Raw material - Metal Sheet
Semi- finished - 150 parts and any new part
BOM - 150 BOM of each semi-finished.
Item quantity depends on volume*density * scrap % .
This calculation with be out of SAP.
Since you come to know semi finished produced only after it's completed, you have to confirm individual orders.
As per your description, you are doing bottom-up planning . Normally it should be Top - Down.
Give a thought..