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Continuous Casting Mould Powders

• satyendra
• June 16, 2013
• 245 Comments
• continuous casting, Fly ash based powder, mould, mould powder, Pre-fused powder, Synthetic powder, viscosity,

Continuous Casting Mould Powders

Continuous casting mould powders are used primarily to facilitate the passage of liquid steel through the mould of the continuous casting machine. It is also known by several other names such as mould powder, casting powder, mould flux, mould flux slag, or mould flux powder. Mould powder plays an important role in the continuous casting of liquid steel and is one of the most influential and critical factors in the stability of the casting process and for the smooth casting of the liquid steel.

The mould powder improves the performance of the casting process and reduces the surface defects. The main functions of the mould powder are (i) to protect liquid steel against oxidation, (ii) to provide lubrication for the solidifying steel, (iii) to control, optimize, and insulate the heat transfer from the liquid steel to the mould and the ambient in horizontal and vertical directions, (iv) to absorb the inclusions from the liquid steel to produce cleaner cast steel product, and (v) to provide chemical protection to the liquid steel from oxidation and other undesired reactions. The high basicity of the mould powder increases its capability to assimilate non-metallic inclusions.

When continuous casting of liquid steel started in 1950s, lubrication and heat transfer between the steel shell and the mould were provided by the use of the rape seed oil. In the early days when the continuous casting of liquid steel started, liquid steel was cast in open stream using rapeseed type oils to lubricate the mould. However, the surface quality of the cast products was poor due to the failure of the rapeseed type oils to provide adequate thermal insulation, protection against steel reoxidation and failure to act as a flux for impurities such as alumina. With the advent of submerged entry nozzle (SEN) in the continuous steel casting, use of oils presented a new problem in addition, since the frozen steel platelets, skulls would grow around the submerged nozzle until they became big enough to break the SEN.

The mould powders have been developed in an attempt to overcome the problems encountered with rapeseed type oils. This development has been considered by many as ‘trial and error’ development. Mould powders based on fly ash were introduced by 1968. Initially, mould powders were physical mixtures of materials such as fly ash, blast furnace (BF) slag, fluorspar, alkali carbonates and cryolite. These powders have a very positive effect on the heat losses and the super heat temperatures of liquid steel could be reduced. These powders were also found to ‘wet’ both the mould and the shell. These powders also had positive effect on casting parameters which in turn improved the quality of the cast product. However, fly-ash based powders used to lubricate and protect the liquid steel were not efficient since the fly ash has a varying composition. Hence, there was a necessity to blend the fly ash carefully with limestone, soda ash, and fluorite (CaF2) so as to have reproducible composition of the mould powder. This has led to the development of synthetic mould powder around early 1980s. The continuous development of mould powder since its first introduction has now made continuous casting of many varied sections of steel as routine.

Today, many types of mould powders with different compositions and shapes (granular, powder form, and extruded powders) are produced to suit the casting of diverse steel grades and sizes. The factors which influence the properties of mould powders are (i) chemical composition, (ii) mineralogical composition, (iii) grain size composition, (iv) manufacturing process, (v) drying or roasting method, and (v) free carbon content. Each shape and type of powder has its own advantages and disadvantages, such as price, health issues, flow-ability, thermal insulation, and melting rate. The choice of powder requires a deep knowledge of the casting process, composition of liquid steel, desired and feasible preferences, and characteristics of the process and the product.

Mould powders are mechanical blends of various metallic oxides and fluorides containing small amounts of carbon to regulate their fusion rate. During melting of the powder, the oxides and fluorides react to form a liquid which produces complex oxides and oxy-fluorides upon cooling. The extent to which these phases affect lubrication and heat transfer properties of the slag depends on the chemical composition of the original powder. With such a large number of chemical components in the mould powder, it is difficult to compare performance of the different powders on a compositional basis.

The composition of the mould powders differs according to the application, steel grade, and the desired product. However, some components are considered to be the main constituents of the mould powders. The components which characterize the composition are (i) a mixture of CaO and SiO2 which is around 70 % of the composition with CaO in the range of 22 % to 45 % and SiO2 in the range of 17 % to 56 %,  (ii) CaO/SiO2 ratio normally in the range of 1 to 1.3 with some powders have basicity ratio as 0.8, (iii) MgO around 0 % to 10 %, (iv) Fe2O3 around 0 % to 6 %, (v) Al2O3 around 0 % to 13 %, (vi) Na2O around 0 % to 25 %, (vii) K2O around 0 % to 2 %,  (viii) fluorine around 2 % to 15 %, (ix) TiO2, B2O3, ZrO2, Li2O, and MnO which are added to the powders according to the need of the application and hence the amount can vary a lot, and  (x) C  around 2 % to 20 %.

Fluorine (F) in mould powder has a great influence on the mould powder properties and also has hazardous effect on the environment. The advantages of fluorine are (i) it lowers the melting point of mould powder and hence, enhances the lubricating property of mould powder, (ii) it decreases the viscosity of the mould powder and hence helps in formation of continuous and stable slag film, and (iii) it enhances the crystallization of powder film. The disadvantages of fluorine are (i) there are emission of volatile component like SiF4 and NaF, and (ii) it reacts with cooling water leading to the formation of HF.

The viscosity, solidification point, melting point, and slagging speed are considered important properties for the mould powders. The mould powders are required to have low viscosity, low liquidus temperature and melting rate which matches with the speed of the continuous casting. Sometimes it becomes necessary to give attention to the density and chemistry of the mould powder during the casting of certain grades of steel. The thermal insulating property of mould powder is controlled through the bulk density, particle size, and carbon type. The effect of chemical composition on some of the major properties of mould powder is shown in Tab 1.

 There are specific requirements of mould powder to suit the steel composition. These requirements are given below.

• Low carbon aluminum killed steel requires mould powder which can absorb Al2O3 inclusion without any adverse effect on the viscosity. Mould powder is required to have good insulating properties, good absorption of non metallics, and stable properties. The stability of the mould powder is the ability to absorb Al2O3 without any adverse effect on the viscosity.
• Carbon steels with carbon in the range of 0.1 % to 0.18 % are prone to cracking. High solidification temperature of the mould powder reduces heat through mould. For adequate lubrication, low viscosity of the mould powder is needed.
• Carbon steels with carbon content more than 0.18 % also needs mould powders of low viscosity, low density, and low melting point. These powders are to have good insulating properties, correct carbon content, and good lubrication capability.
• Ultra low carbon steels needs mould powders which can absorb non metallic inclusions, can improve insulation, can provide good lubrication, and have stable properties and minimal slag entrapment. Casting powder is not to cause carbon pick up in the steel.

Mould powders today are broadly classified into three groups namely (i) fly ash based, (ii) synthetic, and (iii) pre-fused or fritted material. Fly ash based mould powders are mechanical blends of raw materials such as bauxite, lime, fluorspar etc. with powdered fly ash as the major component. Synthetic powders are simply blends of powdered raw materials in desired proportions. Pre-fused mould powders, normally used for the casting of aluminum killed deep drawing quality steels, are blends which are melted and sized after the mechanical mixing of the raw materials. Mould powders are supplied in different forms namely powders, granulated, extruded and expanding granules. Each type of mould powder has its own advantages and disadvantages related to cost, flowability, thermal insulation, meting rate, and health hazards. Powders are cheaper than granular product but fine powders are having health hazard issues. There is also some inhomogeneity in supplies since fines tend to settle to the bottom of the container. Different types of mould powders are described below.

Fly ash powders – These powders are mechanical blends in which powdered fly ash is a significant component of the mix. In these powders, fly ash is blended with different minerals. Since the fly ash composition varies a lot, this has restricted the production and use of this type of powder.

Synthetic powders – These powders are mechanical blends of many fine powdered minerals. These are made with high shear mix. It is desirable to use raw materials with similar melting points. It is preferable to use minimum numbers of raw materials to achieve the required composition since it simplifies the quality assurance. Also those raw materials which have health hazard issues are not to be used. 

Pre-fused powders – These powders have a sizable portion of materials which have been pre-melted and sized. Introduction of pre-fused powders is done to improve the uniformity and the chemical composition.

Granular powders – These powders have the shape of spherical or extruded granules. These granules have much lesser dust than mould powders. Spherical granules are particularly suitable for automatic application. Granulated mould powders are produced by spray drying while extruded powders are produced by extrusion. These powders have better quality control and lesser health hazard issues. Expanding granules contain an expanding agent. During heating of the granules, the expanding agent alters the shape of the granules and reduces the flowability of the powder on the top of the mould.

Starter powders – These are sometimes used at the beginning of the casting for providing a quickly formed slag pool. These powders have low melting points, have high sodium oxide (Na2O) content and frequently contain exothermic agents such as calcium silicide and a small amount of carbon (less than 1 %).  Starter powders are to be used only when there is a necessity.

During continuous casting, the liquid steel is continuously poured out from the tundish into the oscillating mould. The copper mould is cooled with water. In order to protect the liquid steel from oxidation and sticking to the mould, the mould powder is used. The mould powder is continuously added on the top surface of the liquid steel inside the mould which can be done manually or automatically. This mould powder forms a layer having a total depth of 100 mm to 150 mm. Due to the high temperature of the liquid steel, well above the melting point of the powder, a temperature gradient forms in the vertical direction through the powder. On the top there is the newly added mould powder, forming a powder layer. On the addition of the mould powder different layers of mould powder which are formed are shown schematically in Fig1. The mechanism of the formation of these layers is described below.

• The mould powder heats up and loses some carbon by reaction with oxygen. The removal of water takes place as the temperature rises and the mould powder forms a sintered layer.
• The mould powder melts at a definite rate and forms sintered (mushy) and liquid layers. The liquid layer acts as a reservoir to supply liquid powder to the strand. This liquid pool is to be deeper than the stroke length to ensure good lubrication.
• The mould powder forms a solid powder film through the first infiltration of liquid powder into the mould / strand gap. This solid powder film is glassy in nature and is typically 2 mm to 4 mm thick. This solid film subsequently crystallizes in the high temperature regions adjacent to strand.
• The mould powder forms a liquid powder film typically of 0.1 mm thickness. This liquid slag is drawn down into the gap along the steel shell and lubricates the strand. This lubrication prevents the steel from adhering to the mould thus removing a cause of the strand break out.

The powder on the meniscus normally consists of four layers namely (i) an un-reacted, un-melted, dark powder layer on the top, (ii) a sintered, semi-reacted layer, (iii) a mushy zone in which the mould powder is melting, and (iv) a molten slag layer directly on the liquid steel. The lubrication process is almost completely carried out within this last mentioned layer and depends on many factors. Normally fluxes with lower viscosity and / or melting temperature tend to provide lower friction, better lubrication properties, and hence prevent sticking.

It is clear from Fig 1 that moving from liquid steel to the external surface of the powder, there are three different layers which can be defined according to their state of aggregation and physical state. Each of them exists for a certain temperature range. On the other side, moving from the liquid steel towards the cooled mould, other layers arise, which can be however predicted by the melting curve. The properties of the powder film dictate the main functions of strand lubrication and mould heat transfer. According to the chemical composition and physical properties, two main mechanisms can in turn take place namely crystallization and vitrification. The formation of crystals is favourable for a homogeneous and controlled (horizontal) heat transfer during casting, which is required in order to prevent the formation of surface cracks.

But the mould powders which are directly exposed to liquid steel also experience an instantaneous heating which is able to provide thermal conditions which are very far from thermodynamic state. Powders normally have a glassy behaviour in this case.

Fig 1 Schematic diagram of different layers of mould powder in the mould

Process of mould powder functioning 

The lower part of the mould powder starts to sinter and forms a sintered layer. However, the powder which is in closest contact with the liquid steel melts down due to the high temperature and results in a liquid powder pool on top of the liquid steel inside the mould. At the narrow and wide faces of the mould, the liquid powder penetrates the narrow gap between the steel meniscus and the mould wall. A solid, glassy slag layer is formed when liquid slag is quenched onto the water-cooled mould wall. If more poder is added onto this layer, or when preheating takes place, there is a likelihood of crystalline phases being formed in the layer. Due to the higher rate of heat transfer at the upper part of the mould, a slag layer called rim is formed on the top of meniscus and around the mould. The solid slag layer structure is part of controlling the horizontal heat flux, i.e. the cooling rate of the steel, directly relates to the number of surface defects occurring on the final steel product. The temperature on the solid steel surface is above the melting point of the mould powder throughout the length of the mould. As a result, a liquid powder layer is present between solid powder layers and the steel shell (strand). This layer is crucial to maintain a low strand / mould friction, and hence avoiding sticking of the steel shell

The layers of liquid steel which are closest to the mould start to solidify. The quality and characterizations of this shell is of outmost importance. Hence, it is essential to control and optimize the stability and quality of the solidified shell. This can be done by adjusting the casting speed, mould oscillation speed, heat transfer, and mould powder properties such as melting rate, composition and viscosity etc. The mould powder covering the liquid steel forms a liquid powder layer. Above this layer is a carbon-rich sintered layer and above this the unfused powder. The mould powder is expected to fulfill several functions such as (i) protection of metal against oxidation by air, (ii) thermal insulation to prevent partial solidification at the surface, (iii) absorption of inclusions rising up to the surface, (iv) lubrication of the contact between the metal and the mould, and (v) allowing of homogeneous heat transfer between the strand and the mould as per the casting conditions.

During the oscillation of the mould, the liquid powder formed at the surface of the liquid steel infiltrates between the steel strand and the mould to act as a lubricant and also to regulate the heat extraction from the strand to the mould. If the heat is dissipated too slowly, too thin a steel shell is formed by the strand and a ‘breakout’ can occur, i.e. the steel shell ruptures, just below the mould since it cannot sustain the ferro-static pressure of the liquid steel. On the other hand, if the rate of heat removal is too high, longitudinal cracks can appear in the cast product.

Mould powder characterization

Physical characterization plays an important role in the selection procedure and the operational evaluation. In general, the chemical composition, the viscosity including the start of crystallization, and the melting behaviour are considered for the characterization of the mould powder.

Viscosity – The viscosity of a mould powder influences the infiltration of mould powder during casting. In general, infiltration increases with a decreased viscosity of the mould powder for the same operating conditions. Operating windows for the viscosity are mainly based on rules of thumb, but other demands such as the control of powder entrapment also play an important role when defining the required viscosity of a mould powder.

Melting behaviour – The melting behaviour of a mould powder strongly influences both the liquid pool depth and the sensitivity towards rim / bear formation. The melting behaviour can be described by the melting trajectory and the melting speed. In both cases, additions of free carbon are considered to be a principal factor. The other main parameter is the flow condition in the mould i.e. the meniscus stability during casting. The liquid pool depth results from the balanced values of the feeding and the infiltration of the mould powder.

Melting trajectory – The melting trajectory of the mould powder is determined using a hot stage microscope. Results are normally given as values for the softening, the melting and the flow temperatures.

Melting speed – The melting speed of mould powders is determined using the so-called softening method. With this method, the displacement of a pre-pressed cylinder of mould powder is measured as a function of time at a fixed temperature (1400° deg C). The method yields qualitative results which can be related to the mould powder composition i.e. the free carbon content of a mould powder.