iOS 11.0.3 on iPhone 6S, WIFI turns on automatically.

I have an iPhone 6S with iOS 11.0.3. When I pick up the phone WIFI is invariably on. I turn it off and seemingly automatically WIFI is again on the next time I pick up the phone. I'd like to have control over when it is turned on in an attempt to conserve the battery. Is it possible to disable this automatic enabling of WIFI?