The Casting Powders Book

This chapter provides an introduction into the use of casting powders in the continuous casting of steel. It includes brief descriptions of the following:Finally, definitions are given of various terms used in this book.

It is essential to lubricate the shell since inadequate lubrication leads to defects in the steel product (e.g. longitudinal cracks, sticker breakouts and star cracks). The liquid layer of the slag film, formed between the shell and the mould, lubricates the newly formed shell; the lubrication increases with increasing liquid slag thickness (d_l). Lubrication is usually represented by the powder consumption (Q_s in units of kg slag (or powder) m⁻²) which is related to liquid film thickness (d_l). However, there are several terms used for powder consumption and these terms are interrelated (e.g. Q_s, Q_t and Q_MR). The frictional forces acting on the shell are highest in the centre of slabs and thus slabs need more lubrication. The required powder consumption, Q_s increases with increasing distance from the corner and thus Q _s ^slab  > Q _s ^bloom  > Q _s ^billet . The required powder consumption can be calculated from the relation, Q _s ^req  = 2/(R* – 5) where R* = {2(w + t)/w · t} =  (surface area/volume) of the mould. However, the powder consumption, Q_s, is also affected by other parameters, namely, the casting speed (V_c), slag viscosity (η), the break temperature of the slag and the oscillation frequency (f) and stroke (s). There is general agreement that Q_s decreases with increasing casting speed and viscosity (e.g. empirical rules, Q _sreq ^slag  = 0.55/ η^0.5 · V_c). There is some dispute with regard to the effect of f, s and T_br but most plant studies indicate that Q _{s req} ^slag decreases as f, s and T_br increase. The required values of powder consumption and viscosity can be calculated for the given casting conditions using empirical rules. The predictions of a mathematical model indicate that slag infiltration into the model/ strand channel occurs when the mould and slag rim are descending but little powder consumption occurs when the mould is ascending. The changes in mould direction are accompanied by periods of confused flow in the mouth of the channel and little slag infiltration occurs in these periods. Frictional forces and the factors affecting them are also discussed; it was found that liquid friction increased with increasing mould dimensions, slag viscosity, casting speed and (V_m − V_c). Plots of liquid friction (F_l) versus casting speed exhibit a minimum since F_l increases with increasing V_c but decreases with decreasing (V_m − V_c).

The condition of the shell is paramount in continuous casting. The heat transfer from the shell is important because it determines how thick the shell is and, consequently, how strong the shell is. Four aspects of the heat transfer are considered here, namely, (i) horizontal heat transfer, (ii) shell solidification, (iii) vertical heat transfer and (iv) the variability in heat transfer. The horizontal heat transfer occurs across the slag film separating the shell from the mould. Heat is transferred by two mechanisms, lattice conduction and radiation conduction. The latter is usually controlled by (i) manipulation of the amounts of glassy (f_gl) and crystalline phases (f_crys) in the slag film and (ii) to a lesser extent, by incorporating transition metal oxides into the mould powder to absorb the IR radiation. The key properties of the slag film are (i) the thickness of solid slag film (which is dependent on the solidification or break temperature) and (ii) the fraction of crystalline phase (f_crys) formed in the film (which (a) reflects the IR radiation and (b) creates an interfacial resistance, (R_Cu/sl) during crystallisation). In practice, all of these factors increase with increasing basicity (C/S) of the mould slag. Other factors like the effect of casting conditions (e.g. casting speed, metal flow pattern) on the heat flux (q_hor) are discussed. The factors affecting shell thickness (d_shell) are discussed below. There are two regimes controlling shell growth (i) a period of slower growth for the initial period t ≤ 0.05 s and (ii) a subsequent period of linear growth of d_shell which exhibits a linear relation with t^0.5. Vertical heat transfer is controlled through (i) the depth of the bed, (ii) use of exothermic mould powders, (iii) to a less amount, by the particle size of the powder and (iv) by use of electromagnetic braking (EMBr) in the mould which reduces the efficiency of vertical heat transfer. Gaseous convection is shown to be a major contributor to the vertical heat flux (q_vert) and the permeability of the powder bed is a key factor affecting q_vert. Local variations in heat flux are an issue because an uneven shell can lead to longitudinal cracking. The various causes of local variations in shell thickness are discussed.

The mould slag plays a key part in the continuous casting process. The mould slag carries out a series of tasks and it is essential that the slags perform each of these tasks efficiently. This chapter looks at these tasks and analyses the key factors affecting performance and suggests ways in which the slag can be manipulated to perform efficiently. The following processes are examined: (1) Thermal insulation of the powder bed and vertical heat transfer (2) Melting rate of the mould powder (3) Formation of a slag pool (4) Control of powder consumption and lubrication of the shell (5) Control of Horizontal heat transfer and the effect of the thickness of the solid slag film (6) Control of crystalline and glass phases in the slag film and (7) Control of both horizontal and vertical heat transfer in order to delay solidification of the shell. The proposed actions are summarised in a table.

Continuous casting is a complex process affected by a number of factors and it is necessary to optimise these factors (e.g. shell lubrication and heat extraction) to minimise product defects and process problems. The mould powder is expected to alleviate these problems. However, the casting conditions have a pronounced effect on factors like slag infiltration and heat transfer and this requires careful selection of mould slag properties to provide optimum casting conditions. The effect of individual, casting variables (e.g., casting speed) on the casting process is discussed in this chapter and the following casting variables are examined and discussed: (i) mould characteristics (e.g., dimensions, taper, coatings) (ii) oscillation characteristics (iii) casting speed (iv) metal flow characteristics (v) metal-level variations (vi) fluctuations (in metal flow, mould level, etc. (vii) the application of electromagnetic devices (viii) steel grade being cast (ix) water flow rate. The effects of these individual factors are discussed and how the mould powder can be used to optimise the casting process.

The mould powder must carry out a series of tasks. Arguably, the most important of these tasks is the formation of a slag film which provides the optimum level of lubrication and heat extraction from the shell. These properties are determined by the (i) mould dimensions (ii) the casting conditions and (iii) the steel grade being cast. Empirical rules have been developed to express the properties providing good casting performance; these include the required values for the powder consumption, slag viscosity and the break temperature but the fraction of crystal phase in the slag film (f_crys) should also be cited. The various types of mould powders are described, e.g. starter, exothermic and prefused powders. Most mould powders in use are denoted as Conventional; the properties of these powders would be expected to be (i) consistent with those derived from empirical rules and (ii) to contain fluorides to form cuspidine in the slag film. However, there are a number of specialist powders which have been developed to carry out specific tasks or to cover special steel grades or casting conditions. The specialist mould powders include the following: F-free powders; C-free powders, Non-Newtonian powders, and fluxes used to cast high–speed, thin slabs and round billets and powders used to cast steels with high Al and rare earth contents and stainless steels. Although the properties of all these fluxes are consistent with those predicted by the empirical rules, developments have been made to deal with special features for each type of powder.

Mould powders for ingot casting have, in many ways, the same function as mould powders for continuous casting. However, in some aspects the properties and compositions must meet requirements other than those for continuous casting. After the introduction of continuous casting on a bigger scale and adaptation of mould powder for this technology, the development of ingot casting powder has mainly been based on experience derived from the development of CC mould powders. Thus, there has been little development in casting powder research and ingot casting technology for more than 30 years. The remaining steel grades that are cast by the ingot process route today tend to be high-grade, high-quality steels and there is a new, re-awakened interest in the development of both ingot casting technology and mould powders for ingot casting. Today most ingot casting is carried out with killed steel cast into bottom end up (BEU) ingots. Thus, the most important properties of the mould powders are good flowability and good thermal insulation. The creation of a mould powder slag is important but the thickness and composition of this must be designed according to the steel grade being cast and the process conditions which require careful control of the composition and melting speed. Most ingot powders are still produced with fly ash as a base but the use of synthetic mould powders is growing because of its greater versatility in composition. Granulated powders for ingot casting are also attracting a growing interest, since they are more environmentally friendly, causing much less dust during casting. They also give better flowability, compared to loose synthetic powders, and open up the opportunity of continuous feeding during casting.

It is difficult to get a complete knowledge of the composition of mould powders because the source and mixture of the raw materials is considered by the manufacturers to be confidential. Most raw materials are open-pit-mined minerals and can vary considerably in composition from batch to batch. For that reason, the mould powder is composed of a number of different raw materials where price optimisation is also an important factor. This chapter provides information on how mould powders are produced, its constituents, and how the quality and properties are controlled by the steel plants in order to get a better understanding of the mould powder and how it affects the casting process. The data sheet supplied by the manufacturer gives little information of value when it comes to the performance of the powder in the mould and when the given chemical composition of the mould powder can be achieved in an endless number of combinations of raw materials.

In this chapter, the properties and the structure of the slags used in continuous casting are collated, analysed and discussed. The required property values of mould slags for successful casting are determined by the mould dimensions, the casting conditions and the steel grade being cast; these can be calculated using empirical rules. Required values of the powder consumption (Q _s ^req ) viscosity (η^req), break temperature (T _br ^req ) and the fraction crystal in the slag film (f_crys) are the key properties. However, other properties such as the interfacial tension are important when dealing with slag entrapment, thus a large amount of property data for mould slags are needed. Mould slags have been classified here into two groups (i) Conventional slags used in continuous casting and (ii) Specialised slags, which includes F-free slags, non-Newtonian slags, calcium aluminate (CA) type slags, etc. Some properties are very dependent on the structure of the slags. Thus, published data on slag structures for all types of slag have been collated and analysed. Routines, developed to estimate the various properties, are outlined and reviewed. The data for the following properties of the various mould slag families have been reviewed (i) viscosity (ii) Liquidus (T_liq), Break (T_br) and Glass transition (T_g) temperatures (iii) Thermal conductivities (iv) Surface and Interfacial tensions (v) Densities and thermal expansion coefficients (vi) Heat capacity and enthalpy and (vii) Optical properties. In addition, the crystallisation process in mould slags and the ability of the liquid slag to dissolve inclusions are reviewed and discussed. The requirements and the properties of slags used in continuous casting and ingot casting are compared and discussed.

The various factors influencing the selection of mould powder compositions are discussed in this chapter. Mould powders contain two types of component (i) the minerals which form the slag film and (ii) Carbon which controls the melting rate. Both are vital to the successful performance of the mould powder. The composition of the mould powder is determined by the mould dimensions, the casting conditions and the steel grade being cast. Empirical rules have been developed to provide the required values of the powder consumption (Q _s ^req ), the viscosity (η^req) and the break temperature (T _br ^req ) for satisfactory casting (i.e. free from longitudinal cracking and sticker break-outs). More than 85% of mould powders conform to these empirical rules. However, other factors can also influence the powder selection process. These include All of these problems are discussed in detail and possible solutions are outlined.

The quality of the cast product is affected by any defects present. The causes of each individual defect are discussed, along with the factors affecting their formation and proposed treatment to minimise the severity of the defect. The following defects and process problems are covered in this chapter: (i) longitudinal cracks, (ii) longitudinal corner cracks, (iii) sticker breakouts, (iv) oscillation marks, (v) transverse cracks, (vi) star cracks, (vii) depressions, (viii) overflows, (ix) entrapment of slag, gas and inclusions, (x) formation of scales on the surface of the steel, (xi) Carbon pick-up by the steel, (xii) SEN erosion and (xiii) fluoride emissions. These defects are discussed from the viewpoint of how the mould powder can help to alleviate the problem.