Advanced lightweight materials have increasingly replaced mild steel to meet the OEMs’ demand for increased fuel economy and crash-safety requirements. The latest of these, the third generation (Gen 3) of advanced high-strength steels (AHSS) was the steel industry’s answer to OEMs’ requests for a material that has excellent formability — but without the additional expense of hot-stamping — and with a minimum 980 MPa tensile strength.
Just as with previous AHSS offerings, there is no “heat and beat” with the new steels; they need to be removed and replaced. But the good news for collision repairers is this new steel promises to have repairability similar to existing high-strength steels, said John Catterall, vice president of the American Iron and Steel Institute’s automotive program.
Catterall also leads the Automotive Applications Council, a group of member steel producers, in automotive research, education and technology-transfer activities. He coordinates the steel member company collaborations with the Auto/Steel Partnership, which includes AISI members, automaker representatives from FCA, Ford, and GM, and other steel-related consortia.
“The OEMs basically said it would be nice to have some very high-strength parts that we could just cold-stamp in our regular presses and dies that we use for other parts, to be able to stamp a complex part and yet have it be very strong,” he said.
The base material of Gen 3 steels is ductile for good formability at a lower cost, but out of the die it has higher strength because it work-hardens as it is stamped, Catterall said.
The new Ford Bronco incorporates four Gen 3 parts in the floor structure. Catterall thinks wherever martensite press-hardened steel parts are used in today’s vehicles are likely candidates for using Gen 3 parts instead in future models, including roof rails and B pillars. For repairers, the new material may be a little easier to remove and require fewer consumables.
“The generation three, I think, will probably be a little easier to cut out of the vehicle. It’s not quite a martensite; it’s what is called a retained austenite material, and when you form it or hit it with speed, the retained austenite turns into martensite. But the overall hardness won’t be quite as hard when you remove it from a vehicle.”
Various types of steel are specified for multiple needs
So why, then, is there a need for so many different types of steel throughout the vehicle?
“A lot of it is driven by crash requirements,” Catterall said, pointing out the shallow offset frontal impact test was a prime catalyst for change.
“As the speeds of those tests have gone up, the expectation of survivability and basically wanting to create this safety cage around the passenger with those very high strength PHS [pressure-hardened steel] and martensite parts has driven up the strength requirements of the panels.”
Gen 3 steels offer the added benefit of work-hardening during the crash event, Catterall said.
“OEMs like the B pillars to be strong but maybe have a little give in the bottom so that you get a little bit of intrusion in the bottom, but you don’t get anything above basically the [occupant’s] leg. It will give the engineer a bit more ductility in the bottom of the B pillar.
“For some of the PHS parts, they do what’s called Ductibor®, where even though it’s stamped in one die, they cool the top portion of the die at the regular rate and then they slow down the cooling rate at the bottom. The material of the bottom of the B pillar is at a lower strength to get what they call the ‘soft zone.’”
But not every panel usually contributes to crash safety, he noted, including exterior panels such as the roof, fenders, rear quarter panels, and floor panels.
“They’ve stayed on the lower end of the strength, with the mild steels, the 210 MPa, or the 240 bake-hardenables,” he said. “And those have very good paintability. The richer the chemistries get, the harder the paintability of materials. It’s just the fact when you go to a higher strength. So you’ll always have these milder steels being used on the exterior panels.”
Front and rear rails are designed to deform to absorb crash energy and are typically of dual-phase steel, in the 780 to 980 MPa range.
“In the early days, they were HSLA [high-strength, low-alloy] materials that in the past could have perhaps been pulled back out,” Catterall said. “But now, these dual-phase steels actually get their strength from heat-treating. And that’s the case with a lot of the high-strength steels, which become stronger through heat-treating through the manufacturing process of the steel, and in some cases from the hot-stamping process of the part. So if you were to apply any heat to those materials after the fact, you would actually reduce their properties, as the heat has a softening effect. That’s why you have the trend now with most OEMs recommending to remove and replace.”
Improved roof-crush and side-impact performance requirements have meant parts have required hot-stamping to be of the proper strength and still be drawn in a deep die, he said.
“That is a very hard, very strong, low-ductility material once it’s formed. And again, if you apply any heat to those, it has a softening effect.”
The lower amount of heat, compared to a MIG/MAG welder’s plug weld, is also why adaptive squeeze-type resistance spot welders are required for most replacements.
“We’ve studied other approaches, such as drilling a hole and using a rivet, but it’s got to be a rivet in combination with an adhesive [rivet-bonding], and with that you can actually get back to the strength of a spot weld. You could do a puddle (plug) weld, but there’s just a little bit more variation in that, and you run the risk of applying more heat than you would want to.”
Auto/Steel Partnership works with OEMs to develop repairability procedures
Catterall acknowledged that the advancement in materials has made it difficult for repairers to keep pace with the changes.
“We’ve had at least an additional 100 grades of sheet steel in the last two decades, so it’s been a hard job for the repair shops because it seems like it’s something new every time. There’s certainly a lot more research needed prior to beginning a repair.”
AISI’s Auto/Steel Partnership works with FCA, Ford, and GM to develop repairability procedures.
“I think the cross-pollination is good for the repair community,” Catterall said. The Partnership recently completed a repairability project for repairing Gen 3 steels, with plans to present its findings at AISI’s Great Designs in Steel symposium, scheduled for May 19, 2021. Until then, its findings will not be publicized, he said.
“Obviously, the adaptive spot welding will be great, and some of the other techniques can be made to work. You’ve just got to be careful with with heat input and how you go about repairing it when you’re MIG welding it, because you can get what’s called liquid metal embrittlement with the third-gen advanced high-strength steels.”
