Study On Behaviors Of Hot Deformation And Phase Transformation Of A Nb-bearing Miroalloying Steel

Increasing of the content of Nb in microalloyed steel is an important developing direction to reduce the product cost of low carbon and low alloy high strength steel.In Nb-bearing steels,the effectiveness of Nb on microstructure and mechanical properties is achieved through the dissolution and precipitation of Nb in the steels during thermo-mechanical control process(TMCP).The effectiveness of Nb on microstructure and mechanical properties increases with the increasing of Nb content in the steels.Thus,The study of the effectiveness of Nb in high-Nb steels is of great significance to the development of low cost and low carbon microalloying steels.In this thesis,the behaviours of austenite grain growth,hot deformation,recrystallization and phase transformation of a low-C and high-Nb steel have been studied on a Gleeble-3500 thermal simulator.The behaviours of dissolution and precipitation of Nb have been investigated by means of quantitative analysis method of dissolved Nb.Furthermore,the effects of dissolution and precipitation of Nb on microstructure evolution and mechanical properties have been discussed.The reheating temperature,that the austenitic grain size began to rapidly grow up with the increase of heating temperature,is 1150 ?C.Which is according to the temperature of the increasing of the Nb content.The temperature,that the Nb content rapidly increases with the increase of heating temperature,is 1100 ?C.The Nb-carbonitride cannot be completely dissolved during the reheating process,which significantly inhibits the growth of austenite grain.The austenite grain size still remains below 100 ?m,when the heating temperature reaches to 1200 ?C.The dissolved Nb can significantly inhibit the dynamic recrystallization of the austenite.When the austenitizing temperature is at 1250 ?C,the hot deformation activation energy is 465.8 kJ/mol.While,the hot deformation activation energy reduces to 422.9 kJ/mol when the austenitizing temperature decreases to 1150 ?C.Nb can significantly restrain the phase transformation of ferrite,While the deformation of austenite in recrystallization and non-recrystallization zone significantly accelerates the phase transformation,as the result,the CCT curves move to up and left corner.It is only when Nb dissolves in the austenite,the Nb can effectively restrain the phase transformation during cooling process.While,the precipitation of NbC in austenite reduces the dissolved Nb content,which results the decrease of the effectiveness of Nb to restrain the phase transformation,and the CCT curve move to the up and left corner,moreover,the hardness of the product microstructure after cooling reduces.