Application HTP Process to Produce X80 Strip for 2nd West-East Gas Pipeline Project

HTP practice,alloyed with low-carbon (less than 0.06%) and high-niobium (up to 0.11%),has been developed to produce large-wall X80 hot rolled strip for 2nd West-East Gas Pipeline project successfully.In this paper,existing status of niobium at different rolling stage,such as reheating,rough rolling and final product and strengthening effects have been presented and analyzed systematically.Some technological questions concerrned,such as the mixed grain occurred on the delay table,dynamic recrystallization during the finish rolling course,γ→αtransformation and final microstructure obtained at low coiling temperature,have been explored further.Analytical results presented increased recrystallization stop temperature due to high niobium contents can provide the requisite to remove waiting-temperature operation of transfer bar before finish rolling to ensure controlled effect of traditional low-temperature,which is benificial not only to production efficiency,but also to uniformity of austenite microstructure.Secondly,dynamic-recrystallization occurred during finish rolling may reduce the density of dislocation,but will better the grain gradient along the thickness because the dynamic recrystallization proceeds instantly,which is beneficial to reduce the appearance of separation of impact fracture.Finally,the lower coiling temperature than traditional TMCP practice,up to 300℃,is a key factor to obtain low-carbon bainite microstructure composed of acicular ferrite and small amount M/A component,which results in high strength and excellent low-temperature toughness.