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Analysis of Steel Leakage of Continuous Casting Billet
Continuous casting steel leakage is a serious production accident in continuous casting processing. In this paper, the causes of steel leakage are discussed in detail in combination with the phenomenon and causes of continuous casting steel leakage.
The breakout phenomena of billet continuous casting can be divided into angle breakout, middle breakout, pull breakout and start the breakout.
The Causes of Continuous Casting Steel Leakage can be divided into:
1. Steel leakage due to improper operation.
2. The superheat of steel is unreasonable.
3. Steel leakage caused by mold slag.
4. The vibration frequency and amplitude of the crystallizer are unreasonable.
5. The spray water of secondary cooling is unreasonable.
6. The crystallizer assembly is not reasonable.
SPECIFIC ANALYSIS
I. Reasons for Improper Operation
1. Misalignment of the water nozzle in the crystallizer results in uneven temperature cooling of the molten steel in the crystallizer and uneven leakage of the casting billet shell.
2. The steel liquid level is not observed, which will cause the steel liquid to leak too low or the billet to break through after overflow. Or the rhythm is unstable due to various reasons in production, resulting in large fluctuation of drawing speed, deviation of solidification curve from the internal curve of copper tube, uneven thickness of billet shell, corner crack leakage of crystallizer at the later stage of use of the crystallized copper tube, which often occurs in a short time after adjustment of drawing speed.
Therefore, it is necessary to ensure as much stability of drawing speed as possible, and it is impossible to adjust the drawing speed to meet the temperature of molten steel, smelting cycle, and steel supply rhythm. It is necessary to actively ensure the supply and quality of steel to meet the continuous casting demand. The submerged nozzle has a short life and frequent replacements. When replacing, the tundish needs to be raised as a whole.
Other flow secondary nozzles are inserted too shallowly and the liquid level is unstable, which easily causes slag leakage. The raw water nozzle is not matched with the refractories, and cold steel is joined between the upper and lower water nozzles. Oxygen slag blown by small oxygen tubes enters the crystallizer, which easily causes slag leakage.
3. The slag ring in the crystallizer is not fished in time, resulting in slag leakage of the casting billet.
4. Steel leakage due to blockage of water nozzle or mechanical breakage.
II. Unreasonable Superheat of Steel
Crack breakout is closely related to ladle temperature and drawing speed, which ensures that the molten steel has a certain degree of superheating and that the molten steel can be poured out smoothly.
Theoretical research shows that the thickness of the shell at the outlet of the crystallizer decreases by 3% with the increase of superheat of 10 C. Excessive temperature will result in a thin shell and low high-temperature strength.
Once the shell is torn by stress, crack and steel leakage will occur.
III. Steel Leakage Caused by Mold Slag
1) The delay of adding protective slag results in no lubrication between the slab and the copper tube of the crystallizer.
2) The choice of protective slag is unreasonable, i.e. the melting point and dissolving rate are unreasonable.
Molding slag function: insulating heat, preventing secondary oxidation, absorbing inclusions, lubricating billet shell, and crystallizer copper tube, reducing friction resistance. There are many types of mold slag for continuous casting.
(1) It can be divided into SiO2-A2O3-CaO system, SiO2-A2O3-FeO system, and SiO2-A2O3 system according to the chemical composition of the basic families. Na2O system, where the former application Z is common. On this basis, a small number of additives (alkali metal or alkali metal oxide, fluoride, boride, etc.) and carbon materials (carbon black, graphite, coke, etc.) control the melting rate are added.
(2) According to the shape of protective slag, it can be divided into powder slag (mechanical mixing moulding) and granular slag (extruded products are elongated, disc moulded products are round and spray moulded products are hollow round particles).
(3) The raw materials used can be divided into mixing, semi-pre-melting, and pre-melting types.
(4) According to its service characteristics, according to the characteristics of steel, continuous casting equipment, and continuous casting process conditions, it can be divided into various specifications of protection (low, medium, and high carbon steel slag and special steel slag), heating type Open-casting slag, etc.
Selection principle of protective slag for continuous casting crystallizer:
Molten slag for continuous casting crystallizer should follow reasonable melting temperature, melting rate, and melting layer structure in the crystallizer. Stable and suitable viscosity; Adequate capacity to absorb inclusions in the steel.
IV. Molder Vibration Frequency and Amplitude
1. The choice of vibration frequency and amplitude of the crystallizer is not reasonable. No negative slip or small negative slip in the crystallizer results in steel leakage.
2. Vibration simulates arc difference and large sway will generate a shear force on the billet shell, which will affect the lubrication of protective slag and increase the resistance of the billet. From the point of view of heat transfer, large vibration arc-like difference, and sway will increase the air gap nonuniformity between the shell and copper tube, which will lead to the increase of shell thickness difference.
The common causes are unstable vibration caused by damage of steel slag or plate spring on the east side of the vibration frame, serious oscillator sway, severe wear of billet shell on both sides of the inner chamber and inner east corner of the copper tube, and uneven heat transfer at the corners leading to corner crack and steel leakage.
V. Sprinkler Water for Secondary Cooling
The billet shell which has just come out of the crystallizer is hot and unsupported. At this time, uniform strong cooling is required to accelerate the rapid growth of the billet shell. If partial cooling of the upper part of secondary cooling is too weak, steel leakage will occur.
VI. Crystallizer
1) When the crystallizer is assembled, uneven water encounter between the water jacket and the copper pipe causes uneven cooling of the copper pipe and low heat conduction on the side with a small flow rate, resulting in thin steel leakage on the shell of the casting billet.
2) The taper of the copper tube in the crystallizer is unreasonable: the main heat resistance of heat transfer in the crystallizer is the air gap. If the air gap is small, the heat resistance will be small and the air gap will be large. At the early stage of crystallizer use, the internal curve of the copper tube is close to the shell shrinkage curve, with a uniform air gap, uniform heat transfer, and uniform shell thickness.
During use, copper pipes are constantly worn and deformed by heat. At the middle and late stages of service, the total taper becomes smaller, and heat transfer under the bending surface is large, deformation of the copper tube occurs locally, which also increases the inhomogeneity of the billet shell. The shell is prone to bulge in the lower part of the crystallizer. The sampling shows 150 mm.
If the bulge of the 150 mm billet is more than 2 mm, it is easy to deviate from the angle of internal cracking. After crystallizer, the billet shell loses its support, and steel leakage occurs.
Effect of the inner surface of copper pipe: During the use of the copper pipe, excessive and skewed cold steel is placed due to the treatment of steel leakage accident, resulting in scratches on the face and corners with a depth greater than 1 mm.
During the process of steel drawing, the interfacial heat resistance between the shell and wall of copper pipe at the scratched point is large, the shell is thin and prone to depression, and there are obvious cracks at the bottom of the depression. At this time, steel leakage is easy at the cracks if the overheating increases or the pulling speed is increased abruptly.
The quality of copper tubes is poor, especially the falling off of local coating on the face and corner of the copper tube. Increased heat resistance, resulting in unstable heat transfer, easy to causes steel leakage.
In addition, there is a trachoma in the copper pipe blank. With the increase of excessive steel in the crystallizer, the inner surface of the copper pipe wears, and the trachoma leaks out, causing steel hanging and cracking in serious cases.