Square steel tube is the steel tube of equal length of four side. It is made of strip steel after process treatment. Generally, the strip steel is disassembled, flat, rolled, welded to form a round pipe, and then rolled by the round pipe into a square pipe and then cut into the required length.

General Standard and Application of Square 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