APPARATUS FOR IMPROVING CONTINUOUSLY CAST STRANDS
United States Patent 3831660
Method and apparatus are disclosed for improving the structure of continuously cast strand by separating undesirable hot metal inclusions at locations spaced from cooling wall surfaces of a continuous casting mold and removing at least a portion of the superheat of molten metal internally of the shape being cast. Apparatus is provided for delivering hot metal subsurface to molten metal level within a continuous casting mold with a component of motion opposite to the direction of casting, initially confining such metal, and changing its direction of movement for separating undesirable inclusions. Apparatus is provided for introducing solid metal below the slag and molten metal surface into the high temperature zone.
US Patent References:
FLOAT CONTROL VALVE FOR CONTINUOUS CASTING
Baier et al. - May 1970 - 3511304
METHOD OF PRODUCING STEEL BARS BY CONTINUOUS CASTING
Willim et al. - October 1970 - 3536122
CONTINUOUS CASTING PROCESS
Bunting, Jr. - April 1973 - 3726331
FLOAT CONTROL VALVE FOR CONTINUOUS CASTING
Baier et al. - May 1970 - 3511304
METHOD OF PRODUCING STEEL BARS BY CONTINUOUS CASTING
Willim et al. - October 1970 - 3536122
CONTINUOUS CASTING PROCESS
Bunting, Jr. - April 1973 - 3726331
Application Number:
05/349854
Publication Date:
08/27/1974
Filing Date:
04/10/1973
View Patent Images:
Export Citation:
Assignee:
National Steel Corporation (Pittsburgh, PA)
Primary Class:
Other Classes:
164/418
International Classes:
B22D11/111; B22D11/20; B22D41/50; B22D11/11; B22D11/10
Field of Search:
164/82,275,281,57-59
Primary Examiner:
Annear, Spencer R.
Attorney, Agent or Firm:
Shanley, And O'neil
Parent Case Data:
This is a division, of U.S. Pat. application Ser. No. 156,818, filed June 25, 1971 now U.S. Pat. No. 3,746,070.
Claims:
I claim
1. Apparatus for improving the final structure of continuously cast metal in which molten metal is delivered from a source of molten metal into a continuous casting mold, comprising
2. The apparatus of claim 1 in which the sleeve means include wall means defining such passage means for introducing solid metal additives into the continuous casting mold.
3. Continuous casting apparatus for improving the final structure of a continuously cast strand comprising
4. The apparatus of claim 3 including means for determining the temperature of molten metal within the continuous casting mold.
5. The apparatus of claim 4 including means for controlling the feed rate of solid metal into the continuous casting mold.
6. The apparatus of claim 3
7. Continuous casting apparatus for improving the final structure of continuously cast strand comprising
8. The apparatus of claim 7
9. The apparatus of claim 8 further including means for controlling the rate of feeding such continuous-length type solid metal additive.
10. Continuous casting apparatus for improving the final structure of continuously cast strand comprising
11. The apparatus of claim 10 in which
12. The apparatus of claim 1 in which the snorkel means and the sleeve means are unitary.
13. The apparatus of claim 12 including means for suspending the unitary snorkel and the sleeve means above a substantially vertically oriented continuous casting mold.
14. The apparatus of claim 1 including means for suspending the snorkel means and sleeve means independently from above a substantially vertically oriented continuous casting mold.
15. The apparatus of claim 1 further including means for supporting the sleeve means on the continuous casting mold.
1. Apparatus for improving the final structure of continuously cast metal in which molten metal is delivered from a source of molten metal into a continuous casting mold, comprising
2. The apparatus of claim 1 in which the sleeve means include wall means defining such passage means for introducing solid metal additives into the continuous casting mold.
3. Continuous casting apparatus for improving the final structure of a continuously cast strand comprising
4. The apparatus of claim 3 including means for determining the temperature of molten metal within the continuous casting mold.
5. The apparatus of claim 4 including means for controlling the feed rate of solid metal into the continuous casting mold.
6. The apparatus of claim 3
7. Continuous casting apparatus for improving the final structure of continuously cast strand comprising
8. The apparatus of claim 7
9. The apparatus of claim 8 further including means for controlling the rate of feeding such continuous-length type solid metal additive.
10. Continuous casting apparatus for improving the final structure of continuously cast strand comprising
11. The apparatus of claim 10 in which
12. The apparatus of claim 1 in which the snorkel means and the sleeve means are unitary.
13. The apparatus of claim 12 including means for suspending the unitary snorkel and the sleeve means above a substantially vertically oriented continuous casting mold.
14. The apparatus of claim 1 including means for suspending the snorkel means and sleeve means independently from above a substantially vertically oriented continuous casting mold.
15. The apparatus of claim 1 further including means for supporting the sleeve means on the continuous casting mold.
Description:
This invention is concerned with improving the final structure of continuously cast strand. In particular this invention is concerned with improving both surface characteristics and internal texture of continuously cast strands by directional control of the discharge and by control of the temperature of molten metal within a continuous casting mold. The invention is also concerned with increasing casting speeds in continuous casting operations.
The invention will be described in an environment for continuous casting of steel. The ability to consistently produce continuously cast steel suitable, as cast, for reduction to flat rolled product such as tinplate had not been fully developed prior to this invention.
The problem is compounded by the need to cast steels which are killed sufficiently to avoid the violent reaction of rimmed steels in the continuous casting mold. Killing steel most commonly involves aluminum additions. The formation of alumina results. Solidification of alumina and other inclusions at and under the surface of the casting has prevented achieving the surface quality required on the slabs, or other shapes, as delivered from continuous casters. However, it has been discovered that the problem is not merely a matter of surface inclusions. Solidification of metal at the mold walls proceeds sufficiently differently in relation to solidification centrally, that unequal stresses are set up, cracks are produced, and breakouts can occur. Slower casting rates have been tried in an attempt to avoid breakouts but this apparently does not solve the problems associated with a thin shell on the strand as delivered from the mold.
Notwithstanding use of relatively slow casting rates, the driving force and pressure of the molten metal, as introduced into the mold in the prior art, penetrates deeply into the casting, causing agitation and internal piping in the cast product. Also the agitation against the shell being formed and the superheat of such incoming metal are factors in preventing desirable shell thickness growth. This invention provides for increasing shell thickness of a continuously cast strand within the mold while increasing casting speeds over those previously available.
Significant contributions of this invention stem from systematically and uniformly regulating the movement of molten metal being discharged centrally within a continuous casting mold and controlling the superheat level of molten metal within the mold. The agitation caused by mechanical forces of the incoming metal and the separation of inclusions from such metal within the mold are concentrated at a location spaced from the mold walls to increase both surface cleanliness and strand shell thickness.
The accompanying drawings will be used in presenting a more detailed description and bringing out other advantages of the invention. In these drawings:
FIG. 1 is a schematic elevational cross-sectional view of continuous casting apparatus embodying the invention;
FIG. 2 is a schematic cross-sectional plan view of the apparatus embodying the invention;
FIG. 3 is a schematic elevational view, partially in cross section of apparatus embodying the invention showing modified support structure;
FIG. 4 is a schematic elevational cross sectional view of apparatus embodying the invention showing another embodimnt of support structure;
FIG. 5 is a plan view of apparatus embodying the invention showing another embodiment of support structure and adjustment means, and
FIG. 6 is an elevational view, partly in section of the apparatus of FIG. 5 .
In carrying out the objects of the invention, a snorkel device, of special configuration, is used to introduce molten metal in a particular fashion subsurface to the metal level within a continuous casting mold. In the inventive combination, a special, flow-through, "sleeve" structure is used to temporarily confine metal, effecting a change in direction of metal movement at the upper portion of the mold, spaced from the mold walls.
Several differing types of snorkels can be investigated in prior patents, for example in the U.S. Pats. to Junghans No. 2,224,303, No. 2,225,414, and No. 2,243,425 and the U.S. Pat. to Mills et al. No. 3,517,726. Also other patents disclose suspended structures, other than snorkels, within a continuous casting mold; for example Canadian Pat. No. 519,175 and U.S. Pat. No. 3,533,462. However, the prior art has not considered the particular problems undertaken nor the solutions provided by the present invention and none of the prior patents show the particular structures and combinations of the present invention.
In the invention, as shown in FIG. 1, snorkel 10 interconnects tundish 12 with continuous casting mold 14 providing an elongated passage between the two along the longitudinal axis of the snorkel. Stopper 16 controls the flow of molten metal 18 from tundish 12 into such longitudinal passage.
Note that the flow of metal from snorkel 10, i.e., the discharge of molten metal within the casting mold, is controllably directed in an upward direction. This controlled movement results from the orientation of upwardly directed apertures, such as 20 and 22. Also note that metal does not flow from the snorkel axially; axially directed discharge is prevented by snorkel terminating end 26. The flow of metal from apertures, such as 20 and 22, is upwardly and outwardly as indicated by arrows at 28 and 30. The introduction of molten metal and the removal of strand from the mold are controlled to maintain the desired molten metal level in the mold.
Unusual advantages of the invention stem from the use of a confining sleeve-like, open-ended box structure 32 in surrounding relationship to snorkel 10. The coaction between the snorkel and the sleeve, combining upwardly directed discharge and the confinement of metal spaced from the mold walls undergoing a change in direction of movement, helps produce the long sought surface quality in continuous casting. The sleeve 32 acts as a trap for the undesirable inclusions in the metal being cast and keeps such inclusions from solidifying against the mold walls. Also the agitation of the incoming metal is concentrated and essentially confined to the upper part of the mold within the "inclusion trap."
Note in the embodiment of FIG. 2 that snorkel 10 and the inclusion trap structure 32 can be integral and that the cross-sectional configuration of sleeve 32 approximates the cross-sectional configuration of the continuous casting mold. Considering the configuration aspects, an important teaching of the invention is that the peripheral spacing between the inclusion trap 32 and the casting mold 14 is selected so as to avoid freezing of molten metal along the molten metal meniscus 36 between the two structures.
The molten metal discharged from the snorkel and being directed as indicated by arrows 28 and 30 will wash against the inner surfaces of the sleeve structure in an upward direction and be temporarily confined while changing direction of movement within the sleeve. This induced action permits alumina and other inclusions to flow to the surface within the confines of the inclusion trap structure.
Casting powder is added within the inclusion trap to assist in this separation and help form a slag layer 40 within the confines of sleeve 32. Access to the upper end of the sleeve can be maintained to permit removal of inclusion accumulations. Casting powder is also added externally of the sleeve to form a cover layer 41 between the inclusion trap 32 and the mold walls. Both slag layers, internally and externally of the sleeve structure 32, have the purpose of limiting heat radiation from the metal thereby helping to avoid a freeze over. Also, the slag layers help to prevent oxidation across the surface of the molten metal, assist in lubrication and assist in removing inclusions.
One outstanding advantage of the sleeve structure and the upward flow of metal confined within the sleeve structure spaced from the continuous casting mold walls is the substantial elimination of the splashing of metal against the mold walls. As part of this contribution, the invention substantially eliminates the freezing of undesirable inclusions against the chilled mold walls as well as freezing of metal which could cause scaling.
Significant in this regard is the opportunity, provided by the inventive structure for metal to solidify to form the shell in the relatively calm area between the inclusion trap and the mold walls. The agitation of incoming metal at the mold walls in prior practice contributed to solidification and accumulation of many of the inclusions against the cast surfaces as solidifying metal was washed away by the incoming metal.
The strand shell, solidifying against the water cooled mold 14, is shown at 42 in FIG. 1. Substantially eliminated are the inclusions and inferior surface which had previously caused faults in product rolled directly from continuously cast strands unless extensive surface scarfing and the like, are substantially reduced or eliminated by the present invention.
Another important benefit of the inventive sleeve arrangement is the prevention of agitation deep within the molten center of the strand previously caused by the downwardly directed kinetic energy, ferrostatic head, and depth of penetration of the molten metal stream. The unusual aspect of the invention is that this benefit is provided notwithstanding the "flow-through" features of the sleeve maintained by the invention.
Considering this flow-through feature, note the absence of obstacles or barricades preventing downward flow within the mold. The unusual aspect is that this flow-through feature is provided and, at the same time, the momentum and pressure of the metal being cast are dissipated in an upward direction so as to avoid the deep penetration which previously extended 15 to 20 feet within the strand. With such deep penetration pockets of superheated metal could exist deep within the casting causing piping which would not heal.
Referring to FIG. 1, the molten metal flows downwardly within and outwardly from the sleeve as indicated by arrows 44 and 46. A portion of the flow-through metal rises around the outer surfaces of the sleeve and starts the solidification process against the mold walls surrounding the sleeve. It is important that the shell solidification starts in the quiescent, relatively inclusion-free, area between the sleeve structure and the mold walls. There are several additional factors which contribute to the lean rim provided by the invention. Any inclusions which might escape the trap structure tend to rise to the surface and do not readily adhere to the metal shell which has been formed on the mold walls. Any such alumina or similar types of inclusions moving against the sleeve is likely to adhere and accumulate on this refractory structure.
In practice the snorkel and sleeve structures can be unitary as shown in FIG. 2. In this way both structures can be suspended from the tundish by the snorkel sleeve and upper flange as shown in FIG. 1. Other support arrangements can be used without impairing the inventive contributions.
In FIG. 3 the sleeve 50 is supported on the continuous casting mold 14 by sleeve holders 52, 53. The snorkel 54 is suspended from the tundish 12 independently of the sleeve. The latter is connected to the mold 14 by bars such as 52 and 53 spaced about the sleeve and the mold so that access to the area between the sleeve and the mold is available for adding slagging powder, etc. With this type of structure the sleeve would oscillate if the mold oscillates, and its use may be restricted under certain circumstances. Molten metal flows axially downwardly through the axial passage of the snorkel 54 and upwardly from the snorkel outlets as described earlier in relation to FIG. 1.
As is known, heat transfer in conventional continuous casting operations occurs substantially entirely through the mold walls by transfer of heat through the solidifying steel. As the steel solidifies and shrinks away from the mold walls some thermal insulation occurs and the superheat in the metal tends to keep the shell thickness from increasing. However, novel additional heat removal methods and means are provided by the invention.
Included in the overall objective of the invention are improvements in the control of casting speed. An important aspect of the inventive combination which contributes to such purpose is the controlled addition of solid metal within the mold. Also, the location of the solid metal addition is important to the overall objective of the invention. Optimum operational efficiency and optimum effect from the solid metal addition are made possible by the means for introducing solid metal of the present invention.
Referring to FIG. 3, sleeve 50 comprises longitudinally extending walls of refractory material of sufficient thickness to provide the strength and durability required. Such sleeve walls are provided with openings, such as 56 and 58, extending longitudinally throughout the length of such peripheral walls. Through these longitudinally directed apertures, wire, rod, strand, web or the like, for example steel wires 60 and 62, which substantially fill the apertures within the sleeve walls, are fed into the molten metal. It should be noted that such solid metal makes its first substantial contact with the molten metal internally of the mold walls as the metal flows from the sleeve means. This is the ideal location to remove superheat and to control the temperature of metal. Also freeze-up of metal within the apertures is avoided.
Steel wire, or the like, is selectively fed at a predetermined and controlled rate into the metal contiguous to the exit side of the sleeve 50. The objective is to remove sufficient superheat from the metal to provide for more uniform cooling across the strand, decrease erosion of the shell by incoming metal, and to increase the skin thickness of the slab leaving the mold. With steel, the steel added should preferably have a carbon content higher than the steel being cast in order to facilitate dissolution.
The rate of feeding of the steel wire, or other shape, through the sleeve 50 is selected so as to obtain the desired solidification within the mold.
As brought out above, the sleeve means and the snorkel means coact with one another to bring about the special advantages enumerated. The sleeve means serves an additional purpose of providing methods and means for increasing heat removal from the molten metal within the mold at an optimum location. This additional aspect of the present invention functions in conjunction with the previously described coaction between the sleeve means and the snorkel means by virtue of the double purpose of the sleeve means. As the description of the heat removal feature of the present invention proceeds it will be obvious that this function of the sleeve means can be carried out in the absence of the snorkel means. The apertures in the sleeve wall permit the addition of wire, rod, or the like at a location to prevent the erosion of the shell by the superheat of the metal being added.
The present invention teaches the addition of solid metal at a preferred location subsurface to molten metal level in the high temperature zone of the mold to remove heat internally. The total rate of heat removal can be increased and, by this method, controlled more accurately for best results. Adding solid metal, as taught, to a caster operating at conventional casting rates decreases the liquid core depth and the time required for complete solidification. Alternatively, casting speeds can be proportionately increased or an intermediate casting speed can be selected. Melting of the wire is required, of course, for quality reasons. Therefore in the continuous casting of steel, the carbon content of the added solid steel is preferably increased above the carbon content of the metal being cast, e.g. if the steel being cast has a carbon content of about 0.05 percent C. then the steel wire can have a slightly higher carbon content, ranging up to about 0.1 percent C. This will insure dissolution of the added solid metal in the casting and will not significantly change the carbon content of the casting.
In steel casting, the feeding of the wire into the superheated steel also affects the solidification modes. Equiaxed grains tend to be formed and grow at a greater rate while the extent of the dendritic type growth will tend to be suppressed.
Referring to FIG. 3, steel wire 60 can be fed from coil 66 either manually or by feed rate drive means 68. Several factors can affect the rate of feed of the solid metal additive, for example, the desired casting speed, the optimum thickness of the shell 70 within the mold or upon exit from the mold, and uniformity of the shell thickness. However, a dominant factor in controlling the feed rate of added solid metal is the desired temperature level of metal within the mold after leaving the inclusion trap. Such temperature can be determined by a variety of known sensors and measurement steps. Such measurement can be fed into controller 72 for use in controlling the feed rate of the solid metal additive. The heat removal provided per unit weight of metal additive can readily be determined by one skilled in this art. The heat removal required based on the feed rate of molten metal can also be readily determined. Controller 72 can be computer operated. Other data fed into controller 72 can be the carbon content for the steel being cast, shell thickness and the speed of the strand. Output of computer controller 72 can include a signal for controlling solid metal feed rate, a signal for use in selectively controlling casting speed, and the like. Suitable computer and control apparatus for carrying out such teachings of the invention are available in the computer and electronic control art.
The optimum temperature for metal in the mold for particular casting speeds and materials can be predetermined. Controlling the feed rate of solid metal, in either lbs/unit time or lbs/ton cast, manually or by computer, can be responsive to such temperature.
As a representative example in the casting of steels of the coordinated control taught by the present invention, attempts should be made to eliminate the superheat of the steel within the mold so as to arrive at temperatures such as the following for the designated plain carbon steels:
Approximate Carbon Content Temperature ______________________________________ 0.10C 2785°F 0.20C 2770°F 0.40C 2745°F 0.60C 2720°F 0.70C 2700°F ______________________________________
These temperatures are the desired approximate temperatures within the mold below the sleeve 32. Several thermocouples, e.g. inserted through the wall of sleeve 32 can be used to measure and obtain the average temperature at the desired location. Also, knowing the approximate heat losses and the rate of casting, the temperature of the metal in the tundish can be measured and the amount of solid metal additive required to obtain the desired temperature within the mold can be calculated.
As shown in FIG. 2, solid metal additive apertures can be distributed about the periphery of sleeve 32 at locations 80-87. Remaining apertures include thermocouple sensing devices 92, 94 for measuring the temperature of metal within the mold after exit from the sleeve. A plurality of submerged thermocouples is used, distributed about the mold and the average temperature is obtained.
As stated, the temperature of metal in the mold can be calculated with accuracy suitable for operation from a temperature measurement in the tundish taken with temperature means 96 shown in FIG. 3.
Suitable refractory materials for the snorkel means and sleeve means disclosed are well known in the art. Similarly, powder additives and slag forming materials to be added are well known in the continuous casting art. The snorkel should be sealed where connected to the tundish, for example as shown at 97 in FIG. 3, so as to avoid syphoning of air into the metal and introduction of air below the surface of the bath. If desired, cover means (not shown) can be supported across the sleeve.
In FIG. 4 holding bracket means 98 is provided for supporting the sleeve structure. Bracket 98 is attached to tundish 100. In this embodiment the snorkel 102 is also supported by tundish 100. Sleeve 104 is suspended from tundish 100 by a series of spaced chains 106, 108, or other suspension means permitting access to and visibility of the metal and slag surface within the sleeve.
FIGS. 5 and 6 show an alternate embodiment of the invention providing independent support via the tundish means for both a snorkel means and an inclusion trap means.
In the scissors jack type of linkage of FIGS. 5 and 6, linkage arms interconnect tundish 110 and the inclusion trap means 112. Snorkel means 114 is separately connected to the tundish.
As shown in FIG. 6, upper linkage arms 115 and 116 are pivotally connected to the tundish at brackets 117 and 118. The lower linkage arms 119 and 120 are pivotally connected to the inclusion trap means 113 at ring means 121 and 122. The juncturew of the upper and lower linkage arms are interconnected by thread-mounted sleeves 125 and 126 to threaded crank arm 130. Crank mechanism 132 operates to raise and lower the inclusion trap means; the mechanical operation of such linkage means is well known. The raised position of linkage arms is shown in dotted lines at 136.
The embodiment shown permits visibility of and access to the mold area for making additions or raising and lowering of the inclusion trap. Also each of the embodiments provides for solid metal additions through the mold wall as described in relation to FIG. 3.
It is important that operational steps and dimensional aspects be selected for the slab or billet caster such that freezing of the metal across the meniscus between the sleeve and the mold walls is avoided. The size and shape of strands cast vary so that the sleeve is selected in relation to the mold size and product being cast to avoid such freezing. For example, when casting a 32 inch width slab of approximately 9 inch thickness the spacing at lateral ends between the sleeve and the mold walls may be about 5 inches at each end. This space is determined by the factors mentioned. Spacing along each side is effected by the minimum snorkel diameter required to avoid clogging or undue impedance to flow due to accumulation of alumina or the like. In general an overall outside diameter of about 4 inches is generally required for large slab casting of steel.
In practice the inclusion trap means can be raised until after start of the cast. The inclusion trap can be preheated, in the preferred method taught, by being lowered slowly into the superheated metal following the start of casting and proper mold level has been obtained. As the mold fills and withdrawal of the strand commences, the incoming metal is introduced subsurface to the metal level within the mold.
The inclusion trap means and snorkel means make unusual contributions in improving surface characteristics without the use of the solid metal additive teachings. Also a special coaction exists between the sleeve combination and the metal additive teachings since the location of the solid metal additions taught by the invention helps to prevent the erosion of the shell by the superheated molten metal exiting from the sleeve.
The present invention is especially advantageous in the continuous casting of steels suitable for sheets, plate and the like, but has similar advantages in the casting of other steels and metals. These advantages include elimination of surface inclusions and internal porosity, generally improving the quality of product, increasing casting capacity, and reducing breakouts in continuous casting operations.
In describing the invention apparatus has been set forth and methods described for reducing surface inclusions and improving internal characteristics of continuously cast strand by controllably introducing molten metal into a continuous casting mold so as to be discharged subsurface to molten metal level in the mold with a component of motion opposite to the direction of casting, initially confining such metal and reversing the direction of flow of the metal to permit removal of the maximum amount of the metal impurities, and coordinating this operation with, or optionally practicing separately, heat removal internally of the strand. While specific structures have been set forth it will be readily understood that variations in such structures and materials will be readily available to those skilled in the art based on the teachings of the present application. Therefore it is understood that the scope of the invention should not be limited to specific structures shown but is to be determined by reference to the appended claims.
The invention will be described in an environment for continuous casting of steel. The ability to consistently produce continuously cast steel suitable, as cast, for reduction to flat rolled product such as tinplate had not been fully developed prior to this invention.
The problem is compounded by the need to cast steels which are killed sufficiently to avoid the violent reaction of rimmed steels in the continuous casting mold. Killing steel most commonly involves aluminum additions. The formation of alumina results. Solidification of alumina and other inclusions at and under the surface of the casting has prevented achieving the surface quality required on the slabs, or other shapes, as delivered from continuous casters. However, it has been discovered that the problem is not merely a matter of surface inclusions. Solidification of metal at the mold walls proceeds sufficiently differently in relation to solidification centrally, that unequal stresses are set up, cracks are produced, and breakouts can occur. Slower casting rates have been tried in an attempt to avoid breakouts but this apparently does not solve the problems associated with a thin shell on the strand as delivered from the mold.
Notwithstanding use of relatively slow casting rates, the driving force and pressure of the molten metal, as introduced into the mold in the prior art, penetrates deeply into the casting, causing agitation and internal piping in the cast product. Also the agitation against the shell being formed and the superheat of such incoming metal are factors in preventing desirable shell thickness growth. This invention provides for increasing shell thickness of a continuously cast strand within the mold while increasing casting speeds over those previously available.
Significant contributions of this invention stem from systematically and uniformly regulating the movement of molten metal being discharged centrally within a continuous casting mold and controlling the superheat level of molten metal within the mold. The agitation caused by mechanical forces of the incoming metal and the separation of inclusions from such metal within the mold are concentrated at a location spaced from the mold walls to increase both surface cleanliness and strand shell thickness.
The accompanying drawings will be used in presenting a more detailed description and bringing out other advantages of the invention. In these drawings:
FIG. 1 is a schematic elevational cross-sectional view of continuous casting apparatus embodying the invention;
FIG. 2 is a schematic cross-sectional plan view of the apparatus embodying the invention;
FIG. 3 is a schematic elevational view, partially in cross section of apparatus embodying the invention showing modified support structure;
FIG. 4 is a schematic elevational cross sectional view of apparatus embodying the invention showing another embodimnt of support structure;
FIG. 5 is a plan view of apparatus embodying the invention showing another embodiment of support structure and adjustment means, and
FIG. 6 is an elevational view, partly in section of the apparatus of FIG. 5 .
In carrying out the objects of the invention, a snorkel device, of special configuration, is used to introduce molten metal in a particular fashion subsurface to the metal level within a continuous casting mold. In the inventive combination, a special, flow-through, "sleeve" structure is used to temporarily confine metal, effecting a change in direction of metal movement at the upper portion of the mold, spaced from the mold walls.
Several differing types of snorkels can be investigated in prior patents, for example in the U.S. Pats. to Junghans No. 2,224,303, No. 2,225,414, and No. 2,243,425 and the U.S. Pat. to Mills et al. No. 3,517,726. Also other patents disclose suspended structures, other than snorkels, within a continuous casting mold; for example Canadian Pat. No. 519,175 and U.S. Pat. No. 3,533,462. However, the prior art has not considered the particular problems undertaken nor the solutions provided by the present invention and none of the prior patents show the particular structures and combinations of the present invention.
In the invention, as shown in FIG. 1, snorkel 10 interconnects tundish 12 with continuous casting mold 14 providing an elongated passage between the two along the longitudinal axis of the snorkel. Stopper 16 controls the flow of molten metal 18 from tundish 12 into such longitudinal passage.
Note that the flow of metal from snorkel 10, i.e., the discharge of molten metal within the casting mold, is controllably directed in an upward direction. This controlled movement results from the orientation of upwardly directed apertures, such as 20 and 22. Also note that metal does not flow from the snorkel axially; axially directed discharge is prevented by snorkel terminating end 26. The flow of metal from apertures, such as 20 and 22, is upwardly and outwardly as indicated by arrows at 28 and 30. The introduction of molten metal and the removal of strand from the mold are controlled to maintain the desired molten metal level in the mold.
Unusual advantages of the invention stem from the use of a confining sleeve-like, open-ended box structure 32 in surrounding relationship to snorkel 10. The coaction between the snorkel and the sleeve, combining upwardly directed discharge and the confinement of metal spaced from the mold walls undergoing a change in direction of movement, helps produce the long sought surface quality in continuous casting. The sleeve 32 acts as a trap for the undesirable inclusions in the metal being cast and keeps such inclusions from solidifying against the mold walls. Also the agitation of the incoming metal is concentrated and essentially confined to the upper part of the mold within the "inclusion trap."
Note in the embodiment of FIG. 2 that snorkel 10 and the inclusion trap structure 32 can be integral and that the cross-sectional configuration of sleeve 32 approximates the cross-sectional configuration of the continuous casting mold. Considering the configuration aspects, an important teaching of the invention is that the peripheral spacing between the inclusion trap 32 and the casting mold 14 is selected so as to avoid freezing of molten metal along the molten metal meniscus 36 between the two structures.
The molten metal discharged from the snorkel and being directed as indicated by arrows 28 and 30 will wash against the inner surfaces of the sleeve structure in an upward direction and be temporarily confined while changing direction of movement within the sleeve. This induced action permits alumina and other inclusions to flow to the surface within the confines of the inclusion trap structure.
Casting powder is added within the inclusion trap to assist in this separation and help form a slag layer 40 within the confines of sleeve 32. Access to the upper end of the sleeve can be maintained to permit removal of inclusion accumulations. Casting powder is also added externally of the sleeve to form a cover layer 41 between the inclusion trap 32 and the mold walls. Both slag layers, internally and externally of the sleeve structure 32, have the purpose of limiting heat radiation from the metal thereby helping to avoid a freeze over. Also, the slag layers help to prevent oxidation across the surface of the molten metal, assist in lubrication and assist in removing inclusions.
One outstanding advantage of the sleeve structure and the upward flow of metal confined within the sleeve structure spaced from the continuous casting mold walls is the substantial elimination of the splashing of metal against the mold walls. As part of this contribution, the invention substantially eliminates the freezing of undesirable inclusions against the chilled mold walls as well as freezing of metal which could cause scaling.
Significant in this regard is the opportunity, provided by the inventive structure for metal to solidify to form the shell in the relatively calm area between the inclusion trap and the mold walls. The agitation of incoming metal at the mold walls in prior practice contributed to solidification and accumulation of many of the inclusions against the cast surfaces as solidifying metal was washed away by the incoming metal.
The strand shell, solidifying against the water cooled mold 14, is shown at 42 in FIG. 1. Substantially eliminated are the inclusions and inferior surface which had previously caused faults in product rolled directly from continuously cast strands unless extensive surface scarfing and the like, are substantially reduced or eliminated by the present invention.
Another important benefit of the inventive sleeve arrangement is the prevention of agitation deep within the molten center of the strand previously caused by the downwardly directed kinetic energy, ferrostatic head, and depth of penetration of the molten metal stream. The unusual aspect of the invention is that this benefit is provided notwithstanding the "flow-through" features of the sleeve maintained by the invention.
Considering this flow-through feature, note the absence of obstacles or barricades preventing downward flow within the mold. The unusual aspect is that this flow-through feature is provided and, at the same time, the momentum and pressure of the metal being cast are dissipated in an upward direction so as to avoid the deep penetration which previously extended 15 to 20 feet within the strand. With such deep penetration pockets of superheated metal could exist deep within the casting causing piping which would not heal.
Referring to FIG. 1, the molten metal flows downwardly within and outwardly from the sleeve as indicated by arrows 44 and 46. A portion of the flow-through metal rises around the outer surfaces of the sleeve and starts the solidification process against the mold walls surrounding the sleeve. It is important that the shell solidification starts in the quiescent, relatively inclusion-free, area between the sleeve structure and the mold walls. There are several additional factors which contribute to the lean rim provided by the invention. Any inclusions which might escape the trap structure tend to rise to the surface and do not readily adhere to the metal shell which has been formed on the mold walls. Any such alumina or similar types of inclusions moving against the sleeve is likely to adhere and accumulate on this refractory structure.
In practice the snorkel and sleeve structures can be unitary as shown in FIG. 2. In this way both structures can be suspended from the tundish by the snorkel sleeve and upper flange as shown in FIG. 1. Other support arrangements can be used without impairing the inventive contributions.
In FIG. 3 the sleeve 50 is supported on the continuous casting mold 14 by sleeve holders 52, 53. The snorkel 54 is suspended from the tundish 12 independently of the sleeve. The latter is connected to the mold 14 by bars such as 52 and 53 spaced about the sleeve and the mold so that access to the area between the sleeve and the mold is available for adding slagging powder, etc. With this type of structure the sleeve would oscillate if the mold oscillates, and its use may be restricted under certain circumstances. Molten metal flows axially downwardly through the axial passage of the snorkel 54 and upwardly from the snorkel outlets as described earlier in relation to FIG. 1.
As is known, heat transfer in conventional continuous casting operations occurs substantially entirely through the mold walls by transfer of heat through the solidifying steel. As the steel solidifies and shrinks away from the mold walls some thermal insulation occurs and the superheat in the metal tends to keep the shell thickness from increasing. However, novel additional heat removal methods and means are provided by the invention.
Included in the overall objective of the invention are improvements in the control of casting speed. An important aspect of the inventive combination which contributes to such purpose is the controlled addition of solid metal within the mold. Also, the location of the solid metal addition is important to the overall objective of the invention. Optimum operational efficiency and optimum effect from the solid metal addition are made possible by the means for introducing solid metal of the present invention.
Referring to FIG. 3, sleeve 50 comprises longitudinally extending walls of refractory material of sufficient thickness to provide the strength and durability required. Such sleeve walls are provided with openings, such as 56 and 58, extending longitudinally throughout the length of such peripheral walls. Through these longitudinally directed apertures, wire, rod, strand, web or the like, for example steel wires 60 and 62, which substantially fill the apertures within the sleeve walls, are fed into the molten metal. It should be noted that such solid metal makes its first substantial contact with the molten metal internally of the mold walls as the metal flows from the sleeve means. This is the ideal location to remove superheat and to control the temperature of metal. Also freeze-up of metal within the apertures is avoided.
Steel wire, or the like, is selectively fed at a predetermined and controlled rate into the metal contiguous to the exit side of the sleeve 50. The objective is to remove sufficient superheat from the metal to provide for more uniform cooling across the strand, decrease erosion of the shell by incoming metal, and to increase the skin thickness of the slab leaving the mold. With steel, the steel added should preferably have a carbon content higher than the steel being cast in order to facilitate dissolution.
The rate of feeding of the steel wire, or other shape, through the sleeve 50 is selected so as to obtain the desired solidification within the mold.
As brought out above, the sleeve means and the snorkel means coact with one another to bring about the special advantages enumerated. The sleeve means serves an additional purpose of providing methods and means for increasing heat removal from the molten metal within the mold at an optimum location. This additional aspect of the present invention functions in conjunction with the previously described coaction between the sleeve means and the snorkel means by virtue of the double purpose of the sleeve means. As the description of the heat removal feature of the present invention proceeds it will be obvious that this function of the sleeve means can be carried out in the absence of the snorkel means. The apertures in the sleeve wall permit the addition of wire, rod, or the like at a location to prevent the erosion of the shell by the superheat of the metal being added.
The present invention teaches the addition of solid metal at a preferred location subsurface to molten metal level in the high temperature zone of the mold to remove heat internally. The total rate of heat removal can be increased and, by this method, controlled more accurately for best results. Adding solid metal, as taught, to a caster operating at conventional casting rates decreases the liquid core depth and the time required for complete solidification. Alternatively, casting speeds can be proportionately increased or an intermediate casting speed can be selected. Melting of the wire is required, of course, for quality reasons. Therefore in the continuous casting of steel, the carbon content of the added solid steel is preferably increased above the carbon content of the metal being cast, e.g. if the steel being cast has a carbon content of about 0.05 percent C. then the steel wire can have a slightly higher carbon content, ranging up to about 0.1 percent C. This will insure dissolution of the added solid metal in the casting and will not significantly change the carbon content of the casting.
In steel casting, the feeding of the wire into the superheated steel also affects the solidification modes. Equiaxed grains tend to be formed and grow at a greater rate while the extent of the dendritic type growth will tend to be suppressed.
Referring to FIG. 3, steel wire 60 can be fed from coil 66 either manually or by feed rate drive means 68. Several factors can affect the rate of feed of the solid metal additive, for example, the desired casting speed, the optimum thickness of the shell 70 within the mold or upon exit from the mold, and uniformity of the shell thickness. However, a dominant factor in controlling the feed rate of added solid metal is the desired temperature level of metal within the mold after leaving the inclusion trap. Such temperature can be determined by a variety of known sensors and measurement steps. Such measurement can be fed into controller 72 for use in controlling the feed rate of the solid metal additive. The heat removal provided per unit weight of metal additive can readily be determined by one skilled in this art. The heat removal required based on the feed rate of molten metal can also be readily determined. Controller 72 can be computer operated. Other data fed into controller 72 can be the carbon content for the steel being cast, shell thickness and the speed of the strand. Output of computer controller 72 can include a signal for controlling solid metal feed rate, a signal for use in selectively controlling casting speed, and the like. Suitable computer and control apparatus for carrying out such teachings of the invention are available in the computer and electronic control art.
The optimum temperature for metal in the mold for particular casting speeds and materials can be predetermined. Controlling the feed rate of solid metal, in either lbs/unit time or lbs/ton cast, manually or by computer, can be responsive to such temperature.
As a representative example in the casting of steels of the coordinated control taught by the present invention, attempts should be made to eliminate the superheat of the steel within the mold so as to arrive at temperatures such as the following for the designated plain carbon steels:
Approximate Carbon Content Temperature ______________________________________ 0.10C 2785°F 0.20C 2770°F 0.40C 2745°F 0.60C 2720°F 0.70C 2700°F ______________________________________
These temperatures are the desired approximate temperatures within the mold below the sleeve 32. Several thermocouples, e.g. inserted through the wall of sleeve 32 can be used to measure and obtain the average temperature at the desired location. Also, knowing the approximate heat losses and the rate of casting, the temperature of the metal in the tundish can be measured and the amount of solid metal additive required to obtain the desired temperature within the mold can be calculated.
As shown in FIG. 2, solid metal additive apertures can be distributed about the periphery of sleeve 32 at locations 80-87. Remaining apertures include thermocouple sensing devices 92, 94 for measuring the temperature of metal within the mold after exit from the sleeve. A plurality of submerged thermocouples is used, distributed about the mold and the average temperature is obtained.
As stated, the temperature of metal in the mold can be calculated with accuracy suitable for operation from a temperature measurement in the tundish taken with temperature means 96 shown in FIG. 3.
Suitable refractory materials for the snorkel means and sleeve means disclosed are well known in the art. Similarly, powder additives and slag forming materials to be added are well known in the continuous casting art. The snorkel should be sealed where connected to the tundish, for example as shown at 97 in FIG. 3, so as to avoid syphoning of air into the metal and introduction of air below the surface of the bath. If desired, cover means (not shown) can be supported across the sleeve.
In FIG. 4 holding bracket means 98 is provided for supporting the sleeve structure. Bracket 98 is attached to tundish 100. In this embodiment the snorkel 102 is also supported by tundish 100. Sleeve 104 is suspended from tundish 100 by a series of spaced chains 106, 108, or other suspension means permitting access to and visibility of the metal and slag surface within the sleeve.
FIGS. 5 and 6 show an alternate embodiment of the invention providing independent support via the tundish means for both a snorkel means and an inclusion trap means.
In the scissors jack type of linkage of FIGS. 5 and 6, linkage arms interconnect tundish 110 and the inclusion trap means 112. Snorkel means 114 is separately connected to the tundish.
As shown in FIG. 6, upper linkage arms 115 and 116 are pivotally connected to the tundish at brackets 117 and 118. The lower linkage arms 119 and 120 are pivotally connected to the inclusion trap means 113 at ring means 121 and 122. The juncturew of the upper and lower linkage arms are interconnected by thread-mounted sleeves 125 and 126 to threaded crank arm 130. Crank mechanism 132 operates to raise and lower the inclusion trap means; the mechanical operation of such linkage means is well known. The raised position of linkage arms is shown in dotted lines at 136.
The embodiment shown permits visibility of and access to the mold area for making additions or raising and lowering of the inclusion trap. Also each of the embodiments provides for solid metal additions through the mold wall as described in relation to FIG. 3.
It is important that operational steps and dimensional aspects be selected for the slab or billet caster such that freezing of the metal across the meniscus between the sleeve and the mold walls is avoided. The size and shape of strands cast vary so that the sleeve is selected in relation to the mold size and product being cast to avoid such freezing. For example, when casting a 32 inch width slab of approximately 9 inch thickness the spacing at lateral ends between the sleeve and the mold walls may be about 5 inches at each end. This space is determined by the factors mentioned. Spacing along each side is effected by the minimum snorkel diameter required to avoid clogging or undue impedance to flow due to accumulation of alumina or the like. In general an overall outside diameter of about 4 inches is generally required for large slab casting of steel.
In practice the inclusion trap means can be raised until after start of the cast. The inclusion trap can be preheated, in the preferred method taught, by being lowered slowly into the superheated metal following the start of casting and proper mold level has been obtained. As the mold fills and withdrawal of the strand commences, the incoming metal is introduced subsurface to the metal level within the mold.
The inclusion trap means and snorkel means make unusual contributions in improving surface characteristics without the use of the solid metal additive teachings. Also a special coaction exists between the sleeve combination and the metal additive teachings since the location of the solid metal additions taught by the invention helps to prevent the erosion of the shell by the superheated molten metal exiting from the sleeve.
The present invention is especially advantageous in the continuous casting of steels suitable for sheets, plate and the like, but has similar advantages in the casting of other steels and metals. These advantages include elimination of surface inclusions and internal porosity, generally improving the quality of product, increasing casting capacity, and reducing breakouts in continuous casting operations.
In describing the invention apparatus has been set forth and methods described for reducing surface inclusions and improving internal characteristics of continuously cast strand by controllably introducing molten metal into a continuous casting mold so as to be discharged subsurface to molten metal level in the mold with a component of motion opposite to the direction of casting, initially confining such metal and reversing the direction of flow of the metal to permit removal of the maximum amount of the metal impurities, and coordinating this operation with, or optionally practicing separately, heat removal internally of the strand. While specific structures have been set forth it will be readily understood that variations in such structures and materials will be readily available to those skilled in the art based on the teachings of the present application. Therefore it is understood that the scope of the invention should not be limited to specific structures shown but is to be determined by reference to the appended claims.