1. What Press Hardening Is
Press hardening — also called hot stamping or hot forming — is a sheet metal process where the blank is heated to fully austenitize the steel, then formed and quenched in a water-cooled die within a few seconds. The die does double duty: it shapes the sheet and it extracts heat fast enough to transform the austenite into martensite. What comes out is a structural part at 1300–1600 MPa tensile with almost no springback.
The name matters for buyers: "press hardening", "hot stamping" and "hot forming" describe the same family of processes, and the steel families carry names like 22MnB5, Usibor1500P, T1500HS+AS and their equivalents. If you see those on a certificate, you are in press-hardening territory.
1.1 Direct vs indirect route
Direct press hardening: the flat blank is heated, transferred to the die, and formed and quenched in one stroke. It is the most common route for parts that can be formed from a flat blank in one hit.
Indirect press hardening: the blank is cold pre-formed (usually with a separate pre-forming die), then heated, then finish-formed and quenched. The indirect route is used when the geometry is too deep or complex to form from a flat blank in one hot stroke. It costs more in tooling and handling and buys the ability to make those geometries.
1.2 Why heat it at all?
Two reasons. First, formability: at austenitizing temperature the steel flows like a much softer material, so deep, sharp features form without cracking and without the springback that plagues cold forming of high-strength steel. Second, the heat treatment: the very same heating that makes the steel formable sets up the martensitic transformation that makes the part strong. Cold stamping of 1500 MPa-grade steel would need enormous press forces, brutal die wear, and still produce springback; press hardening sidesteps all three.
2. The Process, Step by Step
- Blanking. The coil is slit and blanked to the developed shape of the part. For coated grades, the blank edges are exposed steel — a detail that matters downstream.
- Heating (austenitizing). The blank is heated in a roller-hearth furnace, typically to 880–950°C (about 930°C is the familiar nominal) for a soak long enough to fully austenitize. Bare steel needs a protective atmosphere to limit scale; Al-Si coated steel is largely protected by its coating.
- Transfer. The hot blank is moved to the press, usually by robot or automated transfer. Every second of transfer cools the blank — typical transfer windows are counted in seconds, and the die must close while the blank is still above the transformation start temperature.
- Forming and die quenching. The die closes, forms the part, and the water-cooled tooling extracts heat at high rate. The blank is held under pressure until it cools below the martensite finish temperature — typically a few seconds of dwell.
- Trimming and finishing. The hardened part is too strong for conventional trimming, so edges and holes are cut by laser, or by trimming dies for simpler shapes. Some parts get shot blasting if bare steel was used.
For a buyer of the strip, the practical consequence of this chain is a supply requirement: the coil must be consistent in chemistry, thickness and coating, because the entire downstream process is tuned to one material behavior.
3. Materials: Boron Steel and the Al-Si Coating
3.1 Why boron
The base steel of press hardening is a low-carbon steel with a small boron addition — 22MnB5 sits at roughly 0.22% carbon with manganese and boron as the hardenability players. Boron is the star: even a few parts per million at the grain boundaries dramatically delays the ferrite and pearlite transformations, which is exactly what lets a thin sheet transform to martensite during a fast die quench. Without boron, the same cooling would produce a soft, mixed structure and the part would not reach its strength.
3.2 Why the Al-Si coating
At 930°C a bare steel surface oxidizes quickly. Scale on the part would have to be blasted off, and the surface would decarburize, softening the finished part. The aluminum-silicon (Al-Si) coating — the "+AS" suffix in grade names like 22MnB5+AS — solves both: it forms a stable diffusion layer that keeps oxygen off the steel and holds carbon in. After forming, the coating remains on the part, ready for adhesive bonding and painting in the body shop.
3.3 The grade family
The press hardening grades stocked and supplied by HS-FINEB — drawn from the Baosteel product families we carry — include 22MnB5+AS, Usibor1500P, T1500HS+AS, T1500P, AC1000HS and AC1500HS+AS, HC950/1300HS+AS, CR1200/2000HS+AS and others. The naming pattern carries information: the number is the nominal tensile class (1500 MPa, 2000 MPa), "HS" marks the hot-stamping family, and "+AS" marks the Al-Si coating. A 1500-class part and a 2000-class part need different furnace settings and different die cooling — another reason to keep the grade explicit on every order.
4. The Temperature–Time Curve
Press hardening is a race against the transformation diagram. The table below is the typical sequence for a 22MnB5-class part — the original data point of this guide. Values are typical and must be confirmed against the steel supplier's CCT data and the line's qualification records.
| Stage | Temperature (typ.) | Time (typ.) | What is happening |
|---|---|---|---|
| Furnace heating | → ~880–950°C | 3–10 min (soak) | Full austenitization; coating diffusion |
| Transfer | ~900 → ~750°C | 3–10 s | Blank cools; must stay above Ar3 |
| Forming + die quench | ~750 → ~200°C | 5–15 s dwell | Forming, then martensite transformation |
| Die open / ejection | ~150–200°C | — | Part below Ms, safely removed |
| Resulting structure | Martensite | — | ~1300–1600 MPa tensile, 45–50 HRC |
Typical values for orientation. The critical requirement is cooling rate above the transformation nose — for 22MnB5-class steel this is typically in the 20–50°C/s range during the quench. Confirm with the material supplier's CCT diagram.
5. The Die: Cooling Is the Process
The press-hardening die is part tooling, part heat exchanger. Cooling channels are drilled close to the forming surface, and chilled water is pumped through them at controlled flow and temperature. Design faults show up as uneven hardness across the part:
- Channel density must match the local cooling demand — tight radii and thick sections need more channels.
- Water temperature and flow are process parameters, logged like press tonnage, not set-and-forget.
- Hot spots produce soft zones where the cooling rate fell below the critical value; those zones fail crash tests unpredictably.
- Die surface condition matters because the hot blank is in direct contact — coating transfer and wear change the heat transfer coefficient over the die's life.
The strip buyer's connection to all this is indirect but real: die validation is done on production material, and a coil with different coating weight or chemistry than the validation coil will shift the cooling response. Keep the supplier consistent, or re-validate.
6. Press Hardening vs Cold Stamping of AHSS
Cold-stamped advanced high-strength steels (AHSS) — the DP and CP families — and press-hardened steel compete for the same crash-structure jobs, and the choice is not always press hardening.
| Dimension | Cold stamped AHSS (e.g. DP, CP) | Press hardened (22MnB5-class) |
|---|---|---|
| Achievable strength | ~600–1180 MPa class | ~1300–2000 MPa class |
| Springback | Significant, hard to control | Minimal — formed hot, no elastic recovery |
| Formability at target strength | Limited — strong steel is hard to form | High — formed in soft austenitic state |
| Cycle / energy | Cold, fast, low energy | Furnace + dwell, higher energy |
| Edge trimming | Conventional dies | Laser or dedicated trimming |
| Typical use | Floor pans, rails, complex panels | B-pillars, roof rails, impact beams |
The practical pattern in body-in-white design: press hardening for the parts that must absorb crash energy at minimal gauge, cold-stamped AHSS for the parts where geometry complexity and cost win. The two families are complements, not substitutes.
7. Common Defects and How to Prevent Them
- Uneven cooling / soft spots. Hot spots in the die or insufficient dwell leave ferrite or bainite instead of martensite. Prevention: cooling channel design, water temperature control, and hardness mapping of first articles.
- Over-long transfer time. The blank cools below the forming window before the die closes — the part either does not fill or transforms too early. Prevention: automated transfer within seconds and a logged transfer time.
- Coating damage. The Al-Si layer can crack or be scraped off during handling or forming, exposing bare steel that scales and decarbs. Prevention: careful blank handling, coating-compatible tooling, and inspecting blanks before loading.
- Decarburization of bare steel. Without coating or atmosphere control, the surface loses carbon. Prevention: protective furnace atmosphere or coated grade; verify surface hardness.
- Incomplete austenitization. Short soak or low furnace temperature leaves undissolved carbides and a soft, mixed structure. Prevention: furnace calibration and soak-time logging.
8. Frequently Asked Questions
Short answers here; the full schema FAQ follows below.
- Is press hardening the same as hot stamping? Yes — the terms are used interchangeably for the same forming-plus-quench process.
- Can press-hardened parts be welded? Yes, though the martensitic structure and the Al-Si coating need controlled welding parameters; resistance spot welding is standard in automotive body shops.
- What gauge range is typical? Automotive press hardening typically runs ~0.8–2.5 mm sheet and strip; the exact range depends on the part and the line.