To extract the private key for each of your public address from Bitcoin-Qt, type below command under the console tab.
dumpprivkey BitcoinPublicAddress
In place of
BitcoinPublicAddress you have to mention your Bitcoin public key address.
When dumping multiple private keys this command might be better:
dumpwallet C:\path\to\dump.txt
This command does dump all private keys into a .txt file.
But note that the private keys will be saved in this .txt file in
plain text. Never execute this command on an (eventually) compromised computer.