U.S. patent application number 15/115185 was filed with the patent office on 2016-12-01 for steel wire for springs having excellent fatigue properties, and spring.
This patent application is currently assigned to KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.). The applicant listed for this patent is KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.). Invention is credited to Masaki KAIZUKA, Sei KIMURA, Akihiro OWAKI, Masaki SHIMAMOTO, Yosuke SHINDO, Tomoko SUGIMURA.
Application Number | 20160348221 15/115185 |
Document ID | / |
Family ID | 53757144 |
Filed Date | 2016-12-01 |
United States Patent
Application |
20160348221 |
Kind Code |
A1 |
SUGIMURA; Tomoko ; et
al. |
December 1, 2016 |
STEEL WIRE FOR SPRINGS HAVING EXCELLENT FATIGUE PROPERTIES, AND
SPRING
Abstract
A steel wire rod for a spring includes: C: 0.2 to 1.2%, Si: 1.0
to 3%, Mn: 0.1 to 2%, Cr: 3% or less (not inclusive of 0%), Al:
0.0002 to 0.005%, Ca: 0.0002 to 0.002%, Ti: 0.0003 to 0.010%, and
balance: iron and unavoidable impurities. An average composition of
oxide-based inclusions having a minor axis of 1 .mu.m or more which
are present in a cross section satisfies CaO: 35% or less,
Al.sub.2O.sub.3: 40% or less, SiO.sub.2: 30 to 95%, MgO: 8% or
less, MnO: 5% or less, TiO.sub.2: 3 to 10%, and
CaO+Al.sub.2O.sub.3+SiO.sub.2+MgO+MnO+TiO.sub.2.gtoreq.80%. The
number of oxide-based inclusions having a minor axis of 2 .mu.m or
more which are present in the cross section is more than 0.002
inclusions/mm.sup.2.
Inventors: |
SUGIMURA; Tomoko; (Hyogo,
JP) ; SHIMAMOTO; Masaki; (Hyogo, JP) ; SHINDO;
Yosuke; (Hyogo, JP) ; KAIZUKA; Masaki; (Hyogo,
JP) ; OWAKI; Akihiro; (Hyogo, JP) ; KIMURA;
Sei; (Hyogo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.) |
Kobe-shi |
|
JP |
|
|
Assignee: |
KABUSHIKI KAISHA KOBE SEIKO SHO
(KOBE STEEL, LTD.)
Kobe-shi
JP
|
Family ID: |
53757144 |
Appl. No.: |
15/115185 |
Filed: |
January 29, 2015 |
PCT Filed: |
January 29, 2015 |
PCT NO: |
PCT/JP2015/052595 |
371 Date: |
July 28, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/50 20130101;
C22C 38/002 20130101; C22C 38/34 20130101; C22C 38/28 20130101;
C22C 38/46 20130101; C21D 2211/004 20130101; C22C 38/02 20130101;
C21D 8/06 20130101; C22C 38/04 20130101; C22C 38/06 20130101; C22C
38/24 20130101; C22C 38/38 20130101 |
International
Class: |
C22C 38/50 20060101
C22C038/50; C22C 38/34 20060101 C22C038/34; C22C 38/28 20060101
C22C038/28; C22C 38/00 20060101 C22C038/00; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/46 20060101 C22C038/46; C22C 38/24 20060101
C22C038/24 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 29, 2014 |
JP |
2014-014633 |
Claims
1. A steel wire rod for a spring excellent in fatigue properties,
comprising: C: 0.2 to 1.2%, where % means mass %, hereinafter the
same, unless otherwise indicated, Si: 1.0 to 3%, Mn: 0.1 to 2%, Cr:
3% or less but not inclusive of 0%, Al: 0.0002 to 0.005%, Ca:
0.0002 to 0.002%, Ti: 0.0003 to 0.010%, and balance: iron and
unavoidable impurities, wherein an average composition of
oxide-based inclusions having a minor axis of 1 .mu.m or more which
are present in a cross section parallel to a longitudinal direction
of a steel satisfies, by mass %, CaO: 35% or less and inclusive of
0%, Al.sub.2O.sub.3: 40% or less and inclusive of 0%, SiO.sub.2: 30
to 95%, MgO: 8% or less and inclusive of 0%, MnO: 5% or less and
inclusive of 0%, TiO.sub.2: 3 to 10%, and
CaO+Al.sub.2O.sub.3+SiO.sub.2+MgO+MnO+TiO.sub.2.gtoreq.80%, and the
number of oxide-based inclusions having a minor axis of 2 .mu.m or
more which are present in the cross section is more than 0.002
inclusions/mm.sup.2.
2. The steel wire rod for a spring according to claim 1, wherein
the average composition of the oxide-based inclusions further
satisfies, by mass %, ZrO.sub.2: less than 1% but not inclusive of
0% and Na.sub.2O: less than 5% and inclusive of 0%.
3. The steel wire rod for a spring according to claim 1, further
comprising at least one of Ni: 0.5% or less but not inclusive of
0%, Cu: 0.5% or less but not inclusive of 0% and V: 0.5% or less
but not inclusive of 0%.
4. A spring obtained using the steel wire rod for a spring
according to claim 1.
5. The steel wire rod for a spring according to claim 2, further
comprising at least one of Ni: 0.5% or less but not inclusive of
0%, Cu: 0.5% or less but not inclusive of 0% and V: 0.5% or less
but not inclusive of 0%.
6. A spring obtained using the steel wire rod for a spring
according to claim 2.
7. A spring obtained using the steel wire rod for a spring
according to claim 3.
8. A spring obtained using the steel wire rod for a spring
according to claim 5.
Description
TECHNICAL FIELD
[0001] The present invention relates to a steel wire rod for a
spring excellent in fatigue properties and a spring.
BACKGROUND ART
[0002] With increase in requirement for weigh reduction and high
output of automobiles and the like, improvement of fatigue
properties has been required in springs such as valve springs and
suspension springs. Further improvement of fatigue properties has
been required also in steel wire rods for springs, such as rolled
steels used as materials thereof and drawn wire rods obtained by
subjecting the rolled steels to wire drawing. In particular, a
request for improvement of fatigue properties is very strong in
steel wire rods for valve springs.
[0003] In the steel wire rods for springs, which require high
fatigue strength, it is necessary to reduce as possible nonmetallic
inclusions becoming start points of wire breakage or fatigue
breakage, or to reduce the size thereof. For example, in steels for
valve springs, a technology has been proposed in which inclusions
are controlled to a system containing
SiO.sub.2--Al.sub.2O.sub.3--CaO--MgO--MnO and the like, using
so-called "Si-killed steel" which deoxidizes using Si so as not to
form Al.sub.2O.sub.3 harmful to fatigue properties, thereby
refining the inclusions. In addition, there have been proposed a
method of controlling composition of nonmetallic inclusions to a
low-melting-point region, thereby refining the inclusions, a method
of extending inclusions during hot rolling, thereby fragmenting the
inclusions, and the like.
[0004] For example, Non-Patent Document 1 describes that, in a
steel for a valve spring, deformation during hot working is
accelerated by controlling a composition of inclusions to a
CaO--Al.sub.2O.sub.3--SiO.sub.2-based or
MnO--Al.sub.2O.sub.3--SiO.sub.2-based amorphous stabilized
composition, thereby preventing start points of fatigue breakage
from being formed to improve fatigue properties.
[0005] The present inventors also variously propose steel wire rods
for springs excellent in fatigue properties and the like. For
example, Patent Document 1 proposes a technology in which the whole
inclusions have a low melting point and are easily deformable, and
hard SiO.sub.2 is less likely to be formed, even when phase
separation occurs at the time of heating before hot rolling or
during hot rolling. Further, Patent Document 2 proposes a
technology in which fragmentation of inclusions during hot rolling
is accelerated by forming many fine grains in the inclusions to
enhance refinement thereof. Furthermore, Patent Document 3 proposes
a technology in which at least one of LiO.sub.2, Na.sub.2O and
K.sub.2O is allowed to be positively contained, in order to
decrease the melting point and viscosity of composite oxide-based
inclusions to be formed, thereby finally refining the
inclusions.
[0006] In addition, Patent Document 4 describes that addition of
ZrO.sub.2 as an unconventional oxide component contributes to
securing of an amorphous phase. Further, Patent Document 5
describes that oxide-based inclusions are finely fragmented by
allowing B.sub.2O.sub.3 to be contained in a composite oxide (for
example, such as a CaO--Al.sub.2O.sub.3--SiO.sub.2-based composite
oxide or a CaO--Al.sub.2O.sub.3--SiO.sub.2--MgO-based composite
oxide), thereby being able to remarkably improve wire drawability
and fatigue strength.
PRIOR ART DOCUMENTS
Patent Documents
[0007] Patent Document 1: Japanese Patent No. 4134204 [0008] Patent
Document 2: Japanese Patent No. 4347786 [0009] Patent Document 3:
Japanese Patent No. 4423050 [0010] Patent Document 4:
JP-A-2010-202905 [0011] Patent Document 5: JP-A-2009-263704
Non-Patent Document
[0011] [0012] Non-Patent Document 1: Tsuyoshi Mimura, 182.sup.nd
and 183.sup.rd Nishiyama Memorial Technical Lecture "Inclusion
Control and High Cleanliness Degree Steel Production Technology",
edited by The Iron and Steel Institute of Japan, Tokyo, 2004, p.
125
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0013] However, for example, in the above Patent Document 3, Li
having strong deoxidizing force is positively added as a formation
origin of oxide-based inclusions. However, there is a problem that
control of the LiO.sub.2 concentration is difficult, because Li is
easily vaporized. Further, in the above Patent Document 3, there is
difficulty in production, as described as "the Li.sub.2O
concentration in inclusions cannot be measured by a conventional
EPMA (Electron Probe X-ray Micro Analyzer), and therefore, an
analytical method according to SIMS (Secondary Ion Mass
Spectrometry) has been uniquely developed". Furthermore, from
experimental results of the present inventors, it has been revealed
that ZrO.sub.2 described in the above Patent Document 4 and
B.sub.2O.sub.3 described in the above Patent Document 5 may
conversely deteriorate fatigue properties.
[0014] The present invention has been made in view of the above
circumstances, and an object thereof is to provide a steel wire rod
for a spring extremely excellent in fatigue properties, and a
spring.
Means for Solving Problems
[0015] The steel wire rod for a spring excellent in fatigue
properties in the present invention, which is capable of solving
the problems includes:
[0016] C: 0.2 to 1.2% (% means mass %, hereinafter the same, unless
otherwise indicated),
[0017] Si: 1.0 to 3%,
[0018] Mn: 0.1 to 2%,
[0019] Cr: 3% or less (not inclusive of 0%),
[0020] Al: 0.0002 to 0.005%,
[0021] Ca: 0.0002 to 0.002%,
[0022] Ti: 0.0003 to 0.010%, and
[0023] balance: iron and unavoidable impurities, and
[0024] an average composition of oxide-based inclusions having a
minor axis of 1 .mu.m or more which are present in a cross section
parallel to a longitudinal direction of a steel satisfies, by mass
%, CaO: 35% or less (inclusive of 0%), Al.sub.2O.sub.3: 40% or less
(inclusive of 0%), SiO.sub.2: 30 to 95%, MgO: 8% or less (inclusive
of 0%), MnO: 5% or less (inclusive of 0%), TiO.sub.2: 3 to 10%, and
CaO+Al.sub.2O.sub.3+SiO.sub.2+MgO+MnO+TiO.sub.2.gtoreq.80%, and
[0025] the number of oxide-based inclusions having a minor axis of
2 .mu.m or more which are present in the cross section is more than
0.002 inclusions/mm.sup.2.
[0026] In particular, it is preferred that the average composition
of the oxide-based inclusions satisfies: CaO: 10 to 35%,
Al.sub.2O.sub.3: 10 to 40%, SiO.sub.2: 30 to 70%, MgO: 8% or less
(inclusive of 0%), MnO: 5% or less (inclusive of 0%), TiO.sub.2: 3
to 10%, and
CaO+Al.sub.2O.sub.3+SiO.sub.2+MgO+MnO+TiO.sub.2.gtoreq.80%, and the
number of the oxide-based inclusions having a minor axis of 2 .mu.m
or more which are present in the cross section is more than 0.002
inclusions/mm.sup.2.
[0027] In the steel wire rod for a spring, the average composition
of the oxide-based inclusions may further satisfy, by mass %,
ZrO.sub.2: less than 1% (not inclusive of 0%) and Na.sub.2O: less
than 5% (inclusive of 0%).
[0028] The steel wire rod for a spring may further include Ni: 0.5%
or less (not inclusive of 0%), and Cu: 0.5% or less (not inclusive
of 0%).
[0029] The steel wire rod for a spring may further include V: 0.5%
or less (not inclusive of 0%).
[0030] The spring excellent in fatigue properties, which is capable
of solving the problems, is obtained using the steel wire rod for a
spring.
Advantageous Effects of the Invention
[0031] According to the present invention, chemical components of a
steel wire rod for a spring, and a composition and the number of
oxide-based inclusions are appropriately controlled. Therefore, a
steel wire rod for a spring extremely excellent in fatigue
properties can be obtained.
MODE FOR CARRYING OUT THE INVENTION
[0032] In order to provide a steel wire rod for a spring having
extremely excellent fatigue properties, the present inventors have
made studies also after disclosure of the above Patent Documents 1
to 3 and the like. Conventionally, for improvement of fatigue
properties, it is effective to refine oxide-based inclusions by
fragmentation due to extension during hot rolling, and there has
been proposed a method of controlling a composition of oxide-based
inclusions obtained by Si deoxidization to a SiO.sub.2-containing
composition in which amorphousness is relatively stable, for
example, SiO.sub.2--CaO--Al.sub.2O.sub.3--MgO--MnO or the like. In
that case, as a means for realizing refinement even when the
inclusions are crystallized, the technology of Patent Document 2 is
proposed as a method for controlling a crystallization state (a
fine grain phase is precipitated without being completely
crystallized).
[0033] However, according to subsequent studies of the present
inventors, it has been found that even when the size is so fine as
to usually scarcely become a start point of failure, a void occurs
at an interface between steel as a matrix and the oxide-based
inclusion depending on a crystallization form, and may become the
start point of failure under severer test conditions. On that
basis, it has been found that amorphousness of the oxide-based
inclusions can be kept more stable by allowing TiO.sub.2 to be
contained in the oxide-based inclusions based on
SiO.sub.2--CaO--Al.sub.2O.sub.3--MgO--MnO as reported in the above
Patent Documents, thereby obtaining high fatigue properties.
Thereby, high fatigue properties can be realized in a composition
in which analysis or control of the oxide-based inclusions is
easier. Here, in the oxide-based inclusions in which TiO.sub.2 is
allowed to be contained, based on
SiO.sub.2--CaO--Al.sub.2O.sub.3--MgO--MnO, an average composition
thereof satisfies, by mass %,
CaO+Al.sub.2O.sub.3+SiO.sub.2+MgO+MnO+TiO.sub.2.gtoreq.80%. In
addition to the effect due to the oxide-based inclusions based on
SiO.sub.2--CaO--Al.sub.2O.sub.3--MgO--MnO as reported in the above
Patent Documents, the effect due to TiO.sub.2 is exerted by
containing 80% or more of these in total.
[0034] Then, in order to exert the above effect due to TiO.sub.2,
it has been found that it is useful to allow Ti to be contained
within a range of from 0.0003 to 0.010% as a component in steel,
thus completing the present invention.
[0035] In the present invention, the reason why fatigue properties
are improved by allowing a predetermined amount of TiO.sub.2 to be
contained in the above oxide-based inclusions is not clear in
detail. However, it is considered as follows.
[0036] That is, containing of TiO.sub.2 in the above oxide-based
inclusions obtained by Si deoxidization causes separation into two
phases of a TiO.sub.2-concentrated phase (A phase) and a
SiO.sub.2-concentrated phase (B phase). The reason for the
separation into two phases is considered because TiO.sub.2 has a
property to be separated from SiO.sub.2 as two liquid phases at a
molten steel stage. As a result, the SiO.sub.2 concentration in the
SiO.sub.2-concentrated phase (B phase) is increased to suppress
crystallization of gehlenite, spinel (MgO.Al.sub.2O.sub.3) and the
like, which is liable to occur in Si-killed steel. On the other
hand, also in the TiO.sub.2-concentrated phase (A phase), the
liquidus temperature is decreased to suppress crystallization by
containing TiO.sub.2 in the oxide-based inclusions. As a result, it
is presumed that it becomes possible to enhance amorphous stability
of the oxide-based inclusions.
[0037] On the other hand, Patent Documents 1 to 5 described above
do not disclose the above characterizing portion in the present
invention. For example, in the above Patent Document 4, TiO.sub.2
is mentioned as an impurity of the inclusions. However, it is not
described at all that fatigue properties are improved by
controlling the TiO.sub.2 amount within a predetermined range as in
the present invention. As a matter of fact, in all of Examples of
the above Patent Document 4, examples of containing impurities such
as FeO and TiO.sub.2 in an amount of 1.0% are only disclosed, and
this does not provide the effect of improving fatigue properties
due to addition of TiO.sub.2 (see No. 14 in the table described
later). In the first place, the above Patent Document 4 is
different from the present invention in the composition of the
oxide-based inclusions in that ZrO.sub.2 is contained in an amount
of 1% or more.
[0038] The present invention is described in detail below.
[0039] As described above, in the present invention, Ti is
contained within a range of 0.0003 to 0.010% as a component in
steel and TiO.sub.2 is contained in the above oxide-based
inclusions of SiO.sub.2--CaO--Al.sub.2O.sub.3--MgO--MnO within a
range of 3 to 10%.
[0040] In this description, the steel wire rod for a spring
includes both a steel after rolling (rolled steel) and a drawn wire
rod obtained by subjecting the rolled steel to wire drawing. In the
present invention, these are called the "steel wire rod" as a
whole.
[0041] In this description, the oxide-based inclusions mean
oxide-based inclusions in which oxide forming elements such as Ca,
Al, Si, Ti, Mn, Mg, Na, Cr and Zr bond to oxygen. The above
oxide-based inclusions can be observed under an electron microscope
and measured by an energy dispersive X-ray spectrometry (EDX) or a
wavelength-dispersive X-ray spectrometry (WDX). The details of
measuring methods will be described later.
[0042] First, the composition of the oxide-based inclusions is
described. As described above, in the present invention, for the
oxide-based inclusions contained in steel, the average composition
of the oxide-based inclusions having a minor axis of 1 .mu.m or
more which are present in a cross section parallel to a
longitudinal direction of the steel satisfies, by mass %, CaO: 35%
or less (inclusive of 0%), Al.sub.2O.sub.3: 40% or less (inclusive
of 0%), SiO.sub.2: 30 to 95%, MgO: 8% or less (inclusive of 0%),
MnO: 5% or less (inclusive of 0%), TiO.sub.2: 3 to 10%, and
CaO+Al.sub.2O.sub.3+SiO.sub.2+MgO+MnO+TiO.sub.2.gtoreq.80%, and
preferably satisfies CaO: 10 to 35%, Al.sub.2O.sub.3: 10 to 40%,
SiO.sub.2: 30 to 70%, MgO: 8% or less (inclusive of 0%), MnO: 5% or
less (inclusive of 0%), TiO.sub.2: 3 to 10%, and
CaO+Al.sub.2O.sub.3+SiO.sub.2+MgO+MnO+TiO.sub.2.gtoreq.80%. The
number of the oxide-based inclusions having a minor axis of 2 .mu.m
or more which are present in the above cross section is preferably
more than 0.002 inclusions/mm.sup.2.
[0043] First, the composition of the oxide-based inclusions is
described. As described above, in the present invention, in the
oxide-based inclusions contained in steel, the average composition
of the oxide-based inclusions having a minor axis of 1 .mu.m or
more which are present in a cross section parallel to a
longitudinal direction of the steel satisfies, by mass %, CaO: 35%
or less (inclusive of 0%), Al.sub.2O.sub.3: 40% or less (inclusive
of 0%), SiO.sub.2: 30 to 95%, MgO: 8% or less (inclusive of 0%),
MnO: 5% or less (inclusive of 0%), TiO.sub.2: 3 to 10%, and
CaO+Al.sub.2O.sub.3+SiO.sub.2+MgO+MnO+TiO.sub.2.gtoreq.80%, and the
number of the oxide-based inclusions having a minor axis of 2 .mu.m
or more which are present in the above cross section is more than
0.002 inclusions/mm.sup.2.
[CaO: 35% or Less]
[0044] CaO is a basic oxide, and when it is contained in SiO.sub.2
of an acidic oxide, the liquidus temperature of the oxide is
decreased to have an effect of suppressing crystallization of the
oxide-based inclusions. It may therefore be contained in the
inclusions. The content thereof is desirably 10% or more, and more
preferably 15% or more. However, when the CaO content is too high,
the oxide-based inclusions are crystallized. Therefore, the upper
limit thereof is 35% or less. The upper limit of the CaO content is
preferably 30% or less.
[SiO.sub.2: 30 to 95%]
[0045] SiO.sub.2 is an acidic oxide and a component which is
essential for making the oxide-based inclusions amorphous. In order
to effectively exert such an effect, the lower limit of the
SiO.sub.2 content is 30% or more. The lower limit of the SiO.sub.2
content is preferably 40% or more. However, when the SiO.sub.2
content is more than 95%, extensibility of the above inclusions is
decreased to cause easy formation of voids, resulting in
deterioration of fatigue properties. Therefore, the upper limit of
the SiO.sub.2 content is 95% or less, preferably 70% or less, and
more preferably 50% or less.
[Al.sub.2O.sub.3: 40% or Less]
[0046] Al.sub.2O.sub.3 is an amphoteric oxide, and when it is
contained in SiO.sub.2 of an acidic oxide, the liquidus temperature
of the oxide is decreased to have an effect of suppressing
crystallization of the oxide. It may therefore be contained in the
inclusions. The content thereof is desirably 10% or more, and more
preferably 20% or more. On the other hand, when the upper limit of
the Al.sub.2O.sub.3 content is more than 40%, an Al.sub.2O.sub.3
crystal phase such as corundum is crystallized in molten steel and
during a solidification process, or a MgO.Al.sub.2O.sub.3 crystal
phase such as spinel is crystallized together with MgO. Further,
these crystal phases are formed in a rolling temperature range.
These solid phases are hard and remain as coarse inclusions to
deteriorate fatigue properties. From such a viewpoint, the upper
limit of the Al.sub.2O.sub.3 content is required to be 40% or less,
and is preferably 30% or less.
[MgO: 8% or Less (Inclusive of 0%)]
[0047] MgO is not an essential component in the present invention,
but has an effect of controlling a SiO.sub.2-based oxide to an
optimum composition to decrease the melting point thereof. In order
to effectively exert such an action, the lower limit of the MgO
content is preferably 0.2% or more. However, when the MgO content
is too large, the melting point of the SiO.sub.2-based oxide is
increased, or MgO-based crystals are formed. For this reason, the
upper limit thereof is 8% or less. It is preferably 5% or less, and
more preferably 3% or less.
[MnO: 5% or Less (Inclusive of 0%)]
[0048] Similarly to the above case of MgO, MnO is also not an
essential component in the present invention. However, MnO has an
effect of decreasing the melting point of the SiO.sub.2-based
oxide. In order to effectively exert such an action, the lower
limit of the MnO content is preferably 0.1% or more, and more
preferably 0.5% or more. However, in a high Si steel containing Si
in an amount of 1.0% or more as in the present invention, it is not
realistic to control MnO to an excessively high concentration.
Therefore, the upper limit of the MnO content is 5% or less.
[TiO.sub.2: 3 to 10%]
[0049] TiO.sub.2 is an oxide component which is the characteristics
feature in the present invention. As described above, containing of
TiO.sub.2 in SiO.sub.2 of the acidic oxide causes separation into
two phases of the TiO.sub.2-concentrated phase (A phase) and the
SiO.sub.2-concentrated phase (B phase), and both phases have an
action of suppressing crystallization. As a result, it can be
realized that crystallization of SiO.sub.2-containing oxide-based
inclusions obtained in Si-killed steel during hot working is
suppressed, and that occurrence of voids at interfaces between the
steel and the oxide-based inclusions is suppressed, thereby further
improving fatigue properties. Such effects are obtained by
controlling the lower limit of the TiO.sub.2 content to 3% or more.
Therefore, the TiO.sub.2 content is 3% or more. It is preferably 4%
or more, and more preferably 5% or more. However, when the
TiO.sub.2 content is too large, a TiO.sub.2-based oxide is formed
alone as a crystal phase. Therefore, fatigue properties are
decreased. For this reason, the upper limit of the TiO.sub.2
content is 10% or less. It is preferably 8% or less, and more
preferably 7% or less.
[CaO+Al.sub.2O.sub.3+SiO.sub.2+MgO+MnO+TiO.sub.2.gtoreq.80%]
[0050] In the present invention, it is necessary to control the
contents of the respective oxides as described above and to control
the total of these contents to 80% or more, thereby keeping
amorphousness of the oxide-based inclusions to improve fatigue
properties. The larger total amount of the above oxides is better,
and preferably 90% or more. It is most preferably 100%.
[0051] As described above, the oxide-based inclusions contained in
the steel wire rod for a spring in the present invention are
basically CaO, Al.sub.2O.sub.3, SiO.sub.2, MgO, MnO and TiO.sub.2,
and the balance is impurities. The above impurities include, for
example, impurities unavoidably contained in a production process
and the like. The above impurities can be contained to such an
extent that desired fatigue properties are obtained without
adversely affecting a crystallization state or form of the
oxide-based inclusions. However, in relation to the total amount of
the above oxide-based inclusions, the total amount of the
impurities is required to be controlled to at most 20%.
[0052] The above impurities include, for example, ZrO.sub.2,
Na.sub.2O, Cr.sub.2O.sub.3 and the like. Of these, when the
concentration of ZrO.sub.2 in the oxide-inclusions is increased,
crystallization of the above inclusions is accelerated to
deteriorate fatigue properties. It is therefore preferably
decreased as much as possible. The ZrO.sub.2 content is preferably
less than 1%, more preferably 0.5% or less, and most preferably not
contained. Further, Na.sub.2O has a wide allowance, compared to the
above ZrO.sub.2, so that it may be contained in an amount of about
5%.
[Number of Oxide-Based Inclusions Having a Minor Axis of 2 .mu.m or
More which are Present in a Cross Section Parallel to a
Longitudinal Direction of Steel: More than 0.002
Inclusions/mm.sup.2]
[0053] In the present invention, it is necessary to control the
contents of the respective oxides and the total amount thereof, and
to satisfy that the number of the oxide-based inclusions having a
minor axis of 2 .mu.m or more is more than 0.002
inclusions/mm.sup.2. Thereby, high fatigue properties are secured,
and homogeneity is also improved. The number of the oxide-based
inclusions having a minor axis of 2 .mu.m or more is preferably
0.005 inclusions/mm.sup.2 or more, more preferably 0.01
inclusions/mm.sup.2 or more and still more preferably 0.05
inclusions/mm.sup.2 or more. The above "oxide-based inclusions" as
used herein mean oxide-based inclusions in which oxide forming
elements such as Ca, Al, Si, Ti, Mn, Mg, Na, Cr and Zr bond to
oxygen, and should not be limited to the above-mentioned oxides
(CaO, Al.sub.2O.sub.3, SiO.sub.2, MgO, MnO and TiO.sub.2). Further,
of the above oxide-based inclusions, the ones having "a minor axis
of 2 .mu.m or more" are particularly specified, because the
oxide-based inclusions having a minor axis of less than 2 .mu.m
have relatively little adverse effects on fatigue properties.
[0054] Components in steel are described below.
[C: 0.2 to 1.2%]
[0055] C is an element necessary for securing predetermined
strength, and in order to effectively exert such properties, the C
content is 0.2% or more. It is preferably 0.5% or more. However,
when the C content is excessive, steel becomes brittle and
therefore does not become practical. Therefore, the upper limit
thereof is 1.2% or less. The upper limit of the C content is
preferably 0.8% or less, and more preferably 0.7% or less
[Si: 1.0 to 3%]
[0056] Si is an important element contributing to high
strengthening of the steel wire rod for a spring and improvement of
fatigue properties. Further, it is also a useful element for
enhancing softening resistance and improving setting resistance.
Furthermore, Si is an essential element also for controlling to a
composition of desired oxide-based inclusions. In order to
effectively exert such actions, the Si content is 1.0% or more. The
Si content is preferably 1.4% or more, and more preferably 1.8% or
more. However, when the Si content is excessive, hard pure
SiO.sub.2 may possibly be formed during solidification, and surface
decarburization and surface flaws are increased to deteriorate
fatigue properties in some cases. For this reason, the upper limit
of the Si amount is 3% or less. It is preferably 2.4% or less, and
more preferably 2.2% or less.
[Mn: 0.1 to 2%]
[0057] Mn is an element acting as a deoxidizing agent and
additionally increasing hardenability, thereby contributing to the
enhancement of strength. In order to effectively exert such
actions, the lower limit of the Mn content is 0.1% or more. It is
preferably 0.5% or more. However, when the Mn amount is excessive,
toughness and ductility are deteriorated. For this reason, as the
upper limit thereof, it is 2% or less. It is preferably 1% or
less.
[Cr: 3% or Less (not Inclusive of 0%)]
[0058] Cr is an element for improving matrix strength of the steel
wire rod for a spring by solid solution strengthening. Further,
similarly to the case of Mn, Cr also effectively acts on
improvement of hardenability. The Cr amount is preferably 0.5% or
more, and more preferably 0.9% or more. However, when Cr is
excessive, the steel wire rod for a spring tends to become brittle
to increase sensitivity of the oxide-based inclusions. Therefore,
fatigue properties are deteriorated. Then, the upper limit of the
Cr amount is 3%. As the upper limit of the Cr amount, it is
preferably 2% or less, and more preferably 1% or less.
[Al: 0.0002 to 0.005%]
[0059] When the Al content is increased and particularly exceeds
0.005%, the production amount of hard oxides mainly composed of
Al.sub.2O.sub.3 is increased, and the oxides remain as coarse
oxides even after further reduction. Therefore, fatigue properties
are decreased. Accordingly, the Al content is 0.005% or less,
preferably 0.002% or less, and more preferably 0.0015% or less.
However, when the Al content is less than 0.0002%, the
Al.sub.2O.sub.3 content in the oxide-based inclusions is
excessively decreased to form crystal phases containing a large
amount of SiO.sub.2. Accordingly, the lower limit of the Al content
is 0.0002% or more, and preferably 0.0005% or more.
[Ca: 0.0002 to 0.002%]
[0060] Ca is a component contained in the steel wire rod by slag
refining for controlling the composition of the oxide-based
inclusions. In the present invention, it is an effective element
for controlling the CaO content in the oxide-based inclusions to
suppress crystallization of the oxide-based inclusions, thereby
improving fatigue properties. In order to exert such an effect, the
Ca content is 0.0002% or more, preferably 0.0003% or more, and more
preferably 0.0005% or more. However, when the Ca content is
excessive and is more than 0.002%, the ratio of CaO becomes too
high, resulting in crystallization of the oxides. Accordingly, the
Ca content is 0.002% or less, preferably 0.001% or less, and more
preferably 0.0008% or less.
[Ti: 0.0003 to 0.010%]
[0061] Ti is an element which is the characteristic feature in the
present invention. A predetermined amount of Ti is added to
appropriately control the TiO.sub.2 content in the oxide-based
inclusions, thereby more enhancing amorphous stability to further
improving fatigue properties. In order to obtain such an effect,
the Ti content is required to be 0.0003% or more. It is preferably
0.0005% or more, and more preferably 0.0008% or more. However, when
the Ti content is increased to be more than 0.010%, the
TiO.sub.2-based oxide is formed alone as a crystal phase.
Accordingly, the Ti content is 0.010% or less. It is preferably
0.0050% or less, and more preferably 0.0030% or less.
[0062] The elements in steel used in the present invention are as
described above, and the balance is iron and unavoidable
impurities. The above unavoidable impurities include, for example,
elements introduced depending on situations of raw materials,
materials, production facilities and the like, such as S, P, H and
N.
[0063] Further, in the present invention, the following selective
components may be contained.
[Ni: 0.5% or Less (not Inclusive of 0%)]
[0064] Ni is an effective element for suppressing decarburization
of ferrite formed in hot rolling at the time of producing the steel
wire rod for a spring or in heat treatment at the time of producing
the spring. Further, Ni has an action to increase toughness of the
spring after hardening and tempering. As the lower limit of the Ni
amount, it is preferably 0.05% or more, more preferably 0.15% or
more, and still more preferably 0.2% or more. On the other hand,
when the Ni amount is excessive, the residual austenite amount is
increased during hardening and tempering treatment to decrease the
tensile strength. For this reason, as the upper limit of the Ni
amount, it is preferably 0.5% or less, and more preferably 0.3% or
less.
[Cu: 0.5% or Less (not Inclusive of 0%)]
[0065] Cu is an effective element for suppressing decarburization
of ferrite formed during hot rolling at the time of producing the
steel wire rod for a spring or in heat treatment at the time of
producing the spring. Therefore, it may be contained in an amount
of 0.05% or more. As the upper limit thereof, it is preferably 0.5%
or less, and more preferably 0.3% or less.
[V: 0.5% or Less (not Inclusive of 0%)]
[0066] V bonds to carbon, nitrogen or the like to form fine
carbide, nitride or the like, and an element useful for improving
hydrogen brittleness resistance and fatigue properties. Further, V
is an element contributing to improvement of toughness, proof
stress, setting resistance and the like by refinement effect of
grains. As the lower limit of V amount, it is preferably 0.05% or
more, and more preferably 0.10% or more. However, when the V amount
is excessive, the amount of carbide that is not soluted in
austenite during heating for hardening is increased, resulting in
difficulty to obtain sufficient strength and hardness.
Additionally, coarsening of nitride is brought about to cause easy
occurrence of fatigue breakage. Further, when the V amount is
excessive, the residual austenite amount is increased to decrease
the hardness of the spring. For this reason, as the upper limit of
the V amount, it is preferably 0.5% or less, and more preferably
0.4% or less.
[0067] A method for producing the steel wire rod for a spring in
the present invention is described below. In the present invention,
it is important to produce it, particularly paying attention to
respective steps of a smelting step and hot working so as to obtain
the desired composition and number of oxide-based inclusions.
However, the other steps are not particularly limited, and methods
usually used for the production of steel wire rods for springs can
be appropriately selected and used.
[0068] The preferred smelting step and hot step used in the present
invention are as follows.
(Smelting Step)
[0069] First, deoxidization by Si is performed, and C, Si, Mn, Cr,
Ti, Al, Ni and V are added so as to provide the composition
specified in the present invention. Thereafter, slag refining is
performed using CaO--SiO.sub.2-based slag according to a
conventional method, thereby controlling to a composition of
CaO--Al.sub.2O.sub.3--SiO.sub.2--MgO--MnO--TiO.sub.2. At this time,
the above slag is fully suspended in molten steel, thereby being
able to adjust the number of oxide-based inclusions having a minor
axis of 2 .mu.m or more to a predetermined range. In the present
invention, a predetermined amount of TiO.sub.2 is contained as the
oxide-based inclusions. However, a controlling method thereof is
also not particularly limited, and it should be sufficient to add
Ti during smelting so that the Ti amount in the steel is controlled
within a range of 0.0003 to 0.010%, based on a method usually used
in the technical field in the present invention. A method for
adding Ti is not particularly limited, and for example, an
iron-based alloy containing Ti may be added to perform adjustment,
or the Ti concentration in molten steel may be controlled by
controlling a slag composition.
[Hot Step]
[0070] After a cast slab obtained is heated in a heating furnace at
1100 to 1300.degree. C., blooming is performed at 900 to
1200.degree. C. Thereafter, hot rolling is performed at 800 to
1100.degree. C. to a desired diameter.
[0071] The steel rod for a spring in the present invention is thus
obtained. However, after the above hot rolling, wire drawing may be
further performed to obtain the steel wire rod for a spring. Wire
drawing conditions are not particularly limited, and a method
usually used can be employed.
[0072] The steel wire rod for a spring in the present invention is
very useful as a material for a processed product requiring high
fatigue properties. The above processed products include, for
example, springs such as valve springs to be used in engines or
suspensions of automobiles, clutch springs, brake springs and
suspension springs; steel wires such as steel cords; and the
like.
[0073] A production method of the above spring is not particularly
limited, and the spring can be produced according to a conventional
method. Specifically, the above steel wire rod for a spring is
annealed as needed, and thereafter subjected to stripping
treatment, lead patenting treatment, wire drawing and oil tempering
treatment to produce the spring.
[0074] The present invention is described below in more detail with
reference to examples, but the present invention should not be
construed as being limited by the following examples, changes may
be made without departing from the spirit described above and
later, and these should be understood to be included in the
technical scope of the present invention.
EXAMPLES
Production of Cast Slab
[0075] Using a small-sized melting furnace having a volume of 150
kg/1 ch, test steels having various chemical components shown in
the following Table 1 were smelted, and cast slabs of 245 mm
diameter.times.480 mm were prepared. In smelting, using a crucible
of a MgO-based refractory during smelting, at least one of Ni and V
was added as needed, as well as C, Si, Mn and Cr, and adjustment to
a predetermined concentration was performed. Thereafter, Ti and Ca
were added in this order, and each concentration of Ti and Ca was
adjusted. In this example, a Ni--Ca alloy was used as Ca to be
added to molten steel, and a Fe--Ti alloy was used as a Ti source.
The chemical components of the cast slabs thus obtained are shown
in Table 1.
TABLE-US-00001 TABLE 1 Test Chemical Components (mass %) Steel
Balance: Iron and Unavoidable Impurities No. C Si Mn Cr Al Ca Ti Ni
V 1 0.60 2.00 0.90 0.90 0.0005 0.0005 0.0018 0.25 0.10 2 0.55 1.45
0.87 0.70 0.0005 0.0003 0.0015 -- -- 3 0.63 1.40 0.60 0.65 0.0007
0.0005 0.0020 -- 0.09 4 0.60 2.20 0.50 1.75 0.0010 0.0005 0.0028
0.20 0.30 5 0.60 2.00 0.90 0.90 0.0003 0.0002 0.0025 0.25 0.10 6
0.60 2.00 0.90 0.90 0.0010 0.0005 0.0025 0.25 0.10 7 0.60 2.00 0.90
0.90 0.0008 0.0008 0.0007 0.25 0.10 8 0.60 2.00 0.90 0.90 0.0008
0.0008 0.0005 0.25 0.10 9 0.60 2.00 0.90 0.90 0.0020 0.0010 0.0006
0.25 0.10 10 0.60 2.00 0.90 0.90 0.0008 0.0005 0.0020 0.25 0.10 11
0.65 2.00 0.90 0.90 0.0008 0.0005 0.0040 0.25 0.10 12 0.63 1.40
0.60 0.65 0.0003 0.0015 0.0020 -- 0.09 13 0.60 2.00 0.90 0.90
0.0010 0.0010 0.0020 0.25 0.10 14 0.63 1.40 0.60 0.65 0.0005 0.0005
0.0002 -- 0.09 15 0.60 2.00 0.90 0.90 0.0003 0.0005 0.0130 0.25
0.10 16 0.60 2.00 0.90 0.90 0.0005 0.0005 0.0020 0.25 0.10 17 0.60
2.00 0.90 0.90 0.0005 0.0005 0.0018 0.25 0.10 18 0.60 2.00 0.90
0.90 0.0005 0.0005 0.0018 0.25 0.10
[0076] Each cast slab obtained was heated in a heating furnace at a
temperature of 1100 to 1300.degree. C., and then, blooming was
performed at 900 to 1200.degree. C. Thereafter, hot rolling was
performed at 830 to 1100.degree. C., thereby obtaining a hot-rolled
steel having a diameter of 8.0 mm.
(Measurement of the Composition and Number of Oxide-Based
Inclusions)
[0077] For the hot-rolled steel (diameter: 8.0 mm) thus obtained,
one micro sample having L (L is a length in a rolling direction) of
20 mm or more was cut out in a longitudinal direction thereof
(corresponding to the rolling direction) so as to include a central
axis of the hot-rolled steel, and a cross section including the
above central axis was polished. This polished surface was observed
using an electron probe X-ray micro analyzer (EPMA, trade name
"JXA-8500F") manufactured by JEOL Datum Ltd., and for oxide-based
inclusions having a minor axis of 1 .mu.m or more, the component
composition was quantitatively analyzed. The observation area of
the polished surface was from 100 to 1000 mm.sup.2, and the
component composition in a center part of the oxide-based
inclusions was quantitatively analyzed by characteristic X-ray
wavelength dispersion spectrometry. Elements to be analyzed were
Ca, Al, Si, Ti, Mn, Mg, Na, Cr and Zr. The average composition of
the oxide-based inclusions was determined by previously determining
the relationship between the X-ray intensity of each element and
the element concentration, as a calibration curve by using a known
substance, and quantifying the element amount contained in each
sample from the X-ray intensity obtained from the above oxide-based
inclusions to be analyzed and the above calibration curve, followed
by conversion to the oxide. In Ti oxides, a plurality of valences
can be taken, but all are calculated as TiO.sub.2.
[0078] Further, of the oxide-based inclusions which were present in
the above polished surface, the value obtained by dividing the
number of oxide-based inclusions having a minor axis of 2 .mu.m or
more by the above observation area (100 to 1000 mm.sup.2) was taken
as the inclusion number of oxide-based inclusions
(inclusions/mm.sup.2).
[Fatigue Strength Test (Breakage Ratio)]
[0079] The above hot-rolled steel (diameter: 8.0 mm) was stripped
and wire-drawn to a diameter of 7.4 mm, followed by patenting and
cold wire drawing to a diameter of 4.0 mm. Then, oil tempering
treatment was performed by continuously performing oil quenching
and tempering in a lead bath at about 450.degree. C., and
thereafter, a wire of 4.0 mm diameter.times.650 mm length was
obtained. After the wire thus obtained was subjected to treatment
equivalent to strain relieving annealing at 400.degree. C., shot
peening was performed, and low temperature annealing at 200.degree.
C. was performed. Thus, a test piece for measuring fatigue strength
was prepared.
[0080] The above test piece was tested using a Nakamura-type
rotational bending tester with nominal stress: 970 MPa, rotational
speed: 4000 to 5000 rpm, and the number of times of stoppage:
2.times.10.sup.7 times. Of broken test pieces, the number A of test
pieces broken from inclusions as start points and the number B of
test pieces in which the above test was stopped because the
predetermined number of times of stoppage was attained were each
measured, and the breakage ratio was determined by the following
equation.
Breakage ratio (%)=[A/(A+B)].times.100
[0081] These results are shown in Table 2. The test No. in Table 2
shows that the test steel No. in Table 1 having the same number was
used.
TABLE-US-00002 TABLE 2 Average Composition of Oxide-Based
Inclusions (mass %) Number of Oxide-Based Total Inclusions Having
Minor Breakage Test (CaO + Al.sub.2O.sub.3 + SiO.sub.2 + Axis of 2
.mu.m or More Ratio No. CaO Al.sub.2O.sub.3 SiO.sub.2 MgO MnO
TiO.sub.2 Na.sub.2O ZrO.sub.2 MgO + MnO + TiO.sub.2)
(inclusions/mm.sup.2) (%) 1 20 19 47 4 1 6 97 0.050 20 2 22 19 40 6
3 7 97 0.050 23 3 23 17 50 1 2 7 100 0.050 23 4 18 22 46 4 1 6 97
0.050 20 5 13 13 64 3 1 4 98 0.200 20 6 25 23 35 5 1 5 3 94 0.050
20 7 22 20 45 4 1 6 98 0.060 23 8 34 12 43 4 0 5 98 0.015 27 9 15
25 45 5 1 6 97 0.030 27 10 20 22 48 0 2 6 98 0.100 27 11 12 12 64 3
0 8 99 0.100 27 12 40 14 35 3 1 6 99 0.050 67 13 29 29 25 4 1 7 95
0.040 67 14 23 18 50 1 2 1 95 0.050 33 15 20 19 40 4 1 15 99 0.040
57 16 20 19 47 4 1 6 97 0.001 43 17 0 8 78 0 2 10 98 0.010 30 18 0
1 90 0 3 5 0.5 99 0.010 30
[0082] All of Test Nos. 1 to 11, 17 and 18 in Table 2 satisfy the
chemical component composition and oxide composition specified in
the present invention, and it is known that they are excellent in
fatigue properties.
[0083] On the other hand, Test Nos. 12 to 16 did not satisfy any
one of the requirements in the present invention. Therefore,
fatigue properties were decreased.
[0084] In Test No. 12, the Si amount and the Al amount were within
the ranges in the present invention, but the Si amount and the Al
amount were relatively low. Therefore, the CaO amount in the
oxide-based inclusions was increased to cause a decrease in fatigue
properties
[0085] In Test No. 13, the Ti amount and the Al amount were within
the ranges in the present invention, but the amounts thereof were
relatively larger than in the other examples. Therefore, the
SiO.sub.2 amount in the oxide-based inclusions was decreased to
cause a decrease in fatigue properties.
[0086] In Test No. 14, the Ti amount was small. Therefore, the
TiO.sub.2 amount in the oxide-based inclusions was decreased to
cause a decrease in fatigue properties.
[0087] In Test No. 15, the Ti amount was large. Therefore, the
TiO.sub.2 amount in the oxide-based inclusions was increased to
cause a decrease in fatigue properties.
[0088] In Test No. 16, stirring during slag refining was weaker
than in the other examples. Therefore, suspension became
insufficient, and the number of oxide-based inclusions having a
minor axis of 2 .mu.m or more was decreased to cause a decrease in
fatigue properties
[0089] Although the present invention has been described in detail
and by reference to the specific embodiments, it is apparent to one
skilled in the art that various modifications or changes can be
made without departing from the spirit and scope of the present
invention.
[0090] This application is based on Japanese Patent Application No.
2014-014633 filed on Jan. 29, 2014, the content of which is
incorporated herein by reference.
INDUSTRIAL APPLICABILITY
[0091] The steel wire for a spring in the present invention is more
excellent in fatigue properties than the conventional one, and
suitable for valve springs, suspension springs and the like.
* * * * *