Microstructure and Precipitation Behavior in Heat Affected Zone of Nitrogen-Enhanced Microalloyed Steel Containing V and Ti

Three steels:0.05%C-0.1%V-0.01%N (steel V-LN), 0.05%C-0.1%V-0.02%N (steel V-HN), 0.05%C-0.1%V-0.02%N-0.01%Ti (steel V-HN-Ti), all essentially vanadium microalloyed steels were subjected to simulating the microstructure of a coarse grained heat affected zone (CGHAZ). This involved reheating to 1350°C, rapid cooling to room temperature, including four cooling rates of t8/5 equals to 7.5s, 20s, 40s, 100s, as the relationship of heat input and t8/5 was calculated by Quiksim software. The microstructure and precipitation of vanadium and titanium carbon nitrides were studied in this paper. The results showed that the microstructure consists of granular bainite and some side plate ferrite in the grain boundary when the steels were produced with the highest heat input. As the heat input decreased, there appeared numerous polygonal ferrite and grain boundary ferrite, and the size apparently grows up. For the limitation of cooling time, vanadium carbon nitrides can't precipitate sufficiently, but as titanium was contained, the unmelted or precipitated TiN on cooling absorbed some fraction of nitrogen in the matrix and make more precipitate positions for the round shaped V(C,N), so many useful round particles can be seen in titanium-contained steel, most of them were around TiN.