Powder functions and properties

A tundish covering powder is placed on top of molten steel in the tundish.
In general, the tundish covering powder should have the following functions:

Tundish powders are mainly divided in two categories: acid and basic. The main component of acid powders is SiO₂ while lime (CaO) is the main components of basic powders. When using an acid covering powder the steel cleanness can be weakened by oxygen pick-up – reoxidation – from the covering powder because the oxygen potential of an acid slag is by far higher than that of the steel.
For high grade steels basic tundish covering powder is preferred. It can get mixed without great problem with the rest of the basic ladle slag. Basic covering powders in the tundish are also beneficial for the lining because they do not readily react with basic refractories.
Tundish covering powders with high basicity still have problems when casting long sequences without changing tundish because the covering layer becomes hard in a few hours. This creates big problems when the steel level in tundish varies, for instance when changing ladle, or when blocking the stopper rod, as well as causing problems in sampling and measuring temperature in the tundish.
Usually, basic powders are used together with acid powders (the so-called double covering layer) in order to fulfil the three functions listed above as follows:

Both in case of one powder (acid) or two powders (basic + acid), the total covering layer becomes a complex multi-layer system.
Along the layer thickness there is a variation of physical status, chemical composition and structure of the oxide system.
During the service a continuous variation of this system occurs, with consequent degradation of performance: loss of thermal insulating ability and other desirable abilities like inclusion adsorption and low oxidising tendency.
The degradation is caused by dissolution of refractory materials (stopper rod, ladle shroud and tundish lining), inclusion capture and, over all, ladle slag carry over. This is in many cases a limit for the casting sequence for several steel grades.
Thus, in many cases the performance of the covering powder is a bottle neck for steelmaking productivity and a problem for steel quality. Nevertheless, a complete physical and chemical knowledge of the mechanisms acting and controlling the properties of the covering system has not yet been achieved.

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State of the art

Considering that the primary function of the tundish cover powders is threefold:

the tundish powder performance during use depends on melting and solidification temperature, viscosity, thermal insulation, refractory compatibility and oxide absorption capability.

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Thermal Insulation and re-oxidation prevention

In tundish, provision of thermal insulation and prevention of re-oxidation is obtained by:

Thermal insulation

It has been reported that the cover powder could loose its thermal insulation ability and allow the slag to crust because of temperature and composition gradient through the covering layer. Different theories of crust formation have been proposed. For example, according to one theory, firstly the crust forms at the top surface of the liquid slag layer and then it grows upwards to the powdery layer because of the thermal gradient.
The fairly fluid slag can wet the grains of the tundish cover powder, causing them to agglomerate. Along the thickness of the tundish powder layer there will be a point where the powdery layer, on its point of fusion, is in contact with the slag on its point of solidification, hence creating a rigid layer. This rigid raft on the steel surface causes problems for taking tundish samples as well as to the movement of the ladle shroud. On the chemical side, it is observed practice that increasing C level in Alumino-Silicate slags changes the heavy rigid nature of the crust to the highly viscous but plastically deformable layer but the exact reason for this behaviour is not known.
At present, the exact mechanism of crust formation has not been quantified as well as the effect of external oxidation, cast sequence length, ladle slag carryover and internal chemical reaction.
In the past, acid powders, i.e. rice husks rich in SiO₂ (85-90%), were used as an excellent thermal insulator but it was found that they are not well suited for inclusion absorption. Moreover, in order to prevent steel re-oxidation, some acid tundish fluxes also contain Carbon, this could cause a problem of C-pick up by steel. Similarly H pick up by steel is also reported when using acid fluxes.
Basic powders enriched in CaO and/or MgO for improving thermal insulating ability have been used, but these powders are less efficient to control steel re-oxidation.
It has also been reported that the insulation properties of basic powders can be improved by changing their chemistry. For example, it is shown that the addition of TiO₂ to CaO-Al₂O₃-SiO₂ powders reduces the radiative component of heat transfer, which provides a means of improving insulation in the tundish slag and decreasing the possibility of crusting. However, using TiO₂-including tundish powders, could be problematic when casting some special steel grades more sensitive to carbide formation.

Reoxidation prevention

One of the main functions of tundish powders is to prevent the reoxidation of the steel melt.
This reoxidation is due to:

The prevention of extrinsic reoxidation of the steel requires a continuous impermeable barrier between the steel and the atmosphere. Normally, powders with fusion temperature lower than the temperature of the liquid steel are fed when the tundish is only partially filled in order to limit the exposure time of the molten steel surface to the oxidising atmosphere.
The steel reoxidation in tundish is mainly at the boil area around the ladle shroud; this re-oxidation is responsible for powder chemistry changes.
An inappropriate slag chemistry, for instance with high levels of reducible oxides (such as FeO, MnO and SiO₂) becomes a source of oxygen through redox reactions and oxygen transport. Hence tundish powder chemistry must be adjusted to maintain low levels of reducible oxides. For example, in non-desulphurised Al-killed steel, reoxidation products (FeO, MnO, SiO₂ etc.) are present in addition to Al₂O₃. In this case the slag becomes a source of reoxidation through its reaction with the dissolved aluminium in the steel and also by acting in a transport mechanism for moving oxygen from air through highly oxidised slag. There is a co-relation between FeO+MnO contents of the tundish slag and the steel total oxygen contents.
It is also possible that redox reactions (as shown below) occur at the steel surface which may transfer silicon and manganese to the steel melt hence increases the Si and Mn to an undesired level:

2Mn + 3SiO₂   2MnO.SiO₂ + Si

4Al + (3+x) SiO₂     2Al₂O₃.xSiO₂ + 3Si

It is also recognised that the change in composition of tundish slag depends on the steel chemistry in the ladle before tapping to tundish.

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Inclusion absorption

An important role of avoiding contamination of the steel and facilitating further separation of inclusions is given to the tundish. The idea of increasing retention time for inclusion floatation in the tundish has been pursued by gradually increasing the tundish capacity.
In general, non-metallic inclusions will be removed from the liquid steel if they come in contact with the free surface (the interface between liquid steel and tundish cover powder). The physical, thermophysical properties of the interface are very important in assimilating the floating inclusions. For example, the viscosity of tundish flux at the interface plays an important part in absorbing inclusions. Too high a viscosity (mainly acid powders) limits inclusion absorption capability while too low a viscosity (mainly basic powders) can lead to entrainment of the flux into the steel during unstable conditions, particularly if the metal level drops too low causing vortexing. It is also noted that Al₂O₃ addition increases the viscosity of the flux so initially high Al₂O₃ fluxes should not be used. Since the prime requirement for absorbing most of the deoxidisation and reoxidation products is the presence of liquid slag, it is recommended the use of low melting fluid basic fluxes (sometime termed “Active powders”) from the start.
This is best achieved by insuring that the initial melting and solidification temperatures are well below steelmaking temperatures.
The other side of the picture is that unfortunately any flux, which has a high capacity for absorbing inclusions, will also tend to be aggressive towards tundish refractories (particularly Al₂O₃ based). For the same reason, mould powders are not used as tundish powders since they contain high amount of fluidisers such as calcium fluoride (CaF₂). In order to minimise refractory corrosion, it has been suggested to use either:

Low interfacial tension between the tundish slag and steel is suggested to enhance inclusion assimilation. However, a very low interfacial tension between steel and molten tundish powder (mainly due to high S and O in steel) results in a Marangoni effect. In this case, non-metallic inclusions may transfer back to the body of the liquid steel.
Some work on the operational parameters and processing has also been carried out in the past to improve the inclusion assimilation. For example, increasing the residence time in the tundish by incorporating weirs and dams in the tundish geometry has been shown to increase the chance of inclusion floatation out to the interface.

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Present situation and open problems

The simple solution of achieving good thermal insulation and inclusion assimilation capability of tundish powders is to keep their chemistry constant during the operation, but it is very difficult under practical dynamic conditions.
The slag chemistry can be controlled to some extent. Possible actions, currently pursued, are:

All of these actions are based on thermodynamic considerations and practical experience.