Steel strip for steel profiles -
requirements for the feedstock

In the production of cold-formed sections and welded hollow sections, the feedstock accounts for 50 to 75 per cent of the cost of the finished product. This means that strip parameters determine the profitability of production to a greater extent than the output of the line itself. At the same time, it is in roll forming that material characteristics which go unnoticed in other processes are exposed most sharply: camber, coil set and variations in yield strength between batches.

A profile that leaves the line twisted or bowed is rarely the result of incorrect settings. Far more often it is caused by material that should never have been fed to that line in the first place.

Standards for the finished product

Requirements for profiles are set out in two separate standards, depending on whether the cross-section is open or closed.

EN 10219 covers wall thicknesses up to 40 mm and specifies the permitted grades: non-alloy steels S235JRH, S275J0H, S275J2H, S355J0H, S355J2H and S355K2H, as well as fine-grain steels in normalised and thermomechanically rolled conditions. The letter H in the grade designation indicates that the steel is intended for hollow sections.

Strip grades used as feedstock

The choice of feedstock follows from the intended use of the profile and falls into three groups.

Hot-rolled structural strip to EN 10025-2, in grades S235JR and S355J2, used for load-bearing profiles with thicker walls. The yield strength is 235 and 355 MPa respectively for thicknesses up to 16 mm.

Hot-rolled strip for cold forming to EN 10111, in grades DD11 to DD14, intended for cross-sections that require greater formability.

High-yield-strength strip to EN 10149-2, in the MC series grades. For S355MC the minimum yield strength is 355 MPa, the tensile strength 430-550 MPa and the A80 elongation at least 19 per cent in the rolling direction. This group makes it possible to reduce wall thickness while retaining load-bearing capacity, which translates into lower product weight.

Hot-dip coated strip to EN 10346, in grades DX51D to DX57D and structural grades S220GD to S550GD. DX51D has a tensile strength of 270-500 MPa and an A80 elongation of at least 22 per cent, over a thickness range of 0.45 to 3.00 mm. The structural grades offer yield strengths from 220 to 550 MPa, with elongation decreasing as strength increases: at least 20 per cent for S220GD and 16 per cent for S350GD.

Cold-rolled strip to EN 10130, in grades DC01-DC06, in thicknesses from 0.3 to 3.0 mm, used where surface quality and dimensional accuracy of the profile matter.

What determines material behaviour on the line

Roll forming differs from press brake bending in that the material is deformed gradually, in successive stands, along the entire length of the strip. Every shape imperfection of the strip is reproduced along the whole profile.

The technical literature identifies four key shape defects of the feedstock: centre buckle, coil set, crossbow and camber. Strip flatness is explicitly described as a critical parameter for achieving the correct profile shape.

Uniformity of properties is a further factor. Variations in thickness and yield strength within a batch cause uneven bending across the strip width and, as a result, twist, bow and unpredictable springback. This problem is difficult to correct with settings, because an adjustment made for one section of the strip worsens the result for the next.

The practical consequence: in roll forming, repeatability matters more than a tight tolerance in itself. Material with stable parameters from coil to coil allows operation without changeovers, while material with widely scattered parameters forces corrections at every coil change.

Formability and bend radii

Roll forming achieves smaller bend radii than press bending with the same material, because the deformation is distributed over many passes. In high-strength materials the difference can be threefold. For the highest-strength grades, a minimum radius of around three to four times the material thickness is taken as a general rule, while values for specific grades are given in the manufacturers' material data sheets.

The choice between a grade with a higher yield strength and a more formable grade is therefore decided by the geometry of the cross-section. A profile with gentle bends allows a stronger and thinner material. A profile with sharp corners and numerous embossments requires material with higher elongation, even at the cost of a greater wall thickness.

Longitudinal welding of hollow sections

In the production of tubes and hollow sections by high-frequency induction welding, the positioning of the strip edges ahead of the welding zone is crucial. Non-parallel edge presentation is one of the most frequently cited process problems, and its significance grows as wall thickness decreases and higher-strength grades are adopted. Optical inspection systems measure edge alignment to an accuracy of the order of hundredths of a millimetre.

From a purchasing perspective, this means that for hollow section feedstock not only the strip width tolerance matters, but also the edge quality after slitting: no burrs, uniformity and no edge waviness.

Galvanised feedstock or galvanising after fabrication

Corrosion protection of a profile can be achieved in two ways, and the choice has both technological and cost implications.

The reach of cathodic protection at a cut edge depends on material thickness and is effective mainly in thin sheet. For thicker walls and for profiles welded after fabrication, hot-dip galvanising has the advantage, as it covers welds and holes.

Galvanised feedstock has one significant operational advantage: it removes an external stage from the production chain, and with it the transport time and waiting time for the galvanising plant. In series production this can be an argument as important as the durability of the coating itself.

Customer segments and their requirements

Cold-formed and welded hollow sections go to several segments with clearly different priorities. In racking systems, dimensional repeatability and load-bearing capacity at minimum weight are what count. In agricultural structures and mounting systems for photovoltaic installations, corrosion resistance and cost stability dominate. In the manufacture of trailers and vehicle bodies, the combination of strength and formability is important, as cross-sections can be complex. In lightweight and finishing structures, surface quality comes to the fore.

This diversity means that a single profile manufacturer usually works with several feedstock grades at the same time, and correctly matching material to product has a direct impact on margin.

What to specify when ordering

The material specification for profiles should cover the grade together with the standard, thickness, strip width with a defined tolerance, flatness and camber requirements, surface condition and coating type with coating mass, coil inner diameter and the maximum coil weight permitted by the uncoiler. It is also worth specifying whether welded coils are acceptable and what edge quality is required after slitting.

In continuous production, the greatest value lies in the repeatability of these parameters from one delivery to the next. Material that complies with the standard, but comes from a different source each time with a different spread of properties, generates costs in the form of changeovers and scrap which do not appear in the price per tonne, but do appear in the profit and loss account. With recurring demand, it is therefore worth defining the parameters once and maintaining them in subsequent orders.

Available steel grades, coating types and formats of coils, slit strip and sheets are presented in the HEN-STOL steel sheet offer, with separate pages for galvanized steel sheet, cold-rolled steel sheet and hot-rolled steel sheet.

See how we can help

HEN-STOL supplies slit strip in hot-rolled, cold-rolled and coated grades, with the width selected for the product to limit scrap during profiling. Send us the specification and a description of the profile, and we will check availability and propose the most favourable option.

+48 664 700 160hs@henstol.pl

Frequently asked questions

Sources

1. EN 10162, Cold rolled steel sections - Technical delivery conditions - Dimensional and cross-sectional tolerances
2. EN 10219-1 and EN 10219-2, Cold formed welded structural hollow sections of non-alloy and fine grain steels
3. EN 1993-1-3, Design of steel structures - Cold-formed members and sheeting
4. EN 10111, EN 10130, EN 10149-2 and EN 10346, technical delivery conditions for flat products
5. EN ISO 1461, Hot dip galvanized coatings on fabricated iron and steel articles
6. SSAB, product data sheets for coated products to EN 10346
7. AHSS Insights, Roll Forming