采用冷弯实验、等温热处理结合硬度测量、光学金相技术和透射电子显微术观测分析了微合金钢中贝氏体组织在A.以下温度受热时的组织演化历程.结果表明,提高微合金钢的碳铌浓度积可以显著延缓贝氏体组织趋于平衡的演变,阻碍钢的软化进程.冷变形在增加钢的初始硬度的同时也加快了随后的受热过程中钢的软化和显微组织向平衡组织演化的进程.张应变和压应变这两种不同类型的应变对组织演变的促进效果几乎相同.冷变形提高了钢中位错密度,也加剧了位错分布的不均匀性,后者是预应变加速组织演变的直接原因.在等温受热过程中贝氏体组织的演变首先是贝氏体板条内的位错发生重新分布,在板条边界附近形成高位错密度区,并同时伴随有低位错密度区出现,然后低位错密度区跨越板条边界形成铁素体晶核并不断吞噬高位错密度区,最终完全演变为平衡组织多边形铁素体.
Evolution of bainite in low carbon Nb-bearing microalloyed steels isothermally held below A1 temperature was investigated by means of cold bending test, isothermal heat treatment.hardness measurement, optical microscopy and transmission electron microscopy (TEM).It is found that the evolution of bainite towards equilibrium and accompanied softening can be markedly slowed down by increasing of concentration product of carbon and niobium in the microalloyed steels.The effect works no matter whether the steel is subjected to pre-deformation.Cold deformation enhances the initial hardness of the steels while it largely accelerates softening and evolution of bainite towards equilibrium during post-heating.Almost same effects are observed for tensile strain and compressive strain.Cold deformation results in increasing of dislocation density and inhomogeneous dislocation distribution, the latter is the direct reason for pre-strain to promote microstructural evolution.The evolution of bainite during isothermal holding starts by redistribution of dislocations inside bainite laths, resulting in high density dislocations gathering along lath boundaries accompanied by occurring of zones with low density dislocations, followed by the zones with low dislocation density traveraing lath boundaries and ferrite nucleating.The growth of ferrite by swallowing up the zones with high density dislocations and finally the microstructure evolves into polygonal ferrite.
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