MicroDicom is free for non-commercial use. If you are a medical student, you can use it for free. When you run it for the first time, it will ask you to register MicroDicom. Select the desired option.
This post talks about what is an STP file and how you can open such files in Windows 11/10. STP or STEP is a 3D assembly file saved in Standard for the Exchange of Product Data (STEP) format.