CRCA for sheet metal parts: grade, thickness and formability

A CRCA part is specified three times: once by the grade on the drawing, once by the thickness the mill delivers, and once by the tool that forms it. When those three disagree, the press does not fail loudly. The part springs back 2°, the flange finishes 0.3 mm short, or a panel shows a band of orange-peel strain after powder coating, and the drawing is blamed for a material decision.

CRCA — cold rolled, closed annealed — is the default sheet for OEM brackets, panels and housings because it holds a tight thickness band, a smooth surface and enough elongation to bend and shallow-draw. It is also the grade most often specified by habit. "CRCA, 2 mm" is not a specification: it leaves the grade, the thickness tolerance and the forming window undefined, and the supplier fills the gap with whatever coil is on the floor.

This page gives the grade table, the thickness rule and the forming numbers that close that gap. Every value is either published in a standard or worked from the geometry of the part.

Which CRCA grade should the drawing call out?

Match the grade to the strain the part will see. A bracket with one or two 90° bends runs on D grade. A panel drawn at a draw ratio above about 1.8 needs DD grade. Commercial O grade is for flat covers and shallow flanges where ductility is not doing any work.

The column behind the part decision is elongation, and behind that the plastic strain ratio r, measured to ASTM E517 on a tensile specimen cut at 0°, 45° and 90° to the rolling direction. A drawing-quality sheet at r = 1.8 resists thinning while the flange feeds inward. A D-grade bracket bent in a V-die does not care. A 90 mm deep housing drawn from D grade will thin at the punch nose and split where a DD blank of the same shape would run.

Grade also sets the price direction. Every step from O to EDD costs more per kilogram and buys a narrower forming window. A part that stabilises a D-grade split by adding one bend station to the tool is usually cheaper over the programme than the same part moved wholesale to DD grade. Price the two routes against each other before the die is cut.

Surface finish is the other grade-linked property. Cold rolled sheet is skin-passed on a ground roll, which gives the matt or bright finish named on the order and a surface roughness in the 0.6–1.6 µm Ra band depending on the roll texture. A visible panel needs the finish called out, because a matt roll and a bright roll take powder differently and two coils from different mills will not match under a 70 µm coat.

What thickness does the part actually need?

Thickness is usually chosen for stiffness, then checked for weight. Bending stiffness rises with the cube of thickness, so small thickness changes buy large deflection changes. Take a bracket arm 100 mm long and 50 mm wide, carrying 100 N at the free end, with a second moment of area I = b·t³/12.

At 2.0 mm, I = 50 × 8 / 12 = 33.3 mm⁴. Deflection is δ = F·L³ / (3·E·I) = 100 × 1,000,000 / (3 × 200,000 × 33.3) = 5.0 mm. At 2.5 mm, I rises to 65.1 mm⁴ and the same load gives 2.56 mm, a 49% reduction. The arm weighs 78.5 g at 2.0 mm and 98.1 g at 2.5 mm, because 10,000 mm³ of steel at 7.85 g/cm³ is 78.5 g.

That trade is the whole thickness argument in one line: 25% more thickness buys half the deflection and costs 25% more material. Going the other way, dropping 2.0 mm to 1.5 mm cuts weight by 25% and increases deflection by 137%, because (2.0/1.5)³ = 2.37.

The available band constrains the choice. CRCA is commonly rolled from about 0.4 mm to 3.0 mm; above that the mill route changes to hot-rolled sheet, which brings mill scale and a wider thickness tolerance. Ganesh cuts and forms CRCA across the 0.8–40 mm custom-part range, but the CRCA coil itself is a thin-sheet product, and a 6 mm drawing is a hot-rolled drawing whatever the title block says.

Sheet is bought by weight and consumed by area, which is where the thickness decision reaches procurement. One tonne of 2.0 mm CRCA at 7.85 g/cm³ covers 1,000 ÷ 15.7 = 63.7 m². The same tonne at 2.5 mm covers 1,000 ÷ 19.63 = 51.0 m², 20% less area for the same money per kilogram. A stiffness-driven move from 2.0 mm to 2.5 mm therefore costs 20% more per part in material as well as 25% more in weight, which is the number to put against the deflection saving.

Thickness tolerance is a separate decision from nominal thickness, and it is covered in full in the thickness tolerance guide. The short version: a 2.0 mm callout with no tolerance class can arrive anywhere in a band near ±0.10 mm, which is 5% of the section and enough to move every bend on the part.

How do grade and thickness set the bend?

Bend allowance is the arc length of the neutral axis through the bend: BA = π/180 × θ × (R + K·t). For a 90° bend, an inside radius of 2 mm, 2 mm sheet and K = 0.44, that is 1.5708 × (2 + 0.88) = 4.52 mm.

K is not a constant of nature. It moves with the R/t ratio, the tooling and the material, from about 0.33 at a tight radius to 0.50 at a generous one. K = 0.44 is a working value for mild steel at R/t = 1; the K-factor guide covers the measurement, and the bend allowance method covers the flat-pattern arithmetic. Cut the blank from a nominal K and the first article will tell you whether the tool agrees.

The minimum inside radius follows the grade. For D grade at 2 mm, 0.5t (1 mm) is safe across the grain and 1.5t (3 mm) is the cautious value along the grain. DD and EDD grades will take a tighter radius before the outer fibre necks. The limit is not the tool: it is the outer-fibre strain, which is t/(2R + t). At R = t that is 2/(4 + 2) = 0.33, just past the 0.2–0.3 uniform elongation of mild steel, which is why a 1t bend needs a drawing-quality grade and a good die.

Springback follows yield strength and the R/t ratio. A 2 mm D-grade bend at R/t = 1 springs back roughly 1–2°; open the radius to R/t = 5 and the same bend returns 4–5°. That is why a part with a 10 mm radius and a 90° callout needs either overbend in the tool or a coining pass, and why the springback guide treats it as a tool-correction number rather than a material property.

Draw ratio decides whether the part is a bend job or a draw job. It is blank diameter divided by cup diameter. An 80 mm cup from a 140 mm blank is 1.75 and sits inside DD territory. A 100 mm cup from a 200 mm blank is 2.0 and needs EDD, a radiused punch nose and a blank-holder force high enough to stop the flange wrinkling. Above about 2.2 the part is usually two draws with an interstage anneal, and CRCA is the wrong starting point for a single-operation tool.

Which defects come from the material rather than the tool?

Four failures on a CRCA part point back to the coil before they point to the press.

Stretcher strains appear as bands on a shallow-formed or flat panel, described above. They are a strain-ageing effect and the fix is material control or IF grade.

Earing is a wavy top edge on a drawn cup, caused by the normal anisotropy of rolled sheet. It is measured as ear height divided by cup height and is typically 2–4% on drawing-quality CRCA. Add a trimming operation or accept the ear in the blank allowance.

Orange peel is a rough surface after stretching, from a coarse grain structure. It shows through paint and cannot be polished out. It points to annealing practice at the mill, not to the tool.

Galling is pick-up on the punch or die radius, usually from poor lubrication or insufficient die clearance. It marks the part and accelerates tool wear. Clearance of 8–12% of thickness per side and a drawn-quality lubricant are the standard controls.

Cracks at a bend are different again: they usually start at a sheared edge, where the cut face has a burr and a work-hardened layer. Deburring before forming, or moving the bend line away from the edge, is cheaper than changing grade.

What does Ganesh run for CRCA parts?

CRCA is the mainstay of the press shop: mechanical presses from 20 T to 300 T, 250 T punching, single-stage and progressive tools designed and built in the in-house tool room, with checking fixtures built alongside the tool. Formed tolerance is ±0.5 mm typical, tighter on critical features by agreement, and first article inspection or PPAP documentation is available where the programme requires it.

Blanks and short-run parts run on the 12 kW fibre laser with a 40 ft × 16 ft bed, which takes a 2,500 × 1,250 mm sheet without re-clamping, or on the 6 kW and 3 kW support machines. Formed parts run on the 200 T, 5-axis CNC press brake with a 4,450 mm bed and Delem control, to 10 mm thickness. Welded CRCA assemblies run through three CLOOS robotic MIG cells or the 30+ manual MIG, spot and SPM stations, with fixtures built in-house.

Coating is in-house: seven-tank pretreatment, a 16 ft × 16 ft shot blast chamber, an 18 ft × 14 ft electrostatic powder booth, a wet paint booth and a curing oven, with dry film thickness and cross-hatch adhesion measured and recorded. The business has manufactured since 1992, holds ISO 9001:2015 and ISO 14001:2015, and runs five units in Mumbai (Vasai) and Nagpur.

When CRCA is the wrong sheet

CRCA is the wrong choice for a part that lives wet. A bracket under a genset canopy in a coastal installation, or a panel that collects condensate, will rust from the cut edge whatever the coating, and a galvanised or stainless blank removes the argument. The corrosion and finish guide sets out the exposure categories.

It is also wrong for a structural member that carries load above roughly 300 MPa yield, where hot-rolled E350 or an HSLA grade does the job at the same thickness. And it is wrong above 3 mm, where hot-rolled sheet is the mill product and the route changes to blasting before coating.

We do not roll CRCA, we do not anneal after forming, and we do not supply coil. Where the drawing needs an interstage anneal for a deep draw, that is a different process route and the quotation should say so rather than absorb it.

What to put on the drawing

• Grade and standard reference, for example IS 513 D, not the word CRCA alone.
• Nominal thickness with the tolerance class, and the coil width if the nest depends on it.
• Inside bend radii, and whether the bend line runs across or along the grain.
• The hole-to-bend distance for any hole within 3t of a bend.
• The critical features that locate the part in the assembly, with their tolerances.
• Surface finish: pretreatment system, powder or paint, colour and dry film thickness range.
• The general tolerance class from ISO 2768-1, plus any tighter geometric callout.
• Annual volume, release pattern and expected part life, so the tool decision can be priced.

Send the drawing, grade and volume to request a quote. The materials and grades hub carries the rest of the grade and finish guides, and the custom parts capability page shows the press and tool range behind a CRCA quotation.