Burring structural member
A burring structural member resistant to fracture even if the burring structural part is bent back is provided. In the burring structural member of the present disclosure, a thickness T of the sheet-shaped part is 2.0 mm or more, a Charpy impact value vE(0) of the sheet-shaped part at 0° C. is 50 J/cm 2 or more, a BCI value calculated by BCI=vE(0)/Lc based on the Charpy impact value vE(0) (J/cm 2 ) and a maximum length Lc (μm) of a crack at the inside bend of the bent wall part of the burring structural parts is 2.5 or more, and a curvature radius R (mm) of the inside bend of the bent wall part is larger than R1 calculated by R1=25/BCI−1.5.
1 . A burring structural member of high strength steel comprising:
a burring hole;
a burring wall part provided around the burring hole and having a vertical wall part and a bent wall part connected to the vertical wall part; and
a sheet-shaped part provided around the burring wall part and connected to the bent wall part,
wherein a thickness T of the sheet-shaped part is 2.0 mm or more,
an indicator relating to toughness and cracking at an inside bend of the bent wall part Charpy impact value vE(0) of the sheet-shaped part at 0° C. is 50 J/cm 2 or more,
a BCI value calculated by the following formula (1) based on the Charpy impact value vE(0) and a maximum length Lc of a crack at an inside bend of the bent wall part is 2.5 or more, and
a curvature radius R of the inside bend of the bent wall part is larger than R1 calculated by the following formula (2):
BCI
=
vE
(
0
)
/
Lc
(
1
)
R
1
=
2
5
/
BCI
-
1.5
(
2
)
where, the unit of vE(0) is J/cm 2 , the unit of Lc is μm, and
the unit of R1 is mm, and
wherein R/BCI defines a curve and identifies pass values and fail values of R1 calculated in the formula (2), and
the curvature radius R is selected based on the pass values identified by the curve.
2 . The burring structural member according to claim 1 , wherein
the curvature radius R of the bent wall part is 0.5 mm or more and 10.0 mm or less.
3 . The burring structural member according to claim 1 , wherein
the thickness T of the sheet-shaped part is 2.0 mm or more and 8.0 mm or less.
4 . The burring structural member according to claim 1 , wherein
a tensile strength TS of the sheet-shaped part is 780 MPa or more.
5 . The burring structural member according to claim 4 , wherein
the tensile strength TS of the sheet-shaped part is 980 MPa or more.