Randomization of ferrite/austenite orientation relationship and resultant hardness increment by nitrogen addition in vanadium-microalloyed low carbon steels strengthened by interphase precipitation

Interphase precipitation of nano-sized alloy carbides is recently used to strengthen low carbon steels for its excellent contributions to strength and formability. The effects of nitrogen addition on the hardness of vanadium-microalloyed low carbon steels were investigated by considering both the dispersion of interphase precipitation and the ferrite/austenite crystallography. Three-dimensional atom probe analysis reveals that interphase precipitation of vanadium carbide is hardly affected by increasing the nitrogen content, although the nanohardness of ferrite is slightly increased. Another important factor determining the overall hardness of ferrite is found to be the ferrite/austenite crystallography. At lower transformation temperature, nitrogen addition reduces the amount of Widmanstatten ferrite and bainite, which are formed in absence of interphase precipitation. Instead, relatively harder allotriomorphic and idiomorphic grain boundary ferrite without Kurdjumov-Sachs orientation relationship against austenite are formed extensively.