Development of High Strength Hot Rolled Steel Strip for Sour Service

摘要

The increased use of natural gas as an energy source has been continuous driven industry demand for everincreasing strength in gas transmission pipeline materials in order to achieve safe and economic transportation of natural gas. In particular, linepipe steel for sour service needs to have good Hydrogen Induced Cracking (HIC) and Sulfide Stress Corrosion Cracking (SSCC) resistance as a result of steady development in wells with increasingly severe environments. Also, there is a strong need for total cost reduction in pipeline systems by applying higher strength linepipe grade steels. Therefore, this paper discusses the possibility of a higher strength sour resistance steel such as grade X70. The suitable microstructure was compared against each other among polygonal ferrite with pearlite microstructure, polygonal ferrite with acicular ferrite, and acicular ferrite. The typical composition of the Nb-V-Ti type steel used for grade X65 was modified by adding of Cr or Mo. It is shown that the Cr containing acicular type microstructure steel has good HIC resistance even for grade X70. The successful mill-trial production of this newly developed X70 grade with high sour resistance is introduced.