Advanced Materials for Lightweighting Cars: What’s New in Steel?
When talking about reducing our collective carbon footprint, the conversation quickly turns to transportation. Transportation is responsible for 28 percent of greenhouse gas emissions in this country, second only to electricity generation. With utilities switching from coal to cleaner sources, transportation could conceivably advance to the number one spot if the Obama administration hadn’t passed a new set of corporate average fuel economy (CAFE) standards requiring each manufacturer’s fleet to average 54.5 mpg by the year 2025.
Despite advances in powertrain technology, including such things as hybrids, plug-ins, all-electrics, and fuel cells, carmakers determined that they could not achieve this goal without taking the additional step of significantly reducing vehicle weight. This requires taking a fresh look at the way that cars are constructed, with an eye toward models that are lighter, but no less safe, or comfortable, or affordable than today’s cars.
Over the next several weeks, we will examine the use of various materials used in the construction of automobiles, including steel, aluminum, polymers, and magnesium, to try and gain an understanding of their relative merits in reaching this goal.
We begin the series with steel, which has been the material of choice since the earliest days of the industry, primarily due to its strength, formability, and relatively low cost. Some have suggested that we must move away from steel to achieve the new weight reduction targets, though two things are clear at the outset.
First, moving forward, there will likely be a mix of materials that are optimal for each of the many components and applications found within an automobile. Second, steel is certainly not going away without a fight, and as I think you will see, there is ample reason to believe that there will still be plenty of steel in the cars of 2025.
Blake Zuidema, director of global R&D for automotive product applications for ArcelorMittal, the world’s largest steel and mining company, told me two game-changing facts about steel. First, recent advances in steel-making have resulted in high-strength steels that are significantly stronger than their predecessors.
“Ten years ago structural steel had a yield strength of 270-350 MPa,” said Zuidema, whose company is a leader in automotive sheet steel technology. ”Today’s advanced high-strength steels (AHSS) can go as high as 1,500 to 1,700 MPa. These new steels have evolved using a combination of new formulations and alloys, as well as different processing and treatment techniques. The newest steels are not only stronger, but they also tend to be more formable. This gives steel producers a rich and diverse portfolio of material options that can be custom-tailored to meet the requirements of each application.”
That last part about custom-tailoring turns out to be the key to the second fact. Steel producers like ArcelorMittal and others have become increasingly involved in the early stages of the design process in an extended enterprise-type model, wherein the suppliers are brought in as soon as the requirements are known, rather than waiting for the design team to develop a specification. Instead of simply providing material as they once did, steel companies are now providing solutions.
“The key to lightweighting is not just substituting a lighter material or a stronger material,” said Zuidema. ”Those things only get you so far. You must do that in a holistic design optimization program to get maximum structural efficiency out of the vehicle. Keep in mind that most vehicle’s current structures have been based on the grades of steel, and the strengths of steel that were available 10, 20, and in some cases even 30 years ago.”
So it’s really a whole new ballgame. And it involves new teammates, in the sense that the OEMs are now working more closely with steel suppliers than they had in the past. It also changes the way the game is played. “In years passed, we would have started product development in a pilot steel-making laboratory, mixing together different alloys and seeing what kind of properties we would get and then looking for applications,” said Zuidema. ”Now we start with the applications. We determine using our CAE tools what properties are needed and feed that into our product development process.”
In essence, they work together with their OEM partners to redesign the structural load paths to take advantage of these higher strength steels, resulting in lighter designs.
“We’ve seen automakers, in the last year or so, come up with new vehicle body designs that are 15 to 18 percent lighter than the designs that came before them,” he said. “These were all achieved with steels that are available today and are in commercial production, though they might not have been available a few years back.”
This level of weight reduction, when used in conjunction with high-efficiency powertrains, improved aerodynamics, and other advances, is sufficient to achieve the new mileage goals, as verified by the computer models used by Environmental Protection Agency (EPA) and National Highway Traffic Safety Administration (NHSTA). That is without taking full advantage of all the weight-reduction opportunities ArcelorMittal has identified.
Steel has been used to lighten body structure, closures, bumpers, and engine cradles. There are also opportunities for using steel in suspensions. Advanced bar steels can be used for making lighter springs and stamped high-strength steel suspension control arms, which, in many cases, match the weight of an aluminum control arm. Improvements in axles and drive shafts, using both advanced steels and multi-walled tubes, provide substantial weight reductions. Manufacturers also have done a great deal of work on doors so that they can now match the weight of an aluminum door.
Zuidema suggested that the main driving force was not so much technological but rather a change in process that allowed them to focus on requirements. Many of the improvements in steel had evolved over several decades and are only now being exploited. Some examples include dual phase steels, with its excellent energy absorbing and fatigue strengthening properties, which came about as the result of changes in both heat treatment and alloy formulation. This makes them well suited for structural and safety components. Transformation induced plasticity (TRIP) steels bring these same properties to complex parts due to their high formability.
High-strength martensitic grades, which were originally used for bumpers and side-impact door beams, are now used throughout the body structure to improve lightweighting.
Stiffness is maintained despite smaller cross-sections, primarily by paying attention to connections, redesigning joints, using novel manufacturing techniques like tailor-welded blanks and hydro-forming, and in some cases using high modulus adhesives.
“There is another benefit we haven’t talked about yet,” Zuidema said. “That’s greenness. Pound for pound, it requires far less energy and far less CO2 to produce steel than aluminum, magnesium or carbon fiber. In fact, each pound of aluminum requires about five times as much CO2, each pound of carbon fiber requires about 10 times as much CO2, and each pound of magnesium requires about 15 times as much CO2.
“So even if these materials could produce a lighter car, with lower carbon emission during the use phase of their lifecycle, this would not offset the additional carbon emissions generated during the manufacturing process, starting from raw materials.”
This is a very important consideration of which most people are unaware. Indeed, the public needs to become better informed about lifecycle impacts.
Otherwise we could shoot ourselves in the foot by mandating lower tailpipe emissions and ending up with solutions that have higher lifecycle emissions.