he addressed in presentation, from what i remember it had a lot to do with cost. Like $130k/ton with carbon vs $3k/ton with steel. He mentioned a few other factors. Also it allowed them to build outside, not sure if t that is true in production.
Both characteristics combine to give a lighter rocket than one built out of Al / carbon fiber.
The price is just the cherry on top.
The number of rings is roughly 35 for Starship, with about ten vertical seams each.
For easier thinking, line the vertical seams up and you see that this gives 50m length per vertical seam from bottom to top of Starship. So there are a total of 500m for that.
35 rings with ~28m circumference add up to about 1km of seams between them.
Now if they can use rolls twice as wide and only a single vertical weld each, then that's 500m between the rings and 50m for a single vertical seam. 550m of welds there instead of 1.5m before. This should speed things up quite a lot and reduce the cost of labour.
Raptor production is the real bottleneck, though.
They are shooting for one per day, no? Based on the differences identified in the three hanging under the Boca Chica vehicle now, it’s still in development.
No, they'll do a helical weld, exactly like large pipes (like this: http://www.xysteelpipe.com/upload/201512118145340710.jpg)
You basically need to do that as the steel rolls off the press, though, which is why they didn't do that for Mk I. They'll need to cold roll the steel onsite.
Composite tanks on few rockets being flown with them are made in one of a kind, purpose built autoclaves