Browsing through some steel supplier sites and a few other catalogues, it indeed seems C70 is a plain carbon steel, 0.7 % being roughly the carbon content. There is a whole family of low to high carbon steels ranging up from C10, which has a minimal amount of carbon. This actually looks to be the Italian UNI standard, and comparable to AISI, e.g. UNI C60 ~ AISI 1060. Similar/same type names are also in the German DIN standards, so it could be that as well, depending on the case. It should signify the same thing. The Italian steel producer Arvedi mentioned on their site (now offline), for a range of C20-C70, to expect 0.2-0.8 % C and 0.4-0.9 % Mn.