U.S. patent application number 15/599497 was filed with the patent office on 2017-12-14 for maraging steel.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). The applicant listed for this patent is Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). Invention is credited to Zhuyao CHEN, Takeo MIYAMURA, Shigenobu NAMBA.
Application Number | 20170356070 15/599497 |
Document ID | / |
Family ID | 58671326 |
Filed Date | 2017-12-14 |
United States Patent
Application |
20170356070 |
Kind Code |
A1 |
MIYAMURA; Takeo ; et
al. |
December 14, 2017 |
MARAGING STEEL
Abstract
Disclosed is a mar aging steel containing, in combination in
mass percent, C in a content from greater than 0% to 0.02%, Mn in a
content from greater than 0% to 0.3%, Si in a content from greater
than 0% to 0.3%, Ni in a content of 10% to 13%, Mo in a content of
0.5% to 3.5%, Co in a content of 9% to 12%, Cr in a content of 1.5%
to 4.5%, Ti in a content of 1.5% to 4.5%, and Al in a content of
0.01% to 0.2%, where the total content of Mo and Ti is 5.0 mass
percent or less, and the ratio ([Mo]/[Ti]) of the Mo content [Mo]
to the Ti content [Ti] is 1.0 or less, with the remainder
consisting of iron and inevitable impurities.
Inventors: |
MIYAMURA; Takeo; (Kobe-shi,
JP) ; NAMBA; Shigenobu; (Kobe-shi, JP) ; CHEN;
Zhuyao; (Kobe-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) |
Kobe-shi |
|
JP |
|
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
Kobe-shi
JP
|
Family ID: |
58671326 |
Appl. No.: |
15/599497 |
Filed: |
May 19, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/004 20130101;
C22C 38/06 20130101; C22C 38/28 20130101; C21D 6/02 20130101; C22C
38/22 20130101; C22C 38/04 20130101; C21D 1/18 20130101; C22C
38/001 20130101; C22C 38/02 20130101; C22C 38/44 20130101; C22C
38/50 20130101; C21D 6/004 20130101; C21D 2211/008 20130101; C21D
6/007 20130101; C22C 38/105 20130101; C22C 38/52 20130101 |
International
Class: |
C22C 38/10 20060101
C22C038/10; C22C 38/04 20060101 C22C038/04; C22C 38/28 20060101
C22C038/28; C22C 38/00 20060101 C22C038/00; C22C 38/50 20060101
C22C038/50; C22C 38/22 20060101 C22C038/22; C22C 38/06 20060101
C22C038/06 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 8, 2016 |
JP |
2016-114225 |
Claims
1. A maraging steel comprising, in mass percent: C in a content
from greater than 0% to 0.02%; Mn in a content from greater than 0%
to 0.3%; Si in a content from greater than 0% to 0.3%; Ni in a
content of 10% to 13%; Mo in a content of 0.5% to 3.5%; Co in a
content of 9% to 12%; Cr in a content of 1.5% to 4.5%; Ti in a
content of 1.5% to 4.5%; and Al in a content of 0.01% to 0.2%, a
total content of Ti and Mo being 5.0% or less, a ratio ([Mo]/[Ti])
of the Mo content [Mo] to the Ti content [Ti] being 1.0 or less,
with the remainder consisting of iron and inevitable
impurities.
2. The maraging steel according to claim 1, wherein the maraging
steel has: a phosphorus (P) content from greater than 0% to 0.01%;
a nitrogen (N) content from greater than 0% to 0.01%; and a sulfur
(S) content from greater than 0% to 0.01%, where P, N, and S are
present in the inevitable impurities.
3. The maraging steel according to claim 1, wherein the maraging
steel has a surface hardness in terms of Vickers hardness of 400 Hv
or more.
4. The maraging steel according to claim 2, wherein the maraging
steel has a surface hardness in terms of Vickers hardness of 400 Hv
or more.
Description
FIELD OF INVENTION
[0001] The present invention relates to managing steels.
BACKGROUND OF INVENTION
[0002] Gas turbines and steam turbines for use in thermal power
facilities each include a rotor and blades, where the rotor acts as
a rotating shaft. The rotor functionally supports the blades and
transmits turning force (torque) to a generator. The rotor, as to
be exposed to a high-temperature environment at about 500.degree.
C., is made of any of heat-resisting materials. Such heat-resisting
materials are mainly selected from ferritic heat-resisting steels
and Ni-based alloys.
[0003] Exemplary practically used ferritic heat-resisting steels
for rotors are high-chromium ferritic steels such as 12%-Cr steels.
The high-chromium ferritic steels, however, are significantly
inferior in strength at high temperatures (high-temperature
strength) to Ni-based alloys, which are expensive. Austenitic
stainless steels, which are widely used as general heat-resisting
materials, are not suitable as materials for rotors, which are
large-scale members. This is because the austenitic stainless
steels have high coefficients of thermal expansion, although they
have high-temperature strength lying midway between that of the
ferritic heat-resisting steels and that of the Ni-based alloys. In
addition to these materials, exemplary known heat-resisting
materials for rotors include precipitation-hardened iron-based high
heat-resistance alloys (superalloys) such as "A286", and
precipitation-hardened ferritic heat-resisting steels such as
maraging steels.
[0004] Of these heat-resisting materials, the maraging steels are
materials strengthened (hardened) by aging precipitation of
martensitic phases and intermetallic compounds and are produced via
quenching and aging heat treatments. The maraging steels are
significantly superior in high-temperature strength to ferritic
heat-resisting steels. Disadvantageously, however, the maraging
steels have lower toughness when designed to have such a chemical
composition and to be subjected to a heat treatment under such
conditions as to offer high strength at high temperatures. In
particular, steels for rotors of gas turbines and steam turbines
for use in thermal power facilities require excellent toughness,
because high heat stress is generated when the temperatures of the
steels fall down to mom temperature during suspension of
operation.
[0005] There have been proposed various techniques so as to improve
both strength and toughness. For example, Japanese Patent No.
5362995 proposes a stainless steel alloy including, by weight:
0.002% to 0.015% carbon (C), 2% to 15% cobalt (Co), 7.0% to 14.0%
nickel (Ni), 8.0% to 15.0% chromium (Cr), 0.5% to 2.6% molybdenum
(Mo), 0.4% to 0.75% titanium (TO, less than 0.5% tungsten (W), less
than 0.7% aluminum (Al), with the balance essentially iron (Fe) and
incidental elements and impurities. The alloy avoids copper (Cu) as
an alloying constituent, has a lath martensite microstructure
having undergone predetermined treatments, and has a volume
fraction of retained austenite of less than 15%, essentially
without topologically close packed (TCP) intermetallic phases. In
the alloy, the carbon (C) is in a dispersion of 0.02% to 0.15% by
volume TIC carbide particles. The alloy further includes a
dispersion of intermetallic particles primarily of Ni.sub.3Ti.eta.
phase as a strengthening phase.
[0006] Japanese Unexamined Patent Application Publication (JP-A)
No. 2015-61932 proposes a maraging steel excellent in fatigue
characteristics. The maraging steel has a chemical composition
including, in mass percent: C in a content of 0.015% or less, Ni in
a content of 12.0% to 20.0%, Mo in a content of 3.0% to 6.0%, Co in
a content of 5.0% to 13.0%, Al in a content of 0.01% to 0.3%, Ti in
a content of 0.2% to 2.0%, 0 in a content of 0.0020% or less, N in
a content of 0.0020% or less, and Zr in a content of 0.001% to
0.02%, with the balance being Fe and unavoidable impurities.
[0007] JP-A No. Hei04(1992)-59922 proposes a method for producing a
maraging steel. The method includes subjecting a maraging steel to
a recrystallization solution treatment, an unrecrystallized
solution treatment, and an aging heat treatment. The maraging steel
contains, in mass percent, C in a content of 0.05% or less, Si in a
content of 0.2% or less, Mn in a content of 0.2% or less, Pin a
content of 0.05% or less, S in a content of 0.05% or less, Ni in a
content of 10.0% to 21.0%, Co in a content of 9.5% to 15.0%, Mo in
a content of 3.0% to 12.0%, Ti in a content of 0.2% to 1.6%, Al in
a content of 0.30% or less, and Bin a content of 0.0005% to
0.0020%, and the managing steel has undergone hot forming. In the
method, the recrystallization solution treatment is performed as a
two-stage treatment including heating in a temperature range of
from 1000.degree. C. to 1180.degree. C. for one minute or longer,
cooling at a cooling rate of 20.degree. C./min or more, and further
heating in a temperature range of from 800.degree. C. to
950.degree. C. for one minute or longer, and then cooling.
SUMMARY OF INVENTION
[0008] The technique described in Japanese Patent No. 5362995
allows a stainless steel alloy to have higher strength and better
toughness by adjusting the chemical composition and microstructure
of the alloy. The technique evaluates strength and room-temperature
toughness, but fails to evaluate strength at high temperatures of
about 500.degree. C., to which high temperatures the present
invention is to be applied.
[0009] The technique described in JP-A No. 2015-61932 offers
excellent fatigue strength by refinement of TiN inclusions, but
fails to evaluate strength at high temperatures of about
500.degree. C., to which high temperatures the present invention is
to be applied. Maraging steels offer more excellent
high-temperature strength as compared with ferritic heat-resisting
steels. The maraging steels, however, do not always maintain such
excellent high-temperature strength as intact when controlled to
have higher fatigue strength and better toughness.
[0010] JP-A No. Hei04(1992)-59922 mentions that a maraging steel
having strength, toughness, and ductility at better levels is
obtained by appropriately controlling the heat treatment
conditions. However, the technique described in this literature
also fails to evaluate strength at high temperatures to which the
present invention is to be applied, as with the technique described
in Japanese Patent No. 5362995 and JP-A No. 2015-61932.
[0011] The present invention has been made under these
circumstances and has an object to improve the toughness of a
maraging steel which is more inexpensive as compared with Ni-based
alloys and has higher strength at high temperatures as compared
with ferritic heat-resisting steels and to provide a maraging steel
having high-temperature strength and room-temperature toughness
both at excellent levels.
[0012] The present invention has achieved the object and provides,
in an embodiment, a maraging steel containing, in combination, in
mass percent, C in a content from greater than 0% to 0.02%, Mn in a
content from greater than 0% to 0.3%, Si in a content from greater
than 0% to 03%, Ni in a content of 10% to 13%, Mo in a content of
0.5% to 3.5%, Co in a content of 9% to 12%, Cr in a content of 1.5%
to 4.5%, Ti in a content of 1.5% to 4.5%, and Al in a content of
0.01% to 0.2%, with the remainder consisting of iron and inevitable
impurities. In the maraging steel, the total content of the Ti and
Mo is 5.0% or less, and the ratio ([Mo]/[Ti]) of the Mo content
[Mo] to the Ti content [Ti] is 1.0 or less.
[0013] The maraging steel according to the present invention
preferably has a phosphorus (P) content from greater than 0% to
0.01%, a nitrogen (N) content from greater than 0% to 0.01%, and a
sulfur (S) content from greater than 0% to 0.01%, where P, N, and S
are present in the inevitable impurities. The maraging steel
preferably has a surface hardness in terms of Vickers hardness of
400 Hv or more.
[0014] The present invention can actually provide a maraging steel
which not only has excellent high-temperature strength by aging
precipitation of intermetallic compounds, but also offers good
room-temperature toughness by controlling the chemical composition
and microstructure. The maraging steel as above offers excellent
high-temperature strength and good room-temperature toughness and
is very useful typically as materials for rotors for use in thermal
power facilities. The maraging steel, when applied to materials for
rotors for use in thermal power facilities, gives rotors which are
inexpensive and still have lighter weights as compared with
conventional Ni-based alloy rotors, and can contribute to improved
generation efficiency and thereby to CO.sub.2 emission control.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0015] The inventors of the present invention made investigations
from various different angles so as to actually provide a maraging
steel which features compatibility between high-temperature
strength and room-temperature toughness. In particular, to achieve
the high-temperature strength, the inventors made intensive
investigations on how the chemical composition and the
microstructure state determined by aging heat treatment after
quenching affect the room-temperature toughness.
[0016] In regular maraging steels, precipitates to perform
precipitation strengthening are generally intermetallic compounds
mainly containing Mo. Assume that a maraging steel has such a
chemical composition as to tend to form such intermetallic
compounds. In this maraging steel, a Laves phase including
Fe.sub.2Mo, which is a binary intermetallic compound, tends to form
upon aging heat treatment of the maraging steel. The maraging
steel, when containing a larger amount of the Laves phase, tends to
readily have lower toughness. In particular, the aging heat
treatment of materials to form rotors, which are large-sized
members, is performed at a high temperature for a long time, and
thereby causes the compound to be readily formed in a large amount,
and this lowers the toughness.
[0017] The inventors then hit on an idea that conversion of the
intermetallic compounds mainly containing Mo into such
intermetallic compounds as not to adversely affect toughness may
actually provide good toughness without occurrence of the
above-mentioned problems. After further investigations, the
inventors found such a chemical composition as to form
intermetallic compounds mainly including Ti, such as Ni.sub.3Ti
intermetallic compound. The present invention has been made on the
basis of these findings.
[0018] A maraging steel having the chemical composition specified
in the present invention, when subjected to an aging heat treatment
under predetermined conditions, has a microstructure in which
finely divided martensite is dispersed in a ferritic phase in which
the Ni.sub.3Ti intermetallic compound is precipitated. This
maraging steel offers such properties as to offer a surface
hardness in terms of Vickers hardness of 400 Hv or more.
[0019] As apparent Am the above-mentioned concept, appropriate
settings of, among the chemical composition, in particular the Mo
and Ti contents and the relationship between them are important in
the maraging steel according to the present invention. Conversion
into the precipitates of intermetallic compounds as above requires
appropriate settings of not only the contents of Mo and Ti and the
total contents of them, but also the ratio ([Mo]/[Ti]) of the Mo
content [Mo] to the Ti content [Ti]. Reasons for the settings of
these factors are as follows.
[0020] Mo: 0.5% to 3.5%, Ti: 1.5% to 4.5%
[0021] Molybdenum (Mo) and titanium (Ti) form precipitates of
various intermetallic compounds mainly containing these elements
and are useful for higher strength and better toughness of the
steel. To offer these advantageous effects effectively, the steel
is controlled to contain Mo in a content of 0.5% or more and Ti in
a content of 1.5% or more, and preferably contains Mo in a content
of 1.0% or more and Ti in a content of 2.0% or more.
[0022] However, the steel, if having an excessively high Mo
content, may suffer from the formation of a larger amount of FeMo,
which adversely affects toughness. To eliminate or minimize this,
the Mo content is controlled to 3.5% or less, preferably 3.0% or
less, and more preferably 2.5% or less. The steel, if having an
excessively high Ti content, may suffer from insufficient
room-temperature durability. To eliminate or minimize this, the Ti
content is controlled to 4.5% or less, preferably 4.0% or less, and
more preferably 3.5% or less.
[0023] Total Content of Mo and Ti: 5.0% or less, Ratio ([Mo]/[Ti]):
1.0 or less
[0024] In addition to the settings of the Mo and Ti contents as
above, providing of intermetallic compounds formed in the steel
mainly including not Mo, but Ti requires the control of the total
content of Mo and Ti to 5.0% or less, and the control of the ratio
([Mo]/[Ti]) to 1.0 or less.
[0025] Increase or decrease of the total content of Mo and Ti
causes toughness and high-temperature strength to vary in a
trade-off manner. To keep toughness and high-temperature strength
in balance, the total content of Mo and Ti is controlled to 5.0% or
less. The steel, if having a total content of Mo and Ti of greater
than 5.0%, has satisfactory high-temperature strength, but fails to
surely have toughness, because of excessive amounts of precipitated
various intermetallic compounds. The total content is preferably
4.0% or less, and more preferably 3.0% or less. The total content
in to terms of lower limit is inevitably 2.0% or more on the basis
of the contents of the respective elements, but is preferably 2.2%
or more.
[0026] In contrast, the steel, if having a ratio ([Mo]/[Ti])
(namely, mass ratio) of the Mo content [Mo] to the Ti content [Ti]
of greater than 1.0, fails to surely have toughness, because of a
larger proportion of the Laves phase. The ratio ([Mo]/[Ti]) is
preferably 0.8 or less, and more preferably 0.6 or less. The ratio
([Mo]/[Ti]) in terms of lower limit is 0.11 or more on the basis of
the respective contents, but is preferably 0.2 or more, and more
preferably 0.3 or more.
[0027] The settings of the total content of Mo and Ti and the ratio
([Mo]/[Ti]) of the Mo content [Mo] to the Ti content [Ti] within
the predetermined ranges allows the steel to have toughness and
high-temperature strength both at satisfactory levels. However, the
aging heat treatment, if performed at an excessively high
temperature and/or for an excessively long time, may fail to give
sufficient high-temperature strength. To eliminate or minimize
this, the temperature and time conditions in the aging are
preferably controlled so as to allow the steel to have a surface
Vickers hardness of 400 Hv or more, as mentioned below.
[0028] In the maraging steel according to the present invention, at
least Mo and Ti are to be controlled as mentioned above, but, in
addition to these elements, elements such as C, Mn, Si, Ni, Co, Cr,
and Al are to be controlled within appropriate ranges. Reasons for
the settings on these elements are as follows.
[0029] C: from greater than 0% to 0.02%
[0030] Carbon (C) forms carbides in a high-temperature environment
to allow the steel to have high-temperature strength and
high-temperature creep strength at higher levels. However, the
carbon content should be minimized so as to maximize the
precipitation of intermetallic compounds mainly containing Ti. The
steel, if having an excessively high carbon content of greater than
0.02%, may contrarily have lower toughness because of formation of
TiC in a larger amount. The carbon content in terms of upper limit
is preferably 0.015% or less, and more preferably 0.010% or less.
The carbon content in terms of lower limit is preferably 0.001% or
more, and more preferably 0.005% or more, so as to allow carbon to
offer basic actions.
[0031] Mn: from greater than 0% to 0.3%
[0032] Manganese (Mn) has a deoxidation action in molten steel. The
element offers the advantageous effect more with an increasing
content of the element. To offer the advantageous effect
effectively, the Mn content is preferably controlled to 0.005% or
more. The Mn content in terms of lower limit is more preferably
0.010% or more, and furthermore preferably 0.015% or more. However,
the steel, if having an excessively high Mn content of greater than
0.3%, may fail to include the martensitic phase after quenching,
due to increased stability of the austenitic phase. The Mn content
in terms of upper limit is preferably 0.2% or less, and more
preferably 0.1% or less.
[0033] Si: from greater than 0% to 0.3%
[0034] Silicon (Si) has a deoxidation action in molten steel, as
with Mn. This element, even when present in a trace amount,
effectively allows the steel to have better oxidation resistance.
To offer these advantageous effects effectively, the Si content is
preferably controlled to 0.005% or more. The Si content in terms of
lower limit is preferably 0.010% or more, and furthermore
preferably 0.015% or more. However, the steel, if having an
excessively high Si content, may suffer from impaired ductility
because of excessive work hardening. To eliminate or minimize this,
the Si content is controlled to 0.3% or less. The Si content in
tams of upper limit is preferably 0.2% or less, and more preferably
0.1% or less.
[0035] Ni: 10% to 13%
[0036] Nickel (Ni) is an austenitic phase-stabilizing element which
is necessary for austenitization of the microstructure in heating
before quenching. This element also allows Ti to be precipitated as
the Ni.sub.3Ti intermetallic compound and thereby allows the steel
to have more satisfactory high-temperature strength. To offer these
advantageous effects, the Ni content is contained to 10% or more.
The Ni content is preferably 10.5% or more, and more preferably
11.0% or more. However, the steel, if having an excessively high Ni
content of greater than 13%, may cause higher cost and may cause
austenite to remain after quenching. The Ni content in terms of
upper limit is preferably 12.5% or less, and more preferably 12.0%
or less.
[0037] Co: 9% to 12%
[0038] Cobalt (Co) is dissolved as a solute in the steel to offer
solid-solution strengthening. To offer the advantageous effect, the
Co content is controlled 9% or more. The Co content in terms of
lower limit is preferably 9.5% or more, and more preferably 10.0%
or more. However, the steel, if having an excessively high Co
content, may cause higher cost and may have impaired ductility due
to excessively increased strength. To eliminate or minimize these,
the Co content in terms of upper limit is controlled to 12% or
less, and is preferably 11.5% or less, and more preferably 11.0% or
less.
[0039] Cr: 1.5% to 4.5%
[0040] Chromium (CO is necessary for better oxidation resistance of
the maraging steel. To offer good oxidation resistance, the Cr
content is controlled to 1.5% or more. The Cr content in terms of
lower limit is preferably 2.0% or more, and more preferably 2.5% or
more. However, the steel, if having an excessively high Cr content,
may be embrittled due to the formation of o phases in a
high-temperature environment in which the steel is used as a
product. To eliminate or minimize this, the Cr content in terms of
upper limit is controlled to 4.5% or less, and is preferably 4.0%
or less, and more preferably 3.5% or less.
[0041] Al: 0.01% to 0.2%
[0042] Aluminum (Al) has a deoxidation action in molten steel, as
with Mn. To offer the advantageous effect, the Al content is
controlled to 0.01% or more. The Al content in terms of lower limit
is preferably 0.02% or more, and more preferably 0.03% or more.
However, the steel, if having an excessively high Al content, may
suffer from formation of coarse inclusions derived from Al. To
eliminate or minimize this, the Al content is controlled to 0.2% or
less, and is preferably 0.1% or less, and more preferably 0.05% or
less.
[0043] The chemical composition specified in the present invention
is as described above, with the remainder being iron and inevitable
impurities. Of the inevitable impurities, P, N, and S are
preferably decreased to levels as mentioned below. The impurities
excluding P, N, and S may include low-melting point impurity metals
derived from scrap raw materials, such as Sn, Pb, Sb, As, and Zn.
These elements, however, lower grain-boundary strength during hot
working and in use in a high-temperature environment and are
desirably minimized in content.
[0044] P: from greater than 0% to 0.01%
[0045] Phosphorus (P) is an inevitably-contaminated impurity, and
causes the steel to have lower weldability with an increasing
content thereof. From this viewpoint, phosphorus is preferably
minimized, and the phosphorus content is controlled to preferably
0.01% or less, more preferably 0.005% or less, and furthermore
preferably 0.001% or less.
[0046] N: from greater than 0% to 0.01%
[0047] Nitrogen (N) is also an inevitably-contaminated impurity,
fixes Ti as nitrides, and lowers the amounts of formed
intermetallic compounds that contribute to higher strength, where
Ti is contained as an essential element in the steel according to
the present invention. From this viewpoint, nitrogen is preferably
minimized, and the nitrogen content is controlled to preferably
0.01% or less, more preferably 0.005% or less, and furthermore
preferably 0.001% or less.
[0048] S: from greater than 0% to 0.01%
[0049] Sulfur (S) is also an inevitably-contaminated impurity and
impairs hot workability necessary typically for forging, with an
increasing content thereof. From this viewpoint, sulfur is
preferably minimized, and the sulfur content is controlled to
preferably 0.01% or less, more preferably 0.005% or less, and
furthermore preferably 0.001% or less.
[0050] The maraging steel according to the present invention has a
chemical composition as mentioned above. The steel having the
chemical composition can be easily obtained by adjusting
proportions of raw materials as appropriate via melting. Ingots
obtained by ingot making may be subjected to homogenization or
soaking (hereinafter also referred to "soaking treatment") as
needed subjected to hot working to adjust its shape, and then
subjected to an appropriate quenching heat treatment and a
subsequent aging heat treatment.
[0051] When the ingots are those obtained by ingot making, the
soaking treatment eliminates or minimizes solidifying segregation
of the ingots, by holding the ingots in a temperature range of
typically from 1250.degree. C. to 1300.degree. C. for about 10
hours. The hot working may be performed while heating the work at a
temperature of about 1000.degree. C. or higher.
[0052] The steel obtained by subjecting an ingot to the soaking
treatment and hot working is subjected to quenching so as to form a
martensitic phase. The heating temperature in quenching, namely,
the heating temperature before cooling is controlled within such a
temperature range that the entire steel becomes an austenitic phase
and that precipitates undergo solutionization. The steel according
to the present invention having the chemical composition as above
is preferably subjected to quenching performed at a heating
temperature of 900.degree. C. or higher, more preferably
950.degree. C. or higher, and furthermore preferably 1000.degree.
C. or higher. However, quenching, if performed at an excessively
high heating temperature, may cause the austenitic phase to
coarsen, and this may impede the formation of finely divided
martensite. From this viewpoint, the heating temperature in
quenching is controlled to preferably 1150.degree. C. or lower,
more preferably 1100.degree. C. or lower, and furthermore
preferably 1050.degree. C. or lower.
[0053] Cooling in quenching is preferably performed via air cooling
or water cooling. Cooling in a temperature range down to 80.degree.
C., which is lower than the martensitic transformation start
temperature Ms, is preferably performed at a cooling rate of
5.degree. C./hr or more. The cooling rate in this temperature range
is more preferably 10.degree. C./hr or more, and furthermore
preferably 20.degree. C./hr or more. However, the cooling rate has
a ceiling with respect to such large-sized steels and is about
100.degree. C./hr or less.
[0054] The steel, in which the martensitic phase is formed in the
above manner, has very high strength, but has low ductility and
toughness, and thus requires an aging heat treatment so as to
adjust balance between strength and toughness, where the aging heat
treatment corresponds to a tempering heat treatment.
[0055] The aging heat treatment is performed in such a temperature
range as not to increase the austenitic phase, namely, at a
temperature lower than the Ac.sub.3 transformation temperature. For
the managing steel having the chemical composition as above, the
upper limit temperature is 675.degree. C. Accordingly, the
temperature and holding time of the aging heat treatment are
controlled in a temperature range lower than 675.degree. C. so that
the steel has a surface Vickers hardness of 400 Hv or more.
[0056] The aging heat treatment is not limited in temperature and
holding time, except for the temperature upper limit. However, the
aging heat treatment, typically when performed at a set temperature
of 650.degree. C., can stably give a sufficient hardness when
performed for a holding time of 3 hours or shorter. To allow the
aging heat treatment to proceed effectively at that temperature,
the holding time is preferably at least one hour or longer, and is
more preferably 1.5 hours or longer.
[0057] The present invention will be illustrated in further detail
on operation and advantageous effects thereof, with reference to
several examples below. It should be noted, however, that the
examples are by no means intended to limit the scope of the present
invention; and that various modifications and changes in design
without deviating from the spirit and scope of the present
invention described herein all fall within the technical scope of
the present invention.
Examples
[0058] Steels A to I having chemical compositions given in Table 1
were heated and melted using a vacuum induction furnace, cast into
20-kg ingots, subjected to a soaking treatment at 1280.degree. C.
for 12 hours, and further subjected to hot forging to be processed
into steels having a size of 60 mm in width by 15 mm thickness by L
in length.
TABLE-US-00001 TABLE 1 Chemical composition* (in mass percent)
Steel C Si Mn P S Ni Cr Co Mo Ti Al N A 0.009 0.018 0.009 0.005
0.001 11.9 3.1 9.8 1.9 2.0 0.07 0.001 B 0.006 0.008 0.010 0.004
0.001 12.0 3.1 9.8 1.0 2.0 0.09 0.001 C 0.015 0.056 0.130 0.008
0.001 11.3 2.2 11.3 2.0 2.8 0.05 0.008 D 0.012 0.182 0.094 0.003
0.001 11.6 2.5 10.3 0.8 2.3 0.06 0.003 E 0.008 0.087 0.209 0.009
0.001 12.1 2.7 10.8 2.4 2.5 0.04 0.002 F 0.014 0.116 0.165 0.004
0.002 10.8 2.9 9.9 1.9 2.6 0.05 0.005 G 0.011 0.143 0.055 0.005
0.001 11.0 2.8 10.1 3.0 1.7 0.04 0.003 H 0.013 0.221 0.245 0.007
0.001 10.5 3.4 11.0 1.6 4.2 0.02 0.008 I 0.003 0.014 0.012 0.002
0.001 12.1 3.0 10.3 5.0 2.0 0.10 0.002 *Remainder: iron and
inevitable impurities excluding P, S. and N
[0059] The obtained steels were heated at 1000.degree. C. for 15
minutes, subjected to quenching via water-immersion cooling, and
each subjected to an aging heat treatment in a temperature range of
from 650.degree. C. to 700.degree. C. for a time range of from 2 to
30 hours, under one of four conditions (a), (b), (c), and (d) as
follows.
[0060] Aging Heat Treatment Conditions
[0061] (a) At a temperature of 650.degree. C. for a holding time of
3 hours
[0062] (b) At a temperature of 650.degree. C. for a holding time of
30 hours
[0063] (c) At a temperature of 700.degree. C. for a holding time of
30 hours
[0064] (d) At a temperature of 650.degree. C. for a holding time of
2 hours
[0065] Table 2 presents the steel type and the aging heat treatment
condition each employed in Tests Nos. 1 to 12, together with the
total content of Mo and Ti, and the ratio ([Mo]/[Ti]).
TABLE-US-00002 TABLE 2 Total content (in Aging heat Test mass
percent) of Ratio treatment number Steel Mo and Ti ([Mo]/[Ti])
condition 1 A 3.9 0.95 (a) 2 B 3.0 0.50 (a) 3 B 3.0 0.50 (d) 4 C
4.8 0.71 (a) 5 D 3.1 0.35 (a) 6 E 4.9 0.96 (a) 7 F 4.5 0.73 (a) 8 G
4.7 1.76 (a) 9 H 5.8 0.38 (a) 10 I 7.0 2.50 (a) 11 I 7.0 2.50 (b)
12 I 7.0 2.50 (c)
[0066] From the above-prepared steels, flanged round bar test
specimens each including a gauge portion of 6 mm in diameter by 30
mm in length were prepared, subjected to high-temperature tensile
tests at 500.degree. C. in accordance with the method prescribed in
Japanese Industrial Standard (JIS) G 0567:2012, to determine a 0.2%
yield strength as a high-temperature strength. A sample, when
having a 0.2% yield strength as measured of 750 MPa or more, is
judged to surely have excellent high-temperature strength
[0067] From the above-prepared steels, full-size 2-mmV notch Charpy
test specimens in conformity with JIS Z 2242:2005 were prepared,
subjected to Charpy impact tests to measure Charpy impact values at
0.degree. C., on the basis of which toughness was evaluated. The
present invention is to improve toughness at room temperature of
about 25.degree. C. A sample, when having good toughness at
0.degree. C., can be judged to also have good toughness at room
temperature. On the basis of these, toughness was evaluated at
0.degree. C. A sample, when having a Charpy impact value as
measured of 10.0 J/cm.sup.2 or more, can be judged to offer more
excellent toughness as compared with conventional managing steels.
The Charpy impact value is preferably 15.0 J/cm.sup.2 or more, and
more preferably 17.0 J/cm.sup.2 or more.
[0068] The above-prepared steels, namely, steels after the aging
heat treatment, were subjected to mirror-like finishing via
mechanical polishing, followed by measurements of surface Vickers
hardness at a load of 500 g. A sample steel, when having a surface
Vickers hardness of 400 Hv or more, can be judged to have excellent
surface hardness.
[0069] Evaluation results on the high-temperature strength, Charpy
impact value, and Vickers hardness are presented in Table 3.
TABLE-US-00003 TABLE 3 Test High-temperature Charpy impact Vickers
hardness number strength (MPa) value (J/cm.sup.2) (Hv) 1 835 12.3
443 2 781 17.6 430 3 847 15.8 458 4 856 10.2 486 5 801 21.9 441 6
859 11.4 479 7 848 13.6 468 8 833 6.8 456 9 892 8.2 497 10 940 5.9
520 11 783 5.8 462 12 736 9.7 386
[0070] These results give considerations as follows. Samples of
Tests Nos. 1 to 7 are examples which meet all conditions specified
in the present invention and are found to offer excellent
high-temperature strength and to have better toughness. These
samples are also found to have sufficiently high steel surface
hardness after the aging heat treatment.
[0071] In contrast, samples of Tests Nos. 8 to 12 are comparative
examples which do not meet one or more of the conditions specified
in the present invention and offer at least one of high-temperature
strength, toughness, and surface hardness at poor level.
[0072] Specifically, the sample of Test No. 8 is a sample using
Steel G, which has a ratio ([Mo]/[Ti]) of the Mo content to the Ti
content of out of the range specified in the present invention.
This sample offers lower toughness even it has undergone an aging
heat treatment under appropriate conditions.
[0073] The sample of Test No. 9 is a sample using Steel H, which
has a total content of Mo and Ti of out of the range specified in
the present invention. This sample offers lower toughness even it
has undergone an aging heat treatment under appropriate
conditions.
[0074] The sample of Test No. 10 is a sample using Steel I, which
has a total content of Mo and Ti and a ratio ([Mo]/[Ti]) both out
of the ranges specified in the present invention. This sample
offers lower toughness even it has undergone an aging heat
treatment under appropriate conditions.
[0075] The sample of Test No. 11 is a sample using Steel I, which
has a total content of Mo and Ti and a ratio ([Mo]/[Ti]) both out
of the ranges specified in the present invention. In addition, this
sample has undergone an aging heat treatment for an excessively
long holding time. In this sample, the aging heat treatment
condition causes the sample to have lower toughness, although it
does not so much affect the high-temperature strength and the
surface hardness.
[0076] The sample of Test No. 12 is a sample using Steel I, which
has a total content of Mo and Ti and a ratio ([Mo]/[Ti]) both out
of the ranges specified in the present invention. In addition, this
sample has undergone an aging heat treatment at an excessively high
temperature for an excessively long holding time. This sample
offers lower toughness, is in the state of over-aging, and has a
high-temperature strength and a surface hardness not meeting the
predetermined conditions (criteria).
* * * * *