Effects Of Niobium And Heat Treatment On Microstructure And Mechanical Properties Of Low Carbon Cast Steel

Microalloyed steels are widely used in construction,mining,petroleum,shipping and automobile for their high strength,good ductility and toughness and superior weldability as compared to carbon steels.Former studies indicated that microalloying elements,such as niobium,vanadium and titanium,had a pronounced effect on grain refining and thereby improving the strength of microalloyed cast steels.However,most of them shown that the improvement of strength was at the expense of toughness.In the present work,a series of low carbon microalloyed cast steels with different contents of niobium were produced and then treated under different normalizing and tempering temperatures.Then,characterization techniques such as optical microscopy(OM),electron back scatter diffraction(EBSD),scanning electron microscopy(SEM),transmission electron microscopy(TEM)and selected area electron diffraction(SAED),together with tensile and room temperature Charpy-V notch impact tests were utilized to study the effects of niobium microalloying element and different heat treatment processes on evolution of microstructure and mechanical properties of low carbon cast steels.The research results are as follows:(1)Grain and microstructure of the low carbon cast steel could be refined signif-icantly by niobium microalloying element.When the amount of niobium was below 0.044wt%,the grains of low carbon cast steel would become finer with higher niobium content.When the amount of niobium was above 0.044wt%,the role of microalloying element niobium in grain refining would become less effective with higher niobium content.As compared to the Nb-free cast steel,the grains of microalloyed cast steels with different amount of niobium could be refined by 20.8?34.6%.The microalloyed cast steel with 0.044wt%niobium tended to have the finest microstructure and the average grain size was about 19?m.(2)Graining refining and precipitation hardening were the main strengthening mechanisms in Nb-microalloyed low carbon cast steels.Microalloying element niobium could promote the precipitation of NbC particles with a diameter in the range of 1?15nm during the process of normalizing and tempering.Besides,niobium in solid solution could also contribute to refinement of the distance between two adjacent ferrite and cementite in the pearlite.(3)The microstructure and mechanical properties of Nb-microalloyed cast steel varied with different normalizing and tempering temperatures.The Nb-microalloyed cast steel had the finest grain and microstructure when it was normalized at 900?and then tempered at 550?,both for 2h.The grains of Nb-microalloyed cast steel would be coarsened and the mechanical properties would be deteriorated by increasing either the normalizing or tempering temperature when they were higher than 900? and 550?.Normalizing temperature tended to have a more obvious influence on variation of the grains and microstructure of Nb-microalloyed cast steel as compared to tempering temperature.(4)The steel with 0.044wt%niobium had the optimal combination of strength,ductility and toughness after they were normalized at 900? and then tempered at 550?,both for 2h.As compared with Nb-free steel,the strength and toughness of Nb-microalloyed steel with 0.044wt%Nb could be improved from 290MPa and 485MPa to 350MPa and 520MPa,respectively,while the room temperature impact energy could be improved by 25.6%with remained ductility.