Rolling mill roll for a hot train of rolls

The object of the invention is to improve the performance of the rolls of roughing and finishing stands of a hot strip train or mill and in particular to retard as far as possible the banding phenomenon, namely the phenomenon of the incrusting of hard oxides or scale formed at the conventional rolling temperatures. For this purpose, the roll is made, at least in the working region thereof, from a chrome steel in which the ratio between the respective contents of chromium and carbon is between 7 and 12 and in which the content of equivalent carbon is between 1.5 and 1.7%. This content of equivalent carbon is calculated as the sum of the content of carbon and the content of chromium, the latter being given a coefficient 0.05.

The present invention relates to rolling mill rolls for in particular 
roughing stands and the first finishing stands of a hot strip train or 
mill. 
As concerns the rolls of these hot train stands, there is encountered a 
problem termed "banding" which consists in a phenomenon of the embedding 
in the surface of the rolls of hard scale or oxides which come from the 
rolled strip and are formed on this surface at the high rolling 
temperatures. These oxides are torn away from the surface of the roll, 
sometimes carrying along therewith certain parts of the metal, and are 
thereafter rolled in the following stands with the strip and result in a 
deterioration of the quality of this strip owing to the occurrence of 
surface defects termed "gravel". This is a serious problem since it leads 
either to a lowering of the rolling temperatures so as to limit the 
oxidation, which is not desirable, or the frequent replacement of the 
rolls which reduces the overall output of the rolling mill train. 
In an effort to retard this phenomenon, a first solution consists in making 
the rolls from chrome iron, it being possible to employ chromium contents 
of from 12 to 20%, whereas the carbon content is of the order of 2.5%. 
This first solution resulted in a certain improvement, in particular as 
concerns the hardness of the roll. However, this improvement was found to 
be insufficient since with such chrome irons carbides are formed in the 
material of the roll which break and result in cracks which facilitate the 
encrusting of the oxides or scale. 
In a second solution in an effort to solve this problem, it was attempted 
to employ steels containing between 1.3 to 1.9% of carbon and roughly 
between 1 and 1.5% of chromium, with a certain number of alloy elements 
moreover for improving the hardness. However, here again, this solution 
was not found to be absolutely satisfactory since the oxide which is 
formed on the surface of the roll is fragile and easily scales off and 
this increases the risk of adhesion between the roll and the strip and 
facilitates the encrusting phenomenon since, in scaling off, the oxide 
tears away a part of the metal. In an effort to overcome this drawback, an 
intense lubrication must be provided and this complicates the installation 
and increases the cost. Moreover, such rolls have a hardness which is 
insufficient to oppose the banding or encrusting phenomenon. 
The problem that the invention is intended to solve is consequently to 
provide rolling mill rolls which are intended to equip mainly roughing 
stands or the first finishing stands of a hot strip train or mill, it 
being required that these rolls have an increased duration of utilization 
and be less susceptible to the encrusting phenomenon, notwithstanding the 
fact that these rolls operate under particularly difficult conditions. 
The prescribed solution is based on an original approach which consists in 
associating more closely than in the past the carbon and chromium contents 
and in finding between these two elements an optimum ratio for the 
contemplated application. For this purpose, the invention provides a 
rolling mill roll whose surface is made from chrome steel and in which the 
ratio between the respective contents of chromium and carbon is between 7 
and 12 and in which the content of equivalent carbon is between 1.5 and 
1.7%, this content of equivalent carbon being calculated as the sum of the 
carbon content and the chromium content, the latter being given a 
coefficient of 0.05 said chrome steel excluding boron and tungsten apart 
from any traces of boron and tungsten as impurities. 
These conditions result in carbon contents of between 0.9 and 1.3% and in 
chromium contents of between 6 and 15%, the other alloy elements, such as 
manganese, silicon, nickel, molybdenum, etc . . . , being chosen in 
accordance with the desired hardness. 
In contrast to the situation with high chrome irons, the carbon content 
remains limited, which permits the avoidance of the formation of 
intergranular carbides of the type M.sub.7 C.sub.3 which no heat treatment 
can put in solution. The chromium content is sufficient to obtain a matrix 
which is rich in chromium but however remains within such limits that the 
chromium oxide can form to a certain extent and constitute a protective 
lubricating layer which constitutes a positive element. It is therefore 
essential to conform to the aforementioned ratios and magnitudes. 
It was found that a roll having such a composition has a much better 
structure than a chrome iron or a steel of the adamite type, in particular 
as concerns the secondary carbides which are extremely fine and dispersed.

A roll according to the invention may be made by a composite casting method 
and preferably by a centrifugal casting method, this type of method having 
the desired effect as concerns the refining and distribution of the 
carbides. By way of example, such a roll may have the following 
composition: 
C: 0.96 
Mn: 0.58 
Si: 1.4 
Ni: 0.62 
Cr: 11.5 
Mo: 1.13 
In this composition, the ratio Cr/C is in the neighbourhood of 12 and the 
content of equivalent carbon equals 1.535. 
This composition is of course that of the working surface or outer region 
of the roll whose heart or core may be made for example from nodular iron. 
After centrifugal casting, the roll is subjected to a suitable heat 
treatment comprising, for example, a simple quenching in air from a 
temperature of 1,000.degree. C., if desired with a stage or step in the 
course of the cooling; these treatments are conventional and are 
consequently not described in more detail. 
Such a roll has a hardness higher than 70 shore C. 
Tests were carried out, in particular by using such a roll with a chrome 
iron roll of conventional design and composition C: 2.9--Si: 0.6--Mn: 
0.91--Cr: 18.9-- Ni: 0.92--Mo: 1.45. These two rolls were mounted 
together, while regularly reversing their positions, in the F3 stand of a 
hot continuous train or mill. It was found that, after an equivalent 
period of operation, the roll according to the invention had no 
geometrical wear and no trace of encrusting with oxide or scale, whereas 
the conventional roll already has a marked commencement of banding. 
The object of the invention is therefore attained and a roll having in its 
working region the claimed composition has in the particularly difficult 
conditions of utilization encountered in roughing stands and in the first 
finishing stands of a hot train or mill, a behaviour and performance which 
are very substantially improved relative to known rolls, so that the 
overall output of the train or mill can be increased in the same 
proportions.