Reduction of Slivers DueTo Nonmetallic Inclusions inContinuous Casting

This is one of the major defects observed in steel slabs that appear as an extra 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.

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 depending upon its EBT life.

Figure  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. Itis 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.

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