Continuous Casting Machine Lubrication: Why Steel Mills Use Dual-Line Systems (Application Note)

The Caster Is a Worst-Case Environment for Grease Delivery

A slab or billet continuous caster is one of the longest lubricated machines in any plant. From mold exit to the point where the strand is fully solid — the metallurgical length — a caster typically runs 10 to 40 metres, and the lubrication system must cover all of it plus the run-out roller tables downstream.

The environment attacks the lubricant and the delivery hardware from several directions at once:

• Radiant heat. In the secondary cooling zone the strand surface is typically between 800 and 1,000 °C. Segment roll bearings sit only tens of millimetres from that surface, so grease near the strand sees severe thermal stress even though the bearing itself is water-cooled.
• Water, everywhere. Secondary cooling sprays the strand directly. Washout, emulsification and water ingress into bearing housings are routine, not exceptional.
• Scale and contamination. Iron oxide scale works into seals and fittings and abrades any exposed pipework.
• Point count and distance. A typical slab caster carries 12–36 segments and roughly 300–400 grease points once the mold oscillator, withdrawal straightener, dummy-bar system and roller tables are included. Dual-line architectures in heavy industry scale past 1,000 points without a fundamental redesign.

Every one of those points feeds a spherical roller bearing on a segment roll, guide roll or table roll. A single bearing that runs dry can seize a roll, mark or crack the slab surface, and force an unplanned segment change — an expensive event on a machine that earns money only while it is casting.

Why Dual-Line Became the Steel Mill Standard

Three architectures exist for centralized grease distribution — single-line, progressive, and dual-line lubrication systems — and caster builders and mill maintenance departments converged on dual-line for reasons that map directly onto the environment above:

• Fault tolerance by design. Dual-line metering valves are driven by alternating pressure in two main lines. Each valve strokes independently when the line is pressurized. If one outlet is blocked or a line to one bearing is cut by scale, the rest of the system keeps delivering. In a progressive divider, by contrast, one blocked point mechanically stops the whole block sequence.
• Long main lines. Dual-line systems are engineered for high working pressure — metering valves such as the ZVB and VSG families are rated to 400 bar (40 MPa) — which overcomes the flow resistance of stiff NLGI 2 grease pushed through tens of metres of steel pipe. Caster systems in practice operate anywhere from about 80 to 350 bar depending on line length and grease grade.
• High point counts and easy expansion. Extra valve sections or whole valve groups can be teed into the two main lines without re-engineering the pump station. This matters when a caster is upgraded with additional table rolls or a new segment.
• Compatibility with stiff, water-resistant greases. The high-pressure positive-displacement metering of a dual-line valve handles NLGI 1–2 calcium-sulfonate greases that lighter-duty systems struggle to push.

A Typical Zoned Layout on a Slab Caster

Mills rarely treat the caster as one lubrication zone. The machine is divided into areas with different heat loads, water exposure and bearing duty, and each area gets its own metering valve groups and cycle settings. A representative layout looks like this:

These figures are typical engineering ranges consistent with a 300–400 point machine; the actual count on any given caster depends on roll diameter, segment design and table length. Two layout principles hold across nearly all installations:

• Per-segment valve groups. Each segment carries its own dual-line group mounted on or beside the segment frame. When a segment is swapped during a maintenance outage, only its quick-disconnects are opened — the main lines stay pressurized and the rest of the machine keeps its lubrication schedule.
• Valves outside the radiant zone. Metering blocks are mounted behind heat shields or on the cold side of the strand, with only the feed lines and bearing fittings crossing into the hot zone. FKM (Viton) seal variants, rated to +120 °C, are standard practice anywhere near the spray chamber.

Selecting the Metering Valve: ZVB vs VSG

The two valve families most commonly specified on caster dual-line systems cover the same pressure class but differ in how the dose is set:

• ZVB dual-line valves are hydraulically operated, fixed-output metering valves. Each section is factory-calibrated to one of ten metering volumes between 0.03 and 3.0 cm³ per half-cycle, with 1 to 8 outlets per block. Working range is 20–400 bar, and the valve tolerates greases from NLGI 000 to NLGI 3. Fixed doses make ZVB sections tamper-proof — a practical advantage on a caster floor where a well-meaning adjustment can quietly double a segment’s grease consumption.
• VSG metering blocks offer stepless adjustment from zero up to 1.5 cm³ per outlet per stroke (0.14 mL per turn of the metering screw), 35–400 bar working range, 2–8 outlets, and a G3/8 main-line inlet — a larger bore than the G1/4 of the smaller VSKH/VSKV series, which keeps pressure drop low on long main-line runs feeding many points. Maximum outlet back-pressure is 30 bar, and FKM seal versions cover the +120 °C locations.

A common pattern is VSG groups in the spray-zone segments — where bearing sizes and therefore dose requirements vary within one segment — and fixed-output ZVB groups on the oscillator and roller tables, where doses are uniform and locked settings are preferred.

Grease Selection for Caster Duty

The metering hardware only works as well as the grease it pushes. Caster bearing grease has three non-negotiable properties:

• High-temperature stability. Calcium-sulfonate complex greases have become the default for continuous caster bearings, with dropping points around 300 °C — far above what conventional lithium greases offer — and good structural stability near the strand.
• Water resistance. Secondary cooling guarantees large-scale water contact. Calcium-sulfonate thickeners combine inherent water washout resistance with corrosion protection, which is precisely why the metals industry adopted them for caster and roll-stand duty.
• Pumpability at NLGI 1–2. NLGI 2 is the most common choice in the segments; NLGI 1 or softer is used where main lines are long or ambient temperatures drop, because the dual-line pump and 400-bar valves can only push what the pipe friction allows. High-viscosity mineral base oils (roughly ISO VG 460 class) provide the film thickness segment bearings need at low speeds and high loads.

Whatever grease is chosen, it must be qualified against the metering valves’ operating window — both the ZVB (NLGI 000–3) and VSG (NLGI 0–2) families have defined grease ranges, and exceeding them is a common root cause of incomplete valve strokes.

Dual-Line vs Progressive on a Caster: The Honest Trade-off

Dual-line is not superior in every respect, and engineers should be clear about what they give up:

• Monitoring. A progressive divider’s mechanical sequence gives positive blockage indication — if the block stops cycling, the monitor alarms immediately. A blocked outlet on a dual-line valve is silent. Caster installations compensate with end-of-line pressure switches on each main-line branch, cycle counters at the pump station, and grease-consumption logging per zone; newer systems add per-point monitoring at the valve.
• Dose precision. Progressive dividers meter very small, highly repeatable volumes. For the relatively large doses caster bearings consume, dual-line metering accuracy is more than adequate — but it is not the right tool for microlitre oil dosing.
• Hybrid layouts. Many mills combine both: dual-line main lines cover the long distances, and small progressive dividers sit downstream of dual-line valves inside dense segment roll packs. This gets dual-line’s fault tolerance and reach with progressive metering density at the point of use.

For a broader comparison of the architectures, see our overview of centralized lubrication systems.

Practical Design and Maintenance Recommendations

• Pipe, not hose, in the hot zone. Use steel main lines and high-temperature fittings near the spray chamber; reserve flexible hose for the oscillator and other moving connections.
• Specify FKM seals near the strand. The +120 °C FKM variants of ZVB/VSG valves cost little more and survive locations where standard NBR seals harden and leak.
• Instrument the ends of the lines. A pressure switch at the far end of each main-line branch is the cheapest reliable confirmation that every valve in that branch has cycled.
• Discipline shutdowns and restarts. Segment changes and caster outages are when air pockets and water enter the system. A written procedure for depressurizing, isolating, refilling and re-verifying each zone — such as the one in our guide to steel mill lubrication shutdown and restart procedures — prevents the classic post-outage failure pattern.
• Trend grease consumption per zone. A rising or falling consumption trend in one zone is the earliest indicator of a leaking line, a stuck valve or a grease that no longer matches the season — long before a bearing complains.