Hello Texol
I think the material standard is BS EN 10025 , formerly BS4360. Although I don't have a full copy of this specification, the material is a carbon/manganese steel and the chemistry for S275 is :-
Carbon <=0.18%
Manganese <= 1.50%
Sulphur & Phosphorus 0.035% max.
Nitrogen <= 0.009%
S275R = BS4360 Grade 43B
S275J0 = BS4360 Grade 43C
S275J2G3 = BS4360 Grade 43D
S275J2G4 No BS4360 equivalent
Hope this helps.
Jimsan