Hot Ductility and Precipitation Analysis of 700 MPa Nb-Ti-bearing High Strength Steel Slab

The influence of the addition of Nb and Ti on the hot ductility and the precipitation of continuous casting 700 MPa Nb-Ti-bearing high strength steel slabs was researched. Its high temperature mechanical properties were tested by Gleeble -3500. The morphology characteristics of fracture surface of the tensile specimen and the precipitation were observed and analyzed by scanning electron microscopy （SEM）and energy dispersive spectrometer （EDS）. Thermo-calc software was employed to calculate the species and starting temperature of precipitation during the solidification of the steel slabs. The brittle temperature zone Ⅲ of the 700 MPa Nb-Ti-bearing high strength steel slabs was observed to widen and extend to high temperature area （725~925 ℃） with the addition of Nb,according to the result of high-temperature tensile test. It was calculated that （Nb,Ti） carbonitride were precipitated during solidification of 700 MPa Nb-Ti-bearing high strength steel. Nb C-containing face-centered cubic phase was precipitated in large amount at about 1 100 ℃ which led to the rapid reduction of hot ductility of the slab. The hot ductility was further decreased due to the occurrence of phase transformation γ→ α at about 835 ℃. When eutectoid transformation was finished,the hot ductility of the slab was recovered. The parameters can be provided as reference for reducing crack in the 700 MPa Nb-Ti-bearing high strength steel slabs through this research.