Analysis on the Causes of Breakout of Continuous Casting Billets

Breakout of continuous casting is a serious production accident of continuous casting. This article discusses the causes of breakout in more detail in combination with a comprehensive analysis of the phenomenon and causes of breakout in continuous casting.
From the billet continuous casting, the breakout phenomenon can be divided into: corner crack breakout, middle breakout, pull-off breakout, and start-up breakout. From the analysis of the causes of continuous casting breakout, it can be divided into: 1. Breakout caused by improper operation. 2. The overheating of steel is unreasonable. 3. Breakout caused by mold powder. 4. The mould vibration frequency and amplitude are unreasonable. 5. The second cold spray water is unreasonable. 6. The mold assembly is unreasonable.

detailed analysis:
one. Causes of improper operation
1. Misalignment of the nozzle of the mold causes uneven cooling of the molten steel in the mold, resulting in uneven thickness of the cast slab shell.
2. The liquid level of steel is not checked, causing the liquid steel to be too low to break out or to cause the steel billet to break and break out. Or due to various reasons in the production, the rhythm is unstable, which leads to large fluctuations in the pulling speed, and the solidification curve deviates from the curve of the inner cavity of the copper tube, which is prone to uneven thickness of the blank shell. Angle cracking often occurs within a short period of time after the drawing speed is adjusted. Therefore, the drawing speed should be as stable as possible. The drawing speed cannot be adjusted to adapt to the molten steel temperature, smelting cycle and steel supply rhythm. Instead, the molten steel supply should be actively ensured. The quality of molten steel meets the needs of continuous casting; the submerged nozzle has a short life and is frequently replaced. The tundish must be raised as a whole during replacement. The insertion depth of the nozzle in other flows is too shallow, and the liquid level is unstable, which is easy to cause slag and steel leakage. The refractory material of the original nozzle is not matched, and the cold steel is connected between the upper and lower nozzles, and the oxide slag formed by blowing and burning with a small oxygen tube enters the mold, which is easy to cause the leakage of the slag.
3. The slag ring in the crystallizer is not caught in time, which causes the slag coil of the cast slab to break out.
4. The nozzle is blocked or mechanically broken, causing leakage.

two. Unreasonable overheating of steel
The cracked steel leakage is closely related to the temperature of the tundish and the pulling speed, which ensures that the molten steel has a certain degree of overheating and can ensure that the molten steel is poured smoothly. Theoretical studies have shown that for every 10℃ increase in superheat, the thickness of the mold shell at the exit of the mold will decrease by 3%. If the temperature is too high, the mold shell will be thin and the high temperature strength will be low. Cracks occur.

three. Breakout caused by mold powder
1) The mold slag was not added in time, resulting in no lubrication between the casting billet and the mold copper tube.
2) The selection of mold powder is unreasonable, that is, the melting point and melting rate are unreasonable.

The role of mold protective slag: heat insulation, prevent secondary oxidation, absorb inclusions, lubricate the blank shell and mold copper tube, reduce frictional resistance. There are many varieties of mold powder for continuous casting.

(1) According to the chemical composition of the base, it can be divided into: SiO2-A2O3-CaO series, SiO2-A2O3-FeO, SiO2-A2O3. Na2O series, the former is the most common application. On this basis, a small amount of additives (alkali metal or alkali metal oxides, fluorides, borides, etc.) and carbon materials (carbon black, graphite, coke, etc.) that control the melting rate are added.
(2) According to the shape of mold powder, it can be divided into powdery slag (mechanical mixing molding) and granular slag (extruded products are elongated, products formed by disc method are round, and products formed by spray method are hollow. Round particles).
(3) According to the raw materials used, it can be divided into raw material mixed type, semi-pre-melted type and pre-melted type.
(4) According to its use characteristics, according to the characteristics of steel grades, continuous casting equipment and continuous casting process conditions, it can be divided into various specifications of protection (low, medium and high carbon steel mold slag and special steel special slag), heating type opening Pouring slag and so on.
The selection principle of mold powder for continuous casting:
The mold powder of continuous casting mold should have a reasonable melting temperature, melting speed and melting layer structure in the mold; stable and suitable viscosity; sufficient ability to absorb the inclusions in the steel.

Four. Mold vibration frequency and amplitude
1. Unreasonable selection of mould vibration frequency and amplitude. There is no negative slippage or small negative slippage in the mold causing steel breakout.
2. Vibration imitating arc difference and large deflection will produce shear force on the blank shell, affect the lubrication of the mold powder, and increase the drawing resistance. From the perspective of heat transfer, the vibration imitates the arc difference and the large amount of deflection will increase the unevenness of the air gap between the blank shell and the copper tube, resulting in an increase in the difference in the thickness of the blank shell. The general reason is that the steel slag on the east side of the vibrating frame or the damage of the leaf spring causes the vibration to be unstable, the mold deflection is serious, the billet shell wears the two sides of the inner cavity of the copper tube and the inner east corner seriously, and the uneven heat transfer at the corner causes the corner. Cracked steel.
five. Second cold spray water
The temperature of the shell of the cast slab just out of the mold is high and it loses support. At this time, uniform and strong cooling is required to promote the rapid growth of the shell. If the upper part of the secondary cooling is too weakly cooled, a reflow phenomenon will occur, resulting in steel breakout.


six. Crystallizer
1) When the mold is assembled, the uneven water between the water jacket and the copper tube causes uneven cooling of the copper tube, and the heat conduction on the side with small flow is low, resulting in a thin breakout of the cast slab shell.
2) Unreasonable taper of mold copper tube: The thermal resistance of mold heat transfer is mainly the air gap. If the air gap is small, the thermal resistance is small, and if the air gap is large, the thermal resistance is large. In the early stage of use of the mold, the inner cavity curve of the copper tube is relatively close to the contraction curve of the blank shell, the air gap is uniform, the heat transfer is uniform, and the thickness of the blank shell is also relatively uniform. During use, the copper pipe is constantly worn and deformed by heat. In the middle and late stages of use, the total taper becomes smaller, and the heat transfer under the meniscus is large, and the copper tube is locally deformed, which also increases the unevenness of the blank shell. The billet shell is prone to bulging in the lower part of the mold. Sampling shows that the bulge of 150mm×150mm cast billet is more than 2mm prone to deviation angle internal cracking, and the billet shell loses support after exiting the mold, which is prone to breakout. The influence of the inner surface of the copper pipe: During the use of the copper pipe, excessive and skewed cold steel was placed due to the breakout accident, causing scratches on the face and corners with a depth greater than 1mm. During the drawing process, the blank shell was scratched. The thermal resistance between the copper tube and the wall is large, the billet shell is thin, and dents are easy to appear, and there are obvious cracks at the bottom of the dents. At this time, if the overheating degree increases or the pulling speed is suddenly increased, it is easy to leak steel at the cracks. The quality of the copper pipe is poor, especially the partial coating on the face and corners of the copper pipe falls off. Increased thermal resistance, resulting in unstable heat transfer, and easy to cause leakage. In addition, there are trachoma in the copper tube blank. As the amount of steel passing through the mold increases, the inner surface of the copper tube is worn out, the trachoma leaks, and the phenomenon of steel hanging occurs, which may cause cracks and leaks in severe cases.

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Reference source: Internet
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Post time: Sep-08-2021