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Hot Rolled Steel

As you may have already guessed, hot rolled steel is steel that is in fact rolled while hot.

Hot rolled steel profiles are manufactured by heating a particular steel alloy until it is red-hot at which point the molten material is squeezed through rollers (aka: mills) which impart a particular shape and size (just like your old Play-Doh Fun Factory).

Through this process of heating and milling the steel takes on a mill scale finish. Bluish-black in color with an uneven & flaky texture, mill scale is iron oxide that coats the surface of hot rolled steel. Some folks like the industrial look of the mill scale and there are ways to protect that character from corrosion with a topcoat finish. If your project requires a finish other than the factory mill scale- paint, powder coat, blackening, etc- the mill scale will have to be removed. Some methods we have employed to removed mill scale include pickling and oiling, sand blasting, and/or DA sanding prior to applying a final finish.

Mill Scale on Hot Rolled Flat Bar

Hot rolled steel is readily available in a wide variety of shapes and sizes– from flat bar to pipe, tubing, sheet, channel, angle, wide flanges and bar stock– the list goes on. One thing to note about hot rolled steel forms is that similar to the aforementioned Play-Doh, it’s near impossible to get a nice crisp, perfectly square edge. Squeezing the red-hot steel through the rollers produces forms with slightly eased edges which can be cut or ground square, but if square is your goal you may want to consider cold rolled steel.

Bent Hot Rolled Steel Bar (note the slightly smooshie edges)

Discussion

3 thoughts on “Hot Rolled Steel

  1. hoping you can help with a few steel questions…
    1. Can either cold rolled or hot rolled steel be blackened? Someone was saying that there is a difference and one is more difficult for some reason.
    2. I’ve had fabricators tell me that based on who they buy their steel from, the same tube stock will either be black or almost clear resembling stainless…what’s up with that? Is there a way you can specify this depending on what you want?
    3. Lastly, a fabricator recently tried to charge our client 48K just to blacken 200LF of handrail…WTF? I think we spec’d prestoBlack, but why would they think they could charge that much? thanks for your help – sage

    Posted by sage | 05/22/2011, 11:41 am
    • Hi Sage!

      We’ve decided you win a gold star for being our first blog responder (aside from dads or husbands)!

      Here’s our best hack at answering your questions below:

      1. Both cold and hot rolled steel can be blackened. In order to blacken hot rolled steel, however, you have to remove the mill scale (and that may be the “difficulty” your source is referring to).

      Mill scale can be removed in a number of ways, and often multiple methods are combined to achieve the desired finish. If we’re working with sheet, or plate under 1/2″ thick we typically specify HRPO (hot rolled pickled & oiled). The pickling removes the mill scale, and if we want to achieve an even finer finish we may have the steel DA’d (dual action sanded) before blackening.

      Steel plate over 1/2″ cannot be pickled, so to remove the mill scale we have those goods sandblasted with a fine grit medium (red sand)– but this does impart a somewhat “orange-peely” texture.

      2. Tube stock and its various colors– our best guess on this one is that your fabricator may be talking about the difference between cold and hot rolled steel. Mill scale is blue-black in color and is present only on hot-rolled steel. Cold rolled steel profiles do not develop mill scale and they do have a more crisp, clean, & shiny steel appearance similar to a stainless alloy.

      (We will ask our resident steel experts what other causes there may be for such drastic variations in coloration.)

      3. Holy tamole! 48K for 200LF of handrail! That’s $240 a foot and it does sound steep indeed. We can’t really speak to the, “what were they thinking?!?” aspect of this inquiry, but the cost may be based on what needed to be done to that handrail before blackening– removal of mill scale via pickling, sand blasting, DA’ing, what-have-you– these things add up! But we agree, 48K is just silly- we would recommend getting a couple competitive quotes. As to the method of blackening– neither of our finishers have any experience with the Presto Black product– in fact, you may get better pricing if you let your fabricator blacken with their product of choice (just ask them to submit a sample on similar steel stock prior to final finishing so you know what to expect).

      Hope that helps!

      Posted by steelforgirls | 05/23/2011, 10:06 am

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  1. Pingback: Black Oxide/ Blackening « steel for girls - 12/13/2011

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