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!