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Slab center crack
The liquid cavity in the center of the billet shrinks around the solidification point, which is caused by the layering or bulging of the billet and is also expressed as the center segregation line. The central crack appears at the center of the slab section, parallel to the wide face, and an opening-like defect can be seen on the section. The main reasons for the central crack are: uneven solidification shell due to uneven cooling of the mold and changes in the fluctuation of the pulling speed; uneven strong cooling of the solidification channel at the end of the solidification; the inaccuracy of the roll gap opening to the arc, Unreasonable and abnormal pressure occurred in the solidification channel to stop the replenishment of molten steel. The quality of the molten steel, the excessive heat of the molten steel, the speed of pulling, the unsatisfactory temperature, and the expansion of the opening of the roll gap all have an impact on the central crack.
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Slab intermediate crack
The internal cracks are caused the cracks caused by the tensile stress on the solidification interface due to the repeated temperature recovery of the surface temperature of the slab, the opening of the roll gap, the arc accuracy of the stick, and the continuous straightening of the bending. It continues to expand and extends to the resistance stress that the high strength of the shell can withstand. The intermediate cracks are perpendicular to the drawing direction, and most of them are scattered in the places where the shape distortion of the casting billet is large. The variation of the pulling speed, the accuracy of the roll gap to the arc and the reduced amount will all affect the intermediate cracks in the continuous casting billet.
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Slab corner cracks
Crack formation occurs within 250 mm below the meniscus of the mold and extends at the solid-liquid interface. The corner of the billet is two-dimensional heat transfer, the earliest solidification, the first shrinkage, the first air gap is formed, and then the heat transfer is slow, the thickness of the formed billet shell is thin, and the shape of the billet is deformed. The influence of , causes cracks at the weakest point of the shell. The internal crack at the corner is parallel to the drawing direction, and there are short internal cracks around the corner. The composition of molten steel, the fluctuation of the liquid level of the mold and the cooling strength of the secondary cooling will all affect the cracks in the corner of the continuous casting billet.
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Slab triangle crack
On the transverse section of the slab, the microstructure can be seen after grinding and low-magnification inspection. The columnar crystals develop inwards with vertical wide surface and narrow surface respectively. When the columnar crystal “bridges”, when the boundary line and the narrow surface The edge will form a triangular region of the narrow columnar crystal region, and the cracks grown in this region are called triangular cracks.
The liquidus temperature of the Q235 series steel is 1515°C, and the casting temperature of the continuous casting machine and the content of S fluctuate greatly between different furnaces. When the content of S is 0.16%-0.20%, the temperature of the middle bag is as high as 1560 Above °C, the steel must be drawn at a reduced speed. The content of S is the most important factor that directly affects the generation of cracks. It is also the most important reason affecting the triangular crack. By reducing the superheat of the tundish, the temperature of the tundish should be controlled below 1545°C as much as possible, which is one of the methods to eliminate the cracks in the triangle zone. The uneven secondary cooling is one of the important reasons for the generation of cracks in the triangle area. When the narrow surface encounters strong cooling, the billet surface is poorly cooled, and the repeated temperature of the billet is too high, which can lead to triangle cracks. The excellent secondary cooling effect can also reduce the generation of triangular cracks. The maintenance of the sector of the casting machine, the accuracy of the arc of the casting machine, and the accuracy of the roll gap opening parameters must be strictly implemented, which is conducive to eliminating cracks in the triangle area.
