Reduced Nonmetallic Inclusions in Steel Using Next Generation Disposable Tundish Lining

William. N. Porter VGH Refractories & Equipment LLC

Mason, Ohio, USA

porter@vgh-ag.com

The Tundish as a Metallurgical Vessel Well Known Practices

• Tundish furniture for increased residence time and floatation of nonmetallic inclusions.

• Dry vibratable refractory working lining for reduced hydrogen ppm pickup vs. tundish sprays that contain > 25% H2O.

• High temperature preheating for less heat lost from steel during casting for improved steady state conditions.

Tundish Working Lining Refractory

• Both tundish spray and organically bonded tundish dry vibratable refractory utilize several binders. The initial binder will burn out at temperatures well below steel casting temperatures; so a low temperature binder and a high temperature binder must be utilized.

• Both tundish spray refractory and organically bonded tundish dry vibratable refractory contain fluxes such as boron and silicate based additions to bond the refractory aggregate at steel casting temperatures.

Novel Refractory Binder SystemReduces Formation of Nonmetallic Inclusions

• Eliminates need for multiple binders.• Does not burn out during heat up; stable over entire

casting temperature range.• Does not require addition of water, organic resins or

sugars, boron based or silicate based fluxes.• Less reactive with Oxygen or Aluminum in steel for

reduced formation of Alumina and Spinel inclusions.• Higher hot strength for improved erosion resistance.• Lower thermal conductivity for less heart lost from

steel to tundish.

Tundish as Source of Reoxidation of Steel

• Mantovani, Moraes, da Silva et al (2013) observed three interactions at the tundish refractory/liquid steel interface:

1. Steel infiltration into the open porosity of the tundish refractory. This is most apparent with the wet tundish spray materials.

2. Oxidized layer of solidified steel (FeO formation) at the steel/refractory interface. This is apparent in both tundish spray and tundish dry vibratable refractory linings.

3. Nonmetallic oxide formation in the liquid steel phase nearest to the steel/refractory interface: Al2O3, MgO; SiO2, MnO, and MgAl2O4 spinel.

Tundish as Source of Reoxidation of Steel

• Mantovani, Moraes, da Silva et al postulate that the formation of the nonmetallic inclusions observed in the tundish are due to solid phase reactions.

• Given the melting points of MgO are well above steel casting temperatures and much greater than 1700 oC (3100 oF), we postulate that liquid formation occurs in the tundish refractory throughout the steel casting period.

• Our research indicates that the presences of boron based fluxes such as boric oxide, borosilicate or boric acid in the refractory lowers the melting point forming a liquid surface on each of the MgO refractory grains at the hot face of the working lining.

Tundish as Source of Reoxidation of Steel

• Gan, Chen, He, et al (2012) have published a typical formulation for the tundish working lining refractory material containing:

• Magnesia (MgO) as main aggregate.• Borax (Na2B4O7·5H2O) as a flux.

• Silica Powder (SiO2) for formation of glassy binder phase.• Phenolic Resin as a low temperature binder.

• Our research indicates the addition of boron and silica forms low melting liquid phases for faster liquid-liquid reactions at the refractory steel interface.

Tundish as Source of Reoxidation of Steel

• Buoro and Romanelli (2012) have shown that the ideal tundish working lining refractory must be of low oxygen potential and not be so reactive to alter the steel composition.

• Buoro and Romanelli (2012) have also shown that the formation of MgAl2O4 spinel inclusion can also be related to the thermal conductivity and the depth of sinter of the refractory.

• With addition of fluxes and silicates there is an increase in densification at the refractory hot face for higher thermal conductivity, greater depth of sinter and increased potential for reactions at the steel/refractory interface.

Tundish as Source of Reoxidation of Steel

• Pal, Bharati, Krishna (2012) et al have shown that sintering behavior of the tundish refractory impacts phase transformation and thermal conductivity. Greater sintering in the refractory promotes greater formation of nonmetallic inclusions.

• They state the ideal tundish refractory should exhibit high strength and low thermal conductivity over the full range of casting sequences and that attention to following leads to reduced formation of nonmetallic inclusions:

• Binder Selection• Grain Sizing• Pore Formation • Refractory Quality

Formation of Inclusions

MgO(Refractory) + B2O3(Refractory) + FeO(Interface)+ Al(Steel)

TO

Al2O3(Steel) + MgAl2O4(Steel)

New Dry and Inorganic Bonded Tundish Refractory

• No fluxes or silica additions.• Conventional cement type bond forms when a

hydrous magnesia salt is exposed to heat.• The magnesia salt serves as a catalyst for formation

of strong bonds that are stable and do not form liquid phases over the entire casting sequence.

• Depth of sinter and thermal conductivity of the refractory are reduced.

• Hot or Cold Installation.

New Dry and Inorganic Bonded Tundish Refractory

Hot Tundish Installation

• Residual heat from previous cast is sufficient heat source to form the inorganic bond.

• Additional benefits include:– Time savings for faster

tundish turnaround. No time lost for cooling the tundish down or heating it back up.

– Energy savings. No added energy is required.

Hot Tundish Installation

• Open bottom form is set into the warm tundish.

• Refractory material is placed into the gap between the form and the tundish.

• After 45-60 minutes of curing time the form can be removed.

• Flow control devices (stopper rods, SENs, tundish furniture) can be installed during the curing period.

• After curing, the tundish is ready to be sent to the caster.

Bulk Hopper for Quick Installation

Cold Tundish Installation

• Similar to current installation for dry vibratable refractory.

• Uses same form as DV which includes a heat source.

• Material is placed into the gap between form and tundish.

• Unlike dry vibratable installation, form vibration may not be required.

• Form can be removed after 45-60 minutes of curing at 230 oC (450 oF).

Importance of Thermal Conductivity

• Many studies agree that thermal conductivity of the tundish refractory may affect the steel quality:

• Temperature lost from steel super heat to heat the refractory is greater when the refractory has a higher conductivity.• Thermal conductivity is generally proportional to

density; higher density imparts higher conductivity.• Higher conductivity increases the depth of sinter.• Flux additions increase both the sintered density and

the depth of sintering for higher thermal conductivity.

Importance of Thermal ConductivityDry and Inorganic Tundish

Importance of Thermal ConductivityDry and Inorganic Tundish

• Elimination of fluxes and replacement of glassy bond with refractory cement bond achieves:

• Reduced Δ T. Less heat lost from steel during casting.• Observed super heat variation from start to end of

casting is less than 1.5 oC (3 oF).• Lower temperature at tundish working lining refractory

to tundish back up lining refractory (castable).– Less reaction between working lining and back up lining.– Cleaner deskulling.– Longer life of tundish back up lining.

The Use of Olivine

• Olivine is a naturally occurring magnesia silicate material where MgO and SiO2 are locked in tight forsteritic bonds.

• The use of Olivine as a substitute for magnesia in conventional tundish spray and dry vibratable has not met with great success due to increased fluxing of the olivine by the fluxes used as binders versus Magnesia.

• This increased fluxing not only negatively impacted density and thermal conductivity, but also reduced corrosion resistance of the refractory during casting.

The Use of Olivine

CONCLUSIONS

• Steel cleanliness is improved:– Formation of nonmetallic inclusions reduced.– Hydrogen ppm in the mold is reduced.

• Improved steady state casting conditions.– Less temperature lost from steel during casting.

• Improved castability with less nozzle clogging.

• Addition of olivine is made possible by elimination of fluxes in the refractory:– Reduced reaction rates between refractory and steel.– Less nonmetallic formation than an all MgO lining.– Lower cost per ton of steel cast.

Thank You