Please tell me about the reliable, sturdy smart phones available below Rs 6000.

Below 6000 .. if you really want to go for reliable and sturdy then I think you should avoid brand like Micromax, Lava, Karbonn , videocon etc as they are just Chinese rebranded.. assembled in India phones. If you are ok with Nokia then Nokia Asha series is well suited in your budget , you can look into Nokia Asha 208 and also 311