HIGH STRENGTH MICRO ALLOYED STEEL SEAMLESS PIPE FOR SOUR SERVICE AND HIGH TOUGHNESS APPLICATIONS

Abstract

Micro alloyed steels with a yield strength of at least 485 MPa (70 ksi) with outstanding toughness behavior, good weldability and improved sulphide stress cracking resistance for line pipes, for applications as process pipes, flowlines or risers in the oil and gas industry. Tubular products, such as seamless pipes, can be made from this steel. A process for manufacturing tubular products can be performed with this steel. The seamless pipes can be used for line pipes, flowlines and risers in the oil and gas industry.

Claims

1. A steel presenting a yield strength greater than or equal to 485 MPa and having a chemical composition consisting of, in weight %, relative to the total weight of said chemical composition, 0.05C0.10 0.15Si0.35 1.20Mn1.50 0.02Cr0.10 0.10<Mo0.30 0.015Al0.040 0.002N0.012 0.10Ni0.30 0.02V0.06; 0.01Nb0.03 0.001Ti0.025 the balance of the chemical composition of said steel being constituted by Fe and one or more residual elements, including P, S, B, Ca, Cu and mixtures thereof; and the chemical composition of said steel satisfying the following formula between C, Cr, Mo, Nb, V and Ti, the contents of which are expressed in weight %,
C+Cr+Mo+Nb+V+Ti0.55formula.

2. The steel according to claim 1, wherein said steel presents a yield strength between 495 and 675 MPa.

3. The steel according to claim 1, wherein it said steel presents a yield strength greater than or equal to 555 MPa, and below 675 MPa.

4. The steel according to claim 1, wherein its chemical composition contains in weight %, relative to the total weight of said chemical composition: 0.06C0.08.

5. The steel according to claim 1, wherein its chemical composition contains in weight %, relative to the total weight of said chemical composition: 0.23Si0.31.

6. The steel according to claim 1, wherein its chemical composition contains in weight %, relative to the total weight of said chemical composition: 1.35Mn1.45.

7. The steel according to claim 1, wherein its chemical composition contains in weight %, relative to the total weight of said chemical composition: 0.06Cr0.08.

8. The steel according to claim 1, wherein its chemical composition contains in weight %, relative to the total weight of said chemical composition: 0.16Mo0.26.

9. The steel according to claim 1, wherein its chemical composition contains in weight %, relative to the total weight of said chemical composition: 0.03V0.05.

10. The steel according to claim 1, wherein its chemical composition contains in weight %, relative to the total weight of said chemical composition: 0.02Nb0.028.

11. The steel according to claim 1, wherein its chemical composition contains in weight %, relative to the total weight of said chemical composition: 0.005Ti0.020.

12. The steel according to claim 1, wherein its chemical composition satisfies the following formula between Cr and Mo, the contents of which are expressed in weight %,
Cr+Mo0.35formula.

13. The steel according to claim 1, wherein its chemical composition satisfies the following formula between Nb and V, the contents of which are expressed in weight %,
Nb+V0.07formula.

14. The steel according to claim 1, wherein the amounts of the residual elements, expressed in weight %, relative to the total weight of said chemical composition are as follows: P0.012 S0.003 B0.0005 Ca0.004 Cu0.12.

15. A seamless pipe, made from a steel according to claim 1.

16. The seamless pipe according to claim 15, wherein steel yield strength is between 555 MPa and 740 MPa, pipe wall thickness is between a 15.1 and 35 mm, and the steel chemical composition contains in weight %, relative to the total weight of said chemical composition: 0.16Mo0.26.

17. The seamless pipe according to claim 15, wherein steel yield strength is between 485 MPa and 635 MPa, pipe wall thickness is between 9.3 and 40 mm, and steel chemical composition contains in weight %, relative to the total weight of said chemical composition: 0.10Mo0.21.

18. A process for manufacturing a seamless pipe comprising the following successive steps: (a) providing a steel having a chemical composition as defined in claim 1, and hot forming a seamless pipe thereof (b) cooling down the seamless pipe obtained at (a) to room temperature, (c) heating up the cooled seamless pipe obtained at (b) to an austenitization temperature ranging from 890 to 970 C. to obtain an austenitized seamless pipe, and then quenching said austenitized seamless pipe down to ambient temperature to obtain a quenched seamless pipe, (d) heating up the quenched seamless pipe obtained at (c) to a tempering temperature ranging from 610 to 680 C. before keeping said seamless pipe at the temperature and then cooling said seamless pipe down to ambient temperature to obtain a quenched and tempered seamless pipe.

19. A process according to the claim 18, wherein the tempering temperature ranges from 630 to 665 C.

20. A method of using a seamless pipe as defined in claim 15 for line pipes, said method comprising a step of installing said seamless pipe as process pipes, flowlines or risers in an oil and gas industry application.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0160] FIG. 1a and FIG. 1b show graphs representing all individual values of hardness measurements performed on different positions (bottom and head ends of the pipe), as well as on cross section (outer wall, mid wall and inner wall) for each pipe (A1, 25.4 mm), (A2, 25.4 mm), (A3, 25.4 mm), (A1, 15.9 mm) and (A2, 15.9 mm).

[0161] FIG. 2a and FIG. 2b show graphs representing the yield strength values measured for each pipe (A1, 25.4 mm), (A2, 25.4 mm), (A3, 25.4 mm), (A1, 15.9 mm) and (A2, 15.9 mm).

[0162] FIG. 3 shows graphs representing all individual values of hardness measurements performed for each pipe (A4, 25.4 mm) and (A5, 25.4 mm).

[0163] FIG. 4 shows graphs representing the yield strength values measured for each pipes (A4, 25.4 mm) and (A5, 25.4 mm).

EXAMPLES

I. Example 1X80

[0164] 1. Tested Steels

[0165] The following compositions of seamless pipe's steels according to the present invention (A1), (A2) and (A3) have been prepared from the elements indicated in the table 1 below, the amounts of which are expressed as percent by weight, relative to the total weight of the chemical composition.

TABLE-US-00001 TABLE 1 Steel A1 A2 A3 Chemical composition C 0.075 0.062 0.075 (Unit: mass %, Si 0.273 0.277 0.298 Balance: Fe and Mn 1.428 1.432 1.417 residual elements) Cr 0.062 0.065 0.078 Mo 0.172 0.215 0.241 Al 0.027 0.025 0.025 N 0.005 0.0072 0.0088 Ni 0.167 0.18 0.164 V 0.039 0.045 0.050 Nb 0.0215 0.0209 0.0236 Ti 0.0055 0.0052 0.0055 P 0.006 0.009 0.01 S 0.0017 0.0023 0.0019 B 0.0004 0.0004 0.0004 Ca 0.0009 0.0012 0.0015 Cu 0.029 0.05 0.04 Pcm 0.181 0.173 0.190 Ceq 0.381 0.381 0.404 C + Cr + Mo + Nb + V + Ti 0.372 0.410 0.4988 Nb + V 0.060 0.065 0.065 Nb + V + Ti 0.065 0.071 0.070 Cr + Mo 0.234 0.280 0.319

[0166] 2. Protocol

[0167] The steels (A1), (A2) and (A3) having the chemical compositions described in the table 1 above have been heated and then hot formed into seamless steel pipes of the desired dimensions by hot working using a rolling mill at 1250 C.

[0168] The seamless steel pipes thus obtained have a wall thickness (WT) equal to 15.9 mm or 25.4 mm.

[0169] After hot forming, the seamless steel pipes have undergone the following process conditions summarized in the table 2, with:

[0170] AT: Austenitization temperature in C.

[0171] At: Austenitization time in seconds

[0172] TT: Tempering temperature in C.

[0173] Tt: Tempering time in seconds

[0174] The following steps, defined in table 2 and correspond to steps (c) and (d) of the process of the present invention.

TABLE-US-00002 TABLE 2 Steel (WT) AT ( C.) At (s) TT ( C.) Tt (s) A1 (25.4 mm) 920 900 640 2400 A2 (25.4 mm) 920 900 644 2400 A3 (25.4 mm) 920 900 664 2400 A1 (15.9 mm) 920 900 644 2400 A2 (15.9 mm) 920 900 653 2400

[0175] The mechanical properties, fracture mechanics and SSC behavior have then been evaluated for each steel (A1), (A2) (A3) according to the following methods.

[0176] 3. Hardness Behavior

[0177] The hardness behavior has been evaluated according to the standard ISO 6507-1.

[0178] Each pipe (A1, 25.4 mm), (A2, 25.4 mm), (A3, 25.4 mm), (A1, 15.9 mm) and (A2, 15.9 mm) has been cut in its transversal direction and divided in four quadrants. Four indentations on its outer wall, mid wall and inner wall have been performed on both bottom and head ends of the pipe. In other words, 16 measurements have been performed for each wall at the bottom end of the pipe and 16 others have been performed at the head end.

[0179] The results are shown on FIG. 1a (representing the results obtained for pipes (A1, 25.4 mm), (A2, 25.4 mm) and (A3, 25.4 mm)) and FIG. 1b (representing the results obtained for pipes (A1, 15.9 mm) and (A2, 15.9 mm)).

[0180] Each graph represents all individual values of hardness measurements performed on different positions (bottom and head ends of the pipe), as well as on cross section (outer wall, mid wall and inner wall) for each pipe (A1, 25.4 mm), (A2, 25.4 mm), (A3, 25.4 mm), (A1, 15.9 mm) and (A2, 15.9 mm).

[0181] The results thus obtained clearly show that pipes made of steel according to the present invention comply with the requirements of DNVGL-ST-F101. Indeed, for both thicker (FIG. 1a) and thinner (FIG. 1b) walls, all hardness values are well below 250 HV10. This good level is even improved for the thinner walls (FIG. 1b) since these values are mostly below 230 HV10.

[0182] 4. Tensile Properties

[0183] The tensile properties have been evaluated according to the standard ASTM A370.

[0184] The tensile properties, represented by the yield strength, are shown on FIG. 2a (representing the results obtained for pipes (A1, 25.4 mm), (A2, 25.4 mm) and (A3, 25.4 mm)) and FIG. 2b (representing the results obtained for pipes (A1, 15.9 mm) and (A2, 15.9 mm)).

[0185] Each graph represents the yield strength values measured for each pipe (A1, 25.4 mm), (A2, 25.4 mm), (A3, 25.4 mm), (A1, 15.9 mm) and (A2, 15.9 mm).

[0186] The results thus obtained clearly show that pipes made of steel according to the present invention present a yield strength greater than or equal to 555 MPa (80 ksi). In other words, the steel according to the present invention corresponds to the grade X80.

[0187] 5. HIC Resistance

[0188] The HIC test, corresponding to the test according to NACE TM 0284 method, consists in immersing the test pipes in a solution (solution A) of 100% H.sub.2S. The testing duration is 96 hours. All specimens (A1, 25.4 mm), (A2, 25.4 mm), (A3, 25.4 mm), (A1, 15.9 mm) and (A2, 15.9 mm) passed with no cracks, namely, crack sensitivity ratio (CSR), crack length ratio (CLR) and crack thickness ratio (CTR) are all zero.

[0189] 6. SSC Resistance

[0190] The SSC resistance corresponds to the sulphide stress corrosion cracking resistance. This SSC resistance has been evaluated for each pipe (A1, 25.4 mm), (A2, 25.4 mm) (A3, 25.4 mm), (A1, 15.9 mm) and (A2, 15.9 mm) according to the three following methods: [0191] the first SSC test, corresponding to standard NACE TM0177-2016 Method A, consists in immersing the pipes under axial load in an aqueous solution (Solution A), which consists of 5.0 wt. % of sodium chloride and 0.5 wt. % glacial acetic acid dissolved in distilled water. The pH of the solution before or after H.sub.2S saturation was in the range of 2.6 and 2.8. The solution temperature is 24 C. The testing duration is 720 hours, and the applied stress is 90% of the actual yield strength. [0192] the second SSC test, corresponding to Four point bend testASTM G-39 standard, consists in immersing the pipes under load in an aqueous solution (Solution A), which consists of 5.0 wt. % of sodium chloride and 0.5 wt. % glacial acetic acid dissolved in distilled water. The pH of the solution before or after H.sub.2S saturation was in the range of 2.6 and 2.8. The solution temperature is 24 C. The testing duration is 720 hours, and the applied stress is 90% of the actual yield strength.

[0193] The absence of failure was then determined.

[0194] After 720 hours, the pipes according to the present invention presented no failures and no secondary cracks across the section. All the pipes show an excellent sulphide stress cracking resistance and can therefore be used in severe sour conditions.

[0195] 7. Test after Straining and Ageing

[0196] The steel composition (A1) according to the present invention has been prepared from the elements indicated in the table 1 above. The steel (A1) thus obtained has then been hot formed according to the protocol previously mentioned in example 1 and the two following pipes (A1a) and (A1b) have been produced.

[0197] After quenching and tempering, the pipe (A1b) has been aged and deformed according to the following protocol:

[0198] The full wall strip specimen (A1b) has been submitted to uniaxial tensile stress causing a plastic deformation of 2% (A1b-2%) or of 5% (A1b-5%). The pipes have then been aged at 250 C. for 1 hour. This protocol is intended to simulate the impact of reeling on the mechanical properties of the pipe at low temperatures.

[0199] The mechanical properties, fracture mechanics and SSC behavior of the pipe (A1a) and of the aged and deformed pipes (A1b-2%) and (A1b-5%) have then been evaluated according to the following methods.

7.1. Hardness Behavior

[0200] The hardness behavior has been evaluated according to the standard ISO 6507-1, as previously mentioned in example 1.

[0201] The results obtained with (A1b-2%), having a wall thickness of 15.9 mm, are mentioned in the table 3 here below. The following values correspond to the means of the four measurements of two of the four quadrants for each ends.

TABLE-US-00003 TABLE 3 Hardness (HV10) A1b-2% Outer wall Mid wall Inner wall Pipe Quadrant 1 215 220 237 trailing end Quadrant 2 223 224 240 Pipe Quadrant 1 245 219 229 leading end Quadrant 2 219 224 246

[0202] The results thus obtained clearly show that even after straining and ageing, pipes made of steel according to the present invention comply with the requirements of DNVGL-ST-F101. Indeed, all hardness values are below 250 HV10.

7.2. Impact Energy Test

[0203] For each pipe (A1a) and (A1b-5%), impact energy values have been determined according to the Charpy impact test ASTM E23Type A.

[0204] Three notches located on different positions across the wall thickness (outer wall, mid wall and inner wall) were performed on pipes (A1a) and (A1b-5%). The pipes were then placed at different temperatures ranging from 10 C. to 80 C.

[0205] The absorbed energy (Joule) and the shear area (%) were then measured after impact Charpy tests at different temperatures.

[0206] The results are given in the table 4 below for pipe (A1a) and pipe (A1b-5%). The results shown in the table correspond to the mean values of the three measurements using full size specimens.

TABLE-US-00004 TABLE 4 Charpy Impact Test Test T Absorbed energy (J) Shear area (%) ( C.) A1a A1b-5% A1a A1b-5% 80 348 166 100 60 60 343 251 100 100 30 377 272 100 100 10 385 280 100 100

[0207] The results thus obtained show that pipes made of steel according to the present invention present no brittle fracture even at 80 C. for the as-quenched and tempered conditions (A1a).

[0208] In addition, even after a severe uniaxial deformation of 5% followed by ageing at 250 C. for 1 h, this good ductile behavior is kept for temperatures down to 60 C. (A1b-5%).

7.3. Crack Tip Opening Displacement (CTOD)

[0209] This method is used to determine when the crack starts to propagate.

[0210] Three specimen were cut on each pipes (A1a), (A1b-2%) and (A1b-5%) using an electrical discharging machine (EDM) in order to guarantee narrow notches from which fatigue pre-cracks can extend by fatigue. The tests and results evaluations followed the standard BS 7448-1. The tested pipes had a wall thickness of 25.4 mm.

[0211] DNVGL-ST-F101 specification establishes a minimum CTOD value of 0.150 mm for the design temperature.

[0212] The CTOD values are given in the table 5 here below.

TABLE-US-00005 TABLE 5 CTOD CTOD values (mm) Steel T ( C.) notch 1 notch 2 notch 3 mean A1a 20 C. 1.24 1.32 1.30 1.29 A1a 60 C. 1.28 1.27 1.20 1.25 A1b-2% 60 C. 1.19 1.28 1.20 1.23 A1b-5% 60 C. 1.08 1.13 1.10 1.10

[0213] The results thus obtained show that the CTOD values of the steel according to the present invention are much higher than 0.150 mm. Indeed, even at 60 C., the CTOD values are higher than 1.10 mm.

[0214] In addition, all the pipes (A1a), (A1b-2%) and (A1b-5%) present a ductile fracture mode.

[0215] Both impact energy and CTOD tests show that the steel of the present invention present an excellent resistance to stress. Even after deformation and ageing, the pipes remain tough for temperatures down to 60 C.

7.4. SSC Resistance

[0216] The SSC resistance has been evaluated for each pipes (A1a, 25.4 mm), (A1a, 15.9 mm) and (A1b-5%) according to the methods previously mentioned in example 1.

[0217] After 720 hours, none of the pipes presented neither failures nor secondary cracks. All the pipes show an excellent sulphide stress cracking resistance, even after straining and ageing, and can therefore be used in severe sour conditions.

II. Example 2X70

[0218] 1. Tested Steels

[0219] The following compositions of seamless pipe's steels according to the present invention (A4) and (A5) have been prepared from the elements indicated in the table 1 below, the amounts of which are expressed as percent by weight, relative to the total weight of the chemical composition.

TABLE-US-00006 TABLE 6 Steel A4 A5 Chemical composition C 0.071 0.062 (Unit: mass %, Si 0.272 0.277 Balance: Fe and Mn 1.419 1.432 residual elements) Cr 0.072 0.065 Mo 0.107 0.120 Al 0.029 0.025 N 0.0051 0.0072 Ni 0.168 0.18 V 0.040 0.045 Nb 0.0213 0.0209 Ti 0.0023 0.0052 P 0.007 0.009 S 0.0016 0.0023 B 0.0005 0.0004 Ca 0.0013 0.0012 Cu 0.029 0.05 Pcm 0.172 0.173 Ceq 0.364 0.381 C + Cr + Mo + Nb + V + Ti 0.2962 0.3136 Nb + V 0.0552 0.0613 Nb + V + Ti 0.0572 0.0636 Cr + Mo 0.179 0.185

[0220] 2. Protocol

[0221] The steels (A4) and (A5) having the chemical compositions described in the table 6 above have been heated and then hot formed into seamless steel pipes of the desired dimensions by hot working using a rolling mill at 1250 C.

[0222] The seamless steel pipes thus obtained have a wall thickness (WT) equal to 25.4 mm.

[0223] After hot forming, the seamless steel pipes have undergone the following process conditions:

[0224] AT: Austenitization temperature at 920 C.

[0225] At: Austenitization time during 900 seconds

[0226] TT: Tempering temperature at 650 C.

[0227] Tt: Tempering time during 2400 seconds

[0228] The mechanical properties, fracture mechanics, HIC and SSC behavior have then been evaluated for each steel (A4) and (A5) according to the following methods.

[0229] 3. Hardness Behavior

[0230] The hardness behavior has been evaluated according to the standard ISO 6507-1, according the method described for Example 1.

[0231] The results are shown on FIG. 3.

[0232] The results thus obtained clearly show that pipes made of steel according to the present invention comply with the requirements of DNVGL-ST-F101. All hardness values are below 240 HV10.

[0233] 4. Tensile Properties

[0234] The tensile properties have been evaluated according to the standard ASTM A370.

[0235] The tensile properties, represented by the yield strength, are shown on FIG. 4. Each graph represents the yield strength values measured for each pipes (A4, 25.4 mm) and (A5, 25.4 mm).

[0236] The results thus obtained clearly show that pipes made of steel according to the present invention present a yield strength greater than or equal to 485 MPa (70 ksi).

[0237] 5. HIC Resistance

[0238] Pipes (A4, 25.4 mm) and (A5, 25.4 mm) passed the HIC test, corresponding to the test according to NACE TM 0284 method, with no cracks after 96 hours, namely, crack sensitivity ratio (CSR), crack length ration (CLR) and crack thickness ratio (CTR) are all zero.

[0239] 6. SSC Resistance

[0240] Pipes (A4, 25.4 mm) and (A5, 25.4 mm) passed the SSC test, corresponding to the test according to NACE TM0177-2016 Method A, with no failures and no secondary cracks after 720 hours. All the pipes showed an excellent sulphide stress cracking resistance and can therefore be used in severe sour conditions.

[0241] 7. Test after Straining and Ageing

[0242] The steel composition (A4) and (A5) according to the present invention has been prepared from the elements indicated in the table 6 above. The steel thus obtained has then been hot formed according to the protocol previously mentioned in example 2. After quenching and tempering, the pipe has been aged and deformed according to the following protocol: an uniaxial tensile stress causing a plastic deformation of 5% was applied to the full wall pipe, and then the pipes have then been aged at 250 C. for 1 hour.

7.1. Hardness Behavior

[0243] The hardness behavior has been evaluated according to the standard ISO 6507-1, as previously mentioned in example 1.

[0244] The maximum hardness measurement obtained was 240HV10. The results thus obtained clearly show that even after straining and ageing, pipes made of steel according to the present invention comply with the requirements of DNVGL-ST-F101. Indeed, all hardness values are well below 250 HV10.

7.2. Impact Energy Test

[0245] For pipes according to Steel (A4), impact energy values have been determined according to the Charpy impact test ASTM E23Type A, using full size specimen.

[0246] The results are given in the table 7 below for pipe (A4) without straining and ageing, and pipe (A4-5%) after straining and ageing. The results shown in the table correspond to the mean values of the three measurements using full size specimens.

TABLE-US-00007 TABLE 7 Charpy Impact Test Test T Absorbed energy (J) Shear area (%) ( C.) A4 A4-5% A4 A4-5% 80 390 175 100 70 60 405 287 100 100 30 400 298 100 100 10 403 297 100 100

[0247] The results thus obtained show that pipes made of steel according to the present invention present no brittle fracture even at 80 C. for the as-quenched and tempered conditions.

[0248] In addition, even after a severe uniaxial deformation of 5% followed by ageing at 250 C. for 1 h, this good ductile behavior is kept for temperatures down to 60 C.

7.3. Crack Tip Opening Displacement (CTOD)

[0249] Three specimen were cut on each pipes (A4) and (A4-5%), and respectively (A5). The tests and results evaluations followed the standard BS 7448-1. The tested pipes had a wall thickness of 25.4 mm.

[0250] The CTOD values are given in the table 8 here below.

TABLE-US-00008 TABLE 8 CTOD CTOD values (mm) Steel T ( C.) notch 1 notch 2 notch 3 mean A4 20 C. 1.35 1.36 1.34 1.35 A4 60 C. 1.23 1.20 1.27 1.23 A5 60 C. 1.31 1.38 1.32 1.33 A5 60 C. 1.24 1.18 1.21 1.21 A4-5% 20 C. 1.22 1.18 1.15 1.18 A4-5% 60 C. 1.02 1.08 1.11 1.07

[0251] The results thus obtained show that the CTOD values of the steel according to the present invention are much higher than 0.150 mm as established by DNVGL-ST-F01 specification. Indeed, even at 60 C., the CTOD values are higher than 1 mm.

[0252] In addition, all the pipes (A1a), (A1b-2%) and (A1b-5%) present a ductile fracture mode.

[0253] Both impact energy and CTOD tests show that the steel of the present invention present an excellent resistance to stress. Even after deformation and ageing, the pipes remain tough for temperatures down to 60 C.

7.4. SSC Resistance

[0254] After 720 hours, none of the pipes (A4) and (A4-5%) presents a failure nor secondary cracks. All the pipes show an excellent sulphide stress cracking resistance, even after straining and ageing, and can therefore be used in severe sour conditions.