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(11) | EP 0 178 334 B1 |
| (12) | EUROPEAN PATENT SPECIFICATION |
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| (54) |
Martensitic stainless steels for seamless steel pipe Rostfreie martensitische Stähle für nahtlose Rohre Aciers inoxydables martensiques pour tubes d'acier sans soudure |
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| Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention). |
[0001] This invention relates to the use of martensitic stainless steels for the manufacture of seamless steel pipe such as oilwell pipe and pipeline tubing. Martensitic stainless steels, two representative grades of which are SUS 410 and SUS 420 (Japan Industrial Standard [JIS] designations), have excellent corrosion resistance in highly corrosive environments containing CO2, These materials are thus regarded as excellent candidates for use in oil-well pipe, geothermal well pipe, and pipeline tubing.
[0002] The strength of oil-well pipe is normally required to be least equivalent to that of American Petroleum Institute (API) standard L80 grade steel (yield strength ? 80 ksi). Pipeline tubing should generally have a strength at least equivalent to that of API standard X60 grade steel (yield strength ? 60 ksi).
[0003] Martensitic stainless steels having a variety of strengths can be obtained by the application of specific types of heat treatment, such as quench-tempering, normalizing-tempering, or just tempering. However, it is known that the resistance to stress-corrosion cracking of martensitic stainless steels in CO2-containing environments falls when tempering is performed at a temperature of less than 600°C. We tempered the martensitic stainless steels in Table 1 at various temperatures and cut out test pieces 120 mm long by 20 mm wide by 3 mm thick. These pieces were subjected to U-bend tests at a bending radius of 15 mm in a 3.5% aqueous solution of NaCI heated to 80°C and having a carbon dioxide partial pressure of 1 atmosphere. As shown in Table 1, each of these steels developed stress-corrosion cracking when tempered at less than 600°C, but none demonstrated stress-corrosion cracking when tempered at 600°C or more (a cross "X" in Table 1 indicates the presence of stress-corrosion cracking; an open circle "0" indicates the absence of stress-corrosion cracking).
Hence, to achieve good resistance to stress-corrosion cracking in CO2-containing environments, an important property of martensitic stainless steels, it is clear that the steel must be tempered at a temperature of at least 600°C.
[0004] In the EP-A-0 081 592 a martensitic stainless steel material for disc brake of motorcycle is known, which consists of 0.04 to 0.10% by weight of C + N; 0 to 0.5% by weight of Si; 1.0 to 2.5% by weight of Mn; 10.0 to 14.5% by weight of Cr; 0 to 0.5% by weight of Ni; 0 to 0.5% by weight of Cu and the remainder thereof being Fe. The fabricating of the disc brakes is performed by melting the steel and subjecting to hot rolling and annealing to prepare a hot rolled annealed sheet. The sheet is then hardened by applicating a temperature of 800 to 1050°C.
[0005] As well known, the strength of martensitic stainless steels decreases as the ferrite content of the steel structure increases. When the ferrite content at 1200°C exceeds 40%, the ferrite content in the normal quenching or normalizing temperature range of 900-1000°C rises to 20% or more, making it difficult to achieve the high strength required in pipeline tubing and oil-well pipe by temperature at 600°C or more. Accordingly, to allow tempering to be performed at the temperatures of 600°C or more necessary to impart good resistance to stress-corrosion cracking, and at the same time satisfy the high-strength requirements for use in pipeline tubing and oil-well pipe, martensitic stainless steels must be composed of not more than 40% ferrite at 1200°C.
[0006] Compositions in which the austenite phase (which becomes martensite at room temperature) exists in combination with a ferrite phase comprising 20-30% of the composition have the worst hot workability. when the amount of ferrite is about 40%, the hot workability is about the same as that of austenitic single-phase steels (which become martensitic single-phase steels at room temperature or below the Ms point). The hot workability rises sharply with increasing ferrite content above this point. Thus, because martensitic stainless steels with a ferrite content of 40% or less at 1200°C have inferior hot workability, their use in the production of high-strength seamless steel pipe by the processing described below tends to result in defects, complicating pipe manufacture.
[0007] Seamless stainless steel pipe is generally produced either by an inclined rolling method such as the plug mill or mandrel mill process, or by a hot extrusion method, of which the Ugine-Sejournet and Erhart pushbench processes are typical. However, certain types of matensitic stainless steels (namely, those with a ferrite content of 40% or less at 1200°C), have poor hot workability. When seamless steel pipe is manufactured from these steels by a cross rolling process such as the plug mill process or the mandrel mill process, defects arise on both the outside and inside walls of the pipe during piercing of the billet on a piercing mill. For this reason, seamless pipe made of this type of steel is generally produced by a hot extrusion process, such as the Ugine-Sejournet process.
[0008] However, when a hot extrusion process is employed and the billet directly pierced (a process known direct piercing), a billet length 5-7 times the diameter results in a greater eccentricity in the wall thickness of the pipe. This makes it difficult to produce long pipe. A partial solution to this problem is provided by a process for producing long pipe that makes use of what is known as the expansion method. This method consists of mechanically opening a guide hole in the center of the billet, then extending the hole. However, even with the use of this expansion method the billet length is still limited to only about 15 billet diameters. Another problem concerns the glass lubricant used in the Ugine-Sejournet hot extrusion process. This must be peeled off following rolling, a process that is both time-consuming and costly.
[0009] The limits on billet length inherent in the Ugine-Sejournet and other hot extrusion processes make it impossible to raise producitivity above a certain level. Moreover, the use of short billets inevitably results in a low yield and is therefore also disadvantageous in terms of cost. In contrast, both the plug mill and mandrel mill processes involve piercing the billet on a piercing mill that utilizes the Mannesmann effect. These processes permit the manufacture of longer pipe than is possible by the Ugine-Sejournet and other hot extrusion processes. These processes are thus known to be advantageous in terms of productivity and cost. However, as indicated above, certain types of martensitic stainless steels are not suitable for use in the production of seamless pipe on account of the formation of defects during pipe manufacture.
[0010] Object of the invention is to enable the practical application of the plug mill and mandrel mill process in martensitic stainless steels having a ferrite content of 40% or less at 1200°C, for the manufacture of seamless steel pipe without formation of defects during pipe fabrication at high productivity and low cost.
[0011] The object is solved by the use of martensitic stainless steel consisting of not more than 0.30% by weight of C, not more than 1.0% by weight of Si, not more than 2.0% by weight of Mn, 11-14% by weight of Cr, 0.005―0.10% by weight of AI, and not more than 0.10% by weight of N, wherein the amount of P is held to no more than 0.02% by weight and the amount of S to no more than 0.003% by weight, the remainder thereof being Fe and unavoidable impurities, said steel having a ferrite content of not more than 40% by weight at 1200°C for manufacture of seamless steel pipe by the Mannesmann plug mill or mandrel mill process.
[0012] This invention also contemplates the use of martensitic stainless steel consisting of not more than 0.30% by weight of C, not more than 1.0% by weight of Si, not more than 2.0% by weight of Mn, 11-14% by weight of Cr, 0.005―0.10% by weight of Al, not more than 0.01% by weight of N, and one or more elements selected from the group consisting of up to 3.5% by weight of Ni, up to 2.0% by weight of Cu, up to 2.5% by weight of Mo, up to 0.10% by weight of Nb, and up to 0.10% by weight of V, the remainder thereof being Fe and unavoidable impurities, said steel having a ferrite content of not more than 40% by weight at 1200°C for manufacture of seamless steel pipe by the Mannesmann plug mill or mandrel mill process.
[0013] The present invention furthermore contemplates the use of martensitic stainless steel consisting of not more than 0.30% by weight of C, not more than 1.0% by weight of Si, not more than 2.0% by weight of Mn, 11-14% by weight of Cr, 0.005―0.10% by weight of Al, not more than 0.10% by weight of N, and one or more elements selected from the group consisting of rare earth elements, Ca, and B, the amount of the rare earth elements ranging from 4 x (% of S) to 20 x (% of S), that of Ca from 1 x (% of S) to 10 x (% of S), and that of B from 0.001 to 0.008% by weight, wherein the amount of P is held to no more than 0.02% by weight and the amount of S to no more than 0.003% by weight, the remainder thereof being Fe and unavoidable impurities, said steel having a ferrite content of not more than 40% by weight at 1200°C, for manufacture of seamless steel pipe by Mannesmann plug mill or mandrel mill process.
[0014] Lastly, the invention also contemplates the use of martensitic stainless steels consisting of not more then 0.30% by weight of C, not more than 1.0% by weight of Si, not more than 2.0% by weight of Mn, 11-14% by weight of Cr, 0.005―0.10% by weight of AI, not more than 0.10% by weight of N, and one or more elements selected from the group consisting of up to 3.5% by weight of Ni, up to 2.0% by weight of Cu, up to 2.5% by weight of Mo, up to 0.10% by weight of Nb, and up to 0.10% by weight or less of V, as well as one or more elements selected from the group consisting of the rare earth elements, Ca, and B, the amount of the rare earth elements ranging from 4 x (% of S) to 20 x (% of S), that of Ca from 1 x (% of S) to 10 x (% of S), and that of B from 0.001 to 0.008% by weight, wherein the amount of P is held to no more than 0.02% by weight and the amount of S to no more than 0.003% by weight, the remainder thereof being Fe and unavoidable impurities, said steel having a ferrite content of not more than 40% at 1200°C, for manufacture of seamless steel pipe by Mannesmann plug mill or mandrel mill process.
[0015] The martensitic stainless steels of the present invention shall now be described in detail. We shall begin by explaining the reasons for each of the limits placed on the contents of the basic constituents in the steels used in the invention.
[0016] Carbon is necessary for strength. However, because corrosion resistance declines at a carbon content, in excess of 0.30%, the upper limit on carbon content has been set at 0.30%.
[0017] Chromium markedly increases corrosion resistance in CO2-containing environments. The addition of at least 11% is intended to prevent pitting and crevice corrosion. However, this element also is ferrite- forming. The addition of more than 14% therefore increases the amount of ferrite, making it difficult to achieve the desired strength under the heat conditions (tempering temperature) necessary to preserve resistance of the steel to stress-corrosion cracking. The range in the chromium content was thus set at 11-14%.
[0018] Silicon is effective as a deoxidizer and should preferably be included at a level of at least 0.2%. A content of more than 1.0% reduces toughness, so we set an upper limit of 1.0%.
[0019] Manganese acts to strengthen the steel and improve toughness. Therefore a content of not less than 0.5% is desirable. However, no further improvement in strength and tougness can be gained from the addition of more than 2.0%. Hence, the Mn content was set at not more than 2.0%.
[0020] Aluminum acts as a deoxidizer, reducing the level of oxygen in the steel by oxygen fixation, enhancing the hot workability. The stabilizing effects of AI addition are not observed at less than 0.005%; these effects reach a saturation level at 0.10%. The content of AI was thus limited to a range of from 0.005% by 0.10%.
[0021] Nitrogen increases the strength and corrosion resistance of the steel up to a concentration of 0.10%, beyond which additional improvement is not observed. For this reason, the N content was set at not more than 0.10%.
[0022] Sulfur is present in the steel as an undesirable impurity that severely reduces the hot workability of the steels in the present invention. This adverse effect is particularly large during piercing of the billet on a piercing mill when the seamless pipe is fabricated by the plug mill process or the mandrel mill process. A sulfur level in excess of 0.003% makes if difficult to produce scratch-free seamless pipe, which is why the S content must be held to no more than 0.003%.
[0023] P is another impurity unavoidably present in steels. It produces a marked deterioration in the hot workability of the steel at high temperatures of 1200°C and above. This element causes scratch formation on the inside wall of the tube during piercing of the billet on a piercing mill. Piercing without scratch formation is difficult at a P level about 0.02%, and so this must be held to 0.02% or less.
[0024] In addition to the above basic constituents, the martensitic stainless steels used in the present invention may also contain one or more elements selected from the group consisting of Ni, Cu, Mo, Nb, and V, and one or more elements chosen from the group consisting of rare earth metals, Ca, and B. The reasons for the limits set to the contents for each of these are given below.
[0025] Nickel: Ni increases corrosion resistance. The addition of more than 3.5%, however, produces no further improvement in corrosion resistance. Because Ni is expensive, the upper limit of addition was set at 3.5%.
[0026] Molybdenum: Mo enhances the strength and corrosion resistance of the steel. At levels of under 0.01 %, this effect is not fully exhibited, while the addition of more than 2.5% produces no corresponding increase in effect. Because Mo too is a high-priced metal, Mo addition was limited to a range of from 0.01 % to 2.5%.
[0027] Niobium: Nb increases steel strength, but is ineffective at levels of under 0.01 %. The addition of over 0.10% fails to produce additional improvement. The range of Nb addition was thus set at 0.01-0.10%.
[0028] Vanadium: V increases the strength of the steel, but is not effective at levels below 0.1%. Further improvement does not result from the addition of more than 0.10%. The range in the content of V was thus set at 0.01-0.10%.
[0029] Copper: Copper improves the strength and corrosion resistance of the steel. However, because the addition of more than 2.0% reduces hot workability, the upper limit on copper addition was set at 2.0%.
[0030] Rare earth metals (REM), calcium: The rare earth metals and calcium are powerful sulfide-forming elements. The formation of the sulfides of rare earth metals or calcium reduces the amount of sulfur in solid solution within the steel, thereby improving the hot workability of the steel. However, when the amount of rare earth elements is four times as great as the sulfur content (wt%) or the amount of calcium less than equivalent to the amount of sulfur, this effect is minor. On the other hand, when the level of rare earth elements is greater than 20 times, or the level of calcium greater than 10 times, the amount of sulfur, this effect reaches a saturation point and the oxides and sulfides of these elements may even have the opposite effect of increasing surface defects. For these reasons, we limited the amount of rare earth metals to a range of from 4 x (%S) to 20 x (%S), and the amount of calcium to a range of from 1 x (%S) to 10 x (%S).
[0031] Boron: The addition of a trace amount of boron improves the hot workability of the steel. This effect does not arise at a level of less than 0.001 %. However, the addition of more than 0.008% of boron has the opposite effect of reducing the hot workability at temperatures of 1200°C and over. We therefore limited boron addition to a range of 0.001-0.008%.
[0032] We have also set a ferrite content for the steels of the present invention of 40% or less at 1200°C. This is because, as we have seen above, even the plug mill and mandrel mill processes can be used to manufacture seamless pipe without hindrance or defects from steels having a ferrite content of more than 40% at 1200°C. The ferrite content (%) at 1200°C is defined by Eq. (1) below:
Ferrite content (%) =
-195 x (%C) - 185 x (%N) - 18 x (%Ni) - 6 x (%Cu) -
5 x (%Mn) + 5 x (%Si) + 12 x (%Cr) + 16 x (%V) +
5 x (%Mo) + 6 x (%Nb) + 5 x (%Ti) + 50 x (%AI) 120 - (1)
[0033] We shall now describe a method for fabricating seamless steel pipe using the martensitic stainless steels of the present invention.
[0034] First, a bloom obtained by continuous casting or blooming is rolled into a round billet. This billet is then heated to a given temperature, preferably from 1200 to 1250°C, and pierced and rolled by means of the Mannesmann plug mill process or the Mannesmann mandrel mill process. When the Mannesmann plug mill process is employed, the billet is first pierced on a piercer, then rolled by an elongator, a plug mill, a reeler and a sizer, in that order. When the Mannesmann mandrel mill process is used, the billet is first pierced on a piercer then rolled respectively on a mandrel mill and hot-stretch reducer. Following this, the pipe is heat-treated either in a batch-type furnace or by induction heating. This heat treatment may consist of quench-tempering, normalize-tempering, or simply tempering. This gives steel pipe of the desired strength. In the case of oil-well pipe, it is more common to first increase the wall thickness at the tube ends by upsetting, the heat-treat.
[0035] Table 2 shows the chemical constituents and whether or not defects were formed for a number of examples illustrating the present invention and several comparative examples. In each case, a billet having a diameter of 175 mm was heated to 1230°C and pierced on a piercer to form a tube with an outside diameter of 185 mm and a wall thickness of 19.76 mm. The inside and outside walls of the tube were examined. An "X" in the table denotes that defects such as scratches or cracks were found on the tube wall. An open circle 0 indicates that no defects were observed, or only minor faults of no practical consequence noted. The ferrite (%) shown in Table 1 shows the ferrite contents (%) at 1200° which was calculated using Eq. (1). If the computed value was negative, this was indicated in the table as 0. The rare earth metals used in the examples shown in Table 2 consisted primarily of cesium (approx. 50%).
[0036] The P and S levels of specimens 21-26 (comparative examples) in Table 2 all exceed the upper limits of the ranges set in the present invention. Defects were observed in each of these specimens. Similarly, the levels of the rare earth metals, B, and Ca in specimens 27-29 (comparative examples) all exceeded the upper limits defined above. Defects were again noted in each of these.
[0037] As we have indicated in the preceding explanation, the martensitic stainless steels used in seamless steel tube in accordance with the present invention raises the hot workability, and especially the hot piercability, of steels having ferrite contents of 40% or less at 1200°C, despite the difficulty previously encountered in manufacturing seamless pipe from such steels by a plug mill or a mandrel mill process. This is achieved by holding down the P and the S contents of the steel. As a result, seamless steel pipe need no longer be manufactured by a hot extrusion process, and can now be manufactured free of defects by a plug mill or a mandrel mill process. Because this permits the use of plug mill and mandrel mill processes in the production of seamless pipe from this type of steel, higher productivity can be achieved, along with increased yield and reduced costs.
[0038] This invention also provides the use of martensitic stainless steels for manufacturing seamless steel pipe wherein, in addition to restricting the levels of P and S, one or more elements selected from the rare earth metals, calcium, and boron are added, further increasing the hot piercability of the steel. This permits the manufacture of defect-free seamless steel pipe by means of a mandrel mill or plug mill process.
1. Use of martensitic stainless steel consisting of not more than 0.30% by weight
of C, not more than 1.0% by weight of Si, not more than 2.0% by weight of Mn, 11-14%
by weight of Cr, 0.005―0.10% by weight of Al, and not more than 0.10% by weight of
N, wherein the amount of P is held to no more than 0.02% by weight and the amount
of S to no more than 0.003% by weight, the remainder thereof being Fe and unavoidable
impurities, said steel having a ferrite content of not more than 40% by weight at
1200°C for manufacture of seamless steel pipe by the Mannesmann plug mill or mandrel
mill process.
2. Use of martensitic stainless steel consisting of not more than 0.30% by weight
of C, not more than 1.0% by weight of Si, not more than 2.0% by weight of Mn, 11-14%
by weight of Cr, 0.005―0.10% by weight of Al, not more than 0.01 % by weight of N,
and one or more elements selected from the group consisting of up to 3.5% by weight
of Ni, up to 2.0% by weight of Cu, up to 2.5% by weight of Mo, up to 0.10% by weight
of Nb, and up to 0.10% by weight of V, wherein the amount of P is held to no more
than 0,02% by weight and the amount of S to no more than 0.003% by weight, the remainder
thereof being Fe, and unavoidable impurities, said steel having a ferrite content
of not more than 40% by weight at 1200°C for manufacture of seamless steel pipe by
the Mannesmann plug mill or mandrel mill process.
3. Use of martensitic stainless steel consisting of not more than 0.30% by weight
of C, not more than 1.0% by weight of Si, not more than 2,0% by weight of Mn, 11-14%
by weight of Cr, 0.005―0.10% by weight of AI, not more than 0.10% by weight of N,
and one or more elements selected from the group consisting of rare earth elements,
Ca, and B, the amount of the rare earth elements ranging from 4 x (% of S) to 20 x
(% of S), that of Ca from 1 x (% of S) to 10 x (% of S), and that of B from 0.001
to 0.008% by weight, wherein the amount of P is held to no more than 0.02% by weight
and the amount of S to no more than 0.003% by weight, the remainder thereof being
Fe and unavoidable impurities, said steel having a ferrite content of not more than
40% by weight at 1200°C, for manufacture of seamless steel pipe by Mannesmann plug
mill or mandrel mill process.
4. Use of martensitic stainless steel consisting of not more then 0.30% by weight
of C, not more than 1.0% by weight of Si, not more than 2.0% by weight of Mn, 11-14%
by weight of Cr, 0.005-0.10% by weight of Al, not more than 0.10% by weight of N,
and one or more elements selected from the group consisting of up to 3.5% by weight
of Ni, up to 2.0% by weight of Cu, up to 2.5% by weight of Mo, up to 0.10% by weight
of Nb, and up to 0.10% by weight or less of V, as well as one or more elements selected
from the group consisting of the rare earth elements, Ca, and B, the amount of the
rare earth elements ranging from 4 x (% of S) to 20 x (% of S), that of Ca from 1
x (% of S) to 10 x (% of S), and that of B from 0.001 to 0.008% by weight, wherein
the amount of P is held to no more than 0.02% by weight and the amount of S to no
more than 0.003% by weight, the remainder thereof being Fe and unavoidable impurities,
said steel having a ferrite content of not more than 40% at 1200°C, for manufacture
of seamless steel pipe by Mannesmann plug mill or mandrel mill process.
1. Verwendung von martensitischem, rostfreien Stahl, der aus nicht mehr als 0,30 Gew.-%
an C, nicht mehr als 1,0 Gew.-% an Si, nicht mehr als 2,0 Gew.-% an Mn, 11-14 Gew.-%
an Cr, 0,005-0,1'0 Gew.-% an AI und nicht mehr als 0,10 Gew.-% an N, worin die Menge
an P auf nicht mehr als 0,02 Gew.-% und die Menge an S auf nicht mehr als 0,003 Gew.-%
eingehalten wird, der Rest dessen Fe sowie unvermeidbare Verunreinigungen sind, bestehlt,
wobei der genannte Stahl einen Ferritgehalt von nicht mehr als 40 Gew.- % bei 1200°C
hat, für die Herstellung eines nahtlosen Stahlrohres mittels des Mannesmann-Stopfen-
oder Dornwalzverfahrens.
2. Verwendung von martensitischem, rostfreien Stahl, der aus nicht mehr als 0,30 Gew.-%
an C, nicht mehr als 1,0 Gew.-% an Si, nicht mehr als 2,0 Gew.-% an Mn, 11-14 Gew.-%
an Cr, 0,005-0,10 Gew.-% an Al, nicht mehr als 0,10 Gew.-% an N und einem oder mehreren
Elementen, die aus der sich aus bis zu 3,5 Gew.-% an Ni, bis zu 2 Gew.-% an Cu, bis
zu 2,5 Gew.-% an Mo, bis zu 0,10 Gew.-% an Nb und bis zu 0,10 Gew.-% an V zusammensetzenden
Gruppe ausgewählt sind, worin die Menge an P auf nicht mehr als 0,02 Gew.-% und die
Menge an S auf nicht mehr als 0,003 Gew.-% eingehalten wird, der Rest dessen Fe sowie
unvermeidbare Verunreinigungen sind, besteht, wobei der genannte Stahl einen Ferritgehalt
von nicht mehr als 40 Gew.-% bei 1200°C hat, für die Herstellung eines nahtlosen Stahlrohres
mittels des Mannesmann-Stopfen- oder Dornwalzverfahren.
3. Verwendung von martensitischem, rostfreien Stahl, der aus nicht mehr als 0,30 Gew.-%
an C, nicht mehr als 1,0 Gew.-% an Si, nicht mehr als 2,0 Gew.-% an Mn, 11-14 Gew.-%
an Cr, 0,005-0,10 Gew.-% an Al, nicht mehr als 0,10 Gew.-% an N und einem oder mehreren
Elementen, die aus der sich aus Seltenerdelementen, Ca und B zusammensetzenden Gruppe
ausgewählt sind, worin die Menge an Seltenerdelementen von 4 x (% von S) bis 20 x
(% von S), diejenige an Ca von 1 x (% von S) bis 10 x (% von S) und diejenige an B
von 0,001 bis 0,008 Gew.-% reicht, worin die Menge an P auf nicht mehr als 0,02 Gew.-%
und die Menge an S auf nicht mehr als 0,003 Gew.-% eingehalten wird, der Rest dessen
Fe und unvermiedbare Verunreinigungen sind, besteht, wobei der genannte Stahl einen
Ferritgehalt von nicht mehr als 40 Gew.-% bei 1200°C hat, für die Herstellung eines
nahtlosen Strahlrohres mittels des Mannesmann-Stopfen- oder Dornwalzverfahrens.
4. Verwendung von martensitischem, rostfreien Stahl, der aus nicht mehr als 0,30 Gew.-%
an C, nicht mehr als 1,0 Gew.-% an Si, nicht mehr als 2,0 Gew.-% an Mn, 11-14 Gew.-%
an Cr, 0,005―0,10 Gew.-% an Al, nicht mehr als 0,10 Gew.-% an N und einem oder mehreren
Elementen, die aus der sich aus bis zu 3,5 Gew.-% an Ni, bis zu 2 Gew.-% an Cu, bis
zu 2,5 Gew.-% an Mo, bis zu 0,10 Gew.-% an Nb und bis zu 0,10 Gew.-% oder weniger
an V zusammensetzenden Gruppe ausgewählt sind, wie auch aus einem oder mehreren Elementen
der sich aus Seltenerdelementen, Ca und B zusammensetzenden Gruppe ausgewählt sind,
wobei die Menge as Seltenerdelementen von 4 x (% von S) bis 20 x (% von S), diejenige
von Ca von 1 x (% von S) bis 10 x (% von S) und jiejenige von B von 0,001 bis 0,008
Gew.-% reicht, worin die Menge an P auf nicht mehr als 0,02 Gew.-% und die Menge an
S auf nicht mehr als 0,003 Gew.-% eingehalten wird, der Rest dessen Fe und unvermeidbare
Verunreinigungen sind, besteht, wobei der genannte Stahl einen Ferritgehalt von nicht
mehr als 40% bei 1200°C hat, für die Herstellung eines nahtlosen Stahlrohres mittels
des Mannesmann-Stopfen- oder Dornwalzverfahrens.
1. Utilisation d'acier inoxydable martensitique comprenant au plus 0,30% en poids
de C, au plus 1,0% en poids de Si, au plus 2,0% en poids de Mn, 11 à 14% en poids
de Cr, 0,005 à 0,10% en poids de AI, et au plus 0,10 en poids de N, la proportion
de P étant maintenue à au plus 0,02% en poids et la proportion de S à au plus 0,003%
en poids, le complément consistant en Fe et des impuretés inévitables, ledit acier
ayant une teneur en ferrite de 40% en poids au plus à 1.200°C pour la fabrication
de tube d'acier sans soudure par le procédé Mannesmann avec fraise à queue ou fraise
à mandrin.
2. Utilisation d'acier inoxydable martensitique comprenant au plus 0,30% en poids
de C, au plus 1,0% en poids de Si, au plus 2,0% en poids de Mn, 11 à 14% en poids
de Cr, 0,005 à 0,10% en poids de AI, et au plus 0,10 en poids de N, et un ou plusieurs
éléments choisis dans le groupe comprenant jusqu'à 3,5% en poids de Ni, jusqu'à 2%
en poids de Cu, jusqu'à 2,5% en poids de Mo, jusqu'à 0,10% en poids de Nb, et jusqu'à
0,10% en poids de V, la proportion de P étant maintenue à au plus 0,02% en poids et
la proportion de S à au plus de 0,003% en poids, le complément étant constitué par
Fe et des impuretés inévitables, ledit acier ayant une teneur en ferrite de 40% en
poids au plus à 1.200°C pour la fabrication de tuyau d'acier sans soudure par le procédé
Mannesmann avec fraise à queue ou fraise à mandrin.
3. Utilisation d'acier inoxydable martensitique comprenant au plus 0,30% en poids
de C, au plus 1,0% en poids de Si, au plus 2,0% en poids de Mn, 11 à 14% en poids
de Cr, 0,005 à 0,10% en poids de AI, et au plus 0,10 en poids de N, et un ou plusieurs
éléments choisis dans le groupe comprenant des éléments des terres rares allant de
4 x (% de S) à 20 x (% de S), celle de Ca de 1 x (% de S) à 10 x (% de S), et celle
de B de 0,001 à 0,008% en poids, la proportion de P étant maintenue à 0,02% en poids
au plus et la proportion de S à 0,003% en poids au plus, le complément étant constitué
par Fe et des impuretés inévitables, ledit acier ayant une teneur en ferrite de 40%
en poids au plus à 1.200°C, pour la fabrication de tuyaux d'acier inoxydable par le
procédé Mannesmann avec fraise à queue ou fraise à mandrin.
4. Utilisation d'acier inoxydable martensitique comprenant au plus 0,30% en poids
de C, au plus 1,0% en poids de Si, au plus 2,0% en poids de Mn, 11 à 14% en poids
de Cr, 0,005 à 0,10% en poids de AI, et au plus 0,10 en poids de N, et un ou plusieurs
éléments choisis dans le groupe comprenant jusqu'à 3,5% en poids de Ni, jusqu'à 2%
en poids de Cu, jusqu'à 2,5% en poids de Mo, jusqu'à 0,10% en poids de Nb, et jusqu'à
0,10% en poids ou moins de V, ainsi qu'un ou plusieurs éléments choisis dans le groupe
comprenant les éléments des terres rares, Ca et B, la proportion des éléments des
terres rares allant de 4 x (% de S) à 20 x (% de S), celle de Ca de 1 x (% de S) à
10 x (% de S), et celle de B de 0,001 à 0,008% en poids, la proportion de P étant
maintenue à 0,02% en poids au plus et à la proportion de S à 0,003% en poids au plus,
le complément étant constitué par Fe et des impuretés inévitables, ledit acier ayant
une teneur en ferrite de 40% en plus à 1.200°C, pour la fabrication de tube d'acier
sans soudure par le procédé Mannesmann avec fraise à queue ou fraise à mandrin.