Been doing a bit of googling for it, and no results.
The Japanese steel specifications would generally be "JIS" specs, and don't include an MY.
In looking through one of the Japanese mill's products, they used proprietary names for everything (IE, initials of the mill were the first few letters). Could the "MY" denote a particular mill's products?
this material is a S460G3+M it is usually a thermomecanical structural steel.
Yield strength = 460 Mpa
Tensil strength = 550-700 Mpa
This is a EN 10225 steel
But it is important to have a protective primer coat for protection.