| Improving the surface quality is the burning question of TSCR strip taking the place of cold-rolled sheet. The slab surface cracks will degenerate the quality of hot-rolled coil, even worse affect the production schedule badly and disrupt production Rhythm. In January-April, 2009, as the casting causes cracks and rotten side defects larger than 1.6% of total SS400 coil producted by FTSR in Tang Steel. Based on the practical production, studied working conditions impacting crack generate and growth, use of mathematical simulation method coupled with heat and stress calculate steel solidification in mould. Besides, a model is established to study the solidification shrinkage.Through observation of microstructure near crack found that cracks formed in the high temperature zone. At first intergranular cracks below slab shell came into being, then grew to surface, eventually formed surface longitudinal crack. Impurity elements segregation in interdendritic and deterioration steel high temperature plastic property. Created favorable conditions for micro-cracks formation and development.Mathematical simulation results show that steel has a wide and rapidly growth two-phase region during solidification, makeing columnar crystal well-developed and coarse grain. Stress calculation results show that internal tensile stress reach to 13-17MPa in two-phase region, and the closer to shell, the greater of SEQV. It has the condition for microcracks generation and growth in Mould. Control heat transfer in mould can uniform temperature distribution, thereby reduce stress and enhance the plastic property of shell.Using statistical analysis method finds that adopting the following measures can reduce the extent of longitudinal cracks: Floor controling C & Si Content,minimizeing S & P content,making w[Mn]≥0.020%;tundish superheat controlling at 17-32℃, and to ensure a smooth casting speed at 3.9-4.2m·min-1; choosing mould slag melting point at 1150-1160℃, viscosity range of 0.12-0.14Pa·s, R =0 .8-1.0, toner and graphite composite, and w[C]>7%.At the end, a function of node displacement ratio with temperature is obtained. |
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