Many new high-tech products fit that bill, but their price tags are often too high for mainstream vehicles. To help solve the problem, automakers turn to high-strength steel.
It’s been used in cars since the 1970s, when a fuel crisis prompted automakers to start shedding weight in their vehicles, and gained favour in the 1980s as better high-strength steel was developed.
“Metallurgy is like being a chef”
Faced with today’s stricter fuel, emissions, and crash standards, companies are using even more of it, and steel suppliers are working to produce even stronger versions.
“Basic steel got stronger by adding certain alloys, making it perhaps 50 per cent stronger than it was before,” says Ron Krupitzer, vice-president of automotive applications at the Steel Market Development Institute in Michigan. “It was useful for applications like bumpers, or some of the structural parts in vehicles.
“Until we moved forward in the last 15 years, that had been fine, but now there’s such a growing emphasis on weight reduction that we’ve had to introduce even higher-strength and better-forming grades, so we can make as many parts as possible out of higher-strength steel than before.”
These new grades are called Advanced High-Strength Steel, or AHSS. Steel’s tensile strength is measured in megapascals (MPa), with normal mild steel generally coming in at around 300 MPa. Krupitzer says that high-strength steel commonly ranges from 590 to 780 MPa, while the new AHSS can go as high as 1,800 MPa.
These highest grades would be primarily used for parts that have to stand up to crash impacts, such as bumpers and door beams.
High-strength steel bumper of the 2015 Ram truck
The Right Mix
Three factors go into steel strength, the first one being the ingredients. Adding manganese, silicon, boron, or other alloys will strengthen the final product. “Metallurgy is like being a chef,” Krupitzer says. “You have many ingredients, and it comes out with different properties.”
Second is the way the steel is processed. It used to be common to mix individual batches of molten steel and cast them into chunks, but this can produce variation in the finished product. High-strength steel is processed in a continuous flow, so that every bit of the slab goes through the same computer-controlled heating and cooling treatment—“thermal history,” as it’s known. This creates a metallurgy microstructure inside the steel that gives it its strength.
Finally, the steel is further strengthened by “working hardening.” Stamping the steel into its final shape, and even the heat from baking a coat of paint on top, alters its structure and makes it even stronger.
AHSS is more expensive than conventional steel. But because it’s so much stronger, a part can be made with much less of it and still be as strong as a thicker, heavier one made of conventional steel.
“In many cases, you can get the weight out for no extra cost,” Krupitzer says. “The steel costs more per tonne, but you use fewer tonnes, maybe 20 to 35 per cent, so you buy much less. That’s a big difference that would cover a lot of the cost premium.”
Why not More?
So if it’s so much stronger, and you use much less, why not make the whole car out of it? It comes down to analyzing each component and determining the best product for each one. If brute strength isn’t required, then light but less-expensive materials are used. “An aluminum hood will almost always be lighter than a steel hood, and there’s no load or strength requirement for it,” Krupitzer says. “High-strength steel doesn’t give you as big an advantage where there’s little structure.”
A potential future vehicle made with high strength steel
High-strength steel is the go-to product for collision protection, where it’s used to absorb or deflect crash energy without adding the bulk of conventional steel. That said, it isn’t a case of one-steel-fits-all. A car needs to crumple progressively, which helps to absorb and dissipate the crash energy that can kill you if it’s transferred through the cabin and into your body. Automakers use different steel strengths in different components, putting the crush-and-crumple products toward the outside of the car, and the strongest stuff around the cabin to reduce intrusion.
One of the newer products is TRIP steel, an acronym for Transformation-Induced Plasticity. This steel crushes to absorb energy, but is extremely strong and durable. One of its most important characteristics for the automotive industry is that it can be easily worked, making it an excellent choice for complicated components that can be difficult to form from conventional high-strength steels.
While aluminum has to be separated by grade for the most efficient and value-added recycling, AHSS can simply be tossed into the melting pot with other scrap steel to form new steel. The alloys in some experimental AHSS formulations could potentially require sorting in the future, but for now, the alloy concentrations in high-strength steel are low enough that they don’t present any issues.
New Learning Curve
AHSS does present a learning curve for body shops during repairs, since many grades can’t be heated and straightened, which would alter their strength. Instead, damaged components have to be replaced with new ones, using special methods for cutting and welding them.
The steel producers also have to work closely with automakers, since new steel grades and formulations have to be tested for such things as corrosion resistance, paint adhesion, surface quality, and the way it needs to be stamped or welded. “We can’t just make a new steel and ship it to the car companies,” Krupitzer says. “We’ve been moving up the ladder with tensile strength, and each level has to go through the various tests the car companies do before we get the stamp of approval. You can satisfy the weight reduction and keep the car steel-intensive. Car companies are desperate to put in light weight to double their fuel economy.”