This article describes the requirements of the continuous casting mold for the quality of molten steel, pouring temperature, and tapping temperature.
Continuous casting mold copper tubes place very strict requirements on the quality of molten steel.
Continuous casting molten steel quality
Molten steel temperature: The requirements for continuous casting of molten steel are: low superheat, stable and uniform.
Purity of molten steel: Minimize the content of harmful impurities (such as S, P) and inclusions to ensure the smooth operation of the casting machine and improve the quality of the cast slab. If the S content in the molten steel is greater than 0.03%, longitudinal cracks in the cast slab will easily occur. The high inclusion content in the molten steel will easily cause the accumulation of internal arc inclusions in the cast slab of the arc casting machine, affecting product quality.
Composition of molten steel: Ensure that the alloy elements added to the molten steel are evenly distributed and the composition is controlled within a narrow range to ensure the stability of product performance.
To ensure the pourability of molten steel, it is necessary to maintain a suitable and stable temperature of the molten steel and a degree of deoxidation to satisfy the pourability of the molten steel. For example, if aluminum is deoxidized, the Al2O3 inclusion content in the molten steel will be high and the fluidity will be poor, which will easily cause the tundish nozzle to become clogged and interrupt pouring.
Therefore, it is necessary to accurately and appropriately control the temperature, composition and purity of continuous casting molten steel according to product quality and continuous casting process requirements. The rhythmic and balanced supply of molten steel of acceptable quality to the continuous casting machine is the primary condition for smooth continuous casting production.
Requirements for pouring temperature of molten steel for continuous casting
Reasonable selection of pouring temperature is one of the basic parameters of continuous casting. If the pouring temperature is too low, the molten steel will become sticky and inclusions will not float easily; the molten steel will condense on the surface of the mold, causing defects on the surface of the cast slab; the nozzle will freeze and pouring will be interrupted. If the pouring temperature is too high, 1) the refractory material will be severely eroded and the inclusions in the steel will increase; the molten steel will absorb oxygen and nitrogen from the air; the shell of the mold shell will be thin and easy to leak; the columnar crystals of the cast slab will be developed and the central segregation will be aggravated. .
If an inappropriate pouring temperature can barely be poured during mold casting, it will cause trouble during continuous casting (such as leakage and frozen water inlets). Therefore, the temperature of molten steel in continuous casting is much stricter than that in mold casting. The requirements for continuous casting molten steel temperature are:
(1) Low superheat. On the premise of ensuring smooth pouring, the superheat should be controlled as low as possible. The billet is generally controlled at 20~30°C.
(2) Uniform. In fact, the temperature of the molten steel in the ladle is low at the top and bottom, and the temperature in the middle is high. This will cause the temperature of the molten steel in the tundish to be low at both ends and high in the middle, which is not conducive to the control of the pouring process. Therefore, the temperature of the molten steel in the ladle is required to be uniform up and down. .
(3) Stable. The temperature of each furnace of molten steel supplied during continuous pouring should not fluctuate too much and should be kept within the range of 10°C.
Determination of pouring temperature
The continuous casting pouring temperature refers to the temperature of the molten steel in the tundish. The pouring temperature of molten steel includes two parts: one is the solidification temperature of molten steel (also called liquidus temperature), which varies with different steel types. The second is the superheat of molten steel, which is the value exceeding the solidification temperature. Let TC represent the pouring temperature, TL represent the liquidus temperature, and ΔT represent the superheat of molten steel, then:
There are different formulas for calculating TL. The commonly used formulas are as follows:
For example, after alloying Q235 steel (original A3 steel), the composition of molten steel in the ladle is: C0.15%, Si0.25%, Mn0.45%, P0.025%, S0.025%. Substitute each component into the formula to get:
In other words, the starting solidification temperature of molten steel is 1518°C. For C=0.10~0.20% steel, the solidification temperature of molten steel generally fluctuates between 1510~1520℃.
The determination principle of superheat degree ΔT is related to product quality. For medium-thick plates, in order to reduce internal cracks and central segregation of the cast slab, ΔT is preferably low (10~15℃). In this way, the liquidus temperature is calculated based on the steel type, and the pouring temperature can be obtained by adding the superheat, which is the target temperature of the molten steel that needs to be maintained in the tundish during the pouring process. Practice has proved that controlling the target temperature of molten steel in the tundish is a key process parameter to ensure the output of the continuous casting machine and the quality of the cast slab. It must be given full attention.
Determination of tapping temperature
After the target temperature of the molten steel in the tundish is determined, how to determine the tapping temperature of the steelmaking furnace? The tapping temperature can be expressed as:
ΔT1 is the tapping temperature loss. The empirical data on the temperature loss of converter tapping is: for converters larger than 50t, the tapping time is 3 to 6 minutes, and the average temperature drop is 10°C/min. For converters smaller than 50t, the tapping time is 2~4min, and the average temperature drop is 15℃/min; the general tapping temperature drop is 40~60℃.
ΔT2 is the temperature drop of molten steel during argon blowing and stirring (or other outside-furnace treatment) (the argon blowing time in the converter is generally 3~5 minutes). The temperature drop of molten steel during argon blowing is related to the steel ladle capacity and argon blowing time. The temperature drop caused by blowing argon is 4~6℃/min.
ΔT3 is the temperature drop of molten steel during transportation and resting time of the ladle. The natural temperature drop of the molten steel in the ladle is related to the quality of the refractory material lining the ladle, the addition of covering agent or capping to the ladle, etc. Generally it is 1~1.5℃/min.
ΔT4 is the temperature drop of molten steel during the pouring process, which is generally less than 1℃/min.
Taking a 50t steel ladle from a certain factory as an example, the steel type is Q235, and the calculated value is TL=1510℃. The measured temperature losses of molten steel at each stage are: ΔT1=60℃, ΔT2=30℃, ΔT3=6℃, ΔT4=45℃, so T is:
In other words, the tapping temperature is 1681°C.
The temperature drop of the molten steel in the ladle at each stage after tapping can be actually measured with an inserted thermocouple, and statistical analysis can be performed to obtain an average value. Then make a chart or table to guide production.
Measures to reduce temperature drop during ladle process
The most prominent factors affecting the temperature drop during the ladle process are the ladle volume, ladle lining material and usage conditions. Production practice shows that the following insulation measures are effective:
The steel ladle is equipped with an insulation layer to reduce the heat dissipation loss of the ladle lining. For example, if a 110t ladle is equipped with a 30mm thick insulation layer, the temperature drop rate will be 20~40% lower on average than that without the insulation layer. 40 minutes after tapping, the temperature of the molten steel in the ladle will drop by 17~20°C.
High-temperature baking of ladles: For example, if a 70t ladle adopts a rapid baking device, the lining temperature of the ladle can reach over 850℃ after baking for 15 minutes, and the average temperature drop of the baked ladle is reduced from 80~90℃ to 30~60℃.
Red envelope tapping: speed up ladle turnover and increase ladle lining temperature. The tapping of 35t steel ladle red envelope can reduce the tapping temperature by 17°C on average.
Carbonized rice husks or insulation materials are added to the surface of the ladle to reduce heat loss.
The ladle is covered. On the one hand, this can effectively keep the ladle warm for a long time, and can also keep the slag on the liquid steel surface in a liquid state, which facilitates slag cleaning after injection. In addition, it can reduce the heat dissipation of the ladle lining and increase the temperature of the ladle.
Adopting the above measures can reduce the temperature drop during the ladle process and is beneficial to the stability of the temperature of the molten steel in continuous casting.
Several measures to adjust the temperature of molten steel
In actual production, due to factors such as raw materials and operations, the tapping temperature is often not accurately controlled and is often higher than the predetermined tapping temperature. In order to meet the requirements of continuous casting pouring temperature, the temperature of molten steel is adjusted after tapping. The general method is: (1) Stirring method. Blow argon gas into the top or bottom of the ladle to stir the molten steel to make the temperature and composition of the upper and lower parts of the ladle uniform.
(2) Stirring + cold scrap steel. While blowing and stirring, add light and clean scrap steel, and use the scrap steel to melt and absorb heat to cool down. If the temperature of molten steel decreases by 1°C, 0.7kg/t of scrap steel needs to be added.
If the steel is tapped at the predetermined target temperature, the molten steel must be refined outside the furnace. Due to the heat loss during the treatment, the required tundish pouring temperature cannot be guaranteed. In this case, heat compensation needs to be carried out in the ladle. The methods used are:
(1) Arc heating method: Use graphite electrodes to generate high-temperature arcs (4000°C) to heat molten steel. The larger the steel package capacity, the higher the heating efficiency. For example, for a 20t ladle, the heating efficiency is 30%; for a 250t ladle, the heating efficiency is 75%, and the heating rate is 3~6°C/min.
(2) Induction heating method: The AC magnetic field generated by the coil is used to generate an induced electric potential in the molten steel to heat the molten steel. The heating efficiency can reach 70%, and the heating rate is 2.5℃/min.
(3) Plasma heating method: Gas (such as argon, nitrogen) will become plasma when heated to high temperature. High-temperature plasma (temperature can reach over 3000°C) is used to heat molten steel. The heating rate is 5~6°C/min, and the thermal efficiency can reach 70~80%.