EP0059289B1

EP0059289B1 - Tuyère

Info

Publication number: EP0059289B1
Application number: EP19810305994
Authority: EP
Inventors: Minoru Kitamura; Shinji Koyama; Shuzo Ito; Masahiko Ohgami; Hideo Matsui; Isamu Hirose; Hideaki Fujimoto; Tsuyoshi Yasui
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 1980-12-20
Filing date: 1981-12-21
Publication date: 1985-04-10
Also published as: AU540689B2; DE3169921D1; KR890003014B1; AU7868781A; EP0059289A1

Description

This invention relates to a gas blowing tuyere for use in the bottom or side walls of molten metal containers such as metal refining furnaces or ladles and the like.
There are many types of containers for holding molten metal for refining, lagging, storing, transporting or for other purposes. For example, in addition to LD converters, there are many types of converters, including LF furnaces, VAD furnaces, AOD furnaces, ASEA-SKF furnaces and RH and DH vacuum melters. Among known molten metal containers other than refining furnaces are ladles, metal mixers, mixer cars and the like. Most of such molten metal containers require the contents to be stirred either constantly or intermittently. Of the various conventional mechanical and gas stirring systems, the present inventors conducted an extensive study of gas stirring particularly in refining processes in top and bottom- blown LD converters using oxygen for top blowing and an inert gas or oxygen wrapped in a cooling gas for bottom blowing, and arrived at some conclusions. More specifically, the inventors have succeeded in producing a tuyere construction which permits one to set or vary the blowing gas flow rate over a wide range and to reduce the erosion of the tuyere itself and the surrounding refractory material to a significant degree. Further, experiments on the tuyere construction according to the present invention have revealed that it is also useful for molten metal containers other than LD converters.
The converters which are designed to blow pure oxygen into molten metal are generally classified into either a top blowing type or a bottom blowing type, of which the top blowing type has hitherto been more popular although both have long histories. However, the bottom blowing type converters are becoming increasingly accepted recently as they allow one to utilize the stirring effect of the rising streams of the bottom blown gas. It has also been shown that the metallurgical reactions are improved to a significant degree as a result of the positive stirring action of the rising gas streams on the molten steel and slag, as compared with a pure oxygen top blowing type converter. Therefore, there is now a trend to replace the top blowing type converters by the bottom blowing type. The present inventors have been pursuing a study of top and bottom blowing converters in an attempt to develop a new refining process which uses both top and bottom blown gases in such a manner as to secure the advantages of bottom blowing whilst retaining the merits of top blowing, for instance, its versatility in refining.
In the research on the top and bottom blowing converters, one of the following systems was adopted:

1. A system in which several per cent to several tens per cent of the total quantity of the feed oxygen was blown in to the container through the bottom; or
2. A system in which all of the feed oxygen was used for top blowing, an inert gas being blown in at the bottom at a relatively low flow rate (e.g., at a rate of 0.01 to 0.2 Nm³/min per ton of a charge).

The increase of the stirring action by the use of bottom blowing causes the following effects:

A. The composition and temperature of the molten bath are maintained uniform throughout the furnace, increasing the likelihood of attaining a desired composition at the end.
B. The efficiency of the consumption of oxygen in decarburization reactions is increased, thereby lowering the consumption of the refining oxygen.
C. The percentages of the T. Fe (total iron) component in the slag at the end is reduced, improving the yield of steel.
D. The O-content of the steel is reduced and the Mn-content is increased at the end. Therefore, it becomes possible to reduce the amount of A1 and Fe-Mn which are added to adjust the composition.
E. The dephosphorization capacity of the slag is improved allowing one to reduce the prime consumption of a subsidiary material like calcined lime.

Although the above-mentioned effects which improve the metallurgical reactions are largely influenced by the flow rate of the bottom blown gas, the improvements only increase significantly up to a flow rate of approximately 0.05 Nm³/min per ton of molten steel in the case of an inert gas bottom-blowing system, with no great improvement in the effects even if the flow rate of the bottom blowing gas is increased further. Rather, in the case of a high carbon steel with a C-content greater 0.60% at the end of the blowing, the T. Fe content of the slag is reduced considerably by the end of the blowing giving rise to a problem of degraded dephosphorization capacity. We have discovered that the advantages of bottom blowing can be acquired without causing the above-mentioned problems, by restricting the flow rate of the bottom blowing gas to about 0.1 Nm³/min per ton of molten steel when refining a high carbon steel.
Conventional tuyeres for use with bottom blowing comprise (I) a tuyere consisting of a single tube and (II) a tuyere consisting of concentric double tubes. The former is used for blowing an inert gas alone, whilst the latter is used for blowing oxygen through the inner tube and a protecting or cooling gas through the outer tube. These tuyeres, however, have the following drawbacks when used for blowing an inert gas. Referring to FIGURE 1 which illustrates a conventional single tube tuyere 1 embedded in a refractory bottom wall 2 of a furnace, the molten steel 5 in the vicinity of the bottom wall 2 is partly solidified by the initial cooling action of the blown gas, forming a mushroom shaped plug of base metal as indicated at 3. The blown gas is injected into the molten steel 5 through narrow gas passages 4 which are formed in the mushroom 3, and rises up through the molten steel 5 in the form of bubbles 6. In some cases, however, the gas passages 4 are too narrow or too few causing an increase of resistance and the blowing is often blocked so that the gas is not blown in a stable manner. In order to avoid this problem, the back pressure of the gas in the tuyere has to be raised to a level higher than 10 kg/cm²G in the case of a single tube tuyere, although the exact pressure depends on the static pressure of the molten steel. On the other hand, if the flow rate of the blowing gas is to be maintained at a value lower than 0.1 Nm³/min per ton of molten steel as mentioned hereinbefore, it becomes necessary to make the tuyere hole diameter smaller. In order to satisfy these requirements in a process which involves control of the flow rate of the blowing gas over a wide range, there are imposed further restrictions, i.e., a pressure increase to a range over 10 kg/cm²G for stable blowing operation and the use of blowing apparatus which are calibrated to an extremely high pressure.
In an attempt to solve these problems, we have conducted experiments extensively, using a tuyere of concentric double tubes (FIGURE 2) instead of the above-mentioned single tube tuyere, and found that a predetermined stable gas flow rate can be provided by maintaining the back pressure of the gas in the outer tube at a predetermined high level, without reducing too much the diameter of the opening of the inner tube.
The double tube type of tuyere has been effective particularly for simultaneously blowing a gas and powder or the like, in addition to the injection of a large quantity of oxygen or other gas. However, even the concentric double tube type of tuyere has a problem in that the gas flow from the inner tube has a large influence and in some cases the blowing operation is thereby rendered instable. As is clear from the results of experiments described with reference to FIGURES 3 and 4, the concentric double tube type of tuyere is unsuitable for blowing in a gas at a relatively low flow rate or for blowing in the gas at a controlled flow rate over a wide range.
More specifically, the graphs of FIGURES 3 and 4 show variations in the gas flow rates of a concentric double tube tuyere which is mounted in a bottom wall of a converter, oxygen being blown in through the inner tube and a cooling CnHm gas being blown in through the outer tube, the blowing gas pressure being measured in the pipes in the vicinity of the tuyere. Although no large variations in flow rates are observed in FIGURE 3 the blowing becomes unstable with extremely large variations in the inner and outer tube pressure Ip and Op in the case of FIGURE 4 where the flow rate of oxygen through the inner tube is about 1/2.5. It will be understood therefrom that the use of a concentric double tube tuyere does not solve the above-mentioned problems.
A further problem with conventional tuyeres is that the refractory wall around the tuyere is eroded because the upward passage of large gas bubbles injected from the tuyere causes violent downward flow of molten metal which scours the refractory wall and thereby erodes it.
In view of these circumstances, it has been concluded that conventional tuyeres are not good enough for blowing in an inert gas or oxygen.
Conventional tuyeres are illustrated in UK Patent Specifications 2 002 818 and 1 498 482 and in the Transactions of Iron and Steel Institute 20, No. 2, 1981, B-67.
Japanese Laid Open Utility Model Specification No. 55-142554 and 54-110608, Japanese Laid Open Patent Specification No. 50-87908 and Japanese Patent Publication Specification Nos. 43-29843 and 49-21002 are cited here as references of interest.
The present invention provides a blowing tuyere in a bottom or side wall of a molten metal container for blowing a gas thereinto, said tuyere comprising a cylindrical core body located at the centre of said tuyere and an outer tube mounted concentrically around said core body with a gap of a width t to form an annular blowing passage therebetween, characterised in that said core body and said outer tube are arranged to satisfy the following conditions:

where d is the diameter of said core body and D is the outer diameter of said outer tube.
The blowing tuyere of the invention is capable of improved uniform and stable continuous gas blowing operations and is adapted to reduce the erosion of the surrounding refractory walls by the injected gases which would otherwise shorten the service life of the container in which it is mounted.
The above and other features of the present invention will become apparent from the following description of preferred embodiments of the invention taken in conjunction with the accompanying drawings (FIGURES 1 to 4 referring to conventional constructions already referred to) in which:

FIGURE 1 is a sectional view of a conventional tuyere,
FIGURE 2 is a perspective view of a conventional concentric double tube tuyere,
FIGURE 3 is a graph plotting the pressure of gas within the tubes of a concentric double tube tuyere in a high flow rate blowing operation,
FIGURE 4 is a graph plotting the pressure of gas within the tuyere tubes of a conventional concentric double-tube tuyere in a low flow rate blowing operation,
FIGURE 5 is a sectional view of a single-annular blowing tuyere according to the present invention,
FIGURE 6 is a perspective view of another embodiment of the present invention, which is in the form of a double annular blowing tuyere,
FIGURE 7 is a graph plotting the pressure of gas within the tube of a double annulus or dual type annular tuyere of the invention in a high flow rate blowing operation,
FIGURE 8 is a graph plotting the pressure of gas within the tubes of a dual type annular tuyere of the invention in a low flow rate blowing operation,
FIGURE 9 is a graph plotting the occurrences and non-occurrences of tuyere blockage in relation to the gap width and tuyere back pressure,
FIGURE 10 is a graph showing the depth of erosion of a single type annular tuyere with respect to the number of refining charges,
FIGURE 11 is a graph showing the depth of erosion of a dual type annular tuyere in relation to the number of refining charges,
FIGURE 12 is a chart of the gas flow rate in respect of time in an example of one process,
FIGURE 13 is a graph showing the relationship between the blowing pressure and the gas flow rate of single type annular tuyere of the invention, and
FIGURES 14 and 15 are graphs showing the relationship of the width t of the gap between a core body and an outer tube, the diameter d of the core body, the outer diameter D of the outer tube, and the gas blowing conditions.

FIGURE 5 shows in section a representative single type annular tuyere of the invention for blowing an inert gas, the tuyere comprising a cylindrical core body 9 comprising an inner tube 7 filled with a refractory material 9 and an outer tube 8 which is disposed concentrically around the inner tube 7 with an appropriate gap therebetween forming an annular blowing passage. The outer tube 8 has a lower bulged portion 8 with a blowing gas inlet 10 at the lower end thereof. A flange 11 seals with and projects from the body of the outer tube 8 at a position slightly above the bulged portion 8 thereby to secure the tuyere to a shell 12 of a molten metal container. Thus, an inert gas which enters the outer tube 8 in the direction of arrow A through the gas inlet 10 rises up the bulged portion 8 as indicated by arrow B and leaves the tuyere through an annular nozzle 13 formed between the inner and outer tubes 7 and 8. In this instance, a mushroom 3 of solid metal may be formed over the tuyere so that the inert gas is released into molten steel 5 through gas passages 4 and rises up in the form of small bubbles 6.
In a tuyere of such construction, if the refractory core material 9 were removed and the tuyere were in the form of a simple double tube construction as in FIGURE 2, the bubbles which are released from the inner tube 7 would be of a large diameter and would be naturally increased in size and would have a greater mechanical effect and hence more severe scouring thereby accelerating the erosion of the refractory walls of the furnace. However, as the inner cavity of the inner tube 7 is filled with a refractory material 9 so that the gas is blown solely through the annular nozzle 13 between the inner and outer tubes 7 and 8, the size of the bubbles are reduced so that they do not accelerate the erosion of the refractory walls. In order to reduce the scouring it is desirable to make the width of the gap between the inner and outer tubes 7 and 8 as small as possible, more particularly to make the width of the gap smaller than 3 mm, and preferably smaller than 2 mm.
In FIGURE 6 which illustrates another embodiment of the present invention in a perspective view, an outermost or second outer tube 18 is disposed concentrically around the first outer tube 8 with a small gap therebetween. Thus, there is formed a dual annular tuyere, which will be hereinafter referred to as a dual type annular tuyere. In this instance, it is possible to blow two different gases through the respective annular tuyere nozzles. Thus in a refining process, for example, one may blow pure oxygen through the inner tuyere nozzle and an inert gas or a cooling gas through the outer tuyere nozzle.
In FIGURE 6 the dual type annular tuyere is shown as having a cylindrical central core body comprising the inner tube 7 filled with a refractory material but in an alternative construction the central core body may comprise a round solid rod of a refractory material or ceramic or other filler material.
Another important effect of this blowing tuyere according to the present invention is that the flow rate of the blowing gases can be controlled over a range which is much broader than the ranges of the conventional tuyere. FIGURES 7 and 8 show the results of experiments on the dual type annular tuyere of FIGURE 6 similar to the experiments which produced the results of FIGURES 3 and 4. More specifically, as shown in FIGURES 7 and 8, the blow-in gas pressure remains stable even when the flow rate of oxygen gas is reduced to about 1/2.5 in contrast to the performance of the conventional concentric double tube tuyere of FIGURE 2 shown in FIGURES 3 and 4. The stability of the inner pressure I_P (i.e. the gas pressure inside the first outer tube 8) during blowing at the low flow rate by the dual type annular tuyere (FIGURE 8) is regarded as indicating the stability of the blowing gas pressure in a low flow rate blowing operation by the single type annular tuyere (FIGURE 5). Although the reasons for these phenomena are not entirely clear, a stable blowing operation is possible in a relatively low flow rate range without lowering the tuyere back pressure because the gas velocity at the nozzle end of the tuyere is higher. Further, the reduction in size of bubbles of the gases leaving the nozzle of the tuyere is believed to contribute to the reduction of scouring which takes place in the vicinity of the nozzle end of the tuyere due to the production of large bubbles when a tuyere of conventional concentric double tube construction is used.
Where the single type annular tuyere is employed for bottom blowing of an inert gas, for example, in a refining process of a high carbon steel, the gas is blown in at a flow rate of about 0.05 Nm3/min.ton. On the other hand, for refining a low carbon steel, it is possible to blow in the gas at a flow rate as high as 0.1 to 0.15 Nm³/min-ton so as to make maximum use of the improving effect of the process.
The range over which the flow rate may be controlled varies depending upon the tuyere design. For example, stable blowing operation is possible in the range of 0.02 to 0.057 Nm³/min.ton if one employs a pair of single type annular tuyeres each having an inner core tube of 15.5 mm outside diameter and a gap of 1.8 mm between the inner and outer tubes, and one controls the blowing gas pressure as represented by the tuyere back pressure in the range of about 5.5 to 18.0 kg/cm^2. When using a tuyere with an inner tube of 30 mm outside diameter and a gap of 1.8 mm width stable operation is possible in the range of about 0.02 to 0.093 Nm³/min .ton under the same blowing conditions. Thus, the blowing tuyere according to the present invention permits one to control the flow rate easily up to a ratio of the maximum to minimum flow rate of approximately 3 to 1 or even 5 to 1, an unexpected attainment as compared with the conventional tuyeres in which the ratio is 1.5 to 1 or 2.0 to 1 at the most.
When oxygen gas is blown in, it produces CO gas of double the quantity by reaction with C in the molten steel bath according to the reaction
so that the effective stirring effects of the blow-in gas is doubled. It follows that the stirring force can be controlled to a quintuplicate level by the use of a tuyere which is capable of controlling the maximum to minimum flow rate to a value 2.5 to 1 times as mentioned hereinbefore. Thus the dual type annular tuyere has extremely favorable characteristics for use with top and bottom blown converters.
The graph of FIGURE 9 shows the results of experiments studying the liability of the tuyere to blockage by varying the gap width and back pressure of the tuyere in a refining process using a 240-ton converter with a pair of single type annular tuyeres of FIGURE 5 embedded in the bottom wall. In this figure, a solid black circle indicates the occurrence of tuyere blockage while a white or blank circle denotes that there was no blockage. The blank circle also indicates that a stable blowing operation was possible without tuyere blockage over several hundred charges. Straight lines B and C are guide lines which indicate the boundaries of the regions of the blank and solid black circles. In other words, the safe region is on the higher back pressure side or narrower gap side of these lines. Further, the straight line A corresponds to the static pressure of molten steel so that in some cases the back pressure of the tuyere can be lowered to a level close to that line. In such a case, however, the back pressure should be increased as promptly as possible in order to secure a desired gas flow rate. In the same figure, curve D indicates the condition where the calculated value of linear gas velocity at the nozzle end of the tuyere reaches the sonic level (i.e. speed of sound) in a blowing operation using an Ar (argon) gas blowing tuyere over a length of about 1,200 mm, while curve E is at a pressure level which is 2 kg/cm² less than the curve D.
The number of charges and depth of erosion of the tuyere in blowing operations carried out at pressures higher than curve D or at least higher than curve E is shown in FIGURE 10. In this regard, it has been found that the amount of erosion of the double tube tuyere, which is about 1.05 mm/CH (i.e. mm per charge) can be reduced to about half, namely, to about 0.46 mm/CH by the use of the tuyere shown in FIGURE 5. The amounts of erosion of the refractory material during blowing operations using the concentric double tube tuyere and the dual type annular tuyere are shown in FIGURE 11 for the purpose of comparison. It will be seen that the erosion of the refractory material is also reduced by approximately one half when an annular tuyere is used in place of a conventional double tube tuyere.
FIGURE 12 is a chart showing the variations, with respect to time, of the gas flow rate during a refining operation for each charge using an inert gas and a single type annular tuyere. In the experiment, N₂ (nitrogen) gas was blown into the converter before charging the molten pig iron, and the blowing gas was changed to Ar as soon as the charging was finished to start the refining, in order to prevent N₂ from dissolving in the molten steel during the refining process. The blowing gas was changed back again to N₂ when the refining was finished.
FIGURE 13 graphically illustrates an example of flow rate control using the single type annular tuyere. As shown, it is possible to control the flow rate of the blowing gas in a stable manner between 3.0 and 8.0 Nm³/min by controlling the blowing gas pressure in a broad range of about 5.2 to 15.4 kg/_cm ²G.
As described above, the annular tuyere according to the present invention broadens the range over which the flow rate can be controlled and prolongs the life of the refractory walls in the vicinity of the tuyere. However, a further study including pilot tests on annular tuyeres of various dimensions has revealed that the scouring due to the back flow of molten metal could increase in some cases depending upon the tuyere design. We have therefore established a range of dimensions for a preferred tuyere design of the invention.
The annular tuyere according to the present invention is constructed to satisfy the following conditions

where t is the width of the gap between the core body and outer tube of the tuyere, d is the diameter of the core body and D is the outer diameter of the outer tube.
In designing an annular tuyere of the above construction, it is also necessary to take into account the pressure of the injecting fluid as well as the dimensions of the tuyere and the blowing pressure such that a sonic velocity is attained after an isoentropic change. Thus, in general, the velocity of the blowing gas as it passes through the tuyere suddenly increases and reaches the sonic velocity at the nozzle end of the tuyere. At that point, if the frictional pressure loss is large, the blowing gas expands too much and becomes unstable due to the generation of exfoliated flows and waves of condensation and rarefaction. It is known that the coefficient of flow rate through a tuyere (in other words, the coefficient of the stirring flow) varies depending upon the opening angle of the tuyere nozzle. The coefficient is about 0.75 in the case of a straight tuyere of the type shown in FIGURE 5. It is therefore considered that the lower limit of the stable blowing velocity of the above-mentioned tuyere is about 75% of the sonic velocity.
On the other hand, to increase the blowing gas flow rate of the annular tuyere, it is desirable to enlarge the outside diameter D of the tuyere and the width t of the gap. The frictional loss within the tuyere particularly is reduced by enlargement of the width t of the gap so that the pressure of the gas when the flow is at the sonic level is less.
Therefore, for a given flow rate of blown gas, there is a close correlation between the outer tuyere diameter D and the width t of the gap and between the outer tuyere diameter D and the core diameter d. FIGURE 14 shows the relationship between the dimensional factors of the tuyere and the erosion of the refractory material surrounding the tuyere. In FIGURE 14, the chain line is a subsonic line (75% of sonic velocity) for an operation in which gas is blown through one tuyere nozzle at a rate of 0.08 Nm³/min per ton of molten steel, while the solid line is a subsonic line (do.) for an operation in which gas is blown through one tuyere hole at a rate of 0.06 Nm³/min per ton of molten steel. The extent of erosion of the refractory material around the tuyere is indicated by a blank circle (for a loss lower than 0.4 mm/charge), a half-black circle (for a loss of 0.4 to 0.6 mm/charge) and a solid black circle (for a loss greater than 0.6 mm/charge). It is known from the data of FIGURE 14 that, in order to keep the blowing in the subsonic range, the tuyere should have a smaller ratio of t/D when the ratio d/D is high or vice versa.
FIGURE 15 is similar to FIGURE 14 but restricted to the part in which there is less erosion which is determined from the data given in FIGURE 14. As shown in FIGURE 15, the usable ratio of d/D is limited to less than 0.4 since otherwise vacuum is developed in the gas flow at the nozzle end of the tuyere and the molten steel tends to flow into the tuyere if there is even a slight outer disturbance, coupled with increases in the amount of erosion and a trend towards increasing scouring by molten metal. On the other hand, d/D ratios smaller than 0.1 are also excluded as the tuyere becomes increasingly like the single tube tuyere of FIGURE 1 (although the adverse effect of the vacuum portions is reduced). Similarly, the tuyere loses the characteristics inherent to the annular tuyere of the invention if the ratio t/D becomes greater than 0.08 performing similarly to the conventional double tube type tuyere. Further, a t/D ratio smaller than 0.02 reflects an extremely small gap width which is unacceptably difficult to manufacture. Accordingly, t/D ratios greater than 0.08 as well as t/D ratios smaller than 0.02 are excluded from the preferred range of the present invention.
As mentioned hereinbefore, it is desirable to determine the values of t/d and d/D in an inversely proportional relation and to exclude the range of t/D > -0.11 d/D + 0.11 where the amount of erosion increases. Thus the hatched area of FIGURE 15 defines the preferred range of the present invention which allows control of the flow rate of the blowing gas over a broad range and at the same time reduces the erosion of the refractory material in the vicinity of the blowing tuyere to a minimum. Although the above description has been directed to the dimensions of the tuyere of FIGURE 5, it is to be understood that the same applies to the dual type annular tuyere of FIGURE 6 except for the dimensions of the outer tube. Furthermore, more concentric tubes with gaps therebetween may be provided around the second outer tube 8 if desired for blowing further gases.
As is clear from the foregoing description, the present invention makes it possible to carry out a stable and safe continuous gas blowing operation when gas stirring is required for molten metals in various containers by providing an annular blowing tuyere or tuyeres in the bottom or side walls of the containers. Moreover, the tuyere of the invention can reduce the erosion of the refractory material by scouring to a considerable degree, so that, if applied to converters, it helps to eliminate problems which prevent the adoption of top and bottom blown refining processes.

Claims

1. A blowing tuyere for embedding in a bottom or side wall of a molten metal container for blowing a gas thereinto, said tuyere comprising a cylindrical core body (9) located at the centre of said tuyere and an outer tube (8) mounted concentrically around said core body (9) with a gap of a width t to form an annular blowing passage therebetween, characterised in that said core body (9) and said outer tube (8) are arranged to satisfy the following conditions:

where d is the diameter of said core body (9) and D is the outer diameter of said outer tube (8).

2. A blowing tuyere as claimed in claim 1, characterised in that said outer tube (8) forms a first outer tube (8), said gap t forms a first annular blowing passage, a second outer tube (18) is mounted concentrically around said first outer tube (8) with a gap of a predetermined width to form a second annular blowing passage therebetween.

3. A blowing tuyere as claimed in claim 2, characterised in that said tuyere comprises a plurality of outer tubes arranged concentrically around said core body (9) with gaps between them to form a corresponding number of annular blowing passages therebetween.

4. A blowing tuyere as claimed in any of claims 1 to 3 characterised in that the width t of the gap between said centre core body (9) and said outer tube (8) or said first outer tube (8) is smaller than 3 mm.