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.