As you all know, when we hit the refresh button, the entire page starts to reload. But I dont that, I only want to refresh some portions, which need to be refresh in case of a need.
I know, I can use ajax to reload a piece of content. But I want to attach this ajax function while the user clicks the refresh button or presses F5 or presses Ctrl+F5.
Don't do this, don't attempt to do this, you're breaking the behavior that the user has come to expect from their browser, this should not be overridden in any way. Nothing outside of the frame containing your page should be affected by your code, that's the user's browser, not yours.
Refresh has a purpose, they want to re-load the content, what if your script breaks, what if it's a JavaScript error? They may need a browser refresh to fix the issue...this is one of a thousand valid reasons, none of which should behave unexpectedly.
Disregarding the why you shouldn't above, you can't do this, not in every browser...for exactly the reasons I provided above plus many more.