[Technical Field of the Invention]
[0001]
The present invention relates to a case-hardening steel and a case-hardened
steel member, and particularly to a case-hardening steel, which has excellent cold
forgeability and capable of obtaining excellent temper softening resistance after a
carburizing treatment or a carbonitriding quenching and tempering treatment, and is
suitable for components for automobiles, construction machines, and industrial
machines, and a case-hardened steel member.
Priority is claimed on Japanese Patent Application No. 2013-087857, filed
April 18, 2013, the content of which is incorporated herein by reference.
[Related Art]
[0002]
Transmission gears used in automobiles, construction machines, etc. and
reduction gears used in industrial machines, etc. are mainly configured by gears.
These components can be obtained by using a medium carbon alloy steel such as JIS
SCr 420, JIS SCM 420, etc. as a material, forming the material into the shape of a
component by hot forging, cutting, cold forging, or a combination of these processes
and then subjecting the material to a surface-hardening treatment such as carburized
quenching and tempering, etc.. Among the components, the component formed by
cold forging is subjected to spheroidizing annealing before cold forging in order to
improve die life by softening of the material. The problem when the cold forging is
performed is to prevent cracks from initiating during the cold forging and improve die
- 1 -
life. Accordingly, when both suppression of formation of inclusions serving as an
origin of crack initiation and softening of material can be achieved, the cost for cold
forging can be reduced.
On the other hand, there has been a strong demand for increasing the strength
of gears to achieve high output performance and improvement of fuel efficiency of
automobiles, etc... In the related art, in order to increase the strength of these
components, a technique of improving the tooth root bending fatigue strength of gears,
which is a big problem in high-strengthening, has been developed. However, in
recent years, along with the expansion of application of hard shot peening capable of
rapidly increasing tooth root bending fatigue strength, the main point of the problem to
achieve high-strengthening of gears is shifted from improvement of tooth root bending
fatigue strength to improvement of pitching strength.
[0003]
In order to enhance (improve) the pitching strength, it is effective to improve
the temper softening resistance of a carburized layer in a gear. As a method of
improving temper softening resistance, a technique of improving the components of
steel is proposed. For example, in Patent Document 1, it is disclosed that when the
amounts of Si, Cr, and Mo are defined and the total amount of these elements is more
than a predetermined value, temper softening resistance increases. However, when
the total amount of these elements increases, the hardness of the material before cold
forging is increased and deformation resistance is increased. In addition, for example,
in Patent Document 2, it is disclosed that when the amount of Si is more than 0.15%,
deformation resistance during cold forging is increased. As described above,
generally, when the amount of each component of steel (particularly, Si) is increased,
the effect of improving temper softening resistance is obtained. However, the
- 2 -
hardness of the material is increased. That is, improving temper softening resistance
and ensuring cold forgeabihty are in a trade-off relationship. Therefore, it has been
desired to develop steel achieving both high temper softening resistance and high cold
forgeabihty. In Patent Document 3, there is disclosed a method of realizing both high
temper softening resistance and high cold forgeabihty by increasing temper softening
resistance by increasing the amounts of Si and Cr and then limiting the total amount of
Si, Mn, Cr, and Mo to a value that is determined by a predetermined relational
expression or less. However, the technique disclosed in Patent Document 3 does not
consider prevention of crack initiation during cold forging. Therefore, there is a
problem of that cracks initiate from inclusions when large inclusions are present in the
portion in which the working ratio increases during cold forging and there is still much
room for improvement. In Patent Documents 4 to 11, a steel for machine structure in
which the size of the inclusions is limited is disclosed. However, there is no
description of cold forging in any of the above Patent Documents. In Patent
Document 12, there is disclosed a steel bar and a wire rod achieving both high cold
forgeability and high machinability by limiting the size of sulfide-based inclusions,
oxide-based inclusions, nitride-based inclusions, or composite inclusions thereof.
However, there is no description of a technique of improving temper softening
resistance. In Patent Document 13, a steel for vacuum carburizing or vacuum
carbonitriding is disclosed. In the steel, the maximum equivalent circle diameter of
oxides, composite inclusions mainly composed of oxides and nitrides, and composite
inclusion mainly composed of nitrides, which is expressed by [(7tLW/4) 5] when a
cumulative distribution function estimated by an extreme value statistical method is
99%, is 35 urn or less. However, applying vacuum carburizing or vacuum
carbonitriding is premised in Patent Document 33.
- 3 -
Accordingly, there is need for a steel which both improves temper softening
resistance and ensures cold forgeability (prevention of crack initiation and prevention
of an increase in the material hardness).
[Prior Art Document]
[Patent Document]
[0004]
[Patent Document 1] Japanese Unexamined Patent Application, First
Publication No. 2003-231943
[Patent Document 2] Japanese Unexamined Patent Application, First
Publication No. H6-29924I
[Patent Document 3] Japanese Unexamined Patent Application, First
Publication No. 2006-199993
[Patent Document 4] Japanese Unexamined Patent Application, First
Publication No. 2001-234275
[Patent Document 5] Japanese Unexamined Patent Application, First
Publication No. 2001-131685
[Patent Document 6] Japanese Unexamined Patent Application, First
Publication No. 2001-131686
[Patent Document 7] Japanese Unexamined Patent Application, First
Publication No. 2003-269460
[Patent Document 8] Japanese Unexamined Patent Application, First
Publication No. 2006-63402
[Patent Document 9] Japanese Unexamined Patent Application, First
Publication No. 2007-289979
[Patent Document 10] Japanese Unexamined Patent Application, First
_ 4 _
Publication No. 2004-143550
[Patent Document 11 ] Japanese Unexamined Patent Application, First
Publication No. 2005-154886
[Patent Document 12] Japanese Unexamined Patent Application, First
Publication No. 2007-63589
[Patent Document 13] Japanese Unexamined Patent Application, First
Publication No. 2010-150566
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0005]
The present invention has been made in consideration of the above
circumstances and an object thereof is to provide a case-hardening steel having
excellent cold forgeability and temper softening resistance and a case-hardened steel
member made from the case-hardening steel.
In the present invention, excellent temper softening resistance means that the
hardness of a carburized layer after tempering at 300°C is higher than the hardness of
JIS SCr 420 or JIS SCM 420.
[Means for Solving the Problem]
[0006]
In order to solve the above-described problem, the present inventors have
conducted an intensive investigation on adjustment of the chemical composition to be
suitable for improving temper softening resistance and control of size of inclusions,
which is required to prevent crack initiation during cold forging. As a result, it has
been found that cracks can be prevented from initiating during cold forging due to the
fact that (i) Si and Cr have a significant action of increasing the temper softening
- 5 -
resistance of a carburized layer, (ii) the hardness of the steel after spheroidizing
annealing depends on the total amount of Si, Mn, Cr and Mo and the contribution rate
of each element is different, and (iii) the size of non-metallic inclusions present in the
steel, particularly, the size of sulflde-based inclusions is appropriately limited, etc., and
the present invention has been completed.
The gist of the present invention is as follows.
[0007]
(1) According to an aspect of the invention, there is provided a case-hardening
steel includes, as a chemical composition, by mass%:
C: 0.05% to 0.30%;
Si: 0.40% to 1.5%;
Mn:0.2%to 1.0%;
S: 0.001% to 0.050%;
Cr: 1.0% to 2.0%;
Mo: 0.02% to 0.8%;
Al: 0.001% to 0.20%;
N: 0.003% to 0.03%;
Nb:0%to0.i0%;
Cu:0%to0.2%;
Ni:0%to 1.5%;
V:0%to0.20%;
Ca:0% to 0.0050%;
Mg: 0% to 0.0050%;
Sb: 0% to 0.050%;
P: limited to 0.030% or less;
- 6 -
O: limited to 0.0020% or less;
Ti: limited to 0.005% or less; and
a balance consisting of Fe and impurities,
wherein the following Equations (a) and (p) are satisfied,
in inclusion evaluation using an extreme value statistical method, when an
estimated area S is 30,000 mm , an estimated value of the maximum size (Warea)s of
sulfide-based inclusions present in the estimated area S is 49 um or less and an
estimated value of the maximum size (Varea)ox of oxide-based inclusions present in the
estimated area S is 80 um or less, and
the number of sulfide-based inclusions having a length of more than 20 urn
and a thickness of more than 2 urn per 1 mm is limited to 200 or less,
12 x Si(%) 4- 25 x Mn(%) + Cr(%) + 2 x Mo(%) < 25 ... (a),
31 x Si(%) + 15 x Mn(%) + 23 x Cr(%) > 50 ... (P),
here, Si(%), Mn(%), Cr(%) and Mo(%) represent the amount of each element
by mass% in Equations (a) and (p).
[0008]
(2) The case-hardening steel according to (1), may include, as the chemical
composition, by mass%:
Nb: 0.015% to 0.10%.
[0009]
(3) The case-hardening steel according to (1), may include, as the chemical
composition, by mass%:
Si: 0.55% to 1.5%.
[0010]
(4) The case-hardening steel according to any one of (1) to (3), may include,
- 7 -
as the chemical composition, by mass%, either or both of,
Cu: 0.001% to 0.2%; and
Ni: 0.001% to 1.5%.
[0011]
(5) The case-hardening steel according to any one of (1) to (4), may include,
as the chemical composition, by mass%:
V: 0.01% to 0.20%.
[0012]
(6) The case-hardening steel according to any one of (1) to (5), may include,
as the chemical composition, by mass%, either or both of,
Ca: 0.0001% to 0.0050%; and
Mg; 0.0001% to 6.0050%.
[0013]
(7) The case-hardening steel according to any one of (1) to (6), may include,
as the chemical composition, by mass%:
Sb: 0.0001% to 0.050%.
[0014]
(8) The case-hardening steel according to any one of (1) to (7),
in which the microstructure has a spheroidized carbide structure.
[0015]
(9) According to another aspect of the invention, there is provided a casehardened
steel member made from the case-hardening steel according to any one of (1)
to (8); in which a surface hardened layer that is formed by a carburizing quenching and
tempering treatment or a carburizing nitriding quenching and tempering treatment.
[Effects of the Invention]
- 8 -
[0016]
According to the embodiments of the present invention, it is possible to
provide a case-hardening steel having higher hardness of a carburized layer after
tempering at 300°C than the hardness of JIS SCr 420 or JIS SCM 420 and excellent
cold forgeability and a case-hardened steel member. That is, it is possible to provide
a case-hardening steel having excellent temper softening resistance and cold
forgeability and providing a case-hardened steel member. In addition, the use of the
case-hardening steel and the case-hardened steel member enables reduction of the
production cost of gears and contribution to high output performance and improvement
of fuel efficiency, etc. for automobiles, construction machines, and industrial machines.
[Brief Description of the Drawing]
[0017]
FIG. 1 is a view showing a pattern of spheroidizing annealing (SA) applied in
examples of the present invention.
[Embodiments of the Invention]
[0018]
As a result of the research, the present inventors have found the following (a)
to (d).
[0019]
(a) The limit of cold forgeability (limit of hardness before cold forging) can
be determined based on indices of the amounts of each of Si, Mn, Cr and Mo in
consideration of the hardness increasing action of each element.
The present inventors have conducted spheroidizing annealing (SA) on plural
types of steel obtained by adding various alloy elements to 0.2% C (steel including C
in an amount of 0.2%) to quantitatively evaluate the effect of each alloy element on
- 9 -
hardness after spheroidizing annealing. When spheroidizing annealing is performed,
carbides in steel constituting pearlite, etc. are spheroidized and the microstructure has a
spheroidized carbide structure. When the carbides are spheroidized, the spacing
between carbides which becomes an obstacle to dislocation motion is increased and the
hardness is decreased. Thus, it is desirable to spheroidize carbides.
As a result of the investigation, the present inventors have found that the
hardness of steel after spheroidizing annealing can be expressed by the form of the left
side of the following Equation (1). The reason that the coefficients of Si and Mn are
relatively high is that these alloy elements are solid-soluted into ferrite and the
hardness of a spheroidizing-annealed material is increased by solute strengthening.
On the other hand, the reason that the coefficients of Cr and Mo are relatively low is
that these alloy elements, which are concentrated in cementite during spheroidizing
annealing or precipitated in the form of alloy carbides, are not so much contribute to
solute strengthening and these carbides relatively contribute little to precipitation
strengthening because the size of the carbide is large.
The present inventors have found that when the value of the left side of the
following Equation (1) is 25 or less, the hardness of the steel after spheroidizing
annealing is not excessively increased, and when the value of the left side of the
following Equation (1) is more than 25, the hardness of the steel after spheroidizing
annealing is excessively increased and thus cold forgeability is deteriorated.
[0020]
12 x Si(%) + 25 x Mn(%) + Cr(%) + 2 x Mo(%) < 25 ... (1)
Here, Si(%), Mn(%), Cr(%) and Mo(%) represent the amounts (mass%) of
each component in steel in Equation (1).
[0021]
- 10 -
(b) The temper softening resistance (hardness after tempering at 300°C) of
steel (particularly, carburized layer) can be expressed by indices of the amounts of each
of Si, Mn and Cr in consideration of the temper softening resistance increasing action
of each element.
Si, Mn and Cr have a significant action of increasing the temper softening
resistance of the carburized layer. This is because when Si, Mn and Cr are contained
in the steel, iron carbides precipitated during tempering are prevented from being
coarsened. In order to quantitatively evaluate the effect of each alloy element, the
present inventors have simulated a carburized layer and conducted tempering on plural
types of steel obtained by adding various alloy elements to 0.8% C at 300°C to
investigate the effect of various alloy elements on hardness after tempering (hardness
after tempering at 300°C). As a result, the present inventors have found that the
action of increasing the hardness of the carburized layer after tempering at 300°C by
each alloy element can be expressed by the form of the left side of the following
Equation (2). In addition, it has been found that when the value of the left side is 50
or more, the hardness after tempering at 300°C is apparently increased compared to the
hardness of a general carburized component, and excellent pitching strength can be
obtained, and when the value is less than 50, the pitching strength is not sufficiently
improved.
[0022]
31 x Si(%) + 15 x Mn(%) + 23 x Cr(%) > 50 ... (2)
Here, Si(%), Mn(%) and Cr(%) represent the amounts (mass%) of each
component in steel in Equation (2).
[0023]
Accordingly, both an increase in temper softening resistance and a decrease in
- 11 -
material hardness (ensuring cold forgeability) can be achieved by containing Si, Mn,
Cr and Mo in a range satisfying the above Equations (1) and (2) at the same time.
[0024]
(c) It is possible to prevent cracks from initiating during cold forging by
limiting the size of non-metallic inclusions (sulfide-based inclusions, oxide-based
inclusions and nitride-based inclusions), particularly, sulfide-based inclusions, present
in the steel.
The large inclusions present in the steel serve as an origin of crack initiation.
Thus, it is necessary to evaluate the distribution condition of the material of the
component in a wide range for stable mass production on an industrial scale. The
presence of the large inclusions serving as an origin of crack initiation can be estimated
by an "extreme value statistical method". The extreme value statistical method refers
to a method of estimating the maximum particle size of inclusions (A/area) present in
the population or an arbitrary area (or volume) by selecting plural test pieces from a
given population, measuring the maximum size of inclusions present in the individual
test pieces by microscopy, and plotting the square root of the area to extreme value
probability paper. As a specific method of applying the extreme value statistical
method to the evaluation of non-metallic inclusions in the steel, for example, the
extreme value statistical method can be performed according to the method described
in non-patent documents, Metal fatigue: effects of small defects and nonmetallic
inclusions, written by Murakami Yukitaka, etc. In the embodiment, the following
method is used, (i) Optical microscope observation is performed in 30 view fields
respectively for each test material by setting the area of one view field (inspection
reference area: So) to, for example, 10 mm x 10 mm, so as to avoid overlapping of the
area So. (ii) The maximum particle size of inclusions present in the respective 30
- 12 -
view fields is measured and the square root of the area (Varea) is plotted to extreme
value probability paper, (iii) The maximum particle size of inclusions (Varea) is
estimated by setting an estimated area S to 30,000 mm .
For the measurement of the inclusions, it is necessary to measure the size of
respective oxide inclusions and sulfide inclusions. This is because the particle
distribution of oxides and the particle distribution of sulfides are different from each
other and the evaluation has to be performed separately. The extreme value statistical
method is relatively simple and has a high reliability.
[0025]
(d) The presence frequency of sulfide-based inclusions is high. Therefore,
in order to prevent cracks from initiating during cold forging, it is necessary to limit
the number of sulfide-based inclusions per unit area of a given size or larger (number
density) in addition to the maximum size estimated by the extreme value statistical
method.
[0026]
Hereinafter, a case-hardening steel according to one embodiment of the
present invention (referred to as a case-hardening steel according to an embodiment in
some cases) and a case-hardened steel member according to one embodiment of the
present invention (referred to as a case-hardened steel member according to an
embodiment in some cases) will be described in detail. First, the reason for limiting
the components of the case-hardening steel according to the embodiment will be
described. The components represent the components of a core which is not affected
by an increase in the amount of carbon due to carburizing of the surface portion. The
symbol % of amounts of components means mass%.
[0027]
- 13 -
(C: 0.05% to 0.30%)
C is an essential element to obtain core strength of a carburizing-quenched
component. In addition, the amount of C determines the hardness of the core and also
affects the depth of an effective hardened layer of a carburized layer. Here, in the
embodiment, the lower limit of the amount of C is set to 0.05%. However, when the
amount of C is excessive, toughness is deteriorated. Therefore, the upper limit of the
amount of C is set to 0.30%. The amount of C is more desirably 0.10% to 0.25%.
[0028]
(Si: 0.40% to 1.5%)
Si is an effective element for improving the temper softening resistance of the
carburized layer. Therefore, the lower limit of the amount of Si is set to 0.40%.
However, when the amount of Si is excessive, the hardness after spheroidizing
annealing is increased and cold forgeability is deteriorated. Therefore, the upper limit
of the amount of Si is set to 1.5%. The amount of Si is desirably 0.45% to 1.0%.
When improving the temper softening resistance while limiting a cost increase, it is
more desirable to set the lower limit of the amount of Si to 0.55%.
[0029]
(Mn: 0.2% to 1.0%)
Mn is an effective element for improving the hardenability of the steel. In
addition, Mn improves hot ductility by fixing S in the steel in the form of MnS and
prevents scratches from being formed in the process of producing steel (continuous
casting and hot rolling). Further, MnS improves machinability. In order to obtain
these effects, the lower limit of the amount of Mn is set to 0.2%. However, when the
amount of Mn is excessive, the hardness after spheroidizing annealing is increased and
cold forgeability is deteriorated. Therefore, the upper limit of the amount of Mn is set
- 14 -
to 1.0%. The amount of Mn is desirably 0.4% to 0.7%.
[0030]
(S: 0.001% to 0.050%)
S has an effect of improving machinability by forming MnS in the steel. In
order to obtain this effect, the lower limit of the amount of S is set to 0.001%.
However, when the amount of S is excessive, the amount of MnS, etc., so-called
sulfide-based inclusions, is increased and the size thereof is coarsened. As described
later, when the number of coarsened sulfide-based inclusions is large, the coarsened
sulfide-based inclusions serve as an origin of crack initiation during cold forging.
Therefore, the upper limit of the amount of S is set to 0.050%. The amount of S is
desirably 0.005% to 0.020%.
[0031]
(Cr: 1.0% to 2.0%)
Cr is an effective element not only for improving hardenability but also for
improving temper softening resistance. Additionally, even when the amount of Cr is
relatively large, the hardness after spheroidizing annealing is less affected by Cr.
Therefore, the lower limit of the amount of Cr is set to 1.0%. However, when the
amount of Cr is more than 2.0%, the effect of improving temper softening resistance is
saturated and thus the upper limit of the amount of Cr is set to 2.0%. The amount of
Cr is desirably 1.3% to 1.6%.
[0032]
(Mo: 0.02% to 0.8%)
Mo is an effective element for improving hardenability. Si, Mn and Cr cause
the hardenability of the surface portion to be deteriorated by being selectively oxidized
in the surface portion of the steel during carburizing heating in some cases. In such
- 15 -
cases, a layer which is not completely hardened during quenching is formed and causes
deterioration in bending fatigue strength and pitching strength. On the other hand,
since Mo is less likely to be oxidized compared to the above elements, Mo is effective
for reducing the incompletely hardened layer of the surface portion. In order to
obtain the effect, the lower limit of the amount of Mo is set to 0.02%. However,
when the amount of Mo is excessive, the hardness after spheroidizing annealing is
increased and cold forgeability is deteriorated. Thus, the upper limit of the amount of
Mo is set to 0.8%. The amount of Mo is desirably 0.05% to 0.5%.
[0033]
(Al: 0.001% to 0.20%)
Al has an effect of refining austenite grains by forming fine nitrides in the
steel. In order to obtain the effect, the lower limit of the amount of Al is set to
0.001%o. However, when the amount of Al is more than 0.20%, the effect becomes
saturated. Therefore, the upper limit of the amount of Al is set to 0.20%. The
amount of Al is desirably 0.015% to 0.050%.
[0034]
(N: 0.003% to 0.03%)
N has an effect of refining austenite grains by forming nitrides with Al, Nb, or
V in the steel. In order to obtain the effect, the lower limit of the amount of N is set
to 0.003%. However, when the amount of N is excessive, the hot ductility of the steel
is deteriorated and scratches are remarkably formed in the process of producing steel
(continuous casting and hot rolling). Therefore, the upper limit of the amount of N is
set to 0.03%. The amount of N is desirably 0.007% to 0.02%.
[0035]
(P: 0.030% or less)
- 16 -
P is an impurity element and is an element which deteriorates the toughness of
the steel. Therefore, the amount of P is limited to 0.030% or less. The amount of P
is desirably limited to 0.020% or less.
[0036]
(0:0.0020% or less)
O is an impurity element and forms oxides with Al, Si, etc.. When the
amount of O is increased, the amount of so-called oxide-based inclusions is increased
and the size thereof is also coarsened. As described later, when coarsened oxidebased
inclusions are present, the coarsened oxide -based inclusions serve as an origin
of crack initiation during cold forging. Therefore, the amount of O is limited to
0.0020%. or less. The amount of O is desirably limited to 0.0015%> or less and more
desirably to 0.0005% or less.
[0037]
(Ti: 0.005% or less)
Ti is an element which enters unavoidably and forms nitrides such as TiN in
the embodiment. When the amount of Ti is increased, the amount of so-called
nitride-based inclusions is increased and the size thereof is coarsened. When
coarsened nitride-based inclusions are present, the coarsened nitride-based inclusions
serve as an origin of crack initiation during cold forging. Therefore, the amount of Ti
is limited to 0.005% or less. The amount of Ti is desirably limited to 0.003%o or less.
[0038]
The case-hardening steel according to the embodiment basically contains the
above-described chemical composition and may further contain the following
components. The following elements are not necessarily contained in the steel.
Therefore, it is not necessary to particularly limit the lower limit of the amount of each
- 17 -
component and the lower limit thereof is 0%.
[0039]
(Cu: 0.2% or less)
Cu is an effective element for improving hardenability similar to Mo. In
addition, Cu is an element which is less likely to be oxidized and an effective element
for reducing the incompletely hardened layer of the surface portion. In order to
obtain these effects, the lower limit of the amount of Cu is desirably set to 0.001%.
However, when the amount of Cu is excessive, the hot ductility of the steel is
deteriorated and defects are remarkably formed in the process of producing steel
(continuous casting and hot rolling). Therefore, the upper limit of the amount of Cu
is set to 0.2%. When Cu is contained in the steel and Ni whose amount is about a half
of the amount of Cu needs to be contained at the same time, the deterioration of hot
ductility is reduced. The amount of Cu is desirably 0.05% to 0.15%.
[0040]
(Ni: 1.5% or less)
Ni is an effective element for improving hardenability similar to Mo and Cu.
In addition, Ni is an element which is less likely to be oxidized and is an effective
element for reducing the incompletely hardened layer of the surface portion. In order
to obtain these effects, it is desirable to set the lower limit of the amount of Ni to
0.001%. However, since Ni is an element which significantly affects the cost, the
upper limit of the amount of Ni is set to 1.5%. The amount of Ni is more desirably
0.05% to 1.0%.
[0041]
(Nb: 0.10% or less)
Nb has the effect of refining austenite grains by forming fine carbides and
- 18 -
nitrides in the steel. In order to obtain the effect, the lower limit of the amount of Nb
is desirably set to 0.001%. Particularly, since austenite grains are likely to be
coarsened when normalizing or annealing is not performed after cold forging, when the
carburizing temperature is a high temperature which is 930°C or higher, etc., it is
effective to increase the amount of Nb carbonitrides to prevent coarsening. Therefore,
it is more desirable to set the lower limit of the amount of Nb to 0.015%. However,
when the amount of Nb is more than 0.10%, the effect becomes saturated. Therefore,
the upper limit of the amount of Nb is set to 0.10%. The upper limit of the amount of
Nb is desirably 0.050%.
[0042]
(V: 0.20% or less)
V has the effect of refining austenite grains by forming fine carbides and
nitrides in the steel. In order to obtain the effect, the lower limit of the amount of V is
desirably set to 0.01%. However, when the amount of V is more than 0.20%, the
effect becomes saturated. Therefore, the upper limit of the amount of V is set to
0.20%. The amount of V is more desirably 0.05% to 0.15%.
[0043]
(Ca: 0.0050% or less)
Ca has the effect of preventing so-called sulfide-based inclusions from serving
as an origin of crack initiation during cold forging by refining so-called sulfide-based
inclusions. In order to obtain the effect, it is desirable to set the lower limit of the
amount of Ca to 0.0001%. However, when the amount of Ca is more than 0.0050%,
the effect becomes saturated. Therefore, the upper limit of the amount of Ca is set to
0.0050%. The amount of Ca is more desirably 0.0005% to 0.0015%.
[0044]
- 19 -
(Mg: 0.0050% or less)
Mg has the effect of preventing so-called sulfide-based inclusions from
serving as an origin of crack initiation during cold forging by refining the sulfide-based
inclusions. In order to obtain the effect, it is desirable to set the lower limit of the
amount of Mg to 0.0001%. However, when the amount of Mg is more than 0.0050%,
the effect becomes saturated. Therefore, the upper limit of the amount of Mg is set to
0.0050%. The amount of Mg is more desirably 0.0005% to 0.0015%.
[0045]
(Sb: 0.050% or less)
Sb has the effect of suppressing decarburization during hot rolling and
spheroidizing annealing. In order to obtain the effect, it is desirable to set the lower
limit of the amount of Sb to 0.0001%. However, when the amount of Sb is more than
0.050%, the effect becomes saturated. Therefore, the upper limit of the amount of Sb
is set to 0.050%. The amount of Sb is more desirably 0.001% to 0.010%.
[0046]
Next, in the case-hardening steel according to the embodiment, the amounts
of Si, Mn, Cr and Mo will be described from the viewpoint of cold forgeability and
temper softening resistance.
[0047]
In the case-hardening steel according to the embodiment, from the viewpoint
of cold forgeability, it is necessary to control the amounts of Si, Mn, Cr and Mo to
satisfy the following Equation (1), that is, to make the value of the left side of the
following Equation (1) be 25 or less. This is because the limit of the cold forgeability
(hardness before cold forging) of a spheroidizing-annealed material has to be
determined in consideration of the effect of each of Si, Mn, Cr and Mo on the hardness
- 20 -
of the spheroidizing-annealed material. In the left side of the following Equation (1),
the reason that the coefficients of each element of Si, Mn, Cr and Mo are different is
that the level of the contribution of the elements to cold forgeability (hardness before
cold forging) is different.
A desirable range of the left side of the following Equation (1) is 24.5 or
lower and a more desirable range thereof is 23 or less.
12 x si(%) + 25 x Mn(%) + Cr(%) + 2 x Mo(%) < 25 ... (1)
[0048]
In addition, in the case-hardening steel according to the embodiment, from the
viewpoint of temper softening resistance, it is necessary to control the amounts of Si,
Mn and Cr to make the value of the left side of the following Equation (2) is 50 or
more. In powertrain components such as gears and CVT, heat is locally generated at
a position in which the powertrain components are brought into contact with other
components by the contact while being used and are subjected to tempering to be
softened. This softening is a controlling factor of the deterioration of pitching fatigue
properties. Accordingly, in order to improve pitching fatigue properties, it is effective
to improve hardness after tempering at 300°C which is the temper softening resistance
of the carburized layer. When the value of the left side of the Equation (2) is 50 or
more, pitching fatigue properties are improved. The value of the left side is desirably
53 or more and more desirably 55 or more.
31 x Si(%) + 15 x Mn(%) + 23 x Cr(%) > 50 ... (2)
[0049]
Next, in the case-hardening steel according to the embodiment, the size and
the number of the sulfide-based inclusions will be described.
[0050]
- 21 -
The sulfide-based inclusions in the embodiment are inclusions containing S
and refer to for example, MnS, CaS, MgS, (Mn,Ca,Mg)S, TiS, Ti(C,S), FeS, etc..
In the case-hardening steel according to the embodiment, when the inclusion
evaluation is performed using an extreme value statistical method, it is required that
the estimated value of (Varea)s which is the maximum size of sulfide-based inclusions
present in the estimated area S = 30,000 mm2 is 49 um or less, and the number of the
sulfide-based inclusions having a length of more than 20 um and a thickness of more
than 2 um per 1 mm is 200 or less.
[0051]
When the steel is subjected to deformation processing by cold forging with a
high working degree and large sulfide-based inclusions are present in the steel, the
interface between the inclusion and the matrix serves as an origin of crack initiation
and finally, cracks grow to large cold forging cracks in some cases. However, as long
as the size of the sulfide-based inclusions is 49 um or less, the inclusions do not serve
as the origin of crack initiation and are harmless. On the other hand, the sulfidebased
inclusions having a size of more than 49 jam serve as an origin of crack initiation.
Therefore, the upper limit of (Varea)s is set to 49 um.
[0052]
Since the amount of the sulfide-based inclusions is large compared to the
amount of oxide-based inclusions or nitride-based inclusions, the presence frequency
of the sulfide-based inclusions is high. In addition, since the sulfide-based inclusions
are stretched into a long and narrow shape by hot working, the sulfide-based inclusions
significantly affect the cold forging cracks. For example, (Varea)s of the sulfidebased
inclusions having a length of 20 urn and a thickness of 2 fj.m is 6.3 um and is
smaller than the maximum size of sulfide-based inclusions (49 fxm) limited in the
- 22 -
above description. However, when the number of the sulfide-based inclusions having
a length of more than 20 iim and a thickness of more than 2 um per 1 mm2 is more
than 200, it is similar to the state in which large inclusions are present and crack
initiation occurs frequently during cold working. Accordingly, regarding the sulfidebased
inclusions, it is necessary to define not only the size of inclusions but also the
number of inclusions whose size is equal to or larger than a predetermined size. That
is, it is necessary to define the number of the sulfide-based inclusions having a length
of more than 20 urn and a thickness of more than 2 um per 1 mm to be 200 or less.
When the length and the thickness of the sulfide-based inclusions or the number of the
sulfide-based inclusions are out of the above ranges, cracks are likely to initiate.
When the size of the sulfide-based inclusions is measured, the major axis is the length
and the minor axis is the thickness.
Regarding MnS having a length of 20 um or less, when the thickness is small,
the limit is not applied to the thickness. However, when considering a case in which
the thickness is very large, for example, a case in which MnS having a length of more
than 20 um is present, the thickness is a length and the length is a thickness, and thus,
the limit is applied to the thickness.
The smaller the size of the sulfide-based inclusions and oxide-based
inclusions is, the more desirable. Thus, the lower limit of the particle size thereof is 0
um. In addition, a smaller number of the sulfide-based inclusions having a length of
more than 20 um and a thickness of more than 2 um are desirable. Thus, the lower
limit of the number density is 0 piece/mm .
[0053]
Next, in the case-hardening steel according to the embodiment, the size of the
oxide-based inclusions will be described.
- 23 -
[0054]
The oxide-based inclusions referred to in the present invention are inclusions
containing 0 and refer to, for example, A1203, CaO, Cr203, MnO, NbO, Si02, MgO,
Zr02, TixOy, Nb205, FeOx, composite compounds thereof, etc.
[0055]
In the case-hardening steel according to the embodiment, it is preferable that
the estimated value of the maximum size (Varea)ox of oxide-based inclusions present in
the estimated area S = 30,000 mm is 80 urn or less in the inclusion evaluation using
the extreme value statistical method.
[0056]
This is because when the steel is subjected to deformation processing by cold
forging with a high working degree and large oxide-based inclusions are present in the
steel the interface between the inclusion and the matrix serves as an origin of crack
initiation and finally, cracks grow to large cold forging cracks in some cases.
However, oxide-based inclusions whose (Varea)ox is 80 um or less do not serve as the
origin of crack initiation and are harmless. On the other hand, the oxide-based
inclusions having a size of more than 80 urn serve as an origin of crack initiation.
Accordingly, it is necessary to define the size of the oxide-based inclusions as
described above.
[0057]
The case-hardened steel member according to the embodiment can be
obtained by subjecting the above-described case-hardening steel to carburizing
quenching and tempering treatment or a carburizing nitriding quenching and tempering
treatment. That is, the case-hardened steel member is made from case-hardening steel.
Therefore, the case-hardened steel member according to this embodiment has
- 24 -
substantially the same chemical composition and inclusions as the chemical
composition and inclusions of the case-hardening steel according to the abovedescribed
embodiment. Accordingly, in order to control the chemical composition
and inclusions of the case-hardened steel member, the case-hardening steel may be
controlled to have a predetermined chemical composition and inclusions.
However, since the case-hardened steel member is subjected to a carburizing
quenching and tempering or a carburizing nitriding quenching and tempering treatment,
a surface hardened layer is provided and the case-hardened steel member is different
from the case-hardening steel in that the surface hardened layer is provided.
[0058]
Preferable conditions for producing the case-hardening steel and the casehardened
steel member according to the embodiment will be described.
[0059]
In the embodiment, in secondary refining, RH vacuum degassing is performed
under the conditions of a total treatment time of 30 minutes or longer including a
treatment time of 15 minutes or longer at a reduced pressure of 1 Torr or lower
(refining process). When refining is performed under the above-described conditions,
the size and the number of the oxide-based inclusions can be controlled to
predetermined ranges. In addition, in the refining process, the chemical composition
is adjusted to be within the above-described preferable range.
[0060]
Next, the molten steel with the chemical composition adjusted in the refining
process is subjected to continuous casting to obtain a slab (casting process). When
continuous casting is used to obtain a slab, it is desirable that the casting rate is set to
0.45 m/min or more. When the casting rate is set to 0.45m/min or more, the size and
- 25 -
the number of the sulfide-based inclusions can be controlled to the above-described
ranges. When the casting rate is less than 0.45m/min, coarse sulfide-based inclusions
are crystallized and precipitated during solidification of the steel. A desirable casting
rate is 0.50 m/min to 1.5 m/min.
Further, when the steel is subjected to casting, it is desirable that the slab is
cooled such that the cooling rate from the liquidus temperature to the solidus
temperature is 5 °C/min to 200 °C/min at a 1/4 portion of the slab in the thickness
direction. When the cooling rate is less than 5 °C/min, coarse sulfide-based
inclusions are precipitated and also the productivity of continuous casting is
deteriorated. Thus, the cooling rate of less than 5 °C/min is not desirable. In
addition, when the cooling rate is more than 200 °C/min, cracks are more likely to
initiate in the slab during continuous casting and thus the cooling rate of more than
200 °C/min is not desirable.
The cooling condition is related to a secondary dendrite arm spacing.
Therefore, by measuring the secondary dendrite arm spacing, the above-described
cooling rate can be calculated. Specifically, the cooling rate can be obtained by
calculation using the following Equitation (3) using the spacing between the secondary
dendrite arms in the solidification structure of the solidified slab in the thickness
direction.
Rc = (?,2/770)("i/a41)... (3)
Re: cooling rate (°C/min), and XI: spacing between secondary dendrite arms
(Mm).
[0061]
A bloom obtained by the above-described casting process is subjected to
blooming to obtain a billet (blooming process). The heating temperature during
- 26 -
blooming is desirably 1240°C or higher since unavoidably formed coarse sulfides are
temporarily solid-soluted in the matrix. The heating temperature is more desirably
1260°C or higher. The reduction of area of the blooming leads to a reduction in the
thickness of the sulfide-based inclusions and thus it is necessary to set the reduction of
area to 40% or more. The reduction of area is desirably 45% or more. In addition,
when the cooling rate during blooming or after blooming is low, the solid-soluted MnS
is precipitated as a coarse sulfide again. Thus, it is necessary to set the cooling rate to
1240°C to 1000°C in the cooling process during blooming or after blooming to
0.7 °C/s or more. The cooling rate is more desirably 1.5 °C/s or more. The cooling
rate is a cooling rate obtained from the actually measured value of the surface
temperature.
[0062]
In order to form the billet into a case-hardening steel (steel bar or wire rod),
steel bar rolling or wire rod rolling is performed. At the time of heating in the steel
bar rolling or wire rod rolling, to prevent MnS from growing and being coarsened, the
heating temperature is desirably set to 1200 °C or lower. The heating temperature is
more desirably 1000°C to 1150°C. In addition, the total reduction of area until the
process of rolling the billet into a steel bar or a wire rod is completed (total reduction
of area in blooming and steel bar rolling or wire rod rolling) is set to 65% or more.
When the total reduction is less than 65%, the thickness reduction along with the
stretching of the sulfide-based inclusions is not sufficient and thus the number of
sulfide-based inclusions having a large thickness which are harmful to cold forging
crack initiation cannot be reduced. A suitable range of the total reduction of area is
90% or more.
[0063]
- 27 -
The above-described case-hardening steel is further subjected to carburizing
quenching and tempering treatment or a carburizing nitriding quenching and tempering
treatment to obtain a case-hardened steel member. The carburizing quenching and
tempering or carburizing nitriding quenching and tempering may be performed by
known methods.
[Examples]
[0064]
Hereinafter, the present invention will be further described using Examples.
Converter molten steels having the compositions (chemical composition)
shown in Tables 1-1 and 1-2 were subjected to RH vacuum degassing under the
conditions shown in Table 2 and subsequently subjected to continuous casting under
the conditions shown in Table 3, and then subjected to soaking as required. Through
blooming, rectangular rolled steels (billets), having a square-shaped cross section
whose length of one side is 162 mm were obtained. The balance in Tablesl-I and 1-2
includes iron and impurities and the blank indicates that the component is intentionally
not added.
- 28 -
[0067]
Next, working was performed by hot rolling under the conditions shown in
Table 4 to form the billets into steel bars and then some of the steel bars were subjected
to spheroidizing annealing (SA) under the conditions of FIG. 1. In addition, regarding
some of the steel bars, steel bars which were not subjected to SA were subjected to hot
forging (heating temperature of 1250°C and an upset rate of 50%) and formed into
disc-like casting raw materials and then the disc-like casting raw materials were
subjected to SA. The other steel bars and some of the casting raw materials were not
subjected to SA. Steel bars and casting raw materials produced in the abovedescribed
manner were used as materials to evaluate various properties.
Then, columnar test pieces having a diameter of 16 mm and a length of 24
mm were taken from the materials by cutting. The columnar test pieces were
subjected to upset cold working under the conditions of an upset rate of 50% and a
strain rate of 1.0. Next, in order to simulate carburizing, the cold-worked columnar
test pieces were heated and retained at 950°C for 5 hours and then immediately watercooled
to freeze the austenite structure after the carburizing simulation as the prior
austenite grain boundary of the martensite structure. Next, the prior austenite grain
structure in the cross section of each of the test pieces which was subjected to the
carburizing simulation in the rolling direction was observed and the JIS grain size
number was measured. A coarse particle was defined as a particle with a grain size
number of 5 or lower in JIS G 0551 and even when one coarse particle was formed in
the view field in the entire cross section, it was determined that coarse particle was
present.
[0068]
The case-hardening steel and the case-hardened steel member of the present
- 32 -
invention may be subjected to SA, however, SA is not necessarily performed. When
cold working is not performed or cold working is possible without performing SA in
actual production of a component, S A may not be performed. In this case, the steel
can be used as high-strength steel.
[0069]
First, the Vickers hardness (measurement load of 10 kgf) at a 1/4 depth
position of the diameter of the steel bar and the casting raw material was measured
according to JIS Z 2244. The number of measurement points is 4 per material and the
average value was obtained. For the steel having a hardness of HV 155 or higher, it
steel was determined that cold forgeability was deteriorated since the deformation
resistance of the steel during cold forging was increased and the die life was
remarkably reduced.
[0070]
At the position in the vicinity of 1/4 of the diameter of the steel bar or at the
position in the vicinity of 1/4 of the diameter of the casting raw material, optica!
microscope observation was performed and inclusion measurement was performed.
The estimated value of Ihe maximum size (Varea)s of sulfide-based inclusion and the
maximum size (Varea)ox of oxide-based inclusions present in the estimated area S =
30,000 mm was set to 10 mm x 10 mm in one visual field area (investigation
reference area: So) and optical microscope observation was performed in 30 view fields
so as to avoid overlapping of the area So- The maximum size (Varea) of inclusions
present in the respective 30 visual fields was measured and plotted to extreme value
probability paper. The maximum size (Varea) of inclusions was determined by
estimating the maximum size of inclusions while setting the estimated area S to 30,000
mm . In the inclusion measurement, each of oxides (oxide-based inclusions) and
- 33 -
sulfides (sulfide-based inclusions) were independently evaluated.
[0071]
The number of sulfide-based inclusions having a length of more than 20 urn
and a thickness of more than 2 um in each visual field was measured while abovedescribed
inclusion measurement was performed. The numbers of the entire 30
visual fields were added and the added value was divided by the total measurement
area (3,000 mm2) to measure the presence number of sulfide-based inclusions having a
length of more than 20 iim and a thickness of more than 2 um per area of 1 mm .
[0072]
Next, as an index of crack initiation in the steel during cold forging, the limit
compression ratio was measured. Test pieces for limit compression ratio
measurement ((|)6 mm x 9 mm, notch configuration: 30°, depth: 0.8 mm, radius of
curvature at tip end: 0.15 mm) were prepared by taking the test pieces from the steel
bars and casting raw materials in a direction parallel to in the longitudinal direction.
In the measurement of the limit compression ratio, cold compression was performed
using a binding die at a speed of 10 mm/min, the compression was stopped when fine
cracks having a length of 0.5 mm or more initiated in the vicinity of the notch, the
compression ratio was calculated when the compression is stopped, and the obtained
compression ratio was set to a compression ratio at crack initiation. The test was
performed (n = 10) at one level to obtain a compression ratio of a cumulative failure
probability of 50% and the obtained compression ratio was set to the index of the limit
working ratio. Since the limit compression ratio of a SA material of JlS-SCr 420 was
about 65%, the steel having a high value of 68% or more, which was apparently higher
than the above value, was determined to have an excellent limit working ratio and
contrarily the steel having a value of less than 68% was determined to have a
- 34 -
deteriorated working ratio.
[0073]
Next, the hardness after tempering at 300°C which is an index of the pitching
resistance of the component after carburizing was measured. In order to measure the
hardness after tempering at 300°C, first, test pieces for carburizing ((p20 mm x 30 mm)
was taken from the steel bars as a material (SA material and non-SA material). Then,
gas carburizing was performed by a gas type converter. The gas carburizing was
performed under the condition of an atmosphere temperature of 950°C and a retaining
time of 5 hours, an atmosphere temperature of 850°C and a retaining time of 0.5 hours,
oil quenching at 130°C, a tempering temperature of 150°C and a retaining time of 90
minutes at a gas potential of 0.8% in order. Next, for the investigation of the surface
structure, a section in the vicinity of the center portion of the test piece in the
longitudinal direction was cut in a direction orthogonal to the longitudinal direction
and a microscope sample having a cross section was prepared. The sample was
subjected to corrosion with 2% nital for structure observation and the surface portion
of the carburized layer was observed with a microscope. The depth of an
incompletely hardened layer formed on the surface portion of the carburized layer (a
layer in which a non-martensite structure mainly composed of pearlite and/or bainite is
present) was measured. When the depth of the insufficient hardened layer was deep,
the pitching properties are deteriorated and the depth of the incompletely hardened
layer of JJS-SCr 420 is about 25 urn. Thus, when the depth of the incompletely
hardened layer was deeper than 25 urn, it was determined that the pitching properties
were insufficiently improved.
In addition, in order to obtain the hardness after tempering at 300°C,
tempering was further performed at a tempering temperature of 300°C for a retaining
- 35 -
time of 90 minutes. Then, a portion in the vicinity of the center portion of the test
piece in the longitudinal direction was cut in a direction vertical to the longitudinal
direction and the Vickers hardness of the cross section was measured. The hardness
measurement position was set to a position having a depth of 50 urn from the surface
and the measurement load was set to 300 gf. Further, the measurement was
performed at 5 points per test piece and the average value was obtained. Since the
temper hardness of JlS-SCr 420 at 300°C is HV 640, the steel having a value of HV
670 or more, which was apparently higher than the above value, was determined to
have excellent pitching properties and the steel having a value of less than HV 670 was
determined to have insufficient pitching properties.
[0074]
In Table 2, the effect of RH conditions was summarized. In RH condition
No. 3-3 of Table 2, both of the total treatment time of RH vacuum degassing and the
treatment time in a reduced atmosphere of 1 Torr or less were out of the desirable
ranges. In addition, in RH condition No. 1-4, the treatment time in a reduced
atmosphere of 1 Torr or less was out of the desirable range. Further, in RH condition
No. 1-B, the total treatment time of RH vacuum degassing was out of the desirable
range. In Production condition Nos. 20, 23, 42, a, b, c, d, e and f adopting these
conditions, floating oxides in the molten steel were not sufficiently removed and the
size of oxide-based inclusions present in the steel bar was large. As a result, the limit
compression ratio was deteriorated. In contrast, in Production Nos. 1, 9 and 2
adopting RH condition Nos. 1-1, 1-2 and 1-A in which the RH conditions were
appropriate, the size of oxide-based inclusions was small and the limit compression
ratio of the S A material was good.
- 36 -
[0076]
In Table 3, the effect of casting conditions was summarized. In Casting
condition No. 2-8 of Table 3, the casting rate was out of the desirable range. In
addition, in Casting condition No. 2-9, a cooling rate from the liquidus temperature to
the soiidus temperature at 1/4 portion of the slab in the thickness direction was low and
thus the size of sulfide-based inclusions present in the steel bar was large. As a result,
all Production Nos. 64, 65, 66 and 67 adopting Casting condition No. 2-8 or 2-9, has a
low limit compression ratio. In contrast, in Production Nos. No.l, 2 and 53 to 58
adopting Casting condition Nos. 2-1 to 2-7 in which the continuous casting condition
was appropriate, the size of sulflde-based inclusions were small and the limit
compression ratio was good.
- 38 -
[0078]
In Table 4, the effect of rolling conditions was summarized. The total
reduction of area of Rolling condition Nos. 3-6 and 3-B of Table 4 was out of the
desirable range. As a result, in Production Nos. 68 and 69 adopting these conditions,
the thickness of MnS was not sufficiently reduced by rolling and thus there were a
large number of sulfide-based inclusions having a large thickness. Accordingly, in
Production Nos.68 and 69, the limit compression ratio was deteriorated. In contrast,
in Production Nos. 1 and 59 to 63 adopting the rolling condition No. 3-1 to 3-5 and 3-
A in which the total reduction of area of hot rolling was appropriate, the thickness was
large, the number of stretched sulfide-based inclusions was small and the limit
compression ratio was also good.
- 40 -
[0080]
In Tables 5-1, 5-2, 6 and 7, the inclusion measurement results and properties
of steels obtained under each production condition are shown. In Tables 5-1, 5-2 and
6, the result of a material which has been subjected to SA and in Table 7, the results of
a material which has not been subjected to SA are shown.
As seen from Tables 5-1, 5-2 and 6, in Production Nos. 1 to 15and 53 to 63in
which all conditions were within the ranges of the present invention, all of the hardness
after SA, the limit compression ratio, the hardness of the carburized layer after
tempering at 300°C, and the thickness of the incompletely hardened layer were
excellent. In addition, in Production Nos. 1, 8, 9 and 11 including Nb, coarse
particles were not observed.
In contrast, in Production Nos. 16 to 52, 64 to 69 and a to f in which at least
one of the chemical composition and the production condition was out of the desirable
range, any of the hardness after SA, the limit compression ratio, the hardness of the
carburized layer after tempering at 300°C, or the thickness of the incompletely
hardened layer did not satisfy the desired value. Further, in Production Nos. 20, 23,
31, 34, 42 and 45, the amount of 0 was large and the maximum Varea of oxide-based
inclusions was out of the range of the present invention. In addition, in Production
Nos. 22, 33 and 44, the amount of S was beyond the range of the present invention,
and thus the maximum Varea of sulfide-based inclusions was out of the range of the
present invention.
As seen from Table 7, even with respect to the material to which S A was not
subjected, in Production Nos. 101 to 115 in which all conditions were within the range
of the present invention, the hardness of the carburized layer after tempering at 300°C
and the thickness of the incompletely hardened layer were excellent. On the hand, in
- 42 -
Production Nos. 116 to 118, 124, 126, 128, 129, 135, 136, 138, 139, 146, 148, 150 and
151 in which at least one of the chemical composition and the production condition
was out of the desirable range, the hardness after tempering at 300°C or the thickness
of the incompletely hardened layer was deteriorated. The same tendency was applied
to the SA material.
- 43 -
the present invention, it is possible to provide a case-hardened steel having excellent
temper softening resistance and cold forgeabihty and a case-hardened steel member.
In addition, the use of the case-hardening steel and the case-hardened steel member.
enables reduction of the production cost of gears and contribution to high output
performance and improvement of fuel efficiency, etc. for automobiles, construction
machines, and industrial machines.

[Document Type] CLAIMS
1. A case-hardening steel comprising, as a chemical composition, by mass%:.
C: 0.05% to 0.30%;
Si: 0.40% to 1.5%;
Mn: 0.2% to 1.0%;
S: 0.001% to 0.050%;
Cr: 1.0% to 2.0%;
Mo: 0.02% to 0.8%;
Al: 0.001% to 0.20%;
N: 0.003% to 0.03%;
Nb:0%to0.10%;
Cu:0%to0.2%;
Ni:0%tol.5%;
V: 0% to 0.20%;
Ca:0% to 0.0050%;
Mg: 0% to 0.0050%;
Sb: 0% to 0.050%;
P: limited to 0.030% or less;
O: limited to 0.0020% or less;
Ti: limited to 0.005% or less; and
a balance consisting of Fe and impurities,
wherein the following Equations (1) and (2) are satisfied,
in inclusion evaluation using an extreme value statistical method, when an
estimated area S is 30,000 mm2, an estimated value of the maximum size (Varea)s of
- 51 -
sulfide-based inclusions present in the estimated area S is 49 urn or less and an
estimated value of the maximum size (Varea)ox of oxide-based inclusions present in the
estimated area S is 80 um or less, and
the number of sulfide-based inclusions having a length of more than 20 urn
and a thickness of more than 2 urn per 1 mm2 is limited to 200 or less,
12 x Si(%) + 25 x Mn(%) + Cr(%) + 2 x Mo(%) < 25 ... (1),
31 x Si(%) + 15 x Mn(%) + 23 x Cr(%) > 50 ... (2),
here, Si(%), Mn(%), Cr(%) and Mo(%) represent the amount of each element
by mass% in Equations (1) and (2).
2. The case-hardening steel according to claim 1, comprising, as the chemical
composition, by mass%:
Nb: 0.015% to 0.10%.
3. The case-hardening steel according to claim 1, comprising, as the chemical
composition, by mass%:
Si: 0.55% to 1.5%.
4. The case-hardening steel according to any one of claims 1 to 3, comprising, as
the chemical composition, by mass%, either or both of,
Cu: 0.001% to 0.2%; and
Ni: 0.001% to 1.5%.
5. The case-hardening steel according to any one of claims 1 to 4, comprising, as
the chemical composition, by mass%:
- 52 -
V: 0.01% to 0.20%.
6. The case-hardening steel according to any one of claims 1 to 5, comprising, as
the chemical composition, by mass%, either or both of,
Ca: 0.0001% to 0.0050%; and
Mg: 0.0001% to 0.0050%.
7. The case-hardening steel according-to any one of claims 1 to 6, comprising, as
the chemical composition, by mass%:
, Sb: 0.0001% to 0.050%.
8. The case-hardening steel according to any one of claims 1 to 7,
wherein the micro structure has a spheroidized carbide structure.
9. A case-hardened steel member made from the case-hardening steel according
to any one of claims 1 to 8, wherein
a surface hardened layer that is formed by a carburizing quenching and
tempering treatment or a carburizing nitriding quenching and tempering treatment.