Cold stamping 1500 and 1700 MPa automotive steels

EVs and 5-star crash performance drive push to stronger steels

To meet the automotive industry’s goals of high-strength, lightweight body structures enabling 5-star crash performance, it’s quite clear that car OEMs plan to further increase their use of higher strength AHSS/UHSS steels. Electric vehicles, in particular, require higher strength materials used in new ways, especially to carry and protect their heavy battery packs in innovative new EV battery enclosures.

For several years, 1.5 GPa steels have been roll formed to make relatively simple profiles for automotive BIW applications. But now, systematic advances in stamping technology are giving BIW designers and production engineers an opportunity to cold stamp 1.5 GPa steels for moderately complex profiles – instead of solely relying on hot stamping.

Cold forming’s advantages for gigapascal steels

Pros of cold forming

1. Up to 28% less expensive: The cold forming process is considerably less costly than hot forming due to hot stamping’s significantly higher operating costs.
2. Much less energy consumption than hot stamping, resulting in both lower energy costs and lower CO₂ footprints.
3. Martensitic steels are available in a wider range of mechanical properties than press-hardening steels (PHS).
4. Cold-formed martensitic parts have better final surface quality than hot-stamped parts.
5. No additional cleaning (shotblasting) needed for cold-stamped parts.
6. Cold-stamped parts can be mechanically pierced and trimmed, while press-hardened parts require more expensive laser operations to avoid hydrogen embrittlement.
7. Martensitic steels are available with cathodic corrosion protection.
8. Martensite grades have better welding properties than PHS due to their lower equivalent carbon content (Ceq or C.E.).

Cold-forming considerations

1. It is not possible to cold form highly complex shapes.
2. Springback and its countermeasures must be planned for – but this is an area where the Japanese and others are proving that it can be done: see the next section and Managing Springback below.

Source: SSAB.

Japan’s early lead on 1.5 GPa cold stamping

“Compared to the rest of the world, the Japanese die manufacturers have more fully developed their dies and tooling to accommodate the higher forces needed for 1500 MPa steels, while controlling springback,” notes Hiroshi Kondo, Japanese auto steel consultant for over 30 years. “All Japanese car OEMs are now evaluating 1500 megapascal steels for cold stamping.”

“Traditionally, Japanese OEMs and their Tier 1s have always been more comfortable with cold stamping as opposed to roll forming or hot stamping,” continues Kondo. “One advantage of cold stamping vs. roll forming is that stamping gives you more freedom with the geometry of the part.”

“And then there are all the advantages of cold stamping vs. hot stamping. Of key importance to the Japanese carmakers is low energy use and fast cycle time – and now, of course, reduced CO₂ footprint – all of which favor cold stamping over hot stamping.”

Which automotive components might be cold-stamped at 1.5 GPa?

For 1500M applications, potential candidates for cold stamping include side intrusion beams, bumper beams, cross beams, and their reinforcements.

“Most rocker panels, rocker panel reinforcements, floor cross beams, and some roof beams can be roll-formed,” says Kenneth Olsson, SSAB Automotive Business Development Specialist. “But there are many parts that, due to their shape, cannot be roll formed.”

“While hot stamping is always an option,” continues Olsson, “it’s more expensive, it’s slower, and if you’re heating your furnaces with fossil fuels – as most manufacturers are – there are CO₂ emissions, which go against the car OEMs’ sustainability goals. So, cold stamping at 1500 MPa strength levels is an exciting, relatively new development.”

Kenneth Olsson.

Impressive trials in cold-stamping 1.5 GPa and 1.7 GPa steels

KIRCHHOFF Automotive has successfully cold stamped this side impact beam prototype using Docol^® 1500M and 1700M grades. The top photos show the formed parts, while the bottom photos show the parts after a 3-point bending test.

For some auto parts, going stronger is straightforward

This side impact beam, cold stamped from Docol^® CR1150Y1400-MS-EG steel, was successfully tested in a serial production die designed for CR950Y1200T-MS-EG. SSAB’s forming experts believe the part could be successfully cold stamped using CR1220Y1500T-MS-EG.

Cold forming avoids hydrogen embrittlement pitfalls

After hot stamping, manufacturers must be extremely careful with trimming and piercing operation, or they might cause delayed fractures. Consequently, lasers are typically needed to pierce or trim hot-stamped parts – and these lasers are more complicated and expensive than conventional mechanical tools.

With cold forming, stampers can use conventional mechanical piercing and trimming tools without the risk of delayed fracture, even on 1.5 GPa steels. And in-line mechanical trimming and piercing are familiar, fast, and cost-effective processes for the stampers.

Managing for cold-stamping springback in ultra-high strength steels

Hot-stamping’s two key advantages are its ability to form very complex shapes and the elimination of springback. But Japanese stampers and others have developed a series of strategies to control springback in cold-stamped parts:

1. Simulations: forming simulations allow designers to optimize the part’s geometry to control for springback and improve the final accuracy of cold-stamped components.
2. Optimization: includes the use of straight bending lines and engineered corner (radius) configurations.
3. Beads: the shape and placement of draw beads for greater springback control.
4. Die geometry: going from a lower to a higher yield-strength steel, like CR1220Y1500T-MS, can require some changes to the geometry of the die to compensate for higher springback.
5. Tools: improved tooling materials, including wear resistance and coatings, to handle the higher die forces.

What other experts are saying about cold stamping UHSS

Martensitic steels provide a cold-formable alternative to hot-formed press hardening steels, as World Auto Steel writes on its martensite webpage.

Using cold stamping allows for the flexibility of considering different strategies when die processing, which may result in reduced springback or incorporating part features not achievable with roll forming. Cold stamping of martensitic steels is not limited to simpler shapes with gentle curvature.

World Auto Steel then shows a photo of a center outer pillar that has been cold-stamped, with a tailor-welded blank of CR1200Y1470T-MS as its upper portion and a CR320Y590T-DP lower portion. They go on to cite:

A study that determined there was a correlation between sheet steel yield strength and the 3-point bending deformation of hat-shaped parts. Based on a comparison of yield strength…CR12001470T-MS has similar performance to hot-stamped PHS-CR1800T-MB and PHS-CR1900T-MB at the same thickness and exceeds the frequently used PHS-CR1500T-MB. For this reason, there may be the potential to reduce costs and even weight with a cold-stamping approach, providing appropriate press, process and die designs are used.

The article goes on to show a cold-stamped cross member reinforcement, in commercial production, made in martensitic 1500 MPa steel:

The varying elevation of this part, combined with a non-uniform cross section at the outermost edges, help control springback, but makes roll forming significantly more challenging if that were the cold-forming approach [instead of its cold-stamped process].

The article concludes with an example of a cold-formed 1500T-MS center roof reinforcement that uses the patented Stress Reverse Forming™ process for improved dimensional accuracy via reduced springback sensitivity.

Comparing 1.5 GPa cold-stamping to 1.5 GPa hot-stamping

Accuracy: With no springback, press-hardened parts can be very precise. With cold-stamping, comprehensive springback management is key to parts accuracy.
Shape of part: Hot stamping is ideal for highly complex part shapes – although cold stamping is making impressive gains parts complexity.
Cycle time: Cold stamping is much, much faster that press hardening.
Energy use: Hot stamping requires rapid heating (to 900°C) and rapid cooling; cold stamping does not, saving money and emissions.
Trimming/Piercing: 1.5 GPa PHS parts require laser trimming and piercing to avoid hydrogen embrittlement. On the other hand, cold-stamped 1500 MPa parts can be mechanically cut and pierced in-line.

Photos courtesy of KIRCHHOFF Automotive.

1400M side impact beam, successfully cold stamped in a die designed for 1200M.

Roll-forming: still going strong(er)

While cold-stamping 1.5 GPa parts is relatively new, (cold) roll-forming UHSS steels has been common for years (in places other than Japan). And designers and Tier 1s have become increasingly sophisticated in their roll-forming applications, pushing the boundaries of what shapes can be made.

For example, Shape Corp. devised roll-forming Docol^® CR1350Y1700-MS-UC steel and then 3D bending it into this roof rail for serial production on Ford models. The result is a low-weight, cost- and energy-efficient component that, with its thinner walls, provides improved visibility for the car occupants.

3D roll forming: a solution looking for problems?

Three-dimensional roll forming has been proposed as an alternative way to make some slightly more complex geometries in UHSS.

SSAB’s crash performance experts proposed the use of 3D roll-forming in 1700M for battery protection in its Docol^® EV Design Concept. Here the design leverages 3D roll-formed (“corrugated”) beams, woven in a mesh pattern, for an exceptionally strong EV battery enclosure base that is one half the conventional height of such structures.

The blue beams below the car’s floor are the “mesh” structure of the battery enclosure. Their cross pattern of 3D roll-formed beams in Docol 1700M reduces the mesh’s height by a factor of 2.

How cold formable is Docol^® CR1220Y1500T-MS?

Docol^® Martensitic 1500 MPa provides improved capabilities for cold forming as demonstrated by:

• Deep-drawing ratio of 2.0
• Hole expansion ratio that is typically 40%
• Guaranteed bending of 4.0*t
• Guaranteed roll-forming of 3.5*t

What’s next for cold forming UHSS?

SSAB carefully follows the introduction and deployment of new cold-forming technologies, knowing that cold-forming efficiencies – with their shorter tack time, simpler presses, and reduced energy use – align well with car manufacturers’ goals for simplified production, reduced cost, and sustainability.

We encourage designers of body-in-white components to involve our forming experts early in your product development for best results. Contact your local Docol^® representative to describe your project’s goals and ambitions.