Next time you are speeding down the motorway spare a thought for the metal box around you - scientists have spent the last decade refining, strengthening and improving it. In a bid to increase passenger safety, fuel economy, and vehicle performance, researchers across Europe have developed a new grade of steel, meeting the ultimate vehicle challenge of low weight and high strength.
© Voestalpine AG, 2012
Ideally a vehicle will be made out a lightweight, strong and ductile material, which is why steel has been the material of choice for car bodies for almost a century. A ductile material can be deformed into shapes without cracking, which is important not only when manufacturing vehicles to desired shapes but also in crash situations, when the material should take the brunt of the force.
In conventional high strength steels, ductility decreases with increasing strength. So, in response to this problem, the steel industry has rapidly developed today's Advanced High Strength Steels (AHSS). Steel is graded for classification depending on its mechanical properties, purpose of use and chemical composition. The grade of steel distinguishes between one used for railway tracks or pipes as well as varying temperature and impact resistances.
After three years of study, funded by European Commission and started in 2004 , scientists at the Steel Division of Austrian company Voestalpine Group have discovered that the most important of the AHSS for crash performance are Dual-Phase (DP) grades. They have managed to combine excellent formability with a remarkable reduction of the vehicle's weight. Advantages that the automotive industry is keen to adopt.
"It's used in the market in a really high amount now," says Dr Andreas Pichler, the project representative. "I would estimate that this kind of steel grade is between ten and twenty percent of the ratio of the body-in-white of the car."
The body-in-white refers to the welded sheet metal of the car before moving parts such as doors, glass and upholstery are added. The form and material of the body-in-white is responsible for the safety of the passengers in a crash, which is why extensive tests are performed to gauge how much force the vehicle can take instead of the passengers.
The team tested different microstructures of DP grade steel, processed industrially and in the lab. The main focus was yield and tensile strength; the amount of force a material can withstand before deforming beyond repair. They also investigated the bendability of the steel and how the material coped when punctured, with an emphasis on the damage to the microstructure during strain, which is often invisible to the naked eye. This microscopic harm could manifest itself at a later date, with dangerous consequences.
In manufacturing vehicles, molten steel needs to be forced into shape under extremely high pressures and parts welded into place. All this work causes stress on the metal. "The problem during processing was reduced. Based on the improved formability, they can use the material for more complex shapes in the car," says Pichler. Greater part complexity leads to fewer parts and ease of manufacturing. Fewer parts mean less welding, saving time and money, as well as fewer weld flanges to hold separate metal parts together, saving weight. Added to the better crash performance with a similar or reduced thickness of steel, the advantages of DP grades are clear.
It takes considerable energy to build a car, and even more to run it. In 2011, 13 million new cars were registered in the EU. Even if a fraction of these vehicles adopted the lighter, stronger DP grade steel not only would millions of drivers be safer behind the wheel but they would consume far less fuel and emit less carbon dioxide.
The next step for ever improved steel is to put new varieties of the metal through its paces, combining the experience of the industry with the innovation of science. With a thorough, systematic understanding of how metal moves and interacts in the processing and crash stages, it is possible to pass on the performance, weight, and cost benefits that these steel grades offer to the consumer.