Use of Graphite in Refractory: Burning or using

 Graphite is a natural limited resources. Because of its unique properties like flaky nature, non wettability with liquid metal and high thermal conductivity along the axis, used in Refractory. Anti oxidant is used along with graphite during manufacturing of refractory to take care of oxidation. But a portion of graphite is burned in the furnace, which can be avoided.

Graphite or Carbon is used largely together with silicon carbide in ramming masses and castables. The function of carbon is prevent premature oxidation of silicon carbide in these refractory products. Depending the time and temperature the carbon is oxidized resting only the SiC to protect the product . If SiC is also oxided then we will have erosoin or ling wear. In consequency the life will be shorted.

Natural graphite consisting of clay material is utilized for the production of refractory blocks, crucibles,sheaths,high temperature lubricant and it is also used as a lining material for ramming the tapholes,and the colloidal graphite is used as mould release compound in foundary practice. Besides which it is also used as reductant for the reduction of metal oxides to produce metals which means it is useful during burning also. In recent years there have been efforts to increase the oxidation resistance of graphite by the addition of anti-oxidants to the surface graphite particles. As against the natural graphite the synthetic and pyrolytic graphite are very pure and highly crystalline substances and because of their outstanding high temperature thermal, electrical conductivity and high thermal strength they are used used in specialised applications such as electrical commutation, spectral elecrodes, heating element,high purity crucibles,thermal seals, metal matrix composites.  

what's the difference between fused zirconia and chemical grade zirconia,

The term fused zirconia refers to the zirconia grains which are melted by heating above their melting point therefore the surface of these grains is in a fused state and it is nearly theoretically dense. One would expect better thermo mechanical properties from fused grains. Where as chemical grade is generally refered to zirconia prepared by chemical precipitation or decomposition of zirconium salts and such grains show high surface porosity hence they are prone to liquid metal wetting.


 Refractoriness is all about purity. A fused grade will contain fewer of the glass forming impurities which reduce the refractoriness of high melting point materials like zirconia. Hence you will have cleaner grain boundaries less prone to chemical attack, high temperature creep etc. In extreme applications, go for fused grade every time. Of course there are grades of fused materials as well, so you have to go by the quantities and types of impurities present in the chemical analysis to be sure that you're getting a quality product.


Price wise:
It can expected the fused grade to be more expensive. But there are different grades of fused zirconia, as with any other fused material. The fused material obtained from the centre of the melted mass is expected to contain the least contaminants and companies will often sell this material at the highest price whilst sell the material obtained from the outside of the melt at a lower price.
Whether it's better depends upon what use of it and how essential the place it is. That is, it could be cost effective to use the lower grade of zirconia in certain applications. Usually any containment vessel is zoned and the highest grade, fused materials only used in the high wear areas.


Crystal Size :
Cristal size is a major difference. Fused ZrO2 is slowly cooled, thus allowing for better formed and larger crystals than chemical ZrO2.


Lattice Size: 
There can not be major difference in the lattice parameters of the fused and chemical grade calcined zirconia but it may vary by from 5to 6% because the chemical grade is expected to have high vacancy or interstitial concentration hence it may show larger lattice parameter.Since in the fused zirconia the imputities are segregated to the surface of grains during solidification therefore it show lattice corresponding to that of pure zirconia crystal.

how Manganese cause Magcarbon refractory erosion?

1) Rich MnO slags can lead to Mn-rich metallic particles and solid solution with Mg (and, indeed, carbon oxydation) at the interface between slag and lining. 
Anyway, this is seldom the first cause of erosion: it generally occurs after the lining has been weared for some other reason (i.e. slags unsaturated in MgO or rich in FeO).


2) Mn presence in converter causes erosion faster, as it behaves acidic, makes the liquid less viscous, penetrates the pores and joints and reacts with MgO at the contact point. For Mn steel, different of configuration of MgC brick is used.


3) it is observed that after a low grade ore(high impurity i.e Mn,Si) converter life is almost reduced and high erosion profile is being observed. 
as my observation high Mn slag is very fluid and do not cover the converter lining after slag splashing. reducing use of iron ore as coolant may help.


MnO + SiO2 = very low melting and corrosive liquid. 

I suppose primary wear area limiiting converter life will be the trunnions. Trunnions can be zoned with higher quality MgO containing bricks - MgO purity should be 97.5 minimum and of largest MgO crystal size available. Graphite should be 10-20% and have coarse flaked quality -- exact amount of graphite is function of sracp charge; hot metal chemistry; gunning practice; slag viscosity etc. A good start would be 15% C. Metal additions should be aluminum and silicon metal which will form carbides for added strength and corrosion resistance. 

Turkish fused MgO is superior to Chinese fused MgO especially for corrision resistance - crystal size is larger and grain chemistry is more homogeneous. 

Some other thoughts: 

The operator should be adding enough lime/limestone/dolomitic lime to maintain slag basciity at a > 3:1 lime:silca ratio and some MgO is helpful to reduce slag liquidity and reactability. 

Reblows will be especialy harmful as added FeO can result and FeO+MnO+SiO2 is a refractory solvent that is very aggressive. So effort should be taken to control reblowing to minimum. 

Overblowing such that temperature is overheated should be controlled. 

Corriosive slag should be slagged off shortly after tap. 

A high purity MgO gun mix and laser readings to identify low spots for added gunning maintenance will extend service life. 

Do practice slag splashing; a special lance is used to inject nitrogen after tap - the slag is made more refractory prior to the nitrogen splasing by addition of dolomitic lime. 

A good strategy would be to plan for Continuous Improvement over several linings rather than thinking that one design change can be a silver bullet. Key is to study the wear profile, identify the wear mechanism and develop new lining design that addresses the wear area and wear meachanism; this should be repeated in several iterations over several linings as in "chaisng the hole". As one area is upgraded the weak link might move to another area of the vessel. EX: An upgrade of trunnions might shift the limiting zone to the cone or the slagline or the charge pad or the tap pad...

phenomenon of Alumina pick up by the liquid mould flux during Continuous Casting and how it affects the quality of cast strands?

The pickup of alumina in the mold causes the formation of calcium aluminates. Since there are a variety of different calcium aluminate morphs(each one with a higher melting temp.) over time the mold flux can and will begin to thicken up and the fluid lubrication on the strand mold faces can be interrupted causing surface defects & breakouts. Old timers used to throw Calcium flourides (spar) into the mold to liquify it but each time you do that you also increase the CA content which in turn will eventually thicken back up. Spar is a bad actor in the mold. In order to stay fluid there is a delicate chemical balance between CA & Al.

Why mag-c bricks is mostly prefer for ladle and EAF, why not spinel refractory?

EAF is all mag-carbon. Why it works well and zoning is done based on carbon level, anti-oxidant, graphite quality and magnesia grain quality. In steel ladles, however, mag-carbon is the solution in the slagline only. If the shop is in an alumina killed process, the common is alumina/magnesia carbon for the barrel and bottom. For silicon killed shops, the norm is dolomite (carbon bond normally) or magnesia carbon. The selection comes down to what works in the application and which technology is most cost effective. The later takes on a regional aspect. In fact one shop in ---- (blastfurnace not EAF) is using a bottom and barrel that is alumina magnesia castable with a mag-carbon slagline, why? Simple they maintain the barrel profile with shotcrete and recasting through many slaglines. In fact they only reline the barrel and bottom from the shell once a year!


MgO-C is more compatible with high lime fluid slag. Spinel is more neutral and performs well in molten metal contact.

In basic steel making process, the slag is high in lime. So the brick need to be compatible with the slag. So MgO.C is one of the brick suitable for ladle metal line & only brick suitable for slag line. Graphite addition provides nonwetting character, spalling resistance and additional corrosion resistance especially against FeO. Other bricks Al2O3-MgO-C (Al killed) & Dolo-C (Si-Killed) bricks are used mostly on metal line of ladle. Some ultra low-C, Al-killed steel customers use Al2O3-MgO castable & Spinel bricks in ladle metal line, prefab/precast ladle bottom depending on their plant practices. In EAF also basic slag & hot spots suits MgO.C bricks. It is also economic against spinel brick.

Benefits of using Steel Fibers and Organic Fibers in Refractory Castables and Monolithics

One of the most effective ways of improving the mechanical and thermal properties of refractory castables and other monolithic refractories is adding in suitable proportions of stainless steel fibers and organic fibers to the castable respectively.

Steel Fibers

Steel fiber reinforced refractory castables are very resistant to the tendency of the material to fall apart on thermal cycling. Stainless steel fibers greatly improve the flexural strength of the castable. And this added increase in ductility contributes significantly to the thermal shock and spalling resistance of the material. The fibers generally used are in size varying between 0.1 to 0.4 mm2 in cross-section & 20-40 mm in length. For monolithic SS is used either high chrome or high chrome nickel steels available in the market with different grades. One reason commonly reported that the thermal shock resistance of castables is greatly increased through addition of SS fibers because these fibers act as crack arresters, preventing cracks propagating. This is also possible that the microcracks caused by a mismatch in thermal expansion coefficients of matrix and fibers dissipate energy from larger cracks propagating as a result of thermal stress. However percentage of these fibers added becomes important because of two reasons as it has a direct impact on the fluidity of the castable, then it may also cause mixing difficult due to fiber-balling when added beyond 3% by volume. Another critical factor will be the maximum application temperature for the castable that those fibers present in the castable can resist oxidation (since these fibers can not perform beyond their melting temperature).

Organic Fibers

An effective means for improving the explosive spalling resistance of a castable is to add organic fibers to the formulation. It has been reported that the composition & concentration of fibers are not as important as melting temperature of the fiber, since these fibers after melting increase permeability at certain temp. & thereby reducing the explosive spalling tendency of the castables. The fibers generally used for this purpose are Polypropylene fibers, Polyester staple fibers, etc.
Because of these different advantages it have been found that both organic and SS fiber reinforced refractory castables provide substantial increase in service life and therefore, a considerable reduction in refractory maintenance cost and furnace down-time.

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.

Reference

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