THE ROLLING PROCESS
[Illustration:
EARLY ROLLS
]
After invention of the puddling furnace with its rather large yield from
the standpoint of those days, Cort about 1783 found the hammering method
unsatisfactory for his purposes and rolls were devised by him to
facilitate working of the larger balls of iron which his furnace
produced.
His rolls were provided with a series of grooves which systematically
reduced the balls of iron to pieces of longer and longer length and
proportionally decreasing diameters. They were power driven and served
very well as long as iron and steel were made in quantities no larger
than those which were produced in the puddling and crucible furnaces.
Quite naturally there was little or no change in the essentials of
rolling mill design until it was forced by the invention of the Bessemer
steel-making process. With that occurrence trouble began. The
open-hearth process followed, and, with the increasingly large steel
outputs of mills using these processes, necessity after necessity
developed which resulted in the highly developed rolling mills of
to-day.
[Illustration: [Mill]]
The “Two-High” Mill
The mill as invented by Cort had but two rolls and these were actuated
by a fly wheel. Turning in one direction continually, the rolls allowed
the piece being rolled to go through in only one direction, i.e., it had
to be returned from the rear to the front side of the mill after every
passage, usually called “pass.” This was done by the “catcher,” a brawny
man at the rear of the mill, seizing the piece, lifting one end bodily
to the top of the upper roll over which it was carried back with more or
less difficulty and awkwardness to the “roller,” who, from the front,
seized and entered it again into the next succeeding or smaller groove
of the rolls.
[Illustration:
TWO-HIGH ROLLS AND EFFECT ON THE PIECE
]
The “Three-High” Mill
[Illustration:
ACTION OF THREE-HIGH ROLLS
]
In 1857 John Fritz was watching his men at their slow and fatiguing work
at the two-high mill of the Cambria (Pa.) Iron Works. The thought struck
him that by adding another or third roll at the top, the piece could
also be given a pass on every trip back to the roller in front. The
rolls would of course pull it through, the work would be less severe on
the men, and, receiving passes in both directions, the piece would
receive the full number in approximately one-half of the time which was
then required, and, more important than all else, it would not have
nearly so long a time to cool and could be finished at a more desirable
temperature—a great advantage.
Strange to say the idea was immediately pronounced impracticable when he
mentioned it and it was necessary for him to go through a long fight to
obtain permission to make a trial.
The experiment was from the first successful but the mill burned one
Saturday night, having supposedly been set afire by workmen who feared
loss of their jobs. Rebuilt and manned by new workmen it ran with
success.
The succeeding ten years saw the “three-high” type of mill come into
extensive use both in America and Europe. Elevating or tilting tables
have since been provided which mechanically raise the piece to the upper
rolls, thereby relieving the workmen of this duty, which, with the great
increase in size of ingots and pieces rolled soon became very arduous.
To-day the “three-high” mill is just as important as ever.
The Reversing Mill
Having a fly wheel for the storing-up of power, the rolls must keep
turning continuously in the established direction. In England,
Nasmyth—the same man who invented the steam hammer—suggested that the
fly wheel be dispensed with and the two-high rolls reversed after each
pass. The piece would go back to the roller’s side receiving work in a
regular pass on the way, just as in the three-high return. The idea was
developed by Mr. Ramsbottom of the London and Northwestern Railway
Company. By the use of powerful enough engines the desired end was
accomplished and this type is now quite generally used for very heavy
ingots which it is not economical to lift by tilting tables in the
three-high mill process. There are certain other advantages also.
The above are the three general types of mills.
“Breaking-Down” the Ingot
Whether it is to be sold in intermediate shapes or further rolled down
into a finished product, all ingots have to be “cogged” or broken down
into intermediate-sized slabs, blooms or billets, for an ingot contains
altogether too much steel for any single plate, rail, rod, or other
finished product.
The cogging or first rolling is accomplished in one of the three types
of rolls already described but now more generally in the reversing mill.
[Illustration:
INGOT COMING OUT OF THE “SOAKING PIT”
]
When the tongs of the big overhead crane have lifted the white-hot ingot
out of the soaking pit it is run back and forth through the rolls which
are forced nearer and nearer together by the “screw-down” man so that
the piece continually becomes thinner and much longer with each pass.
Tables made up of small rollers geared together receive the long piece
as it emerges from the rolls. After each pass they bring the piece back
to the rolls which are now turning in the opposite direction. The table
on the other side repeats the process, the piece being regularly turned
on edge or slid from one side of the rolls to the other by steel guides
which can be raised up between the rollers of the tables where desired.
The direction of these as well as of the rolls themselves is controlled
through levers by two or three men standing at one side of the mill.
Reversing after each pass, with the big ingot apparently turning and
sliding itself into the most advantageous position for the next entry,
the big engine, mill and roll-train seem almost human.
[Illustration:
INGOT IN THE ROLLS
]
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.
[Illustration:
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.
[Illustration:
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.
[Illustration:
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.
[Illustration:
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.
[Illustration:
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.
[Illustration:
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.
Sheets
[Illustration:
THE ROLLING OF SHEETS
]
Most plates are rolled from slabs which are about 36 inches wide, but
“sheets,” which are plates less than ³⁄₁₆″ thick are rolled from much
smaller-sized slabs known as “sheet bars.” After “pulling out” into
sheets these may be folded once or even more times, so that from two to
eight thinner sheets are rolled at once. That they may not weld or stick
together under the heavy pressure, they must be rolled colder than are
single plates. They are later trimmed and pulled apart. Some mills start
sheet bars of smaller size, for each sheet a separate piece, which,
after drawing out somewhat are piled, two, three, four or five high.
With coal or charcoal dust—either dry or mixed with water—between them,
they are heated and rolled, the charcoal and coal dust keeping them from
sticking together. After annealing, pickling, etc., they may be cold
finished in polished rolls or otherwise treated according to the purpose
for which they are intended. After straightening some are galvanized,
others are tinned, blued or painted. Most of them are sold “black,”
i.e., with no coating at all. Terneplate has a coating of 75% of lead
and 25% of tin.
The Rolling of Rails and Structural Shapes
It will be readily understood after reading the above, that, instead of
using plain rolls, mills for rolling steel rails, I beams, channels,
angles, Z bars, rods, etc., must have grooved rolls. For these products
the first pass will be through a groove slightly smaller than the bloom
or billet. Successive passes will be through other grooves in the same
set of rolls which will gradually make smaller and bring more nearly to
the finished shape the piece being rolled.
[Illustration:
RAIL IN THE FINISHING ROLLS
]
Before our eyes the white-hot bloom enters the three-high mill, goes
backward and forward through the rolls and very shortly assumes the
general shape desired. Each pass thereafter brings it nearer to the
finished shape. Rails, for instance, are rolled out from the blooms into
one long rail perhaps 140 feet in length which glides along like a huge
snake to the swiftly revolving “hot” saws which are so spaced that four
33–foot rails are sawed from it at the same time. As the rails pass from
the saws to the cooling bed they are marked by a revolving stamp. When
cool they go to the straightening yard, are straightened, drilled,
inspected and later loaded into cars for shipment.
The production of all kinds of finished rolled iron and steel products
in the United States during the past twenty-eight years is given in the
following table which shows how extensive are our rolling mill
industries and the rapidity of their development.
────┬─────────┬─────────┬───────┬─────────┬──────────┬──────────┬──────────
Year│Iron and │ Plates │ Nail │Wire Rods│Structural│All Other │ Total
│ Steel │ and │ Plate │ │ Shapes │ Finished │Gross Tons
│ Rails │ Sheets │ │ │ │ Rolled │
│ │ │ │ │ │ Prod. │
────┼─────────┼─────────┼───────┼─────────┼──────────┼──────────┼──────────
1887│2,139,640│ 603,355│308,432│ │ │ 2,184,279│ 5,235,706
1890│1,885,307│ 809,981│251,828│ 457,099│ │ 2,618,660│ 6,022,875
1895│1,306,135│ 991,459│ 95,085│ 791,130│ 517,920│ 2,487,845│ 6,189,574
1899│2,272,700│1,903,505│ 85,015│1,036,398│ 850,376│ 4,146,425│10,294,419
1901│2,874,639│2,254,425│ 68,850│1,365,934│ 1,013,150│ 4,772,329│12,349,327
1903│2,992,477│2,599,665│ 64,102│1,503,455│ 1,095,813│ 4,952,185│13,207,697
1905│3,375,929│3,532,230│ 64,542│1,808,688│ 1,660,519│ 6,398,107│16,840,015
1907│3,633,654│4,248,832│ 52,027│2,017,583│ 1,940,352│ 7,972,374│19,864,822
1909│3,023,845│4,234,346│ 63,746│2,335,685│ 2,275,562│ 7,711,506│19,644,690
1911│2,822,790│4,488,049│ 48,522│2,450,453│ 1,912,367│ 7,316,990│19,039,171
1913│3,502,780│5,751,037│ 37,503│2,464,807│ 3,004,972│10,030,144│24,791,243
1915│2,204,203│6,077,694│ 31,929│3,095,907│ 2,437,003│10,546,188│24,392,924
────┴─────────┴─────────┴───────┴─────────┴──────────┴──────────┴──────────
Specifications and Inspection
Customers, of course, have a right to see that their specifications are
lived up to. Though years ago the mills perhaps intentionally sold to
customers products which did not fulfill his specifications to the
letter, it is not generally so to-day. Now the mills’ own inspectors are
commonly more severe in their rejections of products than are the
representatives which the customers themselves send. Not only does the
mill laboratory make careful and accurate analysis of each heat or batch
of steel made, but, after its application to orders, pieces of plate,
shapes or rails rolled from it are examined, gauged, and test bars of
the steel are pulled in the physical testing laboratory.
The mill rightly recognizes that it is for its own interest that the
standard of its product be kept high.
A trip through one of the large steel plants with its furnaces, its
blooming and slabbing mills, its rail, plate, structural and rod mills
is one of the most interesting that can be taken. If the visitor is not
afraid of smoke or dust or of what seems to him an uncomfortable heat on
a warm day he will discover new worlds. No particular attention is paid
to the casual visitor to the plant, but, for those who show real
interest, steel men have a warm welcome, from manager to the sample
boys.