Precipitation behavior of Ti-microalloyed high-strength steel by CSP process

Optical microscopy, electron microscopy and physic-chemical phase analysis, in combination with thermodynamic calculations, were used to study precipitates and their precipitation regularity in Ti-microalloyed high-strength steel produced by CSP process. It is found that lots of spherical TiC and Ti(C,N) nanoparticles with a few of irregular shaped Ti₄C₂S₂ above 100 nm in size exist in the steel with a yield strength of 700 MPa. TiN is completely precipitated from the steel before tandem rolling, but TiC mainly precipitates during coiling and cooling. The mass fractions of MC phases in the steel without Mo and the steel containing Mo are 0.049% and 0.043%, respectively. Owing to the addition of molybdenum, the amount of Ti precipitates in the steel containing Mo decreases while the precipitate size becomes smaller. The precipitation strengthening effects of the steel without Mo and the steel containing Mo are 126 MPa and 128 MPa, respectively.