U.S. patent application number 12/735897 was filed with the patent office on 2011-01-06 for steel for induction hardening.
This patent application is currently assigned to NIPPON STEEL CORPORATION. Invention is credited to Toshiharu Aiso, Masayuki Hashimura, Manabu Kubota, Atsushi Mizuno, Hajime Saitoh.
Application Number | 20110002807 12/735897 |
Document ID | / |
Family ID | 42339706 |
Filed Date | 2011-01-06 |
United States Patent
Application |
20110002807 |
Kind Code |
A1 |
Saitoh; Hajime ; et
al. |
January 6, 2011 |
STEEL FOR INDUCTION HARDENING
Abstract
This steel for induction hardening includes: in terms of mass %,
C: 0.40% or more to 0.75% or less; Si: 0.002% or more to 3.0% or
less; Mn: 0.20% or more to 2.0% or less; S: 0.002% or more to 0.1%
or less; Al: more than 0.10% to 3.0% or less; P: 0.030% or less;
and N: 0.035% or less, with the remainder being Fe and inevitable
impurities.
Inventors: |
Saitoh; Hajime; (Tokyo,
JP) ; Aiso; Toshiharu; (Tokyo, JP) ;
Hashimura; Masayuki; (Tokyo, JP) ; Mizuno;
Atsushi; (Tokyo, JP) ; Kubota; Manabu; (Tokyo,
JP) |
Correspondence
Address: |
KENYON & KENYON LLP
ONE BROADWAY
NEW YORK
NY
10004
US
|
Assignee: |
NIPPON STEEL CORPORATION
Tokyo
JP
|
Family ID: |
42339706 |
Appl. No.: |
12/735897 |
Filed: |
January 5, 2010 |
PCT Filed: |
January 5, 2010 |
PCT NO: |
PCT/JP2010/000017 |
371 Date: |
August 23, 2010 |
Current U.S.
Class: |
420/83 ; 420/103;
420/104; 420/117; 420/120; 420/121; 420/123; 420/125; 420/126;
420/127; 420/128; 420/84; 420/87; 420/92 |
Current CPC
Class: |
C22C 38/02 20130101;
Y02P 10/25 20151101; Y02P 10/253 20151101; C22C 38/06 20130101;
C21D 1/42 20130101; C22C 38/04 20130101 |
Class at
Publication: |
420/83 ; 420/87;
420/103; 420/117; 420/120; 420/128; 420/104; 420/92; 420/121;
420/126; 420/123; 420/127; 420/84; 420/125 |
International
Class: |
C22C 38/60 20060101
C22C038/60; C22C 38/06 20060101 C22C038/06; C22C 38/02 20060101
C22C038/02; C22C 38/04 20060101 C22C038/04; C22C 38/00 20060101
C22C038/00; C22C 38/18 20060101 C22C038/18; C22C 38/16 20060101
C22C038/16; C22C 38/14 20060101 C22C038/14; C22C 38/12 20060101
C22C038/12 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 16, 2009 |
JP |
2009-007757 |
Claims
1. A steel for induction hardening comprising: in terms of mass %,
C: 0.40% or more to 0.75% or less; Si: 0.002% or more to 3.0% or
less; Mn: 0.20% or more to 2.0% or less; S: 0.002% or more to 0.1%
or less; Al: more than 0.10% to 3.0% or less; P: 0.030% or less;
and N: 0.035% or less, with the remainder being Fe and inevitable
impurities.
2. The steel for induction hardening according to claim 1, which
further comprises, in terms of mass %, B: 0.0004% or more to 0.005%
or less.
3. The steel for induction hardening according to claim 1, which
further comprises, in terms of mass %, Ti: 0.004% or more to 0.10%
or less.
4. The steel for induction hardening according to claim 1, which
further comprises, in terms of mass %, either one or both of: Cr:
0.05% or more to 1.50% or less; and Mo: 0.05% or more to 0.6% or
less.
5. The steel for induction hardening according to claim 1, which
further comprises, in terms of mass %, either one or both of: Nb:
0.005% or more to 0.2% or less; and V: 0.01% or more to 1.0% or
less.
6. The steel for induction hardening according to claim 1, which
further comprises, in terms of mass %, one or more elements
selected from the group consisting of: Sb: 0.0005% or more to
0.0150% or less; Sn: 0.005% or more to 2.0% or less; Zn: 0.0005% or
more to 0.5% or less; Te: 0.0003% or more to 0.2% or less; Bi:
0.005% or more to 0.5% or less; and Pb: 0.005% or more to 0.5% or
less.
7. The steel for induction hardening according to claim 1, which
further comprises, in terms of mass %, one or two or more elements
selected from the group consisting of: Mg: 0.0002% or more to
0.003% or less; Ca: 0.0003% or more to 0.003% or less; Zr: 0.0003%
or more to 0.005% or less; and REM: 0.0003% or more to 0.005% or
less.
8. The steel for induction hardening according to claim 1, which
further comprises, in terms of mass %, either one or both of: Ni:
0.05% or more to 2.0% or less; and Cu: 0.01% or more to 2.0% or
less.
Description
TECHNICAL FIELD
[0001] The present invention relates to a steel for induction
hardening (inductive quenching) having excellent machinability.
More particularly, the present invention relates to a steel for
induction hardening that is used for gear wheels, parts of
vehicles, such as a Continuously Variable Transmission (CVT) and a
Constant Velocity Joint (CVJ), and the like.
[0002] The present application claims priority on Japanese Patent
Application No. 2009-007757, filed on Jan. 16, 2009, the content of
which is incorporated herein by reference.
BACKGROUND ART
[0003] Conventionally, gear wheels for vehicles are generally
subjected to a surface hardening treatment before been used. As a
surface hardening method, carburization, nitriding, and induction
hardening (inductive quenching) are employed. Among these,
"carburization" aims to harden a matrix with a high toughness by
performing high carburization on the surface layer of a material,
and is mainly applied to materials of gear wheels, parts of
vehicles, such as a CVT and a CVJ, and the like for the purpose of
improved fatigue strength. However, the carburization treatment is
mainly a batch treatment in a gaseous atmosphere. Massive energy
and costs are consumed, since, for example, heating should be
performed at around 930.degree. C. and this temperature should be
maintained for several hours or more. In addition, in actual
operation, the treatment of carburized materials or the like is
vulnerable to the problem of environmental degradation, and in
addition, it is difficult to perform this treatment in-line.
[0004] For such reasons, in order to solve the foregoing problems,
studies were made, aiming to obtain the intended strength
characteristics only by an induction hardening treatment. This is
because the induction hardening is very advantageous in terms of
reduction in surface hardening treatment time, required energy, and
environmental impact.
[0005] As inventions relating to the induction hardening treatment
aiming to solve the foregoing problems, a proposal about a steel
for induction hardening is reported, for example, in Patent
Document 1. This is to provide a steel material in which the area
fraction of martensite is controlled to be in a range of 70% or
more with respect to a microstructure prior to an induction
hardening, by limiting the Si content to be in a range of 0.50% or
less and the Al content to be in a range of 0.10% or less.
According to this technique, the strength is notably improved for a
certainty. However, processability, in particular, machinability is
extremely degraded. Up to the present, as steel materials which are
subjected to carburization in the manufacturing of parts, so-called
case hardened steels such as JIS SCr420 and SCM420, in which the
amount of C is about 0.2%, are used. The greatest reason to use the
steel materials including a small amount of C is to ensure
machinability. After these steel materials are processed into the
parts, they are subjected to carburizing quenching. This results in
a rise in surface hardness; and thereby, the strength of the parts
is enhanced. However, in order to obtain an appropriate surface
hardness for the parts which are subjected to induction hardening,
it is essential to raise the amount of C of the steel material
itself to be in a range of 0.4% or more. In this case, the hardness
of the steel material is increased before the cutting, and thus
machinability degrades. Therefore, a steel material is required
that keeps good machinability even when it is hard due to an
increase in the amount of C. That is, up to the present, in the
technical field in which parts manufactured by carburization are
subjected to induction hardening, the biggest challenge is an
improvement of the machinability of the steel material.
[0006] Observing foregoing inventions aimed at raising
machinability, Patent Document 2 proposed an invention that
improves machinability. This is a BN free-cutting steel in which
large amounts of B and N are included such as 0.0050% or more of B,
and 0.007% or more of N. This technology is applicable to a low-C
steel (i.e., a steel that includes C at a low content) such as JIS
SUM11 which does not need strength but improves only machinability
such as surface roughness. However, if large amounts of B and N are
added to a middle-high-C steel (i.e., a steel that includes C at a
middle or high content) which is the main object of the invention,
hot brittleness becomes notably worse, and thus the manufacture of
the steel materials becomes difficult. In addition, the
characteristics of the steel material notably degrade in terms of
toughness and fatigue strength. Therefore, this is not a suitable
steel.
[0007] Patent Documents 3 and 4 are inventions that have both of
machinability and fatigue strength. In Patent Document 3, the total
fraction of a ferrite microstructure and a pearlite microstructure
is controlled to be in a range of 90% or more by adjusting the
composition, and furthermore, the maximum thickness of the ferrite
structure is controlled to be in a range of 30 .mu.m. Thereby, a
steel having both of machinability and fatigue strength is
provided. Although there are various types of steels in which the
total fraction of the ferrite microstructure and the pearlite
microstructure is in a range of 90% or more, this factor itself
(the controlling of the fraction of microstructures) is
insufficient to improve the machinability. New improvement based on
alloy elements is required. In Patent Document 4, the aspect ratio
of MnS is reduced to be in a range of 10 or less, and furthermore,
induction heating is performed up to the central portion of the
steel material. Thereby, machinability and fatigue strength are
improved. This method of improving machinability and fatigue
strength by lowering the aspect ratio of MnS is a method that has
conventionally been known. However, this method is insufficient,
and new improvement based upon alloy elements is required. In
addition, this method has the problem of limited practical use,
because a limitation is also given to an induction hardening
method. [0008] [Patent Document 1] Japanese Unexamined Patent
Application, First Publication No. 2007-131871 [0009] [Patent
Document 2] Japanese Unexamined Patent Application, First
Publication No. 2007-107020 [0010] [Patent Document 3] Japanese
Unexamined Patent Application, First Publication No. 2006-28598
[0011] [Patent Document 4] Japanese Unexamined Patent Application,
First Publication No. 2007-16271
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0012] The object of the invention is to improve the foregoing
drawbacks of the conventional inventions and to provide a steel for
induction hardening that has excellent machinability. In
particular, the object of the invention is to convert a process of
manufacturing gear wheels, parts used for vehicles such as a CVT
and a CVJ, and the like, from carburizing treatment to induction
hardening treatment.
Means for Solving the Problems
[0013] As a result of detailed examination carried out to solve the
foregoing objects, the inventors found that it is possible to
improve machinability while maintaining strength by increasing the
amount of Al to be much greater than that of conventional steels
and appropriately controlling the amount of Al and the amount of N;
and thereby, the present invention was completed. That is, the
present invention has the following aspects.
[0014] A steel for induction hardening of the invention includes:
in terms of mass %, C, 0.40% or more to 0.75% or less; Si: 0.002%
or more to 3.0% or less; Mn: 0.20% or more to 2.0% or less; S:
0.002% or more to 0.1% or less; Al: more than 0.10% to 3.0% or
less; P: 0.030% or less; and N: 0.035% or less, with the remainder
being Fe and inevitable impurities.
[0015] The steel for induction hardening of the invention may
further include, in terms of mass %, B: 0.0004% or more to 0.005%
or less.
[0016] The steel for induction hardening of the invention may
further include, in terms of mass %, Ti: 0.004% or more to 0.10% or
less.
[0017] The steel for induction hardening of the invention may
further include, in terms of mass %, either one or both of: Cr:
0.05% or more to 1.50% or less; and Mo: 0.05% or more to 0.6% or
less.
[0018] The steel for induction hardening of the invention may
further include, in terms of mass %, either one or both of: Nb:
0.005% or more to 0.2% or less; and V: 0.01% or more to 1.0% or
less.
[0019] The steel for induction hardening of the invention may
further include, in terms of mass %, one or more elements selected
from the group consisting of: Sb: 0.0005% or more to 0.0150% or
less; Sn: 0.005% or more to 2.0% or less; Zn: 0.0005% or more to
0.5% or less; Te: 0.0003% or more to 0.2% or less; Bi: 0.005% or
more to 0.5% or less; and Pb: 0.005% or more to 0.5% or less.
[0020] The steel for induction hardening of the invention may
further include, in terms of mass %, one or two or more elements
selected from the group consisting of: Mg: 0.0002% or more to
0.003% or less; Ca: 0.0003% or more to 0.003% or less; Zr: 0.0003%
or more to 0.005% or less; and REM: 0.0003% or more to 0.005% or
less.
[0021] The steel for induction hardening of the invention may
further include, in terms of mass %, either one or both of: Ni:
0.05% or more to 2.0% or less; and Cu: 0.01% or more to 2.0% or
less.
EFFECTS OF THE INVENTION
[0022] In accordance with the steel for induction hardening of the
present invention, it is possible to improve the machinability
while maintaining the strength of the steel for induction
hardening, because the content of C is in a range of 0.40% or more
to 0.75% or less, and the content of Al is in a range of more than
0.10% to 3.0% or less.
[0023] Therefore, the present invention provides the steel that
makes it possible, in particular, to convert the process of
manufacturing gear wheels, parts which are used for a CVT and a CVJ
for vehicles, and the like, from carburizing treatment to induction
hardening treatment.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a diagram showing the relationship between the Al
content and the lifetime of a steel material.
BEST MODE FOR CARRYING OUT THE INVENTION
[0025] Hereinafter, exemplary embodiments of the invention will now
be described in detail.
[0026] The reason why a steel composition is limited will be
described. Herein, the unit % of the content of the component
represents % by mass.
[0027] C: equal to or more than 0.40% and equal to or less than
0.75%
[0028] C serves to ensure a surface hardness after induction
hardening and to ensure an intended strength of steel (core part).
In the case where the content of C is in a range of less than
0.40%, intended effects due to the above action cannot be obtained.
Meanwhile, in the case where the content of C exceeds 0.75%,
toughness degrades. This causes a manufacturing problem such as the
aging cracking of a rolled steel or the like. Therefore, the
content of C is set to be in a range of 0.40% or more to or 0.75%
or less, and is preferably adjusted to be in a range of 0.50% or
more to 0.65% or less in order to more stably ensure the
above-described effects.
[0029] Al: greater than 0.10% and not greater than 3.0%
[0030] Al will be described in detail, since Al is the most
important element in the steel of the invention.
[0031] A plurality of ingots were prepared by adjusting the
compositions to include: C, 0.50% or more to 0.60% or less; Si:
0.002% or more to 0.80% or less; Mn: 0.50% or more to 0.9% or less;
S: 0.005% or more to 0.1% or less; Al: 0.010% or more to 3.5% or
less; N: 0.001% or more to 0.035% or less; and P: 0.030% or less,
with the balance being Fe and inevitable impurities. Rolled steels
of 50.phi.) were manufactured by using the ingots. The compositions
were adjusted in this fashion so that the hardness of the rolled
steels becomes in a range approximately from 200 to 220 HV.
Disc-like samples of 45.phi..times.15 mm were manufactured from
these materials.
[0032] These samples were subjected to a machinability test (i.e.,
a test method described in "METHOD OF FINDING AND SELECTING
WORKPIECE MATERIAL AND TOOL MATERIAL WITHOUT ERROR" edited by
Katayama Akira, published by The Nikkan Kogyo Shimbun, Ltd., Tokyo,
2007, P. 27), under the conditions presented in Table 1. Holes
having a total depth of 1000 mm were bored in the disk-like samples
at a certain drill rotation rate (m/min). In the case where a drill
is not damaged during the drilling, holes having a total depth of
1000 mm were bored at a faster drill rotation rate, using a new
drill. This operation was carried out until a drill is damaged. And
then, among the rotation rates at which the drills were not
damaged, the maximum rotation rate (i.e., the maximum drilling rate
(m/min) at which the total depth of the hole bored by the drill was
1000 mm) was used in the evaluation of machinability. This is a
test that evaluates the tool lifetime, and it can be understood
that the tool is less easily damaged and the machinability of the
steel is better if the maximum cutting rate is faster.
TABLE-US-00001 TABLE 1 Drilling conditions Drill Drilling rate: 10
to 90 m/min .phi. 3 mm Transport: 0.25 mm/rev NACHI (trade name)
HSS drill: Shape Water soluble cutting oil No. SD3.0 drill,
available from was used NACHI-FUJIKCOSHI CORPORATION
[0033] The test result is presented in FIG. 1. Referring to FIG. 1,
it can be understood that an excellent tool lifetime can be
obtained in the case where the amount of Al exceeds 0.10% and is
not greater than 3.0% (in a range of more than 0.1% to 3.0% or
less).
[0034] From the above test result, the amount of Al for improving
the tool lifetime is set to be in a range of more than 0.1% to 3.0%
or less.
[0035] Although the detailed reason regarding why it was possible
to obtain these interesting results is unclear, the inventors
examined the surface of the tool by EPMA or Auger electron
spectroscopy after the cutting test. The inventors could confirm
that Al.sub.2O.sub.3 was formed on the surface of the tool after
the cutting even though it was not observed on the surface of a new
product tool. From this, it is considered that a hard
Al.sub.2O.sub.3 film was formed through the reaction of Al in the
steel, which was attached to the surface of the tool during the
cutting, with oxygen in the air, oxygen contained in a cutting oil,
or oxygen in a homo-treatment film (Fe.sub.3O.sub.4) on the surface
of a High Speed Steel (HSS) drill (The homo-treatment is also
referred to as a steam treatment, and is a treatment that forms a
steel oxide film having a thickness of several micrometers by heat
treatment in the steam or the like to provide corrosion resistance
or the like to a tool. Refer to "HANDBOOK FOR HEAT TREATMENT,"
edited by The Japan Society for Heat Treatment, published by The
Nikkan Kogyo Shimbun, Ltd., Tokyo, 2000, P. 569).
[0036] Since Al.sub.2O.sub.3 has a hardness of about 3000 HV,
Al.sub.2O.sub.3 is harder than a HSS drill steel (having a hardness
of about 700 HV), and does not wear easily. In addition, generally,
if a steel material is the same material (steel) as the tool,
adhesion occurs in the contact portion; and thereby, the tool is
vulnerable to abrasion (adhesion abrasion). In contrast, if
Al.sub.2O.sub.3 is interposed between these materials, it is
possible to reduce the contact between the same materials (steel);
and thereby, the wear of the tool can be suppressed. Therefore, it
was surmised that the lifetime of the tool was improved since the
Al.sub.2O.sub.3 film prevented the adhesion abrasion.
[0037] Conventional free-cutting steel, which includes Pb, easily
melts due to a rise in temperature during the cutting processing
because Pb has a low melting point of about 330.degree. C. This
causes a lubricating action in the interface between the tool and
cutting scraps; and thereby, adhesion is prevented. In addition,
since ductility is lowered due to the melting of Pb, it is easy to
cause a ductile fracture in a portion surrounding a blade edge.
Therefore, machinability is improved since the deformation
processing energy necessary for the cutting is decreased. However,
this is not preferable in respect to health, and thus a
free-cutting element, which does not include Pb, is required. In a
conventional free-cutting steel, which includes S, machinability is
improved due to two effects. The first effect is a lubricating
action which is created by MnS since MnS is greatly deformed at
high temperatures and is attached to the interface between the tool
and the cutting scraps. The second effect is a promoting of the
ductile fracture attained by the fact that MnS in a portion
contacting the blade edge of the tool becomes the fracture starting
point. However, since MnS is drawn in hot forging, there is a
problem in that mechanical characteristics such as ductile
toughness in a direction perpendicular to the forging direction are
degraded.
[0038] In contrast, in the steel of the present invention, it is
considered that an Al.sub.2O.sub.3 film is formed on the surface of
the tool; and thereby, the wear of the tool can be suppressed.
[0039] In general, if the hardness of a steel material is higher,
the life time of the tool generally decreases. However, when
compared to a steel material having an equal level of hardness, the
steel of the present invention has an effect of increasing the
lifetime of the tool.
[0040] The amount of Al is preferably in a range of 0.11% or more
to 3.0% or less. The amount of Al is more preferably in a range of
0.15% or more to 2.9% or less, and is even more preferably in a
range of 0.2% or more to 1.1% or less.
[0041] This embodiment relates to a steel for induction hardening,
and in general, in the induction hardening, a surface portion of
the steel with a thickness of 2 to 3 mm from the surface is heated
at a temperature within a range of not less than A.sub.1 point
(i.e., a transformation temperature from the ferrite phase (.alpha.
phase) into the austenite phase (.gamma. phase)), and then the
steel is subjected to water-cooling. Due to this, the surface layer
becomes martensite (having a hardness of about 600 HV or more).
[0042] If the content of Al increases, the A.sub.1 point rises. If
the content of Al exceeds 3.0%, phase transformation does not occur
in the induction hardening. Therefore, from the point of the
induction hardening, it is necessary to set the content of Al to be
in a range of 3.0% or less.
[0043] In addition, Al typically acts as a deoxidizing agent and,
as a result, Al.sub.2O.sub.3 remains in the steel at a content in a
range of approximately 0.001% or more to 0.002% or less. Here, the
amount of Al in Al.sub.2O.sub.3 is
27.times.2/(27.times.2+16.times.3). A portion of the remaining Al
is bonded with N; and thereby, AlN is formed. It is considered that
Al in MN hardly forms a solid solution and does not easily react
with oxygen in the atmosphere, oxygen in the cutting lubricant, or
oxygen in the homo-treatment film (Fe.sub.3O.sub.4) on the surface
of the HSS drill.
[0044] Therefore, it is preferred that the content of
solid-solution Al (i.e., the amount of Al except for AlN) exceeds
0.1%. For this, it is preferred that the following relationship be
satisfied:
[% Al]-(27/14).times.[% N]-0.001>0.10%,
[0045] Here, the bracket [ ] in the relationship indicates the
content of an element (by mass percent). In addition, the above
relationship is a formula that can be obtained on the assumption
that all the amount of N in the steel is bonded with Al due to heat
treatments or the like performed in the manufacture of the
steel.
[0046] Si: equal to or more than 0.002% and equal to or less than
3.0%
[0047] Si is an element that is added as a deoxidizing agent in
steelmaking as well as improving the strength of a steel material,
and the content of Si is adjusted according to the required
strength. However, the content of Si is required to be in a range
of 0.002% or more in order to effectuate the deoxidizing action.
Meanwhile, in the case where the content of Si exceeds 3.0%, the
toughness and the ductility of the steel material are lowered and,
at the same time, the machinability of the steel material is
lowered because a number of hard inclusions are generated inside
the steel material. Therefore, the content of Si is set to be in a
range of 0.002% or more to 3.0% or less. The content of Si is
preferably in a range of 0.3% or more to 3.0% or less. The amount
of Si is more preferably in a range of 0.4% or more to 2.5% or
less, and is even more preferably in a range of 0.5% or more to
2.2% or less. If the amount of Si is set to be in a range of 0.6%
or more to 2.1% or less, excellent strength is attained. If the
amount of Si is set to be in a range of 0.8% or more to 2.0% or
less, more excellent strength is attained.
[0048] Mn: equal to or more than 0.20% and equal to or less than
2.0%
[0049] Similar to Si, Mn is an element that improves the strength
of the steel material, and the content of Mn is adjusted according
to the required strength. Therefore, it is required to ensure the
content of Mn in a range of 0.20% or more in order to effectuate
this action. However, in the case where the content of Mn exceeds
2.0%, hardenability is raised excessively. Thus, in the manufacture
of the material, the forming of a bainite microstructure or a
martensite-austenite constituent is promoted; and thereby,
processability is lowered. Therefore, the amount of Mn is set to be
in a range of 0.20% or more to 2.0% or less.
[0050] In the case where the shape of a part is formed by cutting
the steel of the present embodiment and then the part is subjected
to induction hardening, it is preferred that the steel be
relatively soft in processes up to the cutting process and the
steel be hardened to have an intended hardness by the induction
hardening. In order to realize such excellent processability, the
content of Mn is set to be, preferably, in a range of 0.40% or more
to 1.5% or less and, more preferably, in a range of 0.45% or more
to 1.0% or less.
[0051] S: equal to or more than 0.002% and equal to or less than
0.1%
[0052] It is required to include S at a content in a range of
0.002% or more in order to ensure the minimum machinability. In the
case where the content of S exceeds 0.1%, toughness or fatigue
strength is caused to degrade. Therefore, the content of S is set
to be in a range of 0.002% or more to 0.1% or less. In the case of
being used for gear wheels, the content of S is, preferably, in a
range of 0.005 or more to 0.06% or less and, more preferably, in a
range of 0.01 or more to 0.05% or less.
[0053] P: 0.030% or less.
[0054] P serves to degrade the toughness of a hardened layer. In
particular, in the case where the content of P exceeds 0.030%,
toughness is caused to notably degrade. Therefore, the content of P
is set to be in a range of 0.030% or less. The content of P is set
to be, preferably, in a range of 0.0001% or more to 0.030% or less
and, more preferably, in a range of 0.0001% or more to 0.020% or
less.
[0055] N: 0.035% or less
[0056] In the case where the addition amount of N exceeds 0.035%,
it leads to notable degradation in terms of hot brittleness, and
this makes it extremely difficult to manufacture a rolled steel.
Therefore, the content of N is limited to be in a range of 0.035%
or less.
[0057] In addition, N reacts with Al to form AlN; and thereby, N
has an effect to suppress the coarsening of crystal grains. In
general, it is difficult to form such large crystal grains via
induction heating because the heating time is extremely short
unlike a typical heating in a heat treatment furnace. However, in
the case where it is intended to positively promote the refining of
crystal grains, it is preferred that the content of N be set to be
in a range of 0.0001% or more to 0.035% or less. More preferably, N
is added at an amount in a range of approximately 0.001% or more to
0.015% or less. In addition, it is even more preferred that the
amount of N be set to be in a range of approximately 0.002% or more
to 0.007% or less.
[0058] It is preferred that the steel of the present invention
further include the elements, which will be mentioned below,
according to the necessity.
[0059] B: equal to or more than 0.0004% and equal to or less than
0.005%
[0060] B is an element that is important in two points. One point
relates to an action that provides hardenability to the steel. In
the case where the amount of B is in a range of 0.0004% or more, B
sufficiently segregates in austenite grain boundaries; and thereby,
hardenability is developed. In addition, B is practical in terms of
economy, since a very small amount of B can develop the
hardenability and the cost of raw materials of B is inexpensive.
The other point relates to an action that raises the strength of
crystal grain boundaries. If the surface layer is hardened via
induction hardening, it becomes brittle and fractures in the
crystal grain boundaries. B serves to prevent this embrittlement.
Even in this case, it is required that B sufficiently segregates
across the austenite grain boundaries; and therefore, it is
necessary to add B at a content in a range of 0.0004% or more. In
the case where the added amount of B exceeds 0.005%, the steel
material becomes brittle instead. Therefore, the amount of B is set
to be in a range of 0.0004% or more to 0.005% or less.
[0061] Particularly, in the steel of the present invention, since
the content of Al exceeds 0.1%, BN is rarely formed; and therefore,
the above-described actions and effects of B are easy to
obtain.
[0062] The content of B is, preferably, in a range of 0.0005 or
more to 0.004% or less and, more preferably, in a range of 0.001%
or more to 0.0035% or less. In this case, a steel can be realized
which has excellent hardenability as well as excellent mechanical
characteristics.
[0063] Ti: equal to or more than 0.004% and equal to or less than
0.10%
[0064] Ti is an element that is important in two points. One point
relates to an action that refines the diameter of crystal grains
after induction heating. The other point relates to an action that
prevents a decrease in the amount of solid-solution B due to the
forming of BN, by segregating N in the form of TiN. Typically, in
the latter point, the addition amount of Ti is required to be 3.43
times that of N. However, since the steel of the present invention
includes a great amount of Al, such an amount of Ti is not
necessary. In the case where the content of Ti is in a range of
less than 0.004%, both the effects are insufficient. Meanwhile, in
the case where the content of Ti exceeds 0.10%, coarse and large Ti
inclusions are formed and act as the starting point of fatigue
fracture. Therefore, the content of Ti is set to be in a range of
0.004% or more to 0.10% or less.
[0065] The content of Ti is, preferably, in a range of 0.005% or
more to 0.08% or less and, more preferably, in a range of 0.01% or
more to 0.03% or less. In this case, the effect of improving the
hardenability due to solid-solution B can be effectively utilized,
and in addition, the crystal grains can be refined.
[0066] Either one or both of Cr: equal to or more than 0.05% and
equal to or less than 1.50%, and Mo: equal to or more than 0.05%
and equal to or less than 0.6%
[0067] Cr and Mo are elements that improve the strength of the
steel material, and may be included in a certain amount according
to the required strength and the size of parts.
[0068] However, in the case where the content of Cr is in a range
of less than 0.05%, it is impossible to obtain the intended effects
of the above-described actions. Meanwhile, in the case where the
content of Cr exceeds 1.50%, hardenability is raised excessively.
Thus, in the manufacture of a material (steel), the forming of a
bainite microstructure or a martensite-austenite constituent is
promoted; and thereby, processability is lowered. Therefore, if
added, the amount of Cr is set to be in a range of 0.05% or more to
1.50% or less. In particular, in the case where it is required to
easily melt cementite during induction heating so as to uniformize
solid-solution C, it is preferable that the content of Cr be in a
range of 0.05% or more to 0.2% or less.
[0069] The effect cannot be obtained in the case where the amount
of Mo is in a range of less than 0.05%. Meanwhile, in the case
where the content of Mo exceeds 0.6%, hardenability is raised
excessively. Thus, in the manufacture of the material (steel), the
forming of a bainite structure or a martensite-austenite
constituent is promoted; and thereby, processability is lowered.
Therefore, if added, the amount of Mo is set to be in a range of
0.05% or more to 0.6% or less.
[0070] In an attempt to improve the strength of the steel material,
either one or both of Cr and Mo may be added, since Cr and Mo
perform the common action.
[0071] Either one or both of Nb: equal to or more than 0.005% and
equal to or less than 0.2%, and V: equal to or more than 0.01% and
equal to or less than 1.0%
[0072] Carbonitrides of Nb and V precipitate in the steel, and
these carbonitrides have a pinning property to fix crystal grain
boundaries; and thereby, Nb and V serve to refine crystal grains.
This results in a rise in the strength of the crystal grain
boundaries.
[0073] In the case where the amount of Nb is in a range of less
than 0.005%, the amount of precipitates is small; and therefore,
the action of suppressing the growth of grains is insufficient.
Meanwhile, in the case where the amount of Nb exceeds 0.2%, the hot
brittleness of the steel increases, and this makes it difficult to
manufacture the steel. Therefore, the amount of Nb is set to be in
a range of 0.005% or more to 0.2% or less.
[0074] In the case where the amount of V is in a range of less than
0.01%, the amount of precipitates is small; and therefore, the
action of suppressing the growth of grains is insufficient.
Meanwhile, in the case where the amount of V exceeds 1.0%, the hot
brittleness of the steel increases, and this makes it difficult to
manufacture the steel. Therefore, the amount of V is set to be in a
range of 0.01% or more to 1.0% or less.
[0075] Either one or both of Nb and V may be added, since Nb and V
perform the same action.
[0076] Either one of both of Ni: equal to or more than 0.05% and
equal to or less than 2.0%, and Cu: equal to or more than 0.01% and
equal to or less than 2.0%
[0077] Both of Ni and Cu are elements that improve the strength of
the steel material, and may be included in a certain amount
according to the required strength and the size of parts.
[0078] However, in the case where the content of Ni is in a range
of less than 0.05%, it is impossible to obtain the intended effects
of the above-described actions. Meanwhile, in the case where the
content of Ni exceeds 2.0%, hardenability is raised excessively.
Thus, in the manufacture of a material (steel), the forming of a
bainite structure or a martensite-austenite constituent is
promoted; and thereby, processability is lowered. Therefore, the
amount of Ni is set to be in a range of 0.05% or more to 2.0% or
less.
[0079] The intended effects of the above-described actions cannot
be obtained in the case where the content of Cu is in a range of
less than 0.01%. Meanwhile, in the case where the content of Cu
exceeds 2.0%, hardenability is raised excessively. Thus, in the
manufacture of the material (steel), the forming of a bainite
structure or a martensite-austenite constituent is promoted; and
thereby, processability is lowered. Therefore, the amount of Cu is
set to be in a range of 0.01% or more to 2.0% or less. In addition,
in case of including Cu, it is preferred that Ni be also added at
an amount half as large as that of Cu, because Cu also has the
problem of causing hot brittleness.
[0080] In addition, in the case of improving machinability, one or
two elements selected from the group consisting of: Sb: equal to or
more than 0.0005% and equal to or less than 0.0150%; Sn: equal to
or more than 0.005% and equal to or less than 2.0%; Zn: equal to or
more than 0.0005% and equal to or less than 0.5%; Te: equal to or
more than 0.0003% and equal to or less than 0.2%; Bi: equal to or
more than 0.005% and equal to or less than 0.5%; and Pb: equal to
or more than 0.005% and equal to or less than 0.5% may be added in
addition to the respective components as described above.
[0081] Sb: equal to or more than 0.0005% and equal to or less than
0.0150%
[0082] Sb makes ferrite brittle appropriately; and due to this
action, Sb serves to improve machinability. In the case where the
content of Sb is in a range of less than 0.0005%, the effect cannot
be exhibited. In addition, in the case where the content of Sb
exceeds 0.0150%, the macrosegregation of Sb becomes excessive, and
this makes it difficult to manufacture the steel. Therefore, if Sb
is added, the content of Sb is set to be in a range of 0.0005% or
more to 0.0150% or less.
[0083] Sn: equal to or more than 0.005% and equal to or less than
2.0%
[0084] Sn makes ferrite brittle; and due to this action, Sn has
effects of increasing tool lifetime and improving surface
roughness. However, in the case where the content of Sn is in a
range of less than 0.005%, those effects cannot be exhibited. In
addition, in the case where the content of Sn exceeds 2.0%, the
manufacture of the steel becomes difficult. Therefore, if Sn is
added, the content of Sn is set to be in a range of 0.005% or more
to 2.0% or less.
[0085] Zn: equal to or more than 0.0005% and equal to or less than
0.5% Zn makes ferrite brittle; and due to this action, Zn has
effects of increasing tool lifetime and improving surface
roughness. However, in the case where the content of Zn is in a
range of less than 0.0005%, those effects cannot be exhibited. In
addition, in the case where the content of Zn exceeds 0.5%, the
manufacture of the steel becomes difficult. Therefore, if Zn is
added, the content of Zn is set to be in a range of 0.0005% or more
to 0.5% or less.
[0086] Te: equal to or more than 0.0003% and equal to or less than
0.2%
[0087] Te is an element that improves machinability. In addition,
Te forms MnTe, and Te coexists with MnS. Thereby, Te lowers the
deformability of MnS; and due to this action, Te serves to suppress
the drawing of a MnS shape. In this way, Te is an element that is
effective in reducing anisotropy. However, in the case where the
content of Te is in a range of less than 0.0003%, those effects
cannot be exhibited. In addition, in the case where the content of
Te exceeds 0.2%, not only do those effects become saturated but
also the hot ductility is lowered, and this becomes the cause of
faults. Therefore, if Te is added, the content of Te is set to be
in a range of 0.0003% or more to 0.2% or less.
[0088] Bi: equal to or more than 0.005% and equal to or less than
0.5%
[0089] Bi is an element that improves machinability. However, in
the case where the content of Bi is in a range of less than 0.005%,
that effect cannot be obtained. In addition, in the case where the
content of Bi exceeds 0.5%, not only does the effect of improving
machinability become saturated but also the hot ductility is
lowered, and this becomes the cause of faults. Therefore, if Bi is
added, the content of Bi is set to be in a range of 0.005% or more
to 0.5% or less.
[0090] Pb: equal to or more than 0.005% and equal to or less than
0.5%
[0091] Pb is an element that improves machinability. However, in
the case where the content of Pb is in a range of less than 0.005%,
that effect is not confirmed. In addition, in the case where the
content of Pb exceeds 0.5%, not only does the effect of improving
machinability become saturated but also hot ductility is lowered,
and this becomes the cause of faults. Therefore, if Pb is added,
the content of Pb is set to be in a range of 0.005% or more to 0.5%
or less.
[0092] In addition, in the case of controlling the configuration of
MnS, one or more selected from the group consisting of Mg: equal to
or more than 0.0002% and equal to or less than 0.003%; Ca: equal to
or more than 0.0003% and equal to or less than 0.003%; Zr: equal to
or more than 0.0003% and equal to or less than 0.005%; and REM:
equal to or more than 0.0003% and equal to or less than 0.005% may
be added.
[0093] Mg: equal to or more than 0.0002% to and equal to or less
than 0.003%
[0094] Elongated MnS, which is present in a steel part, has a
drawback to cause anisotropy in mechanical characteristics of the
steel part and a drawback that it becomes a fracture starting point
of metal fatigue. Extreme fatigue strength is required depending on
parts, and in such cases, the addition of Mg is effective to
control the configuration of MnS. Mg forms (Mg, Mn)S in the steel;
and thereby, MnS becomes a harder compound. Therefore, drawing does
not occur during the rolling, and thus it becomes possible to
control the configuration. For the purpose of controlling the
configuration of MnS, it is necessary to include Mg at a content in
a range of 0.0002% or more. Meanwhile, in the case where the
content of Mg exceeds 0.003%, Mg coarsens oxides; and thereby,
fatigue strength is degraded against the intention. Therefore, if
Mg is added, the content of Mg is set to be in a range of 0.0002 or
more to 0.003% or less.
[0095] Ca: equal to or more than 0.0003% to and equal to or less
than 0.003%
[0096] Ca is an element that also helps the controlling of the
configuration of MnS. Ca forms (Ca, Mn)S in the steel; and thereby,
MnS becomes a harder compound. Therefore, drawing does not occur
during the rolling, and thus it becomes possible to control the
configuration. For the purpose of controlling the configuration of
MnS, it is necessary to include Ca at a content in a range of
0.0003% or more. Meanwhile, in the case where the content of Ca
exceeds 0.003%, Ca coarsens oxides; and thereby, fatigue strength
is degraded instead. Therefore, if Ca is added, the content of Ca
is set to be in a range of 0.0003 or more to 0.003% or less.
[0097] Zr: equal to or more than 0.0003% to and equal to or less
than 0.005%
[0098] Zr is also an effective element for controlling the
configuration of MnS. Zr forms (Zr, Mn)S in the steel; and thereby,
MnS becomes a harder compound. Therefore, drawing does not occur
during the rolling, and thus it becomes possible to control the
configuration. In order to control the configuration of MnS, it is
necessary to include Zr at a content in a range of 0.0003% or more.
Meanwhile, in the case where the content of Zr exceeds 0.005%, Zr
coarsens oxides; and thereby, fatigue strength is degraded instead.
Therefore, if Zr is added, the content of Zr is set to be in a
range of 0.0003 or more to 0.005% or less.
[0099] REM: equal to or more than 0.0003% to and equal to or less
than 0.005%
[0100] REM is also an effective element for controlling the
configuration of MnS. REM forms (REM, Mn)S in the steel; and
thereby, MnS becomes a harder compound. Therefore, drawing does not
occur during the rolling, and thus it becomes possible to control
the configuration. In order to control the configuration of MnS, it
is necessary to include REM at a content in a range of 0.0003% or
more. Meanwhile, in the case where the content of REM exceeds
0.005%, REM coarsens oxides; and thereby, fatigue strength is
degraded against the intention. Therefore, if REM is added, the
content of REM is set to be in a range of 0.0003% or more to 0.005%
or less.
[0101] In addition, REM indicates a rare earth metal element, which
is one or more selected from among Sc, Y, La, Ce, Pr, Nd, Pm, Sm,
Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.
[0102] The steel of the present invention can realize the
above-described actions and effects in the case where it has the
above described composition. Therefore, the steel of the present
invention is manufactured by known methods, and manufacturing
conditions thereof are not specifically limited. Typically, the
steel of the present invention is a rolled steel that could be
produced by controlling the composition of a slab and subjecting
the slab to hot rolling or cold rolling.
[0103] Although the steel of the present invention is a steel
having extremely excellent machinability, examples of utilizing the
steel (i.e., processes of manufacturing products such as parts and
the like using the steel) may include a case in which the shape of
a part is formed by cutting a rolled steel (i.e., the steel of the
present invention), and then induction hardening is performed, a
case in which a rolled steel is forged, the shape of a part is
formed by cutting the forged rolled steel, and then induction
hardening is performed, a case in which annealing is performed
prior to cutting, and the like. In addition, it is possible to
further raise the hardness of the surface layer after induction
hardening by performing soft nitriding prior to the induction
hardening, and the steel of the present invention can be applied to
such a process. Furthermore, it is possible manufacture parts from
the steel by combining the above-described plurality of processes
with each other.
Examples
[0104] Hereinafter, the present invention will be described in more
detail with respect to Examples. However, the following Examples
are not intended to limit the invention. All changes in design made
in view of the intentions, which will be described later, shall
fall within the technical scope of the present invention.
[0105] Below, the present invention will be described in detail
with respect to Examples.
[0106] Steel bars of 50.phi. were manufactured by melting and
rolling steels presented in Tables 2 to 4.
TABLE-US-00002 TABLE 2 Ingredients of steel material (% by mass)
Sample Other Classification No. C Si Mn P S Al N B Ti Cr Mo Nb V Ni
Cu Pb element(s) Inventive Ex 1A 0.43 0.68 0.42 0.026 0.047 0.150
0.0023 Inventive Ex 1B 0.44 0.71 0.41 0.025 0.040 0.108 0.0023
Comparative Ex 1C 0.45 0.26 0.64 0.017 0.020 0.070 0.0051
Comparative Ex 1D 0.35 0.25 0.65 0.017 0.017 0.110 0.0051 Inventive
Ex 2A 0.50 0.45 0.53 0.011 0.013 0.140 0.0031 0.12 Comparative Ex
2B 0.50 0.45 0.53 0.012 0.015 0.065 0.0031 0.12 Inventive Ex 3A
0.51 0.39 0.52 0.020 0.018 0.110 0.0029 Inventive Ex 3B 0.53 0.05
0.70 0.018 0.052 0.139 0.0044 Inventive Ex 3C 0.55 0.20 0.64 0.013
0.012 0.200 0.0060 0.022 Inventive Ex 3D 0.52 0.59 0.59 0.030 0.048
0.120 0.0062 0.021 Comparative Ex 3E 0.52 0.59 0.59 0.020 0.045
0.052 0.0062 Inventive Ex 4A 0.51 0.72 0.53 0.009 0.020 0.120
0.0050 Sb: 0.01 Comparative Ex 4B 0.51 0.72 0.53 0.009 0.020 0.044
0.0050 Inventive Ex 5A 0.56 0.05 0.55 0.024 0.027 0.123 0.0071 Bi:
0.011 Comparative Ex 5B 0.56 0.05 0.55 0.010 0.022 0.052 0.0130 Bi:
0.011 Inventive Ex 6A 0.65 0.05 0.39 0.014 0.022 1.300 0.0048
Comparative Ex 6B 0.43 0.68 0.42 0.026 0.047 0.150 0.0023
TABLE-US-00003 TABLE 3 Ingredients of steel material (% by mass)
Sample Other Classification No. C Si Mn P S Al N B Ti Cr Mo Nb V Ni
Cu Pb element(s) Inventive Ex 7A 0.55 0.38 0.68 0.026 0.032 0.146
0.0078 Sn: 0.05 Comparative Ex 7B 0.55 0.38 0.68 0.011 0.027 0.065
0.0078 Sn: 0.05 Inventive Ex 8A 0.53 0.29 0.50 0.010 0.018 0.116
0.0077 0.18 Inventive Ex 8B 0.51 0.60 0.80 0.014 0.008 0.160 0.0130
Inventive Ex 8C 0.59 0.29 0.61 0.018 0.013 0.148 0.0065 0.35 0.50
Inventive Ex 8D 0.55 0.53 0.76 0.010 0.018 0.172 0.0026 0.057
Inventive Ex 8E 0.56 0.67 0.66 0.022 0.022 0.128 0.0065 Inventive
Ex 8F 0.59 0.27 0.63 0.021 0.011 0.153 0.0062 0.0025 0.030 0.46
Inventive Ex 8G 0.53 0.67 0.87 0.017 0.051 0.189 0.074 Inventive Ex
8H 0.53 0.60 0.95 0.018 0.050 0.250 0.0038 Comparative Ex 8i 0.59
0.74 0.43 0.021 0.051 0.063 0.0056 Comparative Ex 8J 0.56 0.61 0.70
0.016 0.050 3.320 0.0071 Inventive Ex 9A 0.56 0.58 0.80 0.010 0.032
0.180 0.0043 Zn: 0.06 Comparative Ex 9B 0.56 0.58 0.80 0.015 0.033
0.071 0.0043 Zn: 0.06 Inventive Ex 10A 0.59 0.56 0.73 0.010 0.017
0.136 0.0053 0.0021 Inventive Ex 10B 0.53 0.66 0.97 0.009 0.018
0.195 0.0070 Inventive Ex 10C 0.60 1.25 0.54 0.014 0.023 0.125
0.0041 Comparative Ex 10D 0.60 1.25 0.54 0.011 0.027 0.072
0.0041
TABLE-US-00004 TABLE 4 Ingredients of steel material (% by mass)
Sample Other Classification No. C Si Mn P S Al N B Ti Cr Mo Nb V Ni
Cu Pb element(s) Inventive Ex 11A 0.58 1.42 0.53 0.012 0.032 0.130
0.0061 Zr: 0.001 Inventive Ex 11B 0.60 1.53 0.41 0.016 0.018 0.102
0.0003 Inventive Ex 11C 0.62 1.39 0.55 0.019 0.025 1.250 0.0053
REM: 0.002 Inventive Ex 11D 0.59 1.57 0.58 0.01 0.062 0.155 0.0043
Mg: 0.001 Inventive Ex 11E 0.69 0.58 0.85 0.010 0.013 0.200 0.0160
Inventive Ex 11F 0.64 1.30 0.60 0.009 0.050 0.183 0.0042 Inventive
Ex 11G 0.59 2.02 0.45 0.011 0.025 0.119 0.0044 0.022 Ca: 0.0009
Comparative Ex 11H 0.74 0.20 0.67 0.0025 0.044 0.083 0.0160
Inventive Ex 12A 0.51 0.49 1.80 0.012 0.026 0.107 0.0021 Te: 0.0011
Comparative Ex 12B 0.51 0.49 1.80 0.009 0.003 0.070 0.0021 Te:
0.0011 Inventive Ex 13A 0.51 1.00 2.00 0.012 0.019 1.420 0.0055
0.0019 Inventive Ex 13B 0.61 2.92 0.21 0.018 0.012 0.110 0.0039
Inventive Ex 13C 0.74 0.05 0.50 0.014 0.020 0.184 0.0048 0.21
Comparative Ex 13D 0.55 1.20 0.60 0.013 0.019 0.045 0.0048 0.21
Comparative Ex 13E 0.47 3.30 0.55 0.013 0.013 0.058 0.0053
Comparative Ex 14 0.78 0.20 0.67 0.025 0.044 0.166 0.0073
[0107] The rolled steels were cut, and cross sections thereof were
polished. An average hardness was obtained by measuring the Vickers
hardnesses of three points, positioned in a depth of 12.5 mm from
the surface, under a load of 10 kg.
[0108] Meanwhile, disc-like samples of 45.phi..times.15 mm were
manufactured from these materials (i.e., the rolled steels). These
disc-like samples were subjected to a machinability test under the
conditions presented in Table 1. As described above, the
machinability was evaluated by obtaining the maximum drilling rate
(m/min) by which 1000 mm of the total depth of the hole bored by
the drill was achieved. This is a test that evaluates the tool
lifetime.
[0109] Meanwhile, cylindrical samples of 17.5.phi..times.52.5 mm
were manufactured from these materials. These cylindrical samples
were subjected to induction hardening under conditions by which a
portion within a depth of 2 mm became a hardened layer. Afterwards,
cross sections of the samples were cut and polished, and an average
hardness of the surface layer was obtained by measuring the Vickers
hardnesses of 10 points, positioned in a depth of 0.5 mm from the
surface layer, under a load of 300 g. This is an index that
evaluates the strength of the steel for induction hardening, in
particular, when used for a variety of parts, such as a gear
wheel.
[0110] The results of the average hardness, tool lifetime, and
average hardness of surface layer after induction hardening of the
rolled steels, which were obtained by the tests, are presented in
Tables 5 and 6.
TABLE-US-00005 TABLE 5 Hardness Tool lifetime Surface layer of
steel to rolled hardness after Sample material steel induction
Classification No. (HV) (m/min) hardening (HV) Inventive Ex. 1A 150
to 180 130 650 Inventive Ex. 1B 150 to 180 125 658 Comparative Ex.
1C 150 to 180 110 665 Comparative Ex. 1D 150 to 180 130 587
Inventive Ex. 2A 180 to 200 135 703 Comparative Ex. 2B 180 to 200
115 703 Inventive Ex. 3A 180 to 200 105 710 Inventive Ex. 3B 180 to
200 115 720 Inventive Ex. 3C 180 to 200 110 738 Inventive Ex. 3D
180 to 200 110 715 Comparative Ex. 3E 180 to 200 90 715 Inventive
Ex. 4A 180 to 200 110 712 Comparative Ex. 4B 180 to 200 80 712
Inventive Ex. 5A 180 to 200 125 740 Comparative Ex. 5B 180 to 200
105 740 Inventive Ex. 6A 200 to 220 100 800 Comparative Ex. 6B 200
to 220 80 747 Inventive Ex. 7A 200 to 220 110 735 Comparative Ex.
7B 200 to 220 90 735 Inventive Ex. 8A 200 to 220 95 721 Inventive
Ex. 8B 200 to 220 100 705 Inventive Ex. 8C 200 to 220 100 765
Inventive Ex. 8D 200 to 220 100 736 Inventive Ex. 8E 200 to 220 100
739 Inventive Ex. 8F 200 to 220 100 761 Inventive Ex. 8G 200 to 220
100 725 Inventive Ex. 8H 200 to 220 100 722 Comparative Ex. 8i 200
to 220 75 760 Comparative Ex. 8J 200 to 220 75 590
TABLE-US-00006 TABLE 6 Hardness Tool lifetime Surface layer of
steel to rolled hardness after Sample material steel induction
Classification No. (HV) (m/min) hardening (HV) Inventive Ex. 9A 200
to 220 90 742 Comparative Ex. 9B 200 to 220 70 742 Inventive Ex.
10A 220 to 240 80 760 Inventive Ex. 10B 220 to 240 80 719 Inventive
Ex. 10C 220 to 240 80 768 Comparative Ex. 10D 220 to 240 60 768
Inventive Ex. 11A 240 to 260 60 755 Inventive Ex. 11B 240 to 260 55
768 Inventive Ex. 11C 240 to 260 60 781 Inventive Ex. 11D 240 to
260 60 762 Inventive Ex. 11E 240 to 260 60 824 Inventive Ex. 11F
240 to 260 60 794 Inventive Ex. 11G 240 to 260 55 762 Comparative
Ex. 11H 240 to 260 35 853 Inventive Ex. 12A 240 to 260 55 709
Comparative Ex. 12B 240 to 260 40 709 Inventive Ex. 13A 270 to 280
25 708 Inventive Ex. 13B 270 to 280 20 775 Inventive Ex. 13C 270 to
280 5 853 Comparative Ex. 13D 270 to 280 5 736 Comparative Ex. 13E
270 to 280 5 680 Comparative Ex. 14 Season cracking formed in
rolled steel
[0111] The test results of inventive examples are compared with
those of comparative examples at every hardness level of the rolled
steels. This is because comparison is meaningless unless it is made
between the steel materials having substantially the same hardness,
since machinability is influenced by hardness. The same numeral in
a sample number (Sample No.) indicates the same hardness level of
the rolled steels.
[0112] Sample Nos. 1A and 1B are inventive examples. The tool
lifetimes thereof are excellent. In addition, the hardnesses of
surface layer are HV600 or more; and therefore, these are steel
materials that realize sufficient strength characteristics. Sample
Nos. 1C and 1D are comparative examples. Sample No. 1C is an
example of which the tool lifetime is decreased because the amount
of Al is less than the range of the present invention. Sample No.
1D is an example of which the tool lifetime is excellent because
the amount of Al is within the range of the present invention;
however, the hardness of surface layer after induction hardening is
lowered because the amount of C is less than the range of the
present invention.
[0113] Sample No. 2A is an inventive example. The tool lifetime is
excellent because the amount of Al is within the range of the
present invention and Pb is included. In addition, the hardness of
surface layer is HV600 or more; and therefore, this is a steel
material that realizes sufficient strength characteristics. Sample
No. 2B is a comparative example. This is an example of which the
tool lifetime is decreased because the amount of Al is less than
the range of the present invention.
[0114] Sample Nos. 3A, 3B, 3C, and 3D are inventive examples. The
tool lifetimes thereof are excellent. In addition, the hardnesses
of surface layers are HV600 or more; and therefore, these are steel
materials that realize sufficient strength characteristics. Sample
No. 3E is a comparative example. This is an example of which the
tool lifetime is decreased because the amount of Al is less than
the range of the present invention.
[0115] Sample No. 4A is an inventive example. The tool lifetime is
excellent because the amount of Al is within the range of the
present invention and Sb is also included. In addition, and the
hardness of surface layer is HV600 or more; and therefore, this is
a steel material that realizes sufficient strength characteristics.
Sample No. 4B is a comparative example. This is an example of which
the tool lifetime is decreased because the amount of Al is less
than the range of the present invention.
[0116] Sample No. 5A is an inventive example. The tool lifetime is
excellent because the amount of Al is within the range of the
present invention and Bi is also included. In addition, and the
hardness of surface layer is HV600 or more; and therefore, this is
a steel material that realizes sufficient strength characteristics.
Sample No. 5B is a comparative example. This is an example of which
the tool lifetime is decreased because the amount of Al is less
than the range of the present invention.
[0117] Sample No. 6A is an inventive example. The tool lifetime
thereof is excellent. In addition, the hardness of surface layer is
HV600 or more; and therefore, this is a steel material that
realizes sufficient strength characteristics. Sample No. 6B is a
comparative example. This is an example of which the tool lifetime
is decreased because the amount of Al is less than the range of the
present invention.
[0118] Sample No. 7A is an inventive example. The tool lifetime is
excellent because the amount of Al is within the range of the
present invention and Sn is also included. In addition, the
hardness of surface layer is HV600 or more; and therefore, this is
a steel material that realizes sufficient strength characteristics.
Sample No. 7B is a comparative example. This is an example of which
the tool lifetime is decreased because the amount of Al is less
than the range of the present invention.
[0119] Sample Nos. 8A, 8B, 8C, 8D, 8E, 8F, 8G, and 8H are inventive
examples. The tool lifetimes thereof are excellent, and the
hardnesses of surface layers are HV600 or more; and therefore,
these are steel materials that realize sufficient strength
characteristics. Sample No. 8i is a comparative example. This is an
example of which the tool lifetime is decreased because the amount
of Al is less than the range of the present invention. Sample No.
8J is a comparative example. This is an example of which the tool
lifetime is decreased because the amount of Al is less than the
range of the present invention.
[0120] Sample No. 9A is an inventive example. The tool lifetime is
excellent because the amount of Al is within the range of the
present invention and Zn is also included. In addition, the
hardness of surface layer is HV600 or more; and therefore, this is
a steel material that realizes sufficient strength characteristics.
Sample No. 9B is a comparative example. This is an example of which
the tool lifetime is decreased because the amount of Al is less
than the range of the present invention.
[0121] Sample Nos. 10A, 10B, and 10C are inventive examples. The
tool lifetimes are excellent. In addition, the hardnesses of
surface layers are HV600 or more; and therefore, these are steel
materials that realize sufficient strength characteristics. Sample
No. 10D is a comparative example. This is an example of which the
tool lifetime is decreased because the amount of Al is less than
the range of the present invention.
[0122] Sample Nos. 11A, 11B, 11C, 11D, 11E, 11F, and 11G are
inventive examples. The tool lifetime is excellent because the
amount of Al is within the range of the present invention. In
addition, the hardnesses of surface layers are HV600 or more; and
therefore, these are steel materials that realize sufficient
strength characteristics. Sample No. 11H is a comparative example.
This is an example of which the tool lifetime is decreased because
the amount of Al is less than the range of the present
invention.
[0123] Sample No. 12A is an inventive example. The tool lifetime is
excellent because the amount of Al is within the range of the
present invention and Te is also included. In addition, the
hardness of surface layer is HV600 or more; and therefore, this is
a steel material that realizes sufficient strength characteristics.
Sample No. 12B is a comparative example. This is an example of
which the tool lifetime is decreased because the amount of Al is
less than the range of the present invention.
[0124] Sample Nos. 13A, 13B, and 13C are inventive examples. The
tool lifetimes are excellent. In addition, the hardnesses of
surface layers are HV600 or more; and therefore, these are steel
materials that realize sufficient strength characteristics. Sample
No. 13D is a comparative example. This is an example of which the
tool lifetime is decreased because the amount of Al is less than
the range of the present invention. Sample No. 13E is a comparative
example. This is an example of which the tool lifetime is decreased
due to the increase of the amount of hard inclusions because the
content of Si is more than the range of the present invention.
[0125] Sample No. 14 is a comparative example. This is an example
in which aging cracks are formed after rolling. This is because the
toughness is degraded since the content of C is more than the range
of the present invention.
INDUSTRIAL APPLICABILITY
[0126] The steel for induction hardening of the present invention
can be very suitably used as a steel material that is applied to a
process of manufacturing products, such as parts, in which the
steel is subjected to processing treatment such as cutting, and
then is subjected to induction hardening. In particular, the steel
of the present invention can be very suitably used as a steel for
the manufacture of parts, which are used for a gear wheel, a CVT or
a CVJ for vehicles, or the like, via induction hardening.
* * * * *