U.S. patent application number 14/378553 was filed with the patent office on 2015-01-22 for steel for nitrocarburizing and nitrocarburized component using the steel as material.
The applicant listed for this patent is JFE Bars & Shapes Corporation, JFE Steel Corporation. Invention is credited to Keisuke Ando, Takashi Iwamoto, Shinji Mitao, Yasuhiro Omori, Kunikazu Tomita, Kiyoshi Uwai.
Application Number | 20150020926 14/378553 |
Document ID | / |
Family ID | 48983924 |
Filed Date | 2015-01-22 |
United States Patent
Application |
20150020926 |
Kind Code |
A1 |
Iwamoto; Takashi ; et
al. |
January 22, 2015 |
STEEL FOR NITROCARBURIZING AND NITROCARBURIZED COMPONENT USING THE
STEEL AS MATERIAL
Abstract
According to the present invention, it is possible to obtain
steel for nitrocarburizing having a predetermined chemical
composition, a bainite area ratio exceeding 50% and excellent
machinability by cutting before nitrocarburizing, and having
strength and toughness equivalent to conventional steel, such as
SCr420 carburized steel material, and excellent fatigue properties
after nitrocarburizing.
Inventors: |
Iwamoto; Takashi; (Sendai,
JP) ; Ando; Keisuke; (Sendai, JP) ; Tomita;
Kunikazu; (Sendai, JP) ; Omori; Yasuhiro;
(Tokyo, JP) ; Uwai; Kiyoshi; (Tokyo, JP) ;
Mitao; Shinji; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE Bars & Shapes Corporation
JFE Steel Corporation |
Tokyo
Tokyo |
|
JP
JP |
|
|
Family ID: |
48983924 |
Appl. No.: |
14/378553 |
Filed: |
February 15, 2013 |
PCT Filed: |
February 15, 2013 |
PCT NO: |
PCT/JP2013/000838 |
371 Date: |
August 13, 2014 |
Current U.S.
Class: |
148/318 ;
420/106 |
Current CPC
Class: |
C22C 38/38 20130101;
C22C 38/002 20130101; C22C 38/04 20130101; C22C 38/60 20130101;
C22C 38/06 20130101; C22C 38/001 20130101; C22C 38/02 20130101;
C22C 38/32 20130101; C23C 8/32 20130101; C22C 38/26 20130101; C22C
38/24 20130101; C22C 38/28 20130101; C22C 38/22 20130101; C21D
6/002 20130101; C21D 2211/002 20130101 |
Class at
Publication: |
148/318 ;
420/106 |
International
Class: |
C23C 8/32 20060101
C23C008/32; C22C 38/32 20060101 C22C038/32; C22C 38/28 20060101
C22C038/28; C22C 38/26 20060101 C22C038/26; C22C 38/00 20060101
C22C038/00; C22C 38/22 20060101 C22C038/22; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/38 20060101 C22C038/38; C22C 38/24 20060101
C22C038/24 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 15, 2012 |
JP |
2012-031031 |
Claims
1.-3. (canceled)
4. A steel for nitrocarburizing comprising, in mass %, C: 0.01% or
more and less than 0.10%, Si: 1.0% or less, Mn: 0.5% to 3.0%, Cr:
0.30% to 3.0%, Mo: 0.005% to 0.4%, V: 0.02% to 0.5%, Nb: 0.003% to
0.15%, Al: 0.005% to 0.2%, S: 0.06% or less, P: 0.02% or less, B:
0.0003% to 0.01%, and the balance being Fe and incidental
impurities, and including a microstructure with a bainite area
ratio exceeding 50% before nitrocarburizing.
5. The steel according to claim 4, wherein after nitrocarburizing,
precipitates including V and Nb are dispersed in a bainite
phase.
6. A nitrocarburized component comprising a steel produced by
nitrocarburizing the steel according to claim 4.
7. A nitrocarburized component comprising a steel produced by
nitrocarburizing the steel according to claim 5.
Description
TECHNICAL FIELD
[0001] This disclosure relates to steel for nitrocarburizing and
nitrocarburized components using the steel as material. In
particular, the disclosure relates to steel for nitrocarburizing
that has excellent fatigue properties after nitrocarburizing and is
suitable for use in automobiles and construction equipment and to
nitrocarburized components using the steel as a material.
BACKGROUND
[0002] Since excellent fatigue properties are desired for machine
structural components such as automobile gears, surface hardening
is generally performed. Carburizing treatment, induction quench
hardening and nitriding treatment are well-known forms of surface
hardening.
[0003] With carburizing treatment, carbon is caused to infiltrate
and diffuse into a high-temperature austenite region, yielding a
deep hardening depth. Carburizing treatment is thus useful to
improve fatigue strength.
[0004] However, since heat treatment distortion occurs, it is
difficult to apply carburizing treatment to components that, from
the perspective of noise or the like, require high dimensional
accuracy.
[0005] Induction quench hardening is a process of quenching a
surface part by high frequency induction heating and, like
carburizing treatment, causes degradation of dimensional
accuracy.
[0006] Nitriding treatment is a process to harden a surface by
causing nitrogen to infiltrate and diffuse into a high-temperature
region at or below the Ac.sub.1 critical point. The treatment is
long, taking 50 to 100 hours, and requires removal of a brittle
compound layer on the surface after treatment.
[0007] Therefore, nitrocarburizing treatment has been developed for
nitriding at approximately the same treatment temperature as
nitriding treatment yet in a short time. In recent years,
nitrocarburizing treatment has become commonly used on machine
structural components and the like. During nitrocarburizing
treatment, nitrogen and carbon are simultaneously caused to
infiltrate and diffuse into a temperature region at 500.degree. C.
to 600.degree. C. to harden the surface, making it possible to
reduce the treatment time to half or less that of conventional
nitriding treatment.
[0008] However, whereas it is possible to increase the core
hardness by quench hardening during carburizing treatment,
nitrocarburizing treatment is performed at a temperature at or
below the critical point of steel, thus causing the core hardness
not to increase and yielding nitrocarburized material with poorer
fatigue strength than carburized material.
[0009] To improve the fatigue strength of nitrocarburized material,
quenching and tempering are generally performed before
nitrocarburizing to increase the core hardness. The resulting
fatigue properties, however, cannot be considered sufficient.
Furthermore, this approach increases manufacturing costs and
reduces mechanical workability.
[0010] To address these problems, it has been proposed to form
steel with a chemical composition including Ni, Al, Cr and Ti, to
age-harden the core during nitrocarburizing by Ni--Al and Ni--Ti
intermetallic compounds or by Cu compounds, and to
precipitation-harden nitrides and carbides such as Cr, Al and Ti in
a nitrided layer of the surface (JP 5-59488 A, JP 7-138701 A).
[0011] JP 2002-69572 A discloses cogging steel that contains 0.5%
to 2% of Cu by hot forging and then air cooling the steel to
provide a ferrite-based microstructure with solute Cu,
precipitating the Cu during nitrocarburizing treatment at
580.degree. C. for 120 minutes and, furthermore, concurrently
precipitation-hardening Ti, V and Nb carbonitrides to yield a steel
that, after the nitrocarburizing treatment, has excellent bending
fatigue properties. JP 2010-163671 A discloses steel for
nitrocarburizing having dispersed therein Ti--Mo carbides and
carbides including at least one element selected from the group
consisting of Nb, V and W.
[0012] While the nitrocarburizing steel recited in JP 5-59488 A and
JP 7-138701 A improves bending fatigue strength through
precipitation-hardening of Cu and the like, the resulting
workability cannot be considered sufficient. By requiring the
addition of a relatively large amount of Cu, Ti, V and Nb, the
nitrocarburizing steel recited in JP 2002-69572 A has a high
production cost. The steel for nitrocarburizing recited in JP
2010-163671 A has the problem of high production cost due to the
inclusion of a relatively large amount of Ti and Mo.
[0013] In view of the foregoing, it could be helpful to provide
steel for nitrocarburizing and a nitrocarburized component using
the steel as material, the steel having a low hardness and
excellent mechanical workability before nitrocarburizing while
allowing for an increase in core hardness via nitrocarburizing
treatment and allowing for relatively inexpensive manufacture of
nitrocarburized components with excellent fatigue properties.
SUMMARY
[0014] We intensely studied the effects of the microstructure and
composition of steel on the fatigue properties after
nitrocarburizing of steel. As a result, we discovered that with a
steel material provided with a specific amount of V and Nb in the
steel composition and a bainite-based microstructure before
nitrocarburizing, excellent fatigue properties are obtained after
nitrocarburizing by performing nitrocarburizing treatment on the
steel material while utilizing the rise in temperature to increase
the core hardness by age precipitating fine precipitates in the
core structure other than the nitrocarburized surface part.
[0015] We thus provide:
[0016] [1] A steel for nitrocarburizing comprising, in, mass %, C:
0.01% or more and less than 0.10%, Si: 1.0% or less, Mn: 0.5% to
3.0%, Cr: 0.30% to 3.0%, Mo: 0.005% to 0.4%, V: 0.02% to 0.5%, Nb:
0.003% to 0.15%, Al: 0.005% to 0.2%, S: 0.06% or less, P: 0.02% or
less, B: 0.0003% to 0.01%, and the balance being Fe and incidental
impurities, and including a microstructure with a bainite area
ratio exceeding 50% before nitrocarburizing.
[0017] [2] The steel for nitrocarburizing according to [1], wherein
after nitrocarburizing, precipitates including V and Nb are
dispersed in a bainite phase.
[0018] [3] A nitrocarburized component using the steel for
nitrocarburizing according to [1] or [2] as material.
[0019] It is thus possible to obtain steel for nitrocarburizing,
and nitrocarburized components using the steel as material, that
has excellent machinability by cutting before nitrocarburizing, and
that after nitrocarburizing has strength and toughness equivalent
to conventional steel, such as SCr420 carburized steel material,
and excellent fatigue properties, thus proving extremely useful in
industrial terms.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] Our steels will be further described below with reference to
the accompanying drawings, wherein:
[0021] FIG. 1 is a schematic diagram illustrating the manufacturing
process to manufacture a nitrocarburized component using steel for
nitrocarburizing.
DETAILED DESCRIPTION
[0022] The microstructure, chemical composition and manufacturing
conditions of the steel for nitrocarburizing will be described.
[0023] 1. Microstructure
[0024] The microstructure before nitrocarburizing has a bainite
area ratio exceeding 50%, and the microstructure after
nitrocarburizing has V and Nb precipitates dispersed in a bainite
phase. When a matrix phase before nitrocarburizing is a
bainite-based microstructure with a bainite area ratio exceeding
50%, formation of V and Nb precipitates in the matrix phase is
drastically inhibited compared to a ferrite-pearlite
microstructure. As a result, formation of the V and Nb precipitates
before nitrocarburizing and consequent increased hardness of the
steel can be prevented, thereby improving workability of cutting
generally performed before nitrocarburizing. Furthermore, applying
nitrocarburizing treatment to the steel causes the surface part to
be nitrided and simultaneously age precipitates the V and Nb
precipitates in the core bainite phase other than the nitrided
surface part, thereby increasing the core hardness. Both the
fatigue properties and the strength after nitrocarburizing
therefore dramatically improve.
[0025] Note that the "microstructure with a bainite area ratio
exceeding 50%" contemplated herein refers to the area ratio of the
bainite microstructure (phase) exceeding 50% under cross-sectional
microstructure observation (microstructure observation with a
200.times. optical microscope). The area ratio of the bainite phase
preferably exceeds 60% and even more preferably exceeds 80%.
Moreover, the V and Nb precipitates in the bainite phase are
preferably a dispersion of fine precipitates having a grain size of
less than 10 nm. Furthermore, for sufficient strengthening by
precipitation, 500 or more of the V and Nb precipitates with the
grain size of less than 10 nm preferably exist per 1
.mu.m.sup.2.
[0026] 2. Chemical Composition
[0027] Reasons for the limitations of the chemical composition in
the steel for nitrocarburizing will now be described. The fraction
of each steel component represents mass %.
[0028] C: 0.01% or More and Less Than 0.10%
[0029] Carbon (C) is added for bainite phase formation and to
ensure strength. When the amount of C added is less than 0.01%, the
amount of bainite formed decreases, as does the amount of V and Nb
precipitates, thus making it difficult to ensure strength. On the
other hand, when 0.10% or greater of C is added, the bainite phase
becomes harder, thereby reducing the mechanical workability.
Accordingly, the amount of C added is 0.01% or more and less than
0.10%. C preferably 0.03% or more and less than 0.10%.
[0030] Si: 1.0% or Less
[0031] Silicon (Si) is added for its usefulness in deoxidizing and
bainite phase formation. Adding an amount of Si exceeding 1.0%,
however, deteriorates mechanical workability and cold-rolling
workability due to solid solution hardening of ferrite and bainite
phases. Accordingly, the amount of Si added is 1.0% or less. The
amount is preferably 0.5% or less and more preferably 0.3% or less.
Note that for Si to contribute effectively to deoxidation, the
amount of Si added is preferably 0.01% or more.
[0032] Mn: 0.5% to 3.0%
[0033] Manganese (Mn) is added for its usefulness in bainite phase
formation and in increasing strength. When the amount of Mn added
is less than 0.5%, the amount of bainite phase formed decreases,
and V and Nb precipitates are formed, causing the hardness before
nitrocarburizing to increase and the amount of V and Nb
precipitates formed after nitrocarburizing treatment to decrease.
In turn, this lowers the hardness after nitrocarburizing and makes
it difficult to ensure strength. On the other hand, adding an
amount of Mn exceeding 3.0% deteriorates mechanical workability and
cold-rolling workability. Accordingly, the amount of Mn added is
0.5% to 3.0%. The amount is preferably 0.5% or more and 2.5% or
less, and more preferably 0.6% or more and 2.0% or less.
[0034] Cr: 0.30% to 3.0%
[0035] Chromium (Cr) is added for its usefulness in bainite phase
formation. When the amount of Cr added is less than 0.30%, the
amount of bainite phase formed decreases, and V and Nb precipitates
are formed, causing the hardness before nitrocarburizing to
increase and the amount of V and Nb precipitates formed after
nitrocarburizing treatment to decrease. In turn, this lowers the
hardness after nitrocarburizing and makes it difficult to ensure
strength. On the other hand, adding an amount of Cr exceeding 3.0%
deteriorates mechanical workability and cold-rolling workability.
Accordingly, the amount of Cr added is 0.30% to 3.0%. The amount is
preferably 0.5% or more and 2.0% or less, and more preferably 0.5%
or more and 1.5% or less.
[0036] V: 0.02% to 0.5%
[0037] Vanadium (V) forms fine precipitates along with Nb due to
the rise in temperature during nitrocarburizing and is therefore an
important element to increase core hardness and improve strength.
An added amount of V less than 0.02% does not satisfactorily
achieve these effects. On the other hand, adding an amount of V
exceeding 0.5% causes the precipitates to coarsen. Accordingly, the
amount of V added is 0.02% to 0.5%. The amount is preferably 0.03%
or more and 0.3% or less, and more preferably 0.03% or more and
0.25% or less.
[0038] Nb: 0.003% to 0.15%
[0039] Niobium (Nb) forms fine precipitates along with V due to the
rise in temperature during nitrocarburizing and is therefore an
extremely effective element to increase core hardness and improve
fatigue strength. An added amount of Nb less than 0.003% does not
satisfactorily achieve these effects. On the other hand, adding an
amount of Nb exceeding 0.15% causes the precipitates to coarsen.
Accordingly, the amount of Nb added is 0.003% to 0.15%. The amount
is preferably 0.02% or more and 0.12% or less.
[0040] Mo: 0.005% to 0.4%
[0041] Molybdenum (Mo) causes fine V and Nb precipitates to form
and is effective in improving the strength of the nitrocarburized
material. Mo is therefore an important element. Mo is also useful
for bainite phase formation. To improve strength, 0.005% or more is
added, but since Mo is an expensive element, adding more than 0.4%
leads to increased component cost. Accordingly, the amount of Mo
added is 0.005% to 0.4%. The amount is preferably 0.01% to 0.3% and
more preferably 0.04% to 0.2%.
[0042] Al: 0.005% to 0.2%
[0043] Aluminum (Al) is a useful element to improve surface
hardness and effective hardened case depth after nitrocarburizing
and is therefore intentionally added. Al also yields a finer
microstructure by inhibiting the growth of austenite grains during
hot forging and is thus a useful element to improve toughness.
Therefore, 0.005% or more is added. On the other hand, including
over 0.2% does not increase this effect, but rather causes the
disadvantage of higher component cost. Accordingly, the amount of
Al added is 0.005% to 0.2%. The amount is preferably over 0.020%
and 0.1% or less, and more preferably over 0.020% and 0.040% or
less.
[0044] S: 0.06% or Less
[0045] Sulfur (S) forms MnS in the steel and is a useful element to
improve the machinability by cutting. Including over 0.06%,
however, lessens toughness. Accordingly, the amount of S added is
0.06% or less. The amount is preferably 0.04% or less. Note that
for S to achieve the effect of improving machinability by cutting,
the amount of S added is preferably 0.002% or more.
[0046] P: 0.02% or Less
[0047] Phosphorus (P) exists in a segregated manner at austenite
grain boundaries and lowers the grain boundary strength, thereby
lowering strength and toughness. Accordingly, the P content is
preferably kept as low as possible, but a content of up to 0.02% is
tolerable. The P content is therefore 0,02% or less. Note that
setting the content of P to less than 0.001% requires a high cost.
Therefore, it suffices in industrial terms to reduce the content of
P to 0.001%.
[0048] B: 0.0003% to 0.01%
[0049] Boron (B) effectively promotes bainite phase formation. An
added amount of B less than 0.0003% does not satisfactorily achieve
this effect. On the other hand, adding over 0.01% does not increase
this effect and only leads to higher component cost. Accordingly,
the amount of B added is 0.0003% to 0.01%. The amount is preferably
0.0010% or more and 0.01% or less.
[0050] Note that to achieve the effect of promoting bainite phase
formation, it is preferable that B be present in the steel as a
solute. When solute N is present in the steel, however, the B in
the steel is consumed by formation of BN. B does not contribute to
improved quench hardenability when existing in the steel as BN.
Accordingly, when solute N exists in the steel, B is preferably
added in an amount greater than that consumed by formation of BN,
and the amounts of B (% B) and of N (% N) in the steel preferably
satisfy formula (1) below.
% B.gtoreq.% N/14.times.10.8+0.0003. (1)
[0051] In the steel for nitrocarburizing, after subjection to
forging or when improving machinability by cutting of the
nitrocarburized material, one or more selected from the group of
Pb.ltoreq.0.2% and Bi.ltoreq.0.02% may be added. Note that the
desired effects achieved are not diminished regardless of whether
these elements are added and regardless of their content.
[0052] Furthermore, in the steel for nitrocarburizing, the balance
other than the above added elements consists of Fe and incidental
impurities. In particular, however, Ti not only adversely affects
strengthening by precipitation of V and Nb, but also lowers the
core hardness and therefore is not to be included insofar as
possible. The amount of Ti is preferably less than 0.010% and more
preferably less than 0.005%.
[0053] 3. Manufacturing Conditions
[0054] FIG. 1 is a schematic diagram illustrating the manufacturing
process of manufacturing a nitrocarburized component using steel
for nitrocarburizing according to the present invention.
[0055] In FIG. 1, S1 indicates a manufacturing process of a steel
bar as a material, S2 indicates a transportation process, and S3
indicates the process of finishing the product (nitrocarburized
component).
[0056] Specifically, in the steel bar manufacturing process (S1), a
steel ingot is hot rolled into a steel bar and shipped after
quality inspection. After shipping, the steel bar is transported
(S2), and during the process (S3) of finishing the product
(nitrocarburized component), the steel bar is cut to predetermined
dimensions and subjected to hot forging or cold forging. After
cutting the steel bar into a predetermined shape by drill boring,
lathe turning or the like as necessary, nitrocarburizing treatment
is performed, yielding the final product.
[0057] Alternatively, hot rolling material may be directly cut into
a predetermined shape by lathe turning, drill boring or the like,
with nitrocarburizing treatment then being performed to yield the
final product. In the case of hot forging, cold straightening may
be performed afterwards. Coating treatment such as painting or
plating, may also be applied to the final product. Preferable
manufacturing conditions will now be described.
[0058] Rolling Heating Temperature
[0059] The rolling heating temperature is preferably 950.degree. C.
to 1250.degree. C. This range is adopted to cause carbides
remaining after melting to be present as a solute during hot
rolling, so as not to diminish forgeability due to formation of
fine precipitates in the rolling material (the steel bar which is
the material for the hot forging component).
[0060] In other words, when the rolling heating temperature is less
than 950.degree. C., it becomes difficult for the carbides
remaining after melting to form a solute. On the other hand, a
temperature exceeding 1250.degree. C. facilitates coarsening of the
crystal grains, thus reducing forgeability. Accordingly, the
rolling heating temperature is preferably 950.degree. C. to
1250.degree. C.
[0061] Rolling Finishing Temperature
[0062] The rolling finishing temperature is preferably 800.degree.
C. or more. This temperature is adopted because at a rolling
finishing temperature of less than 800.degree. C., a ferrite phase
forms. Particularly when the next process is nitrocarburizing after
cold forging or cutting, such a ferrite phase is disadvantageous to
obtain a bainite phase with an area ratio exceeding 50% of the
matrix phase after nitrocarburizing. Moreover, at a rolling
finishing temperature of less than 800.degree. C., the rolling load
increases, which degrades the out-of-roundness of the rolling
material. Accordingly, the rolling finishing temperature is
preferably 800.degree. C. or more.
[0063] Cooling Rate
[0064] To prevent fine precipitates from forming before forging,
thereby reducing forgeability, it is preferable to specify the
cooling rate after rolling. In the precipitation temperature range
of fine precipitates of 700.degree. C. to 550.degree. C., it is
preferable to cool the steel bar faster than the critical cooling
rate at which fine precipitates are produced (0.5.degree.
C./s).
[0065] Nitrocarburizing Treatment (Precipitation Treatment)
[0066] The resulting steel bar is then used as material that is
forged and shaped into components by cutting and the like.
Nitrocarburizing treatment is then performed. The temperature for
nitrocarburizing treatment is preferably 550.degree. C. to
700.degree. C. to yield fine precipitates including V and Nb, and
the treatment time is preferably 10 minutes or more. This range is
adopted because at less than 550.degree. C., insufficient
precipitates are obtained, whereas over 700.degree. C., the
temperature enters the austenite region, making nitrocarburizing
difficult. A more preferable range is 550.degree. C. to 630.degree.
C. Furthermore, the treatment time is 10 minutes or more to obtain
a sufficient amount of V and Nb precipitates.
[0067] Note that when hot forging is used, the hot forging is
preferably performed with the heating temperature during hot
forging at 950.degree. C. to 1250.degree. C., with the forging
finishing temperature at 800.degree. C. or more and the cooling
rate after forging exceeding 0.5.degree. C./s for the bainite phase
to exceed 50% in area ratio of the matrix phase after
nitrocarburizing and in order to prevent formation of fine
precipitates from the standpoints of cold straightening and
workability of cutting after hot forging.
EXAMPLES
[0068] Next, our steels are further described by examples.
[0069] Steel samples with the composition shown in Table 1 (steel
samples No. 1 to 17) were obtained by steelmaking in a 150 kg
vacuum melting furnace, then rolling by heating at 1150.degree. C.,
finishing at 970.degree. C., and subsequently cooling to room
temperature at a cooling rate of 0.9.degree. C./s to prepare steel
bars with o 50 mm. No. 17 is a conventional material, JIS SCr420.
Note that P was not intentionally added to any of the steel samples
in Table 1. Accordingly, the content of P in Table 1 indicates the
amount mixed in as an incidental impurity. Furthermore, Ti was
added to steel samples No. 14 and No. 15 but not intentionally
added to steel samples No. 1 to 13 and No. 16 to 17 in Table 1.
Accordingly, the content of Ti in steel samples No. 1 to 13 and No.
16 to 17 in Table 1 indicates the amount mixed in as an incidental
impurity.
[0070] These materials were then heated to 1200.degree. C. and
subsequently hot forged at 1100.degree. C. to a size of o 30 mm.
The materials were cooled to room temperature at a cooling rate of
0.8.degree. C./s, with a portion being cooled at 0.1.degree. C./s
for the sake of comparison.
TABLE-US-00001 TABLE 1 (mass %) Steel Sample No. C Si Mn P S Cr Mo
V Nb Al Ti B N Category 1 0.038 0.07 1.82 0.012 0.020 0.61 0.20
0.18 0.09 0.032 0.001 0.0051 0.0056 Inventive Example 2 0.049 0.18
1.14 0.010 0.017 1.13 0.13 0.13 0.04 0.025 0.002 0.0074 0.0084
Inventive Example 3 0.077 0.24 0.73 0.015 0.020 1.42 0.07 0.29 0.12
0.024 0.002 0.0050 0.0055 Inventive Example 4 0.086 0.29 0.64 0.018
0.034 1.20 0.10 0.14 0.03 0.029 0.003 0.0078 0.0090 Inventive
Example 5 0.089 0.16 0.85 0.013 0.019 0.79 0.20 0.11 0.10 0.037
0.001 0.0068 0.0061 Inventive Example 6 0.050 0.25 1.35 0.019 0.031
1.01 0.05 0.14 0.06 0.025 0.004 0.0055 0.0055 Inventive Example 7
0.170 0.22 0.70 0.017 0.025 1.13 0.19 0.13 0.06 0.024 0.002 0.0069
0.0077 Comparative Example 8 0.081 1.10 3.15 0.014 0.015 0.64 0.14
0.14 0.05 0.029 0.002 0.0055 0.0057 Comparative Example 9 0.079
0.28 0.34 0.018 0.027 1.20 0.07 0.19 0.10 0.028 0.003 0.0053 0.0056
Comparative Example 10 0.069 0.23 1.01 0.016 0.022 0.27 0.09 0.14
0.06 0.028 0.001 0.0057 0.0059 Comparative Example 11 0.048 0.08
1.04 0.011 0.018 0.85 0.003 0.13 0.06 0.026 0.003 0.0059 0.0064
Comparative Example 12 0.073 0.11 0.94 0.011 0.016 1.08 0.12 0.01
0.001 0.025 0.003 0.0060 0.0061 Comparative Example 13 0.040 0.06
1.68 0.014 0.019 1.15 0.10 0.12 0.001 0.030 0.001 0.0049 0.0051
Comparative Example 14 0.039 0.08 1.65 0.014 0.022 1.20 0.08 0.12
0.04 0.029 0.030 0.0048 0.0053 Comparative Example 15 0.037 0.09
1.66 0.012 0.018 1.16 0.12 0.16 0.05 0.025 0.100 0.0045 0.0054
Comparative Example 16 0.065 0.15 1.13 0.010 0.016 0.85 0.10 0.14
0.05 0.004 0.002 0.0058 0.0062 Comparative Example 17 0.220 0.27
0.79 0.014 0.018 1.18 0.001 0.005 0.001 0.027 0.004 0.0001 0.0105
Conventional Example
[0071] The microstructure of the above materials was observed,
hardness was measured, and machinability by cutting was tested.
During microstructure observation, a cross-section was observed
under an optical microscope, and the core microstructure was
identified. For samples in which a bainite phase was present in the
core, the area fraction of the bainite phase in the core was
calculated. Machinability by cutting was assessed by a drill
cutting test. Specifically, hot forging material was sliced to
yield 20 mm thick pieces of test material in which through holes
were bored in five locations per cross section using a JIS
high-speed tool steel SKH51 straight drill with a 6 mm, under the
following conditions: feed rate, 0.15 mm/rev; revolution speed, 795
rpm. Machinability by cutting was assessed by the total number of
holes before the drill could no longer cut.
[0072] Hardness was measured by testing the hardness of the core
using a Vickers hardness tester, with a test force of 100 g.
[0073] For steel samples No. 1 to 16, gas nitrocarburizing
treatment was further applied to the hot forging material, and for
steel sample No. 17, gas carburizing treatment was applied to the
hot forging material. The gas nitrocarburizing treatment was
performed by heating to 570.degree. C. to 620.degree. C. and
retaining for 3.5 h under an atmosphere of
NH.sub.3:N.sub.2:CO.sub.2=50:45:5. The gas carburizing treatment
was performed by carburizing at 930.degree. C. for 3 h, then oil
quenching after retaining at 850.degree. C. for 40 minutes and,
furthermore, tempering at 170.degree. C. for 1 h.
[0074] The microstructure of these heat treatment materials was
observed, hardness measured, precipitates observed, and impact
properties and fatigue properties tested.
[0075] During microstructure observation, a cross-section was
observed under an optical microscope, and the core microstructure
was identified. For samples in which a bainite phase was present in
the core, the area fraction of the bainite phase was
calculated.
[0076] To measure the hardness of the nitrocarburized material and
the carburized material, the core hardness and surface hardness
were measured. The surface hardness was measured at a position 0.02
mm from the surface, and the effective hardened case depth was
measured as the depth from the surface at a hardness of HV 400.
Samples for transmission electron microscopy observation were
created from the cores of the nitrocarburized material and the
carburized material by Twin-jet electropolishing. Precipitates were
observed in the resulting samples using a transmission electron
microscope with an acceleration voltage of 200 kV. Furthermore, the
composition of the observed precipitates was calculated with an
energy-dispersive X-ray spectrometer (EDX).
[0077] The assessment of impact properties was made by performing a
Charpy impact test and calculating the impact value (J/cm.sup.2).
Notched test pieces (R: 10 mm, depth: 2 mm) were used as test
pieces. The notched test pieces were collected from the hot forging
material, and after performing the above-described nitrocarburizing
treatment or carburizing treatment, the collected test pieces were
used in the Charpy impact test.
[0078] The assessment of fatigue properties was made by an Ono-type
rotary bending fatigue test, and the fatigue limit was calculated.
Notched test pieces (notch R: 1.0 mm; notch diameter: 8 mm; stress
concentration factor: 1.8) were used as test pieces. The test
pieces were collected from the hot forging material and, after the
above-described nitrocarburizing treatment or carburizing
treatment, were used in the fatigue test.
[0079] Table 2 shows the test results. No. 1 to 6 are our examples,
No. 7 to 17 are comparative examples, and No. 18 is a conventional
example provided by JIS SCr420 steel.
TABLE-US-00002 TABLE 2 Characteristics Before Characteristics After
Cooling Rate After Nitrocarburizing Nitrocarburizing Treatment
Steel Heat Treatment Core Core Bainite Drill Nitrocarburizing
Surface Sample Corresponding to Hot Hardness Structure Phase Area
Hole Treatment Hardness No. No. Forging (.degree. C./s) HV (1)
Ratio (%) Count Temperature (.degree. C.) HV 1 1 0.8 240 B-based 98
496 605 787 2 2 0.8 244 B-based 92 487 570 795 3 3 0.8 264 B-based
96 441 620 805 4 4 0.8 268 B-based 97 431 590 796 5 5 0.8 266
B-based 92 436 590 784 6 6 0.8 240 B-based 90 495 590 790 7 2 0.1
228 F + P 0 524 590 787 8 7 0.8 290 B-based 94 200 590 799 9 8 0.8
323 M + B 38 89 590 786 10 9 0.8 290 F + P + B 12 193 590 801 11 10
0.8 284 F + P + B 15 198 590 837 12 11 0.8 213 B-based 65 577 590
787 13 12 0.8 252 B-based 96 470 590 795 14 13 0.8 242 B-based 97
491 590 790 15 14 0.8 241 B-based 97 499 590 788 16 15 0.8 244
B-based 98 492 590 795 17 16 0.8 249 B-based 96 479 590 724 18 17
0.8 248 F + P + B 85 449 930.degree. C. .times. 3 h 730
carburizing, 850.degree. C. .times. 40 m retaining then oil
quenching, 170.degree. C. .times. 1 h tempering Characteristics
After Nitrocarburizing Treatment Effective Core Core Bainite Impact
Fatigue Hardened Case Hardness Structure Phase Area Value Strength
No. Depth (mm) HV (1) Ratio (%) (J/cm.sup.2) (MPa) Category 1 0.15
295 B-based 98 12 512 Inventive Example 2 0.17 277 B-based 92 12
476 Inventive Example 3 0.19 324 B-based 96 11 577 Inventive
Example 4 0.17 300 B-based 97 12 525 Inventive Example 5 0.15 294
B-based 92 13 509 Inventive Example 6 0.16 277 B-based 90 12 474
Inventive Example 7 0.15 226 F + P 0 13 347 Comparative Example 8
0.17 319 B-based 94 11 566 Comparative Example 9 0.15 353 M + B 38
12 640 Comparative Example 10 0.17 298 F + P + B 12 13 527
Comparative Example 11 0.17 295 F + P + B 15 13 514 Comparative
Example 12 0.18 232 B-based 65 12 373 Comparative Example 13 0.16
250 B-based 96 12 416 Comparative Example 14 0.17 253 B-based 97 13
423 Comparative Example 15 0.17 251 B-based 97 3 418 Comparative
Example 16 0.19 260 B-based 98 2 450 Comparative Example 17 0.12
279 B-based 96 9 395 Comparative Example 18 1.05 360 Tempered M + B
50 15 470 Conventional Example (1) F: Ferrite, P: Pearlite, B:
Bainite, M: Martensite
[0080] As is clear from Table 2, nitrocarburized materials No. 1 to
6 have better fatigue strength than the material resulting from
carburizing, quenching, and tempering the conventional example (No.
18). As for workability of drill cutting, the material before
nitrocarburizing treatment in No. 1 to 6 (hot forging material) has
a level equivalent to or greater than the conventional material in
practical terms. Furthermore, the results of transmission electron
microscopy observation and of testing the precipitate composition
by EDX confirm that the nitrocarburized materials No. 1 to 6
contain 500 or more fine precipitates, including V and Nb, with a
grain size of less than 10 nm dispersed per 1 .mu.m.sup.2 in the
bainite phase. Based on these results, it can be concluded that our
nitrocarburized material exhibits a high fatigue strength due to
strengthening by precipitation based on the above fine
precipitates.
[0081] By contrast, comparative examples No. 7 to 17 have a
chemical composition or a resulting microstructure that are outside
of our scope and thus have worse fatigue strength or drill
workability.
[0082] In particular, No. 7 has low fatigue strength as compared to
our examples due to the slow cooling rate after hot forging. For
No. 7, the results of transmission electron microscopy observation
showed no dispersion of fine precipitates with a grain size of less
than 10 nm, whereas course precipitates with a grain size greatly
exceeding 10 nm were observed. Based on these results, the
coarseness of such resulting precipitates can be considered the
cause of the reduction in fatigue strength. In other words, we
believe that if the cooling rate after hot forging is slow and the
desired bainite phase is not obtained, course precipitates are
formed before nitrocarburizing. The amount of fine precipitates
that form after nitrocarburizing treatment then decreases,
resulting in insufficient strengthening by precipitation.
[0083] No. 8 includes a high amount of C, outside of our range. The
hardness of the bainite phase therefore increases, reducing drill
workability.
[0084] No. 9 includes high amounts of Si and Mn, outside of our
range. The hardness of the hot forging material is therefore high,
reducing the drill workability to approximately 1/5 that of
conventional material.
[0085] No. 10 includes a low amount of Mn, outside of our range. A
ferrite-pearlite microstructure thus forms before nitrocarburizing
(after hot forging), lowering the area ratio of the bainite phase
and forming V and Nb precipitates in the microstructure. The
hardness before nitrocarburizing thus increases, reducing the drill
workability.
[0086] No. 11 includes a low amount of Cr, outside of our range. A
ferrite-pearlite microstructure thus forms before nitrocarburizing
(after hot forging), lowering the area ratio of the bainite phase
and forming V and Nb precipitates in the microstructure. The
hardness before nitrocarburizing thus increases, reducing the drill
workability.
[0087] No. 12 includes a low amount of Mo, outside of our range.
Therefore, few fine precipitates exist after the nitrocarburizing
treatment, and the resulting core hardness is insufficient. The
fatigue strength is therefore lower than the conventional
example.
[0088] No. 13 includes low amounts of V and Nb, outside of our
range. Therefore, few fine precipitates exist after the
nitrocarburizing treatment, and the resulting core hardness is
insufficient. The fatigue strength is therefore lower than the
conventional material.
[0089] No. 14 includes a low amount of Nb, outside of our range.
Therefore, few fine precipitates exist after the nitrocarburizing
treatment, and the resulting core hardness is insufficient. The
fatigue strength is therefore lower than the conventional
material.
[0090] Ti was added to No. 15 and No. 16, thus yielding few
precipitates including V and Nb after the nitrocarburizing
treatment. The resulting core hardness is therefore insufficient,
and the fatigue strength is lower than the conventional material.
Furthermore, the impact value is low.
[0091] No. 17 includes a low amount of Al, outside of our range.
The surface hardness after the nitrocarburizing treatment and the
effective hardened case depth are therefore insufficient, resulting
in a lower fatigue strength than the conventional material.
* * * * *