I don't know about stronger steel, but I have read that when the Allies captured u-boats they were surprised to find that the pressure hulls were thicker, and of course therefore stronger.
As to the claim of causing serious damage at a range of 30-50 meters, Allied sources claimed a lot less:
Quote:
|
The British claimed this DC would split a 0.875 inch (22 mm) hull at 20 feet (6.1 m) and force a submarine to surface at about twice that distance. A minol charge introduced in 1942 increased these distances to 26 feet (7.9 m) and 52 feet (15.8 m), respectively.
|
This note is for the British Mk VII Heavy depth charge.
http://www.navweaps.com/Weapons/WAMBR_ASW.htm