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Press-hardened Steel – strong, tough and safe

10/04/2019

Auto bodies are designed to prevent injuries and save lives when an accident occurs. To protect passengers, while keeping weight to a minimum, requires materials of construction that have both high strength and toughness. Press-hardened steel (PHS) provides these properties and has therefore become the backbone of today's auto body crash structure. As manufacturers push the limits of protection and lightweighting, they are interested in the benefits of molybdenum.


Photo credit: © Volvo Car Group

Automotive vehicle crash safety has dramatically improved over the last few decades, resulting in a substantial decline in accident-related casualties. All cars are rated on standardized crash-testing procedures, with the best performing receiving a 5-star rating, whereas legal standards usually require at least a 3-star rating. Auto makers are torn because designs that increase crash safety tend to add weight to the car body, conflicting with goals of improved fuel economy and reduced CO2 emissions. Stronger materials that add less weight to the structure allow designers to achieve both objectives.

The most difficult situation to design for is the side impact crash. The post between the front and rear door is in close proximity to an occupant's pelvis and shoulder. Therefore, the pillar needs to deform as little as possible without breaking. Instead, transferring the kinetic energy of the crash to areas of the car body that can safely deform and absorb that energy. Traditionally, critical components of a vehicle's side structure have been reinforced with one or two extra shells stamped from relatively thick medium strength steel sheet, but this takes up space required for airbags and sensors. Using stronger steel would eliminate the need for additional reinforcements, but such steels are difficult to form into the shapes required.

While a Swedish company, Hardtech, developed a 'press-hardening' process in the early 1980s, it wasn't until 2004 that Volkswagen made major improvements in process efficiency and designed press-hardened components for its Passat model. Volvo took the technology to an unprecedented level in 2014, with nearly 40% of the body structure of the XC90 being made from press-hardened steel.

For cars, impact resistance, represented by the property called 'toughness', is very important. It means that the steel can absorb more energy in a crash before failing. Unfortunately, many steels suffer a loss of toughness as strength increases. As car makers are now interested in increasing the tensile strength of press-hardening steel towards 2000 MPa, in order to further reduce weight, the interaction between strength and toughness has become increasingly important.

Projects supported by IMOA have clearly established that molybdenum additions to such steels can provide a good combination of strength and toughness. Molybdenum acts much like the 'glue' between the microstructural features within the steel, holding them together under extreme loads. Newly developed press-hardening steel grades are already making use of molybdenum in the alloy. The first commercially available 2000 MPa press-hardening steel (34MnB5) contains 0.15 to 0.2% molybdenum.

Experience has shown that there are still things to learn about molybdenum's beneficial effects on steel properties. But as demand for lighter and safer cars increases, the demand for molybdenum will most likely increase with it.

Read more in MolyReview

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