U.S. patent application number 10/196358 was filed with the patent office on 2003-03-27 for case hardening steel and carburized part using same.
This patent application is currently assigned to NISSAN MOTOR CO., LTD.. Invention is credited to Hanyuda, Tomoki, Hayashi, Takao, Honda, Masatoshi, Kurebayashi, Yutaka, Miura, Naomi, Murakami, Youichi, Nakamura, Tuyoshi, Usuki, Hideki.
Application Number | 20030056859 10/196358 |
Document ID | / |
Family ID | 26618879 |
Filed Date | 2003-03-27 |
United States Patent
Application |
20030056859 |
Kind Code |
A1 |
Hanyuda, Tomoki ; et
al. |
March 27, 2003 |
Case hardening steel and carburized part using same
Abstract
A case hardening steel high in impact strength, consists
essentially of carbon in an amount of from 0.1 to 0.3% by weight,
silicon in an amount of from more than 0.3 to 1.0% by weight,
manganese in an amount of from 0.3 to 1.7% by weight, phosphorus in
an amount of not more than 0.03% by weight, sulfur in an amount of
not more than 0.03% by weight, molybdenum in an amount of not more
than 1.0% by weight, aluminum in an amount of not more than 0.04%
by weight, nitrogen in an amount of not more than 0.03% by weight,
and balance being iron and inevitable impurities. The case
hardening steel meets an equation of [C %]+5([P %]+[S
%]).ltoreq.([Mn %]+[Mo %]+1.8)/8.
Inventors: |
Hanyuda, Tomoki; (Nagoya,
JP) ; Nakamura, Tuyoshi; (Nagoya, JP) ;
Hayashi, Takao; (Kanagawa, JP) ; Usuki, Hideki;
(Kanagawa, JP) ; Honda, Masatoshi; (Nagoya,
JP) ; Kurebayashi, Yutaka; (Nagoya, JP) ;
Miura, Naomi; (Kanagawa, JP) ; Murakami, Youichi;
(Yokohama, JP) |
Correspondence
Address: |
FOLEY AND LARDNER
SUITE 500
3000 K STREET NW
WASHINGTON
DC
20007
US
|
Assignee: |
NISSAN MOTOR CO., LTD.
|
Family ID: |
26618879 |
Appl. No.: |
10/196358 |
Filed: |
July 17, 2002 |
Current U.S.
Class: |
148/319 |
Current CPC
Class: |
C23C 8/22 20130101; C22C
38/32 20130101; C22C 38/22 20130101; C22C 38/26 20130101; C22C
38/18 20130101; C22C 38/04 20130101 |
Class at
Publication: |
148/319 |
International
Class: |
C22C 038/12 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 17, 2001 |
JP |
2001-216990 |
Mar 19, 2002 |
JP |
2002-075624 |
Claims
What is claimed is:
1. A case hardening steel consisting essentially of carbon in an
amount of from 0.1 to 0.3% by weight, silicon in an amount of from
more than 0.3 to 1.0% by weight, manganese in an amount of from 0.3
to 1.7% by weight, phosphorus in an amount of not more than 0.03%
by weight, sulfur in an amount of not more than 0.03% by weight,
molybdenum in an amount of not more than 1.0% by weight, aluminum
in an amount of not more than 0.04% by weight, nitrogen in an
amount of not more than 0.03% by weight, and balance being iron and
inevitable impurities, wherein said case hardening steel meets the
following equation: [C %]+5([P %]+[S %]).ltoreq.([Mn %]+[Mo
%]+1.8)/8 Eq. (1).
2. A case hardening steel as claimed in claim 1, further consisting
essentially of boron in an amount of from 0.001 to 0.005% by
weight, and at lease one of niobium in an amount of from 0.01 to
0.10% by weight and titanium in an amount of from 0.01 to 0.10% by
weight.
3. A case hardening steel as claimed in claim 1, wherein said case
hardening steel has an elemental composition to meet the following
equation: [C %]+5.2([P %]+[S %]).ltoreq.([Mn %]+[Mo %]+3.8)/22+96[B
%]+[austenite grain size number according to JIS G 0551]/111 Eq.
(3)
4. A case hardening steel as claimed in claim 1, further consisting
essentially of at least one selected from the group consisting of
lead in an amount of not more than 0.3% by weight, bismuth in an
amount of not more than 0.15% by weight and calcium in an amount of
not more than 0.1% by weight.
5. A case hardening steel consisting essentially of carbon in an
amount of from 0.1 to 0.3% by weight, silicon in an amount of from
more than 0.3 to 1.0% by weight, manganese in an amount of from 0.3
to 1.7% by weight, phosphorus in an amount of not more than 0.03%
by weight, sulfur in an amount of not more than 0.03% by weight,
aluminum in an amount of not more than 0.04% by weight, nitrogen in
an amount of not more than 0.03% by weight, chromium in an amount
of from more than 0 to 1.6% by weight, and balance being iron (Fe)
and inevitable impurities, wherein the case hardening steel meets
the following equation: [C %]+5([P %]+[S %]).ltoreq.([Mn %]+1.8)/8
Eq. (2).
6. A case hardening steel consisting essentially of carbon in an
amount of from 0.1 to 0.3% by weight, silicon in an amount of not
more than 0.3% by weight, manganese in an amount of from 0.3 to
1.7% by weight, phosphorus in an amount of not more than 0.03% by
weight, sulfur in an amount of not more than 0.03% by weight,
molybdenum in an amount of not more than 1.0% by weight, aluminum
in an amount of not more than 0.04% by weight, nitrogen in an
amount of not more than 0.03% by weight, and balance being iron and
inevitable impurities, wherein said case hardening steel meets the
following equation: [C %]+5([P %]+[S %]).ltoreq.([Mn %]+[Mo
%]+1.8)/8 Eq. (1).
7. A case hardening steel as claimed in claim 6, further consisting
essentially of chromium in an amount of from more than 0 to 1.6% by
weight.
8. A case hardening steel as claimed in claim 6, wherein said case
hardening steel has an elemental composition to meet the following
equation: 80[Si %]+24[Mn %]+33[Mo %]+13.ltoreq.40 Eq. (4)
9. A case hardening steel consisting essentially of carbon in an
amount of from 0.1 to 0.3% by weight, silicon in an amount of not
more than 0.3% by weight, manganese in an amount of from 0.3 to
1.7% by weight, phosphorus in an amount of not more than 0.03% by
weight, sulfur in an amount of not more than 0.03% by weight,
aluminum in an amount of not more than 0.04% by weight, nitrogen in
an amount of not more than 0.03% by weight, and balance being iron
and inevitable impurities, wherein the case hardening steel meets
the following equation: [C %]+5([P %]+[S %]).ltoreq.([Mn %]+1.8)/8
Eq. (2).
10. A carburized part formed of a case hardening steel which
consists essentially of carbon in an amount of from 0.1 to 0.3% by
weight, silicon in an amount of from more than 0.3 to 1.0% by
weight, manganese in an amount of from 0.3 to 1.7% by weight,
phosphorus in an amount of not more than 0.03% by weight, sulfur in
an amount of not more than 0.03% by weight, molybdenum in an amount
of not more than 1.0% by weight, aluminum in an amount of not more
than 0.04% by weight, nitrogen in an amount of not more than 0.03%
by weight, and balance being iron and inevitable impurities,
wherein said case hardening steel meets the following equation: [C
%]+5([P %]+[S %]).ltoreq.([Mn %]+[Mo %]+1.8)/8 Eq. (1), wherein
said carburized part has a hardened layer of carburized case
including fine austenite whose austenite grain size number
according to JIS G 0551 is not smaller than 7.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to improvements in a case hardening
steel and a carburized part using the case hardening steel, and
more particularly to the case hardening steel and carburized part
belonging to ferrous material to be used structural parts and to be
used for parts required to be high in hardness at their surface
layer upon being subjected at their surface layer to a case
hardening treatment such as carburizing, carbonitriding and the
like (including gas carburizing, solid carburizing, liquid
carburizing, salt bath carburizing, plasma carburizing, vacuum
carburizing and the like), the parts including engine parts (such
as piston pin), gears, shafts and the like sued in engines,
transmissions, differentials and the like of an automotive
vehicle.
[0002] Hitherto, case hardening steels have been known and
identified as SCr420H, SCM420H and SNCM420H according to JIS
(Japanese Industrial Standard). However, recently it has been
eagerly required to improve impact strength of parts for power
transmission to meet an increase in power output and
weight-lightening made in transportation machines such as
automotive vehicles or the like. Accordingly, the above case
hardening steels according to JIS seem to be insufficient in impact
strength.
[0003] In order to meet such a requirement, a method of producing a
bevel gear high in impact strength by improving forging and heat
treatment manners has been proposed as disclosed in Japanese Patent
Provisional Publication No. 9-201644. However, this method has
encountered difficulties in which material cost and processing cost
are high. Additionally, the impact strength of the bevel gear
cannot be largely improved.
SUMMARY OF THE INVENTION
[0004] It is an object of the present invention to provide an
improved case hardening steel which can overcome drawbacks
encountered in conventional case hardening steels.
[0005] Another object of the present invention is to provide an
improved case hardening steel which is high in impact strength
without causing a large increase in material cost and processing
cost as compared with conventional case hardening steels, and a
carburized part using the improved case hardening steel.
[0006] As a result of eager studies of the present inventors, it
has been found to overcome the above problems encountered in the
conventional case hardening steels by controlling amounts of
elements of C, Mn, Mo, P and S inherently contained in case
hardening steel and of B and the like within specified content
ranges thereby establishing a suitable balance between crystal
grain size and a carburized case (hardened layer).
[0007] A first aspect of the present invention resides in a case
hardening steel consisting essentially of carbon in an amount of
from 0.1 to 0.3% by weight, silicon in an amount of from more than
0.3 to 1.0% by weight, manganese in an amount of from 0.3 to 1.7%
by weight, phosphorus in an amount of not more than 0.03% by
weight, sulfur in an amount of not more than 0.03% by weight,
molybdenum in an amount of not more than 1.0% by weight, aluminum
in an amount of not more than 0.04% by weight, nitrogen in an
amount of not more than 0.03% by weight, and balance being iron and
inevitable impurities. The case hardening steel meets the following
equation:
[C %]+5([P %]+[S %]).ltoreq.([Mn %]+[Mo %]+1.8)/8 Eq. (1).
[0008] A second aspect of the present invention resides in a case
hardening steel consisting essentially of carbon in an amount of
from 0.1 to 0.3% by weight, silicon in an amount of from more than
0.3 to 1.0% by weight, manganese in an amount of from 0.3 to 1.7%
by weight, phosphorus in an amount of not more than 0.03% by
weight, sulfur in an amount of not more than 0.03% by weight,
aluminum in an amount of not more than 0.04% by weight, nitrogen in
an amount of not more than 0.03% by weight, chromium in an amount
of from more than 0 to 1.6% by weight, and balance being iron (Fe)
and inevitable impurities. The case hardening steel meets the
following equation:
[C %]+5([P %]+[S %]).ltoreq.([Mn %]+1.8)/8 Eq. (2).
[0009] A third aspect of the present invention resides in a case
hardening steel consisting essentially of carbon in an amount of
from 0.1 to 0.3% by weight, silicon in an amount of not more than
0.3% by weight, manganese in an amount of from 0.3 to 1.7% by
weight, phosphorus in an amount of not more than 0.03% by weight,
sulfur in an amount of not more than 0.03% by weight, molybdenum in
an amount of not more than 1.0% by weight, aluminum in an amount of
not more than 0.04% by weight, nitrogen in an amount of not more
than 0.03% by weight, and balance being iron and inevitable
impurities. The case hardening steel meets the following
equation:
[C %]+5([P %]+[S %]).ltoreq.([Mn %]+[Mo %]+1.8)/8 Eq. (1).
[0010] A fourth aspect of the present invention resides in a case
hardening steel consisting essentially of carbon in an amount of
from 0.1 to 0.3% by weight, silicon in an amount of not more than
0.3% by weight, manganese in an amount of from 0.3 to 1.7% by
weight, phosphorus in an amount of not more than 0.03% by weight,
sulfur in an amount of not more than 0.03% by weight, aluminum in
an amount of not more than 0.04% by weight, nitrogen in an amount
of not more than 0.03% by weight, and balance being iron and
inevitable impurities. The case hardening steel meets the following
equation:
[C %]+5([P %]+[S %]).ltoreq.([Mn %]+1.8)/8 Eq. (2).
[0011] A fifth aspect of the present invention resides in a
carburized part formed of a case hardening steel which consists
essentially of carbon in an amount of from 0.1 to 0.3% by weight,
silicon in an amount of from more than 0.3 to 1.0% by weight,
manganese in an amount of from 0.3 to 1.7% by weight, phosphorus in
an amount of not more than 0.03% by weight, sulfur in an amount of
not more than 0.03% by weight, molybdenum in an amount of not more
than 1.0% by weight, aluminum in an amount of not more than 0.04%
by weight, nitrogen in an amount of not more than 0.03% by weight,
and balance being iron and inevitable impurities. The case
hardening steel meets the following equation:
[C %]+5([P %]+[S %]).ltoreq.([Mn %]+[Mo %]+1.8)/8 Eq. (1).
[0012] Additionally, the carburized part has a hardened layer of
carburized case including fine austenite whose austenite grain size
number according to JIS G 0551 is not smaller than 7.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a cross-sectional view of a gear specimen used in
Experiment 1 for evaluating performance of case hardening steels
according to the present invention;
[0014] FIG. 2 is a graphic representation showing a heating pattern
for carburizing hardening and tempering for obtaining the gear
specimen of FIG. 1;
[0015] FIG. 3 is a plan view illustrating an impact test by using a
drop impact tester, in Experiment 1;
[0016] FIG. 4 is a graph showing the relationship between the
impact torque (Nm) and the frequency (times) of application of
impact load, in connection with the impact test in Experiment
1;
[0017] FIG. 5A is a cross-sectional view of an example of a gear
specimen used in the impact test in Experiment 2;
[0018] FIG. 5B is a cross-sectional view similar to FIG. 5A but
showing another example of the gear specimen;
[0019] FIG. 6 is a graph showing the relationship between the
(cold) forging load ratio and the hardness upon undergoing the
spheroidizing annealing, for the steels of Examples and Comparative
Examples in connection with Experiment 2; and
[0020] FIG. 7 is a graph showing the relationship between the (100
times) impact strength ratio and the value of [(left side)-(right
side) of Eq. (1)], for the steels of Examples and Comparative
Examples in connection with Experiment 2.
DETAILED DESCRIPTION OF THE INVENTION
[0021] The present invention is based on the present inventors'
findings that, in order to improve impact strength of a case
hardening steel, it is effective to control the amounts of elements
of C, Mn, Mo, P and S inherently contained in the case hardening
steel and of B and the like within specified content ranges thereby
establishing a suitable balance between crystal grain size and a
carburized case (hardened layer) corresponding to an effective case
depth. In other words, the present invention depends on the present
inventors' knowledge that the impact strength of case hardening
steel can be improved upon strong contribution of decreasing the
amount of P and S as impurity elements in place of addition of a
large amount of Mo and the like which are high in cost, addition of
Mn in place of Mo, addition of B, and refining crystal grain.
[0022] A first embodiment of a case hardening steel according to
the present invention consists essentially of carbon (C) in an
amount of from 0.1 to 0.3% by weight, silicon (Si) in an amount of
from more than 0.3 to 1.0% by weight, manganese (Mn) in an amount
of from 0.3 to 1.7% by weight, phosphorus (P) in an amount of not
more than 0.03% by weight, sulfur (S) in an amount of not more than
0.03% by weight, molybdenum (Mo) in an amount of not more than 1.0%
by weight, aluminum (Al) in an amount of not more than 0.04% by
weight, nitrogen (N) in an amount of not more than 0.03% by weight,
and balance being iron (Fe) and inevitable impurities.
Additionally, the case hardening steel is prepared to meet the
following equation:
[C %]+5([P %]+[S %]).ltoreq.([Mn %]+[Mo %]+1.8)/8 Eq. (1).
[0023] Here, C of component elements of the case hardening steel
functions to increase the hardness of the case hardening steel
obtained after carburizing hardening and to increase the strength
of a carburized (component) part. The content of C is within a
range of from 0.1 to 0.3% by weight. If the content of C is smaller
than 0.1% by weight, the effect of addition of C is insufficient.
If the content of C exceeds 0.3% by weight, the resultant case
hardening steel is lowered in toughness and impact strength.
[0024] Si promotes intergranular oxidation after carburizing and
may lower the strength of the resultant case hardening steel, and
therefore it is preferable that the case hardening steel contains
not more than 0.3% by weight of Si. However, the content of Si may
not be limited to not more than 0.3% in a heat treatment such as
vacuum carburizing or a plasma carburizing which can suppress the
intergranular oxidation. It is to be noted that an excessive
content of Si largely degrades machinability and cold forgeability
of the case hardening steel, and therefore the upper limit of the
Si content is set at 1.0% by weight.
[0025] Mn is an element which is effective for improving
hardenability of the case hardening steel. The lower limit of the
Mn content is set at 0.3% by weight for the reason why austenite in
a suitable amount is necessary to be retained after carburizing in
order to improve the toughness of the case hardening steel. An
excessive Mn content over 1.7% by weight lowers the cold
forgeability of the case hardening steel and promotes the
intergranular oxidation after the carburizing.
[0026] P is an element which functions to lower the toughness of a
carburized case (hardened layer) of the case hardening steel.
Particularly in case that the content of P exceeds 0.03% by weight,
lowering in impact strength of the case hardening steel becomes
conspicuous. Additionally, P is an impurity element, and therefore
it is preferable that the P content approaches 0% by weight as much
as possible.
[0027] S is an elelment which also functions to lower the toughness
of the carburized case (hardened layer). Similarly to P, if the
content of S exceeds, lowering in impact strength of the case
hardening steel becomes conspicuous. Additionally, S is also an
impurity element, and therefore it is preferable that the S content
approaches O % by weight as much as possible.
[0028] Mo is an element which is effective for improving the
hardenability of the case hardening steel and effective for
improving the toughness of the carburized case (hardened layer).
The Mo content is excessive so as to exceed 1.0% by weight, such
effects become saturated.
[0029] Al reacts with N in the case hardening steel to form AlN
thereby being effective to prevent coarsening of austenite grain
size during carburizing. If the Al content exceeds 0.04% by weight,
the effect of preventing the grain size coarsening becomes
saturated. Additionally, for the similar reason, the content of N
exceeds 0.03% by weight, the effect of preventing the grain size
coarsening becomes saturated.
[0030] It will be understood that Fe occupies an almost whole part
of the case hardening steel of the present invention, other than
the above-discussed components (elements). The case hardening steel
of the present invention further contains Cu, O and the like as
inevitable impurities.
[0031] The above equation Eq. (1) is a formula for optimizing the
contents of C, P, S, Mn and Mo in the case hardening steel in order
to suppress formation and propagation of crack at crystal boundary
which crack serves as a starting point of breaking. In other words,
the toughness of the carburized case (hardened layer) obtained
after carburizing can be increased by addition of suitable amounts
of Mn and Mo, while the toughness of the crystal boundary can be
increased by reducing the contents of P and S as the impurities, so
that the case hardening steel can be improved in impact
strength.
[0032] A second embodiment of a case hardening steel according to
the present invention consists essentially of carbon (C) in an
amount of from 0.1 to 0.3% by weight, silicon (Si) in an amount of
from more than 0.3 to 1.0% by weight, manganese (Mn) in an amount
of from 0.3 to 1.7% by weight, phosphorus (P) in an amount of not
more than 0.03% by weight, sulfur (S) in an amount of not more than
0.03% by weight, aluminum (Al) in an amount of not more than 0.04%
by weight, nitrogen (N) in an amount of not more than 0.03% by
weight, optionally chromium in an amount of from more than 0 to
1.6% by weight, and balance being iron (Fe) and inevitable
impurities. Additionally, the case hardening steel is prepared to
meet the following equation:
[C %]+5([P %]+[S %]).ltoreq.([Mn %]+1.8)/8 Eq. (2).
[0033] The case hardening steel of this embodiment is similar in
composition and in effects to be produced, to that of the first
embodiment with the exception that Mo is not contained. In this
embodiment, Mn is added in place of Mo thereby omitting use of Mo
which is high in cost. Additionally, the case hardening steel of
this embodiment may contain not more than 1.6% by weight of Cr. Cr
is an element which is effective for improving the hardenability of
the case hardening steel. However, addition of an excessive amount
of Cr may invite embrittlement of crystal grain boundary, and
therefore the Cr content is preferably not more than 1.6% by
weight. The lower limit of the Cr content is decided in accordance
with a required hardenability and therefore is not particularly
set.
[0034] Next, a third embodiment of the case hardening steel
according to the present invention will be discussed. This case
hardening steel consists essentially of carbon (C) in an amount of
from 0.1 to 0.3% by weight, silicon (Si) in an amount of not more
than 0.3% by weight, manganese (Mn) in an amount of from 0.3 to
1.7% by weight, phosphorus (P) in an amount of not more than 0.03%
by weight, sulfur (S) in an amount of not more than 0.03% by
weight, molybdenum (Mo) in an amount of not more than 1.0% by
weight, aluminum (Al) in an amount of not more than 0.04% by
weight, nitrogen (N) in an amount of not more than 0.03% by weight,
and balance being iron (Fe) and inevitable impurities.
Additionally, the case hardening steel is prepared to meet the
above equation Eq. (1).
[0035] Thus, the case hardening steel of the third embodiment is
similar to the case hardening steel of the first embodiment with
the exception that the Si content is not more than 0.3% by weight.
Si is an element which is effective for improving the hardenability
of the case hardening steel; however, addition of an excessive
amount of Si promotes intergranular oxidation after carburizing
thereby inviting lowering in strength Qf the case hardening steel.
Accordingly, the Si content is limited to not more than 0.3% by
weight.
[0036] Next, a fourth embodiment of the case hardening steel
according to the present invention will be discussed. This case
hardening steel consists essentially of carbon (C) in an amount of
from 0.1 to 0.3% by weight, silicon (Si) in an amount of not more
than 0.3% by weight, manganese (Mn) in an amount of from 0.3 to
1.7% by weight, phosphorus (P) in an amount of not more than 0.03%
by weight, sulfur (S) in an amount of not more than 0.03% by
weight, aluminum (Al) in an amount of not more than 0.04% by
weight, nitrogen (N) in an amount of not more than 0.03% by weight,
and balance being iron (Fe) and inevitable impurities.
Additionally, the case hardening steel is prepared to meet the
above equation of Eq. (2). Thus, the case hardening steel of this
embodiment is the same as that of the second embodiment with the
exception that the Si content is not more than 0.3% by weight and
no Cr is contained.
[0037] The above case hardening steels of the third and fourth
embodiments may contain not more than 1.6% by weight of Cr. Cr is
an element which is effective for improving the hardenability of
the case hardening steel. However, addition of an excessive amount
of Cr may invite embrittlement of crystal grain boundary, and
therefore the Cr content is preferably not more than 1.6% by
weight. The lower limit of the Cr content is decided in accordance
with a required hardenability and therefore is not particularly
set.
[0038] Preferably, the above case hardening steels of the third and
fourth embodiments have an elemental composition to meet the
following equation:
80 [Si %]+24[Mn %]+33[Mo %]+13.ltoreq.40 Eq. (4)
[0039] With this elemental composition, the hardness of the
material (case hardening steel) before cold forging can be lowered
thereby making it possible to lower deformation resistance and
improve deformability of the material and additionally to lower a
pressing load (or cold forging load) during cold forging. In other
words, cold forgeability of the case hardening steel can be
improved by preparing the case hardening steel to meet the above
equation Eq. (4).
[0040] The case hardening steels of the first to fourth embodiments
may contain boron (B) in an amount of from 0.001 to 0.005% by
weight, niobium (Nb) in an amount of from 0.01 to 0.10% by weight
and/or titanium (Ti) in an amount of from 0.01 to 0.10% by weight.
The above-mentioned B is an element which is effective for
improving the hardenability of the case hardening steel, and also
effective for strengthening grain boundary of the carburized case
(hardened layer) upon its segregation at the grain boundary of the
carburized case (hardened layer). In order to obtain such effects,
addition of not less than 0.001% by weight of B is preferable.
However, addition of B in an amount exceeding 0.005% by weight is
not preferable because not only the effect of improving
hardenability becomes saturated but also hot or cold machinability
is degraded.
[0041] It will be understood that at least one of the
above-mentioned Nb and Ti may be contained in the case hardening
steel. In case that the case hardening steel contains both Nb and
Ti, each of Nb and Ti is preferably contained in an amount of from
0.01 to 0.10% by weight. Nb and Ti react with C and N to form
carbide and nitride thereby preventing coarsening of austenite
crystal grain. If the content of Nb or Ti is less than 0.01% by
weight, it is difficult to obtain a sufficient effect of preventing
the crystal grain coarsening. If the content of Nb or Ti exceeds
0.10% by weight, the effect of preventing the crystal grain
coarsening becomes saturated.
[0042] It is preferable that the case hardening steels of the first
to fourth embodiments have an elemental composition to meet the
following equation:
[C %]+5.2([P %]+[S %]).ltoreq.([Mn %]+[Mo %]+3.8)/22+96[B
%]+[austenite grain size number according to JIS G 0551]/111 Eq.
(3)
[0043] With this elemental composition, impurities at crystal grain
boundary can be removed under the effect of addition of B, thereby
achieving strengthening the grain boundary. Additionally, crystal
grain size becomes small, thereby suppressing breaking at crystal
grain boundary.
[0044] Particularly in case of meeting the above equations Eq. (1)
and Eq. (3), achievement can be made on optimizing the contents of
C, P, S, Mn and Mo for suppressing the formation and propagation of
crack at crystal grain boundary which crack serves as the breaking
starting point, and on reinforcement of crystal grain boundary
under the effects of addition of B and crystal grain refining. In
other words, addition of a suitable amount of Mn and Mo is
preferable to improve the toughness of the carburized case
(hardened layer) after carburizing, and decreasing the contents of
P and S as impurities is preferable to improve the toughness of
crystal grain boundary. These concepts lead to the limitation of
the equation Eq. (1). It is preferable that impurities at crystal
grain boundary are removed under the effect of addition of B
thereby to achieve strengthening of grain boundary. Additionally,
it is also preferable that crystal grain size is lowered thereby
preventing breaking at grain boundary. These concepts lead to the
limitation of the equation Eq. (3).
[0045] Furthermore, the case hardening steels of the embodiments 1
to 4 may contain lead (Pb) in an amount of not more than 0.3% by
weight, bismuth (Bi) in an amount of not more than 0.15% by weight
and/or calcium (Ca) in an amount of not more than 0.1% by weight.
Pb, Bi and Ca may be contained in any combinations. These elements
are effective for improving machinability of the case hardening
steel; however, not only the machinability improving effect may
become saturated but also the toughness may lower if the Pb content
exceeds 0.3% by weight, the Bi content exceeds 0.15% by weight or
the Ca content exceeds 0.1% by weight.
[0046] A carburized (component) part according to the present
invention will be discussed. The carburized part is formed of the
above-mentioned case hardening steel and has a carburized case
(hardened layer) includes fine austenite whose austenite grain size
number according to JIS (Japanese Industrial Standard) G 0551 is
not smaller than 7. Such refining crystal grain size is
accomplished during carburizing and effective for improving
resistance to the crack propagation upon input of impact. If
austenite in the carburized case is not so refined that the
austenite grain size number is smaller than 7, the carburized part
cannot obtain an excellent impact strength characteristics.
[0047] Thus, the case hardening steel of the present invention is
used as parts whose surface layer requires a high hardness, such as
gears, shafts and the like in a transmission, a differential and
the like of an automotive vehicle.
EXAMPLES
[0048] The present invention will be more readily understood with
reference to the following Examples in comparison with Comparative
Examples; however, these Examples are intended to illustrate the
invention and are not to be construed to limit the scope of the
invention. Additionally, although the case hardening steels of
Examples and Comparative Examples are directed to gears, it will be
understood that the principle of the present invention are not
limited to gears and therefore may be applied to all machine
structural parts which particularly regard impact strength
characteristics as important.
[0049] Experiment 1
[0050] Steels A to I, K to M and R of Examples (according to the
present invention) and steels N to Q of Comparative Examples (not
according to the present invention) in an amount of 150 Kg were
produced in a usual manner under vacuum melting. The steels A to R
and the steels N to Q had chemical compositions shown in Table 1.
The steel N of Comparative Example corresponded to a conventional
case hardening steel identified as SCr420H according to JIS.
Subsequently, each of these steels was subjected to rolling and
normalizing in a usual manner, and thereafter was machined to a
gear shape having a module of 1.5 as shown in FIG. 1. The gear
shaped steel had an outer diameter (corresponding to addendum
circle) of 64.5 mm and a width (axial dimension) of 26 mm as
illustrated in FIG. 1. Thereafter, each gear shaped steel was
subjected to carburizing hardening and tempering in a heating
pattern as shown in FIG. 2, followed by finish machining, thereby
obtaining gear specimen 10 shown in FIG. 1.
[0051] An impact test was conducted on each gear specimen 10 by
using a drop impact tester as shown in FIG. 3. With the impact
tester, gear specimen 10 was fixedly mounted on a first shaft and
engaged with opposed gear 12 fixedly mounted on a second shaft
supported by a supporting base 14. A torque arm 16 has a base end
section fixedly mounted on the first shaft. A free end section of
the torque arm 16 has a position 18 to which impact load was
repeatedly applied so as to apply impact (load) torque (Nm) to gear
specimen 10. In this impact test, the frequency or number (times)
of application of the impact load to torque arm 16 at a time when
breaking of the gear specimen 10 had occurred was measured. This
measurement of the frequency of impact load application was made
plural times by changing the impact torque to be applied to the
gear specimen, thereby obtaining an upper group of data for each
gear specimen of Example and a lower group of data for each gear
specimen of Comparative Example. In the upper group of data, each
black dot indicates the measured frequency of impact load
application at a value of the impact torque. In the lower linear
data, each light triangle indicated the measured frequency of
impact load application at a value of the impact torque.
[0052] From each of the upper group of data and the lower group of
data in FIG. 4, an impact (load) torque (Nm) applied to the gear
specimen in case that the (measured) frequency of impact load
application was 100 times (at which the gear specimen was broken)
was determined from a relational expression between the impact
torque and the frequency of impact load application, i.e. in a
manner using a dotted arrow as illustrated in FIG. 4. The dotted
arrow is drawn from a straight line representing the upper or lower
group of data. The thus determined impact torque is referred to as
"100 times impact strength (Nm)". The 100 times impact strength for
each of the gear specimens of Examples and Comparative Examples is
shown in Table 2.
[0053] Additionally, austenite grain size of the gear specimens of
Examples and Comparative Examples were determined by a judgment
method using crossover line segments, according to JIS G 0551. The
thus determined austenite grain size of the gear specimens are
shown in Table 2.
[0054] As apparent from the experimental results shown in Table 2,
the gear specimens formed of the steels A to I, K to M and R of
Examples meet either one of the above equations Eq. (1) and Eq. (2)
and the equation Eq. (3) by optimizing balance between the impurity
elements and the added elements, and therefore are high in impact
strength as compared with those formed of the steels of Comparative
Example. In contrast, the gear specimen formed of the steel N of
Comparative Example cannot meet the above equations Eq. (2) and Eq.
(3) and therefore are low in impact strength. The gear specimen
formed of the steel 0 of Comparative Example contains much Cr and
cannot meet the above equations Eq. (2) and Eq. (3), and therefore
is low in impact strength. The gear specimen formed of the steel P
of Comparative Example is lower than 7 in the grain size number and
cannot meet the above equations Eq. (1) and Eq. (3), and therefore
low in impact strength. The gear specimen formed of the steel Q is
lower than 7 in the grain size number and cannot meet the above
equations Eq. (2) and Eq. (3), and therefore is low in impact
strength.
[0055] Experiment 2
[0056] Steels 1 to 3 of Examples (according to the present
invention) and steels 4 to 9 of Comparative Examples (not according
to the present invention) in an amount of 150 Kg were produced in a
usual manner under vacuum melting. The steels A to R and the steels
N to Q had chemical compositions shown in Table 3. The steels 1 to
3 met all the equations Eq. (1), Eq. (3) and Eq. (4), whereas the
steels 4 to 9 cannot meet at least one of the equations Eq. (1),
Eq. (3) and Eq. (4). The steel 8 of Comparative Example
corresponded to a conventional case hardening steel identified as
SCM418H according to JIS.
[0057] Subsequently, each of the steels of Examples and Comparative
Examples was subjected to rolling in a usual manner and formed into
a bar material, and thereafter underwent cutting, spheroidizing
annealing, shot blasting, a treatment for forming lubricating
coating, and cold teeth forging. Thereafter, the bar material was
subjected to cutting such as turning or the like so as to be formed
into a gear of the final shape as shown in FIG. 5A. The bar
material might be formed into the final shape of a gear as shown in
5B. The gear of the final shape was then subjected to carburizing
hardening and tempering, followed by finish grinding, thereby
obtaining a gear specimen of each of the steels 1 to 3 of Examples
and the steels 4 to 9 of Comparative Examples, as shown in FIG.
5A.
[0058] The gear of the final shape shown in FIG. 5A or 5B may be
produced by another production method in which the steel is formed
into a certain blank shape under cold forging and thereafter
subjected to turning and gear cutting. Concerning the steels 1 to 3
of Examples, forming of the gear may be sufficiently accomplished
even if the spheroidizing annealing as a softening heat treatment
made before the cold forging is omitted.
[0059] The impact test was conducted on each of the gear specimens
of the steels 1 to 3 of Examples and steels 4 to 9 of Comparative
Examples by using the drop impact tester in the same manner as that
for the gear specimens in Experiment 1, in which the 100 times
impact strength was measured for each gear specimen. Then,
calculation was made for each gear specimen to determine a ratio of
the 100 times impact strength of each gear specimen to the 100
times impact strength of the gear specimen of the steel 8 of
Comparative Example (corresponding to the case hardening steel
SCM418H according to JIS) on the assumption that the 100 times
impact strength of the steel 8 was 100. This ratio was referred to
as "100 times impact strength ratio" and shown in Table 4.
[0060] Additionally, in order to determine cold forgeability of
each of the gear specimens of the steels 1 to 3 of Examples and the
gear specimens of the steels 4 to 9 of Comparative Examples, a
press load (or cold forging load) applied to the bar material for
each gear specimen was measured during the above cold teeth forging
by using a load cell equipped with a press work machine. It will be
understood that the cold forgeability is excellent as the press
load is low. Then, calculation was made for each gear specimen to
determine a ratio of the press load of each gear specimen to the
press load of the gear specimen of the steel 8 of Comparative
Example (corresponding to the case hardening steel SCM418H
according to JIS) on the assumption that press load of the steel 8
was 100. This ratio was referred to as "cold forging load ratio"
and shown in Table 4. Furthermore, each of the gear specimens of
the steels 1 to 3 of Examples and the steels 4 to 9 of Comparative
Examples was subjected to measurement of Rockwell hardness
(B-scale). The measured Rockwell hardness (HRB) of the gear
specimens were shown in Table 4.
[0061] As apparent from the experimental results shown in Table 4,
it is confirmed that the steels 1 to 3 of Examples meet the
equations Eq. (1), Eq. (3) and Eq. (4) and therefore are excellent
both in cold forgeability and impact strength. In contrast, it is
confirmed that the steels 4, 5 and 9 of Comparative Examples meet
the equations Eq. (1) and Eq. (3) and therefore excellent in impact
strength; however, they cannot meet the equation Eq. (4) and
therefore are inferior in cold forgeability. The steels 7 and 8 of
Comparative Examples meet the equations Eq. (1) and Eq. (4) and
cannot meet the equation Eq. (3), and therefore are inferior in
impact strength.
[0062] While the present invention has been discussed particularly
on examples of gears, it will be appreciated that the principle of
the present invention may be applied to all machinery structural
parts in which impact strength is particularly regarded as
important.
[0063] As appreciated from the above, according to the present
invention, the amounts of elements of C, Mn, Mo, P and S inherently
contained in case hardening steel and of B and the like are
controlled within specified content ranges thereby establishing a
suitable balance between crystal grain size and a carburized case
(hardened layer). This can provide the case hardening steel high in
impact strength without large increase in material cost and
processing cost, and the carburized part using the thus improved
case hardening steel.
[0064] The entire contents of Japanese Patent Applications
P2001-216990 (filed Jul. 17, 2001) and P2002-075624 (filed Mar. 19,
2002) are incorporated herein by reference.
[0065] Although the invention has been described above by reference
to certain embodiments and examples of the invention, the invention
is not limited to the embodiments and examples described above.
Modifications and variations of the embodiments and examples
described above will occur to those skilled in the art, in light of
the above teachings. The scope of the invention is defined with
reference to the appended claims.
1 TABLE 1 C Si Mn P S Cr Mo B Al N Nb Ti Pb Bi Ca Alloy of Example
A 0.18 0.20 1.45 0.008 0.009 -- 0.15 -- 0.030 0.012 -- -- -- -- --
B 0.19 0.25 0.74 0.007 0.006 1.01 -- -- 0.029 0.013 -- -- -- -- --
C 0.19 0.31 1.47 0.010 0.008 -- 0.25 -- 0.029 0.011 -- -- -- -- --
D 0.20 0.43 0.71 0.007 0.008 1.03 -- -- 0.030 0.013 -- -- -- -- --
E 0.18 0.08 0.50 0.009 0.006 1.48 -- -- 0.031 0.012 -- -- -- -- --
F 0.18 0.06 0.50 0.009 0.010 1.47 -- 0.0018 0.029 0.008 0.090 0.033
-- -- -- G 0.18 0.10 0.65 0.011 0.017 0.96 0.40 0.0018 0.034 0.007
0.046 0.035 -- -- -- H 0.19 0.07 0.30 0.012 0.015 0.30 0.75 0.0019
0.032 0.008 0.051 0.037 -- -- -- I 0.18 0.06 1.48 0.011 0.015 1.12
0.00 0.0014 0.033 0.007 0.070 0.040 -- -- -- K 0.18 0.09 0.48 0.007
0.012 1.00 0.41 0.0015 0.030 0.009 0.050 0.028 0.09 -- -- L 0.18
0.09 0.49 0.009 0.011 1.03 0.41 0.0015 0.029 0.009 0.034 0.036 --
0.07 -- M 0.19 0.10 0.47 0.009 0.012 1.02 0.40 -- 0.029 0.010 -- --
-- -- 0.03 R 0.20 0.23 0.90 0.008 0.007 -- -- -- 0.032 0.011 -- --
-- -- -- Alloy of Compr. Example N 0.21 0.20 0.76 0.023 0.015 1.11
-- -- 0.032 0.012 -- -- -- -- -- O 0.18 0.43 0.35 0.011 0.017 2.62
-- -- 0.033 0.009 -- -- -- -- -- P 0.19 0.07 0.83 0.020 0.020 1.08
0.35 -- -- -- -- -- -- -- -- Q 0.18 0.06 0.50 0.011 0.014 1.47 --
0.0013 0.029 0.008 -- -- -- -- --
[0066]
2 TABLE 2 Austenite (left side) - (right side) (left side) - (right
side) 100 times impact grain size of Eq. (1) of Eq. (2) strength
(Nm) Alloy of Example A 8.0 (1) - 0.160 -0.049 12663 B 7.5 (2) -
0.063 -0.016 12518 C 8.1 (1) - 0.160 -0.040 12653 D 8.3 (2) - 0.039
-0.002 12397 E 9.1 (2) - 0.033 -0.019 12454 F 8.9 (2) - 0.013
-0.170 13123 G 8.8 (1) - 0.036 -0.147 12813 H 11.0 (1) - 0.031
-0.172 13024 I 9.4 (1) - 0.100 -0.144 13008 K 8.3 (1) - 0.061
-0.153 13001 L 9.0 (1) - 0.058 -0.155 13017 M 9.5 (1) - 0.039
-0.133 12939 R 9.0 (2) - 0.063 -0.017 12530 Alloy of Compr. Example
N 8.4 (2) - 0.080 0.125 11917 O 8.0 (2) - 0.051 0.065 12134 P 5.1
(1) - 0.018 0.126 11914 Q 4.3 (2) - 0.070 0.000 12289 Eq. (1): [C
%] + 5[P % + S %] .ltoreq. ([Mn %] + [Mo %] + 1.8) / 8 Eq. (2): [C
%] + 5.2[P % + S %] .ltoreq. ([Mn %] + [Mo %] + 3.8) / 22 + 96[B %]
+ [JIS austenite grain size number] / 111
[0067]
3 TABLE 3 C Si Mn P S Cr Mo B Nb Alloy of Ex- ample 1 0.19 0.07
0.41 0.006 0.017 0.97 0.15 0.0014 0.05 2 0.18 0.05 0.74 0.007 0.012
1.09 0.01 0.0017 0.05 3 0.18 0.10 0.50 0.010 0.018 1.00 0.20 0.0015
0.05 Alloy of Compr. Ex- ample 4 0.17 0.07 0.59 0.010 0.017 0.92
0.41 0.0013 0.05 5 0.18 0.07 0.40 0.008 0.015 0.95 0.40 0.0015 0.05
6 0.19 0.07 1.44 0.009 0.015 0.97 0.00 -- 0.00 7 0.19 0.06 0.82
0.008 0.014 1.07 0.41 -- 0.00 8 0.19 0.19 0.79 0.013 0.017 0.97
0.16 -- 0.00 9 0.21 0.15 0.60 0.015 0.020 1.20 0.25 0.0030 0.07
[0068]
4 TABLE 4 Cold forgeability Impact strength Left side Hardness Cold
forging (left side) - (right side) (left side) - (right side) 100
times impact Austenite of Eq. (4) (HRB) load ratio of Eq. (1) of
Eq. (3) strength ratio grain size Alloy of Example 1 33.4 74 100
-0.220 -0.095 124 7.99 2 35.1 73 98 -0.234 -0.163 128 7.97 3 39.6
79 104 -0.273 -0.095 124 8.00 Alloy of Compr. Example 4 46.3 85 117
-0.315 -0.105 126 8.04 5 41.4 82 109 -0.260 -0.126 123 8.05 6 40.2
81 105 -0.335 0.005 110 7.95 7 38.0 77 102 -0.299 0.004 108 7.97 8
39.4 79 100 -0.304 0.058 100 8.00 9 47.7 88 117 -0.296 -0.179 130
7.95 Eq. (1): [C %] + 5[P % + S %] .ltoreq. ([Mn %] + [Mo %] + 1.8)
/ 8 Eq. (3): [C %] + 5.2[P % + S %] .ltoreq. ([Mn %] + [Mo %] +
3.8) / 22 + 96[B] + [JIS grain size number] / 111 Eq. (4): 80[Si %]
+ 24[Mn %] + 33[Mo %] + 13) .ltoreq. 40
* * * * *