Hot-rolled mild steel parts: grade, thickness and formability

Hot rolled mild steel is the material a part moves to when cold rolled sheet runs out: above 3 mm, in a structural member, or in a frame that will be blasted and painted rather than finished as rolled. The trade is blunt. Hot rolling buys thickness and yield strength, and it gives back surface finish, thickness control and a clean edge.

A 6 mm E250 bracket is not a scaled-up CRCA bracket. The mill scale has to come off before welding and coating, the thickness band is roughly twice as wide, and the bend force at the full 4,450 mm bed can exceed the brake rating if the die opening is chosen by habit. Those three consequences decide the route more than the grade does.

This page covers the grades a buyer will see, the thickness range and its tolerance, the bend force arithmetic at 6 mm, and the drawing notes that keep a hot rolled part from being quoted as if it were cold rolled.

Which hot rolled mild steel grade should the drawing call out?

Use E250 for general fabrication and move up only when the stress calculation asks for it. IS 2062 E250, ISO 4995 HR235 and EN 10025-2 S235JR are the same class of structural sheet: about 235–250 MPa minimum yield, 410–540 MPa tensile and 23% elongation at 6 mm. E300 and E350 raise the yield and narrow the weld window.

The grade decision is a stress decision, and the number is the calculated stress at the chosen thickness, not the yield of the steel. A guard that deflects 8 mm under a 500 N lean is failing on stiffness at 250 MPa yield, and moving to E350 changes the deflection by nothing because the modulus is the same 200 GPa for all of them. A lifting frame at 330 MPa working stress is a different case: E250 is not allowed there and E350 is the grade that keeps the section.

Carbon equivalent is the weldability number. CE = C + Mn/6 + (Cr + Mo + V)/5 + (Cu + Ni)/15. A typical E250 with 0.18% carbon and 1.4% manganese gives CE = 0.18 + 0.233 = 0.41. E350 at 0.20% carbon and 1.5% manganese gives 0.45. Below 0.40, 6 mm sheet welds without preheat at normal shop temperature. At 0.45 and 20 mm, a 100 °C preheat is the normal precaution, and the weld procedure should say so.

What thickness does hot rolled sheet come in?

Hot rolled sheet runs from about 1.6 mm to 20 mm in the coil and cut-length products a fabricator buys, with plate above that. Ganesh cuts to 20 mm on the 12 kW fibre laser and to 30–40 mm on the iron worker for plate work, which covers the range where hot rolled replaces cold rolled.

The thickness band is wider than cold rolled at the same nominal, roughly twice as wide. At 6 mm, a normal mill band is of the order of ±0.3 mm, against ±0.10 mm at 2 mm cold rolled. That matters less for a frame than for a flat pattern, because the bend allowance at 6 mm is 1.5708 × (9 + 2.64) = 18.3 mm at R = 1.5t and K = 0.44, and a 0.3 mm thickness step moves it by 0.21 mm. On a 6 mm part with two bends, that is 0.4 mm of flat length, which is usually inside the assembly tolerance and outside the flatness tolerance of a long panel.

Mass climbs quickly with thickness. At 7.85 g/cm³, 1 m² of 6 mm sheet is 47.1 kg, 10 mm is 78.5 kg and 20 mm is 157 kg. A 2.4 m × 1.2 m sheet at 6 mm is 2.88 m² and 136 kg, which is a two-person lift and a crane beyond that. Handling belongs in the route, not in the quote notes.

Coil set and camber are more visible on hot rolled than on cold rolled. A coil that has been stored on its side holds a curve, and a long part cut from the outside of the coil inherits it. Cutting the blank with the camber direction alternating across the nest spreads the error instead of stacking it.

How much force does a 6 mm bend need?

Air bending force is F = 1.42 × σ × L × t² / V, where σ is tensile strength, L is bend length, t is thickness and V is the die opening. For 6 mm E250 at 450 MPa, a 1,000 mm bend and a V opening of 8t = 48 mm:

F = 1.42 × 450 × 1,000 × 36 ÷ 48 = 479,250 N, or 479 kN, which is 48.8 tf per metre.

Across the full 4,450 mm bed that is 2,133 kN, or 217 tf. The brake is rated 200 T, so the job is over capacity at that die opening even though the material is ordinary and the thickness is inside the machine's 10 mm limit. Opening the die to 64 mm, which is 10.7t, drops the load to 359 kN/m and 1,600 kN over the bed, or 163 tf, and the job fits.

The alternative is to shorten the bend. At 48.8 tf/m, a 200 T rating allows 4.10 m of bend, so the same part can run on a shorter tool if the geometry permits. Either way, the die opening and the bend length are the two numbers that decide whether a 6 mm job is a 200 T job, and neither is printed on the drawing.

Springback at 6 mm is small in angle but visible in a long part. At R/t = 1.5 and 450 MPa tensile, a 90° bend returns about 1–2°, which the tool corrects by overbend. The bigger effect is along the bend line: a 4,450 mm bend needs crowning or shimming to keep the middle of the part from opening, because the ram and the bed deflect under 160–220 tf. The long-panel bending guide covers the compensation.

Edge condition decides whether the bend cracks. A sheared or laser-cut edge at 6 mm has a work-hardened zone and, on a sheared edge, a burr. Bending with the burr on the outside of the bend puts a stress riser exactly where the outer fibre is at maximum strain. Deburr the bend line, or orient the burr to the inside, before the bend.

How does mill scale change the route?

Mill scale is a layer of iron oxides, a few micrometres to about 20 µm thick, formed at the rolling temperature. It is cathodic to the steel beneath it, so a scratch that exposes bare steel makes the steel the anode and the scale the cathode, and corrosion runs under the scale. That is why paint over scale fails in sheets rather than spots.

ISO 8501-1 defines the rust grades A to D and the preparation grades Sa, St and Fl. For a coated hot rolled part, Sa 2½ is the normal specification: no visible scale, rust, paint or foreign matter, with only light shadows permitted. The surface profile after blasting is typically 40–70 µm, which is what the coating keys into and what the dry film thickness gauge reads over if it is not zeroed on the blasted surface.

Ganesh runs a 16 ft × 16 ft shot blast chamber ahead of the seven-tank pretreatment line, so a blasted and coated hot rolled part can run as one route. The blast also removes the laser oxide from the cut edge, which is where a coating adheres worst if it is left on.

Welding is the other place scale matters. The joint faces must be ground to bright metal before MIG or spot welding, because scale traps slag, causes incomplete fusion and adds oxygen to the arc. A fabricator who welds through scale produces a weld that passes a visual check and fails a bend test.

Which defects come from the sheet rather than the process?

Edge cracking at a bend is the most common. It comes from a combination of a tight radius, a sheared edge and a grade with low elongation at thickness. Open the radius, deburr, or bend across the grain.

Lamellar tearing appears in thick plate at a welded T-joint, where the through-thickness ductility of the steel is exhausted and the plate separates parallel to the surface. It is a steel cleanliness and joint-design issue, and it is why a 20 mm T-joint needs a weld procedure rather than a habit.

Centre-line segregation in a thick plate shows up as a band of inclusions that cracks when the plate is bent or welded. It is a mill quality issue, and the mill certificate with the ladle analysis is the evidence.

Coil set and camber are handling defects, corrected by re-levelling or by nesting the blanks so the error alternates. They are not a reason to reject the heat.

Scale patches left after a short blast show as dull grey areas under the coating. They are a process defect, caught at the blast inspection and not at final paint.

What does Ganesh run for hot rolled parts?

Hot rolled parts are laser cut on the 12 kW fibre laser with a 40 ft × 16 ft bed to 20 mm, with 6 kW and 3 kW support machines for thinner work, and cut or punched on the iron worker to 30–40 mm for plate. Bending runs on the 200 T, 5-axis press brake with a 4,450 mm bed and Delem control to 10 mm. Welding runs through three CLOOS robotic MIG cells and 30+ manual MIG, spot and SPM stations, with fixtures designed and built in-house.

Blasting and coating run in-house: a 16 ft × 16 ft shot blast chamber, seven-tank pretreatment, an 18 ft × 14 ft electrostatic powder booth, a wet paint booth and a curing oven, with dry film thickness and cross-hatch adhesion recorded. Formed tolerance is ±0.5 mm typical, tighter on critical features by agreement, and first article inspection or PPAP documentation is available.

Ganesh has manufactured since 1992 under ISO 9001:2015 and ISO 14001:2015, with five units in Mumbai (Vasai) and Nagpur and an effluent treatment plant for the surface treatment waste.

When hot rolled is the wrong material

Hot rolled is wrong for a visible panel. The surface carries scale pits, a rougher profile and a wider thickness band, and no amount of powder hides a 0.3 mm thickness step on a flat door. Cold rolled sheet exists for that surface.

It is wrong below about 1.6 mm, where the mill product is cold rolled and a thin hot-rolled callout is usually a drafting error. It is wrong for a deep draw, where the lower elongation and the scale both work against the punch. And it is wrong for an uncoated wet part: hot rolled steel carries no more corrosion protection than CRCA, and its rougher, scaled surface holds moisture and pits.

We do not roll plate, we do not heat treat after welding, and we do not galvanise in-house. A hot rolled frame that needs stress relief after welding or a zinc coating after fabrication is a subcontracted step, and the quotation should name it.

What to put on the drawing

• Grade and standard, for example IS 2062 E250, with the impact-test requirement if the duty is dynamic.
• Nominal thickness with the tolerance class, and the plate or sheet size if the nest depends on it.
• Bend lines, inside radii and the grain direction, with 1.5t across and 2t along the grain as the default.
• Weld symbols, the weld procedure reference and the preheat requirement for the grade and thickness.
• Surface preparation to ISO 8501-1 Sa 2½, and the coating system with dry film thickness.
• The features that are machined after fabrication, with their tolerances separated from the general ones.
• The general tolerance class from ISO 2768-1, plus any tighter geometric callout.
• Annual volume and the largest part size in the release, so the bed and the blast chamber can be checked.

Send the drawing, grade and release volume to request a quote. The materials and grades hub carries the grade and corrosion guides, and the laser cutting capability page shows the thickness range the 12 kW machine covers.

Related reading: the 1050 aluminium corrosion and finish compatibility and the 1050 aluminium for sheet metal parts: grade and formability take the same problem further.