Window safety grill requires special attention. Any normal iron is generally easy to cut and so, hard iron should be used. With technology, any rod can be broken, however a hard steel will be tough nut for burgler.
It is hard steel with more amount of carbon
This steel is one of the best quality and used as for window safety grill
Its price is a bit higher than normal steel used for grill
It is less susceptible for rusting due to carbon presence
Ensure quality steel
Look of grill is your choice. However, ensure the gap such way that any small baby head should not be stuck
https://www.metalsupermarkets.co.uk/difference-between-black-steel-and-bright-drawn-steel/
https://www.quora.com/What-are-the-main-uses-for-bright-mild-steel
https://pearlitesteel.com/difference-between-stainless-steel-and-mild-steel/