Strengthening Mechanism And Usage Performance Of 590MPa Grade V Micro-alloyed Wheel Steel

The automobile industry is developing towards the direction of lightweight.The wheel,as an important part of the automobile,not only requires reducing energy consumption in actual production,but also needs to have excellent usage performance while ensuring the intensity.In this paper,following the principle of low-cost and high-performance,the chemical composition design and industrial trial production of 590 MPa grade V micro-alloyed wheel steels were carried out and the strip after rolling not only had good microstructure and mechanical properties,but also had excellent forming properties,welding properties and fatigue properties.The main contents and research results of this paper are as follows:(1)Based on the idea of low-carbon V micro-alloying,the composition design of 590 MPa grade wheel steels was carried out.The content of Mn was appropriately increased on the basis of the ordinary C-Mn steel and the content of N was reasonably controlled to promote the precipitation of V.Added as few small amounts of alloying elements as possible to reduce the cost under the condition of guaranteeing the strength.After the industrial trial production,properties met the requirement and the impact toughness of the strip after rolling in service temperature range was good with a good match of strength and toughness.(2)The microstructure of V micro-alloyed wheel steels were polygonal ferrite and a small amount of pearlite.The ferrite matrix contained a large number of diffusion approximately circular precipitation V(C,N)with an average size of 8.73 nm and a relative precipitated volume fraction of 65.5%.The main precipitation form of the test steel were phase precipitation and general precipitation and the precipitation effect was good.Among strengthening mechanisms,the strengthening contribution of fine grain was 232.4 MPa,solid solution was 179 MPa,precipitation was 96 MPa,dislocation and phase change were 42.6～67.6 MPa.(3)According to the requirement of wheel steel structure,the forming property of V micro-alloyed wheel steels was evaluated through cold bending and hole expanding tests.The cold bend property was qualified with no visible macroscopic cracks and good surface quality of the specimen.The hole expansion property of the test steel was excellent with an average limit expanding rate of 108.9%and the expanding fracture belonged to ductile fracture.(4)The welding property of V micro-alloyed wheel steels was studied by flash butt welding.Series of process tests showed that the test steel had a large weldable range,good joints’quality and welding process properties.The optimal welding process parameters of the test steel were:flash current 48°/582.0 A,upset current 44°/516.6 A.Under the optimal process,the microstructure of weld seam was composed of ferrite side plate,acicular ferrite,bainite ferrite and martensite.The microstructure of coarse grain zone was acicular ferrite and granular bainite while the microstructure of fine grain zone was dominated by fine ferrite.The joint tensile fracture occurred in the base metal and the maximum hardness of the joint appeared at the weld and gradually decreased from the weld to the base metal.The change rule of lowtemperature impact toughness in each area of the joint was:fine grain zone>coarse grain zone>weld seam>base metal.The cold bending property of the joint was qualified.(5)The fatigue property of V micro-alloyed wheel steels was studied by high-cycle fatigue test.Under the stress ratio R=-0.1 and the circular base 107,the conditional fatigue limit of the test steel was 506.2 MPa and the fatigue strength ratio was 0.77.The relationship between high stress level and fatigue life is as follows:σ=710.5-28.81g N.Under low stress levels,the fatigue crack source was inclusion and there were few fatigue striations and secondary cracks in the crack growth zone.Under high stress levels,the source of the fatigue crack was persistent slip bands and there were clear fatigue striations and secondary cracks in the crack growth zone.It was typical ductile fracture that occurred in the transient fracture zone.