2002 年 42 巻 7 号 p. 708-716
In this study, the surface crack sensitivities in samples of a Nb-bearing steel and a high Ni grade steel from continuous casting slabs have been examined by simple compression tests at various temperatures between 700-1100°C using a metallography method to evaluate the severity of surface cracks, and corresponding microstructure in the two grades of steel has been investigated. The results show that most specimen have cracks on the hoop surface after 50% compression in height in the temperature range of interest, and the critical hoop strain obtained from the metallography examination gives a clear cracking tendency for the two grades of steel. Microstructure observation revealed that the static precipitation of TiNb(CN)before deformation and thin ferrite film along grain boundaries are important for controlling surface crack sensitivity for the Nb-bearing steel, while coarse grains with flat boundaries, and grain-boundary precipitation of Cu2S and flake-like Ti(CN) at lower temperature, is responsible for the high crack sensitivity in high Nisteel between 700-1100°C, which implies Cu, S, Ti and N content should be kept as low as possible in thisgrade of steel, and surface temperatures of continuous casting slabs at the straightening point should be above 970°C for Nb-bearing steel while 980°C for high Ni steel to avoid transverse cracking.
