Making steel high strength means lightening the load for carmakers, and applications are extending to stamped and rolled components.

In modern vehicle manufacturing the demand for lightweight, high-strength steels, including the latest dual-phase materials, is increasing because of the overall blend of structural, crash performance and weight benefits they offer. The attraction of these steels is that they give vehicle engineers a good balance between strength and formability. However, they do behave differently to traditional steel grades and require significant expertise to implement successfully.

Before the metallurgists and chemists reach for their pens to write letters of complaint, it should be remembered that lightweight steel is something of a misnomer. As they would be quick to remind us, the weight of a given mass of steel will not change, but by making steel stronger it is possible to use less for a given component, making it lighter. So high-strength steels are essentially the same as lightweight steels, all it means is that it is stronger so you can use thinner material.

Structure and safety

“There has been pressure over the past 15 years to get vehicles lighter, but in general they are still getting heavier,” says Professor Jon King, Director of Corus Automotive. “The reasons are that there are more features, there is more structure, due to increasing safety legislation, and the actual vehicles tend to get bigger from generation to generation. The best example of that is the Golf platform: the current Polo is about the same size as the Golf Mk1, and the Golf Mk5 is about 60 per cent heavier for the three basic reasons.”

When you add that to the increasing environmental concerns, it is clear why the automakers are calling out for highly formable, high-strength steels. “There is a bit of conflict there,” King continues, “so we have got to try and keep the structural and safety performance, while trying to optimise and reduce weight in the structure.

It is fair to say that if materials had not made advances over the past 10-15 years then cars would be even heavier than they are today, so it’s that level of weighting that we have contributed to.”

The current trend is to use high-strength steel where you can, and probably about 25 per cent of the structure can make sensible use of high-strength steel; this in locations where strength is more important than stiffness. The main areas where you can deploy these high-strength steels is the front crash structure, where you want better crash energy management, and then the passenger cell, where you need to make the passenger cell as resistant to crush and deformation as you can to protect the occupants.

The steel revolution

According to Ron Krupitzer, Vice President of Automotive Applications for the American Iron and Steel Institute, the use of advanced high-strength steels has been gradually gaining momentum over the past 15 years. “There have been high-strength steels available in the past, most of these got their strength by alloying,” he says. “The new advanced-strength steels get their strength by manipulating the microstructure, by tempering and crunching to produce a mix of microstructure.

“You will get a lot of steel makers selling their own brand named steel. These are categorised by dual-phased steels – steels that include some martensite in them, that is probably the strongest phase of plain steel that you can get – and TRIP (Transformation Induced Plasticity) steel. These steels change their microstructure as they are crushed. In other words, if you start to load and form the steel it actually gets stronger; then you have the martensitic steels.

“The revelation is that the steels are now finding their way into stamped and rolled section formations in the body that heretofore they were not used in. In the past you could find some very high-strength martensitic steels in door beams and things like that, but today the dualphase steels are in the front and rear compartment rails, the rocker sections and cross members. It is proliferating through the car, primarily because of its higher strength and the ever increasing requirements of the car in terms of passenger safety compartment, and the energy absorbing capabilities around the passenger compartment.”

These steels are still almost exclusively used in structural parts rather body panels although Ford did experiment with a short run of door outer panels and these had very good resistance to dents. The problem with dual-phase as body outers is that it is produced as a hot-dipped galvanised product, while the carmakers really want an electro galvanised product on the outer skin as it is smoother and so they get a better painted surface.”

Manufacturing concerns

High-strength steels are ordinary in the sense that they are fully recyclable at the end of their life, and they are for the most part very weldable; the welding parameters have to change with the welding characteristics of the material.

The biggest issue for the automaker is the formability. Typically mild steel might have as much as 30-35 per cent elongation, which would make it highly formable – deep draw parts are not problem at all with that material.

However, the older high-strength steels might have formability from 10-25 per cent elongation, which can be a problem. The new dual-phase and TRIP steels can get that elongation back up to around 30 per cent.

“The trick was to produce a high-strength steel that was also formable. That said, it takes more force obviously because it is higher strength steel, so the press has to have more force,” Krupitzer explains. “You also have to be careful and match the force requirements of forming with your design, so there is a little discipline about forming.

“You will also hear people talk about springback.

Obviously the high-strength materials have the characteristic where, when you form them to the net shape they really want to revert back to flat, and they do that a little more than conventional mild steels. Again, it takes a little bit of a different approach to the design of the product and the design of the dies to accommodate that.”

High-strength in action

Two new vehicles have recently received accolades for pioneering the use of high-strength steels in new vehicles: Mercedes for the S Class and Chrysler for the Dodge Caliber.

The new Mercedes S Class was awarded the EuroCarBody Award for its pioneering innovations in the development and production of car bodies with its all-new body concept which relies heavily on high-strength steels.

Approximately half of all body components are made from highstrength or advanced high-strength steels delivering maximum rigidity at minimum weight.

The use of highstrength steels and design techniques also improve the static torsional rigidity of the body structure by 12 per cent compared to the previous S Class model. Torsional rigidity is one indicator of safe and comfortable handling.

“The innovative bodyshell of the new S Class forms one of the foundations for the safety, comfort and longevity offered by our new top model,” says Dr Thomas Weber, Member of the Board of Management, responsible for the Mercedes groups Research and Technology Division.

When Dodge engineers set out to develop a new compact car, they looked to advanced high-strength steels to save weight and improve crash performance. The result is the 2007 Dodge Caliber which is lightweight while meeting stringent impact requirements. The Caliber is deemed an affordable, boldly-styled five-door vehicle that combines the features of a sport utility vehicle with the profile and performance of a coupe. The first Calibers began rolling off the production line in January 2006.

Underneath its edgy exterior, the Caliber relies on highstrength steels. Forty per cent of its body structure is made of high-strength and hot-stamped steel – making it one of few current production models to utilise such a high level of advanced steel technology.

“The higher weight-to-strength ratio of high-strength steel allowed us to develop a body-in-white safety cage that meets more aggressive front-, side-, and rearimpact requirements, but without the added weight of regular steel,” explains Matt Liddane, Chief Engineer – Dodge Caliber. “Hot-stamped steel used on the A- and B-pillar and roof-rail reinforcements reduced Caliber’s weight overall by 44 pounds [20kg].”

In a first for Chrysler Group vehicles, the Caliber uses a tailor-rolled, hot-stamped steel B-pillar. The twopiece pillar includes a lower section made from mild strength steel, which maximises energy absorption.

This is combined with a tailor-rolled ultra high-strength steel upper section designed to enhance occupant safety.

“We used high-strength, lightweight and soundabsorbent materials from across the globe in Dodge Caliber,” Liddane adds. “Combined, these materials give Caliber excellent impact performance and body stiffness, as well as a smooth, quiet ride.”

Caliber’s use of advanced steels extends to the front and rear rail, tunnel reinforcements and floor cross members.

The steel components combine with the body structure to absorb and control impact energy in the case of highspeed frontal impact. The rails can handle greater impact loads.

Caliber also features a hydroformed front closure and upper cross member – the first high-production volume compact car to do so. Previously, this technology was exclusive to trucks and sports utility vehicles, but advances in hydroforming technologies allow for smaller diameter tubes and thinner walls, making it possible to apply this technology to compact vehicles.

Another Galaxy

One company that Corus has worked closely with recently is Ford. Together they have developed and implemented the latest grades of high strength steels into several key applications in the new Galaxy.

The collaborative work has seen Corus combine its material expertise and computer simulation techniques to help Ford identify areas where material selection can be optimised for a number of key parts for the rear structure of the vehicle. Importantly, improved application of high strength steels during the early design and engineering phase has provided opportunities to reduce development time and costs as well as improve vehicle crash performance.

Collaborative projects undertaken by Corus with Ford engineers at the carmaker’s Merkenich Technical Centre in Germany included forming feasibility studies on the rearfloor, rear-cross member and the heel-kick panels. The study on the rear-floor panel looked at opportunities to reduce the gauge, and therefore the weight of the panel, whilst ensuring that the complex panel shape was feasible to press.

Corus was also involved in a detailed parts integration study of the rear-floor panel. In this study, Corus showed that it was possible to use just one part for the floor panel instead of the originally planned two, allowing Ford to save on tooling, process and manufacturing costs.

For the heel-kick and rear-cross member panels, Corus again demonstrated that it was possible to down-gauge in order to reduce weight, replacing high-strength low alloy steels with dual-phase material whilst retaining the same impact performance for these panels.

As part of its collaborative work, Corus also employed its materials analysis simulation technique called Forming to Crash (or F2C) to help Ford engineers evaluate the crash performance of key parts such as the rear longitudinals made from its dual-phase material.

By using computer crash analysis techniques, Ford engineers were able to optimise the design of these parts during the Galaxy’s development process.

According to Krupitzer: “We have talked a little bit with the automakers about the future and of course we understand some of the requirements and there is a material type that has been experimented with in Europe at Thyssenkrup called TWIP [Twining Induced Plasticity].

“Essentially it acts like a stainless steel: it’s very highly formable, it’s very strong and it has got a lot of alloy in it.

We don’t know a lot about it yet. We listen to the OEMs and they ask for more strength and more formability, they want to be able to make all sorts of parts for the car, and include more high-strength steel. The way they think they will be able to do that is by improving the ability to form a product and TWIP can do that.”