U.S. patent application number 17/437505 was filed with the patent office on 2022-05-19 for steel plate and method for manufacturing the same.
This patent application is currently assigned to JFE STEEL CORPORATION. The applicant listed for this patent is JFE STEEL CORPORATION. Invention is credited to Shigeki KITSUYA, Shusaku OTA, Yuya SATO, Tomoyuki YOKOTA.
Application Number | 20220154303 17/437505 |
Document ID | / |
Family ID | |
Filed Date | 2022-05-19 |
United States Patent
Application |
20220154303 |
Kind Code |
A1 |
SATO; Yuya ; et al. |
May 19, 2022 |
STEEL PLATE AND METHOD FOR MANUFACTURING THE SAME
Abstract
An object is to provide a steel plate having excellent
deformability in the central portion in the thickness direction and
a method for manufacturing the steel plate. A steel plate having a
chemical composition containing, by mass %, C: 0.01% to 0.15%, Si:
0.01% to 1.00%, Mn: 0.10% to 2.00%, P: 0.010% or less, S: 0.0050%
or less, Al: 0.002% to 0.100%, Ni: 5.0% to 10.0%, N: 0.0010% to
0.0080%, and with a balance being Fe and incidental impurities and
a percentage reduction of area of 30% or more in a tensile test in
the thickness direction performed on the central portion in the
thickness direction.
Inventors: |
SATO; Yuya; (Tokyo, JP)
; KITSUYA; Shigeki; (Tokyo, JP) ; OTA;
Shusaku; (Tokyo, JP) ; YOKOTA; Tomoyuki;
(Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE STEEL CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
JFE STEEL CORPORATION
Tokyo
JP
|
Appl. No.: |
17/437505 |
Filed: |
February 25, 2020 |
PCT Filed: |
February 25, 2020 |
PCT NO: |
PCT/JP2020/007377 |
371 Date: |
September 9, 2021 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C22C 38/22 20060101 C22C038/22; C22C 38/16 20060101
C22C038/16; C22C 38/14 20060101 C22C038/14; C22C 38/10 20060101
C22C038/10; C22C 38/08 20060101 C22C038/08; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; C21D 8/02 20060101
C21D008/02; C21D 6/00 20060101 C21D006/00; B21B 3/02 20060101
B21B003/02 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 13, 2019 |
JP |
2019-045433 |
Claims
1. A steel plate having a chemical composition comprising, by mass
%: C: 0.01% to 0.15%, Si: 0.01% to 1.00%, Mn: 0.10% to 2.00%, P:
0.010% or less, S: 0.0050% or less, Al: 0.002% to 0.100%, Ni: 5.0%
to 10.0%, N: 0.0010% to 0.0080%, and a balance being Fe and
incidental impurities, wherein when a test specimen of the steel
plate is subjected to a tensile test along a thickness direction of
the test specimen, a percentage reduction of an area of the test
specimen is 30% or more.
2. The steel plate according to claim 1, wherein the chemical
composition further comprises, by mass %, at least one group
selected from the group consisting of Group A and Group B: Group A:
at least one selected from the group consisting of: Cr: 0.01% to
1.50%, Mo: 0.03% to 1.0%, Nb: 0.001% to 0.030%, V: 0.01% to 0.10%,
Ti: 0.003% to 0.050%, B: 0.0003% to 0.0100%, and Cu: 0.01% to
1.00%, and Group B: at least one selected from the group consisting
of: Sn: 0.01% to 0.30%, Sb: 0.01% to 0.30%, W: more than 0% to
2.00%, Co: more than 0% to 2.00%, Ca: 0.0005% to 0.0050%, Mg:
0.0005% to 0.0050%, Zr: 0.0005% to 0.0050%, and REM: 0.0010% to
0.0100%.
3. (canceled)
4. A method for manufacturing the steel plate according to claim 1,
the method comprising heating a slab having the chemical
composition to a temperature that is in a range of 1000.degree. C.
or higher and 1300.degree. C. or lower and hot rolling the heated
slab, including finish rolling at a reduction ratio of 3 or more
and, and performing that each at least two rolling passes of final
three rolling passes at a rolling shape factor of 0.7 or more.
5. A method for manufacturing the steel plate according to claim 2,
the method comprising heating a slab having the chemical
composition to a temperature that is in a range of 1000.degree. C.
or higher and 1300.degree. C. or lower and hot rolling the heated
slab, including finish rolling at a reduction ratio of 3 or more,
and performing at least two rolling passes of final three rolling
passes at a rolling shape factor of 0.7 or more.
Description
[0001] The present application relates to a steel plate which can
preferably be used as structural steel used in a cryogenic
environment for, for example, a liquefied gas storage tank and to a
method for manufacturing the steel plate. In particular, the
disclosed embodiments relate to a steel plate which is excellent in
terms of mechanical properties and, especially, deformability in
the central portion in the thickness direction and to a method for
manufacturing the steel plate. Here, in the present application,
the term "steel plate" denotes a steel plate having a thickness of
6 mm to 80 mm.
BACKGROUND
[0002] A steel plate which is used in a cryogenic environment for,
for example, a liquefied gas storage tank is required to have not
only satisfactory strength as a steel plate but also toughness at
cryogenic temperatures. For example, in the case where a steel
plate is used for a liquefied natural gas (LNG) storage tank, it is
necessary to achieve excellent toughness at temperatures equal to
or lower than -164.degree. C., which is the boiling point of LNG.
In the case where the steel material is poor in terms of
low-temperature toughness, there is a risk in that it is not
possible to maintain the safety of a structure used as a cryogenic
storage tank. Therefore, the steel plate to be used is strongly
required to have improved low-temperature toughness. In response to
such a requirement, a Ni-containing steel plate having a retained
austenite microstructure such as a 7% Ni steel plate or a 9% Ni
steel plate, in which embrittlement does not occur at cryogenic
temperatures, is used.
[0003] To achieve excellent low-temperature toughness, Patent
Literature 1 discloses a method for stabilizing a retained
austenite microstructure, which tends to be unstable at low
temperatures, by decreasing the grain size of untransformed
austenite and by generating lattice defects to decrease the Mf
temperature. In addition, Patent Literature 2 discloses steel for
cryogenic temperatures having an excellent CTOD property in a
welded heat affected zone including a weld toe obtained by
adjusting the contents of Si, Al, and N and by controlling the Fe
content in residue after a simulated thermal recycle test has been
performed. In addition, Patent Literature 3 discloses a steel plate
whose yield strength, tensile strength, and toughness at cryogenic
temperatures are equal to or higher than predetermined values and
which is excellent in terms of safety against fracturing and a
method for manufacturing the steel plate.
CITATION LIST
Patent Literature
[0004] PTL 1: International Publication No. 2007/034576
[0005] PTL 2: International Publication No. 2007/080646
[0006] PTL 3: Japanese Unexamined Patent Application [0007]
Publication No. 2011-241419
SUMMARY
Technical Problem
[0008] For example, in the case of a structure used as a cryogenic
storage tank which is used in a cryogenic environment, a T-shaped
joint is formed around a weld zone between a bottom panel and a
side panel. Since stress applied to a steel material increases with
an increase in the size of the tank, the steel material is required
to have satisfactory deformability in the thickness direction from
the viewpoint of safety. Therefore, achieving satisfactory
deformability in the central portion in the thickness direction, in
which such a property tends to be particularly poor, is
required.
[0009] However, in the case of conventional Ni-containing steel
plates including those according to Patent Literature 1 to Patent
Literature 3, since no consideration is given to deformability in
the central portion in the thickness direction, it may be said that
deformability in the central portion in the thickness direction is
not sufficiently achieved.
[0010] The disclosed embodiments have been completed in view of
such a problem, and an object of the disclosed embodiments is to
provide a steel plate having excellent deformability in the central
portion in the thickness direction and a method for manufacturing
the steel plate.
Solution to Problem
[0011] To achieve the object described above, the present inventors
diligently conducted investigations regarding the chemical
composition and manufacturing method of a Ni-- containing steel
plate which can preferably be used as structural steel for use in a
cryogenic environment and, as a result, established the following
knowledge.
[0012] 1. By controlling a percentage reduction of area in a
tensile test in the thickness direction performed on the central
portion in the thickness direction, it is possible to improve the
deformability of the central portion in the thickness
direction.
[0013] 2. In finish rolling in a hot rolling process, by
controlling a reduction ratio to be 3 or more and by controlling a
rolling shape factor to be 0.7 or more in each of at least two
rolling passes of the final three rolling passes, since it is
possible to inhibit the generation of casting defects and coarse
grains in the central portion in the thickness direction to obtain
a homogeneous grain size distribution throughout a steel plate, it
is possible to improve a tensile property in the thickness
direction (percentage reduction of area) in the central portion in
the thickness direction.
[0014] 3. Regarding the tensile property in the thickness direction
(percentage reduction of area) in a tensile test in the thickness
direction performed on the central portion in the thickness
direction, the percentage reduction of area decreases with an
increase in the number of casting defects and coarse MnS particles
having a major axis of 100 .mu.m or more in the central portion in
the thickness direction. In addition, the percentage reduction of
area decreases with an increase in the number of coarse prior
austenite grains having a circle-equivalent diameter of 100 .mu.m
or more. This is because, since stress concentration occurs at the
positions of casting defects, coarse MnS particles, and coarse
prior austenite grains, such positions may become fracture
origins.
[0015] 4. By controlling the S content to be 0.0050% or less and by
decreasing the amount of central segregation through soft reduction
when continuous casting is performed, since it is possible to
decrease the number of casting defects and coarse MnS particles, it
is possible to further improve the tensile property in the
thickness direction (percentage reduction of area) in the central
portion in the thickness direction.
[0016] The disclosed embodiments have been completed by further
conducting investigations into the knowledge described above, and
the subject matter of the disclosed embodiments is as follows.
[0017] [1] A steel plate having a chemical composition containing,
by mass %, C: 0.01% to 0.15%, Si: 0.01% to 1.00%, Mn: 0.10% to
2.00%, P: 0.010% or less, S: 0.0050% or less, Al: 0.002% to 0.100%,
Ni: 5.0% to 10.0%, N: 0.0010% to 0.0080%, and with a balance being
Fe and incidental impurities, and a percentage reduction of area of
30% or more in a tensile test in a thickness direction performed on
a central portion in the thickness direction.
[0018] [2] The steel plate according to [1] , wherein the chemical
composition further contains, by mass %, one, two, or more selected
from Cr: 0.01% to 1.50%, Mo: 0.03% to 1.0%, Nb: 0.001% to 0.030%,
V: 0.01% to 0.10%, Ti: 0.003% to 0.050%, B: 0.0003% to 0.0100%, and
Cu: 0.01% to 1.00%.
[0019] [3] The steel plate according to [1] or [2] , wherein the
chemical composition further contains, by mass %, one, two, or more
selected from Sn: 0.01% to 0.30%, Sb: 0.01% to 0.30%, W: 0% (not
inclusive) to 2.00%, Co: 0% (not inclusive) to 2.00%, Ca: 0.0005%
to 0.0050%, Mg: 0.0005% to 0.0050%, Zr: 0.0005% to 0.0050%, and
REM: 0.0010% to 0.0100%.
[0020] [4] A method for manufacturing a steel plate, the method
comprising heating a slab having the chemical composition according
to any one of [1] to [3] to a temperature of 1000.degree. C. or
higher and 1300.degree. C. or lower and performing hot rolling on
the heated slab in such a manner that finish rolling is performed
with a reduction ratio of 3 or more and that each of at least two
rolling passes of final three rolling passes is performed with a
rolling shape factor of 0.7 or more.
Advantageous Effects
[0021] According to the disclosed embodiments, it is possible to
obtain a steel plate having excellent deformability in the central
portion in the thickness direction. The steel plate according to
the disclosed embodiments contributes significantly to improving
the safety of steel structures used in a cryogenic environment such
as a liquefied gas storage tank, which has a significant effect on
the industry.
DETAILED DESCRIPTION
[0022] Hereafter, the disclosed embodiments will be described.
Here, the disclosed embodiments are not limited to the specific
embodiments below.
[0023] First, the chemical composition of the steel plate according
to the disclosed embodiments and the reasons for the limitations on
the chemical composition will be described. Here, % used when
describing the chemical composition denotes mass %, unless
otherwise denoted.
[0024] C: 0.01% to 0.15%
[0025] C is effective for increasing strength, and it is necessary
that the C content be 0.01% or more to realize such an effect. It
is preferable that the C content be 0.03% or more. On the other
hand, in the case where the C content is more than 0.15%, since C
is segregated in the central portion in the thickness direction,
the precipitation of Cr carbides and Nb--, V--, and Ti-based
carbides is excessively promoted, which results in a deterioration
in low-temperature toughness and percentage reduction of area.
Therefore, the C content is set to be 0.15% or less. It is
preferable that the C content be 0.10% or less.
[0026] Si: 0.01% to 1.00%
[0027] Since Si functions as a deoxidizing agent, Si is necessary
for a steel-making process. In addition, Si is effective for
increasing the strength of a steel plate through solid solution
strengthening by forming a solid solution in steel. To realize such
effects, it is necessary that the Si content be 0.01% or more. On
the other hand, in the case where the Si content is more than
1.00%, there is a deterioration in weldability and surface quality.
Therefore, the Si content is set to be 1.00% or less. It is
preferable that the Si content be 0.5% or less or more preferably
0.3% or less.
[0028] Mn: 0.10% to 2.00%
[0029] Mn is an element which is effective for increasing strength
by improving the hardenability of a steel plate. To realize such an
effect, it is necessary that the Mn content be 0.10% or more. It is
preferable that the Mn content be 0.40% or more. On the other hand,
in the case where the Mn content is more than 2.00%, since central
segregation is promoted, there is a deterioration in cryogenic
toughness and percentage reduction of area in a tensile test in the
thickness direction performed on the central portion in the
thickness direction, and stress corrosion cracking occurs. In
addition, in the central portion in the thickness direction, the
formation of coarse MnS particles having a major axis of 100 .mu.m
or more, which become fracture origins, is promoted, and there is
thus a significant deterioration in percentage reduction of area in
a tensile test in the thickness direction. Therefore, the Mn
content is set to be 2.00% or less. It is preferable that the Mn
content be 1.00% or less.
[0030] P: 0.010% or less
[0031] In the case where the P content is more than 0.010%, since P
causes a deterioration in grain-boundary strength as a result of
being segregated at grain boundaries, thereby becoming a fracture
origin, there is a deterioration in percentage reduction of area in
a tensile test in the thickness direction performed on the central
portion in the thickness direction. Therefore, it is preferable
that the P content be as small as possible, and the P content is
set to be 0.010% or less.
[0032] S: 0.0050% or less
[0033] S forms MnS in steel, thereby causing a significant
deterioration in low-temperature toughness and percentage reduction
of area in a tensile test in the thickness direction performed on
the central portion in the thickness direction. Therefore, it is
preferable that the S content be as small as possible, and the S
content is set to be 0.0050% or less. It is preferable that the S
content be 0.0020% or less.
[0034] Al: 0.002% to 0.100%
[0035] Since Al functions as a deoxidizing agent, Al is most
commonly used in a deoxidizing process performed on molten steel.
In addition, by fixing solid solution N in steel to form AlN, Al is
effective for inhibiting a deterioration in toughness by decreasing
the amount of solid solution N. To realize such effects, it is
necessary that the Al content be 0.002% or more. It is preferable
that the Al content be 0.010% or more or more preferably 0.020% or
more. On the other hand, in the case where the Al content is more
than 0.100%, since Al is diffused in a weld metal when welding is
performed, there is a deterioration in the toughness of the weld
metal. Therefore, the Al content is set to be 0.100% or less. It is
preferable that the Al content be 0.070% or less or more preferably
0.060% or less.
[0036] Ni: 5.0% to 10.0%
[0037] Ni is an element which increases the strength of a steel
plate and which is significantly effective for improving the
low-temperature toughness of a steel plate by stabilizing retained
austenite. Since Ni is an expensive element, steel plate cost
increases significantly with an increase in the Ni content.
Therefore, the Ni content is set to be 10.0% or less. On the other
hand, in the case where the Ni content is less than 5.0%, there is
a deterioration in the strength of a steel plate, and it is not
possible to form stable retained austenite at low temperatures,
which results in a deterioration in the low-temperature toughness
and strength of a steel plate. Therefore, the Ni content is set to
be 5.0% or more. It is preferable that the Ni content be 6.0% to
9.0%.
[0038] N: 0.0010% to 0.0080%
[0039] Since N is an austenite-stabilizing element, N is an element
which is effective for improving cryogenic toughness. In addition,
by combining with Nb, V, and Ti to be finely precipitated in the
form of nitrides or carbonitrides, which function as trap sites of
diffusive hydrogen, N is effective for inhibiting stress corrosion
cracking. To realize such effects, it is necessary that the N
content be 0.0010% or more. It is preferable that the N content be
0.0020% or more. On the other hand, in the case where the N content
is more than 0.0080%, since the formation of an excessive number of
nitrides or carbonitrides is promoted, there is a decrease in the
amount of solid solution elements, which results in a deterioration
in not only corrosion resistance but also toughness and percentage
reduction of area in a tensile test in the thickness direction
performed on the central portion in the thickness direction.
Therefore, the N content is set to be 0.0080% or less. It is
preferable that the N content be 0.0060% or less.
[0040] In the disclosed embodiments, to further improve strength
and low-temperature toughness, one, two, or more selected from Cr:
0.01% to 1.50%, Mo: 0.03% to 1.0%, Nb: 0.001% to 0.030%, V: 0.01%
to 0.10%, Ti: 0.003% to 0.050%, B: 0.0003% to 0.0100%, and Cu:
0.01% to 1.00% may be added as needed in addition to the
indispensable constituents described above.
[0041] Cr: 0.01% to 1.50%
[0042] Cr is an element which is effective for increasing strength.
To realize such an effect, the Cr content is set to be 0.01% or
more, in the case where Cr is added. On the other hand, since Cr
may be precipitated in the form of nitrides, carbides,
carbonitrides, and the like when rolling is performed, such
precipitates become origins at which corrosion and fracturing
occur, which results in a deterioration in low-temperature
toughness. Therefore, in the case where Cr is added, the Cr content
is set to be 1.50% or less. It is preferable that the Cr content be
1.00% or less.
[0043] Mo: 0.03% to 1.0%
[0044] Mo is an element which is effective for decreasing the
temper embrittlement susceptibility of a steel plate and for
increasing the strength of a steel plate without causing a
deterioration in low-temperature toughness. To realize such
effects, the Mo content is set to be 0.03% or more, in the case
where Mo is added. It is preferable that the Mo content be more
than 0.05%. On the other hand, in the case where the Mo content is
more than 1.0%, there is a deterioration in low-temperature
toughness. Therefore, in the case where Mo is added, it is
preferable that the Mo content be 1.0% or less or more preferably
0.30% or less.
[0045] Nb: 0.001% to 0.030%
[0046] Nb is an element which is effective for improving the
strength of a steel plate. To realize such an effect, the Nb
content is set to be 0.001% or more, in the case where Nb is added.
It is preferable that the Nb content be 0.005% or more or more
preferably 0.007% or more. On the other hand, in the case where the
Nb content is more than 0.030%, since coarse carbonitrides are
precipitated, such precipitates become fracture origins, which may
result in a deterioration in a tensile property in the thickness
direction in the central portion in the thickness direction. In
addition, there is coarsening of the precipitate, which may result
in a deterioration in the toughness of a base material. Therefore,
in the case where Nb is added, the Nb content is set to be 0.030%
or less. It is preferable that the Nb content be 0.025% or less or
more preferably 0.022% or less.
[0047] V: 0.01% to 0.10%
[0048] V is an element which is effective for improving the
strength of a steel plate. To realize such an effect, the V content
is set to be 0.01% or more, in the case where V is added. It is
preferable that the V content be 0.02% or more or more preferably
0.03% or more. On the other hand, in the case where the V content
is more than 0.10%, since coarse carbonitrides are precipitated,
such carbonitrides may become fracture origins. In addition, there
is coarsening of the precipitates, which may result in a
deterioration in the toughness of a base material. Therefore, in
the case where V is added, the V content is set to be 0.10% or
less. It is preferable that the V content be 0.09% or less or more
preferably 0.08% or less.
[0049] Ti: 0.003% to 0.050%
[0050] Ti is an element which is effective for improving the
strength of a steel plate as a result of being precipitated in the
form of nitrides or carbonitrides. To realize such an effect, the
Ti content is set to be 0.003% or more, in the case where Ti is
added. It is preferable that the Ti content be 0.005% or more or
more preferably 0.007% or more. On the other hand, in the case
where the Ti content is more than 0.050%, there is coarsening of
the precipitates, which may result in a deterioration in the
toughness of a base metal. In addition, since coarse carbonitrides
are precipitated, such precipitates may become fracture origins.
Therefore, in the case where Ti is added, the Ti content is set to
be 0.050% or less. It is preferable that the Ti content be 0.035%
or less or more preferably 0.032% or less.
[0051] B: 0.0003% to 0.0100%
[0052] B is an element which is effective for improving the
strength of a base material. To realize such an effect, the B
content be 0.0003% or more, in the case where B is added. On the
other hand, in the case where the B content is more than 0.0100%,
since coarse B-based precipitates are formed, there is a
deterioration in toughness. Therefore, in the case where B is
added, the B content is set to be 0.0100% or less. It is preferable
that the B content be 0.0030% or less.
[0053] Cu: 0.01% to 1.00%
[0054] Cu is an element which is effective for improving the
strength of a steel plate because of an improvement in
hardenability. To realize such an effect, the Cu content is set to
be 0.01% or more, in the case where Cu is added. On the other hand,
in the case where the Cu content is more than 1.00%, there is a
deterioration in the low-temperature toughness of a steel plate,
and there may be a deterioration in the surface quality of a steel
slab which has been cast. Therefore, in the case where Cu is added,
the Cu content is set to be 1.00% or less. It is preferable that
the Cu content be 0.30% or less.
[0055] Moreover, in the disclosed embodiments, one, two, or more
selected from Sn: 0.01% to 0.30%, Sb: 0.01% to 0.30%, W: 0% (not
inclusive) to 2.00%, Co: 0% (not inclusive) to 2.00%, Ca: 0.0005%
to 0.0050%, Mg: 0.0005% to 0.0050%, Zr: 0.0005% to 0.0050%, and
REM: 0.0010% to 0.0100% may be added as needed.
[0056] Sn: 0.01% to 0.30%
[0057] Sn is an element which is effective for improving corrosion
resistance. Although such an element is effective, even in the case
where its content is low, the Sn content is set to be 0.01% or
more, in the case where Sn is added. However, in the case where the
Sn content is high, there is a deterioration in weldability and
toughness, and there is also a disadvantage from a cost point of
view. Therefore, in the case where Sn is added, the Sn content is
set to be 0.30% or less. It is preferable that the Sn content be
0.25% or less.
[0058] Sb: 0.01% to 0.30%
[0059] Sb is, like Sn, an element which is effective for improving
corrosion resistance. Although such an element is effective, even
in the case where its content is low, the Sb content is set to be
0.01% or more, in the case where Sb is added. However, in the case
where the Sb content is high, there is a deterioration in
weldability and toughness, and there is also a disadvantage from a
cost point of view. Therefore, in the case where Sb is added, the
Sb content is set to be 0.30% or less. It is preferable that the Sb
content be 0.25% or less.
[0060] W: 0% (not inclusive) to 2.00%
[0061] W is, like Sn and Sb, an element which is effective for
improving corrosion resistance. Since such an element is effective,
even in the case where its content is low, W may be added in an
amount of more than 0%. However, in the case where the W content is
high, there is a deterioration in weldability and toughness, and
there is also a disadvantage from a cost point of view. Therefore,
in the case where W is added, the W content is set to be 2.00% or
less. It is preferable that the W content be 0.50% or less.
[0062] Co: 0 (not inclusive) to 2.00%
[0063] Co is, like Sn, Sb, and W, an element which is effective for
improving corrosion resistance. Since such an element is effective,
even in the case where its content is low, Co may be added in an
amount of more than 0%. It is preferable that the Co content be
0.10% or more. However, in the case where the Co content is high,
there is a deterioration in weldability and toughness, and there is
also a disadvantage from a cost point of view. Therefore, in the
case where Co is added, the Co content is set to be 2.00% or less.
It is preferable that the Co content be 1.50% or less.
[0064] Ca: 0.0005% to 0.0050%
[0065] Since Ca is an element which is effective for the
morphological control of inclusions such as MnS, Ca may be added as
needed. The expression "morphological control of inclusions"
denotes a case where elongated sulfide-based inclusions are made
into granular inclusions. Through such morphological control of
inclusions, it is possible to improve a tensile property in the
thickness direction, toughness, and sulfide stress corrosion
cracking resistance in the central portion in the thickness
direction. To realize such effects, the Ca content is set to be
0.0005% or more, in the case where Ca is added. It is preferable
that the Ca content be 0.0010% or more. On the other hand, in the
case where the Ca content is high, since there is an increase in
the amounts of nonmetallic inclusions, there may be a
deterioration, rather than an improvement, in a tensile property in
the thickness direction in the central portion in the thickness
direction. Therefore, in the case where Ca is added, the Ca content
is set to be 0.0050% or less. It is preferable that the Ca content
be 0.0040% or less.
[0066] Mg: 0.0005% to 0.0050%
[0067] Since Mg is, like Ca, an element which is effective for the
morphological control of inclusions such as MnS, Mg may be added as
needed. Through such morphological control of inclusions, it is
possible to improve a tensile property in the thickness direction,
toughness, and sulfide stress corrosion cracking resistance in the
central portion in the thickness direction. To realize such
effects, the Mg content is set to be 0.0005% or more, in the case
where Mg is added. It is preferable that the Mg content be 0.0010%
or more. On the other hand, in the case where the Mg content is
high, since there is an increase in the number of nonmetallic
inclusions, there may be a deterioration, rather than an
improvement, in a tensile property in the thickness direction in
the central portion in the thickness direction. Therefore, in the
case where Mg is added, the Mg content is set to be 0.0050% or
less. It is preferable that the Mg content be 0.0040% or less.
[0068] Zr: 0.0005% to 0.0050%
[0069] Since Zr is, like Ca and Mg, an element which is effective
for the morphological control of inclusions such as MnS, Zr may be
added as needed. Through such morphological control of inclusions,
it is possible to improve a tensile property in the thickness
direction, toughness, and sulfide stress corrosion cracking
resistance in the central portion in the thickness direction. To
realize such effects, the Zr content is set to be 0.0005% or more.
It is preferable that the Zr content be 0.0010% or more. On the
other hand, in the case where the Zr content is high, since there
is an increase in the number of nonmetallic inclusions, there may
be a deterioration, rather than an improvement, in a tensile
property in the thickness direction in the central portion in the
thickness direction. Therefore, in the case where Zr is added, the
Zr content is set to be 0.0050% or less. It is preferable that the
Zr content be 0.0040% or less.
[0070] REM: 0.0010% to 0.0100%
[0071] Since REM are, like Ca, Mg, and Zr, elements which are
effective for the morphological control of inclusions such as MnS,
REM may be added as needed. Through such morphological control of
inclusions, it is possible to improve a tensile property in the
thickness direction, toughness, and sulfide stress corrosion
cracking resistance in the central portion in the thickness
direction. To realize such effects, the REM content is set to be
0.0010% or more. It is preferable that the REM content be 0.0020%
or more. On the other hand, in the case where the REM content is
high, since there is an increase in the number of nonmetallic
inclusions, there may be a deterioration, rather than an
improvement, in a tensile property in the thickness direction in
the central portion in the thickness direction. Therefore, in the
case where REM are added, the REM content is set to be 0.0100% or
less.
[0072] Here, the remainder is Fe and incidental impurities.
[0073] In addition, the steel plate according to the disclosed
embodiments has deformability represented by a percentage reduction
of area of 30% or more in a tensile test in the thickness direction
performed on the central portion in the thickness direction. Here,
the term "percentage reduction of area" denotes, when a tensile
test is performed, the ratio (.DELTA.S/S (%)) of the amount of
reduction in the cross-sectional area .DELTA.S of a test specimen
after the test to the cross-sectional area S of the test specimen
before the test. By controlling the percentage reduction of area to
be 30% or more, it is possible to achieve satisfactory
deformability in the central portion in the thickness direction. In
the disclosed embodiments, it is preferable that the percentage
reduction of area be 35% or more. Here, it is possible to achieve
the percentage reduction of area according to the disclosed
embodiments by controlling the soft reduction condition applied for
casting and/or the conditions applied for finish rolling described
below.
[0074] In addition, in the disclosed embodiments, in the central
portion in the thickness direction, it is preferable that the
number of MnS particles having a major axis of 100 pm or more be 10
pieces/mm.sup.2 or less and that prior austenite grains have a
circle-equivalent diameter of less than 100 .mu.m. This is because,
since stress concentration occurs at the positions of casting
defects, coarse MnS particles, and coarse prior austenite grains,
such positions may become fracture origins. Here, it is possible to
form the desired MnS by controlling the soft reduction condition
applied for continuous casting described below.
[0075] In addition, in the disclosed embodiments, the expression
"central portion in the thickness direction" denotes a position
located at 1/2 of the thickness, and the percentage reduction of
area and the sizes of MnS particles and prior austenite grains are
determined by using the determination methods described in EXAMPLES
below.
[0076] Hereafter, the manufacturing conditions according to the
disclosed embodiments will be described. Here, in the description
below, the term "temperature (.degree. C.)" denotes the temperature
in the central portion in the thickness direction.
[0077] The method for manufacturing the steel plate according to
the disclosed embodiments includes heating a slab having the
desired chemical composition to a temperature of 1000.degree. C. or
higher and 1300.degree. C. or lower and performing hot rolling on
the heated slab in such a manner that finish rolling is performed
with a reduction ratio of 3 or more and that each of at least two
rolling passes of the final three rolling passes is performed with
a rolling shape factor of 0.7 or more.
[0078] Reheating temperature of steel material: 1000.degree. C. or
higher and 1300.degree. C. or lower
[0079] The steel material is reheated to dissolve precipitates in a
microstructure and to homogenize a grain size distribution and the
like, and the reheating temperature is set to be 1000.degree. C. or
higher and 1300.degree. C. or lower. In the case where the
reheating temperature is lower than 1000.degree. C., since
precipitates such as A1N are not sufficiently dissolved, and since
there is coarsening of such precipitates when reheating is
performed, such precipitates become fracture origins, which results
in the desired percentage reduction of area in a tensile test in
the thickness direction not being achieved. On the other hand, in
the case where the reheating temperature is higher than
1300.degree. C., there is a deterioration in toughness due to an
increase in grain size, and there is an increase in manufacturing
costs. Therefore, the reheating temperature is set to be
1300.degree. C. or lower. It is preferable that the reheating
temperature be 1250.degree. C. or lower or more preferably
1200.degree. C. or lower. Here, it is preferable that the reheating
time be 1 hour to 10 hours.
[0080] Reduction ratio in finish rolling: 3 or more When finish
rolling in a hot rolling process is performed, by controlling the
reduction ratio ((slab thickness)/(final thickness)) to be 3 or
more, it is possible to homogenize a grain size distribution by
promoting recrystallization, and it is possible to render casting
defects such as minute voids known as porosity harmless by
eliminating the voids through pressure compression. Moreover, since
it is possible to form the desired microstructure in the hot rolled
steel plate by inhibiting the central segregation of Mn, P, S, and
the like, it is possible to achieve the desired tensile property in
the thickness direction. In the case where hot rolling is performed
with a reduction ratio of less than 3, since it is not possible to
form the desired microstructure, for example, due to coarse grains
remaining in the microstructure and due to the above-described
casting defects and central segregation being insufficiently
rendered harmless, it is not possible to achieve the desired
percentage reduction of area in a tensile test in the thickness
direction. Therefore, the reduction ratio is set to be 3 or more.
It is preferable that the reduction ratio be 4 or more or more
preferably 5 or more.
[0081] Rolling shape factor in each of at least two rolling passes
of final three rolling passes in finish rolling: 0.7 or more
[0082] By performing at least two rolling passes of three final
rolling passes, in which the material properties are finally
determined, with a rolling shape factor of 0.7 or more, it is
possible to render casting defects harmless with certainty, and it
is possible to homogenize a grain size distribution by inhibiting
coarse grains from remaining in the whole steel plate, and in
particular, in the central portion in the thickness direction. As a
result, there is an improvement in percentage reduction of area in
a tensile test in the thickness direction performed on the central
portion in the thickness direction. Here, the term "rolling shape
factor (ld/h.sub.m)" denotes {(contact length between rolling roll
and steel plate (roll contact arc length: 1d)}/{average thickness
calculated from thickness on the roll entry side and thickness on
the roll exit side: h.sub.m}, which is calculated by using equation
(1).
1d/h.sub.m={R(h.sub.i-h.sub.o)}.sup.1/2/{(h.sub.i+2h.sub.o)/3}
(1)
Here, [0083] R: roll radius of each rolling pass [0084] h.sub.i:
thickness on the entry side of each rolling pass [0085] h.sub.o:
thickness on the exit side of each rolling pass
[0086] In the case where the number of passes having a rolling
shape factor of 0.7 or more is less than 2, since it is not
possible to form the desired microstructure, for example, due to
coarse grains remaining in the microstructure and due to the
above-described casting defects being insufficiently rendered
harmless, it is not possible to achieve the desired percentage
reduction of area in a tensile test in the thickness direction
performed on the central portion in the thickness direction.
Therefore, the number of rolling passes having a rolling shape
factor of 0.7 or more is set to be at least 2. Here, to increase
the rolling shape factor, it is sufficient that the diameters of
rolling rolls be increased or that the rolling reduction be
increased.
[0087] Although there is no particular limitation on the
manufacturing conditions other than those described above, it is
preferable that the following conditions be applied.
[0088] Soft reduction in casting
[0089] In the disclosed embodiments, it is preferable that slab be
subjected to soft reduction when continuous casting is performed.
In the disclosed embodiments, by performing soft reduction, it is
possible to further inhibit coarse MnS particles having a major
axis of 100 .mu.m or more and coarse prior austenite grains having
a circle-equivalent diameter of 100 .mu.m or more from remaining in
the central portion in the thickness direction. Regarding the
specific condition applied for soft reduction, it is preferable
that the reduction gradient be 0.1 mm/m or more on the upstream
side of the final solidification position of a slab.
[0090] Cooling start temperature after hot rolling
[0091] In the disclosed embodiments, there is no particular
limitation on the cooling start temperature after hot rolling has
been performed, and it is preferable that the cooling start
temperature be 1000.degree. C. or lower and 500.degree. C. or
higher.
[0092] Cooling method after hot rolling
[0093] In the disclosed embodiments, there is no particular
limitation on the cooling method used after hot rolling has been
performed, and an appropriate method such as an air cooling method
or a water cooling method may be used. To achieve necessary
properties such as strength and low-temperature toughness, water
cooling such as spray cooling, mist cooling, or laminar cooling may
be performed after hot rolling has been performed.
[0094] Heat treatment after hot rolling
[0095] In the disclosed embodiments, although a final product may
be obtained by performing cooling after hot rolling has been
performed, it is preferable that a heat treatment be further
performed to achieve necessary properties such as low-temperature
toughness. As a heating treatment, it is preferable that a
tempering treatment be performed after hot rolling has been
performed. In addition, a quenching-tempering treatment, in which a
quenching treatment is also performed before a tempering treatment
is performed, may be performed. In addition, an inter-critical
quenching-tempering treatment, in which a tempering treatment is
performed after an inter-critical quenching has been performed, may
be performed. Moreover, a quenching-inter-critical
quenching-tempering treatment, in which an inter-critical quenching
is performed between a quenching treatment and a tempering
treatment, may be performed. It is preferable that at least one of
the manufacturing treatments described above be performed.
[0096] It is preferable that the quenching temperature be equal to
or higher than the Ac.sub.3 transformation temperature and
1000.degree. C. or lower. It is preferable that the inter-critical
quenching temperature be equal to or higher than the M.sub.I
transformation temperature and lower than the Ac.sub.3
transformation temperature. It is preferable that the tempering
temperature be 500.degree. C. to 650.degree. C.
[0097] Here, the Ac.sub.1 transformation temperature and the
Ac.sub.3 transformation temperature are respectively calculated by
using equations (2) and (3) below.
Ac.sub.1 (.degree.
C.)=750.8-26.6C+17.6Si-11.6Mn-22.9Cu-23Ni+24.1Cr+22.5Mo-39.7V-5.7Ti+232.4-
Nb-169.4A1 (2)
Ac.sub.3 (.degree.
C.)=937.2-436.5C+56Si-19.7Mn-16.3Cu-26.6Ni-4.9Cr+38.1Mo+124.8V+136.3Ti-19-
.1Nb+198.4Al (3)
[0098] Here, each of the atomic symbols in equations (2) and (3)
above denotes the content (mass %) of the corresponding element and
is assigned a value of 0 in the case where the corresponding
element is not contained.
EXAMPLES
[0099] Molten steels having the chemical compositions given in
Table 1 were prepared and made into slabs, and the slabs were then
made into steel plates having a thickness of 12 mm to 70 mm under
the manufacturing conditions given in Table 2. Here, regarding soft
reduction, the reduction gradient was 0.20 mm/m in the case of
sample Nos. 1 to 30, 0.07 mm/m in the case of sample No. 31, and
0.10 mm/m in the case of sample No. 32.
[0100] The obtained steel plates were subjected to the tests
described below.
[0101] (Mechanical Properties in Thickness Direction)
[0102] Regarding tensile properties, the steel plate was processed
into a test specimen having the shape of Type A so that the
thickness direction of the steel plate was the tensile direction,
and a tensile test was performed in accordance with JIS G 3199.
Regarding low-temperature toughness, a test specimen which had been
taken so that the thickness direction of the steel plate was the
tensile direction was cooled to a temperature of -196.degree. C. in
liquefied nitrogen, and then subjected to a Charpy impact test in
accordance with JIS Z 2242 to derive absorbed energy vE.sub.-196 at
a temperature of -196.degree. C.
[0103] In the disclosed embodiments, a case of a yield strength
(YS) of 585 MPa or more, a tensile strength (TS) of 690 MPa or
more, a percentage reduction of area after breaking (the ratio of
the amount of reduction in the cross-sectional area .DELTA.S of the
test specimen after a tensile test to the cross-sectional area S of
the test specimen before the tensile test) of 30% or more, and a
vE.sub.-196 of 34 J or more was judged as satisfactory.
[0104] (Microstructure)
[0105] A test specimen for microstructure observation was taken
from the obtained steel plate so that a position located at 1/2 of
the thickness was an observation position. The test specimen was
embedded in a resin so that the cross section perpendicular to the
rolling direction was an observation surface and subjected to
mirror polishing. Subsequently, the test specimen was etched in
picric acid and observed by using a SEM at a magnification of 200
times to obtain the SEM image of a microstructure at the position
located at 1/2 of the thickness. The photographic images obtained
in five fields of view were analyzed by using an image analyzer to
derive the number density of MnS particles having a major axis of
100 .mu.m or more and the maximum value of the circle-equivalent
diameter of prior austenite grains.
[0106] The results obtained as described above are given in Table
2.
TABLE-US-00001 TABLE 1 Steel Chemical Composition (mass %) Code C
Si Mn P S Al Ni N Ti Cr B Cu Mo V W A 0.08 0.22 0.69 0.001 0.0019
0.027 8.1 0.0019 B 0.05 0.23 0.50 0.007 0.0011 0.030 9.5 0.0036 C
0.02 0.43 1.65 0.001 0.0004 0.020 9.3 0.0024 D 0.05 0.02 0.85 0.003
0.0006 0.035 7.1 0.0055 0.011 E 0.07 0.10 0.18 0.002 0.0009 0.026
6.6 0.0035 0.25 F 0.12 0.43 1.06 0.006 0.0012 0.031 9.3 0.0024 G
0.06 0.08 0.85 0.002 0.0004 0.036 5.2 0.0026 0.50 0.13 H 0.05 0.06
0.55 0.009 0.0005 0.022 7.6 0.0029 0.0007 I 0.04 0.04 0.91 0.005
0.0007 0.022 8.2 0.0026 0.05 J 0.03 0.03 0.25 0.002 0.0003 0.021
9.1 0.0018 0.45 K 0.04 0.05 0.85 0.003 0.0006 0.020 6.7 0.0031 L
0.11 0.07 0.41 0.005 0.0005 0.032 8.9 0.0021 M 0.02 0.11 0.71 0.006
0.0009 0.021 9.3 0.0024 N 0.05 0.07 0.61 0.003 0.0006 0.033 7.1
0.0055 O 0.04 0.11 0.85 0.007 0.0007 0.010 8.4 0.0024 P 0.45 0.03
0.85 0.003 0.0012 0.022 7.1 0.0026 Q 0.11 0.12 3.50 0.007 0.0010
0.025 5.5 0.0044 0.21 0.19 R 0.12 0.43 1.65 0.050 0.0012 0.025 5.5
0.0026 0.31 S 0.07 0.23 0.91 0.009 0.0100 0.036 9.3 0.0035 0.06 T
0.04 0.43 0.25 0.006 0.0012 0.031 1.5 0.0026 0.15 U 0.05 0.14 0.41
0.009 0.0007 0.034 7.6 0.0151 V 0.06 0.15 0.85 0.002 0.0004 0.031
9.1 0.0026 0.50 0.15 Ac.sub.1 Ac.sub.3 Transfor- Transfor- Steel
Chemical Composition (mass %) mation mation Code Nb Sn Sb Co Mg Ca
Zr REM Temperature Temperature A 554 691 B 524 672 C 521 677 D
0.0006 571 719 E 0.0021 587 734 F 0.0023 524 647 G 630 770 H 0.0009
566 710 I 546 697 J 535 683 K 583 732 L 0.011 537 654 M 0.03 527
677 N 0.10 575 725 O 0.10 547 688 P 562 541 Q 588 692 R 0.0012 598
730 S 0.0011 520 669 T 715 905 U 567 720 V 543 670 Underlined
values are outside the scope of the disclosed embodiments.
TABLE-US-00002 TABLE 2 Nunber of Rolling Passes with a Rolling
Product Heating Shape Factor of Quenching Mate- Thick- Soft Reduc-
Temper- Reduc- 0.7 or More in Temper- Sample rial ness Reduc- tion
ature tion Final 3 Rolling Treatment ature No. Code (mm) tion
Gradient (.degree. C.) Ratio Passes after Rolling Heat Treatment
(.degree. C.) 1 A 30 Done 0.20 1100 6 3 Water Cooling None -- 2 B
50 Done 0.20 1200 4 2 Air Cooling Quenching-Tempering 685 3 C 30
Done 0.20 1050 7 2 Water Cooling Inter-critical --
Quenching-Tempering 4 D 50 Done 0.20 1200 5 3 Water Cooling
Tempering -- 5 E 30 Done 0.20 1100 5 3 Air Cooling Quenching-
Inter-critical 753 Quenching-Tempering 6 F 30 Done 0.20 1250 6 2
Water Cooling Inter-critical -- Quenching-Tempering 7 G 50 Done
0.20 1050 3 3 Water Cooling Tempering -- 8 H 50 Done 0.20 1100 4 2
Air Cooling Quenching-Inter-critical 735 Quenching-Tempering 9 I 30
Done 0.20 1150 5 3 Water Cooling Quenching-Tempering 724 10 J 30
Done 0.20 1100 6 3 Air Cooling Quenching-Tempering 900 11 K 50 Done
0.20 1200 6 2 Water Cooling Tempering -- 12 L 50 Done 0.20 1200 5 2
Water Cooling Quenching- Inter-critical 687 Quenching-Tempering 13
M 30 Done 0.20 1050 5 2 Air Cooling Quenching-Tempering 712 14 N 30
Done 0.20 1250 6 2 Air Cooling Quenching-Tempering 762 15 O 30 Done
0.20 1050 6 3 Water Cooling Tempering -- 16 P 50 Done 0.20 1200 4 3
Air Cooling Tempering -- 17 Q 30 Done 0.20 1200 6 2 Water Cooling
Tempering -- 18 R 50 Done 0.20 1050 4 3 Water Cooling Tempering --
19 S 30 Done 0.20 1150 5 3 Air Cooling Quenching-Tempering 712 20 S
50 not Done -- 1250 3 2 Water Cooling Tempering -- 21 T 30 Done 020
1100 5 3 Air Cooling Quenching-Tempering 850 22 U 50 Done 0.20 1200
6 2 Water Cooling Tempering -- 23 D 30 Done 0.20 1050 6 0 Water
Cooling Tempering -- 24 D 50 Done 0.20 1250 2 2 Water Cooling
Tempering -- 25 L 50 Done 0.20 850 3 3 Water Cooling Tempering --
26 L 50 Done 0.20 1350 3 3 Water Cooling Tempering -- 27 A 50 not
Done -- 1050 3 2 Water Cooling Quenching-Tempering 750 28 B 12 Done
0.20 1100 15 5 Air Cooling Quenching- Inter-critical 800
Quenching-Tempering 29 V 70 Done 0.20 1100 3 3 Air Cooling
Quenching- Inter-critical 780 Quenching-Tempering 30 B 50 Done 0.20
1150 3 1* Water Cooling Tempering -- 31 A 40 Done 0.07 1100 4 2
Water Cooling None -- 32 A 40 Done 0.10 1100 4 2 Water Cooling None
-- Inter- Maximum Circle- Number Density of critical Tempering
Equivalent MnS Particle Quenching Temper- Diameter of Having a
Major Axis Percentage Sample Temperature ature Prior Austenite of
100 .mu.m or more YS TS vE.sub.196 Reduction of No. (.degree. C.)
(.degree. C.) Grain (.mu.m) (pieces/mm.sup.2) (MPa) (MPa) (J) Area
(%) Class 1 -- -- 25 7 950 1150 76 70 Example 2 -- 526 30 5 720 790
210 80 Example 3 611 524 15 9 705 760 205 80 Example 4 -- 575 21 3
710 770 84 45 Example 5 674 592 20 1 680 741 211 70 Example 6 600
530 16 6 736 810 184 80 Example 7 -- 637 24 2 721 774 201 60
Example 8 655 574 34 3 719 759 189 70 Example 9 -- 555 26 2 724 771
144 65 Example 10 -- 580 34 1 664 820 75 45 Example 11 -- 594 20 3
654 709 175 70 Example 12 617 549 19 2 741 780 106 45 Example 13 --
540 23 2 721 767 81 40 Example 14 -- 589 24 3 716 771 71 40 Example
15 -- 562 35 3 711 736 188 55 Example 16 -- 578 25 4 700 1120 25 15
Comparative Example 17 -- 605 26 14 771 902 54 20 Comparative
Example 18 -- 616 24 9 701 755 15 15 Comparative Example 19 -- 539
36 15 751 802 16 20 Comparative Example 20 -- 550 130 20 766 830 15
10 Comparative Example 21 -- 640 24 5 400 560 6 70 Comparative
Example 22 -- 588 35 2 716 740 19 25 Comparative Example 23 -- 580
150 4 716 765 54 15 Comparative Example 24 -- 580 140 3 689 732 41
15 Comparative Example 25 -- 560 26 2 711 744 44 15 Comparative
Example 26 -- 560 120 3 701 729 31 20 Comparative Example 27 -- 570
30 12 750 780 79 30 Example 28 600 520 18 3 805 856 105 70 Example
29 670 580 30 6 620 705 206 50 Example 30 -- 500 100 5 685 715 86
25 Comparative Example 31 -- -- 25 14 885 1085 69 30 Example 32 --
-- 24 14 878 1060 71 35 Underlined values are outside the scope of
the disclosed embodiments. *Done in the final rolling pass
[0107] The examples of the disclosed embodiments (sample Nos. 1 to
15, 27 to 29, 31, and 32) had a percentage reduction of area of 30%
or more and excellent strength and low-temperature toughness. On
the other hand, the comparative examples (sample Nos. 16 to 26 and
30), which were outside the scope of the disclosed embodiments,
were poor in terms of at least one of percentage reduction of area,
strength, and low-temperature toughness.
* * * * *