Control Of Transverse Corner Cracks On Low Carbon Micro-alloyed Continuous Casting Slabs

Transverse corner crack is one of the common slab surface defects. As its high occurrence ratio, transverse corner cracks became a major problem affecting slab quality and continuous casting process for low carbon micro-alloyed steel. In the current thesis, the low carbon micro-alloyed steels which are produced by the 2# slab continuous caster of a steel company in China is studied. Firstly, the macroscopic and microscopic morphology of transverse cracks on slabs were studied. Secondly, research on occurrence temperature ranges of corner crack was done. Thirdly, the hot ductility study under different test conditions was presented. After that, the slab temperature fields were simulated. Finally, some industrial experiments of optimizing secondary cooling were done to control transverse corner cracks.Three dimensional observation of the transverse corner cracks was done by Micro-CT. The crack tip morphology, the crack growth along the grain boundary and the relationship between the inside cracks were observed. It is more realistic to reflect the concrete condition of the cracks in the slab than two dimensional observation. The cracks were happened along austenite grain boundaries together with some proeutectoid ferrite thicker than 50μm. Some iron oxide layers were found inside the cracks. The microstructure was refined near cracks. And no decarburization layer was found. All the microstructure analysis results show the cracks studied were happened during the secondary cooling zones.A method is proposed to predict the occurrence temperature range of the transverse corner cracks by studying the microstructure. Including the relationship between crack and dendrite, decarburization and grain refinement near crack, and the symmetry of microstructure.The hot ductility tests results showed that both adjusting carbon content and adding titanium could significantly improve the ductility of the studied steels. The generation of AlxTiyOz at high temperatures can act as the nucleation sites for dimples to promote ductile fracture. The amount of detrimental AlN can be reduced due to the formation of AlxTiyOz and TiN. Industrial tests showed that the addition of titanium can effectively control the occurrence of transverse corner cracks on slab.The ductility trough of coupled cooling was wider than that of slow cooling but narrower than fast cooling. During cooling process, a slow cooling rate guaranteed coarse precipitates that were beneficial for ductility, while fast cooling was preferred for refining microstructures. The coupled cooling method of a slow cooling followed by a fast cooling provided coarse precipitates and refined microstructure. Thicker ferrite film resulted in a worse ductile at the temperature range that ferrites generated only along austenite grain boundaries. When the intergranular ferrite film thickness was larger than 5μm, it had a nearly linear relationship with RA value, on the contrary, it is almost no effect.According to the calculated temperature at slab corner, the cooling water flow rate and cooling strategy were optimized by adjusting the cooling water flow rate at each spray cooling zone to avoid the embrittling temperature range at the bending and straightening segments of the caster. As a result, the transverse corner cracks were successfully weakened.The slab temperature fields were simulated for the vertical bending type continuous caster of a steel plant in China, a new secondary cooling scheme of a weak cooling intensity before the bending segment and followed by an strong cooling intensity at the straightening segments was suggested. The industrial test results showed that the new method was proved to be effective in eliminating transverse corner cracks on slabs without deteriorating the quality of slab. The optimized secondary cooling method replaced the original one to control the actual production.