U.S. patent application number 17/394830 was filed with the patent office on 2021-11-25 for steel sheet for hot press formed member having excellent painting adhesion and post-painting corrosion resistance, and method for manufacturing same.
The applicant listed for this patent is POSCO. Invention is credited to A-Ra CHO, Seong-Woo KIM, Jin-Keun OH, Hyeon-Jeong SHIN.
Application Number | 20210362472 17/394830 |
Document ID | / |
Family ID | 1000005756885 |
Filed Date | 2021-11-25 |
United States Patent
Application |
20210362472 |
Kind Code |
A1 |
OH; Jin-Keun ; et
al. |
November 25, 2021 |
STEEL SHEET FOR HOT PRESS FORMED MEMBER HAVING EXCELLENT PAINTING
ADHESION AND POST-PAINTING CORROSION RESISTANCE, AND METHOD FOR
MANUFACTURING SAME
Abstract
Provided is a steel sheet for a hot press formed member having
excellent painting adhesion and post-painting corrosion resistance,
and a method for manufacturing the same. A steel sheet for hot
press forming according to one aspect of the present invention
comprises a base steel sheet and a plated layer formed on a surface
of the base steel sheet, wherein the ratio of an area occupied by
pores to the entire area of a surface layer portion may be 10% or
more in a cross section of the surface layer portion observed when
the plated layer is cut in a thickness direction thereof.
Inventors: |
OH; Jin-Keun; (Gwangyang-si,
KR) ; KIM; Seong-Woo; (Gwangyang-si, KR) ;
SHIN; Hyeon-Jeong; (Incheon, KR) ; CHO; A-Ra;
(Incheon, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
POSCO |
Pohang-si |
|
KR |
|
|
Family ID: |
1000005756885 |
Appl. No.: |
17/394830 |
Filed: |
August 5, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16617798 |
Nov 27, 2019 |
11141953 |
|
|
PCT/KR2018/006259 |
May 31, 2018 |
|
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17394830 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
Y10T 428/12993 20150115;
C23C 30/00 20130101; C22C 38/20 20130101; C22C 38/001 20130101;
C22C 38/22 20130101; C22C 38/28 20130101; C23F 17/00 20130101; C22C
38/24 20130101; C22C 30/00 20130101; C22C 38/26 20130101; Y10T
428/12757 20150115; B32B 15/013 20130101; C21D 6/008 20130101; B21D
35/00 20130101; C23C 2/06 20130101; C22C 38/06 20130101; C22C 38/08
20130101; C23C 26/00 20130101; C22C 38/54 20130101; C23C 2/04
20130101; C21D 9/46 20130101; C22C 38/04 20130101; C23C 2/12
20130101; C22C 38/50 20130101; C22C 38/002 20130101; Y10T 428/12979
20150115; B21D 35/005 20130101; C22C 38/40 20130101; B32B 15/18
20130101; C22C 38/18 20130101; C23C 28/322 20130101; C23C 30/005
20130101; B32B 15/01 20130101; C22C 38/32 20130101; C22C 38/16
20130101; C22C 38/02 20130101; C22C 38/12 20130101; C23C 28/321
20130101; B21D 22/022 20130101; B32B 15/043 20130101; C22C 38/38
20130101; B32B 15/04 20130101; C22F 1/04 20130101; Y10T 428/12958
20150115; C22C 38/58 20130101; C22C 21/00 20130101; C23C 28/023
20130101; C22C 38/46 20130101; Y10T 428/12972 20150115; B32B 15/011
20130101; B21D 22/02 20130101; C23C 2/26 20130101; C22F 1/002
20130101; C23C 2/40 20130101; B32B 15/012 20130101; Y10T 428/12799
20150115; C22C 38/48 20130101; C22C 38/42 20130101; C21D 6/005
20130101; C22C 38/44 20130101; C22C 38/14 20130101; C23C 2/28
20130101 |
International
Class: |
B32B 15/01 20060101
B32B015/01; C22C 38/00 20060101 C22C038/00; C22C 38/02 20060101
C22C038/02; C22C 38/04 20060101 C22C038/04; C22C 38/06 20060101
C22C038/06; C22C 38/28 20060101 C22C038/28; C22C 38/32 20060101
C22C038/32; C23C 2/12 20060101 C23C002/12; C23C 2/28 20060101
C23C002/28; B21D 22/02 20060101 B21D022/02; C21D 9/46 20060101
C21D009/46; C22F 1/00 20060101 C22F001/00; C22F 1/04 20060101
C22F001/04; C21D 6/00 20060101 C21D006/00; C23C 2/40 20060101
C23C002/40; C23F 17/00 20060101 C23F017/00; B21D 35/00 20060101
B21D035/00; C23C 26/00 20060101 C23C026/00; C23C 28/02 20060101
C23C028/02; C23C 30/00 20060101 C23C030/00; C23C 28/00 20060101
C23C028/00; B32B 15/18 20060101 B32B015/18; C23C 2/04 20060101
C23C002/04; B32B 15/04 20060101 B32B015/04; C23C 2/26 20060101
C23C002/26; C23C 2/06 20060101 C23C002/06; C22C 30/00 20060101
C22C030/00; C22C 38/14 20060101 C22C038/14; C22C 38/48 20060101
C22C038/48; C22C 38/40 20060101 C22C038/40; C22C 38/18 20060101
C22C038/18; C22C 38/42 20060101 C22C038/42; C22C 38/44 20060101
C22C038/44; C22C 38/08 20060101 C22C038/08; C22C 38/16 20060101
C22C038/16; C22C 38/24 20060101 C22C038/24; C22C 21/00 20060101
C22C021/00; C22C 38/38 20060101 C22C038/38; C22C 38/46 20060101
C22C038/46; C22C 38/20 20060101 C22C038/20; C22C 38/26 20060101
C22C038/26; C22C 38/22 20060101 C22C038/22; C22C 38/58 20060101
C22C038/58; C22C 38/12 20060101 C22C038/12; C22C 38/50 20060101
C22C038/50; C22C 38/54 20060101 C22C038/54 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 1, 2017 |
KR |
10-2017-0068651 |
Aug 10, 2017 |
KR |
10-2017-0101563 |
Claims
1. A steel sheet for a hot press formed member having excellent
painting adhesion and post-painting corrosion resistance,
comprising: a base steel sheet; and a plated layer formed on a
surface of the base steel sheet, wherein a ratio of an area
occupied by pores to an entire area of a surface layer portion is
10% or more in a cross-section of the surface layer portion
observed when the plated layer is cut in a thickness direction
thereof, and wherein the composition of the base steel sheet
further comprises, by wt %, one or more among a sum of one or more
selected from a group consisting of Cr, Mo, and W: 0.01 to 4.0%, a
sum of one or more selected from a group consisting of Ti, Nb, Zr,
and V: 0.001 to 0.4%, Cu+Ni: 0.005 to 2.0%, Sb+Sn: 0.001 to 1.0%,
and B: 0.0001 to 0.01%.
2. The steel sheet for the hot press formed member having excellent
painting adhesion and post-painting corrosion resistance of claim
1, wherein a ratio of an area occupied by pores to an entire area
of a surface layer portion is 15% or more in a cross-section of the
surface layer portion observed when the plated layer is cut in a
thickness direction thereof.
3. The steel sheet for the hot press formed member having excellent
painting adhesion and post-painting corrosion resistance of claim
1, wherein the plated layer is an aluminum alloy plated layer.
4. The steel sheet for the hot press formed member having excellent
painting adhesion and post-painting corrosion resistance of claim
3, wherein the aluminum alloy plated layer has an average content
of Fe of 30 wt % or more.
5. The steel sheet for the hot press formed member having excellent
painting adhesion and post-painting corrosion resistance of claim
3, wherein the aluminum alloy plated layer has an average content
of Fe of 40 wt % or more.
6. The steel sheet for the hot press formed member having excellent
painting adhesion and post-painting corrosion resistance of claim
1, wherein the base steel sheet has a composition including, by wt
%, carbon (C): 0.04 to 0.5%, silicon (Si): 0.01 to 2%, manganese
(Mn): 0.01 to 10%, aluminum (Al): 0.001 to 1.0%, phosphorus (P):
0.05% or less, sulfur (S): 0.02% or less, nitrogen (N): 0.02% or
less, and a balance of iron (Fe) and inevitable impurities.
7. A manufacturing method of a steel sheet for a hot press formed
member having excellent painting adhesion and post-painting
corrosion resistance, comprising operations of: aluminum plating a
surface of a base steel sheet and coiling to obtain an aluminum
plated steel sheet; annealing the aluminum plated steel sheet to
obtain an aluminum alloy plated steel sheet; and cooling the
aluminum alloy plated steel sheet, wherein an amount of the
aluminum plating is 30 to 200 g/m.sup.2 based on one side of the
steel sheet, coiling tension is 0.5 to 5 kg/mm.sup.2 during
coiling, the annealing is performed for 30 minutes to 50 hours in a
heating temperature range of 550 to 750.degree. C. in a batch
annealing furnace, when heating is performed from room temperature
to the heating temperature during the annealing, an average
temperature increase rate is 20 to 100.degree. C./h, and a
temperature increase rate in a section from the heating temperature
-50.degree. C. to the heating temperature is 1 to 15.degree. C./h,
and a difference between an atmosphere temperature and a
temperature of the steel sheet in the batch annealing furnace is 5
to 80.degree. C.
8. The manufacturing method of the steel sheet for the hot press
formed member having excellent painting adhesion and post-painting
corrosion resistance of claim 7, wherein the base steel sheet has a
composition including, by wt %, carbon (C): 0.04 to 0.5%, silicon
(Si): 0.01 to 2%, manganese (Mn): 0.01 to 10%, aluminum (Al): 0.001
to 1.0%, phosphorus (P): 0.05% or less, sulfur (S): 0.02% or less,
nitrogen (N): 0.02% or less, and a balance of iron (Fe) and
inevitable impurities.
9. The manufacturing method of the steel sheet for the hot press
formed member having excellent painting adhesion and post-painting
corrosion resistance of claim 8, wherein the composition of the
base steel sheet further comprises, by wt %, one or more among a
sum of one or more selected from a group consisting of Cr, Mo, and
W: 0.01 to 4.0%, a sum of one or more selected from a group
consisting of Ti, Nb, Zr, and V: 0.001 to 0.4%, Cu+Ni: 0.005 to
2.0%, Sb+Sn: 0.001 to 1.0%, and B: 0.0001 to 0.01%.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to a steel sheet for a hot
press formed member having excellent painting adhesion and
post-painting corrosion resistance, and a method for manufacturing
the same.
BACKGROUND ART
[0002] In recent years, due to depletion of petroleum energy
resources and high interest in environmental protection,
regulations on improving the fuel efficiency of automobiles are
becoming stronger.
[0003] In terms of materials, reducing a thickness of a steel sheet
used in automobiles is one method for improving the fuel efficiency
of automobiles; however, reducing the thickness of a steel sheet
may cause problems in the safety of automobiles, such that the
strength of the steel sheet should be supported.
[0004] Thus, demand for high-strength steel sheets has been
continuously generated, and various kinds of steel sheets have been
developed. However, since these steel sheets have high strength in
themselves, there is a problem that workability thereof is poor.
That is, since a product of strength and elongation for each grade
of steel sheet tends to always have a constant value, when the
strength of the steel sheet increases, there may be a problem that
elongation, an index of workability, decreases.
[0005] In order to solve this problem, a hot press forming method
has been proposed. The hot press forming method is a method of
forming a low temperature structure, such as martensite, in a steel
sheet by forming at a high temperature suitable for forming and
then quenching the steel sheet at a low temperature to increase the
strength of the final product. In this case, there is an advantage
that the problem of workability may be significantly reduced when
manufacturing a member having high strength.
[0006] However, according to the above-described hot press forming
method, there may be a problem in that a surface of the steel sheet
may be oxidized, since the steel sheet needs to be heated to a high
temperature, and thus, a process of removing an oxide from the
surface of the steel sheet after the press forming should be
added.
[0007] In order to solve this problem, the disclosure, U.S. Pat.
No. 6,296,805 has been proposed. In the above-described disclosure,
the steel sheet subjected to aluminum plating is used in a process
of hot press forming or heating and quenching after room
temperature forming (briefly, post-heat treatment). Since an
aluminum plated layer is present on the surface of the steel sheet,
the steel sheet is not oxidized at the time of heating.
[0008] However, even if the aluminum plated layer is present on the
surface thereof such that the steel sheet is not oxidized at the
time of heating, a member obtained after heating and forming is
still exposed to a corrosive environment.
[0009] Particularly, in the process of heating the plated steel
sheet, base iron diffuses into the aluminum plated layer, and a
hard Fe--Al-based plated layer is formed on the surface of the
steel sheet. In the case of the Fe--Al-based plated layer, because
it is hard and fragile, there is a concern that cracks may occur in
the plated layer, and thus the base steel sheet may be exposed to a
corrosive environment.
[0010] In order to prevent this, the hot press formed member is
formed with a painting layer, which is required to have excellent
painting adhesion.
DISCLOSURE
Technical Problem
[0011] An aspect of the present disclosure is to provide a steel
sheet for a hot press formed member having excellent painting
adhesion and post-painting corrosion resistance.
[0012] Subjects of the present disclosure are not limited to the
above issue and it may be understood from an overall content of the
present specification, and it will be understood by those skilled
in the art that there is no difficulty in understanding additional
subjects of the present disclosure.
Technical Solution
[0013] According to an aspect of the present disclosure, a steel
sheet for a hot press formed member includes a base steel sheet and
a plated layer formed on a surface of the base steel sheet. A ratio
of an area occupied by pores to an entire area of a surface layer
portion may be 10% or more in a cross-section of the surface layer
portion observed when the plated layer is cut in a thickness
direction thereof.
[0014] In an embodiment of the present disclosure, a ratio of an
area occupied by pores to an entire area of a surface layer portion
may be 15% or more in a cross-section of the surface layer portion
observed when the plated layer is cut in a thickness direction
thereof.
[0015] In an embodiment of the present disclosure, the plated layer
may be an aluminum alloy plated layer.
[0016] In an embodiment of the present disclosure, the aluminum
alloy plated layer may have an average content of Fe of 30% by
weight or more.
[0017] In an embodiment of the present disclosure, the aluminum
plated layer may have an average content of Fe of 40% by weight or
more.
[0018] In an embodiment of the present disclosure, the base steel
sheet may have a composition including, by wt %, carbon (C): 0.04
to 0.5%, silicon (Si): 0.01 to 2%, manganese (Mn): 0.01 to 10%,
aluminum (Al): 0.001 to 1.0%, phosphorus (P): 0.05% or less, sulfur
(S): 0.02% or less, nitrogen (N): 0.02% or less, and a balance of
iron (Fe) and inevitable impurities.
[0019] In an embodiment of the present disclosure, the composition
of the base steel sheet may further include, by wt %, one or more
among, a sum of one or more selected from a group consisting of Cr,
Mo and W: 0.01 to 4.0%, a sum of one or more selected from a group
consisting of Ti, Nb, Zr and V: 0.001 to 0.4%, Cu+Ni: 0.005 to
2.0%, Sb+Sn: 0.001 to 1.0%, and B: 0.0001 to 0.01%.
[0020] According to an aspect of the present disclosure, a
manufacturing method of a steel sheet for a hot press formed member
having excellent painting adhesion and post-painting corrosion
resistance includes operations of: aluminum plating a surface of a
base steel sheet and coiling to obtain an aluminum plated steel
sheet; annealing the aluminum plated steel sheet to obtain an
aluminum alloy plated steel sheet; and cooling the aluminum alloy
plated steel sheet. An amount of the aluminum plating is 30 to 200
g/m.sup.2 based on one side of the steel sheet, and coiling tension
is 0.5 to 5 kg/mm.sup.2 during coiling. The annealing is performed
for 30 minutes to 40 hours in a heating temperature range of 550 to
750.degree. C. in a batch annealing furnace. When heating is
performed from room temperature to the heating temperature during
the annealing, an average temperature increase rate is 20 to
100.degree. C./h, an average temperature increase rate in a section
of 400 to 500.degree. C. is 1 to 15.degree. C./h, a temperature
increase rate in a section from a heating temperature of
-50.degree. C. to a heating temperature is 1 to 15.degree. C./h, a
difference between an atmospheric temperature in the batch
annealing furnace and a temperature of the steel sheet is 5 to
80.degree. C., and cooling may be performed at a rate of 50.degree.
C./h or less to 500.degree. C. in the operation of cooling the
aluminum alloy plated steel sheet.
[0021] In an embodiment of the present disclosure, the base steel
sheet may include a composition, including, by wt %, carbon (C):
0.04 to 0.5%, silicon (Si): 0.01 to 2%, manganese (Mn): 0.01 to
10%, aluminum (Al): 0.001 to 1.0%, phosphorus (P): 0.05% or less,
sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less, and a
balance of iron (Fe) and inevitable impurities.
[0022] In an embodiment of the present disclosure, the composition
of the base steel sheet may further include, by wt %, one or more
among, a sum of one or more selected from a group consisting of Cr,
Mo and W: 0.01 to 4.0%, a sum of one or more selected from a group
consisting of Ti, Nb, Zr and V: 0.001 to 0.4%, Cu+Ni: 0.005 to
2.0%, Sb+Sn: 0.001 to 1.0%, and B: 0.0001 to 0.01%.
[0023] According to an aspect of the present disclosure, a
manufacturing method of a steel sheet for a hot press formed member
includes operations of: aluminum plating a surface of a base steel
sheet and coiling to obtain an aluminum plated steel sheet;
annealing the aluminum plated steel sheet to obtain an aluminum
alloy plated steel sheet; and cooling the aluminum alloy plated
steel sheet. An amount of the aluminum plating is 30 to 200
g/m.sup.2 based on one side of the steel sheet, and coiling tension
is 0.5 to 5 kg/mm.sup.2 during coiling. The annealing performed for
30 minutes to 40 hours in a heating temperature range of 550 to
750.degree. C. in a batch annealing furnace. When heating is
performed from room temperature to the heating temperature at the
time of annealing, an average temperature increase rate is 20 to
100.degree. C./h, an average temperature increase rate in a section
of 400 to 500.degree. C. is 1 to 15.degree. C./h, a temperature
increase rate in a section from a heating temperature of
-50.degree. C. to a heating temperature is 1 to 15.degree. C./h, a
difference between an atmospheric temperature in the batch
annealing furnace and a temperature of the steel sheet is 5 to
80.degree. C., and cooling may be performed at a rate of 50.degree.
C./h or less to 500.degree. C. in the operation of cooling the
aluminum alloy plated steel sheet.
[0024] In an embodiment of the present disclosure, the base steel
sheet may include a composition, including, by wt %, carbon (C):
0.04 to 0.5%, silicon (Si): 0.01 to 2%, manganese (Mn): 0.01 to
10%, aluminum (Al): 0.001 to 1.0%, phosphorus (P): 0.05% or less,
sulfur (S): 0.02% or less, nitrogen (N): 0.02% or less, and a
balance of iron (Fe) and inevitable impurities.
[0025] In an embodiment of the present disclosure, the composition
of the base steel sheet may further include, by wt %, one or more
among, a sum of one or more selected from a group consisting of Cr,
Mo and W: 0.01 to 4.0%, a sum of one or more selected from a group
consisting of Ti, Nb, Zr and V: 0.001 to 0.4%, Cu+Ni: 0.005 to
2.0%, Sb+Sn: 0.001 to 1.0%, and B: 0.0001 to 0.01%.
Advantageous Effects
[0026] According to one aspect of the present disclosure, since a
steel sheet for hot press forming includes pores in a surface layer
portion, surface roughness of the member obtained after hot press
forming may be greatly increased, resulting in excellent painting
adhesion, and as a result, excellent post-painting corrosion
resistance may be obtained.
DESCRIPTION OF DRAWINGS
[0027] FIG. 1 is a cross-sectional image of a cutting plain of a
plated layer of a steel sheet according to an embodiment of the
present disclosure.
[0028] FIG. 2 is a composition profile obtained by analyzing the
plated layer of the steel sheet manufactured according to Inventive
Example 1 using a GDS analyzer.
[0029] FIG. 3 is a scanning electron image (backscattered electron
image) of a cross-section of the plated layer of the steel sheet
manufactured by Inventive Example 1 taken by a scanning electron
microscope.
[0030] FIG. 4 is a composition profile obtained by analyzing a
plated layer of a steel sheet manufactured according to Inventive
Example 2 using a GDS analyzer.
[0031] FIG. 5 is a scanning electron image (backscattered electron
image) of a cross-section of the plated layer of the steel sheet
manufactured by Inventive Example 2 taken by a scanning electron
microscope.
[0032] FIG. 6 is a composition profile obtained by analyzing a
plated layer of a steel sheet manufactured according to Comparative
Example 1 using a GDS analyzer.
[0033] FIG. 7 is a scanning electron image of a cross-section of
the plated layer of the steel sheet manufactured by Comparative
Example 1 taken by a scanning electron microscope.
[0034] FIG. 8 is a composition profile obtained by analyzing a
plated layer of a steel sheet manufactured according to Comparative
Example 2 using a GDS analyzer.
[0035] FIG. 9 is a scanning electron image of a cross-section of
the plated layer of the steel sheet manufactured by Comparative
Example 2 taken by a scanning electron microscope.
[0036] FIG. 10 is a composition profile obtained by analyzing a
plated layer of a steel sheet manufactured according to Comparative
Example 3 using a GDS analyzer.
[0037] FIG. 11 is a scanning electron image of a cross-section of
the plated layer of the steel sheet manufactured by Comparative
Example 3 taken by a scanning electron microscope.
BEST MODE FOR INVENTION
[0038] Hereinafter, the present disclosure will be described in
detail.
[0039] In the present disclosure, a member refers to a component
manufactured by hot press forming or a material for the component.
In addition, a steel sheet means a steel sheet before hot press
forming, and the steel sheet may be wound during a manufacturing
process to have a coil form, and in this case, the steel sheet is
called as a coil.
[0040] FIG. 1 is an image by observing a cutting plane of a plated
layer of a steel sheet according to an embodiment of the present
disclosure. As can be seen in the figure, the steel sheet of the
present disclosure is composed of a based steel sheet and a plated
layer formed on a surface of the base steel sheet, and has a
plurality of pores in a surface layer portion of the plated layer.
This is a phenomenon that can not be observed in a conventional
aluminum plated steel sheet for hot press forming. In the
conventional aluminum plated steel sheet, pores are hardly
generated in the surface layer portion by molten aluminum plating,
but in the steel sheet according to an embodiment of the present
disclosure, a plurality of pores are generated in the surface layer
portion of the plated layer. In the present embodiment, the surface
layer portion means a region within 10 .mu.m depth from the surface
(if the surface layer is rough, the depth is measured from each
point of the rough surface).
[0041] If a plurality of pores are included in the surface layer
portion of the plated layer, when the steel sheet is heated to a
high temperature and press-formed, a portion of pores of the
surface layer portion are opened by a stress applied during press
working, which serves to increase roughness of the surface of the
plated layer.
[0042] A hot press formed member obtained by hot press forming an
aluminum plated steel sheet is subjected to alloying of the surface
thereof. Since a resulting obtained alloy layer is relatively
stable compared to a non-alloyed aluminum plated layer, reactivity
with phosphate is weak, and there is little room for improving
painting adhesion only by a common phosphate treatment. Of course,
roughness during alloying increases in the hot press forming
process, painting adhesion itself may be improved at a certain
level, but there is a limitation in the improvement thereof.
[0043] Therefore, in the present embodiment, in order to improve
this, by forming pores in the plated layer in the steel sheet
operation as described above, it contributes to improve the
roughness by collapse of the pores in the press forming in the
future.
[0044] To this end, an area ratio of an area occupied by pores to
an overall area of a surface layer portion may be 10% or more in a
cross-section of the surface layer portion observed when the plated
layer of the steel sheet is cut in a thickness direction thereof,
or may be 15% or more. In this case, when the steel sheet is
subjected to hot press forming, the surface roughness may be
improved, such that painting adhesion and post-painting corrosion
resistance may be greatly improved. Although an upper limit of the
surface roughness does not need to be particularly limited in terms
of paint adhesion or corrosion resistance after painting, a ratio
of the pores may be determined to be 70% or less or 60% or less.
Although there may be various methods for measuring the ratio of
pores, in one embodiment of the present disclosure, a method for
measuring a proportion of a portion where the pore exists by using
an image analyzer may be used.
[0045] In the present disclosure, in order to form a plated layer
having a high ratio of pores in the surface layer portion thereof
on the surface of the steel sheet, the plated layer may be an
aluminum alloy plated layer, and in one embodiment, the plated
layer may be an Al--Fe alloy plated layer. According to one
embodiment of the present disclosure, the Al--Fe alloy plated layer
may be obtained by alloying an Al plated steel sheet under
appropriate conditions. That is, in the present embodiment, when
the Al plated steel sheet is heated under appropriate conditions,
diffusion occurs between Al of the plated layer and Fe of the base
steel sheet, and Al and Fe are alloyed, which use a phenomenon in
which a plurality of pores are formed in the surface layer portion
in the process.
[0046] In this case, in order to form pores, an average content of
Fe of the plated layer may be 30 wt % or more, more preferably 40
wt % or more, and most preferably 50 wt % or more. That is, since
sufficient alloying has to occur to obtain pores in the surface
layer portion, an average content of Fe of the plated layer may be
30% wt %, 40% wt %, or 50% wt % or more. An upper limit of the
average content of Fe does not need to be particularly determined,
but may be set to be 80% wt % or less when considering an
efficiency of alloying. Here, the average content of Fe refers to
an average Fe content in the entire plated layer, and there may be
various measuring methods, but in the present embodiment, the
average content of Fe may be used as a value by integrating a Fe
content curve according to the depth (thickness) appearing when
analyzing the surface of the plated layer and an interface of the
steel sheet by a glow discharge emission spectrometry (GDS) method
and then dividing it by the thickness of the plated layer. There
may be various criteria for determining an interface between the
plated layer and the steel sheet, but in the present embodiment, a
point at which the Fe content is 92% of a base Fe content from GDS
results may be defined as an interface between the plated layer and
the steel sheet.
[0047] The steel sheet is a steel sheet for hot press forming, and
if used for hot press forming, a composition thereof is not
particularly limited. However, according to one aspect of the
present disclosure, by wt % (hereinafter, unless noted otherwise,
it is necessary to note that the composition of the steel sheet and
the plated layer is based on weight), the steel sheet and the
plated layer of the present disclosure may have the composition
including, by wt %, C: 0.04 to 0.5%, Si: 0.01 to 2%, Mn: 0.01 to
10%, Al: 0.001 to 1.0%, P: 0.05% or less, S: 0.02% or less, and N:
0.02% or less.
[0048] C: 0.04 to 0.5%
[0049] C may be added in an appropriate amount as an essential
element for increasing the strength of a heat treatment member.
That is, in order to secure sufficient strength in the heat
treatment member, the C may be added in an amount of 0.04% or more.
In one embodiment, a lower limit of the C content may be 0.1%.
However, if the content thereof is too high, in the case of
manufacturing a cold rolled material, when the hot rolled material
is cold-rolled, the strength of a hot rolled material is so high
that cold rollability is greatly inferior, and spot weldability is
also greatly reduced. Thus, C may be added in an amount of 0.5% or
less to secure sufficient cold rollability and spot weldability. In
addition, the C content may be limited to 0.45% or less or 0.4% or
less.
[0050] Si: 0.01 to 2%
[0051] Si not only needs to be added as a deoxidizer in
steelmaking, but also suppresses the formation of a carbide which
most affects the strength of the member for hot press forming, and
in the hot press forming, Si serves to secure residual austenite by
concentrating carbon to the grain boundaries of martensite lath
after forming martensite. Therefore, Si may be added in an amount
of 0.01% or more. In addition, when aluminum plating is performed
on the steel sheet after rolling, an upper limit may be set to be
2% in order to secure sufficient plating properties. In one
embodiment of the present disclosure, the Si content may be limited
to 1.5% or less.
[0052] Mn: 0.01 to 10%
[0053] Mn may be added in an amount of 0.01% or more in order to
secure a solid solution strengthening effect and to lower a
critical cooling rate for securing martensite in the member for hot
press forming. In addition, the Mn content may be 10% or less in
terms of securing workability of the hot press forming process by
appropriately maintaining the strength of the steel sheet, reducing
manufacturing costs, and improving spot weldability, and in one
embodiment of the present disclosure, Mn may be included in an
amount of 9% or less, or 8% or less.
[0054] Al: 0.001 to 1.0%
[0055] Al may be added in an amount of 0.001% or more since Al
deoxidizes in steelmaking, together with Si, to increase
cleanliness of steel, Al may be added in an amount of 0.001% or
more. In addition, the content of Al may be 1.0% or less in order
to prevent an Ac3 temperature from becoming too high so that
heating required during hot press forming may be performed in an
appropriate temperature range.
[0056] P: 0.05% or Less
[0057] P is present as an impurity in steel, and the smaller the
content thereof is, the more advantageous. Therefore, in one
embodiment of the present disclosure, P may be included in an
amount of 0.05% or less. In another embodiment of the present
disclosure, P may be limited to 0.03% or less. Since less P is an
advantageous impurity element, there is no need to particularly set
an upper limit of the content thereof. However, in order to
excessively lower the P content, there is a possibility that
manufacturing costs may increase, and in consideration thereof, a
lower limit thereof may be set to be 0.001%.
[0058] S: 0.02% or Less
[0059] S is an impurity in steel, and the maximum content is 0.02%
(preferably 0.01% or less) since S is an element that deteriorates
ductility, impact characteristics, and weldability of a member. In
addition, since manufacturing costs may increase when a minimum
content thereof is less than 0.0001%, in one embodiment of the
present disclosure, a lower limit of the content thereof may be
0.0001%.
[0060] N: 0.02% or Less
[0061] N is an element included as an impurity in steel, and in
order to reduce sensitivity for crack generation during slab
continuous casting, and to secure impact characteristics, the lower
the content is, the more advantageous, and thus, N may be included
in an amount of 0.02% or less. Although a lower limit does not need
to particularly determined, the N content may be set to be 0.001%
or more in one embodiment of in consideration of an increase in
manufacturing costs, or the like.
[0062] In the present disclosure, if necessary, in addition to the
above-described steel composition, one or more of a sum of one or
more selected from a group consisting of Cr, Mo, and W: 0.01 to
4.0%, a sum of one or more selected from a group consisting of Ti,
Nb, Zr and V: 0.001 to 0.4%, Cu+Ni: 0.005 to 2.0%, Sb+Sn: 0.001 to
1.0%, and B: 0.0001 to 0.01% may further be added.
[0063] A sum of one or more selected from a group consisting of Cr,
Mo and W: 0.01% to 4.0%
[0064] Cr, Mo, and W may improve hardenability and secure grain
refinement and the strength through a precipitation strengthening
effect, such that one or more these may be added in an amount of
0.01% or more, based on the total content. In addition, in order to
secure weldability of the member, the content thereof may be
limited to 4.0% or less. In addition, when the content of these
elements exceeds 4.0%, a further increase in an effect is also not
great, so when the content thereof is limited to 4.0% or less, it
is also possible to prevent an increase in costs due to the
addition of additional elements.
[0065] A sum of one or more selected from a group consisting of Ti,
Nb, Zr and V: 0.001 to 0.4%
[0066] Ti, Nb, and V are effective in improving the steel sheet of
the heat treatment member by forming fine precipitates, and in
stabilizing retained austenite and improving impact toughness by
grain refinement, such that it (they) may be added in an amount of
0.001% or more of one or more based on the total content. However,
if an added amount exceeds 0.4%, an effect thereof is not only
saturated, but also an increase in costs by the addition of
excessive ferroalloy may be caused.
[0067] Cu+Ni: 0.005 to 2.0%
[0068] Cu and Ni are elements forming fine precipitates to improve
strength. In order to obtain the above-described effects, a sum of
one or more these elements may be 0.005% or more. However, if the
value exceeds 2.0%, a cost increases excessively, so an upper limit
thereof is 2.0%.
[0069] Sb+Sn: 0.001 to 1.0%,
[0070] Sb and Sn may be concentrated on a surface during an
annealing heat treatment for Al--Si plating to suppress the
formation of a Si or Mn oxide on the surface to improve plating
properties. Sb and Sn may be added in an amount of 0.001% or more
in order to obtain such an effect. However, if an added amount
exceeds 1.0%, since besides an excessive ferroalloy cost, solid
solution at slab grain boundaries may cause coil edge cracks during
hot rolling, an upper limit is 1.0%.
[0071] B: 0.0001 to 0.01%
[0072] B is an element that can not only improve hardenability but
also be segregated in an old austenite grain boundary, and suppress
brittleness of the member for hot forming due to grain boundary
segregation of P or/and S by addition of a small amount. Therefore,
B may be added in an amount of 0.001% or more. However, if a
content exceeds 0.01%, the effect is not only saturated, but also
causes brittleness in hot rolling, so an upper limit thereof may be
0.01%, and in one embodiment, the content of B may be 0.005% or
less.
[0073] Iron and inevitable impurities may be mentioned as a
remainder other than the above-mentioned elements, and the element
that can be included in the steel sheet for hot forming is not
particularly limited.
[0074] Hereinafter an example of manufacturing method of steel for
hot press forming is disclosed. However, a method of manufacturing
a steel sheet for hot press forming described below is a mere
example and it does not mean that the steel sheet for hot press
forming of the present disclosure should be manufactured by the
present manufacturing method, and it is to be noted that any
manufacturing method meets the claims of the present disclosure and
there is no problem in implementing each embodiment of the present
disclosure.
[0075] The steel sheet of the present disclosure may be obtained by
using a hot-rolled or a cold-rolled steel sheet, by performing
molten aluminum plating on the surface of the base steel sheet, and
performing an annealing treatment on the plated steel sheet.
[0076] [Aluminum Plating Process]
[0077] In an embodiment of the present disclosure, a process of
preparing a base steel sheet, aluminum plating a surface of the
base steel sheet under appropriate conditions and coiling is
performed to obtain an aluminum plated steel sheet (coil).
[0078] Aluminum Plating of the Surface of the Base Steel Sheet in a
Plating Amount of 30 to 200 g/m.sup.2 Per Side
[0079] An aluminum plating treatment may be performed on a surface
of the rolled steel sheet. Aluminum plating may usually include
AlSi plating (which may contain 80% or more of Al and 5 to 20% of
Si, and additional elements as required), named as a type I, and
any plating containing 90% or more of Al and additional elements as
required, named as a type II. Hot dip aluminum plating may be
performed to form a plated layer, and an annealing treatment may be
performed on the steel sheet before plating. A suitable plating
amount when plating is 30 to 200 g/m.sup.2 based on one side. If
the plating amount is too large, it may take an excessive time to
alloy to the surface, on the contrary, if the plating amount is too
small, it may be difficult to obtain sufficient corrosion
resistance.
[0080] Coiling Tension after Plating is Set to be 0.5 to .about.5
kg/mm.sup.2
[0081] When a coil is obtained by coiling the steel sheet after
plating, coiling tension of the coil may be adjusted. According to
the adjustment of the coiling tension of the coil, an alloying
behavior and a surface quality of the coil may be changed during a
subsequent annealing treatment.
[0082] [Annealing Treatment]
[0083] An aluminum plated steel sheet obtained by the
above-described process is subjected to annealing under the
following conditions to obtain an aluminum alloy plated steel
sheet.
[0084] Performing Annealing for 30 Minutes to 50 Hours in a Range
of 550 to 750.degree. C. in a Batch Annealing Furnace
[0085] An aluminum plated steel sheet (coil) is heated in a batch
annealing furnace. When heating the steel sheet, it is desirable
that a heat treatment target temperature and a holding time be
maintained for 30 minutes to 50 hours in a range of 550 to
750.degree. C. based on a temperature of the steel sheet (in the
present disclosure, a highest temperature at which a material
reaches in this temperature range is called as a heating
temperature). Here, the holding time is the time after a coil
temperature reaches a target temperature until the start of
cooling. In one embodiment of the present disclosure, when alloying
is not sufficiently performed, a plated layer may be peeled off
during roll leveling, such that a heating temperature may be
550.degree. C. or higher for sufficient alloying. In addition, the
heating temperature may be 750.degree. C. or less in order to
prevent excessive generation of oxides on a surface layer and to
secure spot weldability. In addition, in order to sufficiently
secure the plated layer and prevent a decrease in productivity, the
holding time may be set to be 30 minutes to 50 hours. In one
embodiment of the present disclosure, a temperature of the steel
sheet may have a pattern in which the temperature continues to rise
without a cooling process until a heating temperature is
reached.
[0086] Heating to a Heating Temperature with an Average Time
Increase Rate of 20 to 100.degree. C./h
[0087] When heating the steel sheet at the above-described heating
temperature, in order to secure sufficient productivity and to
uniformly alloy the plated layer on all steel sheets (coils), the
average temperature increase rate may be 20 to 100.degree. C./h
based on the steel sheet (coil) temperature for an entire
temperature section (a section from room temperature to a heating
temperature). In addition, an overall average temperature increase
rate may be controlled in the above numerical range, but in one
embodiment of the present disclosure, as described later, a
temperature increase rate of a specific temperature section may be
also controlled to achieve the object of the present disclosure. In
another embodiment of the present disclosure, the average
temperature increase rate of the entire temperature section may be
set to be 70.degree. C./h.
[0088] Heating at an Average Temperature Increase Rate of 1 to
15.degree. C./h in a Section of 400 to 500.degree. C. at the Time
of Temperature Increase
[0089] In one embodiment of the present disclosure, in order to
secure sufficient productivity while preventing rolling oil
remaining in the temperature range in which the rolling oil mixed
during rolling is vaporized to cause surface stains, it may be
heated at the average temperature increase rate of 1 to 15.degree.
C./h in the section of 400 to 500.degree. C. at the time of
temperature increase. In one embodiment of the present disclosure,
a lower limit of the average temperature increase rate in the
section of 400 to 500.degree. C. at the time of the temperature
increase may be 4.degree. C./hr, and in another embodiment, a lower
limit of the average temperature increase rate in the section of
400 to 500.degree. C. at the time of the temperature increase may
be also 5.degree. C./hr.
[0090] Heating at an Average Temperature Increase Rate of 1 to
15.degree. C./h in a Section from the Heating Temperature
-50.degree. C. to the Heating Temperature
[0091] In order to secure sufficient productivity while preventing
sticking during alloying (surface defects where the surfaces of the
coils are alloyed and stuck) and allowing sufficient pores to form,
heating at an average temperature increase rate of 1 to 15.degree.
C./h in a section from a heating temperature of -50.degree. C. to a
heating temperature during temperature increase may be performed.
In one embodiment of the present disclosure, a lower limit of the
average temperature increase rate in the section may be set to be
4.degree. C./h, and in another embodiment, a lower limit of the
average temperature increase rate of the section may be set to be
5.degree. C./h.
[0092] A difference between an atmospheric temperature and a
temperature of the steel sheet in a batch annealing furnace is 5 to
80.degree. C.
[0093] In general, heating of the batch annealing furnace uses a
method of heating the steel sheet (coil) by increasing the
atmosphere temperature in the annealing furnace, rather than a
method of directly heating the steel sheet (coil). In this case,
the difference between the atmosphere temperature and the
temperature of the steel sheet may not be avoided. However, the
difference between the atmosphere temperature and the coil
temperature may be 80.degree. C. or less based on a time point at
which the heat treatment target temperature is reached in order to
significantly reduce variations in materials and plating quality
for each position in the steel sheet. It is ideal that the
temperature difference should be as small as possible, but since
this slow down the temperature increase rate, and thus it may be
difficult to meet the overall average temperature increase rate,
the temperature difference may be 5.degree. C. or more in
consideration thereof. Here the temperature of the steel sheet
means a temperature measured in a bottom part of the charged steel
sheet (coil) (meaning the lowest portion of the coil), and the
atmosphere temperature means a temperature measured at a center of
the internal space of the heating furnace.
[0094] [Cooling Process]
[0095] After Annealing, Cooling at a Rate of 50.degree. C./h to
500.degree. C.
[0096] After maintaining the target temperature for a certain time,
the aluminum alloy plated steel sheet (coil) is cooled. As a
cooling method, various methods such as furnace cooling, air
cooling, water cooling, and the like may be applied. There is no
particular limitation on the average cooling rate of the entire
cooling section, and it may be rapidly cooled to improve
productivity. However, in order to prevent sticking defects and
secure material uniformity and to form pores sufficiently, the
cooling rate of the temperature section to 500.degree. C. after
heating may be 50.degree. C./h or less. A lower limit is not
particularly limited, but may be 1.degree. C./h or more in
consideration of productivity.
BEST MODE FOR INVENTION
[0097] Hereinafter, the present disclosure will be described more
specifically through embodiments. It should be noted, however, that
the following embodiments are intended to illustrate the present
disclosure in more detail and not to limit the scope of the present
disclosure. The scope of the present disclosure is determined by
the matters set forth in the claims and the matters reasonably
inferred therefrom.
EXAMPLE
[0098] Manufacturing a Steel Sheet
Inventive Example 1
[0099] A cold-rolled steel sheet for hot press forming having the
composition of Table 1 below was prepared. A surface of the steel
sheet was plated with a type 1 plating bath having an Al-9% Si-2.5%
Fe composition. During plating, the amount of plating was adjusted
to 70 g/m.sup.2 per side, and a coil was wound by adjusting coiling
tension after plating to 2.2 kg/mm.sup.2.
TABLE-US-00001 TABLE 1 Additional Element C Si Mn Al P S N element
Content(%) 0.21 0.2 1.3 0.03 0.01 0.003 0.005 Ti 0.03, B 0.002, Cr
0.2
[0100] The plated steel sheet was heated to 650.degree. C., under
the following conditions in a batch annealing furnace.
[0101] An overall average temperature increase rate to 650.degree.
C.: 20.degree. C./h
[0102] An average temperature increase rate in a temperature
section of 400 to 500.degree. C.: 10.degree. C./h
[0103] An average temperature increase rate in a temperature
section of 600 to 650.degree. C.: 10.degree. C./h
[0104] A temperature difference between an atmosphere and a coil at
a heating temperature: 30.degree. C.
[0105] After heating, the plated steel sheet was maintained at the
same temperature for 10 hours, and the steel sheet was then cooled
at an average cooling rate of 40.degree. C./h to 550.degree. C.,
and then cooled at an average cooling rate of 55.degree. C./h to
100.degree. C. to obtain a steel sheet for hot press forming.
[0106] As a result of analyzing the plated layer of the steel sheet
using a GDS analyzer, a composition profile having a form shown in
FIG. 2 could be obtained, and an average Fe content calculated
based on this was 51.5 wt %. A cross-sectional form of the steel
sheet, as shown in FIG. 3, was formed with a plated layer formed on
an outer surface of a base steel sheet, and it can be confirmed
that an area ratio of pores formed in a portion corresponding to a
surface layer portion from the surface of the formed plated layer
to a point of 10 .mu.m in a thickness direction was 22.8%.
Inventive Example 2
[0107] A surface of the steel sheet having the composition shown in
Table 1 above was plated with a type I plating bath having an Al-9%
Si-2.5% Fe composition. During plating, an amount of plating was
adjusted to 80 g/m.sup.2 per side, and a coil was wound by
adjusting the coiling tension after plating to 2 kg/mm.sup.2.
[0108] The plated steel sheet was then heated to 700.degree. C.
under the following conditions in a batch annealing furnace.
[0109] An overall average temperature increase rate to 700.degree.
C.: 20.degree. C./h
[0110] An average temperature increase rate in a temperature
section of 400 to 500.degree. C.: 12.degree. C./h
[0111] An average temperature increase rate in a temperature
section of 650 to 700.degree. C.: 8.degree. C./h
[0112] A temperature difference between an atmosphere and a coil at
a heating temperature: 40.degree. C.
[0113] After heating, the plated steel sheet was maintained at the
same temperature for 1 hour, and the steel sheet was then cooled at
an average cooling rate of 30.degree. C./h to 500.degree. C., and
then cooled at an average cooling rate of 57.degree. C./h to
100.degree. C. to obtain a steel sheet for hot press forming.
[0114] As a result of analyzing the plated layer of the steel sheet
using a GDS analyzer, a composition profile of the form as shown in
FIG. 4 could be obtained, and the average Fe content calculated
based on this was 53.7 wt %. A cross-sectional form of the steel
sheet, as shown in FIG. 5, was formed with a plated layer formed on
an outer surface of a base steel sheet, and it can be confirmed
that an area ratio of pores formed in a portion corresponding to a
surface layer portion from the surface of the formed plated layer
to a point of 10 .mu.m in a thickness direction was 28.5%.
Comparative Example 1
[0115] An aluminum plated steel sheet subjected to plating with the
same condition as that of Inventive Example 1 only but not heating
and cooling was Comparative Example 1.
[0116] As a result of analyzing a plated layer of the steel sheet
using a GDS analyzer, a composition profile having a form shown in
FIG. 6 could be obtained, and an average Fe content calculated
based on this was 23.6 wt %. As shown in FIG. 7, it can be
confirmed that a cross-sectional form of the steel sheet had a
plated layer formed on an outer surface of the base steel sheet,
and almost no pores were formed in a portion corresponding to a
surface layer portion from the surface of the formed plated layer
to a point of 10 .mu.m in the thickness direction thereof. An area
ratio of the formed pores was 0%.
Comparative Example 2
[0117] An aluminum plated steel sheet, which is subjected to
plating with the same condition as that of Inventive Example 2 only
but not heating and cooling was Comparative Example 2.
[0118] As a result of analyzing a plated layer of the steel sheet
using a GDS analyzer, a composition profile of a form as shown in
FIG. 8 was obtained, and an average Fe content calculated based on
this was 21 wt %. As shown in FIG. 9, it can be confirmed that a
cross-sectional form of the steel sheet had a plated layer formed
on an outer surface of the base steel sheet, and almost no pores
were formed in a portion corresponding to a surface layer portion
from the surface of the formed plated layer to a point of 10 .mu.m
in the thickness direction thereof. An area ratio of the formed
pores was 0%.
Comparative Example 3
[0119] A surface of the steel sheet having the composition shown in
Table 1 above was plated with a type I plating bath having an Al-9%
Si-2.5% Fe composition. During plating, a painting amount was
adjusted to 90 g/m.sup.2 per side, and a coil was wound by
adjusting coiling tension after plating to 2 kg/mm.sup.2.
[0120] The plated steel sheet was then heated to 650.degree. C.
under the following conditions in a batch annealing furnace.
[0121] An overall average temperature increase rate to 650.degree.
C.: 50.degree. C./h
[0122] An average temperature increase rate in a temperature
section of 400 to 500.degree. C.: 10.degree. C./h
[0123] An average temperature increase rate in a temperature
section of 600 to 650.degree. C.: 70.degree. C./h
[0124] A temperature difference between an atmosphere and a coil at
a heating temperature: 30.degree. C.
[0125] After heating, the steel sheet was maintained at the same
temperature for 10 hours, and the steel sheet was then cooled to an
average cooling rate of 45.degree. C./h to 500.degree. C., and then
cooled to an average cooling rate of 60.degree. C./h to 100.degree.
C. to obtain a steel sheet for hot press forming.
[0126] As a result of analyzing the plated layer of the steel sheet
using a GDS analyzer, a composition profile having a form as shown
in FIG. 10 could be obtained, and an average Fe content calculated
based on this was 48.4 wt %. As shown in FIG. 11, a cross-sectional
form of the steel sheet had a plated layer formed on an outer
surface of the base steel sheet, and it can be confirmed that an
area ratio of pores formed in a portion corresponding to a surface
layer portion from the surface of the formed plated layer to a
point of 10 .mu.m in the thickness direction thereof was 3.5%.
[0127] Hot Press Forming
[0128] The steel sheets of Inventive Examples 1 and 2, and
Comparative Examples 1 to 3 were heated to 950.degree. C., and
maintained at the above-described temperature for 5 minutes, and
then subjected to hot press forming in which it was quenched while
pressurized by a press to obtain a hot press formed member.
[0129] A cross-section of the obtained member was observed to
observe the surface roughness Ra, and the results thereof were
shown in Table 2 below.
TABLE-US-00002 TABLE 2 Division Surface roughness(Ra) Inventive
Example 1 2.01 Inventive Example 2 2.23 Comparative Example 1 1.12
Comparative Example 2 1.27 Comparative Example 3 1.48
[0130] As can be seen in Table 2, in Inventive Example 1 and
Inventive Example 2, surface roughness (Ra) was 2.01 and 2.23
.mu.m, respectively, and in Comparative Example 1, Comparative
Example 2, and Comparative Example 3, a surface roughness (Ra) was
only 1.12, 1.27, and 1.48 .mu.m.
[0131] Phosphate treatment and electrodeposition painting were
performed on the member obtained from each of Examples and
Comparative Examples, and crosses were formed on the surface of the
steel sheet, and then a cyclic corrosion test was performed to
observe a degree of forming a blister on the crosses. The cyclic
corrosion test was conducted for 24 hours with 1 cycle, 2 hours of
wet atmosphere exposure-2 hours of salt water spray exposure-1 hour
of drying-6 hours of wet atmosphere exposure-2 hours of drying-6
hours of wet atmosphere exposure-2 hours of drying-3 hours of
cooling, and maintained a total of 50 cycles. In both Inventive
Examples 1 and 2, a maximum width of the blister is 1 mm or less,
while in Comparative Examples 1, 2, and 3, a maximum width of the
blister is 3.2, 2.9, and 2.4 mm, respectively, and it could be
confirmed that corrosion resistance after painting is inferior,
compared to that of the Inventive Example.
[0132] Thus, advantageous effects of the present disclosure could
be confirmed.
* * * * *