[MrForbes]

Another feature of steel is that lots of folks can weld it, even me! and I have little skill.

We have made two robots with steel frames. As mentioned, you want to use relatively thin steel...16 gauge square tubing is good, and easy to find. But also as mentioned, we don't know what it is you're trying to make, exactly. Sketches are very helpful when asking questions like this.

btw, most cars have been made mostly of steel for about a century. It works.