Patent ID: 11969779
Assignee: NIPPON STEEL CORPORATION
Field: Machine tools (Mechanical engineering)
Classification: CPC B  F | IPC B  F

Claim 0:
1. 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).