GB2142262A

GB2142262A - Continuous casting of steels

Info

Publication number: GB2142262A
Application number: GB08317869A
Authority: GB
Inventors: William G Wilson
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-07-01
Filing date: 1983-07-01
Publication date: 1985-01-16
Also published as: GB8317869D0; GB2142262B

Abstract

A reactive metal such as aluminium, titanium, zirconium or a rare earth entrained in an inert gas is injected into the molten steel such as to prevent clogging of the tundish nozzle. A flux such as cryolite may be added where the reactive metal is injected upstream of the nozzle. Alternatively, the reactive metal may be injected through a hollow stopper rod, or below the tundish nozzle e.g. into a pouring tube. Where the reactive metal is added to the melt bath in the ladle or tundish, inert gas injected down the tundish hollow stopper rod carries a flex such as cryolite which acts to dissolve deposits, arising from the reactive metal addition, from the tundish nozzle.

Description

SPECIFICATION Continuous casting of steels The invention relates to continuous casting of steels and is particularly concerned with the prevention of plugging of the tundish nozzles of continuous casting machines by incisions derived from reactive metal additives.

Farrel and Hilty showed in their paper presented at the 1971 Electric Furnace Conference that the oxides and/or oxysulphides of certain elements frequently added to steel for the purpose of deoxidizing or desulphurizing are deposited in the tundish nozzle as the steel flows into the continuous casting mould.

These deposits form a plug in the nozzle and stop the flow of steel. The elements that Farrel and Hilty investigated were: aluminium, titanium, zirconium, rare earths (REs) and, in some instances, silicon.

The problem has been largely overcome when casting steels containing aluminium by the use of stopper rods in the tundish to control the flow of steel and large nozzles that are not readily clogged by precipitation of alumina. However, this is an imperfect solution, and the reaction products deposited in the bores of the tundish nozzles are frequently required to be removed by mechanical or chemical means during the continuous casting.

The solution applied to aluminium is less than satisfactory with additions of titanium, zirconium, or REs because these other materials have a greater tendency to plug nozzles than aluminium. Titanium, zirconium and REs all have a higher affinity for oxygen and sulphur than aluminium, so the cleanness of continuously cast steel is enhanced Iby addition of proper amounts of these elements.

Steel cleanness can be related to ductility or ability to stretch under Woad.

According to the invention, a reactive metal such as titanium, aluminium, zirconium and REs or an alloy of two or more thereof, i5 introduced into the molten steel entrained in an inert gas so that inclusions derived from the reactive metals do not plug the tundish nozzle. The reactive metal may be injected either immediately below the tundish nozzle into a snorkel delivering steel to the mould, or down a hoilow tundish stopper rod, or below the tundish nozzle.Thus they do not have time to react with the oxygen and sulphur in the steel and plug the tundish nozzle. If some inclusions are formed, their melting point can be reduced by addition of a flux so that the melting point of the resulting mixture is lower than the temperature of the steel going through the nozzle. Inclusions that are liquid do not plug the nozzle.

When metal particles, such as alloy addi- tions, are added to molten steel, the first thing that happens is that the cold particles extract heat from the surrounding steel and the steel solidifies into a shell around a metal particle.

A finite time is necessary for the shell around the particle to remelt, and only after that does the alloy particle melt or go into solution. A further finite time elapses before the alloy goes completely into solution in the steel.

Once in solution, these alloys react with the impurities in the steel, such as oxygen and sulphur, and form inclusions or solid reaction products that plug the tundish nozzle. The times necessary for most of the alloys added to dissolve the steel shell formed when they are first added, to go into solution and react with the impurities in the steel to form tundish nozzle plugging inclusion are long enough to prevent tundish nozzle blocking inclusions forming .

if the alloys are added through a port in the snorkel used location for stirring, neither the reactive metals nor their reaction products with impurities in the steel ever comes in contact with the tundish nozzle.

If the solution time of the reactive alloy is short and their rate of reaction with the impurities is swift, nozzle plugging can occur, but such inclusions can be prevented from attaching to the bore of the tundish nozzle if their melting point is reduced by adding a flux with the reactive metal. As an example, if a low melting point alloy of REs were added, and the alloy went into solution swiftly and reacted with the impurities to form solid inclusions capable of nozzle blockage while the steel was still in the nozzle, one of the compounds formed would be rare earth oxide, (RE203), whose melting point exceeds 3000 F. It is possible to lower the melting point of the RE203 so formed by reaction with a flux such as cryolite, Na2AIF6. One such compound formed when cryolite and RE203 are reacted together is Na2 (RE)F4 whose melting point is less than 1 900 F, far below the temperature of the steel going through the tundish nozzle (2800-2900 F). Cryolite is not the only compound capable of fluxing RE203; others include halide salts of one kind or another.

The flux necessary for the inclusions is fluid at the temperature of the steel going through the tundish nozzle, and can be introduced into the system by coating the alloy particle with tlhe flux. This can be accompiished by dissolving the flux in a liquid such as water, immersing the alloy in a concentrated solution of the flux, and drying leaving a coating of the flux on the particle. A simpler procedure is to make an intimate mixture of fine particles of the alloy and flux so that low melting point reaction products of the alloy and flux are formed.

Where continuously cast steel must have the ultimate in cleanness, it is preferable to add the reactive metal either in the ladle or in the tundish to allow a great time for the flotation of the reaction products from the steel. However, the steel going through the tundish nozzle wili contain RE oxides or oxysulphides that can precipitate in the bore of the nozzle and stop the flow of steel through the noule. In such instances, a flux such as cryolite can be entrained in the stream of inert gas being injected down the tundish stopper rod. A portion of this flux, when dissolved in the steel just prior to its entry into the tundish nozzle, will contact the high melting point compounds previously precipitated on the nozzle to reduce their melting point below the temperature of the steel going through the tundish nozzle, thus dissolving the deposits from the bore of the nozzle.

Claims

1. A method of continuous casting of steels which comprises introducing a reactive metal entrained in an inert gas into the molten steel so that inclusions derived from the reactive metal do not plug the tundish nozzle.

2. A method according to claim 1 in which the reactive metal is aluminium, titanium, zirconium, a rare earth, or an alloy of two or more thereof.

3. A method according to claim 1 or claim 2 in which the reactive metal is injected into the molten steel immediately below the tundish nozzle into a snorkel delivering molten steel to the mould of the continuous casting system.

4. A method according to claim 1 or claim 2 in which the reactive metal is injected into the molten steel in the tundish nozzle through a hollow nozzle stopper rod.

5. A method according to claim 1 or claim 2 in which the reactive metal is injected below the tundish nozzle.

6. A method according to any preceding claim in which the reactive metal is in particulate form and coated with a flux.

7. A method according to claim 6 in which the flux is cryolite.

8. A method of continuous casting of steels according to any preceding claim substantially as herein described.

9. Steel continuously cast by a method according to any preceding claim.