Rectangle tube is a hollow rectangular steel, also known as flat tube, flat tube. It is a rectangular section shape steel made of hot rolled or cold rolled strip steel or coil plate after cold bent processing and formed by high frequency welding.

General Standard and Application of Rectangular Steel Tube
1ASTM A500Cold Formed Welded and Seamless Carbon Steel Structural Tubing in Rounds and Shapes
2ASTM A501Hot-Formed Welded and Seamless Carbon Steel Structural Tubing
3ASTM A511Seamless Staniless Steel Mechanical Tubing
4ASTM A513Electric-Resistance-Welded Carbon and Alloy Steel Mechanical Tubing
5ASTM A554Welded Stainless Steel Mechanical Tubing 
6ASTM A618Hot-Formed Welded and Seamless High-Strength Low-Alloy Structural Tubing
7EN 10210Hot Finished Structural Hollow Section of Non-alloy and Fine Grain Structural Steels
8EN 10219Cold Formed Welded Structural Hollow Sections of Non-alloy and Fine Grain Steel
9EN 10305-5Steel Tubes for Precision Applications-Welded and Cold Sized Square and Rectangular Tubes