+1, almost certainly hi-tensile steel. The best indicator is the seat post diameter.
As for the age, it would appear to be newer than 1980. The rear derailleur appears to be a Positron 400. My literature lists these as 5 speed models through 1980. I don't have the 1981 literature, but starting in 1982 they were listed with 5 and 6 speed versions and were available through at least 1984. There should be a double letter date code on the back of the derailleur that will narrow down the year.
The serial number on the bicycle is very interesting. It uses the location but not the format used by Bianchi in the 1970s. It's also does not match their 1980's serial numbers. Nor does it match their 1980's Japanese contractor used for most entry level models. I'm assuming, based on the serial number and Italian threading, that this is a subcontracted, European market model.