| MC(M=Nb,V,Ti et.al)carbide plays an important role in refining grain size and precipitation strengthening due to its small size and dispersed distribution,which has a significant effect on improving the mechanical properties of iron and steel materials.In recent years,a complex MC type carbide was obtained by adding small amount of(<0.5wt%)Mo or W into the low carbon steel containing Nb,Ti and V,which has better grain refinement and precipitation strengthening effect compared to pure MC carbide.However,it is not clear that the precipitation behavior of the complex carbide,especially the influence of Mo and W on its nucleation and coarsening in steel.In this thesis,the effects of Mo and W on the formation energy of MC carbide and the interfacial energy of MC/Fe system are calculated by the first-principle calculations.The results show that the addition of Mo or W increases the formation energy of MC carbide,while decreases the interfacial energy of MC/Fe.The results of the density of states show that the incoporation of Mo or W into MC carbide can promote the electron orbital hybridization between atoms on both sides of the interface(including C2p-Fe3d orbitals and Mo3d-Fe3d orbitals),this is the main reason for the decrease of the energy of the interface system.In order to verify the results of the first principle,two kinds of steels,Nb-Mo and Ti-W,were designed,and fabricated used for the thermal deformation experiment of austenite zone.The results show that a small amount of Mo(～2-10at.%)enters into NbC in Nb-Mo low carbon steel,forming a complex carbide(Nb,Mo)C,they have smaller size and larger volume fraction than NbC in Nb steel.With the prolongation of time,the content of Mo in the complex carbide decreases.In the Ti-W in low carbon steel,a kind of dispersed complex ultrafine(1～6nm)carbide riched in W was formed,which has NaCl-type crystal structure with average lattice parameters of 3.75 A.The amount of precipitation reached the maximum when holding time reaches 60s,which directly leads to the occurence of precipitation hardening peak.With the prolongation of time,the size of part of complex carbides increases rapidly,while the part of small sized carbides is maintained.The contents of W and Fe decreased and that of Ti increased with the growth of complex carbide.The W element of the grown particles segregated on the outlayer to form a 1-2nm thick W riched layer.In order to utilize the precipitation strengthening effect of the fine carbide in Ti-W system,thin plate hot rolling process was adopted to develop the high strength and high plastic steel.The results show the Ti-W steel with yield strength 732MPa,ultimate tensile strength 807MPa,total elongation 24%and uniform elongation 15%was obtained by coiling at 600 ℃.The yield strength(802MPa)and tensile strength(839MPa)after isothermal holding at 650l0 for 5h were increased partly compared with those of hot rolling.The formation of a large number of fine dispersed complex(Ti,W,Fe)C particles is crucial for the high strength of hot rolled steel plate.In addition,the restart of precipitation of fine carbides at the defects(e.g.dislocations)of matrix during tempering is the main reason for the increase of strength.Combined with the first principles calculation and experimental results,to reduce the interfacial energy,Mo(or W)and Fe elements are generally enriched in the MC particles during early stage of precipitation,thus promoting the nucleation of precipitates.As the particles grow,the carbides enter the coarsening stage and the main reason for coarsening resistance in terms of energy is the decrease of the interfacial energy.However,due to the particles grow up,Mo or W elements are enriched in the surface of the particles,forming a Mo or W enriched layer with the thickness of 1-2nm,hindering the incoporation of Nb and Ti atoms and eventually inhibiting the coarsening of MC particles,which is the micro reason of coarsening resistance. |
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