FIELD
[0001]
The present invention relates to a spot welded joint obtained by superposing and spot
welding a plurality of-steel sheets and a spot welding method suitable for obtaining that spot
welded joint. In particular, it relates to a spot welded joint excellent in tensile strength e~en when
10 comprised of a plurality of steel sheets including one or more high strength steel sheets and to a
spot welding method suitable for obtaining that spot welded joint.
[0002]
In recent years, in the automotive field, to lower fuel consumption and cut emissions of
C02, reduction of the weight of car bodies has been sought. Further, to improve collision safety,
15 increasing the strength of car body members has been pursued. To satisfy these demands, use of
high strength steel sheets for car bodies and their parts etc. is effective. In this regard, when
assembling car bodies and attaching parts, spot welded joints fonned by superposing and spot
welding a plurality 6f'steet · shed:Sltave been used.
20
[0003]
In a spot welded joint, the tensile strength is an important property. "Tensile strength''
includes tensile shear strength (TSS) measured by applying a tensile load in a shear direction and
cross tensile strength (CTS) measured by applying a tensile load in a peeling direction. Note that,
the methods for measurement of the tensile shear strength and cross tensile strength are
prescribed in JIS Z 3136 and JIS Z 3137.
25 [0004]
The CTS of a spot welded joint formed by superposing a plurality of steel sheets with
tensile strengths which are not that high, for example, steel sheets having 270 MPa to 600 MPa
or so tensile strength, increases along with the increase in the tensile strength of the steel sheets
forming the joint, so problems seldom arise relating to the tensile strength of the joint.
30 [0005]
However, the CTS of a spot welded joint formed by a plurality of steel sheets including at
least one steel sheet with a high tensile strength, for example, a steel sheet having 750 MPa or
more tensile strength, will not increase even if the tensile strengths ofthesteel sheets forming the
joints increase or else will decrease. The drop in the deformation abilities of the steel sheets and
3 5 · consequent increase in concentration of stress at ·a weld zone and hardening of the weld zone and
consequent drop in toughness of the weld zone are the reasons for this. For this reason,
1
5
improvement of the CTS at a spot welded joint formed by a plurality of steel sheets including at
least one steel sheet with a high tensile strength, that is, a "high strength steel sheet", is being
sought.
[0006]
As the method of securing strength and toughness of a spot welded joint formed
including a high strength steel sheet, for example, the methods described in PTLs I to 6 may be
mentioned.
[0007]
PTL I describes a method of supplying a temper current after a certain time elapses from
I 0 when a main current for making the steel sheets melt and bond finishes being supplied so as to
thereby anneal a bonded zone of the spot welded joint (nugget zone and heat affected zone) and
cause the hardness to decrease.
[0008]
However, with this method, the martensite transformation has to be made to substantially
15 end before supplying the temper current. For this reason, a long cooling time becomes required
after the end of the supply of the main current. Furthermore, in the spot welded joint obtained by
this method, the nugget is softened so the shear strength falls.
[0009)
PTL 2 describes a method of tempering a weld zone after welding by a heating means
20 separate from welding, that is, high frequency heating. However, with this method, a separate
step becomes required after welding and the work procedure becomes complicated. Further, with
this method, special equipment is necessary for utilizing high frequency waves. Still further, in
the spot welded joint obtained by this method, the nugget is softened and the shear strength falls.
[0010]
25
30
PTL 3 describes the method of forming a nugget by main welding, then supplying a later
current of the main weld current or more. However, with this method, if extending the later
current supply time, in the obtained spot welded joint, the nugget size just expands: the structure
becomes the same as that of ordinary welding.
[0011]
PTL 4 describes a method of spot welding steel sheets with a tensile strength of 440 MPa
or more. In this method, the chemical composition of the steel sheet is restricted to CxP~0.0025,
P: 0.015% or less, and S: 0.01% or less. Further, after welding, the weld zone-is-heat treated at
300°C for 20 minutes or so. However, in this method, the steel sheets which can be used are
limited. Further, in this method, a long time is required for welding and the productivity is
35 therefore low.
[0012]
2
PTL 5 describes a spot welded joint formed including a high strength steel sheet (tensile
strength: 750 to 1850 MPa, carbon equivalent Ceq: 0.22 to 0.55 mass%) and defined in
microstructure of the outer layer area of the nugget and the average particle size and number
--
density of carbides in the microstructure. However, when fractured at the outside of the nugget,
5 since the structure of the nugget does not contribute to the strength at all, definition of the
microstructure is meaningless.
[0013]
-- PTL 6 describes a method of spot welding steel sheets-with a tensile strength of 900 to
18'50 MPa and a sheet thickness of-1.8 to 2.8 mm. In this method, after welding, a current of 0.5
10 time to 0.9 time the weld current is later supplied for 0.3 time to 0.5 time the weld time.
However, in this method, the time between the main welding and the later supply of current is-~
not sufficiently studied. This does not contribute to improvement of the joint strength.
[CITATION LIST]
15 [PATENTLITERATURE]
[0014]
PTL l: Japanese Patent Publication No. 2002-l 03048A
PTL 2: Japanese'PatenfPublication No. 2009-l2580IA
PTL 3: Japanese Patent Publication No. 2010-115706A
20 PTL 4: Japanese Patent Publication No. 2010-059451A
PTL 5: W02011/025015A
PTL 6: Japanese Patent Publication No. 2011-005544A
25 [TECHNICAL PROBLEM]
[0015]
SUMMARY
Due to the above such backdrop, in the past, in a spot welded joint formed from a
plurality of steel sheets including at least one high strength steel sheet with a high tensile
strength, the toughness easily became insufficient and a sufficiently high cross tensile strength
30 was difficult to secure.
[0016]
The-present invention was made in consideration of the abovg..,situation and has as its
object the provision of a spot welded joint by which a sufficiently high cross tensile strength
(CTS) is obtained even if including at least one high strength steel sheet with a high tensile
35 strength, specifically a high strength steel sheet with a tensile strength of750 MPa to 2500 MPa,
and a spot welding method suitable for obtaining that spot welded joint.
3
5
[SOLUTION TO PROBLEM]
[0017]
· · -Trr~olve-tn-e-a:bove problem, the inventors· engaged in intensive studies on a metho-d-of · --
.solving the above problem from a metallurgical perspective and a mechanical perspective. As a
result, they learned that with just securing the toughness of an inside of a melted then solidified
part (below, referred to as a "nugget"), at the time of a cross tensile test, even if low load fracture
occurring inside the nugget can be suppressed, low luad fracture occurring in the surroundings of
the nugget, that is, the heat affected zone, cannot be suppressed.
10 [0018]
Further, they discovered that to obtain a spot welded joint with a high reliabil-ity, it is
necessary to improve the structure not only inside of the nugget, but also at the surroundings of
the nugget, and that for this, it is possible to control the structure of the heat affected zone
surrounding the solidified area after a solidified area is formed in the melted part so as to thereby
15 suppress low load fracture occurring at the heat affected zone, and thereby completed the present
invention.
[0019)
That is, the aspeets oflhe present invention include:
(1) A spot welded joint formed by superposing and spot welding a plurality of steel sheets,
20 wherein
25
at least one steel sheet of the plurality of steel sheets is a high strength steel sheet with a
tensile strength of750 MPa to 2500 MPa;
a carbon equivalent Ceq of the high strength steel sheet expressed by the following
formula (A) is 0.20 mass% to 0.55 mass%;
in the superposing of the plurality of steel sheets, the high strength steel sheet is arranged
at an outermost side, the high strength steel sheet arranged at the outermost side being defined as
a "high strength steel sheet S 1 ",
in a cross-section cut along the sheet thickness direction of the steel sheet while passing
through a center of a welding mark formed on the surface of the steel sheet due to the spot
30 welding,
when defining superposed surfaces of the high strength steel sheet S 1 and another steel
sMet superposed with that high strength steel sheet S 1 as a ~lane A" and
when defining a plane passing through a point of one-half of a distance between a
crossing point of a line L 1 in the sheet thickness direction passing through the center of the
35 nugget-and a line showing the end of the nugget at the high strength steel sheet.S 1 side and a
crossing point of the plane A and the line Ll and parallel to the plane A as a "plane B",
4
a square area, which has sides of 30 J.lm centered about a crossing point of a line L2
separated from a tangent at any position on a nugget end line NEL sandwiched between the plane
A and the plane Bin the line showing the end of the nugget by 250 J.lm to a heat affected zone
side and parallel to that tangent and a line L3 passing through that any position and vertical to
5 the line L2, is contained in a heat affected zone having the high strength steel sheet S 1 as a base
material; and
10
in the square area, an average value of widths of blocks comprised of lath martensite is
0.5 to 7.0 J.lm:
Ceq=[C]+[Si]/30+[Mn]/20+2[P]+4[S] ···(A)
in formula (A), [C], [Si], [M.n], [P], and [S] respectively being contents ofC, Si, Mn, P,
and S (mass%). --
(2) A spot welded joint formed by superposing and spot welding a plurality of steel sheets,
wherein
at least one steel sheet of the plurality of steel sheets is a high strength steel sheet with a
15 tensile strength of 750 MPa to 2500 MPa;
a carbon equivalent Ceq of the high strength steel sheet expressed by the following
formula (A) is 0.20 mass% to 0.55 mass«»/o;
in the superposing 'Of 'the plurality of steel sheets, the high strength steel sheet is .not
arranged at an outermost side, the high strength steel sheet not arranged at the outermost side
20 being defined as a "high strength steel sheetS l ",
in a cross-section cut along the sheet thickness direction of the steel sheet while passing
through a center of a welding mark formed on the surface of the steel sheet due to the spot
welding,
a square area, which has sides of 30 J.lm centered about a crossing point of a line L2
25 separated from a tangent at any position on a nugget end line NEL showing the end of the nugget
having the high strength steel sheet S 1 as a base material in the line showing the end of the
nugget by 250 J.lm to a heat affected zone side and parallel to that tangent and a line L3 passing
through that any position and vertical to the line L2, is contained in a heat affected zone having
the high strength steel sheet S 1 as a base material; and
30 in the square area, an average value of widths of blocks comprised of lath martensite is
0.5 to 7.0 J.lm:
Ceq=[ C]+[Si]/30+[Mn]/20+ 2[P]+4[S] · · ·(A+
in formula (A), [C], [Si], [Mil], [P], and [S] respectively being contents of C, Si, Mn, P:and
S (mass%).
35 [0020]
(3) A spot welding method superposing and spot welding a plurality of steel sheets,
5
wherein
at least one steel sheet of the plurality of steel sheets is a high strength steel sheet with a
tensile strength of 750 MPa to 2500 MPa;
a carbon equivalent Ceq of the high strength steel sheet expressed by the following
5 formula (A) is 0.20 mass% to 0.55 mass%; and,
10
in the superposing of the plurality of steel sheets, the high strength steel sheet is arranged
at an outermost side, the high strength steel sheet arranged at the outermost side being defined as
a "high strength steel sheet S 1 ",
the method comprising
a main welding step of supplying current to the superposed plurality of steel sheets
through-welding electrodes and
a heating control step comprising, after the main welding step, maintaining the assembly
at a temperature of the Ms point of the high strength steel sheet S 1 or more while, in a crosssection
cut along the sheet thickness direction of the steel sheet while passing through a center of
15 a welding mark formed on the surface of the steel sheet due to the main welding step,
when defining superposed surfaces of the high strength steel sheet S l and another steel
sheet superposed with that high strength steel sheet S l as a 4'plane A" and
when defining a plane passing through a point of one-half of a distance between a
crossing point of a line L 1 in the sheet thickness direction passing through the center of the part
20 to form a nugget and a line showing the end of the part to form the nugget at the high strength
steel sheet Sl side and a crossing point of the plane A and the line Ll and parallel to the plane A
as a "plane B'',
controlling a square area contained in a heat affected zone having the high strength steel
sheet S 1 as a base material, which square area has sides of 30 Jlffi centered about a crossing point
25 of a line L2 separated from a tangent at any position on a nugget end line NEL sandwiched
between the plane A and the plane B in the line showing the end of the part for forming the
nugget by 250 Jlm to a heat affected zone side and parallel to that tangent and. a line L3 passing
through that any position and vertical to the line L2, to be heated in a range of an Ar3 point [°C]
ofthe high strength steel sheet Sl to (Ar3 point+400)[°C] for 0.04 second to 5.0 seconds:
30 Ceq=[C]+[Si]/30+[Mn]/20+2[P]+4[S] ···(A)
in formula (A), [C], [Si], [Mn], [P], and [S] respectively being contents ofC, Si, Mn, P,
and S (mass%).
( 4) A spot welding method superposing and spot welding a plurality of steel sheets, wherein
at least one steel sheet of the plurality of steel sheet is a high strength steel sheet with a
35 tensile strength of 750 MPa to 2500 MPa;
a carbon equivalent Ceq of the high strength steel sheet expressed by the following
6
formula (A) is 0.20 mass% to 0.55 mass%; and
in the superposing of the plurality of steel sheets, the high strength steel sheet is not
arranged at an outermost side, the high strength steel sheet not arranged at the outermost side
-----------being-defined as a ''high strength steelsheet Sl", --------
5 the method comprising
a main welding step of supplying current to the superposed plurality of steel sheets
through welding electrodes and
a heating control--step comprising, after the main welding step, maintaining-the assembly
at a temperature of the Ms point of the high strength steel sheet S I or more while, in a cross-
I 0 section cut along the sheet thickness direction of the steel sheet while passing through a center of
--a welding mark formed on the surface of the steel sheet due to the main welding step,
controlling a square area contained in a heat affected zone having the high strength steel
sheet S I as a base material, which square area has sides of 30 J.Lm centered about a crossing point
of a line L2 separated from a tangent at any position on a nugget end line NEL showing the end
I5 of the part for forming the nugget having the high strength steel sheetS 1 as a base material in the
line showing the end of the part for forming the nugget by 250 J.Lm to a heat affected zone side
and parallel to that tangent and a line LJ passing through that any position and vertical to the line
L2, to be heated iri a· range of an Ar3 point (°C] of the high strength steel sheet Sl to (Ar3
point+400)[°C] for 0.04 second to 5.0 seconds:
20 Ceq=[C]+[Si]/30+[Mn]/20+2(P]+4[S] ···(A)
in formula (A), [C], [Si], [Mn], [P], and [S] respectively being contents ofC, Si, Mn, P,
and S (mass%).
[ADVANTAGEOUS EFFECTS OF INVENTION]
25 [0021]
According to the present invention, it is possible to provide a spot welded joint by which
a sufficiently high cross tensile strength (CTS) is obtained even if including at least one high
strength steel sheet with a tensile strength of 750 MPa to 2500 MPa, and a spot welding method
suitable for obtaining that spot welded joint.
30 BRIEF DESCRIPTION OF DRAWINGS
[0022]
[FIG. I] FIG. I is a schematic cross-sectional view in a sheet thickness direction of-a-spot
welded joint according to the present embodiment.
[FIG. 2] FIG. 2 is a schematic view for explaining blocks in a martensite structure forming a heat
35 affected zone of a spot welded joint according to the present embodiment.
[FIG. 3A] FIG. 3A is aschematic view for explaining a square area for measuring the widths of
7
blocks in a cross-section in a sheet thickness direction of a spot welded joint obtained by
superposing two steel sheets in the spot welded joint according to the present embodiment when
a high strength steel sheet is arranged at an outermost side in that spot welded joint.
[FIG. 3B] FIG. 3B is a schematic view for explaining a square area for measuring the widths of
5 blocks in a cross-section in a sheet thickness direction of a spot welded joint obtained by
superposing three steel sheets in the spot welded joint according to the present embodiment
when a high strength steel sheet is arranged at an outermost side in that spot welded joint.
[FIG. 3C] FIG.--3-C is a schematic view for explaining a square area for measuring the widths of
blocks in a cross-section in a sheet thickness direction of a spot welded joint obtained by
10 superposing three steel sheets in the spot welded joint by according to the present embodiment
when a high strength steel sheet is not arranged -at an outermost side in that spot welded joint.
[FIG. 4] FIG. 4 is a view showing one example of the arrangement of two steel sheets and
welding electrodes at the time of starting spot welding.
[FIG. 5] FIG. 5 is a view for explaining temperature histories applied to the square area shown in
15 FIG. 3A to FIG. 3C in a heating control step in a spot welding method according to the present
embodiment.
[FIG. 6] FIG. 6 is an SEM photograph showing a martensite structure in a square area of a spot
welded joint according 'to an-invention example.
[FIG. 7] FIG. 7 is an SEM photograph showing a martensite structure in a square area of a spot
20 welded joint according to a comparative example.
DESCRIPTION OF EMBODIMENTS
[0023]
Below, the present invention will be explained based on the embodiment shown in the
25 drawings in the following order:
1. Spot Welded Joint
1.1. High Strength Steel Sheets
1.2. Heat Affected Zone
1.3. Steel Sheets Other Than High Strength Steel Sheets
30 2. Spot Welding Method
2.1. Main Welding Step
2.2. Heating Control St~
3. Etfects of Present Embodiment
4. Modifications
35 [0024]
1. Spot Welded Joint
8
5
The spot welded joint 1 according to the present embodiment, as shown in FIG. 1, is
-comprised of base materials of two steel sheets 1A and 1B superposed and with parts ofthe
superposed parts melted and bonded by spot welding thereby making the steel sheet lA and steel
sheet 1-B-integrally-jeined. In-the present embodiment, at least one·ofthe steel-sheets lA and-1-B-is
a later explained high strength steel sheet with a high tensile strength.
[0025]
The plurality of steel sheets forming the spot welded joint according to the present
embodiment are not particularly limited so long as the steel sheetsinclude one or more high
strength steel sheets. That is, all of the steel sheets forming the spot welded joint may be high
10 strength steel sheets or just one may be a high strength steel sheet
[0026]
Further, FIG. 1 shows a spot welded joint obtained by joining two steel sheets, but it may
also be a spot welded joint obtained by joining three or more steel sheets. In this case as well, it
is sufficient that at least one of the steel sheets be a high strength steel sheet. Further, when
15 superposing three or more steel sheets~ the three or more steel sheets may differ in sheet
thickness. Further, when superposing three or more steel sheets, at least two steel sheets may be
the same in sheet thickness.
(0027)
Further, the type of the steel sheet is not particularly limited. For example, a dual-phase
20 structure type (for example, structure including martensite in ferrite or structure including bainite
in ferrite), work induced transformation type (structure including residual austenite in ferrite),
hardened type (martensite structure), fine grain crystal type (mainly ferrite structure), or any
other type of steel is possible.
25
[00281
1.1. High Strength Steel Sheet
In the present embodiment, the tensile strength of the high strength steel sheet is 750
MPa to 2500 MPa. As explained above, if forming a spot welded joint including a high strength
steel sheet with a high tensile strength, for example, a tensile strength of 750 MPa or more,
despite using the high strength steel sheet, the cross tensile strength (CTS) of the spot welded
30 joint ends up decreasing.
[0029]
If the tsasile strength of the high strength steel sheet is less than '7-W MPa, basically the
cross tensile strength (CTS) is high and, further, the load on the spot welded joint is small.
Therefore, few problems arise relating to the joint strength. Accordingly, in the present
35 embodiment, the high strength steel sheet is made a steel sheet having a 750 MPa or more tensile
strength where the above-mentioned such problem is manifested.
9
[0030]
If the tensile strength of the high strength steel sheet exceeds 2500 MPa, it becomes
difficult to suppress a drop·and variation in joint strength. Furthermore, along with this,
deterioration of the fracture mode at the weld zone and occu-rrenc-e of defects and cracks insiae- --
5 the nugget become difficult to suppress. Accordingly; the tensile strength of the high strength
steel sheet is made 2500 MPa or less.
[0031]
Carbon Equivalent Ceq
In the present-embodiment, the carbon equivalent Ceq ofthe high strength steel sheet is
10 0.20 mass% to 0.55 mass% in range. If the carbon equivalent Ceq is less than 0.20 mass%, the
lower limit value 750 MPa or-more tensile strength of the tensile strength of the above--mentioned
high strength steel sheet tends not to be obtainable. On the other hand, if the carbon
equivalent Ceq exceeds 0.55 mass%, the tensile strength tends to exceed the upper limit value
2500 MPa of the tensile strength of the above-mentioned high strength steel sheet, so this is not
15 preferable. Note that, the carbon equivalent Ceq is expressed by the following formula (1):
Ceq=[C)+[Si]/30+[Mn ]/20+2[P]+4[S] · · ·(1)
(C], [Si], [Mn], (P], and (S] are respectively the contents ofC, Si, Mn, P, and S (mass%).
(0032)
1.2. Heat Affected Zone
20 The spot welded joint 1 according to the present embodiment is comprised including such
a high strength steel sheet but has excellent tensile strength, in particular, cross tensile strength
(CTS).
[0033]
As shown in FIG. 1, at the joined part of the spot welded joint 1, there is a nugget
25 (melted, then solidified part) 3 formed by melting, then solidification, due to spot welding.
Around the nugget, there is a heat affected zone (HAZ) 4 which is heated by the spot welding to
the Ac 1 point [°C] to less than the melting temperature and then cooled, resulting in the base
materials changing in structure and thereby forming a structure different from the structures of
the base materials.
30 [0034]
The spot welded joint 1 according to the present embodiment improves the structure of
the heat affected zone 4 and suppr~sses plug fracture occurring at- the heat affected zone along
with the increase in the tensile strength of the steel sheets forming the base materials to thereby
realize improvement of the CTS. Below, the improved structure of the heat affected zone 4 will
35 be explained in detail.
(0035)
10
The heat affected zone 4 is formed in the step where current is supplied to the steel sheets
IA and IB at the time of spot welding where the structures of the base materials are heated by
the resistance heat generated due to the supply of current, then are cooled. At the time of this
- ---- - - ------------------------
cooling, if the temperature of the heat affected zone 4 falls below the Ms point of the base
5 materials, a martensite structure starts to be formed in the structure of the heat affected zone 4.
[0036]
Therefore, the structure ofthe heat affected zone 4 ofthe spot welded joint I after the end
of spot welding is mainly comprised of a martensite structure. In the present embodiment, by
controlling this martensite structure, the structure of the heat affected zone 4 is improved,
I 0 fracture at the heat affected zone is suppressed, and improvement of the CTS is realized.
[0037]
Specifically, in a heat affected zone 4 having a high strength steel sheet as a base
material, there is a martensite structure. As shown in FIG. 2, in a martensite structure 10, fine
lath martensite having substantially the same crystal orientations gather together to form blocks
15 11. The blocks 11 have elongated shapes. As a parameter of size of the blocks 11, usually the
widths of the blocks ll (lengths in short side directions) are measured. In the present
embodiment, in a specific square area inside the heat affected zone 4, the blocks ll formed from
the lath martensite are relativel)llarge in widths. The average value is 0.5 to 7.0 J.UD., preferably 2
to 6 J..Lm.
20 [0038]
In the present embodiment, the martensite structure of the heat affected zone having a
high strength steel sheet as a base material is controlled and the widths of the blocks are
increased to the above range to thereby improve the toughness of the heat affected zone 4. As a
result, fracture at the heat affected zone is suppressed. Even in a spot welded joint including a
25 high strength steel sheet, the CTS can be improved.
[0039]
The range of the widths of the blocks does not have to be satisfied in the heat affected
zone 4 as a whole. It is sufficient that it be satisfied in a specific square area of the heat affected
zone. Therefore, when the widths of the blocks are within the above range, this means that the
30 structure is improved to an extent where the structure of the heat affected zone 4 as a whole can
contribute to improvement of the CTS, that is, to an extent where plug fracture at the heat
affected zone 4 is suppressed. Note that, the specific SEf.Yare area is determined as follows.
[0040]
Below, the case where in a spot welded joint comprised of a plurality of steel sheets
35 superposed, a high strength steel sheet is arranged at an outermost side and the case where a high
strength steel sheet is not arranged. at the outermost side will be explained separately. Usually,
1I
5
fracture occurs from a location including the steel sheet at an outermost side of the spot welded
joint. First, the case where a high strength steel sheet is arranged at an outermost side will be
explained.
- -[0041-]-------
FIG. 3A shows a cross-section in a spot welded joint obtained by superposing two steel
sheets which is cut along the sheet thickness direction of the steel sheet while passing through a
center of a welding mark able to be seen as a welded part. In FIG. 3A, the steel sheet 1A is the
high strength steel sheet S1, while the-steel sheet 1B is a steel sheet other than a high strength
steel sheet. Further, as clear from FIG. 3A as well, in the present embodiment, in a spot welded
10 joint obtained by superposing two steel sheets, the high strength steel sheet is always arranged at
an outermost side.
[0042]
In this cross-section, the superposed surfaces of the high strength steel sheet S 1 and the
other steel sheet 1B superposed on that high strength steel sheet Sl are defined as the "plane A".
15 Further, the plane passing through the point of one-half of a distance between the crossing point
E between the line L 1 in the sheet thickness direction passing through the center 0 of the nugget
Oand the line showing the end of the nugget at the high strength steel sheet S I side and tbe
crossing point of the plane' A and the line Ll (in FIG. 3A, the center 0 of the nugget) and
parallel to the plane A is defined as the "plane B". The end of the nugget sandwiched between
20 the plane A and plane B in the line showing the end of the nugget 3 is defined as the "nugget end
line NEL". In FIG. 3A, the nugget end line NEL is shown by a bold line.
[0043]
Assuming a tangent TL at any position on the nugget end line NEL prescribed in this
way, the crossing point X between the line L2 separated from this tangent TL to the heat affected
25 zone side by 250 J..Lm and parallel to that tangent TL and the line L3 passing through that any
position and vertical to the line L2 is found. Further, the square area SA of sides of 30 J..Lm
centered on this crossing point X may be made the area for measurement of the widths of the
blocks. This square area is included in the heat affected zone 4 having the high strength steel
sheet S 1 as a base material.
30 [0044]
Next, a spot welded joint obtained by superposing three or more steel sheets wherein a
high strength steel sheet is arranged at an outSR11ost side will be explained.
[0045]
FIG. 3B shows a cross-section of a spot welded joint obtained by superposing three steel
3 5 sheets which is cut along the sheet thickness direction of the steel sheets while passing through a
center of a welding mark able to be seen as a welded part. In FIG. 3B, the steel sheet 1A is the
12
high strength steel sheet S I, while the steel sheets I B and I C are steel sheets other than the high
strength steel sheet.
[0046]
---------- ---- ---rn-this- c-ross~-section, the -su-perposed s-urfaces of the high stre-ngth- ste_e_l_ sh-e-et-- s-1-- a-no--toe------
5 other steel sheet 1B superposed with that high strength steel sheet S1 is defined as the "plane A".
Further, the plane passing through the point of one-half of the distance between the crossing
pointE of the line L1 in the sheet thickness direction passing through the center 0 of the nugget
and the end of the nugget atthe high. strength steel sheet S1 side and the crossing point-e ufthe
plane A and the line L1 and parallel to the plane A is defined as the "plane B". In the line
10 showing the end of the nugget 3, the end of the nugget sandwiched between the plane A and
pl-ane B is defined as the "nugget end line NEL". In FIG. 3B-,the nugget end line NEL is shown
by a bold line.
[0047]
Assuming a tangent TL at any position on the nugget end line NEL prescribed in this
15 way, the crossing point X between the line L2 separated from this tangent TL to the heat affected
zone side by 250 f.liD and the line L3 passing through that any position and vertical to the line L2
is found. Further, the square area SA with sides of 30 f.ID-1. centered on this crossing point X may
be made the area·for measuring the widths of the blocks. This square area is contained in the heat
affected zone 4 having the high strength steel sheet S 1 as a base material.
20 (0048)
Next, a spot welded joint obtained by superposing three or more steel sheets wherein a
high strength steel sheet is not arranged at an outermost side will be explained.
[0049]
FIG. 3C shows a cross-section in a spot welded joint obtained by superposing three steel
25 sheets which is cut along the sheet thickness direction of the steel sheet while passing through
the center of a welding mark able to be seen as a welded part. In FIG. 3C, the steel sheet 1B is
the high strength steel sheet S1, while the steel sheets 1A and 1C are steel sheets other than the
high strength steel sheet.
30
[0050]
In this cross-section, in the. line showing the end of the nugget 3, the end of the nugget
having the high strength steel sheet S 1 as the base material is shown as the nugget end line NEL.
In FIG. 3C, the nugget end line NEb-is shown by the bold line. Assuming a tangent TL at anyposition
on this nuggetend line NEL, the crossing point X between the line L2 separated from
this tangent TL to the heat affected zone side by 250 J.lm and parallel to that tangent TL and the
35 line LJ passing through that any position and vertical to the line L2 is found. Further, the square
area SA of sides of 30 J.!m centered on this crossing point X may be made the area for
13
measurement of the widths of the blocks. This square area is included in the heat affected zone 4
having the high strength steel sheet S 1 as a base material.
[0051]
Note that, in the square area SA, the width of a block may be measured in the following
5 _ way. EBSD (Electron Back Scatter Diffraction) patterns of the corresponding portions in the
SEM are used to obtain an orientation map.
Further, from the K-S relation, the old austenite grain boundaries are judged. The smallest unit
with a difference of-orientation in the grains of a difference of 15° or more is-judged. The size of
the narrower width of that shape is defined as the "width of the block".
10 [0052]
15
The high strength steel sheet of the base material of the heat affected zone at which the
above width of the block is measured preferably satisfies the following requirements in addition
to the above-mentioned tensile strength and carbon equivalent.
(0053]
Sheet Thickness
The sheet thickness of the high strength steel sheet is not particularly limited, but for
example may be the sheet thickness of the high strength steel sheets generally used in car bodies
of automobiles ett:'"(O:s·mm to 3.2 mm) or so. However, along with an increase in the thickness
of the high strength steel sheet, the stress concentration around the nugget increases, so the
20 thickness of the high strength steel sheet is preferably 2.6 mm or less.
[0054]
Chemical Composition
It is sufficient to select a chemical composition enabling the tensile strength of the abovementioned
high strength steel sheet (750 MPa to 2500 MPa) to be secured. The steel member
25 after spot welding is mainly used in the automotive field etc. If considering this, the chemical
composition of the high strength steel sheet is preferably the following chemical composition.
Note that, below,"%" means mass%.
[0055]
C: 0.07 mass% to 0.45 mass%
30 C is an element raising the tensile strength of steel. The greater the content of C in the
steel, the higher the strength of the nugget can be made. However, if the content ofC in the steel
is less than 0.07 mass%, a '7~0 MPa or more tensile strength is difficult to obtain. On -t-he other
hand, if the content ofC in the steel exceeds 0.45 mass%, the workability of the high strength
steel sheet falls. Therefore, the content ofC of the high strength steel sheet is preferably 0.07
35 mass% to 0.45 mass%.-
[0056]
14
Si: 0.001 mass% to 2.50 mass%
Si is an element raising the strength of steel due to solution strengthening and structure
strengthening. However, if the content of Si in steel exceeds 2.50 mass%, the workability of the
steelfalls.-E>n-the other hand, reducingthe content ofSi in the steel industrially-to less thanO~OOt ---
5 mass% is technically difficult. Therefore, the content of Si of the high strength steel sheet is
preferably 0.001 mass% to 2.50 mass%.
[0057]
Mrr:-0.8 mass% to 5.0 mass%
Mn is an element raising the strength of steel. However, if the content of Mn in steel
10 exceeds 5.0 mass%, the workability of the steel falls. On the other hand, if the content ofMn in
steel is less than 0.8 mass%, it is difficult to-obtain a 750 MPa or more tensile strength.
Therefore, the content of Mn of the high strength steel sheet is preferably 0.8 mass% to 5.0
mass%.
[0058]
15 P: 0.03 mass% or less
P is an element making a nugget brittle. If the content of P in steel exceeds 0.03 mass%,
the inside of the nugget easily crackS and it is difficult to obtain a sufficiently high joint strength.
Therefore, the corttenf'ofP of a high strength steel sheet is preferably 0.03 mass% or less. Note
that reducing the content ofP in steel to less than 0.001 mass% is not preferable cost-wise.
20 Therefore, the content ofP of the high strength steel sheet is preferably 0.001 mass% or more.
However, the content of P of the high strength steel sheet may also be made less than 0.001
mass%.
[0059]
S: 0.01 mass% or less
25 S is an element making a nugget brittle. Further, S is an element bonding with Mn to
form coarse MnS which obstructs the workability of steel. If the content of S in steel exceeds
0.01 mass%, the inside of the nugget easily cracks and it is difficult to obtain a sufficiently high
joint strength. Further, the workability of the steel falls. Therefore, the content ofS of the high
strength steel sheet is preferably 0.01 mass% or less. Note that reducing the content of Sin steel
30 to less than 0.0001 mass% is not preferable cost-wise. Therefore, the content ofS ofthe high
strength steel sheet is preferably 0.0001 mass% or more. However, the content ofS of the high
strength steel sheet-may also be made less than 0.0001 mass%.
35
[0060]
N: 0.01 mass% or less
N is an element forming coarse nitrides and degrading the workability of steel. Further, N
is an element causing the formation of blowholes at the time of welding. If the content ofN in
15
steel exceeds 0.01 mass%, degradation of the workability of the steel and formation of blowholes
become remarkable. Therefore, the content ofN of a high strength steel sheet is preferably 0.0 I
mass% or less. Note that reducing the content ofN in steel to less than 0.0005 mass% is not
prefer-;ble -~-~-~1-=-~i~e. Therefore,-the contentoCN of the high strength steel sheet is preferably _________ _
5 0.0005 mass% or more. However, the content ofN of the high strength steel sheet may also be
made less than 0.0005 mass%.
(0061]
0: 0.01 mass% or less
0 is an element forming oxides and degrading the workability of steel. If the content of 0
10 in steel exceeds 0.01 mass%, the deterioration ofthe workability of steel becomes remarkable.
Therefore, the content ofO of the high strength steel sheet is preferably 0.01 mass% or less-;-Note
that reducing the content ofO in the high strength steel sheet to less than 0.0005 mass% is
not preferable cost-wise. Therefore, the content of 0 of the high strength steel sheet is preferably
0.0005 mass% or more. However, the content ofO of the high strength steel sheet may also be
15 made less than 0.0005 mass%.
[0062]
AI: 1.00 mass% or less
AI is an element stabilizing ferrite and has the effect of suppressing the precipitation of
cementite at the time of bainite transformation. For this reason, it is included for control of the
20 steel structure. Further, Al also functions as a deoxidizing material. On the other hand, AI easily
oxidizes. If the content of Al exceeds 1.00 mass%, the inclusions increase, whereby the steel
easily degrades in workability. Therefore, the content of Al of the high strength steel sheet is
preferably 1. 00 mass% or less.
25
30
(0063]
The high strength steel sheet may also selectively contain the following elements
according to need in addition to the above main elements:
[0064]
Ti: 0.005 mass% to 0.20 mass%
Nb: 0.005 mass% to 0.20 mass%
V : 0.005 mass% to 0.20 mass%
Ti, Nb, and V are elements which contribute to the rise in strength of steel due to at least
one of prSGipitation strengthening,. fine grain strengthening by sup~ssion of growth of ferrite
crystal grains, and dislocation strengthening by suppression of recrystallization. However, in
each of these elements, ifthe content in the high strength steel sheet is less than 0.005 mass%, it
35 is difficult for the effect of addition to be manifested. On the other hand, if the content in steel
exceeds 0.20 mass%, the workability of steel is obstructed. Therefore, the contents of these
16
elements in the high strength steel sheet are in all cases preferably 0.005 mass% to 0.20 mass%.
[0065]
B: 0.0001 mass% to 0.01 mass%
B-is-an elem-ent-controlling the steel structure and strengthening steel. However, ifth-e
5 content ofB in steel is less than 0.0001 mass%, it is difficult for the effect of addition to be
manifested. On the other hand, ifthe content ofB in steel exceeds 0.01 mass%, the effect of
addition become saturated. Therefore, the content of B in the high strength steel sheet is
preferably 0.0001 mass% to 0.01 mass%.
[0066]
10 Cr: 0.01 mass% to 2.0 mass%
Ni: 0.01 mass%-t0- 2.0 mass%
Cu: 0.01 mass% to 2.0 mass%
Mo: 0.01 mass% to 0.8 mass%
Cr, Ni, Cu., and Mo are elements contributing to improvement of the strength of steeL
15 These elements can be used in place of: for example, part of the Mn (strength improving
element). However, in each of these elements, if the content in the high strength steel sheet is
less than 0.0 l mass%, there is no contribution to improvement of strength.
[0067)
Therefore, the contents of these elements in the high strength steel sheet are in all cases
20 preferably 0.0 l mass% or more. On the other hand, if the contents of Cr, Ni, and Cu in steel
exceed 2.0 mass% and if the content of Mo in steel exceeds 0.8 mass%, obstacles arise at the
time of pickling or at the time of hot working. Therefore, the contents of Cr, Ni, and Cu in the
high strength steel sheet is preferably 2.0 mass% or less. Further, the content of Mo in the high
strength steel sheet is preferably 0.8 mass% or less.
25 [0068]
At least one type ofCa, Ce, Mg, and REM (rare earth metal): Total of0.0001 mass% to
1.0 mass%
Ca, Ce, Mg, and REM are elements reducing the size of oxides after deoxidation or the
size of sulfides present in hot rolled steel sheet and contributing to improvement of the
30 workability of steel. However, ifthe contents of these elements in steel is in total less than
0.0001 mass%, it is difficult for the effect of addition to be manifested. On the other hand, ifthe
--GOntents of these elements in steel in total exceeds 1.0 mas-s!»/o, the workability of the steel falls·.
Therefore, the contents of these elements in the high strength steel sheet are preferably in total
0.0001 mass% to 1.0 mass%.
35 [0069]
Note that, an REM is an element belonging to the lanthanide series. An REM and Ce can
17
be added to molten steel as mischmetal at the steelmaking stage. Further, in addition to La and
Ce, lanthanoid series elements may also be included compositely.
[0070]
The balance in a high strength steel sheet other than the above elements may be Fe and
5 unavoidable impurities. Note that the above-mentioned Cr~ Ni, Cu~ Mo, B~ Ti~ Nb~ and V are
allowed to be included in trace amounts of less than the above lower limit values as impurities.
Further, Ca, Ce, Mg, La, and REM are also allowed to be included in trace amounts of less than
the above lower limit values of the total-content as impurities.
10
[0071]
Plating
A plating layer may also be formed on the surface of the high strength steel sheet.
Further, a plating layer may also be formed on the surface of a steel sheet superposed on the high
strength steel sheet. As the type of the plating layer, for example~ a Zn-based, Zn-Fe-base~ ZnNi-
based, Zn-Al-based~ Zn-Mg-base~ Pb-Sn-based, Sn-Zn-base~ Al-Si-based~ or other layer
15 may be mentioned.
[0072]
As the high strength steel sheet provided with a Zn-based plating layer, for example, a
hot dip galvannealed ~I sheet, hot dip galvanized steel sheet, electrogalvanized steel sheet, etc.
may be mentioned. If a plating layer is formed on the surface of the high strength steel sheet, the
20 spot welded joint exhibits. excellent corrosion resistance. When the plating layer is a
galvannealed layer on the surface of the high strength steel sheet, an excellent corrosion
resistance is obtained an~ further, paint adhesion becomes excellent.
[0073]
The basis weight of the plating layer is not particularly limited. It is preferable to make
25 the basis weight of the plating layer at one surface of the high strength steel sheet 100 g/m2 or
less. If the basis weight at one surface of the high strength steel sheet exceeds 100g/m2,
sometimes the plating layer becomes an obstacle at the time of welding. The plating layer may
be formed on only one surface of the high strength steel sheet or may be formed on both
surfaces. Note that, the surface of the plating layer may be formed with an inorganic or organic
30 film (for example, a lubrication film etc.) etc.
[0074]
1.3. Steel Sheet Other Than High Strength-Steel Sheet
The type of a steel sheet superposed with the high strength steel sheet (steel sheet other
than the high strength steel sheet). is not particularly limited. It may be a steel type different from
35 the steel type of the high strength steel sheet or may be the same steel type. As a steel sheet ·other
than the. high strength steel sheet, for example, a mild steel sheet may be illustrated.
18
[0075]
The tensile strength of a steel sheet other than the high strength steel sheet is not
particularly limited. In the case of a steel member used in the automotive field etc., the tensile
-strengtninay-oe selected in accordance with-the steel member used. For example, the steersn-eeT-
5 may be made one with a tensile strength of less than 750 MPa.
[0076]
10
The carbon equivalent Ceq of a steel sheet superposed with the high strength steel sheet
is not particularly limited.
[0077]
The thickness of a steel sheet other than the high strength steel sheet is not particularly
limited-.-The thicknesses of the superposed plurality of steel sheet-s- may also be different from
each other. Note that, generally, the thickness of a steel sheet is 6 mm or less.
[0078]
The chemical composition of a steel sheet other than the high strength steel sheet is not
15 particularly limited. Further, a stee~ sheet other than the high strength steel sheet may also be
plated. The conditions relating to the plating layer are the same as the case of the high strength
steel sheet.
20
[0079]
2. Spot Welding Method
Below, the optimal spot welding method for obtaining the above spot welded joint will be
explained in detail.
(0080)
The spot welding method according to the present embodiment has a main welding step
performed for supplying current between steel sheets and joining the steel sheets and a heating
25 control step performed for controlling the structure of the heat affected zone after the main
welding step.
[0081]
2.1. Main Welding Step
FIG. 4 is a view showing one example of the arrangement of two steel sheets, including
30 at least one high strength steel sheet, and the welding electrodes when starting spot welding. As
shown in FIG. 4, first, the steel sheets 1A and 1B are superposed so that the sheet surfaces face
each other. Next, the superposed steel sh~ets 1A and lB are sandwiched from above and below
by welding electrodes 2A-and 2B while supplying current through the welding electrodes 2A and
2B and, if necessary, applying pres·sure. Due to the resistance heat generated by the supply of
35 current, the parts of the steel'sheets lA and 1B supplied with current melts and the steel sheet 1A
and the steel sheet 1B are integrally joined. Right after the main welding ends, the melted part is
19
cooled and the assembly starts to solidify from the outer circumference. When the melted part
completely solidifies, the result becomes the nugget 3. Further, around the nugget 3, a heat
affected zone 4 with a structure of the base material changed due to the resistance heat is formed.
That is, after the main welding ends, the nugget 3 and heat affected zone 4 such as shown in FIG.
5 1 are formed and the steel sheet 1A and steel sheet 1B are integrally joined. Note that, in the
present embodiment, in FIG. 1, one or both of the steel sheets 1A and 1B are the abovementioned
high strength steel sheet.
(0082]
The welding conditions in the main welding step (weld current value, current supply time
10 (weld time)) are not particularly limited and may be set according to the thicknesses of the steel
-- sheets, the desired nugget size, etc. Further, if applying-pressure at the time of welding, the
pressing force may also be suitably set in accordance with the welding conditions etc.
(0083]
Regarding the spot welding equipment, the conventional general spot welding equipment
15 can be used as is. Further, for the welding electrodes etc. as well, the conventional welding
electrodes can be used as is. The power supply is not particularly limited. An AC power supply,
DC inverter, AC inverter, etc. may be used.
[00841
2.2. Heating Control Step
20 In the. present embodiment, a heating control step is performed after the main welding
step. In this heating control step, heat treatment supplying heat to the square area shown in FIG.
3A to FIG. 3C to hold it at a temperature within a predetermined temperature range for a
predetermined time is performed to control the structure of the heat affected zone 4. At this time,
that square area has to be maintained at a temperature over the Ms point of the high strength steel
25 sheet S1 until that heat treatment ends.
[0085]
Specifically, this heat treatment is treatment for holding the above square area at a
temperature over the Ms point of the high strength steel sheet S 1 after the end of the main
welding while making the time of holding the assembly at a temperature in the range of the Ar3
30 point [°C] ofthe high strength steel sheet S1 to (Ar3+400) [°C] a total of0.04 second to 5.0
seconds. Preferably, it heats the assembly to 1000 [°C] to 1250 [°C] in range and makes the time
holding the assembly in this range a total of 0.04 second to 5.0 seconds. Further, it is prefsrable
to heat the assembly to the Ar3 point [°C] to the (Ar3 point+400) [°C] in range and make the
time holding the assembly. in this range a total of0.2 second to 2.0 seconds. Furthermore, it is
35 more preferable to heat the assembly to 1050 [°C] to 1200 [°C] in range and make the time
holding the assembly in this range a total of 0.6 second to 1.0 second. By performing such heat
20
5
treatment, the martensite structure of the heat affected zone after cooling is controlled and it
becomes easy to enlarge the widths of the blocks of groups of lath martensite with the same
crystal orientations to the above-mentioned range .
. [0086]- - -- -
If satisfying the above heat treatment conditions, in the range of the Ar3 point [°C] to the
(Ar3 point +400) [°C], the temperature may be constant or may fluctuate.
[0087]
Further, if a temperature of over the Ms point is maintained afterihe end of the main
welding, the temperature of the square area shown in FIG. 3A to FIG. 3C may fall below the Ar3
10 point until the time of holding the assembly at a temperature inside the range of the Ar3 point
[°C] to (Ar3 point+400) [°C] becomes a total of 0.04 second to 5.0 seconds.
[0088]
Furthermore, right after the end of the main welding, it is preferably to hold the assembly
at a temperature in the range of the Ar3 point [°C] to the (Ar3 point+400) [°C] and make the
15 holding time 0.04 second to 5.0 seconds without allowing that temperature to become outside
that range. This is because after the end of the main welding, by holding the assembly at the Ar3
point (°C) to the (ArJ point+400) roc) without once making the temperature of the square area
lower than the Ar3 point (°C]~ the block widths can easily be made larger.
20
[0089]
Note tha~ if the temperature of the square area shown in FIG. 3A to FIG. 3C exceeds the
(Ar3 point+400) [°C], there is a possibility of the part of the nugget 3 near the part supplied with
current remelting, so the temperature of the square area is preferably not more than the (Ar3
point+400) [°C].
[0090]
25 FIG. 5 shows the temperature histories applied to the square area shown in FIG. 3A to
FIG. 3C in the heating control step. In FIG. 5, the temperature histories shown by THI and TH2
satisfy the above-mentioned heat treatment conditions. The martensite structures after cooling
can be controlled and the average values of the widths of the blocks can be made ones within the
above range. Note that, in the temperature history shown by TH2, sometimes the temperature
30 becomes lower than the Ar3 point temperature, but it will never become lower than the Ms point
temperature until the time for holding the assembly at a temperature in the range of the Ar3 point
[°C] to the (Ar3 point4=4·00) [°C] becomes a total of0.04 second to 5.0 seconds.+herefore, the
temperature history shown by TH2 satisfies the above-mentionedheat treatment conditions and
enables the effects of the present invention to be manifested.
35 [0091]
\.
On the other hand, in the temperature history shown by TH3, before the "heat treatment
21
making the time for holding the assembly at a temperature in the range of the Ar3 point [°C] to
(Ar3 point+400) [°C] a total of0.04 second to 5.0 seconds" ends, the temperature ofthe square
area falls to a temperature of the Ms point or less, so after that even if satisfying the "heat
treatment making the time for holding the assembly at a temperature in the range of the Ar3
5 point [°C] to (Ar3 point+400) [°C] a total of0.04 second to 5.0 seconds", the desired martensite
structure is not obtained.
(0092]
- In the present embodiment, the Ar3 point temperature may be calculated using the
following formula (2):
10 Ar3=902-527(C]-62[Mn ]+60(Si] · · · (2)
wherein, (C], [Mn], and [Si] respectively are contents (mass%) of carbon, manganese, and
silicon.
[0093]
After the heating control step ends, the joined assembly of the steel sheet 1A and steel
15 sheet 1B is cooled whereby a spot welded joint according to the present embodiment is obtained.
(0094)
The cooling after the heating control step is not particularly limi~ but may be made
equal to the coolirilfiri ·the' conventional spot welding method. Note that, after satisfying the
above-mentioned heat treatment, the cooling rate down to the Ms point preferably is made a
20 cooling rate where no pearlite transformation or bainite transformation etc. occurs. The cooling
rate from the Ms point on is preferably relatively slow .
. [0095)
In the above heating control step, the heating method is not limited so long as a method
enabling realization of the above heat treatment. However, in the above heat treatment, after the
25 main welding, from the viewpoint of the need to heat the assembly while maintaining a
temperature over the Ms point of the high strength steel sheetS 1, ease of temperature control,
work efficiency, etc., the heating control step is preferably performed by later supply of current
supplying current after the main welding. The conditions for later supply of current should be set
so as to satisfy the above heat treatment.
30 [0096]
3. Effects of Present Embodiment
In the-above embodiment, while a spot welded joint containing~ne or more high strength
steel sheets, by controlling the structure of not only the nugget but also the heat affected zone, it
is possible to make the widths of the blocks of the martensite structure within the above range
35 and, as a result, it is possible to suppress low load fracture at the heat affected zone and improve
the CTS.
22
[0097]
To obtain such a structure, after the main welding, it is sufficient to perform heat
treatment holding the temperature of a specific area of the heat affected zone in the abovementioned
range for the above-mentioned time. At this, in the period after the main welding to
5 when the heat treatment ends, the temperature of the square area has to be made a temperature of
over the Ms point of the high strength steel sheet. Due to such heat treatment, the widths of the
blocks in the martensite structure after cooling become larger and the average value becomes
within the above-mentioned range.
10
[0098]
4. Modification
In the above-mentioned embodiment, after the main welding step, a heating step- is
performed, but a pre-current supplying step may be performed before the main welding step as
well. By performing the pre-current supplying step, the steel sheet is made to soften and, in the
main welding step, the "spattering" occurring due to the inside pressure of the molten metal
15 exceeding the outside pressure acting on the corona bond can be suppressed.
[0099)
Above, an embodiment of the present invention was explained, but the present invention
is not limited in any way to the· move-mentioned embodiment and can be changed in various
ways within a scope not departing from the gist of the present invention.
20 EXAMPLE 1
25
[0100]
Below, the present invention will be explained based on more detailed examples, but the
present invention is not limited to these examples.
(0101]
Steel sheets A, B, C, D, E, and F shown in Table 1 were prepared. Note that, the carbon
equivalent "Ceq" in Table 1 is the value calculated by the formula (1 ), while the Ar3 point
shown in Table 1 is the value calculated by the formula (2). The steel sheets A to F are steel
sheets containing the above-mentioned chemical compositions within the ranges of the abovementioned
upper and lower limits.
30 [0102]
23
[Table I]
Steel Steel type Tensile Sheet Ceq[%] Ar3 [°C] Plating
sheet strength thickness
[mm]
A Cold rolled 980 MPa 1.6 1- 0.41 740'' None
steel sheet class
B Cold rolled 1180 MPa 1.2 0.42 720 Zinc type
-
steel sheet class
c Hot rolled 980 MPa 2.6 0.31 820 None
steel sheet class
D Hot stamp 1470 MPa 1.8 0.34 850 Aluminum
class
E Hot stamp 2000 MPa 1.4 0.45 710 None
class
F Cold rolled 270 MPa 0.8 0.05 880 Zinc type
steel sheet --elass
(0103)
In the combinations shown in Table 2, two or three steel sheets were superposed and
5 welded by main welding using a servo gun type welding machine~ then the heat treatment shown
in Table 2 was performed in the heating control step. The heat treatment, in the same way as the
main welding, was controlled using a servo gun type welding machine so that the square area
shown in FIG. JAto FIG. JC was heated under predetermined conditions.
(0104)
I 0 In the heat treatment of Table 2, if that square area is heated so as to satisfy the
temperature range prescribed in the present invention (Ar3 point [°C] to (Ar3 point+400)[°C]),
"S" is shown in the column of "Temperature range"~ while if not satisfying it, "NS" is shown.
Further, if that square area is heated so as to satisfy the holding time prescribed in the present
invention at the predetermined temperature range (0.04 second to 5.0 seconds), "S" is shown in
15 the column of "Holding time", while if not satisfying it, "NS" is shown. If the square area is
maintained at the temperature of the Ms point or more after the main welding until the heat
treatment is finished~··"S'~'·is shown in the column of"Holding at Ms point or more", while if not
satisfying it, "NS" is shown. That is, if all of the columns of "Temperature range", "Holding
time", and "Holding at Ms point or more" are "S", the joint is in the scope of the present
20 invention, while if even one is "NS", it is outside the scope of the present invention.
[0105]
The obtained spot welded joint was measuregJor block widths in the square area shown
in FIG. 3A to FIG. 3C and the average--value was calculated. The block widths were measured bythe
above-mentioned method. The measurement results are shown in Table 2.
25 [0106]
Further, the method prescribed in JIS Z 3137 was used to measure the CTS (cross tensile
24
strength) of the spot welded joints. In the case of the combination of three or more steel sheet
(Sample Nos. 13 to 16 and 20 to 22), for Sample Nos. 13, 14, and 20 to 22, the CTS's at the
combinations where fracture easily occurred ("A-B" and "F-B") were measured, while for
-- -
Sample Nos. 15 and 16, the CTS's of both of"A-F" and "B-F" were measured. The CTS's of the
5 spot welded joints of the same combinations were calculated as the rate of improvement of the
CTS with the results of measurement indexed to the CTS of a spot welded joint just welded by
main welding and not heat treated (as 100%). The results of measurement are shown in Table 2.
In the present examples, a sample with a rate of improvement of the CTS of 150% or more was-judged
to be good.
10 (0107]
Further, a SEM photograph showing the martensite structure-at a square area of the spot
welded joint of an invention example of Sample No. 2 is shown in FIG. 6, while a SEM
photograph showing the martensite structure at a square area of the spot welded joint of a
comparative example of Sample No.5 is shown in FIG. 7.
15 [0108]
25
[Table 2]
Heating step Properties
Combination
Heat treatment Block
CTS
Test y- _ _ Holding width -- --of-steel emp- CTS improveno.
sheets t Holding at Ms average
era ure f . t [kN] value
ment
tme pom [%] range
or more [Jlm]
1 A-A No later heat treatment 0.3 5.2 100
2 A-A s s s 2.1 10~8 208
3 A-A s s NS 0.4 5.4 104
4 A-A s NS s 0.3 5.7 110
5 A-A NS s s 0.2 4.5 87
6 A-A s NS NS 0.4 5.7 110
7 B-C No later heat treatment 0.3 4.3 100
- 8 B-C s s s 2.8 7.9 184
9 B-C NS NS s 8.5 3.8 88
10 D-D No later heat treatment 0.2 4.7 100
11 D-D s s s 3.5 12.5 266
12 D-D s NS s 0.4 5.8 123
13 A-B-F No later heat treatment 0.4
5.7(AB
100
interface)
14 A-B-F s s s 0.9 8.7 (AB
153
interface)
0.3(8)
7.2(F-B
100
interface)
15 A-F-B No later heat treatment
8.5(F-A
0.4(A) 100
interface)
4.5
12.5(F-B
174
16 A-F-B s s s interface)
3.8
l5.8(F-A
186
interface)
17 E-E No later heat treatment 0.3 3.7 100
18 E-E s s s 3.5 8.9 241
19 E-E s NS s 0.4 4.2 114
20 F-B-F No later heat treatment 0.2
7.2(F-B
100
interface)
21 F-B-F s s s 2.4
12.8(F-B
178
interface)
22 F-B-F s NS NS 0.2
7.6(F-B
108
interface)
[0109]
From Table 2, it could be c--enfirmed that while the block widths in the square area of-a-
5 spot welded joint which was not heat treated were small, as shown in FIG. 6, the block widths in
the square area of a spot welded joint which was heat treated as prescribed by the present
invention became large and, as a result, the CTS was greatly improved.
[0110]
26
------ --
Further, it could be confirmed that even if performing heat treatment, if the heat treatment
prescribed by the present invention was not satisfactory, the block widths became outside the
scope of the present invention and as a result the rate of improvement of the CTS was not
sufficient. Note that, Sample No. 9 had too large a size of the blocks deemed the units of fracture
5 (average value of widths of blocks), so the fracture toughness fell. As a result, the CTS became
lower than the reference.
INDUSTRIAL APPLICABILITY
(0111]
According to the present invention, it is possible to obtain a spot welded joint sufficiently
10 high in joint strength and high in reliability. Accordingly, the present invention has high
industrial applicability in industries using spot welding as a manufacturing technique.

CLAIMS
Claim 1. A spot welded joint formed by superposing and spot welding a plurality of steel
sheets, wherein
at least one steel sheet of the plurality of steel sheets is a high strength steel sheet
5 with a tensile strength of 750 MPa to 2500 MPa;
a carbon equivalent Ceq of the high strength steel sheet expressed by the
following formula (A) is 0.20 mass% to 0.55 mass%;
in the superposing of the plurality of steel sheets, the-high strength steel sheet is
arranged at an outermost side, the high strength steel sheet arranged at the outermost side being
10 defined as a "high strength steel sheetS 1 ",
in a cross-section cut along-the sheet thickness direction of the steel sheet while
passing through a center of a welding mark formed on the surface of the steel sheet due to the
spot welding,
when defining superposed surfaces of the high strength steel sheet S 1 and another
15 steel sheet superposed with that high strength steel sheet S 1 as a "plane A" and
when defining a plane passing through a point of one-half of a distance between a
crossing point of a line L 1 in the sheet thickness direction passing through the center of the
nugget and a line showing the'end of the nugget at the high strength steel sheetS 1 side and a
crossing point of the plane A and the line L1 and parallel to the plane A as a "plane B",
20 a square area, which has sides of 30 J.liD centered about a crossing point of a line
L2 separated from a tangent at any position on a nugget end line NEL sandwiched between the
plane A and the plane Bin the line showing the end of the nugget by 250 J.1l1l to a heat affected
zone side and parallel to that tangent and a line L3 passing through the any position and vertical
to the line L2, is contained in a heat affected zone having the high strength steel sheet S 1 as a
25 base material; and
in the square area, an average value of widths of blocks comprised of lath
martensite is 0.5 to ·7.6 ~m: ·
Ceq=[C]+[Si]/30+[Mn]/20+2[P]+4[S] ···(A)
in the formula (A), [C], [Si], [Mn ], [P], and [S] respectively being contents of C,
30 Si, Mn, P, and S (mass%).
Claim 2.-A spot welded joint formed by superposing and spot weldiBg a plurality of steel
sheets, wherein
at least one steel sheet of the plurality of steel sheets is a high strength steel sheet
35 with a tensile strength of750 MPa to 2500 MPa;
a carbon equivalent Ceq of the high strength steel sheet expressed by the
28
following formula (A) is 0.20 mass% to 0.55 mass%;
in the superposing of the plurality of steel sheets, the high strength steel sheet is
not arranged at an outermost side, the high strength steel sheet not arranged at the outermost side
-------------
being defined as a "high strength steel sheetS I",
5 in a cross-section cut along the sheet thickness direction of the steel sheet while
passing through a center of a welding mark formed on the surface of the steel sheet due to the
spot welding,
a square area, which has sides of 30 J..Lm centered about a crossing point of a line
L2 separated from a tangent at any position on a nugget end line NEL showing the end of the
10 nugget having the high strength steel sheet S 1 as a base material in the line showing the end of
the nugget by 250 J..Lm to a heat-affected zone side and parallel to that tangent and a line L-3-
passing through the any position and vertical to the line L2, is contained in a heat affected zone
having the high strength steel sheet S 1 as a base material; and
in the square area, an average value of widths of blocks comprised of lath
15 martensite is 0.5 to 7.0 J.Lm:
20
Ceq=[C]+[Si]/30+[Mn]/20+2[P]+4[S] ···(A)
in the fonnula (A), [C], (Si], [Mn], [P], and (S] respectively being contents ofC,
Si, Mn, P, and S (masSOio)."
Claim 3. A spot welding method superposing and spot welding a plurality of steel sheets,
wherein
at least one steel sheet of the plurality of steel sheets is a high strength steel sheet
with a tensile strength of750 MPa to 2500 MPa;
a carbon equivalent Ceq of the high strength steel sheet expressed by the
25 following formula (A) is 0.20 mass% to 0.55 mass%; and
30
in the superposing of the plurality of steel sheets, the high strength steel sheet is
arranged at an outermost side, the high strength steel sheet arranged at the outermost side being
defined as a "high strength steel sheet S 1 ",
the method comprising
a main welding step of supplying current to the superposed plurality of steel
sheets through welding electrodes; and
a heating control step comprising, after the main w-@-!ding step, maintaining the
assembly at a temperature ofthe Ms. point ofthe high strength st~el sheet S1 or more while, in a
cross-section cut along the sheet thickness direction of the steel sheet while passing through a
35 center of a welding mark formed on the surface of the steel sheet due to the main welding step,
when defining superposed surfaces of the high strength steel sheet S 1 and another
29
steel sheet superposed with that high strength steel sheet S 1 as a "plane A" and
when defining a plane passing through a point of one-half of a distance between a
crossing point of a line L 1 in the sheet thickness direction passing through the center of the part
------- ------------------------------ --------- -
to form a nugget and a line showing the end of the part to form the nugget at the high strength
5 steel sheet S 1 side and a crossing point of the plane A and the line L 1 and parallel to the plane A
as a "plane B",
controlling a square area contained in a heat affected zone having the high
strength steel sheet S 1 as a base material, which-square area has sides of 30 J.lm centered about a
crossing point of a line L2 separated from a tangent at any position on a nugget end line NEL
10 sandwiched between the plane A an.d the plane Bin the line showing the end of the part for
forming the nugget by--l-50 f.!m to a heat affected zone side and parallel to that tangent and a line
L3 passing through the any position and vertical to the line L2, to be heated in a range of an Ar3
point [°C] ofthe high strength steel sheet S1 to (Ar3 point+400)[°C] for 0.04 second to 5.0
seconds:
15 Ceq=[C]+[Si]/30+[Mn ]/20+2[P]+4[S] ···(A)
in the formula (A), [C], [Si], [Mn ], [P], and [S] respectively being contents of C,
S~ Mo., P, and S (mass%).
Claim 4. A spot welding method superposing and spot welding a plurality of steel sheets,
20 wherein
25
at least one steel sheet of the plurality of steel sheets is a high strength steel sheet
with a tensile strength of 750 MPa to 2500 MPa;
a carbon equivalent Ceq of the high strength steel sheet expressed by the
following formula (A) is 0.20 mass% to 0.55 mass%; and
in the superposing of the plurality of steel sheets, the high strength steel sheet is
not arranged at an outermost side, the high strength steel sheet not arranged at the outermost side
being defined as a "high strength steel sheet S 1 ",
the method comprising
a main welding step of supplying current to the superposed plurality of steel
30 sheets through welding electrodes; and
35
a heating control step comprising, after the main welding step, maintaining the
-assembly at a temperature of the Ms ·point of the high stnmgth steel sheet S 1 or more while, in a
cross-section cut along the sheet thickness direction of the steel sheet while passing through a
center of a welding mark formed on the surface of the steel sheet due to the main welding step,
controlling a square area contained in a heat affected zone having the high
strength steel sheet S 1 as a, base material, which square area has sides of 30 J.lm centered about a
30
crossing point of a line L2 separated from a tangent at any position on a nugget end line NEL
showing the end of the par for forming the nugget having the high strength steel sheet S I as a
base material in the line showing the end of the part for forming the nugget by 250 J-Lm to a heat
affected zone side and parallel to that tangent and a line L3 passing through the any position and
5 vertical to the line L2, to be heated in a range of an Ar3 point [°C] of the high strength steel sheet
Sl to (Ar3 point+400)[°C] for 0.04 second to 5.0 seconds: