Dual Phase Leads Revival

After years of losing ground in automotive applications to aluminum, composites and other materials, steel is winning business back with lighter and higher-strength grades.

The principal driver is Dual Phase steel, a relatively new high-strength steel with properties that can be “tuned” for different strength levels and processing methods.

“Dual Phase clearly is leading the way,” says Jody Shaw, U.S. Steel Corp. technical industry manager-product technology automotive.

Dual Phase steel is less expensive than another advanced high-strength steel, Transformation Induced Plasticity (TRIP) steels, he says. It's also easier to fabricate, especially in regards to welding, and is a very good absorber of energy.

Even as steel sheds its Rust Belt image for the technological limelight, the material's best strength remains its low price.

“I hate to say it, but the biggest lever we do have isn't our fantastic technology, it's our lower cost,” says Ron Krupitzer, senior director-automotive applications for the American Iron and Steel Institute.

“It's hard to beat. So we still play that card every time.”

General Motors Corp., the world's largest auto maker, is aggressively pursuing advanced high-strength steels, specifically Dual Phase and martensitic steel.

At the Management Briefing Seminars in Traverse City, MI, in August, GM's Jody N. Hall, engineering group manager-GM Metal Fabricating Div., says use of these advanced steels will increase tenfold over the next six model years, while conventional “mild” steel is phased out.

More than 50% of the body structure of these new vehicles by 2010 will be made from advanced steels that are as much as three times stronger than mild steel.

In comparison, the '92 Pontiac Grand Am was predominantly mild steel, with 5% high-strength steel. GM recognizes it must look past mild steels, but cost — almost double that of steel — is keeping the auto maker from fully embracing aluminum. The light metal also is more difficult to form and weld than steel, Hall says.

European consumers are quicker to pay the premium for an aluminum-intensive vehicle because the payback time for a lighter, more fuel-efficient vehicle is one or two years, compared with five to seven years in North America, says Hall. That is longer than the average person owns a vehicle, she points out.

Nevertheless, additional weight is being piled on the typical vehicle in the quest to improve crashworthiness, reduce emissions and add creature comforts.

GM has decided to use several different types of advanced high-strength steels to help control weight issues.

While not as light as aluminum, high-strength steels are far less expensive than aluminum and can be fabricated into much lighter and more versatile structures than mild steels.

The current Epsilon platform (Malibu, Malibu Maxx, Saab 9-3 and Opel Vectra) saw the amount of mild steel cut in half.

Dual Phase steels crush well and are especially suited for body structures designed for crash-energy management, while martensitic steels resist crushing and are ideal for use for occupant-protecting safety cages.

“We think steel is the answer for structures for improved performance,” Hall tells attendees of the conference.

However, she points out that high-strength steels bring with them their own special set of challenges, including problems with “springback” and “sidewall curl,” that makes it difficult for fabricated components to hold their shape.

The advanced steels also cause metal-forming dies to wear out quickly.

Hall says these issues all are being worked on by the Auto/Steel Partnership, a consortium of auto makers and steel suppliers.
— with Drew Winter