TECHNICAL FIELD
[0001] The disclosure relates to high-Mn steel that is suitable for structural steel used
in an extremely low-temperature environment such as a storage tank of liquefied gas,
in particular, high-Mn steel excellent in toughness at low temperature, and a production
method therefor.
BACKGROUND
[0002] To use a hot-rolled steel plate in a structure for a storage tank of liquefied gas,
the steel plate needs to have high strength and excellent toughness at extremely low
temperature because the structure is used at extremely low temperature. For example,
when the hot-rolled steel plate is used for a storage tank of liquefied natural gas,
excellent toughness needs to be guaranteed at the boiling point of the liquefied natural
gas, that is, -164 °C or lower. When a steel material has poor low-temperature toughness,
the safety as a structure for an extremely low temperature storage tank may not be
maintained. Thus, there is a growing demand for steel materials with improved low-temperature
toughness that are applied to such a structure.
[0003] In view of the demand, austenitic stainless steel which has austenite as a structure
of a steel plate, the austenite showing no brittleness at extremely low temperature,
9 % Ni steel, or five thousand series aluminum alloys have been conventionally used.
However, the alloy cost and production cost are high, and thus there is a demand for
steel materials which are inexpensive and excellent in extremely low-temperature toughness.
[0004] As new steel materials replacing conventional steel for extremely low temperature,
for example,
JP 2016-196703 A (PTL 1) proposes using, as structural steel used in an extremely low temperature
environment, high-Mn steel added with a large amount of Mn which is relatively inexpensive
and an austenite-stabilizing element.
[0005] PTL 1 proposes a technique of properly controlling the austenite grain size to prevent
carbides generated in crystal grain boundaries from becoming an origin of fracture
and a propagation path of cracks. The technique may provide high-Mn steel which exhibits
excellent low-temperature toughness in a base metal and a heat-affected zone after
welding.
CITATION LIST
Patent Literature
SUMMARY
(Technical Problem)
[0007] In an application such as the aforementioned structure for a storage tank of liquefied
gas, it is important to guarantee ductility in addition to low-temperature toughness.
Specifically, when such a structure is made, steel materials to be used in the structure
need to have high workability. Additionally, excellent ductility is required in such
an application. The technique of PTL 1, however, does not verify ductility. Further,
the high-Mn steel material of PTL 1 has a thickness of about 15 mm to 50 mm, but for
example, in an application such as a longitudinal member, the thickness is required
to be less than 15 mm, in particular 10 mm or less. When such a thin plate is produced,
the technique exemplified in paragraph 0040 of PTL 1 in which hot rolling at 950 °C
or more and subsequent accelerated cooling are performed tends to cause deflection
and strain in an obtained steel plate, which requires an extra process such as shape
adjustment, thus deteriorating productivity.
[0008] It could thus be helpful to propose a method of further imparting excellent ductility
to high-Mn steel exhibiting excellent low-temperature toughness of base metal and
heat-affected zone. Further, it could be helpful to propose a method of producing
a thin plate of such high-Mn steel without deflection or strain. As used herein, the
"exhibiting excellent low-temperature toughness" means that the absorbed energy vE
-196 °C in a Charpy impact test at -196 °C is 100 J or more.
(Solution to Problem)
[0009] To achieve the aforementioned objects, we conducted extensive study on high-Mn steel
as to various factors determining the chemical composition of a steel plate and a
production method therefor to discover the following:
- a. High-Mn austenite steel has no occurrence of brittle fracture at extremely low
temperature. Such fracture occurs from grain boundaries. This suggests that for improving
low-temperature toughness of high-Mn steel, it is effective to coarsen crystal grains
to thereby decrease grain boundaries which become an origin of fracture and a propagation
path.
- b. Further, we newly discovered that nonmetallic inclusions become an origin of fracture
and a propagation path of cracks to thus adversely affect low-temperature toughness
and ductility. Thus, by properly controlling the Cr content added to high-Mn steel,
and limiting the contents of Ti and Nb that are inevitably mixed, the increase in
crystal grain boundaries and the formation of excessive nonmetallic inclusions that
become an origin of fracture are avoid.
- c. On the other hand, simply coarsening of crystal grains deteriorates yield stress.
Further, when a thin plate with a thickness of less than 15 mm is produced, an obtained
steel plate tends to have deflection and strain. Therefore, to sufficiently guarantee
yield stress as structural steel and leave no deflection or strain in a steel plate,
it is necessary to properly control hot rolling conditions in producing a steel plate.
[0010] This disclosure is based on these discoveries and further investigation conducted
by the inventors. The primary features of this disclosure are as follows.
- 1. High-Mn steel comprising:
a chemical composition containing (consisting of), in mass%,
C: 0.30 % or more and 0.90 % or less,
Si: 0.05 % or more and 1.0 % or less,
Mn: 15 % or more and 30 % or less,
P: 0.030 % or less,
S: 0.0070 % or less,
Al: 0.01 % or more and 0.07 % or less,
Cr: 0.5 % or more and 7.0 % or less,
N: 0.0050 % or more and 0.0500 % or less, and
O: 0.0050 % or less,
Ti: less than 0.005 %, and
Nb: less than 0.005 %, with the balance being Fe and inevitable impurities;
a microstructure containing austenite as a matrix phase and nonmetallic inclusions
with an area fraction of less than 5.0 %; and
a yield stress of 400 MPa or more and an absorbed energy (vE-196) of 100 J or more.
The nonmetallic inclusions are nonmetallic inclusions in a structure test of JIS G0202,
and specifically refer to A type inclusions, B type inclusions, and C type inclusions
that are described in JIS G0202.
- 2. The high-Mn steel according to 1., wherein the chemical composition further contains,
in mass %, at least one selected from
Cu: 0.01 % or more and 1.00 % or less,
Ni: 0.01 % or more and 1.00 % or less,
Mo: 2.0 % or less,
V: 2.0 % or less,
W: 2.0 % or less,
Ca: 0.0005 % or more and 0.0050 % or less,
Mg: 0.0005 % or more and 0.0050 % or less, and
REM: 0.0010 % or more and 0.0200 % or less.
- 3. A production method of high-Mn steel comprising: heating a steel material having
the chemical composition according to 1. or 2. to a temperature range of 1100 °C or
more and 1300 °C or less; hot rolling the steel material with a rolling finish temperature
of 800 °C or more and less than 950 °C to obtain a hot-rolled steel plate; and then
subjecting the hot-rolled steel plate to cooling treatment at an average cooling rate
of 1.0 °C/s or more from a temperature at or above (the rolling finish temperature
- 100 °C) to a temperature range of 300 °C or more and 650 °C or less.
Each temperature range is a surface temperature of the steel material or the steel
plate.
- 4. The production method of high-Mn steel according to 3. comprising: after the cooling
treatment, heating the steel plate to a temperature range of 300 °C or more and 650
°C or less and then cooling the steel plate.
(Advantageous Effect)
[0011] According to the present disclosure, it is possible to provide high-Mn steel excellent
in low-temperature toughness and ductility. When the high-Mn steel is welded, both
a base metal and a heat-affected zone have excellent low-temperature toughness. Therefore,
our high-Mn steel largely contributes to the improvement of the safety and the service
life of a steel structure used in an extremely low temperature environment such as
a tank for a storage tank of liquefied gas, and has industrially significant effects.
Further, our production method does not decrease productivity or increase the production
cost, and thus is excellent in economic efficiency.
DETAILED DESCRIPTION
[0012] Our high-Mn steel will be described in detail below.
[Chemical Composition]
[0013] The chemical composition of our high-Mn steel and the reasons for the limitations
thereof are described first. In the description of the chemical composition, "%" denotes
"mass%" unless otherwise noted.
C: 0.30 % or more and 0.90 % or less
[0014] C is an inexpensive austenite-stabilizing element, and an important element to obtain
austenite. To obtain this effect, the C content needs to be 0.30 % or more. On the
other hand, a C content beyond 0.90 % generates excessive Cr carbides, deteriorating
low-temperature toughness. Therefore, the C content is set to 0.30 % or more and 0.90
% or less. In particular, from the perspective of stabilizing austenite, the lower
limit of the C content is preferably 0.36 %, and more preferably 0.40 %. Further,
from the perspective of preventing deterioration in low-temperature toughness, the
upper limit of the C content is preferably 0.80 %, and more preferably 0.66 %. For
the preferable C content, the upper limits and the lower limits can be arbitrarily
combined. Thus, for example, the C content is preferably set to 0.36 % or more and
0.80 % or less, and more preferably 0.40 % or more and 0.80 % or less.
Si: 0.05 % or more and 1.0 % or less
[0015] Si acts as a deoxidizer, is necessary for steelmaking, and is effective at increasing
hardness of a steel plate by solid solution strengthening when dissolved in steel.
To obtain such an effect, the Si content needs to be 0.05 % or more. On the other
hand, a Si content beyond 1.0 % deteriorates weldability. Therefore, the Si content
is set to 0.05 % or more and 1.0 % or less. In particular, from the perspective of
obtaining a steel plate with increased hardness, the lower limit of the Si content
is preferably 0.07 %, more preferably 0.23 %, further preferably 0.26 %, and still
further preferably 0.51 %. Further, from the perspective of preventing deterioration
in weldability, the upper limit of the Si content is set to 0.8 %, more preferably
0.7 %, further preferably 0.6 %, and still further preferably 0.5 %. For the preferable
Si content, the upper limit and the lower limit can be combined. Thus, for example,
the Si content is preferably set to 0.07 % or more and 0.8 % or less, 0.23 % or more
and 0.7 % or less, and more preferably 0.26 % or more and 0.6 % or less. Further,
the Si content is preferably 0.07 % or more and 0.5 % or less.
Mn: 15.0 % or more and 30.0 % or less
[0016] Mn is a relatively inexpensive austenite-stabilizing element. In the disclosure,
Mn is an important element for achieving both strength and extremely low-temperature
toughness. To obtain such effects, the Mn content needs to be 15.0 % or more. On the
other hand, a Mn content beyond 30.0 % does not increase the effect of improving extremely
low-temperature toughness, but increases alloy cost. Further, such a high Mn content
deteriorates weldability and cuttability, and further promotes segregation as well
as the occurrence of stress corrosion cracking. Therefore, the Mn content is set to
15.0 % or more and 30.0 % or less. In particular, from the perspective of stabilizing
austenite, the lower limit of the Mn content is preferably 16.0 %, more preferably
18.0 %, and further preferably 19.0 %. Further, from the perspective of preventing
deterioration in the low-temperature toughness, the upper limit of the Mn content
is preferably 29.0 %, and more preferably 28.0 %. For the preferable Mn content, the
upper limits and the lower limits can be arbitrarily combined. Thus, for example,
the Mn content is preferably set to 16.0 % or more and 29.0 % or less, and more preferably
18.0 % or more and 28.0 % or less.
P: 0.030 % or less
[0017] When a P content is beyond 0.030 %, P segregates to grain boundaries and becomes
an origin of stress corrosion cracking. Therefore, the upper limit of the P content
is 0.030 %, and desirably, the P content is kept as small as possible. Therefore,
the P content is set to 0.030 % or less. Further, from the perspective of decreasing
the origin of stress corrosion cracking, the upper limit of the P content is preferably
0.028 % or less, and more preferably 0.024 % or less. Excessively reducing P, however,
involves high refining cost and is economically disadvantageous. Therefore, the lower
limit of the P content is preferably set to 0.002 %, and more preferably 0.005 %.
S: 0.0070 % or less
[0018] S deteriorates base metal low-temperature toughness and ductility. Therefore, the
upper limit of the S content is 0.0070 %, and desirably, the S content is kept as
small as possible. Therefore, the S content is set to 0.0070 % or less. From the perspective
of preventing deterioration in base metal low-temperature toughness and ductility,
the upper limit of the S content is preferably 0.0060 % or less. Excessively reducing
S, however, involves high refining cost and is economically disadvantageous. Therefore,
the lower limit of the S content is preferably set to 0.001 % or more. The S content
is preferably set to 0.0020 % or more and 0.0060 % or less.
Al: 0.01 % or more and 0.07 % or less
[0019] Al acts as a deoxidizer and is used most commonly in molten steel deoxidizing processes
to obtain a steel plate. To obtain such an effect, the Al content needs to be 0.01
% or more. On the other hand, when the Al content is beyond 0.07 %, Al is mixed into
a weld metal portion during welding, deteriorating toughness of the weld metal. Therefore,
the Al content is set to 0.07 % or less. Therefore, the Al content is set to 0.01
% or more and 0.07 % or less. In particular, from the perspective of obtaining an
effect as a deoxidizer, the lower limit of the Al content is preferably 0.02 %, more
preferably 0.046 %, and further preferably 0.052 %. Further, from the perspective
of toughness of weld metal, the upper limit of the Al content is preferably set to
0.065 %, and more preferably 0.06 %. For the Al content, the upper limit and the lower
limit can be combined. Thus, for example, the Al content is preferably set to 0.02
% or more and 0.06 % or less.
Cr: 0.5 % or more and 7.0 % or less
[0020] Cr is an element which stabilizes austenite with an appropriate amount of addition
and is effective at improving extremely low-temperature toughness and base metal strength.
To obtain such effects, the Cr content needs to be 0.5 % or more. On the other hand,
a Cr content beyond 7.0 % generates Cr carbides, deteriorating low-temperature toughness
and stress corrosion cracking resistance. Therefore, the Cr content is set to 0.5
% or more and 7.0 % or less. In particular, from the perspective of improving extremely
low-temperature toughness and base metal strength, the lower limit of the Cr content
is preferably 1 % or more, more preferably 1.2 %, and further preferably 2.0 %. Further,
from the perspective of low-temperature toughness and stress corrosion cracking resistance,
the upper limit of the Cr content is preferably set to 6.7 % or less, more preferably
6.5 % or less, and further preferably 6.0 %. For the preferable Cr content, the upper
limits and the lower limits can be arbitrarily combined. Thus, for example, the Cr
content is preferably set to 1.0 % or more and 6.7 % or less, and more preferably
1.2 % or more and 6.5 % or less. To further improve stress corrosion cracking resistance,
the Cr content is further preferably 2.0 % or more and 6.0 % or less.
N: 0.0050 % or more and 0.0500 % or less
[0021] N is an austenite-stabilizing element and an element which is effective at improving
extremely low-temperature toughness. To obtain such an effect, the N content needs
to be 0.0050 % or more. On the other hand, the N content beyond 0.0500 % coarsens
nitrides or carbonitrides, deteriorating toughness. Therefore, the N content is set
to 0.0050 % or more and 0.0500 % or less. In particular, from the perspective of improving
extremely low-temperature toughness, the lower limit of the N content is preferably
0.0060 % or more, more preferably 0.0355 %, and further preferably 0.0810 %. Further,
from the perspective of preventing deterioration in toughness, the upper limit of
the N content is preferably set to 0.0450 % or less, and more preferably 0.0400 %
or less. For the preferable N content, the upper limits and the lower limits of the
N content can be arbitrarily combined. Thus, for example, the N content is preferably
set to 0.0060 % or more and 0.0400 % or less.
O: 0.0050 % or less
[0022] O deteriorates extremely low-temperature toughness because of the formation of oxides.
Therefore, the O content is set to 0.0050 % or less. From the perspective of preventing
deterioration in toughness, the upper limit of the O content is preferably 0.0045
% or less. Further, the lower limit of the O content is preferably 0.0023 % or more.
For the preferable O content, the upper limits and the lower limits of the O content
can be arbitrarily combined. Thus, for example, the O content is preferably set to
0.0023 % or more and 0.0050 % or less.
Ti: less than 0.005 %, and Nb: less than 0.005 %
[0023] Ti and Nb form carbonitrides with a high melting point in steel to prevent coarsening
of crystal grains, then becoming an origin of fracture and a propagation path of cracks.
In particular, in high-Mn steel, Ti and Nb hinder structure control for enhancing
low-temperature toughness and improving ductility, and thus, need to be intentionally
limited. Specifically, Ti and Nb are components which is inevitably mixed from raw
materials into steel, and Ti of 0.005 % or more and 0.010 % or less and Nb of 0.005
% or more and 0.010 % or less are typically mixed. Thus, according to the method described
below, it is necessary to avoid inevitable mixing of Ti and Nb and limit the content
of each Ti and Nb to less than 0.005 %. By limiting the content of each Ti and Nb
to less than 0.005 % to eliminate the adverse effect of carbonitrides, excellent low-temperature
toughness and ductility can be guaranteed. Therefore, from the perspective of excellent
low-temperature toughness and ductility, the content of each Ti and Nb is preferably
set to 0.004 % or less, and more preferably 0.003 % or less.
[0024] The balance other than the aforementioned components includes Fe and inevitable impurities.
The inevitable impurities include H, and a total content of 0.01 % may be allowed.
[Structure]
Microstructure which has austenite as its matrix phase
[0025] When a steel material has a body centered cubic (bcc) crystal structure, the steel
material may cause brittle fracture in a low temperature environment, and thus, is
not suitable for use in a low temperature environment. When the steel material is
assumed to be used in a low temperature environment, the steel material is required
to have, as its matrix phase, an austenite structure which has a face centered cubic
(fcc) crystal structure. As used herein, "have austenite as its matrix phase" means
that area ratio of austenite phase is 90 % or more. When area ratio of austenite phase
is 90 % or more, low-temperature toughness can be guaranteed. The balance other than
austenite phase is ferrite or martensite phase. However, even a small amount of ε
martensite deteriorates low-temperature toughness. Thus, as the microstructure which
has austenite as a matrix phase according to the disclosure, a structure is preferable
which substantially has no ε martensite phase. Specifically, to guarantee low-temperature
toughness, volume fraction of ε martensite is preferably less than 1.0 %, more preferably
less than 0.5 %, and further preferably less than 0.1 %.
Area ratio of nonmetallic inclusions: less than 5.0 %
[0026] As to nonmetallic inclusions, A type means sulfide inclusions, B type means cluster-type
inclusions, and C type means granular oxide inclusions. When a large amount of these
nonmetallic inclusions exist in steel, they become an origin of fracture, deteriorating
extremely low-temperature toughness and ductility. Therefore, area fraction of the
inclusions in total needs to be limited to 5 % or less, and preferably 4 % or less.
Accordingly, the control of the chemical composition as stated above and the following
production method need to be performed.
[0027] Further, when area ratio of austenite phase is 90 % or more and area fraction of
nonmetallic inclusions is less than 5.0 %, steel exhibiting extremely low-temperature
toughness and good ductility can be provided.
[0028] When the above requirements are satisfied, properties desired in the disclosure can
be obtained. For example, when high-Mn steel is subjected to welding treatment, low-temperature
toughness of a heat-affected zone is particularly a matter of concern. However, when
high-Mn steel satisfying the aforementioned requirements is used, the microstructure
of the heat-affected zone has austenite as its matrix phase, and the grain size of
the austenite is an equivalent circular diameter of 50 µm or more. Thus, low-temperature
toughness is guaranteed in the heat-affected zone.
[0029] Specifically, coarsening of crystal grains is effective at guaranteeing low-temperature
toughness in austenite steel. The same applies to a heat-affected zone. For example,
to obtain a value of 100 J or more as an absorbed energy of a Charpy impact test at
-196 °C, the biggest crystal grain size of the microstructure needs to be 50 µm or
more, which is achieved by using high-Mn steel satisfying the aforementioned requirements.
[0030] In the disclosure, to further improve strength and low-temperature toughness, in
addition to the essential elements described above, the following elements can be
contained as necessary.
At least one of Cu: 0.01 % or more and 1.00 % or less, Ni: 0.01 % or more and 1.00
% or less, Mo: 2.0 % or less, V: 2.0 % or less, W: 2.0 % or less, Ca: 0.0005 % or
more and 0.0050 % or less, Mg: 0.0005 % or more and 0.0050 % or less, and REM: 0.0010
% or more and 0.0200 % or less.
Cu: 0.01 % or more and 1.00 % or less, Ni: 0.01 % or more and 1.00 % or less, Mo,
V, and W: 2.0 % or less
[0031] Cu, Ni, Mo, V, and W stabilize austenite and improve base metal strength. To obtain
such an effect, Cu and Ni are each preferably contained in an amount of 0.01% or more,
and Mo, V, and W are each preferably contained in an amount of 0.001 % or more. On
the other hand, when the content of each Cu and Ni is beyond 1.00 %, or the content
of each Mo, V, and W is beyond 2.0 %, coarse carbonitrides are generated, which may
become an origin of fracture, and additionally increase production cost. Therefore,
when these alloying elements are contained, the content of each Cu and Ni is preferably
1.00 % or less, and the content of each Mo, V, and W is preferably 2.0 % or less.
The content of each Cu and Ni is more preferably 0.05 % or more and 0.70 % or less.
The content of each Mn, V, and W is more preferably 0.003 % or more and 1.7 % or less.
Ca: 0.0005 % or more and 0.0050 % or less, Mg: 0.0005 % or more and 0.0050 % or less,
REM: 0.0010 % or more and 0.0200 % or less
[0032] Ca, Mg, and REM are elements useful for morphological control of inclusions, and
can be contained as necessary. The morphological control of inclusions means granulating
elongated sulfide-based inclusions. The morphological control of inclusions improves
ductility, toughness, and sulfide stress corrosion cracking resistance. To obtain
such effects, Ca and Mg are preferably contained in an amount of 0.0005 % or more
and REM is preferably contained in an amount of 0.0010 % or more. On the other hand,
when these elements are contained in a large amount, not only the amount of nonmetallic
inclusions may be increased, ending up deteriorating ductility, toughness, and sulfide
stress corrosion cracking resistance, but also an economic disadvantage may be entailed.
[0033] Therefore, when Ca and Mg are contained, the content of each element is set to 0.0005
% or more and 0.0050 % or less. When REM is contained, the content is set to 0.0010
% or more and 0.0200 % or less. Preferably, the Ca content is set to 0.0010 % or more
and 0.0040 % or less, the Mg content is set to 0.0010 % or more and 0.0040 % or less,
and the REM content is set to 0.0020 % or more and 0.0150 % or less.
[0034] Our high-Mn steel can be obtained from molten steel having the aforementioned chemical
composition which is prepared by steelmaking using a publicly-known method such as
using a converter and an electric heating furnace. In addition, the high-Mn steel
may also be subjected to secondary refinement in a vacuum degassing furnace. During
the secondary refinement, to limit the contents of Ti and Nb which hinder suitable
structure control within the aforementioned range, it is necessary to prevent Ti and
Nb from being inevitably mixed from raw materials or the like into steel and decrease
the contents of Ti and Nb. For example, by decreasing the basicity of slag in the
refining stage, these alloy elements are concentrated in the slag to be discharged,
thus decreasing Ti and Nb concentrations in a final slab product. Further, a method
may be used in which oxygen is injected for oxidation, floating and separating alloy
of Ti and Nb during circulation. Subsequently, it is preferable to make the steel
into a steel material such as a slab having a determined size by a publicly-known
steel making method such as continuous casting or ingot casting and blooming.
[0035] Further, production conditions are defined to make the aforementioned steel material
into a steel material exhibiting excellent low-temperature toughness.
Heating temperature of a steel material: 1100 °C or more and 1300 °C or less
[0036] To coarsen crystal grains of the microstructure of the steel material, the heating
temperature before hot rolling is set to 1100 °C or more. Further, when the lower
limit of the heating temperature of the steel material is less than 1100 °C, the amount
of nonmetallic inclusions in steel is increased, thus deteriorating extremely low-temperature
toughness and ductility. However, the heating temperature beyond 1300 °C may trigger
local melting. Thus, the upper limit of the heating temperature is set to 1300 °C.
The temperature is controlled based on surface temperature of the steel material.
Rolling finish temperature: 800 °C or more and less than 950 °C
[0037] A steel ingot or a billet is heated and subsequently subjected to hot rolling. To
make coarse crystal grains, it is preferable to increase the cumulative rolling reduction
at high temperature. However, the crystal grains become excessively coarse in a temperature
range of 950 °C or more, and thus desired yield stress cannot be obtained. Therefore,
final finish rolling of at least one pass at less than 950 °C is necessary. On the
other hand, hot rolling at low temperature refines the microstructure and excessively
introduces working strain, thus deteriorating low-temperature toughness. Therefore,
the lower limit of the rolling finishing temperature is set to 800 °C.
[0038] Average cooling rate from a temperature at or above (rolling finish temperature -
100 °C) to a temperature range of 300 °C or more and 650 °C or less: 1.0 °C/s or more
[0039] After the hot rolling, cooling is immediately performed. Slowly cooling of the steel
plate after the hot rolling promotes formation of precipitates, thus deteriorating
low-temperature toughness. Cooling the steel plate at a cooling rate of 1 °C/s or
more can prevent formation of these precipitates. Further, excessive cooling distorts
the steel plate, deteriorating productivity. Therefore, the upper limit of cooling
start temperature is set to 900 °C. For the aforementioned reasons, in the cooling
after the hot rolling, the average rate of cooling a surface of the steel plate from
a temperature at or above (rolling finish temperature - 100 °C) to a temperature range
of 300 °C or more and 650 °C or less is set to 1.0 °C/s or more. In the case of air
cooling a thick steel plate having a thickness of 10 mm or less, the cooling rate
is 1 °C/s or more. By air cooling a steel plate having a thickness of 10 mm or less,
the steel plate can be free from strain.
[0040] Further, after the cooling treatment, treatment of heating to a temperature range
of 300 °C or more and 650 °C or less and then cooling can be added as necessary. Specifically,
tempering treatment may be performed to adjust strength of the steel plate.
EXAMPLES
[0041] The present disclosure will be described in further detail below by way of Examples.
Note that the present disclosure is not limited to the following Example.
[0042] Steel slabs having the chemical compositions listed in Table 1 were made by a process
for refining with converter and ladle and continuous casting. Next, the obtained steel
slabs were charged into a heating furnace and heated to 1150 °C, and subsequently,
hot rolled into steel plates having a thickness of 10 mm to 30 mm. Tensile properties
and toughness of the steel plates were evaluated as described below.
(1) Tensile test properties
[0043] JIS NO. 5 tensile test pieces were collected from each steel plate. Then, the test
pieces were subjected to a tensile test in conformity with JIS Z 2241 (1998) to investigate
tensile test properties. In the disclosure, when a test piece had a yield stress of
400 MPa or more and a tensile strength of 800 MPa or more, the corresponding steel
plate was determined to have excellent tensile properties. Further, when a test piece
had a total elongation of 30 % or more upon breakage, the corresponding steel plate
was determined to have excellent ductility.
(2) Low-temperature toughness
[0044] Charpy V-notch test pieces were collected from each steel plate having a plate thickness
of more than 20 mm at a 1/4 position of the plate thickness or from each steel plate
having a plate thickness of 20 mm or less at a 1/2 position of the plate thickness,
in a direction orthogonal to the rolling direction in conformity with JIS Z 2202 (1998).
Then, the test pieces were subjected to Charpy impact test in conformity with JIS
Z 2242 (1998), where three test pieces were used for each steel plate, to determine
absorbed energy at -196 °C and evaluate base metal toughness. When the three test
pieces had an average absorbed energy (vE-
196) of 100 J or more, the corresponding steel plate was determined to have good base
steel toughness.
[0045] The above-described evaluation results are listed in Table 2.
Table 2
Sample No. |
Steel No. |
Sheet thickness |
Production method |
Base metal properties |
Base metal toughness |
HAZ portion |
Remarks |
Slab heating temperature |
Rolling finish temperature |
Cooling start temperature |
Cooling rate (*) |
Re-hcatmg temperatire |
Area ratio of inclusions |
Volume fraction of ε martensie |
Yield stress |
Tensile strength |
Total elongation |
Absorbed energy at -196 °C (vE-196°C) |
Absorbed energy at -196 °C (vE-196°C) |
Largest grain size |
(mm) |
(°C) |
(°C) |
(°C) |
(°C/s) |
(°C) |
(%) |
(%) |
(MPa) |
(MPa) |
(%) |
(J) |
(J) |
(µm) |
1 |
1 |
10 |
1110 |
945 |
872 |
13 |
450 |
0.2 |
0.0 |
480 |
912 |
60 |
100 |
145 |
165 |
Example |
2 |
2 |
15 |
1110 |
835 |
768 |
7 |
- |
4.0 |
0.0 |
510 |
934 |
62 |
108 |
100 |
175 |
Example |
3 |
3 |
20 |
1100 |
890 |
815 |
12 |
- |
2.0 |
0.0 |
487 |
912 |
61 |
113 |
112 |
150 |
Example |
4 |
4 |
25 |
1100 |
910 |
845 |
10 |
- |
0.2 |
0.0 |
446 |
891 |
61 |
124 |
134 |
187 |
Example |
5 |
5 |
10 |
1150 |
860 |
795 |
12 |
500 |
0.2 |
0.1 |
467 |
905 |
62 |
125 |
121 |
50 |
Example |
6 |
6 |
15 |
1150 |
890 |
808 |
5 |
550 |
0.3 |
0.0 |
442 |
899 |
61 |
133 |
129 |
102 |
Example |
7 |
7 |
20 |
1150 |
825 |
785 |
7 |
- |
0.2 |
0.0 |
445 |
903 |
61 |
122 |
128 |
100 |
Example |
8 |
8 |
5 |
1200 |
825 |
740 |
4 |
- |
1.0 |
0.0 |
598 |
987 |
51 |
106 |
108 |
50 |
Example |
9 |
9 |
10 |
1200 |
810 |
730 |
10 |
450 |
0.1 |
0.0 |
484 |
858 |
65 |
112 |
115 |
100 |
Example |
10 |
10 |
15 |
1200 |
870 |
798 |
10 |
- |
0.3 |
0.0 |
466 |
884 |
64 |
127 |
124 |
150 |
Example |
11 |
11 |
15 |
1250 |
860 |
795 |
9 |
500 |
20 |
0.0 |
487 |
840 |
66 |
114 |
106 |
100 |
Example |
12 |
12 |
20 |
1250 |
915 |
860 |
13 |
550 |
0.5 |
0.0 |
435 |
850 |
67 |
113 |
113 |
168 |
Example |
13 |
13 |
25 |
1250 |
860 |
805 |
6 |
- |
1.5 |
0.2 |
441 |
852 |
65 |
109 |
110 |
125 |
Example |
14 |
14 |
30 |
1250 |
815 |
744 |
3 |
300 |
0.1 |
0.0 |
426 |
853 |
61 |
132 |
127 |
109 |
Example |
15 |
15 |
10 |
1300 |
870 |
821 |
1 |
- |
0.1 |
0.0 |
422 |
874 |
61 |
104 |
108 |
115 |
Example |
16 |
16 |
15 |
1300 |
954 |
784 |
7 |
450 |
1.0 |
0.0 |
501 |
926 |
57 |
116 |
114 |
89 |
Example |
17 |
17 |
20 |
1300 |
884 |
810 |
20 |
- |
1.5 |
0.0 |
512 |
934 |
52 |
112 |
116 |
97 |
Example |
18 |
18 |
10 |
1150 |
895 |
810 |
5 |
- |
2.0 |
0.0 |
512 |
922 |
58 |
56 |
61 |
125 |
Comparative Example |
19 |
19 |
15 |
1150 |
815 |
735 |
15 |
- |
0.1 |
1.0 |
471 |
879 |
57 |
121 |
61 |
95 |
Comparative Example |
20 |
20 |
20 |
1150 |
862 |
805 |
5 |
- |
0.5 |
0.0 |
361 |
701 |
51 |
45 |
44 |
80 |
Comparative Example |
21 |
21 |
25 |
1150 |
871 |
820 |
13 |
- |
0.1 |
0.0 |
425 |
871 |
61 |
24 |
26 |
80 |
Comparative Example |
22 |
22 |
10 |
1150 |
900 |
815 |
10 |
- |
4.0 |
0.0 |
497 |
922 |
34 |
46 |
48 |
105 |
Comparative Example |
23 |
23 |
15 |
1150 |
850 |
815 |
5 |
- |
0.2 |
0.5 |
455 |
906 |
60 |
51 |
53 |
65 |
Comparative Example |
24 |
24 |
20 |
1200 |
840 |
770 |
11 |
- |
0.2 |
0.0 |
375 |
716 |
61 |
78 |
65 |
60 |
Comparative Example |
25 |
25 |
5 |
1200 |
920 |
850 |
10 |
- |
5.5 |
0.0 |
467 |
900 |
30 |
42 |
41 |
126 |
Comparative Example |
26 |
26 |
10 |
1200 |
875 |
805 |
5 |
- |
4.5 |
0.0 |
525 |
927 |
54 |
67 |
68 |
78 |
Comparative Example |
27 |
27 |
15 |
1200 |
855 |
790 |
12 |
- |
0.3 |
0.0 |
511 |
916 |
58 |
65 |
41 |
65 |
Comparative Example |
28 |
28 |
15 |
1150 |
896 |
853 |
5 |
- |
3.5 |
0.0 |
456 |
897 |
29 |
70 |
66 |
100 |
Comparative Example |
29 |
29 |
15 |
1150 |
916 |
848 |
4 |
- |
0.5 |
0.0 |
432 |
875 |
25 |
43 |
46 |
125 |
Comparative Example |
30 |
30 |
15 |
1150 |
898 |
805 |
9 |
- |
0.1 |
0.0 |
465 |
912 |
60 |
60 |
58 |
110 |
Comparative Example |
31 |
31 |
15 |
1150 |
908 |
821 |
11 |
- |
0.2 |
0.0 |
448 |
905 |
35 |
47 |
51 |
105 |
Comparative Example |
32 |
32 |
15 |
1150 |
850 |
785 |
4 |
- |
0.1 |
0.0 |
487 |
932 |
55 |
79 |
68 |
75 |
Comparative Example |
33 |
33 |
15 |
1150 |
806 |
726 |
6 |
- |
0.2 |
0.2 |
512 |
936 |
45 |
59 |
61 |
35 |
Comparative Example |
34 |
34 |
15 |
1150 |
827 |
768 |
4 |
|
0.2 |
0.0 |
487 |
909 |
45 |
78 |
76 |
40 |
Comparative Example |
35 |
35 |
15 |
1150 |
878 |
804 |
3 |
- |
0.1 |
0.0 |
439 |
887 |
51 |
69 |
69 |
40 |
Comparative Example |
36 |
36 |
15 |
1150 |
851 |
776 |
7 |
- |
0.1 |
0.0 |
466 |
889 |
43 |
51 |
51 |
35 |
Comparative Example |
37 |
1 |
20 |
1050 |
815 |
724 |
8 |
- |
5.5 |
0.0 |
560 |
1064 |
42 |
78 |
91 |
165 |
Comparative Example |
38 |
2 |
25 |
1250 |
960 |
885 |
9 |
- |
3.5 |
0.1 |
367 |
724 |
61 |
116 |
108 |
175 |
Comparative Example |
39 |
3 |
30 |
1250 |
780 |
695 |
14 |
- |
2.0 |
0.0 |
598 |
987 |
52 |
54 |
81 |
150 |
Comparative Example |
40 |
4 |
10 |
1250 |
835 |
715 |
10 |
- |
0.2 |
0.0 |
437 |
879 |
28 |
65 |
70 |
187 |
Comparative Example |
41 |
5 |
10 |
1250 |
975 |
889 |
8 |
- |
0.2 |
0.0 |
375 |
786 |
64 |
126 |
124 |
50 |
Comparative Example |
42 |
6 |
15 |
1250 |
850 |
800 |
0.5 |
- |
0.3 |
0.0 |
412 |
861 |
28 |
55 |
67 |
102 |
Comparative Example |
43 |
7 |
20 |
1250 |
915 |
865 |
9 |
800 |
0.2 |
0.0 |
316 |
704 |
45 |
119 |
112 |
100 |
Comparative Example |
Note: Underlies are out of the scope of the disclosure.
(*) an average cooling rate from a temperature at or above (rolling finish temperature
- 100 °C) to a temperature range of 300 °C or more and 650 °C or less |
[0046] High-Mn steel of the disclosure was confirmed to satisfy the aforementioned desired
performance (a base metal yield stress of 400 MPa or more, a total elongation upon
breakage of 30 % or more, an average of absorbed energy (vE-
196) of 100 J or more for low-temperature toughness). On the other hand, comparative
examples out of the scope of the disclosure did not satisfy the aforementioned desired
performance in terms of any one or more of total elongation, yield stress, and low-temperature
toughness.
[0047] Further, to evaluate impact absorbed properties of a welded portion, the steel materials
were subjected to heat cycle treatment under conditions of peak temperature of 1400
°C and a cooling rate of 10 °C/s. The results are listed in Table 2. As illustrated
in Table 2, the steel materials according to the disclosure were excellent in welded
portion toughness as well as in base metal toughness at low temperature. Specifically,
in welding which gave input heat of 0.5 kJ/cm to 5 kJ/cm, the maximum crystal grain
size was 50 µm or more, and the absorbed energy in Charpy impact test at -196 °C was
100 J or more.