Carbon fiber round tubes
Carbon fiber tubes can replace steel, but more often them not, it is replacing aluminum, a carbon fiber tube can weigh 1/3rd of an aluminum tube and still have the same or better strength characteristics. Because of this, carbon fiber reinforced tubes are ofter found in applications light weight is crucial, like aerospace, race cars, and recreational sports.

Rectangular tube/ square tube
The most common carbon fiber tubular profile shapes are squares, rectangles, and circular. The rectangular adn square profiles are commonly referred to as a "box beam". Carbon fiber box beams prodvide excellent rigidity to a structure and simulate two parallet I-beams.

Application
Any application where weight is crucial, swiching to carbon fiber will be beneficial. The following are some of the most common uses of carbon fiber tubes:
1. Aerospace beams adn spars
2. Structural components
3. Arrow shafts
4. Bicycle tubes
5. Kayak paddles