采用OM、SEM、TEM、EBSD、拉伸和冲击等分析和检测技术，研究了工业生产的117 mm厚Ni-Cr-Mo-B超厚钢板在厚度方向上微观组织的变化及其对力学性能的影响。结果表明，从表层到芯部，超厚板的屈服强度逐渐降低，表层和芯部的屈服强度分别为798和718 MPa；延伸率变化不大，为20.0%~22.0%；然而超厚板的-60 ℃冲击功变化较大，其中表层、1/8T (T代表板厚)和芯部的冲击功分别为35、160和20 J，使得整个厚度方向上的冲击功变化曲线呈现“M”型。从表层到芯部，超厚板的板条宽度(198.7~500.6 nm)、界面碳化物尺寸(130.6~226.6 nm)和晶内碳化物尺寸(45.8~106.2 nm)均逐渐增加，芯部还存在一定的块状区，板条的细晶强化和碳化物的析出强化效果均减小，使得屈服强度从表层到芯部逐渐降低。从表层到芯部，有效晶粒尺寸先减小后增加，表层(2.2 μm)和芯部(2.7 μm)的有效晶粒尺寸较大，对解理裂纹扩展的阻碍作用较弱，使得表层和芯部的冲击功较低；而1/8T位置具有较小的有效晶粒尺寸(1.7 μm)，对解理裂纹的阻碍作用较强，从而获得较高的冲击功。

With the increasing demand and exploitation depth for offshore oil and gas, offshore platforms are becoming larger and the performance requirements and size for offshore platform of ultra-heavy plates are also increasing. Due to the large plate thickness and the limitation of manufacturing techniques, inhomogeneous microstructures and mechanical properties along thickness direction are great challenges for offshore platform of ultra-heavy plates. In this work, variation of microstructure and its effect on mechanical properties for the 117 mm-thick Ni-Cr-Mo-B industrial ultra-heavy plate were investigated by means of OM, SEM, TEM and EBSD observation, in combination with the tensile and impact toughness test. The results show that yield strength reduces gradually from the surface (798 MPa) to the center (718 MPa) and elongation almost keeps constant around 20.0%~22.0% for the 117 mm-thick plate. It is noted that impact energy at -60 ℃ increases first from 35 J at the surface and reaches its peak 160 J at the depth of 1/8T (T—thickness of plate), and then drops to the minimum about 20 J at the center, which suggests that impact energy curve along the whole section varies sharply and exhibits like letter 'M'. Lath width, boundary carbide size and intragranular carbide size are all gradually increasing from the surface to the center, i.e., from 198.7 nm to 500.6 nm, 130.6 nm to 226.6 nm, 45.8 nm to 106.2 nm, respectively, and there are also some blocky areas at the center, all those indicate that refinement strengthening and precipitation strengthening would decrease, as well as the gradual decrease of yield strength. Also, from the surface to the center, effective grain size (EGS) decreases first and then increases. The surface and the center have larger EGS (2.2 μm and 2.7 μm, respectively), which indicates that they have weaker resistance to cleavage crack and exhibit lower impact energy. However, the 1/8T position has smaller EGS (1.7 μm) while obtains higher impact energy.