SLAG

   You...dont sell slag. so....slag has no economic value in itself. Its importance lies not in the cost at which, if at all, it sells, but…by the ROLEs it plays in steel making.

               Its importance is underestimated/ never gets understood by all. But it is important for those who just start into REAL secondary steelmaking process. That is, only during our formative years as secondary steel makers do we slowly….very slowly…. understand the crucial role of slag. that understanding is absolutely necessary for good performance of secondary steel making process metallurgists to make good heats CONSISTENTLY & ALWAYS.

               Slag must perform a couple of functions SUCCESSFULLY in steel making.

DEOXIDATION:

               A good slag promotes good de-O.

               In general, secondary slag is reducing in nature. Necessary to remove oxygen from liquid steel, which will have lower solubility as the steel solidifies. Unless sufficiently removed, it will lead to poor quality castings through blowhole formations.

               This de-O is done using deoxidisers like C, Si, Mn, Al., in different combinations. Oxygen, from steel, forms corresponding oxides and various compound oxides too, mostly in the form of inclusions.

               Other than oxides, there will be sulphides, nitrides too. Almost all of these inclusions should be removed from steel as much as possible. Here’s where the term CLEAN STEEL figures in. loosely used in steel making contexts, it seems to mean different qualities to different people.

               Unless good amount of inclusions is reduced in steel bath, castability is hampered.

               Unless the liquid steel is cast smoothly in the caster and rolled neatly in the mills, it’s not good steel. Castability becomes an issue in high speed/quality casting.

DESULPHURISATION:

               A good slag promotes de-S.

               De-S follows, de-O. de-S is necessary to have good grade of steel, through better mechanical properties. MnS/ FeS layers formation along the grain boundaries will be reduced during casting/rolling. This gives better mechanical properties.

               Good slag has good amount of free Ca-ions to remove S from FeS. And this Ca is expected to come from lime, rather than from Ca-cored wires. Having sufficient Ca-ions needs controlled slag.

               One must bear in mind that, being surface active elements, O and S vie with each other to combine with Ca, in the most reactive zone of the steel bath- that is slag-metal interface. So a good steel making needs a very good control over this slag-metal interface/reaction zone.

SINK:

               A good slag acts as a sink to the inclusions.

               Slag must be prepared in such a way to accommodate the upcoming inclusions from bath. It should act as a buffer for inclusions. To act as a buffer, it needs to be tailor-made to have perfect combination of various oxides that impart the right viscosity.

               As many may know, thicker slag is tooooo viscous to permit the upcoming inclusions to enter into it, while thinner slag is tooooo fluidic to RETAIN inclusions within itself. Any amount of purging post calcium treatment would not produce good results.

Purging would only be able to float inclusions to the top layers of the steel, but not into the thicker slag-phase. In thinner slag, inclusions would be floated into the thinner-slag-phase but would be drawn/forced back into the steel-phase again as inclusions do not get trapped in the thinner-slag-phase.

If this function of slag fails, then, be ready to get choked suddenly in between the casting sequences, throwing the shop floor into dizzy logistics.

curiously, synthetic slag acts only as this sink, NOT as a desulphurising medium.

BURIED-ARCING:

               A good slag buries the arc, thereby reducing arc flare. Reduced arc flare would lead to reduced refractory damages, giving better ladle life (esp, in slag lining area).

               Buried arc further conserves energy by reducing radiation losses. Much of the heat produced by arc is trapped by slag and gets directed into the heat-bath.

ELECTRODE SPALLING:

               A good slag reduces electrode spalling by reducing harsh, direct arc being established between the hard-steel surface and the electrode tip. This open arc seems to generate a lot of vibrations that may or may not reach the resonance frequencies of the electrodes. Slag contains gases, esp CO/CO2, entrapped; that acts like a cushion to accommodate the vibrations that accompany arcing.

               Thicker, dry slag deflects the arc away.

               Thinner, wet slag may not bury the arc sufficiently and, may permit direct arcing too.

CARBON PICKUP:

               A good slag reduces carbon pickup into the metal from electrode. This may be attributed to thermionic emissions, mechanical erosion of electrode into the metal due to physically uncovered, direct arcing.

               This will become more visible during ULC grades making.

HYDROGEN/NITROGEN PICKUP:

               A good slag reduces hydrogen and nitrogen pickup by providing a good barrier between atmosphere and the steel-bath-top. Thinner slag permits pickup. So it has to be paid attention.

               In some applications, permitted N and H levels will have a limited threshold values.

OTHERs:

               A good slag permits easier deslagging as it does not cling hard to the ladle inner linings, due to optimal fluidity.

               A good slag permits easier penetration of Al/Ca-cored wires to pierce through into the metal-bath.

               A good slag is less reactive towards the ladle refractory linings. You will have better ladle lives.

               A good slag permits easier carbon pickups during pet-coke or coke breeze additions. You can make Hi-C grades easier in LF units.

               It’s true that good steel making is only a good slag making.

               To put it exactly…… if you donot know how to PRACTICALLY make a good slag, then it’s an unfortunate fact that,….you DO NOT KNOW steel making.

Happy steel making!

LADLE FURNACE

     almost 10 years ago, when my career started, i had a tough time operating Ladle Furnaces (LF). problem was, not that study materials were not available, but there were no easy-to-understand, made-simple guidelines available.

     every theoretical book talked about secondary metallurgy, some discussed about thermodynamics and kinetics in depths. but....what i was missing was, a set of guidelines for beginners like me.....i may call it- LF for dummies. none such was available. and...even after so many years, i could not find any book that bridges the gap between theory and practice in secondary metallurgy units-a gap between colleges and shopfloors. beginners need a kick-starter.

     though not many....,there would be certainly a few like me, who would be happy to find a set of hints that would help operating LFs. recently when my boss asked me for a write-up on that i spent almost 2 hours writing that. later i thought i shall bring it to blog-post, as i could not recently find time to write articles of general interests....and also to keep blog alive. so.....an article that is purely technical....

     here it goes....

STAGES of LF treatment in sequential order:

1. Complete homogenization and ceiling temperature.
2. Slag-killing and desulphurization.
3. Chemistry trimming.
4. Calcium treatment.
5. Settling time/soft rinsing.

Points to note during these stages:

1. As soon as the ladle arrives in LF station, arcing for 2 to 3 minutes shall be done to DISSOLVE tapping-addition-lumps floating on top of the bath. Purging (~4-5bar & 30-40NM3/hr) shall be controlled in such a way to concentrate temperature accumulation on top of melt to dissolve those lumps. Care must be taken not to permit arc-flare which would damage slag-lining.

2. Once lumps are dissolved, hi-purging (>10bar & >100 NM3/hr) is started for homogenizing. Then sample, temperature and slag-sample are taken.

3. Sample is sent to lab for initial analysis. Arcing is restarted to soak the ladle. If it’s a circulation ladle, then temperature is raised around 10-20 degrees above LIFTING temperature, depending upon the remaining time in caster. Shorter the remaining time, lesser the ceiling temperature.

4. Slag-sample shall be dipped in water and cooled. Most likely, it would appear black (FeO) & shiny (Silica). Green-tinged-white (Alumina+CaO) & powdery (CaO) slag should be finally achieved to ensure complete Oxygen killing and basicity in the range of 2.8 to 3.2 (facilitates desulpharisation and inclusion retention. This basicity ensures right fluidity).

5. Aluminum is added (in the form of pellets & cubes. Have more surfacearea-to-volume ratio for better kinetics) that stays afloat on top of the bath, over the slag to reduce black-FeO (FeO+Al=> Fe+Alumina). reaction-collision-rate is much accelerated by full rate purging. (Can be added during arcing also, while purging is kept moderate). On addition of lime in right quantity, desulpharisation takes place in parallel and slag becomes white. (Lime-CaO+Al => Ca-ion+alumina) (Ca-ion+FeS => CaS +Fe)

6. Adding CaO balances Silica, changes shiny slag into powdery slag, (increasing basicity=CaO/Silica) giving better sulphur partitioning.

7. In situ, slag fluidity has to be VISUALLY OBSERVED with full argon purging rate, through the sampling door. It should be mushy (soupy), neither too viscous nor watery.

8. Care must be taken not to add excess CaO that produces thick, dry slag, which impedes both desulphurization and inclusion-retention capacity. Insufficient CaO produces thin (more fluidic) slag that gets easily saturated with lesser S (lesser S partitioning, less (%s)/[%s]) & has poor inclusion absorption.

9. Further, this controlled slag ensures buried arc, better heat transfer and thermal yield and reduced arc-flare (which damages slag-lining, leading to ladle-throughs). Arcing at this stage produces humming sound against blasting, burst sounds produced during dry/open arcing & flaring. It shall also be noted, that too-silent arcing may indicate foamy-oxidising-carryover slag that completely buries the arcing sound.

10. Once slag is killed (white slag), aluminium wire shall be fed into the bath (piercing through the top-slag layer) to kill metal oxygen(leftover &limited). Total aluminium added at this stage is the combination, necessary for killing left-over metal oxygen and elemental, dissolved aluminium required in the grade-chemistry. {Total (Al)} = {Dissolved Al} + {bath-oxygen killing Al}.

11. Once the slag is killed, it’s most likely that metal will have lesser free oxygen; because oxygen in slag & metal is in equilibrium. i.e,. if we kill metalbath, oxygen from slag enters into metalbath. And if we kill slag, oxygen from metalbath enters into slag as they are in equilibrium. So when killed-white slag is produced, metal-oxygen will be very less. So requires only little aluminium to kill it (giving lesser aluminium fading effects over longer holding times; more Al recovery %).

12. Trimming additions are done to meet grade-chemistry. Hi-rate purging ensures complete homogenization.

13. Amount of P reversion indicates the amount of converter-slag carryover.

14. Amount of Si and Mn reversion(without external addition) into metal indicates more reducing capacity of the killed-slag. (because additions done before slag-killing is lost to the slag; but when that slag is killed with better deoxidizer-Al,……after FeO reduction, these Silica and MnO are also reduced and return to metal as Si and Mn)

15. Ca-Fe/Ca-Si/Ca-Fe-Al cored wire is fed to form and modify inclusions (C12A7 & MnS) for better castability and rolling properties.

16. Calcium recovery is in the range of only 15-20% during ideal conditions, as it boils during addition. Excess slag depth, dry slag, more bath top distance from wire discharge point, slower (<220m/min) wire feedrate and non-uniform density of powder in the wirecore reduce recovery further (produce unpredictable Ca pickup). Vigorous boiling, splatters of slag &metal and excess turbulence in ladle are indications of Ca being picked into the bath.

17. Though Ca has high affinity towards oxygen (of alumina), it (>30/35ppm) also reacts with S to produce CaS, when S is available in sizeable (>0.01 wt%) quantity. This solid CaS inclusion can lead to choking, worse than non-Ca heats. This calls for inevitable desulphurization before Ca treatment.

18. It should be noted, both excess and insufficient Ca tend to form solid inclusions that choke SESs. Ca ppm should be proportionate (as to produce ~45%Cao- ~55%Alumina) to alumina in the bath to form liquid C12A7. All other ratios produce solid calcium aluminate, making it difficult to hit the minor Ca window that produces liquid calcium aluminate. Hence attempts should be made to attain ~20ppm of Ca by visual observation for cues during Ca feeding and sampling.

19. Just before lifting heat, settling time of 3-5 minutes should be given with soft rinsing (~3-5bar & 25-35 NM3/hr so as to assist floatation of micro inclusions that don’t have sufficient buoyancy to reach slagtop. Lesser pressure produces smaller bubbles that have more surface area to volume ratio, helping in smaller inclusion floatation. Formation of a small (~10-15cm) eye opening on slag top indicates the right argon flowrate.) Unless this step is religiously followed, be sure to have choking, making all the heat making efforts null and void. This helps in floating smaller solid inclusions through assisted buoyancy and coalescence of liquid inclusions into larger ones. Float maximum, keep rest as liquids.

20. Intermittent slag sampling & observation helps in tracking the progress of heat making. Change of slag appearance over time follows this sequence. Shiny, black (oxidizing) slag >>> dark brown >>> red brown >>> deep green >>> light green >>> greenish gray (very short duration) >>> greenish white & powdery (reducing).

21. Si killed heats are much much easier to kill and desulphurise, compared to only Al killed heats. overkilling of slag in only Al killed heats quickly and easily reduces silica from slag, leading to excess Si in the metal through Si reversion, in turn to offgrades.

22. CaO-lime addition for desulfurisation shall be done in smaller quantities in multiple batches. doing so helps in increasing the S partitioning gradually while providing better reaction collisions as the slag is thinner and more fluidic at the beginning. the quotes, "GOOD STEEL MAKING is only GOOD SLAG MAKING"....."take care of the slag, and that will take care of the metal.." all make real sense.

it would be relevant to add, that these observations were made during the operations of SMS-DEMAG supplied twin station LFs with single-swivel gantry. average heat size was 185 T, and those LFs supplied only Al killed steel ladles to thin slab casters that run at an average speed of 5.8mtr/min during 2007 in india. peak speed touched was 7 mtr/min.i.e., that speed was targeted and maintained throughout the heat size of 185T.

abt grades....mostly CG04** cold rolled galvanised sheets, but mettle-testing grades were PP70**- petroleum pipe line grades and LPG grades...every element on higher side. (i was rarely a success in this grade ;-) ). attempts on IF steels with extra-LC are unforgettable.

other specifications about those casters are LCR-liquid core reduction enabled. that was done from 65mm thickness to 55mm thickness, and was done in multiple stages, rather than achieving it in a single shot. slab widths varied from 950mm to as high as 1550 mm. but most of the production was done in 1250mm sized slabs.

certainly, i am grateful to rbv.ramana sir, ss.upadhyay sir, sridharan unni chakkungal sir, zakir sir, giyas, birendar, babban prasad, bakshi and ispat industries as they PATIENTLY helped me learn all these. hints about specific approach to treat VD/VOD heats shall be added later.

happy to engage in any healthy, value-adding discussions. have a great time.