Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
BACKGROUND OF THE INVENTION
Field of the Invention:
The present invention relates to a roll outer layer
material for hot rolling steel which exhibits excellent
resistance to surface deterioration and wear resistance
and elimination of accidents, such as breakage of the
roll during manufacture or rolling, as well as a method
of manufacturing a roll for hot rolling.
Description of the Related Art:
To manufacture hot rolled steel plates or strips, a
slab having a thickness ranging from 130 to 300 mm and
manufactured by the continuous casting process or by
blooming is heated by a heating furnace, and then hot
rolled by a rough rolling mill and a finish rolling mill
to obtain a strip having a thickness ranging from 1.0 to
mm. After the thus-obtained strip is coiled by a
coiler, it is cooled. The cooled strip is subjected to
various finish operations such as cutting, slitting,
marking, bundling etc., in finish lines. In the work
20 roll of a conventional finish rolling mill, a high Cr
cast iron, Adamite, or a Ni grain cast iron or the like
is used as the outer layer material, and a gray cast iron
or a ductile cast iron exhibiting excellent toughness is
used as the inner layer material. The work roll of a
25 conventional finish rolling mill is a composite roll
manufactured by centrifugally casting these outer and
2
_ y
inner layer materials.
In recent years, rolling conditions tend to become
severe and an improvement in the productivity of rolling
has been required. Hence, there has been a demand for
rolling rolls exhibiting higher wear resistance.
To meet such requirements, for example, Japanese
Patent Laid-Open Nos. Sho 60-124407 and Sho 61-177355
have proposed the use of a high V cast iron as the outer
layer material manufactured by the centrifugal casting
process.
However, when used as the outer layer material of a
roll manufactured by centrifugal casting, a high v-cast
iron has a disadvantage in that a V carbide having a
small specific gravity is centrifugally separated, thus
making the characteristics of the roll outer layer
material non-uniform in the direction of thickness
thereof. Hence, it has been proposed in, for example,
U.S. Patent No. 5,316,596 and Japanese Patent Laid-Open
Nos. Hei 4-365836, Hei 5-1350 and Hei 5-339673 to add Nb
to prevent segregation.
More specifically, it has been found that in a
composite carbide of Nb and V, segregation of a carbide
in primary crystal is reduced because the specific
gravity of the composite carbide is higher than that of
the V carbide (specific gravity: 5.77) and closer to the
specific gravity (7.0) of the molten metal than the V
3
carbide. Accordingly, it has been proposed to make a
Nb/V ratio 2.0 or above and basically to not add W in
order to prevent segregation of an eutectic carbide.
Although the above-described high V and high v-Nb
roll outer layer materials are advantageous in terms of
the improvement of the wear resistance; the present
inventors found that a hard V or Nb carbide (MC carbide)
in the high V or high V-Nb material tend to form
projected parts near the surface of the roll during
rolling. Such parts act as spikes and increase the
coefficient of friction between a rolled material and the
surface of the roll. Accordingly, a roll having an outer
layer made of such a high V or high V-Nb material has the
following disadvantages:
(a) Rolling load is increased excessively.
(b) Secondary scale is generated in the surface of a
rolled material due to frictional heat, thus generating
surface roughening of the rolled material.
(c) The roll surface is damaged due to excessive
frictional heat generated during rolling under a high
pressure, thus generating surface roughening of the
rolled material.
Furthermore, since wear resistance of the roll is
improved and the useful life of a roll before roll
changing is thus increased, another problem involving
spalling of the roll surface (hereinafter referred to as
4
"banding") is encountered during rolling. This is due to
fatigue occurred at the portion of the roll located
immediately below the surface thereof.
The above-described problems (a) to (c) are
remarkable in a roll used under an environment of large
heat load, such as a roll for use in a hot rolling stand
in a front stage of finish strip mill.
Furthermore, since wear resistance of the roll is
improved, the load applied to the roll is usually
increased. Thus, there is the possibility that a very
small defect generated in the roll during manufacture
thereof leads to an accident, such as breakage of a roll.
SUMMARY OF THE INVENTION
In view of the aforementioned problems of a
conventional roll outer layer material, an object of the
present invention is to provide a roll outer layer
material which exhibits excellent wear resistance and has
a low coefficient of friction in order to solve the
problems (a) to (c).
Another object of the present invention is to
provide a roll outer layer material which eliminates
banding which would occur when a roll is used over a long
period.
Still another object of the present invention is to
provide a method of manufacturing a roll for hot rolling
which has no defect therein and thus prevents breakage
5
CA 02166258 1997-11-14
during the use of the roll.
The present invention provides a roll outer layer
material for hot rolling which consists essentially, as
analyzed in weight percent, of 2.5-4.0~ C, 6.0-20~ Cr,
2.0-15~ Mo, 3.0-10.0 V, 0.6-5.0~ Nb, 3.0~ or below Si,
3.0~ or below Mn, C, V, Nb and Cr satisfying a formula
(1)
10.5, ~ 6.5~C(~k) - 1.3~V($) - 0.7~Nb(~) <_
2~Cr(~) - 2 ~~~ (1)
with the balance of Fe and incidental impurities.
The present invention further provides a method of
manufacturing a roll for hot rolling, which comprises the
steps of
centrifugally casting an outer layer material
consisting essentially, as analyzed in weight percent, of
2.5-4.0~ C, 6.0-20~ Cr, 3.0-10.0 V, 0.6-5.0~ Nb, 2.0-15~
Mo, 3.0~ or below Si, 3.0~ or below Mn, C, V, Nb and Cr
satisfying a following formula (1):
10.5 < 6.5~C(~) - 1.3~V(~) - 0.7~Nb(~) _<
2~Cr(~S) - 2 ~~~ (1)
with the balance of Fe and incidental impurities;
centrifugally casting graphite steel containing 0.5g
or above of C as an intermediate layer; and
casting an axis material which is a spheroidal
graphite cast iron, a flake graphite cast iron or a
graphite steel.
6
73461-64
CA 02166258 1997-11-14
C is present in the intermediate layer in an amount
which satisfies a following formula (2):
C(~) (intermediate layer) >_ 2.0 - 0.5 [C(%) - 0.2V($) -
O.llNb($)] (outer layer) ~~~ (2)
Other objects and advantages of the invention will
become apparent from the following description taken in
connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a graph illustrating the correlation
between the relations between C, V and Nb with the
coefficient of friction;
Fig. 2 is a graph illustrating the correlation
between C, V, Nb and Cr with the amount of wear;
Fig. 3 illustrates dependency of the generation of a
boundary defect on the amount of C in an intermediate
layer during casting;
Fig. 4 is a graph illustrating the relation between
the number of cracks generated in a carbide due to
fatigue and the amounts of Mo and Cr;
Fig. 5 is a graph illustrating dependency of the
generation of cracks on a Mo/Cr ratio;
Fig. 6 is a graph illustrating comparison between a
roll according to the present invention and a
conventional roll in terms of the rolling load; and
Fig. 7 is a graph illustrating comparison between a
roll according to the present invention and a
7
73461-64
conventional roll in terms of the degree of wear
resistance.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
(A) Basic concept of the invention
First, the basic technical concept of the present
invention will be described.
The structure of a high V or high V-Nb roll outer
layer material is made up of proeutectic carbides
(including V and Nb carbides), eutectic carbides
(including Cr, Mo and Fe carbides) and a base. During
rolling, the less hard base selectively wears, resulting
in projection of the hard proeutectic and eutectic
carbides. An increase in the coefficient of friction
occurs because the proeutectic carbides act as
projections and because shoulders are generated between
the matrix which occupies most of the structure and the
proeutectic carbides. Hence, the frictional force is
reduced to prevent an excessive increase in the rolling
load and generation of secondary scale in the surface of
a rolled material and to prevent generation of surface
roughening of the rolled material either by changing the
amount or shape of the proeutectic carbides or by
lessening the shoulders between the proeutectic carbides
and the portion other than the proeutectic carbides.
It is possible to generate eutectic carbides having
the second highest hardness in the structure of the roll
8
CA 02166258 1997-11-14
outer layer material in which the proeutectic carbides have
the highest hardness. Thus, shoulders between the proeutectic
carbides and the portion other than the proeutectic carbides
can be lessened to reduce the coefficient of friction of the
roll outer layer material by increasing eutectic carbides in
the structure of the roll outer layer material.
Furthermore, in order to reduce fatigue of the roll
surface layer portion and hence restrict banding, the present
inventors consider that strengthening of the proeutectic
carbides, eutectic carbides and matrix structure is effective.
Thus, it is considered that fatigue resistance (banding
resistance) can be improved due to strengthening by solid-
solution or precipitation by increasing the amounts of
adequate alloy elements to be added to the roll.
The roll according to the present invention is
particularly suitable for use in a hot rolling stand in a
front stage of finishing strip mill. This stage performs
rolling at a rolling reduction of 85 to 90~ with the thickness
of a slab, which does not yet enter the finishing strip mill,
and in such a manner that the temperature of the slab at the
outlet side of the each stand is between 850 and 900oC.
9
73461-64
(B) Composition of the roller outer layer material
according to the present invention
C . 2.5 to 4.0~
The addition of C is essential to form a hard
carbide which is required to improve wear resistance of
the roll outer layer material. The presence of less than
2.5~ C makes formation of a sufficient amount of carbide
impossible, thus reducing wear resistance of the roll
outer layer material while increasing the coefficient of
friction thereof which can be a cause of surface
roughening. The addition of more than 4.0~ C has no
significance in terms of the effect of reducing the
coefficient of friction and it reduces wear resistance.
The preferred proportion of C is between 2.8 and
3.5~.
Si : 3.0~ or below
The addition of an adequate amount of Si assures
deoxidation and castability. Since the effect of the
addition thereof is saturated if the proportion exceeds
3~, the proportion is limited up to 3~. The preferred
range is between 0.1 and 1.5~.
Mn . 3.0 or below
The inclusion of Mn is effective to remove S in the
form of MnS and to strengthen the structure. The upper
limit of the proportion thereof is 3~, because the effect
of the addition of Mn is saturated if the proportion
CA 02166258 1997-11-14
exceeds 3~. The preferred proportion is between 0.1 and
1.2~.
Cr . 6.0 to 20~
Cr forms a tough eutectic carbide and thus the
presence thereof is advantageous to improve wear
resistance. A solid-solution of Cr in the matrix
strengthens the matrix structure and improves fatigue
characteristics, and improves adhesion of a oxide layer
on the roll. Thus, the introduction of Cr is mandatory.
The amount of Cr added is 6.0~ or above. However, since
the effect of improving wear resistance is saturated when
the amount of Cr exceeds 20~, the proportion of Cr is
limited up to 20~ to prevent deterioration in resistance
to sticking.
The preferred proportion is between 8.0 and 20~.
Mo : 2.0 to 15~
Mo forms a carbide, like Cr, and thus the inclusion
thereof is advantageous to improve wear resistance.
Further, Mo concentrates in the carbide and strengthens
the carbide. Mo enhances banding resistance. These
effects can be obtained at 2,0~ or above Mo. However,
since the effects are saturated when the proportion
exceeds 15~, the proportion of Mo is limited up to 15~.
Fig. 4 illustrates the relation between the number
of fatigue cracks generated in the surface of a sample
subjected to the double disk slide-contact rolling
11
73461-64
fatigue test and the amounts of Mo and Cr. When Mo
exceeds 2.0~, the number of cracks is halved. Up to
about 15~, as the amount of Mo increases, the number of
cracks decreases. Regarding Cr, the effect of the
addition of Cr increases, as the proportion of Cr
increases.
The preferred proportion of Mo according to the
present invention is between 3 and 12~.
Mo/Cr ratio . 0.25 or above
There is a preferred range for Mo/Cr ratio from the
viewpoint of banding resistance. Fig. 5 shows the
rearranged results shown in Fig. 4 with the Mo/Cr ratio
as the abscissa.
As can be seen from Fig. 5, when Mo/Cr ratio is 0.25
or above, the number of cracks generated greatly
decreases. The number of cracks is minimized when Mo/Cr
ratio is between 0.3 and 1Ø Thus, in order to prevent
banding, Mo/Cr ratio is set to 0.25 or above with a
preferred range being between 0.3 and 1Ø
Ni . 1.0~ or below
The addition of Ni improves hardenability and thus
expands the operation range for quenching. The addition
of Ni is more effective for rolls having a large roll
diameter. Thus, Ni is added when necessary. Since the
presence of more than 1.0~ Ni forms an unstable structure
in which, for example, residual y is present, it is
12
__ ~16~~~8
limited up to 1.0~.
W : 1.0~ or below
In the present invention, the inclusion of W, which
bonds with C to form a hard carbide, has no effect of
improving wear resistance and surface roughening
inhibiting property of the roll (of reducing the
coefficient of friction). Further, the presence of more
than 1.0~ W promotes segregation of carbides (further
promotes segregation during centrifugal casting), and
deteriorates wear resistance and surface roughening
resistance. Thus, the inclusion of W is not generally
necessary in the invention. If W is added, however,
depending on the conditions under which the roll is used,
the preferred W proportion must be 1.0~ or below.
Co . 5~ or below
Since the presence of Co stabilizes the structure at
high temperatures, Co is added if necessary. However,
the effect of Co addition in terms of the improvement of
wear resistance and surface roughening inhibiting
property of the roll is not apparent. The preferred
proportion of Co is limited up to 5~ from the economical
viewpoint.
V : 3.0 to 10.0, Nb :0.6 to 5.0~
V is essential to form a hard MC or M4C3 carbide
which is the most effective substance to improve wear
resistance. The effect of V addition can be discerned at
13
CA 02166258 1997-11-14
3.0~ or above. The presence of more than 10.0 has no
effect and causes a problem during casting, such as a
making micro cavity. Thus, the presence of V is limited
up to 10Ø The preferred proportion is between 4.0 and
7.0~.
Like V, Nb forms an MC carbide which is the
effective substance to improve wear resistance. Further,
Nb restricts segregation of a V carbide, and thus enables
the provision of an outer layer in which the MC carbide
is uniformly dispersed even if the centrifugal casting
process is used to manufacture a roll. The effect of Nb
addition can be discerned at 0.6~ or above. The presence
of up to 5.0$ has no effect and causes a problem during
casting, such as a making micro cavity. Thus, the
presence of Nb is limited up to 5.0~. Further, the
addition of Nb or V alone forms a rough bulk carbide or a
dendritic carbide, and thus greatly deteriorates wear
resistance. Thus, Nb must be added together with V.
The preferred proportion of Nb is between 1.0 and
3.0~.
Parameter : 10.5 _< 6.5~C(~) - 1.3~V(~) - 0.7~Nb(~) <_
2~Cr(~) - 2
The present invention is characterized in that the
composition of a roll exhibiting excellent wear
resistance and surface roughening inhibiting property
satisfies the above-described formula. In a high V-Nb
14
73461-64
CA 02166258 1997-11-14
roll, whereas formation of hard V (Nb) C greatly improves
the wear resistance of the roll, it increases the
coefficient of friction, thus generating surface
roughening of the plates. Hence, the present inventors
had the following formula which calculated the amount of
C consumed by both V and Nb as follows:
(6.5~C(~) - 1.3~V(~) - 0.7~Nb(~)),
studied the relation between the amounts of V and Nb and
the total amount of C added, and correlated the
coefficient of friction with the resultant relation. The
results are shown in Fig. 1. That is, the present
inventors found that a high V-Nb type composition must
satisfy the formula (10.5 ~ 6.5~C(~) - 1.3~V(~) -
0.7~Nb(~)) in order to obtain a coefficient of friction
(about 0.28) similar to that of a high Cr cast iron
suitable for use as a hot rolling stand in a front-stage
of finishing strip mill.
It was observed that in a composition which
satisfies the above-described formula, a reduction in the
amount of Cr greatly deteriorated wear resistance and
fatigue resistance. The present inventors hypothesize
that in a composition which satisfies the above formula
in which large amounts of eutectic carbides are present,
a reduction in the amount of Cr reduces the amount of Cr
distributed into the eutectic carbides, thus lowering
wear resistance. Thus, the present inventors studied the
73461-64
CA 02166258 1997-11-14
relation between 6.5~C(~) - 1.3~V(~) - 0.7~Nb(~)) and the
amount of Cr, and correlated the amount of wear and the
resultant relation. The results are shown in Fig. 2.
That is, it is apparent that the composition, which
satisfies 10.5 _< 6.5~C(~) - 1.3~V(~) - 0.7~Nb(~) and
thus has a low coefficient of friction, must satisfy
6.5~C(~) - 1.3~V(~) - 0.7~Nb(~) <_ 2~Cr(~) - 2 to
provide a roll exhibiting excellent wear resistance.
It is thus apparent that in a high V=Nb roll
exhibiting excellent wear resistance and surface
roughening inhibiting property, a composition must
satisfy 10.5 <_ 6.5~C(~) - 1.3~V(~) - 0.7~Nb(~) <_
2~Cr(~) - 2.
The results shown in Figs. 1, 2 and 4 are based on '
the examples.
(C) The reason why the amount of Cr in the intermediate
layer is limited in the present invention when the
intermediate layer is formed by casting will be described
below.
In a composite three-layer roll, the boundary
portion between the outer layer and the intermediate
layer and the boundary portion between the intermediate
layer and the inner layer are metallurgically joined. A
defect, such as internal shrinkage, must not be present
in any of these boundary portions. The present inventors:I
inspected the presence or absence of a boundary defect,
16
73461-64
_ ~~~~~~8
which could be inspected during manufacture from the
surface of the roll by ultrasonic flaw detection, of a
broken roll and of a unbroken abolished roll by cutting
the roll. They found that internal shrinkage was present
in the boundary portion of the broken roll between the
outer layer and the intermediate layer.
The mechanism of generation of internal shrinkage in
the boundary portion is as follows:
In centrifugal casting, a molten metal forming the
outer layer is first poured into a cooled metal mold.
Solidification starts from the outer side of the molten
metal which is contact with the metal mold. After the
entire molten metal is solidified, a molten metal forming.
the intermediate layer is poured. The poured molten
metal forming the intermediate layer melts the inner side
of the outer layer again. Thereafter, the melted outer
layer and the intermediate layer (in which the outer
" layer component is present as the result of remelting)
are solidified by cooling through the metal mold and
outer layer. At that time, if the melting point of the
outer layer is higher than or equal to the melting point
of the intermediate layer, solidification apparently
starts from the outer side which is close to the mold and
proceeds inwardly. If the melting point of the outer
layer is lower than the melting point of the intermediate
layer, the intermediate layer first solidifies, and then
17
CA 02166258 1997-11-14
the boundary portion between the outer layer and the
intermediate layer solidifies. At that time, internal
shrinkage may be generated depending on the shrinkage rate of
the outer layer. .
The present inventors centrifugally cast (140 G) an
outer layer first and then an intermediate layer, and finally
stationarily cast the axis material (the inner layer . a
spheroidal graphite cast iron), and inspected the presence or
absence of a boundary defect between the inner layer and the
intermediate layer by conducting ultrasonic flaw detection
tests on the outer layer and the intermediate portion from the
outer surface of the roil. It can be seen from Fig. 3 that
the range which does not substantially generate a boundary
detect is determined by
C(~) (intermediate layer) ? 2.0 - 0.5[C(~) - 0.2V(~) -
O.llNb($)] (outer layer) ~~~ (2)
C concentration of the intermediate layer in the
formula (2) is the value obtained during casting, and does not
include the amount of C contained in the intermediate layer as
the result of remelting of the outer layer. C, V and Nb
concentrations of the outer layer in the formula (2) are those
obtained during casting and are equivalent to those of the
manufactured outer layer.
In Fig. 3, the presence or absence of porosity in
18
73461-64
the boundary portion is determined by obtaining a
ultrasonic flaw detection index from the results of the
ultrasonic flaw detection test on the boundary portion
between the outer layer and the intermediate layer in the
manner described below. The ultrasonic flaw detection
index is a product of the total defective area obtained
by ultrasonic flaw detection and the reflection echo peak
rate of a standard defective sample. A reflection echo
peak rate 0.2 or above is regarded as "boundary portion
porosity is present". This is because when a ultrasonic
flaw detection test is conducted on the boundary portion
between the outer layer and the intermediate layer of the
roll, even if the boundary portion is not defective at
all, a reflection echo of about 0.15 is obtained due to a.
difference in the structure between the outer layer and
the intermediate layer.
The standard defective sample from which the
reflection echo peak rate was obtained was made of a
material having a composition which consisted of 4.2~ C,
0.5~ Si, 0.5~ Mn, 7.2~ Cr, 3.1~ Mo, 6.0~ V and 2.2~ Nb.
The sample had a thickness of 100 mm. A defect of 2 mm
was formed at a position of 50 mm in the direction of the
thickness of the sample.
A desirable amount of C in the intermediate layer is
0.5~ or above, although it is restricted by the above-
described formula. Less than 0.5~ C increases the
19
CA 02166258 1997-11-14
viscosity of the molten metal, making it impossible for
the molten metal to be uniformly distributed in the mold
during centrifugal casting. Accordingly, variations in
the amount of melted outer layer become too great, making
the use of a resulting roll impossible.
Examples
Example 1
Samples equivalent to roll outer layer materials and
having chemical compositions shown in Table 1 were each
melted, and quenched at temperatures which started from
1000 °C and then were tempered at 550°C to manufacture
sample materials. Wearing test was two-disk sliding
friction type which employed a sample material of X50 x
10 and the other material of X190 x 15. The test was
conducted by rotating the sample material at 800 rpm in a
state wherein it was pressed against the other material
heated to 900°C under a load of 100 kgf while the sample
material was water cooled. In order to increase the
surface damage of the sample material and to enable
relative evaluation of the coefficient of friction, the
wear test was conducted for 120 minutes at a sliding rate
of 14.2 to examine the amount of wear and the average
coefficient of friction.
The results of the wear test are shown in Table 2.
Samples B-1 to B-8 which do not satisfy the formula n
10.5 <- 6.5~C($) - 1.3~V(~) - 0.7~Nb(~) have
73461-64
CA 02166258 1997-12-O1
coefficients of friction which are about 30~ higher than
that of a high Cr cast iron. If such a material is used
for a hot rolling roll, a rolling load is increased, thus
causing surface roughening of the plate.
Samples D-1 to D-5 which do not satisfy the formula
6.5~C(~) - 1.3~V($) - 0.7~Nb(~ ) S 2~Cr(~) - 2 exhibit
greatly deteriorated wear resistance. In contrast to
these comparative examples, the examples according to the
present invention have coefficients of friction which are
similar to that of a high Cr cast iron, and wear
resistances which are 7 times that of a high Cr cast
iron.
Samples C-1 and C-2 are comparative examples in
which W is not present in the determined amount, and
sample C-3 is a comparative example in which Nb is not
present in the determined amount. It is apparent from
Table 2 that wear resistance of these comparative
examples is greatly reduced.
That is, it is possible according to the present
invention to provide a roll outer layer material for hot
rolling suitable for use in hot rolling finish front
stage which exhibits excellent wear resistance and a low
coefficient of friction and an hence excellent surface
roughening inhibiting property by optimizing the
composition of the outer layer material and by mutually
restricting the amounts of C, Cr, V and Nb.
21
73461-64
~~~~2~8
Table 1 - 1
(Example of the invention)
C Si Mn P S Mo Cr V Nb W Co Ni
A 2.5 0.9 0.5 0.03 0.014.2 8.1 3.5 0.8 - - -
1
A 2.6 0.5 0.4 0.03 0.013.8 6.5 4.3 1.1 - - -
2
A 2.9 0.3 0.5 0.01 0.015.2 7.0 5.1 1.4 - - -
3
A 3.0 0.4 0.6 0.01 0.012.4 8.3 3.6 1.0 - - -
4
A 3.0 0.5 0.5 0.03 0.032.5 8.4 3.5 1.1 - - 0.8
A 3.0 0.3 1.1 0.03 0.026.9 12.43.7 1.2 - - -
6
A 3.0 0.6 0.3 0.02 0.015.1 9.9 4.9 1.4 - - -
7
A 3.1 0.3 0.3 0.02 0.017.4 8.0 4.4 2.2 - - -
8
A 3.2 1.0 0.3 0.03 0.013.6 8.4 4.9 0.9 - - -
9
A10 3.2 0.9 0.9 0.04 0.014.5 9.5 4.2 3.8 - - -
All 3.2 1.2 0.9 0.03 0.017.8 7.8 6.6 2.4 - - -
A12 3.4 0.7 0.5 0.03 0.016.5 7.2 6.8 2.3 - - -
A13 2.9 0.3 0.4 0.05 0.033.5 8.5 5.0 1.4 - - -
A14 2.9 0.4 0.4 0.06 0.033.6 8.5 5.1 1.5 0.6 - -
A15 3.0 0.3 0.4 0.04 0.033.6 8.5 5.0 1.5 - 3.8 -
A16 2.9 0.4 0.4 0.02 0.017.4 18.64.8 1.4 - - -
A17 3.0 0.3 0.3 0.05 0.047.5 18.54.9 1.5 0.5 4.0 -
A18 3.1 0.4 0.5 0.02 0.014.4 14.85.0 1.4 - - -
A19 3.1 0.3 0.4 0.01 0.014.2 12.05.0 1.5 - - -
A20 3.8 1.1 0.6 0.03 0.025.3 13.49.1 3.2 - - -
A21 3.6 0.5 0.3 0.02 0.0111.2 17.36.2 1.8 - - -
A22 3.4 0.3 0.2 0.01 0.018.2 14.65.8 1.6 - - -
22
Table 1 - 2
(Comparative example )
C Si Mn P S Mo Cr V Nb W Co Ni
B 1 2.2 0.6 0.5 0.03 0.013.0 5.7 5.1 1.5 - - -
B 2 2.7 0.6 0.5 0.03 0.014.6 7.2 7.0 1.8 - - -
B 3 2.7 0.5 0.6 0.03 0.012.8 4.1 5.5 2.0 - - -
B 4 2.7 0.6 0.4 0.03 0.024.0 7.8 5.6 1.5 - - -
B 5 2.S 1.5 0.3 0.03 0.022.6 6.0 5.9 1.6 - - -
B 6 3.2 0.3 0.3 0.04 0.027.1 7.8 8.0 2.1 - - -
B 7 3.2 0.3 1.1 0.03 0.026.8 6.2 8.1 4.5 - - -
B 8 2.7 0.5 0.6 0.04 0.032.4 13.1 6.1 1.8 - - -
C 1 3.1 0.3 0.4 0.01 0.014.5 12.5 5.2 1.4 3.6 - -
C 2 2.9 0.3 0.4 0.04 0.023.6 8.6 5.0 1.4 1.9 - -
C 3 3.0 0.3 0.3 0.05 0.033.5 8.5 5.2 0.2 - - -
D 1 2.9 1.2 0.6 0.03 0.013.1 5.1 4.9 1.5 - - -
D 2 3.0 0.8 0.4 0.03 0.012.3 6.6 4.8 1.4 - - -
D 3 2.9 0.4 0.5 0.04 0.033.2 4.5 4.8 0.9 - - -
D 4 3.2 1.3 0.4 0.03 0.011.9 6.9 5.0 0.8 - - -
D 5 3.8 1.1 1.1 0.04 0.013.0 5.7 8.1 4.6 - - -
E 2.80 0.600.800.03 0.012.6017.8 - - - - -
(High
Cr
cast
iron)
23
Table 2 - 1
(Example of the invention)
6.5C - 1.3V -0.7Nb 2Cr -2 Amount Coefficient
of wear of friction
A 1 11.1 14.2 0.21 0.28
A 2 10.5 11.0 0.23 0.29
A 3 11.2 12.0 0.19 0.28
A 4 14.1 14.6 0.24 0.27
A 5 14.2 14.8 0.24 0.27
A 6 13.9 22.8 0.23 0.27
A 7 12.2 17.8 0.19 0.28
A 8 12.9 14.0 0.21 0.28
A 9 13.8 14.8 0.23 0.27
A10 12.7 17.0 0.21 0.28
All 10.5 13.6 0.19 0.29
A12 11.7 12.4 0.19 0.29
A13 11.4 15.0 0.16 0.27
A14 11.2 15.0 0.23 0.29
A15 12.0 15.0 0.16 0.27
A16 11.6 35.2 0.15 0.26
A17 12.1 35.0 0.15 0.27
A18 12.7 27.6 0.15 0.26
A19 12.6 22.0 0.14 0.26
A20 10.6 24.8 0.16 0.29
A21 14.1 32.6 0.17 0.26
A22 13.4 27.2 0.15 0.26
24
2i662~~
Table 2 - 2
(Comparative example)
6.5C - 1.3V - 2Cr - Amount Coefficient
0.7Nb 2 of wear of friction
B 1 6.6 9.4 0.19 0.39
B 2 7.2 12.4 0.18 0.39
B 3 9.0 6.2 0.52 0.37
B 4 9.2 13.6 0.24 0.36
B 5 9.4 10.0 0.39 0.36
B 6 8.9 13.6 0.22 0.38
B 7 7.1 10.4 0.19 0.39
B 8 8.4 24.2 0.48 0.37
C 1 12.4 23.0 0.62 0.31
C 2 11.4 15.2 0.50 0.28
C 3 12.6 15.0 0.54 0.27
D 1 11.4 8.2 0.63 0.28
D 2 12.3 11.2 0.52 0.28
D 3 12.0 7.0 0.67 0.28
D 4 13.7 11.8 0.58 0.27
D 5 11.0 9.4 0.55 0.29
E 18.2 33.6 1.62 0.28
(High Cr
cast iron
CA 02166258 1997-12-O1
Example 2
Composite rolls, having a diameter of 800 mm and a
length of 2400 mm, were each manufactured by centrifugally
casting (140 G) first an outer layer material (having a
thickness of 100 mm) and then an intermediate layer material
(having a thickness of 40 mm), and then by static casting an
inner layer material. Table 3 shows the compositions of the
outer layers and intermediate layers. The inner layer
material of each of the composite rolls was a spheroidal
graphite cast iron. Table 4 shows the boundary portion
porosity index of each roll. Samples G-1 to G-6 are
comparative examples which do not satisfy the formula (2).
C (~) (intermediate layer) ? 2 . 0 - 0 . 5 [C (~) - 0 .2V (~) -
O.llNb(~)] (outer layer) ~~~ (2)
In these comparative examples having high ultrasonic
flaw detection detect indexes, D2 and D5 rolls were broken
during manufacture and G2 to G5 rolls were broken during heat
treatment. Thus, the use of G-1 to G-5 rolls was suspended
because there was the possibility that an accident would occur
during rolling.
The results of the inspection of internal porosity
are shown in Fig. 3. It was found that when the amount of C
in the intermediate layer was a value determined by C(~S) ? 2.0
- 0.5 (C(~) - 0.2V(~) - O.llNb(~)) (components of the outer
layer material), a roll for hot rolling having no
26
73461-64
internal defect could be manufactured.
Table 3 - 1
(Example of the invention)
C Si Mn Mo Cr V Nb Co W
F Outer Layer 2.6 0.9 0.54.0 8.5 3.30.7 -
1
Intermediate 1.2 1.5 0.6- - - - - -
Layer
F Outer Layer 2.6 0.6 0.54.2 6.8 4.30.8 - -
2
Intermediate 1.2 1.6 0.6- - - - - -
Layer
F Outer Layer 3.0 0.3 0.33.5 8.3 4.81.4 - -
3
Intermediate 1.5 2.0 0.8- - - - - -
Layer
F Outer Layer 3.0 0.3 1.15.1 12.2 5.11.6 - -
4
Intermediate 1.5 1.5 0.5- - - - -
Layer
F Outer Layer 3.1 0.6 0.35.1 9.9 3.30.8 - -
Intermediate 1.1 1.6 0.6- - - - - -
Layer
F Outer Layer 3.4 0.3 0.37.4 10.1 5.52.2 - -
6
Intermediate 1.0 1.4 0.7- - - - - -
Layer
F Outer Layer 3.5 1.0 0.77.6 10.5 3.61.1 3.1 0.4
7
Intermediate 0.8 1.5 0.5- - - - - -
Layer
27
_ ~~~s~~
Table 3 - 2
(Comparative Example)
C Si Ma Mo Cr V Nb Co W
G Outer Layer 2.7 0.6 0.5 4.1 8.5 3.4 0.9 - -
1
Intermediate 0.9 1.5 0.6 - - - - - _
Layer
G Outer Layer 2.6 0.5 0.6 4.2 6.6 4.5 0.9 - -
2
Intermediate 1.1 1.6 0.6 - - - - -
Layer
G Outer Layer 2.8 0.3 0.3 5.1 8.5 4.9 1.4 - -
3
Intermediate 0.9 1.4 0.7 - - - - - _
Layer
G Outer Layer 3.1 0.8 0.4 5.1 10.04.3 0.7 - -
4
Intermediate 0.7 1.5 0.5 - - - - - -
Layer
G Outer Layer 3.8 1.3 0.4 7.5 10.25.3 2.3 - -
Intermediate 0.7 1.6 0.6 - - - - - -
Layer
G Outer Layer 2.2 0.3 0.3 2.5 6.2 5.3 1.6 - -
6
Intermediate 1.4 2.0 0.8 - - - - _ -
Layer
28
__ ~lss2~s
Table 4
C Z in 2-0.5(C-0.2V-O.llNb)Boundary
intermediate Layer
Layer porosity
index
F 1.2 1.07 (outer 0 Example
1 layer) of the
invention
F 1.2 1.17 (outer 0.1
2 layer)
F 1.5 1.06 (outer 0
3 layer)
F 1.5 1.10 (outer 0
4 layer)
F 1.1 0.82 (outer 0
layer)
F 1.0 0.97 (outer 0
6 layer)
F 0.8 0.67 (outer 0
7 layer)
G 0.9 1.04 (outer 64 Comparative
1 layer)
Example
G 1.1 1.20 (outer 49
2 layer)
G 0.9 1.17 (outer 138
3 layer)
G 0.7 0.92 (outer 57
4 layer)
G 0.7 0.76 (outer 18
5 layer)
G 1.4 1.52 (outer 6
5 layer)
29
__
Example 3
Samples equivalent to roll outer layer materials and
having chemical compositions shown in Table 5 were made
by melting and casting, and quenched at temperatures
which started from 1000 °C and were then tempered at
550°C to manufacture sample materials. A two-disk
sliding friction test, which employed a sample material
of X50 x 10 and the other material of X190 x 15, was
conducted at a sliding rate of 14.2 for 200 minutes by
rotating the sample at 800 rpm in a state wherein it was
pressed against the other material heated at 800°C under
a pressure of 130 kg while the sample was water cooled.
The surface of each of the samples subjected to the test
was SEM observed to examine the number of cracks
generated in the carbide.
The relationships between the number of cracks
generated and the amounts of Mo and Cr are shown in Fig.
4. The number of cracks generated is reduced by half at
2~ or above of Mo. Up to about 15~ of Mo, the more the
amount of Mo, the less the number of cracks generated.
The number of cracks generated in a 9~ Cr material is
less than that in a 7~ Cr material. Thus, the effect of
Cr addition is discerned.
Cracks in the carbide are generated by synergism of
rolling and heat fatigue and sliding stress during the
test. Thus, the results obtained by the experiments are
21~~2~~
a simulation of the fatigue which will occur in the roll
surface in an actual rolling, and thus clarify that the
materials according to the present invention exhibit
excellent banding resistance.
Table 5
C Si Mn Cr Mo V Nb Mo/Cr
7Z Cr 2.9 0.3 0.3 7.1 1.2 5.9 2.0 0.17 Comparative
Example
material
3.0 0.3 0.3 7.2 2.2 5.8 2.2 0.31 Example of
the
invention
2.9 0.4 0.3 7.2 4.1 6.1 2.1 0.57
9I Cr 3.0 0.3 0.4 9.2 2.3 6.0 2.0 0.25
material
3.1 0.4 0.3 9.1 3.2 6.0 2.0 0.35
3.1 0.3 0.3 9.1 4.8 6.1 2.1 0.53
3.2 0.4 0.3 9.0 7.6 6.0 2.1 0.84
2.9 0.4 0.4 9.1 11.4 6.2 2.0 1.25
12T 3.2 0.4 0.2 12.12.2 5.9 2.0 0.18
Cr
material
3.2 0.4 0.2 12.03.1 5.8 2.2 0.26
3.1 0.3 0.3 12.05.0 6.1 2.1 0.42
3.2 0.4 0.3 12.18.9 6.0 2.0 0.74
3.1 0.4 0.2 12.012.3 6.0 2.0 1.03
15Z 3.2 0.3 0.4 15.02.5 6.1 2.1 0.17
Cr
material
3.2 0.4 0.3 14.93.9 6.0 2.1 0.26
3.3 0.3 0.3 15.014.1 6.2 2.0 0.94
31
CA 02166258 1997-12-O1
Example 4
In the composite rolls shown in Table 3, heat
treatment consisting of quenching which started from 1050°C
and hardening at 550°C was conducted on examples F3 and F4
according to the present invention and comparative example G6.
Each of the rolls obtained was used in the second stand in an
actual hot strip mill.
It was observed in actual rolling that the rolling
load of examples F3 and F4 according to the present invention
was equivalent to that of a high Cr cast iron of a
conventional roll, as shown in Fig. 6, and that wear
resistance of the examples according to the present invention
was at least 6 times that of a high Cr cast iron, as shown in
Fig. 7.
Comparative example G6 involves a material in which
the composition of the outer layer did not satisfy the
restricted range, namely 6.5C(~) - 1.3V(~) - 0.7Nb(~) - 6.29.
The rolling load of comparative example G6 was about 20°s
higher than that of a high Cr cast iron roll, and thus surface
roughening of the rolled material occurred. Wear resistance
of comparative example G6 was inferior to that of the examples
according to the present invention.
It is therefore possible according to the present
invention to provide a roll for hot rolling by centrifugal
casting exhibiting excellent productivity and
32
73461-64
__ 2~.~~02~~
economical properties in which an outer layer thereof
exhibits excellent wear resistance, a low coefficient of
friction and hence an excellent surface roughening
inhibiting property, and excellent banding resistance,
and a greatly reduced breakage accident occurrence during
manufacture or rolling.
33
