Note: Descriptions are shown in the official language in which they were submitted.
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A METHOD OF PRODUCING A STEEL PRODUCT
FIELD OF THE INVENTION
The invention relates to a steel shell for a suction roll and a method of
producing a
steel product, in which method a piece of steel material is worked by a
cutting operation such
as milling, turning and/or drilling.
BACKGROUND OF THE INVENTION
Stainless steel is used in fields in which a high corrosion resistance is
necessary. A
high corrosion resistance may be required in environments within off-shore,
paper and pulp
industry and chemical industry. One example is suction roll shells for paper
machines, that are
manufactured from stainless steel. One type of stainless steel is the so
called duplex steels
that contain ferrite and austenite. Duplex steels are known to combine a high
mechanical
strength and toughness with a good corrosion resistance, in particular in
terms of stress
corrosion and corrosion fatigue. For corrosion resistance as well as
mechanical properties
such as weldability, it is important that the steel is well balanced in terms
of the essential
components austenite and ferrite. In modern development of duplex steels, it
is desired to
have a micro-structure containing 35-65 % ferrite, the remainder being
austenite. In fields
requiring high strength and good corrosion resistance, duplex steels are
increasingly
competing with traditional austenite stainless steels. Such a steel material
is described in
published US Patent Application No. 2003/0172999. The steel material described
in this
publication is a ferrite-austenite stainless steel having a micro-structure
essentially consisting
of 35-65 % by volume ferrite and 35-65 % by volume austenite. The steel in
question has a
chemical composition containing 0.005-0.07 C, 0.1-2.0 Si, 3-8 Mn, 19-23 Cr,
0.15-0.30 N and
0.5-1.7 Ni, in % by weight. Some other components may also be included.
Nitrogen is of considerable importance to the steel described in US
2003/0172999,
since nitrogen is dominant as austenite former and contributes to the strength
of the steel as
well as to its corrosion resistance. For this reason, it was estimated that
the nitrogen content
of the steel should be in the range of 0.15-0.30 %, and preferably in the
range of 0.20-0.24
%. However, it has been previously shown that steel types of such a high
nitrogen content are
poor in cuttability.
Most often, a stainless steel intended to be used for a particular product
must be
subjected to some type of cutting operation, such as milling, turning or
drilling. In themselves,
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,
austenite and duplex stainless steels are poor in cuttability and hence
various measures are
undertaken in order to increase cuttability of the stainless steel. It is
previously known that the
presence of nitrogen in stainless steel decreases cuttability. In for example
US Patent No.
4,769,213, a method is suggested for increasing cuttability of a martensite
stainless steel by
reducing carbon and nitrogen contents such that the total content of carbon
and nitrogen
together is not more than 0.05 % by weight. However, compared with duplex
steels,
martensite steels have a poorer corrosion resistance. For austenite stainless
steels, it is
suggested in US Patent No. 5,482,674 that the content of carbon and nitrogen
should be
reduced such that neither the content of carbon nor the content of nitrogen is
more than about
0.035 % by weight. It is also known that the addition of sulphur may increase
cuttability.
Accordingly, US Patent No. 4,784,828 suggests that sulphur should be added to
an austenite
stainless steel in order to increase cuttability. It is also stated that the
contents of carbon and
nitrogen should be very low, in total up to 0.065 % by weight. However,
compared with duplex
steels, austenite steels have a lower strength.
US Patent No. 4,964,924 suggests use of a martensite stainless steel in a
suction roll,
in that publication, it is stated that since they are difficult to drill,
stainless ferrite-austenite
duplex steels are unsuitable as materials for suction rolls. Instead, it is
suggested that a
stainless steel suitable for a suction roll shell should be of martensite
type, among other things
containing carbon at a % by weight of more than 0 but not more than 0.06,
silicon at a % by
weight above 0 but not more than 2, manganese at a % by weight above 0 but not
more than
2, nickel at 3-6 % by weight, chromium at 14-17 % by weight, molybdenum at 1-3
% by weight
and copper at a A) by weight of from 0.5 to 1.5.
The present invention aims at providing a solution to the problem of finding a
steel
material that exhibits a high strength as well as a good corrosion resistance,
and that
moreover is suitable for cutting operations without having to be subjected to
sulphur addition
treatment. It is also an object of the invention to provide a suction roll
shell with good corrosion
resistance, which is easy to manufacture by cutting operations.
SUMMARY OF THE INVENTION
Surprisingly, the present inventors have found that a steel material of the
type
described in above mentioned US 2003/0172999, not only has a high strength and
a good
corrosion resistance, but that the material in question also is suitable for
cutting operations
such as turning, milling and drilling, without the material in question having
been treated by
addition of sulphur. The inventors have also found that the material in
question is particularly
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suitable as a material for paper machine suction rolls, and that it is
advantageous to
manufacture a suction roll shell of such a material. Accordingly, the
invention relates to a
suction roll shell of this material. The invention can also be understood as a
method for cutting
operations, in particular when manufacturing suction roll shells, but also in
manufacturing of
other products, e.g. rotating machine parts, such as shafts. The invention can
also be defined
in terms of a use of said steel as a workpiece in cutting operation of steel.
Hence, the invention relates to a suction roll shell having a plurality of
through holes.
The suction roll shell according to the invention is made of a stainless
ferrite-austenite steel
having a micro-structure essentially consisting of 35-65 % by volume of
ferrite and 35-65 %
by volume of austenite. The steel composition will be described in greater
detail in the detailed
description.
The invention also relates to a suction roll comprising the inventive suction
roll shell.
According to an aspect of the invention, there is provided a method of
producing a
steel product, which method comprises providing a steel workpiece, and working
of the
workpiece by cutting operation including drilling, wherein the steel of the
workpiece is a
stainless ferrite-austenite steel having a micro-structure consisting of 35-65
% by volume of
ferrite and 35-65 % by volume of austenite, and having a chemical composition
containing
0.005-0.07 C, 0.1-2.0 Si, 3-8 Mn, 19-23 Cr, 0.5-1.7 Ni, 0.15-0.30 N, in % by
weight.
In a preferred embodiment, the cutting operation comprises drilling of at
least one
through hole, and preferably drilling of a plurality of holes. In a
particularly advantageous
embodiment, the method comprises drilling of hundreds of thousands of holes. A
corresponding drilled length is several kilometres. The cutting operation may
also comprise
turning of outside and inside faces of the shell.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows the bending of a blank for a suction roll shell.
Fig. 2 shows a blank having been bent and welded together to form a shell.
Fig. 3 shows schematically a first step of working the shell shown in Fig. 2.
Fig. 4 shows a second step of working the shell.
Fig. 5 shows a completed suction roll shell.
Figs. 6-9 show the result of comparative tests in which the steel used
according to the
invention is compared with other steels in terms of cuttability.
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DETAILED DESCRIPTION OF THE INVENTION
In the following, the manufacturing of suction roll shells is schematically
described.
With reference to Fig. 1, a first step in the manufacturing of a suction roll
shell is shown. As
is shown in Fig. 1, an essentially planar blank 1 is roller bent between two
rollers 2, 3, as is
known as such and need not be described in greater detail herein. After
bending to an
essentially circular shape, the ends of the blank 1 are welded together such
that a weld joint
4 unites the blank 1 to form a segment 9. A plurality of segments are then
united by circular
joints to form a shell that is heat treated after the welding. Fig. 3 shows
how the thus achieved
shell 9 can be subjected to a working operation, such as turning. Fig. 3 shows
a turning tool
5 acting on the face of the shell 9. The object of the turning operation is to
ensure that the face
of the shell 9 is smooth and regular. Fig. 4 shows schematically a subsequent
step in the
manufacturing process, in which the shell 9 is drilled by a drill 6, whereby
the shell is provided
with a number of through holes 7. Fig. 5 shows the completed suction roll
shell 8 with its
circular cylindrical shell 9 and the through holes 7 thereof. Fig. 5 also
shows schematically that
the ends of the suction roll shell 8 can be closed by side covers 10. When the
suction roll shell
8 is used, its interior will be connected to a vacuum source (not shown),
which results in air
being drawn from the outside and in via the through holes 7. Only a few holes
are shown in
the drawings. It should be realised however that in real applications the
number of holes can
be very large, such as 100,000 holes or more. Suction roll shells have
previously been
manufactured from a material sold under the name 3RE60 Avesta SRG. This steel
is a
stainless ferrite-austenite steel that has been improved in respect of
cuttability by sulphur
treatment and that has the following typical composition in % by weight.
0.02
Si 1.50
Cr 18.5
Ni 4.90
Mo 2.80
0.08
0.02
With good results, steel 3RE60 has been used for about 30 years for the
manufacturing of suction roll shells, and about 10 years ago it was provided
with an additive
for improved cuttability and its name was changed to 3RE60 SRG. Nowadays, the
steel is
called 3RE60 Avesta SRG.
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It has now been surprisingly shown that another ferrite-austenite steel exists
that has
in addition a high nitrogen content, and that has equally good or in some
respects even better
cuttability than the cuttability-improved 3RE60 Avesta SRG. This steel has a
microstructure
essentially consisting of 35-65 % by volume of ferrite and 35-65 % by volume
of austenite, and
its chemical composition contains in % by weight:
0.005
Si 0.1-2.0
Mn 3-8
Cr 19-23
Ni 0.5-1.7
0.15-0.3
A steel that is particularly suitable for this application suitably contains:
optionally Mo
and/or W at a total content of no more than 1.0 (Mo + W/2), optionally Cu up
to a maximum
of 1.0 Cu, balance being iron and impurities. For the ferrite and austenite
formers in the alloy,
i.e. chromium and nickel equivalents, the following conditions should
preferably be true:
< Creeq < 24.5
10 < Nieq, where
Creq = Cr + 1.5 Si + Mo + 2 Ti + 0.5 Nb
Nieq = Ni + 0.5 Mn + 30 (C+N) + 0.5 (Cu + Co).
20 In an advantageous embodiment, the steel contains 0.02-0.05 C.
Suitably, the steel
contains 0.18-0.26 N and advantageously 20-23 Cr. In a preferred embodiment,
the steel
contains 0.8-1.70 Ni, and even more preferred 1.35-1.7 Ni.
A steel of this composition is described in published US Patent Application
No.
2003/0172999.
In a particularly advantageous embodiment of the invention, the steel contains
0.22 N,
21.5 Cr, 1.5 Ni, 0.3 Mo, 5 Mn and not more than 0.04 C. Such a steel is sold
by Outokumpu
Stainless AB, Box 74, SE-774 22, AVESTA. This steel is sold by Outokumpu under
the name
LDX 21010. The name is a trademark registered in the European Union.
Accordingly, the LDX
2101 steel is particularly suitable to be used in a suction roll shell.
Particularly suitable
contents of copper and silicon are 0.3 Cu and 0.7 Si, respectively. The
guideline values 0.3
Cu and 0.7 Si (in % by weight) are used for LDX 21010.
Compared with e.g. steel 3RE60 Avesta SRG, the steel of the type mentioned
above
has a relatively high nitrogen content. As it is known that nitrogen tends to
impair cuttability,
it would be expected that cuttability is poorer. However, it has been
surprisingly shown that the
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cuttability of the steel used according to the present invention is
considerably higher than
expected.
Fig. 6 shows the results of a comparative test in which an LDX 21010 steel
were
compared with two other, cuttability-improved, austenite steels called 304L
PRODECO and
316L PRODECO, respectively. The steel 304L PRODECO has the following
composition in
% by weight:
C 0.02
Si 0.5
Mn 1.5
Cr 18.2
Ni 8.4
Mo essentially none
N 0.07
S 0.02
The steel 316L PRODECO has the following composition:
C 0.02
Si 0.5
Mn 1.5
Cr 17.2
Ni 11.2
Mo 2.3
N 0.05
S 0.02
As the nitrogen content of both cuttability-improved austenite steels 304L
PRODEC0
an 316L PRODEC0 is considerably lower than in an LDX 21010 steel, it would
normally be
expected for these steels to be better in cuttability than an LDX 2101 steel.
In turning tests
it was however shown that for an LDX 21010 steel, working time 30 minutes,
high-speed steel
tools being used, a considerably higher cutting speed was possible as compared
with the other
two steels, which is shown in Fig. 6.
Fig. 7 shows the results of an additional comparative test between an LDX
21010 steel
and steels 304L PRODEC0 and 316L PRODECO. Fig. 7 shows a test with a working
time of
15 minutes, in which turning was made by a cutting edge of hard metal. Under
these
circumstances, a cutting speed was achieved for an LDX 21020 steel that was
somewhat
lower in comparison with the other two steels. The difference is however
marginal.
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Fig. 8 shows another test in which the steel LDX 21010 is compared with a
conventional duplex steel sold under the name 2205. This steel, which is more
highly alloyed
than LDX 21010, is standardized (EN 1.4462) and is used in a great number of
applications.
It has no cuttability-improving additives and is not used for this type of
suction roll shells. 2205
has the following composition:
0.02
Si 0.4
Mn 1.5
Cr 22.2
Ni 5.7
Mo 3.1
0.17
0.001
In the test, a comparison was made in terms of the useful life of the tool
when milling
with a cutting edge of hard metal. As is evident from Fig. 8, the useful life
of the tool was
considerably longer when working an LDX 21010 steel as compared with working
of the steel
2005.
Finally, yet another test is shown in Fig. 9. In the test shown in Fig. 9, an
LDX 21010
steel was compared with three other steel types used for suction roll shells,
i.e. 2304 Avesta
SRG, 3RE60 Avesta SRG and 2205 Avesta SRG. All steels designated SRG (Suction
Roll
Grade) are cuttability-improved by sulphur addition. The steel 2304 Avesta SRG
has the
following typical composition:
0.02
Si 0.8
Mn 1.5
Cr 22.7
Ni 4.7
Mo 0.3
0.09
S 0.02
The steel 2205 Avesta SRG has the following typical composition:
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0.017
Si 0.6
Mn 1.35
Cr 22.0
Ni 5.7
Mo 2.9
0.13
0.02
In the test shown in Fig. 9, a comparison was made in respect of the cutting
speed that
can be achieved for a drilled length of 1000 mm without tool failure, in
different materials. As
is evident from Fig. 9, LDX 2101 is considerably better than the cuttability-
improved steels
2205 Avesta SRG and 2304 Avesta SRG, and in this respect it is equally good as
the
cuttability-improved steel 3RE60 Avesta SRG, despite the fact that LDX 2101
contains
considerably more nitrogen than the steel 3RE60 Avesta SRG. It is a clear
technical
advantage if cuttability of the material can be achieved without so called
cuttability-improving
additives such as sulphur, as these lead to a number of drawbacks such as
impaired rollability
and impaired corrosion resistance.
It is to be understood that although the invention has been described in terms
of a
suction roll shell and a method, those are just different aspects of one and
the same invention,
as the method according to the invention is suited to be used for
manufacturing of the suction
roll shell according to the invention.
By the invention, the advantage is obtained, among other things, that the
completed
roll shell achieves a very good corrosion resistance.
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