US3293707A

US3293707A - Methods in continuous casting

Info

Publication number: US3293707A
Application number: US324005A
Authority: US
Inventors: Olsson Erik Allan
Original assignee: Individual
Current assignee: Individual
Priority date: 1960-09-07
Filing date: 1963-11-15
Publication date: 1966-12-27
Anticipated expiration: 1983-12-27
Also published as: GB935046A; DE1290667B

Description

Dec. 27, 1966 E. A. OLSSON METHODS IN CONTINUOUS CASTING 2 Sheets-Sheet 1 Filed Nov. 15, 1963 INVENTOR. ERIC A. OLSSON ATTORNEY-5'.

Dec. 27, 1966 E. A. OLSSON METHODS IN CONTINUOUS CASTING 2 Sheets-Sheet 2 Filed NOV. 15, 1963 INVENTOR. ERIC A. OLSSON M A TTORNE Y5:

United States Patent 3,293,707 METHODS IN C(BNTINUOUS CASTING Erik Allan Olsson, Zurichstrasse 66, Kusnacht, Zurich, Switzerland Filed Nov. 15, 1963, Ser. No. 324,005 Claims priority, application Sweden, Sept. 7, 1960, 8,557 4 Claims. ((11. 22200.1)

This application is a continuation in part of application Serial No. 135,345, filed August 31, 1961, now abandoned.

In continuous casting, molten metal is supplied to the one end of the mold passage of a cooled mold and simultaneously the partly solidified ingot is drawn out of the other end of the mold passage. The portion of the molten metal which comes into contact with the cold wall of the mold is immediately solidified to a thin layer, or skin, which increases in thickness and strength in the direction of ingot withdrawal by heat being conducted away through the wall of the mold. As the ingot is continuously drawn out of the mold, friction arises between the solidified skin layer of the ingot and the wall of the mold and there is also a tendency of a portion of the molten metal and solidified skin to stick to the wall of the mold, especially at the place where the molten metal being poured meets the Wall of the mold. By greasing the wall a reduction of this sticking tendency as well as of the friction between the ingot and the mold is obtained. In spite of greasing, it may, nevertheless occur that a portion of the skin of solidified metal nearest the entry of the melt, where the melt is introduced, sticks to the wall of the mold so that the necessary force to draw out the ingot breaks the solidified skin layer at a place where said layer, because the ingot has shrunk away from the wall of the mold, is weakened in comparison to other places. The risk that such a break in the skin will continue or follow the ingot towards the exit end of the mold is very great, and, for this reason, eruptions of molten metal through the skin of the ingot after it has left the mold have occurred while casting in vertical casting machines. Of course, the same risk occurs in horizontal casting.

In order to avoid this disadvantage it has been proposed many years ago to cast in a reciprocating mold. Herewith the advantage Was achieved that grease could be introduced and distributed in the mold through the existing clearance between the mold and the nozzle which introduces the molten metal and projects somewhat into the mold. Later, several different methods have been proposed to reduce the above mentioned risk of break by means of a movement of the mold. The method which has been used most extensively, is based on a constant and positively guided reciprocating movement (up and down) of the mold which causes a constant change of the contact line between the upper surface of the melt and the wall of the mold while the ingot is drawn away continuously, whereby the stresses on the walls of the mold are reduced and the heat-transfer from the mold is increased. Furthermore, this conventional method enables the surface of the wall of the mold exposed by the reciprocation to be efiiectively greased. By having the mold following the ingot in the casting direction the skin or the wall of the ingot has the opportunity to increase its thickness and strength during a short period in each cycle without being affected by the friction forces which occur when a relative movement takes place between the ingot and the wall of the mold, especially if the wall of the mold is poorly greased. One conventional apparatus for reciprocating the mold includes a frame, or table, supporting the mold and resting on springs which by means of draw-bars and cam-discs are compressed a certain distance, e.g. 20 or 30 mm., with the same velocity as the velocity with which the ingot is drawn out of the mold. The return movement of the mold, however, is many times 3,293,707 Patented Dec. 27, 1966 faster than the casting velocity, so that the synchronous movement of the mold and ingot lasts the greatest possible part of one working cycle. A mold movement which is controlled in this way, and usually driven by gearing from the pinch rollers which draw the ingot out of the mold, cannot under all conditions be in absolute synchronism with the ingot skin in the mold, as the magnitude of the ingot shrinkage changes in dependence on the cooling conditions and the casting velocity. Therefore, it is also customary in some conventional practices to impart to the mold a movement which is somewhat faster than the velocity of the movement of the ingot, whereby the ingot skin within the mold is not subjected to tensile stress during the synchronous movement. But this practice can result in deteriorated surfaces on the ingot because new molten metal can be fed downwardly into the mold, between the mold and the already solidified skin to press said skin inwardly of the ingot and allow the formation of a second skin of later solidified metal which is not integral with the initially solidified portion of the ingot.

The described, positively guided movement of the mold under conventional practices demand special mechanical means which make the whole plant more expensive.

In order to avoid the disadvantages of the above described, conventional methods, it has been proposed to draw the ingot continuously out from the mold as long as the friction, or resistance against relative movement between the solidified metal skin and the mold is below a limit value, the drawing being interrupted when this resistance exceeds said limit value, and the drawing starting again when the resistance has fallen below this value. In this conventional method, the mold is movable against the action of means which exert an upwardly directed force, e.g. compression springs, to prevent the downward movement of the mold together with the ingot and if such movement takes place return the mold to its upper position. Thus the ingot is always subjected to a drawing force corresponding to the friction force of the solidified skin against the mold wall.

A further known method is characterized in that the melt is filled into a stationary mold to a certain level and then the drawing of the ingot takes place until the melt level in the mold has sunk to a certain lower level at which time the drawing is stopped until the melt again has reached the upper level. The disadvantage of this method is that a uniform secondary cooling of the step- Wise fed ingot through the cooling means is difiieult to obtain.

The present invention primarily aims to obviate the above stated disadantages of conventional casting methods and relates to a method of continuous casting in which, irrespective of the casting velocity or the velocity of the withdrawal of the ingot from the mold, an absolute synchronism is obtained between the mold and the ingot within the mold during a predetermined time period, or portion of the movement cycle of the mold, while the ingot skin formed within the mold is periodically subjected to a suitable excess pressure in the casting direction that its velocity is braked to synchronism by the friction and/ or adhesion between the mold and ingot. With the method of the invention, the movement of the mold in both directions can be arranged independent of the casting extraction drive means and can be obtained by independent driving means, e.g. a motor with a cam disc, pneumatic and/ or hydraulic cylinders.

The excess pressure or force applied to the mold may he obtained by balancing, or adjusting of the weight of the mold for an appropriate or desired movement in the casting direction in combination with or independent of other means (e.g. springs, counterweight-s, pneumatic and/or hydraulic cylinders) for the return of the mold in the direction opposite the casting direction.

According to the invention the independent excess force applied to move the mold in the casting direction is adjusted so that the mold free velocity (velocity in the absence of frictional or adhesive resistance to movement exerted by the casting) in said direction does not essentially exceed the maximum casting velocity.

It is also possible to adjust the force acting on the mold in the casting direction, eg, by balancing, or adjusting, the weight of the mold, so that the pressure the mold exerts on the ingot in the casting direction on account of the resistance against relative movement between these two elements prevents appreciable deformation of the ingot.

The novel features that are considered characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of a specific embodiment when read in connection with the accompanying drawings, wherein like reference characters indicate like parts throughout the several figures and in which: a

FIG. 1 is a schematic elevational view, partly in section, of a vertical continuous casting apparatus according to the invention, and

FIGS. 2-9 are sectional diagrams illustrating various condition-s Within an ingot during reciprocation of the mold.

An embodiment of a device for carrying out the method according to the invention is schematically shown in FIG. 1, in which 1 denotes a mold and 2 a frame carrying the mold and supported by fluid pressure cylinders 3.

The frame 2 with the mold 1 tends to move downwardly by its own weight. Molten metal is supplied from the ladle 11 to the mold 1 to form a cast ingot which is continuously withdrawn by pinch rollers 8 driven by independent drive means 12. A magnet valve 4 is either set so that the cylinders 3 communicate with the atmosphere (or a container if liquid is used) via a valve 5, which can be set in relation to pressure or outflow velocity of the pressure medium or so that the pressure medium flows into the cylinders at a pressure which is controlled by a reducing valve 6. The magnet valve is influenced by a limit-position switch 7 at the upper-andlower position respectively of themold. Of course, other systems for changing the flow direction of the pressure medium can be chosen, eg a magnet valve controlled by time switches or time relays, and if for instance air is used the return movement may be divided in two phases, one pressure thrust and thereafter an expansion of the air in the cylinders until the turning position 'of the mold is reached. A directly controlled reversing valve may be used which is reversed in the upper and lower positions of the mold.

The force produced by the weight of the frame 2 and mold 1 in the casting direction is chosen such as to be slightly greater than necessary to move the mold in synchronism with the ingot and such that friction and adhesion of the solidified ingot skin to the mold walls operate as a braking force to keep the mold and ingot in synchronism. The force applied to the mold is not so much greater as to overcome this friction and adhesion and cause the mold to move faster than the ingot in the casting direction. With this application of the greater force to the mold, a longitudinal pressure exists in the unsolidified portions of the ingot, arising out of the friction and adherence of the ingot skin to the mold Walls, such that should shrinkage fissures, or the like, appear in the surface of the ingot, on the withdrawal stroke of the mold, the unsolidified casting material above such fissures under the influence of the longitudinal force will be pressed into the fissures to fill them. The pressure'cylinders serve to move the mold upwardly, or in the stripping direction, the pressure fluid entry to the cylinders being controlled by the switches 7, 7 and magnet valve 4. When the mold is moved in the casting direction, the pressure fluid in cylinders 3 is released gradually by valve 5 so as to balance the weight of the mold and frame. Adjustment of valve 5 also provides an adjustment of the force moving the mold in the casting direction. In vertical casting it is also possible to use mechanical counter weights acting against the weight of the mold. In horizontal casting a force acting in the casting direction must of course be applied to the mold, as its own weight cannot be used in which case mechanical, hydraulic or pneumatic means can be used in similar way as mentioned above, but acting in the casting direction. The value of the force in the casting direction varies with the dimensions of the casting as well as with the grade of the steel or other metal to be used and is determined by investigating the casting as regards surface faults. Should it be observed that the casting has a tendency to be upset, the force must be lowered. On the contrary, if there are transverse fissures it is obvious that the force is too low and/ that the velocity of the mold is too low, so that there occurs a tension stress during the movement of the mold in the casting direction.

As an example it can be mentioned that in casting a rod having a square crosssection of x 80 mm. and comprising carbon-steel of 0.l00.40% carbon it has been found suitable to use a force of about 30 kg.

In FIGS. 2-9 are shown illustrative examples of a casting at various stages of reciprocating mold movement which will serve to explain the advantage according to the present invention of applying a slightly greater force to the mold in the casting direction than necessary to synchronize the mold with ingot withdrawal and allowing the mally this sticking tendency disappears rather soon after the moment the solidified layer is formed, but the friction occurs until the thickness and strength of the layer has grown to such an extent, that the gradual progress of the shrinkage (the decrease of the parameter) gives rise to a clearance between the Wall of the mold and that of the casting. Beyond the described forces, or'stresses which may be termed inner stresses, there is a further tension stress, which occurs due to the positive drawing of the casting out from the mold, i.e. due to the relative movement between the wall of the mold and that part of the casting, which still has contact with the wall of'the mold. The stresses acting in the same direction often result, in transversal fissures in the surface of the casting and, in some cases, the casting is totally broken in a zone, where the solidified layer is weakest. Due to the shrinkage of the casting, which can vary with the casting velocity and With changed cooling conditions, there arises during the following cooling of the casting a longitudinal tension stress between the withdrawal rolls and the contact portion of the casting with the mold, which, of course, is added to said above mentioned stresses In the present invention the mold during a period (down stroke) is subjected to a force greater than that necessary to move synchronously with the skin of the casting, Which is in frictional contact with and sticks to the mold, but is braked by the sticking and friction to move synchronously with the casting.

The braking restraint causes a force in the casting skin in the longitudinal direction of the casting which reduces the stresses causing the fissures. Thus in spite of synchronization a longitudinal pressure is maintained in the ingot casting which overcomes the internal tensile pressure. Beyond this, it is possible to simplify the equipment for the continuous casting and also to use a more favorable speed due to the fact that the stripping stroke can be chosen irrespective of the casting velocity used. Furthermore, there is obtained more favorable conditions for healing fissures, which may occur in the surface of the casting, e.g. during the stripping stroke especially with poor greasing of the wall of the mold.

The mentioned sticking tendency and friction tend to disappear as the ingot moves toward the exit of a mold, which means that the part of the casting which has solidified sufliciently to contract, during the synchronous stroke portion is subjected to longitudinal pressure, at least below the zone between A and B in FIGS. 2-9. This is the zone in which said sticking and friction can hold the mold in equilibrium of movement with the casting continuously leaving the mold. It seems to be self-evident that this pressure may balance the inner tension stresses in the longitudinal direction caused by the above mentioned factors, before a clearance is formed between the solidifying layer and the mold. Thus, stresses are caused, i.e. due to the ferrostatic pressure from the inner, not yet solidified molten metal, which stresses can be counteracted by or partly or wholly balanced by the introduction of a force or pressure in the opposite direction. FIG. 2 shows the mold 1 in its lowest position. The solidified portion 20 of the casting contains molten metal 30, which presses the still flexible wall of the casting against the wall of the mold in the region between A and B. Due to the increasing thickness, strength and shrinkage of the wall 20 of the casting below AB, this lower portion of the wall is released and separates from the wall of the mold, so that a clearance 60 is formed. The molten metal is continuously delivered to maintain the top of the ingot constantly at level A, and in the level B said clearance 60 arises irrespective of the position of the mold. Suppose there are two marks a and b in the mold in the level A and B respectively, when the mold is in its lower end position, according to FIG. 2, and that a mark a is made in the surface of the casting opposite mark a and another mark b opposite the mark b. When the mold rapidly moves from the lowermost position to its uppermost position, as shown in FIG. 3, i.e. the disstance 50 during /2 second, the distances aa and bb' increase rapidly. While a is still in contact with the wall of the mold, new molten metal is supplied above said point and has reached the wall of the mold to which it has a tendency to stick. But this sticking tendency normally disappears very soon. However, the friction between the surface (skin) 20 of the casting and the mold remains in the area from the upper surface of the molten metal down to the level B. The sticking action and the friction brake the subsequent downward movement of the mold, so that a synchronous movement is obtained during a period of about 3 seconds.

As the casting is withdrawn and the mold reciprocates, the mark b will as shown in FIG. 4, drop below the level B, where the friction has ceased, wherefore the solidified layer 20 of the ingot in this level as well as below it, is rapidly subjected to a pressure on down strokes of the mold if the mold is moved with a force greater than necessary for synchronism but restrained or braked to synchronized speed. During the whole synchronous stroke, molten metal is continuously supplied from above, wherefore the zone of sticking and friction moves upwardly relative to the wall of the mold, while simultaneously below this zone, (i.e. in the level B) sticking and friction progressively disappear. Thus said longitudinal pressure on the surface of the casting will continuously occur on mold downstrokes as the contact between the surface of the casting and the mold ceases and the clearance 60 is formed. The tension stresses in the longitudinal direction induced in the unfree shrinkage in the region between A and B will progressively be changed by the desired compressive stresses during the synchronous period. The instant or the position for this change of stresses depends on the shrinkage factor of the cast material. The only way to obtain a corresponding pressure in the longitudinal direction beyond that described above according to the invention, is to use the relative movement between the mold and the casting as mentioned before, i.e. when the mold is actually moved faster than the casting in the casting direction, a method which is not desired by the applicant for the reason previously mentioned.

The preceding four paragraphs described the primary concept of the invention which if followed normally provides a faultless (unfissured) outer layer with the lowest possible inner tension stresses. Should, however, fissures arise during the stripping stroke, the method according to the present invention aids in healing such fissures. The molten metal, which is continuously supplied above the meniscus at a (FIG. 5) at the time for the stripping stroke, is assumed to give such a great tendency of sticking, e.g. on account of poor greasing of the surface of the mold, that the sticking together with the friction present between A and B causes a break 31 (FIG. 5) in the solidified layer 20 of the casting during the upward stroke. Thus the upper part 21 of the layer 20 follows the mold upwardly a distance such that the mark a for a moment is lifted above the surface of the molten metal in the mold. During the synchronous portion of the stroke the mold is, however, filled simultaneously as the mold is moved downwards (FIG. 6) and new molten metal reaches the wall of the mold above a, which metal has the above mentioned sticking tendency and causes together with that part of the casting lying between A and B a resistance against the relative movement. Thus the fissure or break, 31, which is situated in a region, where this resistance does not occur is subjected to a pressure so that the molten metal, which has been penetrated into the fissure, acts as an effective adhesive. Thus the healing takes place under pressure, which results in a good bond.

Should the casting conditions be so bad, that the solidified layer 20 is definitely broken during the stripping stroke,- so that a sleeve 21 of the layer follows the upper end of the mold to its uppermost position, that shown in FIG. 7 will happen. The clearance 32 between sleeve 21, which is connected to the mold and follows the mold during the stripping stroke, and the continuously withdrawn casting is directly filled by new molten metal 33, FIG. 8, which meets the cold wall of the mold, rapidly solidifies and produces the braking effect of the downwards movement tendency of the mold during the synchronous stroke. Thus, as the mold, so to say is hanging on the layer 33 just formed between the wound edges 44 and 45 due to the fact that the sleeve thereabove may in the meantime have loosened, the new layer is pressed against the wound edge 45. Should, however, the friction between the sleeve 21 and the mold be maintained long enough, this sleeve will be pressed downwards against the new layer and thus also the wound edge 44 is healed under pressure (FIG. 8), which is of advantage for the strength during the following stripping stroke. The stresses in the wound edge 44 during the stripping stroke can be considered to be small due to the fact, that the sleeve 21 has had more time to shrink away from the wall of the mold and as no new molten metal is supplied above this sleeve (abovev a) there is no longer any tendency for sticking above the break place. When new molten metal has reached above a, i.e. when a has come below the level A, FIG. 9, it can with certainty be said that the wound edge 44 is subjected to a pressure, when the new formed layer causes the braking effect on the mold.

Although a certain specific embodiment of the invention has been shown and described, it is obvious that many modifications thereof are possible. The invention, therefore, is not to be restricted except insofar as is necessitated 7" I by the prior art and by the spirit of the appended claims.

What is claimed is:

1. A method of continuous casting with a reciprocating mold, comprising the steps of continuously pouring molten metal into one end of a mold open at opposite ends, and continuously withdrawing apartly solidified casting from the opposite end of the mold while applying opposite sequential forces to reciprocate the mold back and forth in the line of casting withdrawal, the reciprocating force applied in the advancing direction of casting during each cycle of reciprocation being of such strength that the mold in absence of friction and sticking resistance should attain a greater velocity than the casting but due to said resistance is braked to an extent that the casting and the mold have substantially the same velocity, whereby during simultaneous movements of the mold and casting in the same direction the mold exerts a longitudinal pressure on the casting to compress any fissures in the skin of the casting.

2. A method according to claim 1, wherein said reciprocating force applied to the mold in the advancing direction of casting during each cycle of reciprocation is so chosen that the mold velocity, in the absence of said friction and sticking resistance, does not essentially exceed a desired highest casting speed.

3. A method according to claim 1, wherein said re ciprocating force applied to the mold in the advancing direction of the casting during each cycle of reciprocation is the weight of the mold modified and partially compensated for so that the longitudinal pressure the mold exerts on the casting in said direction exceeds the force moving the casting by an amount substantially equal to the frictional and sticking resistance against relative movement between the mold and casting, whereby appreciable deformation of the casting is prevented.

4. A method according to claim 1, wherein said reciprocating force applied to the mold in the direction opposite to the advancing direction of the casting is such that the mold achieves a stripping speed independent of the casting speed.

References Cited by the Examiner UNITED STATES PATENTS 11/1938 Junghans 2257.2 X 3/1962 Littlewood 22-57.2

OTHER REFERENCES McBride and Daney: Continuous Casting, Interscience Publishers, 1962, TN 750, M3 C2, pp. 6, 13, 14, 20 and, 21.

J. SPENCER OVERHOLSER, Primary Examizzen,

R. S. ANNEAR, Assistant Examiner.v

Claims

1. A METHOD OF CONTINUOUS CASTING WITH A RECIPROCATING MOLD, COMPRISING THE STEPS OF CONTINUOUSLY POURING MOLTEN METAL INTO ONE END OF A MOLD OPEN AT OPPOSITE ENDS, AND CONTINUOUSLY WITHDRAWING A PARTLY SOLIDIFIED CASTING FROM THE OPPOSITE END OF THE MOLD WHILE APPLYING OPPOSITE SEQUENTIAL FORCES TO RECIPROCATE THE MOLD BACK AND FORTH IN THE LINE OF CASTING WITHDRAWAL, THE RECIPROCATING FORCE APPLIED IN THE ADVANCING DIRECTION OF CASTING DURING EACH CYCLE OF RECIPROCATION BEING OF SUCH STRENGTH THAT THE MOLD IN ABSENCE OF FRICTION AND STICKING RESISTANCE SHOULD ATTAIN A GREATER VELOCITY THAN THE CASTING BUT DUE TO SAID RESISTANCE IS BRAKED TO AN EXTENT THAT THE CASTING AND THE MOLD HAVE SUBSTANTIALLY THE SAME VELOCITY, WHEREBY DURING SIMULTANEOUS MOVEMENTS OF THE MOLD AND CASTING IN THE SAME DIRECTION THE MOLD EXERTS A LONGITUDINAL PRESSURE ON THE CASTING TO COMPRESS ANY FISSURES IN THE SKIN OF THE CASTING.