[Title of Invention] CASE KARDENIKG STEEL &LATERIAL
[Technical Field]
[OOO 17
The present invention relates to a case hardening steel material. More
particularly, the present invention relates to a case hardening steel material
that is low in component cost. moreover is excellent in bending fatigue
strength and wear resistance. and is used suitably as a raw material for a
carburized part such as a belt type continuously variable transmission pulley
shaft (hereinafter. referred to as a "CVT pulley shaft") for a motor vehicle.
[Background Art]
foooal
,Automotive parts, especially parts used for a transmission such as CtTT
pulley shafts, are generally manufactured by surface hardening treatment
such as carburizing and quenching followed by tempering, from the viewpoint
of improving the bending fatigue strength and wear resistance.
Eooo33
In general. the "carburizing and quenching" is a treatment in which
using a low-carbon "case hardening steel" as a raw material steel (base metal
steel). and C has been intrucled and diffused in the austenitic region at a high
temperature of *4c3 point or higher. then the steel is quenched.
tooo41
In recent years, motor vehicles are required to have a lighter weight and
lugher torque. To meet this requirement, the carburized parts such as the
C'c-T pulley sh:lfta must have a higher bending fatigue strength and higher
wear resistance than before. In this description, hereinafter an explanation
may be given by referring to the "CVT pulley shaft" as a representative of the
"carburized part".
Eooo53
)Then large amounts of alloying elements such as Ni, Cr and ~ J aore
added to a case hardening steel, the CVT pulley shaft can exhibit a high
bending fatigue strength and high wear resistance: however, the component
cost is increased by the increased amount of alloying elements.
l000~1
However. both of Ni and 310 are important elements that increase the
depth of a carburized layer and the hardness of a core part (base metal), and
also are elements for improving the temper softening resistance. 31oreover.
both of Ni and hIo also have an effect of improving the hardenability of
carburized laper without increasing the depth of an intergranular oxidation
layer formed on the surface during gal: carburization because these elements
are nonoxidizing elements.
[00071
Therefore, as a "case hardening steel", which serves as a raw material
for CVT pulley shaft, a "chromium-molybdenttm steel" such as SCAI42OH
defined in JIS G 4052 (2008) is often used. However. in view of the situation
of a recent steep rise in 310 cost, there is a greatly increasing demand for a
ease hardening steel material in which the addition amount of &lo is kept 3s
small as poss~blew, hereby the component cost is decreased. and moreover, the
C'L'T pulley shaft car1 be pro\-ided with a high bending fatigue strength ancl
high wear resistance.
~if(4081
_Iccordin~gl_vto. meet the above-described ctemancl, for example. Pate:~t
Documents 1 and 2 propose a "high chromium steel for carbusizing and carbonitriding
treatment" and "method for manufacturing case-hardened product
having high fatigue strengr-h", respectively.
foo091
Specifically, Patent Document 1 discloses a "high chromium steel for
carburizing and carbo-nitriding treatment" obtained by heating a steel
consisting of, by mass percent, C: 0.10 to 0.30%. Si: 0.15% or less, &In: 0.90 to
1.4096, P: 0.015% or less, Cr: 1.25 to 1.70%. -41: 0.010 to 0.050%. Nb: 0.001 to
0.050%, 0: 0.0015% or less, and ,h,': 0.0100 to 0.0200%, further containing, as
necessary, one or more kinds of elements selected from (a) Xi: 0.15% or less
and &lo: 0.10% or less. rb) Ti: 0.005 to 0.01596, and (c) S: 0.005 to 0.035%. Pb:
0.01 to 0.09?6, Bi: 0.04 to 0.2096, Te: 0.002 to 0.050$4, Zr: 0.01 to 0.20%, and Ca:
0.0001 to 0.0100%. and the balance being Fe and unavoiclable impurity
elements, to 1200°C or higher, finishing hot forming such as hot rolling at a
finishing temperature of 800°C or higher. and thereafter cooiing the steel. to
600°C or lower at an average cooling rate of 30GC/min or higher.
too 101
Also, Patent Document 2 discloses ''method for manufacturing casehardened
product having high fatigue strengthv, wherein a steel material
consisting of, by mass ratio. C: 0.10 to 0.30%. >In: 0.50 to 2.096, S: 0.01 to
0.20°h, Cr: 0.50 to 1.5096, Al: 0.02 to 0.10°6, and N: 0.010 to 0.025%, it being
restricted such that Si: 0.10% or less, P: 0.010% or less. and 0: 0.005% or less.
further containing, as necessary. one or more kinds of elements selected &om
(a) 3%: 0.020 to 0.12094 and. Ti: 0.005 to 0.10%, and ib) Xi: 4.0% or less, ,210:
1.0% or less, T-: 1.0% or less. and Cu: 3.0% or less, and the balance being Fe
and unavoidable impurities, is worked into a required product shape ancl is
suhjecteci to carburizing treatment under the condition that the amoiult of
retained austenite in a 0.02-mm outer layer is in the range of 20 to 60% in area
fraction. anti thereafter repented bending stresses in the range of 70 yo
120 kgfi'mn~' (686 to 1136 hipa) in the net maximum stress on the outernlost
surface are applied 10.3 times or less to a stress concentrating part.
[Citation List]
[Patent Document]
i O O l l 1
[Patent Document 11 JP2001- 152284A
[Patent Document 21 JP2-259012A
[Summary of invention]
[Technical Problem]
loo 121
In the technique disclosed in Patent Document 1, although technical
idea that the content of Si is kept low and intergranular oxidation is reduced is
afforded, consideration is not given to the restraint of the depths of a
intergranular oxidation layer and a non-martensitic layer (hereinafter, a
general name of "carburized abnormal layer" may be collectively given), which
decrease the bending fatigue strength ancl wear resistance. Therefore, the
technique disclosed in Patent Document 1 does not necessarily provide parts
such as CITT pulley shafts with ensured high bending fatigue strength and
wear resistance.
LOO 131
In the technique disclosed in Patent Document 2 as well, although
technical idea that the content of Si is restricted to 0.1% or less and
intergranular oxidation is reduced is afforded, consiiferation is not given to the
restraint of the depth of a carburi~ed abnormal layer that decreases the
bending fatigue ~trength. Further, in Patent clocument 2 considerat:on is also
not given to the temper softening resistance of a steel material surface portion
exposed to high temperatures. Therefore. the technique disclosed in Patent
Document 2 as well does not necessarily provide parts such as CVT pulley
shafts with ensured high bending fatigue strength and wear resistance.
loo 1-21
hloreover, in the technique disclosed in Patent Document 2,
consideration is not given to the suppression of formation of coarse MnS, which
becomes a starting point of cracking when a raw material steel is hot-forged
into a desired product shape, and therefore the hot workability is insufficient.
Fui*thermore, since the coarse 3fnS itself decreases the bending fatigue
strength, a desired high bending fatigue strength cannot be ensured in some
cases.
100 133
The present invention has been made in view of the above-described
situation, and accordingly an objective thereof is to provide a case hardening
steel material in which even when 310, which is an expensive element. is not
added, a CVT pulley shaft can be provided with ensured high bending fatigue
strength and wear resistance, w-hicls are evaluated with the case where a raw
material steel is SCh1420E1 of "chromium-molybdenum steel"defined in J1S C
4052 (2008) being a reference, the component cost is low. and moreover. the
hot workability and machinability are excellent.
[Solutiols to Problem]
to0 161
To solve the above-described problems, thc present inventors have
conducted various studies. ,As the result, first, the findings of the follo.cving
items (a) to (d) had been obtained.
[ o o ~ r f
(a) In order to ensure a high bending fatigue strength and high wear
resistance without the adclition of -210. the component composition of steel has
to be made a coniposition capable of suppressing the decrease in hardenability
occurring due to the decrease in 310 content.
LOO 181
ib) Since the decrease in bending fatigue strength occurs due to the
formation of coarse MnS, in order to ensure a high bending fatigue strength,
the formation of coarse hInS has to be suppressed.
Loo191
ic) Coarse AInS becomes a starting point of cracking during hot working.
Therefore. to suppress cracking during hot working as well, coarse SlnS has to
be minimized as much as possible.
i00201
(d) In order to minimize coarse MnS as much as possible. not only the
respective contents of -2ln and S have to be controlled, but also the content
balance between Sln and S has to be optimized. Specifically. the formation of
coarse LlnS can be suppressed by controlling Fnl representecl by a formula of
IFn1 = LlnJS]. in which the element symbol in the formula represents the
content in mass percent of the elemenc, to k25 < Fnl < 851. Therefore, in order
to suppress cracking during hot working while good hot workability is ensured
and also to ensure a high bending fatigue strength. the respective contents of
JjIn and S have to he controlled. and also these contents have to satisf:~ the
above-described r.elatiu~iaflo rmula.
boa 11
-1ccordingly. the present inventors further have conducted various
stuclies of a steel in which the hardenability is ensured so as to offset the
ciccr.ense in ?+lo content, and the 1.espeetive contents of Afn and S and the
balance thereof are optimized to suppress the formation of coarse MnS. As
the result, the findings of the following items (e) to (j) were obtained.
lo0221
(e) X high bending fatigue strength cannot be ensured merely by
suppressing the decrease in hardenability occurring due to the decrease in AIo
content and by suppressing the formation of coarse 3InS. In addition to the
ensuring of hardenability and the suppression of formation of coarse MnS, the
depth of the carburized abnormal layer, that is, the depths of the intergranular
oxidation layer and the non-martensitic layer have to be decreased.
[0023]/
(0 The depths of the intergranular oxiclation la>-er and the nonmartensitic
layer. which are the carburized abnormal layer, can be decreased
by optimizing the content balance of oxidizing elements. especially Cr. Si and
3 . Specifically, the depth of the carburized abnormal layer can be decreased
by controlling Fn2 represented by a formula of [Fn2 = Cr/(Si + 23ln)], in which
tlze element symbol in the formula represents the content hy mass percent of
that element, to L0.90 < Fn2 < 1.201, whereby a high bending fatigue strength
can be ensured.
looa4
ig) In order to ensure a high bending fatigue strength, large-sized hard
inclusions of type 11: and type D measured in conformity to method LA of XSTAIE-
IS-11, that is, thick irrclusions of the inclusions of type B consisting nlainly of
-4l~Qi-basedin clusions and the inclusions of type D consisting mainly of TiSbased
inclusions have to be restrained. This is because the large-sized hard
inclusions of t?-pe B and type D described above become starting points of
fatigue fracture.
~00251
(h) In order to restrain the large-sized hard inclusions of type B and type
D described above. the contents of impurities, especially the contents of Ti and
0 (oxygen) have to be controlled to 0.005% or less and 0.0015% or less;,
respectively. ,Also, in order to restrain the large-sized hard inclusions of type
B and type D, it is desirable that a steel be melted in a vacuum furnace. or in
the case a steel is melted in a converter, secondary refining be repeated, or
electronlagneric stirring be performed during continuous casting.
[00261
!i) In order to steadily ensure good machinability. 20 to 70% of structure
in an area ratio have to be ferrite.
1TooaTl
(j) In order to ensure high wear resistance, it is effective to suppress
temper-softening of the sliding surface. Specifically, the temper softening
resistance is increased by controlling Fn3 represented by a formula of fFn3 =
1.II;Si + 0.70Aln i- Cr], in which the element symbol in the formula reprVesents
the content by mass percent of the element, to [Fn3 2 2.201, whereby high wear
resistance can be ensured.
rooas1
The present invention was completed on the basis of the above-described
findings. and the gists thereof are the case hardening steel materials described
below-.
lo0291
"1) X ease hardening steel material having a chemical cornpostion
consisting of, by mass percent. C: 0.15 to 0.2396, Si: 0.01 to 0.15%. LIn: 0.63 to
0.90%. S: 0.010 to 0.03094, Cr: 1.65 to 1.8096, A1: 0.013 to O.OGO%, and K:
0.0100 to 0.02;50?6, the balance being Fe and impuriries;
Fn1, F n h n d Fn3 ~*eprescntecbly the following Formtila: (I!, (2). and (3)
being 25 < Fnl 185, 0.90 < Fn:! < 1.20, and Fn:3 2 2.20, respectively; and
the contents of P, Ti and 0 in the impurities being P: 0.020% or less, Ti:
0.005% or less, and 0: 0.0015% or less, and
having a structure consisting of 20 to 70% in an area ratio being ferrite:
and
the portion other than the ferrite being one or more kinds of pearlite and
bainite :
Fnl = bIniS ... (1)
Fn2 = Cr/(Si i- 25111) ... (2)
Fn3 = 1.16Si i- 0.703In + Cr ... (3)
wherein, the element symbol in the Formulas (I), (2). and (3) represents the
content by mass percent of the element.
t00301
(2) The case hardening steel material described in the above item (I),
whereln in lieu of a part of Fe, one or more kinds selected from Cu: 0.20% or
less and Xi: 0.20% or less, by mass percent, are contained.
[Advantageous Effects of Invention]
t003 11
The case hardening sreel material of the present invention is low in
component cost, has good hot workability. and also is excellent in
machinability. 5Ioreovr:r. a carb~arizedp art ~nani~facturebdy using this case
hardening steel material as a raw material has a good bending fatigue
strength and good wear resistance, which are evaluated with tlre carburized
part produced by using SCL1I-I;"OES of "chromium-molybdenum steel" defined in
JIS G 4052 (2005) as a raw material steel being a reference. Therefore, the
case hardening steel material of the present invention is usecl suitably as a
raw material of the carburized part such as a CVT pulley shaft, which is
required to have a high bending fatigue strength and high wear resistance to
reduce the weight and to increase the torque.
[Brief Description of Drawings]
COO327
[Figure 11 Figure 1 is a view of a notched Ono type rotating bending fatigue
test specimen used in Examples, showing a rough shape in a state of being cut
out of a steel bar. The unit of dimension in the figure is "mm".
[Figure 21 Figure 2 is views of a block test specimen used in a block-on-ring
test in Examples, showing a rough shape in a state of being cut out of a steel
bar. The unit of dimension in the figure is "mm".
[Figure 31 Figure 3 is a view of a ring test specimen used in a block-on-ring test
in Examples, showing a rough shape in a state of being cut out of a steel bar.
The unit of dimension in the figure is "mrn".
[Figure 41 Figure 1 is a diagram showing a heat pattern of "carb~wizinga nd
quenching - tempering" performed on the test specimens shown in Figures I to
3 in Examples.
[Figure 51 Figure 5 is a vieu- stlowing the finished shape of a notc11i.d Ono type
rotating hending fatigue test specimen used in Examples. The unit of
dimension in the figure is "rnm".
[Figure 61 Figure G is views showing the finished shape of a block test
specimen used in a block-on-ring test in Examples. The unit of dimension In
the figure is "krm" only in the location clescribed as "test surface: Rq = 0.18 to
0.20", and is "mm" in other locations.
EFigure 71 Figuse 7 is a view showing the finished shape of a sing test specimen
used in a block-on-ring test in Examples. The unit of dimension in the figure
is "pm" only in the location described as "test surface: Rq = 0.13 to 0.30". and is
"mm" in other locations.
[Figure 81 Figure 8 is schematic views for explaining a hot compression test
perfortned in Examples, in which Figures 8(a) and 8(b) show the size and
shape of a test specimen before anct after the hot colnpression test, respectively.
The unit of dimension in the figure is "mm".
[Figure 91 Figure 9 is a view for explaining the length of a chip produced in
lathe turning rvork using an XC lathe in Examples.
[Description of Embodiment]
[00331
Hereinbelow, requirements for the present invention are explained in
detail. Here, the symbol "%" for the conrent of each element means "54 by
mass*'.
[0034]
(A) Concerning chemical composition:
C: 0.15 to 0.23O?
C is an element essential for securing the strength of the carburized part
such as a CVT pulley shaft. and therefore 0.15941 or more of C has to be
contained. E-Iov;ever, when the content of C is too high, the hardness
increases, and thereby the machinability is clecreased. In particular, when
the C content is more than 0.23%. the decrease in machinability caused by the
Increase in hardness becomes remarkable. Therefore, the content of C is set
to 0.15 to 0.23%.
Eoo351
In the case where much higher machinability is required. the content of
C is preferably set to 0.22% or less.
[0036]
Si: 0.01 to 0.13%
Si has a hardenability improving function and a deoxidizing function.
--llso, Si has resistance to temper-softening, and has an effect of preventing
surface softening in a situation in which the sliding surface of the CVT pulley
shaft or the like is exposed to a high temperature. In orcler to obtain these
effects, 0.0196 or more of Si has to be contained. However. since Si is an
oxidizing element. when the content thereof increases, Si is selectively oxidized
by a minute amount of H20 or 6 0 2 contained in a carburizing gas, and Si
oxides are formed on rhe steel surface. Therefore, the depths of the
intergranular oxidation layer and the non-martensitic layer. which are the
carburized abnormal layer, increase. The increase in depth of the carburized
abnorislal layer leads to a decrease in bending fatigue strength. Also. when
the Si content increases, not only the temper-softening resisting effect is
saturated, but also the carburizing property is hindered. and further the
machinability is decreased. In particular, when the Si content is more than
0.15%. the decrease in the bending fatigue strength becomes remarkable. and
also the decrease in the machinability becomes remarkable by the increase in
depth of the carburized abnormal layer and the decrease in surface liardness
caused by the hindrance to carburizing property. Therefore. the content of Si
is set to 0.01 to 0.1594.
i[loo3'71
In the case where much higher bending fatigue strength is required. the
content of Si is preferably set to 0.10% or less.
[00381
1In: 0.617 to 0.90%
Lln has a hardenability improving function and a deoxidizing funcrion.
Also, 1In has an effect of suppressing temper-softening. In order to obtain
these effects, the Lln content has to be 0.65% or more. However, when the &In
content increases. the hardness increases, and thereby the machir~ahiliry is
decreased. In particular, when the l l n content is more than 0.90%, the
decrease in machinability caused by the increase in hardness becomes
remarkable. Moreover, since, like Si, hfn is an oxidizing element, when the
content thereof increases, 1In oxides are formed on the steel surface.
Therefore. the depths of the intergranular oxidation layer and the nonmartensitic
layer, which are the carburized abnormal layer, increase. The
increase in depth of the carburized abnormal layer leads to a decrease in
bending fatigue strength. In particular, when the %In content is more than
0.90%, the decrease in bending fatigue strength caused by the increase in
depth of the carburized abnormal layer becomes remarkable. Therefme, the
content of ltln is set to 0.65 to 8.90%. The &In content is preferably set to
0.7096 or more.
[00391
S: 0.010 to 0.030%
S combines with &In to form MnS, and has a function of improving the
macl~inability. In order to cbtain the effect of improving the nlachinabiiity,
the S content has to be 0.010% or niore. On the other hand. when the S
content is more than 0.030%. coarse hfnS is formed, and the hot workability
and bending fatigue strength are decreased. Therefore, the content of S is set
to 0.010 to 0.030%.
Eooj;oi
In order to steadily obtain the above-described effect of improving the
machinability by S. the content of S is preferably set to 0.015% or more.
b041 I
tn the case where much higher hot workability and bending fatigue
>trength are reyuiscd. the content of S is preferably 0.023% or less.
Eoo;?l
Cr: 1.66 to 1.80%
Cr has an effect of improving the hardenability. Cr has resistance to
temper-softening. and also has an effect of preventing surface softening in a
situation in which the sliding surface of the CVT pulley shaft or the like is
exposed to a high temperature. In order to obtain these effects, the Cr
content has to be 1.65% or more. However. when the content of Cr increases,
the hardness increases, and thereby the machinability is decreased. In
particular, when the Cr content is more than 1.80%. the decrease in
machinability caused by the increase in hardness becomes remarkable.
Noreover, since. like Si and &in, Cr is an oxidizing element, when the content
thereof increases. Cr oxides are formed on the steel surface. Therefore, the
depths of the intergranular oxidation layer and the non-martensitic layer,
which are the carhurized abnormal layer, increase. The increase in depth of
the carburized abnormal layer leads to decreases in bending fatigue strength
and wear resistance. In particular, when the Cr content is more than 1.80%,
the decrease in bending fatigue strength caused by the increase in depth of tlae
carburized abnormal layer becomes remarkable. Therefore, the content of Cr
is set to 1.63 to 1.80%.
I00431
In the case where rnuch higher machinability is required. xhe content of
Cr is preferably set to less than 1.80°,/0.
[00441
,-11: 0.015 to 0.060?/;
-41 hiis a deoxidizing function. -4lso. A! combines with S to fvrrn XIN.
find makes crystal grains fine, therefore has a function of strengthening a steel.
'lowever, when the content of A! is less than O.015%, it is dlfficuit to obtain the
above-ciescribcd effects. On the other hand, when the A1 ct~ntcnt is
excessively high, hard anci coarse XlA.3: is formed, and tbevehy the
machinability is decreased. Further, the bending fatigue strength and wear
resistance are also decreased. In particular, when the Al content is more than
0.060°/o, the machinability, hending fatigue strength, and wear resistance
decrease remarkably. Therefore, the content of -41 is set to 0.015 to 0.060%.
The -41 content is preferably 0.020'?6 or more, and also is preferably 0.055% or
less.
[0045]
X: 0.0100 to 0.0250%
?LT makes crystal grains fine by the formation of nitrides. and therefore
has an effect of irrrproving the bending fatigue strength. Tn order to obtain
rhis effect, 0.0100% or more of X has to be contained. However. tt-hen the
content of X is excessively high, coarse nitrides are formed. and thereby the
toughness is decreased. In pa~ticular. when the N content is more than
0.0250%, the toughness decreases remarkably. Therefore, the content of N is
set to 0.0100 to 0.0250%. Tlle N content is preferably 0.0130% or more. and
also is preferably 0.0200% or less.
100461
The case hardening steel material in accordance with the present
invention has a chemical composition consisting of the above-described
elements ranging from C to X. the balance being Fe and impurities, the lat.el--
described conditions of Fnl, Fn2 and Fn3 being met, and the contents of P, Ti.
t-inti 0 (oxygen) in the impurities being restricted to the later-described ranges,
too471
The term "irnpuritieb" in the "Fe and impurities" of the balance me;ins
components tl~ate nter nnixecIly from ore and scrap used as a raw material,
production environments, ancl the like when a steel material is procluced on an
industrial scale.
[ooLrsl
Fnl: 25 to 85
Even if the contents of XIn and S are within the above-described ranges,
when coarse &In§ 1s formed, the decrease in bending fatigue strength occurs.
In order to ensure a high bending fatigue strength, the formation of coarse
&Ins has to be suppressed. hIoreover. since the coarse hInS also becomes a
starting point of cracking during hot working, in order to suppress the
cracking during hot working, coarse hfnS has to be minimized as much as
possible. Therefore, the balance between the contents of hIn and S is
important. and Fnl represented by Formula (1) has to be within a fixed range.
f00491
When Fnl is less than 25, the content of S becomes excessively high, and
the formation of coarse 3InS is unavoidable. On the other hand, *vvhen Fnl is
more than 85, the content of Xln becomes excessively high, and coarse LlnS is
formed in a central segregation zone. Therefore, in both the cases. the
bending fatigue strength is decreased, and moreover, the cracking during hot
working becomes liable to occur. Therefore, Fnl is set so as to be 23 L Fnl 5
8<5.
Zoos01
Fn2: 0.90 to 1.20
In order to provide a high bending fatigue strength without the addition
of JIo. the clepths of the intergranular oxidation layer and the non-martensitic
layer. wl~icha re the carburized abnormal layer, have to be decreased while the
harcienahility is ensured. For this purpose. the contents of Cr. Si ancl Aln of
the oxidizing elenlents are made within the above-described ranges, ancl
additionally. FnZ represented by Formula (2). which indicates the content
balance of these elements. has to be within the range of 0.90 to 1.20.
[OG;TI]
When Fn2 is less than 0.90 or when it is more than 1.20, the depth of the
carburized abnormal layer increases, and thereby the bending fatigue strength
is decreased. Therefore, Fn2 is set so as to be 0.90 < Fn2 L: 1.20.
lo0521
Fn3: 2.20 or more
In order to ensure high wear resistance, it is effective to increase the
temper softening resistance of the sliding surface exposed to a high
temperature. For this purpose, the contents of Si, >In and Cr, which are
elements having an effect of suppressing temper-softening, are rnacle within
the above-described ranges. anci additionally, Fn3 represented by Forn-tula (3).
which indicates the content balance of these elements, has to be 2.20 or more.
When Fn3 is less than 2.20, the wear resistance is decreased. Fn3 is
preferably 2.60 or less.
Eoos3l
Furthermore. in the present invention, the contents of P, Ti and 0 in the
impurities have to be subject to especially strict restriction. The contents of
these elements have to be restricted as follows: P: 0.020% or less. Ti: 0.005% or
less, and 0: 0.0015% or less.
Euoxl
In the following, explanation is given of the restriction of the contents of
these elements.
~00551
P: 0.020% or less
P is an impuriry contained in a steel, and segregates at crystal grain
boundaries and cmbrittles the steel. In particular, when the content of P is
more ehan O.Oi?OO/o. the degree of embrittlennent is remarkable. Therefore, the
content of P is set to 0.02994 o~ less. The content of P in the impurities is
preferably 0.015% or less.
f00561
Ti: 0.005% or less
Ti has a high affinity to N, and therefore combines with N in a steel to
form a D type inclusion TiS, which is a hard and coarse nonmetallic inclusion,
whereby the bending fatigue strength and wear resistance are decreased, and
further the machitiability is decreased. Therefore, the content of Ti in the
impurities is set to 0.005% or less.
foos71
0: 0.0015% or less
0 combines with Si, Xl, and the like in a steel to form oxides. Among
these oxides. especially a B type inclusion A1203 is hard, thus decreases the
machinability. and further decreases the bending fatigue strengrh and wear
resistance. Therefore, the content of 0 in the impurities is set to 0.0015% or
less. The content of O in the impurities is preferably 0.0013% or less.
I00581
In the case hardening steel material in accordance with the present
invention, in lieu of a part of Fe, one or more kinds of elements selected frona
Cu and Xi may be contained as necessary.
Lo0591
In the following, there are explained of the operational advantages and
the reasons for restricting the contents of Cu and Xi, which are optional
elements.
[0060]
Cu: 0.2094 or less
Cu has a function of enhanelng the harclenability. and therefore @u may
be contained to further improve the hardcnability. However. Cu is an
expensive element. and also decreases the hot workability when the content
thereof increaxs. In particular, when the content of Gu is more than 0.20°%,
the hot u~orkability is decreased remarkably. Therefore. the content of Cu,
when contained, is set to 0.20% or less. The content of Cu, when contained, is
preferably 0.15% or less.
Eoo~11
On the other hand, in order to steadily obtain the above-described
harclenability improving effect of Cu. the content of Cu, when contained, is
preferably 0.05c " /a or more.
lo0621
Xi: 0.20% or less
Si has a function of enhancing the hardenability. Nickel has a function
of improving the toughness, 2nd additionally. because of being a nonoxidizing
element, Ni can also strengthen the steel surface without the increase in clepth
of the intergranular oxidation layer during car'nurization. Therefore, to
obtain these effects, Xi may be contained, However, Xi is an expensive
element, so that the excessive addition thereof leads to a rise in component
cost. In particular, when the content of Ni is more than 0.20%, the cost rises
grearly. Therefore, the content of Ni. when contained. is set to 0.20% or less.
The content of Xi, when containecl, is preferably 0.15% or less.
100631
On the other hand, in order to steadily obtain the above-described
characteristics improving effect of Xi, the content of Xi. when contained, is
preferably 0.05% or more.
[006-21
For the Cu and Ni. only any one kind of these elements can be contained,
or two kinds of these elements can be contained compositely. The totai
content of these elements may be 0.40%. but is prefert-rbly 0.3006 or less.
k0065l
(IZ) Concerning micro-structure:
The case hardening steel material of the present invention not only has
the chemical composition described in the above item (,A), but also has to have
a structure consisting of 20 to 70% in an area ratio being ferrite, and the
portion other than the ferrite being one or rnore kinds of pearlite and bainite.
The reason for this is as follotvs.
f0066f
The area ratio of ferrite in the steel material structure exerts an
infltlence on the machinability. When ferrite in the structure is less than 20%
in an area ratio, tool wear cltwing cutting is accelerated, and the machinability
is decreased. On the other hand, when the area ratio of ferrite is more than
7096, chips generated during lathe turning connect, and the chip disposal
ability is deteriorated. In this case as kvell, the machinability is decreased.
Therefore. 20 to 70% of structure in an area ratio is set to be ferrite. The area
ratio of ferrite is preferably 30% or more.
[00671
When martensite is intermixed in the portion other than the ferrite. the
hardness increases. and thereby the machinability is decreased. Therefore,
the portion other than the ferrite is made to have a structure consisting of one
or more kinds of pearlite and bainite.
l006SI
The case hardening steel having the chemical composition described in
the above item (A) can have a structure consisting of 20 to 70% in an area ratio
being ferrite, and the portion other than the ferrite being one or rnore kinds of
pearlite and bainite as described above by the process described below. For
example, after being hot-rollcci or hot-forged, the steel is normalized within
8'70 to !lSOcC, and is allo.cved to cool in the atmospheric air or is cvincZ-cooled
with fan in sucl~a manner that the average cooling rate in the range of 800 to
500°C is 0.1 to 3'C/s.
LO0691
The following exanaples illustrate the present invention more specifically.
Steels 1 to 21 having the chemical compositions given in Table 1 were
melted by using a converter or a vacuum furnace to prepare a cast piece or
ingots.
10017 11
Specifically, for steel 1, the steel was melted by using a 70-ton converter,
and after the component adjustment had been made by performing seconclary
refining two times, the steel was continuously cast to prepare a cast piece.
During continuous casting, inclusions were caused to float and re~llovetl
sufficiently by coiltrolling the electromagnetic stirring.
[(I0 721
For steels 2 to 16 and 18 to 21. after the steels had been melted by using
a 150-kgv acuuna furnace, casting was perforn~edto prepare ingots.
LO0731
For steel 17, after the steel had been melted by using a 150-kg
atmospheric furnace, casting was performeci to prepare an ingot.
Eoo731
Steels 1 to 12 were iteels of inventive examplei who*e chemical
cvmpositians were within the ranges defin~din the present invention.
[001731
On the other hand, both of steels 13 and I9 were steels of comparative
examples in which although the content of each compunclnt element >atisfied
the condition defined in the preoent invention. Fn2 deviateci fro111 rhe conditior~
defined in the present invei~tron, and steel 13 was a strel of comparatit-c
example in which although the content of each component element satisfied
the condition defined in the present invention, Fn3 deviated from the condition
defined in the present; invention. ,Also. both of steels 20 and 21 were steels of
comparative examples in which although the content of each component
element satisfied the condition deilned in the present invention, Fn1 deviated
from the condition defined in the present invention. Further. steels 14 and 16
to 18 were steels of comparative examples in which the content of at least a
component element deviated from the condition defined in the present
invention.
10076j
,-lmong the steels of comparative examples, steel 14 was a steel
corresponding to SCM420H ctefined in JIS C $052 (2008).

From each of the cast piece and ingots. steel bars each having a diameter
of 25 mm and a diameter of 45 mm were produced by the processes ciescribed
in the follotving items [f I and E21.
Eoor;91
111 Blooming:
After being held at 1250°C for two hours, the cast piece was subjected to
blooming, whereby a 130 nzm -square billet was produced.
loo801
t21 Hot working:
The surface defects of the 180 mm-square billet produced by blooming
were removed with a grinder. being held at 1250°C for 50 minutes, and
thereafter the billet was hot-rolled, whereby steel bars each having a diameter
of 25 mm and a diameter of 43 mm were produced.
li~0181
_ifso. each ingot was held at 1250°C for two hours. and thereafter \vt;ls
hot-forged, whereby steel bars each having a diamtlter of 25 nlnl and a
diameter of 45 rnm were produced.
Eoos21
From each 2.5 mm-diameter and 45 mm-diameter steel bars thus
obtained. various test specimens ii-ere preprired by the proceases described in
the following items 131 to f6I.
100831
[:_$I Sornlalizing:
Each 25 mm-diameter steel bar was held at 900'C for one hour, and was
r~orma!izect by being allom-ecf to cool in the atmospheric air.
!0084l
Each 45 mm-diaineter steel bar was held at 900°C for one hour, then
normalized by be~ngal lowed to cool in the atmospheric air for steels 1 to 3 ancl
13 to 15, and was held at 900cC for one hour. then nor~r~alizebdy being winclcooled
with a fan for steels 6 to 12 and 16 to 21.
lsossl
The average cooling rate in the range of 800°C to 500°C in the case
where the 23 mm-diameter steel bar was allowed to cool in the atmospheric air
was 0.89'Cis.
lo0861
The average cooling rate in the range of 800°C to 500°C in the case
where the 45 mm-diameter steel bar was allowed to cool in the atmospheric air
xas 0.46"Cis. 21iso, the average cooling rate in the range of 800°C to 500°C in
the case where the 45 mm-diameter steel bar was wind-coolecl with a fan was
0.85"Cis.
[do871
[4] Machining (rough workiiig or finish working):
From the central portion of each normalized 23 mm-diai~~etsetre el bar. a
notched Ono type rotating bending fatigue test specirnen having a rough shape
shown in Figure 1, a block test specimen for block-on-ring test having a rough
shape shown In Figure 2, and a test specimen for a hot compression test
having a fini~heds hape having a diaineter of 20 Inn1 and a length of 30 mrn
were cut out in parallel with the rolling direction or the forging axis.
Eoo881
-1lsv. from the central portion of the normalizccl 45 rnm-diameter steel
bar. a ring test bpecimen fbr block-on-ring test having a rough shape shown In
Figure 3. ancl a test specmen for a machinability test havlng a diameter of 40
mm anti a length of -130 nim were cut out in parallel with the forging arrls.
[i!039t
All the dimensions of the cut-out test specimens shown in Figures 1. to 3
are expressed in millimeters, and three kinds of inverted triangular finish
marks in the figures are "triangle marks" indicating surface roughness
described in Explanation Table 1 cf JIS B 0601 11982).
[oo9ol
-4 part of each remaining norrnalizecl 25 mm-diameter steel bar was
water-quenched. and thereafter was ttsed for nonmetallic inclusion
examination. The details of the examination method will he described later.
I009 11
[5] Carburizing and quenching - tempering:
,411 of the notched Ono type rotating bending fatigue test specinlen, and
the block test specimen and ring test specimen for block-on-ring test that had
been cut out in the above iten1 141 were subjected to "carburizing inad
quenching - tempering" using the heat pattern shown in Figure 4. The "Cp" in
Figure 4 represents a carbon potential. Also, the "130°C oil quenching"
represents quenching in an oil having an oil temperature of 130°C, and further
the ",lCn represents air cooling.
Eoog~l
The notched Ono type rotating bending fatigue test specimen was
subjected to the above-described treatment In a hung state in which a wire is
allowed to go through a hole formed for hanging. On the other hand, the
block test specimen and ring test specimen for block-on-ring test were
subjectecl to the above-described treatment in 2 state of being placed flat on a
. .
jig above a wire mesh.
100931
The oil quenching wiis performed by putting the test specimen into a
atirised qrrerlching oil so that ytaenuhing is performed uniformly.
[oc;94l
161 Machining (fjnishing work of material subjected to carburizing and
quenching - tempering):
The above-described test specimens subjected to carburizing and
quenching - tempering were finished to prepare the notched Ono type rotating
benciing fatigue test specimen shown in Figure 5, the block test specimen for
block-on-ring test shown in Figure 6, and the ring test specimen for block-onring
test shown in Figure 7.
Lo0951
Tbe dimensions of the test specimens shown in Figures 5 to 7 are
expressed in millimeters excluding the locations described as "test surface: Rq
= 0.10 to 0.20" in Figure 6 and "test surface: Rq = 0.15 tc 0.30" in Figure 7.
Also. as in Figures 1 to 3, three kinds of inverted triangular finish marks in
Figures 5 to 7 are "triangle marks" indicating surface roughness described in
Explanation Table 1 of JIS B 0601 (1982).
[0096]
Also, the "C" attached to the finish mark in Figure 3 is an abbreviation
of working method indicating "grinding" that is defi~lttdi n JIS B 0122 (19178).
!0097l
Further. the "- (swung dash)" is a "\vaveform symbol" that means a base
metal, that is, a surface as is subjected to carburizing and quenching -
temperlng of the above item L.51.
loo981
The "tcqt surface: Rq = 0.13 to 0.20" in Figure G and "test surface: Rq =
0.15 to 11.30" in Figure '7 mean that the root-mean-square roughnesses "Rq"
defined 11.1 JiS B 0601 (2001) are 0.10 to 0.20 ,urn and 0.15 to 0.30 pm.
respectively.
hoosnl
For each of steels 1 to 21, there were conducted examination of microstructure.
examination of hot workability through the hot compression test,
examination of noninetallic inclusions, examination of surface hardness,
examination of core hardness, examination of depth of effective hardened layer,
examination of depth of intergranular oxidatinn layer, examination of depth of
non-martensitic layer, exanxination of fatigue characteristics through the Ono
type rotating bending fatigue test, examination of wear resistance through the
block-on-ring test, and examination of machinability through lathe turning,
lo n ool
Hereinbelow. the details of each of the examinations are explained.
Eoioll
<> Examination of micro-structure:
-A specimen was cut out of the R12 portion ("R" indicates the radius of
steel bar) of the transverse cross section (the surface cut perpendicularly to the
rolling direction or the forging axis) of the normalized 45 mm-diameter steel
bar produced in the above item [31.
To1021
,After the specimen had been embedded in a resin so that the cut surface
was a surface to be examined. the surface was polished into a mirror surface
finish. and was etched with nital. Thereafter, the micro-structure was
observed under an optical microscope at a rnagnihcation of 400. Five optional
vrsual fields were observed, whereby the "phase" was identified, and the area
rario of ferrite was measurer1 by image analysis.
201031
<> Examination of hot workability:
The test specimen for hot compression rest having a diameter of 20 mrn
and n leng~hof 30 mnl, which was prepared as de~cribecli n the above iten1 [dl,
was held at 1200cC for 30 minutes, and then compl.esscd to a height of 3.1.5
mm by using a crank press with the length direction being a height as shown
in Figures 8(a) and 8(b).
EoIo~;]
Figures 8(a) and 8(b) are schematic viecvs showing the size and shape of
test specimen before and after the hot compression test, respectively.
f01051
For each of the steels, five test specimens were subjected to the abovedescribed
compression test using a crank press, and cracks on the outer
peripheral surface were observed visttally. In the case where no crack having
an opening width of 2 mm. or larger was recognized on all of the five test
specimens, it was evaluated that the hot workability was excellent.
[01061
<<3>> Examination of nonn~etallicin clusions:
For the 23 mm-diameter steel bar that was normalized as described in
the above item E31, the remainder of steel bar from which the block test
specimen for block-on-ring test having a rough shape shown in Figure 2 was
cut out was held at 900cC for 30 minutes, and thereafter was water-quenched.
[01071
After being water-quenched, the steel bar was embedded in a resin so
that the longirt~dinalc ross section thereof (the surface cut in parallel wrth the
rolling direction or the forging axis so as to pass through the centerline
thereof) was a surface to be examined, and the surface was polished into a
rnlrrur suvft'ttee finish.
[0108]
Xext. in conforxnity to method --of IA- 1STlILE-E4S-11t, he thicknesses of
thick inclubions of the nonmetallic inclusions of type E3 ancl type D. specifii:aily.
inclusions having a thickness larger than 4 pm and 12 pnz or srnaller and
inclusions having a thickness larger than 8 pm and 13 ym or smaller were
measured. ancl the class judgment of each of the inclusions was made.
to1091
In the following explanation, the nonmetallic inclusions of type B and
type D having a large thickness are called "BH" and "DE-I", respectively.
toll01
<<-2>> Examination of surface hardness and core hardness
By using the notched Ono type rotating bending fatigue test specinlen
subjected to carburrzing and quenching - tempering as described in the above
item [5], the notch portion having a diameter of 8 mrn was transversely cut.
and was embeddeci irt a resin so that the cut surface was a surface to be
examined. Thereafter, the surface was polished into a mirror sur*face finish,
and the surface hardness and the core hardness were examined by using a
micro Vickers hardness tester.
[01111
Specificall::. in confornlity to "Vickers hardness test - Test method"
described in JfS Z 2244 (20091, Vickers hardness (hereinafter. referred to as
"HI"') was measul.ed at ten optional points at a position 0.03 mrn deep from
the surface of test specimen by using a micro Vickers hardness tester,
2pecifically a microhardness tester F5I-700 manufacturrd by FLTURE-TECH,
with the test force being 0.98s. The rneasrrrement values were arithmetically
averaged, and thereby the surface hardness was evaluated.
Ea11aI
Likewise, in conformity to above-described specification of JIS, HT/' was
rneas~~readt ten optional points in the core part. which is a portion of base
metal not affected 113- carburization, by using a micro Vickers harciness tester
with the tes"cfi,rce being 2.941;. The met3suremsnt valr~esw ere arithrni~ticallqaveraged,
and thereby the core harclnes,;: was evalufited.
[0113]
For the block test specimen for block-on-ring test subjected to
carburizing and quenching - tempering as described in the above item [51 as
well, the central portion of the length thereof of 15.7'5 min was transversely cut,
and tvas embedded in a resin so that the cut surface tvas a surface to be
examined. Thereafter, the surface was polished into a mirror surface finish,
and the surface hardness and the core hardness were examined by using a
micro Vickers hardness tester by the same method as that in the case where
the notchecf Ono type rotating bending fatigue test specirnerl was used.
f0ll41
For the block test specimen for block-on-ring test subjected to
carbttrizing and quencl~ing - tempering as described in the above item [5f, in
the case where the test specimen was subjected to treatment ia which it was
tempered at 300°C for one hour by using a vacuum furnace and thereafter was
water-cooled as well, the surface hardness was measured by tile sarne method
as described above.
lo1151
<<3>> Exaaiination of effective hardened layer depth:
By using the resin-embedded test specimens of the notched Ono type
rotatlng bending fatigue rest specimen and the block test specinlen ibr blockon-
ring test used for the examination of surface hardness and core harc2ness in
the above item <> ;after merely being subjected to carburizir~g and
quenching - tempering in the above item [51, the effective hardened layer depth
was examined.
[ ( l l l ~ ]
Specifically, as in the case of examination of surface hardness in the
above item <<-1>>, in conformity to "TLTicker~ hardness test - Test ri~ethocl"
clescrihd in JIS Z 2244 (2009f, HV was meaeurecl in the direction directed
from the lnirror surface finished test specimen surface toward the center by
using a micro Tickers hardness tester with the test force being 2.94s. The
depth from the surface in the case where ElT< was 550 was measured. The
minimum value of the measurement values obtained from 10 optional locations
was made the effective hardened layer depth.
lo1171
<<6>> Examination of intergranular oxidation layer depth and nonmartenbitic
layer depth:
By using the resin-embedded Ono type rotating bending fatigue test
specimen used in the above items <<4>> anci <<5>>, the intergranular
oxidation layer depth and the non-martensitic layer depth were examined.
t01187
Specifically. the test specimen embedded in a resin was polished again.
and the surface part of test specimen, which was in a state of being mirror
surface finkhed and not etched. was observect In 10 optional visual fields under
an optical microscope at a magnification of 1000. An oxidized layer observed
along the grain intergranular in the surface part was defined as the
intergranular oxidation layer, and the depths of these layers were
arithmetically averaged, and thereby the intergranular oxidation layer depth
was evaluated.
[01191
Fetrtber, the Identical test specimen was etched with nital for 0.2 to 2
..clconds. and the surf:~ce part of test specimen was observed in 10 optional
visual fieids under an optical microscope at a mag~lification of 1000.
portion in which the degree of etching was more remarkable than that of the
periphery in the surface part was defined as the non-martensitic layer. and the
tlepths of these layers were arithmetically aver:\gecI, and thereby the nonm;:
rtensitic layer clslyt'n was evaluated.
[01%01
<<'7>> Examination of fatigue characteristics rhrouglr Ono type rotating
bending fatigue test:
By using the Ono type rotating bending fatigue test specimen finished in
the above item [GI, an Ono type rotating bending fatigue test was: conducted
under the following test conditions. The bending fatigue strength was
evaluatecl by the maxlmurn strength at the time \viaen the test specimen did
not rupture in repeatinag number of 10;.
Lolzll
* Temperature: Room temperature
* Atmosphere: in the armospheric air
* Number of rotations: 3000 rpm
Eo~aal
With reference to the value of steel 14, which was the steel
corresponding to SC;\I42OH defined in JIS G 4032 (ZOOS), in the case where the
bending fatigue strength mas 510 MPa or higher, the hencling fatigue
characterlstlcs were evaluated as excellent, and this bending fatigue strength
was defined as the target.
to1231
<<8>> Examination of wear resistance through block-on-ring teat:
By using the block test specimen ant1 n r ~ gte st >pecirnen fix block-onring
test finished in the above item [6l. n bock-on-img test %vas concluctcd
uncler the following test ccnclttlons, and therehy the \%-,?;carre. si.;t;rncc-. lvas
examined.
[0124]
* Load: 1000X
Sliding velocityt-: 0.1 nl/sec
* Lubricatiol~:L uhrictrxing oil for CVT having t x n oil ttlmt~cr;~turoef 9g3C
* Total sliding distance: 8000m
Co1a5l
That is. the block test specimen was pressed against the ring test
specimen rotating in a lubricating oil for CVT, and the block-on-ring test was
conducted until the total siicling distance reached 8000rn. The amount of
wear of the block test specimen after testing was evaluated. -4 stylus type
surface roughness tester in which the radius of stylus tip end was 2 ,um and
the taper angle of circular cone at the tip end was 60" was used. The
maximum depth obtained by moving the stylus of the roughness tester from
the noncontact portion to the contact portion and to noncontact portion
between the block test specimen and the ring test specimen was defined as the
amount of wear.
!01261
Kith reference to the value of steel 14, which was the steel
corresponding to SC?d420H defined in JIS G 4052(2008), in the case where the
amount of wear was 7.0 pm or smaller. the wear resistance was evaluated as
excellent, an3 this amount of wear was defined as the target.
to1271
<<9>> Machinability teat:
The outer peripheral part of the test specimen having a diameter of 30
nm and a length of 430 rnm that had been prepared in the above item [-21 was
I:2the turned by using an S C lathe, and thereby the machinabilitj- was
evaluated.
[0128]
The lathe turning work w:ls performed under the turning conciitions of
cutting ,ipecci: 200 mlmin, infeed: f .3 mm, and feed: 0.3 mmlrev in the ~ a t rpn
which no lubricant was used. By using a tool dynamometer, the
machinability was evaluated by the cutting rksistance and the chip disposal
ability during lathe turning.
to1291
The cutting resistance was evaluated by determining the resultant force
of cutting force, feed force, and thrust force by using the follokving formula.
Cutting resistance = {(cutting force)' + (feed force)" (thrust force)")" 5
When the cutting resistance \\-as 900N or smaller, the cutting resistance was
evaluated as small.
hi3 I 301
The chip disposal ability was evaluated for each steel by selecting a chip
whose chip length shown in Figure 9 was at the maximum from 10 optional
chips after lathe turning and by measuring the length of the selected chip.
The chip disposal ability was evaluated as "excellent (OO)""g,o od (13)an~d' ~
"poor (x)" in the case where the chip length is 5 mm or shorter, in the case
where it is longer than 5 mm and 10 mm or shorter, and in the case where it is
longer than 10 mm, respectively.
[01311
In the ease where the cutting resistance was small, being 90OK or
smalier, and the chip disposal ability was evaluated excellent or good ("00"o r
"O")th,e machinability was evaluated as excellent, and this machinability was
defined as the target.
lolsal
Tables 2 to 4 give the above-dei;cribed examination results collectively.
Tn Table 2, the cooling conditions after the 3.3 mm-diameter steel bar had been
held at 900°C f ~orne hour are clescribed as "allowect to cool in atmospheric air"
and "wincl-coolecl with fan".
Table 2
!1;1v1
than
Inf>t 1
%OO°C for orw hour
'I\-irttl-csix)letvlc rth t'nn
\ti-~ntl-rtult.tl \t l t h I'i3r1
t\~mrl-c~tx)clt~ tithl fitn
'IT~ntl-ctx,lrtnt it h f,ln
\Vintl-cml~tnl 3 t tl f,in
\L intl-c.cx,trif rt ith E:in
'It mtl-cix-detl c\ rth fiin
'It inil-cmli~tilt ith fan
/171ntl-cooleti vr rth fan
It-mil-coolctl t5 11 h f;rn
"F" "P" ixntl "B" In mlcm htructurti toluain rc1~ri.i-en[ fcrrltc, pcawlltc, ant1 1::11111tr
ct1vely
For (#rack in hot workirb~ilry c.olumn In the c:tbc wficrr orlc or rnorc crackc c;lc,h hnvln
ntz ~ l i l t hof 2 Irlm or 1arqi.r wc1rc not r~ct>qnizeotln outer ~ ~ r ~ p h esrusrfItl c~ho f nil tht. tic
pP('ltnenh ;tfttlr compre?.hiotl test "not ocrurrtlti" was c l e s ~ r ~ ba~nd(\ I n the cnie ul.lc.rt? one L
clr;~c-ktu Pr.fJ r~c*c~gnizr"tol r*i-urrptl"w ar itr~cr1i)c~il.
Numt~rrtb:i.l~ ;:IuiPn n onmrtnlltt' rnclui.ions rt31)rc.c 11th c!ii-* ~u~Jgle)dy rnr:1.-urlnq ~nc'lu-if~r
lg tiuc.krltl+ !;irgtar than 1 urn :rntl 12 brn or mitller ;inti ~nciu%lonihl av~ngr hii.l\n~iLi iirqt
X urn 1 .ip m or -rniiltoi~o f nonmt~ttillrcm clu-.ionn of t~pr'l3 irncl type 1) 111 c'ontijrrcitq t
7tl -1 of .\s'l":bl 1.: 1.;- 11
" milt-k rnOtc.~ittc~lr~\: lnt~on from conrll[ton clrfinr(1 In tlrr prc-pnt la1 rntron
-----a=- - - rrX_ -__ail -x L__*---P---m-s---_ uFd*-s-mrj_a
10 1341
[Table 31
[Table 41
Table 4
-1s is apparent from Tables 2 to 4. in test Sos. 1 to 12 satisfying the
conditions defined in the present invention, the steel material had good hut
tvorki~bility and also was excellent in machinability, and moreover, steels 1 to
lhtifficicntil- met the targets of a bending fatigue srrengrh of 510 3IPa or
higher and ;In amount of wear of Y.0 yrn or smaller, which were evaluated with
the case of test So. 1-2 in which steel 1-2 corresponding to SChI-220TI of
"chrorniui~1-rno]iyt:)ctcnt11s:tne 2l" was used as a reference, so that it is clear that
B. high bending fatigue strength and high wear resistance can be ensured.
[013'i1
In contrast, in test Xos. 13 and 15 to 21 of comparative examples
deviating from the conditions defined in the present invention, for either one or
both of the bending fatigue strength and the wear resistance, the targets (that
is, bending fatigue strength: 510 MPa or higher, amount of wear: 7.0 pm or
smaller) defined with the case of test No. 14 in which steel 14 was used as a
reference could not be met. Also, in test Nos. 16 and 17, the hot workability
was low, and the machinability was poor. Further, in test Xo. 18, the
machinability was poor.
[O 1381
That is, in test No. 13, since Fn2, that is, [Cr/(Si + 21Cfn)l of steel 13 was
higher than the range defined in the present invention, the bending fatigue
strength was as low as 490 MPa. and therefr~ret he target could not be met.
[O 1391
In test So. 15, Fn3, that is, [1.16Si + 0.703In + Cr] of steel 15 was lower
than the range defined in the present invention. For this reason. the amount
of wear was as large as 7.8 pm. and therefore the wear resistance was poor.
T01401
In tesr Xo. 16, the contents of Si and 3In of steel 16 were higher than the
values defined in the present invention, and the content of Cr was lower than
the value defined in the present invention. -Also, Fnl, that is, [&ln!SI was
higher than the range defined in the present invention, and moreover, Fn2.
hat is. [Gr!(Si + 23In)l was lower than the range defined in the present
invention. For thi.; reason, the bending fatigue strength was ;as low as 460
AIPa, and therefore the bending fatigue strength was poor. ,Also, a crack
having an opening width of 2 mm or larger was generated by the conlpression
test using a crank press, so that the hot workability was also poor. Further.
since the strutrure was a bainite single-phase structure that does not contain
ferrite at all, the cutting resistance was large, and therefore the machinability
was poor.
[0141]
in test So. 17. all of the contents of S, Ti and 0 of steel 17 were higher
than the values defined in the present invention, and the contents of IIn and
Cr were lower than the values defined in the present invention. ,Also. Fnl.
that is, [;?/Ini'S] was lower than the range defined in the present invention.
moreover, Fn2, that is. [Cr/(Si -t- 2hln)I was lower than the range defined in the
present invention. and further, Fn3, that is. [ l . l ~ Sii 0 .703In + Cr] was lower
than the sange clefined in the present invention. Fc~rt his reason, the bending
fatigue strength -vas as low as 420 MPa. and the amount of wear was as large
as 15.4 ym. Therefore. the bending fatigue strength anif. the wear resistance
were poor. -Also, nonmetallic inclusions of type B of class 2.5 and nonmetallic
inclusions of type D of class 1.0 were observed. Further, a crack having an
opening width of 2 mrn or larger was generated by the con~pressiont est using a
crank press, so that the hot workability was also poor. Also, the area ratio of
ferrite was higher than the range defined in the present invention. so that t11e
chip ciisposal ability was poor, and therefore the rnachinability was poor.
iol-121
In test Xo. 18, the contents of Si, Cr and Ti of deel 18 were higher than
the values defined in the present invention. and moreover. Fn2, that is, [Cr/(Si
+ 21In)l was also higher than the range defined in the present invention.
Therefore, the bending fatigue strength was as low as 430 LlPa. and the target
coulri not be met. ,Use, the area ratio of ferrire was lo~x-er than the range
defined In the present invention, so that the cutting resistance was large, and
therefore the machinability was poor.
l o l - ~ ~ l
In test No. 19, Fn2, that is, [Cr/(Si + 2Mn)l of steel 19 was lower than
the range defined in the present invention. Therefore, the bending fatigue
strength was as low as 490 MPa, and the target could not be met.
[01441
In test No. 20, Fnl, that is, [Mn/S] of steel 20 was lower than the range
defined in the present invention. Therefore, the bending fatigue strength was
as low as 490 MPa, and the target could not be met.
[0145]
In test No. 21, Fnl, that is, [Mn/S] of steel 21 was higher than the range
defined in the present invention. Therefore, the bending fatigue strength was
as low as 490 MPa, and the target could not be met.
[Industrial Applicability]
10 1461
The case hardening steel material of the present invention is low in
component cost, has good hot workability, and also is excellent in
machinability. Moreover, a carburized part manufactured by using this case
hardening steel material as a raw material has a good bending fatigue
strength and good wear resistance, which are evaluated with the carburized
part produced by using SCM420H of "chromium-molybdenum steel" defined in
JIS G 4052 (2008) as a raw material steel being a reference. Therefore, the
case hardening steel material of the present invention is used suitably as a
raw material of the carburized part such as a CVT pulley shaft, which is
required to have a high bending fatigue strength and high wear resistance to
reduce the weight and to increase the torque.
We claim:
[Claim 11
A case hardening steel material having a chemical composition
consisting of, by mass percent, C: 0.15 to 0.23%, Si: 0.01 to 0.15%, Mn: 0.65 to
0.90%, S: 0.010 to 0.030%, Cr: 1.65 to 1.80%, Al: 0.015 to 0.060%, and N:
0.0100 to 0.0250%, the balance being Fe and impurities;
Fnl, Fn2 and Fn3 represented by the following Formulas (I), (2), and (3)
being 25 5 Fnl 5 85, 0.90 r Fn2 I 1.20, and Fn3 2 2.20, respectively; and
the contents of P, Ti and 0 in the impurities being P: 0.020% or less, Ti:
0.005% or less, and 0: 0.0015% or less, and
having a structure consisting of 20 to 70% in an area ratio being ferrite;
and
the portion other than the ferrite being one or more kinds of pearlite and
Fnl = MnIS
wherein, the element symbol in the Formulas (I), (2), and (3) represents the
content by mass percent of the element.
[Claim 21
The case hardening steel material according to claim 1, wherein in lieu
of a part of Fe, one or more kinds selected from Cu: 0.20% or less and Ni:
0.20% or less, by mass percent, are contained.