Ferrite - Pearlite Type Non-transfer Application Technology Research And Tempered Steel

The hot ductility within the forging temperature range of non-quenched and tempered V-Ti steel and V-Nb-Ti steel were investigated by using a Gleeble-1500simulator. The results indicate that both steels show better hot ductility whose area reduction values are more than85%. The hot ductility of V-Nb-Ti steel is a little lower than that of V-Ti steel at the same temperature due to niobium’strong inhibition effect on recrystallization.Austenite dynamic recrystallization behavior of the non-quenched and tempered V-Ti steel and V-Nb-Ti steel were investigated by thermal simulation tests. The results show that dynamic recrystallization is prone to occur at higher temperature and lower strain rate. The fine recrystallized grain size can be obtained at lower deformation temperature and higher strain rate. Compared with V-Ti steel, the V-Nb-Ti steel has finer recrystallization grain size because of the following roles of niobium. Firstly, the niobium solutes play the dragging role when they segregate at the grain boundaries and crystal defects when deformed at high temperature, thus retarding the occurrence of austenite recrystallization. Secondly, the precipitates Nb(C, N)which formed at austenite grain boundaries and dislocation lines play the roles in pining grain boundaries and dislocations when deformed at low temperature, thus preventing their movement and recrystallization process.The dynamic recrystallization grain size model, state chart of dynamic recrystallization and hot processing and plastic instability map of both steels were built through analyzing austenite dynamic recrystallization behavior of the steels.The controlled forging processing and their metallurgical principles of both steels were concluded. The V-Ti steel is suitable for recrystallization-controlled forging process and the following processing parameters are recommended:reheating temperature1130℃, start forging temperature1000℃, finishing forging temperature850℃and strain rate0.4s-1.The V-Nb-Ti steel is suitable for recrystallization, then non-recrystallization controlled forging process and the processing parameters are recommended as follows:reheating temperature1200℃, start forging temperature1000℃, finishing forging temperature850℃and strain rate0.4s-1. The continuous cooling transformation curves of non-quenched and tempered V-Ti steel and V-Nb-Ti steel were established by using Formastor-FⅡ automatic transformation measuring apparatus. The phase transformation laws of both steels were investigated and their controlled cooling temperature range and cooling rate were determined. For V-Ti steel, the controlled cooling temperature range is recommended in the range of740℃～500℃and the cooling rate0.3～1.2℃/s, it belongs to recrystallization controlled forging and cooling process. For V-Nb-Ti steel, the controlled cooling temperature range is720℃～500℃and the cooling rate is0.3-1℃/s, it’s forging process belongs to recrystallization-non recrystallization controlled forging and cooling process. The results show that the higher proportion of ferrite with massive distribution can be obtained at lower cooling rate, thus leading to the better toughness of the steel.The controlled cooling experiments of the forgings of V-Ti steel and V-Nb-Ti steel were conducted to study the effect of different cooling pattern (air cooling and wind cooling) on mechanical property and microstructure of the forgings. The hot forging conditions are as follows:reheating temperature of1130℃for V-Ti steel and1200℃for V-Nb-Ti steel, start forging temperature1000℃～1050℃. The results showed that, wind cooling can significantly improve the tensile strength of V-Ti steel but decrease the impact toughness simultaneously with the finishing forging temperature decreasing and the deformation increasing. While for V-Nb-Ti steel, wind cooling has little effect on its tensile strength, but significantly improves the impact toughness value with the finishing forging temperature decreasing and the deformation increasing.V-Nb complex microalloying refines austenite grain size, makes recrystallization stop temperature increased and promotes intragranular ferrite formation on the MnS-VN compound precipitations, makes pearlite metamorphosis.spherical ferrite precipitations and V-Nb complex non recrystallization rolling make pearlite break, spherical granular.which significantly improve the impact toughness value.