[Technical Field]
[0001] The present invention relates to a hot-dipped galvanized steel sheet, and more particularly,
to a hot-dipped galvanized steel sheet having improved deep drawing properties and
low-temperature adhesive brittleness, and a method of manufacturing the hot-dipped
galvanized steel sheet.
[Background Art]
[0002] In general, steel sheets are galvanized by passing a steel sheet through molten zinc
contained in a bath and solidifying the molten zinc applied to the steel sheet. When
molten zinc applied to a steel sheet solidifies, coarse dendritic crystal grains,
called spangles, are formed on the surface of the molten zinc. Generation of such
spangles is a characteristic of zinc solidification.
[0003] In detail, when molten zinc solidifies, crystals start to rapidly grow from solidification
nuclei in the shape of dendrites to form the basic structure of a zinc plating layer,
and pools of molten zinc remaining between the dendrites are subject to solidification.
Due to this solidification mechanism, spangles are generated. The size of spangles
may be determined by the basic structure of the zinc plating layer formed in an initial
stage of plating.
[0004] Such spangles decrease the adhesive strength of paint on a zinc plating layer and
the corrosion resistance of a steel sheet, and even though a zinc plating layer is
painted, spangles still make the surface of the zinc plating layer uneven and spoils
the appearance of a zinc-plated steel sheet because spangles can be seen through paint.
[0005] Therefore, an inorganic salt solution may be sprayed on a steel sheet before molten
zinc applied to the steel sheet is solidified, so as to minimize the size of spangles
formed on the steel sheet. At this time, the inorganic salt solution is sprayed on
the steel sheet through an electrode disposed on the front side of a nozzle. Since
the inorganic salt solution is sprayed through the electrode, droplets of the inorganic
salt solution are charged with static electricity and are thus easily attached to
the steel sheet by electrical attraction to minify the metallographic structure of
a zinc plating layer. A phosphate solution is widely used as the inorganic salt solution.
[0006] A plating layer having spangles of 150 µm or less can be formed on a steel sheet
by spraying droplets of a phosphate solution charged with electricity as described
above. In this case, the steel sheet can have a aesthetically-pleasing appearance,
improved image clarity after painting, and high corrosion resistance, and the plating
layer can be prevented from breaking into flaking during a press process.
[0007] In addition, when molten zinc solidifies, spangles are formed to have different shapes
depending on how hexagonal crystals of zinc are formed on the surface of a steel sheet.
In other words, since hexagonal crystals of zinc grow in different angles in different
regions of a steel sheet, spangles have different shapes.
[0008] FIG. 1A illustrates a hot-dipped galvanized steel sheet on which spangles having
a size of 150 µm or less are formed, and FIG. 1B illustrates a hot-dipped galvanized
steel sheet on which spangles having a size of 400 µm or greater are formed. Referring
to the hot-dipped galvanized steel sheet of FIG. 1B on which spangles having a size
of 400 µm or greater are formed, relatively large zinc crystals are randomly oriented,
advantageous in terms of brittleness but disadvantageous in terms of the appearance
of the steel sheet.
[0009] However, referring to the hot-dipped galvanized steel sheet of FIG. 1A on which spangles
having a size of 150 µm or less are formed, the spangles have the same crystallographic
orientation in a manner such that the basal planes of zinc, (0001) planes, are parallel
to the surface of the steel sheet. The above-mentioned crystallographic orientation
of zinc in which the basal planes of zinc crystals are parallel to the surface of
the steel sheet is known to be most effective in preventing corrosion, a black patina,
and chemical instability. Until recently, there have been many efforts to improve
such properties.
[0010] For example, Japanese Patent Application Laid-open Publication No.:
1999-100653 discloses a technique for adjusting the size of spangles to be within 60 µm to 1000
µm by spraying mist through a nozzle, and Japanese Patent Application Laid-open Publication
No.:
1996-188863 discloses a technique for adjusting the size of spangles to be 50 µm or less and
the level of surface roughness to be within 0.4 µm to 1.0 µm. In addition,
US Patent No.: 4500561 discloses a technique for decreasing the size of spangles to 1000 µm or less by using
droplets passed through an electric field.
[0011] Many automobile manufacturers have recently attempted to use structural adhesives
for joining steel sheets, in addition to or instead of using common existing welding
methods such as spot welding, for the purpose of reducing manufacturing costs, improving
stability, reducing work time, and making processes eco-friendly.
[0012] Unlike mechanical joining methods such as spot welding, structural adhesives are
used after determining whether plated steel sheets can be joined using the structural
adhesives at a low temperature of -40°C for the case of using automobiles in polar
regions. However, if an adhesive is used for a galvanized steel sheet in which the
(0001) planes of zinc crystals are parallel to the surface of the steel sheet and
thus spangles are not formed, a zinc plating layer may easily be stripped from the
steel sheet at a low temperature of -40°C or during a deep drawing process.
[0013] The brittleness of a zinc plating layer increases if spangles of the zinc plating
layer are small, and the (0001) planes (basal planes) of the zinc plating layer function
as slip planes or cleavage planes. Therefore, if a zinc plating layer formed on a
steel sheet has small spangles or the (0001) planes of zinc crystals of the zinc plating
layer are parallel to the surface of the steel sheet, the zinc plating layer may easily
be stripped from the steel sheet when the steel sheet is impacted.
[0014] Therefore, to deal with the recent methods of joining hot-dipped galvanized steel
sheets using structural adhesives, it is necessary to develop a hot-dipped galvanized
steel sheet having an improved appearance, deep drawing properties, and adhesive brittleness
at a low temperature.
[Disclosure]
[Technical Problem]
[0015] Aspects of the present invention provide a hot-dipped galvanized steel sheet having
improved deep drawing properties and low-temperature adhesive brittleness by controlling
the structure and grain size of a zinc plating layer, and a method of manufacturing
the hot-dipped galvanized steel sheet.
[Technical Solution]
[0016] According to an aspect of the present invention, there is provided a hot-dipped galvanized
steel sheet with improved deep drawing properties and low-temperature adhesive brittleness,
the hot-dipped galvanized steel sheet including a zinc plating layer, wherein grains
of the zinc plating layer have an average particle diameter of 150 µm to 400 µm, and
intensity of preferred orientation of (0001) planes of the zinc plating layer is from
3000 cps (count per second) to 20000 cps.
[0017] According to another aspect of the present invention, there is provided a method
of manufacturing a hot-dipped galvanized steel sheet, the method including: applying
molten zinc to a steel sheet; adjusting the amount of the molten zinc applied to the
steel sheet; spraying an aqueous solution on the steel sheet; cooling the steel sheet;
and performing a skin pass milling process on the steel sheet, wherein the spraying
of the aqueous solution includes spraying electrically charged demi-water (demineralized
water) on the steel sheet.
[Advantageous Effects]
[0018] According to the present invention, owing to the spraying of electrically charged
demi-water and a high reduction ratio of the skin pass milling process, grains of
the zinc plating layer of the hot-dipped galvanized steel sheet can have a reduced
size deviation, the intensity of orientation of the (0001) planes of zinc crystals
of the zinc plating layer can be lowered, and the volume fraction of crystallographic
twins of the zinc plating layer can be increased. Therefore, the hot-dipped galvanized
steel sheet can have improved properties such as deep drawing properties, bending
properties, and adhesive brittleness.
[Description of Drawings]
[0019] FIG. 1A is a schematic view illustrating the crystallographic structure of a hot-dipped
galvanized steel sheet of the related art on which spangles having a size of 150 µm
or less are formed, and FIG. 1B is a schematic view illustrating the crystallographic
structure of a hot-dipped galvanized steel sheet of the related art on which spangles
having a size of 400 µm or greater are formed.
[0020] FIG. 2A is an X-ray analysis graph illustrating the crystallographic orientation
of (0001) planes of a hot-dipped galvanized steel sheet of the related art on which
spangles having a size of 150 µm or less are formed, and FIG. 2B is an X-ray analysis
graph illustrating the crystallographic orientation of (0001) planes of a hot-dipped
galvanized steel sheet of the related art on which spangles having a size of 400 µm
or greater are formed.
[0021] FIGS. 3A and 3B are images illustrating evaluation results of low-temperature adhesive
brittleness of Comparative Samples 1 and Inventive Samples 1.
[Best Mode]
[0022] Hereinafter, the present invention will be described in detail.
[0023] According to embodiments of the invention, crystal grains of a zinc plating layer
of a hot-dipped galvanized steel sheet have an average particle diameter of 150 µm
to 400 µm.
[0024] If the average particle diameter of the crystal grains is less than 150 µm, the hot-dipped
galvanized steel sheet may have a beautiful appearance owing to small spangles but
the zinc plating layer may have unsatisfactory low-temperature adhesive brittleness.
On the other hand, if the average particle diameter of the crystal grains is greater
than 400 µm, even though the zinc plating layer may have satisfactory low-temperature
adhesive brittleness, the hot-dipped galvanized steel sheet may have a poor appearance
and image clarity, and the zinc plating layer may easily be separated from the hot-dipped
galvanized steel sheet during a continuous press process due to coarse spangles.
[0025] In the embodiments of the invention, it may be preferable that the crystal grains
of the zinc plating layer of the hot-dipped galvanized steel sheet have a minimum
diameter of 30 µm and the deviation of the diameters of the crystal grains be 40%
or less of the average particle diameter of the crystal grains.
[0026] If the zinc plating layer includes crystal grains having a diameter of 30 µm or less,
the crystal grains may be more brittle than surrounding crystal grains, and thus cracks
may start from the crystal grains. In addition, when the hot-dipped galvanized steel
sheet is bent, the zinc plating layer may be separated from the hot-dipped galvanized
steel sheet, and thus the formability of the hot-dipped galvanized steel sheet may
be deteriorated.
[0027] As described above, in the embodiments of the invention, it may be preferable that
the deviation of the diameters of the crystal grains of the zinc plating layer be
40% or less of the average particle diameter of the crystal grains. That is, it may
be preferable that the size of spangles formed on the zinc plating layer be uniform
within that range. If the deviation is greater than 40% and thus the size of zinc
crystals is not uniform, when the hot-dipped galvanized steel sheet undergoes plastic
deformation, the zinc plating layer may receive non-uniformly applied force and may
thus be partially separated from the hot-dipped galvanized steel sheet. Therefore,
to prevent problems related to adhesive brittleness, it may be preferable that the
deviation of the diameters of crystal grains be 40% or less of the average particle
diameter of the crystal grains.
[0028] In the embodiments of the invention, it may be preferable that the intensity of preferred
orientation of the (0001) planes of the zinc plating layer of the hot-dipped galvanized
steel sheet be from 3000 cps to 20000 cps (count per second). When the hot-dipped
galvanized steel sheet of the embodiments of the invention was irradiated with X-rays
generated under the conditions of 20 KV and 10 mA, the intensity of preferred orientation
of the zinc plating layer of the hot-dipped galvanized steel sheet was measured to
be from 3000 cps to 20000 cps. In detail, the maximum intensity of the (0001) planes
of zinc crystals was measured to be from 3000 cps to 20000 cps (the tilt angle of
a sample was 5°, and intensity values measured at intervals of 5° in a rotational
angle of 0° to 360° were averaged).
[0029] Referring to FIG. 2A, the intensity of preferred orientation of a hot-dipped galvanized
steel sheet of the related art on which spangles having a size of 150 µm or less are
formed is greater than 20,000 cps, and referring to FIG. 2B, the intensity of preferred
orientation of a hot-dipped galvanized steel sheet of the related art on which spangles
having a size of 400 µm or greater is formed on is less than 3000 cps.
[0030] In the embodiments of the invention, the intensity of preferred orientation of (0001)
planes is adjusted to be within the range of 3000 cps to 20000 cps. If the intensity
of preferred orientation of (0001) planes is less than 3000 cps, it is advantageous
in terms of the brittleness of a zinc plating layer but disadvantageous in terms of
appearance due to coarse spangles. On the other hand, if the intensity of preferred
orientation of (0001) planes is greater than 20000 cps, the appearance of a zinc plating
layer may be good, owing to small spangles but the deep drawing properties and low-temperature
brittleness of the zinc plating layer may be deteriorated.
[0031] In the embodiments of the invention, it may be preferable that the volume fraction
of crystallographic twins of the zinc plating layer of the hot-dipped galvanized steel
sheet be 30% or greater. Crystallographic twins may be present in the zinc plating
layer when the hot-dipped galvanized steel sheet is processed through a skin pass
milling process, and in zinc crystals having a hexagonal close-packed (HCP) structure,
crystallographic twins function as an important plastic deformation mechanism to facilitate
a deep drawing process and improve brittleness characteristics. If the volume fraction
of crystallographic twins of the zinc plating layer is less than 30%, plastic deformation
may be less facilitated, and the workability of the hot-dipped galvanized steel sheet
may be deteriorated particularly when the size of the zinc crystals of the zinc plating
layer is from 150 µm to 400 µm.
[0032] A method of manufacturing a hot-dipped galvanized steel sheet will now be described
in detail according to an embodiment of the invention.
[0033] In an embodiment of the invention, a method of manufacturing a hot-dipped galvanized
steel sheet includes: applying molten zinc to a steel sheet; adjusting the amount
of the molten zinc applied to the steel sheet; spraying an aqueous solution on the
steel sheet; cooling the steel sheet; and performing a skin pass milling process on
the steel sheet.
[0034] The spraying of the aqueous solution is performed by spraying electrically charged
demi-water (demineralized water) on the steel sheet.
[0035] In the embodiment of the invention, the applying of the molten zinc is performed
by passing the steel sheet through a zinc plating solution to attach molten zinc to
the steel sheet. In the embodiment of the invention, the applying of the molten zinc
is not limited to a particular method or process. That is, molten zinc may be applied
to the steel sheet using any zinc plating solution and process conditions that are
commonly used for manufacturing hot-dipped galvanized steel sheets in the art to which
the present invention pertains. The zinc plating solution may include aluminum (Al),
antimony (Sb), and/or lead (Pb). However, the embodiment of the invention is not limited
thereto. The steel sheet may be any kind of steel sheet. That is, any steel sheet
used for manufacturing a hot-dipped galvanized steel sheet in the related art may
be used.
[0036] In the adjusting of the amount of the molten zinc after the applying of the molten
zinc to the steel sheet, the steel sheet is air-wiped to remove an excessive amount
of the zinc plating solution from the steel sheet. The amount of the molten zinc applied
to the steel sheet may be adjusted to be any degree considered appropriate by those
of skill in the art to which the present invention pertains. That is, the amount of
the molten zinc applied to the steel sheet is not limited to any particular degree.
For example, the amount of the molten zinc applied to the steel sheet may be adjusted
according to the purpose of the steel sheet.
[0037] After the adjusting of the amount of the molten zinc applied to the steel sheet,
the spraying of the aqueous solution is performed by spraying electrically charged
demi-water on the steel sheet to solidify the molten zinc. The electrically charged
demi-water is sprayed so as to form a uniform zinc plating layer having uniformly
sized spangles. If a solution is electrically charged and sprayed in the form of mist,
droplets of the solution collide with a molten zinc plating layer and absorb heat
from the molten zinc plating layer to facilitate solidification of the molten zinc
plating layer. However, if an inorganic solution such as a phosphate solution is sprayed,
regions of the molten zinc plating layer colliding with nuclear particles such as
phosphate nuclear particles may lose heat much more quickly than other regions. Thus,
relatively small spangles may be formed on the regions, and relatively large spangles
may be formed on the other regions to increase the deviation of the sizes of the spangles.
[0038] If the deviation in the sizes of spangles is large, the zinc plating layer of the
hot-dipped galvanized steel sheet may not be uniformly stressed during a deep drawing
process, and thus cracks may start from relatively small spangles. In addition, when
the hot-dipped galvanized steel sheet is bent, the zinc plating layer may be separated
from the hot-dipped galvanized steel sheet. That is, a large deviation of the sizes
of spangles may deteriorate the formability of the hot-dipped galvanized steel sheet.
[0039] In the embodiment of the invention, it may be preferable that electrically charged
demi-water be sprayed through a nozzle at a demi-water injection pressure of 0.3 kgf/cm
2 to 5.0 kgf/cm
2, an air injection pressure of 0.5 kgf/cm
2 to 7.0 kgf/cm
2, and a demi-water pressure/air pressure ratio of 1/10 to 8/10.
[0040] If the demi-water is sprayed at a pressure of lower than 0.3 kgf/cm
2, spangles may not be minified. If the demi-water is sprayed at a pressure of greater
than 5.0 kgf/cm
2, pitting marks may be formed on the steel sheet while the steel sheet collide with
droplets of the demi-water, and thus the appearance of the steel sheet may be spoiled.
[0041] It may be preferable that the front side of the nozzle may be charged to have a voltage
of -1 KV to -25 KV. If the front side of the nozzle is charged to have a voltage of
less than -1 KV, electrical attraction may not be sufficient to minify droplets and
spangles. On the other hand, if the front side of the nozzle is charged to a voltage
of greater than -25 KV, spangles smaller than 150 µm may be formed on the zinc plating
layer, and thus deep drawing properties and adhesive brittleness may be deteriorated.
[0042] In the embodiment of the invention, after the spraying of demi-water, a skin pass
milling process is performed on the steel sheet. During the skin pass milling process,
crystallographic twins are formed in the zinc plating layer. The skin pass milling
process may be performed at an elongation of 5% or less.
[0043] In the embodiment of the invention, as described above, it may be preferable that
the skin pass milling process be performed at an elongation of 5% or less. During
the skin pass milling process, crystallographic twins are formed, which function as
an important processing mechanism in zinc crystals having an HCP structure because
the HCP structure has few deformation mechanisms. In addition, owing to physical deformation
by the skin pass milling process, the intensity of preferred orientation of the (0001)
planes of zinc crystals may be lowered. In other words, if the skin pass milling process
is not performed, the bonding between the zinc plating layer and the steel sheet may
not be firm, and the formability of the steel sheet may not be good. On the other
hand, if the skin pass milling process is performed at an elongation of greater than
5%, the properties of the steel sheet may be deteriorated even though the formability
and adhesiveness of the zinc plating layer are improved.
[Mode for Invention]
[0044] An example of the present invention will now be described in detail. However, the
present invention is not limited thereto.
(Example)
[0045] Hot-dipped galvanized steel sheets were treated with a phosphate solution or demi-water
under the conditions shown in Table 1 to adjust the size of spangles. Thereafter,
the steel sheets were treated through a skin pass milling process at an elongation
of 1.0% and a roll pressure of 200 tons to 240 tons, and the adhesive brittleness,
appearance, and image clarity of the hot-dipped galvanized steel sheets were measured
as shown in Table 1.
[0046] The hot-dipped galvanized steel sheets were prepared by performing a hot-dip galvanization
process on soft IF steel sheets having a thickness of 0.67 mm to form zinc plating
layers on the steel sheets at a plating density of 70 g/m
2.
[0047] Sizes and size deviations of spangles formed on the zinc plating layers were measured
and analyzed using an optical microscope and an image analyzer before the hot-dipped
galvanized steel sheets were treated through the skin pass milling process. The measured
and analyzed results are shown in the "spangle size" and "spangle size deviation"
columns of Table 1 below. Adhesive brittleness was measured by bonding two hot-dipped
galvanized steel sheets with an adhesive for automotive structural parts (Sealer Terokal
5089 by Henkel Korea, Ltd.), keeping the bonded hot-dipped galvanized steel sheets
at -40°C, impacting the hot-dipped galvanized steel sheets with a wedge, and observing
separation of zinc plating layers of the hot-dipped galvanized steel sheets. In Table
1, O denotes the case where a zinc plating layer was not stripped off, Δ denotes the
case where 20% or less of a zinc plating layer was stripped off, and X denotes the
case where 50% or more of a zinc plating layer was stripped off. Appearance and image
clarity were measured with the naked eye, and results thereof are denoted as good
(O), fair (Δ), and poor (X) in Table 1.
[Table 1]
No. |
Spraying Solution |
Spangle size (µm) |
Spangle size deviation (µm) |
Preferred orientation (cps) |
Adhesive brittleness |
Appearance |
Image quality |
* CS1 |
Demi-water |
150 |
99 |
28670 |
× |
○ |
○ |
** IS1 |
Demi-water |
250 |
81 |
10190 |
○ |
○ |
○ |
IS2 |
Demi-water |
350 |
126 |
4800 |
○ |
○ |
○ |
IS3 |
Demi-water |
400 |
- |
3253 |
○ |
○ |
Δ |
CS2 |
Phosphate |
150 |
- |
44214 |
× |
○ |
○ |
CS3 |
Phosphate |
250 |
106 |
11850 |
× |
○ |
○ |
CS4 |
Phosphate |
350 |
141 |
4215 |
Δ |
○ |
Δ |
CS5 |
Phosphate |
700 |
- |
1540 |
○ |
Δ |
× |
CS6 |
- |
1000 |
- |
954 |
○ |
× |
× |
*CS: Comparative Sample
**IS: Inventive Sample |
[0048] Referring to Table 1, Inventive Samples treated with demi-water have spangles within
a preferable size range, intensity of preferred orientation within the range of 3000
cps to 20000 cps, size deviations within a preferable range, good adhesive brittleness,
and good appearance.
[0049] Although Comparative Sample 1 treated with demi-water has spangles within a preferable
size range, Comparative Sample 1 has an unacceptably large spangle size deviation,
an unacceptably high degree of intensity of preferred orientation, and poor adhesive
brittleness. Comparative Samples 2 to 5 treated with a phosphate solution have unsatisfactory
adhesive brittleness or appearance. Comparative Sample 6, a general hot-dipped galvanized
steel sheet has poor appearance.
[0050] FIGS. 3A and 3B are images for evaluating adhesive brittleness of Comparative Samples
1 and Inventive Samples 1. Adhesive brittleness was evaluated based on whether a blue
adhesive remained. Referring to Comparative Samples 1 shown in FIG. 3A, as indicated
by circles, an adhesive does not remain after zinc plating layers are fractured. Referring
to Inventive Samples 1 shown in FIG. 3B, an adhesive remains owing to improved adhesive
brittleness.
1. A hot-dipped galvanized steel sheet with improved deep drawing properties and low-temperature
adhesive brittleness, the hot-dipped galvanized steel sheet comprising a zinc plating
layer, wherein grains of the zinc plating layer have an average particle diameter
of 150 µm to 400 µm, and intensity of preferred orientation of (0001) planes of the
zinc plating layer is from 3000 cps (count per second) to 20000 cps.
2. The hot-dipped galvanized steel sheet of claim 1, wherein the grains of the zinc plating
layer have a diameter of 30 µm or greater and a diameter deviation equal to or less
than 40% of the average particle diameter thereof.
3. The hot-dipped galvanized steel sheet of claim 1, wherein the zinc plating layer comprises
30% or more, by volume fraction, of crystallographic twins.
4. A method of manufacturing a hot-dipped galvanized steel sheet, the method comprising:
applying molten zinc to a steel sheet;
adjusting the amount of the molten zinc applied to the steel sheet;
spraying an aqueous solution on the steel sheet;
cooling the steel sheet; and
performing a skin pass milling process on the steel sheet,
wherein the spraying of the aqueous solution comprises spraying electrically charged
demi-water (demineralized water) on the steel sheet.
5. The method of claim 4, wherein the spraying of the electrically charged demi-water
is performed using a nozzle at a demi-water injection pressure of 0.3 kgf/cm2 to 5.0 kgf/cm2 and an air injection pressure of 0.5 kgf/cm2 to 7.0 kgf/cm2.
6. The method of claim 5, wherein the spraying of the electrically charged demi-water
is performed at a demi-water pressure/air pressure ratio of 1/10 to 8/10.
7. The method of claim 4, wherein the skin pass milling process is performed at an elongation
of 5% or less.