Mass Reduction of Vehicles Using Advanced High Strength Steels

The Honda Fit. Image Credits: WorldAutoSteel

Car making is big business, and it’s getting bigger. Current worldwide production is about 65 million cars per year, up from around 40 million at the turn of the century, and the great majority of those vehicles are made of steel.

That steel, though, is far more advanced than the materials of just a few years ago. Meeting the increasingly tough demands of vehicle safety, weight reduction for fuel economy, and manufacturability has led the steel industry to create a new range of 'super steels' for the cars of the future.

The Car of the Future – Lighter, More Efficient, and Made of Steel

At the forefront of these is Advanced High Strength Steel, AHSS, developed by WorldAutoSteel’s member companies, which is proving to be something of a revelation in automobile manufacturing. WorldAutoSteel is the automotive group of the World Steel Association, including 20 major steel producers from around the world.

The typical engineering trade-off in steel selection involves balancing the need for ultimate strength against ductility and workability – stronger steels tend to be stiffer and less ductile, making them harder to form into cars and harder to weld. AHSS is able to retain most of the ductility and workability of lower grades of steel, while offering much greater strength.

The Steel Strength Ductility Diagram. Image Credits: WorldAutoSteel

Where a typical mild steel might have a tensile strength of 300MPa, AHSS can exceed 1500MPa while retaining a maximum elongation of 25%, compared to about 40% for mild steel. The secret is in the microstructure, containing a martensite, bainite, austenite phase rather than ferrite, pearlite, or cementite.

This specialist metallurgy results in steel that can make stronger and lighter cars, while retaining the excellent impact resistance and recyclability of steel.

Wheels of Steel

The body in white (BIW) of a typical modern car uses roughly 350-400kg of steel, forming the shell onto which everything else is attached to create the vehicle. This body has to meet a number of different requirements, some of which may conflict with one another (strength vs lightness for example), while being economical to manufacture and repair and being able to receive coatings for corrosion resistance and, of course, attractive colours.

Steel has been the material of choice since car making began, offering an unbeatable combination of versatility, strength, and affordability, but the metal used today is very different from that in the first production line for the Model T Ford in the early 20th century.

As recently as 2007, more than half the BIW of a typical car would be made from conventional mild steel. Today this 'plain vanilla' metal accounts for less than 30% of the chassis, and within a few years it could almost disappear.

In its place, car manufacturers are using a range of advanced grades of steel including AHSS, developed to offer specific combinations of qualities for particular parts of the car. For example, the greatly improved safety of modern cars means that components like roll over protection structures are now routinely incorporated into a BIW. This safety-critical element must be very strong to resist crash loads and protect occupants.

Conversely, body panels like wings and door skins need to be stiff in order to resist denting, while the front and rear impact protection areas – popularly called the crumple zones – are made of steel that will deform predictably to absorb energy. A typical modern car will employ around half a dozen different grades of steel in different locations around the body.

Mass Reduction of Vehicles Using Advanced High Strength Steels

North American Truck of the Year 2014 - the Chevrolet Silverado

Engineers comb the new VW Passat AHSS body structure at the 2014 Aachen Body Engineering Days

Light is Right

One of the biggest driving forces in car development these days is the desire to reduce vehicle weight. A lighter car will use less fuel, will accelerate and brake more efficiently, and probably cost less to manufacture.

Manufacturers have worked for decades to reduce weight, and recently have been using AHSS to achieve some quite startling improvements.

Traditional American cars are sometimes thought of as big gas-guzzling beasts, but this is changing. In 2014 the North American Truck of the Year was named as the Chevrolet Silverado, a pick-up of conventional (large) proportions. But this new model is 90kg lighter than its predecessor, thanks to the application of Advanced High Strength Steel.

Even more impressive is the similar weight reduction made by Volkswagen on its 2015 model Passat. This popular five-seater is the epitome of the German manufacturer's approach to efficient manufacturing, having evolved over eight models to be one of its biggest sellers. It's quite astonishing that VW was able to shave 85kg off the weight of the new model through the use of AHSS. That's the equivalent of a typical adult passenger.

The possibilities extend even into the micro car category. The tiny Honda Fit uses advanced steels for more than a quarter of its body shell, saving 20kg of mass and further boosting its already impressive fuel economy.

The Future's Light

No discussion of car manufacturing is complete without acknowledging that the vehicles are likely to change more radically in the years ahead than they ever have in the past. In particular, the internal combustion engine is likely to be superseded by new forms of propulsion, most probably electric motors powered by batteries or fuel cells.

It might be thought that these futuristic types of vehicles would be made of some different material, but in fact it seems likely that steel will continue to dominate – and for very good environmental and economic reasons. The two main alternatives are advanced composite materials such as carbon- or glass-fibre reinforced plastics, and aluminum.

Fibre-reinforced plastics offer excellent strength-to-weight properties, but remain expensive to manufacture. Their longevity in service is not as good as metal, and their recyclability is poor. It is interesting to note that the energy cost (and therefore the carbon footprint) for the production of a typical vehicle component in fibre-reinforced plastic is about four times that of the equivalent steel vehicle.

Aluminum also offers good strength and somewhat lower weight. Its main disadvantage is its high manufacturing cost and heavy energy requirement, giving it a carbon footprint roughly 10% higher than that of plastics and about six times more than steel.

This becomes increasingly significant as cars become lighter and use new low-carbon power sources. In a conventional car, manufacturing accounts for 15-20% of lifetime emissions. In a zero-emission vehicle, that becomes more like 80%, depending on the energy source for the powertrain. In other words, the ecological advantages of steel are actually increased when coupled with low-carbon power systems.