Casting nozzle and method of casting using same

Abstract

 To overcome the problem of carbon pick-up in the casting of ultra-low carbon steel, a casting nozzle is provided which is formed of a material comprising 70 to 98% by weight of at least one refractory material other than carbon or graphite, and 2 to 30 weight % of boron nitride.

Description

[0001] The present invention relates to a nozzle for casting ultra low carbon steel. Amongst the various types of new steels which are of interest, an ultra low carbon steel has been proposed which contains less than 15 ppm carbon. As it is currently desirable to cast both killed-steel and oxygen-containing steel by the continuous casting process, a need has arisen for a casting nozzle suitable for casting ultra low carbon steels by the continuous casting process.

[0002] As a consequence of the stringent quality requirements of modern steels, it has become necessary to use nozzles made of refractory materials which can ensure the continuous quality requirements described.

[0003] We have found that if alumina-graphite nozzles, which are used for casting conventional, carbon-containing steels, are used for casting ultra-low carbon steels. carbon pickup becomes apparent either as the nozzle is worn away by the molten steel or through decarburization of the nozzle itself. Thus, we have found that conventional alumina-graphite nozzles are not suitable for use with ultra low carbon steels. Fused silica nozzles do not contain carbon. However, these nozzles are susceptible to wear, especially by high manganese steel.

[0004] It is one object of the present invention to obviate or mitigate the problems of carbon pickup and excessive wear and provide a nozzle suitable for the continuous casting of ultra low carbon steels.

[0005] In accordance with the present invention, there is provided a casting nozzle formed of a material comprising 70 to 98 weight % of a refractory material other than carbon or graphite, and 2 to 30 weight % of boron nitride.

[0006] Preferably, the refractory material is selected from sintered or electrically fused magnesium, non-stabilized or semi-stabilized zirconia, alpha-alumina, beta-alumina, mullite, sillimanite, clay materials, fused silica, Si₃N₄, B₄C and SiC.

[0007] It is also preferable to add Si, Al or the like to the above composition to promote sintering and prevent oxidation of the nozzle material.

[0008] Conveniently, an inorganic bonding agent is mixed with the composition, e.g in an amount of from 5 to 20 weight % of the composition. For example, there may be used as an inorganic bonding agent, clay (e.g. bentonite), waterglass, aluminium phosphate etc.

[0009] Typically the nozzle of the present invention is made by moulding the present composition followed by sintering in a reducing atmosphere.

[0010] If more than 30% boron nitride is used, then the nozzle is easily oxidized to B₂0₃ and wear of the nozzle is increased. If less than 2% boron nitride is used, the desirable characteristics of boron nitride, i.e. chemical inertness and high thermal conductivity are not shown and thermal shock spalling is increased.

[0011] The present invention will be illustrated by the following examples (all percentages are by weight in the composition excluding the-inorganic bonding agent unless otherwise specified).

Composition 1

[0012] 

Composition 2

[0013] 

Composition 3

[0014] 

Composition 4

[0015] 

Composition 5

[0016] 

Comparative Example

[0017] 

[0018] The respective compositions and bonding agents mentioned above are added with an appropriate amount of water and kneaded in a kneader for 40 minutes. The mixtures are dried in a drying furnace at 80°C for 12 hours so that the residual water content goes down to 1% to 3%, and thereafter crushed by a crusher to particles smaller than lmm.

[0019] Rubber molds are filled with the thus prepared materials and molded under a molding pressure of 1,000 kg/cm² in order to form immersion nozzles of predetermined shapes. The molding products are then sintered in a tunnel furnace whose ceiling temperature is 1250°C, and manufactured into predetermined shapes.

[0020] The chemical ingredients of the final products according to the above Compositions are shown in the following table:-

[0021] Conventional alumina-graphite nozzle composition (A)

[0022] Conventional alumina-graphite nozzle composition (B)

[0023] An immersion nozzle according to Composition 1 was compared with the conventional nozzle of Composition A When using each nozzle four times with a 250 short ton (227 tonne) ladle for 240 minutes, the wear of the present nozzle was 0.021 mm/minute while the wear of the conventional nozzle A was 0.022 mm/minute. As for the pickup of carbon, the present nozzle showed no carbon pick up while the conventional nozzle showed a carbon pick up of 11 ppm by wt.

[0024] An immersion nozzle according to Composition 2 was compared with the conventinal nozzle of composition B. When using each nozzle five times with a 300 short ton (272 tonne) ladle for 250 minutes, the wear of the present nozzle was 0.015 mm/minute while the wear of the conventional nozzle be was 0.020 mm/minute. As for the pickup of carbon, the present nozzle had a carbon pick up of only 1 ppm by wt while the conventional nozzle B had a carbon pickup of 10 ppm by wt.

[0025] A long nozzle according to the Composition 3 was used between a ladle and a tundish six times with a 280 short ton (254 tonne) ladle for 300 minutes. The wear was 0.023 mm/minute and there was no carbon pickup.

[0026] An immersion nozzle according to Composition 4 was compared with the conventional nozzle of compositon A. When using each nozzle four times with a 250 short ton (227 tonne) ladle for 240 minutes, the wear of the nozzle of Composition 4 was 0.023 mm/minute and the wear of the conventional nozzle of Composition A was 0.022 mm.minute. As for the pickup of carbon, the nozzle of Composition 4 showed no carbon pickup while the conventional nozzle of Composition A had a carbon pickup of 11 ppm by wt.

[0027] An immersion nozzle according to Composition 5 was compared with the Comparative Example. When using each nozzle five times with a 300 short ton (272 tonne) ladle, the wear of the nozzle of the Comparative Example was so substantial that the nozzle broke off for the third time while the nozzle of Composition 5 was capable of casting five times for 250 minutes. The wear thereof was 0.020 mm/minute and the carbon pickup was 2 ppm by wt.

Claims

1. A casting nozzle formed of a material comprising from 70 to 98 weight % of at least one refractory material other than carbon or graphite, and 2 to 30 weight % of boron nitride.

2. A casting nozzle as claimed in claim 1, wherein said at least one refractory material is selected from sintered or electrically fused magnesium, non-stabilized or semi-stabilized zirconia, alpha-alumina, beta-alumina, mullite, sillimanite, clay material, fused silica, Si₃N₄, _B4 C, and SiC.

3. A casting nozzle as claimed in claim 1, wherein at least one additive is included in the material to promote sintering and prevent oxidation of the nozzle.

4. A casting nozzle as claimed in any preceding claim, wherein the material contains an inorganic bonding agent.

5. A casting nozzle as claimed in claim 4 wherein said inorganic bonding material is present in an amount from 5 to 20%.

6. A method of casting an ultra-low carbon steel using a casting nozzle formed from a material comprising from 70 to 98 weight % of a conventional refractory material other than carbon or graphite, and 2 to 30 weight % of boron nitride; and optionally including at least one additive to promote sintering and reduce oxidation and an inorganic bonding agent.