Technical Practice of Electric Furnace Production of Hot-rolled Silicon Steel Continuous Casting Billets

The electric furnace is equipped with molten iron and refining outside the furnace to produce silicon steel continuous casting billets.

Keywords: electric furnace; silicon steel; continuous casting billet

Introduction

Hot-rolled silicon steel has strict chemical composition requirements, and the electromagnetic properties of the produced continuous cast billet after rolling must meet the requirements. This article summarizes the basic methods of process control in our plant’s smelting and continuous casting processes.

Overview of tooling

A steelmaking plant currently has two 30-ton ultra-high-power DC electric arc furnaces, two 40-ton LF ladle refining furnaces, and a 4-machine, 4-stream R6m Rokop continuous casting machine. After years of technological transformation, it now has the ability to produce 500,000 tons of continuous casting slabs per year. Especially after the blast furnace was put into operation in 2004, the electric furnace adopted the process of mixing molten iron, which greatly enhanced the steelmaking plant’s product development capabilities, with more than 60 product specifications. The main process flow of the steelmaking plant is shown in Figure 1.

Figure 1 Main process flow of steelmaking plant

Characteristics of hot-rolled silicon steel

The most important electromagnetic properties of silicon steel require low iron loss (hysteresis + eddy current) and high magnetic induction. Chemical elements that are beneficial to reducing iron loss include Si, Al, Cu, and P (but the brittleness increases), and harmful elements that increase iron loss are C, Mn, and S. [O], [H], and [N] in steel are harmful gases, and SiO₂ and Al₂O₃ inclusions in steel are harmful.

Technical difficulties in producing hot-rolled silicon steel continuous casting billets in electric furnaces

The control of chemical components C, Si, S, and P requires controlling the carburization phenomenon in each process to prevent C and Si from going out of scope. S should be controlled as low as possible, and P should be controlled appropriately.

Control of gas content in steel. Most of the oxygen in steel exists in the state of inclusions, and only a small part is dissolved in a-Fe. Both forms of oxygen have a great impact on electromagnetic properties. They not only increase iron loss, reduce magnetic permeability and magnetic induction intensity, but also cause magnetic aging. The harmful effects of nitrogen are similar to those of oxygen. Hydrogen increases the coercive force of silicon steel sheets and increases iron losses. Smelting requirements [0]≤60ppm, [H]≤4ppm, [N]≤70ppm

Control of non-metallic inclusions. Since non-metallic inclusions are non-ferromagnetic substances and hinder the movement of magnetic domains, non-metallic inclusions have a negative impact on the performance of finished silicon steel sheets, especially iron loss.

The nozzle of the tundish is easily clogged, and silicon steel molten steel with a silicon content of 1.80% to 2.80% has poor flow properties at high temperatures. However, if the aluminum content of ferrosilicon added to silicon steel is too high, the molten steel will easily become sticky. The SiO₂ contained in silicon steel can combine with Al₂O₃ to form mullite (3Al₂O₃2SiO₂) into linear crystals, forming a chain network in the steel to form a soft diaphragm. When the molten steel flows through the nozzle brick, these fibrous crystals stick to the nozzle to form nodules.

Silicon steel has a tendency to absorb hydrogen. The molten steel of silicon steel has a higher hydrogen content than the molten steel of carbon structural steel, which can easily cause microscopic bubbles to form inside the cast slab. In severe cases, it may cause edge rot during rolling and be scrapped. Continuous casting billets are prone to cracks, and the thermal conductivity of silicon steel is not bad. If the billet shell is thinned if the casting speed is too fast, it is easy to produce bulges, and even longitudinal cracks at the corners and steel leakage accidents. Therefore, the drawing speed control is lower than that of ordinary low carbon steel. Due to the poor thermal conductivity of silicon steel, the cast slab must be cooled slowly. Cooling too fast will cause cracks.

Production operation control practice

Smelting operation control

Manufacturer’s required chemical composition (%)

Steel typeCSiMnPSCrNiCu
DR510≤0.06 2.3~2.80.15~0.35≤0.035≤0.020≤0.10≤0.10≤0.10

Preparation before production

Ensure that the water-cooling systems of electric furnaces and refining furnaces do not leak, and that alloys and raw materials are fully dry to prevent molten steel [H] from exceeding the standard. Use scrap steel with clean, low rust and low copper content. The electric furnace is required to be used more than 20 times to prevent the magnesia carbon brick furnace lining from being difficult to decarburize in the later stage of oxidation. The number of times the ladle is used is required to be more than 10 times to prevent the magnesia-alumina carbon brick lining from bursting out of carbon and causing carbon excess.

Smelting operations

Each furnace mixes 25 to 35 tons of molten iron, the oxidation temperature is ≥1550°C, and the average decarburization speed is ≥0.03%/min to ensure full degassing and inclusion removal in the DC furnace. The oxygen purity requirement is ≥98%, and the tapping composition requires carbon between 0.02% and 0.03%, phosphorus ≤0.020%, and sulfur ≤0.030%. The accuracy of the carbon is controlled by static boiling for 2 to 3 minutes before tapping. The eccentric bottom tapping of the DC furnace must prevent slag from affecting the recovery rate of the refined alloy and the deoxidation operation. The tapping temperature of the DC furnace should be controlled between 1600 and 1620°C. To prevent a large temperature drop from affecting the physical and chemical reactions of the refining, the temperature should not be high to prevent a low recovery rate of ferrosilicon. The baking temperature of the bale before refining and steel joining is required to be no less than 900°C. The alloy is added to the bag in advance and baked together with the big bag until dark red. In order to prevent [ALs] in the steel from exceeding the standard, it is required to use low aluminum ferrosilicon (aluminum content less than 1%). When 1/4 of the steel is tapped, 1kg/t of calcium silicon alloy is added for pre-deoxidation to improve the alloy recovery rate. Generally, the ferrosilicon recovery rate is 91% to 93%. Since the alloy has been added to the package in advance, the sales volume is required to be accurately controlled between 48 and 53 tons to prevent excessive silicon production due to too little steel production.

The operation of the refining station requires good control of argon gas, good deoxidation operations, and good control of the composition of molten steel. The purity of molten steel will seriously affect the electromagnetic properties of silicon steel. The amount of refining slag is required to be controlled at about 800kg. Excessive slag amount will worsen the chemical reaction of molten steel. Too small slag amount will be detrimental to the temperature rise of molten steel and adsorption of inclusions and cause air intake during the smelting process. Lime requires a certain degree of activity, CaO content ≥ 80%, and alkalinity above 2.5. Use ferrosilicon powder or calcium silicate powder for diffusion deoxidation to ensure that the white slag time is above 20 minutes, and control the entire refining time to 40 to 50 minutes. , the final deoxidation is based on the [ALs] condition of the molten steel and the calcium silicate line is fed at 70~100M/furnace to ensure 0.14<[Ca]%/[AL]%<0.18, so that the deoxidation product generates liquid calcium aluminate (12CaO.7AL₂O₃) at the same time It also enables AI₂O₃ to be spherified and eliminates clogging of the water inlet. Controlling the soft argon blowing time to be greater than 8 minutes is beneficial to the removal of inclusions in the molten steel and the uniform composition and temperature of the molten steel.

In order to meet the performance of hot-rolled silicon pots, the ingredients are required to meet the standards provided by the manufacturer, and it is also required that P0.30% ~ 0.35%, S/Si≤ 0.01, [ALs] 0.003 ~ 0.005%. Because the DC electric arc furnace has a better phosphorus removal effect, the molten steel is generally 0.001% to 0.025% during refining. It is required to use phosphorus iron to adjust the phosphorus to the specification when refining the slag white.

Continuous casting operation control

Preparation before production

Choose a good mold slag. Because silicon steel is easily oxidized, the generated oxides contain more oxides, resulting in a larger fluctuation range in the composition of the mold slag. The mold slag is required to have a low melting point and a fast melting speed to increase the consumption of slag and reduce the melting rate. The AL₂O₃ concentration of the mold slag in the mold, and to prevent the deterioration of the composition of the mold slag from forming a slag ring in the mold and affecting the quality of the cast slab, our factory’s mold slag uses a special mold slag produced by Xixia Tongyu, with a free carbon content of 8%~ 10%, melting point 1100±30℃, mold slag and other raw materials for continuous casting should be well dried. The tundish is made of magnesium coating material. It must be baked with gas for 24 hours before use. The operating temperature must be greater than 700°C. Before drawing steel, the arrangement of the two cold water nozzles must be adjusted to the spray angle and coverage area to ensure that the inner arc water spray volume is approximately 1/2~1/3 of the arc, the water spray volume of the side arc is 2/3 of the outer arc.

Casting operation

The superheat is controlled at 10~20℃, and the first two packages are controlled at 20~25℃ to prevent low-temperature condensation. Silicon steel has good high-temperature plasticity. During the steel pouring process, it is easy to produce deformation, bulging, and in severe cases, leakage. Therefore, the casting speed is controlled at 0.8 to 1.2m/min. See Table 2for details.

Superheat degree ℃ Pulling speed m/min Section mm2 Sizing nozzle mm
≤101.2-1.1220*160∮16
11~151.1-1.0
16~201.0-0.9
21~250.9-0.8

During the water distribution operation, the crystallizer water volume is controlled at 110~130m³/h, and the water pressure is 0.8~0.9MPa. Silicon steel has poor thermal conductivity and the secondary cold water uses weak cooling to automatically distribute water to prevent cracks caused by uneven cooling of the slab.

Silicon steel contains high silicon content and is easy to be re-oxidized, so the whole process of protective pouring must be adopted. The long nozzle of the large package uses a @65 inner diameter quartz protective sleeve, and the immersed nozzle uses a φ25 inner diameter quartz nozzle. The immersion depth of the nozzle is 60 – 80mm. The liquid level height is not less than 450mm, and carbon-free covering agent is used for heat preservation. Silicon steel enters the brittle stage below 300°C. The low-temperature thermal conductivity is only equivalent to 1/20 of carbon structural steel. If it is cooled too quickly, cold cracking will occur. Our factory uses heat to be sent to the steel rolling heating furnace, but the heat is not sent in time. The steel billets are placed in the middle of the billet stack and slowly cooled.

 Conclusion

Our factory has solved defects such as internal cracking, shedding, bulging, and inclusions of silicon steel billets through the control of smelting, out-of-furnace refining, and continuous casting processes. The problem of casting nodules in the fixed-diameter nozzle has been solved technologically, and the electromagnetic properties of the silicon steel sheet also meet the manufacturer’s requirements.

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