Rolling-Mill
ROLLING-MILL, a term which includes several types of machines used for producing the sectional forms (fig. i) in which wrought iron and steel are required for the use of boiler-makers, platers and bridge-builders, and for constructional work generally. The production of wrought iron has been a diminishing industry for many years, while that of steel increases. Though the plant employed for both is alike in essential principles of design, the growth in the use of steel has revolutionized the practice, chiefly on account of the more massive dimensions in which steel sections are rolled. Iron sections are relatively small, and many are produced by piling, i.e. by building up with small portions of malleable puddled metal. There is no limit in reason to the dimensions in which steel sections can be rolled, and they are never piled, however large, but always rolled from soh'd cast ingots.
When steel ingots are rolled into sectional forms the reduction in transverse dimensions is very great. The work begins at nearly a white heat, and continues until a low red is reached. Obviously the stresses to which the material is subjected are very severe. For this reason the process of reduction has to be effected very gradually, and especially so in those cases where reduction is being done in two directions at right angles with each other, as in channel sections (fig. 6) and joist or beam sections (figs. 7 and 8).
It might be thought, since steel is always cast previously to rolling, that it might be cast at once into the sectional forms required. But sound results could not be obtained in this way, because the gases occluded in the metal form blow-holes which are sources of weakness. The material itself, even in the solid portions, is not homogeneous. By removing the head of the ingot where the blow-holes chiefly congregate and rolling the remainder at a white or red heat, the metal is improved by consolidation, and by the work done upon it. To this practice there is no exception.
Rolling-mills are known as " two-high," or " three-high," according as two or three rolls are mounted one over the othel FIG. I. Forms of the Principal Rolled Sections, i, 2, Flats. 3, Flat with bevelled edges. 4, 5, Flats with rounded edges. 6, Bulb bar. 7, Wedge bar. 8, Scree or -grate bar. 9, Square. 10, Triangular. II, Hexagonal. 12, Round. 13, Oval. 14, Hollow half-round. 15, Half-round. 16, Convex. 17, Square-edged convex. 18, Vee. 19, O.G. 20, Angle iron. 21, Square root, or square throat angle. 22, Round-backed angle. 23, Unequal-sided angle. 24, Acute angle. 25, Obtuse angle. 26, Bulb angle. 27, Tee. 28, Bulb tee. 29, 30, Beams or joists, or girders, or H-irons. 31, Channel. 32, Zed. 33, Cruciform section. 34, Pillar section. 35, Troughing. 36, 37, 38, Railway rail. 39, Tramway rail. 40, Heavy crane rail.
(figs. 2 and 3). In the two-high type the two rolls revolve in opposite directions, so that an ingot, slab or bloom presented to the entering side is drawn in and between the rolls, which reduce its thickness. In the case of rolls which are two perfectly plain cylinders (plate-rolls) the shape produced is that of broad, long and flat plates or sheets. Several passages (passes) are required to effect the reduction required, because this must be gradual. To regulate the amount the top roll is set down bodily by means of screws pressing on its bearings which slide in the end supports (housings). In the case of plate-rolls, which are plain cylinders, this setting down must be equal at each end. The mass of the top roll is balanced, to avoid shock when a plate is entering. The rolls are made of cast iron, and are either grain rolls or chilled rolls. The first are formed from a tough strong grade of iron, the quality which is used for all the roughing down and general work. The second are made of a highly mottled iron, cast against a cold mould (chill) of cast iron, by which a steely surface is obtained. These are used for fine finishing, or for imparting a polished surface to a section already nearly reduced to size in grain rolls. In later heavier practice, rolls of cast steel and forged steel are becoming common. They are more costly than iron, but more durable and much lighter for equal strength. They are essential in armour plate rolls. The length of rolls should not exceed about four times their diameter, for otherwise they are liable to spring and produce plates thicker at the centre than towards the edges.
From this elementary design several types are derived. In the two-high mill it is clear that if the direction of the rotation of the rolls is always the same, then the plate being rolled must be taken back after each " pass " to the front of the rolls. Hence there is one " lost pass " for every reduction in thickness. This is the case in the " pull-over " mill, nearly obsolete. In the two-high reversing mill, introduced to avoid this " lost pass," as soon as a plate has gone through, the direction of rotation of the rolls is reversed, and the plate is rolled again on the backward journey, so avoiding the lost ROLLING-MILL aotLe* tor uw FIG. 2. General Arrangement of 12-in. Merchant and Guide-Mill Plant. (Thomas Perry & Son Ltd., Bilston.)
A, First roughing rolls. B, Second ditto. C, Guide rolls for ovals or diamonds. D, Ditto for rounds or squares. E, Driving pinions. ' Engine, 30 in. X 22 in. cylinder, directcoupled to rolls. Runs from loo to 180 revolutions per minute to suit work. The shears are used for cutting the smaller sections, the hot saw for cutting the merchant iron.
FIG. 3. 12-in. Merchant Guide-Mill and Engine. Four-set mill.
A, B, Three-high sets. C, Works either three-higher two-high; a, being a dummy roll. D, Twohigh set (guide rolls). E, Coupling pieces. F, Housings. G, Pinions. The mill is capable of rolling rounds, squares, flats, angles, tees or similar sections by changing the rolls. The guide rolls D are used for small sections, and the second set B for merchant iron (larger sections).
pass. An alternative is the threehigh mill, in which three rolls are used. Here the plate is run through the lower rolls and back through the upper ones, so that there is no reversal of direction of the mill as a whole, but the lower and upper 'rolls draw the plates in opposite directions (see also IRON AND STEEL, 129).
Plate-Mills. In Great Britain platemills are generally two-high reversing mills, in America three-high mills. Another difference is that in British practice two stands of rolls are used, in America one only. In the two-stand design there are two sets of rolls coupled endwise, one set being grain-rolls for roughing, and the other chilled rolls for finishing. Sets of live rollers conduct the plates to and from the separate rolls. The plate-mills proper are those which roll from } in. to about 2 in. thick. Armour plate-mills are a special design for massive plates and sheet-mills are for thin plates or sheets having a less thickness than } in. Armour plate-mills are of twohigh reversing type usually, with forged steel rolls. They are of immense proportions, the rollers ranging from 10 to 14 ft. in length, by from 3 to 4 ft. in diameter. In sheet-mills, on the other hand, the rolls seldom exceed 30 in. in diameter, and they are chilled. The size of sheet-mills has within the last few years been considerably increased (since the introduction of steel sheets), and all new mills are made from 28 to 30 in. diameter. The mills are of the twohigh type and are almost the only instance of the retention in present practice of the non-reversing mill. It is found more convenient in this case than the reversing or the three-high mills, because two men roll two pieces at once, one handing over a sheet just rolled to his fellow just as the latter has entered a sheet between the rolls on his side. Strip-mills are a smaller but similar type, used for rolling the thin narrow strips required for the hoops of barrels, ties for cotton bales, etc. The details of these mills cannot be discussed here, nor the numerous arguments in favour of the two systems. English practice retains the two-high reversing mill for all heavy work, the exceptions being those just noted. American practice retains the three-high mill.
Grooved Rolls. In the mills designed for rolling various sectional forms the same distinction between two-high and three-high remains, but new problems arise. By " sectional forms " is meant all those which are i*>t plates and sheets, such as bars of round and square section, angles, channels, rails and allied sections (fig. l), for the production of which grooved rolls are required. _ The shapes and proportions of these grooves are such that reduction is effected very gradually. When metal is squeezed or hammered, one effect is to spread it laterally, since the metal cannot be appreciably squeezed in on itself. But the lateral extension is very much less than the longitudinal. The most marked effect of reduction in thickness is extension in length. But as there is some lateral extension, three courses are open: one is to gauge the exact amount of width required for extension ; another is to turn a bar over at intervals in order to exercise pressure on the portions extended laterally and obliterate them (open passes) ; and a third is to allow the extensions to take the form of fin to be cut off subsequently (closed passes). The first is generally impracticable. The second can be illustrated by diagrams representing roll sections.
The work of reduction is generally divided between three sets of rolls. The first are the cogging-, or blooming-rolls, as they are termed in America, in which ingots are reduced to blooms with dimensions suitable for rolling the various sections. In these an ingot of say 14 in. square may be reduced to a bloom of 6 in. square. The grooves form rectangular sections (box passes). The top roll being raised, the ingot is passed through the largest groove; then the roll is lowered and it is passed through a second time. Then it is turned round through 90 and re-rolled. Afterwards the same processes are gone through till the last groove is reached. There is a great difference between, say, a plate and a rail, but the cogging-rolls have to be so designed as to produce blooms for varied forms. There are three principal forms: the box just noticed, the gothic and the diamond (fig. 4), all open passes. For plates, A, Box Pass.
FIG. 4. B, Gothic Pass.
C, Diamond Pass.
provision is made in " slabbing " rolls for roughing out, first in a box pass, and then in a broad flat groove, alternating with the square groove for correction of the edges. Gothic passes and diamond passes produce blooms which are subsequently used for various shapes having little resemblance to each other. These shapes are simple, and little difficulty arises in the work of drawing down. The rolls make 40 to 50 revolutions per minute; the difference in the area of the cross section (draught) between adjacent grooves is from 20 to 25 %.
The formative rolls for finished sections are of two classes: roughing and finishing. The roughing-rolls approximate much more closely to the finished sections than the cogging-rolls, but the aim is to make them do duty for a wide range of sections, in order to change them as seldom as possible. Thus the gothic pass (fig. 4) will serve alike for rolling square or round bars. Finishing rolls must be changed for every different section, except when slight differences in thicknesses only are made in the webbed portion of a rolled section. With the exception of rounds, sections are usually roughed and finished in closed passes that is, the bar is wholly enclosed by the rolls. The groove in the lower roll is flanked by collars slightly deeper than the enclosed bar. These enter into grooves turned on the upper roll, and between them the bar Is confined (fig. 5). It passes through a succession of these grooves, FIG. 5. Pair of Rolls for producing Angle Sections. (Thomas Perry & Son Ltd., Bilston.)
being diminished in area and extended at each pass. A certain amount of fin is squeezed out, and this is obliterated in the succeeding pass, and more formed, until in the finishing pass the amount of reduction is very slight, a surface finish being the principal result.
Since but a slight amount of lateral extension occurs, it follows that the reduction wholly or mainly in the vertical plane is the most favourable condition. Rounds, squares and flats are wholly reduced in this way and offer no difficulty. The most unfavourable section is the joist or girder, the channels, tees and rails fojlow, and after these the various angles. In rolling a channel or a girder section (figs. 6, 7, 8), a square bloom is taken, and passed in succession through closed passes. The first produce shallow grooves in FIG. 6. Reduction of Channel Section.
FIG. 7. Reduction of Girder Section in Roughing Rolls.
FIG. 8. Reduction of Girder Section in Finishing Rolls.
the opposite faces, gradually deepening until the insides of the flanges assume a definite slope. The angle of slope becomes gradually lessened, and the thicknesses of web and flanges, and also the radius in the corners, are reduced. At the same time the width over the flanges is being gradually increased. While this is going on, the fibres of the flanges, are being strained, because the rolls run at a higher speed at their peripheries than next the body. The metal is being violently thrust and drawn in different ways, so that while economy has to be studied by reducing the number of passes, as much as possible, undue stress must be avoided by making the reductions as easy as is practicable. These things cannot be put into a formula, but the roll-turners work by experience and empirical rules gathered by long practice. In order to avoid these deep groovings, and also severe lateral thrusts on the rolls, angle sections are always rolled with the slope of the flanges approximately equalized; so too are zeds (fig. I, No. 32). The reduction is then effected with the minimum of stress to the metal. Variations are readily made in the thicknesses of rolled sections without changing the rolls, by simply varying the distance between their centres. This is effected by the adjustment of the top roll (fig. 5). Differences in thickness are made in j^ths of an inch, up to a maximum of about J in. Another detail of design in closed passes is so to shape the rolls as to make any pass obliterate the fin produced in the previous groove. Sometimes sections are turned over to effect this, but often the bodies of the rolls are turned of suitable diameters to produce the result. Guards are required to prevent the bars from becoming wrapped round the rolls (" collaring "). With the same object the upper roll is always made larger in diameter than the lower. Its speed is therefore slightly greater than that of the lower one. This stretches the plate or bar very slightly on the upper side, and so imparts a downward movement to it towards the floor, which is what is required. The difference in diameter varies with circumstances, ranging from th to about I in. Besides the standard types of mills noticed, the two-high and three-high, there are special mills. The merchant mill simply denotes either one of the above types used for the production of flat bars. The continuous mills are special designs for rolling small rods to be drawn into wire. In these there are several pairs of rolls placed in series, so that the billet is rolled from one stand to others in succession without re-heating. There are a number of different designs, _one of which is the Belgian looping mill, so called because the rod is bent backward and forward in the form of the letter S in its passage through adjacent sets of rolls. In another design a flying shear is employed, which automatically cuts off billets from the bar while the latter is travelling at the rate of 6 or 8 ft. per second.^ (J. G. H.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)
