JP2006342400A - Steel for high strength spring, and heat treated steel wire for high strength spring - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heat treated steel wire for a spring subjected to cold coiling, in which sufficient atmospheric strength and coiling workability can be made consistent, and having a tensile strength of ≥2,000 MPa, and to provide steel for a spring used for the steel wire. <P>SOLUTION: The heat treated steel sheet for a high strength spring has a composition comprising, by mass, 0.5 to 0.9% C, 1.0 to 3.0% Si, 0.1 to 1.5% Mn, 1.0 to 2.5% Cr, >0.15 to 1.0% V and ≤0.005% Al, and in which the content of N is limited to ≤0.007%, and further comprising one or two kinds selected from 0.001 to <0.01% Nb and 0.001 to <0.005% Ti, and has a tensile strength of ≥2,000 MPa. Further, regarding cementite based spheroidal carbide and alloy based spheroidal carbide occupying in a microscopic cross section, the occupancy area ratio of the ones having a diameter of the equivalent circle of ≥0.2 μm satisfies ≤7%, and the density of the ones having a diameter of the equivalent circle of ≥0.2 μm satisfies ≤1 piece/μm<SP>2</SP>, also, old austenite grain size number is ≥10, and the content of retained austenite is ≤15 mass%. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a spring steel that is cold-coiled and has high strength and high toughness, and a heat-treated steel wire for springs.

  With the reduction in weight and performance of automobiles, springs have also been strengthened, and high-strength steel having a tensile strength exceeding 1500 MPa after heat treatment is used for the springs. In recent years, steel wires having a tensile strength exceeding 1900 MPa are also required. This is to ensure a material hardness that does not hinder the spring even if it is slightly softened by heating, such as strain relief annealing or nitriding during the manufacture of the spring.

  In addition, it is known that the surface layer hardness is increased by nitriding and shot peening, and the durability in spring fatigue is remarkably improved. However, the sag characteristics of the spring are not determined by the surface layer hardness. Strength or hardness is greatly affected. Therefore, it is important to make it a component that can maintain the internal hardness very high.

  As a method, elements such as V, Nb, and Mo are added to form fine carbides that are dissolved by quenching and precipitated by tempering, thereby restricting the movement of dislocations and improving sag resistance. There is an invention (for example, see Patent Document 1).

  On the other hand, in the method of manufacturing a coil spring of steel, the coil is heated and coiled to the austenite region of the steel, and then hot coiling in which quenching and tempering is performed, and high-strength steel wire that has been previously quenched and tempered are coiled cold. There is cold coiling. Cold coiling can use oil tempering or high-frequency treatment, which can be rapidly heated and cooled at the time of steel wire production, making it possible to reduce the prior austenite grain size of the spring material, resulting in fracture characteristics. An excellent spring can be manufactured. In addition, since equipment such as a heating furnace in the spring production line can be simplified, there is an advantage for the spring manufacturer that the equipment cost is reduced, and in recent years, the springs have been coldened. Although the suspension spring uses a thick steel wire as compared with the valve spring, cold coiling is introduced for the above advantages.

  However, when the strength of the steel wire for cold coiling springs is increased, it is often broken during cold coiling and cannot be formed into a spring shape. Until now, since strength and workability are not compatible, it has been necessary to achieve both strength and workability by methods such as heating coiling and quenching and tempering after coiling, which are industrially disadvantageous.

  In addition, when cold-coiling a high-strength heat-treated steel wire and nitriding to ensure strength, it is effective to add a large amount of so-called alloy elements such as V and Nb that precipitate fine carbides in the steel. Has been considered. However, if it is added in a large amount, it cannot be dissolved by heating at the time of quenching, but grows coarsely and becomes a so-called undissolved carbide, which causes breakage during cold coiling. For this reason, there are also technologies that focus on undissolved carbides.

  There is an invention in which performance is improved by controlling not only such alloy elements but also carbides centering on cementite, which are present in large amounts in steel (see, for example, Patent Document 2).

JP-A-57-32353 JP 2002-180198 A

  An object of the present invention is to provide a heat-treated steel wire for springs having a tensile strength of 2000 MPa or more that can be cold-coiled and achieve both sufficient atmospheric strength and coiling workability, and a spring steel for use in the steel wire.

  The inventors have found that by controlling N, which has not been attracting attention until now, the formation of undissolved carbides can be suppressed even when alloying elements are added, and toughness and workability can be ensured. We have developed a heat treated steel wire for springs.

  That is, the gist of the present invention is as follows.

(1) In mass%,
C: 0.5 to 0.9%
Si: 1.0-3.0%
Mn: 0.1 to 1.5%
Cr: 1.0-2.5%
V: more than 0.15 to 1.0% or less Al: 0.005% or less,
N: limited to 0.007% or less,
Further, Nb: containing 0.001 to less than 0.01%, Ti: less than 0.001 to 0.005% of one or two,
A steel for high-strength springs, the balance being iron and inevitable impurities.

(2) Furthermore, in mass%,
W: 0.05-0.5%
Mo: 0.05-0.5%
The steel for high strength springs according to (1), characterized by containing one or two of the following.

(3) Furthermore, in mass%,
Ni: 0.05-3.0%,
Cu: 0.05 to 0.5%
Co: 0.05-3.0%
B: 0.0005 to 0.006%
The steel for high-strength springs according to (1) or (2), comprising one or more of the following.

(4) Furthermore, in mass%,
Te: 0.0002 to 0.01%,
Sb: 0.0002 to 0.01%
Mg: 0.0001 to 0.0005%:
Zr: 0.0001 to 0.0005%
Ca: 0.0002 to 0.01%
Hf: 0.0002 to 0.01%
The steel for high-strength springs according to any one of (1) to (3), comprising one or more of the following.

(5) having the steel component according to any one of (1) to (4) above,
Regarding the cementite-based spherical carbide and the alloy-based spherical carbide having a tensile strength of 2000 MPa or more and occupying the speculum surface,
Occupied area ratio of circle equivalent diameter 0.2μm or more is 7% or less,
The existence density of the circle equivalent diameter of 0.2 μm or more is 1 piece / μm ² or less,
And the prior austenite grain size number is 10 or more, the residual austenite is 15% by mass or less,
A heat-treated steel wire for high-strength springs, characterized in that

  The steel of the present invention has a high strength of 2000 MPa or more by reducing the occupied area ratio, abundance density, austenite grain size, and retained austenite amount of cementite-based and alloy-based spherical carbides in steel wires for cold coiling springs. In addition, it is possible to manufacture a spring having high strength and excellent fracture characteristics while ensuring coiling properties.

  The inventor has invented a steel wire that secures coiling characteristics sufficient for manufacturing a spring by controlling the shape of carbide in the steel by heat treatment while defining chemical components to obtain high strength.

  Details are shown below. First, the reason why the chemical components and component ranges of the high strength spring steel are limited will be described.

C: 0.5 to 0.9%
C is an element having a great influence on the basic strength of the steel material, and is set to 0.5 to 0.9% so that a sufficient strength can be obtained as compared with the conventional steel. If it is less than 0.5%, sufficient strength cannot be obtained. In particular, even when nitriding for improving the spring performance is omitted, 0.5% or more of C is necessary to ensure sufficient spring strength. If it exceeds 0.9%, substantial hypereutectoid precipitation occurs, and a large amount of coarse cementite is precipitated. This simultaneously reduces the coiling characteristics.

  In addition, the relationship with the microstructure is also closely related, and since the number of carbides is less than 0.5%, the area ratio of the area where the carbide distribution is locally smaller than other parts (hereinafter referred to as the carbide dilute area) increases. It is difficult to obtain sufficient strength and toughness or coiling property (ductility). Therefore, it is preferably 0.55% or more, and more preferably 0.6% or more from the viewpoint of balance between strength and coiling.

  On the other hand, when the amount of C is large, alloy-type and cementite-type carbides tend to be difficult to dissolve by heating during quenching. When the heating temperature in heat treatment is high or the heating time is short, strength and coiling There are many cases where sex is insufficient.

  Further, the undissolved carbide also affects the carbide lean region. When C in the steel forms an undissolved carbide, the substantial C in the matrix decreases, so that the carbide lean region area ratio increases as described above. There is also a thing.

  It is known that when the amount of C increases, the martensite form during tempering is a typical lath martensite in medium carbon steel, whereas when the amount of C is large, the form changes to lens martensite. It has been. As a result of research and development, it was found that the carbide distribution in the tempered martensite structure formed by tempering lenticular martensite is lower than that in the case of tempering lath martensite. Therefore, by increasing the amount of C, the carbide lean region may increase due to an increase in lens martensite and undissolved carbide.

  Therefore, preferably 0.7% or less. More preferably, by setting it to 0.65% or less, it is possible to reduce the carbide lean region relatively easily.

Si: 1.0-3.0%
Si is an element necessary for ensuring the strength, hardness and sag resistance of the spring, and when it is small, the necessary strength and sag resistance are insufficient, so 1.0% was made the lower limit. Si also has the effect of spheroidizing and refining the carbide-based precipitates at the grain boundaries, and positively adding it has the effect of reducing the grain boundary occupation area ratio of the grain boundary precipitates. However, adding too much will not only cure the material, but will also embrittle. Therefore, in order to prevent embrittlement after quenching and tempering, the upper limit was made 3.0%.

  Furthermore, Si is an element that contributes to temper softening resistance, and is preferably added in a certain amount in order to produce a high-strength wire. Specifically, it is preferable to add 2% or more. On the other hand, in order to obtain stable coiling properties, the content is preferably 2.6% or less.

Mn: 0.1 to 1.5%
Mn is frequently used because it deoxidizes and fixes S in steel as MnS, and also enhances hardenability and sufficiently obtains the hardness after heat treatment. In order to ensure this stability, 0.1% is made the lower limit. In order to prevent embrittlement due to Mn, the upper limit was made 2.0%. Further, in order to achieve both strength and coiling properties, 0.3 to 1% is preferable. In the case where priority is given to coiling, it is effective to make it 1.0% or less.

Cr: 1.0-2.5%
Cr is an effective element for improving hardenability and temper softening resistance. Furthermore, it is an element effective not only for ensuring the tempering hardness but also for increasing the surface hardness after nitriding and the depth of the hardened layer in the nitriding treatment as found in recent high-strength valve springs. However, if the amount added is large, not only the cost is increased, but cementite that is found after quenching and tempering is coarsened. It also has the effect of stabilizing and coarsening the alloy carbide. As a result, since the wire becomes brittle, there is also an adverse effect that breakage easily occurs during coiling. Therefore, when Cr is added, the effect is not clear unless it is 0.1% or more. Further, the upper limit is set to 2.5% at which embrittlement becomes remarkable.

  However, in the present invention, carbide is finely controlled by defining N, so that a large amount of Cr can be added, so the addition amount is set to easily obtain high strength.

  When nitriding is performed, the hardened layer by nitriding can be deepened by adding Cr. Therefore, addition of 1.1% or more is preferable, and addition of 1.2% or more is preferable in order to make it suitable for nitriding for a high-strength spring which is not conventional.

  Since Cr inhibits the dissolution of cementite by heating, especially when C> 0.55% and the amount of C increases, suppressing the amount of Cr can suppress the formation of coarse carbides, and it is easy to achieve both strength and coiling properties. Therefore, it is preferable that the amount added is 2.0% or less. More preferably, it is about 1.7% or less.

V: More than 0.15 to 1.0%
V can be used for hardening of the steel wire at the tempering temperature or hardening of the surface layer at the time of nitriding because of secondary precipitation hardening in which carbides are precipitated and hardened during tempering. Further, it is effective in suppressing the coarsening of the austenite grain size due to the formation of nitrides, carbides, and carbonitrides, and it is preferable to add them. However, until now, nitrides, carbides and carbonitrides of V have been generated even at austenitizing temperature A3 or higher of steel, so if the solid solution is insufficient, they remain as undissolved carbides (nitrides). It was easy to do. This undissolved carbide not only causes breakage during spring coiling, but also consumes “V”, reducing the effect of improving the temper softening resistance and secondary precipitation hardening due to the added V. Will reduce the performance. Therefore, it has been considered preferable to be 0.15% or less industrially.

  However, in the present invention, by controlling the amount of N, the formation of V-based nitrides, carbides and carbonitrides at the austenitizing temperature A3 or higher can be suppressed, so that a large amount of V can be added accordingly. Thus, the amount of V added was set to exceed 0.15% and not more than 1.0%. If the added amount is 0.15% or less, the effect of adding V, such as improving the hardness of the nitrided layer and increasing the depth of the nitrided layer, is small, and sufficient fatigue durability over conventional steel cannot be ensured. Moreover, when the added amount exceeds 1.0%, coarse undissolved inclusions are generated, and the toughness is reduced. Like Mo, an overcooled structure is likely to be formed, which causes breakage at the time of cracking or wire drawing. Cheap. For this reason, the upper limit was set to 1.0%, which is industrially stable and easy to handle.

  Since nitrides, carbides, and carbonitrides of V are generated even at an austenitizing temperature A3 or higher of the steel, they tend to remain as undissolved carbides (nitrides) when the solid solution is insufficient. Therefore, considering the current industrial nitrogen amount control capability, it is industrially preferably 0.5% or less, and more preferably 0.4% or less.

  On the other hand, since the surface hardening treatment by nitriding is most heated to a temperature of 300 ° C. or higher, it is necessary to add more than 0.15% in order to suppress hardening of the outermost layer and softening of internal hardness by nitriding, Is preferably added at 0.2% or more.

Al: 0.005% or less Al is a deoxidizing element and affects oxide formation. In particular, in a high-strength valve spring, a hard oxide centering on Al ₂ O ₃ tends to be a starting point of fracture, so this must be avoided. For that purpose, it is important to strictly control the amount of Al. In particular, when the tensile strength of the heat-treated steel wire exceeds 2100 MPa, strict control of the oxide-forming element is essential in order to reduce the fatigue strength variation.

In the present invention, Al is defined as 0.005% or less. This is because if it exceeds 0.005%, an oxide mainly composed of Al ₂ O ₃ is likely to be formed, so that breakage caused by the oxide occurs and sufficient fatigue strength and quality stability cannot be ensured. Further, when a high fatigue strength is required, the content is preferably 0.003% or less.

N: Limiting to 0.007% or less In the present invention, control of N is a major point. In the present invention, a strict limit value is defined as N ≦ 0.007%. This is because the role of N is newly noticed in the spring steel, and the effect of N control and the reason for the definition in the present invention will be described below.

  In steel, the influence of N is (1) exists as solid solution N in ferrite, and hardens ferrite by suppressing the movement of dislocation in ferrite. (2) Ti, Nb, V, Al, B, etc. Alloy elements and nitrides are produced, affecting steel performance. The mechanism will be described later. (3) It affects the precipitation behavior of iron-based carbides such as cementite and affects the steel material performance.

  In spring steel, since the strength is secured by alloy elements such as C, Si, and V, the effect of hardening solute N is not large. On the other hand, when cold working (coiling) of the spring is considered, the movement of the dislocation is suppressed, so that deformation of the processed part is suppressed and the processed part becomes brittle. Let

  Further, among the specified elements in claim 1, V generates precipitates at a high temperature in the steel. The chemical component is mainly nitride at high temperature, and changes its form into carbonitride and carbide with cooling. Therefore, the nitride formed at high temperature tends to be a precipitation nucleus of V carbide. This tends to generate undissolved carbide during heating in the patenting or quenching process, and further, because it becomes a nucleus, it is easy to grow its size.

Further, from the viewpoint of cementite, the tempering temperature is tempered at 300 to 500 ° C. from the required strength in the high-strength spring like this time. In spring steel, iron-based carbides produced during tempering due to its characteristic component system change in a complex manner to ε-carbides and θ-carbides (so-called cementite Fe ₃ C). Therefore, it affects the mechanical properties such as ductility of steel. N also affects the carbide formation, and the smaller the amount of N, the better the ductility and toughness at 350 to 500 ° C.

  In the present invention, the amount of N is limited to N ≦ 0.007% in order to reduce the harmfulness of such N. Further, as will be described later, any one or two of Ti and Nb are added in a small amount.

  Originally, if the N amount can be suppressed to 0.003% or less, good performance can be obtained without adding any one or two of Ti and Nb, but industrially stable to 0.003% or less. This is disadvantageous in terms of manufacturing costs. Therefore, a trace amount of either one or two of Ti and Nb is added.

  When Ti or Nb is added, these elements generate nitrides at a high temperature, so that the substantial solute nitrogen is reduced. Therefore, the same effect as that obtained by reducing the N addition amount can be obtained. Therefore, you may increase the upper limit of the addition amount of N amount. However, if the amount of N exceeds 0.007%, the amount of nitrides of V, Nb or Ti increases, resulting in an increase in undissolved carbides and an increase in hard inclusions such as TiN. Since the fatigue endurance characteristics and coiling characteristics are reduced, the upper limit of the N amount is limited to 0.007%.

  That is, even when either one or two of Ti and Nb are added, too much N amount or too much Ti or Nb will produce Ti or Nb nitrides and become harmful. Therefore, the addition amount of Ti or Nb needs to be made very small.

  Therefore, the upper limit of the N amount is preferably 0.005% or less, and more preferably 0.004% or less.

  Such precise N control suppresses the embrittlement of ferrite and suppresses the formation and growth of undissolved carbides by suppressing the formation of V-based nitrides. Moreover, toughness can be improved by controlling the form of the iron-based carbide.

  That is, when N exceeds 0.007%, V-based nitrides are easily generated, a large amount of undissolved carbides are generated, and the steel becomes brittle depending on the form of ferrite and carbides.

  Even when Ti or Nb is added in this way, it is preferably 0.005% or less in view of the ease of heat treatment and the like. In addition, although the lower limit of the N amount is preferably smaller, 0.0015% or more is preferable in consideration of the manufacturing cost and the ease in the denitrification step because it is likely to be mixed from the atmosphere such as a steelmaking step. .

Nb: 0.001 to less than 0.01% Nb generates nitrides, carbides, and carbonitrides, and nitrides are generated at a higher temperature than V. Therefore, N in the steel is consumed by generating Nb nitride during cooling, and V-based nitride generation can be suppressed. As a result, since generation of V-based undissolved carbide can be suppressed, temper softening resistance and workability can be ensured.

  Further, it can be used for hardening of the steel wire at the tempering temperature and hardening of the surface layer during nitriding in addition to suppressing the coarsening of the austenite grain size by the Nb-based carbonitride. However, if the amount added is too large, undissolved carbides with Nb-based nitrides as the core tend to remain, so a large amount should be avoided. Specifically, when the Nb addition amount is less than 0.001%, the added effect is hardly recognized. On the other hand, if it is 0.01% or more, the addition of a large amount generates coarse undissolved inclusions and lowers the toughness, and like Mo, it tends to cause a supercooled structure and easily causes breakage during cracking or wire drawing. Therefore, it was made less than 0.01% which is easy to handle industrially and stably.

  FIG. 1 shows the results of measuring the impact values of the materials having chemical components shown in Table 1, and shows the results of measuring the impact values of samples A and B heat-treated by the method of Examples described later. As shown in FIG. 1, it can be seen that the steel with Nb added slightly to control N has a higher impact value overall.

Ti: 0.001 to less than 0.005% In the present invention, when Ti is added, the addition amount is 0.001% or more and less than 0.005%. Since Ti is a deoxidizing element and a nitride and sulfide-forming element, it affects the generation of oxides, nitrides and sulfides. Accordingly, since a large amount of addition tends to generate hard oxides and nitrides, if added inadvertently, hard carbides are generated and fatigue durability is reduced. Like Al, especially in high-strength springs, the stability of variation in fatigue strength is lowered than the fatigue limit of the spring itself, and if the Ti content is large, the incidence of fracture due to inclusions increases, so it is necessary to control the amount. And less than 0.005%.

  On the other hand, since Ti produces TiN at a high temperature in the molten steel, sol. N works to reduce N. In the present invention, by limiting N, it is a technical point to suppress the generation of V-based nitrides and further suppress the growth of V-based undissolved carbides. Therefore, if N is consumed at a temperature equal to or higher than the V-based nitride formation temperature, the growth of the V-based nitride and the V-based carbonitride that grows at the time of cooling with the core can be suppressed. That is, by adding Ti, the amount of N that substantially binds to V is reduced, so that the temperature at which V-based nitrides are formed can be lowered, and further V-based undissolved carbides can be suppressed.

Therefore, the addition of a large amount of Ti should be avoided from the viewpoint of Ti-based undissolved carbonitride and oxide formation, but a small amount of addition can lower the V-based nitride formation temperature, rather reducing undissolved carbide. it can. The amount added is 0.001% or more, and if it is less than 0.001%, there is no effect of N consumption, there is no V-based undissolved carbide suppressing effect, and no workability improving effect is seen. However, the amount of Ti added is preferably 0.003% or less.
The steel of the present invention has the above-described components as basic components, and can further contain components for improving the properties of the steel. That is, one or two of W and Mo are further added to enhance the temper softening resistance.

W: 0.05-0.5%
W not only improves hardenability, but also generates carbides in the steel and increases strength, and is effective for imparting temper softening resistance. Therefore, it is preferable to add as much as possible.

  W produces carbides at a lower temperature than Ti, Nb, etc., and thus hardly produces undissolved carbides.

  Moreover, temper softening resistance can be provided by precipitation hardening. That is, the internal hardness is not greatly reduced even in nitriding or strain relief annealing.

  If the addition amount is 0.05% or less, no effect is seen. If the addition amount is 0.5% or more, coarse carbides are formed. On the other hand, mechanical properties such as ductility may be impaired. 0.5%. Furthermore, if considering the ease of heat treatment, 0.1 to 0.4% is preferable. In particular, addition of 0.15% or more is more preferable in order to obtain maximum temper softening resistance while avoiding adverse effects such as a supercooled structure immediately after rolling.

Mo: 0.05-0.5%
Mo improves hardenability and precipitates as carbides at a temperature about the tempering or nitriding temperature, so that it can provide temper softening resistance. Therefore, even after tempering at a high temperature or heat treatment such as strain relief annealing or nitriding that is performed in the process, high strength can be exhibited without being softened. This suppresses a decrease in the internal hardness of the spring after nitriding and facilitates hot setting and strain relief annealing, so that the fatigue characteristics of the final spring are improved. That is, the tempering temperature when controlling the strength can be increased. This increase in the tempering temperature is advantageous in reducing the grain boundary occupation area ratio of the grain boundary carbide. That is, the grain boundary carbide precipitated in a film shape is spheroidized by tempering at a high temperature, and the grain boundary area ratio is reduced. Mo produces Mo-based carbides separately from cementite in steel. In particular, since the precipitation temperature is lower than V or the like, there is an effect of suppressing the coarsening of the carbide. If the amount added is 0.05% or less, no effect is observed. However, if the addition amount is large, an overcooled structure is likely to be generated by softening heat treatment before rolling or wire drawing, and it is liable to cause breakage or breakage during wire drawing. That is, at the time of wire drawing, it is preferable to draw the steel material in advance after forming a ferrite-pearlite structure by a patenting treatment.

  However, Mo is an element that greatly imparts hardenability, so if the addition amount increases, the time until the end of the pearlite transformation becomes longer, and a supercooled structure is likely to occur during cooling after rolling or in the patenting process, and disconnection occurs during wire drawing. If it does not cause or break, and exists as an internal crack, the properties of the final product are greatly degraded. If Mo exceeds 0.5%, the hardenability increases and it becomes difficult to make a ferrite-pearlite structure industrially, so this is the upper limit. In order to suppress the production of a martensite structure that lowers manufacturability in a manufacturing process such as rolling or wire drawing, and to facilitate rolling and wire drawing stably industrially, it is preferably 0.4% or less. More preferably, it is about 0.2%.

Furthermore, when W and Mo are compared with V, Nb, and Ti, which also have the effect of strengthening the temper softening resistance, V, Nb, and Ti generate nitrides as described above, and it is easy to grow carbide using them as nuclei. On the other hand, since W and Mo hardly generate nitrides, they can be added without being affected by the amount of N to enhance the softening resistance. That is, although the softening resistance can be strengthened even with V, Nb, and Ti, the addition amount is naturally limited in order to strengthen the softening resistance while avoiding undissolved carbides. Therefore, it does not generate undissolved carbides, and when higher softening resistance is required, it does not generate nitrides, precipitates carbides at a relatively low temperature, and the addition of W or Mo that functions as a precipitation strengthening element is extremely It is valid.
In addition, when one or more of Ni, Cu, Co, and B are used to achieve both strength and workability, matrix reinforcement is required when the optimum balance between softening resistance and workability by carbide control cannot be obtained. To ensure strength.

Ni: 0.05-3.0%
Ni improves the hardenability and can increase the strength stably by heat treatment. In addition, the ductility of the matrix is improved to improve the coilability. However, quenching and tempering increase the retained austenite, which is inferior in terms of sag and material uniformity after spring forming. If the added amount is 0.05% or less, no effect is observed in increasing strength and improving ductility. On the other hand, the addition of a large amount of Ni is not preferable, and if it is 3.0% or more, the adverse effect of increasing the retained austenite becomes remarkable, and the effect of improving hardenability and ductility is saturated, which is disadvantageous in terms of cost and the like.

Cu: 0.05 to 0.5%
About Cu, decarburization can also be prevented by adding Cu. In order to reduce the fatigue life of the decarburized layer after spring processing, efforts have been made to reduce it as much as possible. When the decarburized layer becomes deep, the surface layer is removed by a peeling process called peeling. Moreover, it has the effect of improving corrosion resistance like Ni. By suppressing the decarburized layer, the fatigue life of the spring and the peeling process can be omitted. The decarburization suppressing effect and corrosion resistance improving effect of Cu can be exhibited at 0.05% or more, and even if Ni is added as described later, if it exceeds 0.5%, it tends to cause rolling flaws due to embrittlement. Therefore, the lower limit is set to 0.05% and the upper limit is set to 0.5%. Although the mechanical properties at room temperature are hardly impaired by the addition of Cu, when adding Cu exceeding 0.3%, the billet surface may be cracked during rolling in order to deteriorate the hot ductility. Therefore, it is preferable that the amount of Ni added to prevent cracking during rolling is [Cu%] <[Ni%] according to the amount of Cu added. In the range of Cu 0.3% or less, no rolling flaws occur, so there is no need to regulate the amount of Ni added for the purpose of preventing rolling flaws.

Co: 0.05-3.0%
Although Co may reduce the hardenability, it can improve the high temperature strength. Moreover, in order to inhibit the production | generation of a carbide | carbonized_material, it has the effect | action which suppresses the production | generation of the coarse carbide | carbonized_material which becomes a problem by this invention. Therefore, the coarsening of the carbide containing cementite can be suppressed. Therefore, it is preferable to add. When added, the effect is small at 0.05% or less. However, if added in a large amount, the hardness of the ferrite phase increases and the ductility is lowered, so the upper limit was made 3.0%. Industrially, stable performance can be obtained at 0.5% or less.

B: 0.0005 to 0.006%
B is effective for cleaning hardenability improving elements and austenite grain boundaries. By adding B, elements such as P and S that segregate at the grain boundaries to reduce toughness, and destructive properties are improved. At that time, if B is combined with N to generate BN, the effect is lost. The lower limit of the amount added is 0.0005% at which the effect becomes clear, and the upper limit is 0.0060% at which the effect is saturated. However, since even a slight amount of BN is embrittled, it is necessary to give sufficient consideration not to generate BN. Therefore, it is preferably 0.003 or less, and more preferably, it is effective to fix free N with a nitride-forming element such as Ti or Nb to obtain B: 0.0010 to 0.0020%.

  These Ni, Cu, Co and B are mainly effective for strengthening the ferrite phase of the matrix. In order to achieve both strength and workability, it is an effective element for securing strength by matrix strengthening when the optimum balance between softening resistance and workability by carbide control cannot be obtained.

  Furthermore, one or more of Te, Sb, Mg, Zr, Ca, and Hf is an element that controls the form of oxides and sulfides when further higher performance and stability of performance are required. Add as

Te: 0.0002 to 0.01%
Te has the effect of spheroidizing MnS. If it is less than 0.0002%, the effect is not clear, and if it exceeds 0.01%, the toughness of the matrix is reduced, hot cracking occurs, and the adverse effect of reducing fatigue durability becomes significant. The upper limit is 0.01%.

Sb: 0.0002 to 0.01%
Sb has the effect of spheroidizing MnS, and if it is less than 0.0002%, the effect is not clear. If it exceeds 0.01%, the toughness of the matrix is reduced, hot cracking occurs, and fatigue durability is increased. Since the harmful effect of lowering becomes significant, the upper limit is made 0.01%.

Mg: 0.0001 to 0.0005%
Mg forms an oxide in the molten steel having a temperature higher than the MnS formation temperature, and already exists in the molten steel when MnS is formed. Therefore, it can be used as MnS precipitation nuclei, whereby the distribution of MnS can be controlled. Also, the number distribution of Mg-based oxides is more finely dispersed in molten steel than Si and Al-based oxides often found in conventional steels. Therefore, MnS with Mg-based oxides as the core must be finely dispersed in steel. It becomes. Therefore, even if the S content is the same, the MnS distribution differs depending on the presence or absence of Mg, and the addition of them makes the MnS particle size finer. The effect is sufficiently obtained even in a minute amount, and if Mg is added, MnS is refined. However, if it exceeds 0.0005%, hard oxides are easily generated, and sulfides such as MgS also start to be generated, leading to a decrease in fatigue strength and a decrease in coiling properties. Therefore, the amount of Mg added is set to 0.0001 to 0.0005%. When used for a high-strength spring, the content is preferably 0.0003% or less. Although these elements are trace amounts, about 0.0001% can be added by using a large amount of Mg-based refractories. Moreover, the amount of Mg added can be controlled by carefully selecting an auxiliary material and using an auxiliary material having a low Mg content.

Zr: 0.0001 to 0.0005%
Zr is an oxide and sulfide generating element. In spring steel, oxides are finely dispersed, so that it becomes MnS precipitation nuclei as in Mg. As a result, the fatigue durability is improved and the coilability is improved by increasing the ductility. If less than 0.0001%, the effect is not seen, and even if added over 0.0005%, the formation of hard oxide is promoted. Become. In addition, addition of a large amount generates nitrides and sulfides such as ZrN and ZrS in addition to oxides, and decreases manufacturing troubles and fatigue durability characteristics of the spring. Further, when used for a high-strength spring, the amount added is preferably 0.0003% or less. Although these elements are in trace amounts, they can be controlled by carefully selecting auxiliary materials and precisely controlling refractories.

  For example, when Zr refractories are frequently used in places such as ladle, tundish, nozzle, etc. where they are in contact with molten steel for a long time, about 1 ppm can be added to about 200 t of molten steel. Furthermore, it is only necessary to add auxiliary materials so as not to exceed the specified range while taking this into consideration.

  The analysis method of Zr in steel can be measured by ICP after collecting 2 g from a portion not affected by the surface scale of the steel material to be measured and processing the sample in the same manner as in JIS G 1237-1997 Annex 3. At this time, the calibration curve in ICP is set so as to be suitable for a very small amount of Zr.

Ca: 0.0002 to 0.01%
Ca is an oxide and sulfide-forming element. In spring steel, by making MnS spherical, the length of MnS as a fracture starting point such as fatigue can be suppressed and rendered harmless. The effect is not clear if it is less than 0.0002%, and even if added over 0.01%, not only the yield is bad, but also sulfides such as oxides and CaS are produced, manufacturing troubles and springs The fatigue endurance characteristics are deteriorated, so the content is made 0.01% or less. This addition amount is preferably 0.001% or less.

Hf: 0.0002 to 0.01%
Hf is an oxide-generating element and serves as a precipitation nucleus of MnS. Therefore, Zr is an oxide and sulfide generating element by fine dispersion. In spring steel, oxides are finely dispersed, so that it becomes MnS precipitation nuclei as in Mg. As a result, the fatigue durability is improved and the coilability is improved by increasing the ductility. The effect is not clear if it is less than 0.0002%, and even if added over 0.01%, not only the yield is low, but also oxides, nitrides such as ZrN, ZrS, and sulfides are produced, and in production. Of 0.01% or less, because the trouble and the fatigue endurance characteristics of the spring are reduced. This addition amount is preferably 0.003% or less.

  Below, the preferable content range of another component is demonstrated.

P: 0.015% or less Although P and S are not included in the claims, restrictions are necessary. P hardens the steel but further segregates and embrittles the material. In particular, P segregated at the austenite grain boundaries causes a delayed fracture or the like due to a drop in impact value or hydrogen penetration. Therefore, it is better to have less. Therefore, P: 0.015% or less is preferable because the embrittlement tendency becomes significant. Furthermore, when the tensile strength of the heat-treated steel wire is high such that it exceeds 2150 MPa, it is preferably less than 0.01%.

S: 0.015% or less If S is present in the steel as in the case of P, the steel is embrittled. Although the effect is reduced as much as possible by Mn, since MnS also takes the form of inclusions, the fracture characteristics are lowered. Particularly in the case of high-strength steel, it may break down from a small amount of MnS, and it is desirable to reduce S as much as possible. It is preferable to set it as 0.015% or less from which the bad influence becomes remarkable.

  Furthermore, when the tensile strength of the heat-treated steel wire is high such that it exceeds 2150 MPa, the content is preferably less than 0.01%.

t-O: 0.0002 to 0.01
In the steel, there are oxides and solid solution O generated through the deoxidation step. However, when this total oxygen amount (t-O) is large, it means that there are many oxide inclusions. If the size of the oxide inclusions is small, the spring performance is not affected, but if a large amount of large oxide is present, the spring performance is greatly affected.

  If the oxygen content exceeds 0.01%, the spring performance is remarkably lowered, so the upper limit is preferably made 0.01%. Further, it is sufficient if the amount of oxygen is small, but even if it is less than 0.0002%, the effect is saturated, so it is preferable to set this as the lower limit.

  In consideration of the ease of practical deoxidation process and the like, it is desirable to adjust to 0.0005 to 0.005%.

Tensile strength 2000 MPa or more If the tensile strength is high, the fatigue characteristics of the spring tend to be improved. Even when a surface hardening treatment such as nitriding is performed, higher fatigue characteristics and sag characteristics can be obtained if the basic strength of the steel wire is high. On the other hand, if the strength is high, the coiling property is lowered, and the spring manufacturing becomes difficult. Therefore, it is important not only to improve the strength but also to impart ductility that can be coiled at the same time.

  The strength of the steel wire is required from the viewpoints of fatigue, sag, etc., but the tensile strength TS ≧ 2000 MPa is the lower limit. Further, when applied to a high-strength spring, higher strength is desirable, preferably 2200 MPa or more, and for application to a high-strength spring, it is preferable to increase the strength within a range that does not impair the coiling property. .

Undissolved carbide C and other so-called alloy elements such as MnTi, V, and Nb are added to obtain high strength, but when a large amount of elements that form nitrides, carbides, and carbonitrides are added, they are not dissolved. Carbide tends to remain. Undissolved carbide is generally spherical, and is mainly composed of alloy elements and cementite.

  FIG. 2 shows a typical observation example. 2A is an example of observation of undissolved carbide by a scanning electron microscope, FIG. 2B is an example of elemental analysis by X-ray of alloy-based insoluble carbide X, and FIG. 2C is an example of cementite-based insoluble carbide Y. An example of elemental analysis by X-ray is shown. According to this, the steel has two types of matrix needle-like structure and spherical structure. In general, it is known that steel forms a martensitic needle-like structure by quenching and generates carbides by tempering to achieve both strength and toughness. However, in the present invention, as shown in FIGS. 2 (a) X and Y, not only the needle-like structure but also a lot of spherical structures may remain. This spherical structure is undissolved carbide, and its distribution greatly affects the performance of the spring steel wire. Therefore, the undissolved carbide here means not only the so-called alloy-based spherical carbide (X) in which the above alloy forms nitrides, carbides, carbonitrides, but also cementite-based spherical carbides mainly composed of Fe carbide (cementite). (Y) is included.

  FIGS. 2B and 2C show examples of analysis by EDX attached to the SEM. The conventional invention focuses only on carbides of alloy elements such as V and Nb, an example of which is shown in FIG. 2B, in which the Fe peak is relatively small in the carbide, and the alloy peak (in this example, V) is large. Strictly speaking, many of these alloy-based carbides (X) become composite carbides with nitrides (so-called carbonitrides), so here, these alloy-based carbides and nitrides and their composite alloy-based spherical precipitates are used. Collectively referred to as alloy-based spherical carbide.

In the present invention, not only conventional alloy element-based spherical carbides but also precipitation of so-called cementite-based carbides in which Fe ₃ C having an equivalent circle diameter of 3 μm or less and an alloy element are slightly dissolved as shown in FIG. I found that the form was also important. When achieving both high strength and workability higher than those of conventional steel wires as in the present invention, if there are many cementite-based spherical carbides of 3 μm or less, the workability is greatly impaired. Hereinafter, such a spherical carbide having Fe and C as main components as shown in FIG. 2C will be referred to as a cementite-based spherical carbide.

  Note that these results can be obtained by the replica method using a transmission electron microscope.

  These spherical carbides are considered to be carbides that are not sufficiently dissolved in quenching and tempering by oil tempering or induction treatment, and are spheroidized and grown or reduced in the quenching and tempering step. Carbide of this size does not contribute to the strength and toughness due to quenching and tempering, but rather deteriorates. That is, the addition of C, which should be a source of strength by fixing C in the steel, is simply wasted and becomes a stress concentration source by further coarsening, thus lowering the mechanical properties of the steel wire.

  Therefore, the following regulations are important in order to eliminate the adverse effects caused by the following provisions regarding the alloy-based spherical carbide and cementite-based spherical carbide occupying the mirror surface.

Occupied area ratio of circle equivalent diameter 0.2μm or more is 7% or less,
The existence density of the circle equivalent diameter of 0.2 μm or more is 1 piece / μm ² or less,
When steel is cold-coiled after quenching and tempering, undissolved spherical carbide affects its coiling characteristics, that is, bending characteristics up to fracture. In order to obtain high strength, it has been common to add not only C but also a large amount of alloy elements such as Cr and V. However, the strength is too high and the deformability is insufficient to deteriorate the coiling characteristics. There was a harmful effect. The cause may be coarse carbides precipitated in the steel.

  These steel-in-steel and cementite carbides can be observed by subjecting a mirror-polished sample to etching such as picral or electrolytic etching, but the detailed observation and evaluation of the dimensions and the like is 3000 times using a scanning electron microscope. It is necessary to observe at the above high magnification, and the alloy-based spherical carbide and cementite-based spherical carbide used here have an equivalent circle diameter of 0.2 μm or more. Normally, carbide in steel is indispensable for securing the strength and resistance to temper softening of steel, but its effective particle size is 0.1 μm or less, and conversely, if it exceeds 1 μm, the strength and austenite particle size are reduced. Does not contribute, but merely deteriorates the deformation characteristics. However, this importance is not recognized so much in the prior art, and attention is paid only to alloy type carbides such as V and Nb, and carbides having an equivalent circle diameter of 3 μm or less, particularly cementite type spherical carbides, are considered to be harmless.

  These alloy-based and cementite-based carbides are electrolytically etched into a mirror-polished sample, and 10 or more fields are observed at 10,000 times with a scanning electron microscope. If the area ratio of spherical carbide exceeds 7%, the workability is extremely poor. Therefore, this is the upper limit.

In addition, in the case of an alloy system and cementite-based spherical carbide having an equivalent circle diameter of 0.2 μm or more, which is a subject of the present invention, not only the size but also the number is a major factor. Therefore, the scope of the present invention is defined in consideration of both. In other words, the number of spherical carbides having an equivalent circle diameter of 0.2 μm or more is very large, and if the density of presence on the microscopic surface exceeds 1 piece / μm ² , the coiling characteristics deteriorate significantly, so this is the upper limit.

  Since the influence of a dimension will become large when the dimension of a carbide | carbonized_material exceeds 3 micrometers, it is desirable that there is no thing exceeding this.

Old austenite grain size number 10 or more In steel wires based on a tempered martensite structure, the prior austenite grain size has a great influence on the basic properties of steel wires along with carbides. That is, the smaller the prior austenite grain size, the better the fatigue characteristics and coiling properties. However, even if the austenite grain size is small, the effect is small if the carbide is contained more than specified. In general, to reduce the austenite particle size, it is effective to lower the heating temperature during quenching, but this increases the undissolved spherical carbide. Therefore, it is important to finish the steel wire with a good balance between the carbide content and the prior austenite grain size. Here, when the carbide satisfies the above-mentioned regulations, if the prior austenite grain size number is less than 10, sufficient fatigue properties and coiling properties cannot be obtained, so the previous austenite grain size number is defined as 10 or more.

  Further, for application to a high-strength spring, finer particles are more preferable. By setting the number 11 or even 12 or more, both high strength and coiling can be achieved.

Residual austenite is 15 mass% or less. Residual austenite often remains in the vicinity of segregated portions, old austenite grain boundaries, and regions sandwiched between subgrains. Residual austenite becomes martensite due to work-induced transformation, and when it is transformed during spring forming, a local high hardness portion is generated in the material, and rather, the coiling characteristics as a spring are lowered. Also, recent springs will try to strengthen the surface by plastic deformation such as shot peening and setting. However, if there is a manufacturing process that includes multiple processes that apply plastic deformation in this way, the work-induced martensite that occurs at an early stage will cause fracture strain. Reduce the workability and the breaking characteristics of the spring in use. In addition, even when industrially unavoidable deformations such as cracks are introduced, they are easily broken during coiling.

  Furthermore, in the heat treatment such as nitriding and strain relief annealing, the mechanical properties are changed by gradually decomposing, resulting in problems such as lowering the strength and lowering the coiling property.

  Therefore, the workability is improved by reducing the retained austenite as much as possible and suppressing the formation of work-induced martensite. Specifically, when the amount of retained austenite exceeds 15% (mass%), the sensitivity to hammering and the like becomes high and breaks easily in coiling and other handling, so the amount is limited to 15% or less.

  The amount of retained austenite varies depending on the amount of alloying elements such as C and Mn added and the heat treatment conditions. Therefore, it is important to enhance not only the component design but also the heat treatment conditions.

  When the martensite generation temperature (start temperature Ms point, end temperature Mf point) is low, martensite is not generated and retained austenite is likely to remain unless the temperature is sufficiently low during quenching. Water or oil is used in industrial quenching, but the suppression of retained austenite requires advanced heat treatment control. Specifically, it is necessary to maintain the cooling refrigerant at a low temperature, maintain a low temperature as much as possible after cooling, and ensure a long transformation time to martensite. Since it is processed in a continuous line industrially, the temperature of the cooling refrigerant easily rises to near 100 ° C., but is preferably maintained at 60 ° C. or lower, and more preferably at 40 ° C. or lower. Furthermore, in order to sufficiently promote martensitic transformation, it is necessary to hold in the cooling medium for 1 s or more, and it is important to secure a holding time after cooling.

  Furthermore, in addition to the definition of these carbides and the like, a structure in which the distribution of carbides is smaller than other parts should be avoided. Specifically, in the lens martensite and its tempered structure, the distribution of carbides is smaller than in other parts and the microstructure is inhomogeneous, which adversely affects fatigue strength and workability.

Evaluation Items In order to evaluate the applicability to the spring of the present invention, the tensile strength, the hardness after annealing, the impact value, and the drawing measured by a tensile test are shown as evaluation items. The tensile strength is directly related to the durability of the spring, and the higher the strength, the higher the durability.

  In addition, the diaphragm simultaneously measured at the time of measuring the tensile strength indicates the plastic deformation behavior of the material, and is an evaluation index of workability (coiling characteristics) to the spring. This indicates that the larger the aperture, the easier it can be processed, but generally the aperture decreases as the strength increases. From the example of the conventional steel, it is known from the evaluation in this wire diameter that when the restriction exceeds 30%, it is difficult to cause a failure in industrial mass production even in other wire diameters. The prepared specimen was quenched and tempered to a material of φ13 mm so as to exceed approximately 2200 MPa, then prepared with a JIS Z 2201 No. 9 specimen, tested in accordance with JIS Z 2241, and tensioned from its breaking load. Intensity was calculated.

  In recent years, the surface layer is often subjected to a hardening treatment by nitriding to increase the strength of the spring. In nitriding, the spring is heated to 400 to 500 ° C. in a nitriding atmosphere gas, and the surface layer is cured by holding for about several minutes to 1 hour. At that time, since the inside into which nitrogen does not enter is heated, it is annealed and softened. Since it is important to suppress this softening, the hardness after annealing simulating nitriding was set as an evaluation item of the softening resistance.

  Furthermore, Charpy impact value was used as an evaluation item in order to evaluate the workability and fracture resistance of the material. In general, a material having a good impact value is considered to have good fracture resistance including fatigue characteristics. Further, since a brittle material is also inferior in workability, a material having high toughness is considered to be excellent in workability. In this example, the Charpy impact value of a material subjected to the same heat treatment as that for measuring the tensile strength after quenching and tempering was measured. Since the Charpy impact value is also affected by the austenite grain size, the austenite grain size of the same material was also measured.

  The Charpy impact test piece was U-notched to 2 mm from a heat-treated material having a diameter of 13 mm to a so-called half-size (5 × 10 mm cross section) material.

  The spring has a smaller diameter of about 4 mm and finishes heat treatment in a relatively short time. Therefore, it is known that undissolved carbides are likely to remain and workability is reduced. Therefore, in the examples of the present invention, patenting-drawing was performed to obtain a diameter of 4 mm, the wire-drawn material was heat-treated, and the carbide distribution and austenite grain size were measured. In general, if the heating temperature is low and the time is short, the austenite grain size becomes small, but undissolved carbides tend to increase. Since the result appears in the tensile strength and elongation, both of them were evaluated. Since the cross-sectional area is small in a small diameter material of φ5 mm or less, the plastic deformation behavior clearly shows a difference in elongation than the drawing.

  Details of the heat treatment conditions of the evaluation material will be described below.

  In the tensile test, a test piece having a parallel part φ6 mm was prepared in accordance with JIS, and the tensile strength and elongation were measured.

The amount of retained austenite was mirror polished after quenching and tempering and measured by X-ray.
Furthermore, the hardness after annealing was mirror-polished after the heat treatment, the Vickers hardness at a position of 1/2 the radius from the surface was measured at three points, and the average value was taken as the hardness after annealing.

Material production method (Wire-rod)
Invention Example 16 of the present invention produced billets by rolling after melting in a 2 t-vacuum melting furnace. At that time, in the invention example, it was held at a high temperature of 1200 ° C. or higher for a certain time. Thereafter, in each case, the billet was rolled to φ13 mm.

  In other examples, after melting in a 16 kg vacuum melting furnace, forging to φ13 mm × 600 mm by forging and then heat treatment. In this case as well, after being kept at a high temperature of 1200 ° C. or higher for a certain time, heat treatment was performed so as to obtain a predetermined strength.

Heat treatment method When there is no description in the preparation of the evaluation test piece, after 1200 ° C. × 15 min → air cooling, after heating at 950 ° C. for 10 minutes, put into a lead bath heated to 650 ° C., and further after heating at 950 ° C. × 10 min In an example of the invention, the tempering temperature was adjusted so that the tensile strength exceeded 2200 MPa. Tensile strength, drawing, and Charpy impact value were measured in this heat treatment.

  Although the tempering temperature varies depending on the chemical component, the present invention was heat-treated according to the chemical component so that the tensile strength was 2200 MPa or more. On the other hand, the comparative example was heat-treated so as to match the tensile strength.

  Furthermore, the softening resistance was evaluated by annealing at 400 ° C. for 20 minutes imitating nitriding and measuring the hardness.

Further, in the case of φ4 mm wire for carbide evaluation, unless otherwise specified, after 1200 ° C. × 15 min → air cooling, φ10 mm was obtained by cutting, heated at 950 ° C. for 10 minutes, and then put into a lead bath heated to 650 ° C. Further, the diameter was reduced by drawing to φ4 mm, and after heating at 950 ° C. × 5 min, it was put into a 60 ° C. oil bath and quenched, and then the tempering temperature was adjusted so that the tensile strength exceeded 2200 MPa. In addition, the stress that could exceed the number of load cycles of 10 ⁷ in the Nakamura rotary bending test was defined as fatigue strength.

  Tables 2 to 9 show the chemical composition of the present invention and comparative steel when processed at φ4 mm, the cementite-based carbide dilute area ratio, the alloy / cementite-based spherical carbide occupation area ratio, and the cementite with an equivalent circle diameter of 0.2 to 3 μm. Spherical carbide abundance density, cementite spherical carbide abundance density greater than 3μm equivalent circle diameter, maximum oxide diameter, prior austenite grain size number, retained austenite amount (% by mass), resulting tensile strength, hardness after annealing Figure 2 shows the aperture measured by impact value and tensile test. That is, Tables 2 and 3 show chemical components of Invention Examples Nos. 1 to 25, and Tables 4 and 5 show chemical components of Invention Examples Nos. 26 to 51. Table 6 shows chemical components of Comparative Examples Nos. 52 to 77. Table 7 shows the characteristics of Invention Examples Nos. 1 to 25 and Table 8 of Invention Examples 26 to 51 with and without drawing. Table 9 shows the characteristics of Comparative Examples No. 52 to 77 with and without drawing.

Explanation of Comparative Example In the invention example, even heat treated material without wire drawing shows good performance in impact value, softening resistance after annealing, tensile properties, etc., and even heat treated material after wire drawing is specified by tensile properties, carbide distribution, etc. However, in the following examples, it does not exhibit sufficient performance because it falls outside the specification.

  Examples 52 and 53 are cases in which neither Ti nor Nb is contained. Since a large amount of V and Cr is added, undissolved carbides having nitrides as nuclei are formed. Elongation after wire is low and processability is reduced.

  In Examples 54 and 55, Ti and Nb are added, but N is excessive, and undissolved carbides with nitrides as nuclei are formed. Therefore, elongation after drawing and drawing in a tensile test is low, and processing is performed. Reduce sex.

  Examples 56 to 59 are examples in which Ti is added and N is fixed as TiN, and the amount of Ti added is excessive, so that the adverse effects caused by TiN become healthy. Therefore, the inclusion distribution increases, and as a result, the elongation after drawing or drawing in the tensile test is low, and the workability is lowered.

  Especially Example 57 is a case where many undissolved carbide | carbonized_materials were produced | generated by making the heating temperature at the time of hardening low.

  In Examples 60 to 62, Nb was added, but since the addition amount was excessive, a large amount of undissolved carbide was observed, and elongation after drawing or wire drawing in a tensile test was low, thereby reducing workability. .

  In Examples 63 and 64, since Al is excessive, the oxide becomes large, and the fatigue characteristics are deteriorated.

  In Examples 65 and 66, the amount of V added is too small. In this case, the hardness after annealing simulating nitriding is low, and the prior austenite grain size tends to be coarse, and the fatigue characteristics are deteriorated. Furthermore, in actual nitridation, compared with the invention example in which a prescribed amount of V was added, the surface layer hardness was low and the nitridation depth was shallow even in the same nitridation time.

  In Examples 67 and 68, the amount of Cr added is too small, and the hardness after annealing simulating nitriding is low, or the surface hardened layer at the time of nitriding treatment becomes thin, resulting in deterioration of fatigue characteristics.

  In Examples 69 to 71, the cooling temperature at the time of quenching is high and the cooling time is short, and the amount of retained austenite increases. Therefore, the hardness after annealing is insufficient, and the handling is practically slight. Since the periphery of the heel is embrittled by stress-induced transformation, workability is reduced.

  Examples 72 and 73 are examples in which the heating temperature at the time of quenching was too high, the prior austenite grain size was increased, the impact value was lowered, and the fatigue characteristics were lowered.

  Examples 74 to 77 are cases where C or Si is less than specified, and the tensile strength after annealing is lowered, so that fatigue strength cannot be ensured.

It is a figure explaining the effect (relationship between tempering temperature and Charpy impact value) of Nb addition when N is reduced. (A) is a photograph which substitutes for a drawing and shows an example of observation of undissolved carbide by a scanning electron microscope. (B) is an X-ray elemental analysis example of an alloy-based undissolved carbide X, and (c) is an X-ray elemental analysis example of a cementite-based undissolved carbide Y.

Explanation of symbols

X Alloy-based undissolved carbide Y Cementite-based undissolved carbide

Claims (5)

% By mass
C: 0.5-0.9%
Si: 1.0-3.0%,
Mn: 0.1 to 1.5%
Cr: 1.0-2.5%,
V: more than 0.15 to 1.0% or less,
Al: containing 0.005% or less,
N: limited to 0.007% or less,
Further, Nb: containing 0.001 to less than 0.01%, Ti: less than 0.001 to 0.005% of one or two,
A steel for high-strength springs, the balance being iron and inevitable impurities. Furthermore, in mass%,
W: 0.05-0.5%
Mo: 0.05-0.5%
The high strength spring steel according to claim 1, comprising one or two of the above. Furthermore, in mass%,
Ni: 0.05-3.0%,
Cu: 0.05 to 0.5%,
Co: 0.05-3.0%
B: 0.0005 to 0.006%
The high strength spring steel according to claim 1 or 2, characterized by containing one or more of the above. Furthermore, in mass%,
Te: 0.0002 to 0.01%,
Sb: 0.0002 to 0.01%,
Mg: 0.0001 to 0.0005%,
Zr: 0.0001 to 0.0005%,
Ca: 0.0002 to 0.01%,
Hf: 0.0002 to 0.01%
The steel for high-strength springs according to any one of claims 1 to 3, wherein one or more of them are contained. Regarding the cementite-based spherical carbide and the alloy-based spherical carbide having the steel component according to any one of claims 1 to 4 and having a tensile strength of 2000 MPa or more and occupying a specular surface,
Occupied area ratio of circle equivalent diameter 0.2μm or more is 7% or less,
The existence density of the circle equivalent diameter of 0.2 μm or more is 1 piece / μm ² or less,
And the prior austenite grain size number is 10 or more, the residual austenite is 15% by mass or less,
A heat-treated steel wire for high-strength springs, characterized in that

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