can a grinder cut threw mm steal

Could someone advise me as to the best type of steel to specify that would be be most resistant to a cordless grindercutting disk attack I have looked at Abrasion resistant steel and case hardening but I am a lowly IT guy who has no such knowledge I can go up to 3 or 4 mm thick and the box section is approx 20cm x 20cm x 30cm