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.
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.