Tailored Properties by Smart Microstructural Adjustments

by | Sep 6, 2017

A proposal to extend the traditional two-dimensional material performance map to a multi-dimensional view incorporating local formability, such as hole expansion ratio or bending angle during the development of future automotive steel grades.

Over the years, the development of advanced high strength steels was focused on improving strength and ductility as the main drivers for lightweight design. Hereby, ductility is a measure of global formability determined during tensile testing, e.g. total elongation. However, nowadays car manufacturers are challenged within their part forming processes more and more with problems of local formability, which cannot be described simply by the traditional measures gained during tensile testing. It is commonly accepted that simply improving ductility does not consequently solve this problem of local formability. In fact, the question is at which spatial position of the automotive part which kind of formability, i.e. local or global, is crucial for a successful forming operation. Hence, the traditional development of modern steel grades in terms of improving the two properties strength and ductility is not sufficient anymore.

A team of researchers of German steel producer Salzgitter proposed in a recent article in steel research international to broaden the view in order to envision further properties, like measures for local formability, during the development of future automotive steel grades. Based on a historical review on the development of sheet steel material for automotive application, they showed a first idea to extend the traditional two-dimensional material performance map (“banana diagram”) to a multi-dimensional view incorporating local formability like hole expansion ratio or bending angle. This unveils possible new material classes which can be termed as “Generation 4.0” AHSS.

Tailored formability seems to become the future challenge. How this property could be influenced and adjusted during industrial processing of hot dip galvanized dual phase steels is also described in the article. In addition to the chemical composition, the annealing parameters are especially key to modifying the microstructure in order to produce tailored properties in terms of pronounced elongation, hole expansion ratio or even a balanced state of both.

“Generation 4.0” is coming up and accompanying the 3rd generation AHSS, which in turn opens up to the car manufacturers a new dimension for lightweight design in future vehicles by using highly sophisticated materials with exceptional tailored properties – the right material at the right place with the specifically adjusted properties.

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