The effect of carbides size on fracture toughness of HSLA
In view of the abnormal tensile properties of HSLA,the fracture morphology and micro-structure of the abnormal samples were observed and analyzed by means of SEM and EDS.It was found that the abnormal tensile proper-ties were caused by large-sized niobium carbide phases in the steel.The structural parameters and elastic properties of nio-bium carbide were calculated using first principles theory,and the calculated results were in good agreement with the ex-perimental results.The calculation results show that the structural parameters obtained by the generalized gradient approxi-mation method are very accurate,and the relative error of the lattice constant of niobium carbide is only 0.669%.Niobi-um carbide is a brittle carbide with poor compressibility and low binding energy,and its elastic properties are quite differ-ent from those of G50.Therefore,the mechanism that causes the abnormal mechanical properties of the sample steel is that after the material is stretched,the large-sized NbC is first broken due to the stress concentration,and finally breaks.The thermodynamic calculation results show that NbC precipitates is prone to form during production,its precipitated temperature is high.For the addition of Nb elements in G50,it is necessary to adjust the appropriate timing and control the amount of addition to avoid the formation of large-sized NbC,which may affect the mechanical properties.
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