Development of high-strain L485 steel plate for deep-sea pipelines
Thread Content
I. Background and Issues of the Study Marine oil and gas resources are an important part of petroleum and natural gas resources, and represent a key area for the future exploitation of such resources. As global energy consumption continues to rise and land-based oil and gas resources decline, oil and gas exploration has shifted from land to the oceans, resulting in an increasing proportion of oil and gas resources coming from marine sources. It is reported that currently, marine oil and gas reserves account for 30%-40% of the world’s total reserves, and deep-sea oil and gas production is set to increase by more than 300% over the next decade. A large portion of marine oil and gas resources is found in deep-water areas at depths of 1,000 meters or even more than 2,000 meters, requiring the construction of specialized deep-sea oil and gas extraction and transportation systems for exploitation ； Among them, deep-sea oil and gas pipelines, as a major form of oceanic oil and gas storage and transportation, serve as an essential lifeline for the development and production of offshore oil and gas fields. They represent the safest, most economical, and least environmentally damaging method of transporting oil and gas. Deep-sea pipelines are exposed to various complex and harsh conditions during installation and operation, such as plastic deformation, extremely high water pressures, and oceanic currents; therefore, the materials used must possess high strength and toughness, as well as a range of comprehensive properties including resistance to strain, crush, and fatigue ； In other words, the steel plates used for deep-sea pipelines are required to possess comprehensive technical properties such as thick walls, high strength, high toughness, and a low yield ratio, as well as high uniform deformation rates, high strain hardening indices, and high fatigue strength. At the same time, they must also meet the requirements for manufacturing straight-seam submerged arc welded pipes with a large thickness-to-diameter ratio. In recent years, certain progress has been made in the research and development of materials such as steel plates and steel pipes for offshore oil and gas pipelines in our country. However, these products are mainly designed based on stress considerations; the manufacturing and application of steel plates and pipes suitable for deep-sea oil and gas pipelines, which can withstand high strain levels, remain unexplored. II. Approaches to Solving the Problem and Technical Solutions: Addressing the specific requirements of high-strength L485 steel plates for use in deep-sea pipelines, this project is based on research into mechanisms such as microstructural characteristics and plate/tube dimensions, as well as product design. It focuses on overcoming the key manufacturing techniques related to these steel plates, while also addressing the technical challenges associated with their application. This approach follows a research framework and technical pathway that emphasizes fundamental research, key technologies, product development, and application technologies, thereby laying a solid foundation for the development of high-strength L485 steel plates suitable for deep-sea pipelines. The detailed research approach and technical route are shown in Figure 1.
II. Major innovative achievements: The project has developed high-strain L485 steel plates for deep-sea pipelines, JCOE steel pipes with a large thickness-to-diameter ratio, as well as the key manufacturing technologies for them. Core manufacturing technologies for thick-walled, high-strain L485 pipeline steel plates have been innovatively developed, including a multi-stage gradient high-permeability rolling technique based on temperature gradients and grain control; a uniform cooling technique regulated by differences in cooling rate and temperature drop, along with specialized cooling control models; and microstructure control techniques centered on dynamic/static recrystallization grain refinement, austenite flattening, deformation-induced ferrite formation, and control of hard phase structures. Based on product design and manufacturing technology development, the strain capacity has been significantly enhanced through further precise control of the phase ratio, phase distribution, and substructure of the microstructure. This ensures that the material possesses sufficient strain reserve after forming and expansion, thereby guaranteeing the bending performance of steel pipes and circumferential welds. As a result, a leading technology for controlling high strain capacity in marine pipeline pipes has been developed, addressing the key technical challenges related to matching and controlling the strength, plasticity, and toughness of high-strain L485 steel plates and pipes, and enabling the production of high-strain L485 steel plates for deep-sea pipelines. 1. High-permeability rolling technology based on temperature gradients: A temperature-controlled deformation technique was developed based on the theory of the relationship between temperature and plasticity as well as deformation resistance. This technique utilizes rapid cooling of the billet during the rolling process to increase the temperature gradient in the thickness direction of the billet, thereby enhancing the effect of rolling-induced deformation penetration. It addresses the key technical challenges associated with difficult grain refinement in the center of thick-walled pipeline steel and poor uniformity of the microstructure across the thickness section.
2. Multi-stage high-strain rolling technology based on grain control: By taking into account the technical characteristics of high-strain L485 steel plates used for deep-sea pipelines, recrystallization thermodynamic analysis, and the capabilities of the production line equipment, a low-temperature high-deformation process for the final stage of rough rolling was developed. The key point is that, in combination with high-permeability rolling, the early stage of rough rolling at low temperatures focuses on roll reduction control, while the later stage of rough rolling focuses on pass deformation rate control, with the pass deformation rate showing an increasing trend ； The rough rolling holding zone and pass intervals are cooled in an appropriate manner to ensure high-permeation rolling. 3. Uniform cooling technology controlled by differences in cooling rate and temperature drop. By taking into account the heat dissipation characteristics of steel plates during the rolling process, and through research on the relationships between temperature, microstructure, precipitated phases, and properties during the post-rolling and accelerated cooling stages of thick-gauge, high-strain L485 pipeline steel, a uniform cooling technology controlled by differences in cooling temperature drop was developed ； By designing the cooling system curves, ideal temperature changes are achieved in different areas of the steel plate during accelerated cooling, thereby addressing the issue of uneven microstructural properties caused by temperature differences before accelerated cooling, and enabling effective control over the microstructure and properties.
4. Thick-walled, high-strain L485 steel plates for deep-sea pipelines: This project was the first to develop L485 high-strain steel plates with a thickness of 31.8 mm for use in JCOE steel pipes with a diameter of φ559 mm. These products possess comprehensive technical properties such as strength, ductility, toughness, strain resistance, compressive strength, and fatigue resistance, marking a breakthrough in China’s development of key materials for deep-sea, high-strain marine pipelines – from nothing to something tangible. Among them, the longitudinal yield ratio of the steel plate is ≤0.75, the longitudinal uniform elongation rate is ≥11%, the longitudinal n-value is ≥0.11; the transverse DWTT of the steel plate (average value at -10°C) is ≥90%, and the ductile-to-brittle transition temperature of DWTT is below -20℃ ； The lateral impact energy CVN (average at -20°C) is ≥190 J, and the impact energy toughness-transition temperature is below -60°C, meeting the requirements for materials used in deep-sea oil and gas transmission in China.
5. Large-thickness-to-diameter ratio JCOE high-strain L485 steel pipes for deep-sea pipelines: In China, for the first time, high-strain L485 grade JCOE straight-seam steel pipes suitable for deep-sea pipelines were successfully manufactured, with a thickness-to-diameter ratio exceeding 0.056. In contrast, the thickness-to-diameter ratio of high-grade straight-seam submerged arc welded pipes available both domestically and internationally rarely exceeds 0.050. The steel pipes possess good toughness and strain resistance, with their longitudinal and transverse strengths meeting the requirements specified in DNVGL-ST-F101 and GB/T9711. The longitudinal yield strength ranges from 495 to 550 MPa, the tensile strength ranges from 610 to 690 MPa, the yield-to-tensile strength ratio is ≤0.82, the uniform elongation is ≥7.5%, and the strain hardening index is >0.11 ； The transverse yield strength of the tube body is 500–560 MPa, and its tensile strength is 620–695 MPa. The transverse DWTT at 0°C is ≥90%, and the ductile-to-brittle transition temperature at 85% DWTT is below -20°C. The transverse impact energy of the tube body, measured as CVN at -20°C, is ≥190 J; the impact energy of the weld/hot affect zone, also measured as CVN at -20°C, is ≥150 J. The CTOD value of the weld at 0°C is ≥0.20 mm, and the HV10 value is ≤260. These comprehensive performance indicators are at the leading level both domestically and internationally.