Analysis of the causes of inclusions in continuous casting billets and improvement measures

The main reasons for non-metallic inclusions in continuous casting billets are personnel operation problems such as slag entrainment during the casting process and untimely slag discharge from the tundish. Poor protective pouring sealing effect, unreasonable nozzle immersion depth, nozzle deflection, etc. increase the secondary oxidation of molten steel. The superheat of molten steel is high, the life of the slag retaining wall is short, and the design of the nozzle slag line is unreasonable. Through standardized operations, preventing secondary oxidation of molten steel, reducing the superheat of the tundish from 30 to 50 ℃ to 20 to 35 ℃, promoting the floating of inclusions and other improvement measures, the internal inclusion ratio of ordinary carbon steel has been reduced from 5.2 times per 10,000 tons of steel to 0.3 times .

Keywords: continuous casting billet; inclusions; protective casting; superheat

Introduction

Hansteel’s 1# continuous caster is an eight-machine, eight-stream arc continuous caster used to produce small billets (150 mm × 150 mm). It currently mainly produces plain carbon steel and low alloy steel. Since the continuous casting machine was put into operation in February 2012, the internal inclusions of ordinary carbon steel have seriously affected the quality of the continuous casting billet, causing serious economic losses to the company. In view of the problem of inclusions in the continuous casting billet, the production process of the 1# continuous casting machine was deeply analyzed, and the reasons for the large-area billet inclusions were gradually understood. By standardizing personnel operations, controlling the sources of inclusions, improving the metallurgical effect of the tundish and the quality of refractory materials, we have effectively controlled inclusions in ordinary carbon steel and improved the cleanliness of the cast slab.

Analysis of the causes of inclusions in continuous casting billets

Hansteel’s 1# continuous caster is an eight-machine, eight-stream arc continuous caster with the ability to produce 150-200 mm square billets and Φ210mm round billets. It is currently mainly used to produce billets with a cross-section of 150 mm × 150 mm. Continuous casting machine parameters: continuous casting machine arc radius R10 m, vibration frequency 75~240 times/min, crystallizer passing length 900 mm, intermediate bag capacity 45 t, flow spacing 1250 mm, working speed 2.0~3.2 m/min .

In view of the inclusion problem inside the continuous casting billet, the production process and rolling situation of the continuous casting billet are tracked. Due to the wide range of sources and complex composition of inclusions, inclusions are first divided into endogenous inclusions and exogenous inclusions based on 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 corrosion products of refractory materials in ladles and tundishes, and involved slag and protective slag.

Personnel operation issues

1) The liquid level of the tundish and the crystallizer is unstable, and slag is frequently collected during the pouring process. In the early stages of production, due to unskilled personnel, there were serious loopholes in the pouring process: ① Large fluctuations in the crystallizer liquid level caused uneven growth of the shell, and a large number of inclusions were also involved in the solidified shell. ② The working layer of the tundish of the #1 continuous casting machine uses magnesium dry materials. In the early stage of production, due to unskilled personnel and frequent flow control of the large ladle water port, the liquid level in the tundish was unstable, which accelerated the erosion of the dry materials by the molten steel. The washed dry materials seriously pollute the molten steel. In March 2017, the liquid level control of the VI stream crystallizer in heat 217870 was unstable and fluctuated greatly, and the liquid level entrained slag, causing obvious defects in the cast slab.

2) The tundish is not discharged in time. Plain carbon steel has a large amount of slag in the molten steel itself, in order to increase the metal yield. Stop pouring when slag is deposited over a large area at the ladle nozzle. The thickness of the slag layer on the liquid surface of the tundish is very large, sometimes reaching 250 mm, which leads to serious process accidents. The untimely discharge of slag from the tundish is mainly manifested in the following aspects: ① It is difficult for the liquid surface covering agent to absorb the inclusions of the molten steel in the tundish. ② During normal pouring, the normal liquid level in continuous casting is 850 mm. During the process of changing ladles and waiting for steel, the liquid level in the tundish drops to 550 mm, and the actual depth of molten steel is 300 mm. When the liquid level is < 300 mm, a vortex is formed at the water inlet, and a large amount of inclusions flow into the crystallizer and condense in the primary green shell.

3) When the crystallizer oil and protective slag are mixed, inclusions with larger hardness are formed and are difficult to remove.

Irregular use of protective pouring

Reasons such as poor sealing effect of protective pouring, unreasonable immersion depth of immersed nozzle, deflection of nozzle, etc. 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 irregular. ① The installation of the long nozzle and immersed nozzle of the large bag is asymmetrical, which seriously erodes the inner wall of the refractory material, and creates a gap between the immersed nozzle and the quick-change water nozzle, which easily absorbs air and affects the effect of protective pouring. ② The 1# continuous casting machine uses a straight immersed nozzle with a nozzle height of 660 mm, and the subcontractor does not use a lifting device. When the liquid level in the mold fluctuated sharply, the immersion depth of the immersion nozzle was adjusted invisibly. The immersion depth of the immersion nozzle was too deep, and the high-temperature molten steel washed away the lower shell of the mold, making the shell thinner and prone to steel leakage. At the same time, inclusions are difficult to float and the immersion depth is too shallow, which can easily stir the crystallizer liquid level, causing slag entrainment and affecting the quality of the steel billet.

2) The addition of protective slag is unreasonable. The personnel’s operation is very random, the amount of protective slag added at one time is large, the liquid surface of the crystallizer is frequently exposed, and the molten steel entrains slag, which affects the effect of protective pouring. The slag fishing is not standardized. The operator habitually uses a slag fishing rod to stir the liquid level of the crystallizer, causing the liquid slag to flow into the space between the copper tube of the crystallizer and the primary shell.

3) The sealing effect of the long spout of the large package is poor. When installing the long nozzle, if the seal between the long nozzle and the sliding nozzle of the ladle is not tight, or the nozzle is not installed correctly, it will cause air to be sucked in and cause secondary oxidation of the molten steel. And because the inner hole of the shroud is larger than the inner hole of the sliding nozzle, the molten steel cannot fill the inner hole. At this time, a negative pressure zone is formed at the top of the shroud, and air is continuously sucked in from the gap, causing secondary oxidation of the molten steel. In the early stages of protective pouring, the sealing effect of the nozzle is poor, and the secondary oxidation of molten steel is serious, forming a large number of non-metallic inclusions, which are distributed along the inner arc of the billet. Low-magnification analysis shows that the center of the slab is loose at level 0.5, the cracks at the corners are at level 0.5, and the non-metallic inclusions are at level 2.0. It is concluded that non-metallic inclusion defects are the main factor affecting the quality of steel billets.

High temperature of molten steel

The following conditions during the production process will cause the temperature of molten steel to increase: 1) There are many flow interruptions during the pouring process, low pulling speed and long pouring cycle. 2) The intermediate tank has a capacity of 40 t, a molten pool depth of 1 m, a flow spacing of 1.25 m, and a length of 10 m. Due to the long pouring cycle of a heat and the large length of the tundish, the superheat of the tundish is controlled to be high in order to avoid low pouring temperature, condensation and dead zones caused by I and VIII flows in the later stage. The 1# continuous casting machine mainly produces plain carbon steel and low alloy steel, and the superheat of molten steel is generally controlled at 30 ~50°C. Excessively high temperature of molten steel will cause many hazards: 1) Refractory materials such as ladle lining, tundish dry material and immersed nozzle are severely corroded and pollute the molten steel. 2) The temperature difference between the liquid core inside and outside the green billet shell is large, which increases the crack ratio and degree of the billet.

The life of the slag retaining wall is short, which affects the floating inclusions.

In order to make full and effective 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 33 hours, and the life of the slag retaining wall is only 15 hours. The floating effect of inclusions in the later stage of pouring is poor.

The design of the nozzle slag line is unreasonable and breaks frequently.

Hansteel adopts a straight immersed nozzle with a length of 660 mm, and a slag line with a length of 100 mm is installed at the lower end of the immersed nozzle. There is no lifting device during the use of the tundish. The liquid level in the crystallizer is manually controlled and fluctuates greatly. When the liquid level rises, it is easy to corrode the aluminum carbon body above the slag line, causing the nozzle to break.

Measures to control inclusions in continuous casting billets

Standard operation

1) Improve the effect of argon blowing and promote the floating of non-metallic inclusions. The composition and temperature distribution of the molten steel from the converter to the ladle are uneven, and there are a large number of non-metallic inclusions and gases. Argon blowing can promote the floating of non-metallic inclusions and gases to a certain extent and improve the purity of the molten steel. . Through experiments, the following adjustments were made to the argon blowing parameters: 1) Change the argon blowing time from 7 minutes to 8 to 9 minutes. 2) Standardize the argon gas pressure and flow rate. When blowing argon, the blowing pressure and flow rate should be controlled in such a way that the molten steel is not exposed and tumbling.

2) Stabilize the liquid level of the tundish and crystallizer. The liquid level of the mold is controlled at ±5 mm; the liquid level of the tundish is ≥800 mm during normal pouring, the liquid level is ≥600 mm when replacing the ladle, and the liquid level of the tundish is ≥300 mm when the molten steel is tight.

3) Control the thickness of the intermediate slag layer. Ordinary carbon steel is organized to discharge slag every 6 hours. When the slag layer is >100 mm, slag discharge is organized in advance to improve the ability of the tundish covering agent to absorb inclusions and avoid slag entrapment at the nozzle.

Prevent secondary oxidation of molten steel

Large particle inclusions generated by secondary oxidation of molten steel during the pouring process, some float, and some may remain in the steel, becoming an important source of inclusions in the steel. Large particle inclusions in steel bring great harm to product quality. To improve the cleanliness of steel, it is necessary to reduce inclusions in steel, especially large particle inclusions. The key is to prevent secondary oxidation of molten steel during the pouring process. .

1) Standardize the use of protective slag. ① Mold powder should be added after the pouring is basically completed (complete the immersed nozzle operation within 10 seconds after adding). ② Mold powder should be added sparingly, frequently and evenly; ③ When the liquid level fluctuates, the addition of mold powder should be completed during the rise of the liquid level; ④ Mold powder should not be added to the corners of the crystallizer.

2) Correctly install the long nozzle and submersible nozzle to improve the sealing effect of the nozzle. ① Install a fire-resistant fiber sealing ring or metal ring between the long nozzle and the sliding nozzle, and use inert gas argon to seal and protect the injection flow. Create a positive pressure at the top of the long nozzle, isolate the injection flow from the air, reduce the amount of air inhaled, and avoid secondary oxidation. ② Before installing the long nozzle, the sticky steel in the bowl of the long nozzle should be cleaned in time, and the long nozzle should be aligned to ensure sealing and ensure that the steel flow is not re-oxidized.

Controlling the superheat of the tundish

1) Add an insulation layer to the ladle to reduce the heat loss of the ladle lining. 2) Red envelope tapping speeds up ladle turnover and increases ladle lining temperature. 3) Add carbonized rice husk or insulation material to the surface of the ladle to reduce heat loss. 4) The ladle is covered to effectively insulate the ladle for a long time and increase the temperature of the ladle. 5) Improve the argon blowing effect, even out the temperature of molten steel, and avoid the problem of uneven temperature in the tundish before, during and after the same heat. After a series of control measures, the superheat of the tundish was reduced from 30 ~ 50 ℃ to 20 ~ 35 ℃.

Promote the floating of inclusions

The large capacity and deep melt pool of the tundish are used to promote the floating of inclusions. The original magnesium slag retaining wall is changed to aluminum to improve the life of the slag retaining wall and synchronize with the tundish. Eliminate the dead zone at the bottom of the tundish, improve the flow trajectory of molten steel, make the molten steel flow along the steel slag interface, shorten the floating distance of inclusions, and promote the absorption of inclusions by the mold slag.

The use of retaining walls can also limit the strong vortex caused by the impact of ladle injection to a local area, preventing turbulent diffusion from causing surface fluctuations to cause internal slag entrapment in the molten steel.

Adjust the installation position of the slag line

After long-term on-site tracking and analysis, it was found that the main corrosion part of the immersed nozzle is 100 to 130 mm from the lower end of the nozzle. The slag line that is resistant to molten steel erosion is installed at 0 to 100 mm from the lower end, causing the nozzle to break. In order to take advantage of the zirconia material, the installation position of the slag line was moved up 50mm, which solved the problem of the immersed nozzle breaking.

Conclusion

Through standardized operations, the argon blowing time is controlled between 8 and 9 minutes, and the argon gas flow and pressure are controlled to reduce secondary oxidation of molten steel; the slag layer is controlled to be more than 100 mm and the slag is discharged in time. When using mold powder correctly, add it sparingly, frequently, and evenly to ensure that the mold powder does not enter the corners of the mold. Control the superheat of the tundish from 30 to 50 ℃ to 20 to 35 ℃. The large capacity and deep melt pool of the tundish are used to promote the floating of inclusions and improve the metallurgical effect of the tundish. Adjust the slag line of the immersed nozzle and increase the position of the slag line by 50 mm. After taking a series of optimization and improvement measures, the internal inclusion ratio of ordinary carbon steel has been reduced from 5.2 times per 10,000 tons of steel to the current 0.3 times per 10,000 tons of steel, which has contributed to the production and efficiency of the continuous casting machine of Hansteel Steelmaking Plant.

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