Water will freeze if it gets below 32*F, slightly lower if it is constantly moving. If the temperature drops below that, the steel pins (being the furthest from the relatively warmer water and being a good conductor) will freeze first.
Any automatic watering system you plan to use in sub freezing weather will need to be heated, and preferably insulated to retain the heat.
I plan to use aquarium or stock tank in heaters my watering system. I will circulate the heated water through a loop of pvc pipe with the nipples just downstream from the heater, and emptying into an insulated cooler which may have another heater in it. These will be controlled by a thermostat to maintain the water temperature at or above 35*F.
This will all be inside the barn so will not get quite as cold as outside, but still was down to 0*F or so this past winter.