In recent years, improvement of the fuel economy of automobiles has been sought from the
viewpoint of restrictions on emission of hothouse effect gases accompanying measures against
global warming. High strength steel sheet is being increasingly used for lightening the weight of
15 car bodies and securing safety in collision. In particular, recently, the need for ultra-high strength
steel sheet with a tensile strength of 980 MPa or more has been rising.
[0003]
Hot dip galvanized steel sheet used for automobile parts is being asked to be improved in not
only strength, but also press formability, weldability, and various other aspects of workability
20 required for forming parts. Specifically, excellent bendability is being sought from steel sheet from
the viewpoint of press formability.
[0004]
PTL 1 discloses steel sheet in which B is contained at the steel sheet surface layer part
mainly in a precipitated state and inside of the steel sheet mainly in a solid solution state so as to
25 improve the bendability.
[0005]
PTL 2 discloses high strength steel sheet excellent in delayed fracture resistance of a cut
end face and steel sheet base material having a martensite single phase structure, having a region
with a KAM value (kernel average misorientation value) of a value of 1ﾟ or more comprising
30 50% or more of the total, and having a maximum tensile residual stress in the surface layer
region down to the 1/4 depth position of sheet thickness from the surface of 80 MPa or less.
[0006]
As art for the improvement of the bendability of high strength steel sheet, for example, PTL
3 describes high strength cold rolled steel sheet with a surface layer part mainly comprised of
35 ferrite which is produced by decarburization of the steel sheet. Further, PTL 4 describes ultrahigh strength cold rolled steel sheet having a soft layer at the surface layer part which is
2
produced by annealing for decarburization of steel sheet.
[CITATION LIST]
[PATENT LITERATURE]
5 [0007]
[PTL 1] WO2017/002883
[PTL 2] Japanese Unexamined Patent Publication No. 2015-155572
[PTL 3] Japanese Unexamined Patent Publication No. 10-130782
[PTL 4] Japanese Unexamined Patent Publication No. 5-195149
10
SUMMARY
[TECHNICAL FIELD]
[0008]
Further, high strength steel sheet used for auto parts is being required to not break due to
15 deformation by collision after being formed into a part. In particular, the steel sheet used for auto
parts has to be excellent in not only bendability before press forming, but also bendability after
plastic strain is introduced by press forming. In particular, keeping down the load drop due to
fine cracks formed at the time of deformation by collision is sought from steel sheet for
automobile use. However, improvement of the bendability after plastic strain is introduced has
20 not necessarily been sufficiently studied up to now.
[0009]
Therefore, the present invention has as its object the provision of steel sheet excellent in
tensile strength and improved in bendability after plastic working and a method of production
thereof.
25
[SOLUTION TO PROBLEM]
[0010]
The inventors engaged in repeated intensive studies for solving the above problem and as a
result discovered that it is necessary to inhibit the formation and propagation of cracks after
30 plastic working at the time of bending deformation for suppressing fracture in collision.
Specifically, the inventors discovered that it is important to keep down the bending angle at the
time of maximum load in a VDA bending test and a load drop after the maximum load. Further,
the inventors discovered that it is possible to improve the bendability after plastic working by, as
a means, forming a suitable deboronized layer at the surface layer part. The present invention
35 was perfected based on these findings. The present invention includes the following aspects.