STEEL MATERIAL FOR LINE PIPES, METHOD FOR PRODUCING THE SAME, AND METHOD FOR PRODUCING LINE PIPE

Abstract

A method for producing a steel material for line pipes including heating a steel having a specific composition to a temperature of 1000 C. to 1200 C.; performing hot rolling such that a cumulative rolling reduction ratio in a non-recrystallization temperature range is 60% or more, a cumulative rolling reduction ratio in a temperature range of (a rolling finish temperature +20 C.) or less is 50% or more, and a rolling finish temperature is the Ar.sub.3 transformation point or more and 790 C. or less; subsequently performing accelerated cooling from a temperature of the Ar.sub.3 transformation point or more, at a cooling rate of 10 C./s or more, to a cooling stop temperature of 200 C. to 450 C.; and then performing reheating such that the temperature of a surface of the steel plate is 350 C. to 550 C. and the temperature of the center of the steel plate is less than 550 C.

Claims

1. A method for producing a steel material for line pipes, the steel material having a tensile strength of 570 MPa or more, a compressive strength of 440 MPa or more, and a thickness of 30 mm or more, the method comprising heating a steel having a composition containing, by mass, C: 0.030% to 0.10%, Si: 0.01% to 0.30%, Mn: 1.0% to 2.0%, Nb: 0.005% to 0.050%, Ti: 0.005% to 0.025%, and Al: 0.08% or less, the composition further containing one or more elements selected from, by mass, Cu: 0.5% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 0.5% or less, and V: 0.1% or less, wherein a Ceq value represented by Formula (1) is 0.350 or more, a Pcm value represented by Formula (2) is 0.20 or less, and an Ar.sub.3 transformation point represented by Formula (3) is 750 C. or less, with the balance being Fe and inevitable impurities, to a temperature of 1000 C. to 1200 C.; performing hot rolling such that a cumulative rolling reduction ratio in a non-recrystallization temperature range is 60% or more, such that a cumulative rolling reduction ratio in a temperature range of (a rolling finish temperature +20 C.) or less is 50% or more, and such that a rolling finish temperature is the Ar.sub.3 transformation point or more and 790 C. or less, the rolling finish temperature being an average temperature of a steel plate; subsequently performing accelerated cooling from a temperature of the Ar.sub.3 transformation point or more, at a cooling rate of 10 C./s or more, to a cooling stop temperature of 200 C. to 450 C., the cooling stop temperature being an average temperature of the steel plate; and then performing reheating such that the temperature of a surface of the steel plate is 350 C. to 550 C. and such that the temperature of the center of the steel plate is less than 550 C.,
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 (1)
Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10 (2)
Ar.sub.3( C.)=910310C80Mn20Cu15Cr55Ni80Mo (3) wherein, in Formulae (1) to (3), the symbol of each element represents the content (mass %) of the element and is zero when the composition does not contain the element.

2. A method for producing a line pipe having a tensile strength of 570 MPa or more, a compressive strength of 440 MPa or more, and a thickness of 30 mm or more, the method comprising cold forming a steel material for line pipes produced by the method according to claim 1 into a steel pipe-like shape; joining butting edges to each other by seam welding; and subsequently performing pipe expansion at an expansion ratio of 1.2% or less to produce a steel pipe.

3. A steel material for line pipes, the steel material having a tensile strength of 570 MPa or more, a compressive strength of 440 MPa or more, and a thickness of 30 mm or more, the steel material comprising a composition containing, by mass, C: 0.030% to 0.10%, Si: 0.01% to 0.30%, Mn: 1.0% to 2.0%, Nb: 0.005% to 0.050%, Ti: 0.005% to 0.025%, and Al: 0.08% or less, the composition further containing one or more elements selected from, by mass, Cu: 0.5% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 0.5% or less, and V: 0.1% or less, wherein a Ceq value represented by Formula (1) is 0.350 or more, a Pcm value represented by Formula (2) is 0.20 or less, and a Ar.sub.3 transformation point represented by Formula (3) is 750 C. or less, with the balance being Fe and inevitable impurities, the steel material further comprising a metal microstructure composed primarily of bainite, wherein an area fraction of polygonal ferrite at a position of plate thickness is 10% or less, an area fraction of martensite-austenite constituent at the position of plate thickness is 5% or less, and an average grain size of bainite at a position of plate thickness is 10 m or less,
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 (1)
Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10 (2)
Ar.sub.3( C.)=910310C80Mn20Cu15Cr55Ni80Mo (3) wherein, in Formulae (1) to (3), the symbol of each element represents the content (mass %) of the element and is zero when the composition does not contain the element.

4. The steel material for line pipes according to claim 3, wherein a ratio of compressive strength to tensile strength is 0.748 or more, and wherein a hardness measured at a position 1.5 mm from an inner surface of a steel pipe is HV 260 or less.

5. A method for producing a line pipe having a tensile strength of 570 MPa or more, a compressive strength of 440 MPa or more, and a thickness of 30 mm or more, the method comprising cold forming a steel material for line pipes according to claim 3 into a steel pipe-like shape; joining butting edges to each other by seam welding; and subsequently performing pipe expansion at an expansion ratio of 1.2% or less to produce a steel pipe.

6. A method for producing a line pipe having a tensile strength of 570 MPa or more, a compressive strength of 440 MPa or more, and a thickness of 30 mm or more, the method comprising cold forming a steel material for line pipes according to claim 4 into a steel pipe-like shape; joining butting edges to each other by seam welding; and subsequently performing pipe expansion at an expansion ratio of 1.2% or less to produce a steel pipe.

Description

EXAMPLES

[0106] Slabs were manufactured from steels (Steel types A to K) having the chemical compositions described in Table 1 by a continuous casting process. Steel plates (Nos. 1 to 26) having a thickness of 35 to 40 mm were manufactured from the slabs. Steel pipes were manufactured from the steel plates by the UOE process. Seam welding was performed by four-wire submerged arc welding such that one welding path is formed on both of the inner and outer surfaces of the seam. The heat input during the welding was selected from the range of 20 to 80 kJ/cm in accordance with the thickness of the steel plate. Table 2 summarizes the conditions under which the steel plates were produced and the condition under which the steel pipes were produced (expansion ratio).

TABLE-US-00001 TABLE 1 Ar.sub.3 Steel Composition (mass %) Ceq Pcm transformation type C Si Mn Nb Ti Al Cu Ni Cr Mo V value .sup.(1) value .sup.(2) point .sup.(3) Remark A 0.050 0.150 1.80 0.028 0.012 0.030 0.020 0.200 0.250 0.100 0.020 0.439 0.171 727 Invention B 0.060 0.230 1.75 0.020 0.015 0.033 0.200 0.365 0.159 740 example C 0.095 0.040 1.55 0.025 0.010 0.020 0.200 0.393 0.187 741 D 0.065 0.050 1.60 0.030 0.011 0.025 0.150 0.150 0.200 0.030 0.398 0.170 748 E 0.060 0.040 1.90 0.025 0.020 0.033 0.050 0.300 0.050 0.410 0.167 718 F 0.050 0.050 1.85 0.028 0.010 0.030 0.150 0.150 0.300 0.020 0.442 0.171 731 G 0.025 0.100 1.58 0.028 0.020 0.025 0.600 0.005 0.329 0.118 743 Comparative H 0.080 0.150 1.80 0.030 0.013 0.033 0.200 0.250 0.200 0.200 0.490 0.213 704 example I 0.055 0.210 1.53 0.012 0.032 0.180 0.150 0.352 0.155 765 J 0.140 0.150 1.55 0.025 0.011 0.028 0.200 0.200 0.425 0.236 728 K 0.065 0.350 1.80 0.030 0.015 0.025 0.200 0.200 0.100 0.010 0.414 0.188 723 *The underlined values are outside the scope of the present invention. .sup.(1) Ceq = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5 .sup.(2) Pcm = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 .sup.(3) Ar.sub.3 = 910 310 C 80 Mn 20 Cu 15 Cr 55 Ni 80 Mo

TABLE-US-00002 TABLE B Cumulative rolling reduction ratio Below (rolling Non- finish Rolling Cooling Cooling Reheating Ar.sub.3 Heating recrystallization temperature + finish start Cooling stop temperature Expansion Steel transformation Thickness temperature temperature 20 C.) temperature temperature rate temperature ( C.) ratio No. type point ( C.) (mm) ( C.) range (%) (%) ( C.) ( C.) ( C./s) ( C.) Reheating facility Surface Center (%) Remark 1 A 727 40 1050 75 70 760 755 25 280 Induction heating furnace 470 420 0.8 Invention 2 A 727 40 1030 75 55 765 750 20 300 Induction heating furnace 450 400 0.8 example 3 A 727 40 1040 75 75 780 770 20 320 Induction heating furnace 450 400 0.8 4 A 727 40 1060 75 70 765 755 27 280 Induction heating furnace 360 320 0.8 5 A 727 40 1050 75 70 760 750 22 310 Induction heating furnace 520 470 0.8 6 B 740 35 1100 80 75 775 770 30 260 Gas combustion furnace 460 450 0.8 7 C 741 35 1060 75 75 775 765 35 420 Induction heating furnace 520 470 1.0 8 D 748 35 1100 75 70 770 760 30 300 Induction heating furnace 500 450 1.0 9 E 718 35 1050 75 70 775 765 28 280 Induction heating furnace 470 420 1.0 10 F 731 40 1050 75 70 755 745 32 260 Induction heating furnace 460 420 0.6 11 A 727 40 950 75 70 770 760 20 280 Induction heating furnace 460 420 0.8 Comparative 12 A 727 40 1250 75 70 765 760 25 290 Induction heating furnace 470 450 0.8 example 13 A 727 40 1050 55 55 760 750 25 270 Induction heating furnace 450 400 0.8 14 A 727 40 1040 75 45 765 760 25 310 Induction heating furnace 460 410 0.8 15 A 727 40 1030 75 70 725 720 20 320 Induction heating furnace 450 400 0.8 16 A 727 40 1050 75 70 800 790 30 300 Induction heating furnace 480 420 0.8 17 F 731 40 1070 75 75 760 750 20 500 Induction heating furnace 540 510 0.8 18 F 731 40 1040 75 70 770 760 30 280 Induction heating furnace 320 280 0.8 19 F 731 40 1050 75 70 760 750 25 290 Induction heating furnace 600 550 0.8 20 A 727 40 1050 75 70 760 755 25 280 None 0.8 21 F 731 40 1040 75 75 760 750 26 290 Induction heating furnace 460 410 1.6 22 G 743 35 1060 75 70 775 770 25 310 Induction heating furnace 480 430 0.8 23 H 704 40 1030 75 70 760 750 20 320 Induction heating furnace 450 400 1.0 24 I 765 35 1050 80 75 785 770 20 280 Induction heating furnace 420 380 0.8 25 J 728 40 1080 75 75 765 755 25 310 Induction heating furnace 490 460 0.8 26 K 723 35 1030 75 70 770 760 25 290 Induction heating furnace 470 420 0.8 *The underlined values are outside the scope of the present invention.

[0107] To determine the tensile properties of the steel pipes produced as described above, a full-thickness test piece in the circumferential direction of the pipe was taken from each of the steel pipes as a test piece for tensile test and the tensile strength of the test piece was measured by a tensile test. In a compression test, a test piece having a diameter of 20 mm and a length of 60 mm was taken from the inner surface-side portion of each of the steel pipes in the circumferential direction of the pipe and the 0.5% compressive proof strength of the test piece was measured as a compressive yield strength.

[0108] A DWTT test piece was taken from each of the steel pipes in the circumferential direction of the pipe. Using the DWTT test piece, the temperature at which the percent ductile fracture reached 85% was determined as 85% SATT.

[0109] For determining the HAZ toughness of the joint, the temperature at which the percent ductile fracture reached 50% was determined as vTrs. The position of the notch was determined such that the fusion line was located at the center of the notch root of the Charpy test piece and the ratio between the weld metal and the base metal (including weld heat-affected zone) at the notch root was 1:1.

[0110] For determining the hardness of each of the steel pipes at a position 1.5 mm from the surface, the hardness of the steel pipe was measured at randomly selected 20 positions spaced at intervals of 10 mm in the circumferential direction of the steel pipe at a depth of 1.5 mm below the inner surface of the steel pipe using a Vickers hardness tester with a load of 10 kgf (98 N) and the average thereof was calculated.

[0111] For determining metal microstructure, a sample was taken from the inner surface-side portion of each of the steel pipes at a position of plate thickness. The sample was etched using nital after polishing, and the metal microstructure was observed using an optical microscope. The area fractions of bainite and polygonal ferrite were calculated by image analysis of 3 photographs captured at a 200-fold magnification. For observing MA, the sample used for measuring the area fractions of bainite and polygonal ferrite was subjected to nital etching and then electrolytic etching (two-step etching). Subsequently, the metal microstructure was observed with a scanning electron microscope (SEM). The area fraction of MA was calculated by image analysis of 3 photographs captured at a 1000-fold magnification. The average grain size of bainite was determined by a linear analysis using a micrograph obtained by taking a sample from the inner surface-side portion of each of the steel pipes at a position of plate thickness, etching the sample using nital after polishing, and observing the metal microstructure using an optical microscope.

[0112] Although the metal microstructures of the steel pipes are determined in Examples, the results may be considered as the metal microstructures of the respective steel plates (steel materials).

[0113] Table 3 shows the metal microstructures and mechanical properties measured.

TABLE-US-00003 TABLE 3 Metal microstructure Plate thickness 1/4 position Area Mechanical properties Area fraction of Plate thickness HV (10 kg) at DWTT HAZ Area fraction of martensite- 1/2 position Tensile Compressive Compressive position 1.5 mm property toughness Steel fraction of polygonal austenite Bainite grain strength strength strength/tensile from steel 85% vTrs No. type bainite (%) ferrite (%) constituent (%) Balance size (m) (MPa) (MPa) strength pipe surface SATT ( C.) ( C.) Remark 1 A 94.5 3.7 0.9 6.5 620 511 0.824 228 27 35 Invention 2 A 93.8 4.5 1.3 8.5 612 519 0.848 230 22 37 example 3 A 98.3 0.0 1.5 9.2 634 563 0.888 235 17 35 4 A 94.9 1.7 3.2 6.0 648 487 0.752 254 30 37 5 A 94.8 3.5 0.5 7.3 603 476 0.789 214 26 35 6 B 91.6 4.8 3.2 7.5 586 469 0.800 216 25 50 7 C 91.2 4.5 4.1 6.5 591 538 0.910 212 33 25 8 D 89.5 8.0 1.3 7.2 613 471 0.769 216 28 27 9 E 98.0 0.0 1.4 6.5 610 486 0.796 226 22 40 10 F 95.5 2.3 1.1 7.0 620 562 0.906 225 23 37 11 A 98.5 0.0 1.2 6.0 562 470 0.836 210 32 38 Comparative 12 A 97.5 0.0 1.7 18.0 732 633 0.864 256 5 38 example 13 A 95.1 2.3 1.5 17.0 622 503 0.809 226 5 37 14 A 98.0 0.0 1.1 17.5 623 549 0.882 240 0 36 15 A 74.5 22.0 2.5 6.0 565 425 0.752 209 32 37 16 A 97.9 0.0 1.5 20.0 646 554 0.858 255 0 37 17 F 93.2 2.0 1.3 , P 9.0 580 429 0.740 205 15 39 18 F 93.2 0.0 6.8 6.5 672 436 0.648 266 25 39 19 F 96.3 0.0 1.4 7.5 582 369 0.634 212 17 41 20 A 86.4 6.0 7.6 8.2 669 376 0.561 252 17 41 21 F 96.5 2.0 1.2 6.5 619 430 0.695 228 25 41 22 G 96.9 2.5 0.3 7.0 526 434 0.825 197 20 53 23 H 95.6 0.0 2.3 , P 6.0 673 582 0.865 250 33 5 24 I 86.0 11.0 2.1 8.7 561 417 0.744 208 14 42 25 J 90.8 1.5 6.1 , P 6.2 614 439 0.714 226 25 0 26 K 94.3 0.0 5.7 6.5 600 437 0.728 223 23 28 * The underlined values are outside the scope of the present invention. * In the above table, and P denote cementite and pearlite, respectively.

[0114] In Table 3, all of Nos. 1 to 10 had a tensile strength of 570 MPa or more; a compressive strength of 440 MPa or more; as for DWTT property, a 85% SATT of 10 C. or less; and a HAZ toughness of 20 C. or less. That is, all of Nos. 1 to 10 were evaluated as good. Moreover, in all of Nos. 1 to 10, the ratio of compressive strength to tensile strength was 0.75 or more and hardness at a position 1.5 mm from the surface of the steel pipe was HV 260 or less. This is effective for producing steel pipes having suitable roundness further stably.

[0115] In contrast, in Nos. 11 to 21, although the composition fell within the scope according to aspects of the present invention, the production method was outside the scope of the present invention and therefore the intended microstructure was not formed. As a result, Nos. 11 to 21 were evaluated as poor in terms of any of tensile strength, compressive strength, and DWTT property. In Nos. 22 to 26, the chemical composition was outside the scope of the present invention. As a result, Nos. 22 to 26 were evaluated as poor in terms of any of tensile strength, compressive strength, DWTT property, and HAZ toughness. In Nos. 18 and 19, the production conditions during reheating were outside the scope of the present invention. As a result, Nos. 18 and 19 were evaluated as poor in terms of the ratio of compressive strength to tensile strength and hardness at a position 1.5 mm from the surface of the steel pipe.

[0116] According to aspects of the present invention, a high-strength steel pipe of API-X70 grade or more which has excellent low-temperature toughness and an excellent DWTT property may be produced. Therefore, the steel pipe according to aspects of the present invention may be used as deep-sea line pipes that require high collapse resistant performance.