Casting Powders

Introduction Tundish and mould powders have significant influence on both the quality and the constancy of casting operations. This is the reason why all powders have to be specially tailored to perform in accordance with the most disparate conditions characterizing each process plant, as: steel grades produced, sequence length, different types and mould sizes, casting speed, quality standard requirements.

Tundish Powders

Tundish Functions Steel reservoir during ladle changes Steel cleanliness Steel temperature control Steel composition fine tuning

Tundish Specifications

Tundish Powders Main Functions to act as a thermal insulator to act as a barrier between air and the liquid steel to prevent reoxidation to assimilate the inclusions that separate from the steel

Reoxidation Causes and Prevention extrinsic oxidation exposure of the liquid steel to the atmosphere (particularly in the turbulent area around the ladle shroud) chemical or intrinsic oxidation chemical reactions inside the melt

developing and retaining a reasonably thick slag barrier between molten steel atmosphere

and external oxidising

Tundish Powders Acid powders (mainly based on SiO2)making an excellent thermal insulation but neither capable against re-oxidation renor participating to the inclusion absorption

Basic powders (mainly based on CaO)forming a liquid layer with high inclusions absorption capability and efficient barrier against re-oxidation rebut not acting as an efficient thermal insulator

ACID Tundish PowdersBeneficial

excellent thermal insulationDisadvantages

When using an acid covering powder steel cleanness can be weakened by oxygen pick-up (re-oxidation) pick(rein fact, the oxygen potential of an acid slag is by far higher than that of the steel

BASIC Tundish PowdersBeneficial For high grade steels basic tundish covering powder should be preferred: It can get mixed without great problem with the rest of the basic ladle slag It is also beneficial for the lining because does not readily react with basic refractories Disadvantages the covering layer becomes hard in a few hours when casting long sequences impairing the insulation ability

DOUBLE COVERING LAYERUsually, basic powders are used together with acid powders at first, the basic (low melting) powder is put on the steel forming a liquid layer then, an acid powder is added (forming the top powdery insulating layer)

IMPORTANCE OF PHYSICAL AND THERMOTHERMOPHYSICAL PROPERTIES OF THE INTERFACE IN ASSIMILATING THE FLOATING INCLUSIONS For example Too high a viscosity (mainly acid powders) limits inclusions absorption capability Too low a viscosity (mainly basic powders) can lead to entrainment of the flux into the steel during unstable conditions

HOW TO MINIMISE REFRACTORY CORROSION at the start low viscosity slags with low Al2O3 (although these are aggressive towards refractories, but soon become high in viscosity) or use two fluxes: a simple lime-alumina flux in the pouring area lime(so that refractory erosion is minimised) and a more complex flux covering all other areas

Mould MetallurgyThermal exchange Fluid dynamicsUniformity Intensity Thermal exchange Interaction with powders SEN shape Thermal exchange Lubrication Inclusion caption Mechanical properties at high temperature Shrinkage coefficient Thermal diffusivity Oscillation mark depth Powder consumption Friction force

Powders

Steel characteristics

Mould oscillation

Mould Thermal Exchange

Effect of steel composition on heat transfer

Effect of thermal flux amount on Longitudinal cracking

Mould Thermal ExchangeIncreasing casting speed decreases the heat extracted from the steel for weight unit

Q ! Q0 Kv ! AThermal flux

Q

Qo ! A

I

Qo

Q, Qo ! Thermal Flux

A ! Slab Sectionv ! Casting speed

! Steel density

! Steel mass flowCasting speed

Mould Fluid Dynamics

Steel Characteristics

Effect of steel composition on heat transfer

Relation between steel carbon content and shrinkage coefficent

Mould Oscillation

Definition of the negative and positive strip time

Effect of Frequency, Stroke and Casting Speed on NSTCASE 1: NTS = 0.8515 10 5 0 0 -5 -10 -15 1 2 3 Position Velocity Vc

CASE 2: Doubled frequency NTS = 0.4515 10 5 0 0 -5 -10 -15 1 2 3 Position Velocity Vc 25 20 15 10 5 0 -5 0 -10 -15 -20 -25

CASE 3: Doubled Stroke NTS = 1.30

1

2

3

Position Velocity Vc

Oscillation Mark Depth

Powder Consumption

Powder consumption in function of positive strip time

Powder Consumption vs C Content

No Sinusoidal Oscillation

Comparison between sinus and deformed cycles

Inverse OscillationWhen growing casting speed, the aim is to obtain, through the stroke increase and frequency decrease, an approximately constant NST for low OSM depth and, in the same time, a relatively high PST for good powder consumption.

Functions of Mould PowdersChemical protection Thermal insulation Lubrication between mould and steel shell Uniform heat transfer between steel and mould Absorption of inclusions

Schematic Representation of the Various Slag Layers Formed in the Mould

Mould Powders Working SequenceMelting of powder Formation of a molten poolInterrupted slag flow

Infiltration of molten slag into mould/strand gap Effect on shell formation Formation of solid and liquid slag films

Sticker breakout

Heat transfer beetwen strand and mouldHigh q Variable q

Liquid lubrication of the strandHigh F

Surface defects

Mould Powders

Mould heat transfer in function of crystallization temperature

Best working conditions to have both good lubrication and thermal exchange

The Physical Properties of the Casting Flux are one of the Keys for the Casting Process. Melting rate Melting Temperature Viscosity vs Temperature Break Temperature Crystallisation Temperature

Melting RateCarbon, in the form of either carbon black, graphite or coke dust is added to the mould powder.

Carbon particles are non-wetting to molten nonslagconsequently it slows down the agglomeration of molten slag globules.

Thus, for higher free C content, more time is required to agglomerate and slower is the melting rate.

ViscosityThe viscosity of the casting powder is a key physical property since it influences: lubrication between mould and steel shell molten powder entrapment (lower with higher viscosity) depth of oscillation marks (lower with higher viscosity) SEN erosion (higher with lower viscosity)

Viscosity Al2O3 additions cause increase in viscosity since they result in a prone polymerised slag Li2O, Na2O and CaF2 cause a decrease in viscosity since they result in a more depolymerised melt

Break Temperature Is the temperature below which there is a marked change in viscosity and represents the point where liquid lubrication starts to break down Break temperatures are obtained from viscosity measurements during the cooling cycle

Break Temperatures are Dependent Upon the Cooling Rate and are Usually Quoted for a Cooling rate of 10C/min 10

highhigh-viscosity fluxes such as that used in the billet-casting billethave no break temperature since they form super-cooled liquids super-

Crystallisation Temperature It is the temperature where the flux starts solidifying (formation of crystalline phase) and is sometimes called Solidification temperature. It is usually measured during cooling cycle in Differential Thermal Analysis (DTA) or Differential Scanning Calorimetry (DSC) measurements.

Melting TemperatureIt is the temperature where flux becomes totally fluid and is usually measured on the heating cycle in DTA/DSC experiments

Differential Thermal Analysis are generally made when a new powder is designed. Usually, the characteristic temperatures (softening, melting and fluidity) are determined by means of Heating Microscope

How to Read a Mould Powder Technical Data Sheet Chemical Analysis 1The basicity index (BI = CaO/SiO2) gives an idea of the slag tendency to crystallisation: crystallisation: BI > 1.0 generally means a certain tendency to crystallisation BI < 1.0 generally means that the powder is prone to form a glassy film between mould and steel

How to Read a Mould Powder Technical Data Sheet Chemical Analysis 2The Free Carbon content indicates the melting rate of the powder: the lower is the free C the higher is the melting rate the higher is the molten pool depth (keeping constant operating parameters and powder viscosity)

How to Read a Mould Powder Technical Data Sheet Physical Analysis 1Softening, Melting and Fluidity Temperatures determined by Heating Microscope too high fluidity temperature can determine a so big slag rim that can inhibit the slag inflow into the channel between mould and steel

How to Read a Mould Powder Technical Data Sheet

Physical Analysis 2Viscosity as a function of temperature Generally, the viscosity has to be lower for higher casting speeds to guarantee a good infiltration (consumption) and, in turn, a good lubrication