US3540517A - Method of direct strip casting on a coated drum - Google Patents

Description

United States Patent [72] Inventors Witold M. Wojcik Fredonia, New York;

Eugene A. Mlzikar, Pittsburgh, Pennsylvania May 27, I968 Continuation-in-part of Ser. No. 470,532, July 8, 1965, abandoned.

Nov. 17, 1970 Jones & Laughlin Steel Corporation Pittsburgh, Pennsylvania a corporation of Pennsylvania Appl. No. Filed Patented Assignee METHOD OF DIRECT STRIP CASTING ON A COATED DRUM 5 Claims, 5 Drawing Figs.

US. Cl. 164/72, 164/87 Int. 1 822d 11/06 Field of Search 164/72, 73,

References Cited UNITED STATES PATENTS Wagner Foley et al. Schuh et al. Brennan Kitada Rochester FOREIGN PATENTS Germany Great Britain Primary Examiner-J. Howard Flint, Jr. Assistant Examiner-R. Spencer Annear Attorney- Brown, Murray, Flick and Peckham 164/87 164/282X 164/72 164/276X 164/72X 164/138X ABSTRACT: in the casting of metal strip against a generally drum-shaped metal chill, the problem of the formation of unwanted hills and hollows in the surface of the metal strip opposite to the surface thereof that is in contact with the chill itself is avoided by applying to the chill, prior to its immersion in the bath of molten metal, a particulate refractory material in the form of a thin coating.

Patented Nov. 17, 1970 3,540,517

Fig.3A.

PRIOR ART F ig.3B.

PRIOR ART REFRACTORY OXIDE COATING INVENTORS WITOLD M. WOJCIK and 30 H9. 4. EUGENE A. MIZIKAR ATTORNEY eristics between the forming METHOD OF DIRECT STRIP CASTING ON A COATED DRUM CROSS-REFERENCES TO RELATED APPLICATIONS BACKGROUND OF THE INVENTION 1. Field of the Invention I This invention relates to the casting of metal strip. More .u'ticularly, the invention is concerned with the casting of w-carbon steel strip suitable for subsequent rolling, and is :scribed hereinafter with particular reference to that materi- 2. Description of the Prior Art Steel strip is conventionally manufactured by casting mol-. -n steel of the desired composition into ingots and then ducing the ingot thickness by successive hot rollings. A large innage of steel is cold rolled to final gage, but the starting raterial for this cold rolling is hot band, so-called, which is rip hot rolled to an intermediate gage. Attempts have been iade to eliminate the casting of the steel into ingots by connuously casting the steel through a mold. Attempts have also een made to eliminate both the casting of the steel into ingots nd some of the rolling, by casting the steel directly into a pair f rolls or moving belts, to produce a cast product requiring ttle further reduction. While these attempts have'had some egree of success with bar shapes, they have not resulted in a ommercial process for the production of strip of any conderablc width.

A seemingly attractive process for the direct production of ide thin strip is that of solidifying the steel against a chill, that a surface much colder than the molten steel, and stripping r removing it therefrom. Such processes exemplified in -'agner U.S. Pat. No. l,025,848 and Sendzimir US. Pat. No. 074,812, both of which produce strip by rotating the chill .irface in a pool of molten metal. A difficulty with previously nown processes of this type nough to be easily removed from a chill, it is not at all is that if steel strip is cast thin I is a continuation-in-part of copending U.S. 5

2 of metal strip cast on achill surface resulting in a pronounced hill-and-hollow pattern is believed to be caused by partial separation of the cast strip'from the chill during the initial stages of solidification. This partial separation is usually in the form of a depression or irregular channel (i.e., the air spaces mentioned above) and appears to be associated with simultaneous fracturing of the thin metal skin. Insulating gases,

either generated by the solidifying metal or provided by the atmosphere, fill the depressions. As a result, the reduced rate of heat transfer through the gaseous layer in the area of the hollow depressions or air spaces results in a dip or hollow in the outer surface of the cast strip.

The major requirement of the chill surface, which is usually cooled by a fluid medium, is that it have a coefficient of thermal conductivity sufficient to withdraw heat rapidly from the molten metal and thus form the strip on the surface of the chill. It has been found that the aforesaid hill-and-hollow network pattern on the outer surface of the strip can be suppressed if a pattern of distributed point indentations is deliberately imposed on the solidifying skin. Such indenting of solidifying surface of the cast steel can be accomplished by knurling the surface of the chill; and this is the subject of copending US. Pat. application Ser. No. 410,066, filed Nov.

niform in thickness and breaks up in subsequent rolling. That previous attempts at directly casting steel strip onto a chill urface have resulted in strip that has a severe surface disconnuity condition characterized in a pronounced hill-and-holiw pattern.

As an overall object, the present invention seeks to provide process for casting thin metal strip, particularly steel strip, hich is sufficiently uniform in thickness to withstand sub- :quent rolling to a desired gage.

Another object of the invention is to provide a process. for asting thin metal strip on a chill surface treated such that niform heat transfer from the molten metal to the chill sursee is achieved along the length and width of the strip. As will e seen, this results in a uniform strip thickness.

A further object of the invention is to provide a process for asting metal strip on a chill surface coated with refractory metal oxide particles to achieve esults in uniform thickness along the length of the strip.

In order to produce strip of uniform thickness on a chill surace, it is necessary to provide uniform heat transfer characstrip and the 'chill while it is imnersed in the molten metal. In this respect, the formation of he strip begins with the creation of a very thin solidified skin in the chill surface. In order to increase the thickness of the trip from this initial skin, it is necessary to transfer heat from :he molten metal to the chill; and as the heat is transferred, successive layers of metal solidify on the initial skin and the strip increases in thickness until it emerges from themolten metal bath. 1f the aforesaid initial skin is not in uniform contact with the chill (i.e., if air spaces or shallow air spaces form between the two), the heat transfer will be greater at the areas of contact and less at those areas where the air spaces form under the initial skin. Thus, localized variation in the thickness uniform heat transfer, which 10, 1964 now US. Pat. No. 3,345,738, issued Oct. 10, 1970, and assigned to the Assignee of the present application.

Processes are known, e.g., from U.S. Pat. No. 2,745,151, which rely upon having both surfaces of a strip to be produced come into contact with a chill or the chills, so that the final solidification takes place within the interior of the strip that is formed, and the hill-and-hollow problem mentioned above is, therefore, not encountered. Other processes are known, for example, U .S, Pat. No. 2,399,606, that relate to the production of solidified metal of relatively thin crosssectional thickness by the unidirectional withdrawal of heat therefrom, but these processes have, prior to the instant invention, been limited to ones involving centrifugal-casting, such that the above-mentioned hill-and-hollow problem is inherently solved. Moreover, centrifugal casting is obviously unsuitable for the task of producing in substantial quantity a length of metal in strip sheet form.

The prior art with respect to the casting of metal, and particularly the casting of steel, is replete with teachings of the use of various materials as mold coating agents, but the teachings of these patents relate, without exception so far as the Applicants are aware, to practices for modifying the character of the surface of the metal in contact with the mold or, in some cases, the microstructure of a thin skin of metal nearest to the mold surface.

BRIEF SUMMARY OF THE INVENTION In accordance with the present invention, variations in cast strip thickness are suppressed by applying a thin coating of refractory metal oxide particles to the'chill surface prior to immersion of the chill in the metal bath. For example, the refrac tory material may be a zircon mold wash, an alumina suspension, a magnesia suspension, a silica suspension or any combination of such materials. If the refractory coating is sprayed or brushed on a c hill surface which is above 2l2F., the coating will dry immediately after contact. Assuming that the size of the major portion of the refractory particles is above a predetermined minimum value, the refractory coating, in ef fect, breaks up the large direct contact areas which occur on an uncoated surface into much smaller ones, such that the undersurface of the strip appears more or less stippled. This results in uniform heat transfer and substantially uniform strip much higher than those for interfaces of liquid metal with DESCRIPTION OF THE DRAWINGS The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings which form a part of this specification, and in which:

FIG. 1 is a schematic illustration of direct strip casting apparatus employing the principles of the invention;

FIG. 2 is a cross-sectional view of the casting drum shown in FIG. 1;

FIGS. 3A and 3B are cross-sectional views showing the formation of metal strip with the apparatus of FIG. 1 on an uncoated or smooth chill surface; and

FIG. 4 is a cross-sectional view of metal strip formed in accordance with the present invention wherein the metal chill is coated with refractory metal oxide.

DESCRIPTION OF THE PREFERRED EMBODIMENT With reference now to the drawings, and particularly to FIGS. I and 2, the drum casting apparatus shown includes a refractory-lined crucible containing molten metal 12, such as molten steel. Above the crucible 10 is the casting drum proper 14 which, as best shown in FIG. 2, is formed from a metal shell 16, the sides of which are coated with refractory metal 18 so as to prevent the formation of solidified metal at these areas. The periphery of the shell 16, however, is exposed to form a chill surface 20. Suitable means, not shown, are provided for cooling the chill surface to maintain its temperature well below that of the molten steel. While the temperature of the chill surface 20 may vary between about 70F. and 1,200F., it is preferably maintained above 212F. for reasons which will hereinafter be explained.

The drum 14 rotates about an axis 22 in the direction of arrow 24. As it rotates, the chill surface 20 will enter the hot metal bath [2 at 26 and, due to its lower temperature, will extract heat from the molten metal. Consequently, a, solidified metal skin will form on the chill surface 20; and this solidified layer will build up in thickness as the surface 20 progresses through the molten bath 12 until it emerges at point 28. After emerging from the bath 12, the formed strip 30 is stripped from the chill surface 20 as by passing it over a roller 32.v

The effect of attempting to cast strip material on the chill surface 20 when it is not coated with a refractory metal oxide is shown in FIGS. 3A and 3B. In FIG. 3A, the area of the surface 20 is shown immediately after it enters the metallic bath 12 at 26. Thus, as the chill surface 20 initially enters the bath, a thin metal skin 34 initially forms thereon. The formation of the skin 34, however, is not uniform along the length of the chill surface 20. Rather, there are areas 36 in direct contact with the chill surface 20, these areas being separated by shallow depressions 38 filled with gas. In order to solidify the strip on the chill surface 20 it is, of course, necessary to transfer heat from the molten metal bath 12 to the chill surface 20 along the direction of the arrow 40. The final thickness of the strip is dependent upon the rate of heat transfer as well as the length of time that the chill surface 20 remains in the molten bath 12. Furthermore, it will be appreciated that the heat transfer characteristics at areas 36 are much better than at areas 38. The result is shown in FIG. 3B, illustrating the approximate cross-sectional configuration of the strip 30 as it emerges from the molten metal bath 12 with no refractory oxide coating applied to the chill surface 20. Thus, the strip thickness above areas 36 is much greater than that above areas 38, due to the fact that heat is transferred from the molten metal bath to the chill surface 20 much more efficiently and rapidly at areas 36. The areas 38 do not extend entirely across the width of the chill surface 20 and, consequently, a hill-and-valley or -hollow pattern is achieved, as shown in FIG.

FIG. 4 illustrates the cross-sectional configuration of the strip as it emerges from the metal bath with a refractory oxide coating 40 applied to the chill surface 20 in accordance with the present invention. Note that the underside of the strip 3%? is, more or less, stippled. This stippled effect is uniform across the entirety of the chill surface 20. Hence, the large direct contact areas 36 shown in FIGS. 3A and 3B are broken up into much smaller ones exemplified by the stipple effect 40, resulting in much more uniform heat transfer characteristics.

The refractory oxide coating may be applied to the chill surface 20 as a suspension in aqueous solution as by means of nozzle 42', however, the invention is not necessarily limited to aqueous solutions. Alumina (A1 0 (silica Slo magnesia (MgO) and ground fire clay have all been found to be satisfactory oxidesflOn the other hand, iron oxide (Fe O and grit-- phite are unsuitable. Thus, the oxide must be of the refractory metal type.

If an attempt is made to spray the oxide particles onto the chill surface 20 simply as a suspension in aqueous solution, and without the addition of a binder, the temperature of the chill surface 20 cannot be higher than 300F. Temperatures above 300F. without the addition of a binder cause the formation of bubbles, rapid evaporation and uneven distribution of the oxide coating. Temperatures above 300F., however, may be employed to achieve a more uniform and superior oxide coating if the oxide particles are suspended in an aqueous solution containing as a binder zircon wash, ammonium phosphate or aluminum phosphate. While the zircon wash is a satisfactory binder, it can be used only at temperatures less than about 400F. The ammonium phosphate and aluminum phosphate binders, on the other hand, can be used at temperatures up to about 900F. and higher.

The major portion of the refractory metal oxide in aqueous solution must be larger than 200 mesh. This is illustrated, for example, in the following Table 1 showing the results of tests conducted with aluminum oxide, silicon dioxide, magnesium oxide and fire clay.

TABLE I.EFFEOT OF PARTICLE SIZE ON REFRACTORY OXIDE COATING FOR CHILL SURFACES Sample Condition of No. Coating Outer Surface 1 to +200 mesh A1203-.." Nearly even. 2 200 to +325 mesh A1 0 Shallow hill and hollow 72 7 100 t +200 11 A1 0 pattem' a o mes 1 3 --{2s% -2o0 to +325 mesh A;.} 4 -100 to 200 mesh Si02.-.- 0- 5 200 to +325 mesh S102 Shallow hill and hollow pattern.

6 l.00 to +200 mesh MgO Nearly even.

-200 to +325 mesh MgO Shallow hill and hollow pattern.

100 to +200 mesh fire clay... Nearly even. 200 to +325 mesh fire clay Shallow hill and hollow pattern.

*Chill surtacesmooth stainless steel; chill temperature-700 It bath temperaturo2,810 F.; carbon content of bath-0.10%; approximate thickness of strlp-0.25O inch.

tion again results, notwithstanding the fact that the remainder of the material is smaller than 200 mesh. Therefore, the major portion of the refractory particles should be greater than 200 requirements without departing from the spirit and'scope of the invention.

We claim:

1. A process for producing directly cast metal strip comprising bringing molten metal into contact with an internally TABLE IL-EFFECT OF CHILL COATINGS ON THICKNESS UNIFORMITY OF STEEL STRIP Chill Bath Carbon Mean temp., temp., content, thickness, Sample No. Type of chill Coating F. F. percent inch Condition of outer surface OVl-l None 70 2, 825 .09 Deep hill and hollow pattern. OVl-3 d 0.2 mill of rapeseed oil 70 2, 805 .09 Deep hill and hollow pattern.

0.4 mill of rapeseed oil 70 2, 810 .09 Shallow hill and hollow pattern. 0.6 mill of rapeseed oil 70 2,800 .09 Do.

Lime 70 2,810 06 D0. Z11c011 70 2,800 .09 Nearly even. 00% A1203, 10% z1rcon 70 2, 800 10 Do. 90% A1 03, 10% SiO- 70 2, 825 10 D0. 90% A1103, 10% zircon. 1 800 2,810 .09 Shallow hill and valley.

50% A120 50% SiOg 800 2, 800 .0!) Shallow hill and valley pattern. 00% A1103, 10% zircon. 800 2, 810 .09 Nearly even. 50% A1 03, 50% S102 800 2,800 .09 Do. Chrome, smooth 90% A1103, 10% zircon 800 2, 800 00 Do. OXXl-ti. Stainless, smooth 100% A120 AlPO, binder. 400 2, 825 .00 D0- 0kX1-4 0 100% Algoa, NILPO; binder. 400 2, 815 09 DO. OXXlll-G 100% MgO, AlPO binderv 500 2, 820 .10 D0. XX 100% M110, NII4IO; binder. 500 2, 825 10 D0. OXXIV- 100% fire clay, AlPO; l)inder 200 2, 820 .00 Do. OXlV-L 90% F0304, 10% zircon. 70 2, 815 .10 Moderate 111115 and hollows. 0X113. 00% graphite, 10% zircon..." 70 2, 810 .09 D0- Note that the chill temperature has very little effect on the eventual surface condition of the strip. However, it is preferable to employ a stainless steel chill rather than copper at higher temperatures (i.e., above 300F.), at which temperature superior oxide coatings containing a binder can be formed on the chill surface as mentioned above. Note than in Samples OXl-2 and OXl-3 a copper chill coated in accordance with the teachings of the invention and having a temperature of about 800F. produced somewhat unsatisfactory results in that a shallow hill and valley pattern occurred on the surface of the strip. At lower chill temperatures exemplified by Sample Nos. 0V1 1 1-3, OlX-3 and OX-l, however, a nearly even surface condition resulted. In addition to stainless steel chill surfaces, chromium plated chill surfaces also operate satisfactorily at higher temperatures as exemplified, for example, by Sample No. 0X1-6.

Thus far, the invention has been described in connection with coatings which must be continually renewed as by nozzle 42 for each revolution of the drum 14. In certain cases, however, it is possible to apply a permanent refractory coating to the drum. Such a coating may be formed by flame spraying 50 aluminide particles coated with nickel and mixed with A1 0; or Zr0 onto the chill surface. As a specific example, a mixture of percent nickel aluminide and 70 percent A1 0 is satisfactory for flame spraying purposes. The resulting coating is tough and adherent, but nevertheless breaks up the large contact areas on the chill surface into smaller ones as m entioned above such that the cast strip is uniform in thickness along its length.

Although the invention has been shown in connection with certain specific examples, it will be readily apparent to those 60 skilled in the art that various changes may be made to suit cooled metal chill that is generally drum-shaped and is partly immersed in a bath of said metal in molten form so as to cause rapid solidification of a skin of metal in strip form against the exterior cylindrical surface of said chill by unidirectional flow of heat through said skin from said molten metal to said chill, and withdrawing the strip so formed from said chill, with the formation of substantial hills and hollows in the surface of said metal strip opposite the surface thereof which is in contact with said chill being avoided by applying to said chill prior to its immersion in said bath of molten metal a particulate refrac tory material in the form of a thin coating.

2. A process as defined in claim 1, characterized in that said metal is steel.

3. A process as defined in claim 1, characterized in that said refractory material is selected from the group consisting of aluminum oxide, silicon dioxide, zirconium dioxide, magnesium oxide, and tire clay, said refractory material being in form of granules with the major portion of said granules being 200 mesh or larger in size.

4. A process as defined in claim 3, characterized in that said refractory material is applied to a portion of said chill that is about, by reason of the rotation of said chill about an axis parallel to the surface of said bath, to enter said bath, in the form of a suspension in aqueous solution, the aqueous solution incorporating a binder selected from the group consisting of zircon, ammonium phosphate and aluminum phosphate, the temperature of the portion of the surface of the chill to which said suspension in aqueous solution is applied being at a temperature above 212F.

5. A process as defined in claim 3, characterized in that said refractory material is incorporated into a binder such that a permanent oxide coating is formed on said chill.

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