U.S. patent application number 16/765615 was filed with the patent office on 2020-09-24 for non-magnetic austenitic stainless steel having excellent corrosion resistance and manufacturing method therefor.
The applicant listed for this patent is POSCO. Invention is credited to Ja Yong Choi, Hak Kim, Ji Soo Kim, Young-Jong Seo.
Application Number | 20200299816 16/765615 |
Document ID | / |
Family ID | 1000004938078 |
Filed Date | 2020-09-24 |
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United States Patent
Application |
20200299816 |
Kind Code |
A1 |
Kim; Ji Soo ; et
al. |
September 24, 2020 |
NON-MAGNETIC AUSTENITIC STAINLESS STEEL HAVING EXCELLENT CORROSION
RESISTANCE AND MANUFACTURING METHOD THEREFOR
Abstract
Disclosed is a non-magnetic austenitic stainless steel with
excellent corrosion resistance which is applicable to an
environment requiring corrosion resistance along with excellent
non-magnetic properties, and manufacturing method thereof. The
non-magnetic austenitic stainless steel with excellent corrosion
resistance according to an embodiment of the present disclosure
includes, in percent (%) by weight of the entire composition, C:
0.05% or less, Si: 1.0% or less, Mn: 0.5 to 2.0%, Cr: 16 to 24%,
Ni: 10 to 16%, N: 0.2% or less, the remainder of iron (Fe) and
other inevitable impurities, and satisfies a following equation
(1).
Ni.gtoreq.-2.7-5.8*C-1.77*Si-0.066*Mn+0.893*Cr+1.05*Mo-0.88*Cu-13.8*N
(1)
Inventors: |
Kim; Ji Soo; (Pohang-si,
Gyeongsangbuk-do, KR) ; Kim; Hak; (Pohang-si,
Gyeongsangbuk-do, KR) ; Choi; Ja Yong; (Pohang-si,
Gyeongsangbuk-do, KR) ; Seo; Young-Jong; (Pohang-si,
Gyeongsangbuk-do, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
POSCO |
Pohang-si, Gyeonhdangbuk- do |
|
KR |
|
|
Family ID: |
1000004938078 |
Appl. No.: |
16/765615 |
Filed: |
August 10, 2018 |
PCT Filed: |
August 10, 2018 |
PCT NO: |
PCT/KR2018/009162 |
371 Date: |
May 20, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/001 20130101;
C21D 2211/005 20130101; C21D 8/0205 20130101; C22C 38/04 20130101;
C21D 2211/001 20130101; C22C 38/02 20130101; C22C 38/42 20130101;
C22C 38/44 20130101; C21D 8/0226 20130101; C22C 38/54 20130101 |
International
Class: |
C22C 38/42 20060101
C22C038/42; C22C 38/44 20060101 C22C038/44; C22C 38/54 20060101
C22C038/54; C22C 38/00 20060101 C22C038/00; C22C 38/02 20060101
C22C038/02; C22C 38/04 20060101 C22C038/04; C21D 8/02 20060101
C21D008/02 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 6, 2017 |
KR |
10-2017-0166443 |
Claims
1. A non-magnetic austenitic stainless steel with excellent
corrosion resistance, the austenitic stainless steel comprising, in
percent (%) by weight of the entire composition, C: 0.05% or less,
Si: 1.0% or less, Mn: 0.5 to 2.0%, Cr: 16 to 24%, Ni: 10 to 16%, N:
0.2% or less, the remainder of iron (Fe) and other inevitable
impurities, and wherein the austenitic stainless steel satisfies a
following equation (1), and has a permeability of 1.02.mu. or less.
Ni.gtoreq.-2.7-5.8*C-1.77*Si-0.066*Mn+0.893*Cr+1.05*Mo-0.88*Cu-13.8*N
(1) (Ni, C, Si, Mn, Cr, Mo, Cu, N mean the content (% by weight) of
each element)
2. The austenitic stainless steel of claim 1, further comprising:
in percent (%) by weight, Cu: 3.0% or less.
3. The austenitic stainless steel of claim 1, further comprising:
in percent (%) by weight, Mo: 4.0% or less.
4. The austenitic stainless steel of claim 1, further comprising:
in percent (%) by weight, B: less than 0.01%.
5. The austenitic stainless steel of claim 1, wherein the stainless
steel satisfies a calculated .delta.-ferrite fraction represented
by the following equation (2) of 0% or less.
161*{[Cr+Mo+1.5*Si+18]/[Ni+30*(C+N)+0.5*(Cu+Mn)+36]+0.262}-161 (2)
(Cr, Mo, Si, Ni, C, N, Cu, Mn mean the content (% by weight) of
each element)
6. The austenitic stainless steel of claim 1, wherein the stainless
steel satisfies a pitting resistance equivalent number (PREN)
represented by the following equation (3) of the range of 20 to 30.
Cr+3.3*Mo+30*N-Mn+Si (3) (Cr, Mo, N, Mn, Si mean the content (% by
weight) of each element)
7. The austenitic stainless steel of claim 1, wherein the stainless
steel satisfies a .sigma. phase formation index represented by the
following equation (4) of the range of 18 to 24. Cr+Mo+3*Si (4)
(Cr, Mo, Si mean the content (% by weight) of each element)
8. The austenitic stainless steel of claim 1, wherein the stainless
steel has a permeability of 1.012.mu. or less.
9. The austenitic stainless steel of claim 1, wherein an average
grain size of the stainless steel is 1.012.mu. or less.
10. A manufacturing method of a non-magnetic austenitic stainless
steel with excellent corrosion resistance, the manufacturing method
comprising: hot rolling the slab comprising, in percent (%) by
weight of the entire composition, C: 0.05% or less, Si: 1.0% or
less, Mn: 0.5 to 2.0%, Cr: 16 to 24%, Ni: 10 to 16%, N: 0.2% or
less, the remainder of iron (Fe) and other inevitable impurities
and satisfying a .sigma. phase formation index represented by the
following equation (4) of the range of 18 to 24; and performing a
solution heat treatment of the hot rolled material. Cr+Mo+3*Si (4)
(Cr, Mo, Si mean the content (% by weight) of each element)
11. The manufacturing method of claim 10, wherein the slab
satisfies a following equation (1), and satisfies a calculated
.delta.-ferrite fraction represented by a following equation (2) of
0% or less.
Ni.gtoreq.-2.7-5.8*C-1.77*Si-0.066*Mn+0.893*Cr+1.05*Mo-0.88*Cu-13.8*N
(1) 161*{[Cr+Mo+1.5*Si+18]/[Ni+30*(C+N)+0.5*(Cu+Mn)+36]+0.262}-161
(2) (Ni, C, Si, Mn, Cr, Mo, Cu, N mean the content (% by weight) of
each element).
12. The manufacturing method of claim 10, wherein the solution heat
treatment is performed at 1,100 to 1,150.degree. C. for 60 to 120
seconds.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to a non-magnetic austenitic
stainless steel, and more particularly, to a non-magnetic
austenitic stainless steel with excellent corrosion resistance
applicable to an environment requiring corrosion resistance
together with non-magnetic properties, and a manufacturing method
thereof.
BACKGROUND ART
[0002] Austenitic stainless steel, represented by STS304, has good
corrosion resistance, and exhibits a non-magnetic austenite
structure in annealing heat treatment, and is used as a
non-magnetic steel in various devices. However, there are cases
where cold working is performed depending on the application, and
when cold working is applied to STS304 steel, due to the phase
transformation to deformation induced martensite structure, it is
difficult to maintain non-magnetic properties, which limits the
application to materials.
[0003] Therefore, STS316L-based steel grades with higher austenite
stability than STS304 are used for non-magnetic applications.
However, in the case of STS316L-based steel grades, the Mo content
is high, so a secondary phase such as .sigma. or .delta.-ferrite is
often present in the austenite matrix, and since the solidification
starts from .delta.-ferrite during continuous casting of STS316L
steel, it is difficult to decompose the secondary phases due to
high Cr and Mo content in the center segregation region in the
continuous casting slab, and thus the secondary phases tend to
remain after hot rolling and final heat treatment.
[0004] When the secondary phases remain, it acts as a cause of
increased magnetism in the area, and adversely affects the function
of the device. Therefore, there is a need for a material capable of
maintaining non-magnetic properties without these secondary
phases.
[0005] Patent Document 1 refers to a high-strength non-magnetic
austenitic stainless steel that maintains non-magnetic properties
even after severe cold working and can significantly improve
elastic limit stress by aging treatment.
[0006] However, the stainless steel of Patent Document 1 has a Mn
content of 2 to 9%, so it is feared that corrosion resistance is
reduced due to Mn, and its application is limited in applications
requiring corrosion resistance. For the stabilization of the
austenite phase, Ni-equivalent range was proposed to refer to
maintaining non-magnetic properties even after cold working.
However, since .delta.-ferrite, which affects non-magnetic
properties, is not mentioned, it is necessary to solve the
deterioration of non-magnetic properties due to .delta.-ferrite
formation.
[0007] (Patent Document 0001) Korean Patent Publication No.
10-2015-0121061 (Oct. 28, 2015)
DISCLOSURE
Technical Problem
[0008] Therefore, it is an aspect of the present invention to
provide a highly corrosion-resistant austenitic stainless steel
with excellent non-magnetic properties by suppressing
.delta.-ferrite formation during solidification by solving the
above problems.
Technical Solution
[0009] In accordance with an aspect of the present disclosure, a
non-magnetic austenitic stainless steel with excellent corrosion
resistance includes, in percent (%) by weight of the entire
composition, C: 0.05% or less, Si: 1.0% or less, Mn: 0.5 to 2.0%,
Cr: 16 to 24%, Ni: 10 to 16%, N: 0.2% or less, the remainder of
iron (Fe) and other inevitable impurities, and satisfies a
following equation (1), and has a permeability of 1.02.mu. or
less.
Ni.gtoreq.-2.7-5.8*C-1.77*Si-0.066*Mn+0.893*Cr+1.05*Mo-0.88*Cu-13.8*N
(1)
[0010] Ni, C, Si, Mn, Cr, Mo, Cu, N mean the content (% by weight)
of each element.
[0011] The austenitic stainless steel may further include: in
percent (%) by weight, Cu: 3.0% or less.
[0012] The austenitic stainless steel may further include: in
percent (%) by weight, Mo: 4.0% or less.
[0013] The austenitic stainless steel may further include: in
percent (%) by weight, B: less than 0.01%.
[0014] The austenitic stainless steel may satisfy a calculated
.delta.-ferrite fraction represented by the following equation (2)
of 0% or less.
161*{[Cr+Mo+1.5*Si+18]/[Ni+30*(C+N)+0.5*(Cu+Mn)+36]+0.262}-161
(2)
[0015] Cr, Mo, Si, Ni, C, N, Cu, Mn mean the content (% by weight)
of each element The austenitic stainless steel may satisfy a
pitting resistance equivalent number (PREN) represented by the
following equation (3) of the range of 20 to 30.
Cr+3.3*Mo+30*N-Mn+Si (3)
[0016] Cr, Mo, N, Mn, Si mean the content (% by weight) of each
element The austenitic stainless steel may satisfy a .sigma. phase
formation index represented by the following equation (4) of the
range of 18 to 24.
Cr+Mo+3*Si (4)
[0017] Cr, Mo, Si mean the content (% by weight) of each
element
[0018] The austenitic stainless may have a permeability of
1.012.mu. or less.
[0019] The average grain size of the stainless steel may be 70
.mu.m or less.
[0020] In accordance with an aspect of the present disclosure, a
manufacturing method of a non-magnetic austenitic stainless steel
with excellent corrosion resistance, includes: hot rolling the slab
comprising, in percent (%) by weight of the entire composition, C:
0.05% or less, Si: 1.0% or less, Mn: 0.5 to 2.0%, Cr: 16 to 24%,
Ni: 10 to 16%, N: 0.2% or less, the remainder of iron (Fe) and
other inevitable impurities and satisfying a .sigma. phase
formation index represented by the following equation (4) of the
range of 18 to 24; and performing a solution heat treatment of the
hot rolled material.
[0021] (4) Cr+Mo+3*Si Cr, Mo, Si mean the content (% by weight) of
each element The slab may satisfy a following equation (1), and
satisfy a calculated .delta.-ferrite fraction represented by a
following equation (2) of 0% or less.
Ni.gtoreq.-2.7-5.8*C-1.77*Si-0.066*Mn+0.893*Cr+1.05*Mo-0.88*Cu-13.8*N
(1)
161*{[Cr+Mo+1.5*Si+18]/[Ni+30*(C+N)+0.5*(Cu+Mn)+36]+0.262}161
(2)
[0022] The solution heat treatment may be performed at 1,100 to
1,150.degree. C. for 60 to 120 seconds.
Advantageous Effects
[0023] High corrosion-resistant non-magnetic austenitic stainless
steel according to an embodiment of the present disclosure can be
applied to a variety of non-magnetic components used in various
devices.
[0024] In addition, since the non-magnetic property is determined
by the components without an additional process of heat-treating
the material for a long time in order to remove the magnetism by
.delta.-ferrite, it is possible to provide non-magnetic austenitic
stainless steel with a simple manufacturing process.
[0025] In addition, it is possible to prevent roughness
deterioration due to an orange peel defect on the surface of the
steel sheet.
DESCRIPTION OF DRAWINGS
[0026] FIG. 1 is a graph illustrating a correlation of permeability
according to a difference between a Ni content and a Ni correction
value (Ni.sub.adj).
BEST MODE
[0027] A non-magnetic austenitic stainless steel with excellent
corrosion resistance according to an embodiment of the present
disclosure includes, in percent (%) by weight of the entire
composition, C: 0.05% or less, Si: 1.0% or less, Mn: 0.5 to 2.0%,
Cr: 16 to 24%, Ni: 10 to 16%, N: 0.2% or less, the remainder of
iron (Fe) and other inevitable impurities, and satisfies a
following equation (1), and has a permeability of 1.02.mu. or
less.
Ni.gtoreq.-2.7-5.8*C-1.77*Si-0.066*Mn+0.893*Cr+1.05*Mo-0.88*Cu-13.8*N
(1)
[0028] Ni, C, Si, Mn, Cr, Mo, Cu, N mean the content (% by weight)
of each element.
MODES OF THE INVENTION
[0029] Hereinafter, the embodiments of the present disclosure will
be described in detail with reference to the accompanying drawings.
The following embodiments are provided to transfer the technical
concepts of the present disclosure to one of ordinary skill in the
art. However, the present disclosure is not limited to these
embodiments, and may be embodied in another form. In the drawings,
parts that are irrelevant to the descriptions may be not shown in
order to clarify the present disclosure, and also, for easy
understanding, the sizes of components are more or less
exaggeratedly shown.
[0030] Also, when a part "includes" or "comprises" an element,
unless there is a particular description contrary thereto, the part
may further include other elements, not excluding the other
elements.
[0031] An expression used in the singular encompasses the
expression of the plural, unless it has a clearly different meaning
in the context.
[0032] Hereinafter, it describes a non-magnetic austenitic
stainless steel which can secure non-magnetic properties even if it
is manufactured in a normal process without requiring an additional
process for decomposing .delta.-ferrite by controlling the content
of .delta.-ferrite present in the microstructure of the steel and
has superior corrosion resistance compared to commonly used STS316L
stainless steel.
[0033] The present disclosure provides austenitic stainless steel
which exhibits excellent non-magnetic properties only by
controlling the alloy element components even without an additional
heat treatment process, and a manufacturing method thereof.
[0034] A non-magnetic austenitic stainless steel with excellent
corrosion resistance according to an embodiment of the present
disclosure includes, in percent (%) by weight of the entire
composition, C: 0.05% or less, Si: 1.0% or less, Mn: 0.5 to 2.0%,
Cr: 16 to 24%, Ni: 10 to 16%, N: 0.2% or less, the remainder of
iron (Fe) and other inevitable impurities, and satisfies a
following equation (1).
Ni.gtoreq.-2.7-5.8*C-1.77*Si-0.066*Mn+0.893*Cr+1.05*Mo-0.88*Cu-13.8*N
(1)
[0035] Hereinafter, the reason for the numerical limitation of the
alloy component element content in the embodiment of the present
disclosure will be described. In the following, unless otherwise
specified, the unit is % by weight.
[0036] The content of C is 0.05% or less.
[0037] C is a strong austenite phase stabilizing element and is an
effective element for increasing material strength by solid
solution strengthening. However, when the content is excessive, it
is easily combined with a carbide-forming element such as Cr
effective for corrosion resistance at the ferrite-austenite phase
boundary, thereby lowering the Cr content around the grain
boundaries to reduce corrosion resistance. Therefore, the content
of C is limited to 0.05% or less. In order to minimize the risk of
carbide precipitation, which can inhibit corrosion resistance, it
is desirable to limit the content of C to 0.03% or less.
[0038] The content of Si is 1.0% or less.
[0039] Si, which also acts as a ferrite phase stabilizing element,
is effective in improving corrosion resistance, but when it is
excessive, it promotes precipitation of intermetallic compounds
such as .sigma. phase, thereby reducing mechanical properties and
corrosion resistance related to impact toughness, and is limited to
1.0% or less.
[0040] The content of Mn is 0.5 to 2.0%.
[0041] Mn is an austenite phase stabilizing element such as C and
Ni, which can improve the N solubility, and is added by 0.5% or
more. However, an increase in the Mn content is undesirable when
corrosion resistance is required because it is involved in the
formation of inclusions such as MnS, so it is preferable to limit
the Mn content to 2.0% or less in order to secure corrosion
resistance.
[0042] The content of Cr is 16.0 to 24.0%.
[0043] Cr is the most contained element of the corrosion resistance
enhancing element of stainless steel, and must be included by 16%
or more for the expression of corrosion resistance. However, Cr is
a ferrite stabilizing element. As the Cr content increases, the
ferrite fraction increases. Therefore, in order to obtain a
non-magnetic property, since a large amount of Ni must be
contained, the cost increases, and the formation of the .sigma.
phase is promoted, causing a decrease in mechanical properties and
corrosion resistance. Therefore, it is preferable to limit the Cr
content to 24% or less.
[0044] The content of Ni is 10.0 to 16.0%.
[0045] Ni is the most powerful element of the austenite phase
stabilizing element and must be contained by 10% or more to obtain
non-magnetic properties. However, since the increase in Ni content
is directly related to the increase in the price of raw materials,
it is preferable to limit Ni content to 16% or less.
[0046] The content of N is 0.2% or less.
[0047] N is an element useful for stabilizing the austenite phase
as well as improving corrosion resistance in a chlorine atmosphere.
However, it is preferable to limit N content to 0.2% or less,
because the hot workability is reduced when a large amount is added
to lower the yielding percentage of steel.
[0048] In addition, according to an embodiment of the present
disclosure, by weight %, Cu of 3.0% or less may be further
included.
[0049] Cu has the advantage of improving corrosion resistance in a
sulfuric acid atmosphere, so it can be selectively added. However,
in the chlorine atmosphere, there is a disadvantage of reducing the
pitting resistance and lowering the hot workability, so it is
limited to 3.0% or less.
[0050] In addition, according to an embodiment of the present
disclosure, by weight %, Mo of 4.0% or less may be further
included.
[0051] The content of Mo is 4.0% or less.
[0052] Mo is an element useful for improving corrosion resistance,
and can be expected to improve corrosion resistance, so it can be
selectively added. When adding, it is preferable to add 2.0% or
more. However, Mo is a ferrite stabilizing element, and when added
in large amounts, it is difficult to obtain a non-magnetic property
due to an increase in the ferrite fraction and, in addition, the
formation of the .sigma. phase is promoted, leading to a decrease
in mechanical properties and corrosion resistance. Therefore, the
content of Mo is limited to 4.0% or less.
[0053] In addition, according to an embodiment of the present
disclosure, by weight %, B may be further included less than
0.01%.
[0054] The content of B is less than 0.01%.
[0055] B has the effect of improving the hot workability, so it can
be added in a range of less than 0.01%. However, when it is added
more than that, since a low melting point boride compound is formed
and rather hot workability is lowered, it is preferable to limit to
less than 0.01%.
[0056] In various devices that use non-magnetic properties of
steel, the permeability of the steel applied to the parts must be
1.02.mu. or less for normal device operation. In order to satisfy
this, it is necessary to control the fraction of .delta.-ferrite
formed during solidification of the steel.
[0057] In general, .delta.-ferrite present in the microstructure of
austenitic stainless steel becomes magnetic due to the
characteristics of the structure having a body-centered cubic
structure, and austenite does not become magnetic due to the
face-centered cubic structure. Therefore, it is possible to obtain
a magnetic property of a desired size by controlling the fraction
of .delta.-ferrite, and in the case of non-magnetic steel, it is
necessary to make the fraction of .delta.-ferrite as low as
possible or eliminate the fraction of .delta.-ferrite.
[0058] The fraction of .delta.-ferrite present in the
microstructure of austenitic stainless steel can be determined by
the content of various alloying elements, as shown in equation (2),
which will be described later. In particular, it is possible to
reduce the .delta.-ferrite fraction by adding an austenite
stabilizing element. Since the Ni content is useful for stabilizing
austenite without deteriorating other physical properties, the Ni
content can be controlled to suppress the formation of
.delta.-ferrite.
[0059] The Ni correction formula (hereinafter, Ni.sub.adj) means a
minimum Ni content that prevents .delta.-ferrite from being formed
in a given composition component, and can be expressed as
follows.
-2.7-5.8*C-1.77*Si-0.066*Mn+0.893*Cr+1.05*Mo-0.88*Cu-13.8*N
[Ni.sub.adj]
[0060] When the Ni content contained in the actual steel is greater
than the value of Ni.sub.adj, .delta.-ferrite cannot be formed,
thereby exhibiting non-magnetic properties. That is, in order to
satisfy the non-magnetic property, it means that the content of Ni
contained in the steel should be greater than Ni.sub.adj combined
with the content of C, Si, Mn, Cr, Mo, Cu, and N components.
Ni.gtoreq.-2.7-5.8*C-1.77*Si-0.066*Mn+0.893*Cr+1.05*Mo-0.88*Cu-13.8*N
(1)
[0061] FIG. 1 is a graph illustrating a correlation of permeability
according to a difference between a Ni content and a Ni.sub.adj
value. Referring to FIG. 1, it can be seen that when the difference
between the Ni content and the Ni.sub.adj value included in the
steel is positive, the permeability of the steel satisfies 1.02.mu.
or less.
[0062] However, Ni is an expensive alloy element, and the more Ni
is added, the higher the cost. Therefore, it is preferable to make
the difference between the actual Ni content and the Ni.sub.adj
value less than 8%.
[0063] In addition, according to an embodiment of the present
disclosure, the non-magnetic austenitic stainless steel with
excellent corrosion resistance may satisfy the calculated
.delta.-ferrite fraction represented by the following equation (2)
of 0% or less.
161*{[Cr+Mo+1.5*Si+18]/[Ni+30*(C+N)+0.5*(Cu+Mn)+36]+0.262}-161
(2)
[0064] The equation (2) is a formula that can predict the
.delta.-ferrite content of steel through the content of each
component when producing austenitic stainless steel in a normal
steelmaking process. When the fraction of .delta.-ferrite
calculated through equation (2) is 0% or less, the non-magnetic
property to be achieved in the present disclosure may be
satisfied.
[0065] The non-magnetic austenitic stainless steel of the present
disclosure according to the equations (1) and/or (2) may exhibit a
permeability of 1.02.mu. or less, and more preferably 1.012.mu. or
less, thereby realizing a completely non-magnetic property.
[0066] On the other hand, in order to improve corrosion resistance
of steel, it is effective to add alloy elements that improve
corrosion resistance, such as Cr, Mo, Si, and N. In addition, when
a large amount of Mn is added, since water-soluble inclusions such
as MnS in steel are formed and corrosion resistance is lowered, it
is necessary to control the Mn content.
[0067] In general, as an index indicating corrosion resistance of
austenite stainless steel, pitting resistance equivalent number
calculated by a combination of Cr, Mo, and N contents is applied.
However, as described above, since the contents of Mn and Si also
greatly affect corrosion resistance of steel, a new pitting
resistance equivalent number considering these elements is also
required.
[0068] According to an embodiment of the present disclosure,
non-magnetic austenitic stainless steel with excellent corrosion
resistance may satisfy a pitting resistance equivalent number
(PREN) value represented by the following equation (3) of the range
of 20 to 30.
Cr+3.3*Mo+30*N-Mn+Si (3)
[0069] The present inventors have found that the pitting resistance
equivalent number including Mn and Si content represented by
equation (3) well reflects the corrosion resistance of steel, and
have confirmed that when the range of equation (3) is 20 to 30,
corrosion resistance may be equal to or higher than that of
conventional STS316L.
[0070] However, when the content of Cr, Mo, and Si increases, not
only does the cost increase, but the formation of the .sigma. phase
promotes brittleness, and a Cr and Mo depletion region is formed,
which adversely affects corrosion resistance. Therefore, it is
necessary to set an appropriate Cr, Mo, Si content range that can
minimize the formation of the .sigma. phase while obtaining desired
corrosion resistance.
[0071] According to an embodiment of the present disclosure, the
non-magnetic austenitic stainless steel with excellent corrosion
resistance may satisfy the .sigma. phase formation index
represented by the following equation (4) of the range of 18 to
24.
Cr+Mo+3*Si (4)
[0072] When the .sigma. phase formation index is less than 18, the
Cr and Mo content is so low that it is difficult to secure
corrosion resistance of the steel, so it is limited to 18 or
more.
[0073] By limiting the .sigma. phase formation index to 24 or less,
the .sigma. phase fraction can be controlled to less than 1.0%,
more preferably to 0.8% or less. When the .sigma. phase formation
index is greater than 24, decreased corrosion resistance and
brittle material degradation due to excessive .sigma. phase
formation may occur. Non-magnetic properties may be further
improved by securing a low .sigma. phase fraction.
[0074] On the other hand, the .sigma. phase formation control can
suppress the formation of the .sigma. phase by controlling the
alloy component composition as in the present disclosure, but the
formed .sigma. phase may also be decomposed by controlling the
solution heat treatment conditions. For the decomposition of the
.sigma. phase, it is effective to anneal for a long time at a high
temperature, but in this case, the possibility of causing an orange
peel defect on the surface increases due to excessive grain size
growth. Here, the orange peel defect refers to a defect in which
unevenness of roughness occurs on the surface when the steel is
formed by coarse grain size, thereby damaging the beautiful
surface.
[0075] According to an embodiment of the present disclosure, the
average grain size of non-magnetic austenitic stainless steel with
excellent corrosion resistance may be 70 .mu.m or less.
[0076] For general 300-based austenitic stainless steel, solution
heat treatment is performed at about 1,100.degree. C. for about 60
to 100 seconds. In order to lower the incidence of defects in
orange peel during molding, the average grain size of stainless
steel should be 70 .mu.m or less, and for this purpose, the average
grain size of the non-magnetic austenitic stainless steel with
excellent corrosion resistance of the present disclosure may be
controlled to 70 .mu.m or less by performing solution heat
treatment of the hot rolled material at 1,100 to 1,150.degree. C.
for 60 to 120 seconds.
[0077] Hereinafter, it will be described in more detail through a
preferred embodiment of the present disclosure.
Example
[0078] After the steel having the alloy composition shown in Table
1 was dissolved in a vacuum induction furnace, hot rolling was
performed, and solution heat treatment was performed to prepare a
hot rolled sheet having a thickness of 6 mm.
TABLE-US-00001 TABLE 1 composition (wt %) C Si Mn Cr Ni Mo Cu N B
S1 0.030 0.45 1.3 15.8 15.8 0.0 1.5 0.070 0.0026 S2 0.034 0.46 1.2
18.5 15.4 0.0 1.2 0.080 0.0045 S3 0.025 0.52 1.2 20.4 16.0 0.0 1.3
0.080 0.0035 S4 0.028 0.47 1.4 24.3 15.4 0.0 1.6 0.070 0.0026 S5
0.021 0.45 1.3 18.6 14.3 0.0 2.1 0.090 0.0025 S6 0.024 0.46 1.1
17.9 14.5 2.8 0.0 0.080 0.0025 S7 0.026 0.43 1.2 17.6 10.5 0.0 2.5
0.120 0.0026 S8 0.030 0.42 1.2 18.6 13.5 4.2 2.6 0.090 0.0034 S9
0.037 0.48 1.3 18.1 12.6 2.1 1.5 0.220 0.003 S10 0.032 0.45 1.3
18.2 13.5 2.5 1.6 0.100 0.002 S11 0.026 0.45 1.2 17.9 14.2 2.6 1.4
0.050 0.0021 S12 0.028 0.46 0.5 18.1 13.8 0.0 0.0 0.070 0.0026 S13
0.029 0.47 1.3 18.4 13.9 0.0 0.6 0.080 0.0026 S14 0.027 0.41 2.2
18.1 13.9 0.0 0.0 0.080 0.0032 S15 0.024 0.42 1.2 18.1 14.5 2.1 0.3
0.080 0.0025 S16 0.026 0.75 1.3 18.4 14.0 2.0 0.4 0.090 0.003 S17
0.035 1.12 1.1 18.7 13.9 2.3 0.6 0.110 0.0026
[0079] In some of the S1 to S17 steel types listed in Table 1, the
solution heat treatment conditions were varied to change the grain
size, and when the steel with each grain size was formed, the
incidence of orange peel defects was investigated and shown in
Table 2 below.
TABLE-US-00002 TABLE 2 orange average peel solution heat treatment
grain incidence temperature(.degree. C.) time(sec) size(.mu.m) (%)
Inventive 1 S2 1,150 90 23.48 <1.0 Example 2 S3 1,150 90 30.45
<1.0 3 S5 1,150 90 25.63 <1.0 4 1,100 90 22.89 <1.0 5
1,150 180 50.88 3.2 6 1,180 90 53.48 3.5 7 S6 1,150 90 30.89
<1.0 8 S7 1,150 90 38.52 <1.0 9 S10 1,150 90 23.58 <1.0 10
S11 1,150 90 25.25 <1.0 11 S12 1,150 90 26.84 <1.0 12 S13
1,150 90 21.35 <1.0 Comparative 13 S1 1,150 90 21.93 <1.0
Example 14 S4 1,150 90 27.56 <1.0 15 S5 1,180 120 74.58 15.8 16
1,180 180 86.72 22.9 17 S8 1,150 90 24.69 <1.0 18 S9 1,150 90
23.75 <1.0 19 S14 1,150 90 26.84 <1.0 20 S17 1,150 90 31.24
<1.0
[0080] As shown in Table 2, the S5 steel grades used in Comparative
Examples 15 and 16 satisfy all the components of the present
disclosure, but the solution heat treatment temperature was
performed at 1,180.degree. C. exceeding 1,150.degree. C. for 120
seconds or more and the average grain size exceeded 70 .mu.m, and
the orange peel defect after molding was more than 15%.
[0081] As the solution heat treatment temperature changed, the
grain size of the steel changed. As the heat treatment temperature
and time increased, it was found that the average grain size
increased. When the average grain size was 70 .mu.m or more, it was
found that the incidence of orange peel defects was 15% or more,
significantly increasing compared to other heat treatment
conditions.
[0082] In addition, for the S1 to S17 steel grades described in
Table 1, the calculated values according to equations (1) to (4),
permeability, and .sigma. phase fraction were measured and are
shown in Table 3 below.
TABLE-US-00003 TABLE 3 measured measured .delta.-ferrite .sigma.
phase equation fraction equation permeability equation equation
fraction (1) (%) (2) (.mu.) (3) (4) (%) Inventive 1 S2 4.8 0 -11.9
1.003 20.2 19.9 0.06 Example 2 S3 3.9 0 -7.0 1.004 22.1 22.0 0.11 3
S5 4.5 0 -10.4 1.003 20.5 20.0 0.16 4 4.5 0 -10.4 1.004 20.5 20.0
0.08 5 4.5 0 -10.4 1.003 20.5 20.0 0.06 6 4.5 0 -10.4 1.004 20.5
20.0 0.15 7 S6 0.4 0 -1.7 1.005 28.9 22.1 0.09 8 S7 2.3 0 -8.2
1.012 20.4 18.9 0.03 9 S10 1.2 0 -3.4 1.004 28.6 22.1 0.06 10 S11
1.1 0 -1.5 1.003 27.2 21.9 0.17 11 S12 2.3 0 -7.0 1.006 20.2 19.5
0.05 12 S13 2.9 0 -8.4 1.001 20.0 19.8 0.11 Comparative 13 S1 7.7 0
-20.1 1.001 17.7 17.2 0.03 Example 14 S4 -0.1 0.9 5.2 1.042 25.5
25.7 0.97 15 S5 4.5 0 -10.4 1.003 20.5 20.0 0.05 16 4.5 0 -10.4
1.003 20.5 20.0 0.12 17 S8 -0.3 0.3 2.5 1.026 34.4 24.1 0.94 18 S9
2.4 0 -10.2 1.002 30.8 21.6 0.17 19 S14 2.6 0 -9.8 1.002 18.7 19.3
0.03 20 S17 1.8 0 0.0 1.028 28.7 24.4 0.84
[0083] As shown in Table 3, when the Ni-Ni.sub.adj value
represented by the equation (1) is positive, it was found that
permeability satisfies 1.02.mu. or less, particularly, the examples
of the present disclosure satisfy 1.012.mu. or less. When the
calculated .delta.-ferrite fraction according to equation (2) is 0%
or less, it was found that the measured .delta.-ferrite fraction
was 0%. In addition, when the .sigma. phase formation index is 24
or more, the .sigma. phase fraction is 0.8% or more, which is close
to 1.0%, indicating that the .sigma. phase fraction is
significantly increased compared to other steel types.
[0084] Comparative Example 13 did not satisfy the corrosion
resistance requirement due to the low PREN (Eq. (3)) value due to
the S1 steel grade having insufficient Cr content, and the .sigma.
phase formation index (Eq. (4)) was also less than 18.
[0085] Comparative Example 14 does not satisfy the equations (1)
and (2) due to the S4 steel grade containing excessive Cr, and has
a high .sigma. phase fraction, so the permeability was measured to
be 1.042.mu., and the desired non-magnetic property of the present
disclosure was not satisfied. The .sigma. phase formation index
also exceeded 24, indicating that the .sigma. phase fraction was
close to 1.0%. It was found that the .sigma. phase formation was
promoted by the increase of the Cr content, thereby forming a Cr
depletion region.
[0086] Comparative Example 17 does not satisfy the equations (1)
and (2) due to the S8 steel grade containing excessive Mo, and has
a high .sigma. phase fraction, so the permeability was measured to
be 1.026.mu., and the desired non-magnetic property of the present
disclosure was not satisfied. The .sigma. phase formation index
also exceeded 24, indicating that the .sigma. phase fraction was
close to 1.0%. It was found that the .sigma. phase formation was
promoted by the increase of the Mo content, thereby forming a Mo
depletion region. Through this, it was confirmed that when adding
additional Mo, it should be added at 4.0% or less.
[0087] Comparative Example 18 did not meet the corrosion resistance
requirement due to the high PREN value due to the S9 steel grade
containing N excessively.
[0088] Comparative Example 19 did not satisfy the corrosion
resistance requirement due to the low PREN value due to the S14
steel grade containing excessive Mn, and it was found that
corrosion resistance was not secured due to inclusion formation due
to an increase in the Mn content.
[0089] In Comparative Example 20, the .sigma. phase formation index
exceeded 24 due to the S17 steel containing excessive Si, and the
permeability was high as 1.028.mu. despite satisfying equations (1)
and (2) due to the .sigma. phase, which is a magnetic secondary
phase. It was confirmed that Si is effective in improving corrosion
resistance, but when it is excessive, it promotes precipitation of
intermetallic compounds such as .sigma. phase, thereby lowering
corrosion resistance and non-magnetic properties, and thus should
be added at 1.0% or less.
[0090] While the present disclosure has been particularly described
with reference to exemplary embodiments, it should be understood by
those of skilled in the art that various changes in form and
details may be made without departing from the spirit and scope of
the present disclosure.
INDUSTRIAL APPLICABILITY
[0091] The austenitic stainless steel according to the present
disclosure may be applied as a non-magnetic component of various
electronic devices, and may secure non-magnetic properties without
an additional process such as heat treatment for a long time.
* * * * *