Analysis On Crack In Continuous Casting Slab Of High Strength Automobile Sheet

Cracks are the common defects of continuous casting slab and occur frequently in the production of continuous casting slab of high strength automobile sheet. There is a serious impact on the quality of the subsequent rolling process and the qualified rate of the final product. Crack defects of the continuous casting slab not only become an important factor affecting the quality of high strength automobile sheet, but also a certain influence on the steel production and the performance of contracts, or even cause serious economic losses. With the development of hot delivery and hot charging technology, the production of continuous casting slab without defects become more and more important. So it is necessary to study and analyze the problems of the current common defects of continuous casting slab. In order to improve the quality of continuous casting products to meet the practical production requirements, crack defects of continuous casting slab should be controlled and reduced.The quality of continuous casting slab of high strength automobile sheet was analyzed in this study. According to the test results and the analyses, the causes of cracks in the continuous casting slab had been analyzed. Nine experimental samples were from continuous casting slabs of automobile sheet in practical production. The continuous casting slabs were subjected to macroscopic examination and classification based on the defects. Organization and morphology around the cracks were observed through the optical microscopy and the scanning electron microscopy. Components surrounding the cracks could be analyzed in the EDS spectrometer. The main research results are listed as follows.(1)1#and2#continuous casting slab defects were transverse corner cracks. There are AlN inclusions in the angle of1#continuous casting slab. It may lead to greatly reduce the ductility of the steel and increase the brittleness and cracking susceptibility of steel, eventually exacerbate the formation of cracks. In addition, the sulfur precipitation increased the possibility of crack formation.2#continuous casting slab crack appeared on the deep chatter marks. There was CaO-SiO2-Al2O3based slag in the2#continuous casting slab. It is due to a large mold level fluctuation and unreasonable vibration parameters, stress concentration near the chatter marks caused the corner crack.(2)3#continuous casting slab defects was transverse surface crack. There was a serious decarburization phenomenon in the surface of transverse surface cracks. It showed that crack occurred at a relatively high temperature and in the low ductility temperature ranges. It was easy to cause crack sensitivity. When the secondary cooling intensity was too large or the cooling water flow rate was too high, it may cause uneven cooling of the slab surface which caused the defects of transverse surface crack.(3)4#and5#continuous casting slab defects were longitudinal cracks. TiN, Ti(C,N) and TiO2of4#continuous casting slab gathered on the steel slag interface and led to the bad performance of mould powder. Mould powder flowed into5#casting slab. They led to uneven heat transfer, which may cause stress concentration where the solidified shell was thin. When the stress of the solidified shell was more than the tensile strength of a grain boundary, casting slab micro cracks may be produced. After entered the secondary cooling zone, micro cracks would extend to obvious longitudinal cracks.(4)6#and7#slab defects were central cracks which belonged to internal cracks. There was Al2O3-K2O-Na2O-based component in the cracks. They formed in the crystallizer before entering the secondary cooling zone. Casting temperature of the first piece of slab in the first furnace casting tundish was higher than others.6#and7#Continuous casting slab couldn’t completely solidify in the metallurgical length. They produced ductile fracture surfaces and triggered center cracks during solidification shrinkage.(5)8#and9#slab defects were side cracks and formed in the crystallizer. It was associated with the inflows of mould powder. The cracks extended in the stage of cooling and straightening, eventually formed a longer crack.