Continuous Casting Billet Defects - Continuous Casting Machine

Continuous casting is the process of converting liquid steel into a solid product mainly in the form of thick or thin slabs, blooms or billets.

It is an advanced steelmaking technology that allows the production of cast products with infinite lengths of the desired cross-section. 

The continuous billet casting process requires strict specifications. Many advanced production and control techniques are observed and adopted, but still the possibility of defects in the product cannot be completely excluded.

Factors

Continuous casting billet defects are formed during the production process due to several factors. For example:

material-related factors, 

the casting speed and temperature, 

the crystallizer vibration, 

the casting powder, 

the phase change of solute element bias coefficient

mechanical and thermal stresses, etc.

Factor Characteristics

Material-related factors

Include the δ to γ phase transition, high sulfur and low manganese/sulfur ratios, high oxygen potential of the steel, high superheat and the presence of inclusions in the steel.

During the δ (ferrite) to γ (austenite) transformation, volume changes and deformations occur, which can reduce the rate of heat transfer from the solidifying shell to the mold and result in non-uniform thickness of the cured steel shell.

Steels with a carbon content of 0.09% to 0.15%, known as peritectic steels, are prone to the formation of defects, usually longitudinal cracks.

Casting Speed

High casting speeds reduce the thickness of the cold zone, which may break under extreme conditions.

High casting temperatures (higher superheat) increase the surface temperature of the cast billet.

The skin formed during casting becomes superheated, which then generates thermal and tensile stresses, which leads to the formation of cracks and defects in the skin.

Casting Temperature

The casting temperature of the steel must be kept above the liquid phase line temperature.

The difference between the casting temperature and the liquid phase line temperature is called superheat.

Superheating of the steel plays an important role in the formation of defects and must be controlled.

Vibration

Regular vibration of the mold prevents molten metal from sticking to the mold.

At low vibration frequencies of the mold, the skin can break or surface cracks and vibration marks can form.

The formation of defects can be avoided by increasing the vibration frequency of the crystallizer and by a stable casting speed.

Mechanical

Mechanical stresses are caused by friction, ferrostatic pressure, bending and straightening operations and roll pressure.

Mechanical deformation can occur due to insufficient lubrication and during the strand straightening process. Acts in longitudinal and transverse directions.

Thermal Stress

Thermal stresses are caused by uneven cooling in the die or secondary areas.

When the strand cools rapidly, thermal stresses act in the transverse direction, which is the cause of the formation of defects.

Controlling the water flux on the surface of the impact strand and minimizing reheating of the strand can reduce thermal stresses.

Continuous Casting Billet Defects

Longitudinal Cracks

They are formed in the direction of extraction of the steel. Causes:

1. uneven primary cooling in the mold.

2. turbulence of the steel and changes of the curved lunar surface in the mold.

3. uneven or very strong secondary cooling.

4. uneven conductivity coefficients of thermal variations along the length of the crystallizer leading to advanced wear of the crystallizer.

5. casting of steels with high superheat.

6. high-speed casting.

7. the use of casting powders with improper characteristics.

Transverse Cracks

These cracks usually occur as a result of tension in the longitudinal direction of the strand.

Usually, these cracks are ground within the permissible specified limits as long as they are not deep.

Reasons:

1. thermal stress; 

2. changes in the level of the bending moon; 

3. presence of deflection at the bottom of the vibrating mark; 

4. friction of the cast billet in the mold with transverse cracks.

Angular Cracks

These are cracks that exist at the edges of the cast steel product.

Due to high temperature variations in the molten steel, higher aluminum content in the steel, higher sulfur content in the steel, uneven edge temperatures, excessive friction at the edges due to uneven distribution of the casting powder during the casting process, and low superheat steel.

Star Cracks

These cracks are very small and are caused by the fragility of the strand at high temperatures.

They are only visible on unscaled surfaces.

The surface is usually locally ground to remove the defect.

Strong local cooling and the presence of copper at the austenite grain boundaries cause star-shaped cracks.

To avoid star cracks in cast products, it is necessary to.

1. make the correct correlation between the jet stream and the casting speed.

2. a homogeneous layer of molten casting powder between the casting stream and the crystallizer

3. moderate secondary cooling of the cast billet to avoid increased thermal stress.

Dents

These are localized deformations of the casting surface. 

The depressions can be longitudinal or transverse.

Longitudinal depressions look like shallow grooves along the length of the casting.

They occur as a result of uneven heat transfer in the mold.

These depressions can be controlled by uniform cooling in the mold, centering of the steel jet in the mold, controlling fluctuations in the mold steel level, using a casting powder with the proper viscosity and melting characteristics, and regular monitoring of the mold.

The degree and uniformity of mold wear. Transverse depressions may occur periodically along the length of the strand.

Low carbon, high manganese clad crystalline steels and stainless steels are more sensitive to this defect.

Transverse depressions may be caused by crystallizer level fluctuations, large amounts of casting powder and steel turbulence in the sub-curved liquid surface.

These depressions are controlled by controlling the mold steel level, having the proper mold taper, using casting powders with the proper viscosity and melting characteristics, and proper positioning of the input nozzle and its holder.

Air Holes

These are cavities in the outer surface of the cast product and are usually associated with inclusions.

They are caused by the presence of gases in the steel, the humidity and quality of the casting powder, variations in the level of the crystallizer, and the presence of moisture in the refractory lining of the intermediate ladle.

The gas cavities are controlled by adequate deoxidation of the steel, use of dry casting powders, use of casting powders adapted to the grade, temperature and casting speed of the steel, control of the fluctuation of the crystallizer level, control of the depth of immersion of the spout, avoidance of high superheat to control and avoid slag generation around the nozzle.

Interruption of The Physical Continuity of The Casting Product

This defect occurs when the casting process is suspended.

It usually occurs when the heat changes during the sequential operation mode.

This defect is caused by a short interruption of the pouring process, when the pouring speed changes suddenly, due to changes in the intermediate ladle steel temperature, changes in the steel level in the crystallizer, changes in the level of alumina in the spout tooth grooves due to high temperatures, or changes in the casting pattern.

Corrective measures are to maintain a constant continuous casting speed, a narrow range of intermediate ladle temperature variations and a steel level in the intermediate ladle within the specified limits.

Slag Point Defects

This defect is caused by the penetration of the ladle slag into the casting.

The reason for this is the high slag content in the middle ladle, the elevated active oxygen content in the steel, the lowering of the steel level in the middle ladle leading to slag entering the crystallizer and the high viscosity of the casting powder.

Through the above specific analysis of the defects I believe we can understand more.

Although there are many continuous casting billet defects, it's better to avoid these problems when using and purchasing.

You are welcome to contact us at any time to discuss or any demand for continuous casting machine.

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