Mandrel, bent tube, and method and apparatus for producing bent tube

A bent tube made of steel that comprises a bending portion that is two-dimensionally bent in a rotary draw bending operation and a straight tube portion, wherein:

• when a wall thickness of the bending portion is represented by t1 (mm), a wall thickness of the straight tube portion is represented by t2 (mm), an outside diameter of the straight tube portion is represented by D1 (mm), a bending radius of the bending portion is represented by R (mm), and a bending angle of the bending portion is represented by θ(°),
• t2/D1:
• 0.005 to 0.3,
• t2:
• 0.5 to 30,
• D1:
• 15 to 700,
• R:
• 1 D1 to 6 D1,
• θ:
• 0 to 180,
• a maximum thickness reduction ratio:
• 14.5% or less,
• a maximum flattening ratio:
• 8.0% or less, and
• a hardness ratio between straight tube portion and bending portion:
• 8 to 50%;
• where,
• the maximum thickness reduction ratio:
• a maximum value of {(t2−t1)/t2}×100(%),
• the maximum flattening ratio:
• a maximum value of {(maximum value of outside diameter−minimum value of outside diameter)/D1} of the bending portion×100(%), and
• the hardness ratio between straight tube portion and bending portion:
• {(average value of values obtained by measuring 5 points under conditions of a 1 mm pitch in a longitudinal direction of the tube with Hv (10 kg) at a center portion of an outside of bend in the longitudinal direction of the tube in an L cross-section of the tube)—(average value of values obtained by measuring 5 points under conditions of a 1 mm pitch in a longitudinal direction of the tube with Hv (10 kg) at the straight tube portion in the L cross-section of the tube)}/(average value of values obtained by measuring 5 points under conditions of a 1 mm pitch in a longitudinal direction of the tube with Hv (10 kg) at the straight tube portion in the L cross-section of the tube).