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.