CLAD CAST METAL STRIP

Abstract: Clad cast steel strip or sheet (8) and process for production thereof in which molten carbon steel (1) is introduced between at least two steel strips (2, 3) to at least partially bond the strips to the metal. The molten carbon steel (1) is introduced between the steel strips (2, 3) under conditions such that the strips (2, 3) do not delaminate during subsequent mechanical operations, and the process is preferably conducted in an inert-oxidizing atmosphere.

Description: CLAD CAST METAL STRIP The present invention relates to clad cast thin metal strip, and to a process for its production. In particular, the present invention includes an energy effici¬ ent, continuous casting procedure capable of producing a hot band crystalline steel strip having a thickness between about 0.03 to 0.375 inches. This application is a continuation-in-part of se¬ rial number 547,682 filed November 1, 1983. It has been determined that title in and to the present invention shall be in the inventor, Ralph L. Shene- man, subject to the reservation to the Government of a non-exclusive, irrevocable, royalty-free license with power to grant licenses for all governmental purposes. BACKGROUND OF THE INVENTION The primary metal industry in the United States is faced with economic problems today stemming from the nature of primary metal processing operations, which are highly energy intensive and require large capital invest¬ ment. In addition, in much of the industry, productivity has slipped behind overseas competition. In the formation of hot band steel strip mater¬ ial, the energy costs are quite significant. At present, the process utilized in the formation of thin hot band steel strip or sheet usually comprises casting of large slabs of steel (generally S to 16" thick) followed by a number of hot rolling operations (e.g., reheating of the cast slabs and rolling) and cold rolling operations which reduce the thickness of the slab in stages until the desired thin band steel strip is produced. This procedure __ * •_ * _>-. * is energy intensive. The expenditure of energy during the hot rolling operations greatly contributes to the cost of manufacture. Accordingly, a process which would reduce the energy expenditure by eliminating some or all of the hot rolling operations would obviously be of substantial econom¬ ic benefit to the steel industry. With this object in mind, the steel industry has been looking to develop a feasible and economic continuous casting process useful in the production of thin hot band steel strips . Examples of various thin film continuous casting procedures proposed for the production of steel strips includes those set forth in published European patent applications 040,070; 040,072; and 040,009. However, all of these procedures are limited in practice to the production of extremely thin, (e.g., .008 in.) strip steel. Attempts to cast steel strip of any substantial thickness (e.g., 0.064 in.) have not been successful because of the tendency of the molten metal to adhere to the casting wheel. In addition, the surface opposite the casting wheel solidifies in a roughened condition unacceptable for cold rolling. In addition, these methods achieve limited throughput, as most of the heat reduction from approxi¬ mately 2900°F molten to approximately 1600-1900°F (present hot mill exit temperature range) must be transferred to the cooling wheel, or other cooling device, in order to achieve adequate solidification for subsequent handling into ten¬ sion wound coil. At present, these problems have substantially prevented the development of any viable continuous casting procedure for hot band steel strip. Accordingly, casting of thin hot band steel strips or sheets with any substantial PI thickness range (generally, 0.03 to 0.375 inches) has not been accomplished in spite of the many millions of dollars expended by the industry. The process of the present inven¬ tion is directed to a solution to these problems . The continuous casting procedure of the present invention pro¬ duces thin hot band steel strip or sheet having a substan¬ tial thickness (generally 0.03 to 0.375 inches, e.g. 0J_3 to 0.125 inches) which can be immediately cold rolled on both sides to provide good finished surfaces . SUMMARY OF THE INVENTION It has now been found, according to the present invention, that it is possible to provide an energy efficient process for producing thin metal strip or sheet by a continuous casting procedure. The rapidly solidified resulting thin metal strip or sheet has finer grain struct¬ ure and is capable of being cold rolled to a final gauge. Hereafter, the product of the invention is referred to as thin metal strip, hot band metal strip, or composite thin metal strip, and it is to be understood that those expressions, in so far as they include the word "strip", do not imply any width or thickness limitations with respect to the thin metal strip product. According to one aspect of the present invention, there is provided a process for continuously producing thin metal strip, comprising the steps of providing at least two metal strips, and introducing molten metal between the metal strips to at least partially bond the metal strips and the molten metal together. The molten metal is intro¬ duced between the metal strips under conditions such that the metal strips do not delaminate during subsequent mechan- ical operations, and it is preferred to conduct the process in an inert gas environment, to minimize oxidation. In a preferred aspect, the process of the present invention comprises providing at least two metal strips, preferably stainless steel strips, feeding those metal strips to a gap between a first and second quench surface opposed one to the other, maintaining an inert atmosphere about the area defined by at least the gap and the first and second quench surfaces, and joining the metal strips one to the other at the gap during contact with the first and second quench surfaces by providing a layer of molten metal, preferably carbon steel, therebetween. The strips and molten metal are kept in contact with at least one of the first and second quench surfaces for a time sufficient to at least insure partial solidification of the molten metal and at least partial bonding of the strips to the molten metal. The partial bond must be of sufficient strength to insure that the strips do not delaminate during subsequent mechanical operations. The partially bonded strips are then cooled further by contact with at least a third quench or cooling surface to permanently bond the metal strips. In a preferred embodiment of the process of the present invention, the joining of the metal strips is accomplished by providing a layer of rheocast metal slurry between the strips. In a further preferred embodiment of the present invention the hot band crystalline metal strip is produced by a continuous casting procedure utilizing a molten metal or a rheocast slurry source. OMPI In a still further preferred embodiment of the present invention, the hot band metal strip produced is selected to have a thickness in the range of about .03 to .375 inch . In another preferred embodiment of the present invention, the outer two metal strips are selected to have a thickness of about .008 inch (mils) and the resulting hot band metal strip produced has a thickness in the range of about .070 to 080 inch. In still another preferred embodiment of the present invention, the outer two metal strips are selected to have a thickness of between about .012 to .040 inch and the resulting hot band metal strip produced has a thickness in the range of 0.120 to 0.375 inch. In a further preferred embodiment of the present invention, the molten to solid ratio is above about 3-4. Preferably, the molten to solid ratio is above 4-2. For hot band metal strip product having a thickness in the range of about .070 to .375 inch the ratio is generally above about 3.4, preferably above about 5-0. In a further preferred embodiment of the process of the present invention, the hot band metal strip produced is cold rolled to a thickness of between about .008 to 0.040 inches and a portion of cold rolled product is recycled for use as the outer strip in the process of the present invention. In another preferred embodiment of the present invention, the metal strips are fed into the gap between the first and second quench surfaces at a speed of about 500 to 8000 inches/minute. Most preferably, the strip speed is between about 500 to 1900 inches/ minute. In still another preferred embodiment of the pre¬ sent invention, the quench surfaces have a cooling fluid continuously passing through them. In another preferred embodiment of the present invention, the process includes positioning two interme¬ diate endless metal support belts between the outer metal strips and at least the first and second quench surfaces. In still another preferred embodiment of the pre¬ sent invention the composition of the molten metal is selected to be different than the composition of the outer metal strips. In a still further preferred embodiment of the present invention, the composition of the molten metal is selected to be the same as the composition of the outer strips. In a further preferred en odiment of the present invention, the quench surfaces ar*. selected to included opposed wheels, spaced apart one from the other, each roller having an annular flange at one end, the flanges being located at opposite ends of the rollers, respec¬ tively. These flanges facilitate the confinement of the molten or rheocast metal layer. In a still further preferred embodiment of the present invention, the feeding of the metal strips, the joining of the metal strip and initial cooling of the molten or rheocast layer are performed in an inert atmos¬ phere such as argon, nitrogen or helium, _______ OMPI In another preferred embodiment of the present invention, the metal strips and the molten or rheocast layer are selected to consist of carbon-steel. In still another preferred embodiment of the pre¬ sent invention, the two metal strips are selected to have a thickness of from about .008 to .040 inches. In a still preferred embodiment of the present invention, the molten metal bath is maintained in a .tempera¬ ture range of about 2900°F to 298θ°F. According to another preferred aspect of the pre¬ sent invention, the process is conducted so as to minimize entrapment of ambient air or inert gas in the molten metal as the molten metal is introduced between the metal strips. Such entrapment is minimized in order to reduce the forma¬ tion of bubbles or metal oxides either in the area of the fusion bond between the metal strips and the molten metal or within the molten metal, which would reduce the bonding integrity of the thin strip product and create voids in the cast structure of the core. This is achieved by providing a barrier for shielding the molten metal from ambient air or inert gas as the molten metal is introduced between the metal strips. Preferably, the metal strips form at least part of the barrier, and act as a curtain to shield the molten metal from the ambient air or inert gas. According to this embodiment, the metal strips are fed in sliding contact past a tundish nozzle tip into contact with first and second quench surfaces, with the metal strips traveling a distance between the point of contact with the tundish nozzle tip and the point of contact with the quench sur¬ faces such that distortion or bursting of the metal strips is avoided. This procedure also prevents or minimizes gene- OMPI ration of molten metal turbulence in the presence of ambient air or inert gas, which would result in such atmospheres being entrapped in the molten metal and poss¬ ibly giving rise to voids in the resulting thin strip product. Turbulence in the molten metal might also result in increased heat transfer from the molten metal to the clad metal strips, and thereby enhances the possibility of melt-through of the molten metal through the clad strip. It is a further preferred feature in the process of the present invention to remove metal oxide present on the metal strips prior to introducing the molten metal between the metal strips . This is generally achieved by passing the steel strips through a pickling acid bath to remove the oxide deposits and improves the bonding integ¬ rity and tensile strength of the resulting thin strip product. The thin metal strip produced by the process of the invention forms another aspect of the present inven¬ tion. According to a further aspect of the present invention, there is provided a clad cast metal strip com¬ prising a metal core and exterior metal cladding bonded to the core, with the metal cladding and the core defining a diffusion interface therebetween. The grain size is larger in the core than at the diffusion interface in view of the fact that the rate of solidification of molten metal when cooling and bonding to the strips to give the diffusion interface is higher than the rate of solidification of metal towards the center of the core. In a further aspect of the present invention, there is provided an apparatus for manufacturing thin metal -^tJRl_T strip, comprising at least two quench surfaces defining a gap therebetween for receiving at least two metal strips, feed means for feeding at least two metal strips in spaced- apart configuration into the gap and into contact with at least one of said spaced-apart quench surfaces, and means for supplying molten metal between the metal strips as the metal strips are fed between the quench surfaces and into the gap. The process of the present invention substan¬ tially reduces the amount of energy required in the produc¬ tion of thin strip hot band steel. The estimated U.S. market for thin strip (.030 inch thick) steel product is 10 million tons per year and as much as 30 million ton in all thicknesses of strip and sheet. The process of the present invention has potential energy saving of as much as 5 million BTU/ton. The direct casting procedure of the pre¬ sent invention enables improvement in the surface condition of the thin strip. In previous melt drag casting pro¬ cedures, the side of the strip which did not contact the quench roll had irregularities. The process of the present invention, which joins two strips with a maximum amount of molten metal, yields a product having two good external surfaces which retain the surface characteristics of the incoming strip. Finally, the use of two thin outer strips eliminates the sticking problem associated with the prior attempts to continuously cast strip steel because in the present process the molten metal does not contact the quench roll. The process of the present invention is premised upon the use of a superior method of transferring the heat from the molten metal while retaining the molten metal in strip form in a thickness range of .030 to .300 inches. Rapid solidification occurs over a time period typically 50 seconds or less, more usually 20 seconds or less, e.g. 5 to 10 seconds, with a temperature drop from about 2800 F to about 2400°F. Rapid heat transfer occurs between the molten metal and the clad strip while bonding also occurs. The metal-to-metal contact between molten metal with a tempera¬ ture of approximately 2800°F and cladding strip with a temperature of less than 500 results in rapid equalization of the composite strip temperature at about 2400 where it is solid. It is not necessary to extract large amounts of heat from the composite product at the time of solidifi¬ cation with the proper molten to solid ratio in the clad cast process. But rather, solidification occurs by equaliza¬ tion of the temperatures of the strip and molten metal being rolled into the proper gauge composite strip, sheet or plate. Rolling and solidification occur simultaneously at a rapid rate by the clad cast process. The rapid equalization of the composite strip temperature is also important from the standpoint of avoid¬ ing wavyness or warping in the final composite strip. If rapid temperature equalization is not obtained, and the metal strips cool at a different rate (typically a higher rate of cooling) than the rate of cooling of the core, then warping or wavyness in the strip may result, which is undesirable. Such warping or wavyness may be reduced by placing a tension on the composite strip after it is completely solidified. The tension may be placed on the strip as it is pulled through the apparatus (for example by arranging the exit speed of the composite strip to be slightly greater than the entry speed), which tension is high enough to eliminate or substantially reduce warping or wavyness without giving rise to melt-through problems. In this way, a more uniform product is obtained and subsequent rolling to reduce thickness is more readily facilitated. The melt drag process and similar processes which extract heat from molten metal by brief contact between the cooling drum and the molten metal are limited in the thickness of metal which can be rapidly solidified. In these practices, heat extraction from the molten metal to the cooling drum is the predominant source of solidifica¬ tion. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and form a part of the specification illustrate pre¬ ferred embodiments of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings: Figure 1 is a schematic side view of a first preferred embodiment of the present invention. Figures 2a and 2b are schematic side view of two preferred embodiments of the present invention. Figures 3a and 3b are cross sectional views of two preferred configurations for the quenching surfaces used in the process of the present invention, taken along the lines III-III in Figure 1. Figure 3c is a schematic side view of an arrange¬ ment for edge crimping of thin metal strip products. Figure 4 is a schematic side view of a third preferred embodiment of the present invention. Figure 5 is a schematic side elevation illu¬ strating a fifth preferred embodiment of the present inven¬ tion. Figure 6 is a schematic view illustrating the internal structure of clad cast metal strip of the present invention. DETAILED DESCRIPTION OF THE INVENTION Reference will now be made in detail to the present preferred embodiments of the invention, which are illustrated in the accompanying drawings. With reference to Figure 1, metal strips 2 and 3, having a thickness of .008 inch, are provided from suitable sources 90 (e.g., coiled on reels) for feeding into gap 9 located between quenching surfaces (e.g., wheels) 4 and 5. The area defined by gap 9 and wheels 4 and 5 is maintained under inert atmospheric conditions by passing an inert gas into the chamber (not shown) containing bath 1 and wheels 4 and 5. Preferably, the inert gas is selected from the group consisting of N 2 , Ar, He. It should be understood that the sources for strips 2 and 3 may also be placed in the chamber. The term "inert" as used herein means not giving rise to any conditions or products which would adversely affect the structure, strength and/or integrity of the thin strip product of this invention. The term "inert atmos¬ pheric conditions" preferably means non-oxidizing or sub¬ stantially non-oxidizing conditions. New strips 2 and 3 may also be provided by utilizing conventional continuous casting procedures such as those set forth in detail in published European Patent " υRE_ξ Applications 040,070; 040,072, and 040,009 herein incor¬ porated by reference. Molten metal from bath 1 is distributed between metal strips 2 and 3 at gap 9, while strips 2 and 3 are in contact with quench surfaces 4 and 5- The ratio of molten to solid is maintained at above 3-4- Strips 2 and 3 and the molten metal layer must remain in contact with at least one quench wheel 4 and/or 5 for a time sufficient to enable partial solidification and cooling of the molten layer enabling strips 2 and 3 to join, one to the other, in sufficient strength to ensure that strips 2 and 3 will not delaminate during subsequent mechanical operations. Joined strips 2 and 3 are further cooled by contact with a third cooling wheel 6 and support surface(s) 7 to ensure permanent bonding of strip 2 and 3 producing a thin hot band crystalline metal strip 8 suitable for imme¬ diate cold reduction or coiling. The thickness of strip 8 is between about .03 to .125 in. Most preferably, the thickness is between about .070 to .080 inch. Support surfaces 7 are also fluid cooled (e.g., water or steam) to aid in the solidification of the molten metal layer. In addition, cool air blast may be supplied to the partially bonded metal strip at appropriate locations. For example, air blast may be positioned between adjacent support surfaces 7- In a preferred embodiment of the present inv. n- tion a rheocast metal slurry may be utilized as the means of joining strips 2 and 3 • Accordingly, molten metal bath 1 may be replaced by a conventional rheocast slurry source. The strip speed for strips 2 and 3 may be within the range of 500 to 8000 inches/min. Most preferably, 500 to 1900 inches/minute should be utilized. Quench surfaces , 5 and 6, respectively, are preferably fluid (e.g., water or steam) cool wheels capable of maintaining the outer surfaces of strips 2 and 3 at a temperature low enough to maintain the integrity of the strips and to provide proper heat transfer from the laminate. In a further preferred embodimen of the present invention, the metal strips 2 and 3 are carbon-steel and the molten metal bath is carbon steel maintained at a temperature of between about 2900 to 2980 F. In a still further preferred embodiment of the present invention, the distribution of the molten metal or rheocast metal slurry between strips 2 and 3 is performed in an inert atmosphere such as He, Ar and N~. It should be understood that metal strips 2 and 3 may be selected of various thicknesses depending upon the ultimate thickness desired in the resulting thin band crystalline metal strip 8. For example, strips 2 and 3 should have a thickness of between aabout .012 and .016 inch for the production of thin band strip 8 having a thickness in the range of .120 and .125 inch. The critical factor in the process of the present invention is to maintain the molten to solid ratio below the point where melt through of strips 2 and 3 occur. The molten solid ratio must be maintained above 3-4 and preferably is about 4.2. Figures 2a and 2b illustrate alternative embodi¬ ments of the process of the present invention.: The cor¬ responding reference numerals appearing in Figures 1, 2a and 2b designate corresponding elements. "gtJRE_t * Referring to Figure 2a, continuous metal support belt 15 is substituted for support wheels 7 shown in Figure 1. The operation of the apparatus proceeds in accordance with the description of Figure 1, supra. However, belt 15 is positioned about cooling wheels 13 and located between feed strip 3 and quenching surface 4. Belt 15 providing continuous support for the partially bonded feed strips 2 and 3 as they emerge from gap 9 and come into contact with quench wheel 6. Of course, cool air blast may be supplied to support belt 15 on its exposed side as it passes about quench wheel 6. Figure 2b illustrates an embodiment which is similar to that shown in Figure 2a, except that an upper support belt 115 is provided which passes around the periph¬ eries of wheels 5, 6 and 113. In addition support surfaces 7 are provided around cooling wheel 6, and may be fluid cooled (e.g., with water) to aid in the solidification of the molten metal layer, as described above in connection with Figure 1. During fabrication of the thin metal strip it is most vulnerable to melt-through in the area identi¬ fied by the letter X in Figure 2b as the thin composite metal strip with molten metal between the metal strips 2 and 3 progresses around cooling wheel 4 and into contact with cooling wheel 6. The upper support belt 115 provides support for the thin metal strip as it progresses through the area X of greatest vulnerability, and thereby lessens the chance of any melt-through of the molten metal through the metal strips 2 and 3- In view of the greater distance over which the metal strip 2 travels, as opposed to the metal strip 3 between cooling wheels 4 and 6, and consequently the greater amount of time during which the metal strip 2 is unsupported by the wheels 5 or 6, it is desirable for the -16- thickness of the strip 2 to be slightly greater than the thickness of the strip 3. For example, the thickness of strip 2 may be between 0.18 to 0.25 inch, typically 0.19 to 0.21 inch, and the thickness of strip 3 may be 0.008 to 0.16 inch, typically 0.12 to 0.16 inch. As discussed above, the thin metal strip is subject to its greatest vulner¬ ability to melt-through in the area designated by the letter X and the chance of melt-through -is reduced by utilizing thicker metal strip for the strip 2 than for the strip 3• A further feature which can be appreciated from Figures 1, 2a and 2b is that the composite thin metal strip, after passing between quench surfaces 4 and 5 moves along a tortuous multi- deflectional path around wheels 4, 6 and 13• As the thin metal strip moves along this tortuous path, it is subject to the constant directional force of gravity while the cast core is still molten, and this results in any bubbles and/or slag in the molten metal, typically non-metallic solid particles such as aluminates and silicates, to be more evenly dispersed throughout the molten metal, and not to collect and be deposited in the interface between the core and the clad strips, as solidifi¬ cation occurs. Such an accumulation of bubbles or particu¬ late slag material in the molten-clad strip interface would impair the bonding Integrity of the thin strip product, and Increase the risk of delamination during subsequent mechan¬ ical working of the product. In commercial practice, solidi¬ fication of the core metal is expected to be completed along the surface of cooling wheel 6 (wheel 6 could be as large as 27 feet in diameter), where maximum through-put will be realized. Thus, by causing the thin metal product to move through a tortuous multi-deflectional path while under the influence of gravity, any entrapped gas bubbles and/or solid impurities in the molten metal having a different specific gravity or weight to that of the molten metal will be dispersed throughout the molten metal as solidification occurs. The solid particles (e.g., non-metallic aluminates and silicates) have a similar effect as gas bubbles in that they form voids in the continuity of the solidified core of the thin metal product, and must be dispersed to prevent accumulations of such particles or voids which might give rise to weaknesses in the tensile strength of the thin metal product or starting sources of delamination. Figures 3(a) and 3(b) are cross-sectional illustrations of two preferred embodiments of quench wheels 4 and 5. Wheels 4 and 5 comprise opposed circular surfaces spaced apart one from the other, forming gap 9. Wheel 4 has an annular flange 11. Wheel 5 has an annular flange 10. Flanges 10 and 11 are located at opposite ends of rolls 4 and 5, respectively. The function of flanges 10 and 11, respectively, is to aid in the confinement of the molten metal or rheocast metal slurry material during distribution between metal strips 2 and 3- Gap 9 provided between flanges 10 and 11 respectively, provides an area where strips 2 and 3 and molten or rheostat metal are maintained during the partial solidification and bonding of strips 2 and 3. It should be understood that other configurations for quenching surfaces 4 and 5 may be utilized. For example, molten bath end control of the laminated material is possible. In operation, the strips 2 and 3 are wider than the distance between flanges 10 and 11, so that edges 120, 122 are crimped at 124 through an angle of approximately 90 . In this way, molten metal is prevented from leaking out between edges 120, 122, and a thin strip product of even width and thickness is subsequently obtained by trim¬ ming the edges to remove edges 120 from 122 after cooling and solidification of the molten metal. In Figure 3(b), flanges 10 and 11 force edges 120, 122 closer together without crimping through an angle of approximately 90° as occurs in the embodiment illustrat¬ ed in Figure 3(a). In order to insure that molten metal does not leak out between edges 120, 122, a floating seal arrangement 126 is provided which urges the edges 120, 122 towards each other to prevent leakage of metal between edges 120, 122. The result of the edges 120, 122 being urged towards one another by the floating seal arrangement 126 is that very little molten metal is allowed to pene¬ trate between edges 102, 122, so that generally only a film thickness of molten metal is present between edges 120, 122. This, in turn, results in solidification of the thin film of metal almost instantaneously to seal edges 120, 122 together. The floating seal arrangement 126 is shown in more detail in Figure 3(c), wherein a floating seal 128 is urged against edge 120 of strip 2 by an adjustable pressure roller 130. In this way, a satisfactory seal between edges 120, 122 is produced without the necessity of crimping the ssttrriippss 22,, 33 tthhrroouugghh 9900° oorr ooff uuttiilliizziinngg a separate crimping apparatus to facilitate such deformation, Figure 4 illustrates another preferred embodiment of the present invention. Metal strips 20 and 22 are positioned to enter gap 23 located between quench surfaces 25 and 27. Positioned between strip 20 and 22 and quench surface 25 and 27 are oveable endless metal support belts 29 and 31, respectively. Belts 29 and 31 are maintained at OMPI substantially the same temperature as quench surfaces 25 and 27 and are mounted about quench surfaces 25, 27, 33, 35, 37 and 39 to provide an endless moveable support surface for strips 20 and 22. Supplemental quench surfaces (wheels 41(a) and 4Kb)) may be provided between quench sur¬ faces 25 and 33, and 27 and 37, respectively. In Figure 4, the thin metal strip does not tra¬ verse a tortuous path as described above in connection with the embodiments illustrated in Figures 1, 2a and 2b, but insteai moves along an approximately straight path between quench surfaces 25, 27, 33, 41(a), 4Kb) and 35. Such an embodiment is particularly useful when "clean" steel is utilized as the molten material, i.e. steel which does not contain significant amounts of slag and has not been sub¬ jected to turbulence or other conditions giving rise to bubbles in the molten metal, since, in that instance, the need to disperse the bubbles or slag throughout the molten metal is lessened. When molten metal containing slag or bubbles is utilized, then it is preferred to employ an arrangement illustrated in Figures 1, 2(a) and 2(b) in order to obtain the desired even dispersion of the bubbles and/or impurities throughout the molten metal as solidifica¬ tion occurs, as discussed in detail earlier. In operation, metal strips 20 and 22 having a thickness of about .008 inch are fed into gap 23 located between quench surfaces 25 and 27. Strips 20 and 22 are cooled and supported by endless belts 29 and 31 positioned about surfaces 25, 27, 33, 35, 37 and 39- Molten metal (43) is fed into gap 23 into contact with strips 20 and 22. The ratio of molten to solid is maintained at above 3-4. Strips 20 and 22 having molten metal 41 therebetween is passed through gap 23 between quench surface 25 and 27 while in contact with cooled endless support belts 29 and 31. The olten layer remains in contact with support belts 29 and 31 for a time sufficient to ensure at least partial solidi¬ fication of the molten metal and the formation of a bond at least strong enough to ensure that strips 20 and 22 do not delaminate. The resulting cast strip 43 has a thickness in the range of .070 to .080 inch. Figure 5 illustrates a further preferred embodi¬ ment in which metal strips 50, 52 are advanced past a tundish 53 in sliding contact with the tundish nozzle tip 54 towards a gap 56 between quench surfaces 58, 60. By adjustment of the angle Y of the two incoming clad strips 50,- 52, usually 5 to 25° relative to the tundish wall 55, the strips touch or bear upon the tundish nozzle tip 54 at contact points 62, 64 before the clad strips come into contact with the quench surfaces 58, 60 on the opposite sides of the two clad strips. The distance between the contact points 62, 64 an contact points 66, 68 where the strips contact the quench surfaces 58, 60 is variable and can be controlled, and is generally in the range 0.05 to 2 inches, usually no more than 0.1 inch, depending on the thickness of the strips 50, 52. The distance Is suffici¬ ently short as compared with the thickness of the strips 50, 52 to ensure that the strips 50, 52 have sufficient tensile strength to prevent bulging or bursting under the head pressure of molten metal 70 entering the gap 56 through the nozzle tip 54- The head pressure of the molten metal is controlled by the depth X of the molten metal in the tundish. Another function of this distance between con¬ tact points 62, 64 and contact points 66, 68 Is to prevent the nozzle tip 54 from bearing downward, in the direction of the metal flow and onto the quench surfaces 5^>, 60 through the strips 50, 52. This prevents the nozzle tip 54 from being swept into pinch point 5. The downward head pressure of the molten metal flowing from the tundish 53 through the nozzle tip 54 operates to reliably and continu¬ ously fill the region between the nozzle tip 54 and the pinch point 55 to continuously provide a layer of molten metal having a depth at least equal to the depth X referred to above. According to the above described arrangement, the clad strips form a barrier between the molten metal flowing from the tundish nozzle tip 54 into the gap 56 and shield the molten metal from ambient air or the inert gas atmosphere and thereby minimize entrapment of inert gas or air in the molten metal as it passes into the gap 56. The reduced exposure of the molten metal to oxygen in the ambient air reduces the formation of metal oxides which, in turn, improves the bonding integrity and tensile strength of the resulting product, and reduces any tendency for the product to undergo delamination when subjected to further mechanical operations such as rolling and bending. If an inert gas atmosphere is present, then the use of the metal strips 50, 52 as a barrier or curtain prevents or minimizes entrapment of inert gases in the molten metal which subsequently can give rise to voids in the metal after it solidifies. A further advantage arising from the arrangment illustrated in Figure 5 is that the entry of the strips 50, 52 past the tundish nozzle tip 54 reduces molten metal turbulence in the presence of ambient air or inert gases which would result in such atmospheres being entrapped in the molten metal, and giving rise to oxide metal formation or void formation, as described above. In the description of Figure 1 above, it was indicated that it is preferred to house the apparatus, especially that portion of the apparatus where the molten C metal is introduced between the strips 2, 3, in a chamber so that the metal can be introduced between the metal strips in an inert gas atmosphere. However, when the above- described arrangement is used where the metal strips form a barrier and the process is conducted under a shroud of inert gas in the region where the molten metal is intro¬ duced between the metal strips, then sufficient protection is generally afforded by the shroud, and it -is possible to dispense with the chamber for housing the apparatus. The prevention of oxide entry into the molten metal can be further accomplished by passing the strips 50, 52 through a pickling acid bath in order to remove metal oxide present on the strip. Pickling is preferably con¬ ducted immediately prior to feeding the metal strips 50, 52 into the gap 56, and this removes metal oxide deposits adhering to the surface of the strips 50, 52. Figure 6 illustrates in cross section the clad cast metal strip of the invention. The strip comprises a metal core 70 and exterior metal cladding 72, 74 bonded to the core 70 by a fusion bond generally identified by the numeral 76. The fusion bond 76 is formed as the strips 50, 52 come into contact with the molten metal and at least a molecular thickness of the surface layer of the metal strip melts to give rise to bonding upon cooling and solidifica¬ tion of the molten metal. The fusion bond 76 defines a diffusion interface 78 between the cladding 72, 74 and the core 70, and the grain structure 80 in the core is general¬ ly larger than the grain structure 82 in the diffusion interface in view of the fact that the metal in the diffusion interface underwent more rapid cooling than the metal towards the center of the core. As indicated earlier, according to another aspect of the present invention, there is provided an apparatus for manufacturing thin metal strip of the invention, and this is illustrated in Figures 1 through 4 of the present application. Referring to Figure 1, the apparatus comprises at least two quench surfaces 4,5 defining a gap 9 there¬ between for receiving at least two feed metal strips 2, 3- The apparatus also includes feed means 90 for feeding the at least two feed metal strips 2, 3 in spaced-apart con¬ figuration into gap 9 and into contact with at least one of said quench surfaces 4, 5 - Means 1 is also provided for supplying molten metal between the feed strips 2,3 as the strips 2, 3 are fed between quench surfaces 4, 5 into gap 9. The process of the present invention produces a 'hot band crystalline metal strip which has sufficient strength and thickness to enable immediate cold rolling of the strip. In addition, the product can be partially recycled reducing substantially the energy expenditure as¬ sociated with past hot band strip formation techniques. The potential energy savings by the use of the process are as much as 10 million BTU/ton. The direct casting procedure of the present invention produces a continuous strip product having both surfaces retaining the original surface charac¬ teristics of the metal strips while simultaneously elimi¬ nating the sticking problem associated with previous cast¬ ing techniques. The outer strips prevent molten metal con¬ tact with the cooling surfaces, thereby eliminating the sticking problem associated with prior casting techniques. A critical aspect of the present invention is that the two metal strips and molten metal are retained in contact with the quenching surfaces under appropriate conditions and for a time sufficient to produce an adequate bond between these strips so that delamination of the strips will not occur during the subsequent operation such as cold rolling. The foregoing description of preferred embodi¬ ments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. For example, the angle shown in Figure 1 may be varied from 45 to 70°. In addition, the molten metal may be the same as that of the metal strip, for example carbon steel in each instance, or the molten metal used may be different to that of the metal strip, for example copper cladding on a steel core. Alternatively, the metal strip may be of a different compo¬ sition to the molten metal, for example the strip may be of an alloy of the metal present in the core, e.g., stainless steel cladding on a carbon steel core, or a high purity aluminum alloy on a high tensile strength- aluminum alloy core. Finally, the quench surface (wheels) may be varied in size. For example, wheels 4 and 5 shown in Figure 1 may be made as large as wheel 6. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled In the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.

Claims: I CLAIM 1. A process for continuously producing thin metal strip, said process comprising: (a) providing at least two metal strips; and (b) introducing molten metal between said metal strips to at least partially bond said metal strips and said molten metal together, said molten metal being introduced between said metal strips under conditions such that said metal strips do not delami¬ nate during subsequent mechanical operations. 2. A process according to claim 1, wherein said metal strips are fed to a gap between a first quench surface and a second quench surface. 3. A process according to claim 2, wherein said molten metal is introduced between said metal strips to provide a molten layer of metal between said strips during contact of said strips with said first and second quench surfaces. 4. A process according to claim 3, wherein said metal strips remain in contact with at least one of said quench surfaces for a time sufficient to ensure at least partial solidification of said molten metal and at least partial bonding together of said strips and molten metal. 5. A process according to claim 1, and including the step of cooling said partially bonded strips to form a permanently bonded thin metal strip. 6. A process according to claim 5, wherein said cool¬ ing is conducted by contacting said partially bonded strips with a thir-3. quenching surface. ( OMPI 7. A process according to claim 1, wherein said molten metal is introduced between said metal strips in the presence of an inert atmosphere. 8. A process according to claim 7, wherein said inert atmosphere is selected from the group consisting of argon, nitrogen and helium. 9- A process according to claim 8, wherein said inert atmosphere is argon. 10. A process according to claim 1, and including the step of minimizing entrapment of ambient air or inert gas in said molten metal as said molten metal is introduced between said metal strips. 11. A process according to claim 10 where said entrap¬ ment of ambient air or melt gas in said molten metal is minimized by minimizing generation of turbulence in said molten metal as said molten metal is introduced between said metal strips. 12. A process according to claim 10, wherein said entrapment is minimized by providing a barrier for shield¬ ing said molten metal from said ambient air or inert gas atmosphere as said molten metal is introduced between said metal strips. 13. A process according to claim 12, wherein said metal strips form at least part of said barrier. 14- A process according to claim 13, wherein said metal strips are each fed in sliding contact with a tundish nozzle tip into contact with a first and second quenching surface, said metal strips traveling a distance between said tip and said first and second quench surfaces such that distortion or bursting 'of said metal strips during passage between said tip and said first and second quench surfaces is minimized. 15- A process according to claim 1, wherein metal oxide present on said metal strips is removed prior to introducing said molten metal between said metal strips. 16. .". process according to claim 15, wherein said metal oxice is removed by passing said metal strips through a pickling acid bath. 17. A process according to claim 1, wherein said metal strips comprise the same metal as the metal of the molten metal . 18. A process according to claim 17, wherein said metal strips are comprised of stainless steel, and said molten metal is comprised of carbon steel. 19- A process according to claim 1, wherein said metal strips comprise a different metal to that of said molten metal . 20. A process according to claim 19, wherein said metal strips are comprised of copper, and said molten metal is comprised of carbon steel. 21. A process according to claim 1, wherein said metal strips are partially bonded by introducing a rheocast metal slurry between said strips. 22. A process according to claim 21, wherein said metal strips and said rheocast metal slurry consist of carbon-steel. _ OMPI -28- 23. A process according to claim 1, wherein the molten to solid ratio is above 3-4. 24. A process according to claim 1, wherein the molten to solid ratio is above 4.2. 25. A process according to claim 24 wherein the result¬ ing thin metal strip has a thickness in the range of .070 to .375 inch. 26. A process according to claim 1, wherein said metal strips of step (a) have a thickness of about .008 to .040 inches. 27. A process according to claim 1, wherein said thin metal strip has a thickness between about .03 to .375 inches. 28. A process according to claim 27, wherein said thin metal strip is cold rolled to a thickness of about .008 to 0.040 inches and recycled to provide said feed strip. 29. A process according to claim 2, wherein said metal strips are fed into said gap between said quenching sur¬ faces at a speed of about 500 to 8000 inches/ minute. 30. A process according to claim 29, wherein said speed is between about 500 to 1900 inches/minute. 31. A process according to claim 2, wherein said quench surfaces are provided with an internal cooling fluid. 32. A process according to claim 31, wherein said cooling fluid is water or steam. 33. A process accordin-g to claim 31, wherein said quench surfaces are maintained at a temperature low enough to prevent fatiguing of the metal . 34. A process according to claim 2, wherein the quench surfaces comprise at least two quench wheels having two endless metal support belts positioned between said wheels and the outer metal strips. 35- A process according to claim 34, wherein said quench surfaces are maintained in inert atmosphere. 36. A process according to claim 35, wherein said inert atmosphere is selected from the group consisting of Ar, N , and He. 37. A process according to claim 1, wherein said metal strips have a composition different than the composition of the molten metal. 38. A process according to claim 1, wherein said strips and molten metal are carbon steel and the molten metal is maintained in a temperature range of 2900° to 2980°F. 39. A process according to claim 1 and further includ¬ ing moving said metal strip with molten metal therebetween along a tortuous path to effect even dispersion of gas bubbles and/or impurities present in said molten metal throughout said molten metal as said molten metal solidi¬ fies. 40. A process according to claim 2, wherein said metal strips are moved past said first and second quench surfaces under a tension sufficient to prevent or reduce warping in the thin metal strip in order to facilitate subsequent rolling to reduce the thickness of the thin metal strip . 41. A thin metal strip produced by the process of claim 1. 42. Clad cast metal strip comprising a metal core and exterior metal cladding bonded to said core, said metal cladding and said core defining a diffusion interface there¬ between, said diffusion interface having a grain size smaller than that of said metal core. 43- Clad cast metal strip according to claim 9, wherein said metal core is comprised of carbon-steel, and said exterior metal cladding is comprised of stainless steel . 44. Apparatus for manufacturing thin metal strip, said apparatus comprising: (a) at least two quench surfaces defining a gap therebetween for receiving at least two metal strips; (b)feed means for feeding at least two metal strips in spaced-apart configuration into said gap and into contact with at least one of said quench sur¬ faces; and (c) means for supplying molten metal between said metal strips as said metal strips are fed into said gap and into contact with said quench surfaces. OMPI

Inventors: SHENEMAN RALPH L, SHENEMAN, RALPH, L.
Applicants: SHENEMAN RALPH L, SHENEMAN, RALPH, L.
Application year: 1984
Main classification: B22D19
IPC codes: B22D11