COMPLETE SPECIFICATION
(See section 10, rule 13)
"FERRITIC STAINLESS STEEL FOR AUTOMOTIVE EXHAUST
SYSTEM, WHICH HAVE EXCELLENT CORROSION
RESISTANCE AGAINST CONDENSATE, MOLDABILITY, AND
HIGH-TEMPERATURE OXIDATION RESISTANCE, AND
METHOD FOR MANUFACTURING SAME"
POSCO of (Goedong-dong) 6261, Donghaean-ro
Nam-gu, Pohang-si Gyeongsangbuk-do 790-300,
Republic of Korea
The following specification particularly describes the invention and the manner
in which it is to be performed.
[DESCRIPTION]
[Invention Title]
FERRITIC STAINLESS STEEL FOR AUTOMOTIVE EXHAUST SYSTEM,
WHICH HAVE EXCELLENT CORROSION RESISTANCE AGAINST
5 CONDENSATE, MOLDABILITY AND HIGH-TEMPERATURE OXIDATION
RESISTANCE, AND METHOD FOR MANUFACTURING SAME
[Technical Field]
The present invention relates to a ferritic stainless steel for automotive
exhaust system, and a method of manufacturing the same, and more particularly, to a
10 ferritic stainless steel which is used in parts of the automotive exhaust system to
improve condensate corrosion resistance, and has excellent formability and high
temperature oxidation resistance, and a method of manufacturing the same.
[Background Art]
In general, parts of an automotive exhaust system are classified into a hot part
15 and a cold part according to a temperature of an exhaust gas. The automotive
components of the hot parts include an exhaust manifold, a converter, a bellows, etc.
These are primarily used at a temperature of 600 °C or more, and thus are required to
have excellent high temperature strength, thermal fatigue, high temperature salt
corrosion properties, etc. On the other hand, the cold parts are used at a
20 temperature of 400 °C or less, such as a muffler for decreasing the noise of an
automotive exhaust gas. Due to condensate corrosion properties resulting from a
sulfur (S) component in an automotive fuel, exterior rust/corrosion properties
according to the use of a snow removal salt in winter, and the like, materials such as
1
stainless (or STS) 409, 409L, 439, 436L and the like are used for the cold parts of the
automotive exhaust system.
Particularly, a material of stainless 409 or 409L is a steel grade which
includes Cr at 11%, has C and N stabilized by Ti, prevents sensitization of the weld
5 and has excellent formability. The above-described steel grade is primarily used at
a temperature of 700 °C or less, has a certain degree of corrosion resistance with
respect to components of condensed water generated in the automotive exhaust
system, and thus is a steel grade which is most frequently used.
Meanwhile, stainless 439 has C and N stabilized by Ti, and has about 17% Cr.
10 Further, stainless 436L is steel which is obtained by adding about 1% Mo to stainless
439, and has excellent condensate corrosion properties and corrosion resistance
properties.
Meanwhile, the S component in automobile fuel components is concentrated
in 5042-, and thus is changed to a highly corrosive sulfuric (H2SO4) atmosphere
15 having a pH of 2 or less. When a stainless 409L material, which is most frequently
used, is used for an automotive muffler material used in an area where a large
amount of the S components are contained in gasoline components as described
above, corrosion caused by condensed water or the like occurs. In this case, there is
a problem in that it is difficult for the automotive to meet the warranty of a car
20 manufacturer. Accordingly, the use of a high Cr-based stainless material such as
stainless 439, 436L or the like including a Cr component at 17% or more is gradually
increasing, currently. However, a resource price of this material is gradually
increasing, and thus there is a need for the development of a stainless material which
is manufactured by adding other elements instead of adding expensive elements such
2
as Mo or the like to have condensate corrosion properties which are at least equal to
a material of stainless 439 or 436L.
Meanwhile, an aluminum-plated stainless steel plate which is manufactured
by hot-dip coating a stainless 409L material with aluminum has been developed and
5 used in order to improve exterior rust/corrosion properties attributable to the
occurrence of surface oxidation or surface discoloration. However, in general, a
passivation film is present on a surface of a stainless steel plate, and the passivation
film degrades the wetting properties of hot dipping. Accordingly, there is a
problem in that aluminum plating is difficult to be performed without a separate
10 treatment process such as atmosphere control.
[Disclosure]
[Technical Problem]
The present invention relates to a ferritic stainless steel for automotive
exhaust system having excellent corrosion resistance, particularly condensate
15 corrosion resistance and formability, and a method of manufacturing the same. The
objective of the present invention is directed to providing the ferritic stainless steel
having improved condensate corrosion resistance under the environment of the use of
a high sulfur fuel and excellent formability, by adding a small amount of an alloying
element of Sn, or Sn and Cu to the ferritic stainless steel, and the method of
20 manufacturing the same.
Further, the objective of the present invention is directed to providing the
ferritic stainless steel, which has condensate corrosion resistance at least equal to that
of a stainless 439 or 436L material of which corrosion resistance is improved by
adding an element such as Mo or the like to an existing 17Cr alloy base, and which
3
may also be manufactured without adding an expensive element such as Mo or the
like thereto, and the method of manufacturing the same.
Further, the objective of the present invention is directed to providing the
ferritic stainless steel, which is plated with aluminum in order to improve condensate
5 corrosion resistance, formability, and high temperature oxidation resistance
properties, and the method of manufacturing the same.
[Technical Solution]
According to an aspect of the present invention, there is provided a ferritic
stainless steel having excellent condensate corrosion resistance, formability and high
10 temperature oxidation resistance, which includes, by wt%, C: more than 0 and 0.01%
or less, Cr: 9 to 13%, Si: 0.5 to 1.0%, Mn: more than 0 and 0.5% or less, P: more
than 0 and 0.035% or less, S: more than 0 and 0.01% or less, Ti: 0.15 to 0.5%, N:
more than 0 and 0.01% or less, Sn: 0.05 to 0.5%, and Fe and unavoidable impurities
as a remainder, wherein Sn concentrated at a surface part of a stainless steel is 10
15 times or more than Sn concentrated at a base part.
According to an embodiment of the present invention, 0.05 to 0.15% Sn may
be included at the base part of the stainless steel, and 0.5 to 1.5% Sn may be included
in a part extending from the surface of the stainless steel to a depth of 0 to 1μm in a
depth direction.
20 According to the embodiment of the present invention, when 0.1% Sn is
included at the base part of the stainless steel, at least 1% Sn may be included in a
part extending from the surface of the stainless steel to a depth of 0.1 i.tm in a depth
direction.
4
According to the embodiment of the present invention, the stainless steel may
further include 0.5 to1.0% Cu. Here, it is preferable that (5Sn+Cu) is in the range
of 0.5 to 2.0.
According to the embodiment of the present invention, the stainless steel
5 preferably has (Si+Ti)/(C+N) in the range of 50 to 90.
According to the embodiment of the present invention, a surface of the
stainless steel may be hot-dip coated with aluminum at a thickness of 200 p.m or less.
According to the embodiment of the present invention, the stainless steel may
have a maximum corrosion depth of 0.25 mm or less measured using a JASO M609-
10 91 method which is a method for measuring corrosion resistance under the
environment of condensed water.
According to the embodiment of the present invention, the stainless steel may
have a corrosion area ratio of 5 % or less, which is measured after 3 cycles were
repeated according to a JASO M611-92 method, and a maximum corrosion depth of
15 0.25 mm or less, which is measured after 100 cycles were repeated, the JASO M611-
92 method is a method for measuring exterior rust of automotive exhaust system.
According to another aspect of the present invention, there is provided a
method of manufacturing a ferritic stainless steel having excellent condensate
corrosion resistance, formability and high temperature oxidation resistance, including
20 cold rolling and annealing a ferritic stainless steel plate which includes, by wt%, C:
more than 0 and 0.01% or less, Cr: 9 to 13%, Si: 0.5 to 1.0%, Mn: more than 0 and
0.5% or less, P: more than 0 and 0.035% or less, S: more than 0 and 0.01% or less,
Ti: 0.15 to 0.5%, N: more than 0 and 0.01% or less, Sn: 0.05 to 0.5%, and Fe and
unavoidable impurities as a remainder; and removing Si oxide included in an
25 annealing scale through primary neutral salt electrolytic pickling and secondary
5
sulfuric acid electrolytic pickling. According to the embodiment of the present
invention, the cold rolling and annealing is performed in a temperature range of 980
to 1,020 °C.
According to the embodiment of the present invention, it is preferable that
5 conditions of the primary neutral salt electrolytic pickling include a temperature in
the range of 70 to 90 °C and a neutral salt concentration in a range of 150 to 250 g/L,
and conditions of the secondary sulfuric acid electrolytic pickling include a
temperature in the range of 30 to 50 °C and a sulfuric acid concentration in a range of
65 to 85 g/L.
10 Further, according to the embodiment of the present invention, a surface of
the stainless steel may be hot-dip coated with aluminum at a thickness of 200 [im or
less.
According to still another aspect of the present invention, there is provided a
method of manufacturing a ferritic stainless steel having excellent condensate
15 corrosion resistance, formability and high temperature oxidation resistance, including
heating a ferritic stainless steel slab, which includes, by wt%, C: more than 0 and
0.01% or less, Cr: 9 to 13%, Si: 0.5 to 1.0%, Mn: more than 0 and 0.5% or less, P:
more than 0 and 0.035% or less, S: more than 0 and 0.01% or less, Ti: 0.15 to 0.5%,
N: more than 0 and 0.01% or less, Sn: 0.05 to 0.5%, and Fe and unavoidable
20 impurities as a remainder, in a temperature range of 1,100 to 1,240 °C; performing
hot rolling and annealing in a temperature range of 1,030 to 1,070 °C after hot rolling
the slab; performing cold rolling and cold annealing in a temperature range of 980 to
1,020 °C; preprocessing an underlying metal of a stainless steel after the cold rolling
and annealing; preheating and heating; and hot-dip coating with aluminum.
6
According to the embodiment of the present invention, the preprocessing of
the stainless steel may be performed using an electrolytic cleaning method, in which
the stainless steel is bathed in a caustic soda solution having a concentration in a
range of 2 to 5% in a temperature range of 60 to 80 °C in a high temperature bath
5 tank to remove foreign substances such as oil or the like from a surface of the
underlying metal of the stainless steel under conditions of an electrolytic process, and
final cleaning is performed in a high temperature cleaning tank.
According to the embodiment of the present invention, in the preheating and
heating of the stainless steel, the preheating may be performed under conditions of a
10 preheating temperature of 530 °C or more and oxygen concentration of 20 ppm or
more such that a composite oxide layer of activated Fe and Cr is formed on a
preprocessed surface of the underlying metal, the stainless steel may be heated under
conditions of a heating temperature in the range of 900 to 1,000 °C, a hydrogen
concentration of 30% or more, and a dew point temperature in the range of -30 to -
15 45 °C, and cooled to a temperature in the range of about 630 to 730 °C for annealing
such that the composite oxide layer of activated Fe and Cr is reduced.
According to the embodiment of the present invention, in the hot dip coating
the stainless steel with aluminum, an annealed underlying metal may be plated in a
molten aluminum plating bath in a temperature range of 600 to 700 °C, and a plated
20 steel plate may be cooled at a cooling rate in a range of 20 to 40 °C/sec to a
temperature of 350 °C or less to manufacture a plated steel plate.
According to the embodiment of the present invention, composition of the
molten aluminum plating bath may include Al at 88 to 92% and Si at 8 to 11%, and
the stainless steel may be hot-dip coated with aluminum at a thickness of 200 μm or
25 less.
7
[Advantageous Effects]
According to the embodiment of the present invention, the ferritic stainless
steel having improved condensate corrosion properties and formability can be
provided to parts of an automotive exhaust system.
5 Further, according to the embodiment of the present invention, a component
of the automotive exhaust system having condensate corrosion properties at least
equal to that of a stainless 439 or 436L steel in an area such as China or the like in
which an existing high sulfur fuel is used can be manufactured without increasing a
manufacturing cost.
10 Further, according to the embodiment of the present invention, the ferritic
stainless steel for the automotive exhaust system having economic efficiency and
excellent formability can be provided without adding expensive alloying elements
thereto.
Further, according to the embodiment of the present invention, the ferritic
15 stainless steel for the aluminum plated steel plate for the automotive exhaust system
having improved condensate corrosion properties and high temperature oxidation
resistance can be provided.
[Description of Drawings]
FIG. 1A is a graph illustrating anodic polarization characteristics of steel with
20 the addition of Sn according to an embodiment of the present invention and steel
without the addition of Sn in a condensate solution of automotive exhaust system.
FIG. 1B is a graph illustrating anodic polarization characteristics of steel with
the addition of Cu according to the embodiment of the present invention and steel
8
without the addition of Cu in the condensate solution of the automotive exhaust
system.
FIG. 2 is a graph illustrating a change in a concentration of Sn at a surface
part and a base part of a ferritic stainless steel according to the embodiment of the
5 present invention.
FIG. 3 shows graphs and pictures of a result of analyzing cold rolling and
annealing scale components of a comparative steel containing 11Cr and steel
containing 11Cr-0.1Sn according to the embodiment of the present invention in a
depth direction, and a result of observing pictures of the surfaces of the steels and
10 analyzing surface components of the steels after immersing the steels in an about
5%-sulfuric acid solution for 24 hours.
FIG. 4 shows a graph illustrating a result of a simulated corrosion test of an
condensed water environment of the automotive exhaust system according to the
addition ratio of 5Sn+Cu, in which Sn and Cu both are added to steel having 11%-Cr.
15 FIG. 5 shows a graph illustrating a result of measuring an elongation rate to
determine formability according to the addition ratio of (5Sn+Cu) in the steel having
11%-Cr.
FIG. 6 shows a graph illustrating a measurement result of an intergranular
corrosion test according to the addition ratio of (SI+Ti)/(C+N) in the 11Cr steel.
20 FIG. 7 shows structural images illustrating a formation position and shape of
SiO2 at a vicinity of a surface part of the comparative steel and the invention steel.
FIG. 8 shows structural images for comparing measurement results of
exterior rust/corrosion resistance in a state in which the steel is continuously heated
such as in the automobile exhaust system.
9
FIG. 9 is a graph illustrating a result of measuring corrosion resistance
according to the addition of alloying elements of Sn and Cu and Al plating under the
condensed water environment of the automotive exhaust system.
[Modes of the Invention]
5 Hereinafter, the present invention will be described in detail in conjunction
with the drawings. Advantages and features of the present invention, and the
method of achieving the same will be obvious with reference to embodiments along
with the accompanying drawings which are described below. Alloy components of
the ferritic stainless steel as described above and a method of manufacturing the
10 stainless steel will be described below, and the improvement effect of condensate
corrosion resistance and formability according to the embodiment of the present
invention will be described in detail in conjunction with the drawings.
Conventionally, a variety of techniques for improving condensate corrosion
resistance and formability of parts of an automotive exhaust system have been
15 suggested. First, Japanese Laid-open Patent Publication No. 2009-174036 discloses
a stainless steel material for improving the properties of the passivation film by
adding, by weight% (wt%), 0.01 to 0.2% Si, 13 to 22% Cr, and 0.001 to 1% Sn
thereto. However, in the case of the above-described laid-open patent, an annealing
process at a temperature in the range of 200 to 700 °C for 1 minute or more is
20 required to improve the properties of the passivation film, and thus oxidation occurs
particularly at a heated portion such as the automotive exhaust system, resulting in a
decrease in pitting/corrosion resistance and rust/corrosion resistance.
Moreover, Japanese Laid-open Patent Publication No. 1994-145906 discloses
that corrosion resistance at least equal to that of a stainless steel having 17Cr may be
25 ensured by adding 0.3 to 2.0% Cu and 0.06 to 0.5% P and without adding Mo to a
10
stainless steel. However, Cu and P are solid solution strengthening elements, and
when a large amount of these elements are added to a stainless steel, formability is
degraded. Unless a material applied to components of the automotive exhaust
system has sufficient formability in addition to corrosion resistance, the material may
5 not be applied.
Furthermore, Japanese Laid-open Patent Publication No. 2008-190003
discloses a technique to improve crevice corrosion properties of a stainless steel by
adding at least one type of 0.005 to 2% Sn and 0.005 to 1% Sb thereto. However,
since the above-described report is determining rust/corrosion properties in a
10 chlorine atmosphere with a neutral environment, the stainless steel of the abovedescribed
report may not exhibit sufficient corrosion resistance in an acidic
atmosphere such as the condensed water environment in the automotive exhaust
system, and corrosion occurs at a portion such as exhaust system due to the influence
of oxidation.
15 The present invention intends to suggest a ferritic stainless steel for the
exhaust system, which may exhibit corrosion properties close to that of an existing,
relatively expensive steel grade of stainless steel 439, but which may also be
manufactured at a low cost, by synthetically designing components such as Cr, Si, Sn,
Cu, Ti, C, N, or the like to improve condensate corrosion resistance and formability,
20 and the method of manufacturing the ferritic stainless steel. To this end, hereinafter,
alloy composition according to the embodiment of the present invention will be
described first.
First, the present invention provides a ferritic stainless steel including, by
wt%, C: 0.01% or less, Cr: 9 to 13%, Si: 0.5 to 1.0%, Mn: 0.5% or less, P: 0.035% or
11
less, S: 0.01% or less, Ti: 0.05 to 0.5%, Sn: 0.05 to 0.5%, and Fe and unavoidable
impurities as a remainder.
Particularly, the stainless steel according to the embodiment of the present
invention includes 0.05 to 0.5% Sn, and Sn concentrated at a surface part of a
5 stainless steel is 10 times or more than Sn concentrated at a base part. The surface
part of the stainless steel ranges from an outermost surface portion to a depth of 1 um
in a depth direction, and preferably, to a depth of 0.1 um.
Further, according to the embodiment of the present invention, 0.05 to 0.15%
Sn is included at the base part of the stainless steel, but is included at 0.5 to 1.5% in
10 the surface part of the stainless steel. Accordingly, Sn concentrated at the surface
part is 10 times or more than Sn concentrated at the base part. According to the
embodiment of the present invention, preferably, when Sn is included at 0.1% at the
base part of the stainless steel, at least 1% Sn is included at the surface part of the
stainless steel.
15 Further, according to the embodiment of the present invention, 0.5 to 1 %
Cu may be further included, by wt%: Here, it is preferable that the stainless steel
has (5Sn+Cu) in the range of 0.5 to 2.0.
Further, according to the embodiment of the present invention, it is intended
to improve condensate corrosion properties and formability of the stainless steel by
20 adjusting (Si+Ti)/(C+N) in the stainless steel to be in the range of 50 to 90.
Hereinafter, the reason for limiting the compositional range of the alloying
elements will be described in detail.
First, according to the embodiment of the present invention, C and N are Ti(C,
N) carbonitride forming elements, and are present as interstitial alloying elements.
25 When the contents of C and N as described above are increased, C and N solid
12
solutions which are not formed as Ti(C, N) carbonitride lead to the degradation of an
elongation rate and low temperature impact properties of a material, and when used
at 600 °C or less for an extended period of time after welding, a Cr23C6 carbide is
generated, and thereby intergranular corrosion may occur.
5 Accordingly, it is preferable to adjust the content range of each C and N to
0.01% or less. Meanwhile, when the contents of C and N are increased and a large
amount of Ti is added, rigid inclusion increases, and thus many surface defects such
as a scab are generated, nozzle clogging occurs during continuous casting, and an
elongation rate and impact properties decrease due to the increase of a C and N solid
10 solution, and thus the contents of C and N are limited to 0.02% or less.
Although Si is conventionally added to improve weld zone corrosion
resistance, a pitting potential, and oxidation resistance, according to the embodiment
of the present invention, a Si component suppresses diffusion and segregation of Cr
at a temperature in the range of 400 to 700 °C, preventing intergranular corrosion.
15 According to the embodiment of the present invention, at least 0.5% or more of Si is
added so as to improve a pitting potential, and oxidation resistance, and intergranular
corrosion resistance properties. When 1.0 % or more of Si is included, a problem
such as an increase in rigid Si inclusions, surface defects, or the like is caused, and
thus the content of Si is limited not to be greater than a maximum of 1.0%.
20 When the content of Mn increases, precipitates such as MnS or the like are
formed, decreasing pitting corrosion resistance. However, an excessive decrease in
Mn causes an increase in refining costs or the like, and thus the content of Mn is
limited to 0.5% or less.
P and S form intergranular segregation and MnS precipitates to decrease hot
25 workability, and thus the contents of P and S are preferably as small as possible.
13
However, an excessive decrease in P and S causes an increase in refining costs or the
like, and thus the content of P is limited to 0.035% or less, and the content of S is
limited to 0.01% or less.
Cr is an essential element for securing corrosion resistance. When the
5 content of Cr is low, condensate corrosion resistance decreases, and when the content
of Cr is excessively high, corrosion resistance or the like is improved, and strength
increases, but an elongation rate and impact properties decrease, and thus the content
of Cr is limited to the range of 9 to 13%.
Ti is an effective element which prevents the occurrence of intergranular
10 corrosion by fixing C and N. However, when the ratio of (Si+Ti)/(C+N) is low,
intergranular corrosion occurs in weld zone or the like, decreasing corrosion
resistance. Accordingly, the content of Ti is limited to at least 0.15%. However,
an excessive addition of Ti leads to an increase in rigid inclusions, and thus many
surface defects such as a scab are formed, and nozzle clogging occurs during
15 continuous casting. Therefore, the content of Ti is limited to 0.5% or less.
Particularly, when (Si+Ti)/(C+N) is controlled to the range of 50 to 90,
intergranular corrosion may be prevented, and corrosion resistance may be improved.
Referring to FIG. 4, the critical range of intergranular corrosion occurrence is shown.
It will be described in detail below.
20 Sn is an essential element for securing target corrosion resistance of the
present invention. In order to secure target corrosion resistance of the present
invention, an addition of at least 0.05% Sn or more is required.
However, an excessive addition of Sn causes a decrease in hot workability
and manufacturability, and thus the upper limit of Sn is limited to 0.5%. According
25 to the embodiment of the present invention, when about 0.1% Sn is added, Sn is
14
added to a surface of a passivation layer of the stainless steel, and thus pitting
corrosion resistance increases, and Sn suppresses the formation of Si02 oxide on a
scale surface generated in the process of cold or hot rolling and annealing, thus
increasing the process efficiency of cold or hot rolling and annealing. Related
5 details will be described below.
Cu is an essential element for securing target corrosion resistance of the
present invention. Cu is an element having excellent sulfuric acid corrosion
resistance, but having low chloride corrosion resistance. In order to secure target
corrosion resistance of the present invention, an addition of at least 0.5% Cu is
10 required. The addition of 1% Cu or more causes a decrease in hot workability and
problems in a manufacturing process, and thus the content of Cu is limited to 1% or
less. According to the embodiment of the present invention, when Cu is added,
corrosion resistance may be improved under a sulfuric acid atmosphere.
Further, according to the embodiment of the present invention, the test result
15 of FIGS. 4 and 5 shows that (SSn+Cu) in the range of 0.5 to 2.0 is effective in
improving condensate corrosion properties and formability.
Next, the effect of Sn and Cu according to the embodiment of the present
invention will be described in detail in conjunction with FIGS. IA and 2B.
FIGS. 1A and 2B are graphs illustrating a comparison of anodic polarization
20 properties of steel with the addition of alloying elements such as Sn, Cu, Cr, or the
like according to the embodiment of the present invention and steel without the
addition of the alloying elements in a condensate solution of the automotive exhaust
system.
In order to test the anodic polarization properties in a condensate solution of
25 the automotive exhaust system, high purity ferritic stainless steel including Cr at 11%
15
and a simulated solution of condensed water of the automotive exhaust system (CF:
500 ppm, 5042-: 5,000 ppm, pH: 3.0) are prepared. Here, the solution has a
temperature of 50 °C. In this case, the anodic polarization properties of the ferritic
stainless steel with the addition of Sn or Cu and stainless steel with the addition of Sn
5 or Cu are compared. First, steel including Cr at 11% has a constant corrosion
potential of about -600 mV regardless of the addition of Sn as shown in FIG. 1A.
However, as the addition amount of Sn gradually increases, an activated current
density tends to gradually decrease, and secondary-activated dissolution behavior is
shown. From the result, it is determined that precipitates are formed on a surface of
10 the steel after Sn is dissolved, decreasing an activated current density, and that Sn
has excellent corrosion resistance under an 5042- containing environment. Further,
referring to FIG. 1B, when Cu is added, a corrosion potential of about -630 mV is
increased to about -560 mV, and thereby a corrosion potential is increased about 70
mV. A corrosion potential increases due to Cu, and thus corrosion resistance
15 relatively increases. An activated current density about 5 to 10 times decreased
according to the addition of Cu, and the corrosion current densities at a passivation
area of steel with the addition of Cu and steel without the addition of Cu were
identical. The result shows that a compact passivation film was formed. As a
result, it may be determined that, when an alloying element such as Sn or Cu is
20 added to high purity ferritic steel, a corrosion potential increases, an activated current
density decreases, and starting potential decreases. Further, it was found that the
steel has excellent corrosion resistance in the acidic environment in which pH is
about 3 and a large amount of S042- is contained. The environment in which a pH
is 3, and 5,000 ppm of 5042- and ions are contained may show a corrosion
25 tendency of a muffler material of the automotive exhaust system in China, India,
16
Latin America and Russia, where a large amount of an S component, such as about
500 ppm, is included in gasoline components.
Meanwhile, FIG. 2 is a graph illustrating that Sn is concentrated at a surface
of the stainless steel according to the embodiment of the present invention, and
5 thereby condensate corrosion resistance is improved. Particularly, FIG. 3 shows a
result of measuring the content of Sn at a surface part and base part of steel having
11Cr-0.1Sn, which is the invention steel according to the embodiment of the present
invention. The surface part may be defined as a part extending from an outermost
surface layer of the stainless steel to a depth of 1 gm in a depth direction as described
10 above. I Iowever, FIG. 2 illustrates a result of measuring the content of Sn at a part
extending from an outermost surface layer of the stainless steel to a depth of 0.1 gm
in a depth direction. Further, the measurement result of the base part represents a
measurement result at a depth of 500 gm in a depth direction. In the steel with the
addition of about 0.1% of Sn according to the embodiment of the present invention,
15 the content of Sn at the surface part of the steel is more than about 1%, and as
compared to 0.1% of Sn at the base part, Sn concentrated at the surface part is 10
times or more than Sn concentrated at the base part. When a concentrated layer in
which Sn is concentrated is present at a surface, excellent sulfuric acid corrosion
resistance may be provided. A thermal diffusion of Sn occurs prior to those of the
20 other elements in the cold rolling and annealing process, and thus Sn is concentrated
at a surface of the steel. According to the embodiment of the present invention,
conditions of surface concentration of Sn may be controlled by controlling heat
treatment conditions in the cold rolling and annealing process as described above.
Further, when Sn is concentrated at a surface as described above, an oxide of Fe, Cr,
25 or the like generated in the cold rolling and annealing process is sufficiently
17
dissolved and removed by subsequent neutral salt electrolytic and sulfuric acid
electrolytic pickling processes. On the other hand, Sn concentrated at a surface is
not dissolved and removed in the neutral salt electrolytic and sulfuric acid
electrolytic pickling processes, and thus may remain in a concentrated state at a
5 surface.
FIG. 3 shows graphs and pictures of a result of analyzing the cold rolling and
annealing scale components of a comparative steel containing 11Cr and steel
containing 11Cr-0.1Sn according to the embodiment of the present invention in a
depth direction, and a result of observing pictures of surfaces of the steels and
10 analyzing surface components of the steels after immersing the steels in an
approximate 5%-sulfuric acid solution for 24 hours.
As illustrated in the drawing, a cold rolling and annealing scale of the
comparative steel has a thickness of about 0.1 um, and the scale mainly includes Fe
and Cr oxides, and a small amount of Si oxide. Meanwhile, an annealing scale of
15 the invention steel according to the embodiment of the present invention mainly
includes Fe and Cr oxides, but has a smaller amount of Si oxide than that in the
comparative steel. Further, in the invention steel according to the embodiment of
the present invention, about 1% Sn or more is concentrated at a part extending from a
surface to a depth of about 0.1 p.m in a depth direction, and thus Sn concentrated at a
20 surface part of a stainless steel is 10 times or more than Sn concentrated at a base
part. Referring to a result of analyzing components of a cold rolling and annealing
scale in a depth direction after neutral salt-sulfuric acid electrolytic pickling is
performed on the invention steel containing 11Cr-0.1Sn according to the
embodiment of the present invention, a Fe and Cr annealing scale was removed, but
25 Si oxide still remained in the comparative steel. On the other hand, a Fe and Cr
18
annealing scale were removed, and no Si oxide was observed at a surface in the
invention steel. Further, at least 1% Sn or more is concentrated at a surface.
Referring to a result of observing pictures of surfaces of the steels after the
comparative steel and the invention steel are immersed in a 5%-sulfuric acid solution
5 for 24 hours after cold rolling and pickling as described above, it may be seen that a
surface of the comparative steel was corroded. On the other hand, a surface of the
invention steel was not corroded.
As a result of analyzing components in a surface after an immersion
corrosion test, it was observed that about 1% Sn was concentrated. From the result,
10 it may be determined that Sn concentrated at a surface suppresses the formation of Si
oxide in the cold rolling and annealing process, and thereby sufficient pickling may
be achieved only by neutral salt-sulfuric acid electrolytic pickling. Further, an Sn
layer concentrated at a surface remains without being removed by cold rolling and
neutral salt-sulfuric acid electrolytic pickling processes, and thus the steel may have
15 excellent sulfuric acid corrosion resistance. As may be seen from the result, when
Sn is added to a base part, the formation of Si oxide in an annealing scale formed
during the cold rolling and annealing process may be suppressed.
Meanwhile, FIG. 4 shows a result of comparing a result of a corrosion test in
a simulated condensed water environment of the automotive exhaust system
20 according to the addition ratio of 5Sn+Cu, in which Sn and Cu are added to the steel
including 11% Cr according to the embodiment of the present invention, for 11Cr
(STS 409) and 17Cr (STS 439), which are used in a cold part such as a muffler part
of the automotive exhaust system. A condensate solution used in the present test is
prepared according to a JASO-611-92 method of the Japanese automobile standards
25 association. In the test, 10 ml of the solution was supplied to a specimen at 90 °C
19
every 6 hours in air in which the specimen was completely dried after 5 hours, and
this was repeated as a one cycle. After 80 cycles, a corrosion oxide of the specimen
was removed in a boiling 60% nitric acid solution, and then a corrosion depth was
measured. A maximum corrosion depth was defined after measuring a depth of 30
5 portions of the specimen. Referring to FIG. 4, under the environment of condensed
water, the maximum corrosion depth of a steel grade of STS 409 which is an 11Cr
steel was about 0.45 mm, and the maximum corrosion depth of a steel grade of STS
439 which is a 17Cr steel was about 0.25 mm. In the case of a steel grade of STS
409 which is a 11Cr steel, when the addition amount of 5Sn+Cu increased, a
10 corrosion depth rapidly decreased, and when 5Sn+Cu is 0.5 or more, a corrosion
depth was smaller than that of a steel grade of STS 439 which is a 17Cr steel.
However, when 5Sn+Cu was 0.5 or less, the addition amount of alloys are
insufficient, and thus a corrosion depth at a level of that of a steel grade of STS 439
which is a 17Cr steel was not observed. Meanwhile, when 5Sn+Cu is increased to 2
15 or more, a corrosion depth in the environment of condensed water decreases, and
thus the steel may have excellent corrosion resistance, but there may be a problem in
formability and manufacturability due to an excessive addition of alloys.
FIG. 6 is a result of comparing elongation rate measurement results of a steel
grade of 11Cr STS 409 and a steel grade of 17Cr STS 439 which are used in the cold
20 part such as the muffler part of the automotive exhaust system in order to determine
formability of the steel including 11% Cr according to the addition ratio of (5Sn+Cu).
An elongation rate of a steel grade of 11Cr STS 409 was about 36%, and an
elongation rate of a steel grade of 17Cr STS 439 was about 30%. An elongation
rate required for a processing site of the cold part of the automotive exhaust system is
25 about 30% or more. When the amount of 5Sn+Cu added to 11Cr STS 409
20
increased, an elongation rate tended to linearly decrease. When 5Sn+Cu increased
to 2 or more, an elongation rate decreased to 30% or less. In general, Sn and Cu
have been known to decrease hot workability during hot working. However, the
inventors of the present invention have found that, since Sn rapidly diffuses at a hot .
5 working temperature range, when the addition amount is less than about 0.5% Sn,
hot workability may not decrease. In terms of formability, Sn and Cu are solid
solution strengthening elements, and thus are known to decrease an elongation rate
by increasing the strength of a material. However, it was determined for high purity
ferritic stainless steel, when the addition amounts of Cr, Sn and Cu are suitably
10 adjusted, for example, such that 5Sn+Cu is 2 or less, formability may be ensured and
corrosion resistance may be improved without a decrease in an elongation rate.
Meanwhile, FIG. 6 shows an evaluation result of an intergranular corrosion
test according to the addition ratio of (SI+Ti)/(C+N) to 11Cr. In the weld zone
intergranular corrosion test, a sensitizing heat treatment was performed with respect
15 to a GTA (TIG) welded specimen at 500 °C for 10 hours in air and the specimen was
quenched to simulate a temperature atmosphere of the automotive exhaust system.
Thereafter, according to a modified Strauss test method, after Cu balls were laid on a
lower part of a 6% CuSO4 + 0.5% H2SO4 solution, the specimen was immersed in the
boiling solution for 24 hours, a sectional structure of the specimen was observed and
20 a 1U-bend test was performed to evaluate intergranular corrosion. GTA (TIG)
welding was performed using a DC type welder (maximum welding current 350A)
and a bead on plate. Welding conditions are as below: welding current: 110A,
welding speed: 0.32m/min, Tungsten electrode diameter: 2.5mm, electrode tip angle:
1000, arc length 1.5mm, and shielding gas: Ar (151/min).
21
The inventors of the present inventor have found that Si is an effective
element for preventing intergranular corrosion at a sensitizing temperature in the
range of about 400 to 700 °C, which is a condition for operating the automotive
exhaust system.
5 With respect to an 11Cr stainless steel, when the ratio of (SI+Ti)/(C+N) is 50
or less, intergranular corrosion occurred. On the other hand, when the ratio of
(SI+Ti)/(C+N) is 50 or more, intergranular corrosion did not occur. On the other
hand, when the ratio of (SI+Ti)/(C+N) is 90 or more, intergranular corrosion did not
occur, but an amount of Si and Ti alloys increased, and thereby formability decreased
10 to 30% or less, or there were many restrictions in a manufacturing process, such as
surface cracking and nozzle clogging during a productiOn process.
Further, according to the embodiment of the present invention, ferritic
stainless steel having the alloy compositions as , described above is plated with
aluminum, and thereby further improved high temperature oxidation resistance and
15 condensate corrosion properties may be obtained. It will be described in detail with
reference to FIGS. 8 and 9.
First, FIG. 8 shows structural images for comparing measurement results of
exterior rust/corrosion resistance in a state in which steel is continuously heated such
by a simulated salt solution for snow removal or the automobile exhaust system.
20 This shows an evaluation result of exterior rust/corrosion resistance of a material of
the automotive exhaust system. In general, stainless steel (STS) 436L which is
17Cr-IMo steel has excellent exterior rust/corrosion resistance, and thus is frequently
used for a material of the automotive exhaust system. However, as shown in FIG.
8B or 8C, oxidation and red corrosion are generated in a short time at a part which is
25 continuously heated by a simulated salt solution for snow removal (5% NaCl+5%
22
CaCl2) or the automotive exhaust system, and thus an exterior appearance is
damaged. When Sn is added at 0.1% to 11Cr steel, exterior rust/corrosion
resistance is relatively improved as compared to that of a 11Cr steel, but red
corrosion occurs in a short time at a part which is continuously heated by the
5 simulated salt solution for snow removal or the automotive exhaust system.
However, in the case of the specimen which is 11Cr-0.1Sn steel plated with
aluminum, it may be determined that red corrosion does not occur at a part which is
continuously heated by the simulated salt solution for snow removal or the
automotive exhaust system.
10 FIG. 9 is a graph illustrating a result of measuring corrosion resistance under
the condensed water environment of the automotive exhaust system according to the
addition of alloying elements of Sn and Cu, and Al plating. A condensate solution
used in the present test is prepared according to a JASO-611-92 method of Japan
automobile standards association. In the test, 10 ml of the solution was supplied to
15 a specimen at 90 °C every 6 hours in air in which the specimen was completely dried
after 5 hours, and this was repeated as a one cycle. After 80 cycles, a corrosion
depth was measured for evaluation. A maximum corrosion depth was defined after
measuring a depth of 30 portions of the specimen. Referring to the maximum
corrosion depth, when Sn and Cu were added to 11Cr STS 409L, the corrosion depth
20 rapidly decreased. The corrosion depth also gradually decreased according to the
addition amount of Sn and Cu. Referring to the maximum corrosion depth, when
Sn is added at 0.05 to 0.5%, preferably at 0.1 to 0.5% to 11Cr, the corrosion depth
similar to that of 17Cr or 17Cr-lMo was shown. Further, when the steel is plated
with Al, it may be determined that the corrosion depth is significantly smaller than
25 that of STS 436L which is 17Cr-1Mo stainless steel.
23
From the results of FIGS. 8 and 9 as described above, the inventors of the
present invention determined that, when an alloying element of Sn or Sn and Cu is
added to steel and the steel is plated with Al, a steel grade for the exhaust system
which has excellent condensate corrosion resistance as compared to STS 436L under
5 the environment in which a large amount of the S component is contained in gasoline
components, and by which no red corrosion occurs in a part which is continuously
heated by an automotive exhaust system, may be developed.
Next, the manufacturing method according to the embodiment of the present
invention will be described.
10 (Slab heating temperature condition)
For the ferritic stainless steel according to the embodiment of the present
invention, slab heating temperature condition is controlled. Preferably, the slab
heating temperature condition is in the range of 1,100 to 1,240 °C. In general, the
slab heating temperature is limited to the range of 1,100 to 1,240 °C such that a grain
15 size is reduced to improve toughness and an r value, and to ensure formability and
processability. When the slab heating temperature is 1,100 °C or less, a sticking
defect occurs, that is, a surface part of a material is detached from the material and
attached to a rolling roll upon hot rolling. Further, when the slab heating
temperature is 1,240 °C or more, a grain size of a product is coarsened, and thereby
20 toughness and an r value decrease. Accordingly, the slab heating temperature is
preferably limited to the range of 1,100 to 1,240 °C.
(Hot rolling and annealing temperature condition)
According to the embodiment of the present invention, the hot rolling and
annealing temperature is in the range of 1,030 to 1,070 °C based on a strip
25 temperature. Regarding the hot rolling and annealing temperature in manufacturing
24
conditions for the stainless steel according to the embodiment of the present
invention, when the steel is annealed at a relatively low temperature within the range
in which recrystallization is performed, a recrystallized grain size is reduced after
annealing, and thus an r-bar value of a final cold annealed plate is excellent.
5 However, when the hot rolling and annealing temperature is 1,030 °C or less,
recrystallization is insufficiently performed, and thus formability and an elongationrate
decrease, and when the hot rolling and annealing temperature is 1,070 °C or
more, toughness of a hot annealed coil decreases, and thus there is a concern of strip
breakage during the manufacturing process, a grain size is coarsened, and thus an
10 orange peel defect may be generated upon molding. Accordingly, annealing is
preferably performed at a temperature in the range of 1,030 to 1,070 °C to improve
toughness and an r value.
(Cold rolling and annealing temperature)
According to the embodiment of the present invention, the cold rolling and
15 annealing temperature is in the range of 980 to 1,020 °C based on a strip temperature.
With respect to the stainless steel according to the embodiment of the present
invention, when the cold rolling and annealing temperature is 980 °C or less,
recrystallization after annealing is insufficiently performed, and thus an elongation
rate and formability may decrease. Further, when the cold rolling and annealing
20 temperature is 1,020 °C or more, a grain size is coarsened, and thus an orange peel
defect may be generated upon molding. Accordingly, cold rolling and annealing is
preferably performed in the above-described range to improve high temperature
strength by reducing the grain size of precipitates.
(Cold rolling and pickling condition)
25
The ferritic stainless steel according to the embodiment of the present
invention has a specific cold rolling and pickling condition. Particularly, after cold
rolling and annealing, Si oxide included in an annealing scale is removed through
primary neutral salt electrolytic pickling and secondary sulfuric acid electrolytic
5 pickling. In an existing pickling process contrasting with the present invention,
pickling was performed through mixed acid soaking after neutral salt electrolytic
pickling and secondary sulfuric acid electrolytic pickling.
FIG. 7 shows structural images for illustrating a difference in cold rolling and
pickling conditions according to a type of SiO2 formation of an existing steel grade
10 of STS 409 which is the comparative steel and the invention steel according to the
embodiment of the present invention. In the case of the comparative steel, as
shown in FIG. 7, coarsened SiO2 may be seen at the vicinity of a surface of the
stainless steel. Accordingly, the existing steel grade of STS 409 necessarily
requires a pickling process through mixed acid soaking in addition to neutral salt
15 electrolytic pickling and secondary sulfuric acid electrolytic pickling. However, the
above-described mixed acid soaking causes serious environmental issues due to the
use of nitric acid and hydrofluoric acid. Meanwhile, in the case of the invention
steel according to the embodiment of the present invention, a relatively smaller
amount of Si oxide is formed as compared to that of the comparative steel, and the Si
20 oxide is formed in a thin and continuous strip shape. Particularly, in the case of the
invention steel including 11Cr-0.1Sn according to the embodiment of the present
invention, referring to a result of analyzing components of a cold rolling and
annealing scale in a depth direction after neutral salt-sulfuric acid electrolytic
pickling is performed on the steel, a Fe and Cr annealing scale was removed, and no
25 Si oxide is observed in a surface of the steel.
26
Particularly, in the case of the invention steel, referring to a result of
observing a surface picture after the invention steel is immersed in a 5% sulfuric acid
solution for 24 hours, the surface is not corroded, and as a result of a surface
component analysis after an immersion corrosion test, about 1% Sn is concentrated at
5 the surface. From the result, it may be determined that Sn concentrated at a surface
suppresses the formation of Si oxide in the cold rolling and annealing process, and
thereby sufficient pickling may be achieved only by neutral salt-sulfuric acid
electrolytic pickling. Further, an Sn layer concentrated at a surface remains without
being removed by a cold rolling and neutral salt-sulfuric acid electrolytic pickling
10 processes, and thus the steel may have excellent sulfuric acid corrosion resistance.
As a result, it may be determined that, when Sn is added to a base part, the formation
of Si oxide in an annealing scale formed during the cold rolling and annealing
process may be suppressed, and sufficient pickling may be achieved only by typical
neutral salt-sulfuric acid electrolytic pickling. The pickling conditions according to
15 the embodiment of the present invention preferably include a temperature in the
range of 70 to 90 °C and a neutral salt concentration in the range of 150 to 250 g/L,
as a typical pickling condition for a ferritic stainless steel. Further, secondary
sulfuric acid electrolytic pickling conditions preferably include a temperature in the
range of 30 to 50 °C and a sulfuric acid concentration in the range of 65 to 85 g/L.
20 Subsequently, aluminum plating conditions and processes according to the
embodiment of the present invention will be described. The process of plating
aluminum on a surface of the stainless steel may be performed after both the cold
rolling and pickling processes, or may also be performed before the cold rolling and
pickling processes. According to the embodiment of the present invention, the
25 aluminum plating process includes preprocessing of an underlying metal, preheating
27
and heat cracking, and plating with aluminum, as typical hot-dip aluminizing. A
typical hot-dip aluminizing process may be used for the following preprocessing,
preheating and heating, and plating.
(Preprocessing of underlying metal)
5 Preprocessing of the underlying metal is performed in order to remove
foreign substances from the surface of the underlying metal and induce an immediate
surface reaction under an oxidizing atmosphere of a front end preheating zone by
heating, which is the next process, after removing. Preprocessing is preferably
electrolytic cleaning. The steel is bathed in a caustic soda solution having a
10 concentration in the range of 2 to 5% in a high temperature bath tank at a
temperature in the range of 60 to 80 °C. Subsequently, foreign substances such as
oil or the like are removed from the surface of the underlying metal under conditions
of an electrolytic process, and final cleaning is performed in a high temperature
cleaning tank.
15 (Preheating and heat cracking)
Preheating is performed under conditions of a preheating temperature of
530 °C or more and an oxygen concentration of 20 ppm or more such that a
composite oxide layer of activated Fe and Cr is formed on the preprocessed surface
of the underlying metal, and the stainless steel is heated under conditions of a heating
20 temperature in a range of 900 to 1,000 °C, a hydrogen concentration of 30% or more,
and a dew point temperature in the range of -30 to -45 °C, and cooled to a
temperature in a range of about 630 to 730 °C for annealing such that the composite
oxide layer of activated Fe and Cr is reduced.
(Hot dip aluminizing)
28
After the annealed underlying metal is plated in a molten aluminum plating
bath at a temperature in the range of 600 to 700 °C, and the plated steel plate is
cooled at a cooling rate in the range of 20 to 40 °C/sec to a temperature of 350 °C or
less to manufacture a plated steel plate. Composition of the molten aluminum
5 plating bath include Al at 88 to 92% and Si at 8 to 11%. The underlying metal of
the stainless steel is continuously immersed in the composition of the molten
aluminum plating bath, and is controlled to have a plating layer with a suitable
thickness. Preferably, plating is performed such that the plating layer of the plated
steel has a thickness of 200μm or less
10
(Examples)
Hereinafter, the present invention will be described in detail in conjunction
with examples.
An ingot having a thickness of 120 mm is prepared by dissolving ferritic
15 stainless steel having the compositions represented in Table 1 in a vacuum induction
furnace with a capacity of 50 kg. The prepared ingot was hot rolled at a
temperature in the range of 1,100 to 1,200 °C to manufacture a hot rolled plate
having a thickness of 3.0 mm. Thereafter, after annealing and pickling the hot
rolled steel plate, cold rolling was performed such that a plate thickness of a cold
20 rolled plate is 1.2 mm, and a pickling process was performed, and thereby the steel
plate was used for the evaluation of corrosion resistance and mechanical properties.
Compositions in the range defined by the present invention, and compositions out of
the range defined by the present invention were used. Compositions of the
comparative steel were set to be similar to those of a steel grade of 11Cr STS 409
25 and those of a steel grade of 17Cr STS 439.
29
The following Table 1 represents a composition table for the stainless steel
according to the embodiment of the present invention.
(Table
N
o.
Clas
sific
ation
C Si Mn P S Cr Ti Sn N Cu Mo
(Si+Ti)/
(C+N)
5Sn+
Cu
0.0 0.6 0.2 0.0 0.0 11. 0.17 0.09 0.00 0.0
1 0.0 57.2 0.5
05 54 13 3 03 14 5 9 95
0.0 0.5 0.2 0.0 0.0 11. 0.21 0.17 0.00 Q.5
2 0.0 62.2 1.4
05 68 11 2 03 19 0 4 75
0.0 0.8 0.2 0.0 0.0 11. 0.20 0.00 0.6
3 0.05 0.0 82.9 0.9
06 95 07 3 03 10 8 73 8
0.0 0.5 0.2 0.0 0.0 0.21 0.09 0.00 0.5
4 Inve 13 0.0 55.8 1.1
ntion
06 64 03 1 03 2 9 79 6
0.0 0.5 0.2 0.0 0.0 12. 0.17 0.18 0.00 0.0
5 steel 0.0 52.2 1.0
06 17 03 2 03 17 7 5 73 0
0.0 0.5 0.2 0.0 0.0 12. 0.17 0.28 0.00 0.0
6 0.0 52.2 1.5
06 17 03 2 03 57 7 5 73 0
0.0 0.5 0.1 0.0 0.0 0.28 0.07 0.00 0.6
7 13 0.0 55.0 1.0
06 20 93 3 03 3 5 86 3
0.0 0.6 0.2 0.0 0.0 11. 0.21 0.06 0.00 0.8
8 0.0 86.0 1.2
04 93 12 2 03 19 0 1 65 4
9 0.0 0.7 0.1 0.0 0.0 12. 0.25 0.21 0.00 0.7 0.0 71.1 1.8
30
06 85 93 3 03 40 3 2 86 3
1 0.0 0.4 0.2 0.0 0.0 11. 0.20 0.03 0.00 0.4
0.0 46.0 0.6
0 05 00 13 2 03 13 7 0 82 1
0.0 1.1 0.1 0.0 0.0 9.9 0.12 0.07 0.00 0.0
11 0.0 93.3 0.4
05 81 09 3 03 8 5 1 90 0
0.0 0.5 0.2 0.0 0.0 11. 0.25 0.75 0.00 0.2
12 0.0 57.6 4.0
06 14 07 2 03 10 2 0 73 3
0.0 0.5 0.5 0.0 0.0 9.5 0.18 0.08 0.00 0.0
13 0.0 45.3 0.4
07 40 09 2 03 0 5 9 90 0
0.0 0.4 0.2 0.0 0.0 11. 0.17 0.00 0.00 0.1
14 Corn 0.0 44.0 0.2
parat
07 95 13 3 03 13 4 0 82 6
0.0 0.6 0.2 10.0 0.0 9.8 0.20 0.05 0.00 1.8
15 ive 0.0 62.2 2.1
steel
06 50 04 3 03 5 8 0 78 5
0.0 1.0 0.2 0.0 0.0 10. 0.20 0.03 0.00 0.0
16 0.0 105.6 0.2
06 86 13 2 03 98 3 0 62 0
0.0 0.4 0.1 0.0 0.0 13. 0.10 0.31 0.00 1.0
17 0.0 36.6 2.6
05 08 09 2 03 98 5 0 90 9
0.0 1.2 0.2 0.0 0.0 12. 0.21 0.79 0.00 2.0
18 0.0 107.1 6.0
06 12 07 2 03 50 3 5 73
0.0 0.5 0.5 0.0 0.0 12. 0.15 0.10 0.00 1.1
19 0.0 55.3 . 1.7
04 64 09 3 03 45 5 9 90 2
0.0 0.5 0.1 0.0 0.0 11. 0.27 0.00 0.00 0.0
1 20 0.0 54.3 0.0
07 52 93 2 03 00 4 0 82 0
31
0.0 0.5 0.2 0.0 0.0 17. 0.21 0.00 0.00 0.0
21 1.1 47.6 0.0
07 00 29 2 03 65 4 0 80 0
Table 2 shows a result of measuring the occurrence of GTA-welded zone
intergranular corrosion, a corrosion depth under the environment of condensed water,
and an elongation rate of the high purity ferritic stainless steel according to the
5 embodiment of the present invention.
[Table 21
No. Classification
Occurrence of
intergranular
corrosion
(0 X)
Corrosion depth
under environment
of condensed water
(mm)
Elongation
rate
(%)
Developed
steel
0 0.24 33.0
2 0 0.16 34.0
3 0 0.17 33.0
4 0 0.18 33.0
5 0 0.21 33.0
6 0 0.16 31.0
0 0.20 33.0
8 0 0.21 32.0
9 0
i
0.18 34.0
32
10
Comparative
steel
X 0.27 27.0
11 © 0.31 37.8
12 * 0.13 26.0
13 X 0.33 36.0
14 X 0.44 35.6
15 © 0.15 28.5
16 * 0.57 34.6
17 X 0.16 27.5
18 © 0.08 24.5
19 © 0.17 30.1
20 © 0.45 36.0
121 CI 0.25 30.0
(Weld zone intergranular corrosion test)
First, according to a modified Strauss test method, after Cu balls were laid on
a lower part of a 6% CuSO4 + 0.5% H2SO4 solution, a specimen was immersed in a
5 boiling solution for 24 hours, a sectional structure of the specimen was observed and
a 1U-bend test was performed to investigate the formation of cracks (R=20. The
case in which no crack is formed after the 1U-bend test is represented by "*' which
indicates no occurrence of intergranular corrosion, and the case in which cracks are
formed after the 1U-bend test is represented by "X" which indicates the occurrence
10 of intergranular corrosion. GTA (TIG) welding was performed using a DC type
welder (maximum welding current 350A) and a bead on plate. Welding conditions
33
are as below: welding current: 110A, welding speed: 0.32m/min, Tungsten electrode
diameter: 2.5mm, electrode tip angle: 1000, arc length 1.5mm, and shielding gas: Ar
(151/min).
(Evaluation of corrosion resistance under environment of condensed water)
5 For the evaluation of corrosion resistance under the environment of
condensed water, 10 ml of the condensate solution manufactured according to a
JASO-611-92 method of Japan automobile manufacturers association was supplied
to a specimen at 90 °C every 6 hours in air in which the specimen was completely
dried after 5 hours, and this was repeated as a one cycle. After 80 cycles, a
10 corrosion oxide of the specimen was removed in a boiling 60% nitric acid solution,
and then a corrosion depth was measured. Here, the condensate solution having a
Cl- concentration of 50 ppm and a 5042- concentration of 5,000 pmm was used after
analyzing components of an automotive muffler used in China. After the test,
corrosion resistance was evaluated using a maximum corrosion depth which is
15 defined after measuring a depth of 30 portions of the specimen.
The invention steel has a maximum corrosion depth of 0.25 mm or less.
The existing well-known JASO-611-92 method was used as the corrosion evaluation
method in the embodiment of the present invention, and details will be omitted.
Further, according to the embodiment of the present invention, the stainless
20 steel also has a maximum corrosion depth of 0.25 mm or less measured using a
JASO M609-91 method which is a method for measuring corrosion resistance under
the environment of condensed water. According to the method of evaluating
corrosion resistance, spraying was performed on the stainless steel at pH 7.0 using a
salt spray method for 1 hour, the stainless steel was dried for 2 hours, and this was
25 repeated by 10 cycles to obtain a final value of a corrosion depth. The maximum
34
corrosion depth of the invention steel was 0.25 mm or less. The existing wellknown
JASO M609-91 method was used as the corrosion evaluation method in the
embodiment of the present invention, and details will be omitted.
(Formability evaluation)
5 Further, according to the embodiment of the present invention, the
formability evaluation was performed by preparing a JIS 13B tensile specimen using
a cold rolled steel plate having a thickness of 1.2 mm to measure an elongation rate.
The above-described evaluation result may be seen in detail through a result
of measuring the occurrence of GTA-welded zone intergranular corrosion, a
10 corrosion depth under the environment of condensed water, and an elongation rate of
the high purity ferritic stainless steel according to the embodiment of the present
invention in Table 2.
Referring to Tables 1 and 2, Specimen Nos. 1 to 9 are based on components
of the invention steel according to the embodiment of the present invention, and
15 Specimen Nos. 10 to 21 are related to the comparative steel. According to the weld
zone intergranular corrosion test, intergranular corrosion occurred at the steels of
Specimen Nos. 10, 13, 14 and 17.
In the case of Specimen Nos. 1 to 9 which are the invention steels, weld zone
intergranular corrosion did not occur, and the corrosion depth under the environment
20 of condensed water was less than 0.25 mm. Further, all the elongation rates were
more than 30% in the formability evaluation.
The comparative steel of Specimen No. 10 includes 0.4% Si and 0.03% Sn,
and thus is out of the scope of the present invention. Further, it may be determined
that the ratio of (SI+Ti)/(C+N) is 50 or less. Accordingly, intergranular corrosion
25 may occur in the comparative steel 10.
35
Meanwhile, the comparative steels of Specimen Nos. 13, 14 and 17 also have
Mn, Si, Sn, Cr, Ti or the like which are out of the scope of the present invention, and
have the ratio of (SI+Ti)/(C+N) of 50 or less, as well.
Accordingly, intergranular corrosion may also occur in the comparative steels
5 of Specimen Nos. 13, 14 and 17.
Meanwhile, the comparative steel of Specimen No. 11 has Si and Ti which
are out of the scope of the present invention, and also has 5Sn+Cu out of the scope of
the present invention. Although intergranular corrosion did not occur in the
comparative steel of Specimen No. 11, it may be determined that the corrosion depth
10 under the environment of condensed water is 0.31 mm.
In the case of the comparative steels of Specimen Nos. 12, 15, 17, 18 and 19,
the excess amount of alloys such as Sn, Cu, Si or the like are added, and thus the
formability is 30% or less as compared to the invention steel.
Further, in the case of the comparative steels of Specimen Nos. 10, 11, 13, 14,
15 16 and 20, the corrosion depth under the environment of condensed water is 0.25 mm
or more.
In the case of the comparative steel of Specimen No. 21, the ratio of
(Si+Ti)/(C+N) is 50 or less, but the content of Cr is 17%, and no intergranular
corrosion occurred in the intergranular corrosion test. However, the content of Cr is
20 out of the scope of the present invention, and the content of Cr as described above
decreases economic efficiency.
It is to be appreciated that those skilled in the art can change or modify the
embodiments without departing the technical concept of this invention.
Accordingly, it should be understood that above-described embodiments are
25 essentially for illustrative purpose only but not in any way for restriction thereto.
36
Thus the scope of the invention should be determined by the appended claims and
their legal equivalents rather than the specification, and various alterations and
modifications within the definition and scope of the claims are included in the claims.
37
[CLAIMS]
[Claim 1]
A ferritic stainless steel having excellent condensate corrosion resistance
properties, formability and high temperature oxidation resistance, comprising: by
5 wt%, C: more than 0 and 0.01% or less, Cr: 9 to 13%, Si: 0.5 to 1.0%, Mn: more than
0 and 0.5% or less, P: more than 0 and 0.035% or less, S: more than 0 and 0.01% or
less, Ti: 0.15 to 0.5%, N: more than 0 and 0.01% or less, Sn: 0.05 to 0.5%, and Fe
and unavoidable impurities as a remainder,
wherein Sn concentrated at a surface part of a stainless steel is 10 times or
10 more than Sn concentrated at a base part.
[Claim 2]
The ferritic stainless steel of claim 1, wherein 0.05 to 0.15% Sn is included at
a base part of the stainless steel, and 0.5 to 1.5% Sn is included in a part extending
from a surface of the stainless steel to a depth of 0 to 1 lam in a depth direction.
15 [Claim 3]
The ferritic stainless steel of claim 1, further comprising 0.5 to1.0% Cu.
[Claim 4]
The ferritic stainless steel of claim 3, wherein the stainless steel has (5Sn+Cu)
in a range of 0.5 to 2.0.
20 [Claim 5]
The ferritic stainless steel of any one of claim 1 or 4, wherein the stainless
steel has (Si+Ti)/(C+N) in a range of 50 to 90.
38
[Claim 61
The ferritic stainless steel of any one of claim 1 to 5, wherein a surface of the
stainless steel is hot-dip coated with aluminum.
[Claim 71
5 The ferritic stainless steel of claim 6, wherein the surface hot-dip coated with
the aluminum has a thickness of 200 1.tm or less.
[Claim 81
The ferritic stainless steel of claim 5, wherein the stainless steel has a
maximum corrosion depth of 0.25 mm or less measured using a JASO M609-91
10 method which is a method for measuring corrosion resistance under an environment
of condensed water.
[Claim 91
The ferritic stainless steel of claim 5, wherein the stainless steel has a
corrosion area ratio of 5% or less measured after 3 cycles of a JASO M611-92
15 method were performed, and a maximum corrosion depth of 0.25 mm or less
measured after 100 cycles of the JASO M611-92 method were performed, the JASO
M611-92 method is a method for measuring corrosion of an automobile exhaust gas
system.
[Claim 101
20 A method of manufacturing a ferritic stainless steel having excellent
condensate corrosion resistance properties, formability and high temperature
oxidation resistance, comprising: cold rolling and annealing a ferritic stainless steel
39
plate which includes, by wt%, C: more than 0 and 0.01% or less, Cr: 9 to 13%, Si:
0.5 to 1.0%, Mn: more than 0 and 0.5% or less, P: more than 0 and 0.035% or less, S:
more than 0 and 0.01% or less, Ti: 0.15 to 0.5%, N: more than 0 and 0.01% or less,
Sn: 0.05 to 0.5%, and Fe and unavoidable impurities as a remainder; and
5 removing Si oxide included in an annealing scale through primary neutral salt
electrolytic pickling and secondary sulfuric acid electrolytic pickling.
[Claim 11]
The method of claim 10, wherein Sn concentrated at a surface part of the
ferritic stainless steel is 10 times or more than Sn concentrated at a base part.
10 [Claim 12]
The method of claim 11, wherein 0.05 to 0.15% Sn is included at a base part
of the ferritic stainless steel, and 0.5 to 1.5% Sn is included in a part extending from
a surface of the ferritic stainless steel to a depth of 0 to 1 1.tm in a depth direction.
(Claim 131
15 The method of claim 10, wherein the ferritic stainless steel further includes
0.5 to 1.0 % Cu.
[Claim 14]
The method of claim 13, wherein the ferritic stainless steel has (5Sn+Cu) in a
range of 0.5 to 2.0.
20 [Claim 15]
The method of any one of claim 10 to14, wherein the ferritic stainless steel
includes (Si+Ti)/(C+N) in a range of 50 to 90.
40
[Claim 16]
The method of claim 10, wherein the cold rolling and annealing is performed
in a temperature range of 980 to 1,020 °C.
[Claim 17]
5 The method of claim 10, wherein conditions of the primary neutral salt
electrolytic pickling include a temperature in a range of 70 to 90 °C and a neutral salt
concentration in a range of 150 to 250 g/L, and conditions of the secondary sulfuric
acid electrolytic pickling include a temperature in a range of 30 to 50 °C and a
sulfuric acid concentration in a range of 65 to 85 g/L.
10 [Claim 18]
The method of any one of claim 10 to 17, wherein a surface of the ferritic
stainless steel is hot-dip coated with aluminum at a thickness of 200 [irn or less.
[Claim 19]
A method of manufacturing a ferritic stainless steel having excellent
15 condensate corrosion resistance, formability and high temperature oxidation
resistance, comprising:
heating a ferritic stainless steel slab, which includes, by wt%, C: more than 0
and 0.01% or less, Cr: 9 to 13%, Si: 0.5 to 1.0%, Mn: more than 0 and 0.5% or less,
P: more than 0 and 0.035% or less, S: more than 0 and 0.01% or less, Ti: 0.15 to
20 0.5%, N: more than 0 and 0.01% or less, Sn: 0.05 to 0.5%, and Fe and unavoidable
impurities as a remainder, in a temperature range of 1,100 to 1,240 °C;
performing hot rolling and annealing in a temperature range of 1,030 to
1,070 °C after hot rolling the ferritic stainless steel slab;
41
performing cold rolling and cold annealing in a temperature range of 980 to
1,020 °C;
preprocessing an underlying metal of a ferritic stainless steel after the cold
rolling and annealing;
5 preheating and heating; and
hot-dip coating with aluminum.
[Claim 20]
The method of claim 19, wherein the preprocessing of the ferritic stainless
steel is performed using an electrolytic cleaning method, in which the ferritic
10 stainless steel is bathed in a caustic soda solution having a concentration in a range of
2 to 5% in a temperature range of 60 to 80 °C in a high temperature bath tank to
remove foreign substances such as oil or the like from a surface of the underlying
metal of the ferritic stainless steel under conditions of an electrolytic process, and
final cleaning is performed in a high temperature cleaning tank.
15 [Claim 21]
The method of claim 19, wherein, in the preheating and heating of the ferritic
stainless steel, the preheating is performed on the ferritic stainless steel under
conditions of a preheating temperature of 530 °C or more and an oxygen
concentration of 20 ppm or more such that a composite oxide layer of activated Fe
20 and Cr is formed on a preprocessed surface of the underlying metal, the ferritic
stainless steel is heated under conditions of a heating temperature in a range of 900 to
1,000 °C, a hydrogen concentration of 30% or more, and a dew point temperature in
the range of -30 to -45 °C, and cooled to a temperature in a range of about 630 to
42
730 °C for annealing such that the composite oxide layer of activated Fe and Cr is
reduced.
[Claim 221
The method of claim 19, wherein, in the hot-dip coating with aluminum of
5 the ferritic stainless steel, an annealed underlying metal is plated in a molten
aluminum plating bath in a temperature range of 600 to 700 °C, and a plated steel
plate is cooled at a cooling rate in a range of 20 to 40 °C/sec to a temperature of
350 °C or less to manufacture a plated steel plate.
[Claim 231
10 The method of claim 22, wherein composition of the molten aluminum
plating bath include Al at 88 to 92% and Si at 8 to 11%, and the ferritic stainless steel
is hot-dip coated with aluminum at a thickness of 200 1.1m or less.
Dated this 16th day of June, 2015
PRI 1JKA CHOPRA
OF K S PARTNERS
ATTORNEY FOR THE APPLICANT(S)
IN/PA-1218
43
[ABSTRACT]
The present invention relates to a ferritic stainless steel having excellent
corrosion resistance against condensate, moldability, and high-temperature oxidation
resistance, wherein the ferritic stainless steel is capable of being manufactured in an
5 economically advantageous manner without adding expensive alloying elements.
Provided is a ferritic stainless steel having excellent corrosion resistance against
condensate and moldability, and comprising, by weight%, C: greater than 0 and
0.01% or less, Cr: 9 to 13%, Si: 0.5 to 1.0%, Mn: greater than 0 and 0.5% or less, P:
greater than 0 and 0.035% or less, S: greater than 0 and 0.01% or less, Ti: 0.15 to
10 0.5%, N: greater than 0 and 0.01% or less, Sn: 0.05 to 0.5%, and the remainder is Fe
and inevitable impurities, wherein Sn concentrated at the surface part of the stainless
steel is 10 times or more than Sn concentrated at the base part.
44