prof. a k lahiri -plenary lecture- some aspects for clean steel production

7/22/2019 Prof. a K Lahiri -Plenary Lecture- Some Aspects for Clean Steel Production

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Some aspects for clean steel production

A K lahiri

Former Professor IISc, Bangalore, [email protected]

Abstract: Often large number of small Al2O3inclusions are formed in the

tundish. As such these are not detrimental to steel but they clog the nozzle

resulting asymmetric flow in mold and mold flux entrapment. Proper tundish

slag along with electromagnetic break can help to overcome this problem.

Introduction

Steel with desired low value of impurities like Phosphorus, Sulfur, total

oxygen, Nitrogen, Hydrogen and in some grade even carbon is known as

clean steel. Among these impurities the behavior of oxygen is different in

two ways. After Ladle refining, the other impurities remain almost

unchanged but oxygen may increase due to absorption from air or pick up

from refractory and slag. Secondly other impurities remain in solution but a

significant part of oxygen is present as oxide inclusion so oxygen content is

always specified as total oxygen. Since significant part of oxygen is present

as inclusion, liquid steel is not homogeneous with respect to oxygen.

For number grades both total oxygen and inclusion size should be within a

limit. For sheets in tin plate, total oxygen


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Oxygen Pick up from air

During liquid steel pouring of from ladle to tundish, the shroud preventsexposure of metal stream to atmosphere in the steady state operation. But

during initial filling up or during ladle change over, metal is exposed to

ambient air. This leads to pick up of oxygen and nitrogen from atmosphere.

Fig. 1 shows the change in oxygen level as observed by Solhed1. When

pouring starts, there is significant oxygen pick up by metal, which leads to

overall increase in oxygen level in tundish. At the outlet oxygen decreases

Figure 1: Variation of total oxygen content at the inlet, middle and outlet

with time. Ladle was changed at 40th

and 83rd

minute.

monotonically from ~50ppm at the start to ~15ppm at the time of ladle

change over. Again immediately after ladle change over, it increases to a

high value and then monotonically decreases. This trend is present for

second change over as well confirming significant oxygen pick up from

ambient air. This leads to more inclusion in first slab and immediately after

ladle change over. It sometimes leads to cluster of iron oxide, Fig2.

0

2 5

5 0

7 5

1 0 0

1 2 5

1 5 0

1 7 5

0 2 0 4 0 6 0 8 0 1 0 0T i m e ( m i n )

O

(ppm)

I n l e t

M id d l e

O u t le t

0

50

100

150

200

0 4 8 12


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Fig.2 Cluster of Iron oxide in the first slab

Tundish covering slag

Figure 3 schematically shows that covering slag in contact with steel is

liquid but that in contact with atmosphere is solid. The liquid slag prevents

oxygen transfer from air and readily absorbs oxide inclusion. So the ideal

tundish slag should cover steel surface with a liquid slag from the beginning

Fig.3 Schematic diagram of tundish covering slag in a tundish

so that it prevents oxygen diffusion from atmosphere and can absorb the

inclusion. But in reality this is rarely the case. If liquid slag is formed by the

end of first ladle tapping, it is considered as a good covering slag sincesometimes it is formed only during tapping of third or fourth ladle. Besides

slag should have high dissolution rate and solubility for the inclusions and

should not react with the dissolved elements present in steel. Because of

these requirements recently Hollappa et al2used thermodynamic modeling

along with laboratory experiment to develop chemistry of active tundish

slag. Although the technique is useful to design the chemistry of ideal

tundish covering slag but its usage may be limited due to two reasons: firstly

it has to be cost effective and secondly number of factors including particle


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size distribution of the powder controls the liquid slag formation. So it will

require practical modification. Both acidic as well as basic fluxes are used

as covering slag. Kijac et al3

showed that basic slag is more effective thansilico aluminate type slag in removing Al2O3inclusions but even basic slag

eventually gets saturated with Al2O3 .

Slag metal interaction

Slag and metal form two separate layers but slag metal interface may not be

smooth and a mixed domain may exist close to interface because of highly

turbulent flow due to Kelvin-Helmholtz interfacial instability. The stability

condition for the present system is given by4,5

.

sm

smsm

sm

sm

gUU


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region above the metal phase. The thickness of mixing zone is about 100m

close to inlet but in other areas, the size is smaller. The thickness of mixedzone in Fig.4 is about 30. Mixed zone is layered with Al2O3inclusions in

the upper part. Most of the inclusions present here are less than 10m.

Similar observation was reported by Solhed6 earlier. Fig.5 shows the

interface. It shows that in microscopic scale the interface is not flat and

. Figure 5 slag metal interface1

inclusions are present at the interface. Small globular slag lumps and

inclusions could be seen close to interface. Fig.6 shows that these inclusions

are CaO, Al2O3 and CaO.Al2O3. The presence of CaO.Al2O3 inclusions

suggest that at the interface Al2O3 reacts with CaO present in slag to form

these inclusions. Small Al2O3inclusions might have formed by the reaction

of aluminum with FeO in slag or dissolved oxygen. Since the sizes of

globular slag lumps are very small, and no big lump was observed, the origin

of these lumps could be interfacial instability or due to the impact of highenergy small eddies present in the metal. Solhed et al

7 showed by

mathematical modeling that the recirculation flow at the inlet region can also

lead to slag entrapment in metal and metal entrapment in slag. However only

small fraction of these can reach the outlet.

It is well known that ladle slag infiltrates into the tundish. This leads to

increase in CaO even in silico aluminate type tundish covering slag besides


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it increases FeO content of tundish slag resulting in formation of Al2O3

inclusions.

Fig.6 Inclusions close to interface. Whitish ones are Al2O3and black ones

are CaO and partly whitish and partly blackish ones are CaO.Al2O31.

Table 2 shows inclusion count of samples from ladle furnace and twolocations of tundish. The increase of small size inclusion


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Refractory metal interaction

Refractory is another source of oxygen pick up. MgO based refractories usedin tundish contain olivine which is a solid solution of fayalite (Fe2SiO4) and

forsterite (Mg2SiO4). These can react with de-oxidized steel according to

following reactions:

Fe2SiO4= 2Fe + 2O + SiO2

Mg2SiO4+ SiO2= 2(MgSiO3)

2Al + 3O = Al2O3

MgO + Al2O3= MgAl2O4

Pack et al8studied used tundish refractory by electron probe micro analysis,

TEM and back scattered electron images. They found olivine where its

faylite component is reduced, Fig.7 Besides they observed Mg2SiO3 and

MgAl2O4and concluded that all the above mentioned reactions take place in

the tundish.

Fig.7 TEM of olivine in contact with liquid steel containing 0.04%Al8

Mould

Although under ideal operating condition, inclusions particularly Al2O3should get removed in the mould but in reality sometimes a significant

amount of exogenous inclusion gets trapped into metal. Fluid flow

characteristic in the mould has a major role in it.


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Absorption of inclusion

Mold flux picks up about 4% Al2O3. Part of it is de-oxidation product andpart of it is formed by the reaction:

1.5 SiO2(flux) + [Al] = Al2O3+ 1.5 [Si]

However in case of special steel which contains high Al, pick up of Al2O3is

much higher and thereby the resulting flux will have different property

compared to original flux.

SEN clogging

It is well established that Al2O3 inclusions deposit on inner wall of nozzle

and clog it. Fig.8 shows original as well as clogged nozzle as reported by

Rodl et al9. It shows that inclusions deposit primarily at the discharge end of

the nozzle and blocks the port partly.

Fig 8 Nozzle clogging9

Fig 9 shows that discharge from the port of clogged nozzle and flow in the

mold is highly asymmetric and complex. Asymmetry is very significant


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Fig 9 Computed result of clogged nozzle9

on the top surface where a region on one side has a high velocity. Longs10

model showed that only inclusions below 15m can deposit on the nozzle.

Al2O3 inclusions deposited on SEN sometimes get dislodged and gets

entrapped in metal as large inclusion. Figure 10 shows one such case. It isinteresting to note that inclusions in the cluster are


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Longs result. This along with the observation of Rodl9 that number of

inclusions, not total inclusions, play a major role in clogging suggest that

Al2O3 formed in tundish due to oxygen influx has a major role in clogging.

Flux entrapment

Fig. 11 shows schematically shows top surface of mold. The wave shown on

the top is not a standing wave but it oscillates. Because of the shear force

Fig 11 Schematic diagram of flux metal interface in the mould

at the slag metal interface liquid flux can get entrained either because of

wave instability or vortex formation as shown in Fig 12(a) and (b)respectively. These happen only at high metal velocity at the meniscus.

(a) (b)

Fig 12 Flux entrainment by sharing and vortex formation11

Possibility of slag entrapment by these mechanism becomes less for high

viscosity of mold slag.

Besides the above two mechanisms of slag entrainment, solid flux gets

entrained in the metal if amplitude of wave is more than the liquid slag layer


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thickness. In this case part of meniscus metal becomes bald as shown in Fig

13(a) thereby when solid flux is added there, it directly gets entrained in

metal, Fig 13(b)12

. This can be avoided by maintaining proper thickness ofliquid flux layer.

(a) (b)

Fig 13 Flux entrainment due to balding of meniscus (a) balding (b) Flux

entrainment

It has been pointed out earlier that nozzle clogging leads to asymmetric flow

and can lead to significant disturbances in meniscus. This could lead to flux

entrapment. Sometimes it is found that even at the apparently steady casting

condition strand quality is below par. This could be attributed to asymmetric

flow through the ports due to nozzle clogging.

Flux can get entrapped in metal due to Kelvin Helmoltz instability although

there is no direct evidence for it.

Discussion

Improved tundish design helped in reducing large inclusion from the steel.

But due to various mechanisms of oxygen input in the tundish, a large

number of fine Al2O3 inclusions


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with high Al2O3absobing capacity. Basic slag satisfy these requirements but

more research is needed to see that slag formation is early enough.

Ca treatment is widely used to prevent the nozzle clogging. But investigation

of Rodl9

showed that some CaO form CaO.2Al2O3which has melting point

of 1705C and these inclusions deposit on SEN.

The deposition of inclusion on SEN makes the fluid flow in the meniscus

complex and leads to high meniscus velocity which may lead to flux

entrainment. Only reduction of velocity at the meniscus can prevent flux

entrainment. So electro-magnetic break and SEN design to give swirling

flow are the available options. A simpler option of course is the increase of

port diameter but there is practical limitation to this.

Conclusion

Suitable tundish flux development which will lead to early liquid slag

formation in tundish along with high Al2O3absorption capacity will go long

way in producing cleaner steel.

References

1. Henrik Solhed, Pranesh Dayal, A.K.Lahiri : Int Conf on Continuous

Casting Past, Present & Future, Jamshedpur, 24- 25th October 2005

2. L. Hollappa et al : Steel Res Int. published on line on 18thFeb.2013

3. J. Kijac et al : Metalurgija vol 43 2004 p 59

4. S. Chandrasekhar: Hydrodynamics and Hydromagnetic stability, Dover

Pub. N.Y.,1961 p485

5. Y.Chung and A.W.Cramb: Metall Mater. Trans. Vol.31B (2000) p957

6. H.Solhed, L. Jonsson and P.Jonsson : Metall Mater. Trans. Vol.33B

2002 p17

7. H.Solhed, L. Jonsson and P.Jonsson : Steel Res. Int. 2008 vol 79 p 348

8. A. Pack, S.Hoernes and Th Walther: Proc. 6th

Int. congress on Applied

Mineralogy, Gottingen, Germany, 17-19 July 2000 p518

9. S. Rodl et al : New strategies for clogging prevention for improved

productivity of Steel, EU report 2012

10.M. Long et al ISIJ Int. 2010 vol 50 p712

11.D Gupta and AK Lahiri: Ironmaking- Steelmaking 1996, vol 23 p361

12.D Gupta and AK Lahiri:Metall Trans.B, 1996 vol 27B p 695

prof. a k lahiri -plenary lecture- some aspects for clean steel production

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