U.S. patent application number 16/345102 was filed with the patent office on 2019-09-19 for structural material.
This patent application is currently assigned to HITACHI, LTD.. The applicant listed for this patent is HITACHI, LTD.. Invention is credited to Hisamitsu HATOU, Matahiro KOMURO, Kazuya NONOMURA.
Application Number | 20190284676 16/345102 |
Document ID | / |
Family ID | 62109685 |
Filed Date | 2019-09-19 |
![](/patent/app/20190284676/US20190284676A1-20190919-D00000.png)
![](/patent/app/20190284676/US20190284676A1-20190919-D00001.png)
![](/patent/app/20190284676/US20190284676A1-20190919-D00002.png)
![](/patent/app/20190284676/US20190284676A1-20190919-D00003.png)
United States Patent
Application |
20190284676 |
Kind Code |
A1 |
KOMURO; Matahiro ; et
al. |
September 19, 2019 |
STRUCTURAL MATERIAL
Abstract
For providing a structural material having improved fatigue
strength and abrasion resistance, provided is a structural material
containing iron and carbon, including: a first layer formed of
pearlite; a second layer formed of a mixed phase of martensite and
a carbide; and a third layer formed of a carbide, in order from a
center to a surface of the structural material. The carbide of the
third layer is represented by MC (where M is one element among Ti,
V, Nb, Mo, Ta, and W), and the structural material has a
concentration gradient in which an M element concentration is
decreased from the surface to the center of the structural
material.
Inventors: |
KOMURO; Matahiro; (Tokyo,
JP) ; HATOU; Hisamitsu; (Tokyo, JP) ;
NONOMURA; Kazuya; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HITACHI, LTD. |
Tokyo |
|
JP |
|
|
Assignee: |
HITACHI, LTD.
Tokyo
JP
|
Family ID: |
62109685 |
Appl. No.: |
16/345102 |
Filed: |
October 25, 2017 |
PCT Filed: |
October 25, 2017 |
PCT NO: |
PCT/JP2017/038449 |
371 Date: |
April 25, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C23C 8/54 20130101; C23C
8/62 20130101; C21D 1/06 20130101; C22C 38/00 20130101; C23C 8/02
20130101; C23C 8/80 20130101; C23C 8/40 20130101; C23C 8/32
20130101; C22C 38/44 20130101; C23C 8/22 20130101 |
International
Class: |
C23C 8/32 20060101
C23C008/32; C23C 8/22 20060101 C23C008/22 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 8, 2016 |
JP |
2016-217728 |
Claims
1. A structural material containing iron and carbon, comprising: a
first layer formed of pearlite, a second layer formed of a mixed
phase of martensite and a carbide, and a third layer formed of a
carbide, in order from a center to a surface of the structural
material, wherein the carbide of the third layer is represented by
MC (wherein M is one element among Ti, V, Nb, Mo, Ta, and W), and
the structural material has a concentration gradient in which an M
element concentration is decreased from the surface to the
center.
2. The structural material according to claim 1, wherein the
carbide of the second layer is represented by M.sub.3C (wherein M
is one element among Ti, V, Nb, Mo, Ta, and W), and in the second
layer, a spherical carbide is dispersed in the martensite to form
the mixed phase.
3. The structural material according to claim 1, wherein the third
layer contains a carbonitride.
4. The structural material according to claim 1, wherein the third
layer has a Vickers hardness in a range of 1500 to 3500.
Description
TECHNICAL FIELD
[0001] The present invention relates to a surface modification
method for increasing fatigue strength of low alloy steel and a
structural material manufactured using the method.
BACKGROUND ART
[0002] As a method of controlling texture for improving mechanical
properties, corrosion resistance, and functionality in an
iron-based material, a surface hardening method has been reviewed.
For example, PTL 1 discloses a rolling member manufactured using a
surface hardening method.
CITATION LIST
Patent Literature
[0003] PTL 1: JP 2006-009145 A
SUMMARY OF INVENTION
Technical Problem
[0004] The rolling member of PTL 1 is provided with a first
quenched hardened layer having martensite as a matrix phase; and a
second quenched hardened layer formed in a deeper layer than the
first quenched hardened layer, in which cementite is dispersed in a
matrix phase containing at least one of a martensite phase and a
bainite phase in which carbon is solid-solubilized. PTL 1 is aimed
at strength improvement; however, particularly additional
improvement in terms of fatigue strength and abrasion resistance is
being required.
Solution to Problem
[0005] The present invention provides a structural material
containing iron and carbon, including: a first layer formed of
pearlite; a second layer formed of a mixed phase of martensite and
a carbide; and a third layer formed of a carbide, in order from a
center to a surface of the structural material. The carbide of the
third layer is represented by MC (wherein M is one element among
Ti, V, Nb, Mo, Ta, and W), and the structural material has a
concentration gradient in which an M element concentration is
decreased from the surface to the center of the structural
material.
Advantageous Effects of Invention
[0006] By adopting a three-layer structure as such, a structural
material having improved fatigue strength and abrasion resistance
can be provided. By applying the structural material to a sliding
material, or the like, a lighter weight, a longer lifespan, and
improved reliability of the entire structural material can be
achieved, as compared with the related art.
BRIEF DESCRIPTION OF DRAWINGS
[0007] FIG. 1 is an optical micrograph of a section.
[0008] FIG. 2 is a Vickers hardness distribution.
[0009] FIG. 3 is an X-ray diffraction pattern.
[0010] FIG. 4 is a schematic diagram of a sectional texture.
[0011] FIG. 5 is a result of a fatigue test.
DESCRIPTION OF EMBODIMENTS
[0012] As a representative inexpensive structural material, low
alloy steel can be mentioned. The low alloy steel is an iron
carbon-based material having a content of transition metal elements
such as Cr or Mo as an additive element of 20 wt % or less. The low
alloy steel is a material having an A1 transformation point and
having a carbide form being greatly different between a high
temperature side and a low temperature side of the transformation
point. That is, in a higher temperature side than the A1
transformation point, .gamma. (austenite) or a carbide is grown,
and in a lower temperature side than the A1 transformation point, a
(ferrite) and a carbide can be formed, and a layered carbide is
easily grown when being maintained in the low temperature side.
Here, the carbide has M.sub.3C as a main, wherein M is a transition
metal element such as Fe, Cr, or Mo.
[0013] For spheroidizing of the carbide, the layered carbide is
partially dissolved in a .gamma. phase and aggregated, whereby
braking of the layered carbide is progressed for surface energy
reduction so that the layered carbide becomes closer to a sphere
from an elliptical shape. Carbon other than the spheroidized
carbide is solid-solubilized in .gamma.. Pearlite is grown from the
.gamma. phase at an A1 point or less to form a layered carbide
between carbides, and is further heated to a higher temperature
side than the A1 point, whereby the layered carbide is broken and
aggregated. Heating and cooling are repeated between the high
temperature side and the low temperature side of the A1 point,
thereby growing the spherical carbide. Then, the pearlite refers to
a texture which is a layered structure and formed by alternating
Fe(.alpha.-Fe) and Fe3C of a bcc structure.
[0014] The spherical carbide is grown as described above and the
matrix is rapidly cooled in a temperature range in which the matrix
is a .gamma. phase, whereby the matrix can be transformed from the
.gamma. phase into a mixed phase of martensite and a residual
.gamma. phase. For forming MC (wherein M is at least one of Ti, V,
Nb, Mo, Ta, and W) which is a high-hardness carbide, on the outside
of a martensite layer, a slurry of powder containing an M element
thereof is applied on a surface of a structural material at a
thickness of 10 to 100 .mu.m before a carburization process. The
slurry is a mixed body of amorphous powder having a powder diameter
of 10 .mu.m or less and an alcohol solvent, and applied several
times in the air. Thereafter, the solvent is evaporated by drying
and heating, and heating is performed in a .gamma. stable
temperature range of the structural material. By the heating, some
of M elements are diffused into a base material and forms a
concentration gradient. Next, MC is formed by primary carburization
and at the same time carbon is diffused also to a base material
side.
[0015] The M element is diffused from the surface of the base
material to about 10 .mu.m, whereby close adhesion of MC growing
later can be increased, and growth of .gamma. crystal grains in the
base material side by diffusion of the M element can be
simultaneously suppressed, and a carbide containing the M element
grows in a grain boundary and in grains to contribute to
miniaturization of a lathe or a packet of martensite simultaneously
to suppress crack propagation.
[0016] Use of nitrogen-containing gas during or after carburization
allows nitrogen martensite and a carbon nitrogen compound to grow,
thereby contributing to an increase of tempering softening
resistance or prevention of deformation by quenching temperature
lowering.
[0017] In the present invention, a layer constitution from the
outermost surface to the inside of the structural material is
changed as follows. On the outermost surface, an MC (wherein M is
at least one of Ti, V, Nb, Mo, Ta, and W) or MCN-based carbonitride
(wherein M is at least one of Ti, V, Nb, Mo, Ta, and W) is grown,
and the thickness thereof is 10 .mu.m or more. In order to increase
close adhesion, a diffusion layer of the M element is formed in the
inner side of this high-hardness carbide or a carbon nitrogen
compound, and the matrix is martensite. In the martensite, a
spherical carbide is dispersed and grows. The martensite is
decreased toward a center part of the structural material, and
becomes pearlite or a mixed phase of ferrite and pearlite. In the
present invention, the carbide is dispersed in the martensite
matrix, and the concentration of the M element contained in the
carbide is high on the surface side and low in the center side. In
addition, the texture of martensite becomes fine as the surface
layer.
[0018] The life of sliding units may be greatly improved by forming
the M element-containing carbide.
[0019] In Examples 1 to 4, an SCM420 material was used to describe
the characteristics of the structural material; however, the effect
of improving a lifespan can be obtained by including iron and a
carbide.
[0020] Further, the SCM420 material is defined as having a
composition of C: 0.18 to 0.23, Si: 0.15 to 0.35, Mn: 0.60 to 0.90,
P: 0.030 or less, S: 0.030 or less, Ni: 0.25 or less, Cr: 0.90 to
1.20, and Mo: 0.15 to 0.25, according to the JIS specification, and
in Examples 1 to 4, one alloy steel satisfying the range was
selected and used.
Example 1
[0021] Ti is pulverized in a methanol solvent to form an amorphous
form having a Ti particle diameter of 1 to 50 .mu.m. Methanol
containing 10 wt % of Ti pulverized powder is applied on the
surface of an SCM420 material (0.2 wt % of C, 0.3% of Si, 0.7% of
Mn, 0.1% of Cu, 0.1% of Ni, 1.1% of Cr, 0.2% of Mo, and the balance
of Fe) in the air. The thickness of Ti after application is 200
.mu.m. Methanol as a solvent is evaporated and dried, and inserted
in a carburizing furnace. The inside of the carburizing furnace is
vacuum-exhausted to 1 Pa and then substituted with Ar to clean the
atmosphere in the furnace.
[0022] The partial pressure of H.sub.2O and O.sub.2 is decreased
and whether the time change is minimized is confirmed, and then
heating is started. Heating is performed to 1100.degree. C. at a
rate of 10.degree. C./min and the temperature is maintained for 1
hour, and acetylene (C.sub.2H.sub.2) gas is introduced. Diffusion
is performed from the surface to the inside of the structural
material by the introduction of the acetylene gas. In addition, Ti
on the surface of the structural material and a part of Ti diffused
from the surface of the structural material to the inside are
carbonized. By the carbonization, Ti becomes TiC or TiCN. The
concentration of carbon is higher on the surface than in the
inside. Ti is diffused at a temperature of 1100.degree. C. and
diffused into the grain boundary or into the grains of SCM420.
[0023] A part of diffused Ti forms TiC by the diffusion of carbon
by acetylene introduction. The TiC does not continuously grow in
the .gamma. grain boundary of SCM420 and grows in a granular form.
A total carbon supply amount at 1050.degree. C. is 1.2 wt % in the
SCM420 side in the interface between TiC and SCM420.
[0024] TiC and SCM420 carburized at 1100.degree. C. are reheated to
850.degree. C. and carburized again by acetylene gas. A total
carbon supply amount at 850.degree. C. is smaller than the supply
amount at 1050.degree. C., and the concentration of carbon in the
SCM420 side in the interface between TiC and SCM420 is 1.4 wt %. By
the carburization, a spherical carbide is grown in the grain. After
the carburization, oil quenching and tempering are performed. An
oil temperature is 100.degree. C. and a tempering temperature is
160.degree. C.
[0025] The texture of the structural material manufactured by the
process has a three-layer structure in order from the center to the
surface in a depth direction. A first layer is formed of pearlite,
a second layer is formed of a mixture of martensite and a carbide,
and a third layer is formed of a carbide. The first layer further
includes ferrite, or may further include a mixture of pearlite and
ferrite.
[0026] As compared with the case of carburization quenching and
tempering without forming a carbide, a fatigue life measured by a
rotating bending fatigue test is 20 times longer in the present
Example. A factor causing the high fatigue life is represented
below. TiC or TiCN is formed on the surface at a thickness of 200
.mu.m and the hardness is 1500 to 3000 Hv. A region in which TiC or
TiCN is dispersed in SCM420 is formed between SCM420 and TiC-based
films, thereby increasing close adhesion of TiC or TiCN. Around the
granular TiC, martensite and residual austenite are formed, and the
hardness of martensite is 800 Hv. Granular TiC is not confirmed in
the center of the material, and grown by diffusion of Ti and
carbon, and an amount of TiC is decreased from the surface to a
center part, and as the carbide Fe.sub.3C-based is increased rather
than TiC-based. By having the constitution, a high-hardness TiC or
TiCN-based film which is difficult to be abraded is formed in the
surface layer, and occurrence and propagation of cracks are
suppressed by dispersed high-hardness TiC and fine martensite
around the high-hardness TiC, and Fe.sub.3C and martensite around
the high-hardness TiC.
Example 2
[0027] On the surface of an SCM420 material (0.2 wt % of C, 0.3% of
Si, 0.7% of Mn, 0.1% of Cu, 0.1% of Ni, 1.1% of Cr, 0.2% of Mo, and
the balance of Fe), Al is deposited at a thickness of 50 .mu.m, and
Ti is plated by an exchange reaction of Al and Ti in a solvent
containing Ti acetylacetate and fluorine. The SCM420 material on
which the Ti plated film is grown is inserted to a carburizing
furnace and heated to 1100.degree. C. By heating to 1100.degree.
C., interdiffusion proceeds between Ti and SCM420, and Ti is
diffused into the grain boundary or into the grain of SCM420 and
partially becomes TiC. After Ti is diffused, the temperature is
lowered to 1050.degree. C. and acetylene is introduced. An amount
of introduced acetylene depends on a depth and a texture requiring
hardness (amount of carbide or the like); however, up to 1.2 wt %
of carbon is introduced in a .gamma. region. By the primary
carburization, martensite is partially grown in the vicinity of the
surface layer of SCM420 by nitrogen gas cooling. At this cooling
rate, carbides more than 10 m do not grow in the grain boundary of
SCM420.
[0028] After cooling, heating is performed to a temperature of an
A1 point (codeposition transformation point) or more, and a carbide
is precipitated in the .gamma. crystal grain. From the precipitated
carbide, carbon is diffused to the inside of SCM420, and acetylene
is introduced for suppressing a decrease in a carbon concentration,
thereby diffusing carbon from the surface. Due to the introduction
of acetylene, the carbide is grown, so that a granular carbide is
dispersed and grown in a .gamma. crystal grain boundary or into the
grain, and the particle diameter becomes 0.5 to 2 .mu.m. Here,
carbon is diffused into SCM420 through TiC.
[0029] The texture of the structural material manufactured by the
process has a three-layer structure in order from the center to the
surface in a depth direction. A first layer is formed of pearlite,
a second layer is formed of a mixture of martensite and a carbide,
and a third layer is formed of a carbide. The first layer further
includes ferrite, or may further include a mixture of pearlite and
ferrite.
[0030] When the specimen of the present Example is compared with an
SCM420 specimen in which only martensite is formed in a surface
layer by a rotating bending test at 1200 MPa, the number of
repeated fractures of the specimen of the present Example is 50
times larger. As such, it was confirmed in the fatigue test that
the lifespan is prolonged. The structural material in the present
Example can be applied to a member to which a long lifespan is
essential as a sliding material, and particularly, can be used in
components such as torque transmission components, repeating
operation units, or rotating units for automobiles.
[0031] The constitution required for securing a fatigue life as in
the present Example is represented in the following.
[0032] (1) The structural material has two carbides. A first
carbide is a compound having TiC as a main component, and a second
carbide is a compound mainly having M.sub.3C (wherein M is Ti, Cr,
or Mo), and the structural material has at least two carbides.
[0033] (2) The first carbide is formed as a layered form on the
outermost surface of the structural material and a TiCN-based
compound or a TiN-based compound is included in the portion
thereof.
[0034] (3) The Vickers hardness of VC, VCN, or VN is in a range of
1500 to 3500.
[0035] (4) A thickness of the first carbide is in a range of 10
.mu.m or more and 200 .mu.m or less.
[0036] (5) For securing close adhesion between the second layer and
the third layer, at least one of the constituent elements of the
first carbide has a concentration gradient toward a center part,
while at least one of the constituent elements diffused into the
first layer and the second layer grows as a compound in the grain
boundary or in the grain.
[0037] The above constitution is further described. 1) The carbide
is formed as at least two kinds of a compound having TiC as a main
component and a carbide mainly having M.sub.3C (wherein M is Ti,
Cr, or Mo). Since Ti is applied on the surface and then a heat
treatment process is initiated, Ti is in a large amount on the
surface and in a small amount in the inside of the material. Since
a diffusion distance of Ti is about 100 .mu.m at 1100.degree. C., a
concentration of Ti becomes about 10:1 on the surface of the base
material and in the depth of 100 .mu.m. In the vicinity of the
surface of SCM420 as the base material, the concentration of Ti is
greatly changed, and thus, a volume ratio of TiC and TiCN-based
compounds is decreased. At a depth of 10 .mu.m from the surface of
the base material, M.sub.3C as well as TiC and TiCN, is confirmed.
M in M.sub.3C contains Cr, Mo, or Ti.
[0038] A distance in a depth direction from which the concentration
gradient of Ti is confirmed is in a range of 10 .mu.m to 200 .mu.m.
The concentration gradient can be confirmed by composition analysis
of a section by a scanning electron microscope, Auger analysis, or
the like. By forming the concentration gradient of Ti, close
adhesion between a base material and a TiC-based layered film can
be secured, whereby the fatigue life is improved by 50 times longer
than the case without TiC, by the TiC-based film which is not
peeled off at 2000 MPa.
[0039] Though Ti is applied in the present Example, VC, NbC, MoC,
TaC, or WC is formed by applying V, Nb, Mo, Ta, or W, and a carbide
in which these elements are substituted for M in M.sub.3C is formed
in the inside of the base material.
[0040] 2) A carbide having TiC as a main component is in a layered
form on the outermost surface, some TiCN-based or TiN-based
compounds are confirmed, and a volume ratio in the layered carbide
becomes TiC>TiCN>TiN.
[0041] 3) The Vickers hardness of TiC, TiCN, or TiN is in a range
of 1500 to 3500, and the thickness thereof is 10 .mu.m or more and
200 .mu.m or less. When the Vickers hardness of the layered carbide
is less than 1500, cracks easily occur, so that an effect of
improving a lifespan in the rotating bending test is not
significant. In the rotating bending test at 2000 MPa, when the
case in which only martensite is formed is defined as 1, the case
in which carburization is performed by controlling a determined
concentration or texture without using Ti becomes 5, and the case
in which a TiC-based layered carbide is formed after Ti is applied
and at the same time a Ti diffusion layer is formed for securing
close adhesion becomes 10 to 50. For the dramatic improvement of
lifespan, the Vickers hardness of the layered carbide (such as TiC)
or the layered carbonitride (such as TiCN), or the layered nitride
(such as TiN) needs to be 1500 or more, and for a 50 times longer
lifespan, the Vickers hardness of 2500 to 3000 is preferred.
[0042] 4) For securing close adhesion of the surface layer carbide,
at least one of the constituent elements of the surface layer
carbide has a concentration gradient of 0.5%/.mu.m to 10%/.mu.m for
Ti toward the center part of the base material. In addition, Ti
which is diffused into SCM420 grows as a compound in the grain
boundary or in the grain, and some form cementite referred to as
M.sub.3C.
[0043] In addition to the improved lifespan in the rotating bending
fatigue test, an effect of improving a lifespan is confirmed also
in a pitching test in a roller test. The same effect can be
confirmed for the base material other than SCM420 without
limitation of the composition as long as the base material is
versatile FeC-based alloy steel such as low alloy steel or case
hardening steel.
Example 3
[0044] On a surface of an SCM420 material (0.2 wt % of C, 0.3% of
Si, 0.7% of Mn, 0.1% of Cu, 0.1% of Ni, 1.1% of Cr, 0.2% of Mo, and
the balance of Fe), V powder is mixed at 10 wt % in a methanol
solvent and pulverized by a ball mill. The V powder is in a range
of 0.01 .mu.m to 10 .mu.m, and a slurry formed of methanol and Ti
powder is formed. This slurry is applied on the SCM420 material and
dried, and the thickness of the applied film is 50 .mu.m. The
SCM420 material on which the V applied film is formed is inserted
to a carburizing furnace, and heated to 1200.degree. C. By heating
to 1200.degree. C., counter diffusion between V and SCM420
proceeds, so that V is diffused into the grain boundary or into the
grain of SCM420 and partially becomes VC. After V is diffused, the
temperature is lowered to 1050.degree. C. and acetylene is
introduced. An amount of introduced acetylene depends on a depth
and a texture requiring hardness (amount of carbide or the like);
however, up to 1.2 wt % of carbon is introduced in a .gamma.
region. By the primary carburization, martensite is partially grown
in the vicinity of the surface layer of SCM420 by argon gas
cooling. At this cooling rate, carbides more than 10 .mu.m do not
grow in the grain boundary of SCM420.
[0045] After cooling, heating is maintained at 700.degree. C.,
which is just below an A1 point (codeposition transformation
point), for 1 hour and then heating is performed to a temperature
of the A1 point or more, and a carbide is precipitated in the
.gamma. crystal grain. From the precipitated carbide, carbon is
diffused to the inside of SCM420, and acetylene is introduced for
suppressing a decrease in a carbon concentration, thereby diffusing
carbon from the surface. Due to the introduction of acetylene, the
carbide is grown, so that a granular carbide is dispersed and grown
in a .gamma. crystal grain boundary or into the grain, and the
particle diameter becomes 0.5 to 2 .mu.m. Here, carbon is diffused
into SCM420 through VC.
[0046] The texture of the structural material manufactured by the
process has a three-layer structure in order from the center to the
surface in a depth direction. A first layer is formed of pearlite,
a second layer is formed of a mixture of martensite and a carbide,
and a third layer is formed of a carbide. The first layer further
includes ferrite, or may further include a mixture of pearlite and
ferrite.
[0047] When the specimen of the present Example is compared with an
SCM420 specimen in which only martensite is formed in a surface
layer by a rotating bending test at 1200 MPa, the number of
repeated fractures of the specimen of the present Example is 50
times larger. As such, it was confirmed in the fatigue test that
the lifespan is prolonged. The structural material in the present
Example can be applied to a member to which a long lifespan is
essential as a sliding material, and particularly, can be used in
components such as torque transmission components, repeating
operation units, or rotating units for automobiles.
[0048] The constitution required for securing a fatigue life as in
the present Example is represented in the following.
[0049] (1) The structural material has two carbides. A first
carbide is a compound having TiC as a main component, and a second
carbide is a compound mainly having M.sub.3C (wherein M is Ti, Cr,
or Mo), and the structural material has at least two carbides.
[0050] (2) The first carbide is formed as a layered form on the
outermost surface of the structural material and a VCN-based
compound or a VN-based compound is included in the portion
thereof.
[0051] (3) The Vickers hardness of VC, VCN, or VN is in a range of
1500 to 3500.
[0052] (4) A thickness of the first carbide is in a range of 10
.mu.m or more and 200 .mu.m or less.
[0053] (5) For securing close adhesion between the second layer and
the third layer, at least one of the constituent elements of the
first carbide has a concentration gradient toward a center part,
while at least one of the constituent elements diffused into the
first layer and the second layer grows as a compound in the grain
boundary or in the grain.
[0054] The above constitution is further described. 1) The carbide
is formed as at least two kinds of a compound having VC as a main
component and a carbide mainly having M.sub.3C (wherein M is V, Cr,
or Mo). Since V is applied on the surface and then a heat treatment
process is initiated, V is in a large amount on the surface and in
a small amount in the inside of the material. Since a diffusion
distance of V is about 100 .mu.m at 1100.degree. C., a
concentration of V becomes about 10:1 on the surface of the base
material and in the depth of 100 .mu.m. In the vicinity of the
surface of SCM420 as the base material, the concentration of V is
greatly changed, and thus, a volume ratio of VC and VCN-based
compounds is decreased. At a depth of 10 .mu.m from the surface of
the base material, M.sub.3C as well as VC and VCN, is confirmed. M
in M.sub.3C contains Cr, Mo, or V.
[0055] A distance in a depth direction from which the concentration
gradient of V is confirmed is in a range of 10 .mu.m to 200 .mu.m.
The concentration gradient can be confirmed by composition analysis
of a section by a scanning electron microscope, Auger analysis, or
the like. By forming the concentration gradient of Ti, close
adhesion between a base material and a VC-based layered film can be
secured, whereby the fatigue life is improved by 50 times longer
than the case without VC, by the VC-based film which is not peeled
off at 2000 MPa.
[0056] Though V is applied in the present Example, an MC compound
such as TiC, NbC, MoC, TaC, WC, or (Ti, Cr) C is formed by applying
at least one or more of Ti, Nb, Mo, Ta, W, or Cr, and a carbide in
which these elements are substituted for M in M.sub.3C is formed in
the inside of the base material.
[0057] 2) A carbide having VC as a main component is in a layered
form on the outermost surface, some VCN-based or VN-based compounds
are confirmed, and a volume ratio in the layered carbide becomes
VC>VCN>VN.
[0058] 3) The Vickers hardness of VC, VCN, or VN is in a range of
1500 to 3500, and the thickness thereof is 10 .mu.m or more and 200
.mu.m or less. When the Vickers hardness of the layered carbide is
less than 1500, cracks easily occur, so that an effect of improving
a lifespan in the rotating bending test is not significant. In the
rotating bending test at 2000 MPa, when the case in which only
martensite is formed is defined as 1, the case in which
carburization is performed by controlling a determined
concentration or texture without using V becomes 5, and the case in
which a VC-based layered carbide is formed after V is applied and
at the same time a V diffusion layer is formed for securing close
adhesion becomes 10 to 50. For the dramatic improvement of
lifespan, the Vickers hardness of the layered carbide (such as VC)
or the layered carbonitride (such as VCN), or the layered nitride
(such as VN) needs to be 1500 or more, and for a 50 times longer
lifespan, the Vickers hardness of 2500 to 3000 is preferred.
[0059] 4) For securing close adhesion of the surface layer carbide,
at least one of the constituent elements of the surface layer
carbide has a concentration gradient of 0.5%/.mu.m to 10%/.mu.m for
V toward the center part of the base material. In addition, V which
is diffused into SCM420 grows as a compound in the grain boundary
or in the grain, and some form cementite referred to as
M.sub.3C.
[0060] In addition to the improved lifespan in the rotating bending
fatigue test, an effect of improving a lifespan is confirmed also
in a pitching test in a roller test. The same effect can be
confirmed for the base material other than SCM420 without
limitation of the composition as long as the base material is
versatile FeC-based alloy steel such as low alloy steel or case
hardening steel.
Example 4
[0061] Ti is pulverized in a methanol solvent to form an amorphous
form having a Ti particle diameter of 1 to 50 .mu.m. Methanol
containing 10 wt % of Ti pulverized powder is applied on the
surface of an SCM420 material (0.2 wt % of C, 0.3% of Si, 0.7% of
Mn, 0.1% of Cu, 0.1% of Ni, 1.1% of Cr, 0.2% of Mo, and the balance
of Fe) in the air. The thickness of Ti after application is 20
.mu.m. Methanol as a solvent is evaporated and dried, and inserted
in a carburizing furnace. The inside of the carburizing furnace is
vacuum-exhausted to 1 Pa and then substituted with Ar to clean the
atmosphere in the furnace.
[0062] The partial pressure of H.sub.2O and O.sub.2 is decreased
and whether the time change is minimized is confirmed, and then
heating is started. Heating is performed to 1100.degree. C. at a
rate of 10.degree. C./min and the temperature is maintained for 1
hour, and acetylene (C.sub.2H.sub.2) gas is introduced. By the
introduction of the acetylene gas, Ti and a part of Ti diffused
from the surface of SCM420 are carbonized, and carbon is diffused
into the surface of SCM420. A carbon concentration is surface
Ti>SCM420 surface, and Ti becomes TiC or TiCN. Ti is diffused at
a temperature of 1100.degree. C. and diffused into the grain
boundary or into the grains of SCM420. A part of the diffused Ti
forms TiC or (Ti, Cr, Mo).sub.3C, by diffusion of carbon due to the
introduction of acetylene. The TiC does not continuously grow in
the .gamma. grain boundary of SCM420, and grows in a granular form.
A total carbon supply amount at 1050.degree. C. is 1.2 wt % in the
SCM420 side of the TiC/SCM420 interface.
[0063] TiC/SCM420 carburized at 1100.degree. C. is reheated to
850.degree. C. and carburized again by acetylene gas. A total
carbon supply amount at 850.degree. C. is smaller than the supply
amount at 1050.degree. C., and the concentration of carbon in the
SCM420 side of the TiC/SCM420 interface is 1.4 wt %. By the
carburization, a spherical carbide is grown in the grain. N.sub.2
gas is introduced during the carburization process to perform
nitrification after carburization, and oil quenching and tempering
are performed. An oil temperature is 100.degree. C. and a tempering
temperature is 160.degree. C.
[0064] The texture of the structural material manufactured by the
process is a layered carbide/a mixture of carbide and
martensite/pearlite/a mixed phase of ferrite and pearlite, in a
depth direction from the surface to the center part. A thickness of
layered carbide is 20 .mu.m as confirmed in the photograph of the
section in FIG. 1. The part of a mixed phase of martensite and
spherical carbide is in a range to 40 to 50 .mu.m from the
interface between the layered carbide and the base material. In
this range, a concentration gradient of Ti is confirmed. A carbon
concentration in the surface layer part is 12 to 17 wt %, but a
carbon concentration in the mixed phase of martensite and carbide
is 1.0 to 1.7 wt %, and a concentration of Ti forming the carbide
is decreased. The concentration gradient of Ti is 1 wt %/.mu.m in
the mixed phase of martensite and carbide, and the concentration
gradient is increased to the vicinity of the layered carbide.
[0065] As compared with the case of carburization quenching and
tempering without forming a carbide, a fatigue life measured by a
rotating bending fatigue test is 20 times longer in the present
Example. A factor causing the high fatigue life is represented
below. TiC or TiCN is formed on the surface at a thickness of 20
.mu.m and the hardness is 1550 to 1800 Hv, as shown in FIG. 2. The
TiC or TiCN and TiNO can be confirmed by X-ray diffraction patterns
as shown in FIG. 3, and the region in which carbide is dispersed in
SCM420 is formed between SCM420 and a TiC-based film, thereby
increasing close adhesion of TiC or TiCN. Around the granular TiC,
martensite and residual austenite are formed, and the hardness of
martensite is 750 Hv. Granular TiC is not confirmed in the center
of the material, and grown by diffusion of Ti and carbon, and an
amount of TiC is decreased from the surface to a center part, and
as the carbide, cementite which is Fe.sub.3C-based is increased
rather than TiC-based. By having the constitution, a high-hardness
TiC or TiCN-based film which is difficult to be abraded is formed
in the surface layer, and occurrence and propagation of cracks are
suppressed by dispersed high-hardness TiC and fine martensite
around the high-hardness TiC, and Fe.sub.3C and martensite around
the high-hardness TiC.
Example 5
[0066] The section schematic diagram of the present Example is
shown in FIG. 4. A layer including MC carbide is formed in the
outside of SCM420 as a base material. A mixed phase of M.sub.3C
carbide and martensite is formed in the base material side of MC
carbide (1). In FIG. 4, M.sub.3C carbide (3) having a spherical
shape can be seen and is dispersed in martensite (2). Pearlite is
formed in the further inner side (inner side) of the mixed phase. M
represented by MC carbide contains at least one of Ti, Nb, Ta, V,
W, and Mo, and an M element forming the MC carbide is contained
also in M.sub.3C. Fe in the M.sub.3C carbide has a higher
concentration than Fe in the MC carbide.
[0067] When the specimen of the present Example is compared with an
SCM420 specimen in which only martensite is formed in a surface
layer by a rotating bending test at 1200 MPa, the number of
repeated fractures of the specimen of the present Example is 50
times larger. As such, it was confirmed that the lifespan in the
fatigue test was prolonged, and thus, the treatment process and the
material constitution in the present Example can be applied to a
member to which a long lifespan is essential as a sliding material,
and particularly, can be used in components such as torque
transmission components, repeating operation units, or rotating
units for automobiles.
[0068] The improvement of the fatigue life can be achieved by
having the texture constitution of FIG. 4 also in an iron
carbon-based structural material other than SCM420.
Example 6
[0069] The results of the fatigue test are shown in FIG. 5. Upon
comparison with gas carburization, a surface pressure at the same
fatigue cycle is increased 1.3 times. In addition, at the same
surface pressure, the lifespan is increased 10 times or more. As
such, it was confirmed that the lifespan in the fatigue test was
prolonged, and thus, the treatment process and the material
constitution can be applied to a member to which a long lifespan is
essential as a sliding material, and particularly, can be used in
components such as torque transmission components, repeating
operation units, or rotating units for automobiles.
[0070] In the present embodiment, MC is present in the outermost
surface, and M.sub.3C is present in martensite. Since MC has a
higher hardness than M.sub.3C, and has excellent abrasion
resistance, MC is needed in the outermost surface layer. A carbon
concentration is lowered in the inside to disperse M.sub.3C.
REFERENCE SIGNS LIST
[0071] 1 MC carbide [0072] 2 Martensite [0073] 3 M.sub.3C carbide
[0074] 4 Pearlite
* * * * *