STEEL FOR CARBURIZING OR CARBONITRIDING USE
TECHNICAL FIELD
[0001]
The present invention relates to a steel for carburizing or carbonitriding
use. More particularly, the present invention relates to a steel that has an
excellent property in preventing grain coarsening at the time of carburizing or
carbonitriding, and in addition has an excellent bending fatigue strength after
carburizing or carbonitriding, and is suitable as a steel for a starting material
of parts, such as gears, pulleys, and shafts.
BACKGROUND ART
[0002]
In many cases, parts such as gears, pulleys, and shafts for motor vehicles
and industrial machinery are manufactured by roughly shaping them by hot
forging or cold forging, by subjecting them to machining and thereafter to
casehardening by carburizing quenching or carbonitriding quenching.
However, at the said casehardening treatment, if austenite grains before
quenching are coarsened, there easily arise problems that the fatigue strength
as a part decreases and that the amount of distortion at the quenching time
increases.
[0003]
Generally, it has been thought that, as compared with cold forged parts,
in hot forged parts, the austenite grains are less liable to be coarsened at the
time of carburizing or carbonitriding.
[0004]
In recent years, however, with the progress of hot forging technique, hot
forging has frequently been carried out in various temperature ranges^ and
7-
thus the number of hot forged parts with the austenite grains coarsened at the
time of carburizing or carbonitriding has increased.
[0005]
Therefore, there has been demanded a hot rolled steel bar or wire rod in
which the austenite grain coarsening can be stably prevented at the heating
time in the process of carburizing or carbonitriding even if hot forging is carried
out in various temperature ranges, and techniques concerning steels and
producing methods therefor have been proposed in the Patent Literatures 1 to
3.
[0006]
To be concrete, the Patent Literature 1 discloses "a grain stabilized
carburizing steel" characterized in that the steel with limited amounts of sol.Al
and N and a limited ratio of "sol.Al/N" is heated to a temperature of 1200°C or
more and thereafter is hot worked.
[0007]
The Patent Literature 2 discloses "a steel for high temperature
carburizing having an excellent property in high temperature carburizing, and
a hot forged member for high temperature carburizing" characterized in that
the elements such as Al, Nb and N are contained in a specific amount
respectively, and the precipitation amounts of Nb(C, N) and A1N after hot
rolling are restricted, and moreover the microstructure after hot rolling is also
restricted.
[0008]
The Patent Literature 3 discloses "a steel for gears" in which Si: 0.1% or
less and P: 0.01% or less and so on are regulated and which provides highly
reliable gears having high strength and high toughness.
LIST OF PRIOR ART DOCUMENT
PATENT LITERATURES
3
[0009]
Patent Literature V JP 56-75551 A
Patent Literature 2 : JP 2001-279383 A
Patent Literature l: JP 60-21359 A
DISCLOSURE OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[0010]
In the techniques disclosed in the aforementioned Patent Literatures 1 to
3, it could not necessarily be said that in the case where hot forging is carried
out in various temperature ranges, the austenite grain coarsening can be stably
prevented at the time of heating in the process of carburizing or carbonitriding.
[0011]
In the technique proposed in the Patent Literature 1, the steel is heated
to a temperature of 1200°C or more, and thereafter is hot worked. However,
in the hot forging in mass production, many parts are heated to a temperature
less than 1200°C. Therefore, the said Patent Literature 1 does not propose a
technique in which the austenite grain coarsening can be stably prevented at
the time of carburizing or carbonitriding even in the case where hot forging is
carried out in various temperature ranges.
[0012]
In the technique proposed in the Patent Literature 2, precipitates except
Nb(C, N) and A1N are not considered. Therefore, it cannot necessarily be said
that in the case where hot forging is carried out in various temperature ranges,
the austenite grain coarsening can be stably prevented at the time of heating
process for carburizing or carbonitriding.
[0013]
In the technique proposed in the Patent Literature 3, the austenite grain
coarsening at the time of heating process for carburizing or carbonitriding is
•J*
not considered. Therefore, it cannot necessarily be said that high bending
strength can be stably ensured.
[0014]
The present invention has been made in view of the aforementioned
situation, and accordingly the objective thereof is to provide a steel for
carburizing or carbonitriding use in which the said austenite grain coarsening
can be stably prevented, when the steel is heated in the process of carburizing
or carbonitriding, especially when the steel is heated at a temperature of 980°C
or less for three hours or less even if being hot forged in various temperature
ranges, especially being hot forged after being heated to 1050 to 1300°C, and in
addition an excellent bending fatigue strength after carburizing or
carbonitriding can be ensured. The steel according to the present invention is
suitable as a steel for a starting material of parts that are roughly formed by
hot forging.
[0015]
With regard to the present invention, in the case where two or more
austenite grains having a grain size number of 4 or less exist in the field of 10
mm2, it is judged that the austenite grains are coarsened.
MEANS FOR SOLVING THE PROBLEMS
[0016]
So far, it has been known that as disclosed in the Patent Literature 1 and
the Patent Literature 2, by limiting the amounts of sol.Al and N and the ratio of
"sol.Al/N", and the precipitation amounts of Nb(C, N) and A1N, the austenite
grain coarsening can be prevented at the time of heating process for carburizing
or carbonitriding.
[0017]
However, with regard to the aforementioned techniques, it cannot
necessarily be said that in the case where hot forging is carried out in various
-*?
5
temperature ranges, the austenite grain coarsening can be stably prevented
when the steel is heated for carburizing or carbonitriding at a temperature of
980°C or less.
[0018]
The present inventors made investigations and studies repeatedly into
the influence of the chemical composition, especially the contents of Al, Ti and
0, which are the elements being liable to form relatively coarse precipitates, on
a steel in which the said austenite grain coarsening can be stably prevented
even if the steel is heated at a temperature of 980°C or less in the process of
carburizing or carbonitriding in the case where hot forging is carried out in
various temperature ranges.
[0019]
As a result, the present inventors obtained the following findings (a) to
(e).
[0020]
In the description below, the said "carburizing or carbonitriding" is
sometimes referred simply to as "carburizing". And in addition, unless
otherwise noted, the "heating for carburizing" means "heating at a temperature
of 980°C or less for carburizing."
[0021]
(a) With regard to steels for carburizing use, if once austenite grains
become coarse, from the stage of a bloom to the stage before carburizing, the
austenite grain coarsening is liable to occur at the time of carburizing.
Therefore, even in the case where hot forging is carried out in various
temperature ranges, in order to stably prevent the austenite grain coarsening
at the time of carburizing, it is necessary to suppress the occurrence of
austenite grain coarsening in all the stages from the stage of a bloom to the
stage before carburizing.
[0022]
(b) Among the production processes from the stage of a bloom to the stage
of a carburized part, AI2O3 and TiN have an effect of preventing the austenite
grain coarsening in the case where the heating temperature is 1200°C or more.
However, AI2O3 and TiN are liable to become coarse precipitates: and in the
case where the amounts of the coarse precipitates become large, bending
strength decreases.
[0023]
(c) In order to control the amounts and sizes of the said AI2O3 and TiN
precipitates, it is useful to estimate the contents of Al, O, Ti and N from the
formulas based on the respective solubility products.
[0024]
(d) In order to stably prevent the austenite grain coarsening at the time
of carburizing even in the case where hot forging is carried out in various
temperature ranges, it is necessary to increase the content of Al in addition to
the above item (c).
[0025]
(e) In order to increase the bending strength, it is effective to decrease the
content of Si in addition to the prevention of the austenite grain coarsening at
the time of carburizing.
[0026]
The present invention has been accomplished on the basis of the
above-described findings. The main points of the present invention are the
steels for carburizing or carbonitriding use shown in the following (l) and (2).
[0027]
(l) A steel for carburizing or carbonitriding use characterized in that the
steel consists of, by mass %J
C: 0.1 to 0.3%,
1
Si: 0.01 to 0.15%,
Mn: 0.6 to 1.5%,
S'- 0.012 to 0.05%,
Cr: 0.5 to 2.0%,
Al: 0.030 to 0.050%,
Ti: 0.0006 to 0.0025%,
N= 0.010 to 0.025%, and
O: 0.0006 to 0.0012%, and
the balance of Fe and impurities,, wherein P and Nb among the impurities are
P: 0.025% or less and Nb: 0.003% or less respectively,
and further the following formulas (l) and (2) are satisfied:
-5.0 < log (Ti x N) < -4.4 ... (l),
-12.5 < log (A12x 03) < -11.7 ...(2),
wherein each element symbol in the above formulas (l) and (2) represents the
content by mass percent of the element concerned.
[0028]
(2) The steel for carburizing or carbonitriding use according to the above
(l), which contains, by mass percent, at least one element selected from the
elements shown below in lieu of a part of Fe:
Mo: 0.5% or less,
Ni: 1.5% or less, and
Cu: 0.4% or less.
[0029]
The term "impurities" so referred to in the phrase "the balance of Fe and
impurities" indicates those elements which come from the raw materials such
as ore and scrap, and/or the production environment when the steel is produced
on an industrial scale.
ADVANTAGEOUS EFFECTS OF THE INVENTION
8
[0030]
The steel for carburizing or carbonitriding use in accordance with the
present invention can stably prevent the austenite grain coarsening, when the
said steel is heated in the process of carburizing or carbonitriding, especially
when the steel is heated at a temperature of 980°C or less for three hours or
less even if being hot forged in various temperature ranges, especially being hot
forged after being heated to 1050 to 1300°C, and in addition can ensure an
excellent bending fatigue strength after carburizing or carbonitriding.
Therefore, the steel according to the present invention can be suitably used as a
steel for a starting material of parts, such as gears, pulleys, and shafts, that are
roughly formed by hot forging.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031]
[Fig. l] Figure 1 is a view showing a shape of a notched Ono type rotating
bending fatigue test piece used in the EXAMPLES. In this figure, the units of
the dimensions are "mm".
[Fig. 2] Figure 2 is a diagram showing the heat pattern of "carburizing
quenching" carried out on the test piece shown in Figure 1 in the EXAMPLES.
The "CP" in this Figure 2 represents carbon potential.
MODE FOR CARRYING OUT THE INVENTION
[0032]
In the following, all of the requirements of the present invention are
described in detail. In the following description, the symbol "%" for the
content of each element means "% by mass".
[0033]
C: 0.1 to 0.3%
C (carbon) is an essential element for ensuring the core strength of a part
subjected to carburizing quenching or carbonitriding quenching, and thus
q
obtaining the target bending fatigue strength. In the case where the content
of C is less than 0.1%, it is insufficient to achieve the said effect. On the other
hand, if the content of C exceeds 0.3%, the amount of distortion of the part
subjected to carburizing quenching or carbonitriding quenching increases
remarkably. Therefore, the content of C is set to 0.1 to 0.3%. The content of
C is preferably 0.18% or more, and preferably 0.23% or less.
[0034]
Si: 0.01 to 0.15%
Si (silicon) is an element having an effect of enhancing the hardenability.
However, if the content of Si is less than 0.01%, the above effect is insufficient.
On the other hand, Si increases the intergranularly oxidized layers at the time
of carburizing treatment or carbonitriding treatment. In particular, if the
content of Si exceeds 0.15%, the said intergranularly oxidized layers increase
remarkably and the bending fatigue strength deteriorates! and thus the
bending fatigue strength cannot meet the target of the present invention
mentioned later. Therefore, the content of Si is set to 0.01 to 0.15%. The
content of Si is preferably 0.05% or more, and preferably 0.10% or less.
[0035]
Mn: 0.6 to 1.5%
Mn (manganese) has a great effect of enhancing the hardenability and is
an essential element for ensuring the core strength at the time of carburizing
quenching or carbonitriding quenching, and thus for obtaining the target
bending fatigue strength. In the case where the content of Mn is less than
0.6%, the above effect is insufficient. If the content of Mn exceeds 1.5%, the
said effect is saturated! and thus the cost increases. Therefore, the content of
Mn is set to 0.6 to 1.5%. The content of Mn is preferably 1.1% or less, and
more preferably 0.9% or less.
[0036]
10
S: 0.012 to 0.05%
S (sulfur) is an element which combines with Mn to form MnS, and
improves the machinability. The said MnS has an effect of preventing the
austenite grain coarsening in the case where the heating is carried out at high
temperatures. However, if the content of S is less than 0.012%, the above
effects are insufficient. On the other hand, if the content of S increases, coarse
MnS is liable to be formed, and it tends to degrade the bending fatigue strength.
In particular, if the content of S exceeds 0.05%, the bending fatigue strength
degrades remarkably. Therefore, the content of S is set to 0.012 to 0.05%.
The content of S is preferably 0.02% or less.
[0037]
Cr: 0.5 to 2.0%
Cr (chromium) is an effective element for improving the bending fatigue
strength because of having an effect of enhancing the hardenability. In the
case where the content of Cr is less than 0.5%, the target bending fatigue
strength cannot be obtained. If the content of Cr exceeds 2.0%, the said effect
is saturated; and thus the cost increases. Therefore, the content of Cr is set to
0.5 to 2.0%. The content of Cr is preferably 0.9% or more, and preferably 1.3%
or less.
[0038]
Al: 0.030 to 0.050%
Al (aluminum) has a deoxidizing action. In addition, Al is liable to form
A1N by combining with N; and thus Al is an effective element for preventing the
austenite grain coarsening at the time of heating for carburizing. If the
content of Al is less than 0.030%, it is impossible to prevent the austenite grain
coarsening stably. In the case where austenite grains are coarsened, the
bending fatigue strength deteriorates. On the other hand, if the content of Al
exceeds 0.050%, coarse oxides are liable to be formed, and it tends to degrade
-)%?
II
the bending fatigue strength. Therefore, the content of Al is set to 0.030 to
0.050%. The content of Al is preferably 0.045% or less, and more preferably
0.040% or less.
[0039]
Ti: 0.0006 to 0.0025%
Ti (titanium) is liable to form hard and coarse TiN by combining with N.
However, Ti is an effective element for preventing the austenite grain
coarsening in the case where the heating is carried out at high temperatures.
If the content of Ti is less than 0.0006%, it is impossible to prevent the
austenite grain coarsening stably. In the case where austenite grains are
coarsened, the bending fatigue strength deteriorates. On the other hand, if
the content of Ti exceeds 0.0025%, the bending fatigue strength deteriorates
remarkably. Therefore, the content of Ti is set to 0.0006 to 0.0025%. The
content of Ti is preferably 0.0008% or more, and more preferably 0.0010% or
more. In addition, the content of Ti is preferably 0.0020% or less.
[0040]
N: 0.010 to 0.025%
N (nitrogen) is liable to form TiN and A1N by combining with Ti and Al,"
and thus N is an effective element for preventing the austenite grain
coarsening at the time of heating for carburizing. If the content of N is less
than 0.010%, it is impossible to prevent the austenite grain coarsening stably.
On the other hand, if the content of N exceeds 0.025%, in the steel making
process, stable mass production becomes difficult to achieve. Therefore, the
content of N is set to 0.010 to 0.025%. The content of N is preferably 0.014% or
more, and preferably 0.020% or less.
[0041]
O: 0.0006 to 0.0012%
0 (oxygen) is liable to form hard and coarse AI2O3 by combining with Al.
However, 0 is an effective element for preventing the austenite grain
coarsening in the case where the heating is carried out at high temperatures.
If the content of 0 is less than 0.0006%, it is impossible to prevent the austenite
grain coarsening stably. In the case where austenite grains are coarsened, the
bending fatigue strength deteriorates. On the other hand, if the content of 0
exceeds 0.0012%, the bending fatigue strength deteriorates remarkably.
Therefore, the content of O is set to 0.0006 to 0.0012%. The content of O is
preferably 0.0009% or less.
[0042]
log(TixN):-5.0to-4.4
The said TiN is effective in preventing the austenite grain coarsening in
the case where the heating is carried out at high temperatures. If the log (Ti x
N) is less than -5.0, even if the contents of Ti and N are in the aforementioned
ranges, it is impossible to prevent the austenite grain coarsening stably. In
the case where austenite grains are coarsened, the bending fatigue strength
deteriorates. On the other hand, if the log (Ti x N) exceeds -4.4, the bending
fatigue strength deteriorates remarkably.
[0043]
Therefore, it is necessary to satisfy the formula (l), that is to say, the
formula of [-5.0 < log (Ti x N) < -4.4].
[0044]
The log (Ti x N) is preferably -4.9 or more, and preferably -4.6 or less.
[0045]
log (Al2 x 03): -12.5 to -11.7
The said AI2O3 is effective in preventing the austenite grain coarsening in
the case where the heating is carried out at high temperatures. If the log (Al2
x O3) is less than -12.5, even if the contents of Al and O are in the
aforementioned ranges, it is impossible to prevent the austenite grain
coarsening stably. In the case where austenite grains are coarsened, the
bending fatigue strength deteriorates. On the other hand, if the log (Al2 x O3)
exceeds -11.7, the bending fatigue strength deteriorates remarkably.
[0046]
Therefore, it is necessary to satisfy the formula (2), that is to say, the
formula of [-12.5 < log (Al2 x O3) <-11.7].
[0047]
The log ((Al2 x O3) is preferably -12.4 or more, and preferably -12.0 or
less.
[0048]
One of the steels for carburizing or carbonitriding use of the present
invention consists of the elements mentioned above, and the balance of Fe and
impurities, wherein P and Nb among the impurities are P: 0.025% or less and
Nb"- 0.003% or less respectively.
[0049]
Hereunder, P and Nb among the impurities are explained.
[0050]
P: 0.025% or less
P (phosphorus) is an element that segregates at grain boundaries and is
liable to make the grain boundaries brittle. If the content of P exceeds 0.025%,
the bending fatigue strength deteriorates. Therefore, the content of P among
the impurities is set to 0.025% or less. The content of P among the impurities
is preferably set to 0.020% or less.
[0051]
Nb: 0.003% or less
Nb (niobium) is an element being liable to form Nb(C, N) by combining
with C and N. The said Nb(C, N) is sometimes effective in preventing the
austenite grain coarsening at the time of carburizing. However, at various
heating temperatures for hot forging, it rather sometimes promotes austenite
grain coarsening at the time of carburizing, and in the case where the content
of Nb exceeds 0.003%, the said coarsening is liable to occur. Therefore, the
content of Nb among the impurities is set to 0.003% or less. The content of Nb
among the impurities is preferably set to 0.001% or less.
[0052]
Another of the steels for carburizing or carbonitriding use of the present
invention contains at least one element selected from Mo, Ni and Cu in lieu of a
part of Fe.
[0053]
Hereunder, the effects of containing Mo, Ni and Cu, which are optional
elements, and the reasons for the restriction of content thereof are explained.
[0054]
Mo: 0.5% or less
Mo (molybdenum) has a great effect of enhancing the hardenability and
is an effective element for increasing the bending fatigue strength, so that Mo
can be contained according to need. However, if the content of Mo exceeds
0.5%, the said effect is saturated; and thus the cost increases. Therefore, if Mo
is contained, the content of Mo is set to 0.5% or less. When Mo is contained,
the content of Mo is preferably 0.4% or less.
[0055]
In the case where Mo is contained, in order to stably achieve the above
effect of increasing the bending fatigue strength due to the improvement in
hardenability of Mo, the content of Mo is preferably 0.02% or more, and more
preferably 0.05% or more.
[0056]
Ni: 1.5% or less
Ni (nickel) has an effect of enhancing the hardenability and is an
effective element for increasing the bending fatigue strength, so that Ni can be
contained according to need. However, if the content of Ni exceeds 1.5%, the
said effect is saturated; and thus the cost increases. Therefore, if Ni is
contained, the content of Ni is set to 1.5% or less. When Ni is contained, the
content of Ni is preferably 0.8% or less.
[0057]
In the case where Ni is contained, in order to stably achieve the above
effect of increasing the bending fatigue strength due to the improvement in
hardenability of Ni, the content of Ni is preferably 0.1% or more, and more
preferably 0.2% or more.
[0058]
Cu: 0.4% or less
Cu (copper) has an effect of enhancing the hardenability and is an
effective element for increasing the bending fatigue strength, so that Cu can be
contained according to need. However, if the content of Cu exceeds 0.4%, hot
ductility decreases, and thus hot workability deteriorates remarkably.
Therefore, if Cu is contained, the content of Cu is set to 0.4% or less. When Cu
is contained, the content of Cu is preferably 0.3% or less.
[0059]
In the case where Cu is contained, in order to stably achieve the above
effect of increasing the bending fatigue strength due to the improvement in
hardenability of Cu, the content of Cu is preferably 0.1% or more, and more
preferably 0.2% or more.
[0060]
With regard to the aforementioned Mo, Ni and Cu, only one or a
combination of two or more elements can be contained. The total amount of
-jrthese
elements can be 2.4% or less! however the said total amount is preferably
1.0% or less.
[0061]
In case of mass production on an industrial scale, in order to reduce the
content of O, and in addition to regulate it within the desired ranges, for
example, in the stage of a secondary refining of steelmaking process, it is
recommended to use a LF (Ladle Furnace) equipment and an RH
(Ruhrstahl-Heraeus) equipment and in addition to condition the respective
treating time in the said each equipment.
[0062]
In the following, the present invention is explained in detail by referring
to examples.
EXAMPLES
[0063]
The Steels a to z and Steels A to I having the chemical composition shown
in Table 1 and Table 2 were melted by using a vacuum melting furnace and cast
to 150 kg ingots respectively.
[0064]
The Steel b, Steel c, Steel f, Steel i, Steel j , Steel m, Steels o to s, Steel v,
Steel y, Steels A to F, Steel H and Steel I in Table 1 and Table 2 are steels
having the chemical composition being within the range regulated by the
present invention.
[0065]
The Steel a, Steel d, Steel e, Steel g, Steel h, Steel k, Steel 1, Steel n, Steel
t, Steel u, Steel w, Steel x, Steel z and Steel G are steels of comparative
examples having the chemical composition being out of the condition regulated
by the present invention.
[0066]
\1
The Steel a is a steel corresponding to the SCr420H specified in JIS G
4052 (2008).
[0067]
[Table 1]
/
/
/
/
/
/
/
/
/
f — > H o o o t - c - r H o i > t o m e - o 3 ( N c o o o
i H O r H r H O O O ' H ' - t O O O O i H r H O O
n o o o o o o o o o o o o o o o o o
^ O O O O O O O O O O O O O O O O O fl
d d d d d d d d d d d d d d d d d .2
m * * * -P
a : ri
-J3 a
H H t - o o i o u i m t - o o t - t o i o t o o o i o ^ m o 5
? 17 r H r - l r H r - I ^ H r H r - 4 t H i H r H r H i - ( i H r H r - ( > H ( M .3
ft ^ o o o o o o q o o o o o o o o o o ^
S d d d d d d d d d d d d d d d d d a
•«
H J?
H r-t*-*r H t — l p ~ * r - t r - t r H i - H r - 4 r H r - ( , ' H ! H r ^ , ~ ~ * *H _ro • ^ J QO OO Oo Oo OOOoOOOoOOOOOOOoOOOOOOOOO OO T0)j
ch^H d d d d d d d d o d d d d d d o d ^
^ ' ^ ' - ' V V V V V V V V ^
tn . a)
( D O ) U
rs rt H i a u s n o o o o n f c o i U H n o s o i o u s u i i o .
p™ "<< 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 . q
>> d d d d d d d d d d d d d d d o ' dS
o
• ^ V S I I I I I I I I I I I I I I I I I S
a S 8
o
' g g I I I I I I I I I I I 1 I I 1 1 1 .H
P. H
I £
" ( g l l l l l l l l l l l l l l l l l , ^ I :
3 (ft O iH r-J iH r-l rH r-l iH t - j H r t H H H H i H H .2
r& p l r i H H H H H ' H H H T 4 H H H H i - i r i 3 o i
i - l r - t i - H r - t i - t r - t i - t r H t H i - l i - l i - l i - l f - t e ^ t H r H
w o o o o o o q o o o o o o o o o o »>
o ' d o ' d d d d d o o o ' d c i d o o o 'P
^
^ N H d l N O l H C O T f t D H t O U J r i l f i l l l W -S
r H i H r H i - l i H r H i - 4 r H r - l i H t - 4 i H i - l i - l r - l r - l i H a
P n o o o o o o o o o o o o o o o o o 3
d d d d d d d d d d d d d d d d d °
f1-5c BM ci-ootooowo oiono^o^oto-otqBoioooiqooto-ocqoo omoHq oi Nq qcqoqh ao * £
S d d d d d d d d d d d d d d r - i r - i r - J 'M
w
0>
i O l f l - * C 0 0 1 » O 0 1 0 ) 0 0 O 0 ) O 0 1 > ( t l ( 0 O
j ^ i N O r - j T - i o o r H o q o r H q i H q q q q a
d d d d d d d d d d d d d d d d d £
-n
© i H r H i - l © i - t i - l i H e * T - l i - l i - I C « 3 i - l i - i e q » - t ij
d d d d d d d d d d d d d d d d d 2
_j a
a)
t—i jg
iLJ . L
[0068]
[Table 2]
w
O H O l H N O l O O H m f i n N O O T I S O f f l M
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[0069]
The above each ingot was heated at 1250°C for 4 hours, and thereafter
was forged to prepare a steel bar having a diameter of 50 mm at a finishing
temperature of 950°C or more.
[0070]
Four test pieces having a length of 90 mm were cut out from the said each
steel bar having a diameter of 50 mm. Subsequently, in order to simulate hot
forging, the four test pieces were heated at respective temperatures of 1300°C,
1200°C, 1100°C, and 1050°C for one hour. Thereafter, after 15 seconds from
when the each test piece was taken out from the furnace, the said test piece was
compressed by 70% in the height direction of the cylindrical shape, and
subsequently was stood to cool in the atmosphere, and thus cooled to room
temperature.
[0071]
The thus obtained test pieces were further heated at 930°C for one hour,
and then were stood to cool in the atmosphere, and thus cooled to room
temperature.
[0072]
Next, in order to simulate heating for carburizing, the thus obtained each
test piece was cut into four equal pieces in the longitudinal cross sectional
direction, and the divided test pieces were respectively held at temperatures of
950°C, 980°C, 1010°C, and 1040°C for three hours, and thereafter were cooled
to room temperature by water cooling.
[0073]
The cut plane of the thus obtained each test piece was removed by a
thickness of 1 mm, and the said cut plane was mirror-like polished and was
etched with a picric acid saturated aqueous solution to which a surface-active
agent was added. Subsequently, with regard to the above each etched plane,
oA
randomly selected ten visual fields were observed by using an optical
microscope at a magnification of 100 times to examine the state of the
occurrence of austenite grain coarsening.
[0074]
The size of each visual field in the above examination was set to 1.0 mm x
1.0 mm. In the case where it was found by this examination that two or more
austenite grains having a grain size number of 4 or less, specified in JIS G 0551
(2005), did exist in the field of 10 mm2, it was judged that the austenite grains
were coarsened.
[0075]
The target of the effect of preventing the austenite grain coarsening was
made such that austenite grains are not coarsened when the test piece is
heated at a temperature of 980°C or lower for three hours in the above
simulation of heating for carburizing.
[0076]
Tables 3 and 4 show the above investigation results of the state of the
occurrence of austenite grain coarsening together with the temperature at
which the test piece was heated to simulate hot forging.
[0077]
[Table 3]
Table 3
Heating Austenite grain. Heating Austenite grain
Steel temperature coarsening temperature Steel temperature coarsening temperature
at forging (°C) at forging (°C)
£0) £C)
1300 1010 1300 1010
* a 1200 1040 j 1200 1040
1100 1010 1100 1010
1060 # 980 1050 1010
1300 1040 1300 # 950
b 1200 >1040 * k 1200 1010
1100 1040 1100 1010
1050 1010 1050 10W
1300 1040 1300 # 950
c 1200 1040 * 1 1200 1010
U00 1U1U 1100 1010
1050 1010 1050 1010
1300 1040 1300 >1040
* d 1200 1040 m 1200 1040
1100 1010 1100 1010
1060 1010 1050 1010
1300 1010 1300 1010
* e 1200 1010 * n 1200 1040
1100 # 950 1100 1010
1050 # 950 1050 1010
1300 >1040 1300 1010
f 1200 1040 o 1200 1040
1100 1010 1100 1010
1050 1010 1050 1010
1300 1040 1300 1040
* g 1200 1010 p 1200 1040
1100 # 980 1100 1010
1050 # 950 1050 1010
1300 >1040 1300 1040
* h 1200 1040 q 1200 1040
1100 1010 1100 1010
1050 1010 1060 1010
1300 >1040 1300 1040
i 1200 1040 r 1200 1040
1100 1010 1100 1010
I 1050 | 1010 I | 1050 I 1010
The mark * denotes that the steel is falling outside the conditions regulated by the present
invention.
The mark # denotes falling short of the target in the present invention.
[0078]
[Table 4]
Table 4
Heating Austenite grain Heating Austenite grain
Steel temperature coarsening temperature Steel temperature coarsening temperature
at forging (°C) at forging (°C)
(°C) K!)
1300 1010 1300 1040
s 1200 1040 A 1200 >1040
1100 1010 1100 1040
1050 1010 1050 1010
1300 # 980 1300 1040
* t 1200 1010 B 1200 >1040
1100 1010 1100 1040
1050 1010 1050 1010
1300 # 950 1300 1040
* u 1200 # 980 C 1200 >1040
1100 1010 1100 1040
1050 1010 1050 1010
1300 1010 1300 1040
v 1200 1040 D 1200 >1040
1100 1010 1100 1040
1050 1011) 1050 1010
1300 1010 1300 1040
* w 1200 1040 E 1200 >1040
1100 1010 1100 1040
1050 1010 1050 1010
1300 1010 1300 1040
* x 1200 1040 F 1200 >1040
1100 1010 1100 1010
1050 1010 1051) 1010
1300 >1040 1300 1040
y 1200 1040 * G 1200 >1040
1100 1010 1100 # 980
1050 1010 1060 # 950
1300 # 950 1300 1040
* z 1200 1010 H 1200 >1040
1100 1010 1100 1010
| 1050 | # 980 1050 1010
~"-—•- 1300 1040
" ^_____^ I 1200 >1040
- — - ^ ^ ^ 1100 1010
*~--^"-^J I 1050 | 1010
The mark * denotes that the steel is falling outside the conditions regulated by the present
invention.
The mark # denotes falling short of the target in the present invention.
[0079]
In addition, with regard to each steel, on the basis of the investigation
results of the occurrence of austenite grain coarsening, the steel bar thereof
having a diameter of 50 mm was heated at one of temperatures shown in the
following <1> to <3> (to be concrete, the temperature described in the column of
"Heating temp." in Table 5, mentioned later) for 0.75 hours, and was then hot
^ 11)
forged at a finishing temperature of 950°C or more into steel bar having a
diameter of 30 mm, and thereafter the obtained steel bar was stood to cool in
the atmosphere, and thus cooled to room temperature.
[0080]
<1> In Tables 3 and 4, in the case of the steels that could achieve the
aforementioned target of the effect of preventing the austenite grain coarsening
(that is to say, the steels having no description of 980°C and 950°C that were a
temperature of 980°C or less in the column of "Austenite grain coarsening
temperature"): 1200°C.
[0081]
<2> In Tables 3 and 4, in the case of the steels having one of the
descriptions of 980°C and 950°C in the column of "Austenite grain coarsening
temperature": the "Heating temperature at forging" in the case of the
"Austenite grain coarsening temperature" of either 980°C or 950°C.
Incidentally, if there are two "Heating temperatures at forging" corresponding
to the above condition, the lower temperature of them.
[0082]
<3> In Tables 3 and 4, in the case of the steels having both of the
descriptions 980°C and 950°C in the column of "Austenite grain coarsening
temperature": the "Heating temperature at forging" in the case of the
"Austenite grain coarsening temperature" of 950°C.
[0083]
The each steel bar having a diameter of 30 mm obtained in the above
described manner was further heated at a temperature of 930°C for one hour,
and thereafter was stood to cool in the atmosphere, and thus cooled to room
temperature.
[0084]
Notched Ono type rotating bending fatigue test pieces having the shape shown
in Figure 1 were produced from the central portion of the said each steel bar
having a diameter of 30 mm by machining. In Figure 1, the units of the
dimensions are "mm".
[0085]
By using a gas carburizing furnace, the above test pieces were subjected
to carburizing quenching under the condition shown in Figure 2. Thereafter,
they were tempered at 170°C for 1.5 hours. The "CP" in Figure 2 represents
carbon potential.
[0086]
For the purpose of removing heat treating distortion from each test piece,
both grip portions of each test piece were finished to have a diameter of 15 mm.
Thereafter, the Ono type rotating bending fatigue test was carried out at room
temperature by using the said finished test pieces.
[0087]
The Ono type rotating bending fatigue test at room temperature was
carried out in accordance with a common test method except for the following
conditions^ number of used test piece: eight, and number of revolutions: 3000
rpm. The maximum stresses where the test pieces did not rupture in the
number of cycles of 10 x 104, and of 10 x 107 were defined as the "Medium cycle
rotating bending fatigue strength", and as the "High cycle rotating bending
fatigue strength", respectively.
[0088]
The target value of the said rotating bending fatigue strength was
defined to be 10% or more than the references of "100", that is to say, to be 110
or more, where the said references were defined by using the "Medium cycle
rotating bending fatigue strength" and the "High cycle rotating bending fatigue
strength" after carburizing quenching-tempering treatment of the said Steel a *
corresponding to the SCr420H being common as a versatile steel.
[0089]
The investigation results of the aforementioned Ono type rotating
bending fatigue test at room temperature are shown in Table 5 together with
the 0.75-hours-heating temperatures used in heating the steel bars having a
diameter of 50 mm on the basis of the aforementioned categories <1> to <3>.
[0090]
[Table 5]
Table 5
Heating Rotating bending Heating Rotating bending
Steel temp. fatigue strength. Steel temp. fatigue strength
(°C) Medium cycle High cycle (°C) Medium cycle High cycle
* a 1050 $ 100 $ 100 s 1200 115 115
b 1200 125 120 * t 1300 # 100 # 100
c 1200 115 110 *u 1300 # 9 5 # 100
* d 1200 # 105 # 105 v 1200 110 115
* e 1050 # 9 5 #105 * w 1200 # 105 # 100
f 1200 120 115 * x 1200 # 105 # 105
* g 1050 # 100 # 105 y 1200 120 120
* h 1200 110 #100 * z 1300 # 100 110
i 1200 115 110 A 1200 125 125
j 1200 120 115 B 1200 130 130
* k 1300 # 105 # 100 C 1200 135 130
* 1 1300 # 105 # 100 D 1200 125 125
m 1200 120 115 E 1200 140 130
* n 1200 # 105 # 105 F 1200 115 115
o 1200 115 115 * G 1050 # 105 # 105
p 1200 120 125 H 1200 130 125
q 1200 120 120 I | 1200 | 130 | 130
r 1200 120 120 "^
The column of "Heating temp." denotes that the steel bar having a diameter of 50 mm was
heated at the shown temperature for 0.75 hours.
The column of "Medium cycle" denotes the fatigue strength at 1.0x10* cycles.
The column of "High cycle" denotes the fatigue strength at l.OxlO7 cycles.
The mark * denotes that the steel is falling outside the conditions regulated by the present
invention.
The mark $ denotes the reference of evaluation.
The mark # denotes falling short of the target in the present invention.
[0091]
As apparent from Tables 3 to 5, the steels satisfying the conditions
regulated by the present invention can achieve the target of the effect of
preventing the austenite grain coarsening, and the target bending fatigue
strength (the medium cycle rotating bending fatigue strength and the high
cycle rotating bending fatigue strength).
[0092]
In contrast, the steels of the "comparative examples" falling outside the
conditions regulated by the present invention cannot achieve one or both of the
said targets, that is to say, target of the effect of preventing the austenite grain
coarsening, and the target bending fatigue strength (the medium cycle rotating
bending fatigue strength and the high cycle rotating bending fatigue strength).
INDUSTRIAL APPLICABILITY
[0093]
The steel for carburizing or carbonitriding use in accordance with the
present invention can stably prevent the austenite grain coarsening, when the
said steel is heated in the process of carburizing or carbonitriding, especially
when the steel is heated at a temperature of 980°C or less for three hours or
less even if being hot forged in various temperature ranges, especially being hot
forged after being heated to 1050 to 1300°C, and in addition can ensure an
excellent bending fatigue strength after carburizing or carbonitriding;
accordingly, the steel according to the present invention can be suitably used as
a steel for a starting material of parts, such as gears, pulleys, and shafts, that
are roughly formed by hot forging.
•2$

%0 We claim:
1. A steel for carburizing or carbonitriding use characterized in that the
steel consists of, by mass %C: 0.1 to 0.3%,
Si: 0.01 to 0.15%,
Mn: 0.6 to 1.5%,
S: 0.012 to 0.05%,
Cr: 0.5 to 2.0%,
Al: 0.030 to 0.050%,
Ti: 0.0006 to 0.0025%,
N: 0.010 to 0.025%, and
0: 0.0006 to 0.0012%, and
the balance of Fe and impurities, wherein P and Nb among the impurities are
P: 0.025% or less and Nb: 0.003% or less respectively,
and further the following formulas (1) and (2) are satisfied:
-5.0