JP5440933B2 - Immersion nozzle and continuous casting method using the same - Google Patents

Description

  The present invention relates to an immersion nozzle for continuous casting of steel and a continuous casting method using the same.

  Continuous casting of steel is performed while pouring molten steel supplied from a tundish into a continuous casting mold using an immersion nozzle. A discharge hole is provided in the lower part of the immersion nozzle, and the molten steel is discharged from the discharge hole into the mold.

  In order to prevent nozzle clogging, an inert gas such as argon gas is blown into the immersion nozzle, and the bubbles are diffused in the mold on the discharge flow. Further, non-metallic inclusions such as alumina, which is a deoxidation product, also ride on the discharge flow and diffuse into the mold, and when these bubbles and non-metallic inclusions adhere to the solidification interface, surface defects are generated.

  For example, as shown in FIG. 1, when the immersion nozzle 2 is arranged at the center of the mold 1 and the molten steel is discharged from the discharge holes 3 formed on both sides of the lower part, as shown by the thick arrows in the drawing, A strong discharge flow is generated in the longitudinal direction of the mold 1. For this reason, bubbles and non-metallic inclusions are carried to the end portion of the mold 1 on the discharge flow and easily reach the solidification interface. As a result, as shown in the graph of FIG. 2, the surface defect occurrence rate at the edge portion of the slab increases. The surface defect occurrence index used in this specification is a value obtained by indexing the frequency of occurrence of defects (mainly sliver defects) due to continuous casting appearing on the surface of a coil produced by rolling continuously cast slabs. is there.

  As shown in FIG. 3, it is also widely known that a static magnetic field is applied to the molten steel in the mold 1 by an electromagnetic brake 4 to attenuate the downward discharge flow. In this case, bubbles and non-metallic inclusions float on the molten metal surface 5 more rapidly than in the case of FIG. 1 and are less likely to adhere to the solidification interface, so that the occurrence of surface defects in the entire slab can be suppressed.

  However, as shown in FIGS. 3 and 4, when the electromagnetic brake 4 is used, a counter flow called an MHD counter flow is generated around the immersion nozzle 2 by the electromagnetic fluid force. This MHD counterflow is generated on both sides in the longitudinal direction of the mold 1, flows toward the discharge holes 3, collides with the side surface of the immersion nozzle 2, and becomes a strong upward flow along the inner wall surface of the mold 1. For this reason, bubbles and non-metallic inclusions easily adhere to the solidified surface of the central portion, and the surface defect occurrence rate at the center portion of the slab increases as shown in the graph of FIG.

  As described above, when an electromagnetic brake is used, surface defects at the edge portion can be suppressed, but it is not easy to prevent surface defects due to bubbles and non-metallic inclusions at the center portion. Therefore, in Patent Document 1, a surface defect is generated by making the discharge hole of the immersion nozzle face upward and providing a box whose lower surface is opened so as to surround the discharge hole, and raising the molten steel flow and then making it a downward flow. An invention to prevent is disclosed. However, according to the present invention, the structure of the nozzle portion is complicated and maintenance is not easy.

  Patent Document 2 discloses an invention in which a swirling flow is formed by providing a baffle plate above the discharge hole of the submerged nozzle to prevent the molten steel flow from staying in the center portion. However, this invention is based on the premise that no electromagnetic brake is used, and the effect of reducing the surface defect occurrence rate at the edge portion of the slab is considered to be small.

  Further, Patent Document 3 discloses an invention in which a baffle plate is provided above the discharge hole of the submerged nozzle and a strong swirl flow is generated with the discharge hole being inclined with respect to the baffle plate. However, in the present invention, it is considered that bubbles and non-metallic inclusions easily reach the edge portion of the mold by riding on the swirling flow, and the effect of reducing the surface defect occurrence rate at the edge portion of the slab is considered to be small.

JP 2003-266155 A JP 63-2335050 Japanese Patent Laid-Open No. 5-146851

  Therefore, the object of the present invention is to solve the above-mentioned conventional problems, and to effectively reduce the surface defect occurrence rate of the center part of the slab due to the MHD counterflow generated when using the electromagnetic brake, and the structure is simple. And an immersion nozzle that is easy to maintain and a continuous casting method using the same.

The immersion nozzle of the present invention, which has been made to solve the above-mentioned problems, prevents dispersion extending in the longitudinal direction of the continuous casting mold on both sides of the nozzle body for pouring molten steel into the continuous casting mold while applying an electromagnetic brake. A plate is mounted across the discharge hole, the upper end height of the dispersion prevention plate is between the upper end of the discharge hole and the hot water surface, and the lower end height of the dispersion prevention plate is between the lower end of the discharge hole and the lower end of the nozzle body. It is characterized by being between .

  In addition, it is preferable that said immersion nozzle shall make the protrusion amount of the horizontal direction from the nozzle main body of a dispersion prevention board into 10 mm or more and 300 mm or less. Moreover, it is preferable that the discharge angle of the discharge hole is between 20 ° upward and 60 ° downward.

  Furthermore, the continuous casting method of the present invention uses the above-mentioned immersion nozzle, pours molten steel into the continuous casting mold while applying an electromagnetic brake, and causes the opposing flow toward the nozzle body to be melted by a pair of right and left dispersion prevention plates. It is characterized by being directed toward. Note that the static magnetic field applied by the electromagnetic brake is preferably 500 G or more at the discharge hole position.

  According to the present invention, the MHD counterflow generated when using the electromagnetic brake and directed toward the immersion nozzle is guided by the dispersion preventing plate extending in the longitudinal direction of the continuous casting mold formed on both sides of the nozzle body and directed toward the molten metal surface. Make it rise. Since this upward flow mainly flows between the pair of dispersion preventing plates, bubbles and non-metallic inclusions are prevented from adhering to the solidification interface and are captured by the powder on the molten metal surface. For this reason, the surface defect occurrence rate can be greatly reduced. Moreover, since the immersion nozzle of the present invention has a relatively simple structure, the manufacturing cost and the maintenance cost are low.

It is the top view and front view which show a prior art. It is a graph of the surface defect generation index in the case of FIG. It is the top view and front view which show other prior art (electromagnetic brake adoption). It is explanatory drawing of the reversal flow in the case of FIG. It is a graph of the surface defect generation index in the case of FIG. It is the top view and front view which show embodiment of this invention. It is explanatory drawing of the inversion flow in embodiment of this invention. It is dimension explanatory drawing of a dispersion | distribution prevention board. It is a graph of the surface defect generation index in the embodiment of the present invention.

Embodiments of the present invention will be described below.
6 to 8 are explanatory views of the immersion nozzle according to the present invention. As in the prior art, 1 is a mold for continuous casting, 2 is an immersion nozzle, 3 is a discharge hole formed on both sides of the lower part of the immersion nozzle 2, 4 is an electromagnetic brake for applying a static magnetic field to the molten steel in the mold 1, and 5 is a molten metal surface.

  In the present invention, the dispersion preventing plates 6 are attached to both sides of the nozzle body of the immersion nozzle 2. The dispersion prevention plate 6 is a refractory flat plate having the same quality as the nozzle body, and is disposed on both sides of the discharge hole 3. The direction is the longitudinal direction of the mold 1. In this embodiment, two flat plates are arranged in parallel, but they may be arranged obliquely so that the outer side far from the immersion nozzle 2 opens slightly. The attachment method to the nozzle body is arbitrary, and for example, joining with a mortar or fixing with a ceramic fixture can be appropriately employed.

  The dispersion prevention plate 6 serves to guide the MHD counterflow toward the discharge hole 3 and to float toward the molten metal surface 5, so that the vertical width W is at least a width that can cover the discharge hole 3. To do. Therefore, the upper end height of the dispersion preventing plate 6 is set between the upper end of the discharge hole 3 and the molten metal surface 5, and the lower end height of the dispersion preventing plate 6 is set between the lower end of the discharge hole 3 and the lower end of the nozzle body. It is preferable. If the upper end height of the dispersion preventing plate 6 exceeds the molten metal surface 5, the refractory forming the dispersion preventing plate 6 is likely to deteriorate due to the reaction with the molten metal powder, and above the molten metal surface 5. This is not preferable because a temperature difference occurs between the exposed portion protruding to the inside and the portion immersed in the molten metal. If the lower end height of the dispersion preventing plate 6 is lower than the lower end of the nozzle body, it is not preferable because it is difficult to manufacture and handle the immersion nozzle 2.

  The horizontal protrusion amount L of the dispersion preventing plate 6 from the nozzle body is preferably 10 mm or more and 300 mm or less. If the protruding amount L is less than 10 mm, the function of guiding the MHD counterflow is insufficient. Further, even if the protruding amount L exceeds 300 mm, the rectification effect is not improved, and there is a risk of causing an increase in manufacturing cost or trouble due to breakage. The thickness t of the dispersion preventing plate 6 is not particularly limited, but may be set so as to have a service life equivalent to that of the nozzle body in the molten steel flow, for example, 10 mm or more.

  Note that the discharge angle α of the discharge hole 3 of the immersion nozzle 2 is preferably between 20 ° upward and 60 ° downward. If the discharge angle α exceeds 20 ° upward, boiling of the hot water surface 5 occurs, which is not preferable. On the other hand, when the angle exceeds 60 ° downward, the depth of penetration of the discharge flow becomes too deep and it is difficult to float, and bubbles and non-metallic inclusions tend to adhere to the solidification interface, which is not preferable. Especially conducive to internal defects.

  When the immersion nozzle 2 with the dispersion prevention plate 6 configured as described above is used and the molten steel is continuously cast using the electromagnetic brake 4, the MHD counterflow toward the discharge hole 3 is applied to the two dispersion prevention plates 6. Ascending toward the hot water surface 5 through the sandwiched central flow path, it becomes difficult to make contact with the solidification interface of the center portion. For this reason, as shown in the graph of FIG. 9, the surface defect occurrence index can be significantly reduced.

  As described above, the present invention is premised on the use of the electromagnetic brake 4, and the reduction of the surface defect occurrence index at the edge portion is largely due to the electromagnetic brake 4. For this reason, it is preferable that the static magnetic field applied with an electromagnetic brake shall be 500 G or more.

Table 1 shows examples according to the present invention together with comparative examples.
Molten steel was poured from the immersion nozzle 2 into a mold 1 having a short side of 250 mm and a long side of 1600 mm, and continuously cast at a casting speed of 2.5 m / min. The dispersion prevention plate was cast by being attached to an immersion nozzle having each discharge angle while setting the upper end position, the lower end position, and the protrusion amount of the dispersion prevention plate to the dimensions shown in Table 1. The slab after casting was rolled, and defects due to continuous casting that appeared on the coil surface were inspected to confirm the effect of the present invention.

  According to the results of Examples 1 to 13, the surface defect index was 0.5 or less, and a very good surface quality could be achieved. The MHD counterflow toward the discharge hole 3 of the submerged nozzle 2 rises toward the molten metal surface 5 through the central flow path sandwiched between the two dispersion prevention plates 6 and is less likely to contact the solidification interface at the center. It is thought.

In Reference Examples 14 and 15, although the vertical length of the dispersion preventing plate 6 is short and the effect of preventing the dispersion of bubbles and inclusions is relatively low, the surface defect index is 1.0 or less, and there is no dispersion preventing plate 6 Contrast with the example. In Example 16, the protrusion amount of the dispersion preventing plate 6 was as short as 2 mm, but the surface defect index was 1.0 or less, which was superior to the comparative example without the dispersion preventing plate 6. In Example 17, the protrusion amount of the dispersion preventing plate 6 was as long as 400 mm, but the surface defect index was 1.0 or less, which was superior to the comparative example in which the dispersion preventing plate 6 was not provided. In Example 18, the angle of the discharge hole was upward 30 °, and the molten metal surface fluctuation was relatively large, but the surface defect index was 1.0 or less, which was superior to the comparative example without the dispersion preventing plate 6. It was.

  Example 19 is a comparative example in which the angle of the discharge hole is 70 ° downward and the discharge flow becomes strong downward, and there is concern about internal quality, but the surface defect index is 1.0 or less and there is no dispersion prevention plate 6 Contrast with it. Example 20 is a case where the immersion nozzle according to the claims of the present application and an electromagnetic stirrer are used in combination, and the best surface quality among the examples by the synergy effect with the shell washing effect near the molten steel meniscus by the electromagnetic stirrer. It became.

Comparative example

  Casting was carried out under the same casting conditions as in the Examples, using an immersion nozzle having no dispersion prevention plate. In Comparative Examples 21 and 22 in which the static magnetic field generated by the electromagnetic brake is weak, the strong discharge flow generated in the longitudinal direction of the mold 1 causes air bubbles and non-metallic inclusions carried on the discharge flow to be carried to the end of the mold 1. Thus, the surface quality at the edge portion of the slab was lowered, and the surface defect index was larger than 1. In Comparative Examples 23 to 25 in which the static magnetic field generated by the electromagnetic brake was increased, the surface quality at the width center was lowered due to the MHD counter flow, and the surface defect index was larger than 1.

DESCRIPTION OF SYMBOLS 1 Mold 2 Immersion nozzle 3 Discharge hole 4 Electromagnetic brake 5 Molten surface 6 Dispersion prevention board

Claims (5)

A dispersion preventing plate extending in the longitudinal direction of the continuous casting mold is attached to both sides of the nozzle body for pouring molten steel into the continuous casting mold while applying an electromagnetic brake, with the discharge hole interposed therebetween,
An immersion nozzle characterized in that the upper end height of the dispersion prevention plate is between the upper end of the discharge hole and the molten metal surface, and the lower end height of the dispersion prevention plate is between the lower end of the discharge hole and the lower end of the nozzle body. . Immersion nozzle according to claim 1, wherein the amount of protrusion of the horizontal direction from the nozzle body of distributed prevention plate, characterized in that a 10mm or 300mm or less. Immersion nozzle according to claim 1, wherein the discharge angle of ejection Deana, characterized in that the between the upward 20 ° and down 60 °.   Using the immersion nozzle according to any one of claims 1 to 3, molten steel is poured into a mold for continuous casting while applying an electromagnetic brake, and a counter flow toward the nozzle body is made to a molten metal surface by a pair of right and left dispersion prevention plates. A continuous casting method characterized by being directed toward.   The continuous casting method according to claim 4, wherein a static magnetic field applied by an electromagnetic brake is set to 500 G or more at a discharge hole position.

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