DESCRIPTION
Title of Invention: Steel Part Excellent In Temper
Softening Resistance
Technical Field
0001 The present invention relates to a part made of
steel (in particular case hardened steel) such as a
pulley or gear for a continuous variable transmission for
automobile applications where in the process of use, the
temperature in the environment of the part rises and it
is necessary to suppress a drop in hardness of the
surface of the part.
Background Art
0002 For example, pulleys and gears for continuous
variable transmissions of automobile applications are
usually formed using JIS SCr420, SCM420, or other case
hardening use chromium steel or chrome-molybdenum steel
which is then gas carburized and tempered for use.
0003 In recent years, demands for prevention of global
warming through the reduction of COz emissions and for
conservation of resources have been growing stronger.
Reduction of the weight of vehicles is being sought for
further improvement of fuel economy. For this reason,
unit sizes have to be reduced or the load which can be
transmitted has to be increased for the same size of
units. In such a case, the load applied to each part
becomes greater than in the past, so further higher
strength is demanded.
Further, the temperature in the usage environment of the
parts rises to about 300°C, so the surface of the parts
falls in hardness and pitting fracture occurs. Therefore,
to improve the pitting fatigue resistance (improve the
surface pressure fatigue strength, increase the surface
pressure, etc.), case hardened steel with excellent
temper softening resistance is sought.
- 2 -
0004 As opposed to this, to improve the temper
softening resistance, various methods of increasing the
content of Si etc. have been proposed. For example,
SCr420 steel and SCM420 have an Si content of 0.15 to
0.35%.
0005 PLT 1 proposes a gear made of steel containing
Si: 0.5 to 1.5%, Mn: 0.3 to 1%, and Cr: 0.2 to 0.5% which
is carburized, quenched, and tempered to give a content
of C of 0.6% or more. However, the inventors evaluated
this and as a result found that the content of Cr was
small, so a sufficient temper softening resistance could
not be obtained.
0006 PLT 2 proposes a gear comprised of steel
containing Si: 1% or less, Mn: 1% or less, and Cr: 1.5 to
5.0% which is carburized, quenched, and tempered to give
a content of C of 0.7 to 1.3%.
PLT 3 discloses steel which contains Si: 0.7 to 1.5%, Mn:
0.5 to 1.5%, and Cr: 0.1 to 3.0% and which contains
Si+0.2Cr in over 1.3%.
0007 However, the inventors evaluated these and as
result found that the steels described in PLTs 2 and 3
have large contents of Mn and Cr and that if
carburization results in the content of C also rising,
the amount of residual austenite becomes too great.
Therefore, a sufficient temper softening resistance was
not necessarily obtained. Further, if Cr exceeds 1.5%,
there was the problem that proeutectoid cementite ends up
being produced at the stage of holding before quenching
after carburization (about 850°C) and the strength ends up
deteriorating. Retention before quenching is generally
employed for the purpose of reducing the quench
distortion.
0008 PLT 4 discloses steel to which Si: 0.4 to 1.5%,
Mn: 0.3 to 2.0%, Cr: 0.5 to 3.0%, and Ti: 0.02 to 0.2%
are added. However, the inventors evaluated this and
found that a sufficient temper softening resistance was
not necessarily obtained. Further, there was a concern
over breakage of the gear starting from coarse TiC
unavoidably formed to some extent due to the addition of
Ti.
0009 PLT 5 discloses a part comprised of steel
containing Si: 0.5 to 1.5%, Mn: 0.2 to 1.5%, and Cr: 0.5
to 1.5% which is carburized, quenched, and tempered to
give a content of C of 0.8 to 1.2%, then again heated to
the austenite region to thereby cause carbides with an
average particle size of 1.2 μm or less to precipitate by
an area ratio of 2 to 8% and which is adjusted to a
solute carbon content in the matrix of 0.60 to 0.95%.
However, the inventors evaluated this and as a result
found that the temper softening resistance of the matrix
could not necessarily be said to be sufficient.
0010 PLT 6 proposes a rolling member characterized by
comprising steel containing Si: 1.0 to 5.0% into which at
least one of Si, Al, Be, Co, P, and Sn is made to diffuse
and permeate and treated at the surface layer by
carburization, carbonitridation, and/or nitridation, then
quenching or quenching and tempering. This does not
consider the residual austenite at all. Further, a
diffusion permeation step is newly required and leads to
deterioration of productivity and increase in production
costs.
Citations List
Patent Literature
On PLT 1: Japanese Patent Publication (A) No. 2003-
27142
PLT 2: Japanese Patent Publication (A) No. 7-
242994
PLT 3: Japanese Patent Publication (A) No. 2003-
231943
PLT 4: Japanese Patent Publication (A) No. 2001-
329337
PLT 5: Japanese Patent Publication (A) No. 2005-
68453
- 4 -
PLT 6: Japanese Patent Publication (A) No. 2004-
107709
Summary of Invention
Technical Problem
0012 The arts disclosed in PLTs 1 to 6 result in
insufficient temper softening resistance and the
inability to maintain the carburized layer surface
hardness at 300°C. Further, the arts disclosed in PLTs 1
to 6 require the addition of a new production process and
have problems of a drop in productivity, new capital
investment, etc.
Therefore, the present invention was made to solve this
problem and provides a case hardened steel part which is
excellent in temper softening resistance. Specifically,
the indicator of the pitting fatigue strength (surface
pressure fatigue strength of the gear), that is, the
"temper softening resistance at 300°C", is defined as the
carburized layer surface hardness when performing gas
carburization, quenching, and tempering, then again
holding at 300°Cx2 hours, then tempering. The object is
the provision of case hardened steel with a "temper
softening resistance" of HV700 or more.
Solution to Problem
0013 The inventors engaged in repeated intensive
studies for solving the above problems and as a result
discovered that to secure a sufficient temper softening
resistance at 300°C, it is necessary to satisfy all of the
following matter.
0014 (a) The content of Si in the steel be 0.7% or
more.
(b) The content of C in the carburized layer be made 0.9
to 1.1% and a hyper eutectic content of C be caused by
carburization.
(c) Carbides with an outside diameter of 0.1 μm or more
- 5 -
(proeutectoid cementite or spherical carbides) not be
allowed to be present after carburization.
(d) An amount of residual austenite after carburization,
quenching, and tempering be made less than 30%. In other
words, the rest of the structure other than residual
austenite becomes tempered martensite, so this is
synonymous with tempered martensite of 70% or more.
0015 FIG. 1 shows the effects of the content of Si and
the content of C on the hardness after 300°C tempering in
the state satisfying the above (b), (c), and (d). As
clear from FIG. 1, the inventors clarified that the
hardness after 300°C tempering in the hyper eutectic state
has a large dependency on the Si content. Normally,
carburization is performed to the vicinity of the
eutectic point where the content of C is 0.8%.
0016 The inventors raised the carbon potential of the
carburized layer to make the C hyper eutectic. Normally,
if raising the carbon potential too much, the drop in the
Ms points results in the amount of residual austenite
increasing too much and the hardness required as a part
becoming unable to be secured any longer. Further,
proeutectoid cementite and spherical carbides end up
precipitating. These become starting points of fracture
and therefore are not good.
0017 In the present invention, the inventors
discovered that by limiting the range of ingredients of
various types of alloy elements, it is possible to
prevent proeutectoid cementite and spherical carbides
from precipitating. The amount of precipitation of fine s
carbides of the nanosize order which precipitate in the
first stage of tempering of martensite increases by the
amount of increase of carbon and the temper softening
resistance at 300°C can be maintained. This is the point
of the present invention.
0018 Summarizing this, by using high carbon to prevent
the presence of proeutectoid cementite and spherical
- 6
carbides, it is possible to ensure a greater presence of
nanosize order s carbides in the martensite than usual.
Further, by making the material high in Si, even when the
s carbides are tempered from 300°C, transition to
cementite becomes difficult. Due to these synergistic
effects, the temper softening resistance is greatly
improved.
As will be understood from FIG. 1, the inventors learned
from experiments that to secure a temper softening
resistance of HV700, the carburized layer should have a
content of C of 0.9 to 1.1% and a content of Si of 0.7%
or more,
0019 Further, the inventors discovered that to stably
cause carburization to the hyper eutectic content of C
(0.9 to 1.1%), from the viewpoint of prevention of
sooting in the gas carburization, the amount of Si has to
be made 1.0% or less. That is, they learned that the
suitable range of the content of Si is 0.7% to 1.0%. They
learned that range of content of Si is much narrower than
the ranges described in the above PLTs.
0020 Further, the inventors discovered that to use gas
carburization to obtain a hyper eutectic content of C
(0.9 to 1.1%) and further to prevent the precipitation of
proeutectoid cementite even if holding before quenching
at about 850°C, the content of Cr has to be made 1.5% or
less. However, if the content of C is small, there is the
possibility that the temper softening resistance will be
made to drop, so at least 1.0% is necessary. Therefore,
the suitable range of the Cr content becomes 1.0 to 1.5%.
The inventors learned that the range of content of C is
much narrower compared with the range described in the
above PLTs.
0021 Still further, the inventors learned that to
quench and temper a steel part', which is gas carburized to
obtain a hyper eutectic content of C (0.9 to 1.1%) and to
make the residual austenite amount less than 30%, it is
- 7 -
necessary to make the total amount of Mn, Cr, Mo, and Ni
2.50 or less so as to suppress the drop in the Ms point.
0022 The present invention was made based on the above
new discoveries. The gist of the present invention is as
follows.
0023 (1) A steel part excellent in temper softening
resistance which is comprised of steel containing as
chemical ingredients, by mass%, of
C: 0.1 to 0.3%,
Si: 0.7 to 1.0%,
Mn: 0.2 to 1.5%,
Cr 1.0 to 1.5%,
S: 0.001 to 0.15%,
Al: 0.001 to 1%, and
N: 0.001 to 0.03%,
satisfying a total content of Mn and Cr of 2.5% or less,
and
having a balance of iron and unavoidable impurities, and
which is carburized, quenched, and tempered on its
surface,
the steel part characterized by having a content of C at
a part at a 50 μm depth from the surface of 0.9 to 1.1%,
not containing proeutectoid cementite or spherical
carbides of an average diameter (average size) of 0.1 μm
or more or even if containing it, having an area ratio of
it of less than 1%, and, further, not containing residual
austenite or even if containing it, having an area ratio
of it of less than 30% (or having an area ratio of
martensite of 70% or more).
The area ratio of proeutectoid cementite or spherical
carbides of an average diameter of 0.1 pm or more is an
area ratio of less than 1% in a part of a depth of 50 pm
from the surface in range of 10 pm square on a plane
parallel to the surface.
Further, the area ratio of the residual austenite
similarly is an area ratio of less than 30% in a part of
- 8 -
a depth of 50 μm from the surface in range of 10 μm
square on a plane parallel to the surface. The structure
other than the residual austenite is also martensite, so
in other words this is synonymous with martensite being
present at an area ratio of 70% or more.
Further, the area ratio can be obtained by photographing
the microstructure by a scan type electron microscope at
X2000, then analyzing it by an image analysis system.
(2) A steel part excellent in temper softening
resistance as set forth in the above (1) characterized by
the steel further containing one or more chemical
ingredients, by mass%, of
Mo: 0.02 to 0.6%,
Ni: 0.1 to 1.0%, and
B: 0.0002 to 0.005%, and
having a total content of Mn, Cr, Mo, and Ni satisfying
2.5% or less.
(3) A steel part excellent in temper softening
resistance as set forth in the above (1) or (2)
characterized by the steel further containing one or more
chemical ingredients, by mass%, of
Ti: 0.01 to 0.05%,
Nb: 0.001 to 0.04%, and
V: 0.02 to 0.1%.
(4) A steel part excellent in temper softening
resistance as set forth in any one of the above (1) to
(3) characterized by the steel further containing one or
more chemical ingredients, by mass%, of
Ca: 0.0002 to 0.005%,
Zr: 0.0003 to 0.005%,
Mg: 0.0003 to 0.005%, and
REM (rare earth metals): 0.0001 to 0.005%.
(5) A pulley for a continuous variable transmission of a
belt type excellent in temper softening resistance using
the steel part as set forth in any one of the above (1)
to (4).
9
Advantageous Effect of Invention
0024 According to the present invention, case hardened
steel of a "temper softening resistance" of HV700 or more
is obtained. It is possible to obtain a gear or other
part excellent in pitting fatigue resistance at a high
temperature. This is a case hardened steel part which is
extremely excellent in temper softening resistance. If
using a case hardened steel part, it becomes possible to
increase the strength of a pulley or gear etc. for a
continuous variable transmission for automobile
applications and possible to reduce the size of the
parts. Due to this, it is possible to lighten the weight
of vehicles and therefore improve the fuel economy and
reduce the emissions of CO2 and possible to prevent global
warming and conserve on resources. The effect in industry
according to the present invention is extremely
remarkable.
Brief Description of Drawings
0025 FIG. 1 is a view showing the effects of the
content of Si and the content of C on the hardness after
300°C tempering.
Description of Embodiments
0026 Below, as the mode for working the present
invention, a case hardened steel part excellent in temper
softening resistance will be explained in detail.
0027 First, the reasons for limitation of the chemical
ingredients of the steel of the present invention will be
explained. Below, unless otherwise indicated, mass% in
the composition is simply described as "%".
0028 C: 0.1 to 0.3%
C is an element which is effective for imparting the
necessary strength to steel, but if less than 0.1%, it is
not possible to secure the necessary strength, while if
over 0.3%, the hardness of the material before machining
increases and the machineability deteriorates, so the
10 -
content must be made 0.1 to 0.3% in range. The suitable
range is 0.15 to 0.25%.
0029 Si: 0.7 to 1.0%
Si is the most important element in the present invention
steel. That is, Si is the most effective element for
improving the softening resistance in the 300°C
temperature region.
The Ms point of the carburized layer (martensite
transformation temperature) falls if the content of C is
made high. For this reason, tempering causes an increase
in the residual austenite. The inventors learned from
FIG. 1 that when making the content of C of the
carburized layer the hyper eutectic 0.9 to 1.1%, the
suitable range of Si is 0.7% or more.
However, Si is also an element which impairs the
carburization ability. From the viewpoint of prevention
of sooting in the gas carburization furnace, it was
learned by experience that to stably cause a high content
of C of the hyper eutectic 0.9 to 1.1%, the amount of Si
has to be given an upper limit of 1.0%.
0030 For the above reasons, the content of Si has to
be made 0.7 to 1.0% in range. The suitable range of the
lower limit of the content of Si is, from the viewpoint
of securing the temper softening resistance, preferably
0.75%, more preferably 0.8%. The upper limit of Si is
determined by the sooting limit, so considering a safety
margin, 0.98% is preferable and 0.95% is more preferable.
0031 Mn: 0.2 to 1.5%
Mn is an element which is effective for deoxidation of
the steel and an element which is effective for imparting
the necessary strength and quenchability to the steel. If
Mn content is less than 0.2%, the effect cannot be
sufficiently obtained. Further, if over 1.5%, the effect
becomes saturated. Not only that, the hardness of the
material before machining increases and the
machineability degrades. For this reason, the content has
to be made 0.2% to 1.5% in range. The lower limit of the
- 11 -
Mn content, from the viewpoint of drawing out the effect
and considering a safety margin, is preferably 0.3%, more
preferably 0.4%. The upper limit of the Mn content, from
the viewpoint of economy due to saturation, is preferably
1.40, more preferably 1.3%.
0032 Further, Mn is an element which lowers the Ms
point of the steel. In addition, Ni, Cr, Mo, etc. are
also elements which lower the Ms point. When such
elements promoting a drop in the Ms point are added in a
large amount, transformation to martensite is sometimes
suppressed and the amount of residual austenite after the
gas carburization exceeds 30%. For this reason, it is
necessary to limit the total amount of such elements
promoting a drop in the Ms point to 2.5% or less. From
the viewpoint of suppressing the drop in the Ms point,
the amount is preferably 2.3%, more preferably 2.0% or
less.
0033 Cr: 1.0 to 1.5%
Cr is an element which, upon addition, is effective for
imparting strength and quenchability to the steel. To
obtain these effects, 1.0% or more is necessary.
However, to prevent precipitation of proeutectoid
cementite under carburization conditions giving a hyper
eutectic content of C (0.9 to 1.1%) and even if holding
before quenching at about 850°C, the amount of Cr has to
be made less than 1.5%. For the above reasons, the Cr
content has to be made 1.0 to 1.5% in range. The lower
limit of the Cr content, from the viewpoint of drawing
out the effect and considering a safety margin, is
preferably 1.05%, more preferably 1.1%. The upper limit
of the Cr content, from the viewpoint of suppressing
precipitation of cementite, is preferably 1.4%, more
preferably 1.3%.
0034 In the past, the problem as pointed out that Cr
forms oxides together with Si at the surface of a treated
object at the time of gas carburization and lowers the
carburization ability (for example PLT 1). However, the
12 -
present invention is characterized by performing gas
carburization controlled to a high content of C (0.9 to
1.1%). In this case, the concentration of oxygen in the
carburizing atmosphere falls 20 to 50% compared with the
general gas carburization targeting a content of C at the
surface of the treated object of 0.8% (for example, the
concentration of oxygen in the furnace during gas
carburization is 1.8x10-20 atm at the time of a carbon
potential of 0.8, is 1.4x10-20 atm at the time of a carbon
potential of 0.9, is 1.1x10-20 atm at the time of a carbon
potential of 1.0, and is 0.9x10-20 atm at the time of a
carbon potential of 1.1). For this reason, it was
confirmed that the trouble of Cr forming oxides is not
manifested and the content of Cr was in the range of 1.0
to 1.5%.
0035 S: 0.001 to 0.15%
S forms MnS in the steel and is added for the purpose of
improving the machineability due to this. If S content is
less than 0.001%, the effect is insufficient. On the
other hand, if over 0.15%, the effect is saturated and
rather grain boundary segregation is caused and grain
boundary embrittlement is invited. For the above reasons,
the content of S has to be made 0.001 to 0.15% in range.
When stressing the machineability, the suitable range is
0.005 to 0.15%, while when stressing the grain boundary
strength, the suitable range is 0.001 to 0.030%. When
stressing both the machineability and the grain boundary
strength, the suitable range is 0.005 to 0.070%.
0036 Al: 0.001 to 1%
Al is added for the purpose of deoxidation and
improvement of the machineability. If less than 0.001%,
the effect of deoxidation is insufficient. Al (all Al in
steel) partially bonds with N and precipitates as AlN,
while the remainder remains as 'solute Al. The solute Al
effectively acts to improve the machineability, but when
stressing the machineability, over 0.05% is necessary,
3-
but if over 1%, the transformation characteristics are
greatly affected, so the upper limit was made 1.0%. When
stressing the machineability, the suitable range is 0.05
to 1%, while when not stressing the machineability that
much, the suitable range is 0.02 to 0.05%.
0037 N: 0.001 to 0.03%
N is an unavoidably contained element, but also has the
effect of refining the crystal grains by formation of
compounds with Al and N, so 0.001% or more is required.
However, if over 0.03%, the forgeability is remarkably
impaired, so 0.03% as made the upper limit. The suitable
range is 0.004 to 0.015%. The more suitable range is
0.005 to 0.012%.
0038 P: limited to 0.03% or less
P is an unavoidably contained impurity element. It
segregates at the grain boundaries to cause a drop in
toughness, so has to be limited to 0.03% or less. The
suitable range is 0.015% or less. The more suitable range
is 0.01% or less.
0039 0: limited to 0.005% or less
0 is an unavoidably contained impurity element. It is an
element which segregates at the grain boundaries to
easily cause grain boundary embrittlement and which forms
hard oxide-based inclusions in the steel to easily cause
embrittlement fracture. 0 has to be limited to 0.005% or
less. The suitable range is 0.0025% or less. The more
suitable range is 0.001% or less.
0040 As explained above, Mn, Cr, Mo, and Ni are all
elements which cause the Ms point to drop. The Ms point
falls along with an increase in the total content of
these elements resulting in a tendency for increase of
the residual austenite after carburization, quenching,
and tempering. In the present invention, the residual
austenite amount has to be made 30% or less. For this
reason, the total content of Mn, Cr, Mo, and Ni has to be
2.5% or less. From the viewpoint of suppressing the drop
in the Ms point, it is preferably 2.3% or less, more
14 -
preferably 2.0% or less.
0041 Next, in claim 2 of the present invention, there
is provided claim 1 in addition to which, to improve the
quenchability, one or more of Mo, Ni, and B is included.
0042 Me: 0.02 to 0.6%
Mo, by addition, has the effect of imparting strength and
quenchability to the steel, but if less than 0.02%, the
effect is insufficient. However, if over 0.6% is added,
the effect becomes saturated. Not only that, the hardness
of the material before machining increases and the
machineability is degraded, so the amount has to be made
0.02% to 0.6% in range. The suitable range has a lower
limit of 0.15 and a upper limit of 0.25%. The more
preferable range has a lower limit of 0.17 and an upper
limit of 0.23%.
0043 Ni: 0.1 to 1.0%
Ni, by addition, has the effect of imparting strength and
quenchability to the steel, but if less than 0.1%, the
effect is insufficient. However, if over 1.0% is added,
the effect becomes saturated. Not only that, the hardness
of the material before machining increases and the
machineability is degraded in accordance with the amount
of addition. Therefore, Ni has to be 0.1% to 1.0% in
range. The suitable range is 0.2 to less than 0.5%, more
preferably a range of 0.2 to 0.3%.
0044 B: 0.0002 to 0.005%
B, by addition, has the effect of imparting strength and
qUenchability to the steel, but to obtain that effect,
0.0002% or more is necessary. However, if adding over
0.005% of B, the effect becomes saturated and conversely
deterioration of the impact strength and other
detrimental effects are feared, so its content has to be
made 0.005% or less in range. The suitable range is
0.0010 to 0.003%, more preferably a range of 0.0015 to
0.0025%.
0045 Next , in claim 3 of the present invention, there
is provided claim 1 or 2 wherein in addition one or more
15 -
of Ti, Nb, and V is contained for the purpose of refining
the crystal grains.
0046 Ti: 0.01 to 0.05%
Ti bonds with the C and S in the steel to form fine TiC
and TiCS of several nanometers to several tens of
nanometers in size and therefore is added to refine the
austenite crystal grains at the time of carburization.
However, if Ti content is less than 0.01%, the effect is
insufficient. However, if adding Ti over 0.05%, due to
the loss of C due to TiC, the hardness after 300°C
tempering falls, so the upper limit was made 0.05%. The
suitable range is 0.015 to 0.04%, more preferably a range
of 0.02 to 0.03%.
0047 Nb: 0.001 to 0.04%
Nb bonds with the C and N in the steel to form fine
Nb(CN) of several nanometers to several tens of
nanometers in size and is an element which is effective
for suppressing coarsening of the crystal grains, but to
obtain that effect, 0.001% or more is necessary. However,
if over 0.04% is added, due to the loss of C due to
Nb(CN), the hardness after 300°C tempering falls, so the
upper limit was made 0.04%. For the above reasons, its
content has to be made 0.001% to 0.04% in range. The
suitable range is 0.005 to 0.03%, more preferably arange
of 0.01 to 0.025%.
0048 V: 0.02 to 0.1%
V bonds with the C and N in the steel to form fine V(CN)
of several nanometers to several tens of nanometers in
size and is an element which is effective for suppressing
coarsening of the crystal grains, but to obtain that
effect, 0.02% or more is necessary. However, if over 0.1%
is added, due to the loss of C due to V(CN), the hardness
after 300°C tempering falls, so the upper limit was made
0.1%. For the above reasons, its content has to be made
0.02 to 0.1% in range. The suitable range is 0.03 to
0.08%, more preferably a range of 0.05 to 0.07%.
1.6 -
0049 Next, in claim 4 of the present invention, there
is provided any one of claims 1 to 3 in addition to
which, to improve machineability, one or more of Ca, Zr,
Mg, and an REM are contained.
0050 Ca: 0.0002 to 0.005%
Ca lowers the melting point of the oxides and causes
softening due to the rise in temperature under a cutting
environment, so improves the machineability, but if less
than 0.0002%, there is no such effect, while if over
0.005%, CaS is produced in a large amount and reduces the
machineability, so the amount of Ca is preferably made
0.0002 to 0.005%. The suitable range is 0.0005 to 0.004%,
more preferably a range of 0.001 to 0.003%.
0051 Zr: 0.0003 to 0.005%
Zr is a deoxidizing element. It produces oxides, but also
produces sulfides, so is an element having an
interrelationship with MnS. It is effective for
improvement of the machineability. Zr-based oxides easily
form the nuclei for crystallization/precipitation of MnS.
For this reason, they are effective for control of
dispersion of MnS. As the amount of addition of Zr, to
aim for spheroidization of MnS, addition of over 0.003%
is preferable, but to cause fine dispersion, conversely
addition of 0.0003 to 0.005% is preferable. For the final
product, the latter is preferable from the viewpoint of
stability of quality in product (yield of ingredients
etc.). That is, 0.0003 to 0.005% for causing fine
dispersion of MnS is realistically preferable. If 0.0002%
or less, the effect of addition of Zr is not recognized
much at all. The suitable range is 0.0005 to 0.004%,
while a range of 0.001 to 0.003% is more preferable.
0052 Mg: 0.0003 to 0.005%
Mg is a deoxidizing element and produces oxides, but also
produces sulfides, so is an element having an
interrelationship with MnS. It is effective for
improvement of the machineability. Mg-based oxides easily
form the nuclei for crystallization/precipitation of MnS.
Further, the sulfides become composite sulfides of Mn and
Mg, whereby deformation is suppressed and the sulfides
become spherical. For this reason, this is effective for
control of dispersion of MnS, but if less than 0.0003%,
there is no such effect, while if over 0.005%, a large
amount of MgS is formed and the machineability falls, so
the amount of Mg is preferably made 0.0003 to 0.005%. The
suitable range is 0.0005 to 0.004%, more preferably a
range of 0.001 to 0.003%.
0053 REM (rare earth metals): 0.0001 to 0.005%
REM (rare earth metals) is an element which, by addition,
has the action of causing fine dispersion of MnS and is
effective for improvement of the machineability. To
obtain that effect, 0.0001% or more is necessary.
However, if over 0.005%, the effect is saturated. The
suitable range is 0.0005 to 0.003%. The suitable range is
0.0005 to 0.004%, more preferably a range of 0.001 to
0.003%.
0054 Next, the limitation of the area ratio of the
cementite and carbides inside the carburized layer will
be explained. To obtain the temper softening resistance
indicator of the carburized layer, that is, the
carburized layer surface hardness (Vicker's hardness) at
300°C, it is sufficient to suppress the formation of
proeutectoid cementite and spherical carbides and instead
cause the precipitation of fine s carbides on the nanosize
order. This is because, due to this, no starting points
of fracture are formed and the temper softening
resistance at 300°C can be maintained. Therefore, by
defining the amounts of proeutectoid cementite and
spherical carbides, it is possible to maintain the
carburized layer surface hardness.
0055 The inventors discovered, for the carburized
layer surface hardness, it is preferable that
proeutectoid cementite and spherical carbides of an
average diameter of 0.1 μm or more not be formed at the
18 -
part of a depth of 50 μm from the carburized layer
surface and that even if formed, the ratio of the area
(area ratio) be suppressed to less than 1%.
0056 Further, to secure the strength of the carburized
layer, the residual austenite amount should be suppressed
and martensite should be formed. For this reason, the
inventors discovered that it is preferable that residual
austenite not be present at the part of a depth of 50 μm
from the carburized layer surface and that even if
present, it be suppressed to less than 30%. The reason
for limitation in this way is as follows:
0057 A case hardened steel part which has been gas
carburized oxidizes from the surface due to the fine
amount of oxygen in a carburization atmosphere (about 10
to 20 atm). At this time, alloys of Si, Mn, Cr, etc.
become oxides. Due to the loss from the steel (alloy
loss), formation of an incomplete hardened layer of about
20 μm to 40 μm becomes unavoidable. This incomplete
hardened layer becomes worn at the extremely initial
stage of the time of part use, so in the strength of the
part, the properties of the carburized layer directly
under it become the dominating factor. As an indicator of
the properties of the carburized layer directly under it,
the proeutectoid cementite and spherical carbides and the
residual austenite amount at the part of 50 μm depth from
the surface were defined.
0058 The present invention uses steel increased in Si
to 0.7% or more and makes the content of C 0.9 to 1.10,
preferably over 0.95 to 1.1%, that is, hyper eutectic. At
this time, if malting the state one in which proeutectoid
cementite and spherical carbides are not substantially
present and carburizing, quenching, and tempering to give
a residual austenite amount of less than 30%, the temper
softening resistance is greatly improved. The residual
austenite amount should be 0%.
0059 If the content of C of the carburized layer is
19 -
less than 0.9%, the hardness after 300°C tempering
sometimes cannot satisfy HV700 or more, so the lower
limit of the content of C was made 0.9%. If the content
of C is over 1.1%, proeutectoid cementite precipitates at
the stage of retention before quenching at about 850°C
performed after carburization. Therefore, the upper limit
of the content of C was made 1.1%. C is present as s
carbides and thereby contributes to the improvement of
the temper softening resistance. If C bonds as
proeutectoid cementite and spherical carbides, s carbides
are decreased by that amount, so it is preferable to
establish a state in which proeutectoid cementite and
spherical carbides are not substantially present.
0060 Here, proeutectoid cementite is cementite which
precipitates from austenite before eutectic
transformation when cooling hyper eutectic steel from a
high temperature and is defined by JIS G 0201, "Spherical
carbides" mean carbides which became spherical. Here, it
means a size which can be identified by an optical
microscope (average diameter found by average value of
long diameter and short diameter of particles) of 0.1 μm
or more. If proeutectoid cementite and spherical carbides
of a diameter of 0.1 μm or more are present in an area
ratio of less than 1% (including 0%), a similar effect as
the state in which they are not present is obtained.
0061 The area ratio of proeutectoid cementite and
spherical carbides of an average diameter of 0.1 pm or
more is an area ratio of less than 1% at a part of a
depth of 50 pm from the surface in a range of 10 μm
square on a plane parallel to the surface.
0062 The area ratio can for example by obtained by
photographing the microstructure by a scan type electron
microscope by a X2000 power, then measuring the area
ratio by an image analysis system. The method of
measurement of the area ratio is not limited to the above
method. The method is not an issue if proeutectoid
20 -
cementite and spherical carbides can be identified.
0063 As shown in FIG. 1, the inventors discovered that
the hardness after 300°C tempering in the hyper eutectic
state is largely dependent on the content of Si. Further,
when carburizing the steel by a high content of C of the
hyper eutectic state (content of C of 0.9 to 1.1%), the
microstructure becomes a dual phase of martensite and
residual austenite. Further, by establishing gas
carburization conditions reducing the amount of residual
austenite of the carburized layer (that is, reducing the
amount of martensite), it is possible to increase the
presence of s carbides in the martensite to more than the
usual content of C of 0.8%. Further, by making the
content of Si high, transition to cementite is difficult
even when the s carbides are quenched at 300°C. Due to
these synergistic effects, the temper softening
resistance is greatly improved. At this time, the Ms
point falls due to the higher C, so it is sufficient to
restrict the total content of Mn, Cr, Mo, and Ni to 2.5%
or less so that the Ms point does not fall too much.
0064 Further, the gas carburization method is not
particularly limited so long as it is possible to obtain
the case hardened steel part of the present invention
explained above. However, to stably make the content of C
at the part of 50 μm depth from the surface 0.9 to 1.1%
in range, it is preferable to secure a carburization time
of 30 minutes or more. The oil temperature at the time of
quenching after gas carburization may be made either the
general hot quenching one such as 130°C or so-called cold
quenching one such as 60°C. The inventors compared 130°C
oil quenched parts and 60°C quenched parts and discovered
that parts oil quenched at 130°C became greater in amount
of residual austenite the higher the percentage. However,
by suitably setting the quenching conditions, it becomes
possible to adjust the amount of residual austenite to
less than 30%, so the oil temperature is not limited.
2
0065 The gas carburization for auto part applications
is generally performed after carburization at a
temperature of around 950°C by lowering the temperature to
about 850°, holding before quenching, then quenching by
oil. However, in part, so-called secondary quenching is
performed by quenching by heating to the austenite region
of 850°C or more after carburization for the purpose of
refining the crystal grains. The content of C in the gas
carburization of the present invention is the hyper
eutectic state of 0.9 to 1.1%, so unlike the general
eutectic C content (0.80), if heating to the 850°C or
higher austenite region again after carburization, there
is the problem that spherical carbides precipitate.
Therefore, secondary quenching is not recommended. If
desiring to perform secondary quenching at all costs, the
reheating temperature has to be raised to 900°C or more,
preferably 930°C, and the carbides precipitated at the
time of reheating have to again be made to form a solid
solution in austenite.
0066 Rather, in the case of case hardened steel
according to the present invention, the fact that it is
possible to make fine s carbides precipitate in the
martensite structure even without secondary quenching is
also a major feature.
0067 That is, if in the range of ingredients of the
present invention, by using carburization conditions at
the hyper eutectic state as with general gas
carburization, no proeutectoid cementite and spherical
carbides is substantially present and the amount of
residual austenite can be made less than 30%.
Examples
0068 Below, the present invention will be specifically
explained by examples. Note that these examples are for
explaining the present invention and do not limit the
range of the present invention.
22 -
0069 Steel ingots having the chemical ingredients
shown in Table 1 were forged, then soaked and normalized,
then worked to prepare 15x45 mm rod shaped test pieces in
two types of steel. The rod shaped test pieces were
carburized in a conversion type gas carburization furnace
(propane conversion) at 930°Cx5 hours, then held before
quenching at 850°Cx30 minutes, then oil quenched at 130°C.
Here, the carbon potential with respect to pure iron in
the carburization was made 0 .9 to 1.2% so as to adjust
the content of C in the vicinity of the surface (50 μm
depth from the surface) of the test pieces to 0.8 to
1.1%.
0070 After oil quenching at 130°C, the test pieces were
quenched at 150°Cx2 hours to obtain test pieces
corresponding to actual case hardened steel parts. Here,
one test piece out of every two was electrolytically.
polished to a 50 μm depth from the surface, then measured
for amount of residual austenite by a micro X-ray
diffraction system.
0071 Further, a piece was cut from the center part in
the axial direction and measured for content of C at the
part of a 50 μm depth from the surface by EPMA. Further,
the inventors examined the state of presence of carbides
under a scan type electron microscope at X2000. If
proeutectoid cementite and spherical carbides of an
average grain diameter of 0.1 μm or more are present in
an area ratio of 1% or more, they are judged "present". A
micro Vicker's hardness meter (load 300 gf) was used to
measure the hardness at a part of 50 μm depth from the
surface. As explained above, the area ratio can be
obtained by for example photographing the microstructure
by a scan type electron microscope at an X2000 power,
then analyzing it by an image analysis system.
0072 The results of measurement of the amount of
residual austenite and the content of C and the results
23 -
of investigation of the state of presence of the carbides
are shown in Table 2. The other of each two test pieces
was further tempered at 300°Cx2 hours to reproduce the
environment of an actual part. After that, a piece was
cut from the center part in the axial direction and the
part of 50 μm depth from the surface as measured for
hardness by a micro Vicker°s hardness meter. The
measurement results are shown in Table 2.
0073 As shown in Table 2, Test Nos. 1 to 32 of the
invention examples had hardnesses after tempering at 300°C
in the vicinity of the surface (50 gm depth from the
surface) of HV700 or more and were excellent in temper
softening resistance.
0074 As opposed to this, Test No. 33 of the
comparative example had a hardness after tempering at
300°C in the vicinity of the surface (50 μm depth from the
surface) of a low HV685. This was because the Si of the
steel material was less than 0.7% and consequently the
temper softening resistance became insufficient.
0075 Test No. 34 of the comparative example had a
hardness after tempering at 300°C in the vicinity of the
surface (50 gm depth from the surface) of a low HV696.
This was because the Si of the steel material exceeded
1.0% and consequently the carburization ability fell and
because the content of C in the vicinity of the surface
(50 μm depth from the surface) was less than 0.9% and
therefore the temper softening resistance was
insufficient.
0076 Test No. 35 of the comparative example had a
hardness after tempering at 300°C in the vicinity of the
surface (50 gm depth from the surface) of a low HV680.
This was because the Cr of the steel material was lower
than 1.0% and consequently the temper softening
resistance was insufficient.
0077 Test No. 36 of the comparative example had a
24 -
hardness after tempering at 300°C in the vicinity of the
surface (50 μm depth from the surface) of a low HV671.
This was because the Cr of the steel material was over
1.5% and consequently, when holding before quenching for
850°Cx30 minutes, proeutectoid cementite formed and the
proeutectoid cementite caused a reduction in E carbides
and therefore the temper softening resistance was
insufficient.
0078 Test No. 37 of the comparative example had a
hardness after tempering at 300°C in the vicinity of the
surface (50 μm depth from the surface) of a low HV675.
This was because the total content of Mn, Cr, Mo, and Ni
of the steel material exceeded 2.5%, so the amount of
residual austenite of the material tempered at 150°C
became 30% or more and therefore the temper softening
resistance was insufficient.
0079 Test No. 38 of the comparative example had a
hardness after tempering at 300°C in the vicinity of the
surface (50 μm depth from the surface) of a low HV671.
This was because the total content of Mn and Cr of the
steel material exceeded 2.5%, so the amount of residual
austenite of the material tempered at 150°C became 30% or
more and therefore the temper softening resistance was
insufficient.
E 4 ' Z - - - - S ' 0 - - - 86'0 T'0 TOO.O £TO.O ZTO.O TEO.0 6TO'0 90-T E'0 8'0 SZ'0 •xa -dmo0 Ob
T6'T - - - - - - - - - 91'0 T00'0 600.0 ZT0'0 TEO'0 610'0 IZ'T 0S-0 9'0 Z'0 xa •dmoQ 6£
9'Z - - - - - - - - - - 100'0- 510'0 310"0 ££0'0 Si0'0 31'1 84'0 91'0 Z'0 xa -dmo0 9£
85'3 - - - - - - - - 8'0 ZZ'0 100'0 ET0'0 10'0 9Z0'0 410'0 TO'T 99'C 6.0 61'0 xa •dm00 LE
ZE'Z - - - - - - - - - 91'0 100'0 110"0 STO"0 9£0'0 STO'0 9'1 9S'0 61'0 TZ'0 'xa °dmo0 9E
S9'1 - - - - - - - - - 91'0 100'O 910'0 ZTO'0 EO'0 £10'0 66'0 99.0 S6.0 Z"0 •xa • dmo0 S£
9L'T - - - - - - - - 91.0 100'0 SiO'O ZTO.O 6Z 0'0 910"0 90-T SS-0 90-T Z'0 xa ° mod 4E
96'1 - - - - - - - - - 91'0 100'0 600.0 10'0 £0'0 410'0 60'1 9L'0 49'0 12'0 'xa -dmo0 E£
44'2 - - - 2000 - - - - - - 100'0 610'0 110'0 £0'0 9T0'0 90'1 69'1 91'0 61'0 'x9 •AUI 39
LB'T 330010 - - - - - - - - 02'0 TOO'0 SiO"O ZTO'O TE0'0 £10'0 Z'T £560 66'0 £Z'0 'xa nul T£
80'2 - STOO'0 - - - - - - - 80'0 100'0 910"0 110'0 620'0 SEO'0 TO-T 69'0 S8'0 3'0 'XD AUI 0£
EB'i 1100'0 - - - £00 - - - LT'0 100'0 910"0 TO'O £E0'O 010'0 11'1 55'0 16'0 Z'0 'xa AUI 6Z
90"Z - - - 500'0 - - - - - 91.0 100'0 ETO'0 110.0 90'0 90'0
- -
ZE'1 82.0 6.0 Z'0 -X0 AUI BE
66'1 - - - - T'0 - 20'0 - - 91.0 1000 ZTO'0 900.0 T£0'0 TT 67 0 SZ'T £S"0 46'0 91'0 •xa AUI LZ
89'1 - - - - ZO'0 - - - - 100'0 STO'O 31040 990'0 £0'0 EO'T S9"0 96.0 92'0 'xa AUI 93
18"1 - - - - - 60'0 - 63000 - 91'0 100.0 STO"0 110'0 £Z6'0 STO'0 T-1 SS-0 98.0 61'0 xa -AUI 9Z
1L T - - - - - 0 - - - ST'O 100'0 STO'0 10'0 860.0 90'0 906T TS-0 8.0 91'0 -xa •AUI 43
LO'Z - - - - - - SO"0 - - 91'0 T00'0 410.0 400'0 SZO'0 620'0 SZ'1 99.0 08'0 0"0 •xa AUI EZ
£6'T - - - - - - 10'0 - - BT40 100'0 410'0 S00'0 990.0 ZO.0 T"1 S9.0 S6.0 £Z-O 'xa •AUI ZZ
S9'T - - - - - - - 9100"0 SO'0 100'0 £10'0 110'0 SZO.O 830'0 1"T S'0 S8"0 Z'0 xa •AUI TO
03 3 - - - - 90'0 - - - 86.0 - 100.0 £TO"0 ZTO"0 SZO'O STO"0 90'1 13"0 91.0 LZ'O •xa AUI OZ
16'1 - - - - - - - - to - 100'0 610'0 110"0 £EO'O 610.0 9E'T Sb'0 96'0 4240 •xa AUI 61
CUE - - - - - - - - - 9'0 T00'0 000'0 ZT0"0 £0'0 90'0 8E°T S£'0 8'0 Z'0 xa AUI 81
8'0 - - - 20010 - - - - - ZO.O Too40 610'0 310"0 101'0 £00.0 £1'T 99'0 8'0 81'0 xa AUT LT
8'1 - - - - - ZO'0 £0"0 - - - S00'O £TO.O 600'0 E0060 910'0 56'1 9E'0 SL'0 31'0 xa AUI 91
9141 - - - - - - - - - 91'0 100'0 E0.0 310"0 1E060 590.0 90'1 90/0 96'0 93'0 xa -7UI 91
96'1 - - - - L0'0 - - - - S1'0 T0060 610'0 630'0 99.0 STO'0 93'1 S9'0 B'0 12'0 xa •AUI 41
Z5'Z - - - - - - - - £60 LT'0 £00'0 STO'0 100.0 500'0 800.0 UT 99'0 61.0 ZZ'0 xa •AUI E1
L6"T - - 5000'0 - - - - - - 91'0 100.0 SZO'0 ZTO"0 96'0 900'0 9Z 'T SS'0 T8'0 L1'0 xa •AUI 31
TL'1 - - - - - - - 100'0 - - Z00'0 STO'O 900'0 100'0 STO'0 TT'T 9'0 SB'0 Z6 0 xa AUI 11
SL 'T 6000'0 - - - - TO'0 - - - Z'0 100'0 510'0 ST O"0 990'0 901'0 T'L S4'0 98'0 61'0 'xa AUI OT
8£'Z - - - 100'0 - - - - 66'0 ST 0 10001 900 0 TTO'0 60"0 100'0 Z'T 99'0 B'0 91'0 xa •A01 6
LZ"Z - - - - - - ZO'0 - - 91'0 100.0 STO'0 400'0 930'0 £10'0 9"T 19.0 96'0 Z"0 'xa -AU1 g
S " Z - - - - - - - -
T E O
T S'T SL 'O 81'0 "XU AUI L
98'1 - - - - - - - 3300"0 - 90'0 100'0 TTO'0 210'0 ZEO"0 910'Q S'1 Z'0 8.0 Z'0 •xa •AUI 9
164T - - - - - - - - - 910 1000 6100 TTO'0 EO'0 900'0 SZ'1 S'0 T TZ'0 xa •AUI S
LUZ - - - - - - - - 32'0 Z"0 100.0 ETO'O ZTO'0 9£0'0 ZO'0 £"1 99'0 L'0 £Z'0 •xa AUl 4
68'1 - 9000'0 - - - - - - - . 40.0 100'0 STO'0 910"0 CO-0 STO'0 ST-1 9.0 S6"0 E'0 xa -AUI £
3 6 ' 1 - - - - - - - - - 9 1 " 0 T 0 0 ' 0 STO'0 ZT040 630'0 900'0 ZZ'T 9S'0 06'0 1'0 •xa AUI Z
T6"T - - - - - - - - - 91'O 000'0 600'0 Z10"0 TEO'.0 510'0 TZ"i 0S'O 8.0 Z'0 'xa -AUI 0
(gssem )
TN put oW
Gi32I bW sZ u0 A 4N TS H TN OK 0 d N TV S 10 UW T5 0 S52T0
IN
^saS
'ux 3o 4unom2 T24oy ( oss2m ) s]ua Tpasbul
T GT4ps 0800
- 26 -
0081 Table 2
est No. lass
Content of C
at 50 pm depth
from the
surface
(mass% )
Presence of outside
diameter 0.1 pm or
more proeutectoid
cementite or
spherical carbides
(area ratio)
Amount of residual
austenite at 50 pm
depth from the
surface ( mass%)
Hardness after
tempering at 300 °C
at 50 μm depth
from the surface
(HV)
1 Inv. ex. 0.9 None 22 704
2 Inv, ex. 1 None 24 720
3 Inv. ex. 0.9 None 15 715
4 Inv. ex. 0.9 None 19 703
5 Inv. ex. 0.9 None 16 716
6 Inv. ex. 1 None 10 715
7 Inv. ex. 0.9 None 29 704
9 Inv. ex. 1 None 26 722
9 Inv. ex. 1 None 20 714
10 Inv. ex. 0 . 9 None 14 708
11 Inv. ex. 1 . 1 None 24 721
12 Inv. ex. 1 None 21 716
13 Inv. ex. 0 . 9 None 16 706
14 Inv. ex. 0 . 9 None 13 704
15 Inv. ex. 1 None 15 720
16 Inv. ex. 1 . 1 None 20 718
17 Inv. ex. 1 None 22 713
18 Inv. ex. 0 . 9 None 14 706
19 Inv. ex. 1 None 20 720
20 Inv. ex. 1 . 1 None 24 720
21 Inv. ex. 1 None 14 718
22 Inv. ex. 1 None 20 722
23 Inv. ex. 0 . 9 None 10 710
24 Inv. ex. 1.1 None 14 723
25 Inv. ex. 1 None 20 719
26 Inv. ex. 1.1 None 24 730
27 Inv. ex. 1 None 19 722
28 Inv. ex. 0 . 9 None 13 711
29 Inv. ex. 1 None 20 723
30 Inv, ex. 0 . 9 None 24 709
31 Inv. ex. 1 . 1 None 24 727
32 Inv. ex. 0 . 9 None 26 705
33 Comp. ex. 0.9 None 17 685
34 Comp. ex . 0.8 None 16 696
35 Comp. ex . 1 None 24 680
36 Comp. ex . 1
Proeutectoid
cementite present
(1%)
25 671
37 Comp. ex. 1 None 33
_
675
38 Comp. ex . 1 None 34 671
39 Comp. ex . 1.2
Proeutectoid
cementite present
(1%)
24 684
40 Comp. ex . 0.9
Spherical carbides
presence (1%)
20 670
Industrial Applicability
0082 The case hardened steel part according to the
present invention can be utilized for auto parts or
machine parts. In particular, when used for auto parts,
pulleys, gears, etc. for continuous variable
transmissions can be made higher in strength and the
parts can be made smaller in size. Due to this, it is
possible to achieve a reduction of the weight of the
27 -
vehicle and consequent improvement of fuel efficiency and
reduction of the emission of CO2 and prevent global
warming and conserve resources. The effect in industry of
the present invention is extremely remarkable.
- 28 -
CLAIMS
Claim 1
A steel part excellent in temper softening
resistance which is comprised of steel containing as
chemical ingredients, by mass%, of
C: 0.1 to 0.3%,
Si: 0.7 to 1.0%,
Mn: 0.2 to 1.5%,
Cr: 1.0 to 1.5%,
5: 0.001 to 0.15%,
Al: 0.001 to 1%, and
N: 0.001 to 0.03%,
satisfying a total content of Mn and Cr of 2.5% or less,
and
having a balance of iron and unavoidable impurities, and
which is carburized, quenched, and tempered on its
surface,
said steel part characterized by having a content of C at
a part at a 50 μm depth from the surface of 0.9 to 1.1%,
not containing proeutectoid cementite or spherical
carbides of an average diameter of 0.1 pm or more or even
if containing it, having an area ratio of it of less than
1%, and, further, not containing residual austenite or
even if containing it, having an area ratio of it of less
than 30%.
Claim 2
A steel part excellent in temper softening
resistance as set forth in claim 1, characterized by the
steel further containing one or more chemical
ingredients, by mass%, of
Moe, 0.02 to 0.6%,
Ni: 0.1 to 1.0%, and
B: 0.0002 to 0.005%, and
having a total content of Mn, 'Cr, Mo, and Ni satisfying
2.5% or less.
Claim 3
A steel part excellent in temper softening
-- 29 -
resistance as set forth in claim l or 2, characterized by
the steel further containing one or more chemical
ingredients, by mass%, of
Ti: 0.01 to 0.05%,
Nb: 0.001 to 0.04%, and
V: 0.02 to 0.1%.
Claim 4
A steel part excellent in temper softening
resistance as set forth in claim 1 or 2 characterized by
the steel further containing one or more chemical
ingredients, by mass%, of
Ca: 0.0002 to 0.005%,
Zr: 0.0003 to 0.005%,
Mg: 0.0003 to 0.005%, and
REM (rare earth metals): 0.0001 to 0.005%.
Claim 5
A steel part excellent in temper softening
resistance as set forth in claim 3 characterized by the
steel further containing one or more chemical
ingredients, by mass%, of
Ca: 0.0002 to 0.005%,
Zr: 0.0003 to 0.005%,
Mg: 0.0003 to 0.005%, and
REM (rare earth metals): 0.0001 to 0.005%.