U.S. patent application number 15/558963 was filed with the patent office on 2018-08-30 for age-hardenable steel, and method for manufacturing components using age-hardenable steel.
This patent application is currently assigned to Nippon Steel & Sumitomo Metal Corporation. The applicant listed for this patent is DENSO CORPORATION, NIPPON STEEL & SUMITOMO METAL CORPORATION. Invention is credited to Tomomitsu FUKUOKA, Tatsuya HASEGAWA, Masashi HIGASHIDA, Toshimasa ITO, Taizo MAKINO, Kouji MORITA, Tadashi NISHIWAKI, Motoki TAKASUGA, Yoshihiro TANIMURA, Masato YUYA.
Application Number | 20180245172 15/558963 |
Document ID | / |
Family ID | 56919036 |
Filed Date | 2018-08-30 |
United States Patent
Application |
20180245172 |
Kind Code |
A1 |
YUYA; Masato ; et
al. |
August 30, 2018 |
AGE-HARDENABLE STEEL, AND METHOD FOR MANUFACTURING COMPONENTS USING
AGE-HARDENABLE STEEL
Abstract
Age hardenable steel is low in hardness after hot forging,
providing a machine part with the desired fatigue strength and
yield strength by aging treatment, and high in toughness after
aging treatment, comprising C: 0.09 to 0.20%, Si: 0.01 to 0.40%,
Mn: 1.5 to 2.5%, S: 0.001 to 0.045%, Cr: over 1.00% to 2.00%, Al:
0.001 to 0.060%, V: 0.22 to 0.55%, N: over 0.0080 to 0.0170%, and a
balance of Fe and impurities, where an area rate of bainite
structures is 80% or more, an effective V ratio (amount of
dissolved V/total amount of V) is 0.9 or more, a P and Ti in the
impurities is P: 0.03% or less and Ti: less than 0.005%, and the
chemical composition is one where the following F1 is 1.00 or less
and the F2 is 0.30 or more:
F1=C+0.1.times.Si+0.2.times.Mn+0.15.times.Cr+0.35.times.V
F2=-4.5.times.C+Mn+Cr-3.5.times.V
Inventors: |
YUYA; Masato; (Tokyo,
JP) ; TAKASUGA; Motoki; (Tokyo, JP) ; MAKINO;
Taizo; (Tokyo, JP) ; HIGASHIDA; Masashi;
(Tokyo, JP) ; HASEGAWA; Tatsuya; (Tokyo, JP)
; MORITA; Kouji; (Kariya-shi, JP) ; ITO;
Toshimasa; (Kariya-shi, JP) ; FUKUOKA; Tomomitsu;
(Kariya-shi, JP) ; NISHIWAKI; Tadashi;
(Kariya-shi, JP) ; TANIMURA; Yoshihiro;
(Kariya-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL & SUMITOMO METAL CORPORATION
DENSO CORPORATION |
Tokyo
Kariya-shi, Aichi |
|
JP
JP |
|
|
Assignee: |
Nippon Steel & Sumitomo Metal
Corporation
Tokyo
JP
Denso Corporation
Kariya-shi, Aichi
JP
|
Family ID: |
56919036 |
Appl. No.: |
15/558963 |
Filed: |
March 16, 2016 |
PCT Filed: |
March 16, 2016 |
PCT NO: |
PCT/JP2016/058391 |
371 Date: |
October 19, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/44 20130101;
C21D 6/02 20130101; C21D 6/004 20130101; C22C 38/04 20130101; C22C
38/001 20130101; C21D 8/005 20130101; C22C 38/38 20130101; C21D
9/525 20130101; C22C 38/20 20130101; C21D 8/065 20130101; C22C
38/48 20130101; C22C 38/58 20130101; C21D 6/005 20130101; C22C
38/06 20130101; C22C 38/46 20130101; C21D 9/0068 20130101; C22C
38/42 20130101; C22C 38/50 20130101; C21D 2211/002 20130101; C21D
1/18 20130101; C21D 6/008 20130101; C22C 38/002 20130101; C22C
38/22 20130101; C22C 38/02 20130101; C22C 38/24 20130101 |
International
Class: |
C21D 8/00 20060101
C21D008/00; C21D 6/00 20060101 C21D006/00; C21D 1/18 20060101
C21D001/18; C22C 38/58 20060101 C22C038/58; C22C 38/50 20060101
C22C038/50; C22C 38/48 20060101 C22C038/48; C22C 38/46 20060101
C22C038/46; C22C 38/44 20060101 C22C038/44; C22C 38/42 20060101
C22C038/42; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C22C 38/00 20060101
C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 16, 2015 |
JP |
2015-052557 |
Claims
1. Age hardenable steel comprising, by mass %, C: 0.09 to 0.20%,
Si: 0.01 to 0.40%, Mn: 1.5 to 2.5%, S: 0.001 to 0.045%, Cr: over
1.00% to 2.00%, Al: 0.001 to 0.060%, V: 0.22 to 0.55%, N: over
0.0080 to 0.0170%, and a balance of Fe and impurities, the P and Ti
in this impurities being P: 0.03% or less and Ti: less than 0.005%,
wherein an area rate of bainite structures is 80% or more, an
effective V ratio (amount of dissolved V/total amount of V) is 0.9
or more, and a chemical composition is one where the F1 expressed
by the following formula (1) is 1.00 or less and the F2 expressed
by the following formula (2) is 0.30 or more:
F1=C+0.1.times.Si+0.2.times.Mn+0.15.times.Cr+0.35.times.V (1)
F2=-4.5.times.C+Mn+Cr-3.5.times.V (2) where, in the above formulas
(1) and (2), the element symbols mean the contents of the elements
by mass %.
2. Age hardenable steel comprising, by mass %, C: 0.09 to 0.20%,
Si: 0.01 to 0.40%, Mn: 1.5 to 2.5%, S: 0.001 to 0.045%, Cr: over
1.00% to 2.00%, Al: 0.001 to 0.060%, V: 0.22 to 0.55%, Mo: 0.9% or
less, N: over 0.0080 to 0.0170%, and a balance of Fe and
impurities, the P and Ti in this impurities being P: 0.03% or less
and Ti: less than 0.005%, wherein an area rate of bainite
structures is 80% or more, an effective V ratio (amount of
dissolved V/total amount of V) is 0.9 or more, and a chemical
composition is one where the F1' expressed by the following formula
(1') is 1.00 or less and the F2' expressed by the following formula
(2') is 0.30 or more:
F1'=C+0.1.times.Si+0.2.times.Mn+0.15.times.Cr+0.35.times.V+0.2.times.Mo
(1') F2'=-4.5.times.C+Mn+Cr-3.5.times.V-0.8.times.Mo (2') where, in
the above formulas (1') and (2'), the element symbols mean the
contents of the elements by mass %.
3. The age hardenable steel according to claim 1 further comprising
one or more of Cu: 0.3% or less and Ni: 0.3% or less.
4. The age hardenable steel according to claim 1, further
comprising one or more of Ca: 0.005% or less and Bi: 0.4% or
less.
5. A method of production of part using age hardenable steel
comprising: a forging step of heating age hardenable steel
according to claim 1 at 1100 to 1350.degree. C. for 0.1 to 300
minutes, then forging it so that a surface temperature after finish
forging becomes 900.degree. C. or more, then cooling it down to
room temperature while making the average cooling speed in a
temperature region from 800 to 400.degree. C. a speed of 10 to
90.degree. C./min, a machining step machining the steel after
forging, and an aging treatment step holding the steel after
machining in the temperature region from 540 to 700.degree. C. for
30 to 1000 minutes.
6. The age hardenable steel according to claim 2 further comprising
one or more of Cu: 0.3% or less and Ni: 0.3% or less.
7. The age hardenable steel according to claim 2, further
comprising one or more of Ca: 0.005% or less and Bi: 0.4% or
less.
8. The age hardenable steel according to claim 3, further
comprising one or more of Ca: 0.005% or less and Bi: 0.4% or
less.
9. A method of production of part using age hardenable steel
comprising: a forging step of heating age hardenable steel
according to claim 2 at 1100 to 1350.degree. C. for 0.1 to 300
minutes, then forging it so that a surface temperature after finish
forging becomes 900.degree. C. or more, then cooling it down to
room temperature while making the average cooling speed in a
temperature region from 800 to 400.degree. C. a speed of 10 to
90.degree. C./min, a machining step machining the steel after
forging, and an aging treatment step holding the steel after
machining in the temperature region from 540 to 700.degree. C. for
30 to 1000 minutes.
10. A method of production of part using age hardenable steel
comprising: a forging step of heating age hardenable steel
according to claim 3 at 1100 to 1350.degree. C. for 0.1 to 300
minutes, then forging it so that a surface temperature after finish
forging becomes 900.degree. C. or more, then cooling it down to
room temperature while making the average cooling speed in a
temperature region from 800 to 400.degree. C. a speed of 10 to
90.degree. C./min, a machining step machining the steel after
forging, and an aging treatment step holding the steel after
machining in the temperature region from 540 to 700.degree. C. for
30 to 1000 minutes.
11. A method of production of part using age hardenable steel
comprising: a forging step of heating age hardenable steel
according to claim 4 at 1100 to 1350.degree. C. for 0.1 to 300
minutes, then forging it so that a surface temperature after finish
forging becomes 900.degree. C. or more, then cooling it down to
room temperature while making the average cooling speed in a
temperature region from 800 to 400.degree. C. a speed of 10 to
90.degree. C./min, a machining step machining the steel after
forging, and an aging treatment step holding the steel after
machining in the temperature region from 540 to 700.degree. C. for
30 to 1000 minutes.
Description
TECHNICAL FIELD
[0001] The present invention relates to age hardenable steel. More
specifically, it relates to steel for production of a machine part
for automobiles, industrial machinery, and construction machinery
which is worked into a predetermined shape by hot forging and
machining, is treated for age hardening (below, simply referred to
as "aging treatment"), and has the desired strength and toughness
secured by this aging treatment. Further, the present invention
relates to such a method of production of a part using age
hardenable steel.
BACKGROUND ART
[0002] From the viewpoints of raising the engine output, lightening
the weight aiming at improvement of the fuel economy, etc., machine
parts for automobiles, industrial machinery, construction
machinery, etc. are required to be high in fatigue strength. If
just providing steel with a high fatigue strength, it is possible
to easily achieve this by utilizing alloy elements and/or heat
treatment to raise the hardness of the steel. However, in general,
machine parts are formed by hot forging, then are machined to
finish them to predetermined product shapes. For this reason, the
steel used as a material for machine parts must be provided with
high fatigue strength together with sufficient machinability
simultaneously.
[0003] In general, the fatigue strength becomes better the higher
the hardness of the material. On the other hand, in the
machinability, the machining resistance and the tool life tend to
become more inferior the higher the hardness of the material.
Furthermore, among the parts forming the engine, precision shaped
machine parts are required to remain unchanged in dimensions during
use. Depending on the environment of use, these precision shaped
machine parts can be instantaneously subjected to higher loads
compared with the loads of the extents of usual use. Yield strength
is also required so that the dimensions do not change in the face
of such loads.
[0004] Therefore, various arts have been disclosed which are able
to provide fatigue strength, yield strength, and machinability all
together by keeping the hardness low at the shaping stage where
good machinability is required while raising the hardness by aging
treatment at the final product stage where strength is
required.
[0005] For example, Japanese Patent Publication No. 2006-37177A
(PLT 1) discloses "age hardening steel" obtained by rolling,
forging, or solutionization of steel in which the precipitation
strengthening elements of Mo and V are contained in amounts limited
by specific formulas, and cooling between a temperature of
800.degree. C. to 300.degree. C. by an average cooling rate of 0.05
to 10.degree. C./sec, having an area rate of bainite structures of
50% or more and a hardness of 40 HRC or less before aging
treatment, and, after aging treatment, having a hardness of 7 HRC
or more higher than the hardness before aging treatment.
[0006] Japanese Patent Publication No. 2011-236452A (PLT 2)
discloses, as steel excellent in hot forgeability and machinability
after hot forging and able to be raised in strength by age
hardening after machining, bainite steel containing specific
amounts of Mo and V as precipitation hardening elements.
[0007] Japanese Patent Publication No. 2000-17374A (PLT 3)
proposes, as age hardening type high strength bainite steel for hot
forging use, age hardening type high strength bainite steel
characterized by having a yield point or 0.2% yield strength of 900
MPa or more obtained by hot rolling or hot forging steel containing
Mo and V, then cooling it according to the steel components, making
the hardness 400 HV or less, making the structure a bainite rate of
70% or more, making the old austenite grain size 80 .mu.m or less,
then machining or plastic forming the steel according to need and
applying aging treatment.
[0008] Japanese Patent Publication No. 2013-245363A (PLT 4)
describes steel promising both a high machinability and a high
fatigue strength which is adjusted in the contents of the alloy
elements to satisfy specific parameter formulas and thereby
relatively reduce the content of Mo while making the hardness
before aging treatment after hot forging 290 HV or less and making
the hardness after aging treatment 325 HV or more.
[0009] WO2012/161323A (PLT 5) discloses a steel part for machine
structure use using cooling and heat treatment after hot forging to
optimize the shapes of V carbonitrides and shapes of bainite
structures having a precipitation strengthening ability and provide
machinability, fatigue strength, and toughness all together.
[0010] Japanese Patent Publication No. 2013-213254A (PLT 6)
discloses steel for cold forging and nitriding use excellent in
cold forgeability and chip removal ability after cold forging and
able to provide a cold forged nitrided part with a high core
hardness, high surface hardness, and deep effective hardened layer
depth.
SUMMARY OF INVENTION
Technical Problem
[0011] By using aging treatment to cause fine secondary phase
particles to precipitate in steel, it is possible to obtain a high
fatigue strength and yield strength. In this regard, the steel
strengthened by aging treatment is lowered in toughness.
[0012] Steel lowered in toughness rises in notch sensitivity. If
the notch sensitivity rises, the fatigue strength of the steel
becomes easily affected by fine surface flaws.
[0013] Further, steel low in toughness suffers from faster
progression of fractures and larger scale fractures once fatigue
cracks occur.
[0014] Furthermore, if steel becomes too low in toughness, it
becomes difficult to correct strain caused in the hot forging by
cold working.
[0015] The steel disclosed in PLT 1 can be adjusted in the contents
of the alloy elements so as to satisfy specific parameter formulas
to obtain a high age hardening ability, but the toughness is not
considered at all.
[0016] The steel disclosed in PLT 2 adjusts the contents of the
alloy elements to satisfy specific parameter formulas so as to
relatively reduce the content of Mo while making the hardness
before aging treatment 300 HV or less after hot forging and making
the hardness after aging treatment 300 HV or more. In this regard,
however, the steel is not sufficiently designed to be raised in
toughness after aging treatment.
[0017] The steel disclosed in PLT 3 has a C content kept low at
0.06 to 0.20%, but the V content is an extremely high 0.51 to
1.00%, so the steel is remarkably strengthened by age hardening but
is not excellent in toughness.
[0018] The steel disclosed in PLT 4 is not sufficiently designed to
raise the toughness and the yield strength after aging
treatment.
[0019] The steel disclosed in PLT 5 is not sufficiently designed to
raise the yield strength after aging treatment.
[0020] The steel disclosed in PLT 6 is low in N content, so
nitrides are insufficiently produced and as a result an excellent
yield strength is not obtained.
[0021] Therefore, an object of the present invention is to provide
age hardenable steel satisfying the following <1> to
<3>:
<1> A low hardness after hot forging related to the machining
resistance and tool life. Note that, in the following explanation,
the hardness after the above hot forging will be referred to as the
"hardness before aging treatment". <2> The ability to provide
a machine part with a desired fatigue strength and yield strength
by aging treatment. <3> A high toughness after aging
treatment.
[0022] Specifically, an object of the present invention is to
provide age hardenable steel having a hardness before aging
treatment of 340 HV or less, a fatigue strength explained later
after aging treatment of 480 MPa or more, a 0.2% yield strength,
found by the offset method using a prescribed plastic strain amount
of 0.2% in a tensile test conducted using a tensile test piece of
14A of the JIS having a .PHI.6 parallel part, of 800 MPa or more,
and further having an absorption energy at 20.degree. C. after
aging treatment, evaluated by a Charpy impact test conducted using
a U-notched standard test piece having a notch depth of 2 mm and a
notch bottom radius of 1 mm described in JIS Z 2242, of 25 J or
more.
Solution to Problem
[0023] Findings (a) to (d)
[0024] The inventors engaged in surveys and studies relating to the
chemical composition, structure, and effective V ratio (amount of
dissolved V/total amount of V) and the values calculated by
formulas using contents of specific elements so as to solve the
above problem. Specifically, they investigated the conditions for
obtaining good toughness even with steel giving a high fatigue
strength and yield strength by causing the precipitation of fine
secondary phase particles in the steel due to aging. As a result,
they obtained the following findings (a) to (d).
[0025] (a) Limitation of Chemical Composition (C, V, Mo, and
Ti)
[0026] The elements for causing deterioration of toughness after
aging treatment are C, V, Mo, and Ti. Among these, Ti bond with N
and/or C to form TiN and/or TiC. If TiN and/or TiC precipitates,
the fatigue strength sometimes becomes higher, but the toughness is
made to greatly fall. The intensity of the action of Ti in lowering
the toughness is extremely large compared with those of V and Mo
which are the elements contributing to precipitation strengthening
as V. For this reason, Ti must be limited as much as possible.
[0027] C forms cementite in steel and can become the starting point
for cleavage fracture. Even if treating steel containing an amount
of V or Mo excessive with respect to the amount of C by aging, part
of the cementite remains. Both V and Mo cause precipitation of
carbides at the same crystal planes of the matrix along with aging
treatment and thereby assist the progression of cleavage fractures
and cause deterioration of the toughness. Therefore, to raise the
toughness, it is necessary to reduce the contents of C, V, and
Mo.
[0028] (b) Limitation of Structure
[0029] To raise the toughness, it is necessary to make the majority
of the structure fine bainite. Furthermore, making the difference
in orientation between blocks forming the bainite greater is also
essential for improving the toughness. If the difference in
orientation between blocks is small, even if the size of the blocks
is refined, the effect of raising the toughness is not sufficiently
obtained. To enlarge the difference in orientation between blocks,
it is necessary to enlarge the driving force at the time of bainite
transformation and promote the formation of nuclei of blocks with
large differences in orientation. To obtain these effects, the
contents of C, Mn, Cr, and Mo have to be increased.
[0030] However, C and Mo have the effect of refining the structure
and raising the toughness and the action of precipitating as
cementite or carbides and lowering the toughness. Overall, C
greatly lowers the toughness and Mo slightly lowers the
toughness.
[0031] (c) Limitation of Effective V Ratio
[0032] To utilize the precipitation strengthening by V to the
maximum extent, it is necessary to limit the effective V ratio,
defined as the amount of dissolved V to the total amount of V. The
effective V ratio being small means the ratio of the amount of V
contributing to precipitation strengthening is small and the
strengthening ability is small and is not preferable. There is no
upper limit of the effective V ratio. The closer to 1, the
better.
[0033] (d) Limitation of Values Calculated by Formulas Using
Contents of Specific Elements and Limitation of Amount of Ti
[0034] To impart sufficient toughness to age hardenable steel
having a high strength, the contents of C, Mn, Cr, V, and Mo have
to be controlled so that the value expressed by (2) or (2') showing
an indicator of toughness after aging treatment explained later
becomes a specific value or more. Furthermore, the content of Ti
has to be made a specific value or less so that inclusions and
precipitates harmful to toughness are not contained in the
steel.
[0035] Findings (e) to (g)
[0036] Next, the inventors engaged in further surveys and studies
relating to the values calculated by the formulas using the
chemical composition and contents of specific elements.
Specifically, they adjusted the components of steel able to secure
toughness after aging and investigated the conditions relating to
the hardness before aging and the hardness after aging and the age
hardening ability expressed by the difference of the same. As a
result, they obtained the findings of the following (e) to (g).
[0037] (e) Limitation of Values Calculated by Formulas Using
Contents of Specific Elements
[0038] If the contents of C, Si, Mn, Cr, V, and Mo are controlled
so that the value expressed by the later explained formula (1) or
the formula (1') becomes a specific range, the hardness before the
above aging treatment can be kept from excessively rising. For this
reason, when machined under various conditions, machinability
enabling industrial mass production can be expected.
[0039] (f) Limitation of Chemical Composition (Mo, V, C)
[0040] If reducing the contents of Mo, V, and C so as to raise the
toughness after aging, the driving force in precipitation of V
carbonitrides at the time of aging becomes smaller. For this
reason, the fine precipitates formed due to aging become fewer and
the hardness and yield strength after aging become lower.
[0041] (g) Limitation of Chemical Composition (Mn, Cr)
[0042] If increasing the contents of Mn and Cr so as to raise the
toughness after aging, the hardenability becomes higher and the
hardness before aging becomes harder. With such a structure, the
structure easily recovers at the time of aging, so the margin of
increase of the hardness due to aging easily becomes smaller. If
the hardenability becomes higher, the mobile dislocation density
remaining in the matrix after aging also easily becomes greater, so
obtaining a high yield strength becomes difficult.
[0043] Findings (h) to (j)
[0044] Next, the inventors engaged in further surveys and studies
relating to the chemical composition. Specifically, they focused on
the fact that even if raising the toughness after aging by reducing
the contents of C, V, and Mo and increasing the contents of Mn and
Cr, to cause sufficient amounts of precipitation strengthening
particles to precipitate and obtain a sufficient age hardening
ability and high yield strength, it is necessary to increase the
precipitation strengthening ability per unit V amount. Further, the
inventors engaged in various studies on techniques for increasing
the precipitation strengthening ability of V and obtained the
following findings (h) to (j).
[0045] (h) Limitation of Chemical Composition (C, N)
[0046] By utilizing precipitation strengthening by V to the maximum
extent, it is sufficient to raise the driving force for
precipitation of V carbonitrides. For this purpose, it is necessary
to sufficiently secure the amount of C and amount of N able to be
utilized for precipitation of V carbonitrides in a range not
obstructing toughness.
[0047] (i) Limitation of Chemical Composition (N)
[0048] The bonding force between N and V is larger than the bonding
force between C and V, so the effect of promoting the precipitation
of V carbonitrides is larger with N than with C.
[0049] (j) Limitation of Chemical Composition (C, N)
[0050] If the contents of C and N become too great, V does not
enter a solution even by heating at the time of hot forging or ends
up precipitating in the austenite region during forging. For this
reason, if overly increasing the contents of C and N, conversely
the precipitation strengthening ability falls.
[0051] The present invention was made based on the above findings
(a) to (j) and has as its gist the following:
[0052] [1] Age hardenable steel comprising, by mass %, C: 0.09 to
0.20%, Si: 0.01 to 0.40%, Mn: 1.5 to 2.5%, S: 0.001 to 0.045%, Cr:
over 1.00% to 2.00%, Al: 0.001 to 0.060%, V: 0.22 to 0.55%, N: over
0.0080 to 0.0170%, and a balance of Fe and impurities, the P and Ti
in this impurities being P: 0.03% or less and Ti: less than 0.005%,
wherein an area rate of bainite structures is 80% or more, an
effective V ratio (amount of dissolved V/total amount of V) is 0.9
or more, and the chemical composition is one where the F1 expressed
by the following formula (1) is 1.00 or less and the F2 expressed
by the following formula (2) is 0.30 or more:
F1=C+0.1.times.Si+0.2.times.Mn+0.15.times.Cr+0.35.times.V (1)
F2=-4.5.times.C+Mn+Cr-3.5.times.V (2)
where, in the above formulas (1) and (2), the element symbols mean
the contents of the elements by mass %.
[0053] [2] Age hardenable steel comprising, by mass %, C: 0.09 to
0.20%, Si: 0.01 to 0.40%, Mn: 1.5 to 2.5%, S: 0.001 to 0.045%, Cr:
over 1.00% to 2.00%, Al: 0.001 to 0.060%, V: 0.22 to 0.55%, Mo:
0.9% or less, N: over 0.0080 to 0.0170%, and a balance of Fe and
impurities, the P and Ti in this impurities being P: 0.03% or less
and Ti: less than 0.005%, wherein an area rate of bainite
structures is 80% or more, an effective V ratio (amount of
dissolved V/total amount of V) is 0.9 or more, and the chemical
composition is one where the F1' expressed by the following formula
(1') is 1.00 or less and the F2' expressed by the following formula
(2') is 0.30 or more:
F1'=C+0.1.times.Si+0.2.times.Mn+0.15.times.Cr+0.35.times.V+0.2.times.Mo
(1')
F2'=-4.5.times.C+Mn+Cr-3.5.times.V-0.8.times.Mo (2')
where, in the above formulas (1') and (2'), the element symbols
mean the contents of the elements by mass %.
[0054] [3] The age hardenable steel according to [1] or [2] further
comprising one or more of Cu: 0.3% or less and Ni: 0.3% or
less.
[0055] [4] The age hardenable steel according to any one of [1] to
[3], further comprising one or more of Ca: 0.005% or less and Bi:
0.4% or less.
[0056] [5] A method of production of a part using age hardenable
steel comprising a forging step of heating age hardenable steel
according to any one of [1] to [4] at 1100 to 1350.degree. C. for
0.1 to 300 minutes, then forging it so that a surface temperature
after finish forging becomes 900.degree. C. or more, then cooling
it down to room temperature while making the average cooling speed
in a temperature region from 800 to 400.degree. C. a speed of 10 to
90.degree. C./min, a machining step machining the steel after
forging, and an aging treatment step holding the steel after
machining in the temperature region from 540 to 700.degree. C. for
30 to 1000 minutes.
Advantageous Effects of Invention
[0057] The age hardenable steel of the present invention has a
hardness before aging treatment of 340 HV or less. Further, a
machine part using the age hardenable steel of the present
invention has a fatigue strength of 490 MPa or more due to aging
treatment performed after machining. Further, the machine part has
a toughness (absorption energy at 20.degree. C. after aging
treatment evaluated by a Charpy impact test performed using a
standard test piece with a U-notch of a notch depth of 2 mm and a
notch bottom radius of 1 mm) of 25 J or more. Furthermore, the
machine part has a yield strength of 800 MPa or more. For this
reason, the age hardenable steel of the present invention can be
extremely suitably used as a material for a machine part of
automobiles, industrial machinery, construction machinery, etc.
BRIEF DESCRIPTION OF DRAWINGS
[0058] FIG. 1 A view showing the correlation between a steel
material hardness before aging and an F1 value
[0059] FIG. 2 A view showing the relationship between a Charpy
impact value of a steel material after aging and an F2 value.
DESCRIPTION OF EMBODIMENTS
[0060] Below, the requirements of the present invention will be
explained in detail. Note that the "%" of the contents of the
elements mean "mass %".
[0061] Age Hardenable Steel
Essential Components
[0062] C: 0.09 to 0.20%
[0063] C is an important element in the present invention. C bonds
with V to form carbides and strengthen the steel. However, if the
content of C is under 0.09%, carbides of V become harder to
precipitate, so the desired strengthening effect cannot be
obtained. On the other hand, if the content of C becomes too great,
the amount of C not bonding with V or Mo forming carbides with Fe
(cementite) increases, so the toughness ends up being degraded.
Therefore, the content of C was made 0.09 to 0.20%. The content of
C preferably is made 0.10% or more, more preferably made 0.11% or
more. Further, the content of C preferably is made 0.18% or less,
more preferably is made 0.16% or less.
[0064] Si: 0.01 to 0.40%
[0065] Si is useful as a deoxidizing element at the time of
steelmaking and simultaneously has the action of improving the
strength of the steel by dissolving in the matrix. To sufficiently
obtain these effects, Si has to be made 0.01% or more in content.
However, in steel containing Mn and Cr in large amounts, if the
content of Si becomes excessive, sometimes the amount of residual
austenite of the structure after hot forging becomes too great and
deformation becomes greater during aging treatment. Therefore, the
content of Si was made 0.01 to 0.40%. The content of Si preferably
is made 0.05% or more. Further, the content of Si preferably is
made 0.35% or less, more preferably 0.30% or less.
[0066] Mn: 1.5 to 2.5%
[0067] Mn has the effect of improving the hardenability and making
the structure bainite. Furthermore, it has the effect of lowering
the bainite transformation temperature and thereby refining the
bainite structure to improve the toughness of the matrix. Further,
Mn has the action of forming MnS in steel to improve the chip
removal ability at the time of machining. To sufficiently obtain
these effects, Mn has to be made at least 1.5% in content. However,
Mn is an element which easily segregates at the time of
solidification of the steel, so if the content becomes too great,
the fluctuation in hardness in a part after hot forging unavoidably
becomes larger. Therefore, the content of Mn was made 1.5 to 2.5%.
The content of Mn preferably is made 1.6% or more, more preferably
is made 1.7% or more. Further, the content of Mn preferably is made
2.3% or less, more preferably is made 2.1% or less.
[0068] S: 0.001 to 0.045%
[0069] S bonds with Mn in the steel to form MnS and improves the
chip removal ability at the time of machining, so has to be made
0.001% or more. However, if the content of S becomes greater,
coarse MnS increase and the toughness and fatigue strength are
degraded. In particular, if the content of S exceeds 0.045%, the
fall in toughness and fatigue strength becomes remarkable.
Therefore, the content of S was made 0.001 to 0.045%. The content
of S preferably is made 0.005% or more, more preferably is made
0.010% or more. Further, the content of S preferably is made 0.040%
or less, more preferably is made 0.035% or less.
[0070] Cr: Over 1.00% to 2.00%
[0071] Cr, like Mn, has the effect of raising the hardenability and
making the structure bainite. Furthermore, it lowers the bainite
transformation temperature to refine the bainite structure.
Furthermore, it has the effect of lowering the ease of movement of
grain boundaries to refine the austenite grain size at the time of
hot forging and as a result refine the bainite structure after
transformation. Cr has the effect of raising the toughness of the
matrix through the effects of these in refining the bainite
structure. To sufficiently obtain these effects, it must be
included in over 1.00%. However, if the content of Cr is over 2.0%,
the hardenability becomes larger and the hardness before aging
treatment sometimes exceeds 340 HV. Therefore, the content of Cr
was made over 1.00% to 2.00%. The content of Cr preferably is made
1.10% or more. Further, the content of Cr preferably is made 1.80%
or less, more preferably is made 1.60% or less.
[0072] Al: 0.001 to 0.060%
[0073] Al is an element having a deoxidizing action. To obtain this
effect, 0.001% or more in content is required. However, if Al is
excessively contained, coarse oxides are formed and the toughness
falls. Therefore, the content of Al was made 0.001 to 0.060%. The
content of Al preferably is made 0.050% or less.
[0074] V: 0.22 to 0.55%
[0075] V is the most important element in the steel of the present
invention. At the time of aging treatment, V bonds with C to form
fine carbides and thereby has the action of raising the fatigue
strength. Further, when the steel contains Mo, V has the effect of
combining with Mo and precipitating due to aging treatment and
further raising the age hardening ability. To obtain these effects,
V has to be made 0.22% or more in content. However, if the content
of V becomes excessive, even in the heating at the time of hot
forging, undissolved carbonitrides easily remain inviting a drop in
toughness. Further, if the content of V becomes excessive,
sometimes the hardness before aging treatment ends up becoming
higher. Therefore, the content of V was made 0.22 to 0.55%. The
content of V preferably is under 0.45%, more preferably is made
0.40% or less. Further, the content of V preferably is made 0.25%
or more, more preferably is made 0.27% or more.
[0076] N: Over 0.0080 to 0.0170%
[0077] N has the effect of promoting the precipitation of V
carbonitrides at the time of aging and raising the yield strength.
To sufficiently obtain this effect, the content of N has to be made
over 0.0080%. However, if the content of N exceeds 0.0170%, at the
time of hot forging, the V carbonitrides fail to enter a solution
and at the next time of aging and precipitation of a sufficient
amount of fine V carbonitrides becomes difficult, so the yield
strength falls. Therefore, the content of N was made over 0.0080 to
0.0170%. The content of N preferably is made 0.0090% or more, more
preferably is made 0.0100% or more. Further, the content of N
preferably is made 0.0160% or less, more preferably is made 0.0150%
or less.
[0078] The age hardenable steel of the present invention is
comprised of the above elements from C to N and a balance of Fe and
impurities, the P and Ti in the impurities are P: 0.03% or less and
Ti: less than 0.005%, the area rate of the bainite structure is 80%
or more, and the effective V ratio (amount of dissolved V/total
amount of V) is 0.9 or more.
[0079] Impurities
[0080] The "impurities" indicate elements which enter from the
starting materials of the ore and scraps and the manufacturing
environment etc. when industrially producing ferrous metal
materials.
[0081] P: 0.03% or Less
[0082] P is an element contained as an impurity and not preferable
in the present invention. That is, P segregates at the grain
boundaries to thereby cause a drop in toughness. Therefore, the
content of P was made 0.03% or less. The content of P preferably is
made 0.025% or less.
[0083] Ti: Less than 0.005%
[0084] Ti is an element contained as an impurity and is
particularly not preferable in the present invention. That is, Ti
bonds with N and/or C to form TiN and/or TiC to invite a drop in
toughness. In particular, if the content becomes 0.005% or more,
the toughness greatly deteriorates. Therefore, the content of Ti
was made less than 0.005%. To secure a good toughness, the content
of Ti preferably is made 0.0035% or less.
[0085] Structure
[0086] In the age hardenable steel of the present invention, the
area rate of the bainite structure is 80% or more. Here, the area
rate of the bainite structure means the area rate in the case of
observing a metal structure at a position from 1/3 depth to 1/2
depth of thickness from the surface of the steel material with an
optical microscope. If making the area rate of the bainite
structure 80% or more, the precipitation of V is suppressed, the
effective V ratio becomes larger, and a high fatigue strength and
0.2% yield strength can be obtained.
[0087] Effective V Ratio
[0088] The effective V ratio (amount of dissolved V/total amount of
V) is 0.9 or more. Here, the "effective V ratio" means the amount
of dissolved V in the total amount contained in the steel. If the
effective V ratio is 0.9 or more, the amount of V carbonitrides
precipitating during the aging treatment becomes greater and a high
fatigue strength and 0.2% yield strength can be obtained.
[0089] Optional Components
[0090] Next, the optional components able to be contained in the
age hardenable steel of the present invention will be referred
to.
[0091] Mo: 0.9% or Less
[0092] Mo, like V, is an element with a relatively low
precipitation temperature of carbides and suitable for age
hardening. Mo has the action of raising the hardenability and
making the structure after hot forging bainite and of increasing
the area rate. Mo has the action of forming carbides together with
V to increase the age hardening ability in steel containing 0.22%
or more of V. For this reason, Mo may be included in accordance
with need. However, Mo is an extremely expensive element, so if the
content becomes greater, the cost of manufacture of the steel
increases and the toughness also falls. Therefore, the amount of Mo
when included was made 0.9% or less. The amount of Mo when included
preferably is made 0.75% or less, more preferably is made 0.60% or
less, and still more preferable is less than 0.50%. On the other
hand, to stably obtain the above effect of Mo, the amount of Mo
when included desirably is made 0.05% or more, more desirably is
made 0.10% or more.
[0093] Cu: 0.3% or Less
[0094] Cu has the action of improving the fatigue strength. For
this reason, Cu may be included according to need. However, if the
content of Cu becomes greater, the hot workability falls.
Therefore, the amount of Cu when included was made 0.3% or less.
The amount of Cu when included preferably is made 0.25% or less. On
the other hand, to stably obtain the effect of raising the fatigue
strength of Cu, the amount of Cu when included is desirably made
0.1% or more.
[0095] Ni: 0.3% or Less
[0096] Ni has the action of improving the fatigue strength.
Furthermore, Ni also has the action of suppressing the drop in hot
workability due to Cu. For this reason, Ni may be included in
accordance with need. However, if the content of Ni becomes
greater, the cost swells and, in addition, the above effect is also
saturated. Therefore, the amount of Ni when included was made 0.3%
or less. The amount of Ni when included preferably is made 0.25% or
less. On the other hand, to stably obtain the above effects of Ni,
the amount of Ni when contained desirably is made 0.1% or more.
[0097] The above Cu and Ni may be included as just one type of
either of the same or as two types combined. The total content of
the elements when included can be made 0.6% of the case where the
contents of Cu and Ni are at their respective upper limit
values.
[0098] Ca: 0.005% or Less
[0099] Ca has the action of lengthening the tool life. For this
reason, Ca may be included in accordance with need. However, if the
content of Ca becomes larger, coarse oxides are formed and the
toughness is lowered. Therefore, the amount of Ca when included was
made 0.005% or less. The content of Ca when included is preferably
made 0.0035% or less. On the other hand, to stably obtain the
effect of Ca on increasing tool life, the amount of Ca when
included is desirably made 0.0005% or more.
[0100] Bi: 0.4% or Less
[0101] Bi has the action of lowering the machining resistance and
increasing the tool life. For this reason, Bi may be included in
accordance with need. However, if the content of Bi becomes
greater, it causes a drop in the hot workability. Therefore, the
amount of Bi when included was made 0.4% or less. The amount of Bi
when included is preferably made 0.3% or less. On the other hand,
to obtain the effect of Bi in prolonging the tool life, the amount
of Bi when included is desirably made 0.03% or more.
[0102] The above Ca and Bi may be included as just one type of
either of the same or as two types combined. The total content of
the elements when included can be made 0.405% of the case where the
contents of Ca and Bi are at their respective upper limit values,
but is preferably made 0.3% or less.
[0103] Values Calculated by Formulas Using Contents of Specific
Elements: F1 (F1') and F2 (F2')
[0104] The age hardenable steel of the present invention satisfies
the conditions of the above-mentioned chemical composition
(essential components and optional components), structure, and
effective V ratio. Further the values F1 (F1') and F2 (F2')
calculated by formulas using contents of specific elements has to
be 1.00 or less and 0.30 or more respectively.
[0105] First, the value F1 (F1') calculated by a formula using
contents of specific elements will be explained.
[0106] That is, when optional elements from Mo to Bi are not
contained, F1 expressed by
F1=C+0.1.times.Si+0.2.times.Mn+0.15.times.Cr+0.35.times.V (1)
is 1.00 or less and when one or more optional elements from Mo to
Bi are contained, F1' expressed by
F1'=C+0.1.times.Si+0.2.times.Mn+0.15.times.Cr+0.35.times.V+0.2.times.Mo
(1')
is 1.00 or less.
[0107] Note that, in the above formula (1) and formula (1'), the
element symbols mean the contents of those elements in mass %.
[0108] F1 and F1' are indicators showing the hardness before aging
treatment. If the age hardenable steel of the present invention
satisfies the conditions relating to the above F1 or F1', the
hardness before aging treatment does not become too high, the
machining resistance at the time of machining does not become
large, and longer tool life is realized.
[0109] F1 and F1' are preferably 0.97 or less, more preferably 0.95
or less. Further, F1 and F1' are preferably 0.60 or more, more
preferably 0.65 or more.
[0110] FIG. 1 is a graph showing the relationship between the
hardness before aging (ordinate; HV) and the F1 values of various
types of steel (abscissa). As clear from the graph of FIG. 1, a
strong primary positive correlation is found between the two. If
F11.00 or less, it is judged that the hardness before aging 340
HV.
[0111] Next, the value F2 (F2') calculated by a formula using
contents of specific elements will be explained.
[0112] That is, when optional elements from Mo to Bi are not
contained, F2 expressed by
F2=-4.5.times.C+Mn+Cr-3.5.times.V (2)
is 0.30 or more and when one or more optional elements from Mo to
Bi are contained, F2' expressed by
F2'=-4.5.times.C+Mn+Cr-3.5.times.V-0.8.times.Mo (2')
is 0.30 or less.
[0113] Note that, in the above formula (2) and formula (2'), the
element symbols mean the contents of those elements in mass %.
[0114] F2 and F2' are indicators showing the toughness after aging
treatment. That is, by just satisfying the condition of F1 or F1',
sometimes the toughness after aging treatment falls and the
targeted toughness cannot be secured, so it is necessary to
separately prescribe F2 and F2'.
[0115] FIG. 2 is a view showing a relationship between a Charpy
impact value of a steel material after aging and an F2 value. As
shown in this figure, a positive correlative relationship is
observed between the Charpy impact value (J) after aging treatment
and the F2 value (abscissa). When F2 or F2' is less than 0.30,
toughness after aging treatment is not sufficiently obtained. To
obtain a yield strength of 800 MPa or more while securing the
targeted toughness, it is necessary to make the contents of the
above alloy elements within the prescribed ranges, satisfy the
conditions of F1 or F1', and satisfy the conditions of F2 or
F2'.
[0116] F2 and F2' preferably are 0.45 or more, more preferably are
0.60 or more.
[0117] Note that, if F2 becomes larger, often the hardness before
aging also becomes larger. However, so long as F1 is controlled to
1.00 or less, even if F2 is large, the hardness before aging will
not become too large and the machinability will not be
degraded.
[0118] Accordingly, there is no need to particularly set an upper
limit for F2. Similarly, if F1' is 1.00 or less, there is no need
to particularly set an upper limit for F2'.
[0119] Method of Production of Age Hardenable Steel
[0120] The method of production of the age hardenable steel of the
present invention is not particularly limited. A general method may
be used to smelt the steel and adjust the chemical composition.
[0121] Method of Production of Part Using Age Hardenable Steel
[0122] Below, one example of the method of production of a machine
part for an automobile, industrial machinery, construction
machinery, etc. using as a material the age hardenable steel of the
present invention produced in the following way will be shown.
[0123] First, a material used for hot forging (below, referred to
as a "material for hot forging use") is prepared from steel with a
chemical composition adjusted to the above-mentioned range. As the
material for hot forging use, a billet obtained by blooming from an
ingot, a billet obtained by blooming from a continuously cast
material, or steel rods obtained by hot rolling or hot forging
these billets etc. can be used.
[0124] Next, the material for hot forging use is hot forged and
further is machined to finish the worked material to a
predetermined part shape. Note that the hot forging is for example
performed by heating the material for hot forging use to 1100 to
1350.degree. C. for 0.1 to 300 minutes, then allowing the surface
temperature after the finish forging to fall to 900.degree. C. or
more, then cooling down to room temperature by an average cooling
rate of 10 to 90.degree. C./min in the temperature region from 800
to 400.degree. C.
[0125] Furthermore, the thus cooled worked material was further
machined to finish it into a predetermined part shape.
[0126] Finally, the worked material was supplied to aging treatment
to obtain a machine part for an automobile, industrial machinery,
construction machinery, etc. provided with the desired
characteristics. The aging treatment is, for example, performed in
the temperature region from 540 to 700.degree. C., preferably from
560 to 680.degree. C. The holding time of the aging treatment is
adjusted by the size (mass) of the machine part for soaking, but
can be made 30 to 1000 minutes.
Example 1
[0127] The Steels 1 to 27 of the chemical compositions shown in
Table 1 were smelted with a 50 kg vacuum melting furnace. The
Steels 1 to 17 in Table 1 are steels with chemical compositions
within the ranges prescribed in the present invention. On the other
hand, the Steels 18 to 27 in Table 1 are steels with chemical
compositions outside the conditions prescribed by the present
invention.
TABLE-US-00001 TABLE 1 Steel Components type mass % (balance: Fe
and impurities) name C Si Mn P S Cu Ni Cr Al 1 0.13 0.11 1.63 0.012
0.018 <0.01 <0.01 1.11 0.021 2 0.12 0.06 2.16 0.010 0.015
0.11 0.13 1.22 0.022 3 0.10 0.30 2.00 0.011 0.013 <0.01 <0.01
1.20 0.018 4 0.13 0.34 1.85 0.012 0.016 <0.01 <0.01 1.35
0.025 5 0.13 0.20 1.81 0.012 0.015 0.01 0.01 1.38 0.025 6 0.16 0.06
1.55 0.008 0.023 <0.01 <0.01 1.52 0.005 7 0.16 0.10 1.55
0.009 0.022 0.21 0.11 1.23 0.016 8 0.12 0.19 1.71 0.006 0.006 0.01
0.01 1.20 0.019 9 0.12 0.20 1.72 0.005 0.005 0.01 0.01 1.21 0.018
10 0.13 0.20 1.75 0.011 0.016 0.01 0.01 1.20 0.022 11 0.10 0.30
1.81 0.011 0.015 0.01 0.01 l.03 0.03 12 0.10 0.20 1.77 0.010 0.014
<0.01 <0.01 1.25 0.026 13 0.11 0.20 1.78 0.010 0.016 <0.01
<0.01 1.50 0.029 14 0.14 0.15 1.65 0.010 0.014 <0.01 <0.01
1.11 0.031 15 0.11 0.20 1.77 0.010 0.014 <0.01 <0.01 1.49
0.026 16 0.12 0.06 2.25 0.022 0.024 0.01 0.01 1.12 0.011 17 0.12
0.31 1.86 0.011 0.020 <0.01 <0.01 1.75 0.023 18 0.16 0.34
2.25 0.015 0.015 <0.01 <0.01 1.70 0.024 19 0.14 0.20 1.59
0.015 0.033 <0.01 <0.01 1.02 0.029 20 0.16 0.16 1.55 0.021
0.029 <0.01 0.11 1.03 0.011 21 0.15 0.19 1.82 0.013 0.019
<0.01 <0.01 1.29 0.020 22 0.08 0.20 1.75 0.010 0.019 <0.01
<0.01 1.39 0.026 23 0.23 0.05 1.79 0.010 0.021 <0.01 <0.01
1.49 0.026 24 0.13 0.10 1.61 0.010 0.021 <0.01 <0.01 1.13
0.026 25 0.13 0.11 1.65 0.005 0.018 <0.01 <0.01 1.11 0.022 26
0.11 0.10 1.59 0.010 0.024 <0.01 <0.01 1.02 0.025 27 0.11
0.33 1.55 0.01 0.020 <0.01 <0.01 0.20 0.019 Area rate Steel
Components of bainite Eff. F1 P2 type mass % (balance: Fe and
impurities) structures V or or name V N Mo Ti Others (%) ratio F1'
P2' 1 0.31 0.0130 0.10 <0.001 +0.005Nb 100 0.99 0.76 0.99 2 0.35
0.0161 0.05 0.001 100 0.98 0.87 1.58 3 0.45 0.0090 <0.01
<0.001 100 0.99 0.87 1.18 4 0.39 0.0099 <0.01 <0.001 100
0.99 0.87 1.25 5 0.32 0.0115 0.39 <0.001 100 0.99 0.91 1.17 6
0.29 0.0089 0.35 <0.001 100 0.99 0.88 1.06 7 0.30 0.0126 0.05
0.001 100 0.98 0.78 0.97 8 0.31 0.0126 0.30 0.001 +0.001Ca 100 0.98
0.83 1.05 9 0.30 0.0146 0.30 0.001 +0.015Bi 100 0.98 0.83 1.10 10
0.32 0.0146 0.30 0.001 100 0.98 0.85 1.01 11 0.40 0.0155 0.29
<0.001 +0.018Nb 100 0.98 0.84 0.78 12 0.35 0.0151 0.14 0.002
+0.018Nb 100 0.97 0.81 1.23 13 0.42 0.0149 0.15 <0.001 100 0.98
0.89 1.20 14 0.23 0.0111 0.49 <0.001 100 0.99 0.83 0.93 15 0.32
0.0158 0.20 <0.001 +0.018Nb 100 0.98 0.86 1.49 16 0.35 0.0133
0.01 <0.001 100 0.99 0.87 1.60 17 0.42 0.0129 0.32 <0.001 100
0.98 1.00 1.34 18 0.41 0.0126 0.12 <0.001 48 0.98 1.07 1.70 19
0.45 0.0126 0.16 <0.001 100 0.98 0.82 0.28 20 0.45 0.0110
<0.01 <0.001 100 0.98 0.80 0.29 21 0.39 0.0111 <0.01 0.012
100 0.97 0.86 1.07 22 0.30 0.0129 0.11 <0.001 100 0.99 0.79 1.64
23 0.23 0.0103 0.15 <0.001 100 0.98 0.93 1.32 24 0.30 0.0021
0.09 <0.001 +0.005Nb 100 0.99 0.75 1.03 25 0.30 0.0229 0.08
<0.001 +0.005Nb 100 0.89 0.76 1.06 26 0.30 0.007 0.06 <0.001
100 0.99 0.71 1.02 27 0.25 0.0135 0.01 <0.001 69 0.86 0.57
0.37
[0128] The ingots of the various steel were heated at 1250.degree.
C., then were hot forged to steel rods of diameters of 60 mm. The
hot forged steel rods were cooled to room temperature in the
atmosphere. After that, these were heated at 1250.degree. C. for 30
minutes, then, envisioning forging to part shapes, were hot forged
to steel rods with a diameter of 35 mm while surface temperatures
of the forging rods at the time of finishing was kept from 950 to
1100.degree. C. After the hot forging, all of the rods were cooled
to room temperature in the atmosphere. The cooling rate at the time
of allowing the rods to cool in the atmosphere was measured after
by burying a thermocouple near R/2 of the steel rods hot forged
under the above conditions ("R" indicates the radius of the steel
rods), again raising the temperature to near the finish temperature
in hot forging, then allowing the rods to cool in the atmosphere.
The average cooling rate in the temperature region from 800 to
400.degree. C. after forging measured in this way was about
40.degree. C./min.
[0129] For each steel, from part of the steel rods hot forged to a
diameter of 35 mm, then cooled down to room temperature, in the
state not subjected to aging treatment (that is, in the state as
cooled), the two end parts of the steel rods were cut off by 100 mm
in length, then test pieces were cut out from the remaining center
parts and were investigated for hardness before aging
treatment.
[0130] On the other hand, for each steel, the remainder of the hot
forged steel rods were treated for aging by holding them at 600 to
630.degree. C. for 60 to 180 minutes, the two end parts of the
steel rods were cut off by 100 mm in length, then test pieces were
cut out from the remaining center parts and were investigated for
hardness after aging treatment. Further, for each steel, test
pieces were cut out from the steel rods and were investigated for
absorption energy in a Charpy impact test, fatigue strength, and
yield strength after aging treatment.
[0131] The hardness was measured in the following way. First, a
steel rod was cross-cut, was buried in resin so that the cut
surface became the measured surface, then was polished to a mirror
finish to prepare a test piece. Next, based on "Vickers Hardness
Test--Test Method" in JIS Z 2244 (2009), 10 points near the R/2
Part of the measured surface ("R" indicating the radius) were
measured for hardness with a test force of 9.8N. The values of the
above 10 points were arithmetically averaged to obtain the Vickers
hardness. When the hardness before the aging treatment was 340 HV
or less, it was judged that mass production was industrially
possible even with parts machined under various conditions. This
was made the target. The test piece after measurement of the
hardness was corroded with Nital and observed for structure,
whereupon the structure of the test piece of each steel was also
mainly bainite with some MA structures mixed in.
[0132] The toughness after aging treatment was evaluated by a
Charpy impact test conducted using a U-notched standard test piece
with a depth of notch of 2 mm and notch bottom radius of 1 mm. When
the absorption energy at a test temperature of 20.degree. C. was 25
J or more, it was judged sufficiently high. This was made the
target.
[0133] The fatigue strength was investigated by fabricating an Ono
type rotating bending fatigue test piece with a diameter of the
parallel part of 8 mm and length of 106 mm. That is, the above test
piece was taken so that the center of the fatigue test piece
becomes the R/2 part of a steel rod. An Ono type rotating bending
fatigue test was conducted eight times under conditions of room
temperature, the atmosphere, and a stress ratio of -1. The maximum
value of the stress amplitude up to 1.0.times.10.sup.7 repetitions
while not fracturing was made the fatigue strength. If the fatigue
strength was 490 MPa or more, it was judged that the fatigue
strength was sufficiently high and this was made the target.
[0134] A tensile test was conducted using a tensile test piece of
14A of JIS having a .PHI.6 parallel part, the 0.2% yield strength
was found by the offset method using a prescribed plastic strain of
0.2%, and the yield strength was made equal to this. When the yield
strength was 800 MPa or more, it was judged sufficiently high and
this was made the target. Table 2 shows the results of the
surveys.
TABLE-US-00002 TABLE 2 Before After aging Steel aging Fatigue Yield
Impact Test type Hardness Hardness strength strength value no. name
HV HV MPa MPa J A1 1 265 301 510 815 66 A2 2 309 330 530 885 77 A3
3 300 335 530 925 61 A4 4 293 329 520 884 50 A5 5 319 353 565 955
49 A6 6 320 352 550 940 38 A7 7 310 350 545 941 40 A8 8 299 331 535
886 50 A9 9 295 330 540 883 45 A10 10 300 333 540 899 42 A11 11 294
336 540 889 36 A12 12 291 316 525 862 75 A13 13 300 332 540 915 58
A14 14 295 323 535 861 39 A15 15 296 335 525 890 80 A16 16 305 330
520 890 66 A17 17 335 364 545 960 54 B1 18 351 360 560 978 46 B2 19
291 344 535 940 22 B3 20 310 355 560 955 19 B4 21 301 349 535 953 9
B5 22 280 293 485 790 84 B6 23 326 339 520 895 16 B7 24 266 292 485
775 72 B8 25 263 282 470 744 66 B9 26 260 280 475 748 81 B10 27 261
273 460 715 65
[0135] As clear from Table 2, in the case of the "invention
examples" of Test Nos. A1 to A17 having the chemical composition,
structure, and effective V ratio (amount of dissolved V/total
amount of V) prescribed in the present invention and the values
calculated using the formulas using the contents of specific
elements, the hardness before aging treatment becomes 340 HV or
less, while due to aging treatment, the fatigue strength becomes
510 MPa or more, the yield strength becomes 815 MPa or more, and
the absorption energy in the Charpy impact test becomes 36 J or
more. For this reason, all of the target values are achieved, so
both strength and toughness can be realized after aging treatment
and the hardness before aging treatment is also low, so a fall in
the machining resistance and a longer tool life can be
expected.
[0136] As opposed to this, in the case of the "comparative
examples" of the Test Nos. B1 to B10 outside that which is
prescribed in the present invention, at least one of the targeted
performances cannot be obtained.
INDUSTRIAL APPLICABILITY
[0137] The age hardenable steel of the present invention can secure
a suitable hardness before aging treatment (340 HV or less) and
promises a drop in machining resistance and longer life of tools.
Further, if using the age hardenable steel of the present
invention, due to the aging treatment performed after machining, a
suitable fatigue strength (490 MPa or more), yield strength (800
MPa or more), and impact value (25 J or more) can be secured
together. For this reason, the age hardenable steel of the present
invention can be extremely suitably used as a material for a
machine part in automobiles, industrial machinery, construction
machinery, etc.
* * * * *