1996 Volume 36 Issue 2 Pages 235-240
The study confirms the effect of lowering manganese (Mn) content of sour service line pipe steels on improving the resistance to hydrogen induced blister cracking (HIBC), following which thermo-mechanical control process (TMCP) is considered as an effective tool to strengthen and toughen the low manganese steels. The HIBC is preventable when the Mn content is less than 0.4 mass% even for the steels containing an order higher sulfur than latest ordinary steels for sour service. After confirming the effect of lowering Mn, the Mn content is estimated to obtain the required strength for line pipe steels. Presupposing ordinary rolling facilities available for the TMCP, 0.5 mass% of Mn is necessary to strengthen a steel microalloyed with Nb and Ti. Applying heavy reduction (60%) and rapid cooling (80°C/s) to the 0.5Mn-Nb-Ti steel refines the microstructure and consequently improves simultaneously both yield strength and Charpy transition temperature. As a result of studying the TMCP variables to achieve such grain-refinement, necessary TMCP conditions to obtain the better strength-transition temperature balance are the rolling finish temperature higher than the Ar3 transformation temperature of the steel, the heavy reduction which accumulates equivalent strain of more than 0.9 at 850°C, and the rapid cooling rate of more than 60°C/s.
