Chapter 27 DEVELOPMENT OF HIGH STRENGTH HIGH STRAIN LINEPIPE STEELS

In order to achieve safety and reliability of long-distance gas transmission pipeline installed in seismic and permafrost region, it is quite important to apply high strength linepipes with sufficient strain capacity against buckling and weld fracture by large ground movement. Grades X70-X80 linepipe steels with bainite-MA (Martensite-Austenite constituent) microstructure have been developed by applying advanced plate manufacturing technologies with the combination of accelerated cooling (AcC) and heat treatment on-line process (HOP™). This microstructure shows higher strain capacity which has low yield to tensile (Y/T) ratio and high elongation in the longitudinal direction even after pipe coating heat treatment. In this paper, the material design concept of Grades X70-X80 high strength high strain linepipe steels are introduced. In Grade X70 high strain linepipe steels, adequate MA volume fraction was required even in lower carbon equivalent (CE) level compared with Grade X80 high strain linepipe steels. Mn and Ni addition was useful to obtain the adequate MA volume fraction efficiently, resulting in superior strain capacity which has low Y/T ratio even after the external coating. High strength with Grade X70 level and high elongation have been achieved by control of homogeneous material property such as hardness distribution. Advanced accelerated cooling device (S-OLAC™-A) enables it to obtain the homogeneous microstructure because of homogeneous temperature control after accelerated cooling. Higher elongation has been achieved by lower surface hardness and homogeneous hardness control. In developed Grade X80 high strain linepipe steels, MA volume fraction and its size distribution were controlled simultaneously to achieve higher strength, low Y/T ratio and higher Charpy absorbed energy in full ductile fracture region. It is very important to control MA morphologies to achieve these mechanical properties. HOP™ type bainite-MA dual phase linepipe steels show the highest strain capacity even after pipe coating heat treatment. On the other hand, AcC type bainite-MA dual phase linepipe steels have the highest Charpy absorbed energy because of lower MA volume fraction and finer size of MA by applying appropriate accelerated cooling stopping temperature. Detailed laboratory test results and actual mill test data will be introduced in the presentation.