Resistance spot weld, resistance spot welding method, resistance spot welded joint, and method for manufacturing resistance spot welded joint

Patent metadata

Field	Value
Publication number	US-12410492-B2
Application number	US-202017614424-A
Country	US
Kind code	B2
Filing date	Mar 4, 2020
Priority date	May 28, 2019
Publication date	Sep 9, 2025
Grant date	Sep 9, 2025

Abstract

A resistance spot weld, a resistance spot welding method, a resistance spot welded joint, and a method for manufacturing the resistance spot welded joint. In the resistance spot weld, at least one of steel sheets is a high-strength steel sheet having a prescribed chemical composition. In at least one of a nugget edge region corresponding to a faying interface, the nugget edge region has a metal microstructure including tempered martensite as a main phase. The hardness Hv of the nugget edge region and the hardness Hmw of martensite in the nugget as a whole satisfy formula (4). In at least one of a strong HAZ region corresponding to the faying interface, the hardness Hh of the strong HAZ region and the hardness Hmh of martensite in a prescribed steel sheet satisfy formula (8).

Hv
≤

Hmw
-
40

(
4
)

Hh
<

Hmh
-
25

(
8
)

First claim

The invention claimed is:

1. A resistance spot weld in a welded member prepared by resistance spot-welding at least two overlapping steel sheets, the resistance spot weld comprising a nugget that comprises a nugget edge region,
wherein
at least one of the steel sheets is a high-strength steel sheet having a chemical composition comprising, in mass %:
C: 0.05 to 0.6%,
Si: 0.1 to 3.5%,
Mn: 1.5 to 10.0%, and
P: 0.1% or less,

two points on a boundary of the nugget at intersections of the boundary of the nugget with a faying interface between the steel sheets are defined as a first edge and a second edge or when a gap is present at the faying interface between the steel sheets, two points on the boundary of the nugget at intersections of the boundary of the nugget with a straight line Y positioned at a midpoint of the gap and parallel to surfaces of the steel sheets are defined as the first edge and the second edge,
a length of a line segment X connecting the first edge and the second edge is defined as D (mm),
on the line segment X:
a point O is positioned between the first edge and a center of the nugget, and
a point P is positioned between the second edge and the center of the nugget,

the nugget edge region is a region which is located inside the nugget and in which a distance L (mm) from the first edge to the point O and the distance L (mm) from the second edge to the point P satisfy formula (1) below:
0< L≤ 0.25× D   (1)

in at least one of the nugget edge region corresponding to the faying interface:
the nugget edge region has a metal microstructure including tempered martensite as a main phase, and
a hardness Hv of the nugget edge region and a hardness Hmw of martensite in the nugget as a whole satisfy formula (4) below, the hardness Hmw is computed using formulas (2) and (3) below:

Hmw
=

884
×
Cw
×

(

1
-

0.3
×
C
⁢

w
2

)

+
294

(
2
)

Cw
⁡

(

%
⁢

⁢
by
⁢

⁢
mass

)

=

∑

i
=
n

⁢

C
i

⁢

V
i

∑

i
=
n

⁢

V
i

(
3
)

Hv
≤

Hmw
-
40

(
4
)

where in formulae (2) and (3):
Cw (% by mass): a content of C per unit volume in the nugget,
C i (% by mass): a content of C in each of the overlapping steel sheets,
V i (mm 2 ): a molten area of each of the steel sheets in a region surrounded by the boundary of the nugget and the line segment X in a cross section taken in a sheet thickness direction and passing through a center of the nugget, and
n: a number of overlapping steel sheets,

in the steel sheet that is on an upper side of the faying interface and/or the steel sheet that is on a lower side of the faying interface:
points q are intersections of a straight line Z, which is parallel to the faying interface, with the boundary of the nugget,
points r are positions on the straight line Z in a heat-affected zone,
a region which is located inside the heat-affected zone, in which a distance M (mm) between the straight line Z and the faying interface in the sheet thickness direction satisfies formula (5) below, and in which a distance T (mm) from each of the points q to a corresponding one of the points r satisfies formula (6) below is defined as a strong HAZ region:
M=D/ 20  (5)

0< T≤D/ 10  (6)

in at least one of the strong HAZ region corresponding to the faying interface:
a hardness Hh of the strong HAZ region and a hardness Hmh of martensite in the steel sheets satisfy formula (8) below, and
the hardness Hmh is computed using formula (7) below:
H mh= 884×C h ×(1−0.3×C h 2 )+294  (7)

where in formula (7), Ch (% by mass): a content of C in the steel sheet on the upper side of the faying interface or the content of C in the steel sheet on the lower side of the faying interface, and
Hh<Hmh− 25  (8).

2. The resistance spot weld according to claim 1 , wherein an area fraction of carbides in the tempered martensite is more than 20%.

3. The resistance spot weld according to claim 2 , wherein an average grain size of the carbides is 300 nm or less.

4. The resistance spot weld according to claim 1 , wherein the high-strength steel sheet has a tensile strength of 780 MPa or more.

5. A resistance spot welding method for producing the resistance spot weld according to claim 1 , the method comprising:
forming a weld by perfo

Assignees

• Jfe Steel Corp

Inventors

• Endo Reiko
• Matsushita Muneo
• Matsuda Hiroshi

Classifications

• C22C38/04

  containing manganese · CPC title

• C22C38/02 Primary

  containing silicon · CPC title

• C22C38/002

  containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60 · CPC title

• C21D2211/008

  Martensite · CPC title

• C21D6/008

  containing Si · CPC title