| With the improvement of the national economy,the output of automobiles is rapidly increasing.At the same time,automotive steel also faces huge challenges in terms of environment,safety and energy.Therefore,in order to reduce energy consumption and make cars lighter,the development of high-performance automotive steel is an important research direction in the steel industry.In this paper,two kinds of Nb-V-Ti-Mo high-strength steels were researched by studying the recrystallization behavior of high-temperature austenite,continuous cooling transformation behavior,isothermal ferrite transformation behavior and laboratory hot rolling process.The effects of microstructure on the properties of steel were investigated.The main works and conclusions are as follows:(1)The single compression experiment was conducted to study the dynamic recrystallization flow stress curve of high temperature austenite.The recrystallization activation energy was solved,and the constitutive equation with a strain compensation was established.The results revealed that the deformation temperature is higher,the lower the deformation resistance and the higher the deformation rate,the higher the deformation resistance;the dynamic recrystallization activation energies of A steel and B steel are 392.688kJ/mol and 381.419kJ/mol,respectively.V,Nb and Mo play a significant role in recrystallization;the correlation between the constitutive equation model value and the experimental value is more than 99%,and the absolute error is smaller than 5.(2)By continuous cooling transformation experiments,the continuous cooling transformation behaviors of A and B steels were studied,and static and dynamic CCT curves were drawn.The results revealed that under the undeformed condition,Nb,V and Mo can inhibit the ferrite transformation,which results in the shift of ferrite transformation zone to the right direction.The deformation obviously restrains the bainite transformation.It makes the increase in phase transformation starting point and the decrease in phase transformation end point.With the increase in cooling rate,the ferrite volume decreases gradually and the bainite increases gradually.When the cooling rate is greater than 40?/s,the ferrite transformation is prohibited and all microstructures are bainite.(3)Isothermal phase transformation experiments were used to study the ferrite transformation during the isothermal process,and the TTT curve was drawn.The results revealed that with the increase in isothermal time,the grain size of ferrite gradually coarsens;the microhardness of ferrite increases first and then decreases with the extension of isothermal time,reaching a peak at 37s,and the maximum hardness of A steel and B is highest at 625?and 575?,respectively.The hardness of B steel is higher.The "nose temperature" of A and B steel are both 600?.(4)Through hot rolling experiments,the effect of parameters was studied.The results revealed that:A steel under the condition that heating temperature is in the range of 1205?1230?,final rolling temperature is in the range of 870?900?,cooling rate is in the range of 42?57?/s and final cooling temperature is in the range of 600?620?,had a good toughness and tensile strength.The microstructure is composed of bainite-and ferrite.Tensile strength is above 800MPa and impact energy at-40?0? was 20?60J.The optimum parameters of B steel are also obtained that heating temperature is 1205?,final rolling temperature is 900?,cooling rate is in the range of 42?57?/s and final cooling temperature is in the range of 570?620?.The microstructure was composed of bainite and ferrite,which has good strong and plasticity.Tensile strength is above 800MPa and the elongation is about 21%. |
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