1
[Name of Document] DESCRIPTION
[Title of the Invention] STEEL PART FOR MACHINE STRUCTURAL USE
AND MANUFACTURING METHOD THEREOF
[Technical Field]
5 [0001] The present invention relates to a steel part for machine structural
use of a transportation machine such as an automobile, an industrial machine,
and the like and a manufacturing method thereof, and particularly relates to a
steel part for machine structural use having high fatigue strength and high
toughness without its machinability being deteriorated and a manufacturing
10 method thereof This application is based upon and claims the benefit of
priority of the prior Japanese Patent Application No. 2011-118350, filed on
May 26, 2011, the entire contents of which are incorporated herein by
reference.
[Background Art]
15 [0002] Conventionally, in many cases, high strength and high toughness
have been given to a machine structure part for an automobile, an industrial
machine, and the like in a manner that a steel product such as a bar steel is hot
forged into a part shape and then is reheated to be subjected to thermal
refining of quenching and tempering. In recent years, in terms of a reduction
20 in manufacturing cost, an omission of a thermal refining process of quenching
and tempering has been promoted, and as shown in Patent Document 1 and
the like, for example, there has been proposed a non-heat-treated steel to
which high strength and high toughness can be given even though it remains
being hot-forged. However, that both high fatigue strength and excellent
25 machinability are accomplished is actually to be an obstacle to the application
of the high strength and high toughness non-heat-treated steel to a steel part
2
for machine structural use.
[0003] Generally, the fatigue strength relies on tensile strength, and as the
tensile strength is increased, the fatigue strength is increased. On the other
hand, the increase in tensile strength deteriorates the machinability. Many of
5 the steel parts for machine structural use need to be cut after being hot forged,
and the cutting cost accounts for most of the manufacturing cost of the part.
The deterioration of machinability caused by the increase in tensile strength
causes the significant increase in manufacturing cost of the part. Generally,
when the tensile strength exceeds 1200 MPa, the machinability deteriorates
10 significantly and the manufacturing cost is increased drastically, and thus it is
practically difficult to achieve the high strength in excess of the above
strength. Thus, in the parts for machine structural use, the increase in cutting
cost caused by the deterioration of machinability is a bottleneck in achieving
the high fatigue strength, and a technique of accomplishing both the high
15 fatigue strength and the excellent machinability is required.
[0004] As conventional knowledge of securing machinability even
though the steel part is high in strength, in Patent Document 2, for example, it
has been proposed that a large amount of V is added to a steel, V carbonitride
that has precipitated by an aging treatment is attached to a tool surface at the
20 time of machining to protect the tool, which is effective for preventing tool
abrasion. However, a large amount of V is needed in order to secure the
machinability, and due to the steel being a high alloy, hot ductility is
significantly poor. In the case when such a steel is used, there is caused a
problem of occurrence of cracking and flaws to occur at the time of casting
25 and flaws at the time of subsequent hot working, namely at the time of hot
rolling of a bar steel and hot forging of a part.
3
[0005] As a means of accomplishing both the high fatigue strength and
the excellent machinability, it is effective to improve the ratio of the fatigue
strength to the tensile strength, namely an endurance ratio (the fatigue
strength/the tensile strength). In Patent Document 3, for example, it has
5 been proposed that it is effective to turn a structure mainly composed of
bainite to decrease high-carbon martensite island and retained austenite in the
structure. However, the endurance ratio is 0.56 or less at the most, there is a
limit to increase the strength without deteriorating the machinability, and the
fatigue strength and the tensile strength are both low.
10 [0006] Further, in Patent Document 4, for example, it has been proposed
that it is effective to turn a structure into a fine ferrite-bainite structure after
molding by warm forging in a temperature zone of 800 to 1050°C and to
cause V carbonitride to precipitate by a subsequent aging treatment.
Generally, there is shown a tendency for the toughness to decrease when the
15 achievement of high endurance ratio is accomplished, but by the warm
forging, the ferrite-bainite structure is made fine, and thereby the toughness is
improved. However, in the steel part for machine structural use requiring
toughness, the improvement of toughness is small. Further, in the warm
forging in the temperature zone of 800 to 1050°C, a forging load is large to
20 thereby decrease the life of a mold significantly, and thus the production is
difficult to be performed industrially.
[0007] Further, in Patent Documents 5 and 6, for example, there has been
proposed a method of increasing strength by causing Ti carbide and V carbide
to precipitate in a steel. However, when Ti is contained, Ti turns into nitride
25 at high temperature preferentially to carbide, and thereby coarse Ti nitride is
formed, and Ti nitride does not contribute to precipitation strengthening and
4
further significantly decreases an impact value.
[Prior Art Document]
[Patent Document]
[0008] [Patent Document 1] Japanese Laid-open Patent Publication No.
5 Hl-198450
[Patent Document 2] Japanese Laid-open Patent Publication No.
2004-169055
[Patent Document 3] Japanese Laid-open Patent Publication No.
H4-176842
10 [Patent Document 4] Japanese Patent No. 3300511
[Patent Document 5] Japanese Laid-open Patent Publication No.
2011-241441
[Patent Document 6] Japanese Laid-open Patent Publication No.
2009-84648
15 [Disclosure of the Invention]
[Problems to Be Solved by the Invention]
[0009] The present invention has an object to provide a steel part for
machine structural use whose fatigue strength and toughness are improved
without its machinability being deteriorated by controlling a structure in the
20 part in subsequent cooling and a heat treatment even with ordinary hot forging,
and a manufacturing method thereof.
[Means for Solving the Problems]
[0010] In the present invention, it was found to obtain a steel part for
machine structural use having high Charpy absorbed energy and a high
25 endurance ratio and having its fatigue strength and toughness improved
without its machinability being deteriorated in a maimer that, after hot forging.
5
by cooling a hot-forged steel product at a relatively fast cooling rate, the main
structure is caused to turn into fine bainite, and then V carbide is caused to
precipitate in the bainite structure by an aging treatment to control the size
and dispersed state of V carbide, and the present invention was completed.
5 [0011] The gist of the present invention is as follows.
[0012] (1) A steel part for machine structural use made of a steel
containing,
in mass%,
C: 0.05 to 0.20%,
10 Si: 0.10 to 1.00%,
Mn: 0.75 to 3.00%,
P: 0.001 to 0.050%,
S: 0.001 to 0.200%,
V: 0.05 to 0.20%,
15 CnO.Ol to 1.00%,
Al: 0.001 to 0.500%, and
N: 0.0080 to 0.0200%, and
a balance being composed of Fe and inevitable impurities, in which
a steel structure contains a bainite structure having an area ratio of 95% or
20 more,
a bainite lath width is 5 |im or less,
V carbide having an average grain diameter of not less than 4 nm nor more
than 7 nm dispersedly exists in the bainite structure, and
an area ratio of V carbide in the bainite structure is 0.18% or more.
25 (2) The steel part for machine structural use according to (1), in which
the steel further contains one type or two types or more of, in mass%.
6
Ca: 0.0003 to 0.0100%,
Mg: 0.0003 to 0.0100%, and
Zr: 0.0005 to 0.1000%.
(3) The steel part for machine structural use according to (1) or (2), in which
5 the steel further contains one type or two types of, in mass%,
Mo: 0.01 to 1.00%, and
Nb: 0.001 to 0.200%.
(4) The steel part for machine structural use according to (1), in which
Charpy absorbed energy at 20°C is 80 J/cm or more and an endurance ratio is
10 0.60 or more.
(5) A manufacturing method of a steel part for machine structural use
includes:
heating a steel product containing, in mass%,
C: 0.05 to 0.20%,
15 Si: 0.10 to 1.00%,
Mn: 0.75 to 3.00%,
P: 0.001 to 0.050%,
S: 0.001 to 0.200%,
V: 0.05 to 0.20%,
20 Cr: 0.01 to 1.00%,
Al: 0.001 to 0.500%, and
N: 0.0080 to 0.0200%, and
a balance being composed of Fe and inevitable impurities to not lower than
1100°C nor higher than 1300°C and hot forging the steel product;
25 after the hot forging, cooling the hot-forged steel product at an average
cooling rate down to 300°C set to be not less than 3°C/second nor more than
7
120°C/second; and
after the cooling, performing an aging treatment within a temperature range of
not lower than 550°C nor higher than 700°C.
[Effect of the Invention]
5 [0013] According to the present invention, it becomes possible to provide
a steel part for machine structural use having high fatigue strength and high
toughness without increasing cutting cost by selecting a steel component
range, a structure form, and a heat treatment condition, which is extremely
effective industrially.
10 [Mode for Carrying out the Invention]
[0014] The present inventors earnestly examined a steel component range,
a structure form, and a heat treatment condition with respect to the
above-described object, and consequently obtained the following pieces of
knowledge (a) to (d),
15 [0015] (a) The structure is caused to turn into a bainite structure having
an area ratio of 95% or more, and is caused to turn into a microstructure in
which a bainite lath width is 5 |Lim or less, and then by an aging treatment,
fine V carbide is caused to disperse in the bainite structure, and thereby an
endurance ratio higher than that of a conventional non-heat-treated steel can
20 be obtained. By the aging treatment, fine V carbide precipitates, and thereby
tensile strength and fatigue strength both increase. However, when the
temperature of the aging treatment becomes higher than a certain temperature,
V carbide is coarsened and the tensile strength stops increasing, but the
fatigue strength fixrther increases. As a result, when the temperature of the
25 aging treatment becomes higher than a certain temperature, the endurance
ratio improves.
8
[0016] (b) As long as the structure is the bainite structure having an area
ratio of 95% or more, and is the microstructure in which the bainite lath width
is 5 |i,m or less, it is possible to obtain the high toughness and high endurance
ratio in which U-notch Charpy absorbed energy at 20°C is 80 J/cm^ or more
5 and the endurance ratio is 0.60 or more. In a conventional non-heat-treated
steel (with its endurance ratio of 0.48 or so), improving the endurance ratio to
be 0.60 or more means to, in the case of the tensile strength being 1100 MPa,
for example, improve the fatigue strength by about 130 MPa or more without
increasing the tensile strength. Machinability strongly relies on the tensile
10 strength. As long as it is possible to improve only the fatigue strength
without increasing the tensile strength, the fatigue strength is improved
without deteriorating the machinability and both the excellent machinability
and the high fatigue strength are accomplished.
[0017] (c) A steel product to which low C, high N and V are added is hot
15 forged and molded, and then an average cooling rate down to 300°C is set to
fall within a range of not less than 3°C/second nor more than 120°C/second,
and thereby a desired fine bainite structure can be obtained even with the
ordinary hot forging.
[0018] (d) When Ti is contained in the steel, Ti turns into nitride at high
20 temperature preferentially to carbide, and thereby coarse Ti nitride is formed,
and Ti nitride does not contribute to precipitation strengthening and further
significantly decreases an impact value. In contrast to this, as for V, its
dissolution amount at the time when the steel is austenitized is large, and even
though part of V turns into nitride, the amount of nitride is small, by the aging
25 treatment, most of dissolved V turns into V carbide to precipitate, and a large
amount of precipitation strengthening can be obtained.
9
[0019] The present invention was completed for the first time by further
repeated examinations based on these pieces of knowledge.
[0020] Hereinafter, the present invention will be explained in detail.
First, there will be explained reasons for limiting the above-described steel
5 component range of the steel part for machine structural use. Here, "%" of
the component means mass%.
[0021] C: 0.05 to 0.20%
C is an important element that determines the strength of the steel.
For sufficiently obtaining the strength as the part, the lower limit is set to
10 0.05%. The alloy cost is low as compared with other alloy elements, and as
long as it is possible to add C in large amounts, the alloy cost of tiie steel
product can be reduced. However, when a large amount of C is added, at the
time of bainite transformation, retained austenite and martensite island in
which C is concentrated are formed at boundaries of laths, and the toughness
15 and the endurance ratio decrease, and thus the upper limit is set to 0.20%.
[0022] Si: 0.10 to 1.00%
Si is an effective element as an element that increases the strength of
the steel and as a deoxidizing element. For obtaining these effects, the lower
limit is set to 0.10%. Further, Si is an element that promotes ferrite
20 transformation, and when the upper limit exceeds 1.00%, ferrite is formed at
grain boundaries of prior austenite and the fatigue strength and the endurance
ratio significantly decrease, and thus the upper limit is set to 1.00%.
[0023] Mn: 0.75 to 3.00%
Mn is an element that promotes the bainite transformation, and is an
25 important element for turning the structure into bainite in a cooling process
after hot forging. Further, Mn has an effect of improving the machinability
10
by bonding to S to form sulfides, and also has an effect of maintaining the
high toughness by suppressing the growth of austenite grains. For exhibiting
these effects, the lower limit is set to 0.75%. On the other hand, when Mn in
an amount in excess of 3.00% is added, the hardness of a base metal increases
5 to make the steel brittle, and thus the toughness decreases and the
machinability deteriorates significantly. The upper limit is set to 3.00%.
[0024] P: 0.001 to 0.050%
As for P, 0.001% or more is ordinarily contained in the steel as an
inevitable impurity, and thus the lower limit is set to 0.001%. Then, P that is
10 contained is segregated at grain boundaries of prior austenite and the like to
significantly decrease the toughness, and thus the upper limit is limited to
0.050%. It is preferably 0.030% or less, and is more preferably 0.010% or
less.
[0025] S: 0.001 to 0.200%
15 S has an effect of improving the machinability by forming sulfides
with Mn, and also has an effect of maintaining the high toughness by
suppressing the growth of austenite grains. For exhibiting these effects, the
lower limit is set to 0.001%. However, although S depends also on the
amount of Mn, when S is added in large amounts, anisotropy in mechanical
20 properties such as the toughness is increased, and thus the upper limit is set to
0.200%.
[0026] V: 0.05 to 0.20%
V is an element effective for increasing the strength and the endurance
ratio by forming carbide to strengthen the bainite structure by precipitation.
25 For sufficiently obtaining the above effect, the content of 0.05% or more is
required. On the other hand, when the content exceeds 0.50%, the effect is
11
saturated and the alloy cost is increased, and further hot ductility significantly
decreases to thus cause a problem of occurrence of flaws at the time of hot
rolling of the bar steel and hot forging of the part, hi the present invention,
emphasis is placed on the hot ductility and the economic efficiency, in
5 particular, and thus the range of V is set to 0.05 to 0.20%.
[0027] Cr: 0.01 to 1.00%
Cr is an element effective for promoting the bainite transformation.
For obtaining the effect, 0.01% or more of Cr is added, but even though Cr is
added in excess of 1.00%, the effect is saturated and the alloy cost is only
10 increased. Thus, the content of Cr is set to 0.01 to 1.00%.
[0028] Al: 0.001 to 0.500%
Al is effective for maintaining the high toughness by suppressing
deoxidation and the growth of austenite grains. Further, Al has an effect of
preventing tool abrasion by bonding to oxygen at the time of machining to be
15 attached to a tool surface. For exhibiting these effects, the lower limit is set
to 0.001%. On the other hand, when the upper limit exceeds 0.500%, a large
number of hard inclusions are formed, and the toughness, the endurance ratio,
and the machinability all decrease/deteriorate. Thus, the upper limit is set to
0.500%.
20 [0029] N: 0.0080 to 0.0200%
N is an element that forms nitrides with various alloy elements such as
V and Al, maintains the high toughness even though the strength is increased
by suppressing the growth of austenite grains and making the bainite structure
fine, and is further important for obtaining the high endurance ratio. For
25 obtaining the above effect, the lower limit is set to 0.0080%. On the other
hand, when the upper limit exceeds 0.0200%, the effect is saturated. Further,
12
the hot ductility significantly decreases to thus cause a problem of occurrence
of flaws at the time of hot rolling of the bar steel and hot forging of the part,
and thus the upper limit is set to 0.0200%.
[0030] Ca: 0.0003 to 0.0100%, Mg: 0.0003 to 0.0100%, and Zr: 0.0005
5 to 0.1000%
In the present invention, Ca, Mg, and Zr are not mandatory. One
type or two types or more of Ca: 0.0003 to 0.0100%, Mg: 0.0003 to 0.0100%,
and Zr: 0.0005 to 0.1000% may also be contained.
[0031] Ca, Mg, and Zr each have an effect of forming oxides to be
10 crystallization nuclei of Mn sulfides and uniformly and finely dispersing Mn
sulfides. Further, each of the elements has an effect of being solid-dissolved
in Mn sulfides to decrease the deformability of Mn sulfides and suppressing
the extension of the shape of Mn sulfides after rolling and hot forging to
decrease the anisotropy in the mechanical properties such as the toughness.
15 For exhibiting these effects, the lower limit of each of Ca and Mg is set to
0.0003% and the lower limit of Zr is set to 0.0005%. On the other hand,
when Ca and Mg each exceed 0.0100% and Zr exceeds 0.1000%, a large
number of hard inclusions such as these oxides and sulfides are formed
thereby, and the toughness and the endurance ratio decrease, and the
20 machinability deteriorates. Thus, the upper limit of each of Ca and Mg is set
to 0.0100% and the upper limit of Zr is set to 0.1000%.
[0032] Mo: 0.01 to 1.00% andNb: 0.001 to 0.200%
In the present invention. Mo and Nb are not mandatory. One type or
two types of Mo: 0.01 to 1.00% and Nb: 0.001 to 0.200% may also be
25 contained.
[0033] Mo and Nb each are an element effective for increasing the
13
strength and the endurance ratio by forming carbide to strengthen the bainite
structure by precipitation, similarly to V. For obtaining the above effect, the
lower limit of Mo is set to 0.01% and the lower limit of Nb is set to 0.001%.
Even though Mo and Nb are each added more than necessary, the effect is
5 saturated and the increase in alloy cost is only caused. Thus, the upper limit
of Mo is set to 1.00% and the upper limit of Nb is set to 0.200%.
[0034] Next, there will be explained reasons for limiting the steel
structure of the steel part for machine structural use of the present invention.
[0035] The bainite structure having an area ratio of 95% or more
10 The reason why the structure is defined to be the bainite structure
having an area ratio of 95% or more is because if the main structure is the
bainite structure, the steel has the high toughness and high endurance ratio,
but in the case when, in an area ratio, 5% or more of ferrite, retained austenite,
and martensite island, which are the remaining structures of the steel, exists,
15 the toughness and the endurance ratio significantly decrease. As these
remaining structures are smaller and smaller, the toughness and the endurance
ratio are higher, and the bainite structure is preferably 97% or more in an area
ratio.
[0036] The bainite lath width being 5 ^m or less
20 Further, the reason why the bainite lath width is defined to be 5 \xm or
less is because if the width exceeds 5 |Lim, the structure is the bainite structure
that is transformed at relatively high temperature, coarse cementite
precipitates at lath boundaries, and the toughness and the endurance ratio are
low. As the lath width is narrower, the structure is the bainite structure that
25 is transformed at low temperature, the size of cementite also becomes smaller,
and the steel has the higher toughness and higher endurance ratio. Thus, the
14
bainite lath width is preferably set to 3 |im or less.
[0037] V carbide having an average grain diameter of not less than 4 nm
nor more than 7 nm dispersedly existing in the bainite structure
The reason why the average grain diameter of V carbide in the bainite
5 structure is defined to be 4 nm or more is because if the average grain
diameter is less than 4 nm, the steel has the high fatigue strength, but at the
same time, the tensile strength is also high and the value of the endurance
ratio is decreased, thus making it impossible to accomplish both the high
fatigue strength and the excellent machinability. Further, the reason why the
10 upper limit value of the average grain diameter of V carbide is defined to be 7
nm is because if the average grain diameter exceeds 7 nm, not only the tensile
strength but also the fatigue strength significantly decreases, thus making it
impossible to accomplish the high fatigue strength.
[0038] The area ratio of V carbide in the bainite structure being 0.18% or
15 more
Further, the reason why the area ratio of V carbide in the bainite
structure is defined to be 0.18% or more is because if the area ratio is less
than 0.18%, the amount of precipitation strengthening is small and the
endurance ratio is low.
20 [0039] Incidentally, in the case of Mo and Nb being contained, in
addition to V carbide. Mo carbide and Nb carbide each having an average
grain diameter of not less than 4 nm nor more than 7 nm also dispersedly exist
in the bainite structure. In the case, in the bainite structure, the total area
ratio of V carbide. Mo carbide, and Nb carbide is 0.18% or more.
25 [0040] Next, there will be explained a manufacturing method of the steel
part for machine structural use of the present invention.
15
[0041] First, the steel product (bar steel, steel plate, or the like)
containing the above-described chemical composition and the balance being
composed of Fe and inevitable impurities is heated to not lower than 1100°C
nor higher than 1300°C to be hot forged. The reason why it is defined that
5 the steel product made of the above-described chemical composition is heated
to not lower than 1100°C nor higher than 1300°C is to sufficiently dissolve V,
Mo, and Nb in the steel by the heating prior to the hot forging. Here, V, Mo,
and Nb that are dissolved turn into carbides of V, Mo, and Nb in a subsequent
aging treatment to dispersedly precipitate in the bainite structure. When the
10 heating temperature is lower than 1100°C, it is not possible to sufficiently
dissolve V, Mo, and Nb in the steel, and the amount of precipitation
strengthening in the subsequent aging treatment is small and thus the fatigue
strength and the endurance ratio decrease. On the other hand, when the
heating temperature is increased more than necessary in excess of 1300°C, the
15 growth of austenite grains is promoted and the structure that is transformed in
a subsequent cooling process is coarsened, and thus the toughness and the
endurance ratio decrease. Thus, the heating temperature of the steel product
is set to be not lower than 1100°C nor higher than 1300°C.
[0042] After being hot forged, the hot-forged steel product is cooled at an
20 average cooling rate down to 300°C set to be not less than 3°C/second nor
more than 120°C/second. The reason why the average cooling rate down to
300°C is defined to be not less than 3°C/second nor more than 120°C/second
is to turn the structure into the bainite structure having an area ratio of 95% or
more and to set the bainite lath width to be 5 |Lim or less. In a temperature
25 range of lower than 300°C, the bainite ratio and the bainite lath width that are
defined in the present invention do not change by the cooling rate, so that the
16
cooling rate from the temperature after the hot forging down to 300°C is
limited. When the average cooling rate is less than 3°C/second, ferrite
having an area ratio of 5% or more is formed along grain boundaries of prior
austenite, and fiirther the bainite lath width exceeds 5 |im to thus significantly
5 decrease the toughness, the fatigue strength, and the endurance ratio. On the
other hand, when the average cooling rate exceeds 120°C/second, retained
austenite and martensite island having an area ratio of 5% or more are formed
at boundaries of bainite laths to thus significantly decrease the toughness and
the endurance ratio (fatigue strength/tensile strength).
10 [0043] After the cooling, the aging treatment is performed in a
temperature range of not lower than 550°C nor higher than 700°C. The
reason why it is defined that the aging treatment is performed at not lower
than 550°C nor higher than 700°C is because fine V carbide. Mo carbide, and
Nb carbide are caused to precipitate in the bainite structure by this aging
15 treatment to strengthen the bainite structure by precipitation to thereby obtain
the high fatigue strength and high endurance ratio. When the aging
treatment temperature is lower than 550°C, the precipitation amount of V
carbide. Mo carbide, and Nb carbide is small and the sufficient amount of
precipitation strengthening cannot be obtained and thus the fatigue strength
20 and the endurance ratio are both low, or V carbide. Mo carbide, and Nb
carbide sufficiently precipitate and the steel has the high fatigue strength but
at the same time, the tensile strength is also high and thus the endurance ratio
is low. The lower limit of the heat treatment temperature is set to 550°C.
On the other hand, when the treatment temperature exceeds 700°C, V carbide,
25 Mo carbide, and Nb carbide are coarsened, thereby making it impossible to
obtain the sufficient amount of precipitation strengthening, the tensile strength
17
and the fatigue strength are both low, and thus the high fatigue strength cannot
be accomplished. Thus, the upper limit is set to 700°C. Within the
above-described defined temperature range, as the aging treatment
temperature is higher, the endurance ratio is improved, and thus the aging
5 treatment temperature is preferably 600°C or higher and is more preferably
set to 650°C or higher.
[0044] Incidentally, the present invention makes it possible to obtain the
steel part for machine structural use having the high fatigue strength and high
toughness, but for sufficiently securing the machinability, the tensile strength
10 is desirably set to 1200 MPa or less.
[Example]
[0045] The present invention will be explained according to examples.
Incidentally, these examples are to explain the technical reasons and effects of
the present invention and are not intended to limit the scope of the present
15 invention.
[0046] Steels each having a chemical composition shown in Table 1 and
being 100 kg were melted in a vacuum melting fiimace. Each of the steels
was rolled to a bar steel having a diameter of 55 mm, and then a test piece for
forging was cut out of each of the bar steels and was heated to a heating
20 temperature shown in Table 1 to be hot forged. After the hot forging, as a
cooling method down to 300°C, oil cooling, water cooling, or air cooling was
performed, the cooling rate was controlled, and then, at lower than 300°C, air
cooling was performed. The average cooling rate was obtained by dividing
the value obtained by subtracting 300°C fi-om the temperature of the test piece
25 after being hot forged by the time required for cooling the test piece down to
300°C after the hot forging. Thereafter, at each of aging temperatures shown
18
in Table 1, the aging treatment was performed. Incidentally, each underline
part in Table 1 is a condition outside the range of the present invention.
[0047] From each of middle portions of these forged products, a No. 14
tensile test piece of JIS Z 2201, a No. 1 rotating bending fatigue test piece of
5 JIS Z 2274, and a 2 mm U-notched impact test piece of JIS Z 2202 were
obtained, and the tensile strength, the Charpy absorbed energy at 20°C, and
the fatigue strength were obtained. Here, the fatigue strength was defined to
be the stress amplitude when at a rotating bending fatigue test, the test piece
was endured without being fi*actured by 10 rotations. Further, the ratio of
10 the obtained fatigue strength to the obtained tensile strength was obtained as
the endurance ratio (the fatigue strength/the tensile strength).
[0048] From a 1/4 thickness portion, of each of the forged products, in
the L direction, a test piece for structure observation was obtained. The area
ratio of bainite was calculated in a manner that the test piece was polished to
15 have a mirror finished surface and then was subjected to repeller etching, and
the structures of ferrite, martensite island, and the like, being the remaining
portion other than bainite, were confirmed, an optical photomicrograph of 500
magnifications was taken at 10 visual fields of each of the test pieces, and
then was image-analyzed. Further, as for the bainite lath width, the test
20 piece was polished again to have a mirror finished surface and then was
subjected to nital etching, and a scanning electron photomicrograph of 5000
magnifications was taken at 10 visual fields of each of the test pieces, the lath
widths in 10 places of each of the visual fields were measured, and the
average value of the lath widths was obtained. As for the average grain
25 diameter of carbide, the test piece was finished into a thin film by
electropolishing, and then by a transmission electron microscope, a
19
transmission electron photomicrograph of 15000 magnifications was taken at
10 visual fields of each of the test pieces, an area of each of alloy carbides of
V, Mo, and Nb observed in the photomicrographs was obtained by image
analysis, a circle-equivalent diameter of each of the areas was calculated, and
5 the average value of the circle-equivalent diameters was obtained. Further,
the area ratio of the precipitates was calculated from the total area of alloy
carbides occupied in the observation area. Incidentally, the identification of
carbide was performed by analysis of a selected area electron diffraction
pattern by using a transmission electron microscope, or by elemental analysis
10 by energy dispersive X-ray spectroscopy.
[0049] In each of present invention examples of No. 1 to 23, the structure
is the bainite structure having an area ratio of 95% or more and is the
microstructure in which the lath width is 5 |nm or less, and the aging treatment
temperature is 550°C or higher, so that the steel causes carbide having an
15 average grain diameter of not less than 4.4 nm nor more than 6.9 nm to
sufficiently precipitate therein and has the high toughness and high endurance
ratio in which the Charpy absorbed energy at 20°C is 97 J/cm or more and
the endurance ratio is 0.60 or more. The tensile strength is 1200 MPa or less
in order to secure the machinability, but as is clear from the comparison with
20 the equivalent tensile strength, the higher strength is achieved rather than a
ferrite-pearlite non-heat-treated steel in a conventional example of No. 36.
[0050] In contrast to this, in comparative examples of No. 24 and 25, the
content of C or Si is large, and further No. 34 and 35 each fall within the
defined steel composition range, but the average cooling rate is outside the
25 definition and a large amount of the remaining portion of ferrite, retained
austenite, and the like exists at boundaries of bainite laths, and ftirther in No.
20
35, the bainite lath width is large, and the Charpy absorbed energy and the
endurance ratio are low. In No. 26 and 28, the steel composition or the heat
treatment condition is outside the definition, and thus the sufficient
precipitation strengthening caimot be obtained and the endurance ratio is low.
5 In No. 26, 27 and 31, the alloy elements are added more than necessary, and
thus the Charpy absorbed energy is low. In No. 29 and 30, Ti is contained
and the Charpy absorbed energy is low, and further in No. 30, the sufficient
precipitation strengthening cannot be obtained and the endurance ratio is low.
In No. 32, the steel causes fine carbide to precipitate therein in large amounts
10 and has the high fatigue strength but the tensile strength is also high, and thus
the endurance ratio and the Charpy absorbed energy are both low. In No. 33,
the aging treatment temperature is higher than the defined aging treatment
temperature and the average grain diameter of carbide is in excess of 7 nm,
which is coarse, and thus the strength and the endurance ratio are low.
15 [0051] As is clear fi-om the above, the present invention examples in
which the conditions defined in the present invention are all satisfied are each
more excellent in toughness and fatigue property than the comparative
examples and conventional example.
[0052]
20 [Table 1]
25
21
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22
[Name of Document] What is claimed is
[Claim 1] A steel part for machine structural use made of a steel
containing,
in mass%,
5 C: 0.05 to 0.20%,
Si: 0.10 to 1.00%,
Mn: 0.75 to 3.00%,
P: 0.001 to 0.050%,
S: 0.001 to 0.200%,
10 V: 0.05 to 0.20%,
Cr: 0.01 to 1.00%,
Al: 0.001 to 0.500%, and
N: 0.0080 to 0.0200%, and
a balance being composed of Fe and inevitable impurities, wherein
15 a steel structure contains a bainite structure having an area ratio of 95% or
more,
a bainite lath width is 5 fim or less,
V carbide having an average grain diameter of not less than 4 nm nor more
than 7 nm dispersedly exists in the bainite structure, and
20 an area ratio of V carbide in the bainite structure is 0.18% or more.
[Claim 2] The steel part for machine structural use according to claim 1,
wherein
the steel further contains one type or two types or more of, in mass%,
Ca: 0.0003 to 0.0100%,
25 Mg: 0.0003 to 0.0100%, and
Zr: 0.0005 to 0.1000%.
23
[Claim 3] The steel part for machine structural use according to claim 1
or 2, wherein
the steel further contains one type or two types of, in mass%,
Mo: 0.01 to 1.00%, and
5 Nb: 0.001 to 0.200%.
[Claim 4] The steel part for machine structural use according to claim 1,
wherein
Charpy absorbed energy at 20°C is 80 J/cm or more and an endurance ratio is
0.60 or more.
10 [Claim 5] A manufacturing method of a steel part for machine structural
use, comprising:
heating a steel product containing, in mass%,
C: 0.05 to 0.20%,
Si: 0.10 to 1.00%,
15 Mn: 0.75 to 3.00%,
P: 0.001 to 0.050%,
S: 0.001 to 0.200%,
V: 0.05 to 0.20%,
Cr: 0.01 to 1.00%,
20 Al: 0.001 to 0.500%, and
N: 0.0080 to 0.0200%, and
a balance being composed of Fe and inevitable impurities to not lower than
1100°C nor higher than 1300°C and hot forging the steel product;
after said hot forging, cooling the hot-forged steel product at an average
25 cooling rate down to 300°C set to be not less than 3°C/second nor more than
120°C/second; and
24
after said cooling, performing an aging treatment within a temperature range
of not lower than 550°C nor higher than 700°C.