A crack-resistant offshore riser x65m steel plate and a method of producing the same

Abstract

This invention discloses a crack-resistant X65M steel plate for marine risers and its production method. The chemical composition of the steel (by weight percentage) is: C=0.06%–0.07%, Si=0.15%–0.25%, Mn=1.56%–1.6%, P≤0.015%, S≤0.003%, Alt=0.02%–0.45%, Ti=0.012%–0.02%, Nb=0.035%–0.04%, Cr=0.18%–0.2%, Cu=0.1%–0.15%, B≤0.0005%, with the balance being Fe and unavoidable impurities. The steel plate thickness is 65–68 mm. The process steps include smelting, heating, controlled cooling of intermediate billets, temperature-controlled rolling, post-rolling zoned controlled cooling, and plate stacking cooling. Excellent strength and toughness matching can be achieved even at large thicknesses without adding precious alloying elements such as Mo and Ni. Its strength performance meets the requirements of X65 steel grade and can be widely used in various subsea pipeline projects.

Description

Technical Field

[0001] This invention belongs to the field of heavy plate production technology, and relates to a crack-resistant X65M steel plate for marine risers and its production method. Background Technology

[0002] Offshore risers are core equipment for deep-sea oil and gas development, serving as pipelines connecting floating offshore platforms (FPSOs) and semi-submersible platforms to subsea wellheads and manifolds. Their operation requires withstanding extreme loads such as high pressure, corrosion, ocean currents, and vortex-induced vibrations, necessitating a balance of rigidity and toughness, excellent resistance to fatigue fracture and crack arrest, and high wall thickness, high toughness, excellent fracture resistance (DWTT) performance, and high CTOD value. This type of steel is increasingly widely used in the manufacture of offshore risers, with X65M steel (thickness 50mm and above) becoming the mainstream material in the industry.

[0003] Given the numerous advantages of thicker walls, downstream industries have a strong demand for even thicker pipe walls. However, increasing the thickness of risers primarily leads to a sharp increase in the technical difficulty of steel plate production. To achieve both sufficient strength and toughness, marine risers often use low-carbon components, with a carbon mass fraction of ≤0.12%. The thicker the pipe wall, the more difficult it is to guarantee strength. Although adding expensive strengthening elements such as Mo and Cr can improve strength by increasing hardenability, promoting bainification, and through solid solution strengthening, it will lead to a deterioration in toughness and crack resistance, and will also significantly increase the production cost of materials, which is not conducive to the promotion and application of these materials by downstream industries.

[0004] Currently, the research on X65 steel grade in the steel industry is relatively mature. However, there are very few reports on high-performance thick-walled X65 steel with a thickness of more than 60mm, especially with low alloy cost additions and good strength and toughness matching, crack resistance and crack arrest ability. Meanwhile, the demand for such products from downstream industries is becoming increasingly strong. CN120485639A discloses an X65 grade pipeline steel and its preparation method, which uses a low-carbon, low-manganese composition and requires the addition of expensive Mo element to reach 0.11-0.19%, resulting in high costs and limited production thickness, which cannot be extended to 60mm and above. CN118374737A discloses a high-ductility, high-toughness, fatigue-resistant X65 grade catenary riser steel plate and its production method, with a production thickness ≤40mm, making it difficult to extend to greater thicknesses of 60mm and above. CN116555670A discloses a fatigue-resistant marine riser steel and its preparation method, which requires the addition of expensive Mo and Ni elements, resulting in excessively high costs. CN118726841A discloses a thick-walled X65 grade pipeline steel and its manufacturing method, using expensive alloy Ni to reach a cost of 0.20% to 0.25%, and this process lacks special control measures in the slab heating and controlled rolling and cooling stages, making it difficult to achieve a balance between strength and toughness when extending the thickness to above 60mm. Currently, there is another type of acid-resistant pipeline in the industry, X65MS, which can meet the requirements of strength and toughness matching, and has good corrosion resistance and crack arrest ability. However, this type of steel often uses an ultra-low carbon composition with a C mass fraction of ≤0.05%, and requires the addition of expensive alloying elements such as Mo, Cr, and Ni for strengthening and toughening, which is costly. At the same time, due to the characteristics of the ultra-low carbon composition, it is difficult to meet the steel grade strength requirements in the thickness direction when producing a thickness exceeding 60mm. It is usually suitable for extreme and special environments, and cannot be widely promoted and used due to cost and specification limitations.

[0005] Therefore, it is of urgent practical significance to develop an X65M steel pipe that is cost-effective, combines strength and toughness, has good crack resistance (including good crack initiation and crack arrest properties), and has a maximum thickness of 68mm to meet the requirements of marine engineering applications. Summary of the Invention

[0006] The present invention aims to provide a crack-resistant X65M steel plate for marine risers and its production method. The steel plate is 65-68mm thick. Under the premise of meeting the requirements of steel grade strength, low temperature impact toughness and excellent crack resistance, it can achieve economical production without adding expensive alloying elements such as Mo and Ni.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A type of crack-resistant X65M steel plate for marine risers, the steel's chemical composition by weight percentage is C=0.06%~0.07%, Si=0.15%~0.25%, Mn=1.56%~1.6%, P≤0.015%, S≤0.003%. Alt=0.02%~0.45%, Ti=0.012%~0.02%, Nb=0.035%~0.04%, Cr=0.18%~0.2%, Cu=0.1%~0.15%, B≤0.0005%, with the balance being Fe and unavoidable impurity elements; the steel plate grade is X65M, thickness 65~68mm, with yield strength stable at 460~500MPa, tensile strength stable at 560~600MPa, elongation A50≥36%, yield strength ratio≤0.85; hardness≤210HV10; impact energy KV8≥400J at -45℃, CTOD value≥1.0 at -10℃, and drop weight DWTT shear area ratio≥85% at -15℃.

[0008] A method for producing crack-resistant X65M steel plate for marine risers, comprising the following steps: S1 smelting: Molten steel is refined by LF and vacuum treated by VD / RH, and then protected and cast into a 300mm thick continuous casting billet. The low magnification of the billet is 1.5 for Class C of the national standard.

[0009] S2 slab heating: The highest furnace temperature during the heating process is 1180℃, and the tapping temperature is 1140~1180℃.

[0010] S3 controlled rolling: Two-stage controlled rolling is adopted. After rough rolling, the intermediate billet is cooled by intermediate water cooling; the starting temperature of finish rolling is ≤ 840℃.

[0011] S4 controlled cooling: post-rolling zoned cooling is adopted; the steel plate reddening temperature after cooling is 400-450℃.

[0012] S5 Post-rolling cooling: After the steel plate is cooled to 300-350℃ on the cooling bed, it is lifted down for slow cooling. The stacking time is ≥36h, and the stacking is dismantled after the steel plate temperature is ≤50℃.

[0013] Furthermore, S2 slab heating: cold billets are loaded into the furnace, the temperature of the soaking zone is 1180℃, and the soaking time is ≥60min.

[0014] Furthermore, S3 controlled rolling: during rough rolling at a rolling temperature ≥1130℃, the reduction in two consecutive passes is >35mm, the intermediate billet thickness is ≥2 times the finished product thickness +30mm; the intermediate water cooling time for the intermediate billet is ≤100s, and the surface temperature of the intermediate billet after cooling is ≤900℃; the finishing rolling temperature is 790~810℃, and the maximum reduction rate per pass during the finishing rolling process is 9%.

[0015] Furthermore, S4 controls the cooling process: the inlet water temperature is 740-750℃, the cooling roller speed is 0.5-0.6m / s, and a zoned cooling method is adopted. The high-pressure zone accounts for 1 / 4 of the total cooling time and 40% of the total water volume; the low-pressure zone accounts for 3 / 4 of the total cooling time and 60% of the total water volume, with an average cooling rate of 6-8℃ / s to cool to the target red color.

[0016] The inventive measures and principles of this invention: Step S1 Smelting: Low carbon and low impurity + multi-element microalloy synergistic strengthening. Through the design of low carbon bainite microstructure, Nb-Ti composite fine grain strengthening, Mn-Cr solid solution strengthening and hardenability improvement synergistic effect, replacing Mo, Ni and other precious alloys, reducing production costs.

[0017] Step S2: Slab heating: After the slab has been fully cooled and hydrogen has been released, it is loaded into the furnace to avoid hydrogen-induced cracking in the finished product due to excessive hydrogen in the steel. The maximum furnace temperature does not exceed 1180℃ to ensure the refinement of the original austenite grains. Above this temperature, the grain size tends to coarsen sharply, which will deteriorate the low-temperature toughness and drop hammer performance. The soaking temperature is 1180℃ and the soaking time is ≥60min to ensure that Nb, Ti and other alloys are fully dissolved and to ensure the fine grain strengthening effect.

[0018] Step S3 Controlled Rolling: During rough rolling at a rolling temperature ≥1130℃, the reduction in two consecutive passes is >35mm. This is to achieve a single-pass large reduction in the rough rolling stage (recrystallization zone) to break up austenite while simultaneously promoting recrystallization, thus enhancing austenite refinement. Intermediate water cooling is used for the intermediate billet because, in addition to the traditional advantages of reducing waiting time and improving production efficiency, it can also significantly improve process performance. The proposed cooling rate is 3-5℃ / s, with a cooling temperature ≤900℃. This achieves the following beneficial effects: First, rapid cooling of the intermediate billet in the high-temperature range increases the austenite supercooling, making it easier to form fine ferrite or bainite structures during subsequent rolling, promoting phase transformation refinement and thus improving strength and toughness. Second, it reduces the temperature rise caused by deformation heat during rolling, preventing excessive austenite grain growth, retaining more deformation energy, providing more nucleation sites for subsequent phase transformation, and inhibiting dynamic recrystallization. Third, it prevents abnormal local grain growth caused by uneven temperature, ensuring microstructure uniformity. By employing a water-cooling process for intermediate billets, the grain refinement effect in the non-recrystallized zone during the finishing rolling stage can be significantly improved, while simultaneously enhancing strength and low-temperature impact toughness. Therefore, the addition of precious metal elements such as Mo, Ni, and Nb can be reduced while ensuring performance. The intermediate billet thickness is ≥2 times the finished product thickness + 30 mm, the finishing rolling temperature is 790-810℃, and the maximum pass reduction rate during finishing rolling is 9%. This is to ensure sufficient cumulative reduction in the non-recrystallized zone, flattening and elongating the austenite, increasing the density of dislocations and nucleation zones, and providing the initial microstructure conditions for further grain refinement during subsequent rapid cooling.

[0019] Step S4: Controlled cooling. The initial cooling water temperature is 740–750℃, lower than the ferrite precipitation temperature (Ar3) of the hypoeutectoid steel (770℃). This ensures sufficient ferrite soft phase precipitation before initial cooling, thereby improving the toughness and crack arrest resistance of the microstructure. Simultaneously, it avoids insufficient austenite to transform into bainite hard phase during rapid cooling due to excessively low initial cooling temperature, resulting in lower strength. Final cooling is carried out at 400–450℃ and a cooling rate of 6–9℃ / s, with a cooling roller speed of 0.5–0.6 m / s and a cooling time of 40–48 s. High-pressure and low-pressure zoned cooling is employed, with the high-pressure zone accounting for 1 / 4 of the total cooling time and 40% of the total water volume, and the low-pressure zone accounting for 3 / 4 of the total cooling time and 60% of the total water volume. This ensures that the steel plate undergoes transformation within the bainitic region, achieving a microstructure dominated by bainite and ferrite throughout its thickness. Meanwhile, due to the large thickness of the steel plate, there is a relatively slow gradient process in the cooling of the steel plate surface and the heat exchange in the core. Sufficient cooling time must be ensured. By using two sections of the cooling system, namely the high-pressure section (10-12s) and the low-pressure section (30-36s), a total cooling time of 40-80s can be ensured to guarantee sufficient heat exchange in the thickness direction, improve the cooling rate and hardenability of the core, and achieve sufficient bainite structure in the core position with a maximum thickness of 68mm. With the addition of a small amount or even no addition of precious alloying elements such as Mo and Cr, it can support the strength requirements of X65 steel grade.

[0020] Step S5 Post-rolling stacking cooling: After the steel plate is cooled to 300-350℃ on the cooling bed, it is lifted down for slow cooling. The stacking time is ≥36h. The stacking is removed after the steel plate is ≤50℃. This is to further release hydrogen into the steel plate, eliminate hydrogen-induced crack defects, improve the flaw detection quality, and avoid the deterioration of related physical and chemical properties caused by hydrogen-induced defects.

[0021] This invention relates to an economical, crack-resistant X65M steel plate for marine risers and its production method, achieving the following beneficial effects: 1) It does not require the use of ultra-low carbon components with a C mass fraction of ≤0.05%, which can reduce the difficulty of steelmaking. Through the extreme rolling process, the grains can be refined, and high toughness and high crack resistance, including DWTT and CTOD performance, can be achieved. At the same time, it does not require the addition of expensive Mo and Ni precious alloys to achieve a balance between strength and toughness, and has the ability to produce at a lower cost.

[0022] 2) It can stably produce specifications with a maximum thickness of 68mm, and the delivery width can exceed 4100mm. Its yield strength is stable at 460-500MPa, tensile strength is stable at 560-600MPa, elongation A50 ≥36%, yield strength ratio ≤0.85; hardness ≤210HV10; impact energy KV8 ≥400J at -45℃, CTOD value ≥1.0 at -10℃, and drop weight DWTT shear area ratio ≥85% at -15℃.

[0023] 3) By innovatively utilizing the intermediate water cooling device in the basic equipment of the medium and heavy plate production line, and by formulating a reasonable cooling model to water cool the intermediate billet, the grains can be significantly refined, further enhancing the grain refinement effect of the finished product after rapid cooling, while improving strength and toughness.

[0024] 4) The rolling process employs sectional cooling, which provides sufficient heat exchange in the thickness direction to achieve the transformation of bainite and acicular ferrite structures, thereby strengthening the microstructure and ensuring that the steel grade strength requirements are met even when the thickness reaches 68mm. Detailed Implementation Example 1

[0025] An economical, crack-resistant X65M steel plate for marine risers and its production method are disclosed. The steel's weight percentage composition is C=0.06%, Si=0.19%, Mn=1.60%, P=0.011%, S=0.0011%, Alt=0.039%, Ti=0.016%, Nb=0.037%, Cr=0.19%, Cu=0.11%, B=0.0003%, with the balance being Fe and unavoidable impurities. The steel plate is of grade X65M and has a thickness of 65mm. The process steps include: S1 smelting: Molten steel is refined by LF and vacuum treated by RH, and then protected and cast into a 300mm thick continuous casting billet. The low magnification of the billet is 1.5 for Class C of the national standard.

[0026] S2 slab heating: Cold billet is loaded into the furnace. The highest furnace temperature during the heating process is 1180℃, the soaking temperature is 1180℃, the soaking time is 78min, and the tapping temperature is 1162℃.

[0027] S3 controlled rolling: The reduction in the last two passes of roughing rolling is 38mm and 39mm respectively. The intermediate slab temperature after roughing rolling is 1135℃, and the intermediate slab thickness is 160mm. After roughing rolling, the intermediate slab is cooled by intermediate water cooling (IC) for 80 seconds, and the surface temperature of the slab after cooling is 895℃. The starting rolling temperature of finishing rolling is 840℃, the maximum reduction per pass of finishing rolling is 9%, the final rolling temperature is 806℃, and the water immersion temperature is 748℃.

[0028] S4 controlled cooling: Post-rolling zoned cooling is adopted; the roll speed is 0.55m / s, the water flow rate in the high-pressure zone with a length of 6m is 1800L / s, and the water flow rate in the low-pressure zone with a length of 18m is 2580L / s; the average cooling rate is 7.2℃ / s, and the steel plate reddening temperature after cooling is 435℃.

[0029] S5 Post-rolling stacking cooling: After the steel plate is cooled to 300-350℃ on the cooling bed, it is hoisted down for slow cooling, and the stacking time is 52 hours.

[0030] The performance of the sampled steel is shown in Table 1. Example 2

[0031] An economical, crack-resistant X65M steel plate for marine risers and its production method are disclosed. The steel's weight percentage composition is C=0.06%, Si=0.21%, Mn=1.59%, P=0.012%, S=0.0015%, Alt=0.027%, Ti=0.015%, Nb=0.038%, Cr=0.2%, Cu=0.1%~0.15%, B=0.0004%, with the balance being Fe and unavoidable impurities. The steel plate is of grade X65M and has a thickness of 68mm. The process steps include: S1 smelting: Molten steel is refined by LF and treated by RH vacuum, and then cast into a 300mm thick continuous casting billet under protective conditions. The low magnification of the billet is 1.5 for Class C of the national standard.

[0032] S2 slab heating: Cold billet is loaded into the furnace. The highest furnace temperature during the heating process is 1180℃, the soaking temperature is 1180℃, the soaking time is 94min, and the tapping temperature is 1166℃.

[0033] S3 controlled rolling: The reduction in the last two passes of roughing rolling is 36mm and 38mm respectively. The intermediate slab temperature after roughing rolling is 1140℃, and the intermediate slab thickness is 170mm. After roughing rolling, the intermediate slab is cooled by intermediate water cooling (IC) for 100s, and the surface temperature of the slab after cooling is 880℃. The starting rolling temperature of finishing rolling is 820℃, the maximum reduction per pass of finishing rolling is 9.2%, the finishing rolling temperature is 799℃, and the water immersion temperature is 741℃.

[0034] S4 controlled cooling: Post-rolling zoned cooling is adopted; the roll speed is 0.5m / s, the water flow rate in the high-pressure zone with a length of 6m is 1800L / s, and the water flow rate in the low-pressure zone with a length of 18m is 2580L / s; the average cooling rate is 6.7℃ / s, and the red-hot temperature of the steel plate after cooling is 421℃.

[0035] S5 Post-rolling stacking cooling: After the steel plate is cooled to 300-350℃ on the cooling bed, it is hoisted down for slow cooling, and the stacking time is 58 hours.

[0036] The performance of the sampled steel is shown in Table 1.

[0037] Table 1 Instance Performance Test Results .

Claims

1. A crack-resistant X65M steel plate for marine risers, characterized in that: The chemical composition of the steel, by weight percentage, is C = 0.06%–0.07%, Si = 0.15%–0.25%, Mn = 1.56%–1.6%, P ≤ 0.015%, and S ≤ 0.003%. Alt=0.02%~0.45%, Ti=0.012%~0.02%, Nb=0.035%~0.04%, Cr=0.18%~0.2%, Cu=0.1%~0.15%, B≤0.0005%, with the balance being Fe and unavoidable impurity elements; the steel plate is grade X65M, with a thickness of 65~68mm, a yield strength stable at 460~500MPa, a tensile strength stable at 560~600MPa, an elongation A50≥36%, a yield strength ratio ≤0.85; hardness ≤210HV10; impact energy KV8≥400J at -45℃, CTOD value ≥1.0 at -10℃, and drop hammer DWTT shear area ratio ≥85% at -15℃.

2. A crack-resistant X65M steel plate for marine risers and its production method, characterized in that... The process steps include: S1 smelting: Molten steel is refined by LF, vacuum treated by VD / RH, and then cast into a 300mm thick continuous casting billet under protective conditions. The low magnification of the billet is 1.5 for Class C of the national standard. S2 slab heating: The highest furnace temperature during the heating process is 1180℃, and the tapping temperature is 1140~1180℃; S3 controlled rolling: Two-stage controlled rolling is adopted. After rough rolling, the intermediate billet is cooled by intermediate water cooling; the starting temperature of finish rolling is ≤ 840℃. S4 controlled cooling: Post-rolling zoned cooling is adopted; the steel plate reddening temperature after cooling is 400-450℃. S5 Post-rolling cooling: After the steel plate is cooled to 300-350℃ on the cooling bed, it is lifted down for slow cooling. The stacking time is ≥36h, and the stacking is dismantled after the steel plate temperature is ≤50℃.

3. The method for producing a crack-resistant X65M steel plate for marine risers according to claim 1, characterized in that... S2 slab heating: Cold billet is loaded into the furnace, the temperature of the soaking zone is 1180℃, and the soaking time is ≥60min.

4. The method for producing a crack-resistant X65M steel plate for marine risers according to claim 1, characterized in that... S3 controlled rolling: During roughing at a rolling temperature ≥1130℃, the reduction in two consecutive passes is >35mm, and the thickness of the intermediate billet is ≥2 times the finished product thickness +30mm; the intermediate water cooling time for the intermediate billet is ≤100s, and the surface temperature of the intermediate billet after cooling is ≤900℃; the finishing rolling temperature is 790~810℃, and the maximum reduction rate per pass during the finishing rolling process is 9%.

5. A method for producing a crack-resistant X65M steel plate for marine risers according to claim 1: characterized in that... S4 controlled cooling: water inlet temperature 740~750℃, cooling roller speed 0.5~0.6m / s, adopting zoned cooling, the high pressure section accounts for 1 / 4 of the total cooling time and 40% of the total water volume; the low pressure section accounts for 3 / 4 of the total cooling time and 60% of the total water volume, with an average cooling rate of 6~8℃ / s to cool to the target red color.