Hello Kingpin,
You decide yourself what the functional unit of your analysis is. It is very possible to model the environmental impacts of a process, in fact it happens all the time as a part of larger analyses.
Choose a unit of analysis, and scope: This could be one stamped sheet of x type steel, of x thickness, with x surface treatment. Remember to do a full scoping per the standard you have chosen.
Gather LCI data: Gather all the inputs and outputs necessary to fulfill the unit of analysis. In the given case it will focus on a sheet of steel, so remember to include that as well.
Calculate LCIA results: Model in OpenLCA, and gather your results.
Interpret: Investigate what the results tell you. Divide the processes into raw materials and process, to investigate the impacts from just stamping. Just remember that stamping is a part of a life cycle, and that it produces a product with a functional unit. The process itself, without the steel, is not actually anything.
I hope this was helpful. Good luck.