Why did churchhill impose the iron curtain?

He did not, Stalin did. Churchill just named it.

Surprisingly, it wasn't Churchill who came up with the name - it was the Nazi minister of Propaganda, Joseph Goebbels.

An evil man, Goebbels, but not all evil men are stupid. A saying of his is still the motto of all propagandists - "it doesn't matter what is true, only what is believed."