Continuous cooling transformation behavior of X70 pipeline steel

摘要

The continuous cooling transformation behaviors were researched on X70 pipeline steel through two pass deformation and non-deformed austenite using Gleeble-3500 thermal mechanical simulator, and static continuous cooling transformation curve and dynamic continuous cooling transformation curve were measured through thermal dilation method and metallographic method. The influence of cooling rate and deformation parameters on microstructure was analyzed. The results show that the hot deformation accelerates the acicular ferrite and polygonal ferrite phase transformation, increases the starting transformation temperature and the finishing transformation temperature significantly, and shifts the CCT curve moving upward to the left side corner. Acicular ferrite is obtained in practice using accelerated cooling rate after deformation Acicular ferrite can be obtained in wider range of cooling rates, and microstructure and island structure is finer through hot deformation. The comprehensive mechanical properties of steel depended on the final microstructure and grain refinement. In order to obtain high performance pipeline steel, composition design and the application of controlled rolling and controlled cooling technology had become a key, especially the latter which had decisive influence on the product's final texture and comprehensive performance in modern iron and steel material production. Along with the development of the modern rolling equipment as well as the in-depth study of controlled rolling and controlled cooling technology, the application of controlled rolling and controlled cooling technology had become an important means of fully excavating the potential of the material properties based on the existing material chemical compositions and it had got the extensive attention from modern iron and steel materials research field. The continuous cooling transformation curve (CCT) of steel reflected the changing relation between supercooling austenite transformation product, quantity and hardness under different cooling speed. At present, there were more reports on the study of the continuous cooling transformation of pipeline steel. But owing to the great influence of chemical composition and rolling processes on CCT curve, and in order to accurately describe the texture evolution of the continuous cooling process of X70 pipeline steel produced by 1700 rolling of Tangshan Steel and Iron Company, Gleeble-3500 thermal simulation machine was used to study static continuous cooling phase transition and double lane dynamic continuous cooling phase transition respectively to determine the corresponding CCT curve in order to provide theoretical basis for the optimization of rolling process.