Influence of cooling rate on phase transformation and precipitation behavior of Ti-bearing steel in continuous cooling process

The influence of cooling rate on microstructural evolution and precipitation behavior in Ti, Ti–Nb and Ti–Mo low-carbon steels during the continuous cooling process was studied by dilatometer method, optical microscopy, and transmission electron microscopy. The results indicated that austenite transformation temperature decreased with the increasing cooling rate in three steels. The addition of Nb and Mo promoted bainite and martensite transformation and improved the hardenability of steels. In addition, precipitates formed in deformed austenite and ferrite can be observed simultaneously. Deformation in the austenite non-recrystallization zone can introduce a large number of deformation bands, and then, the precipitates preferentially nucleated in these deformation bands. In the following process, randomly distributed precipitates and interphase precipitates will be formed in ferrite. The precipitates formed in deformed austenite obey Kurdjumov–Sachs orientation relationship with the matrix, while the precipitates formed in ferrite obey Baker–Nutting orientation relationship with the matrix. The addition of Nb and Mo in Ti-bearing steels decreased the precipitates size and increased the number density of precipitates and then improved the precipitation hardening. And the effect of Mo addition is more obvious than that of Nb addition.