Curving rectangular bars can be done several ways:
the “hard way”, that is, against the strong axis (if it were rolled into a ring, it would look like a washer);
the “easy way”, that is, against the weak axis (it would look like a belt);
helically, as in a railing for circular stairs, or
off-axis like a cone segment.
Bars can be either true mill bars or bars sheared from plate. For most applications, either type will work fine. But when it comes to curving bars in any way but the easy way, true mill bars roll better. The reason is that true mill bars tend to be more consistent dimensionally, a trait that can be critical when they are being curved. When the rolls on a section bender are set to curve a bar, any variation in the width of the bar can cause the bar to bend without uniform curvature.