JP2000263198A - Method for continuously casting molten steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the internal defect caused by enclosure of mold powder by setting the spouting quantity of molten steel so as to become the equal flowing speed at near both short side walls in a mold and pouring the molten steel into the mold from each spouting hole. SOLUTION: The immersion nozzle having the spouting holes 5 faced to both short side walls 6 in the mold, is set so as to shift this center axis in the direction along the side walls 5 from the center of the mold, and the molten steel is poured into the mold from each spouting hole 5. At this time, the spouting quantity from the immersion nozzle is decided so that the flowing speeds of the molten steel at near the short side walls 6 equal, and the molten steel is poured into the mold. In this way, periodical variation of the flow velocity on the surface of the molten steel can be restrained. As a means for adjusting the spouting quantity, e.g. the areas of the faced spouting holes 5 may be to mutually change in the immersion nozzle. That is, many immersion nozzles having different cross sectional areas of the faced spouting holes 5, are prepared and the flow velocity sensors are arranged at near both short side walls 6 in the mold, and the testing operations are executed to find the suitable immersion nozzle.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for continuously casting molten steel.

[0002]

2. Description of the Related Art In order to mass-produce steel slabs, molten steel is continuously poured into a water-cooled cylindrical mold (often rectangular in cross section), the outer periphery solidifies, and the interior is still molten. A so-called “continuous casting method” in which an unfinished cast slab in a state is gradually extracted from the mold and cooled is widely used. During the continuous casting, the surface of the molten steel is covered with a powdery substance (referred to as mold powder) having a specific composition in order to keep the temperature of the molten steel and lubricate the mold wall.

As shown in FIG. 4, molten steel is injected into a casting mold through a nozzle 3 attached to the bottom of the tundish 1 with the molten steel 2 stored in an intermediate container of the tundish 1. Since the nozzle 3 is immersed in the molten steel 2 in the mold 4, the nozzle 3 is called an immersion nozzle 3, and has a plurality of discharge holes 5 of the target molten steel 2 on its shaft as shown in FIG. 3. ing. At present, the immersion nozzle 3 is surrounded by a short side wall 6 and a long side wall 7 and arranged at the center of a mold 4 having a rectangular cross section, and the molten steel 2 flows toward the short side walls 6 on both sides. In general, the molten steel 2 is discharged from the discharge hole 5 so that As a result, a symmetric molten steel flow is formed in the mold 4 with respect to the immersion nozzle 3.

However, according to a recent study, even if an attempt is made to form such a symmetrical flow of molten steel 2, the flow velocity of the molten steel flow fluctuates periodically with respect to time as shown in FIG. It became clear that the period was not the same for the left and right flows (this state is called drift). This is due to the fact that the molten metal surface vibrates up and down in a long-term cycle under the influence of pulling out the slab 9 from the mold 4 and pouring the molten steel 2. When the flow velocity of the molten steel 2 becomes significantly higher than the average flow velocity, the mold powder 8 is caught in the molten steel 2 at the upper part of the mold, causing surface defects of the produced slab 9, and the molten steel 2 is formed at the lower part of the mold. It was also clarified that the non-metallic inclusion solidified while deeply penetrating into the inside, causing internal defects of the slab 9. Therefore, in the continuous casting of the molten steel 2, it has been desired to minimize the periodic fluctuation of the flow velocity on the surface of the molten steel and the accompanying drift as much as possible. For example, Japanese Unexamined Patent Publication No. Hei 4-322858 discloses a method of detecting a drift on the surface of molten steel in a mold using a flow rate sensor and moving the position of the immersion nozzle 3 from the center of the mold 4 to an arbitrary position so as to cancel the drift. To disclose the technology. Further, Japanese Patent Application Laid-Open No. Hei 4-322859 discloses a technique of subjecting molten steel to electromagnetic stirring so as to cancel the drift detected in the same manner.

However, the technique described in Japanese Patent Application Laid-Open No. 4-322858 requires a device for periodically moving the position of the immersion nozzle 3, and has a problem in mechanical reliability and controllability. Further, in the technology described in Japanese Patent Application Laid-Open No. Hei 4-32859, the stirrer becomes large-scale,
There was a problem of economics.

[0006]

SUMMARY OF THE INVENTION In view of the above circumstances, according to the present invention, a slab having less surface defects due to entrainment of mold powder, less internal defects due to nonmetallic inclusions and gas intrusion than before can be obtained. It is an object of the present invention to provide a method for continuously casting molten steel.

[0007]

Means for Solving the Problems In order to achieve the above object, the inventor studied periodic fluctuations of molten steel in a mold. And the result was embodied in the present invention.

That is, according to the present invention, an immersion nozzle having discharge holes opposed to both short side walls of a mold is arranged with its central axis shifted from the center of the mold in a direction along the long side wall. In the continuous casting method of molten steel in which molten steel is injected into a mold, a discharge amount of the molten steel is set so as to have an equal flow rate near both short side walls, and molten steel is injected into the mold from each discharge hole. A continuous casting method for molten steel.

According to the present invention, the periodic fluctuation of the molten steel surface flow velocity can be suppressed much more than before. As a result, it has become possible to obtain a slab having few surface defects due to entrainment of mold powder, non-metallic inclusions and internal defects due to invasion of gas.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention will be described below on the basis of the circumstances leading to the invention.

First, the inventor has studied diligently on the periodic variation of the surface of molten steel in a mold. And, since the field forming the flow is bilaterally symmetrical with respect to the immersion nozzle, it is convinced that slight fluctuation of the discharge flow from the immersion nozzle is promoted, and this fluctuation is confirmed. To prevent this, we thought that it was only necessary to create a symmetrical flow of molten steel in the mold, and searched for prior art.

As a result, Japanese Patent Application Laid-Open No. 58-224051 has existed as a technique for forming an asymmetrical molten steel flow. That is, one immersion nozzle having a plurality of asymmetric discharge holes is displaced from the center of the mold in a direction along the long side wall, and the molten metal is supplied evenly into the mold. However, this technique is to supply molten steel evenly to the left and right asymmetrical molten steel injection volume in the mold formed by shifting the position of the immersion nozzle, and to reduce the periodic fluctuation of the surface velocity of molten steel. It did not suppress.

[0013] The inventor has continued the experiment.
If the velocities of the discharge streams colliding with both short side walls of the mold are made equal to each other, it is not clear why, but as shown in FIG. 1, it has been found that periodic fluctuations in the surface velocity of the molten steel 2 can be suppressed. That is, the discharge amount of the molten steel 2 from the immersion nozzle 3 is determined so that the molten steel 2 is injected into the mold 4 so that the flow velocity of the molten steel 2 near the both short side walls 6 becomes equal.

The means for adjusting the discharge amount may be any means. For example, the areas of the discharge holes 5 of the immersion nozzle 3 facing each other may be changed.
That is, a number of immersion nozzles 3 having different cross-sectional areas of the opposing discharge holes 5 are prepared in advance, and a flow sensor (not shown) of molten steel is arranged near both short side walls 6 of the mold 4.
By performing the test operation, a suitable immersion nozzle 3 can be found. Instead of this test operation, the determination may be made by an off-line water model experiment or further by a computational method.

[0015]

EXAMPLE An immersion nozzle (two-hole nozzle) in which two discharge holes of molten steel having different cross-sectional areas were provided at positions opposed to the central axis was prepared. And these immersion nozzles,
The length of the short side wall 6 is 220 mm and the length of the long side wall 7 is 1
An 800 mm flat section was placed in a rectangular mold. The arrangement position is a position shifted by 200 mm to the right from the center of the mold. Then, molten steel made of extremely low carbon steel was continuously discharged from the discharge holes at an average flow velocity of 1.0 m / sec, and casting was performed. At this time, near the two short side walls of the mold, flow velocity sensors (immersion rod type, round bar) which measure the force received from the molten steel flow by the load cell The flow rate was constantly measured.

The fluctuation of the surface flow velocity at each immersion nozzle is
The maximum flow rate and the minimum flow rate are shown in Table 2. From Table 2,
N where the flow velocities near both short side walls of the discharge flow became almost equal
o. It is clear that the variation of the surface flow velocity is smallest when casting is performed using the nozzle 3. That is, the variation is remarkably reduced as compared with the case where the conventional nozzle (No. 5) having the same two discharge hole cross-sectional areas is used.

The occurrence of surface defects and internal defects of the cast steel slab was examined. As a result, as shown in Table 2, No. corresponding to the implementation of the continuous casting method according to the present invention.
It became clear that the defect occurrence index was the smallest when three nozzles were used.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

As described above, according to the present invention, the periodic fluctuation of the surface velocity of the molten steel can be suppressed much more than before. As a result, a steel slab having few surface defects due to powder entrainment, internal defects due to nonmetallic inclusions and gas intrusion has been obtained.

[Brief description of the drawings]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view showing a flow velocity fluctuation on a molten steel surface when a continuous casting method of molten steel according to the present invention is carried out, (a) showing a flow velocity fluctuation, and (b) showing a nozzle arrangement state.

FIG. 2 is a diagram showing a flow velocity fluctuation on the surface of molten steel when a conventional continuous casting method of molten steel is performed.
(B) shows the arrangement of the nozzles.

FIG. 3 is a perspective view showing a general immersion nozzle.

FIG. 4 is a diagram illustrating a situation in which continuous casting of general molten steel is performed.

[Explanation of symbols]

 Reference Signs List 1 tundish (intermediate container) 2 molten steel 3 immersion nozzle (nozzle) 4 mold 5 discharge hole 6 short side wall 7 long side wall 8 mold powder 9 steel slab (slab)

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Toshitane Matsukawa 1-chome, Kawasaki-dori, Mizushima, Kurashiki-shi, Okayama Pref. Kawasaki-dori 1-chome (without address) Kawasaki Steel Corporation Mizushima Works F-term (reference) 4E004 FB02 FB10 MB20 NB01 NC01

Claims (1)

[Claims] An immersion nozzle having discharge holes opposed to both short side walls of a mold is arranged so that a central axis thereof is shifted from a center of the mold along a long side wall, and molten steel is poured into the mold from each discharge hole. In the continuous casting method of molten steel, the discharge flow of the molten steel is set to have the same flow rate near both short side walls, and molten steel is injected from each discharge hole into the mold. Continuous casting method of molten steel.