I have personally changed the straps on my Tortue a couple of times and never wondered what material the screws are made of. I stopped changing my straps lately because when I asked the boutique for new screws, they insisted on taking my watch in (not for service but more for “observation”). I didn’t understand nor agree to why they needed to do that when all I wanted was to purchase a set of new screws.
Here’s my Tortue.

Have a great weekend and stay safe!