hole 3 is arranged at almost center of the furnace bottom and on the circle surrounding this, for example, eight pieces of the tuyeres 2 are set at almost equal intervals. By this method, stirring of the molten steel by gad blowing is effectively executed and the decarburization is quickly progressed and particularly in the case the cross sectional area in the furnace enlarges and the molten steel surface lowers as the wear of the refractory progresses, this is advantageous.

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to an improvement in a method for refining stainless steel, and also to a refining apparatus used to carry out the method. [Prior art] A technology known as the AOD process is to inject Ar gas and O2 gas into a molten metal containing stainless steel containing Cr to perform refining centered on decarburization while minimizing oxidation of Cr. Hiro is being implemented. Although various improvements and changes have been made to this AOD process, the basic operation is to use a converter-like tiltable smelting furnace, such as a GRAF furnace, installed horizontally below the sidewall. The above gas is blown through one or several tuyeres, usually using a double-tube tuyere, with Ar from the outer tube and 02 from the inner tube, and stirring by the gas and decarburization by the 02. Become. As is well known, the decarburization speed of molten steel is C=2
．． In the relatively high carbon range of 0-0.3%, O2 supply is rate-limiting. That is, in the high carbon region, C is abundant everywhere in the molten steel, so the speed of 02 supply is the dominant factor that determines the speed of decarburization. However, C=0.
In the low carbon region of 3% or less, C is not abundantly present in molten steel, and therefore the diffusion rate of C or O2 becomes rate limiting. That is, in this region, how efficiently the C in the molten steel and the injected O2 are stirred and encountered is the dominant factor that determines the speed of decarburization. By the way, as shown in FIG. 6, in the AOD process, high-temperature molten steel flows violently, so that the refractories of the furnace wall and hearth are considerably consumed. The wear and tear of large furnace wall components increases the furnace inner diameter, and the expansion of the furnace inner diameter causes a drop in the melt level. It will be easily understood that a decrease in the hot water level reduces the distance from the tuyere tip to the hot water level, that is, the path length of the stirring gas, which reduces the efficiency of stirring. Also noteworthy is the wear and tear of the hearth. Worn out of the hearth directly leads to a lowering of the hot bed surface, and since the height of the tuyere is constant, the relative height of the tuyere to the height of the hot water level becomes significantly higher. It is also easily understood that this is another major factor that reduces the efficiency of stirring. If this is seen in FIG. 6, as the initial hot water level S decreases to the undawn hot water level S-, the relative height of the tuyere increases from the initial H''/H to the undawn B-/B. The reduction in stirring efficiency delays the completion of decarburization and lengthens the time required for refining, which not only leads to the consumption of refractories but also to the use of expensive Ar.
It is known to those skilled in the art that this causes a number of problems, including increased gas consumption, increased oxidation of Cr, and increased amount of metallic Si (used in the form of ferrosilicon) added in subsequent steps for its reduction. This is something that engineers know. In addition, from the perspective of the consumption rate of refractories, when tuyeres are installed in the lateral direction of the furnace wall, the part directly above the tuyere is likely to be damaged due to the concentration of stirring gas, and even if the refractories in other parts are intact, local damage may occur. Repairs are required due to excessive wear and tear, which is often uneconomical. [Problem to be Solved by the Invention 1] One purpose of the present invention is to solve the above-mentioned problems in the decarburization refining of stainless steel represented by the AOD process, to efficiently stir molten steel by blowing gas,
It is an object of the present invention to provide a refining method that can advantageously perform refining by shortening the time required for decarburization and reducing consumption of refractories. Another object of the present invention is to provide a refining apparatus suitable for carrying out the above-mentioned refining method.

[Means for Solving the Problems] The stainless steel refining method of the present invention involves blowing Ar gas and O2 gas into molten steel containing Cr stored in a refining furnace to decarburize while minimizing oxidation of Cr. In a stainless steel refining method, the gas is injected by blowing the gas in a substantially vertical direction through a plurality of tuyere provided at the bottom of the refining furnace,
It is characterized by completing the required decarburization within a shortened time under high stirring efficiency. The stainless steel refining apparatus of the present invention is shown in FIGS. 1 and 2.
As shown in the figure, in a stainless steel refining S@ consisting of a furnace body (1) with a steel outer shell (11) and a refractory lining (12), Ar is injected into molten steel. It is characterized in that a plurality of tuyeres (2) for gas and 02 gas are provided at the bottom of the furnace so that the gases are ejected in a substantially vertical direction. This refining equipment can, of course, be configured to tilt like a conventional AOD furnace, but as shown in the figure, a sliding nozzle-type tapping port (3) is provided approximately in the center of the furnace bottom. The above tuyeres (2
), in the illustrated example, eight pieces may be arranged at approximately equal intervals so that steel can be tapped without tilting. [Function] The immersion tuyere, which blows refining gas into molten steel, is normally installed vertically at the bottom of the converter, but A
In the OD furnace, as described above, the tubes are only provided horizontally on the side wall, and no attempt has been made to provide a plurality of tubes vertically on the bottom of the furnace. The reason for this is probably due to the consideration of minimizing the use of expensive Ar gas. In other words, since conventional AOD furnaces were often tilted not only for tapping but also for slag removal, sampling, or temperature measurement, it was possible to install the immersion tuyere at a position above the molten steel lake surface when tilting. At least while the tuyere is above the hot water surface, it is possible to make the gas blown out from the tuyere not Ar but N2, or even air. For this reason, even when a plurality of horizontal tuyeres are used, they are usually arranged side by side on one side of the furnace body. In view of the fact that recent advances in steelmaking technology have made tapping, sampling, and temperature measurement possible without tilting the furnace body, the inventors believe that it is necessary to blow out Ar gas from the tuyere, which is constantly immersed. However, if the time required for refining can be shortened by improving stirring efficiency, the amount of Ar gas consumed may actually be reduced.Based on this idea, multiple tuyeres were arranged vertically at the bottom of the furnace. It was investigated. The purpose of arranging the tuyeres perpendicular to the hearth is to change the position of the tuyere as the hearth wears out, and to ensure the length of the stirring gas bath.The purpose of arranging multiple tuyeres is to: The purpose is to greatly promote the diffusion of C and 02 in the molten steel. The study began with the dissolved oxygen method using a water model. A nozzle (4sφ
), attach a nozzle (4-hole φ, 3-hole φ, or 2-hole φ) that blows vertically from the bottom upward (hereinafter referred to as "bottom blow J"), fill it with water, and blow in 02 gas to remove oxygen from the water. After saturation, Ar gas was blown in to drive out the oxygen in the water, and the rate of decrease was measured. (Water model) d(02%)/dt=-k (02%>(Decarburization reaction) d(C%)/dt=-k(C%) Different diameters (4M, 3mm, 2's) 8 nozzles
In an experiment using one nozzle at a time, it was found that the larger the nozzle, the smaller the value of k. This is because the gas flow rate is the same, so the nozzle outlet pressure P1 depends on the nozzle diameter.
This is thought to be due to the difference in the stirring energy shown by the following formula, W=aQ,Q n (P1/P2) (
(P2 is atmospheric pressure) It was found that to examine the influence of the number of tuyeres, it was necessary to make comparisons with the same Pl. Therefore, 4Mr1φX2, 3sφ×4 and 2Mφ
The graph of FIG. 5 shows the results of an experiment using a combination of X8 wires and using almost the same Pl in the above formula. This graph shows that it is better to blow from the bottom than from the side, and that using a large number of nozzles is more effective. According to the observation of the water model, in the case of side blowing and in the case of bottom blowing with two nozzles, a dead zone occurs where stirring is not thorough, and with four side blowing, this almost disappears.
It was found that the problem was completely resolved after 8 tubes. Based on the prediction that the reaction rate kI is determined by the path length of the stirring gas, we calculated the value of k by changing the position of the nozzle, and found that the ratio of depth h from the water surface to k and water depth H It was confirmed that there was a linear relationship between h/H. Next, the consumption of the refractory lining in an actual device is determined by the reaction rate k
In order to investigate the effect on k, vessels having the shapes shown in FIGS. 7 and 9 were prepared according to the changes in the cross section of the furnace interior shown in FIG. 6, and k was measured using the dissolved oxygen method. The results are shown in the graphs in Figures 8 and 10, and there is not much difference in the early stages of furnace operation, when the refractories are new, but in the early hours, when the refractories are worn out, repairs are required. At this stage, the difference becomes larger, and the advantage of bottom blowing over side blowing becomes clear. If this is roughly organized, it will be as shown in Figure 11. [Example] Based on the above findings, a test was conducted using an actual device. A test AOD furnace with a capacity of 0.4 tons was modified, and the inner diameter of the inner tube was 3.6M in the hearth as shown in Figures 1 and 2.
8 double tubes with an outer tube inner diameter of 6.0 # were provided. Molten steel 310 containing 1.85% C and 18% Cr
Decarburization refining was performed on 0Ky under the conditions shown in Table 1. (The unit of gas amount is N, ll/min) 1st
Table 2 The results are shown in Table 2 in comparison with data obtained when a conventional side-blown tuyere was installed and used in the above experimental reactor. Second
In the table, Δ[C]/Δ[C] is the ratio of the amount of decarburization to the amount of chromium oxidation. Table 2 If we pay attention to the 02 efficiency in Table 2, it becomes clear that the effects of the present invention are particularly significant in the low carbon region. The time required to reduce the C content in molten steel to 0.01% was 41.8 minutes, compared to the time required of 46°1 minute using a conventional side-blown tuyere. ,9.
This corresponds to a 3% reduction. The amount of Or oxidized during decarburization was reduced by 17%, and the amount of ferrosilicon introduced in the subsequent reduction step was also reduced accordingly. When this furnace used conventional side-blown tuyeres, it was necessary to replace the refractory after 50 charges on average, but with the use of bottom-blown tuyeres, the refractories remained stable even after 70 charges. It withstood use.

【Effect of the invention】

If stainless steel is refined according to the method of the present invention,
Molten steel can be effectively stirred by blowing gas, and decarburization proceeds quickly and is completed in a shortened time. Therefore, the amount of Ar gas and O2 consumed can be reduced. Since the amount of Or oxidized during refining is also reduced, the amount of metal 3i to be input for its reduction in a later step can also be reduced. The reduction in time results in a reduction in the amount of refractory consumed during one charge. These benefits work together to increase productivity and reduce costs in stainless steel refining. The apparatus of the present invention enjoys the merits of the above-mentioned refining method, and in particular, the apparatus having more than four blades, for example, eight blades, has no dead space when stirring molten steel, and the stirring High efficiency. The superiority of the apparatus of this preferred embodiment over the conventional apparatus is particularly apparent when the refractory is worn out, the furnace cross section expands, and the hot water level decreases. If a device having a sliding nozzle at the bottom of the furnace is used and sampling, temperature measurement, and tapping are performed without tilting the furnace body, the time-saving effect can be more clearly obtained.

[Brief explanation of the drawing]

1 and 2 show a typical example of the refining apparatus of the present invention, in which FIG. 1 is a vertical sectional view, and FIG.
FIG. FIGS. 3 and 4 show an experimental device that was used to complete the present invention, with FIG. 3 being a longitudinal sectional view and FIG. 4 being a plan view. FIG. 5 is a graph showing data from experiments conducted using FIGS. 3 and 4. FIG. FIG. 6 is a longitudinal sectional view of the AOD furnace showing the state of the refractory and the position of the molten steel surface at the beginning and end of use of the AOD furnace. 7 and 9 are longitudinal cross-sectional views of experimental equipment similar to that shown in FIGS. 3 and 4. FIGS. 8 and 9 are graphs similar to FIG. 5 showing data from experiments conducted using the apparatus of FIGS. 7 and 9, respectively. FIG. 11 is a graph that organizes and shows the data in FIGS. 8 and 10. 1...Furnace body 11...Steel membrane 12...Refractory lining 2...
Tuyere 3: Sliding nozzle type tapping port patent applicant Daido Steel Co., Ltd.

Claims (3)

[Claims] (1) In a stainless steel refining method that involves blowing Ar gas and O_2 gas into molten steel containing Cr stored in a refining furnace to perform decarburization while minimizing oxidation of Cr, the blowing of the above gases is performed. is carried out by ejecting gas almost vertically through a plurality of tuyeres provided at the bottom of the smelting furnace, and is characterized by completing the required decarburization within a shortened time under high stirring efficiency. Refining method. (2) In a stainless steel refining equipment consisting of a furnace body with a steel outer shell lined with refractory material, multiple tuyeres are installed at the bottom of the furnace for Ar gas and O_2 gas to be blown into the molten steel. A refining device characterized by being installed so that gas is ejected in an almost vertical direction. (3) The refining apparatus according to claim 2, wherein a sliding nozzle for tapping steel is provided at approximately the center of the hearth bottom, and the tuyeres are arranged at approximately equal intervals on a circumference surrounding the sliding nozzle.