GB1585189A

GB1585189A - Process for the continuous casting of steel

Info

Publication number: GB1585189A
Application number: GB53729/77A
Authority: GB
Original assignee: Concast AG; Belipar SA
Current assignee: SMS Concast AG; Belipar SA
Priority date: 1976-12-28
Filing date: 1977-12-23
Publication date: 1981-02-25
Also published as: BE862396A; BR7708668A; SE7714350L; IN148728B; IT1089234B; FI773932A; ES466021A1; CA1099891A; CH609263A5; FI61138C; YU306477A; JPS5384826A; FI61138B; FR2375934A1; DE2758103A1

Abstract

In the continuous casting of steel with an open casting stream between a tundish and a downstream mould (1), a multi-phase mixture of liquid inert gas and a particulate substance is introduced as a layer (7) onto the surface (5) of the molten metal in the mould. Casting powder together with the inert gas as the multi-phase mixture is applied directly to the surface of the molten metal during the coating, at a distance from the casting stream (2), via a common line (6). This distance is greater than half the diameter (8) of coating stream. The wave (9) caused by the casting stream prevents the casting powder from being swept into the liquid casting head (3). <IMAGE>

Description

(54) PROCESS FOR THE CONTINUOUS CASTING OF STEEL (71) We, CONCAST AG., a Swiss Corporation of Tödistrasse 7, 8027, Zurich, Switzerland and Belipar SA, a Body Corporate organized under the laws of Luxembourg of 11, Boulevard Prince Henri, Luxembourg, Luxembourg, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates to a process for the continuous casting of steel and employing a tundish with bottom-pouring means and an open-ended mould, the surface of the molten metal in the mould being covered by a multiphase mixture of liquified inert gas and a particulate substance.

In the continuous casting of steel it is known to place casting powder on the surface of the molten metal, the powder that is in direct contact with the liquid steel melting and exercising a lubricating action between the forming strand and the wall of the mould. When casting strands of larger size, the steel is customarily fed to the mould through a casting tube, which protects the steel from contact with the atmosphere and allows the steel to be introduced into the mould below the level of the molten metal in the mould, and protects a surface layer of casting powder or slag from the poured stream.

When casting smaller sizes of strand, e.g.

billets, it is, however, hardly possible to use casting tubes since, because of the crosssections in question and the necessary wall thickness of the casting tube, there is too little space between the tube and the cooled wall of the mould. This results in the formation at this point of a bridge of unfused casting powder and/or of solidified steel which can lead to stoppages in the casting operation. If, however, casting is carried out using casting powder but without a casting tube, there results the disadvantage that particles of slag are drawn into the interior of the strand by the stream of molten steel, and the steel is thus contaminated. Therefore, smaller sizes of product are normally cast using oil as the lubricant. However, when oil is used then on the one hand the surface of the molten metal in the mould is no longer protected against atmospheric oxygen, and on the other hand, the lubricating action, particularly when casting free-cutting steels is unsatisfactory since the oil is decomposed by the high temperature, and the carboncontaining residues do not provide sufficient lubrication. This results in surface defects which lead to cracks, particularly during further shaping operations such as rolling and, possibly, drawing, and results in products that are unusable or are capable of meeting only lower quality standards. Furthermore, the effect of the oil can result in the occurrence of what are known as pin-prick pores which likewise have a deleterious effect upon the surface of the strand. A further disadvantage encountered in the casting of free-cutting steels, especially lead-bearing steels, and when using oil, is that during casting intermittent small eruptions accompanied by spurts of hot metal from the mould can occur, so that the men operating the installation are at risk.

When casting powder and a casting tube are used in the casting of larger sizes of strand such as blooms and slabs, one of the functions of the casting powder is to protect the surface of the molten metal in the mould against atmospheric oxygen and to absorb impurities contained in the molten steel. The movements of the surface of the molten metal necessitate, for this purpose, a layer of casting powder having a thickness of approximately 1 cm. Only that part of the casting powder in direct contact with the surface of the molten metal liquifies and serves to carry out lubrication. A layer of caked casting powder is still present between this liquid layer and the pulverulent top layer. Oscillatory movement of the mould not only draws liquid but also caked and pulverulent casting powder into the gap between the wall of the mould and the molten or solidified steel. This mixture of casting powder has a less efficient lubricating effect than liquified casting powder, and this results in a strand surface of poorer quality and having pro nounced oscillation marks and casting pow der inclusions. Oscillation marks promote the formation of cracks and lead to increased scarfing losses. In the case of stainless steels, casting powder inclusions lead to increased grinding losses.

It is also known to use a homogeneous multi-phase mixture when continuously cast ing a metal into a mould with the poured stream of metal open i.e. uncovered. In this procedure a particulate substance, for example particles of soot, intended to improve the lubricating effect along the wall of the mould is introduced into a liquified inert gas. In this system, the multi-phase mixture is fed into the poured stream of metal by what is known as a phase separator. The multi-phase mixture reaches the surface of the molten metal in the mould along with the poured stream and protects the metal against the action of the atmosphere. Because of the introduction of the multi-phase mixture into the mould by way of the poured stream of metal and parti cularly when fairly large quantities of particu late substance are used, the particles thereof are carried into the casting head by the energy of the poured stream, and this leads to im purities. Also, when using this procedure for applying the multi-phase mixture to the poured stream of metal, trouble can occur due to external influences, such as for ex ample draught. The solid particles of the multi-phase mixture block the relatively small openings in the nozzles of the phase separator so that the protective effect of the poured stream is unsatisfactory and this has a disadvantageous effect on the quality of the cast material. In the case of larger sizes of product, even distribution of the multi-phase mixture and therefore of the solid particles over the surface of the molten metal in the mould by way of the poured stream is not ensured, and this leads to uneven lubrication along the wall of the mould. Wavering of the poured stream following bottom-pouring cannot be prevented for various reasons, for example irregularities in the pouring nozzles.

This can result in the phase separator being sprayed with steel and this leads to trouble in -supplying the multi-phase mixture and to -stoppages in the casting operation.

The object of the present invention is to provide a method of applying a multi-phase mixture that, on the one hand, ensures trouble-free feed of the mixture, and, on the other hand, results in a high degree of purity of the cast steel and an improved surface.

This object is achieved by the provision in accordance with the invention of a process for the continuous casting of steel which comprises pouring molten steel through a bottom pouring orifice in a tundish into an open-ended continuous casting mould and covering the surface of the molten metal in the mould with a multi-phase mixture of liquefied inert gas and a particulate substance, wherein the multi-phase mixture comprises a suspension in the liquefied inert gas of solid particles which fuse after separation from the mixture when in contact with the molten steel, and said mixture is fed directly onto the surface of the molten metal in the mould by means independent of the poured steel.

Surprisingly, it has been found that the feeding of the multi-phase mixture directly onto the surface of the molten metal can largely prevent non-metallic particles from being carried into the casting head. The solid particles only perform the function of providing lubrication which is improved by the fusing of these solid particles. The inclusion of solid particles in the surface of the strand can be prevented. The multi-phase mixture takes over the function of providing protection against atmospheric oxygen. Troubles at the feed apparatus due to spatter from the poured stream can no longer occur.

When casting with an open stream of poured metal the multi-phase mixture is preferably applied at a distance from the poured stream that is greater than half the diameter of this stream at the metal surface of the mould. When the poured stream strikes the surface of the molten metal in the mould a wave is formed in the zone around the poured stream, and the height and flow of this wave form a small protective wall which prevents the solid particles from entering the zone in the immediate vicinity of the poured stream and from being carried by this stream into the casting head. The open stream of poured material between the intermediate container and the surface of the molten metal is protected against atmospheric oxygen by the vaporizing inert gas. The impact of the poured stream on the surface of the molten metal eliminates a large part of the kinetic energy, so that the depth of flow in the sump is reduced and this has a favourable effect upon the quality of the cast steel.

When casting larger sizes of strand and using an open poured stream and the downwardly directed flow necessitated thereby, pronounced cooling of the steel in the zone of the surface of the molten metal in the mould cannot be prevented, and this leads to the formation of a top crust and therefore to considerable reduction in quality in the cast strand. This disadvantage can be eliminated by conveying the steel emerging from the pouring orifice in the bottom of the tundish into the mould and releasing it therein below the surface of the molten metal thereby causing it to flow towards and along said surface.

The multi-phase mixture preferably consists of 1-15 kg of liquified inert gas and 0 01 0 2 kg of casting powder for each tonne of cast steel. Such a small quantity of casting powder has the effect of permitting only liquid casting powder to reach the zone between the wall of the mould and the steel, and this results in a good lubricating action. The gas present in the zone of the wall of the mould prevents casting powder, contained in the liquid nitrogen, from reaching the wall of the mould so that the oscillatory movement does not cause any unfused casting powder to become mixed with the liquid casting powder, and this prevents the powder from being occluded in the surface of the strand. Thus, losses arising in subsequent dressing operations, such as grinding losses in the case of stainless steel, are reduced. The liquid casting powder performs the additional function of absorbing non-metallic inclusions rising to the surface of the molten metal.

When casting billets it is possible to dispense with oil lubrication and this leads to elimination of the above-described disadvantages particularly when casting lead-bearing freecutting steels.

In order to achieve an even distribution of the multi-phase mixture over the surface of the molten metal when producing strands of larger size it is preferred to feed the multiphase mixture directly onto the surface of the molten metal at several places.

Advantageously the liquified inert gas is nitrogen.

Some embodiments of the invention will now be described by way of example, reference being made to the accompanying drawings in which: Figure 1 is a vertical section on line I-I of Figure 2 through a continuous casting mould and illustrates the supply of the multi-phase mixture to the surface of the molten metal in the mould; Figure 2 is a plan view of the Figure 1 arrangement, and Figure 3 is a section through part of a tundish having a casting tube and through part of a mould of a second embodiment.

Referring to Figure 1, molten steel flows from a tundish, not illustrated, as an open poured stream 2 into a billet mould 1. The mould 1 is oscillated during casting. In the zone of the casting head 3 the steel commences to solidify to form a crust 4, the thickness of which increases along the length of the strand. During casting, a multi-phase mixture is fed directly onto the surface 5 of the molten metal by way of a pipe 6, this mixture consisting of liquid inert gas and, suspended therein, fusible solid particles which are advantageously in the form of casting powder. A mixture of liquified inert gas with a little casting powder suspended therein and of deliquified inert gas froms on the surface 5 of the molten metal as a layer 7. Advantageously 1--15 kg of liquified inert gas, for example nitrogen, is supplied for each tonne of steel. The quantity of casting powder suspended in the above-mentioned quantity of gas is 0 010 2 kg. The liquid nitrogen is fed to a pipe 6 through a phase separator 30 as described in DT-OS 26 06 871. The casting powder is introduced into the pipe 6 with the aid of a low-pressure powder distributor 31, a consistent suspension being important. The quantity of multi-phase mixture supplied to the surface 5 of the molten metal is regulated by a valve 32; Instead of casting powder, other fusible solid particles capable of forming a suspension, for example glass powder, paraffin wax etc., can be added to the liquified inert gas. The outlet of the pipe 6 is located just above the surface of the molten metal and at a distance from the poured stream 2 that is greater than half the diameter of this stream in the zone of the surface of the molten metal. Advantageously, the multiphase mixture is applied near the wall of the mould, for example, at a distance of 1--4 cm therefrom.

As shown in Figure 2, the poured stream 2 has a diameter 8. It should be mentioned that the poured stream 2 does not always have the ideal circular cross-section. When the poured stream 2 strikes the surface 5 of the molten metal, a wave 9 forms in a zone 10 around the area of impact of the poured stream, the zone 10 terminating at a distance from the poured stream 2 that is approximately equal to half the diameter of said stream. This wave, caused by the kinetic energy of the poured stream, and the inert gas which vaporizes around the poured stream, prevents the casting powder from reaching the zone immediately adjacent the poured stream and from being carried into the molten casting head 3 by this stream. As the liquified inert gas vaporizes, casting powder is deposited on the surface of the molten metal. As this is happening, the heat given off by the molten casting head causes the powder to fuse so that it can be drawn, as a film, into the space between the solidified crust 4 and the wall of the mould. Since the unliquified casting powder in the liquid nitrogen does not reach the walls of the mould because of the vaporization, the oscillatory movement of the mould 1 cannot cause any unliquified casting powder to find its way between the steel and the wall of the mould. Thus it is possible, both to maintain an optimum lubricating action with a small quantity of casting powder and to prevent non-metallic particles from being carried into the casting head 3.

Furthermore, the inert gas provides protection against atmospheric oxygen both for the surface of the molten metal and, in its gaseous form, for the poured stream issuing from the intermediate vessel.

Figure 3 shows a tundish 15 having a bottom-pouring orifice 16. The steel issuing from the orifice 16 is carried below the sur face 5 of the molten metal in a slab mould 20 by means of a casting pipe 17. The casting pipe 17 has two upwardly directed bores 21 drilled therein which impart to the steel a flow 22 which is directed towards and along the surface 5 of the molten metal. The liquid nitrogen with the casting powder suspended therein is fed directly onto the surface 5 of the molten metal through two pipes 6 and 6'.

If required the multi-phase mixture can also be applied at additional places. As a result of the flow 22 hot steel is always supplied to the surface of the molten metal so that a top crust cannot form, while at the same time non-metallic inclusions can settle in the liquid layer of slag lying on the surface of the molten metal.

WHAT WE CLAIM IS 1. A process for the continuous casting of steel which comprises pouring molten steel through a bottom pouring orifice in a tundish into an open-ended continuous casting mould and covering the surface of the molten metal in the mould with a multi-phase mixture of liquefied inert gas and a particulate substance, wherein the multi-phase mixture comprises a suspension in the liquefied inert gas of solid particles which fuse after separation from the mixture when in contact with the molten steel, and said mixture is fed directly onto the surface of the molten metal in the mould by means independent of the poured steel.

2. A process according to Claim 1, wherein the steel is poured as an open stream and the multi-phase mixture is fed onto the surface of the molten metal in the mould at a distance from the poured stream that is greater than half the diameter of said stream at the metal surface in the mould.

3. A process according to Claim 1, wherein the steel poured through the bottom pouring orifice of the tundish is released below the surface of the molten metal in the mould and is caused to flow towards and along said surface.

4. A process according to any one of Claims 1 to 3, wherein the multi-phase mixture consists of 1-15 kg of liquefied inert gas and 0-010 2 kg of casting powder per tonne of cast steel.

5. A process according to any one of Claims 1 to 4, wherein the multi-phase mixture is fed directly onto the surface of the molten metal at several places.

6. A process according to any one of Claims 1 to 5, wherein the liquified inert gas is liquid nitrogen.

7. A process for the continuous casting of steel according to Claim 1 and substantially as hereinbefore described.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. face 5 of the molten metal in a slab mould 20 by means of a casting pipe 17. The casting pipe 17 has two upwardly directed bores 21 drilled therein which impart to the steel a flow 22 which is directed towards and along the surface 5 of the molten metal. The liquid nitrogen with the casting powder suspended therein is fed directly onto the surface 5 of the molten metal through two pipes 6 and 6'. If required the multi-phase mixture can also be applied at additional places. As a result of the flow 22 hot steel is always supplied to the surface of the molten metal so that a top crust cannot form, while at the same time non-metallic inclusions can settle in the liquid layer of slag lying on the surface of the molten metal. WHAT WE CLAIM IS

1. A process for the continuous casting of steel which comprises pouring molten steel through a bottom pouring orifice in a tundish into an open-ended continuous casting mould and covering the surface of the molten metal in the mould with a multi-phase mixture of liquefied inert gas and a particulate substance, wherein the multi-phase mixture comprises a suspension in the liquefied inert gas of solid particles which fuse after separation from the mixture when in contact with the molten steel, and said mixture is fed directly onto the surface of the molten metal in the mould by means independent of the poured steel.