JP2010070815A - Ladle for refining molten steel and method for refining molten steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ladle for refining a molten steel, which has a plug for blowing a gas for stirring at the bottom and can markedly enhance a stirring power compared to a conventional ladle, and to provide a method for refining the molten steel by using the ladle for refining the molten steel and strongly stirring the molten steel therein. <P>SOLUTION: The ladle 1 for refining the molten steel has two or more blowing plugs 4 for blowing the gas in the bottom. When an inner diameter of a side wall in the bottom of the ladle is defined as D, at which the blowing plug is installed, a distance between the centers of the adjacent blowing plugs is D/6 to D/4. The method for refining the molten steel includes stirring the molten steel 5 accommodated in the ladle under a stirring condition that the power density &epsi; for stirring the molten steel calculated by the expression of &epsi;=(0.0285&times;Q<SB>Ar</SB>&times;T/W<SB>m</SB>)&times;log[1+(760&times;H)/(148&times;P)] is 1,000 W/ton or more. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  TECHNICAL FIELD The present invention relates to a molten steel refining ladle having a stirring gas blowing plug at the bottom, and a molten steel refining method using the molten steel refining ladle.

  In recent years, quality requirements for steel products have become stricter than ever, and in order to cope with this, a ladle that performs decarburization, deoxidation, desulfurization, inclusion removal and final component adjustment of molten steel in a ladle. Refining is generally carried out. In order to perform this ladle refining efficiently, it is important to stir the molten steel in the ladle. Therefore, there are various methods such as the use of electromagnetic force, suction into a vacuum tank, and gas blowing from the immersion lance. A stirring method is performed. Among them, a gas stirring method in which a blowing plug is placed at the bottom of the ladle and stirring gas is blown from the blowing plug is efficient and easy in terms of equipment. Has been.

  In this bottom blowing gas stirring method, a blowing plug made of porous brick is generally provided at the bottom of the pan, and an inert gas such as Ar gas or nitrogen gas is blown into the molten steel from the blowing plug to flow the molten steel.・ Stirring. In this case, in order to obtain optimum stirring conditions, the number of blowing plugs is increased to increase the gas flow rate, or the position of the blowing plug is specified to control the stirring strength and stirring efficiency. ing.

In particular, as in a VOD furnace (Vacuum Oxygen Decarburization Furnace), oxygen gas is supplied under reduced pressure for decarburization and refining, and then oxides generated during decarburization and refining are reduced with reducing agents such as Al and Fe-Si alloys. In the process of reducing and deoxidizing by adding NO, the effect of gas stirring on decarburization efficiency and carbon concentration at the time of decarburization, and oxygen concentration in molten steel after reduction is large. Attempts have been made to regulate the flow rate (see, for example, Patent Document 1). It has also been attempted to arrange a plurality of blowing plugs (see, for example, Patent Document 2).
JP-A-1-287218 Japanese Utility Model Publication No. 61-55055

  Improvements in molten steel stirring technology in ladle refining are important due to the increasing demands on the cleanliness of molten steel due to the increasingly demanding quality of steel products that are becoming more and more severe year by year. It is clear that this is a technical development.

  However, in the conventional ladle refining method described above, due to the restriction of the blowing plug installed at the bottom of the ladle, it is difficult to secure a large gas flow rate, and the blowing due to the physical restrictions on the bottom of the ladle. The place and number of plugs are limited, and it has been difficult to increase the stirring gas flow rate significantly.

  In addition, even if a blow plug that can blow a large flow rate of gas is developed, the large flow rate of gas flow increases operational troubles such as increased splash of molten steel when the blown gas leaves the molten steel. In addition, since the slag present on the molten steel is highly viscous, bubbles do not escape from the slag, causing slag forming, resulting in large-scale equipment due to the scattering of high-temperature materials to the refining equipment There was a risk of trouble. Further, as in Patent Document 2, when the blowing plug is disposed in the vicinity of the side wall of the ladle, the stirring effect is promoted, but there is a problem that the refractory on the side wall of the ladle becomes worn out.

  Thus, with the conventional bottom blowing gas stirring method, it has been said that it is difficult to improve the stirring of molten steel beyond the current level.

  The present invention has been made in view of the above circumstances, and an object thereof is to disperse molten steel by gas when the molten steel is released in a ladle for molten steel refining having a stirring gas blowing plug at the bottom. In addition to providing a ladle for molten steel refining that can significantly increase the agitation strength compared to conventional methods without increasing the slag, a method for refining molten steel under strong agitation using this ladle for molten steel refining Is to provide.

  The present inventors conducted research and studies to solve the above problems. As a result, by arranging two or more blowing plugs and defining the relative position of each of the two or more blowing plugs, the flow form of the molten steel that could not be controlled only by increasing the stirring gas flow rate was taken. We obtained the knowledge that it was possible to control to a form effective for refining such as decarburization reaction of molten steel without suppressing the damage of the pan refractory and causing troubles such as scattering of molten steel.

  The present invention has been made based on the above knowledge, and a ladle for molten steel refining according to the first invention is a ladle for molten steel refining having two or more blowing plugs for gas blowing at the bottom. The distance between the centers of adjacent blowing plugs is D / 6 or more and D / 4 or less, where D is the inner diameter of the side wall of the bottom of the ladle where the blowing plug is installed. Is.

  In the ladle for molten steel refining according to the second invention, in the first invention, at least one of the blowing plugs is installed at a position separated from the side wall of the ladle bottom by D / 6 or more. It is a feature.

  The molten steel refining method according to the third invention is a molten steel agitating power density calculated by the following formula (1) of the molten steel accommodated in the ladle for molten steel refining described in the first or second invention is 1000 W. It is characterized by stirring with Ar gas under stirring conditions of at least / ton.

In Equation (1), ε is the molten steel stirring power density (W / ton), Q _Ar is the total flow rate of the bottom-blown Ar gas (NL / min), T is the molten steel temperature (K), and W _m is the molten steel mass ( T), h is the depth of the molten steel (cm), and P is the atmospheric pressure (torr).

  According to the ladle for molten steel refining according to the present invention, the gas bubbles blown from the respective blow plugs interfere with each other, thereby adding a horizontal flow to the rising of the gas bubbles, thereby As a result, the rising speed of the gas in the vertical direction is reduced, and as a result, even if the stirring gas supply amount is increased, the swell of molten steel in the ladle is suppressed, and the amount of molten steel scattered by the gas when leaving the molten steel decreases. The supply amount of the stirring gas can be increased, and strong stirring of the molten steel becomes possible, and industrially beneficial effects such as shortening the processing time in ladle refining and the accompanying decrease in the temperature drop of the molten steel are brought about.

  The present invention will be specifically described below. First, the background to the present invention will be described.

  The inventors of the present invention conducted a water model experiment for the purpose of remarkably increasing the stirring efficiency in comparison with the prior art in a ladle for molten steel refining having a plurality of stirring gas blowing plugs at the bottom. In the experiment, the installation position of the blowing plug was changed and the gas supply amount was changed. The following knowledge was obtained from this water model experiment.

  When the stirring gas is blown from the blowing plug installed at the bottom of the ladle for molten steel refining, the gas rises in the molten steel, and the molten steel is agitated as this gas rises. When coming out from the atmosphere into the atmosphere, the hot water surface or slag in the ladle rises due to the viscosity of the molten steel and slag. At this time, if the inner diameter of the side wall of the bottom of the ladle where the blowing plugs are installed is D, the distance between the blowing plugs is made larger than D / 4, and each of them at a position separated from D / 4. When gas is blown from the blow plug, the gas bubbles from the blow plugs rise vertically above the blow plug without interfering with each other, increasing the flow rate of gas blow from the blow plug. It has been found that the rise of the molten steel surface increases with that, that is, the amount of molten steel scattered by the gas when the molten steel is released increases according to the gas supply amount.

  On the other hand, when the distance between each blowing plug is set to a distance shorter than D / 6, the effect of arranging a plurality of blowing plugs is reduced, similar to the case where gas is supplied from one blowing plug. As a result, the rise of the molten steel surface in the ladle is larger than when installed at a position more than D / 4, depending on the number of blowing plugs and the gas supply amount. I found out that In other words, it was found that the amount of molten steel scattered by the gas when the molten steel is released further increases.

  On the other hand, when the distance between each blowing plug is within the range of D / 6 or more and D / 4 or less, the gas bubbles blown from the respective blowing plugs interfere with each other, and the gas bubbles It was found that the vertical flow of gas on the surface of the molten steel was reduced by the addition of a horizontal flow to the levitation, and the rise of the molten steel in the ladle was suppressed. In other words, even if the amount of stirring gas supplied is increased to increase the molten steel stirring power density, the rise of molten steel in the ladle is suppressed, and the amount of molten steel scattered by the gas when leaving the molten steel is significantly less than in the past. It turns out that it is possible.

  Moreover, when a blowing plug is installed in the position below D / 6 from the side wall of the ladle bottom part, since the space | interval of a ladle side wall and a blowing plug is short, gas floats in the vicinity of a ladle side wall. It was also confirmed that the molten steel flow velocity in the vicinity of the ladle side wall increased, and the ladle refractory was severely damaged. That is, it was also found that it is preferable to install at a position away from the ladle side wall by D / 6 or more.

  The present invention is based on the above experimental results, and the ladle for molten steel refining according to the present invention is a ladle for molten steel refining having two or more blowing plugs for gas blowing at the bottom, wherein the blowing plug When the inner diameter of the side wall of the bottom of the ladle is set to D, the distance between the centers of the blowing plugs is D / 6 or more and D / 4 or less. In this case, it is preferable that at least one of the blowing plugs is installed at a position away from the side wall of the bottom of the ladle by D / 6 or more.

  Next, an example of a ladle for molten steel refining according to the present invention and a method for refining molten steel using the ladle will be described.

  FIG. 1 is a schematic side sectional view showing an example of a ladle for molten steel refining according to the present invention, FIG. 2 is a schematic plan view of the ladle for molten steel refining shown in FIG. It consists of an iron shell 2 that forms an outer shell and a refractory 3 that is constructed inside the iron shell 2, and is attached to the refractory 3 at the bottom of the ladle 1 for molten steel refining (also called “laying”). A plurality of blowing plugs 4 that are fitted and made of porous bricks are installed. Here, an example in which three plugs 4 are installed is shown. In FIG. 2, the side wall of the ladle 1 for molten steel refining is omitted. Moreover, the code | symbol 5 in FIG. 1 is molten steel, and 6 is slag.

  Each blowing plug 4 is connected to a gas introduction pipe (not shown), and a rare gas such as Ar gas or He gas or a non-oxidizing gas such as nitrogen gas is used as a stirring gas from each blowing plug 4. It is blown into the inner surface side of the ladle 1 for molten steel refining. In this case, when the inner diameter of the side wall at the bottom of the ladle 1 for molten steel refining is D, each blowing plug 4 is set so that the distance d between the centers of the blowing plugs 4 is D / 6 or more and D / 4 or less. Plug 4 is installed.

  A specific installation method of the blowing plug 4 will be described with reference to FIGS. 3 and 4. FIG. 3 shows an example in which two blowing plugs 4 are installed and the first blowing plug 4a is installed at the center position of the ladle 1 for molten steel refining. In this case, the second blowing plug 4b needs to be installed in a region A surrounded by an arc having a radius D / 6 centered on the first blowing plug 4a and an arc having a radius D / 4. And may be installed at an arbitrary position in the area A. In this case, since the first blowing plug 4a is installed at the center position of the ladle 1 for molten steel refining, the second blowing plug 4b is naturally separated from the ladle side wall by D / 6 or more. It will be installed in a different position.

  It is preferable to install the blowing plug 4 at a position away from the ladle side wall by D / 6 or more. When the blowing plug 4a is installed at a position shifted from the center position of the ladle 1 for molten steel refining, It is preferable to consider the installation position of the blowing plug 4a so that the plug 4b is installed at a position away from the ladle side wall by D / 6 or more. In this case, as a matter of course, the blowing plug 4a is also preferably installed at a position separated from the ladle side wall by D / 6 or more.

  FIG. 4 shows an example in which three blowing plugs 4 are installed and the first blowing plug 4a is installed at the center position of the ladle 1 for molten steel refining. The second blowing plug 4b needs to be installed in a region A surrounded by an arc having a radius D / 6 centered on the first blowing plug 4a and an arc having a radius D / 4. What is necessary is just to install in the arbitrary positions of the area | region A. In the third blowing plug 4c, a region B surrounded by an arc having a radius D / 6 centered on the second blowing plug 4b and an arc having a radius D / 4 overlaps the region A. It is necessary to install in the position, and it should just install in arbitrary positions of this overlapping position. FIG. 4 shows an example in which the center positions of the blow plugs 4 are arranged to form a regular triangle.

  Also in this case, since the first blowing plug 4a is installed at the center position of the ladle 1 for molten steel refining, the blowing plug 4b and the blowing plug 4c are naturally separated from the ladle side wall by D / 6 or more. In the case where the first blowing plug 4a is installed at a position shifted from the center position of the ladle 1 for molten steel refining, the blowing plug 4b and the blowing plug 4c are provided. It is preferable to consider the installation position of the blowing plug 4a so that it is installed at a position away from the ladle side wall by D / 6 or more. In this case, as a matter of course, the blowing plug 4a is also preferably installed at a position separated from the ladle side wall by D / 6 or more.

  Although not shown, even when four blowing plugs 4 are installed, each blowing plug 4 may be arranged so that the distance d between each blowing plug is D / 6 or more and D / 4 or less. .

  In refining the molten steel 5 using the ladle 1 for molten steel refining according to the present invention configured as described above, the amount of molten steel scattered by the agitating gas can be suppressed, so that the gas agitation strength is increased compared to the conventional case. Therefore, it is preferable to stir under stirring conditions in which the molten steel stirring power density is 1000 W / ton or more. The upper limit of the molten steel stirring power density need not be specified, but when the molten steel stirring power density becomes higher than a certain level, the refining reaction rate control is different from the stirring of the molten steel, that is, the mass transfer rate control on the molten steel side (for example, (Chemical reaction rate limiting), and the meaning of raising the stirring strength beyond that is lost. Therefore, the upper limit of the molten steel stirring power density of about 5000 W / ton may be set.

The molten steel stirring power density when Ar gas is used as the stirring gas is given by the following equation (1). In Equation (1), ε is the molten steel stirring power density (W / ton), Q _Ar is the total flow rate of the bottom-blown Ar gas (NL / min), T is the molten steel temperature (K), and W _m is the molten steel mass ( T), h is the depth of the molten steel (cm), and P is the atmospheric pressure (torr).

Therefore, based on the mass (W _m ) of the molten steel 5 contained in the ladle 1 for molten steel refining, the depth (h) of the molten steel 5, the pressure (P) of the atmosphere, and the molten steel temperature (T), The total flow rate (Q _Ar ) of the bottom blown Ar gas at which the molten steel stirring power density (ε) is 1000 W / ton or more is obtained, and each Ar gas flow rate that is equal to or higher than the obtained total flow rate (Q _Ar ) of Ar gas is injected. What is necessary is just to blow Ar gas into the molten steel 5 from each blowing plug 4 by distributing to the plug 4. The blowing flow rate from each blowing plug 4 does not need to be equal, and if the total blowing flow rate from each blowing plug 4 is equal to or greater than the total Ar gas flow rate (Q _Ar ) obtained. Good.

  Thus, according to the ladle 1 for molten steel refining according to the present invention, the gas bubbles blown from the respective blow plugs 4 interfere with each other, thereby adding a horizontal flow to the rising of the gas bubbles. As a result, the vertical ascent rate of gas on the surface of the molten steel decreases, and as a result, even if the gas supply amount is increased, the rise of the molten steel 5 in the ladle is suppressed, and the amount of molten steel scattered by the gas when the molten steel 5 is released Therefore, the amount of gas supply for stirring can be increased as compared with the conventional case, and the molten steel 5 can be strongly stirred, and the ladle refining can be efficiently performed.

Three blowing plugs are installed at the bottom of a ladle for molten steel refining with a capacity of 190 tons in the arrangement shown in FIG. 4 (d = D / 5), and 185 ton stainless steel is placed in the ladle for molten steel refining. Molten steel (Cr content: 18% by mass) was accommodated, and decarburization refining was performed in a VOD furnace while Ar gas was being blown from a blow plug (example of the present invention). The molten steel temperature (T) at this time is 1650 ° C. (= 1923 K), the atmospheric pressure (P) in the VOD furnace is 3.8 torr, the molten steel height (h) is 210 cm, and the molten steel stirring power density (ε) is 1000 W. The total flow rate (Q _Ar ) of the bottom blowing Ar gas was determined by the equation (1) so as to be equal to or higher than / ton, and the Ar gas blowing flow rate from each blowing plug was 750 NL / min. The molten steel stirring power density (ε) at this time is 1636 W / ton. Further, the top blown oxygen gas for decarburization was supplied at 25 Nm ³ / min at the initial stage of refining, and was supplied at 18 Nm ³ / min after the carbon concentration in the molten steel reached 0.06% by mass.

  In addition, for comparison, the three blowing plugs are disposed within D / 6 from the ladle side wall and the distance between the centers of the three blowing plugs is less than D / 6. Deposition of stainless steel (Cr content: 18% by mass) in a VOD furnace by using a ladle for molten steel refining and blowing oxygen gas and stirring Ar gas under the same conditions as above. Refining was carried out (comparative example).

  FIG. 5 shows the results of measuring the rising height of the molten steel in the ladle during refining using a laser optical distance meter. In FIG. 5, the swell height in the comparative example is displayed as an index, and when the stirring gas flow rate is the same, the swell height is about 40% lower in the present invention example than in the comparative example. I was able to confirm.

  FIG. 6 is a diagram showing the transition of the carbon concentration in the molten steel during decarburization refining in comparison with the present invention example and the comparative example. In the present invention example, since stirring is efficient, the carbon concentration in the molten steel is It has been found that the decarburization refining time can be shortened by about 5 minutes.

  Moreover, FIG. 7 shows the wear situation of the slag line part (part contacting the slag) of the ladle for molten steel refining after 10 times of continuous decarburization refining of stainless steel in the VOD furnace. It is a figure which shows the investigated result, and in FIG. 7, it displays with the index | index based on the refractory material loss amount in a comparative example. As shown in FIG. 7, it was confirmed that the refractory was prevented from being melted in the example of the present invention.

It is a side section schematic diagram showing an example of a ladle for molten steel refining concerning the present invention. It is a schematic plan view of the ladle for molten steel refining shown in FIG. It is a figure which shows the example of the specific installation position of the blowing plug in this invention. It is a figure which shows the other example of the specific installation position of the blowing plug in this invention. It is a figure which shows the rising height of the molten steel in a ladle under refining in contrast with the example of this invention, and a comparative example. It is a figure which shows transition of the carbon concentration in molten steel during decarburization refining in contrast with the example of the present invention and the comparative example. It is a figure which shows the wear condition of the slag line part of the ladle for molten steel refining in contrast with the example of this invention, and a comparative example.

Explanation of symbols

1 Ladle for steel refining 2 Iron skin 3 Refractory 4 Blow plug 5 Molten steel 6 Slag

Claims (3)

  A ladle for molten steel refining having two or more blowing plugs for gas blowing at the bottom, and the adjacent blowing plug when the inner diameter of the side wall of the ladle bottom where the blowing plug is installed is D A ladle for molten steel refining, wherein the distance between the centers of the steel is D / 6 or more and D / 4 or less.   The ladle for molten steel refining according to claim 1, wherein at least one of the blow plugs is installed at a position separated from the side wall of the ladle bottom by D / 6 or more. The molten steel accommodated in the ladle for molten steel refining according to claim 1 or 2 is subjected to Ar gas under stirring conditions where the molten steel stirring power density calculated by the following equation (1) is 1000 W / ton or more. A method for refining molten steel, comprising stirring.

In Equation (1), ε is the molten steel stirring power density (W / ton), Q _Ar is the total flow rate of the bottom-blown Ar gas (NL / min), T is the molten steel temperature (K), and W _m is the molten steel mass ( T), h is the depth of the molten steel (cm), and P is the atmospheric pressure (torr).

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