Method and apparatus for rolling strip or plate

A method of processing large slabs into memory free strip or plate comprises the steps of: rolling the large slabs into strip or plate on a rolling mill with a finishing temperature above 1340.degree. F.; in-line cooling the strip or plate to a temperature in the range 900.degree. F. to 650.degree. F. with the strip or plate laid out on a flat cooling conveyor; slowing the speed of the strip or plate to speeds typical of cut-to-length lines; side trimming and/or cutting the strip or plate to lengths at temperatures above 500.degree. F.; cooling the strip or plate to a temperature below 300.degree. F.; and stacking the strip or plate.

BACKGROUND OF THE INVENTION 
The economics of steel strip and plate production favor the processing of 
large slabs, sometimes referred to as "jumbo" and "junior jumbo" slabs. 
These slabs are too large to be used to roll a single strip or plate. 
Typically, jumbo slabs weigh approximately 30 tons, junior jumbo slabs 
from 10 to 15 tons and normal or pattern slabs 1 to 2 tons. Pattern slabs 
are just large enough to produce a single strip or plate following end 
cropping. 
When the product of a pattern slab emerges from the hot rolling mill, it is 
typically transferred perpendicular to the rolling direction onto a 
parallel cooling bed where it remains flat until cooled below the brittle 
temperature range (about 500.degree. F. to 300.degree. F.). Because it is 
never coiled, the strip or plate cannot suffer the disadvantage of coil 
memory or uncoiling flaws, such as coil break. On the other hand, the 
economics of rolling pattern slabs and cooling on a parallel cooling bed 
are limited. For example, the hot rolling mill must be set up over and 
over again for each different pattern slab introduced to the mill. The 
ends of every strip or plate must be cropped reducing the yield from many 
pattern slabs versus the yield from one or several jumbo or junior jumbo 
slabs. The capacity of the parallel cooling bed varies depending upon the 
width of the strip or plate transferred to it due to the uniform placement 
of the "dogs" that pull the plates over the bed. Unless the strip or plate 
width is almost an exact multiple of the spacing between the "dogs", the 
cooling bed is less than 100% covered reducing its throughput. 
The current practice for rolling jumbo slabs and junior jumbo slabs is to 
hot coil the strip or plate and then to place the coils in storage for at 
least about three days and sometimes weeks before uncoiling, leveling and 
passing the strip or plate to a cut-to-length line. The three-day cooling 
period is required because if the coil is unwound while still in the 
temperature range of 650.degree. F. to 100.degree. F., the strip or plate 
will suffer from what is known as coil breaks. Coil breaks are visible and 
undesired metallurgical deformations of the strip that cannot be corrected 
by further processing. 
While the three-day cooling period alleviates or overcomes the problem of 
coil breaks, it results in another drawback known as coil memory. Once a 
coil is allowed to cool below 650.degree. F., the strip or plate 
"remembers" the curved shape of the coil even after leveling and cutting. 
Leveling (processing through a series of small rolls positioned 
alternately above and below the strip or plate to alternately pull each 
surface beyond the yield point) does so in part by balancing the residual 
stress in the plate around a neutral axis. This results in trapped 
stresses. On subsequent cutting of the plate or strip, these stresses can 
produce a shape defect. (This effect is directly related to strip or plate 
thickness for a given coil diameter.) Also, cooling the coils for from 
three days to several weeks creates a process inventory that ties up 
valuable working capital. 
The coil memory problem can be avoided by never forming and cooling as a 
finished coil. A process coiling with flat-pass finishing has been 
proposed for product rolled from large slabs but not without using a 
parallel cooling bed and cutting the strip or plate as it emerges from the 
finishing pass into a length that the width of the cooling table can 
accommodate. 
It is an advantage, according to this invention, to roll jumbo or junior 
jumbo slabs to strip or plate with in-line cooling (without using a 
parallel cooling bed) and without experiencing the drawbacks of coil break 
and coil memory. 
SUMMARY OF THE INVENTION 
Briefly, according to this invention, there is provided a method of 
processing large slabs into memory free strip or plate. The method 
comprises first rolling a large slab to strip or plate on a hot mill with 
a finishing temperature above 1350.degree. F., that is, above the 
eutectoid temperature of 727.degree. C. (1340.degree. F.). The next step 
comprises in-line cooling of the strip or plate to a temperature in the 
range of 900.degree. F. to 650.degree. F. with the slab laid out on a flat 
cooling conveyor. Next, the strip or plate is side trimmed and/or cut to 
length at temperatures above 500.degree. F. Following cooling to a 
temperature below about 300.degree. F., the strip or plate is piled. 
According to one embodiment of this invention, immediately following hot 
rolling, the strip or plate is coiled at temperatures above 1350.degree. 
F. at the finishing speed and then uncoiled at a speed to facilitate side 
trimming and cutting to length. Preferably, cooling after uncoiling takes 
place in a short water cooling section. 
According to yet another embodiment of this invention, a runout table is 
provided which is long enough to receive the entire strip or plate rolled 
from a large slab so that after the tail of the strip or plate has emerged 
from the rolling mill, it may be slowed to speeds at which side trimming 
and cutting to length are practical.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, the general process, according to this invention, is 
illustrated. Following hot rolling of strip or plate 10 from large slabs, 
for example, in a Steckle Mill with hot coilers upstream and downstream 
from the mill, the strip or plate emerges from the last pass at finishing 
speeds, say, 1,500 feet per minute. Most of the downstream processing is 
more easily carried out at slower speeds, say, 500 feet per minute. Thus, 
a staging step 11, that is, slowing the rolling mill product after the 
entire strip or plate has emerged from the mill, takes place either before 
or after a first in-line cooling step 12. The first in-line cooling step 
has for its purpose to reduce the temperature of the strip or plate to 
near 650.degree. F. The strip at this temperature is not yet to the 
brittle range (500.degree. F. to 300.degree. F.). The strip or plate is 
then side trimmed and/or cut to length at temperatures above the brittle 
range. A second in-line cooling step 14 reduces the temperature of the 
strip or plate to below 300.degree. F. and the product is stacked at 15. 
According to one method embodiment as described with reference to FIG. 2, 
large slabs are first hot rolled into strip or plate during a hot rolling 
step 20. This is followed by a step for coiling 21 at temperatures above 
the austenite to ferrite transition range (about 1340.degree. F. 
(727.degree. C.)). In the next step, the coils are transferred to a 
cut-to-length line and uncoiled at 22 while still above the transition 
temperature. The coiling and uncoiling provide for staging since there 
typically is a period of about 5 minutes (that is, the time it takes to 
roll a slab in a sequence of passes to a finish coil) between coil 
transfers. The uncoiling can take place at about one-tenth the speed of 
coiling. 
The uncoiled strip or plate may then be passed to an optional hot 
flattening step 23 before it is passed along to a second in-line cooling 
step 24. This cooling step may, for example, be a laminar flow water 
cooling bank of known construction. The number of headers supplying 
cooling water needed for cooling are much reduced compared to laminar flow 
cooling banks used for cooling strip or plate as it emerges from the 
rolling mill due to the large difference in speeds of the strip or plate 
as it moves through the cooling banks. Ideally, the in-line cooling 
reduces the temperature of the strip or plate to near but not below 
500.degree. F. The next step 25 is side trimming or cutting to length 
(CTL) while the strip or plate is still above the brittle temperature 
range. A second in-line cooling step 26 follows reducing the temperature 
of the strip or plate below 300.degree. F. An optional precision leveling 
step 27 may precede a step 28 for stacking the strip or plate. 
Referring now to FIG. 3, there is illustrated a cut-to-length line for the 
practice of the method embodiment described with reference to FIG. 2. The 
strip or plate enters coiler 31 from the hot strip mill 30 and runout 
table 32. A laminar flow cooler may be positioned on the runout table 32 
to cool the surface just sufficiently to control scale formation. The coil 
is then transferred to uncoiler 33 and may optionally uncoil into hot 
flattener 34. The strip or plate then proceeds to a first water cooler 35, 
for example, a laminar flow water cooler. A runout table following the 
first water cooler conveys the strip or plate to a side trimmer 36 and/or 
cut-to-length shear 37. The strip or plate is then conveyed to the stacker 
39. Optionally, a skin pass mill 301 may be positioned after the first 
water cooler 35. Optionally, a recoiler may be positioned after the side 
trimmer, although if the recoiler is used, it would be a departure from 
the methods according to this invention. Typically, a shear gauge 303 is 
placed downstream of the second water cooler 38. Optionally, a precision 
leveler 304 is located just before the stacker 39. 
This invention is based on the concept of charging a hot coil from the hot 
strip mill or the Tippins patented coil plate process or coiled directly 
into a newly-designed, cut-to-length line. This coil transfer can occur 
automatically through a coil transfer device or by manually operated 
mobile transfer equipment. 
This invention not only contemplates a new type of cut-to-length line, but 
also new operating practices in both the hot rolling mill and 
cut-to-length line. As the hot strip or plate exits the roll bite of the 
last finishing pass on the hot rolling mill, the material surface is 
reduced in temperature or chilled by water cooling to stop oxide scale 
formation. However, the material will be kept above 1340.degree. F. (the 
eutectoid decomposition temperature) to retain the as-rolled austenitic 
phase prior to hot mill finish coiling. The finished hot coil must then be 
transferred to the cut-to-length line and the entire coil must be uncoiled 
in a controlled sequence to control the rate of temperature change below 
1340.degree. F., the point at which the high temperature phase begins 
forming the low temperature phase and the final material properties. If 
the transfer is not accomplished in the correct time-temperature window, 
the coil must be diverted to a coil cooling area for conventional 
processing. The cut-to-length line might also be designed to accept cold 
coils produced under the conventional practice. 
The above-described process sequence allows for net elongation of the strip 
at the optional skin pass rolling step, thereby creating the opportunity 
to bring all parallel "fibers" of the metal to the same length or state of 
strain. The process critical part of the invention is to take a transverse 
section of the strip through a well-controlled, time-temperature (or 
cooling rate) sequence in a precisely controlled fashion to create 
transformed metal with consistent properties. This is achieved by 
carefully controlling the uncoiling speed and water flow in the laminar 
flow cooler for any given strip or plate thickness. The cut-to-length line 
may be fully automated and designed to automatically cool the strip at the 
appropriate rate to the appropriate temperature to ensure accurate control 
of the material's metallurgical properties. 
A summary of the described typical cooling practice for the new invention 
is described below. 
______________________________________ 
PROCESS DESCRIPTION TEMPERATURE* 
______________________________________ 
Rolling Mill Last pass finish 
1450.degree. F.-1750.degree. F. 
temperature (typical) 
Laminar Flow/Coiler 
Surface chill to 
1350.degree. F.-1450.degree. F. 
stop scale (surface) 
Water Cool Phase transformation 
1350.degree. F.-900.degree. F. 
Side Trim/Shear 
Warm range 900.degree. F.-500.degree. F. 
Water Cool Brittle range 500.degree. F.-300.degree. F. 
Precision Level 
Finish 300.degree. F.-100.degree. F. 
______________________________________ 
*mean body temperature, except where noted. 
Referring to FIG. 4, another method embodiment is illustrated. This method 
comprises a hot rolling step 40 followed by an in-line cooling step 41 and 
the second in-line cooling step 42. In some embodiments, the first and 
second in-line cooling steps can be combined as one. An optional hot 
flattening step 46 may take place between the first and second in-line 
cooling steps. After the strip has been cooled to near 650.degree. F., it 
is side trimmed and/or cut to length in a step 43. A final in-line water 
cooling step 44 may precede a step 45 for stacking the strip or plate. 
Also, an optional precision leveling step 47 may precede the stacking 
step. Finally, if the side trim step does not include a cut-to-length 
step, a shearing step 48 is required just preceding the stacking step. 
Referring now to FIG. 5, there is illustrated an in-line cooling and 
cut-to-length line for practice of the method embodiment described with 
reference to FIG. 4. Rolling mill 50 delivers the strip or plate to a 
laminar flow cooler 51 and then to an optional hot leveler 52. The strip 
or plate is then passed to a very long in-line air cooling conveyor 53. At 
the end of the air cooling conveyor, side trimmer 54 and shear 55 are 
arranged to side trim and cut to length the strip or plate. A cooling tank 
56 further cools the side trimmed and sheared plate prior to passing 
through an optional precision leveler 57. A shear 58 is positioned to cut 
the strip or plate to length if it has not been cut to length with shear 
55 and finally the strip or plate is fed to stacker 59. 
With the apparatus described with reference to FIG. 5, the plate is rolled 
at the rougher and finishing mill utilizing junior jumbo or jumbo slabs. 
The slabs are rolled straight away or cross rolled to width as required. 
Typically, the finished thickness will range from 3/16 inch to 1 inch. As 
the product is rolled from the finishing mill on the last pass, the plate 
is run through the laminar flow cooling system which cools the workpiece 
to a targeted temperature to set the physical properties, cools the 
surface of the plate to stop the growth of scale and provides additional 
cooling for shedding heat energy in the workpiece. The targeted 
temperatures of the plate emerging from the finishing mill and going onto 
the cooling bed are set forth in the following table for some typical 
grades and thicknesses. 
______________________________________ 
FINISHING AND TARGETED TEMPERATURES 
Cooling 
Mill Bed Entry 
Min. Max. Min. Max. Finish 
Targeted 
Grade Gauge Gauge Width Width Temp. Temp. 
______________________________________ 
A36 1012 
0.1870 0.1880 22.000 
104.000 
1575 1075 
A635 1012 
0.1800 0.7500 22.000 
104.000 
1600 1175 
X-42 0.2500 0.4050 72.000 
96.000 
1575 1200 
A572 42 0.1800 0.7500 22.000 
104.000 
1575 1150 
X-52 0.2500 0.5000 72.000 
96.000 
1575 1175 
X-60 0.2500 0.5000 72.000 
96.000 
1425 950 
X-65 0.2500 0.3750 72.000 
96.000 
1425 1075 
X-70 0.2400 0.5250 72.000 
96.000 
1425 1075 
______________________________________ 
After emerging from the laminar flow cooling bed, the workpiece may travel 
through an existing leveler (optional) onto a disc-type conveyor which 
comprises a linear cooling conveyor to move the workpiece slowly to the 
cut-to-length line. 
When the workpiece arrives at the cut-to-length line, the temperature of 
the plate is approximately 650.degree. F. which is an acceptable 
temperature for side trimming, leveling and cutting to length. An optional 
water cooling trough similar in design to a push-pickling line may be used 
to submerge the plate to remove heat therefrom. Upon exiting the water 
cooling trough, the workpiece proceeds to an inner blowoff (not shown), 
optional precision levelers and an optional cut-on-the-fly shear. Daughter 
plates cut to a length up to, say, 72 feet, travel to a stack which may 
consist of an inverted magnetic roller table with an end stop and are 
dropped onto a stack formed under the table. The stacks are then removed 
onto a side transfer mechanism for access to overhead cranes or mobile 
carriers. The time in minutes to reduce the strip or plate from 
1200.degree. F. to 625.degree. F. depending upon the thickness of the 
strip or plate is set forth in the following table. 
______________________________________ 
Thickness (inch) 
Time (minutes) 
______________________________________ 
.0800 2.215 
.100 2.771 
.125 3.46 
.135 3.74 
.187 5.19 
.250 6.92 
.375 10.38 
______________________________________ 
The required linear cooling conveyor length is the product of the number of 
spaces on the cooling conveyor and the plate length which results in a 
relationship that is independent on gauge given by L (m)=1.1, P where P is 
the process throughput rate in metric tons per hour as shown in the 
following chart. 
______________________________________ 
Linear Cooling 
Throughput Rate Bed Length 
Metric Tons/Hour 
Cycle Time (min.) 
Feet 
______________________________________ 
100 7.25 360 
150 4.84 541 
200 3.62 722 
______________________________________ 
As can be seen, the methods of hot rolling as described herein and the 
in-line cooling and cut-to-length lines as described herein enable the 
processing of very large slabs while avoiding the use of parallel cooling 
beds and overcome the problems associated with coiling and uncoiling at 
low temperatures. A key feature of this process is the recognition that 
each unit process, such as hot rolling or coiling, has a definite time 
period. A pacing time for the finishing mill can be chosen without a 
significant loss of production because the unit operations after hot 
rolling provide a retention time consistent with the overall process 
pacing and proper product cooling which is chosen to achieve the desired 
properties. 
Having thus described our invention with the detail and particularity 
required by the Patent Laws, what is desired protected by Letters Patent 
is set forth in the following claims.