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New supersteels help the automotive industry meet environmental targets

SSAB Tunnpl : 18 August, 2004  (New Product)
The time is fast approaching for the automotive industry to conform to its agreement with the EU Commission concerning reduced carbon dioxide emissions. No later than 2008, the aver-age emissions from new cars will have to be below 140 grams per kilometre. In terms of fuel consumption, this amounts to 6 litres of petrol per 100 kilometres, a figure that few of today's family cars can achieve. To meet the future limits, cars will need to be developed to be lighter and more fuel-efficient.
At the same time, the demands of buyers for comfort, safety, performance and reasonable price will have to be considered. One way of achieving all of this, is by increasing the use of the advanced high strength steels which have undergone significant development in recent years.

The agreement with the EU is now spurring carmakers to accelerate their development of new fuel-efficient models. Time is running out. In 2008, a mere four years away, the new rules will be com-ing into force. It is generally agreed that the targets are important, since road traffic is one of the biggest sources of carbon dioxide emissions and therefore much is expected of carmakers, by the general public and regulatory bodies alike.

Lower weight is essential
Technical development must progress on several fronts. One aspect is that more efficient engines and transmission systems and reduced friction of moving parts will be needed. Car buyers may have to compromise on their performance and comfort requirements. The trend towards the increas-ing weight of cars must be reversed. This applies particularly to medium-sized and large cars and the increasingly popular SUV's. Cars in these categories are often well equipped as standard, and the equipment levels have been constantly rising.

'Much of today's standard equipment was regarded as luxury options only a few years ago,' con-siders Kenneth Olsson, who is responsible for Automotive Technical Marketing at SSAB Tunnplåt. 'But views have now changed and equipment such as electric windows, ABS, air conditioning and electric door mirrors is now contributing to better driving conditions and improved safety. More equipment however, also increases the weight of the vehicle, which in turn leads to higher fuel con-sumption and increased emissions.'

Benefits of steel
Regardless of the standard equipment, the choice of material is of major importance to the weight of a car. The body weight of an average passenger car accounts for roughly one fifth of the total weight. This is followed by the engine and transmission at 18 percent and then by the chassis, dampers and suspension at 12 percent. New materials such as advanced high strength steels can be used to reduce much of the weight of many parts.

'Steel is a very appropriate material for the load-bearing and energy-absorbing parts of a vehicle,' explains Kenneth Olsson. 'Strength, fatigue properties, formability and price have always favoured steel, and today's advanced high strength steels now offer yet another strong argument. A body made of high strength steels basically weighs as much as an equivalent aluminium body.'

So carmakers can cut the vehicle weight by a modest investment in new resources, whilst being able to retain much of the expertise and equipment they already have,' adds Kenneth Olsson. 'This makes it easier to achieve the environmental targets within the specified time and at reasonable cost.'

Important technical properties
Kenneth Olsson feels that steel is often compared to aluminium in a misleading way.
'The materials are totally different and have different properties. Admittedly aluminium has much lower specific gravity, which means that a certain volume of aluminium weighs less than the same volume of steel, but many other material properties come into play within a structure.'

The technical properties of steel offer benefits compared to those of aluminium for example higher static and dynamic strength and also better fatigue strength. In a structure subjected to a variety of forces, these properties ensure a longer useful life and better impact resistance.

From an economical viewpoint, the fact that steel can be machined and formed using conventional methods and machines is important. Steel can also be of value from an environmental aspect.

'Compared to aluminium, less energy is needed to produce steel,' explains Kenneth Olsson. 'Both materials are recyclable, but the benefit of steel is that mixed fractions can be used in recycling. It is also easier to remove impurities when re-melting steel, than it is when re-melting aluminium, and steel can be recycled an endless number of times without quality being impaired.'

Interest is growing
Interest in advanced high strength steels is growing all the time.
'It all began in earnest a few years ago when the ULSAB concept study was presented,' recalls Kenneth Olsson. 'This demonstrated the great opportunities available, and the proportion of high strength steels used for cars has increased continuously since then. Several carmakers are also using high strength steel as a sales argument aimed at car buyers.'

Dual phase steels in particular are gaining ground among carmakers. Characteristic features of these steels are low yield strength in relation to the tensile strength, good weldability and excellent stretch-forming properties. Viewed overall, dual phase steels are well suited for vehicle production.

'The name reveals that the steel characteristics are partially due to the two phases that determine the properties of the steel,' explains Kenneth Olsson. 'Immediately after the steel has been produced, its properties make the material easy to work with. Deformation and bake hardening during curing of the paint cause the steel to harden further and become even stronger. This can be used both in production and for collision protection parts.'

Quenching in water
The secret behind dual phase steels lies in its production method. The properties are achieved by a special method of continuous annealing and subsequent quenching in water. The method is based entirely on heat and water, which makes it very environmentally friendly. The steel acquires a spe-cial microstructure consisting of hard martensite that imparts strength to the steel and soft ferrite that accounts for its formability. This enables high strength to be achieved even though the content of alloying elements is very low. The steels are therefore easy to weld and form.

Side collision protection weighs 1.8 kg
The Italian Frigostamp company has put the benefits of dual phase steel to use in a side collision protection beam developed for different Fiat models as well as vehicles from other manufacturers. The beams are specifically designed for cars where the latest EuroNCAP regulations dictate the safety requirements. By using advanced high strength steels, designers have developed a lightweight beam with very modest outside dimensions.

'In a crash, a beam is deformed in a very short space of time,' says Paolo Cavallo at Altair Engi-neering, the company involved in the development work. 'The deformation and energy absorption must be uniform throughout the crash process, and the intrusion of the door into the interior must not be too large. The very high forces involved must be arrested within the space of a few centime-tres.'

The solution to the problem was a beam made of advanced high strength, dual phase steel with a tensile strength of 1000 MPa. The cross-section of the beam is in the form of an open double-channel profile that is relatively simple to produce. The weight of the entire beam is no more than 1.8 kg.

'The Frigostamp door beam demonstrates the opportunities offered by advanced high strength steels to designers and carmakers,' says Kenneth Olsson. 'The beam is very thin and whilst it feels extremely light, it still provides effective protection a variety of side collisions. This impressive demonstration shows how effective protection can be provided by using such a small amount of material.'

Car seats, mountings and body
The next step will be to increase the proportion of advanced high strength steels in cars. The propor-tion of conventional steels used in vehicles is still high and so there is great potential for saving weight by switching to high strength steels.

'The opportunities are enormous, and we already have the techniques for using high strength steels effectively,' continues Kenneth Olsson. 'But developments are not confined to replacing conven-tional steels with high strength steels. Advanced high strength steels are also replacing aluminium, which has been used in car seats, for instance. The objective here is to maintain low weight while reducing costs, which is possible when using steels. In addition, high strength steels enable a com-pact design to be achieved.'

In the Volvo XC90 car, for instance, a complicated mounting problem for the centre row of seats has been solved by means of a floor mounting manufactured from advanced high strength steel.

The mounting concealed in the floor under the seat, must absorb the whole of the force exerted in a collision by the three occupants of the centre row of seats. The most extreme crash tests have pro-duced forces of up to 6 tonnes.

'The backrest cannot be secured to the sides of the car body,' recalls Martin Wallström, design en-gineer at Volvo Car and the man responsible for the development of the new mounting frame. 'To achieve a sufficiently high standard of comfort for all passengers, the backrest of the centre row of seats must fold forward to enable passengers to enter and leave at the rear of the car. The backrest cannot be secured to the sides of the body in the usual way and in addition, the seat belt reel for the centre passenger is secured to the backrest. The floor mounting must be capable of taking up all of these forces without any side fixings at all.'

The solution to the problem was a frame in which the front beam is open and is made of advanced high strength steel with a tensile strength of 1200 MPa. The rear beam that takes up most of the force is of box section and is made of hardened boron steel with an even higher tensile strength. The two side beams that comprise the crash boxes of the structure are pressed of high strength steel with a yield strength of 600 MPa.

Solution to stricter future environmental demands
The proportion of advanced high strength steels used in the production of car bodies is steadily in-creasing, but there is still more work to be done. The ULSAB-AVC project demonstrated the oppor-tunities available, and presented a complete car that meets the low carbon dioxide emission re-quirements of the EU.

'The ULSAB-AVC was a success and became a spearhead for the entry of modern advanced high strength steels into the automotive industry,' considers Kenneth Olsson. 'Admittedly high strength steels have long been in use in car production, but cars with bodies made entirely of advanced high strength steels will also be safer than today's cars. This will be part of the solution for meeting the tough future environmental and safety demands.'
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