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. Numerous mechanisms for CET have been proposed, and can be mainly divided into two types namely i mechanical blocking hard blocking , and ii solutal blocking soft blocking.
Mechanical blocking mechanism considers the blocking effect of the equiaxed grains on columnar grains that if the equiaxed grains are large enough or their volume fraction is large enough, CET will occur, whereas solutal blocking mechanism takes the solutal interaction into account that if the solute rejected from the equiaxed grains is sufficient to dissipate the undercooling at the columnar front, CET can take place.
Steel ingots have internal discontinuities. They contain non-metallic particles of different chemical composition and size, as well as sites with different chemical composition of the steel. The process of ingot solidification and the internal discontinuities are affected by a number of factors. These factors are i shape of the ingot or the mould, ii cooling rate or the rate of solidification, iii size of the ingot, iv temperature of liquid steel and the speed of casting, and v chemical composition of the steel.
On the solidification of alloys liquid steel , solute is partitioned between the solid and liquid to either enrich or deplete the inter-dendritic regions. This naturally leads to variations in the composition on the scale of micro-metres micro-segregation. Macro-segregation, however, refers to chemical variations over length scales approaching the dimensions of the casting, which for large ingots may be of the order of centimeters or metres.
Micro-segregation can be removed by homogenization heat treatments, but it is practically impossible to remove the macro-segregation because of the distances over which species are required to move.
Almost all macro-segregation is undesirable since the chemical variations can lead to changeable microstructural and mechanical properties. The effects of macro-segregation are critically important in such applications and the ability to predict segregation severity and location is highly sought after.
The first examinations of macro-segregation phenomena in steel ingots were carried out many decades ago and since then the understanding of the processes leading to segregation has improved considerably. However, the macro-segregation can still be observed in ingots made today.
The macro-segregation can be in the form of A-segregation, V-segregation and negative base segregation as shown in Fig 2.
Up to the mids, solute buildup at the tips of advancing solid interface has been believed to be the predominant underlying cause of macro-segregation phenomena in ingots. However, this reason has since been demonstrated to be erroneous by numerous theoretical and experimental investigations. It is now well recognised that the majority of solute is rejected sideways from a growing dendrite, enriching the mushy zone, and that build up in front of dendrite tips is negligible in this regard.
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. The movement of enriched liquid and depleted solid can occur through a number of processes as shown in the Fig 2 and described below.
This is because of the different solubility of elements in liquid and solid steel. There are two types of defects namely i external defects or surface defects, and ii and internal defects. The surface defects are external scales, longitudinal tears, transverse, skewed, or zigzag tears, cracks, cold shuts, superficial voids, and Slag and sand nodes on the surface. Other than segregation, the internal defects are pipe formation, blow holes, flakes, and exogenous and endogenous inclusions.
Some of these defects are described below. Pipe formation — Liquid steel contracts on solidification. The volumetric shrinkage leads to formation of pipe. In killed steels pipe formation occurs toward the end of solidification. Fig 3 shows primary and secondary pipe in narrow end up mould and in wide end up mould while casting killed steel. Only primary pipe can be seen in wide end up mould. Rimming and semi-finished steels show very less tendency for pipe formation.
Wide end up moulds show smaller pipe as compared with narrow end up mould. The portion of ingot containing pipe has to be discarded which affects yields. 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. The steel is then teemed poured into a series of ingot molds. After the ingots solidify, the ingot molds are stripped and the ingots are placed in soaking pits for heating and to equalize the internal and external temperature.
These elements will react readily with oxygen, forming metallic oxides. Metallic oxides float in the melt and can be removed with the slag. The three basic types of steel ingots produced in industry today, represent three different degrees to which the steel melt is deoxidized. Rimmed steel, usually low carbon steel, is deoxidized to the least degree. In rimmed steel gases released during the process form spherical blow holes within the material, particularly along the outer rim.
Impurities will tend to segregate more towards the center of the casting. Material flaws in rimmed steel due to impurities can be an issue.
The goal when producing rimmed steel is an ingot with a sound outer skin. Blow holes within the metal may be closed up in latter forming processes. They are acceptable as long as they do not break through the outer skin, becoming exposed to the atmosphere.
Rimmed steel has little piping if any, since the space taken by the gas pockets negates the space taken by shrinkage due to solidification. Semi-killed steel is partially deoxidized by the addition of oxygen combining elements. Since semi killed steel is not completely oxidized, gases still form in the melt and create blowholes. Gas porosity is much less than rimmed steel and is usually more prevalent in the upper portion of the ingot. A semi killed steel ingot may have a little bit of piping.
The semi-killed manufacturing process is an economical method of creating steel ingots. Killed steel is produced by the full oxidation of the metal melt. Enough oxygen combining elements are added to the melt that the formation of metal oxides eats up all of the oxygen preventing the development of gas. Metallic oxides can form solid inclusions or combine with the slag, and can be scooped out with or along side it.
The molten steel will sit quietly when the ingot is poured, hence the name killed. Killed steel has excellent chemical and mechanical properties that are uniform throughout the material. The severity and distribution of this, depends on several factors including the quality of steel, superheat at the time of pouring, method of pouring whether direct or indirect and the dimensions and taper of the mould. Such cavitation is also associated with impurities, which concentrate, by segregation forming an undesirable distribution of undesirable elements in the final product.
It is thus important to influence the amount and position of cavitation by reducing it to a minimum and locating it where it will be least harmful in the solidified ingot. 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.
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