Production method of economical thick-wall X70 pipeline steel

Abstract

The invention discloses a production method of economical thick-wall X70 pipeline steel. The steel comprises chemical components in percentage by mass as follows: 0.08%-0.10% of C, 0.10%-0.25% of Si, 1.50%-1.60% of Mn, smaller than or equal to 0.015% of P, smaller than or equal to 0.0015% of S, 0.055%-0.060% of Nb, 0.008%-0.020% of Ti, 0.20%-0.25% of Cr, 0.05%-0.12% of Ni, smaller than or equal to 0.0003% of B and the balance of Fe and inevitable impurities. The low-temperature drop-weight brittle failure pipeline steel is suitable for an international general component system, adopts a microalloy component system of medium carbon, medium manganese, Nb and Cr, breaks through the microalloy component design of low carbon, medium manganese, Nb and Mo of conventional pipeline steel, and is matched with new heating, rolling and cooling processes, so that the problem of low-temperature drop-weight brittle failure of the pipeline steel of the medium carbon component system is solved; the requirement that the drop hammer shearing area ratio of the medium-carbon component system X70-grade pipeline steel with the wall thickness of 8-40 mm at the temperature of-30 DEG C reaches 85%-100% is met, and other performance indexes meet the requirements of the national standard, the Russian standard, the American standard and the Australian standard.

Description

Technical Field

[0001] The invention belongs to the technical field of pipeline steel manufacturing and relates to a production method of economical thick-wall X70 pipeline steel. Background Art

[0002] With the continued growth of energy demand, pipeline steel, a core material for oil and gas pipeline construction, has seen rapid development in its production technology. This continuous advancement in pipeline steel production technology is primarily driven by the demand for improved material performance within the pipeline engineering sector. With the emergence of complex and extreme environments such as earthquakes, freeze-thaw cycles, debris flows, and desertification, oil and gas pipeline projects must meet higher pressure transmission requirements while also requiring stable operation under harsh conditions such as low temperatures, significant geological deformation, deep-sea environments, and acidic media. These demands are driving continuous innovation and development in pipeline steel production technology. Pipeline steel is developing towards high strength, high toughness, corrosion resistance, and excellent weldability. Furthermore, the specificity of its use scenarios and the pursuit of extreme service performance are driving innovation and breakthroughs in its production technology.

[0003] Existing pipelines are almost all designed with low-carbon compositions. This presents two challenges: first, as the carbon content decreases, the addition of other precious alloys increases dramatically to maintain strength, undoubtedly increasing steel manufacturing costs; second, the drop hammer performance of thick-walled pipelines with high-carbon compositions remains largely unresolved.

[0004] Chinese patent CN119956242A discloses an "X70 pipeline steel and its preparation method." The steel plate composition is designed with a low carbon content of 0.03-0.045%, which does not meet the ≥0.08% carbon content requirement for overseas projects. Chinese patent CN119464939A discloses a "high-efficiency, high-quality production method for triple-length X70 oil pipeline steel plates." The steel plate composition is designed with a low carbon content of 0.05%-0.07%, which also does not meet the ≥0.08% carbon content requirement for overseas projects. Chinese patent CN118531319A discloses an "economical X70 grade acid-resistant pipeline steel and its manufacturing method." The steel plate composition is designed with a low carbon content of 0.04%-0.06%, which also does not meet the ≥0.08% carbon content requirement for overseas projects. Chinese patent CN118326268A discloses "A High-Plasticity, Toughness, and Fatigue-Resistant X70-Grade Catenary Riser Steel Plate and Its Production Method." The steel plate's composition is designed to have a low carbon content of 0.020% to 0.040%, which falls short of the 0.08% or higher carbon content requirement for overseas projects. Almost all of these patents utilize a low-carbon composition system, which not only increases steel consumption during the steelmaking process but also requires the addition of precious alloys to compensate for the strength loss associated with low carbon content, impacting manufacturing costs and reducing market competitiveness.

[0005] Therefore, under the premise of the current serious overcapacity in steel production, if medium-carbon pipeline steel, especially economical thick-walled X70 pipeline steel, can be developed, it will not only reduce manufacturing costs but also expand the international market. Summary of the Invention

[0006] The present invention aims to provide an economical production method for thick-walled X70 pipeline steel, producing medium-carbon X70-grade pipeline steel with a wall thickness of 8-40 mm, and solving the problem of low-temperature drop hammer brittle fracture of medium-carbon pipeline steel. The method requires that the drop hammer shear area ratio at -30°C reach 85%-100%.

[0007] Implementation of the present invention: A method for producing economical thick-walled X70 pipeline steel, wherein the chemical composition of the steel is as follows by mass: C=0.08%-0.10%, Si=0.10%-0.25%, Mn=1.50%-1.60%, P≤0.015%, S≤0.0015%, Nb=0.055%-0.060%, Ti=0.008%-0.020%, Cr=0.20-0.25%, Ni=0.05-0.12%, B≤0.0003%, with the remainder being Fe and unavoidable impurities; the method comprises the following key process steps: (1) Steelmaking and billet preparation: large-section billets 300~350×2280~2500mm, steelmaking and pouring superheat 24~30℃, billet low-power acceptance according to Man standard level 1 or above; (2) Heating: heating temperature 1100~1130℃, soaking time 80~100min; (3) Rolling: Rough rolling adopts two-stage controlled rolling. After the steel plate is widened, it is swing-cooled at the entrance of the roughing mill, air-cooled for 180~240s, and then the subsequent rolling passes are completed at 940~960℃; the intermediate billet is set at 90~130mm, the finishing rolling start temperature is 800~830℃, the cooling start temperature is 700~740℃, and the final cooling temperature is 360~420℃.

[0008] The innovations and beneficial effects of the present invention are as follows: (1) The use of large-section billets can effectively increase the compression ratio and refine the grains. At the same time, high superheat casting improves the low-magnification quality of the billet and reduces the segregation in the center of the billet, which is beneficial to improving the low-temperature drop hammer performance of medium-carbon pipeline steel.

[0009] (2) The microalloy composition system of medium carbon + medium manganese + Nb + Cr is adopted, which breaks through the microalloy composition design of low carbon + medium manganese + Nb + Mo of conventional pipeline steel. Combined with unconventional heating temperature + soaking time, the original austenite grain size is effectively controlled. The rough rolling adopts two controlled rolling, air cooling after widening, deformation, crystallization and recovery, and then rolling and deformation, which further refines the grain size.

[0010] (3) The Ar3 steel of this composition system is 30~50℃ lower than the Ar3 steel of the low carbon composition system pipeline steel, and a lower finishing rolling start temperature can be used, thereby further refining the grain size and core structure, and improving the low temperature drop hammer performance of low temperature thick wall pipelines. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 This is the metallographic structure diagram of the steel produced in Example 1. DETAILED DESCRIPTION

[0012] The present invention is further described below with reference to the embodiments.

[0013] Example 1: Production method of 36mm thick wall X70M pipeline steel The chemical composition of the steel is C=0.10%, Si=0.122%, Mn=1.51%, P=0.012%, S=0.0012%, Nb=0.056%, Ti=0.012%, Cr=0.24%, Ni=0.08%, B=0.0001%, and the balance is Fe and unavoidable impurities. The key process steps are as follows: (1) Steelmaking and billet preparation: large-section billet 300×2300mm, steelmaking and pouring superheat 26℃, billet low-multiple Man standard level 1.

[0014] (2) Heating: Heating temperature 1120℃, soaking time 85min.

[0015] (3) Rolling: Rough rolling adopts two-stage controlled rolling. After the steel plate is widened, it is swing-cooled at the entrance of the roughing mill, air-cooled for 205s, and then the subsequent rolling is completed at 945℃; the intermediate billet is set to 102mm, the finishing rolling start temperature is 825℃, the cooling start temperature is 730℃, and the final cooling temperature is 380℃.

[0016] The mechanical properties of the obtained steel plates are shown in Table 1.

[0017] Example 2: Production method of 40mm thick wall X70M pipeline steel The chemical composition of the steel is C=0.09%, Si=0.22%, Mn=1.55%, P=0.010%, S=0.0010%, Nb=0.057%, Ti=0.015%, Cr=0.24%, Ni=0.10%, B=0.0002% by mass, with the remainder being Fe and unavoidable impurities. The key process steps are as follows: (1) Steelmaking and billet preparation: large-section billet 320×2500mm, steelmaking and pouring superheat 28℃, billet low-power acceptance according to Man standard level 1.

[0018] (2) Heating: Heating temperature 1120℃, soaking time 95min.

[0019] (3) Rolling: Rough rolling adopts two-stage controlled rolling. After the steel plate is widened, it is swing-cooled at the entrance of the roughing mill, air-cooled for 220s, and then the subsequent rolling is completed at 945℃; the intermediate billet is set to 125mm, the finishing rolling start temperature is 805℃, the cooling start temperature is 705℃, and the final cooling temperature is 365℃.

[0020] The mechanical properties of the produced steel plates are shown in Table 1.

[0021] Table 1 Mechanical properties test results of X70M steel plates produced in the examples .

Claims

1. A method for producing economical thick-walled X70 pipeline steel, characterized by: The chemical composition of the steel is as follows by mass: C = 0.08% ~ 0.10%, Si = 0.10% ~ 0.25%, Mn = 1.50% ~ 1.60%, P ≤ 0.015%, S ≤ 0.0015%, Nb = 0.055% ~ 0.060%, Ti = 0.008% - 0.020%, Cr = 0.20-0.25%, Ni = 0.05 ~ 0.12%, B ≤ 0.0003%, and the balance is Fe and unavoidable impurities; The key process steps include: (1) Steelmaking and billet preparation: large-section billets 300~350×2280~2500mm, steelmaking and pouring superheat 24~30℃, billet low-power acceptance according to Man standard level 1 or above; (2) Heating: heating temperature 1100~1130℃, soaking time 80~100min; (3) Rolling: Rough rolling adopts two-stage controlled rolling. After the steel plate is widened, it is swing-cooled at the entrance of the roughing mill, air-cooled for 180~240s, and then the subsequent rolling passes are completed at 940~960℃; the intermediate billet is set at 90~130mm, the finishing rolling start temperature is 800~830℃, the cooling start temperature is 700~740℃, and the final cooling temperature is 360~420℃.

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