Study On The Structure Characteristics And Mechanical Properties Of The Sheaf-Like Bainite In The Steels

The microstructure morphology and mechanical properties of the intermediate transformation products austenitized at different temperatures in the Fe-0.05C-0.90Si-0.25Mo-1.70Mn-0.35Ni(ultra-low carbon steel),Fe-0.80C-1.32Si-1.06Cr-0.45Mn(high carbon steel)and Fe-1.52C-1.97Cr-0.76Si-0.73Mn-0.09Mo(ultra-high carbon steel)steels were investigated by X-ray diffraction,optical microscope,transmission electron microscopy(TEM),hardness,tensile strength and impact toughness test,in order to obtain the new knowledge about the universality of the sheaf-like bainite and provide the theory basis for the application of the sheaf-like bainite in engineering field.The results of the microstructure morphology analysis showed that the sheaf-like bainite consisting of the ferrite laths arranged in paralle and separated by the remained austenite can be formed in all three experiment steels.For the ultra-low carbon steel without Nb,Ti elements,the sheaf-like bainite can be formed in intermediate transformation process under the condition of austenitized temperature far above Ac3 and not experienced high temperature deformation.The austenitization at temperature above Accm can be the formation condition of the sheaf-like bainite in the high carbon steel.For the ultra-high carbon steel,when the austenitizion temperature closed to Accm,the sheaf-like bainite can be formed.The results researched by TEM showed that the fine microstructure of the sheaf-like bainite in the different carbon content steels was of little difference.With the carbon content increasing,the ferrite lath became finer.The width of the ferrite lath was about dozens of nanometers in the ultra-high carbon steel.The bainitic ferrite was of the continuous lath in both the ultra-low carbon steel and the ultra-high carbon steel.While in the high carbon steel,the bainitic ferrite consisted of the subunits,and width of a ferrite lath was about 100 nm.The effects of the sheaf-like bainite on mechanical properties of the experiment steels were researched.The results indicated that the sheaf-like bainite was important effect on the mechanical properties of the experiment steels.With the content of the sheaf-like bainite increasing,the properties of tensile strength and impact toughness of the experiment steels became better.For both the high carbon and the ultra-high carbon experiment steel with lower toughness,the nanoscale sheaf-like bainite can improve the toughness significantly,and make their mechanical properties excellent.The intermediate transformation products in the experiment steels austenitized at different temperature were studied.It can be obtained that the sheaf-like bainite existed in the steel universally.