Welded joint and automobile member
A welded joint comprising a first steel sheet and a second steel sheet. A first plating layer is provided on a surface of the first steel sheet facing the second steel sheet, no plating layer is present on or a second plating layer is provided on a surface of the second steel sheet facing the first steel sheet, and a boundary plating layer is provided between the first steel sheet and the second steel sheet in a range of 0.5 mm from an end part of the corona bond toward an outside of the spot welded part. A higher tensile strength of a tensile strength of the first steel sheet and a tensile strength of the second steel sheet is 780 MPa or more, and an area ratio of a MgZn 2 phase at the cross-section of the boundary plating layer is 10% or more.
1 . A welded joint comprising a first steel sheet, a second steel sheet, and a spot welded part joining the first steel sheet and the second steel sheet, wherein,
a first plating layer is provided on a surface of the first steel sheet facing the second steel sheet,
no plating layer is present on or a second plating layer is provided on a surface of the second steel sheet facing the first steel sheet,
the spot welded part has a nugget and corona bond,
a boundary plating layer is provided between the first steel sheet and the second steel sheet from an end part of the corona bond to 0.5 mm from the end part of the corona bond in a direction toward an outside of the spot welded part,
a higher tensile strength of a tensile strength of the first steel sheet and a tensile strength of the second steel sheet is 780 MPa or more,
an area ratio of a MgZn 2 phase at a cross-section of the boundary plating layer is 10% or more, the cross-section of the boundary plating layer being a cross-section of the welded joint running through a center of the nugget and along a thickness direction of the first steel sheet, and
the first plating layer and the second plating layer satisfy the following Relation I:
0.30≥[(Mg composition of the first plating layer(mass %))×(amount of deposition of the first plating layer(g/m 2 ))+(Mg composition of the second plating layer(mass %))×(amount of deposition of the second plating layer(g/m 2 ))]/[(Zn composition of the first plating layer(mass %))×(amount of deposition of the first plating layer(g/m 2 ))+(Zn composition of the second plating layer(mass %))×(amount of deposition of the second plating layer(g/m 2 ))]≥0.06 Relation I
where, if no second plating layer is present, the Mg composition, Zn composition, and amount of deposition of the second plating layer are 0.
2 . The welded joint according to claim 1 , wherein
the area ratio of the MgZn 2 phase at the cross-section of the boundary plating layer is 60% or more.
3 . The welded joint according to claim 2 , wherein
at least one oxide with a long diameter of 0.5 μm or more is present in the cross-section of the boundary plating layer, wherein the long diameter represents a length of a largest line segment cutting across the at least one oxide.
4 . An automobile member comprising the welded joint of claim 3 , wherein
the welded joint comprises the second plating layer,
the first steel sheet is arranged at the outer side of the vehicle,
the second steel sheet is arranged at the inner side of the vehicle, and
the Mg composition of the second plating layer is lower than the Mg composition of the first plating layer.
5 . The welded joint according to claim 2 , wherein
the welded joint has an internal oxide layer of a depth of 1.5 μm or more and 20.0 μm or less on the surface side of the first steel sheet facing the second steel sheet.
6 . An automobile member comprising the welded joint of claim 5 , wherein
the welded joint comprises the second plating layer,
the first steel sheet is arranged at the outer side of the vehicle,
the second steel sheet is arranged at the inner side of the vehicle, and
the Mg composition of the second plating layer is lower than the Mg composition of the first plating layer.
7 . An automobile member comprising the welded joint of claim 2 , wherein
the welded joint comprises the second plating layer,
the first steel sheet is arranged at the outer side of the vehicle,
the second steel sheet is arranged at the inner side of the vehicle, and
the Mg composition of the second plating layer is lower than the Mg composition of the first plating layer.
8 . The automobile member according to claim 7 , wherein
the [Mg composition of first plating layer (mass %)]/[Zn composition of first plating layer (mass %)] is larger than 0.30.
9 . The welded joint according to claim 1 , wherein
at least one oxide with a long diameter of 0.5 μm or more is present in the cross-section of the boundary plating layer, wherein the long diameter represents a length of a largest line segment cutting across the at least one oxide.
10 . The welded joint according to claim 9 , wherein
the welded joint has an internal oxide layer of a depth of 1.5 μm or more and 20.0 μm or less on the surface side of the first steel sheet facing the second steel sheet.
11 . An automobile member comprising the welded joint of claim 10 , wherein
the welded joint comprises the second plating layer,
the first steel sheet is arranged at the outer side of the vehicle,
the second steel sheet is arranged at the inner side of the vehicle, and
the Mg composition of the second plating layer is lower than the Mg composition of the first plating layer.
12 . An automobile member comprising the welded joint of claim 9 , wherein
the welded joint comprises the second plating layer,
the first steel sheet is arranged at the outer side of the vehicle,
the second steel sheet is arranged at the inner side of the vehicle, and
the Mg composition of the second plating layer is lower than the Mg composition of the first plating layer.
13 . The automobile member according to claim 12 , wherein
the [Mg composition of first plating layer (mass %)]/[Zn composition of first plating layer (mass %)] is larger than 0.30.
14 . The welded joint according to claim 1 , wherein
the welded joint has an internal oxide layer of a depth of 1.5 μm or more and 20.0 μm or less on the surface side of the first steel sheet facing the second steel sheet.
15 . An automobile member comprising the welded joint of claim 14 , wherein
the welded joint comprises the second plating layer,
the first steel sheet is arranged at the outer side of the vehicle,
the second steel sheet is arranged at the inner side of the vehicle, and
the Mg composition of the second plating layer is lower than the Mg composition of the first plating layer.
16 . The automobile member according to claim 15 , wherein
the [Mg composition of first plating layer (mass %)]/[Zn composition of first plating layer (mass %)] is larger than 0.30.
17 . An automobile member comprising the welded joint of claim 1 , wherein
the welded joint comprises the second plating layer,
the first steel sheet is arranged at the outer side of the vehicle,
the second steel sheet is arranged at the inner side of the vehicle, and
the Mg composition of the second plating layer is lower than the Mg composition of the first plating layer.
18 . The automobile member according to claim 17 , wherein
the [Mg composition of first plating layer (mass %)]/[Zn composition of first plating layer (mass %)] is larger than 0.30.