IL44406A - Process and apparatus for continuous casting of metals using an oscillatable mould - Google Patents

Abstract

Continuous casting of metal in which molten metal contacts a cooled axial mold wall surface to form a skin increasing in thickness in a direction toward a discharge opening of the mold. At least a portion of the last-mentioned mold surface increases in cross-sectional size over its length in the direction of the discharge opening. The casting is relatively withdrawn from the mold at an average casting rate during the casting operation and also the casting and the mold are oscillated relatively to one another in a particular manner. The maximum velocity capability of oscillation is greater than the casting rate. During oscillation and relative movement of the mold in the casting direction, a substantially constant, compressive loading of the last-mentioned mold surface portion on the skin of the casting is maintained throughout a portion of the last-mentioned movement. Concurrently with the relative movement of the mold in the casting direction, the resistance of the casting to mold movement is allowed to slow the mold velocity to approximately that of the casting, so as to minimize hot tearing of the skin of the casting. [US3857437A]

Description

444062 mana na'^T np'X'1? jpnm T^nn Process and apparatusafor continuous' casting of metals using an oscillatable mould TECIMICOH INSTRUMENTS CORPORATION 0/ 42562 File 1792-A This Invention relates to a process and apparatus for continuous casting of metals, and relates more particularly to such method and apparatus which is improved to provide increased casting rates and smoother surface finish of castings.

Heretofore, casting rates have been restricted in part by inefficient dissipation of heat from continuous castings within copied molds in which such castings are at least partially solidified to molded shapes. Such low heat issipation has been caused by lack of effective cooling contact of such molds with solidifying castings' therein.

The surface characteristics of castings made . by prior art continuous casting techniques have also lacked desirable uniformity and smoothness due to hot tearing of the solidifying casting skin within such molds. Hot tearing results from resistance between the mold and thin solidifying skin upon movement of the casting relatively to the' mold such as to create tensile stresses in the skin greater than the tensile strength of the skin. . Hot tearing may also result in a phenomenon, known as a "breakout" such as to cause cessation of the casting operation.

It Is known that mold oscillation during continuous casting of metal is advantageous for minimizing local heat stresses in a casting mold, . and such oscillation is shown and described, for example, in Junghans United States Patent vertical casting apparatus reciprocating the mold in a special manner, namely so that the mold is moved a. given distance in the casting direction at the same speed as the casting is withdrawn from the mold. Mold oscillation has also been utilized in horizontal continuous casting.

It is also known that in particular horizontal cast-r ing apparatus utilizing a stationary mold as shown and desrr cribed in Webbere and Williams United States Patent 3 , 642 , 058 issued February 15 , 1972 , the mold may have a tubular internal axial surface portion provided with a taper opening in the . casting direction to facilitate release of the casting by the mold on withdrawal of the casting, the taper being located, adjoining a molten metal delivery nozzle. Oscillating molds have been known to have a similar taper but in an opposite direction and, at least in part, for approximating the cross sectional shrinkage of the casting on progressive longitudinal cooling of the casting within the mold in the casting direct.' tion. That is, the internal axial mold surface follows generally such shrinkage in an attempt to increase heat transfer between the. casting and the mold in the area of the last-mentioned taper. The stationary mold of aforementioned Patent 3 , 642 , 058 also is provided with a taperec¾ surface portion of the last-^mentioned type, located adjoining the first-mentioned taper and extending therefrom in the casting direction.

When a mold of the oscillating type such as described above is moved relatively to the casting in a direction opposite the casting direction, the thin solidifying skin of the casting in contact with the mold is placed in tension longitudinally which may result in hot tearing. On the other pressure of the mold on the solidifying skin, such as to enhance heat transfer from the casting, is relieved. On the last-mentioned movement of the mold, the mold tends to lose contact with the casting skin unless hot tearing has previously occurred, and the tear has adhered to the mold.

The present invention includes the use of an oscillating mold in such casting of metals, which mold has a partir cular internal configuration throughout at least a portion thereof which, when the mold is oscillated, co-acts with the casting forming within the mold to minimize hot tearing of the casting skin and maximize heat transfer from the casting.

.. The' invention contemplates improvement in techniques of continuous casting of metals, applicable to vertical and horizontal casting operations and casting operations utilizing molds of the open-ended type and of the type having a closed end or a plug. It contemplates improved cooperation between a molten metal delivery nozzle and an open-ended mold utilized in continuous, casting of metals.

One object of the invention is to increase casting rates by improved cooling of a casting within and by a mold while at the same time improving the surface characteristics of such castings by effectively reducing hot tearing within a mold. Moreover the invention contemplates in this respect improved structure for heading any hot tearing.

Another object is to provide contact and axial com pression between the mold and the thin solidifying skin of the casting on each movement of the mold in the casting direction during oscillation to repair any hot tearing and enhance, heat transfer from the casting, while avoiding longitudinal o the mold In the opposite direction.

Still anpther object is to allow the resistance of the casting to movement of the mo].d in the casting direction to slow the mold from a greater velocity to approximately the casting rate or velocity, and maintain substantially constant the axial pressure of the mold on the casting throughout a portion of the movement of the mold in the casting direction.

According to the present invention there is provided a. process for the continuous casting of an elongated metal article by withdrawing molten metal from a source and causing said metal to solidify during casting, said process including the steps of casting metal from the source at an average cast-r ing speed, causing said metal partially to solidify during casting and Intermittently compressing the external surface of the partially solidified casting in both radial and axial directions .

According to the present invention there is further provided an apparatus for carrying out the method which appara-r tus includes a source of molten metal and a casting mold hav-ing an upstream end in communication with said source, wherein the cross 'sectional area of the internal surface of the mold increases at least part way along its length from the upstream end and wherein said mold is oscillatible along its length.

Other features of the invention will be apparent from the following detailed description of the presently preferred forms of the invention. Such features include the continuous casting of metal in which molten metal contacts a cooled axial wall surface to form a skin increasing in thick- ' ness in a direction toward a discharge opening of the mold. in cross-sectional size over its length in the direction of the discharge opening. The casting lis relatively withdrawn from the mold at an average casting rate during the casting operation and also the casting and the mold are oscillated relatively to one another In a particular manner. The maximum velocity capability of oscillation is greater than the casting rate. During oscillation and relative movement of the mo^Ld in the casting direction, substantially constant, compressive loading of the last-mentioned surface ' portion on the skin of .the casting Is maintained throughout a portion of the last-mentioned movement. Concurrently, with movement of the mold in the castin direction, resistance betwee the skin of the casting and the mold is allowed to slow the mold to approxi- " mately the velocity of the casting rate, so as to minimize hot tearing of the skin of the casting.

The invention will now be described in greater detail with reference to. he accompanying drawings, in which: Fig. ' 1 is a fragmentary schematic view of apparatus for continuous casting of metals, embodying the invention; ;.. '· ■ Fig. 2. is an enlarged broken and fragmentary view of the mold in cross section which may be employed in such apparatus and illustrates a particular internal gurface con--figuration of a portion of the mold, omitting details of the water Jacketing; Fig. 3 is an enlarged median sectional view of the apparatus of Fig. 1 illustrating the. latter in operation, omitting certain structural details of Fig. 1; Fig. 4 is a fragmentary diagrammatic view illustrating certain parts of the mold oscillating mechanism in certain Pig. 5 is a view similar to Fig. 4 but showing the parts in different relative positions, during such an oscillation cycle.

In the drawings, there is shown casting apparatus which is oriented horizontally for horizontal casting of metals but which may be used in a vertical orientation for a vertical casting if desired. In the form shown by way of example and with particular reference to Pigs. 1 and 3, a tundish, indicated generally at 10, is provided for receipt, as from a non-illustrated ladle of molten metal to form a reservoir for such molten metal, having a refractory lining element 12 and a molten metal delivery tube or nozzle 14 also of a refractory material. As shown in Pig. 3» the discharge end of the nozzle 14 is surrounded by a copper collar 16 which is secured to the tundish as by being bolted thereto as at .18. The collar is provided with a passageway 20 for circulation therethrough of a coolant such as water. Intermediate a portion of the collar 16 and the discharge portion of the nozzle 14 is a sleeve 22 formed of a heat-resistance material such as boron nitride, for example, and which may have the cross-sectional configuration shown in Pig. 3· The sleeve 22 may be held in axi£lly fixed relation to the collar 16 by suitable keys 24 and by abutment with the collar 16 in the manner shown, The sleeve 22 forms a continuation of the nozzle 14 for the discharge of molten metal.

The nozzle construction thus far described cooperates in this illustrated form with an open-ended mold 26 having in the wall structure thereof a passage 28 for the circula- · is movable axially on oscillation with reference to the nozzle structure. In the illustrated form the mold is slidingly supported on support element 30. The tundish may be supported in a conventional manner not shown. The cavity of the mold 26 has the desired cross-sectional shape of the casting to be produced. Suitable bearings 32 are interposed between the slidable mold 26 and the collar 16.

The forward end portion of the sleeve 22 of the nozzle structure which extends forwardly beyond the collar, has an outer peripheral surface 34 which is inclined in the manner shown to provide a cam surface for spring-biased sealing segments 36 interposed, between the sleeve 22 and the mold 26 and spaced forwardly of the collar 16. The sealing segments 36, which are formed of a heat-resistant material such as boron '. nitride or graphite coact internally with the mold and with one another around the interior^ of the mold for sealing purposes and each has a surface of cpmplemental shape to the ' cam surface 3 for sliding engagement therewith so that the seal may move generally inwardly toward and outwardly from the center line of the mold. A plurality of compression springs are provided urging the sealing segments 36 outwardly against the mold, the springs being indicated at 38. Each spring 38 may have one end thereof socketed in the collar 16 and the other end thereof socketed in the corresponding seal^ ing segment 36. The desired and selected maximum distance of oscillation of the mold in engagement with the seals-.36 is represented by the solid line position and the broken line position of the mold shown in Fig. 3.

It is to be noted as shown in Pig. 3 that the seals - structure and in the positions shown form a continuation of the surface 40 of the sleeve 22. Owing to the construction and arrangement of the seals 36, the seals, which are spring, biased, have a considerable forward and rearward component of movement capability on the cam surface 34 of the sleeve 22.

The seals prevent the passage of molten metal rearwardly there-past while allowing hot gases from molten metal to pass and escape in the last-mentioned direction.

As indicated in Pig. 3 molten metal 42 from the tundish 10 exits therefrom into the mold 26 through the above-described nozzle structure in a manner such that the molten metal contacts the cooled internal axial surface of the mold and solidifies thereagainst to form a skin 44 which, as the casting operation proceeds , increases in thickness in the cast-ing direction: which is to the right as viewed, in Fig. 3. The casting is indicated generally at 46. At the commencement of · a casting operation, : the usual dummy bar, inserted into the mold through the open end thereof remote from the tundish 10 and on which the molten metal solidifies, is withdrawn by the usual pinch rollers to commence the issuance of the casting from the mold. It will also be understood that the usual jets of cooling water are impinged on the casting along a portion of its length as it leaves the mold to solidify the casting further. . The dummy bar, the pinch rollers and the water jets are not shown.

The internal axial surface, of the tubular wa^l structure of the mold is provided with an axial taper opening in the casting direction and located at least within the area of the internal axial surface which is exposed or in contact little relative tensile strength, that is in an area where the skin is relatively newly formed. In the form illustrated by way of example in Pig. 2 the last-mentioned taper, indicated at 48, extends from the end of the mold which when the mold is assembled on the above-described nozzle structure is closest to the tundish 10. When the mold is assembled in the last- ■ mentioned manner the taper 48 extends in the casting direction beyond the nozzle structure, and the aforementioned seals 36, cooperate with the surface portion 48.

It is desirable that the taper 48 terminate in the casting direction a distance short of the discharge end of the mold, so that the internal axial surface of the mold may have forwardly and adjoining the taper 48 an axial surface portion which is untapered or which may have a taper converging in the casting direction or a combination of such surface por- . tions wherein the converging taper, hot shown, Is downstream of the untapered surface portion indicated at 50, to approximate the cross sectional shrinkage of the casting in the qasting direction for better heat transfer from the casting in this area of the casting where the casting skin is relatively thick and strong as compared to the aforementioned thin portion of the skin 44. .

The invention and the aforementioned configuration of the internal axial surface portion of the mold is applicable to the casting of metals Utilizing a mold of the type having a closed end or mold cavity bottom as illustrated arid described in United States Patent 3, 517*725 wherein the casting direction is in the direction toward the tundish from the mold. As shown and described therein the mold and the tundish are se arated durin a castin o eration formin a casting having a solidified skin or shell around a molten core, and the mold and the casting are oscillated relatively to one another. Molten metal, flowing through the core of the casting and entering the mold cavity, enters the cavity adjacent the bottom thereof and forms a thjn axial skin near the mold bottom which skin increases in thickness in the casting direction as the , casting proceeds. The casting J,s withdrawn . from the mold by solidification on a starting device supported on the tundish. For the purpose of describing and claiming the invention as applied to the casting of metals utilizing a mold of the closed end type, the molten metal inlet of the mold is considered as distinct and separate from the open discharge end of the mold. The molten metal .inlet is considered as being adjacent the bottom of the mold within the mold cavity.

With reference to the Oscillation of the mold, the mold has a maximum velocity capability in the casting or forward direction which is greater than the casting rate. The mold velocity in the forward direction is the sum of the casting rate, the component due to deformation of the casting by the mold and longitudinal shrinkage of. the casting.

Another feature of the mold oscillation is that the casting, during such forward movement, slows the mold to approximately the casting rate or velocity by the resistance of the skin of the casting to movement of the mold, Various devices or mechanisms such as an air cylinder may be utilized for ac-complishment of these . features . For example, limit switches may be utilized to fix the length of the stroke. However, the presently preferred oscillation mechanism, one form. of which is illustrated, utilizes instead - of an: .air 'cylinder:,1 a driver fdr.:.the mold whlch" i is In the form of a timed cam' or. crank.'·..:■. Eurt-herr;. the maximum compressive load of the mold on the skin of the casting lengthwise of the latter is predetermined, durin any portion of a cast and, may be adjusted during the cast. In addition; such compressive loading on. the skin may be. main-, tained substantially constant throughout a por- . tion of the forward stroke of the mold.

The general organization of the drive for oscillating the mold is shown diagrammatically in Pig. 1 , wherein a cam or crankwheel 52 is driven through suitable reduction gearr ing from a prime mover 54 . A link, indicated generally at 56 , self-adjusting as to length, interconnects the cam or crankwheel 52 with a flange 8 in fixed relation to the mold 26. In the form shown, the driving wheel 52 generates a sine wave but the wheel 52 may be so constructed as to gener-r ate other wave forms. In the form shown for illustrative purposes only, one end of the link 56 is pivoted (Pigs, 4 and 5 ) eccentrically to, the wh,eel 52 as at 60.. The other end of the link 56 is pivoted to the flange 58 of the mold as at 62. The motor 54 , which is of the. adjustable' speed type, drives the wheel 52 at a predetermined constant speed.

, The link 56 ,. best shown in Pigs. 4 and 5 > includes a pneumatic cylinder 64 having at one end thereof a rod extension: 66 fixed to the cylinder 64. The distal end of the rod 66 has the aforementioned pivot connection 60 to the wheel 52 . The link 56 also includes a driven member in the form of piston 68 in the cylinder 64, provided with a piston rod 70 having the a orementioned pivot connection 62 with the flange 58 of the mold. A source of compressed gas, not shown has an output 72 to a pressure regulator 74 and a gas line 76 connects the cylinder 64 and the regulator 74 . The pres-sure regulator 74 > which. is of a conventional type, not only governs the input to the cylinder 64' through the line 76 but has the function of. relieving pressure in the cylinder 64 through the line 76 to maintain a constant pressure therein regardless of the position of the piston 68 in the c linder.. He ce a line 6 is a two-wa as line.

The cylinder 64 arid piston 68 form a constant-rate air spring, the pressure of which is adjustable by the regulator 74. It will be understood that the torque of the wheel 52 is greater than the maximum anticipated desired load- ing of the air spring. The load of the air spring represents the. maximum load with which the mold 26 is moved in the forward direction, and may be changed during the casting operation to accommodate changing thermal conditions in the mold if desired.' The compressive loading of the mold on the forward stroke, is always sufficient to maintain intimate contact between the mold taper 48 and the casting for heat transfer from the latter. The load on the mold may also be adjusted in accordance with the particular metal being cast.

During the early part of a casting operation, owing - to thermal conditions in the mold, the casting skin may be relatively thin and weak and require less of a. load than, during a later part of the casting operation as when the temperature of the metal being poured has decreased by the amount of superheat lost as during the pouring of molten metal from , a ladle into the tundish over a period of time, say one hour for example. Upon such a reduction in the superheat in the metal, the molten metal may solidify faster in the formation of the casting skin. . .

The speed of the forward oscillation stroke is. in part always greater per. unit length than; the . casting rate at the same unit length. The difference may not be significant, but if operating conditions require it, the difference may be substantial. As is customary, the speed or rate of withdrawal of the casting,from the mold is adjustable and, as reviousl indicated the speed of the: driving wheel 52 may be adjusted by adjusting the speed of the motor 54. Usuall the proportional difference between the forward oscillation stroke of the. mold and the withdrawal rate of the casting is maintained throughout a casting operation. That is, when the castin speed is increased during a casting operation, the speed of the driving wheel .52 is increased correspondingly.

As has been made clear, the compressive load of the mold on the casting is through the taper 48 of the mold bearing on the skin of the casting during the forward stroke, and the load is relieved on the rearward stroke. The cam. or cranks-wheel 52 rotates in the direction of the arrow of Figs. 4 and 5> and at the instant in time in which the pivot 60 is in. the position of Fig. 4, the mold is at rest, the mold having been returned to the fullest extent of its rearward movement from a forward position by the pull of the cylinder 64 on the piston.68.

At the commencement of the forward stroke , that is from the position of the pivot 60 of Fig. 4 moving toward the position, of the pivot 60 of Fig. 55 the moId accelerates in the forward direction and catches up to the speed of the casting rate. It may pass the speed of the casting rate on acceleration, provided that the skin of the casting in. contact with the. taper 48 of the mold is sufficiently soft to be deformed by the mold, as the pivot 60 approaches the position of Fig..5 from the position of Fig. 4. If the casting ,s¾in, as the mold travels thereover, is sufficiently soft to be deformed by the compressive loading on the casting skin throughout the entire area of the skin which is contacted by In a forward , direction, with the piston 68 remaining in the cylinder-bottoming position of Fig. 4.

However> the resistance of the casting skin, to move-ment of the mold in a forward direction is usually such as to be greater than the compressive loading on the casting dictated by the air pressure in the cylinder 64, and this condition is shown in Fig. 5· During the angular movement of the driving wheel 52 from the position of Fig. 4 to that, of Fig. 5» the resistance of the casting to the mold movement has slowed the velocity of the mold to approximately that of the casting rate, and accordingly the piston 68 is displaced to the extent shown by way of. example in Fig. 5· When the mold is slowed to the casting :rate in this manner, the compressive loading on the casting remains substantially con-stant through a portion of the. forward mold stroke.

Because of the action of the air spring formed by the cylinder 64 and.the piston 68, the mold taper. 8 may re?-main in contact with the casting for a period of time after the velocity component of the cam on' the forward stroke has slowed to a speed below that of the casting, as the driving. wheel 52 rotates from the angular position of Fig. 5 through a further angle in a clock-wise direction thereof. Thus, the action of the air spring, when the piston 68 has been displaced in the cylinder 64 by resistance of the casting to mold movement, is. such as to attempt to effect the maximum,, forward stroke of the mold.

It is to be. noted that the length 'of .the forward stroke during an oscillation cycle s dependent upon the ' relative velocities of the casting and the mold and the re-sistance of t e castin to the mold movement. Further it is % made clear from the foregoing :that the casting, through Its resistance to mold movement on the forward stroke, itself changes the character of mold oscillation during a casting operation such as average mold velocity on the last-mentione stroke, effectively tending to achieve a character of . mold oscillation suitable to the then prevailing particular conditions of the casting operation including thermal conditions . During changing thermal conditions of a casting operation, the maximum compressive loading of the mold on the casting will remain constant, provided' that the regulated · air pressure in the cylinder 6 is not adjusted by the regulator 74. ; While the mold taper 48 is in contact with the rela-? tively thin skin of the casting there is substantial heat transfer from the casting. This Improved cooling of a casting within and by the mold makes possible an increase in casting rates while at the same time improving the surface characteristics of such casting by effectively reducing hot tearing within the mold. Further, the remaining objects of the Invention are achieved.

Claims (12)

File 1792-A

1. A process for the continuous ; casting of an elongated metal article by withdrawing molten metal from a source and causing said metal to solidify during casting, said process including the steps of casting metal from the source at an average casting speed,, causing said metal, partially to solidify during casting and intermittently compressing the ex* ternal surface of the partially solidified casting in both radial and axial directions.

2. A process according to claim 1, wherein the compression of the partially solidified casting, is effected periodicall .

3. A process according to claim 1 or 2, wherein the partially solidified casting is compressed along its length at a rate not substantially greater than the casting rate.

4. A process according to claim 3» wherein the rate of compression is controlled by the resistance of the solidifying cast metal.

5. Apparatus for carrying out the process of claim 1, and including a source for molten metal and a casting mold having an upstream end in communication with said source, wherein the cross sectional area of the internal surface of the mold increases at least part way alon its length from a the upstream end and wherein said mold is pscillat/ble along its length.

6. Apparatus according to claim 5, wherein tphe mold is an open ended mold.

7. · Apparatus according to claim 5 or 6, wherein a motor is coupled to the mold through a drive linkage to oscillate said mold.

8. Apparatus according to claim 7> wherein...said drive linkage includes a crank wheel, a rod connected thereto, a pneumatic ..cylinder at the end of said rod remote from said crank wheel, a piston displaceable within said cylinder and having a piston rod projecting axially from the cylinder end remote from the connecting rod and connected at its distal end with said mold and a regulated compressed gas. supply communicating with the compression chamber of said pneumatic cylinder .

9. Apparatus according to ciaim 8 wherein the crank wheel is rotatable to oscillate the mold at a rate greater than the rate at which the casting metal issues from the source.

10. Apparatus according to any of claims 5 to 9, wherein a pouring sleeve is connected around the outlet of the source and projects within the upstream end of the mold and wherein elements provide a seal between said sleeve and mold to prevent egress of molten metal.

11. Apparatus according to claim 3.0, wherein the sealing elements are spring biased wedges .. "

12. A process for the continuous casting of an elongated metal article substantially as hereinbefore .described. 13· Apparatus for the continuous casting of an elongated metal article substantially as hereinbefore described with reference to the accompanying drawings. ¾ ΑΛ