Effects of Soluble Ti and Zr Content and Austenite Grain Size on Microstructure of the Simulated Heat Affected Zone in Fe–C–Mn–Si Alloy

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The effect of soluble Ti and Zr contents on the microstructure in a simulated heat affected zone of a Fe-0.05mass%C-1.5mass%Mn-0.20mass%Si alloy was studied. This was done as a function of the austenite grain size as well as the number and size of TiN, Ti2O3, MgO(+Ti2O3), TiN(+MgO) and ZrO2(+Ti2O3) particles under different peak temperature (1300 and 1400°C), peak holding time (0, 60 and 600 s) and a constant time of cooling (70 s) from 800 to 500°C. The effectiveness of inclusion phases for intragranular ferrite (IGF) formation was studied from the area fraction of IGF for a given austenite grain size and soluble Ti content. For a given austenite grain size, the primary ferrite decreases and the IGF increases with an increased soluble Ti content up to about 100 ppm. After going through a maximum the replacement of IGF by lath bainitic ferrite occurs with an increased soluble Ti content. It is found that the effect of soluble Zr content on the microstructure is much stronger than that of a soluble Ti content. The area fraction of IGF decreases gradually with an increase in the austenite grain size (50–1000 μm) for a given soluble Ti content (50–400 ppm). By using the hardenability parameter based on the non-equilibrium grain-boundary segregation model, it is observed that the IGF formation is encouraged in the following order: TiN>{Ti2O3, MgO(+Ti2O3)}>ZrO2(+Ti2O3).