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

Abstract

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 δ 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 ≈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 °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.