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Clever design keeps small items in place

SSAB Tunnpl : 26 November, 2003  (New Product)
The ability to seek out new opportunities is what distinguishes Teresa Amman, who is in charge of the Materials Supply Department at the American Su-Dan Corporation. Clever thinking in design and interest in new materials have resulted in the company being selected as one of the candidates for the Swedish Steel Prize 2003. The company's nominated design is a spring strip made of advanced high strength steel to replace an earlier design for which hardened steel was envisaged.
Halved weight and thickness, virtually no rejections in production, and the elimination of heat treatment have substantially improved the production economy and also opened the door to a number of new development projects in the future.

The Su-Dan Corporation has been developing and producing parts for passenger cars since the mid-1960s. The Head Office of the Corporation is just outside the automotive city of Detroit in the USA, and the company has six production plants located in Orion, Rochester Hills and in Belton, South Carolina. The proximity to the carmaker giants GM, Ford and Chrysler is obvious. The company's production covers the main areas of pressing and stamping of sheet steel parts, casting of plastic parts, and also assembly, including welding and riveting, and a great deal more. The parts produced include handles, brackets, bonnet locks, and also frames and lifting devices for sunroofs. Production is highly automated.

Work for the future
A few years ago, the company began planning for the future, which resulted in a new department being formed for advanced design, research and development. This is fundamental to the strategy of the company, which aims to become a partner to customers in development, design and production. The development work has moved the Su-Dan Corporation a number of notches up in the value chain, and the technical specialists of the company have played a decisive role in its cooperation with many of the big customers.

This is how the steel strip project now nominated for the Swedish Steel Prize first began.

Thin, strong and resilient
Su-Dan was entrusted with a task of developing a concealed steel strip for a storage compartment in the new Chevrolet 4x4 Equinox model due to be released on the market early next year.

The storage compartment is located on the sides of the propeller shaft bracket inside the car. The compartment is covered by stretch plastic fabric and is intended for maps and other small items which means it is subjected to intensive use and hard wear. At the top edge, the compartment fabric will be tensioned by means of the concealed steel strip that must be thin, strong and resilient. The demands made on this small strip are intense, since it is of crucial importance to the form and dura-bility of the storage compartments.

40 percent rejection rate
'We began by making this strip of hardenable steel,' recalls Teresa Amman. 'The results were dis-astrous. Stamping and forming went well, but the strips distorted during heat treatment. We had to reject 40 percent of the strips after the costly heat treatment process. An additional problem was that after fitting, the strips distorted when the compartment was in use, and did not spring back cor-rectly.'

So the company had to find an alternative. They began by modifying production, but soon also by trying to find other materials.

'Our partners were of great help,' says Teresa Amman. 'Good cooperation between materials ex-perts and our own engineers led to the choice falling on ultra-high strength steel. We found a cold-reduced, dual-phase steel with a tensile strength of 1200 MPa.'

Su-Dan engineers were concerned about the bendability of the ultra-high strength steel, as to ensure smooth production, the steel had to be bendable in all directions.

'But we were given bendability guarantees and assistance in solving some production problems.' continues Teresa Amman. 'And it all worked out well in the end.'

Half the weight
The original part was made of 1 mm thick hardenable steel. In the initial tests, the ultra-high strength steel strip was made of the same thickness. But that proved to be too stiff and the spring-back was too high. The properties of the steel are optimised by reducing the thickness to 0.5 mm, which also halved the weight.

'At that stage, we did not have the time to modify the tools to suit the halved thickness. So we bent two thicknesses of strip together,' says Teresa Amman.

Now, in the autumn of 2003, as the start of production approaches, all settings have been adjusted and everything runs smoothly. The first cars will be rolling off the production line after the turn of the year, and it is important that nothing goes wrong on the production line. Su-Dan is sure it will not.

With an eye to leadership in the US
The Su-Dan Corporation has high ambitions, and a leading position in the US is within its reach. It is clearly apparent that nothing is left to chance in the company to achieve this. Teresa Amman takes us for a guided tour among the presses in the large, light and remarkably quiet workshops. The work for the future included raising production efficiency, which is also reflected in the improved working conditions. This is obviously a company that regards its personnel as its foremost resource. As an example, everyone without exception wears protective goggles and ear protectors.

The operators now work in groups. All presses have gradually been equipped with sensors that quickly stop the press if something should go wrong. For particularly sensitive parts, quality inspec-tion takes place by digital sensing. The work is aimed at zero defect tolerance. No carmaker can afford recalls on a popular model just because a faulty part has been detected.

Interesting future
The work on the new ultra-high strength steel strip has opened the door to exciting development projects. One of the Su-Dan Corporation conference rooms has an interesting exhibition of all con-ceivable variants of grab handles and support handles.

The exhibition includes typical handles, but the company has surveyed the market and found a total of about 2000 different plastic handles with a load-bearing metal core. In most of these, the core is made of pressing grade mild steel. There is obviously scope for development here.

'The ultra-high strength steel has demonstrated the opportunities we could offer many of our cus-tomers,' declares Teresa Amman. 'The combination of advanced high strength steels and our prod-uct and production expertise could no doubt contribute to many American cars being lighter in the future.'
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