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.
