Solingen steel isn't a type of steel like 440C, M4, AUS 8, etc. It is more or less a set of manufacturing processes that originated in the Solingen area. An old type of manufacturing guild. The original guilds specialized in sword and blade crafting. Here
is a short blurb about the origin. They steel used could be anything they have/had available at the time. The steel just went through whatever guild proprietary techniques they were/are using at the time. Here
is a bit deeper look at the history.
Solingen stamped products have a mythic sort of quality associated with them. As to the validity of such a lofty reputation, that is up to the individual to determine. Historically, they were about the only game around at certain points and the fact that they were able to crank out relatively large quantities for the times. I'm sure some products were better than others as it was a conglomeration of guilds initially that each marched to the beat of their own drums.