Effects of Addition of Titanium and Boron on Columnar Austenite Grain in Carbon Steel

Effects of titanium and boron addition on columnar austenite grain structure of S45C steel have been investigated in casting experiments with different cooling rates of 4.50 and 16.67 degrees C/s. Without addition of these elements, the columnar austenite grains develop over the whole sample under the present casting conditions. The addition of titanium and boron induces formation of equiaxed austenite grains and, importantly, fully equiaxed austenite structure was observed in the sample with 0.2 mol% Ti and 0.4 mol% B. The microstructural observations indicated that this behavior of austenite structure stems from the columnar-to-equiaxed transition of ferrite dendrite structure. The addition of these elements, furthermore, leads to refinement of the columnar austenite grains, which is ascribable to pinning effect of boron nitride and titanium carbonitride.