The plaster/silica investment mixture used for casting silver, gold and other non-ferrous metals won't work for steel - the temperature is too hot, so the plaster would burn, causing a lot of gas to evolve in your casting. Actually, this would also be a problem even with non-ferrous metals, using the method you describe which attempts to short-cut the process by using the pour-cup as a crucible.
You'd either have to figure out something else to substitute for the gypsum plaster, or use the ceramic shell method, which is considerably different although it's still called lost wax investment casting.
Really, it's not worth setting up just for a few small parts. I'd say send them out to a foundry that specializes in investment casting of steel.
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