Mills for Semi-Finished Materials

MILLS FOR SEMI-FINISHED MATERIALS Continuous Mills.—Four classes of semi-finished materials are manufactured in such tonnage that special mills are set up to pro duce them, namely: sheet bars, skelp, strip, and wire rods. These products are the raw material for special manufacturing plants often located at a distance from the steel furnaces. Sheet bars are worked into tinplate, galvanized, and uncoated sheet ; pipe is made from skelp; strip is narrow sheet for hoops and bands; and rods are drawn into wire, from which all kinds of fencing, screening, cable, nails, and springs are made. Sheet bars and skelp are made in bar mills, frequently located immediately beyond a blooming mill, which take a 6 in. square bloom hot from the shear, and roll into the thin narrow plate without reheating. Such a plan requires the most rapid handling to complete the rolling be fore too much heat is radiated. This is done by placing a succes sion of two-high roll stands in tandem, within a few feet of each other, and guiding the bar automatically from one roll to the next. Since each pass elongates the piece, the speed of each roll is higher than the preceding, and the relation between speed and reduction must be nicely adjusted so the hot bar will neither buckle nor be stretched at any stage. A succession of eight or ten such rolls, with perhaps three pairs of vertical rolls to control the width— interspersed in the train—can produce standard width 8 in. sheet bars or skelp from 6 to 20 in. wide (suitable for forming into pipe up to 6 in. diameter), and from to in. thick, at the rate of 4,000 to 20,009 tons per week depending upon the sectional area of the bars manufactured. Since a mill of this sort can easily pro duce material far too long to ship, flying shears are set beyond the finishing roll to cut the bars to length. Many designs have been made, capable of square cutting a rapidly moving bar. Sheet bars are sometimes quenched by travelling down a long table under a series of water spray nozzles. Interesting piling devices are used.

Sheet Mills.

Sheets are in. thick or less, whereas plates have sufficient mass to retain heat until rolled to proper gauge, and a surface covered with mill scale is quite good enough for most pur poses. Sheets, on account of their thinness and the excellent sur face demanded by such finishing operations as galvanizing or lacquering, must be rolled in an entirely different manner. Many sheet mills are still operated by hand after the methods originated in Wales two centuries ago (see TIN) and have larger and more rugged equipment, generally producing material 18 to 3o gauge thick, whereas tin plate is smaller in area and thinner-28 to 38 gauge. The 193os saw a replacement of many hand mills by a few continuous sheet mills, especially in the American industry.

Sheet bars are cut with length slightly greater than the width of the required sheet, heated to a dull red 85o° C, and rolled sideways through a two-high mill with massive cylindrical rolls, 32 in. diameter. One workman catches the bars as they come through and passes them back to another over the top roll; a third screws down the top roll gradually to continue the reduction. The crew passes two bars back and forth very dextrously; when these bars have been flattened to about %2 in. one is placed on top of the other, the top roll raised, and the pair rolled down until each is about twice as thick as the desired sheet. In the best practice, one pass is then given in the mill to be used for finishing, in order to "shape" the pack to this mill.

By this time, reheating is necessary; the pair is opened, that is the two sheets are separated, and placed in a furnace until the temperature is up to about 800° C. Low temperatures and a smoky flame are necessary to prevent surface oxidation in the furnace. A pair of these sheets are then further rolled to the re quired thickness in a finishing mill, more accurate in construction and requiring more skill to operate. For thin gauges, sheets are folded double and rolled in packs of four or even eight. Frequent opening and reheating is necessary in the latter stages of the process. They are then straightened and sheared to size.

Many surface finishes are required by customers who manufac ture a multitude of objects of sheet metal. The sheets may be annealed either by being passed slowly through a long furnace with hot central zone—blue annealing—or by being piled in a tight box containing a prepared atmosphere to protect the steel from tarnish and heated for hours at a considerably lower tem perature—box annealing ; or the cold sheets may he given several passes through accurate rolls for a smooth surface and a some what stiffer temper. Interspersed with these operations, the oxide or mill scale may be removed by pickling (q.v.) in weak sulphuric acid. Finally the cleaned cold-rolled sheets may be dipped in white metal for a protective coat, wiped, cooled, polished and boxed. Coating with zinc is called galvanizing; tin or tin sheet is thin steel dipped in pure tin; terne sheet is dipped in a lead-tin alloy.

Strip and Continuous Sheets.

Desire to eliminate the large amount of labour required for sheet working by the above de scribed process has led to the recent production of sheets in con tinuous mills. Since 190o it had been common to roll strip up to 3 in. wide and 16 gauge or less in thickness, in very long coils in a series of roll stands placed close together; mills have been built for wider and wider strip-sheet, until sheet 8 ft. wide was pro duced commercially in America in 1939, but no thinner than 16 gauge. In a narrow strip, hoop or cotton-tie mill, a continuous heating furnace feeds flat bars about the width of the finished piece, endwise into the first of a series of six tongue and groove rolls, perhaps so in. diameter. One edger may be installed after the third roll, and the remainder then be flat rolls. Then the bar passes through a series of three very accurate finishing rolls, some times being bent sharply, scraped, and air blasted before entering the last roll to remove the scale. These finishing rolls are cylin drical, and the guides are shifted sidewise at frequent intervals, to prevent the chilled strip from wearing the roll-surface irregularly; speeds are watched closely; the first roughing roll is very slow, the last very fast; in fact the trailing end of the billet may still be in the furnace when the front end is being reeled, 200 'ft. away, at the speed of i,000 ft. per minute. One great advantage of such close-set continuous mills is that the time of passage from furnace to finishing roll is the same for all parts of the billet; consequently the temperature at the last pass is uniform, and the thickness after cooling is also quite uniform. Finished strip is automatically turned on edge, looped back and forth on a broad slow moving conveyor belt passing through a muffle; emerging it is cool enough for reeling, cutting to length and bundling. About 4o men are required to operate such a strip mill producing from so to 20 tons of strip per hour, depending upon its width and gauge. Such strip has a surface covered with mill scale, or if this is scraped off, a blued colour formed during slow cooling. For higher finishes and stiffer tempers, the strip may be pickled and cold rolled rather slowly to accurate gauge and smooth surface.

In the new continuous sheet mills, slabs weighing up to a ton and a half, the product of a blooming mill, are reheated to the proper temperature and delivered to the first roll stand, where jets of high pressure, superheated steam strike the hot slab just as it enters the rolls, thus breaking loose most of the scale.

This roll stand is set somewhat further from its successors than the space between later stands. Some of this table is utilized for a turning mechanism, so that the slab can be turned sidewise, since its original length should represent the width of the strip. Heavy machinery also holds the slab down while its edges are squeezed to the correct distance apart. The slab is also held here a little time to radiate down to the necessary temperature; the latter is fixed by the speed of the remaining stands and the desired delivery temperature. Once started on its way, the slab passes through three or four heavy, four-high roll stands com prising what is known as the roughing train, separated far enough so the slab is entirely free in its passage along the roller table from stand to stand. This roughing train will reduce the thick ness of the 41 to 5-in. slab to somewhere around 1 in., and cor respondingly increase its length. It is usual for these stands also to have vertical edging rolls set at their entering side to keep the width of the hot metal correct. The work rolls in all these four-high mills are relatively small in diameter and have very heavy, massive backing-up rolls. Next in train comes a series

of six four-high stands set fairly close together, each one rotating at a faster speed than its predecessor, the last one delivering sheet of required thickness from, say, 4 in. down to -A in., at a speed of 3o miles an hour. The metal emerges from the last stand before the trailing end is clear of the first, so safe passage without breaking requires finger-tip control of the speed of the mills, as well as accurate and ready adjustment of the clearance be tween rolls—that is, the relative amount of reduction, stand to stand. All of these controls are electrical in nature and are centred in a pulpit where the operators are in full view of the entire operation. Temperature of delivery of the sheet is im portant for many applications. Instantaneous and automatic optical pyrometers, indicating and recording temperatures not only of the slab at the beginning, but of the finished sheet at the end, are essential items of equipment. Sometimes the hot finished sheet is rapidly cooled by water sprays; at other times the sheet is delivered as hot as possible to the coiling devices and the tightly wound coil is cooled quite slowly.

This sheet-mill product may be cut or slit into such sizes as make it available for other operations, such as thin plate or thick sheet for tankage or barrels, or it may be slit into narrow strips suitable for making into pipe. However, if it is to be further rolled into thinner sheet, the mill scale is pickled off, and then the material is rolled through four-high cold rolling mills, usually set three in tandem. Reduced finally to the proper thickness and hardness, the coil is uncoiled through a leveller and automatic shear which cuts and piles the sheets, after which they are sent to the annealing department for proper annealing. This is now done in controlled atmosphere so that there is no scaling and even little discoloration. After a final pass through a four-high stand ("skin rolling and levelling"), the sheets are resquared, in spected, oiled, and shipped.

Pipe.

Skelp, the flat bars from which small sized pipe is made, is rolled in a continuous mill specially designed to produce bars true in thickness, width, and properly bevelled on the edges. Frequently the finishing stand is adjustable, and a skilled work man varies the roll spacing slightly in accordance with his esti mate of the temperature of the metal issuing, thus compensating for the different amounts of contraction during cooling. Skelp is cut 20 ft. long, one end pointed, and then charged into a flat hearth, properly designed to produce a uniform white heat throughout; when reaching the welding temperature the metal appears to be covered with a shiny grease, which is molten iron oxide. The operator then catches the pointed end in tongs, and by means of a draw bench (see WIRE) draws the white hot bar through a welding bell (fig. 5-A) forming the flat bar into a tube and pressing the butted ends together into a true weld all in one operation. Further operations reduce the pipe, but only slightly, to the correct size, straighten it, remove the scale, shape and thread the ends, proof test it, and finally cover it with a protec tive coating.

Butt welding is done on pipe from 1- to 3 in. nominal inside diameter. Common pipe from 2 to 18 in. is more usually made of flat plate by lap welding (see fig. 5-B). Plates are piled into a furnace, heated to a dull red, and run one by one through scarfing rolls, which form the necessary bevel on the edges for an over lapping seam. The hot steel is then drawn through a forming bell which curls it up into a tube (fig. 5-A). After reheating to the welding temperature this tube is caught between a pair of rolls and rolled over a manganese steel mandrel (fig. 5-B), and the joint welded by the high heat and great pressure at the point of contact. When the length has been rolled, the bar which has been holding the mandrel in position is withdrawn from the far end of the pipe, and the latter rolled away for the finishing operations al ready enumerated. It requires only a few seconds to weld a standard length pipe by either of these processes. Welded pipe over 3o in. in diameter is made by hammer welding or more recently by automatic arc-welding. In the former, the wide plate is curled on plate bending rolls and the overlapping joint heated and forged shut in sections. During the heating cycle two water gas burners play a flame on a portion of the joint from both inside and outside ; when the correct temperature is reached the lower burner is replaced by an anvil and the upper by a pneumatic hammer, and the hot spot welded and hammered down flat (fig. 5-C). The next section is similarly treated. After welding the length is re-rolled for shape and annealed, the ends sized and shaped, and the pieces tested and dipped in anti-corrosive liquid.

For arc-welding, the edges of the plate are planed to a bevelled V opening (fig. 5-F), the pipe bent to shape in rolls, clamped between long copper bars, and the bevel filled with metal from a slowly travelling wire, from which an electric arc is struck to the steel plate below. Details of this and other welding methods are given in the article on WELDING.

Seamless

Tubing.—In America, seamless tubing is preferred for all purposes where the internal pressure is high and service severe, and a rapidly increasing amount of such pipe is being made in diameters up to 12 in. First, a round billet at full red heat is pierced by inclined rolls and mandrel. As sketched in fig. 5-E, two 36 in. rolls of special shape are set side by side, with axes crossing and about o° askew. The shape is such that the rolls grip the end of the ingot and spin it forward slowly, forcing it over a mandrel set between them. The operation is such that the mandrel seeks and maintains a truly axial position. After pierc ing the inner and outer surfaces are smoothed and the tube brought to exact size by several rollings over a mandrel, as in lap welding. Further operations from straightening to shipping are performed with considerable care. Not only is such pipe more reliable for the lack of any welded seam, but the piercing operation is so strenuous that nothing but metal of good quality can endure it. Therefore the tubing can safely withstand a considerably higher pressure than the best of welded pipe. Seamless tubes after cold drawing to thin walls are much used for structural members on bicycles, automobiles and aircraft, and for shaping into metal furniture and machine parts. Cold drawing is done as sketched (fig. 5-D ). A tube somewhat too large and thick in wall has the end swaged down slightly. All oxide is then pickled off, the tube washed, dried and dipped in tallow or other lubricant. The pointed end is thrust through a round die, and gripped by draw bench tongs. Next, a mandrel is thrust through the tube from the far end until it has a position directly within the die, and then the tube drawn forcibly through the annular opening thus formed. In this way the outer and inner diameter may both be controlled. Each draw hardens the metal and makes it more brittle ; after a few such passes, if the required size is not yet reached, the pipe is given a low temperature anneal to relieve the cold-working strains, and the process continued.

Rods and Wire.

As indicated above, wire rods are frequently rolled from 2 in. billets, 3o ft. long, on continuous mills, and the sixteenth or finishing pass delivers a 0.2 in. round to reels at a speed of 45 m. an hour. Such a mill will produce 15 tons per hour, with a complete labour force of 45 men. Other arrangements utilize a continuous mill for breaking down a 4 in. billet, and about a dozen loops fed by hand in a cross-country mill to finish. Rod reels are dumped automatically, and the rod cold-drawn through dies, with frequent annealings and other intermediate operations, into wire of various diameters, finishes and tempers. The operations are described more fully under WIRE. Most of this wire is stranded into cable or wire rope, woven into screens, net ting, or fencing or made into nails. All such is done by auto matic machinery so economically that the increase in cost of nails, for instance, over wire is 15 cents per Ioo lb., which not only includes manufacture, but the cost of the keg as well.