Analysis on the Causes of Inclusions in Continuous Casting Billets

Due to the wide range of sources and complex composition of inclusions, inclusions are firstly classified into endogenous inclusions and exogenous inclusions according to their generation methods. Endogenous inclusions mainly refer to non-metallic inclusions produced by the molten steel itself; external inclusions mainly refer to inclusions brought in during the pouring process, such as ladle, tundish refractory erosion, involved slag and mold slag, etc.

Personnel operation problems:

1) The liquid level of the tundish and the mold is unstable, and the slag is frequently entrained during the pouring process.

In the early stage of production, due to the unskilled operation of personnel, there are serious loopholes in the pouring process:

①The large fluctuation of the liquid level of the crystallizer causes the uneven growth of the billet shell, and a large number of inclusions are also involved in the solidified billet;

②The working layer of the tundish of the continuous casting machine uses magnesium dry material. In the early stage of production, due to the unskilled operation of the personnel and the frequent flow control of the large ladle nozzle, the liquid level of the tundish is not stable, which accelerates the erosion of the molten steel on the dry material, and is The flushed dry material seriously pollutes the molten steel. Caused billet defects are obvious.

2) The slag discharge of the tundish is not timely.

The molten steel of ordinary carbon steel has a large amount of slag. In order to improve the metal recovery rate, the pouring is stopped when the ladle nozzle has a large area of slag. The untimely slag discharge of the tundish is mainly manifested in the following aspects:

①The liquid surface covering agent is difficult to absorb the inclusions of molten steel in the tundish.

② During normal pouring, the normal liquid level of continuous casting is 850mm. During the process of changing the ladle and waiting for steel, the liquid level of the tundish is reduced to 550mm, and the actual depth of molten steel is 300mm. When the liquid level is less than 300mm, a vortex is formed at the nozzle, and a large amount of inclusions flow into the crystal. The device is condensed in the primary body.

3) Mixing mold oil and mold slag will form inclusions with high hardness and difficult to remove.

Irregular use of protective pouring

The poor sealing effect of protective pouring, unreasonable immersion depth of immersion nozzle, and deflection of nozzle seriously affect the effect of protective pouring and increase the secondary oxidation of molten steel.

1) The installation accuracy of the nozzle is poor and the use is not standardized.

①The installation of the long nozzle and the immersion nozzle is asymmetrical, which seriously scours the inner wall of the refractory material, and creates a gap between the immersion nozzle and the quick-change nozzle, which is easy to absorb air and affects the effect of protection and pouring.

②The continuous casting machine uses a straight-type immersion nozzle with a nozzle height of 660mm. The tundish truck does not use a lifting device. When the liquid level of the mold fluctuates rapidly, the immersion depth of the immersion nozzle is adjusted invisibly, and the immersion nozzle is immersed. If the depth is too deep, the high temperature molten steel will wash the billet at the lower part of the mold, making the billet thin and easy to break out. At the same time, the inclusions are difficult to float, and the immersion depth is too shallow, which is easy to agitate the liquid level of the mold, resulting in slag entraining and affecting the quality of the steel.

2) The addition of mold residue is unreasonable.

The personnel operation is more arbitrary, the amount of mold slag added at one time is large, the liquid level of the mold is frequently exposed, and the molten steel is entrained with slag, which affects the effect of protection and pouring; the slag fishing is not standardized. The operator habitually uses the slag scraper to stir the liquid level of the mold, and the broken liquid slag flows between the copper tube of the mold and the primary billet.

3) The sealing effect of the long nozzle of the large bag is poor.

When installing the long nozzle, if the long nozzle and the ladle sliding nozzle are not tightly sealed, and the nozzle is not installed correctly, it will cause air inhalation and secondary oxidation of molten steel. And because the inner diameter of the long nozzle is larger than that of the sliding nozzle, the molten steel cannot fill the inner hole. At this time, a negative pressure area is formed at the top of the long nozzle, and air is continuously sucked from the gap, resulting in secondary oxidation of molten steel. In the initial stage of protective casting, the sealing effect of the nozzle is poor, the secondary oxidation of molten steel is serious, and a large number of non-metallic inclusions are formed, which are distributed along the inner arc of the billet. The low magnification analysis shows that the loose center of the cast damage is 0.5, the corner crack is 0.5, and the non-metallic inclusions are 2.0. Therefore, it is concluded that the defect of non-metallic inclusions is the main factor affecting the quality of the billet.

high temperature of molten steel

The following conditions during the production process will cause the temperature of molten steel to rise:

1) During the pouring process, there are many times of blocking and interruption, low pulling speed and long pouring cycle;

2) The capacity of the tundish, the depth of the molten pool, the flow spacing, etc., due to the long pouring cycle and long tundish length of a heat, in order to avoid the low pouring temperature in the later stage, the formation of dead zones due to slippage and I, VII flow, the tundish passes through the tundish. Heat control is high.

The continuous casting machine mainly produces ordinary carbon steel and low alloy steel, and the superheat degree of molten steel is generally controlled at 30-50 °C.

Excessive molten steel temperature can cause many hazards:

1) The refractory materials such as ladle lining, tundish dry material and submerged nozzle are seriously eroded and pollute the molten steel;

2) The temperature difference between the inner and outer liquid cores of the primary billet is large, which increases the crack ratio and degree of the billet.

The service life of the slag retaining wall is low, which affects the floating of inclusions

In order to make full use of the tundish volume and promote the floating of inclusions, two slag retaining walls with a height of 1m were installed in the buffer zone of the tundish. The life of the tundish is 33h, the life of the slag retaining wall is only 15h, and the effect of the inclusions floating in the later stage of pouring is poor.

Unreasonable design of nozzle slag line and frequent fractures

The straight-tube immersion nozzle with a length of 660m is adopted. The slag line is installed at the lower end of the immersion nozzle with a length of 100mm. There is no lifting device during the use of the tundish. The aluminum carbon body above the slag line causes the nozzle to break.

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