Research On Controlling Microstructures And Mechanical Properties Of X80 Pipeline Steel Excellent Deformabaility

The characteristic of pipeline steel with excellent deformation is consisted by dual phase. It is designed by ultra-low carbon multiple micro-alloying and thermo-mechanical controlled rolling process.We can obtain bainite-ferrite dual microstructures or bainite-martensite/austenite microstructures.In this paper, a Gleeble-3500 simulator is used to simulate heat treatment process in order to research X80 pipeline steel with excellent deformation. The relationship between the different volume fraction of bainite(VB) in different process and mechanical properties were systematically investigated by OM, SEM, TEM and so on. The results indicate that the testing steel is consisted mainly by bainite and ferrite microstructures. Because of the multiple effections of process parameters, the microstructures and the mechanical properties of the testing steel is different, including the volume fraction of bainite, hardness, strength, elongation, work hardening exponent and so on. The TEM analysis shows that the fine precipitates of Nb and Ti make strong precipitation hardening effect.In order to research the influences of process parameters on the microstructures of the testing steels in two schemes, some parameters are controlled, including austenitizing temperature, holding temperature, holding time and cooling rate. The study indicates that higher austenitizing temperature, higher holding temperature, lower holding time and higher cooling rate are good for increasing the volume fraction of bainite. However, lower austenitizing temperature is good for fine grain.The study also indicates that the hardness and the strength of the testing steel is increasing with the increasing of the volume fraction of bainite while the elongation and the work hardening exponent is decreasing with the increasing of the volume fraction of bainite. Therefore, the plastic of the testing steel becomes worse.Compared with the two schemes, the properties of the testing steel obtained in lower austenitizing are preferred to the properties of the testing steel obtained in higher austenitizing.