This has to do with the 'N' at the end ofr P355N. The standard EN10028-3 says:

6.1 Classification

6.1.1 The steel grades covered by this European Standard are given in four qualities:

 

a) the room temperature quality (P ... N),
b) the elevated temperature quality (P...NH),
c) the low temperature quality (P...NL1) and
d) the special low temperature quality (P...NL2).

Use P355NH in case of design conditions with a warmer temperature than room temperature.