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Yes, a diamond-tipped tool can cut through steel.
i think that it is quite difficult to cut mild steel because of its stiffness/strength
because it is made of super steel
yes
Cutting torches will cut stainless steel, but you are likely to be left with a discoloured edge to the cut. Which you may or may not like. A laser cutter will produce a cleaner cut, and smaller discolouration.