U.S. patent application number 15/103660 was filed with the patent office on 2016-10-20 for method of manufacturing ferrous metal component.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. The applicant listed for this patent is TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Shinichi HIRAMATSU, Koji INAGAKI, Takaaki KANAZAWA.
Application Number | 20160305007 15/103660 |
Document ID | / |
Family ID | 52273362 |
Filed Date | 2016-10-20 |
United States Patent
Application |
20160305007 |
Kind Code |
A1 |
HIRAMATSU; Shinichi ; et
al. |
October 20, 2016 |
METHOD OF MANUFACTURING FERROUS METAL COMPONENT
Abstract
A method of manufacturing a ferrous metal component includes:
performing an element removal treatment on a workpiece formed of a
ferrous metal material; and performing a surface hardening
treatment on the workpiece through a carburizing treatment after
the element removal treatment. In this method, the element removal
treatment is performed under a condition of a higher temperature
and a lower pressure than in the carburizing treatment.
Inventors: |
HIRAMATSU; Shinichi;
(Toyota-shi, JP) ; INAGAKI; Koji; (Toyota-shi,
JP) ; KANAZAWA; Takaaki; (Nisshin-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TOYOTA JIDOSHA KABUSHIKI KAISHA |
Toyota-shi, Aichi-ken |
|
JP |
|
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-shi, Aichi-ken
JP
|
Family ID: |
52273362 |
Appl. No.: |
15/103660 |
Filed: |
December 8, 2014 |
PCT Filed: |
December 8, 2014 |
PCT NO: |
PCT/IB2014/002806 |
371 Date: |
June 10, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 1/773 20130101;
C23C 8/02 20130101; C21D 3/02 20130101; C21D 9/0068 20130101; C22C
38/00 20130101; C23C 8/22 20130101; C21D 1/06 20130101 |
International
Class: |
C23C 8/22 20060101
C23C008/22; C21D 1/773 20060101 C21D001/773; C21D 3/02 20060101
C21D003/02; C21D 1/06 20060101 C21D001/06; C23C 8/02 20060101
C23C008/02; C21D 9/00 20060101 C21D009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 12, 2013 |
JP |
2013-257520 |
Claims
1. A method of manufacturing a ferrous metal component, the method
comprising: performing an element removal treatment on a workpiece
formed of a ferrous metal material, wherein in the element removal
treatment, an element which forms an oxide on a surface of the
workpiece during a carburizing treatment is evaporated from the
surface of the workpiece; and performing a surface hardening
treatment on the workpiece through a carburizing treatment after
the element removal treatment, wherein the element removal
treatment is performed under a condition of a higher temperature
and a lower pressure than in the carburizing treatment.
2. (canceled)
3. The method according to claim 1, wherein in the element removal
treatment, the element is evaporated from the surface of the
workpiece in a vacuum.
4. The method according to claim 1, wherein the element is at least
one of Mn, Si, and Cr.
5. The method according to claim 1, wherein after the element
removal treatment, the higher temperature of the element removal
treatment is decreased to a temperature of the carburizing
treatment to perform the carburizing treatment.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method of manufacturing a
ferrous metal component in which a metal surface is hardened using
a carburizing treatment and particularly to a technique in which a
brittle grain boundary oxidation layer formed on the metal surface
is suitably reduced during the carburizing treatment.
[0003] 2. Description of Related Art
[0004] For example, in a ferrous metal component such as a steel
component containing Fe as a major component, a technique of
embedding carbon into a metal surface through, for example, a gas
carburizing treatment and then enhancing the hardness of the metal
surface by quenching is known. For example, Japanese Patent
Application Publication No. 05-171348 (JP 05-171348 A) discloses
such a ferrous metal component.
[0005] However, in JP 05-171348 A, it is known that oxygen
contained in carburizing gas during the gas carburizing treatment
infiltrates into a grain boundary of a material surface of the
ferrous metal component and forms a brittle grain boundary
oxidation layer by binding to an element such as Si, Mn, or Cr
contained in the material surface. To deal with this, in JP
05-171348 A, the formation of the grain boundary oxidation layer is
reduced by reducing the content of Si, Mn, or Cr contained in the
ferrous metal component.
SUMMARY OF THE INVENTION
[0006] The ferrous metal component disclosed in JP 05-171348 A has
a limit in reducing the content of the element such as Si, Mn, or
Cr and thus has a problem in that the fatigue strength of the
ferrous metal component decreases due to the grain boundary
oxidation layer formed by the element such as Si, Mn, or Cr
contained in the surface of the ferrous metal component.
[0007] The invention provides a method of manufacturing a ferrous
metal component in which the fatigue strength can be improved by
suitably reducing a grain boundary oxidation layer which is formed
during a carburizing treatment.
[0008] As a result of various kinds of analysis and investigation,
the present inventors have found the following facts. That is, it
was found that an element such as Mn, Si, or Cr is evaporated, that
is, an element removal phenomenon occurs from a surface of a
workpiece formed of a ferrous metal material under a condition of a
higher temperature and a lower pressure than in a carburizing
treatment. In addition, typically, this element removal phenomenon
has a negative image as in the case of "decarburization". However,
it was found that, conversely, by causing this element removal
phenomenon to occur before a carburizing treatment on the basis of
the technical knowledge relating to carburizing, the formation of a
grain boundary oxidation layer can be suitably inhibited in a
subsequent carburizing treatment. The invention has been made based
on the above-described findings.
[0009] A method of manufacturing a ferrous metal component
according to an aspect of the invention includes: performing an
element removal treatment on a workpiece formed of a ferrous metal
material; and performing a surface hardening treatment on the
workpiece through a carburizing treatment after the element removal
treatment. In this method, the element removal treatment is
performed under a condition of a higher temperature and a lower
pressure than in the carburizing treatment.
[0010] In the method of manufacturing a ferrous metal component
according to the aspect, the element removal treatment is performed
under a condition of a higher temperature and a lower pressure than
in the carburizing treatment. Therefore, before the carburizing
treatment, an element which causes ann oxide to be formed during
the carburizing treatment is evaporated from the surface of the
workpiece. Accordingly, a grain boundary oxidation layer which is
formed on the surface of the workpiece during the carburizing
treatment can be suitably removed, and the fatigue strength of the
ferrous metal component can be improved.
[0011] According to the aspect, in the element removal treatment,
before the carburizing treatment, an element which forms an oxide
on a surface of the workpiece during the carburizing treatment may
be evaporated from the surface of the workpiece. Accordingly, a
grain boundary oxidation layer which is formed on the surface of
the workpiece during the carburizing treatment can be suitably
removed.
[0012] According to the aspect, in the element removal treatment,
the element may be evaporated from the surface of the workpiece in
a vacuum. Accordingly, a grain boundary oxidation layer which is
formed on the surface of the workpiece during the carburizing
treatment can be suitably removed.
[0013] According to the aspect, the element may be at least one of
Mn, Si, and Cr. Therefore, in the element removal treatment, at
least one of Mn Si, and Sr having a relatively high vapor pressure
is evaporated from the surface of the workpiece. Accordingly, a
grain boundary oxidation layer which is formed on the surface of
the workpiece during the carburizing treatment can be suitably
removed.
[0014] According to the aspect, after the element removal
treatment, the higher temperature of the element removal treatment
may be decreased to a temperature of the carburizing treatment to
perform the carburizing treatment. Accordingly, after the element
removal treatment, the carburizing treatment can be continuously
performed suitably.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] Features, advantages, and technical and industrial
significance of exemplary embodiments of the invention will be
described below with reference to the accompanying drawings, in
which like numerals denote like elements, and wherein:
[0016] FIG. 1 is a diagram illustrating a shaft which is
manufactured using a method of manufacturing a ferrous metal
component according to the invention and illustrating a
configuration of an apparatus used for manufacturing the shaft;
[0017] FIG. 2 is a flow chart illustrating manufacturing processes
of the shaft of FIG. 1;
[0018] FIG. 3 is a diagram illustrating temperature conditions and
pressure conditions in a desiliconizing and demanganizing process
and a gas carburizing process illustrated in FIG. 2;
[0019] FIG. 4 is a diagram illustrating the results of Experiment I
in which, by using test pieces formed of the same material as the
shaft of FIG. 1, the contents (mass %) of Si, Mn, and Cr contained
in surfaces of the test pieces were measured under different
conditions of a temperature (.degree. C.), a pressure (Pa), and a
holding time (min) in the desiliconizing and demanganizing process
of FIG. 2;
[0020] FIG. 5 is a diagram illustrating the contents of Mn and Si
contained in surfaces of test pieces formed of the same, material
as the shaft of FIG. 1, the test pieces including a test piece (a
shaft according to Example 1) which was manufactured through the
desiliconizing and demanganizing process and the gas carburizing
process of FIG. 2 and a test piece (a shaft according to
Comparative Example 1) which was manufactured through only the gas
carburizing process of FIG. 2;
[0021] FIG. 6 is a diagram illustrating the thicknesses of grain
boundary oxidation layers which were formed in the test piece (the
shaft according to Example 1) and the test piece (the shaft
according to Comparative Example 1) of FIG. 5;
[0022] FIG. 7 is a diagram illustrating a part of the surface of
the test piece (the shaft according to Comparative Example 1) of
FIG. 5;
[0023] FIG. 8 is a diagram illustrating a part of the surface of
the test piece (the shaft according to Example 1) of FIG. 5;
[0024] FIG. 9 is a diagram illustrating the fatigue strengths of
the test piece (the shaft according to Example 1) and the test
piece (the shaft according to Comparative Example 1) of FIG. 5;
and
[0025] FIG. 10 is a diagram corresponding to FIG. 1 illustrating a
method of manufacturing a ferrous metal component according to
another example of the invention.
DETAILED DESCRIPTION OF EMBODIMENTS
[0026] Hereinafter, Example 1 of the invention will be described
with reference to the drawings. In the drawings of the following
Example 1, each part is appropriately simplified and modified, and
the dimension, shape, and the like thereof may be not accurately
illustrated.
[0027] FIG. 1 is a diagram illustrating a ferrous metal component
to which the invention is suitably applied, that is, a shaft 10
which is included in, for example, a belt-type continuously
variable transmission (CVT) used in a vehicle. The shaft 10 is
formed of a ferrous metal material containing Fe as a major
component, for example, formed of low carbon steel such as steel or
case-hardened steel having a C content of 0.02% to 2.14% (wt %). A
surface of the shaft 10 is carburized by a gas carburizing
apparatus 12 of FIG. 1; as a result, the surface is hardened.
[0028] Here, the gas carburizing apparatus 12 will be described. As
illustrated in FIG. 1, the gas carburizing apparatus 12 includes: a
heat treatment chamber 16 that is formed of a heat insulating
material 14 and accommodates the shaft 10; a jig 18 that fixes and
supports the shaft 10 in the heat treatment chamber 16; a heater 20
that heats the inside of the heat treatment chamber 16; a mass flow
controller 24 that measures and controls a flow rate of nitrogen
gas flowing from a supply device 22, which supplies, for example,
nitrogen gas, into the heat treatment chamber 16; and a
pressure-reducing pump 26 that evacuates the inside of the heat
treatment chamber 16 to reduce an internal pressure of the heat
treatment chamber 16. Therefore, in the gas carburizing apparatus
12, the shaft 10 can be held in the heat treatment chamber 16 under
a condition of a relatively high temperature and a relatively low
pressure by the heater 20 and the pressure-reducing pump 26 which
are included in the gas carburizing apparatus 12. In addition, the
temperature of the shaft 10 is decreased by nitrogen gas which is a
cooling gas supplied from the supply device 22, and thus the shaft
10 is cooled. In addition, the gas carburizing apparatus 12 is
provided with a carburizing gas supply device (not illustrated)
which supplies carburizing gas into the heat treatment chamber 16.
During a gas carburizing treatment, the carburizing gas is supplied
from the carburizing gas supply device continuously. The
carburizing gas is prepared by, for example, mixing a source gas
such as propane gas, town gas, natural gas, or charcoal gas with
air at a predetermined ratio and heating the mixture to be
decomposed.
[0029] In addition, here, a method of manufacturing the shaft 10
according to the Example 1, that is, manufacturing processes P1 to
P5 will be described using FIG. 2.
[0030] As illustrated in FIG. 2, first, in a forging process P1, a
workpiece formed of a ferrous metal material (steel material) of,
for example, SCR420 which is case-hardened steel is formed into a
predetermined shape by, for example, forging.
[0031] Next, in a preheating (annealing) process P2, the workpiece
formed in the forging process P1 is annealed to be softened.
[0032] Next, in a machining process P3, the workpiece softened in
the preheating process P2 is cut into the same shape as the shaft
10 by machining.
[0033] Next, in a desiliconizing and demanganizing (element
removal) process P4, the shaft 10 which is the workpiece cut in the
machining process P3 is arranged in the gas carburizing apparatus
12 and is held under a condition of a higher temperature and a
lower pressure than in a gas carburizing process (carburizing
process) P5 described below, for example, under a condition of an
internal temperature T (.degree. C.) of the heat treatment chamber
16 of 1000.degree. C. to 1300.degree. C. and a vacuum, that is, an
internal pressure P (Pa) of the heat treatment chamber 16 of 100 Pa
to 1000 Pa for a predetermined time t (min) of, for example, 5
minutes to 30 minutes. As a result, an element such as Mn, Si, or
Cr having a relatively high vapor pressure which is contained in
the surface of the shaft 10 is evaporated. In the desiliconizing
and demanganizing process P4, a vacuum represents a pressure being
sufficiently lower than the atmospheric pressure, for example,
about 100 Pa to 1000 Pa. The pressure P (100 Pa to 1000 Pa) of the
desiliconizing and demanganizing process P4 is sufficiently lower
than a pressure condition (higher than 1 kPa and 10 kPa or lower)
of, for example, a vacuum carburizing treatment of the related
art.
[0034] Next, in the gas carburizing process P5, carbon is embedded
into the surface of the shaft 10 havihg the surface, from which the
element such as Mn, Si, or Cr is evaporated in the desiliconizing
and demanganizing process P4, by carburizing gas at a gas
carburizing temperature of about 930.degree. C. as illustrated in
FIG. 3. Next, the carburized shaft 10 is rapidly cooled and
quenched. As a result, the shaft 10 in which the fatigue strength
is improved by the surface being hardened is manufactured. In the
gas carburizing apparatus 12, as illustrated in FIG. 3, after the
desiliconizing and demanganizing process P4, a temperature is
decreased to, a gas carburizing temperature of for example, about
930.degree. C. to perform the gas carburizing process P5. In
addition, the gas carburizing process P5 is performed under an
internal pressure of the heat treatment chamber 16 of about
1.0.times.10.sup.5 Pa, that is, the atmospheric pressure as
illustrated in FIG. 3.
[0035] The gas carburizing apparatus 12 includes a mechanism of
holding the inside of the heat treatment chamber 16 at a high
temperature and a low pressure (vacuum) before carburizing, in
addition to a mechanism of carburizing and quenching the shaft 10.
Therefore, when the manufacturing processes P1 to P5 of the Example
1 are performed, that is, when the desiliconizing and demanganizing
process P4 and the gas carburizing process P5 are performed, it is
not necessary that a new device which holds the shaft 10 under a
condition of a high temperature and a low pressure, for example, in
the desiliconizing and demanganizing process P4 be added in
addition to a gas carburizing apparatus of the related art which
carburizes and quenches the shaft 10. Therefore, the manufacturing
cost can be significantly reduced.
Experiment I
[0036] Here, Experiment I which was performed by the present
inventors will be described. Experiment I was performed in order to
verify the fact that the amounts of Si, Mn, and Cr evaporated from
the surface of the shaft 10 can be suitably increased, that is, the
contents of Si, Mn, and Cr contained in the surface of the shaft 10
can be suitably reduced by changing conditions of the temperature T
(.degree. C.), the pressure P (Pa), and the holding time t (min) in
the desiliconizing and demanganizing process P4.
[0037] In Experiment I, the desiliconizing and demanganizing
process P4 was performed under 16 kinds of conditions, that is,
under Condition 1 to Condition 16, in which: test pieces formed of
the same material as the shaft 10, that is, SCR 420 and having a
predetermined shape (for example, .phi.18 mm.times.50 mm) were
used; the temperature T (.degree. C.) was changed in a range of
1000.degree. C. to 1300.degree. C., that is, was 1000.degree. C.,
1100.degree. C., 1200.degree. C., or 1300.degree. C. as illustrated
in FIG. 4; the pressure P (Pa) was changed in a range of 100 Pa to
1000 Pa, that is, was 100 Pa, 200 Pa, 500 Pa, or 1000 Pa; and the
holding time t (min) was changed in a range of 5 minutes to 30
minutes, that is, was 5 minutes, 10 minutes, 15 minutes, or 30
minutes. The contents of Si, Mn, and Cr in the surfaces of the test
pieces corresponding to the shafts 10 on which the desiliconizing
and demanganizing process P4 was performed under Condition 1 to
Condition 16 were measured.
[0038] In Experiment I, as illustrated in FIG. 5, the sum
(10Si+Mn+Cr) of 10 times of the Si content (mass %), the Mn content
(mass %), and the Cr content (mass %) per unit mass at a depth of 6
.mu.m from the surface of the test piece corresponding to the shaft
10 is represented by the content y (mass %) of Si, Mn, and Cr in
the surface of the test piece corresponding to the shaft 10. In
addition, the content (mass %) of Si, Mn, and Cr per unit mass at a
depth of 6 .mu.m from the surface of the test piece was measured by
glow discharge optical emission spectroscopy.
[0039] Hereinafter, the results of Experiment I will be described
using FIG. 4. As illustrated in FIG. 4, the content y (mass %) of
Si, Mn, and Cr of the test piece was 2 (mass %) or less under
Condition 8, Condition 9, and Condition 13, which was relatively
small. Therefore, it is considered that, by performing the
desiliconizing and demanganizing process P4 under Condition 8,
Condition 9, and Condition 13, the content y (mass %) of Si, Mn,
and Cr contained in the surface of the shaft 10 can be suitably
reduced.
[0040] In addition, by performing multiple regression analysis
using the experimental results of Condition 1 to Condition 16
illustrated in FIG. 4, a relational expression (1) between the
temperature T (.degree. C.), the pressure P (Pa), and the holding
time t (min) in the desiliconizing and demanganizing process P4 and
the content y (mass %) of Si, Mn, and Cr in the surface of the test
piece corresponding to the shaft 10 is obtained. y (mass
%)=-0.0018.times.T (.degree. C.)+0.0001.times.P (Pa)-0.024.times.xt
(min)+6.47677 . . . (1)
[0041] It can be considered from the relational expression (1)
that, in the desiliconizing and demanganizing process P4, the
element such as Si, Mn, or Cr is suitably evaporated from the
surface of the shaft 10 by increasing the temperature T (.degree.
C.), the element such as Si, Mn, or Cr is suitably evaporated from
the surface of the shaft 10 by reducing the pressure P (Pa), and
the element such as Si, Mn, or Cr is suitably evaporated from the
surface of the shaft 10 by increasing the holding time t (min).
Typically, when the content y (mass %) of Si, Mn, and Cr in the
surface of the shaft 10 is 2 (mass %) or less, the thickness of a
grain boundary oxidation layer A (refer to FIG. 7) which is formed
on the surface by the carburizing treatment can be inhibited to be
6.0 .mu.m or less, and thus a decrease in fatigue strength can be
inhibited. Therefore, it is considered that, for example, by
setting the temperature T (.degree. C.), the pressure P (Pa), and
the holding time t (min) in the desiliconizing and demanganizing
process P4 such that the content y (mass %) of Si, Mn, and Cr in
the surface of the shaft 10 is 2 (mass %) or less, the grain
boundary oxidation layer A which is formed on the surface of the
shaft 10 in the gas carburizing process P5 can be suitably
inhibited.
Experiment II
[0042] Here, Experiment II which was performed by the present
inventors will be described. Experiment II was performed in order
to verify the effect of the desiliconizing and demanganizing
process P4 on the shaft 10 in the manufacturing processes P1 to P5
of FIG. 2, that is, the effect of the desiliconizing and
demanganizing process P4 on the grain boundary oxidation layer A
which is formed on the surface of the shaft 10. In Experiment II,
the effect of the grain boundary oxidation layer A, which is formed
on the shaft 10, on the fatigue strength of the shaft 10 was also
verified.
[0043] In Experiment II, the thicknesses (.mu.m) of the grain
boundary oxidation layers A, which were formed on test pieces
formed of the same material as the shaft 10, that is, formed of
SCR420 and having a predetermined shape (for example, .phi.18
mm.times.50 mm), were measured, the test pieces including: a test
piece (desiliconizing and demanganizing+gas carburizing)
corresponding to the shaft 10 according to Example 1 on which the
desiliconizing and demanganizing process P4 and the gas carburizing
process P5 were performed; and a test piece (only gas carburizing)
corresponding to the shaft 10 according to Comparative Example 1 on
which only the gas carburizing process P5 was performed without
performing the desiliconizing and demanganizing process P4. In
addition, the fatigue strengths, that is, the nominal stresses a
(MPa) of the test piece corresponding to the shaft 10 according
Example 1 and the test piece corresponding to the shaft 10
according to Comparative Example 1 were measured. In the
desiliconizing and demanganizing process P4, the element removal
treatment was performed under, for example, the above-described
Condition 8. In addition, in Experiment II, a test piece formed of
the same material as the shaft 10, that is SCR 420 and having a
predetermined shape (for example, .phi.18 mm.times.50 mm) was
prepared, only the gas carburizing process P5 was performed thereon
without performing desiliconizing and demanganizing process P4, and
a finishing process of removing the surface of the test piece, that
is, the grain boundary oxidation layer A by machining was performed
thereon. As a result, a test piece (gas carburizing+finishing)
corresponding to the shaft 10 according to Comparative Example 2
was prepared. Using this test piece corresponding to the shaft 10
according to Comparative Example 2, the fatigue strength was
measured.
[0044] Hereinafter, the results of Experiment II will be described
using FIGS. 5 and 9. As illustrated in FIG. 5, in the test piece
(the shaft 10 according to Example 1), the contents (mass %) of Si
and Mn contained in the surface of the test piece were suitably
reduced as compared to the test piece (the shaft 10 according to
Comparative Example 1). FIG. 5 illustrates the results of the
above-described measurement using glow discharge optical emission
spectroscopy.
[0045] In addition, as illustrated in FIG. 6, the thickness of the
grain boundary oxidation layer A which was formed in the test piece
(the shaft 10 according to Example 1) was 4 um, and the thickness
of the grain boundary oxidation layer A which was formed in the
test piece (the shaft 10 according to Comparative Example 1) was 20
um. In the test piece (the shaft 10 according to Example 1), the
grain boundary oxidation layer A was suitably reduced as compared
to the test piece (the shaft 10 according to Comparative Example
1). The measured values of the thicknesses of the grain boundary
oxidation layers A of FIG. 6 were obtained by measuring, for
example, the grain boundary oxidation layers of the surface of the
test piece (the shaft 10 according to Example 1) and the surface of
the test piece (the shaft 10 according to Comparative Example 1)
illustrated in FIGS. 7 and 8 using an optical microscope. The
thickness of the grain boundary oxidation layer A is defined as the
depth from the surface of the test piece at which a grain boundary
is observed. In FIG. 8, the grain boundary oxidation layer A was
not observed.
[0046] In addition, as illustrated in FIG. 9, when the number of
repetitions Nf was about 10.sup.7, the nominal stress .sigma. of
the test piece (the shaft 10 according to Example 1) was about 580
MPa, the nominal stress .sigma. of the test piece (the shaft 10
according to Comparative Example 2) was about 575 MPa, and the
nominal stress .sigma. of the test piece (the shaft 10 according to
Comparative Example 1) was about 515 MPa. Therefore, the fatigue
strength of the test piece corresponding to the shaft 10 according
to Example 1 was suitably higher than that of the test piece
corresponding to the shaft 10 according to Comparative Example 1.
The measurement results of FIG. 9 were obtained using, for example,
the Ono-type rotary bending fatigue test apparatus.
[0047] According to the results of Experiment II, as illustrated in
the measurement results of FIG. 5, in the test piece corresponding
to the shaft 10 according to Example 1 on which the desiliconizing
and demanganizing process P4 was performed, the contents (mass %)
of Si and Mn contained in the surface of the test piece was
suitably reduced as compared to the test piece corresponding to the
shaft 10 according to Comparative Example 1 on which the
desiliconizing and demanganizing process P4 was not performed.
Therefore, it is considered that Si and Mn, which cause oxides
(SiO, MnO) to be formed on the surface of the shaft 10 in the gas
carburizing process P5, was evaporated from the surface of the
shaft 10 through the desiliconizing and demanganizing process
P4.
[0048] In addition, according to the results of Experiment II, as
illustrated in the measurement results of FIGS. 5 and 6, in the
test piece corresponding to the shaft 10 according to Example 1 on
which the desiliconizing and demanganizing process P4 was
performed, the contents of Si and Mn contained in the surface of
the test piece was suitably reduced, and the thickness of the grain
boundary oxidation layer A formed in the test piece was suitably
reduced, as compared to the test piece corresponding to the shaft
10 according to Comparative Example 1 on which the desiliconizing
and demanganizing process P4 was not performed. Therefore, it is
considered that, by evaporating Si and Mn from the surface of the
shaft 10 in the desiliconizing and demanganizing process P4, the
contents of Si and Mn in the surface of the shaft 10 were suitably
reduced, the amount of O, which was contained in carburizing gas,
binding to Si and Mn in the subsequent gas carburizing process P5
was reduced, and thus the grain boundary oxidation layer A formed
on the shaft 10 was reduced.
[0049] In addition, according to the results of Experiment II, as
illustrated in measurement results of FIGS. 6 and 9, the fatigue
strength of the test piece corresponding to the shaft 10 according
to Example 1 in which the thickness of the grain boundary oxidation
layer A was relatively thin (4 .mu.m) was higher than that of the
test piece corresponding to the shaft 10 according to Comparative
Example 1 in which the thickness of the grain boundary oxidation
layer A was relatively thick (20 .mu.m). In addition, the fatigue
strength of the test piece corresponding to the shaft 10 according
to Comparative Example 2 in which the desiliconizing and
demanganizing process P4 was not performed and the grain boundary
oxidation layer A was removed by the finishing process was higher
than that of the test piece corresponding to the shaft 10 according
to Comparative Example 1. Therefore, it is considered that, by
reducing the thickness of the grain boundary oxidation layer A
formed on the shaft 10, the fatigue strength of the shaft 10 was
improved. In addition, it is considered that, in the manufacturing
processes of the shaft 10 according to Example 1, in the finishing
process, which was performed on the test piece corresponding to the
shaft 10 according to Comparative Example 2, of removing the grain
boundary oxidation layer A by machining, the manufacturing cost was
higher than that in the desiliconizing and demanganizing process
P4. Therefore, it is considered that, in the manufacturing
processes P1 to P5 of the shaft 10 according to Example 1, the
manufacturing cost was suitably suppressed as compared to the
manufacturing processes of the shaft 10 according to Comparative
Example 2 in which the desiliconizing and demanganizing process P4
was not performed and the finishing process was performed.
[0050] In the manufacturing processes P1 to P5 of the shaft 10
according to Example 1, before the gas carburizing process P5, the
desiliconizing and demanganizing process P4 is performed under a
condition of a higher temperature and a lower pressure than in the
gas carburizing process P5. Therefore, before the gas carburizing
process P5, the element such as Si, Mn, or Cr which causes an oxide
to be formed during the gas carburizing process P5 is evaporated
from the surface of the shaft 10. Accordingly, the grain boundary
oxidation layer A formed on the surface of the shaft 10 during the
gas carburizing process P5 can be suitably reduced, and the fatigue
strength of the shaft 10 can be improved.
[0051] In addition, in the manufacturing processes P1 to P5 of the
shaft 10 according to Example 1, in the desiliconizing and
demanganizing process P4, before the gas carburizing process P5,
the element such as Si, Mn, or Cr which causes an oxide to be
formed on the surface of the shaft 10 during the gas carburizing
process P5 is evaporated from the surface of the shaft 10.
Accordingly, the grain boundary oxidation layer A which is formed
on the surface of the shaft 10 during the gas carburizing process
P5 can be suitably reduced.
[0052] In addition, in the manufacturing processes P1 to P5 of the
shaft 10 according to Example 1, in the desiliconizing and
demanganizing process P4, the element such as Si, Mn, or Cr which
causes an oxide to be formed on the surface of the shaft 10 during
the gas carburizing process P5 is evaporated from the surface of
the shaft 10 in a vacuum in which the pressure was sufficiently
lower than the atmospheric pressure, that is, under a pressure of
100 Pa to 1000 Pa. Accordingly, the grain boundary oxidation layer
A which is formed on the surface of the shaft 10 during the gas
carburizing process P5 can be suitably reduced, and the fatigue
strength of the shaft 10 can be improved.
[0053] In addition, in the manufacturing processes P1 to P5 of the
shaft 10 according to Example 1, the element which causes an oxide
to be formed during the gas carburizing process P5 is Mn, Si, or
Cr. Therefore, in the desiliconizing and demanganizing process P4,
the element such as Mn, Si, or Cr having a relatively high vapor
pressure is evaporated from the surface of the shaft 10.
Accordingly, the grain boundary oxidation layer A which is formed
on the surface of the shaft 10 during the gas carburizing process
P5 can be suitably reduced.
[0054] In addition, in the manufacturing processes P1 to P5 of the
shaft 10 according to Example 1, after the desiliconizing and
demanganizing process P4, a temperature is decreased to a
temperature of the gas carburizing process P5 of about 930.degree.
C. to perform the gas carburizing process P5. Accordingly, after
the desiliconizing and demanganizing process P4, the gas
carburizing process P5 can be continuously performed suitably.
[0055] Next, another example of the invention will be described. In
the following description, the same components as in the
above-described Example 1 are represented by the same reference
numerals, and the description thereof will not be repeated.
[0056] Manufacturing processes of a ferrous metal component
according to this example are substantially the same as the
manufacturing processes P1 to P5 of the shaft 10 according to
Example 1, except that a gear 28 which is a driving component used
in, for example, a vehicle is manufactured instead of the shaft 10
according to Example 1. A gas carburizing apparatus 12 according to
the example illustrated in FIG. 10 has a slightly different shape
from the as carburizing apparatus 12 according to Example 1
illustrated in FIG. 1. For example, a heater 20 and a jig 18
according to the example have different shapes from Example 1 but
are functionally the same as in Example 1.
[0057] In the manufacturing processes of the gear 28 according to
the example, similarly to the effects of the above-described
Example 1, in the desiliconizing and demanganizing process P4, a
grain boundary oxidation layer A which is formed on a surface of
the gear 28 during the gas carburizing process P5 can be suitably
reduced, and thus the fatigue strength of the gear 28 can be
improved. In addition, in order to manufacture the gear 28,
typically, a shot peening process for improving the fatigue
strength is performed. However, in the manufacturing processes of
the gear 28 according to the example, since the fatigue strength
can be suitably improved, the shot spinning process is not
necessary. Accordingly, the manufacturing cost of the gear 28 can
be significantly reduced.
[0058] Hereinabove, the examples of the invention have been
described with reference to the drawings, but the invention is also
applicable to other embodiments.
[0059] In the manufacturing processes P1 to P5 of the shaft 10
according to the examples, in the desiliconizing and demanganizing
process P4, the element such as Si, Mn, or Cr having a high vapor
pressure is suitably evaporated from the surface of the shaft 10.
However, other elements may be evaporated from the surface of the
shaft 10. In addition, by evaporating at least one element of Mn,
Si, and Cr from the surface of the shaft 10, the formation of the
grain boundary oxidation layer A can be inhibited, and thus the
fatigue strength of the shaft 10 can be improved.
[0060] In addition, in the manufacturing processes P1 to P5 of the
shaft 10 according to the examples, in the desiliconizing and
demanganizing process P4, by performing multiple regression
analysis using the results of Experiment I illustrated in FIG. 4,
the relational expression (1) between the temperature T (.degree.
C.), the pressure P (Pa), and the holding time t (min) in the
desiliconizing and demanganizing process P4 and the content y (mass
%) of Si, Mn, and Cr in the surface of the shaft 10 is obtained.
However, for example, when the material of the shaft 10 is changed
from SCR420 to another one, a new relational expression may be
obtained by multiple regression analysis after performing
Experiment I with the same method as the example.
[0061] In addition, in the above-described examples, the shaft 10
and the gear 28 used in a vehicle are used as examples of a ferrous
metal component. However, the invention is suitably applicable to
other ferrous metal components. That is, the invention is suitably
applicable to any ferrous metal component on which a carburizing
treatment is performed. In addition, in the above-described
examples, the shaft 10, that is, the ferrous metal component is
formed of the ferrous metal material containing Fe as a major
component, for example, formed of a steel material having a C
content of 0.02% to 2.14% (wt %). However, the ferrous metal
component may be formed of pure iron having a C content of 0.02%
(wt %) or less.
[0062] The above-described examples are merely exemplary, and
various modifications and improvements may be added to the
invention based on the knowledge of a person skilled in the
art.
* * * * *