Cracking in Roll Forming

1. Definition

Cracking in roll forming means fractures or splits that open during (or immediately after) progressive bending—most often along the outer fiber of a bend where tensile strain peaks. It is a scrap-critical defect distinct from cosmetic paint microcracks, though both can appear together on pre-painted coil.

2. Where Cracks Appear

• Outer surface of sharp corners / ribs
• Slit edges at flanges that stretch around bends
• Punched hole edges after subsequent forming
• Along inclusions or prior damage lines

3. Mechanism

Industry guides state cracking when tensile stress/strain at the outer bend surface exceeds the material’s ductility. Higher-strength steels generally show lower total elongation and reduced bendability (larger required R/t). Work hardening during progressive bends further consumes remaining ductility.

4. Primary Causes

5. High-Strength / AHSS Notes

AHSS roll-forming guidance highlights lower elongation (examples in trade literature: DP grades much lower than mild steel; martensitic grades lower still). Recommended practice themes:

• More stations than mild steel flowers
• Larger minimum inside radii (AHSS Insights often cites ~3–4× thickness depending on grade; other AHSS guides discuss R/t bands such as ~3–5 for severe grades—always use grade data)
• FEA to screen strain before tooling freeze
• Check mill structural ratings—forces rise with strength and thickness squared (rule of thumb in AHSS roll-forming notes)

Microstructure matters: some dual-phase steels show good uniform elongation yet poorer edge stretchability/bendability than certain other high-yield grades with more uniform microstructures—do not rank only by elongation %.

6. Edge Quality

Burrs, nicks, and shear-affected edges from slitting initiate cracks. Specify and inspect slit quality; consider edge conditioning for crack-sensitive AHSS programs.

7. Coated Coil Cracks

Paint or zinc can crack cosmetically before the substrate splits. Treat cosmetic coating cracks under PPGI criteria; treat base-metal splits as structural scrap. See Pre-painted Steel and Bend Radius pages.

8. Prevention

1. Increase inside radius / r/t
2. Add stations; reduce per-pass angle
3. Buy adequate elongation / correct temper
4. Improve slit edges
5. Avoid over-closing gaps to “force” sharp corners
6. Simulate severe AHSS flowers
7. Use compatible lubrication (does not raise certificate elongation, but reduces surface damage)

9. Inspection

• Raking light on outer corners
• Dye penetrant / magnification for critical AHSS
• Compare against limit samples
• Attach heat ID to every cracked coupon

10. Boundaries

• Base-metal crack ≠ paint microcrack.
• Cracking ≠ edge wave (different mechanism).
• No universal R/t for all steels is asserted as a single number here.

11. Buyer / Engineer FAQ

Why did soft trial coil pass but production cracked?

Production heat often has higher yield / lower elongation. Prove on the hardest expected temper.

Will more oil stop cracks?

Not if r/t exceeds ductility. Lube helps marking/friction, not missing elongation.

Is AHSS unsuitable for roll forming?

No—AHSS is widely roll-formed with larger radii, more stands, and stronger mills.

Crack only at punched holes?

Hole edges are stress raisers; check punch quality and subsequent stretch.

Can straightener cause cracks?

Aggressive leveling can, but bend-radius cracks dominate classic cases.

Relation to FLD?

FLD maps stretch necking; bend cracking is often an outer-fiber elongation / r/t problem. Use both concepts appropriately.

12. Troubleshooting Checklist

1. Photograph crack location (outer fiber? edge?)
2. Measure actual inside radius and thickness
3. Read elongation / yield on MTC
4. Inspect slit edge
5. Count stations vs severity
6. Decide: radius / flower / coil / edge fix

13. Plant Practice Notes

Never approve a flower on the softest sample coil when purchasing will ship full-hard or high-yield structural heats.

For painted parts, train inspectors to tag “paint only” vs “steel split” differently in the scrap code system.

• Keep cracked coupons with heat tags in a red box for weekly review
• Log ambient temperature—cold coil can worsen paint cracks
• Do not grind cracks and ship

1. Stop the line when crack rate exceeds the control plan
2. Contain inventory
3. Root-cause with the checklist
4. Correct and re-prove
5. Update PO windows if coil family is the cause

Design reminder: write minimum inside radius on the customer drawing when crack risk is structural.

Tooling reminder: high-strength coils wear corner radii on rolls—worn tools can quietly tighten effective r and start cracking campaigns.

14. Additional Engineering Notes

Metalforming references correlate bendability with total elongation: lower elongation steels need larger R/t. That correlation guides RFQs but does not replace bend trials on your flower.

HSLA grain direction sensitivity means blanked parts mixed into roll-form programs need orientation control; coil-fed longitudinal bends have a fixed grain relationship—manage via grade and r/t instead.

• Archive FEA strain plots for AHSS launches
• Compare cracked heats against elongation histograms
• Re-check after any intentional sharpening of roll corners

If cracks appear only after a lubricant brand change on identical coil, still check whether operators also closed gaps “to get the angle”—multiple variables often move together.