Intragranular Ferrite Formation Behavior From Non-Metallic Compounds in Steels

Research interest is focused on ferrite formation from non-metallic compounds in steels, which has attracted many researchers’ attention owing to its potential capability to refine microstructure in heat affected zone of steel welds. Single crystalline grains of various non-metallic compounds such as B1 type compounds and titanium oxides were embedded insides steels, and the austenite-to-ferrite transformation of the steels from the compounds was examined after isothermal holding or continuous cooling at temperatures/cooling rate of interest. It was confirmed that ferrite formation behavior was affected by various factors such as transformation temperature, chemical compositions of the compounds, composition of the steels, residual strain induced around the compounds, and crystal orientation relationships between ferrite, prior austenite and compounds. The Baker-Nutting (B-N) orientation relationship played a dominant role in ferrite formation from B1 compounds at higher transformation temperature, but some low-index orientation relationships other than the B-N orientation relationship were also observed as the transformation temperature decreased, which led to an increase in ferrite formation having the Kurdjumov-Sachs (K-S) orientation relationship with prior austenite. Also, chemistry of B1 compounds was found to exert an influence to the ferrite formation behavior from the compounds in spite of their similar misfit against ferrite. Ferrites from TiO had larger misorientation angles from the exact B-N orientation relationship than other B1 compounds, and thus tended to have the K-S orientation relationship with prior austenite at lower transformation temperatures, resulting in a good development of acicular ferrite. Experimental results on the ferrite formation behaviors from other oxides such as Ti2O3 and the effect of the chemistry and residual strain are shown and discussed further.