1.This slide is about causes of breakouts during continuous casting of steel and remedies about the same 2. It will help to reduce breakouts problem during continuous casting of steel up to 80%

1. Date : 22th April, 2017
Breakouts During Continuous Casting Of Steel

2. 2
CAUSES OF BREAKOUTS DURING CONTINUOUS CASTING
 Lack of lubrication during solidification process
 Air entrapment like Hydrogen, Nitrogen etc
 Heat transfer during solidification is not proper
 Higher TDS content of water for primary cooling of mould
 Sudden change in mold design
 Level of liquid metal in the mold
 Sudden change in mode of oscillation during process
 Re- oxidation of metal and Aluminum Addition

3. 3
CAUSES OF BREAKOUTS DURING CONTINUOUS CASTING
 Breakout during strand withdrawal
 Higher TDS of water during secondary cooling
 Improper heating of tundish
 Low Carbon Equivalent (Ceq ) of the molten steel
 Higher phosphorous content in the steel

4. 4
EFFECT AND REMEDIES OF POOR LUBRICATION AND AIR ENTRAPMENT
 Lack of lubrication and air entrapment during solidification :
 Powder consumption depends upon 1) Casting Speed 2) Carbon content of steel 3)
Temperature in the mould 4) superheat of steel ( Temperature above solidifying
temperature )
 Mould oscillation and its frequency also affect lubricating properties of flux powder
 Low powder consumption leads to minimum slag thickness, which leads to air entrapment
in to the mould which decreases casting powder lubricating properties
 Sources of Hydrogen are : 1) Tundish covering agents 2) Casting Powder 3) Atmosphere
4) Water used for billet cooling
 Low carbon content and also higher nitrogen content in the steel ( which comes during
purging of metal ) increases hydrogen pick up possibilities from free or combine moisture
in the casting powder

5. 5
EFFECT AND REMEDIES OF POOR LUBRICATION AND AIR ENTRAPMENT
 Hydrogen in steel can be controlled by CO bubbles which are formed by the reaction of
Carbon and Oxygen
 Formation rate of CO bubbles is controlled by aluminum addition
 Hydrogen gets dissolve in liquid steel du to its higher solubility and it gets increases if
content of oxygen in metal decreases
 Hydrogen have high solubility in liquid phase and its decreases with decrease in
temperature of steel
 Hydrogen gets evacuated towards the walls of the mould ,where it gets trapped in the slag
film causes molten flux to crystallize, increasing viscosity of the flux
 But we need low viscosity of flux for proper lubrication

6. 6
EFFECT AND REMEDIES OF POOR LUBRICATION AND AIR ENTRAPMENT
 If the viscosity of flux is increased then liquid friction will increased which may lead to
sticking of the molten metal to the mold
 Humidity of casting powder is the one source of hydrogen if the pre-heating of casting
powder has not done properly or it is not get mixed properly
 This solid slag reduces the heat transfer rate due to which proper heat extraction does not
happen

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EFFECT AND REMEDIES OF POOR LUBRICATION AND AIR ENTRAPMENT

8. 8
EFFECT AND REMEDIES OF POOR LUBRICATION AND AIR ENTRAPMENT
 REMEDIES :
 Hydrogen content should not be more than 7 PPM i.e. 0.0007 % and Nitrogen should not
be greater than 100 PPM i.e. 0.01 %
 To remove moisture content in casting powder it should be heated between 70-80 °C
and should be stirred after certain period of time
 Casting Powder addition should not be too high because free carbon content in it controls
the melting rate of flux by providing inert barrier between solid particles and liquid metal
 At the beginning of the sequence a large amount of powder should be added to cover the
steel then only the powder consumed by the strand has to be replenished and during the
last heat of the sequence it is a practice to slowly minimize the powder addition

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EFFECT AND REMEDIES OF POOR LUBRICATION AND AIR ENTRAPMENT
Wearing of mold should be very low which will happen only when lubrication is good
 Thickness of mold slag should be high which will help for good lubrication
Casting Powder composition should vary with casting speed and composition of steel then
and then it will give proper lubricating properties under present condition
 If quality of used casting powder is low then it gets entrapped into the molten steel below
the meniscus resulting in improper lubrication

10. 10
H₂
H
N₂ CH₄ H₂o
Entrapped
air

11. 11
EFFECT AND REMEDIES OF POOR HEAT TRANSFER
 Improper heat transfer occurs due to :
 Lack of lubrication due to anything which we have seen earlier
 Air entrapment in the mould will change the mode of heat transfer from conduction to
radiation which will result in decrease in the heat flux ( Rate of heat energy transfer )
 Higher TDS content of water
a) Increase in TDS content of water, increases impurities in the water
b) These impurities will react with mould surface and will form the layer on the
surface of mould which will result in improper heat transfer
c) Improper heat transfer will cause sticking of metal at the mould surface

12. 12
EFFECT AND REMEDIES OF POOR HEAT TRANSFER
Air Gap
Heat
Transfer By
Radiation

13. 13
EFFECT AND REMEDIES OF POOR HEAT TRANSFER
d) e.g. oxygen present in water will react with cupper mould and it will form the
layer of cupper oxide slowly on the mould surface during primary cooling
 TDS content of water :
 TDS less than 300 ppm is Excellent
 TDS level in between 300-600 ppm is good
 TDS level that is in between 600-900 ppm is fair
 TDS level that is in between 900-1200 ppm poor
 REMEDIES :
 Softening of water should be done time to time to minimize the TDS and PH level of
water

14. 14
Clˉ
O₂
Clˉ
O₂
Ca⁺ Ca⁺
Corrosion
layer

15. 15
EFFECT AND REMEDIES OF POOR HEAT TRANSFER
 Sudden change in mold design:
 Increase in casting speed plus higher metal tonnage production may reduce mold life
affecting it earlier than its normal life
 Increasing mold life causes deformation of mold which leads to increase in taper loss
 If taper provided does not match the requirements ,which would cause air gap to form
between solidified shell and mold
 This air gap acts as a heat resistant in mold heat transfer system
 It generally hampers the formation of the shell of required thickness eventually leading to
breakout
 An excessive taper of the mold increases the resistance to withdrawal that results in
increased mold wear and lowering heat extraction which causes metal surface temperature
to increase

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EFFECT AND REMEDIES OF POOR HEAT TRANSFER
 Sudden change in mold design:
 This increases ferrostatic pressure at surface of metal which induces tensile strain at the
surface in the corner causing cracks which will then leads to breakouts
 Improper alignment of the mold has an impact on heat transfer mechanism influencing
occurrence of breakouts
 Difference in inlet and outlet temperature ( ∆T ), Pressure and flow rates has bearing on
the mold cooling
 Chocking in the mold cooling system due any reason like trash in water which will lower
the flow of water in pipe which will cause increase in pressure, reduction in flow rate
affect heat extraction rate and causes breakout
 Large difference in inlet and outlet temperature leads to sticking of the strand to mold
which causes breakouts

17. 17
CHANGE IN LEVEL OF METAL IN THE MOLD
 Change in level of metal in mold :
 The metal level in the mold needs to be maintained 70-80 % of the mold height
 If it drops below this level, then skin formed on the subsequently added metal will be less
which will leads to breakouts
 Drop in the level of metal leads to entrapment of the slag in to the metal causing breakout
 overflow of the mold involves sticking of the metal at the upper part causing difficulty in
strand withdrawal which after sometimes leads to breakout

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CHANGE IN MODE OF OSCILLATION
 Change in mode of oscillation during process :
 For proper lubrication we want non-sinusoidal oscillation in which either positive strip
time is higher or negative strip time is higher
 Positive strip time is that time in which mold moves in upward direction and negative strip
time is that time in which mold moves in downward direction
 We generally use higher negative strip time because during this period tensile force
changes from tensile to compressive and due this lubrication becomes good and metal
sticking problem get reduced
 If this two time becomes equal for any reason then mode of oscillation changes from non-
sinusoidal to sinusoidal which we definitely don’t need
 Due to this change in mode of oscillation powder consumption becomes less and
lubrication will no be done properly

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CHANGE IN MODE OF OSCILLATION
Sinusoidal Waveform
Non-Sinusoidal Waveform
Negative Strip Time
( ɳ ) =
𝑽𝒄−𝑽𝒎
𝑽𝒄
X 100
Where, Vc = Casting Speed
Vm = 2 X h X n
h = Stroke Length
n = Mold Frequency

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RE-OXIDATION AND ALUMINUM ADDITION
 Re- oxidation and Aluminum addition :
 Re- Oxidation can happen due to flaring open stream as in open casting
 We add aluminum in the molten metal for De-oxidation purpose and to maintain purity of
steel by this
 If injection of aluminum wire is not proper then it will react with oxygen and form
aluminum oxide ( Al2O3 ) in higher amount
 This Aluminum oxide increases crystallization temperature of slag by increasing its
viscosity
 This aluminum oxide infiltrate in the strand forming slag spots
 This slag entrapment paves way towards improper lubrication between the strand and
mold causing sticking of the strand to the mold resulting in a strand discontinuity which
further leads to breakouts

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BREAKOUT DURING STRAND WITHDRAWAL
 Breakout during strand withdrawal :
 If the shell has not had the time to solidify to the required thickness or the metal is too hot
in the primary cooling zone
 Which means that final solidification takes place well below the straightening rolls
 During this time chances of shell breakout increases due to stresses applied during
straightening

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HIGHER TDS OF WATER DURING SECONDARY COOLING
 Higher TDS of water during secondary cooling :
 If water quality during secondary cooling is not good then it reduces the heat extraction
rate from shell
 Due to this variation in temperature gradients occur which induces thermal stresses and
strains on the surface of steel
 These stresses and strains generate cracks on the surface of steel which may further leads
to breakouts
 Increase in content of inclusion in secondary cooling water increases chances of scale
losses from steel, which will affect the quality of steel

23. 23
O₂
O₂
O₂
O₂ Ca⁺
Ca⁺
Ca⁺
Ca⁺
Secondary
cooling
zone

24. 24
IMPROPER HEATING OF TUNDISH
 Improper heating of tundish :
 Tundish focus on continuous improvement of many quality related parameters such as
1) Fluid dynamics 2) Thermal Insulation 3) Inclusion floatation 4) Hydrogen pic up
 If for any reason there is reduction in pre-heating time of tundish then chances of skull
formation increases
Skull = Solidified ( Steel + Slag )
 If pre-heating of tundish is done properly then chances of clogging of pouring nozzle due
to iron oxide and aluminum oxide is reduced
 If pre-heating of tundish is done at low temperature, then high temperature of molten
metal may give thermal shock to the refractory lining of tundish

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IMPROPER HEATING OF TUNDISH
 Improper heating of tundish :
 Due to this crack gets generated and pre-mature wear of tundish lining may occur
 If pre-heating of tundish is not done properly then chances of hydrogen and nitrogen pic
up from tundish lining may increases which leads to breakouts in mold
 For proper insulation in tundish, pre-heating of tundish should be done properly otherwise
it will increase inclusions in steel who may erode tundish lining by affecting it earlier than
normal

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EFFECT AND REMEDIES OF LOW CARBON EQUIVALENT
 Carbon Equivalent (Ceq ) of the molten steel :
 Carbon Equivalent = % C + 0.04% Mn + 0.1% Ni + 0.07% N
– 0.14% Si – 0.04% Cr –0.1% Mo
 Lower Carbon Equivalent decrease the reducing atmosphere in the mold which is must
needed for the good lubrication
 Lower Ceq especially susceptible to breakouts because of strong contraction from Fe-ɗ and
Fe-ϒ promotions and early solidification of metal
 Early Solidification gives thin shell formation of steel due to improper heat transfer of
metal which will leads to sticking of metal to the mould

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EFFECT OF HIGHER PHOSPHOROUS CONTENT IN THE STEEL
 Higher Phosphorous content in steel :
 Higher phosphorous content in steel gives rise to generation of Iron Phosphide (Fe3P)
phase during solidification.
 This Iron Phosphide phase is very brittle in nature and gets segregated during solidification
it may break and metal will gets stuck to the mould

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Thank you !