GB2110971A - Immersion nozzle for continuous casting - Google Patents

Abstract

In order to prevent blockage of the nozzle (1) during casting, the inner diameter of the lower nozzle portion (3) is greater than the inner diameter of the upper nozzle portion (2), a step at the border between the upper and lower nozzle portions (2, 3) being at least 3 mm in radial height (t) and having a step face (4) whose angle ( theta ) with respect to a line perpendicular to the central axis of the lower end of the upper nozzle portion (2) is in the range from +60 to -60 degrees. The part of the nozzle bore below the step face (4) is constituted by zirconia-graphite and the remainder by alumina-graphite. <IMAGE>

Description

SPECIFICATION Immersion nozzle for continuous casting This invention relates to an immersion nozzle for continuous casting used for the purpose of preventing the oxidation of molten steel for the control of the molten steel flow in the mould and for the prevention of splash and inclusion of slag when pouring steel from the tundish to the mould.

Immersion nozzles for continuous casting used in the prior art have generally come to be nozzles of alumina-graphite or fused quartz for obtaining good quality in cast slabs as well as for smoothness in the process of continuous casing of molten steel.

However, when using such an immersion nozzle for the continuous casting of low-carbon aluminiumkilled steel with an aluminium content of over 0.02 wt.%, oxidized inclusions build up on the inner walls of the immersion nozzle and frequent blockages develop and this has become an obstacle to large continuous casting operations.

As far as preventing the blockage of the nozzle is concerned we have advanced to the study of materials and gas blowing. But there are many cases where the pinholes etc. caused by gas blowing exert a negative influence on the quality of the steel. As for the former, it may be thought of as generally effective to use zirconia-graphite, but from the aspect of spall resistance, its disposition is limited to below the meniscus line, the nozzle 1 being as shown in Figure 1. However, in this case there is also a weak point in that it is easy to have unusual abrasive melting losses due to the flow of molten steel at the inner tube boundary 5 of a zirconiagraphite lower nozzle portion 3 and an aluminagraphite upper nozzle portion 2, because of the formation of a low melting point compound of zirconia-alumina as a result of molten steel decarburation.

Upon examining an immersion nozzle for continuous casting after use, one would find that fine, minute particles of a-alumina, calcium-aluminate type materials, and congealed matrix largely make up the blocking materials of the above type.

The present inventors, as a result of researching the blocking structures, have ascertained the process of the blocking action, as follows.

Namely, when the molten steel that is flowing down the tube of the immersion nozzle for continuous casting is suddenly cooled, it reduces the solubility of the oxygen in the steel and thus the oxygen that is present in a state of equilibrium at high temperatures is released, and this released oxygen reacts with the aluminium or calcium in the steel and creates calcium aluminate crystals or alumina. They have discovered that these fine crystals congeal together and build up on the inner wall of the nozzle, finally leading to the blockage of the nozzle. They also discovered that the blockage forms espcially easily from the meniscus line down and that a closure in this area is the cause of operational problems.

As a result of various studies the inventors have desired an immersion nozzle for continuous casting that has the capability of preventing the aforementioned blockage during casting by combining zirconia-graphite, which has the special property of being resistant to the deposition of materials, in the structural body with a step made in the inner face of the nozzle in the vicinity of expected blockage.

The present invention provides an immersion nozzle for continuous casting having the inner diameter of the lower part of the nozzle greater than the inner diameter of the upper part with the step being over 3 mm at the boundary, the inclination angle of the step face at the lower end of the upper nozzle being between + or - 60 degrees from a perpendicular to the central axis of the nozzle.

Preferably the area below the step face is made of zirconia-graphite and the other sections are made of alumina-graphite.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which Figure lisa sectional view of a prior art immersion nozzle for continuous casting, referred to above; Figure 2 and Figure 3 are respective sectional views of two embodiments of an immersion nozzle for continuous casting according to the present invention; and Figure 4 is an eniarged detail of an immersion nozzle according to the invention.

An immersion nozzle for continuous casting according to the present invention is shown in Figures 2 and 4, the inner diameter of the lower nozzle portion 3 being greater than the inner diameter of the upper nozzle portion 2, the bore of the nozzle having a step face 4 at the interface zone between the upper and lower portions 2,3, which are of alumina-graphite and zirconia-graphite respectively.

The step face 4 in the present invention is not limited to the configuration as shown in Figure 2, but it is acceptable if the step face 4 has an inclination angle 6 (Figure 4) of from +60 to -60 degrees with respect to a perpendicular to the centre line of the lower end of the upper nozzle portion 2. For example, the step face 4 may have the configuration shown in Figure 3 (in which 0 is negative).

If the angle of inclination 6 of the step face 4 were more than +60 degrees there would be a strong tendency for the molten steel to flow laminarly between the upper and lower nozzle portions and the product of the reaction between zirconia and alumina would cause damage to the inner wall in the vicinity of the step face. If the angle 0 of the step face were less than -60 degrees, it would be easy for the front part of the step face to erode or even break off during use. The radial height tofthe step between the upper and lower nozzle portions is 3 mm or over.

If the step height were less than 3 mm the results would not be satisfactory.

It is possible to dispose the step face at any position in orderto effectively prevent the blockage, which is especially notable below the meniscus line, that causes difficulties in production.

Thus, what we are offering is an immersion nozzle for continuous casting comprising a nozzle that has greatly improved anti-corrosion and anti-blocking properties by providing a step at the border between the zirconia-graphite and alumina-graphite combination in the inner nozzle face that mitigates the force of the molten steel flow in order to prevent the abrasive melting loss that occurs from a reaction of an aggregate at the said border.

Claims (3)

1. An immersion nozzle for continuous casting having an upper nozzle portion and a lower nozzle portion, in which the inner diameter of the lower nozzle portion is greater than the inner diameter of the upper nozzle portion, a step at the border between the upper and lower nozzle portions being at least 3 mm in radial height and having a step face whose angle with respect to a line perpendicular to the central axis of the lower end of the upper nozzle portion is in the range from +60 to -60 degrees.

2. An immersion nozzle as claimed in claim 1, in which the part of the nozzle bore below the step face is constituted by zirconia-graphite and the remainder is constituted by alumina-graphite.

3. An immersion nozzle for continuous casting, substantially as described with reference to, and as shown in, Figure 2 or Figure 3 of the accompanying drawings.