>>>So, the strength of a forging is needed.<<<
No requirement stated so far leads to any such conclusion.
You have not stated a tolerance on the linear dimensions controlling the axial size of the various steps. You have not stated a tolerance for concentricity or colinearity. Your drawing does not state a standard by which to interpret it.
Forgings of that general shape and size tend to be hollowed out somewhat to reduce the shipping weight, if nothing else.
My first guess at the cheapest process for producing the stated quantities and sizes is to buy circles flame-cut from thick plate by a steel distributor, edge-clamp them at the bottom to a plate, and let a CNC vertical mill gnaw on them, orbiting end mills, basically unattended, to produce the central hole and all of the reduced diameter steps, then flip the part onto a pilot in the center hole, clamp it with a stud in the pilot, and machine the OD last.
That's based on WAGs about the missing tolerances; a proper drawing could change my recommendations.
Mike Halloran
Pembroke Pines, FL, USA