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.