Ingot Defects

Today, more than 90% of all steel semi-finished products are continuously cast. Ingot casting production is increasingly concentrated on special alloys and products, which can only be produced by this process and where all of the typical quality issues associated with ingot casting are accepted. Steel ingots are subsequently subject to further processing steps, the most important of which is forging.

Steel ingots are subsequently subject to further processing steps, the most important of which is forging. There is no doubt that proper quality control and cost savings throughout the whole production process are key factors for a competitive production. The quality of the as-cast ingot is the starting point for all of the subsequent heat treatment and deformation steps. There is a need for a through-process methodology to predict possible defects and to optimize the whole process chain such that the best possible quality and lowest reject rate is obtained.

Most major quality problems in ingots originate from the casting process. Defects like shrinkage, porosity, segregation, non-metallic inclusions and cracks are initiated during teeming of the liquid steel and/or during solidification in the mould. There are various parameters of the casting process that can be modified in order to limit defects and, if not completely prevent their existence, reduce their number and appearance so that the product fulfils the quality specification.

Defects in ingot casting:

Shrinkage and porosity
The specific solidification pattern of ingots leads to a characteristic shrinkage appearance. There is always a shrinkage cavity in the hot top, but it has to be assured that this primary shrinkage does not extend into the block. In case of an unfavourable solidification pattern, shrinkage can also appear inside the block, far below the hot top. Dissolved gases can also influence porosity development in a steel ingot.
In many cases, problems with centre-line porosity are reported. This porosity is small in comparison to the hot top shrinkage cavity and is found along a line in the centre of the block.
Porosity in ingot casting is influenced by various factors like insulating powder, hot top insulation, hot top geometry, ingot height and diameter (H/D), ingot conicity and so on. Depending on the size and position of porosity, it is possible to close them in subsequent hot deformation process, e.g. forging. Casting process simulation can be applied to optimize the casting process to prevent porosity from being formed. If its presence is inevitable, it is of importance to transfer information about the size and position of the porosity to the deformation simulation. There, it is possible to determine the forging process parameters that are required to close the porosity or to maximize the yield of the final product.

Macrosegregation
Segregation is an in inhomogeneity of the concentrations of alloying elements and impurities in the steel. Macrosegregation is differentiated from microsegregation dependent on the particular scale at which the concentration differences are observed. Most alloying elements are more soluble in the liquid phase than in the solid phase. Thus, as the metal solidifies, alloying elements in the mushy zone (solidifying liquid-solid mixture) are rejected from the growing solid dendrites into the neighbouring interdendritic liquid. This liquid becomes increasingly enriched with alloying elements as solidification proceeds. On the scale of the dendrites (tens to hundreds of microns), segregation results in a nonuniform solute distribution in and between the dendrite arms. This is termed microsegregation.
The movement of liquid melt or the liquid-solid mixture during solidification lead to a spread of these micro-scale concentration differences over larger areas up to the scale of the whole ingot or parts of it. The resulting inhomogeneities in concentration are called macrosegregation.
Macrosegregation can result in a cast ingot with regions having a composition quite different from the nominal value, either being higher (positive segregation) or being lower (negative segregation). A state-of-the art model to simulate thermo-solutal convection and macrosegregation is described in literature. This model has been extensively applied to the simulation of the solidification of heavy steel castings. It has also proven to give good results in application to heavy ingots.
Segregations lead to locally lower material properties. The locally chemistry variations can lead to a different thermochemical behaviour, e.g. when it comes to forming precipitates or local hot spots that induce shrinkage.

Inclusions
Cleanliness is a topic of basic importance in steel production. Non-metallic (oxidic or sulphuric) inclusions lead to a local degradation in mechanical properties. For example, they decrease the ductility and fracture toughness of the steel.
Large inclusions are a reason for fatigue problems. When brittle oxide inclusions, embedded in the steel matrix, are exposed to deformation during rolling or forging, they can lead to cracks. Inclusions close to the ingot surface can cause various kinds of surface defects.
Inclusions can have various sources. They can be deoxidation products (carried over from the ladle), slag particles, refractory particles, particles from hot top insulation or casting powder, reoxidation products generated during casting or precipitates from solidification and cooling. The inclusion size ranges from 10 µm up to around 1 mm. It is known that the majority of those inclusions that are most critical for the steel properties are caused by reoxidation during casting. Experimental observations have shown that the fraction of reoxidation inclusions is between 60% and 83% of the total.
The most common oxygen source for reoxidation is the exposure of the free melt surface to air during teeming. An important factor is a proper shrouding of the steel when being poured from the ladle.
Nevertheless, it is of decisive importance to take care of the flow of liquid steel in the runner system and inside the mould. In many cases, the flow is highly turbulent and the steel melt enters the mould through the nozzle with a high velocity. Particularly at the beginning of the teaming process, this can lead to heavy splashing.
With casting process simulation, the flow during teeming for a combination of parameters like runners and nozzle geometries, teeming rates and temperatures, etc. can be investigated. Potential actions for process optimization can be investigated virtually. Particles can be monitored during the filling process and during their movement caused by convective flow during solidification. The formation, growth and agglomeration of reoxidation particles can be followed. Thus, a proper prediction of steel cleanliness for the cast product is possible. A properly designed casting process should reduce the number of inclusions, particularly of large ones, inside the solidified cast ingot. The majority of inclusions should float up into the top slag. The information about which kind of inevitable inclusions are to be expected at which position of the block should be transferred to the deformation simulation.

Cracks
During cooling, residual stresses build up in the different layers of the solidifying ingot. The stress development strongly depends on the heat transport properties of the steel and the cooling rate. If the already solidified outer shell cools down and thus shrinks rapidly in comparison to the inner area, cracks can occur. In order to prevent these cracks, there is a need to properly control the cooling rate and/or for a hot transfer to the deformation (forging or rolling) process.
The stresses that are built up in the ingot and also in the mould can be simulated. By comparison of these stresses with the known material properties, it is possible to predict cracks that are initiated during casting. Based on further information from solidification, the simulation can also identify regions in danger of hot tearing. It is very advantageous for the optimization of the forged product to transfer information about cracks potentially formed during casting to the forging simulation.

Reference: I. Hahn, M. Schneider, J. Terhaar, J. Jarolimeck, R. Sauermann, Quality Prediction of Cast Ingots, MagmaSoft, 2012.

Copyright © 2017 by Steel Data. All Rights Reserved.