Why press hardening steel is “hot” now for auto designers

We interview two of SSAB’s experts on press hardening steels to find out about:

• Where car OEMs are now using 1800MPa and 2000MPa hot-stamped boron steels in their auto bodies.
• The design freedom that current PHS production technologies provide, including “soft cell” components.
• The advantages (e.g., better shape accuracy, complex part geometries) and disadvantages (cost, energy, slow speed) of hot stamping.
• The development story of Docol^® 2000 PHS.
• A comparison of UHSS steels: PHS vs. Martensitic.

What can you tell us about the new VDA standard for 1900MPa press hardening steel?

It’s a brand new standard but already car manufacturers have used it to develop their own OEM standards. The VDA name for the new PHS grade is CR1900T-MB-DS, which corresponds to SSAB’s grade Docol^® PHS 2000, which goes up to a tensile strength of two gigapascals, or 2000MPa.

That’s the strongest steel available — where are car designers using it?

There are many different possible applications, but high on the list are EV battery enclosures, where no intrusion is allowed — which is particularly challenging for the side impact crash test. So automotive OEMs will be using PHS 2000MPa steel in safety components like cross beams in the floor area, between the side rocker panels, to protect the EV battery.

Do you think the new Docol^® PHS 2000MPa could also be used on other applications where PHS 1500MPa has traditionally be used?

Oh yes, PHS 2000 — and PHS 1800 — isn’t just for EVs: it will be useful for ICE (internal combustion engine) cars as well. It can be used wherever designers either need maximum steel strength or weight reduction by using thinner walls.

The classic automotive application for boron press hardening steels is the vehicle’s A and B pillars. But PHS is also used in roof rails, sidewall members, roof and dash panel cross members, as well as the reinforcement of doors, windshield pillars, and floors. You’ll note that the current designs of these hot-stamped parts tend to feature highly complex geometries, which is where PHS steels excel.

And another advantage of using hot-stamped steels is that a single part can have different strength levels — so the part behaves in a specific way during a crash, right?

Right. A hot stamping die can be segmented to have different quenching processes. For example, to create a PHS “soft cell” you don’t quench the entire part. The covered, unquenched part will have a lower strength level which, in turn, can absorb high energy during a crash. The quenched segment of the part, which has higher tensile strength, will withstand high forces. There has been a lot of development in recent years on PHS soft cells. For example, making a soft cell for the lower part of the car’s B pillar.

Tailor blanks are another way to get “segmented behavior” in a PHS part, right?

Yes. A tailor rolled blank is a strip of steel that is cold rolled to have different thicknesses across the width of the strip. So you can specify were you want the strip to be thicker and where you want it to be thinner, based on your ultimate requirements for the part and how you want it to perform.

A tailer welded blank can be even more varied: you can weld together strips of PHS that have different thickness, or even weld a PHS steel strip to a non-PHS steel strip.

All of these choices reminds me of what you call the “design freedom” of PHS.

Think about it: you have a boron steel that you heat and stamp at 900°C. At that temperature, you can readily hot stamp the PHS into very complex shapes with deep sections. So with that kind of design freedom, car designers can be more creative with their components. Maybe that means leveraging PHS’s high tensile strength to design lighter parts. Maybe design freedom translates into few parts — parts consolidation. And maybe design freedom means parts that perform better in crash tests.

Hot-stamped parts can have better final part shape accuracy, right?

Well, the standard augment for heat treatable steel is that you have little or no springback and that, in turn, leads to better shape accuracy. That sticks in many people’s minds as AHSS tensile strengths get higher and higher and springback has the potential to increase.

But on the other hand, we’ll also understand better than ever — and can better predict and control — springback in cold formed car components, even when using gigapascal steels.

Of course, a lot depends on your part’s design: rather than just assuming you must pay more for using PHS steels to avoid springback, talk to your AHSS supplier early on in your design concept process. Many design concepts, with slight modifications and the proper sequence of production steps, can be successfully cold formed with very good final shape accuracy.

Docol^® PHS 2000 steel is unusual because it was originally developed for one customer, right?

Yes. We always work closely with our customers. But in this case the customer, Gestamp, approached us with a very specific request: “Can you develop a press hardening steel with 2000 megapascal tensile strength but with the ductility of 22MnB5 (1500MPa tensile strength)? We want use it in our idea for a lighter bumper.” We saw the value of a 2000 MPa steel for other safety components too, so we said yes.

And developing a hot-stamping steel has its own unique challenges, correct?

That’s right. As you know, with press hardened steels the fabricator develops the final mechanical properties of the steel as they heat, form, and then quench the steel. And many different factors come into play: How long is the steel in their furnace and at what temperature? What cooling media are they using in the tool? What is the contact pressure between the tool and the material?

And all of these OEM and Tier 1 hot-stamping processes are proprietary — each one can process the PHS steel differently. Gestamp doesn’t even share their hot-stamping information with us! (laughs) But we had to make a PHS 2000 steel that will work for multiple car OEMs. So it was quite challenging.

To get the PHS 2000 to the required ductility levels, SSAB was left to focus on the steel’s chemical composition and our processing systems in our mills. That and providing customers with the recommended furnace temperature and the cooling rate — the minimum cooling rate necessary to achieve the steel’s martensitic microstructure and therefore its ultimate tensile strength.

And what did Gestamp do?

They would test our materials using their press hardening tools. Their feedback and tests were extremely valuable as we made iterations to the PHS 2000 steel. Working so closely with them really sped up our learning and theirs.

What was the outcome?

The Gestamp PHS 2000 steel bumper is 17% lighter and still cost-effective. So it was a win-win for both Gestamp and SSAB.

A Quick Comparison of Ultra High Strength Steels