cold rolled vs hot rolled

Cold rolled has been worked cold, has a better surface finish and grain structure. Depending on how much rolling it had, it may be slightly stronger before hitting it's yield point.
Hot rolled is usually cheaper, comes covered with mill scale. Higher end hot rolled can be "Pickled and Oiled" to remove the scale. If you are machining things, hot rolled is your choice since cold rolled has a tougher "skin" that tends to warp your piece as you cut into it.
If the piece in hand has the bluish mill scale on it, it is hot rolled. If the surface is smooth, it can be either. For small projects buy whatever will work for your project. Hot rolled will likely be cheaper but you need to deal with the scale.
Emmo wrote:

Hot and cold rolled are first rolled out from a red hot ingot. Possibly you have seen short scenes from rolling mills in automobile commercials. It is quite impressive to see a ribbon of red hot steel running along conveyor rolls. Hot roll when formed to its final shape is clipped to length and set on cooling racks. The black scale is iron oxide formed on the surface. Cold rolled steel or cold finish is the same bar run through more processes. The scale is removed in an acid bath then the bar is drawn through dies with a precise dimension giving it that nice shiny oxide free finish. The final drawing cold gives the cold finish a finer grain and its yield strength is higher. There is a small difference in ultimate tensile strength. Keep in mind that some cold finished material may not be mild steel if you pick it up as scrap. The same goes for hot rolled steel. You could pickup an alloy steel and not know just by looking. HRS and CRS are weldable if they are mild steel. Randy
I am not clear on the differences between cold rolled steel and hot rolled steel.
When would I use which?
Are there different requirements to weld them?
I make furniture, sculpture, gates and the like, nothing architectural so far - do I need to care which I am getting?
I buy a lot of scrap - is there a way to tell whether a piece I pick up is cold-rolled or hot-rolled?
Thanx for answering this basic question...
-- __ "All it took was all I had..."

"Emmo" wrote: (clip) I make furniture, sculpture, gates and the like, nothing architectural so far - do I need to care which I am getting? (clip) ^^^^^^^^^^^^^^^ I do a lot of stuff like that, so I will give you MY answer. I usually like hot rolled better, because the appearance doesn't change if you heat it and forge it to shape. And, a weld will look the same as the metal next to it. Even after painting, the surface of a cold rolled bar with a shape forged onto the end will show up differently.