Induction heating in a hot reversing mill for isothermally rolling strip product

The present invention provides a hot reversing rolling mill for isothermally reducing a metal strip product. The mill includes a hot reversing mill stand with a pair of coilers positioned on opposite sides of the mill stand. At least one heater is positioned between one of the coilers and the mill stand with at least one strip cooling unit positioned between one of the coilers and the mill stand. A sensor is provided for sensing a strip parameter indicative of strip temperature as well as a controller for controlling each of the heating and cooling units in response to the sensed parameter. The rolling mill of the present invention can be utilized for isothermally rolling multiple passes of a metal strip product on the hot reversing mill.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to using induction heating to achieve 
isothermal temperature conditions during the rolling of steel slabs and 
the like into a strip or plate product through a hot reversing rolling 
mill. 
2. Prior Art 
It has been recognized for many years that strip shape is dependent on many 
factors, including the temperature at which hot rolling takes place. This 
dependence on temperature relates not only on the minimum temperatures 
needed for hot rolling to achieve the desired metallurgical properties, 
but also on any head to tail temperature differential which occurs and 
which then may change rolling conditions and result in shape problems. 
These temperature differentials are inherent in the rolling of the strip 
via a hot reversing mill because of the temperature decay which takes 
place over time and the difference in exposure time to ambient conditions 
at various positions along the product being rolled. Not only is there a 
temperature drop at the respective ends of the coil, but the extreme head 
and tail positions of the product realize greater heat decay because the 
lack of a heat reservoir ahead and to the rear of the head and tail 
positions, respectively. In addition, there tends to be a temperature 
buildup in the middle from frictional forces, and all of these conditions 
vary with the width of the strip as well. 
In addition to strip shape, thickness tolerances must be maintained through 
such techniques as roll bending and automatic gauge control. These 
techniques may change rolling conditions and, thus, actual problem of 
temperature differentials. 
A number of rolling methods and apparatus have been tried and are employed 
to correct these shape problems. Many such efforts are directed to 
correcting the shape should it be less than desired. Other solutions 
address the cause of the problem and attempt to reduce the head to tail 
temperature differential in the first place. These include tapered slabs, 
tapered rolling, coil boxes upstream of the rolling mill and zoom rolling 
wherein the speed of rolling is accelerated to create frictional heat 
energy to increase the temperature of the tail of the workpiece. 
However, there remains a need for a method and apparatus which go to the 
root cause of the problem, namely, the change in temperature which takes 
place in a slab product being rolled to strip thickness on existing hot 
reversing mills. 
Induction heaters for heating metal products are known as shown in U.S. 
Pat. No. 4,751,360 and 4,407,486. Additionally, the use of induction 
heaters for side edge heating of metal strip is shown in U.S. Pat. No. 
4,627,259. Induction heating used in conjunction with continuous mills is 
known as shown in U.S. Pat. No. 5,133,205. This prior art does not address 
the issue of rolling on a hot reversing mill. 
An object of this invention is to achieve isothermal rolling temperatures 
throughout any given pass through a reversing mill stand or stands through 
use of induction heating. By isothermal rolling temperature, it is meant 
as reasonably constant as possible so as to have a negligible effect on 
resultant shape. 
It is also an object of this invention to achieve these isothermal rolling 
temperatures by also cooling the strip being rolled, as needed, in advance 
of the roll bite. 
It is also an object of this invention to monitor temperature or 
temperature-dependent functions such as roll force so as to provide 
control loops for achieving the isothermal rolling conditions. 
SUMMARY OF THE INVENTION 
The objects of the present invention are carried out in a hot rolling 
process on a rolling mill including at least one hot reversing stand 
having coiler units on opposite sides thereof. The product is converted to 
strip by passing the product back and forth through the stand in 
successive passes to reduce the thickness of the work product and coiling 
the product during the rolling process when it reaches a coilable 
thickness. The improvement of the present invention comprises providing an 
induction heating unit between at least one of the coiler units and the 
hot reversing stand together with sensing mill loads on the hot reversing 
stand and monitoring the sensed loads to compare them with predetermined 
load differentials. These loads are a function of the rolling temperature. 
The induction heater is utilized to heat portions of the product, as 
needed, in response to the sensed loads to maintain constant hot rolling 
temperature throughout the product being rolled. The present invention may 
additionally provide a cooling unit, such as a laminar flow cooling spray, 
for cooling various portions of the work strip. The induction heating unit 
of the present invention is preferably expandable to adjust to the 
appropriate size of the strip being worked upon. Isothermal rolling 
conditions are met by either heating or cooling portions of the strip as 
the case may be. 
These and other advantages of the present invention will be clarified in 
the description of the preferred embodiment taken together with the 
attached figures.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Rolling with automatic gauge control of the front and tail ends of the 
workpiece can produce a sheet product with extremely small variation in 
strip thicknesses; however, the rolling function does induce internal 
stresses into the front and tail portion of the strip. These stresses are 
not apparent during the hot rolling process. However, after the workpiece 
cools and it is cut into pieces for fabrications, it is likely that the 
sheet will deform as the internal stresses are released which, of course, 
makes the ends of the workpiece unfit for use. FIG. 1 is a representation 
of separating forces for each of nine passes through a single stand hot 
reversing mill according to conventional rolling procedures. The magnitude 
of the separating force over any pass is directly related to the 
resistance of deformation of the material being worked. The resistance of 
deformation, in turn, is inversely related to the temperature of the 
workpiece. FIG. 2 illustrates the temperature profile of the last pass, 
pass nine, through the single stand rolling mill illustrated in FIG. 1. 
FIGS. 1 and 2 clearly demonstrate the inverse relationship between the 
separating force and the temperature of the workpiece. Consequently, a 
measurement of the separating force will provide a substantially accurate 
measurement of the temperature of the workpiece. 
Due to the nature of hot reversing mills of the prior art, the front and 
back ends of the workpiece are inherently colder than the center portion. 
This characteristic is best illustrated in FIG. 2 where the temperature of 
the ends of the workpiece is shown substantially lower than that of the 
center. Although not specifically illustrated, a chart of the temperature 
for each of the previous eight passes would show similar decays in the 
temperature of the ends to a somewhat lesser degree. Referring to FIG. 1 
illustrating the separating forces, which are inversely proportioned to 
the temperature of the workpiece, it can be seen that as early as pass 
three the end pieces of the workpiece are exhibiting a loss of rolling 
temperature (i.e., an increase in the separating force measured). As 
demonstrated in subsequent passes, this characteristic is increased with 
each subsequent pass. Further, an attempt to add heat to the strip prior 
to the last pass of the workpiece does not cure the problem since the 
workpiece has been deformed and worked and certain stresses may have been 
added and not necessarily dissipated in each of the previous passes. 
As described herein, the present invention will monitor the rolling forces 
and, consequently, temperature and take corrective action to automatically 
add heat energy to the ends of the workpiece and cool the center of the 
workpiece as required for each pass so that as the workpiece rolls into 
the roll bite of the rolling mill, the mill will roll the workpiece in an 
isothermal condition throughout its length. The rolling of the workpiece 
under isothermal conditions helps increase the metallurgical properties of 
the resulting product, and decrease the end spread and non-uniform gauge 
presented and most importantly prevents the growth of internal stresses in 
the workpiece. 
Both the head to tail temperature differential and the absolute rolling 
temperatures must be controlled to achieve optimum properties. By 
responding to early drifts in temperatures, it is possible to correct both 
the absolute temperature and any differential in the workpiece on a 
pass-by-pass basis. This temperature can be simulated on a screen as an 
inverse function of mill load and automatically controlled. 
FIG. 3 illustrates a hot rolling mill 10 for isothermally reducing a metal 
strip product 12 according to the present invention. The mill 10 includes 
a four-high reversing hot mill stand 14 positioned on the pass line 16 for 
the strip product 12. A pair of coiler furnaces 18 is positioned on 
opposite sides of the mill stand 14. It will be understood that a two 
stand mill may likewise be used in such an arrangement. 
An induction heating unit 20 is positioned between each coiler furnace 18 
and the mill stand 14. The induction heating unit 20 is formed of two 
side-by-side portions adjustable relative to each other to provide a width 
adjustment. The induction heating unit 20 is operable to quickly add a 
significant amount of heat energy to the strip product. Two independent 
high frequency power units 20 with heating coils should be applied to both 
sides of the rolling mill stand 14 for the most flexible operation which 
would include heating the strip before it enters the roll bite and also 
before it leaves the roll bite. However, for economy of capital cost, the 
inducting heating coils (that actually heat the workpiece) can be powered 
from one power unit which would shift energy alternatively to the 
induction coils for heating the strip before it enters the roll bite only. 
A cooling spray 22 is additionally positioned between each coiler furnace 
18 and the mill stand 14. The cooling spray 22 should preferably be 
actuated by a quick- acting valve. The cooling spray 22 may be water or 
other conventional cooling fluid for use in cooling metal strip products. 
A force sensor 24 is attached to the rolling mill stand 14 for sensing the 
load or separating force thereon. The force sensor 24 is coupled to a 
controller 26 which controls the operation of each heating unit 20 and 
cooling spray 22. The sensor 24 may measure temperature, horsepower or 
other similar parameters. 
In operation, the force sensor 24 measures the separating force on the work 
product during the pass. This sensed parameter is indicative of the 
temperature of the workpiece as described above. Additionally, the 
difference between the sensed parameter and a predetermined value is 
determined by the controller 26. The cooling spray 22 or heating unit 20 
is activated by the controller 26, as appropriate, to modify the 
temperature condition of this portion of the strip. 
The induction heating units, which are adjustable for product width, are 
generally operated over a short length of the strip, often as short as 
5-25 feet at either the head and/or the tail of a coil. On the other hand, 
the laminar flow cooling is generally used to cool down the middle of the 
coil which has received a heat buildup from the frictional rolling forces. 
The system according to the present invention provides a feedback control 
loop which allows the process to be a self-learning adaptive process as 
the workpiece is rolled. By rolling according to the present method, the 
wide variations in separating force and, consequently, temperature 
illustrated in FIG. 1 can be eliminated or reduced. This isothermal 
rolling will result in improved metallurgical properties as well as gauge 
and other associated parameters throughout the final product. 
The heating units 20 may be positioned below the pass line 16 between rolls 
of the roller table. The cooling sprays 22 may be positioned above the 
pass line 16 to allow for gravity assist for the cooling sprays. 
It should be apparent to those of ordinary skill in the art that various 
modifications may be made to the present invention without departing from 
the spirit and scope thereof. Consequently, the scope of the present 
invention is intended to be defined by the attached claims.