You can define UOM conversion in the item master.
In your case:
Default UOM = Meter, assuming you want to see the Stock of Steel Tube in Meters. Stock Entry will always be posted in Default UOM
Then define multiple UOMs and Conversion, set Purchase Default UOM as 6 Meter (you can even set this as Kg and define the Meter to Kg conversion in the UOM table)
Define BOM in terms of Lengths
This way Raw Material will be calculated in terms of Meters considering all the Sub Assemblies, check the Exploded Material Requirement. You can then post Stock entry of the actual scrap at the end of day or each operation.