Note: Descriptions are shown in the official language in which they were submitted.
CA 03047237 2019-06-14
[DESCRIPTION]
[Invention Title]
AUSTENITIC STEEL MATERIAL HAVING SUPERB SURFACE
CHARACTERISTIC, AND METHOD FOR PRODUCING SAME
[Technical Field]
[0001] The present disclosure relates to an austenitic steel
having excellent surface characteristics, and a method of
manufacturing the same.
[Background Art]
[0002] The present disclosure relates to an austenitic steel
material, excellent in terms of wear resistance, used in the
mining, transportation and storage fields in oil and gas
industries such as for industrial machinery, structural
materials, a steel material for slurry pipes, a steel material
in sour service, and a method for producing the same, and more
particularly to an austenite steel material, having excellent
surface characteristic, and excellent in wear resistance,
toughness, and corrosion resistance, and the like, as well as
ductility, and a method for producing the same.
[0003] The austenitic steel may be used for various purposes
due to characteristics thereof such as process hardenability,
non-magnetic properties, and the like. Particularly, as
carbon steel having ferrite and martensite as amain structure
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has a limitation in properties thereof, an austenitic steel has
been increasingly used as a substitutable material which may
overcome disadvantages of the carbon steel.
[0004] In particular, due to the growth of the mining industry
as well as the oil and gas industries, wear of a steel material
used in mining , transportation, refining, and storage processes,
is becoming a significant problem. Particularly, as the
development of oil sands as a source of fossil fuel to replace
petroleum has been regularized, wear of a steel material, caused
by a slurry including oil, gravel, sand, and the like, is a main
cause of an increase in production costs. Therefore, demand
for the development and application of a steel material
excellent in terms of wear resistance and toughness has
significantly increased.
[0005] Hadfield steel has been used extensively as a
wear-resistant component for various industries due to its
excellent wear resistance. It has continuously and steadily
tried to increase austenite structure and wear resistance by
containing a high content of carbon and a large amount of
manganese in order to increase the wear resistance of the steel
material.
[0006] However, the high content of carbon the Hadfield steel
may cause network-type carbides to form at high temperatures
along the austenite grain boundaries, thereby drastically
lowering the properties of the steel material, particularly
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ductility. In order to suppress the precipitation of the
network-type carbides, a method of making high manganese steel
by a quenching or a solution treatment at a high temperature
and then cooling it rapidly to room temperature after hot
processing has been proposed.
[0007] However, the Hadfield steel has excellent wear
resistance in a general mechanical wear environment, but it is
difficult to exhibit excellent wear resistance in an
environment where corrosion wear is accompanied, so that it is
difficult to apply to a harsh environment in which complex wear
occurs.
[0008] Recently, austenitic wear resistant steel has been
developed to ensure corrosion resistance in consideration of
the above-described problems. However, deterioration of
toughness due to precipitation of carbides may be a problem in
austenitic wear resistant steel having a very high content of
carbon. Further, segregation due to alloying elements such as
manganese, carbon, and the like, in solidification may be
inevitably generated in the case of an ingot or casting piece
of the high manganese steel. In this case, the occurrence of
the segregation may be further exacerbated in post-processing
such as hot-rolling, or the like. As a result, partial
precipitation of carbides may occur in the form of a network
along a segregation zone, deepened in a final product. As a
result, non-uniformity of a microstructure may be promoted, and
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properties thereof may be deteriorated. Therefore, studies
have mainly been made to prevent deterioration of the properties
due to precipitation of carbides, mainly in austenitic wear
resistant steel.
[0009] Another problem may be non-uniform oxidation occurring
on the surface. Such non-uniform oxidation may especially
occur along the grain boundaries, causing cracks in the slab
reheating process, and propagating the cracks during the
rolling process in which stress occurs, to deteriorate the
surface characteristics of the final product. Cracks on the
surface may cause premature failure in product bending or
tensioning process, and may reduce wear resistance thereof.
[0010] (Prior art document)
[0011] (Patent Document 1) Korean Patent Publication No.
2010-0106649
(Disclosure]
[Technical Problem]
[0012] An aspect of the present disclosure is to provide an
austenitic steel having excellent surface characteristics with
improved surface quality by suppressing non-uniform oxidation.
[0013] Another aspect of the present disclosure is to provide
a method for producing an austenitic steel having superb surface
characteristics with improved surface quality by suppressing
non-uniform oxidation.
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[Technical Solution]
[0014] According to an aspect of the present disclosure, an
austenitic steel having excellent surface characteristics,
includes, by weight, carbon (C): 0.6% to 1.3%, manganese (Mn):
14% to 22%, copper (Cu): 5% or less (excluding 0%), chromium
(Cr): 5% or less (excluding 0%), silicon (Si): 1.0% or less
(excluding 0%), aluminum (Al): 0.5% or less (excluding 0%),
phosphorus (P): 0.1% or less (including 0%), sulfur (S): 0.02%
or less (including 0%), a remainder of iron (Fe), and inevitable
impurities, wherein a microstructure of the austenitic steel
comprises, by area, 5% or less of carbide and a remaining
austenite structure, and a surface defect size of the austenitic
steel is 0.3 mm or less.
[0015] According to another aspect of the present disclosure,
a method for producing an austenitic steel having excellent
surface characteristic, includes: reheating a slab to a
temperature of 1000 C or higher to 1150 C or lower, the slab
comprising, by weight, carbon (C): 0.6% to 1.3%, manganese (Mn):
14% to 22%, copper (Cu): 5% or less (excluding 0%), chromium
(Cr): 5% or less (excluding 0%), silicon (Si): 1.0% or less
(excluding 0%), aluminum (Al): 0.5% or less (excluding 0%),
phosphorus (P): 0.1% or less (including 0%), sulfur (S): 0.02%
or less (including 0%), a remainder of iron (Fe) , and inevitable
impurities; hot-rolling the reheated slab at a finishing
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rolling temperature of 850 C to 950 C to obtain a hot-rolled
steel; and cooling the hot-rolled steel to a temperature of
600 C or lower at a cooling rate of 5 C/s or higher.
[Advantageous Effects]
[0016] According to an aspect of the present disclosure, an
austenitic steel having excellent surface characteristics may
be provided.
[0017] Through the above, it may be applied to fields requiring
wear resistance in mining, transportation, storage, or
industrial machinery fields in the oil and gas industry, in
which wear is generated in a large amount and wear resistance
is thus required, due to excellent wear resistance. In
particular, the application range to a field requiring an
excellent surface quality may be expanded. In
addition,
productivity and efficiency may be expected to increase due to
reduced surface repair of products in the light of the
production of steel material.
[Description of Drawings]
[0018] FIG. 1 is a photograph of structures of Inventive Steel
3 and Comparative Steel 5.
[Best Mode for Invention]
[0019] The inventors of the present disclosure have been
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studying steels having excellent strength and wear resistance
as compared with conventional steel materials used in technical
fields requiring wear resistance. In this connection, it is
recognized that, in the case of high manganese steels, excellent
strength and elongation specific to an austenitic steel
material may be secured; and, when the process hardening rate
is improved, the hardness may be increased due to the process
hardening of the material itself in the wear environment, and
excellent wear resistance may be ensured. On the basis of this
recognition, the present disclosure is completed.
[0020] Further, it is recognized that, in order to improve the
poor surface characteristics, which is a problem of
conventional austenitic wear-resistant steels, by deriving the
reheating conditions of the slab before hot-rolling to suppress
the non-uniform oxidation, wear resistant steel having
excellent wear resistance and superb surface characteristics
may be produced.
[0021] Hereinafter, an austenitic steel having excellent
surface characteristics according to a preferred aspect of the
present disclosure will be described.
[0022] The austenitic steel having excellent surface
characteristics according to a preferred aspect of the present
disclosure may include, by weight, carbon (C): 0.6% to 1.3%,
manganese (Mn): 14% to 22%, copper (Cu): 5% or less (excluding
0%), chromium (Cr): 5% or less (excluding 0%), silicon (Si):
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1.0% or less (excluding 0%), aluminum (Al): 0.5% or less
(excluding 0%), phosphorus (P): 0.1% or less (including 0%),
sulfur (S): 0.02% or less (including 0%), a remainder of iron
(Fe), and inevitable impurities, wherein a microstructure of
the austenitic steel comprises, by area, 5% or less of carbide
and a remaining austenite structure, and a surface defect size
of the austenitic steel is 0.3 mm or less.
[0023] First, steel components and component ranges will be
described.
[0024] C: 0.6wt% to 1.3wt% (hereinafter, referred to as
[0025] Carbon (C) maybe an austenite stabilizing element which
may improve a uniform elongation rate, and may be advantageous
to improving strength and process hardenability. When the
content of carbon is less than 0.6%, it maybe difficult to form
stable austenite at room temperature such that there may be the
problem in which it may be difficult to secure sufficient
strength and process hardenability. When the content thereof
exceeds 1.3%, a large amount of carbides may be precipitated
such that a uniform elongation rate may decrease, and it may
thus be difficult to secure an excellent elongation rate,
resulting in a decrease in wear resistance and occurrence of
early breakage. To improve wear resistance, it is preferable
to increase a content of carbon to the maximum, but there is
a limit to the carbon solubility and there is a concern about
deterioration of the properties. Therefore, an upper limit
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thereof is preferably limited to 1.3%.
[0026] Therefore, the C content is preferably limited to 0 . 6
to 1.3%.
[0027] A more preferable content of C may be 0.6 to 1.25%.
[0028] Mn: 14% to 22%
[0029] Manganese (Mn) may be a very important element that
stabilizes austenite and improves the uniform elongation. To
obtain austenite as the main structure in the present disclosure,
Mn is preferably contained in an amount of 14% or more.
[0030] When the content of Mn is less than 14%, the stability
of the austenite may decrease, and a martensitic structure may
be formed. As a result, when the austenite structure is not
sufficiently secured, it may be difficult to secure a sufficient
uniform elongation rate. When the content thereof exceeds 22%,
not only production costs may increase, but there may also be
problems of deterioration of corrosion resistance, difficulty
in the manufacturing process, and the like, due to the addition
of manganese.
[0031] Therefore, the Mn content is preferably limited to 14
to 22%.
[0032] Cu: 5% or less (excluding 0%)
[0033] Copper (Cu) may have a significantly low solid solution
degree in carbides, and may slowly disperse in austenite such
that copper may be concentrated on a carbide interfacial surface
nucleated with austenite. Therefore, copper may interfere
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with dispersion of carbon such that copper may effectively slow
down the growth of carbides, and may thus have an effect of
preventing the formation of carbides. Further, copper may help
to improve corrosion resistance. When the content of Cu exceeds
5%, there may be a problem of deteriorating the hot workability
of the steel. Therefore, an upper limit thereof is preferably
limited to 5%. The content of copper for obtaining the
above-described carbide-suppressing effect is more preferably
0.05% or more.
[0034] More preferably, the content of Cu maybe 0.05% to 3.0%.
[0035] Cr: 5% or less (excluding 0%)
[0036] Chromium may be dissolved in the austenite to increase
the strength of the steel material up to the proper amount of
addition. Chromium may be also an element that improves the
corrosion resistance of steel material. Chromium may be a
carbide element, and may be also an element that reduces
toughness by forming carbides in the austenite grain boundary.
Therefore, it is preferable to determine the content of chromium
added in the present disclosure in consideration of
relationships with carbon and other elements to be added
together. In order to prevent the formation of carbide, an
upper limit of Cr content is preferably limited to 5%. When
the Cr content exceeds 5%, it may be difficult to effectively
inhibit the formation of chromium-based carbides in the
austenite grain boundary, and thus the impact toughness may be
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reduced. Therefore, the chromium content is preferably
limited to 5% or less.
[0037] Silicon (Si): 1.0% or less (excluding 0%), aluminum
(Al): 0.5% or less (excluding 0%)
[0038] Aluminum (Al) and silicon (Si) may be components which
may be contained as deoxidizers during a steelmaking process.
The steel material of the present disclosure may include
aluminum (Al) and silicon (Si) within the above defined range.
[0039] Phosphorus (P): 0.1% or less (including 0%) ,sulfur (S):
0.02% or less (including 0%)
[0040] Phosphorus (P) and sulfur (S) may be representative
impurities, which may cause deterioration in quality when added
in excess. Therefore, it is preferable that phosphorus (P) is
limited to 0.1% or less, and sulfur (S) is limited to 0.02% or
less.
[0041] The remainder of the invention may be iron (Fe) and other
inevitable impurities. In a general steel manufacturing
process, inevitable impurities may be inevitably added from raw
materials or a surrounding environment, and thus, impurities
may not be excluded.
[0042] A person skilled in the art may be aware of the
impurities, and thus, the descriptions of the impurities may
not be provided in the present disclosure.
[0043] An austenite steel material according to one preferred
aspect of the present disclosure may have a microstructure
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comprising 5% or less of carbide and the remainder austenite
structure, and the surface defect size may be 0.3 mm or less.
More preferably, the surface defect size may be 0.2 mm or less.
[0044] When the content of the carbide exceeds 5%, the carbide
may surround the grain boundaries, and there may be a
possibility that the elongation and the impact toughness may
be drastically reduced.
[0045] When the surface defect size exceeds 0.3 mm, the
generated surface cracks may propagate during further
processing to cause early breakage, or there is a problem in
guaranteeing the target final product thickness.
[0046] The proposed surface defect size may be defined, for
example, as a distance from a point in which the crack starts
to a point in which the crack stops.
[0047] Hereinafter, a method for producing an austenitic steel
having excellent surface characteristics according to another
preferred embodiment of the present disclosure will be
described.
[0048] According to another aspect of the present disclosure,
there is provided a method of manufacturing an austenitic steel
having excellent surface characteristics, may include:
reheating a slab to a temperature of 1000 C or higher to 1150 C
or lower, the slab comprising, by weight, carbon (C): 0.6% to
1.3%, manganese (Mn): 14% to 22%, copper (Cu): 5% or less
(excluding 0%), chromium (Cr): 5% or less (excluding 0%),
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silicon (Si): 1.0% or less (excluding 0%), aluminum (Al): 0.5%
or less (excluding 0%), phosphorus (P): 0.1% or less (including
0%), sulfur (S): 0.02% or less (including 0%), a remainder of
iron (Fe), and inevitable impurities; hot-rolling the reheated
slab at a finishing rolling temperature of 850 C to 950 C to
obtain a hot-rolled steel; and cooling the hot-rolled steel to
a temperature of 600 C or lower at a cooling rate of 5 C/s or
higher.
[0049] Reheating Slab
[0050] Before hot-rolling, a slab may be reheated at a
temperature of 1000 C or higher to 1150 C or lower. Reheating
at 1000 C or higher is necessary to ensure a sufficient
temperature in hot-rolling, and it is essential to reheat at
1150 C or lower to suppress surface uneven oxidation of the high
Mn steel slab.
[0051] Hot-Rolling
[0052] As described above, the reheated slab maybe hot-rolled
to obtain a hot-rolled steel plate having a finish rolling
temperature of 850 C to 950 C.
[0053] When the finish rolling temperature is lower than 850 C,
carbide may precipitate and the uniform elongation may be
lowered, and the microstructure may be pancaked to cause
non-uniform stretching due to anisotropy of structure. When
the finishing rolling temperature exceeds 950 C, the rolling
finishing temperature may be too high and it may be difficult
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to hit the target temperature in the actual process.
[0054] Cooling
[0055] The hot-rolled steel obtained through the hot-rolling
may be cooled to a temperature of 600 C or lower at a cooling
rate of 5 C/s or higher.
[0056] When the cooling rate is less than 5 C/s, or the cooling
stop temperature is more than 600 C, carbides may precipitate
and elongation may decrease . The rapid cooling proces s may also
help to ensure high employment of C and N elements in the matrix.
Therefore, it is preferable that the cooling is carried out to
a temperature of 600 C or lower at a cooling rate of 5 C/s or
higher. The cooling rate is more preferably 10 C/s or higher,
and more preferably 15 C/s or higher.
[Mode for Invention)
[0057] In the description below, an example embodiment of the
present disclosure will be described in greater detail. It
should be noted that the exemplary embodiments are provided to
describe the present disclosure in greater detail, and to not
limit the scope of rights of the present disclosure. The scope
of rights of the present disclosure may be determined on the
basis of the subject matters recited in the claims and the
matters reasonably inferred from the subject matters.
[0058] (Example)
[0059] Slabs satisfying the composition and composition ranges
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as shown in Table 1 below were prepared as hot-rolled steel
plates having a thickness of 12 mm through reheating and rolling
conditions shown in Table 2 below.
[0060] Then, the microstructure, yield strength, uniform
elongation, and impact toughness of each of the manufactured
hot-rolled steel sheets were measured, and the results are shown
in Table 3 below. The surface defect size of the hot-rolled
steel plate was measured and shown in Table 3 below.
[0061] Inventive Steel 3 and Comparative Steel 5 were observed
for their structure, and the results are shown in FIG. 1.
[0062] [Table 1]
Component Composition (wt%)
Mn Si Al Cr Cu
'IS' 0.64 16.9 0.08 0.057 4.7 1.5 0.022
0.009
IS2 0.81 18.1 0.014 0.119 2.5 1.3 0.023 0.006
IS3 1.09 21.5 0.31 0.041 3.3 0.06 0.016
0.004
IS4 1.22 14.7 0.091 0.262 0.09 0.35 0.012 0.016
2CS1 0.33 15.2 0.017 0.08 0.023 0.025 0.013 0.007
CS2 1.35 15.8 0.098 0.044 0.11 0.1 0.017 0.005
CS3 0.65 12.2 0.046 0.041 0.22 0.15 0.013 0.003
CS4 1.11 18.6 0.16 0.076 5.8 0.09 0.018
0.009
CS5 1.23 19.1 0.15 0.08 1.1 0.09 0.015
0.006
CS6 0.64 16.4 0.11 0.041 1.8 0.9 0.017
0.008
CS7 0.61 18.3 0.11 0.045 1.8 0.9 0.014
0.005
CS8 0.75 17.6 0.11 0.041 1.8 0.9 0.017
0.008
[0063] 'IS: Inventive Steel, 2CS: Comparative Steel
[0064] [Table 2]
Reheating & Rolling Conditions
Reheating Finish Rolling Cooling Rate Cooling
Stop
Temp. ( C) Temp. ( C) ( C/s) Temp. ( C)
lIS1 1145 940 26 385
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IS2 1108 915 16 200
IS3 1056 901 32 475
IS4 1023 869 41 275
2CS1 1110 876 22 495
CS2 1125 899 19 425
CS3 1130 920 27 355
CS4 1134 925 19 375
CS5 1231 925 19 375
CS6 1105 825 25 390
CS7 1140 912 3.5 435
CS8 1125 947 23 670
[0065] 'IS: Inventive Steel, 'CS: Comparative Steel
[0066] [Table 3]
Surface Yield Impact
Microstructure Elongat
Defect Size Strength Toughness
(y:austenite) ion (%)
(mm) (MPa) (J@-40 C)
0.24mm or
lIS1 y+carbide 5% or less 453 50 199
less
0.11mm or
IS2 y+carbide 5% or less 411 60 227
less
0.05mm or
IS3 y+carbide 5% or less 500 53 208
less
_
0.13mm or
IS4 y+carbide 5% or less 523 47 116
less
2CS1 y+carbide 5% or less - 270 48 87
CS2 y+carbide 9.1% - 581 19 27
CS3 y+martensite 380 33 19
CS4 y+carbide 11.8 - 607 17 22
More than
CS5 y+carbide 5% or less 564 31 121
0.3mm
CS6 y+carbide 6.1% - 420 36 42
C57 y+carbide 6.9% - 431 53 33
CS8 y+carbide 7.2% - 520 43 29
[0067] 'IS: Inventive Steel, 2CS: Comparative Steel
[0068] As shown in Tables 1 to 3, Inventive Steels 1 to 4
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satisfied both the composition range and the manufacturing
conditions, and Inventive Steels 1 to 4 showed good surface
characteristics.
[0069] Comparative Steel I showed that sufficient strength did
not be secured because C was very low.
[0070] Comparative Steel 2 showed that formation of carbide
increased, due to excessive addition of C, and values for
elongation and impact toughness rapidly decreased.
[0071] In Comparative Steel 3, a stable austenite phase was
not formed due to the insufficient Mn content, and martens ite
was formed, and a value for impact toughness rapidly decreased.
[0072] Comparative Steel 4 showed that elongation and impact
toughness rapidly decreased, due to excessive carbide formation
when the Cr content was exceeded.
[0073] Comparative Steel 5 showed that the reheating
temperature exceeded the reference value, and a relatively
large defect occurred on the surface of the product.
[0074] Comparative Steels 6 to 8 showed that the conditions
such as the rolling finishing temperature, the cooling rate,
and the cooling stop temperature failed to fall within the scope
of the present disclosure, and the impact toughness rapidly
decreased, due to excessive precipitation of carbide.
[0075] As shown in FIG. 1, Comparative Steel 5 having a
relatively high reheating temperature had large cracks formed
on the surface thereof. In Inventive Steel 3 having a
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relatively low temperature reheating temperature, it can be
seen that the surface layer was uniform, and no relatively large
defect occurred.
[0076] While example embodiments have been shown and described
above, the scope of the present disclosure is not limited
thereto, and it will be apparent to those skilled in the art
that modifications and variations could be made without
departing from the scope of the present invention as defined
by the appended claims.
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