This appears to be the back material for a hacksaw blade (for bimetal blades). It is unused, but has some mill scale from hot rolling, giving it a slightly dirty appearance. There's a possibility that the box and contents don't match. (I'm also unsure, as I purchased it at auction.) It's likely carbon steel/spring steel, equivalent to SUP6~SK5. Please purchase only if you understand this. If you know more, I'd appreciate it if you could tell me in the comments!