Year

2015

Degree Name

Doctor of Philosophy

Department

Faculty of Engineering and Information Sciences

Abstract

The thin solidifying shell in the meniscus region of a continuous caster deforms due to severe contraction in steels of near-peritectic composition, thereby exacerbating the risks of cracking and in the extreme, breakouts. The main purpose of the present study was to contribute to an improved understanding of the fundamentals underpinning the solidification behaviour of selected steels of industrial importance and to relate these fundamentals to industrial practice. A further aim was to explore the potential of countermeasures aimed at avoiding or at least minimizing surface defects that originate as a result of the occurrence of the peritectic phase transition.

Several experimental techniques were employed to simulate various aspects of solidification events that occur in the meniscus region of an operating continuous caster. In-situ observations of solidification in addition to subsequent solid-state phase transformations were made by employing a concentric solidification technique in a high-temperature laser-scanning confocal microscope (HTLSCM) in real time and at temperature. It is possible by this technique to effectively simulate early solidification events occurring in an operating continuous caster and the solidification behaviour of a series of steel of near-peritectic composition and of industrial importance have been assessed and compared. Differential scanning calorimetry (DSC) was employed to determine the exact phase relationships in the selected steels as a function of temperature as an aid to the interpretation of the outcomes of the solidification studies. A potential countermeasure that can be taken in industrial practice to counteract uneven solidification is to texture the mould surface. In order to explore and evaluate the effect of different mould surface textures on reducing the unevenness of the solidifying, a simple dip-test was developed, followed by more sophisticated experiments and analyses in a continuous casting mould simulator. In parallel to the texture studies in the mould-simulator certain aspects of the interaction between mould, strand and mould-flux have also been assessed. The stresses encountered in and development of these stresses in the course of solidification, were assessed by the use of a submerged split-chill contraction (SSCC) technique. In an attempt to relate the fundamental studies to operational practice, an in-plant study of off-corner crack formation was conducted.

The solidification studies conducted in a high-temperature laser-scanning microscope revealed that the velocity of the liquid/delta-ferrite interface decreases with the extent of segregation occurring in the early stages of solidification. An increase in alloy content of steels and/or an increase in cooling rate exacerbate segregation during solidification. It is understood that the migration of the interface requires the diffusion of large amounts of solute across the interface, implying that the delay in shell growth in the very early stage of solidification in mould might be attributed to this segregation. This is an important observation since it means that the initial rate of solidification in industrial practice is a strong function of steel composition, but that the influence of alloying elements on the rate of solidification can be quantified through experimental simulation studies. The rate of the solid-state delta-ferrite to austenite phase transformation in the presence of liquid phase is significantly higher in hypo-peritectic steels than in the other steels investigated, even at low cooling rates. The higher undercooling of these steels before nucleation of austenite initiates has been attributed to the pre-existence of a higher fraction of primary solidified deltaferrite in the steels of hypo-peritectic composition. However, it is not the fraction of pre-existing delta-ferrite as such that causes the deeper undercooling, but rather the higher solute concentration gradients that develop in the delta-ferrite phase in these steels that leads to an increased rate of the subsequent delta-ferrite to austenite phase transformation.

In order to illustrate the ill-effects of uneven solidification in a thin-slab caster in industrial practice, the case of off-corner cracking of titanium bearing steels close to peritectic composition was selected for further study. This in-plant study revealed that off-corner cracks can occur as a result of excessive growth of austenite grains caused by the peritectic phase transition and/or by the precipitation of fine TiN precipitates at grain boundaries.

Experiments conducted by the simple dip-test have shown that textured surfaces with 2 mm interval grooves provided the best mould surface to produce even solidification. These grooves also provided the most extensive shell growth through the improved contact between shell and textures mould surface. This observation is attributed to the effect of grooves on the dispersion of deformation sites in the thin shell. In mould simulator experiment, with the application of mild cooling mould flux the growth of shell in hypo-peritectic steel became much evener. Also with mild cooling much thicker shell grew through more intimate contact between shell and mould. The mould simulator experiments revealed that the use of a ‘mild cooling mould flux’ provided conditions conducive to high shell growth rates and even shell formation in casting hypo-peritectic steel. This observation is also attributed to the intimate contact between shell and mould as a result of the reduced heat extraction rate achieved with the ‘mild cooling mould flux’.

In industrial practice it is difficult to predict the solidification behaviour of a specific steel design. The experiments conducted by DSC techniques provided convincing evidence that the susceptibility to the ill-effects of the peritectic phase transition of a given steel cannot be predicted by currently available thermodynamic software or by the calculation of the carbon equivalent alone. It is necessary to determine the absolute values of the phase boundaries as a function of temperature. It was shown that DSC technique is a tool for the identification of steels that will be subject to undergoing the peritectic transformation during continuous casting. It was found that in the case of at least one steel, DSC measurements proved it to be of hypo-peritectic composition whereas thermodynamic software predicted it to be of hyper-peritectic composition.

It was possible to measure the stress developing in the course of solidification in each of the steels investigated by the use of the SSCC test. It was also found that the effect of a reduction in heat transfer by the use of a zirconium oxide coating in the SSCC test on decreasing the contraction forces is more pronounced in the case of hypo-peritectic steels.

In summary: An attempt was made to explore practical measures (or the directions towards improvement) aimed at minimizing defects resulting from solidification events in high-speed continuous casting processes such as uneven shell formation, depressions, cracks and even breakouts. Specific attention was paid to the high stresses that result from the massive-type of phase transformation occurring in the meniscus region. The present study confirmed that it is necessary to gain a comprehensive understanding of the factors, fundamental and operational, that impact on solidification in order to predict the solidification behaviour of steels that fall within the peritectic composition range. Steel composition, casting speed and cooling rate play dominant roles. Two distinct occurrences close to the liquid/solid interface in a solidifying steel shell in a continuous casting mould were identified as of paramount importance: A massive type of deltaferrite to gamma-austenite transformation with planar liquid/solid interface morphology in the very initial stages of solidification in the meniscus region, which is caused by solute elements build-up in the liquid ahead of the growing solid/interface (partitioning) and a diffusionally driven deltaferrite to gamma-austenite transformation upon further solidification in the region of dendritic growth as a result of smoothed concentration gradients. Hot-spots need to be avoided at all cost and the development of a thin shell of even thickness in the region below the meniscus is a pre-requisite to preventing casting defects.

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