Effect by Nb Content on Microstructure and Strength/Toughness of X70 LSAW Pipe

The strength and toughness of the X70 LSAW pipes with 0.046%Nb and 0.075%Nb respectively are tested，and the microstructure of pipe body is analyzed. The analysis reveals that along with the rise of the Nb content，the ave- rage grain size of the pipe body is getting reduced，and the microstructure changes from the PF+B particle structure into the structure mainly composed by B particle；the transversal tensile stress-strain curves are all typical dome shape，and all the tensile properties are up to the applicable requirement，whereas the yield strength and tensile strength of the pipe body with 0.075%Nb content are rather high；in case the Nb contents are the same，the higher the C content is，the higher both the yield strength and the tensile strength will be；the steel pipe with 0.075%Nb content shows better low- temperature impact toughness，while the ductile-brittle transition curve of the steel pipe with 0.046%Nb content shows higher upper platform energy；the HAZ of the steel pipe with high Nb content and low C content also has much better low temperature toughness. Furthermore the effects by the contents of Elements C and Nb and the microstructure types，and the precipitated phase and the dislocation density on the strength and toughness of the X70 linepipe steel are discussed.