Solving the Steel Industry's Blind Spot: Real-Time, Multi-Depth Tundish Temperature Monitoring

For decades, continuous casting operations have accepted a critical process limitation: a significant thermal “blind spot” inside the tundish. Maintaining precise control over the molten steel temperature and bath level is fundamental to ensuring high-quality steel. Yet the industry has relied almost exclusively on manual, disposable, single-point dip probes.

This traditional methodology provides only a fleeting snapshot — a single data point at one specific depth and moment in time. Manual tests are labor-intensive, create hazardous working conditions by exposing operators to extreme radiant heat, and crucially, miss dynamic events occurring between measurements, such as ladle exchanges or subtle temperature stratification.

A groundbreaking development detailed in a recent paper published in AIST Transactions^[1] has overcome these limitations. This work successfully validated the first-ever industrial deployment of a multiplexed, multi-depth sapphire Fiber Bragg Grating (sFBG) temperature sensor array capable of withstanding the brutal conditions of a steelmaking tundish.

This is not a theoretical concept — this ruggedized system survived 6.5 consecutive heats immersed in an active slab caster tundish at the SSAB Iowa facility, providing continuous, real-time, multi-depth thermal data.

Engineering for the Industrial Inferno: The Sapphire Advantage

The primary obstacle to deploying advanced sensors in molten steel is the extreme environment. Temperatures exceed 1,600°C, and the molten slag is highly corrosive. Conventional silica-based optical fibers fail rapidly under these conditions due to dopant diffusion and hydrogen darkening. Similarly, standard point thermocouples are prone to thermoelectric drift and chemical attack.

The solution necessitated a material overhaul, utilizing single-crystal sapphire optical fibers. With a melting point exceeding 2,000°C and inherent chemical inertness, sapphire provides the robust substrate required for high-temperature industrial sensing — the same fiber class covered by the FBG calibration tool on this site.

However, placing a delicate fiber-optic assembly inside a vibrating, high-stress steelmaking vessel requires more than just resilient fiber; it demands revolutionary packaging.

The Innovation: A Ruggedized, Multiplexed Probe

The core innovation centered on re-engineering an existing Vesuvius commercial tundish probe system. The objective was to integrate a custom sFBG array alongside a standard platinum B-type thermocouple for direct comparison, as shown in Figure 1 below.

The central fabrication challenge was housing the 125 µm optical fiber within the protective sheath alongside the thermocouple. This was achieved by precision wheel-grinding a standard alumina tube, removing half its profile to create a D-shaped configuration. This ingenious modification provided the necessary internal clearance for the single-crystal sapphire tube sheath containing the sFBG array.

A 3-sFBG array was subsequently inscribed onto a 1-meter sapphire fiber, with specific inter-sensor spacing. This cascaded multiplexing approach — the same principle behind point-by-point vs. line-by-line FBG inscription — allowed simultaneous temperature monitoring at three distinct depths along a single fiber.

The Power of Distributed Sensing: Unprecedented Process Visibility

The industrial trial at SSAB Iowa demonstrated that the packaged 3-sFBG probe provided continuous, accurate data streams, vastly superior to intermittent dip tests. This new level of visibility — the same principle behind why a continuous thermal profile shows what a single point can't — yielded three critical insights.

Continuous Thermal Profiling

Unlike the single snapshot of a dip test, the system generated a continuous spatial-temporal thermal map of the tundish, tracking transient thermal fluctuations throughout entire casting sequences.

Real-Time Slag-Line Tracking

The uppermost sensor (sFBG₁) was positioned precisely at the steel-slag interface. It successfully captured slag-line variations and steel level fluctuations during critical periods like ladle exchanges.

Detection of Superheat Stratification

The distributed layout identified a persistent vertical temperature gradient within the tundish. Hotter steel consistently remained deeper in the bath, while significantly colder steel was measured near the surface.

Reliance on surface measurements alone — as with traditional dip probes — can lead to significant underestimation of the bulk bath temperature, potentially causing operators to waste energy by overheating ladles.

Implications for the Steel Industry: Quality, Safety, and Automation

The successful industrial validation of this sapphire FBG distributed temperature sensing (DTS) system represents a paradigm shift for metallurgical process control.

Enhanced Product Quality

By providing a complete thermal profile rather than isolated moments, steelmakers can refine thermal management, reduce variability in casting temperatures, and mitigate quality defects such as inclusion entrapment and macrosegregation.

Operational Savings and Safety

Continuous, deep-immersion monitoring allows operators to safely reduce superheat targets, potentially lowering electrical energy consumption at the heating station by 1.2–2.5%. Furthermore, it reduces personnel exposure to hazardous manual measurements.

The Foundation for Digital Twins

This high-resolution, real-time spatial data stream is the critical missing component for advanced process modeling, feeding data to CFD-based casting digital twins. This paves the way for closed-loop automation of casting speeds and cooling parameters, ultimately stabilizing the solidification front and improving overall steel cleanliness.

Conclusion

This transition from intermittent guesswork to continuous, data-driven knowledge marks a significant step forward in the digital transformation of the steel industry.

Reference

1. F. Mumtaz et al., “On-Line Multidepth Thermal Profiling and Level Tracking in a Continuous Caster Tundish Using Distributed Sapphire FBG Sensors,” AIST Transactions, vol. 23, no. 8, Oct. 2026. DOI: 10.33313/TR/1026