Specification
[Title of the Invention] SPOT WELDED JOINT
[Technical Field of the Invention]
[OOOl]
The present invention relates to a spot welded joint formed by overlapping
steel sheets which is used in a vehicle field and the like and which has a tensile
strength of 750 MPa to 2500 MPa.
Priority is claimed cin Japanese Patent Application No. 2012-100324, filed on
April 25,2012, the content of which is incorporated herein by reference.
[Related Art]
[0002]
Recently, in a vehicle field, a reduction in weight of a vehicle body for
reduction of fuel consumption and C02 emissions, or high rigidity of the vehicle body
for improvement in collision safety has been in demand. To satisfy the demand, it is
effective to use a high-strength steel sheet in the vehicle body or components.
However, spot welding is mainly used for assembly of the vehicle body, attaching of
the components, and t h l~ike . Strength in a joint portion is always problematic in the
spot welding of the steel sheet (high-strength steel sheet) having a high tensile strength.
[0003]
In a joint (also, referred to as a "spot welded joint") that is formed by
overlapping steel sheets and performing spot welding, a tensile strength and a fatigue
strength are important characteristics. Particularly, a tensile shear strength (TSS) that
is obtained by performing measurement by applying a tensile load in a shear direction,
and a cross tension strength (CTS) that is obtained by a cross tensile test in which
measurement is performed by applying a tensile load in a peeling direction are
important in the spot welded joint. In addition, methods of measuring the TSS and
the CTS are defined in JIS Z 3 136 and JIS Z 3 137, respectively.
[0004]
In a steel sheet in which the tensile strength is equal to or higher than 270
MPa and lower than 750 MPa, the CTS of the spot welded joint increases along with
an increase in the tensile strength of the steel sheet, and thus a problem related to the
strength of the spot welded joint is less likely to occur. However, in a steel sheet in
which the tensile strength exceeds 750 MPa, even when the tensile strength of the steel
sheet increases, the CTS does not increase or a value of the CTS conversely decreases,
and thus the strength of the spot welded joint may be deficient in some cases.
LO0051
In general, in a case of a high-strength steel sheet, a degree of stress
concentration on a nugget increases due to a decrease in deformability of the steel
sheet, and in addition to this, toughness of a nugget decreases by quench hardening,
and thus the CTS tends to decrease. Therefore, improvement of the CTS has been
demanded in the spot welded joint of the high-strength steel sheet having a tensile
strength of 750 MPa or higher.
[0006]
With regard to the spot welded joint of the high-strength steel sheet, as a
method of securing the strength and the toughness, a two-step energizing method in
which heating energizing is performed after main energizing may be exemplified.
For example, Patent Document 1 discloses a method in which tempering energizing is
performed after the passage of a predetermined amount of time fiom completion of the
main energizing, and a nugget portion and a heat-affected zone of the spot welded joint
are annealed to decrease hardness of the spot welded joint. In addition, Patent
Document 2 discloses a method in which after forming a nugget by main energizing,
post heating energizing is performed with a current value that is equal to or higher than
a main energizing current value. In a spot welded joint that is lormed based on the
above-described methods, satisfactory characteristics are shown in comparison to a
spot welded joint formed by single step energizing in which energizing is not
performed after main energizing.
[0007]
However, these methods have a problem in which producticity is deteriorated
by lengthening a post heating energizing time, a problem in which peeling fracture
tends to occur inside the nugget due to softening of the nugget, and a problem in which
a stable high joint strength is not obtained.
[OOOS]
In addition, as a method of securing the strength and the toughness in the spot
welded joint of the high-strength steel sheet, a method of heating a welded portion
after welding by using a different heating unit may be exemplified. For example,
Patent Document 3 discloses a method of performing a tempering treatment by heating
a welded portion with a high frequency after welding.
[0009]
However, this method has a problem in which a separate process is necessaly
after welding, and thus a working sequence becomes complicated, a problem in which
a special apparatus is necessary to use the high frequency, and a problem in which
peeling fracture tends to occur inside the nugget due to softening of the welded portion
which is caused by a tempering treatment.
[OO lo]
Patent Document 4 discloses a method of increasing strength in a peeling
1
direction of the spot welded joint in a case of spot welding a steel sheet having a
tensile strength of 440 MPa or higher. In the method, with regard to a component
composition of the steel sheet, CxP is defined as 0.0025 or less, P is defined as 0.015%
or less, and S is defined as 0.01% or less. In addition, a heat treatment is performed
~ with respect to a welded portion after welding at 30OoC for 20 minutes. However,
I
this method has problems in that an applicable steel sheet is limited, and a long time is
necessary for welding, and thus productivity is low
Patent Document 5 discloses a spot welded joint obtained by using a highstrength
steel sheet (a tensile strength: 750 MPa to 1850 MPa, and a carbon equivalent
Ceq: 0.22% by mass to 0.55% by mass) in which a microstructure in an outer layer
region of a nugget, and an average particle size and a number density of carbides in the
microstructure are defined. I-Iowever, a joint strength is largely affected by a
segregation state of impurities (brittle elements) in the joint, and thus it is difficult to
obtain a highly reliable joint strength only with a definition of the microstructure
[Prior Art Document]
[Patent Document]
[OO 121
[Patent Document 11 Japanese Unexamined Patent Application, First
Publication No. 2002-103048
[Patent Document 21 Japanese Unexamined Patent Application, First
Publication No. 2010-1 15706
[Patent Document 31 Japanese Unexamined Patent Application, First
Publication No. 2009-125801
[Patent Document 441 Japanese Unexamined Patent Application, First
Publication No. 2010-05945 1
[Patent Document 51 PCT International Publication No. W02011/025015
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0013]
Accordingly, an object of the invention is to stably secure a sufficiently high
joint strength, particularly, CTS in spot welding of a steel sheet having a tensile
strength of 750 MPa to 2500 MPa in which deficiency of toughness tends to occur and
it is difficult to secure a sufficient CTS, and to provide a highly reliable spot welded
joint in which the CTS is stably secured.
[Means for Solving the Problem]
[0014]
The present inventors have made a thorough investigation on a metallurgical
reason why a highly reliable spot welded joint having a sufficiently high CTS is not
obtained only with the definition relating to a microstructure in the spot welded joint
disclosed in Patent Document 5. As a result, it was proved that continuous
segregation of a brittle element such as P and S in a nugget formed by spot welding has
an adverse effect on mechanical characteristics of the spot welded joint, and the
segregation of the brittle element, which has an adverse effect on the mechanical
characteristics of the spot welded joint, is not mitigated only with the definition of the
microstructure.
[00 151
It is diEcult to avoid segregation during solidification of the brittle element in
the spot welding, and thus the present inventors have made a thorough investigation to
resolve the segregation of the brittle element through a heat treatment in a cooling
process during welding. As a result, they have obtained the following findings.
Specifically, when an edge of the nugget is solidified once after energizing of the spot
welding, and then the nugget is heated by allowing a current to flow through the
nugget under necessary conditions, it is possible to mitigate segregation of the brittle
element at the edge of the nugget, and thus the CTS of the spot welded joint is
improved.
[0016]
The invention has been made in consideration of the above-described findings,
and the gist of the invention is as follows.
[OO 171
(1) A spot welded joint according to an aspect of the present invention is a
spot welded joint formed by overlapping a plurality of steel sheets, wherein with
respect to a rectangular planar region of 100 pmx 100 pm, which is perpendicular to a
sheet surface of the plurality of the steel sheets and which centers around an inner
point spaced away by 100 pm in a direction parallel with the sheet surface from an
edge of a nugget along the direction parallel with the sheet surface, the nuggct being
formed in an overlapped face of the plurality of the steel sheets, in a case of measuring
a P concentration in terms of % by mass along the direction parallel with the sheet
surface and a direction perpendicular to the sheet surface at a pitch of 1 pm to obtain
measured values of the P concentration at l00x 100 of measurement points, and in a
case of repetitively calculating an average value of the measured values of the P
concentration at 20 of the measurement points which are adjacent to each other and
which are arranged in a row in the direction parallel with the sheet surface among the
1 0 01~0 0 of the measurement points while shifting each one point along the direction
parallel with the sheet surface and the direction perpendicular to the sheet surface to
obtain 81 ~ 1 0o0f the average values, the number of the average values, which are more
than two times an average value of all of the measured values of the P concentration at
the 100x100 of the measurement points, among the average values is 0 to 100.
(2) In the spot welded joint according to (I), a tensile strength of the plurality
of the steel sheets may be 750 MPa to 2500 MPa.
(3) In the spot welded joint according to (1) or (2), when an average sheet
thickness in terms of mm of the plurality of steel sheets which are overlapped is set as
h, a diameter of the nugget in terms of rnm may be 3 xh'" to 7xhln
[Effects of the Invention]
[ooao]
According to the invention, it is possible to provide a highly reliable spot
welded joint due to a high CTS.
[Brief Description of the Drawings]
[0021]
FIG. 1A is a view illustrating a metallographic structure of a cross-section of a
nugget in which a metal flow on the cross-section of the nugget which is perpendicular
to an overlapped face of a spot welded joint having a high CTS according to the
invention is caused to appear by using an aqueous picric acid solution.
FIG. 1B is a view illustrating a metallographic structure of a cross-section of a
nugget in which a metal flow on the cross-section of the nugget which is perpendicular
to an overlapped face of a single step welded joint formed by a typical manufacturing
method is caused to appear by using the aqueous picric acid solution.
FIG. 2A is a view schematically illustrating the metallographic structure of the
cross-section of the nugget which is shown in FIG. 1A.
FIG. 2B is a view schematically illustrating the metallographic structure of the
cross-section of the nugget which is shown in FIG. 1B.
FIG. 3 is a view illustrating an example of an energizing pattern in spot
welding.
FIG. 4A is a view illustrating six results which are obtained by analyzing a P
concentration (% by mass) irt a region y shown in FIG. 2A at a visual field or250
pmx250 pm using an FE-EPMA and which are arranged in a direction (overlapped
interface direction) parallel with a sheet surface of a steel sheet.
FIG. 4B is a view illustrating six results which are obtained by analyzing the P
concentration (% by mass) in a region y shown in FIG. 2B at a visual field of 250
pmx250 pm using the FE-EPMA and which are arranged in a direction (overlapped
interface direction) parallel with a sheet surface of a steel sheet.
FIG. 5 is a view illustrating a method of obtaining an average value of the P
concentration.
FIG. 6 is a vicw illustrating a method of obtaining the average valuc orthe P
concentration.
[Embodiments of the Invention]
[0022]
In a spot welded joint formed by overlapping steel sheets according to an
embodiment, with respect to a rectangular planar region of 100 pmx100 pm, which is
perpendicular to a sheet surface of the steel sheets and which centers around an inner
point spaced away by 100 pm in a direction parallel with the sheet surface from an
edge of a nugget, the nugget being formed in the steel sheets, in a case of measwing a
P concentration in terms of % by mass along a direction parallel with-the sheet surface
and a sheet thickness direction at a pitch of 1 pm to obtain measured values of the P
concentration at 100x100 of measurement points, and in a case of repetitively
calculating an average value of the measured values at 20 of the measurement points
which are adjacent to each other and which are arranged in a row in the direction
parallel with the overlapped sheet surface among the 100x100 of the measurement
points while shifting each one point along the direction parallel with the sheet surface
and the sheet thickness direction to obtain 81 x100 of the average values of the
measured values at 20 of the measurement points which are adjacent to each other, the
number of the average values, which are more than two times an average P
concentration in the nugget among the average values is 0 to 100. The spot welded
joint according to this embodiment has a sufficiently high joint strength, particularly, a
sufficiently high CTS.
[0023]
Hereinafter, this embodiment will be described with reference to the attached
drawings.
[0024]
The present inventors performed spot welding of steel sheets with variously
changing energizing conditions to prepare various spot welded joints. In addition, a
nugget of each of the spot welded joints was cut in a direction perpendicular to an
overlapped face of a steel sheet, and a cross-section of a nugget was corroded with an
aqueous picric acid solution to observe a metallographic structure of the cross-section
of the nugget. As a result, the present inventors obtained the following findings. In
a case of performing the spot welding according to an energizing pattern shown in FIG.
3, a spot welded joint as shown in FIG. 1A in which a metallographic structure in the
central portion of a nugget and a metallographic structure in an edge of the nugget are
different from each other is obtained in many cases. The spot welded joint frequently
has the above-described P concentration distribution. In addition, the spot welded
joint having the above-described P concentration distribution has the high CTS.
[0025]
Details of the energizing pattern shown in FIG. 3 are as follows. First, main
energizing (primary energizing) is performed with a current value I, for an energizing
time tw while applying a welding pressure to steel sheets. Next, energizing is
performed with a current value I, lower than the current value I, for an energizing time
t, to solidify an edge of the nugget. Continuously, post energizing (secondary
energizing) is performed with a current value I, for an energizing time t,. In the post
energizing, the solidified edge of the nugget is heated or retained at a high temperature.
The welding pressure applied to steel sheets is released at a point of time after the
passage of a predetermined amount of time from completion of the post energizing.
Typical single step energizing includes a process of melting the steel sheets
through welding energizing to form a nugget. However, the typical single step
energizing does not include a process of solidifying the nugget after welding
energizing, and a process of heating the nugget through post energizing, and thus a
metallographic structure of the nugget wholly becomes an as-solidified microstructure.
In contrast, when energizing is performed with the current value I, lower than the
current value I, to solidify the edge of the nugget once, and then appropriate post
energizing is performed, it is considered that a metallographic structure of the edge of
the nugget is modified by heat during the post energizing, and thus toughness is
improved.
[0027]
FIGS. 1A and 1B illustrate a corroded structure of a cross-section of a nugget
which is perpendicular to an overlapped face of the spot welded joint. FIG. 1A
illustrates a cross-section of a spot welded joint which has a high CTS and is spot-
1 welded by the energizing pattern shown in FIG. 3, and FIG. 1B illustrates a
macrostructure of a cross-section of a single step welded joint which has a standard
CTS obtained by a typical single step welded joint.
[0028]
According to investigation on a corroded structure of the cross-section of the
nugget, it is confirmed that in the spot welded joint shown in FIG. 1A in which the
CTS is improved, a structure of the central portion of the nugget and a structure of an
edge of the nugget are apparently different fiom each other. The structure of the edge
of the nugget is considered as a modified structure which is solidified after the main
energizing with the current value I, for the energizing time t,, and is heated or retained
at a high temperature by the post energizing with the current value I, for the energizing
time tp. The structure of the central portion of the nugget is considered a nonmodified
structure which is not solidified with the current value I, for the errcrgizing
time t, after the main energizing, and is retained in an as-solidified state after being
melted at the time of completion of the post energizing. Here, the reason for the
solidification only at the edge of the nugget is that heat of the edge of the nugget
mainly migrates to the outside of the nugget for the energizing time t,. Heat at the
central portion of the nugget does not sufficiently migrate to the outside for the
energizing time t,, and thus thc central portion of the nugget is not solidified for the
energizing time t,.
In the single step welded joint, as shown in FIG. lB, a difference between a
structure of the central portion of the nugget and a structure of an edge of the nugget is
not found. This is considered to be because the energizing with the current value I,
that is lower than the current value I, for the energizing time t,, and the post energizing
with the current value I, for the energizing time t, are not performed, and thus all
regions become an as-solidified metallographic structure.
[0029]
With regard to the spot welded joint in which the C'TS is high, a
microstructural change of the edge of the nugget, which is confirmed by causing a
metal flow to appear; is considered to reflect a variation in a solidifying segregation
state of a brittle element.
[0030]
Accordingly, the present inventors have analyzed a concentration of a
segregated element by using an FE-EPMA (refer to "development of a wavelength
dispersion type submicron EPMA equipped with a field emission electron gun",
Materia Japana, The Japan Institute of Metals and Materials, February 2007, Vol. 46,
No. 2, pages 90 to 92) which is capable of performing accurate quantitative z~tralysisin
a submicron region.
[003 11
In FIGS. 2A and 2B, a P concentration (% by mass) in a region y (region
including an inner region of the nugget, a white region, an edge of the nugget, and a
heat-affected zone) including an edge of the nugget is analyzed with an FE-EPMA
under the following conditions with which solidifying segregation can be easily
detected.
Acceleration voltage: 15 kV
Beam current: 0.5 pA
Beam staying time per one pixel: 60 ms
Number of pixels: 250x250
Visual field: 1 pmx 1 pm
[0032]
In addition, to measure the P concentration with higher accuracy, a plurality of
points at the inside of the 1 pmx 1 pm region may be measured with a narrower
electron beam diameter, and the resultant measured values may be averaged.
It is known that among segregated elements, P and S have an adverse effect
on mechanical characteristics of the spot welded joint, but in this embodiment,
segregation of P is analyzed. The reason is that in general, in a high-strength steel
sheet field, a steel sheet in which an amount of P is larger than that of S is in
widespread use. However, in a material in which the amount of S is larger than that
of P, similarly, it is possible to evaluate a modified state of a metallographic structure
of the nugget by evaluating S instead of P.
[0033]
FIGS. 4A and ill3 illustrate a P concentration (% by mass) distribution in the
region y shown in FIGS. 2A and 2B. In FIGS. 4A and 4B, six images of results
obtained by analyzing the P concentration distribution at a visual field of 250 pmx250
pm are arranged in parallel with each other. Here, a portion in which the P
concentration is high is indicated with white color. FIG. 4A illustrates an analysis
result of the spot welded joint which is obtained by the energizing pattern shown in
FIG. 3 and in which the CTS is high, and FIG. 4B illustrates an analysis result of the
spot welded joint (single step welded joint) obtained by single step welding. As
shown in FIG. 4A, in the spot welded joint according to this embodiment, a site in
which a high P-concentration region extends along a direction parallel with the sheet
surface is not confirmed at an edge of the nugget. In the single step welded joint
shown in FIG. 4B, many sites in which the high P-concentration region extends along a
direction parallel with the sheet surface are confirmed at an edge of the nugget.
[0034]
As described above, at the edge of the nugget of the typical spot welded joint,
linear P solidifying segregation having a length of several tens of micrometers or
longer is confirmed along a direction parallel with the sheet surface. It is considered
that the linear solidifying segregation decreases crack extension resistance from an
interface of steel sheets. To improve the CTS of the spot welded joint, it is necessary
for the P solidifying segregation not to be continuous along a direction parallel with the
sheet surface.
1003 51
When evaluating a segregation state of P at the edge of the nugget by the
present inventors, with respect to a rectangular planar region of 100 pmx100 pm,
which is perpendicular to the sheet surface and which centers around an inner point
spaced away from the edge of the nugget by 100 pm in a direction parallel wilt1 the
sheet surface in the analysis results shown in FIGS. 4A and 4B, the P concentration in
terms of % by mass is measured along the direction parallel with the sheet surface and
the sheet thickness direction at a pitch of 1 pm to obtain measured values of the P
concentration at 1 OOx 100 measurement points. In addition, an average value of the
measured values of the P concentration at 20 of the measurement points which are
adjacent to each other and which are arranged in a row in the direction parallel with the
overlapped sheet surface, among the 100x100 of the measurement points is repetitively
calculated while shifting each one point along the direction parallel with the sheet
surface and the sheet thickness direction to calculate 8 1 x 100 of the average values of
the measured values at 20 adjacent measurement points. The average values are
compared with a value that is two times an average P concentration of the above.
described rectangular planar region. An average P concentration value of the
I rectangular planar region is set as an average value of the measured values of the P
concentration at the 100x100 of the measurement points.
[0036]
The reason for evaluating only the rectangular planar region of 100 pmx 100
pm which is perpendicular to the sheet surface and which centers around the inner
point spaced away from the edge of the nugget by 100 pm in a direction parallel with
the sheet surface is as follows. In a cross tensile test for measuring the CTS, the edge
of the nugget and a stable crack growing region present at the periphery of the edge
bear a load, and thus modification of a broad region including the edge of the nugget
and the periphery of the edge is effective to improve the CTS. In the spot welded
joint according to this embodiment, modification is performed across the broad region.
However, the width of the stable craclc growing region is different depending on the
sheet thickness or the kind of steel, and a flat solidification structure may occur at the
outermost edge of the nugget. Here, the flat solidification structure represents a
structure that is formed by solidification of a molten metal without an occurrence of
dendritic growth at a solid-liquid interface. Accordingly, the region to which a
peeling load concentrates and which is considered as a dendrite solidification region is
preferable for evaluation of modification in the stable crack growing region regardless
of the kinds of steel or the sheet thickness.
To illustrate continuity of segregation in a direction parallel with the sheet
surface, as shown in FIGS. 5 and 6, it is necessary to obtain an average of the P
concentration at measurement points arranged in a row in a direction parallel with the
sheet surface. In a case where dot-shaped segregation is present, a measurement
point in which the P concentration is high is confirmed during concentration
measurement, but an average value of the P concentration at a plurality of
measurement points arranged in a row in a direction parallel with the sheet surface is
low. In a case where linear segregation along a direction parallel with the sheet
surface is present, the average value of the P concentration at the plurality of
measurement points arranged in a row in a direction parallel with the sheet surface is
high. Accordingly, it is possible to detect only the linear segregation that has an
effect on the CTS by obtaining an average value of the P concentration at the
measurement points that are arranged in a row in a direction parallel with the sheet
surface.
Here, in a case where a length of a section for obtaining the average is too
short, the linear segregation and the dot-shaped segregation are apt to be counted as the
same one, and thus it is difficult to detect only the linear segregation. In a case where
the length of the section for obtaining the average is too long, it is difficult to
sufficiently detect the linear segregation having a length which is shorter than the
section length and which may have an effect on the CTS. In general, when the length
of the linear segregation exceeds 20 p, the CTS is affected, and thus it is preferable
that the length for obtaining the average be 20 pm.
[0038]
In addition, the reason for setting a comparison target of the average value of
the P concentration to a value that is two times the average P concentration in the
provided nugget is that even in a base metal in which segregation is not present, a
variation of approximately *SO% is present in the P concentration in accordance with a
location. In order for an increase in the average value of the P concentration to be
determined as an increase in concentration due to segregation during solidification, it is
necessary to set the comparison target of the average value of the P concentration to a
value that is two times the average P concentration in the nugget.
[0039]
Among 8 100 average values of the 20 measurement points which are adjacent
to each other, which are arranged in a row in a direction parallel with the sheet surface,
and which are obtained in this manner, when the number of the average values, which
are more than two times the average value of all of the measured values of the P
concentration in the 100x100 of the measurement points is 100 or less, it is found that
the CTS is very high. In this case, segregation of P is mitigated in the x region of FIG.
2A, and thus it is considered that toughness is satisfactorily maintained in the vicinity
of an intersection, which becomes a starting point of a fracture, between a pressure
welding portion (corona bond) of steel sheets and the nugget, and an inner region
thereof.
[0040]
It is considered that performing the post energizing after the main energizing
may improve the segregation in the edge region of the nugget due to the following
reasons.
[0041]
When lowering a current, the nugget that is formed by the main energizing
starts to be solidified from the edge of the nugget. At this time, a brittle element such
as P segregates to a boundary of a dendrite during solidification of the nugget. In the
case of single step energizing spot welding, or in a case where two step energizing is
performed without any time to solidify the nugget between ihe main energizing and the
post energizing, the segregated brittle element remains in a final structure, and thus the
segregated brittle element becomes a cause of embrittlemeni of the nugget.
[0042]
In this embodiment, a structure of the nugget and a structure at the periphery
of the nugget are heated by the post energizing after passage of the energizing time t,
from completion of the main energizing. At this time, depending on the condition of
the post energizing, linearly segregated P is homogenized due to diffusion.
Alternativery, at this time, depending on the condition of the post energizing, the
concetration of the linearly segragated P decreases or the linearly segragated P is
segmentated due to re-segregation into a grain boundary of fine crystal grains ihat are
generated by reverse transformation.
[0043]
In a case where temperature rise due to the post energizing is low, or in a case
where the heating time due to the post energizing is short, the diffusion of the
segregated P or re-segregation of the segregated P into a grain boundary of thc fine
crystal grains does not sufficiently occur. In addition, in a case where temperature
rise due to the post energizing is high, or in a case where the heating time due to the
post energizing is long, a volume of a melted portion at the center of the nugget
increases, and thus the edge of the solidified nugget is re-melted. Therefore, the edge
of ibe nugget has a typically as-solidified structure after completion of the post
energizing, and thus solidifying segregation remains.
[0044]
A diameter of the nugget of the spot welded joint according to this
embodiment is in a range of 3xh1" mm to 7xh1" mm with respect to an average sheet
thickness h of steel sheets that are subjected to spot welding in consideration of
strength that is necessary in practical use in the spot welded joint of a steel sheet
having a tensile strength of 750 MPa to 2500 MPa. When the diameter of the nugget
exceeds a diameter of an electrode front end, expulsion tends to occur during spot
welding, and thus it is preferable that the diameter of the nugget be set to be equal to or
less than the diameter of the zlectrode front end.
[0045]
(Tensile Strength of Steel Sheet)
Generally, the further a tensile strength of the steel sheet increases, the further
a load operating on the spot welded joint increases, and the further stress concentration
to a welded portion increases. Accordingly, it is necessary for the spot welded joint
to have a high joint strength.
[0046]
Typically, if the tensile strength of the steel sheet exceeds 750 MPa, even
when the tensile strength of the steel sheet further increases, the CTS does no1 increase
or, in contrast, a value of the CTS decreases. However, in the spot welded joint
according to this embodiment, segregation of P is mitigated, and thus even in a case of
being formed from a steel sheet having a tensile strength of 750 MPa or higher, it is
possible to maintain a satisfactory CTS.
[0047]
The upper limit of the tensile strength of the steel sheet is not particularly
defined. However, in a case where the tensile strength is high, (i) it is difficult to
suppress a decrease or a deviation in the strength of the spot welded joint, or (ii) it is
difficult to suppress a defect or a crack inside the nugget. Accordingly, it is
preferable that the tensile strength of the steel sheet be 2500 MPa or lower.
Accordingly, the spot welded joint according to this embodiment is preferable
for spot welding of a steel sheet having a tensile strength of 750 MPa to 2500 MPa.
[0048]
(Component Composition of Steel Sheet)
A component composition of a steel sheet that is a target in the spot welded
joint according to this embodiment is not particularly limited. As a steel shcet
material, a cold-rolled material or a hot-rolled material such as a known hot stamp
material and a super-high tension material may be used. However, specifically, a
steel sheet having the following component composition is preferable.
[0049]
For example, a steel sheet which has a basic composition including, in terms
of % by mass, C: 0.08% to 0.40%, Si: 0.01% to 2.50%, Mn: 0.8% to 3.0%, P: 0.05% or
less, S: 0.01% or less, N: 0.01% or less, 0: 0.007% or less, and Al: 1.00% or less, the
remainder being Fe and unavoidable impurities, and further including an element
selected from the following groups (a) to (c) as necessary is preferable to m;~liufacture
the spot welded joint according to this embodiment.
[0050]
(a) One or more kinds of elements selected from the group consisting of Ti:
0.005% to 0.10%, Nb: 0.005% to 0.10%, and V: 0.005% to 0.10%
(b) One or more kinds of elements selected from the group consisting of B:
0.0001% to 0.01%, Cr: 0.01% to 2.0%,Ni: 0.01% to 2.0%, Cu: 0.01% to 2.0%, and
Mo: 0.01% to 0.8%
(c) One or more ltinds of elements selected from Ca, Ce, Mg, and REM in a
I
total amount of 0.0001% to 0.5%
[005 11
In addition, particularly, P and S segregate in a dendrite to decrease the CTS,
and thus when it is desirable to obtain a high CTS, it is preferable that the amount of
the elements be made to be relatively small.
[0052]
Hereinafter, the reason for limiting the component composition will be
described. In addition, % represents % by mass.
[0053]
C: 0.08% to 0.40%
C is an element that increases a tensile strength of steel. When an amount of
C is large, an amount of C in the nugget is also large, and thus a final structure tends to
be hard. When the amount of C is less than 0.08%, it may be difficult to obtain a
tensile strength of 750 MPa or higher, and hardness of the nugget may be low, and thus
the TSS may be apt to be lowered. On the other hand, when the amount of C exceeds
0.40%, workability may deteriorate, and the nugget may be too hard and may be brittle.
Accordingly, it is preferable that the amount of C be 0.08% to 0.40%, and more
preferably 0.10% to 0.31%.
[0054]
Si: 0.01% to 2.50%
Si is an element that increases the strength of steel through solid-solution
strengthening and structure strengthening. When the amount of Si exceeds 2.50%,
worlcability of steel may decrease. On the other hand, it is industrially difficult to
reduce the amount of Si to an amount less than 0.01%. Accordingly, it is preferable
1
that the amount of Si be 0.01% to 2.50%, and more preferably 0.05% to 1.20%.
[0055]
Mn: 0.8% to 3.0%
Mn is an element that increases hardenability of steel. When an amount of
Mn exceeds 3.0%, formability of steel may deteriorate. On the other hand, when the
amount of Mn is less than 0.8%, it may be difficult to obtain a tensile strength of 750
MPa or higher. Accordingly, it is preferable that the amount of Mn be 0.8% to 3.0%,
and more preferably 1 .O% to 2.5%.
[0056]
P: 0.05% or less
P is an element that segregates and makes the spot welded joint brittle.
When an amount of P exceeds 0.05%, even when the segregation of P is mitigated, it
may be difficult to obtain a sufficient CTS. In addition, the less the amount of P is.
the more preferable. However, it is industrially difficult to reduce the amount of P to
an amount less than 0.001%, and thus 0.001% is the substantial lower limit of the
amount of P. More prcrerably, the amount of P is 0.03% or less.
[0057]
S: 0.01% or less
S is an element which segregates to malce the spot welded joint brittle, and
which forms coarse MnS and deteriorates the workability of steel. When an amount
of S exceeds 0.01%, the workability of steel may deteriorate, and cracking may tend to
occur inside the nugget, and it may be difficult to obtain a suficient CTS. In addition,
the less the amount of S is, the more preferable. However, when the amount of S is
set to be less than 0.0001%, the manufact~~rincgo st increases. Accordingly, 0.0001%
is the substantial lower limit of the amount of S. More preferably, the amount of S is
0.006% or less.
[OOSS]
N: 0.01% or less
N is an element which forms coarse nitrides and deteriorates formability, and
which becomes a cause of generating blow holes during welding. When an amount
of N exceeds 0.01%, the deterioration of the formability and the generation of the blow
holes may be significant. In addition, the less the amount of N is, the more preferable.
However, when the amount of N is set to be less than 0.0005%, the manufacturing cost
increases, and thus 0.0005% is the substantial lower limit of the amount of N. More
preferably, the amount of N is 0.007% or less.
[0059]
0: 0.007% or less
0 is an element that forms oxides and deteriorates the formability of steel.
When the amount of 0 exceeds 0.007%, the deterioration ofthe formability may be
significant. In addition, the less the amount of 0, the more preferable. However,
when the amount of 0 is set to be less than 0.0005%, the manufacturing cost increases.
Accordingly, 0.0005% is the substantial lower limit of the amount of 0. More
preferably, the amount of 0 is 0.005% or less.
[0060]
Al: 1.00% or less
A1 is a deoxidizing element and is an element that stabilizes ferrite and
suppresses precipitation of cementite. When the amount of A1 exceeds 1.00%,
inclusions may increase, and t h ~f~orsm ability may deteriorate. It is not necessary to
define the lower limit of the amount of Al. However, excessive reduction in an
amount of A1 is not practical in consideration of the refining cost, and thus the lower
limit of the amount of A1 is set to 0.001%.
[0061]
(a) One or More Kinds of Elements Selected from Group Consisting of Ti:
0.005% to O.lO%,Nb: 0.005% to 0.10%, and V: 0.005% to 0.10%
Ti, Nb, and V are elements that strengthen steel through precipitation
strengthening, fine grain strengthening by suppression of growth in ferrite crystal
grains, and dislocation strengthening by suppression of recrystallization. When an
amount of any element is less than 0.005%, a containing effect is not sufficient.
When the amount of any element exceeds 0.10%, the formability of steel may
deteriorate. More preferably, the amount of each of Ti, Nb, and V is 0.01% to 0.08%.
[0062]
(b) One or More Kinds of Elements Selected from Group Consisting of B:
0.0001% to 0.01%, Cr: 0.01% to 2.0%,Ni: 0.01% to 2.0%, Cu: 0.01% to 2.0%, and
Mo: 0.01% to 0.8%
B is an element that controls a structure and strengthens steel. When an
amount of B is less illan 0.0001%, strength may not be sufficiently improved. When
the amount of B exceeds 0.01%, a containing effect is saturated. Cr, Ni, Cu, and Mo
are elements that contribute to the improvement of the strength of steel. When an
amount of any element is less than 0.01%, a containing effect may not be sufficient.
When the amount of any element exceeds 2.0%, a problem such as the occurrence of
craclcing may occur during pickling or hot rolling. The amount of B is more
preferably 0.0003% to 0.08%, the amount of each of Cr, Ni, and Cu is more preferably
0.02% to 1.0%, and thc amount of Mo is more preferably 0.015% to 0.6%.
[0063]
(c) One or More Kinds of Elements Selected from Ca, Ce, Mg, and REM in
Total Amount of 0.0001% to 0.5%
Ca, Ce, Mg, and REM are elements which reduce dimensions of oxides after
deoxidation and dimensions of sulfides in a hot-rolled steel sheet and which improve
formability. When the total amount of these elements is less than 0.0001%, a
containing effect may be small. When the total amount of these elements exceeds
0.5%, the formability of steel may deteriorate. In addition, the REM is a lanthanoidseries
element, and the REM and Ca are contained in a form of misch metal at a steel
making step. More preferably, the total amount of Ca, Ce, Mg, and REM is 0.0002%
to 0.3%.
[0064]
(Method of Manufacturing Spot Welded Joint)
As described above, the spot welded joint according to this embodiment,
which has the above-described P concentration distribution, can be obtained by
modifying the edge of the nugget of the spot welded joint through the post energizing.
Manufacturing conditions, under which the spot welded joint according to this
embodiment is formed, basically conform to the energizing pattern shown in ITG. 3.
[0065]
In a case of performing energizing according to the energizing pattern shown
in FIG. 3, the following temperature history is assumed. Specifically, the edge of the
nugget is solidified after the main energizing, and then the nugget is reheated or
retained at a high temperature in a range in which the solidified edge is not re-melted.
It is not necessary to rapidly cool the edge of the nugget duxing solidification of the
edge of the nugget, and thus it is considered that a current value may be lowered in
order for a heat input to be sufficiently lowered.
[0066]
However, when the time taken to solidify the edge of the nugget is lengthened,
or when the nugget in a normal state with a lowered current value is large, a solidified
region from the edge becomes narrow, and thus a low current value is preferable.
[0067]
Specifically, the spot welded joint according to this embodiment is obtained in
the following sequence. Hereinafter, a unit of the current value is set as kA, and a
unit of time is set as msec.
[0068]
A current value I, and an energizing time t, with which, when typical one
step energizing spot welding with respect to overlapped steel sheets (average sheet
thiclmess: h [mm]) is performed, a predetermined nugget diarncter axh'" [mm] is
obtained is experimentally obtained. In general, a nugget shape is approximately
circular. However, in a case where the nugget shape is not approximately circular, on
the assumption of a circle having the same area as that of the nugget, a diameter of the
circle is set as the predetermined nugget diameter axh'" [mm]. Continuously, the
spot welded joint is prepared by the following processes.
100691
(Main Energizing Process)
First, the typical welding energizing is performed with the current value I, for
energizing time t,. With regard to a welding pressure during the welding energizing,
1960xh to 3920xh IN] is set as a standard with respect to an average sheet thickness h.
(Solidification Process)
After the common welding energizing, energizing is retained under pressure
with a current value I, satisfying Expression (I) for an energizing time t, satisfying
Expression (2).
051,5(1/2)"~x1, ...... (I)
a2x{2x(1,2/12)+1}x h0.8xI, ...... (3)
1 ~ ~ ~ t , 5 1 2...~...t (,4 )
[0070]
The current value I, to heat the edge of the nugget or to retain the edge of the
nugget at a high temperature during the post energizing process is required to be higher
than the current value I, which solidifies the edge of the nugget during the
solidification process. In a case of performing energizing with a low current during
the post energizing process, heat release to an electrode or a sheet is dominant in
comparison to heat generation, and thus heat necessay to diffuse P that is subjected to
solidifying segregation to the edge of the nugget is not sufficiently obtained.
Therefore, time is taken for diffusion of P, and thus the above-described case is not
appropriate in practical use.
[0071]
In the post energizing process, in a case of performing energizing with a high
current for a long time, re-melting occurs, and thus modification of the edge does not
occur.
In the post process, one current value and one energizing time are defined, but
the post energizing pattern may be complicated as long as re-melting does not occur.
In this case, it is preferable to determine the current value in order for an average
current value (effective value) in each process to satisfy Expression (3) and Expression
(4). In a case of using a cornplicated current pattern in each ofthe main energizing
process in which a joint portion is melted, the solidification process in which the edge
of the nugget is solidified, and the post energizing process in which the edge of the
nugget is heated or is retained at a high temperature, it is preferable to determine the
current value by using an effective value with respect to the pattern.
[0072]
Next, in the spot welded joint obtained by the above-described processes, a
segregation state of P at the edge of the nugget is evaluated. The evaluation is
performed as follows. First, with respect to a rectangular planar region of 100
pmx 100 pm, which is pc~pendicularto the sheet surface and which centers a1 ound an
inner point spaced away from the edge of the nugget by 100 pm in a direction parallel
with the sheet surface, the P concentration in terms of % by mass is measured along a
direction parallel with the sheet surface and a direction perpendicular to the sheet
surface at a pitch of 1 pm to obtain measured values of the P concentration at 100x100
of the measurement points. In addition, an average value of the measured values of
the P concentration at 20 of the measurement points which are adjacent to each other
and which are arranged in a row in a direction parallel with the overlapped sheet
surface, among the l0Ox 100 of the measurement points is repetitively calculated while
shifting-each one point along the direction parallel with the sheet-surface and the
direction perpendicular to the sheet surface to obtain 81 x 100 of the average values of
the measured values at 20 of the adjacent measurement points. It is determined
whether or not among the obtained average values, the number of average values that
are more than two times an average P concentration (% by mass) in the nugget is 100
or less. When the number of average values that are more than two times the average
P concentration (% by mass) in the nugget is 100 or less, it can be determined that the
spot welded joint according to this embodiment, in which segregation of P is mitigated,
is obtained. When perfonning the spot welding based on the energizing conditions
under which the spot welded joint is obtained, it is possible to obtain the spot welded
joint according to this embodiment by using the steel sheet in which the spot welded
joint is obtained.
[0073]
Even when the manufacturing is performed while satisfying the abovedescribed
tonditions, the segregation of P may not be mitigated. In this case, the
following situations may be considered. Specifically, the current value during the
post energizing process IS deficient, and thus modification of the edge of thc nugget is
not sufficiently performed. In addition, the current value during the post energizing
process is too large, and thus the solidified region of the edge of the nugget is remelted.
As a result, the modification of the edge of the nugget is not performed. In
these cases, a metal flow of a cross-section of the nugget is observed, and energizing
conditions are changed based on observation results to optimize manufacturing
processes.
In a case where the segregation of P is not mitigated, but an apparent
difference between a structure of the central portion of the nugget and a structure of the
edge of the nugget is confirmed by the observation of the metal flow on the crosssection,
it is considered that the modification of the edge of the nugget is performed,
but the modification is deficient. As a cause of the deficiency in the modification, a
case in which the heat input during the post energizing process is deficient, or a case in
which the solidification time between the main energizing process and the post
energizing process is too long and thus a temperature of the nugget is lowered may be
considered. In each cases, the heat input is deficient in comparison to an amount of
heat necessary to raise a tern~eratureo f the nugget to a temperature capable of
effectively subjecting the nugget to a heat treatment. Accordingly, it becomes close
to the conditions under which the segregation is mitigated by increasing I, or t, or by
decreasing t,. The increasing of I, or t, is performed to increase the heat input, and
the decreasing oft, is performed to reduce a solidification region to be thermally
treated during the post energizing process, and to input heat in order for the edge of the
nugget to enter a relatively high temperature state, thereby easily raising a temperature
of the nugget to a temperature at which the heat treatment is effectively performed.
[0074]
In a case where the segregation of P is not mitigated, and an apparent
difference between the structure of the central portion of the nugget and the structure
of the edge of the nugget is not confirmed by the observation of the metal flow of the
cross-section, it can be considered that the solidified region at the edge of the nugget is
re-melted. This is caused by too much heat input during the post energizing process,
and thus it becomes close to the conditions under which the segregation is mitigated by
decreasing I, or t, or by increasing t,. The decreasing of I, or t, is performed to
decrease the heat input, and the increasing oft, is performed to increase an amount of
heat dissipation to the outside from the nugget before initiation of the post energizing,
thereby reducing an effect of the heat input due to the post energizing. When the
manufacturing processes are optimized by repeating the change of the energizing
conditions and the observation of the metal flow of the cross-section, it is possible to
obtain the spot welded joint according to this embodiment. When performing the
spot welding based on the energizing conditions under which the spot welded joint is
obtained, it is possible to obtain the spot welded joint according to this embodiment by
using the steel sheet in which the spot welded joint is obtained.
With regard to optirilization of the manufacturing process in a case where
even when performing welding based on recommended conditions, the segregation of
P is not reduced, it is not necessary to satisfy Expression (1) to Expression (4).
[Examples]
[0075]
Next, examples of the invention will be desctibed. Manufacturing
conditions in examples represent a conditional example that is employed to confirm
applicability and an effect of the invention, and the invention is not limited to the
conditional example. The invention may employ various conditions as long as the
object of the invention is accomplished without departing from the gist of thc invention.
[0076]
(Example 1)
Steel sheets A to D shown in Table 1 were prepared, and the same kind of
steel sheets were overlapped. Then, spot welding was performed with respect to the
overlapped steel sheets by using a servo gun type welding machine to prepare test
specimens of the spot welded joint. At that time, conditions of the main energizing,
and time (squeeze time) from pressurizing to energizing and pressuring retention time
after post energizing were made to be constant in combinations of different kinds of
steel.
In steel sheets A to C, the amount of C was 0.21% by mass, and Ceq was
0.34%. In steel sheet D, the amount of C was 0.19% by mass, and the Ceq was
0.32%. The Ceq represents a carbon equivalent, and is obtained by the following
expression.
Ceq=[C]+[Si]/3O+[Mn]/20+2x[P]+4x[S]
In Table, a site indicated by "-" represents that energizing is not performed.
[0077]
[Table 11
[0078]
In addition, a preliminary experiment was performed before the spot welding,
and on the basis of results of the preliminary experiment, an amount of main
energizing during the spot welding was set to a value (4.5xti" to 5xtin) with which a
diameter of the nugget became 4.5 to 5 times the square root of the sheet thickness of
one steel sheet, and then the welding was performed under conditions in which the post
energizing was changed.
[0079]
In addition, analysis of a cross-section of the nugget and measurement of the
cross tension strength (CTS) were performed for each condition. In the combinations
of the steel sheets, whether or not the segregation mitigating conditions of the
invention were satisfied and results obtained by evaluating the CTS are shown in Table
2.
In addition, a CTS improvement rate shown in Table 2 represents a rate of an
amount of increase or decrease in CTS to CTS of spot welded joints (A-6, B-4, C-4,
and D-5) in which only the main energizing was performed. In addition, whether or
not reduction of segregation was performed was determined as follows. With respect
to a rectangular planar region of 100 pmx100 pm, which is perpendicular to the sheet
surface and which centers around an inner point spaced away from the edge of the
nugget by 100 pm in a direction parallel with the sheet surface, the P concentration in
terms of % by mass was measured along the direction parallel with the sheet surface
and the sheet thickness direction at a pitch of 1 pm to obtain measured values of the P
concentration at 100x100 of the measurement points. In addition, an average value of
the measured values of the P concentration at 20 of the measurement points which are
adjacent to each other and which are arranged in a row in the direction parallel with the
overlapped sheet surface, among the 100x100 ofthe measurement points is calculated
while shifting each one point along the direction parallel with the sheet surface and the
sheet thickness direction to calculate 81 ~ 1 0o0f the average values of the measured
values at 20 of the measurement points which are adjacent to each other. Then,
whether or not reduction of segregation was performed was determined on the basis
whether or not among the average values, the number of the average values that are
more than two times an average value (average P concentration) of all of thc measured
values of the P concentration in the 1 0 01~0 0 of the measurement points is 0 to 100.
In samples in which the number of the average values exceeding the average P
concentration was 0 to 100, it was considered that the reduction of segregation was
performed, and thus a symbol " G (Good) was given to these samples. A symbol "B"
(Bad) was given to samples in which the number of the average values exceeding the
average P concentration was 100 or more.
[0080]
[Table 21
[0081] -
As shown in Table 2, in Inventive Examples in which the segregation state of
P satisfied the standard of the invention, the CTS value was high even in the same
sheet combination and the same nugget diameter, and thus a highly reliable spot
welded joint was obtained. On the other hand, samples that did not satisfy the
standard of the invention were spot welded joints in which the CTS value was low.
[Industrial Applicability]
[0082]
As described above, according to the invention, it is possible to provide a spot
welded joint in which a fracture shape is satisfactory and reliability is high.
Accordingly, the invention has high industrial applicability.
[Brief Description of the Reference Symbols]
[0083]
1 : lOOx 100 measurement points
2: 8 1 x 100 of the average values
TABLE 1
[Table 21
rn
Crl
4
6x3
-4
E+
[Documet~t-Type] CLAIMS --
[Claim 11
A spot welded joint formed by overlapping a plurality of steel sheets,
wherein with respect to a rectangular planar region of I00 pmx 100 pm, which
is perpendicular to a sheet surface of the plurality of the steel sheets and which centers
around an inner point spaced away by 100 pn in a direction parallel with the sheet
surface from an edge of a nugget along the direction parallel with the sheet surface, the
nugget being formed in an o~erlappedfa ce of the plurality of the steel sheets,
in a case of measuring a P concentration in terms of % by mass along the
direction parallel with the sheet surface and a direction perpendicular to the sheet
surface at a pitch of 1 pm to obtain measured values of the P concentration at 1 00x 100
of measurement points, and
in a case of repetitively calculating an average value of the measured values
of the P concentration at 20 of the measurement points which are adjacent to each other
and which are arranged in a row in the direction parallel with the sheet surface among
the 100x100 of the measurement points while shifting each one point along the
direction parallel with the sheet surface and ihe direction perpendicular to ihc sheet
surface to obtain 81 xl00 of the average values,
the number of the average values, which are more than two times an average
value of all of the measured values of the P concentration at the 1 0 01~0 0 of the
measurement points, among the average values is 0 to 100.
[Claim 21
The spot welded joint according to Claim 1,
wherein a tensile strength of the plurality of the steel sheets is 750 MPa to
2500 MPa.
The spot welded joint according to Claim 1,
wherein when an average sheet thickness in terms of mm of the plurality of
the steel sheets which are overlapped is set as h, a diameter of the nugget in terms of
mm is 3xh'" to 77x1".