中文摘要：

采用光学显微镜、扫描电子显微镜和透射电子显微镜对热轧态和回火态AH80DB低碳贝氏体钢的显微组织、马氏体/奥氏体（M/A）岛、第二相的析出行为以及晶界取向差、有效晶粒尺寸进行研究,揭示回火后低碳贝氏体钢冲击韧性得到改善的原因.结果表明：两种试样的组织均由板条状贝氏体、粒状贝氏体和针状铁素体组成,其中回火态试样中针状铁素体组织较多.热轧态钢中存在较大尺寸M/A岛且呈方向性分布,大角度晶界比例占17.33%,有效晶粒尺寸为3.57μm;而回火态钢中M/A岛的尺寸较小,大角度晶界比例增加3.43%,有效晶粒尺寸减小0.56μm.热轧态钢中析出相主要是（Nb,Ti）C,尺寸在50~150 nm之间,回火态试样中析出较多细小的球状（Nb,Ti）C析出相,尺寸在10 nm左右.

英文摘要：

The microstructure,martensite / austenite（ M / A） islands,precipitated phases,high-angle grain boundaries and effective grain size of hot rolled and tempered low carbon bainitic steel AH80 DB were studied by scanning electron microscopy（ SEM）,transmission electron microscopy（ TEM） and electron back-scattered diffraction（ EBSD）. The reason for the improvement in impact toughness of the low carbon bainitic steel after tempering was discovered. The results show that the microstructures of both the samples mainly consist of lath bainite,granular bainite and acicular ferrite. There is more acicular ferrite in the tempered steel. Coarse M / A islands are found in the hot rolled steel and its distribution is directional,the ratio of large-angle grain boundaries is 17. 33%,and the average effective grain size is 3. 57 μm. Nevertheless,the size of M / A islands decreases after tempering. Besides,the ratio of largeangle grain boundaries increases by 3. 43% and the average effective grain size decreases by 0. 56 μm after tempering. The precipitated phase of the hot rolled steel is（ Nb,Ti） C with the size of 50-150 nm,while much small and spherical precipitates with the size of about 10 nm appear after tempering.