Study On Microstructure Transformation And Strengthening Mechanism Of Vanadium Microalloyed Carbon Non - Quenched And Tempered Steel

It is an effective way to enhance strength and toughness of microalloyed medium carbon forging steel with ferrite plus pearlite microstructure through proper application of microalloying technology and controling forging parameters. In this paper, different V contents of medium carbon microalloyed forging steels 38MnVS for fracture splitting connecting rod. Austenite grain growth behavior, static and dynamic CCT curves as well as mechanical properties in the as-forged condition of the tested steels were studied. The conclusions obtained are as follows:The austenite grain of 38MnVS steel grows up exponentially with the increase of reheating temperature, while austenite grains grow up in a parabolic way as the soaking time is prolonged. Austenite grain size decreases and its coarsening temperature increases sharply with the increase of V content. However, the increase in V content has little effect on the isothermal grain growth index. Measured data combined with the grain growth kinetics theory was used to establish a mathematical model which could predict the austenite grain growth behavior of steel 38MnVS with different V contents.Both static and dynamic CCT curves of 38MnVS steel included obviously ferrite-pearlite, bainite and martensite phase regions. With increasing V content, Ac3 and Aci temperatures increase gradually, and phase transformation regions for ferritic-pearlite, bainite and martensite were gradually shifted to the right side and low cooling rate of the diagrams. Compared to the static CCT curve, the starting temperature of ferrite-pearlite transformation in the dynamic CCT curve was significantly increased, as well as bainite and martensite phase regions. With increasing V content, the degree of the increase in starting temperature of transformation was further increased. The hardness of the tested steels increases nearly linearly with increasing cooling rate before the formation of bainite, and then the hardness exhibits a sharp increase with further increasing cooling rate. The increase of V content results in significant increase of hardness and this tendency enhances with increasing cooling rate until the formation of bainite. There exists an optimum cooling rate to obtain an appropriate fraction of fine precipitates to get the highest precipitation hardening effect for the testes steels with ferrite-pearlite microstructure.For the 38MnVS steels in the as-forged air cooling condition, both the volume fraction of ferrite and the precipitation particles with size less than 10 nm increase, the ferrite grain size and pearlite interlamellars pacing decrease with increasing V content. Both hardness and microhardness ratio of ferrite and pearlite increase with increasing V content, while toughness decreases drastically as the V content increases higher than 0.2%. It was found that the yield strength does not depend completely on ferrite. A predictive equation for yield strength suitable for V microalloyed medium carbon forging steel was developed by the combination of the Hall-Patch relationship, coefficient of solid solution strengthening and the Ashby-Orowan model and experimental data in the literatures.