Analysis of a model for the formation of fold-type oscillation marks in the continuous casting of steel

This paper investigates the different possible behaviours of a recent asymptotic model for oscillation-mark formation in the continuous casting of steel, with particular focus on how the results obtained vary when the heat transfer coefficient ($m$), the thermal resistance ($R_{mf}$) and the dependence of the viscosity of the flux powder as a function of temperature, $\mu _{f}\left ( T\right ),$ are changed. It turns out that three different outcomes are possible: (I) the flux remains in molten state and no solid flux ever forms; (II) both molten and solid flux are present, and the profile of the oscillation mark is continuous with respect to the space variable in the casting direction; (III) both molten and solid flux are present, and the profile of the oscillation mark is discontinuous with respect to the space variable in the casting direction. Although (I) gave good agreement with experimental data, it suffered the drawback that solid flux is typically observed during actual continuous casting; this has been rectified in this work via alternative (II). On the other hand, alternative (III) can occur as a result of hysteresis-type phenomenon that is encountered in other flows that involve temperature-dependent viscosity; in the present case, this manifests itself via the possibility of multiple states for the oscillation-mark profile at the instants in time when solid flux begins to form and when it ceases to form.

Formation of Surface Depression during Continuous Casting of High-Al TRIP Steel

High aluminum transformation-induced plasticity (TRIP) steels offer a unique combination of high tensile strength and ductility, high impact energy absorption and good formability. The surface of the slab is prone to depressions and longitudinal cracks during continuous casting due to the high Al content in steels. Surface depressions of the 1.35 wt.% Al-TRIP steel slab in a steel works were investigated by scanning electronic microscopy (SEM) and mold fluxes with different Al2O3/SiO2 ratios were researched by thermodynamic calculations and high-temperature static balance experiments. The results show that some micro-cracks were distributed along the grain boundary in the surface depression of the slab. Inclusions containing K and Na, which were probably from mold flux, were found in the depression samples. Meanwhile, the components of reactive mold flux showed significant variation in their chemical composition during the continuous casting process of the Al-TRIP steel. A large number of depressions and irregular oscillation marks on the Al-TRIP steel slab surface were generated due to serious deterioration in the physical properties of the mold flux. Since the TRIP steel is a typical hypo-peritectic steel, the overly large thermal contraction and volume contraction during initial solidification is the intrinsic cause of surface depression. The change of mold flux properties during casting aggravates the formation of depressions.

Measurements of surface velocity and level fluctuation in an actual continuous wide slab casting mold

In the current work, a kind of nailboard made of three rows of nails was designed to simultaneously measure the surface velocities and level fluctuations of the loose side, the center plane and the fixed side in an actual continuous wide slab casting mold. Seven nailboard dipping experiments were conducted to obtain both the transient and time-averaged behaviors near the meniscus. The distribution of disturbed oscillation marks (OMs) on the surface of corresponding slabs (in common with nailboard experiments strand) was systematically studied. In order to investigate the relationship between disturbed OMs and entrapment of inclusions or slag to the solidified shell, samples were taken at 10 locations on one relevant slab, and they were polished 2 mm each time. Inclusions larger than 10 µm were analyzed using the optical microscope. The level fluctuates with the continuous casting processing. The level positions on the fixed side and loose side may not be consistent. Surface level near the submerged entry nozzle (SEN) and the narrow face fluctuates on a higher position, while on the region between the SEN and the narrow face, the level fluctuates on a lower position. Time-history of the level fluctuation on the loose side and the centerline is synchronous, while the flow fluctuates in another pace on the fixed side. Root-mean-square (RMS) value of level fluctuation varies along the mold width. There are peak values at the locations near the SEN and narrow face, where slag entrapment may be induced. The OMs are more likely to be disturbed at the middle and 1/4 location in mold width. There is a big difference in entrapment of inclusions at the solidified shell between loose side and fixed side. On the fixed side, the distribution of number density of inclusions is in agreement with number distribution of disturbed OMs on meniscus, while more inclusions were entrapped at narrow faces on the loose side.

On the formation of fold-type oscillation marks in the continuous casting of steel

Asymptotic methods are employed to revisit an earlier model for oscillation-mark formation in the continuous casting of steel. A systematic non-dimensionalization of the governing equations, which was not carried out previously, leads to a model with 12 dimensionless parameters. Analysis is provided in the same parameter regime as for the earlier model, and surprisingly simple analytical solutions are found for the oscillation-mark profiles; these are found to agree reasonably well with the numerical solution in the earlier model and very well with fold-type oscillation marks that have been obtained in more recent experimental work. The benefits of this approach, when compared with time-consuming numerical simulations, are discussed in the context of auxiliary models for macrosegregation and thermomechanical stresses and strains.

Impact of Oscillation Parameters on Surface Quality of Cast Billets

The paper is focused on impact of different oscillation parameters on surface quality of peritectic steel grades cast into billets 150x150 mm. Hydraulic oscillation used for this purpose was temporarily installed on one strand of the billet caster. Hydraulic oscillation enables, in comparison to ordinary used electromechanical oscillation, flexible set-up of basic parameters of the oscillation cycle (negative strip time and its ratio). Proper oscillation mode is capable to assure regular oscillation marks development, good lubrication in the mould and adequate compression of the solidifying shell. Impact of an oscillation mode providing negative strip time 0.085 s and its ratio -50 % on surface quality of cast billets is compared with standard oscillation mode applied on strands equipped with electromechanical oscillation characterized with variable negative strip time between 0.084 and 0.096 s and fixed negative strip ratio to -14 %.

Background of Branched Crack Formation in Surface Zone of Continuously-Cast Ti-Nb Microalloyed Steel Slab

Branched cracks are undesirable and dangerous, and under the oxidized surface of a continuously-cast slab they are mostly difficult to identify. They can cause the formation of defects in thick sheet or on thin strip. This work deals with the study of the background to the formation of branched cracks in a continuously-cast slab, at pulling rate 0.43 m.min-1. The results of the work show that branched cracks extended to a depth of 10 mm below the slab surface and occurred mostly below oscillation marks. Cracks were mostly present in the edge parts of the slab, where they were also deeper in comparison with locations in the middle of the slab width. Microstructural analysis confirmed heterogeneity of ferrite grain sizes in the slab surface skin. The microstructure was formed predominantly of polyhedral ferritic-pearlitic grains, and on the ferritic grain boundaries the presence of tertiary cementite was observed. In the area around the cracks mainly non-equilibrium microstructure of acicular character was observed. A typical feature of these cracks after their opening is intercrystalline fracture. Wavelength-dispersive X-ray analysis of a sample with a branched crack showed chemical heterogeneity of harmful elements, mainly S and As. Large amounts of these elements point to their significant segregation. The formation of branched cracks is conditioned not only by segregation of impurities, but possibly also by the presence of undesirable brittle cementite networks. The presence of cementite was confirmed not only by microstructural analysis, but also diffraction of hard X-ray radiation. Precipitation of microalloying elements in connection with the cementite can lead to higher probability of surface crack formation, as also confirmed in this study.Keywords: slab, branched cracks, oscillation marks, segregation, precipitation