U.S. patent application number 12/457268 was filed with the patent office on 2009-12-10 for steel for nitrocarburizing use, steel product for nitrocarburizing use and crankshaft.
This patent application is currently assigned to Sumitomo Metal Industries, Ltd.. Invention is credited to Makoto EGASHIRA, Hiroaki TAHIRA.
Application Number | 20090304543 12/457268 |
Document ID | / |
Family ID | 41400485 |
Filed Date | 2009-12-10 |
United States Patent
Application |
20090304543 |
Kind Code |
A1 |
EGASHIRA; Makoto ; et
al. |
December 10, 2009 |
Steel for nitrocarburizing use, steel product for nitrocarburizing
use and crankshaft
Abstract
A steel for nitrocarburizing use, which comprises by mass
percent, C: more than 0.45% to not more than 0.60%, Si<0.50%,
Mn: more than 1.30% to not more than 1.70%, P.ltoreq.0.05%, S: 0.02
to 0.10%, Cr.ltoreq.0.30% and N: more than 0.007% to not more than
0.030%, and which further contains one or two elements selected
from Al: more than 0.010% to not more than 0.10% and Ti: more than
0.005% to not more than 0.035%, with Al+Ti being 0.015% or more,
with the balance being Fe and impurities, wherein V among the
impurities is not more than 0.010%, and further satisfies the
following 2 formulas, has high fatigue strength and excellent
straightenability after the nitrocarburizing treatment, without
performing the expensive heat treatment of quenching and tempering.
Consequently, they are suitable as raw materials for
nitrocarburized components: fn1=1.25C+Mn-0.1Cr, fn2=N-0.45Al-(
1/22)Ti.
Inventors: |
EGASHIRA; Makoto;
(Kitakyushu-shi, JP) ; TAHIRA; Hiroaki; (Osaka,
JP) |
Correspondence
Address: |
CLARK & BRODY
1090 VERMONT AVENUE, NW, SUITE 250
WASHINGTON
DC
20005
US
|
Assignee: |
Sumitomo Metal Industries,
Ltd.
|
Family ID: |
41400485 |
Appl. No.: |
12/457268 |
Filed: |
June 5, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61129155 |
Jun 6, 2008 |
|
|
|
Current U.S.
Class: |
420/84 ; 420/104;
420/87; 420/91 |
Current CPC
Class: |
C22C 38/02 20130101;
C23C 8/32 20130101; F16C 2204/64 20130101; C22C 38/04 20130101;
C23C 8/56 20130101; C22C 38/001 20130101; F16C 3/06 20130101 |
Class at
Publication: |
420/84 ; 420/87;
420/91; 420/104 |
International
Class: |
C22C 38/00 20060101
C22C038/00; C22C 38/60 20060101 C22C038/60; C22C 38/42 20060101
C22C038/42; C22C 38/18 20060101 C22C038/18 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 6, 2008 |
JP |
2008-148677 |
Claims
1. A steel for nitrocarburizing use, which comprises by mass
percent, C: more than 0.45% to not more than 0.60%, Si: less than
0.50%, Mn: more than 1.30% to not more than 1.70%, P: not more than
0.05%, S: 0.02 to 0.10%, Cr: not more than 0.30% and N: more than
0.007% to not more than 0.030%, and which further contains one or
two elements selected from Al: more than 0.010% to not more than
0.10% and Ti: more than 0.005% to not more than 0.035%, with Al+Ti
being 0.015% or more, with the balance being Fe and impurities,
wherein V among the impurities is not more than 0.010%, "fn1"
expressed by the following formula (1) is not less than 1.90, and
"fn2" expressed by the following formula (2) is more than 0:
fn1=1.25C+Mn-0.1Cr (1), fn2=N-0.45Al-( 1/22)Ti (2); wherein each
element symbol in the formulas (1) and (2) represents the content
by mass percent of the element concerned.
2. The steel for nitrocarburizing use according to claim 1, which
further contains by mass percent, Ca: not more than 0.005%.
3. The steel for nitrocarburizing use according to claim 1, which
further contains by mass percent, one or two elements selected from
Cu: not more than 0.3% and Ni: not more than 0.2%.
4. A steel product for nitrocarburizing use, which has a chemical
composition by mass percent, C: more than 0.45% to not more than
0.60%, Si: less than 0.50%, Mn: more than 1.30% to not more than
1.70%, P: not more than 0.05%, S: 0.02 to 0.10%, Cr: not more than
0.30% and N: more than 0.007% to not more than 0.030%, and which
further contains one or two elements selected from Al: more than
0.010% to not more than 0.10% and Ti: more than 0.005% to not more
than 0.035%, with Al+Ti being 0.015% or more, with the balance
being Fe and impurities, wherein V among the impurities is not more
than 0.010%, "fn1" expressed by the following formula (1) is not
less than 1.90, and "fn2" expressed by the following formula (2) is
more than 0; and has a microstructure which comprise
ferrite-pearlite phase with a ratio of the ferrite of 10% or less:
fn1=1.25C+Mn-0.1Cr (1), fn2=N-0.45Al-( 1/22)Ti (2); wherein each
element symbol in the formulas (1) and (2) represents the content
by mass percent of the element concerned.
5. The steel product for nitrocarburizing use according to claim 4,
whose chemical composition further contains by mass percent, Ca:
not more than 0.005%.
6. The steel product for nitrocarburizing use according to claim 4,
whose chemical composition further contains by mass percent, one or
two elements selected from Cu: not more than 0.3% and Ni: not more
than 0.2%.
7. A crankshaft manufactured by using the steel product for
nitrocarburizing use according to claim 4.
8. A crankshaft manufactured by using the steel product for
nitrocarburizing use according to claim 5.
9. A crankshaft manufactured by using the steel product for
nitrocarburizing use according to claim 6.
10. The steel for nitrocarburizing use according to claim 2, which
further contains by mass percent, one or two elements selected from
Cu: not more than 0.3% and Ni: not more than 0.2%.
11. The steel product for nitrocarburizing use according to claim
5, whose chemical composition further contains by mass percent, one
or two elements selected from Cu: not more than 0.3% and Ni: not
more than 0.2%.
Description
[0001] This application is a formal application of the U.S.
Provisional Application No. 61/129,155 filed on Jun. 6, 2008.
TECHNICAL FIELD
[0002] The present invention relates to a steel for
nitrocarburizing use, a steel product for nitrocarburizing use, and
a crankshaft produced by using the said steel product for
nitrocarburizing use. More specifically, the present invention
relates to: a steel for nitrocarburizing use, which is used as a
raw material for a machine component such as a crankshaft and/or a
connecting rod for automobiles, industrial machines, construction
machines and so on which has high fatigue strength and excellent
straightenability, and which is produced by subjecting the said
steel to, for example, hot forging into an intended shape, followed
by normalizing and nitrocarburizing; a steel product for
nitrocarburizing use, which is produced by working the said steel
for nitrocarburizing use into an intended shape by various methods;
and a crank shaft produced by using the said steel product for
nitrocarburizing use.
BACKGROUND ART
[0003] Conventionally, a crankshaft, a connecting rod, and the like
for automobiles, industrial machines and construction machines have
been produced by subjecting a steel to, for example, hot forging
into an intended shape, followed by heat treatment of quenching and
tempering for obtaining a fine microstructure, and then a
nitrocarburizing treatment to mainly enhance fatigue strength.
[0004] Generally, the above-mentioned "nitrocarburizing treatment"
is a technique of simultaneously allowing the invasion and
diffusion of N and C in the temperature range of 500 to 600.degree.
C. in order to harden the surface of a steel. The said
"nitrocarburizing treatment" is particularly advantageous in
improving fatigue strength, as compared with a "nitriding
treatment" whose purpose is mainly improving wear resistance, and
accordingly, the nitrocarburizing treatment has been spread
rapidly.
[0005] Subjecting a steel to a nitrocarburizing treatment may cause
strain, with the result that a dimensional precision of a machine
component may deteriorate. After nitrocarburizing treatment, a
straightening treatment for bending is frequently performed on a
machine component, in particular, to such as a crankshaft and/or a
connecting rod. Therefore, an excellent straightenability is also
demanded for a machine component in which a nitrocarburizing
treatment is performed (hereinafter, the said machine component is
referred to as a "nitrocarburized component").
[0006] On the other hand, in recent years, a demand for reducing
the cost of heat treatment has been increasing. Accordingly, the
use of a steel which can be used in a hot forged condition; in
other words, the use of a non-heat treated steel has been
increasing.
[0007] However, generally, a microstructure of a non-heat treated
steel product is a coarse ferrite-pearlite (a composite
microstructure of pearlite and ferrite), that is to say, a soft
ferrite phase exists in the non-heat treated steel product.
Accordingly, in many cases, it may be difficult to ensure high
fatigue strength, if a nitrocarburized component is made of the
non-heat treated steel product.
[0008] Therefore, it has been proposed an approach of producing a
non-heat treated steel having a fine microstructure by containing
various alloy elements, and enhancing fatigue strength of a
nitrocarburized component by increasing the hardness of a soft
ferrite phase. However, merely increasing the content of an alloy
element may increase the hardness of the outer layer of the steel
product subjected to nitrocarburizing treatment. As the result, a
degradation of the said straightenability is unavoidable.
[0009] Moreover, in recent years, a demand for reducing the weight
of an automobile has been increasing particularly in the aspect of
reducing the amount of CO.sub.2 in the exhaust gas. Accordingly,
there is an increasing demand for a crankshaft having a still
higher fatigue strength than ever. However, the conventional
non-heat treated steel has failed to sufficiently satisfy the said
demand.
[0010] In view of these circumstances, there is an increasing
demand for a steel for nitrocarburizing use which is capable of
suppressing the heat treatment cost of quenching and tempering, and
also having both the "high fatigue strength" and the "excellent
straightenability" described in the following and have not been
provided in a conventional non-heat treated steel.
[0011] "High fatigue strength": fatigue strength of 460 MPa or more
by the Ono type rotating bending fatigue test using a notched
fatigue test specimen, which is suitable for evaluating fatigue
strength of a machine component having a complicated shape such as
a crankshaft, at room temperature in the atmosphere.
[0012] "Excellent straightenability": there is no crack on the
surface of a machine component until a large bending displacement
is applied, or the crack length is sufficiently short, even if
there is a crack. Concretely, there is no crack, or a crack length
of 0.1 mm or less is observed by a bending test at room temperature
in the atmosphere, using a test piece of 20 mm in diameter, which
is described later.
[0013] Therefore, the Patent Documents 1 to 4 propose various
steels for nitrocarburizing use in order to satisfy the
aforementioned demand. Moreover, the Patent Documents 5 to 7
propose non-heat treated steels suitable for an engine component
such as a crank.
[0014] Specifically, the Patent Document 1 discloses "a low-alloy
steel for nitrocarburizing use which is capable of obtaining high
strength and high toughness after nitrocarburizing treatment, which
contains by weight %, C: 0.4 to 0.7%, Si: not more than 1.0%, Mn:
0.8 to 2.0%, Cr: not more than 0.2%, Al: not more than 0.05%, Ti+V:
not less than 0.02% to not more than the content of [C/6]%, and
which further contains, according to need;
[0015] [a] P: not more than 0.015%
and/or
[0016] [b] at least one element selected from S: not more than
0.15%, Pb: not more than 0.3%, Bi: not more than 0.3%, Se: not more
than 0.1% and Ca: 0.0005 to 0.0100%; with the balance being
substantially Fe, and N among the impurities of 0.007% or less
being allowed".
[0017] The Patent Document 2 discloses "a non-heat treated forging
steel for nitrocarburizing use, which has a steel composition by
weight ratio, C: 0.30 to 0.60%, Si: 0.05 to 1.50%, Mn: 0.20 to
2.00%, P: not more than 0.02%, S: not more than 0.04%, Cr: not more
than 0.30%, Al: not more than 0.005%, and N: 0.01 to 0.02%, and
which further contains, according to need, at least one element
selected from the group consisting of P: 0.02 to 0.07%, S: 0.04 to
0.10%, Ca: 0.0003 to 0.003% and Pb: 0.01 to 0.20%, with the balance
being Fe and inevitable impurities, wherein a content of V as an
impurity element is not more than 0.01%".
[0018] The Patent Document 3 discloses a "steel for a machine
component use, which contains by weight %, a content ratio of Fe:
not less than 90%, C: 0.35 to 0.5%, Si: 0.01 to 0.3%, Mn: 0.6 to
1.8%, Cu: 0.01 to 0.5%, Ni: 0.01 to 0.5%, Cr: 0.01 to 0.5%, Al:
0.001 to 0.01% and N: 0.005 to 0.025%, and which further contains,
according to need, one element or two or more elements selected
form Pb: not more than 0.30%, S: not more than 0.20%, Ca: not more
than 0.01%, Bi: not more than 0.30%, Ti: not more than 0.02%, Zr:
not more than 0.02% and Mg: not more than 0.01%".
[0019] The term "machine component" refers to a "machine component
subjected to a surface hardening treatment by nitrocarburizing,
wherein a Vickers hardness of the surface layer part in the
standard position corresponding to a depth of 50 .mu.m from a
member surface of the said machine component is from 340 to 460 HV;
a Vickers hardness of an inner region having a substantially
constant hardness, where the influence of nitrocarburizing is not
affected, is from 190 to 260 HV; and an effective case depth from
the member surface, where the Vickers hardness is set to 270 HV, is
adjusted to not less than 0.3 mm".
[0020] The Patent Document 4 discloses a "steel for machine
structural use having excellent seizure resistance and fatigue
strength, and produced by performing nitrocarburizing treatment on
a steel, which contains by weight ratio, C: 0.22 to 0.55%, Si: 0.05
to 0.80%, Mn: 0.50 to 1.50%, P: not more than 0.025%, S: 0.04 to
0.08%, Ni: not more than 0.15%, Cr: not more than 0.15%, Mo: not
more than 0.05%, Cu: not more than 0.15%, Al: 0.002 to 0.018%, Ti:
not more than 0.0030%, V: 0.01 to 0.06%, Nb: not more than 0.0030%,
B: not more than 0.0005% and N: 0.0080 to 0.0200%, and which
further contains, according to need, one element or two or more
elements selected from Ca: 0.0010 to 0.0120%, Pb: 0.04 to 0.40%,
Bi: 0.05 to 0.50%, Te: 0.05 to 0.35% and Se: 0.05 to 0.35%, and
satisfies the condition of Ni+Mo+Cu.ltoreq.0.25%,
Ti+Nb+B.ltoreq.0.0040%, with the balance being Fe and impurities,
wherein the steel surface has a compound layer of not less than 12
.mu.m on the average".
[0021] The Patent Document 5 discloses a "non-heat treated steel
for hot forging use having excellent wear resistance, which
contains by weight unit,
[0022] C: 0.40 to 0.70%, Si: not more than 0.50%, Mn: 0.90 to
1.80%, Cr: 0.05 to 1.00%, s-Al: 0.010 to 0.045% and N: 0.005 to
0.025%, and which further contains, according to need, one element
or two or more elements selected from Pb: not more than 0.030%, S:
not more than 0.20%, Te: not more than 0.030%, Ca: not more than
0.01% and Bi: not more than 0.30%, with the balance being Fe and
impurities, wherein the microstructure after hot-forging is
ferrite+pearlite, and the area ratio of pro-eutectoid ferrite is
not more than 10%".
[0023] The Patent Document 6 discloses a "non-heat treated steel
for hot forging use having excellent wear resistance, which
contains by mass %, C: 0.3 to 0.8%, Mn: 0.3 to 2.0% and Si: 0.5 to
2.5%, and which further contains, according to need, one or more
elements of one or more groups selected from [a] to [d]:
[0024] [a] one or more elements selected from the group consisting
of V: not more than 0.4%, Nb: not more than 0.15% and Ti: not more
than 0.15%;
[0025] [b] Cr: not more than 1.5%;
[0026] [c] Al: not more than 0.04%; and
[0027] [d] one or more elements selected from the group consisting
of S: not more than 0.12%, Pb: not more than 0.3%, Zr: not more
than 0.2%, Ca: not more than 0.01%, Te: not more than 0.1% and Bi:
not more than 0.1%, wherein: the F value defined by
F=Si+(Mn/3.5)+3V+2.5Nb+2.5Ti is not less than 1.0; and the number
of oxide type inclusion with a mean grain size is not less than 20
.mu.m per 300 mm.sup.2 of detected area is ten or less in an
arbitrary vertical section".
[0028] The Patent Document 7 discloses a "non-heat treated steel
for hot forging use, which contains by mass %, C: 0.30 to 0.80%,
Si: 0.1 to 2.5%, Mn: 0.30 to 2.0%, Al: 0.001 to 0.06%, N: 0.005 to
0.10%, P: not more than 0.30%, S: not more than 0.12%, Cr: not more
than 1.0%, Cu: not more than 0.3% and Ni: not more than 0.3%, and
which further contains, according to need, one or more elements
selected from Pb: not more than 0.3%, Zr: not more than 0.2%, Ca:
not more than 0.010%, Te: not more than 0.10% and Bi: not more than
0.1%, with the balance being Fe and inevitable impurities,
wherein:
[0029] the steel satisfies the relations:
Si+3.4Mn+19.5P-13.4S+2.7Cr.gtoreq.3.5;
C+1.1Mn-1.9Si+1.5Cu+1.8Ni+0.6Cr.ltoreq.2.6; and
[0030] the steel has tensile strength from 600 to 900
N/mm.sup.2".
[0031] Patent Document 1: Japanese Unexamined Patent Publication
No. 64-25949
[0032] Patent Document 2: Japanese Unexamined Patent Publication
No. 8-170146
[0033] Patent Document 3: Japanese Unexamined Patent Publication
No. 2004-162161
[0034] Patent Document 4: Japanese Unexamined Patent Publication
No. 7-18379
[0035] Patent Document 5: Japanese Unexamined Patent Publication
No. 2000-265242
[0036] Patent Document 6: Japanese Unexamined Patent Publication
No. 2000-328193
[0037] Patent Document 7: Japanese Unexamined Patent Publication
No. 9-310152
DISCLOSURE OF INVENTION
Problems to be Solved by the Invention
[0038] In order to ensure both the high fatigue strength and the
straightenability, the technique described in the above-mentioned
Patent Document 1 is directed to ensure the toughness of a surface
hardened layer after the nitrocarburizing treatment and also to
improve the strength of the core portion by minimizing the contents
of Cr, Al, V and Ti which may deteriorate the straightenability;
and the addition of Mn to compensate for strength deterioration
will minimize the contents of the above-mentioned elements.
However, the "straightenability" in the said Patent Document 1 is
evaluated based on a measure of bending flexure which can be
obtained when a large crack has occurred in the bending test using
a test piece with a thickness as small as 5 mm. Accordingly, the
steel proposed in the Patent Document 1 has failed to ensure the
"straightenability", in the case where both the "high fatigue
strength" and the "excellent straightenability" as described above
are required.
[0039] Without performing a thermal refining treatment, by limiting
the contents of N, Cr and V, the technique disclosed in the Patent
Document 2 allows for ensuring both the fatigue strength and the
straightenability substantially equivalent to those of a steel
which will be performed a thermal refining treatment. However, the
"fatigue strength" in the Patent Document 2 is evaluated by using a
smooth fatigue test specimen. Accordingly, the steel proposed in
the said Patent Document 2 has failed to particularly ensure the
"fatigue strength" under such a circumstance that the weight
reduction in automobiles has been required in the aspect of
reducing the amount of CO.sub.2 in the exhaust gas, and a demand
for a crankshaft having a still higher fatigue strength than ever
has been increasing, and both the "high fatigue strength" and the
"excellent straightenability" as described above are required.
[0040] In order to ensure both the fatigue strength and the
straightenability, the technique disclosed in the Patent Document 3
is directed to controlling a hardness distribution after the
nitrocarburizing treatment by controlling the contents of C, Mn,
Si, Cr, Cu and Ni. However, even with use of the steel proposed in
the said Patent Document 3, both the "high fatigue strength" and
the "excellent straightenability" as described above could not be
ensured.
[0041] The steel disclosed in the Patent Document 4 is directed to
increasing the thickness of a compound layer to be formed after the
nitrocarburizing treatment by reducing the contents of Ni, Cu, Mo
and V so as to ensure both the seizure resistance and the fatigue
strength. However, the steel proposed in the Patent Document 4 does
not take into account the straightenability after the
nitrocarburizing treatment. Accordingly, the said steel disclosed
in the Patent Document 4 has failed to particularly ensure the
"straightenability", in the case where both the "high fatigue
strength" and the "excellent straightenability" as described above
are required.
[0042] The steel disclosed in the Patent Document 5 is a non-heat
treated steel, having high wear resistance in a hot forged
condition, without performing a surface hardening treatment such as
high frequency induction hardening, nitrocarburizing and so on.
Accordingly, even if the steel proposed in the said Patent Document
5 is subjected to nitrocarburizing, the intended fatigue strength
could not be obtained, not to mention the intended
straightenability.
[0043] The technique disclosed in the Patent Document 6 is directed
to providing a non-heat treated steel for hot forging use having
excellent wear resistance, whose microstructure comprises
ferrite-pearlite, by dissolving Si and Mn in the ferrite, and also
precipitating fine carbonitrides such as V, Nb, Ti and so on in the
said ferrite. The steel proposed in the Patent document 6 does not
take into account the nitrocarburizing. Accordingly, if the steel
proposed in the Patent Document 6 is subjected to nitrocarburizing
treatment, not only the "excellent straightenability" as described
above but also the said intended fatigue strength could not be
obtained.
[0044] The technique disclosed in the Patent Document 7 is directed
to providing a non-heat treated steel for hot forging use which
has, in a non-heat treated condition, high proof strength and
fatigue property substantially equivalent to those of a steel
containing V or the like, and which has excellent machinability, by
controlling the chemical compositions of the steel to enhance the
proof strength. However, the steel proposed in the Patent document
7 also does not take into account the nitrocarburizing. And
consequently, if the steel proposed in the Patent Document 7 is
subjected to nitrocarburizing treatment, not only the said
"excellent straightenability" but also the said intended fatigue
strength could not be obtained.
[0045] Accordingly, it is an objective of the present invention to
provide a steel for nitrocarburizing use, which has the high
fatigue strength and the excellent straightenability after the
nitrocarburizing treatment, and is used suitably as a raw material
for a machine component, such as a crankshaft and/or a connecting
rod for automobiles, industrial machines, construction machines and
so on, without performing the expensive heat treatment of quenching
and tempering; a steel product for nitrocarburizing use, which is
produced by working the said steel for nitrocarburizing use into an
intended shape by various methods; and a crankshaft produced by
using the said steel product for nitrocarburizing use.
[0046] The specific fatigue property and straightenability after
the nitrocarburizing treatment, which are attained by the present
invention, are as follows.
[0047] Fatigue property: fatigue strength of 460 MPa or more by the
Ono type rotating bending fatigue test, using a notched fatigue
test specimen shown in FIG. 1, at room temperature in the
atmosphere.
[0048] Straightenability: there is no crack, or the crack length is
0.1 mm or less, in the case where a load is applied to a test
specimen of 20 mm in diameter in a state that a strain gauge is
attached to the longitudinal middle portion of the said test
specimen, with two fulcrums being spaced away from each other by
the distance of 70 mm, until a reading of the strain gauge reaches
17000.mu. (corresponding to 1.7% of bending strain) by a 3-point
bending method at room temperature in the atmosphere.
Means for Solving the Problems
[0049] In order to accomplish the above objective, the present
inventors first investigated how to omit the heat treatment of
quenching and tempering.
[0050] (a) By reducing the precipitation of soft ferrite included
in the microstructure as much as possible and forming a fine
microstructure, the high fatigue strength can be ensured for a
steel product which is not subjected to the heat treatment of
quenching and tempering.
[0051] (b) However, a large amount of soft ferrite may precipitate
in the case of a steel in a hot forged condition, that is to say, a
non-heat treated steel, and a fine microstructure may not be
obtained.
[0052] (c) By using normalizing in place of the heat treatment of
quenching and tempering, it is possible to reduce the heat treating
cost to approximately one-half of that required for quenching and
tempering. In this case, as compared with non-heat treated steel,
soft ferrite precipitation is somewhat suppressed, and a fine
microstructure is formed to a certain degree. However, the degree
of refinement of the ferrite phase is not sufficient.
[0053] Accordingly, the present inventors investigated
microstructures in detail by melting various kinds of steels. By
evaluating the fatigue strength and the straightenability after the
nitrocarburizing treatment, the present inventors also investigated
the influences of microstructures on the fatigue strength and the
straightenability. As a result of the said investigation, the
following findings (d) to (h) were obtained.
[0054] (d) Simply increasing the content of C, which is an
indispensable element for forming carbides, fails to sufficiently
suppress the generation of pro-eutectoid ferrite at the time of
cooling the steel from an austenite temperature region during
normalizing. However, increasing the content of Mn as an
austenite-forming element as well as increasing the C content, and
limiting the content of Cr as a ferrite-forming element suppresses
the generation of pro-eutectoid ferrite.
[0055] (e) By controlling the contents of C, Mn and Cr so that the
value of "fn1", which is expressed by the formula (1), becomes not
less than 1.90, the ratio of ferrite is suppressed to not more than
10%;
fn1=1.25C+Mn-0.1Cr (1).
[0056] (f) Furthermore, in the case where the grain diameter of
austenite (hereinafter, referred to as "prior-austenite") in
normalizing is coarse, the ferrite-pearlite to be formed after
cooling may become coarse, and therefore, the high fatigue strength
can not be ensured. However, in the case where the prior-austenite
grain diameter is small, fine ferrite-pearlite is obtained, and the
fatigue strength and the straightenability are significantly
improved.
[0057] (g) A coarsening of the prior-austenite grain diameter is
suppressed by processing a steel into an intended shape by hot
forging and so on, which is followed by normalizing, and by
utilizing a pinning effect of nitrides of Ti and/or Al during the
said normalizing. In particular, at a normalizing temperature
region between 800 and 900.degree. C., the prior-austenite grain
diameter is sufficiently reduced to approximately 5 to 45
.mu.m.
[0058] (h) Containing, by mass %, N: more than 0.007% to not more
than 0.030%, and containing one or two elements selected from Al:
more than 0.010% to not more than 0.10% and Ti: more than 0.005% to
not more than 0.035%, with Al+Ti being 0.015% or more, wherein the
contents of N, Al and Ti satisfy a requirement that the value of
"fn2", which is expressed by the formula (2) is more than 0,
obtains the said pinning effect by nitrides of Ti and/or Al in the
above-mentioned item (g);
fn2=N-0.45Al-( 1/22)Ti (2).
That is to say, it is necessary not only to contain the proper
amounts of N, Al and Ti, but also to satisfy the requirement that
the value of "fn2" is more than 0, in order to contain a sufficient
amount of N which is able to form sufficient nitrides of Al and/or
Ti which can ensure the pinning effect.
[0059] As mentioned above, if "normalizing" is adopted as a heat
treatment after hot forging, it can reduce the heat treating cost
to approximately one-half of the one required for "quenching and
tempering".
[0060] The present invention has been accomplished on the basis of
the above-described findings. The main points of the present
invention are steels for nitrocarburizing use shown in the
following (1) to (3), steel products for nitrocarburizing use shown
in the following (4) to (6), and crankshafts shown in the following
(7) to (9).
[0061] (1) A steel for nitrocarburizing use, which comprises by
mass percent, C: more than 0.45% to not more than 0.60%, Si: less
than 0.50%, Mn: more than 1.30% to not more than 1.70%, P: not more
than 0.05%, S: 0.02 to 0.10%, Cr: not more than 0.30% and N: more
than 0.007% to not more than 0.030%, and which further contains one
or two elements selected from Al: more than 0.010% to not more than
0.10% and Ti: more than 0.005% to not more than 0.035%, with Al+Ti
being 0.015% or more, with the balance being Fe and impurities,
wherein V among the impurities is not more than 0.010%, "fn1"
expressed by the following formula (1) is not less than 1.90, and
"fn2" expressed by the following formula (2) is more than 0:
fn1=1.25C+Mn-0.1Cr (1),
fn2=N-0.45Al-( 1/22)Ti (2);
[0062] In the formulas (1) and (2), each element symbol represents
the content by mass percent of the element concerned.
[0063] (2) The steel for nitrocarburizing use according to the
above (1), which further contains by mass percent, Ca: not more
than 0.005%.
[0064] (3) The steel for nitrocarburizing use according to the
above (1) or (2), which further contains by mass percent, one or
two elements selected from Cu: not more than 0.3% and Ni: not more
than 0.2%.
[0065] (4) A steel product for nitrocarburizing use, which has a
chemical composition by mass percent, C: more than 0.45% to not
more than 0.60%, Si: less than 0.50%, Mn: more than 1.30% to not
more than 1.70%, P: not more than 0.05%, S: 0.02 to 0.10%, Cr: not
more than 0.30% and N: more than 0.007% to not more than 0.030%,
and which further contains one or two elements selected from Al:
more than 0.010% to not more than 0.10% and Ti: more than 0.005% to
not more than 0.035%, with Al+Ti being 0.015% or more, with the
balance being Fe and impurities, wherein V among the impurities is
not more than 0.010%, "fn1" expressed by the following formula (1)
is not less than 1.90, and "fn2" expressed by the following formula
(2) is more than 0; and has a microstructure which comprise
ferrite-pearlite phase with a ratio of the ferrite of 10% or
less:
fn1=1.25C+Mn-0.1Cr (1),
fn2=N-0.45Al-( 1/22)Ti (2);
[0066] In the formulas (1) and (2), each element symbol represents
the content by mass percent of the element concerned.
[0067] (5) The steel product for nitrocarburizing use according to
the above (4), whose chemical composition further contains by mass
percent, Ca: not more than 0.005%.
[0068] (6) The steel product for nitrocarburizing use according to
the above (4) or (5), whose chemical composition further contains
by mass percent, one or two elements selected from Cu: not more
than 0.3% and Ni: not more than 0.2%.
[0069] (7) A crankshaft manufactured by using the steel product for
nitrocarburizing use according to the above (4).
[0070] (8) A crankshaft manufactured by using the steel product for
nitrocarburizing use according to the above (5).
[0071] (9) A crankshaft manufactured by using the steel product for
nitrocarburizing use according to the above (6).
[0072] The above-mentioned "ferrite-pearlite phase" represents a
composite microstructure of ferrite and pearlite. The ferrite,
which is 10% or less by the "ratio" in the microstructure,
represents the ferrite which forms the said "ferrite-pearlite
phase" together with pearlite, and does not embrace the one which
forms pearlite with cementite.
[0073] The above-mentioned inventions (1) to (3) related to the
steels for nitrocarburizing use, the inventions (4) to (6) related
to the steel products for nitrocarburizing use and the inventions
(7) to (9) related to the crankshafts are referred to as "the
present invention (1)" to "the present invention (9)",
respectively, or collectively referred to as "the present
invention".
EFFECTS OF THE INVENTION
[0074] The steels for nitrocarburizing use of the present invention
have high fatigue strength and excellent straightenability after
the nitrocarburizing treatment, without performing the expensive
heat treatment of quenching and tempering. Accordingly, the said
steels for nitrocarburizing use of the present invention are
suitable as raw materials for nitrocarburized components such as
crankshafts and/or connecting rods for automobiles, industrial
machines, construction machines and so on.
BRIEF DESCRIPTION OF THE DRAWING
[0075] FIG. 1 shows the shape of a notched fatigue test specimen
used in the Ono type rotating bending fatigue test at room
temperature in the atmosphere in the examples.
BEST MODE FOR CARRYING OUT THE INVENTION
[0076] In the following, all of the requirements of the present
invention are described in detail. The symbol "%" for the content
of each chemical composition represents "% by mass".
[0077] (A) Chemical Composition
[0078] C: more than 0.45% to not more than 0.60%
[0079] C is bonded to Fe and precipitates as cementite, and
consequently forms lamellar pearlite and reduces the ratio of soft
ferrite. Thus, C has an effect of enhancing fatigue strength. In
order to obtain such effects, a content of C more than 0.45% is
necessary. However, an excessive C content, in particular, a
content of C exceeding 0.60% may increase the hardness of a steel
product and thereby causes deterioration of machinability.
Therefore, the content of C is set to more than 0.45% to not more
than 0.60%. The lower limit of the C content is preferably
0.48%.
[0080] Si: less than 0.50%
[0081] Si is contained as an impurity in a steel. Si may increase
the hardness of a surface layer and deteriorate the
straightenability. A content of Si not less than 0.50% may
significantly deteriorate the straightenability. Furthermore, the
content of Si not less than 0.50% may increase the ratio of ferrite
in the microstructure and thereby deteriorates fatigue strength. In
view of this, it is necessary to set the content of Si to less than
0.50%. As far as the content of Si is less than 0.50%, Si has
substantially no effect on the ratio of ferrite in the
microstructure. However, in order to suppress the increase in the
ratio of ferrite stably and surely, the content of Si is preferably
set to not more than 0.45%.
[0082] Mn: more than 1.30% to not more than 1.70%
[0083] Mn has an effect of ensuring high fatigue strength by
lowering the ratio of ferrite in the microstructure, as an
austenite-forming element, and by increasing the hardness of a base
material, as a solid-solution strengthening element. In order to
obtain such effect, it is necessary to contain more than 1.30% of
Mn. However, an excessive Mn content, in particular, a content of
Mn exceeding 1.70% may increase the hardness of a steel product and
thereby causes deterioration of the straightenability and
machinability. Therefore, the content of Mn is set to more than
1.30% to not more than 1.70%. The lower limit of the Mn content is
preferably 1.35% and the upper limit thereof is preferably
1.65%.
[0084] P: not more than 0.05%
[0085] P is an impurity element including in a steel and may
promote the occurrence of intergranular brittle fracture by
segregating on the grain boundaries. In particular, a content of P
exceeding 0.05% may significantly increase the occurrence of the
said intergranular brittle fracture. In view of this, the content
of P is set to not more than 0.05%. The content of P is preferably
not more than 0.045%.
[0086] S: 0.02 to 0.10%
[0087] S is an effective element for improving the machinability of
steel. In order to obtain this effect, it is necessary to contain
not less than 0.02% of S. However, an excessive S content may cause
deterioration of hot workability and fatigue strength. In
particular, a content of S exceeding 0.10% may significantly
deteriorate the said hot workability and fatigue strength.
Therefore, the content of S is set to 0.02 to 0.10%. The upper
limit of the S content is preferably 0.07%.
[0088] Cr: not more than 0.30%
[0089] Cr is contained as an impurity in a steel, and may increase
the hardness of a surface layer after nitrocarburizing treatment
and deteriorate the straightenability. Furthermore, as a
ferrite-forming element, Cr may increase the ratio of ferrite in
the microstructure and thereby deteriorates fatigue strength. In
particular, a content of Cr exceeding 0.30% may significantly
deteriorate the said straightenability and fatigue strength. In
view of this, the content of Cr is set to not more than 0.30%. It
is desirable that the Cr content be reduced as low as possible.
[0090] N: more than 0.007% to not more than 0.030%
[0091] N is bonded to Al and/or Ti, and forms fine nitrides of Al
and/or Ti, thereby contributing to refine the prior-austenite grain
diameter and has an effect of improving fatigue strength and the
straightenability. In order to obtain this effect, it is necessary
to contain more than 0.007% of N. However, in industry, it is
difficult to add more N than 0.030% as a content thereof. And
moreover, in such a case, for instance, the gas bubbles may
generate in the ingot, which may impair the quality of the steel.
Therefore, the content of N is set to more than 0.007% to not more
than 0.030%. The lower limit of the N content is preferably 0.010%
and the upper limit thereof is preferably 0.025%.
[0092] One or two elements selected from Al: more than 0.010% to
not more than 0.10% and Ti: more than 0.005% to not more than
0.035%, with Al+Ti being 0.015% or more
[0093] Al and Ti precipitate as nitrides in s steel, and the
prior-austenite diameter can be refined by the pinning effect of
these nitrides. As the result, fatigue strength and the
straightenability improve. In order to obtain such effects, it is
necessary to contain one or two elements selected from Al: more
than 0.010% to not more than 0.10% and Ti: more than 0.005% to not
more than 0.035%, with Al+Ti being 0.015% or more.
[0094] However, an excessive Al content may cause precipitation of
nitrides during nitrocarburizing treatment, thereby increasing the
hardness of a surface layer. As a result, the straightenability may
be impaired. In particular, a content of Al exceeding 0.10% may
significantly deteriorate the said straightenability. Furthermore,
a content of Ti exceeding 0.035% may cause coarsening of the
carbonitrides of Ti. As a result, not only the straightenability
may be deteriorated, but also the effect of refining the
prior-austenite diameter may not be obtained, thereby deteriorating
fatigue strength.
[0095] Therefore, in the present invention, one or two elements
selected from Al: more than 0.010% to not more than 0.10% and Ti:
more than 0.005% to not more than 0.035% are contained, with Al+Ti
being 0.015% or more.
[0096] In the present invention, the upper limit of the [Al+Ti]
content may be 0.135% with the content of Al being 0.10% and the
content of Ti being 0.035%.
[0097] In the present invention, Al and Ti may not be necessarily
contained in combination, that is to say, either one of Al and Ti
may be exclusively contained in the content of not less than
0.015%.
[0098] In the present invention, at least as far as Al is
contained, the lower limit of the Al content is preferably 0.015%
and the upper limit thereof is preferably 0.070%.
[0099] In the present invention, it is most preferable to contain
Al+Ti of not less than 0.015% in combination, on the condition of
Al: more than 0.010% to not more than 0.10% and Ti more than 0.005%
to not more than 0.035%.
[0100] Value of fn1: not less than 1.90
[0101] Even if the contents of C, Mn and Cr fall within the
aforementioned ranges, in other words, satisfy the requirements
that C: more than 0.45% to not less than 0.60%, Mn: more than 1.30%
to not more than 1.70%, and Cr: not more than 0.30%, when the value
of "fn1" expressed by the formula (1) is less than 1.90, the ratio
of ferrite in the ferrite-pearlite phase exceeds 10%, and
therefore, the intended high fatigue strength cannot be
obtained:
fn1=1.25C+Mn-0.1Cr (1);
[0102] Therefore, the value of "fn1" expressed by the formula (1)
is set to not less than 1.90. The value of "fn1" is preferably not
less than 1.93. The upper limit of the value of "fn1" may be close
to 2.45, wherein the contents of C and Mn are respectively 0.60%
and 1.70%, which are the upper limits thereof, and the Cr content
is close to 0.
[0103] Value of fn2: more than 0
[0104] Even if the contents of N, Al and Ti fall within the
aforementioned ranges, in other words, satisfy the requirement that
N: more than 0.007% to not more than 0.030%, and further satisfy
the requirements that one or two elements selected from Al: more
than 0.010% to not more than 0.10% and Ti: more than 0.005% to not
more than 0.035%, with Al+Ti being 0.015% or more, when the value
of "fn2" expressed by the formula (2) is not more than 0, the said
pinning effect by nitrides of Al and/or Ti cannot be obtained, and
the prior-austenite grain diameter may coarsen, and therefore the
intended high fatigue strength and excellent straightenability can
not be obtained:
fn2=N-0.45Al-( 1/22)Ti (2);
[0105] Therefore, the value of "fn2" expressed by the formula (2)
is set to more than 0. The value of "fn2" is preferably not less
than 0.0005. The upper limit of the value of "fn2" may be 0.0293,
wherein the content of N is 0.030%, which is the upper limit
thereof, A.sup.1 is not contained, and the content of Ti is
0.015%.
[0106] In the present invention, it is necessary to suppress the
content of V among the impurities in the following manner.
[0107] V: not more than 0.010%
[0108] V may increase the hardness of a surface layer and degrade
the straightenability. In particular, a content of V exceeding
0.010% may significantly degrade the straightenability. Moreover, a
content of V more than 0.010% may increase the ratio of ferrite in
the microstructure, thereby deteriorating fatigue strength. In view
of this, the content of V among the impurities is not more than
0.010%.
[0109] From the reasons mentioned above, the steel for
nitrocarburizing use according to the present invention (1) is
defined as the one containing C, Si, Mn, P, S, Cr, N, Al and Ti
within the aforementioned ranges, with the balance being Fe and
impurities, wherein V among the impurities is not more than 0.010%,
the "fn1" expressed by the said formula (1) is not less than 1.90,
and the "fn2" expressed by the said formula (2) is more than 0.
[0110] The chemical composition of the steel product for
nitrocarburizing use according to the present invention (4) is also
defined as the one containing C, Si, Mn, P, S, Cr, N, Al and Ti
within the aforementioned ranges, with the balance being Fe and
impurities, wherein V among the impurities is not more than 0.010%,
the "fn1" expressed by the said formula (1) is not less than 1.90,
and the "fn2" expressed by the said formula (2) is more than 0.
[0111] The steel for nitrocarburizing use of the present invention
may further contain Ca, according to need. The chemical composition
of the steel product for nitrocarburizing use of the present
invention also may further contain Ca, according to need.
[0112] In the following, Ca as an optional element will be
explained.
[0113] Ca: not more than 0.005%
[0114] Ca, if added, has the effect of improving the machinability
of steel. In order to obtain this effect, Ca may be added. However,
an excessive Ca content may cause deterioration of hot workability
and fatigue strength. In particular, a content of Ca exceeding
0.005% may significantly deteriorate the said hot workability and
fatigue strength. Therefore, the content of Ca is set to not more
than 0.005%.
[0115] On the other hand, in order to obtain the effect of
improving machinability by Ca sufficiently, the lower limit of the
Ca content is preferably set to not less than 0.0005%.
[0116] From the above-mentioned reason, the steel for
nitrocarburizing use according to the present invention (2) is
defined as the one which further contains Ca: not more than 0.005%
in addition to the steel for nitrocarburizing use according to the
present invention (1).
[0117] The chemical composition of the steel product for
nitrocarburizing use according to the present invention (5) is also
defined as the one which further contains Ca: not more than 0.005%
in addition to the chemical composition of the steel product for
nitrocarburizing use according to the present invention (4).
[0118] The steel for nitrocarburizing use of the present invention
may further contain Cu and/or Ni, according to need. The chemical
composition of the steel product for nitrocarburizing use of the
present invention also may further contain Cu and/or Ni, according
to need.
[0119] In the following, Cu and Ni as optional elements will be
explained.
[0120] Cu: not more than 0.3%
[0121] Cu, if added, has the effect of ensuring the further higher
fatigue strength by increasing the hardness of a base material as a
solid-solution strengthening element. In order to obtain this
effect, Cu may be added. However an excessive Cu content may
increase the hardness of a steel product and thereby causes
deterioration of machinability. In particular, a content of Cu
exceeding 0.3% may significantly deteriorate machinability.
Therefore, the content of Cu set to not more than 0.3%.
[0122] In order to obtain the effect of enhancing the fatigue
strength by Cu sufficiently, the lower limit of the Cu content is
preferably set to not less than 0.05%.
[0123] Ni: not more than 0.2%
[0124] Ni, if added, also has the effect of ensuring the further
higher fatigue strength by increasing the hardness of a base
material as a solid-solution strengthening element. In order to
obtain this effect, Ni may be added. However an excessive Ni
content may increase the hardness of a steel product and thereby
causes deterioration of machinability. In particular, a content of
Ni exceeding 0.2% may significantly deteriorate machinability.
Therefore, the content of Ni set to not more than 0.2%.
[0125] In order to obtain the effect of enhancing the fatigue
strength by Ni sufficiently, the lower limit of the Ni content is
preferably set to not less than 0.05%.
[0126] From the reasons mentioned above, the steel for
nitrocarburizing use according to the present invention (3) is
defined as the one which further contains one or two elements
selected from Cu: not more than 0.3% and Ni: not more than 0.2% in
addition to the steel for nitrocarburizing use according to the
present invention (1) or (2).
[0127] The chemical composition of the steel product for
nitrocarburizing use according to the present invention (6) is also
defined as the one which further contains one or two elements
selected from Cu: not more than 0.3% and Ni: not more than 0.2% in
addition to the chemical composition of the steel product for
nitrocarburizing use according to the present invention (4) or
(5).
[0128] (B) Microstructure
[0129] A coarsening of the prior-austenite grain diameter can be
suppressed by processing the steel, which satisfies the chemical
composition requirements described in the item (A), into the
intended shape by hot forging, followed by normalizing. The process
forms a microstructure being ferrite-pearlite phase with a ratio of
the ferrite of 10% or less, without coarsening the microstructure
of the steel product. And thereby the intended high fatigue
strength can be obtained.
[0130] In view of the above, the microstructure of the steel
products for nitrocarburizing use according to the present
inventions (4) to (6) is defined as the one being ferrite-pearlite
phase with a ratio of the ferrite of 10% or less.
[0131] As already described above, the "ferrite-pearlite phase"
represents a composite microstructure of ferrite and pearlite. The
ferrite of 10% or less by the "ratio" in the microstructure
represents the ferrite which forms the said "ferrite-pearlite
phase" together with pearlite, and does not encompass the one which
forms pearlite with cementite. In order to obtain the high fatigue
strength, it is desirable that the ferrite ratio in the
microstructure be reduced as much as possible. However, the ferrite
ratio of approximately 1% is the lower limit in the industrial
scale production by normalizing.
[0132] From the reasons mentioned above, each of the crankshafts
according to the present inventions (7) to (9) is defined as the
one which is respectively manufactured by using the steel product
for nitrocarburizing use according to the present inventions (4) to
(6).
[0133] At the heating temperature of 800 to 900.degree. C. on the
said "normalizing" treatment, a microstructure of a fine
ferrite-pearlite phase whose ferrite ratio is 10% or less can be
ensured. Therefore, in that case, fatigue strength and the
straightenability can be extremely improved.
[0134] The heat treating cost for "normalizing" is reduced to
approximately one-half of that required for "quenching and
tempering".
[0135] A nitrocarburized component such as a crankshaft and/or a
connecting rod for automobiles, industrial machines, and
construction machines, as well as the crankshafts according to the
present inventions (7) to (9), can be manufactured by using the
steel product for nitrocarburizing use which is obtained by working
the steel for nitrocarburizing use according to the present
invention into the intended shape in several methods.
[0136] Specifically, for instance, the steel product for
nitrocarburizing use of the present invention can be obtained by:
subjecting a cast bloom or a steel ingot of the steel for
nitrocarburizing use of the present invention, or a semi-finished
steel product manufactured from the said cast bloom or steel ingot
to hot working into the intended shape, followed by normalizing.
Then, the said nitrocarburized component can be obtained by
machining the steel product for nitrocarburizing use into a
predetermined shape, followed by nitrocarburizing.
[0137] In the aforementioned process, the hot working condition for
the intended shape is not particularly defined. However, it is
preferable to set the heating temperature before hot working for
the intended shape to 1100 to 1300.degree. C. Moreover, the hot
working is preferably terminated at 1100 to 900.degree. C., and
after the hot working, cooling is preferably performed in the
atmosphere.
[0138] Also, the normalizing condition is not necessarily defined.
However, it is preferable to set the heating temperature for
austenitization to 800 to 900.degree. C.
[0139] Moreover, the condition for nitrocarburizing treatment is
not necessarily defined. An ordinary method such as gas
nitrocarburizing, salt bath nitrocarburizing and plasma
nitrocarburizing may be conveniently used. Any of the above methods
can form stably and homogeneously an approximately 20 .mu.m thick
compound layer and a diffusion layer immediately beneath the said
compound layer on the surface of a nitrocarburized component.
[0140] In order to obtain the intended nitrocarburized component by
the gas nitrocarburizing method, for example, the treatment may be
performed in an atmosphere of 570.degree. C., which contains a
mixture of endothermic gas (RX gas) and ammonia gas at a ratio of
1:1, for approximately 3 hours, followed by cooling in an oil of
100.degree. C.
[0141] In the following, the present invention is described in
detail by referring to examples.
EXAMPLES
[0142] The steels 1 to 18 having the chemical compositions shown in
Table 1 were melted by use of a vacuum melting furnace with a
volume of 180 kg and made into ingots.
[0143] The steels 1 to 6, 17 and 18 shown in Table 1 are steels
whose chemical compositions fall within the range regulated by the
present invention. On the other hand, the steels 7 to 16 are steels
of comparative examples whose chemical compositions are out of the
range regulated by the present invention.
TABLE-US-00001 TABLE 1 Chemical composition (% by mass) Balance: Fe
and impurities Steel C Si Mn P S Cu Ni Cr V Al Ti Al + Ti N Ca fn1
fn2 1 0.52 0.21 1.49 0.019 0.049 -- -- 0.16 0.002 0.007 0.016 0.023
0.014 -- 2.124 0.0101 2 0.53 0.22 1.51 0.014 0.022 -- -- 0.14 0.002
-- 0.015 0.015 0.012 -- 2.159 0.0113 3 0.52 0.20 1.52 0.019 0.048
-- -- 0.16 0.001 0.022 -- 0.022 0.015 -- 2.154 0.0051 4 0.51 0.19
1.34 0.018 0.052 -- -- 0.19 0.003 0.021 0.005 0.026 0.013 -- 1.959
0.0033 5 0.54 0.22 1.54 0.017 0.025 -- -- 0.17 0.002 0.022 0.009
0.031 0.015 -- 2.198 0.0047 6 0.53 0.20 1.50 0.020 0.050 -- -- 0.15
0.002 0.025 0.003 0.028 0.015 0.0024 2.148 0.0036 7 0.54 0.20 *0.79
0.015 0.054 -- -- 0.16 0.003 0.004 0.013 0.017 0.013 -- *1.149
0.0124 8 0.53 0.20 1.45 0.016 0.051 -- -- 0.15 0.002 *0.004 *0.003
*0.007 0.014 -- 2.098 0.0121 9 0.52 0.21 1.50 0.021 0.052 -- --
0.16 *0.051 0.006 0.017 0.023 0.014 -- 2.134 0.0105 10 0.55 0.25
1.34 0.025 0.045 -- -- 0.17 0.003 0.023 0.002 0.025 0.009 -- 2.011
*-0.0019 11 0.49 0.22 1.31 0.017 0.030 -- -- 0.25 0.004 0.022 0.003
0.025 0.015 -- *1.898 0.0050 12 *0.30 0.19 1.41 0.015 0.042 -- --
0.09 0.002 0.015 0.002 0.017 0.011 -- *1.776 0.0042 13 0.58 0.22
*1.20 0.011 0.031 -- -- 0.05 0.001 0.021 0.007 0.028 0.017 -- 1.920
0.0072 14 0.55 0.23 *1.82 0.022 0.028 -- -- 0.14 0.002 0.024 0.003
0.027 0.014 -- 2.494 0.0031 15 0.53 0.25 1.32 0.018 0.044 -- --
0.13 0.003 0.012 0.001 *0.013 0.008 -- 1.970 0.0026 16 0.55 *0.75
1.32 0.020 0.020 -- -- 0.09 0.002 0.005 0.015 0.020 0.016 -- 1.999
0.0131 17 0.53 0.20 1.50 0.020 0.050 0.20 0.10 0.15 0.002 0.025
0.005 0.030 0.017 -- 2.148 0.0055 18 0.56 0.25 1.58 0.020 0.050
0.20 0.10 0.17 0.001 0.025 0.005 0.030 0.017 -- 2.263 0.0055 fn1 =
1.25C + Mn--0.1Cr fa2 = N--0.45Al-(1/22)Ti The mark * indicates
falling outside the conditions regulated by the present
invention.
[0144] The thus-obtained steel ingots were heated to a temperature
of 1200.degree. C. and hot forged into round bars having a diameter
of 60 mm on a condition that the finishing temperature of hot
forging fell to between 1050 and 1000.degree. C. After the said hot
forging, the round bars were subjected atmospheric cooling.
[0145] Then, the round bars having a diameter of 60 mm of steels 1
to 18 were normalized, that is to say, they were heated to
880.degree. C. and held for one hour at that temperature, and then
subjected air cooling.
[0146] Test specimens for microstructure observation having a 10
mm.times.10 mm section and 5 mm long were taken from the R/2
portion of each normalized round bar ("R" representing the radius
of round bar) having a diameter of 60 mm in the direction parallel
to the longitudinal direction thereof. Then, each of the test
specimens was embedded in a resin in such a manner that the 10
mm.times.10 mm surface served as a test plane. After mirror-like
polishing, in order to investigate the microstructure and area
ratio of ferrite, the polished surfaces were etched with nital, and
each etched surface was observed by an optical microscope at a
magnification of 100 times.
[0147] It is well-known-that ferrite precipitates as "pro-eutectoid
ferrite" on the prior-austenite grain boundaries. Accordingly, a
pearlite surrounded by the pro-eutectoid ferrite was assumed to be
a prior-austenite grain; the average area of prior-austenite grains
was calculated in the unit of mm.sup.2, and the average diameter of
prior-austenite grains was also calculated based on the definition
regulated in JIS G 0551 (2005) "Steels--Micrographic determination
of the apparent grain size".
[0148] Moreover, the notched Ono type rotating bending fatigue test
specimens shown in FIG. 1 and the test specimens for evaluating the
straightenability, having a diameter of 20 mm and a length of 300
mm, were taken from the R/2 portion of each normalized round bar
("R" representing the radius of round bar) having a diameter of 60
mm in the direction parallel to the longitudinal direction
thereof.
[0149] Subsequently, the above-mentioned two kinds of test
specimens were subjected to nitrocarburizing treatment, that is to
say, they were held in the atmosphere of 570.degree. C. which
contains a mixture of RX gas and ammonia gas at a ratio of 1:1 for
3 hours, followed by cooling in the oil of 100.degree. C.
[0150] The said nitrocarburized test specimens, having the shape of
FIG. 1, were tested for fatigue strength by the Ono type rotating
bending fatigue test, which was carried out at room temperature in
the atmosphere.
[0151] The target in the said fatigue test was to ensure a fatigue
strength of not less than 460 MPa.
[0152] Moreover, the said nitrocarburized test specimens, having a
diameter of 20 mm and a length of 300 mm, were tested for
evaluating the straightenability by the following bending test.
That is to say, the bending test was carried out by measuring the
crack length when a load was applied to the test specimens until a
reading of the strain gauge reached 17000.mu. (corresponding to
1.7% of bending strain) by a 3-point bending method at room
temperature in the atmosphere, with two fulcrums being spaced away
from each other by the distance of 70 mm. In the case where the
crack was too large to carry out the strain measurement with the
strain gauge, the said bending test was interrupted before the
reading reached 17000.mu..
[0153] The target in the said bending test was no cracking or a
crack length of not more than 0.1 mm.
[0154] The investigated results of the above-mentioned
microstructure, ferrite ratio, prior-austenite grain diameter,
fatigue strength, and the degree of strain and crack length as
evaluation criteria of the straightenability are shown in Table 2.
The testing Nos. 9 and 14 in Table 2 represent that the said
bending test was interrupted when the reading of the strain gauge
reached 15000.mu. and 14500.mu., respectively.
TABLE-US-00002 TABLE 2 Microstructure Fatigue Bending property
Testing Ratio of Prior-austenite grain strength Reading of the
Crack length No. Steel Phase ferrite (%) diameter (.mu.m) (MPa)
strain gauge (.mu.) (mm) 1 1 F + P 3 40 510 17000 0 2 2 F + P 3 43
500 17000 0 3 3 F + P 6 18 520 17000 0 4 4 F + P 8 21 510 17000 0 5
5 F + P 4 22 530 17000 0 6 6 F + P 7 25 505 17000 0 7 *7 F + P #28
42 $390 17000 0.02 8 *8 F + P 3 70 $360 17000 $0.11 9 *9 F + P #11
22 480 $15000 $0.45 10 *10 F + P 5 52 $380 17000 $0.12 11 *11 F + P
#26 38 $360 17000 0.008 12 *12 F + P #35 28 $360 17000 0 13 *13 F +
P 9 31 $440 17000 0 14 *14 F + P 4 22 510 $14500 $0.23 15 *15 F + P
8 61 $390 17000 $0.12 16 *16 F + P #19 39 $430 17000 $0.16 17 17 F
+ P 3 25 530 17000 0 18 18 F + P 2 21 560 17000 0 In the column of
microstructure, "F" and "P" denote ferrite and pearlite
respectively. The mark * indicates falling outside the chemical
compositions regulated by the present invention. The mark #
indicates falling outside the condition of microstructure regulated
by the present invention. The mark $ indicates falling short of the
target value in the present invention.
[0155] As is apparent from Table 2, regarding the testing Nos. 1 to
6, 17 and 18 using the said steels Nos. 1 to 6, 17 and 18, which
are the steels for nitrocarburizing use according to the present
invention, no crack was observed, and the excellent
straightenability was obtained even after a load was applied to the
test pieces until the reading of the strain gauge reached 17000.mu.
by the said bending test. Table 2 also obviously shows that fatigue
strengths of the said testing numbers were from 505 to 560 MPa,
which were higher than 460 MPa, and the said testing numbers had
the excellent fatigue strength.
[0156] On the contrary, the comparative testing Nos. 7 to 16
falling out of the conditions regulated by the present invention
could not satisfy the target of the present invention, namely,
having both the "excellent straightenability" and the "high fatigue
strength".
[0157] Specifically, regarding the testing No. 7, the Mn content of
steel 7 was 0.79%, which is smaller than the value defined by the
present invention. As a result, the ratio of ferrite in the
ferrite-pearlite phase was as high as 28%. Therefore, the fatigue
strength was as low as 390 MPa, which falls short of the target
value.
[0158] Regarding the testing No. 8, the total content of Al and Ti
of steel 8 was 0.007%, which is smaller than the value defined by
the present invention. Also, the value of "fn1" expressed by the
said formula (1) was 1.149, which does not satisfy the condition
defined by the present invention. As a result, the said testing No.
8 satisfies neither the target of the fatigue strength nor the
target of the straightenability.
[0159] Regarding the testing No. 9, the V content of steel 9 was
0.051%, which exceeds the value defined by the present invention.
Also, the ratio of ferrite in the ferrite-pearlite phase was as
high as 11%. Furthermore, a crack of 0.45 mm was generated at a
point of time when a load was applied to 15000.mu. on the reading
of the strain gauge. Therefore, the straightenability was
significantly poor.
[0160] Regarding the testing No. 10, the individual chemical
components of steel 10 fell within the range defined by the present
invention. However, the value of "fn2" expressed by the said
formula (2) was -0.0019, which does not meet the condition defined
by the present invention. As a result, the said testing No. 10
satisfies neither the target of the fatigue strength nor the target
of the straightenability.
[0161] Regarding the testing No. 11, the individual chemical
components of steel 11 fell within the range defined by the present
invention. However, the value of "fn1" expressed by the said
formula (1) was 1.898, which does not meet the condition defined by
the present invention. As a result, the ratio of ferrite in the
ferrite-pearlite phase was as high as 26%. Therefore, the fatigue
strength was as low as 360 MPa, which falls short of the target
value.
[0162] Regarding the testing No. 12, the C content of steel 12 was
0.30%, which is smaller than the value defined by the present
invention. Furthermore, the value of "fn1" expressed by the said
formula (1) was 1.776, which does not meet the condition defined by
the present invention. As a result, the ratio of ferrite in the
ferrite-pearlite phase was as high as 35%. Consequently, the
fatigue strength was as low as 360 MPa, which falls short of the
target value.
[0163] Regarding the testing No. 13, the Mn content of steel 13 was
1.20%, which is smaller than the value defined by the present
invention. As a result, the fatigue strength was as low as 440 MPa,
which falls short of the target of the present invention.
[0164] Regarding the testing No. 14, the Mn content of steel 14 was
1.82%, which exceeds the value defined by the present invention. As
a result, a crack of 0.23 mm was generated at a point of time when
a load was applied to 14500.mu. on the strain gauge. Therefore, the
straightenability was significantly poor.
[0165] Regarding the testing No. 15, the total content of Al and Ti
of steel 15 was 0.013%, which is smaller than the value defined by
the present invention. As a result, the said testing No. 15
satisfies neither the target of the fatigue strength nor the target
of the straightenability.
[0166] Regarding the testing No. 16, the Si content of steel No. 16
was 0.75%, which is higher than the value defined by the present
invention. As a result, the ratio of ferrite in the
ferrite-pearlite phase was as high as 19%. Consequently, the
fatigue strength was as low as 430 MPa and a crack length was 0.16
mm in the case where a load was applied to 17000.mu. on the strain
gauge, that is to say, the said testing No. 16 satisfies neither
the target of the fatigue strength nor the target of the
straightenability.
[0167] The present invention has been described as above in detail
referring to examples, but the present invention is not restricted
to the foregoing examples. As far as an arrangement satisfies the
requirements of the present invention, although not disclosed as
examples, the arrangement is embraced by the present invention.
INDUSTRIAL APPLICABILITY
[0168] The steels for nitrocarburizing use of the present invention
have high fatigue strength and excellent straightenability after
the nitrocarburizing treatment, without performing the expensive
heat treatment of quenching and tempering. Consequently, they are
suitable as raw materials for nitrocarburized components such as
crankshafts and/or connecting rods for automobiles, industrial
machines, construction machines and so on.
* * * * *