KR20220087845A - Steel wire rod, steel wire, and manufacturing method thereof for ultra-high strength springs - Google Patents

Abstract

Disclosed herein are a wire rod for an ultra-high strength spring that can be applied as a motorcycle suspension spring, a steel wire, and a method for manufacturing the same.
According to an embodiment of the disclosed ultra-high strength spring wire rod by weight, C: 0.5 to 0.7%, Si: 0.4 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.2 to 0.6%, P: 0.015% or less , S: 0.010% or less, Al: 0.01% or less, N: 0.01% or less, O: 0.005% or less, the remainder contains Fe and unavoidable impurities, in an area of 1 mm ² of the center of the cross-section perpendicular to the longitudinal direction, wt% Therefore, the ratio of the area satisfying at least one of C>0.8%, Si>0.9%, Cr>0.8%, and Mn>0.8% may be 5% or less.

Description

Wire rod for ultra-high strength spring, steel wire and its manufacturing method {STEEL WIRE ROD, STEEL WIRE, AND MANUFACTURING METHOD THEREOF FOR ULTRA-HIGH STRENGTH SPRINGS}

The present invention relates to a wire rod for an ultra-high strength spring, a steel wire, and a method for manufacturing the same. More particularly, the present invention relates to a wire rod for an ultra-high strength spring that can be applied as a motorcycle suspension spring, a steel wire, and a method for manufacturing the same.

Similar to the automobile material market, the motorcycle market is continuously reducing weight or restructuring. Recently, the demand for high-strength spring steel is increasing by replacing the dual-type suspension used in existing motorcycles with the mono-type.

The existing spring steel used in motorcycle suspension is a hard steel wire, and it lacks strength and fatigue resistance to be used in monotype suspension. Accordingly, the use of tempered martensite (TM) tissue steel for automobiles has been reviewed, but automobile suspension springs have difficult management standards, are difficult to manufacture, and are expensive, so it is difficult to apply them to motorcycle suspension springs. .

In order to solve this problem, there is a method of reducing the alloy content and lowering the tempering temperature. However, when the alloy content is reduced and the tempering temperature is lowered, it is difficult to secure ductility at high strength, and as a result, the workability is insufficient. In order to solve this problem, segregation control is required to ensure sufficient ductility even at a low tempering temperature.

In addition, recently, with the development of IT heat treatment (Induction heat treatment) technology, water cooling It was possible to secure sufficient hardenability even by using it, and it was possible to achieve the desired strength while lowering the content of alloying elements contained in the steel. However, the material deviation of spring steel manufactured through IT heat treatment is very severe depending on the degree of segregation. In particular, high-strength steel tends to have more severe material deviation due to segregation. Therefore, it is necessary to control segregation in order to secure high strength and a stable reduction in area when IT heat treatment is applied.

Korean Patent Publication No. 1995-0018545 (published date: July 22, 1995)

In order to solve the above problems, the present invention is to provide a high-quality ultra-high-strength spring wire, a steel wire, and a method for manufacturing the same that can secure high strength and high cross-sectional reduction ratio due to less material deviation after IT heat treatment.

As a means for achieving the above object, the wire rod for ultra-high strength spring according to an embodiment of the present invention is, by weight, C: 0.5 to 0.7%, Si: 0.4 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.2 to 0.6%, P: 0.015% or less, S: 0.010% or less, Al: 0.01% or less, N: 0.01% or less, O: 0.005% or less, the remainder contains Fe and unavoidable impurities, and a cross section perpendicular to the longitudinal direction In an area of 1 mm ² at the center of the , the proportion of an area satisfying at least one of C > 0.8%, Si > 0.9%, Cr > 0.8%, and Mn > 0.8% in weight % may be 5% or less.

In the wire rod for ultra-high strength spring according to an embodiment of the present invention, the thickness of the surface ferrite decarburized layer may be 1 μm or less.

In addition, as another means for achieving the above object, the method for manufacturing a wire rod for ultra-high strength spring according to an example of the present invention is, by weight, C: 0.5 to 0.7%, Si: 0.4 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.2 to 0.6%, P: 0.015% or less, S: 0.010% or less, Al: 0.01% or less, N: 0.01% or less, O: 0.005% or less, remainder molten steel containing Fe and unavoidable impurities continuous casting to provide a bloom, and rolling the bloom into a steel piece, and then rolling a wire rod, wherein in the continuous casting step, the casting speed is 0.48 to 0.54 m/min, and the total rolling reduction during continuous casting It may include light pressure reduction in an amount of 15 to 35 mm.

The method for manufacturing a wire rod for ultra-high strength spring according to an example of the present invention is characterized in that it is rolled to 4 mm or less for each of the light-pressured rolling rolls, and the cumulative reduction amount is 60% or more when the solidification fraction is 0.6 or more. have.

In addition, as another means for achieving the above object, the steel wire for an ultra-high strength spring according to an example of the present invention is, by weight, C: 0.5 to 0.7%, Si: 0.4 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.2 to 0.6%, P: 0.015% or less, S: 0.010% or less, Al: 0.01% or less, N: 0.01% or less, O: 0.005% or less, the remainder contains Fe and unavoidable impurities, as an area fraction , containing at least 90% tempered martensite, in an area of 1 mm ² at the center of the cross-section perpendicular to the longitudinal direction, in wt%, one of C > 0.8%, Si > 0.9%, Cr > 0.8%, and Mn > 0.8% The ratio of the area satisfying the above may be 5% or less.

In the steel wire for ultra-high strength spring according to an embodiment of the present invention, in a cross section perpendicular to the longitudinal direction, the hardness deviation in the remaining area except for the area having a depth of 0.5 mm or less from the outermost surface may be 25 Hv or less.

The steel wire for ultra-high strength spring according to an embodiment of the present invention may have a tensile strength of 1750 to 2200 MPa, and a reduction in area of 40% or more.

In addition, as another means for achieving the above object, the method for manufacturing an ultra-high strength spring steel wire according to an example of the present invention is, by weight, C: 0.5 to 0.7%, Si: 0.4 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.2 to 0.6%, P: 0.015% or less, S: 0.010% or less, Al: 0.01% or less, N: 0.01% or less, O: 0.005% or less, the remainder including Fe and unavoidable impurities, Wire rod having a ratio of 5% or less of an area satisfying at least one of C > 0.8%, Si > 0.9%, Cr > 0.8%, and Mn > 0.8%, in weight %, in an area of 1 mm ² at the center of the cross-section perpendicular to the longitudinal direction It may include a step of freshening, heating to 900 to 1000° C. within 10 seconds, water cooling at high pressure, tempering by heating to 400 to 500° C. within 10 seconds, and water cooling.

The present invention can secure high strength even at a low alloy content compared to conventional spring steel wire, and can secure an excellent cross-sectional reduction rate by reducing central segregation, so that it can be applied to products requiring a low spring index. , a steel wire and a manufacturing method thereof can be provided.

The present invention can provide a high-quality, ultra-high-strength spring wire, a steel wire, and a method for manufacturing the same that can ensure high strength and high cross-sectional reduction due to less material variation after IT heat treatment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following describes preferred embodiments of the present invention. However, the embodiment of the present invention may be modified in various other forms, and the technical idea of the present invention is not limited to the embodiment described below. In addition, the embodiments of the present invention are provided in order to more completely explain the present invention to those of ordinary skill in the art.

The terms used in this application are only used to describe specific examples. Therefore, for example, a singular expression includes a plural expression unless the context clearly requires it to be singular. In addition, terms such as "comprises" or "including" as used in the present application are used to clearly indicate that the features, steps, functions, components, or combinations thereof described in the specification exist, and other features It should be noted that the use is not intended to preliminarily exclude the existence of elements, steps, functions, components, or combinations thereof.

On the other hand, unless otherwise defined, all terms used herein should be regarded as having the same meaning as commonly understood by those of ordinary skill in the art to which the present invention belongs. Accordingly, unless explicitly defined herein, specific terms should not be construed in an unduly idealistic or formal sense. For example, a singular expression herein includes a plural expression unless the context clearly dictates otherwise.

In addition, in this specification, "about", "substantially", etc. are used in or close to the numerical value when manufacturing and material tolerances inherent in the stated meaning are presented, and are used in a precise sense to help the understanding of the present invention. or absolute figures are used to prevent unreasonable use by unconscionable infringers of the mentioned disclosure.

The present invention is to provide a wire rod, a steel wire, and a method for manufacturing the same for an ultra-high strength spring that can ensure high strength and excellent cross-sectional reduction ratio due to small material deviation after IT heat treatment through optimization of element content that promotes segregation and high rolling reduction during playing.

The wire rod for ultra-high strength spring according to an embodiment of the present invention is, by weight, C: 0.5 to 0.7%, Si: 0.4 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.2 to 0.6%, P: 0.015% or less , S: 0.010% or less, Al: 0.01% or less, N: 0.01% or less, O: 0.005% or less, the remainder contains Fe and unavoidable impurities.

Hereinafter, the reason for limiting the alloy composition of the ultra-high strength spring wire rod will be described in detail. The reason for limiting the alloy composition of the ultra-high strength spring steel wire is the same as that of the wire rod, so it is omitted for convenience.

The content of C is 0.5 to 0.7% by weight.

C is an element added to secure product strength. When the C content is less than 0.5% by weight, the desired strength and Ceq cannot be secured. Accordingly, when the steel material is cooled, the martensitic structure is not completely formed, so it may be difficult to secure strength, and even if an intact martensitic structure is formed, it may be difficult to secure the desired strength. If the C content exceeds 0.7% by weight, impact properties may be deteriorated and quenching cracks may occur during water cooling.

The content of Si is 0.4 to 0.9% by weight.

Si is used for deoxidation of steel and is an element advantageous for securing strength through solid solution strengthening. In the present invention for securing strength, silicon may be added in an amount of 0.4 wt% or more. However, when Si is excessively added, it may segregate in the center, causing a difference in hardness between the center and the surface layer, may cause surface decarburization, and may have difficulty in processing the material. In consideration of this, the upper limit of the Si content in the present invention may be limited to 0.9 wt%.

The content of Mn is 0.3 to 0.8% by weight.

Mn is a hardenability improving element, and is one of the essential elements for forming a high-strength tempered martensite steel. In order to secure strength, in the present invention, manganese may be added in an amount of 0.3% by weight or more. However, when Mn is added excessively, it is segregated in the center, causing a difference in hardness between the center and the surface layer, and there is a fear that toughness may be lowered in tempered martensitic steel. In consideration of this, the upper limit of the Mn content in the present invention may be limited to 0.8% by weight.

The content of Cr is 0.2 to 0.6% by weight.

Cr, together with Mn, is effective in improving hardenability and improves strength in tempered martensitic steel. To this end, in the present invention, Cr may be added in an amount of 0.2 wt% or more. However, since Cr is a segregation promoting element like Si and Mn, there is a risk of hardness deviation due to central segregation when excessively added. In consideration of this, the upper limit of the Cr content in the present invention may be limited to 0.6% by weight.

The content of P is 0.015% by weight or less.

Since P is an element that segregates at grain boundaries to reduce toughness and reduces resistance to delayed hydrogen fracture, it is preferable to exclude it as much as possible from steel. In the present invention, the upper limit may be limited to 0.015% by weight.

The content of S is 0.010% by weight or less.

Like P, S is segregated at grain boundaries to reduce toughness and to form MnS to lower the hydrogen delayed fracture resistance, so it is preferable to be excluded as much as possible from steel. In the present invention, the upper limit may be limited to 0.010% by weight.

The content of Al is 0.01% by weight or less.

Al is a powerful deoxidizing element that can remove oxygen in steel and improve cleanliness. However, when Al is added, there is a problem of reducing fatigue resistance by forming Al ₂ O ₃ inclusions. Accordingly, in the present invention, the upper limit may be limited to 0.01% by weight.

The content of N is 0.01% by weight or less.

N is combined with Al or V in steel to form coarse AlN or VN precipitates that are not dissolved during heat treatment. Accordingly, in the present invention, the upper limit may be limited to 0.01% by weight.

The content of O is 0.005% by weight or less.

O may combine with Al to form coarse inclusions. Accordingly, in the present invention, the upper limit may be limited to 0.005% by weight.

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. Since these impurities are known to any person skilled in the art of a conventional manufacturing process, all details thereof are not specifically mentioned in the present specification.

The present invention controls the central segregation of the wire rod in order to secure an excellent cross-sectional reduction ratio at high strength after IT heat treatment. The wire rod for ultra-high strength spring according to an example contains at least one of C > 0.8%, Si > 0.9%, Cr > 0.8%, and Mn > 0.8%, in weight%, in an area of 1 mm ² of the center of the cross-section perpendicular to the longitudinal direction. A ratio of a satisfactory area may be 5% or less.

When the above-mentioned area ratio exceeds 5%, hardness deviation occurs in the cross section perpendicular to the longitudinal direction after IT heat treatment, which also causes material deviation, so that it is not possible to secure a sufficient reduction in area at the target strength.

In addition, according to the present invention, it is possible to suppress the surface decarburization phenomenon through the low Si alloy composition. According to an example, the thickness of the surface ferrite decarburized layer of the wire rod may be 1 μm or less. When the thickness of the surface ferrite decarburized layer exceeds 1 μm, there is a fear that additional processes such as carburizing treatment for securing high strength must be performed.

Hereinafter, a method for manufacturing a wire rod for an ultra-high strength spring according to the present invention will be described in detail. The wire rod for ultra-high strength spring according to an embodiment of the present invention is manufactured by continuously casting molten steel having the above-described alloy composition to prepare a bloom, rolling the bloom into a steel piece, and then rolling the wire rod. Hereinafter, each manufacturing step will be described.

The present invention controls the continuous casting step in addition to the control of the alloy composition described above in order to minimize the central segregation of the wire rod. According to an example, during continuous casting, the casting speed may be 0.48 to 0.54 m/min, and the total rolling reduction may be reduced to 15 to 35 mm during continuous casting. The amount of cooling water is appropriately adjusted so that solidification can be completed to the point where light pressure reduction is completed.

If the casting speed is too slow, solidification is completed before under light pressure, and the ratio of liquid phase compared to solid phase is too low, so it is difficult to secure the segregation removal effect under light pressure. On the other hand, if the casting rate is too fast, the ratio of the liquid phase to the solid phase is too high, which is not preferable because segregation is formed according to the solidification shrinkage. In consideration of this, according to an example of the present invention, the casting speed is controlled to 0.48 to 0.54 m/min.

If the total reduction is too small during light pressure, it is difficult to secure the segregation removal effect under light pressure. On the other hand, if it is too excessive, the above-described effect is saturated, and the performance equipment is burdened. In consideration of this, according to an example of the present invention, the total amount of reduction under light pressure is controlled to be 15 to 35 mm.

According to an example of the present invention, each of the rolling rolls to be lightly pressed is rolled to 4 mm or less, and when the solidification fraction is 0.6 or more, the cumulative reduction amount may be 60% or more. The solidification fraction means a ratio of the weight of the molten steel that has become a solid phase to the total weight of the molten steel.

The bloom prepared by the above-described process may be manufactured into a wire rod by rolling a steel piece and then rolling a wire rod.

Hereinafter, a method for manufacturing an ultra-high strength spring steel wire according to the present invention will be described in detail. The steel wire for ultra-high strength spring according to the present invention satisfies the alloy composition described above, and in an area of 1 mm ² at the center of the cross-section perpendicular to the longitudinal direction, in weight%, C > 0.8%, Si > 0.9%, Cr > 0.8%, and Mn It is manufactured by drawing a wire rod having an area ratio of 5% or less satisfying at least one of > 0.8%, heating it, cooling it with water under high pressure, then tempering it, and then cooling it with water. Hereinafter, each manufacturing step will be described.

In the drawing step of the present invention, the above-described wire rod may be drawn up to a wire diameter of 16 mm or less so as to be applied to a motorcycle suspension spring and manufactured as a steel wire.

Subsequently, in the heating step of the present invention for QT heat treatment of the drawn steel wire, the fresh steel wire is heated to the 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 for austenitization. can When the heating time to the target temperature of 900 to 1000°C exceeds 10 seconds, crystal grains grow and it is difficult to secure desired physical properties. If the holding time is less than 5 seconds, the pearlite structure may not be transformed into austenite, and if it exceeds 60 seconds, the grains may be coarsened.

In the present invention, 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 water cooling can be performed at a high pressure enough to remove the boiling film of the austenitized steel wire in the previous step. At this time, when cooling is performed by oil cooling instead of water cooling, the desired strength cannot be secured due to the low Ceq. In addition, if the pressure is not high enough to remove the boiling film during water cooling, the possibility of quenching cracks increases.

The step of tempering in the present invention is a step of heating martensite, which is the main structure of the water-cooled steel wire, and tempering it with tempered martensite. The step of tempering may be heated to 400 to 500° C. within 10 seconds and then maintained within 30 seconds. If the tempering temperature is less than 400 ℃, toughness is not secured, so processing is difficult and the risk of product damage increases, and if it exceeds 500 ℃, the strength is lowered. In addition, if the temperature is not heated within 10 seconds to the above-described temperature range during tempering, coarse carbides are formed and there is a risk of deterioration of toughness, so it is preferable to rapidly heat within 10 seconds.

In the present invention, the means for heating to the quenching temperature and the means for tempering upon heating utilize IT heat treatment to sufficiently harden the surface during subsequent water cooling by rapid heating.

Thereafter, the tempered steel wire is cooled with water to room temperature to manufacture an ultra-high strength spring steel wire.

The steel wire for ultra-high strength spring according to an embodiment of the present invention is, by weight, C: 0.5 to 0.7%, Si: 0.4 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.2 to 0.6%, P: 0.015% or less , S: 0.010% or less, Al: 0.01% or less, N: 0.01% or less, O: 0.005% or less, the remainder contains Fe and unavoidable impurities, as an area fraction, tempered martensite contains 90% or more, In an area of 1 mm ² of the center of the cross-section perpendicular to the longitudinal direction, the proportion of the area satisfying at least one of C > 0.8%, Si > 0.9%, Cr > 0.8%, and Mn > 0.8%, in weight %, may be 5% or less have.

In addition, in the steel wire for ultra-high strength spring according to an example of the present invention, in a cross section perpendicular to the longitudinal direction, the hardness deviation in the remaining area except for the area having a depth of 0.5 mm or less from the outermost surface may be 25 Hv or less. If the hardness deviation exceeds 25Hv, it is not possible to secure a sufficient reduction in section.

In addition, the steel wire for ultra-high strength spring according to an embodiment of the present invention may have a tensile strength of 1750 to 2200 MPa, and a reduction in area of 40% or more.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

{Example}

Molten steel having the alloy composition shown in Table 1 was cast with bloom at the casting speed and total light reduction in Table 2, and then a wire rod having a diameter of 9 mm was manufactured by rolling the steel slab and the wire rod.

The segregation area in Table 2 was derived by EPMA analysis at the center 1mm ² of the cross-section perpendicular to the longitudinal direction of the manufactured wire rod. 'C segregation area' in Table 2 means a ratio of an area satisfying C > 0.8 wt% in an area of 1 mm ² at the center of a cross-section perpendicular to the longitudinal direction. The 'Si segregation area' refers to a ratio of an area satisfying Si > 0.9 wt% in an area of 1 mm ² at the center of a cross section perpendicular to the longitudinal direction. 'Cr segregation area' means a ratio of an area satisfying Cr > 0.8 wt% in an area of 1 mm ² at the center of a cross section perpendicular to the longitudinal direction. The 'Mn segregation area' refers to a ratio of an area satisfying Mn > 0.8 wt% in an area of 1 mm ² in the center of a cross-section perpendicular to the longitudinal direction.

Alloy composition (wt%) C Si Mn Cr P S Al O N Comparative Example 1 0.55 1.41 1.01 0.65 0.011 0.004 <0.003 <0.005 <0.01 Comparative Example 2 0.56 0.25 0.75 0.75 0.01 0.005 <0.003 <0.005 <0.01 Comparative Example 3 0.61 0.79 0.63 0.55 0.01 0.004 <0.003 <0.005 <0.01 Invention Example 1 0.58 0.78 0.62 0.43 0.009 0.005 <0.003 <0.005 <0.01 Invention example 2 0.62 0.61 0.41 0.52 0.011 0.005 <0.003 <0.005 <0.01

casting speed
(m/min) gun
light pressure
(mm) C segregation
area
(%) Si segregation
area
(%) Mn segregation
area
(%) Cr segregation
area
(%) Comparative Example 1 0.50 25 < 1 5.4 6.2 < 1 Comparative Example 2 0.52 25 < 1 1.2 < 1 < 1 Comparative Example 3 0.56 10 12 13 11 12 Invention Example 1 0.52 25 < 1 < 1 < 1 < 1 Invention example 2 0.52 25 < 1 < 1 < 1 < 1

Thereafter, the wire rods in Table 1 were manufactured with a steel wire having a diameter of 8 mm, and heat treatment was performed under the conditions shown in Table 3 below. Austenitization heat treatment-high pressure water cooling-tempering-water cooling was performed in the order.

The hardness deviation refers to the hardness deviation when hardness is measured at 10 points or more in the remaining area except for the area having a depth of 0.5 mm or less from the outermost surface in a cross section perpendicular to the longitudinal direction.

austenitization
temperature
(℃) tempering
temperature
(℃) Hardness deviation
(Hv) section reduction rate
(%) The tensile strength
(MPa) Comparative Example 1 950 430 35.2 38 2020 Comparative Example 2 950 430 12.1 48 1,710 Comparative Example 3 950 430 45.1 25 1,801 Invention Example 1 950 430 18.1 48 1,820 Invention example 2 950 430 10.1 47 1,790

Referring to Tables 1, 2, and 3, in Inventive Examples 1 and 2, which satisfy the alloy composition and manufacturing conditions of the present invention, in the cross section perpendicular to the longitudinal direction after IT heat treatment, the remaining regions except for the region having a depth of 0.5 mm or less from the outermost surface The hardness deviation was 25 Hv or less, the tensile strength was 1750 to 2200 MPa, and the section reduction ratio was 40% or more.

On the other hand, Comparative Example 1 had a high content of Si and Mn, so hardness deviation due to Si and Mn segregation in the center occurred, and thus the cross-sectional reduction ratio was 40% or less.

Comparative Example 2 did not secure the target tensile strength due to the low Si content.

In Comparative Example 3, the center segregation occurred due to the too fast casting speed and insufficient light pressure reduction, and accordingly, the hardness deviation increased, and the section reduction rate was 40% or less.

In the above description, exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and those of ordinary skill in the art will not depart from the concept and scope of the following claims. It will be appreciated that various modifications and variations are possible.

Claims (8)

By weight%, C: 0.5 to 0.7%, Si: 0.4 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.2 to 0.6%, P: 0.015% or less, S: 0.010% or less, Al: 0.01% or less, N: 0.01% or less, O: 0.005% or less, the remainder contains Fe and unavoidable impurities,
Seconds in which the proportion of the area satisfying at least one of C > 0.8%, Si > 0.9%, Cr > 0.8%, and Mn > 0.8%, in weight %, in an area of 1 mm ² at the center of the cross-section perpendicular to the longitudinal direction is 5% or less Wire rod for high-strength springs. According to claim 1,
A wire rod for ultra-high strength springs with a surface ferrite decarburized layer thickness of 1 μm or less. By weight%, C: 0.5 to 0.7%, Si: 0.4 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.2 to 0.6%, P: 0.015% or less, S: 0.010% or less, Al: 0.01% or less, N: 0.01% or less, O: 0.005% or less, the rest is Fe and the steps of providing a bloom by continuously casting molten steel containing unavoidable impurities; and
Including; after rolling the bloom to the steel strip, and then the wire rod;
In the continuous casting, the casting speed is 0.48 to 0.54 m/min, and the method for manufacturing a wire rod for ultra-high strength spring comprising light-reducing to a total rolling reduction of 15 to 35 mm during continuous casting. 4. The method of claim 3,
the pressure,
A method of manufacturing a wire rod for ultra-high strength spring, characterized in that it is rolled to 4 mm or less for each rolling roll, and the cumulative rolling reduction is 60% or more when the solidification fraction is 0.6 or more. By weight%, C: 0.5 to 0.7%, Si: 0.4 to 0.9%, Mn: 0.3 to 0.8%, Cr: 0.2 to 0.6%, P: 0.015% or less, S: 0.010% or less, Al: 0.01% or less, N: 0.01% or less, O: 0.005% or less, the remainder contains Fe and unavoidable impurities,
As an area fraction, it contains 90% or more of tempered martensite,
Seconds in which the proportion of the area satisfying at least one of C > 0.8%, Si > 0.9%, Cr > 0.8%, and Mn > 0.8%, in weight %, in an area of 1 mm ² at the center of the cross-section perpendicular to the longitudinal direction is 5% or less Steel wire for high-strength springs. 6. The method of claim 5,
In a cross section perpendicular to the longitudinal direction, a steel wire for ultra-high-strength springs with a hardness deviation of 25Hv or less in the remaining areas except for the area of 0.5mm or less in depth from the outermost surface. 6. The method of claim 5,
A steel wire for ultra-high strength springs with a tensile strength of 1750 to 2200 MPa and a reduction rate of 40% or more. Drawing the wire according to claim 1;
heating to 900 to 1000°C within 10 seconds;
water cooling under high pressure;
Tempering by heating to 400 to 500 ℃ within 10 seconds; and
A method of manufacturing a steel wire for an ultra-high strength spring comprising a; water cooling.