Apparatus for removing non-metalic inclusions in molten metal

Abstract: A system for removing non-metallic inclusions in molten steel composed of a tundish and a coil device, wherein the tundish is an intermediate vessel that receives molten steel from a ladle, cleans the same by removing non-metallic inclusions therein, and feeds molten steel so cleaned into a mold. In order to remove non-metallic inclusions, the tundish has a circulation tank and a floating tank. A coil device is provided around the circulation tank of the tundish for circulating molten steel in the circulation tank. The tundish and the coil device are separate bodies, and are constructed so as to move relative to each other. Molten steel in the circulation tank of the tundish is circulated in a horizontal direction by virtue of the magnetic field generated by the coil device and is formed to have a parabolic, concaved surface so as to force non-metallic inclusions in the molten steel to float to the parabolic, concaved surface of molten steel for removal with a suitable means. Molten steel so cleaned flows from the circulation tank into the floating tank, where non-metallic inclusions still remaining in so-cleaned molten steel float in a calm current therein. Molten steel cleaned therein is then poured via the bottom portion of the floating tank into a mold. Since the tundish and the coil device are separate bodies, coil devices can be fewer than tundishes, resulting in lower equipment costs, and easier and less-time-consuming operations in replacement and repair of tundishes.

Description: FIELD OF THE INVENTION The present invention relates to an apparatus for removing non-metallic foreign matter in a molten metal. BACKGROUND ART In a production technology for high class sheets, removal of non-metallic foreign matter or impurity at molten steel state is critical technology for determining fraction defective of the products. It is recent trend in molten steel purification technologies, (1) to increase size of an intermediate container, i.e. tundish, between a ladle and a mold in a continuous casting to prolong a period to maintain the molten steel in the tundish with expecting floating up of the foreign matter; (2) to provide gates in multi-stages in the tundish for controlling flowing route of the molten steel to prolong period to maintain the molten steel in the tundish; and (3) in the mold, to prevent mold powder generated by molten steel flow from a discharge opening of a nozzle from penetrating by modifying configuration of an immersion nozzle to control flow of the molten steel within the mold. However, with these method, satisfactory improvement of the quality cannot be obtained. Particularly, the quality at the non-steady pouring, so-called as ladle exchange, is a level creating a problem. Therefore, as approaches, as disclosed in Japanese Unexamined Patent Publications (Kokai) Nos. 58-22317, 55-107743, 01-312024 and 02-217430, there are methods to generate a horizontal swirl flow of the molten metal to float up the foreign matter. These technology provides centrifugal force by horizontal rotation to the molten metal and the non-metallic foreign matter so as to concentrate the non-metallic foreign matter toward the swirl center due to difference of specific weights to separate by promoting collision, absorption and aggregation. This technology can achieve an improvement in the foreign matter separation effect in comparison with the methods simply prolonging dwell period or controlling molten steel flow path in the tundish. In other words, when an equal separation capacity is required, the last-mentioned method may provide advantage in significant reduction of the size of the tundish. On the other hand, the above-mentioned method for purifying the molten steel employing horizontal swirl flow as disclosed in Japanese Unexamined Patent Publications 58-22317 or 55-107743, the following problems can be encountered. (1) When the molten steel is horizontally rotated, the outer circumferential portion of the molten steel proturburates in parabolic fashion, the height of which is proportional to square of the radius and rotation speed. Therefore, increasing of the radius results in substantial increase of the height of the facility. In addition, in order to drive all of the molten steel for horizontal rotation, substantially large electromagnetic coil is required to increase cost for facility to make it impractical. (2) Reduction of rotational radius may be desirable in viewpoint of requirement for the facility. However, reduction of the capacity of the tundish may make it impossible to accomplish buffer function for realizing ladle replacing. (3) Due to penetration of air into the molten metal resulting from swirl flow, air oxidation of the upper surface of the molten metal or melting of refractory will be simultaneously progressed to abruptly increase the non-metallic foreign matter generated in the container and to flow out the large size non-metallic foreign matter. As a solution to this, it becomes necessary to use expensive refractory having high wear resistance in the overall area of the container and to seal the overall container with gas or so forth, to cause rising of the cost. An object of the invention is to provide an apparatus for removing non-metallic foreign matter in a molten metal for effectively and economically realizing separation and removal of the non-metallic foreign matter in the molten metal. According to the invention, an apparatus for removing a non-metallic foreign matter in a molten metal, in which horizontal swirl flow is provided for the molten metal for separating and removing the non-metallic foreign matter in the molten metal, comprises a swirl flow bath receiving the molten metal and flowing the molten metal in horizontal swirl fashion, and a floatation bath provided with a flowing out opening in communication with the swirl flow bath and floating up the non-metallic foreign matter in the molten metal, the swirl flow bath having a dimension satisfying h ≥ 0.47 x q 1/3 t m ≥ 2 h : minimum molten steel level in the swirl flow bath (m); q: molten steel flowing out amount from the floatation bath (ton/min); and t m : average dwell period of the molten steel in the swirl flow bath (min). Also, according to the present invention, an apparatus for removing a non-metallic foreign matter in a molten metal, in which horizontal swirl flow is provided for the molten metal for separating and removing the non-metallic foreign matter in the molten metal, comprises a swirl flow bath receiving the molten metal and flowing the molten metal in horizontal swirl fashion, and a floatation bath provided with a flowing out opening in communication with the swirl flow bath and floating up the non-metallic foreign matter in the molten metal, the swirl flow bath and the floatation bath having a dimensions determined based on h derived as defined above, satisfying h = q x t m ρ x π x r 2 - (r x ω) 2 4g H = q x t c ρ(a x b + π x r 2 ) + (r x ω) 2 4g + q x t m ρ x π x r 2 h : minimum molten steel level in the swirl flow bath (m); H: maximum molten steel level in the swirl flow bath (m); q: molten steel flowing out amount from the floatation bath (ton/min); t m : average dwell period of the molten steel in the swirl flow bach (min); ρ: specific weight of the molten steel (ton/m 3 ); r: radius of the swirl flow bath (m); ω: horizontal rotation speed in the swirl flow bath (rad/min); g: acceleration of gravity (m/min 2 ) t c : maximum interrupting period of pouring to the swirl flow bath (min) a: vertical dimension of the floatation bath (m); and b: lateral dimension of the floatation bath (m). BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagrammatic illustration of a non-metallic foreign matter removing apparatus having a swirl flow bath and a floating up bath of the invention, in which (a) is a plan view and (b) is a cross section; Fig. 2 is an illustration showing configuration of a meniscus when a molten metal is horizontally rotated; Fig. 3 is a diagrammatic illustration showing condition below molten metal surface upon replacing of a ladle; Fig. 4 is a diagrammatic illustration showing dimensions of the facility according to the present invention used in the embodiment, in which (a) is a plan view and (b) is a cross section; Fig. 5 is an illustration showing a relationship between a radius of the swirl flow bath and a maximum molten metal level in case of a facility solely having the swirl flow bath; and Fig. 6 is an illustration showing results of experiments performed on the embodiment. Hereafter will be discussed in detail an apparatus for removing non-metallic foreign matter in a molten metal according to the present invention. As will be set out in detail, the apparatus for removing the non-metallic foreign matter in the molten steel comprises separately constructed tundish and coil. An apparatus (tundish) 50 for removing the non-metallic foreign matter in the molten metal, according to the present invention, includes a swirl flow bath 41 and a floatation bath 42. To the swirl flow bath 41, the molten steel is poured from the ladle (not shown) though a nozzle 43 as indicated by an arrow in Fig. 1. The poured molten steel is preferably flown in the horizontal swirl fashion by a rotating or shifting field generating device 44. By this, the non-metallic foreign matter in the molten steel or the non-metallic foreign matter due to melting of the refractory of the tundish 50 is separated and floated on the parabolic swirl flow in the swirl flow bath. The molten steel thus purified flows into the floatation bath 42 through a communication opening 45 at the bottom of the swirl flow bath 41. The residual non-metallic foreign matter in the statically placed molten steel floats up in the floatation bath 42 and chus separated. The molten steel thus further purified is poured into the mold (not shown) via a discharge output 46 and produced as a casted product. It has been desired to optimally design the non-metallic foreign matter removing apparatus having such swirl flow bath and the floatation bath. Especially, a problem is encountered in the height of the swirl flow bath due to parabolic proturburance of the molten steel by the swirl flow in the time range of steady state, namely while the molten steel is poured into the swirl flow bath from the ladle. Also, it is important to prevent the non-metallic foreign matter floating on the swirl flow bath from flowing out to the mold through the discharge opening 46 via the communication opening 45 of both baths in a time range of non-steady state, namely while the molten steel is only flowing out through the discharge opening during ladle replacement. More particularly, prevention of the above-mentioned problem to be encountered in the non-steady state is absolutely necessary. As a result of energetic study in design of the non-metallic foreign matter removing apparatus in view of the problems as set forth above, the inventors have found the following condition through computer simulation, water model experiments and preliminary experiments in the scale of actual facility. The conditions are as expressed by the following equations (1), (2), (3) and (4). Methods of derivation of these formulae will be discussed herebelow. When the molten metal is horizontally rotated, the surface thereof is formed into the parabolic configuration relative to the static bath surface 49, as shown in Fig. 2. The height ΔH of the proturburance is expressed by the following equation: ΔH = (r x ω) 2 2g where r: radius of the swirl flow bath (m); ω: horizontal rotation speed in the swirl flow bath (rad/min); g: acceleration of gravity (m/min 2 ). On the other hand, at the ladle replacement, by the flowing out of the molten steel, the molten steel level in the container will be lowered in a magnitude as expressed in the following formula: ** q x t c ρ(a x b + π x r 2 ) where q: molten steel flowing out amount (ton/min) from the floatation bath (ton/min); t c : maximum pouring interruption period for the swirl flow bath (min); a: vertical dimension of the floatation bath (m); b: lateral dimension of the floatation bath (m); ρ: specific weight of the molten steel (ton/m 3 ). On the other hand, in order to achieve foreign matter separating and removing effect by the horizontal rotation, the necessary molten steel level required for certainly maintaining the necessary minimum average dwell period t m (= amount of molten steel in the swirl flow bath ÷ molten steel flowing out amount at unit period) in the swirl flow bath can be expressed by the following formula: q x t m ρ x π x r 2 Accordingly, with taking the buffer function during ladle replacement, the necessary maximum molten steel level H (see Fig. 3) in the swirl flow bath while the molten steel is steadily flowing in and out, becomes the height of the sum of the minimum molten steel level, the proturburance height of the molten steel surface and the level lowering magnitude during ladle replacement and can be expressed by the following equation. It should be noted that, in Fig. 3, 47 denotes the molten steel level in the floatation bath corresponding co the minimum molten steel level in the swirl flow bath, and 48 denotes a molten steel level corresponding to the maximum molten steel level in the swirl flow bath. H = q x t c ρ(a x b + π x r 2 ) + (r x ω) 2 4g + q x t m ρ x π x r 2 On the other hand, the minimum molten steel level h (see Fig. 3) required during ladle replacement can be expressed by the following equation. h = q x t m ρ x π x r 2 - (r x ω) 2 4g Here, the necessary minimum average dwell period in the swirl flow bath and the necessary minimum molten steel level necessary for achieving foreign matter separating and removing effect by the horizontal rotation are obtained through a water model experiments. As a result, it has been found that the necessary minimum average dwell period t m is 2 min irrespective of the molten steel flowing out velocity, and the necessary minimum molten steel level h min is proportional to 1/3 power of the molten steel flowing out velocity and can be expressed by the following equation: h min = 0.47 x q 1/3 By this, the following conditions are found for achieving the foreign matter separating and removing effect with maintaining the buffer function of the molten steel in the ladle replacement: h ≥ 0.47 x q 1/3 t m ≥ 2 Namely, in order to prevent the non-metallic foreign matter from reaching the mold from the swirl flow bath via the discharge opening of the floatation bath, it becomes necessary to satisfy the formulae (1) and (2). In the range satisfying the formulae (1) and (2), the range of radius of the swirl flow bath satisfying the minimum molten steel level required in the non-steady state, such as ladle replacement and so forth is determined by the equation (3). By selecting the radius of the swirl flow bath within the range of the radius, as shown in the equation (4), at which the necessary maximum molten steel level becomes minimum, it becomes possible to design the non-metallic foreign matter removing apparatus with minimum height of the facility with achieving the targeted non-metallic foreign matter separating and removing effect. According to the present invention, the apparatus for effectively removing the non-metallic foreign matter which can be a cause for defects in the products, such as sheet can be formed without excessive enlarging of the facility. Furthermore, by employing the apparatus, the non-metallic foreign matter can be steadily removed even in the non-steady state, such as during ladle replacement and so forth to lower the fault ratio of the product and to enable substantial improvement of the yield. Also, as a result, it becomes possible to produce highly purified steel without requiring significant equipment investment and at low cost. It should be noted that when the molten steel is poured into the swirl flow phase in the tundish from the nozzle of the ladle, it can be poured to the swirl center of the swirl flow phase or at a desired position offset from swirl center. Also, the nozzle of the ladle may be submerged or not submerged into the swirl flow phase in the tundish. Concrete discussion for the present invention will be given herebelow in terms of an example. (Example) One example (invention) of the non-metallic foreign matter removing apparatus having the swirl flow bath of the invention and the floatation bath, which is minimized and optimized from the formulae (1) and (8) in order to satisfy the operating condition shown in table 1, is illustrated in Fig. 4 with a dimensions (unit: mm). Conversely, under the condition of table 1, in case of the facility (comparative example) illustrated in Fig. 2, which does not have the floatation bath, with taking the minimum molten metal level being higher than or equal to 0.5 m (= 0.47 x 1.2 1/3 ), in order to certainly maintain 3 minutes of the set dwell period in the swirl flow bath, the height has to be determined based on the constraint of set dwell period in the swirl flow bath in case that the swirl flow bath radius is smaller than or equal to 0.46m, and based on the constraint of the minimum molten metal level upon the ladle replacement in case that the swirl flow bath radius is greater than or equal to 0.46m, from the formulae (1) and (3). Therefore, in case of the comparative example, the height of the facility as illustrated in Fig. 5 is required. Even at the minimum height in Fig. 5, the maximum level of the molten steel reaches 1.52m. Therefore, the height of the facility has to be approximately 400 mm higher than the example of the invention illustrated in Fig.4. Increasing of the height of the tundish causes substantial increase of the cost for facility due to increasing of the height of building. Also, when it is applied to the existing continuous casting facility, it often becomes impossible to realize due to constraint of the facility. Furthermore, when the radius of the swirl flow bath to minimize the facility is employed, only about 4 tons of molten steel can be obtained to encounter the problem to make it difficult to certainly maintain the molten steel level. In contrast to this, according to the example of the invention, it becomes possible not only to lower the necessary height than that in the comparative example, but also to adjust the molten steel capacity by the size of the floatation bath. In the experiments, number of non-metallic foreign matter was measured by analyzing the samples obtained at the discharge opening during casting in the condition illustrated in Fig. 4 and the table 1. In Fig. 6, there is shown the comparison of the ratio of the non-metallic foreign matter in cases swirl flow in the swirl flow bath is provided and not provided. From Fig. 6, it can be seen that the substantial amount of the non-metallic foreign matter in the molten steel can be removed by the removing apparatus according to the present invention and the effect can be maintained even at the ladle replacement. Item Content Ladle Capacity 100 tons Kind of Casting Steel Ferrite type Stainless Steel (SUS 430) Molten Steel Flowing Out Amount 1.2 tons/min Molten Steel Rotation Speed in Swirl Flow Bath 60 r.p.m. (120 πrad/min) Ladle Replacing Period 2 min Set Dwell Period in Swirl Flow Bath 3 min INDUSTRIAL APPLICABILITY It is very important for supplying purified molten steel, from which the non-metallic foreign matter is removed from the molten steel, to the mold. In order to purify the molten steel, the tundish is provided with the swirl flow bath and the floatation bath. With the coil arranged on the circumference of the swirl flow bath, the molten steel is flown in swirl fashion to float up the non-metallic foreign matter to the surface of the molten steel and the floated non-metallic foreign matter is removed. The molten steel removed the non-metallic foreign matter flows out to the floatation bath. With the static flow in the floatation bath, the residual non-metallic foreign matter float up. The molten steel thus purified is supplied to the mold from the bottom of the floatation bath. With such system, degree of removal of the non-metallic foreign matter in the molten steel can be significantly improved in comparison with that in the prior art. On the other hand, the tundish and the coil are formed separately to have the construction allowing relative displacement to each other. Therefore, number of the coil can be smaller than the number of tundish to contribute lowering of the cost for facility. Also, since the tundish is formed separately from the coil and is movable relative to the later, the regular replacing operation of the tundish, repairing of the lining refractory brick of the tundish can be done easily and in short period.

Claims: An apparatus (50) for separating and removing a nonmetallic foreign matter in a molten metal, comprising a swirl flow bath (41) receiving the molten metal and flowing the molten metal in horizontal swirl fashion, and a floatation bath (42) provided with a flowing out opening (46) in communication with said swirl flow bath (41) and floating up the non-metallic foreign matter in the molten metal, characterized in that said swirl flow bath (41) has a dimension satisfying h ≥ 0.47 × q 1/3 t m ≥ 2 wherein: h: minimum molten steel level in the swirl flow bath(41)(m); q: molten steel flowing out amount from the floatation bath (42) (ton/min); and t m : average dwell period of the molten steel in the swirl flow bath (41) (min). An apparatus (50) for separating and removing a nonmetallic foreign matter in a molten metal, comprising a swirl flow bath (41) receiving the molten metal and flowing the molten metal in horizontal swirl fashion, and a floatation bath (42) provided with a flowing out opening (46) in communication with said swirl flow bath (41) and floating up the non-metallic foreign matter in the molten metal, characterized in that said swirl flow bath (41) and said floatation bath (42) have dimensions determined based on h derived by h ≥ 0.47 × q 1/3 satisfying h = q × t m ρ×π× r 2 - ( r ×ω) 2 4 g H = q × t c ρ( a × b +π× r 2 ) + ( r ×ω) 2 4 g + q × t m ρ×π× r 2 wherein: h: minimum molten steel level in the swirl flow bath(41)(m); H: maximum molten steel level in the swirl flow bath(41)(m); q: molten steel flowing out amount from the floatation bath (42) (ton/min); t m : average dwell period of the molten steel in the swirl flow bath (41) (min); ρ: specific weight of the molten steel (ton/m3); r: radius of the swirl flow bath (41) (m); ω: horizontal rotation speed in the swirl flow bath (41) (rad/min); g: acceleration of gravity (m/min2); t c : maximum interrupting period of pouring to the swirl flow bath (41) (min); a: vertical dimension of the floatation bath (42) (m); and b: lateral dimension of the floatation bath (42) (m).

Inventors: KITOAKA HIDENARI, MIKI YUJI, MORIWAKI SABURO, NABESHIMA YUKI, OGURA SHIGERU, ONISHI MASAYUKI, SAKURAI MITSURU, SAKURAYA TOSHIKAZU, TANINO YOZO, TERASHIMA TSUKASA, TOMIYAMA YOSHIRO, YASUKAWA NOBORU, KITAOKA, H., MIKI, Y., MORIWAKI, S., NABESHIMA, Y., OGURA, SHIGERU, ONISHI, M., SAKURAI, M., SAKURAYA, T., TANINO, YOZO, TERASHIMA, T., TOMIYAMA, Y., YASUKAWA, N., KITAOKA, H., TECHN. RES. DIV. OF KAWAS. ST.CORP., MIKI, Y., TECHN. RES. DIV. OF KAWAS. STEEL CORP., MORIWAKI, S., CHIBA WORKS OF KAWASAKI STEEL CORP., NABESHIMA, Y., CHIBA WORKS OF KAWAS. STEEL CORP., OGURA, SHIGERU, CHIBA WORKS OF KAWAS. STEEL CORP., ONISHI, M., CHIBA WORKS OF KAWAS. STEEL CORP., SAKURAI, M., CHIBA WORKS OF KAWASAKI STEEL CORP., SAKURAYA, T., TECH. RES. DIV. OF KAWAS. STEEL CO., TANINO, YOZO, CHIBA WORKS OF KAWASAKI STEEL CORP., TERASHIMA, T., CHIBA WORKS OF KAWAS. STEEL CORP., TOMIYAMA, Y., CHIBA WORKS OF KAWASAKI STEEL CORP., YASUKAWA, N., CHIBA WORKS OF KAWASAKI STEEL CORP.
Applicants: KAWASAKI STEEL CO, KAWASAKI STEEL CORPORATION
Application year: 1992
Main classification: B22D11
IPC codes: B22D11