This is a question that has little published research. I have been researching into this general area for the last couple of months and I am entering into a project that will look into the feasibility of different welding processes and how they apply to welding Magnesium to carbon steel and aluminum.
The current status of the project is finding applications that would be useful to industry. If you need answers to your question, let me know what alloy and size of magnesium along with what size and type of carbon steel. If there is a specific joint configuration that you must use, let me know that as well. I will try to incorporate this into my specific research and let you know what your options are.

Wayne

I had a contact, in January from Spain, in order to solve, I think, an automotive welding Mg/ C-steel application with thickness between 1 and 5mm.

I madeno contact with this person since then. I thing it will be interesting for you and your project to contact Alberto González in albreg@cidaut.es.

Andrade

Thank you for your response. I will attempt to contact Alberto Gonzalez regarding joining magnesium to carbon steel.

Wayne