X80 pipeline steel with good strain-aging performance, pipeline tube and method for producing same

Abstract

A X80 pipeline steel with good strain-aging performance comprises (wt. %): C: 0.02-0.05%; Mn: 1.30-1.70%; Ni: 0.35-0.60%: Ti: 0.005-0.020%; Nb: 0.06-0.09%; Si: 0.10-0.30%; Al: 0.01-0.04%; N≤0.008%; P≤0.012%; S≤0.006%; Ca: 0.001-0.003%, and balance iron and unavoidable impurities.

Claims

1. An X80 pipeline steel with a strain-aging resistance, consisting of chemical elements in percentage by mass: 0.02-0.05% of C, 1.30-1.70% of Mn, 0.35-0.60% of Ni, 0.005-0.020% of Ti, 0.06-0.09% of Nb, 0.10-0.30% of Si, 0.01-0.04% of Al, N≤0.008%, P≤0.012%, S≤0.006%, 0.001-0.003% of Ca, Cr≤0.30 wt %, and the balance being Fe and other inevitable impurities, and wherein the microstructure of the steel is polygonal ferrite+ acicular ferrite+ bainite, wherein the phase portion of said polygonal ferrite is 25-40%; wherein after an aging test being carried out under temperature-maintaining conditions of 200° C. for a period of 5 minutes, the steel has a longitudinal yield strength of 510-630 MPa, a tensile strength of 625-770 MPa, a uniform elongation of ≥6% and a yield ratio of ≤0.85, and the tensile curve of the steel appears as a dome-shaped continuous curve.

2. The X80 pipeline steel of claim 1, wherein a body of said X80 pipeline steel has a circumferential yield strength of 560-650 MPa and a tensile strength of 625-825 MPa.

3. A line pipe made of the X80 pipeline steel of claim 1.

4. A method for manufacturing the X80 pipeline steel of claim 1, comprising the steps of smelting, casting, casting slab heating, staged rolling, delayed rate-varying cooling and pipe making.

5. The method for manufacturing the X80 pipeline steel of claim 4, wherein in said casting step, continuous casting is used, and a ratio b which is defined as the thickness of the steel slab after the continuous casting to the thickness of the steel plate after the completion of the staged rolling is ≥10.

6. The method for manufacturing the X80 pipeline steel of claim 4, wherein in said casting slab heating step, the steel slab is reheated at a T Kelvin temperature, T=7510/(2.96−log [Nb][C])+30, wherein [Nb] and [C] respectively represent the contents in percentage by mass of Nb and C.

7. The method for manufacturing the X80 pipeline steel of claim 4, wherein said staged rolling step comprises a first rolling stage and a second rolling stage, and the steel slab is rolled to a thickness of 4t.sub.plate−0.4t.sub.slab in the first rolling stage, wherein t.sub.plate represents the thickness of the steel plate after the completion of the rolling step, and t.sub.slab represents the thickness of the steel slab after the continuous casting.

8. The method for manufacturing the X80 pipeline steel of claim 7, wherein the start rolling temperature of said first rolling stage is 960-1150° C., and the start rolling temperature of said second rolling stage is 740-840° C.

9. The method for manufacturing the X80 pipeline steel of claim 7, wherein at least two passes in said first rolling stage have a single pass reduction of ≥15%, and at least two passes in said second rolling stage have a single pass reduction of ≥20%.

10. The method for manufacturing the X80 pipeline steel of claim 7, wherein the finish rolling temperature of said second rolling stage is Ar3 to Ar3+40° C.

11. The method for manufacturing the X80 pipeline steel of claim 4, wherein in said delayed rate-varying cooling step, the steel plate after the completion of the rolling is first air-cooled and hold for 60-100 s to reach 700-730° C., and wherein ferrite at a phase proportion of 25-40% is precipitated.

12. The method for manufacturing the X80 pipeline steel of claim 11, wherein in said delayed rate-varying cooling step, after the precipitation of the ferrite at a phase proportion of 25-40%, the steel plate is water-cooled rapidly to 550-580° C. at a cooling rate of 25-40° C./s, and then further water-cooled slowly at a cooling rate of 18-22° C. %, with the final cooling temperature being 320-400° C.

13. The method for manufacturing the X80 pipeline steel of claim 4, wherein in said pipe making step, the O-moulding compression ratio is controlled at 0.15-0.3%, and the E-moulding diameter expansion ratio is controlled at 0.8-1.2%; wherein the O-moulding compression ratio=(the width of the steel sheet before moulding−the perimeter of the natural plane after O moulding)/the width of the steel sheet before moulding; and the E-moulding diameter expansion ratio=(the perimeter of the outer diameter of the steel pipe after diameter expansion−the perimeter of the outer diameter of the steel pipe before diameter expansion)/the perimeter of the outer diameter of the steel pipe before diameter expansion.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram of the delayed rate-varying cooling process in the method for manufacturing the X80 pipeline steel with good strain-aging resistance of the present invention.

(2) FIG. 2 is a metallographic diagram of the X80 pipeline steel with good strain-aging resistance of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

(3) The X80 pipeline steel with good strain-aging resistance, the line pipe and the manufacturing method for the pipe of the present invention are further explained and described below in conjunction with the description of the drawings and specific examples; however, the explanation and description do not constitute an inappropriate limitation to the technical solution of the present invention.

(4) X80 line pipes of Examples A1-A6 are manufactured according to the following steps, wherein the contents in percentage by mass of various chemical elements in the X80 line pipes of Examples A1-A6 are as shown in Table 1:

(5) 1) Smelting: molten steel is smelted and refined, with the proportions in percentage by mass of various chemical elements in the steel being as shown in Table 1;

(6) 2) Casting: a continuous casting method is used, and the ratio of the thickness of the steel slab after the continuous casting to the thickness of the steel plate after the completion of rolling is ≥10;

(7) 3) Casting slab heating: the steel slab is reheated at a T Kelvin temperature, T=7510/(2.96−log [Nb][C])+30, wherein [Nb] and [C] respectively represent the contents in percentage by mass of Nb and C; 4) Staged rolling step:

(8) 4i) first rolling stage (rough rolling): the start rolling temperature is 960-1150° C., the single pass reductions of at least two passes are ensured to be ≥15% and the thickness of the steel slab in rolling is controlled at 4t.sub.plate−0.4t.sub.slab, wherein t.sub.plate represents the thickness of the steel plate after the completion of the rolling step, and t.sub.slab represents the thickness of the steel slab after the continuous casting;

(9) 4i) second rolling stage (finishing rolling): the start rolling temperature is 740-840° C., the single pass reductions of at least two passes are ensured to be ≥20%, and the finish rolling temperature is Ar3 to Ar3+40° C.;

(10) 5) Delayed rate-varying cooling: the steel plate after the completion of the rolling is first air-cooled and hold for 60-100 s to reach 700-730° C. so that ferrite is precipitated at a phase proportion of 25-40%, and after the precipitation of the ferrite at a phase proportion of 25-40%, the steel plate is water-cooled rapidly to 550-580° C. at a cooling rate of 25-40° C./s, and then further water-cooled slowly at a cooling rate of 18-22° C. %, with the final cooling temperature being 320-400° C.; FIG. 1 shows the schematic diagram of the delayed rate-varying cooling process, and it can be seen from FIG. 1 that after the completion of the rolling of the steel plate, the steel plate undergoes air-cooling and temperature-holding phase 1, rapid water-cooling phase 2 and slow water-cooling phase 3 of different cooling rates.

(11) 6) Pipe making: the O-moulding compression ratio is controlled at 0.15-0.3%, and the E-moulding diameter expansion ratio is controlled at 0.8-1.2%.

(12) For the specific process parameters involved in the various steps of the above-mentioned manufacturing method in detail, reference can be made to Table 2.

(13) Table 1 lists the contents in percentage by mass of the various chemical elements for making the pipeline steels of Examples A1-A6.

(14) TABLE-US-00001 TABLE 1 (wt. %, the balance being Fe and inevitable impurities other than N, P and S) Serial number C Mn Ni Ti Nb Si Al Ca N P S Cr PF* (%) A1 0.030 1.70 0.60 0.017 0.08 0.30 0.033 0.0019 0.006 0.008 0.002 0.30 30 A2 0.040 1.65 0.49 0.014 0.075 0.30 0.030 0.0013 0.005 0.010 0.003 0.30 33 A3 0.045 1.68 0.50 0.009 0.06 0.25 0.030 0.0022 0.004 0.009 0.005 0.25 35 A4 0.045 1.50 0.45 0.012 0.06 0.20 0.025 0.0020 0.004 0.009 0.002 0.10 34 A5 0.045 1.40 0.40 0.011 0.06 0.20 0.030 0.0027 0.004 0.008 0.003 0.20 36 A6 0.050 1.35 0.35 0.008 0.06 0.15 0.020 0.0025 0.003 0.006 0.003 0.15 40 *Note: PF (%) is the phase proportion of a polygonal ferrite in a microstructure.

(15) Table 2 lists the process parameters of the method for manufacturing the X80 line pipes in Examples A1-A6.

(16) TABLE-US-00002 TABLE 2 Staged rolling First rolling stage Plate thickness Single after pass the reductions completion of two of the larger first passes Second rolling stage Steel Reheating rolling Start in Start slab Plate Heating stage rolling multiple rolling Serial thickness thickness Casting temperature (4t.sub.plate − temperature Rolling passes temperature Rolling number (mm) (mm) R* T* (K) 0.4t.sub.slab) (° C.) pass (%) (° C.) pass A1 250 17.5 14.3 1376 87 1060 7 17, 15 830 15 A2 300 22 13.6 1400 110 1080 7 16, 15 800 13 A3 300 28.6 10.5 1388 120 1055 5 15, 15 770 9 A4 300 25.4 11.8 1388 120 1063 5 15, 15 780 11 A5 300 22 13.6 1388 110 1042 7 16, 15 800 13 A6 300 21 14.3 1400 105 1026 7 16, 15 800 13 Staged rolling Second rolling stage Single pass reductions of two Delayed rate-varying cooling larger Temperature Pipe making passes after Rapid E-moulding in Finish Air rapid water Slow Final O-moulding diameter multiple rolling cooling Holding water cooling cooling cooling compression expansion Serial passes temperature time temperature cooling rate rate temperature ratio ratio number (%) (° C.) (s) (° C.) (° C.) (° C./s) (° C./s) (° C.) (%) (%) A1 23, 21 760 60 730 550 40 21 320 0.20 1.0 A2 22, 20 740 80 700 570 35 21 340 0.25 0.9 A3 20, 20 730 67 710 550 25 18 360 0.30 0.9 A4 20, 20 740 100 700 570 27 19 390 0.30 0.9 A5 22, 20 740 80 700 580 35 21 360 0.25 0.9 A6 23, 21 740 73 700 580 37 21 400 0.25 1.0 *Note: 1) R is the ratio of the thickness of a steel slab after continuous casting to the thickness of the steel plate after the completion of rolling; and 2) Heating temperature T = 7510/(2.96 − log[Nb][C]) + 30, wherein [Nb] and [C] respectively represent the contents in percentage by mass of Nb and C.

(17) The mechanical properties of the above-mentioned X80 line pipes as obtained after testing are shown in Table 3, and Table 3 lists the various mechanical property parameters of the line pipes in Examples A1-A6.

(18) Table 3 lists the various mechanical property parameters of the X80 line pipes in Examples A1-A6.

(19) TABLE-US-00003 TABLE 3 Transversal Transversal Longitudinal Longitudinal Impact yield tensile Transversal yield tensile Uniform work DWTT strength strength yield strength strength Longitudinal elongation Tensile at at Serial Rt0.5 Rm ratio Rt0.5 Rm yield Uel curve −45° C. −45° C. number (MPa) (MPa) Y/T (MPa) (MPa) ratio Y/T (%) shape (J) SA % A1 611 712 0.86 564 699 0.81 7.4 Doom-shaped 226 100 A2 586 708 0.83 550 686 0.80 8.1 Doom-shaped 240 96 A3 575 677 0.85 530 670 0.79 8.2 Doom-shaped 200 85 A4 584 684 0.85 556 670 0.83 7.9 Doom-shaped 214 87 A5 570 686 0.83 540 686 0.79 8.3 Doom-shaped 231 92 A6 579 688 0.84 542 673 0.81 8.1 Doom-shaped 241 93

(20) It can be seen from Table 3 that the X80 line pipes in Examples A1-A6 herein have a higher yield strength and tensile strength, wherein the transversal yield strengths are ≥575 MPa, the transversal tensile strengths are ≥677 MPa, the longitudinal tensile strengths are ≥530 MPa, and the longitudinal tensile strengths are ≥670 MPa. Moreover, the X80 line pipes further have a good low temperature toughness, an impact work at −45° C. reaching 200 J or greater and a uniform elongation Uel reaching 7.4% or greater. In particular, the line pipes in Examples A1-A6 herein further have excellent low temperature fracture toughness resistance and can still meet DWTT performance SA %≥85% at an extremely low temperature of −45° C.

(21) FIG. 2 shows the microstructure of the pipeline steel in Example A4, and it can be seen from FIG. 2 that the microstructure of the pipeline steel is a polygonal ferrite (PF)+acicular ferrite (AF)+bainite (B) composite microstructure plate, in which the polygonal ferrite (PF) has a phase proportion of 34%.

(22) An aging test is carried out on the line pipes in Examples A1-A6 under temperature-maintaining conditions of 200° C. for a period of 5 min, to simulate the aging process in anti-corrosion coatings. The mechanical property parameters of the X80 line pipes as obtained after the aging treatment are as shown in Table 4.

(23) TABLE-US-00004 TABLE 4 Transversal Transversal Longitudinal Longitudinal yield tensile Transversal yield tensile Uniform Impact DWTT strength strength yield strength strength Longitudinal elongation at at Serial Rt0.5 Rm ratio Rt0.5 Rm yield Uel Tensile −45° C. −45° C. number (MPa) (MPa) Y/T (MPa) (MPa) ratio Y/T (%) curve shape (J) SA % A1 629 715 0.88 561 703 0.80 6.1 Doom-shaped 214 100 A2 601 710 0.85 559 696 0.80 7.2 Doom-shaped 236 93 A3 589 696 0.85 546 683 0.80 7.6 Doom-shaped 211 85 A4 610 695 0.88 563 679 0.83 6.9 Doom-shaped 216 89 A5 600 689 0.87 560 694 0.81 7.3 Doom-shaped 221 90 A6 608 691 0.88 559 690 0.81 7.1 Doom-shaped 223 90

(24) In conjunction with the contents of Tables 3 and 4, it can be seen that compared with the various mechanical property parameters of the X80 line pipes shown in Table 3, the yield strength and the tensile strength of the X80 line pipes after the aging treatment (e.g., simulated coating at 200° C.) both are increased, the yield ratio is slightly increased, and the uniform elongation is slightly reduced, which can still meet performance requirements for strain-based designs. In addition, when the above-mentioned X80 line pipes undergo a tensile strength test, the tensile curve shape is still dome-like, and no yield platform appears, which also correspondingly indicates that the X80 line pipes in Examples A1-A6 herein have good strain-aging resistance.

(25) It is to be noted that the examples listed above are merely specific examples of the present invention, and obviously the present invention is not limited to the above examples and can have many similar changes. All variations which can be directly derived from or associated with the disclosure of the invention by a person skilled in the art should be within the scope of protection of the present invention.