U.S. patent application number 11/548080 was filed with the patent office on 2007-06-07 for spring steel wire rod excellent in pickling performance.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). Invention is credited to Fujio Koizumi, Nao YOSHIHARA.
Application Number | 20070125455 11/548080 |
Document ID | / |
Family ID | 37591555 |
Filed Date | 2007-06-07 |
United States Patent
Application |
20070125455 |
Kind Code |
A1 |
YOSHIHARA; Nao ; et
al. |
June 7, 2007 |
SPRING STEEL WIRE ROD EXCELLENT IN PICKLING PERFORMANCE
Abstract
A spring steel wire rod according to the present invention is a
spring steel wire rod wherein: the spring steel wire rod contains
C: 0.35% to 0.7% (by mass, the same is hereunder applied), Si: 1.5%
to 2.50%, Mn: 0.05% to 1.0%, Cr: 0.05% to 1.9%, Cu: 0.05% to 0.7%,
Ni: 0.15% to 0.8%, P: 0.02% or less (excluding 0%), S: 0.02% or
less (excluding 0%); and the balance being Fe and unavoidable
impurities, wherein the ratio of Si to Cu (Si/Cu) is in the range
of 4 or more; and the difference between the concentration of Cu in
the surface layer and the concentration of Cu in the steel is 0.50%
or less and the difference between the concentration of Ni in the
surface layer and the concentration of Ni in the steel is 1.00% or
less. The present invention makes it possible to improve pickling
performance in a spring steel wire rod containing Si abundantly and
further Cr, Cu, and Ni.
Inventors: |
YOSHIHARA; Nao; (Kobe-shi,
JP) ; Koizumi; Fujio; (Kobe-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
Kobe-shi
JP
|
Family ID: |
37591555 |
Appl. No.: |
11/548080 |
Filed: |
October 10, 2006 |
Current U.S.
Class: |
148/332 ;
420/91 |
Current CPC
Class: |
C22C 38/42 20130101;
C21D 8/065 20130101; C22C 38/34 20130101; C22C 38/50 20130101; C21D
9/02 20130101; C22C 38/04 20130101; C22C 38/46 20130101 |
Class at
Publication: |
148/332 ;
420/091 |
International
Class: |
C22C 38/42 20060101
C22C038/42 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 18, 2005 |
JP |
2005-334845 |
Claims
1. A spring steel wire rod comprising: C: 0.35% to 0.7% (by mass,
the same is hereunder applied unless otherwise specified); Si: 1.5%
to 2.50%; Mn: 0.05% to 1.0%; Cr: 0.05% to 1.9%; Cu: 0.05% to 0.7%;
Ni: 0.15% to 0.8%; P: 0.02% or less (excluding 0%); and S: 0.02% or
less (excluding 0%), wherein the ratio of Si to Cu (Si/Cu) is in
the range of 4.0 or more, and the difference between the
concentration of Cu in the surface layer and the concentration of
Cu in the steel is 0.50% or less and the difference between the
concentration of Ni in the surface layer and the concentration of
Ni in the steel is 1.00% or less.
2. The spring steel wire rod according to claim 1, wherein said
spring steel wire rod further contains at least one element
selected from among the group comprising: V: 0.07% to 0.4%; Ti:
0.01% to 0.1%; and Nb: 0.01% to 0.1%.
3. A spring fabricated from the spring steel wire rod according to
claim 1.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a spring steel wire rod
excellent in pickling performance, and more specifically a
technology for improving pickling performance of a spring steel
wire rod containing Si abundantly and further Cr, Cu, and Ni. A
spring steel wire rod according to the present invention is
favorably used for: a valve spring used for the engine of an
automobile or the like; a clutch spring; a brake spring; a
suspension spring such as a stabilizer spring or a torsion bar
spring; and other applications.
BACKGROUND OF THE INVENTION
[0002] The chemical composition of a spring steel used for a valve
spring, a suspension spring, or the like is stipulated in JIS G3565
to JIS G4801 for example and an appropriate steel type is used in
accordance with the type of spring design. In recent years, as the
size and weight of a spring reduce with the reduction of exhaust
gas and fuel consumption, the design stress of the spring increases
and it is desired to provide, for example, a spring steel wire rod
which can realize a high strength of about 1,600 MPa or higher in
terms of the tensile strength of a spring material wire (a quenched
and tempered material). Further, in order to improve the durability
in the air which is one of the important characteristics of a
spring, the improvement of proof stress is also desired and a steel
wire rod abundantly including Si and Cr as alloying elements which
are capable of improving the proof stress by solid solution
hardening tends to be used. In addition, in order to improve
corrosion fatigue resistance which is another important
characteristic of a spring, it is attempted to add Cu and Ni and
thus improve corrosion resistance.
[0003] A spring is generally fabricated by heating a billet;
subjecting the hot-rolled wire rod (the rolled wire rod) to filming
treatment (surface coating treatment) by applying a lubricant to
the surface if necessary; thereafter drawing the wire rod to a
prescribed diameter; and applying forming (hot forming or cold
forming). The heating is usually carried out in an oxidizing
atmosphere and hence an oxidation layer comprising Fe oxide called
"rolling scale" or "scale" is formed on the surface of a rolled
wire rod. When a rolled wire rod bearing such scale is used for the
fabrication of a spring, surface defects and the like appear, thus
the quality deteriorates, and hence pickling treatment is applied
before the drawing treatment in order to remove the scale.
[0004] FIG. 1 shows a photograph of a cross section of a rolled
wire rod bearing scale on the surface of the steel containing Si
abundantly and further Cr, Cu, and Ni, the photograph being taken
with an Fe-SEM. The case corresponds to the case No. E-1 in the
Example which will be described later. As shown in FIG. 1, the
scale comprises, in order from the surface side, hematite
(Fe.sub.2O.sub.3), magnetite (Fe.sub.3O.sub.4), wustite (FeO), and
fayalite (2FeO.SiO.sub.2). Between the steel (the base steel) and
the scale, subscale wherein Si and Cr condense is further
formed.
[0005] Among those, fayalite is a low melting point oxide which is
seen when a steel abundantly containing Si is used and is a
material hard to exfoliate through ordinary pickling treatment. For
example, when a steel containing Si and Cr is heated to a
temperature exceeding the eutectic temperature of fayalite and
wustite (about 1,170.degree. C.), a dense melt phase wherein those
oxides are intricately interwound with each other is formed and
then, when it is further heated to a temperature of 1,200.degree.
C. or higher, Cr intrudes into the aforementioned melt phase and
the fayalite, condenses at the interface with the base steel, and
forms a condensed layer (the detail will be described later) The Cr
once condensed is very hard to remove through the succeeding
processes and deteriorates pickling performance. When the pickling
performance deteriorates, scale remains after pickling treatment,
hence the adhesiveness with a lubricant applied on the surface
(used for surface coating treatment) lowers, and wire breakage may
undesirably occur during drawing. Even when wire breakage does not
occur, cracks (crevices) may occur during drawing and breakage may
occur during spring forming (cold coiling) Those problems are
alleviated, for example, by prolonging the pickling treatment time
and completely removing scale. In this case however, since the time
of dipping in a pickling solution increases, the attack to the base
steel by acid intensifies, the surface roughness lowers, and
finally the durability comes to deteriorate even in the air.
Further, since a part of hydrogen generated during the attack to
the base steel by acid diffuses rapidly in the steel and is
absorbed therein, the amount of stored hydrogen increases, that
causes the embrittlement (hydrogen embrittlement) of the steel
material, and wire breakage occurs during drawing in some
cases.
[0006] Further, although it is not shown in FIG. 1, a layer wherein
Cu and Ni are concentrated is also formed between the steel (the
base steel) and the scale. The concentrated layer is seen when a
steel contains Cu and Ni. It is known that: metals such as Cu and
Ni which are nobler than iron are not oxidized by heating; hence
such metals concentrate at the interface between scale and a base
steel or in fayalite; and surface cracks caused by red shortness
appear. For example, the melting point of Cu is about 1,080.degree.
C. and a steel material is usually heated to a temperature higher
than the melting point of Cu (about 1,100.degree. C. to
1,250.degree. C. for example) before hot-rolled, and hence it is
estimated that: Cu in a molten state (of a liquid phase) is formed
at the interface between scale and the base steel; the Cu intrudes
into grain boundaries of the base steel and concentrates; the steel
material does not withstand shearing stress or tensile stress any
more during hot rolling; and surface cracks caused by red shortness
appear. Such a phenomenon is called liquid metal melt embrittlement
and it is understood that the phenomenon resultantly deteriorates
pickling performance.
[0007] Various methods are proposed in order to prevent: Cu from
concentrating at a base steel interface; and surface cracks caused
by red shortness from appearing.
[0008] For example, JP-A No. 297026/1994 describes a method for
preventing surface cracks by: adding Si in a steel; making Si exist
in scale during the heating process preceding hot rolling; forming
low melting point oxide liquid scale of an SiO.sub.2--FeO system;
and thus taking the Cu molten liquid into the scale.
[0009] JP-A No. 223523/2004 describes a technology related to a
method of heating a Cu containing steel material, the technology
being proposed by the same applicants as those of JP-A No.
297026/1994. The patent application describes a method for
controlling the temperature of a heating atmosphere and the oxygen
concentration in the atmosphere with regard to the conditions of
the heating before hot rolling as a method which makes it possible
to prevent surface cracks caused by the concentration of Cu without
such change of a steel composition as described in JP-A No.
297026/1994.
[0010] As stated above, although a spring steel wire rod containing
Si abundantly and Cr, Cu, and Ni is used mostly in order to improve
proof stress and corrosion fatigue resistance, there is concern
that, in the case of such a spring steel wire rod, Cu concentrates
at the interface between scale (particularly fayalite) and the base
steel and hence pickling performance deteriorates.
[0011] However, a method for sufficiently improving the pickling
performance of such a spring steel wire rod is not disclosed yet.
JP-A No. 223523/2004 stated above describes a technology for
preventing surface cracks caused by the concentration of Cu without
the abundant addition of Si and by the technology the improvement
of proof stress by the addition of Si cannot be obtained.
Meanwhile, JP-A No. 297026/1994 stated above, as it is indicated in
the section of "Prior Art" of JP-A No. 223523/2004 stated above
which is proposed afterward, has the problems in that: a steel
material to which Si is added has poor detachability of scale; the
scale is hardly exfoliated and removed even when descaling with
high-pressure water is applied before rolling; the surface
properties deteriorate, for example scale remains and the surface
of the steel material turns red; and, when a pickling process is
applied thereafter, the scale hardly dissolves in the pickling
process and hence the cost of the pickling process increases and
the productivity lowers.
SUMMARY OF THE INVENTION
[0012] The present invention has been established in view of the
above situation and the object thereof is to provide a spring steel
wire rod having improved pickling performance in the case of a
spring steel wire rod containing Si abundantly and further Cr, Cu,
and Ni.
[0013] A spring steel wire rod according to the present invention
which can solve the above problems is a spring steel wire rod
wherein: the spring steel wire rod contains C: 0.35% to 0.7% (by
mass, the same is hereunder applied unless otherwise specified),
Si: 1.5% to 2.50%, Mn: 0.05% to 1.0%, Cr: 0.05% to 1.9%, Cu: 0.05%
to 0.7%, Ni: 0.15% to 0.8%, P: 0.02% or less (excluding 0%), and S:
0.02% or less (excluding 0%), wherein the ratio of Si to Cu (Si/Cu)
is in the range of 4.0 or more; and the difference between the
concentration of Cu in the surface layer and the concentration of
Cu in the steel is 0.50% or less and the difference between the
concentration of Ni in the surface layer and the concentration of
Ni in the steel is 1.00% or less.
[0014] In a preferable embodiment, a spring steel wire rod
according to the present invention further contains at least one
element selected from among the group comprising V: 0.07% to 0.4%,
Ti: 0.01% to 0.1%, and Nb: 0.01% to 0.1%.
[0015] A spring according to the present invention which can solve
the above problems is a spring fabricated from any one of the
spring steel wire rods stipulated above.
[0016] A spring steel wire rod according to the present invention
is excellent in pickling performance since the concentration of Cr
and Ni in the surface layer thereof is considerably suppressed and
the thickness of scale is very thin. When a spring is fabricated
from a spring steel wire rod according to the present invention, a
spring excellent in surface properties can be provided since the
scale is easily exfoliated through a pickling process.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is an Fe-SEM photograph showing a cross section of a
rolled wire rod bearing scale.
[0018] FIG. 2 is a graph showing the relationship between a Cu
concentration (%) and a distance from a surface.
[0019] FIG. 3 is a graph showing the relationship between a Ni
concentration (%) and a distance from a surface.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] The present inventors have earnestly studied in order to
improve the pickling performance of a spring steel wire rod
containing Si abundantly and further Cr, Cu, and Ni. As a result,
as it will be explained later in detail, the present inventors have
found that, by properly controlling particularly, (i) a descaling
process preceding hot rolling, and (ii) a hot-rolling process, the
concentration of Cu and Ni on the wire rod surface (particularly
the concentration of Cu and Ni in fayalite) is suppressed; the
difference between the concentration of Cu in the surface layer and
the concentration of Cu in the steel (hereunder may be abbreviated
to ".DELTA.Cu") and the difference between the concentration of Ni
in the surface layer and the concentration of Ni in the steel
(hereunder may be abbreviated to ".DELTA.Ni") are remarkably
lowered; and thereby pickling performance outstandingly improves.
And the present inventors have established the present
invention.
[0021] Specifically, (i) water shower of a prescribed high pressure
is applied in a descaling process preceding hot rolling. By so
doing, Cu taken into fayalite is exfoliated and the amount of
concentrated Cu in a surface layer can be reduced. Further, (ii)
water shower of a prescribed high pressure is applied in a
hot-rolling process. By so doing, the concentration of Cu in
fayalite formed during hot rolling can be considerably
suppressed.
[0022] The present invention is hereunder explained in detail.
[0023] As stated above, a spring steel wire rod according to the
present invention is a spring steel wire rod wherein: the spring
steel wire rod comprises C: 0.35% to 0.7%, Si: 1.5% to 2.50%, Mn:
0.05% to 1.0%, Cr: 0.05% to 1.9%, Cu: 0.05% to 0.7%, Ni: 0.15% to
0.8%, P: 0.02% or less (excluding 0%), S: 0.02% or less (excluding
0%), and the balance consisting of Fe and unavoidable impurities;
the ratio of Si to Cu (Si/Cu) is in the range of 4.0 or more; and
the difference between the concentration of Cu in the surface layer
and the concentration of Cu in the steel is 0.50% or less and the
difference between the concentration of Ni in the surface layer and
the concentration of Ni in the steel is 1.00% or less.
[0024] In the present specification, the term "steel wire rod"
means a steel material (a rolled material) which is fabricated by
heating and thereafter hot-rolling a billet into a linear shape,
which is not yet subjected to pickling treatment.
[0025] Firstly, components in a steel are explained.
C: 0.35% to 0.7%
[0026] C is an element which contributes to the enhancement of
strength (hardness) after being quenched and tempered and improves
durability in the air. When a C content is less than 0.35%, the
above functions cannot be effectively exhibited. On the other hand,
when a C content exceeds 0.7%, toughness and ductility deteriorate,
cracks tend to propagate and durability lowers, and thus corrosion
resistance is also badly affected. A preferable C content is in the
range of 0.39% to 0.54%.
Si: 1.5% to 2.50%
[0027] Si is an element which contributes to the enhancement of
strength as a solid solution hardening element and also can improve
proof stress. When a Si content is less than 1.5%, the strength of
a matrix is insufficient. However, when a Si content exceeds 2.50%,
the ratio of fayalite in scale increases and pickling performance
deteriorates. A preferable Si content is in the range of 1.70% to
2.1%.
Mn: 0.05% to 1.0%
[0028] Mn is an element which enhances the hardenability of a
steel. Mn is added by 0.05% or more in order to effectively exhibit
such a function. When Mn is added in excess of 1.0% however, the
hardenability increases, a supercooled structure tends to form, and
drawability deteriorates. Further, when an annealing process is
applied after hot rolling before pickling treatment with the aim of
softening a wire rod in the same way as "a spring forming process
(c)" which will be described later, the cost unavoidably increases.
A preferable Mn content is in the range of 0.12% to 0.8%. In the
present invention, as stated later, measures such as the reduction
of a S content, the addition of another sulfide-forming element
such as Cu, and others are taken in order to prevent MnS which
functions as the origin of fracture from forming.
Cr: 0.05% to 1.9%
[0029] Cr is an element which makes rust formed on a surface
amorphous and dense under corrosive conditions, contributes to the
improvement of corrosion resistance, and effectively affects the
enhancement of hardenability like Mn. Cr is added by 0.05% or more
in order to exhibit such functions. When a Cr content exceeds 1.9%
however, a supercooled structure tends to form during cooling after
rolling and drawability deteriorates. When an annealing process is
applied after hot rolling before pickling treatment with the aim of
softening a wire rod in the same way as "a spring forming process
(c)" which will be described later, the cost unavoidably increases.
A preferable Cr content is in the range of 0.15% to 1.75%.
Cu: 0.05% to 0.7%
[0030] Cu is an element which is electrochemically nobler than iron
and has the function of improving corrosion resistance. Cu is added
by 0.05% or more in order to effectively exhibit such a function.
When a Cu content exceeds 0.7% however, the amount of Cu
concentrated in a base steel surface layer, at the interface
between a base steel and rolling scale, and in fayalite increases
and pickling performance deteriorates. A preferable Cu content is
in the range of 0.20% to 0.5%.
Ni: 0.15% to 0.8%
[0031] Ni has the functions of enhancing toughness after quenching
and tempering, making rust forming on a surface amorphous and dense
under corrosive conditions, and thus improving corrosion
resistance. Further, Ni has also the function of suppressing
ferrite decarburization occurring before and during rolling. Ni is
added by 0.15% or more in order to effectively exhibit such
functions. When a Ni content exceeds 0.8% however, hardenability
increases and a supercooled structure tends to form after rolling.
In addition, the amount of retained austenite increases and spring
hardness lowers. A preferable Ni content is in the range of 0.25%
to 0.55%.
P: 0.02% or Less (Excluding 0%)
[0032] P segregates at prior austenite grain boundaries, embrittles
the grain boundaries, and lowers resistance to delayed fracture.
For that reason, a P content should be as small as possible. In the
present invention, the upper limit of a P content is set at 0.02%
in view of industrial production.
S: 0.02% or Less (Excluding 0%)
[0033] S segregates at prior austenite grain boundaries, embrittles
the grain boundaries, and lowers resistance to delayed fracture.
For that reason, a S content should be as small as possible. In the
present invention, the upper limit of a S content is set at 0.02%
in view of industrial production.
Ratio of Si to Cu (Si/Cu): 4.0 or more
[0034] In the present invention, not only the amounts of Si and Cu
are regulated respectively but also the lower limit of a ratio
Si/Cu is regulated. By the regulation, fayalite formed by the
addition of Si and the amount of Cu concentrated at the interface
between a base steel and scale by the addition of Cu are properly
controlled, and hence the Cu concentration into the fayalite is
suppressed and .DELTA.Cu lowers. As a result, pickling performance
improves (refer to the after-mentioned Example). The upper limit of
a ratio Si/Cu is set at 50 from the contents of Si and Cu described
above. A preferable ratio Si/Cu is in the range of 4.5 to 35.
[0035] A spring steel wire rod according to the present invention
contains the aforementioned components and the balance thereof
consists of Fe and unavoidable impurities.
[0036] In the present invention, with the aim of further enhancing
resistance to hydrogen embrittlement, it is preferable to further
contain at least one element selected from among the group
comprising V: 0.07% to 0.4%, Ti: 0.01% to 0.1%, and Nb: 0.01% to
0.1%. The elements are hereunder explained in detail.
V: 0.07% to 0.4%
[0037] V is an element which forms fine carbide and nitride and
contributes to the improvement of resistance to hydrogen
embrittlement. Further, V enhances fatigue resistance. In addition,
V improves toughness and proof stress by the crystal grain
fractionizing effect and also contributes to the improvement of
corrosion resistance and resistance to permanent set in fatigue. It
is preferable to add V by 0.07% or more in order to effectively
exhibit such functions. When V is added in excess of 0.4% however,
the amount of carbide not dissolved in austenite increases during
heating for hardening, sufficient strength and hardness cannot be
obtained, the amount of retained austenite also increases, and the
hardness of a spring lowers. A yet preferable V content is in the
range of 0.1% to 0.2%.
Ti: 0.01% to 0.1%
[0038] Ti is an element which fractionizes prior austenite crystal
grains after quenching and tempering and is effective in the
improvement of resistance to hydrogen embrittlement. Further, Ti
has the function of enhancing durability in the air. It is
preferable to add Ti by 0.01% or more in order to effectively
exhibit such functions. When Ti is added excessively however,
coarse nitride tends to precipitate and durability in the air
deteriorates. For that reason, it is preferable to set the upper
limit of a Ti content at 0.1%. A yet preferable Ti content is in
the range of 0.04% to 0.09%.
Nb: 0.01% to 0.1%
[0039] Nb is an element which forms fine precipitate comprising
carbide, nitride, sulfide, and a composite compound thereof and
thus contributes to the improvement of resistance to hydrogen
embrittlement. Further, toughness and proof stress also improve by
the crystal grain fractionizing effect. It is preferable to add Nb
by 0.01% or more in order to effectively exhibit such functions.
When Nb is added in excess of 0.1% however, the amount of carbide
not dissolved in austenite increases during heating for hardening
and an intended tensile strength cannot be obtained. A yet
preferable Nb content is in the range of 0.02% to 0.05%.
[0040] The components in a steel according to the present invention
have been explained above.
Difference Between the Cu Concentration in a Surface Layer and the
Cu Concentration in a Steel (.DELTA.Cu): 0.50% or Less
[0041] In the present invention, .DELTA.Cu is controlled to a low
level of 0.50% or less. As stated above, although most of the scale
(the primary scale) formed through heating can be removed through
ordinary descaling treatment applied before hot rolling, the scale
(the secondary scale) formed during hot rolling and during cooling
after hot rolling, particularly in the case of such a steel
containing Si and Cu in quantities as a steel according to the
present invention, cannot be easily removed through ordinary
descaling treatment, Fe diffuses in the scale, and thereby Cu
precipitates at the interface between the scale (fayalite) and the
base steel or in fayalite. As a result, pickling performance
deteriorates. In the present invention, as it will be explained
later in detail, since not only a descaling process preceding hot
rolling but also a finish-rolling process is properly controlled,
.DELTA.Cu can be controlled to a low level. The lower the value
.DELTA.Cu is, the better. For example, a value .DELTA.Cu is
preferably 0.45% or less, and yet preferably 0.40% or less.
[0042] Here, the method for measuring "a Cu concentration in a
surface layer" is explained with reference to FIG. 2. FIG. 2
comprises the graphs obtained by using a specimen prepared as
stated below and measuring a Cu content from a surface to the
center of the interior by EPMA quantitative linear analysis under
after-mentioned conditions; and shows the relationship between a Cu
concentration (%) and a distance from the surface. Here with regard
to Fe, the point where the Fe strength (cps) of X-ray comes to the
maximum when the EPMA quantitative linear analysis is applied in
the same way as stated above is defined as a base steel interface
(an interface between scale and a base steel). The region between a
surface and a base steel interface is defined as "a surface layer"
and the maximum value of the Cu content in the surface layer is
defined as "a Cu concentration in a surface layer." The state of
such a surface layer varies also in accordance with components in a
steel, fabrication conditions of a wire rod, and others but the
surface layer contains at least fayalite.
[0043] EPMA measuring apparatus: An X-ray microanalyzer "JXA-8800
RL" made by JEOL Ltd. was used,
[0044] Specimen: prepared by embedding a steel material bearing
scale into resin, mirror-finishing a cross section thereof
perpendicular to the rolling direction with an abrasive, and
thereafter applying vapor deposition of osmium in order to keep
electric conductivity,
[0045] Accelerating voltage: 15 kV,
[0046] Irradiation current: 0.3 .mu.A, and
[0047] Quantitative linear analysis: measured at 300 spots in total
at distribution intervals of 1 .mu.m.
Difference Between the Ni Concentration in a Surface Layer and the
Ni Concentration in a Steel (.DELTA.Ni): 1.00% or Less
[0048] In the present invention, .DELTA.Ni is also controlled to a
low level of 1.00% or less. Ni is an element which contributes to
the improvement of corrosion resistance like Cu as stated above and
is the same as Cu also in the fact that Ni precipitates at the
interface between scale (fayalite) and a base steel or in fayalite
in a heating process and a hot-rolling process. As a result,
pickling performance deteriorates. In the present invention
therefore, it is necessary to suppress the concentration of Ni in
the same way as Cu. The lower the value .DELTA.Ni is, the better.
For example, a value .DELTA.Ni is preferably 0.90% or less, and yet
preferably 0.85% or less.
[0049] Here, the term "a Ni concentration in a surface layer" means
the maximum Ni content obtained by using a specimen prepared by the
same method as used in the measurement of "a Cu concentration in a
surface layer" stated above and measuring a Ni content in the range
of 0.3 mm from the surface toward the center of the interior by the
EPMA quantitative linear analysis under the same conditions as
described above. FIG. 3 shows the relationship between a Ni
concentration (%) and a distance from a surface.
[0050] .DELTA.Cu and .DELTA.Ni which characterize the present
invention have been explained above.
[0051] In a spring steel wire rod according to the present
invention, it is preferable to control the thickness and the
composition of scale in an appropriate manner as stated below and
by so doing pickling performance is further improved.
(Thickness of Scale)
[0052] A preferable thickness of scale is 40 .mu.m or less. As it
will be explained hereunder in detail, it is preferable that the
thickness of scale is basically in the range of 5 to 35 .mu.m in
consideration of the exfoliation of scale caused by the crevices
(cracks) formed in the scale.
[0053] It sometimes happens that crevices of a micron level form in
scale, for example, during cooling after rolling and handling of a
rolled wire rod. As crevices increase, scale tends to exfoliate
more easily from the surface of a base steel and hence it is
estimated that pickling performance improves. In general, as the
thickness of scale is reduced, the strength of the scale is lowered
and crevices are likely to form. However, if the thickness of scale
is reduced excessively, the ductility of the scale itself is
improved, the internal stress is reduced, and hence crevices are
decreased. For those reasons, it is preferable that the thickness
of scale is in the above range.
(Composition of Scale)
[0054] It is preferable that the composition of scale basically
satisfies the ranges of, in volume percentage, 2% to 10% (yet
preferably 3% to 7%) as fayalite, 2% to 20% (yet preferably 10% to
18%) as wustite, 35% to 70% (yet preferably 37% to 50%) as
magnetite, and 20% to 60% (yet preferably 30% to 55%) as hematite.
As stated above, in the present invention, the composition of scale
is controlled so that the percentage of fayalite which is hardly
exfoliative may be reduced and the percentages of wustite and
magnetite which are excellent in scale detachability may be
increased, and hence pickling performance is further improved.
[0055] A spring steel wire rod according to the present invention
has been explained above.
[0056] A method for fabricating such a spring steel wire rod as
stated above is explained hereunder.
[0057] A method for fabricating a spring steel wire rod includes
(i) a heating process, (ii) a soaking process, (iii) a descaling
process preceding a hot-rolling process, and (iv) a hot-rolling
process. In the present invention in particular, for example,
prescribed high-pressure water shower is applied in (iii) a
descaling process preceding a hot-rolling process in order to
rapidly remove scale including fayalite, and prescribed cooling
water shower is applied in (iv) a hot-rolling process in order to
prevent Cu from concentrating in fayalite formed during hot
rolling, and thereby it has been possible to suppress .DELTA.Cu and
.DELTA.Ni to considerably low levels. As shown in the Example to be
described later, by the present invention, even though a steel
containing Si abundantly and Cr, Cu, and Ni is used, the
concentration of Cu and Ni in a surface layer is considerably
suppressed, the thickness of scale is reduced, and hence it is
possible to provide a spring having a tensile strength of about
1,600 MPa or more, excellent in surface properties.
[0058] Each process is hereunder explained in detail.
(i) Heating Process
[0059] Heating conditions are not particularly limited but
basically a billet is heated to a temperature in the range of
700.degree. C. to 1,000.degree. C. (750.degree. C. to 900.degree.
C.) at a heating rate of 10.degree. C./min or higher. When a
heating rate is lower than 10.degree. C./min, the concentration of
Cr into a surface layer cannot be prevented effectively. A higher
heating rate is desirable and a preferable heating rate is
15.degree. C./min or higher. Further, when a heating temperature
exceeds the above range, the concentration of Cu and Ni advances
and the amounts of Cu and Ni in the surface layer increase. In
contrast, when a heating temperature is lower than the above range,
heating is insufficient and hence rough rolling fails.
(ii) Soaking Process
[0060] In the process, basically it is preferable to soak a billet
for 20 to 60 minutes (yet preferably 30 to 50 minutes) at
1,050.degree. C. to 1,250.degree. C. (yet preferably 1,100.degree.
C. to 1,200.degree. C.). The soaking conditions are determined so
as to prevent Cu and Ni from concentrating in a surface layer and
suppress the progress of grain boundary oxidation. For example, the
concentration of Cu and Ni tends to advance when a soaking
temperature or a soaking time exceeds the above range, and in
contrast the concentration of Cu into austenite crystal grain
boundaries advances when a soaking temperature or a soaking time is
lower than the above range.
[0061] In the present invention, a heating temperature in a heating
process and a soaking temperature in a soaking process are not
necessarily identical. In the Example which will be described later
for example, the soaking temperature is higher than the heating
temperature by about 200.degree. C. to 300.degree. C. and the
reason is that the temperature at the soaking rises during the
residence time after heating before soaking.
(iii) Descaling Process Preceding Hot-rolling Process
[0062] In the process, in order to rapidly remove scale containing
fayalite, it is preferable to apply high-pressure water shower for
about 1 to 10 seconds (yet preferably 3 to 7 seconds) with a water
pressure in the range of about 80 kgf/mm.sup.2 (nearly 785 MPa) to
160 kgf/mm.sup.2 (nearly 1,569 MPa), and yet preferably about 100
kgf/mm.sup.2 (nearly 981 MPa) to 120 kgf/mm.sup.2 (nearly 1,176
MPa). Thereby it is possible to carry out the succeeding
hot-rolling process soon. When a water pressure of the shower is
lower than 80 kgf/mm.sup.2, the thickness of scale increases and
that may undesirably cause surface defects caused by bite during
hot rolling to appear, and the Cu concentration in a surface layer
to increase. On the other hand, when a water pressure of the shower
exceeds 160 kgf/mm.sup.2, the temperature of a billet before hot
rolling lowers and rolling is hardly carried out.
[0063] In a descaling process preceding a hot-rolling process, the
descaling means is not limited to such high-pressure water shower
as stated above and, for example, mechanical descaling such as shot
blasting may be adopted.
(iv) Hot-rolling Process
[0064] In the process, prescribed cooling water shower is applied
in order to prevent Cu from concentrating into fayalite formed
during hot rolling and further control the composition of scale
properly.
[0065] Specifically, cooling with the shower is applied in a
finish-rolling process after the application of rough rolling. In
general, the flow rate of the shower water is preferably in the
range of 100 to 200 t/hr, and yet preferably 120 to 180 t/hr. When
the flow rate of the shower water is less than 100 t/hr, the
intended functions of removing scale (fayalite) and reducing
.DELTA.Cu and .DELTA.Ni are not effectively exhibited. On the other
hand, when the flow rate of the shower water exceeds 200 t/hr, a
steel material is excessively cooled and a supercooled structure
undesirably precipitates.
[0066] A preferable finish-rolling temperature is basically in the
range of 800.degree. C. to 1,000.degree. C. (yet preferably
900.degree. C. to 950.degree. C.) mainly in order to properly
control the thickness and the composition of scale.
[0067] Further, from the same viewpoint as above, it is preferable
to control a cooling rate to, for example, 4.degree. C./sec to
20.degree. C./sec (yet preferably 6.degree. C./sec to 15.degree.
C./sec) in the temperature range of a temperature after the
completion of the finish rolling to about 700.degree. C. When a
cooling rate is lower than 4.degree. C./sec in the above
temperature range, the thickness and the like of scale increase and
pickling performance deteriorates. On the other hand, when a
cooling rate exceeds 20.degree. C./sec in the above temperature
range, the retention time in the temperature range decreases, the
percentage of wustite formed in the temperature range is reduced,
and hence pickling performance deteriorates.
[0068] In the present invention, besides such a spring steel wire
rod as stated above, a spring fabricated from such a steel wire rod
is also included. By the present invention, scale flaws called red
scale do not appear at all and it is possible to produce a spring
which is outstandingly excellent in surface properties and has
further improved fatigue resistance.
[0069] A method for fabricating a spring is not particularly
limited and it is possible to adopt an ordinarily used method in an
appropriate manner. As typical examples, a spring can be fabricated
through any one of the spring forming processes (a) to (c)
below.
[0070] (a) Pickling.fwdarw.surface coating
treatment.fwdarw.drawing.fwdarw.quenching and tempering (oil
tempering),
[0071] (b) Lead patenting (LP).fwdarw.pickling.fwdarw.surface
coating treatment.fwdarw.drawing.fwdarw.oil tempering, and
[0072] (c) Annealing.fwdarw.pickling surface coating
treatment.fwdarw.skin shaving (SV).fwdarw.pickling.fwdarw.surface
coating treatment.fwdarw.drawing.fwdarw.oil tempering.
[0073] As shown in the Example which will be described later, by
the present invention, a rolled wire rod having low .DELTA.Cu and
.DELTA.Ni values can be obtained and hence a spring outstandingly
excellent in surface properties can be obtained by any of the
methods described in the above spring forming processes (a) to
(c).
[0074] The treatment methods described in the above spring forming
processes (a) to (c) are not particularly limited and it is
possible to adopt an ordinarily used method in an appropriate
manner. As typical examples, pickling treatment can be processes by
dipping a steel material either in a 5% to 25% H.sub.2SO.sub.4
solution in the temperature range of 60.degree. C. to 90.degree. C.
or in a 5% to 15% HCl solution in the temperature range of
20.degree. C. to 50.degree. C.
EXAMPLE
[0075] The present invention is hereunder explained in detail on
the basis of examples. However, the examples below do not limit the
present invention and any arbitrary modification in the range not
deviating from the afore- and after-mentioned tenor of the present
invention is also included in the technological scope of the
present invention.
[0076] Various types of steels shown in Table 1 (steel types A to
K, the balance consisted of Fe and unavoidable impurities) of 150
kg each were fabricated by melting and refining in a small vacuum
melting furnace. Each of the steels was subjected to hot forging
and a billet of 155 cm square was fabricated. Thereafter steel wire
rods 13.5 mm in diameter were fabricated under the heating,
soaking, and hot-rolling conditions described in the wire rod
forming processes 1 to 6 shown below. Among the wire rod forming
processes 1 to 6, the wire rod forming processes 1 and 2 satisfied
all the fabrication conditions stipulated in the present invention
and represents invention examples, and the wire rod forming
processes 3 to 6 did not satisfy any of the fabrication conditions
stipulated in the present invention and represents comparative
examples.
(Wire Rod Forming Process 1)
[0077] A billet was heated to about 900.degree. C. at a heating
rate of 15.degree. C./min; thereafter soaked for 50 minutes at
about 1,150.degree. C., and subjected to descaling by applying
high-pressure water shower for about 5 seconds under a water
pressure of 100 kgf/mm.sup.2 (nearly 981 MPa). Successively, the
billet was subjected to rough rolling, thereafter subjected to
finish rolling (the finish-rolling temperature was 920.degree. C.)
while being cooled with the shower of 150 t/hr and, after the
completion of the finish rolling, cooled at a cooling rate of
7.degree. C./sec in the temperature range up to about 700.degree.
C. The coiling temperature was 875.degree. C.
(Wire Rod Forming Process 2)
[0078] A billet was heated to about 800.degree. C. at a heating
rate of 20.degree. C./min, thereafter soaked for 30 minutes at
about 1,100.degree. C., and subjected to descaling by applying
high-pressure water shower for about 7 seconds under a water
pressure of 150 kgf/mm.sup.2 (nearly 1,471 MPa). Successively, the
billet was subjected to rough rolling, thereafter subjected to
finish rolling (the finish-rolling temperature was 950.degree. C.)
while being cooled with the shower of 130 t/hr, and, after the
completion of the finish rolling, cooled at a cooling rate of
6.degree. C./sec in the temperature range up to about 700.degree.
C. The coiling temperature was 925.degree. C.
(Wire Rod Forming Process 3)
[0079] A billet was heated to about 1,050.degree. C. at a heating
rate of 15.degree. C./min; thereafter soaked for 60 minutes at
about 1,280.degree. C.; and subjected to descaling by applying
high-pressure water shower for about 10 seconds under a water
pressure of 50 kgf/mm.sup.2 (nearly 490 MPa). Successively, the
billet was subjected to rough rolling; thereafter subjected to
finish rolling (the finish-rolling temperature was 1,000.degree.
C.) while being cooled with the shower of 50 t/hr; and, after the
completion of the finish rolling, cooled at a cooling rate of
2.degree. C./sec in the temperature range up to about 700.degree.
C. The coiling temperature was 980.degree. C.
[0080] The wire rod forming process 3 represents a comparative
example which did not satisfy the fabrication conditions stipulated
in the present invention and all the processes of: the heating
process (the heating temperature was too high); the soaking process
(the soaking temperature was too high); the descaling process
preceding the hot-rolling process (the water pressure was too low);
and the hot-rolling process (the finish-rolling temperature was too
high, the flow rate of the shower water was too low, and the
cooling rate in the temperature range of the finish-rolling
temperature to 700.degree. C. was too low) deviated from the
conditions stipulated in the present invention.
(Wire Rod Forming Process 4)
[0081] A billet was subjected to heating, soaking, and descaling
preceding rolling in the same way as the aforementioned wire rod
forming process 3. Successively, the billet was subjected to finish
rolling, cooling, and coiling in the same way as the aforementioned
wire rod forming process 2.
[0082] The wire rod forming process 4 and the after-mentioned wire
rod forming process 6 represents comparative examples which did not
satisfy the fabrication conditions stipulated in the present
invention and the processes of: the heating process (the heating
temperature was too high); the soaking process (the soaking
temperature was too high); and the descaling process preceding the
hot-rolling process (the water pressure was too low) deviated from
the conditions stipulated in the present invention.
(Wire Rod Forming Process 5)
[0083] A billet was subjected to heating, soaking, and descaling
preceding rolling in the same way as the aforementioned wire rod
forming process 1. Successively, the billet was subjected to finish
rolling, cooling, and coiling in the same way as the aforementioned
wire rod forming process 3.
[0084] The wire rod forming process 5 represents a comparative
example which did not satisfy the fabrication conditions stipulated
in the present invention and the hot-rolling process (the
finish-rolling temperature was too high, the flow rate of the
shower water was too low, and the cooling rate in the temperature
range of the finish-rolling temperature to 700.degree. C. was too
low) deviated from the conditions stipulated in the present
invention.
(Wire Rod Forming Process 6)
[0085] A billet was subjected to heating, soaking, and descaling
preceding rolling in the same way as the aforementioned wire rod
forming process 3. Successively, the billet was subjected to finish
rolling, cooling, and coiling in the same way as the aforementioned
wire rod forming process 2.
[0086] Each of the steel wire rods thus fabricated was evaluated by
measuring the values of .DELTA.Cu and .DELTA.Ni by the
aforementioned methods, and measuring the pickling performance and
the scale thickness in the following manner.
(Evaluation of Pickling Performance)
[0087] A fabricated steel wire rod was cut into test pieces 100 mm
in length each and the test pieces were subjected to the following
pickling test (beaker test) with the number of samples (n) per test
being three. The test here was carried out under the same
conditions as pickling treatment adopted in commercial
production.
[0088] Acid solution: 15% sulfuric acid solution, containing an
inhibitor (cationic amine derivative) by 0.5% to prevent a base
steel from dissolving and ferrous iron by 20 g/L as an iron
component,
[0089] Dipping condition: for 10 minutes at 60.degree. C.
[0090] Then a scale exfoliation ratio after pickling was measured
as described below. In the present example, the term "a scale
exfoliation ratio after pickling" was defined by the percentage of
the ratio of scale exfoliated at pickling (B) to the ratio of the
originally existing scale (A which will be described later) (B/A x
100 (%)): A (%)=[(W.sub.0-W.sub.1)/W.sub.0].times.100, (1)
wherein
[0091] A was the ratio of originally existing scale (the ratio of
scale to a steel wire rod),
[0092] W.sub.0 represents the weight (g) of the steel wire rod
before being dipped (as rolled, bearing scale), and
[0093] W.sub.1 represents the weight (g) of the steel wire rod
after dipped in the aforementioned acid solution at 60.degree. C.
until the existing scale was completely exfoliated; and B
(%)=[(W.sub.01-W.sub.2)/W.sub.01].times.100, (2) wherein
[0094] B was the ratio of the scale exfoliated after pickled under
the aforementioned conditions,
[0095] W.sub.01 represents the weight (g) of the steel wire rod
before being dipped (as rolled), and
[0096] W.sub.2 represents the weight (g) of the steel wire rod
after dipped in the aforementioned acid solution under the
aforementioned dipping condition.
[0097] In the above expressions (1) and (2), although both W.sub.0
and W.sub.01 represents the weight of an as-rolled steel wire rod,
different reference characters were used in order to clarify that
those were the weights of different test pieces (as-rolled steel
wire rods) fabricated under identical conditions. That is because A
and B could not be measured using an identical test piece.
[0098] In the present invention, when a scale exfoliation ratio
measured as stated above was 100%, the steel wire rod was judged as
excellent in pickling performance (as acceptable:.largecircle.)
(Thickness of Scale)
[0099] The thickness (the maximum thickness) of scale on each of
the fabricated steel wire rods was measured on the basis of a
photograph (3,000 magnifications) taken with an Fe-SEM (a field
emission type scanning electron microscope S-4500 made by Hitachi,
Ltd.).
[0100] In the present invention, when the thickness of scale
measured as stated above was 40 .mu.m or less, the steel wire rod
was judged as acceptable.
(Fabrication of a Spring Steel Wire)
[0101] Next, a spring steel wire (an oil-tempered wire) 4.0 mm in
diameter was fabricated from each of the aforementioned steel wire
rods under the following conditions (corresponding to the
aforementioned spring forming process (a));
[0102] Surface coating treatment.fwdarw.drawing (dry wire
drawing).fwdarw.oil tempering (heating temperature: 930.degree. C.,
quenching oil temperature: 70.degree. C., tempering temperature:
450.degree. C., and cooling after tempering: water cooling).
(Surface Roughness)
[0103] Next, in order to evaluate the surface properties of a
spring fabricated as stated above, surface roughness (Ry, maximum
roughness) was measured in accordance with JIS B0601 1994.
[0104] In the present invention, when surface roughness (Ry)
measured as stated above was 18.0 .mu.m or less, the spring was
judged as "excellent in surface properties
(acceptable:.largecircle.)."
[0105] Those results are shown in Tables 2 and 3. TABLE-US-00001
TABLE 1 Steel type C Si Mn Cr Cu Ni P S V Ti Nb Si/Cu A 0.35 1.71
0.14 0.06 0.05 0.16 0.018 0.019 -- -- -- 34.2 B 0.40 1.82 0.19 1.04
0.19 0.54 0.008 0.020 -- -- -- 9.6 C 0.41 1.80 0.18 1.09 0.21 0.51
0.010 0.012 0.160 0.070 -- 8.6 D 0.48 1.96 0.76 0.20 0.21 0.32
0.008 0.006 0.150 0.073 -- 9.3 E 0.48 2.10 0.18 1.20 0.50 0.70
0.003 0.006 -- 0.080 -- 4.2 F 0.46 1.95 0.16 1.05 0.39 0.60 0.005
0.009 0.107 0.065 -- 5.0 G 0.60 1.52 0.52 1.85 0.37 0.2 0.010 0.013
0.165 -- -- 4.1 H 0.46 2.42 0.18 1.06 0.58 0.71 0.013 0.006 0.325
-- 0.030 4.2 I 0.61 1.56 0.54 0.06 0.40 0.22 0.018 0.005 -- -- --
3.9 J 0.59 1.51 0.52 0.06 0.61 0.68 0.019 0.007 -- -- -- 2.5 K 0.64
2.54 0.88 0.65 0.64 0.26 0.019 0.007 -- 0.080 -- 4.0
[0106] TABLE-US-00002 TABLE 2 Cu concentration Ni concentration
Steel Surface layer In steel .DELTA.Cu Surface layer In steel
.DELTA.Ni Scale thickness Scale exfoliation Ry No. type Code (%)
(%) (%) (%) (%) (%) (.mu.m) ratio (%) Judgment (.mu.m) Judgment 1 A
A-1 0.41 0.05 0.36 0.17 0.16 0.01 31 100 .largecircle. 12.2
.largecircle. 2 A A-4 0.44 0.05 0.39 1.21 0.16 1.05 30 98 X 19.8 X
3 A A-3 0.58 0.05 0.53 0.18 0.16 0.02 32 97 X 2.17 X 4 B B-1 0.19
0.19 0.00 0.58 0.54 0.04 32 100 .largecircle. 13.8 .largecircle. 5
B B-4 0.21 0.19 0.02 1.71 0.54 1.17 18 93 X 25.4 X 6 B B-3 0.78
0.19 0.59 1.21 0.54 0.67 20 92 X 26.4 X 7 C C-1 0.25 0.21 0.04 0.65
0.51 0.14 19 100 .largecircle. 14.5 .largecircle. 8 C C-2 0.38 0.21
0.17 0.52 0.51 0.01 21 100 .largecircle. 15.8 .largecircle. 9 C C-3
0.45 0.21 0.24 1.59 0.51 1.08 36 93 X 19.7 X 10 C C-4 0.74 0.21
0.53 0.53 0.51 0.02 33 95 X 19.0 X 11 D D-1 0.21 0.21 0.00 0.35
0.32 0.03 33 100 .largecircle. 15.2 .largecircle. 12 D D-2 0.22
0.21 0.01 0.38 0.32 0.06 35 100 .largecircle. 14.8 .largecircle. 13
D D-3 0.28 0.21 0.07 1.39 0.32 1.07 36 95 X 22.5 X 14 D D-4 0.78
0.21 0.57 1.25 0.32 0.93 37 92 X 27.8 X
[0107] TABLE-US-00003 TABLE 3 Cu concentration Ni concentration
Scale Steel Surface layer In steel .DELTA.Cu Surface layer In steel
.DELTA.Ni Scale thickness exfoliation ratio Ry No. type Code (%)
(%) (%) (%) (%) (%) (.mu.m) (%) Judgment (.mu.m) Judgment 15 E E-1
0.25 0.19 0.06 1.55 0.70 0.85 23 100 .largecircle. 15.2
.largecircle. 16 E E-6 0.22 0.19 0.03 1.89 0.70 1.19 25 89 X 22.5 X
17 E E-3 0.72 0.19 0.53 0.71 0.70 0.01 21 91 X 19.8 X 18 E E-2 0.21
0.19 0.02 1.61 0.70 0.91 18 100 .largecircle. 14.3 .largecircle. 19
F F-1 0.40 0.39 0.01 0.85 0.60 0.25 33 100 .largecircle. 10.1
.largecircle. 20 F F-4 0.45 0.39 0.06 1.92 0.60 1.32 36 90 X 25.4 X
21 F F-3 0.95 0.39 0.56 0.91 0.60 0.31 38 92 X 26.8 X 22 F F-2 0.62
0.39 0.23 1.01 0.60 0.41 28 100 .largecircle. 12.1 .largecircle. 23
G G-1 0.39 0.37 0.02 0.52 0.20 0.32 38 100 .largecircle. 15.8
.largecircle. 24 G G-4 0.40 0.37 0.03 1.62 0.20 1.42 35 90 X 30.5 X
25 G G-3 0.99 0.37 0.62 0.39 0.20 0.19 36 88 X 34.8 X 26 H H-1 0.59
0.58 0.01 0.75 0.71 0.04 22 100 .largecircle. 14.9 .largecircle. 27
H H-5 0.65 0.58 0.07 1.99 0.71 1.28 28 92 X 28.2 X 28 H H-3 1.21
0.58 0.63 0.98 0.71 0.27 29 95 X 26.3 X 29 I I-1 1.91 0.40 1.51
0.55 0.22 0.33 33 85 X 39.8 X 30 J J-1 2.52 0.61 1.91 0.95 0.68
0.27 21 84 X 42.1 X 31 K K-1 0.66 0.64 0.02 0.52 0.26 0.26 15 41 X
38.5 X 32 K K-2 0.64 0.64 0.00 0.58 0.26 0.32 12 32 X 37.1 X
[0108] Among the steel types A to K shown in Table 1, the steel
types A to H are the examples which satisfy the steel compositions
stipulated in the present invention, the steel types I and J are
the comparative examples wherein the ratios (Si/Cu) are lower than
the range stipulated in the present invention, and the steel type K
is the comparative example wherein the Si content exceeds the range
stipulated in the present invention.
[0109] In Tables 2 and 3, for example, the code "A-1" represents
the case of using the steel type A shown in Table 1 and adopting
the wire rod forming process 1 as the fabrication method, and the
code "A-2" represents the case of using the steel type A shown in
Table 1 and adopting the wire rod forming process 2 as the
fabrication method. The same is applied to other codes.
[0110] The results shown in Tables 2 and 3 lead to the following
consideration.
[0111] Firstly, the cases of Nos. 1, 4, 7, 8, 11, 12, 15, 18, 19,
22, 23, and 26 are the invention examples wherein the values
.DELTA.Cu and .DELTA.Ni stipulated in the present invention satisfy
the ranges stipulated in the present invention and, in any of the
cases, the scale exfoliation ratio is 100% and the pickling
performance is outstandingly excellent. Further, as a result of
examining the compositions of the scale of the wire rods by the
X-ray diffraction method, it was confirmed that the composition was
controlled in the desirable range in any of the cases (not shown in
the tables). Furthermore, the surface properties of the
oil-tempered wires fabricated from the aforementioned wire rods
were good. In addition, the tensile strength of the oil-tempered
wires was measured in accordance with JIS Z2241 and resultantly it
was confirmed that any of the oil-tempered wires had a high
strength of about 1,900 to 2,100 MPa or more (not shown in the
tables).
[0112] In contrast, the cases of Nos. 2, 3, 5, 6, 9, 10, 13, 14,
16, 17, 20, 21, 24, 25, 27, and 28 are the comparative examples
wherein springs are fabricated by adopting any of the wire rod
forming processes 3 to 6 which deviate from the fabrication
conditions stipulated in the present invention, the value .DELTA.Cu
or .DELTA.Ni deviates from the range stipulated in the present
invention, and hence the scale exfoliation ratio is low and
intended pickling performance cannot be obtained.
[0113] Further, the surface properties of the oil-tempered wires
fabricated from such wire rods also deteriorate.
[0114] Further, the cases of Nos. 29 to 32 are the comparative
examples wherein the compositions of the steels deviate from the
requirement of the present invention, thus the value .DELTA.Cu or
.DELTA.Ni deviates from the range stipulated in the present
invention, and hence the scale exfoliation ratio is low and
intended pickling performance cannot be obtained. Further, the
surface properties of the oil-tempered wires fabricated from such
wire rods also deteriorate.
[0115] Here, in the present examples, the spring steel wire rods
were fabricated through the aforementioned spring forming process
(a). However, the spring forming process is not limited to the
spring forming process (a) and it has been experimentally confirmed
that a spring steel wire rod outstandingly excellent in surface
properties can be obtained even when, for example, the
aforementioned spring forming process (b) or (c) is adopted.
* * * * *