The short answer
On most ERW lines, square and rectangular tube is made by round-to-square forming. The strip is formed and welded into a round tube, called the mother tube, exactly as for round tube. The sizing section then shapes it, stand by stand, into a square or rectangle, and the Turk's heads finish the section. So a square tube mill is normally an ERW tube mill with sizing rollers for square and rectangular sizes, not a different machine. The same mill makes round, square and rectangular tube, and a square or rectangular size shares the forming rollers of its mother tube.
How round-to-square forming works
The strip passes the same breakdown and fin passes as for round tube and is welded round by HF welding. In the sizing section, the tube is pressed into shape a little at each stand: the centerline of the tube stays fixed while the lower rollers are raised slightly from stand to stand, so the round section turns gradually into a square or rectangle. Two Turk's heads after the sizing stands straighten the tube and finish the flat faces and corners. The whole process is described in ERW Pipe Making Process: From Steel Coil to Finished Tube.
Because the tube is welded while it is still round, the welding area works exactly as it does for round tube.
Round-to-square or direct square forming
Direct square forming bends the strip toward the square shape from the first stands, instead of making a round tube first. It is a different process route and needs the corresponding mill and tooling configuration, so it should not be assumed to use the same mill set-up or rollers as round-to-square forming. Our standard tube mill lines make square and rectangular tube by round-to-square forming. We have also built mills for direct square forming; this is agreed for each project.
Sharing rollers with the mother tube
A square or rectangular tube is shaped from the round tube with the same perimeter, its mother tube. It therefore shares the mother tube's forming rollers and needs only its own set of sizing rollers, often called a half set. If you already make the mother tube, adding the matching square or rectangular size often needs only that set of sizing rollers. How roller sets are counted and shared is explained in Tube Mill Rollers: Roller Sets, Sharing and Size Changes.
Strip width, wall thickness and corners
- Strip width: as for round tube, it is estimated from the outside perimeter, here the sum of the four sides, and set exactly by the roller design. The estimate and the weight per meter are in ERW Tube Mill Calculations: Strip Width and Weight per Meter.
- Wall thickness: shaping the corners takes more force than rounding, so on the same mill the wall limit for square and rectangular tube is usually lower than for round tube. On the YJ-50, for example, round tube goes up to 3.0 mm wall and square tube up to 2.5 mm.
- Corners: the corner radius depends on the wall thickness and the sizing roller design. Product standards for structural hollow sections set limits on it, so tell us the standard your tube must meet.
- Weld seam: the seam lies on a flat face. It can be close to a corner, but not right on it.
Square and rectangular sizes on our 12 models
InGen MillTek's 12 YJ models make square tube from 8 × 8 mm to 215 × 215 mm and rectangular tube from 10 × 6 mm to 250 × 180 mm, with walls from 0.4 to 8.5 mm. Each model covers part of this range, from square 8 × 8–20 × 20 mm on the YJ-20 to 110 × 110–215 × 215 mm on the YJ-273. Larger sizes, such as 250 × 250 mm and 300 × 300 mm square tube, and other custom models are available on request. The full table is on our Forming & Sizing Units and ERW Tube & Pipe Mill Lines pages.
One model may not cover a wide square range: square tube from 20 × 20 to 60 × 60 mm, for example, falls across two standard models. Worked examples are in Step 1 of 10 Steps to Specify an ERW Tube Mill: Tube Size to Full Line.
What to send
- Every square and rectangular size with its wall thickness, and the round sizes you also make
- Material grade with yield and tensile strength
- The product standard, and any corner radius or straightness requirement
- Which sizes you run regularly and how often you change size
- Target line speed, cut length and packing
Frequently asked questions
Prepared by the InGen Engineering Team. Values are reference values and are confirmed in the technical proposal and technical agreement.
- Is a square tube mill a different machine from a round tube mill? Usually not. Most square and rectangular tube is made on an ERW tube mill by round-to-square forming, with sizing rollers for the square sizes.
- Can one tube mill make round, square and rectangular tube? Yes, within its size and wall range and with the matching sizing rollers.
- Do square tubes need a full roller set? No. A square or rectangular tube shares the forming rollers of its mother tube and needs one set of sizing rollers, a half set.
- Why is the wall limit lower for square tube? Shaping the corners takes more force than rounding, so the same mill reaches a lower wall thickness in square and rectangular sizes.
- What square sizes can you make? Our 12 models cover square tube from 8 × 8 to 215 × 215 mm and rectangular tube from 10 × 6 to 250 × 180 mm; larger sizes are available on request.
- How much does a square tube making machine cost? It is configured for your sizes and output and quoted for each configuration; rollers are quoted separately for each size.
About this article
Prepared by the InGen Engineering Team. This is a general technical guide, not a specification or performance guarantee for any project. Model ranges are the catalogue standard series; final parameters are set in the technical proposal and technical agreement.