Advanced High-Strength Steels:
Right Choice for Vehicle Greenhouse Gas Emission Reduction

As environmental and climate change concerns escalate, pressure is being applied in to the automotive to reduce the greenhouse gas (GHG) emissions its products contribute to the environment. Consequently, concerted measures are being taken to reduce vehicle weight for increased fuel efficiency. But at the same time, it is critical to continually improve vehicle safety to meet ever increasing crash requirements around the world, a need that is in direct opposition to weight reduction.

So, in the quest for safer and lighter cars, automotive and steel companies are rapidly introducing new advanced high-strength steels (AHSS) for body structures. AHSS alloys include high tensile strengths--500 MPa or greater--with good formability, and include grades like dual phase (DP), complex phase (CP), transformation-induced plasticity (TRIP) and some martensitic steels. The high strengths of these materials allow for mass-efficient designs for improved fuel economy, while simultaneously increasing crashworthiness. Unlike many competitive materials, AHSS can accomplish these objectives with little or no overall cost to the manufacturer. Several full-vehicle concept designs and subsystem concept designs, such as the steel industry’s UltraLight family of research, have demonstrated 25 percent mass savings over conventional steel designs while improving crash performance, without increasing cost.

The body-in-white for the 2008 Mercedes C-Class contains 70 percent high-strength steels and 20 percent advanced high-strength steels.

The advantages of these materials for meeting automotive manufacturers’ goals are well recognized by the design community and, consequently, are being incorporated into nearly every new vehicle design in increasing percentages and strength levels. Steel makes up more than 50 percent of today’s vehicles and the predominant material of vehicle bodies-in-white, which is the body structure plus doors, hoods, decklid or hatch. Today it is becoming common place for as much as 60 percent or more of the body-in-white to be manufactured using high-strength steels (HSS) and AHSS.

Many vehicles on the road today contain significant amounts of AHSS, one of which is the 2008 Mercedes C-Class whose body-in-white contains 70 percent high-strength steels and 20 percent advanced high-strength steels. Mercedes-Benz states in an S-Class media release that advanced high-strength  steels are “indispensable when it comes to meeting the stringent Mercedes requirements with respect to durability and safety.”

Addressing GHG Issue
Globally, existing or proposed regulations regarding vehicle greenhouse gas (GHG) emissions address only the use (driving) phase of a vehicle’s total life cycle. From this perspective it is easily understood that, assuming all other things are equal, a lighter weight vehicle results in reduced fuel consumption and consequently reduced use phase GHG emissions. Material choices, therefore, that result in the lowest mass vehicle may be preferred if one considers only a vehicle’s use phase. Automakers are consequently looking to alternative materials to further reduce weight.

But as a recent Life Cycle Assessment study completed by the University of California Santa Barbara Bren School of Environmental Science (commissioned by WorldAutoSteel, IISI’s Automotive Group) reveals, to truly reduce the vehicle footprint on the environment, consideration beyond fuel efficiency must be given. The study compares conventional steel vehicles to AHSS- and aluminium-intensive vehicles over all vehicle life cycle phases—that is material production, vehicle production, use (or driving) and vehicle end-of-life recycling. This Life Cycle Assessment (LCA) reveals that material selection is a critical aspect to the GHG emissions during the vehicle material production phase. As shown in the following table, alternatives to steel produce 5 to 10 times as much GHG emissions during their production. For example, GHG emissions from steel production consist of only carbon dioxide whereas GHG emissions from aluminium consist of carbon dioxide and potent perfluorocarbons (PFCs).

Consequently, applications of alternative materials front-load the environment with more GHG emissions resulting from the production of the material than the steel application they replace. In the case where the alternative material results in reduced use phase fuel consumption, this achievement may not be sufficient to offset the upfront loading of GHG emissions in the material production phase. Vehicles built with alternative materials may net more GHGemissions during their lives than AHSS-intensive vehicles. 

In two examples calculated using the UCSB study’s LCA model, an optimised AHSS vehicle design was compared to a conventional mild steel design. (Figure 1) The effect of 25% mass reduction in the body-in-white was to reduce CO₂ emissions in both the material stages and vehicle use phase so that the total life cycle emissions of the vehicle are reduced by 3%.   And, this is accomplished with little or no cost increase.

Figure 1

The UCSB model also compared an optimized aluminium design with the AHSS design (Figure 2). Although some additional mass savings can be achieved with aluminium, the increase of CO₂ equivalent emissions from the material production phase more than offsets the reductions generated in the use phase. The vehicle’s total life cycle emissions are increased by 2%. To add insult to injury, this environmental burden also comes with a significant (nearly 60%) cost increase.

Figure 2

Steel is Still the Right Choice
Using AHSS to reduce vehicle weight creates a win-win situation in that fuel efficiencies can be achieved due to reduced weight with no increase to GHG emissions, even potentially decreasing GHG. AHSS provides for light weight automotive solutions that are light on cost, light on the environment and light on natural resources, providing peace of mind and unmatched safety for automotive manufacturers and consumers. As automakers address the climate change impact of their products, steel is still the right choice for vehicle applications.