[CriticalCathed]

I don't know about his general claim, but a detail within his comment is right.

Smaller grain size leading to stronger material -- this is a result of more grain boundaries as the scale of the grain goes down. Hall-Petch strengthening.[0] A secondary effect is that with smaller grains oriented in random directions the metal is more resistant in general from stresses in all directions; whereas with larger grains you tend to get weakness in a particular direction (along the slip planes.)

This is common knowledge, at least it's basic material physics that I learned in college.

[0] https://en.wikipedia.org/wiki/Grain_boundary_strengthening

[kwk1]

The GP comment is claiming the opposite--that larger grain size results in a stronger material.

[elif]

Actually I said strong is too ambiguous of a term to use. For instance, do you want it "strong" enough to not bend under x force at y temperature, or do you want it "strong" enough to bend rather than shear at z force?

It is more practical to discuss the hardness and ductility at specific temperatures, as well as its ability to keep its carbon content under those temperature conditions.