Claims
- 1. A method of open channel strip casting molten material through a casting nozzle onto a rotating substrate with reduced freezing of the molten material along refractory surfaces of said casting nozzle and for providing improved uniform molten material flow onto said substrate, said method comprising the steps of:
- a) providing a container vessel having a refractory floor and refractory sidewalls for holding said molten material;
- b) providing a casting nozzle having refractory walls connected to said container vessel;
- c) providing a cooled rotating substrate which is spaced from nozzle sufficiently far to insure said nozzle does not contact said substrate and close enough to said nozzle to insure the molten material does not leak between said nozzle and said substrate;
- d) providing a nozzle casting weir which is positioned about 0.25 to about 2 inches (about 6 to about 50 mm) from said substrate, said weir having a bottom central gap between said weir and said nozzle floor up to about 0.75 inches (about 20 mm) and bottom weir edges tapered to increase the gaps at said edges to provide an increased localized flow of molten material along said nozzle refractory walls to increase the volume of localized hot material to reduce sticking and provide a more uniform casting flow across the width of said nozzle.
- 2. The method of claim 1 wherein said container vessel has a flat refractory bottom floor.
- 3. The method of claim 1 wherein said container vessel has a refractory floor sloped upwardly towards said substrate at an angle of about 30.degree. to 60.degree..
- 4. The method of claim 1 wherein said weir edge gap is about at least 15% more than said central gap opening.
- 5. The method of claim 1 wherein said central weir portion is 90 to 95% of the total weir width.
- 6. The method of claim 1 wherein said rear wall of said weir is tapered.
- 7. The method of claim 1 wherein said sidewalls of said weir and nozzle are tapered.
- 8. The method of claim 6 wherein said rear wall taper is from 15.degree. to 75.degree..
- 9. The method of claim 7 wherein said sidewall taper is from 80.degree. to 90.degree..
- 10. The method of claim 1 wherein said casting material is a ferrous material.
- 11. The method of claim 1 wherein said substrate is rotated at a speed of 50 to 5,000 feet per minute (about 15 to 1500 meters per minute) and said cast strip is about 0.001 to 0.1 inches (0.025 to 2.5 mm) thick.
- 12. The method of claim 1 wherein said casting flow is pressurized to further increase the flow rates by providing said container vessel with a cover having an opening through which a pressurizing gas is introduced.
- 13. An apparatus for open channel strip casting comprising:
- a) a container vessel for storing molten material;
- b) a cooled rotating substrate;
- c) a refractory nozzle connected to said container vessel and positioned about 0.001 to about 0.03 inches (about 0.025 to about 0.75 mm) from said substrate, said nozzle having an outer surface conforming to the shape of said substrate; and
- d) a weir positioned within said nozzle at about 0.25 to about 2 inches (about 6 to about 50 mm) from said substrate and spaced about 0.05 to 0.75 inches (about 1 to about 19 mm) above the nozzle floor in the central portion and spaced at least about 15% further from the floor at the edges of said weir.
- 14. The apparatus of claim 13 wherein said weir has a tapered rear wall.
- 15. The apparatus of claim 14 wherein said rear taper is from 15.degree. to 75.degree..
- 16. The apparatus of claim 13 wherein said weir has tapered sidewalls.
- 17. The apparatus of claim 16 wherein said taper is from 80.degree. to 90.degree..
- 18. The apparatus of claim 13 wherein said nozzle has a sloped floor.
- 19. The apparatus of claim 13 wherein the edges of said weir have a gap above said nozzle floor which is at least 25% larger than at said central portion of said weir.
- 20. The apparatus of claim 13 wherein said central portion of said weir is at least 90% of said total length.
- 21. The apparatus of claim 13 wherein a container vessel weir is provided in said container vessel to control slag and improve the flow of molten metal into the container vessel.
- 22. The apparatus of claim 13 wherein additional pressurizing means are provided to increase the flow of molten metal through the nozzle.
- 23. The apparatus of claim 21 wherein a roof is provided with said container vessel to pressurize said molten metal flow.
- 24. The apparatus of claim 14 wherein said weir rear wall is tapered at an angle of 30.degree. to 60.degree..
FIELD OF THE INVENTION
The Government of the United States of America has right in this invention pursuant Contract No. DE-FC07-88ID12712 awarded by the U.S. Department of Energy.
The present invention relates to a system for the continuous casting of thin strip or foil which may be crystalline or amorphous. The system uses a casting method wherein the melt pool is not contained by the casting nozzle on its upper surface and provides an improved flow of molten material from a pool onto a cooled rotating substrate.
Continuous casting molten strip requires the critical control of bath conditions if the strip is to be uniform. The temperature of the molten material, the length of pool contact with the rotating substrate, the flow rates within the nozzle, and the bath composition must all be controlled precisely if the cast strip is to be uniform. Any slag on the bath surface must be restrained.
Prior strip casting methods for regulating the flow of molten material have varied widely depending on the casting method. The melt overflow method relies mainly on the height of the molten pool and its proximity to the rotating substrate. The method uses a nozzle which is open at one end and does not contain the top surface of the pool. Weirs, dams or baffles in the pouring box have been used to prevent the flow of slag onto the substrate, control initial filling of the vessel and control the height of the pool. The rotating speed of the substrate and the strip thickness produced will determine the flow rate from the pool.
Baffles have been provided in the center of the pool near the substrate to slow the flow of metal in the middle to approximate the edge conditions where the sidewalls restrict the flow rates. The center of a flowing stream will always flow fastest with uniform conditions because there are fewer obstructions to retard flow.
Another important consideration to develop uniform cast strip is the ability to control turbulence which is related to flow rates and edge conditions. It has been proposed by some that turbulence may help reduce ripples in the bath and some nozzles were sloped downward at the lip to induce turbulence. U.S. Pat. No. 4,819,712 stated that a transverse horizontal bar was placed in the flow path below the melt surface and closely adjacent the casting surface to induce turbulence and help reduce ripples. It was concluded, however, that turbulence was immaterial and the bar was removed.
Another important influence on the cast strip uniformity is the shape of the nozzle adjacent the rotating substrate. U.S. Pat. No. 4,819,712 developed a downwardly sloped or curved lip in the discharge area of the tundish. A great change in flow direction in the meniscus area was thought to minimize ridges in the cast strip.
Slag control is required for uniform composition and strip thickness. As far back as U.S. Pat. No. 2,383,310, people have used a device to control the slag layer during strip casting. However some modern casting systems have used only a contoured tundish lip without weirs or baffles such as U.S. Pat. No. 4,819,712.
Another example of flow control in strip casting is U.S. Pat. No. 4,715,428 which uses partially submerged plates 36 to develop uniform flow. These plates baffle or dampen the flow to obtain uniform flow across the width of the tundish and restrain the flow of surface oxides and slag.
U.S. Pat. No. 4,828,012 argued U.S. Pat. No. 4,715,428 reference did not suggest the use of these plates for the control of channeling and temperature control. The '012 patent used two diverging walls (48 and 50) in combination with a central baffle 46 and a flow restricting dam 52. This combination of diverting and dividing walls created a submerged opening 54 which controlled flow, temperature and strip uniformity. Opening 54, the distance between the floor of the tundish and the bottom of the dam 52, was preferably slightly less than the maximum depth of the liquid metal pool adjacent the casting substrate. The only example was for casting aluminum strip and no details were provided on opening 54.
U.S. Pat. No. 4,865,117 is another melt drag process which shows the use of various weir designs to control the molten metal supply for strip casting. The position of the weirs or dams determines if their function is to control slag on the surface of the bath, provide a source of molten metal or modify the flow of molten metal. The weir closest to the drum may be used to control the melt level and the length of melt contact with the drum. The contact length is very important in the melt drag process to control the strip thickness. The use of a weir positioned near the drum could be used to meter the liquid metal as an orifice but far better control was found to be provided by using a gas knife to control melt thickness. U.S. Pat. No. 4,865,117 uses weir 5 to control the height of the metal bath and the length of contact of the melt with the drum, which is related to strip thickness. Weir 5 may be closely spaced to the drum to act as a metering orifice.
U.S. Pat. No. 4,751,957 shows the use of weirs to provide surge chambers which provide a uniform supply of molten metal for strip casting. The weir may be vertically adjusted to provide a uniform depth for continuous casting. U.S. Pat. No. 4,751,957 shows the use of a weir 72 to meter the flow at a point along the drum where there is no longer a molten pool. In effect, the air knife shown as the invention replaced the prior art weir 72.
Another weir design is represented by World Patent Publication No. 87/02284. A series of weirs are shown which control the flow of molten metal onto a grooved wheel.
U.S. Pat. No. 4,399,860 is a melt drag process which contains the molten metal on one side of a meniscus pool by the rotating substrate or wheel. The wheel drags the melt onto the wheel to form a continuous strand. One of the orifices shown has a fanning arrangement to provide more molten metal at the lateral edge portions to produce strip having improved edge equality. The process has been limited in line speed by the restricted flow conditions along the refractory walls in the pouring nozzle area. This reduces the localized flow rate of molten metal into the meniscus pool area and creates a condition which causes freezing of the molten metal along the refractory surfaces.
The attempts to overcome the flow restrictions with strip casting have included nozzles with enlarged openings at the edges to provide more molten metal at the edges, such as in U.S. Pat. No. 4,399,860. However, this solution does not employ an open pool of metal between the orifice and the wheel. The teachings are related to very thin foil and do not have the flexibility to produce a wide range of product thicknesses and provide a long contact between the meniscus pool and the wheel.
The prior work to control metal flow for the production of thin metal strip has not been completely successful due to the lack of control of metal flow in the pool adjacent the substrate. Prior melt overflow casting systems have suffered from the molten material freezing along the refractory surfaces in the pool discharge area. The quality of the cast strip in terms of uniform gage and surface has not been entirely successful in the past. The present invention has improved the uniformity of composition and thickness. The present invention has overcome the prior casting difficulties and provided a method and means to produce uniform cast strip using the open channel casting process.
The open channel method for strip casting involves contact between a single cooling wheel or belt and an open melt pool. The melt pool is partially contained between the cooling wheel and the pouring nozzle. A stable meniscus forms between the molten pool and the casting substrate to the extent that there is no melt leakage at the point of initial contact. The melt pool is controlled to provide a more rapid localized flow near the rotating substrate and a higher volume of hot metal along the refractory bottom and sidewall joints than is found in melt overflow casting methods. The present invention does not contain the top surface of the pool with the nozzle and provides a critically controlled weir which drastically changes the casting process from melt overflow. The present invention has minimized freeze-ups and improved the uniformity of strip cast compared to the melt overflow process.
The metal flow is essentially under a very low head condition where the major driving force is the pumping action from the rotating substrate. The molten pool is modified by increasing the localized flow of the hottest metal available to the contact areas with the refractory containment using an improved nozzle-weir design. The localized metal flow rate is increased from previous systems to prevent premature solidification and freezing near the substrate. The pool metal will have a circulation pattern which is attributed to these flow conditions. The system may include a sloped nozzle weir wall in the rear which improves the flow into the casting pool. Further flow improvements result from a tapered sidewall in the casting area adjacent the substrate. The channel under the nozzle weir in the casting pool must be controlled to provide the desired clearance with the bottom of the nozzle. Optimum conditions are provided when the gap under the nozzle weir is increased at the edges to provide larger volumes of hot metal along the bottom and in the corners of the nozzle and more rapid local flow rates of hot metal in the areas where freeze-ups along the refractory surfaces are most likely to occur.
It is a principal object of the present invention to provide a system which produces a uniform cast strip in a wide range of the thicknesses and widths. It is also an object of the present invention to provide a system which improves the localized flow of molten metal into the pool by controlling the slopes of the nozzle weir and nozzle walls in combination with the gap beneath the nozzle weir.
Another object of the present invention is to improve the circulation of molten metal in the nozzle to reduce thermal gradients and improve the uniformity of composition while containing the upper slag level.
A still further object of the present invention is to provide the hottest molten metal possible to the pouring nozzle adjacent the substrate to drastically reduce the rate of freeze-ups. The volume and flow rates of hot metal into these potential freeze areas will be increased.
Other objects and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments and related drawings.
US Referenced Citations (10)
Non-Patent Literature Citations (1)
| Entry |
| Three-Dimensional Modeling of Melt Flow in Tundishes for Strip Casting-A. K. Sinha, Y. Sahai & R. C. Sussman, presented at International Symposium on Casting of Near Net Shape Products in Honolulu, Hawaii--Nov. 13-17 1988. |
Patent Grant
5063990
