JP2024521927A - Spring wire rod, steel wire, spring with improved strength and fatigue limit, and manufacturing method thereof - Google Patents

Abstract

The present invention provides a wire rod, steel wire, and spring that are excellent in strength and processability, can be easily nitrided even at high temperatures, and have improved nitriding properties and fatigue limit, and a manufacturing method thereof. [Solution] The alloy consists of, by weight, C: 0.6-0.7%, Si: 2.0-2.5%, Mn: 0.2-0.7%, Cr: 0.9-1.5%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.01% or less, Mo: 0.25% or less, W: 0.25% or less, V: 0.05%-0.2% or less, Nb: 0.05% or less, with the remainder being Fe and other unavoidable impurities, and is characterized in that Mn+Cr≦1.8%, 0.05at%≦Mo+W≦0.15at%, and in a central 1mm2 area of a cross section perpendicular to the length direction, the proportion of an area that satisfies one or more of C>0.85%, Si>3.0%, Mn>0.8%, and Cr>2.0% by weight is 10% or less. [Selected Figure] None

Description

The present invention relates to spring wire rod, steel wire, springs, and methods for manufacturing the same, and more specifically to ultra-high strength spring steel of the 2,200 MPa class, which has excellent strength and workability, is easy to nitride even at high temperatures, and has improved nitriding properties and fatigue limit, and relates to methods for manufacturing the same.

As vehicle weight reductions lead to a continuous demand for lighter automotive parts, springs used in automotive transmissions and engine valves are also required to be continuously stronger. However, as the strength of spring materials increases, the wire diameter becomes thinner and the sensitivity to inclusions increases, lowering the fatigue limit. In other words, there is a limit to how much the fatigue limit can be improved by improving strength. To overcome this, spring manufacturers have attempted to increase the fatigue limit of spring materials by maintaining strength and improving surface hardness through nitriding treatment.

Normally, nitriding is carried out at 500°C or higher for other parts, but in the case of spring steel, nitriding is carried out at 420-460°C to prevent a decrease in strength, and a long heat treatment of 10 hours or more is carried out to ensure sufficient nitrogen penetration depth.
Since the tempering heat treatment temperature for normal spring steel is 450°C or less, many spring steels lose strength significantly when heat treated for a long time at 420 to 450°C, and therefore high alloy materials containing elements capable of forming carbides and improving softening resistance must be used. However, although the addition of large amounts of carbide-forming elements such as Mo and V can suppress the loss of strength during nitriding, it can also cause the formation of a low-temperature structure due to segregation in the center, which can cause a problem of a decrease in the reduction of area.

In addition, since the spring material is repeatedly subjected to high-temperature heat treatment during the manufacturing process, control of the prior austenite grain size (PAGS) becomes an issue, and a technology for controlling carbides during the heat treatment process is also required.
In addition, spring manufacturers desire to perform the nitriding treatment at as high a temperature as possible in order to shorten the nitriding treatment time, and at the same time, they require high-strength wire rod that does not affect on-site productivity.
For this reason, there is a demand for the development of wire rods and steel wires which are excellent in quality such as strength and workability, and at the same time, have improved nitriding properties and fatigue limit.

Korean Patent Publication No. 10-2000-0043776

The present invention has been made to solve the above problems, and its purpose is to provide wire rod, steel wire, and springs that are excellent in strength and workability, can be easily nitrided even at high temperatures, and have improved nitriding properties and fatigue limit, as well as a manufacturing method for the same.

The present invention provides a spring wire rod having improved strength and fatigue limit, which comprises, by weight, C: 0.6-0.7%, Si: 2.0-2.5%, Mn: 0.2-0.7%, Cr: 0.9-1.5%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.01% or less, Mo: 0.25% or less, W: 0.25% or less, V: 0.05% to 0.2% or less, Nb: 0.05% or less, with the balance being Fe and other unavoidable impurities, and satisfies Mn+Cr≦1.8%, 0.05at%≦Mo+W≦0.15at%, and has a thickness of 1mm at the center of a cross section perpendicular to the longitudinal direction. In the ^steel sheet, the proportion of the area that satisfies, by weight, one or more of C>0.85%, Si>3.0%, Mn>0.8%, and Cr>2.0% is 10% or less.

The wire rod preferably contains, in terms of area fraction, 80% or more of pearlite structure, with the remainder being bainite structure or martensite structure.
The wire may have an average prior austenite grain size of 20 μm or less.
The wire preferably has carbonitrides with a maximum diameter of 15 μm or more distributed at less than 2 particles/cm ² in a cross section parallel to the length direction within a surface depth of 1 mm.
The wire preferably has a tensile strength of 1,400 MPa or less and a cross-sectional area reduction rate of 35% or more.

To achieve the above object, the manufacturing method of the present invention for spring wire with improved strength and fatigue limit comprises, by weight percentage, C: 0.6-0.7%, Si: 2.0-2.5%, Mn: 0.2-0.7%, Cr: 0.9-1.5%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.01% or less, Mo: 0.25% or less, W: 0.25% or less, V: 0.05%-0.2% or less, Nb: 0.05% or less, the remainder being Fe and It is characterized by including the steps of: continuously casting molten steel consisting of copper and other unavoidable impurities to prepare a bloom; heating the bloom at a temperature of 1,200°C or higher and rolling it into a billet; heat treating the billet at a temperature of 1,030°C or higher and rolling it into a wire at a temperature of 1,000°C or lower; winding the wire at a temperature of 800 to 900°C; and cooling the wound wire at a rate of 0.5 to 2°C/s.

Preferably, the continuous casting step includes soft reduction with a total reduction of 20 mm or more.
The soft reduction may be performed by rolling each roll 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 spring steel wire according to the present invention, which has improved strength and fatigue limit and which is intended to achieve the above object, is characterized by comprising, by weight percentage, C: 0.6-0.7%, Si: 2.0-2.5%, Mn: 0.2-0.7%, Cr: 0.9-1.5%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.01% or less, Mo: 0.25% or less, W: 0.25% or less, V: 0.05%-0.2% or less, Nb: 0.05% or less, the balance being Fe and other unavoidable impurities, satisfying Mn+Cr≦1.8%, and satisfying 0.05at%≦Mo+W≦0.15at%, and containing, by area fraction, 85% or more of tempered martensite structure and the balance being austenite structure.

The steel wire preferably has an average grain size of prior austenite of 15 μm or less.
The steel wire may have carbonitrides with a maximum diameter of 15 μm or more distributed at less than 2 particles/cm ² in a cross section parallel to the length direction within a depth of 1 mm from the surface.
It is preferable that the number of carbides is 10 to 50 per ¹⁰⁰ μm2 area, the maximum diameter of the carbides is 5 to 50 nm, and the content of V or Nb is 10 at % or more.
The steel wire may have a tensile strength of 2,100 MPa or more and a cross-sectional reduction rate of 45% or more.

To achieve the above object, the method for manufacturing a spring steel wire with improved strength and fatigue limit according to the present invention includes the steps of subjecting the wire to LP heat treatment; drawing the LP heat-treated wire to prepare a steel wire; and subjecting the steel wire to QT heat treatment; the LP heat treatment step includes a first austenitizing step of heating the wire to 950-1100°C within 3 minutes and maintaining the temperature for 3 minutes or less; and passing the first austenitized wire through a lead bath at 650-700°C within 3 minutes.

The time required for completing pearlite transformation in the LP heat treatment step is preferably less than 130 seconds.
The method may further include a step of subjecting the wire to an LA heat treatment before the LP heat treatment, and the LA heat treatment may further include a step of heat treating at 650 to 750° C.; and a step of pickling.
The QT heat treatment step may include a second austenitizing step of heating to 900-1000°C within 3 minutes and maintaining for 3 minutes or less; a first oil quenching step at 70°C or less; a tempering step of heating to 450-550°C within 3 minutes and maintaining for 3 minutes or less; and a second oil quenching step at 70°C or less.

The spring with improved strength and fatigue limit according to the present invention, which achieves the above objective, is characterized by being composed of, by weight percent, C: 0.6-0.7%, Si: 2.0-2.5%, Mn: 0.2-0.7%, Cr: 0.9-1.5%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.01% or less, Mo: 0.25% or less, W: 0.25% or less, V: 0.05%-0.2% or less, Nb: 0.05% or less, with the remainder being Fe and other unavoidable impurities, satisfying Mn+Cr≦1.8%, 0.05at%≦Mo+W≦0.15at%, and having a fatigue limit of 700MPa or more that can withstand 10 million repeated stresses.

In order to achieve the above object, a method for manufacturing a spring having improved strength and fatigue limit according to the present invention includes the steps of cold forming the steel wire into a spring shape; subjecting the formed spring to stress relief heat treatment; and nitriding the spring at a temperature of 420 to 450° C. for 10 hours or more.
In order to achieve the above object, the method of manufacturing a spring having improved strength and fatigue limit according to the present invention is characterized in that the fatigue limit after the nitriding treatment is increased by 10% or more.

According to one aspect of the present invention, it is possible to provide a wire rod, a steel wire, a spring, and a manufacturing method thereof, which suppress the generation of low-temperature structures in the center by reducing central segregation and ensure an excellent area reduction rate while at the same time ensuring a tensile strength of 2,200 MPa or more.
According to another aspect of the present invention, by controlling the crystal grain size and the number of precipitates, it is possible to provide a wire rod, a steel wire, and a spring having improved nitriding characteristics and fatigue limit, and a method for manufacturing the same.
The various and beneficial advantages and effects of the present invention are not limited to the above-mentioned contents, but can be more easily understood in the course of describing specific embodiments of the present invention.

The spring wire rod according to the present invention, which has improved strength and fatigue limit, comprises, by weight, C: 0.6-0.7%, Si: 2.0-2.5%, Mn: 0.2-0.7%, Cr: 0.9-1.5%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.01% or less, Mo: 0.25% or less, W: 0.25% or less, V: 0.05% to 0.2% or less, Nb: 0.05% or less, with the balance being Fe and other unavoidable impurities, and satisfies Mn+Cr≦1.8%, and 0.05at%≦Mo+W≦0.15at%, and has a central 1mm In the area ² , the ratio of the area which satisfies, by weight percentage, one or more of C>0.85%, Si>3.0%, Mn>0.8%, and Cr>2.0% is 10% or less.

In the following, preferred embodiments of the present invention will be described. However, the embodiments of the present invention may be modified into various other forms, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those having average knowledge in the art.
The terms used in this application are merely used to describe specific examples. Therefore, singular expressions include plural expressions unless the context clearly requires singularity. In addition, it should be noted that the terms "include" or "comprise" used in this application are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to preliminarily exclude the presence of other features, steps, functions, components, or combinations thereof.

In addition, unless otherwise defined, all terms used in this specification should be considered to have the same meaning as that generally understood by a person having ordinary knowledge in the technical field to which the present invention belongs. Therefore, unless clearly defined in this specification, a specific term should not be interpreted in an overly ideal or formal sense. For example, in this specification, a singular expression includes a plural expression unless there is a clear exception in the context.
In addition, in this specification, the terms "about,""substantially," and the like are used to mean a numerical value or a approximation thereof when manufacturing and material tolerances inherent in the referred meaning are given, and are used to prevent unscrupulous infringers from unfairly taking advantage of the disclosure in which precise or absolute numerical values are referred to in order to aid in the understanding of the present invention.

The spring wire rod having improved strength and fatigue limit according to one embodiment of the present invention is composed, by weight, of C: 0.6-0.7%, Si: 2.0-2.5%, Mn: 0.2-0.7%, Cr: 0.9-1.5%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.01% or less, Mo: 0.25% or less, W: 0.25% or less, V: 0.05% to 0.2% or less, Nb: 0.05% or less, with the remainder being Fe and other unavoidable impurities.
The reasons for limiting the range of each alloy element will be explained below. In the following, unless otherwise specified, the unit is weight percent.

The C content is 0.6 to 0.7%.
C is an element that improves the strength of a material, and can be added in an amount of 0.6% or more to ensure sufficient strength of the material. However, if the C content is excessive, the impact properties are significantly reduced after QT (Quenching & Tempering) heat treatment, and the possibility of low-temperature structures occurring during wire production is significantly increased, which may result in deterioration of the quality of the wire. In addition, if the C content is excessive, the time required for LP heat treatment, which is one of the manufacturing processes for steel wire, is significantly increased, resulting in reduced productivity. In consideration of this, it is preferable to limit the upper limit of the C content to 0.7%.

The Si content is 2.0 to 2.5%.
Silicon is not only used for deoxidizing steel, but is also an element advantageous in ensuring strength through solid solution strengthening, and can be added in an amount of 2.0% or more to suppress the decrease in strength during nitriding and improve the deformation resistance of the spring. However, if the silicon content is excessive, it may induce surface decarburization, which may deteriorate the workability of the material. In consideration of this, it is preferable to limit the upper limit of the silicon content to 2.5%.

The Mn content is 0.2 to 0.7%.
Mn is an element that improves hardenability, and can be added in an amount of 0.2% or more to ensure the hardenability and high-strength tempered martensite structure of the material and to fix S as a Mn compound to render it harmless. However, if the Mn content is excessive, there is a risk of quality deterioration due to segregation. Taking this into consideration, it is preferable to limit the upper limit of the Mn content to 0.7%.

The Cr content is 0.9 to 1.5%.
Cr, together with Mn, is an element that improves hardening ability, and can be added in an amount of 0.9% or more to improve the softening resistance of steel during nitriding. However, if the Cr content is excessive, it significantly reduces the toughness of the steel wire and promotes the generation of low-temperature structures during cooling of the wire rod. In consideration of this, it is preferable to limit the upper limit of the Cr content to 1.5%.

The P content is 0.015% or less.
Since P is an element that segregates at grain boundaries and reduces the toughness of the material and the resistance to hydrogen-induced delayed fracture, it is preferable to remove it from the steel material as much as possible. Taking this into consideration, it is preferable to limit the upper limit of the P content to 0.015%.

The S content is 0.01% or less.
Like P, S not only segregates at grain boundaries to reduce toughness, but also forms MnS, which may reduce hydrogen-delayed fracture resistance. In consideration of this, it is preferable to limit the upper limit of the S content to 0.01%.

The Al content is 0.01% or less.
Al is a strong deoxidizing element and can remove oxygen from steel to increase cleanliness, but it may form _Al2O3 inclusions and reduce fatigue resistance. In consideration of this, it is preferable to limit the upper limit of the Al content to _0.01 %.

The N content is 0.01% or less.
N combines with impurities, Al, or V to form coarse AlN or VN precipitates that do not dissolve during heat treatment. Taking this into consideration, it is preferable to set the upper limit of the N content to 0.01%.

The Mo content is 0.25% or less.
Mo is an element that improves the softening resistance of the nitriding material, forms carbides together with V, and increases the strength during tempering. Mo also forms MC carbides and maintains the strength of the material even during long-term heat treatment. However, if the Mo content is excessive, the formation of pearlite structure is suppressed, and there is a risk that the quality of the wire will deteriorate due to the formation of a low-temperature structure after rolling the wire. In addition, if the Mo content is excessive, pearlite transformation is suppressed even during LP heat treatment before wire drawing, and the pearlite transformation time is increased, resulting in a significant decrease in productivity. In consideration of this, it is preferable to limit the upper limit of the Mo content to 0.25%.

The W content is 0.25% or less.
W, together with Mo, is an element that can improve the softening resistance of a nitriding material, and like Mo, it forms MC carbides and can maintain the strength of the material even during long-term heat treatment. However, if the W content is excessive, there is a risk that the formation of pearlite will be suppressed and the formation of low-temperature structures will be promoted in the wire rod. In consideration of this, it is preferable to limit the upper limit of the W content to 0.25%.

The V content is 0.05 to 0.2%.
V, together with Mo, is an element that improves the softening resistance of materials for nitriding, and can form carbides to increase strength during tempering and maintain strength even during long-term nitriding. In addition, unlike Mo and W, V has a high carbide solid solution temperature, so it plays a role in maintaining the prior austenite grain size. V accelerates pearlite transformation, so it can suppress low-temperature structures during wire production. V also shortens the isothermal transformation time during LP heat treatment, so it can improve productivity during the steel wire manufacturing process, and can be added in an amount of 0.05% or more. However, if the V content is excessive, there is a risk of forming coarse carbonitrides during the wire production process, so the temperature of the heating furnace must be increased during the rolling of the wire. In consideration of this, it is preferable to limit the upper limit of the V content to 0.2%.

The Nb content is 0.05% or less.
Nb is a carbonitride forming element and has a higher solid solution temperature than V, and therefore has a better effect of controlling the prior austenite grain size than V. However, if the Nb content is excessive, there is a risk of the prior austenite grain size becoming coarse. In consideration of this, the upper limit of the Nb content is preferably set to 0.05%, and when the prior austenite grain size is controlled through the manufacturing process, the addition of Nb can be omitted.

The remaining component other than the above composition is iron (Fe). However, in normal manufacturing processes, unintended impurities may be unavoidably mixed in from the raw materials or the surrounding environment, and these cannot be eliminated. These impurities are known to any engineer of normal manufacturing processes, so all of their contents will not be mentioned in this specification.

The wire rod having improved strength and fatigue limit according to an embodiment of the present invention can satisfy, in weight percent, Mn+Cr≦1.8%.
If the sum of Mn and Cr exceeds 1.8%, a low-temperature structure such as bainite or martensite is generated during the cooling process of the wire rod, and the time to complete pearlite transformation during LP heat treatment may be extended. In addition, if the sum of Mn and Cr exceeds 1.8%, the carbon equivalent (Ceq) increases significantly, and the amount of W and Mo added is limited, so that it is not possible to prevent the strength of the material from decreasing during nitriding. In addition, if the carbon equivalent increases, the pearlite transformation time increases, and a complete pearlite structure is not secured during the cooling process of the wire rod, which results in a problem of increased LP heat treatment time and reduced productivity.
In addition, the wire having improved strength and fatigue limit according to an embodiment of the present invention may satisfy 0.05 at%≦Mo+W≦0.15 at%, where at% means atomic weight%.
If the sum of the atomic percentages of Mo and W is less than 0.05 atomic percent, the strength loss during nitriding is not suppressed, making it difficult to use as a nitrided steel. On the other hand, if the sum of the atomic percentages of Mo and W exceeds 0.15 atomic percent, the carbon equivalent increases, and the pearlite transformation time is delayed, resulting in a problem of reduced productivity.
The reason for controlling the content to atomic % is that Mo and W form carbides in the form of MC (M=Mo or W, C=carbon) and contribute to improving strength, so that Mo and W correspond to the carbides in a 1:1 ratio.

The wire rod according to an embodiment of the present invention can ensure that the pearlite transformation completion time during LP (Lead Patenting) heat treatment is less than 130 seconds. Here, the LP heat treatment process includes heating at 950 to 1100° C. and then quenching to 650 to 750° C. If the pearlite transformation completion time during LP heat treatment exceeds 130 seconds, a problem of reduced productivity occurs.
Furthermore, the wire rod having improved strength and fatigue limit according to an embodiment of the present invention may include a pearlite structure having an area fraction of 80% or more.
In the wire rod having improved strength and fatigue limit according to an embodiment of the present invention, the prior austenite average grain size is preferably 20 μm or less. If the prior austenite average grain size exceeds 20 μm, the time of the LP heat treatment process increases, and the workability of the wire rod deteriorates.

In the wire rod having improved strength and fatigue limit according to an embodiment of the present invention, the percentage of an area of ¹ mm2 at the center of a cross section perpendicular to the longitudinal direction that satisfies one or more of C>0.85%, Si>3.0%, Mn>0.8%, and Cr>2.0% by weight is preferably 10% or less.
If the area ratio exceeds 10%, the quality of the material is degraded, such as the generation of low-temperature structures due to central segregation, and the reduction of area (RA) is degraded after the steel wire is manufactured, which leads to problems of reduced workability and increased frequency of breakage during spring processing. Also, if the area ratio exceeds 10%, there is a risk of the carbide effect being reduced due to the concentration of carbide-forming elements in the central area.
In the wire having improved strength and fatigue limit according to an embodiment of the present invention, carbonitrides having a maximum diameter of 15 μm or more may be distributed at less than 2 particles/ ^cm2 in a cross section parallel to the length direction within a surface depth of 1 mm.
If carbonitrides of 15 μm or more exist on the surface of the wire, there is a risk of fatigue damage occurring in the material. Therefore, it is preferable that carbonitrides of 15 μm or more with a maximum diameter are present in a number of less than 2 pieces/ ^cm2 in a cross section parallel to the longitudinal direction within a surface depth of 1 mm.
The wire rod having improved strength and fatigue limit according to one embodiment of the present invention may have a tensile strength of 1,400 MPa or less and a reduction in area (RA) of 35% or more.

Next, a method for producing a spring wire having improved strength and fatigue limit according to an embodiment of the present invention will be described.
According to one embodiment of the present invention, a method for producing a spring wire having improved strength and fatigue limit comprises the following components by weight: C: 0.6-0.7%, Si: 2.0-2.5%, Mn: 0.2-0.7%, Cr: 0.9-1.5%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.01% or less, Mo: 0.25% or less, W: 0.25% or less, V: 0.05% to 0.2% or less, Nb: 0.05% or less, the remainder being Fe and the like. The method includes the steps of: continuously casting molten steel containing the above-mentioned components and inevitable impurities to prepare a bloom; heating the bloom at a temperature of 1,200°C or higher and then rolling it into a billet; heat-treating the billet at a temperature of 1,030°C or higher and then rolling it into a wire at a temperature of 1,000°C or lower; coiling the rolled wire at a temperature of 800 to 900°C; and cooling the coiled wire at a rate of 0.5 to 2°C/sec.
The reasons for limiting the component ranges of each alloying element are as described above, and each manufacturing step will be described in more detail below.

According to one embodiment of the present invention, the continuous casting step includes soft reduction with a total reduction of 20 mm or more.
Soft reduction refers to a method of casting a slab having an unsolidified layer at the final stage of solidification in a continuous casting machine while gradually reducing it with a collection of reduction rolls at a total reduction amount and reduction speed equivalent to the sum of the solidification shrinkage amount and the thermal shrinkage amount. Here, the total reduction amount is the reduction amount from the start of reduction to the end of reduction. If the total reduction amount is less than 20 mm, it is difficult to ensure the effect of removing segregation by soft reduction, so in order to minimize segregation of the wire rod, it is preferable to control the total reduction amount of soft reduction to 20 mm or more.
According to an embodiment of the present invention, the soft reduction is preferably performed by rolling each roll to 4 mm or less, and when the solidification fraction is 0.6 or more, the cumulative reduction amount is 60% or more. The solidification fraction means the ratio of the weight of the molten steel that has become a solid phase to the weight of the entire molten steel.
If the casting speed is too slow, solidification is completed before soft reduction, and the ratio of the liquid phase to the solid phase is extremely small, making it difficult to ensure the effect of removing segregation by soft reduction. On the other hand, if the casting speed is too fast, the ratio of the liquid phase to the solid phase becomes extremely high, which is undesirable because segregation occurs due to solidification shrinkage. Therefore, when the solidification fraction is 0.6 or more, it is necessary to control the casting speed so that the reduction amount is 60% or more.

The amount of cooling water used is appropriately adjusted so that solidification is completed until the point where soft reduction is completed. Mold-EMS (Mold Electro Magnetic Stirrer) and Strand-EMS can be set according to the conventional spring steel conditions or arbitrarily depending on the equipment.
Unlike ordinary spring wires, spring steel for nitriding treatment contains a large amount of high alloying elements, and therefore, it is necessary to control the carbonitrides inside. Therefore, according to one embodiment of the present invention, the bloom prepared as described above is heated at a temperature of 1,200° C. or more, and then rolled into a billet, thereby minimizing the carbonitrides inside.
The billet may then be heat treated at a temperature of at least 1,030°C and then rolled into wire at a temperature of at most 1,000°C.

If the heat treatment temperature of the billet is less than 1030°C, the V component in the material does not dissolve sufficiently, and the carbide cannot be solid-dissolved, resulting in a problem of reduced softening resistance in the final product. The step of rolling into a wire rod is preferably performed at a temperature of 1000°C or less so that the coiling temperature can be 900°C or less.
The rolled wire can then be coiled at a temperature of 800-900°C.
If the temperature difference between the rolling step and the coiling step is large, there is a risk of F decarburization occurring severely due to localized excessive cooling. In consideration of this, it is preferable that the coiling step of the rolled wire is performed at a temperature of 800 to 900°C.
This allows the wound wire to be cooled at a rate of 0.5 to 2° C./s.

Unlike ordinary spring wires, spring steel for nitriding treatment contains many high alloying elements, so it is necessary to suppress the low-temperature structure. If the wound wire is cooled at a rate of less than 0.5°C/s, decarburization may occur. On the other hand, if the cooling rate exceeds 2°C/s, the low-temperature structure may cause the material to break.
Next, a spring steel wire having improved strength and fatigue limit according to one embodiment of the present invention will be described.

A spring steel wire with improved strength and fatigue limit according to one embodiment of the present invention contains, by weight, C: 0.6-0.7%, Si: 2.0-2.5%, Mn: 0.2-0.7%, Cr: 0.9-1.5%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.01% or less, Mo: 0.25% or less, W: 0.25% or less, V: 0.05% to 0.2% or less, Nb: 0.05% or less, with the remainder being Fe and other unavoidable impurities.
The spring steel wire having improved strength and fatigue limit according to one embodiment of the present invention preferably satisfies Mn+Cr≦1.8%.
The spring steel wire having improved strength and fatigue limit according to an embodiment of the present invention can satisfy 0.05 at %≦Mo+W≦0.15 at %.
The reason for limiting the component range of each alloying element is as described above.

In addition, the spring steel wire having improved strength and fatigue limit according to an embodiment of the present invention may include, in terms of area fraction, 85% or more of a tempered martensite structure and the remaining austenite structure.
In the spring steel wire having improved strength and fatigue limit according to one embodiment of the present invention, the average prior austenite grain size may be 15 μm or less.
In the spring steel wire having improved strength and fatigue limit according to one embodiment of the present invention, it is preferable that the percentage of an area satisfying one or more of C>0.85%, Si>3.0%, Mn>0.8%, and Cr>2.0%, by weight, is 10% or less in a central ^{1 mm2} area of a cross section perpendicular to the longitudinal direction.

If the area ratio exceeds 10%, the quality of the material deteriorates due to the generation of low-temperature structures due to segregation in the center, and the workability decreases, resulting in problems such as an increase in the frequency of breakage during processing of the steel wire into springs. If the area ratio exceeds 10%, the carbide effect decreases due to the concentration of carbide-forming elements in the center.
In the spring steel wire having improved strength and fatigue limit according to an embodiment of the present invention, the number of carbonitrides having a maximum diameter of 15 μm or more in a cross section parallel to the longitudinal direction within a surface depth of 1 mm is preferably less than 2 per 100 mm length.
If carbonitrides of 15 μm or more are present on the surface of the steel wire, there is a risk of fatigue failure in the material. It is preferable that the number of carbonitrides present per 100 mm length is less than 2 in a cross section parallel to the longitudinal direction within a surface depth of 1 mm.

In the spring steel wire having improved strength and fatigue limit according to one embodiment of the present invention, the number of carbides is 10 to 50 per 100 ^μm2 area, the maximum diameter of the carbides is 5 to 50 nm, and the content of V or Nb is 10 at% or more.
In the case of carbides containing V or Nb, when they start to grow to 10 nm or more, they grow containing not only V but also other carbide-forming elements such as Cr and Mo. Therefore, it is necessary to appropriately allocate the carbide-forming elements that are utilized for suppressing the growth of prior austenite grains and for precipitation hardening.
When the number of carbides having a maximum diameter of 5 to 50 nm is less than 10, it is difficult to control the prior austenite grain size. On the other hand, when the number of carbides having a maximum diameter of 5 to 50 nm exceeds 50, the amount of carbides having a diameter of 5 nm or less available for precipitation hardening is reduced, and the tensile strength of the steel wire may be reduced.
The spring steel wire having improved strength and fatigue limit according to one embodiment of the present invention has a tensile strength of 2,100 MPa or more and a reduction in area (RA) of 45% or more.

Next, a method for producing a spring steel wire having improved strength and fatigue limit according to an embodiment of the present invention will be described.
A method for manufacturing a spring steel wire according to an embodiment of the present invention includes the steps of: subjecting a wire rod according to an embodiment of the present invention to an LA heat treatment; subjecting the wire rod to an LP heat treatment; drawing the wire rod to prepare a steel wire; and subjecting the steel wire to a QT heat treatment.

First, the wire according to the embodiment of the present invention is subjected to low temperature annealing (LA) at 650 to 750°C.
Although not limited thereto, the LA heat treatment step is preferably performed within 2 hours since the carbide becomes coarse as the process time increases, making it difficult to control the carbide in the subsequent process. The strength of the wire rod is reduced to 1,200 MPa or less by the LA heat treatment, and the LA heat treatment step can be omitted as necessary.

Thereafter, the wire that has been subjected to the LA heat treatment is pickled and then subjected to an LP (Lead Patenting, LP) heat treatment.
The LP heat treatment may include a first austenitizing step of heating to 950-1100°C within 3 minutes and maintaining the temperature for 3 minutes or less, and a step of passing the first austenitized wire through a lead bath at 650-700°C within 3 minutes.
By performing an austenitizing process in which the material is heated to 950 to 1100°C within 3 minutes and then maintained at that temperature for 3 minutes or less, it is possible to secure an austenitic structure and at the same time further dissolve the carbides that have become coarse in the LA process.

Next, the first austenitized wire is quenched in a lead bath at 650 to 750°C for 3 minutes or less, and isothermally transformed to obtain a pearlite structure. When the temperature of the lead bath is less than 650°C, a low-temperature structure can be formed. On the other hand, when the temperature of the lead bath exceeds 750°C, carbides become coarse and strength decreases.
Thereafter, the LP heat-treated wire is drawn to prepare a steel wire. At this time, the prepared steel wire may have a wire diameter of 5 mm, and in order to ensure that the wire diameter of the steel wire is 2 mm or less, the LP heat treatment is further performed.
Thereafter, the prepared steel wire may be subjected to a QT heat treatment process to ensure a tempered martensite structure.

According to an embodiment of the present invention, the QT heat treatment step may include a second austenitizing step of heating to 900-1000°C within 3 minutes and maintaining the temperature for 3 minutes or less; a first oil quenching step of heating to 450-550°C within 3 minutes and maintaining the temperature for 3 minutes or less; and a second oil quenching step of heating to 70°C or less.
In the QT heat treatment step, the austenitizing temperature can be 900 to 1000° C. so as to maintain the fine carbides precipitated during the LP heat treatment. Although not limited thereto, it is preferable that the austenitizing process in the QT heat treatment step is performed within 6 minutes.
If the tempering temperature in the QT heat treatment step is less than 450°C, the nitriding temperature is low, and further carbide formation cannot be induced, resulting in a problem of reduced toughness. On the other hand, if the tempering temperature in the QT heat treatment step is more than 550°C, sufficient strength cannot be secured.

Next, a spring having improved strength and fatigue limit according to an embodiment of the present invention will be described.
A spring with improved strength and fatigue limit according to one embodiment of the present invention consists, by weight, of C: 0.6-0.7%, Si: 2.0-2.5%, Mn: 0.2-0.7%, Cr: 0.9-1.5%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.01% or less, Mo: 0.25% or less, W: 0.25% or less, V: 0.05% to 0.2% or less, Nb: 0.05% or less, with the remainder being Fe and other unavoidable impurities, and satisfies Mn+Cr≦1.8% and 0.05at%≦Mo+W≦0.15at%.
The reason for limiting the component range of each alloying element is as described above.

The spring according to an embodiment of the present invention has a fatigue limit increased by 10% or more after nitriding. Here, the fatigue limit means a limit at which a spring can withstand repeated loads of 10 million times or more during a fatigue test after designing the spring.
The spring according to an embodiment of the present invention may have a fatigue limit of 700 MPa or more, which is sufficient to withstand repeated stresses of 10 million times.
In the spring according to one embodiment of the present invention, the change in strength before and after nitriding is preferably 15% or less, and the nitriding temperature is preferably 430° C. or more.
A method for manufacturing a spring with improved strength and fatigue limit according to an embodiment of the present invention will now be described.
A method for manufacturing a spring having improved strength and fatigue limit according to an embodiment of the present invention includes the steps of cold forming a steel wire according to an embodiment of the present invention into a spring; subjecting the formed spring to a stress relieving heat treatment; and subjecting the formed spring to a nitriding treatment.

The steel wire according to an embodiment of the present invention can be improved in fatigue limit by nitriding before the shot peening step in the spring manufacturing process. If the nitriding temperature is too low, nitrogen cannot penetrate the surface accurately, while if the nitriding temperature is too high, the core hardness of the material decreases and the desired material strength cannot be ensured. In consideration of this, the nitriding process is preferably performed at a temperature of 420 to 450°C for 10 hours or more.
The present invention will be described in more detail below based on examples. However, the description of such examples is merely for illustrating the implementation of the present invention, and the present invention is not limited by the description of such examples. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred from them.
{Example}

Blooms were produced by continuous casting with a total reduction of 10-25 mm for the various alloy composition ranges shown in Table 1 below. The produced blooms were homogenized at 1,200°C, heat treated at 1,050°C, and then hot rolled to a final wire diameter of 6.5 mm while lowering the temperature to 850°C to produce wire with a final diameter of 6.5 mm. The rolled wire was then coiled at 800-900°C and cooled at a rate of 1°C/s.

Table 2 below shows the at% contents of W+Mo and the total soft reduction amount of the examples and comparative examples. The segregation areas in Table 2 below were derived by analyzing the central ¹ mm2 area of the cross section perpendicular to the length direction of the manufactured wire rod. "C segregation area" in Table 2 means the percentage of the area satisfying C>0.85 wt% in the central 1 ^mm2 area of the cross section perpendicular to the length direction. "Si segregation area" means the percentage of the area satisfying Si>3.0 wt% in the central ^{1 mm2} area of the cross section perpendicular to the length direction. "Mn segregation area" means the percentage of the area satisfying Mn>0.8 wt% in the central 1 ^mm2 area of the cross section perpendicular to the length direction. "Cr segregation area" means the percentage of the area satisfying Cr>2.0 wt% in the central 1 mm2 area of the cross section perpendicular to the length direction. The segregation area was measured using an electron microanalyzer (Electron Probe X-ray Micro Analyzer, EPMA) with a model name of EMPA-1600.

Referring to Table 2 above, Examples 1 and 2 satisfied the alloy composition and total soft reduction amount proposed by the present invention, and as a result, the sum of the C, Si, Mn, and Cr segregation areas was able to be formed at 10% or less. In contrast, in Comparative Example 1, the total soft reduction amount was 10 mm, which is less than 20 mm, and as a result, the sum of the C, Si, Mn, and Cr segregation areas reached 30%.

The tensile strength, area reduction (RA), central low-temperature structure, average prior austenite grain size, pearlite structure, and number of carbonitrides of the manufactured wire rod are shown in Table 3. The average prior austenite grain size, pearlite structure, and number of carbonitrides were measured using a scanning electron microscope (SEM) of model JEOL, JSM-6610LV.
In Table 3 below, "◯" means that the low temperature structure exceeds 20% in terms of area fraction, and "×" means that the low temperature structure is 20% or less in terms of area fraction.
The pearlite structure in Table 3 below means the number of test specimens in which pearlite structure was detected in 80% or more of area fraction when a 3 m wire rod was divided into 8 equal parts to prepare 8 test specimens, and the microstructure of each test specimen was measured in a cross section perpendicular to the length direction.
The number of carbonitrides in Table 3 below is the number of carbonitrides having a maximum diameter of 15 μm or more when a wire rod having a length of 10 cm is divided into 10 equal parts to prepare 10 test pieces having a length of 1 cm, and the microstructure of a cross section parallel to the length direction is measured within a surface depth of 1 mm.

Referring to Table 3, in Examples 1 and 2, low-temperature structures were not formed in the center, and prior austenite was formed with an average grain size of 20 μm or less. In addition, in Examples 1 and 2, of the eight test pieces, six or more test pieces had pearlite structures of 80% or more, and the tensile strength was 1400 MPa or less, which showed excellent workability. In addition, in Examples 1 and 2, no carbonitrides were formed on the surface.
In contrast, Comparative Example 1 had a tensile strength exceeding 1400 MPa, a cross-sectional area reduction rate of 35% or less, deteriorated workability, and a low-temperature structure was formed in the center. In addition, in Comparative Example 1, only five of the eight test pieces had a pearlite structure of 80% or more, and a pearlite structure of 80% or more was not uniformly formed.

In Comparative Example 2, referring to the alloy composition in Table 1, V was added in an amount of less than 0.05%, so that the average prior austenite grain size was coarsened to 24 μm, exceeding 20 μm.
In Comparative Example 3, the tensile strength was 1510 MPa, and the area reduction rate was only 10%, which not only deteriorated the workability but also formed a low-temperature structure in the center. In addition, in Comparative Example 3, only two of the eight test pieces had a pearlite structure of 80% or more, which means that the pearlite structure was not sufficiently formed.
Next, the examples and comparative examples were subjected to LA heat treatment at 720°C for 2 hours, pickled, and then subjected to LP heat treatment. The LP heat treatment was performed by heating to the first austenitizing temperature within 3 minutes, and the remaining time was performed according to the conditions in Table 4 below. Table 4 below also shows the pearlite transformation time of the examples and comparative examples during the LP heat treatment. The pearlite transformation time was measured by deriving a TTT (Time-Temperature-Transformation) curve through a dilatometry experiment.

In Examples 1 and 2, the pearlite transformation times were measured as 110 seconds and 105 seconds, respectively, which were less than 130 seconds, and the productivity was excellent. In contrast, in Comparative Example 3, the pearlite transformation time was measured as 130 seconds, and the productivity was so low that on-site production was difficult.
Next, the LP heat-treated examples and comparative examples were wire-drawn to produce steel wires with a wire diameter of 3 mm. The produced steel wires were subjected to a second austenitization and a first quenching, and then tempered and a second quenching to obtain QT steel wires. The steel wires were heated to the second austenitization temperature within 3 minutes, and the first and second quenching were performed in oil at 60°C. The remaining conditions were as shown in Table 5 below.

Table 6 below shows the tensile strength, area reduction (RA) and number of carbides of the manufactured QT steel wire. Here, the number of carbides means the number of carbides with a maximum diameter of 5-50 nm and a V or Nb content of 10 at% or more in an area of ¹⁰⁰ μm2. The number of carbides is the average value of the measurements taken at eight arbitrary points of an area of ¹⁰⁰ μm2 on the wire surface using a FEI Tecnai OSIRIS transmission electron microscope (TEM).

Referring to Table 6, Examples 1 and 2 have excellent tensile strength of 2200 MPa or more and at the same time, have a cross-sectional area reduction rate of 45% or more. Also, Examples 1 and 2 have 10 to 50 carbides.
In contrast, in Comparative Example 1, the cross-sectional area reduction rate was only 32%, and the number of carbides exceeded 50. In Comparative Example 2, the tensile strength was degraded to 2200 MPa or less, and the number of carbides was less than 10, resulting in a problem of difficulty in controlling the average grain size of prior austenite. In Comparative Example 4, the tensile strength was degraded to 2200 MPa or less, and the number of carbides exceeded 50.
Next, the QT steel wire was cold-formed into a spring shape, and the formed spring was then heat-treated and then nitrided at 420 to 450°C.

Table 7 below shows the occurrence of breakage during the formation of the springs, their fatigue limits, and the fatigue limits after the nitriding treatment.
The fatigue limits before and after the nitriding treatment were measured under the conditions of a stress ratio R (tensile capacity/compressive capacity)=-1 and a test speed of 30 to 60 Hz.
In Table 7 below, "x" means that the spring was not damaged during molding, and "o" means that the spring was damaged during molding.

Examples 1 and 2 were excellent in workability, did not break, and had a fatigue limit of 650 MPa or more before nitriding, indicating that they had excellent fatigue limits. Furthermore, Examples 1 and 2 had a fatigue limit of 750 MPa or more after nitriding, indicating that the fatigue limit increased by 10% or more after nitriding compared to before nitriding, indicating that they had excellent nitriding characteristics.
In contrast, in Comparative Examples 1 and 2, the workability decreased and breakage occurred, and the fatigue limit after the nitriding treatment increased by less than 10% compared to before the nitriding treatment.
In Comparative Example 4, although no breakage occurred during processing of the spring, the fatigue limit after the nitriding treatment did not increase by 10% or more compared to before the nitriding treatment, and the nitriding characteristics were deteriorated.

According to the disclosed embodiment, by optimizing the alloy composition and manufacturing conditions, excellent tensile strength and area reduction rate are ensured while improving nitriding characteristics and fatigue limit, making it applicable to materials such as automobile transmission gears and engine valves.

According to one example of the present invention, it is possible to provide spring wire, steel wire, springs, and manufacturing methods thereof that have improved strength and fatigue limit.

Claims (15)

In weight percent, it is composed of C: 0.6 to 0.7%, Si: 2.0 to 2.5%, Mn: 0.2 to 0.7%, Cr: 0.9 to 1.5%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.01% or less, Mo: 0.25% or less, W: 0.25% or less, V: 0.05% to 0.2% or less, Nb: 0.05% or less, with the remainder being Fe and other unavoidable impurities.
Mn + Cr ≦ 1.8%,
0.05 at%≦Mo+W≦0.15 at% is satisfied;
A spring wire having improved strength and fatigue limit, characterized in that in a central ¹ mm2 area of a cross section perpendicular to the longitudinal direction, the proportion of an area which satisfies one or more of C>0.85%, Si>3.0%, Mn>0.8%, and Cr>2.0%, by weight, is 10% or less. A spring wire rod with improved strength and fatigue limit as described in claim 1, characterized in that it contains 80% or more pearlite structure by area fraction, with the remainder being bainite structure or martensite structure. A spring wire having improved strength and fatigue limit as described in claim 1, characterized in that the average prior austenite grain size is 20 μm or less. 2. The spring wire having improved strength and fatigue limit according to claim 1, characterized in that carbonitrides having a maximum diameter of 15 μm or more are distributed at less than 2 particles/ ^cm2 in a cross section horizontal to the length direction within a surface depth of 1 mm. The spring wire with improved strength and fatigue limit described in claim 1, characterized in that the tensile strength is 1,400 MPa or less and the cross-sectional reduction rate is 35% or more. a step of continuously casting a molten steel containing, by weight, 0.6-0.7% C, 2.0-2.5% Si, 0.2-0.7% Mn, 0.9-1.5% Cr, 0.015% or less P, 0.01% or less S, 0.01% or less Al, 0.01% or less N, 0.01% or less Mo, 0.25% or less W, 0.05%-0.2% or less V, 0.05% or less Nb, the remainder being Fe and other unavoidable impurities, to prepare a bloom;
heating the bloom to a temperature of at least 1,200° C. and then rolling the bloom into a billet;
heat treating the billet at a temperature of 1,030° C. or more and then rolling the billet into a wire at a temperature of 1,000° C. or less;
A method for manufacturing a spring wire having improved strength and fatigue limit, comprising: winding the wire at a temperature of 800 to 900° C.; and cooling the wound wire at a rate of 0.5 to 2° C./s. The method for manufacturing spring wire rod according to claim 6, characterized in that the continuous casting step includes soft reduction with a total reduction of 20 mm or more. The soft reduction is
8. The method for manufacturing a spring wire rod having improved strength and fatigue limit according to claim 7, wherein each rolling roll is rolled to 4 mm or less, and when the solidification fraction is 0.6 or more, the cumulative reduction is 60% or more. In weight percent, it is composed of C: 0.6 to 0.7%, Si: 2.0 to 2.5%, Mn: 0.2 to 0.7%, Cr: 0.9 to 1.5%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.01% or less, Mo: 0.25% or less, W: 0.25% or less, V: 0.05% to 0.2% or less, Nb: 0.05% or less, with the remainder being Fe and other unavoidable impurities.
Mn + Cr ≦ 1.8%,
0.05 at%≦Mo+W≦0.15 at% is satisfied;
A spring steel wire having improved strength and fatigue limit, characterized in that it contains, in terms of area fraction, 85% or more of a tempered martensite structure and the remainder of an austenite structure. A spring steel wire with improved strength and fatigue limit as described in claim 9, characterized in that the average prior austenite grain size is 15 μm or less. 10. A spring steel wire having improved strength and fatigue limit according to claim 9, characterized in that carbonitrides having a maximum diameter of 15 μm or more are distributed at less than 2 particles/ ^cm2 in a cross section horizontal to the length direction within a surface depth of 1 mm. The number of carbides is 10 to 50 per 100 ^μm2 area,
10. The spring steel wire having improved strength and fatigue limit according to claim 9, wherein the carbide has a maximum diameter of 5 to 50 nm and a V or Nb content of 10 at % or more. A spring steel wire with improved strength and fatigue limit as described in claim 9, characterized in that the tensile strength is 2,100 MPa or more and the area reduction rate is 45% or more. LP heat treatment of the wire according to any one of claims 1 to 5;
The LP heat-treated wire is drawn to prepare a steel wire; and the steel wire is subjected to a QT heat treatment.
The LP heat treatment step includes:
A method for producing a spring steel wire having improved strength and fatigue limit, comprising: a first austenitizing step of heating the wire to 950 to 1100°C within 3 minutes and maintaining the temperature for 3 minutes; and a step of passing the first austenitized wire through a lead bath at 650 to 700°C within 3 minutes. In the LP heat treatment step,
15. The method for producing a spring steel wire having improved strength and fatigue limit according to claim 14, characterized in that a pearlite transformation completion time is less than 130 seconds.