Keeping it steel

Can steel really compete with aluminium and composite materials when it comes to lightweighting? Justin Cunningham talks to a company that says it can.

With the need to reduce CO2 emissions becoming ever more prevalent, one of the main drivers now is to reduce the weight of structures. From a composite fuselage to an aluminium chassis, many industries are turning their back on steel as the material of choice.

Steel has a long legacy in engineering and has long been the material of choice. It can be easily shaped and joined to produce strong and versatile structures. But, more recently, steel has become viewed as a bit old fashioned, a heavy and dirty material that has less place in 21st Century engineering.

For example, the demand on engineers to provide weight saving on vehicle structures has seen many naturally turn to aluminium. The difficulties in welding and manufacturing aluminium, its greater CO2 footprint during production and disposal and the problems of repair have lbeen seen as hurdles that need to be overcome and not insurmountable. These are tradeoffs that many OEMs are considering taking on the chin, to reduce the all important CO2 per km of its fleet, which ultimately is the main design driver.
"There is a great misconception about carbon footprint," says Greg Ludkovsky, Vice President of Global R&D at ArcelorMittal. "It is a story that still has not been properly highlighted. The amount of CO2 it takes to produce and process aluminium means it can take decades of driving to equalise the carbon footprint compared to steel.

"The idea that you buy aluminium from one country, pollute the air there so a car will produce slightly less CO2 in the country it is driven; the whole notion is totally ridiculous. It seems to be a bit of a desperate solution that is going to cost a lot of money. And intelligent application of steel can actually get you the same result."

To prove the point, ArcelorMittal, carried out a project called 'S-in motion' with the aim of creating lighter, safer and greener vehicles of the future using steel. The result saw the weight of the body-in-white and chassis reduced by 19%.

The company used a number of different steels including its high strength Usibor. Due to its strength, 1500MPa, it is difficult to cold form. However, the company has found that using a hot stamping process allows the production of complex shapes and can deliver strength precisely in the areas where it is needed.

One of its most remarkable breakthroughs was a one piece door ring made out of its hot stamped 1500MPa Usibor, but in the lower portion where it might have an impact and a higher level of ductility is needed for smooth energy absorption, the company put another of its hot stampable steels called Ductibor. This allowed strength and ductility to be accurately positioned in the areas that required it.

"To accomplish this, we developed a totally new way of laser welding the materials," says Ludkovsky. "In today's automotive community everyone is trying to adapt this approach because it is one move and takes out 20% of weight.

"It is very expensive to take weight out of a structure and costs a lot of money per kg that is removed if you substitute steel for another material. But, through our solution carmakers can get that major reduction in weight at a neutral cost. What we have established is that most of the time, and certainly when it comes to automotive structures, we can create solutions using steel that are at least as light as aluminium, and we do it at a significantly lower cost.

"Our goal is to show that steel is fully capable of competing with aluminium and deliver solutions that are significantly more cost effective and do not require major re tooling and do not take more out of the pockets of customers."

Author
Justin Cunningham

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