Effect of Mo on precipitation behavior of composite carbonitrides in Nb-V-N microalloyed steel
The precipitation behavior of microalloyed carbonitrides in steel is directly related to its strength.For the purpose of revealing the precipitation rule of MX(M=Nb,V and Mo;X=C and N)composite carbonitrides in Nb-V-Mo-N quaternary microalloyed steels with different Mo contents,the precipitation thermodynamic phase diagram,driving force and interfacial energy of MX second phase in austenite and ferrite matrix,as well as the solid solution rule of alloying elements in austenite under the equilibrium condition were calculated using Thermo-Calc software.Besides,the precipitation characteristics of MX carbonitrides under non-equilibrium condition were also studied by field emission scanning electron microscope,transmission electron microscope and electron probe microscope ana-lyzer.The results showed that the MX phases in the 0Mo steel were Nb-rich high-temperature precipitates(FCC_A1#1)and V-rich intermediate-temperature precipitates(FCC_A1#2).The(V,Mo)C low-temperature precipitates(MC_ETA)were also formed in 0.26Mo and 0.50Mo steels.With increasing Mo content from 0 to 0.50%,the precipitation temperature and the maximum precipitation amount of FCC_A1#1 and FCC_A1#2 did not change significantly,whereas the precipitation of MC_ETA phase was promoted.Moreover,the submicron Nb-rich MX particles precipitated in the high-temperature austenite region could pin the grain boundaries and induce the het-erogeneous nucleation of acicular ferrite.The nanosized V-rich MX particles interphase precipitated in the intermediate-temperature two-phase region and randomly precipitated in the low-temperature ferrite region inter-acted with dislocations and effectively hindered their movement.In addition,the microstructure of 0Mo experimental steel at room temperature was composed of polygonal ferrite and pearlite.With increasing Mo content from 0 to 0.26%,the proportions of polygonal ferrite and pearlite were decreases,promoting the formation of acicular ferrite and a small amount of bainite.When the Mo content further increased to 0.50%,the proportion of ferrite was obvi-ously decreased while that of bainite was increased.The ferrite grain size decreased from(10.2±0.42)µm to(6.7±0.42)µm.The microhardness increased from(223±13.8)HV0.1 to(318±19.9)HV0.1,and the yield strength and tensile strength enhanced from(496±5.3)MPa and(618±9.7)MPa to(554±6.7)MPa and(823±13.3)MPa,respectively.By exerting the synergistic effect of various alloying elements in Nb-V-Mo-N quaternary microalloyed system,the precipitation of MX carbonitride was optimized,consequently improving the strength of structural steel.
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