Method of levelling two-layered clad metal sheet

A method for preventing the camber of a two-layered clad metal sheet having a base layer and a covering layer of different metals which exhibit different amounts of thermal contraction. The method comprises developing a temperature difference .DELTA. T expressed by the following formula between the base layer and the covering layer during a hot levelling, by providing a greater cooling effect before or during levelling to the layer which exhibits the greater thermal contraction than to the layer which exhibits the smaller thermal contraction: EQU .DELTA.T=f (.DELTA..alpha., .alpha., a, To) where, PA1 .DELTA..alpha.: the difference in thermal expansion coefficient between both metals PA1 a: the clad ratio (ratio of covering layer thickness to total sheet thickness) PA1 To: hot leveller inlet temperature (.degree.C.) PA1 .alpha.: mean thermal expansion coefficient of both metals. Since the layer which exhibits a greater thermal contraction is forcibly cooled before or during a hot levelling adequately and by a required amount, the clad metal sheet does not exhibit any substantial camber after cooled down to the room temperature.

TECHNICAL FIELD 
The present invention relates to a method of levelling a two-layered clad 
metal sheet. 
BACKGROUND ART 
Two-layered clad metal sheets are known which have a base layer of a carbon 
steel clad with a covering layer of, for example, stainless steel, cupro 
nickel, etc.. The production of such a two-layered clad metal sheet 
encounters the following problem. Two layers of different metals have 
different amounts of thermal contraction when the clad sheet is cooled 
after levelling by a hot leveller. Therefore the clad metal sheet after 
cooled down to the room temperature exhibits a camber in such a manner 
that the metal layer of the greater thermal expansion coefficient is 
disposed on the radially inner side of the clad metal sheet. 
More specifically, in ordinary process for rolling thick metal sheets, the 
hot sheet after the rolling is levelled by a leveller in order to remove 
any shape defect in the sheet, e.g., center buckle, edge wave, breadthwise 
camber and lengthwise camber. The hot-levelled sheet is then cooled on a 
cooling table. Any shape defect which is generated during the cooling is 
also levelled by a leveller after the cooling, whereby a flat sheet is 
obtained. 
In the case of a two-layered clad steel sheet composed of a base sheet 1A 
and a covering layer 1B which have different values of thermal expasion 
coefficient, a breadthwise camber, which is much greater than that 
experienced by ordinary steel sheet, is caused during the cooling after 
the hot levelling, as shown in FIG. 22. In case of the two-layered clad 
sheet, a difference in the value of the thermal expansion coefficient 
exists between the base layer which is, for example, a carbon steel and a 
covering layer which is, for example, a stainless steel, as will be seen 
from FIG. 23. In consequence, two layers of the clad sheet exhibit 
different amounts of thermal contraction during the cooling down to the 
room temperature after the hot levelling, resulting in a large breadthwise 
camber. The amount y of the camber is maximized when the clad sheet has 
been cooled down to the room temperature. In the case of a two-layered 
clad sheet composed of a base layer of a carbon steel and a covering layer 
of a stainless steel, the camber amount y reaches 300 to 400 mm, although 
this amount y varies depending on the conditions such as the levelling 
temperature, sheet thickness, sheet width and the clad ratio, i.e., the 
ratio of the thickness of the covering layer to the total thickness of the 
clad sheet. 
This heavy camber after the hot levelling causes the following 
inconveniences. 
(a) It is difficult to convey the sheet by table rollers, when the sheet 
has a heavy camber. 
(b) The sheet has to experience an impractically large number of passes 
during a subsequent cold levelling. 
(c) If the camber is extremely heavy, it is practically impossible to level 
the sheet by an ordinary cold leveller. 
The present inventors have already proposed, in Japanese Patent Unexamined 
Patent No. 42122/1984, a method for levelling a two-layered clad sheet, 
which has been successfully carried out. 
According to this proposed method, the layer having the greater thermal 
expansion coefficient is forcibly cooled before or during the hot 
levelling so as to create a temperature difference between two layers, and 
the sheet is levelled in this state, so that the clad sheet shows only a 
small camber when cooled to the room temperature. 
More specifically, referring to FIG. 23, the covering layer exhibits a 
greater amount of thermal contraction than that of the base layer. Thus, 
the base layer exhibits a thermal contraction .DELTA..epsilon.c when 
cooled down from a temperature Tc to the room temperature, whereas the 
covering layer exhibits the same thermal contraction .DELTA..epsilon.s 
when cooled to the room temperature from a temperature Ts which is below 
the temperature Tc. Thus, if the covering layer is cooled down forcibly to 
the temperature Ts while the base layer is maintained at the temperature 
Tc and the sheet is levelled in this state, both layers exhibit the same 
amount of thermal contraction when they are cooled down to the room 
temperature, thus the generation of the camber after cooling is prevented 
substantially. 
According to this method, a negative camber is generated in the clad sheet 
immediately after the hot levelling, as a result of a uniformalization of 
the temperature, i.e., the transfer of heat from the base layer to the 
covering layer. However, this negative camber is gradually decreased as 
the temperature is lowered, and a substantially flat state is obtained 
when the sheet has been cooled down to the room temperature. In 
consequence the load in the subsequent cold levelling is reduced or, in 
some cases, eliminates the necessity for the cold levelling. 
Actually, however, there are a variety of types of two-layered clad metal 
sheets requiring this method. These clad metal sheets have different 
values of thickness, width, clad ratio and the material of the covering 
layer. This means that the above-mentioned proposed method cannot equally 
apply to the variety of clad metal sheets. 
Moreover the above-mentioned proposed method encounters the following 
problems. 
(a) The clad metal sheet immediately after the hot levelling exhibits a 
negative large camber. This undesirably impedes the convey of the clad 
metal sheet by table rollers, with a result that the production efficiency 
is impaired seriously. 
(b) In order to forcibly cool the layer of the greater thermal contraction 
amount during the hot levelling, it is necessary that the hot leveller 
must equip a cooling device. The size and the capacity of the cooling 
device, however, must be small in the hot leveller. Thus, it is very 
difficult to attain the desired temperature difference (Tc - Ts) between 
two layers, particularly when the amount of camber is large, as in the 
case where the levelling temperature the clad ratio and the sheet 
thickness are high, high and thin respectively. 
(c) The steel sheet has a greater tendency of shape defect such as camber 
than ordinary steel sheet consisting of a single layer, not only during 
the finish rolling but also during the subsequent convey. Therefore, a 
longer time is wasted until the hot levelling is commenced, so that the 
temperature, at which the hot levelling is started, tends to be lowered 
undesirably. In such a case, the levelling temperature may further come 
down as a result of the forcible cooling conducted during the hot 
levelling. Consequently, as the yield stress of the base layer becomes 
high, it is difficult to impart the desired plastic deformation by the hot 
levelling. In such a case, the positive camber, that the layer of the 
greater thermal contraction constitutes the inner side, appears 
immediately after the hot levelling, and this camber further grows as the 
sheet is cooled down to the room temperature. 
Accordingly, an object of the invention is to provide a method which 
prevents the camber of a two-layered clad metal sheet at the room 
temperature. 
DISCLOSURE OF THE INVENTION 
According to a first aspect of the invetion, there is provided a method for 
preventing the camber of a two-layered clad metal sheet having a base 
layer and a covering layer of different metals which exhibit different 
amounts of thermal contraction, the method comprising: developing a 
temperature difference .DELTA.T expressed by the following formula between 
the base layer and the covering layer during a hot levelling, by providing 
a greater cooling effect before or during the levelling to the layer which 
exhibits the greater thermal contraction than to the layer which exhibits 
the smaller thermal contraction: 
EQU .DELTA.T=f (.DELTA..alpha., .alpha., a, To) 
where, 
.DELTA..alpha.: the difference in thermal expansion coefficient between 
both metals 
a: the clad ratio (ratio of covering layer thickness to total sheet 
thickness) 
To: hot leveller inlet temperature (.degree.C.) 
.alpha.: mean thermal expansion coefficient of both metals. 
According to a second aspect of the invention, there is provided a method 
for preventing the camber of a two-layered clad metal sheet having a base 
layer and a covering layer of different metals which exhibit different 
amounts of thermal contraction, wherein a greater cooling effect is 
imparted to the layer which exhibits the greater thermal contraction than 
to the layer which exhibits the smaller thermal contraction by upper and 
lower water-cooling means before or during a hot levelling, the method 
comprising: computing the temperature difference during the levelling 
between the upper and lower surfaces of the clad metal sheet necessary for 
preventing the final camber of the clad metal sheet when the sheet is 
cooled to the room temperature; and controlling the density of the cooling 
water applied by the water-cooling means and the velocity at which the 
clad metal sheet passes through the hot leveller, in such a manner that 
the actual temperature difference measured by upper and lower thermometers 
disposed in the hot leveller coincides with the computed temperature 
difference. 
According to a third aspect of the invention, there is provided a method 
for preventing the camber of a two-layered clad metal sheet having a base 
layer and a covering layer of different metals which exhibit different 
amounts of thermal contraction, wherein a greater cooling effect is 
imparted to the layer which exhibits the greater thermal contraction than 
to the layer which exhibits the smaller thermal contraction by upper and 
lower water-cooling means before or during a hot levelling, the method 
comprising: computing the temperature difference during the levelling 
between the upper and lower surfaces of the clad metal sheet necessary for 
preventing the final camber of the clad metal sheet when the sheet is 
cooled to the room temperature; and controlling the density of the cooling 
water between the upper and lower water-cooling means and the velocity at 
which the clad metal sheet passes through the hot leveller, in such a 
manner that the actual temperature difference measured by upper and lower 
thermometers disposed in the hot leveller coincides with the computed 
temperature difference; predicting the expected final amount of camber at 
the room temperature from information obtained from the clad metal sheet 
at the outlet side of the hot leveller after a uniformalization of the 
temperature; correcting the result of computation of the temperature 
difference of the next clad metal in accordance with the predicted final 
amount of camber. 
According to a fourth aspect of the invention, there is provided a method 
for preventing the camber of a two-layered clad metal sheet having a base 
layer and a covering layer of different metals which exhibit different 
amounts of thermal contraction, wherein a greater cooling effect is 
imparted to the layer which exhibits the greater thermal contraction than 
to the layer which exhibits the smaller thermal contraction by upper and 
lower water-cooling means before or during a hot levelling, the method 
comprising: setting the difference in the density of the cooling water 
between the upper and lower water-cooling means and the velocity of the 
sheet in the hot leveller which are necessary for preventing the final 
camber of the clad metal sheet when the sheet is cooled to the room 
temperature; and controlling the upper and lower water-cooling means and 
the sheet velocity in accordance with the setting values. 
According to a fifth aspect of the invention, there is provided a method 
for preventing the camber of a two-layered clad metal sheet having a base 
layer and a covering layer of different metals which exhibit different 
amounts of thermal contraction, wherein a greater cooling effect is 
imparted to the layer which exhibits the greater thermal contraction than 
to the layer which exhibits the smaller thermal contraction by upper and 
lower water-cooling means before or during a hot levelling, the method 
comprising: setting the difference in the density of the cooling water 
between the upper and lower water-cooling means and the velocity of the 
sheet in the hot leveller which are necessary for preventing the final 
camber of the clad metal sheet when the sheet is cooled to the room 
temperature; controlling the upper and lower water-cooling means and the 
sheet velocity in accordance with the setting values; predicting the 
expected final amount of camber at the room temperature from information 
obtained from the clad metal sheet at the outlet side of the hot leveller 
after a uniformalization of the temperature; correcting the result of 
computation of the density of the cooling water and the sheet velocity of 
the next clad metal in accordance with the predicted final amount of 
camber. 
According to a sixth aspect of the invention, there is provided a method 
for preventing the camber of a two-layered clad metal sheet composed of a 
base layer and a covering layer of different metals having different 
values of thermal expansion coefficient, the method comprising: forcibly 
cooling, before or during hot levelling, the layer which exhibits the 
greater thermal contraction so as to develop a predetermined temperature 
difference during the levelling between the upper and lower surfaces of 
the clad metal sheet; and further forcibly cooling, after the hot 
levelling, the layer so as to decrease a negative camber which occurs due 
to a uniformalization of the temperature at the outlet side of the 
leveller. 
According to a seventh aspect of the invention, there is provided a method 
for preventing the camber of a two-layered clad metal sheet composed of a 
base layer and a covering layer of different metals having different 
values of thermal expansion coefficient, the method comprising: heating 
the layer which exhibits the smaller thermal contraction before or during 
hot levelling, while forcibly cooling the layer which exhibits a greater 
thermal contraction before or during the hot levelling so as to develop a 
predetermined temperature difference between the upper and lower surfaces 
of the clad metal sheet during the hot levelling.

THE BEST MODE FOR CARRYING OUT THE INVENTION 
Preferred embodiments of the invention will be described hereinunder with 
reference to the accompanying drawings. 
[First Embodiment] 
The first levelling method in accordance with the invention will be 
explained hereinunder. 
A two-layered clad steel sheet with a base layer of a carbon steel and a 
covering layer constituted by a stainless steel, having a total thickness 
of 25 mm and a width of 2850 mm, was used as a representative of the 
two-layered clad metal sheet to which the invention pertains. Four factors 
were selected with the clad steel sheet: namely, the difference 
.DELTA..alpha. in the thermal expansion coefficient between the base layer 
of the carbon steel and the covering layer of the stainless steel, mean 
thermal expansion coefficient .alpha. of two metals, clad ratio a, i.e., 
the ratio of the thickness of the covering layer to the total sheet 
thickness, and the sheet temperature To at the hot leveller inlet. A 
plurality of clad sheets of the above-stated specification were prepared 
and subjected to hot levelling in which various temperature difference 
values between the upper and lower surfaces of the clad sheet were 
imparted by means of a water-cooling type cooling device, by varying one 
of the factors while maintaining other factors at respective standard 
values. The results of the hot levelling are shown in FIGS. 1A to 1D. The 
standard conditions were: .DELTA..alpha.=0.4.times.10.sup.-5 
(1/C.degree.), .alpha.=1.6.times.10.sup.-5 (1/C.degree.), a=0.3 and 
To=400.degree. C. 
In these Figures, marks "X" represents the presence of positive camber with 
the layer having greater thermal expansion coefficient constituting the 
inner side at the room temperature, "+" represent the presence of negative 
camber with the layer of greater thermal expansion coefficient 
constituting the outer side at the room temperature, and "o" indicates 
that the clad is flat at the room temperature. 
From FIGS. 1A to 1D, it will be seen that, in order that the final camber 
of the clad sheet is reduced to zero at the room temperature, i.e., the 
substantially flat state of the clad sheet, it is necessary that the 
temperature difference .DELTA.T applied between the upper and lower 
surfaces of the clad sheet during the hot levelling meets the condition of 
the following function: 
EQU .DELTA.T=f (.DELTA..alpha., .alpha., a, To) (1) 
where, .DELTA..alpha. represents the difference in the thermal expansion 
coefficient between both metals, .alpha. represents the mean thermal 
expansion coefficient of both metals; a represents the clad ratio (ratio 
of covering layer thickness to total sheet thickness), and To represents 
the hot leveller inlet temperature. 
Using the data shown in FIGS. 1A to 1D, the function (1) shown above can be 
reformed as follows: 
##EQU1## 
where, K.sub.1 to K.sub.4 are proportional constants. Therefore, the 
temperature difference is expressed by the following formula (2): 
EQU .DELTA.T=K.sub.o 
.multidot..DELTA..alpha./.alpha..multidot.a(1-a).multidot.T.sub.o (2) 
From the data shown in FIGS. 1A to 1D, it is understood that the constant 
Ko preferably ranges between 4 and 6, in order that the clad steel sheet 
is flat, i.e., the camber is zero, at the room temperature. However, the 
first embodiment of the levelling method of the invention provides an 
appreciable effect as compared with the case where the first embodiment is 
not conducted, provided that the constant Ko ranges between 1 and 11. The 
condition expressed by the formula (2) is an example of the formula which 
is composed of respective factors. Thus, the first embodiment of the 
invention can include the temperature control conducted in other formula 
which is composed of these factors. 
The data shown in FIGS. 1A to 1D have been obtained when the clad sheet 
composed of a base layer of a carbon steel and a covering layer of a 
stainless steel is used as the two-layered clad metal sheet. The inventors 
have confirmed, however, the tendency explained heretofore observed with 
this clad sheet apply generally to other ordinary two-layered clad metal 
sheets. 
In FIG. 8, a reference numeral 101 denotes the clad steel sheet, 102 
denotes a hot leveller and 103 denotes a cooling device. 
A description will be made hereinunder as to the result of experimental 
levelling conducted in accordance with the first embodiment of the 
invention, in comparson with the result of the levellig in accordance with 
the conventional levelling method. The experiment was conducted on three 
types of clad steel sheet having a carbon steel base layer and a stainless 
steel covering layer with a total sheet thickness and sheet width of 20 mm 
and 3000 mm, respectively. The three types of sheets had different clad 
ratios of 10%, 30% and 50%, respectively. In the conventional method, the 
clad sheets were hot-levelled by the apparatus shown in FIG. 8 such that a 
constant temperature difference is developed between the upper and lower 
of the clad sheet. On the other hand, the levelling method of the 
invention was carried out while controlling the temperature difference in 
accordance with the formula (2) which is one of the forms derived from the 
function of the formula (1). After the experimental levelling, the amounts 
of camber were compared between the clad sheets levelled by the 
conventional method and the clad sheets levelled in accordance with the 
invention. 
More specifically, in the conventional levelling method, the clad sheets 
were fed into the hot leveller at a hot leveller inlet temperature of 
700.degree. C., and the hot levelling was conducted while cooling the clad 
sheet from one side of thereof such that the levelling is finished to 
obtain a flat state of the clad sheets with the stainless steel layer and 
the carbon steel layer maintained at 500.degree. C. and 600.degree. C., 
respectively, for all of the three types of clad sheets having the clad 
ratios of 10%, 30% and 50%. 
On the other hand, in the levelling method in accordance with the 
invention, the temperature difference between the upper and lower surfaces 
of the clad sheet was controlled in accordance with the formula (2) 
mentioned above: namely, the clad sheet of the 10% clad ratio was 
hot-levelled to the flat state while the stainless steel layer and the 
carbon steel layer were held at 580.degree. C. and 620.degree. C., 
respectively. In the case of the clad sheet of 30% clad ratio, the hot 
levelling was finished at the stainless steel layer temperature and the 
carbon steel layer temperature of 500.degree. C. and 600.degree. C., 
respectively. Finally, the clad sheet of 50% clad ratio was finished at 
the stainless steel layer temperature and carbon steel layer temperature 
of 460.degree. C. and 580.degree. C., respectively. FIGS. 9A and 9B show 
the temperature difference between the upper and lower surfaces of the 
clad sheets immediately after the levelling in accordance with the 
invention, and the change in the amount of camber of the clad sheets after 
the levelling in accordance with the invention. Similar data obtained with 
the clad sheets levelled by the conventional method are shown in FIGS. 10A 
and 10B. 
Three types of clad sheet levelled by the conventional levelling method 
showed almost the same amount of camber of about 180 mm at a moment about 
1 minutes after the completion of the levelling as a result of the 
uniformalization of the temperature, due to the fact that a constant 
temperature difference was maintained between the upper and lower surfaces 
of the clad sheet during the levelling. However, these three types of 
sheets having different clad ratios exhibit different amounts of camber as 
they are further cooled. Namely, altough the clad sheet having the clad 
ratio of 30% exhibits a final camber of substantially zero at the room 
temperature, the clad sheet of the 10% clad ratio showed a large negative 
camber of 100 mm, because the negative camber imparted by the forcible 
cooling during the levelling cannot be completely extinguished. On the 
other hand, the clad sheet having 50% clad ratio showed a positive camber 
of 35 mm, due to the large difference in the amount of thermal expansion 
between both layers. 
In contrast, three types of clad sheet levelled in accordance with the 
invention showed different amounts of camber at the moment about 1 minute 
after the levelling as a result of the uniformalization of the 
temperature, i.e., the transfer of heat from the base layer, due to the 
fact that these sheets were levelled with temperature differences between 
the upper and lower surfaces. Namely, the clad sheets having the clad 
ratios of 10%, 30% and 50% showed, respectively, the negative cambers of 
70 mm, 180 mm and 210 mm, at the above-mentioned moment. However, as the 
sheets were further cooled, the amounts of camber converged and the camber 
was substantially prevented finally, regardless of the clad ratio. 
Thus, in the first embodiment of the levelling method of the invention, the 
temperature difference between the upper and lower surfaces of the clad 
sheet is controlled in accordance with the condition expressed by the 
formula (2) mentioned before or during the levelling, so that a 
substantially flat state of the clad sheet is finally obtained for various 
values of factors such as the sheet thickness, clad ratio, material of the 
covering layer and the temperature at which the levelling is commenced. 
This in turn eliminates the necessity for cold levelling which heretofore 
has to be conducted after cooling in the conventional process, for all 
types of two-layered clad metal sheet. 
It will be clear to those skilled in the art that the first embodiment of 
the levelling method of the invention can theoretically apply to all types 
of two-layered clad metal sheet having a variety of combinations of metals 
of different thermal expansion coefficients, not only to the clad steel 
sheet mentioned hereinbefore. 
As has been described, according to the first embodiment of the levelling 
method of the invention, the temperature difference which is to be 
developed across the thickness of the two-layered clad metal sheet is 
determined taking into account the factors which affect the camber of the 
clad metal sheet, so that the camber is prevented at the room temperature 
without fail. In other words, in the first embodiment of the invention 
described hereinbefore, the metal layer which exhibits greater amount of 
thermal contraction is cooled optimumly such as to ensure the flat state 
of the two-layered clad metal sheet at the room temperature. 
[Second Embodiments] 
The second and third levelling methods in accordance with the invention 
will be described hereinunder. 
FIG. 2 schematically shows a levelling system 10, as well as a block 
diagram of the control system for the levelling system 10, suitable for 
use in carrying out the second and third embodiments of the invention. 
A two-layered clad steel sheet 11 is composed of a base layer of a metal 
having, for example, a comparatively small thermal contraction amount 
(small thermal expansion coefficient), e.g., a carbon steel, and a 
covering layer having a comparatively large thermal contraction amount 
(large thermal expansion coefficient), e.g., a stainless steel. The clad 
sheet 11 is rolled by a rolling mill, hot-levelled by a leveller 12 and 
then conveyed to a subsequent step of a process by means of a table 
roller. 
The hot leveller 12 has a plurality of upper and lower hot leveller rolls 
13 which are arranged in a staggered manner, and cooling headers 14 
arranged on the upper and lower sides of the path of the levelled clad 
sheet at positions between adjacent upper leveller rollers and between 
adjacent lower leveller rollers. The cooling headers 14 are designed and 
arranged such that the covering layer having the greater thermal expansion 
coefficient is cooled more strongly than the base layer having smaller 
thermal expansion coefficient, so as to maintain, during the hot 
levelling, a temperature difference necessary for preventing the camber of 
the clad sheet 11 after the latter is cooled to the room temperature. 
If the hot levelling is conducted with the covering layer of the clad sheet 
directed downwardly, therefore, the cooling heads 14 should be arranged 
such as to provide a greater cooling effect to the lower side of the clad 
sheet than to the upper side of the same. 
In the embodiments shown in FIG. 2, since the covering layer having the 
greater thermal expansion coefficient is directed upwardly, the cooling 
headers are arranged to provide a greater cooling effect to the upper side 
of the clad sheet than to the lower side thereof. Thus, in the embodiments 
shown in FIG. 2, the cooling headers 14 may be arranged only on the upper 
side of the path of the clad sheet, such as to face the covering layer 
having the greater thermal expansion coefficient. 
That is, the levelling may be conducted by using only the cooling headers 
disposed on the upper side of the path of the clad sheet, while the 
cooling headers disposed under the path of the clad sheet is not used or 
omitted. Similarly, when the levelling is conducted with the covering 
layer having the greater thermal expansion coefficient directed 
downwardly, the cooling headers may be arranged only at the lower side of 
the path of the clad sheet. 
The levelling system 10 has a temperature difference computing device 15 
which is adapted to compute the temperature difference .DELTA.T between 
the upper and lower surface of the clad sheet 11 necessary for preventing 
the camber of the clad sheet 11 at the room temperature, in accordance 
with the formula (1), practically the formula (2) mentioned before, from 
various data stored in a line computer 16, such as the size of the clad 
sheet 11, difference in thermal expansion coefficient between the base 
layer and the covering layer, mean thermal expansion coefficient .alpha. 
of both metals, clad ratio a and so forth, as well as the temperature To 
of the clad sheet 11 at the inlet side of the hot leveller 12 as measured 
by a thermometer 17. 
The application of the computed temperature difference .DELTA.T to the 
steel sheet 11 is practically conducted by adjusting the period of the 
water cooling on the steel sheet 11, as well as the adjustment of the 
difference in the heat transfer coefficient between the upper and lower 
surfaces. The influences of the water cooling period and the difference in 
the heat transfer coefficient on the temperature difference .DELTA.T are 
shown by diagrams in FIGS. 3 and 4, respectively. Referring to FIG. 3, the 
temperature difference between the upper and lower surfaces of the clad 
sheet 11 is increased as the time elapses. This suggests that the 
temperature difference between the upper and lower surfaces of the clad 
sheet 11 is controllable by adjusting the period of the water cooling. On 
the other hand, FIG. 4 shows that the greater the difference in the heat 
transfer coefficient between both sides of the clad sheet, the higher the 
speed of increase of the temperature difference between both sides of the 
clad sheet. This suggests that the temperature difference between the 
upper and lower surfaces of the clad sheet is controllable also by means 
of the difference in the heat transfer coefficient between the upper and 
lower surfaces of the clad sheet. The adjustment of the water cooling 
period and the difference in the heat transfer coefficient in the actual 
hot-leveller 12 requires a control of the following two factors: 
(a) To control the velocity of the clad sheet in the hot leveller 12 such 
as to vary the period of stay of each point on the clad sheet 11 in the 
cooling region, thus varying the water cooling period. 
(b) To adjust the flow rate of water while taking into account the area of 
the water cooling region, so as to vary the density of the cooling water 
spray on both sides of the clad sheet, i.e., to vary the amout of water by 
which each point on the steel sheet is contacted per unit time and unit 
area, thus varying the difference in the heat transfer coefficient between 
the upper and lower surfaces of the clad sheet. 
Namely, from the practical point of view, the controls (a) and (b) 
mentioned above are rather easy to conduct, as the measures for 
controlling the temperature difference .DELTA.T between the upper and 
lower surfaces of the clad sheet. These two controls (a) and (b), however, 
are not exclusive. For instance, the cooling period can be varied by 
varying the effective length of the cooling region, by arranging the 
levelling device to have a considerable length as shown in FIG. 6 and 
effecting an on-off control of the cooling water nozzles which are 
arranged along the length of the levelling device. The cooling headers 
disposed at the lower side of the path of the clad sheet in the 
arrangement shown in FIG. 6 may be omitted, as in the case of the 
arrangement shown in FIG. 2. The difference in the heat transfer 
coeficient between the upper and lower surfaces of the clad sheet can be 
varied also by changing other factors such as the size of the nozzle ports 
of the cooling water nozzles and the state of the cooling medium applied, 
e.g., change from mist cooling to spray cooling and further to laminar 
cooling. Such alternative measures, however, requires a significant change 
or modification in the equipment. Other possible measures such as an 
adjustment of the cooling water temperature or a forcible heating of the 
layer having smaller thermal contraction amount also require a substantial 
variation in the equipment. 
In the levelling system 10 in the described embodiments, therefore, the 
temperature difference computing device 15 computes the temperature 
difference .DELTA.T between the upper and lower surfaces of the clad 
sheet, and delivers instruction signals to a setting device 18 of the 
water density and sheet velocity, the instruction signals representing the 
cooling water densities, i.e., flow rates QU and QD which are to be 
provided by the upper and lower cooling headers, as well as the velocity V 
of the clad sheet through the hot leveller 12, necessary for developing 
the desired temperature difference .DELTA.T between the upper and lower 
surfaces of the clad sheet 11. To this end, the setting device of the 
water density and sheet velocity beforehands stores, in the form of 
numerical data, table or chart, the relationships between the temperature 
difference .DELTA.T and the water flow rates QU, QD and the sheet velocity 
V, for the clad steel sheets of various sizes, materials and clad ratios, 
so that the setting device 19 can sets the water flow rates QU and QD, as 
well as the velocity V, necesary for imparting the desired temperature 
difference between the upper and lower surfaces of the clad steel sheet 11 
to be levelled. 
In response to the instruction signals given by the setting device 18, the 
levelling system 10 operates a cooling water flow rate controller 19 and a 
sheet velocity controller 20, thereby controlling the flow rates of the 
cooling water from the upper and lower cooling headers and the velocity at 
which the clad sheet passes through the hot leveller 12. 
The levelling system 10 also has an upper thermometer 21 and a lower 
thermometer 22 which are disposed in the hot leveller 12 and adapted to 
measure the temperatures TU and TD of the obverse and reverse sides of the 
clad steel sheet 11, respectively. The thermometers 21 and 22 deliver 
signals representing the temperatures TU and TD to the setting device 18. 
Upon receipt of these signals, the setting device 18 performs a feedback 
control in such a manner that the measured temperature difference (TU-TD) 
between the upper and lower surfaces of the clad steel sheet coincides 
with the command temperature difference .DELTA.T computed by the device 15 
for computing the temperature difference, through controlling the 
operation of the water flow rate controller 19 which in turn controls the 
water flow rates from the upper and lower cooling headers, and controlling 
also the operation of the velocity controller 20 which controls the 
velocity at which the clad sheet 11 passes through the hot leveller 12. 
The levelling system 10 may be arranged such that, after the completion of 
the levelling, it measures the final amount of the camber of the clad 
sheet 11 at the room temperature, and suitably varies the amounts of 
control of the cooling water flow rates and/or the sheet velocity in 
accordance with the measured final value of the camber, thus allowing the 
succeeding clad sheet to be controlled at higher accuracy. Actually, 
however, such a feedback control is impractical because of too long time 
required for the clad sheet to be cooled down to the room temperature. On 
the other hand, the present inventors have confirmed that there are fixed 
relationships between the final amount of the camber and the clad sheet 
temperature and amount of camber of the clad sheet 11 at the outlet of the 
hot leveller 12, i.e., in the state immediately after the uniformalization 
of the temperature. 
FIG. 5 shows the relationship between the amount of camber and the mean 
temperature of the clad sheet having a total sheet thickness of 20 mm, 
sheet breadth of 3000 mm and the clad ratio of 30%, as observed after the 
temperature uniformalization. As will be seen from this Figure, the same 
material exhibits the same gradient of change of the camber in relation to 
temperature. This means that the final amount of the camber at the room 
temperature can be predicted provided that the amount of camber and the 
temperature after the temperature uniformalization are measured. 
In view of the above, the levelling system 10 incorporates an outlet 
thermometer 23 and a camber meter 24 disposed at the outlet side of the 
hot leveller 12, so as to measure the temperature Tm and the amount 
.DELTA.ym of the camber of the clad steel sheet 11 after the temperature 
uiformalization, and to deliver the thus measured temperature Tm and the 
amount .DELTA.ym of the clad steel sheet to a final camber computing 
device 25. The final camber computing device 25 computes the final amount 
.DELTA.yf of the clad steel sheet at the room temperature, using the 
measured temperature Tm and the amount .DELTA.ym of the camber, as well as 
the information derived from the line computer, such as the sheet 
thickness, sheet breadth, clading ratio and the materials of the base and 
covering layers. The result of this computation can be fed back to the 
temperature difference computing device 15. The temperature difference 
computing device 15 is adapted to correct the result of computation of the 
temperature difference .DELTA.T in such a manner that the computed final 
amount .DELTA.yf of the camber becomes zero. By virtue of this function, 
the levelling system 10 can perform the optimum control of the levelling 
operation, thus reducing the final amount of the camber at the room 
temperature substantially to zero. 
The correction of the temperature difference .DELTA.T computed by the 
temperature difference computing device 15 is conducted, for example, in 
accordance with the following manner. The final amount of camber computed 
on condition that the temperature difference .DELTA.T is zero, i.e., on 
condition that the levelling method of the invention is not carried out, 
is represented by yo. On the other hand, the actual final amount of 
camber, obtained when the temperature difference is set at TR so as to 
prevent the final amount of camber, is represented by yR. That is, the 
reduction in the final amount of camber computed on the basis of the 
temperature difference is yo, while the actual reduction obtained with the 
same temperature difference is yo-yR. Therefore, a correction coefficient 
KT is obtained from the following formula (3). 
EQU KT=yo/(yo-yR) (3) 
By multiplying the temperature difference .DELTA.T computed for the next 
clad sheet by the correction coefficient KT, therefore, it is possible to 
effect a more adequate levelling for the next clad sheet. 
The method of the invention, however, does not essentially requires the 
correction of the computed temperature difference .DELTA.T by the final 
camber computing device 25. 
As will be understood from the foregoing description, according to the 
second and the third embodiments of the invention, it is possible to 
obtain a flat clad sheet having no camber after the cooling, regardless of 
the factors such as the sheet thickness, clad ratio, material of the 
covering layer and the temperature at which the levelling is commenced. 
This in turn eliminates the necessity for the cold levelling which 
heretofore has been necessarily employed following the hot levelling, for 
all types of two-layered clad steel sheet. 
It will be understood by those skilled in the art that the described second 
and third embodiments are theoretically applicable not only to the 
described clad steel sheet but also to any types of two-layered clad metal 
sheet having various combinations of metals of different thermal expansion 
coefficients. 
[Third Embodiments] 
The fourth and fifth levelling methods in accordance with the invention 
will be described hereinunder. 
FIG. 7 schematically shows a levelling system 110, as well as a block 
diagram of the control system for the levelling system 110, suitable for 
use in carrying out the fourth and fifth embodiments of the invention. 
A two-layered clad steel sheet 111 is composed of a base layer of a metal 
having, for example, a comparatively small thermal contraction amount 
(small thermal expansion coefficient), e.g., a carbon steel, and a 
covering layer having a comparatively large thermal contraction amount 
(large thermal expansion coefficient), e.g., a stainless steel. The clad 
sheet 111 is rolled by a rolling mill, hot-levelled by a hot leveller 112 
and then conveyed to a subsequent step of a process by means of table 
rollers. 
The hot leveller 112 has a plurality of upper and lower hot leveller rolls 
113 which are arranged in a staggered manner, and cooling headers 114 
arranged on the upper and lower sides of the path of the levelled clad 
sheet at positions between adjacent upper leveller rollers and between 
adjacent lower leveller rollers. The cooling headers 114 are designed and 
arranged such that the covering layer having the greater thermal expansion 
coefficient is cooled more strongly than the base layer having smaller 
thermal expansion coefficient, so as to maintain, during the hot 
levelling, a temperature difference necessary for preventing the camber of 
the clad sheet 111 after the latter is cooled to the room temperature. In 
the embodiments shown in FIG. 7, since the covering layer of the stainless 
steel having the greater thermal expansion coefficient is directed 
upwardly, the cooling headers are arranged to provide a greater cooling 
effect to the upper side of the clad sheet than to the lower side thereof. 
Thus, in the embodiments shown in FIG. 7, the cooling headers 114 may be 
arranged only on the upper side of the path of the clad sheet, such as to 
face the covering layer having the greater thermal expansion coefficient. 
That is, the levelling may be conducted by using only the cooling headers 
disposed on the upper side of the path of the clad sheet, while the 
cooling headers disposed under the path of the clad sheet is not used or 
omitted. Similarly, when the levelling is conducted with the covering 
layer having the greater thermal expansion coefficient directed 
downwardly, the cooling headers may be arranged only at the lower side of 
the path of the clad sheet. 
The levelling system 110 has a device 115 for setting water density and 
sheet velocity which stores, in the form of formulae or chart, the 
difference in the water density between the upper and lower cooling 
headers 114 and the velocity of the clad sheet in the hot leveller 112 
which are necessary for preventing the final camber of the clad sheet 111 
at the room temperature, for a variety of values of factors such as the 
size, materials, and clad ratio of the steel sheet 111, as well as the hot 
leveller inlet temperature. 
Namely, the levelling system 110 computes the temperature difference 
.DELTA.T between the upper and lower surfaces of the clad sheet 111 
necessary for preventing the camber of the clad sheet 111 at the room 
temperature, in accordance with the formula (1), practically the formula 
(2) mentioned before, from various data such as the size, materials and 
the clad ratio of the clad steel sheet 111, as well as the temperature of 
the clad sheet 111 at the inlet side of the hot leveller 112. The 
levelling system 110 also stores, within the setting device 115, the 
difference in the water density between the upper and lower cooling 
headers 114, i.e., between the flow rates QU and QD, as well as the sheet 
velocity V in the hot leveller 112, necessary for imparting the 
temperature difference .DELTA.T. 
Thus, the setting device 115 sets the water flow rates QU and QD from the 
upper and lower water headers 114 and the sheet velocity V in the hot 
leveller 112 which are necessary for preventing the final camber of the 
clad sheet 111 at the room temperature, in accordance with the data from 
the line computer 116 such as the size of the clad sheet 111, materials of 
the base and covering layers and the clad ratio, as well as the 
temperature To of the clad sheet 111 at the inlet side of the hot leveller 
112 as measured by the thermometer 117. 
The levelling system 110 of the described embodiments, therefore, delivers 
instruction signals to a water density controller 118 and to a sheet 
velocity controller 119, instruction signals representing the cooling 
water densities, i.e., flow rates QU and QD which are to be provided by 
the upper and lower cooling headers and the velocity V of the clad sheet 
through the hot leveller 112, in accordance with the values set by the 
setting device 115, thereby controlling the water flow rates QU, QD and 
the sheet velocity V. 
The levelling system 110 may be arranged such that, after the completion of 
the levelling, it measures the final amount of the camber of the clad 
sheet 111 at the room temperature, and suitably varies the amounts of 
control of the cooling water flow rates and/or the sheet velocity in 
accordance with the measured final value of the camber, thus allowing the 
succeeding clad sheet to be controlled at higher accuracy. Actually, 
however, such a feedback control is impractical because of too long time 
required for the clad sheet to be cooled down to the room temperature. On 
the other hand, the present inventors have confirmed that there are fixed 
relationships between the final amount of the camber and the clad sheet 
temperature and the amount of camber of the clad sheet 111 at the outlet 
of the hot leveller 112, i.e., in the state immediately after the 
temperature uniformalization, as explained before in connection with FIG. 
5. 
In view of the above, the levelling system 110 incorporates an outlet 
thermometer 120 and a camber meter 121 disposed at the outlet side of the 
hot leveller 112, so as to measure the temperature Tm and the amount 
.DELTA.ym of the camber of the clad steel sheet 111 after the temperature 
uniformalization, and to deliver the thus measured temperature Tm and the 
amount .DELTA.ym of the clad steel sheet to a final camber computing 
device 112. The final camber computing device 112 computes the final 
amount .DELTA.yf of the clad steel sheet at the room temperature, using 
the measured temperature Tm and the amount .DELTA.ym of the camber, as 
well as the information derived from the line computer, such as the sheet 
thickness, sheet breadth, clad ratio and the materials of the base and 
covering layers. The result of this computation can be fed back to the 
setting device 115. The device 115 is adapted to correct the result of 
computation of the water flow rates Q and the sheet velocity V in such a 
manner that the computed final amount .DELTA.yf of the camber becomes 
zero. By virtue of this function, the levelling system 110 can perform the 
optimum control of the levelling operation, thus reducing the final amount 
of the camber at the room temperature substantially to zero. 
The correction of the water flow rates QU, QD from the upper and lower 
cooling headers and the sheet velocity V computed by the setting device 
115 is conducted, for exampe, in accordance with the following manner. 
Referring first to the water flow rates from the upper and lower cooling 
headers, the final amount of camber computed on condition that the 
difference in the water flow rate is zero, i.e., on condition that the 
levelling method of the invention is not carried out, i.e., QU=QD, is 
represented by yo. On the other hand, the actual final amount of camber, 
obtained when the water flow rate difference is set at .DELTA.QR 
(=QUR-QDR) so as to prevent the final amount of camber, is represented by 
yR. That is, the reduction in the final amount of camber computed on the 
basis of the flow rate difference .DELTA.QR is yo, while the actual 
reduction obtained with the same flow rate difference is yo-yR. Therefore, 
a correction coefficient KQ is obtained from the following formula (4). 
EQU KQ=yo/(yo-yR) (4) 
By multiplying the water flow rate difference .DELTA.Q computed for the 
next clad sheet by the correction coefficient KQ, therefore, it is 
possible to effect a more adequate levelling for the next clad sheet. 
Referring now to the sheet velocity V, the final amount of camber computed 
on condition that the levelling method of the invention is not carried out 
is represented by yo. On the other hand, the actual final amount of 
camber, obtained when the sheet velocity is set at VR so as to prevent the 
final amount of camber, is represented by yR. That is, the reduction in 
the final amount of camber computed on the basis of the sheet velocity VR 
is yo, while the actual reduction obtained with the same sheet velocity is 
yo-yR. Therefore, a correction coefficient KV is obtained from the 
following formula (5). 
EQU KV=(yo-yR) / yo (5) 
By multiplying the sheet velocity V computed for the next clad sheet by the 
correction coefficient KV, therefore, it is possible to effect a more 
adequate levelling for the next clad sheet. 
The method of the invention, however, does not essentially requires the 
correction of the computed water flow rates QU, QD and the sheet velocity 
V by the final camber computing device 122. 
In addition, the setting device 115 may be arranged such as to control 
either one of the water density and the sheet velocity, while maintaining 
the other constant. 
A description will be made hereinunder as to the result of experimental 
levelling conducted in accordance with the first embodiment of the 
invention, in comparson with the result of the levellig in accordance with 
the conventional levelling method. The experiment was conducted on three 
types of clad steel sheet having a carbon steel base layer and a stainless 
steel covering layer having a total sheet thickness and sheet width of 20 
mm and 3000 mm, respectively. The three types of sheets had different clad 
ratios of 10%, 30% and 50%, respectively. In the conventional method, the 
clad sheets were hot levelled by the apparatus shown in FIG. 7 while 
maintaining the cooling water density and the sheet velocity at constant 
levels. On the other hand, the levelling method of the invention was 
carried out while controlling the cooling water density and the sheet 
velocity. After the experimental levelling, the amounts of camber were 
compared between the clad sheets levelled by the conventional method and 
the clad sheets levelled in accordance with the invention. 
More specifically, in the conventional levelling method, the clad sheets 
were fed into the hot leveller at a hot leveller inlet temperature of 
700.degree. C., and the hot levelling was conducted while maintaining the 
sheet velocity and the cooling water density at consant levels of 30 m/min 
and 700 l/m.sup.2 min to obtain a flat state of the clad sheets for all of 
the three types of clad sheets having the clad ratios of 10%, 30% and 50%. 
On the other hand, in the levelling method in accordance with the 
invention, either one of the cooling water density and the sheet velocity 
was controlled while maintaining the other constant. In the first case 
where the sheet velocity was maintained at the constant level of 30 m/min, 
the cooling water density was changed such that the clad sheet of the 10% 
clad ratio was hot-levelled to the flat state by cooling at the cooling 
water density of 300 l/min. In the case of the clad sheet of 30% clad 
ratio, the hot levelling was conducted with the cooling water density of 
700 l/m.sup.2 min. Finally, the clad sheet of 50% clad ratio was finished 
with the cooling water density of 1000 l/m.sup.2 min. In the case where 
only the sheet velocity was controlled, the cooling water density was 
maintained constant at 700 l/m.sup.2 min and the sheet velocity was 
controlled at 60 m/sec for the clad sheet of 10% clad ratio, 30 m/min for 
the clad sheet of 30 % clad ratio and 10 m/min for the clad sheet of 50% 
clad ratio, thus obtaining flat states of the clad sheets. FIG. 11 shows 
the change in the amounts of camber of the clad sheets after the levelling 
in accordance with the invention, while FIG. 12 shows the change in the 
amounts of camber after the levelling by the conventional method. 
Three types of clad sheet levelled by the conventional levelling method 
showed almost the same amount of camber of about 180 mm after the 
completion of the levelling as a result of the temperature 
uniformalization, i.e., the transfer of heat from the base layer, due to 
the fact that a constant temperature difference was maintained between the 
upper and lower surfaces of the clad sheet during the levelling. However, 
these three types of sheets having different clad ratios exhibit different 
amounts of camber as they are further cooled. Namely, although the clad 
sheet having the clad ratio of 30% exhibits a final camber of 
substantially zero at the room temperature, the clad sheet of the 10% clad 
ratio showed a large negative camber of 100 mm, because the negative 
camber imparted by the forcible one-side cooling during the levelling 
cannot be completely extinguished. On the other hand, the clad sheet 
having 50% clad ratio showed a positive camber of 35 mm, due to the large 
difference in the amount of thermal expansion between both layers. 
In contrast, three types of clad sheet levelled in accordance with the 
invention showed different amounts of camber after the levelling as a 
result of the temperature uniformalization, due to the fact that the 
cooling water density and the sheet velocity were controlled during the 
levelling. Namely, the clad sheet having the clad ratios of 10% showed 
negaive cambers of 70 mm (water density controlled) and 80 mm (sheet 
velocity controlled). The clad sheet of 30% clad ratio showed a negative 
camber of 180 mm in both cases. Finally, the clad sheet of 50% clad ratio 
showed negative cambers of 210 mm (water density controlled) and 200 mm 
(sheet velocity controlled). However, as the sheets were further cooled, 
the amounts of camber converged and the camber was substantially prevented 
finally, regardless of the clad ratio. 
When the total clad sheet thickness is as small as 10 mm or below, although 
the control of the temperature difference between the upper and lower 
surfaces of the clad sheet can be conducted satisfactorily by the control 
of the cooling water density, the control by the sheet velocity cannot 
provide a large temperature difference. Namely, if the sheet velocity is 
set at a too low level in order to develop a large temperature difference, 
the clad sheet may be cooled excessively, so that the sheet cannot be 
completely flattened by the hot leveller. In the case where the clad sheet 
thickness is small, therefore, the control should be done mainly by the 
control of the water density. 
As will be understood from the foregoing description, according to the 
fourth and the fifth embodiments of the invention, it is possible to 
obtain a flat clad sheet having no camber after the cooling, regardless of 
the factors such as the sheet thickness, clad ratio, material of the 
covering layer and the temperature at which the levelling is commenced. 
This in turn eliminates the necessity for the cold levelling which 
heretofore has been necessarily employed following the hot levelling, for 
all types of two-layered clad steel sheet. 
It will be understood by those skilled in the art that the described fourth 
and fifth embodiments are theoretically applicable not only to the 
described clad steel sheet but also to any types of two-layered clad metal 
sheet having various combinations of metals of different thermal expansion 
coefficients. 
[Fourth Embodiment] 
The sixth levelling method in accordance with the invention will be 
described hereinunder. 
FIG. 13 shows general arrangement of a production line for producing a 
two-layered clad metal sheet, e.g., a two-layered clad steel sheet 211, to 
which the sixth embodiment of the levelling method in accordance with the 
invention is applied. The two-layered clad steel sheet 211 is composed of 
a base layer and a covering layer. For instance, the base layer is 
constituted by a carbon steel which exhibits a comparatively small amount 
of thermal contraction, while the covering layer is constituted by a 
stainless steel which exhibits a comparatively large amount of thermal 
contraction. The clad steel sheet 211 is rolled by a rolling mill 212, 
hot-levelled by a hot leveller 213 and then conveyed by table rollers 214 
to the next step of a production process. 
As will be seen from FIG. 14, the hot leveller 213 has hot leveller rolls 
215 arranged on the upper and lower sides of the path of the clad sheet 
211, and a plurality of cooling headers 216 arranged between adjacent 
lower leveller rolls 215. The layer which exhibits the greater amount of 
thermal contraction, i.e., the covering layer of the stainless steel, is 
directed downwardly, and the cooling headers 216 are arranged to face this 
layer, so as to forcibly cool the covering layer of the clad steel sheet 
211 during the hot levelling through the hot leveller rolls 215, thereby 
developing between the base layer and the covering layer a temperature 
difference which is necessary for restraining the camber of the clad steel 
which may otherwise appear when the clad steel sheet 211 is cooled to the 
room temperature. As will be seen from FIG. 15, a plurality of table 
rollers 214 are disposed at the outlet side of the hot leveller 213, and 
cooling headers 217 are disposed between adjacent table rollers 214. 
The cooling headers 217 are so arranged as to further cool the covering 
layer of the clad steel sheet 211 immediately after the hot levelling, 
thereby suppressing the tendency of such a breadthwise camber that the 
covering layer constitutes the outer side, as a result of transfer of the 
heat to the covering layer from the base layer which is not cooled 
forcibly in the hot leveller 213. 
In this embodiment, the covering layer of the clad steel sheet 211 
immediately after the hot levelling by the hot leveller 213 is forcibly 
cooled so as to realize a greater amount of thermal contraction in the 
covering layer than in the base layer, thereby preventing such a camber of 
the clad steel sheet that the covering layer constitutes the outer side 
which may otherwise appear immediately after the hot levelling. 
The arrangement of this embodiment may be modified in such a manner that 
pinch rolls 218 are arrangd at the outlet side of the hot leveller 213 and 
cooling headers 219 in place of the cooling headers 217 mentioned above 
are disposed between the adjacent rollers of the pinch roll 218. Thus, in 
this embodiment, the hot-levelled clad sheet 211 is subjected to an 
additional forcible cooling in which the clad sheet is cooled at its one 
side while it is being restrained by the rollers of the pinch roll 218 or, 
alternatively, the additional forcible cooling is effected on one side of 
the hot-levelled clad sheet 211 while the latter is being conveyed by 
table rollers without being restrained in such a manner as to develop a 
large temperature difference between the upper and lower surfaces of the 
hot levelled clad steel sheet 211. This additional forcible cooling 
generates a thermal stress in the hot-levelled clad sheet 211, which in 
turn produces a compressive plastic deformation in the base layer which 
has a lower yield stress, whereby the amount of camber of the clad steel 
sheet at the room temperature is reduced. 
The additional forcible cooling on one side of the clad steel sheet by the 
cooling headers 217 or 219 after the hot levelling need not be conducted 
for a long time. Namely, the additional forcible cooling may be terminated 
when the camber after the hot levelling has been reduced to a level of 
about 100 mm which does not substantially hinder the convey of the clad 
steel sheet after the hot levelling. In addition, the plastic deformation 
in the clad steel sheet takes place only when the material exhibits a low 
yield stress, i.e., only when the material temperature is high. For these 
reasons, the additional forcible cooling may be finished only in a short 
period of time. 
The extent of the forcible cooling conducted during the hot levelling may 
be such that it can develop a temperature difference which is large enough 
to materially prevent the final camber when the clad steel sheet is cooled 
down to the room temperature. The extent of the forcible cooling conducted 
after the hot levelling also may be such that it can materially reduce the 
negative camber after the hot levelling. The method of the described 
embodiment, therefore, may be modified such that the forcible cooling is 
effected on both sides of the clad steel sheet at different rates or 
cooling power levels. 
Although two-layered clad steel sheet is mentioned specifically, it will be 
clear to those skilled in the art that the described embodiment is 
applicable to various two-layered clad metal sheet composed of different 
metals having different values of thermal expansion coefficient. 
A practical example of this embodiment will be explained hereinunder. Clad 
steel sheets having a covering layer of stainless steel, having a total 
thickness of 20 mm, breadth of 3000 mm and clad ratio of 30%, were 
levelled by three types of levelling method: namely, a conventional method 
I in which no forcible cooling was effected, a conventional method II in 
which one-side cooling was effected only during the hot levelling, and a 
method of the embodiment in which the one-side cooling was effected both 
during and after the hot levelling. The results of the test hot levelling 
were as follows. 
In the conventional method II and in the method of the embodiment, the clad 
steel sheets composed of a base layer of a carbon steel and a covering 
layer of a stainless steel were hot-levelled the following way. During the 
hot levelling, cooling water was sprayed onto the covering layer of the 
clad steel sheet so as to cool the stainless steel constituting the 
covering layer, by means of water spraying heads disposed between adjacent 
hot leveller rollers. The temperature of the clad steel sheet was 
650.degree. C. at the inlet side of the hot leveller. The temperatures of 
the stainless steel covering layer on the lower side of the clad steel 
sheet and the carbon steel constituting the base layer on the upper side 
of the clad steel sheet were 520.degree. C. and 600.degree. C., 
respectively, immediately after the hot levelling. In the method of the 
embodiment of the invention, a further cooling was effected on the clad 
steel sheet after the hot levelling. This additional forcible cooling was 
commenced at a moment 15 seconds after the completion of the hot 
levelling, and was maintained for 20 seconds thereafter. The additional 
cooling was effected by applying cooling water to the covering layer of 
the stainless steel by cooling water spray headers disposed between 
adjacent table rollers which are arranged at the outlet side of the hot 
leveller. In contrast, in the cnventional method I, the hot levelling was 
completed at a uniform temperature of 630.degree. C., without employing 
the one-side cooling by water. FIG. 17 shows the change in the amounts of 
camber observed after the hot levelling, while FIG. 18 shows the changes 
in the temperatures of the base layer of the carbon steel and the covering 
layer of the stainless steel. In the clad steel sheet after the hot 
levelling by the conventional method I, the covering layer of the 
stainless steel showed a greater thermal contraction than the base layer 
of the carbon steel because both layers are maintained at the same high 
temperature during the hot levelling and then air-cooled. In this clad 
steel sheet, therefore, a large camber of 240 mm was left after the 
cooling to the room temperature. 
In the case of the clad steel sheet hot levelled by the conventional method 
II, the temperature difference maintained during the hot levelling was 
uniformalized in a short time after the hot levelling, so that the base 
layer of the carbon steel exhibited greater thermal contraction than the 
covering layer of the stainless steel, resulting in a negative camber of 
-170 mm. However, as the clad steel sheet is further air-cooled, the 
camber was gradually decreased due to greater thermal contraction 
exhibited by the base layer of the stainless steel, and only a small 
camber of 50 mm was left finally after the cooling down to the room 
temperature. In contrast, in the clad steel sheet treated in accordance 
with the method of the embodiment, the camber after the hot levelling did 
not exceed -80 mm, thanks to the one-side cooling by water after the hot 
levelling. In addition, the amount of camber finally left in the clad 
steel sheet after cooling down to the room temperature was substantially 
zero. 
As has been described, according to the sixth embodiment of the invention, 
the layer which makes greater thermal contraction is forcibly cooled not 
only during the hot levelling but also after the hot levelling. It is, 
therefore, possible to suppress the generation of a negative camber which 
may otherwise appear after the hot levelling, thereby facilitating the 
convey of the two-layered clad metal sheet after the hot levelling, and to 
minimize the final camber after the cooling down to the room temperature. 
This also reduces the load during a cold levelling. In consequence, the 
hot levelling method of the sixth embodiment greatly contributes to the 
improvement in the efficiency of production of the clad metal sheets of 
the kind described. 
[Fifth Embodiment] 
The seventh levelling method in accordance with the invention will be 
described hereinunder. 
FIG. 19 shows general arrangement of a production line for producing a 
two-layered clad metal sheet, e.g., a two-layered clad steel sheet 301, to 
which the levelling method of the seventh embodiment is applied. The 
two-layered clad steel sheet 301 is composed of a base layer and a 
covering layer. For instance, the base layer is made of a cabon steel 
which makes a comparatively small thermal contraction, while the covering 
layer is made of a stainless steel which exhibits a comparatively large 
thermal contraction. With the base layer and the covering layer directed 
upwardly and downwardly, respectively, the clad steel sheet 301 is rolled 
by a rolling mill 302 and conveyed by table rollers 304 while the base 
layer thereof is heated by burners 303. The clad steel sheet 301 is then 
levelled by a hot leveller 305 and sent to a next step of the process. The 
hot leveller 305 has upper and lower hot leveller rolls 306, burners 303 
disposed between adjacent upper hot leveller rolls 306 and cooling spray 
nozzles 307 disposed between adjacent lower hot leveller rolls 306. The 
heating of the base layer of the clad steel sheet conducted by the burners 
303 before and during the hot levelling is intended for maintaining the 
yield stress of the base layer at a low level during the hot levelling, 
and for maintaining a sufficiently large temperature difference between 
the base layer and the covering layer. On the other hand, the forcible 
cooling by the cooling spray nozzles 304 during the hot levelling is 
conducted for the purpose of developing a large temperature difference 
between the base layer and the covering layer of the clad steel sheet 301. 
The heating which is effected on the layer of smaller thermal cntraction 
may be conducted by any one of the following three modes: (1) to effect 
the heating both before and during the hot levelling; (2) to effect the 
heating only before the hot levelling; and (3) to effect the heating only 
during the hot levelling. 
On the other hand, the forcible cooling effected on the layer of greater 
thermal contraction may be conducted either by (1) applying the cooling 
water both before and during the hot levelling or (2) applying the cooling 
water only during the hot levelling. 
A practical example of this embodiment will be explained hereinunder. 
Clad steel sheets having a covering layer of a stainless steel, having a 
tickness of 20 mm, breadth of 3000 mm and a clad ratio of 30%, were 
finished by the finish rolling stand of the rolling mill, at a 
comparatively low temperature of 800.degree. C. The clad steel sheets were 
then subjected to different types of hot levelling: namely, a conventional 
method I in which no heating nor cooling is effected before and during the 
hot levelling, a conventional method II in which the covering layer of the 
stainless steel is forcibly cooled only during the hot levelling, and a 
method of the invention in which, before the hot levelling, only the 
heating of the base layer by the burners is conducted and, during the hot 
levelling, both the heating of the base layer by the burners and the 
cooling of the covering layer by the water spray nozzles are conducted. In 
all cases, the temperatures of the base layer of the carbon steel and the 
covering layer of the stainless steel were substantially equal. Namely, in 
the case of the conventional methods I and II, the temepratures of the 
base layer and the covering layer were 550.degree. C., while, in the case 
of the method of the embodiment, the temperatures were 580.degree. C. As 
to the temperature at the outlet side of the hot leveller, the clad steel 
sheet treated by the conventional method I showed an equal temperature of 
500.degree. C. both at the base and covering layers thereof, whereas, in 
the clad steel sheet treated in accordance with the conventional method 
II, the base and covering layers showed temperatures of 450.degree. C. and 
400.degree. C., respectively. On the other hand, the clad steel sheet 
treated by the method of the embodiment showed temperatures of 510.degree. 
C. and 410.degree. C., respectively, at its base and covering layers. 
FIG. 21 shows the changes in the temperatures of the base layer of the 
carbon steel and the covering layer of the stainless steel in relation to 
time after the hot levelling, for each of the clad steel sheets treated by 
these three different methods, while FIG. 21 shows changes in the amounts 
of camber in these clad steel sheets after the hot levelling. 
The clad steel sheet treated by the conventional method I exhibited a flat 
shape immediately after the hot levelling. However, since the hot 
levelling is effected while both sides of the clad steel sheet are 
maintained at the same temperature, the covering layer of the stainless 
steel exhibits a greater thermal contraction than the base layer of the 
carbon steel during the cooling, thus leaving a large final camber of 200 
mm after cooling down to the room temperature. 
The clad steel sheet treated by the conventional method II was not 
completely flattened by the hot leveller and exhibited a camber of 50 mm 
immediately after the hot levelling. However, the camber was changed into 
negative camber of -80 mm as a result of the temperature uniformalization. 
Thereafter, the camber was gradually decreased as the cooling further 
proceeds, due to the greater amount of thermal contraction exhibited by 
the covering layer of the stainless steel. Finally, a positive camber of 
80 mm was left in the clad steel sheet after cooling down to the room 
temperature. 
In contrast, the clad steel sheet treated in accordance with the method of 
the embodiment was flat in the state immediately after the hot rolling. 
Then, as the cooling proceeds, the clad steel sheet exhibited a negative 
camber of -150 mm, but the final camber after the cooling down to the room 
temperature was substantially zero. 
Thus, according to the seventh embodiment of the invention, it is possible 
to restrain the generation of the final camber in the two-layered clad 
metal sheet after cooling down to the room temperature, even if the final 
rolling temperature of the clad metal sheet and the interval between the 
rolling and the hot levelling are changed. This in turn reduces the load 
of a cold levelling. 
As has been described, according to the seventh embodiment of the levelling 
method of the invention, the hot levelling is conducted while the layer of 
smaller thermal contraction and the layer of the greater thermal 
contraction are forcibly heated and cooled, respectively. By this method, 
it is possible to reduce the yield stress of the base layer during the hot 
levelling, thus obtaining a flat state of the clad metal sheet immediately 
after the hot levelling, while developing a sufficiently large temperature 
difference between the base layer and the covering layer, whereby the 
generation of substantial camber in the clad metal sheet after cooling 
down to the room temperature is avoided without fail.