How to distinguish the quality defects of billets?

How to distinguish the quality defects of billets?

An article by LMM Group

“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 defects of the slab: it refers to whether there are defects on the surface of the slab such as cracks, slag inclusions and pores. They are related to the casting temperature, the casting speed, the casting powder(mold flux) performance and the crystallizer liquid level stability.

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.

Fourth, the appearance properties of the slab: refers to whether the shape and size of the slab meet the specified requirements,
It is related to the size, surface condition and uniformity of cooling of the mold cavity.

“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.”

Next, we will introduce the classification of slab defects.

There are three kinds of slab defects.

First: surface defects
Surface defects are divided into:
Surface transverse cracks, longitudinal cracks, corner cracks, homomorphic lines, subcutaneous channels, subcutaneous pores, surface depressions, etc.

First: surface defects
Surface defects are divided into:
Surface transverse cracks, longitudinal cracks, corner cracks, homomorphic lines, subcutaneous channels, subcutaneous pores, surface depressions, etc.

Third: shape defect

For the billet, it is the diamond change (off square). For the slab, it is bulging.

The following describes the common casting blank defects in the form of legends

Transverse surface cracks in the casting blank surface

Transverse cracks generally appear at the vibration mark “valley”, which is invisible.When thecasting blank is straightened, under the effect of tensile stress, the vibration mark curve force effect causes some small cracks to appear in the production and then spread to transverse cracks.

How to prevent transverse cracks?

  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)

Corner cracks in surface defects of casting blank

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.

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

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.

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

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.

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

An article by LMM Group

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