TITLE OF INVENTION
CASE HARDENING STEEL AND MANUFACTURWG METHOD THEREOF
5 rield of the Invention
[0001]
The present invention relates to a case hardening steel and a manufacturing
method thereof in which carburizing and quenching is perfomled after hot forming such
as hot forging, cold forming such as cold forging or form rolling, cutting, and the like
10 have been performed.
Priority is claimed on Japanese Patcnt Application No 2010-226478, tiled
October 6,2010, the content orwhich is incorporated herein by reference.
Description of Related Art
Since rotating parts such as gears or bearings, or rotation transmission parts such
as constant velocity joints or shafts need hardness in their surfaces, carburiz~nga nd
quenching is performed on these parts. For example, these carburized parts are
-
manufactured by forming medium carbon alloy steel for mechai~ical structural use, which
20 is defined in JIS G 4052, JTS G 4 104, JIS G 4105, JIS G 4106, or the like, into a
predetermined shape through plastic foiming such as hot ihrging, wain1 forging, cold
forging, or form rolling, or cutting, and by carburizing and quenching the formed steel.
[0003]
When the carburized parts are manufactured, accuracy of the shape of the parts
25 may be deteriorated by heat treatment distortion due to the carb~~izinangd quenching
2
Particularly, in parts such as a gears or constant velocity joints, heat treatment distortion
becomes the cause of noise or vibration and may decrease fatigue characteristics at t11e
contact surface. Moreover, in shafts or the like, if bending is increased by the heat
treatment distortion, power transmission efficiency or fatigue characteristics are
5 adversely affected. A major cause of the heat treatment distortion is coarse grains
which are nonuniformly generated by heating while the carburizing and quenching is
being performed.
[0004]
Previously, after forging, the occurrence of coarse grains has been suppressed by
10 performing annealing before the carburizing and quenching. I-Iowever, there is a
problem in that the manufacturing costs increases if the annealing is performed.
Moreover3 since a high s~xfacep ressure is applied on rotating parts such as a gears or
bearings, deep carburizing is performed. In the deep carburizing, in order to shoiten
carbuizing time, a carburizing temperature which generally is abo~9~3t0 °C is increased
15 up to a temperature range of 990 to 1090°C. Thereby, in deep carbu~rizingc, oarse grains
are easily generated.
[0005]
In order to suppress occurrence of the coarse grains when the carburizing and
quenching is performed, the quality of the case hardening steel, that is, the quality of the
20 material before the plastic forming, is important. In order to suppress coarsening of
crystal grains at high temperatures, fine precipitates are cffective, and a case hardening
steel which uscs precipitates of Ni and Ti, AIN, or the like has been suggested (for
example, Patent Citations 1 to 5).
25 Patent Citation
3
[0006]
[Patent Citation I] Japanese Unexamined Patent Application, First Publication
NO. H11-335774
[Patent Citation 21 Japanesc Unexamined Patent Application, First Publication
5 No. 2001-303144
[Patent Citation 31 Japanese Unexamined Patent Application, First Publication
No. 2004-183064
[Patent Citation 41 Japanese Unexamined Patent Application, First P~lblication
No. 2004-204263
10 [Patent Citation 51 Japanese Unexamincd Patent Application, First Publication
No. 2005-240175
SUMMARY OF THE INVENTION
Problems to bc Solved by the Invention
15 [0007]
However, if the fine precipitates are used to suppress the occurrence of the
coarse grains, tllc case hardening steel is hardened by precipitation strengthening.
Moreover, the case hardening steel is also hardened by the addition of the alloying
elements that generate the precipitates. Thereby, in steel which can prevent the coarse
20 grains from being generated at high temperatures, a decrease in cold formability with
respect to cold forging, cutting, or the like can arise as new problems.
[oooe]
Pal-ticularly, the cutting is a processing which requires high accuracy close to the
final shape, and a slight increase in hardness significantly influences the accuracy of the
25 cutting. Therefore, when the case hardening steel is used, it is vely important not only
4
to prevent occtrrence of the coarse grains but also to view machinability (ease of cutting
of a material). Conventionally, it is lmown that addition of machinability improvemerit
elements such as Pb or S is effective in order to improve the machinability.
[0009]
5 I-Iowever, Pb is an ei~vironmentallyh azardous substance, and the addition of Pb
to steels is becoming limited in view of the importance of environmental techi~ology.
Moreover, S forms MnS or the like in the steel and improves machinability. However,
coarse MnS which is elongated by hot forming, easily becomes the starting point of a
fract~lrew hen rolling, hot forging, or cold forging is perfomled, which becomes the cause
10 of processing defects in many cases. Thereby, the addition of a large amount of S easily
decreases formability and forgeability at the time of hot rolling and cold rolling, or easily
decreases mechanical properties such as rolling fatigue.
[001 01
In the present invention, in order to be applied in carburized pai-ts which need
15 good fatigue characteristics, pai-ticularly, beariilg parts, rotating parts, gears, or the like
which need rolling fatigue characteristics, it is possible to provide the case hardening
steel which has an excellent characteristics preventing coarse grains, an excellent cold
formability, an excellent machinability, and an excellent fatigue characteristics after the
carburizing and quenching; and the manufacturing method thereof. Hcre, the case
20 hardening steel is used after the hot forn~ings uch as the hot forging, the cold forming
such as the cold forging or the form rolling, the cutting, and the carburizing and
quenching are performed
Methods for Solving the Problem
25 [OOll]
The inventors have intensively studied to solve the above problems. As a
result, if the carburizing and quenching is performed to the steel to whish Ti is added,
Ti-based precipitates act as the starting point of the fatigue fracture, and fatigue
characteristics, particularly, the rolling fatigue characteristic are easily deteriorated
5 Therefore, the inventors have obtained the following findings and completed the present
invention. First, if the Ti-based precipitates are finely dispersed by limiting the amount
of N, increasing a hot rolling temperature, or the like, it is possible to strike a balance
between both the characteristics preventing coarse grains and fatigue characteristics.
Moreover, adding S to the steel is effective in improving the machinability. However, it
10 is important to control the size and shape of sulfides by adding Ti. In addition, since 'Ti
also forms the sulfide and combines with MnS, Ti is effective in refinement of MnS.
[OOI 21
The summery of the present invention is as follows.
[0013]
15 (1) A case hardening steel according to an aspect ofthc prcselit invention
includes: by mass%, as a chemical composition, C: 0.1% to 0.5%, Si: 0.01% to 1.5%,
limited to 0.2% or less, N: limited to 0.0050% or less, P: limited to 0.025% or less, 0:
-
limited to 0.0025% or Icss, and the balance of iron and inevitable impurities, wherein the
20 number d of sulfide having an equivalent circle diameter more than 5 pm per 1 mm2 and
a mass percentage [S] of S satisfy: d 5 500 x [S1 + 1.
[0014]
(2) The case hardening steel according to (I), may f~u-theirn clude, by mass%, as
the chemical composition, at least one sclected from: Nb: less than 0.04%, Mo: 1.5% or
25 less, Ni: 3.5% or less, V: 0.5% or less, B: 0.005% or less, Ca: 0.005% or less, Mg:
6
0.003% or less, and Zr: 0.005%) or less.
[0015]
(3) In the case hardening steel according to (2), [AI]/[Ca] which is a ratio of a
mass perccntage [All olAl to a mass percentage [Ca] of Ca may be 1 or more and 100 or
5 ltss.
[OO 161
(4) In the case hardening steel according to any one or (1) to (3). the maxim~rsn
equivalent circle diamcter D pm of the sulfide and the mass percentage [S] of S may
satisfy: D 5 250 x [S] t 10.
10 [00 171
(5) In the case hardening steel according to ally one of (1) to (4), the amount of
Mn may be 1 .O% or less, and [Mn]l[S] which is a ratio of a mass percentage [S] of S to a
mass percentage [Mn] of Mn may be 100 or less.
[00 181
15 (6) In the case hardening steel according to any one of (1) to (S), the ratio of
bainite may be 30% or lcss in the microstmcture.
[00 191
(7) In the case hardening steel according to any one of(1) to (6), the maximum
equivalent circle diameter of Ti-based precipitates may be 40 pm or lcss.
20 [ooao]
(8) A method of manufacturing a case hardening steel according to another
aspect of the present invention includes, casting steel having a chemical composition
which contains: by Inass %, C: 0.1% to 0.5%, Si: 0.01% to 1.5%, Mn: 0.3% to 1.8%, S:
0.001% to 0.15%, Gr: 0.4% to 2.0%, Ti: 0.05% to O.2%, Al: limited to 0.2% or less, N:
25 limited to 0.0050% or less, P: limited to 0.025% or less, 0: limited to 0.0025% or less,
7
and the balance of Fe and inevitable impurities, at an average cooling rate of 12 to 100
"Glmin; maintaining the steel in a soalcing temperature range of 1250°C to 1 320°C for 3
to 180 min; hot-rolling the steel so that a finish rolling is performed in a finishing
temperature range of 840°C to 1000°C after heating the steel to a temperature range of
5 11 50°C to 1320°C; and cooling the steel so that the average cooling rate in a temperature
range of 800°C to 500°C is 1 "CI s or less.
[0021]
(9) In the method of manufacturing the case hardening steel according to (8), the
chemical composition may fiirther contain, by mass %, at least one selecled from Nb:
10 less than 0.04%, Mo 1.5% or less, Ni: 3.5% or less, V: 0.5% or less, B: 0.005% or less,
Ca: 0.005% or less, Mg: 0.003% or less, and Zr: 0.005% or less.
[0022]
(10) In the method of manufact~uingt he case hardening steel according to (9),
IAl]/[Ca] which is a ratio of a mass percentage [All of Al to a mass percentage [Ca] of
15 Ca may be 1 or more and 100 or less.
[0023]
(11) In the method of manufacturing the case hardening steel accord~ngto any
one of (-8 ) to (1 O), the amount of Mn may be 1.0% or less, and [Mn]l[S] which is a ratio
of a mass percentage [S] of S to a mass percentage [Mn] of Mn may be 100 or less.
20
Effects of the Invention
[0024]
The case hardening steel according to the present invention has excellent fatigue
characteristics after thc carburizing and quenching, and excellent lhrmability such as
25 forgeability, machinability, or the like. That is, in the case hardening steel according to
?
the present invention, in the hot forging and the subsecluent cutting, improved formability
is oblained, coarsening of the crystal grain can be suppressed even though carburizing is
performeci under a condition of higher tenlperature and shorter time than conventional at
the time of the carburizing, and improved fatigue characteristics can be obtained.
5 &,doreove~in; the case hardening steel according to the present invention, cold
deformation characteristics are improved even when the cold forging is performed,
abnormal grain growth of the crystal grain in the carburizing can be suppressed even
when normalizing after the cold forging is sltipped, and deterioration in accuracy of
dimension by quenching distortion and deterioration in the fatigue strength caused by this
10 are significantly decreased. In addition, in the case hardening steel according to the
present invention, the conventional problem that the machinability decreases if various
alloyi~lge leme~ltsa re added so as to prevent the occurrence of coarse grains is solved,
high accuracy in the pan shape can be achieved, and tool life becomes longer.
[00'25]
15 That is, in the parts in which the case hardening steel according to the present
invention is used as the material, even when high temperature carburizing is performed
or normalizing is sltipped before the carburizing, it is possible to prevent coarse grains
from being generated, sufficient strength characteristics such as rolling fatigue
-
characteristics or the like, can be obtained, and therefore, the present invention
20 significantly contributes to the industry.
[0026]
Speckfically, when the case hardening steel according to the present invention is
used, processes shown in FIG. 1 are assumed. In addition, when hot forging is
performed, carburizing is perfornicd at a higher temperature than conventional alter
25 cutting, and the carb~~izinisg c ompleted for a shorter time than conventional. In
9
addition, when cold forging is performed, in order to avoid an abnormal grain growth at
the time of the carb~rizingi,n general, normalizing is performed after the cold forging
However, when the case hardening steel according to the present invention is used, the
normalizing can be skipped, and high performance can be achieved with carhurized parts
5 such as gears or bearings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027]
FIG. 1 is a diagram showing an example of an outline in a process of hot
10 forming (hot forging) or cold rorming (cold forging), cutting, and carburizing and
quenching which are assumed when a case hardening steel according to the prescnt
invention is used.
FIG. 2A is a diagram illustrating a balance between machinability and cold
formability of the case hardening steel when the amo~mot f S and a morphology of
15 sulfide are changed in a steel equivalent to SCr 420.
FIG. 2B is a diagram illustrating a balance between machinability and cold
formablity of the case hardening steel when the amount of S and a morphology of sulfide
are changed in a steel equivalent to SCM 420. -
FIG. 3 shows a diagram showing a position in which cooling rate is measured
20 during solidification of steel.
FIG. 4. is a diagram of a test piece which is used in an upsetting test in which hot
forging is assumed
FIG. 5 is a diagram of a test piece which is used in an upsetting test in which
cold forging is assumed.
25 FIG. 6 is a diagram showing an example of a relationship bctween an average
10
cooling rate in a bloom and an average area of MnS
FIG. 7 is a flow chart showing an example of a method of manuhctuing the
case hardening steel according to an embodiment of the present invention.
5 DETAILED DESCRIPTlON OF THE INVENTION
[ooas]
Coarsening of crystal grains due to carburizing and quenching is prevented by
s~~ppressinggra in growth using precipitates as pinning particles. Particularly, finely
precipitating Ti-based precipitates which are mainly composed of TiC and TiCS during
10 cooling after hot forming are significantly effective in preventing occurrence of coarse
grains. Moreover, in order to prevent occurrence or coarse grains, it is preferable to
finely precipitate Nb-based precipitates such as NbC in a case hardening steel
[0029]
However, if the amount of N contained in steel is largc, coarse TiN gcnerated in
15 caqting is not dissolved in heating of hot rolling and hot forging, and may remain in large
quantities. If the coarse TiN remains in the steel, Tic, TiCS, and NbC are precipitated
by TiN acted as precipitation nuclei at the time of the carburizing and quenching, which
may hinder fine dispersion of the precipitates. Therefore, in order to prevent occurrence
-
ofthe coarse grains at the time of the carburizing and quenching by fine Ti-based
20 precipitates or Nb-based precipitates, it is important to dccrease the amount of N and to
dissolve the Ti-based precipitates or the Nb-based precipitates during heating in hot
forming.
[0030]
In a method of manufacturing the case hardening steel, after a steel is cast by
25 controlliiig solidification rate (cooling rate: 12 to 100 'Clmin) in continuous casting, first,
11
it is necessary to uniformly heat the steel in a heating temperature of 1250°C to 1320°G
so that precipitates of Ti, Nb and A1 are dissolved in the steel. Particularly, it is
inlportant to increase the heating temperature of the hot forming such as hot rolling or
hot forging to 1 150°C to 1320°C and to dissolve the Ti-based puecipitatcs and the
5 Nb-based precipitates in the steel. Next, after the hot forming, that is, after the hot
rolling or hot forging, it is necessary to perforin a slow cooling at a cooling rate of 1 "CIS
or less in a precipitation temperature range of the Ti-based precipitates and the Nb-based
precipitates. As a resrrlt, it is possible to finely disperse the Ti-based precipitates and
the Nb-based precipitates in the case hardening steel. In addition, if ferrite grains in the
10 steel before the carburizing and quenching are too the, coarse grains are easily generated
during the carburizing heating. Thereby, in order to not generate the fine ferrites, it is
necessary to control a finishing temperature of the hot rolling or the hot forging to 840°C
to 1000°C.
[003 11
15 Moreover, when the case hardening steel according to the present invention is
processed into part shapes such as a gears, for example, as shown in FIG. 1, before the
carburizing and quenching after the bloom subjected to the continuous casting is rolled,
the hot forging or the cold forging and the cutting (in the case of gears, gear forming is .-
performed by gear cntting) are perlormcd. At this time, sulfide such as MnS decreases
20 cold forgeability. IIowever, the sulfide is significantly effective in cutting (for example,
gear cutting). That is, the sulfide in the case hardening steel (workpiece material)
suppresses change in the tool shape due to abrasion of a cutting tool, and therefore, the
sulfide exhibits an effect which extend the so-called tool life. Particularly, in the case of
precise shapes such as gears, if the cutting tool lik is shori, it is impossible to stably fo~nl
1%
the gear shape. Thereby, the cutting tool life influences not only the manufactcrring
efficiency or the costs but also the shape accuracy of the parts.
[0032]
Therefore, in order to enhance machinability, it is preferable to generate the
5 sulfide in the steel. On the other hand, in the hot rolling or the hot forging, particularly,
in many cases, sulfide such as coarse MnS is elongated. Moreover, if the size (length)
of the sulfide increases, there is a high probability that the sulfide is found as defects in
the parts, and performance in the part is decreased. Therefore, it is important to control
not only the size of the sulfide, but also the shape of the sulfide so that the sulfide is not
10 elongated. Moreover, in order to snppress coarsening of the sulfide, it is preferable to
control the solidification rate during the casting, The cooling rate (average cooling rate)
at the time of casting greatly influences the size of MnS, the size of MnS decreases as the
cooling rate increases, and on the contrary, the size of MnS increases as the cooling rate
decreases. Thereby, as described below, from the standpoint of the size of MnS, the
15 cooling rate should be increased. On the other hand, with the fast cooling rate, cracks
are generated on the surface of the bloom, and therefore, in some cases, problems occurs
during casting, or it is liecessary to remove defects by conditioning after the casting.
[0033]
-~
in order to effectively and finely generate the sulfide mainly including MnS, a
20 range in the solidification cooling rate (average solidification cooling rate) is controlled
to 12 "Cimin to 100 OCimin, When the cooling rate is less than 12 "Clmin, since the
solidification is too slow, the crystallized sulfide mainly including MnS coarsens, and it
is difficult to finely disperse the sulfide so as to satisfy Equation 2 described below. In
addition, when the cooling rate is more than 100 "Glmin, the density of the sulfide
25 mainly including tlne MnS generated is sat~uatedh, ardness of the bloonl (steel before
13
rolling) increases, and there is a concern that cracks may be generated. Accordingly, the
cooling rate during the casting needs to be 12 "Clmin to 100 'Clmin. Particularly, in
order to more reliably and finely disperse the sulfide, it is preferable that the cooling rate
during casting be 15 "Clmin to 100 'Clmin. The cooling rate can be obtained by
5 controlling the size of a mold cross section, a casting rate, or thc like by appropriate
values. This cooling control can be applied to both the continuous casting method and
an ingot-malting method.
[0034]
Here, the solidification cooling rate means a rate when being cooled from a
10 liquidus temperature to a solidus temperature on a center line in a width of thc bloon~a nd
in a portion (114 portion) of 114 in thickness of the bloom in a cross section (cross section
perpendicular to casting direction) ofthe bloom shown in FIG. 3. The solidification
cooling rate can be obtained by Equation 1 below &om a secondary dendrite arm spacing
of a solidification microstructrre in the cross section of the bloom after the solidification.
15 [0035]
Rc = (hzl740)- 110.41 (Equation 1)
Here, Rc means the solidification cooling rate ("Clmin), A2 means the spacing
(pm) of- t he secondary dendrite arm.
[0036]
20 In order to decrease the soft sulkides such as MnS by a chemical composition
control in steel, adding Ti to the steel and generating the Ti-based sulfide such as TiCS
are effective. However, if the soft MnS decreases, the added S does not contribute to
improvement of machinability. Therefore, in order to improve the machinability, it is
importaut to control the size and the shape of the soft sulfide in the molten steel to which
;4
not only S but also Ti is added. Thus, it is preferable to control the size and shape orthe
sulfide by adding Ti required for suppressing the grain growth and refining the sulfide,
and controlling the amount of S.
[0037]
5 The machinability and the cold formability will be further described.
[0038]
During the cold forming, the sulfide mainly including MnS is deformed and
becomes a starting point of fractures. Parlicularly, coarse MnS decreases cold
forgeability such as limiting compressibility. Moreover, if the MnS in the steel coarsens,
10 anisotropy in characteristics of the steel is generated according to the shape of the MnS.
In order to apply the case hardening steel to various and complicated parts, stable
mechanical properties are required it1 all directions. Thereby, in the case hardening steel
according to the present invention, it is preferable to refine the sulfide mainly including
MnS and to control the shape of the sulfide to a substantially spherical shape. In
15 addition, it is preferable that a change in the shape before and after the cold forming w~ch
as forging be decreased.
[0039]
On the other hand, fiorn the standpoint of machinability, it is important to
-~
increase the amount of S. The tool life during machining is improved by adding S, and
20 the effect is determined by the total amount of S and is not easily subjected to the
influence of the shape of sulfide. Thereby, both the cold forgeability and the
machinability (tool life) can be achieved by increasing the amount of the added S and
conlrolling the shape of the sulfide. In the case hardening steel, it important not only to
prevent coarse grains ftom being generatcd during the carburizing and quenching but
25 also to secure the cold formability and the macliinability. If the amount of S increases,
15
the machinability is improved, but the cold formability decreases. Here, in the case of
comparing the steel including the same amount of S, it is also important to secure further
improved cold formablity.
[0040]
5 FIGS. 2A and 2B shows a relationship between the machinabilily and :he cold
formability in the case hardening steel having good pinning characteristics which
suppress the coarse grains from being geiierated during the carbwizing and quenching.
Here, in FIG. 2A, the amount of S is changed in a steel equivalent to SCr 420.
Moreover, in FIG. 2B, the amount of S is changed in a steel equivalent to SCM 420 in
10 which Mo is added to the steel equivalent to SCr 420. In the present invention, it is
possible to achieve both hot or cold forgeability (limiting conlpressibility) and
machinability (drill machinability VLlooo)w hile maintaining good pinning characteristics
(generation temperature of coarse grains is more than 1000°C). In FIGS. 2A and 2B, a
balance between the machinability and the cold formability is improved as the steel is
15 positioned in the upper right, and the balance is changed according to the lund of the
steel (particularly, the amount of element which enhances hardenability).
[0041]
Hereinafter, the case hardening steel accordiug to an embodiment of the prescnt
invention will be described in detail. First, chemical components will be described.
20 Hereinafter, mass % (the amount of chemical component) in a chemical composition is
denoted by only %.
[0042]
[Cl
C is an elenlent which increascs strength of the steel. In order to secure
25 sufficient tensile strength, the amount of C needs to be 0.1% or more, and is preferably
16
0.15% or more. On the other hand, if the asnoimt of C is more than 0.5%, the cold
formability is deteriorated by significant hardening, and therefore, the amount of C needs
to be 0.5% or less. Moreover, iir order to secure toughness of the core after carburizing,
it is preferable that the amount of C be 0.4% or less and it is more preferable that the
5 amount of C be 0.3% or less.
[0043]
[Sil
Si is an element which is effective in deoxidation of steel and the amount of Si
needs to be 0.01% or more. Moreovel; Si is an element which strengthens the steel and
10 improves hardenability, and it is preferable that the amount of Si be 0.02% or more. 111
addition, Si is an element which is effective in increasing grain boundary strength, and Si
is an element which is en'ective in extending scrvice life of bearing parts and rotating
parts by suppressing the microstructure change or deterioration of the material in the
rolling fatigue process. Thereby, in a case of obtaining higher strength, it is more
15 preferable that the amount of Si be 0.1% or morc. Pahcularly, in order to enhance
rolling fatigue strength, it is preferable that the amount of Si be 0.2% or more.
100441
On the other hand, if the amount or Si is more than 1.5%, cold formability such
-
as cold lorging is deteriorated by hardening, therefore the amount of Si needs to be 1.5%
20 or less. Moreover, in order to enhance cold formability, it is preferable that the amount
of Si be 0.5% or less. Particularly, when the cold forgeability is emphasizcd, it is
preferable that the amonnt of Si be 0.25% or less.
[004 51
[Mill
25 Mn is a11 elenicnt which is effective in deoxidation of the steel and enhaices
17
stlength and hardenability 01 the steel and the amount o1M1 needs to be 0.3% 01 more.
On the other hand, if the amount of Mn is more than 1.8%, cold forgeability is
deteriorated due to an increase in the hardness, therefore the amount of Mn needs to be
1.8% or less. A preferable range of the amount of Mn is 0.5 to 1.2%. Moreover, when
5 cold forgeability is emphasized, it is preferable that thc amount of Mn be 0.75"/0 or less.
In addition, Mn is an element which improves hardenability, However, in an aspect of
generation of the sulfide, Mn is an element which generates MnS in the steel along with
S. Mn has an effect which hardens the steel by increasing a fraction of bainite from an
aspect of hardenability, and Mn decreases cold forgeability or machinability ftom an
10 aspect of formability. Thereby, in the aspecl of generation of the sulfide, if the amount
of Mn increases and [Mn]/[SI which is a ratio of an amount [S] of S with rcspect to an
aniounl [Mn] of Mn increases, coarse MnS is easily generated. Particularly, in order to
decrease the fraction of bainite and sufficiently secure cold forgeability, it is preferable
that the amount of Mn be 1.0 or less and [Mn]/[S] be 100 or less. Morcover, [Mn]/[S]
15 may be 2 or more.
[0046]
[Sl
S is an element which forms MnS in the steel and improves machinability. In
-
order to enhance the machinability, the amount of S needs to be 0.001% or more and it is
20 preserable that the amount of S be 0.01% or more. On the other hand, if the amount of
S is more than 0.1576, intergranular embrittlement is generated by grain boundary
segregation, therefore the amount of S necds to be 0.15% or less. In addition, for
considering a high strength part, it is preferable that the amount of S be 0.05% or less.
Moreover, in regard to strength, cold fo~mabilitya, nd the stability, it is more preferable
25 that the amount of S be 0.03% or less.
Moreover, conventionally, in the bearing parts and the rotating parts, since MnS
deteriorates the rolling fatigue life, it was considered that there is a need to dccrease S.
However> the inventors fonnd that the aniount of S greatly influences machinability for
5 the improvement, and the shape of the sullide greatly influences cold formabiliiy for the
improvement. In the embodiment, the shape of the sulfide is controlled by the addition
of Ti or Nb, the control of cooling rate (solidification cooling rate) at the time of
solidification, and heating for soaliing. Ti forms complex sulfide including Mn and the
complex sulfide does not extend like simple MnS. Moreover, if the solidification
10 cooling rate decreases, coarse MnS is generated in the liquid phase before the
solidification is completed. In addition, since uniform heating generates the complex
sulfide or flnely generates MnS which is precipitated from the solute Mn and solute S,
the heating for soalting is important. Since MnS is not sufficiently generated at a low
temperature, FeS or the lilte is generated, the steel is embrittled, and the required amount
15 of MnS cannot be secured. Thereby, it is preferable that the amount of S be 0.01% or
more. When machinability is emphasized, it is inore preferable that the amount of S be
0.02% or more.
[0048]
20 Cr is an effective element which improves strength and hardenability of the steel
and the amount of Cr needs to be 0.4% or more. In addition, in the bearing parts and
the rotating parts, Cr increases the amount of residual y on the snrface after cab~~rizing,
suppresses the microstructure change and the material deterioration in the rolling fatigue
process, and iherefore is effective in an extended servicc life. Thereby, it is preferable
25 that the amount of Cr be 0.7% or more and it is more preferable that the amount of Cr be
19
1 .O% or more. On the other hand, if 2.0% or more of Cr is added to the steel, cold
lormability is deteriorated due to increase of hardness, therefore the amount of Cr needs
to be 2.0% or less. In order to enhance cold forgeability, it is preferable that the amount
of Cr be 1.5% or less.
5 [0049]
[Ti1
Ti is an element which generates precipitates such as carbide, carbosulfide,
nitride in the steel; In order to prevent coarse grains from being generated during the
carburizing and quenching using fine Tic and TiCS, the amount of Ti needs lo be 0.05%
10 or more and it is preferable that the amount of Ti be 0.1% or more. On the other hand,
if more than 0.2% of Ti is added to the steel, since cold formability is significantly
deteriorated by the precipitation hardening, the amount of Ti needs to be 0.2% or less.
Moreover, in order to improve rolling fatigue characteristics by controlling the
precipitation of TiN, it is preferable that the amount of Ti be 0.15% or less. In addition,
15 it is possible to refine the precipitates of MnS by adding Ti.
[0050]
[All
A1 is a deoxidizing agent and the amount of A1 is preferably 0.005% or more.
Ho\vevel; the amount of Al is not limited to this. On the other hand, if the amount of Al
20 is more than 0.2%, AIN is not dissolved by heating of hot forming and remains in the
steel. Thereby, coarse A1N acts as precipitation nuclei of precipitates of Ti or Nb, and
geueration of fine precipitates is inhibited. In order to prevent coarsening of crystal
grains during the carburizing and quenching, the amount of Al needs to be 0.2% or less.
If the amount of A1 is a range of 0.05% or less, heat treatment characteristics during
25 nolrmalizing or carburizing and quenching are not greatly changed compared to the
20
conventional steel, theiefore for practical purposes, it is preferable that the amount of Al
be 0.05% or less. On the other hand, since Al has an effect which improves
machinability, in order to obtain inore improved machinability, it is preferable that the
amo~mot fAl be 0.03% or more. Ifthe balance between the heat treatment
5 characteristics and the machinability is considered, it is preferable that thc amount of A1
be 0.15% or less.
[0051]
If coarse AlN remains during heating of hot forming, similar to TiN, the coarse
A1N inhibits generation of flne particles which act as pinning particles. Therefore,
10 realistically, limiting the precipitation amount of AlN included in the case hardening steel
is effective. Ilthe precipitation amount of AID1 is excessive, since coarse grains are
easily generated during the carbmizing and quenching, the precipitation amount of AlN
of the case hardening steel is prcferably limited to 0.01% or less and is more preferably
limited to 0.005% or less.
15 [0052]
In order to suppress the precipitation amount of A1N of the case hardening steel,
promoting the solution Ireat treatment by increasing heating temperature oS11oL forming is
effective. Since the temperature at whichA1N is dissolved in the steel is lower than the
-
temperature at which 'TiN is dissolved, A1N is more easily dissolved during heating ol the
20 hot rolling compared to TiN. In the embodiment, since the amount of N of the case
hardening steel is limited, if the steel is heated to the temperature at which thc AlN is
dissolved, Ti-based precipitates and Nb-based precipitates can also be dissolved.
Specifically, since the steel is sufficiently heated in thc hcat treatment of the
25 very early stage such as a stage immediately after casting and AlN is dissolved, harmf~il
2 1
influences in the subsequent rolling, forging, and carburizing can be suppressed
Thereby, the bloom is sufficiently heated to 1250°C or more and held (soalted) at a stage
in which a billet or the like is manufactured from a bloom. The higher temperature
(soaking temperature) is preferable, and it is preferable that the steel is heated at a
5 temperature more than 1250°C and soalted. If the soalcing temperature is more than
1350°C, since materials of a heating furnace such as a refractory are significantly
damaged, the soalcing temperature needs to be 1320°C or less.
[0054]
Moreover, during hot forming after the rolling or at the time of the subsequent
10 cooling, the precipitation rate or the gromh rate ofA1N is slower compared to those of
Ti-based precipilates and Ni-based precipitates. Thereby, by preventing rcsidrlal of AlN
during heating of hot forming, the precipitation amount of AIN which is included in the
case hardening steel can be decreased, and it is possible to prevent coarse grains from
being generated d~uingca rburizing and cluenching using fine Ti-based precipitates and
15 Nb-based precipitates.
[0055]
Moreover, the precipitation amount of AlN can be measured by perfolming
chemica-l analysis of extraction residue of the steel. The extraction residue is extracted
by dissolving the steel in bromine methanol solution and by filtering the solution with a
20 filter of 0.2 ym. In addition, even when the filter of 0.2 pm is used, since the filter
generates clogging by precipitates at the filtering process, line precipitates of 0.2 pm or
less are also extracted.
[0056]
[Nl
22
N is an element which generates nitride. In order to suppress generation of
coarse TiN or AlN, the amount of N is limited to be 0.0050% or less. This is because
the coarse TiN or AIN acts as precipitation nuclei of Ti-based precipitates mainly
including Tic or TiCS, Nb-based precipitates mainly including NbC, or the like and
5 inhibits dispersion of fine precipitates. Thereby, it is preferable that the amount of N be
0.0040% or less and it is more preferable that the amount of N be 0.0035% or less. Thc
lower limit of the amount of N is not particularly required to be limited and is 0%.
[0057]
[PI
10 P is an impurity and is an element which increases deformation resistance during
cold forming and deteriorates toughness. If excessive P is contained in the steel, cold
forgeability is deteriorated. Therefore, it is necessary that the amount of P is limited to
0.025% or less. Moreover, in order to improve fatigue strength by suppressing
embrittlement of the clystal grain boundary, it is preferable that the amount of P be
15 0.015% or less. The lower limit of the amount ofP is not partieulaily required to be
limited and is 0%.
[0058]
[Ol
-
0 is an impurity, forms oxide inclusions in the steel, and damages fo~mability.
20 Therefore, the amo~mot f O is limited to 0.0025% or less. In addition, since the case
hardening steel ofthe embodiment contains Ti, oxide inclusions including Ti ale
generated, and TiC is precipitated on the oxide inclusioris which act as the precipitation
nnclei. If the oxide inclusions increases, generation of fine Tic during hot foi~l~inmga y
be suppressed. Thereby, in order to suppress coarsening orthe crystal grains during the
25 carburizing and quenching by finely dispersing the Ti-based precipitates mainly
23
including Tic and TiCS, it is preferable that the amount of 0 be limited to 0.0020% or
Icss. Moreover, in the bearing pats and thc rotating parts, rolling fatigue f r a c t ~m~ eay
be generated from the oxide inclusions which act as the starting point. Thereby, when
the case hardening steel is applied to the bearing parts or the rotating parts, in order to
5 improve the rolling lifetime, it is more preferable that the amount of 0 be limited to
0.0012% or less. The lower limit of the amount of 0 is not particularly required to be
limited and is 0%.
[0059]
Moreover, the chemical composition which includes the above-descr~bedb asic
10 chemical components (basic elements), and the balance of Fe and inevitable impurities is
the basic composition according to the present invention. However, in addition to the
basic composition (instead of a portion of Fe in the balance), the chemical composition
may f~u-theri nclude the lbllowing elements (optional elements) if necessary in the
present invention. Morcove~e, ven though the optional elements are inevitably mixed
15 into the steel, the elements do not damage the effects according to the present
embodiment.
[Nbl -
In addition to the above-described basic elements, in order to suppress
20 occurrence of coarse grains during the carburizing and quenching, similar to Ti, it is
preferable to add Nb which generates carbonitride
Similar to Ti, Nb is a1 element which combines with G and N in the steel and
generates carbonitride. According to addition of Nb, thc effect which suppresses
25 occurrence of the coarse grains due to the Ti-based precipitates is further remarkable.
2 4
Evcn though the amount of the added Nb is minute, compared to the case where Nb is
not added, Nb is significantly more effective for preventing the coarse grains. This is
because Nb is d~ssolvedin the Ti-based precipitates and suppresses coarsening of the
Ti-based precipitates. In order to suppress occurrence of coarse grains at the time of
5 heating of the carbruizing and quenching, it is prefevablc that the amount of Nb be
0.005% or more. However, the amount of Nb is not limited thereto. On the other hand.
if excessive Nb of 0.04% or morc is added to the steel, in the hot forming, the steel is
embrittled, and the excessive Nb causes flaws easily. In addition, in the cold forming,
the steel is hardened and cold forgeability, machinability, or carburizing characteristics
10 may be deteriorated. Therefore, it is preferable that thc amount of Nb be less than
0.04%. When cold formability such as cold forgeability and machinability are
emphasized, it is more prererable that the a ~ n o ~o~f nNtb be less than 0.03%. Moreoverr,
when carburization is emphasized in addition to the formability, it is preferable that the
anlount of Nb bc lcss than 0.02%.
In addition, it is linown that even a minute amount of Nb influences hot ductility,
and in the steel used in gears, the hot ductility becomes morc sensitive to thc amount of
Nb. Thereby, addition of Nb is effective in the control of Ti-based precipitates or
-
microstructures. Howevel; also from the standpoint of ductility in rolling or hot
20 forming such as hot forging, the addition of Nb should be controlled. In this way, since
the effect olthe addition of Nb is seen by the addition ofNb of 0.005% or more,
excessive addition of Nb such as more than 0.04% should be avoided. In addition, in a
case wherc alloy cost is decreased, it is not necessary to intentionally add Nb, and the
lower limit of thc amount of Nb is 0%.
25 [0063]
25
Moreover, in order to achieve both characteristics of preventing coarse grains
(pinning characteristics) and formability, it is preferablc to adjust a total of Nb amount
[Nb] and Ti amount [Ti]. The preferable range of [Ti] + mb] is 0.07% or more and less
than 0.17%. Particularly, in parts in which high temperature carburizing or cold forging
5 is applied, a more preferable range of [Ti] + mb] is more than 0.09% and less than
0.17%.
[0064]
In addition, in order to improve the strength or hardenability of the steel, one or
more selected from Mo, Ni, V, and B may be added.
10 [0065]
LMol
Mo is an element which enhances strength and hardenabiiity of the steel and
may be added in the steel, il'necessary. Also in order to improve the extended service
life by increasing the amount of thc residual y of the surface layer of the carburized paits
15 and further by suppressing the microstructure change and the material deterioration at the
rolling latigue process, Mo is effective. However, ilrnore than 1.5% of Mo is added to
the steel, machinability and cold forgcability may be deteriorated due to an increase of
hardness. Therefore, it is preferable that the amount of Mo be 1.5% or less. Since Mo -
is an expensive element, from the standpoint of the manufacturing costs, it is preferable
20 that the amount of Mo be 0.5% or less. In this way, in order to decrease the alloy cost,
it is not necessary to intentionally add Mo to the steel, and the lower limit of the amount
of Mo is 0%. In addition, when Mo is added and used, it is preferable that the amount
of Mo be 0.05% or more and it is more preferable ihat the amount of Mo be 0.1% or
more.
25 [0066]
26
[Nil
Similar to Mo, Ni is an element which is effective in improvement of strength
and hardenability ofthe steel and may be added to the steel, ifnecessary. However, if
more than 3.5% of Ni is added to the steel, since machinability and cold forgeability are
5 deteriorated due to an increase of hardness, it is preferable that the amount of NI be 3.5%
or less. Since Ni also is an expensive element, from the standpoint of the
manufacturing costs, it is prelerable that the amount of Ni be 2.0% or less and it is more
preferable that the amount of Ni be 1 .O% or less. In this way, in order to decrease the
alloy cost, it is not necessary to intentionally add Ni to the steel, and the lower limit oF
10 thc amo~mot f Ni is 0%. In addition, when Ni is added and used, it is preferable that the
amount of Ni be 0.1% or more and it is more prelerable that the amount of Ni be 0.2% or
more.
[0067]
[Vl
15 V is an element which improves the strength and the hardeilab~l~itlyd issolved
in the steel and may be added to the steel, if necessary. If the amount of V is more than
0.5%, since the machinability and the cold forgeability are deteriorated due lo an increase
of hardness, it is preferable that the amount of V be 0.5% or less and it is more preferable
that the amount of V be 0.2% or less. In order to decrease the alloy cost, it is not
20 necessary to intentionally add V to the steel and the lower limit of the amount of V is 0%.
In addition, when V is added and used, it is preferable that the amount olV be 0.05% or
more and it is more prel'erable that the amount of V be 0.1% or more.
[006S]
P I
B is an element which enhances the hardenability of thc steel by addition of a
2 7
minute amount and may be added to the steel, if necessary. Moreover, B generates iron
boron carbide in a cooling process after hot rolling, increases growth rate of ferrite, and
promotes softening. In addition, B improves the grain boundary strength of the
carb~~izepcalr ts and also is effective in improvement of fatigue strength and impact
5 strength. However, if more than 0.005% of B is added to the steel, the above effect is
saturated and the impact strength is deteriorated, therefore it is preferable that the amount
of B be 0.005% or less and it is more preferablc that the amount ol:B be 0.003% or less.
In order to decrease the alloy cost, it is not necessary to intentionally add B to the steel,
and the lower limit of the amount of B is 0%.
10 [0069]
In addition, in order to control deoxidation and the shape of the sulfide, one or
more selected from Ca, Mg. and Zr may be added.
[0070]
[Gal
15 Ca is a deoxidizing element wh'ich generates oxide in the steel and ma.y be added
to the steel, if necessary. In general, oxide in the steel due to deoxidation of A1 is A1203.
Since A1203 is hard, A1203 has harmful influences which decrease machinability
However, if Ca is added, A1203 which is a basic oxide and Ca generate Al-Ca based
-
complex oxide and the steel can be slightly softened. Thercby, a decrease in
20 machinability can be suppressed due to deoxidation of Al. Moreover, also in the steel
malting stage, adhesion of All03 to the refractory can be suppressed, and harmful
influences such as nozzle clogging can be suppressed.
In addition, since Ca slightly hardens MnS due to the fact that Ca and MnS
generate complex sulfide, elongation of MnS during rolling or forging is suppressed, and
25 craclts which is formed by thc sulfide which acts as their starting point during cold
28
forging c'm be s~lppressed, However, if too much Ca is added to the steel, since a large
amount of CaS is generated and the stcel becomes hard, machinability is adversely
affected. In this way, Ca is an elenlent effective in both aspects of control of oxide as an
countermeasure against erosion and control of sulfide as a measure against forgi~lgc rack.
5 In order to obtain an eSSect of Ca addition, the amount of Ca is preferably 0.0003% or
more, more preferably 0.0005% or more, and most preferably 0.0008% or more.
Moreover, from the standpoint of machinability, the amount of Ca is preferably 0.005%
or less, more preferably 0.003% or less, and most preferably 0.002% or less. In
addition, in order to decrease the alloy cost, it is not necessary to intentionally add Ca to
10 the stcel, and the lower limit of the amount of Ca is 0%.
A ratio of the amount of A1 [All with rcspect to the amount of Ca [Ca] also is
important. If the [AI]I[Ca] indicating the ratio 1s exbeniely small, deoxidation due to Al
is insufficient, and Ca is consumed as oxide. In this casc, the effect of Ca wit11 respect
to the control of the sulfide is insufficient. On the contrary, if [Al]I[Cal is extremely
15 large, an effect of Ca with respect to the control of oxide is insufficient. Therefore, in
the case where Ca is added to the steel, a range of [Al]/[Ca] is preferably 1 or more and
100 or less and more preferably 6 or more and 100 or less.
[0071]
[Mzl and [Zrl
20 Mg and Zr are elenlents which generate oxide and sulfide and may be added to
the steel, if necessary. Since Mg and Zr control deformability of MnS, Mg and Zr
suppress the elongation of PvZnS due to hot lorming. Particularly, even though only
minute amounts of Mg and Zr are contained in the steel, a significant effect is exhibited.
In addition, in ordel to stabilize the amount of Mg and Zr in the steel, it is pieSerable to
25 control the amount of Ng or the amount of Zr depending on the reflactory inclrrding Mg
29
or Zr.
Mg is an elemcut which generates oxide and sulfide. Complex sulfide (Mn,
Mg)S including Mn, MIS, or the like are generated due to the fact that Mg is contained
in the steel, and elongation of MnS can be suppressed. A minute amount of Mg is
5 effective in the control of the shape of MnS, when Mg is added to the steel and
formability is enhanced, therefore it is preferable that the amount of Mg be 0.0002% or
more. In addition, oxide of Mg is finely dispersed ;md acts as a nucleation site of the
sulfide such as MnS. When generation of coarse sulfide is suppressed using oxide of
Mg, it is preferable that the amount of Mg be 0.0003% or more. Moreovel; if Mg is
10 added to the steel, the sulfide is slightly hard and is difticult to elongate by hot forming.
In order to control the shape of the sulfide so as to improve machinability and not to
damage cold fornlability, it is preferable that the amount of Mg be 0.0005% or more.
Moreover, the hot forging has an effcct which uniformly disperses the fine sulfide and is
effective in improvenlcnt of cold formability. In addition, in order to decrcase the alloy
15 cost, it is not necessary to intentionally add Mg to thc stecl, and the lower limit of the
amount of Mg is 0%.
On the other hand, since oxide of Mg easily floats on the molten stccl, the yield
is low, and from the standpoint of the manufacturing cost, it is preferable that the amount
of Mg he 0.003% or less. Moreover, if Mg is excessively added, a large amount of
20 oxide is generated in the molten steel, which may generate problenls in thc steel malcing
such as adhesion to the refractory or ~lozzlec logging. Therefore, it is more preferable
that the amount of Mg be 0.001% or less.
Zr is an element which generates nitride in addition to oxide and sulfide. If a
minute an~ounot f Zr is added to the molten steel, Zr is combined with Ti in moltell steel
25 and fine oxide, sulfide, and nitride are generated. Therefore, the addition of Zr is
30
significa~itlye ffective in the control of inclusions and precipitates. When Zr is added to
the steel, the morphology of inclusions is controlled, and the formability is enhanced, and
therefore it is preferable that the amount olZr be 0.000'2% or more. Moreovel; oxide,
sulfide, and nitride including Zr and Ti act as precipitation nuclei of MnS during
5 solidification. Zr and Ti penetrate to MnS which is precipitated in the periphery of the
oxide, the sdfide, and the nitride which include Zr and Ti, and deformability decreases
Therefore, in order to suppress deformation of MnS by adding Zr and prevent elongation
of MnS due to hot forming, it is preferable that the amount or Zr be 0.0003% or more.
On the other hand, since Zr is an expensive element, from the standpoint of the
10 manufacturing cost, it is preferable that the amount of Zr be 0.005% or less and it is more
preferable that the amount of Zr be 0.003% or lcss. Moreover, in order to decreasc the
alloy cost, it is not necessary to intentionally add Zr to the steel, and the lowcr limit oS
the amount of Zr is 0%.
[0072]
15 As described above, the case hardening steel according to the present
embodiment has the chemical composition which consists of the above-described basic
elements, and the balance of Fe and inevitable impurities, or the chemical co~liposition
which consists of the above-described basic elements, at least one selected from the -
above-described optional elements, and the balance Fe and inevitable impurities.
[Sulfide]
Since MnS is effective in improvement of machinability, it is necessary to secure
the number density. On the other hand, since the elongated coarse MnS damages cold
formability, it is necessary to control the size and the shape of MnS. The inventors
25 examined a relationship between characteristics regarding the sulfide, such as the amount
3 1
of S and the size and the shape of MnS, and formability, such as inachinability and cold
formability. As a rcsult, il' the average equivalent circle diameter of MnS which was
observed by an optical microscope was more than 5 pxu, it was found that the MnS
became the starting point in which cracks are generated d ~ ~ icnolgd forming. The
5 average equivalent circle diameter of MnS is a diameter of a circle which has the same
area as that of MnS and can be obtained by image analysis.
[0074]
Next, the inventors examined influences by distribution of the sulfide. Sulfide
such as MnS in hot rolled material having a diameter of 30 mnl was observed by a
10 scanning electron microscope, the relationship between characteristics of the sulfide such
as the size, the aspect ratio, and the number density and formability such as cold
formability and machinability was established. The observation of the sulfide was
performed at 112 radius portion (portion between the surface and center of hot rolled
material) of a cross section parallel to the rolling direction. 10 fields of view each
15 having an area of 50 Ltm x 50 pm were observed, and the equivalent circle diameter, the
aspect ratio, and the number of the sulfide-based inclusions in the fields of view were
obtained. In addition, the fact that the inclusions were sulfide was observed by energy
dispersive X-ray a~~alysaitsta ched to a scanning electron microscope.
[0075]
20 The number of the sulfides having an average equivalent circle diameter more
than 5 pm was nmeas~red, and the number density d was obtained by dividing the value
by the measured area. If the sulfide is finely dispersed, the sulfide can act as pinning
particles at the time of an austenite grain grow&h dunng the carburizing. Accordingly, i[
the numbel density ol'relativrly large sulfide having the equivalent circle diameter of 5
25 p1n or more is small, there is much fine sulfidc. Thus, it is possible to achieve both
12
formability with respect to forging, cutting, or the lilce, and carburizing characteristics
and fatigue characteristics. Since the number density d (nunnber/mm2) of the sulfide
(particles (number) per 1 mm2 of sulfides having equivalent circlc diameter more than 5
pm) is subjected to the influence of the amount of S, in order to achieve both the
5 machinability and the cold formability, from various tests regarding a relationship
between the number density d of the sulfide ar~dth e amount of S [S], it was found that
the number density d (number/mm2) of the sulfide was recluired to satisfy the lollowing
experimental Equation 2.
[0076]
10 d<5OO[S]+1 (Equation 2)
(Here, IS] indicates the amount (mass%) of S.)
In addition, in MnS and the complex sulfide of Mn and Ti, the sulfide of the
maximum size acts as the fracture starting point in a region to which a load is applied at
the time of the defolmation in the forging, of being used as the parts, and of the fatigue
15 after the carburizing. The trcnd is subjected to the infli~enceo f the amount of S, and if
the amount of S increases, the inaxinium size oC the sulfide increases. The maximum
sulfide which includes not only Ti-based sulfide but also Mn-based sulfide (MnS) having
small amount of Ti should be considcred.
[0077]
20 The inventors perfo~nledv arious tests regarding the relationship between the
amount of S and the maximum sulfidc size. As a result, when the maximum equivalent
circle diameter D (pm) of the observed sulfide satisfies the following Equation 3, it was
confirmed that good forgeability (hot and cold) could be obtained and good fatigue
characteristics could bc obtaixred compared to thc steel having the same amount of S
25 D 2 250 [S] + 10 (Equation 3)
3 3
(Here, [S] indicates the amount (mass %) of S.)
LO0781
In the embodiment, the size of the sulfide can be controlled so that the
maximurn equivalent circle diameter D (ym) of the sulfide satisfies Equation 3 by
5 perfolming a chemical composition control from the casting stage.
If D (ym) is more than 250 [S] + 10, forgeability and fatigue characteristics
decrease, and only the same performance as the conventional steel containing the same
amourit of S may bc exhibited. Therelore, it is preferable that the upper limit of D (pm)
be 250 [S] + 10.
10 [0079]
[Ti-basecl precipitates]
In addition, if coarse Ti-based precipitates are present in the steel, the
precipitatcs act as the starting point of contact fatigue fracture, and fatigue characteristics
may be deteriorated. Contact fatigue strength is a required characteristic of the
15 carburized paits and includes rolling fatigue characteristic and surface fatigue strcngth.
In order to enhance the coiltact fatigue strength, it is preferable that the maximum
equivalent circle diameter (maximum diameter) of the observed Ti-based precipitates be
less than - 40 ym.
[0080]
20 Next, microstructure of the case hardening steel according to the embodiment
will be described.
[0081]
[Bainite]
It is preferable that a ratio of bainite in the microstructure of the case hardening
25 steel be limited to 30% or less. This is because it is preferable to generate fine
34
precipitates in the grain boundary in order to prevent coarse grains from being generated
during the carburizing and quenching. That is, if the ratio ol the bainite which is
generated during cooling after the hol forming is more than 30% in the microstructure, it
is difficult to precipitate Ti-based precipitates and Nb-based precipitates in a phase
5 interface. Moreover, suppressing the ratio of the bainite to 30% or less is effective in
improvement of cold formability or inachinability. In addition, lilce high temperature
carburizing or the lilte, in a case where conditions with respect to prevention of coarse
grains are strict, it is prckrable that the ratio of the bainite be limited to 20% or less, and
it is more preferable that the ratio be limited to 10% or less. In addition, when the high
10 temperature carbu~izinga fter cold forging is performed, or the lilte, it is preferable that
the ratio of the bainite be limited to 5% or less.
[0082l
[Ferrite grains]
If ferrite grains of the case hardening steel are too fine, the coarse grains arc
15 easily generated during the carburizing and quenching. This is becausc austenite grains
are excessively coarsened during the carburizing and quenching. Particularly, if a grain
size number of ferrite is more than 11 which is defined in JIS G 055 1 (2005), coarse
grains are easily generated. On the other hand, if the grain size number ollemite of the
case hardening steel is less than 8 which is defined in JIS G 0551, ductility is deterioratcd,
20 and cold formability may be adversely affected. Therefore, il is preferable that the grain
size nuinber of ferrite of the case hardening steel be within a range of 8 to 11 which are
defined in JIS C 0551. If the amoiu~ot f S increases, the sulfide increases, number of
the lerrite grains which are generated on the nucleus of the sulfide increases. Therefore,
the ferrite grains tend to be fine.
25 [0083]
35
[Manufactruing MethodlSolidification cooling rate]
Next, a method of manufacturing the case hardening steel according to an
embodiment of the present invention will be described.
[0084]
5 Steel is prepared as molten steel through a general n~cthodu sing a converter, an
electric furnace, or the like, adjustment of chemical components in the steel is perlhrmed,
the steel is subjected to a casting process and a billeting process if necessary, and a steel
is obtained. A wire rod or a steel bar is manufactured by performing hot forming, that is,
hot rolling or hot forging with respect to the steel.
10 [0085]
Many sulfides in the steel are generated before the solidif~catior(ii n the molten
steel) or during the solidiiication, and the size of thc sulfide is greatly influenced by
cooling rate during the solidification. The elnbodiment uses a method other than the
conventional method by paying attention in that a thermal history before and after the
15 solidification influences generation and growth of thc sulfide. That is, in order to
prevent coarsening of the sulfide, it is important to control the cooling rate dduring the
solidification. The cooling rate during the solidification is defined as the ~oolingra te in
112 portion (a position indicated by a solid circle, that is, a position X of TI4 from the
surface in the direction of a bloom thicliness T) of a distance froin a bloom surface 3 to a
20 center line in a bloom thicliness T on a center line (Wl2) of a bloom width W on a bloom
cross-section 2 ol'a bloom 1 shown in FIG. 3.
[0086]
In order to control generation of the sulfide mainly including MnS or TiS, it is
preferable to cont~oal range of solidification ~oolingra te (average solidification cooling
25 rate). Specifically, in order to supprcss coarsening of the sulfide, the cooling rate during
3 6
the solidification needs to be 12 "Clmin or more, and it is preferable that the cooling rate
be 15 "Clmin or more. In addition, as described above, the cooling rate during the
solidification can be confinned from the secondary arm spacing of dendrite. When the
cooling rate is less thaa 12"C/min, the solidification is too slow, the crystallized sulfide
5 mainly including MnS or TiS is coarsened, and the sulfide is difficult to be finely
dispersed. On the other hand, when the cooling rate is more than 100 "Clmin, the
number density of the fine sulfide mainly including MnS is satnrated, the hardness of the
bloon~in creases, and there is a concern that cracks may be generated. Accordingly, the
cooling rate during the casting needs to be 12 to 100 'Clmin. Moreover, in order to
10 more reliably prevent the craclc of the bloom, the cooling rate during the casting is
preferably 50 "Clmin or less and more preferably 20 "Clmin or less.
[0087]
This cooling rate can be obtained by controlling size of a mold cross section,
casting rate, or the lilce lo appropriate values. Moreover, the cooling control can be
15 applied to both the co1:tinuous casting method and the ingot-malting method.
Moreover, sinci: it is considered that MnS is crystallized in the liqilirl phase in
the vicinity of a solidification point of the steel, the size of MnS decreases as the cooling
rate increases, and the size of MnS increases as the cooling rate decreases. Thereby, in
the embodiment compared to the cooling conditions of the conventior:al continuous
20 casling machine and the conventional method of manufacturing thc production modcl
ingot, the molten steel is solidified by an extremely fast cooling rate, and the size of MnS
is suppressed so as to be small.
FIG. 6 shows an example of a relationship between average cooling rate in the
bloom and an average area oIMnS in the case oT controlling the cooling rate by adjusting
17
the casting conditions of the mold size, the cooling conditions, or the like while
considering the relationship between the casting condition and the cooling rate duriug the
conventional continuous casting or the casting of the production model ingot in casting
tests. As shown in FIG. 6, if the average cooling rate of the bloom is increased, the
5 average area of MnS (that is, average equivalent circle diameter) can be decreased.
Here, in order to increase the cooling rate during the solidification, a method
which decreases the mold size can be adopted as a simple method. However, in this
method, it is difficult to maintain the quality of the product. That is, when the size of
the bloom decreases, since a reduction by rolling from the bloom to the rolled product
10 (steel bar) decreases, it is difficult to obtain effects of high quality of crimping of gas
defects, homogenization of segregation, or the like by the rolling, and many defects or
segregatious easily remain in the product (case hardening steel). Thereby, in this case,
the inhomogeneous portion due to the defects or the segregations acts as the starting
point of the fixture and irregularity is generated in the hardenability. Therefore, the
15 quality of the case hardening steel may be deteriorated.
[0088]
The bloom is reheated as it is and the case hardening steel is manufactured by
performing the hot forming, or the steel obtained from the bloom by a billeting process is
reheated and the case hardening steel is manufactured by performing hot forming. In
20 gencral, thc bloom is formed into a billct by billeting, the billet is reheated after being
cooled in room temperaturc, and the case hardening stcel is manufactured. Moreover, in
the manufacturing of the parts such as gears, hot forging may be added.
[0089]
[Manufacturing MethodISoalting-Rolling-Forging]
25 In orcler to alleviate alloy clement-concentratcd portion in the bloom even after
3 8
the solidification is completed, the bloom is placed under as high temperature as possible,
and embrittlement elernents such as P and Mn should be uniformly diffused. Thereby,
the temperature of the blooin is maintained at 600°C or more after the casting7 the bloom
is directly inserted into a heating furnace at the billeting. In addition, the bloom is
5 placed during 20 minutes or more at high temperature of 1200°C or morc in the billeting,
and diffusion of P, Mn, and S is promoted. In addition, the heating and the holding have
an effect which dissolves Ti-based and Nb-based precipitates.
[0090]
Afler the solidification, when the bloom or the ingot which is cooled to room
10 te~ilperatureo nce is used, the bloom or the ingot is reheated up to 1250°C to 1320°C and
placed in thc temperature range during 3 minutes or more, and it is preferable that alloy
elements such as P, Mn, or Cr are sufficiently difk~lseda nd Ti-based and Nb-based
nitrides which are precipitated in the solidification process are dissolved in the steel. As
describe above, since the heating for soalting generates complex sulfide including Ti, Mn,
15 or the lilce or finely generates MnS which is precipitated from the solute Mn and solute S,
the heating for soaking is important. Since the sulfide is not sufficiently generated at
low temperature, FeS or the like is generated, the steel is embrittled, and the required
amountof MnS cannot be secured. Therefore, the temperature (holding temperature)
needs to be 1250°C or more. On the other hand, if the holding temperature is more than
20 1320°C, since the refractory in the industrial furnace is severely damaged and the heat
treatment is difklcult to stabilize, the holding temperature needs to be 1320°C or less.
[0091]
In order to sufficiently dissolve the con~po~mdas h, olding time (soalting time)
needs to be 3 illinutes or more after reaching the temperature, and it is preferable that the
3 9
holding time be 10 minutes or more. Paiticularly, in order to stably exhibit the effects,
industrially, it is more preferable that the holding time be 20 minutes or more. In
addition, when a large amount of alloy elements are contained or it is necessary to
dissolve the alloy elements at a high temperature, the holding time is preferably as long
5 as possible. However, if tlie holding time is more than 180 minutes, since damages to
the material surface increases and damages to the refractory also increases, the holding
time needs to be 180 minutes or less, and industrially, it is preferable that the holding
time be 120 minutes or less.
~00921
10 Moreover, also in a so-called rolling of product (hot forming and hot rolling) in
wliich the billet is rolled to a product diameter, if the heating temperature is less than
1150°C, Ti-based precipitates, Nb-based precipitates, and AlN cannot be dissolved in the
steel, and coarse Ti-based precipitates, coarse Nb-based precipitates, and coarse AIN
remain in the steel. In order to disperse fine Ti-based precipitates and Nb-based
15 precipitates in the case hardening steel after the hot forming and suppress generation of
the coarse grains during the carburizing and quenching, the heating temperat~lren eeds to
be 1150°C or more. The lower limit of appropriate heating temperature is 11 80°C. If
the heating temperature is more than 1320°C, since the refractory of the industrial
a
heating hmace is severely damaged and it is difficult to perform the heat treatment in a
20 stable manner, it is impol-tant that the heating temperattux be 1320°C or less.
Considering load on the heating furnace, it is preferable that the temperature of the
heating furnace be 1300°C or less. In order to uniformly hold the temperature of the
steel and dissolve precipitates in the steel, it is preferable that the holding time in rolling
of the product be 10 minutes or more. From the standpoint ofproductivity, it is
40
preferable that the holding time be 60 minutes or less
100931
If a finishing temperature of the hot forming is less than 84OoC, crystal grains of
fenite become fine, and coarse grains are easily generated during the carburizing and
5 quenching. If the finishing temperature is more than 1 000"C, the steel is hardened and
cold formability is deteriorated. Therefore, the finishing temperature of the hot forming
is controlled to 840°C to 1000°C. Moreover, a preferable range of the finishing
temperature is 900°C to 970°C, and a more preferable range of the finishing Lcrnperat~re
is 920°C to 950°C.
10 [0094]
In order to finely disperse the Ti-based precipitates and the Nb-based
precipitates, cooling conditions alter the hot forming are important. The tcmperature
range in which the precipitation of the Ti-based precipitates and the Nb-based
precipitates is promoted is 500°C to 800°C. Therefore, the steel is gradually cooled at
15 an average cooling rate of 1 "Clsecond or less in the temperature range from 800°C to
50OoC, and generation of the Ti-based precipitates and the Nb-based precipii:~tes is
promoted. If the average cooling rate is more than 1 "Clsecond, the time in which the
steel p13ses through the precipitation temperature range of the Ti-based precipitates and
the Nb-based precipitates is decreased, and the amount oC fine precipitates is insufficient.
20 Moreover, if the average cooling rate increases, the ratio of bainite increases in the
microstructure. In addition, if the average cooling rate increases. since the cast.
hardening steel is hardened and cold formability is deteriorated, it is preferable that the
average cooling rate be 0.7 "Clsecond or less. Moreover, as the method which
decreascs the average cooling rate, there is a metllod in which a heat insulation cover or a
4 l
heat insulation cover having a heat source is disposed behind (downstream of) the rolling
line and slow cooling is performed.
Moreover, for reference, FIG. 4 shows a flow cha-t of an example of a method of
manufacturing the case hardening steel accordiilg to the embodiment.
5 [0095]
[Carburizing]
Next, a method of manufacturing (a method of applying case hardening steel) a
carburized part according to an embodiment of the present invention will be described
The case hardening steel of the embodiment can be applied to either a part
10 which is man~lufacturedin the cold forging process or a part which is manufactured in the
hot forging process. For example, as the hot forging process, there is a process of hot
rorging of a steel bar, heat treatment such as normalizing if necessary, cutting,
carburizing and quenching, and grinding if necessary. By using the case hardening steel
of thc embodiment, for example, hot forging is perlhrmed at a heating temperature of
15 1150°C or more, ihercafter, normalizing is pcrfomied if necessary. I herefore, even
when high temperature carburizing is performed at a low temperature range of 950°C to
1090°C, generation of coarse grains can be suppressed. For example, in thc case of
bearing parts and rotating parts, even when high temperature carburizing is performed, an
excellent rolling fatigue characteristics can be obtained.
20 [0096]
Conditions of the carburizing and quenching are not particularly limited. In the
bearing parts 01 rotating parts, when a high rolling fatigue lifetime is emphasized, it is
preferable that carbon potential be set to 0.8% to 1.3%. In addition, carbonitriding, in
which nitsiding is pelformed in the course of diffusion process after the carburizing, is
25 effective in the rolling fatigue lifetime. In this case, a condition in which nitrogen
42
concentration (nitrogen potential) ofthe surfaces of parts is a range oL0.2% to 0.6% is
appropriate. Effects which suppress the microstructure change and the material
deterioration at the rollir~gf atigue process of the bearing parts or the rotating parts by
adding Si, Cr, and optional Mo is particularly great when residual austenite (residual y) in
5 the surface layer of the part after carburizing is 30% to 40%. In order lo control ihe
amount of the residual y of the surface layer of the part to a range of 30% to 40%,
carbonitriding is effective. At this time, it is preferable that the carbonitriding be
performed so that the nitrogen concentration of the surlace layer of the part is a range of
0.2% to 0.6%. By selecting the carbonitriding conditions, a large amount of flne Ti (C,
10 N) is precipitated in the carburized layer and the rolling fatigue lifetime is improved.
[Examples]
[0097]
Hereinafter, the prcsent invention will be described in detail based on examples.
[009R]
15 Steels including chcmical compositions shown in Tables 1 to 3 wcre prepa~eda s
molten steel in a vacuum melting furnace and cast by thc average solidification rate of 12
to 20 "Clmin excluding Nos. 54 to 56. Blanks in chemical components of 'l'ables 1 to 3
mean th-a t the chemical components are not intentionally added, and underlines mean that
the conditions of the chemical components of the present invention are not satisfled. In
20 addition, the balance of thc chemical components shown in Tables 1 to 3 is iron (Fe) and
inevitable impurities. Solidification cooling rate of the bloom was previously adjusted
based on data which establish relationships between the cooling conditions and the
solidification cooling rate when blooms having varlous sizes were cast. It was
confirmed that the solidification cooling rate in the actual bloom was within a range of
25 12 to 20 'Clmin by the secondary arm spacing of dendrite. The confirmed positions are
43
shown in FIG. 3, Billeting was performed to some of the blooms if necessary.
100991
In Tables 4 to 6, maximum equivalerit circle diameters (maximum size and
maximum diameter) D of the sulfides in the steel, density d of sullides more than 0.5 Ltm
5 (riumber density), and maximum equivalent circle diameters of Ti-based precipitai-es
(maximum size and maximum diameter) are shown. Here, ~mderlinesi n Tables 4 to 6
mean that the conditiolls of the density d of the sulfide of the present invention are not
satisfi ed. The maxirnum equivaleilt circle diameters of the Ti-based precipitates and the
maximum equivalent circle diameters D of the sulfides were predicted by ail extreme
10 valuc statistic method. That is, the maxim~mdl iameters of the Ti-based precipitates,
grain diameter distributions and maximum diameters of the sulfides were obtained by the
following. Microstrusturcs of the steel were observed by an optical inisroscope, and the
precipitates were determined fiom contrast in the microstr~~ctures. In addition,
precipitates were identified by using a scanning electron microscope and an energy
15 dispersive X-ray spcctroscopic analyzer (ED§). From a cross-section including a
longitudinal direction of a test piece described below, 10 ground test pieces each having
length 10 mm x width 10 mln were manufactured, predetermined positions ol'the ground
test pieces were photographed at a magnification of 100 times by an optical microscope, -
10 fields of view each having an image of a measurement reference area (region) of 0.9
20 mtn2 were prepared. The distribution in the grain size and the maximum diameter of the
sulfides, and the maximum diameter of the Ti-based precipitates were detected in the
observed tlclds of view (image). These sizes (diameter) were converted to the
cquivalent circle diameter which indicated a diametcr of a circle having the same area as
the arca of precipitate.

claimed :
1. A case hardening steel con~prising:
by mass%, as a chemical composition,
5 C: 0.1% to 0.5%,
Si: 0.01% to 1.5%
Mn: 0.3% to 1.8%,
S: 0.001% to 0.15%,
Cr: 0.4% to 2.0%,
10 Ti: 0.05% to 0.2%,
Al: limited to 0.2% or less,
N: limited to 0.0050% or less,
P: limited to 0.025% or less,
0: limited to 0.0025% or less, and
15 a balance of Fe and inevitable impurities,
wherein a number d of a sulfide having ail equivalent circle diameter more than
5 pm per 1 mm2 and a nlass percentage [S] of S satisfy: d S 500 x [S] + 1.
a
2. The case hardening steel according to claim 1, fi~rtherc omprising.
20 by mass%, as the chemical composition, at least one selected fi-0111:
Nb: less than 0.04%,
Mo: 1.5% or less,
Ni: 3.5% or less,
V: 0.5% or less,
25 B: 0.005% or less,
Ca: 0.005% or less,
Mg: 0.003% or less, and
Zr: 0.005% or less.
5 3. The case hardening steel according to claim 2,
wherein an [Al]/[Ca] which is a ratio of a mass percentage [All of A1 to a mass
percentage [Ca] of Ca is 1 or more and 100 or less.
4. The case hardening steel according to claim 1 or 2,
10 wherein a maximum equivalent circle diameter D pm of the sulfide and the mass
percentage [S] olS satisfy: D 5 250 x [S] + 10.
5. The case hardening steel according to claim 1 or 2,
wherein ail amount of Mn is 1.0% or less, and a [Mn]l[S] which is a ratio of a
15 mass percentage [S] ol S lo a mass pcrcentage [Mn] of Mn is 100 or less
6. The case hardening steel according lo claim 1 or 2,
wherein a ratio of bainite is 30% or less in a microsbucture
20 7. The case hardening steel according to claim 1 or 2,
wherein a maximum equivalent circle diameter of Ti-based precipitates is 40 ym
or less.
8. Anlethod of manufacturing a case hardening steel, the method comprising:
25 casting a steel having a chemical composition which contains: by mass%, C:
00 2n
0.1%to0.5%, Si: 0.0!%to 1.5%, Mn: 0.3%io 1.8%, S: 0.001% ie il,I5%, Ci: 0.4% io
2.056, Ti: 0.05% to 0.296, A!: limited lo 0.2% or less, lu': limited to 0.0050'% or less, P:
limi-ted to 0.025% or less, 0: limited to 0.0025% or less, and the balance of iron and
inevitable impurities, at an average cooling rate of 12 to 100 OCImin;
5 soaking the steel for 3 to 180 min in a soalting temperature range of 1250°C to
hot-rolling the steel so that a finish rolling is performed in a linishing
temperature range of 840°C to 1000°C after heating the steel in a temperature range of
1150°C to 1320°C; and
I. > 6;oolir:g the steel so ti1:ii ar; aireragi: i-cjoiiilg r.;itc Iri a ie.ii~per;i',a!cr ailg; o:,io li:X'C:
to 500°C is 1 "CI s or less.
9. The method of man~lfactmiugt he case hardening steel according to claim 8,
wherein the cherilical composition further contains: by mass %, at least one
15 selected from: Nb: less than 0.04%, Mo: 1.5% or less, Ni: 3.5% or less, V: 0.5% or less,
B: 0.005% or less, Ca: 0.005% or less, Mg: 0.003% or lei< and Zr: 0.005% dr less.
10. The case hardenilig steel according lo claim 9>
-w herein an [Al]/lCa] which is a ratio of a mass percentage [All of A1 to a mass
--
20 percentage [Ca] oi'Ca is 1 or more and 100 or less.
11. The case hardening steel acco~dingto claim 8 or 9,
wherein an amount of M11 is 1 .O% or less, and a [Mn]/[S] which is a ratio of a mass percentage[S]of S to a muss percentage [Mn]of Mn or less.