Development Of Mo-Ti-Ce Microalloyed New Air-cooled Bainite Steel For Shovel Teeth

The shovel tooth which has high performance not only has a long service life,but also can greatly improve the working efficiency of excavator,and fundamentally reduce its cost consumption.At present,the actual service life of domestic excavator shovel teeth is usually only 3～4 weeks,and the consumption is up to 5 million every year,which forces our country still needs to import a large number of expensive shovel teeth every year.In order to change this phenomenon,the research of the new type of domestic steel and its key preparation technology has attracted much attention.In the context of the new era,with the increasing awareness of green production,energy conservation and emission reduction,bainite steel has been widely valued and studied due to its low cost,short production cycle and excellent comprehensive performance.In this dissertation,the objects of the research are strengthen and improve the comprehensive properties of bainitic steel by adding Mo and Ce elements to the matrix,so as to obtain excellent shoveling steel.And the effect of Mo and Ce were added separately and simultaneously on tissue evolution,phase transition point and second phase precipitation after Mo and Ce were studied.At the same time,Ti as microalloying element was introduced to further improve the strength and toughness of bainite steel.Finally,with the help of Ce to improve the precipitation of titanium-containing type and reduction of size,by means of precipitation strengthening,fine grain strengthening and process optimization,the win-win goal of strength and toughness of bainite steel is achieved.Research indicates:The addition of Mo and Ce affects the type of precipitation of the second phase in bainite steel.The addition of rare earth Ce changes the main precipitation form of the second phase containing titanium in steel from adding Mo C₂S₂Ti₄to Ti（C,N）.Both adding rare earth element Ce and Mo can increase the precipitation amount of Titanium carbonitride in bainite steel,reduces its precipitation size,and makes titanium better play the role of precipitation strengthening and fine grain strengthening.Of which Mo increased the proportion of Titanium nitrite by 13.12%,and the average particle size decreased by0.043μm,Ce increased the precipitation proportion of Titanium carbonitride by 28.97%,and the average particle size decreased by 0.104μm.The addition of Mo and Ce in the bainite steel reduces the initial transformation temperature of the bainite.Mo reduces the initial transformation temperature of the bainite by 9.69℃,and the rare earth Ce reduces the initial transformation temperature of the bainite by 57.20℃.The reduction of bainite transformation temperature will be beneficial to the formation of bainite microstructure and the increase of bainite steel toughness,which is shown in the following aspects:the addition of Mo increased the proportion of granular bainite in the microstructure by 4.86%,the martensite phase decreased by 0.41%,and the residual austenite increased by 0.68%.The addition of rare earth Ce increased the proportion of granular bainite phase in the structure by 7.34%,martensite phase decreased by 3.3%,retained austenite increased by 0.36%,and the addition of Mo and Ce reduced the grain size by 0.03μm and 0.15μm respectively,and the hardness increased by 54.5HV and27HV respectively.Through the comparison of the comprehensive properties of the bainite steel after isothermal quenching and normalizing+tempering heat treatment,can conclude that,normalizing and tempering heat treatment can make bainite steel which with Mo and Ce added simultaneously has good strength,hardness,and good toughness at the same heat treatment temperature.That is after normalizing at 1080℃and tempering at 250℃for90min,the hardness of bainite steel is 463.9HV,tensile strength is 1318.6MPa,yield strength is 1064.3MPa,reduction of area is 7.67%,elongation is 10.3%,andα_kuis18.45J/cm².Due to the high content of the second phase in bainite steel,the above properties other than impact toughness have reached the numerical requirements specified in the industry standard.The optimized heat treatment of bainite steel with improved composition（0.08%Ti）was conducted by normalizing and tempering,the optimized microstructure of bainite steel is the same as the as-cast group,all are lath bainite and granular bainite.The best heat treatment temperature of bainite steel is normalizing at 1030℃and tempering at 200℃for120min,under the heat treatment condition,impact absorb energy of bainite steel is 39.8J,the ratio of shear to cleavage on the fracture accounted for 50%respectively,hardness is564.3HV,tensile strength is 1384.7MPa,the fracture mode of impact and tensile is ductile fracture.The performance of the optimized bainite steel has reached the requirements specified in the industry standard,and its wear resistance is better than that of the shovel teeth in a certain factory.