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Stamped out

Hot stamping ArcelorMittal’s Usibor steel for lightweighting in the automotive industry

Automakers continually search for processes and materials that will allow them to make lighter, more cost-effective vehicles. However, with 30,000-plus automobile crash fatalities per year in the United States alone, there is also a pressing goal to make cars safer.

Steel has been the material of choice since the beginning of the automobile industry.

Car and truck companies have used it in various forms to achieve safety and strength standards while working to comply with ever-changing regulations where fuel efficiency is concerned. However, it’s these regulations that are helping to push innovation in material and processing, particularly by the two largest steel producers in the auto industry today, ArcelorMittal and U.S. Steel.

Since the 1970s, federal fuel efficiency standards have pushed automakers to innovate. The Obama administration tightened regulations in 2011, pushing fuel economy standards to 54.5 miles per gallon by 2025 for cars and light-duty trucks. The current administration has its eyes on rolling that back, but regardless, there are cost efficiencies to be gaied by improving the materials and processes used in making cars, so it’s likely that innovative research will continue.

Hot stamped

Despite being developed four decades ago and slow to catch on, hot stamping, a process where steel sheets are pressed into shape after being heated, is increasingly being adopted by more car manufacturers.

Ludovic Dormegny, PHS product development manager at ArcelorMittal, explained in a recent webinar on lightweighting that hot stamping was introduced to the auto industry in 1977 and was first used at Saab. Since then, the hot stamping market has continuously grown and really started taking off in 2011 due to a booming demand in China.

The ArcelorMittal hot stamping process involves heating a sheet of steel up to 900 degrees C, then transferring it to a die where the part, usually a body structure, is stamped into shape. By heating the steel to such an extreme degree, the crystal structures change from ferrite to austenite in a process called austenization, which hardens the metal. Hot stamping offers greater strength for lighter-weight parts.

Specifically, hot stamped parts are created for the BIW phase of manufacturing. BIW, or “body in white,” refers to the frame or skeleton of the car before any components have been added or paint applied. Oddly, the “white” portion of that indicates the primer used on the skeleton, yet the most common primer color used is gray.

Automakers are attracted to this process because it allows for more complex geometries and offers good geometric accuracy. Hot stamping also offers high weight-saving potential and utilizes low press forces to form the parts.

“It started in Europe and North America,” Dormegny says. “It’s reaching a similar explosion in the Chinese market. We expect robust demand in other regions.”

Special steel

Dormegny says in 2008, Usibor 1500 AS, which is a boron alloy steel with an aluminum silicon coating, was used in around one percent of BIW production. Usibor 1500 AS penetration in BIW is projected to reach 10 percent in 2020 and more than 15 percent in 2025.

Aside from greater strength, the other benefits of using Usibor are that there is no sandblasting required, no scales or immediate steps between heating and stamping, and no decarburization.

Usibor is also a preferred steel material because it has superior resistance to perforating corrosion, is more flexible than alternative materials and requires a lower investment.

Perhaps the best reason for using Usibor and the hot stamping method is that the parts are lighter and stronger than alternative steel materials and processes.

According to ArcelorMittal, Usibor and another hardened steel product called Ductibor allow manufacturers to achieve excellent compromises between strength and ductility, making substantial weight reduction possible.

“High mechanical properties of press hardened steel grades can be integrated into multi-material parts to reduce the need for reinforcements,” Dormegny says.

The current uses for these hardened steels include safety and structural components:

  • Front and rear bumper beams
  • Door reinforcements
  • Windscreen upright reinforcements
  • B-pillar reinforcements (reinforcements found in the middle of a vehicle)
  • Floor and roof reinforcements
  • Roof and dash panel cross members

Dormegny says the automobile market is becoming more global and that more automakers are looking to increase performance in terms of lightweighting.

“We are confident that new solutions will be easier to introduce,” he says, “new solutions that are more efficient.”

The adoption of Usibor and Ductibor is expected to grow from 1 million tons in 2013 to 3 million tons in 2018. Getting to this point has been a rather slow ramp up, from zero tons in 2006 to 500 tons in 2011, mainly in Europe and the United States. However, globalization and booming demand in China have helped to really push the materials and the process to where it is today – 300 hot stamping lines in total.

“We do not expect any slowing down,” Dormegny claims.

Dormegny says that by 2025, more than 15 percent of BIW is expected to be comprised of Usibor 1500 AS, due in part because it is a good material for anti-intrusion parts.

Other advantages the advanced high-strength steel offers include:

  • No cold forming step required
  • Most parts can be hot stamped in a single press stroke
  • No surface cleaning
  • No intermediate step between heating and stamping
  • No decarburization
  • No scale
  • No sandblasting

Dormegny says that today and in the coming years, more than 80 percent of the BIW hot stamped parts will be made of an AS coated Usibor or Ductibor, due in part because it offers an excellent compromise between anti-intrusion performance and corrosion.

Another advantage is ductility. Ductility is a measure of a metal’s ability to withstand deformation with failing. In metallurgical terms, think of it as playing tug of war with a piece of steel with each team pulling at the ends of the material.Generally, press hardened steels are able to reach higher strength levels but with limited ductility, Ductibor products are press hardened steels with optimized ductility.

Where Usibor is great for anti-intrusion, Ductibor is equally as outstanding for energy absorption. When it deforms, it does so in a “very stable way,” according to ArcelorMittal. Most major car manufacturers have adopted both products and multiple applications have gone into serial production.

With Ductibor and Usibor being used to make B pillars, the company claims automakers can achieve a cost savings of 11 percent with a weight reduction of 14 percent. The rear rail part sees a cost savings of 20 percent and a weight savings of 32 percent.

“We don’t know what the cars will look like in the future,” Dormegny says, “but we hope they will be made with Usibor and Ductibor.”

ArcelorMittal