Specialist high-strength steels are helping engineers and designers save weight while meeting stringent performance and safety targets

When you talk to carmakers around the world the theme of high-strength steels as the preferred material for carmakers in terms of performance, safety, environmental impact and cost comes up time and again.

Manufacturers moving into high-strength steels come from all corners of the globe and include Fiat and General Motors among their newest converts. Using a mix of highstrength steels, Fiat has already achieved reduced vehicle weight, increased vehicle performance and crash resistance, without impacting on costs. Like all vehicle manufacturers, the company is focusing on weight reduction to help meet its emission reduction target of 140 grams per kilometre. It is currently number two of the top five EU companies with the best performance on CO2 emissions.

GM is also making extensive use of high-strength steels. By 2012, 88 per cent of its vehicle body structures will be composed of a mix of high-strength, advanced highstrength and ultra-high-strength steels.  One example of how multiple grades of high-strength steel can be used to meet design and performance targets is Honda’s 2007 Acura MDX. Over 53 per cent of the steel used is high strength – more than four times the amount used in its predecessor. Helping to reduce vehicle weight while increasing rigidity and crashworthiness, the steel is specified in different grades for use in different parts. It is not, however, all good news for high-strength steels, with some vehicle and equipment manufacturers expressing concern over the formability of some high-strength steels, calling for new forming capabilities to enable them to create more exciting vehicle designs.

A future for steel
For many years the smart money has been on aluminium and plastics as the material of the future, a situation that has yet to transpire. “Steel has undergone a huge evolution in the past few years and the same has happened to steel processing technologies,” Rosanna Serra, engineer of materials at Fiat, says. “Just think, today the performances of forged steels are eight times higher than they were ten years ago. These performances would have been unthinkable with the previous steel grades and, at this point, aluminium is simply not competitive any longer.”

The evolution of high-strength varieties and grades that were once seen as revolutionary are now commonplace. Looking to the future, Serra can see a day, by the middle of the next decade, when over three quarters of the car will be made from high-strength materials. “From what we can see today, steels are continually gaining ground, and if we manage to use steels with higher yield strength in the body panels as well, we could reach 75-80 per cent. “Within the present 67 per cent, the use will concentrate more on the so-called advanced-high-strength steels (extraand ultra-high-strength steels). Imagine that the DP600, which was regarded as a top product a few years ago, has now become a regular commodity.

“Today, the material weight/production cost ration is no longer a linear process. Rather, weight seems to have acquired a higher value than in the past, since it is a direction function of CO2 emissions.” In these days of high environmental awareness, steel, once considered as a liability, has achieved certain green credentials. “The environment is a crucial issue today and it will be the same in the future,” Serra adds. “By using highstrength steels, the body weight of the Grande Punto has been cut by 32kg. If we consider the whole product lifespan, this corresponds to a fuel saving of 150 litres and up to 400kg less CO2 emissions. The Grande Punto car body is made of extra and ultra-high-strength steel.”

Steel innovations
With their partner SSAB Swedish Steel, Fiat is pushing ahead with innovative research into new grades of steel – a delicate balance between cost and performance. Today, the limit is represented by roll-forming and deep-drawing multi-phase steels up to 1000Mpa as well as by hotforging Boron steels up to 1400Mpa. Every steel maker is preparing their own winning product: some are thinking about austenite instead of ferrite, others are concentrating on lowering the steel density while keeping the same mechanical properties; some want to increase the Young’s modulus while others are already producing cold-rolling DP trial lots up to 1400Mpa.

Finally, some steelmakers are focussing their attention on presently used products (i.e. DP600-DP800-DP1000) and are trying to improve their cold-forming properties. “Steelmakers know very well how to solve the technical problems encountered in the development of these products, and costs will not depend so much on raw materials and production cycles as on how many tonnes of scrap will be produced to obtain a tonne of useable product,” Serra says. “Recyclability has become a key issue in the choice of the material, together with other performance properties such as cost, strength and weight. Steels are obviously among the most recyclable materials because of the existence of a self-financing scrap recycling industry.”

Flat carbon steel
According to ArcelorMittal’s Thierry Renaudin, there are four prime drivers from automotive OEMs when they are purchasing flat carbon steels – the traditional requirements for strength and stiffness, allied with increased corrosion resistance and enhanced surface finish.

“As far as flat carbon steel goes, we have a very wide offer with the following main focuses. One of the things, of course, is to provide the customer with a high level of strength as well as very good formability in order to save weight on very complex shape. Another area of focus is to provide a product that increases stiffness, always with the view to save weight on the panel. The main way that we can increase the stiffness is with the use of sandwich materials.  “Another area of focus is to provide steel with overall corrosion resistance in order for customers to either improve the cover guarantee that they can provide for their own customer, or to decrease the cost of the prevention methods to improve the corrosion resistance of the structure.  “And finally there is the development of products to improve the surface aspect of the panels to provide the customer with better looking cars, or to decrease the cost of putting so much treatment on the surface of the structure, to provide good looking cars.”

Like most steel makers, ArcelorMittal is still focussing its efforts on DP, Trip and Multiphase steel and trying to improve the compromise between the strength resistance and formability of the steel. “So far we have a pretty good coverage of customer needs at the level of strength of 800Mpa and we are moving towards higher textile strength and with special textile tricks to even 1000 or even 1200,” Renaudin continues.

Quiet Steel
One of the more interesting products that the company is developing is Quiet Steel: a suite of engineered multi-layer composites with various engineered viscoelastic cores sandwiched between two layers of steel. This viscoelastic core is a ‘tunable’ formulation, allowing vehicle designers to attenuate specific frequencies depending on where in the vehicle they use the steel. The steel laminate may be further engineered with corrosion resistance or other value added processing. When this initial processing is complete, customers may form, stamp, and weld as they would any standard steel.

Of course, ArcelorMittal cannot work in isolation of its major customers so close collaboration is the norm as they continue to push the technological boundaries. “When we have a product we work very early in the process with our customers asking them to what extent this product matches their needs,” Renaudin explains. “On top of that we are developing our own set of tools in order to help the customers implement our product into their processes. You know that when we want to implement a higher grade of steel into stamping processes we very frequently have to cope with the spring back issue during the stamping process. So we are developing our own set of tools to minimise, predict and compensate the spring back that occurs when stamping very high-strength steels.”

Smooth high strength
Another project that the company is involved with is the development panels that are resistant to the dents and scrapes of everyday motoring life. Renaudin says: “We try to develop products with enhanced stiffness, because even if we can use higher-strength steels in these sorts of panels, we are always limited by decreasing the thickness and the stiffness requirement. So we have been developing specific products, especially sandwich products. The last product we developed was a smooth steel made out of one sheet of very high-strength steel, usually a DP500. It is 0.5mm thick and includes a polymer glued to it or around 0.3 or 0.4mm. So we end up with a panel of 0.9mm thickness which is very light.

“When we talk about plain panels, corrosion is not an issue anymore,” continues Renaudin. “However, when we talk about welded or cut edges there is still a need for protection in the vehicle. So usually this protection is achieved by implementing what is in the sealant in special areas of the vehicle. We are trying to develop a product to improve the corrosion resistance of the steel even in these difficult areas. We are in the process of developing a new family of products with a zinc magnesium coating obtained by a vacuum deposition and a further covering of 1.5 micrometre of magnesium on top of 7.5micrometres of zinc. We apply this treatment on the zinc coating itself, and the result is a sub-layer mixing of zinc and magnesium of 3.5 micrometres which provides very good corrosion performance.” Much like his colleague at Fiat, Renaudin admits that weight reduction is still the biggest demand from their customers and that demand is growing. “Obviously the recent talks between the European Commission and European carmakers have put this issue on the top list of the agenda,” he confirms. “Environmental issues are present more than ever, so we feel that customers really want more weight saving on their vehicles. However, they want weight saving at a costefficient price. “We feel that steel has very good role to play in that game, because with all the steel that we are developing we feel that we can really help the OEMs to reach their targets in terms of weight saving.”

What about actual cars themselves? This year’s Frankfurt Show featured an array of innovative applications of steel on cars, such as Audi and BMW, as well as major Tier One supplier Johnson Controls. A joint development team from Johnson Controls and ThyssenKrupp Steel has come up with a revolutionary new concept for a cockpit structure. The new solution is not only more than 20 per cent lighter than a benchmark production cockpit, it also costs less to produce. At the same time it takes up less space and thus opens up new design possibilities for the front passenger side. The benchmark used by the project team was a very well designed cockpit from a highvolume, lower mid-class segment car.

‘Instrument panel upper’ is the technical term for the plastic support structure that accommodates items such as instruments, air conditioning system, radio, glove compartment and airbags. The structure is supported and stabilised by a cross-car beam. This beam – usually steel, sometimes aluminium or magnesium – connects the two A-pillars and thus runs the full width of the vehicle. The design of the new cockpit structure was based on the realisation that the strength of a metal beam is mainly needed in the steering column area. For this reason, the new solution features a steel beam that only extends to the middle of the car.

Cross-car beam
The team took an integrated approach focused not only on optimising individual system components but also on improving interfaces, such as that between the cross-car beam and the steering column, or between the cross-car beam and the plastic module. The cross-car beam is made from a T3 profile from ThyssenKrupp Steel. It is fastened to the driverside A-pillar and to a vertical bar bolted to the floor panel and the cowl.

T3 profiles are flangeless tubes with a cross section which can vary over the length of the component, changing for example from cylindrical to conical to rectangular. They can also integrate secondary design elements such as recesses or projections. These function- and stress-optimised near-net-shape profiles are produced on a unique line at Thyssen Krupp, which curls flat blanks and then laser welds them into tubes. This profiling process is seen as an economic alternative to hydroforming, which is the method commonly used to give tubular parts their final shape. The technology also demonstrates its strengths in the new cockpit structure: at the connecting points to the A-pillar the cross-car beam is round with a relatively small diameter for welding, but it quickly widens so that its diameter and thus stiffness is greatest at the point where the steering column is attached. In this area, the profile has a rectangular cross section so that the steering column bracket can fasten snugly to the flat lower face. The advantage of the T3 profile is that, instead of the seven parts used by the benchmark structure to attach the steering column, now just one is needed. That alone represents a weight saving of 1.8kg.

Steel solutions
As the plastic support structure has to span half the vehicle’s width without the support of a metal crosscar beam, the project team developed a new hybrid connection which – like the solution for attaching the steering column – has been registered for patent. To provide a firm bond between the steel and plastic, perforated metal plates are moulded into the plastic parts. During assembly, these plates are laser-welded to the cross-car beam. Air ducts, airbag connection and the rear wall of the glove compartment are integrated directly into the plastic panel.

So, with so many new products on the market, and research and development in both materials and processes marching on apace, what are the automotive companies searching for in their research efforts? “What I really wish for is a steel that is able to provide solutions both for structural engineers wanting top performances in terms of stiffness, crash and fatigue resistance, and for designers wanting to be free to choose the shapes. Together this will give rise to steel sheeting which can create a car body offering the most aesthetic appearance of the car,” Serra concludes.

Manganese-boron steels for tailored blanks

ThyssenKrupp are the first hot-stamped tailored blanks to be used in a production car. The laser-welded blanks are being used by Audi in the platform of the A5 Coupe and the new A4.

Car manufacturers are using hot stamping more and more frequently whenever high demands on crash performance have to be reconciled with weight reduction efforts. The process uses special manganese-boron steels, which reach a strength of roughly 1,500Mpa as a result of hot stamping. This is significantly more than offered by the highest-strength steels used for conventional cold stamping. Hot-stamped parts can therefore be made with thinner walls, which reduce the weight of the body without detracting from passenger safety. In hot stamping, sheets of manganese-boron steel are first heated in a furnace to temperatures of 880-950ºC, and then formed into parts in a special stamping press while at the same time undergoing very rapid cooling at a rate of more than 30 kelvins per second.

The high temperature of the blank during stamping guarantees excellent formability, while the subsequent rapid cooling together with the alloying elements manganese and boron produces an extremely hard microstructure in the material. In addition, the hot stamped parts are virtually free of residual stresses and therefore extremely accurate. To prevent scaling in the furnace, hot stamping steels are protected either by a silicon-silicon coating or other coating systems such as x-tec. Audi uses steel with a silicon-silicon coating for the A4 and A5.

For all its advantages hot stamping still has limitations when it comes, not just to withstanding impacts, but also partially absorbing the energy occurring in a crash. To meet this kind of demand components need a precisely defi ned amount of residual elongation for energy absorption. The residual elongation of manganese boron steel hot stamped parts is around fi ve per cent and thus in some cases too low. Audi solved this problem by using tailored blanks from Duisburg consisting of both manganese boron steel and a conventional microalloyed deep drawing steel.