Which kind of steel is this ?
 

The blade (cca 60 cms long and originally very rusty), was mechanically cleaned, plunged into disodium salt of EDTA and again cleaned with a very fine steel brush, than cleaned in demi water and treated wit silicon oil.

I think the steel is not mechanical damascus, actually, I don´t know what to think about it. If I cleaned it too much deep, the structure(design) disappeared on some places (tip and near the tip). What is this ? I heard such design could be result of tempering (quenching), but I am not sure. (the handle is original except of the central rhomb rivet which is new).

Martin, I do not see/think it has any particular pattern, although I see one weak streak, but hard to tell. extensive pitting and mechanical cleaning do weaken the pattern and make it harder to etch.

I have to say it looks like mono steel to me, simply exhibiting aging due to exposure to dust/dirt etc.

The type is known to have a twist core centre within the very narrow fullers but nothing gives it away if it is there.

These blades are often layered, which etching may reveal. But it isn't an intentional pattern-weld. I'd leave it as is or perhaps clean lightly.