U.S. patent application number 13/519343 was filed with the patent office on 2012-11-15 for austenite steel material having superior ductility.
This patent application is currently assigned to POSCO. Invention is credited to Hyun-Kwan Cho, Jong-Kyo Choi, Soon-Gi Lee, Hee-Goon Noh.
Application Number | 20120288396 13/519343 |
Document ID | / |
Family ID | 44227000 |
Filed Date | 2012-11-15 |
United States Patent
Application |
20120288396 |
Kind Code |
A1 |
Lee; Soon-Gi ; et
al. |
November 15, 2012 |
AUSTENITE STEEL MATERIAL HAVING SUPERIOR DUCTILITY
Abstract
Provided is an austenite steel having excellent ductility
including 8 wt % to 15 wt % of manganese (Mn), 3 wt % or less
(excluding 0 wt %) of copper (Cu), a content of carbon (C)
satisfying relationships of 33.5C+Mn.gtoreq.25 and
33.5C-Mn.ltoreq.23, and iron (Fe) as well as unavoidable impurities
as a remainder. According to an aspect, austenite is stabilized and
generation of carbides in a network form at austenite grain
boundaries is inhibited by adding copper (Cu) favorable to
inhibition of carbide formation with respect to manganese and
appropriately controlling contents of carbon and manganese, and
thus, high economic efficiency may also be achieved while ductility
and wear resistance are improved.
Inventors: |
Lee; Soon-Gi; (Pohang,
KR) ; Choi; Jong-Kyo; (Pohang, KR) ; Cho;
Hyun-Kwan; (Pohang, KR) ; Noh; Hee-Goon;
(Pohang, KR) |
Assignee: |
POSCO
Pohang, Kyungsangbook-do
KR
|
Family ID: |
44227000 |
Appl. No.: |
13/519343 |
Filed: |
December 28, 2010 |
PCT Filed: |
December 28, 2010 |
PCT NO: |
PCT/KR10/09393 |
371 Date: |
June 27, 2012 |
Current U.S.
Class: |
420/74 ; 420/75;
420/76 |
Current CPC
Class: |
C22C 38/04 20130101;
C22C 38/20 20130101; C22C 38/38 20130101; C22C 38/16 20130101 |
Class at
Publication: |
420/74 ; 420/76;
420/75 |
International
Class: |
C22C 38/16 20060101
C22C038/16; C22C 38/14 20060101 C22C038/14; C22C 38/28 20060101
C22C038/28; C22C 38/20 20060101 C22C038/20 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 28, 2009 |
KR |
10-2009-0132105 |
Dec 23, 2010 |
KR |
10-2010-0133641 |
Claims
1. An austenite steel having excellent ductility comprising: 8 wt %
to 15 wt % of manganese (Mn); 3 wt % or less (excluding 0 wt %) of
copper (Cu); a content of carbon (C) satisfying relationships of
33.5C+Mn.gtoreq.25 and 33.5C-Mn.ltoreq.23; and iron (Fe) as well as
unavoidable impurities as a remainder.
2. The austenite steel having excellent ductility of claim 1,
wherein the steel further comprises 8 wt % or less (excluding 0 wt
%) of chromium (Cr).
3. The austenite steel having excellent ductility of claim 1,
wherein the steel further comprises 0.05 wt % or less (excluding 0
wt %) of titanium (Ti) and 0.1 wt % or less (excluding 0 wt %) of
niobium (Nb).
4. The austenite steel having excellent ductility of claim 3,
wherein yield strength of the steel is 500 MPa or more.
5. The austenite steel having excellent ductility of claim 3,
wherein the steel further comprises 0.002 wt % to 0.2 wt % of
nitrogen (N).
6. The austenite steel having excellent ductility of claim 1,
wherein a microstructure of the steel comprises austenite having an
area fraction of 95% or more.
7. The austenite steel having excellent ductility of claim 6,
wherein magnetic permeability of the steel is 1.01 or less at a
tensile strain of 20%.
8. The austenite steel having excellent ductility of claim 2,
wherein a microstructure of the steel comprises austenite having an
area fraction of 95% or more.
Description
TECHNICAL FIELD
[0001] The present invention relates to austenite steels having
excellent wear resistance, corrosion resistance, or non-magnetic
performance as well as ductility, as steels used for industrial
machines and in structures requiring ductility and wear resistance,
superconducting application devices and general electric devices
requiring non-magnetic properties, in the mining, transportation,
and storage sectors as well as in oil and gas industries such as
steel for a expansion pipe, steel for a slurry pipe, or sour
resistant steel.
BACKGROUND ART
[0002] Recently, demand for austenitic steels (non-magnetic steels)
for use as structural materials in superconducting application
devices, such as a linear motor car track and a fusion reactor, and
general electric devices, has increased. A typical example of
non-magnetic steel is AISI 304 (18Cr-8Ni base) austenitic stainless
steel. However, AISI 304 austenitic stainless steel may be
uneconomical because the yield strength thereof is low and large
amounts of expensive elements, such as Cr and Ni, are included
therein. In particular, with respect to a structural material
requiring stable non-magnetic properties according to a load, such
austenitic steels may exhibit magnetic properties due to a
ferromagnetic ferrite phase induced by deformation-induced
transformation and thus, there may be limitations in the uses and
applications thereof.
[0003] High-manganese austenitic steels have been continuously
developed, in which expensive nickel in the austenitic stainless
steels is replaced by manganese. With respect to the high-manganese
austenitic steels, it is essential to secure stability of an
austenite structure through appropriate changes in contents of
manganese and carbon. In the case that the content of manganese is
high, a stable austenite structure may be obtained even with a low
content of carbon. However, in the case that the content of
manganese is low, a large amount of carbon must be added for
austenitization. As a result, carbides are precipitated by forming
a network along austenite grain boundaries at high temperatures and
the precipitates may rapidly decrease physical properties of the
steel, in particular, ductility.
[0004] In order to inhibit the precipitation of carbides having a
network form, a method of performing a solution treatment at a high
temperature or manufacturing high-manganese steel by rapid cooling
to room temperature after hot working has been suggested. However,
in the case that the steel is thick or changes in manufacturing
conditions are not facilitated as in the case in which welding is
essentially accompanied, the precipitation of carbides having a
network form may not be inhibited and as a result, physical
properties of the steel may rapidly deteriorate. Also, segregation
due to alloying elements, such as manganese and carbon, inevitably
occurs during solidification of an ingot or billet of
high-manganese steel and segregation becomes severe during
post-processing such as hot rolling. Eventually, partial
precipitation of carbides occurs in a network form along an
intensified segregation zone in a final product, thereby promoting
non-uniformity of a microstructure and deteriorating physical
properties.
[0005] In order to inhibit the precipitation of carbides in the
segregation zone, increasing the content of manganese may be a
method generally used. However, this may eventually cause increases
in an alloy amount and manufacturing costs, and thus, research into
the addition of elements effective in inhibiting carbide formation
with respect to manganese has been required for resolving the
foregoing limitations. Also, since a level of corrosion resistance
of high-manganese steel may decrease in comparison to that of a
general carbon steel due to the addition of manganese, applications
in fields requiring corrosion resistance may be limited, and thus,
research into improving corrosion resistance of high-manganese
steel has also been required.
DISCLOSURE
Technical Problem
[0006] An aspect of the present invention provides an alloy having
improved ductility and wear resistance by stabilizing austenite
through appropriate control of contents of carbon and manganese and
economically inhibiting generation of carbides in a network form
that may be formed at austenite grain boundaries.
Technical Solution
[0007] According to an aspect of the present invention, there is
provided an austenite steel having excellent ductility including: 8
wt % to 15 wt % of manganese (Mn); 3 wt % or less (excluding 0 wt
%) of copper (Cu); a content of carbon (C) satisfying relationships
of 33.5C+Mn.gtoreq.25 and 33.5C-Mn.ltoreq.23; and iron (Fe) as well
as unavoidable impurities as a remainder.
[0008] At this time, the steel may further include 8 wt % or less
(excluding 0 wt %) of chromium (Cr).
[0009] Also, the steel may further include 0.05 wt % or less
(excluding 0 wt %) of titanium (Ti) and 0.1 wt % or less (excluding
0 wt %) of niobium (Nb).
[0010] Yield strength of the steel may be 500 MPa or more.
[0011] The steel may further include 0.002 wt % to 0.2 wt % of
nitrogen (N).
[0012] A microstructure of the steel may include austenite having
an area fraction of 95% or more.
[0013] Magnetic permeability of the steel may be 1.01 or less at a
tensile strain of 20%.
Advantageous Effects
[0014] According to an aspect of the present invention, austenite
is stabilized and generation of carbides in a network form at
austenite grain boundaries is inhibited by adding copper (Cu)
favorable to inhibition of carbide formation with respect to
manganese and appropriately controlling contents of carbon and
manganese, and thus, ductility and wear resistance may be improved
and corrosion resistance of steel may also be improved through the
addition of chromium (Cr).
DESCRIPTION OF DRAWINGS
[0015] The above and other aspects, features and other advantages
of the present invention will be more clearly understood from the
following detailed description taken in conjunction with the
accompanying drawings, in which:
[0016] FIG. 1 is a graph showing composition ranges of carbon and
manganese of the present invention;
[0017] FIG. 2 is a photograph showing an example of a
microstructure of a steel sheet according to the present invention;
and
[0018] FIG. 3 is a photograph showing another example of a
microstructure of a steel sheet according to the present
invention.
BEST MODE
[0019] The present invention may provide an austenite steel having
excellent ductility by stabilizing austenite and inhibiting
generation of carbides in a network form at austenite grain
boundaries through controlling contents of carbon, manganese, and
copper in a component system.
[0020] According to an aspect of the present invention, there is
provided a steel having excellent ductility including 8 wt % to 15
wt % of manganese (Mn), 3 wt % or less (excluding 0 wt %) of copper
(Cu), a content of carbon (C) satisfying relationships of
33.5C+Mn.gtoreq.25 and 33.5C-Mn.ltoreq.23, and iron (Fe) as well as
unavoidable impurities as a remainder.
[0021] Manganese (Mn): 8 wt % to 15 wt %
[0022] Mn, as the most important element added to a high-manganese
steel as in the present invention, is an element acting to
stabilize austenite. In consideration of a content of carbon
controlled for improving non-magnetic properties in the present
invention, Mn may be included in an amount of 8% or more so as to
stabilize austenite. That is, in the case that a content of Mn is 8
wt % or less, an austenite structure may not be sufficiently
obtained because ferrite, a ferromagnetic phase, becomes a main
structure. Also, in the case that the content of Mn is greater than
15 wt %, a stable austenite structure may not be maintained because
unstable .epsilon.-martensite is formed and easily transformed into
ferrite according to deformation. As a result, magnetic properties
may increase and fatigue properties may deteriorate, and also, a
decrease in corrosion resistance, difficulty in a manufacturing
process, and increases in manufacturing costs may be obtained due
to the excessive addition of manganese.
[0023] Carbon (C): 33.5C+Mn.gtoreq.25 and 33.5C-Mn.gtoreq.23
[0024] C is an element that allows an austenite structure to be
obtained at room temperature by stabilizing austenite and has an
effect of increasing strength and wear resistance of steel. In
particular, carbon functions to decrease Ms or Md, a transformation
point from austenite to martensite by a cooling process or
working.
[0025] A content of C in the present invention may simultaneously
satisfy relationships of 33.5C+Mn.gtoreq.25 and 33.5C-Mn.ltoreq.23
and content ranges of carbon and manganese controlled in the
present invention may be confirmed in FIG. 1. In the case that a
value of 33.5C+Mn is less than 25, an alpha-martensite structure, a
ferromagnetic phase, may be formed because stabilization of
austenite is insufficient, and thus, a sufficient amount of an
austenite structure may not be obtained. In the case that a value
of 33.5C-Mn is greater than 23, carbides are excessively formed at
grain boundaries because the content of C becomes excessively high,
and thus, physical properties of a material may rapidly
deteriorate. Therefore, the contents of carbon and manganese are
required to be controlled in the foregoing ranges and as a result,
sufficient austenite may be secured and the inhibition of carbide
formation may be possible. Therefore, ductility and non-magnetic
properties may be improved.
[0026] Copper (Cu): 3 wt % or Less (Excluding 0 wt %)
[0027] Cu has very low solubility in carbide and low diffusivity in
austenite, and thus, is concentrated at an interface between the
austenite and the nucleated carbide. As a result, Cu effectively
delays growth of the carbide by inhibiting diffusion of carbon and
eventually, has an effect of inhibiting carbide formation. However,
since hot workability of steel may be decreased in the case that a
content of Cu is greater than 3 wt %, an upper limit thereof may be
limited to 3 wt %. In particular, in order to sufficiently obtain
the effect of inhibiting carbide formation, Cu may be added to an
amount of 0.3 wt % or more, and for example, it is more effective
to maximize the foregoing effect in the case that Cu is added in an
amount of 2 wt % or more.
[0028] At this time, corrosion resistance of the steel may be
additionally improved by further including 8 wt % or less
(excluding 0 wt %) of chromium (Cr).
[0029] Chromium (Cr): 8 wt % or Less (Excluding 0 wt %)
[0030] In general, manganese is an element decreasing corrosion
resistance of steel and corrosion resistance of the steel having
the foregoing range of Mn may be lower than that of a general
carbon steel. However, in the present invention, corrosion
resistance may be improved by the addition of Cr. Also, ductility
may be increased by stabilizing austenite through the addition of
Cr having the foregoing range and strength may also be increased by
solution strengthening.
[0031] In the case that a content of Cr is greater than 8 wt %,
manufacturing costs may not only increase, but also, resistance to
sulfide stress corrosion cracking may be decreased by forming
carbides along grain boundaries as well as carbon dissolved in a
material and a sufficient fraction of austenite may not be obtained
due to formation of ferrites. Therefore, an upper limit thereof may
be limited to 8 wt %. In particular, in order to maximize the
effect of improving corrosion resistance, Cr may be added in an
amount of 2 wt % or more. Corrosion resistance is improved by the
addition of Cr and thus, the steel of the present invention may be
widely used in a steel for a slurry pipe or sour resistance
steel.
[0032] Also, yield strength of the steel may be further improved by
including 0.05 wt % or less (excluding 0 wt %) of titanium (Ti) and
0.1 wt % or less (excluding 0 wt %) of niobium (Nb) and thus, the
steel having a yield strength of 500 MPa or more may be
obtained.
[0033] Titanium (Ti): 0.05 wt % or Less (Excluding 0 wt %)
[0034] Ti combines with nitrogen to form TiN and thus, exhibits an
effect of increasing yield strength of steel by inhibiting growth
of austenite grains at high temperatures. However, in the case that
Ti is added excessively, physical properties of the steel may be
deteriorated due to coarsening of titanium precipitates. Therefore,
an upper limit thereof may be limited to 0.05 wt %.
[0035] Niobium (Nb): 0.1 wt % or Less (Excluding 0 wt %)
[0036] Nb is an element increasing strength through dissolution and
precipitation hardening effects, and in particular, may improve
yield strength through grain refinement during low-temperature
rolling by increasing a recrystallization stop temperature (Tnr) of
steel. However, in the case that Nb is added in an amount of
greater than 0.1 wt %, physical properties of the steel may be
rather deteriorated due to formation of coarse precipitates.
Therefore, an upper limit thereof may be limited to 0.1 wt %.
[0037] Also, in the case that the steel further includes 0.002 wt %
to 0.2 wt % of nitrogen (N), the effect of the present invention
may be further improved.
[0038] Nitrogen (N): 0.002 wt % to 0.2 wt %
[0039] Nitrogen is an element stabilizing austenite with carbon and
also, may improve strength of steel through solution strengthening.
In the case that unstable austenites are formed, N greatly
deteriorates physical properties and non-magnetic properties by
inducing deformation induced transformation into
.epsilon.-martensite and .alpha.-martensite according to
deformation. Therefore, physical properties and non-magnetic
properties of the steel may be improved by stabilizing austenite
through appropriate addition of nitrogen.
[0040] In the case that a content of N is less than 0.002 wt %, the
effect of stabilization may not be anticipated, and in the case
that the content of N is greater than 0.2 wt %, physical properties
of the steel may be deteriorated due to formation of coarse
nitrides.
[0041] Therefore, the content of N may be limited to a range of
0.002 wt % to 0.2 wt %. For example, in the case that N is added to
an amount of 0.05 wt % or more, non-magnetic properties may be more
effectively improved through the stabilization of austenite.
[0042] In the present invention, iron (Fe) and other unavoidable
impurities are included as a remainder. However, since unintended
impurities may be inevitably incorporated from raw materials or a
surrounding environment during a typical steelmaking process, the
unintended impurities may not be excluded. Since the unintended
impurities are obvious to those skilled in the art, detailed
descriptions thereof are not particularly provided in the present
specification.
[0043] Austenite is a main phase in the steel of the present
invention having the foregoing composition and austenite may be
included in an area fraction of 95% or more. In the case that the
foregoing composition is satisfied, a targeted fraction of an
austenite structure may be obtained without performing rapid
cooling (water cooling) in order to inhibit grain boundary carbide
precipitation, a limitation in a typical steel. That is, a targeted
microstructure may be formed in the steel almost without dependency
on a cooling rate and as a result, high ductility and wear
resistance may be obtained. Also, corrosion resistance may be
improved through the addition of Cr having the foregoing range and
strength may be improved through solution strengthening.
[0044] Further, the steel may have a magnetic pearmeability of 1.01
or less at a tensile strain of 20%. In the present invention,
non-magnetic properties are improved by stably securing austenite,
and in particular, excellent non-magnetic properties may be
obtained by allowing very low magnetic permeability to be obtained
even at a tensile strain of 20% through the addition of nitrogen.
For example, non-magnetic properties may be further improved by
controlling magnetic permeability to have a value of 1.005 or less
at a tensile strain of 20%.
[0045] In the present invention, a slab satisfying the foregoing
component system may be manufactured according to a typical method
of manufacturing steel, and for example, the slab of the present
invention may be manufactured by rough rolling and finishing
rolling after reheating the slab and then cooling.
[Mode for Invention]
[0046] Hereinafter, the present invention will be described in
detail, according to an embodiment. However, the following
individual examples are merely provided to allow for a clearer
understanding of the present invention, rather than to limit the
scope thereof.
Embodiment
[0047] Slabs satisfying component systems and composition ranges
described in Tables 1 and 4 were manufactured through a series of
hot rolling and cooling processes, and microstructures,
elongations, strengths, and magnetic permeabilities thereof were
then measured, and the results thereof are presented in the
following Table 2. The results of corrosion rate tests according to
dipping experimentations are presented in Table 3 below and weight
losses of samples in accordance with wear experimentations (ASTM
G65) are presented in Table 4 below.
TABLE-US-00001 TABLE 1 Category (wt %) C Mn Cu Cr Ti Nb N 35.5C +
Mn 35.5C - Mn Inventive 0.66 10 1.06 -- -- -- -- 32 12 Example 1
Inventive 0.83 9.98 1.08 -- -- -- -- 38 18 Example 2 Inventive 0.5
14 0.37 -- -- -- -- 31 3 Example 3 Inventive 0.79 10.84 1.21 --
0.017 0.021 -- 37 16 Example 4 Inventive 0.63 10.25 1.12 1.5 -- --
-- 31 11 Example 5 Inventive 0.93 11.05 1.34 1.47 -- -- -- 42 20
Example 6 Inventive 0.83 9.92 1.28 0.98 -- -- -- 38 18 Example 7
Inventive 0.92 12.01 0.71 1.23 -- -- -- 43 19 Example 8 Inventive
0.6 14.25 0.26 5.07 -- -- -- 34 6 Example 9 Inventive 0.72 12.54
2.35 2.07 -- -- -- 37 12 Example 10 Inventive 0.79 11.2 1.38 2.53
0.014 0.02 -- 38 15 Example 11 Inventive 0.82 10.95 0.95 3.15 0.016
0.02 -- 38 17 Example 12 Inventive 0.64 12.12 1.37 1.85 0.015 0.018
0.13 34 9 Example 13 Comparative 0.39 9.94 -- -- -- -- -- 23 3
Example 1 Comparative 0.9 10 -- -- -- -- -- 40 20 Example 2
Comparative 0.2 17 -- -- -- -- -- 24 -10.3 Example 3 Comparative
1.2 10 -- -- -- -- -- 50 30 Example 4 Comparative 0.9 10 3.5 -- --
-- -- 40 20 Example 5 Comparative 0.9 10.1 1.25 10 -- -- -- 40 20
Example 6 Comparative 0.05 19 -- -- -- -- -- 21 -17 Example 7
Comparative 0.02 17 0.5 1.2 -- -- -- 18 -16 Example 8
TABLE-US-00002 TABLE 2 Magnetic Magnetic permeability Austenite
Yield permeability (after 20% fraction Elongation strength (before
tensile Category (area %) (%) (MPa) deformation) strain) Inventive
98 22.5 376 1.002 1.012 Example 1 Inventive 99 25.6 357 1.002 1.01
Example 2 Inventive 99 27.3 362 1.001 1.009 Example 3 Inventive 99
26.4 574 1.001 1.002 Example 4 Inventive 99 25.7 395 1.002 1.01
Example 5 Inventive 99 28.7 402 1.002 1.01 Example 6 Inventive 99
28.4 386 1.002 1.01 Example 7 Inventive 99 27.6 392 1.001 1.009
Example 8 Inventive 99 35.6 472 1.001 1.009 Example 9 Inventive 100
37.2 630 1.002 1.002 Example 10 Inventive 99 28.1 592 1.002 1.01
Example 11 Inventive 99 30.6 605 1.002 1.01 Example 12 Inventive 99
32.2 577 1.001 1.003 Example 13 Comparative 65 4 336 5 or more Non
Example 1 measurable Comparative 78 4.6 352 1.001 Non Example 2
measurable Comparative 68 32 303 1.002 5 or more Example 3
Comparative 72 4.3 358 1.002 Non Example 4 measurable Comparative
Non Non Non Non Non Example 5 measurable measurable measurable
measurable measurable Comparative 72 3.8 520 1.002 Non Example 6
measurable Comparative 41 31 297 1.002 5 or more Example 7
Comparative 38 27 312 1.002 5 or more Example 8
TABLE-US-00003 TABLE 3 Corrosion rate (mm/year) 3.5% NaCl, 0.05M
Category 50.degree. C., 2 weeks H.sub.2SO.sub.4, 2 weeks Inventive
Example 5 0.12 0.42 Inventive Example 6 0.11 0.41 Inventive Example
7 0.12 0.42 Inventive Example 8 0.12 0.42 Inventive Example 9 0.06
0.33 Inventive Example 10 0.06 0.35 Inventive Example 11 0.09 0.40
Inventive Example 12 0.07 0.37 Inventive Example 13 0.11 0.43
Comparative Example 1 0.14 0.48 Comparative Example 2 0.16 0.48
Comparative Example 3 0.15 0.47 Comparative Example 4 0.16 0.48
Comparative Example 5 Non measurable Non measurable Comparative
Example 6 0.03 0.27 Comparative Example 7 0.15 0.45 Comparative
Example 8 0.14 0.43
TABLE-US-00004 TABLE 4 Category (wt %) Weight C Mn Si Ni Cu Cr Ti
Nb N loss (g) Inventive 0.66 10 1.06 -- -- -- -- 0.59 Example 1
Inventive 0.83 9.98 1.08 -- -- -- -- 0.61 Example 2 Inventive 0.5
14 0.37 -- -- -- -- 0.65 Example 3 Inventive 0.79 10.84 1.21 --
0.017 0.021 -- 0.63 Example 4 Inventive 0.63 10.25 -- -- 1.12 1.5
-- -- -- 0.65 Example 5 Inventive 0.93 11.05 -- -- 1.34 1.47 -- --
-- 0.59 Example 6 Inventive 0.83 9.92 -- -- 1.28 0.98 -- -- -- 0.58
Example 7 Inventive 0.92 12.01 -- -- 0.71 1.23 -- -- -- 0.57
Example 8 Inventive 0.6 14.25 -- -- 0.26 5.07 -- -- -- 0.61 Example
9 Inventive 0.72 12.54 -- -- 2.35 2.07 -- -- -- 0.54 Example 10
Inventive 0.79 11.2 -- -- 1.38 2.53 0.014 0.02 -- 0.57 Example 11
Inventive 0.82 10.95 -- -- 0.95 3.15 0.016 0.02 -- 0.58 Example 12
Inventive 0.64 12.12 -- -- 1.37 1.85 0.015 0.018 0.13 0.62 Example
13 Comparative 0.45 0.6 0.25 -- -- -- -- -- -- 0.75 Example 9
Comparative 0.066 1.5 0.2 0.15 -- 0.1 0.012 0.04 -- 1.32 Example 10
Comparative 0.36 1.5 0.26 -- -- 0.2 0.011 0.012 -- 0.9 Example 11
Comparative 0.9 12 0.5 -- -- -- -- -- -- 0.59 Example 12
[0048] Inventive Examples 1 to 13 were steels satisfying the
component systems and composition ranges controlled in the present
invention and it may be understood that deterioration of physical
properties due to grain boundary carbide formation were not
obtained even by slow cooling. Specifically, since area fractions
of austenite were 95% or more and magnetic permeabilities were
stably maintained even at a tensile strain of 20%, non-magnetic
properties as well as elongations and yield strengths were
excellent. Also, since weight losses of the samples were low, wear
resistance may be secured.
[0049] In particular, in Inventive Examples 5 to 13, it may be
understood that corrosion resistances were also improved because
corrosion rates were slow in the corrosion evaluation tests
according to additional addition of Cr. That is, it may be
confirmed that Inventive Examples 5 to 13 had effects of improving
corrosion resistance better than those of Inventive Examples 1 to 4
in which Cr was not added. Further, it may be understood that
Inventive Example 10 had a better effect of improving corrosion
resistance, because Cu was added to an amount of 2 wt % or more, a
more desirable amount. Also, in Inventive Examples 4 and 11 to 13,
yield strengths were improved by further additions of Ti and Nb,
and thus, were 500 MPa or more.
[0050] In contrast, Comparative Example 1 had a value of 33.5C+Mn
of 23, which did not correspond to the range controlled in the
present invention. A content of carbon as an austenite-stabilizing
element was insufficient and as a result, targeted austenite
structure and elongation were not obtained due to formation of a
large amount of martensites.
[0051] Also, in Comparative Example 2, contents of manganese and
carbon corresponded to the ranges controlled in the present
invention. However, since a large amount of carbides were formed
along gain boundaries due to copper not being added, austenite was
formed in an area fraction of less than 95%. Thus, it may be
confirmed that targeted microstructure and elongation may not be
obtained.
[0052] Further, Comparative Example 3 had a value of 33.5C+Mn of
24, which did not correspond to the range controlled in the present
invention. In particular, since .epsilon.-martensite, a semi-stable
phase, was formed due to a high manganese content, an austenite
structure having a targeted area fraction may not be obtained.
Since the semi-stable .epsilon.-martensite phase was easily
transformed into deformation-induced martensite during subsequent
deformation, very high magnetic permeability may be obtained at a
tensile strain of 20%. Thus, it may be confirmed that non-magnetic
properties were poor.
[0053] Comparative Example 4 had a value of 33.5C-Mn of 30, which
did not correspond to the range controlled in the present
invention. In particular, since carbides having a network form
formed at grain boundaries due to excessive addition of carbon,
austenite was formed in an amount of less than 95%. Thus, a
targeted microstructure may not be obtained and as a result,
elongation was very low.
[0054] In Comparative Example 5, contents of manganese and carbon
corresponded to the ranges controlled in the present invention.
However, since hot workability was rapidly deteriorated due to the
addition of Cu in an amount above the range controlled in the
present invention, severe cracks were generated during hot working,
and thus, a sound rolled material may not be obtained. As a result,
measurements were not possible through experimentations.
[0055] In Comparative Example 6, contents of manganese and carbon
also corresponded to the ranges controlled in the present
invention. However, since Cr carbides precipitated along grain
boundaries due to addition of Cr in an amount above the range
controlled in the present invention, a targeted fraction of
austenite may not be obtained, and as a result, it may be confirmed
that ductility was deteriorated.
[0056] In Comparative Examples 7 and 8, values of 33.5C+Mn were
respectively 21 and 18, which deviated from the range of the
present invention. In particular, since .epsilon.-martensite, a
semi-stable phase, was excessively formed due to a high manganese
content and a low C content, a fraction of austenite was very low.
As a result, the semi-stable .epsilon.-martensite was easily
transformed into deformation-induced .alpha.-martensite, a
ferromagnetic structure, during deformation to increase magnetic
permeability and thus, it may be confirmed that non-magnetic
properties were poor.
[0057] Comparative Example 9 had a composition of AISI 1045 steel,
a general carbon steel for machine structural use. Since a content
of Mn was very low and Cu was not added, a weight loss of the
sample according to the wear test was 0.75 g, and it may be
confirmed that a wear amount was relatively larger than those of
Inventive Examples.
[0058] Comparative Example 10 had a composition of API X70 grade
steel. Likewise, since a content of Mn was very low and Cu was not
added, a weight loss of the sample was greater than 1 g, and it may
be confirmed that wear resistance was very poor.
[0059] Comparative Example 11 had a composition of API K55 grade
steel. Likewise, since a content of Mn was very low and Cu was not
added, a weight loss of the sample was 0.9 g, and it may be
confirmed that wear resistance was very poor.
[0060] Comparative Example 12 was a high-manganese austenitic
Hadfield steel widely used as a wear resistant steel. Since
contents of C and Mn were sufficient, weight loss according to the
wear test was 0.59 g, and thus, excellent wear resistance
properties were obtained. However, since the inhibition of carbide
formation was not facilitated due to no addition of Cu and water
cooling must be performed after a long austenitization treatment at
a high temperature in order to inhibit the carbide formation, there
may be a limitation in a thickness of applied steel and there may
have many constraints in manufacturing steel such as difficulty in
using in a weld structure. Also, since Cr was not added, corrosion
resistance targeted in the present invention may not be
secured.
[0061] FIG. 2 is a micrograph of a steel sheet manufactured
according to Inventive Example 1 and FIG. 3 is a micrograph of a
steel sheet manufactured according to Inventive Example 5. Since
almost all structures were austenitic, it may be confirmed that
stabilization of austenite may be effectively achieved by control
of the component system and the composition range of the present
invention.
[0062] While the present invention has been shown and described in
connection with the exemplary embodiments, it will be apparent to
those skilled in the art that modifications and variations can be
made without departing from the spirit and scope of the invention
as defined by the appended claims.
* * * * *