Same steel used in the Speedmaster and Seamaster. If you're worried about scratching the crystal, do what I did and get a model with the Sapphire glass instead of Hesalite. Waaaaay more expensive to replace if damaged, and not polishable like Hesalite, but it takes a beating and doesn't scratch easily. Micro-scratches drive me up a tree so I'd be polishing them out every hour.
I've found my co-axial (44.25mm auto-winding) Speedy to be very robust. I'm not super careful with my watches, but I do remove them if I'm doing anything like building a rock wall, at the gym, etc. Mine still looks new.
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