Steel the One

The way Roy Platz remembers it, the kick in the butt felt by everyone in the U.S. steel industry came in the form of the 1984 Pontiac Fiero with its Enduraflex plastic exterior body panels hung on a drivable metal space frame.

“That really got our attention,” Platz says. “Our reaction was out of proportion with the importance of the car. But it signaled to us that if we wanted to keep our share of the automotive market, we had to earn it. “The Fiero was simply a catalyst that brought us together as an industry and we’ve been moving positively ever since. We now understand that last year’s birdie is this year bogey. Continuous improvement is standard operating procedure.”

Platz is currently director of Ispat Inland’s marketing department, but has worked in research and quality control and also spent more than a decade selling steel in Detroit — all with Inland.

“Right now the auto industry is in a situation where they need a new approach for driving out costs. They need to simplify, eliminate parts and processing. And they still need to meet crashworthiness, weight and NVH objectives.”

“Suppliers have already been squeezed about as much as they can be on price. The real savings now will be realized by taking costs out of the system,” believes Platz. “Hopefully programs like GMs Epsilon platform will bring good results. It’s certainly an ambitious attempt.”

Featured on the January cover of Automotive Industries, the Epsilon platform will be used for more than a dozen models to be produced at up to eight manufacturing plants for eight different GM brands. The global architecture allows for regional tailoring to meet various market needs or manufacturing requirements.

The Epsilon platform is a showcase for steel, the Chevy Malibu version of the platform was recently featured in a presentation by Michael Weber, body structures engineering group manager for the Epsilon platform at GM an American Iron & Steel Institute seminar. The Malibu BIW (body-in-white) structure is blend of low carbon, solution strengthened, bake hardened, dual phase and high strength low alloy grades of steel. Low carbon steel still accounts for 41 percent of the 677 307 kg) structure but is used in different strengths and minimum thicknesses to meet strength and weight objectives. Nearly 50 percent of the BIW structure is of steel less than 1 mm in thickness.

Low carbon steel is mainly used in stiffness dominant upper structure parts and also close out panels in the underbody. Thickness is minimized in all cases. Lightening holes are used to eliminate any unstressed and therefore unneeded metal.

Bodyside inners, for example, are made from low carbon laser welded blanks. The front section is 2.1 mm thick with a strength 180 Mpa while the larger rear section is 1.05 mm thick with strength of 110 Mpa. The carbon steel, of course, has excellent formability. Laser welded blanks are also used to make up the front and rear rails, again varying the thickness of the steel on both members for weight savings.

Parts in primary load paths on the Malibu make heavy use of 300 and 340 Mpa steels. Dual phase steel is applied to the rocker section to increase compressive strength for key crash members.

High-strength, low-alloy steel is used to strengthen the underbody in local areas and it also addresses load transfers in rocker and crush box areas. Rocker bulkheads, lower hinge reinforcement and roof bow are also of HSLA steel. Load transfer is the chief purpose.

Work on the Epsilon platform began about six years ago. Would it be much different if the starting point were 2003 instead of 1997?

“It’s an engineer’s job to improve things, right? Well I think we definitely could. I think could simplify quite a bit by moving to the of much more high strength steel, perhaps as high as 80 percent of the entire platform,” says Weber “I think we would relegate low carbon steels to close out panels and that’s about it,” he says. “As we rework this current platform we will be adding a great deal of high strength steel. I’m sure of that.”

What about the potential of the so called TRIP (Transformation Induced Plasticity) steels? “They seem to have great potential,” Weber tells AI, “but there are some weldability and joining issues to be deal with and availability is also still a question. So even if you were starting on a new program today I don’t think you would use them. In a few more years, sure. Progress is certainly being made.”

TRIP steels have a microstructure with retained austenite, ferrite and martensite. They are attractive because they have high workhardening potential and they have good formability relative to tensile strength. They have very high springback. But they also require high forces for forming and secondary forming is problematic. DaimlerChrysler plans some use of TRIP steels in its next A-, Sand C-class vehicles.

“The real problem with TRIP steels is that we are in kind of a chicken and egg mode,” says Ispat Inland’s Platz. “Auto builders won’t go out a limb because they need to have at least two sources. And they won’t design an automobile they can’t build.

“See, if it’s a global platform they need two sources in different parts of the world. So it gets kind of complicated. And, naturally, steel companies haven’t committed because they currently have no customers.”

But it’s going to happen. Platz thinks. And he believes we’ll see it first with Japanese auto builders in the 2006 time frame. “We used to think of ductility and strength as being a linear relationship,” he adds. “Now we’re seeing strength levels rise on the new advanced high strength steels and TRIP grades and we still have adequate ductility. Weight savings will be remarkable. I believe the benefits are simply too great for it not to happen.” In its quiver, steel now has many arrows.

They include the long bow favorite low carbon grades. For cross bow marksmen, we have the HSLA steels at yield strengths roughly from 220-500 Mpa; the complex phase steels from 370-890 Mpa; the dual phase steels from 270-700 Mpa; the TRIP steels from 380-780 Mpa; the ultra high strength steels from 550-1500 Mpa; and hot stamped steels up to 1200 Mpa yield strengths.

Importantly, steel is process friendly, has excellent manufacturability, notes Platz. Weldability may be the single largest advantage but it is also simpatico with other conventional metalworking and finishing processes. Newer processes, like hydroforming and laser welding have yet to be fully exploited. The opportunities are significant. Uniformity of mechanical properties is extremely important in developing vehicles where performance conforms to the design intent. This is particularly important in crashworthiness. Through processes like continuous annealing, the uniformity of steel can be closely controlled.

“All in all, I think auto builders are taking a much more holistic approach and that’s the only way they are going to drive costs out of their vehicles,” says Platz. “Not that alternative materials don’t have their place, but steel has plenty of room to evolve from today’s material of choice into tomorrow’s material of choice. I like our bets. I like our chances.”

• Used in stiffness dominant upper structure parts

• Package space allocated for sufficient noise control countermeasures
  
• Local panel stiffness optimized with beads, shape and minimal unsupported areas
  
• “Crash dominant” stiff global geometry reduces noise energy transmission.
  
• Isolator interfaces designed to strict mobility targets.
  
• High noise radiation areas damped with melt-on damping material.

LASER WELDED BLANK

BODY SIDE INNER:

Composite laser welded blank with:
 – different strengths
 – different thickness
 – All low carbon for formability

Benefits include:

• Reduced part count
  
• Reduced weld count
  
• Dimensional control
  
• Properties tailored for function and formability.