US3482621A - Apparatus for continuous casting of steel utilizing a closed chamber between a tundish and a reciprocatable mold - Google Patents

Description

Dec. 9. 1969 M. D. HALLIDAY 3,482,621

APPARATUS FOR CONTINUOUS CASTING OF STEEL UTILIZING A CLOSED CHAMBER BETWEEN A TUNDISH AND A RECIPROCATABLE MOLD Original Filed Nov. 22, 1965 6 Sheets-Sheet l PIP/0E 427' 9. 1969 I 3,482,621 APPARATUS FOR commuous CASTING OF STEEL UTILIZING A CLOSED I. M. D. HALLJDAY CHAMBER BETWEEN A TUNDISH AND A RECIPROCATABLE MOLD Original Filed Nov. 22. 1965 6 Sheets-Sheet 2 Dec. 9. 1969 l. M. D. HALLIDAY 3,432,621

APPARATUS FOR CONTINUOUS CASTING OF STEEL UTILIZING A CLOSED CHAMBER BETWEEN A TUNDISH AND A RECIPRQCATABLE MOLD Original Filed Nov. 22, 1965 s Sheets-Sheet 5 Dec. 9. 1969 A I. M. D. HALLIDAY 3,482,621

APPARATUS FOR CONTINUOUS CASTING OF STEEL UTILIZING A CLOSED CHAMBER BETWEEN A TUNDISH AND A RECIPROCATABLE MOLD Original Filed Nov. 22. 1965 6 Sheets-Sheet 4 Dec. 9. 1969 I. M. D. HALLIDAY 3, ,6

APPARATUS FOR CONTINUOUS CASTING OF STEEL UTILIZING A CLOSED CHAMBER BETWEEN A TUNDISH AND A RECIPROCATABLE MOLD Original Filed Nov. 22,-1965 .6 Sheets-Sheet 5 Dec. 9. 1969 I. M. D. HALLIDAY 3,

APPARATUS FOR CONTINUOUS CASTING OF STEEL UTILIZING A CLOSED CHAMBER BETWEEN A TUNDISH AND A RECIPROCATABLE MOLD Original Filed MW. 22. 1965 6 Sheets-Sheet 6 United States Patent 3,482,621 APPARATUS FOR CONTINUOUS CASTING OF STEEL UTILIZING A CLOSED CHAMBER BE- TWEEN A TUNDISH AND A RECIPROCAT- ABLE MOLD Iain M. D. Halliday, Barrow-in-Furness, England, assignor to The United Steel Companies Limited, Sheflield, England, a British company Original application Nov. 22, 1965, Ser. No. 508,895, now Patent No. 3,402,757, dated Sept. 24, 1968. Divided and this application Nov. 15, 1967, Ser. No. 703,496

Int. Cl. B22d 11/10 US. Cl. 164281 5 Claims ABSTRACT OF THE DISCLOSURE An apparatus for continuous casting of steel includes a tundish and reciprocatable mold, and a closed chamber between the tundish and mold. The closed chamber includes three parts, one part rigid with the tundish, another rigid with the mold and a third part intermediate the two parts which during the reciprocation of the mold is in effect rigid with the mold and makes sliding contact with the other. Gas inlets and outlets allow the chamber to be filled with gas. Sealing rings seal joints between the parts. The intermediate part may be a dished ring which makes sliding contact with a pouring tube on the tundish and with a flange on an angular member attached to the mold.

RELATED APPLICATIONS This application is a divisional application of Ser. No. 508,895 filed Nov. 22, 1965, now Patent No. 3,402,757.

In the continuous casting of steel, the metal in commonly poured into a tundish from a ladle and runs from the tundish into an open-ended mold. As the metal stream flows from the ladle to the mold there is a tendency to oxidation of the stream which it is desirable to avoid. This tendency shows particularly with chromium-containing steels, aluminum-stabilised steels and other steels containing alloying elements which have a high afiinity for oxygen. When carbon and low-alloy steels are cast, there is not so much oxidation but there is still a strong tendency for minute pinholes to form at the surface of the cast metal or immediately below the surface. The mechanism of pinhole formation is not known. The formation of pinholes can, however, be greatly reduced or virtually eliminated by feeding aluminum wire into the metal stream as this enters the mold, but this is of litt e effect in eliminating oxidation and it is undesirable to make any aluminum additions whatever to some steels.

In studying the origin of inclusions in continuously cast steel, particularly oxide inclusions, we have found that about 90% of the oxide in the cast steel is derived from oxygen pick-up by the metal during pouring between the tundish and the mold. Thus, non-metallic oxide material and any extraneous slag carried over in the metal from the ladle to the tundish tends to separate out at the tundish, only a very small proportion of such material passing through the tundish nozzle.

It is well known that in the casting of stainless steels it is necessary to surround the pouring stream by reducing gas such as propane so as to prevent the formation of chromium oxides. We have also found that such gas must be used in casting other steels that include elements having a high afiinity for oxygen. When aluminum is used to prevent pinhole formation, the formation of aluminum oxide is found to be particularly rapid even with considerable amounts of propane gas blanketing. As a result, the cast products tends to have a very high proportion of alumina particles embedded in its surface, some of these particles being as much as /8 to in. in size.

We have found that it is possible to obtain castings sub- 3,482,621 Patented Dec. 9, 1969 stantially free from oxide formed during the pouring and from pinholes without the use of aluminum by pouring the steel from a tundish or other vessel into the mold through a closed chamber if certain steps are employed. These steps, comprise purging the closed chamber with a reducing gas such as propane or butane or with an inert gas such as argon or nitrogen before the start of a cast; permitting metal to flow through the chamber to the mold while the chamber is substantially filled with the reducing or inert gas; as the mold becomes filled with metal with a consequent tendency to substantial increase in the gas pressure in the closed chamber, reducing this pressure, advantageously substantially to atmospheric pressure, by permitting gas to escape at a greater rate than it enters; and as the casting proceeds passing reducing gas through the chamber at a rate such as to ensure that the oxygen concentration in the chamber is not greater than 250 parts per million while maintaining the pressure in the chamber substantially constant.

The important feature is the low concentration of oxygen, and preferably this is maintained at less than parts per million, better still at less than 50 parts per million, and if possible at no more than 20 parts per million. To ensure this the chamber must be truly closed, but even so oxygen tends to be introduced into the chamber with the molten metal, and minor leakages may occur during casting; it is therefore necessary to maintain a continuous flow of reducing gas through the chamber throughout the cast. It is of course also necessary first to purge the chamber.

The choice of the gas used depends to some extent upon the nature of the steel that is being cast. For ordinary carbon and other steels prone to pinhole formation, when the main problem is the prevention of pinholes, propane gas or other reducing gas or gas mixtures is normally preferred, as it is also for steels, containing easily oxidised alloy elements such as aluminum, but not prone to hydrogen embrittlement. In the casting of stainless steels and other steels sensitive to hydrogen embrittlement, an inert gas such as argon or nitrogen may be used. In fact, for such steels nitrogen is generally preferred on the grounds of cost, but since commercial nitrogen contains about 200 parts per million of oxygen the concentration of oxygen in the closed chamber may be up to 250 parts per million. Under these conditions some oxidation of the chromium contained in the steels may occur. If this oxidation is to be minimised, the gas that flows through the closed chamber during the casting may be a mixture of an inert gas such as nitrogen and a reducing gas such as propane or other reducing gas in a concentration such that the sum of the hydrogen and water vapour contents does not amount to more than 5,000 parts per million by volume. Again a mixture of nitrogen and a small proportion of carbon monoxide, say up to 5%, may be used.

In the purging of the closed chamber it is naturally desirable to use as cheap a gas as possible, particularly if the purging gas is flowing for some time before the start of a cast. Although it is convenient to purge with the gas that is to be passed through the chamber subsequently, it may be more economical to effect the purging with one gas and to change over to another directly after the casting begins.

As the mold fills with metal at the beginning of a cast, the pressure rises in the closed chamber. If there be any substantial gas pressure in the closed chamber it hinders the downward flow of the metal, and it is even possible that the gas will bubble upwards through the metal into the metal in the tundish or other pouring vessel and endanger the operators. It is to avoid these effects that when the fiow of metal is started the pressure in the chamber must be allowed to fall or atleast must not be allowed to rise appreciably above atmospheric. This can be done by increasing the opening of an outlet valve or opening an additional vent valve. It is highly desirable that dur ing the casting the rate of flow of the metal should be substantially constant, and accordingly the pressure in the closed chamber must be maintained substantially constant, since if it varies it will affect the rate of flow.

In practice the molds used in the continuous casting of steel are nearly always reciprocated in relation to the casting during the operation.

The stroke is normally small, but of course the reciprocation causes the volume of the closed chamber to vary cyclically with consequential variation of the pressure throughout in the same way.

FIG. 1 is a schematic view of the apparatus of this invention.

FIGS. 2, 3 and 4 are detailed schematic views of one form of the apparatus, FIG. 2 being taken on line 22 of FIG. 3, FIG. 3 being taken on line 33 of FIG. 2, and FIG. 4 being taken on line 44 of FIG. 2.

FIG. 5 is a plan view of another construction.

FIG. 6 is a vertical central sectional view through a third construction.

The closed chamber may take various forms and in designing it account must be taken of the reciprocation of the mold. One simple design that is well known in the art is shown diagrammatically in FIGURE 1 of the accompanying drawings. Here a tundish 1 with a nozzle 2 let into its base carries a fitting 3 on its underside. The mold is shown at 4, and a fitting 5 is united to the top of it in air-tight fashion. The fittings 3 and 5 are interconnected by a ring 6 of flexible material such as asbestos fabric impregnated with rubber to render it impermeable to gas. The edges of this ring are tightly sealed in the fittings 3 and 5. Alternatively the flexible ring 6 may be made of thin sheet metal resistant to the heat to which it is subjected.

The chamber thus formed within the fittings 3 and 5 and the ring 6 has a gas inlet 7 to which a valve-controlled pipe for the supply of gas is connected, a gas outlet 8, and a valve-controlled vent 9. In addition there is a silica window 10 through which the metal stream may be observed.

A long length of pipe 11 which may be from A2 to 1% inch in diameter conveys away the effiuent gas from the chamber. The length of this pipe 11 should be sufficient to ensure that the increase in volume when the mold makes a downstroke cannot result in atmospheric oxygen being sucked into the closed chamber. This increase in volume during a mold downstroke is compensated for to some extent by the supply of gas to the chamber, but at high reciprocation rates it is still necessary to provide some length of pipe 11. Preferably a valve is included in the pipe 11 to aid control of the gas pressure within the enclosure.

In operation, metal poured into the tundish from a ladle, to flow through the nozzle 2, is of such a design as to produce a rod-like stream. Before the pouring of the metal begins, the chamber is purged with gas supplied through the inlet 7. This gas passes out not only through the outlet 8 and pipe 11, but also through the nozzle 2. When pouring begins, the metal flows from the tundish through the nozzle 2 as a rod-like stream through the closed chamber into the mold, the base of which is initially closed by a dummy bar. As the metal begins to rise in the mold, the pressure in the chamber begins to rise, and the vent 9 is opened. When the metal reaches a predetermined level in the mold, the dummy bar is lowered so that the strand begins to form and is continuously withdrawn, and this vent 9 is closed again.

It is found in practice that it is somewhat ditficult to prevent leakage of oxygen into the chamber when any such flexible ring as 6, which may be regarded as a bellows, is used, and it is much preferred to make the closed chamber of improved construction, and in fact the preferred constructions of the closed chamber are in themselves part of the present invention.

One of the features which it is necessary to take into account in designing a suitable closed chamber is that the falling stream must be accurately located in relation to the mold. In a continuous casting machine the position of the mold is elfectively determined by the mechanism which carries it and reciprocates it. It is therefore necessary initially to locate the tundish in the right position in relation to the mold, but in addition it is often found that a stream initially falling as desired tends to wander from the desired path. This may be caused, for example, by the freezing of a very small piece of metal on the inside wall or lower end of the nozzle through which the metal is flowing with the result that the point of entry of the stream into the mold may deviate as much as 2 inches from the desired point. This may actually cause some of the metal not to enter the mold at all but to strike the upper edge which the mold cavity makes with the mold top plate, thereby leading to a major breakout of metal below the mold unless the machine is stopped immediately. It is therefore occasionally and sometimes frequently necessary to adjust the position of the tundish during a cast to take account of this. It follows that for the purposes both of initial location of the tundish and of possible adjustment of the position of the tundish during casting a rigid connection between the tundish and the mold must be avoided.

The preferred forms of closed chamber according to the invention may be regarded as consisting essentially of three parts. One of these parts is composed of one or more members rigid with the tundish, and another of one or more members rigid with the mold. The third part consists of an intermediate member or set of members which during the reciprocation of the mold is effectively rigid with one of the other parts but which is capable of movement in relation to the one or other set of members to allow lateral adjustment to be effected. The joints between the three parts are made by means of sealing rings.

Referring first to FIGURES 2, 3 and 4, the underside of a tundish is shown at 1 and carries a bracket 12 which supports a member 13, a tight seal being made between the two by a sealing ring 14. The member 13 is tubular and water-cooled by water supplied through a pipe 52. The top of the mold is shown at 4 and carries a fitting 15 with a flange 16. The fitting 15 is bolted to the mold by bolts 17 and a tight joint is made by a sealing ring 18. A dished ring 19, forming an intermediate member, bears on the flange 16 and closely surrounds the member 13. A joint is made between the ring 19 and the member 13 by two sealing rings 20 and another joint is made between the member 19 and the flange 16 by a sealing ring 21. The ring 19 is held in position by a clamping ring 22, which has an internal thread engaging with a thread on the fitting 15.

In this construction the bracket 12 and member 13 are at all times rigid with the tundish, and the member 15 is rigid with the mold. During casting the intermediate member formed by the ring 19 is rigid with the fitting 15 and therefore with the mold and slides in relation to the member 13 while a tight seal is maintained by the two rings 20. If lateral adjustment is required, the tundish and the parts rigid with it are moved together with the intermediate member 19, the clamping ring first being loosened by a handle 29 and later retightened.

Gas is introduced in this construction through an inlet 24 and leaves through an outlet 25, whilst a similar outlet 53 is used to connect a pressure-measuring instrument to the interior of the chamber. A pipe such as that shown at 11 in FIGURE 1 is attached to the outlet 25.

A safety device is provided in the form of an aluminum plate 26 which seals an opening in the fitting 15 and which melts if the closed chamber becomes filled with metal.

This plate 26 is urged into the sealing position against a sealing ring 27 by springs 28 and can yield if any excess gas pressure should build up in the closed chamber.

The modified construction shown in FIGURE 5 differs from that in FIGURES 2 to 4 only in that the fitting is made in two parts 30 and 31 held together by clamping bolts 32. These two parts meet along a plane XX which is perpendicular to the centre lines of the window 10 and the aluminum plate 26. The fitting 15 can thus easily be removed if, as sometimes happens, the closed chamber becomes flooded with metal.

The construction shown in FIGURE 6 differs fairly substantially from those previously described. Again there is a bracket 12 on the underside of a tundish, and this carries a ring 34 by which a further ring 35 is held tightly in position to make a seal with the bracket 12 through a sealing ring 14. The ring 35 and a lower ring 36 bolted to it engage round a part-spherical surface of a water-cooled tubular member 37, which forms one intermediate member. A tight seal is made between the part-spherical surface of the member 37 and the rings 35 and 36 by two sealing rings 38 with a spacer ring 39 between them.

A lower fitting 40 rigid with the top of the mold is welded to a ring 41 to which a ring 42 is bolted. An annular member 43, forming a second intermediate memher, is interposed between these two rings 41 and 42 on the one hand and the tubular member 37 on the other hand.

On its outer surface the annular member 43 presents a part-spherical face to the rings 41 and 42, and a tight seal is made here by two rings 44 with a spacer 45 between them. At its inner face, the annular member 43 fits closely around the outer face of the intermediate member 37, making a sliding joint with sealing rings 46. These are tightened by a gland ring 48 which is screwed down onto the annular member 43, whilst a ring 49 acts as a spacer between the rings 46.

In operation the member 37 is effectively held clamped within the rings 35 and 36, and the reciprocating movement of the mold is accommodated by sliding movement between the member 43 and the outer cylindrical surface at the lower end of the intermediate member 37, while an effective seal is also maintained by the rings 44 and the fitting 42 between the member 43 and the fitting 40.

Gas is supplied to the closed chamber shown in FIG- URE 6 through an inlet 24 and leaves through an outlet not shown, which may, for example, be formed in the wall of the fitting 40, and to which a pipe is attached as in the other constructions.

A pouring tube 47 may be provided to receive the metal flowing from the tundish and discharge it into the mold. Such a tube is used if it is desired to control the rate of metal discharge through the nozzle by a stopper. When the nozzle is partially opened or closed by such a stopper the metal stream tends to fall irregularly and frequently with a bell-shaped spread, and the pouring tube 47 acts primarily as a conduit to prevent the metal striking the walls of the member 37, and spreading to the rings 38. No pouring tube is required if the metal stream is always rod-shaped.

Lateral adjustment of the point of discharge of the stream of metal into the mold is effected either by lateral movement of the tundish on its supports, or by tilting the tundish so as to incline the axis of the discharging metal stream or, if there is a tube 47, the axis of this tube. Such tilting movement is accommodated by the part-spherical surfaces on the members 37 and 43.

Because of the freedom allowed for angular movement by the sealing rings 38 and 44 and the part-spherical surfaces on the members 37 and 43 against these rings press, the arrangement of FIGURE 6 is particularly suitable, and indeed particularly designed, for incorporation in a casting machine in which the mold is curved and reciprocates in an are so that its movement is not simply vertical but also includes a substantial lateral component. Such 6 curved reciprocating motion of the mold is accommodated as to its vertical component by the sliding rings 46 and as to its lateral component by the movement of the rings 38 and 44 over the corresponding part-spherical surfaces of the members 37 and 43.

The fitting 40 shown in FIGURE 6 may be fabricated in two parts which fit tightly together, as also may the member 37 with suitable modification to the water-cooling passages.

The gas outlet from the closed chamber, or the additional vent if one is provided, may be closed under automatic control after being opened at the start of a cast. Thus the valve in the outlet or in the pipe attached to the outlet may be moved to a set position in response to a signal derived from the level of the metal in the mold, or the additional vent may be closed by such a signal. The,signal may be given, for example, by a detector of the gamma ray type.

The. gas pressure in the closed chamber during the casting may be maintained substantially constant under automatic control in that a valve in the gas inlet, or preferably in the gas outlet, may be controlled by a signal given by the pressure in the chamber and smoothed to prevent the pulsations caused by the reciprocation of the mold from having any effect.

I claim:

1. An apparatus for use in continuous casting of steel, comprising: a tundish, a reciprocal mold, and a closed chamber interposed between and connected to the mold and the tundish so that metal may flow in a stream from the tundish to the mold without contacting the closed chamber, the chamber including three parts, namely, a first part having a member rigid with the tundish, a second part having a member rigid with the mold, and a third part, movably and sealingly mounted with respect to said first and second parts, effectively rigid with one of the parts transversely to the axis of reciprocation of the mold and making movable sliding sealing contact with the other of the parts so that reciprocation of the mold along a vertical axis may be accommodated and the position of the mold and tundish adjusted relative to one another along a horizontal axis, the closed chamber having a gas inlet and gas outlet.

2. An apparatus according to claim 1 in which joints between the three parts are sealed by sealing rings.

3. An apparatus according to claim 2 in which the intermediate member includes a dished ring that makes sliding contact with a pouring tube fixed to the tundish and also with a flange on an annular member fixed to the mold.

4. An apparatus according to claim 2 in which the third part of the chamber presents part-spherical faces to the first and second parts.

5. An apparatus according to claim 4 in which the mold is curved.

References Cited UNITED STATES PATENTS 2,825,104 3/1958 Jones 164-281 X 2,905,989 9/1959 Black 164-155 2,369,233 2/1945 Hopkins 164-82 X 2,837,791 6/1958 Tessmann 16464 3,050,798 8/1962 Chambers 16464 FOREIGN PATENTS 1,160,289 2/1958 France.

688,955 3/1953 Great Britain.

908,902 10/ 1962 Great Britain.

947,626 l/ 1964 Great Britain.

I. SPENCER OVERHOLSER, Primary Examiner R. SPENCER ANNEAR, Assistant Examiner US. Cl. X.R. 164-259

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