Hi and thank you for your question! 

Generally, all interviews at TKMC consist of a part that is about your personality, your soft skills and your personal fit and a second part with a case study.

Preparation time depends on many factors, e.g. how advanced you are w.r.t. case studies. If you are familiar with assessment centers and case studies, I think a week is enough to freshen up any knowledge regarding that. Additionally, I would do some research on ThyssenKrupp (what are the different business units, what are current news etc.). 

In the end, it's important to be yourself and stay calm. Small mistakes are never the problem, it's how you deal with them that makes all the difference. 

Good luck!