[Original Articles] How to distinguish the quality defects of billets?

[vc_row][vc_column][vc_column_text]The quality of continuous casting slab determines the quality of final product, so in a broad sense, the quality of continuous casting slab refers to the severity of slab defects allowed by qualified products.

Its meaning includes four aspects:

• The first is the purity of billet: it refers to the quantity, shape and distribution of inclusions in steel, which depends on the purity of molten steel. Therefore, select the appropriate refining method, protect the whole casting process, and reduce the content of inclusions in steel as much as possible.

• The second is the surface defect of the slab: it refers to whether there are cracks, slag inclusions and air holes on the surface of the slab, which is related to the casting temperature, casting speed, the performance of the protective slag and the stable factors of the mould liquid level.

• The third is the internal defects of the slab: it refers to whether the slab has the correct solidification structure, and the degree of internal cracks, segregation, porosity and other defects. The key to ensure the quality of the slab is reasonable secondary cooling and strict alignment of the support system.

• The fourth is the appearance and property of the slab: it refers to whether the shape and size of the slab meet the specified requirements, which is related to the size of the mold cavity, the surface state and the uniformity of cooling.

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In order to obtain good billet quality, we can adopt different industrial technologies in ladle, tundish, mould and secondary cooling zone according to different steel grades and product requirements. Control the quality of slab. In order to eliminate the defects of the slab, or to control the defects of the slab within the allowable range without affecting the quality of the slab.

[/vc_column_text][/vc_column][/vc_row][vc_row][vc_column][vc_column_text]Next, we will introduce the classification of the defects of the lower blank. There are three kinds of slab defects

• First: surface defectsSurface defects are divided into: surface transverse cracks. Longitudinal crack. Corner crack, homomorphic striae. Subcutaneous passage. Subcutaneous stomata. Surface depression, etc.
• Second: internal defectsIncluding intermediate cracks. Subcutaneous crack. Press down the crack. Inclusion. Central cracks and segregation, etc,
• Third: shape defectFor square billet, it is deviating (de Square), for slab, it is bulging

[/vc_column_text][/vc_column][/vc_row][vc_row][vc_column][vc_column_text]The common defects of slab are introduced in the form of legend

• Transverse cracks in surface defects of slab

The transverse crack usually appears in the vibration mark Valley, which is invisible. When the slab is straightened, under the action of tensile stress, the force effect of vibration mark curve causes some small cracks in the production to expand to the transverse crack

 [/vc_column_text][/vc_column][/vc_row][vc_row][vc_column][vc_column_text]How to prevent transverse crack

1. Adopt high frequency (200-400 times / min J, small amplitude (2.4mm) crystallizer to reduce vibration trace depth
2. The straightening temperature is generally controlled above 950c to avoid the brittleness temperature (700.900c)

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• Corner cracks in surface defects of slab

Corner cracks appear in the corner 10-15mm and also in the adjacent corner, and even cause steel leakage in serious cases. The reason is that the solidification speed of the bottom heat transfer molten steel is faster than other positions, the early formation of air gap in the primary blank shell forms the growth of heat resistance in the heat preservation layer, and corner cracks appear in the weak part under stress, The corner crack is mainly in the gap, so the corner of the copper mould tube is generally made into a fillet type with a certain arc, in order to force the growth of the shell and prevent the corner crack.

[/vc_column_text][/vc_column][/vc_row][vc_row][vc_column][vc_column_text]How to prevent corner crack

1. Proper copper mould tube taper and fillet radius
2. The thickness of the water gap on the four sides of the copper mould tube is uniform and evenly cooled
3. The copper mould tube flow shall be aligned

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• Billet shape defect – square removal

Square removal is a diagonal angle greater than 90 degrees, a diagonal angle less than 90 degrees, the diagonal length difference, square removal is accompanied by corner cracks, sometimes there are some red spots, we call them red spots, red spots are mainly some low melting point, melt impurities, square removal is mainly located in the four sides of the copper mould tube, the cooling shell thickness and shrinkage are not the same, some parts of the mold wall contact well, some places It is caused by the poor contact between the square and the copper mould tube.

[/vc_column_text][/vc_column][/vc_row][vc_row][vc_column][vc_column_text]How to prevent shape defect – square breaking

1. Reasonable copper mould tube structure to prevent inner wall deformation
2. The cooling water passages on the four sides of the knot device should be evenly spaced
3. Foot roller or cooling plate is set at the lower end of the copper mould tube
4. The four surfaces in the secondary cooling zone shall be uniformly cooled

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• Internal defects of slab – loose center

When it is thrown away along the central line, there is a small gap near the central part, which is called central looseness. Central looseness is the product of small ingot fixing mode, because it is the central part, the molten steel can not be supplemented in time.

[/vc_column_text][/vc_column][/vc_row][vc_row][vc_column][vc_column_text]How to prevent internal defects – Central looseness

1. According to steel grade. Determination of proper drawing speed by steel temperature and secondary cooling strength
2. Optimize the secondary cooling technology, select the right nozzle, increase the cold water pressure and reduce the specific water volume without steel leakage
3. Select appropriate pouring temperature and adopt low temperature and rapid injection
4. Increase the compression ratio by electromagnetic stirring with rare earth elements

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