Analysis on the Causes of Steel Breakout of Continuous Casting Billets

Breakout in continuous casting is a serious production accident in continuous casting. In this paper, combined with a comprehensive analysis of the phenomenon and causes of breakout in continuous casting, the reasons for breakout are discussed in detail.

From the billet continuous casting breakout phenomenon can be divided into: corner crack breakout, middle breakout, pull breakout, start breakout. From the analysis of the reasons of continuous casting breakout, it can be divided into: 1. Breakout caused by improper operation. 2. The superheat of the steel is unreasonable. 3. Steel breakout caused by mold slag. 4. The vibration frequency and amplitude of the crystallizer are unreasonable. 5. The second cold spray water is unreasonable. 6. The mold assembly is unreasonable.

 A. Caused by improper operation

1. The nozzle of the mold is not centered, which causes the temperature of the molten steel in the mold to cool unevenly, resulting in uneven thickness of the cast billet shell.

2. The liquid level of the steel is not checked, which causes the molten steel to be too low to break out or to cause the steel billet to break and break out after the steel overflows. Or due to various reasons of production, the rhythm is unstable, resulting in large fluctuations in the pulling speed, the solidification curve deviates from the inner cavity curve of the copper tube, and the thickness of the billet shell is prone to unevenness. Angle cracks often occur in a short period of time after the adjustment of the pulling speed. Therefore, it is necessary to ensure the stable pulling speed as much as possible. It is not possible to adjust the pulling speed to adapt to the temperature of molten steel, the smelting cycle and the rhythm of steel supply, but to actively ensure the supply of molten steel. and molten steel quality, to meet the needs of continuous casting; the submerged nozzle has a short life and frequent replacement, and the tundish needs to be raised as a whole when replacing it. The insertion depth of other flow nozzles is too shallow, and the liquid level is unstable, which is easy to cause slag 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 oxidized slag formed by blowing and burning with a small oxygen pipe enters the mold, which is easy to cause slag leakage.

3. The slag ring in the mold is not caught in time, resulting in slag leakage of the casting billet.

4. The nozzle is blocked or mechanically broken, resulting in steel leakage.

B. Unreasonable superheat of steel

Crack breakout is closely related to the temperature of the tundish and the pulling speed, which ensures that the molten steel has a certain degree of superheat and can ensure that the molten steel is poured smoothly. Theoretical research shows that the thickness of the shell at the outlet of the mold decreases by 3% for every 10 °C increase in the superheat degree. If the temperature is too high, the shell of the mold will be thin and the strength at high temperature will be low. Crack breakout occurs.

C. Steel breakout caused by mold slag

1) The addition of mold flux is not timely, resulting in no lubrication between the casting billet and the copper tube of the mold.

2) The choice of mold residue is unreasonable, that is, the melting point and dissolution rate are unreasonable.

The role of mold powder: heat insulation, prevent secondary oxidation, absorb inclusions, lubricate the shell and mold copper tube, and reduce frictional resistance. There are many varieties of mold flux 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 carbonaceous materials (carbon black, graphite and coke, etc.) that control the melting rate are added.

(2) According to the shape of mold slag, it can be divided into powder slag (mechanical mixing molding), granular slag (extrusion molding products are elongated, disc molding products are circular, spray molding products are hollow round particles).

(3) According to the raw materials used, it can be divided into original material mixed type, semi-premelted type and pre-melted type.

(4) According to its use characteristics, according to the characteristics of steel types, continuous casting equipment characteristics and continuous casting process conditions, it can be divided into various specifications of protection (low, medium and high carbon steel mold slag and special slag for special steel), heating type open Slag, etc.

Selection principle of mold flux for continuous casting:

The mold flux for continuous casting should follow a reasonable melting temperature, melting speed and melting layer structure in the mold; stable and suitable viscosity; sufficient ability to absorb inclusions in steel.