I think you can recognize between hot and cold rolled steel.. the following site says cold rolled still are smooth grey while hot rolled steel are bluish-grey..
Why does Cold Finished Steel have a smooth grey finish, while Hot Rolled Steel has a rough, blue-grey finish?
Last week we had delivery of cold rolled steel purlins which I took as galvanized or galvalume.. so I rejected it because you are not supposed to have galvalume contact with other metal, concrete because the zinc-aluminum can oxidize.. so I rejected all the items delivered.. in the following picture.. the 4 purlins on the left are cold-rolled (see the smooth grey).. the 5 purlins on the right are hot rolled (just delivered a while ago).. final caliper thickness says 1.7mm..
Quoting from the above url which says it's fragile to bend any existing hot rolled into cold rolled purlins.. then how can you say purlins below 3mm are cold rolled.. they can break if bent cold so I think purlins below 3mm and above 1mm are hot rolled.. the url says:
"Why does Cold Finished Steel have a smooth grey finish, while Hot Rolled Steel has a rough, blue-grey finish?
Cold Finished Steels are just that - the final rolling is done when the steel is cold (room temperature), the whole operation bathed in oil, so the finished product is unoxidized, the grey of the actual steel, and as smooth as the rollers that do the processing.
Hot Rolled Steels are just that - They are heated up red-hot and pushed through rollers that squeeze the metal, literally squishing it into a particular profile, depending on the shape of the rollers. The process takes a long time, and because the steel is so hot for so long in th open air of the steel mill, the surface of the metal has has a long time to oxidize, producing a thick, tough oxide scale with the characteristic blue grey finish of the final product.
Why are there so few shapes available in Cold Finished Steel and so many in Hot Rolled Steel?
Steel is a very tough material when it is “cold” (meaning “room temperature”). When it is bent, hammered (such as in cold forging), or deformed in any way steel can actually harden in the area of distortion and begin to crack or weaken. Only if it is heated past a particular temperature (usually a red-heat) does it become plastic enough to be bent, formed hammered, and squeezed with relative ease and without damaging the metal. For example, few of us would be able to bend a bar of 1” thick steel at room temperature no matter how hard we tried, but if the middle of the bar were heated with a torch until it glowed a healthy cherry red color, most of us would have very little trouble bending the bar back upon itself until both ends touched. And when it cooled it would regain its toughness and strength in that bent condition and we would no longwr be able to bend it.
This means that the intricate profiles apparent in, say, a Hot Rolled beam or channel is only possible to form if the steel is iin the red-hot condition. If these types of profiles were attempted in the cold condition both the metal and the equipment used to shape the metal would be ruined.
This means that only relatively simple shapes like flats, hexagons or rounds are available in Cold Finished Steels. However, one of the great advantages of this process is that the resulting products is a much finer surface (that is bare of oxidation) and sharp corners to the profiles of Cold Finished products as opposed to the rounded and rough finish of Hot Rolled Steels."
Anyway. I have one problem.. the hardiflex limit is 1.6mm base metal thickness.. my purlins or C-sections are 1.7mm base metal thickness when measured using accurate caliper.. can you give a metal object that is 0.1mm thick for comparison.. how thin is 0.1mm thick.. how much percentage can it affect when you insert teksscrews??
