Heating System And Hot Deformation Parameters On The Microstructure And Properties Of X80 Pipeline Steel Impact Studies

Reheating condition affects the prior-austenite and solution of the micro-alloy elements directly in conventional hot rolling process. Moreover, it influences recrystallized austenite size, precipitation amount and state of carbonitride.The key point of hot rolling is to control rolling and cooling, for the different parameters influence room-temperature microstructure and the quality of product. It is very important to research reheating condition and hot deformation in laboratory for the practice production.In this thesis, the steel used in the test is a kind of low carbon high Ni X80 grade continuous casting slab. At first, effect of reheating temperature on prior-austenite size was studied; Next, the influence of TMCP (Thermo Mechanical Controlled Processing) parameters (deformation temperatures, deformation amount, final cooling temperatures and cooling rates) on the microstructures and properties are analyzed by using of Gleeble-1500D thermo-mechanical simulation machine; At last, effect of Mo on microstructures and properties of tested steel was obtained by comparing with the other no-containing Mo steel. As follows are the conclusions.(1) The prior-austenite grains grow with increase of reheating temperature, and coarse and asymmetric grains appear at 1200℃, but Nb has dissolved around 1150℃. In order to insure that Nb dissolve adequately and no obvious coarse grains in microstructures, the reheating technology of the steel should be controlled in 1 to 3 hours at about 1180℃.(2) The TMCP parameters have a great influence on the phase transformation of X80 pipeline steel. Under different processing conditions, the changes of the microstructures and M-A appear special rules. Along with the increase of cooling rates, microstructures of tested steel change from polygonal ferrite and quasi-polygonal ferrite to fine acicular ferrite and M-A islands are refined and dispersed at the same time. Along with the decrease of deformation temperatures, microstructures gradually refined and quasi-polygonal increase, but quantity of M-A islands decreases and M-A grains become small and smooth. Along with the increase of deformation amount, microstructures gradually refined and quasi-polygonal grains become small but quantity increases, amount of M-A islands increases and M-A grains become small and smooth. Along with the decrease of final cooling temperatures, microstructures of tested steel gradually refined, but quantity of M-A islands increases and grains become claviform.(3) Mo is able to not only refine microstructures, boost producing of acicular ferrite, and increase quantity of M-A, but also make transformation line of acicular ferrite in CCT curves right shift and extend cooling rates range of obtaining acicular ferrite.