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In recent years the trend in oil and gas transmission pipelines has been toward higher operating pressures. This trend, while the desire to keep steel costs low, has resulted in an increased demand for large diameter heavy wall X80 with good low temperature toughness. It is well known that improving the low temperature toughness with increasing wall thickness of the pipeline is very difficult. To overcome the difficulty of producing consistent low temperature toughness in heavy wall pipe Shougang Steel Research in cooperation with the Shougang Steel Qinhuangdao China (Shouqin) 4.3 m heavy wide plate mill research was conducted. This paper describes the background, composition design and process strategy to produce good low temperature toughness in heavy wall API plate. The importance of the slab reheating schedule and recrystallized rolling process/schedule that occurs during the roughing process will be discussed. The effect of per pass reductions and work roll speed rotation on the strain introduced was analyzed by means of the numerical simulation technology. Furthermore, the center thickness microstructure and low temperature toughness of plate under the different rolling schedules were researched. The results showed a low reheating temperature and slow rough rolling speed should be implemented. The per pass reductions during recrystallized rough rolling should be increased in a steady fashion, with special emphasis on the reduction of the final roughing pass prior to the intermediate hold (transfer thickness for finishing). When the final roughing pass had a per pass reduction of more than 15%, the main microstructure of plate consists of uniform (surface to center) fine ferrite/acicular ferrite with a small volume fraction of M-A constituent. This fine uniform microstructure results in good low temperature fracture toughness in heavier plate thicknesses. Results of this research and development work will be discussed.

Along with the increasing demand of oil and natural gas by various world economies, the operating pressure of the pipeline is also increasing. Large diameter heavy wall X80 pipeline steel is widely used in the long distance high pressure oil and gas transportation in China today. In addition, development of X90/X100 has begun in earnest to support the growing energy needs of China. With the wide use of X80 steels, the production technology of this grade has become technically mature in the industry. Shougang Group Qinhuangdao Shouqin Metal Materials Co., Ltd. (SQS) since 2008 has been steadily developing heavier thicknesses and wider plate widths over the years. This development has resulted in stable mass production of X80 pipeline steel plate in heavy wall thicknesses for larger pipe OD applications. The technical specifications of X80 heavy wall thickness and X90/X100 14.8–19.6 mm wall thicknesses, large OD (48″) requiring wide steel plates for the 3rd West-to-East Natural Gas Transmission Pipeline Project and the third line of Kazakhstan-China Main Gas Pipeline (The Middle Asia C Line) and the demonstration X90/X100 line (part of the 3rd West-East Project) in China required changes to the SQS plate mill process design. Considering the technology capability of steelmaking and the plate mill in SQS, a TMCP+OCP (Optimized Cooling Process) was developed to achieve stable X80 and X90/X100 mechanical properties in the steel plates while reducing alloy content. This paper will describe the chemistry, rolling process, microstructure and mechanical properties of X80 pipeline steel plates produced by SQS for 52,000 mT of for the 3rd West-to-East Natural Gas Transmission Pipeline Project and 5,000 mT for the Middle Asia C Line Project along with 1000 tons of 16.3 mm X90/X100 for the 3rd West-East demonstration pipeline. The importance of the slab reheating process and rolling schedule will be discussed in the paper. In addition, the per pass reductions logic used during recrystallized rough rolling, and special emphasis on the reduction of the final roughing pass prior to the intermediate holding (transfer bar) resulting in a fine uniform prior austenite microstructure will be discussed. The optimized cooling (two phase cooling) application after finish rolling guarantees the steady control of the final bainitic microstructure with optimum MA phase for both grades. The plates produced by this process achieved good surface quality, had excellent flatness and mechanical properties. The pipes were produced via the JCOE pipe production process and had favorable forming properties and good weldability. Plate mechanical properties successfully transferred into the required final pipe mechanical properties. The paper will show that the TMCP+OCP produced X80 heavy wall and 16.3 mm X90 wide plates completely meet the technical requirements of the three pipeline projects.

In this study, a certain amount of Cu was added into tentative steel to introduce novel Cu-rich nanoprecipitates, thus enhancing strength yet without sacrificing toughness. This type of precipitates was quite different from previous ε-Cu, and was a novel type of Cu-rich nanoprecipitates, which contained more than 50% Cu. The microstructure, mechanical properties and precipitates of the steels aged at 550 °C for different holding times and were carefully examined. The microstructure of the tested steel was mainly bainite and gradually evolved into equilibrium state after aging. Mechanical properties results showed that after being aged at 550 °C for 10 min, the steel can have an excellent mechanical property combination of strength and toughness. In addition, a large amount of tiny precipitates was uniformly distributed in the matrix of the aging steels, and their size kept at nanoscale. In particular, when the steel was aged at 550 °C for 10 min, it produced the largest number of tiny precipitates of this type. This type of Cu-rich nanoprecipitates emerging from the steel aged at 550 °C for 10 min also brought about a remarkable antibacterial property. It revealed that novel Cu-rich precipitates not only have positive effects on strength and toughness, but also played an important role in antibacterial properties.

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