Reductionof Slivers DueTo Nonmetallic Inclusions inContinuous Casting

Reductionof Slivers DueTo Nonmetallic Inclusions in Continuous Casting



Abstract

Determination of non-metallic inclusions is important for a steel makers and

customers. To improve the steel quality or understand the effect of cleanness on

production it is necessary to measure the cleanliness. These non-metallic inclusions give

rise to slivers during continuous casting. Primary deoxidation in Ladle Furnace produces

calcium aluminates with high melting point which remains as solid in the steel making

temperature causing non metallic inclusions and thereby V crack slivers and line type

slivers. The reasons for the above were investigated in Ispat Industries Ltd applying Six

Sigma methodology and SEM analysis. The effect of tap oxygen, powder entrapment, N2

pick up, moisture in Tundish and other parameters on sliver defect was studied

Introduction

Continuous Casting has evolved as an important production process leading to

improvement in the yield, quality, productivity and economics of steel production in the

world. A good quality product with high productivity is an essential requirement of a

modern continuous caster that necessitates identifying the critical factors responsible for

defects and ensuring implementation of possible remedial measures for production of

defect free casting [10].In CSP process there is no scope of inspection and grinding

stroke scarfing of slab before rolling. After caster, slab directly enters the tunnel furnace.

However there is a high pressure descaler after tunnel furnace and before mill. Slivers as

shown in fig.1 (a,b,c) are one of the major defects observed in steel slabs that appear as

anextra layer on the surface of cast slabs. The generation of slivers is mainly due to the

nonmetallic inclusion(NMI) of the liquid steel. The sources of generation of these NMI

start from EAF tapping and continue till Continuous casting in different sections. The

slivers are divided in to two types: one is FeO and the second is Al2O3.

(a) (b) (c)

Figure 1: Different type of Sliver photos: (a) dark band [9], (b)peeled-off skin parallel

to the rolling direction [9],(c) extra surface layer.

Tunnel furnace may also contribute slivers for high percentage of oxygen percentage

generating scales which may some time not removed in high pressure De-scalar which

later appears like slivers after rolling.

During Tapping:

The Al2O3 is primarily generated duringtapping from EAF to deoxidize the oxygen

present in the steel bath. During this period, the O 2 pick up takes place due to its exposure

to atmosphere generating more Al2O3. Generally, 180 tons liquid steel takes around 3-5

minutesto complete its tapping depend ing upon its EBT life

Figure 2: Ladle furnaces inside the reactions [1]

Figure 2 shows the chemical reaction that occurs inside the ladle furnace, formation of

calcium aluminate and its effect in slabs. The formations of CaO.Al2O3 (CA) complex

compounds and propertiesare shown below table no 1:

Table no 1: Chemical reactions in ladle furnace

Reactions [12] Compound (C-

CaO,A-Al2O3)

Melting

point, °C

Density,

gm/cc

3CaS +19Al2O3 = 3(CaO.6Al2O3) + 2Al + 3S C6A 1833 3.38

12CaS + 7(CaO.6Al2O3) = 19(CaO.2Al2O3) +

8Al + 12SC2A 1755 2.91

3CaS + 4(CaO.2Al2O3) = 7(CaO.Al2O3) + 2Al

+ 3SCA1590 2.88

15CaS + 33(CaO.Al2O3) = 4(12CaO.7Al2O3) +

10Al + 15S C12A7 1395 2.83

Effect of phosphorus reversion:

The electric arc furnace slag has high contents of FeO and MnO. It is well-known that

high levels of those oxides produce a harmful effect on steel cleanliness, bringing about

an increase in the total oxygen content of the steel.

FeO and MnO in Slag- An important source of reoxidation is the carryover slag from

the EAF to the ladle, which contain a high content of FeO and MnO. These oxides react

with the dissolved aluminum to generate alumina in liquid steel, owing to the strong

favorable thermodynamics of the following reactions [1]:

3FeO (l) +2Al =Al2O3 +3Fe (l)?Go = -853700+239.9T (J mol - 1) (1)

3MnO +2Al =Al2O3 +3Mn (l) ?Go = -337700+1.4T (J mol - 1) (2)

The higher the FeO and MnO content in the ladle slag, the greater is the potential for

reoxidation and the corresponding generation of alumina inclusions. Many slivers in the

final product have been traced to reoxidation that originated from FeO in the ladle slag

[2, 3,4].

Many countermeasures were adopted to lower these FeO and MnO contamination which

are shown below:

1.Minimized slag carryover from EAF to ladle during tapping

2.Increased aim turndown carbon

3.Avoiding reblowsfor minimizing the dissolved oxygen content in the steel

thereby reducing the amount of FeO in the furnace slag [2].

4.Ladle slag reduction treatment [2,4,7]

By minimizing slag carryover, together with adding a basic ladle slag and basic lining

to lower the ladle slag to less than 1-2% FeO+MnO, can reduce total oxygen to 10 ppm

for Low carbon aluminium killed steel. [5] Another way to lower the FeO+MnO content

of the ladle slag is to add a slag conditioner (i.e. slag reduction or deoxidation treatment),

which is a mixture of aluminum and burnt lime or limestone.

Casting Speed:

Casting speed plays a major role for generating slivers. Slivers may occur when

there is entrapment of casting powder or mould powder. It mainly occurs when there is a

variation in the casting speed thereby more chances of entrapment of these powders

generating slivers.

inclusions creating slivers appear from different sources [10]:

1.Liquid steel cleanliness in the Tundish mainly resulting from secondary

metallurgy practice and Tundish metallurgy including steel flow control

2.Mould slag entrainment in the mould and entrapment by the solidifying shell

3.Re-Oxidation during continuous casting by air or refractory materials

4.Oxide form due to iron oxide being trapped and subject to high temperature,

which can occur between the CC and up to the hot metal reversing rougher.

Chancesof generation of inclusion due to air entrapment:

Figure4: Source of air entrapment

Slivers Observed at

Casting speed variation

The above figure 4 shows the places where there were chances of air entrapment from

EAF tapping to Tundish region generating Al2O3 and FeO inclusions

Table 1 Origin of Al2O3 and FeO from different places

Sources of Reoxidation :Ladle to Tundish

S.

No Parameters O2 pickup

Al2O3 / FeO

generation

1

Carryover slag (average %P reversion is

0.002 and %FeO is 21) - 272 Kg of FeO

2

N2 pickup from ladle lifting to casting

start

<4 ppm

1ppm of O2 2ppm of Al2O3

3

In Tundish moisture H2 pick up is

4 ppm of H2

32ppm of O2 64ppm of Al2O3

4

Ladle exchange time Silicon pick up is

>100ppm Si (Grade change from Si to Al

killed steels [11]

100ppm of O2

200of Al2O3

(almost 400ppm

N2 equivalent)

5

During tapping time through EBT

(Tapping time is around 3-4 mints) 10ppm of O2 20ppm of Al2O3

6

Ladle transformation form EBT station

to Ladle treatment position

(Transformation time is 5 mints) 10ppm of O2 20ppm of Al2O3

7

During ladle open with lance

at casting station [11]

10ppm of O2

20ppm of Al2O3,

FeO

8 Shroud leakage [11]2ppm of O2 4ppm of Al2O3

Observations:

The photographs of different types of Slivers(V type and line type) before and after SEM

analysis were shown below

Figure 5: Photos of (a) v-crack and (b) line crack taken from surface of the coil

a b

SEM Photos:

Figure 6: SEM photographs of Line type slivers in LCAKS

Figure7: SEM photos for V-slivers in LCAKS

Fig no 6 shows the homogeneity is not uniform where the oxide inclusion defect

occurred. Fig no 7 shows the generation of external oxides.

Results and Discussions :

a b

Data

No sliver sliver

1565

1560

1555

1550

1545

1540

1554.71

1552.47

Boxplot of Tundish Temp by status of the defect

O2,ppm

Tap Oxygen-No Sliver Tap Oxygen-Sliver

1000

900

800

700

600

904.69

728.203

Boxplot of Tap Oxygen-Sliver, Tap Oxygen-No Sliver

c d Figure 8: Six sigma analysis result of box plots for slivers: a. Tap Oxygen, b. Tundish

temperature, c. stopper rod fluctuations, d. High Al lifting.

Tap Oxygen: The tap oxygen ppm showed a significant effect on the formation of the

slivers. Below 750 ppm tap oxygen, probability of slivers defect is very low.The amount

of Aluminium added during tapping is mainly determined by the dissolved oxygen

concentration after the melting process. High oxygen concentration requires more

aluminium for deoxidation. Increased aluminium addition results in the formation of an

increased amount of deoxidation products, i.e. alumina inclusions.

Argon Flow rate and time: Argon purging plays a prominent role in the homogenization

of temperature, composition and in the floatation of the inclusions in the ladle metallurgy.

During the refining process there are two types of argon stirring

i.A strong stirring to favor desulphurization, macro inclusion floating

and thermal homogeneity

ii.A Soft Stirring to agglomerate, floating inclusions and a morphologic

modification of those which have not floated, through Ca treatment.

The appropriate argon stirring values are:

? For reduction of temperature of liquid steel: 18-30 Nm3/hr

? For homogenizing stirring: 15-18 Nm3/hr

? For enhanced deoxidation/desulphurization: 9-12 Nm3/hr

? For removing impurities and mild s tirring: 3-6 Nm3/hr

Stopper Rod fluctuations: NMI which remains as solid during the steel making

temperature sticks to the submerged entry nozzle. There will be a rise in stopper rod

which will be flushed out after adding CaSi powder. Therefore the NMI which got stuck

to the SEN will entrap the slab surface causing slivers

Data

No sliver sliver

85

80

75

70

65

60

70.0973

67.7976

Boxplot of stopper rod position by status of defect

% Al lifting

Al Lifting-No Sliver Al Lifting-Sliver

0.07

0.06

0.05

0.04

0.03

0.02

0.0454412

0.0378268

Boxplot of Al Lifting-Sliver, Al Lifting-No Sliver

Table 2 Possible re asons and actions taken for Slivers [13]

Source Reason Effect Action

Isolation of Liquid steel

Single most important source of

oxygen pick up is during transfer

from ladle to Tundish or during

its residence time in Tundish.

Reoxidation effects will be decreased by

1.Minimizing stream break up

2.Minimizing the air entrapment

1.Effective covering of Tundish

2.Lowering of Shroud

Ladle purging Oxygen PPM is high

Over purging

1.As crusted slag that covers the steel

does not allow the much heat to liberate,

therefore the stirring flow is increased to

accelerate the process which causes slag

entrainment.

2.The same is the case for short purging

periods where excessive flows are used

when slag is not crusted. The excessive

flow lowers the temperature of the steel

very quickly, but at the expense of slag

entrainment

1.Avoid excessive purging

2.Avoid over purging (long

time)

3.Avoid overused ladle

Tundish metallurgy and

casting Start up

1.Oxygen PPM is high

2.Air entrainment

1. Very long Mould filling times mean

that fluid properties are weak as large

part of the steel is being exposed to

ambient air.

1. Mould filling time should be

low.2.Avoid air entrainment

during pouring through

shroud.(proper mechanism

covering or insulation to be

made).3.Optimum inert gas

bubbling through stopper

Inert gas protection Inert gas protection while

opening the slide gate

Tundish temperature

So that viscosity of the steel will

decrease which increases the inclusion to

float as less resistance to inclusion float

at higher temperature

Tundish temperature to

be high

Casting speed

Most of the time casting speed

is sudden increasing/decrease

time it happening.

Variation of casing speed

0.582

0.105

0.000 0.000 0.004

0.000 0.000

0.000

0.435

0.000

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

Dec'08 Jan'09 Feb'09 March'09 April'09

% Internal rejection (MTS/NP) % Custmer rejection

Figure 9: Trend chart after taking actions for sliver

SEM results:

Figure10: Coil no 2-0170 and type of defect is FeO sliver)

SEM morphology for figure10 shows dark region spectrum showing presence of FeO

and Gray region spectrum shows light slag entrapment with non uniform distribution of

non metallic inclusions. More amount of cavitations is observed, other than some large

pores found on the inter particle boundaries and triple particle junctions, which may have

originated during solidification of particles from semi molten state.

Figure 11: Coil no 2-0798, and type of defect is Al2O3 sliver.

SEM morphology for Fig 11 shows large number of uniformly distributed Al particles

with globular form with some flattened regions. The grains are mostly equi- axed with

little mismatch between the particles. Amount of cavitations is less compare to FeO sliver

shown in Fig 4. By comparison it shows that FeO sliver is more flattened regions, which

might have been formed during solidification of molten particle that ha ve been fused

together in lumps, where as Al2O3 sliver shows spheroidal shapes of different diameter,

which might have been formed due to breaking or fragmentation of bigger particles

during solidification.

Conclusions

This work has done withsix sigma methodology and SEM analysis. The basis of

inclusion generation, the effect of tap oxygen and slag carry over the non metallic

inclusions and thereby in generating slivers, casting speed, stopper rod fluctuations, LF

chemistry were discussed. Origin of Al2O3 and FeO from different places during the steel

making process were also included. Possible reasons and remedies were given. By taking

care of tapping Oxygen in EAF, minimizing the P reversion for controlling the FeO,

minimizing the LF processing time, control the oxygen pickup in different sections,

minimizing the soft purging flow, to protect the inert gas sealing at shroud and control the

casting speed variation sliver defect can be eliminated.

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