JP2002180196A - High strength spring steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide spring steel in which high strength and coiling properties are reconciled after heat treatment. SOLUTION: The spring steel has a composition containing, by mass, 0.4 to 0.8% C, 0.9 to 3.0% Si, 0.1 to 2.0% Mn, <=0.015% P, <=0.015% S, <=2.0% (inclusive of 0%) Cr and 0.001 to 0.007% N, and the balance iron with inevitable impurities. In its microstructure after hot rolling, the density of cementite spheroidal carbides with a diameter of the equivalent circle of 0.2 to 3 μm is <=0.5 pieces/μm2, and the density of cementite spheroidal carbides with a diameter of the equivalent circle of >3 μm is <=0.005 pieces/μm2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to steel having high strength and high toughness after heat treatment and used as a spring for automobiles and general machinery.

[0002]

2. Description of the Related Art As automobiles are becoming lighter and more sophisticated, springs are becoming stronger. High-strength steels having a tensile strength exceeding 1600 MPa after heat treatment are used for springs. In recent years, steels having a tensile strength exceeding 1900 MPa have been used.

[0003] In a method of manufacturing a coil spring using steel, the steel is heated to an austenite region and coiled, and thereafter,
There are hot coiling in which quenching and tempering is performed and cold coiling in which high-strength steel wire in which steel has been quenched and tempered in advance is cold-coiled. In each case, the basic strength of the spring is determined by quenching and tempering. Therefore, for spring steel, it is important to design components in consideration of the properties after quenching and tempering.

[0004] Specifically, Japanese Patent Application Laid-Open No. Sho 57-3
No. 2353 discloses that the addition of elements such as V, Nb, and Mo improves the hardenability and generates fine carbides precipitated by tempering, thereby restricting the movement of dislocations and improving the sag resistance. And

[0005] However, in the steel manufacturing process, heating is performed many times as in a converter, casting, billet rolling, and wire rod rolling, and simultaneously cooling to room temperature is performed many times. In such a case, the added C
Carbide-forming elements such as r, V, Nb, and Mo harden the steel and tend to remain in the steel as coarse carbides. In particular, when aiming for a high strength exceeding 1900 MPa in tensile strength, the amount of addition of these alloying elements increases, so that a large amount of carbide remains. Until now, JP-A-11-6033
In Japanese Patent Application Laid-Open Publication No. H10-157, there is an invention that focuses on carbides such as Cr, V, Nb, and Mo (hereinafter, these are referred to as alloy-based carbides) and defines their sizes. However, it is not these fine carbides that actually control the strength of the steel, but the behavior of iron carbide, that is, the carbide mainly composed of cementite (hereinafter referred to as cementite-based carbide). Important for steel.

As for the grain size of alloy-based carbides, for example, Japanese Patent Application Laid-Open No. 10-251804 discloses an invention that focuses on the average grain size of Nb and V-based carbides. There is a description that there is a concern that an abnormal structure may be caused by cooling water (paragraph 0015), and the dry rolling is substantially recommended. This is a non-stationary operation industrially, and is presumed to be clearly different from normal rolling, suggesting that even if the average grain size is controlled, unevenness in the surrounding matrix structure causes rolling trouble. are doing. Therefore, it is shown that control of the average particle size of alloy carbides such as V and Nb-based carbides is industrially insufficient.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a spring steel which can be manufactured industrially and which can impart strength and coiling properties for a spring after quenching and tempering.

[0008]

Means for Solving the Problems The inventors of the present invention have succeeded in achieving high strength and high coilability after quenching and tempering by reducing the size of carbides in steel, particularly cementite, which has not been noticed in conventional spring steels. This led to the development of steel.

That is, the present invention provides the following spring steel.

(1) In mass%, C: 0.4 to
0.8%, Si: 0.9 to 3.0%, Mn: 0.1 to
2.0%, P: 0.015% or less, S: 0.015% or less, Cr: 2.0% or less (including 0%), N: 0.00
1 to 0.007%, the balance including iron and unavoidable impurities, and a microequivalent diameter of 0.2 to 0.2 in the microstructure after hot rolling.
A spring steel having an existing density of 0.5 μm / μm ² or less of cementite-based spherical carbides of 3 μm, and 0.005 / μm ² or less of cementite-based spherical carbides having a circle equivalent diameter of more than 3 μm.

(2) Further, in mass%, W: 0.05
1.0%, Co: 0.05 to 5.0%, 1 or 2 types
The spring steel according to the above (1), comprising a seed.

(3) Further, in mass%, Ti: 0.0
05-0.1%, Mo: 0.05-1.0%, V: 0.
The spring steel according to the above (1) or (2), characterized in that the spring steel contains one or more of 0.05 to 0.7% and Nb: 0.01 to 0.05%.

(4) Further, in mass%, B: 0.00
The spring steel according to any one of the above (1) to (3), containing 0.05 to 0.006%.

(5) Further, in mass%, Ni: 0.0
The spring steel according to any one of the above (1) to (4), comprising one or two of 5 to 5.0% and Cu: 0.05 to 0.5%.

(6) Further, in mass%, Mg: 0.0
The spring steel according to any one of the above (1) to (5), which contains 002 to 0.01%.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The inventor obtains high strength by defining appropriate chemical components, and controls the shape of carbide in steel by heat treatment to ensure sufficient coiling characteristics when manufacturing a spring. Invented spring steel.

The details will be described below. First, the reason for defining the chemical composition of steel will be described.

C is an element that has a large effect on the basic strength of the steel material.
And If the C content is less than 0.4%, sufficient strength cannot be obtained, and other alloying elements must be added in a larger amount.
If it exceeds 0.8%, a large amount of coarse cementite is precipitated after ordinary rolling, so that the toughness is significantly reduced. This reduction in toughness simultaneously reduces the coiling properties. In addition, there is an industrial problem such as the necessity of setting a high heat treatment temperature in the spring steel manufacturing process and making high frequency processing difficult.

Si is an element necessary for securing the strength, hardness and sag resistance of the spring. If the amount is small, the necessary strength and sag resistance are insufficient, so the lower limit is 0.9%. . Si has the effect of spheroidizing and refining the carbide-based precipitates at the grain boundaries, and has the effect of reducing the area occupied by the grain boundary precipitates by actively adding them. However, when added in a large amount, the material not only hardens, but also becomes brittle. Therefore, to prevent embrittlement after quenching and tempering, 3.0%
Was set as the upper limit.

Mn improves the hardenability and hardens the matrix. Further, S present in steel is replaced with MnS
And S can be rendered harmless. Further, it is an element that can secure the strength without producing carbide with respect to the behavior of carbide which is particularly noted in the present invention. Therefore, the lower limit is set to 0.1% in order to fix S as MnS. In order to ensure strength, Mn is preferably 0.5% or more. Also M
The upper limit is set to 2.0% in order to prevent embrittlement due to n.

Cr is an element effective for improving hardenability and tempering softening resistance. When nitriding is performed to harden the spring surface and improve spring fatigue strength, the higher the amount of Cr, the shorter the nitriding time , The hardened layer becomes deeper and the maximum hardness tends to increase. Therefore, when nitriding is assumed, C
It is preferred to add r. However, a large amount of addition not only causes an increase in cost but also coarsens cementite observed after quenching and tempering. As a result, care must be taken because the wire becomes brittle and thus easily breaks during coiling. In particular, Cr is contained in the cementite precipitated after rolling.
Is a solid solution, stabilizes cementite, and is likely to be undissolved during quenching and heating. This point has a great effect on oil-tempered wires, high-frequency heating materials, and the like. Therefore, the upper limit is set to 2.0%, at which the cementite becomes difficult to form a solid solution during quenching and heating during the production of the spring, and the heat treatment during the production of the spring or the spring steel wire becomes extremely difficult.

N easily forms a nitride when an element which generates a nitride such as V or Nb is added. They facilitate the formation of carbonitrides. Since these carbonitrides become pinned particles that suppress austenite grain growth during quenching, they are effective in reducing the austenite grain size. For such a purpose, 0.001% or more of N is added. On the other hand, excessive N causes coarsening of nitrides and carbonitrides and carbides formed with nitrides as nuclei.
%.

P hardens the steel, but also causes segregation and embrittles the material. In particular, P segregated at the austenite grain boundary causes a delayed fracture due to a decrease in impact value or penetration of hydrogen. Therefore, the smaller the better. Therefore, the content is limited to 0.015% or less at which the tendency of embrittlement becomes remarkable.

S also embrittles the steel when it is present in the steel, like P. The effect is minimized by Mn.
Since nS also takes the form of inclusions, the destruction characteristics are degraded.
In particular, in the case of high-strength steel, a small amount of MnS may cause destruction, and it is desirable to reduce S as much as possible. The upper limit is set to 0.015% at which the adverse effect becomes significant.

W has the function of improving the hardenability and generating carbides in the steel to increase the strength. Particularly, it is important because coarsening of cementite and other alloy-based carbides can be suppressed. If the addition amount is less than 0.05%, no effect is observed, and if it exceeds 1.0%, coarse carbides are formed, and mechanical properties such as ductility may be rather impaired. 1.0%.

Although Co reduces the hardenability, it can ensure the strength at high temperatures. In addition, since it inhibits the formation of carbides, it has a function of suppressing the formation of coarse carbides which is a problem in the present invention. If it is less than 0.05%, the effect is small, and if it exceeds 5.0%, the effect is saturated.
05 to 5.0%.

Although W and Co have different mechanisms in steel, both have an effect of reducing cementite-based carbides. Therefore, when steel has a high C content as in the present invention, cementite is refined. And it is effective for easy solid solution. Therefore, one or two of W and Co are added.

Ti, Mo, V and Nb precipitate as nitrides, carbides and carbonitrides in steel. Therefore, if one or more of these elements are added, these precipitates are formed and tempering softening resistance can be obtained, and heat treatment such as tempering at a high temperature and strain relief annealing or nitriding applied in the process can be performed. High strength can be exhibited without softening even after passing through. This suppresses a decrease in the internal hardness of the spring after nitriding and facilitates hot setting and strain relief annealing, thereby improving the final fatigue characteristics of the spring.
However, if Ti, Mo, V and Nb are added in too large amounts, their precipitates become too large and combine with carbon in the steel to form coarse carbides. This reduces the amount of C that should contribute to the increase in the strength of the steel wire, and the strength corresponding to the added amount of C cannot be obtained. Furthermore, since coarse carbides are a source of stress concentration, they are easily broken by deformation during coiling.

For Ti, the precipitation temperature of the nitride is high,
Already precipitated in molten steel. Further, since the bonding force is strong, it is also used for fixing N in steel. When B is added, it is necessary to add N as much as possible in order to prevent B from becoming BN. So Ti by N
Is preferably fixed. The addition amount of Ti is the minimum necessary addition amount 0.005 which can make the austenite particle size fine.
% As the lower limit, and the upper limit of 0.1%, at which the precipitate size does not adversely affect the fracture characteristics, was set as the upper limit.

By adding 0.05 to 1.0% of Mo, hardenability can be improved and tempering softening resistance can be given. That is, the tempering temperature for controlling the strength can be increased. This is advantageous in reducing the area occupied by the grain boundary carbides. That is, it is effective in tempering the grain boundary carbide precipitated in the form of a film at a high temperature to make the grain boundary spherical, thereby reducing the grain boundary area ratio. Mo generates Mo-based carbides separately from cementite in steel. In particular, since the deposition temperature is lower than that of V or the like, there is an effect of suppressing coarsening of carbide. If the amount is less than 0.05%, no effect is observed, and if it exceeds 1.0%, the effect is saturated.

V can be used for hardening a steel wire at a tempering temperature and for hardening a surface layer at the time of nitriding, in addition to suppressing coarsening of austenite grain size by forming nitrides, carbides and carbonitrides. If the addition amount is less than 0.05%, the effect of the addition is hardly recognized, and if it exceeds 0.7%, coarse undissolved inclusions are formed, and the toughness is reduced.

Similarly, Nb can be used for hardening of a steel wire at a tempering temperature and hardening of a surface layer at the time of nitriding, in addition to suppressing coarsening of austenite grain size due to formation of nitrides, carbides and carbonitrides. Since Nb generates fine carbides even at higher temperatures than V, Mo, etc., Nb is a very effective element that has a large effect on the reduction of the austenite grain size during the production of heat-treated steel wires, even if its addition amount is very small. If the content is less than 0.01%, almost no effect is recognized, and if it exceeds 0.05%, coarse undissolved inclusions are formed and the toughness is reduced.
05%.

B is known as a hardenability improving element. Further, it is effective for cleaning austenite grain boundaries.
That is, by adding elements such as P and S that segregate at the grain boundaries and lower the toughness, B is rendered harmless and the fracture characteristics are improved. At that time, if B combines with N to form BN, the effect is lost. The amount of addition is 0.0
The lower limit was 005%, and the upper limit was 0.006% at which the effect was saturated.

Ni improves the hardenability and can stably increase the strength by heat treatment. Further, the ductility of the matrix is improved to improve the coilability. Further, the corrosion resistance of the spring is improved, which is effective for a spring used in a corrosive environment. If the amount of addition is less than 0.05%, no effect on increasing strength and ductility is recognized, and if it exceeds 5.0%, the effect is saturated and disadvantages are caused in terms of cost and the like.

Regarding Cu, decarburization can be prevented by adding Cu. Efforts have been made to reduce the decarburized layer as much as possible to reduce the fatigue life after spring processing. When the decarburized layer becomes deep, the surface layer is removed by peeling called peeling. It also has the effect of improving the corrosion resistance, similarly to Ni.

Accordingly, by suppressing the decarburized layer, the fatigue life of the spring can be improved and the peeling step can be omitted.
The effect of suppressing decarburization of Cu and the effect of improving corrosion resistance can be exhibited at 0.05% or more. As described later, even if Ni is added, if it exceeds 0.5%, it is likely to cause rolling flaws due to embrittlement. Therefore, the lower limit is 0.05% and the upper limit is 0.5%
And The addition of Cu hardly impairs the mechanical properties at room temperature, but when Cu is added in excess of 0.3%, the hot ductility is degraded, so that the billet surface may crack during rolling. 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.

Mg is an oxide forming element, and forms an oxide in molten steel. The temperature range is higher than the MnS formation temperature, and is already present in the molten steel at the time of MnS formation.
Therefore, it has been found that it can be used as a precipitation nucleus of MnS, whereby the distribution of MnS can be controlled. That is, since Mg-based oxides are more finely dispersed in molten steel than Si and Al-based oxides often found in conventional steels, MnS with Mg-based oxides as nuclei is finely dispersed in steel. Therefore, even with the same S content, MnS
The distributions are different and the MnS particle size becomes finer when they are added. The effect can be sufficiently obtained even with a very small amount. If Mg is 0.0002% or more, MnS becomes finer. However, since 0.01% or more hardly remains in molten steel, 0.01% is considered to be the upper limit industrially. Therefore, the added amount of Mg is set to 0.0002 to 0.01%. This Mg is effective in improving corrosion resistance, delayed fracture, and preventing rolling cracks due to the effects of MnS distribution and the like. It is desirable to add Mg as much as possible, and the preferable addition amount is 0.0005 to 0.01%.

The manufacturing problems of the spring steel which is aimed at higher strength than the conventional one in the present invention will be described.
In a spring, the strength is increased by quenching and tempering. However, in a conventional component system, the tempering temperature has to be lowered, and it is generally brittle and cannot withstand practical use. In the production by cold coiling, since coiling is performed after quenching and tempering, the coil is broken during coiling. Therefore, it is common practice to slightly increase the amount of C or to add an alloy element. However, when alloying elements such as Cr and V are increased, segregation occurs, and a melting point is locally lowered in a concentrated portion, so that cracks are easily generated. This is considered to be one of the causes of the flaw during rolling.

The carbides which should be noted in the present invention will be further described. When considering the performance of steel, the form of carbides in steel becomes important. The carbides in the steel referred to herein are cementite found in the steel after heat treatment in the steel and carbides dissolved in the alloy elements thereof (hereinafter, both are collectively referred to as cementite) and alloy elements such as Nb, V, and Ti. Carbides and carbonitrides (hereinafter referred to as alloy-based carbides). These carbides mirror-polished the steel wire,
It can be observed by etching.

FIG. 1 shows a micrograph of a typical example of a quenched and tempered structure. According to this, two types of carbides, pearlite-like or plate-like precipitates and spherical carbides 1, are recognized in the steel. The spring steel is cast, rolled into a billet shape, cooled once to room temperature, and then rolled to a wire size according to an order. Furthermore, in the production of spring steel, quenching and tempering is performed, but pearlite-like or plate-like cementite easily dissolves in solid form, but carbides stabilized by spheroidization do not easily dissolve in the quenching and tempering step in the next step. The strength corresponding to the added amount of C cannot be secured, or the ductility during coiling is reduced. It also causes rolling flaws during wire rolling.

Since the remaining carbide does not contribute to the strength and toughness due to quenching and tempering at all, it not only wastes the added C by fixing C in the steel, but also becomes a source of stress concentration because it becomes a source of stress concentration. It becomes a factor to lower the mechanical properties. Since this spherical carbide did not form a solid solution during reheating after cooling (such as wire rod rolling and quenching during spring production), the carbide grew spherically. Therefore, it is preferable that the amount is as small as possible immediately after rolling the wire rod. In particular, this spherical carbide further grows and becomes coarse by heat treatment after rolling such as oil tempering. From such a viewpoint, there is a great possibility that even a carbide having a circle-equivalent diameter of 3 μm or less which is normally not a problem will cause a problem. In the present invention, it has been found that cementite containing Fe and C as main components, which has not been noticed so far, is no exception. This coarse undissolved carbide not only exerts an effect until the spring is manufactured, but also causes flaws during the rolling.

The cementite-based carbides include those in which alloy elements such as Cr and Mo are solid-dissolved in cementite. Generally, cementite in which these are dissolved is stabilized and hardly dissolved. As a feature of detection, when a carbide that appears by etching is analyzed by EDX, Fe and C are detected as main components, and an alloy element in solid solution is sometimes detected. Hereinafter, such carbides containing Fe and C as main components are referred to as cementite-based carbides, and those having a spherical shape are particularly referred to as cementite-based spherical carbides.

FIGS. 2A and 2B show examples of analysis of carbide by EDX attached to the SEM. The same analysis result can be obtained by the replica method using a transmission electron microscope. The conventional invention uses V, Nb added to obtain high strength.
Attention is paid only to alloy element-based carbides, such as those shown in FIG. 2A, and one example thereof is a feature in that the Fe peak in the carbide is very small in FIG. However, in the present invention, as shown in FIG.
Attention was paid to precipitation of a cementite-based spherical carbide in which Fe ₃ C of μm or less and an alloy element thereof were slightly dissolved in the alloy. In the case of achieving both high strength and workability higher than conventional steel wires as in the present invention, if there are many cementite-based spherical carbides of 3 μm or less, workability is greatly impaired.

In the case of coiling after quenching and tempering steel, cementite-based spherical carbide affects its coiling properties, that is, its bending properties up to fracture. Until now, not only C but also a large amount of alloying elements such as Cr and V have been added in order to obtain high strength. In this case, a large amount of coarse spherical carbides is generated, which causes deterioration of the coiling characteristics and generation of rolling flaws. As shown in FIG. 2 (b), what affects rolling flaws and coiling is a so-called cementite-based spherical carbide in which Fe ₃ C and its alloying element are slightly dissolved, and are present in large quantities. If the steel wire grows coarsely, it promotes the generation of cracks in rolling and reduces the mechanical properties, particularly the coiling properties, of the steel wire after heat treatment.

These spherical carbides can be observed by subjecting a mirror-polished sample to etching such as picral, but the carbides of interest here have a circle equivalent diameter of 0.2 to
For observation and detailed observation evaluation of the size and the like of the cementite-based spherical carbide of 3 μm, use of a scanning electron microscope
It is necessary to observe at a high magnification of 0 or more. Conventionally, fine carbides in steel have been indispensable for securing the strength of the steel and the resistance to tempering softening, but the present inventors have found that the effective particle size is 0.1 μm or less in circle equivalent diameter, and conversely It has been found that if it exceeds 1 μm, it does not contribute to the reduction of the strength or the austenite grain size, but merely deteriorates the deformation characteristics.

In the present invention, when the size of the cementite-based spherical carbide (equivalent circle diameter) is 3 μm or less, not only the size but also the number is a major factor. Therefore, the scope of the present invention has been defined in consideration of both of them. That is, the equivalent circle diameter is 0.2 to 3
Even if it is as small as μm, the number is very large, and if the existence density on the speculum surface exceeds 0.5 / μm ² , the coiling characteristics will be significantly deteriorated.

Further, if the size of the carbide exceeds 3 μm, the influence of the size becomes greater, and if the existence density on the speculum surface exceeds 0.005 / μm ² , the deterioration of the coiling characteristics becomes remarkable.

Even if these remain immediately after hot rolling, they are not easily melted by various heat treatments in the subsequent drawing-spring manufacturing process. Therefore, in the microstructure after rolling, the equivalent circle diameter is 0.2.
存在 3 μm cementite spherical carbide existing density is 0.5
Particles / μm ² or less, and the existing density of cementite-based spherical carbide having an equivalent circle diameter of more than 3 μm was 0.005 particles / μm ² or less.

For the rolling of the wire rod, continuous casting → billet rolling →
Taking a wire rod rolling or continuous casting → wire rolling process, to become a temperature lower than the A ₁ transformation point between each step, already carbides after continuous casting are precipitated. Therefore, in order to reduce the cementite-based spherical carbide remaining after wire rod rolling, it is necessary to perform heating for billet rolling and heating for wire rolling at a sufficiently high temperature and for a long time to allow the coarse carbides to form a solid solution. There is.

[0050]

EXAMPLES Table 1 shows examples of the present invention and comparative examples.

In Example 1 of the present invention, a billet was produced by continuous casting of a product refined by a 250 t converter.
In other examples, billets were produced by rolling after melting in a 2t-vacuum melting furnace. At that time, in the invention example, 12
The temperature was kept at a high temperature of 00 ° C. or more for a certain period of time. After that, in each case, the billet was rolled to φ8 mm,
4 mm. On the other hand, the comparative example was rolled under normal rolling conditions and subjected to wire drawing.

Since the present invention has excellent properties different from those of the prior art in terms of rolling flaws and properties after quenching and tempering after rolling, the evaluation was made based on properties immediately after rolling and after quenching and tempering. The flaws immediately after rolling were visually observed for rolling flaws.

After the wire was drawn to φ4 mm by wire drawing, it was quenched by passing through a radiant furnace and immediately quenched in oil, and further passed through molten Pb to perform tempering, so-called quenching and tempering. .

In the oil tempering treatment, the wire drawing material was continuously passed through the heating furnace, and the heating furnace passage time was set so that the steel internal temperature was sufficiently heated. If the heating is insufficient, insufficient quenching occurs, and sufficient strength cannot be achieved. In this embodiment, the heating temperature is 950 ° C. and the heating time is 15
0 sec and a quenching temperature of 50 ° C. (oil tank). Furthermore, tempering was performed at a tempering temperature of 400 to 550 ° C. and a tempering time of 1 min to adjust the strength. The heating temperature during quenching and tempering and the resulting tensile strength in the air atmosphere were as specified in Table 1, and the tensile strength was adjusted to about 2100 MPa.

In the examples, semesters considered important in the present invention are shown.
Spheroidal carbides in steels containing lintite are also described.
Was. Evaluation of the size and number of carbide
Polished to the mirror surface in the longitudinal section of the as-heat treated steel wire,
Furthermore, it is slightly etched with picric acid and carbonized.
Things were raised. Carbide dimensions at the optical microscope level
Because measurement is difficult, hot rolled wire and 1 / 2R part of steel wire
Scanning electron microscope, magnification: 5,000 times, random 10
A photograph of the field of view was taken. And from that photo it becomes spherical
Scanning carbide (cementite spherical carbide)
X-ray microanalyzer attached to the microscope
While confirming that it is cementitious,
The number was measured using an image processing apparatus. Use that data
Calculate the equivalent circle diameter and density of each spherical carbide
Was. The total measurement area is 3088.8 μm ^TwoIt is.

The tensile properties were measured using JIS Z 22019 test pieces in accordance with JIS Z 2241, and the tensile strength was calculated from the breaking load.

The ductility was evaluated by a notch bending test. FIG. 3 shows the outline of the notch bending test. The procedure was as follows. As shown in FIG.
A groove (notch) 2 having a maximum depth of 30 μm is formed at right angles to the longitudinal direction of the steel wire with a punch having a tip radius of 50 μm, and both sides are supported so that the maximum tensile stress is applied to the groove, and a load 3 is applied to the center. A three-point bending deformation for deformation was added.
The bending deformation was continued to be applied until breaking from the notch, and the bending angle at the time of breaking was measured. The measurement angle is as shown in FIG. 3B, and the larger the measurement angle (θ), the better the coiling characteristics. Empirically, coiling is difficult at a notch bending angle of 25 ° or less in a φ4 mm steel wire.

In the invention examples, the dimensions of the spherical carbides after rolling were small, preventing rolling flaws, and exhibited high strength and good notch bending characteristics after quenching and tempering. However, the comparative examples suggested that the notch bending properties were inferior and the coiling properties were inferior. Rolling flaws were also observed, indicating that rolling was difficult.

Further, as shown in Table 2, the wire rod was rolled at a diameter of 15 mm,
Chemical composition of the present invention and comparative steel when drawn at 12 mm and oil-tempered, presence or absence of rolling flaws, equivalent circle diameter of 0.2 to
The carbide existence density of 3 μm, the carbide existence density of more than 3 μm in equivalent circle diameter, tensile strength, and coiling characteristics (drawing in a tensile test) are shown.

For φ12 mm, annealing was performed at 400 ° C. for 20 minutes to simulate the strain relief annealing at the time of manufacturing an oil temper and a spring, and the tensile strength was adjusted to 1950 to 2000 MPa.

From the evaluation results of the φ12 mm steel wire shown in Table 2, the comparative material having a carbide existing density outside the specified range has a small drawing which is an index of the coilability, and has a carbide having an equivalent circle diameter of more than 3 μm after rolling. 0.005 pieces / μm ²
When the above is observed, it remains after the heat treatment, and the drawing is reduced. Therefore, it is considered that the coilability is poor.

[0062] Tables 1 and 2 also indicate that steel flaws outside the scope of the claims of the present invention basically have rolling flaws that are likely to remain. It is thought that the cause is that cementite-based spherical carbides are also greatly involved, and cementite-based spherical carbides exceeding the requirements of the present invention have been detected from the results of observation from a healthy part after rolling. This means that even if the steel is heated multiple times to austenite, if carbides remain in the structure before it, flaws are easily formed in subsequent processes, such as wire rolling, coiling after oil tempering, and hot coiling. It may indicate that

[0063]

[Table 1]

[0064]

[Table 2]

[0065]

The steel of the present invention can be industrially manufactured in spite of having a component system capable of increasing the strength by making the precipitation of carbide including cementite in the steel a controllable component. . After the heat treatment, high strength springs can be manufactured. In particular, the strength of a cold coiled spring can be increased to 1900 MPa or more, and a spring having high coiling properties, high strength and excellent breaking characteristics can be manufactured.

[Brief description of the drawings]

FIG. 1 is a micrograph of steel showing a quenched and tempered structure.

FIG. 2 is a diagram showing an example of analysis of a cementite-based spherical carbide.

FIG. 3 is a view showing a notch bending test method.

[Explanation of symbols]

 1 Spherical carbide 2 Groove (notch) 3 Load θ Measurement angle

Claims (6)

[Claims] C. 0.4 to 0.8 in mass%
%, Si: 0.9 to 3.0%, Mn: 0.1 to 2.0
%, P: 0.015% or less, S: 0.015% or less, C
r: 2.0% or less (including 0%), N: 0.001 to
0.007%, containing the balance iron and unavoidable impurities, and has an equivalent circle diameter of 0.2 to 3 μm in the microstructure after hot rolling.
The density of cementite-based spherical carbides is 0.5 / μm
A spring steel characterized in that the density of cementite-based spherical carbides having an equivalent circle diameter of not more than ² and a diameter of more than 3 μm is not more than 0.005 / μm ² . 2. W: 0.05-1.% By mass%.
2. The spring steel according to claim 1, further comprising one or two of 0% and Co: 0.05 to 5.0%. 3. 3. The composition according to claim 1, further comprising:
0.1%, Mo: 0.05-1.0%, V: 0.05-
0.7%, Nb: one or two of 0.01 to 0.05%
The spring steel according to claim 1, wherein the spring steel contains at least one kind. 4. Further, B: 0.0005 to 5% by mass.
4. The composition according to claim 1, which contains 0.006%.
The spring steel according to any one of the above. 5. The method according to claim 1, further comprising:
The spring steel according to any one of claims 1 to 4, comprising one or two of 5.0% and Cu: 0.05 to 0.5%. 6. Mg: 0.0002 by mass%.
6. The composition according to claim 1, wherein the content is 0.01 to 0.01%.
The spring steel according to any one of the above.