Methods of improving the deep drawing properties of austenitic stainless steels

Austenitic stainless steels have been developed for various purposes because of their combination of corrosion resistance with outstanding mechanical properties. Besides the remarkable strength properties, they are also known for their superior formability behaviour. Especially the stretch forming characteristics outstrip those of conventional ferritic steels by far, which is supposed to be explained by transformation of the cubic face centred austenitic phase to the cubic body centred respectively hexagonal martensite during straining, which is called TRIP-effect (Transformation Induced Plasticity). With regard to light-weight constructions and safety solutions especially in the automotive and transportation industry austenitic stainless steels with excellent cold formability gain much interest [1-4]. However, with a change of the stress state from stretch-forming to deep drawing, austenitic stainless steels lose their favourable combination of properties. That means that specific new applications, i.e. parts with high deep drawing depths, for those austenitic stainless steels would be suitable for, cannot by now be produced by using the deep drawing process. Furthermore, problems with deep drawing parts may occur as using unstable high strength austenitic stainless steels, which is expressed in a phenomenon called delayed cracking and in an increased springback behaviour [5-9]. The first aim of this project was to perform a quantitative evaluation of the above mentioned influencing parameters concerning the deep drawability of austenitic stainless steels. Therefore, various steel grades, including austenitic, partly austenitic and conventional ferritic deep drawing steels, had been examined, which differ in chemical composition, content of austenitic fraction, mechanical properties and topography. The second aim was to carry out formability benchmark tests and FEM-modelling of pilot components in order to achieve a more comprehensive description of press shop behaviour of the steel grades. The third aim was the development and production of an "ideal" austenitic stainless steel for deep drawing and stretch forming operations with an increment of the limiting drawing ratio from the current level of 2.0 up to ≥ 2.15. In this way the application potential of austenitic stainless steels should be enlarged, above all in the automotive industry.