A steel tube mill has to hold size shift after shift, on a strip that never quite matches the last coil. The parts that decide whether it does are the roll tooling, the weld box and the cut-off — and those are exactly the parts we build ourselves, on one site, under one management.
Built Around Your Product
- Round, square and rectangular steel tube from ø8 mm to ø355 mm.
- Mild steel, galvanized and pre-painted strip.
- Line speeds and stand counts set to your wall thickness and output.
- Single stations supplied to upgrade a mill you already run.
Tell us the tube sizes you sell and the output you need per shift; we will propose the HG model and the tooling, and quote the stations separately so you can see what each one costs.
A Steel Tube Mill Is a Strip Machine First
Before anything is formed, the coil has already decided what the mill can do. Strip width, strip thickness and steel grade set the pass line, the weld power, the sizing stands and the saw. Most of the arguments about “the mill is not holding size” start here, at the entry end, not at stand seven.
Three Numbers That Come Before the Machine
- Strip width in millimetres, and the tolerance your slitter can hold on it.
- Strip thickness and the thickness tolerance of the coil, not the nominal figure.
- Yield strength and surface condition — pickled, cold rolled, galvanized or pre-painted.
A mill quoted without these three is a mill quoted for a tube that does not exist yet. Every HG model we build is configured from them.
Strip Width Is Calculated, Not Estimated
For round tube the developed width is the circumference at the mean diameter, so W = 3.1416 × (OD − t). The figure below is the theoretical width; the pass designer adds or subtracts a weld allowance of a few tenths of a millimetre depending on wall and process.
| Tube OD (mm) | Wall (mm) | Theoretical strip width (mm) |
|---|---|---|
| 25.4 | 1.2 | 76.0 |
| 48.3 | 2.0 | 145.5 |
| 60.3 | 2.0 | 183.1 |
| 114.3 | 3.0 | 349.7 |
| 219.1 | 4.0 | 675.9 |
| 355.6 | 6.0 | 1098.4 |
The tolerance matters as much as the number. A coil slit 0.5 mm wide arrives as an external fin the scarfing tool has to fight; a coil slit 0.5 mm narrow arrives as an under-filled weld and a groove on the inside of the tube. If your slitter holds ±0.5 mm, say so at enquiry stage — the pass design can absorb it, but only if the designer knows.
Square and rectangular tube uses the same idea on a different section: the developed width is the perimeter of the finished profile, less the four corner allowances, plus the weld allowance. Corner radius is a customer decision that has to be on the drawing.
Hot Rolled, Cold Rolled, Galvanized: Four Different Behaviours
Hot rolled, pickled and oiled (HRC / HRPO)
The workhorse for structural and general tube. Scale and surface variation are normal, so the forming rolls need a slightly generous entry and the weld area needs clean edges. Width tolerance is usually the widest of the four.
Cold rolled (CRC)
Tighter thickness tolerance and a better surface, which is why furniture and automotive tube is usually cold rolled. It also forms more easily, which is a trap: it is easy to over-bend the first passes and then fight edge wave for the next six stands.
Galvanized (Z120 to Z275)
Two adjustments, not one. First, the corner radius: zinc coating cracks at tight radii, so a galvanized job needs a larger radius than the same size in black steel. Second, the rolls: zinc picks up on a rough roll surface and builds a layer that changes the pass. Galvanized work needs polished rolls and a cleaning routine that black steel does not.
Pre-painted
The coating is a product in itself. Rolls must be polished, the strip must run on the film side without sliding, and any coolant or lubricant that attacks the paint is out. If you plan to run pre-painted coil on the same mill as black steel, plan a roll change with the schedule.
Wall-to-Diameter Ratio Decides How Many Stands You Need
Dividing wall by diameter gives the number that shapes the machine more than any other. Thin wall needs speed and precise edge alignment; heavy wall needs power, more stands and a slower line.
| t / D | What it changes | Typical product |
|---|---|---|
| under 1.5% | Narrow weld window, high line speed, edge alignment critical | Furniture tube, thin-wall conduit |
| 1.5% to 5% | The normal band — most lines are designed here | Scaffold, water pipe, general structural |
| 5% to 8% | Slower speed, more forming stands, heavier sizing | Heavy structural, piling, mechanical tube |
| over 8% | Special pass design, high weld power, careful cooling | Bearing tube, roller shell, thick-wall mechanical |
Springback Follows Yield Strength
S355 springs back roughly twice as far as S235 at the same bend radius. A roll set cut for mild steel will run oval on high-strength coil, and no amount of sizing-stand pressure will make it round without marking the surface. If you change coil supplier mid-project, tell the roll designer before the first coil is opened — a re-cut pass is cheaper than a month of sorting.
The Entry End Is Where the Seconds Go
Coils run from about 3 t to 10 t. A coil change with a shear and butt welder plus a correctly sized accumulator is invisible to the mill; without them it is a stop every coil. Eight coils a shift means eight stops, and eight stops is half a shift of output. Sizing the accumulator is not a detail — it is a capacity decision, and we size it against your coil weight and your coil handling equipment, not against a catalogue page.
What We Need to Quote a Steel Tube Mill
- Tube sizes: minimum and maximum OD with the wall for each.
- Steel grade and surface (black, pickled, galvanized, pre-painted).
- Coil width, thickness tolerance and maximum coil weight you can supply.
- Output required per shift, and the shift pattern.
- Cut length, cut tolerance and end finish.
- Power supply available on site, and whether water treatment is in place.
Send those six and the reply is a configuration with the stations priced separately — so you can see what each part of the line costs, and buy the station you need first if the budget is staged.
From the Knowledge Centre
- Strip Edge Quality and the Weld Defects It Causes
- Specifying a Tube Mill Line: Ten Numbers for the First Meeting
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