Research on the Accurate Control of Initial and Finishing Cooling Temperatures for X70 Pipeline Steels

Abstract:

Different cooling parameters, including the initial cooling temperature, finishing cooling temperature and cooling rate, have a significant effect on the final microstructures and properties for pipeline steels. In present work, Gleeble-1500 thermal-mechanical simulator was used to investigate the microstructural evolution of X70 pipeline steels under different cooling conditions, and the microstructures obtained were analyzed using optical microscope and transmission electron microscope. The experimental results showed that when the initial and finishing cooling temperatures are controlled in the range of 740~760°C and 500~520°C respectively, the microstructure of X70 pipeline steels reveals a proper content (about 12%) of pre-eutectoid ferrite besides acicular ferrite and M/A (Martensite/Austenite) island, which can guarantee an optimum combination of strength and ductility. In contrast to the conventional way using CCT curve, this methodology shows a higher accuracy and operability, and used in industrial production to achieve good effect.