DESCRIPTION
Title oflnvention: NON-THERMAL REFINED SOFT-NITRIDED COMPONENT
TECHNICAL FIELD
[0001]
The present invention relates to a non-thermal refined soft-nitrided component.
Specifically, the present invention relates to a non-thermal refined soft-nitrided
component having a high bending fatigue strength and an excellent bending
straightening property, produced by being subjected to a soft-nitriding treatment without
being subjected to a thermal refining treatment of quenching-tempering after being
forged and machined into a predetermined shape, such as a crankshaft or a connecting
rod used in automobiles, industrial machines, and construction machinery, etc. More
specifically, the present invention relates to a non-thermal refined soft-nitrided
component having an excellent bending straightening property and a high bending
fatigue strength of750 MPa or more in a bending fatigue test.
[0002]
The "non-thermal refined soft-nitrided component" denotes a component
subjected to a soft-nitriding treatment without being subjected to a
"quenching-tempering treatment" that is a so-called "thermal refining treatment" after
being machined. In the following description, the "component subjected to the
soft-nitriding treatment" is referred to simply as a "soft-nitrided component".
BACKGROUND ART
[0003]
In production of automobile components that require a high fatigue strength
and a high wear resistance, treatments, such as an induction hardening treatment and a
soft-nitriding treatment that are a casehardening treatment, are carried out after forging
and machining in most cases.
[0004]
2
The "soft-nitriding treatment" performs a cementation treatment on nitrogen
and carbon at a temperature of an A1 transformation point or less, and has such major
characteristics that have a low heat-treatment temperature and a smaller heat treatment
strain than that in the "induction hardening treatment". A "compound layer" (layer
formed of precipitated nitride such as Fe3N) observed as a white portion through etching
using nital is formed in a surface layer of the component subjected to the soft-nitriding
treatment. A "diffusion layer" is formed between the above compound layer and a
base metal (base material).
[0005]
The soft-nitriding treatment causes a small heat treatment strain, but cannot
eliminate this strain, and thus brings not a small bad influence on dimensional accuracy.
Particularly, even a slight deterioration of dimensional accuracy becomes a crucial
matter in a crankshaft or the like that is a rotational shaft component. Hence, it is
required to perform bending-straightening after the soft-nitriding treatment so as to
improve the dimensional accuracy.
[0006]
Unfortunately, cracks may be generated from the surface layer if the
soft-nitrided component is subjected to the bending-straightening. Hence, a
soft-nitrided component, such as a crankshaft, is required to experience no cracks even
if being subjected to bending-straightening, that is, to have an excellent bending
straightening property as well as a high bending fatigue strength.
[0007]
In the following description, the soft-nitrided component may be represented
by a crankshaft in some cases.
[0008]
Because of current demand for consideration to the environments, a crankshaft
that is a major component of an engine is also oriented to reduction in weight and size
without exclusion, and has been required to have an extremely high bending fatigue
strength of750 MPa or more, for example.
[0009]
3
In the light of cost reduction, resource saving, and others, there has been
increased demand for a non-thermal refined crankshaft without being subjected to a
"quenching-tempering treatment" (thermal refining treatment) during the production
thereof.
[0010]
In order to secure the above bending fatigue strength of 750 MPa or more in a
non-thermal refined crankshaft, it is required to set hardness at a position of 0.05 mm
from the surface of the component (also referred to as a "surface-layer hardness",
hereinafter) to be at least 400 or more in terms of a Vickers hardness (referred to as a
"HV hardness", hereinafter) after the soft-nitriding treatment.
[0011]
However, in the case of setting the HV hardness at a position of 0.05 mm from
the surface of the crankshaft to be 400 or more, cracks are likely to be generated in the
surface layer if the bending straightening is performed. Conducting a bending fatigue
test on such a crankshaft results in fatigue fractures initiated from the above cracks.
[0012]
In addition, as described above, there has been increased demand for further
reduction in weight of a crankshaft, and thus further more flexibility has been required
in crankshaft shape designing. Consequently, steel material for a crankshaft is required
to have a bending straightening property high enough for a crankshaft having a shape
likely to exhibit a greater bending than that in a conventional art during the
soft-nitriding to be bending-straightened.
[0013]
Accordingly, there has been extremely strong demand for a crankshaft having a
sufficient bending straightening property in addition to a bending fatigue strength as
high as 750 MPa or more.
[0014]
To meet the above demand, for example, Patent Document 1 discloses a
"non-thermal refined steel for soft-nitriding", wherein the steel contains, in mass%, C:
0.2 to 0.6%, Si: 0.05 to 1.0%, Mn: 0.25 to 1.0%, S: 0.03 to 0.2%, Cr: 0.2% or less, s-Al:
4
0.045% or less, Ti: 0.002 to 0.010%, N: 0.005 to 0.025%, and 0: 0.001 to 0.005%, and
further contains one or more types of elements selected from Ph: 0.01 to 0.40%, Ca:
0.0005 to 0.0050%, and Bi: 0.005 to 0.40% if necessary, satisfies conditions:
0.12xTi%<0%<2.5xTi%, and 0.04xN%<0%<0.7xN%, and includes a balance made of
Fe and unavoidable impurities, wherein a micro-structure after hot forging is a mixed
structure of ferrite and perlite.
[0015]
Patent Document 2 discloses a "crankshaft" made of a steel whose surface is
subjected to a nitriding treatment or a soft-nitriding treatment, the crankshaft including a
pin section and a journal section, wherein the steel contains, as an alloy element, C: 0.07
mass% or more to 0.12 mass% or less, Si: 0.05 mass% or more to 0.25 mass% or less,
Mn: 0.1 mass% or more to 0.5 mass% or less, Cu: 0.8 mass% or more to 1.5 mass% or
less, Ni: 2.4 mass% or more to 4.5 mass% or less, Al: 0.8 mass% or more to 1.5 mass%
or less, Ti: 0.5 mass% or more to 1.5 mass% or less, and further contains one or more
types of elements selected from S: 0.01 mass% or more to 0.10 mass%, Ca: 0.0010
mass% or more to 0.0050 mass% if necessary, and includes a balance made of Fe and
unavoidable impurities; and each steel specimen taken from a center part of the steel
that is affected by no influence of the nitriding treatment is subjected to a solid solution
·treatment at 1200°C for one hour, and thereafter, is cooled at an appropriate cooling
speed of 0.3°C/seconds or more to 1.5°C/seconds or less within a temperature range
from 900°C or more to 300°C or less, thereby setting a ratio of bainite in the steel
micro-structure to be 80% or more, and setting the HV hardness to be 200 or more to
300 or less; each internal hardness of the pin section and the journal section that are
subjected to the nitriding treatment or the soft-nitriding treatment is set to be 350 or
more to 500 or less in terms of the HV hardness; and the HV hardness at a position of
0.05 mm from the surface is 650 or more to 950 or less.
[0016]
In Patent Document 3, the present inventors have proposed a "non-thermal
refined nitrided crankshaft" wherein a steel material of a base metal contains, in mass%,
C: 0.25 to 0.60%, Si: 0.10 to 1.0%, Mn: 0.60 to 2.0%, P: 0.08% or less, S: 0.10% or less,
5
Al: 0.05% or less, Cr: 0.20 to 1.0%, and N: 0.0030 to 0.0250%, includes a balance made
of Fe and impurities, and satisfies 40-C+2Mn+5.5Cr;?;43.0; and the HV hardness at a
depth of0.05 mm from the surface is 380 to 600, and at least each of a pin fillet section,
a journal fillet section, and a pin section has a compound-layer depth of 5 J.tm or less.
[0017]
This non-thermal refined nitrided crankshaft may further contain one or more
types of elements selected from Cu, Ni, Mo, V, Ti, and Ca, and in this case, it is
necessary to satisfy [40-C+2Mn+5.5Cr+26Mo;?;43.0].
[0018]
In Patent Document 4, the present inventors have further proposed a "thermal
refined soft-nitrided component", wherein a steel material of a base metal contains, in
mass%, C: 0.25 to 0.40%, Si: 0.10 to 0.35%, Mn: 0.60 to 1.0%, P: 0.08% or less, S:
0.10% or less, Al: 0.05% or less, Cr: 0.30 to 1.10%, and N: 0.0030 to 0.0250%, and
includes a balance made of Fe and impurities; and the HV hardness at a position of 0.05
mm from the surface is 400 to 600, and a compound-layer depth at a stress concentrated
region is 5 J.tm or less.
[0019]
The thermal refined nitrided component may further contain one or more types
of elements selected from Cu, Mo, V, Ni, and Ti.
LIST OF PRIOR ART DOCUMENTS
PATENT DOCUMENT
[0020]
Patent Document 1: JP2002-226939A
Patent Document 2: JP2007-177309A
Patent Document 3: JP2012-26005A
Patent Document 4: JP2011-42846A
DISCLOSURE OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
6
[0021]
With the composition in the invention of Patent Document 1, it is hard to attain
a sufficient surface-layer hardness. Hence, the bending fatigue strength is too low to
satisfy 750 MPa, as shown in an embodiment of Patent Document 1.
[0022]
With the composition in the invention of Patent Document 2, the surface-layer
hardness after the soft-nitriding treatment becomes too high, as shown in an
embodiment thereof. Hence, it is hard to tell that a sufficient bending straightening
property is secured during performing the bending straightening treatment.
[0023]
With the composition in the invention of Patent Document 3, it is possible to
attain a high fatigue strength and a high bending straightening property, as shown in an
embodiment thereof. However, a crankshaft has been oriented to reduction in weight
and size, and requires more severe fatigue strength and bending straightening property.
[0024]
The thermal refined soft-nitrided component disclosed in Patent Document 4 is
excellent in bending straightening property after the soft-nitriding treatment, and has a
bending fatigue strength as high as 800 MPa or more in the bending fatigue test.
Accordingly, this component is usable as a component, such as a crankshaft, in
automobiles, industrial machines, and construction machinery, for example, and
contributes to reduction in weight and size. Unfortunately, in the invention of Patent
Document 4, subsequent to the machining, a thermal refining treatment of quenching
and tempering is required before the soft-nitriding treatment.
[0025]
An object of the present invention, which has been made in order to solve the
problems above, is to provide a non-thermal refined soft-nitrided component excellent
in bending straightening property, and having a fatigue strength as high as 750 MPa or
more in the bending fatigue test.
MEANS FOR SOLVING THE PROBLEMS
7
[0026]
In order to solve the aforementioned problems, the present inventors have
conducted various studies. As a result, the following points 1) to 7) were found.
[0027]
1) A thin sheet specimen was collected from a surface layer of each steel
material subjected to the soft-nitriding treatment, and a tension test was conducted on
each specimen; and as a result, specimens whose compound layers were removed
exhibited greatly enhanced tension in the tension test, compared with that of specimens
whose compound layers were not removed.
[0028]
2) As a result of observation on a fracture surface of each thin sheet specimen
after the tension test, the specimens whose compound layers were not removed had
fracture surfaces where brittle fractures were generated in the compound layers, thus
initiating cracking; contrary to this, the specimens whose compound layers were
removed had ductility fracture surfaces.
[0029]
3) If the compound layer in the surface layer of the steel material subjected to
the soft-nitriding treatment is removed, the fracture morphology during the
bending-straightening is changed from the brittle fractures starting from the compound
layer to the ductility fractures. Accordingly, it is possible to enhance the bending
straightening property of the soft-nitrided component.
[0030]
4) Meanwhile, in the bending fatigue strength, there is little difference between
before and after the removal of the compound layer. In the case of the non-thermal
refined soft-nitrided component, if the hardness at a position of 0.05 mm from the
component surface is 400 or more in terms of the HV hardness, and if the hardness at a
position of 1.0 mm from the component surface (also referred to as an "internal
hardness", hereinafter) is 200 or more in terms of the HV hardness, it is possible to
stably attain a high bending fatigue strength of 750 MPa or more.
[0031]
8
5) In the non-thermal refmed component, an endurance ratio (fatigue
strength/tensile strength) of the base metal is lower than that in the thermal refined
component. Therefore, the non-thermal refmed component has a lower fatigue
strength of the base metal than that of the thermal refined component even if the
non-thermal refined component has an internal hardness equivalent to that of the
thermal refined component. Particularly, if the non-thermal refined soft-nitrided
component has an internal hardness of less than 200 in terms of the HV hardness, in
spite of having a high surface-layer hardness of 400 or more in terms of the HV
hardness, fractures initiated from the internal part are caused in the fatigue test, which
makes it hard to attain a fatigue strength as high as 750 MPa or more.
[0032]
6) It is possible to substantially secure a sufficient bending straightening
property by removing the compound layer in the surface layer of the soft-nitrided
component even if the surface-layer hardness after the soft-nitriding treatment is 400 or
more in terms of the HV hardness.
[0033]
7) However, in the case of a crankshaft shape that requires a high bending
straightening property, if the surface-layer hardness of the soft-nitrided component
becomes more than 480 in terms of the HV hardness, it may be hard to attain a
sufficient bending straightening property even ifthe compound layer is removed.
[0034]
The present invention has been accomplished based on the above findings, and
the gist lies in a non-thermal refined soft-nitrided component as follows.
[0035]
( 1) A non-thermal refined soft-nitrided component having a compound layer in
a surface layer of a steel material of a base metal, chemical composition of the steel
material of the base metal containing: in mass%, C: 0.25 to 0.40%; Si: 0.10 to 0.35%;
Mn: more than 2.0% to 2.8% or less; N: 0.0030 to 0.0250%; Cu: 0 to 1.0%; Mo: 0 to
0.3%; Ni: 0 to 0.5%; Ti: 0 to 0.020%; and a balance being Fe and impurities, the
impurities including P: 0.08% or less; S: 0.10% or less; Al: 0.05% or less; and Cr: less
9
than 0.20%, wherein an HV hardness at a position of 0.05 mm from the surface is 400 to
480, an HV hardness at a position of 1.0 mm from the surface is 200 or more, and a
compound-layer depth at a stress concentrated region is 5J.lm or less.
[0036]
(2) The non-thermal refined soft-nitrided component as set forth in the above
(1 ), wherein the steel material of the base metal contains, in mass%, one or more types
of elements selected from Cu: 0.05 to 1.0% and Mo: 0.05 to 0.3%.
[0037]
(3) The non-thermal refined soft-nitrided component as set forth in the above
(1) or (2), wherein the steel material of the base metal contains, in mass%, one or more
types of elements selected from Ni: 0.05 to 0.5% and Ti: 0.005 to 0.020%.
[0038]
The term "impurities" denotes those impurities which come from ores and
scraps as row materials, manufacturing environments, and so on during industrially
producing steel materials.
[0039]
The "stress concentrated region" denotes a region where fatigue fractures are
generated due to bending or cracking is caused while carrying out the bending
straightening. As a specific example thereof, if the "non-thermal refined soft-nitrided
component" is a crankshaft having a shape as shown in Figure 1, the "stress
concentrated region" represents a "pin fillet section" or a "journal fillet section" of the
crankshaft.
ADVANTAGEOUS EFFECTS OF THE INVENTION
[0040]
The non-thermal refined soft-nitrided component of the present invention is
excellent in bending straightening property after the soft-nitriding treatment, and has a
bending fatigue strength as high as 750 MPa or more in the bending fatigue test;
therefore, this non-thermal refined soft-nitrided component is usable as a component,
such as a crankshaft, in automobiles, industrial machines, and construction machinery,
10
and is capable of realizing reduction in weight and size of components used threrein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0041]
[Figure 1] Figure 1 is a drawing exemplifying a part of a crankshaft as a non-thermal
refined soft-nitrided component, and explaining a "pin fillet section" and a "journal fillet
section" equivalent to a "stress concentrated region" of the crankshaft.
[Figure 2] Figure 2 is a drawing showing a shape of a grooved Ono-type rotating
bending fatigue test specimen used in Example; and a unit of measurement in the
drawing is "mm".
[Figure 3] Figure 3 is a drawing showing a shape of a four-point bending test specimen
used in Example; and a unit of measurement in the drawing is "mm".
MODE FOR CARRYING OUT THE INVENTION
[0042]
Each requirement of the present invention will be described in detail,
hereinafter. It should be noted that "%" for a content of each element denotes
"masso/o".
[0043]
(A) Chemical composition of steel material of base metal:
C: 0.25 to 0.40%
C has an action to improve the internal hardness and the surface-layer hardness,
and enhance the bending fatigue strength. The C content is required to be 0.25% or
more in order to attain a desired bending fatigue strength. However, an excessive C
content results in an excessively high surface-layer hardness, so that it is hard to attain a
sufficient bending straightening property even if the compound-layer depth at the stress
concentrated region is 5 J.lm or less. Hence, the C content is set to be 0.25 to 0.40%.
The C content is preferably 0.28% or more, and preferably 0.38% or less.
[0044]
Si: 0.10 to 0.35%
11
Si is an element necessary for deoxidation during melting the steel, and the Si
content of at least 0.10% is required for obtaining the above effect. However, an
excessive content of Si causes excessive deterioration of the bending straightening
property even if the compound-layer depth at the stress concentrated region is 5 ~m or
less. Hence, the Si content is set to be 0.10 to 0.35%. The Si content is preferably
0.15% or more, and preferably 0.30% or less.
[0045]
Mn: more than 2.0% to 2.8% or less
Mn is an element having a deoxidizing action similar to Si. Mn also has an
action to improve the internal hardness, and increase a solute nitrogen content in the
surface layer during the soft-nitriding to improve the surface-layer hardness, thereby
enhancing the bending fatigue strength. In order to exert this effect, the Mn content is
required to be more than 2.0%. On the other hand, the Mn content of more than 2.8%
causes an excessively high surface-layer hardness, which excessively deteriorates the
bending straightening property even if the compound-layer depth at the stress
concentrated region is 5 ~m or less. Accordingly, the Mn content is set to be more
than 2.0% to the 2.8% or less. The Mn content is preferably 2.2% or more, and
preferably 2.7% or less.
[0046]
N: 0.0030 to 0.0250%
N is an element to improve the bending fatigue strength and the bending
straightening property. In order to attain this effect, the N content is required to be
0.0030% or more. On the other hand, the N content of more than 0.0250% rather
saturates this effect. Accordingly, theN content is set to be 0.0030 to 0.0250%. The
N content is preferably 0.0080% or more, and preferably 0.0220% or less.
[0047]
One of the steel materials of the base metal of the non-thermal refined
soft-nitrided component according to the present invention contains the C toN elements,
and a balance made of Fe and impurities, wherein P, S, AI, and Cr among the impurities
are set such that P: 0.08% or less, S: 0.10% or less, Al: 0.05% or less, and Cr: less than
12
0.20%.
[0048]
P: 0.08% or less
P is an impurity contained in the steel, and deteriorates the bending fatigue
strength. Particularly, the P content of more than 0.08% significantly deteriorates the
bending fatigue strength. Accordingly, the P content is set to be 0.08% or less. It is
preferable to set the P content to be 0.04% or less.
[0049]
S: 0.10% or less
S is an impurity contained in the steel. An effect to improve machinability
can be attained by actively containing S. However, the S content of more than 0.10%
significantly deteriorates the bending fatigue strength and the bending straightening
property. Accordingly, the S content is set to be 0.10% or less. It is preferable to set
the S content to be 0.08% or less. In order to attain the effect to improve machinability,
it is preferable to set the S content to be 0.04% or more.
[0050]
Al: 0.05% or less
Al is an impurity contained in the steel. An excessive Al content deteriorates
the bending straightening property. Particularly, the S content of more than 0.05%
significantly deteriorates the bending straightening property even if the compound-layer
depth at the stress concentrated region is 5 ).lm or less. Accordingly, the Al content is
set to be 0.05% or less. The Al content is preferably 0.03% or less.
[0051]
Cr: less than 0.20%
Cr is an impurity contained in the steel. Containing Cr may excessively
increase the surface-layer hardness, which deteriorates the bending straightening
property; thus it is preferable to set the Cr content to be as small as possible.
Accordingly, the Cr content is set to be less than 0.20%. The Cr content is preferably
0.10% or less.
[0052]
13
Another of the steel materials of the base metal of the non-thermal refined
soft-nitrided component according to the present invention contains one or more types
of elements selected from Cu, Mo, Ni, and Ti instead of part of Fe.
[0053]
Each operational effect of Cu, Mo, Ni, and Ti that are optional elements, and a
reason for limitation of each content will be described, hereinafter.
[0054]
Cu and Mo may be contained for the purpose of enhancing the bending fatigue
strength. Detailed description regarding this will be provided as below.
[0055]
Cu: 0 to 1.0%
Cu is ·an element to improve the internal hardness, and enhance the bending
fatigue strength. Hence, Cu may be contained. However, the Cu content of more
than 1.0% deteriorates hot workability. Accordingly, the amount of Cu to be contained
is set to be 1.0% or less. The amount of Cu is preferably 0.4% or less, and more
preferably 0.3% or less.
(0056]
In order to stably attain the above effect, it is preferable to set the amount of Cu
to be 0.05% or more, and more preferably 0.1% or more.
(0057]
Mo: 0 to 0.3%
Mo has an action to strengthen ferrite, and improve the internal hardness to
enhance the bending fatigue strength. Hence, Mo may be contained. However, an
excessive Mo content of more than 0.3% rather saturates the above effect, only to
deteriorate economic efficiency. Accordingly, the amount of Mo to be contained is set
to be 0.3% or less. The amount ofMo is preferably 0.2% or less.
(0058]
In order to stably attain the above effect, the amount of Mo is preferably 0.05%
or more, and more preferably 0.1% or more.
[0059]
14
Any one type selected from Cu and Mo, or two types selected from Cu and Mo
in combination may be contained. The total content of these elements may be 1.30%,
and preferably 0.30% or less.
[0060]
Ni and Ti may be contained for the purpose of enhancing the bending
straightening property.
hereinafter.
[0061]
Ni: 0 to 0.5%
Detailed description regarding this will be provided,
Ni is an element to improve toughness, and enhance the bending straightening
property. Accordingly, Ni may be contained. However, the Ni content of more than
0.5% rather saturates the above effect, only to deteriorate the economic efficiency.
Hence, the amount ofNi to be contained is set to be 0.5% or less. The amount ofNi is
preferably 0.3% or less, and more preferably 0.2% or less.
[0062]
In order to stably attain the above effect, the amount ofNi is preferably 0.05%
or more, and more preferably 0.08% or more.
[0063]
In the case of containing Cu, it is likely to cause hot cracking called ''Cu
checking", and in order to prevent this, it is preferable to contain Cu in combination
with Ni in a manner as to satisfy Ni/Ct20.5.
[0064]
Ti: 0 to 0.020%
Ti is an element that forms nitride, and refines grains to hinder propagation of
cracking during the bending-straightening, thereby enhancing the bending straightening
property. Accordingly, Ti may be contained. However, the Ti content of more than
0.020% generates coarse nitride, and significantly deteriorates the bending straightening
property even if the compound-layer depth at the stress concentrated region is 5 1-Lm or
less. Accordingly, the amount of Ti to be contained is set to be 0.020% or less. The
amount ofTi is preferably 0.015% or less.
15
[0065]
In order to stably attain the above effect, the amount ofTi is preferably 0.005%
or more.
(0066]
Any one type selected from Ni and Ti, or two types selected from Ni and Ti in
combination may be contained. The total content of these elements may be 0.520%,
and preferably 0.30% or less.
[0067]
(B) Hardness (surface-layer hardness) at a position of 0.05 mm from the
surface:
In the non-thermal refined soft-nitrided component according to the present
invention, the HV hardness at a position of 0.05 mm from the surface should be 400 to
480.
[0068]
It is possible to secure a high bending fatigue strength of 750 MPa or more if
the HV hardness at a position of 0.05 mm from the surface of the non-thermal refined
soft-nitrided component, that is, the HV hardness in the surface layer is 400 or more, the
HV hardness at a position of 1.0 mm from the surface of the component, that is, the HV
hardness at the internal part is 200 or more, and the compound-layer depth at the stress
concentrated region is 5 J.lm or less. However, if the HV hardness in the surface layer
is more than 480, in the case of using a crankshaft shape to likely cause a greater
bending than that of a conventional art during the soft-nitriding, it may be hard to attain
a practically sufficient bending straightening property even if the compound-layer depth
at the stress concentrated region is 5 J.lm or less.
[0069]
Accordingly, in the non-thermal refined soft-nitrided component according to
the present invention, the HV hardness at a position of 0.05 mm from the surface is set
to be 400 to 480. The HV hardness at a position of 0.05 mm from the surface is
preferably 410 or more, and preferably 470 or less.
[0070]
16
(C) Hardness (internal hardness) at a position of 1.0 mm from the surface:
In the non-thermal refined soft-nitrided component according to the present
invention, the HV hardness at a position of 1.0 mm from the surface of the component
should be 200 or more.
[0071]
In the non-thermal refined soft-nitrided component, since the endurance ratio
of the base metal is lower than that in the thermal refined soft-nitrided component, the
fatigue strength of the base metal becomes lower than that in the thermal refined
soft-nitrided component even if the non-thermal refined soft-nitrided component has an
internal hardness equivalent to that of the thermal refined soft-nitrided component.
Consequently, in the non-thermal refined soft-nitrided component, in the case of having
an HV hardness of less than 200 at the internal part, even if the non-thermal refined
soft-nitrided component has an internal hardness equivalent to that of the thermal
refined soft-nitrided component, and also has a surface-layer hardness as high as 400 or
more in terms of the HV hardness, fatigue fractures initiated from the internal part may
be caused, which makes it hard to attain a high fatigue strength of750 MPa or more.
[0072]
Accordingly, in the non-thermal refined soft-nitrided component according to
the present invention, the HV hardness at a position of 1.0 mm from the surface is set to
be 200 or more. The HV hardness at a position of 1.0 mm from the surface 1s
preferably 210 or more, and preferably 320 or less in the light of machinability.
[0073]
(D) Compound-layer depth at the stress concentrated region:
In the non-thermal refined soft-nitrided component according to the present
invention, the compound-layer depth at the stress concentrated region should be 5 f.lm or
less.
[0074]
By setting the compound-layer depth at the stress concentrated region to be
thinner, it is possible to improve the bending straightening property without
deteriorating the bending fatigue strength, but it is hard to expect significant
17
improvement of the bending straightening property if the compound layer whose depth
is more than 5 J..l.m still remains.
[0075]
Accordingly, in the non-thermal refined soft-nitrided component according to
the present invention, the compound-layer depth at the stress concentrated region is set
to be 5 J..l.m or less. The compound-layer depth at the stress concentrated region is
preferably 3 J..l.m or less, and it is most preferable to have no compound layer, that is,
have a compound-layer depth of 0 IJ.m.
[0076]
Such a component that satisfies the above (B) to (D) can be obtained by
machining a hot forging that satisfies the chemical composition specified by the present
invention, for example, and thereafter, subjecting the machined hot forging to the
soft-nitriding treatment to retain this hot forging for two hours in an atmosphere where
an RX gas and an ammonia gas are mixed at a mixture ratio of 1 : 1 at a temperature of
600°C, and then cooling the machined hot forging in an oil having a temperature of
90°C, and subsequently, grinding the stress concentrated region through machining
using a lapping machine or the like.
[0077]
The above mentioned "RX gas" is one type of a modified gas, and represents a
brand name ofthis gas.
[0078]
Specifically, representing a crankshaft as an example of the non-thermal
refined soft-nitrided component, for example, this crankshaft can be obtained in such a
manner that a starting material that satisfies conditions on the chemical composition
specified by the present invention is hot-forged into a crankshaft, this crankshaft is
machined, and thereafter this crankshaft is subjected to the soft-nitriding treatment to
retain the crankshaft for two hours in an atmosphere where an RX gas and an ammonia
gas are mixed at a mixture ratio of 1: 1 at a temperature of 600°C, and then cooled in an
oil having a temperature of 90°C, and subsequently, the pin fillet section and the journal
fillet section are ground through machining using a lapping machine or the like.
18
[0079]
The present invention will be described in more detail usmg Example,
hereinafter.
EXAMPLE
[0080]
Each of Steels A to K having respective chemical compositions shown in Table
1 was melt in a 70t convertor, subjected to continuous casting, and further subjected to
blooming into a cast piece having a cross sectional dimension of 180 mmx 180 mm.
[0081]
Subsequently, each cast piece was hot-forged under the conditions that a
heating temperature was 1200°C, and a finishing temperature was 1 000 to 1 050°C into
a steel bar having a diam~ter of 90 mm. Each steel bar after the hot-forging was
cooled in the atmosphere down to a room temperature through allowing cooling.
[0082]
In Table 1, each of Steels A to G is an example having chemical composition
within the range specified by the present invention, and each of Steels H to K is an
example having chemical composition out of the range specified by the present
invention.
[0083]
[Table 1]
19
Table 1
Steel
Chemical composition (mass%) Balance: Fe and impurities
c Si Mn p s Cu Ni Cr Mo AI Ti N
A 0.27 0.20 2.70 0.015 0.084 - - 0.06 - 0.012 - 0.0055
B 0.36 0.14 2.25 0.020 0.042 - - 0.14 - 0.024 - 0.0063
c 0.30 0.25 2.50 0.014 0.060 - - 0.07 - 0.019 0.010 0.0150
D 0.32 0.18 2.42 0.011 0.055 0.20 - 0.18 - 0.008 - 0.0095
E 0.29 0.15 2.10 0.010 0.077 - 0.10 0.14 - 0.011 - 0.0063
F 0.32 0.14 2.48 0.012 0.061 0.18 0.08 0.18 - 0.010 - 0.0085
G 0.31 0.21 2.10 0.011 0.062 - - 0.12 0.10 0.008 - 0.0145
H * 0.10 0.12 2.20 0.022 0.045 - - 0.10 - 0.009 - 0.0101
I 0.37 0.11 * 1.25 0.010 0.044 - - 0.11 - 0.005 - 0.0122
J 0.32 0.20 * 3.20 0.019 0.055 - - 0.14 - 0.012 - 0.0063
K 0.33 0.18 2.52 0.014 0.060 - - *0.34 - 0.022 - 0.0050
A mark (*) represents deviation from the chemical composition specified by the present invention.
20
[0084]
Each steel bar having a diameter of 90 mm obtained in this manner was heated
up to a temperature of 1200°C, and then hot-forged at a finishing temperature of 1000 to
1050°C into a steel bar having a diameter of 50 mm. Each finished steel bar was
cooled down to a room temperature through allowing cooling in the atmosphere.
[0085]
Some of the steel bars, each having a diameter of 50 mm in Steel A were
further subjected to a normalizing treatment to austenitize the steel bar under the
conditions that a heating temperature is 880°C, and a retaining time period is 60 minutes,
and subsequently, cool the steel bar in the atmosphere through allowing cooling.
[0086]
In each of Steels A to K, a grooved Ono-type rotating bending fatigue test
specimen having a shape shown in Figure 2 was cut out in parallel to the forging axis
from an R/2 part ("R" represents a radius of a steel bar) of each steel bar as hot-forged
having a diameter of 50 mm, and a four-point bending test specimen having a shape
shown in Figure 3 was also cut out in the same manner as this grooved Ono-type
rotating bending fatigue test specimen.
[0087]
Similarly, in Steel A, a grooved Ono-type rotating bending fatigue test
specimen having the shape shown in Figure 2 was cut out in parallel to the forging axis
from the R/2 part of each steel bar having a diameter of 50 mm, which was further
subjected to the normalization, and a four-point bending test specimen having a shape
shown in Figure 3 was also cut out in the same manner as this grooved Ono-type
rotating bending fatigue test specimen.
[0088]
In the test specimen in Figure 2, the groove bottom of the R3 is equivalent to
the stress concentrated region. Similarly, in the test specimen in Figure 3, the notch
bottom of the R3 is equivalent to the stress concentrated region.
[0089]
Each grooved Ono-type rotating bending fatigue test spec1men and each
21
four-point bending test specimen, which were obtained in the above manner, were
respectively subjected to the soft-nitriding treatment to retain each test specimen for two
hours in an atmosphere where the RX gas and the ammonia gas are mixed at a mixture
ratio of 1: 1 at a temperature of 600°C; and thereafter, was cooled in the oil having a
temperature of 90°C.
[0090]
In Test No. 1 to Test No. 12, subsequent to the soft-nitriding treatment,
electrolytic grinding was further carried out at the groove bottom of each grooved
Ono-type rotating bending fatigue test specimen, and at the notch bottom of each
four-point bending test specimen, with a target grinding depth of 0.03 mm under the
following conditions.
[0091]
[0092]
- Electrolytic solution: perchloric acid (HC104): acetic acid (CH3COOH) = 1 :9
-Current value: 0.14A
- Grinding area:
Ono-type rotating bending fatigue test specimen: 160 mm2
Four-point bending test specimen: 96 mm2
-Grinding time period:
Ono-type rotating bending fatigue test specimen: 970 seconds
Four-point bending test specimen: 590 seconds
In Test No. 14 to Test No. 16, subsequent to the soft-nitriding treatment,
electrolytic grinding was further carried out at the groove bottom of each grooved
Ono-type rotating bending fatigue test specimen, and the riotch bottom of each
four-point bending test specimen, with a target grinding depth of 0.015 mm under the
following conditions.
[0093]
-Electrolytic solution: perchloric acid (HCl04): acetic acid (CH3COOH) = 1 :9
- Current value: 0.14A
- Grinding area:
22
[0094]
Ono-type rotating bending fatigue test specimen: 160 mm2
Four-point bending test specimen: 96 mm2
- Grinding time period:
Ono-type rotating bending fatigue test specimen: 490 seconds
Four-point bending test specimen: 300 seconds
Using the specimens as soft-nitrided (Test No. 13) and the specimens further
subjected to the electrolytic grinding after the soft-nitriding treatment (Test No. 1 to Test
No. 12, and Test No. 14 to Test No. 16) obtained in the above manner, a study of the
bending fatigue strength by the Ono-type rotating bending fatigue test, and a study of
the bending straightening property by the four-point bending test were respectively
carried out.
[0095]
In addition, usmg the specimens as soft-nitrided (Test No. 13) and the
specimens subjected to the electrolytic grinding after the soft-nitriding treatment (Test
No. 1 to Test No. 12, and Test No. 14 to Test No. 16) for the Ono-type rotating bending
fatigue test and the four-point bending test, the surface-layer hardness (i.e., hardness at a
position of 0.05 mm from the surface of each specimen), and the internal hardness (i.e.,
hardness at a position of 1.0 mm from the surface of each specimen) as well as the
compound-layer depth at the notch bottom were studied, respectively.
[0096]
The details of each study will be described, hereinafter.
[0097]
(1) Study ofbending fatigue strength:
The Ono-type rotating bending fatigue test was carried out at a room
temperature, in the atmosphere, under completely reversed bending at a rotational rate
of 3000 rpm so as to study the bending fatigue strength (referred to as "o"W",
hereinafter).
[0098]
The target crw was set to be 750 MPa or more.
23
[0099]
(2) Study of bending straightening property:
A strain gauge of 2 mm was adhesively bonded to the notch bottom of each
four-point bending test specimen, and bending-straightening strain was applied to this
specimen until the gauge was broken. A read value of the gauge at the moment when
the gauge was broken was evaluated as the bending straightening property.
[01 00]
The target value of the bending straightening property was set to be 22000 1..1.
(equivalent to the bending-straightening strain of2.2%) or more.
[0101]
(3) Surface-layer hardness and internal hardness:
Each Ono-type rotating bending fatigue test specimen was embedded in resin
in a manner as to set a groove-bottom longitudinal sectional portion at the R3 to be a
target surface to be examined, and each four-point bending test specimen was embedded
in resin in a manner as to set a notch-bottom longitudinal sectional portion at the R3 to
be a target surface to be examined, and then, each target surface was polished to be
mirror-finished; and subsequently, the surface hardness and the internal hardness were
respectively studied on the target surface of each specimen using a Vickers hardness
meter.
[0102]
Specifically, in conformity to the "Vickers hardness test - Test method"
described in JIS Z 2244, the HV hardness at any six points at a position of 0.05 mm and
at any six points at a position of 1.0 mm from the 3R of the groove bottom, and the HV
hardness at any six points at a position of 0.05 mm and at any six points at a position of
1.0 mm from the 3R of the notch bottom were respectively measured for each specimen
with a test force of2.94N using a Vickers hardness meter, and the measured values were
arithmetically averaged to evaluate the surface-layer hardness and the internal hardness,
respectively.
[0103]
(4) Compound-layer depth:
24
The compound-layer depth was studied using each of the test specimens
embedded in the resin that were used in the above (3).
[0104]
Specifically, each test specimen embedded in the resin was polished once again,
etched with nita!, and then any five visual fields at the groove bottom of the R3 and any
five visual fields at the notch bottom of the R3 were respectively observed with an
optical microscope with magnification of 400x; and portions observed to be white were
determined as the "compound layers", and the depths of these layers were measured,
and arithmetically averaged as the compound-layer depth.
[0105]
[0106]
[Table 2]
Results ofthe above studies are all shown in Table 2.
25
Table 2
Grooved Ono-type rotating bending fatigue test
Four-point bending test specimen
specimen
Test
Steel Normalizing
No. Compound-! Surface-layer Internal Compound-! Surface-layer Internal Bending
ayerdepth hardness hardness
crw
ayerdepth hardness hardness straightening
(~m) (HV hardness) (HV hardness) (MPa)
(~) (HV hardness) (HV hardness) property(~)
1 A No 1 458 234 820 1 456 233 26000
2 B No 3 418 243 800 3 415 240 31700
3 c No 0 434 237 810 0 430 235 29400
4 D No 2 458 245 820 2 461 244 25500
5 E No 0 418 220 790 0 414 220 32000
6 F No 0 464 246 830 0 462 246 25100
7 G No 2 453 229 820 2 455 228 26700
8 A Yes 1 428 214 770 1 427 212 30200
9 *H No 2 457 • 187 #690 2 460 *189 26100
10 • I No 1 • 332 203 #600 I • 333 202 44000
11 *J No 1 • 526 271 840 1 • 520 274 # 16200
12 *K No 3 • 520 254 840 2 • 518 255 # 17100
13 B No • 19 448 242 820 • 20 443 242 # 15400
14 c No • 12 455 236 810 *11 449 236 # 18700
15 F No *9 480 246 820 *8 480 244 # 16400
16 G No *8 471 230 820 *8 470 231 # 12000 I
I
Normalizing conditions are such that the heating temperature: 880°C, and the retaining time period: 60 minutes. I
A mark (*) represents deviation from the chemical composition specified by the present invention.
A mark(#) represents that the value does not satisfy the target value.
26
[0107]
As shown in Table 2, in the cases of Test No. 1 to Test No. 8 that satisfy the
conditions specified by the present invention in the chemical composition of the steel
material of the base metal, the surface-layer hardness, the internal hardness, and the
compound-layer depth at the stress concentrated region, it is apparent that the target
values of the crw and the bending straightening property were both satisfied, and these
cases are excellent in bending fatigue characteristics and bending straightening property.
[0108]
To the contrary, in the cases of Test No. 9 to Test No. 12, the respective
chemical compositions of Steel H to Steel K deviate from the conditions specified by
the present invention, and thus these cases are poorer in bending fatigue characteristics
or bending straightening property.
[0109]
Specifically, in the case of Test No. 9, the C content of Steel H that is the steel
material of the base metal is less than the range specified by the present invention.
Consequently, the internal hardness of the Ono-type rotating bending fatigue test
specimen is as low as 187 in terms of the HV hardness, and the crw does not satisfy the
target value of 750 MPa or more; thus this case is poor in bending fatigue
characteristics.
[0110]
In the case of Test No. 10, the Mn content of Steel I that is the steel material of
the base metal is less than the range specified by the present invention. Consequently,
the surface-layer hardness of the Ono-type rotating bending fatigue test specimen is as
low as 332 in terms of the HV hardness, and the crw does not satisfy the target value of
750 MPa or more; thus this case is poor in bending fatigue characteristics.
[0 111]
In the case of Test No. 11, the Mn content of Steel J that is the steel material of
the base metal is more than the range specified by the present invention. Consequently,
although the compound-layer depth is as small as 1 ).liD, the surface-layer hardness of
the four-point bending test specimen is as high as 520 in terms of the HV hardness, and
27
the bending straightening property does not satisfy the target value of 22000 J..l. or more
in terms of the read value of the gauge; thus this case is poor in bending straightening
property.
[0112]
In the case of Test No. 12, the Cr content of Steel K that is the steel material of
the base metal is more than the range specified by the present invention. Consequently,
although the compound-layer depth is as small as 2 J..l.m, the surface-layer hardness of
the four-point bending test specimen is as high as 518 in terms of the HV hardness, and
the bending straightening property does not satisfy the target value of 22000 J..l. or more
in terms of the read value of the gauge; thus this case is poor in bending straightening
property.
[0113]
In the cases of Test No. 13 to Test No. 16, the compound-layer depth of the
four-point bending test deviates from the condition specified by the present invention;
thus these cases are poor in bending straightening property.
[0114]
In the case of Test No. 13, although Steel B that is the steel material of the base
metal has chemical composition within the range specified by the present invention, the
compound-layer depth of the four-point bending test specimen is as high as 20 J..l.m, and
the bending straightening property does not satisfy the target value of 22000 J..l. or more
in terms of the read value of the gauge; thus this case is poor in bending straightening
property.
[0115]
In the case of Test No. 14, although Steel C that is the steel material of the base
metal has chemical composition within the range specified by the present invention, the
compound-layer depth of the four-point bending test specimen is as high as 11 J..l.m, and
the bending straightening property does not satisfy the target value of 22000 J..l. or more
in terms of the read value of the gauge; thus this case is poor in bending straightening
property.
[0116]
28
In the case ofTest No. 15, although Steel F that is the steel material ofthe base
metal has chemical composition within the range specified by the present invention, the
compound-layer depth of the four-point bending test specimen is as high as 8 f.Lm, and
the bending straightening property does not satisfy the target value of 22000 f.!. or more
in terms of the read value of the gauge; thus this case is poor in bending straightening
property.
[0117]
In the case of Test No. 16, although Steel G that is the steel material of the base
metal has chemical composition within the range specified by the present invention, the
compound-layer depth of the four-point bending test specimen is as high as 8 f.Lm, and
the bending straightening property does not satisfy the target value of 22000 f.!. or more
in terms of the read value of the gauge; thus this case is poor in bending straightening
property.
INDUSTRIAL APPLICABILITY
[0 118]
The non-thermal refined soft-nitrided component of the present invention is
excellent in bending straightening property after the soft-nitriding treatment, and has a
bending fatigue strength as high as 750 MPa or more in the bending fatigue test;
therefore, this non-thermal refined soft-nitrided component is usable as a component,
such as a crankshaft, in automobiles, industrial machines, or construction machinery,
and this component is capable of attaining reduction in weight and size.

We claim:
[Claim 1]
A non-thermal refined soft-nitrided component having a compound layer in a
surface layer of a steel material of a base metal, chemical composition of the steel
material of the base metal containing: in mass%,
C: 0.25 to 0.40%;
Si: 0.10 to 0.35%;
Mn: more than 2.0% to 2.8% or less;
N: 0.0030 to 0.0250%;
Cu: 0 to 1.0%;
Mo: 0 to 0.3%;
Ni: 0 to 0.5%;
Ti: 0 to 0.020%; and
a balance being Fe and impurities, the impurities including P: 0.08% or less; S:
0.10% or less; AI: 0.05% or less; and Cr: less than 0.20%,
wherein
an HV hardness at a position of 0.05 mm from the surface is 400 to 480,
an HV hardness at a position of 1.0 mm from the surface is 200 or more,
and
a compound-layer depth at a stress concentrated region is 5J.lm or less.
[Claim 2]
The non-thermal refined soft-nitrided component according to claim 1, wherein
the steel material of the base metal contains, in mass%, one or more types of
elements selected from Cu: 0.05 to 1.0% and Mo: 0.05 to 0.3%.
[Claim 3]
The non-thermal refined soft-nitrided component according to claim 1 or 2,
wherein
the steel material of the base metal contains, in mass%, one or more types of elements selected from Ni: 0.05 to 0.5% and Ti: 0.005 to 0.020%.