【Invention Title】
Zn ALLOY PLATED STEEL SHEET HAVING EXCELLENT
PHOSPHATABILITY AND SPOT WELDABILITY AND METHOD FOR
5 MANUFACTURING SAME
【Technical Field】
The present disclosure relates to a zinc alloy plated
steel sheet having excellent phosphatability and spot
10 weldability and a method of manufacturing the same.
【Background Art】
Recently, a zinc plated steel sheet has been widely used
in household appliances, automobiles, and the like, so there
15 is increasing demand for zinc plated steel sheets. In order to
increase the plating adhesion of a zinc plated steel sheet,
excellent phosphatability has been required therein. However,
in a zinc plated steel sheet according to the related art, during
solidification of zinc plated on a surface of a steel sheet,
20 a zinc crystal grain, referred to as a spangle, may be formed,
and such a spangle may remain on a surface of a steel sheet after
solidification, so there is a disadvantage in that
phosphatability may be inferior.
To negate such a disadvantage, a plating technique of
Page 3
mixing various added elements to a plating layer has been
proposed. As a representative example, a zinc alloy plated steel
sheet, improving phosphatability of a steel sheet by forming
a Zn-Mg-Al-based intermetallic compound by adding an element
5 such as aluminum (Al), magnesium (Mg), and the like, to a plating
layer, may be cited. However, in such a Zn-Mg-Al-based
intermetallic compound in a zinc alloy plated steel sheet, a
melting point thereof is rather low, so melting occurs easily
during welding. Thus, there is a disadvantage in that spot
10 weldability of a plated steel sheet may be deteriorated.
【Disclosure】
【Technical Problem】
An aspect of the present disclosure may provide a zinc
15 alloy plated steel sheet having excellent phosphatability and
spot weldability and a method of manufacturing the same.
The object of the present invention is not limited to the
above description. Additional objects and advantages of the
invention will be set forth in part in the description which
20 follows, and those of ordinary skill in the art will readily
understand the additional objects of the present invention from
this application.
【Technical Solution】
Page 4
According to an aspect of the present disclosure, a zinc
alloy plated steel sheet having excellent phosphatability and
spot weldability is provided, the zinc alloy plated steel sheet
including a base steel sheet and a zinc alloy plating layer,
5 wherein the zinc alloy plating layer includes, by wt%, 0.5% to
2.8% of Al and 0.5% to 2.8% of Mg, with a remainder of Zn and
inevitable impurities, a sectional structure of the zinc alloy
plating layer includes a Zn single phase structure of more than
50% by area percentage and a Zn-Al-Mg-based intermetallic
10 compound of less than 50%, and a surface structure of the zinc
alloy plating layer includes a Zn single phase structure of 40%
or less by area percentage and a Zn-Al-Mg-based intermetallic
compound of 60% or more.
According to another aspect of the present disclosure,
15 a method of manufacturing a zinc alloy plated steel sheet
includes: preparing a zinc alloy plating bath including, by wt%,
0.5% to 2.8% of Al and 0.5% to 2.8% of Mg, with a remainder of
Zn and inevitable impurities; immersing a base steel sheet in
the zinc alloy plating bath, and obtaining a zinc alloy plated
20 steel sheet by performing plating; gas wiping the zinc alloy
plated steel sheet; primary cooling the zinc alloy plated steel
sheet at a primary cooling rate of 5°C/sec or less (excluding
0°C/sec) to a primary cooling end temperature of more than 380°C
to 420°C or less, after the gas wiping; maintaining the zinc
25 alloy plated steel sheet at a constant temperature for at least
Page 5
one second at the primary cooling end temperature, after the
primary cooling; and secondary cooling the zinc alloy plated
steel sheet at a secondary cooling rate of 10°C/sec or more to
a secondary cooling end temperature of 320°C or less, after the
5 maintaining the zinc alloy plated steel sheet at a constant
temperature.
【Advantageous Effects】
According to an exemplary embodiment in the present
10 disclosure, a zinc alloy plated steel sheet has excellent
phosphatability and excellent spot weldability.
【Description of Drawings】
FIG. 1 is scanning electron microscope (SEM) images of
15 a cross-sectional structure of a zinc alloy plated steel sheet
according to an exemplary embodiment.
FIG. 2 is SEM images of a surface structure of a zinc alloy
plated steel sheet according to an exemplary embodiment.
FIG. 3 is images of a surface of a zinc alloy plated steel
20 sheet according to an exemplary embodiment, after the zinc alloy
plated steel sheet is phosphate-treated.
【Best Mode for Invention】
The inventors of the present invention conducted various
25 studies in order to simultaneously improve the phosphatability
Page 6
and spot weldability of a zinc alloy plated steel sheet, and
the following findings were obtained.
(1) As a microstructure of a surface portion of a zinc
alloy plating layer, a large amount of a Zn-Al-Mg-based
5 intermetallic compound is secured, so phosphatability may be
improved.
(2) On the other hand, the Zn-Al-Mg-based intermetallic
compound has a low melting point, so spot weldability may be
inhibited.
10 (3) To improve the spot weldability, as a microstructure
of a zinc alloy plating layer, it is necessary to secure a large
amount of a structure with a high melting point. To this end,
it is preferable to secure a large amount of a Zn single phase
structure.
15 (4) In order to obtain both (1) and (3) described above,
a large amount of a Zn single phase structure is secured as a
microstructure in a cross-sectional portion of a zinc alloy
plating layer (a cross-sectional structure), while a large
amount of a Zn-Al-Mg-based intermetallic compound is secured
20 as a microstructure in a surface portion of the zinc alloy
plating layer (a surface structure). Therefore, a zinc alloy
plated steel sheet simultaneously having excellent
phosphatability and spot weldability may be provided.
Hereinafter, an aspect of the present disclosure, a zinc
25 alloy plated steel sheet having excellent phosphatability and
Page 7
spot weldability, will be described in detail.
An aspect of the present disclosure, a zinc alloy plated
steel sheet, includes a base steel sheet and a zinc alloy plating
layer. In an exemplary embodiment, a type of the base steel sheet
5 is not particularly limited, and may be, for example, a
hot-rolled steel sheet or a cold-rolled steel sheet, used as
a base of a zinc alloy plated steel sheet according to the related
art. However, in the case of the hot-rolled steel sheet, a large
amount of oxidized scale may be formed on a surface thereof,
10 and the oxidized scale lowers plating adhesion, so a problem
in which plating quality is lowered may occur. Thus, it is more
preferable to use a hot-rolled steel sheet, from which oxidized
scale is removed in advance by an acid solution, as a base. On
Meanwhile, the zinc alloy plating layer may be formed on one
15 or both sides of the base steel sheet.
The zinc alloy plating layer may include, by wt%, 0.5%
to 2.8% of Al and 0.5% to 2.8% of Mg, with a remainder of Zn
and inevitable impurities.
Mg in the zinc alloy plating layer is an element playing
20 a major role in improving corrosion resistance and
phosphatability of a plating steel sheet by forming a
Zn-Al-Mg-based intermetallic compound as Mg reacts with Zn and
Al in a plating layer. If the content of Mg is significantly
low, corrosion resistance of a plating layer may not be improved
25 and a sufficient amount of a Zn-Al-Mg-based intermetallic
Page 8
compound in a surface structure of a plating layer may not be
secured, so a problem in which an effect of improvement of
phosphatability is not sufficient may occur. Thus, a lower limit
of the content of Mg in the zinc alloy plating layer is preferably
5 0.5 wt%, more preferably 0.6 wt%, and most preferably 0.8 wt%.
However, if the content of Mg is excessive, an effect of
improvement of phosphatability may be saturated, and dross,
related to Mg oxide, is formed in a plating bath, so a problem
in which plating properties are deteriorated may occur.
10 Furthermore, a large amount of a Zn-Al-Mg-based intermetallic
compound in a cross-sectional structure of a plating layer is
formed, so a problem in which spot weldability decreases may
occur. Thus, an upper limit of the content of Mg in the zinc
alloy plating layer is preferably 2.8 wt%, more preferably 2.5
15 wt%, and most preferably 2.0 wt%.
Al in the zinc alloy plating layer is an element playing
a major role in improving the phosphatability of a plating steel
sheet by forming a Zn-Al-Mg-based intermetallic compound as Al
reacts with Zn and Mg in a plating layer, while inhibiting
20 formation of Mg oxide dross in a plating bath. If the content
of Al is significantly low, a Mg dross formation inhibitory
ability may be insufficient, and a sufficient amount of a
Zn-Al-Mg-based intermetallic compound in a surface structure
of a plating layer may not be secured, so a problem in which
25 an effect of improvement of phosphatability is insufficient may
Page 9
occur. Thus, a lower limit of the content of Al in the zinc alloy
plating layer is preferably 0.5 wt%, more preferably 0.6 wt%,
and most preferably 0.8 wt%. However, if the content of Al is
excessive, problems, in which an effect of improvement of
5 phosphatability is saturated and durability of a plating device
is adversely affected as a plating bath temperature increases,
may occur. Furthermore, a large amount of a Zn-Al-Mg-based
intermetallic compound is formed in a cross-sectional structure
of a plating layer, so a problem in which spot weldability
10 decreases may occur. Thus, an upper limit of the content of Al
in the zinc alloy plating layer is preferably 2.8 wt%, more
preferably 2.5 wt%, and most preferably 2.0 wt%.
Meanwhile, as described above, in order to improve
phosphatability and spot weldability of a zinc alloy plated
15 steel sheet simultaneously, it is necessary to appropriately
control a position distribution of a Zn single phase structure
and a Zn-Al-Mg-based intermetallic compound in a plating layer.
In this case, the Zn-Al-Mg-based intermetallic compound may be
at least one selected from the group consisting of a Zn/Al/MgZn2
20 ternary eutectic structure, a Zn/MgZn2 binary eutectic
structure, a Zn-Al binary eutectic structure, and an MgZn2
single phase structure.
A cross-sectional structure of the zinc alloy plating
layer preferably includes, by area percentage, a Zn single phase
25 structure of more than 50% (excluding 100%), more preferably
Page 10
a Zn single phase structure of 55% or more (excluding 100%),
and most preferably a Zn single phase structure of 60% or more
(excluding 100%). Here, the cross-sectional structure refers
to a microstructure observed in a cut section of a zinc alloy
5 plating layer, when a zinc alloy plated steel sheet is cut
vertically, that is, in a sheet thickness direction from a
surface thereof. As described above, as an area percentage of
a Zn single phase structure in a cross-sectional structure is
higher, it is advantageous in improving spot weldability. Thus,
10 in an exemplary embodiment, only a lower limit of an area
percentage of a Zn single phase structure in a cross-sectional
structure for securing desired spot weldability is limited, and
an upper limit thereof is not particularly limited. The
remainder, except for the Zn single phase structure, is formed
15 of a Zn-Al-Mg-based intermetallic compound.
A surface structure of the zinc alloy plating layer
preferably includes, by area percentage, a Zn-Al-Mg-based
intermetallic compound of 60% or more (excluding 100%), more
preferably a Zn-Al-Mg-based intermetallic compound of 70% or
20 more (excluding 100%), and most preferably a Zn-Al-Mg-based
intermetallic compound of 75% or more (excluding 100%). Here,
the surface structure refers to a microstructure observed in
a surface of a zinc alloy plated steel sheet. As described above,
as an area percentage of a Zn-Al-Mg-based intermetallic
25 compound in a surface structure is higher, it is advantageous
Page 11
in improving phosphatability of a zinc alloy plated steel sheet.
Thus, in an exemplary embodiment, only a lower limit of an area
percentage of a Zn-Al-Mg-based intermetallic compound in a
surface structure for securing desired phosphatability is
5 limited, and an upper limit thereof is not particularly limited.
The remainder, except for the Zn-Al-Mg-based intermetallic
compound, is formed of a Zn single phase structure.
According to an example, when an area percentage of a Zn
single phase structure of the cross-sectional structure is a,
10 and an area percentage of a Zn single phase structure of the
surface structure is b, a ratio of b to a (b/a) is 0.8 or less,
preferably 0.5 or less, and more preferably 0.4 or less. As
described above, the ratio of an area percentage of the Zn single
phase structure is appropriately controlled, so desired spot
15 weldability and phosphatability may be secured simultaneously.
A method of controlling a position distribution of the
Zn single phase structure and the Zn-Al-Mg-based intermetallic
compound in a plating layer, described above, may be provided
as various methods, so that the method of controlling the
20 position distribution thereof is not particularly limited.
However, by way of example, as will be described later, when
a plating layer in a molten state is cooled, a two-step cooling
method is introduced, so the position distribution described
above may be obtained.
25 Additionally, the contents of Al, Fe, and the like,
Page 12
solid-dissolved in a Zn single phase structure, are
appropriately controlled, so corrosion resistance of a zinc
alloy plated steel sheet may be further improved.
According to the related art, as an area percentage of
5 a Zn single phase structure is high, it is known that corrosion
resistance of a zinc alloy plated steel sheet is lowered, in
this regard, because, due to a corrosion potential difference
between the Zn single phase structure and the Zn-Al-Mg-based
intermetallic compound, local corrosion occurs in the Zn single
10 phase structure under a corrosive environment. Thus, research
is underway to inhibit a fraction of a Zn single phase structure
and to significantly increase a fraction of a Zn-Al-Mg-based
intermetallic compound, in a technical field, in which
excellent corrosion resistance is required.
15 However, in an exemplary embodiment, rather than by
inhibiting a fraction of a Zn single phase structure, by
significantly increasing the contents of Al, Fe, and the like,
solid-dissolved in a Zn single phase structure, a corrosion
potential difference between the Zn single phase structure and
20 the Zn-Al-Mg-based intermetallic compound is lowered, so as to
improve corrosion resistance of a zinc alloy plated steel sheet.
In detail, a Zn single phase structure is allowed to contain
Al and Fe to be supersaturated, so as to improve corrosion
resistance of a zinc alloy plated steel sheet.
25 On a phase diagram, a solid solution limit of Al with
Page 13
respect to Zn is 0.05 wt% and a solid solution limit of Fe with
respect to Zn is 0.01 wt%. Here, a case, in which a Zn single
phase structure contains Al and Fe to be supersaturated, refers
to a case, in which a Zn single phase structure includes more
5 than 0.05 wt% of Al and more than 0.01 wt% of Fe.
According to an example, the Zn single phase structure
may include 0.8 wt% or more of Al, and preferably 1.0 wt% or
more of Al.
According to an example, the content of Al contained in
10 the zinc alloy plating layer is c, and the content of Al contained
in the Zn single phase structure is d, a ratio of d to c (d/c)
may be 0.6 or more, and preferably 0.62 or more.
According to an example, the Zn single phase structure
may include 1.0 wt% or more of Fe, and preferably 1.5 wt% or
15 more of Fe.
When a Zn single phase structure contains Al and Fe to
be supersaturated, an effect of improvement of corrosion
resistance may be obtained. However, when the contents of Al
and Fe are controlled to be within the range described above,
20 an effect of significant improvement of corrosion resistance
may be obtained.
Meanwhile, as the contents of Al and Fe contained in a
Zn single phase structure are higher, it is advantageous in
improving corrosion resistance. Thus, in an exemplary
25 embodiment, an upper limit of the contents of Al and Fe is not
Page 14
particularly limited. However, if the sum of the contents of
Al and Fe is significantly high, workability of a zinc alloy
plated steel sheet may be deteriorated. In terms of preventing
deterioration of workability, the sum of the contents of Al and
5 Fe contained in the Zn single phase structure may be limited
to 8.0 wt% or less, and preferably 5.0 wt% or less.
According to an example, the Zn single phase structure
may include 0.05 wt% or less (including 0 wt%) of Mg. On a phase
diagram, a solid solution limit of Mg with respect to Zn is 0.05
10 wt%. Here, a case, in which 0.05 wt% or less (including 0 wt%)
of Mg is included, refers to a case, in which a Zn single phase
structure includes a solid solution limit or less of Mg.
As a research result of the present inventors, Mg
contained in a Zn single phase structure has no significant
15 effect on corrosion resistance of a zinc alloy plated steel
sheet. However, if the content of Mg is excessive, workability
of a zinc alloy plated steel sheet may be deteriorated. Thus,
it is preferable to manage the content of Mg contained in a Zn
single phase structure to a solid solution limit or less.
20 Here, a method of measuring concentrations of Al, Fe, and
Mg, contained in a Zn single phase structure, is not
particularly limited, and a following method may be used by way
of example. In other words, after a zinc alloy plated steel sheet
is vertically cut, a cross-sectional image thereof is taken at
25 a magnification of 3,000 times on a field emission scanning
Page 15
electron microscope (FE-SEM), and an energy dispersive
spectroscopy (EDS) is used to point-analyze a Zn single phase
structure, so concentrations of Al, Fe, and the like, may be
measured.
5 The method of controlling the contents of Al, Fe, and the
like, solid-dissolved in a Zn single phase structure, described
above, may be provided as various methods, and is not
particularly limited in an exemplary embodiment. However, by
way of example, as will be described later, a plating bath
10 insertion temperature of a base steel sheet and a plating bath
temperature are appropriately controlled, or a cooling method
during primary cooling is appropriately controlled, so the
contents of Al, Fe, and the like, described above, may be
obtained.
15 As described previously, a zinc alloy plated steel sheet
according to an exemplary embodiment described above may be
manufactured in various methods, and a method of manufacturing
the same is not particularly limited. However, the zinc alloy
plated steel sheet may be manufactured in a following method
20 by way of example.
First, after a base steel sheet is prepared, surface
activation of the base steel sheet is performed. The surface
activation allows a reaction between the base steel sheet and
a plating layer during hot dipping which will be described later
25 to be activated. As a result, the surface activation also has
Page 16
a significant effect on the contents of Al, Fe, and the like,
contained in a Zn single phase structure. However, the surface
activation is not necessarily performed, and may be omitted in
some cases.
5 In this case, an arithmetical average roughness Ra of the
base steel sheet, having been surface activated, may be 0.8 μm
to 1.2 μm, more preferably 0.9 μm to 1.15 μm, and most preferably
1.0 μm to 1.1 μm. Here, the arithmetical average roughness Ra
refers to an average height from a centerline (an arithmetical
10 mean line of profile) to a cross-sectional curve.
When the arithmetical average roughness Ra of a base steel
sheet is controlled to be within the range described above, it
is helpful in controlling the contents of Al, Fe, and the like,
contained in a Zn single phase structure to be within a desired
15 range.
A method of activating a surface of the base steel sheet
is not particularly limited, and surface activation of the base
steel sheet may be performed, for example, in a plasma treatment
or an excimer laser treatment. During the plasma treatment or
20 the excimer laser treatment, specific process conditions are
not particularly limited, and any device and/or condition may
be applied as long as a surface of a base steel sheet is uniformly
activated.
Thereafter, after a zinc alloy plating bath including,
25 by wt%, 0.5% to 2.8% of Al and 0.5% to 2.8% of Mg, with a remainder
Page 17
of Zn and inevitable impurities is prepared, a base steel sheet
is immersed in the zinc alloy plating bath, and a zinc alloy
plated steel sheet is obtained by performing plating.
In this case, a plating bath temperature is preferably
5 440°C to 460°C, and more preferably 445°C to 455°C. In addition,
a surface temperature of a base steel sheet entering a plating
bath is higher than the plating bath temperature, by preferably
5°C to 20°C, and by more preferably 10°C to 15°C. Here, the
surface temperature of a base steel sheet entering a plating
10 bath refers to a surface temperature of a base steel sheet
immediately before or immediately after immersing the base
steel sheet into a plating bath.
The plating bath temperature and the surface temperature
of a base steel sheet entering a plating bath have a significant
15 influence on development and growth of a Fe2Al5 inhibition layer
formed between a base steel sheet and a zinc alloy plating layer,
and have a significant influence on the contents of Al and Fe
eluted in a plating layer, thereby having a significant
influence on the contents of Al, Fe, and the like, contained
20 in a Zn single phase structure.
The plating bath temperature is controlled to be within
a range of 440°C to 460°C, and the surface temperature of a base
steel sheet entering a plating bath is controlled to be higher
than the plating bath temperature by 5°C to 20°C. Thus, the
25 contents of Al, Fe, and the like, contained in a Zn single phase
Page 18
structure may be appropriately secured.
Next, gas wiping is applied to the zinc alloy plated steel
sheet to adjust a plating adhesion amount. In order to smoothly
control a cooling rate and prevent surface oxidation of a
5 plating layer, the wiping gas is preferably a nitrogen (N2) gas
or an argon (Ar) gas.
In this case, a temperature of the wiping gas is preferably
30°C or more, more preferably 40°C or more, and most preferably
50°C or more. According to the related art, a temperature of
10 the wiping gas is controlled to be within a range of -20°C to
room temperature (25°C) in order to significantly increase
cooling efficiency. However, in order to significantly increase
the contents of Al, Fe, and the like, contained in a Zn single
phase structure, it is preferable to control a range of the
15 temperature of the wiping gas to be increased.
Next, the zinc alloy plated steel sheet is primarily
cooled. Primary cooling is an operation for sufficiently
securing a Zn single phase structure as a microstructure
observed in a cut cross section of a zinc alloy plating layer.
20 During the primary cooling, a cooling rate is preferably
5°C/sec or less (excluding 0°C/sec), more preferably 4°C/sec
or less (excluding 0°C/sec), and most preferably 3°C/sec or less
(excluding 0°C/sec). If the cooling rate exceeds 5°C/sec,
coagulation of a Zn single phase structure begins from a surface
25 of a plating layer, whose temperature is relatively low. Thus,
Page 19
a Zn single phase structure in a surface structure of the plating
layer may be excessively formed. Meanwhile, as the cooling rate
is slow, it is advantageous to secure a desired microstructure,
so a lower limit of the cooling rate is not particularly limited
5 during the primary cooling.
Moreover, during the primary cooling, a cooling end
temperature is preferably more than 380°C to 420°C or less, more
preferably 390°C or more to 415°C or less, and most preferably
395°C or more to 405°C or less. If the cooling end temperature
10 is 380°C or less, coagulation of a Zn single phase structure
and coagulation of a portion of a Zn-Al-Mg-based intermetallic
compound occur, so a desired structure may not be obtained.
Meanwhile, if the cooling end temperature exceeds 420°C,
coagulation of a Zn single phase structure may insufficiently
15 occur.
Thereafter, the zinc alloy plated steel sheet is
maintained at a constant temperature, such as the primary
cooling end temperature.
When the zinc alloy plated steel sheet is maintained at
20 a constant temperature, the holding time is preferably at least
one second, more preferably 5 seconds or more, and most
preferably at least 10 seconds. An alloy phase having a low
coagulation temperature is provided to maintain a liquid phase
and to induce partial coagulation of only a Zn single phase.
25 Meanwhile, as a constant temperature holding time is longer,
Page 20
it is advantageous to secure a desired microstructure, so an
upper limit of the constant temperature holding time is not
particularly limited.
Thereafter, the zinc alloy plated steel sheet is
5 secondarily cooled. Secondary cooling is an operation for
sufficiently securing a Zn-Mg-Al-based intermetallic compound
as a microstructure observed in a surface of a zinc alloy plated
steel sheet, by coagulating a remaining liquid-phase plating
layer.
10 During the secondary cooling, a cooling rate is preferably
10°C/sec or more, more preferably 15°C/sec or more, and most
preferably 20°C/sec or more. As described above, during the
secondary cooling, rapid cooling is performed, so coagulation
of a remaining liquid-phase plating layer may be induced in a
15 surface portion of a plating layer, whose temperature is
relatively low. Thus, a Zn-Mg-Al-based intermetallic compound
may be sufficiently secured as a surface structure of the
plating layer. If the cooling rate is less than 10°C/sec, a
Zn-Mg-Al-based intermetallic compound may be excessively
20 formed in a cross-sectional structure of a plating layer, and
a plating layer may be stuck on an upper roll of a plating device,
and the like, and then may be dropped off. Meanwhile, as the
cooling rate is increased, it is advantageous to secure a
desired microstructure, so an upper limit of the cooling rate
25 is not particularly limited during the secondary cooling.
Page 21
Moreover, during the secondary cooling, a cooling end
temperature is preferably 320°C or less, more preferably 300°C
or less, and most preferably 280°C or less. When the cooling
end temperature is in the range described above, complete
5 coagulation of a plating layer may be achieved. A change in a
temperature of a steel sheet thereafter does not affect a
fraction and a distribution of a microstructure of a plating
layer, so is not particularly limited.
Hereinafter, the present invention will be described more
10 specifically by way of examples. It should be noted, however,
that the following examples are intended to illustrate and
specify the present invention and not to limit the scope of the
present invention. The scope of the present invention is
determined by the matters described in the claims and matters
15 able to be reasonably inferred from the claims.
【Mode for Invention】
(Exemplary embodiment 1)
After a low carbon cold-rolled steel sheet having a
20 thickness of 0.8 mm, a width of 100 mm, and a length of 200 mm
was prepared as a test piece for plating, that is, a base steel
sheet, the base steel sheet was immersed in acetone, and then
was ultrasonic cleaned to remove foreign substances such as
rolling oil present on a surface, and the like. Thereafter, a
25 surface of the test piece for plating was plasma treated so as
Page 22
to control an arithmetical average roughness Ra in a range of
1.0 μm to 1.1 μm. Thereafter, in a hot dipping site according
to the related art, after a 750°C reduction atmosphere heat
treatment performed to secure mechanical properties of a steel
5 sheet was performed, the base steel sheet was immersed in a
plating bath having a composition in Table 1 to manufacture a
zinc alloy plated steel sheet. In this case, regarding every
exemplary embodiment, a plating bath temperature was uniformly
450°C, and a surface temperature of a base steel sheet entering
10 the plating bath was uniformly 460°C. Thereafter, respective
zinc alloy plated steel sheets, having been manufactured, had
gas wiping applied thereto with a nitrogen (N2) gas at 50°C to
control a plating adhesion amount to 70 g/m2 per side, and
cooling was performed under the conditions of Table 1.
15 Thereafter, a cross-sectional structure and a surface
structure of the zinc alloy plated steel sheet were observed
and analyzed, and a result thereof is illustrated in Table 2.
A microstructure of a plating layer was observed by a FE-SEM
(SUPRA-55VP, ZEISS). For example, the cross-sectional
20 structure is taken at a magnification of 1,000 times and the
surface structure is taken at a magnification of 300 times. A
microstructure fraction was analyzed using an image analysis
system.
Thereafter, the phosphatability and spot weldability of
25 the zinc alloy plated steel sheet were evaluated, and a result
Page 23
thereof is illustrated in Table 2.
Phosphatability was evaluated by the following method.
First, prior to phosphate treatment, respective zinc
alloy plated steel sheets, having been manufactured, were
5 degreasing treated. In this case, an alkaline degreasing agent
was used as a degreasing agent, and a degreasing treatment was
performed in a 3 wt% aqueous solution at 45°C for 120 seconds.
Thereafter, after washing and surface modifying, the zinc alloy
plated steel sheet was immersed in a phosphate treatment liquid,
10 heated to 40°C for 120 seconds, to form a zinc phosphate-based
coating film. Thereafter, with respect to the zinc
phosphate-based coating film, having been formed, a size of a
crystal and uniformity of a coating film were evaluated. A size
of a phosphate crystal was determined, as a surface was observed
15 at a magnification of 1,000 times using a scanning electronic
microscope (SEM), five large crystal sizes within a field of
view were averaged, and five fields of view were checked and
then were averaged.
Spot weldability was evaluated by the following method.
20 A Cu-Cr electrode having a tip diameter of 6 mm was used
to allow a welding current of 7 kA to flow, and welding was
continuously performed under conditions of a current carrying
time of 11 Cycles (Here, 1 Cycle refers to 1/60 seconds, the
same as above) and a holding time of 11 Cycles with a welding
25 force of 2.1 kN. When a thickness of a steel sheet is t, based
Page 24
on a spot in which a diameter of a nugget is smaller than 4√t,
spotting immediately before the spot was set as continuous
spotting. Here, as the continuous spotting is greater, spot
weldability is greater.
5
【Table 1】
No
.
Plating bath
composition
(wt%)
Primary cooling
condition
Constant
temperatu
re
maintenan
ce
condition
Secondary
cooling
condition
Remark
Al Mg Cooling
rate
(°C/s)
End
tempera
ture
(°C)
Maintaini
ng time
(s)
Cooling
rate
(°C/s)
End
tempera
ture
(°C)
1 0.2 - 2 400 10 20 280 Comparative
Example 1
2 0.5 0.7 2 400 10 20 280 Comparative
Example 2
3 0.8 0.9 2 400 10 20 280 Inventive
Example 1
4 1 1 2 400 10 20 280 Inventive
Example 2
5 1 1 12 - - 12 280 Comparative
Example 3
6 1.2 1.2 12 - - 12 280 Comparative
Example 4
Page 25
7 1.3 1.4 12 400 10 12 280 Inventive
Example 3
8 1.6 1.6 2 400 10 20 280 Inventive
Example 4
9 1.6 1.6 12 - - 12 280 Comparative
Example 5
10 2.5 2.5 2 400 10 20 280 Inventive
Example 5
11 3 3 2 400 10 20 280 Comparative
Example 6
Here, in Comparative Examples 3 through 5, without distinguishing primary cooling
and secondary cooling, cooling is performed at the same speed to a secondary cooling
end temperature.
【Table 2】
No
.
Cross-sectional
structure (area%)
Surface structure
(area%)
Phosphat
e crystal
size (μm)
Continu
ous
spottin
g
Remark
Zn
single
phase
Zn-Al-Mg
-based
intermet
allic
compound
Zn
single
phase
Zn-Al-Mg
-based
intermet
allic
compound
1 100 0 100 0 9.5 650 Comparative
Example 1
2 97 3 83 17 8.9 630 Comparative
Example 2
3 93 7 36 64 2.4 610 Inventive
Page 26
Example 1
4 91 9 21.3 78.7 2.1 600 Inventive
Example 2
5 92 8 53.8 46.2 6.8 650 Comparative
Example 3
6 89 11 62 38 4.1 610 Comparative
Example 4
7 73 27 14 86 1.8 615 Inventive
Example 3
8 62 38 17 83 1.8 580 Inventive
Example 4
9 85 15 41.6 58.4 5.3 600 Comparative
Example 5
10 61 39 11 89 2.2 580 Inventive
Example 5
11 21 79 7.2 92.8 1.9 200 Comparative
Example 6
Referring to Table 2, in a case of Inventive Examples 1
through 5 satisfying all the conditions of the present invention,
it is confirmed that phosphatability and spot weldability are
5 excellent simultaneously. On the other hand, in the case of
Comparative Examples 1 through 5, spot weldability was
excellent, but an area fraction of a Zn-Al-Mg-based
intermetallic compound in a surface structure was low, so it
was confirmed that phosphatability was inferior. In the case
10 of Comparative Example 6, phosphatability was excellent, but
an area fraction of a Zn single phase structure in a
Page 27
cross-sectional structure is low, so it was confirmed that spot
weldability was inferior.
Meanwhile, FIG. 1 is SEM images of a cross-sectional
structure of a zinc alloy plated steel sheet according to an
5 exemplary embodiment. Respective images (a) through (f) of FIG.
1 are SEM images of cross-sectional structures according to
Comparative Example 1, Inventive Example 2, Comparative Example
3, Inventive Example 4, Comparative Example 5, and Comparative
Example 6. In addition, FIG. 2 is SEM images of a surface
10 structure of a zinc alloy plated steel sheet according to an
exemplary embodiment. Respective images (a) through (f) of FIG.
2 are SEM images of surface structures according to Comparative
Example 1, Inventive Example 2, Comparative Example 3,
Inventive Example 4, Comparative Example 5, and Comparative
15 Example 6.
Moreover, FIG. 3 illustrates a surface, after a zinc alloy
plated steel sheet according to an exemplary embodiment was
phosphate-treated and the surface thereof was observed.
Respective images (a) through (e) of FIG. 3 illustrate surfaces,
20 after steel sheets according to Comparative Example 1,
Inventive Example 2, Comparative Example 3, Inventive Example
4, and Comparative Example 5 were phosphate-treated and the
surfaces thereof were observed. Referring to FIG. 3, it is
visually confirmed that uniformity of a coating film according
25 to Inventive Examples 1 and 4 is excellent.
Page 28
(Exemplary embodiment 2)
In Table 3, the content of each alloying element contained
in a Zn single phase structure of a zinc alloy plated steel sheet
5 according to an exemplary embodiment 1 and a corrosion
resistance evaluation result are illustrated.
In this case, for measurement of the content of each
alloying element contained in a Zn single phase structure, after
a zinc alloy plated steel sheet was vertically cut, a
10 cross-sectional image thereof was taken at a magnification of
3,000 times on a FE-SEM, and a EDS is used to point-analyze a
Zn single phase structure, so the content of each alloying
element was measured.
Moreover, for corrosion resistance evaluation, after
15 each zinc alloy plated steel sheet was charged in a salt spray
tester, the red rust occurrence time was measured by an
international standard (ASTM B117-11). In this case, 5% salt
water (at a temperature of 35°C, pH 6.8) was used, and 2ml/80cm2
of salt water was sprayed per hour.
20
【Table 3】
No. Plating bath
composition
(wt%)
Alloy content of Zn
single phase
structure (wt%)
d/c Salt
water
sprayi
ng time
Remark
Page 29
(h)
Al Mg Al Fe Mg
1 0.8 0.9 1.69 1.8 0.02 2.11 530 Inventive
Example1
2 1 1 1.38 2.3 0.01 1.38 610 Inventive
Example2
3 1.3 1.4 1.84 2.5 0.02 1.41 600 Inventive
Example3
4 1.6 1.6 1.71 2.1 0.02 1.06 650 Inventive
Example4
5 2.5 2.5 1.62 3.2 0.01 0.648 780 Inventive
Example5
c refers to the content of Al contained in a zinc alloy plating layer, and
d refers to the content of Al contained in a Zn single phase structure.
Referring to Table 3, in a case of Inventive Examples 1
through 5 satisfying all the conditions of the present invention,
the salt water spraying time was 500 hours or more, so it was
5 confirmed that corrosion resistance was excellent.
While the present disclosure has been particularly shown
and described with reference to exemplary embodiments thereof,
but is not limited thereto. It will be apparent to those skilled
in the art that various changes and modifications thereof may
10 be made within the spirit and scope of the present disclosure,
and therefore, it is to be understood that such changes and
modifications belong to the scope of the appended claims.

【WE CLAIM:】
【Claim 1】
A zinc (Zn) alloy plated steel sheet, the zinc alloy plated
5 steel sheet comprising a base steel sheet and a zinc alloy
plating layer,
wherein the zinc alloy plating layer includes 0.5 wt% to
2.8 wt% of aluminum (Al) and 0.5 wt% to 2.8 wt% of manganese
(Mn), with a remainder of Zn and inevitable impurities,
10 a cross-sectional structure of the zinc alloy plating
layer includes, by area percentage, a Zn single phase structure
of more than 50% (excluding 100%) and a Zn-Al-Mg-based
intermetallic compound of less than 50% (excluding 0%), and
a surface structure of the zinc alloy plating layer
15 includes, by area percentage, a Zn single phase structure of
40% or less (excluding 0%) and a Zn-Al-Mg-based intermetallic
compound of 60% or more (excluding 100%).
【Claim 2】
20 The zinc alloy plated steel sheet of claim 1, the zinc
alloy plating layer includes 0.8 wt% to 2.0 wt% of Al and 0.8
wt% to 2.0 wt% of Mg, with a remainder of Zn and inevitable
impurities.
Page 31
【Claim 3】
The zinc alloy plated steel sheet of claim 1, wherein,
when an area percentage of the Zn single phase structure of the
cross-sectional structure is a, and an area percentage of the
5 Zn single phase structure of the surface structure is b, a ratio
of b to a (b/a) is 0.8 or less.
【Claim 4】
The zinc alloy plated steel sheet of claim 1, wherein the
10 Zn-Al-Mg-based intermetallic compound is at least one selected
from the group consisting of a Zn/Al/MgZn2 ternary eutectic
structure, a Zn/MgZn2 binary eutectic structure, a Zn-Al binary
eutectic structure, and a MgZn2 single phase structure.
15 【Claim 5】
The zinc alloy plated steel sheet of claim 1, wherein the
Zn-Al-Mg-based intermetallic compound is at least one selected
from the group consisting of a Zn/Al/MgZn2 ternary eutectic
structure, a Zn/MgZn2 binary eutectic structure, a Zn-Al binary
20 eutectic structure, and a MgZn2 single phase structure.
【Claim 6】
The zinc alloy plated steel sheet of claim 1, wherein the
Zn single phase structure includes 0.8 wt% or more of Al.
Page 32
【Claim 7】
The zinc alloy plated steel sheet of claim 1, wherein,
when the content of Al contained in the zinc alloy plating layer
5 is c, and the content of Al contained in the Zn single phase
structure is d, a ratio of d to c (d/c) is 0.6 or more.
【Claim 8】
The zinc alloy plated steel sheet of claim 1, wherein the
10 Zn single phase structure contains 1 wt% or more of iron (Fe).
【Claim 9】
The zinc alloy plated steel sheet of claim 1, wherein the
sum of the contents of Al and Fe contained in the Zn single phase
15 structure is 8 wt% or less.
【Claim 10】
The zinc alloy plated steel sheet of claim 1, wherein the
Zn single phase structure includes 0.1 wt% or less of Mg
20 (including 0 wt%).
【Claim 11】
A method of manufacturing a zinc alloy plated steel sheet,
the method comprising:
Page 33
preparing a zinc alloy plating bath including 0.5 wt% to
2.8 wt% of Al and 0.5 wt% to 2.8 wt% of Mg, with a remainder
of Zn and inevitable impurities;
immersing a base steel sheet in the zinc alloy plating
5 bath, and obtaining a zinc alloy plated steel sheet by
performing plating;
gas wiping the zinc alloy plated steel sheet;
primary cooling the zinc alloy plated steel sheet at a
primary cooling rate of 5°C/sec or less (excluding 0°C/sec) to
10 a primary cooling end temperature of more than 380°C to 420°C
or less, after the gas wiping;
maintaining the zinc alloy plated steel sheet at a
constant temperature for at least one second at the primary
cooling end temperature, after the primary cooling; and
15 secondary cooling the zinc alloy plated steel sheet at
a secondary cooling rate of 10°C/sec or more to a secondary
cooling end temperature of 320°C or less, after the maintaining
the zinc alloy plated steel sheet at a constant temperature.
20 【Claim 12】
The method of claim 11, further comprising:
activating a surface of the base steel sheet, before the
base steel sheet is immersed in the zinc alloy plating bath.
25 【Claim 13】
Page 34
The method of claim 12, wherein the activating a surface
of the base steel sheet is performed by a plasma treatment or
an excimer laser treatment.
5 【Claim 14】
The method of claim 12, wherein an arithmetical average
roughness Ra of the base steel sheet, having been surface
activated, is 0.8 μm to 1.2 μm.
10 【Claim 15】
The method of claim 11, wherein a temperature of the zinc
alloy plating bath is from 440°C to 460°C.
【Claim 16】
15 The method of claim 11, wherein a surface temperature of
the base steel sheet entering the zinc alloy plating bath is
higher than a temperature of the zinc alloy plating bath by 5°C
to 20°C.
20 【Claim 17】
The method of claim 11, wherein the zinc alloy plating
bath includes 0.8 wt% to 2.0 wt% of Al and 0.8 wt% to 2.0 wt%
of Mg, with a remainder of Zn and inevitable impurities.
Page 35
【Claim 18】
The method of claim 11, wherein a temperature of a wiping
gas is 30°C or more, during the gas wiping.
【Claim 19】
5 The method of claim 11, wherein the primary cooling rate
is 3°C/sec or less (excluding 0°C/sec).
【Claim 20】
The method of claim 11, wherein the primary cooling end
temperature is from 400°C or more to 410°C or less.
10 【Claim 21】
The method of claim 11, wherein the zinc alloy plated steel
sheet is maintained at the primary cooling end temperature for
at least 10 seconds, during the maintaining the zinc alloy
plated steel sheet at a constant temperature.
15 【Claim 22】
The method of claim 11, wherein the secondary cooling rate
is 20°C/sec or more.