Research On Effects Of Hot Deformation Behaviore Of Pipeline Steels For High Pressure Gas Transportaion

The purpose of this article is to research the effect of Nb in the HTP (High Temperature Processing) steels. The solute Nb and the original austenitic grain growth behavior, dynamic and static recrystallization behavior, strain-induced precipitation of four kinds of high Nb micro-alloyed steels were studied by means of optical microscopy (OM), Gleeble-3500 thermal mechanical simulator, transmission electron microscopy (TEM), the inductive coupled plasma emission spectrometer (ICP-AES), furthermore, the relationship of them was discussed.The results show that austenitic grain of high Nb steels has a low growth tendency. The original austenitic grain grows up quickly when the reheating temperature is above 1150℃, which is corresponding with the temperature of Nb fast dissolution. The content of Nb in austenite is related to the content of Nb in steels. The content of solid solution Nb increases along with the content of Nb in steels. However, the solution ratio decreases as the content of Nb and C increased. If the additive content of Nb in steels is exorbitant, its ability to restraint the austenitic grain growth tendency becomes weakly.Dynamic recrystallization is inhibited by Nb, which mainly depends on the content of solid solution Nb. When the content of solid solution Nb increases from 0.073% to 0.093%, dynamic recrystallization activation energy increases form 385 kJ/mol to 471 kJ/mol. Strain-induced Nb(C,N) precipitation in the steels strongly inhibites static recrystallization. On the static recrystallization percent curves, the start time of recrystallization is restrained corresponded with the time of Nb precipitated quickly; the precipitated time is along with the content of Nb and C.The test results show that the design of high Nb steels should consider the content of Nb and C at the same time. When the content of C is in the range of 0.036~0.06%, the content of Nb should be controlled in the range of 0.09~0.10%. In those conditions, the no-recrystallization temperature of the steels can be increased remarkably, meanwhile, the ratio of solution Nb can reach more than 80%, which avoid the waste of resources.