JP2000239797A - Steel for spring excellent in workability and production of steel wire for spring - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide steel for a spring combining both characteristics of SV properties and rod drawability which are important in the process of producing a spring and to provide a method for producing a steel wire having excellent spring characteristics by using the steel for a spring. SOLUTION: This is the steel for a spring subjected to rolling or subjected to softening and annealing treatment thereafter, which satisfies the following mechanical characteristics and preferably has a metallic structure of essentially consisting of pearlite or ferrite and pearlite, in which the fractional ratio of the supercooled structure is <=10%, the Vickers hardness in the cross-section is <=380, and the standard deviation σ is <=20, where tensile strength (the maximum value) <=1200 MPa, and 30% (the minimum value) <= drawing value <=70% (the maximum value).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a spring steel used for structural parts such as an engine, a clutch, a fuel injection device and a suspension of an automobile, and a material for the above-mentioned various structural parts using the spring steel. The present invention relates to a method for producing various spring steel wires.

[0002]

2. Description of the Related Art In manufacturing springs used for various applications as described above, a cutting process (scalping: hereinafter referred to as "S") for removing surface defects (scratch or decarburized layer) generated on a rolled wire rod is performed.
V), and after that, it is generally tempered by a patenting process using lead or the like and then subjected to wire drawing.

In recent years, there has been a demand for an improvement in linear speed in order to increase productivity. However, when the linear speed is increased, an SV jig (chipper) is required.
The burden on the chipper becomes excessive, causing chipper chipping, and there is a problem that cutting chips are not discharged smoothly and a part that is not shaved remains, especially in high strength spring steel having a high material strength. There is a strong demand for a material that does not cause the above-described problems during SV processing.

[0004] In order to solve such a problem, a method of reducing the material strength by performing a soft annealing treatment has been implemented. However, the above problem cannot be completely solved only by the soft annealing. In order to solve these problems, the metallographic structure and mechanical properties before shaving are considered to have a great effect. In reality, however, research focusing on the metallic structure and mechanical properties before shaving has been made. Absent.

[0005] Even when the same type of spring steel is used, when manufacturing a spring that does not require very high fatigue characteristics, a so-called "raw drawing wire" that draws directly from a rolled wire without SV treatment is used. Has been done. As a method for producing a wire rod to be used for the raw drawing, for example, JP-A-57-1
No. 16727, No. 63-118013, JP-A-3-7
No. 9719 and the like have proposed several methods.

On the other hand, in the recent technology relating to the spring manufacturing method, (A) 72 is used in place of lead patenting after SV processing.
A method in which heating is performed for a short time at a temperature of 3 to 1023 K to soften the surface hardened layer (for example, Japanese Patent Application Laid-Open No. 7-188745).
), (B) In the gas phase, at a heating temperature of 823 to 973K, and the heating temperature T (K) and the heating time t (minute) are T ×
A method of performing heat treatment under a condition that satisfies the relationship of Δt = 6700 to 12000 (K · min) (for example, Japanese Patent Application Laid-Open No. 8-31154)
No. 7) has been proposed.

In these techniques, a so-called "non-heat treatment" is carried out while almost leaving a rolled structure. However, under such a circumstance, not only a good SV property but also a raw drawability drawn from the rolled structure is obtained. It is necessary to secure these two characteristics.

Also, depending on the wire drawing maker, the SV wire may be manufactured in either the process with or without the SV treatment.
It is necessary to ensure both characteristics of the drawability and the drawability. However, no consideration has been given to the compatibility of these characteristics at all, and in the technology proposed so far, the SV property and It is not enough to secure both properties of the raw drawability.

[0009]

DISCLOSURE OF THE INVENTION The present invention has been made under such a circumstance, and an object of the present invention is to provide a spring steel having both the SV property and the draw-draw property which are important in the spring manufacturing process. Another object of the present invention is to provide a useful method for producing a spring steel wire exhibiting good spring characteristics using the spring steel.

[0010]

The spring steel of the present invention which has achieved the above object is a spring steel which has been rolled or softened and annealed under the following conditions after rolling, and which has not been subjected to skinning treatment. The point is to satisfy the following mechanical properties in the state. Tensile strength (maximum value) ≤ 1200 MPa, 30% (minimum value) ≤ drawn value ≤ 70% (maximum value) Soft annealing treatment: In the gas phase, at a heating temperature of 873 to 1023K, and a heating temperature T (K) And heating time t (min)
Heat treatment that satisfies the relationship of T × Δt = 7300 to 15000 (K · min).

[0011] In the spring steel of the present invention, as other characteristics, (1) the metal structure is mainly composed of pearlite or ferrite and pearlite, and the supercooled structure has a structure of 10% or less; 2) The standard deviation σ of the Vickers hardness in the cross section is 20 or less, and (3) The maximum value of the Vickers hardness (Hv) in the cross section is 380 or less. It is preferable because it is more excellent as spring steel.

Further, the manufacturing method of the spring steel wire according to the present invention is as follows.
The gist is that a spring steel satisfying the above requirements is used, and after any of the following processes (a) to (c) is performed after the scalping process, an oil tempering process is performed.

(A) A patenting process is performed after the scalping process, and (b) 823 to 9 in the gas phase after the scalping process.
At the heating temperature of 73K, the heating temperature T (K) and the heating time t (minute) are T × Δt = 670
Heat treatment is performed within a range satisfying the relationship of 0 to 12000 (K · min). (C) After shaving, short-time heating is performed at 723 to 1023 K to soften the surface hardened layer.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present inventors have studied from various angles in order to solve the above problems. As a result, the present inventors have found that the above object can be brilliantly achieved by appropriately adjusting the mechanical properties of the spring steel after rolling or after rolling and further subjected to soft annealing treatment, and completed the present invention. did. Hereinafter, each requirement defined in the present invention will be described in detail.

In the present invention, the tensile strength and the aperture value are defined in appropriate ranges as described above, and the reasons for limiting these ranges are as follows.

In performing SV processing on a rolled steel wire rod for a spring, as can be seen from the fact that the SV processing is a type of cutting, if the material strength is too high, an excessive load is applied to the chipper even if the wire speed is not increased. As a result, the chipper may be chipped and skinning may not be performed, or a streak-like defect may occur in the wire, and the wire may not be used as a wire product. In order to avoid such inconvenience, it is necessary to soften the material subjected to the SV treatment, and for that purpose, the maximum value of the tensile strength is set to 1200.
Mpa or less.

Also, when performing wire drawing, if the tensile strength is too high, wire breakage is likely to occur. Particularly, if a very hard supercooled structure such as martensite or bainite is present in the metal structure, it is used as a starting point. Since disconnection occurs, the tensile strength should be kept low, and from such a viewpoint, the maximum value of the tensile strength should be kept to 1200 MPa or less, more preferably 1100 MPa or less. The lower limit of the tensile strength is not particularly limited, but is preferably 900 MPa or more in order to enhance the chip disposability during the SV treatment described below.

On the other hand, as another factor influencing the SV processability, there is a problem of the discharge of chips generated during the SV process. In other words, when the cutting ability of the chips is poor, a phenomenon in which the chips stay in the vicinity of the chipper and become entangled with the wire occurs, and the chip presses the wire at the chipper portion. In other words, uniform surface flaws cannot be removed. Therefore, it is necessary to improve the cutting performance of the chips so that the chips can be easily discharged. For this purpose, it is considered that the effective means is not to excessively increase the toughness and ductility of the material. . So from this point of view,
In the present invention, the maximum value of the drawing value, which is an index of the toughness of the material, is determined to be 70% or less. More preferable maximum aperture value is 60%
It is as follows.

On the other hand, even when the tensile strength is kept low, if a coarse pearlite structure having poor toughness and ductility (a structure having a large lamellar spacing) is present, chevron cracks may occur in the structure and cause disconnection. There is. Therefore, it is necessary to ensure a certain degree of ductility, and the minimum value of the aperture value, which is the index, is determined to be at least 30% or more. In order to ensure high drawability stably, it is desirable that the minimum aperture value is 40% or more.

In the above description, the reason for determining the tensile strength and the reduction ratio in the as-rolled state has been explained. A steel which has been subjected to a tempering treatment so as to satisfy the above-mentioned requirements of tensile strength and drawing value by performing appropriate softening and annealing treatment can be similarly effectively used as a softened and annealed spring steel.

In the present invention, the softening annealing condition is such that the heating temperature T is in the range of 873 to 1023 K, and the heating temperature T (K) and the heating time t (minute) are “T × Δt = 7300”.
を 満 た す 15000 (K · minute) ”, and the reason for defining these conditions is as follows.

That is, when annealing is performed in a high temperature range, cementite in the wire becomes spherical and a chevron crack is generated at the center of the drawn wire. When the reduction in area due to the drawn wire increases, the wire is easily broken. Further, even when the cementite was annealed at a relatively low temperature that did not cause spheroidization, disconnection might occur during drawing, and it was confirmed that this disconnection was caused by insufficient ductility of the rolled wire. That is, when the annealing temperature is relatively low, the rolled structure is inherited as it is after the heat treatment, and when the structure of the rolled wire is mainly coarse pearlite having poor ductility, the above-mentioned chevron cracks are generated in the drawing process after annealing. It is easy to do so and it leads to disconnection.
Further, even if the wire is not broken during the wire drawing, if the spring is formed with the above-mentioned chevron cracks inside, the wire may be broken.

Based on these findings, the results of pursuing the relationship between the heating temperature and the heating time in the heat treatment after rolling as a requirement for enabling the wire drawing and the spring working to be performed smoothly without causing disconnection or the like. , Leading to the above relational expression.

The aperture value (Reductio) defined in the present invention is
n of Area) is a value obtained by dividing a difference between the minimum cross-sectional area A ′ after the test piece is broken and the original cross-sectional area A by the original cross-sectional area A in a tensile test in%. Aperture value = 100 × (AA ′) / A].

In the present invention, by controlling the above-mentioned mechanical characteristics, it is possible to obtain a product having both the characteristics of the SV processing and the grooving as shown in FIG. That is, FIG. 1 shows that the mechanical properties (tensile strength and reduction value) of the steel material are
This is a graph showing the effect on processability,
The mark has good properties in both the drawability and the SV processability (for the evaluation criteria, refer to the examples described later). The mark x indicates a spring lacking one or both of the above mechanical properties. It indicates that it is steel.
In FIG. 1, the tensile strength is the maximum value of each spring steel material,
The aperture value is plotted as the maximum or minimum value of each spring steel material.

The present inventors have made further studies to further improve the properties of the steel material even after completing the above invention. As a result, they have found that it is preferable to satisfy the following requirements in order to secure more stable manufacturability.

First, even in the case of performing raw drawing,
It has been found that the variation of the microstructure and the like in the cross section of the spring steel has a considerable influence, and that the variation in hardness in the cross section should be minimized. That is, if the variation in Vickers hardness in the cross section of the spring steel wire rod is large, a difference occurs in the deformability in the cross section, causing breakage under severe manufacturing conditions, or causing breakage during spring molding. However, it has been confirmed that if the variation in hardness in the cross section is suppressed as much as possible, such problems can be reduced. Therefore, in the present invention, the standard deviation σ of the Vickers hardness in the cross section
Is preferably suppressed to 20 or less, more preferably 15 or less.

On the other hand, when performing the SV treatment, if there is a part having a high strength, the load on the chipper is increased in the high strength part, the wear amount of the chipper is increased, and the diameter of the chipper is increased. In some cases, it may not be possible to obtain the target allowance. However, since the tensile strength indicates the strength of the entire cross section of the steel material, it is not possible to grasp a partial change in strength within the cross section.

Therefore, in order to accurately control such a partial change in strength in the cross section, a study was repeated with a focus on the hardness in the cross section. As a result, in addition to the above tensile strength, a steel wire rod for spring was used. It has been found that it is extremely effective to suppress the maximum value of the Vickers hardness (Hv) in the cross section of 380 or less, more preferably 370 or less.

By the way, when a super-cooled structure such as martensite or bainite is present in the metal structure, the hardness is reduced by performing the softening annealing treatment, so that a certain degree of drawability is ensured. However, the softened supercooled structure is different from the pearlite structure etc. in terms of hardness, even though the deformability by wire drawing is different,
It has been found that under extremely severe wire drawing conditions, breakage occurs and chevron cracks, which cause breakage during spring forming, occur.

According to further studies by the present inventors, the probability of occurrence of such a problem becomes lower as the fraction of the supercooled structure existing in the metal structure becomes smaller.
It became clear that the fraction should be controlled to 10% or less, preferably 5% or less, and the structure should be controlled substantially to pearlite or pearlite + ferrite.

The following methods were used as the methods for measuring the hardness and microstructure defined in the present invention.

First, the hardness was measured by measuring at least four points from the positions D / 16, D / 8, and D / 4 and the position D / 2 (here, D / 2) in the cross section of the spring steel wire rod. Means the diameter of the wire), and 13 or more points were picked up and measured by Vickers hardness measurement method (JISZ 2244). For the measurement of the microstructure, the cross section of the spring steel wire was observed with an optical microscope, and the area ratio of the supercooled structure was determined. It is preferable to use an image analyzer for measuring the area ratio.

[0034] The type of spring steel wire to be used in the present invention is not particularly limited, but is generally a spring steel wire which is subjected to an SV treatment or a drawing treatment in a manufacturing process.
JIS standard G 3522, G 3560, G 356
An example is a steel wire that is processed into a spring steel wire, such as SWOSC-V defined in No. 1 or the like.

There is no particular limitation on the chemical composition of the spring steel according to the present invention. In short, excellent mechanical properties (tensile strength, elongation, drawing) are easily obtained, and during the wire drawing. It may be determined by comprehensively judging characteristics such as not to deteriorate workability due to excessive work hardening. Specifically, those having the following chemical component composition are preferable.

That is, the preferred chemical composition of the spring steel of the present invention is C: 0.38 to 0.85%.
(Meaning by mass%, the same applies hereinafter), Si: 0.25 to 2.1
0%, Mn: 0.2-1.0%, and P <
0.035%, S <0.035%, and if necessary, Cr: 0.65 to 1.5%, Mo: 0.1 to
0.5%, V: 0.05 to 0.50%, Ni: 0.2 to
0.5%, Nb: 0.02 to 0.50%, Ti: 0.0
Examples of the steel material include one or more selected from the group consisting of 2 to 0.09% and Cu: 0.10 to 0.30% in a total amount of about 2.5% or less. The remaining components are Fe and unavoidable impurities, but it is also possible to use a steel material positively containing other elements depending on required characteristics.

Means for satisfying the requirements defined in the present invention is not particularly limited, but a preferable means is, for example, that the segregation of steel material is C _max / C ₀ (maximum value / ladle value) of 1.2 or less. The temperature after finish rolling and immediately before winding in hot rolling is 850 ° C. (1
123K) or less, and the cooling rate after winding is [Ps
Point (pearlite transformation start temperature) + 15 ° C.] to [Pf point (pearlite transformation end temperature) −15 ° C.]
Cooling at 4 ° C./sec is effective.

After performing the SV treatment using the spring steel of the present invention, any one of the following treatments (a) to (c) was performed, and then the oil tempering treatment was performed. It was confirmed that steel wire for industrial use could be manufactured. Further, the oil-tempered wire is represented by D / d (D: average diameter of spring, d:
A winding test was performed under the condition of (wire diameter) = 2, and it was confirmed that the workability of the spring could be secured without any breakage.

(A) After patenting, a patenting process is performed. (B) After patenting, 823-
At a heating temperature of 973K, the heating temperature T (K) and the heating time t (minute) are T × Δt = 6700 to 12000 (K ·
(C) After the shaving treatment, short-time heating is performed at 450 to 750 ° C. (723 to 1023 K) to soften the surface hardened layer.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following Examples are not intended to limit the present invention, and are appropriately modified within a range that can be adapted to the purpose described above and below. And they are all included in the technical scope of the present invention.

[0041]

EXAMPLE A JIS SUP12 spring steel (Si-Cr spring steel) having the chemical components shown in Table 1 below was rolled to a diameter of 8.0 mm to produce a wire rod. At this time, the segregation of the steel slab was performed using a steel sheet having a _Cmax / C0 of _1.0 to 1.5, and the temperature immediately before winding after finishing rolling was set to 80 at the time of rolling.
0 to 1050 ° C. (1073 to 1323 K), and the cooling rate after winding was 0.1 to 10 ° C./sec. Further, in order to change the characteristic value, the temperature is set to 600 to 700 ° C. (873
９７973K), and the annealing treatment was performed while changing the holding time in the range of 2 to 5 hours.

[0042]

[Table 1]

The obtained wire was cut in the cross-sectional direction, embedded and polished, and then measured for Vickers hardness. After embedding and polishing the cross section, the structure was observed with an optical microscope after acid corrosion, and the fraction of the fine structure was measured by image processing. In the hardness measurement, D / 16, D / 8, and D / 4 were respectively set at four places as described above,
/ 2 (D: diameter of wire rod) were measured at 13 points. The mechanical properties of the wire are 100
This sample was taken, and the tensile strength and the drawn value were measured using a tensile tester.

After the obtained wire was subjected to pickling and zinc phosphate film treatment, SV treatment and wire drawing were examined. At this time, the SV processability was D1 die: 7.7 mm
φ, SV chipper: Performed using 7.4 mmφ. The linear velocity is usually in the range of 50 to 70 m / min. However, in this experiment, in order to easily cause problems such as chipping of the SV chipper and to make the effect of the present invention more clear,
The test was performed at a linear velocity of 0 m / min. In addition, linear velocity 80m / m
For steel materials that did not have a problem in the
The effect of the invention was further confirmed by increasing the speed to 0 m / min.

For evaluation of drawability, a drawing die having an approach angle of about 12 ° is generally used.
In this experiment, an approach angle of 20 ° was adopted so that the effect of the present invention could be more clearly displayed by easily causing a cross section of a cuppy. Further, with respect to a steel material having no problem with a 20 ° die, the effect of the present invention was confirmed using a die having an approach angle of 30 ° which is a severe condition which is hardly used industrially.

Further, a wire having a diameter of 3.35 mm and a strength of 1,9 mm, which could be drawn under each of the drawability evaluation conditions, was used.
An oil-tempered wire of 50 MPa was produced, and a winding test was performed at D / d = 2, and the effect (winding property) of the invention was confirmed by the presence or absence of breakage. Table 2 below shows the mechanical properties of steel materials.
The evaluation results are shown in Table 3 below. Table 2 below
The evaluation criteria for the SV property, wire drawing property and winding property shown in Table 1 are as follows.

(SV property) 問題: No problem Δ: No problem in SV property, but slightly increased diameter ×: Occurrence of problems such as chipper chipping (drawability) ：: Wire drawing possible at 80% or more of area reduction rate ×: disconnection occurred (woundability) ○: no breakage ×: breakage occurred

[0048]

[Table 2]

[0049]

[Table 3]

From these results, the following can be considered.

First, regarding the evaluation of the SV property, in the comparative examples C-1, C-2 and C-5 having high tensile strengths, chipper chipping occurred when the linear velocity was 80 m / min.
For other steel materials, the linear velocity is set to 100 m / mi.
When the value was increased to n, in the case of C-4, the chips were entangled with the wire at the chipper portion, and the chips pressed the wire at the chipper portion, so that portions not shaved on the SV material occurred.

For B-3 and B-5, no problem occurred during the SV treatment, but at the beginning and end of the SV treatment, a chipper diameter increase of +0.05 mm was recognized. On the other hand, in other steel materials, not only does not cause a problem in the SV processability, but the increase in the chipper diameter is also increased by + 0.0%.
It was good within 2 mm.

On the other hand, in the evaluation of drawability performed using a dice having an approach angle of 20 °, C-1, C-3 and C-
In the case of the material having a high tensile strength or a low drawing value of 5, the disconnection of the cappy occurred. Other materials could be drawn up to a diameter of 3.35 mm, and were subjected to a winding test after working on an oil-tempered wire. As a result, a Cappie-shaped breakage was observed in C-2.

Then, A-1, A-2, B-1 to B-6
It was confirmed that the steel materials C and C-4 exhibited excellent drawability. Therefore, the effect of the present invention was confirmed by using these steel materials and performing wire drawing using a die having an approach angle of 30 °.

As a result, in B-1 and B-6, the cut-off line occurred until the area reduction rate exceeded 80%. When a winding test was performed on A-1, A-2, and B-2 to B-5 after forming into an oil-tempered wire, the winding test was performed on B-2, B-4, and B-5. Breakage occurred.

From the above results, in order to satisfy both the SV property and the raw drawability, A-1, A-2 and B-1 to B-1
It turned out that the characteristic of -6 was required at a minimum. Also,
To ensure stable manufacturability under any conditions, A
It has been found that it is preferable to have the characteristics of -1 and A-2.

Further, a steel material having the same chemical composition as described above was used, and a rolled material having an increased tensile strength as shown in Table 4 by changing the rolling conditions variously was used. Physical properties and SV properties were examined in the same manner for the wire rods whose strength and aperture value were adjusted, and the results shown in Table 4 were obtained.

[0058]

[Table 4]

In the experimental example shown in Table 4, 155 mm
A wire having a reduced tensile strength of 8.0 mm is used to increase the tensile strength, and is subjected to a softening annealing treatment to reduce the tensile strength and increase the drawing value.

As is evident from Table 4, even in the case of a wire rod having an excessively high tensile strength in a rolled state and a low drawing ratio, a softening and annealing treatment is performed under suitable conditions to obtain a proper tensile strength and a proper drawing value. , Especially the softened and annealed wires 5 and 7 show excellent SV properties.

[0061]

Industrial Applicability The present invention is constituted as described above, and it is possible to obtain a spring steel which secures both SV properties and draw and draw properties which are important in a spring manufacturing process. By performing the treatment under predetermined conditions using, a spring steel wire exhibiting excellent spring characteristics can be provided.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of mechanical properties (tensile strength, reduction value) of a steel material on workability.

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Nobuhiko Ibaraki 2 Nadahama-Higashi-cho, Nada-ku, Kobe Kobe Steel Works Kobe Works F-term (reference) 4K043 AA02 AB04 AB05 AB10 AB15 AB25 AB26 AB27 AB28 BA04 BA05 BA06 DA00 DA05 FA03 FA07 FA11

Claims (6)

[Claims] 1. A rolled spring steel which satisfies the following mechanical properties and has excellent workability. Tensile strength (maximum value) ≦ 1200 MPa, 30% (minimum value) ≦ aperture value ≦ 70% (maximum value) 2. The spring steel according to claim 1, wherein the spring steel has been soft-annealed after rolling under the following conditions. Soft annealing treatment: In the gas phase, at a heating temperature of 873 to 1023 K, and at a heating temperature T (K) and a heating time t (minute).
Heat treatment that satisfies the relationship of T × Δt = 7300 to 15000 (K · min). 3. The metal structure is mainly composed of pearlite or ferrite and pearlite, and the fraction of the supercooled structure is 10%.
The steel for spring according to claim 1 or 2, wherein: 4. The spring steel according to claim 1, wherein the standard deviation σ of Vickers hardness (Hv) in the cross section is 20 or less. 5. The spring steel according to claim 1, wherein the maximum value of Vickers hardness (Hv) in the cross section is 380 or less. 6. The steel for spring according to claim 1, which is subjected to the following (a) after shaving or wire drawing.
A method for producing a steel wire for a spring, which comprises performing an oil tempering treatment after performing any one of the treatments (c) to (c). (a) A patenting process is performed after the scalping process. (b) After the scalping process, a heating temperature of 823 to 973 K in a gas phase, a heating temperature T (K) and a heating time t (minute).
However, heat treatment is performed within a range that satisfies the relationship of T × Δt = 6700 to 12000 (K · min). (C) After shaving, short-time heating is performed at 723 to 1023 K to soften the surface hardened layer.