COOLING ROLLS FOR PRODUCING RAPIDLY SOLIDIFIED METAL STRIP SHEETS

Abstract: Cooling rolls are disclosed, which are adapted to produce rapidly solidified metal strip sheets through receiving a falling stream of a metal melt, and rapidly cooling and solidifying it. Each of the cooling rolls comprises a roll base body (1) and a sleeve (2) which is fitted around a barrel periphery of the roll base body and forms a cooling water flow path (6) between the roll base body and the sleeve. The sleeve is only partially tightly fixed to the roll base body. End portions of the sleeve are joined to the roll base body in such a soft structure that movement of the sleeve in the roll axial direction due to the thermal expansion is not interrupted at end portions of the sleeve.

Description: The present invention relates to cooling rolls for producing metal strip sheets by rapidly solidifying molten metal and is concerned with advantageously producing sound strip sheet products by reducing to the utmost the heat crown inevitably occurring at the outer peripheral surface of the cooling roll during the step of cooling-solidification of the molten metal. The technique of continuously producing rapidly solidified metal strip sheets by directly feeding molten metal on to the surface of a cooling roll and rapidly cooling and solidifying it has been widely used to produce amorphous alloys by means of a single roll or by means of double rolls. However, since the molten metal is cooled to not more than its solidification point or not more than its crystallization temperature by rapidly extracting heat from it, the temperature of the outer peripheral surface of the roll with which the molten steel is brought into contact increases and consequently the cooling roll thermally expands. At that time, a temperature gradient is developed in the axial direction of the roll between the metal-contacting portion and the portion which does not contact the molten metal, so that the roll surface is deformed into a barrel-like shape having a larger curvature and exhibits a so-called heat crown. In the rapidly liquid-solidifying method using a single roll, a nozzle having a narrow slit-like shape is generally used, and its tip is spaced from the surface of the roll by a narrow distance of about 0.1 to 0.5 mm. Thus, when the dimension of the nozzle slit, the peripheral speed of the roll, and the pressure for injecting the molten metal are set to be constant, the thickness of the strip sheet is largely influenced by the gap between the nozzle tip and the roll surface. Therefore, if a heat crown is formed at the outer peripheral surface of the roll, the gap between the nozzle tip and the roll surface becomes narrower at the central portion of the strip sheet. Accordingly, there is the inconvenience that the thickness of the strip sheet is smaller at its central portion and larger at its edge portions. In order to solve thickness variations in strip sheets due to the above heat crown, Japanese Patent Application Laid-open Nos. 56-68,559, 59-54,445, 57-112,954 and 58-135,751 proposed techniques by which the temperature distribution is uniformized by varying the cooling power between the central portion and the end portions of the roll by suitably selecting the number, dimension and shape of cooling channels so as to enhance the cooling power at the widthwise central portion of the roll as compared with that at the end portions thereof with a view to preventing the occurrence of the heat crown. Each of these techniques involves increasing the amount of heat to be extracted from the central portion of the roll by relatively increasing the amount of cooling water or the cooling area at the central portion as compared with the end portions thereof. However, when utilising the above method it is necessary to exchange the cooling roll when it is desired to produce strip sheets of different thickness and, as mentioned later, even if the temperature distribution is made uniform in the roll axial direction, this does not mean that thermal expansion is uniformized and the heat crown is diminished. Japanese Patent Application Laid-open No. 59-229,263 proposed a technique of mechanically grinding off the thickness difference, due to the thermal expansion, between the central portion and the end portions of the roll. However, although such a technique is not impossible in theory, not only is large equipment provided with a precision mechanism necessary but also this technique is impractical since it requires precision polishing of the rolled surface during pouring of the molten metal. Thus, it is actually inapplicable. Japanese Patent Publication No. 60-51,933 (U.S. Patent Application No. 115,517, filed on January 25, 1980) proposed a technique in which cooling channels are formed inside a metal sleeve in parallel with the roll axial direction to make the thermal expansion in the roll radial direction constant and to lessen the heat crown. In this technique, it is necessary to provide a plurality of cooling water channels in parallel with the roll axial direction and spaced at intervals in the circumferential direction, a cooling water retaining portion on the water feed side, and a cooling water retaining portion on the water discharge side at the axial ends of the roll. Therefore, a fixing mechanism is necessary at the roll central portion. However, this technique places its emphasis upon the radial heat expansion of the roll and the accompanying radial thermal stress only; it utterly fails to consider the importance of the thermal expansion in the roll axial direction which is addressed by the present invention. Furthermore, the fixing mechanism at the roll central portion becomes complicated and a high dimensional precision is also required in the fitting between the inner surface of the roll and the shaft end portions. Thus, extremely precise machining becomes necessary. In addition, this technique has the disadvantage that heat expansion is not improved to a satisfactory degree despite the substantial machining and the high cost. As mentioned above, in the case of the single roll method, the cooling roll is deformed to a barrel-like shape during the casting process, and the gap between the nozzle tip and the roll surface becomes narrower at the central portion. As a result, the strip sheet becomes thinner at the central portion thereof. Thus when producing amorphous alloy strip sheets, it is extremely difficult to appropriately correct the thickness distribution across the strip sheet in the widthwise direction during succeeding rolling, etc. In the above-mentioned Japanese Patent Publication No. 56-68,559 and Japanese Patent Application Laid-open Nos. 59-54,445, 57-112,954 and 58-135,751, the method is controlled such that the temperature distribution in the roll axial direction is uniformized over the whole width of the strip sheet by appropriately devising the water cooling structure inside the cooling roll. In other words, these techniques are based on the assumption that, if the temperature distribution is uniform, the amount of thermal expansion becomes uniform so that no heat crown occurs. However, it has been found, by examination of the heat crown-occurring mechanism in experiments and computer simulations, that this assumption is not justified and that heat crown cannot be suppressed to a satisfactorily low degree by uniformly controlling the temperature distribution. That is, it was found experimentally and thorough simulations that, when rapidly solidified metal strip sheets were cast by using a cooling roll as shown in Fig. 2 of the accompanying drawings in which heat insulating portions are formed in the roll axial direction by cutting deep grooves in the sleeve 3 mm outside a strip sheet width of 100 mm to make the heat flux from the surface of the sleeve flow in the roll radial direction only, the temperature on the surface of the sleeve was highly uniform inside the deep grooves. However, the amount of thermal expansion and the measured thickness distribution of the rapidly solidified metal strip produced were almost the same as in the case of using a rapidly cooling roll of conventional type in which the sleeve surface temperature becomes higher at the centre in the roll axial direction. Thus, extremely inadequate results were only obtained. From the above experimental facts, it was concluded that the heat crown problem could not effectively be solved by the prior art techniques which take into account the surface temperature of the roll only. The present invention has been developed in view of the above-mentioned circumstances, and has, as its object, the provision of a cooling roll for the production of rapidly solidified metal strip sheets, which cooling roll can reduce to the utmost the heat crown occurring at its outer peripheral surface during the rapid cooling-solidification step and effectively give good quality rapidly solidified strip sheets having no variations in thickness. GB-A-2-075 150 discloses a carrier roll useful for high temperature service in the continuous casting of billets and blooms and conveying plates. The roll comprises a sleeve mounted on a shaft. The sleeve is shrink-fitted onto the shaft such that it is tightly fitted to the shaft at the central portion and less tightly fitted to the shaft at the end portions. However there is no suggestion that the sleeve is fixed to the shaft solely in its central portion. Moreover there is no suggestion that the roll is suitable for receiving and rapidly cooling and solidifying a molten metal. DE-A-1-527 650 discloses a carrier roll comprising a sleeve mounted on a roll body provided with cooling water passages. The sleeve may be secured to the roll body by shrinkage or by screws, bolts, etc. However there is no suggestion that the sleeve is fixed to the roll body solely in its central portion or that it is suitable for use as a casting roll in continuous casting. According to the present invention, there is provided a cooling roll adapted to produce rapidly solidified metal strip sheets by receiving a falling stream of molten metal and rapidly cooling and solidifying it, which cooling roll comprising a roll base body and a sleeve which is fitted around the periphery of the roll base body and defines a cooling water flow path for cooling the roll, characterised in that the sleeve is fixed to the periphery of the roll base body only at its central portion and its end portions are sealed to the roll base body in a manner such that movement of the sleeve in the roll axial direction due to thermal expansion is not interrupted at the end portions. These and other objects, constituent features and advantages of the present invention will be appreciated upon reading of the following description of the invention when taken in conjunction with the attached drawings, with the understanding that some modifications, variations and changes of the same can be made by the skilled person in the art to which the invention pertains without departing from the spirit of the invention or the scope of the claims appended hereto. For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, wherein: Figs. 1(a) to 1(c) are sectional views showing preferred embodiments of cooling rolls constructed in accordance with the present invention; Fig. 1(d) is a sectional view of another cooling roll in accordance with the present invention; Fig. 2 is a sectional view of a conventional cooling roll; Fig. 3 is a graph showing the relationship between molten metal feed time and thermal expansion of the roll surfaces for a cooling roll of the present invention and a conventional cooling roll; and Fig. 4 is a graph illustrating the influences of the length over which the roll sleeve is tightly fixed to the roll and the pouring width upon the heat crown. First, the history of the present invention will be explained. When a molten metal is rapidly solidified upon being contacted with the surface of a cooling roll, the roll itself gradually reaches a higher temperature unless heat extracted from the molten metal is transferred into cooling water. Consequently, it becomes impossible to cool the fresh molten metal which is succeedingly fed. Therefore, in order to effectively cool the molten metal, the roll is preferably designed as a double structure consisting of a roll base body and a metallic sleeve so as to provide channels for internal cooling water. Also, in this way, a metal having higher heat conductivity, which is advantageous in extracting heat, can be used for the surface of the roll and the outer peripheral surface is easy to exchange or repair when it has become worn. The present invention is aimed at preventing of occurrence of heat crown due to heat expansion by making the sleeve upon which the molten metal is injected substantially non-restrained by the roll base body, apart from at its central portion, in the roll axial direction. Detailed analysis has revealed that the heat crown, i.e the deformation of the outer periphery of the sleeve into a barrel-like shape owing to thermal expansion, is caused by the fact that the outer peripheral surface of the sleeve swells because the thermal expansion in the roll axial direction is mechanically restrained at the connection between the sleeve and the roll base body or at ends of the sleeve rather than the fact that the amount of the radial thermal expansion varies in the roll axial direction due to the temperature distribution of the roll surface in the roll axial direction. Based on the above analysis, the present inventors have newly developed a cooling roll construction which restrains swelling in the roll radial direction, that is toward the outer peripheral surface of the sleeve, by releasing the thermal expansion of the metallic sleeve in the roll axial direction without restraining the axial thermal expansion of the sleeve at the axial end portions thereof and allows only the essential radial thermal expansion toward the outer peripheral surface of the sleeve. Thus, they have accomplished the present invention. That is, the present invention involves a cooling roll which is adapted to produce rapidly solidified metal strip sheets by receiving a falling stream of a metal melt, and forcedly rapidly cooling and solidifying it, which roll comprises a roll base body and a sleeve fitted around the barrel periphery of the roll base body and forming a cooling water flow path, for example, between the sleeve and the roll base body, for cooling the roll. The sleeve is only partially tightly fixed to the roll base body and is joined to the roll base body at the end portions of the sleeve in such a way that movement of the sleeve in the roll axial direction due to thermal expansion is not interrupted at the end portions of the sleeve. Preferably, the central portion of the sleeve (i.e. about the middle third of the metallic sleeve) is tightly fixed to the roll base body. [The term "tightly fixed portion (or length)" is used throughout the specification and claims to mean the portion (length) of the sleeve which is tightly fixed to the roll base body]. Referring to Figs. 1(a) through 1(c), reference numerals 1 and 2 denote the roll base body and the sleeve which may be made of copper or a copper base alloy, respectively. The sleeve 2 is fitted around the barrel-shaped roll base body 1. The sleeve 2 is tightly fixed to the roll base body 1 by shrink fitting or the like to a part only thereof, that is at central portion "A" in Fig. 1. On the other hand, the sleeve is spaced from the roll base body 1 at portion "B" located away from central portion "A" towards the roll axial ends and at end portion "C" i.e. the sleeve end portion is engaged with a soft structure without the sleeve 2 being in direct contact with the roll base body 1. That is, a sealing member 2 such as an O-ring or a gasket prevents cooling water from leaking at the sleeve end portions C, while it absorbs the expansion in the sleeve axial direction together with a buffer plate 4. The sealing member 3 is supported by a side guide 5 attached to the end portion of the roll base body 1. Reference numerals 6, 7 and 8 are cooling water channels, a metal melt, and a pouring nozzle, respectively. In Fig. 1(a), the sleeve 2 is tightly fixed to the barrel periphery of the roll base body at the centre by means of two flanges projecting inwardly from the inner peripheral surface of the sleeve 2. In Fig. 1(b), the sleeve is tightly fixed around the roll base body by one inner peripheral projection. In Fig. 1(c), a cooling water path is formed around the roll base body and the sleeve is tightly fixed against two flanges on the peripheral surface of the roll base body. As a tightly fixing method, shrinkage fitting is particularly advantageously employed among others. However, the invention is not restricted to it. Thus, the roll base body and the sleeve may, for example, be joined together by using a key or mechanically. In order to prevent heat from dissipating into the air through the end faces of the sleeves 2 and to make the temperature distribution uniform in the sleeve axial direction, it is particularly preferable, as shown in Fig. 1(a), that buffer plate 4 having high heat insulating effect is inserted between the end face of the sleeve 2 and the side guide 5. As such a heat insulating material, asbestos or polytetrafluoro ethylene (such as that known by the Trade Name Teflon) is preferable. In Fig. 1(d) there is shown another embodiment of a cooling roll according to the present invention. This embodiment is constituted such that a cooling water path is provided inside the metallic sleeve and water is fed or discharged from the sides. In this embodiment, the sleeve is also tightly fixed to the roll base body at the central portion "A" only by shrinkage fitting. Next, the effects obtained when a cooling roll according to the present invention is used will be explained below with reference to the following experimental data. By using a cooling roll with the sleeve structure shown in Fig. 1(a) according to the present invention and the conventional cooling roll shown in Fig. 2, the change in thermal expansion with the lapse of time were examined when actually producing rapidly solidified strip sheets and the results are shown in Fig. 3 for comparison purposes. At that time, the width of the nozzle slit (in the roll axial direction) for ejecting the molten metal and the length of the sleeve were set at 100 mm and 105 mm, respectively. In the case of the conventional sleeve shrinkage fitting structure, the difference in the amount of thermal expansion between the sleeve central portion and the portion located 15 mm from the end of the sleeve, that is the heat crown, was about 220 µm and the sleeve was deformed into a barrel-like shape. On the contrary, when the cooling roll according to the present invention was used, the value was as small as about 20 µm. Thus, according to the present invention, the heat crown was reduced to not more than 1/10 of that obtained conventionally. It is clear that the sleeve axial end non-restraint method according to the present invention has an extremely high effect in restraining the heat crown of the cooling roll. What is intended by the present invention is that the heat crown is eliminated by absorbing the expansion of the sleeve in the axial direction. The heat crown can be suppressed to an extremely small level by only partially tightly fixing the sleeve to the roll base body. In the prior art technique, the heat extracting effect has been improved by feeding a large amount of cooling water (not less than 100 m³/hr) to lower the roll surface temperature and reduce the amount of thermal expansion. On the other hand, according to the present invention, even if the amount of cooling water for cooling the sleeve is lessened to a remarkably smaller level as compared with the prior art technique, (for instance around 3 to 5 m³/hr) the absolute value of the thermal expansion will become larger but the difference in thermal expansion between the central portion and the end portions of the sleeve, that is the heat crown, is smaller so that variations in the thickness of the resulting products are not more than 2 µm. As mentioned above, the present invention also has the advantage that the large amount of cooling water as required in the prior art technique is not necessary. Further, it has been revealed that, when the gap between partitions of the sleeve and the outer periphery of the roll base body is not more than 1 mm in the non-restraint zones in the cooling roll structure, cooling water preferentially flows through the cooling water channel. If the gap is more than 1 mm, the amount of cooling water passing through the gap increases so that the cooling water has a reduced tendency to flow through the cooling water channel. Thus, it is preferable to suppress the gap at the cooling water partitions between the sleeve and the roll base body to not more than 1 mm. Furthermore, it is preferable that the distance between the axial end of the sleeve and the side guide is set at not less than a value of (▵Txαxℓ)/2 in which ▵T, α and ℓ are the maximum temperature of the sleeve, the coefficient of linear thermal expansion of the sleeve and the axial length of the sleeve, respectively. If the width of the seal at the sleeve end face can be increased, the space may be arbitrarily increased. Next, the influences of the tightly fixed length upon the heat crown were examined, and the results are shown in Fig. 4 which is a graph of the tightly fixed length and the width of the poured melt. As is evident from Fig. 4, when the tightly fixed length between the roll base body and the sleeve exceeds 60% of the width of the rapidly cooled strip sheet products, the heat crown cannot fully be eliminated. For instance, when a rapidly solidified metal strip sheet of 100 mm in width is produced according to the single roll method and the tightly fixed length exceeds 60% of the width of the strip sheet, the heat crown is 100 µm or more and the difference in the thickness of the products is 3 µm or more. It was also found that when strip sheets having a width of 200 mm or more were produced and the tightly fixed length exceeds 100 mm, the heat crown exceeds 100 µm even if the tightly fixed length is less than 60% of the width of the product. Therefore, it is preferable that the tightly fixed length between the sleeve and the roll base body is not more than 60% of the width of the rapidly solidified metal strip sheet, and is about 100 mm at a maximum. As mentioned in the foregoing, the present invention is different from the prior art techniques, and is mainly aimed at releasing the heat expansion in the roll axial direction. The present invention has been studied from this standpoint of view. The heat crown was extremely effectively suppressed by making the axial end portions of the metallic sleeve substantially free from restraint by the roll base body, while variations in the thickness could be reduced to an almost ignorable level. Preferably, the temperature distribution at the surface of the cooling roll in the roll axial direction is made uniform so that the heat crown is further reduced. Thus, the distribution of the thermal expansion in the roll radial direction is uniformized in the roll axial direction. More particularly, deep grooves serving as a portion for effectively insulating heat in the roll axial direction may be provided just outside of the pouring portion as shown in Fig. 1(b) or a heat insulating plate such as an asbestos plate may be inserted between the metallic sleeve and the side guide. The present invention will be explained in more detail with reference to the following example. It is given merely as an illustration of the invention, but is not intended to be interpreted so as to limit the scope of the invention. Example 1 By using a cooling roll constructed as in Fig. 1(a) in which the length of the sleeve in the roll axial direction was set at 155 mm and the tightly fixed length in the central portion was 40 mm, molten metal was injected on to the surface of the cooling roll through a nozzle slit of width 150 mm and an Fe-B-Si base amorphous alloy strip sheet was produced according to the single roll method. The heat crown at the outer peripheral surface of the sleeve during the injection (expressed as the difference in thermal expansion between the central portion and the portion located 15 mm toward the central portion from the end portion) was as small as 40 µm. At that time, the average thickness of the strip sheet was 21 µm with a longitudinal deviation of ±1 µm and a thickness difference as small as 2 µm. Comparative Example 1 By using a conventional cooling roll as in Fig. 2 in which the length of the sleeve in the roll axial direction was 200 mm and the sleeve was restrained by the cooling roll body over its entire width apart from in the region of cooling channels, an Fe-B-Si base amorphous alloy strip sheet was prepared in the same manner as in Example 1. The heat crown at the outer peripheral surface of the sleeve during the injection was as large as 350 µm. At that time, the thickness of the resulting strip sheet was 16 µm at the widthwise central portion, and 25 µm at the edge portion with a thickness difference as large as 9 µm. Further, numerous holes penetrating the widthwise central portion of the strip sheet over the entire thickness were formed. As has been described above, according to the present invention, the deformation of the cooling roll to a barrel-like shape due to the heat crown during the production of rapidly solidified metal strip sheets is solved by a completely novel method which is different from the conventional technique, that is, by releasing the thermal expansion of the sleeve in the roll axial direction while the axial end portions of the sleeve are substantially not restrained by the roll base body. Thus, the deviation in thickness in the strip sheets can largely be reduced without necessitating complicated changes in the roll structure. Therefore, the invention is of great interest in the industrial field.

Claims: A cooling roll adapted to produce rapidly solidified metal strip sheets by receiving a falling stream of molten metal (7) and rapidly cooling and solidifying it, which cooling roll comprising a roll base body (1) and a sleeve (2) which is fitted around the periphery of the roll base body and defines a cooling water flow path (6) for cooling the roll, characterised in that the sleeve is fixed to the periphery of the roll base body only at its central portion (A) and its end portions (C) are sealed to the roll base body in a manner such that movement of the sleeve in the roll axial direction due to thermal expansion is not interrupted at the end portions. A cooling roll as claimed in claim 1, wherein the length of the central portion of the sleeve which is fixed to the roll base body is not more than 60% of the width of the rapidly solidified metal strip sheet and the width of the rapidly solidified metal strip sheet is not more than 100 mm. A cooling roll as claimed in claim 1 or 2 wherein the end portions of the sleeve are sealed by means of an O-ring (3) supported by a side guide (5) on the roll base body. A cooling roll as claimed in any one of claims 1 to 3 wherein the central portion (A) of the sleeve is constituted by two inwardly projecting flanges. A cooling roll as claimed in any one of claims 1 to 3 wherein the central portion (A) of the sleeve is constituted by a single inwardly projecting flange. A cooling roll as claimed in any one of claims 1 to 3 wherein the roll base body includes two peripheral flanges against which the central portion (A) of the sleeve is tightly fixed.

Inventors: KOBAYASHI, SHINJI C/O TECHNICAL RESEARCH DIVISION, MORITO, NOBUYUKI C/O TECHNICAL RESEARCH DIVISION, SATO, TORU C/O TECHNICAL RESEARCH DIVISION
Applicants: KAWASAKI STEEL CORPORATION
Application year: 1987
Main classification: B22D11
IPC codes: B22D11