The effect of Ti addition on as-cast austenite (g) structure of S45C steel has been investigated. The ascast g structure without the Ti addition consists of coarse columnar grains. The Ti addition leads to formation of equiaxed g-grains and also grain refinement of the g structure. Fully equiaxed and very fine g-grain structure forms in a limited range of Ti addition of 0.13 to 0.17 wt%. The thermodynamic calculation of phase diagram showed that Ti carbonitride crystallizes as a primary phase in this composition range. The formation of the very fine equiaxed g-grain structure originates from the columnar-to-equiaxed transition (CET) in d-dendrite solidification induced by the primary Ti carbonitride particles. These particles act as the nucleation sites of the equiaxed d-dendrite. The experimental results are suggestive of existence of finer Ti carbonitride particles retarding the grain growth of g-phase after the peritectic transformation, which leads to the refinement of the as-cast g structure.KEY WORDS: carbon steel; casting; austenite; grain refinement; columnar-to-equiaxed transition; titanium carbonitride.
1373© 2008 ISIJ cally represents the steel mold employed in the present experiments. The cast in the mold was cooled down to the room temperature and it was then subjected to the metallographic investigations.The cast ingot was a rectangular prism with a thickness of 20 mm, a width of 40 mm and a height of 40 mm. The metallographic observation was performed on a horizontally sliced plane at the height of 20 mm. Both the edge regions of the width direction were excluded in the microstructural investigations. It may be then allowed to take into account only the one-dimensional heat flow in the thickness direction from the inward of the sample to the mold wall in our discussion. As demonstrated later in the next section, in fact, the columnar g-grains and columnar d-dendrites develop along the heat flow in the observation area. The g-grain size was measured after polishing and etching in 3 %-nital solution. The average sizes across long axis and short axis of columnar g-grains and the average size of equiaxed g-grains were determined by graphical analysis of the optical microscope image. The d-dendrite structure was also investigated after etching in Oberhöffer's solution. The primary and secondary dendrite arm spacing and the center-to-center spacing of equiaxed dendrites were determined by the microstructural observation. The fine inclusion particles were examined by means of Scanning Electron Microscope (SEM) and were analyzed by Electron Probe Micro-Analyzer (EPMA).
Results and DiscussionThe g structure in the sample without titanium addition is shown in Fig. 2(a). It is to be noted that the columnar g grains develop from the surface to the inward of the ingot, which should be in accordance with the opposite direction of heat flow in the present casting condition. The equiaxed g-grains form in the center region of the ingot. The former microstructure, i.e., d-dendrite structure is demonstrated in Fig. ...