Cold rolled steel destined for galvanizing is annealed because cold rolling leaves the strip hard, brittle and full of stored strain energy. Annealing, in the form of recrystallization heat treatment, is what converts a work-hardened strip into a soft, ductile, formable substrate whose surface will accept a uniform zinc coating. Without it, the coil may galvanize successfully in appearance while failing in bend tests, cracking during forming, or suffering coating adhesion loss further down the line.
Work Hardening and What Annealing Reverses
Cold rolling reduces thickness by passing the strip between rolls at ambient temperature, with typical reductions of 50-85% between the hot rolled and finished gauge. Because the metal cannot recover at room temperature, the deformation is stored in the crystal lattice. Dislocation density rises sharply, the grains elongate in the rolling direction, and the result is a large increase in hardness and yield strength together with a sharp drop in elongation and toughness. That is work hardening, and it is precisely the condition in which deep forming, profiling and bending would crack the sheet.
Annealing heats the cold rolled strip to a temperature above its recrystallization range and holds it there. In low carbon steel the recrystallization range typically falls between roughly 500 and 650 C, lower at higher cold reduction. During the hold, new strain-free grains nucleate and grow until they replace the deformed structure; the dislocation network is annihilated, hardness falls, and elongation and formability return. Subsequent cooling rate and final temperature govern the balance between strength and ductility, and a light skin pass of about 0.5-1.5% elongation after annealing removes the yield-point elongation that would otherwise cause stretcher strain markings during forming.
Surface Cleanliness Before Dipping
The second reason for annealing in a galvanizing line is surface quality. Rolling oil, iron fines, carbonaceous residue and surface oxide all remain on a cold rolled strip. A clean, reduced and slightly roughened surface is what allows molten zinc to wet the steel, form the iron-zinc intermetallic layer and then build a continuous coating. Annealing in a protective atmosphere such as nitrogen-hydrogen reduces residual oxide and removes lubricant, and the skin pass sets a surface roughness of about 0.8-1.8 micrometre Ra, which gives the mechanical key the zinc coating needs for good adhesion. If the strip goes into the zinc bath with oil or oxide still on it, the coating will not wet properly and will show bare patches, poor adhesion or flaking during bending.
Controlling Mechanical Properties to the Specification
Different end uses need different combinations of strength and ductility, and annealing is the main tool for setting them. Continuous annealing parameters are adjusted to hit the delivery condition required by the applicable standard: cold rolled grades DC01 to DC06 to EN 10130, coated grades DX51D+Z to DX54D+Z to EN 10346 for anything from simple profiling to special deep drawing, or the corresponding cold rolled sheet requirements of ASTM A1008/A1008M and the coated structural grades of ASTM A653/A653M.
| Condition | Typical target | Typical use |
|---|---|---|
| DC01 / DX51D+Z | Yield 140-280 MPa, elongation 28% min | Bending and profiling |
| DC03 / DX52D+Z | Drawing quality, r90 about 1.4 | General forming |
| DC04 / DX53D+Z | Deep drawing quality, r90 about 1.6, n90 about 0.18 | Deep drawn parts |
| ASTM A653/A653M SS Grade 37 | Minimum yield point 255 MPa | Structural coated products |
Continuous Annealing Versus Batch Annealing
Two routes are used, and the choice depends on product mix and volume. Continuous annealing passes the strip through heating, soaking and cooling sections in line with the galvanizing process, all under protective atmosphere, at soaking temperatures commonly around 730-820 C before controlled cooling towards the zinc bath at roughly 450-460 C. It gives high output, uniform properties from head to tail, and is the normal route for high volume automotive and appliance coated products. Batch or hood annealing seals stacked coils under a hood with protective gas and heats them slowly to approximately 600-700 C, then cools at a controlled rate. It costs less per tonne but is slower and less uniform, so it suits smaller lots and coated products with moderate forming requirements.
What Goes Wrong with Incorrect Annealing
Too little heat or too short a hold: work hardening is not fully removed, the strip stays hard and brittle, and cracking appears after galvanizing or during forming.
Too much heat or too long a hold: grains grow excessively, strength and load-bearing capacity fall, and the surface takes on an orange-peel appearance when formed.
Residual oil or oxide: poor zinc wetting, adhesion loss, and zinc layer flaking in pieces during bending.
Wrong surface roughness: uneven zinc pick-up, giving local over-thick or thin coating and inconsistent appearance.
Excess yield-point elongation: visible Lüders band markings on prior-formed parts because the skin pass was insufficient.
FAQ
Q: Why does cold rolled steel need annealing before galvanizing?
Cold rolling work-hardens the strip, leaving it hard, brittle and poorly wetting to molten zinc. Annealing restores ductility and cleanliness so the coating can wet, bond and survive forming.
Q: What is recrystallization in this context?
It is the formation and growth of new strain-free grains above roughly 500-650 C in low carbon steel, which replaces the deformed structure and removes hardening.
Q: What happens if the strip is not annealed at all?
The coil stays hard and brittle, tends to crack when profiled or bent, and the zinc coating adheres poorly because the surface still carries rolling oil and oxide.
Q: What is the difference between continuous and batch annealing?
Continuous annealing treats the strip in line under protective gas at high output with uniform properties, while batch or hood annealing treats stacked coils slowly and is more economical for smaller lots and modest forming requirements.
Q: Why is a skin pass applied after annealing?
A light skin pass of about 0.5-1.5% elongation suppresses yield-point elongation, avoiding stretcher strain markings, and sets the surface roughness needed for coating adhesion.
Q: Can the annealed properties be confirmed from the delivery certificate?
Yes. Yield strength, elongation and, where applicable, r value and n value are stated on the inspection certificate in accordance with the ordered standard and grade.