JP2008030089A - Immersion nozzle for continuous casting of molten steel and continuous casting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a submerged nozzle having a shape capable of stably casting a slab with few surface microcracks due to remelting of a solidified shell and having few defects of non-metallic inclusions caused by mold powder in continuous casting and its casting method I will provide a.
SOLUTION: The discharge port 2 has four holes of 5 to 15 degrees upward from the horizontal, the nozzle cross-sectional diameter of the meniscus 4 on the surface of the molten steel when immersed in the molten steel in the mold 3 is Dm, and the nozzle at the discharge port When the cross-sectional diameter is Dt, Dm> Dt is satisfied, and when the length of the rectangular mold of the continuous casting machine is H and the horizontal is W, Dt of the nozzle cross-sectional diameter at the discharge port 2 is Dt ≦ 0.4. A submerged nozzle 1 for continuous casting of molten steel satisfying × (H · W) ^1/2 .
[Selection] Figure 1

Description

  The present invention relates to an immersion nozzle for continuous casting that can prevent microcracking and powder defects in continuous casting of a steel bloom having a rectangular cross-sectional shape, and a continuous casting method using the immersion nozzle.

  In continuous casting of molten steel, there is a demand for stability of the solidification process in the mold and reduction of non-metallic inclusions in the slab and surface defects of the slab that cause product defects. In continuous casting of molten steel, a refractory immersion nozzle is used as a means for injecting molten steel into a mold. There are various types of submerged nozzles, but when casting a bloom or the like, a submerged nozzle having four obliquely upward discharge ports is often used.

  When continuous casting is performed for a long time, the refractory of the immersion nozzle is melted by the mold powder on the surface of the molten steel in the mold, so use a large diameter immersion nozzle with a thickened refractory wall of the immersion nozzle. Sometimes.

  When a thick-diameter immersion nozzle with such a large thickness is used, the straight flow of the discharge flow from the discharge port becomes stronger. As a result, the molten steel flows toward the upper meniscus in the mold. As a result, the thickness of the solidified shell of the slab bloom was reduced, micro-cracks were generated on the bloom surface, and mold powder flowed into the micro-cracks, thereby generating powder defects and the like in the product.

  Therefore, it is important to stabilize the flow of molten steel inside the mold. For this purpose, various methods for stabilizing the flow of the meniscus by defining the shape of the immersion nozzle have been proposed.

  As a conventional method, a hot water supply port 1 for supplying a molten metal from a tundish, a cylindrical side wall part 2, a bottom wall part 3 covering the lower surface opening of the side wall part, and the side wall part 2 are formed, and the molten metal is put in a mold. By providing a first hot water outlet to be discharged and a second hot water outlet that is formed in the bottom wall portion 3 and adjusts the discharge flow rate from the first hot water outlet, the amount of discharge from the first hot water outlet can be adjusted and solidified. There has been proposed a device that can effectively prevent problems caused by remelting of the shell (see, for example, Patent Document 1).

  However, in this method, when long continuous casting is performed, the refractory in the vicinity of the second pouring gate is melted down, and inclusions are caused by a decrease in the amount of molten steel supplied to the first pouring tap and excessive supply of molten steel from the second pouring tap. May cause entrainment.

  In the immersion nozzle in which one discharge port is arranged at every 90 degrees in the circumferential direction on the side surface of the nozzle tip side and a total of four discharge ports are arranged at equal intervals, The diameters of the discharge ports every other 180 degrees are made the same size, and the opening area of each of the two discharge ports, which are the small diameters of these four discharge ports, has a large diameter. An immersion nozzle for continuous casting in which the opening area of each two discharge ports is 50% to 95% is proposed (see, for example, Patent Document 2).

  Even in this method, in order to perform long continuous casting, it is necessary to use a large-diameter immersion nozzle in which the thickness of the refractory material of the immersion nozzle is increased. When powder flows in, there is a possibility that powder defects and the like are generated in the product.

Japanese Utility Model Publication No. 06-41950 JP 2000-116580 A

  In continuous casting, an immersion nozzle having a shape capable of stably casting a slab with less surface microcracking due to remelting of a solidified shell with less defects of non-metallic inclusions caused by mold powder and a casting method therefor are provided. That is.

The means for solving the above-mentioned problem is that the nozzle diameter of the portion that contacts the meniscus, which is the most eroded portion of the submerged nozzle, is thickened as much as necessary in order to ensure the thickness of the refractory and immersed in the molten steel. The nozzle portion is reduced in diameter so that the discharge flow does not flow upward. That is, in the invention of claim 1, when the nozzle cross-sectional diameter at the meniscus of the molten steel surface when immersed in the molten steel in the mold is Dm, and the nozzle cross-sectional diameter at the discharge port is Dt, Dm> Dt is satisfied, When the length of the rectangular mold of the continuous casting machine is H and the width is W, Dt of the nozzle cross-sectional diameter at the discharge port is Dt ≦ 0.4 × (H · W) ^1/2 (hereinafter, “0.4 √ (H · W) ”)) is satisfied, and it is an immersion nozzle for continuous casting of molten steel.

  In the invention of claim 2, the nozzle has a four-hole discharge port of 5 to 15 ° upward from the horizontal, the nozzle cross-sectional diameter of the meniscus on the surface of the molten steel when immersed in the molten steel in the mold is Dm, and the nozzle at the discharge port When the cross-sectional diameter is Dt, Dm> Dt is satisfied, and when the length of the rectangular mold of the continuous casting machine is H and the width is W, Dt of the nozzle cross-sectional diameter at the discharge port is Dt ≦ 0.4√ It is an immersion nozzle for continuous casting of molten steel characterized by satisfying (H · W).

  According to a third aspect of the present invention, when continuous casting of molten steel is performed with a rectangular mold having a length of H and a width of W, the immersion nozzle of the means of claim 1 or 2 is used. It is a casting method.

  The reason for setting the conditions of the above means will be described. Usually, the immersion nozzle has a discharge port disposed upward from the horizontal for the purpose of replacing molten steel near the meniscus in the mold and supplying temperature. Also, when carrying out a continuous casting method for a long time, the immersion nozzle melts at the meniscus, so it is common to increase the thickness of the nozzle refractory so that it can withstand longer continuous casting. It is.

  However, if the cross-sectional area of the immersion nozzle is larger than the cross-sectional area of the mold, the discharge flow from the immersion nozzle tends to flow linearly. And defects due to mold powder are likely to occur.

  In particular, it is likely to occur when Dt> 0.4√ (H · W) is satisfied, where the vertical dimension of the mold cross section is H, the horizontal dimension is W, and the nozzle sectional diameter at the discharge port of the immersion nozzle is Dt. . Accordingly, when the diameter of the immersion nozzle has to be increased due to the long run, the immersion nozzle diameter at the meniscus is increased, and the nozzle diameter immersed in the molten steel is set to Dt ≦ 0.4√ (H · W). This makes it possible to reduce micro cracks and powder defects on the bloom surface.

  Furthermore, the reason why the direction of the discharge port of the hole of the immersion nozzle is limited to 5 to 15 ° upward from the horizontal will be described. The angle at which the direction of the discharge port of the hole of the immersion nozzle is upward from the horizontal is less than 5 °. Therefore, the flow of molten steel toward the meniscus is reduced, and there is little replacement of molten steel, and there is a concern about the formation of deckles, that is, solidified shells. On the other hand, if the angle at which the outlet of the hole of the immersion nozzle is directed upward from the horizontal exceeds 15 °, the flow to the meniscus is too strong and there is a concern about the entrainment of powder or the like. Further, the reason why the number of holes in the immersion nozzle is four will be described. There are two and four immersion nozzles, but two are for slab continuous casting machines and four are for bloom or billet. Since the slab continuous caster is a horizontally long rectangle, the two-hole immersion nozzle is ineffective in controlling flow by reducing the diameter of the immersion portion and suppressing wrinkles. Therefore, a 4-hole immersion nozzle suitable for bloom is used.

  As described above, by applying the immersion nozzle of the means of the present invention to continuous casting of molten steel, steel with few powder defects due to micro cracks on the bloom surface in the slab bloom by continuous casting and inflow of mold powder is cast. Thus, the present invention has excellent effects.

  BEST MODE FOR CARRYING OUT THE INVENTION The best mode for carrying out the present invention will be described through Example 1 below with reference to tables and drawings. First, the immersion nozzle 1 for continuous casting according to the present invention will be described. The immersion nozzle 1 satisfies Dm> Dt when the nozzle cross-sectional diameter of the meniscus 4 on the surface of the molten steel when immersed in the molten steel in the mold 3 is Dm and the nozzle cross-sectional diameter on the discharge port 2 side is Dt. Further, when the vertical length of the rectangular mold 3 is H and the horizontal width is W, the immersion nozzle 1 according to the present invention has the specifications in the inventive examples 1 to 7 of the following example 1, and the discharge of the lower part of the nozzle in these examples The nozzle cross-sectional diameter Dt on the outlet 2 side satisfied Dt ≦ 0.4√ (H · W).

  Furthermore, the immersion nozzle 1 for continuous casting of this invention can have the discharge port 2 of 4 holes of 5-15 degrees upwards from a horizontal in a nozzle lower end part, as shown in FIG. This immersion nozzle 1 satisfies Dm> Dt when the nozzle cross-sectional diameter of the meniscus 4 on the surface of the molten steel when immersed in the molten steel in the mold 3 is Dm, and the nozzle cross-sectional diameter on the discharge port 2 side is Dt. Furthermore, when the length of the rectangular mold 3 is H and the width is W, the immersion nozzle 1 in the present invention has four holes in the discharge port 2 and the direction of these holes is defined. The nozzle cross-sectional diameter Dt on the discharge port 2 side in the lower part of the nozzle satisfies Dt ≦ 0.4√ (H · W). It was something to do.

  Further, when continuously casting molten steel (steel type is bearing steel (JIS standard SUJ2)) by a vertical continuous casting machine, the immersion nozzle 1 having the specifications of Invention Examples 1 to 7 of the following Example 1 is used as a tundish nozzle. In use, molten steel was continuously cast with a rectangular mold 3 of length H and width W to obtain a bloom with few powder defects caused by microcracking or mold powder flow on the surface of the bloom, which is a slab.

  Hereinafter, detailed examples will be described through Comparative Examples 1 to 7 and Invention Examples 1 to 7 in Table 1. When the slab is cast at a casting speed of 0.5 m / min by continuous casting, the mold size is H / W, and the specifications and conditions of the immersion nozzle are the meniscus diameter Dm, the immersion part diameter, and the microcracking. Table 1 shows the index and powder inclusion index. In all the examples, the direction of the outlet of the hole of the immersion nozzle was 10 ° upward from the horizontal, and the number of holes was four.

In Table 1, the microcracking index shows the total length of wrinkles per standard unit length of the bloom after casting. A case where there is no wrinkle is 0, and a relative value is shown with the total wrinkle length of Comparative Example 1 being 100. Furthermore, the powder inclusion index is specifically determined by cutting out a test piece of 40 mm x 40 mm and a length of 70 mm centered at a position of 40 mm from the center of a steel piece obtained by rolling a 490 x 380 mm bloom to φ167 mm and slimming them. Number of non-metallic inclusions of 37 μm or more extracted by dissolution of slime after dissolution and containing Na ₂ O, SiO ₂ , Al ₂ O ₃ and CaO which are mold powder components obtained using EPMA Is shown as an index with Comparative Example 1 being 100. Any index is desirably 80 or less.

  In Table 1, as in Comparative Examples 1, 2, and 5, with a stepped immersion nozzle having a meniscus diameter Dm larger than the immersion part diameter Dt, and the immersion part diameter Dt is not less than 0.4√ (H · W), Further, even in a stepless immersion nozzle in which the meniscus portion diameter Dm is the same as the immersion portion diameter Dt as in Comparative Examples 3, 4, 6, and 7, the immersion portion diameter Dt is 0.4√ (H · W). When the value is greater than or equal to the value, the discharge flow is directed upward, so that the micro crack index on the bloom surface and the powder inclusion index are relatively high.

  That is, in each of Comparative Examples 1 to 7, the immersion part diameter Dt of the nozzle outlet of the molten steel immersion part is larger than the value of 0.4√ (H · W), and the microcracking index on the bloom surface is 85 at a minimum. The powder inclusion index is also a minimum of 80, which is higher than that of the present invention.

  On the other hand, by setting the immersion part diameter Dt of the nozzle outlet of the molten steel immersion part to be equal to or less than 0.4√ (H · W) as in Invention Examples 1 to 7, the discharge flow is maintained within an appropriate range. As a result, the remelting of the solidified shell is reduced. As a result, the microcracking index on the bloom surface is 78 at the maximum and the powder inclusion index is 79 at the maximum, both of which are lower than 80, and the microcracking on the bloom surface is low. It can be seen that steel with few powder defects due to the inflow of mold powder was able to be cast.

It is a schematic diagram of the immersion nozzle of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Immersion nozzle 2 Discharge port 3 Rectangular mold 4 Meniscus Dm Meniscus side nozzle diameter Dt Discharge port side nozzle diameter H Vertical of rectangular mold W Horizontal of rectangular mold

Claims (3)

When the nozzle cross-sectional diameter at the meniscus on the surface of the molten steel when immersed in the molten steel in the mold is Dm and the nozzle cross-sectional diameter at the discharge port is Dt, Dm> Dt is satisfied, and the rectangular mold of the continuous casting machine is When H and W are W, the nozzle cross-sectional diameter Dt at the discharge port is expressed as Dt ≦ 0.4 × (H · W) ^1/2 (hereinafter, “0.4√ (H · W)”. Submerged nozzle for continuous casting of molten steel characterized by satisfying.   When the nozzle cross-section diameter of the meniscus on the surface of the molten steel when immersed in molten steel in the mold is Dm, and the nozzle cross-sectional diameter at the discharge outlet is Dt. , Dm> Dt is satisfied, and when the length of the rectangular mold of the continuous casting machine is H and the width is W, Dt of the nozzle cross-sectional diameter at the discharge port satisfies Dt ≦ 0.4√ (H · W). An immersion nozzle for continuous casting of molten steel.   A continuous casting method for molten steel using the immersion nozzle according to claim 1 or 2 when performing continuous casting with a rectangular mold having a length of H and a width of W.

Images