S45 is 0.45% carbon steel so does heat treat, it will flame harden as is and can be cased with caution, if deeper harening is required
it is basically the same as AISI 1045 steel but with a much higer & wider band of acceptiable impurities.
Basically it is a downgraded 1045
Most of the Asian JIS standards are the same as AISI ones but with higher impurities so they can be made from scrap remelt and still be within specifications without the need to purify or dilute the impurities to AISI levels.
It is becoming a big problem for steel industries world wide as more & more Asian steels saturate the market
China runs it's own standards that are even more dirty
Because of the high impuritiy levels it can not be nitrided ( in any form ) or cryogenically heat treated ,
It can not be through hardened either
Aussie sort of equivalent is 1045 under AS 1442 and is heat treated the same .