US2743494A

US2743494A - Method for the continuous casting of metal

Info

Publication number: US2743494A
Application number: US391135A
Authority: US
Inventors: Rossi Irving
Original assignee: Continuous Metalcast Co Inc
Current assignee: Continuous Metalcast Co Inc
Priority date: 1953-11-09
Filing date: 1953-11-09
Publication date: 1956-05-01
Anticipated expiration: 1973-05-01

Description

United States Patent O METHOD FOR THE CONTINUOUS CASTING OF METAL Irving Rossi, Morristown, N. J., assignor to- Continuous Metalcast Co., Inc., Wilmington, Del., a corporation of Delaware No Drawing. Application November 9, 1953, Serial No. 391,135

2 Claims. (Cl. 21-2001) This invention relates to the art of the continuous casting of metals.

'Continuous casting, as the name implies, connotes the continuous shaping or forming of liquid metal as it is cooled and solidified to form a casting of indefinite length. Ideally, the liquid metal should be shaped by a mold which is stationary with respect to the metal which it contains, as in static mold casting. From the theoretical point of view, therefore, the ideal form of mold for continuous casting would be one of unlimited length, but since this cannot exist in practice, other devices have been utilized.

Thus, it has been proposed to use endless supports such as revolving drums, wheels and the like, or endless moving bands or endless chains of mold sections which join together to form a mold at the start of the solidification process and separate at its conclusion to release the solidifiedv metal. Since the surfaces of such movable supports can remain stationary with respect to the metal during along the axis of the casting during the entire casting operation. This method of continuous casting was foreshadowed by Pehrson as early as 1914, U. S. Patent No. 1,088,171. Pehrson disclosed a mold which is activated by a cam or eccentric which imparts a simple harmonic motion thereto. As the mold advances a rocker clamp grips the casting causing. it to advance with the mold to the end of its advancing stroke. Then on the return stroke of the mold, the casting is released fromthe mold, but

is held stationary by another clamp until the mold begins its next stroke.

The Pehrson process never achieved commercial success, and the following features may be noted as possibly or probably contributing to its failure. First, the advancing movement of the casting is intermittent. As disclosed by Pehrson, the casting advances only during one half of the total casting time. It remains stationary during the other half of the casting time. Second, the advancing movement of the casting is at a variable rate. Since the casting is advanced by a simple harmonic motion, the rate of advance increases until the middle of the stroke and then decreases until the end of the stroke. Third, the relative motion between the mold and casting which takes place on the return stroke of the mold is also at a variable rate, being at a maximum at the middle of the return stroke.

Other unfavorable factors may also have contributed to Pehrsons failure, but it may now be said that the above, at least would be considered unfavorable in view of present knowledge.

A more successful method of utilizing a reciprocating mold is that proposed by Junghans, U. S. Patent No. 2,135,183. Junghans proposed that the casting be withdrawn continuously at a constant rate, and that the mold be advanced with the casting at the same rate as the casting so that there is no relative motion therebetween during the advancing stroke of the mold. Then, at the end of the advancing stroke, the mold is quickly retracted, at an increased, but uniform rate, so that the return stroke requires much less time than the advancing stroke. In commerical operation, the ratio of the time of the advancing stroke to the time of the retracting stroke is usually of the order of three to one, so that the speed of the retracting stroke is three times the speed of withdrawal of the casting. I,

The Junghans process has been used commercially with great success, starting in Germany in 1936, in the United States in 1938 and subsequently in many other countries of the world. It is unquestionably true that a vastly greater tonnage of metal has been cast successfully by the Junghans process than by any other continuous casting process now known. The major portion of that tonnage has been in the non-ferrous field, however, casting nonferrous metals and alloys such as brass, aluminum and aluminum alloys. Only recently has the use of the process been extended to the casting of steel, and here, as anticipated, new problems have arisen.

One quite serious problem which has arisen is the problem of increasing the casting rate to a level which is highly economical in the casting of steel. Another serious problem is one which arises as a direct consequence of increase in the casting rate, namely, the problem of obtaining satisfactory surface characteristics.

One of the characteristics of castings produced by the J unghans process is the presence of rings extending around the casting in the surface thereof. These rings are spaced at distances equal to the total advance of the casting between successive strokes of the mold. That is, if the total advance of the casting (moving continuously at a constant rate) is ten inches between the beginning of one advancing stroke of the mold and the beginning of the next su cceeding advancing stroke, the rings will be found to be spaced at ten inch intervals.

The rings are characterized by a roughened exterior surface, frequently with surface cracking, and frequently with evidence of bleeding, i. e. the leaking of molten metal through a lesion in formerly solidified metal, with subsequent solidification of the leaking metal. The crystal structure of the metal lying just under the rings is also irregular and disturbed. The width of the rings, i. e. the distance lengthwise of the casting through which these effects may be observed varies depending on the conditions of the casting operation. With extreme care and operating at a low casting rate, the effects may be minimized, but in geenral, the width of the rings is related to the time of the return stroke of the mold. That is, if the return stroke consumes one fourth of the time of the complete cycle, the rings will be found to cover at least one fourth of the surface of the casting.

In the case of non-ferrous metals, these eflYects have been undesirable, but not too serious. in many cases, despite the surface imperfections the castings could be rolled, extruded or otherwise processed without difiiculty. In other cases a light scalping or other surface conditioning operation was sufiicient to remove all objectionable surface imperfections. In the case of steel, however, such surface imperfections cannot be tolerated, and it is not economically feasible to remove the imperfections by scalping. Moreover, the economical continuous casting of steel demands a far greater casting rate than is customary or desirable in casting nonferrous metals, and it has been found that the increased casting rate greatly magnifies the difiiculty. Thus, in casting non-ferrous metals a casting rate of thirty to sixty inches per minute is usually adequate, and at these speeds, the surface imperfections are tolerable in non-ferrous metals. In casting steel, on the other hand, casting rates as high as two hundred inches per minute have already been successfully achieved with the lunghans process, but this success is tempered by the fact that at these speeds, the surface imperfections within the ring areas are extremely bad. Between successive rings, the surface is good and the interior crystalline structure is excellent.

It is an object of the present invention to provide a method of continuous casting, and particularly a method of mold reciprocation, which will mitigate the harmful effects of casting at high rates and reduce the sur face imperfections to a tolerable level.

Other objects and advantages of the invention will appear hereinafter.

In my early attempts to correct this difiiculty, the logical solution appeared certainly to be to increase the speed of the return stroke of the mold in order to minimize the time period during which the mold is moving in the opposite direction from the mold. Thereby, I reasoned, the width and probably also the unevenness of the rings would be reduced to tolerable proportions.

Therefore, I proposed to change the time ratio of the advancing stroke to the return stroke from the usual three to one to four, five and even six to one, the latter being almost the limit of mechanical practicability. Much to my surprise, however, the rings, and the surface imperfections evidenced thereby bccame progressively worse instead of better. In some cases, instead of becoming narrower as might reasonably be expected, the rings widened until they occupied almost 50% of the area of the casting and the area within the rings was characterized by roughness, wrinkles, cracks, and bleed mg. l

According to the present invention, therefore, in order to solve the problem, I have adopted the novel and seemingly illogical expedient of reducing the speed of the re' turn stroke of the mold to a point such that the speed of the return stroke is never faster than the speed of the advancing stroke, and in many cases will be considerably slower than the speed of the advancing stroke. On the advancing stroke, the mold will move with the casting at the same speed as the casting, and the casting will move continuously at a constant rate. On the return stroke of the mold, the casting will continue to move continuously at a constant rate, while the mold moves in the opposite direction at a constant rate.

The contrast between the Junghans process and the method herein proposed may be illustrated as follows. It may be assumed that steel is being cast and that the casting rate (i. e. the rate at which the casting moves continuously) is two hundred inches per minute. According to the Junghans process, and assuming the usual time ratio of advancing stroke to return stroke of three to one, since the mold will advance at the rate of two hundred inches per minute, it must return at the rate of six hundred inches per minute. And, since the casting continues to move at the rate of two hundred inches per minute during the return stroke of the mold, the actual differential is six hundred plus two hundred, or eight hundred inches per minute.

If, in order to overcome the difiiculty, it is attempted to increase the speed of the return stroke to give a time ratio of, say, six to one, the return stroke would be at the rate of twelve hundred inches per minute, and the actual differential of twelve hundred plus two hundred would be fourteen hundred inches per minute.

According to the present invention, on the other hand, wherein the speed of the return stroke is never greater than the speed of the advancing stroke, and again assuming a casting rate of two hundred inches per minute, it will be seen that the maximum actual differential will be two hundred inches per minute (the maximum return speed of the mold) plus two hundred inches per minute (the speed of the casting), or four hundred inches per minute. This, it will be observed, is exactly one half of the actual differential which would exist on the return stroke in the Junghans process.

If desired, however, the speed of the return stroke of the mold may be reduced even further. Thus, assuming again a casting speed of two hundred inches per minute, the return stroke of the mold may be at the rate of one hundred inches per minute or even fifty inches per minute, in which cases the actual differentials would be three hundred inches per minute or two hundred and fifty inches per minute, respectively.

In practice, the actual differential may be adjusted by the operator after inspection of the casting and taking into account the quality of the surface required during the casting of the particular metal which is being cast. In some cases greater perfection of surface is required than in others, but the operator, applying the principles of this invention, and bearing in mind that the optimum condition for casting is the period when the mold and casting are moving together, will be able to set the speed of the return stroke to the maximum consistent with the surface requirements of the metal being cast, but always less than the casting speed.

It will be understood that the invention may be var iously modified and embodied within the scope of the subjoined claims.

I claim as my invention:

1. A method for the continuous casting of metal which comprises pouring molten metal continuously at a constant rate into one end of a casting mold, withdrawing solidified metal continuously at a constant rate of at least two hundred inches per minute from the other end of said mold, advancing said mold with said solidified metal at the same rate at which the solidified metal is being withdrawn, so that there is no relative motion between the solidified metal and the mold during the advancing stroke of the mold, and retracting said mold at a constant rate which is never greater than the rate at which the mold is advanced.

2. A method for the continuous casting of metal as claimed in claim 1 in which the rate at which the mold is retracted is susbtantially less than the rate at which the mold is advanced.

References Cited in the file of this patent UNITED STATES PATENTS 2,135,183 Junghans Nov. 1, 1938