STEEL SHELL FOR A SUCTION ROLL AND A METHOD OF PRODUCING A STEEL PRODUCT

Abstract

A suction roll shell having a plurality of through holes 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 has a composition containing, among other things, 0.005-0.07 C and 0.15-0.30 N, in % by weight.

Description

TECHNICAL FIELD

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 modem 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 och 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 their selves, 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 U.S. Pat. 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 U.S. Pat. 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, U.S. Pat. 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.

U.S. Pat. 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 % 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.

ACCOUNT 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 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.

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.

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 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.

C  0.02
Si 1.50
Cr 18.5
Ni 4.90
Mo 2.80
N  0.08.
S  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.

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:

C  0.005
Si 0.1-2.0
Mn 3-8
Cr 19-23
Ni 0.5-1.7
N  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:

20<Cr_eeq≦24.5

10<Ni_eq, where

Cr_eq=Cr+1.5 Si+Mo+2 Ti+0.5 Nb

Ni_eq=Ni+0.5 Mn+30(C+N)+0.5(Cu+Co).

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 2101®. 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 2101®.

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 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 2101® steel were compared with two other, cuttability-improved, austenite steels called 304L PRODEC® and 316L PRODEC®, respectively. The steel 304L PRODEC® 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 PRODEC® 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 PRODEC® an 316L PRODEC® is considerably lower than in an LDX 2101® 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 2101® 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 2101® steel and steels 304L PRODEC® and 316L PRODEC®. 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 2102® steel that was somewhat lower in comparison with the other two steels. The difference is however marginal.

FIG. 8 shows another test in which the steel LDX 2101® is compared with a conventional duplex steel sold under the name 2205. This steel, which is more highly alloyed than LDX 2101®, 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:

C  0.02
Si 0.4
Mn 1.5
Cr 22.2
Ni 5.7
Mo 3.1
N  0.17
S  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 2101® 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 2101® 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:

C  0.02
Si 0.8
Mn 1.5
Cr 22.7
Ni 4.7
Mo 0.3
N  0.09
S  0.02

The steel 2205 Avesta SRG has the following typical composition:

C  0.017
Si 0.6
Mn 1.35
Cr 22.0
Ni 5.7
Mo 2.9
N  0.13
S  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.

Claims

1-19. (canceled)

20. A suction roll shell with a plurality of drilled through holes, wherein the shell is formed of a plurality of segments united by circular joints and subjected to a working by turning and cutting in order to have through holes in the shell, and the shell is made of a stainless ferrite-austenite steel having a microstructure essentially 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, and wherein the steel does not contain additives for improving cuttability.

21. A suction roll shell according to claim 20, wherein the steel contains up to 1.0 (Mo+W/2) and up to 1.0 Cu, balance being iron and impurities, and for the chromium and nickel equivalents 20<Creq<24.5,10<Nieq,where Creq=Cr+1.5 Si+Mo+2 Ti+0.5 Nb,and Nieq=Ni+0.5 Mn+30(C+N)+0.5 (Cu+Co)

22. A suction roll shell according to claim 20, wherein the steel contains 0.02-0.05 C.

23. A suction roll shell according to claim 20, wherein the steel contains 0.18-0.26 N.

24. A suction roll shell according to claim 20, wherein the steel contains 20-23 Cr.

25. A suction roll shell according to claim 20, wherein the steel contains 0.8-1.70 Ni.

26. A suction roll shell according to claim 25, wherein the steel contains 1.35-1.7 Ni.

27. A suction roll shell according to claim 20, wherein the steel contains 0.22 N, 21.5 Cr, 1.5 Ni, 0.3 Mo, 5 Mn, no more than 0.04 C.

28. A suction roll, comprising a suction roll shell according to claim 20.

29. A suction roll shell according to claim 20, wherein the steel contains 0.3 Cu and 0.7 Si.

30. A suction roll shell according to claim 20, wherein the drilled through holes can be drilled with an uncoated high-speed twist drill for a drilled length of 1000 m at a cutting speed of 36 m/min with tool failure.