how to turn off 3G which is by default ON in iOS 5.1.1

i want to turn off 3G in my iphone 4S which is by default ON. when ever i reboot my phone, it automatically turns on, which affect on battery life,
so when ever i forget to turn it off. my battery hardly lasts for a whole day.

is there any solution for this ?
i have unlocked iphone 4S 64GB iOS 5.1.1