Jim Stewart wrote in message ws.com...
A couple of weeks ago at the recycling center, a
welder was unloading some 1/2" square steel bar.
I asked him if it was mild steel and he replied
"yes, old mild steel, the best" I've heard
references to "old steel" being better than "new
steel" and I've never understood what the difference
is.

The important thing to remember here is "new" steel is not made it is
recycled. The chances of finding a "hard" spot cuased by the scrap mix
are slim and none. The steel gets stirred at the LMF , then again by a
baffle in the tundish then is poured via nozzle into a mold, then the
temp is equalized in a long furnace before rolling.
I would guess that the biggest difference is in the making of steel/ I
have now idea how steel is made, but I do know that thanks to the rust
belt we will have a cheap source of new steel for many years.