How Cold Formed Structural Steel Tube Is Made and Why the Manufacturing Route Affects Structural Behavior
Cold formed structural steel tube is everywhere in modern construction — building frames, canopy structures, purlins, secondary beams, sign supports, light industrial buildings. It’s economical, it’s widely stocked, and it looks like a simple, uniform product. The manufacturing process, though, leaves a fingerprint on the material that shows up in how the section behaves under load — particularly in compression and in connections — in ways that aren’t obvious from looking at the finished product.
Understanding the production route helps make sense of why the design codes treat cold-formed sections differently from hot-finished ones, why corner properties matter in some calculations, and why certain applications require hot-finished sections even when cold-formed sections are cheaper and more readily available.
From Flat Strip to Finished Tube
The production of cold formed structural steel tube starts with hot-rolled flat strip — a coil of steel that arrives at the tube mill in widths and thicknesses that correspond to the perimeter and wall thickness of the finished section. The strip has been produced at a steel mill from a cast slab, hot-rolled to thickness, and coiled for transport.
At the tube mill, the strip is uncoiled and fed through a series of forming rolls. The forming stand sequence progressively bends the flat strip into a circular cross-section — starting from the edges, working inward, until the strip forms a complete open cylinder with the two edges brought together at the top. This continuous roll-forming process happens at room temperature, which is what makes it “cold” forming relative to hot-finishing. No external heat is applied during the forming operation itself.
The two edges of the formed strip are then welded together. For structural hollow sections to EN 10219, the weld is typically made by high-frequency induction (HFI) welding — sometimes called high-frequency electrical resistance welding (HF-ERW). An induction coil surrounds the tube just ahead of the weld point and induces a current that heats the strip edges to forge-welding temperature; the heated edges are then pressed together by squeeze rolls, creating a solid-phase weld without filler metal. The weld bead — both external and internal — is typically removed by scarfing tools immediately after welding, leaving a smooth seam.
For round sections, the process stops here. For square and rectangular sections, the round tube passes through a second set of forming rolls — the sizing section — that converts the circular cross-section into the square or rectangular shape. This second cold-forming operation is where the corners of the finished section receive the highest plastic strain: the material at the corners has been bent twice (once during the initial strip-forming and again during the squaring operation), which produces more work hardening and more residual stress in the corner regions than in the flat faces.
What Cold Forming Does to the Steel
Cold forming permanently deforms the steel beyond its yield point. The material at the corners of a finished square or rectangular section has been strained well into the plastic range. This strain hardening increases the local yield strength at the corners — corner material in a cold-formed section typically has higher yield strength than the base strip — but it also reduces elongation and ductility. The steel has “used up” some of its capacity to deform before fracture in the forming process.
Residual stresses are the other major outcome. During forming, the outer surface of each bend is in tension and the inner surface is in compression. When the forming force is removed, the material tries to spring back, but the cross-section geometry prevents full springback. The result is a locked-in residual stress pattern: the corners have tensile residual stresses on the outside surfaces and compressive on the inside, and the flat faces have a more complex pattern that reverses from the squaring operation.
For compression members, residual stresses that are tensile in the outer fiber are unfavorable for buckling resistance. They effectively reduce the load at which the material at the most-stressed fiber reaches yield, which is the trigger for the inelastic buckling that governs slender compression members. This is why Eurocode 3 assigns cold-formed hollow sections to the more conservative buckling curve c, compared to curve a for hot-finished sections where the normalizing treatment has relieved the residual stresses.
The Weld Seam
The HFI weld seam in a cold-formed structural tube is a forge weld — no filler metal, no cast structure in the weld zone, no heat-affected zone in the conventional sense. The weld is achieved by heating the edges and pressing them together, which creates a solid-phase bond between the two edges of the strip. The mechanical properties across a properly made HFI weld are typically close to the base material properties because the microstructure is similar — it’s a deformed and recrystallized zone, not a cast-and-solidified structure like a fusion weld.
EN 10219 does not require non-destructive examination of the weld seam by default. This is a significant difference from API 5L PSL2 line pipe, which mandates seam weld UT. For most building structure applications, where the design loads are well within the weld capacity and the service conditions don’t create fatigue cycling or aggressive chemistry, the absence of mandatory NDE is acceptable. For crane booms, offshore secondary structures, or fatigue-sensitive applications, specifying HFI seam weld inspection as a supplementary requirement to EN 10219 is worth considering.
What the Mill Certificate Shows
The mechanical properties on an EN 10219 mill test certificate — yield strength, tensile strength, elongation — are measured from a flat face specimen, not from the corner. This is the standard test location defined in the standard. The corner material has different properties: higher yield strength (due to work hardening), lower elongation. The certificate values don’t capture this variation.
For most structural design purposes this is acceptable — Eurocode 3’s rules for cold-formed sections already account for the corner behavior through the buckling curve and section classification rules. Where it matters is in calculating the increased yield strength at the corners for local buckling in thin-walled sections (EN 1993-1-3 provides a method for using the enhanced corner properties in class 4 cross-sections) and in assessing the ductility demands on the material in seismically designed frames, where the material elongation capacity is a factor in rotation capacity calculations.
The manufacturing route is visible in the finished product if you know where to look: the corner radius, the seam weld line (usually detectable on the flat face or identifiable by slight surface variation), and the consistency of wall thickness around the perimeter. None of these are defects — they’re the expected results of the cold-forming process. They’re also the reason why specifying the correct standard for the application, and understanding what the standard does and doesn’t control, matters more than it might appear from looking at a stack of structural tube in a warehouse.
