Shape consistency in AHSS auto parts: towards a comprehensive approach

As AHSS/UHSS steel grades continue to increase in strength — simultaneously allowing optimized but complex geometries (for example, to help achieve crashworthiness) and thinner gauges (for lightweighting) — it’s become more challenging for part manufacturers to produce consistently shaped auto components.

OEM tolerances for part shape can be as rigorous as ±0.2 to 0.5 mm to ensure precision robotic welding and the ability to pack part cavities with an ever-increasing amount of electronics, cables, and sensors. When parts don’t have a consistent shape, part manufacturers face four losses:
1) The waste of production resources forming the out-of-shape parts.
2) The waste of labor resources sorting out the out-of-shape parts.
3) The waste of material generating the scrap and, much more significantly, trying to troubleshoot why “in-spec” AHSS/UHSS materials can produce out-of-tolerance shapes.
4) The waste of planning resources and overall lower production efficiency due to production slowdowns and stoppages.

If there is nothing wrong about your forming process, could your out-of-compliance shapes be the result of inconsistencies in your AHSS/UHSS steel?

AHSS/UHSS consistency includes, obviously, the thickness and the flatness of the steel. Less obvious are other considerations. For example, inconsistencies in material properties (yield strength, tensile strength, elongation, etc.) can lead to problems such as severe stresses that, in turn, result in flatness issues. Inconsistencies can also lead to unwanted or unexpected behavior in springback. An AHSS/UHSS steel may fulfill all its specifications for dimensions and mechanical properties — but the parts manufacturer still ends up with out-of-tolerance part shapes. What’s going on here?

In the above situation, the parts manufacturer may first ask themselves: Is there a trend to the out-of-tolerance parts? If yes, can we compensate by making multiple adjustments to our forming equipment? 

If yes, then the question becomes does our compensating adjustments work over the full length of the AHSS/UHSS coil? (In other words, are there variations within a coil?) And are there variations from one coil to the next coil? 

There almost certainly will be variations in coils coming from different AHSS/UHSS suppliers. Now the parts manufacturer must have one set of tools for each AHSS/UHSS supplier. This approach quickly gets complicated and vulnerable to inconsistencies each time the equipment is adjusted.

The takeaway: The more consistent the AHSS material, the easier it is to get the final part with the required tolerances.

Inconsistent AHSS/UHSS flatness is a major contributor to parts becoming out-of-tolerance for their specified shapes. Out-of-tolerance shape issues include profile straightness, end flare, positional control problems, and punched features and datum holes not aligning properly from the front to the back of the part. Can precision levelers help?

Precision levelers have been used to address flatness issues in AHSS coils, including coil set, crossbow, and so-called “heat pockets” problems. Precision levelers have their place in metalworking, but to quote a recent WorldAutoSteel article:

"Correcting edge waves and distortion requires exceeding the yield strength of the (AHSS) steel strip at levels of 50% or more of the cross section of the strip; this may increase the potential for workhardening the steel strip, negatively affecting the formability . . . . The progressively higher yield strength for AHSS steels are challenging the capabilities of straighteners and precision levelers that were not designed for flattening these high-strength materials."

The progressively higher yield strength for AHSS steels are challenging the capabilities of straighteners and precision levelers that were not designed for flattening these high-strength materials.

Precision levelers are also very large pieces of equipment that are difficult to place in front of blanking operations. However, the alternative of outsourcing the precision leveling has its own drawbacks: The part manufacturer cannot be sure that the mechanical properties of the AHSS/UHSS steel have been preserved (e.g., no workhardening) by the outsourced leveler.

SSAB is continually refining our mill processes so recipients of our Docol® AHSS/UHSS coils won’t need to use a precision lever. For example, for the problem of “heat pockets” — the less-than-flat conditions that can result from internal stresses during the quenching process — we’ve developed several proprietary steps at our mills to reduce those stresses. 

SSAB also offers its own, very precise conversion of AHSS/UHSS coils into sheets or slit coils, done to our Docol specifications, so work hardening, and the resulting reduction to ductility,  is minimized. Customers find this service particularly useful for our higher strength martensitic grades.

“So many times, you will see problems that cannot be fully addressed by the auto OEM’s specifications,” says José Puente Cabrero, SSAB Product Manager for Docol Cold-Rolled AHSS. “When you look at the entire production process, the AHSS specifications are maybe 20% of the challenges that a material actually needs to address.
 
“For example, we can be discussing the OEM’s requirement for elongation or bendability of a part and discover that what the OEM really is concerned about is how the part behaves during a crash test. Now that’s something that’s very difficult to express in a simple specification.

“Instead of just specifications, SSAB, as an AHSS supplier, really needs a comprehensive understanding of the part’s role. We need to understand how that part will interact with surrounding parts. How will it be joined? What crash tests are relevant? What are the loads of those crash tests? What are the relevant criteria in those tests?

“The ideal scenario is when the OEM, the parts manufacturer, and the AHSS/UHSS producer work together in the very early stage of the car design. In other words: What are the OEM’s ultimate goals for this part? If we know the OEM’s end goals, we can work backwards, so  together we figure out the optimized geometries, the forming, and which AHSS steel to use.

“SSAB’s experience is that when we come across a very strict tolerance, we have to ask: ‘What is behind this requirement?’ Is the OEM attempting to address a production problem — like shape consistency — by raising supplier specifications?

“What works better for everyone — for the entire process and the end results — is when, at the very initial designing steps for a part, you’re also thinking about which material you’ll use, and how it’ll be formed, and how it will respond to those forming processes. When designers think like that, their design becomes more optimized — and so will the forming and the final part’s shape consistency.”

Puente Cabrero continues, “The more consistent we make our Docol AHSS, the more problems that will be avoided all along the whole supply chain and assembly.

“Our continual goal as an AHSS manufacturer is to achieve consistency over mass production runs. SSAB has some inherent advantages: Our iron ore source, from northern Sweden, is extremely high quality and consistent. And each Docol grade is always run with great repeatability — regardless of which SSAB mill it’s produced in.

“Our customers tell us that Docol AHSS is already very consistent. But this is the age of digitalization: so we’re preparing to offer more. For example, can we go beyond today’s industry standard of providing FLCs (forming limit curves)? What if we could accurately predict AHSS properties so we could create digital twins for our customers? 

“Imagine: Digital twins that are specific to each of our AHSS coils, so part producers could, by using the twin, change tooling parameters to get even more precise final shape consistency.

“Stay tuned,” says Puente Cabrero.