Effects Of Nb On Isothermal Transformation Of Pearlite In The 60Si2Mn Spring Steel

With the transformation from infrastructure to high-end manufacturing in China,the demand for high strength of machinery steels manufacturing is bound to rise rapidly.To obtain a good match between strength and toughness,mechanical manufacturing steel will generally add some alloying elements.Nb is a strong carbide forming element,which is the most effective one of the strengthening elements,however,Nb as an alloying element has not been fully researched in medium and high carbon machinery steel.In this paper,60Si2Mn spring steel was used as the object of study,the effects of different Nb contents on the microstructure and properties after rolling and air cooling were studied.In addion,the influence of Nb content and other factors on the microstructure evolution of pearlite under undeformed and deformed conditions were researched,while the characterization and analysis were performed by metallographic microscope,scanning electron microscopy,transmission electron microscopy,Vickers hardness and universal electronic tensile testing machine.Firstly,the experimental steels with different contents of Nb were hot-rolled and air-cooled,the effects of different Nb contents on the microstructure and properties of the hot-rolled strip were investigated.The results show:The results show that:(1)the addition of Nb can significantly refine grains and promote ferrite transformation and the degradation of pearlite;(2)When the content of Nb increased to 0.026%,the tensile and yield strength of the steel increased,but with the Nb content further increased to 0.05%,the tensile and yield strength of steel decreased.Secondly,For different Nb content of the experimental steel for different time insulation at suitable temperature,The evolution of pearlite structure during isothermal process was studied.The results show:(1)Nb has an inhibitory effect on the transformation of pearlite,and the pearlite degradation is more and more obvious with the prolongation of holding time;(2)The microhardness of pearlite microstructures was significantly improved by the addition of 0.026%Nb,but this trend was weakened with the further addition of Nb;(3)The addition of Nb can also make the mechanical properties of experimental steel becomes more stable.Finally,The test steels with different contents of Nb were subjected to different deformation and heat treatment at 900?,The influence of deformation mode on the transformation law of pearlite in isothermal process of different Nb content test steel was studied.The results show:(1)The greater the degree of deformation,the lamellar spacing of the pearlite and the more obvious the degradation of the pearlite structure;(2)The longer the holding time,the greater the degree of pearlite degradation;(3)The effect of deformation mode on pearlite degradation is not obvious(4)TEM analysis shows that the increase of deformation degree and the extension of holding time can increase the precipitation of Nb,indicating that the degradation of pearlite may be related to the formation of Nb precipitates.For comprehensive analysis,in order to obtain better mechanical properties in 60Si2Mn spring steel,the Nb content should be controlled at about 0.026%.