U.S. patent application number 14/406405 was filed with the patent office on 2015-05-28 for ni-containing steel plate.
The applicant listed for this patent is JFE STEEL CORPORATION. Invention is credited to Nobuyuki Ishikawa, Shinichi Miura, Yukio Shimbo.
Application Number | 20150147222 14/406405 |
Document ID | / |
Family ID | 49996885 |
Filed Date | 2015-05-28 |
United States Patent
Application |
20150147222 |
Kind Code |
A1 |
Miura; Shinichi ; et
al. |
May 28, 2015 |
NI-CONTAINING STEEL PLATE
Abstract
An object of the present invention is to provide an
Ni-containing steel plate which is low in cost and has excellent
low-temperature toughness. In view of the object, the Ni-containing
steel plate of the present invention has a chemical composition
containing by mass % C: 0.01% to 0.15%, Si: 0.02% to 0.20%, Mn:
0.45% to 2.00%, P: 0.020% or less, 5: 0.005% or less, Al: 0.005% to
0.100% Ni: 5.0 to 8.0%, and the balance being Fe and incidental
impurities, and has a microstructure containing less than 1.7% by
volume fraction of retained austenite when cooled to liquid
nitrogen temperature, and having an average grain size of crystal
grains surrounded by high-angle grain boundaries with an
orientation difference of 15.degree. or more of 5 .mu.m or less by
equivalent circle diameter.
Inventors: |
Miura; Shinichi;
(Kurashiki-shi, JP) ; Shimbo; Yukio;
(Kawasaki-shi, JP) ; Ishikawa; Nobuyuki;
(Fukuyama-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE STEEL CORPORATION |
Tokyo |
|
JP |
|
|
Family ID: |
49996885 |
Appl. No.: |
14/406405 |
Filed: |
July 18, 2013 |
PCT Filed: |
July 18, 2013 |
PCT NO: |
PCT/JP2013/004399 |
371 Date: |
December 8, 2014 |
Current U.S.
Class: |
420/83 ; 420/112;
420/119; 420/84; 420/92 |
Current CPC
Class: |
C22C 38/005 20130101;
C21D 6/001 20130101; C21D 8/0226 20130101; C22C 38/46 20130101;
C22C 38/16 20130101; C21D 8/0263 20130101; C22C 38/06 20130101;
C22C 38/08 20130101; C22C 38/12 20130101; C22C 38/14 20130101; C22C
38/04 20130101; C22C 38/02 20130101; C22C 38/40 20130101; C21D 9/46
20130101; C22C 38/002 20130101 |
Class at
Publication: |
420/83 ; 420/84;
420/92; 420/112; 420/119 |
International
Class: |
C22C 38/46 20060101
C22C038/46; C22C 38/14 20060101 C22C038/14; C22C 38/12 20060101
C22C038/12; C22C 38/00 20060101 C22C038/00; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/16 20060101 C22C038/16; C22C 38/08 20060101
C22C038/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 23, 2012 |
JP |
2012-162335 |
Claims
1. An Ni-containing steel plate having a chemical composition
containing by mass % C: 0.01% to 0.15%, Si: 0.02% to 0.20%, Mn:
0.45% to 2.00%, P: 0.020% or less, S: 0.005% or less, Al: 0.005% to
0.100%, Ni: 5.0% to 8.0%, and the balance being Fe and incidental
impurities, wherein the steel plate has a microstructure containing
less than 1.7% by volume fraction of retained austenite when cooled
to liquid nitrogen temperature, and having an average grain size of
crystal grains surrounded by high-angle grain boundaries with an
orientation difference of 15.degree. or more of 5 .mu.m or less by
equivalent circle diameter.
2. The Ni-containing steel plate according to claim 1, wherein the
chemical composition further contains by mass % at least one
element selected from Cr: 1.00% or less and Mo: 1.000% or less.
3. The Ni-containing steel plate according to claim 1, wherein the
chemical composition further contains by mass % at least one
element selected from Cu: 1.00% or less, V: 0.100% or less, Nb:
0.100% or less, Ti: 0.100% or less, and B: 0.0030% or less.
4. The Ni-containing steel plate according to claim 1, wherein the
chemical composition further contains by mass % at least one
element selected from Ca: 0.0050% or less and REM: 0.0050% or
less.
5. The Ni-containing steel plate according to claim 2, wherein the
chemical composition further contains by mass % at least one
element selected from Cu: 1.00% or less, V: 0.100% or less, Nb:
0.100% or less, Ti: 0.100% or less, and B: 0.0030% or less.
6. The Ni-containing steel plate according to claim 2, wherein the
chemical composition further contains by mass % at least one
element selected from Ca: 0.0050% or less and REM: 0.0050% or
less.
7. The Ni-containing steel plate according to claim 3, wherein the
chemical composition further contains by mass % at least one
element selected from Ca: 0.0050% or less and REM: 0.0050% or
less.
8. The Ni-containing steel plate according to claim 5, wherein the
chemical composition further contains by mass % at least one
element selected from Ca: 0.0050% or less and REM: 0.0050% or less.
Description
TECHNICAL FIELD
[0001] The present invention relates to an Ni-containing steel
plate with excellent low-temperature toughness, in particular to a
steel plate which is suitable for use as members such as storage
tanks for liquefied natural gas.
BACKGROUND ART
[0002] Conventionally, for members such as overland storage tanks
for liquefied natural gas (hereinafter, referred to as LNG) high
Ni-containing steel plates which are excellent in mechanical
properties at low temperatures have been commonly used. In
particular, steel plates composed of high Ni-containing steel which
contains Ni by 9 mass % (hereinafter, referred to as 9% Ni steel)
have been commonly used, and they have, actually been applied in
many cases.
[0003] Regarding 9% Ni steel, considerations on various properties
such as mechanical properties and weldability have been made. For
example, Steel and Iron by Furukimi Osamu, Suzuki Shigeharu, Nakano
Yashifumi, 69(1982)5, S492 (NPL 1) discloses that low-temperature
toughness is improved by reducing the amount of impurity elements
such as P and S. Further, Handbook of Metal, 4.sup.th revised
edition, edited by The Japan Institute of Metals and Materials,
Maruzen, p 801 (NPL 2) discloses that low-temperature toughness is
improved by stabilizing retained austenite. However, since Ni is an
expensive metal, it is desired to reduce Ni content.
[0004] Techniques for obtaining steel plates which can be made to
have an Ni content smaller than that of 9% Ni steel and has good
low temperature toughness are disclosed in for example,
WO2007/034576 (PTL 1), WO2007/080645 (PTL 2), JP2011-214099A (PTL
3). PTL 1 discloses that mechanical properties of a steel plate can
be improved by predetermining the chemical composition of the steel
plate, defining the amount, aspect ratio, and average equivalent
circular diameter of austenite contained in the steel plate, and
manufacturing the steel plate with a method to satisfy such
definitions. Further, PTL 2 discloses that toughness of the
heat-affected zone of a steel plate can be improved when the steel
plate has a predetermined chemical composition and the Fe content
obtained by an extraction residue method after a heat-cycle
simulation test is more than a predetermined value. Further, PTL 3
discloses that a brittle crack-arrest property of steel can be
improved when the steel has a predetermined chemical composition,
with certain textures developed.
CITATION LIST
Patent Literature
[0005] PTL 1: WO2007/034576
[0006] PTL 2: WO2007/080645
[0007] PTL 3: JP2011 -214099A
Non-Patent Literature
[0008] NPL 1: Steel and iron by Furukimi Osamu, Suzuki Shigeharu,
Nakano Yoshifumi, 69(1982)5, S492
[0009] NPL 2: Handbook of Metal, 4.sup.th revised edition, edited
by The Japan Institute of Metals and Materials, Maruzen, p
800-802
SUMMARY OF INVENTION
Technical Problem
[0010] However, the techniques disclosed in PTL 1, 2 and 3 do not
include definitions regarding the amount of austenite at around
-165.degree. C. where the LNG tanks are actually used, and
consideration regarding low-temperature toughness when the
techniques are applied to actual structures were not made. Further,
there were no specific disclosures regarding the manufacturing
method of the steel plates.
[0011] The present invention has been developed in view of such
situation, and an object thereof is to provide an Ni-containing
steel plate which is low in cost and has excellent low-temperature
toughness.
Solution to Problem
[0012] The inventors of the present invention, as a result of
intense investigation for providing an Ni-containing steel plate
with excellent low-temperature toughness, discovered, that by
containing C, Si, Mn, P, S, Al, and Ni as essential elements of a
steel, and setting the amount of retained austenite contained in
the steel after performing sub-zero treatment were cooling is
performed until reaching liquid nitrogen temperature to be less
than 1.7%, and setting the average grain size of crystal grains
surrounded by high-angle grain boundaries with an orientation
difference of 15.degree. or more to 5 .mu.m or less by equivalent
circle diameter, excellent low-temperature toughness can be
achieved even when the Ni content is reduced compared to
conventional 9% Ni steel.
[0013] If the Ni content in steel is reduced to be smaller than
that of 9% Ni Steel, even if retained austenite is stable at room
temperature, it will be unstable at -165.degree. C. where LNG tanks
are used. Further, it is considered that toughness decreases when
retained austenite exists at -165.degree. C., because the retained
austenite is transformed into martensite phase due to deformation
induced transformation, at the tip of a crack formed in the steel
material when the LNG tank fractures. Under the situation, by
reducing the amount of retained austenite remaining after sub-zero
treatment corresponding to -165.degree. C. where LNG tanks are
used, and forming a fine microstructure as described above, it is
assumed that low-temperature toughness can be improved even if the
Ni content in steel is reduced to be smaller than that of
conventional 9% Ni steel.
[0014] The present invention is based on the above discoveries and
it provides the following (1) to (4).
[0015] (1) An Ni-containing steel plate having, a chemical
composition containing by mass % C: 0.01% to 0.15%, Si: 0.02% to
0.20%, Mn: 0.45% to 2.00%, P: 0.020% or less, S: 0.005% or less,
Al: 0.005% to 0.100%, Ni: 5.0% to 8.0%, and the balance being Fe
and incidental impurities, wherein
[0016] the steel plate has a microstructure containing less than
1.7% by volume fraction of retained austenite when cooled to liquid
nitrogen temperature, and having an average grain size of crystal
grains surrounded by high-angle grain boundaries with an
orientation difference of 15.degree. or more of 5 .mu.m or less by
equivalent circle diameter.
[0017] (2) The Ni-containing steel plate according to aspect (1),
wherein the chemical composition further contains by mass % at
least one element selected from Cr: 1.00% or less and Mo: 1.000% or
less.
[0018] (3) The Ni-containing steel plate according to aspect (1) or
(2), wherein the chemical composition further contains by mass % at
least one element selected from Cu: 1.00% or less, V: 0.100% or
less, Nb: 0.100% or less, Ti: 0.100% or less, and B: 0.0030% or
less.
[0019] (4) The Ni-containing steel plate according to any one of
aspects (1) to (3), wherein the chemical composition further
contains by mass % at least one element selected from Ca: 0.0050%
or less and REM: 0.0050% or less.
Advantageous Effect of Invention
[0020] According to the present invention, an Ni-containing steel
plate containing less Ni content compared to 9% Ni steel but having
low-temperature toughness equivalent to that of 9% Ni steel can be
easily manufactured, and an industrially remarkable effect is
provided.
DESCRIPTION OF EMBODIMENTS
[0021] Hereinafter, the Ni-containing steel plate according to the
present invention will be explained in detail and separately based
on chemical composition, microstructure, and manufacturing
method.
[0022] Unless otherwise specified, the indication of "%" regarding
composition shall stand for "mass %".
[0023] (1) Chemical Composition
[0024] First, the chemical composition will be described.
[0025] C: 0.01% to 0.15%
[0026] C is an important element for solid solution strengthening
of steel. If C content is less than 0.01%, sufficient strength
cannot be obtained. On the other hand, adding C in an amount
exceeding 0.15% would cause deterioration of weldability and
workability. Therefore, C content is set to be in the range of
0.01% to 0.15%. Preferably, the range is from 0.03% to 0.10%.
[0027] Si: 0.02% to 0.20%
[0028] Si is an effective element as a deoxidizer in molten steel
and an effective element for solid solution strengthening. If Si
content is less than 0.02%, deoxidizing effect cannot be
sufficiently obtained. On the other hand, adding Si in an amount
exceeding 0.20% would cause problems such as reduction in ductility
and toughness, and an increase of inclusions. Therefore, Si content
is set to be in the range of 0.02% to 0.20% and preferably in the
range of 0.03% to 0.10%.
[0029] Mn: 0.45% to 2.00%
[0030] Mn is an effective element from the viewpoint of ensuring
quench hardenability and enhancing strength. If Mn content is less
than 0.45%, the effect thereof cannot be sufficiently obtained. On
the other hand, adding Mn in an amount exceeding 2.00% would cause
deterioration of weldability. Therefore, Mn content is set to be in
the range of 0.45% to 2.00%, and preferably in the range of 0.55%
to 1.00%.
[0031] P: 0.020% or less
[0032] Although high P content in steel leads to deterioration of
low temperature toughness, the content thereof of 0.020% or less
would be acceptable. Therefore, the upper limit of P content is set
to be 0.020%.
[0033] S: 0.005% or less
[0034] High S content in steel causes precipitation as MnS, and
this, as an inclusion, becomes the fracture generation origin of
high tensile strength steel and leads to deterioration of
toughness. However, if the content thereof is 0.005% or less, it
would cause no problem. Therefore, the upper limit of S content is
set to be 0.005%.
[0035] Al: 0.005% to 0.100%
[0036] Al is an effective element as a deoxidizer in molten steel
and an effective element for improving low-temperature toughness.
If Al content is less than 0.005%, these effects cannot be
sufficiently obtained. On the other hand, if the content thereof
exceeds 0.100%, weldability will decrease. Therefore, Al content is
set to be in the range of 0.005% to 0.100%, and preferably in the
range of 0.020% to 0.050%.
[0037] Ni: 5.0 to 8.0%
[0038] Ni is an important element for the present invention, and it
is an element that enhances quench hardenability and improves
toughness of ferrite matrix. If Ni content is less than 5.0%, these
effects cannot be sufficiently exhibited. On the other hand, if the
content thereof exceeds 8.0%, costs will increase. Therefore, Ni
content is set to be in a range of 5.0% to 8.0%. In addition, from
the viewpoint of further reducing costs, it is desirable for Ni
content to be in the range of 5.0% to 7.5%.
[0039] In addition to the above basic chemical compositions, it is
possible to contain at least one element selected from Cr and Mo,
as a first group of selected components, if necessary, in the
following ranges.
[0040] Cr: 1.00% or less
[0041] Cr enhances quench hardenability and provides an effect of
improving low-temperature toughness by refining martensite phase.
However, if the content thereof exceeds 1.00%, it would cause
deterioration of weldability and an increase in manufacturing
costs. Therefore, when containing Cr, the content thereof is set to
be in the range of 1.00% or less. In order to effectively exhibit
the above effect, it is preferable for the Cr content to be 0.05%
or more, and more preferably in the range of 0.10% to 0.75%.
[0042] Mo: 1.000% or less
[0043] Mo enhances quench hardenability and provides an effect of
improving low-temperature toughness by refining martensite phase.
However, if the content thereof exceeds 1.000%, it would cause
deterioration of weldability and an increase in manufacturing
costs. Therefore, when containing Mo, the content thereof is set to
be in the range of 1.000% or less. In order to effectively exhibit
the above effects, it is preferable for the content thereof to be
0.005% or more, and more preferably in the range of 0.010% to
0.500%.
[0044] Further, in the present invention, it is possible to contain
at least one element selected from Cu, V, Nb, Ti, and B as a second
group of selected components, if necessary, in the following
ranges.
[0045] Cu: 1.00% or less
[0046] Cu is an element that enhances quench hardenability.
However, if the content thereof exceeds 1.00%, it would cause
reduction of hot workability and an increase in costs. Therefore,
when containing Cu, the content thereof is set to be in the range
of 1.00% or less. In order to effectively exhibit the above effect,
it is preferable for the content thereof to be 0.05% or more.
[0047] V: 0.100% or less
[0048] V is an element that precipitates as carbonitride, has an
effect of refining microstructures, and is useful for improving
toughness. However, if the content thereof exceeds 0.100% it would
cause deterioration of weldability. Therefore, when containing V,
the content thereof is set to be in the range of 0.100% or less. In
order to effectively exhibit the above effects, it is preferable
for the content thereof to be 0.005% or more.
[0049] Nb: 0.100% or less
[0050] Nb is an element that precipitates as carbonitride, has an
effect of refining microstructures, and is useful for improving
toughness. However, if the content thereof exceeds 0.100%, it would
cause deterioration of weldability. Therefore, when containing Nb,
the content thereof is set to he in the range of 0.100% or less. In
order to effectively exhibit the above effects, it is preferable
for the content thereof to be 0.005% or more.
[0051] Ti: 0.100% or less
[0052] Ti has an effect of improving toughness by fixing solute N,
which is harmful to toughness, as TiN. However, if the content
thereof exceeds 0.100%, it would cause precipitation of a coarse
carbonitride, and deteriorate toughness. Therefore, when containing
Ti, the content thereof is set to be in the range of 0.100% or
less. In order to effectively exhibit the above effect, it is
preferable for the content thereof to be 0.005% or more, and more
preferably in the range of 0.010% to 0.050%.
[0053] B: 0.0030% or less
[0054] B is an element that enhances quench hardenability when
added to steel by a small amount. However, if the content thereof
exceeds 0.0030%, it would cause deterioration of toughness.
Therefore, when containing B, the content thereof is set to be in
the range of 0.0030% or less. In order to effectively exhibit the
above effect, it is preferable for the content thereof to be
0.0003% or more.
[0055] Further, in the present invention, it is possible to contain
at least one element selected from Ca and REM as a third group of
selected components, if necessary, in the following ranges.
[0056] Ca: 0.0050% or less
[0057] Ca is an element that fixes S and inhibits generation of MnS
which becomes the cause of reduction in toughness. However, if the
content thereof exceeds 0.0050%, it would cause an increase in the
amount of inclusions existing in steel and lead to deterioration of
toughness rather than providing the above effect. Therefore, when
containing Ca, the content thereof is set to be in the range of
0.0050% or less. In order to effectively exhibit the above effect,
it is preferable for the content thereof to be 0.0005% or more.
[0058] REM: 0.0050% or less
[0059] REM (Rare Earth Metal) is an element that fixes S and
inhibits generation of MnS which becomes the cause of reduction in
toughness. However, if the content thereof exceeds 0.0050%, it
would cause an increase in the amount of inclusions existing in
steel and lead to deterioration of toughness rather than providing
the above effect. Therefore, when containing REM, the content
thereof is set to be in the range of 0.0050% or less. In order to
effectively exhibit the above effect, it is preferable for the
content thereof to be 0.0005% or more.
[0060] The balance other than the components described above
includes Fe and incidental impurities.
[0061] (2) Microstructure
[0062] Next, the microstructure will be described.
[0063] The Ni-containing steel plate of the present invention has
the above chemical composition, and also has a microstructure
containing less than 1.7% of retained austenite when cooled to
liquid nitrogen temperature, and having an average grain size of
crystal grains surrounded by high-angle grain boundaries with an
orientation difference of 15.degree. or more of 5 .mu.m or less by
equivalent circle diameter.
[0064] Since the steel plate of the present invention is used
mainly in storage tanks for LNG, the microstructure at -165.degree.
C. where LNG tanks are used is important. Therefore, the
microstructure after sub-zero treatment where the steel plate is
held at liquid nitrogen temperature, is defined. If the amount of
retained austenite remaining after sub-zero treatment is 1.7% or
more by volume fraction, sufficient low-temperature toughness
cannot be obtained. Some reports have been made that retained
austenite improves low temperature toughness. However, for the
Ni-containing steel plate of the present invention, retained
austenite has a harmful effect on toughness. It is considered that
this is due to the fact that, since in Ni-containing steel plate of
the present invention, the Ni content is smaller than the Ni
content in conventional 9% Ni steel, even if retained austenite
exists at -165.degree. C., it is unstable, and if the steel
structure undergoes plastic deformation at the tip of a crack, the
retained austenite transforms into martensite by plasticity-induced
martensite phase transformation. Therefore, the amount of retained
austenite when the steel plate is cooled to liquid nitrogen
temperature is set to be less than 1.7% by volume fraction. This
amount is preferably 1.0% or less, and more preferably 0.5% or
less.
[0065] Further, if the average grain size of crystal grains
surrounded by high-angle grain boundaries with an orientation
difference of 15.degree. or more exceeds 5 .mu.m by equivalent
circle diameter, sufficient low-temperature toughness cannot be
obtained. Therefore, the average grain size of crystal grains
surrounded by high-angle grain boundaries with an orientation
difference of 15.degree. or more is set to be 5 .mu.m or less by
equivalent circle diameter, and preferably 3 .mu.m or less by
equivalent circle diameter.
[0066] (3) Manufacturing Condition
[0067] Next, a preferable manufacturing condition for manufacturing
the steel plate of the present invention having the above described
chemical composition and the above microstructure will be
described. The following manufacturing condition is merely an
example of a condition for manufacturing the Ni-containing steel
plate of the present invention, and as long as the Ni-containing
steel plate of the present invention can be obtained, manufacturing
condition for the present invention is not limited to the following
manufacturing condition.
[0068] In the present invention, it is preferable to heat a slab or
a steel billet having the above described chemical composition at a
temperature range of 900.degree. C. to 1100.degree. C. for 10 hours
or less, and then to subject it to hot rolling at a temperature
range of 870.degree. C. or lower so that the cumulative rolling
reduction ratio is 40% or more and 70% or less and the finisher
delivery temperature is between 700.degree. C. and 820.degree. C.,
and then to subject the obtained hot rolled steel plate to direct
quenching treatment where quenching is immediately performed until
reaching a temperature of 200.degree. C. or lower at a cooling rate
of 5.degree. C./s or more, and then to heat the steel plate to a
temperature range of 500.degree. C. to 650.degree. C. at a heating
rate of 0.05.degree. C./s to 1.0.degree. C./s, and then to subject
the steel plate to tempering by holding the temperature at the same
temperature range for 10 minutes or more and 60 minutes or
less.
[0069] Heating Temperature: 900.degree. C. to 1100.degree. C.,
Heating duration: 10 hours or less
[0070] In a case where the heating temperature is lower than
900.degree. C., coarse AlN which precipitates during the stage of
casting of the steel slab does not dissolve, and toughness
decreases. Further, the following rolling conditions cannot be
substantially satisfied. If the heating temperature exceeds
1100.degree. C, austenite becomes coarse grains and toughness will
decrease. If the heating duration exceeds 10 hours, austenite
grains become coarse and toughness decreases. Therefore, the
heating temperature is set to be between 900.degree. C. and
1100.degree. C., and the heating duration is 10 hours or less.
[0071] Rolling Reduction Ratio: Cumulative Rolling Reduction Ratio
of 40% or more and 70% or less at 870.degree. C. or lower
[0072] If the cumulative rolling reduction ratio in the
non-recrystallized region of austenite at 870.degree. C. or lower
is less than 40%, refinement of martensite phase will not be
sufficient, and toughness decreases. On the other hand, in a case
where the cumulative rolling reduction ratio exceeds 70%, it is
difficult to substantially perform rolling at the following
finisher delivery temperature. Therefore, the rolling reduction
ratio is set to be 40% or more and 70% or less at 870.degree. C. or
lower.
[0073] Finisher delivery temperature: 700.degree. C. to 820.degree.
C.
[0074] If the finisher delivery temperature is lower than
700.degree. C., it results in .alpha.-.gamma. dual phase rolling so
that bainite phase forms, and therefore a desired strength cannot
be satisfied. On the other hand, if the finisher delivery
temperature exceeds 820.degree. C., it becomes substantially
difficult to perform sufficient rolling reduction in the
non-recrystallized region of austenite, a fine microstructure
cannot be obtained, and toughness decreases. Therefore, the
finisher delivery temperature is set to be in the range of
700.degree. C. to 820.degree. C.
[0075] Cooling (Direct Quenching): Start immediately after
rolling
[0076] Cooling (direct quenching) is started immediately after
rolling is finished. If cooling is not immediately started, bainite
phase will generate, and. therefore a desired strength cannot be
satisfied. Therefore, cooling is started immediately after rolling
is finished. Here, "immediately" refers to a point in time within
120 seconds after the completion of rolling.
[0077] Cooling Rate; 5.degree. C./s or more
[0078] In a case where the cooling rate is less than 5.degree.
C./s, transformation to martensite phase will not occur, and a
desirable strength and toughness cannot be obtained. Therefore, the
cooling rate is set to be 5.degree. C./s or more. Preferably, the
cooling rate is 10.degree. C./s or more.
[0079] Cooling Stop Temperature: 200.degree. C. or lower
[0080] In a case where the cooling stop temperature exceeds
200.degree. C., transformation to martensite phase will not Occur
uniformly in the steel plate, and a desirable strength and
toughness cannot be obtained. Therefore, the cooling stop
temperature is set to be 200.degree. C. or lower.
[0081] Tempering Heating Rate: 0.05.degree. C./s to 1.0.degree.
C./s
[0082] In a case where the tempering heating rate is less than
0.05.degree. C./s, the precipitated carbide would become coarse,
and toughness will decrease. On the other hand, in order to perform
rapid short time beating where the tempering heating rate exceeds
1.0.degree. C./s, induction heating facilities and the like will be
required, and costs will increase. Therefore, the tempering heating
rate is set to be in the range of 0.05.degree. C./s to 1.0.degree.
C./s.
[0083] Tempering temperature: 500.degree. C. to 650.degree. C.
[0084] In a case where the tempering temperature is lower than
500.degree. C., toughness improving effect caused by precipitation
of fine carbides such as cementite cannot, be sufficiently
obtained. On the other hand, in a case were the tempering
temperature exceeds 650.degree. C., coarse carbide precipitates,
and toughness decreases. Therefore, the tempering temperature is
set to be in the range of 500.degree. C. to 650.degree. C.
[0085] Tempering Holding Time: 10 minutes or more and 60 minutes or
less
[0086] In a case where the tempering holding time is less than 10
minutes, toughness improving effect caused by precipitation of fine
carbides such as cementite cannot be sufficiently obtained. On the
other hand, in a case where the tempering holding time exceeds 60
minutes, toughness will decrease due to reasons such as
precipitation of a coarse carbide. Further, manufacturing costs
will increase. Therefore, the tempering holding time is set to be
10 minutes or more and 60 minutes or less. Cooling, after tempering
may be performed by either water cooling or air cooling. However,
if the cooling rate is too fast, the temperature difference between
the surface and the inside of the steel plate becomes large and
causes formation of strains inside the steel plate and low
temperature toughness decreases. Therefore, the cooling rate is
preferably 5.degree. C./s or less.
[0087] In the aforementioned manufacturing condition, after direct
quenching, dual phase heat treatment where the steel plate is
heated to a temperature range from 650.degree. C. to 800.degree. C.
at a heating rate of 0.1.degree. C./s to 1.5.degree. C./s, held at
the same temperature range for 10 minutes or more and 60 minutes or
less, and then subjected to quenching until reaching a temperature
of 200.degree. C. or lower at a cooling rate of 5.degree. C./s or
more, may be performed.
[0088] Dual Phase Heat Treatment Heating Rate: 0.1.degree. C./s to
1.5.degree. C./s
[0089] By performing dual phase heat treatment, part of the
microstructure transforms into austenite, and as crystal grains
become fine, tempering proceeds and thereby improves toughness.
However, in a case where the dual phase heat treatment heating rate
is less than 0.1.degree. C./s, austenite grains become coarse and
toughness decreases. Further, since the microstructure generated
after cooling also becomes coarse, toughness decreases. On the
other hand, in a case where the heating rate exceeds 1.5.degree.
C./s, induction heating facilities and the like are required, and
costs increase. Therefore, the dual phase heat treatment heating
rate is set to be in the range of 0.1.degree. C./s to 1.5.degree.
C./s.
[0090] Dual Phase Heat Treatment Temperature: 650.degree. C. to
800.degree. C.
[0091] In a case where the dual phase heat treatment temperature is
lower than 650.degree. C., sufficient austenite reverse
transformation does not occur, and refining effect of the
microstructure cannot be obtained, and therefore a toughness
improving effect cannot be obtained. Further, since the amount of
austenite reverse transformation is small, C easily concentrates in
austenite, and retained austenite increases. On the other hand, if
the dual phase heat treatment temperature exceeds 800.degree. C.,
reverse transformation austenite becomes coarse and toughness
decreases. Further, since the microstructure after cooling becomes
coarse, toughness decreases. Further, manufacturing costs increase.
Therefore, the dual phase heat treatment temperature is set to be
in the range of 650.degree. C. to 800.degree. C. In a case where
the dual phase heat treatment temperature is high, the amount of
reverse transformation austenite increases and the amount of
concentration of C in reverse transformation austenite decreases
compared to a case where the dual phase heat treatment temperature
is low, and therefore the amount of martensite transformation
caused h cooling after dual phase heat treatment increases, and the
amount of retained austenite decreases. Therefore, the dual phase
heat treatment temperature is preferably in the range of
720.degree. C. to 780.degree. C.
[0092] Dual Phase Heat Treatment Holding Time: 10 minutes or more
and 60 minutes or less
[0093] If the dual phase heat treatment holding time is less than
10 minutes, sufficient austenite reverse transformation does not
occur and toughness improving effect caused by refinement of the
microstructure cannot be sufficiently obtained. On the other hand,
in a case where the dual phase heat treatment holding time exceeds
60 minutes, austenite grains become coarse and toughness decreases.
Further, since the microstructure generated after cooling also
becomes coarse, toughness decreases. Since C concentrates in
austenite, retained austenite increases. Manufacturing costs
increase as well. Therefore, the dual phase heat treatment holding
time is set to be 10 minutes or more and 60 minutes or less.
[0094] Cooling Rate after Dual Phase Heat Treatment: 5.degree. C./s
or more
[0095] In a case where the cooling rate is less than 5.degree.
C./s, transformation from austenite to martensite phase will not
occur, and a desirable strength and toughness cannot be obtained.
Further, if the cooling rate is slow, the amount of solute C in
ferrite decreases as the temperature is lowered, and therefore C
moves to austenite from the ferrite surrounding the reverse
transformed austenite, and C concentrates in the austenite and the
austenite tends to remain as retained austenite. Therefore, the
cooling rate is set to be 5.degree. C./s or more. Preferably, the
cooling rate is 10.degree. C./s or more.
[0096] Cooling Stop Temperature after Dual Phase Heat Treatment:
200.degree. C. or lower
[0097] In a case where the cooling stop temperature exceeds
200.degree. C., transformation to martensite phase will not occur
uniformly in the steel plate, and a desirable strength and
toughness cannot be obtained. Further, C concentrates in austenite
and tends to remain as retained austenite. Therefore, the cooling
stop temperature is set to be 200.degree. C. or lower.
[0098] After performing the dual phase heat treatment and cooling,
until reaching 200.degree. C. or lower, tempering is conducted in
the manner previously described. That is, the steel is heated to a
temperature range of 500.degree. C. to 650.degree. C. at a heating
rate of 0.05.degree. C./s to 1.0.degree. C./s, and then subjected
to tempering by holding the temperature at the same temperature
range for 10 minutes or more and 60 minutes or less.
EXAMPLES
[0099] Next, Examples of the present invention will be
described.
[0100] Molten steels with the chemical compositions shown in table
I were obtained by steelmaking in a vacuum melting, furnace and
made into small-sized steel ingots (150 kg). These steels were
heated in the conditions shown in table 2, subjected to hot rolling
until reaching a plate thickness of 7 mm to 50 mm, and then
subjected to quenching just after the rolling. Some of the steel
plates were then subjected to tempering treatment. Regarding the
rest of the steel plates, after quenching, they were subjected to
dual phase heat treatment and then to tempering treatment. The
obtained steel plates were each subjected to a tensile test, a
Charpy impact test, a measurement of austenite volume fraction, and
a measurement of grain size of crystal grains surrounded by
high-angle grain boundaries with an orientation difference of
15.degree. or more, in the manner described below.
TABLE-US-00001 TABLE 1 Steel Chemical Composition (mass %) No. C Si
Mn P S Al Ni Cr Mo Cu V Nb Ti B Ca REM Remarks A 0.06 0.06 1.21
0.005 0.0011 0.035 5.7 -- -- -- -- -- -- -- -- -- Inventive Example
B 0.07 0.09 0.95 0.010 0.0009 0.033 7.2 -- -- -- -- -- -- -- -- --
Inventive Example C 0.05 0.04 0.67 0.003 0.0012 0.029 7.8 -- --
0.12 -- -- -- -- -- -- Inventive Example D 0.09 0.03 1.06 0.009
0.0010 0.028 6.9 0.12 -- -- 0.043 -- -- -- 0.0023 -- Inventive
Example E 0.03 0.05 0.88 0.004 0.0012 0.033 7.4 0.72 -- -- -- -- --
-- -- -- Inventive Example F 0.02 0.06 1.36 0.008 0.0011 0.036 7.6
-- 0.03 -- -- -- -- -- -- -- Inventive Example G 0.05 0.08 0.63
0.006 0.0008 0.024 6.8 -- 0.41 -- -- 0.014 -- -- -- -- Inventive
Example H 0.04 0.07 0.97 0.011 0.0008 0.031 7.3 -- -- 0.23 -- --
0.015 0.0012 -- 0.0018 Inventive Example I 0.06 0.05 1.02 0.005
0.0009 0.030 4.9 -- -- -- -- -- -- -- -- -- Comparative Example The
underlined values are outside the scope of the invention.
[0101] Tensile Test
[0102] From each steel plate, at a position of a half the plate
thickness, and in the roiling direction, a tensile test specimen
having a parallel portion length of 30 mm, GL of 24 mm, a parallel
portion diameter of 6.phi. was collected and subjected to a tensile
test at room temperature. From the obtained stress-strain curve,
tensile strength (TS) and yield strength (YS) were calculated. TS
of 690 MPa or more and YS of 590 MPa or more are each considered as
excellent TS and YS.
[0103] [Charpy Impact Test]
[0104] From each steel plate, at a position of a half the plate
thickness, and in a direction orthogonal to the rolling direction,
V-notch test specimens were collected in accordance with JIS Z2202
(1998) standard, and subjected to a Charpy impact test with 3
specimens per each temperature for each steel plate in accordance
with JIS Z2242 (1998) standard, and absorbed energy at -196.degree.
C. was measured to evaluate base material toughness. Steel plates
with an average value of absorbed energy (vE..sub.196) of 3
specimens of 150 J or more are considered as having excellent base
material toughness.
[0105] [Austenite Volume Fraction]
[0106] Samples collected from each steel plate at a position of a
half the plate thickness in a direction orthogonal to the rolling
direction were subjected to sub-zero treatment for 10 minutes in
liquid nitrogen, and then the austenite volume fraction was
measured by X-ray diffraction.
[0107] [Measurement of Grain Size of Crystal Grains]
[0108] Samples collected from each steel plate at a position of a
half the plate thickness in a direction orthogonal to the rolling
direction were polished and mirror finished, and subjected to EBSP
analysis. Among the obtained data, a high-angle grain boundary
where the orientation difference between two crystal grains
contacting the grain boundary is 15.degree. or more was selected
and the average grain size by equivalent circle diameter of the
region surrounded by the high-angle grain boundary was
obtained.
[0109] The obtained results are shown in Table 2.
[0110] As shown in table 2, it has been confirmed that the
inventive examples have excellent low-temperature toughness whereas
the comparative examples outside the scope of the present invention
have reduced low-temperature toughness.
TABLE-US-00002 TABLE 2 Temp. Dual Dual Cooling Rolling Start of
Cool- Phase Heat Dual Phase Heat Rate Plate Heat- Reduc- Finisher
of Starting Cool- ing Treatment Phase Heat Treatment after Dual
Steel Thick- ing tion Delivery Cool- Cool- ing Stop Heating
Treament Holding Phase Heat Plate Steel ness Temp. Ratio* Temp.
ing** ing Rate Temp. Rate Temp. Time Treatment No. No. (mm)
(.degree. C.) (%) (.degree. C.) (s) (.degree. C.) (.degree. C./s)
(.degree. C.) (.degree. C./s) (.degree. C.) (min) (.degree. C./s) 1
A 25 1050 50 780 34 760 25 150 0.34 750 20 22 2 A 25 1050 60 750 35
730 25 150 0.37 640 20 22 3 A 25 1050 55 730 38 710 25 150 -- -- --
-- 4 B 25 1050 0 900 30 880 25 150 0.33 740 30 22 5 B 25 1050 55
800 34 780 25 150 0.33 740 20 22 6 B 25 1050 55 800 34 780 25 150
0.33 630 30 22 7 B 25 1050 55 800 34 780 25 150 0.33 740 120 22 8 B
25 1050 55 800 34 780 25 150 0.33 740 30 2 9 B 25 1050 55 800 34
780 25 150 0.33 700 30 20 10 B 25 1000 60 750 36 730 25 150 0.31
730 20 22 11 B 25 1000 55 740 37 720 25 150 0.29 720 15 22 12 B 25
1000 50 800 33 780 25 100 -- -- -- -- 13 B 25 1050 60 740 37 720 25
150 -- -- -- -- 14 B 25 1000 55 910 39 870 25 100 -- -- -- -- 15 B
7 1000 65 800 18 780 40 100 0.64 740 10 37 16 B 50 1050 50 780 60
760 12 150 0.19 720 20 9 17 C 25 1050 60 780 34 760 25 150 0.31 730
20 22 18 C 25 1050 55 800 33 780 25 150 -- -- -- -- 19 C 25 1050 55
810 32 790 25 150 -- -- -- -- 20 C 50 1050 60 800 32 780 12 100
0.20 720 20 9 21 D 25 1050 65 750 36 730 25 150 0.35 780 50 22 22 E
25 1200 50 800 33 780 25 100 0.34 750 20 22 23 E 25 950 55 790 33
770 25 150 0.33 740 20 22 24 F 25 1050 60 770 35 750 25 150 0.31
730 30 22 25 F 25 1050 30 800 33 780 25 150 0.19 720 20 22 26 G 25
1050 65 730 37 710 25 100 0.18 730 20 22 27 H 25 1050 55 750 36 730
25 100 0.29 700 20 22 28 H 25 1050 60 780 34 760 25 150 -- -- -- --
29 I 25 1050 60 800 33 780 25 150 0.34 760 20 22 Cooling Ave. Grain
Stop Temp. Size by after Dual Tempering Tempering Cooling Austenite
Equivalent Steel Phase Heat Heating Tempering Holding Rate after
Volume Circle Plate Treatment Rate Temp. Time Tempering TS YS
vE-196 Fraction Diameter No. (.degree. C.) (.degree. C.) (.degree.
C.) (min) (.degree. C./s) (MPa) (MPa) (J) (%) (.mu.m) Remarks 1 100
0.18 570 20 0.4 698 642 225 0.2 2.3 Inventive Example 2 150 0.19
580 20 0.4 711 382 121 2.0 5.6 Comparative Example 3 -- 0.17 560 15
0.4 701 650 156 0.3 3.9 Inventive Example 4 75 0.18 570 15 0.4 722
699 120 0.2 5.8 Comparative Example 5 125 0.19 580 20 0.4 740 715
235 0.2 1.9 Inventive Example 6 125 0.19 580 20 0.4 810 785 56 2.6
3.6 Comparative Example 7 125 0.19 580 20 0.4 822 765 98 1.8 5.1
Comparative Example 8 100 0.19 580 20 0.4 752 722 110 1.8 3.2
Comparative Example 9 350 0.19 580 20 0.4 720 735 103 1.9 2.5
Comparative Example 10 100 0.19 580 25 0.4 731 685 220 0.3 1.4
Inventive Example 11 150 0.19 580 20 0.4 705 615 245 0.3 1.1
Inventive Example 12 -- 0.18 570 40 0.4 715 682 160 0.1 4.3
Inventive Example 13 -- 0.20 590 15 0.4 735 719 152 0.1 4.0
Inventive Example 14 100 0.19 580 20 0.4 730 705 116 0.3 7.3
Comparative Example 15 150 0.53 590 20 1.2 745 719 225 0.2 1.3
Inventive Example 16 100 0.13 600 15 0.2 721 674 167 1.4 2.5
Inventive Example 17 100 0.19 580 20 0.4 749 713 234 0.2 1.4
Inventive Example 18 -- 0.2 590 30 0.4 720 687 174 0.3 4.1
Inventive Example 19 -- 0.14 520 20 0.4 736 701 151 0.1 4.6
Inventive Example 20 100 0.11 560 15 0.2 732 699 173 1.1 2.0
Inventive Example 21 150 0.19 580 20 0.4 726 694 219 0.1 2.0
Inventive Example 22 75 0.17 560 15 0.4 713 689 88 0.5 6.7
Comparative Example 23 100 0.18 570 20 0.4 720 692 239 0.2 1.2
Inventive Example 24 100 0.2 590 20 0.4 721 675 215 0.1 1.5
Inventive Example 25 100 0.19 580 30 0.4 712 653 102 0.3 5.7
Comparative Example 26 150 0.13 600 25 0.4 716 665 258 0.2 1.0
Inventive Example 27 150 0.19 580 20 0.4 721 653 182 1.2 1.4
Inventive Example 28 -- 0.23 630 20 0.4 702 643 176 0.2 4.1
Inventive Example 29 100 0.16 540 20 0.4 675 621 76 0.1 1.9
Comparative Example The underlined values are outside the scope of
the invention. *Cumulative rolling reduction ratio at 870.degree.
C. or lower **Time from when finishing rolling is completed to when
cooling is started
* * * * *