The roll is made from a low alloy steel having the following composition by weight : C: 0.76 to 0.92; Mn: 0.70 to 1.40; Si: 0.70 to 1.40; S.ltoreq.0.020; P.ltoreq.0.025; Ni.ltoreq.0.60; Cr: 1.50 to 2.20; Mo: 0.15 to 0.55; V: 0.08 to 0.25; Cu.ltoreq.0.50; the remainder being iron and accidental impurities.

BACKGROUND OF THE INVENTION 
The present invention relates to forged rolls for cold rolling and more 
particularly to working rolls for rolling iron and steels, non-ferrous 
metals and their alloys at temperatures lower than or equal to 100.degree. 
C. and possibly to backing rolls used in multi-roll rolling mills. 
In order to ensure excellent endurance in service at the lowest cost and 
working rolls must have a number of characteristics in the state of 
utilization, namely: 
1. A high surface hardness of between 90 and 105 Shore C according to the 
products to be rolled. 
2. Great depth of the hardened layer which will permit limiting, or even 
eliminating, retreatments which may be necessary for maintaining the 
desired hardness throughout the given depth of utilization of the roll. 
3. High resistance to wear by abrasion. 
4. A controlled content of residual austenite of the hardened layer; it 
being understood that excessively high contents of residual austenite are 
harmful in that they promote cracking under service stress. 
5. A dendritic structure of the surface layers which is sufficiently 
homogeneous in order to avoid a phenomenon of an extremely fine pitting of 
the sheet which is given the name of "toad skin" or "orange peel" in the 
profession. 
A large number of these characteristics may be regulated by a judicious 
choice of the conditions of manufacture of the cold-rolling rolls and more 
particularly of the heat treating operations: tempering whereby it is 
possible to adjust the hardness of the body of the roll, conventional 
hardening method with heating to a temperature&gt;AC3 of the whole of the 
roll during the austenitization, surface hardening after heating to a 
temperature&gt;AC3 solely of a relatively thin layer, more or less well 
adjusted cooling conditions. 
However, the choice of the grade remains primordial for optimizing the 
required characteristics at the lowest cost. 
The grades used at the present time for cold-rolling working rolls of 
water-hardened forged steel comprise 0.8 to 0.9% carbon, 1.8 to 3.0% 
chromium and other alloy elements and are illustrated by the conventional 
grade 83 CDV7 which has in fact a sufficiently high content of carbon to 
obtain the required high levels of hardness, the contents of Cr, Mo, V are 
sufficient to obtain a correct hardenability and the formation of many 
carbides ensuring good wear resistance. With conventional heat treatments 
followed by an energetic water quenching it is thus possible to obtain 
easily a surface hardness of 103 Shore C, a depth of 15 mm of a hardened 
layer having a hardness of .gtoreq.85 Shore C on rolls having a roll body 
surface diameter of 550 to 650 mm. 
With a surface hardening after induction heating at the frequency of 50 Hz, 
similar surface hardnesses are obtained with however a hardened layer of 
greater depth, namely about 22 mm. 
However, in order to take full advantage of the useful depth of the roll 
body surface, such hardened depths require a minimum of two retreatments. 
These retreatments are expensive and many manufacturers have sought to 
improve the hardenability of the steel so as to obtain hardened layers 
having a depth of about 30 mm, which then limits the number of 
retreatments to a single operation. 
In order to increase this depth, attention has been directed to more highly 
alloyed steels having contents of Cr ranging up to 3% and of Mo up to 
0.5%. Apart from the fact that these alloy elements are expensive, the 
increase in their content has the serious drawback of producing an 
undesirable amount of residual austenite after the martensitic quenching. 
Large amounts of residual austenite may be remedied by a treatment 
subsequent to the quenching consisting in plunging the roll into liquid 
nitrogen (sub-zero treatment), but these treatments are delicate to carry 
out and costly. 
Lastly, the increase in the content of the alloy elements Cr, Mo, V results 
in a banded structure and a dendritic structure which impair the surface 
quality of the rolled products. 
SUMMARY OF THE INVENTION 
An object of the present invention is to overcome these drawbacks while 
providing forged rolls having a hardened layer of great depth. 
The invention also provides a cold-rolling forged roll made from a low 
alloy steel which has the following composition by weight: 
C: 0.76 to 0.92; Mn: 0.70 to 1.40; Si: 0.70 to 1.40; S.ltoreq.0.020; 
P.ltoreq.0.025; Ni.ltoreq.0.60; Cr: 1.50 to 2.20; Mo: 0.15 to 0.55; V: 
0.08 to 0.25; Cu.ltoreq.0.50; the remainder being iron and accidental 
impurities. 
The essential characteristic of the invention resides in the content of Si 
which produces, in association with the Mn, a synergic effect on the 
hardenability of a steel having a low content of alloy element, and in 
particular Mo.

DETAILED DESCRIPTION 
The works of Jatezack and Girardi in the following articles: 
Multiplying factors of the calculation of hardenability of Hypereutectoid 
steels Hardened from 1,700.degree. F.; C. F. Jatezack and D. J. Girardi 
transactions of ASM 1959-51 p 335; and 
Hardenability of high carbon steel; C. F. Jatezack and D. J. Girardi 
Metallurgical transaction--vol 4 Oct. 73 p 2267; 
describe the effect of alloying elements on the hardenability of 
hypereutectoid steels and characterize the hardenability of the various 
grades by the distance from the quenched end of the Jominy point where the 
hardness is 63 RCH or Jominy test specimens austenitized at temperatures 
between ACm+50 and ACm+100. 
The structure corresponding to the hardness of 63 RCH is almost completely 
martensitic with a maximum of 10% bainite, so that the criterion adopted 
is quite representative of the conditions of utilization of the rolls. 
These works show that the heardenability can be increased by using higher 
contents of conventional alloying elements such as Mn, Ni, Cr, V, Si, and 
above all Mo as indicated by the graphs of FIGS. 1a and 1b illustrating 
the multiplying factor F on the distance from the quenched end as a 
function of the content of various indicated elements for a respectively 
normalized and annealed initial structure. 
It is quite clear from these graphs that Mo has the greatest effect and in 
particular an effect greater than Si alone or even combined and greater 
than Mn. 
Now, the applicant has discovered that in contrast to the teachings of 
these works, Mo has an effect on the hardenability which has a maximum for 
relatively small contents. 
These results are given in FIG. 2 which shows graphically the effect of the 
addition elements Mo, Mn and Si on the hardenability of a steel 85 CDV7 
which had been subjected to an austenitization treatment Acm+60.degree. C. 
In this graph, plotted as ordinates is the Jominy distance, i.e. the 
distance in mm to the end of a normalized test specmen (having a diameter 
of 25 mm) in respect of which the Rockwell C hardness (RCH) is higher than 
or equal to 60. 
Further, it is clear that Si has a synergic effect on Mo and above all on 
Mn. 
As a comparison, FIG. 3 shows (hardness as a function of the distance D to 
the quenched end) Jominy curves for a conventional grade which is a steel 
85 CDV7 whose contents of Mn are 0.25 and Si 0.42 and for a range of steel 
grades according to the invention. 
The increase in the hardness at 70 mm from 45 RCH to 63 RCH is particularly 
significant. 
Further, the presence of silicon tends to promote the formation of carbides 
which is advantageous for the wear resistance as has been shown by the 
various laboratory tests carried out. 
On the other hand, there is observed a slight decrease in the carbon 
content of the matrix of the steel and consequently in the maximum 
hardness level which may be obtained: this is not a drawback, since it is 
sufficient to act on the tempering conditions after quenching between 
100.degree. and 200.degree. C. 
The silicon moreover increases the resistance to tempering. Its action can 
therefore only be beneficial when small rolling incidents occur resulting 
in an increase in the superficial temperature of the rolls. 
The absence of a significant influence of the additions of Mn and Si on the 
residual amount of austenite after treatment and on the tensile strength 
of the metal treated at the level of 64 RCH, has been confirmed in the 
range of the chosen contents. The same is true in respect of the dendritic 
structure on the roll body surfaces. The conjugate addition of manganese 
and silicon has been found to be beneficial for the performance of the 
roll in service. 
The following examples are given as an illustration of the invention. 
EXAMPLE 1 
There is made a working cylinder having a roll body surface diameter of 
3.25 mm and a roll body surface length of 1324 mm with a roll body surface 
hardness of 760 Vickers, namely 92 Shore C, intended for the cold rolling 
of rolls of silicon steel. 
This roll is machined from a blank forged from a steel ingot having the 
following composition: 
C 0.83--Mn 1.12--Si 0.89--S 0.009--P 0.012--Ni 0.33--Cr 1.82--Mo 0.25--V 
0.11. 
The final treatment of the roll body surface is carried out by a low 
frequency (50 Hz) surface heating and quenching in water. 
In this way, a 28.5 mm deep hardened layer is obtained. 
As a comparison, a similar roll was made from the conventional grade: 
C 0.83--Mn 0.29--Si 0.33--S 0.007--P 0.014--Ni 0.27--Cr 1.77--Mo 0.24-V 
0.11. 
This roll has, after a low frequency surface hardening, a 20.5 mm deep 
hardened layer. 
Thus, by means of the invention, there is obtained an increase of 40% of 
the depth of the hardened layer in a less expensive grade from the point 
of view of both the constituent elements and the process of manufacture. 
The rolls in the grade of steel according to the invention used in a 
reversible 4-high rolling mill have permitted the rolling of 3,690 metric 
tons instead of 3,100 metric tons for the comparison grade, namely an 
increase of 19%. 
EXAMPLE 2 
There is made a working roll for cold rolling automobile body sheet metal 
having the following characteristics: 
diameter of the roll body surface: 535 mm 
length of the roll body surface: 1676 mm 
intended roll body surface hardness: 830 VH. 
Composition of the metal: C 0.86--Mn 0.96--Si 1.19--S 0.004--P 0.012--Ni 
0.175--Cr 1.66--Mo 0.22--V 0.096. 
The final treatment of the roll body surface is carried out as in Example 
1. 
After detensioning and before adjustment of the hardness, the surface 
hardness is 875 VH. 
The hardened depth, corresponding to a hardness of 700 VH, namely 
substantially 85 Shore C, is 29.6 mm. 
The useful hardened depth of the rolls being 27 mm, the whole of this depth 
can be used before scrapping without retreatment by a rehardening of the 
roll. 
With rolls having a conventional grade 83 CDV7 similar to that of Example 
1, the depth of the hardened layer after low frequency surface hardening 
measured under the same conditions, is 22 mm. This requires a retreatment 
for consummating the whole of the useful depth of the roll. 
It will be clear that, in respect of a roll having the same geometric 
characteristics is before but a roll body surface diameter increased to 
581 mm and a useful depth increased to 50 mm, the grade according to the 
invention limits to one retreatment the total utilization of this depth, 
whereas it is necessary to effect two retreatments with the comparison 
grade. 
The hardened layer of the rolls according to the invention is at least 27 
mm, although it may be greater (28.5 and 29.6 mm having been given as 
depths of hardened layer in the above examples). 
The usual technique for measuring hardness consists in measuring the 
hardness of the roll in a radial direction. 
In the usual practice of the cold rolling, the surface layer of the roll is 
submitted to a (e.g. 6 hours) and before the roll is used again in the 
rolling mill. During that maintenance step, the superficial hardness of 
the roll is measured and the wear of the hardened layer is thus monitored. 
The superficial hardness must be greater than 85 shore C (or 700 HV). 
Thus, the depth of the hardened layer is the depth of the zone of the roll 
where the hardness is at least equal to 85 shore C. Experimentally, it is 
possible to measure the hardness of a test roll or sample every millimeter 
in depth, after a grinding operation in the direction of the axis of the 
roll. 
The amounts of Mn, Si and Mo present in the steel of which the roll of the 
invention is made, is `balanced` in the sense of being selected to be 
within the respective recited ranges. 
More precisely, the inventors have discovered several facts leading to a 
new composition: 
1. Contrary to the knowledge of the prior art, the Mo content has an 
optimum effect on the hardenability when limited to a rather low level 
(&lt;0.5%). Practically, the Mo content must be in the range 0.15-0.55% and 
preferentially about 0.25%. 
2. Si and Mn have a synergic effect by themselves (and with Mo). For 
example, an addition of a total amount of 0.75% Si+0.75% Mn is more 
efficient (as far as the hardenability is concerned) than 1.5% Si or 1.5% 
Mn alone. 
3. Si must be limited to 1.5%, as a content greater than 1.5% modifies the 
form of the carbides in the steel, decreases the wear resistance and 
increases the embrittlement. 
4. Mn must be limited toward 1.5%, principally because of difficulties in 
the steel making process for higher contents. 
An optimal composition of the Mo, Si and Mn content is presently believed 
to be: 
Mo: 0.25%--Si: 1% and Mn 1% 
A broader definition of the Mo, Si and Mn content of the presently 
preferred steel compositon is: 
Mo: 0.15-0.55) 
Mn: 0.7-1.40) with 0.75&lt;Si/Mn&lt;1.25 
Si: 0.7-1.40)