碳化物尺寸对低合金超高强钢断裂韧性的影响

The effect of carbides size on fracture toughness of HSLA

本文针对低合金超高强钢G50拉伸实验结果异常,利用SEM和EDS手段观察分析了异常试样的断口形貌和组织,发现实验结果异常是由于钢中存在大尺寸的碳化铌相。利用第一性原理计算了碳化铌的结构参数和弹性性质。计算结果表明,通过广义梯度近似方法求解结构参数结果十分准确,碳化铌的晶格常数相对误差仅为0.669%。碳化铌为可压缩性差、结合能低的脆性碳化物,与基体材料的弹性性质差异较大。故造成试样钢力学性能异常的原因是材料受到拉伸后,由于大尺寸NbC受到基体钢组织的挤压先产生应力集中后发生破碎,最终断裂。同时热力学计算结果表明,低合金超高强钢在生产过程中容易生成NbC析出相,并且其析出温度高。对于G50中Nb元素的添加应调整合适的加入时机和控制加入量,尽量避免形成大尺寸NbC,影响力学性能。

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.