IMMERSION PIPE FOR CONTINUOUS CASTING OF STEEL

Abstract: An immersion pipe for continuous casting of steel comprises: an immersion pipe body (11) introducing molten steel supplied from a tundish into a continuous casting mold; the immersion pipe body having two exit ports (12a, 12b) located at symmetrically about the vertical center axis of the immersion pipe body at a lower portion thereof and being immersed in the molten steel in the mold; a bore (13) of the immersion pipe body having two sectional areas, one of the two sectional areas at and below the two exit ports being smaller than the other above the two exit ports, through the bore the molten steel passing on; and an inner diameter (16) of the bore at the level of the two exit ports having a length almost equal to a horizontal inner diameter (17) of the two exit ports. A ratio represented by (A)/(B) is from 0.50 to 0.80, where (A) is a sectional area of the bore at and below the two exit ports and (B) is a sectional area of the bore above the two exit ports.

Description: The present invention relates to an immersion pipe for introducing molten steel from a tundish to a mold for continuous casting of steel, and more particularly to a structure of the immersion pipe. Accretion of oxide inclusions to the inwall of an immersion pipe for continuous casting of steel increases as time goes by. This accretion restricts casting time of molten steel, coarsens deoxidized products of a few microns contained in molten steel and induces defects in steel products. An increase of the defect ratio of the products attributable to powder inside a mold has also been ascertained in connection with the recent increase of continuous casting speed. This increase of the defect ratio is closely related with up and down movements of the surface of the molten steel in the mold. The excess movement of the surface of the molten steel over a certain level gives rise to the defects attributable to powder. In the increase of the continuous casting speed, the up-and-down movements of the surface of the molten steel are too hard to be controlled in the range of the optimum levels, and their results are remarkably reflected as the defects of the products. Properly speaking, the forms of the immersion pipe need to be selected individually and elaborately depending on continuous casting speed and width of slabs, because the movements of the surface of the molten steel in the mold are determined by the flow speed and the directions of the molten steel poured into the mold. However, the proceeding of the increase of the inclusions accreted to the inwall of the immersion pipe varies the flow speed and the directions of the molten steel poured into the mold as time goes by and often causes surface defects of the slabs attributable to powder. Fig. 1 shows sectional views illustrating schematically a prior art immersion pipe. Fig. 1(a) is a sectional plan view of an immersion pipe body of the immersion pipe taken on line 2-2, passing through the centers of exit ports 12a and 12b. Fig. 1(b) is a vertical sectional view of the immersion pipe body taken on line 3-3 of Fig. 1(a). Fig. 1(c) is a vertical sectional view of the immersion pipe body taken on line 4-4 of Fig. 1(a). Prior art immersion pipe body 11 has bore 13 of molten steel inside the immersion pipe and two exit ports 12a and 12b facing each other in the lower portion. The sectional area of bore 13 of the molten steel is the same over the length of the pipe. The inner diameters of exit ports 12a and 12b are the same as that of bore 13. Alumina-graphite or zirconium is used for immersion pipe body 11. Referential numeral 14 denotes inclusions accreted to the inwall of the immersion pipe schematically shown, and more particularly, alumina accreted to the inwall of the immersion pipe. The prior art immersion pipe, however, has difficulties in that the inclusions accrete to the inwall of the pipe and the surface defects attributable to powder occur. It is an object of the present invention to provide an immersion pipe having a structure, in which inclusions are not easy to accrete to the inwall of the immersion pipe. In order to attain the object, in accordance with the present invention, an immersion pipe for continuous casting of steel is provided, comprising: an immersion pipe body introducing molten steel supplied from a tundish into a continuous casting mold; said immersion pipe body having two exit ports located symmetrically about the vertical center axis of said immersion pipe body at a lower portion thereof, said immersion pipe body being immersed in the molten steel of the mold and the two exit ports introducing the molten steel into the mold; a bore of said immersion pipe body having two sectional areas, one of the two sectional areas at and below the two exit ports being smaller than the other above the two exit ports, through the bore the molten steel passing on; and an inner diameter of the bore at the level of the exit ports having a length almost equal to a horizontal inner diameter of the exit ports. The object and other objects and advantages of the present invention will become apparent from the detailed description to follow, taken in connection with the appended drawings. Brief Description of the Drawings Fig. 1 shows sectional views illustrating a prior art immersion pipe for continuous casting of steel; Fig. 2 shows sectional views illustrating an immersion pipe for continuous casting of steel of the present invention; Fig. 3 is a graphic representation indicating relation between the reduction ratio represented by (A)/(B) and thickness of alumina accreted to an inwall of the immersion pipe when the immersion pipe of the present invention is used, where (A) is a sectional area of a bore at and below two exit ports and (B) is that above the two exit ports; and Fig. 4 is a graphic representation indicating the comparison of thickness of alumina accreted to the inwall of an immersion pipe at the time of using the immersion pipe for continuous casting of steel of the present invention with that of alumina at the time of using a prior art immersion pipe for continuous casting of steel. Relative to a prior art immersion pipe, we, the inventors, studied relations between casting time and thickness of alumina, i.e. inclusions, accreted to an inwall of the immersion pipe, a flow speed of the molten steel inside the immersion pipe and the thickness of alumina accreted to the inwall, and the amount of argon gas blown in the immersion pipe and the thickness of alumina to the inwall. As a result, the following were recognized: (A) In the direction of the vertical section of immersion pipe body 11 of an immersion pipe taken on line 3-3 of Fig. 1(a), the thickness of alumina is decreased by changing the materials of the immersion pipe body from alumina-graphite into zirconium, by increasing the flow speed of the molten steel inside the immersion pipe body and by increasing the amount of argon blown in the immersion pipe body from a tundish nozzle. (B) In the direction of the vertical section of immersion pipe body 11 taken on line 4-4 of Fig. 1(a), the thickness of alumina accreted to the inwall of the immersion pipe is not decreased because the stagnation of the flow of the molten steel exists, even if the materials for the immersion pipe body are changed from alumina-graphite into zirconium, the flow speed of the molten steel is increased inside the immersion pipe body and the blow amount of argon into the immersion pipe body is increased. (C) The accretion of the inclusions to the inwall of the immersion pipe body in the vertical direction taken on line 4-4 of Fig. 1(a) does not proceed further when the inclusions are accreted to the inwall of the immersion pipe body to a certain extent. This is because the stagnation of the flow is decreased as the accretion of the inclusions goes on in the vertical direction of the immersion pipe body. Based on the abovementioned knowledge, the accretion of the inclusions in the vertical direction of the immersion pipe body taken on line 4-4 of Fig. 1(a) proved to be the same as that in the vertical direction of the immersion pipe body taken on line 3-3 of Fig. 1(a), when the form of the immersion pipe body in the vertical direction was shaped so that the molten steel could not become stagnate. The present invention removes the stagnation in the flow of the molten steel by reducing a sectional area of a bore at and below the two exit ports to less than that above the exit ports. Furthermore, the stagnation in the flow of the molten steel is reduced by making an inner diameter of the bore at the level of the exit ports almost equal to a horizontal inner diameter of the two exit ports located symmetrically about the vertical axis of the immersion pipe. Fig. 2 shows sectional views illustrating an immersion pipe for continuous casting of steel of the present invention. Fig. 2(a) is a sectional plan view of the immersion pipe body 11 taken on line 2-2, passing through the centers of exit ports 12a and 12b. Fig. 2(b) is a vertical sectional view of immersion pipe body 11 of the immersion pipe taken on line 3-3 of Fig. 2(a). Fig. 2(c) is a vertical sectional view of the immersion pipe body taken on line 4-4 of Fig. 2(a). Immersion pipe body 11 of an immersion pipe is made from refractory and provided with exit ports 12a and 12b located symmetrically about the vertical center axis of the immersion pipe body at its lower portion. Exit ports 12a and 12b are circular in shape. The bottom of the immersion pipe body is of a pool shape. When exit ports 12a and 12b are opened, inner diameter 16 of the bore for flowing the molten steel at and below the exit ports is designed to be equal to a horizontal inner diameter 17 of the exit ports. Boring the centers of exit ports 12a and 12b are directed upward relative to a horizontal plane to the vertical center axis of the immersion pipe. So, the exit ports have a center axis with an angle sloping upwards relative to the horizontal line. Furthermore, a line passing through the centers of exit ports 12a and 12b crosses lower end 18 of a reduced portion of the immersion pipe body. Fig. 3 is a graphic representation showing relation between the reduction ratio represented by (A)/(B) and thickness of alumina accreted to the inwall of the immersion pipe body, (A) being a sectional area at and below the exit ports and (B) being a sectional area above the exit ports. Casting conditions are shown below in the case of the reduction ratios being 0.5, 0.6, 0.7 and 0.8: Inner diameter 15 of the bore at and below the exit ports: 75-85 mm Inner diameter 16 of the bore above the portion of the exit ports: 50-65 mm Casting speed: 105-5.0 Ton/min. Casting time: 150-250 min. Material of the immersion pipe body: zirconium lined with alumina-graphite The case that reduction ratio of (A)/(B) is 1.0 is for an example of using the prior art immersion pipe. From Fig. 3, it is recognized that 0.5 or more and 0.8 or less of the reduction ratio is preferable. If the reduction ratio is less than 0.5, solidified metal stops up at the exit ports and therebelow at the beginning stage of casting and the immersion pipe is easily choked. If the ratio is over 0.8, the accretion of alumina inclusions is increased. The reduction ratio (A)/(B) ranges most preferably from 0.55 to 0.7. In Fig. 4, the thickness of alumina accreted to the inwall of an immersion pipe of the present invention is compared with that of a prior art. The thickness of alumina accreted to the inwall according to the present invention reduced to one third of the thickness of alumina accreted to the inwall according to the prior art method. In this example, the exit ports of a circular shape and the pool-shaped bottom portion of the immersion pipe were used but the shapes of the exit ports and of the bottom portion are not necessarily limited to those mentioned above. A square-shaped or an oval exit ports and a convex bottom portion can be also used. According to the immersion pipe of the present invention, in the continuous casting, the stagnation of the molten steel inside the immersion pipe, more particularly, at the portion of the exit ports and in the vicinity thereof is removed and the thickness of alumina accreted to the inwall of the immersion pipe can be reduced. As a result of the reduction of the thickness of alumina accreted to the inwall of the immersion pipe, the quality of slabs and final products can be improved.

Claims: 1. An immersion pipe for continuous casting of steel comprising: an immersion pipe body (11) introducing molten steel supplied from a tundish into a continuous casting mold; said immersion pipe body having two exit ports (12a,12b) located symmetrically about the vertical center axis of said immersion pipe body at a lower portion thereof, said immersion pipe body being immersed in the molten steel of the mold and the two exit ports introducing the molten steel into the mold, characterized by a bore (13) of said immersion pipe body having two sectional areas, one of the two sectional areas at and below the two exit ports being smaller than the other above the two exit ports, through the bore the molten steel passing on; and an inner diameter (16) of the bore at the level of the two exit ports having a length almost equal to a horizontal inner diameter (17) of the two exit ports. 2. The immersion pipe according to claim 1, characterized in that said sectional area at and below the two exit ports represented by (A) and said sectional area above the two exit ports represented by (B) include having a reduction ratio represented by (A)/(B) of 0.50 to 0.80. 3. The immersion pipe according to claim 2, characterized in that the reduction ratio of (A)/(B) includes a range of 0.55 to 0.70. 4. The immersion pipe according to claim 1,2 or 3, characterized in that said inner diameter of the bore at the level of the two exit ports includes being equal to a horizontal diameter of the two exit ports. 5. The immersion pipe according to any one of claims 1 to 4, characterized in that said two exit ports have a circular shape. 6. The immersion pipe according to any one of claims 1 to 5, characterized in that said exit ports have a center axis with an angle sloping upwards relative to a horizontal line. 7. The immersion pipe according to any one of claims 1 to 6, characterized in that said immersion pipe body includes having a bottom with a pool shape.

Inventors: KITAGAWA, TOORU C/O PATENT & LICENSE AND, MASAOKA, TOSHIO C/O PATENT & LICENSE AND, MORI, TAKASHI C/O PATENT & LICENSE AND, OKIMOTO, KAZUTAKA C/O PATENT & LICENSE AND, SUZUKI, MIKIO C/O PATENT & LICENSE AND, TESHIMA, TOSHIO C/O PATENT & LICENSE AND, Kitagawa, Tooru c/o Patent &amp; License and, Masaoka, Toshio c/o Patent &amp; License and, Mori, Takashi c/o Patent &amp; License and, Okimoto, Kazutaka c/o Patent &amp; License and, Suzuki, Mikio c/o Patent &amp; License and, Teshima, Toshio c/o Patent &amp; License and
Applicants: NIPPON KOKAN KABUSHIKI KAISHA
Application year: 1988
Main classification: B22D11
IPC codes: B22D11, B22D41