By Jack Xing, Technical Sales Director, YK Chanshine · Published October 29, 2026 · 12 min read
Short answer: A high-purity MgO dry vibration mix (MgO 85% to 92%) stays stable against dissolved aluminum and manganese. A silica board does not: its SiO2 is reduced by Al and Mn, adding oxygen and silicon to the steel and causing aluminum fade and alumina inclusions. For rebar and merchant bar, a silica board can still be a sensible, low-cost choice. For clean steel, automotive grades, bearing steel, tyre cord and sour-service linepipe, magnesia is mandatory.
Why the Tundish Lining Decides Clean Steel Quality
Clean steel is refined in the ladle. But between ladle and mould, liquid steel spends 15 to 40 minutes inside the tundish. During that time, the steel is in direct contact with the working lining. That contact is either inert or reactive.
If the lining is inert, inclusions float up into the covering flux, total oxygen (T.O.) drops, and the steel arrives at the mould cleaner than it left the ladle. If the lining is reactive, dissolved deoxidizers attack the refractory oxides, new inclusions form continuously, and the steel picks up oxygen. A mill that spends thousands of dollars per heat on secondary metallurgy can lose all of that investment in a 20-minute tundish sequence.
Total oxygen in steel comes from four places inside the tundish:
- Chemical reaction: Dissolved deoxidizers (Al, Mn, Si) reduce oxides in the lining.
- Moisture and binders: Water or organic binders release hydrogen, carbon monoxide and oxygen during heat-up.
- Air ingress: Open joints or loose boards let air draw in through negative pressure zones.
- Physical erosion: Liquid steel washes weak refractory particles into the stream, where they become macro-inclusions.
The Core Chemistry: How Steel Reduces Silica
Silica (SiO2) is thermodynamically less stable than alumina (Al2O3) or magnesia (MgO) at continuous casting temperatures (1520°C to 1580°C). When liquid steel containing dissolved aluminum contacts a silica board, a reduction reaction occurs spontaneously:
This reaction produces three distinct problems on the shop floor:
- Aluminum fade: Soluble aluminum in the steel is consumed by the lining. Steelmakers must either over-alloy with expensive aluminum in the ladle or risk dropping below the minimum [Al] specification during long sequences.
- Silicon pickup: The reduced silicon enters the steel melt. For low-silicon grades, this pickup can push the chemistry out of customer specification.
- Endogenous alumina inclusions: The Al2O3 formed at the refractory hot face does not stay attached to the wall. Turbulent steel flow sweeps fine alumina clusters (2 to 20 μm) into the strand, where they cause nozzle clogging, surface slivers and ultrasonic test failures.
Why MgO Stays Stable
Magnesium oxide (MgO) sits below SiO2 on the Ellingham diagram. Dissolved aluminum and manganese cannot reduce pure MgO at steelmaking temperatures. Instead, any aluminum in contact with MgO forms a thin, dense layer of magnesium aluminate spinel (MgAl2O4):
This spinel layer acts as a solid diffusion barrier. It stops further reaction, prevents steel penetration, and does not shed inclusions into the bath.
Side-by-Side Comparison: Property by Property
| Property | MgO Dry Vibration Mix (MgO 85% to 92%) | Silica-Based Tundish Board |
|---|---|---|
| Main Chemistry | MgO 85–92%, SiO2 ≤ 3.5%, CaO ≤ 2.5%, Fe2O3 ≤ 1.5% | SiO2 75–88%, Al2O3 5–12%, Fe2O3 1.5–3.0% |
| Installation Method | Vibrated dry around a heated former, cured at 220–280°C | Pre-cured boards placed by hand, mortared or wedged |
| Water Addition | 0% (strictly dry installation) | 0% (factory dried, but organic binder holds moisture) |
| Total Oxygen Impact | Stable: T.O. 8 to 18 ppm typical on clean grades | Reactive: T.O. 25 to 55 ppm typical; severe [Al] fade |
| Hydrogen Pickup | Very low (< 0.5 ppm [H]) with inorganic binder | Moderate to high (1.0 to 2.5 ppm [H]) from resin volatilization |
| Sequence Life | Up to 16 to 24+ heats with proper slag conditioning | Typically 1 to 4 heats maximum before board degradation |
| Deskulling Performance | Clean separation from permanent lining after cool-down | Often sticks to permanent lining, high skull removal labor |
Matching Steel Grades to Lining Selection
| Steel Grade | The Main Risk to Cleanliness | Silica Board Performance | MgO Dry Mix Performance |
|---|---|---|---|
| Al-Killed Deep Drawing (LCAK / IF) | [Al] reduction of silica; alumina cluster slivers on exposed panels | Severe quality loss, frequent rejections | Excellent; inert hot face maintains dissolved [Al] |
| Bearing Steel (100Cr6 / 52100) | Macro-inclusions > 15 μm cause contact fatigue failure | Unusable; fails microscopic cleanliness ratings | Standard choice; provides low oxygen and high fatigue life |
| High-Manganese Steel | [Mn] reduces SiO2 forming low-melting rhodonite/spessartite | Rapid board erosion and slag foaming | Resistant; basic MgO withstands high MnO slags |
| Rebar & Structural Carbon Steel | Cost per tonne; sequence turnaround speed | Acceptable; economical for short sequences | Economical only when long sequences (> 8 heats) are run |
Integrated Tundish Flow Furniture
The working lining does not work in isolation. For total oxygen control, high-purity MgO dry mix must be paired with precast flow modifiers:
- Turbostop / Impact Pad: Dissipates incoming ladle stream energy, preventing slag eye opening and air entrainment.
- Weirs & Dams: Redirect steel upward toward the covering flux layer, forcing non-metallic inclusions to float out before reaching the nozzle.
- Well Blocks: Seat the metering nozzles and avoid funnel vortexing during ladle changeovers.
How to Run a Tundish Lining Trial
Do not switch suppliers or materials based on catalogue promises. Measure the difference in your own shop using this 4-step protocol:
- Set a baseline: Sample total oxygen (T.O.) in the tundish on 10 consecutive heats using your current lining.
- Log aluminum fade: Record soluble [Al] from the ladle test vs. the tundish test at heat mid-point.
- Check the deskull: Measure the turnaround time and permanent lining repair depth after removing the tundish skull.
- Calculate net cost: Compare material price against savings from extended sequence length, reduced deoxidizer consumption, and lower downgrades.
Frequently Asked Questions (FAQ): Tundish Refractory Linings & Total Oxygen
Does a silica board always hurt clean steel?
No. For plain carbon and low-alloy grades without tight oxygen limits, a silica board is often acceptable and cheaper. It becomes a problem with Al-killed, Ca-treated, high-Mn and low-T.O. grades, where its SiO2 is reduced by Al and Mn.
Why does silica react with aluminum in the tundish?
Aluminum has a stronger affinity for oxygen than silicon does. Dissolved Al takes oxygen from the SiO2 in the lining, forming Al2O3 and releasing silicon into the steel. The result is Al fade, silicon pickup and new alumina inclusions.
Can a magnesia lining add inclusions?
Yes, if it is wet, porous or poorly sintered, or if the mix contains too much carbon. Particles shed from a weak hot face can become inclusions, and carbon with magnesia can add Mg. A dense, dry, well-sintered face with low carbon avoids most of this.
What MgO content should a tundish dry mix have?
Most grades for clean steel sit between about 85% and 92%. Impurities matter as much as the MgO number. Low SiO2, low CaO and a good grain structure often beat a higher MgO figure with poor impurity control.
How much can the lining lower total oxygen?
There is no fixed number. The lining is one lever among several: shrouding, flow control, cover flux and ladle practice all count. Measure it by heat, on the same grade, before and after the change.
Which steel grades need magnesia and not silica?
Al-killed low carbon, IF, Ca-treated, high-Mn and bearing steel are the usual ones. They either need tight cleanliness, or they attack silica directly.
Does dry mix reduce hydrogen pickup compared with a board?
It can. Dry vibration mix is installed without water, so there is no water to drive off. The binder still matters, so for low-hydrogen and low-carbon grades we choose an inorganic or low-carbon binder.
How do precast impact pads and weirs help total oxygen?
They control where the steel lands and how it flows. A dense precast impact pad avoids erosion at the most turbulent spot, and weirs and dams give inclusions time to float to the flux. They work with the lining, not instead of it.
How do I measure whether a lining change helped?
Compare T.O. in ppm by heat on the same steel grade, before and after. Track Al fade, Si and Mn pickup and inclusion rating at the same sampling points. Check the used lining for erosion and sintered depth.
What data do you need to design a custom tundish lining?
Steel grades and sequence length, T.O. and inclusion targets, tundish size and drawing, current lining and its life, cover flux type, and any clogging or Al fade problems you see today.
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