The remedy for the pipe formation is the use of hot top on the mould. Pipe formation is restricted in the hot top which can be discarded. Use of exothermic materials in the hot top keeps the liquid steel hot in the top portion and pipe formation can be avoided. Another method is to pour extra mass of metal. Fig 3 Pipe formation during solidification of liquid steel in ingot mould. Blow holes — The reason for formation of blow holes in steel ingot is the evolution of gas during solidification of the liquid steel.
Entrapment of gas produces blow holes in the steel ingot. Blow holes located inside the ingot can be welded during rolling. Rimming steels show blow holes due to rimming reaction between C and O2. The rimming reaction produces CO, which when is unable to escape during solidification, produces blow holes. Semi-killed steels also show tendency to the formation of blow holes.
The remedy for the formation of blow-holes is the control of gas evolution during solidification so that blow-hole forms only within the ingot skin of adequate thickness. Non-metallic inclusions — Non-metallic inclusions are inorganic oxides, sulphides and nitrides formed by reaction between metal like iron, titanium, zirconium, manganese, silicon, and aluminum with non-metallic elements like O2, nitrogen, sulphur etc. An inclusion is a mismatch with the steel matrix. Fine size inclusions when distributed uniformly are not harmful.
Non deformable inclusions such as Al2O3 are undesirable. Inclusion modification is the remedy to alleviate the harmful effect of inclusions on properties of steel. Ingot cracks — Surface cracks are formed due to friction between mould and ingot surface.
The improper design of mould taper and corner radius cause surface cracks. Different types of cracks are i transverse cracks, ii longitudinal cracks also known as panel cracks, iii restriction cracks and iv Sub-cutaneous cracks.
Transverse cracks are parallel to the base of ingot and are formed due to longitudinal tension in the ingot skin. As the aspect ratio of the ingot increases, tendency to transverse crack formation increases. Longitudinal cracks are formed due to lateral tension in the skin.
They are parallel to vertical axis of ingot. Alloy steels are more prone to longitudinal cracks than mild steels. Panel crack formation in static-cast steel ingots is a problem that has plagued the steel industry for several decades. Mid-face panel cracks are found exclusively in small, medium C, hypo-eutectoid, and pearlitic steel ingots and usually show a single, continuous, longitudinal fracture down the centre of one of the ingot faces.
Certain alloy steels are particularly prone to this defect and are affected at slightly lower C contents. Off-corner panel cracks frequently form rough oval, discontinuous, crack patterns on the wide faces of large ingots. They affect only low C steels with high Mn content and are usually first observed when they open up during hot rolling. Both types of defect affect only killed, aluminum-treated steels and appear as deep, inter-granular cracks that follow prior austenite grain boundaries.
Restriction cracks can be near the corner radius of the ingot. Smooth corners of the mould and gradual curvature minimize restriction cracks. Sub- cutaneous cracks are internal fissures close to the surface. The cracks are formed due to thermal shocks. Steel ingots and their Casting during Steelmaking satyendra July 20, 0 Comments blow holes , bottom pouring , cast iron mould , cracks , inclusion , ingot casting , pipes , segregation , Steel ingot , top pouring , Steel ingots and their Casting during Steelmaking Ingot casting is a conventional casting process for liquid steel.
The bottom pouring needs a controlled velocity during filling in order to avoid turbulences and, consequently, powder entrapment or reoxidation defects The application of this bottom pouring method for the production of quality ingots has been mainly due to the reduced turbulence of steel in the mould caused by controlled flow of liquid steel in the mould to result in quiet meniscus leading to superior as cast ingot surface, minimal splashing of liquid steel droplets from ladle stream providing freedom from scab type defects, application of steel meniscus during teeming for completely covering the slower teeming rates that reduce turbulence, longitudinal cracks meniscus and maintaining a powder layer throughout the casting formation and minimize laps and ripple marks and finally, process that helped in drastically reducing or even eliminating improved mould life.
Mechanism of solidification of liquid steel in ingot mould The mechanism of the solidification of killed liquid steel in the ingot mould is described below. Liquid steel near the mould walls and bottom is chilled by the cold surfaces and a thin shell or skin is formed on the ingot surface. This surface has a fine equiaxed grains and the skin. The formation of skin results in decrease in rate of solidification.
Due to expansion of mould through the heat transferred from the solidifying steel and contraction of solidified skin an air gap forms between the mould and the skin. This results in decrease in the heat transfer rate, because of the air gap which has a high thermal resistance to heat flow.
The solidification front perpendicular to the mould faces moves inwards and towards the centre as a result columnar grains form next to the chill surface. The columnar crystals rarely extend to the centre of the mould. The central portion of the ingot solidifies as equi-axed grains of bigger size since there is slow rate of solidification in this portion.
Micro-segregation and macro-segregation in steel ingots The general macrostructure that is often seen in the steel ingot can be divided into three distinct zones namely i the outer chill zone with small crystals of approximately equal size, ii the intermediate columnar zone with elongated columnar dendrites, and iii the central equiaxed zone with relatively large equiaxed grains Fig 1.
Fig 1 Three distinct macro-structure zones in steel ingots Besides the referred three zones, a region where the outer columnar dendritic structure transfers to the inner equiaxed grain structure is commonly observed. Fig 2 Macro-segregation phenomenon during solidification in ingot mould All types of macro-segregation are derived from the same basic mechanism which is the mass transfer during solidification.
Convective flows due to density gradients caused by temperature and composition variations in the liquid. The thermal and solutal buoyancy contributions can either aid or oppose each other depending on whether local temperature and concentration fields cause liquid density to increase or decrease. Equiaxed grains in steels are denser than the surrounding liquid and hence they tend to sink. This mechanism, along with convective fluid flow, is a dominant macro-segregation process in the steel ingots.
Flow to account for solidification shrinkage and thermal contraction of the liquid and solid on cooling. Deformation of the solid network due to thermal stresses, shrinkage stresses and metallo-static head the pressure provided by the liquid steel above.
Imposed flows due to pouring, applied magnetic fields, stirring, rotation etc. In order to achieve this VESUVIUS has a range of products and services aimed at reducing undesirable segregation and increasing the yield of sound steel in the final ingot.
These products and services include:. Solutions for Ingot Casting. Solutions for Ingot Casting When steel is poured into the ingot mould, progressive solidification starts from the walls and the base of the mould, moving inwards towards the thermal centre or axis. Want to know more? London Bullion Market Association.
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