Microalloying Non-quenched And Tempered Steel Basic Theory And Application Technology Research

The continuous cooling transformation curves of the tested steels38MnVS and C38N2were researched by the Formastor-F II automatic phase change meter, the law and the influencing factors of the phase change were investigated. The results show that the shape of the continuous cooling transformation curves and the type of the products are approximately similar, only there are some minor differences in the location of the curves, for example, the phase transition temperatures of the tested steel C38N2are lower than that of the tested steel38MnVS. Both the tested steels experienced high temperature change area, medium temperature change area and low temperature change area, their corresponding phase change products are ferrite+pearlite, bainite and martensite, and the critical cooling speed of the microstructure ferrite+pearlite is about1℃/s.The thermoplastic behaviors of the tested steels38MnVS and C38N2were investigated by thermal simulation test enginery Gleeble-1500in the temperature range600℃～1300℃. The results show that the thermoplasticities of the38MnVS and C38N2are all very good, the area reduction of both tested steels are almost more than80%, and the area reduction of the tested steel C38N2is little lower than38MnVS, the machining scope of tested steel C38N2is wider than the tested steel38MnVS, this is because niobium has strong inhibition effect on recrystallization which can prevent the grain boundaries'migration, and carbonitrides exhalation also has the effection.The austenite dynamic recrystallization behavior and static recrystallization behavior of the tested steels38MnVS and C38N2were investigated by thermal simulation test enginery Gleeble-3800with the different deformation systerm. The deformation activation energy, kinetic model, state chart of the dynamic recrystallization, dynamic recrystallization grain size model, hot processing map and instability figure were obtained. The results show that:first, the higher the deformation temperature is and the lower the deformation rate is, the easier the dynamic recrystallization is. Second, the higher the strain rate is and the lower deformation temperature is, it is easier access to the tinier recrystallization grain size. Compared with the test steel38MnVS, the tested steel C38N2which added marginala niobium has more fine austenitic grain size, this is because niobium has the following roles:first, when deformed at high temperature, the solid solution of niobium will play the role of dragging as solute when they cluster at the grain boundaries and the crystal defects; second, when deformed at low temperature, Nb(C,N) which dissolved out at the original austenite grain boundaries and the dislocation line will pin the grain boundaries and dislocations, prevent the movement of the dislocation and grain boundaries, so as to prevent the recrystallization.The controlled forging and controlled cooling processes was carried on by the thermal simulation test enginery Gleeble-3800with the different deformation systerm, the effect of heating temperature, forging temperature, amount of deformation, deformation rate and the cooling speed on the microscopic structure and performance were investigated. The results are as follows:first, increasing the heating temperature and deformation temperature will make the volume fraction of pearlite increase, the size of the pearlite group grow large, the shape of the ferrite changes from block to mesh; second, increasing the deformation amount and the deformation rate will lead to increase the volume fraction of ferrite and weaken the forming of the mesh ferrite, the shape of the ferrite changes from mesh to block; third, along with the increasing of the cooling speed, the volume fraction of ferrite reduces, mesh ferrite is obvious; fourth, the cooling method of urgent colding-slow colding-quick colding is in favor of fining the structure and improving toughness. Manufacturing the cars' steering knuckle with the tested steel38MnVS and C38N2and detecting the steering knuckles' performances. The results show that their various performances have reached cars' steering knuckle's required performance, they can replace the tempering steel40Cr to produce cars' steering knuckle so to attain the objective of shortening the production cycle and reducing the cost.