Internal quality improvement of slabs cast from caster 3, RSP

with soft reduction technology

Author: Prahallad Lenka

Dy. General Manager (operation), SMS-II, RSP, SAIL,

E mail-prahallad.lenka@sailrsp.co.in

Introduction: Rourkela steel plant is one of the five integrated steel plants under SAIL;

the public sector Maharatna Company. Slab caster 3; with most advanced technological controls

has been added under the capacity enhancement project of the steel plant. Online soft reduction

with use of HSA segments helped in improving internal quality of the slabs for plate/structural

grades to a large extent and further improvement is in the process.RSP has already exported 0.22

Million ton of slabs to various countries from the new caster.

For continuous casting of semis like slabs, centre segregation and centre porosity are

usually the main bottleneck to develop high strength steel grades with qualified mechanical

properties. For this reason, the soft reduction technology of continuous casting has been one of

key technologies intensively adopted by modern steel mills world over.

Due to the volume contraction associated with the solidification process of liquid steel, the

shrinkage cavity is produced at the crater of the final solidification region. The volume contraction of

steel always happens upon its solidification .The centre porosity of the castings will also appear in the

case of poor feeding or compensation. The other aspect is that the centerline segregation is always present

in slab castings due to solute redistribution during solidification and fluid flow induced by thermal

convection. The appearance and intensity of center segregation and center porosity or other defects are

normally closely related to steel grades, superheat of liquid steel, casting speed and cooling history.

For these reasons, soft reduction technology is used to compensate the solidification shrinkage of

slab castings through the reduction of roll gaps in the region of centerline mushy zone., Online soft

reduction together with dynamic secondary cooling has been proved to be an effective way to minimize

the center segregation and center porosity through on-line optimization of the cooling and volume

compensation in terms of the variation of casting conditions (fig 1).

. The appearance and intensity of center segregation is related to steel grades, superheat of liquid steel,

casting speed and cooling history. Online soft reduction together with dynamic secondary cooling has

been proved to be an effective way to minimize the center segregation and center porosity through on-line

optimization of the cooling and volume compensation in terms of the variation of casting condition.

Fig1

The online soft reduction minimizes the formation of internal voids in the slab caused by

volume shrinkage, and prevents the inter-dendritic fluid flow of the condensed liquid steel

especially in the transverse direction. On the other hand, the compression produced by the soft

reduction can also promote the condensed liquid steel to flow in the reverse direction of casting,

which may lead to the redistribution of solute elements in the liquid steel. Therefore, the

solidified structure of slab castings is usually more uniform and compact under soft reduction

with reduced centerline segregation and porosity.

The key technology of online soft reduction is used in tandem with DSC; the dynamic

spray water control (or dynamic solidification control) model to accurately predict the location of

the centerline mushy zone and to determine the position( in respect to the segments) of soft

reduction and a total soft reduction amount with the consideration of the different steel

characteristics.

Fig2A (middle piece without soft reduction)

Fig 2B (middle piece with soft reduction)

To improve internal quality of the slabs, the dynamic secondary cooling and soft reduction

control facilities have been commissioned for slab caster 3. The DSC technology in the monitoring mode

runs parallel to the casting process and outputs the following results (figure 3)

• Strand temperatures in thickness- and casting direction• The shell thickness along casting length• Position of the solidification point• Liquid fraction along the casting length

Based on a mathematical physical model of the casting process the monitoring mode lead to an additional

insight into otherwise not observable casting results like complete temperature profiles, shell thickness,

solidification point and solid/liquid fractions

Fig3 (DSC on line screen)

Soft reduction with the dynamic solidification control (DSC) technology has improved the

internal quality of the slabs. The use of soft reduction cast from caster 3 from Mannesmann standard 3 to

standard 1 in respect to centerline segregation and transverse cracks in most of the cases. Some typical

sulphur prints of slab samples from caster 3 have been given here for comparison (fig 4 &5)

Fig 4A without soft reduction

Fig 4B without soft reduction

Fig 5A with soft reduction

Fig 5B with soft reduction

Internal quality improvement of slabs cast from caster 3, RSP

with soft reduction technology

Author: Prahallad Lenka

Dy. General Manager (operation), SMS-II, RSP, SAIL

E mail: prahallad.lenka@sailrsp.co.in , prahalladlenka@gmail.com

Introduction: Rourkela steel plant is one of the five integrated steel plants under SAIL;

the public sector Maharatna Company. Slab caster 3; with most advanced technological controls

has been added under the capacity enhancement project of the plant .Dynamic soft reduction

with use of HSA segments helped in improving internal quality of the slabs for plate/structural

grades to a large extent and further improvement is in the process.

Due to the volume contraction associated with the solidification process of liquid steel,

the shrinkage cavity is produced at the crater of the final solidification region. Another aspect is

the centerline segregation is always present in slab castings due to solute redistribution during

solidification and fluid flow induced by thermal convection. For these reasons, soft reduction

technology is used to compensate the solidification shrinkage of slab castings through the

reduction of roll gaps in the region of centerline mushy zone. The online soft reduction minimizes

the formation of internal voids in the slab caused by volume shrinkage On the other hand, the

compression produced by the soft reduction can also promote the condensed liquid steel to flow

in the reverse direction of casting, which may lead to the redistribution of solute elements in the

liquid steel. Therefore, the solidified structure of slab castings is usually more uniform and

compact under soft reduction with reduced centerline segregation and porosity.

The key technology of online soft reduction is used in tandem with DSC; the dynamic

spray water control (or dynamic solidification control) model to accurately predict the location

of the centerline mushy zone and to determine the position of soft reduction and a total soft

reduction amount with the consideration of the different steel characteristics.

The use of soft reduction technology has improved the internal quality of the slabs cast

from caster 3 from Mannesmann standard 3 to standard 1 in respect to centerline segregation

and transverse cracks.Key words: shrinkage cavity, Centerline Segregation, mushy zone, roll

gap, DSC & HSA segments