What is the impact of mold flux on the quality of continuous casting slabs?

The quality of mold flux mainly affects the surface quality of the cast slab. This article describes the technical aspects of the cast slab surface longitudinal cracks, slag inclusion, and flame cutting for steelmaking and continuous casting.

The mold flux is added to the molten steel surface of the mold. The quality of the mold flux mainly affects the surface quality of the casting slab:

(1) Longitudinal cracks on the surface of cast slab: Longitudinal cracks originate from the unevenness of the thickness of the primary slab in the meniscus area of ​​the mold. The liquid slag on the surface of the molten steel cannot even flow in and distribute around the cast slab, resulting in uneven thickness of the solidified shell. Stress concentration is likely to occur in the thinner part of the blank shell. When the stress exceeds the high temperature strength of the solidified shell, cracks will occur.

The study pointed out that the liquid slag layer on the liquid surface of the mold steel is maintained at 5 to 15 mm, which can significantly reduce the longitudinal cracks on the surface of the slab. Longitudinal cracking is also related to slag viscosity (η), melting speed (tf) and pulling speed (V). Someone pointed out: the larger the η/tf ratio, the smaller the longitudinal fissure index. For example, the slag temperature is 1300℃, η/tf=1, the longitudinal crack index is 6, η/tf=2, and the longitudinal crack index is 0. Some people think that the η·V of continuous casting slab should be controlled within 2~3.5. When the η·V of the billet is controlled at 5, the slag film can be uniform, the heat transfer is stable, the lubrication is good, and the cracks can be significantly reduced.

(2) Slag inclusion: The slag inclusion of the casting billet can be divided into surface slag inclusion and subcutaneous slag inclusion. The size of slag inclusion varies. From a few millimeters to more than ten millimeters, the depth of the slag inclusion on the surface is also different. Slag inclusion seriously harms the surface quality of the product, so it must be removed before thermal processing.

The mold shell is involved in the slag, which is an important source of slag inclusion. If slag spots are formed on the surface of the blank shell, where the thermal conductivity is poor and the condensed shell is thin, a high-temperature “hot spot” is formed, which is one of the reasons for the breakout of the mold shell.

The composition of the slag inclusions on the surface of the casting slab is mainly anorthite and calcein. The A12O3 in these two compounds are both greater than 20%, and their melting points are 1550°C and 1590°C, which are easy to agglomerate the slag. The liquid level in the crystallizer fluctuates too much, the immersion nozzle is inserted too shallow, and the slag will be drawn in by the liquid level turning.

“Steel-making and continuous casting flame cutting high-efficiency and energy-saving technology” has been listed as a key scientific and technological achievement promotion project by the Ministry of Science and Technology. This technology product has been successfully applied in several steel-making enterprises, reversing the high consumption of fuel gas, large cutting slits, and cutting sections in the past. Roughness, high oxygen pressure, excessive dust in the workshop, high noise, heavy environmental pollution, multiple damage to the cutting tool, and high labor intensity of workers, etc., show huge energy-saving power and excellent environmental protection effects. This technology has the following characteristics :

  1. Advanced technology: the flame is concentrated during cutting, and the cutting speed is fast; the cutting section is smooth, the upper edge does not collapse, the lower edge is less slag, and the yield is high; it can be automated, and the cutting and continuous casting speeds are matched.
  2. Saving steel: cutting square billet and slab slits can be kept at about 3mm, and cut damage can be reduced by more than 0.5 kg per ton of steel.
  3. Energy saving: The gas pressure of the energy-saving continuous casting cutting nozzle is 1/2 to 1/3 of other cutting nozzles, and the oxygen pressure is 1/2 of other cutting nozzles, which can save more than 50% of gas and 40-50% of oxygen. , It can automatically cut off and ignite during cutting.

 

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