High-Strength Steel Bending Guide
A practical summary of minimum radius, V-die selection, springback and bending force calculation for crack-free bending of S690QL and S700MC plate.
Bending Fundamentals
High-strength steels behave differently from standard plate: higher yield strength means greater bending force and more pronounced springback. With the right parameters, S690QL and S700MC can be cold-bent safely. Three critical variables: minimum inside radius, bending direction and V-die opening.
Inside Radius (R)
Too small a radius initiates cracks on the outer surface. Given as a multiple of thickness (R = k × t).
Bending Direction
Bending perpendicular to the rolling direction is safest; parallel bending requires a larger radius.
V-Die
The opening (V) is selected according to thickness; a narrow V means higher force and crack risk.
Minimum Inside Bending Radius
The minimum inside radius required to prevent cracking is given as a multiple of thickness (t). Reference values (bend axis perpendicular to the rolling direction):
| Grade | Perpendicular to Rolling Direction | Parallel to Rolling Direction |
|---|---|---|
| S700MC | 1.0 × t | 1.5 × t |
| S690QL | 3.0 × t | 4.0 × t |
| S355 (comparison) | 1.5 × t | 2.0 × t |
Note: Values are for reference; the actual value varies by brand (Strenx 700E generally allows lower radii), thickness, plate surface/edge condition and bending speed. Rely on the manufacturer's bending tables.
Rolling Direction Effect
Rolled plate is anisotropic; its properties vary with the rolling direction. The position of the bend line relative to the rolling direction directly affects the risk of cracking:
- Bending perpendicular to the rolling direction (recommended): The bend line is perpendicular to the rolling direction. Lowest crack risk, smallest radius possible.
- Bending parallel to the rolling direction: A larger radius is required; avoid if possible.
- 45° bending: An intermediate value when geometry makes it unavoidable; verify with a trial bend.
V-Die Opening Selection
On a press brake, the lower die V-opening determines both the inside radius and the required force. High-strength steel requires a wider V-opening:
| Material | V Opening | Resulting Inside Radius (approx.) |
|---|---|---|
| Mild steel (S235/S355) | ≈ 8 × t | ≈ 1.3 × t |
| S690QL / S700MC | 10 – 14 × t | ≈ 1.6 – 2.3 × t |
Rule of thumb: The air-bending inside radius is approximately 16% of the V-opening. Choosing a wide V in high-strength steel reduces both crack risk and tool/press fatigue.
Springback
High yield strength means the material partially opens back up (springback) after bending. In S690QL/S700MC this angle is markedly greater than in mild steel.
- Overbend: Compensate by bending the target angle a few degrees further (e.g. for 90° you may need to bend to ~93–95°).
- Wide V-opening: Increases springback but reduces crack risk; strike a balance.
- Trial bend: Verify the angle with a sample of the same batch/thickness before series production.
- Coining: Gives a more repeatable angle but requires very high force; use with care on HSS.
Bending Force
High-strength steel requires 2–3 times more bending force than mild steel of the same thickness. The air-bending force is approximately:
F (kN/m) ≈ (650 × Rm × t²) / (V × 1000) — Rm: tensile strength (MPa), t: thickness (mm), V: die opening (mm)
This is why the press brake tonnage and load distribution along the length are far more critical for S690QL/S700MC than for mild steel. Always check the press capacity and die strength.
Crack Prevention
Adequate Radius
Do not go below the min. inside radius; it is the most common cause of cracking.
Clean Edge
Burrs and the hardened layer on a laser/plasma cut edge initiate cracks; grind if necessary.
Correct Direction
Position the bend line perpendicular to the rolling direction.
Edge Distance
A hole/cutout too close to the bend line deforms; leave adequate distance.
The quality of the cut edge is critically important in high-strength steel. The thin hardened layer formed during laser cutting can be a crack initiation point in small-radius bending; light grinding along the bend line is recommended for critical bends.
Frequently Asked Questions
How small a radius can S690QL be bent to?
The reference minimum inside bending radius for S690QL is approximately 3 × thickness (when the bend axis is perpendicular to the rolling direction). For bending parallel to the rolling direction, this value rises to ~4 × thickness. The actual value varies by brand and plate condition; optimized grades such as Strenx 700E may allow lower radii.
Why does the rolling direction matter in bending?
Rolled plate has directional (anisotropic) properties. Positioning the bend axis perpendicular to the rolling direction reduces crack risk and allows a smaller radius. When bending parallel to the rolling direction, the material cracks more readily, so a larger radius is required.
How is springback compensated?
Springback is pronounced in high-strength steel. The most common method is to bend the target angle slightly further (overbend); for example, for 90° you may need to bend to 93–95°. A repeatable result is achieved with a wide V-opening, die/angle compensation and a trial bend before series production.
Is S700MC or S690QL easier to bend?
Thanks to its fine-grained TMCP structure and a grade optimized for cold forming, S700MC is generally bent more easily and to a smaller radius. S690QL requires a larger radius and higher force, especially in thick sections. S700MC is preferred for parts requiring heavy bending.
Press Brake Bending Service
We cut and bend your S690QL and S700MC plate to size. Share your bending plan and we will deliver it with the right grade and parameters.