DIFFERENT MACROSTRUCTURE DEFECTS

IN CONTINUOUS CAST STEELS

Soumyabrata Basak-14MS06005James Mathew -14MS06006

Introduction to continuous casting process

Continuous casting of steel is entering a new era of development, not only with respect to its increasing application in the production process, but also in its own evolution as a process and its interaction with other processes in steel manufacture. Continuous casting output has shown an accelerating growth curve. More than 50% of current world steel production is continuously cast, and continuous casting in Japan exceeds 80%.

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The purpose of continuous casting is to bypass conventional ingot casting and to cast to a form that is directly rollable on finishing mills. The use of this process should result in improvement in yield, surface condition, and internal quality of product when compared to ingot-made material.

Continuous casting involves the following sequence of operations:

1. Delivery of liquid metal to the casting strand2. Flow of metal through a distributor into the casting mold.3. Formation of the cast section in a water-cooled copper mold.4. Continuous withdrawal of the casting from the mold.5. Further heat removal to solidify the liquid core from the

casting by water spraying beyond the mold. 6. Cutting to length and removing the cast sections.

General Description of the Process

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The defect can be defined as any deviation from the appearance, form, size, macrostructure or chemical properties provided in the technical standards or other normative documents in force. Defects are detected at the billets reception, by checking their surface quality on the inspection beds, or by checking the macrostructure of the test samples. A defect is not always the result of a single case. Often, the defect is the result of multiple interacting causes, depending on a variable number of parameters. Defects in continuous casting originate from several factors like mould oscillation, mould flux, segregation coefficient of solute elements; phase transformation etc.

  

Macrostructure defects in casting

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DEFECTS

INTERNAL SURFACE SHAPE

• Midway cracks• Triple point cracks• Center line cracks• Diagonal cracks• Centre segregation

and porosity• Casting flux

inclusion• Blow holes• Pipes

• Longitudinal mid face cracks

• Transverse mid face cracks.

• Star cracks

• Longitudinal depression

• Transverse depression

• Rhomboidity

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Cracks are originated in the cast product due to mechanical and thermal stresses

Mechanical stresses are created by friction, ferro static pressure, roll pressure , bending and straightening operation.

It can be reduced by improving control of powder feed rate, resonance in mold, more accurate guidance..

Thermal stresses are created by non-uniform cooling in secondary zone

It can be controlled by Controlling water flux impinging the surface of the strand and minimizing reheating of strand, also air & water mist spray provides more uniform cooling.

Internal defects - Causes

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Blowholes are cavities in the outer surface or in the subsurface zone of the billet , caused by

o Insufficient deoxidation (presence of gases:-hydrogen, nitrogen, oxygen);

o Humidity of the casting powder. Non-metallic inclusions arise because of many physical-

chemical effects that occur in molten and consolidated metal during production.

Segregation refers to non-uniformity of chemical composition. A high degree of positive segregation in the central region is known as centreline segregation.

As a result of segregation liquid is not able to flow freely and compensate for shrinkage, results in porosity

Pipes are shrinkage cavity formed during solidification of fully deoxidized steel.These are normally open.

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Blow holes Triple point cracks

Midway Cracks Inclusion

Centre segregation

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The unevenly removal of the heat in the mould and, therefore, the uneven increase of the strand crust, causing transverse tensions that lead to the strand cracking if the crust is not strong enough (uneven primary cooling);

Turbulent flow of metal and a meniscus level variation in the mould;

Secondary cooling too intense or uneven; Unequal, advanced wear of the mould that leads to a

different thermal conductivity coefficient; High casting temperature

Surface defects - Causes

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Transverse cracks Longitudinal cracks

Star cracks

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Transverse depressions are due to much larger contractions occurred during solidification.

Longitudinal depressions are due to uneven heat transfer in the mould which caused due to-

o The steel level fluctuation in the mould.o The uneven and advanced wear of the mould. Billet Rhomboidity (difference in opposite diagonals)

starts with non-uniform shell solidification in the mould which is due to inconsistent mould cooling causing irregular heat transfer.

Shape defects - Causes

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Transverse depression Longitudinal depression

Rhomboidity

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ASM Handbook vol 15 Wikipedia Defect in steel by Octa Journal www.aws.org_itrends_2004_07_it0704-33

Reference

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Thank You………