Microstructure and Mechanical Properties Evaluation of Microalloyed Low-Carbon Reduced Activation Ferritic/Martensitic Steel

A novel method for reducing the amount of M23C6 carbides, increasing the amount of MX carbonitrides, and decreasing the coarsening rate of M23C6 carbides is put forward to improve the high-temperature properties of reduced activation ferritic/martensitic (RAFM) steel. Scanning electron microscopy, transmission electron microscopy, X-ray diffraction, tensile tests, and impact tests are used to systematically investigate the microstructure evolution and mechanical properties of RAFM (RAFM-0.1C) steel, low-carbon RAFM (RAFM-0.04C) steel, and minor boron and nitrogen microalloyed low-carbon RAFM (RAFM-CBN) steel. Some delta ferrite develops when C decreases from 0.1 to 0.04 wt% and subsequently disappears after 0.01 wt% N addition. The amount and size of M23C6 carbides and dislocation density decrease with the decrease of C (RAFM-0.04C), while the amount of MX carbonitrides is 2-3 times, and dislocation density is approximate to 2 times higher than that of RAFM-0.04C steel after 0.01 wt% N addition (RAFM-CBN). Compared with RAFM-0.1C steel, the yield strength at room temperature and 550 degrees C slightly decreases for RAFM-0.04C steel and considerably increases for RAFM-CBN steel. The contributions of microalloy on strengthening mechanisms of RAFM steel at room temperature are also systematically revealed by combining the experimental and theoretical data.