The Rolling of Steel Plates
It is manifestly impossible in the space at disposal to give in much detail the rolling of many of the better known products. Fortunately it is not necessary, for after description of the making of plate, pipe and tube, and of rod and wire, the rolling of other forms such as rails, bars, and the structural shapes, I beams, channels, angles, Z bars, etc., can well be imagined.
Steel Billets for Forging or Other Purposes
From the contract department to the mill clerk come the orders for plate, with detailed list of sizes and thicknesses, and definite specifications of quality in terms of chemical and physical requirements, etc.
Rolling Ingot into Slabs
After studying these, the clerk makes requisition upon the open-hearth furnace for such tonnages of steel of various compositions as he estimates will give him sufficient stock for his purposes. As soon as possible the steel is made and poured into ingots which are transferred to the soaking pits of the slabbing mill there to await disposition as soon as the chemical laboratory has made analysis of the sample taken and has reported by telephone the result to the clerk who ordered the material. If close enough to the composition he ordered, he sends to the slabbing mill his requisition ordering them to roll and cut the four or six ingots of the “heat” into slabs of definite weights, each one designed for a plate on a customer’s order.
The clerk at the slabbing mill determines to what width and thickness each ingot shall be rolled and in what varying lengths it is to be cut to furnish slabs of the definite weights ordered in the requisition.
After rolling the ingot down to proper width and thickness, the “piped” end is cut off and “discarded.” Slabs are cut and piled in regular order on a little flat steel car on which they are pulled, still red-hot, by the shrieking little dummy engine out from the slabbing mill, through the yard and to the plate mill furnaces, into which they are charged in proper order. Here they remain until they are again white-hot and the plate mill roller is ready for them.
Meanwhile record sheets giving the heat number, the number of the ingot and the weights of the slabs in the order in which they were piled come to the plate mill clerk. From these and the results of analysis of the steel he makes out the rolling orders for the plates to be manufactured.
Slabs from Which Plates Are Rolled
You have heard how difficult it is to get solid ingots and how the top eighth (or sometimes more) of an ingot is usually “piped” and discarded. Now slabs from the balance of the ingot were piled on the car and have been charged in the plate mill furnace. Those from the upper part of the ingot (next to the discarded part) are used for the less exacting qualities of plate. Only the bottom half of the ingot, which of course is the solidest and best, goes into the higher grades of plate, such as “fire box,” the choicest grades of “flange” steel, etc. The third quarter goes into flange stock, “ship” and “tank” plate, the latter representing miscellaneous lower-priced plate which may be used for water tanks, steel flooring, etc. Steel known as “fire box” of course must be of very high grade. It is used for parts of locomotives, etc., which come in contact with and likely will suffer deterioration from flame, smoke, etc. The best “flange” goes into boiler plates and other products which have to stand considerable bending to shape.
In making out his rolling orders the clerk sees that each numbered slab is ordered rolled only into a product for which it is well suited. He has to take into consideration the chemical composition, the probable strength and other physical properties which were definitely named in the specifications of the customer’s order. And, as the physical properties of such steel are mightily affected by temperature and speed of rolling and by rapidity of cooling, he must know mill practice and constantly keep in touch with the results which the physical testing laboratory is getting from bars sheared from such of his plates as have been “pulled” for customers or the inspectors who represent them.
When hot, the slabs one by one and in regular order come to the rolls from the furnace. Following his rolling orders the roller and his helpers put each slab back and forth through the plate mill rolls, first drawing it out to a width a few inches greater than the plate to be sheared from it, and then turning it a quarter around, they draw it out in the rolls until it has come down to the proper thickness or “gauge.”
If the clerk’s computations have been correct the plate will now have the proper length. However, he may have ordered a slab of insufficient weight to make it, particularly if the rolls have become much worn.
It will hardly be realized how much the width and thickness of the plate ordered have to do with the “percentage” of trimmed plate which the mill will get out of the slab ordered. There is a “fish tail” on each end of a rolled plate. On a thin, wide plate this becomes rather serious.
Wherever possible the clerk puts two or three plates end to end and perhaps narrow ones side by side, but he must not exceed the width which the “shears” can “split” nor give the mill such a long plate that it will become too cold to roll or too long to be conveniently handled.
An 84–inch Plate Mill
For diversion the mill men take delight in throwing an extra amount of salt upon the plate to rid it of scale when nervous visitors have come as close to the rolls as their conductor through the mill will bring them. The explosion which comes from the usual amounts is much intensified and it is not at all out of the ordinary to hear shrieks from the women and to see surprised and somewhat dismayed men among the visitors.
Plate mills are usually three-high with tables of small rollers on each side which tilt to feed the plate into the rolls and to receive it on the other side from which it is fed in again, either above or below as the case may be. As the plates must be flat, perfectly plain rolls are used. For plates which are very wide these rolls may be 140 inches or more long and perhaps three feet in diameter.
The rolls, of course, are kept flooded with water to keep them cool. At first thought one would think that the water would cool the plates which are being rolled. It does not materially do so, however, the extreme heat apparently keeping the water from coming in actual contact with them. Thus they are rolled down from the three-inch thick slab to ³⁄₁₆″, ¼″ or ⅜″, and from 6″, 8″, 10″, or 12″ slabs into ½″, ¾″ or possibly 1″ or 1¼″ plates.
The Rolling of Plates
All plates must be rolled very accurately to gauge, allowance of variation often being not over one or two hundredths of an inch. The roller must be a man of experience and of very good judgment for slabs for almost any plate may come of any one of several thicknesses and lengths. He must know his temperatures, speeds of rolling and the amount of reduction given with each pass, and, particularly in case of thin, wide plates, the condition of his rolls, which after two or three days’ wear will produce plates thicker in the middle than at the edges. As the “screw-down” man on top screws the rolls together a little with each successive pass and the “hookers” under the roller’s direction keep the plate entering the rolls properly, he must with his very accurate gauge measure the thickness of the plate as it nears completion. Especially when plates are ordered and paid for by average weight per square foot must he judge accurately the thickness of the center of the plate where he cannot measure, and pull down the edges enough that the finished plate when sheared will average right.
It is fortunate for the steel mill men of this country which does not know the advantages of the metric system that a steel plate one inch thick weighs very close to 40.8 pounds per square foot. This is an easy figure and the clerk, roller, hot bed foreman, weighers and all concerned “think” in terms of a plate one foot square and one inch thick. One-half inch plate, therefore, weighs 20.4 lbs.; ¼″, 10.2 lbs.; and ³⁄₁₆″, 7.66 lbs. per square foot.
As will be seen when we consider wire drawing and cold-drawn seamless tubes, the strength and other physical properties of steel depend first, upon composition, and, secondly, upon temperature at which they are hammered, rolled, or otherwise “worked.” Therefore, plates can be much modified in physical properties by finishing at chosen temperatures. A steel containing .19% of carbon and .45% of manganese, for instance, which in one inch plate should give a tensile strength of around 55,000 pounds per square inch, 58,000 pounds in ½″ or 62,000 pounds in ¼″ when finished at usual temperatures, by slightly “colder rolling” can be made to show a considerably greater strength. Of course, the ductility is somewhat reduced, but, with a moderate amount of cold rolling, it will not be enough to do harm.
All of these and many other details must be not only kept in mind but become second nature to the plate worker.
After the final pass the plates go upon the “hot bed” where they are laid out side by side in the order in which they have been rolled. They must now have marked out upon them the boundaries of the smaller plates or pieces into which they are to be cut. From a duplicate of the roller’s sheet the hot bed foreman marks upon the end of each plate what is to be laid out and boys or men wearing shoes with thick soles of old belting or other cheap non-conducting material go upon them with chalk and “squares” which are somewhat similar to the carpenter’s square but having “legs” six and twelve or fifteen feet long. Though the soles of their shoes smoke from contact with the still hot plate, they very quickly and accurately mark out upon its surface the design which the hot bed foreman has signified.
Usually the plates laid out are rectangular and of standard size but often the boys have to lay out pieces of odd sizes and shapes, and sometimes, what are known as “sketches” have to be drawn using arcs, chords, radii, etc., as the student in geometry draws his geometrical figures. Round plates for boiler heads, tank ends, etc., in plate mill parlance are termed “heads.” These are marked out with string and a piece of chalk. A boy with a pot of white paint follows and paints on the surface of each piece laid out its size, thickness, the customer’s name, the order number and heat number. That the plate can always be identified, even after exposure to severe service or weather conditions, another boy with steel stamps follows and stamps into the steel the heat number.
Cranes with magnets or hooks convey the long plates to the “goose necks” over the small rollers of which they are pulled to the shears where the powerful steam or hydraulically-operated square-edged knife with ease trims the ends and irregular edges along the chalk lines into the sizes marked. Accuracy is everywhere necessary as ¼″ over or under ordered dimensions or a variation of two hundredths of an inch in thickness may and probably will cause rejection of a plate.
After weighing and recording, the plates are conveyed to the shipping yard, where they are loaded by electro-magnets into cars for shipping.
In the plate mill process above described plates anywhere between 30 and 120 inches in width, say, can be rolled. And as mentioned, the more or less irregular edges on the sides are “sheared” off. This extra allowance, which must be given, of course becomes “scrap.”
For plates which can best be rolled in long narrow lengths a “universal” plate mill is often used. This has vertical rolls just back of the two horizontal rolls, which are adjustable so that the plate can be regulated, not only as to thickness, but also as to width as well. Such mills give plates which have to be trimmed on the ends only, the sides being quite smooth.
Loading Plate from the Shipping Yard
The rolling of plate has been described thus in detail that a slight conception can be obtained of the refinement and the minutia which is a necessary part of modern mill practice. American outputs which have grown to as much as several hundred tons per twelve-hour turn, require that every operation move along with “clock-like” precision. But with this immense tonnage and with all of the handicaps of occasional broken rolls and machine parts, electric crane delays, and illness of important men, the work must be and usually is kept up without serious delay.
Modern metallurgical and rolling mill practice is a marvel.