TECHNOLOGY FOCUSBenefits Turn OEMs
Toward Dual-Phase Steel
NORTHFIELD, IL (Oct. 9, 2005) - A new kind of steel is giving automakers the low weight they love with the strength of regular steel and without the cost and manufacturing headaches that can accompany aluminum.
The new material is called "dual-phase" steel because it combines two of the many crystalline structures steel forms into - ferrite and martensite, for you armchair metallurgists. The simplest way to think of it is to remember that steel doesn't have a molecular structure like copper or magnesium, but instead is a mixture with a structure like a cake. One type of steel crystal may be like a pound cake - dense and heavy - while another might be like angel food - soft and moist. Dual-phase steel combines the angel food and pound cakes.
Combining the two crystalline structures gives dual-phase steel great strength found in high-strength alloy steels and an ease of formability that alloy steels such as boron steel don't have.
Automakers are jumping to use dual-phase steel. Already, the material is being used by Ford Motor Co., General Motors Corp. (GM), DaimlerChrysler and other manufacturers because of its cost and crashworthiness. "Crash requirements and safety requirements are the biggest driver for dual-phase steel," says Paul Geck, technical leader at Ford.
US Steel, a manufacturer of DP-600, the type of dual-phase steel being used in the automotive industry, reported that dual-phase steel has "a higher yield to tensile ratio as compared to conventional high-strength, low-alloy (HSLA) steels. This results in a higher capacity to manage vehicle crash energy." Because dual-phase steel bends without breaking, it can provide a great deal of strength and crash resistance.
"Most of the DP-600 is appearing in rocker panels and B-pillars, and side panels versus front panels. You don't want to go too stiff with front panels and rails because you have to get the energy absorption," says Geck. "If you had a lot of flexibility in a front rail you could design the rail to have more absorption with the DP-600. In the longer scenario, we may be seeing more DP-600 in front and rear rail applications." The Mercedes Benz C-class cars are an exception to that and use dual-phase steel in the vehicle's front end.
Another area where dual-phase steel's strength is a benefit is in large, flat panels. "In door outer panels and hoods it could make some sense from a dent-ability perspective. To date Ford is the only company to use dual-phase steel for this purpose and it's very much geometry-related. Typically it's for doors that are fairly flat," says
Geck.
Cost and weight are also a factor in an automaker's decision to use dual-phase steel. Jody N. Hall, engineering group manager at GM's metal fabricating division told a group at a recent GM management briefing seminar in Traverse City, MI, that mild steel use would be phased out in favor of newer steels and that the low cost and weight of dual-phase steel was a large reason behind that phase-out.
"Using the higher grades of steel brings steel into a competitive position with aluminum, but you can always make a vehicle lighter with aluminum," says Geck. But while aluminum may be lighter, it is more expensive and harder to work with.
How hard is it to work with dual-phase steel? There are a few questions involving how fast dies wear out and how OEM's overbend the metal so it is shaped right, but what about in a body shop?
The one thing that everyone stresses is not to use heat when working with dual-phase steel. "We strongly recommend that you do not use heat for stress relief," says Frank Wassilak, damageability engineering technology expert at Ford. Heat can weaken or destroy the crystal structure of dual-phase steel.
Manufacturers are putting their repair methods on their service information Web sites, and they say most other repair procedures are fairly similar to standard methods - at least with the current generation of dual-phase steels. However, there are questions about the kink-versus-bend rule as it applies to dual-phase steels.
"The kink-versus-bend rule is really just a guide, and I don't know if it does or doesn't apply to dual-phase steel. It may be that something that's bent may not be repairable based on its location or the thickness or strength of the steel, but today it still applies for mild steel, high-strength steel and dual-phase steel," says Jason Bartanen, technical development manager at I-CAR. "We definitely will be looking at the repair methods for dual-phase steel in upcoming months and years, that's for sure."
The need to examine and develop repair methods for dual-phase steel is going to become more important as automakers use it in more and more parts of vehicle bodies. Experts of this technology believe repair issues will center around the material's strength, low weight and low cost. "I would expect every new platform that comes out will have a significant amount of dual phase," says
Geck.
