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Nb-V微合金钢在1200 ℃固溶0.5 h后淬火, 在650 ℃回火4 h, 利用SEM和HRTEM 观察显微组织、合金碳化物的形貌特征和精细结构, 用三维原子探针(3DAP)研究合金碳化物中元素分布规律. 结果表明, 淬火微合金钢在650 ℃回火4 h后, 马氏体板条内位错和板条界面因回复而消失, 粗化的合金碳化物分布在原马氏体板条界面和板条内部. 同时, 伴随着合金元素的再分配, 早期析出的圆盘状碳化物沿厚度方向生长, 出现一个与基体 (M_bcc)和原碳化物(P_inner)成半共格关系的新生过渡相(P_outer). 非碳化物形成元素Si和Al主要分布在碳化物/基体界面处; V和Mn主要分布在碳化物内层, 而Mo和Nb分布在整个碳化物区域. 粗化的碳化物是一种具有核心和外壳结构的合金碳化物, 内层主要是V-Mn-Mo-Nb的碳化物, 而外层主要是Mo-Nb的碳化物.

It has been confirmed that the fcc MC-type carbides such as VC and NbC have plate-like morphology and are mutually soluble when precipitated in tempering steel with martensite microstructure. Over-tempering makes the plate--like carbide change to spherical shape because of Ostwald coarsening. As coarsening is strongly linked to the diffusion rate of the carbide-forming elements, it is easy to understand that inhomogeneous structure may be formed when more kinds of elements were added, such as V, Nb and Mo. Besides, non-carbide-forming elements such as Si and Al tend to diffuse toward matrix. The morphology and lattice structure of carbide change simultaneously companying with the compositional redistribution of alloy elements. The detail compositional and nano-structural informations of the carbide can be obtained by 3DAP and HRTEM. In this paper, 3DAP and HRTEM were applied to characterize the composition, morphology and nanostructure of the carbide precipitated during 650 ℃ tempering of as-quenched Nb-V microalloyed steel. The results indicated that the martensite lath morphology was replaced by defect--free polygonal ferrite due to the recovery and recrystallization of the as-quenched microstructure. Simultaneously, the carbide-forming elements Mo and V diffused from smaller carbides to larger ones, resulting in the co-existence of carbides with different sizes and compositions. With the diffusion and redistribution of alloy elements, the prior-formed plate-like carbides grew along the radial direction, and a kind of transition carbide (P_outer) which is semi-coherent with ferrite matrix (M_bcc) was consequently formed. 3DAP constructional atom map demonstrated that Si and Al are rejected from the alloy carbide, whereas Mn and V were inhomogeneously distributed. That is, the coarsening carbide has a core-shell complex nanostructure, the core contains V, Mn, Mo and Nb, and the external shell contains Mo and Nb.