| Under the guidance of the “One Belt and One Road” policy,high speed multiple units,as a business card made in China,shoulder the brand responsibility for export promotion.The railway axle is one of the most important structures in the railway system and its indigenization and the adaptability to the global environment such as cold regions are important links.In high-speed operation,the high-speed railway axles are subjected to dynamic loads and the bearing forces are complex.In order to ensure the safety of railway transportation,high-speed axle steels should ensure that it has a sufficiently high strength,in particular,excellent impact toughness,and has a low ductile-brittle transition temperature.In the present paper,A new process for producing axle steels with excellent mechanical properties was proposed as follow: obtaining refined microstructure by the regulation of heat treatment process and the microalloying design of V,Nb.In this thesis,the microstructure,Austenite grain size and mechanical properties of 0.05 V steel,0.17 V steel and 0.18V-0.04 Nb steel under different quenching and tempering processing parameters were systematically studied,and the strengthening and toughening mechanism of them was discussed.The main results are as follows:The microstructure and mechanical properties evolution of 0.05 V steel was investigated after quenching at 900? and tempering at 600?-675? for 1h or 4h.The results show that: along with the increase of tempering temperature and time,Some martensite laths of 0.05 V steel dissolved and a large amount of alloy carbides precipitated.The strength decreased,while the impact energy increased.The effect of quenching(austenitizing)temperature on the austenite grain size of0.17 V steel and 0.18V-0.04 Nb steel was studied.The results show that:(1)With the increase of temperature,the austenite grains coarsen,but the austenite grain refinement and coarsening tendency of 0.05 V steel,0.17 V steel and 0.18V-0.04 Nb steel decrease in turn.(2)The V in 0.05 V steel is completely solid-solved at 850 temperature quenching and above,and the obtained grains are the coarsest.The VC and(Nb,V)C particles of0.17 V steel and 0.18V-0.04 Nb steel were not completely solid-solved at 850?,so theycan pin austenite grain boundaries effectively.Compared with 0.05 V steel,0.17 V steel and0.18V-0.04 Nb grain refinement and the tendency to coarsening decreases.(3)Due to the higher precipitation temperature of Nb-rich(Nb,V)C particles,0.18V-0.04 Nb steel obtained the finest austenite grains,and the austenite grain size reached 10?m when quenched at 850?.It reaches 8?m in the second quenching.(4)After quenched twice at850?,the prior austenite grains of three test steels were further refined,and 0.17 V steel was the best.The differences in the microstructure and properties of the three test steels under different quenching processes were compared and their toughening mechanisms were discussed.The results show that:(1)The width of the martensite laths of the three test steels were also refined with the reduction of the austenite grain size,and the interface density at large angles increased.(2)Compared with quenching at 900?,the 0.05 V steel quenched at 850? has improved toughness and toughness,while the toughness of 0.17 V steel and 0.18V-0.04 Nb steel has been improved,but the strength has decreased.The reason is that 0.17 V steel and 0.18V-0.04 Nb steel quenched at 850 °C resulted in the coarsening of the nano-precipitate phase in the final structure,and the precipitation strengthening effect was weakened.In general,the quenching temperature at 850?,all three test steels obtained good comprehensive mechanical properties.(3)Compared with one quenching,the strength of the three test steels after two quenchings at 850?decreased,but the toughness was improved.(4)When quenched once or twice at 850?,the microstructure of 0.17 V steel and 0.18V-0.04 Nb steel was refined due to the increase of V content and the addition of trace Nb,and the precipitation strengthening effect was improved,so that the toughness and the low temperature toughness were both better than0.05 V steel. |
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