US2002266A - Method of rolling strip material - Google Patents

Description

May 21, 1935.

METHOD OF ROLLING STRIP MATERIAL Filed Sept. 29, 1954' 2 Sheets-Sheet 1 2 2 5 4 J 5 7 a .9 J0 gg ifl 3 000017 0 01] gnaw gooodgoooog o7oo 0000? 00003 oooog 0000000 I l 17 INVENTOR May 21, 1935. c. A. KRAL 2,002,266

METHOD OF ROLLING STRIP MATERIAL Filed Sept. 29, 1934 2 Sheets-Sheet 2 THICKNESS //v INCHES PA ss 4 044 BER INVENTOR cm 8. KM

CONVEXI ry nv INCHES Patented May 21, 1935 UNITED STATES PATENT OFFICE METHOD OF ROLLING STRIP MATERIAL Charles A. Kral, Steubenville, Ohio Application September 29, 1934, Serial No. 746,221

Claims.

This invention relates to a method of rolling strip material and is herein particularly described as applied to the rolling of wide, thin steel in the formknown as broad strip or strip sheet.

5 Such material is widely used at the present time, both in the hot rolled and cold rolled condition. The hot rolled strip is produced in a hot mill from an ingot 6r a slab and the material thus formed constitutes a valuable article for use either in its then state or as a starting material for cold rolling.

Generally speaking, this material is exceedingly wide in comparison with its thickness. It has been termed high ratio material and has been distinguished from ordinary strip in that it has a width-to-thickness ratio in excess of 400 to 1. A great deal vof attention has been given to the problem of rolling high ratio strip, and especially to the roll contours employed. It has been generally accepted as a fact that some definite relationship must exist between the contours of the successive passes in the mill if the work piece is to track in a straight line therethrough. According to the proponents of one system, the work piece should be originally convex and the. contour of the active passes should be such that the amount of convexity decreases progressively from stand to stand (see Tytus Reissue Patent No.- 16,884). According to the proponents of another system, the work piece should be originally concave and the active passes so arranged that the concavity progressively decreases from stand to stand (see Corrigan and Jenkins Patent No. 1,829,339). According to still a third system, it is proposed that the active passes in each stand shall be to all intents and purposes flat so that the direction of travel of the piece may be controlled by side guides (see Elliot Patent No. 1,754,745). A practical 40 defect of all of these systems is that great care must be exercised in originally determining and maintaining the requisite pass contours in the several stands in order to carry out the intended system of rolling; The contour of the active pass depends, not only on the original contour of the .rolls, but also on a number of other factors. It has been heretofore recognized that the following factors are involved:

(1) The prepared contour of the rolls.

'(2) The temperature of the rolls.

(3) The composition and springiness of the rolls. N

- (4) The spacing of the rolls, or'screw.

(5) The shape, composition and temperature of the work-piece.

In addition to the above,-I have determined that the factor of speed is an important one and should be taken into account if any system, dependent for its success upon the relative contour of successive passes, is adopted. From the 5 foregoing, it will be seen that in practical operation a great deal of experimenting and careful control are essential to the successful operation of any system dependent for its success upon considerations of pass contour. The changing of rolls, in case they are not ground to the proper shape, necessitates a shut-down of the mill and a consequent loss of production. Experimenting with roll settings and the like, as recommended by the proponents of these several systems, is also likely to prove wasteful of time, and hence expensive. Upon to the present time consideration has been given only to the active pass contours when the rolls are new, but in practice the situation is further complicated by the factor of roll wear. All of the work rolls in the millwear during rolling, this being particularly marked in hot rolling. The rolls in the several stands do not wear in the same amount, nor is they original prepared contour of the rolls maintained throughout their life.v In consequence, if the operator is required to maintain a determined relation between active pass contours in the several stands, he must be constantly adjusting his mill to meet the ever-changing situation brought about by roll wear. His difliculties will be fully appreciated when it is considered that certain of the rolls in the mill may be worn out before others, and hence must be changed oftener, so that the mill contains fresh rolls in some stands and worn rolls in another. These diificulties are to be expected regardless of which of the three systems above referred to is adopted.

I have discovered that high ratio strip may be successfully rolled, either hot or cold, and that for practical purposes all considerations of roll contour may be dispensed with, if a sufiicient number of passes is employed and the work is divided between the passes as hereinafter described. A full understanding of my invention 45 requires a short description of continuous rolling generally. In the continuous mill the piece is passed successively from one stand to the next, being reduced from an initial work piece which,

in the case of bars or shapes, is a billet and, in 50 the case of strip, may be a slab or an ingot, and by successive passage through a series of stands arranged in a continuous manner is reduced to the desired size and shape. It is necessary to provide a sufllcient number of stands to insure that the grain refinement resulting from mechanical working is obtained, and it is also necessary to so arrange the reduction in the several stands that the piece travels smoothly from one stand to the. next. In any reducing stand the delivery speed is necessarily greater than the entering speed because the cross sectional area of the material is reduced. It follows that the speed of the -metal progressively increases as it travels through the mill, and the rolls must move at peripheral speeds which progressively increase from pass to pass in order that the piece shall be properly rolled in each stand without piling up or cobbling between passes.

From the foregoing it will be seen that the selection of a schedule of reductions involves several factors but heretofore, so far as I am aware, it has never been known or understood that by properly selecting the reductions in the several passes it is possible to eliminate considerations of pass contour and to roll high ratio material without adjustment for the many factors entering into all pass contour systems. By my invention it is possible to quickly and easily establish a schedule of reductions for the several stands in the mill and by so doing to dispense with all the considerations which must ordinarily be taken into account in order to cause the material to track through the mill.

The principle which I-have established is as followsz-The work piece should be reduced in such an amount in intermediate passes that the curve in which thickness is plotted against pass number lies in an intermediate portion below an exponential curve of like character connecting the first and last points of the first-mentioned curve and representing uniform percentage of reduction per pass. This exponential curve may be readily determined by an equation of the form where T1 is the starting thickness, the number of passes, and F is the reducing factor. The percentage of reduction will be where F is expressed as a percentage. According to my system the thickness of the work piece is drastically reduced in at least two and preferably three successive passes prior to the last reducing pass so that the thickness is less after any such pass than the thickness at that stage of the reduction which would result if a uniform percentage of reduction were obtained in each stand. If this is done, the reduction in the final pass, where the width-thickness ratio of the material is the highest and hence where the problem of guiding is most acute, may be as greator even greater than the reduction as computed by the above formula and yet the concavity, convexity or flatness. of the material from stand to stand follows no rule but the work piece tracks through the mill and an entirely satisfactory product is obtained.

My system should be distinguished from pack rolling systems which, generally speaking, are

concerned only with pieces of relatively short length. A difllculty peculiar to the rolling of strip is that because of the great length of the work piece, any over-rolling, either of thecenter or of the edge, is cumulative to such a degree that the product may be spoiled; whereas in pack rolling the piece is so short that this difllculty is of much smaller consequence. My invention is matic, of a continuous mill in which my invention may be practiced;

Figure 2 is a. side elevation thereof; and Figure 3 is a curve sheet illustrating the reductions taken in each pass and the contour of the material from pass to pass.

The mill illustrated in Figures 1 and 2 comprises a series of reducing stands numbered successively from I to II and three sets of edging rolls I2 placed just in advance of each of stands 3, 4 and 5. Stands I and 2 are 2-high, while stands 3 to II inclusive are all of the now wellknown 4-high type. Stands I and,2 are driven through suitable connections I3 and gearing I4 from a motor I5, while each of stands 3 to II' is provided with an independent motor'drive indicated at IS. The work piece, indicated at W, is supplied either direct from a universal mill or from a heating furnace and is fed successively through the several stands in the'mill. Tables I'I are provided between stands over which the piece travels. The final product may be cropped and/or cut into lengths by a rotary shear I8 and fed tocoilers I9 or run onto a cooling bed 20.

Stand No. II may be considered purely as a finishing pass intended to impart the desired degree of flatness to the product. Stand I0 is properly considered as the last reducing pass and for purposes of the present discussion stand No. I

may also be eliminated because the, work piece is has beenv plotted against the pass numberv and the'thickness at passes 2 to II inclusive is shown. In this case the starting thickness was 1.981" and the final thickness. was .098". These thicknesss are measured at the center of the strip. The strip in this particular example was 37" wide. The thickness at stand I0 maybe arbitrarily taken at .124, this leaving a sufficiently small amount of reduction to be effected in stand II to insure that the final product will have the desired characteristics.- Having the thickness at stand 2 and at stand III, the reduction which would obtain in each pass, were the same percentage of reduction to apply in each case, may be readily de termined by the above formula as follows:

Passes 2 to II] inclusive! total '9 in number. Hencen=9 and .1z4 F .0626, and

The reduction for the successive passes is therefore;l00-70.725=29.275%. Th curve 22 of Figure 3 is an exponential curve re resenting such reduction frompasses 2 to l0 inclusive. It will be noted that the curve 2| does not coincide with the curve 22 but departs materially. therefrom,

particularly at passes 4 to 6 inclusive. The actual thickness of the material after each pass, in this particular example of my invention, is as follows:

After pass No. 2 1.981 After pass No. 3 1.397 After pass No. 4 .868 After pass No. 5 .617 After pass No. 6 .464 After pass No. I Q. .341 After pass No. 8 .236 After pass No. 9 .184 After pass No. In .124

All thicknesses in the above tabulation are at the center of the strip.

In the rolling just described no attempt was made to initially determine or. maintain any fixed relationship between successive active pass contours. However, samples were taken after each pass from 2 to I0 inclusive, and the convexity of the same was determined by subtracting the average thickness of the edge from the thickness at the center. The results of the determination are plotted in Figure 3 by a broken' line 23. The scale for the line 23 appears at the right-hand side of the figure and gives the convexity in inches. It will be noted that there is no rational relationship whatever between the convexities in the several passes. The line 23 is only representative of this particular rolling because my tests show that in other rollings widely differing convexities are obtained. There is no apparent relationship whatever between the lines plotted from different tests. Apparently all of the factors enumerated above as bearing on roll pass contour have come into play but in such different ways and in such different amounts that there is no fixed relationship whatever between the contours of the successive passes. It may therefore be safely said that in my system of rolling consideration of pass contour may be disregarded. I therefore am able to grind an ofthe rolls in the reducing passes to cylindrical form or, if this form is departed from in any pass, to determine the profile of the roll solely by considerations of ultimate flatness of the material and not by considerations of rolling.

I have determined that even with the same set of rolls a mere change in the speed of the mill will cause the active pass contours in successive passes to vary markedly and in amounts difiering one from the other, so that there is not even a rational relationship between the several convexities where the only change has been in the speed of the mill.

The reduction to thicknesses below the exponential curve should be in two and preferably three successive passes, all of which are prior to the last reducing pass. This merely requires a simple screw-down adjustment and a corresponding adjustment of the relative motor speed. It follows that the expensive and time-consuming adjustments of prior systems are entirely eliminated and consequently a greater yield may be obtained from the mill. The amount of skilled supervision ferred embodiment of my invention, it will be understood that the same is not limited thereto but maybe otherwise embodied and practiced within the scope of the following claims.

I claim:

1. In the method of rolling wide thin strip in a continuous mill having a plurality of stands of reducing rolls, the steps consisting in passing a work piece'through the stands in a continuous manner, reducing the thickness of the work piece by single ply passes in each such stand, and in at least two successive passes, which passes are prior to the last reducing pass, drastically reducing the thickness to less than the thickness at that stage of the reduction which would result if a uniform percentage of reduction were taken in each stand to obtainthe same total reduction.

2. In the method of rolling wide thin strip in a continuous mill having a plurality of stands of reducing rolls, the steps consisting in passing a work piece through the stands in a continuous manner, reducing the thickness of the work piece by single ply passes in each such stand, and in at least three successive passes, which passes are prior to the last reducing pass, drastically reducing the thickness to less than the thickness'at that stage of the reduction which would result if a uniform percentage of reduction were taken in each stand toobtain the same total reduction. 3. In the method of rolling wide thin strip in a continuous mill having a plurality of stands of reducing rolls, the steps consisting in heating a work piece to rolling temperature, passing it through the stands in a continuous manner, reducing the thickness of the work piece by single ply passes in each such stand, and in at least two successive passes, which passes are prior to the last reducing pass, drastically reducing the thickness to less than the thickness at that stage of the reduction which would result if a uniform percentage of reduction were taken in each stand to obtain the same total reduction.

4. In the method of rolling wide thin strip in a continuous mill having a plurality of stands of reducing rolls, the steps consisting in passing a work piece through the stands in a continuous manner, reducing the thickness of the work piece by single ply passes in each such stand, and in at least two successive passes, which passes are prior to the last reducing pass, drastically reducing the thickness to less than the thickness at thatstage of the reduction which would result if a uniform percentage of reduction were taken in each stand to obtain the same total reduction, and, subsequent to the reducing passes, subjecting the material in single thickness to a finishing pass.

5. In the method of rolling wide thin strip in a continuous mill having a plurality of stands of reducing rolls, the steps consisting in passing a work piece through the stands in a continuous manner in single ply passes, and reducing the work piece in the intermediate passes so that the curve in which thickness is plotted against pass number lies, in an intermediate portion, sufllciently below an exponential curve of like character connecting the first and last points of the first-mentioned curve and representing uniform percentage of reduction per pass that predetermined relationships between successive active pass contours for the purpose of guiding the work piece through the stands may be dispensed with.

CHARLES A. KRAL.

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