CONTINUOUS STEEL CASTING MACHINE AND METHOD

Abstract: The object of the invention is to provide a continuous metal casting machine in which dimensional changes of the mold during the casting operation are measured and regulated to achieve a desired result in the cast metal. The continuous casting machine includes a copper tubular or plate mold (22) into which molten metal (25) is poured for forming a strand (25b) of high melting point metal, such as steel. A plurality of spray nozzles (29) are spaced around the mold for directing sprays (30) of coolant fluid against the mold. In accordance with the invention, one or more measuring devices (32) are associated with the mold for measuring changes in dimension of the mold in a direction transverse to the longitudinal axis thereof, and controls (36) are connected with the coolant fluid to adjust the rate of heat extraction effected thereby to thus regulate the extent of dimensional change in the mold as molten metal is being cast therein. The method of monitoring and adjusting the extent of dimensional change in a mold of a continuous casting machine as molten metal is being cast therein is also disclosed.

Description: Description Continuous Steel Casting Machine And Method Technical Field: This invention relates to high temperature metal continuous casting machines, and more particularly, to systems for cooling the mold with sprayed coolant fluid and for controlling the extent of expansion and contrac¬ tion of the mold during use. Background Art: In conventional continuous metal casting machines, molten metal is passed through a mold for solidification into a desired shape. As the molten metal passes through ' the mold, an outer shell forms and hardens. In many machines, the mold is vertically oriented and as the metal strand continues to solidify, it is bent through an angle of 90 so that it moves horizontally, and it is subsequent cut into individual segments. In the case of horizontal continuous casting machines, the strand is not bent through the 90 angle but exits the mold horizontally. In both vertical and horizontal machines, the strands are then cut into segments or predetermined lengths. The temperature of molten steel is typically 2850 °F, although with certain grades the temperature may be as low as 2600 °F. In general, although most of the references herein are to steel casting, the inven¬ tion contemplates the casting of any metal or metal alloy whose liquid temperature exceeds 2600 F. The mold which forms the steel strand contains the liquid steel and provides for its initial solidifi¬ cation, that is, hardening of the outer shell. The solidifying strand is extracted continuously from the bottom of the mold at a rate equal to that of the in¬ coming liquid steel at the top, the production rate being determined by the time required for the outer shell to harden sufficiently so as to contain the inner core of liquid steel by the time the strand exits the mold. The liquid steel is cooled in almost all present day casting machines by providing a water system which cir¬ culates a stream of cooling water around the mold. The water enters at the bottom of a pressure-tight vessel which surrounds the mold and travels upwardly in a directi opposite to that of the moving liquid steel. The "counter current" water flow has been found to be adequate for heat transfer in continuous steel casting machines. Such cooling systems embody a baffle jacket closely surround¬ ing the mold, defining an annular space through which the cooling water flows. It has been found in practice that when the mold heats and expands due to direct contact with the molten metal being cast, it has the effect of reducing the annular space available for cooling water circulation between the mold and the baffle jacket. This reduces the rate of heat extraction, permitting further heating and expansion of the mold, thereby further reducing the annular space and the amount of cooling water circulated for cooling. This condi ion -, continues, and in extreme cases can ultimately result in contact between the mold wall and the inside surface of the baffle jacket. When such contac occurs, total meltdown of the mold can result, but in any event, cooling is drastically reduced and the strand quality deteriorates. To attempt to control and maintain the relative positions of the mold wall and the baffle jacket, continuous casting machine designers sometimes weld small spacer bars or pins onto the interior surface of the baffle jacket. This method is relatively unsuccess¬ ful, however, since the mold expands in the area between the spacers, thus further increasing the deformation of the mold. Moreover, such mold deformation will actually cause rhomboidal bulging and other strand shape defor- mations. If mold expansion exceeds 0.0755 inch, the stresses generated in the mold will exceed the yield strength of the mold copper and result in permanent deformation of the mold, further exacerbating the de¬ terioration of strand quality and destroying the mold. In applicant's prior U.S. patent, number 4,494,594, a spray cooling system is disclosed which alleviates many of the problems found in the systems described above. The spray cooling system described in this patent is much more efficient in cooling the mold and controlling its expansion than are the prior art systems described above. As a result, higher quality cast steel strands are achieved when the spray cooling system disclosed in patent 4,494,594 is used. However, neither the baffle jacket design nor the prior spray cooling system disclosed by applicant in its prior U.S. patent 4,494,594 considers the differential expansion zones to be found in a continuous casting machine (meniscus, midsection, lower section) , and the prior devices lack any means for constantly monitoring mold expansion in these zones and adjusting the cooling rate to maintain a predetermined maximum expansion. This inability to control mold expansion with conventional mold cooling systems forces continuous casting machine operators or researchers to accept the adverse effects of uncontrolled mold expansion on the cast structure, and the associated adverse effects on subsequent proces¬ sing operations and finished product quality that can result. Disclosure of the Invention: In accordance with the present invention, a continuous metal casting machine is provided in which one or more parameters of the spray cooling system are defined and established to maintain the desired mold configuration and in which mold distortion is detected and corrected. Applicant has found that by utilizing certain conditions and by modifying other operating conditions as disclosed in prior U.S. patent number 4,494,594 it can accurately control mold expansion and consequently control the quality of the continuously cast strand within the mold. More specifically, applicant»has found that by carefully regulating the coolant directed against a zone in an area extendinq from approximately 25 inches above the meniscus level to approximately 2*. inches below the meniscus level, it can most effectively control the distortion of the mold. In most continuous casting machines for billets, slabs and other shapes, this zone is at a point roughly two to 14 inches below the top of the mold assembly. It is in this meniscus zone that approximately 70% of the heat of the molten metal must be extracted in order to initiate rapid and uniform solidification of the molten metal. Applicant has also determined that the maximum allowable mold expansion during casting with respect to surface and subsurface quality of the cast strand is about 0.0755 inch in directions transverse to the longitudinal axis of the mold, with 0.0550 inch being the preferred limit at the meniscus zone. Expansion of up to 0.0755 inch can be tolerated at the mid and lower zones without detrimental effect to the cast strand. If these maximums are exceeded, the cast strand can have: surface cracks, both transverse and longitudinal; subsurface cracks, from 0.025 inch to 1.250 inch below the surface; rhomboidal bulging and other shape deformations; and excessive oscil¬ lation mark depths (from 0.007 inch to 0.100 inch) . In addition, excessive mold wear can occur. To detect and correct these problems in accordance with the invention, applicant contacts a distortion measuring gauge or gauges to the outside (cooled) surfaces of the mold and monitors the expansion of each face during the casting operation. Expansion- of the mold on any or all desired faces is then controlled within selected limits by selectively adjusting the rate of heat extraction at that face. In achieving this accurate control of mold expan¬ sion during the casting operation, the following parameters are maintained in the spray nozzles: (a) spray angles of the cooling water nozzles at the meniscus zone are selected so as not to exceed 100°; (b) spray nozzle pressure at the meniscus zone is maintained at or above 15 psig; (c) spray pattern overlap at the meniscus zone is designed so that the coverage pattern between adjacent nozzles is uniform per unit area of sprayed surface; and (d) water droplet size in the meniscus zone is maintained in the range of from about 475 microns to about 1450 microns. At the other zones, expansion control is attained following the conditions set forth in applicant's prior U.S. patent 4,494,594. Since so much heat must be extracted from the molte metal at the meniscus zone, the steam barrier generated by the evaporating spray water must be penetrated by the water droplets in order to effectuate rapid and uni¬ form cooling. When the spray angle exceeds about 100 degrees, the force vector of the water droplets perpen- dicular to the surface of the mold to be cooled at the edges of the spray pattern is not sufficient to penetrate the steam barrier. Likewise, the water droplets will not penetrate the steam barrier when the water pressure at the nozzle is less than 15 psig. Moreover, since water droplet size influences the ability of the water spray to penetrate the steam barrier and also the rapidity with which heat extraction occurs, best operating con¬ ditions are obtained when droplet sizes are confined to the range set forth above. Most shape deformations of the cast product are initiated in the mold of the casting machine due to non- uniform expansion of the mold which results in non-uni¬ form contact between the metal being cast and the cooled mold wall. By operating within the above listed para- meters and by monitoring the extent of distortion of the mold wall during casting, it is possible to effect what¬ ever mold expansion conditions are desired by operators or researchers by increasing or decreasing the amount of cooling water sprayed onto the mold. The condition and quality of the cast product is also consequently controlled For example, if desired, one area of the mold can be permitted to expand while another area is simultaneously drawn in, or all mold areas can be controlled to the same degree of expansion/contraction. With the present invention, the operator of the casting machine can monitor expansion conditions through¬ out the casting process and react accordingly to conditions as they may occur within the mold, making minor or major adjustments to the cooling system as required, thereby influencing the quality of the solidifying strand. It is therefore possible to effect complete control of the solidification process on all continuous casting machines, producing billets, bloom, slabs, rounds or other shapes. Moreover, since the operator can monitor mold expansion, and hence strand and mold contact, he now has the ability to control cast strand and mold contact, mold stresses and elevated surface temperatures of the copper mold and thereby directly and beneficially influence mold wear and/or deterioration. Since mold deterioration due to high temperatures of the cast metal occurs most rapidly at the meniscus zone, the ability to continuously monitor and control mold expansion and temperature will allow the operator to directly and beneficially influence mold wear patterns. In slab casting, the mold consists of plates of copper held together at the corners to form a large cavity between the plates. Since in most cases, a slab has a considerably larger cross-sectional area, greater quantitie of heat must be removed and greater ferrostatic pressures must be contained by the copper mold. In conventional mold systems for slab casting, the above facts require further support systems on the back (cooled) surface of the copper plate. The mechanisms that make this support necessary are substantially the same as encountered in casting smaller (billet and/or bloom) cross sections. With the present invention, it is possible for the designer and operator to significantly reduce the back up support systems and in most cases eliminate secondary support systems in the mold altogether. Brief Description of the Drawings: Further objects, features and advantages of the invention will become apparent upon consideration of the following detailed description in conjunction with the drawings, in which like reference characters designate like parts throughout the several views, and wherein: Fig. 1 is a fragmentary view in longitudinal vertical section of the mold and baffle jacket of a prior art system; Fig. 2 is a transverse sectional view of the apparatus of figure 1; Fig. 3 is a view similar to figure 1, showing diagrammatically the effects of mold expansion relative to the coolant fluid space; Fig. 4 is a view similar to figure 2, showing the mold expansion depicted in figure 3; Fig. 5 is a view similar to figure 2 of a further form of prior art system in which spacers are used to attempt to control mold copper expansion and thus to alleviate the reduction of the coolant fluid space upon expansion of the mold; Fig. 6 is a view similar to figure 4 of the system of figure 5, showing the increased distortion of the mold when spacers are used; Fig. 7 is a longitudinal sectional view of a spray cooled mold section of a continuous casting machine incorporating the invention therein; Fig. 8 is a transverse sectional view of the system of figure 7; Fig. 9 is a somewhat enlarged, schematic view of a portion of a mold and a pair of adjacent spray nozzles, showing the overlap of the adjacent sprays to achieve substantially uniform spray coverage per unit area of the surface being sprayed; Fig. 10 is a diagrammatic view in transverse section of a mold in which the expansion of the various areas of the mold is controlled in accordance with the invention; Fig. 11 is a transverse sectional view of a portion of a slab mold, showing the manner in which the copper plates are secured together to form the mold; Fig. 12 is a view similar to figure 11, showing a back-up support system for the mold of figure 11; and Fig. 13 is a view similar to figure 11, schemati- cally illustrating the distortion of the slab mold plates caused by temperature and ferrostatic pressures. Best Mode for Carrying Out the Invention: Referring first to figures 1 through 6 and 11 through 12, prior art systems are depicted generally at 10 and 10', respectively. In figures 1 through 6, a mold 11 extends concentrically within a baffle jacket 12, defining an annular space 13 therebetween for cir¬ culation of coolant water as indicated by arrows A. As shown in figures 3 and 4, when the mold is heated by flow of molten metal therethrough, it expands and thereby decreases the size of the annular space 13, thereby altering the quantity and velocity of the cooling water sweeping over the mold surface and influencing heat transfer. In figures 5 and 6, an arrangement is shown at 14 which is an attempt to overcome the problem illustrated by figures 3 and 4. In this prior art arrangement, pins or spacers 15 are placed against the outer surface of the mold. However, as shown in figure 6, the mold deformation is not prevented by this system and, in fact, the spacers act to further increase the deformation of the mold. The slab mold 10' shown in figures 11 through 13 comprises a plurality of plates 16 bolted together to form the slab mold cavity 17. A back-up support system 18 is bolted onto the copper plates on both sides of the mold, as shown in figure 12. In figure 13, the back-up support system is eliminated and sprays of water are used to control expansion of the plates. For instance, a greater flow of cooling water could be applied in the vicinity of the middle of the mold and a lesser flow applied toward the edges. The present invention is represented generally at 20 in figure 7, and comprises a frame 21 in which a copper mold 22 with an open inlet end and an open out¬ let end is mounted at the top in registry with a central opening 23 through top wall 24 of the frame. In a typical construction, the frame 21 may be made of A-36 steel, and the mold tube may be made of DHP-grade copper. A stream of molten steel 25 is poured into the mold, at a rate relative to the rate of solidifi¬ cation and strand withdrawal, to position the meniscus 25a in the upper region of the mold, i.e. within a range of from about two inches to about 14 inches below the top of the mold. The mold is not connected at its bottom end to the bottom 26 of the frame, but instead simply hangs from its upper end - remaining free of connection with the bottom wall 26. The lower end of the mold is in alignment with an opening 27 in the bottom wall 26, through which the strand is withdrawn. Rather than the baffle jacket and annular space for cooling fluid as depicted in figures 1 through 6, the invention utilizes a plurality of spray pipes 28 spaced around the mold. Each pipe 28 carries a plurality of spray nozzles 29 for forming a spray of water as depicted at 30. Water is supplied to the pipes and nozzles by a supply pipe 31. The spacing of the nozzles relative to the mold and to each other is essentially the same as set forth in applicant's earlier U.S. patent 4,494,594. Further, the selection of nozzle sizes, water flow rate and water (nozzle) pressure are all essentially as set forth in said patent. However, as noted previously herein, the spray angle is set so that it does not go over about 100°, the spray overlap is selected so that the spray coverage is uniform through¬ out the area being sprayed, the droplet size of the spray particles is maintained in the range of from about 475 microns to about 1450 microns, the mold expansion is limited to a maximum of 0.0755 inch per face (0.0550 inch being preferred at the meniscus zone) , and the water pressure is not permitted to go below about 15 psig. With respect to spray overlap, reference is made to figure 9, which depicts the spray overlap in accordance with the invention. - Applicant has observed that approximately 80% of the total water flow produced by the spray nozzles occurs over about 50% of the cover¬ age area - shown here as occurring over a central area C located concentrically within the overall spray pattern 0 produced by the nozzle 29. Thus, the nozzles 29 are adjusted relative to one another and to the mold, taking into consideration the maximum spray angle and the water pressure, so that the central areas C of adjacent sprays just touch one another. In accordance with the invention, one or more distortion measuring instruments 32 are associated with the mold to measure the extent of expansion and contraction of the mold during the casting operation. As shown in figures 7, 8 and 9, these measuring instru¬ ments comprise an elongate arm 33 extending into proximity with the mold surface and having a probe or finger 34 projecting from the end thereof and into contact with the outer surface of the mold. An indicator 35 is connected with the probe to indicate displacements of the probe and hence the mold surface contacted by the probe. The indicator is preferably mounted so as to be readily visible to the operator of the casting machine or linked to a computer for automatic control and adjustment. Other types of deflection measuring instru- ments may be used, if desired. By observing the indicator, the operator can ascertain the extent of distortion or expansion of the mold. As noted pre¬ viously, it is important to maintain the distortion below about 0.0755 inch, and preferably below about 0.0550 inch at the meniscus zone. In order to main¬ tain the expansion of the mold within this limit, the flow of coolant fluid directed against the mold is controlled. This may be accomplished in a number of ways, including a flow controller 36 which may be man¬ ually operated or automatically operated in response to a sensed reading by the indicator (shown at 37 in dot-and-dash lines in figure 7). Further, the spray pipes 28 may be constructed in sections 28a, 28b, etc. , each supplying a number of nozzles 29 and supplied by its own supply pipe 31a, 31b, etc., controlled by a controller 36α, 36b, etc. (shown in dot-and-dash lines in figure 7) . Nozzles may be positioned to direct coolant fluid directly against the face of the mold, as shown in dot- and-dash lines at 38 in figure 8. By maintaining the operating conditions within the parameters described herein and by measuring and controlling the expansion and contraction of the mold during a casting operation, the extent of distortion of respective faces A, B, C and D can be accurately controlled as shown in dot-and-dash lines in figure 10. Consequently, the quality of the cast strand can be controlled and need not be accepted with defects therein as noted earlier herein, such defects being associated with scrap losses incurred during subse¬ quent processing of the cast strand and with the cost of metallurgical claims arising from the shipment of sub-standard finished products to end users. Although the invention has been described with reference to particular embodiments, it is to be under¬ stood that these embodiments are merely illustrative of the application of the principles of the invention. Numerous modifications may be made therein and other arrangements may be devised without departing from the spirit and scope of the invention.

Claims: Claims 1. A machine for continuously casting molten metal, comprising: a frame; a mold carried by the frame and having an open inlet end into which molten metal is poured and an open outlet end through which a solidifying, molded metal strand is withdrawn; means for directing a coolant fluid against the mold surface to extract heat from the mold and the metal being cast therein to initiate solidification of the metal; control means for controlling the coolant fluid flow to obtain the most uniform rate of heat extraction from the metal being cast and to minimize the extent of mold distortion arising from direct contact with the cast metal and thus control the quality of the cast metal strand; and distortion measuring means operatively associated with the mold for -detecting distortion of the mold as the heated metal moves therethrough to permit major or minor adjustments of the coolant fluid control means and thereby control the extent of distortion and further control the quality of the cast metal strand, 2. A continuous casting machine as in claim 1, wherein: the coolant fluid is water, and water spray means are disposed around the mold for directing sprays of water against the mold. 3. A continuous casting machine as in claim 2, wherein: the distortion measuring means include probes in contact with the outer surface of the mold to detect expansion and contraction of the mold in directions transverse to its longitudinal axis. 4. A continuous casting machine as in claim 3, wherein: the control means is operatively connected with the coolant fluid to adjust the flow of coolant fluid to thereby control the expansion of the mold. 5. A continuous casting machine as in claim 4, wherein: the control means is manually operated commens¬ urate with the observed distortion of the mold as detected by the measuring means. 6. A continuous casting machine as in claim 4, wherein: The control means is automatically operated to adjust the flow of coolant in response to the measured distortion of the mold as detected by the measuring means. 7. A continuous casting machine as in claim 2, wherein: the spray means comprises a plurality of spray nozzles spaced relative to one another and to the mold such that substantially uniform spray coverage of the area being sprayed is obtained. 8. A continuous casting machine as in claim 7, wherein: the spray nozzles are constructed such that the spray angle of water sprayed therefrom does not exceed 100 , the spray droplet size is in the range of from about 475 microns to about 1450 microns, and the spray pressure at the nozzle is not less than about 15 psig. 9. A continuous casting machine for continuously casting molten metal, comprising: a frame; a mold carried by the frame and having an open inlet end into which molten metal is poured and an open outlet end through which a solidifying molded metal strand is withdrawn; and spray means for directing coolant water against the surface of the mold to cool the mold and extract heat from the metal being cast therein to initiate solidification of the metal, said spray means including a plurality of spray pipes spaced around the mold and a plurality of spray nozzles carried by each spray pipe, said spray nozzles being constructed and arranged such that the angle of water sprayed therefrom does not exceed 100 , the spray droplet size is in the range of from about 475 microns to about 1450 microns, the spray pressure at the nozzle is not less than about 15 psig, and the sprays from adjacent nozzles overlap sufficiently to obtain substantially uniform spray coverage per unit area of the surface being sprayed. 10. A method of controlling mold expansion in a continuous casting machine having a mold into which molten metal is poured and from which a solidifying metal strand is withdrawn, and in which a coolant fluid is directed against the mold to cool the mold and metal therein to solidify the metal, comprising the steps of: measuring the extent of distortion of the mold during the casting operation; and controlling the coolant fluid in response to the measured mold distortion to thereby control the distortion and consequently control the quality of the cast metal strand. 11. A method as in claim 10, wherein: the coolant fluid is sprayed against the mold; and the flow of sprayed coolant fluid is adjusted to control the distortion of the mold caused by in¬ creased temperature of the mold. 12. A -method as in claim 11, wherein: the coolant fluid is spraved through a plurality of nozzles arranged around the mold such that the angle of fluid sprayed therefrom does not exceed 100 , the spray droplet size is in the range of from about 475 microns to about 1450 microns, the spray pressure at the nozzles is not less than about ^5 psig, and the sprays from adjacent nozzles overlap sufficiently to obtain substantially uniform spray coverage per unit area of the surface being sprayed.

Inventors: KURZINSKI CASS R, KURZINSKI, CASS, R.
Applicants: KURZINSKI CASS R, KURZINSKI, CASS, R.
Application year: 1985
Main classification: B22D11
IPC codes: B22D11