U.S. patent application number 15/577228 was filed with the patent office on 2018-05-31 for hot-rolled steel sheet having excellent composite corrosion resistance to sulfuric acid and hydrochloric acid and manufacturing method therefor.
The applicant listed for this patent is POSCO. Invention is credited to Jong-Hwa KIM, Byoung-Ho LEE, Jeong-Bong YOON.
Application Number | 20180148811 15/577228 |
Document ID | / |
Family ID | 57393247 |
Filed Date | 2018-05-31 |
United States Patent
Application |
20180148811 |
Kind Code |
A1 |
YOON; Jeong-Bong ; et
al. |
May 31, 2018 |
HOT-ROLLED STEEL SHEET HAVING EXCELLENT COMPOSITE CORROSION
RESISTANCE TO SULFURIC ACID AND HYDROCHLORIC ACID AND MANUFACTURING
METHOD THEREFOR
Abstract
Provided are a hot-rolled steel sheet having high composite
corrosion resistance to sulfuric acid and hydrochloric acid, and a
method for manufacturing the hot-rolled steel sheet. According to
an aspect, the hot-rolled steel sheet having high composite
corrosion resistance to sulfuric acid and hydrochloric acid
includes, by wt %, carbon (C): 0.05% to 0.1%, manganese (Mn): 0.5%
to 1.5%, phosphorus (P): 0.02% or less, sulfur (S): 0.02% to less,
aluminum (Al): 0.01% to 0.1%, copper (Cu): 0.2% to 0.6%, antimony
(Sb): 0.05% to 0.1%, and a balance of iron (Fe) and inevitable
impurities, wherein copper (Cu) and antimony (Sb) are concentrated
in a region from a surface to a 500-nm position in a thickness
direction of the hot-rolled steel sheet, and the hot-rolled steel
sheet has a corrosion loss of 2.0 mg/cm.sup.2/hr or less in a
solution of 16.9 volume % sulfuric acid and 0.35 volume %
hydrochloric acid.
Inventors: |
YOON; Jeong-Bong;
(Pohang-si, Gyeongsangbuk-do, KR) ; LEE; Byoung-Ho;
(Pohang-si, Gyeongsangbuk-do, KR) ; KIM; Jong-Hwa;
(Pohang-si, Gyeongsangbuk-do, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
POSCO |
Pohang-si, Gyeongsangbuk-do |
|
KR |
|
|
Family ID: |
57393247 |
Appl. No.: |
15/577228 |
Filed: |
May 28, 2015 |
PCT Filed: |
May 28, 2015 |
PCT NO: |
PCT/KR2015/005381 |
371 Date: |
November 27, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/60 20130101;
C21D 8/02 20130101; C22C 38/16 20130101; C21D 9/46 20130101; C21D
8/0226 20130101; C22C 38/06 20130101; C21D 6/005 20130101; C22C
38/002 20130101; C21D 8/0205 20130101; C22C 38/04 20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C21D 8/02 20060101 C21D008/02; C21D 6/00 20060101
C21D006/00; C22C 38/04 20060101 C22C038/04; C22C 38/00 20060101
C22C038/00; C22C 38/06 20060101 C22C038/06; C22C 38/16 20060101
C22C038/16; C22C 38/60 20060101 C22C038/60 |
Claims
1. A hot-rolled steel sheet having high composite corrosion
resistance to sulfuric acid and hydrochloric acid, the hot-rolled
steel sheet comprising, by wt %, carbon (C): 0.05% to 0.1%,
manganese (Mn): 0.5% to 1.5%, phosphorus (P): 0.02% or less, sulfur
(S): 0.02% or less, aluminum (Al): 0.01% to 0.1%, copper (Cu): 0.2%
to 0.6%, antimony (Sb): 0.05% to 0.1%, and a balance of iron (Fe)
and inevitable impurities, wherein copper (Cu) and antimony (Sb)
are concentrated in a region from a surface to a 500-nm position in
a thickness direction of the hot-rolled steel sheet, and the
hot-rolled steel sheet has a corrosion loss of 2.0 mg/cm.sup.2/hr
or less in a solution of 16.9 volume % sulfuric acid and 0.35
volume % hydrochloric acid.
2. The hot-rolled steel sheet of claim 1, wherein the inevitable
impurities comprise tungsten (W), molybdenum (Mo), cobalt (Co), and
nickel (Ni) in a total amount of less than 10 ppm.
3. The hot-rolled steel sheet of claim 1, wherein the concentrated
copper (Cu) and antimony (Sb) form an oxide layer comprising a
Cu--Sb composite oxide in a corrosive environment containing
sulfuric acid and hydrochloric acid.
4. The hot-rolled steel sheet of claim 3, wherein the oxide layer
has a thickness of 400 nm to 500 nm from the surface of the
hot-rolled steel sheet in the thickness direction of the hot-rolled
steel sheet.
5. A method for manufacturing a hot-rolled steel sheet having high
composite corrosion resistance to sulfuric acid and hydrochloric
acid, the method comprising: reheating a steel slab to 1100.degree.
C. to 1300.degree. C., the steel slab comprising, by wt %, carbon
(C): 0.05% to 0.1%, manganese (Mn): 0.5% to 1.5%, phosphorus (P):
0.02% or less, sulfur (S): 0.02% or less, aluminum (Al): 0.01% to
0.1%, copper (Cu): 0.2% to 0.6%, antimony (Sb): 0.05% to 0.1%, and
a balance of iron (Fe) and inevitable impurities; obtaining a
hot-rolled steel sheet by hot rolling the reheated steel slab and
finish hot rolling the steel slab at a temperature of 850.degree.
C. to 950.degree. C.; rapidly cooling the hot-rolled steel sheet at
a rate of 120.degree. C./s to 150.degree. C./s; coiling the cooled
hot-rolled steel sheet at a temperature of 650.degree. C. to
750.degree. C.; and slowly cooling the coiled hot-rolled steel
sheet at a rate of 30.degree. C./hr to 40.degree. C./hr to a
cooling finish temperature of 350.degree. C. to 400.degree. C.
6. The method of claim 5, wherein during the coiling of the cooled
hot-rolled steel sheet, a surface of the hot-rolled steel sheet
reaches a temperature of 720.degree. C. to 750.degree. C. through a
heat recuperation phenomenon.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to a hot-rolled steel sheet
having high composite corrosion resistance to sulfuric acid and
hydrochloric acid and applicable as a material for thermal power
plant equipment such as desulfurization equipment, denitrification
equipment, preheaters, or parts thereof, and a method for
manufacturing the hot-rolled steel sheet.
BACKGROUND ART
[0002] Steels having resistance to sulfuric acid corrosion or
sulfuric acid-hydrochloric acid composite corrosion are used as
materials for desulfurization equipment and denitrification
equipment in thermal power plants in which sulfuric acid corrosion
or sulfuric acid-hydrochloric acid composite corrosion is serious,
due to sulfuric acid and hydrochloric acid produced by a reaction
between moisture and exhaust gas containing sulfurous acid gas and
chlorine gas generated when fossil fuels such as coal or petroleum
are combusted, pipes of combined cycle power plants, and gas-gas
heater (GGH) heat elements required to be made of relatively thick
steel sheets, etc.
[0003] In general, it has been commonly known that a large amount
of copper (Cu) may be added to steel having resistance to sulfuric
acid-hydrochloric acid composite corrosion so as to decrease the
rate of corrosion in a composite atmosphere of sulfuric acid and
hydrochloric acid, as compared to the rate of corrosion of normal
steels.
[0004] Although copper (Cu) is more effective than other alloying
elements in decreasing the rate of sulfuric acid corrosion, copper
(Cu) added in large amounts causes problems such as the formation
of cracks in steel sheets during a hot rolling process, and thus,
steels to which an appropriate amount of copper (Cu) and other
elements are added have been developed (Japanese Patent Application
Laid-open Publication No. 1997-025536, Japanese Patent Application
Laid-open Publication No. 1998-110237, Korean Patent Application
Laid-open Publication No. 2009-0070249, etc).
[0005] As described above, although the corrosion resistance of
steels resistant to sulfuric acid-hydrochloric acid composite
corrosion is improved as the content of copper (Cu) is increased,
copper (Cu) is a relatively expensive alloying element, and thus
production costs may be increased in proportion to the amount of
copper (Cu). In addition, copper (Cu), having a low melting point,
may be segregated or easily cause cracks, even if only a small
amount of deformation occurs in copper-rich regions. Therefore,
cracks may be easily formed in portions such as slab corners
undergoing a large amount of processing during a continuous casting
process, and thus, surface defects undergoing corrosion earlier
that other portions may remain after a hot rolling process.
[0006] Therefore, there is a need for a method of imparting high
composite corrosion resistance to steel resistant to sulfuric
acid-hydrochloric acid composite corrosion while minimizing the
content of copper (Cu) in the steel.
DISCLOSURE
Technical Problem
[0007] Aspects of the present disclosure may provide a hot-rolled
steel sheet having high corrosion resistance in a composite
corrosive environment containing sulfuric acid and hydrochloric
acid, and a method for manufacturing the hot-rolled steel
sheet.
Technical Solution
[0008] According to an aspect of the present disclosure, a
hot-rolled steel sheet having high composite corrosion resistance
to sulfuric acid and hydrochloric acid may include, by wt %, carbon
(C): 0.05% to 0.1%, manganese (Mn): 0.5% to 1.5%, phosphorus (P):
0.02% or less, sulfur (S): 0.02% to less, aluminum (Al): 0.01% to
0.1%, copper (Cu): 0.2% to 0.6%, antimony (Sb): 0.05% to 0.1%, and
a balance of iron (Fe) and inevitable impurities, wherein copper
(Cu) and antimony (Sb) may be concentrated in a region from a
surface to a 500-nm position in a thickness direction of the
hot-rolled steel sheet, and the hot-rolled steel sheet may have a
corrosion loss of 2.0 mg/cm.sup.2/hr or less in a solution of 16.9
volume % sulfuric acid and 0.35 volume % hydrochloric acid.
[0009] According to another aspect of the present disclosure, a
method for manufacturing a hot-rolled steel sheet having high
composite corrosion resistance to sulfuric acid and hydrochloric
acid may include: reheating a steel slab to 1100.degree. C. to
1300.degree. C., the steel slab including, by wt %, carbon (C):
0.05% to 0.1%, manganese (Mn): 0.5% to 1.5%, phosphorus (P): 0.02%
or less, sulfur (S): 0.02% or less, aluminum (Al): 0.01% to 0.1%,
copper (Cu): 0.2% to 0.6%, antimony (Sb): 0.05% to 0.1%, and a
balance of iron (Fe) and inevitable impurities; obtaining a
hot-rolled steel sheet by hot rolling the reheated steel slab and
finish hot rolling the steel slab at a temperature of 850.degree.
C. to 950.degree. C.; rapidly cooling the hot-rolled steel sheet at
a rate of 120.degree. C./s to 150.degree. C./s; coiling the cooled
hot-rolled steel sheet at a temperature of 650.degree. C. to
750.degree. C.; and slowly cooling the coiled hot-rolled steel
sheet at a rate of 30.degree. C./hr to 40.degree. C./hr to a
cooling finish temperature of 350.degree. C. to 400.degree. C.
[0010] The above-described aspects of the present disclosure do not
include all aspects or features of the present disclosure. Other
aspects or features, and effects of the present disclosure will be
clearly understood from the following descriptions of exemplary
embodiments.
Advantageous Effects
[0011] The present disclosure may provide a hot-rolled steel sheet
having high composite corrosion resistance even though the
hot-rolled steel sheet has lower amounts of alloying elements than
steel sheets of the related art having composite corrosion
resistance to sulfuric acid and hydrochloric acid. In addition, the
hot-rolled steel sheet of the present disclosure may be used as a
material, required to have a relatively thick thickness, for
denitrification equipment and desulfurization equipment of power
plants, exhaust gas pipes of boilers, and preheaters, and may
markedly increase the lifespans of such facilities and
apparatuses.
BEST MODE
[0012] The inventors have repeatedly conducted research into
compositions of steel sheets and methods of manufacturing steel
sheets in order to provide a method of imparting high composite
corrosion resistance to steel resistant to sulfuric
acid-hydrochloric acid composite corrosion while minimizing the
content of copper (Cu) in the steel. As a result, the inventors
have found that if antimony (Sb) is added to a steel sheet as an
alloying element, and cooling conditions after a hot rolling
process and a coiling process are properly controlled, a Cu--Sb
rich layer, guaranteeing high composite corrosion resistance, is
formed on the steel sheet to an appropriate thickness in a
corrosive environment containing sulfuric acid and hydrochloric
acid. Based on this knowledge, the inventors have invented the
present invention.
[0013] Hereinafter, a hot-rolled steel sheet having high composite
corrosion resistance to sulfuric acid and hydrochloric acid will be
described in detail according to an aspect of the present
disclosure.
[0014] First, the alloying composition of the hot-rolled steel
sheet of the present disclosure will be described in detail.
[0015] Carbon (C): 0.05 wt % to 0.1 wt %
[0016] Carbon (C) is effective in increasing the strength of a
steel sheet. If the content of copper (Cu) is less than 0.05 wt %,
it is difficult to obtain a desired degree of strength, and wear
resistance reduces. Conversely, if the content of carbon (C) is
greater than 0.1 wt %, the weldability of the steel sheet markedly
reduces, thereby markedly increasing the possibility of defects
during welding and decreasing the corrosion resistance of the steel
sheet. Therefore, according to the present disclosure, it may be
preferable that the content of copper (Cu) be within the range of
0.05 wt % to 0.1 wt %.
[0017] Manganese (Mn): 0.5 wt % to 1.5 wt %
[0018] Manganese (Mn) dissolves in steel and precipitates sulfur
(S) in the form of manganese sulfide, thereby preventing hot
shortness caused by dissolved sulfur (S) and having a
solid-solution strengthening effect. If the content of manganese
(Mn) is less than 0.5 wt %, manganese sulfide is not sufficiently
precipitated. Thus, hot shortness may be caused by dissolved sulfur
(S), and it may be difficult to obtain a desired degree of
strength. Conversely, if the content of manganese (Mn) is greater
than 1.5 wt %, the above-described effects are saturated, and
product cost markedly increases. Thus, according to the present
disclosure, it may be preferable that the content of manganese (Mn)
be within the range of 0.5 wt % to 1.5 wt %.
[0019] Phosphorus (P): 0.02 wt % or Less
[0020] Phosphorus (P) is an element inevitably added to steel, and
if the content of phosphorus (P) is greater than 0.02 wt %,
composite corrosion resistance may markedly decrease from a desired
value. Therefore, it may be preferable that the content of
phosphorus (P) be within the range of 0.02 wt % or less.
[0021] Sulfur (S): 0.02 wt % or Less
[0022] Sulfur (S) is an element dissolved in steel causing hot
shortness, and thus, the content of sulfur (S) is adjusted to be as
low as possible. If the content of sulfur (S) is greater than 0.02
wt %, there is a high possibility that defects will be formed due
to hot shortness. Therefore, it may be preferable that the content
of sulfur (S) be within the range of 0.02 wt % or less.
[0023] Aluminum (Al): 0.01 wt % to 0.1 wt %
[0024] Aluminum (Al) is an element inevitably added to Al-killed
steel, and it may be preferable that the content of aluminum (Al)
be within the range of 0.01 wt % or greater for the effect of
deoxidation. However, if the content of aluminum (Al) is greater
than 0.1 wt %, surface defects may very likely be formed on the
steel sheet, and the weldability of the steel sheet may be
decreased. Therefore, according to the present disclosure, it may
be preferable that the content of aluminum (Al) be within the range
of 0.01 wt % to 0.1 wt %.
[0025] Copper (Cu): 0.2 wt % to 0.6 wt %
[0026] Copper (Cu) is an element added for composite corrosion
resistance to sulfuric acid and hydrochloric acid. If the content
of copper (Cu) is excessively low, it may be difficult to obtain
desired composite corrosion resistance. Thus, preferably, copper
(Cu) may be added in an amount of 0.2% or greater, and more
preferably in an amount of 0.3% or greater. Although composite
corrosion resistance increases in proportion to the content of
copper (Cu), if the content of copper (Cu) is excessively high, the
increase of corrosion resistance is markedly lowered, and
production costs may be markedly increased. In addition, surface
defects known as star cracks may be formed. Therefore, according to
the present disclosure, preferably, the upper limit of the content
of copper (Cu) may be set to be 0.6 wt %, and more preferably 0.5
wt %.
[0027] Antimony (Sb): 0.05 wt % to 0.1 wt % Together with copper
(Cu), antimony (Sb) is a key element for improving composite
corrosion resistance. In particular, antimony (Sb) forms a Cu--Sb
composite oxide in a corrosive environment, thereby effectively
improving composite corrosion resistance. If the content of
antimony (Sb) is less than 0.05 wt %, it is difficult to obtain the
above-described effects. Conversely, if the content of antimony
(Sb) is greater than 0.1 wt %, the above-described effects are
saturated, and production costs markedly increase. Thus,
preferably, the content of antimony (Sb) may be adjusted to be 0.1
wt % or less.
[0028] The steel sheet includes iron (Fe) and inevitable impurities
in addition to the above-described alloying elements. Although the
addition of elements other than the above-described elements is not
excluded, it may be preferable that the total content of tungsten
(W), molybdenum (Mo), cobalt (Co), and nickel (Ni) be adjusted to
be less than 10 ppm. The reason for this is that these elements may
deteriorate the properties of the hot-rolled steel sheet, for
example, ductility.
[0029] Furthermore, in the hot-rolled steel sheet of the present
disclosure, copper (Cu) and antimony (Sb) may be concentrated in a
region from the surface to a 500-nm position in the thickness
direction of the hot-rolled steel sheet. These elements are
concentrated in the surface of the hot-rolled steel sheet during
manufacturing processes, and if the hot-rolled steel sheet is
exposed to a corrosive environment containing sulfuric acid and
hydrochloric acid, the elements change into a Cu--Sb composite
oxide, thereby markedly improving the corrosion resistance of the
hot-rolled steel sheet.
[0030] In this case, the contents of concentrated copper (Cu) and
antimony (Sb) are not particularly limited. As described below, the
contents of concentrated copper (Cu) and antimony (Sb) may be
adjusted such that an oxide layer having a thickness of 400 nm or
greater from the surface of the hot-rolled steel sheet may be
formed in a corrosive environment containing sulfuric acid and
hydrochloric acid. If the thickness of the oxide layer is less than
400 nm, it may be difficult to obtain a degree of corrosion
resistance intended in the present disclosure. Since corrosion
resistance increases as the thickness of the oxide layer increases,
the upper limit of the thickness of the oxide layer is not
particularly set in the present disclosure. However, if the
thickness of the oxide layer is greater than 500 nm, the effect of
improving corrosion resistance is relatively low when the addition
of large amounts of alloying elements is considered, and production
costs may be excessively increased. Thus, it may be more preferable
that the thickness of the oxide layer be within the range of 400 nm
to 500 nm.
[0031] The hot-rolled steel sheet of the present disclosure has a
corrosion loss of 2.0 mg/cm.sup.2/hr or less in a solution of 16.9
volume % sulfuric acid and 0.35 volume % hydrochloric acid.
[0032] Hereinafter, a method for manufacturing a hot-rolled steel
sheet having high composite corrosion resistance to sulfuric acid
and hydrochloric acid will be described in detail, according to
another aspect of the present disclosure.
[0033] First, a steel slab having the above-described composition
is prepared and reheated to a temperature of 1100.degree. C. to
1300.degree. C. If the reheating temperature is lower than
1100.degree. C., it is difficult to secure a temperature for a
subsequent hot rolling process. Conversely, if the reheating
temperature is higher than 1300.degree. C., copper (Cu) having a
relatively low melting point may melt out, and thus cracks may very
likely be formed in the surface of the steel slab.
[0034] Thereafter, the reheated steel slab is subjected to hot
rolling, and is then subjected to finish hot rolling at a
temperature of 850.degree. C. to 950.degree. C., to obtain a
hot-rolled steel sheet. If the finish hot rolling temperature is
lower than 850.degree. C., the elongation of the hot-rolled steel
sheet is markedly decreased due to elongated grains, and properties
of the hot-rolled steel sheet have directional deviations.
Conversely, if the finish hot rolling temperature is higher than
950.degree. C., austenite grains become coarse, and thus
hardenability markedly increases.
[0035] Thereafter, the hot-rolled steel sheet is rapidly cooled at
a rate of 120.degree. C./s to 150.degree. C./s, based on the
surface temperature of the hot-rolled steel sheet. The rapid
cooling may provide driving force such that alloying elements
improving corrosion resistance may move to the surface of the
hot-rolled steel sheet after a coiling process. If the cooling rate
is less than 120.degree. C./s, the surface temperature of the
hot-rolled steel sheet may be too high to sufficiently drive
oxide-forming elements from the interior to the surface of the
hot-rolled steel sheet, and thus when the hot-rolled steel sheet is
exposed to a composite corrosive environment, oxides may not be
sufficiently formed. Conversely, if the cooling rate is greater
than 150.degree. C./s, the interior temperature of the hot-rolled
steel sheet becomes excessively low, and thus heat recuperation may
not occur to a desired temperature after a coiling process. In this
case, alloying elements effective in forming an oxide layer may not
move smoothly. Therefore, preferably, the cooling rate may be set
to be within the range of 120.degree. C./s to 150.degree. C./s.
[0036] Thereafter, the cooled hot-rolled steel sheet is coiled at a
temperature of 650.degree. C. to 750.degree. C. If the coiling
temperature is lower than 650.degree. C., atoms may not easily move
during the coiling process. As a result, a rich layer may not be
easily formed, and thus an oxide layer may not be formed in a
corrosive environment. That is, corrosion resistance may not be
sufficiently guaranteed. If the coiling temperature is higher than
750.degree. C., heat recuperation occurs to an excessively high
temperature, and thus, defects such as dents may be formed on the
coiled hot-rolled steel sheet. Therefore, the coiling temperature
may preferably be set to be within the range of 650.degree. C. to
750.degree. C.
[0037] In addition, during the coiling process, it may be
preferable that the surface of the hot-rolled steel sheet have a
temperature of 720.degree. C. to 750.degree. C. owing to a heat
recuperation phenomenon. Although the interior temperature of the
hot-rolled steel sheet is adjusted to be within the range of
650.degree. C. to 750.degree. C. through the cooling process, the
surface temperature of the hot-rolled steel sheet is lower than the
range because of rapid cooling. Therefore, the hot-rolled steel
sheet may be allowed to undergo heat recuperation so as to activate
the movement of alloying elements effective in forming an oxide
layer and thus to form a rich layer having a sufficient thickness.
To sufficiently obtain these effects, the surface temperature of
the hot-rolled steel sheet may preferably be 720.degree. C. or
higher after heat recuperation. However, the surface temperature of
the hot-rolled steel sheet will not exceed 750.degree. C. even if
the heat recuperation is sufficient.
[0038] The coiled hot-rolled steel sheet is slowly cooled to a
cooling finish temperature of 350.degree. C. to 400.degree. C. at a
rate of 30.degree. C./hr to 40.degree. C./hr. If the slow cooling
rate is excessively high, copper (Cu) forming a rich layer may not
sufficiently move, and thus it may be difficult to form a rich
layer having a sufficient thickness. Therefore, it may be
preferable that the slow cooling rate be within the range of
40.degree. C./hr or less. However, if the slow cooling rate is less
than 30.degree. C./hr, the size of grains may increase excessively,
and thus, the strength of the hot-rolled steel sheet may decrease.
Thus, the slow cooling rate may preferably be within the range of
30.degree. C./hr to 40.degree. C./hr. In addition, if the cooling
finish temperature is lower than 350.degree. C., properties of the
hot-rolled steel sheet such as ductility may deteriorate, and
productivity may decrease. Conversely, if the cooling finish
temperature is higher than 400.degree. C., a rich layer having a
sufficient thickness may not be formed, and thus the hot-rolled
steel sheet may have poor corrosion resistance. Therefore, the
cooling finish temperature may preferably be within the range of
350.degree. C. to 400.degree. C.
MODE FOR INVENTION
[0039] Hereinafter, the present disclosure will be described more
specifically through examples. However, the following examples
should be considered in a descriptive sense only, and are not for
purposes of limitation. The scope of the present invention is
defined by the appended claims, and modifications and variations
may be reasonably made therefrom.
Examples
[0040] Steel ingots, manufactured through a melting process and
having compositions as illustrated in Table 1 below, were
maintained in a heating furnace for one hour at 1200.degree. C. and
were subjected to a hot rolling process. In this case, finish hot
rolling was performed at 900.degree. C., and hot-rolled steel
sheets having a thickness of 4.5 mm were ultimately manufactured.
Thereafter, the hot-rolled steel sheets were cooled, coiled, and
maintained under the conditions shown in Table 2 below. Thereafter,
the hot-rolled steel sheets were slowly cooled to 380.degree. C. at
a rate of 35.degree. C./h, thereby completing the manufacturing of
the hot-rolled steel sheets.
[0041] To observe corrosion characteristics of the hot-rolled steel
sheets, specimens of the hot-rolled steel sheets were placed in a
solution having a temperature of 60.degree. C. and containing 16.9
volume % sulfuric acid and 0.35 volume % hydrochloric acid for 6
hours, and subsequently, the corrosion loss of each specimen was
measured as shown in Table 2 below.
[0042] In addition, after the immersion in the sulfuric
acid-hydrochloric acid composite corrosive environment, the
thickness of an oxide layer (corrosion-resistant layer) of each
hot-rolled steel sheet was measured as shown in Table 2.
TABLE-US-00001 TABLE 1 Composition (wt %) Steels C Mn P S Al Cu Sb
Inventive 0.075 0.69 0.012 0.009 0.033 0.32 0.08 Steel 1 Inventive
0.068 0.67 0.011 0.009 0.029 0.39 0.06 Steel 2 Inventive 0.074 0.75
0.009 0.01 0.029 0.44 0.05 Steel 3 Comparative 0.069 0.74 0.012
0.011 0.035 0.28 -- Steel 1
TABLE-US-00002 TABLE 2 Corrosion Thickness Cooling Coiling Loss of
Oxide Rate Temperature (mg/cm.sup.2/ Layer Steels (.degree. C./s)
(.degree. C.) hr) (nm) Examples Inventive 130 700 1.8 420 Inventive
Steel 1 Example 1 130 500 4.5 57 Comparative Example 1 10 700 3.8
63 Comparative Example 2 Inventive 130 700 1.6 440 Inventive Steel
2 Example 2 10 700 3.6 69 Comparative Example 3 Inventive 130 700
1.4 460 Inventive Steel 3 Example 3 10 700 3.2 75 Comparative
Example 4 Com- 130 700 8.8 220 Comparative parative Example 5 Steel
1
[0043] As shown in Tables 1 and 2, Inventive Examples 1 to 3
satisfying the alloying composition and manufacturing conditions
proposed in the present disclosure had a corrosion loss of 2.0
mg/cm.sup.2/hr or less in the sulfuric acid-hydrochloric acid
corrosive environment owing to the formation of oxide layers having
a thickness of 400 nm or greater. That is, Inventive Examples 1 to
3 had high corrosion resistance.
[0044] Although Comparative Example 1 satisfied the alloying
composition of the present disclosure, the coiling temperature of
Comparative Example 1 was low, 500.degree. C. Thus, an oxide layer
was not sufficiently formed, resulting in corrosion loss of 4.5
mg/cm.sup.2/hr. That is, Comparative Example 1 had poor corrosion
resistance.
[0045] Although Comparative Examples 2 to 4 satisfied the alloying
composition of the present disclosure, the cooling rate of
Comparative Example 2 to 4 was low, 10.degree. C./s. Thus, oxide
layers were not sufficiently formed, resulting in a corrosion loss
of 3.2 mg/cm.sup.2/hr or greater. That is, Comparative Examples 2
to 4 had poor corrosion resistance.
[0046] Although Comparative Example 5 satisfied the manufacturing
conditions of the present disclosure, antimony (Sb) was not added
to Comparative Example 5, resulting in a corrosion loss of 8.8
mg/cm.sup.2/hr in the sulfuric acid-hydrochloric acid corrosive
environment. That is, Comparative Example 5 had poor corrosion
resistance. The reason for this is that a Cu--Sb composite oxide
having high corrosion resistance was not formed in an oxide
layer.
[0047] While exemplary embodiments have been shown and described
above, it will be apparent to those skilled in the art that
modifications and variations could be made without departing from
the scope of the present invention as defined by the appended
claims.
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