JP2023508314A - Wire rod for ultra-high strength spring, steel wire and manufacturing method thereof - Google Patents

Abstract

An object of the present invention is to provide an ultra-high-strength spring wire, a steel wire, and a method for producing the same, which have excellent workability. A wire rod for an ultrahigh-strength spring according to the present invention has, in % by weight, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, and Mn: 0.3 to 0.8. %, Cr: 0.3 to 0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, Nb: over 0% , 0.04% or less, and the remainder consists of Fe and unavoidable impurities, and the value of the following formula (1) is 0.77 or more and 0.83 or less. Formula (1) C+1/6*Mn+1/5*Cr+1/24*Si In formula (1), C, Mn, Cr and Si mean the content (% by weight) of each element.

Description

TECHNICAL FIELD The present invention relates to an ultra-high-strength spring wire, a steel wire, and a method for producing the same, and more particularly to an ultra-high-strength spring wire, a steel wire, and a method for producing the same, which have excellent workability.

Similar to the automotive material market, the motorcycle market is also undergoing constant weight reduction or structural changes. Recently, the demand for high-strength spring steel has increased as mono-type suspension replaces the dual-type suspension used in conventional bikes.

The conventional spring steel used for motorcycle suspension springs is drawn wire and lacks the strength and fatigue resistance to be used for monotype suspensions. With this in mind, they considered the use of tempered martensite (TM) structure steel for automobiles, but they said that automobile suspension springs have strict control standards, are difficult to manufacture, and are expensive, making it difficult to apply to motorcycle suspension springs. There's a problem.

In particular, a suspension spring for a motorcycle has a smaller suspension size than that for an automobile, and relatively higher workability is required when processing the spring. Also, bike suspension springs are used in relatively thin diameters, making decarburization and cold texture control difficult. Therefore, there is a real need for new high-strength suspension springs that can be used for motorcycle suspensions.

Conventionally, when producing a tempered martensitic structure, steel is heated in a heat treatment furnace and then oil quenched, and manganese and chromium are added to the steel in order to ensure sufficient hardenability. It had to be included at a certain level or higher. Recently, due to the development of induction heat treatment technology, it is possible to secure sufficient hardening ability even if water cooling is used, and it is possible to achieve the desired strength while reducing the content of alloying elements in steel. It became so. However, until now, research on fine hard steel materials that can be applied to motorcycle suspension springs with reduced content of alloying elements using induction heating technology and cooling is still inadequate.

The present invention has been made to solve the above problems, and an object of the present invention is to provide an ultra-high-strength spring wire and steel wire having excellent workability, and a method for producing the same. be.

A wire rod for an ultrahigh-strength spring according to one aspect of the present invention, which has been made to achieve the above object, contains, in weight %, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn : 0.3-0.8%, Cr: 0.3-0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005 % or less, Nb: more than 0%, 0.04% or less, the remainder consisting of Fe and unavoidable impurities, and the value of the following formula (1) is 0.77 or more and 0.83 or less do.

Formula (1) C + 1/6 * Mn + 1/5 * Cr + 1/24 * Si

In the above formula (1), C, Mn, Cr, and Si mean the content (% by weight) of each element.

In the ultrahigh-strength spring wire according to one aspect of the present invention, the total area fraction of bainite and martensite having a hardness of 400 Hv or more is preferably 1% or less on a cross section perpendicular to the longitudinal direction.

In the ultra-high-strength spring wire according to one aspect of the present invention, the decarburized ferrite layer preferably has a thickness of 1 μm or less.

In the ultrahigh-strength spring wire according to one aspect of the present invention, the average size of ferrite crystal grains is preferably 10 μm or less.

In the ultra-high-strength spring wire according to one aspect of the present invention, it is preferable that 1000/mm ² or more Nb-based carbides having a size of 20 nm or less are distributed.

The wire rod for an ultrahigh-strength spring according to one aspect of the present invention preferably has a tensile strength of 1200 MPa or less.

A method for producing a wire rod for an ultra-high-strength spring according to one aspect of the present invention, which has been made to achieve the above object, comprises: C: 0.55 to 0.65%; %, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, Nb: more than 0%, 0.04% or less, the remainder consisting of Fe and unavoidable impurities, and the value of the following formula (1) is 0.77 or more and 0.83 or less. Homogenizing heat treatment at a heating temperature of 900 to 1100° C. within 180 minutes, wire rod rolling at a finish rolling temperature of 730 to Ae3° C., and cooling at a cooling rate of 3° C./s or less. .

Formula (1) C + 1/6 * Mn + 1/5 * Cr + 1/24 * Si

In the above formula (1), C, Mn, Cr, and Si mean the content (% by weight) of each element.

In the method for producing a wire rod for an ultra-high strength spring according to one aspect of the present invention, it is preferable that the amount of deformation is 0.3 to 2.0 in the step of rolling the wire rod.

In the method for producing a wire rod for an ultra-high strength spring according to one aspect of the present invention, it is preferable that the average size of austenite grains before finish rolling is 5 to 15 μm in the stage of wire rod rolling.

In addition, a steel wire for ultra-high strength springs according to one aspect of the present invention, which has been made to achieve the above object, has C: 0.55 to 0.65% and Si: 0.5 to 0.9% by weight. %, Mn: 0.3 to 0.8%, Cr: 0.3 to 0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, Nb: over 0%, 0.04% or less, the rest consists of Fe and unavoidable impurities, and the value of the following formula (1) is 0.77 or more and 0.83 or less, It is characterized by containing tempered martensite in an area fraction of 90% or more.

Formula (1) 0.77 ≤ C + 1/6 * Mn + 1/5 * Cr + 1/24 * Si ≤ 0.83

In the above formula (1), C, Mn, Cr, and Si mean the content (% by weight) of each element.

In the steel wire for ultra-high-strength spring according to one aspect of the present invention, it is preferable that 1000/mm ² or more Nb-based carbides having a size of 20 nm or less are distributed.

In the steel wire for ultra-high strength spring according to one aspect of the present invention, it is preferable that the size of the prior austenite average crystal grain is 10 μm or less.

In the steel wire for ultra-high strength springs according to one aspect of the present invention, the wire diameter is preferably 15 mm or less.

The steel wire for ultra-high strength springs according to one aspect of the present invention preferably has a strength of 1700 MPa or more.

In the steel wire for ultrahigh-strength springs according to one aspect of the present invention, it is preferable that the cross-sectional reduction rate is 35% or more.

In addition, a method for producing an ultrahigh-strength steel wire for springs according to one aspect of the present invention, which has been made to achieve the above object, comprises: C: 0.55 to 0.65%; Si: 0.5 to 0.9%, Mn: 0.3-0.8%, Cr: 0.3-0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less , N: 0.005% or less, Nb: over 0%, 0.04% or less, the rest consists of Fe and inevitable impurities, and the value of the following formula (1) is 0.77 or more and 0.83 or less wire drawing, heating at 900-1000° C., high pressure water cooling, tempering at 400-500° C. and water cooling.

Formula (1) C + 1/6 * Mn + 1/5 * Cr + 1/24 * Si

In the above formula (1), C, Mn, Cr, and Si mean the content (% by weight) of each element.

In the method for producing an ultra-high-strength spring steel wire according to an aspect of the present invention, the heating step may include heating to 900 to 1000° C. within 10 seconds and then holding for 5 to 60 seconds.

In the method for manufacturing a steel wire for ultra-high strength springs according to an aspect of the present invention, it is preferable that the average size of austenite grains after the heating step is 10 μm or less.

In the method of manufacturing the ultra-high-strength spring steel wire according to an aspect of the present invention, the tempering step may include heating to 400 to 500° C. within 10 seconds and holding the temperature within 30 seconds.

According to the present invention, it is possible to provide an ultra-high-strength spring wire in which surface decarburization and low-temperature structure formation are suppressed by using a low- _Ceq and low-Si alloy composition.

Further, according to the present invention, it is possible to provide an ultra-high-strength spring wire rod in which crystal grains are refined by using Nb-based carbide and controlled rolling.

The ultrahigh-strength steel wire for springs according to the present invention has a wire diameter of 15 mm or less and has a small diameter suitable as a steel wire for motorcycle suspension springs.

The steel wire for ultra-high strength springs according to the present invention has a strength of 1700 MPa or more in spite of having a low _Ceq and low Si alloy composition by utilizing induction heating and water cooling, and has ultra-high strength required for motorcycle suspension springs. Physical properties can be secured.

The ultra-high-strength steel wire for spring according to the present invention has an area reduction rate (RA) of 35% or more through grain refinement, and high ductility can be secured. manufactured as

[Best mode for carrying out the invention]
The ultra-high-strength spring wire according to one embodiment of the present invention has, in weight %, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8 %, Cr: 0.3 to 0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, Nb: over 0% , 0.04% or less, the balance being Fe and unavoidable impurities, and the value of the following formula (1) is preferably 0.77 or more and 0.83 or less.

Formula (1) C + 1/6 * Mn + 1/5 * Cr + 1/24 * Si

In the above formula (1), C, Mn, Cr, and Si mean the content (% by weight) of each element.

Embodiments of the present invention will be described below. However, the embodiments of the present invention may be modified in various different forms, and the technical idea of the present invention is not limited to the embodiments described below. Moreover, embodiments of the present invention are provided so that the invention will be more fully understood by those of ordinary skill in the art.

The terms used herein are merely used to describe specific examples. Thus, for example, singular references include plural references unless the context clearly dictates otherwise. Furthermore, terms such as "including" or "comprising" as used herein expressly indicate the presence of the features, steps, functions, components or combinations thereof set forth in the specification. and does not preclude the presence of other features, steps, functions, components or combinations thereof.

On the other hand, unless defined otherwise, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Accordingly, unless expressly defined herein, certain terms should not be interpreted in an overly idealistic or formal sense. For example, the singular references herein include plural references unless the context clearly dictates otherwise.

Also, as used herein, "about," "substantially," and the like are used to mean that numerical value or close to that numerical value when manufacturing and material tolerances inherent in the referenced meaning are presented, and the present invention. Accurate and absolute numerical values are used to prevent unscrupulous infringers from misappropriating disclosures in which precise and absolute numerical values are referred to to aid comprehension.

In order to provide an ultra-high strength spring wire and steel wire having excellent workability, the inventors of the present invention have found an optimum low _Ceq and low Si alloy that can easily suppress surface decarburization and low temperature structure formation. A composition was derived. The ultra-high-strength spring may be produced by cold forming the steel wire disclosed herein at room temperature, and the steel wire may be produced by drawing the wire rod disclosed herein. good.

The ultra-high-strength spring wire according to one embodiment of the present invention has, in weight %, C: 0.55 to 0.65%, Si: 0.5 to 0.9%, Mn: 0.3 to 0.8 %, Cr: 0.3 to 0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, Nb: over 0% , 0.04% or less, and the balance consists of Fe and unavoidable impurities.

The reason why the alloy composition is limited to the above will be specifically described below.

Carbon (C): 0.55 to 0.65% by weight
Carbon is an element added to ensure the strength of the product. If the carbon content is less than 0.55% by weight, the intended strength and low _Ceq cannot be ensured. As a result, the martensite structure is not completely formed when the steel material is cooled, making it difficult to ensure strength. can be difficult. If the carbon content exceeds 0.65% by weight, impact properties may deteriorate and quenching cracks may occur during water cooling. Therefore, according to the invention, the carbon content is controlled between 0.55 and 0.65% by weight.

Silicon (Si): 0.5 to 0.9% by weight
Silicon is used for deoxidizing steel and is an element advantageous for ensuring strength by solid solution strengthening. In order to ensure strength, silicon is added in an amount of 0.5% by weight or more in the present invention. However, if silicon is added excessively, it may cause surface decarburization, which makes it difficult to process the material. Taking this into consideration, the upper limit is limited to 0.9% by weight. Thus, the present invention suppresses surface decarburization by using a low-Si alloy design in which silicon is controlled to 0.9% by weight or less, and ensures sufficient workability.

Manganese (Mn): 0.3 to 0.8% by weight
Manganese is a hardenability-improving element and is one of the essential elements for forming high-strength tempered martensite structure steel. In order to ensure strength, manganese is added in an amount of 0.3% by weight or more in the present invention. However, the upper limit of the manganese content is limited to 0.8% by weight, since excessive manganese content in tempered martensitic steel reduces toughness.

Chromium (Cr): 0.3-0.6% by weight
Chromium, together with manganese, is effective in improving hardenability and improves the corrosion resistance of steel. Therefore, 0.3% by weight or more of chromium is added in the present invention. However, chromium is a relatively expensive element compared to silicon and manganese and increases _Ceq , so the upper limit thereof in the present invention is limited to 0.6 wt%.

Phosphorus (P): 0.015% by weight or less Phosphorus is an element that segregates at grain boundaries to reduce toughness and hydrogen delayed fracture resistance. Its upper limit in the present invention is limited to 0.015% by weight.

Sulfur (S): 0.01% by weight or less Sulfur, like phosphorus, segregates at grain boundaries to reduce toughness and forms MnS, which reduces hydrogen delayed fracture resistance. Maximum exclusion is preferred. Its upper limit in the present invention is limited to 0.01% by weight.

Aluminum (Al): 0.01% by weight or less Aluminum is a strong deoxidizing element and can remove oxygen from steel to improve cleanliness. However, aluminum has the problem that it forms Al ₂ O ₃ inclusions when added and reduces fatigue resistance. This restricts the upper limit in the present invention to 0.01% by weight.

Nitrogen (N): 0.005% by weight or less Nitrogen has the problem of forming coarse AlN or VN precipitates that are not dissolved during heat treatment by combining with aluminum or vanadium in the steel. Accordingly, the upper limit in the present invention is limited to 0.005% by weight or less.

Niobium (Nb): more than 0% by weight, up to 0.04% by weight Niobium is an element that combines with carbon in steel to form Nb-based carbides, refines crystal grains, and improves workability. Niobium is added in an amount exceeding 0% by weight in the present invention in order to improve workability by grain refinement. However, if niobium is added excessively, coarse carbides may be formed and workability may be deteriorated. Niobium may be added in an amount of 0.02% by weight or less from the aspect of improving workability.

The Nb-based carbides formed by adding niobium can be distributed within the structure of the steel wire and wire rod for ultra-high strength springs according to the present invention. The size of the formed Nb-based carbide is preferably 20 nm or less. This is because if the size of the Nb-based carbide exceeds 20 nm, the workability may rather deteriorate. Moreover, it is preferable that the Nb-based carbides are evenly distributed at 1000 pieces/mm ² or more. This is because if the number of Nb-based carbides is less than 1000/mm ² , the crystal grains may not be sufficiently refined. In the Nb-based carbide described above, Nb may be contained at 10 atomic % or more.

The remaining component of the present invention is iron (Fe). However, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in during normal manufacturing processes, and cannot be excluded. The above impurities are known to any person skilled in the normal manufacturing process, so the full content thereof is not specifically mentioned herein.

In addition, the reasons for limiting the alloy composition of the wire described above are the same as the reasons for limiting the alloy composition of the steel wire, and for the sake of convenience, the reasons for limiting the alloy composition of the steel wire are omitted.

Regarding the alloy composition of the wire rod and steel wire of the present invention, in addition to limiting the content of each alloying element according to the conditions described above, these relationships may be further limited as follows.

Value of formula (1): 0.77 or more and 0.83 or less The present invention controls the _Ceq value so as to suppress the surface decarburization and the formation of a low-temperature structure that tend to occur during cooling after wire rolling. The C _eq value is represented by the following formula (1). In the present invention, the value of formula (1) is controlled to 0.77 or more and 0.83 or less in order to suppress surface decarburization and formation of low-temperature structures.

Formula (1) C + 1/6 * Mn + 1/5 * Cr + 1/24 * Si

In the above formula (1), C, Mn, Cr, and Si mean the content (% by weight) of each element.

If the value of formula (1) exceeds 0.83, surface decarburization may occur and a low temperature structure may be formed. On the other hand, when the value of formula (1) is less than 0.77, it is difficult to ensure the target strength.

Hereinafter, a method for manufacturing a wire rod for an ultrahigh-strength spring according to the present invention will be described in detail. A wire rod for an ultrahigh-strength spring according to the present invention is manufactured by subjecting an ingot having the above-mentioned alloy composition and the value of formula (1) to homogenization heat treatment, rolling the wire rod, and then cooling it. Each manufacturing stage will be described below.

In the present invention, the homogenization heat treatment step is performed in a heating furnace at a heating temperature of 900-1100° C. within 180 minutes.

In the present invention, the finish rolling temperature in the stage of wire rod rolling is preferably 730 to Ae3°C. When finish rolling is performed in the temperature range of 730 to Ae3° C., the main structure of the wire rod transforms from austenite to ferrite. In other words, the main structure of the wire before finish rolling is austenite, and the main structure of the wire after finish rolling is ferrite.

In the step of wire rod rolling according to the present invention, crystal grains are refined through controlled rolling, and sufficient workability can be ensured by the refined crystal grains. According to one embodiment, the wire rolling deformation amount is between 0.3 and 2.0. The amount of deformation in the present invention is represented by the following formula.

Change amount = -ln (1 - area reduction rate / 100)

In the above formula, the area reduction rate is defined as ( _{AA 1} )/A*100.

If the amount of deformation during wire rolling is less than 0.3, it is difficult to sufficiently refine the crystal grains, and if the amount of deformation exceeds 2.0, the amount of processing is too high, which is unreasonable during the production process. Therefore, according to the present invention, the deformation amount is preferably controlled between 0.3 and 2.0.

If the wire rod is rolled under the conditions described above, the crystal grains can be refined. According to one embodiment, the average size of the austenite grains before finish rolling is 5-15 μm. In addition, if the average size of austenite grains before finish rolling is refined, the average size of ferrite grains in the final wire structure after subsequent finish rolling and cooling can also be refined.

The cooling step in the present invention cools the wire at a cooling rate of 3° C./s or less. If the cooling rate exceeds 3°C/s, it is difficult to suppress the formation of low-temperature structures.

The ultra-high-strength spring wire according to the present invention manufactured by the alloy composition and manufacturing method described above contains pearlite and ferrite as a microstructure, and according to one embodiment, the area fraction is 60% or more of pearlite, and the remaining It may contain ferrite.

According to the present invention, it is possible to suppress the formation of low-temperature structures through the above-described alloy composition and the low- _Ceq alloy composition that satisfies the range of values of formula (1). A wire rod for an ultrahigh-strength spring according to one embodiment of the present invention contains almost no low-temperature structure on a cross section perpendicular to the longitudinal direction of the wire rod. According to one embodiment, the sum of the area fractions of bainite and martensite having a hardness of 400 Hv or more on a cross section perpendicular to the longitudinal direction (C cross section) may be 1% or less. On the other hand, the low temperature structure in the present invention means bainite and martensite. The ultra-high-strength spring wire of the present invention can ensure sufficient workability by suppressing the formation of low-temperature structures.

According to the present invention, the surface decarburization phenomenon can be suppressed through the alloy composition described above and the low _Ceq and low Si alloy composition that satisfies the range of values of formula (1). According to one embodiment, the thickness of the ferrite decarburized layer of the wire may be 1 μm or less.

According to the present invention, ferrite grains can be refined through Nb-based carbide and controlled rolling. An average size of ferrite grains of the wire according to an embodiment of the present invention may be 10 μm or less. The ultra-high-strength spring wire of the present invention can ensure sufficient workability by refining crystal grains.

A wire rod for an ultra-high strength spring according to an embodiment of the present invention may have a tensile strength of 1200 MPa or less.

The steel wire for ultra-high strength spring according to one embodiment of the present invention contains C: 0.55-0.65%, Si: 0.5-0.9%, Mn: 0.3-0. 8%, Cr: 0.3 to 0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, Nb: 0% Excess, 0.04% or less, the rest consists of Fe and unavoidable impurities, the value of formula (1) is 0.77 or more and 0.83 or less, and the area fraction is 90% tempered martensite It may contain more than

The reason for limiting the alloy composition of the steel wire and the range of the value of formula (1) is the same as the reason for limiting the alloy composition of the wire rod and the range of the value of formula (1) described above. .

Hereinafter, a method for manufacturing a steel wire for ultra-high strength springs according to the present invention will be described in detail. The steel wire for ultra-high-strength springs according to the present invention is manufactured by drawing a wire that satisfies the range of the alloy composition and the value of formula (1) described above, heating it, water-cooling it at high pressure, tempering it, and then water-cooling it. be done. Each manufacturing stage will be described below.

In the present invention, the means for heating to the quenching temperature and the means for tempering utilize induction heating so that rapid heating can be achieved to sufficiently harden the surface during subsequent water cooling. The present invention utilizes induction heating and water cooling in low _Ceq and low Si alloy compositions that satisfy the range of values of the alloy composition and formula (1) described above to reduce the content of alloying elements compared to automotive suspension springs. While lowering it, it will be possible to secure the target ultra-high strength.

In the wire drawing step of the present invention, a steel wire is produced by drawing a wire satisfying the range of the alloy composition and the value of formula (1) described above to a wire diameter of 15 mm or less applicable to a motorcycle suspension spring.

Next, in the heating step of the present invention for QT heat treatment of the drawn steel wire, the drawn steel wire is heated to a quenching temperature of 900 to 1000 ° C. within 10 seconds, and then heated for 5 to 60 seconds. Hold to austenitize the structure of the steel wire. If the heating time to reach the target temperature of 900 to 1000° C. exceeds 10 seconds, crystal grains grow and it is difficult to ensure desired physical properties. If the holding time is less than 5 seconds, the pearlite structure may not transform into austenite, and if it exceeds 60 seconds, the crystal grains may become coarse. Therefore, it is preferable to control the holding time to 5 to 60 seconds. .

In addition, as a result of rapidly heating the drawn steel wire using induction heating, the average size of austenite grains in the austenitized steel wire is refined to 10 μm or less. As a result of finely controlling the austenite grains at this stage, the grains of the final ultra-high strength spring steel wire manufactured through subsequent high-pressure water cooling, tempering, and water cooling are also finely controlled. Accordingly, the ultra-high-strength steel wire for spring according to the present invention has fine crystal grains and excellent workability, and is cold-formed at room temperature to be manufactured as a suspension spring for a motorcycle.

In the present invention, the step of high-pressure water cooling is a step of transforming the main structure of the steel wire from austenite to martensite. At this time, when cooling is performed by oil cooling instead of water cooling, the target strength cannot be secured due to the low _Ceq and low Si alloy composition. In addition, if the pressure is not high enough to remove the boiling film during water cooling, there is a high possibility of quenching cracks during quenching. In addition, in the heating step described above, the steel wire is rapidly heated to the quenching temperature using induction heating, and then rapidly cooled with water in this step to sufficiently harden the surface of the steel wire. The cooling rate during water cooling is 100° C./s or more according to one embodiment.

The step of tempering in the present invention is the step of heating martensite, which is the main structure of the water-cooled steel wire, and tempering it with tempered martensite. The tempering step is maintained within 30 seconds after heating to 400-500° C. within 10 seconds. If the tempering temperature is less than 400°C, the toughness is not ensured, making it difficult to process, and the risk of product breakage increases. If the tempering temperature exceeds 500°C, the strength decreases. . Moreover, if the steel cannot be heated to the above-mentioned temperature range within 10 seconds during tempering, coarse carbides may be formed and the toughness may be lowered. Therefore, rapid heating within 10 seconds is preferable.

Next, the tempered steel wire is water-cooled to normal temperature.

A spring steel wire that satisfies the alloy composition and the value range of formula (1) described above and is manufactured under the manufacturing conditions described above can contain tempered martensite at an area fraction of 90% or more.

Further, in the steel wire for ultra-high strength spring according to one embodiment of the present invention, Nb-based carbides with a size of 20 nm or less are distributed at 1000 pieces/mm ² or more.

In addition, the steel wire for ultra-high strength spring according to one embodiment of the present invention has a prior austenite average crystal grain size of 10 μm or less. Here, prior austenite means the austenite structure of the steel wire after the step of heating the drawn steel wire of the present invention for QT heat treatment.

Further, the ultra-high-strength steel wire for springs according to one embodiment of the present invention has a wire diameter of 15 mm or less, and has a small diameter suitable as a steel wire for motorcycle suspension springs.

In addition, the steel wire for ultra-high strength springs according to an embodiment of the present invention has a strength of 1700 MPa or more, and can ensure ultra-high strength physical properties required for motorcycle suspension springs.

In addition, the steel wire for ultra-high strength springs according to an embodiment of the present invention has a reduction in area (RA) of 35% or more, so that high ductility can be secured. manufactured. In the present invention, Nb can be added to refine the austenite grains before finish rolling during wire rod rolling, so that the area reduction rate (RA) can be further improved. A steel wire for ultra-high strength springs according to a preferred embodiment of the present invention has a reduction in area (RA) of 45% or more.

Hereinafter, the present invention will be described in more detail through examples. However, the following examples are intended to illustrate the present invention in more detail, and are not intended to limit the technical scope of the present invention.

Example After casting a material having the alloy composition shown in Table 1 below into an ingot, performing a homogenization heat treatment at 1100°C, and then rolling a wire rod to a final thickness of 12 mm while decreasing the temperature from 1030°C to 750°C. , and cooled at a rate of 3° C./s to produce a wire rod.

The results shown in Table 2 below are the results of measuring the physical properties of the wires manufactured in the above-described process. The low-temperature structure area fraction in Table 2 means the sum of the area fractions of bainite and martensite on the cross section perpendicular to the longitudinal direction of the wire.

AGS in Table 2 means the average size of austenite grains before finish rolling at the wire rod rolling stage, and was measured using the ASTM E112 standard.

The thickness of the ferrite decarburized layer is the thickness of the ferrite-only layer formed by decarburization on the surface of the steel after wire rod rolling. It measures the vertical distance to a point with the same carbon concentration as the carbon concentration.

The wires in Table 2 were drawn into steel wires with a diameter of 10 mm, heated, and then cooled with high-pressure water. After high-pressure water cooling, tempering was performed, and general water cooling was performed to produce the final steel wire for ultra-high strength springs. In Table 3 below, the heating temperature means the temperature at which the steel wire is heated after wire drawing, and the tempering temperature means the temperature at which the steel wire is tempered after high-pressure water cooling. RA means area reduction rate.

Referring to Tables 1 to 3, invention examples 1 and 2 satisfy the alloy composition, formula (1), and manufacturing conditions of the present invention. As a result, the austenite grains before finish rolling were refined during wire rolling. Moreover, as shown in Table 3, the tensile strength was 1700 MPa or more, and the area reduction rate was 35% or more. On the other hand, Comparative Example 1 had a high Si content and formed a thick ferrite decarburized layer during cooling. In Comparative Example 2, the value of formula (1) was lower than 0.77, and the target strength of 1700 MPa or more could not be secured. In Comparative Example 3, Nb was not added, grain coarsening occurred, and the target average size of austenite grains could not be secured. As a result, the area reduction rate (RA) was lower than that of the Nb-added material.

Although the exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and any person having ordinary knowledge in the relevant technical field can make modifications within the concept and technical scope of the present invention. It can be understood that various modifications and variations are possible within.

The ultra-high-strength spring wire according to the present invention is applied as suspension springs for automobiles, motorcycles, various means of transportation, or springs used in various industrial fields.

Claims (15)

% by weight, C: 0.55-0.65%, Si: 0.5-0.9%, Mn: 0.3-0.8%, Cr: 0.3-0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, Nb: over 0%, 0.04% or less, the balance being Fe and inevitable impurities A wire rod for an ultra-high-strength spring, characterized in that the value of the following formula (1) is 0.77 or more and 0.83 or less.
Formula (1) C + 1/6 * Mn + 1/5 * Cr + 1/24 * Si
(In the above formula (1), C, Mn, Cr, and Si mean the content (% by weight) of each element.) On a cross section perpendicular to the longitudinal direction,
2. A wire rod for an ultrahigh-strength spring according to claim 1, wherein the total area fraction of bainite and martensite having a hardness of 400 Hv or more is 1% or less. 2. The wire rod for an ultrahigh-strength spring according to claim 1, wherein the decarburized layer of ferrite has a thickness of 1 [mu]m or less. 2. The wire for an ultra-high strength spring according to claim 1, wherein the ferrite crystal grains have an average size of 10 [mu]m or less. 2. The wire for an ultra-high-strength spring according to claim 1, wherein 1000 or more Nb-based carbides with a size of 20 nm or less are distributed per mm ² . 2. The wire for an ultra-high-strength spring according to claim 1, having a tensile strength of 1200 MPa or less. % by weight, C: 0.55-0.65%, Si: 0.5-0.9%, Mn: 0.3-0.8%, Cr: 0.3-0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, Nb: over 0%, 0.04% or less, the balance being Fe and inevitable impurities consists of
Homogenizing heat treatment of an ingot having a value of the following formula (1) of 0.77 or more and 0.83 or less at a heating temperature of 900 to 1100 ° C. within 180 minutes;
A step of wire rod rolling at a finish rolling temperature of 730 to Ae3 ° C.;
and cooling at a cooling rate of 3° C./s or less.
Formula (1) C + 1/6 * Mn + 1/5 * Cr + 1/24 * Si
(In the above formula (1), C, Mn, Cr, and Si mean the content (% by weight) of each element.) 8. The method for producing a wire rod for an ultra-high strength spring according to claim 7, wherein the amount of deformation in the step of rolling the wire rod is 0.3 to 2.0. 8. The method for producing a wire rod for an ultra-high strength spring according to claim 7, wherein in the step of rolling the wire rod, the average size of austenite grains before finish rolling is 5 to 15 μm. % by weight, C: 0.55-0.65%, Si: 0.5-0.9%, Mn: 0.3-0.8%, Cr: 0.3-0.6%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.005% or less, Nb: over 0%, 0.04% or less, the balance being Fe and inevitable impurities The value of the following formula (1) is 0.77 or more and 0.83 or less,
A steel wire for an ultra-high-strength spring containing 90% or more of tempered martensite in terms of area fraction.
Formula (1) 0.77 ≤ C + 1/6 * Mn + 1/5 * Cr + 1/24 * Si ≤ 0.83
(In the above formula (1), C, Mn, Cr, and Si mean the content (% by weight) of each element.) The steel wire for an ultra-high strength spring according to claim 10, characterized in that 1000 or more Nb-based carbides with a size of 20 nm ^{or less} are distributed per mm2. 11. The steel wire for an ultra-high strength spring according to claim 10, wherein the average prior austenite crystal grain size is 10 μm or less. 11. The steel wire for ultra-high strength springs according to claim 10, wherein the wire diameter is 15 mm or less. 11. The steel wire for an ultra-high strength spring according to claim 10, having a strength of 1700 MPa or more. 11. The steel wire for ultra-high strength springs according to claim 10, wherein the cross-sectional reduction rate is 35% or more.