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Ultra-high strength steel (UHSS, 980-1500 MPa) is increasingly used in automotive structural parts ??B-pillars, door beams, bumper reinforcements. Forming UHSS on a servo press requires specific techniques to manage springback, edge cracking, and die wear.
## Why UHSS Is Challenging
UHSS has:
- High yield strength (700-1200 MPa) ??large springback
- Low elongation (5-12%) ??prone to edge cracking
- High work hardening rate ??forming force increases rapidly
- High surface hardness ??accelerated die wear
## Servo Press Advantages for UHSS
**Controlled forming speed:**
UHSS is sensitive to forming speed. Too fast causes adiabatic heating and edge cracking. Optimal forming speed: 30-80 mm/s (vs 150-300 mm/s for mild steel).
**Dwell at BDC:**
200-400ms dwell allows stress relaxation, reducing springback by 40-60% compared to no dwell.
**Programmable overbend:**
Servo press can precisely overbend by 1-5mm to compensate for springback, then return to exact BDC position.
## Motion Profile for DP980 B-Pillar
```
TDC ??20mm above material: 250mm/s
20mm ??5mm above: 80mm/s
5mm above ??BDC: 40mm/s
BDC dwell: 300ms
BDC ??TDC: 200mm/s
```
## Die Design Considerations
- Die clearance: 105-108% of material thickness (tighter than mild steel)
- Die radius: minimum 3? material thickness to prevent edge cracking
- Die material: D2 or PM tool steel (not H13 ??too soft for UHSS)
- Die coating: TiAlN or CrN coating extends die life 3-5? on UHSS
## Springback Measurement and Compensation
Measure springback on first article:
1. Form part at nominal BDC
2. Measure angle/geometry after springback
3. Calculate compensation (typically 3-8? for DP980)
4. Adjust BDC position or add overbend to motion profile
5. Iterate until part meets specification
mike_chen_eng
Good point on the servo press selection criteria. One thing I'd add: check the machine's actual cycle time under load, not just the rated speed. We had a press that was spec'd at 60 SPM but only hit that on light forming — under full tonnage on a deep draw, it dropped to 35 SPM. The integrator's test was done with a dummy load, not actual production dies.
Also worth verifying: the repeatability spec under thermal load. A press that's ±0.05mm cold might drift to ±0.15mm after 4 hours of continuous production if the thermal compensation isn't tuned right.