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High-strength steel: Strong enough for car crashes, light enough for fuel efficiency

Automakers face two major challenges when they build vehicles. Cars must be strong enough to protect occupants in a crash, but they must also be as lightweight as possible to meet stringent fuel-economy standards.

One product that addresses both issues is high-strength steel, which itself is constantly being improved for better performance.

“There are different types and grades, with unique microstructural characteristics that engineers build into the product,” says Ron Watkins, president of the Canadian Steel Producers Association. “It’s a tremendous amount of work, but the auto industry is always looking for fuel efficiency.”

Steel becomes stronger through three factors: alloys such as manganese, boron or silicon that are added to the molten steel mix; continuous slab production to ensure each section receives the same heating and cooling treatment; and “work hardening,” when stamping or forming parts, which alters steel’s structure.

By adjusting the ingredients used, or the way the steel is processed, steelmakers can produce various strengths, including the strongest advanced high-strength steel (AHSS). Auto manufacturers may use several grades in a vehicle, depending on each component’s function and where it’s located, for optimum performance.

In a crash, cars need to crumple progressively, gradually absorbing the crash energy so it isn’t transmitted to the occupants. Different steel grades can be used to create this gradual crunch, with the highest-strength steel used in the passenger compartment, which has to remain intact and uncrushed.

AHSS is more expensive than conventional steel, but because it’s so much stronger, automakers use less of it overall. In some cases, a component made of AHSS could be as strong as one made of regular steel, but contain 20 to 35 per cent less material, which helps to offset the high-strength steel’s extra cost. The major benefit is that the car now weighs less, which improves fuel efficiency.

So why not make the whole car out of it? It comes down to selecting the most appropriate product for each component, how strong it needs to be, and the price. A hood doesn’t have a heavy load requirement, for example, so an automaker is more likely to make a lightweight version out of lower-cost aluminum than higher-priced AHSS.

All about steel

  • Automakers first started using high-strength steel in the 1970s, when a gas crisis made fuel efficiency a priority with buyers.

  • Steel strength is measured in megapascals (MPa). Regular steel is around 300 MPa, while high-strength steel usually ranges from 590 to 780. Some advanced steels can go as high as 1,800 MPa.

  •  Steel’s thickness used to be measured in gauge, with higher numbers indicating thinner panels, but it’s now more common to measure it in millimetres.

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