U.S. patent application number 11/401450 was filed with the patent office on 2006-10-12 for crankshaft and method for manufacturing same.
This patent application is currently assigned to HONDA MOTOR CO., LTD.. Invention is credited to Seiji Kobayashi, Yutaka Kurebayashi, Hideki Matsuda, Koki Mizuno, Katsunori Takada, Hisato Takeuchi.
Application Number | 20060225814 11/401450 |
Document ID | / |
Family ID | 37068120 |
Filed Date | 2006-10-12 |
United States Patent
Application |
20060225814 |
Kind Code |
A1 |
Mizuno; Koki ; et
al. |
October 12, 2006 |
Crankshaft and method for manufacturing same
Abstract
A surface of a steel, as a material for a crankshaft, is
nitrocarburized. The steel contains, as alloy elements C having a
content 0.10 mass % or more 0.30 mass % or less, Si having a
content 0.5 mass % or more and 0.3 mass % or less, Mn having a
content 0.3 mass % or more and 1.5 mass % or less, Mo having a
content 0.8 mass % or more and 2.0 mass % or less, Cr having a
content 0.1 mass % or more and 1.0 mass % or less, and V having a
content 0.1 mass % or more and 0.5 mass % or less, with a remainder
consisting of Fe and inevitable impurities. The contents of the
alloy elements fall within ranges: 2.0 mass
%.ltoreq.Mn+Cr+Mo.ltoreq.3.0 mass %, 2.3 mass %.ltoreq.C+Mo+5V
.ltoreq.3.7 mass %, and 2.7 mass %.ltoreq.2.16
Cr+Mo+2.54V.ltoreq.4.0 mass %. If a steel sample extracted from a
central portion of the nitrocarburized steel free from an influence
of the nitrocarburizing treatment is austenitized at 1200.degree.
C. for one hour, and cooled to a room temperature so that a cooling
rate at which the steel sample passes through a temperature range
between 900.degree. C. and 300.degree. C. is 0.5.degree. C./second,
then an area percentage of a bainite structure in steel structures
is 80% or more and a Vickers hardness measured at a cross section
is 260 Hv or more and 330 Hv or less. A surface hardness of a
nitrocarburized layer is 650 Hv or more, a formation depth of the
nitrocarburized layer is 0.3 mm or more, and a hardness of the
central portion is 340 Hv or more. Thereby a crankshaft which is
excellent both in the machinability and in fatigue strength, even
after nitrocarburizing treatment on the surface, is provided.
Inventors: |
Mizuno; Koki; (Wako-shi,
JP) ; Matsuda; Hideki; (Wako-shi, JP) ;
Kobayashi; Seiji; (Wako-shi, JP) ; Takeuchi;
Hisato; (Nagoya-shi, JP) ; Takada; Katsunori;
(Nagoya-shi, JP) ; Kurebayashi; Yutaka; (Tokyo,
JP) |
Correspondence
Address: |
ARENT FOX PLLC
1050 CONNECTICUT AVENUE, N.W.
SUITE 400
WASHINGTON
DC
20036
US
|
Assignee: |
HONDA MOTOR CO., LTD.
|
Family ID: |
37068120 |
Appl. No.: |
11/401450 |
Filed: |
April 11, 2006 |
Current U.S.
Class: |
148/218 ;
148/318 |
Current CPC
Class: |
C23C 8/32 20130101; C23C
8/02 20130101 |
Class at
Publication: |
148/218 ;
148/318 |
International
Class: |
C23C 8/32 20060101
C23C008/32 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 12, 2005 |
JP |
2005-115112 |
Claims
1. A crankshaft consisting of a steel having a surface subjected to
a nitrocarburizing treatment, comprising: a pin; and a journal,
wherein the steel contains, as alloy elements: C having a content
equal to or more than 0.10 mass % and equal to or less than 0.30
mass %, Si having a content equal to or more than 0.05 mass % and
equal to or less than 0.3 mass %, Mn having a content equal to or
more than 0.3 mass % and equal to or less than 1.5 mass %, Mo
having a content equal to or more than 0.8 mass % and equal to or
less than 2.0 mass %, Cr having a content equal to or more than 0.1
mass % and equal to or less than 1.0 mass %, and V having a content
equal to or more than 0.1 mass % and equal to or less than 0.5 mass
%, with a remainder consisting of Fe and inevitable impurities; the
contents of the alloy elements fall within the following ranges:
2.3 mass %.ltoreq.C+Mo+5V.ltoreq.3.7 mass %, 2.0 mass
%.ltoreq.Mn+Cr+Mo.ltoreq.3.0 mass %, and 2.7 mass %.ltoreq.2.16
Cr+Mo+2.54V.ltoreq.4.0 mass %; if a steel sample extracted from a
central portion of the nitrocarburized steel free from an influence
of the nitrocarburizing treatment is austenitized at 1200 degrees
Centigrade for one hour, and cooled to a room temperature so that a
cooling rate at which the steel sample passes through a temperature
range between 900 and 300 degrees Centigrade is 0.5.degree.
C./second, then an area percentage of a bainite structure in steel
structures is equal to or higher than 80 percent and a Vickers
hardness measured at a cross section is equal to or higher than 260
Hv and equal to or lower than 330 Hv, and the pin and the journal
have a surface hardness of a nitrocarburized layer is equal to or
higher than 650 HV, a formation depth of the nitrocarburized layer
is equal to or larger than 0.3 millimeters, and a hardness of the
central portion is equal to or higher than 340 Hv.
2. The crankshaft according to claim 1, wherein a content of Pb is
equal to or less than 0.03 mass %.
3. The crankshaft according to claim 1, wherein the steel contains
one or more of: Nb having a content equal to or more than 0.02 mass
% and equal to or less than 0.2 mass %, Ti having a content equal
to or more than 0.005 mass % and equal to or less than 0.2 mass %,
and Al having a content equal to or more than 0.003 mass % and
equal to or less than 0.1 mass %.
4. The crankshaft according to claim 1, wherein the steel contains
one of or both of: S having a content equal to or more than 0.01
mass % and equal to or less than 0.1 mass %, and Ca having a
content equal to or more than 0.0010 mass % and equal to or less
than 0.010 mass %.
5. A method for manufacturing a crankshaft according to claim 1,
wherein after the steel is hot-forged or hot-forged and subjected
to a solution treatment to have a shape including the pin and the
journal at a temperature equal to or higher than 900 degrees
Centigrade and lower than a melting point of the steel, the steel
is cooled so that the cooling rate of the pin and the journal is
equal to or higher than 0.3.degree. C./second and equal to or lower
than 2.degree. C./second, whereby the area percentage of the
bainite structure in the steel structures is equal to or higher
than 80 percent, and thereafter, the pin and the journal are
subjected to a cutting treatment, furthermore the surface of the
steel is subjected to a nitrocarburizing treatment.
6. The crankshaft according to claim 2, wherein the steel contains
one or more of: Nb having a content equal to or more than 0.02 mass
% and equal to or less than 0.2 mass %, Ti having a content equal
to or more than 0.005 mass % and equal to or less than 0.2 mass %,
and Al having a content equal to or more than 0.003 mass % and
equal to or less than 0.1 mass %.
7. The crankshaft according to claim 2, wherein the steel contains
one of or both of: S having a content equal to or more than 0.01
mass % and equal to or less than 0.1 mass %, and Ca having a
content equal to or more than 0.0010 mass % and equal to or less
than 0.010 mass %.
8. The crankshaft according to claim 3, wherein the steel contains
one of or both of: S having a content equal to or more than 0.01
mass % and equal to or less than 0.1 mass %, and Ca having a
content equal to or more than 0.0010 mass % and equal to or less
than 0.010 mass %.
9. The crankshaft according to claim 6, wherein the steel contains
one of or both of: S having a content equal to or more than 0.01
mass % and equal to or less than 0.1 mass %, and Ca having a
content equal to or more than 0.0010 mass % and equal to or less
than 0.010 mass %.
10. A method for manufacturing a crankshaft according to claim 2,
wherein after the steel is hot-forged or hot-forged and subjected
to a solution treatment to have a shape including the pin and the
journal at a temperature equal to or higher than 900 degrees
Centigrade and lower than a melting point of the steel, the steel
is cooled so that the cooling rate of the pin and the journal is
equal to or higher than 0.3.degree. C./second and equal to or lower
than 2.degree. C./second, whereby the area percentage of the
bainite structure in the steel structures is equal to or higher
than 80 percent, and thereafter, the pin and the journal are
subjected to a cutting treatment, furthermore the surface of the
steel is subjected to a nitrocarburizing treatment.
11. A method for manufacturing a crankshaft according to claim 3,
wherein after the steel is hot-forged or hot-forged and subjected
to a solution treatment to have a shape including the pin and the
journal at a temperature equal to or higher than 900 degrees
Centigrade and lower than a melting point of the steel, the steel
is cooled so that the cooling rate of the pin and the journal is
equal to or higher than 0.3.degree. C./second and equal to or lower
than 2.degree. C./second, whereby the area percentage of the
bainite structure in the steel structures is equal to or higher
than 80 percent, and thereafter, the pin and the journal are
subjected to a cutting treatment, furthermore the surface of the
steel is subjected to a nitrocarburizing treatment.
12. A method for manufacturing a crankshaft according to claim 6,
wherein after the steel is hot-forged or hot-forged and subjected
to a solution treatment to have a shape including the pin and the
journal at a temperature equal to or higher than 900 degrees
Centigrade and lower than a melting point of the steel, the steel
is cooled so that the cooling rate of the pin and the journal is
equal to or higher than 0.3.degree. C./second and equal to or lower
than 2.degree. C./second, whereby the area percentage of the
bainite structure in the steel structures is equal to or higher
than 80 percent, and thereafter, the pin and the journal are
subjected to a cutting treatment, furthermore the surface of the
steel is subjected to a nitrocarburizing treatment.
13. A method for manufacturing a crankshaft according to claim 4,
wherein after the steel is hot-forged or hot-forged and subjected
to a solution treatment to have a shape including the pin and the
journal at a temperature equal to or higher than 900 degrees
Centigrade and lower than a melting point of the steel, the steel
is cooled so that the cooling rate of the pin and the journal is
equal to or higher than 0.3.degree. C./second and equal to or lower
than 2.degree. C./second, whereby the area percentage of the
bainite structure in the steel structures is equal to or higher
than 80 percent, and thereafter, the pin and the journal are
subjected to a cutting treatment, furthermore the surface of the
steel is subjected to a nitrocarburizing treatment.
14. A method for manufacturing a crankshaft according to claim 7,
wherein after the steel is hot-forged or hot-forged and subjected
to a solution treatment to have a shape including the pin and the
journal at a temperature equal to or higher than 900 degrees
Centigrade and lower than a melting point of the steel, the steel
is cooled so that the cooling rate of the pin and the journal is
equal to or higher than 0.3.degree. C./second and equal to or lower
than 2.degree. C./second, whereby the area percentage of the
bainite structure in the steel structures is equal to or higher
than 80 percent, and thereafter, the pin and the journal are
subjected to a cutting treatment, furthermore the surface of the
steel is subjected to a nitrocarburizing treatment.
15. A method for manufacturing a crankshaft according to claim 8,
wherein after the steel is hot-forged or hot-forged and subjected
to a solution treatment to have a shape including the pin and the
journal at a temperature equal to or higher than 900 degrees
Centigrade and lower than a melting point of the steel, the steel
is cooled so that the cooling rate of the pin and the journal is
equal to or higher than 0.3.degree. C./second and equal to or lower
than 2.degree. C./second, whereby the area percentage of the
bainite structure in the steel structures is equal to or higher
than 80 percent, and thereafter, the pin and the journal are
subjected to a cutting treatment, furthermore the surface of the
steel is subjected to a nitrocarburizing treatment.
16. A method for manufacturing a crankshaft according to claim 9,
wherein after the steel is hot-forged or hot-forged and subjected
to a solution treatment to have a shape including the pin and the
journal at a temperature equal to or higher than 900 degrees
Centigrade and lower than a melting point of the steel, the steel
is cooled so that the cooling rate of the pin and the journal is
equal to or higher than 0.3.degree. C./second and equal to or lower
than 2.degree. C./second, whereby the area percentage of the
bainite structure in the steel structures is equal to or higher
than 80 percent, and thereafter, the pin and the journal are
subjected to a cutting treatment, furthermore the surface of the
steel is subjected to a nitrocarburizing treatment.
Description
RELATED APPLICATIONS
[0001] This application claims the priority of Japanese Patent
Application No. 2005-115112 filed on Apr. 12, 2005, which is
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a crankshaft consisting of
a steel that includes a nitrocarburized layer on a surface, and a
method for manufacturing the crankshaft.
[0004] 2. Description of the Related Art
[0005] Patent Literature 1: Japanese Patent Application Laid-Open
No. 10-030632
[0006] Patent Literature 2: Japanese Patent Application Laid-Open
No. 06-128690
[0007] Patent Literature 3: Japanese Patent Application Laid-Open
No. 05-279795
[0008] Patent Literature 4: Japanese Patent Application Laid-Open
No. 05-279794
[0009] An automobile crankshaft is used in an environment in which
a high torsional load and a high bending load repeatedly act on the
crankshaft. The crankshaft is, therefore, required to be excellent
in static strength and fatigue strength. On the other hand, since
the crankshaft is a member quite large in size and complicated in
shape, it is normally and basically manufactured using a non-heat
treated steel that is not quenched and tempered after being
hot-forged. In this case, to ensure strength, it is necessary to
finally perform a hardening treatment on a surface of the steel.
The Patent Literatures 1 to 4 disclose methods using a
nitrocarburizing treatment as the surface hardening treatment. The
nitrocarburizing treatment is a treatment in which a workpiece is
treated in, for example, an ammonia gas atmosphere at a temperature
equal to or lower than the A1 transformation point or generally at
a temperature of about 570 degrees Centigrade, part of carbon as
well as nitrogen is introduced into the steel, nitrides or carbides
are produced, and a surface layer of the steel is thereby hardened.
Such a nitrocarburizing treatment is suited for mass-production of
crankshafts that are large-sized engine parts of an automobile
since the treatment hardly generates strains in the workpiece
differently from a carburizing and quenching method, and does not
require a long time differently from a nitriding method.
[0010] Meanwhile, by performing the nitrocarburizing treatment, it
is expected that a hardness of the surface layer of the crankshaft
is greatly increased by introduction of the nitride thereinto.
However, it is difficult to diffuse nitride into the steel at a
depth equal to or larger than 0.5 millimeters, and an interior of
the crankshaft cannot be reinforced by a nitrocarburizing treatment
normally performed for a few hours. On the contrary and normally,
in the nitrocarburizing treatment, since the heat of the steel is
held at about 600 degrees Centigrade, the interior into which the
nitrogen is not introduced is softened by a thermal history of the
nitrocarburizing and the hardness is rather lower than that before
the treatment.
[0011] On the other hand, if the internal strength of the steel
before the nitrocarburizing treatment is to be increased by
addition of components to the steel or the like, then machinability
of the steel is greatly deteriorated. As a result, efficiency for
machining the steel into a shape of the crankshaft before the
nitrocarburizing treatment is deteriorated, making it difficult to
industrially produce parts. Further, the hardness of the steel
before the nitrocarburizing treatment can be increased by quenching
the steel after machining. However, the quenching treatment deforms
the part and requires a step of removing scales after the quenching
treatment and the like. Thus, the quenching treatment degrades a
quality and considerably increases a machining cost.
[0012] Moreover, an ordinary crankshaft is complicated in shape and
it is essential to cut the steel after hot forging. Conventionally,
at a cutting step after a normalizing treatment, chips generated
are wound around a product or a wear resistance of a tool is
deteriorated by the chips. To prevent these, Pb is normally
contained in the steel as a chip crushability improving element.
However, use of Pb is gradually avoided and being restricted
recently since an environmental preservation attracts increasing
attention on the global scale.
SUMMARY OF THE INVENTION
[0013] It is an object of the present invention to provide a
crankshaft capable of ensuring both excellent machinability and
high fatigue strength despite a nitrocarburizing treatment
performed on a surface thereof, and a method for manufacturing the
crankshaft.
[0014] To attain this object, according to a first aspect of the
present invention, there is provided a crankshaft consisting of a
steel having a surface subjected to a nitrocarburizing treatment,
characterized by comprising:
[0015] a pin; and
[0016] a journal, wherein
[0017] the steel contains, as alloy elements:
[0018] C having a content equal to or more than 0.10 mass % and
equal to or less than 0.30 mass %,
[0019] Si having a content equal to or more than 0.05 mass % and
equal to or less than 0.3 mass %,
[0020] Mn having a content equal to or more than 0.3 mass % and
equal to or less than 1.5 mass %,
[0021] Mo having a content equal to or more than 0.8 mass % and
equal to or less than 2.0 mass %,
[0022] Cr having a content equal to or more than 0.1 mass % and
equal to or less than 1.0 mass %, and
[0023] V having a content equal to or more than 0.1 mass % and
equal to or less than 0.5 mass %, with a remainder consisting of Fe
and inevitable impurities;
[0024] the contents of the alloy elements fall within the following
ranges:
[0025] 2.3 mass %.ltoreq.C+Mo+5V.ltoreq.3.7 mass %,
[0026] 2.0 mass %.ltoreq.Mn+Cr+Mo.ltoreq.3.0 mass %, and
[0027] 2.7 mass %.ltoreq.2.16 Cr+Mo+2.54V.ltoreq.4.0 mass %;
[0028] if a steel sample extracted from a central portion of the
nitrocarburized steel free from an influence of the
nitrocarburizing treatment is austenitized at 1200 degrees
Centigrade for one hour, and cooled to a room temperature so that a
cooling rate at which the steel sample passes through a temperature
range between 900 and 300 degrees Centigrade is 0.5.degree.
C./second, then an area percentage of a bainite structure in steel
structures is equal to or higher than 80 percent and a Vickers
hardness measured at a cross section is equal to or higher than 260
Hv and equal to or lower than 330 Hv, and
[0029] The pin and the journal have a surface hardness of a
nitrocarburized layer is equal to or higher than 650 Hv, a
formation depth of the nitrocarburized layer is equal to or larger
than 0.3 millimeters, and a hardness of the central portion is
equal to or higher than 340 Hv.
[0030] Further, according to a second aspect of the present
invention, there is provided a method for manufacturing the
crankshaft according to the first aspect of the present invention,
characterized in that
[0031] after the steel is hot--forged or hot--forged and subjected
to a solution treatment to have a shape including the pin and the
journal at a temperature equal to or higher than 900 degrees
Centigrade and lower than a melting point of the steel, the steel
is cooled so that the cooling rate of the pin and the journal is
equal to or higher than 0.3.degree. C./second and equal to or lower
than 2.degree. C./second, whereby the percentage of the bainite
structure in the steel structures is equal to or higher than 80
percent, and
[0032] thereafter, the pin and the journal are subjected to a
cutting then the surface of the steel is subjected to a
nitrocarburizing treatment.
[0033] The crankshaft can be constituted to have a structure in
which crank arms arranged at predetermined intervals in a direction
of a rotation axis are alternately coupled by crank journals
arranged so that a central axis of the journals coincides with the
rotation axis and crank pins each having a central axis at a
location away from the rotation axis by a certain distance in a
radial direction.
[0034] The crankshaft and the crankshaft manufacturing method
according to the present invention can reduce the hardness of the
material before the nitrocarburizing treatment and ensure excellent
machinability. In addition, they can attain a high surface hardness
by the nitrocarburizing treatment and harden the interior of the
steel by heat-holding during the nitrocarburizing. Therefore, the
crankshaft and the crankshaft manufacturing method according to the
present invention can also ensure excellent fatigue strength
besides the excellent machinability.
[0035] The crankshaft according to the present invention uses a
steel composition that can improve hardness of the interior of the
steel by heat-holding during the nitrocarburizing. The steel
contains, as essential alloy elements, Mo, V, and Ti. Those
elements are known as carbide generators. In the steel as a
constituent material of the crankshaft according to the present
invention, addition amounts of these elements are adjusted so that
the elements function not only as carbide generators but also
control elements that control structures of a steel matrix or
specifically control the bainite structure if the steel is
air-cooled after being hot-forged to obtain the shape of the
crankshaft.
[0036] By adding the Mo, V, and Ti, if the steel is held at near
the nitrocarburizing treatment temperature, secondary precipitation
strengthening can be expected by the carbides generated by these
elements. However, if a material state before the nitrocarburizing
treatment is turned into a state of a martensitic structure
generated by the quenching and tempering, the hardness of the steel
in material state is excessively increased. This deteriorates the
machinability and often causes problems due to quenching strains.
In addition, the hardness improving effect by the carbide
precipitation is achieved less conspicuously and the significance
of adding the expensive carbide precipitation elements becomes
unclear. According to the present invention, the Mo, V, and Ti
contents are appropriately adjusted, whereby the material state
before the nitrocarburizing is made to be a state of a bainite
structure. As compared with the martensitic structure, the bainite
structure is low in hardness and it is relatively easy to perform a
machining processing including a cutting processing on the bainite
structure. As a result, it is possible to sufficiently increase the
internal hardness by the nitrocarburizing treatment and improve the
fatigue strength while ensuring the same excellent machinability as
the pre-nitrocarburized one. Furthermore, by turning the material
state into the bainite structure, it is possible to suppress
cutting chips becoming continuously connected, and effectively
suppress defects such as winding of the chips around the cutting
tool at the time of cutting the steel into the shape of the
crankshaft.
[0037] A shape stock of the crankshaft is manufactured by the hot
forging or the like. The cooling rate after the hot forging varies
depending on dimensions and the shape of the crankshaft. It is
preferable to cool the steel so that the bainite structure can be
obtained in a wide cooling range. The steel structures of the steel
used for the crankshaft according to the present invention are
adjusted so that the area percentage of the bainite structure in
the steel structures before the nitrocarburizing treatment is equal
to or higher than 80 percent, and so that the hardness is equal to
or higher than 260 Hv and equal to or lower than 330 Hv. However,
in the crankshaft that has been finally nitrocarburized, the
structures before the nitrocarburizing treatment cannot be directly
specified. On the other hand, as a result of calculating the
cooling rate of an actual part, it is confirmed that an average
cooling rate of the pin and the journal, when the material is
cooled after being hot-forged, falls within the range from
0.3.degree. C./second and 2.degree. C./second that are required to
have sufficient strength. Therefore, a steel sample is extracted
from a central portion of the nitrocarburized steel free from an
influence of the nitrocarburizing treatment is austenitized at 1200
degrees Centigrade for one hour, and cooled so that the cooling
rate at which the steel sample passes through a temperature range
between 900 and 300 degrees Centigrade is 0.5.degree. C./second. By
doing so, as long as the area percentage of the bainite structure
in the steel structures is equal to or higher than 80 percent and
the Vickers hardness measured at a cross section is equal to or
higher than 260 Hv and equal to or lower than 330 Hv, it is
possible to attain the percentage of the bainite structure and the
hardness substantially equal in level to those before the
nitrocarburizing treatment in the crankshaft manufacturing steps
including a step of cooling the steel after hot-forging the
steel.
[0038] Reasons for limiting the steel structures and numerical
parameters adopted in the present invention will now be
described.
[0039] "C having a content equal to or more than 0.10 mass % and
equal to or less than 0.30 mass %."
[0040] The C is a necessary element to secure the strength.
However, if the C content is less than 0.10 mass %, the strength
cannot be secured. If the C content exceeds 0.30 mass %, the
hardness of the material before cutting (the material after a hot
forging treatment, a normalizing treatment, or a solution
treatment) is excessively increased. This deteriorates cutting
machinability.
[0041] "Si having a content equal to or more than 0.05 mass % and
equal to or less than 0.3 mass %"
[0042] The Si is contained in the steel as an element serving as a
deoxidizer during smelting steel and serving to improve the fatigue
strength. If the Si content is less than 0.05 mass %, desired
effects cannot be achieved. If a large amount of the Si is added so
that the Si content exceeds 0.30 mass %, the degree of
nitrocarburizing the steel is reduced. As a result, a predetermined
surface hardness cannot be attained.
[0043] "Mn having a content equal to or more than 0.3 mass % and
equal to or less than 1.5 mass %"
[0044] The Mn is an element that plays an important role in the
present invention and that is essential to generate the bainite
structure in the material structures before the nitrocarburizing
treatment. Specifically, the Mn content is adjusted according to
the Cr and Mo contents. If the Mn content is less than 0.3 mass %,
the generation of the bainite structure is unstable. Therefore, it
is necessary to add the Mn at the content equal to or higher than
0.3 mass %. If a large amount of the Mn is added so that the Mn
content exceeds 1.5 mass %, the hardness is increased but, at the
same time, an increase in the internal hardness after the
nitrocarburizing treatment cannot be expected.
[0045] "Mo having a content equal to or more than 0.8 mass % and
equal to or less than 2.0 mass %".
[0046] "V having a content equal to or more than 0.1 mass % and
equal to or less than 0.5 mass %".
[0047] The Mo and V are elements, that play important roles in the
present invention, and that are added so as to increase the
internal hardness by heating during the nitrocarburizing treatment.
The Mo functions to make the internal hardness higher along with
the increase of the addition amount of the Mo. If the Mo content is
equal to or lower than 0.8 mass %, the effect of increasing the
hardness cannot be achieved as intended. If the Mo is added so that
the Mo content exceeds 2.0 mass %, the increase in the internal
hardness can be attained. However, a hardness of the bainite in the
material state is excessively high, thereby deteriorating the
machinability. The V functions to precipitate V carbides and
increase the material hardness by air cooling after the hot
forging. In addition, the V functions to make the internal hardness
higher along with the increase of the addition amount of the V. If
the V content is less than 0.1 mass %, the effect of increasing the
hardness cannot be achieved as intended. If the V is added so that
the V content exceeds 0.5 mass %, the increase in the internal
hardness can be attained. However, the hardness of the bainite in
the material state is excessively high, thereby deteriorating the
machinability.
[0048] "Cr having a content equal to or more than 0.1 mass % and
equal to or less than 1.0 mass %"
[0049] The Cr is also an element that plays an important role in
the present invention. The Cr is added so as to stabilize the
surface hardness after the nitrocarburizing treatment and to attain
the hardness equal to or higher than 650 Hv. If the Cr content is
less than 0.1 mass %, the hardness is insufficiently low after the
nitrocarburizing treatment. If the Cr content exceeds 1.0 mass %,
the effect saturates.
[0050] 2.0 mass %.ltoreq.Mn+Cr+Mo.ltoreq.3.0 mass %,
[0051] "2.3 mass %.ltoreq.C+Mo+5V.ltoreq.3.7 mass %, and
[0052] 2.7 mass %.ltoreq.2.16 Cr+Mo+2.54V.ltoreq.4.0 mass %".
[0053] The present invention is characterized by increasing the
internal hardness by heat-holding during the nitrocarburizing.
However, the effect of increasing the internal hardness cannot be
sufficiently achieved even if the Mo and V (also additionally Ti
and Nb as arbitrarily added elements to be described later) are
simply added as carbide generators. The secondary precipitation of
carbides is a phenomenon that can be recognized in a temperature
range around 600 degrees Centigrade, and is normally used in the
quenching and tempering. However, if the material state is turned
into that of the martensitic structure as done in the quenching and
tempering, it is difficult to increase the internal hardness by the
carbide precipitation, and the hardness higher than that obtained
by the quenching cannot be obtained.
[0054] To solve this disadvantage, according to the present
invention, the material state is made to be the state of the
bainite structure. In this material state, the hardness is low. The
steel is heat-held at the temperature range about 600 degrees
Centigrade so as to make it possible to increase the internal
hardness and to make the hardness higher than that in the material
state. The inventors of the present invention considered addition
amounts of the alloy elements so as to stably obtain the bainite
structure by conducting various investigations. As a result, the
inventors discovered that the contents of the alloy elements of Mn,
Cr, and Mo preferably satisfy the relationship represented by 2.0
mass %.ltoreq.Mn+Cr+Mo .ltoreq.3.0 mass %. The shape stock of the
crankshaft is manufactured by the hot forging or the like. The
cooling rate after the hot forging varies according to the
dimensions and shape of the crankshaft. It is preferable to cool
the steel so that the bainite structure can be obtained in a wide
cooling range. As stated, the average cooling rate of the pin and
the journal when the material is cooled after being hot-forged into
the shape of the crankshaft falls within the range from 0.3
.degree. C./second and 2.degree. C./second that are required to
have sufficient strength.
[0055] As a result of further dedicated studies, the inventors of
the present invention discovered as follows. If a sum of the Mn,
Cr, and Mo contents is set to be less than 2.0 mass % (2.0 mass
%>Mn+Cr+Mo) to obtain the bainite structure in the process of
the hot forging, then the area percentage of the generated bainite
structure is not higher than 80%, and the effect of increasing the
hardness by the heat-holding cannot be expected. On the other hand,
the generation of the bainite structure relates to the C content.
If the sum of the Mn, Cr, and Mo contents is higher than 3.0 mass %
(Mn+Cr+Mo>3.0 mass %), the hardness of the material is
excessively increased, thereby deteriorating the machinability.
[0056] Furthermore, in order to appropriately increase the hardness
by the heat-holding, it is necessary to satisfy the relationship
represented by 2.3 mass %.ltoreq.C+Mo+5V.ltoreq.3.7 mass %. At 2.3
mass %>C+Mo+5V, an allowance for the increased amount of the
hardness by the heat-holding is insufficient. At C+Mo+5V>3.7
mass %, the hardness of the material is excessively increased,
thereby deteriorating the machinability. In addition, in order to
appropriately increase the surface hardness by nitrocarburizing, it
is necessary to satisfy the relationship represented by 2.7 mass
%.ltoreq.2.16 Cr+Mo+2.54V.ltoreq.4.0 mass %. At 2.7 mass %>2.16
Cr+Mo+2.54V, the allowance for the increased amount of the hardness
by the nitrocarburizing is insufficient. At 2.16 Cr+Mo+2.54V>4.0
mass %, the hardness of the material is excessively increased,
thereby deteriorating the machinability.
[0057] The area percentage of the bainite structure in the steel
structures is set equal to or higher than 80 percent and the
Vickers hardness measured at a cross section is set equal to or
higher than 260 Hv and equal to or lower than 330 Hv for the
following reasons. As already stated, it is necessary to turn the
structural state before the nitrocarburizing into that of the
bainite structure so as to increase the internal hardness
simultaneously with the nitrocarburizing treatment. The reason is
as follows. If the steel is heated holding the temperature range
about 600 degrees Centigrade regardless the structure shape before
the nitrocarburizing, carbides are precipitated. However, to
conspicuously increase the hardness by the precipitation
strengthening, it is necessary to turn the structure of the
material state into the bainite structure. In addition, after the
structure subjected to the hot forging is softened by annealing or
the like and machined, the steel is reheated and re-cooled, whereby
the structure can be adjusted and the hardness can be adjusted.
However, the addition of another heat treatment causes a cost
increase. Besides, oxidized scales are generated by heating and
cooling. Therefore, it is preferable to adjust the structure to a
predetermined structure and adjust the hardness to a predetermined
hardness while the steel is being hot-forged. In this case, the
steel is machined in a state after the hot forging. It is,
therefore, necessary to ensure the machinability. In light of the
balance between the machinability and the strength, the Vickers
hardness is set to be equal to or higher than 260 Hv and equal to
or lower than 330 Hv.
[0058] The surface hardness after the nitrocarburizing treatment is
set to be equal to or higher than 650 Hv, a depth of an entire
hardened layer is set to be equal to or larger than 0.3
millimeters, and the hardness of the central portion is set to be
equal to or higher than 330 Hv for the following reasons. Since a
bending stress and a torsional stress repeatedly act on the
crankshaft, the crankshaft needs to have a high bending fatigue
strength and a high torsional fatigue strength. In each of the
fatigue phenomena, a maximum load stress acts on an uppermost
surface of the steel. It is, therefore, important to increase the
hardness of the surface layer so as to improve the fatigue
strength. The higher hardness of the surface layer is more
advantageous. It is confirmed that the surface hardness is
increased by adding Cr, Al and the like in the nitrocarburizing
treatment. If these elements are added, the depth of the hardened
layer tends to be smaller. The inventors of the present invention
performed a strength evaluation using an actual part. As a result,
it was confirmed that the strength is reduced even if the hardness
of the surface layer is increased as long as the depth of the
hardened layer is small. The inventors of the present invention
conducted dedicated studies of the balance between the optimum
hardness of the surface layer and the optimum depth of the hardened
layer so as to ensure sufficiently high fatigue strengths. As a
result, the inventors discovered that it is preferable to set the
surface layer hardness to equal to or higher than 650 Hv (and equal
to or lower than, for example, 950 Hv), the depth of the
nitrocarburized (hardened) layer to be equal to or higher than 0.3
millimeters (and equal to or lower than, for example, 1.5
millimeters), and to set the hardness of the central portion to be
equal to or higher than 330 Hv (and equal to or lower than, for
example, 430 Hv).
[0059] The steel used as the material for the crankshaft according
to the present invention can further contain the following
elements.
[0060] "Nb having a content equal to or more than 0.02 mass % and
equal to or less than 0.2 mass %."
[0061] "Ti having a content equal to or more than 0.005 mass % and
equal to or less than 0.2 mass %."
[0062] Similarly to the Mo, the Nb and Ti function to precipitate
carbides by the heat-holding during the nitrocarburizing and to
thereby increase the internal hardness. Therefore, the Nb and Ti
are added to the steel if it is necessary to do so. However, if the
Nb and Ti are added so that the Nb and Ti contents exceed their
respective upper limits, large-sized crystallized substances are
generated at a forging step in manufacturing the steel by an
ordinary method. As a result, effective Nb and Ti that contribute
to the increase in the internal hardness cannot be obtained. It is,
therefore, preferable to set each of the upper limits of the Nb and
Ti contents to 0.2 mass %. "Al having a content equal to or more
than 0.003 mass % and equal to or less than 0.1 mass %."
[0063] The Al can be added so as to increase the surface hardness.
However, if the Al content is less than 0.003 mass %, the effect of
increasing the surface hardness cannot be conspicuously achieved.
If the Al content is increased, the surface hardness is increased
proportionally. However, if the Al is excessively added, the
diffusion of nitrogen into the steel during the nitrocarburizing is
obstructed and the hardened layer is made shallower. It is,
therefore, preferable to set the upper limit of the Al content to
0.1 mass % so that the Al content can be prevented from adversely
influencing the depth of the hardened layer and so that the Al can
be expected to function only to increase the surface hardness.
[0064] "S having a content equal to or more than 0.01 mass % and
equal to or less than 0.1 mass %."
[0065] "Ca having a content equal to or more than 0.0010 mass % and
equal to or less than 0.010 mass %."
[0066] The S and Ca are elements used to improve the machinability
in the machining of the steel. By dispersing MnS, Ca oxide, and Ca
sulfide into the steel, the machinability is improved. If the S and
Ca contents are less than their respective lower limits, the effect
of improving the machinability cannot be conspicuously achieved. If
they exceed their respective upper limits, a toughness of the steel
is deteriorated.
[0067] The steel used as the material for the crankshaft according
to the present invention may contain elements other than the
above-stated essential components such as Cu, Ni, P, and O within
the range in which the effects of the present invention are not
reduced. The Cu and Ni, contents of which are about 0.10 mass %,
may possibly mixed into the steel as inevitable impurities from
scraps or the like. The P and O are elements that may possibly be
mixed into the steel as inevitable impurities produced at a steel
manufacturing process. Since the P deteriorates the toughness of
the steel, a P content is preferably set to be equal to or less
than 0.0030 mass %.
BRIEF DESCRIPTION OF THE DRAWINGS
[0068] FIG. 1 is a front view of one example of a crankshaft
according to the present invention.
DESCRIPTION OF THE PRESENT INVENTION
[0069] FIG. 1 is a front view of one example of a crankshaft
according to the present invention. The crankshaft 1 is configured
so that crank arms 2 arranged at predetermined intervals in a
direction of a rotational axis O are alternately coupled by a crank
journal 4 arranged so that a central axis of the journal 4
coincides with the rotational axis O, and by crank pins 5 each
having a central axis at a position away from the rotational axis O
by a certain distance. A hole 8 for injecting oil is formed in each
crank pin 5. The two crank arms 2 form a proximal surface formation
portion in which a surface of each crank arm 2 whose surface faces
the adjacent crank arm 2 is a flat proximal surface 2a. A fillet 7
an outside diameter of which is gradually larger as closer to the
proximal surface 2a side, is formed on a protruding proximal end of
the crank journal 4 and the crank pin 5 (an axis-like portion). The
protruding proximal end is concave, so that a stress tends to
concentrate thereon. However, by forming the fillet 7, the
concentration of the stress is relaxed and a bending strength of
the crankshaft 1 can be increased.
[0070] Each of the crank journal 4 and the crank pin 5 is formed
into an axis having a circular cross section. After the steel
having the composition stated above is hot-forged, a
nitrocarburized layer is formed on an entire outer circumference of
the steel. The crankshaft 1 thus configured is manufactured as
follows. Materials are molten, cast, and divided into blocks so as
to obtain the steel having the composition already described above
in detail. Thereafter, the divided steel block is hot-forged and
then air-cooled. By air cooling, a cooling rate at the crank
journal 4 and the crank pin 5 is within the range between
0.3.degree. C./second and 2.degree. C./second. By adopting the
composition, in the steel before the nitrocarburizing, an area
percentage of a bainite in structures before nitrocarburizing is
equal to or higher than 80 percent, and a hardness of the steel is
equal to or higher than260Hv and equal to lower than 330 Hv. Since
the structures mainly consist of the bainite structure, the steel
can be easily machined into a crankshaft shape by cutting. After
the cutting, the resultant member is nitrocarburized in an ammonia
gas atmosphere. The nitrocarburizing treatment is performed at a
temperature equal to or higher than 550 degrees Centigrade and
equal to or lower than 700 degrees Centigrade (e.g., at 600 degrees
Centigrade). Since an interior of the steel is the bainite
structure, fine carbides are conspicuously precipitated during the
nitrocarburizing treatment differently from a quenched structure
consisting of a martensitic structure, thereby improving a strength
of the interior of the steel. A surface hardness of the
nitrocarburized steel is set equal to or higher than 650 Hv, a
depth of an entire hardened layer is set equal to or larger than
0.3 millimeters, and a hardness of a central portion of the steel
is set equal to or higher than 340 Hv. Thereafter, a well-known
cold straightening treatment is performed on the steel using a
straightening roll or the like so as to correct deformations,
strains, and the like of the steel generated by the
nitrocarburizing treatment.
EXAMPLE
[0071] A result of an experiment conducted to confirm the effects
of the present invention will be described.
[0072] A steel having a chemical composition shown in Table 1 below
is molten in a five-ton arc furnace or a 150-kilogram
high-frequency vacuum induction furnace. A resultant steel ingot is
rolled or forged into a round rod having a diameter of 90
millimeters. TABLE-US-00001 TABLE 1 Chemical components (main)
Chemical components (sub) Component formula upper limit 0.30 0.30
1.50 2.00 1.00 0.50 0.20 0.20 0.100 0.10 0.0100 3.70 3.00 4.00
lower limit 0.10 0.05 0.30 0.80 0.10 0.10 0.02 0.01 0.003 0.01
0.0010 2.30 2.00 2.70 C Si Mn Mo Cr V Nb Tl Al S Ca {circle around
(1)} {circle around (2)} {circle around (3)} invention 1 0.12 0.24
0.60 1.50 0.35 0.28 0.05 0.04 3.02 2.45 2.97 2 0.25 0.25 0.81 1.30
0.40 0.30 0.03 0.05 3.05 2.51 2.93 3 0.16 0.12 0.91 1.50 0.40 0.17
0.04 0.06 2.51 2.81 2.80 4 0.16 0.25 0.84 1.60 0.33 0.18 0.04 0.05
2.66 2.77 2.77 5 0.17 0.26 0.40 1.70 0.34 0.22 0.02 0.04 2.97 2.44
2.99 6 0.18 0.26 1.30 1.31 0.35 0.30 0.05 0.04 2.99 2.96 2.83 7
0.18 0.27 0.87 0.90 0.56 0.33 0.07 0.05 2.73 2.33 2.95 8 0.20 0.25
0.69 1.82 0.45 0.30 0.04 0.06 3.52 2.96 3.55 9 0.19 0.29 0.60 1.28
0.20 0.41 0.03 0.05 3.52 2.08 2.75 10 0.20 0.25 0.74 1.43 0.81 0.20
0.04 0.06 2.63 2.98 3.69 11 0.19 0.24 0.75 1.29 0.45 0.18 0.05 0.04
2.38 2.48 2.72 12 0.15 0.25 0.70 1.30 0.67 0.40 0.05 0.04 3.45 2.67
3.76 13 0.19 0.24 0.75 1.30 0.50 0.18 0.05 0.07 0.04 2.39 2.55 2.84
14 0.19 0.29 0.75 1.31 0.56 0.18 0.15 0.02 0.06 2.45 2.62 3.00 15
0.16 0.25 0.94 1.33 0.65 0.20 0.06 0.03 0.04 2.49 2.92 3.24 16 0.19
0.26 0.75 1.32 0.50 0.20 0.16 0.04 0.05 2.51 2.57 2.91 17 0.18 0.25
0.83 0.91 0.46 0.30 0.05 0.04 2.59 2.20 2.67 18 0.19 0.24 0.75 1.24
0.48 0.24 0.07 0.06 2.63 2.47 2.89 19 0.20 0.24 0.76 0.99 0.61 0.25
0.02 0.04 2.44 2.36 2.94 20 0.21 0.23 0.75 1.33 0.71 0.20 0.03 0.05
2.54 2.79 3.37 21 0.19 0.24 0.75 1.30 0.41 0.18 0.04 0.04 0.0035
2.38 2.46 2.64 comparison 1 0.05 0.25 0.80 0.90 0.60 0.20 0.03 0.04
1.95 2.30 2.70 2 0.35 0.15 1.00 1.60 0.50 0.20 0.04 0.05 2.95 3.10
3.19 3 0.21 0.03 0.70 1.20 0.70 0.20 0.03 0.06 2.41 2.60 3.22 4
0.19 0.34 0.80 1.43 0.50 0.20 0.03 0.05 2.62 2.73 3.02 5 0.20 0.25
0.18 1.40 0.40 0.20 0.05 0.04 2.60 1.98 2.77 6 0.22 0.24 2.00 1.30
0.50 0.30 0.05 0.05 3.02 3.60 3.14 7 0.23 0.25 0.50 0.60 0.60 0.20
0.03 0.06 1.83 1.70 2.40 8 0.21 0.24 0.60 2.60 0.60 0.20 0.04 0.05
3.81 3.80 4.40 9 0.18 0.24 0.70 1.30 0.05 0.20 0.03 0.06 2.48 2.05
1.92 10 0.18 0.25 0.50 1.26 1.30 0.20 0.04 0.04 2.47 3.08 4.60 11
0.20 0.26 0.65 1.30 0.50 0.05 0.03 0.04 1.75 2.45 2.51 12 0.21 0.24
0.72 0.75 0.55 0.70 0.04 0.04 4.46 2.02 3.72 13 0.24 0.25 0.70 1.43
0.62 0.40 0.40 0.04 0.06 3.67 2.75 3.79 14 0.25 0.25 0.70 1.50 0.60
0.20 0.30 0.03 0.03 2.75 2.80 3.30 hardness + structure upper limit
330HV -- 430HV -- -- -- lower limit 260HV 650HV 340HV 80% 560 MPa
100 hardness surface hardness central portion after after hardness
after bainite area fatigue machinability forging nitrocarburizing
nitrocarburizing percentage strength index invention 1 272 674 340
81 573 129 2 308 670 391 96 642 106 3 293 656 351 90 589 114 4 290
653 352 89 590 116 5 276 677 343 84 577 126 6 317 659 405 100 658
102 7 290 672 354 87 593 116 8 317 737 427 102 684 102 9 277 651
362 88 605 125 10 319 752 392 100 643 101 11 286 647 343 86 567 118
12 302 760 403 95 657 108 13 289 660 341 87 574 116 14 294 678 350
90 587 113 15 305 704 368 95 612 107 16 291 668 348 88 585 115 17
280 642 341 82 566 123 18 289 665 349 87 587 116 19 280 671 344 87
578 116 20 310 718 375 97 622 105 21 284 639 345 85 563 120
comparison 1 256 646 289 70 481 147 2 366 698 467 100 724 86 3 301
701 357 92 545 109 4 299 580 363 93 605 110 5 268 653 312 75 525
138 6 395 693 522 100 771 60 7 265 614 293 72 489 136 8 361 828 516
121 766 88 9 261 561 306 73 515 141 10 331 849 401 100 654 96 11
279 624 308 81 518 124 12 305 755 454 100 711 107 13 345 762 453
100 709 97 14 335 710 393 100 644 98
[0073] To obtain fundamental characteristics of the steel used for
the crankshaft according to the present invention, the
90-millimeter round rod is hot-forged into a round rod having a
diameter of 45 millimeters. The 45-millimeter round rod is cut to
have a length of 250 millimeters, introduced into an atmospheric
furnace, heated and held at 900 degrees Centigrade for 60 minutes.
The resultant rod is cooled down to a room temperature at the
cooling rate of 0.5.degree. C./second, thereby providing the steel
for an evaluation. In an ordinary crankshaft manufacturing process,
the steel in a state in which the steel is hot-forged is often
used. In this example, with a view of minimizing irregularities
during the forging, a normalizing treatment is additionally
performed. Since it is confirmed that a diameter of each of the
crank pins and the crank journal of the crankshaft is within the
range between 40 millimeters and 50 millimeters, the cooling rate
if the steel is air-cooled after being hot-forged is within the
range between 0.4 degrees Centigrade/second and 0.7 degrees
Centigrade/second, a cooling rate during the normalizing treatment
is controlled to fall within the same range.
[0074] Next, a hardness of a cross section of the central portion
of the round rod having the diameter of 45 millimeters and the
length of 250 millimeters at five points located ten millimeters
under the surface layer are measured using a Vickers hardness
meter. In addition, steel structures are observed at the same
positions, and the area percentage of the bainite structure is
calculated by an image analyzer. Since the steel used as the
material for the crankshaft according to the present invention is
intended to improve the fatigue strength and maintain the high
machinability, a machinability of the 45-millimeter round rod is
evaluated. To evaluate the machinability, a gundrill machinability
of the round rod to simulate machining of the oil hole considered
to be the most important in the machining of the crankshaft is
evaluated. A gundrill drilling is evaluated by setting, as a
machinability index, the number of drilled holes until the round
rod cannot be cut since an abnormal sound is produced or a cutting
tool is broken or damaged. Cutting conditions are: a cemented
carbide gundrill having a diameter of six millimeters, the cutting
speed of 150 m/min, a feed of 0.04 mm/rev, and a hole depth of 60
millimeters. The experimental result is shown in the Table 1.
[0075] Thereafter, hardening characteristics and the fatigue
strength of the nitrocarburized steel are evaluated. Test pieces
having the diameter of 15 millimeters and the length of 210
millimeters are extracted from the round rod having the diameter of
45 millimeters and the length of 250 millimeters thus obtained by
machining. Further, Ono type rotating bending fatigue test pieces
each having a notch formed by the machining are manufactured. The
notch is formed in a central portion of each test piece so as to
have a notch bottom of 1R, a notch bottom diameter of eight
millimeters, and a stress concentration factor (.alpha.) of about
1.8. These Ono type rotating bending fatigue test pieces are
introduced into a gas nitrocarburizing furnace applied for
mass-production, and nitrocarburized at 600 degrees Centigrade for
120 minutes. For each test piece, the internal hardness of the
piece before the nitrocarburizing (at a central position of a cross
section of each test piece), the surface layer hardness after the
nitrocarburizing, the depth of the entire hardened layer, and the
internal hardness are measured by the Vickers hardness meter. For
the round rod, samples for microscopic observation are produced,
structures are corroded by a one-percent-by-mass Nital etching
reagent, and the resultant structures are observed by a microscope.
An average area percentage of the bainite structure in five visual
fields of an optical microscope (a dimension of each visual field
of 1.0.times.1.5 millimeters) is calculated by an image analyzer.
The average area percentage of the bainite structure thus
calculated substantially coincides with an area percentage of the
bainite structure in the steel structures when a small test piece
having dimensions of 10.times.10.times.70 millimeters is cut out
from the central portion of the nitrocarburized round rod free from
the influence of the nitrocarburizing, austenitized at 1200 degrees
Centigrade for one hour, and cooled so that the cooling rate at
which the sample passes through a temperature range between 900 and
300 degrees Centigrade is 0.5.degree. C./second. Further, a
repeated stress at which each test piece is not broken 10.sup.7th
times is measured as a fatigue limit by an Ono type rotating
bending fatigue tester. The result is shown in the Table 1. Namely,
the crankshaft that satisfies the requirement of the present
invention is not only excellent in machinability and high in
fatigue strength although the surface of the steel is subjected to
the nitrocarburizing treatment.
* * * * *