U.S. patent application number 11/296566 was filed with the patent office on 2006-06-22 for carburized component and method of manufacturing the same.
This patent application is currently assigned to Daido Stell Co., Ltd. & Honda Moto Co., Ltd.. Invention is credited to Atsushi Hattori, Takashi Kano, Koki Mizuno, Tomoko Serikawa.
Application Number | 20060130935 11/296566 |
Document ID | / |
Family ID | 36590721 |
Filed Date | 2006-06-22 |
United States Patent
Application |
20060130935 |
Kind Code |
A1 |
Hattori; Atsushi ; et
al. |
June 22, 2006 |
Carburized component and method of manufacturing the same
Abstract
This invention aims to provide a carburized component realizing
a larger strength for power transmission components such as gears,
and a method of manufacturing the same. The carburized component of
this invention, aimed at realizing the object, consists essentially
of, in % by mass and both ends inclusive, C: 0.1-0.30%, Si:
0.80-1.50%, Mn: 0.30-1.20%, Cr: 2.0-5.5%, and the balance of Fe and
inevitable impurities; has a mean C concentration over the range
from the surface of the steel to a depth of 0.2 mm after vacuum
carburization of 1.2% or more and 3.0% or less, and has a ratio of
a carbide area over the range from the surface to a depth of 50
.mu.m of 15% or more and 60% or less, has the carbide precipitated
in a finely dispersed manner so that the carbide having a grain
size of 10 .mu.m or less accounts for 90% or more of the entire
portion, and has a depth of a grain boundary oxide layer below the
surface of 1 .mu.m or less.
Inventors: |
Hattori; Atsushi;
(Nagoya-shi, JP) ; Kano; Takashi; (Nagoya-shi,
JP) ; Serikawa; Tomoko; (Wako-shi, JP) ;
Mizuno; Koki; (Wako-shi, JP) |
Correspondence
Address: |
ARENT FOX PLLC
1050 CONNECTICUT AVENUE, N.W.
SUITE 400
WASHINGTON
DC
20036
US
|
Assignee: |
Daido Stell Co., Ltd. & Honda
Moto Co., Ltd.
|
Family ID: |
36590721 |
Appl. No.: |
11/296566 |
Filed: |
December 8, 2005 |
Current U.S.
Class: |
148/233 ;
148/319 |
Current CPC
Class: |
C23C 8/80 20130101; C23C
8/22 20130101 |
Class at
Publication: |
148/233 ;
148/319 |
International
Class: |
C23C 8/22 20060101
C23C008/22 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 10, 2004 |
JP |
2004-358617 |
Claims
1. A carburized component consisting essentially of, in % by mass
and both ends inclusive, C: 0.1-0.30%, Si: 0.80-1.50%, Mn:
0.30-1.20%, Cr: 2.0-5.5%, and the balance of Fe and inevitable
impurities; having a mean C concentration over the range from the
surface of the steel to a depth of 0.2 mm after vacuum
carburization of 1.2% or more and 3.0% or less, having a ratio of
carbide area over the range from the surface to a depth of 50 .mu.m
of 15% or more and 60% or less, having the carbide precipitated in
a finely dispersed manner so that the carbide having a grain size
of 10 .mu.m or less accounts for 90% or more of the entire portion,
and having a depth of a grain boundary oxide layer of 1 .mu.m or
less.
2. The carburized component as claimed in claim 1, further
comprising either or both of Mo: 0.2 to 1.0% and V: 0.2 to
1.0%.
3. A method of manufacturing a carburized component described in
claim 1, subjecting steel containing the above-described steel
ingredients to a primary carburization at a temperature of Acm or
above, then rapidly cooling the steel to as low as point A1 or
below, and then subjecting the steel to a secondary carburization
at a temperature of point A1 or above and or Acm below.
4. The method of manufacturing a carburized component as claimed in
claim 3, wherein the carburization is carried out by vacuum
carburization at 1,000 Pa or below.
5. The method of manufacturing a carburized component as claimed in
claim 3, further subjecting the steel to peening after the
secondary carburization.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a carburized component and a
method of manufacturing the same.
BACKGROUND ART
[0002] Gears used as power transmission components for automobiles
and so forth are components suffering from dedendum fractures which
occur at the dedendum where bending stress happens, and from
slip-induced fracture (pitting) which occurs in the vicinity of the
pitch point. A technique of carburizing the surface of the
component has, therefore, widely been used for the purpose of
fulfilling characteristics enough for withstanding harsh
conditions, and further improvement has been made by combining
various materials and heat treatments.
[0003] Particularly in recent years, a successful development has
been made on a material capable of suppressing growth of grain
boundary oxide layer and abnormally carburized layer during
carburization, which are understood as being harmful to dedendum
fracture. Another achievement has been made on improvement in the
strength typically by shot peening.
[0004] On the other hand, pitting has also extensively been
investigated, and it has been found out that prevention of
softening of the material is effective to improve the strength.
Gears cause slippage on the tooth surface thereof, and the
repetitive contact generates heat at the portion just under the
tooth surface. Temperature in this state is known to fall in the
range of about 200 to 300.degree. C., and the heat generated herein
supposedly softens the material and consequently results in pitting
fracture. It is therefore believed that prevention of softening in
a temperature range of about 200 to 300.degree. C. is effective for
improving the pitting fracture, and development has been made on
materials added with Si, Cr, Mo and so forth as alloy elements
excellent in the softening resistance in this temperature
range.
[0005] [Patent Document] Japanese Laid-Open Patent Publication
"Tokkaihei" No. 6-158266
[0006] The gear has, however, has been demanded to have a larger
hardness with the recent increases in the output of automobiles and
so forth, but the present situation is that the above-described
material is insufficient for fulfilling the requirements.
[0007] This invention was conceived after considering the
above-described situation, and an object thereof is to provide a
carburized component realizing higher strength for power
transmission components such as gears, and a method of fabricating
said components.
SUMMARY OF THE INVENTION
[0008] Aiming at solving the aforementioned problems, a carburized
component consisting essentially of, in percentages by mass and
both ends inclusive, C: 0.1-0.30%, Si: 0.80-1.50%, Mn: 0.30-1.20%,
Cr: 2.0-5.5%, and the balance between Fe and inevitable impurities;
[0009] has a mean C concentration over the range from the surface
of steel to a depth of 0.2 mm after vacuum carburization of 1.2% or
more and 3.0% or less, has a ratio of carbide area over the range
from the surface to a depth of 50 .mu.m of 15% or more and 60% or
less, has the carbide precipitated in a finely dispersed manner so
that the carbide having a grain size of 10 .mu.m or less accounts
for 90% of the entire portion, and has a depth of a grain boundary
oxide layer below the surface of 1 .mu.m or less.
[0010] It is also allowable to further add either or both of Mo:
0.2 to 1.0% and V: 0.2 to 1.0%.
[0011] This invention has basic features as described below. That
is, a large amount of fine carbide grains are allowed to
precipitate in the surficial portion of the component by
high-concentration vacuum carburization, and to substantially
exclude the surficial grain boundary oxide layer, to thereby raise
the surface hardness and strength. In addition, the temper
softening resistance in the temperature range from about 200 to
300.degree. C. is enhanced by introducing a large amount of Si,
which is realized by the vacuum carburization, and thereby a
desirable level of surface fatigue strength can be obtained. These
features can be obtained only under the appropriately-adjusted
ingredients and conditions as detailed below.
[0012] C: 0.10 to 0.30%
[0013] C is an essential element for ensuring a necessary level of
strength for the component, and is necessary to contain an amount
of 0.10% or more. On the other hand, an excessively large content
thereof increases the hardness of the material, thus degrading the
machinability, and thereby making the machining of the component
difficult. The upper limit is therefore adjusted to 0.30%.
[0014] Si: 0.80 to 1.50%
[0015] Si is an element to be contained as a deoxidizing element
acting in the process of melting and plays an important role in
this invention. The element dissolves into the solid matrix to
thereby raise the temper softening resistance, so that a high level
of surface fatigue strength can be obtained. The element can also
suppress growth of coarse carbide grains, because it shows only a
small solid solubility into the carbide and raises the Si
concentration in the base metal. Moreover, under precipitation of a
large amount of carbide, Si showing only a small solid solubility
into the carbide concentrates in the matrix, and further improves
the temper softening resistance of the matrix. The element is
necessarily contained to an amount of 0.80% or more in order to
obtain this effect. On the other hand, an excessive content of the
element inhibits precipitation and the carburization surface
reaction of the carbide which thereby distinctively degrades the
carburization property, and also degrades the ductility, which
thereby makes cracking more likely to occur in the process of
plastic working. The upper limit of the content is therefore
limited to 1.50%.
[0016] Si is an element promoting oxidation of the grain boundary
in the process of general gas carburization, and the grain boundary
oxidation layer is causative of lowering the impact strength and
fatigue strength of dedendum. The gas carburization therefore
cannot add a large amount of Si, whereas the vacuum carburization
as described in the above can clear the problem of grain boundary
oxidation, and make it possible to obtain a high-Si-content
component.
[0017] Mn: 0.30 to 1.20%
[0018] Mn is an element to be contained as a deoxidizing element
acting in the process of melting, and has an effect of improving
the hardening property, so that it is necessary to contain an
amount of 0.30% or more. In this invention, elements having an
effect of improving the hardening property, such as Cr, are to be
concomitantly contained, wherein the elements such as Cr, capable
of forming the carbide, may sometimes result in only an
insufficient hardening property even under a raised Cr content or
the like, depending on carbide content. It is therefore effective
to adjust the Mn content in order to obtain a necessary level of
hardening property. On the other hand, an excessive content
degrades the machinability due to an increase in the hardness of
the material, thus the upper limit is adjusted to 1.20%.
[0019] Cr: 2.0 to 5.5%
[0020] Cr is an element playing an important role in this
invention. This is necessary to contain an amount of 2.0% or more,
as a carbide-forming element and as an element improving the
hardening property. On the other hand, an excessive content of the
element degrades the machinability due to increased hardness of the
material, and makes a network-structured carbide more likely to be
generated in the grain boundary. The upper limit of the content is
therefore limited to 5.5%.
[0021] Mo: 0.2 to 1.0%
[0022] Mo binds with C, similarly to Cr, to produce the carbide,
and has an effect of improving the pitting strength by raising the
softening resistance over the temperature range from 200.degree. C.
to 300.degree. C. The element is preferably contained to an amount
of 0.2% or more, for the purpose of obtaining these effects. On the
other hand, an excessive content of the element degrades the
machinability due to an increase in hardness of the material, and
increases the material cost. The upper limit of the content is,
therefore, preferably limited to 1.0%.
[0023] V: 0.2 to 1.0%
[0024] V binds with C, similarly to Cr and Mo, to produce the
carbide, and has an effect of improving the pitting strength by
raising the softening resistance, through production of an MC-type
carbide. The element is preferably contained to an amount of 0.2%
or more, for the purpose of obtaining these effects. On the other
hand, an excessive content of the element degrades the
machinability due to an increase in hardness of the material. The
upper limit of the content is, therefore, preferably limited to
1.0%.
[0025] Carburization: Vacuum Carburization (at 1,000 Pa or
Below)
[0026] The carburized component of this invention is subjected to
vacuum carburization. The vacuum carburization makes it possible to
decrease the growth of the grain boundary oxide layer, and is
therefore successful in raising the strength of the carburized
component.
[0027] As described in the above, Si is added as an essential
ingredient. Si is an element promoting the grain boundary oxidation
in the process of the general gas carburization, and such grain
boundary oxidation is causative of reducing the impact strength and
fatigue strength of the dedendum. It is, therefore, extremely
difficult for the general gas carburization to achieve a large Si
content. Whereas the vacuum carburization can, however, suppress
formation of the grain boundary oxide layer, and can readily
realize a high Si content.
[0028] Depth of Grain Boundary Oxide Layer: 1 .mu.m or Less
[0029] The grain boundary oxide layer causes lowering in the
fatigue strength and anti-pitting strength, wherein the degree of
the lowering becomes larger as the depth increases. For the
carburized component of this invention, the depth of grain boundary
layer from the surface of the steel after the vacuum carburization
is adjusted to 1 .mu.m or less.
[0030] Mean C Concentration Up to Depth of 0.2 mm from the Surface:
1.2% or more and 3.0% or Less
[0031] The general carburization is normally carried out as an
eutectic carburization of the surface of steel, targeted at an
eutectic C content of 0.8%. In contrast, this invention is aimed at
improving the anti-pitting property through precipitation of the
carbide in the surficial layer of the steel to thereby enhance the
softening resistance, so that it is necessary to contain C to an
amount of the eutectic C content (0.8%) or more. In addition, the
surface fatigue strength cannot be improved even if the carbide is
allowed to precipitate, unless the carbide is obtained with a
content necessary for improving the softening resistance, so that
it is also necessary to make C contained to an amount sufficient
enough for the improvement.
[0032] From these points of view, the mean C concentration over the
range from the surface of steel to a depth of 0.2 mm (also referred
to as surface C concentration, hereinafter) is adjusted to 1.2% or
more. The reason why the range is defined from the surface of the
steel to a depth of 0.2 mm is that the hardness in such range is
important from the viewpoint of the pitting resistance. On the
other hand, an excessive content results in production of large
carbide grains, and causes insufficient hardening property of the
base material, thereby degrading the strength. The upper limit of
the surface C concentration is therefore limited to 3.0%.
[0033] Ratio of Carbide Area Over the Range from the Surface to a
Depth of 50 .mu.m: 15% or More and 60% or Less
[0034] Precipitation of the carbide raises the surface hardness,
improves the softening resistance over the temperature range from
200.degree. C. to 300.degree. C., and improves the anti-pitting
resistance. A ratio of carbide area over the range from the surface
to a depth of 50 .mu.m of less than 15%, however, cannot fully
improve the softening resistance, and cannot obtain a sufficient
effect of improving the strength. On the other hand, the ratio of
carbide area exceeding 60% can improve the softening resistance,
but lowers the surface fatigue and bending fatigue strength,
because the carbide of a larger grain size is more likely to
precipitate along the grain boundary in a network manner. An
exemplary observation of the obtained carbide is shown in FIG.
4.
[0035] Carbide Precipitated in a Finely Dispersed Manner so that
the Carbide Having a Grain Size of 10 .mu.m or Less Accounts for
90% or More of the Entire Portion.
[0036] The carbide is a hard grain, and may serve as a starting
point of fatigue fracture, similarly to non-metallic inclusions
such as Al oxide and Ti nitride. A smaller carbide is therefore
more preferable, wherein the grain size of which is necessarily
controlled to as small as 10 .mu.m or below, so as not to allow the
carbide to exist as the starting point of fatigue fracture. It is
therefore controlled so that the carbide precipitates in a finely
dispersed manner, so that the carbide having a grain size of 10
.mu.m or less accounts for 90% or more of the entire portion. An
exemplary observation of the obtained carbide is shown in FIG.
4.
[0037] Aiming at manufacturing the above-described carburized
component, a method of manufacturing a carburized component of this
invention subjects the steel containing the above-described steel
ingredients to a primary carburization at a temperature of Acm or
above, then rapidly cools the steel to as low as point A1 or below,
and then subjects the steel to a secondary carburization at a
temperature of point A1 or above and Acm or below. More
specifically, as shown in FIGS. 1A and 1B, the primary
carburization is carried out so as not to precipitate the carbide,
at a temperature of as high as Acm or above, allowing a large solid
solubility limit of C and allowing no carbide to precipitate
(between points "a" and "b"). Next, the steel is rapidly cooled so
as to dissolve C into a solid super-saturated manner (between
points "b" and "c"). Thereafter, the steel is again heated to as
high as point A1 or above, to thereby allow fine carbide nuclei to
uniformly precipitate from the base material super-saturated with C
(between points "d" and "e", see the upper drawing in FIG. 2), and
the steel is further subjected to a secondary carburization so as
to grow the nuclei (between points "e" and "f", see the lower
drawing in FIG. 2). Such multi-stage carburization can realize a
high-C-concentration carburization with a controlled fine
dispersion of the carbide, without allowing the network-structured
carbide to precipitate. In contrast to this, as shown in FIG. 3,
the carburization carried out to as far as the high-C-concentration
region before point Acm makes the network-structured coarse carbide
very likely to produce. The carburization therein is carried out by
vacuum carburization (at 1,000 Pa or below) as described in the
above.
[0038] It is also allowable to subject the steel after the
secondary carburization to peening if necessary, to thereby further
improve the strength. Shot peening (S/P) and water jet peening
(W/J), for example, are applicable to the peening.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] FIGS. 1A and 1B are drawings explaining carburization
involved in the method of manufacturing a carburized component of
this invention;
[0040] FIG. 2 shows a schematic sectional view and a drawing of an
observed section of steel during the carburization shown in FIG.
1;
[0041] FIG. 3 shows a drawing explaining an exemplary carburization
different from this invention, and a drawing of an observed
section; and
[0042] FIG. 4 is a drawing of an observed section of the carburized
component of this invention.
EXAMPLES
[0043] The following paragraphs will describe tests carried out for
confirming the effects of this invention.
[0044] First, each of the steels having chemical compositions
listed in Table 1 was melted in a 150-kg high-frequency vacuum
induction furnace. The obtained steel ingot was rolled or hot
forged so as to produce a 90-mm-diameter round rod, or further
hot-forged, if necessary, so as to obtain steel bar shape having a
diameter of 22 to 32 mm, which was used as a test piece.
[0045] In the compositions of comparative examples listed in Table
1, those departing from the compositional ranges specified by this
invention are indicated by a downward arrow (.dwnarw.), for those
short of the lower limits, an upward arrow ( ), for those exceeding
the upper limits. TABLE-US-00001 TABLE 1 C Si Mn Cr Mo V Remarks
Example 1 0.18 0.98 0.63 2.39 0.00 0.00 Example 2 0.18 0.80 0.50
2.66 0.00 0.00 Example 3 0.19 1.02 0.52 2.52 0.00 0.00 Example 4
0.18 0.97 0.55 3.22 0.00 0.00 Example 5 0.18 1.48 0.55 2.58 0.00
0.00 Example 6 0.19 1.05 0.55 2.12 0.00 0.00 Example 7 0.19 1.08
0.34 2.49 0.00 0.00 Example 8 0.20 1.12 0.35 4.99 0.00 0.00 Example
9 0.19 0.97 0.52 2.50 0.60 0.00 Example 10 0.18 0.97 0.52 2.66 0.00
0.30 Comparative .dwnarw. 0.08 0.96 0.62 2.45 0.00 0.00 Example 1
Comparative .uparw. 0.37 0.97 0.61 2.42 0.00 0.00 Example 2
Comparative 0.18 .dwnarw. 0.40 0.49 2.92 0.00 0.00 Example 3
Comparative 0.19 .uparw. 2.10 0.51 2.44 0.00 0.00 Example 4
Comparative 0.18 1.10 .dwnarw. 0.10 2.16 0.00 0.00 Example 5
Comparative 0.19 0.98 .uparw. 1.74 2.04 0.00 0.00 Example 6
Comparative 0.20 1.02 0.32 .dwnarw. 1.10 0.00 0.00 Example 7
Comparative 0.20 1.13 0.32 .uparw. 6.02 0.00 0.00 Example 8
Comparative 0.20 1.00 0.55 2.54 .uparw. 1.50 0.00 Example 9
Comparative 0.19 0.97 0.53 2.71 0.00 .uparw. 1.50 Example 10
Comparative 0.20 0.22 0.89 1.12 0.00 0.00 JIS-SCR420 Example 11
[0046] The obtained test pieces were subjected to the following
evaluations.
(1) Evaluation of Manufacturability
[0047] The manufacturability was evaluated by measuring the
hardness after annealing.
[0048] A round test piece rod of 32 mm in diameter and 100 mm in
length was subjected to annealing at 920.degree. C. for 1 hour,
further annealed at 760.degree. C. for 5 hours, and the hardness at
the position of R/2 on the transverse section was measured. The
measurement of hardness conforms to JIS Z 2245 (B-scale), with a
target value of HRB90 or smaller
(2) Evaluation of Basic Characteristics of Carburization
(2-1) Method of Carburization
[0049] A round test piece rod 10 mm in diameter and 100 mm in
length was fabricated, as a test piece for carburization property,
from a forged steel bar 22 mm in diameter. The carburization was
carried out in a vacuum carburization furnace, using propane as the
carburization gas, wherein the surface C concentration was
controlled by adjusting flow rate of propane gas, diffusion time,
and carburization temperature. The carburization was carried out at
two levels of conditions so as to achieve a surface C concentration
of 1.5% and 2.5%, respectively.
[0050] As for Example 3, the carburization was carried over the
surface range of C concentration from 0.8 to 3.2%, in order to
investigate influences of the surface C concentration.
[0051] The carburization conditions are as follows.
[0052] Primary Carburization
[0053] The test piece was carburized at 1,100.degree. C. for 70
minutes so as to adjust the C concentration to the topmost surface
to about 1.2%, and then rapidly cooled by cooling gas to a
temperature range as low as 500.degree. C. or below, to thereby
allow C to intrude into the steel to at a high concentration range
so as not to be causative of precipitation of the carbide.
[0054] Secondary Carburization
[0055] The test piece was subjected to the precipitation treatment
by keeping it in the temperature range from 850.degree. C. to
900.degree. C., depending on the target carburization
concentration, further carburized in the temperature range from
850.degree. C. to 1,000.degree. C. for 60 to 90 minutes depending
on the target C concentration, and was hardened by immersing it
into an oil bath kept at 130.degree. C. After the hardening, the
test place was annealed at 180.degree. C. for 120 minutes.
(2-2) Items of Evaluation
[0056] The following paragraphs will describe items of evaluation.
Results of the evaluation are listed in Table 2. Results of Example
3 obtained by varying the surface C concentration are listed in
Table 3.
[0057] Surface C Concentration
[0058] After the carburization, C concentration was measured using
a grinding chip obtained from the surface to a depth of 0.2 mm of
the treated test piece.
[0059] Ratio of Carbide Area
[0060] The transverse section of the carburized and annealed test
piece rod was polished, corroded with picral, to a portion of a
depth of 50 .mu.m from the topmost surface was photographed under a
SEM (at a 3,000.times. magnification of observation), and the ratio
of area was measured by image analysis.
[0061] Size of Carbide
[0062] The test piece was observed under the same conditions as
described in the above, and the area ratio occupied by the carbide
grain sized 10 .mu.m or less was measured.
[0063] Presence or Absence of Network-Structured Carbide
[0064] The test piece was observed under the same conditions as
those described in the above, and presence or absence of the
network-structured carbide was investigated.
[0065] Presence or Absence of Incompletely-Hardened Structure
[0066] The transverse section of the carburized, annealed test
piece rod was polished, corroded with nital, to a portion of a
depth of 50 .mu.m from the topmost surface was photographed under
an optical microscope, and presence or absence of the
incompletely-hardened structure was investigated.
[0067] Depth of Grain Boundary Oxide Layer
[0068] The transverse section of the carburized and annealed rod
test piece was polished, the resultant surface in an uncorroded
state was observed under an optical microscope, and the depth of
the layer appearing as black along the grain boundary at the
topmost surface was measured.
[0069] Temper Softening Resistance
[0070] The carburized and annealed test piece rod was further
annealed at 300.degree. C. for 180 minutes, the transverse section
was polished, and the hardness at a depth of 50 .mu.m from the
topmost surface was measured. The hardness herein conforms to JIS Z
2244 (Hv0.3), wherein a value of Hv750 or above is considered as an
index ensuring a sufficient effect of improving the strength
(.gtoreq.30%: in comparison with SCR420 gas eutectic carburized
steel). TABLE-US-00002 TABLE 2 Carburization (1) [targeted at 1.5%
C.] Ratio of Ratio of area Incompletely- Depth of grain Anneal
Surface C carbide of grains Network- hardened boundary oxide
300.degree. C. temper hardness concentration area .ltoreq.10 .mu.m
structured carbide structure layer hardness Example 1 83 1.62 21%
100% no no no 775 Example 2 81 1.62 20% 93% no no no 751 Example 3
84 1.51 19% 100% no no no 779 Example 4 84 1.53 21% 100% no no no
787 Example 5 89 1.41 16% 100% no no no 760 Example 6 84 1.65 21%
100% no no no 773 Example 7 83 1.61 23% 99% no no no 780 Example 8
88 1.75 29% 94% no no no 807 Example 9 90 1.65 24% 100% no no no
802 Example 10 90 1.61 23% 100% no no no 800 Comparative 79 1.59
22% 100% no no no 776 Example 1 Comparative 92 1.61 22% 100% no no
no 776 Example 2 Comparative 76 1.65 22% 45% yes no no 729 Example
3 Comparative 97 1.31 13% 100% no no no 734 Example 4 Comparative
80 1.62 20% 100% no no no 735 Example 5 Comparative 93 1.64 23%
100% no no no 779 Example 6 Comparative 80 1.51 20% 100% no yes no
743 Example 7 Comparative 91 1.83 32% 91% no no no 814 Example 8
Comparative 99 1.69 26% 94% no no no 814 Example 9 Comparative 100
1.68 25% 100% no no no 816 Example 10 Carburization (2) [targeted
at 2.5% C.] Ratio of Ratio of area Network- Incompletely- Depth of
grain 300.degree. C. Surface C carbide of grains structured
hardened boundary oxide temper concentration area .ltoreq.10 .mu.m
carbide structure layer hardness Remarks Example 1 2.53 40% 97% no
no no 824 Example 2 2.52 52% 93% no no no 835 Example 3 2.45 49%
95% no no no 843 Example 4 2.64 55% 98% no no no 863 Example 5 2.31
35% 94% no no no 828 Example 6 2.47 37% 97% no no no 817 Example 7
2.74 52% 94% no no no 845 Example 8 2.83 58% 92% no no no 875
Example 9 2.56 52% 98% no no no 861 Example 10 2.58 50% 99% no no
no 845 Comparative 2.48 41% 98% no no no 825 Example 1 Comparative
2.51 40% 95% no no no 825 poor machinability Example 2 Comparative
2.61 43% 36% yes no no 825 carbide shape control Example 3 failure,
poor strength Comparative 1.92 27% 99% no no no 800 poor
machinability, Example 4 poor carburization, poor strength
Comparative 2.46 38% 98% no yes no 778 poor hardening, Example 5
poor strength Comparative 2.51 36% 98% no no no 814 poor
machinability Example 6 Comparative 2.44 34% 95% no yes no 796 poor
hardening, Example 7 poor strength Comparative 2.98 66% 81% yes no
no 892 poor machinability, Example 8 carbide shape control failure
Comparative 2.61 56% 95% no no no 875 poor machinability Example 9
Comparative 2.56 54% 98% no no no 860 poor machinability Example
10
[0071] It is known from Table 2 that all of the Examples from 1 to
10 raise no problem in the manufacturability (anneal
hardness.ltoreq.HRB90), show no incomplete-hardened structure,
network-structured carbide and grain boundary oxidation causative
of degradation in the hardness, and give sufficient levels of
temper hardness (.gtoreq.750Hv) at 300.degree. C. In contrast,
Comparative Examples 2, 4, 6 and 8 to 10 show large hardness after
annealing, and raise a problem in the manufacturability.
Comparative Examples 3 and 8 show only insufficient control levels
of fine dispersion of the carbide due to a low Si and a large Cr
content, and production of the network-structured carbide and other
coarse carbide may undesirably degrade the strength. Comparative
Example 4, too large in the Si content, raises a problem in the
manufacturability, inhibits the carburization property, and cannot
allow the carburization to proceed to a sufficient degree.
Comparative Examples 5 and 7, low in the Cr and Mn contents, which
give only poor levels of hardening property, show the
incompletely-hardened structure, and may undesirably degrade the
strength. TABLE-US-00003 TABLE 3 Surface C Ratio of Ratio of area
Network- C Si Mn Cr Mo V concentration carbide area of grains
.ltoreq.10 .mu.m structured carbide Example 3 0.19 1.02 0.52 2.52
0.00 0.00 .dwnarw. 0.80 .dwnarw. 0% .dwnarw. 0% no .dwnarw. 1.11
.dwnarw. 12% 100% no 1.51 19% 100% no 1.99 34% 100% no 2.45 47% 95%
no .uparw. 3.15 .uparw. 67% .dwnarw. 64% yes Incompletely- Depth of
grain 300.degree. C. temper Ratio of surface hardened structure
boundary oxide layer hardness fatigue strength Remarks Example 3 no
no 644 1.09 poor strength no no 734 1.23 poor strength no no 779
1.43 no no 816 1.45 no no 843 1.55 no no 894 1.28 carbide shape
Failure, poor strength
[0072] It is known from Table 3 that the carburization targeted at
a surface C concentration of less than 1.2% is successful in
improving the surface fatigue strength, but unsuccessful in
obtaining a sufficient effect for improving the strength
(.gtoreq.30%). On the other hand, the carburization targeted at a
surface concentration exceeding 3.0% is successful in obtaining a
sufficient level of 300.degree. C. temper hardness, but shows the
network-structured carbide and coarse carbide, and is unsuccessful
in obtaining a sufficient effect for improving the strength.
(3) Evaluation of Surface Fatigue Strength
[0073] The surface fatigue strength was evaluated using a roller
pitting tester, wherein the surface fatigue strength was defined as
the pressure on the load surface not causative of pitting over 107
cycles of the test. More specifically, a 32-mm-diameter round rod
was softened by keeping it heated at 950.degree. C., followed by
gradual cooling, and was then machined to fabricate a roller
pitting test piece having a diameter of test portion of 26 mm. A
roller correspondent to the test piece was configured using SUJ2,
and subjected to quench-and-temper so as to attain a hardness of
HRC61. The radii of curvature of large rollers are 150R and
700R.
[0074] The carburization was simultaneously carried out with the
carburization carried out for basic evaluation of the inventive
steel. A portion of the roller pitting test piece after the
carburization was tempered at 300.degree. C. for 3 hours, and
evaluation was also made on the carbon concentration, ratio of the
carbide area, the maximum carbide size and temper hardness. The
surface fatigue strength of each material was expressed by an
index, assuming the surface fatigue strength of
gas-eutectic-carburized JIS-SCR420 material is 1.0. A sufficient
effect of improving the strength by 30% or more as compared with
gas-eutectic-carburized JIS-SCR420H steel was targeted.
[0075] Results of the evaluation are listed in Table 4.
TABLE-US-00004 TABLE 4 Roller pitting test Incompletely- Depth of
grain Surface C Ratio of Ratio of area Network- hardened boundary
oxide concentration carbide area of grains .ltoreq.10 .mu.m
structured carbide structure layer Example 1 2.04 33% 100% no no no
Example 2 2.13 30% 98% no no no Example 3 1.99 34% 100% no no no
Example 4 2.08 40% 100% no no no Example 5 1.94 31% 97% no no no
Example 6 2.00 32% 98% no no no Example 7 2.34 33% 94% no no no
Example 8 2.49 46% 94% no no no Example 9 2.15 34% 100% no no no
Example 10 2.13 37% 99% no no no Comparative Example 1 2.01 32%
100% no no no Comparative Example 2 2.03 33% 100% no no no
Comparative Example 3 2.20 29% 69% yes no no Comparative Example 4
1.83 24% 100% no no no Comparative Example 5 2.01 30% 96% no yes no
Comparative Example 6 1.95 29% 98% no no no Comparative Example 7
2.10 30% 97% no yes no Comparative Example 8 2.61 54% 88% yes no no
Comparative Example 9 2.19 39% 98% no no no Comparative Example 10
2.12 42% 99% no no no Comparative Example 11 0.78 0% 0% 0% no 8
.mu.m Roller pitting test 300.degree. C. temper Surface fatigue
hardness strength Remarks Example 1 805 1.44 Example 2 798 1.40
Example 3 816 1.45 Example 4 825 1.51 Example 5 800 1.49 Example 6
804 1.46 Example 7 813 1.43 Example 8 841 1.54 Example 9 832 1.52
Example 10 823 1.49 Comparative Example 1 806 0.93 poor strength of
core portion Comparative Example 2 805 1.46 poor machinability
Comparative Example 3 782 1.17 carbide shape control failure, poor
strength Comparative Example 4 781 1.47 poor machnability, poor
carburization Comparative Example 5 766 1.15 poor hardening, poor
strength Comparative Example 6 796 1.41 poor machinability
Comparative Example 7 769 1.18 poor hardening, poor strength
Comparative Example 8 861 1.29 poor machinability, carbide shape
control failure Comparative Example 9 845 1.57 poor machinability
Comparative Example 10 838 1.53 poor machinability Comparative
Example 11 620 1.00 JIS-SCR20 (base steel)-gas carburizaton
[0076] It is known from Table 4 that all of the Examples from 1 to
10 are successful in obtaining sufficient levels (.gtoreq.30%) of
improvement in the strength. In contrast, Comparative Example 1
show only a low strength due to poor strength of the core portion.
Comparative Examples 2, 4, 6, 9 and 10 are successful in
sufficiently improving the strength, but raise a problem in the
manufacturability. Comparative Examples 3 and 8 show growth of the
network-structured carbide and other coarse carbide, and fail in
obtaining sufficient levels of effect for improving the strength.
Comparative Examples 5 and 7, having low contents of Cr and Mn,
show only poor hardening properties as indicated by the
incompletely-hardened structure, and fail in obtaining sufficient
levels of effect for improving the strength.
[0077] As proven by the above-described tests, the carburized
component of this invention was confirmed as having a large amount
of fine carbide grains precipitated in the surficial portion
thereof, as being substantially free from the grain boundary oxide
layer in the surficial portion, and being excellent in the areas of
surface hardness and strength.
[0078] FIG. 1A
primary carburization
secondary carburization
[0079] FIG. 1B, FIG. 3
.gamma. single phase
[0080] FIG. 2
between d-e
precipitation of fine carbide grains
between e-f
growth of carbide grains
* * * * *