PROCESS FOR PRODUCING A HIGH-GRADE STEEL TUBE AND HIGH-GRADE STEEL TUBE

Abstract

A process for producing a high-grade steel tube includes the steps of: providing a tubular blank of an austenitic high-grade steel, wherein the high-grade steel comprises in weight % no more than 0.02% carbon, no more than 1.0% manganese, no more than 0.03% phosphor, no more than 0.015% sulfur, no more than 0.8% silicon, no more than 17.5% t to 18.5% nickel, no more than 19.5% to 20.5% chromium, no more than 6.0% to 6.5% molybdenum, no more than 0.18% to 0.25% nitrogen, no more than 0.5% to 1.0% copper,and a remainder of iron and unavoidable impurities; and cold-forming the blank into a tube.

Claims

1. A method for manufacturing a stainless steel tube comprising the steps of: providing a tubular hollow of an austenitic stainless steel, wherein the stainless steel comprises: carbon in an amount of not more than 0.02 wt.-%, manganese in an amount of not more than 1.0 wt.-%, phosphor in an amount of not more than 0.03 wt.-%, sulfur in an amount of not more than 0.015 wt.-%, silicon in an amount of not more than 0.8 wt.-%, nickel in an amount from 17.5 wt.-% to 18.5 wt.-%, chromium in an amount from 19.5 wt.-% to 20.5 wt.-%, molybdenum in an amount from 6.0 wt.-% to 6.5 wt.-%, nitrogen in an amount from 0.18 wt.-% to 0.25 wt.-%, copper in an amount from 0.5 wt.-% to 1.0 wt.-%, and a remainder of iron and unavoidable impurities; cold forming of the hollow into a tube; coiling the tube; and annealing the coiled tube after the cold forming at a temperature in a range from 1,100 C. to 1,200 C.

2. The method according to claim 1, wherein the coiled tube is annealed at a temperature in a range from 1,115 C. to 1,155 C.

3. The method according to claim 1, wherein the temperature during annealing is such that the annealed and coiled tube has a hardness of 90 HRB or less.

4. The method according to claim 1, wherein the tube is annealed in a vacuum atmosphere at a pressure of less than 6 mbar.

5. The method according to claim 1, wherein the tube is annealed in a shaft oven.

6. The method according to claim 1, wherein the tube is held at a temperature in a range from 1,100 C. to 1,200 C., over a period of time of at least 5 minutes and at most 20 minutes.

7. The method according to claim 1, further comprising the step of shipping the tube in a coiled state.

8. The method according to, wherein after the annealing step, the method further comprises the steps of: decoiling the tube; cold forming the tube; coiling the tube; and annealing the coiled tube again at a temperature in a range from 1,100 C. to 1,200 C.

9. The method according to claim 1, wherein the tube is annealed with a tube coiled to form a ring without a reel or a core.

10. The method according to claim 1, wherein the method further comprises the step of recoiling the annealed coiled tube onto a reel.

11. The method according to claim 1, wherein the tube is cold formed by cold pilger milling or cold drawing.

12. A stainless steel tube comprising; carbon in an amount of not more than 0.02 wt.-%; manganese in an amount of not more than 1.0 wt.-%; phosphor in an amount of not more than 0.03 wt.-%; sulfur in an amount of not more than 0.015 wt.-%; silicon in an amount of not more than 0.8 wt.-%; nickel in an amount from 17.5 wt.-% to 18.5 wt.-%; chromium in an amount from 19.5 wt.-% to 20.5 wt.-%; molybdenum in an amount from 6.0 wt.-% to 6.5 wt.-%; nitrogen in an amount from 0.18 wt.-% to 0.25 wt.-%; copper in an amount from 0.5 wt.-% to 1.0 wt.-%; and a remainder of iron and unavoidable impurities, wherein the stainless steel tube is coiled and in the coiled state has a hardness of less than 90 HRB.

13. The stainless steel tube according to claim 12, wherein the coiled stainless steel tube has a hardness of 80 HRB or less.

14. The stainless steel tube according to claim 12, wherein the coiled stainless steel tube has a strain of at least 35%.

15. A stainless steel tube manufactured by the method according to claim 1.

16. The method according to claim 1, wherein the coiled tube is annealed at a temperature in a range from 1,120 C. to 1,1150 C.

17. The method according to claim 1, wherein the temperature during annealing is such that the annealed and coiled tube has a hardness of 80 HRB or less.

18. The method according to claim 1, wherein the tube is annealed in an inert gas atmosphere in an argon containing atmosphere.

19. The method according to claim 10, wherein the reel is made of wood.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] FIG. 1 shows a flow chart of the method for manufacturing a stainless steel tube according to an embodiment of the present disclosure.

DETAILED DESCRIPTION

[0049] Referring to FIG. 1, in the present example, a hollow made of Sandvik 254 SMO material was formed into a stainless steel tube with an outer diameter of 10 mm and a wall thickness of 1.5 mm by cold pilger milling, was coiled into a ring after the cold pilger milling, wherein the ring was not coiled around a core or a reel, and was soft annealed thereafter.

[0050] The material of the hollow is a high alloyed austenitic stainless steel 254 SMO available from the company Sandvik. This steel fulfills the norm UNS S31254 (254 SMO) of the American Society of Mechanical Engineers (SME) for use in a boiler and high pressure vessel. The material Sandvik 254 SMO exemplarily used for the hollow considered here apart from iron includes 0.011 wt.-% C, 0.45 wt.-% Si, 0.56 wt.-% Mn, 0.022 wt.-% P, less than 0.001 wt.-% S, 20.13 wt.-% Cr, 17.82 wt.-% Ni, 6.09 wt.-% Mo, 0.091 Co, 0.004 wt.-% Ti, 0.51 wt.-% Cu as well as 0.2 wt.-% N.

[0051] The finished tube forms a tube as it is stranded for an off-shore application in further method steps performed outside the tubing plant to form a strand with other tubes.

[0052] The tube after cold pilger milling and coiling was annealed in the coiled state at a temperature of 1,120 C. over a period of time of approximately 10 minutes. The finished tube after cooling down has a hardness of 73 HRB to 77 HRB, a strain of approximately 41%, as well as a tensile strength Rp 0.2 of 370 MPa (N/m.sup.2). Once this tube subsequently is coiled onto a reel of wood or it is recoiled from a ring without a reel onto a reel, the tube on the wooden reel has a hardness of 90 HRB or less.

[0053] In comparison, an uncoiled tube of the same material, Sandvik 254 SMO, annealed at conventional temperatures has a hardness of 96 HRB. This conventional tube thus, after coiling, which further increases the hardness, has a hardness which is significantly too hard for stranding.

[0054] For illustration, the method for manufacturing a stainless steel tube according to the present disclosure is now again briefly summarized with reference to the flow chart of FIG. 1.

[0055] First in step 1, as a raw material, a hollow of an austenitic stainless steel is provided, which in addition to iron includes 0.011 wt.-% C, 0.45 wt.-% Si, 0.56 wt.-% Mn, 0.022 wt.-% P, less than 0.001 wt.-% S, 20.13 wt.-% Cr, 17.82 wt.-% Ni, 6.09 wt.-% Mo, 0.091 Co, 0.004 wt.-% Ti, 0.51 wt.-% Cu as well as 0.2 wt.-% N. This hollow is then cold formed into the finished dimensioned tube by cold pilger milling 2.

[0056] During cold pilger milling 2 a lubricant is applied between the rollers and the tube/the hollow, as well as between the mandrel and the tube/the hollow, thus, this lubricant before annealing must be removed in two steps on the outside 3 as well as on the inside 4. Then the first annealing is effected in step 5. For particular applications, a further cold forming, e.g. by cold drawing, may be performed in step 6. After the second cold forming the steps 3 and 4, i.e. the removal of the lubricant or the degreasing, must be repeated before the tube is annealed again in step 5. After the annealing the tube in step 7 is packaged. This packaging in some embodiments means that the tube is recoiled from a ring onto a reel.

[0057] For the purpose of the original disclosure it is pointed out that all features, as they are apparent for a person skilled in the art form the present specification, from the figures and from the claims, even if they have only been described literally in combination with certain further features may be combined on their own or in arbitrary combination with other combinations of features disclosed herein, as far as those combinations are not explicitly excluded or the technical circumstances make these combinations impossible or useless. A comprehensive, explicit description of all possible combinations of features is only omitted here in order to provide a concise and readable description.

[0058] While the disclosure has been depicted and described in detail in the figure and the previous description this presentation and description is only by way of an example and is not considered as a restriction of the scope of protection as it is defined by the claims. The disclosure is not restricted to the embodiments disclosed.

[0059] Variations of the disclosed embodiments are apparent for a person skilled in the art from the figures, from the description and from the attached claims. In the claims the term comprising does not exclude other elements or steps and the indefinite article a does not exclude a plurality. The mere fact that certain features are claimed in separate claims does not exclude their combination. Reference numbers in the claims are not thought to restrict the scope of protection.