Artillery, Anti-tank guns and Anti-air shouldn`t have any need for special materials.According to the latest WWW ( Japan Single Player #2, minute 54:12) Artillery still needs 1 Tung+2 Steel )
AT-Guns were not researched yet by Daniel but I think they still require 2 Tung +2 Steel as they used to need.
And for the Yamato ... if the japanese don´t have enough steel they just use the oil to produce plastic sheets in place of steel armor, just works the same....
Umh.....plastic Yamato