[Technical Field]
[0001] The disclosure relates to a wire rod and steel wire for ultrahigh strength springs,
and method for manufacturing the same. More particularly, the disclosure relates to
a wire rod and steel wire for ultrahigh strength springs, and method for manufacturing
the same, which may be applied to motorcycle suspension springs.
[Background Art]
[0002] Similar to the car material market, the motorbike market is also constantly undergoing
weight reduction or structural modification. Demand for high-strength spring steel
is growing these days as dual-type suspensions having been used for existing motorbikes
are being replaced with mono types.
[0003] The existing spring steel having been used for motorbike suspension is the hard steel
wire that lacks enough strength and fatigue resistance to be used for the mono-type
suspension. Hence, the steel having a tempered martensite (TM) structure for automobiles
has been considered to be used, but it is hardly applied to the motorbike suspension
spring because the automobile suspension spring has a strict management standard,
gives difficulty in manufacturing and is expensive.
[0004] To solve this problem, there is a way of reducing the alloy content and lowering
tempering temperature. With the reduced alloy content and the lowered tempering temperature,
however, it is difficult to secure ductility at high strength, thereby leading to
a problem of insufficient machinability. To solve this problem, segregation control
for securing sufficient ductility even at low tempering temperature is required.
[0005] Furthermore, with the recent development of induction heat treatment (IT heat treatment)
technology, sufficient hardenability may be secured even with the use of water cooling,
and desired strength may be obtained while the content of an alloy element included
in the steel is lowered. However, spring steel manufactured through IT heat treatment
has a very severe material deviation depending on the degree of segregation. Especially,
the material deviation due to the segregation tends to be more severe the higher the
strength of the steel. Hence, control of segregation is required for securing high
strength and stable reduction ratio of cross-section is required when the IT heat
treatment is applied.
[Disclosure]
[Technical Problem]
[0007] To solve the aforementioned problems, the disclosure provides a high-quality wire
rod and steel wire for ultrahigh strength spring, and method for manufacturing the
same, capable of securing high strength and high reduction ratio of cross-section
with small material deviation after induction heat treatment.
[Technical Solution]
[0008] According to an embodiment of the disclosure, a wire rod for ultrahigh strength spring
includes in percent by weight (wt%), 0.5 to 0.7% of C, 0.4 to 0.9% of Si, 0.3 to 0.8%
of Mn, 0.2 to 0.6% of Cr, 0.015% or less of P, 0.010% or less of S, 0.01% or less
of Al, 0.01% or less of N, 0.005% or less of O, and the remainder having Fe and unavoidable
impurities, wherein a ratio of an area satisfying one or more of C > 0.8%, Si > 0.9%,
Cr > 0.8% and Mn > 0.8%, in wt%, in an area of 1 mm
2 of a center of a cross-section perpendicular to a longitudinal direction is 5% or
less.
[0009] The wire rod for ultrahigh strength spring may include a surface ferrite decarburized
layer having a thickness of 1
µm or less.
[0010] According to an embodiment of the disclosure, a method of manufacturing a wire rod
for ultra-high strength spring includes preparing a bloom by continuous casting of
molten steel including, in wt%, 0.5 to 0.7% of C, 0.4 to 0.9% of Si, 0.3 to 0.8% of
Mn, 0.2 to 0.6% of Cr, 0.015% or less of P, 0.010% or less of S, 0.01% or less of
Al, 0.01% or less of N, 0.005% or less of O, and the remainder having Fe and unavoidable
impurities; and rolling the bloom into billets and then into wire rods, wherein the
continuous casting is performed at a casting speed of 0.48 to 0.54 m/min and comprises
soft reduction with a total amount of reduction of 15 to 35 mm during the continuous
casting.
[0011] The soft reduction may be performed so that each roll rolls to 4 mm or less and a
cumulative amount of reduction is 60% or more when the solidification fraction is
0.6 or more.
[0012] According to an embodiment of the disclosure, a steel wire for ultra-high strength
spring, the steel wire comprising: in percent by weight (wt%), 0.5 to 0.7% of C, 0.4
to 0.9% of Si, 0.3 to 0.8% of Mn, 0.2 to 0.6% of Cr, 0.015% or less of P, 0.010% or
less of S, 0.01% or less of Al, 0.01% or less of N, 0.005% or less of O, and the remainder
having Fe and unavoidable impurities, and for area fraction, 90% or more of tempered
martensite, wherein a ratio of an area satisfying one or more of C > 0.8%, Si > 0.9%,
Cr > 0.8% and Mn > 0.8%, in wt%, in an area of 1 mm
2 of a center of a cross-section perpendicular to a longitudinal direction is 5% or
less.
[0013] The steel wire for ultrahigh strength spring may have hardness variation of 25 Hv
or less in the cross-section perpendicular to the longitudinal direction except for
an area having a depth of 0.5 mm or less from the top surface.
[0014] The steel wire for ultrahigh strength spring may have a tensile strength of 1750
to 2200 MPa and a cross-section reduction rate of 40% or more.
[0015] According to an embodiment of the disclosure, a method of manufacturing a steel wire
for ultrahigh strength spring includes drawing the wire rod including, in wt%, 0.5
to 0.7% of C, 0.4 to 0.9% of Si, 0.3 to 0.8% of Mn, 0.2 to 0.6% of Cr, 0.015% or less
of P, 0.010% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.005% or less
of O, and the remainder being Fe and unavoidable impurities, and having a ratio of
an area satisfying one or more of C > 0.8%, Si > 0.9%, Cr > 0.8% and Mn > 0.8%, in
wt%, in an area of 1 mm
2 of a center of a cross-section perpendicular to a longitudinal direction being 5%
or less, into a wire; heating the wire at 900 to 1000°C within 10 seconds; water cooling
the wire at high pressure; heating and tempering the wire at 400 to 500°C within 10
seconds; and water cooling the wire.
[Advantageous Effects]
[0016] The disclosure may provide a wire rod and steel wire for ultrahigh strength spring,
and method for manufacturing the same, which may secure high strength even with a
lower alloy content than in the traditional spring steel wire and secure an excellent
cross-section reduction ratio with reduced center segregation, and may thus be applicable
to products requiring a low spring index as well.
[0017] The disclosure may provide a high-quality wire rod and steel wire for ultrahigh strength
spring, and method for manufacturing the same, capable of securing high strength and
high cross-section reduction ratio with small material deviation after induction (IT)
heat treatment.
[Best Mode]
[0018] According to an embodiment of the disclosure, a wire rod for ultra-high strength
spring includes in percent by weight (wt%), 0.5 to 0.7% of C, 0.4 to 0.9% of Si, 0.3
to 0.8% of Mn, 0.2 to 0.6% of Cr, 0.015% or less of P, 0.010% or less of S, 0.01%
or less of Al, 0.01% or less of N, 0.005% or less of O, and the remainder having Fe
and unavoidable impurities, wherein a ratio of an area satisfying one or more of C
> 0.8%, Si > 0.9%, Cr > 0.8% and Mn > 0.8%, in wt%, in an area of 1 mm
2 of a center of a cross-section perpendicular to a longitudinal direction is 5% or
less.
[Modes of the Invention]
[0019] Embodiments of the disclosure will now be described. The embodiments of the disclosure,
however, may be modified into many different forms and should not be construed as
being limited to the embodiments set forth herein. The embodiments of the disclosure
are provided to fully convey the idea provided in the disclosure to scope of the invention
to those of ordinary skill in the art.
[0020] Terms as herein used are just for illustration. For example, the singular expressions
include plural expressions unless the context clearly dictates otherwise. It will
be further understood that the terms "comprises" and/or "comprising," when used in
this specification, specify the presence of stated features, integers, steps, operations,
elements, and/or components, but do not preclude the presence or addition of one or
more other features, integers, steps, operations, elements, components, and/or groups
thereof.
[0021] Unless otherwise defined, all terms used herein have the same meaning as commonly
understood by those of ordinary skill in the art to which the disclosure belongs.
Furthermore, unless otherwise clearly defined, a specific term should not be construed
as having overly ideal or formal meaning. It is to be understood that the singular
expression include plural expressions unless the context clearly dictates otherwise.
[0022] Throughout the specification, the word 'about', 'substantially' or the like, is used
to indicate that a numerical value used with the word belongs to a range around the
numerical value, to prevent an unscrupulous pirate from unduly making an advantage
of a description in which the absolute numerical value is mentioned.
[0023] The disclosure provides a wire rod and steel wire for ultrahigh strength spring,
and method for manufacturing the same, which may secure high strength and excellent
cross-section reduction ratio with a small material deviation after induction (IT)
heat treatment through optimization of the content of an element that encourages segregation
and a high amount of reduction during continuous casting.
[0024] In an example of the disclosure, a wire rod for ultrahigh strength spring includes,
in percent by weight (wt%), 0.5 to 0.7% of C, 0.4 to 0.9% of Si, 0.3 to 0.8% of Mn,
0.2 to 0.6% of Cr, 0.015% or less of P, 0.010% or less of S, 0.01% or less of Al,
0.01% or less of N, 0.005% or less of O, and the remainder having Fe and unavoidable
impurities.
[0025] The reason for limiting the alloy composition of the wire rod for ultrahigh strength
spring will now be described in detail. The reason for limiting alloy composition
of a steel wire for ultrahigh strength spring is the same as that of the wire rod,
so the description thereof will be omitted for convenience.
The content of C is 0.5 to 0.7wt%.
[0026] C is an element added to secure strength of the product. When the content of C is
less than 0.5wt%, desired strength and Ceq may not be secured. In this case, it may
be difficult to secure strength because martensite structure is not completely formed
when the steel is cooled, and it may be difficult to secure desired strength even
when an intact martensite structure is formed. When the content of C exceeds 0.7wt%,
impact property may be degraded and quenching cracks may occur during water cooling.
The content of Si is 0.4 to 0.9wt%.
[0027] Si is an element used for deoxidation of steel, and is an advantageous element to
secure strength through solid solution strengthening. In the disclosure, silicon may
be added in an amount of 0.4wt% or more to ensure strength. However, when Si is overly
added, it may be segregated in the center, causing a difference in hardness between
the center and the surface, surface decarburization, and difficulty in processing
the material. Considering this, an upper limit of the content of SI may be limited
to 0.9 wt% in the disclosure.
The content of Mn is 0.3 to 0.8wt%.
[0028] Mn is a hardenability enhancing element and is one of the essential elements for
forming high-strength tempered martensite structure steel. In the disclosure, manganese
may be added in an amount of 0.3wt% or more to ensure the strength. However, when
Mn is overly added, it may be segregated in the center, causing a difference in hardness
between the center and the surface, and is likely to degrade toughness in tempered
martensitic steel. Considering this, an upper limit of the content of Mn may be limited
to 0.8wt% in the disclosure.
The content of Cr is 0.2 to 0.6wt%.
[0029] Cr, along with Mn, is effective in enhancement of hardenability and enhances strength
of tempered martensitic steel. For this, in the disclosure, Cr may be added in an
amount of 0.2wt% or more. However, like Si and Mn, Cr is a segregation promoting element,
which may bring a risk of causing hardness deviation due to center segregation when
overly added. Considering this, an upper limit of the content of Cr may be limited
to 0.6wt% in the disclosure.
The content of P is 0.015wt% or less.
[0030] As P is an element that is segregated at grain boundaries to reduce toughness and
resistance to hydrogen delayed cracking, it is desirable to exclude it from steel
as much as possible. The upper limit may be limited to 0.015wt% in the disclosure.
The content of S is 0.010wt% or less.
[0031] S, like P, is segregated at grain boundaries to reduce toughness, forms MnS to reduce
resistance to hydrogen delayed cracking, so it is desirable to exclude it from steel
as much as possible. The upper limit may be limited to 0.010wt% in the disclosure.
The content of Al is 0.01wt% or less.
[0032] Al is a strong deoxidizing element, which may increase cleanliness by removing oxygen
from steel. On the other hand, when Al is added, it forms Al
2O
3 inclusions and reduces fatigue resistance. Hence, the upper limit may be limited
to 0.01wt% in the disclosure.
The content of N is 0.01wt% or less.
[0033] N is combined with Al or V in steel, causing a problem of forming coarse AIN or VN
precipitates that are not dissolved during heat treatment. Hence, the upper limit
may be limited to 0.01wt% in the disclosure.
The content of O is 0.005wt% or less.
[0034] O may be combined with Al to form coarse inclusions. Hence, the upper limit may be
limited to 0.005wt% in the disclosure.
[0035] The remaining component is iron (Fe) in the disclosure. This may not be excluded
because unintended impurities may be inevitably mixed from raw materials or surroundings
in the normal manufacturing process. These impurities may be known to anyone skilled
in the ordinary manufacturing process, so not all of them are specifically mentioned
in this specification.
[0036] In the disclosure, segregation on the wire rod in the center is controlled to secure
excellent cross-section reduction ratio at high strength after IT heat treatment.
For example, a wire rod for ultrahigh strength spring may have 5% or less of a ratio
of an area satisfying one or more of C > 0.8%, Si > 0.9%, Cr > 0.8% and Mn > 0.8%,
in wt%, in an area of 1 mm
2 of the center of a cross-section perpendicular to the longitudinal direction.
[0037] When the area ratio exceeds 5%, hardness deviation may occur in the cross-section
perpendicular to the longitudinal direction after IT heat treatment, which may lead
to material deviation, thereby failing to secure a sufficient cross-section reduction
ratio at the target strength.
[0038] Furthermore, according to the disclosure, the surface decarburization may be suppressed
by low Si alloy composition. For example, a surface ferrite decarburized layer of
the wire rod may have a thickness of 1
µm or less. When the thickness of the surface ferrite decarburized layer exceeds 1
µm, an additional process such as carburizing treatment may need to be performed to
secure high strength.
[0039] A method of manufacturing a wire rod for ultrahigh strength spring according to the
disclosure will now be described in detail. In an example of the disclosure, the wire
rod for ultrahigh strength spring is manufactured by preparing a bloom by continuous
casting of molten steel having the aforementioned alloy composition, and rolling the
bloom into billets and then into wire rods. The respective manufacturing steps will
now be described.
[0040] In the disclosure, controlled is the aforementioned ally composition as well as the
continuous casting step to minimize center segregation on the wire rod. According
to an example, the continuous casting may be performed at a casting speed of 0.48
to 0.54 m/min and may undergo soft reduction with a total amount of reduction of 15
to 35 mm during the continuous casting. The amount of cooling water is appropriately
adjusted so that solidification may be completed to a point where soft reduction is
completed.
[0041] When the casting speed is too slow, solidification is completed before soft reduction
and the ratio of the liquid phase to the solid phase is too low, so it is difficult
to secure the effect of removing segregation by soft reduction. On the other hand,
when the casting speed is too fast, the ratio of the liquid phase to the solid phase
is too high, leading to undesirable formation of segregation due to solidification
contraction. Considering this, the casting speed is controlled to be 0.48 to 0.54
m/min in an example of the disclosure.
[0042] When the total amount of reduction is too small during the soft reduction, it is
difficult to secure the segregation removal effect by soft reduction. On the other
hand, when it is too much, the effect is saturated, and casting equipment is burdened.
Considering this, the total amount of reduction of soft reduction is controlled to
be 15 to 35 mm/min in an example of the disclosure.
[0043] In an example of the disclosure, soft reduction may be performed so that each role
may roll to 4 mm or less and a cumulative amount of reduction is 60% or more when
the solidification fraction is 0.6 or more. The solidification fraction refers to
a ratio of weight of the molten steel that has become a solid phase to the weight
of the whole molten steel.
[0044] The bloom prepared in the aforementioned procedure may be rolled into billets and
then into wire rods.
[0045] A method of manufacturing a steel wire for ultrahigh strength spring according to
the disclosure will now be described in detail. According to the disclosure, the steel
wire for ultrahigh strength spring is manufactured by wire drawing, heating, water
cooling at high pressure, tempering, and water cooling a wire rod satisfying the aforementioned
alloy composition, having 5% or less of a ratio of an area satisfying one or more
of C > 0.8%, Si > 0.9%, Cr > 0.8% and Mn > 0.8%, in wt%, in an area of 1 mm
2 of the center of a cross-section perpendicular to the longitudinal direction. The
respective manufacturing steps will now be described.
[0046] In the wire drawing step of the disclosure, the wire rod may be drawn to a wire having
diameter of 16 mm or less and manufactured into steel wires to be applied to motorbike
suspension springs.
[0047] Subsequently, in the heating step of the disclosure for QT heat treatment of the
drawn steel wire, the drawn steel wire may be heated to a quenching temperature of
900 to 1000°C within 10 seconds and then maintained for 5 to 60 seconds to heat-treat
the structure of the steel wire to austenite. When the heating time to a target temperature
of 900 to 1000°C exceeds 10 seconds, crystal grains grow and makes it difficult to
secure desired physical properties. When the holding time is less than 5 seconds,
the pearlite structure may not be transformed to the austenite, and when the holding
time exceeds 60 seconds, the crystal grains may be coarsened.
[0048] In the disclosure, the step of water cooling at high pressure is a step of transforming
the main structure of the steel wire from austenite to martensite, and the water cooling
is performed at a pressure as high as to remove the boiling film of the austenitized
steel wire in the previous step. In this case, when cooling is performed as oil cooling
rather than water cooling, the desired strength may not be secured due to low Ceq.
In addition, when the pressure is not as high as to remove the boiling film in the
water cooling, a chance of quenching crack increases during quenching, so water cooling
is preferably performed as highpressure water cooling by spraying water from all directions
at a pressure as high as possible.
[0049] In the disclosure, the tempering step is a step of heating and tempering martensite,
which is the main structure of the water-cooled steel wire, into tempered martensite.
The tempering step may be heated to 400 to 500°C within 10 seconds and then maintained
within 30 seconds. When the tempering temperature is less than 400°C, toughness is
not secured, leading to difficulty in processing and increasing the risk of the product
breaking, and when the temperature exceeds 500°C, strength is reduced, so the tempering
temperature is limited to the above temperature range. In addition, when it is not
heated to the above temperature range within 10 seconds for tempering, coarse carbides
are formed and toughness is likely to deteriorate, so it is desirable to rapidly heat
within 10 seconds.
[0050] In the disclosure, for heating, a means for heating to the quenching temperature
and a means for tempering use IT heat treatment to sufficiently harden the surface
during subsequent water cooling by rapid heating.
[0051] Thereafter, the tempered steel wire is water-cooled to room temperature to manufacture
a steel wire for an ultrahigh strength spring.
[0052] In an example of the disclosure, a steel wire for ultrahigh strength spring includes,
in wt%, 0.5 to 0.7% of C, 0.4 to 0.9% of Si, 0.3 to 0.8% of Mn, 0.2 to 0.6% of Cr,
0.015% or less of P, 0.010% or less of S, 0.01% or less of Al, 0.01% or less of N,
0.005% or less of O, and the remainder including Fe and unavoidable impurities, and
for area fraction, 90% or more of tempered martensite, wherein a ratio of an area
satisfying one or more of C > 0.8%, Si > 0.9%, Cr > 0.8% and Mn > 0.8%, in wt%, in
an area of 1 mm
2 of a center of a cross-section perpendicular to the longitudinal direction may be
5% or less.
[0053] In addition, the steel wire for an ultrahigh strength spring in an example of the
disclosure may have a hardness deviation of 25 Hv or less in other area than an area
having a depth of 0.5 mm or less from the outermost surface in a cross-section perpendicular
to the longitudinal direction. When the hardness deviation exceeds 25 Hv, a sufficient
cross-section reduction ratio may not be secured.
[0054] In addition, the steel wire for an ultrahigh strength spring in an example of the
disclosure may have a tensile strength of 1750 to 2200 MPa and a cross-section reduction
ratio of 40% or more.
[0055] The disclosure will now be described in more detail in the following embodiment of
the disclosure. The following embodiment, however, is an illustrative example to describe
the disclosure in more detail, and should not be construed as limiting the scope of
the disclosure. The scope of the disclosure is defined by the claims and their equivalents.
{Embodiment}
[0056] The molten steel having the alloy composition of Table 1 below was cast into a bloom
at the casting speed and total amount of soft reduction of Table 2, and then manufactured
into a wire rod having a diameter of 9 mm through billet rolling and wire rod rolling.
[0057] Segregation areas in Table 2 were derived by analyzing with an electron probe micro
analyzer (EPMA) in a center area of 1 mm
2 of the cross-section perpendicular to the longitudinal direction of the manufactured
wire rod. 'C segregation area' in Table 2 refers to a ratio of an area satisfying
C > 0.8wt% in the center area of 1 mm
2 of the cross-section perpendicular to the longitudinal direction. 'Si segregation
area' refers to a ratio of an area satisfying Si > 0.9wt% in the center area of 1
mm
2 of the cross-section perpendicular to the longitudinal direction. 'Cr segregation
area' refers to a ratio of an area satisfying Cr > 0.8wt% in the center area of 1
mm
2 of the cross-section perpendicular to the longitudinal direction. 'Mn segregation
area' refers to a ratio of an area satisfying Mn > 0.8wt% in the center area of 1
mm
2 of the cross-section perpendicular to the longitudinal direction.
[Table 1]
|
alloy composition (wt%) |
C |
Si |
Mn |
Cr |
P |
S |
Al |
O |
N |
Comparati ve example 1 |
0.55 |
1.41 |
1.01 |
0.65 |
0.011 |
0.004 |
<0.003 |
<0.005 |
<0.01 |
Comparati ve example 2 |
0.56 |
0.25 |
0.75 |
0.75 |
0.01 |
0.005 |
<0.003 |
<0.005 |
<0.01 |
Comparati ve example 3 |
0.61 |
0.79 |
0.63 |
0.55 |
0.01 |
0.004 |
<0.003 |
<0.005 |
<0.01 |
Inventive example 1 |
0.58 |
0.78 |
0.62 |
0.43 |
0.009 |
0.005 |
<0.003 |
<0.005 |
<0.01 |
Inventive example 2 |
0.62 |
0.61 |
0.41 |
0.52 |
0.011 |
0.005 |
<0.003 |
<0.005 |
<0.01 |
[Table 2]
|
Casting speed (m/min) |
total amount of soft reduction (mm) |
C segregation area (%) |
Si segregation area (%) |
Mn segregation area (%) |
Cr segregation area (%) |
Comparativ e example 1 |
0.50 |
25 |
< 1 |
5.4 |
6.2 |
< 1 |
Comparativ e example 2 |
0.52 |
25 |
< 1 |
1.2 |
< 1 |
< 1 |
Comparativ e example 3 |
0.56 |
10 |
12 |
13 |
11 |
12 |
Inventive example 1 |
0.52 |
25 |
< 1 |
< 1 |
< 1 |
< 1 |
Inventive example 2 |
0.52 |
25 |
< 1 |
< 1 |
< 1 |
< 1 |
[0058] Thereafter, the wire rods in Table 1 were manufactured into steel wires having a
diameter of 8 mm, and heat treatment was performed under the conditions shown in Table
3 below. The austenitizing heat treatment-high pressure water cooling-tempering-water
cooling were performed in sequence. The hardness deviation refers to one when the
hardness was measured at 10 points or more in other area than the area having a depth
of 0.5 mm or less from the outermost surface in the cross-section perpendicular to
the longitudinal direction.
[Table 3]
|
austenitizing temperature (°C) |
tempering temperature (°C) |
hardness deviation (Hv) |
cross-section reduction ratio (%) |
tensile strength (MPa) |
Comparativ e example 1 |
950 |
430 |
35.2 |
38 |
2,020 |
Comparativ e example 2 |
950 |
430 |
12.1 |
48 |
1,710 |
Comparativ e example 3 |
950 |
430 |
45.1 |
25 |
1,801 |
Inventive example 1 |
950 |
430 |
18.1 |
48 |
1,820 |
Inventive example 2 |
950 |
430 |
10.1 |
47 |
1,790 |
[0059] Referring to Tables 1, 2, and 3, the inventive examples 1 and 2 satisfying the alloy
composition and manufacturing conditions of the disclosure satisfied hardness deviation
of 25 Hv or less, tensile strength of 1750 to 2200 MPa, and a cross-section reduction
ratio of 40% or more in the other area than the area having a depth of 0.5 mm or less
from the outermost surface in a cross-section perpendicular to the longitudinal direction
after IT heat treatment. On the other hand, in the comparative example 1, Si and Mn
contents were high, causing hardness deviation due to Si and Mn segregation in the
center, so the cross-section reduction ratio was 40% or less.
[0060] The comparative example 2 did not secure the target tensile strength due to the low
Si content.
[0061] In the comparative example 3, center segregation occurred due to too fast a casting
speed and insufficient amount of soft reduction, and as a result, the hardness deviation
increased and the cross-section reduction ratio was 40% or less.
[0062] Embodiments of the disclosure have thus far been described, but the disclosure is
not limited thereto, and it will be obvious to those of ordinary skill in the art
that various modifications and alterations can be made without deviating from the
scope of the appended claims.
[Industrial Applicability]
[0063] According to an example of the disclosure, a high-quality wire rod and steel wire
for ultrahigh strength spring, and method for manufacturing the same, capable of securing
high strength and high cross-section reduction ratio with small material deviation
after induction heat treatment.
1. A wire rod for ultrahigh strength spring, the wire rod comprising:
in percent by weight (wt%), 0.5 to 0.7% of C, 0.4 to 0.9% of Si, 0.3 to 0.8% of Mn,
0.2 to 0.6% of Cr, 0.015% or less of P, 0.010% or less of S, 0.01% or less of Al,
0.01% or less of N, 0.005% or less of O, and the remainder having Fe and unavoidable
impurities,
wherein a ratio of an area satisfying one or more of C > 0.8%, Si > 0.9%, Cr > 0.8%
and Mn > 0.8%, in wt%, in an area of 1 mm2 of a center of a cross-section perpendicular to a longitudinal direction is 5% or
less.
2. The wire rod for ultrahigh strength spring of claim 1, further comprising: a surface
ferrite decarburized layer having a thickness of 1 µm or less.
3. A method of manufacturing a wire rod for ultrahigh strength spring, the method comprising:
preparing a bloom by continuous casting of molten steel including, in wt%, 0.5 to
0.7% of C, 0.4 to 0.9% of Si, 0.3 to 0.8% of Mn, 0.2 to 0.6% of Cr, 0.015% or less
of P, 0.010% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.005% or less
of O, and the remainder having Fe and unavoidable impurities; and
rolling the bloom into billets and then into wire rods,
wherein the continuous casting is performed at a casting speed of 0.48 to 0.54 m/min
and comprises soft reduction with a total amount of reduction of 15 to 35 mm during
the continuous casting.
4. The method of claim 3, wherein the soft reduction is performed so that each roll
rolls to 4 mm or less and a cumulative amount of reduction is 60% or more when the
solidification fraction is 0.6 or more.
5. A steel wire for ultrahigh strength spring, the steel wire comprising:
in percent by weight (wt%), 0.5 to 0.7% of C, 0.4 to 0.9% of Si, 0.3 to 0.8% of Mn,
0.2 to 0.6% of Cr, 0.015% or less of P, 0.010% or less of S, 0.01% or less of Al,
0.01% or less of N, 0.005% or less of O, and the remainder having Fe and unavoidable
impurities, and
for area fraction, 90% or more of tempered martensite,
wherein a ratio of an area satisfying one or more of C > 0.8%, Si > 0.9%, Cr > 0.8%
and Mn > 0.8%, in wt%, in an area of 1 mm2 of a center of a cross-section perpendicular to a longitudinal direction is 5% or
less.
6. The steel wire for ultrahigh strength spring of claim 5, wherein the steel wire has
a hardness deviation of 25 Hv or less in the cross-section perpendicular to the longitudinal
direction except for an area having a depth of 0.5 mm or less from the outermost surface.
7. The steel wire for ultrahigh strength spring of claim 5, wherein the steel wire has
a tensile strength of 1750 to 2200 MPa and a cross-section reduction ratio of 40%
or more.
8. A method of manufacturing a steel wire for ultrahigh strength spring, the method comprising:
drawing the wire rod of claim 1;
heating at 900 to 1000°C within 10 seconds;
water cooling at high pressure;
heating and tempering at 400 to 500°C within 10 seconds; and
water cooling.