You can add some useful windows keyboard shortcuts to KDE pretty easily. These will only work if you have a keyboard with the windows (or another meta) key, and you have already setup KDE to properly understand the meta key. This will include most users.

Logout (Win+L)

Run KDE's System Settings by running systemsettings, or finding it in KDE's launcher menus. Click on "Keyboard & Mouse". Select "Global Keyboard Shortcuts" from the left column. Select "Run Command Interface" from the KDE component drop down. Now select "Lock Session" and choose a custom key binding. When prompted, press "Win+L", and hit okay. Now you have the ability to lock your screen with only two keypresses.