Continuous casting mould powders


Continuous casting mould powders

Continuous casting (CC) mould powder is used primarily to facilitate the passage of steel through the mould. It is also known by names such as casting powder, mould powder and mould flux. CC mould powder plays an important role in the continuous casting of liquid steels. It is one of the most critical and influential factors for smooth continuous casting of the liquid steel. CC mould powder is sprayed at the top of the liquid steel in the mould either manually or by automatic feeders. CC mould powder after additions in the mould

  • Heat up and loose some carbon by reaction with oxygen
  • Forms a sintered layer
  • Melts at a definite rate and forms sintered (mushy) and liquid layers. The liquid layer acts as a reservoir to supply liquid slag to the strand. This liquid pool should be deeper than the stroke length to ensure good lubrication.
  • Forms a solid slag film through the first infiltration of liquid slag into the mould/strand gap. This slag film is glassy in nature and is typically 2 to 4 mm thick. This slag subsequently crystallizes in the high temperature regions adjacent to strand.
  • Forms a liquid slag 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.

Schematic diagram of different layers of mould powder after its addition in the mould is given in Fig. 1

CC mould flux in mould

Fig 1 Schematic diagram of different layers of mould powder after its addition in the mould

 Besides the function of mould lubrication, CC mould powder has to perform some more basic functions. These other basic functions are enumerated below.

  • Thermally insulate the liquid steel meniscus for preventing premature solidification. Improved thermal insulation increases the temperature in the meniscus region of the liquid steel which helps to make oscillation marks less severe and can reduce sub surface defects such as pin hole.
  • CC mould powder forms three layers namely solid layer, sintered layer and liquid layer. This three layer system acts as a protective barrier and protects the liquid steel meniscus in the mould from reacting with atmospheric gases
  • Absorb inclusions (such as Al2O3 and TiO2) from the liquid steel
  • Provide uniform heat transfer across the infiltrated slag layer between the steel strand and the mould

When continuous casting of liquid steel started in fifties, lubrication and heat transfer between the steel shell and the mould were provided by the use of the rape seed oil. Mould powders based on fly ash were introduced by 1968. 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 had positive effect on casting parameters which in turn improved the quality of the cast product. Since the composition of fly ash is not constant, there is a necessity to blend the fly ash carefully with limestone, soda ash and fluorite so as to have reproducible composition of the CC mould powder. This has led to the development of synthetic CC mould powder.

Types of CC mould powders

Today many types of CC mould powders with different compositions and shapes are available to suit the needs of casting different grades of steels in different cross sections of the castings. The range of chemical compositions of CC mould powders is given in Tab 1.

Tab 1 Range of chemical composition of CC mould powders

Compound/element

Unit

Value

CaO

%

25-45

SiO2

%

20-50

Al2O3

%

0-10

C

%

1-25

Na2O

%

1-20

F

%

4-10

FeO

%

0-6

MnO

%

0-10

MgO

%

0-10

B2O3

%

0-10

BaO

%

0-10

LiO2

%

0-4

TiO2

%

0-5

K2O

%

1-5

CC mould powders are supplied in different forms namely powders, granulted, extruded and expanding granules. Each type of CC mould powder has its own advantages and disadvantages related to cost, flowability, thermal insulation, meting rate and health hazards. CC mould powders can be classified into various types as given 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, it 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 materisls to achieve the required composition since it simplifies the quality assurance. Also those raw materials which are health hazards should not be used.
  • Prefused powders – These powders have a sizable portion that is premelted and sized. Introduction of prefused powders has been 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 (< 1 %).  Starter powders should be used only when there is a necesity.

Powders are cheaper than granular product but fine powders are having health hazards. There is also some inhomogeneity in supplies since fines tend to settle to the bottom of the container.

Important properties of CC mould powders

 The viscosity, solidification point, melting point and slagging speed are considered important properties for CC mould powders. The mould powders should have low viscisity, low liuidus temoerature 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 CC mould powder during the casting of certain grades of steel. The thermal insulating property of CC mould powder is controlled through the bulk density, particle size and carbon types. The effect of chemical composition on some of the major properties of CC mould powder is shown in Tab 2.

Tab 2 Effect of chemical composition on some properties of CC mould powder

Compound/element

Property of CC mould powder

Viscosity

Solidification point

Melting point

CaO

Decrease

Increase

Increase

SiO2

Increase

Decrease

Decrease

CaO2/SiO2

Decrease

Increase

Increase

Al2O3

Increase

Decrease

Increase

Na2O

Decrease

Decrease

Decrease

F

Decrease

Increase

Decrease

Fe2O3

Decrease

Decrease

Decrease

MnO

Decrease

Decrease

Decrease

MgO

Decrease

Decrease

Decrease

B2O3

Decrease

Decrease

Decrease

BaO

Decrease

Decrease

Decrease

LiO2

Decrease

Decrease

Decrease

TiO2

No change

Increase

Increase

K2O

Decrease

Decrease

Decrease

Steel composition and CC mould powder

There are specific requirements of CC mould powder to suit the steel composition. These reuirements 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 should have good insulating properties, good absorption of non metallics and stable properties. Mould powder stability is ability to absorb Al2O3 without any without any adverse effect on 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 more than 0.18 % also needs mould powders of low viscosity, low flux density and low melting point. Mould powders should have good insulating properties, correct carbon content and good lubrication capability.
  • Ultra low carbon steels needs mould powders which can absorb non metallic inclusions, improve insulation, can provide good lubrication, stable properties and minimal slag entrapment. Casting powder should not cause carbon pick up in the steel.