Extra-Large Diameter Thick-Walled Hot-Rolled Seamless Pipes
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2.1 Variety Development History
Extra-large diameter, thick-walled, hot-rolled SMLS pipes come in many varieties and are widely used in various fields. The development of the boiler industry toward high temperatures, high pressures, and large scales has driven the evolution of these steel pipe varieties. In the 1950s and 1960s, China's thermal power units were primarily 5–12 MW low and medium pressure small units, using ordinary SMLS pipes such as steel 10 and 20, while a small amount of low-alloy steel pipes were imported from the Soviet Union. By the mid-1970s, with the construction of thermal power units, China introduced heat-resistant steel pipes produced according to DIN17175 standards, and high-pressure boiler pipes were mostly imported from Germany. In the early 1980s, China introduced subcritical thermal power unit technology from the United States and began using heat-resistant steel pipes produced according to ASME and ASTM standards. Although SMLS pipes used in the low-temperature section were largely domestically produced, high-temperature pipes and large-diameter SMLS pipes still relied on imports. In 1989, the State Council approved the "Nationalization Plan for Steel for Power Generation Equipment," planning to invest 700 million RMB (which later rose to 2 billion RMB) to implement technical transformations in seven steel enterprises, promoting the localization of materials for power generation equipment.
2.2 Production Process and Equipment
2.2.1 Production Process
The production processes for extra-large diameter, thick-walled, hot-rolled seamless steel pipes in China include the large top pipe process, the extrusion pipe process, the cycle rolling pipe process, and the oblique rolling pipe process.
2.2.2 Production Equipment
The manufacture of equipment for extra-large diameter, thick-walled, hot-rolled seamless steel pipes began during the "Eighth Five-Year Plan" period. In 1993, Wuhan 471 Factory (now Wuhan Heavy Industry Casting and Forging) introduced a second-hand large top pipe production line from Sumitomo Corporation of Japan and carried out a technical transformation of the equipment to establish a production line for extra-large diameter, thick-walled, hot-rolled seamless steel pipes. The production line is designed for boiler pipes such as P22 and 12Cr1MoV, with an annual output of 20,000 tons. In 1996, this large pipe jacking unit was put into production, becoming the only unit in China capable of producing thick-walled hot-rolled seamless steel pipes over 508 mm, with an annual output of 4,000 to 6,000 tons and meeting more than 90% of domestic demand. By 2004, the annual output had reached the design target, with an annual profit of over 200 million RMB. Although the technical level was not high, the wall thickness accuracy was low, and the metal recovery rate was about 42%, the unit still had significant benefits due to market monopoly, attracting other companies to follow suit. Hebei Hongrun Nuclear Equipment Technology Co., Ltd. put two large pipe jacking production lines into production in 2006 and 2009, respectively, with annual outputs of 30,000 tons and 50,000 tons. Zhejiang Glos Seamless Steel Pipe Co., Ltd. put a unit with an annual output of 120,000 tons into production in 2010 and innovatively added a diagonal rolling elongation machine to improve wall thickness accuracy. The newly built large pipe jacking units in China increased their annual production capacity by more than 200,000 tons in just a few years, resulting in overcapacity and some units being underused or shut down.
Due to the limitations of large jacking pipe units in producing high-temperature alloy pipes and corrosion-resistant alloy pipes, China began constructing large-tonnage extrusion pipe units. In 2009, Inner Mongolia North Heavy Industry Group constructed a 360 MN extrusion pipe machine with an annual output of 70,000 tons. In 2012, Hebei Hongrun constructed a 500 MN extrusion pipe machine with a designed output of 60,000 tons. Subsequently, the original two large jacking pipe units were essentially shut down. In 2015, Qinghai Kangtai Casting and Forging Machinery Company put a 680 MN extrusion pipe machine into production, but the unit failed to operate normally. Although the extrusion pipe unit has advantages in producing high-alloy and stainless steel pipes, its yield rate is low and its cost is high. To improve the yield rate of carbon steel and low-alloy steel extra-large diameter thick-walled hot-rolled seamless steel pipes, other processes such as two-roller cross-rolling technology were gradually applied.
From 2009 to 2012, China built and put into production several large-diameter thick-walled hot-rolled seamless pipe mills; however, the demand for extra-large diameter thick-walled hot-rolled seamless pipes was limited, leading to serious overcapacity. Especially after 2014, the seamless pipe industry became sluggish, market demand shrank, and some extra-large diameter thick-walled hot-rolled seamless pipe production units experienced varying degrees of under-operation or even shutdown.
Analyzing the characteristics of various large-diameter thick-walled hot-rolled SMLS pipe mills, it is evident that the extrusion pipe mill can produce varieties such as carbon steel pipes, alloy steel pipes, stainless steel pipes, high-temperature alloy pipes, and corrosion-resistant alloy pipes, but the production cost is high. The two-roller oblique rolling pipe mill can produce carbon steel pipes and alloy steel pipes, with high production efficiency and wall thickness accuracy; however, there are difficulties in producing high-alloy steel pipes, stainless steel pipes, high-temperature alloy pipes, and corrosion-resistant alloy steel pipes. The periodic rolling pipe mill can produce carbon steel pipes, alloy steel pipes, and some stainless steel pipes. This unit is suitable for a wider range of steel pipe varieties compared to the oblique rolling pipe mill. While the deformation during rolling is greater, the dimensional accuracy is lower.
Currently, large-diameter two-roller oblique rolling pipe mills and periodic rolling pipe mills have advantages in manufacturing costs and essentially dominate the market for thick-walled carbon steel pipes and alloy steel pipes of 720 mm and above in China. Large-tonnage extrusion pipe units are primarily used to produce medium- and high-alloy steel pipes, stainless steel pipes, high-temperature alloy pipes, and corrosion-resistant alloy pipes with diameters greater than 720 mm. Large jacking pipe units face disadvantages in market competition. In 2010, Wuhan 471 Factory renovated the large jacking pipe unit, added an elongation machine, and improved the wall thickness accuracy and yield rate of the steel pipes.
Due to the limitations of large jacking pipe units in producing high-temperature alloy pipes and corrosion-resistant alloy pipes, China began constructing large-tonnage extrusion pipe units. In 2009, Inner Mongolia North Heavy Industry Group constructed a 360 MN extrusion pipe machine with an annual output of 70,000 tons. In 2012, Hebei Hongrun constructed a 500 MN extrusion pipe machine with a designed output of 60,000 tons. Subsequently, the original two large jacking pipe units were essentially shut down. In 2015, Qinghai Kangtai Casting and Forging Machinery Company put a 680 MN extrusion pipe machine into production, but the unit failed to operate normally. Although the extrusion pipe unit has advantages in producing high-alloy and stainless steel pipes, its yield rate is low and its cost is high. To improve the yield rate of carbon steel and low-alloy steel extra-large diameter thick-walled hot-rolled seamless steel pipes, other processes such as two-roller cross-rolling technology were gradually applied.
From 2009 to 2012, China built and put into production several large-diameter thick-walled hot-rolled seamless pipe mills; however, the demand for extra-large diameter thick-walled hot-rolled seamless pipes was limited, leading to serious overcapacity. Especially after 2014, the seamless pipe industry became sluggish, market demand shrank, and some extra-large diameter thick-walled hot-rolled seamless pipe production units experienced varying degrees of under-operation or even shutdown.
Analyzing the characteristics of various large-diameter thick-walled hot-rolled SMLS pipe mills, it is evident that the extrusion pipe mill can produce varieties such as carbon steel pipes, alloy steel pipes, stainless steel pipes, high-temperature alloy pipes, and corrosion-resistant alloy pipes, but the production cost is high. The two-roller oblique rolling pipe mill can produce carbon steel pipes and alloy steel pipes, with high production efficiency and wall thickness accuracy; however, there are difficulties in producing high-alloy steel pipes, stainless steel pipes, high-temperature alloy pipes, and corrosion-resistant alloy steel pipes. The periodic rolling pipe mill can produce carbon steel pipes, alloy steel pipes, and some stainless steel pipes. This unit is suitable for a wider range of steel pipe varieties compared to the oblique rolling pipe mill. While the deformation during rolling is greater, the dimensional accuracy is lower.
Currently, large-diameter two-roller oblique rolling pipe mills and periodic rolling pipe mills have advantages in manufacturing costs and essentially dominate the market for thick-walled carbon steel pipes and alloy steel pipes of 720 mm and above in China. Large-tonnage extrusion pipe units are primarily used to produce medium- and high-alloy steel pipes, stainless steel pipes, high-temperature alloy pipes, and corrosion-resistant alloy pipes with diameters greater than 720 mm. Large jacking pipe units face disadvantages in market competition. In 2010, Wuhan 471 Factory renovated the large jacking pipe unit, added an elongation machine, and improved the wall thickness accuracy and yield rate of the steel pipes.
