Macro Defects in Steel - Common Continuous Cast defects

Common Continuous Cast defects

Casting is a forming process which converts liquid steel into a solid product. In foundries liquid steel is cast into complex shapes by pouring of liquid steel into a mould in which it sets to the required shape. In steel plants, liquid steel is normally continuously cast in the form of slab (either thick or thin), bloom or billets. Casting defects are defined as those characteristics which create a deficiency or imperfection exceeding quality limits imposed by design and service requirements.

Defects of the continuous cast steel products are formed due to several factors which include material related factors, casting speeds and temperatures, mould oscillation, casting powder, segregation coefficient of solute elements, phase transformation, and mechanical and thermal stresses. Common defects in steel which occurs during the continuous casting are as follows.

Longitudinal cracks – They are formed in the direction of extraction of the steel. The presence of these defects results into the rejection of the steel. Longitudinal cracks occurs mainly due to (i) uneven primary cooling in the mould, (ii) turbulent flow of liquid steel and a meniscus level variation in the mould, (iii) non uniform or very intensive secondary cooling, (iv) variance in thermal conductivity coefficient along the mould length causing unequal, advanced wear of the mould, (v) casting of liquid steel with high superheat, (vi) high speed of casting, and (vii) use of the casting powder with improper characteristics.

Transverse cracks – These cracks usually appear due to the tensions in the longitudinal direction of the strand. The transverse cracks are usually ground within the permissible limits provided they are not deep. These cracks appear due to (i) the thermal stresses, (ii) variation in the meniscus level, (iii) presence of segregation at the bottom of oscillation mark, and (iv) friction of the strand in the mould.

Corner cracks – These are cracks present in the edge of the cast steel product. They appear due to high temperature variations in the liquid steel, higher aluminum content in the steel, higher sulphur level in the steel, non-uniform edge temperature, excess friction in the edges during casting because of non-uniform distribution of casting powder, and lower superheat of the steel.

Star cracks – These cracks are very fine and caused by fragile nature of the strand at high temperatures. They are visible only on scale free surface. The surface is usually ground locally to remove the defect. Intense local cooling and presence of copper at the austenitic grain boundary cause star cracks. To avoid the star cracks in the cast product it is necessary to have (i) correct correlation between the spray flow and the casting speed, (ii) a uniform layer of melted casting powder between the strand and the mould, and (iii) adequate secondary cooling of the strand for avoiding increase of the thermal stress.

Depressions – These are local deformations in the cast surface. Depressions can be longitudinal or transverse. Longitudinal depressions appear like the shallow ditches oriented along the length of the cast product. They occur due to the uneven heat transfer in the mould. These depressions can be controlled by uniform cooling in the mould, by centering of the liquid steel jet in the mould, by controlling the fluctuations of the mould steel level, use of a casting powder with suitable viscosity and melting characteristics, and by regularly monitoring the degree and uniformity of the mould wear. Transverse depressions may occur cyclically along the strand length. The peritectic steels with low carbon and high manganese contents and the stainless steels are sensitive to this defect. The transverse depressions can be caused by the fluctuations in the mould level, large quantity of casting powder, and by the turbulence of steel the sub-meniscus level. These depressions are controlled by controlling the mould steel level, having proper mould taper, use of a casting powder with suitable viscosity and melting characteristics, and proper positioning of the input nozzle and its support.

Blowholes – These are cavities in the outer surface of the cast product and are often associated with inclusions. They are caused by presence of gases in the steel, humidity and quality of the casting powder, variation in the mould level, presence of moisture in the tundish refractory lining. Blowholes are controlled by sufficient de-oxidation of steel, use of dry casting powder, use of casting powder compatible with the grade of steel grade, temperature and casting speed, control of mould level fluctuations, control of nozzle immersion depth, avoiding the high superheat and avoiding slag foaming around the nozzle.

Interruptions - In the physical continuity of the cast product – This defect occurs when there is a pause in the casting process. It often occurs when there is a change of heat during sequence mode of operation. This defect is caused by a short interruption of the casting process and occurs when there is sudden change in casting speed caused by the variations of steel temperature in the tundish, by the variations of steel level in the mould, cogging of the nozzle due to high alumina levels, or by the variations of casting mode. The corrective measures are maintenance of a constant casting speed, a narrow range of temperature variation in the tundish, and steel level in the tundish within the prescribed limits.

Slag spots defects -This defect is caused by the penetration of tundish slag in the cast product. It is caused by high level of slag in the tundish, rise in the active oxygen percentage in the steel, lowering of steel level in the tundish resulting in slag to enter the mould, and high viscosity of casting powder.

Reference: Website http://ispatguru.com/metallurgical-processes-and-defects-in-steel-products, 2017.