Light, Safe, Steel

Gary S. Vasilash

Vehicle engineers are finding that new steels are allowing them to meet requirements for safety and fuel efficiency. Which is not to say they're not using other materials, too.

One of the most eagerly anticipated vehicles of the year is the Ford Fiesta, which is coming to North America after having had notable successes in European and Asian markets, with well over 500,000 units sold of the current generation (Mk VI). Although it is a small car, it is engineered to be a safe one, deploying not only Ford's SPACE Architecture (as in Side Protection And Cabin Enhancement) but an extensive array of cold- and hot-formed, high-strength steel in the body structure. More than 50% of the body structure uses the material. The steels are used in the floor, front rails, beams, and integrated body reinforcement ring.

Dual-phase steels are used for the rocker panels (which are engineered with welded baffles that help absorb impact energy), the side roof arch, and the under-floor support beams (a.k.a., "sled runners").

The A- and B-pillars are produced with ultra-high-strength aluminized boron steel.

Aluminized boron steel?

According to Ron Krupitzer, vice president, Automotive Applications, AISI, aluminum is used as an alloying material in some steels that provides a means by which there is a reduction—as much as 10% in some cases—in the density of the steel. In other words, it is made lighter.

As for the boron, this element is well known for providing hardness and toughness. Krupitzer explains that the material is "wonderful for strength," but because of its characteristics it is more difficult to form. This leads to the requirement for hot forming, wherein the material is heated, formed, then quenched to form martensite, a hard crystalline structure.


Strength & stiffness.

The move is on to develop more and more higher strength steels. As Jody Shaw, manager of Technical Marketing and Product Research, U.S. Steel, notes, "Twenty years ago, the body structures were stiffness-driven, not strength-driven. Stiffness is independent of strength." So back then there were more mild steels used. But in the mean time, there have been increases in legislative requirements regarding crash, which has meant the need for more high-strength steels. Shaw points out, "Back in the ‘80s, these higher strength steels weren't applied because they didn't do anything for you because stiffness was the need." Which, of course, has changed. 

But there is another consideration, which is that of weight. Sure, you could make something stronger by adding brackets and stiffeners and the like, but that adds mass. And just as there are considerations about occupant protection—considerations that take the form of regulations—there are similar issues related to fuel efficiency. Consequently, vehicle engineers are looking for materials that are light but strong. And this not only means steel, but aluminum. What's more, it means that there has to be some considerable engineering going on in order to create structures that can fulfill the requirements in the most efficient manner.


Dynamic drive.

For example, when Acura engineers went to work on developing the 2010 ZDX, one of the things that they paid attention to was what they call "dynamic stiffness," for purposes of driving performance (a lot of this is predicated on chassis loads). This was made all the trickier given the fact that the ZDX has a huge panoramic roof opening, which stretches from the leading edge of the windshield to the trailing edge of the back glass on the tailgate. What's more, this opening also provided a challenge vis-à-vis the government roof crush standard (FMVSS 216), which calls for the ability to handle 1.5 times the curb weight of the vehicle, and a forthcoming standard that will require handling three times curb weight. Acura engineers say that the use of high-strength steel not only allows the vehicle to meet the current requirement, but they expect it to meet the 3x requirement, as well.

Overall, high-strength steel accounts for 48% of the unit body, including applications on the A- and B-pillars, floor sills, roof frame, cross braces, and floor cross members.

But going for additional weight savings led to such things as an aluminum hood, which they estimate saves 15 lb. compared with steel, and an aluminum instrument panel support (it connects the A-pillars to the front floor cross member), which is a structural piece combining extrusions and stampings, saves 14 lb. compared with a steel component.



Another vehicle with a considerable amount of high-strength steel—46% of 340 grade or above—for purposes of both vehicle dynamics and enhanced safety—is the 2010 Accord Crosstour. Like the ZDX, it features what is called by Honda engineers the ACE—or Advanced Compatibility Engineering—body structure, which is designed so that in the case of a frontal collision, crash energy is channeled to both upper and lower structural elements, such as the floor frame rails, side sills, and A-pillars. The Crosstour also utilizes what is called the "Maximum Efficiency Floor," which deploys a network of longitudinal rails, floor cross-members to enhance not only safety, but handling and packaging efficiency, too. What's more, there are high-tensile-strength steel tubular beams in the doors to help minimize intrusion during a crash.



Don't think that using steel for passive safety is a consideration primarily of engineers in the U.S. Consider the structure of the current-generation Renault Mègane. It was engineered to have "programmed deformation" of its structure such that in the event of a collision, the materials used—described as "high, very high and very, very high elastic limit steels"—absorb and dissipate the kinetic energy such that the occupants of the vehicle are subjected to lower deceleration forces, which helps reduce the severity of injury.

BMW is rolling out the sixth generation of its venerable 5 Series Sedan, and it, too, is deploying steel rather extensively and in a specific manner so as to provide what the company is describing as a "safety passenger cell." BMW is deploying high-strength multi-phase steel and ultra-high strength hot-molded steel. The use of the materials is done in such a way that BMW engineers have been able to increase the mean body strength and stiffness by 55% compared to the fifth-generation 5 Series without adding to the overall amount of material deployed (i.e., by using the stronger material, it isn't necessary to use more material in order to achieve the same levels of strength and stiffness).

However, given that the illustration indicates that there is the use of aluminum, too, a word about that is in order. This is the first time that the doors of a 5 Series Sedan are made of aluminum. This use results in a weight save of approximately 50 lb. compared with steel doors. In addition to which, not only are their hang-on panels such as the hood and front side panels made with aluminum, but they're also using it for a number of front and rear axle components. Also, the spring supports are made with pressure-cast aluminum, which are said to make the front section stiffer, and as they are comparatively light, improve the balance of the car, both of which contribute to, BMW says, enhanced driving dynamics. The point being: materials matter.


Steel Standards

According to the ASTM Inter-national ( Committees A01 on Steel, Stainless Steel and Related Alloys and A05 on Metallic-Coated Iron and Steel Products, two ASTM standards have been revised such that the thickness tolerances for both uncoated and coated sheet steel products are now more clear.

The standards in question are:

  • A568/A568M, Specification for Steel, Sheet, Carbon, Structural, and High-Strength, Low-Alloy, Hot-Rolled and Cold-Rolled, General Requirements for
  • A924/A924M, Specification for General Requirements for Steel Sheet, Metallic-Coated by the Hot-Dip Process

Apparently, the previous standard included two sets of tolerance tables for each product, neither of which was the default, so purchasers had to decide which set of tables applied to their purchase. Now there is a worldwide default thickness tolerance table for each standard.

For those who had been using the 3/8-in. edge distance tables, know that these tables have been moved to the Supplementary Requirements section of each of the specifications so that if it is necessary to continue to need to purchase this way, they can invoke the section on their purchase order.