U.S. patent application number 15/770615 was filed with the patent office on 2018-11-08 for zinc alloy plated steel sheet having excellent bending workability and manufacturing method therefor.
The applicant listed for this patent is POSCO. Invention is credited to Jong-Sang KIM, Sang-Heon KIM, Tae-Chul KIM, Min-Suk OH, Bong-Hwan YOO, Hyun-Chu YUN.
Application Number | 20180320260 15/770615 |
Document ID | / |
Family ID | 58743980 |
Filed Date | 2018-11-08 |
United States Patent
Application |
20180320260 |
Kind Code |
A1 |
OH; Min-Suk ; et
al. |
November 8, 2018 |
ZINC ALLOY PLATED STEEL SHEET HAVING EXCELLENT BENDING WORKABILITY
AND MANUFACTURING METHOD THEREFOR
Abstract
Provided are a zinc alloy plated steel sheet and a method for
manufacturing the zinc alloy plated steel sheet. The zinc alloy
plated steel sheet includes a base steel sheet and a zinc alloy
plating layer, wherein the zinc alloy plating layer includes a Zn
single phase structure as a microstructure and a Zn--Al--Mg-based
intermetallic compound, and the Zn single phase structure has a
degree (f) of (0001) preferred orientation expressed by Formula 1
below within a range of 50% or greater. [Formula 1]
f(%)=(I.sub.basal/I.sub.total).times.100 where I.sub.total refers
to the integral of all diffraction peaks of the Zn single phase
structure when an X-ray diffraction pattern is measured within a
range of 2 theta from 10.degree. to 100.degree. using a Cu-K.alpha.
source, and I.sub.basal refers to the integral of diffraction peaks
of the Zn single phase relating to a basal plane.
Inventors: |
OH; Min-Suk; (Gwangyang-si,
KR) ; KIM; Sang-Heon; (Gwangyang-si, KR) ;
KIM; Tae-Chul; (Gwangyang-si, KR) ; KIM;
Jong-Sang; (Gwangyang-si, KR) ; YUN; Hyun-Chu;
(Seoul, KR) ; YOO; Bong-Hwan; (Gwangyang-si,
KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
POSCO |
Pohang-si |
|
KR |
|
|
Family ID: |
58743980 |
Appl. No.: |
15/770615 |
Filed: |
October 26, 2016 |
PCT Filed: |
October 26, 2016 |
PCT NO: |
PCT/KR2016/012098 |
371 Date: |
April 24, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 18/04 20130101;
C23C 2/40 20130101; C23C 2/06 20130101; C23C 2/26 20130101; C23C
2/20 20130101 |
International
Class: |
C23C 2/06 20060101
C23C002/06; C23C 2/40 20060101 C23C002/40; C23C 2/26 20060101
C23C002/26; C23C 2/20 20060101 C23C002/20 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 26, 2015 |
KR |
10-2015-0148793 |
Oct 26, 2016 |
KR |
10-2016-0140342 |
Claims
1. A zinc alloy plated steel sheet comprising a base steel sheet
and a zinc alloy plating layer, wherein the zinc alloy plating
layer comprises a Zn single phase structure as a microstructure and
a Zn--Al--Mg-based intermetallic compound, and the Zn single phase
structure has a degree (f) of (0001) preferred orientation,
expressed by Formula 1 below, within a range of 50% or greater,
f(%)=(I.sub.basal/I.sub.total).times.100 [Formula 1] where
I.sub.total refers to an integral of all diffraction peaks of the
Zn single phase structure when an X-ray diffraction pattern is
measured within a range of 2 theta from 10.degree. to 100.degree.
using a Cu-K.alpha. source, and I.sub.basal refers to an integral
of diffraction peaks of the Zn single phase structure relating to a
basal plane.
2. The zinc alloy plated steel sheet of claim 1, wherein the Zn
single phase structure has a degree (f) of (0001) preferred
orientation, expressed by Formula 1, within a range of 60% or
greater.
3. The zinc alloy plated steel sheet of claim 1, wherein the
Zn--Al--Mg-based intermetallic compound comprises at least one
selected from the group consisting of a Zn/MgZn.sub.2 binary
eutectic structure, a Zn/Al binary eutectic structure, an
MgZn.sub.2 single phase structure, and a Zn/Al/MgZn.sub.2 ternary
eutectic structure.
4. The zinc alloy plated steel sheet of claim 1, wherein an area
fraction of the Zn single phase structure on a surface of the zinc
alloy plating layer is 40% or less (excluding 0%).
5. The zinc alloy plated steel sheet of claim 1, wherein a total
area fraction of a Zn/MgZn.sub.2 binary eutectic structure and a
Zn/Al/MgZn.sub.2 ternary eutectic structure is 50% or greater
(excluding 100%) on a surface of the zinc alloy plating layer.
6. The zinc alloy plated steel sheet of claim 1, wherein an area
fraction of an MgZn.sub.2 single phase structure on a surface of
the zinc alloy plating layer is 10% or less (excluding 0%).
7. The zinc alloy plated steel sheet of claim 1, wherein an average
grain diameter of the Zn single phase structure observed on a
cross-section of the zinc alloy plating layer taken in a sheet
thickness direction is 15 .mu.m or less (excluding 0 .mu.m).
8. The zinc alloy plated steel sheet of claim 1, wherein the zinc
alloy plating layer comprises, by wt %, aluminum (Al): 0.5% to 3%,
magnesium (Mg): 0.5% to 3%, and a balance of zinc (Zn) and
inevitable impurities.
9. The zinc alloy plated steel sheet of claim 1, wherein the zinc
alloy plating layer satisfies Formula 1 below:
1.0<[Mg]/[Al].ltoreq.4.0 [Formula 2] where [Mg] and [Al] refer
to weight percentages (wt %) of corresponding elements,
respectively.
10. The zinc alloy plated steel sheet of claim 1, wherein a number
of black spots per unit area is 0.1/cm.sup.2 or less on a surface
of the zinc alloy plated steel sheet.
11. A method for manufacturing a zinc alloy plated steel sheet, the
method comprising: preparing a zinc alloy plating bath comprising
magnesium (Mg) and aluminum (Al); obtaining a zinc alloy plated
steel sheet by dipping a base steel sheet into the zinc alloy
plating bath to plate the base steel sheet; wiping the zinc alloy
plated steel sheet with gas to adjust a plating weight; and after
adjusting the plating weight of the zinc alloy plated steel sheet,
cooling the zinc alloy plated steel sheet by spraying droplets of
water or an aqueous solution onto the zinc alloy plated steel sheet
and then using air, wherein when the droplets are sprayed, a
droplet spray start temperature ranges from 405.degree. C. to
425.degree. C., a droplet spray stop temperature ranges from
380.degree. C. to 400.degree. C.
12. The method of claim 11, wherein when the droplets are sprayed,
a difference between the droplet spray start temperature and the
droplet spray stop temperature is 15.degree. C. or greater.
13. The method of claim 11, wherein the droplets are sprayed by a
charge spray method to attach the droplets by electrostatic
attraction between the droplets and the zinc alloy plated steel
sheet.
14. The method of claim 11, wherein the droplets are sprayed in an
amount of 50 g/m.sup.2 to 100 g/m.sup.2.
15. The method of claim 11, wherein the aqueous solution is a
phosphate aqueous solution.
16. The method of claim 15, wherein the phosphate aqueous solution
comprises at least one selected from the group consisting of an
aqueous solution of ammonium hydrogen phosphate
((NH.sub.4).sub.2HPO.sub.4), an aqueous solution of sodium ammonium
hydrogen phosphate (NaNH.sub.4HPO.sub.4), an aqueous solution of
zinc dihydrogen phosphate (Zn(H.sub.2PO.sub.4).sub.2), and an
aqueous solution of calcium phosphate
(Ca.sub.3(PO.sub.4).sub.2).
17. The method of claim 15, wherein the phosphate aqueous solution
has a concentration of 0.5 wt % to 5 wt %.
18. The method of claim 11, wherein the zinc alloy plating bath
comprises, by wt %, aluminum (Al): 0.5% to 3%, magnesium (Mg): 0.5%
to 3%, and a balance of zinc (Zn) and inevitable impurities.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to a zinc alloy plated steel
sheet having high bending workability and a method for
manufacturing the zinc alloy plated steel sheet.
BACKGROUND ART
[0002] A zinc plating method for suppressing the corrosion of iron
by cathodic protection has high anti-corrosion efficiency and
economic feasibility, and thus has been widely used in
manufacturing steel materials having high corrosion resistance.
Particularly, hot-dip zinc plated steel sheets, manufactured by
dipping a steel material into molten zinc to form a plating layer,
are obtainable through simple manufacturing processes and are
relatively inexpensive, as compared to electro-zinc plated steel
sheets, and thus, demand therefor has increased in a wide range of
industries, such as the automotive industry, the home appliance
industry, and the construction industry.
[0003] When a hot-dip zinc plated steel sheet is exposed to a
corrosive environment, zinc having a lower oxidation-reduction
potential than iron undergoes corrosion first, and thus, corrosion
of the steel sheet is suppressed by sacrificial corrosion
protection. Along with this, compact corrosion products are formed
on the surface of the steel sheet as zinc of a plating layer is
oxidized, thereby protecting the steel sheet from the corrosive
environment and improving the corrosion resistance of the steel
sheet.
[0004] However, air pollution and corrosive environments have
increased with industrial advances, and regulations on resource and
energy savings have been tightened. Therefore, the need to develop
a steel material having higher corrosion resistance than existing
zinc plated steel sheets has increased.
[0005] In this regard, research has been variously conducted into
techniques for manufacturing zinc alloy-based plated steel sheets
having corrosion resistance improved by adding elements such as
aluminum (Al) and magnesium (Mg) to a zinc plating bath. Techniques
for manufacturing a Zn--Al--Mg-based zinc alloy plated steel sheet,
which is representative of zinc alloy-based plated steel sheets and
manufactured by additionally adding magnesium (Mg) to a Zn--Al
plating composition, have been actively researched.
[0006] However, such a Zn--Al--Mg-based zinc alloy plated steel
sheet has poor bending workability. That is, the zinc alloy plated
steel sheet includes large amounts of Zn--Al--Mg-based
intermetallic compounds in a plating layer thereof as a result of
thermodynamic reaction between zinc (Zn), aluminum (Al), and
magnesium (Mg), and such intermetallic compounds may cause cracks
in the plating layer during a bending process because of high
hardness of the intermetallic compounds, thereby lowering the
bending workability of the zinc alloy plated steel sheet.
DISCLOSURE
Technical Problem
[0007] Aspects of the present disclosure may provide a zinc alloy
plated steel sheet having high bending workability and a method for
manufacturing the zinc alloy plated steel sheet.
[0008] The present disclosure is not limited to the above-mentioned
aspects. Other aspects of the present disclosure are stated in the
following description, and the aspects of the present disclosure
will be clearly understood by those of ordinary skill in the art
through the following description.
Technical Solution
[0009] According to an aspect of the present disclosure, a zinc
alloy plated steel sheet may include a base steel sheet and a zinc
alloy plating layer, wherein the zinc alloy plating layer may
include a Zn single phase structure as a microstructure and a
Zn--Al--Mg-based intermetallic compound, and the Zn single phase
structure may have a degree (f) of (0001) preferred orientation,
expressed by Formula 1 below, within a range of 50% or greater,
f(%)=(I.sub.basal/I.sub.total).times.100 [Formula 1]
[0010] where I.sub.total refers to an integral of all diffraction
peaks of the Zn single phase structure when an X-ray diffraction
pattern is measured within a range of 2 theta from 10.degree. to
100.degree. using a Cu-K.alpha. source, and I.sub.basal refers to
an integral of diffraction peaks of the Zn single phase structure
relating to a basal plane.
[0011] According to another aspect of the present disclosure, a
method for manufacturing a zinc alloy plated steel sheet may
include: preparing a zinc alloy plating bath including magnesium
(Mg) and aluminum (Al); obtaining a zinc alloy plated steel sheet
by dipping a base steel sheet into the zinc alloy plating bath to
plate the base steel sheet; wiping the zinc alloy plated steel
sheet with gas to adjust a plating weight; and after adjusting the
plating weight of the zinc alloy plated steel sheet, cooling the
zinc alloy plated steel sheet by spraying droplets of water or an
aqueous solution onto the zinc alloy plated steel sheet and then
using air, wherein when the droplets are sprayed, a droplet spray
start temperature ranges from 405.degree. C. to 425.degree. C., a
droplet spray stop temperature ranges from 380.degree. C. to
400.degree. C.
Advantageous Effects
[0012] According to one of various effects of the present
disclosure, an embodiment of the present disclosure provides a zinc
alloy plated steel sheet having high bending workability as well as
high corrosion resistance.
[0013] In addition, according to one of various effects of the
present disclosure, the zinc alloy plated steel sheet of the
embodiment has high surface quality.
[0014] In addition, according to one of various effects of the
present disclosure, the zinc alloy plated steel sheet of the
embodiment has high scratch resistance.
DESCRIPTION OF DRAWINGS
[0015] FIG. 1 is views illustrating results of (a) an observation
of a surface microstructure of Inventive Sample 1 and (b) an
observation of a surface microstructure of Comparative Sample
5.
[0016] FIG. 2 is views illustrating results of (a) an observation
of a cross-sectional microstructure of Inventive Sample 1 and (b)
an observation of a cross-sectional microstructure of Comparative
Sample 5.
[0017] FIG. 3 is a view illustrating results of X-ray
diffractometer (XRD) analysis of Inventive Sample 1.
BEST MODE
[0018] Hereinafter, a zinc alloy plated steel sheet having high
bending workability will be described in detail according to an
aspect of the present disclosure.
[0019] According to the aspect of the present disclosure, the zinc
alloy plated steel sheet includes a base steel sheet and a zinc
alloy plating layer. In the present disclosure, the base steel
sheet is not limited to a particular type. For example, a
hot-rolled steel sheet or a cold-rolled steel sheet commonly used
as a base steel sheet of a zinc alloy plated steel sheet may be
used. However, hot-rolled steel sheets have a large amount of
surface oxide scale that lowers plating adhesion and thus plating
quality, and thus a hot-rolled steel sheet from which oxide scale
has been previously removed using an acid solution may be used as
the base steel sheet. In addition, the zinc alloy plating layer may
be formed on one or each side of the base steel sheet.
[0020] The zinc alloy plating layer may include, by wt %, aluminum
(Al): 0.5% to 3%, magnesium (Mg): 0.5% to 3%, and the balance of
zinc (Zn) and inevitable impurities.
[0021] In the zinc alloy plating layer, magnesium (Mg) reacts with
zinc (Zn) and aluminum (Al) and forms a Zn--Al--Mg-based
intermetallic compound, thereby functioning as a key element
improving the corrosion resistance of the zinc alloy plated steel
sheet. If the content of magnesium (Mg) is excessively low, the
Zn--Al--Mg-based intermetallic compound is not present in
sufficient amounts in the microstructure of the zinc alloy plating
layer, and thus corrosion resistance may not be sufficiently
improved. Therefore, the amount of magnesium (Mg) in the zinc alloy
plating layer may be 0.5 wt % or greater, preferably 1.0 wt % or
greater. However, if the content of magnesium (Mg) is excessively
high, the effect of improving corrosion resistance is saturated,
and Mg oxide dross having a negative effect on platability may be
formed in a plating bath. In addition, the Zn--Al--Mg-based
intermetallic compound having high harness may be formed in
excessively large amounts in the microstructure of the zinc alloy
plating layer, and thus bending workability may be lowered.
Therefore, the amount of magnesium (Mg) in the zinc alloy plating
layer may be 3 wt % or less, preferably 2.9 wt % or less.
[0022] Aluminum (Al) suppresses the formation of Mg oxide dross and
reacts with zinc (Zn) and magnesium (Mg) to form the
Zn--Al--Mg-based intermetallic compound in the zinc alloy plating
layer, thereby functioning as a key element improving the corrosion
resistance of the zinc alloy plated steel sheet. If the content of
aluminum (Al) is excessively low, the formation of Mg dross is not
sufficiently suppressed, and the Zn--Al--Mg-based intermetallic
compound is not present in sufficient amounts in the microstructure
of the zinc alloy plating layer, which may result in insufficient
improvements in corrosion resistance. Therefore, the amount of
aluminum (Al) in the zinc alloy plating layer may be 0.5 wt % or
greater, preferably 0.6 wt % or greater. However, if the content of
aluminum (Al) is excessively high, the effect of improving
corrosion resistance is saturated, and the durability of plating
equipment may be negatively affected because of a high plating bath
temperature. Moreover, the Zn--Al--Mg-based intermetallic compound
having high harness may be formed in excessively large amounts in
the microstructure of the zinc alloy plating layer, and thus
bending workability may be lowered. Therefore, the amount of
aluminum (Al) in the zinc alloy plating layer may be 3 wt % or
less, preferably 2.6 wt % or less.
[0023] According to an embodiment, the contents of magnesium (Mg)
and aluminum (Al) in the zinc alloy plating layer may satisfy the
following Formula 2. If [Mg]/[Al] is 1.0 or less, scratch
resistance may deteriorate, and if [Mg]/[Al] is greater than 4.0,
Mg-based dross may be formed in large amounts in a hot-dip plating
bath to lower workability.
1.0<[Mg]/[Al].ltoreq.4.0 [Formula 2]
[0024] where [Mg] and [Al] refer to the weight percentages (wt %)
of corresponding elements, respectively.
[0025] The zinc alloy plating layer may include a Zn single phase
structure as a microstructure and the Zn--Al--Mg-based
intermetallic compound. In the present disclosure, the
Zn--Al--Mg-based intermetallic compound is not limited to a
particular type. However, for example, the Zn--Al--Mg-based
intermetallic compound may include at least one selected from the
group consisting of a Zn/Al/MgZn.sub.2 ternary eutectic structure,
a Zn/MgZn.sub.2 binary eutectic structure, a Zn/Al binary eutectic
structure, and an MgZn.sub.2 single phase structure.
[0026] The inventors have conducted in-depth research into
improving the bending workability of zinc alloy plated steel sheets
and found that if a Zn single phase structure having a hexagonal
close packing (HCP) structure is grown in a (0001) orientation in
the microstructure of the zinc alloy plating layer, ductility
increases owing to easy slippage, and thus cracks are markedly
reduced in a bending process.
[0027] In the present disclosure, to obtain this effect, the degree
(f) of (0001) preferred orientation, expressed by the following
formula 1, may preferably be adjusted to be 50% or greater, more
preferably 60% or greater.
f(%)=(I.sub.basal/I.sub.total).times.100 [Formula 1]
[0028] where I.sub.total refers to the integral of all diffraction
peaks of the Zn single phase structure when an X-ray diffraction
pattern is measured within the range of 2 theta from 10.degree. to
100.degree. using a Cu-K.alpha. source, and I.sub.basal refers to
the integral of diffraction peaks of the Zn single phase structure
relating to a basal plane.
[0029] In addition, the inventors have found that if the Zn single
phase structure coarsely formed in the zinc alloy plating layer is
refined in size, it is also helpful to reduce cracking during a
bending process.
[0030] To obtain this effect of the present disclosure, the average
grain diameter of the Zn single phase structure may be preferably
adjusted to be 15 .mu.m or less, more preferably 12 .mu.m or less,
and even more preferably 10 .mu.m or less. The "average grain
diameter" of the Zn single phase structure refers to the average of
equivalent circular diameters of the Zn single phase structure
measured by observing a thicknesswise cross-section of the zinc
alloy plating layer.
[0031] The zinc alloy plated steel sheet of the present disclosure
has high corrosion resistance and bending workability as well.
[0032] According to an embodiment, the zinc alloy plated steel
sheet of the present disclosure may have a good appearance.
Specifically, the number of black spots per unit area may be equal
to or less than 0.1/cm.sup.2 on the surface of the zinc alloy
plated steel sheet.
[0033] To obtain these effects of the present disclosure, the area
fraction of the Zn single phase structure may preferably be 40% or
less (excluding 0%) on the surface of the zinc alloy plating layer.
That is, the appearance of the zinc alloy plated steel sheet may be
improved by maximizing the fraction of the Zn--Al--Mg-based
intermetallic compound present on the surface of the zinc alloy
plating layer.
[0034] According to an embodiment, the zinc alloy plated steel
sheet of the present disclosure may also have high scratch
resistance.
[0035] According to results of research conducted by the inventors,
if the area fractions of the Zn/MgZn.sub.2 binary eutectic
structure and the Zn/Al/MgZn.sub.2 ternary eutectic structure which
have a layer structure and are present on the surface of the zinc
alloy plating layer are maximized, scratch resistance may be
markedly improved.
[0036] To obtain this effect of the present disclosure, preferably,
the sum of the area fractions of the Zn/MgZn.sub.2 binary eutectic
structure and the Zn/Al/MgZn.sub.2 ternary eutectic structure may
be 50% or greater (excluding 100%), and the area fraction of the
MgZn.sub.2 single phase structure may be 10% or less (including
0%). The MgZn.sub.2 single phase structure has high hardness and
thus causes cracks during a machining process, and thus the area
fraction of the MgZn.sub.2 single phase structure may be adjusted
to be as low as possible.
[0037] The zinc alloy plated steel sheet of the present disclosure
may be manufactured by various methods without limitation. However,
for example, when the zinc alloy plating layer solidifies from a
molten state, the zinc alloy plating layer may be cooled by
spraying droplets thereon and then cooled with air to obtain the
above-described degree of preferred orientation and average grain
diameter.
[0038] In this case, droplets may be sprayed by a charge spray
method to attach the droplets by electrostatic attraction between
the droplets and the zinc alloy plated steel sheet. This charge
spray method may be helpful in forming fine, uniform droplets and
reducing the amount of droplets colliding with and bouncing off the
zinc alloy plated steel sheet after being sprayed on the zinc alloy
plated steel sheet, thereby facilitating rapid cooling of the zinc
alloy plating layer from the molten state and having a positive
effect on the growth of the Zn single phase structure in the (0001)
orientation and refinement of the Zn single phase structure.
[0039] The droplets may be droplets of a phosphate aqueous solution
capable of rapidly cooling the zinc alloy plating layer from the
molten state through an endothermic reaction and thus effective in
growing the Zn single phase structure in the (0001) orientation and
refining the Zn single phase structure. Examples of the phosphate
aqueous solution may include an aqueous solution of ammonium
hydrogen phosphate ((NH.sub.4).sub.2HPO.sub.4), an aqueous solution
of sodium ammonium hydrogen phosphate (NaNH.sub.4HPO.sub.4), an
aqueous solution of zinc dihydrogen phosphate
(Zn(H.sub.2PO.sub.4).sub.2), and an aqueous solution of calcium
phosphate (Ca.sub.3(PO.sub.4).sub.2).
[0040] In addition, the content of the phosphate aqueous solution
may be 1 wt % to 3 wt %. If the content of the phosphate aqueous
solution is less than 1 wt %, the effect of the phosphate aqueous
solution may not be sufficient. If the content of the phosphate
aqueous solution is greater than 3 wt %, the effect of the
phosphate aqueous solution is saturated, and nozzle clogging may
occur in a continuous production process, lowering
productivity.
[0041] In addition, when the droplets may be sprayed at a droplet
spray start temperature of 405.degree. C. to 425.degree. C., and
more preferably 410.degree. C. to 420.degree. C. Here, the term
"droplet spray start temperature" refers to a surface temperature
of the zinc alloy plated steel sheet at the start time of droplet
spraying. If the droplet spray start temperature is less than
405.degree. C., solidification of the Zn single phase structure may
have already started, and thus black spots may be formed on the
surface of the zinc alloy plated steel sheet. Conversely, if the
droplet spray start temperature is greater than 425.degree. C.,
droplets may not effectively undergo an endothermic reaction, and
thus it may be difficult to obtain an intended structure.
[0042] In addition, the droplets may be sprayed at a droplet spray
stop temperature of 380.degree. C. to 400.degree. C., and more
preferably 390.degree. C. to 400.degree. C. Here, the term "droplet
spray stop temperature" refers to a surface temperature of the zinc
alloy plated steel sheet at a point in time at which spraying of
droplets stops. If the droplet spray stop temperature is greater
than 400.degree. C., an endothermic reaction by the droplets may
occur ineffectively, and thus it may be difficult to obtain an
intended structure. Conversely, if the droplet spray stop
temperature is less than 380.degree. C., a Mg.sub.2Zn.sub.11 phase
may be formed due to over cooling while the Zn/MgZn.sub.2 binary
eutectic phase and the Zn/Al/MgZn.sub.2 ternary phase start to
solidify, and thus many black spots may be formed, decreasing the
degree of (0001) preferred orientation of the Zn single phase
structure.
[0043] In addition, the difference between the droplet spray start
temperature and the droplet spray stop temperature may be
15.degree. C. or greater. If the difference is less than 15.degree.
C., the droplets may not undergo an effective endothermic reaction,
and thus it may be difficult to obtain an intended structure.
[0044] In addition, the droplets may be sprayed in an amount of 50
g/m.sup.2 to 100 g/m.sup.2. If the spraying amount of the droplets
is less than 50 g/m.sup.2, the effect of the droplets may be
insufficient, and if the spraying amount of the droplets is greater
than 100 g/m.sup.2, the effect of the droplets may be
saturated.
MODE FOR INVENTION
[0045] Hereinafter, the present disclosure will be described more
specifically through examples. However, the following examples
should be considered in a descriptive sense only and not for
purpose of limitation. The scope of the present invention is
defined by the appended claims, and modifications and variations
reasonably made therefrom.
Example 1
[0046] Low carbon cold-rolled steel sheets each having a thickness
of 0.8 mm, a width of 100 mm, and a length of 200 mm were prepared
as base steel sheets for plating test samples, and then foreign
substances such as rolling oil were removed from the surfaces of
the base steel sheets by dipping the base steel sheets into acetone
and washing the base steel sheets with ultrasonic waves.
Thereafter, a 750.degree. C. reducing atmosphere heat treatment
commonly performed to guarantee mechanical characteristics of steel
sheets in the hot-dipping plating field was performed on the base
steel sheets, and then the base steel sheets were dipped into
plating baths (bath temperature: 460.degree. C.) having
compositions shown in Table 1 below to fabricate zinc alloy plated
steel sheets. Thereafter, each of the zinc alloy plated steel
sheets was wiped with gas to adjust a plating weight to be 70
g/m.sup.2 on each side. Then, the zinc alloy plated steel sheets
were cooled under the conditions shown in Table 1 below and were
cooled with air. Although not shown in Table 1 below, Comparative
Sample 5 was prepared by performing a gas wiping process on a zinc
alloy plated steel sheet fabricated using the same plating bath as
that used to fabricate Inventive Sample 1 to adjust a plating
weight to be 70 g/m.sup.2 on each side, and then cooling the zinc
alloy plated steel sheet using a general cooling device at an
average cooling rate of 12.degree. C./sec until the plating layer
of the zinc alloy plated steel sheet was completely solidified (at
about 300.degree. C. or less).
[0047] Then, the microstructures of the fabricated zinc alloy
plated steel sheets were observed using an FE-SEM (SUPRA-55VP,
Zeiss) as illustrated in FIGS. 1 and 2, and the average grain
diameter of a Zn single phase structure of each of the zinc alloy
plated steel sheets was measured as shown in Table 2 below.
[0048] Thereafter, the degree (f) of (0001) preferred orientation
of the Zn single phase structure was measured using the following
Formula 1, and results thereof are shown in Table 2 below.
f(%)=(I.sub.basal/I.sub.total).times.100 [Formula 1]
[0049] where I.sub.total refers to the integral of all diffraction
peaks of the Zn single phase structure when an X-ray diffraction
pattern was measured within the range of 2 theta from 10.degree. to
100.degree. using a Cu-K.alpha. source, and I.sub.basal refers to
the integral of diffraction peaks of the Zn single phase structure
relating to a basal plane.
[0050] Thereafter, the bending workability of each of the zinc
alloy plated steel sheets was evaluated, and results thereof are
shown in Table 2 below.
[0051] Corrosion resistance was evaluated as follows. A salt spray
test (based on KS-C-0223) was performed on each of the zinc alloy
plated steel sheets to facilitate corrosion, and then the time
taken until the area fraction of red rust on the surface of each
plating layer was 5% was measured.
[0052] Bending workability was evaluated as follows.
[0053] 3T bending was performed on each of the zinc alloy plated
steel sheets, and a 1-mm length of the apex of each bent portion
was observed using an SEM to measure the area fraction of bending
cracks using an image analysis system.
TABLE-US-00001 TABLE 1 Composition of Droplet Droplet plating bath
spray start spray stop Spraying (wt %) temperature temperature
amount No. Al Mg (.degree. C.) (.degree. C.) Droplets (g/m.sup.2)
Notes 1 1.6 1.6 410 390 Aqueous solution 70 *IS 1 of ammonium
hydrogen phosphate, 2 wt % 2 1.6 1.6 420 400 Aqueous solution 70 IS
2 of ammonium hydrogen phosphate, 2 wt % 3 1.6 1.6 430 400 Aqueous
solution 70 **CS 1 of ammonium hydrogen phosphate, 2 wt % 4 1.6 1.6
400 390 Aqueous solution 70 CS 2 of ammonium hydrogen phosphate, 2
wt % 5 1.6 1.6 420 405 Aqueous solution 70 CS 3 of ammonium
hydrogen phosphate, 2 wt % 6 1.6 1.6 410 375 Aqueous solution 70 CS
4 of ammonium hydrogen phosphate, 2 wt % *IS: Inventive Sample,
**CS: Comparative Sample
TABLE-US-00002 TABLE 2 Average grain diameter of Zn Red rust Area
fraction of single phase occurrence bending cracks No. structure
(.mu.m) f (%) time (h) (%) Notes 1 8 63 650 8 *IS 1 2 10 62 645 9
IS 2 3 12 49 640 25 **CS 1 4 14 47 630 38 CS 2 5 15 46 620 40 CS 3
6 16 44 610 42 CS 4 7 18 42 600 45 CS 5 *IS: Inventive Sample,
**CS: Comparative Sample
[0054] Referring to Table 2, Inventive Samples 1 and 2 satisfying
conditions proposed in the present disclosure had high bending
workability.
[0055] However, although Comparative Samples 1 to 5 had high
corrosion resistance, Comparative Samples 1 to 5 had poor bending
workability because the (f) values thereof were less than 50%.
[0056] FIG. 1 is views illustrating results of (a) an observation
of a surface microstructure of Inventive Sample 1 of the present
disclosure and (b) an observation of a surface microstructure of
Comparative Sample 5, and FIG. 2 is views illustrating results of
(a) an observation of a cross-sectional microstructure of Inventive
Sample 1 of the present disclosure and (b) an observation of a
cross-sectional microstructure of Comparative Sample 5.
[0057] FIG. 3 is a view illustrating results of X-ray
diffractometer (XRD) analysis of Inventive Sample 1. In FIG. 3,
peaks denoted with ".largecircle." and ".circle-solid." are all
diffraction peaks of the Zn single phase structure, and the peaks
denoted with ".largecircle." are diffraction peaks of the Zn single
phase structure relating to a basal plane.
Example 2
[0058] Low carbon cold-rolled steel sheets each having a thickness
of 0.8 mm, a width of 100 mm, and a length of 200 mm were prepared
as base steel sheets for plating test samples, and then foreign
substances such as rolling oil were removed from the surfaces of
the base steel sheets by dipping the base steel sheets into acetone
and washing the base steel sheets with ultrasonic waves.
Thereafter, a 750.degree. C. reducing atmosphere heat treatment
commonly performed to guarantee mechanical characteristics of steel
sheets in the hot-dipping plating field was performed on the base
steel sheets, and then the base steel sheets were dipped into
plating baths having compositions shown in Table 3 below to
fabricate zinc alloy plated steel sheets. Thereafter, each of the
zinc alloy plated steel sheets was wiped with gas to adjust a
plating weight to be 70 g/m.sup.2 on each side. Then, the zinc
alloy plated steel sheets were cooled under the same conditions as
Inventive Sample 1 of Example 1.
[0059] Thereafter, the fractions of microstructures observed on the
surface of each of the zinc alloy plated steel sheets were
measured, and the number of black spots on the surface of each of
the zinc alloy plated steel sheets was measured. Results thereof
are shown in Tables 3 and 4.
[0060] Thereafter, a friction test (linear friction test) was
performed by rubbing the surface of each of the zinc alloy plated
steel sheets 20 times with a tool head at a constant pressure. In
the friction test, a target load was 333.3 kgf, a pressure was
3.736 MPa, the tool head traveled 200 mm per rub, and the speed of
the tool head was 20 mm/s.
[0061] After the friction test, a stripping test was performed on
each of the zinc alloy plated steel sheets. Specifically,
cellophane adhesive tape (NB-1 by Ichiban) was attached to a bent
portion of each of the zinc alloy plated steel sheets subjected to
a 10R bending process, and then the cellophane tape was momentarily
separated. Then, the number of plating layer defects was measured
using an optical microscope (magnification: 50 times). Results of
the measurement were evaluated as ".largecircle." when the number
of plating layer defects was 5/m.sup.2 or less, and "X" when the
number of plating layer defects was greater than 5/m.sup.2.
Evaluation results are shown in Table 4 below.
[0062] In addition, after the friction test, each of the zinc alloy
plated steel sheets was inserted into a salt spray tester, and the
time taken until the occurrence of red rust was measured according
to international standard ASTM B117-11. In that time, a 5% salt
solution (35.degree. C., pH 6.8) was sprayed at a rate of 2 ml/80
cm.sup.2 per hour. When the time taken until red rust was present
on a sample was 500 hours or greater, the sample was evaluated as
".largecircle.", and when the time taken until red rust was present
on a sample was less than 500, the sample was evaluated as "X."
Results of the evaluation are shown in Table 4 below.
TABLE-US-00003 TABLE 3 Alloy composition Area fractions of surface
structures (area %) (wt %) Zn/Al/MgZn.sub.2 + No. Al Mg Mg/Al Zn
Zn/MgZn.sub.2 Zn/Al/MgZn.sub.2 MgZn.sub.2 Zn/Al Zn/MgZn.sub.2 Notes
1 0.6 2.3 3.83 28 41 31 0 0 72 *IS A 2 1.5 2.8 1.87 20 57 21 1 1 78
IS B 3 2 2.9 1.45 8 63 28 1 0 91 IS C 4 2.2 2.7 1.23 4 58 34 2 2 92
IS D 5 2.6 2.9 1.12 4 39 51 3 3 90 IS E 6 0 0 -- 100 0 0 0 0 0 **CS
A 7 1.4 1 0.71 82 7 11 0 0 18 CS B 8 2.5 1.2 0.48 6 21 26 46 1 47
CS C 9 5 0 0.00 76 0 0 0 24 0 CS D 10 5 1 0.20 59 9 11 0 21 20 CS E
11 8 3 0.38 13 7 13 18 49 20 CS F 12 55 0 0.00 14 0 0 0 86 0 CS G
Here, surface structures refer to microstructures observed on the
surfaces of zinc alloy plating layers. *IS: Inventive Sample, **CS:
Comparative Sample
TABLE-US-00004 TABLE 4 Results of Results of stripping salt spray
test after test after Number friction test friction test of black
Number Evalua- Red Rust Evalua- spots of defects tion Occurrence
tion No. (/cm.sup.2) (/m.sup.2) results time (hours) results Notes
1 0.05 3 .largecircle. 520 .largecircle. *IS A 2 0.08 2
.largecircle. 550 .largecircle. IS B 3 0.04 4 .largecircle. 600
.largecircle. IS C 4 0.08 3 .largecircle. 650 .largecircle. IS D 5
0.04 2 .largecircle. 580 .largecircle. IS E 6 1.2 2 .largecircle.
120 X **CS A 7 0.8 3 .largecircle. 230 X CS B 8 0.05 23 X 620
.largecircle. CS C 9 1.1 3 .largecircle. 350 X CS D 10 0.6 2
.largecircle. 420 X CS E 11 0.06 15 X 650 .largecircle. CS F 12
0.05 11 X 200 X CS G *IS: Inventive Sample, **CS: Comparative
Sample
[0063] Referring to Table 4, Inventive Samples A to E satisfying
conditions proposed in the present disclosure had good appearance
and high scratch resistance.
[0064] However, each of Comparative Samples A, B, D, and E had poor
appearance because the area fraction of a Zn single phase structure
present on the surface of a plating layer was excessively high, and
each of Comparative Samples A to G had poor scratch resistance
because the area fractions of a Zn/MgZn.sub.2 binary eutectic
structure and a Zn/Al/MgZn.sub.2 ternary eutectic structure are
excessively low.
* * * * *