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The newly developed multiphase high-strength steels with improved formability characteristics mainly owe their beneficial properties to their special microstructural composition. The multiphase structure can in general be classified as a mixture of varying amounts of soft steel phases and distinctly harder ones. As introduction the variety of nowadays multiphase steels is mentioned concerning the process strategy and the resulting range of strength. Focussing on the concept of TRIP steels (TRIP = transformation induced plasticity) the retained metastable austenite as one special steel phase plays an additional role. A microstructural model, which was originally applied to dualphase steels, was successfully used to explain the work hardening of TRIP steels at higher strain rates. The multiphase structure is also interesting in terms of void nucleating mechanisms. As indicated by microstructural observations austenite particles play a double role depending on the grain size.

Newly developed cold and hot rolled TRIP and multiphase steels produced by Thyssen Krupp Stahl possess great potential for use in body-in-white applications. Strength levels range from 500 MPa for the ferritic-bainitic steels to 600 - 800 MPa for dual phase (DP) and TRIP steels to up to 1,000 MPa and more for the complex phase (CP) and TMS steels. At the same time the formability of these steels is considerably better than that of conventional steels. In addition they lead to improved component behavior in terms of crash and dent resistance and to reductions in component weight. As a guide to the use of multiphase steels, this paper presents formability data and discusses use of the steels in try-out and series production.