tl;dr: Steel Productivity research should apply to Cast Steel gear wheel and Casting Steel beam.

The Space Age research for Steel Productivity applies to casting steel plate, but does not apply when direct casting Steel Beams or Steel Gears. Instead, to benefit from the research you must first cast steel plates and then machine them into Beams. Late game, this results in a damning productivity loss by not doing this, effectively ruining the direct casting recipes. Since the base game has no direct casting steel recipes, we can't know if they would've made those recipes be effected by steel prod (other than casting LDS, which has its own productivity research).

Is this unbalanced?
No. Even if casting steel gears is affected by the 300% prod research, it is still strictly worse than casting plates with 300% productivity, and then crafting the plates into gears in an assembler with up to four productivity modules, for up to 600% total productivity (original 300% from plates * 100% prod in second step with four legendary prod mods).

As a minor note, the capitalization for the aforementioned casting recipes is incorrect. The base game recipes only capitalize the first word- "Casting", not the latter "Steel", as seen in "Casting iron gear wheel".