U.S. patent application number 15/535595 was filed with the patent office on 2017-12-21 for steel for low-temperature service having excellent surface processing quality and method for manufacturing same.
The applicant listed for this patent is POSCO. Invention is credited to Sang-Deok KANG, Yong-Jin KIM, Soon-Gi LEE, In-Shik SUH.
Application Number | 20170362675 15/535595 |
Document ID | / |
Family ID | 56150966 |
Filed Date | 2017-12-21 |
United States Patent
Application |
20170362675 |
Kind Code |
A1 |
LEE; Soon-Gi ; et
al. |
December 21, 2017 |
STEEL FOR LOW-TEMPERATURE SERVICE HAVING EXCELLENT SURFACE
PROCESSING QUALITY AND METHOD FOR MANUFACTURING SAME
Abstract
Provided is a steel sheet for low-temperature service, which can
be used at a wide temperature range from low temperature to room
temperature in liquefied gas storage tanks and transport
facilities. The steel sheet for low-temperature service has an
excellent surface processing quality even after a processing
processes is performed, such as a tension process. The a steel
sheet contains manganese (Mn, 15-35 wt %), carbon (C, satisfying
23.6C+Mn.gtoreq.28 and 33.5C--Mn.ltoreq.23), copper (Cu, 5 wt % or
less (excluding 0 wt %)), chrome (Cr, satisfying
28.5C+4.4Cr.ltoreq.57 (excluding 0 wt %)), titanium (Ti, 0.01-0.5
wt %), nitrogen (N, 0.003-0.2 wt %), the balance iron (Fe), and
other inevitable impurities. Ti and N satisfy relational expression
1 below. [Relational expression 1] 1.0.ltoreq.Ti/N.ltoreq.4.5 (Mn,
C, Cr, Ti, and N in the respective expressions mean wt % of
respective ingredient contents).
Inventors: |
LEE; Soon-Gi; (Gwangyang-si,
KR) ; SUH; In-Shik; (Gwangyang-si, KR) ; KIM;
Yong-Jin; (Gwangyang-si, KR) ; KANG; Sang-Deok;
(Gwangyang-si, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
POSCO |
Pohang-si |
|
KR |
|
|
Family ID: |
56150966 |
Appl. No.: |
15/535595 |
Filed: |
December 11, 2015 |
PCT Filed: |
December 11, 2015 |
PCT NO: |
PCT/KR2015/013554 |
371 Date: |
June 13, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/28 20130101;
C21D 9/46 20130101; C22C 38/38 20130101; C21D 6/005 20130101; C22C
38/20 20130101; C21D 8/0226 20130101; C21D 2211/001 20130101; C21D
2211/004 20130101; C22C 38/001 20130101; C21D 8/0205 20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C22C 38/28 20060101 C22C038/28; C21D 8/02 20060101
C21D008/02; C21D 6/00 20060101 C21D006/00; C22C 38/00 20060101
C22C038/00; C22C 38/38 20060101 C22C038/38; C22C 38/20 20060101
C22C038/20 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 24, 2014 |
KR |
10-2014-0189137 |
Claims
1. Steel for low temperature environments having excellent surface
processing qualities, comprising: 15 wt % to 35 wt % of manganese
(Mn), carbon (C) satisfying 23.6C+Mn.gtoreq.28 and
33.5C--Mn.ltoreq.23, 5 wt % or lower of copper (Cu) (excluding 0 wt
%), chrome (Cr) satisfying 28.5C+4.4Cr.ltoreq.57 (excluding 0 wt
%), 0.01 wt % to 0.5 wt % of titanium (Ti), 0.003 wt % to 0.2 wt %
of nitrogen (N), iron (Fe) as a residual component, and inevitable
impurities, wherein Ti and N satisfy Relational Formula 1 below,
1.0.ltoreq.Ti/N.ltoreq.4.5, [Relational Formula 1] where Mn, C, Cr,
Ti, and N in each expression refer to wt % of a content of each
component.
2. The steel for low temperature environments having excellent
surface processing qualities of claim 1, wherein the steel
comprises a TiN precipitate having a size of 0.01 .mu.m to 0.3
.mu.m.
3. The steel for low temperature environments having excellent
surface processing qualities of claim 1, wherein the steel
comprises a TiN precipitate in an amount of 1.0.times.10.sup.7 to
1.0.times.10.sup.10 per 1=.sup.2.
4. The steel for low temperature environments having excellent
surface processing qualities of claim 1, wherein a number of
austenite grains having a size of 200 .mu.m or greater is 5 or less
per 1 cm.sup.2 in a microstructure of the steel.
5. The steel for low temperature environments having excellent
surface processing qualities of claim 1, wherein a microstructure
of the steel comprises austenite in an area fraction of 95% or
greater.
6. The steel for low temperature environments having excellent
surface processing qualities of claim 5, wherein a carbide present
in a grain boundary of austenite is lower than or equal to 5% in an
area fraction.
7. The steel for low temperature environments having excellent
surface processing qualities of claim 1, wherein impact toughness
of the steel is higher than or equal to 41 J at a temperature of
-196.degree. C.
8. A method of manufacturing steel for low temperature environments
having excellent surface processing qualities, comprising:
providing a slab including 15 wt % to 35 wt % of Mn, C satisfying
23.6C+Mn.gtoreq.28 and 33.5C--Mn.ltoreq.23, 5 wt % or lower of Cu
(excluding 0 wt %), Cr satisfying 28.5C+4.4Cr.ltoreq.57 (excluding
0 wt %), 0.01 wt % to 0.5 wt % of Ti, 0.003 wt % to 0.2 wt % of N,
Fe as a residual component, and inevitable impurities, Ti and N
satisfying Relational Formula 1 below; heating the slab at a
temperature of 1050.degree. C. to 1250.degree. C.; and
manufacturing heat-rolled steel by heat rolling the slab that has
been heated, 1.0.ltoreq.Ti/N.ltoreq.4.5, [Relational Formula 1]
where Mn, C, Cr, Ti, and N in each expression refer to wt % of a
content of each component.
9. The method of claim 8, wherein the steel comprises a TiN
precipitate having a size of 0.01 .mu.m to 0.3 .mu.m.
10. The method of claim 8, wherein the steel comprises a TiN
precipitate in an amount of 1.0.times.10.sup.7 to
1.0.times.10.sup.10 per 1 mm.sup.2.
11. The method of claim 8, wherein a number of austenite grains
having a size of 200 .mu.m or greater is 5 or less per 1 cm.sup.2
in a microstructure of the steel.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to steel for low temperature
environments having excellent surface processing qualities and a
method of manufacturing the same.
BACKGROUND ART
[0002] Steel used for storage containers containing liquefied
natural gas, liquid nitrogen, or the like, and used for offshore
platforms and facilities in polar regions may be provided as steel
for low temperature environments maintaining sufficient toughness
and strength even at extremely low temperatures. Such steel for low
temperature environments should have excellent low-temperature
toughness, strength, and magnetic properties, as well as having
relatively low coefficients of thermal expansion and thermal
conductivity.
[0003] Recently, steel (Patent Document 1) having excellent extreme
low temperature properties through the addition of relatively large
amounts of manganese (Mn) and carbon (C), with nickel (Ni)
completely excluded, to stabilize austenite and including aluminum
(Al) have been used. In addition, steel (Patent Document 2) having
excellent low-temperature toughness in such a manner that a mixed
structure of austenite and epsilon martensite is secured by adding
Mn thereto has been used.
[0004] In the case of steel for low temperature environments having
austenite as a main microstructure thereof, relatively large
amounts of C and Mn are added thereto, thereby stabilizing
austenite. However, an addition of C and Mn affects the
recrystallization behavior of austenite, thereby causing partial
recrystallization and nonuniform grain growth in a rolling
temperature range of the related art. Thus, only a specific small
number of austenite grains are significantly grown, thereby causing
significant nonuniformity in the size of austenite grains in a
microstructure.
[0005] In general, in the case of austenite structures having
relatively high contents of C and Mn, deformation behavior is
implemented by slips and twin crystals in a manner different from
general carbon steel. In addition, in the early stage of
deformation, deformation behavior is usually implemented by slips
corresponding to uniform deformation, but twin crystals
corresponding to nonuniform deformation are subsequently
accompanied thereby. When the size of grains is relatively large,
stress required to form twin crystals is reduced, thereby easily
generating twin crystals even in the case of a relatively low
degree of deformation. In a case in which a relatively small number
of coarse grains are present in a microstructure, deformation of
twin crystals occurs in coarse grains in the early stage of
deformation, thereby causing nonuniform deformation. Thus, surface
characteristics of materials may be deteriorated, thereby causing
nonuniform thicknesses of final structures. In detail, in the case
of structures requiring internal pressure resistance by securing
uniform thicknesses of steel, such as low-temperature pressure
vessels, significant problems in structural design and use thereof
occur.
[0006] Thus, in the case of steel, a microstructure of which has
been austenitized by adding C and Mn thereto, steel for extreme low
temperature environments, produced at low cost, which is economical
and has secured structural stability by improving the uneven
surfaces caused by early deformation of coarse grains into twin
crystals is urgently required to be developed.
PRIOR ART DOCUMENT
[0007] Patent Document 1: Korean Patent Application No.
1991-0012277 [0008] Patent Document 2: Japanese Patent Application
No. 2007-126715
DISCLOSURE
Technical Problem
[0009] An aspect of the present disclosure may provide steel for
low temperature environments having excellent surface processing
qualities and a method of manufacturing the same.
Technical Solution
[0010] According to an aspect of the present disclosure, steel for
low temperature environments having excellent surface processing
qualities includes 15 wt % to 35 wt % of manganese (Mn), carbon (C)
satisfying 23.6C+Mn.gtoreq.28 and 33.5C--Mn.ltoreq.23, 5 wt % or
lower of copper (Cu) (excluding 0 wt %), chrome (Cr) satisfying
28.5C+4.4Cr.ltoreq.57 (excluding 0 wt %), 0.01 wt % to 0.5 wt % of
titanium (Ti), 0.003 wt % to 0.2 wt % of nitrogen (N), iron (Fe) as
a residual component, and inevitable impurities. Ti and N satisfy
Relational Formula 1 below.
[0011] According to an aspect of the present disclosure, a method
of manufacturing steel for low temperature environments having
excellent surface processing qualities includes providing a slab
including 15 wt % to 35 wt % of Mn, C satisfying 23.6C+Mn.gtoreq.28
and 33.5C--Mn.ltoreq.23, 5 wt % or lower of Cu (excluding 0 wt %),
Cr satisfying 28.5C+4.4Cr.ltoreq.57 (excluding 0 wt %), 0.01 wt %
to 0.5 wt % of Ti, 0.003 wt % to 0.2 wt % of N, Fe as a residual
component, and inevitable impurities, Ti and N satisfying
Relational Formula 1 below; heating the slab at a temperature of
1050.degree. C. to 1250.degree. C.; and manufacturing heat-rolled
steel by heat rolling the slab that has been heated.
1.0.ltoreq.Ti/N.ltoreq.4.5, [Relational Formula 1]
[0012] where Mn, C, Cr, Ti, and N in each expression refer to wt %
of a content of each component.
[0013] In addition, the foregoing technical solution does not list
an entirety of characteristics of the present disclosure. Various
characteristics of the present disclosure and consequent advantages
and effects will be understood in more detail with reference to
specific exemplary embodiments below.
Advantageous Effects
[0014] According to an aspect of the present disclosure, steel for
low temperature environments having excellent surface processing
qualities even after being processed due to an austenite structure
having uniform particle sizes and a method of manufacturing the
same may be provided.
DESCRIPTION OF DRAWINGS
[0015] FIG. 1A is an image captured using an optical microscope,
illustrating a microstructure of steel for low temperature
environments of the related art.
[0016] FIG. 1B is an image of a cross section of a specimen after
steel for low temperature environments of the related art is
tensioned.
[0017] FIG. 2 is an image captured using an optical microscope,
illustrating a microstructure of steel for low temperature
environments according to an exemplary embodiment in the present
disclosure.
[0018] FIG. 3 is a graph illustrating ranges of carbon (C) and
manganese (Mn) controlled in an exemplary embodiment.
BEST MODE FOR INVENTION
[0019] The inventors recognized that, in the case of steel having
an austenite structure, containing a relatively large amount of
carbon (C) and manganese (Mn), partial recrystallization and grain
growth of the austenite structure occurs in a rolling temperature
range of the related art, thereby generating abnormally coarse
austenite; in general, critical stress required to form a twin
crystal is higher than that of a slip, but in a case in which a
size of a grain is relatively great for the reason described above,
stress required to form the twin crystal is reduced, thereby
causing deformation of the twin crystal in the early state of
deformation, so that a problem in which surface quality may be
degraded due to discontinuous deformation may occur. In addition,
the inventors have conducted in-depth research to solve the problem
described above.
[0020] Thus, the inventors confirmed that steel for low temperature
environments in which fine austenite is uniformly distributed may
be obtained in such a manner that a titanium (Ti)-based precipitate
is properly educed by adding Ti thereto, in order to suppress
significant coarsening of an austenite grain and realized the
present disclosure.
[0021] Hereinafter, steel for low temperature environments having
excellent surface processing qualities according to an exemplary
embodiment will be described in detail.
[0022] According to an aspect of the present disclosure, the steel
for low temperature environments having excellent surface
processing qualities includes 15 wt % to 35 wt % of manganese (Mn),
carbon (C) satisfying 23.6C+Mn.gtoreq.28 and 33.5C--Mn.ltoreq.23, 5
wt % or lower of copper (Cu) (excluding 0 wt %), chrome (Cr)
satisfying 28.5C+4.4Cr.ltoreq.57 (excluding 0 wt %), 0.01 wt % to
0.5 wt % of titanium (Ti), 0.003 wt % to 0.2 wt % of nitrogen (N),
iron (Fe) as a residual component, and inevitable impurities. In
addition, Ti and N satisfy Relational Formula 1 below.
1.0.ltoreq.Ti/N.ltoreq.4.5, [Relational Formula 1]
[0023] where Mn, C, Cr, Ti, and N in each expression refer to wt %
of a content of each component.
[0024] First, an alloy composition of the steel for low temperature
environments having excellent surface processing qualities
according to an exemplary embodiment will be described in detail.
Hereinafter, a unit of each alloying element is wt %.
[0025] Manganese (Mn): 15% to 35%
[0026] Mn is an element playing a role in stabilizing austenite in
an exemplary embodiment. 15% or more of Mn may be contained to
stabilize an austenite phase at extremely low temperatures. In
other words, in a case in which an Mn content is lower than 15%,
when a C content is relatively low, metastable phase epsilon
martensite is formed and easily transformed into .alpha.-martensite
by strain induced transformation at extremely low temperatures,
thereby not securing toughness. In a case in which the C content is
increased to stabilize austenite to prevent the case described
above, physical properties thereof may be dramatically degraded due
to carbide precipitation. Thus, the Mn content may be higher than
or equal to 15%. On the other hand, in a case in which the Mn
content is higher than 35%, a problem in which a corrosion rate of
steel is increased, and economic feasibility is reduced due to an
increase in the Mn content occurs. Thus, the Mn content may be
limited to a range of 15% to 35%.
[0027] Carbon (C): 23.6C+Mn.gtoreq.28 and 33.5C--Mn.ltoreq.23
[0028] C is an element stabilizing austenite and increasing
strength. In detail, C plays a role in reducing M.sub.s and
M.sub.d, transformation points in which austenite is transformed
into epsilon martensite or .alpha.-martensite by a cooling process
or a process. Thus, in a case in which C is insufficiently added,
stability of austenite is insufficient, thereby not obtaining
stable austenite at extremely low temperatures. In addition,
external stress causes strain induced transformation in which
austenite is easily transformed into epsilon martensite or
.alpha.-martensite, and toughness and the strength of steel is
reduced. On the other hand, in a case in which the C content is
significantly high, toughness is dramatically degraded due to
carbide precipitation, and workability is degraded due to a
significant increase in strength.
[0029] In detail, the C content in an exemplary embodiment may be
decided in consideration of a relationship between C and other
elements added thereto. To this end, a relationship between C and
Mn in forming a carbide that the inventor has discovered is
illustrated in FIG. 3. As illustrated in FIG. 3, the carbide is
formed using C. C does not independently affect formation of the
carbide, but affects a tendency to form the carbide in combination
with Mn.
[0030] FIG. 3 illustrates a proper C content. In order to prevent
the carbide from being generated, on a premise that other
components satisfy a range made in an exemplary embodiment, a value
of 23.6C+Mn (in the case of C and Mn, a content of each component
is expressed using wt %) may be controlled to be higher than or
equal to 28. The value refers to a leftward diagonal line in a
hexagonal area in FIG. 3. In a case in which 23.6C+Mn is lower than
28, stability of austenite is decreased, and strain induced
transformation is generated by impacts at extremely low
temperatures, thereby degrading impact toughness. In a case in
which the C content is significantly high, that is, 33.5C-Mn is
higher than 23, an addition of a significant amount of C causes
carbide precipitation, thereby degrading low-temperature impact
toughness. In conclusion, C may be added to satisfy an entirety of
Mn: 15% to 35%, 23.6C+Mn.gtoreq.28, and 33.5C--Mn.ltoreq.23. As
illustrated in FIG. 3, a lowermost limit of the C content is 0%,
within a range satisfying the expression above.
[0031] Copper (Cu): 5% or lower (excluding 0%)
[0032] Cu has significantly low solid solubility in the carbide and
is relatively slow in spreading in austenite, thereby being
concentrated in austenite and at an interface of a nucleated
carbide. Thus, spreading of C is interrupted, thereby effectively
slowing carbide growth. As a result, a generation of the carbide is
suppressed. In addition, Cu stabilizes austenite to improve extreme
low-temperature toughness. However, in a case in which a Cu content
is higher than 5%, hot workability of steel is degraded. Thus, an
uppermost limit may be limited to 5%. In addition, the Cu content
to obtain an effect of suppressing the carbide as described above
may be higher than or equal to 0.5%.
[0033] Chrome (Cr): 28.5C+4.4Cr.ltoreq.57 (excluding 0%)
[0034] Cr plays a role in improving impact toughness at low
temperatures by stabilizing austenite and increasing the strength
of steel through being solubilized in austenite within a range of a
proper content thereof. In addition, Cr is an element improving
corrosion resistance of steel. However, Cr is a carbide element. In
detail, Cr is also an element forming the carbide in an austenite
grain boundary to reduce the impact of low-temperatures. Thus, a Cr
content in an exemplary embodiment may be determined in
consideration of the relationship between C and other elements
added thereto. In order to prevent the carbide from being
generated, on a premise that other components satisfy a range made
in an exemplary embodiment, a value of 28.5C+4.4Cr (in the case of
C and Cr, a content of each component is expressed using wt %) may
be controlled to be lower than or equal to 57. In a case in which
the value of 28.5C+4.4Cr is higher than 57, the generation of the
carbide in the austenite grain boundary is difficult to suppress
effectively, due to significant contents of Cr and C, thereby
causing a problem in which impact toughness at low temperatures is
degraded. Thus, Cr may be added to satisfy 28.5C+4.4Cr.ltoreq.57 in
an exemplary embodiment.
[0035] Titanium (Ti): 0.01% to 0.5%
[0036] Ti is an element forming a TiN precipitate in combination
with nitrogen (N). In an exemplary embodiment, during
high-temperature hot rolling, a portion of the austenite grain may
be significantly coarse. Thus, growth of the austenite grain may be
suppressed by properly educing TiN. To this end, at least 0.01% or
more of Ti is required to be added. However, in a case in which a
Ti content is higher than 0.5%, an effect of growth of the
austenite grain may not be improved anymore. In addition, coarse
TiN is educed, thereby reducing an effect of growth of the
austenite grain. Thus, in an exemplary embodiment, the Ti content
may be limited to a range of 0.01% to 0.5%.
[0037] Nitrogen (N): 0.003% to 0.2 wt %
[0038] In an exemplary embodiment, in order to effectively achieve
a goal of adding Ti described above, N is required to be added
simultaneously. In detail, in order to effectively educe TiN,
0.003% or more of N may be added. However, since solid solubility
of N is lower than or equal to 0.2%, an addition of 0.2% or greater
of N is significantly difficult, and 0.2% or less thereof is
sufficient to educe TiN, thereby limiting an uppermost limit
thereof to 0.2%. Thus, an N content may be limited to a range of
0.003% to 0.2% in an exemplary embodiment.
[0039] A residual component of an exemplary embodiment is Fe.
However, since, in a manufacturing process of the related art,
unintentional impurities may be inevitably mixed from a raw
material or a surrounding environment, unintentional impurities are
unavoidable. Since the impurities are known to those skilled in the
manufacturing process of the related art, descriptions thereof will
not be provided in detail in an exemplary embodiment.
[0040] In addition, a weight ratio of Ti to N, that is, Ti/N, may
satisfy Relational Formula 1 below.
1.0.ltoreq.Ti/N.ltoreq.4.5 [Relational Formula 1]
[0041] In a case in which a Ti/N ratio is controlled to be higher
than or equal to 1.0, solute Ti is combined with N, thereby educing
minute TiN. In addition, since TiN that has been educed using a
method described above is stably present, the growth of the
austenite grain may be effectively suppressed.
[0042] However, in a case in which the Ti/N ratio is higher than
4.5, coarse TiN is crystallized in molten steel, thereby adversely
affecting a property of steel and not obtaining uniform
distribution of TiN. In addition, surplus Ti that has not been
educed to be TiN is present in a state of solid solution, thereby
adversely affecting heat-affected zone toughness. However, in a
case in which the Ti/N ratio is lower than 1.0, an amount of solute
N in a base metal is increased, thereby adversely affecting
heat-affected zone toughness. Thus, the Ti/N ratio may be
controlled to be 1.0 to 4.5.
[0043] In addition, the steel for low temperature environments
according to an exemplary embodiment described above may include
the TiN precipitate having a size of 0.01 .mu.m to 0.3 .mu.m.
[0044] In a case in which a size of the TiN precipitate is less
than 0.01 .mu.m, the TiN precipitate is easily solubilized, so that
an effect of suppressing grain growth becomes insufficient. On the
other hand, in a case in which the size of the TiN precipitate is
greater than 0.3 .mu.m, an austenite grain pinning effect is
reduced, and a coarse size thereof adversely affects toughness.
Thus, the size of the TiN precipitate may be within a range of 0.01
.mu.m to 0.3 .mu.m.
[0045] In addition, the steel for low temperature environments
according to an exemplary embodiment may include the TiN
precipitate in an amount of 1.0.times.10.sup.7 to
1.0.times.10.sup.10 per 1 mm.sup.2.
[0046] In a case in which the TiN precipitate is present in an
amount less than 1.0.times.10.sup.7 per 1 mm.sup.2, a grain pinning
effect is insignificant, thereby not effectively suppressing growth
of a coarse grain. On the other hand, in a case in which the TiN
precipitate is present in an amount greater than 1.0.times.10.sup.7
per 1 mm.sup.2, the size of the TiN precipitate becomes relatively
small, so that the TiN precipitate may be unstable, and impact
toughness of a material thereof may be degraded. Thus, the amount
of the TiN precipitate may be 1.0.times.10.sup.7 to
1.0.times.10.sup.10 per 1 mm.sup.2.
[0047] In addition, the steel for low temperature environments
according to an exemplary embodiment limits the number of coarse
austenite grains having a size of 200 .mu.m or greater in the
microstructure to 5 or less per 1 cm.sup.2.
[0048] Since, in the case of austenite having a grain size less
than 200 .mu.m, stress required to generate the twin crystal is
sufficiently higher than stress required to generate a slip,
nonuniform transformation is not generated within a transformation
rate of steel for low temperature environments of the related art
when a structure is manufactured. Thus, the size thereof may be
limited to 200 .mu.m or greater. In addition, in a case in which
the density of a grain having a size of 200 .mu.m or greater is
greater than 5 per 1 cm.sup.2, due to a relatively high density of
the coarse grain, nonuniform transformation is sufficiently
deteriorated to affect surface qualities. Thus, the density of the
grain having a size of 200 .mu.m or greater may be limited to 5 or
less per 1 cm.sup.2.
[0049] In the meantime, the steel for low temperature environments
according to an exemplary embodiment may include an austenite
structure in an area fraction of 95% or higher. Austenite, a
representative soft structure in which ductile fracture is
generated even at low temperatures, is an essential microstructure
to secure low-temperature toughness and should be included in an
area fraction of 95% or higher. In a case in which austenite is
included in an area fraction of lower than 95%, austenite is not
sufficient to secure low-temperature toughness, that is, impact
toughness of 41 J or greater at a temperature of -196.degree. C.,
so that a lowermost limit thereof may be limited to 95%.
[0050] In addition, the carbide present in the austenite grain
boundary may be lower than or equal to 5% in an area fraction. In
an exemplary embodiment, the carbide is a representative structure
that may be present, beside austenite. The carbide is educed in an
austenite grain boundary and becomes a cause of grain boundary
rupture, thereby degrading low-temperature toughness and ductility.
Thus, an uppermost limit thereof may be limited to 5%.
[0051] Hereinafter, a method of manufacturing the steel for low
temperature environments having excellent surface processing
qualities according to another exemplary embodiment will be
described in detail.
[0052] The method of manufacturing the steel for low temperature
environments having excellent surface processing qualities
according to another exemplary embodiment includes providing a slab
satisfying the alloy composition described above, heating the slab
at a temperature of 1050.degree. C. to 1250.degree. C., and
manufacturing hot-rolled steel by hot rolling the slab that has
been heated.
[0053] Providing a Slab
[0054] The slab satisfying the alloy composition described above is
provided. A reason for controlling the alloy composition is the
same as described above.
[0055] Heating a Slab
[0056] The slab is heated at the temperature of 1050.degree. C. to
1250.degree. C.
[0057] A process described above is performed for the sake of
solution and homogenization of a cast structure, segregation, and
secondary phases generated in a process of manufacturing the slab.
In a case in which the temperature is lower than 1050.degree. C.,
homogenization thereof is insufficient or a temperature of a
heating furnace is significantly low, thereby causing a problem in
which deformation resistance is increased during heat rolling. In a
case in which the temperature is higher than 1250.degree. C.,
partial melting may occur and surface qualities may be degraded in
segregation in the cast structure, and TiN may be crystallized,
thereby not contributing to austenite refinement, but degrading
properties thereof. Thus, a heating temperature of the slab may be
in a range of 1050.degree. C. to 1250.degree. C.
[0058] Manufacturing Hot-Rolled Steel
[0059] The slab that has been heated is heat rolled, thereby
manufacturing the hot-rolled steel.
[0060] In an exemplary embodiment, the alloy composition and the
heating temperature of the slab, described above, may be satisfied,
thereby manufacturing the steel for low temperature environments
having excellent surface processing qualities. Thus, in detail, it
is not necessary to control a condition of the manufacturing
hot-rolled steel and the manufacturing hot-rolled steel may be
performed using a general method.
INDUSTRIAL APPLICABILITY
[0061] Hereinafter, the present disclosure will be described in
more detail through exemplary embodiments. However, an exemplary
embodiment below is intended to describe the present disclosure in
more detail through illustration thereof, but not limit the scope
of rights of the present disclosure, because the scope of rights
thereof is determined by the contents written in the appended
claims and can be reasonably inferred therefrom.
[0062] After a slab satisfying a component system stated in Table 1
below is manufactured in the same manner as a manufacturing
condition stated in Table 2, a microstructure, yield strength, an
elongation rate, Charpy impact toughness at a temperature of
-196.degree. C., or the like, are measured to be stated in Table 2
or Table 3, respectively.
[0063] In Table 3 below, unevenness of surfaces is assessed by
observing surfaces of the steel for low temperature environments
with the naked eye.
TABLE-US-00001 TABLE 1 Weight Ratio of 23.6C + 33.5C - 28.5C +
Classification C Mn Cu Cr N Ti Ti/N Mn Mn 4.4Cr Comparative 0.62
18.12 0.12 0.2 0.012 32.8 2.7 18.6 Example 1 Comparative 0.37 25.4
1.12 3.85 0.018 34.1 -13.0 27.5 Example 2 Comparative 0.61 18.13
1.5 1.25 0.012 32.5 2.3 22.9 Example 3 Comparative 0.31 28.7 0.15
1.32 0.025 0.024 0.96 36.0 -18.3 14.6 Example 4 Comparative 0.45
11.7 0.008 0.07 8.75 22.3 3.4 12.8 Example 5 Comparative 0.37 24.1
1.02 3.5 0.011 0.05 4.55 32.8 -11.7 25.9 Example 6 Inventive 0.58
21.7 0.61 0.55 0.053 0.06 1.13 35.388 -2.3 19.0 Example 1 Inventive
0.45 24.3 0.43 3.08 0.12 0.17 1.42 34.92 -9.2 26.4 Example 2
Inventive 0.39 28.6 0.85 3.45 0.016 0.02 1.25 37.804 -15.5 26.3
Example 3 Inventive 0.44 27.5 0.42 1.62 0.024 0.04 1.67 37.884
-12.8 19.7 Example 4 Inventive 1.1 23.4 1.05 0.87 0.021 0.05 2.38
49.36 13.5 35.2 Example 5
[0064] In Table 1 above, a unit of a content of each element is wt
%.
TABLE-US-00002 TABLE 2 Density of Temperature Coarse of Grain of
200 .mu.m Heating Austenite Carbide TiN or Furnace Fraction
Fraction Size No. of TiN greater Classification (.degree. C.) (%)
(%) (.mu.m) (No./mm.sup.2) (No./cm.sup.2) Comparative 1195 99.1 0.9
10 Example 1 Comparative 1180 99.6 0.4 7 Example 2 Comparative 1200
99 1 8 Example 3 Comparative 1195 98.9 0.8 0.003 1.2 .times.
10.sup.4 7 Example 4 Comparative 1200 82 1 1.25 4.32 .times.
10.sup.5 7 Example 5 Comparative 1195 99.6 0 0.95 5.2 .times.
10.sup.6 9 Example 6 Inventive 1205 99.1 0.8 0.013 5.3 .times.
10.sup.8 0 Example 1 Inventive 1190 99.3 0 0.015 4.2 .times.
10.sup.8 0 Example 2 Inventive 1195 99.4 0 0.022 2.9 .times.
10.sup.8 1 Example 3 Inventive 1198 99.6 0 0.01 5.4 .times.
10.sup.8 0 Example 4 Inventive 1203 98.7 0.8 0.025 2.7 .times.
10.sup.8 0 Example 5
TABLE-US-00003 TABLE 3 Base Metal Yield Tensile Impact Classifica-
Strength Strength Elongation Value Unevenness tion (MPa) (MPa) Rate
(%) (J, -196.degree. C.) of Surfaces Comparative 363 1011 69 83
Occurred Example 1 Comparative 470 931 46 130 Occurred Example 2
Comparative 405 1006 57 81 Occurred Example 3 Comparative 411 912
57 130 Occurred Example 4 Comparative 346 762 12 38 Occurred
Example 5 Comparative 360 926 54 35 Occurred Example 6 Inventive
425 980 67 153 Not Example 1 Occurred Inventive 453 902 58 148 Not
Example 2 Occurred Inventive 468 975 61 165 Not Example 3 Occurred
Inventive 427 980 65 152 Not Example 4 Occurred Inventive 481 971
51 118 Not Example 5 Occurred
[0065] In Inventive Examples 1 to 5, it can be confirmed that a
component system and a composition range controlled in an exemplary
embodiment are satisfied, and high-quality steel for low
temperature environments without uneven surfaces may be obtained in
such a manner that a density of a coarse austenite grain is
controlled to be 5 or less per 1 cm.sup.2 by minute eduction of
TiN, and Inventive Examples 1 to 5 are processed. In addition,
stable austenite in which fraction of austenite in the
microstructure is controlled to be 95% or higher, and fraction of
the carbide is controlled to be lower than 5% may be obtained,
thereby securing excellent toughness at extremely low
temperatures.
[0066] On the other hand, in Comparative Examples 1 to 3, it can be
confirmed that TiN may not be educed, since Ti is not added
thereto, thereby generating a coarse grain and unevenness of
surfaces after Comparative Examples 1 to 3 are processed.
[0067] In detail, in the case of Comparative Example 4, it can be
confirmed that, since the component system and the composition
range controlled in an exemplary embodiment are not satisfied,
ferrite is generated, thereby significantly degrading impact
toughness. In addition, it can be confirmed that, since a size and
the number of TiN controlled in an exemplary embodiment are not
satisfied, the number of coarse grains is increased, thereby
generating unevenness of surfaces.
[0068] In addition, in the case of Comparative Examples 5 to 6, it
can be confirmed that Ti and N within a range controlled in an
exemplary embodiment are added, but the weight ratio of Ti to N and
a size and the number of the TiN precipitate do not satisfy the
range controlled in an exemplary embodiment, so that coarse TiN is
educed, and the coarse grain is significantly generated to generate
unevenness of surfaces after Comparative Examples 5 to 6 are
processed.
[0069] FIG. 1A is an image of the microstructure of steel of the
related art in which a nonideal coarse grain is formed by
coarsening of the austenite grain. FIG. 1B is an image of
unevenness occurring on a surface of steel after steel of FIG. 1A
is tensioned. As such, it can be confirmed that, in a case in which
the austenite grain is coarsened to generate the nonideal coarse
grain in the microstructure of steel, surface qualities are
degraded after a process thereof as described in FIG. 1B. However,
in FIG. 2, illustrating an image of the microstructure of Inventive
Examples, uniform grains without a nonideal coarse austenite grain
is formed, thereby generating excellent surface processing
qualities even after the process thereof.
[0070] While exemplary embodiments have been shown and described
above, it will be apparent to those skilled in the art that
modifications and variations could be made without departing from
the scope of the present invention as defined by the appended
claims.
* * * * *