Q: Why apple store is asking my card details for payment even I am downloading free apps

You need to submit credit card details for apps download be it free or chargeable.

How to Get Apps From the App Store Without a Credit Card

http://ipadhelp.com/ipad-help/how-to-get-free-apps-from-the-app-store-without-a- credit-card/

Creating an iTunes Store, App Store, iBookstore, and Mac App Store account without a credit card

http://support.apple.com/kb/ht2534

If None is not available - On your computer launch iTunes and click "iTunes Store" in the left navigation pane. Click the "down arrow" next to your name at the top right side of the page and click "Account." Enter your username and password and click "View Account" to log into your account information. Next to your Payment Type, click "Edit." Select the "None" button and click "Done." Confirm that your card has been removed by returning to the Apple account information screen. Under Payment Type, it should say that there is no credit card on file.

 Cheers, Tom