US3563298A

US3563298A - Method of continuously casting bars for preventing distortion during solidification of the bars

Info

Publication number: US3563298A
Application number: US693884A
Authority: US
Inventors: Werner Stauffer; Armin Thalmann; Carl Kueng
Original assignee: DER VON MOOS AG; LUZERN AND CONCAST AG
Current assignee: AG DER VON MOOS; DER VON MOOS AG; LUZERN AND CONCAST AG
Priority date: 1966-12-28
Filing date: 1967-12-27
Publication date: 1971-02-16
Anticipated expiration: 1988-02-16
Also published as: CH440569A; GB1173779A; ES349065A1; BE707433A; FR1551365A

Abstract

A continuous casting operation for forming bars is comprised of pouring molten steel into the inlet end of a mold chamber in which the bar is formed and from which the bar is withdrawn while it still has a liquid core. Within the mold chamber the molten steel forms a liquid level, and a first section of the chamber having converging walls commencing at the liquid level and extends downwardly for applying a symmetrical cooling effect to the molten steel and, at the same time, constraining the outer surface of the bar as it contracts and develops a solidified outer layer or crust. The constraint afforded by the walls of the first section prevents a distortion in the shape of the bar as it begins to solidify. From the first section of the chamber the bar passes through a second section where the cooling operation is continued but without any constraint so that a gap develops between the bar and the walls of the chamber. After passage through the second section the bar may be withdrawn from the outlet end of the chamber or, alternatively, it may pass through another section wherein increased cooling is effected and then, if necessary through a further section where the cooling effect is decreased as in the second section. From the outlet the bar is guided to rollers which withdraw it from the mold chamber. Before reaching the rollers, cooling means, such as water sprays, are provided for cooling the casting. The mold chamber is provided with means for circulating a coolant fluid for removing heat from the molten steel within the mold chamber.

Description

United States Patent [72] Inventors Werner Staufl'er, Armin Thalmann, Uster,

and Carl Kueng, Emmenlirucke [21] AppLNo. 693,884 [22] Filed Dec. 27, 1967 [45] Patented Feb. 16,1971 [73] Assignees Aktiengesellschaft Der Von Moos, Schen Eisenwerke Luzern,; Concast AG Zurich, Switzerland [32] Priority Dec. 28, 1966 [33] Switzerland 18686/66 [54] METHOD OF CONTINUOUSLY CASTING BARS FOR PREVENTING DISTORTION DURING SOLIDIFICATION OF THE BARS 6 Claims, 2 Drawing Figs.

[52] U.S.Cl. 164/82, 164/273 [51] Int. Cl. ..B22dll/00 [50] FieIdofSearch ..164/82,89, 273,282, 283,85

[56] References Cited UNITED STATES PATENTS 2,363,695 11/1944 Ruppik 164/82X 2,564,337 8/1951 Maddex... 164/283X 2,564,723 8/1951 Rossi 164/283 3,338,297 8/1967 Foldessy..... 164/282X 3,416,591 12/1968 Babel et al.. 164/85 2,590,311 3/1952 I-Iarter 164/283 N v- -p 3b we 2,601,615 6/1952 Jordan 164/282 3,353,584 11/1967 Atkin 164/89 FOREIGN PATENTS 482,839 7/1953 Italy.

Primary Examiner-Charles W. Lanham Assistant ExaminerR. Spencer Annear Attorney-McGlew & Toren ABSTRACT: A continuous casting operation for forming bars is comprised of pouring molten steel into the inlet end of a mold chamber in which the bar is formed and from which the bar is withdrawn while it still has a liquid core. Within the mold chamber the molten steel forms a liquid level, and a first section of the chamber having converging walls commencing at the liquid level and extends downwardly for applying a sym metrical cooling effect to the molten steel and, at the same time, constraining the outer surface of the bar as it contracts and develops a solidified outer layer or crust. The constraint afforded by the walls of the first section prevents a distortion in the shape of the bar as it begins to solidify. From the first section of the chamber the bar passes through a second section where the cooling operation is continued but without any constraint so that a gap develops between the bar and the walls of the chamber. After passage through the second section the bar may be withdrawn from the outlet end of the chamber or, alternatively, it may pass through another section wherein increased cooling is effected and then, if necessary through a further section where the cooling effect is decreased as in the second section. From the outlet the bar is guided to rollers which withdraw it from the mold chamber. Before reaching the rollers, cooling means, such as water sprays, are provided for cooling the casting. The mold chamber is provided with means for circulating a coolant fluid for removing heat from the molten steel within the mold chamber.

METHOD OF CONTINUOUSLY CASTING BARS FOR PREVENTING DISTORTION DURING SOLIDIFICATION OF THE BARS SUMMARY OF THE INVENTION The present invention is directed to a method of and apparatus for continuously casting a bar by pouring molten steel into a cooled open-ended mold and withdrawing the ,bar from the mold while it still has a liquid core and affording further cooling of the bar after its exits from the mold.

When bars of rectangular cross Section are continuously cast there is a pronounced tendency, which is difficult to control, for the bars to distort angularly and to develop cracks.

In order to compensate for these defects which adversely affect both the quality of the casting as well as any further processing which may be performed on it, it has been known to pass the cast bars between closely set corner rollers after they have left the mold. This arrangement has been successful, to some extent, in squaring off the sides of the casting.

in continuous casting operations, it has also been proposed to use tapered molds for increasing the rate of heat removal and thereby increase the casting rate. The tapered arrangement of the mold is formed by walls converging toward its outlet end and intended to compensate for the shrinkage rate in the bar which is cooled as it passes through the mold. However, because the shrinkage rate in themold is not a linear function of its length tests performed with such molds have shown that the casting is not positively guided until it reaches the end of the mold, and as a result, the formation of peaked corners on the casting is only partly obviated. Moreover, increasing the length of the taper so that thecasting is guided over a greater distance within the mold failed to provide the desired results, and, as a matter of fact, merely caused the casting to rupture because of the increased friction developed.

In any event the above described steps are attempts merely at correcting the resulting defects in the cast bars and do not remove or correct the causes which resulted in the defects. It has been ascertained that purely geometrical design measures cannot prevent the casting from developing the undesired peaked cross section-which is caused by the generation of asymmetrical stresses in the solidifying outer layer resulting from nonuniform cooling within the mold.

Metallographic investigations have disclosed that in continuously cast ingots which displayedthe peaked corner effect the finely crystalline outer layer or crust which was spontaneously solidified at the cooled wall'of the mold is of nonuniform thickness, that is, the layer may be thicker on one corner that on the oppositely disposed corner. Moreover, examination of the shells of continuous castings in which the liquid core has run out, revealed that the nonuniformities in the thickness of the spontaneously formed outer layer determine the progress of solidification which, because of this formation, continues in the same irregular pattern. These differences in thickness of the solidified crust are the direct cause of the irregular and uncontrollable withdrawal of the outer layer from the mold wall when the casting shrinks during further solidification. As the casting shrinks from the mold wall it is usual for two opposite corners to remain in contact with the wall for a longer period than the remaining portions of the outer layer. Because these opposed corners remain in contact with the mold wall, solidification at these points is greatly advanced and the shrinkage below the liquid metal surface in the mold usually begins at these corners. This corner shrinkage causes the original right angular disposition of adjacent sides to become acute and to push the more highly cooled corners into closer contact with the cold walls of the mold. As a consequence, the other corners. which originally had shrunk from the mold wall, are pulled even further away. Accordingly, minor irregularities are formed in the pattern of the initial solidification of the bar which eventually lead to angular distortion and to the develo ment of fissures or cracks in its surface.

Therefore, it is a primary object of the present invention to assure symmetrical conditions of stress during the initial stage of solidification of the casting as it passes through the mold for avoiding angular distortion and crack formations.

Another object of the invention is to provide a mold con struction in which the casting can be properly cooled and constrained during a portion of its passage through the mold.

Still another object of the invention is to provide an arrangement of guiding means for directing the casting from the mold to the rollers which withdraw it from the mold.

Moreover, another object of the invention is to incorporate cooling means with the guiding means at the outlet end of th mold for effecting further cooling of the casting.

A further object of the invention is to provide a method for cooling the casting as it passes through the mold while first providing constraint for its exterior surface and then withdrawing the constraint. These steps may be repeated where necessary for properly shaping the bar and avoiding any asymmetrical development of stresses which would tend to distort the bar.

In the method of the present invention, the molten steel is cooled and its outer layer, which forms a solidified crust, ini

tially is constrained during the passage of the casting through the mold, subsequently the cooling operation is continued, however, the constraint is removed from the casting permitting a gap to develop between the interior surface of the mold and the exterior surface of the casting. Where necessary the steps of cooling first with and then without constraint of the exterior surface of the casting for providing an increased and then a decreased cooling effect may be repeated depending on the various factors which affect the casting operation.

In the apparatus of the present invention vertically extending walls form an open-ended mold chamber having an inlet opening at its upper end and an outlet opening at its lower end and arranged to have a molten steel liquid level near its inlet opening. The first section of the mold chamber commencing at the liquid level has walls which converge downwardly for exerting constraint on the metal as a solidified layer is formed by contact between the metal and the cooled walls of the chamber. The converging or tapered arrangement of the walls affords a smaller transverse cross section for the casting within the mold chamber than normally would be the case where the casting is allowed to shrink without constraint. Due to the constraint afforded in this first section more increased heat removal operation is effected than would be possible if the walls did not converge and the casting were allowed to cool and contract from the walls of the mold. The second section of the mold chamber has walls of a lesser converging attitude than in the first section, and preferably the walls are arranged in parallel relationship. In this section, the casting continues to be cooled and to contract forming a gap between its outer surface and the juxtaposed surface of the mold chamber. Rollers are spaced outwardly from the mold for engaging the surface of the casting and withdrawing it from the mold. Between the rollers and the outlet a plurality of guide members are providedinterspersed with cooling means, such as water sprays, for further cooling the casting.

in an alternative arrangement, a third section and, if necessary, a fourth section may be added to the mold chamber. ln the third section the walls are again arranged in converging relationship so that an increased cooling effect is provided in a manner similar to that achieved in the first section. The fourth section may be added to provide an effect similar to that afforded in the second section with the walls arranged in substantially parallel relationship so that the bar continues to cool but with a reduced cooling effect.

Therefore, the method and apparatus of the present invention overcomes the problems experienced in the prior art by effecting a symmetrical cooling effect on the casting while initially exerting sufficient constraint to prevent the formation of a gap between the outer surface of the casting and the mold wall. Subsequently, the cooling of the casting is continued but without constraint since the outer layer has been adequately formed in the first section of the mold chamber.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which there are illustrated and described preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a vertical sectional view of a continuous casting plant including apparatus embodying the present invention; and

FIG. 2 is an enlarged sectional view of another embodiment of a mold similar to the one disclosed in FIG. 1, which illustrates another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As illustrated in FIG. I, molten steel is poured from a tundish 1 into the open upper end 2a of a water-cooled mold 2. The molten steel is continuously cast into a bar as it passes downwardly through the mold 2 exiting from its lower end 2b. During the passage of the molten steel through the mold an outer layer 3a is solidified, however, it still retains a molten or liquid core 3b as it issues from the outlet end 2b of the mold 2. Spaced downwardly from the outlet end 2b of the mold are rollers 4 which withdraw the continuously cast bar from the mold. Guide means 5 are disposed between the outlet end 2b and the rollers for directing the bar to the rollers. Nozzles 6 are positioned between the guide means 5 for cooling the bar by spraying its outer surface with water. A jacket 7 is disposed about the mold and forms an annular chamber for circulating a coolant fluid through the mold.

Within the upper end of the mold 2 the molten steel forms a liquid metal surface 10. Commencing at the liquid metal surface 10 the mold is divided into an upper first zone 11 and a lower second zone 12. Within the first zone 11 the opposed walls converge forming a downwardly decreasing transverse cross section. The downward taper of the walls in the first zone affords a cross section having a slightly smaller area than would be developed in the casting by normal shrinkage within this same zone. Since the cross section provided by the converging walls is smaller. it affords a constraint on the solidified outer layer or crust of the casting which would not ordinarily take place if the walls were not properly tapered.

The walls within the first zone ll of the mold chamber hold the outer layer of the casting tightly with a uniform pressure on all sides. The combination of the arrangement of the walls in the first zone and the symmetrical cooling effect achieved by circulating a coolant fluid through the chamber formed between the mold 2 and the jacket 7 make it impossible for a gap to develop between the outer surface of the casting and the juxtaposed walls of the mold chamber. However, the encircling pressure exerted on the casting must not be so great as to develop frictional forces which cannot be overcome in withdrawing the casting from the mold. Excessive friction may cause the solidified outer layer to rupture and this, in turn, may result in a break through of the liquid metal core or a complete cleavage of the leading end from the remainder of the casting. It has been found that the shrinkage on each side of the cross section in the first zone 11 of the mold is between 0.4 percent and 0.8 percent. Since no gap can develop in the first zone the entire peripheral surface of the casting is Kept in close contact with the adjacent mold walls. This arrangement insures a high heat transfer rate between the surface of the casting and the mold walls submitting the solidified surface layer of the casting to a symmetrically distributed cooling effect. In other words, all four comers of the casting will have approximately the same temperature and the unavoidable temperature gradient from the comers to the center of each adjoining side of the cross section is approximately the same.

As a result the stresses generated by these temperature gradients are symmetrical with respect to the center of the cross section and do not give rise to any angular distortion. Further, due to the uniform cooling effect achieved, the thickness of the spontaneously solidified layer on the exterior of the casting is symmetrical whereby further solidification within the casting occurs in a symmetrical manner.

The temperature gradient from the corners to the centers of the faces of the casting can be further reduced by covering the outer surface of the mold wall in the region of the corners with an insulating material to reduce the transfer of heat at these locations.

The length of the first zone 11 within the mold chamber depends upon several casting parameters, such as the casting rate, the dimensions of the cross section, the quality of the cast steel and the like. For casting a 10 mm. square section bar at a casting rate of about 3 meters per minute, the length of the first zone 11 may be 200 mm.

If the increased cooling effect achieved in the first zone were continued along the entire length of the mold, the temperature gradient in the solidified layer would be excessive and would tend to cause the formation of small surface fissures. As is well known, such surface fissures are widened by the intense cooling effect in the secondary cooling zone and may even result in the entire casting becoming useless or being exposed to considerable scarfing losses.

In order to avoid these undesirable consequences a second zone 12 affording a less intense cooling effect follows the first zone 11 in the path of the casting through the mold chamber. The lesser cooling effect provided in the second zone as compared to that within the first zone occurs because the casting is not constrained within this second zone. With the absence of any constraint, a gap 13 develops between the surface of the casting and the adjacent mold walls. Preferably, in the second zone the opposed walls are arranged in parallel relationship. If it is desirable to obtain a larger gap 13 between the casting and the interior surface of the mold this can be accomplished by increasing the cross section of the mold within the second zone, this can be accomplished by arranging the walls in diverging relationship which tends to increase the reduction in the cooling effect. Additionally, by varying the termal conductivity of the material forming the mold, in the second zone 12 the cooling effect can also be reduced.

It has been known that the guide means 5 located at the outlet end of the mold 2 are liable to suffer distortion from radiant heat emanating from the casting, and that this distortion effects the maintenance of the gap in the mold. In other words, the width of the gap around the periphery of the casting and hence the cooling effect may vary. To prevent this distortion from developing, it is preferred to provide rollers 15 as the guide means for the casting at the outlet 2b from the mold. Alternatively, cooling plates may be arranged in place of the rollers to provide accurate alignment of the casting in relation to the mold.

Alternatively, a better maintenance of the required gap between the mold and the casting in the zone 12 may be achieved by utilizing the mold 2 which is illustrated in FIG. 2. The mold 2 has a first zone 11 and a shorter second zone 12' than the second zone 12 illustrated in FIG. 1.

A third zone 16 is provided at the outlet end of the second zone 12' in which an increased cooling effect, relative to that provided in zone 12', is achieved by disposing the opposed walls in converging relationship. However, the reduction in cross section within zone 16 is less than that provided in zone 11 since the thickness of the surface layer and the mechanical strength of the casting developed in its passage through the mold to the third zone 16 does not permit the same degree of deformation as occurs in zone 11. Excessive deformation would increase the friction with the undesirable consequences which have already been described. Following the zone 16, another zone 17, similar to zone 12', may be provided. However, in molds of shorter length the final zone 17 may be omitted.

The zone 16 is afforded not only for maintenance of a uniform gap width, but also for the purpose of increasing the casting rate by the addition of a further increased cooling of the casting. Since in the second zone 12' the temperature gradient in the outer layer toward the solidification contour can flatten out because of the lesser cooling effect provided in that zone, the increased cooling effect in zone 16 can be provided without risk of producing inhomogenities in the casting.

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such principles.

We claim:

1. A method of continuously casting bars of square cross section comprising the steps of continuously pouring molten steel into one end of an open-ended laterally enclosed longitudinally extending mold member and establishing a liquid surface therein spaced from the opposite outlet end thereof, dividing the interior of the mold member transversely of its longitudinal direction into a first zone having oppositely disposed surfaces convergently tapering from the liquid sur-. face toward the outlet end and a second zone having oppositely disposed surfaces arranged in parallel relationship extending from the downstream end of the first zone toward the outlet end with the longitudinally extending dimension of the first zone being such that it extends through the portion of the mold member where the casting normally contracts from the surface of the mold member, and continuously removing the cast bar having an exterior solidified layer and a liquid core from the outlet end of the mold member at such a rate that the solidified layer is maintained in contact with the tapered mold walls in said first zone and out of contact with the mold walls in said second zone so that a uniform symmetrical intense cooling effect takes place within the first zone and a lesser cooling effect takes place within the second zone.

2. A method as set forth in claim 1, comprising the step of guiding the bar from the outlet of the mold member.

3. A method as set forth in claim 2, comprising the step of cooling the bar as it exits from the outlet of the mold member.

4. A method as set forth in claim 3, comprising the step of engaging the surface of the bar at a location spaced exteriorly from the outlet of the mold member for withdrawing the bar from the mold member.

5. A method as set forth in claim 1, comprising the step of increasing the cooling effect within the mold member after a gap has been permitted to develop between the mold member and the bar.

6. A method as set forth in claim 5, comprising the step of decreasing the cooling effect within the mold member after increasing the cooling effect.

Claims

2. A method as set forth in claim 1, comprising the step of guiding the bar from the outlet of the mold member.
3. A method as set forth in claim 2, comprising the step of cooling the bar as it exits from the outlet of the mold member.
4. A method as set forth in claim 3, comprising the step of engaging the surface of the bar at a location spaced exteriorly from the outlet of the mold member for withdrawing the bar from the mold member.
5. A method as set forth in claim 1, comprising the step of increasing the cooling effect within the mold member after a gap has been permitted to develop between the mold member and the bar.
6. A method as set forth in claim 5, comprising the step of decreasing the cooling effect within the mold member after increasing the cooling effect.