Bending cracking in galvanized steel is a coating failure rather than a steel failure. When a coated sheet is bent, the outer surface of the bend is stretched while the zinc coating and the brittle iron-zinc alloy layer beneath it are far less ductile than the steel substrate. Cracking starts when the tensile strain at the bend exceeds the elongation capacity of the alloy layer, so every preventive measure described below works either by softening that layer, by thinning it, or by increasing the ductility of the base metal.
Base Metal Selection and Chemistry Control
Low carbon steel is the preferred base metal. Use a substrate with a carbon content of no more than 0.25%, for example Q235 or SPCC, because a hard, brittle matrix with carbon at or above 0.3% concentrates stress during bending. A low carbon base metal gives an elongation of at least 25% and therefore a much wider safe bending window. Two alloy adjustments further improve performance:
Keep silicon below 0.1%. Silicon promotes a brittle iron-silicon-zinc phase at the coating interface, and silicon-bearing steel can show a bending cracking rate around 30% higher than silicon-free steel.
Add 0.01% to 0.05% aluminium to refine the matrix grain size. Refining the grain from roughly 20 micron to 10 micron can increase the number of bends a sheet survives by about 50% before cracking appears.
Zinc Bath Chemistry and Alloy Layer Control
The alloy layer that grows between the steel and the pure zinc layer is the least ductile part of the coating, so it must be kept thin and fine grained.
Add 0.1% to 0.2% aluminium to the zinc bath. Aluminium suppresses excessive growth of the iron-zinc alloy layer so that the coating is dominated by the pure zinc zeta phase, which has an elongation of at least 20% and bends well.
Add 0.05% to 0.1% nickel to refine the alloy layer grains to below 5 micron and reduce brittleness. At a nickel content of about 0.08%, the bending cracking rate can fall by roughly 40%.
Keep the alloy layer below 5 micron in the finished product; a layer thicker than 8 micron is a signal that the bath chemistry or the line parameters need adjustment.
Coating Thickness and Galvanizing Parameters
Thick coatings carry high internal stress and crack first. The target coating thickness therefore depends on sheet gauge.
| Sheet thickness | Target coating thickness | Reason |
|---|---|---|
| Up to 2 mm | 8 to 15 micron | Thin sheet deforms more, so a thinner, more ductile coating is required |
| Above 2 mm | 20 to 30 micron | Thicker sheet tolerates a heavier coating and needs the corrosion reserve |
Stress scales with coating thickness: a 30 micron coating carries roughly twice the bending stress of a 15 micron coating on the same bend radius. Line parameters should be set to limit alloy layer growth:
Zinc bath temperature of 430 to 440 degrees C, which is 20 to 30 degrees C below the traditional 460 degrees C, keeps the iron-zinc alloy layer to about 5 micron instead of 10 micron.
Immersion time of no more than 8 seconds, and no more than 5 seconds for thin sheet. Reducing immersion from 15 seconds to 8 seconds cuts the brittle delta phase thickness by about 30%.
Air cooling after the zinc bath at a rate below 5 degrees C per second. Slow cooling avoids the quenching stress that water cooling introduces and can reduce the bending cracking rate by about 60% compared with water cooling.
Softening Treatments and Bend Test Verification
Where the coating must survive a very tight radius, a post-galvanizing softening step is added.
Annealing at 200 to 250 degrees C for 1 to 2 hours relieves coating stress, dropping residual internal stress from about 150 MPa to below 50 MPa.
Tempering at 200 to 300 degrees C for 1 hour decomposes hard, brittle phases such as the gamma phase, reducing hardness from about 300 HV to below 200 HV and raising the number of bends to failure from roughly 5 to about 15.
Rolling or shot peening introduces a residual compressive stress of at least 100 MPa at the coating surface, which offsets bending tensile stress and can cut the cracking rate by about 25%.
Verification follows GB/T 232, the metallic materials bend test. A pre-production bend test is run with a bend radius R of 2 t, the sheet is bent through 180 degrees, and the coating is accepted when the crack length is below 1 mm with no flaking or spalling. A metallographic section is used to measure the alloy layer, which should remain at or below 5 micron. Where a specific product must be qualified for a very tight radius, finite element analysis of the stress distribution during bending can be used to adjust tooling fillet radii and to identify the areas of stress concentration before tooling is cut.
Frequently Asked Questions
Q: Why does the coating crack while the steel does not?
Because the iron-zinc alloy layer beneath the pure zinc is much less ductile than the steel. The outer fibre of a bend is stretched, and the brittle alloy layer reaches its elongation limit first.
Q: Which base metal should be chosen for bending?
A low carbon steel with carbon no higher than 0.25%, such as Q235 or SPCC, giving an elongation of at least 25%. Steels with carbon at 0.3% or above concentrate stress and crack more readily.
Q: How thick should the zinc coating be?
For sheet up to 2 mm a coating of 8 to 15 micron is preferred; for sheet above 2 mm, 20 to 30 micron. A thicker coating raises internal stress and reduces bending performance.
Q: What does aluminium in the zinc bath do?
At 0.1% to 0.2% it suppresses excessive growth of the iron-zinc alloy layer, so the coating stays dominated by the ductile pure zinc zeta phase.
Q: How is bending quality checked before production?
A GB/T 232 bend test is run at R equal to 2 t with a 180 degree bend, and the coating passes when the crack length stays below 1 mm with no flaking.
Q: Can a heavy galvanized coating be softened after galvanizing?
Yes. Annealing at 200 to 250 degrees C or tempering at 200 to 300 degrees C relieves internal stress and reduces the hardness of brittle coating phases.