What’s the difference between steel plate and steel bar

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I would suggest that you need proper lintel. Consult a structural engineer.

Steel plate is flat different wide plate. Bar is rebar which can be used in foundation or reinforcement of slab . Can Be different sizes from 6mm to up to 48 and more

You should be able to get steel plate cut to size at most steel fabricators. However, it depends on the intended positioning of any steel as to what you should use. You mention stone cast surrounds, so I assume you are discussing the vertical sides to the window and not the lintel. Any structural element you should consult a structural engineer on this. You mention stone cast (as opposed to moulded stone, so assume these are a “concrete with stone dust/fines” from that I’m assuming your builders intention is to put steel plate, which is a flat solid steel fixed externally to your surround held in place by stud bars drilled into the “stone” and fixed with a resin. If I have understood your question, I would feel this would be unsightly and maybe lack any structural use. Perhaps if you posted a drawing or photos a better response would be forthcoming

Try 6mm Flat Bar