The Microstructure Evolution of Nb, Ti Complex Microalloyed Steel during the CSP Process

Abstract

The development of microstructure of Nb,Ti-bearing microalloyed steel during the CSP process was studied. Three samples were taken from the as-cast slab prior to tunnel furnace, intermediate bar after stand F2 and the hot band, respectively. In the as-cast slab, the average austenite grain size is 654 µm with a large size range from 150 to 2000 µm. In the intermediate bar after stand F2, the austenite grains are remarkably refined, but are heterogenous due to the incomplete recrystallization, which are in the size range of 23 to 116 µm. In the hot band is mainly non-polygonal ferrite. Microstructural heterogeneity exists in the hot band. It is attributed to the heterogeneous austenite grain size in the intermediate bar and the less rolling reduction after stand F2. With regards to precipitation, cubic TiN and fine precipitates less than 20nm are commonly observed in the as-cast slab and the intermediate bar. Some complex (Ti,Nb)(C,N) precipitates with a slightly larger size also exist. In the hot band, most particles are complex (Ti,Nb)(C,N) precipitates, in a shape of irregular or cruciform. The fine precipitates which can strengthen the ferrite matrix are seldom seen. These results are in good agreement with the size distribution of the precipitates determined using small angle X-ray scattering method. The chemical phase analysis reveals that 45%Nb of the total and 43%Ti of the total are still in solution in ferrite of the hot band.