Abstract:
A thin cast strip is formed having at least one microstructure selected from the group consisting of polygonal ferrite, acicular ferrite, Widmanstatten, bainite and martinsite, a surface roughness of less than 1.5 microns Ra and a scale thickness of less than about 10 microns by applying a mixture of water and oil on the work rolls of the hot rolling mill, passing the thin cast strip at a temperature of less than 1100° C. through the hot rolling mill while the mixture of oil and water is applied to the work rolls, and shrouding the thin cast strip from the casting rolls through the hot rolling mill in an atmosphere of less than 5% oxygen to form the thin cast strip.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application No. 12/272,207 filed Nov. 17, 2008 now abandoned, which is a continuation of U.S. patent application Ser. No. 11/362,682 filed Feb. 27, 2006 now abandoned, the disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
     This invention relates to cast strip made by a twin roll caster, and the method and apparatus for making such cast strip. 
     In a twin roll caster, molten metal is introduced between a pair of counter-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces, and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the casting rolls. 
     The term “nip” is used herein to refer to the general region at which the casting rolls are closest together. The molten metal may be poured from a ladle through a metal delivery system comprised of a tundish and a core nozzle located above the nip to form a casting pool of molten metal supported on the casting surfaces of the rolls above the nip and extending along the length of the nip. This casting pool is usually confined between refractory side plates or dams held in sliding engagement with the end surfaces of the rolls so as to dam the two ends of the casting pool against outflow. 
     When casting steel strip in a twin roll caster, the strip leaves the nip at very high temperatures on the order of 1400° C. or higher. If exposed to normal atmosphere, it would suffer very rapid scaling due to oxidation at such high temperatures. Therefore, a sealed enclosure is provided beneath the casting rolls to receive the hot strip and through which the strip passes on the way from the strip caster, the enclosure containing an atmosphere which inhibits oxidation of the strip. The oxidation inhibiting atmosphere may be created by injecting a non-oxidizing gas, for example, an inert gas such as argon or nitrogen, or combustion exhaust gases which may be reducing gases. Alternatively, the enclosure may be sealed against ingress of oxygen containing atmosphere during operation of the strip caster. The oxygen content of the atmosphere within the enclosure is then reduced during an initial phase of casting by allowing oxidation of the strip to extract oxygen from the sealed enclosure as disclosed in U.S. Pat. Nos. 5,762,126 and 5,960,855. 
     Cast strip produced by twin roll caster is generally hot rolled in a hot rolling mill after the strip emerges from the caster to shape the thin strip. It is generally understood that to cast strip with acceptable shape, the hot rolling mill is used in connection with a twin roll caster is to provide the desired cross-sectional profile to the cast strip. Still, a surface roughness of 6 to 8 microns Ra with surface micro-cracking was common for cast strip emerging from the hot rolling mill while casting at a standard casting speed of 80 m/min and 16% reduction of the strip by the hot rolling mill.  FIG. 1  is a micrograph showing typical surface roughness of such cast strip emerging from a hot rolling mill in-line with a twin roll caster. With the direction of rolling from left to right, the micrograph shows pronounced lapping on the strip surface (20 to 30 μm deep). The reason or reasons for this surface roughness may be shearing at the strip surface caused by welding of the strip to the work roll surface, imprinting of the texture of the work roll surface onto the surface of the strip, and/or other factors. Moreover, micro-cracking on the surface of the cast strip was a problem. It was possible to reduce the casting speed and the heat rate of the strip to reduce microcracking, but it was uneconomical to reproduce these conditions during production. 
     The microstructure of hot strip mill products is essentially 100% equiaxed ferrite. However, in making a cast strip with a twin roll caster, previous experience was that microstructure was coarse grains of polygonal ferrite, acicular ferrite, and Widmanstatten. It was typical that the microstructure was 30-60% polygonal ferrite, 70-40% Widmanstatten and acicular ferrite. With this microstructure, the typical surface roughness was 4-7 microns Ra. 
     A thin cast strip is provided having at least one microstructure selected from the group consisting of polygonal ferrite, acicular ferrite, Widmanstatten, bainite and martinsite, a surface roughness of less than 1.5 microns Ra and a scale thickness of less than about 10 microns made by the steps comprising: 
     a. assembling a twin roll caster having laterally positioned caster rolls forming a nip between them and a hot rolling mill having work rolls and back-up rolls adjacent the twin roll caster, 
     b. forming a thin cast strip from the nip between the casting rolls of the twin roll caster, 
     c. applying a mixture of water and oil on the back-up rolls of the hot rolling mill, 
     d. passing the thin cast strip at a temperature of less than 1100° C. through the hot rolling mill while the mixture of oil and water is applied to the work rolls, and 
     e. shrouding the thin cast strip from the casting rolls through the hot rolling mill in an atmosphere of less than 5% oxygen forming a cast strip having at least one microstructure selected from the group consisting of polygonal ferrite, acicular ferrite, Widmanstatten, bainite and martinsite, a surface roughness of less than 1.5 microns Ra, and a scale thickness of less than about 10 microns. 
     A thin cast strip may have a surface roughness less than 1.0 microns Ra or less than 0.7 or 0.5 microns Ra. The thin cast strip may have a scale thickness less than 7 or 4 microns. The cast strip may be passed through the hot rolling mill at a temperature less than 1050° C. while a mixture of oil and water is applied to the back-up rolls. The mixture of oil and water may be applied by spraying the upstream and/or low stream surfaces of the upper and/or lower back-up rolls. The mixture of oil and water may be less than 5% oil to form a thin cast strip with a low surface roughness of less than 1.5 microns Ra. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The operation of an illustrative twin roll casting plant in accordance with the present invention is described with reference to the accompanying drawings, in which: 
         FIG. 1  is a micrograph showing typical surface roughness of a cast strip after hot rolling; 
         FIG. 2  is a schematic illustrating a thin strip casting plant having a hot rolling mill for controlling the shape of cast strip; 
         FIG. 3  is an enlarged cut-away side view of the caster of the thin strip casting plant of  FIG. 2 ; 
         FIGS. 4A and 4B  are a schematic diagram showing a system for the application of an oil and water mixture to the rolls of a hot rolling mill; and 
         FIG. 5  is a diagram showing the Average Surface Roughness for Thin Cast Steel Strip, Sequence 2613, made using the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The illustrated casting and rolling installation comprises a twin-roll caster denoted generally by  11  which produces thin cast steel strip  12  which passes into a transient path across a guide table  13  to a pinch roll stand  14 . After exiting the pinch roll stand  14 , thin cast strip  12  passes into and through hot rolling mill  15  comprised of back-up rolls  16  and upper and lower work rolls  16 A and  16 B, where the thickness of the strip reduced. The strip  12 , upon exiting the rolling mill  15 , passes onto a run out table  17  where it may be forced cooled by water jets  18 , and then through pinch roll stand  20  comprising a pair of pinch rolls  20 A and to a coiler  19 . 
     Twin-roll caster  11  comprises a main machine frame which supports a pair of laterally positioned casting rolls  22  having casting surfaces  22 A and forming a nip between them. Molten metal is supplied during a casting campaign from a ladle (not shown) to a tundish  23 , through a refractory shroud to a removable tundish  25  (also called distributor vessel or transition piece), and then through a metal delivery nozzle  28  (also called a core nozzle) between the casting rolls  22  above the nip. 
     Molten steel is introduced into removable tundish  25  from tundish  23  via an outlet of the refractory shroud. The tundish  23  is fitted with a stopper rod and a slide gate valve  34  to selectively open and close the outlet of the shroud  32  and effectively control the flow of molten metal from the tundish  23  to the caster. The molten metal flows from removable tundish  25  through an outlet and optionally to and through the delivery nozzle  28 . 
     Molten metal thus delivered to the casting rolls  22  forms a casting pool above the nip supported by casting roll surfaces  22 A. This casting pool is confined at the ends of the rolls by a pair of side dams or plates  26 , which are applied to the ends of the rolls by a pair of thrusters (not shown) comprising hydraulic cylinder units connected to the side dams. The upper surface of the casting pool (generally referred to as the “meniscus” level) may rise above the lower end of the delivery nozzle  28  so that the lower end of the deliver nozzle is immersed within the casting pool. 
     Casting rolls  22  are internally water cooled by coolant supply (not shown) and driven in counter rotational direction by drives (not shown) so that shells solidify on the moving casting roll surfaces and are brought together at the nip to produce the thin cast strip  12 , which is delivered downwardly from the nip between the casting rolls. 
     Below the twin roll caster  11 , the cast steel strip  12  passes within a sealed enclosure  10  to the guide table  13 , which guides the strip to a pinch roll stand  14  through which it exits sealed enclosure  10 . The seal of the enclosure  10  may not be complete, but is appropriate to allow control of the atmosphere within the enclosure and access of oxygen to the cast strip within the enclosure as hereinafter described. After exiting the sealed enclosure  10 , the strip may pass through further sealed enclosures after the pinch roll stand  14 , including the hot rolling mill  15 . 
     Enclosure  10  is formed by a number of separate wall sections that fit together at various seal connections to form a continuous enclosure wall. These sections comprise a first wall section  41  at the twin roll caster to enclose the casting rolls  22 , and a wall enclosure  42  extending downwardly beneath first wall section  41  to form an opening that is in sealing engagement with the upper edges of a scrap box receptacle  40 . A seal  43  between the scrap box receptacle  40  and the enclosure wall  42  may be formed by a knife and sand seal around the opening in enclosure wall  42 , which can be established and broken by vertical movement of the scrap box receptacle  40  relative to enclosure wall  42 . Seal  43  is formed by raising the scrap box receptacle  40  to cause the knife flange to penetrate the sand in the channel to establish the seal. 
     This seal  43  can be broken by lowering the scrap box receptacle  40  from its operative position, preparatory to movement away from the caster to a scrap discharge position (not shown). Scrap box receptacle  40  is mounted on a carriage  45  fitted with wheels  46  which run on rails  47 , whereby the scrap box receptacle can be moved to the scrap discharge position. Carriage  45  is fitted with a set of powered screw jacks  51  operable to lift the scrap box receptacle  40  from a lowered position, where it is spaced from the enclosure wall  42 , to a raised position where the knife flange penetrates the sand to form seal  43  between the two. 
     Sealed enclosure  10  further may have a third wall section disposed  61  about the guide table  13  and connected to the frame of pinch roll stand  14 , which includes a pair of pinch rolls  50 . The third wall section disposed  61  of enclosure  10  is sealed by sliding seals. 
     Most of the enclosure wall sections  41 ,  42  and  61  may be lined with water cooled panels. Also, scrap box receptacle  40  may be lined with water cooled panels or with a castable refractory lining. In this way, the complete enclosure  10  is sealed prior to a casting operation, thereby limiting access of oxygen to thin cast strip  12 , as it passes from the casting rolls  22  through the pinch roll stand  14  and the hot rolling mill  15 . Initially the strip can take up all of the oxygen from enclosure  10  space by forming heavy scale on an initial section of the strip. However, the sealing enclosure  10  limits ingress of oxygen into the enclosure from the surrounding atmosphere to below the amount of oxygen that could be taken up by the strip. Thus, after an initial start-up period, the oxygen content in the enclosure  10  will remain depleted so limiting the availability of oxygen for oxidation of the strip  12 . In this way, the formation of scale is controlled to a thickness less than  10  microns without the need to continuously feed a reducing or non-oxidizing gas into the enclosure. Of course, a reducing or non-oxidizing gas may be fed through the enclosure walls. However, in order to avoid the heavy scaling during the start-up period, the enclosure  10  can be purged immediately prior to the commencement of casting so as to reduce the initial oxygen level within enclosure  10 , thereby reducing the time period for the oxygen level to stabilize in the enclosure as a result of the interaction of the oxygen in oxidizing the strip passing through it. Thus, illustratively, the enclosure may conveniently be purged with, for example, nitrogen gas. It has been found that reduction of the initial oxygen content to levels of less than 5% will limit the scaling of the strip at the exit from the enclosure  10  to about 10 microns to 17 microns even during the initial start-up phase. In an embodiment of the present invention, the thin cast steel strip has a scale thickness less than about 10 microns, or the scale thickness may be less than 7 or 4 microns, during continuous casting. 
     At the start of a casting campaign, a short length of imperfect strip is produced as the casting conditions stabilize. After continuous casting is established, the casting rolls  22  are moved apart slightly and then brought together again to cause this lead end of the strip to break away in the manner described in Australian Patent 646,981 and U.S. Pat. No. 5,287,912, to form a clean head end of the following thin cast strip  12 . The imperfect material drops into scrap box receptacle  40  located beneath caster  11 , and at this time swinging apron  38 , which normally hangs downwardly from a pivot  39  to one side of the caster as shown in  FIG. 3 , is swung across the caster outlet to guide the clean end of thin cast strip  12  onto the guide table  13  where the strip is fed to the pinch roll stand  14 . Apron  38  is then retracted back to its hanging position as shown in  FIG. 3  to allow the strip  12  to hang in a loop  36  beneath the caster as shown in  FIGS. 2 and 3  before the strip passes onto the guide table  13 . The guide table  13  comprises a series of strip support rolls  37  to support the strip before it passes to the pinch roll stand  14 . The rolls  37  are disposed in an array extending from the pinch roll stand  14  backwardly beneath the caster and curve downwardly to smoothly receive and guide the strip from the loop  36 . 
     The twin-roll caster may be of a kind which is illustrated and described in detail in U.S. Pat. Nos. 5,184,668 and 5,277,243, or 5,488,988. Reference may be made to these patents for construction details, which are no part of the present invention. 
     Pinch roll stand  14  comprises a pair of pinch rolls  50  reactive to tension applied by the hot rolling mill  15 . Accordingly, the strip is able to hang in the loop  36  as it passes from the casting rolls  22  to the guide table  13  and into the pinch roll stand  14 . The pinch rolls  50  thus provides a tension barrier between the freely hanging loop and tension on the strip downstream of the processing line. The pinch rolls  50  also stabilize the position of the strip on the feed table  13 , feeding the strip into hot rolling mill  15 . 
     From the pinch roll stand  14 , the thin cast strip  12  is delivered to the hot rolling mill  15  comprised of upper work roll  16 A and lower roll  16 B. As shown in  FIG. 4 , a preferred embodiment of the present invention comprises spraying a mixture of water and oil on the downstream surfaces of upper and/or lower back-up rolls  16 . An oil reservoir  100  is provided with a heater  101  to maintain the oil at approximately 50° C., but heating is not necessary. The heated oil is transferred through oil transfer lines  103  by fixed displacement pumps  102  to static mixers  104  where the heated oil is mixed with water. 
     Water is supplied from a source  110  to water strip chilling headers  111  and to mill rolls supply lines  112 . A first portion of the water is supplied to spray headers  18  to supply cooling water to cool the hot strip  12  after exiting the hot rolling mill  15 . Typically, the water pressure is reduced through pressure regulator  113  to about 40 psi. Between about 10 and 30 gpm of water is supplied to each static mixer  104  where the water is mixed with about 4 gph of heated oil. 
     The mixed oil and water is then applied to the downstream surfaces (the direction of travel of the thin cast steel strip  12  is shown by arrow  120 ) of upper and/or lower back-up rolls  16  through oil-water nozzles  71 . Alternately, the oil-water mixture may be applied to the upstream or downstream surfaces of the upper and/or lower back-up rolls  16 . 
     Preferably, the temperature of the thin cast steel strip  12  in the hot rolling mill  15  is less than 1100° C., and more preferably less than 1050° C., and most preferably less than 900° C. Also, preferably, the temperature of the thin cast steel strip in the hot rolling mill  15  is above 400° C. 
     The static mixers  104  are standard conventionally available devices. Other forms of mixers may be used provided they are capable of good mixing of the oil and water. 
     In one embodiment, the oil-water mixture is delivered at between 5 and 30 gpm at 40 psi to the upper and lower back-up rolls  16 . Typically the oil-water mixture is delivered to the back-up rolls in this embodiment at about 10 to 20 gpm, with 15 gpm a reasonable setting. The oil-water mixture may comprise less than 5% oil, and in one embodiment comprises 4 parts oil and between 600 parts to 1800 parts water by volume. The oil may be less than 2% or 1% of the mixture. The oil is provided to be mixed with the water generally at less than 15 gph. 
       FIG. 5  shows the Average Surface Roughness (Ra) in microns for thin cast strip steel strip  12  produced using the present invention. As can be seen in  FIG. 5 , the Average Surface Roughness is noticeably lower, about 0.66 to about 1.5 microns with the addition of an oil-water mixture as described above. 
     In one embodiment, the present invention comprises producing thin cast steel strip using the oil-water application described above to produce thin cast steel strip at a rate above 80 meters per minute. 
     While the invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.