Micro-alloy Steel Q345 Cct Curves And Fracture Toughness

The CCT diagrams are believed to be usefull in gaining an insight into microstructures and designing an optimal TMCP during industrial processing of steels. The different dilatometric cures of Continuous Cooling Transformation have determined by Gleeble-1500 thermal mechanical simulate,and combined Metallographic analysis-hardness measurement and Differential Thermal Analysis the CCT diagram is obtained;The microstrcture of the product of the continuous cooling transformation occurring in Q345 and the reason for the difference between Q345 and common steel were investigated.The ductile fracture toughness J_ICand fatigue crack propagation rate of Q345 steel in status of hot rolling,air cooling after 950℃normalizing and wind cooling after 950℃normalizing have been defined.And the microstructure of them were investigated by optical microscope,scanning electronic microscope and transmission electron microscope.1.The results indicate that the product of austenite transformation in different continuous cooling rates include ferrite,pearlite,bainite and martensite,and the range of bainite transformation is very big,it can be found as needle-like bainite when the cooling rate bigger than 0.5℃/s between 450℃to 600℃.2.Moreover,the steel under the treatment of 950℃normalizing have smaller grains and more homogeneous structure,and it can obtain higher ductile fracture toughness J_ICthan those of hot rolling status.But under the quicker cooling rate(wind cooling),it have a higher fatigue crack propagation rate because this can increase the hardness and notch sensitivity of steels.3.The CCT behaviors of the Q345 steel are greatly impacted by alloy additions such as Nb,V,Ti Through comparing with 16Mn steel(without Micro-alloys).The strengthen methods and the effects to fracture toughness of Micro-alloys was analyzed.