DESCRIPTION
Title of Invention: Age Hardening Steel and Method of
Production of Part Using Age Hardening Steel
5
Technical Field
[OOOl] The present invention relates to age hardening
steel, more particularly relates to age hardening steel
which is cut etc. to work it to a predetermined shape,
10 then is treated for age hardening (below, referred to
simply as "aging treatment"). Further, the present
invention relates to a method of production of a part
using such age hardening steel.
Background Art
15 [0002] From the viewpoints of lightening weight aiming
at higher output of an engine and improvement of fuel
efficiency etc., machine parts of automobiles, industrial
machinery, construction machinery, etc. are required to
exhibit high fatigue strength. Steel can be easily
2 0 provided with a high fatigue strength by adding alloy
elements, heat treatment, etc. so as to raise the
hardness of the steel. However, if producing a machine
part by the method of first hot forging to shape a
material, then cutting to finish it to a predetermined
2 5 product shape, a sufficient machinability is also
demanded. That is, at the stage of shaping the machine
part, the steel is required to exhibit machinability,
while at the stage of the final product, the steel is
required to exhibit fatigue strength.
3 0 [0003] To deal with such demands, age hardening steel
which can be kept low in hardness at the shaping stage
and, after that, can be treated to age it to raise the
hardness in the final product stage has been proposed
(for example, see W02010/090238A (PLT I), Japanese Patent
3 5 Publication No. 2012-246527A (PLT 21, Japanese Patent
Publication No. 2011-241441A (PLT 31, Japanese Patent
Publication No. 2012-193416A (PLT 4), and Japanese Patent
No. 5343923B2 (PLT 5)) .
[0004] PLTs 1 and 2 disclose a method of production
controlling the cooling rate after shaping by hot
forging, suppressing the formation of structures other
5 than bainite, keeping down the amount of precipitation of
VC during cooling, and securing the amount of solute V so
as to be able to obtain a sufficient age hardenability.
However, in the method of production described in PLTs 1
and 2, at the time of the cooling step after hot forging,
10 it is necessary to control the cooling rate at each
specific temperature region, there are restrictions in
facilities, apparatuses, etc., and also sometimes
quenching is not possible on an actual production line,
so it was difficult to stably produce age hardening
15 steel.
[OOOS] Therefore, PLTs 3 to 5 propose age hardening
steel for use as a material of a machine part not
requiring strict conditions to be set at the cooling step
after hot forging and enabling cooling by air cooling and
2 0 fans in production.
Summary of Invention
Technical Problem
[00061 As explained above, the steel used as a
material for a machine part is required to be excellent
2 5 in machinability at the stage of production of the
machine part and to be excellent in fatigue strength
after the machine part is completed. When using a method
of production including aging treatment to produce a
machine part, the above demands can be met by steel
3 0 provided with the characteristics of being low in
hardness before the aging treatment and being raised in
hardness after aging treatment. A large difference
between the hardness before the aging treatment and the
hardness after aging treatment (that is, a high age
3 5 hardenability) is preferable for obtaining a machine part
which is excellent in both productivity and fatigue
strength.
[00071 However, the methods of production for
obtaining age hardening steel according to the prior art
require inclusion of a step of quenching the steel. This
quenching step causes an increase in the cost of
5 manufacturing age hardening steel.
[0008] Further, steel treated to age it so as to cause
fine precipitates to disperse in the steel and cause the
strength to rise is known to be greatly degraded in
toughness. If steel is degraded in toughness, the steel
10 rises in notch sensitivity, so if some reason or another
causes a surface flaw to occur at the steel, the steel
falls in low cycle fatigue strength. The "low cycle
fatigue strength" is a characteristic demanded from steel
in which application of stress exceeding the elasticity
15 region is envisioned. The method of production of age
hardening steel disclosed in PLTs 3 to 5 does not require
increase of the cooling rate after hot forging and has
the effect of suppressing the increase in the
manufacturing cost, but it was difficult to obtain steel
2 0 sufficient in toughness after aging treatment.
[0009] The present invention was made in consideration
of such an actual situation and has as its object the
provision of age hardening steel in which the production
conditions are not particularly limited, the
2 5 machinability before aging treatment is excellent,
hardening by aging treatment can be used to stably
improve the fatigue strength, and a drop in toughness due
to aging treatment can be suppressed.
Solution to Problem
3 0 [OOlO] To secure sufficient hardness, fatigue
strength, and low cycle fatigue strength after aging
treatment, it is necessary to suitably control the amount
of formation of carbides, carbonitrides, and other
compounds precipitating due to the aging treatment in
3 5 accordance with the types of the precipitates.
[OOll] Here, the inventors focused on the matters
explained below. V is present forming a solid solution in
steel during the hot forging performed at a general
temperature. The reason is that the starting temperature
for formation of V carbides or V carbonitrides
(precipitation temperature) is low. On the other hand, V
5 is strong in ability to form precipitates (V carbides or
V carbonitrides) at the aging treatment, so is an element
effective for hardening by aging treatment. However, if
the content of N is large, V nitrides are formed at the
time of cooling after hot forging and before aging
10 treatment, the hardness rises before aging treatment, and
the machinability becomes impaired. Based on these
discoveries, the inventors experimented with the
promotion of formation of V carbides or V carbonitrides
after aging treatment and suppression of formation of V
15 nitrides before aging treatment.
[0012] Further, Ti bonds with N and C to form coarse
Ti carbonitrides and, even in a trace amount of a content
of 0.005% or so, causes a great degradation of the
toughness. Therefore, based on these discoveries, the
2 0 inventors experimented with decreasing the Ti content of
steel.
[0013] Further, Nb precipitates in steel as carbides
or carbonitrides in the process of heating and being
worked at the time of hot forging and has the effect of
2 5 refining the austenite crystal grain size due to the
pinning effect, and, after that, refining the bainite
structure in the bainite transformation. Furthermore,
part of the Nb in steel does not precipitate as carbides
or carbonitrides at the time of hot forging but is
3 0 present as solute Nb. This solute Nb precipitates as Nb
carbides or Nb carbonitrides at the time of aging
treatment after hot forging and has the effect of raising
the hardness without inviting a drop in toughness and
thereby achieving an improvement in the low cycle fatigue
3 5 strength and fatigue strength. Based on these
discoveries, the inventors experimented with utilizing Nb
to suppress the drop in toughness due to aging treatment.
[0014] Furthermore, to stably raise the fatigue
strength and stably prevent a drop in toughness after
aging treatment as well, it is necessary to control not
only the types and amounts of formation of carbides,
5 carbonitrides, and other precipitates due to the aging
treatment, but also the form of the inclusions present in
the steel.
[0015] Therefore, the inventors focused on the matters
explained below. REMs are elements which form sulfide-
10 based inclusions or oxide-based inclusions and have the
effect of causing the inclusions to finely disperse and
making the inclusions spherical in shape. However, if the
content of REMs is too great, a drop in the hot ductility
of the steel material at the time of hot rolling or hot
15 forging will end up being caused. Based on this
discovery, the inventors tried to find the suitable REM
content and searched for and determined conditions
enabling stable improvement of the fatigue strength by
hardening by aging treatment and stably preventing a drop
2 0 in toughness after aging treatment.
[0016] The present invention was made based on such a
discovery and has as its gist the following:
[0017] [ll An age hardening steel containing, by
mass%, C: 0.05 to 0.20%, Si: 0.01to 0.50%, Mn: 1.50 to
2 5 2.50%, S: 0.005 to 0.080%, Cr: 0.03 to 1.60%, Al: 0.005
to 0.050%, V: 0.25 to 0.50%, Nb: 0.010 to 0.100%, Ca:
0.0005 to 0.0050%, and REM: 0.001 to 0.05%, limiting P to
0.030% or less, Ti to less than 0.005%, and N to less
than 0.0080%, having a balance of Fe and impurities,
3 0 having an area ratio of a bainite structure of 70% or
more and, furthermore, having a chemical composition
where the following F1 expressed by formula (1) is 0.68
or more, F2 expressed by formula (2) is 0.85 or less, F3
expressed by formula (3) is 0.00 or more, and F4
3 5 expressed by formula (4) is 0.012 to 0.08:.
Fl=C+O.3xMn+0.25xCr . . . (1)
F2=C+O.lxSi+0.2xMn+O.15xCr+0.35xV . . .(2)
F4=10xCa+REM . . . (4)
where the symbols of elements in the above formula (1) to
formula (4) mean the contents by mass% of those elements
5 [QQlE] [2] An age hardening steel containing, by
mass%, C: 0.05 to 0.20%, Si: 0.01 to 0.50%, Mn: 1.50 to
2.50%, S: 0.005 to 0.080%, Cr: 0.03 to 1.60%, Al: 0.005
to 0.050%, V: 0.25 to 0.50%, Nb: 0.010 to 0.100%, Ca:
0.0005 to 0.0050%, and REM: 0.001 to 0.058, having an
10 area ratio of bainite structures of 70% or more,
furthermore, satisfying any one or more of the conditions
of the composition shown by the following to ,
limiting P to 0.030% or less, Ti to less than 0.005%, and
N to less than 0.0080%, having a balance of Fe and
15 impurities, and, furthermore, having a chemical
composition where the following El' expressed by formula
(1' ) is 0.68 or more, F2' expressed by formula (2') is
0.85 or less, and F3' expressed by formula (3') is 0.00
or more, and F4 expressed by formula (4) is 0.012 to
2 0 0.08:
Mo: 0.01 to 1.0%
One or both of Cu: 0.01 to 0.30% and Ni: 0.01% to
0.30%
Bi: 0.01 to 0.400%
2 5 Fl'=C+0.3xMn+O.25xCr+O.6xMo . . . ( 1')
where the symbols of elements in the above formula (1')
3 0 to formula (3') and formula (4) mean the contents by
mass% of those elements
[0019] [31 A method of production of a part using age
hardening steel comprising a forging step of heating age
hardening steel according to [I] or [21 at 1200 to 1250°C
3 5 for 5 to 60 minutes, then forging it so that a surface
temperature after final forging becomes llOO°C or more,
then, after that, cooling it by an average cooling rate
in a 800 to 400°C temperature region of 15 to 60°C/min
down to room temperature, a cutting step of cutting the
forged steel, and an aging treatment step of holding the
5 cut steel in a 540 to 700°C temperature region for 30 to
1000 minutes.
Advantageous Effects of Invention
[0020I According to the present invention, it is
possible to provide age hardening steel which is not
10 particularly limited in production conditions, is
excellent in machinability before aging treatment, can
use the hardening by the aging treatment to stably
improve the fatigue strength, and can keep down a drop in
toughness due to aging treatment. Further, by using the
15 age hardening steel of the present invention as a
material, it is possible to provide a machine part which
is excellent in productivity, excellent in fatigue
strength, and is sufficient in toughness.
[0021] Note that, the age hardening steel of the
2 0 present invention has, as an indicator of the cutting
resistance, a Vicker's hardness before aging treatment of
290 Hv or less. The amount of rise of the Vicker's
hardness (age hardenability, AHv) due to the aging
treatment when making the age hardening steel of the
2 5 present invention a substantially columnar shape with a
diameter of 35 rnm and holding this steel at a temperature
of 620°C for 120 minutes is 30 Hv or more. The age
hardening steel of the present invention after the aging
treatment has a fatigue strength of 425 MPa or more.
3 0 [0022] Further, the age hardening steel of the present
invention after aging treatment has an absorption energy
at 20°C of 503 or more in a Charpy impact test performed
using a standard test piece with a U-notch with a notch
depth of 2 mrn and a notch bottom radius of 1 mm and has a
3 5 low cycle fatigue strength of 520 MPa or more.
[0023] In this way, the age hardening steel of the
present invention can be extremely suitably used as a
material for a machine part of automobiles, industrial
machinery, construction machinery, etc. Its contribution
to industry is extremely remarkable.
5 Brief Description of Drawings
[0024] FIG. 1 is a view showing the shape of a
monoaxial tension and compression type of fatigue test
piece used in the examples. The numerical values in the
figure show the dimensions (units: mm)
10 Description of Embodiments
[0025] Discovery Leading to Derivation of Present
Invention
Below, the chemical composition of the age hardening
steel according to the present embodiment will be
15 explained focusing on the elements important from the
viewpoint of age hardening. Note that, the main
application for the age hardening steel according to the
present embodiment (below, sometimes abbreviated as the
"steel according to the present embodiment") is as the
2 0 material of a machine part produced by a method of
production including hot forging, machining, aging
treatment, etc. Therefore, to explain the features of the
steel according to the present embodiment, sometimes
reference will be made to characteristics of steel after
2 5 hot forging, machining, and aging treatment. However, the
steel according to the present embodiment does not
necessarily require such treatment. That is, the
applications of the steel according to the present
embodiment are not limited to hot forging, machining,
3 0 etc.
[0026] First, in the steel according to the present
embodiment, the fact that the V content has to be made
0.25 mass% or more was discovered by the present
inventors. By making the V content 0.25 mass% or more, it
3 5 is possible to make the amount of carbides of V or
carbonitrides of V formed due to the aging treatment
increase, raise the hardness after aging treatment, and
secure fatigue strength.
V, once forming a solid solution in steel, will not
precipitate until the steel is cooled down to near 850°C
and is strong in ability to form carbides or
5 carbonitrides at the age hardening treatment temperature.
Furthermore, in the steel according to the present
embodiment, in the same way as V, it is also possible to
add Mo which is relatively low in temperature of
precipitation of carbides and can be easily used for age
10 hardening. If making steel containing V in 0.25 mass% or
more further contain Mo, the aging treatment causes
composite carbides of V and Mo or composite carbonitrides
of V and Mo to be formed, so the hardness after aging
treatment rises more.
15 [0027] As explained above, V has the property of not
precipitating once forming a solid solution in the steel
until cooling the steel down to near 85G°C, so is an
element which can remain stably present in the steel in
the solute state. However, V carbides easily precipitate
20 at the phase boundaries when austenite transformed to
ferrite. If the amount of precipitation of V carbides
increases, the amount of solute V decreases. That is, if
a large amount of proeutectoid ferrite is formed during
the cooling after hot forging, V carbides will
2 5 precipitate at the phase boundaries, so it becomes no
longer possible to secure the amount of solute V
necessary for precipitation hardening due to aging
treatment after that. Therefore, to secure a sufficient
amount of solute V in the age hardening steel before
30 aging treatment, it is necessary that in the structure
after hot forging and before aging treatment, the phase
accounting for an area ratio of a 70% or more (below,
sometimes referred to as "main phase") becomes bainite.
Further, to prevent skyrocketing costs of manufacturing a
35 machine part, such control of the structure has to be
performed not by controlling the hot forging conditions,
but by controlling the chemical composition of the steel.
LOO281 The structure after hot forging is closely
correlated with the C, Mn, and Cr improving the
hardenability and, furthermore, the content of Mo. The
present inventors discovered that if making the values of
5 the indicators F1 and El' of hardenability expressed by
the following formula (1) and formula (1') specific
numerical values or more by controlling the contents of
these elements in these formulas, precipitation of a
large amount of proeutectoid ferrite harmful to securing
10 the solute V is suppressed in the usual cooling process
after hot forging (cooling rate 15"C/min to 60°C/min).
That is, the present inventors discovered that by
controlling El and El', the steel structure easily
becomes a structure having bainite as its main phase,
15 that is, becomes a structure including bainite by an area
ratio of 70% or more, so a sufficient amount of solute V
can be secured:
Fl=C+0.3xMn+0.25xCr . ..(I)
Fll=C+O.3 xMn+O.2 5xCrtO. 6xMo . . . (1')
2 0 100291 However, even if making the steel structure a
structure having bainite as its main phase (area ratio of
70% or more) so as to secure a sufficient amount of
solute V, sometimes the hardness before aging treatment
(hardness of structure containing bainite as main phase)
2 5 becomes high. In this case, sometimes a rise in the
cutting resistance of the steel after hot forging is
invited and the machinability falls. The present
inventors studied the method of solving this problem. As
a result, the present inventors discovered that if
3 0 controlling the chemical composition of the steel
according to the present embodiment so that the values of
indicators F2 and F2' of hardness before aging treatment
expressing the contents of C, Si, Mn, Cr, V, and Mo by
the following formula (2) and formula (2') become
35 specific numerical values or less, it is possible to keep
the hardness before aging treatment low and possible to
suppress a rise in the cutting resistance:
F2=C+O.lxSi+0.2xMn+O.15xCr+0.35xV . . . ( 2)
F2'=Ct0.1xSi+0.2xMntO.l5xCr+O.35xV+0.2xMo . . .(2')
[00301 Further, the present inventors produced steel
5 containing 0.25 mass% or more of V and adjusted in
ingredients so that F1 and F2 or F1' and F2' in which the
contents of C, Si, Mn, Cr, Mo, and V are found by the
above formula (1) and formula (2) or formula (1') and
formula (2') satisfy specific ranges of numerical values,
10 hot forged, then treated this steel to age to prepare
samples, and investigated the toughness of the samples.
Specifically, they hot forged and treated the above steel
to age, prepared standard test samples with U-notches
with a notch depth of 2 mm and notch bottom radius of 1
15 mm, ran the test pieces through a Charpy impact test, and
investigated the effects of the ingredients on the
toughness after aging treatment.
[0031] As a result of the above investigations, the
inventors learned that to obtain steel able to suppress a
2 0 drop in toughness due to aging treatment, it is necessary
to control the contents of C, Mn, Cr, V, and Mo of steel
so that the values of indicators F3 and F3' expressed by
the following formula (3) and formula (3') and showing
the toughness after aging treatment become specific
2 5 values or more:
F3=-4.5xC+Mn+Cr-3.5xV . . .(3)
F3'=-4.5xC+Mn+Cr-3.5xV-0.8xMo . . . ( 3')
[0032] If F3 and F3' are large, the toughness of the
steel after aging treatment will be sufficient. Further,
3 0 increasing the contents of C, V, and Mo decreases F3 and
F3'. Therefore, formula (3) or formula (3') mean that the
contents of C, V, and Mo, which are required for
improving the hardness and the fatigue strength after
aging treatment, have to be reduced to suppress a drop in
3 5 toughness due to aging treatment.
LO0331 Furthermore, to achieve both strength and
toughness, it is necessary to utilize elements other than
C, V, and Mo to raise the hardness after aging treatment
and improve the strength.
100341 To suppress a drop in toughness after aging
5 treatment, refining the structure is effective. To retine
the main phase bainite structure, refining the austenite
grain size before bainite transformation is effective. To
refine the austenite grain size, in general inclusion of
Ti is effective, but in the steel according to the
10 present embodiment, this means cannot be used. The
present inventors discovered that Ti forms coarse Ti
carbonitrides causing the steel according to the present
embodiment to degrade in toughness, so even if the Ti
content is a 0.005% or so trace amount, the Ti causes the
15 steel after aging treatment to greatly degrade in
toughness. Therefore, the steel according to the present
embodiment has to be restricted in content of Ti as much
as possible to zero or a specific value or less.
[0035] Further, if inclusions having a detrimental
20 effect on toughness are present in the steel, a
sufficient toughness cannot be obtained. To keep down the
presence of inclusions in steel detrimental to toughness,
the content of S has to be made a specific value or less.
Further, S is also an element which bonds with Mn to form
25 coarse MnS and degrade the toughness, so excessive
addition of S has to be avoided. On the other hand, MnS
is an inclusion essential for securing sufficient
machinability. Therefore, it is not preferable to make
the content of S completely zero. To raise the
3 0 machinability of the steel before aging treatment and
keep down a drop in toughness of the steel due to the
aging treatment, it is necessary to suitably control the
content of S so that the amount of MnS does not become
too great.
35 [0036] The present inventors discovered that inclusion
of Nb is effective as a means for sufficiently raising
the machinability before aging treatment and low cycle
fatigue strength after aging treatment and suppressing a
drop in toughness due to the aging treatment. Nb has the
effect of refining the austenite grain size before
bainite transformation in the same way as Ti.
5 [0037] Nb is an element having the effect of refining
the austenite grain size and has the ability to form a
compound at the aging treatment temperature (secondary
phase). This is because Nb has a higher precipitation
temperature than V and Mo. That is, since Nb has a
10 relatively high precipitation temperature, part of the
contained Nb precipitates as carbides or carbonitrides at
the time of hot forging. The carbides and other Nb
precipitates contribute to the refining of the austenite
grain size.
15 [0038] Steel satisfying the condition of the formula
(I) or formula (1') becoming a specific range has solute
Nb present in it. As explained above, this is because the
main phase of the steel satisfying formula (1) or formula
(1') is the bainite structure and Nb easily forms a solid
2 0 solution in a bainite structure. For this reason, steel
where the formula (1) or formula (1') becomes a specific
range may be treated for aging to make Nb carbides or Nb
carbonitrides precipitate. Further, it was discovered by
the present inventors that even if these Nb-based
2 5 precipitates precipitate, it is possible to raise the
hardness of the steel after aging treatment without
inviting a drop in toughness. Furthermore, it was
discovered by the present inventors that by including Nb,
it is possible to realize steel where excellent low cycle
30 fatigue strength is obtained by refining the bainite
structure and by precipitation strengthening.
[0039] As explained above, the present inventors
obtained the discovery relating to age hardening steel
which is not particularly limited in the production
35 conditions of the steel material, is excellent in the
machinability before aging treatment, is improved in the
fatigue strength by hardening by aging treatment, and can
suppress a drop in toughness after aging treatment.
However, with just the above-mentioned discovery,
sometimes the fatigue strength and toughness after aging
treatment become somewhat low levels though still within
5 the ranges of the desired values.
[0040] Therefore, the present inventors carefully
investigated the mechanism by which the fatigue strength
or toughness becomes lower in level after aging treatment
and as a result discovered that the coarse inclusions
10 contained inside the steel are the cause. That is, they
clarified that by suppressing the formation of such
coarse inclusions, it is possible to stably improve the
fatigue strength after aging treatment and suppress a
drop in toughness after aging treatment.
15 [0041] To keep down coarse inclusions, the present
inventors focused on a REM. A REM has the effect of
forming sulfide-based inclusions or oxide-based
inclusions and making both the sulfide-based inclusions
and oxide-based inclusions finely disperse. However, if
2 0 the REM content is too large, the hot ductility of the
steel material at the time of hot rolling or hot forging
ends up being made to fall.
[0042] However, if just adjusting the REM content,
both the sulfide-based inclusions and the oxide-based
2 5 inclusions cannot stably finely disperse. The present
inventors learned that it is necessary to control the
contents of Ca and a REM so that the value of the control
indicator F4 of the forms of inclusions with a REM and Ca
expressed by the following formula (4) becomes a specific
30 range. The inventors discovered that by doing this, the
sulfide-based inclusions disperse finely in spherical
shapes and the oxide-based inclusions finely disperse:
F4=lOxCa+REM . . . (4)
[00431 If F4 is 0.012 or more, both the sulfide-based
3 5 inclusions and the oxide-based inclusions stably finely
disperse. Further, if F4 is over 0.08, the effect becomes
saturated and the hot ductility of the steel is liable to
be lowered, so it is necessary to make the contents of Ca
and REM suitable.
[0044] The present invention relates to age hardening
steel obtained based on the results of the studies of the
5 present inventors and the findings they obtained
explained above. Below, the requirements of age hardening
steel according to an embodiment of the present invention
will be explained in detail.
100451 Chemical Composition
10 First, the composition of the age hardening steel
according to the present embodiment will be explained
Note that, the "%" of the content of the elements mean
11 mass%".
COO461 Essential Elements
15 C: 0.05 to 0.20%
C is an important element in the present embodiment. C
bonds with V due to the aging treatment to form carbides
and strengthen the steel. However, if the content of C is
less than 0.05%, the force driving the precipitation of V
2 0 carbides becomes smaller and it becomes difficult for V
carbides to precipitate, so the desired strengthening
effect cannot be obtained. On the other hand, if the
content of C is over 0.20%, the C not bonded with V bonds
with Fe to form carbides (cementite) and cause a
25 remarkable degradation of toughness of the steel.
Further, if the content of C exceeds 0.20%, the
concentration of. C concentrating in the austenite also
becomes higher in the middle of the transformation from
austenite to bainite and the structure after bainite
30 transformation becomes partially contaminated by
martensite. If cementite and/or martensite is contained
in steel in this way, the cutting resistance ends up
rising and the machinability falls. Therefore, the
content of C is made 0.05 to 0.20%. The content of C is
3 5 preferably made 0.08% or more, more preferably 0.10% or
more. Further, the content of C is preferably made 0.18%
or less, more preferably 0.16% or less.
[0047] Si: 0.01 to 0.50%
Si is used as a deoxidizing element at the time of
steelmaking and simultaneously has the action of forming
a solid solution in the matrix and improving the strength
5 of the steel. To sufficiently obtain these effects, Si
has to be made 0.01% or more in content. However, if the
content of Si becomes excessive, a rise occurs in the hot
workability and cutting resistance of the steel and a
drop in machinability is invited. In particular, if the
10 content of Si is over 0.50%, a drop in the hot
workability and a rise in the cutting resistance of the
steel become remarkable. Furthermore, Si is liable to
promote the formation of proeutectoid ferrite and
decrease the amount of bainite, so to stably obtain
15 bainite, excessive inclusion of Si is not preferable.
Note that, if proeutectoid ferrite is formed at the stage
of production of the steel, as explained above, V
carbides will precipitate at the phase boundaries of the
proeutectoid ferrite and austenite and it will become
2 0 difficult to secure the amount of solute V required for
precipitation hardening by aging treatment and as a
result the age hardenability of the steel is liable to be
caused to decrease. Therefore, the content of Si is made
0.01 to 0.50%. The content of Si is preferably made 0.06%
2 5 or more. Further, the content of Si is preferably made
0.45% or less, more preferably is made less than 0.35%.
[0048] Mn: 1.50 to 2.50%
Mn has the effect of improving the hardenability and
making the main phase of the structure bainite.
3 0 Furthermore, Mn has the action of causing a reduction in
the bainite transformation temperature and by doing so
has the effect of refining the structure to raise the
toughness of the matrix. Note that, the structure which
accounts for the majority of the volume of the steel will
3 5 be called the "matrix". The matrix of the steel according
to the present embodiment is bainite. Further, Mn has the
action forming MnS in the steel to cause a drop in the
cutting resistance and thereby improve the machinability.
Further, if the amount of Mn is less than 1.50%,
formation of proeutectoid ferrite is promoted and, as
explained above, a reduction in the amount of bainite and
5 drop in the age hardenability are liable to be caused. To
sufficiently obtain these effects, Mn has to be made at
least 1.50% in content. However, Mn is an element which
easily segregates at the time of solidification of the
steel, so the content increases. In particular, if over
10 2.50%, the hardness inside the part after hot forging
will unavoidably greatly vary. Therefore, the content of
Mn is made 1.50 to 2.50%. The content of Mn is preferably
made 1.60% or more, more preferably is made 1.70% or
more. Further, the content of Mn is preferably made 2.30%
15 or less, more preferably is made 2.10% or less.
[0049] S: 0.005 to 0.080%
S bonds with the Mn in the steel to form MnS which causes
a drop in the cutting resistance and improvement of the
machinability. To obtain sufficient machinability, 0.005%
20 or more of S has to be included. However, if the content
of S becomes excessively high, the coarse MnS will
increase and the toughness and fatigue strength are
liable to be degraded. In particular, if the content of S
exceeds 0.080%, the drop in the toughness and the fatigue
25 strength becomes remarkable. Therefore, the content of S
is made 0.005 to 0.080%. The content of S is preferably
made 0.010% or more. Further, the content of S is
preferably made 0.050% or less, more preferably is made
0.030% or less.
3 0 [0050] Cr: 0.03 to 1.60%
Cr has the effect of raising the hardenability and making
the main phase of the structure bainite. Furthermore, Cr
has the action of causing a drop in the bainite
transformation temperature and has the effect of refining
3 5 the structure by doing so and thereby raising the
toughness of the matrix. However, if the content of Cr is
over 1.60%, the hardenability becomes too large and,
depending on the size or location of the part, the
hardness before the aging treatment is liable to become a
Vicker's hardness of over 290 Hv, so sometimes the
cutting resistance will rise and the machinability will
5 fall. Therefore, the content of Cr is made 0.03 to 1.60%.
The lower limit of the content of Cr is preferably made
0.05% or more, more preferably is made 0.10% or more. The
upper limit of the content of Cr is preferably 1.00% or
less, more preferably is made 0.50% or less.
10 [0051] Al: 0.005 to 0.050%
Al is an element having a deoxidizing action. To obtain
such an action, the content has to be made 0.005% or
more. However, if the content of Al is over 0.050%,
coarse oxides are formed and the steel falls in toiighness
15 and fatigue strength. Therefore, the content of Al is
made 0.005 to 0.050%. The content of A1 is preferably
0.040% or less.
COO521 V: 0.25 to 0.50%
V is the most important element in the steel according to
2 0 the present embodiment. V has the action at the time of
aging treatment of bonding with C to form fine V carbides
or of bonding with C and N to form fine V carbonitrides
and thereby raise the strength of the steel after the
aging treatment. Further, V also has the effect of
2 5 precipitating compositely with Mo due to the aging
treatment and further raising the age hardenability of
the steel. To sufficiently obtain these effects, V has to
be made 0.25% or more in content. However, if the content
of V becomes excessive, even in heating at the time of
30 hot forging, undissolved carbonitrides will easily remain
and a drop in toughness will be invited. In particular,
if the content exceeds 0.50%, the drop in toughness will
become remarkable. Further, if the content of V exceeds
0.50%, undissolved carbides will remain and along with
3 5 this sometimes the cutting resistance will end up rising
and the machinability of the steel will also remarkably
fall. Therefore, the content of V is made 0.25 to 0.50%.
The content of V is preferably made less than 0.45%, more
preferably is made 0.40% or less. Further, the content of
V is preferably made 0.27% or more.
[0053] Nb: 0.010 to 0.100%
5 Nb is one of the important elements in the steel
according to the present embodiment. Part of the Nb
contained in the steel precipitates in steel as Nb
carbides or Nb carbonitrides in the process of heating
and being worked at the time of hot forging and causes
10 refinement of the austenite crystal grains due to the
pinning effect. The refinement of the austenite crystal
grains at the time of hot working has the effect of
refining the bainite structure in the bainite
transformation after the end of hot working. Furthermore,
15 part of the Nb in the steel at the time of hot forging
remains present as solute Nb. This solute Nb precipitates
as Nb carbides or Nb carbonitrides at the time of aging
treatment after hot forging so has the effect of raising
the hardness and achieving improvement of the low cycle
2 0 fatigue strength and improvement of the fatigue strength
without inviting a drop in toughness. To sufficiently
obtain these effects, Nb has to be made 0.010% or more in
content. However, if the content of Nb becomes excessive,
undissolved carbonitrides easily remain even at the time
2 5 of heating at the time of hot forging and the effect of
improvement of the hardness after aging treatment and/or
the effect of improvement of the fatigue strength after
aging treatment becomes saturated. Further, if the
content of Nb exceeds 0.100%, along with the remaining
3 0 presence of undissolved carbides or carbonitrides,
sometimes the cutting resistance ends up rising and the
machinability of the steel remarkably falls. Therefore,
the content of Nb is made 0.010 to 0.100%. The content of
Nb is preferably made less than 0.080%, more preferably
35 is made 0.050% or less. Further, the content of Nb is
preferably made 0.020% or more.
100541 Ca: 0.0005 to 0.0050%
Ca is one of the important elements in the steel
according to the present embodiment. The Ca contained in
the steel finely disperses and precipitates in steel as
sulfide-based inclusions or oxide-based inclusions and
5 therefore has the effect of suppressing increase of the
fatigue strength after aging treatment and drop of the
toughness after aging treatment. To sufficiently obtain
this effect, Ca has to be made 0.0005% or more in
content. However, if the content of Ca exceeds 0.0050%,
10 conversely coarse oxide-based inclusions end up being
formed, a drop in the hot ductility and fatigue strength
after aging treatment are caused, and the effect of
suppression of the drop in toughness after aging
treatment cannot be obtained. Therefore, the content of
15 Ca is made 0.0005 to 0.0050%. Note that the content of Ca
is preferably made 0.0010% or more, more preferably is
made 0.0015% or more.
f00551 REM: 0.001 to 0.05%
A REM is one of the important elements in the steel
2 0 according to the present embodiment. A REM contained in
the steel finely disperses and precipitates in the steel
as sulfide-based inclusions or oxide-based inclusions and
thereby has the effect of improving the fatigue strength
after aging treatment and, further, suppressing a drop in
2 5 toughness after aging treatment. To sufficiently obtain
this effect, the REM content has to be made 0.001% or
more. However, if the content of a REM exceeds 0.05%, a
drop in the hot ductility is invited. Therefore, the
content of a REM is made 0.001 to 0.05%. The content of a
3 0 REM is preferably made 0.003% or more, more preferably is
made 0.005% or more.
[00561 The age hardening steel of the present
embodiment is steel comprised of the above-mentioned C,
Si, Mn, S, Cr, Al, V, Nb, Ca, and REM and a balance of Fe
3 5 and impurities, restricting P, Ti, and N in the later
mentioned impurities to P: 0.030% or less, Ti: less than
0.005%, and N: less than 0.0080%, and, furthermore,
having a chemical composition where the F1 expressed by
the above formula (1) is 0.68 or more, the F2 expressed
by the formula (2) is 0.85 or less, the F3 expressed by
the formula (3) is 0.00 or more, and the F4 expressed by
5 the formula (4) is 0.012 to 0.08. Note that, "impurities"
indicates elements entering from the starting material
ore, scraps, the manufacturing environment, etc. when
producing a ferrous material industrially.
[0057] P: 0.030% or less
10 P is an element contained as an impurity and not
preferable in the steel according to the present
embodiment. That is, P segregates at the grain boundaries
and causes a drop in the toughness. In particular, if its
content is over 0.030%, the drop in toughness becomes
15 extremely remarkable. Therefore, the content of P is
limited to 0.030% or less. The content of P is preferably
limited to 0.025% or less. Note that, the effect of the
steel according to the present embodiment is exhibited
without particularly determining the lower limit of the
2 0 content of P. The lower limit value of the P content may
also be made 0%. However, if excessively decreasing the
P, an extreme rise in the cost of dephosphorization is
invited and the result becomes disadvantageous
economically, so the lower limit of the amount of P is
2 5 preferably made 0.005%.
[0058] Ti: less than 0.005%
Ti is an element contained as an impurity and not
preferable in the steel according to the present
embodiment. That is, Ti bonds with the N and C to form
3 0 coarse Ti carbonitrides which invite a drop in toughness.
In particular, if the content becomes 0.005% or more, the
toughness is made to greatly degrade. Therefore, the
content of Ti is limited to less than 0.005%. To suppress
a drop in toughness due to the aging treatment, the
35 content of Ti is preferably limited to 0.0035% or less,
more preferably is limited to 0.0015% or less. The lower
limit value of the Ti content may also be made 0%.
[00591 N: less than 0.0080%
N is an element contained as an impurity and ends up
fixing V as VN so is not preferable in the steel
according to the present embodiment. That is, the V
5 precipitating as VN no longer contributes to age
hardening, so to suppress precipitation of VN, the
content of N must be lowered. To suppress the
precipitation of VN and secure a sufficient amount of
solute V at the stage before the aging treatment, it is
10 necessary to limit the content of N to less than 0.0080%.
The upper limit value of the N content is preferably
0.0070%, 0.0060%, or 0.0050%. The lower limit value of
the N content is 0%.
[0060] Another embodiment of the age hardening steel
15 according to the present embodiment is steel comprising
elements from the above C to REMs, a composition
satisfying one or more of any of the above to ,
and a balance of Fe and impurities, having P, Ti, and N
in the impurities limited to P: 0.03% or less, Ti: less
2 0 than 0.005%, and N: 0.020% or less, and, furthermore,
having a chemical composition where the F1' expressed by
formula (1') is 0.68 or more, F2' expressed by formula
(2') is 0.85 or less, F3' expressed by formula (3') is
0.00 or more, and F4 expressed by formula (4) is 0.012 to
2 5 0.08.
[0061] Optional Elements
Below, the action and effects of any elements shown in
to selectively added in another embodiment of the
age hardening steel according to the present embodiment
3 0 and the reasons for limitation of their contents will be
explained.
lo0621 Mo: 0.01 to 1.0%
Inclusion of Mo is not essential, so the lower limit
value of the Mo content is 0%. On the other hand, Mo has
35 the action of raising the hardenability, making the main
phase of the structure of the steel after hot forging
bainite, and increasing the area ratio of the bainite. Mo
also has the action of forming carbides together with V
to increase the age hardenability in steel containing
0.25% or more of V. For this reason, if necessary, Mo may
also be included. However, Mo is an extremely expensive
5 element, so if the content increases, the cost of
manufacturing the steel increases and furthermore the
toughness also falls. Therefore, the amount of Mo when
included is made 1.0% or less. The amount of Mo when
included is preferably made 0.50% or less, more
10 preferably is made 0.40% or less, still more preferably
is made less than 0.30%. On the other hand, to stably
obtain the above effect of Mo, the amount of Mo when
included is preferably made 0.01% or more, more
preferably is made 0.05% or more, still more preferably
15 is made 0.10% or more.
100631 One or Both of Cu: 0.01 to 0.30% and Ni:
0.01 to 0.30%
Cu and Ni both have the action of increasing the fatigue
strength of steel after aging treatment. For this reason,
2 0 when desiring to obtain a greater fatigue strength, it is
also possible to include these elements in the ranges
explained below.
[0064] Cu: 0.01 to 0.30%
Inclusion of Cu is not essential, so the lower limit
2 5 value of the Cu content is 0%. On the other hand, Cu has
the action of increasing the fatigue strength of the
steel after aging treatment. For this reason, Cu may also
be included in accordance with need. However, if the
content of Cu exceeds 0.30%, the hot workability falls.
3 0 Therefore, the amount of Cu when included is made 0.30%
or less. The amount of Cu when included is preferably
made 0.25% or less. On the other hand, to stably obtain
the above-mentioned effect of increasing the fatigue
strength of Cu, the amount of Cu when included is
35 preferably made 0.01% or more, more preferably is made
0.05% or more, still more preferably is made 0.10% or
more.
[0065] Ni: 0.01 to 0.3%
Inclusion of Ni is not essential, so the lower limit
value of the Ni content is 0%. On the other hand, Ni has
the action of improving the fatigue strength of steel
5 after aging treatment. Furthermore, Ni has the action of
suppressing a drop in the hot workability due to Cu. For
this reason, Ni may also be included in accordance with
need. However, if the content of Ni exceeds 0.30%, the
cost swells and in addition the above effect becomes
10 saturated. Therefore, the amount of Ni when included is
made 0.30% or less. The amount of Ni when included is
preferably made 0.25% or less. On the other hand, to
stably obtain the above effect of Ni, the amount of Ni
when included is preferably made 0.01% or more, more
15 preferably is made 0.05% or more, still more preferably
is made 0.10% or more.
[0066] Note that, the above Cu and Ni may be included
as single types among these or as combination of the two
types. The total content of the elements when included
2 0 may be 0.6% when the contents of Cu and Ni are
respectively the upper limit values.
[0067] Bi: 0.01 to 0.400%
Bi has the action of lowering the cutting resistance and
increasing the machinability of the steel before the
2 5 aging treatment. For this reason, when desiring to obtain
a better machinability, Bi may be included in the range
explained below. .
Inclusion of Bi is not essential, so the lower limit
value of the Bi content is 0%. On the other hand, Bi has
3 0 the action of lowering the cutting resistance of the
steel before the age hardening and the action of
improving the scrap disposability. For this reason, if
necessary, Bi may also be included. However, if the
content of Bi is over 0.400%, a drop in the hot
35 workability is caused. Therefore, the amount of Bi when
included is made 0.400% or less. The amount of Bi when
included is preferably made 0.300% or less. On the other
hand, to stably obtain the above-mentioned effect of
reducing the cutting resistance of Bi and the effect of
improving the scrap disposability, the amount of Bi when
included is preferably made 0.010% or more, more
5 preferably is made 0.030% or more.
[00681 In the above explained embodiment and other
embodiments, the balance besides the above elements is
substantially Fe and unavoidable impurities, but other
elements may be added in trace amounts to an extent not
10 impairing the action and effect of the present invention.
[0069] Formula (1) to Formula (4) and Formula (1') to
Formula (3')
Next, the F1 to F4 expressed by the above-mentioned
formula (1) to formula (4) and the El' to F3' expressed
15 by formula (1') to formula (3') will be explained:
[0070] F1 or El': 0.68 or more
When the age hardening steel according to the present
embodiment does not contain Mo, F1 expressed by
2 0 has to be 0.68 or more. On the other hand, when the age
hardening steel according to the present embodiment
contains Mo, F1' expressed by
has to be 0.68 or more.
2 5 [0071] As already explained, the symbols of elements
in formula (1) and formula (1') mean the contents of
those elements by mass%.
[00721 El and F1' are indicators of the hardenability.
If the amounts of the alloy elements contained in the
3 0 steel satisfy the above ranges and F1 and El' satisfy the
above conditions, even if water cooling or other
accelerated cooling was not performed after hot forging,
the structure after hot forging becomes one having
bainite as its main phase.
35 [0073] When F1 or El' is less than 0.68, proeutectoid
ferrite is mixed in the structure after hot forging and V
carbides precipitate at the phase boundaries, so the
hardness before aging treatment rises or the age
hardenability becomes smaller.
[0074] F1 and F1' are preferably 0.70 or more, more
preferably are 0.72 or more. On the other hand, an
5 excessive increase in the hardenability is liable to
invite a drop in toughness of the steel, so F1 and El'
are preferably 1.00 or less, more preferably are 0.98 or
less.
[0075] F2 or F2': 0.85 or less
10 If the age hardening steel according to the present
embodiment does not contain Mo, F2 expressed by
F2=C+O.IxSi+0.2xMn+O.15xCr+0.35xV ...(2)
must be 0.85 or less. On the other hand, if the age
hardening steel according to the present embodiment
15 contains Mo, F2' expressed by
F2'=C+O.IxSi+O.2xMn+0.15xCr+0.35xV+0.2xMo . . - ( 2 ' )
must be 0.85 or less.
100761 As already explained, the symbols of elements
In formula (2) and formula (2') mean the contents of
2 0 those elements by mass%.
[0077] F2 and F2' are indicators showing the hardness
before aging treatment. Even if steel satisfies the
condition of the above F1 or Fl', if F2 or F2' becomes
within a suitable range, sometimes the hardness before
2 5 aglng treatment becomes too high, the cutting resistance
rises, and a good machinability can no longer be secured.
That is, if F2 or F2' exceeds 0.85, the hardness of the
bainite structure becomes too high. For this reason,
sometimes a rise in the cutting resistance becomes
30 unavoidable and a good machinabillty can no longer be
secured.
[0078] F2 and F2' are preferably 0.82 or less, more
preferably are 0.80 or less. On the other hand, if F2 and
F2' are too low, the hardness after age hardening is
3 5 liable to become insufficient, so F2 and F2' are
preferably 0.55 or more, more preferably are 0.60 or
more.
[0079] F3 or F3': 0.00 or more
If the age hardening steel according to the present
embodiment does not contain Mo, F3 expressed by
F3=-4.5xC+Mn+Cr-3.5xV -..(3)
5 must be 0.00 or more. On the other hand, if the age
hardening steel according to the present embodiment
contains Mo, F3' expressed by
F3'=-4.5xC+Mn+Cr-3.5xV-0.8xMo . . . (3')
must be 0.00 or more.
10 [OOSO] As already explained, the symbols of elements
in formula (3) and formula (3') mean the contents of
those elements by mass%.
[OOSl] F3 and F3' are indicators of the toughness
after aging treatment. That is, even if the conditions of
15 F1 or El' and F2 or F2' were satisfied, if F3 or F3' is
in a suitable range, sometimes the toughness of the steel
after aging treatment falls and the targeted toughness
can no longer be secured. That is, when F3 or F3' is less
than 0.00 (negative number), the toughness after aging
2 0 treatment falls. F3 and F3' are preferably 0.01 or more.
[00821 Note that, if F1 is 0.68 or more and F2 is 0.85
or less, it is not particularly necessary to provide a
limit for the upper limit of F3.
Similarly, if El' is 0.68 or more and F2' is 0.85 or
2 5 less, it is not particularly necessary to provide a limit
for the upper limit of F3'.
[0083] F4: 0.012 to 0.08
In the age hardening steel according to the present
embodiment, F4 expressed by
3 0 F4=lOxCa+REM . . . (4)
must be 0.012 to 0.08.
[0084] As already explained, the symbols of elements
in formula (4) mean the contents of those elements by
mass%.
3 5 [00851 F4 is an indicator showing a control indicator
of the form of inclusions. That is, even if the steel
satisfies the conditions of El or El' and F2 or F2' and
F3 or F3', if F4 is in a suitable range, sometimes the
fatigue strength of the steel after aging treatment will
not stably rise or the drop in toughness after aging
5 treatment cannot be stably suppressed. That is, when F4
is less than 0.012, sometimes sulfide-based inclusions
and oxide-based inclusions cannot be finely dispersed,
the fatigue strength of the steel after aging treatment
will not stably rise, and the drop in toughness after
10 aging treatment cannot be stably suppressed. Therefore,
F4 was made 0.012 or more. To sufficiently obtain the
effect of refining the inclusions, F4 is preferably 0.014
or more. Further, 0.016 or more is more preferable.
Further, the more F4 increases, the more the effect of
15 refining the inclusions can be exhibited, but if F4 is
over 0.08, conversely sometimes coarsening of the oxidebased
inclusions is invited or a drop in the hot
ductility is invited, so the upper limit of F4 was made
0.08 or less. F4 is preferably 0.07 or less, more
2 0 preferably is 0.06 or less.
[0086] Steel Structure (Microstructure)
Next, the steel structure (microstructure) of the age
hardening steel according to the present embodiment will
be explained.
2 5 [0087] As explained above, before the aging treatment,
formation of a large amount of proeutectoid ferrite is
not preferable. Furthermore, from the viewpoint of the
machinability, formation of a large amount of martensite
is also not preferable. For this reason, it is important
30 that the main phase before the aging treatment of the age
hardening steel according to the present embodiment be
made bainite. That is, to secure a sufficient
machinability and solute V, the structure before the
aging treatment must have an area ratio of bainite of 70%
35 or more. Note that, the area ratio of bainite is
preferably 80% or more, while the bainite single phase,
that is, the area ratio of bainite of loo%, is most
preferable. If the area ratio of bainite is less than
loo%, the phases other than the main phase of bainite
includes a ferrite phase, pearlite structure, martensite
structure, etc., but the smaller these phase and
5 structures, the better.
[OOSS] Relationship of Age Hardening Ability,
Machinability, and Fatigue Strength of Steel
If making the steel a diameter 35 nun substantially
columnar shape and defining the amount of rise of the
10 Vicker's hardness of the steel when holding this steel at
a temperature of 620°C for 120 minutes as the age
hardenability of the steel, the lower limit value of the
age hardenability of the steel according to the present
embodiment is preferably 30 Hv, more preferably is 33 Hv,
15 35 Hv, or 40 Hv. Note that, "treatment holding the steel
at a temperature of 620°C for 120 minutes" means general
aging treatment conditions when treating the steel
according to the present embodiment for age hardening to
produce a machine part. If the age hardenability is 30 Hv
2 0 or more, the steel according to the present embodiment
has an excellent machinability before the aging treatment
and has an excellent fatigue strength after the aging
treatment.
[OOSS] Method of Production of Age Hardening Steel and
2 5 Method of Production of (Machine) Part Using Age
Hardening Steel
The method of production of age hardening steel of the
present embodiment is not particularly limited. A general
method may be used to smelt the steel and adjust the
3 0 chemical composition. Below, one example of the method of
production of a machine part in automobiles, industrial
machinery, construction machinery, etc. using as a
material the age hardening steel according to the present
embodiment produced in the above way will be shown.
3 5 [OOSO] First, from steel adjusted in chemical
composition to the above-mentioned range, age hardening
steel used for forming a part (below, referred to as
"intermediate material") is prepared. As the above
intermediate material, a billet obtained by blooming an
ingot, a billet obtained by blooming a continuously cast
material, a steel rod obtained by hot rolling or hot
5 forging these billets, or other such material may be
used. However, at the time of fabrication of the
intermediate material, if holding the material for a
certain time in a temperature region where V carbides
easily precipitate, the age hardenability is liable to be
10 lost. For example, when maintaining the temperature of
the intermediate materials within the range of 540 to
700°C for 30 minutes or more after blooming or after hot
rolling or hot forging, the age hardenability is liable
to be lost. However, if in accordance with the general
15 method, the intermediate material is allowed to stand in
a room temperature environment after blooming or after
hot rolling or hot forging, such a situation will not
arise.
[OOSl] Next, the above intermediate material was hot
2 0 forged and furthermore was cut to finish it into a
predetermined part shape. The above hot forging, for
example, heats the intermediate material at 1200 to 1250°C
for 5 to 60 minutes, then forges it so that the surface
temperature after the final forging becomes llOO°C or
2 5 more, then, after that, makes the average cooling rate in
the 800 to 400°C temperature region 15 to 60°C/min to cool
the material down to room temperature. Such an average
cooling rate is easily obtained by allowing the forged
steel to stand in a room temperature environment.
3 0 However, if the cooling rate is less than 15'C/min, the V
carbides precipitate during cooling and the age
hardenability is liable to become 30Hv or less. After the
material is cooled in this way, it is further cut to
finish it to the desired part shape.
3 5 [GO921 Finally, the roughly shaped material formed to
a predetermined part shape is treated to age it to obtain
a machine part of automobiles, industrial machinery,
construction machinery, etc. provided with the desired
characteristics. The above aging treatment is, for
example, performed in a 540 to 700°C temperature region,
5 preferably a 560 to 680°C temperature region, more
preferably a 580 to 660°C temperature region. The holding
time of this aging treatment is suitably adjusted to, for
example, 30 to 1000 minutes etc. by the size (mass) of
the machine part. If the aging treatment temperature is
10 less than 540°C, V carbides or V carbonitrides cannot be
sufficiently formed and the desired age hardenability of
30Hv cannot be obtained. On the other hand, if the aging
treatment temperature exceeds 700°C, the formed V carbides
or V carbonitrides become coarser, so no longer
15 contribute to hardening and the desired age hardenability
of 30 Hv cannot be obtained. Similarly, if the holding
time is less than 30 minutes, V carbides or V
carbonitrides cannot be sufficiently formed, so the
desired age hardenability of 30Hv cannot be obtained. On
2 0 the other hand, if the holding time exceeds 1000 minutes,
the formed V carbides or V carbonitrides become coarser,
so no longer contribute to hardening and the desired age
hardenability of 30Hv cannot be obtained.
[0093] In the above way, it is possible to produce age
2 5 hardening steel according to the present embodiment and a
machine part using the same as a material.
Examples
[0094] Below, Examples 1 and 2 will be used to explain
the present invention in further detail. The conditions
3 0 in Examples 1 and 2 shown below are examples of
conditions adopted for confirming the workability and
advantageous effects of the present invention. The
present invention is not limited to these examples of
conditions. Further, the present invention can adopt
35 various conditions so long as not departing from the gist
of the present invention and achieving the object of the
present invention.
[0095] Example 1
Each of the Steels 1 to 26 of the chemical compositions
shown in Table 1 was smelted by a 50 kg vacuum melting
5 furnace. The Steels 1 to 13 in Table 1 are steels with
chemical compositions within the ranges prescribed by the
present invention. On the other hand, the Steels 14 to 26
in Table 1 are steels with chemical compositions outside
the conditions prescribed by the present invention. Note
10 that, in the section on Ti, "<0.001" indicates the
content of Ti as an impurity is below the lower limit
value of detection in emission spectroscopy of 0.001%.
[0096] Table 1
[0097] An ingot of each steel was heated at 1250°C,
then hot forged to a diameter 60 mm steel rod. Each hot
forged steel rod was allowed to cool once in the
atmosphere to cool it down to room temperature. After
5 that, furthermore, the steel rod cooled down to room
temperature was heated as an intermediate material to
1250°C and was again hot forged to a diameter 35 mm steel
rod while making the finishing temperature 950°C or more.
This second hot forging was performed for simulating
10 forging to a part shape. The second hot forged steel rod
was allowed to cool in the atmosphere to cool it down to
room temperature. The cooling rate at the time of the
second hot forging was measured using a radiant
thermometer. The average cooling rate after hot forging
15 in the 800 to 400°C temperature region (in Table 2,
indicated as "cooling rate") was 50°C/min in each case.
[0098] In the obtained Steels 1 to 26, some of the hot
forged diameter 35 mm steel rods were trimmed in the
state with no aging treatment (that is, in the state as
2 0 cooled) to cut off 100 mm from each of the two ends of
the steel rods, then test pieces were cut out from the
remaining center parts and were investigated for the
Vicker's hardness before aging treatment and the area
ratio of bainite of the structures. On the other hand,
2 5 the remaining hot forged diameter 35 mm steel rods were
treated to age them by holding them at 62OoC for 120
minutes, then were trimmed at the two end parts of the
steel rods to cut off 100 mm each, test pieces were cut
out from the remaining center parts and were investigated
3 0 for the Vicker's hardness after aging treatment. Further,
in the obtained Steels 1 to 26, test pieces were cut out
from the aging treated steel rods and were investigated
for the absorption energy in Charpy impact tests after
aging treatment and the low cycle fatigue strength and
3 5 fatigue strength.
[0099] The Vicker's hardness was measured in the
following way.
First, a steel rod cut laterally to give a cut crosssection
as the examined surface was buried in a resin.
The examined surface was polished to a mirror finish to
5 prepare a test piece. Next, based on the "Vicker's
Hardness Test - Test Method" in JIS Z 2244 (2009), 10
points near the R/2 part of the examined surface ("R"
indicates the radius) were measured for hardness under a
load of 9.8N. The measured values of hardness at the 10
10 points were arithmetically averaged. The obtained value
was made the Vicker's hardness of the steel rod. Note
that, if the Vicker's hardness before aging treatment was
290 Hv or less, it was judged sufficiently low and was
used as the target. Further, if the difference between
15 the Vicker's hardness after aging treatment and the
Vicker's hardness before aging treatment (below, referred
to as the "amount of hardening AHV") was 30 Hv or more,
the age hardenability was judged sufficiently high and
was used as the target.
2 0 [ 01001 The area ratio of the bainite in the structure
was measured in the following way. First, a test piece
used for measurement of hardness and obtained by burial
in a resin and polishing to a mirror finish was etched by
Nital. The etched test piece was photographed for
2 5 structure at a power of 200X using an optical microscope.
From the obtained photograph, image analysis was used to
measure the area ratio of bainite. If the area ratio of
the bainite was 70% or more, the structure was judged to
be sufficiently converted to bainite and was used as the
3 0 target.
[OlOl] The toughness was measured using a standard
test piece with a U-notch with a notch depth of 2 mm and
a notch bottom radius of 1 mm. If the absorption energy
at 20°C after aging treatment evaluated in this Charpy
35 impact test was 503 or more, it was judged sufficiently
high and was used as the target.
[0102] The fatigue strength was investigated by taking
a monoaxial tension and compression type fatigue test
piece. That is, a smooth fatigue test piece shaped with a
diameter of the parallel part shown in FIG. 1 of 3.4 mm
and a length of the parallel part of 12.7 mm was taken in
5 parallel to the forging direction from the R/2 part of
the steel rod (longitudinal direction of steel rod) and
was tested for fatigue room temperature, in the
atmosphere, under conditions of a stress ratio of 0.05
and a test rate of 10 Hz. The maximum stress where the
10 test piece does not break at lo7 repeated application of
stress under the above conditions was made the fatigue
strength. If the fatigue strength was 425 MPa or more,
the fatigue strength was judged sufficiently high and was
used as the target.
15 [0103] The low cycle fatigue strength was found by the
following method: First, a parallelopiped member of
vertical and horizontal spans in the longitudinal
direction cross-section of respectively 13 mm and a
length of 100 mm was taken from a steel rod in parallel
2 0 with the forging direction (longitudinal direction of
steel rod) so that the sampled portion become the R/2
part of the steel rod. After that, furthermore, a fourpoint
bending test piece provided with a radius 2 mm
semicircular notch at a part at the center in the
2 5 longitudinal direction of one face of that parallelepiped
member (that is, the face having the part for evaluation
of fatigue) was obtained. The low cycle fatigue test was
conducted at room temperature in the atmosphere. It was
performed by performing a four-point bending fatigue test
3 0 under conditions of a stress ratio of 0.1, a distance
between support points of 45 mm, and a test frequency of
5Hz. Under the above conditions, stress was repeatedly
applied. The 5x103th strength was defined as the "low
cycle fatigue strength" for evaluation of the strength.
35 If the low cycle fatigue strength was 520 MPa or more, it
was judged that the low cycle fatigue strength was
sufficiently high and this was used as the target.
[0104] Table 2 shows the results of the above
investigations. Note that, in the column on
"Bainitization", test pieces with a bainite area ratio of
70% or more or meeting the target were indicated as
5 "GOOD", while test pieces of less than 70% or not
reaching the target were indicated as "BAD". Further, in
Table 2, an "absorption energy in a Charpy impact test"
was indicated as the "Charpy absorption energy".
Inv. ex.
[01061 As clear from Table 2, in the case of each of
the "invention examples" of Test Nos. A1 to A13 having
chemical compositions prescribed in the present
invention, it will be understood that the Vicker's
5 hardness HV before aging treatment was 290 or less, the
aging treatment caused hardening to a Vicker's hardness
of 30Hv or more, furthermore, in the Charpy impact test,
the absorption energy was also 50J or more, the fatigue
strength was 425 MPa or more, and the low cycle fatigue
10 strength was 520 MPa or more, that is, the targets were
achieved, and both the fatigue strength and toughness
after aging treatment and the machinability before aging
treatment could be achieved.
[0107] As opposed to this, in the case of the
15 "comparative examples" of Test Nos. B1 to B13 off from
the provisions of the present invention, at least one of
the targeted performances could not be obtained.
[0108] Example 2
Each of the Steels 26 to 39 comprised of the Steel 1 of
2 0 the chemical composition shown in Table 1 changed in at
least one of the Ca content, REM content, and value of F4
was smelted by a 50 kg vacuum melting furnace. The Steel
1 and Steels 27, 30, 33, and 36 to 39 in Table 3 are
steels with chemical compositions within the scope
2 5 prescribed by the present invention. On the other hand,
Steels 28, 29, 31, 32, 34, and 45 in Table 3 are steels
with chemical compositions off from the conditions
prescribed in the present invention. Note that, in the
section on Ti, "<0.001" indicates the content of Ti as an
30 impurity is below the lower limit value of detection in
emission spectroscopy of 0.001%.
[0109] Table 3
[OllO] An ingot of each steel was heated at 1250°C,
then hot forged to a diameter 60 mm steel rod. Each hot
forged steel rod was allowed to cool once in the
atmosphere to cool it down to room temperature. After
5 that, furthermore, the steel rod cooled down to room
temperature was heated as an intermediate material to
1250°C and was again hot forged to a diameter 35 mm steel
rod while making the finishing temperature 950°C or more.
This second hot forging was performed for simulating
10 forging to a part shape. The second hot forged steel rod
was allowed to cool in the atmosphere to cool it down to
room temperature. The cooling rate at the time of the
second hot forging was measured using a radiant
thermometer. The average cooling rate after hot forging
15 in the 800 to 400°C temperature region (in Table 4,
indicated as "cooling rate") was 50°C/min in each case.
[Ollll The obtained Steel 1 and Steels 27 to 39 were
examined in the same way as with the methods performed in
Example 1 so as to investigate the Vicker's hardness
2 0 before aging treatment, the area ratio of bainite in the
structure, the absorption energy in the Charpy impact
test, the low cycle fatigue strength, and the fatigue
strength. Table 4 shows the results of the above
investigations. Note that the notations of Table 4 are
2 5 similar to the notations of Table 2.
[0112] Table 4
Amount of
[0113] As clear from Table 4, in the case of Test Nos.
A27, 30, 33, and 36 to 39 where at least one of the Ca
content, REM content, and value of F4 were made values
more preferable than Test No. Al, the result was a higher
5 fatigue strength compared with Test No. Al. This is due
to the fact that compared with Test No. Al, the sulfidebased
inclusions or oxide-based inclusions are more
finely dispersed.
[0114] As opposed to this, in the case of the
10 "comparative examples" of Test Nos. B28, B29, B31, B32,
B34, and B35 where one or more of the Ca content, REM
content, and the value of F4 were outside the scope of
the present invention; the result was a lower fatigue
strength or toughness compared with Test No. Al. This is
15 due to the fact that compared with Test No. Al, the
sulfide-based inclusions or oxide-based inclusions became
coarser, so a drop in the fatigue strength or a drop in
the toughness was invited.
Industrial Applicability
2 0 [0115] According to the age hardening steel of the
present invention, the hardness before aging treatment is
290 Hv or less so a good machinability can be expected.
Further, if using the age hardening steel of the present
invention, aging treatment performed after cutting causes
2 5 the steel to harden to a Vicker's hardness of 30 Hv or
more, so a 425 MPa or more fatigue strength is obtained.
Furthermore, if using the age hardening steel of the
present invention, the absorption energy at 20°C after
aging treatment is 503 or more and a drop in toughness
30 due to the aging treatment can be sufficiently
suppressed. In addition, if using the age hardening steel
of the present invention, it is possible to make the low
cycle fatigue strength 520 MPa or more. For this reason,
the age hardening steel of the present invention can be
3 5 extremely suitably used as the material for machine parts
of automobiles, industrial machinery, construction
machinery, etc.

CLAIMS
Claim 1. An age hardening steel containing, by
mass%, C: 0.05 to 0.20%, Si: 0.01 to 0.50%, Mn: 1.50 to
2.50%, S: 0.005 to 0.080%, Cr: 0.03 to 1.60%, Al: 0.005
5 to 0.050%, V: 0.25 to 0.50%, Nb: 0.010 to 0.100%, Ca:
0.0005 to 0.0050%, and REM: 0.001 to 0.05%, limiting P to
0.030% or less, Ti to less than 0.005%, and N to less
than 0.0080%, having a balance of Fe and impurities,
having an area ratio of bainite structures of 70% or more
10 and, furthermore, having a chemical composition where the
following F1 expressed by formula (1) is 0.68 or more, F2
expressed by formula (2) is 0.85 or less, F3 expressed by
formula (3) is 0.00 or more, and F4 expressed by formula
(4) is 0.012 to 0.08:
15 Fl=C+0.3xMn+0.25xCr ...(I)
F2=C+O.IxSi+0.2xMn+O.15xCr+0.35xV . . . (2)
F3=-4.5xC+Mn+Cr-3.5xV - - -(3)
F4=1OxCa+REM . . . (4)
where the symbols of elements in the above formula (1) to
2 0 formula (4) mean the contents by mass% of those elements.
Claim 2. An age hardening steel containing, by
mass%, C: 0.05 to 0.20%, Si: 0.01 to 0.50%, Mn: 1.50 to
2.50%, S: 0.005 to 0.080%, Cr: 0.03 to 1.60%, Al: 0.005
to 0.050%, V: 0.25 to 0.50%, Nb: 0.010 to 0.100%, Ca:
2 5 0.0005 to 0.0050%, and REM: 0.001 to 0.05%, having an
area ratio of bainite structures of 70% or more,
furthermore, satisfying any one or more of the conditions
of composition shown by the following to ,
limiting P to 0.030% or less, Ti to less than 0.005%, and
3 0 N to less than 0.0080%, having a balance of Fe and
impurities, and, furthermore, having a chemical
composition where the following El' expressed by formula
(1') is 0.68 or more, F2' expressed by formula (2') is
0.85 or less, and F3' expressed by formula (3') is 0.00
35 or more, and F4 expressed by formula (4) is 0.012 to
0.08:
Mo: 0.01 to 1.0%
One or both of Cu: 0.0i'to'0.30%.anNdi :
0.01% to 0.30%
Bi: 0.01 to 0.400%
Fl'=C-I-0.3xMntO2.5 xCr+0. 6rMo . . . (1')
F2'=C+0.1xSi+0.2xMn+0015~Cr+0035xVtO02~Mo
. . . (2')
F3'=-4.5xC+Mn+Cr-3.5xV-O08xMo . . . ( 3')
F4=1OxCa+REM . . . (4)
where the symbols of elements in the above formula (I) to
formula (3') and formula (4) mean the contents by mass%
of those elements.
Claim 3. A method of procluctioi? of a part usilly age
hardening steel comprising:
a forging step of heating age hardening steel
according to claim I or 2 at 1200 to 1250°C for 5 to 60
minutes, then forging it so tha.t a surface temperature
after final forging becomes llOO°C or more, then, after
that, cooling it by an average cooling rate in a 800 to
400°C temperature reyion of 15 to 60°C/min down to room
temperature,
a cutting step o:t cut-ting -the forged steel, and
an aging treatment step of holding the cut
steel in a 540 to 700°C temperature region for 30 to 1000
2 5 minutes.