Steel flasks are pretty unbreakable but are not as good as glass due to the steel conducting heat from the main body of water to the thermal bridge of the non-vacuum lid.
Performance can be improved by preheating to start off with as much heat as possible, insulting the lid to reduce the rate of heat loss and using a larger volume flask for the gap between the kettle and the start of the route.
For a winter day trip what I do is:
preheat and fill a .5L and 1L flask, then stuff both in with my walking clothes as extra insulation.
After the standard 2hour drive to the start of the route I use the water in the .5L to make a coffee and refill it with the water from the 1L flask. While I drink my coffee I get changed into my walking gear.
Then I stuff the half full 1L flask into my street clothes so that the weak spot (the lid) is well insulated then I can have a hot tea when I get back.
The .5L flask then gets a spare winter sock put over it, leaving most bunched up covering the the lid (the weak spot), and put inside my bag.
The sock both insulates the lid and is a spare should you get a wet foot as happened last week when I discovered a small lochan under deep blanket of virgin snow
