Steel material for high-strength line pipe with high fracture toughness in hydrogen and method for producing the same, and steel pipe for highstrength line pipe and method for producing the same

Abstract

The purpose of the present invention is to provide a high-strength line pipe steel material that has excellent fracture toughness in hydrogen in a high-pressure hydrogen gas environment, and that is suitable for use for a steel structure used in a high-pressure hydrogen gas environment, such as a line pipe for 100% hydrogen gas or for natural gas (natural gas is gas containing a hydrocarbon such as methane and ethane as the main component) containing hydrogen at a hydrogen partial pressure of 1 MPa or more; a method for manufacturing the steel material; a steel tube for high-strength line pipes; and a method for manufacturing the steel tube.　This high-strength line pipe steel material having excellent fracture toughness in hydrogen is characterized by having a specific compositional makeup and a specific structure, having a tensile strength of 520 MPa or more, and having a hydrogen induced crack propagation lower limit KIH of 80 MPa·m1 / 2 or more in a high-pressure hydrogen gas environment of 1 MPa or more.

Description

Underline: outside the scope of the present invention. B: bainite cr>

[0096] Steel Pipe No. Steel material No. Steel No. Area fraction of retained austenite (%) B fraction (%) Maximum grain size of bainite (pm) Number density of inclusions ( / 100 mm2) Tensile strength (MPa) Hydrogen-induced crack growth threshold KIH (MPa ^ / m) Notes Base metal zone 21 21 21 0.0 90.1 17 9 586 140 Inventive example 22 22 22 0.2 94.0 24 13 525 160 Inventive example 23 23 23 0.0 97.0 23 9 548 149 Inventive example 24 24 24 1.5 92.9 40 14 530 58 Comparative example 25 25 25 0.0 90.2 20 10 587 124 Inventive example 26 26 26 0.0 92.8 19 11 554 137 Inventive example 27 27 27 0.0 92.7 17 9 503 90 Comparative example 28 28 28 1.2 91.1 29 8 530 70 Comparative example 29 29 29 1.1 82.1 26 13 501 69 Comparative example 30 30 30 0.0 93.0 30 14 556 70 Comparative example 31 31 31 0.0 90.9 18 10 520 82 Inventive example 32 32 32 1.7 93.6 24 15 700 81 Inventive example 33 33 33 0.3 99.3 19 15 579 81 Inventive example 34 34 34 3.9 90.1 17 12 593 61 Comparative example 35 35 35 0.0 90.9 17 25 609 69 Comparative example 36 36 36 0.2 94.0 19 27 525 59 Comparative example 37 37 37 0.9 93.2 21 9 582 102 Inventive example 38 38 38 1.0 90.4 18 13 601 99 Inventive example Underline: outside the scope of the present invention. B: bainite EXAMPLE 2

[0097] Examples that have verified the advantages of the present invention are described below. In the following Examples, steel materials and steel pipes were produced under the following production conditions and were characterized. The steel Nos. 2, 4, 8, 14, 22, and 33 shown in Tables 1-1 and 1-2 used in Example 1 were used, up to the controlled cooling step was performed under the same conditions as the steel pipes 2, 4, 8, 14, 22, and 33 shown in Example 1 (Tables 2-1 and 2-2) . Steel pipe forming was also performed under the same conditions as Example 1, and the characteristics were evaluated while the dehydrogenation treatment conditions were changed. Table 4 shows the results .

[0098] The dehydrogenation treatment of the steel pipes Nos. 2, 4, 8, 14, 22, and 33 in Example 1 was performed at a dehydrogenation treatment temperature T (ambient temperature) and time shown in Tables 2-1 and 2-2. As shown in Table 4, the dehydrogenation holding time t corresponds to Y and the retention time tc at the temperature Tc at the middle of the sheet thickness corresponds to N, respectively in Table 4.

[0099] For the steel pipes Nos. 2A, 4A, 8A, 14A, 22A, and 33A, the dehydrogenation treatment temperature T was the temperature shown in Table 4, and the holding time to after the temperature To at the middle of the sheet thickness reached the dehydrogenation treatment temperature T shown in Table 4 satisfied the formula (A).

[0100] For the steel pipes Nos. 2B, 4B, 8B, 14B, 22B, and 33B, the dehydrogenation treatment temperature T is the temperature shown in Table 4, but neither the holding time t at the ambient temperature nor the holding time tc after the temperature Tc at the middle of the sheet thickness reaches the dehydrogenation treatment temperature T satisfy the formula (A).

[0101] In Table 4, "Dehydrogenation holding time t is Y" means that the dehydrogenation treatment temperature T (ambient temperature) is a predetermined temperature and the holding time t satisfies the formula (A), and "Dehydrogenation holding time t is N" means that the dehydrogenation treatment temperature T (ambient temperature) is a predetermined temperature, but the holding time t does not satisfy the formula (A). Furthermore, "Holding time tc at steel material center temperature Tc is Y" means that the holding time tc after the temperature Tc at the middle of the sheet thickness reaches a predetermined temperature satisfies the formula (A), and "Holding time tc at steel material center temperature Tc is N" means that the temperature Tc at the middle of the sheet thickness reaches a predetermined temperature, but the holding time tc after Tc reaches a predetermined temperature does not satisfy the formula (A).

[0102] The examination of the fracture toughness in hydrogen and the tensile strength and the evaluation of the microstructure and inclusions were performed in the same manner as in Example 1.

[0103] All of the examples of the present invention satisfied the conditions of a hydrogen-induced crack growth threshold Kih of 80 MPa -m1 / 2 or more and a tensile strength of 520 MPa or more. Among them, the fracture toughness resistance in hydrogen was better when the dehydrogenation treatment was performed under more suitable conditions.

[0104] The steel pipes in Table 4 also showed the same results as the steel materials.

[0105] [Table 4] Steel pipe No. Steel material No. Steel No. Dehydrogenation treatment temperature T (°C) Dehydrogenation holding time t Holding time to at steel material center temperature Tc Area fraction of retained austenite (%) B fraction (%) Maximum grain size of bainite (pm) Number density of inclusions ( / 100 mm2) Tensile strength (MPa) Hydrogen-induced crack growth threshold KIH (MPa ^m) base metal zone Notes 2A 2A 2 200 Y Y 0.2 92.5 23 12 540 199 Inventive example 2 2 2 200 Y N 0.2 92.5 23 12 561 159 Inventive example 2B 2B 2 200 N N 0.2 92.5 23 12 570 134 Inventive example 4A 4A 4 50 Y Y 0.0 98.1 21 12 552 188 Inventive example 4 4 4 50 Y N 0.0 98.1 21 12 552 170 Inventive example 4B 4B 4 50 N N 0.0 98.1 21 12 552 132 Inventive example 8A 8A 8 25 Y Y 0.0 90.6 19 9 662 151 Inventive example 8 8 8 25 Y N 0.0 90.6 19 9 662 100 Inventive example 8B 8B 8 25 N N 0.0 90.6 19 9 662 88 Inventive example 14A 14A 14 500 Y Y 0.0 97.1 22 9 589 141 Inventive example 14 14 14 500 Y N 0.0 97.1 22 9 626 100 Inventive example 14B 14B 14 500 N N 0.0 97.1 22 9 641 89 Inventive example 22A 22A 22 80 Y Y 0.2 94.0 24 13 525 179 Inventive example 22 22 22 80 Y N 0.2 94.0 24 13 525 160 Inventive example 22B 22B 22 80 N N 0.2 94.0 24 13 525 120 Inventive example 33A 33A 33 500 Y Y 0.3 99.3 19 15 540 139 Inventive example 33 33 33 500 Y N 0.3 99.3 19 15 579 81 Inventive example 33B 33B 33 500 N N 0.3 99.3 19 15 588 80 Inventive example B: bainite

Claims

CLAIMS

1. A steel material for a high-strength line pipe with high fracture toughness in hydrogen, the steel material comprising:a chemical composition containing: on a mass percent basis, C: 0 . 02% to 0.15%, Si: 0.01% to 2.0%, Mn: 0.5% to 1.5%, P: 0.0001% to 0.015%, S: 0.0002% to 0.0015%, Al: 0.005% to 0.15%, 0: 0.01% or less, N: 0.010% or less, Nb: 0.10% or less, and H: 0.02 ppm or less, and optionally at least one selected from Ca: 0% to 0.005%, Ni: 0% to 2.0%, Ti: 0% to 0.1%, Cu: 0% to 1.0%, Cr: 0% to 1.0%, Mo: 0% to 0.60%, W: 0% to 1.0%,V: 0% to 0.10%,Zr: 0% to 0.050%,Mg: 0% to 0.01%,REM: 0% to 0.01%,B: 0% to 0.0020%,Ta: 0% to 0.2%,Hf: 0% to 0.2%,Re: 0% to 0.005%,Sn: 0% to 0.3%, andSb: 0% to 0.3%,the remainder being Fe and an incidental impurity element,a metallic microstructure containing bainite and 15 pieces / 100 mm2 or less of inclusions having an aspect ratio of 2.0 or more and a length of 10 pm or more, the bainite in a range from a surface to a middle of a thickness of the steel material having a maximum grain size of 25 pm or less,tensile strength of 520 MPa or more, anda hydrogen-induced crack growth threshold Km in a high-pressure hydrogen gas environment of 1 MPa or more of 80 MPa-m1 / 2 or more.

2. The steel material for a high-strength line pipe with high fracture toughness in hydrogen according to Claim 1, wherein the chemical composition contains, on a mass percentbasis,Ca: 0.0001% to 0.005%,Ni : 0.01% to 2.0%,Ti : 0.005% to 0.1%,Cu: 0.01% to 1.0%,Cr: 0.01% to 1.0%,Mo: 0.01% to 0.60%,W: 0.01% to 1.0%,V: 0 . 01% to 0.10%,Zr: 0.0001% to 0.050%,Mg: 0.0001% to 0.01%,REM: 0.0001% to 0.01%,B: 0.0001% to 0.0020%,Ta: 0.0001% to 0.2%,Hf: 0.0001% to 0.2%,Re: 0.0001% to 0.005%,Sn: 0.0001% to 0.3%, andSb: 0.0001% to 0.3%.

3. The steel material for a high-strength line pipe with high fracture toughness in hydrogen according to Claim 1 or 2, wherein an area fraction of retained austenite is 0% to 3% by area, and an area fraction of the bainite in the range from the surface to the middle of the thickness of the steelmaterial is 90% or more.

4. A method for producing a steel material for a high-strength line pipe with high fracture toughness in hydrogen, the method comprising:a heating step of heating a cast steel having the chemical composition according to Claim 1 or 2 at 1000°C to 1250°C;a hot rolling step of rolling the cast steel heated in the heating step under conditions in which a total rolling reduction in a recrystallization temperature range is 35% or more and 55% or less, a rolling reduction in a final rolling pass in the recrystallization temperature range is 10% or more, a rolling reduction in a final rolling pass at (recrystallization temperature - 80°C) or more is 15% or more, and a finish rolling temperature is an Ara transformation point or higher in terms of a temperature at a surface of a steel sheet; anda controlled cooling step of cooling a hot-rolled steel sheet produced in the hot rolling step under conditions in which a cooling start temperature is the Ara transformation point or higher in terms of a temperature at the surface of the hot-rolled steel sheet, a cooling start time difference between a front end and a rear end of the hot-rolled steel sheet is 50 seconds or less, an average cooling rate from 750°C to 550°C ranges from 15°C / s to50°C / s in terms of a temperature at a middle of a thickness of the steel sheet, and a cooling stop temperature ranges from 250°C to 650°C.

5. A steel pipe for a high-strength line pipe with high fracture toughness in hydrogen, the steel pipe comprising: a chemical composition containing: on a mass percent basis, C: 0 . 02% to 0.15%, Si: 0.01% to 2.0%, Mn: 0.5% to 1.5%, P: 0.0001% to 0.015%, S: 0.0002% to 0.0015%, Al: 0.005% to 0.15%, 0: 0.01% or less, N: 0.010% or less, Nb: 0.10% or less, and H: 0.02 ppm or less, and optionally at least one selected from Ca: 0% to 0.005%, Ni: 0% to 2.0%, Ti: 0% to 0.1%, Cu: 0% to 1.0%, Cr: 0% to 1.0%, Mo: 0% to 0.60%,W: 0% to 1.0%,V: 0% to 0.10%,Zr: 0% to 0.050%,Mg: 0% to 0.01%,REM: 0% to 0.01%,B: 0% to 0.0020%,Ta: 0% to 0.2%,Hf: 0% to 0.2%,Re: 0% to 0.005%,Sn: 0% to 0.3%, andSb: 0% to 0.3%,the remainder being Fe and an incidental impurity element,a metallic microstructure containing bainite and 15 pieces / 100 mm2 or less of inclusions having an aspect ratio of 2.0 or more and a length of 10 pm or more, the bainite in a range from an inner surface to a middle of a thickness of the steel pipe having a maximum grain size of 25 pm or less,tensile strength of 520 MPa or more, anda hydrogen-induced crack growth threshold Km in a high-pressure hydrogen gas environment of 1 MPa or more of 80 MPa-m1 / 2 or more.

6. The steel pipe for a high-strength line pipe with high fracture toughness in hydrogen according to Claim 5, whereinthe chemical composition contains, on a mass percent basis, Ca: 0.0001% to 0.005%, Ni : 0.01% to 2.0%, Ti: 0.005% to 0.1%,Cu: 0.01% to 1.0%,Cr: 0.01% to 1.0%,Mo: 0.01% to 0.60%,W: 0.01% to 1.0%,V: 0 . 01% to 0.10%,Zr: 0.0001% to 0.050%,Mg: 0.0001% to 0.01%,REM: 0.0001% to 0.01%,B: 0.0001% to 0.0020%,Ta: 0.0001% to 0.2%,Hf: 0.0001% to 0.2%,Re: 0.0001% to 0.005%,Sn: 0.0001% to 0.3%, andSb: 0.0001% to 0.3%.

7. The steel pipe for a high-strength line pipe with high fracture toughness in hydrogen according to Claim 5 or 6, wherein an area fraction of retained austenite is 0% to 3%, and an area fraction of the bainite in the range from the inner surface to the middle of the thickness of the steelpipe is 90% or more.

8. A method for producing a steel pipe for a high-strength line pipe with high fracture toughness in hydrogen, the method comprising:a heating step of heating a cast steel having the chemical composition according to Claim 5 or 6 at 1000°C to 1250°C;a hot rolling step of rolling the cast steel heated in the heating step under conditions in which a total rolling reduction in a recrystallization temperature range is 35% or more and 55% or less, a rolling reduction in a final rolling pass in the recrystallization temperature range is 10% or more, a rolling reduction in a final rolling pass at (recrystallization temperature - 80°C) or more is 15% or more, and a finish rolling temperature is an Ara transformation point or higher in terms of a temperature of a surface of a steel sheet;a controlled cooling step of cooling a hot-rolled steel sheet produced in the hot rolling step under conditions in which a cooling start temperature is the Ara transformation point or higher in terms of a temperature at a surface of the hot-rolled steel sheet, a cooling start time difference between a front end and a rear end of the hot-rolled steel sheet is 50 seconds or less, an average cooling rate from 750°C to 550°C ranges from 15°C / s to50°C / s in terms of a temperature at a middle of a thickness of the steel sheet, and a cooling stop temperature ranges from 250°C to 650°C; andany one of a pipe production step of bending the hot-rolled steel sheet and butt-welding both end portions thereof after the controlled cooling step and a pipe production step of forming the hot-rolled steel sheet into a cylindrical shape by cold roll forming and subjecting both circumferential end portions of the cylindrical shape to butt electric resistance welding after the controlled cooling step.