JP2002275591A - Stainless steel for presser bar spring of electronic parts having high strength and excellent fatigue characteristic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide stainless steel for a pressure bar spring of electronic parts which has high strength and excellent fatigue characteristics, and requires thinning. SOLUTION: The stainless steel has a composition containing <=0.15% C, 1.0 to 4.0% Si, <=5.0% Mn, <=0.040% P, <=0.010% S, 4.0 to 10.0% Ni, 12.0 to 18.0% Cr, 0 to 3.5% Cu, 1.0 to 5.0% Mo, <=0.15% N, <=0.02% Nb, <=0.015% Ti, <=0.015% O, >=0.10% C+N and >=3.5% Si+Mo, and the balance substantially Fe, and in which the value of Md(N) defined as Md(N)=580-(520C+2Si+16Mn+16Cr+23Ni+300N+10Mo) lies in the range of 0 to 100. The maximum area of nonmetallic inclusions precipitated into the matrix is preferably controlled to <=40 μm<2> .

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stainless steel which is suitable for a holding spring for electronic parts and has high strength and excellent fatigue characteristics.

[0002]

2. Description of the Related Art Martensitic stainless steel, work hardening stainless steel, precipitation hardening stainless steel and the like have been conventionally used as a material of a holding spring for electronic parts. Martensitic stainless steel is a steel type hardened by rapidly cooling from a high-temperature austenite state and transforming it to martensite, and corresponds to SUS410, SUS402J2, and the like. Although high strength and toughness can be obtained by quenching and tempering, ultra-thin products are deformed by thermal strain during quenching, making it difficult to produce products having a target shape.

In applications where deformation due to thermal strain is regarded as a problem,
A work hardening type austenitic stainless steel represented by SUS301 and SUS304 is used. The work-hardening austenitic stainless steel exhibits an austenitic phase in a solution-treated state, and is hardened by forming work-induced martensite in a subsequent cold rolling step. The strength of the work hardening type austenitic stainless steel depends on the amount of cold work and the amount of martensite, but it is very difficult to adjust the strength only by cold work. Further, when the cold working ratio is significantly increased, the anisotropy of the material increases, and the toughness also decreases.

[0004] Precipitation hardening stainless steel is a type of steel that hardens by aging treatment by adding an element having high precipitation hardening ability, and SUS630 added with Cu and SUS631 added with Al are known. Cu-added precipitation hardening stainless steel is
It exhibits a martensitic single phase after solution treatment, and hardens by subsequent aging treatment.
It stays at about 00 N / mm ² .

[0005] The precipitation hardening stainless steel to which Al is added is obtained by partially transforming a metastable austenite phase formed by solution treatment into martensite by cold working or the like, and then aging treatment to form an intermetallic compound such as Ni ₃ Al. Is precipitated. Hardening due to precipitation of intermetallic compounds results in considerably high strength steel. Incidentally, by positively depositing the martensite layer, the tensile strength is set to 1800 N / m.
Until m ² can be increased.

[0006]

By the way, with the remarkable development of the IT-related field, connectors for electronic components have been reduced in size and thickness, and in mobile phones, mobile communication and the like, downsizing of a holding spring has been required.・ Thinning is strongly required. Further, if the holding spring can be made thinner by further increasing the strength, the weight of the material can be reduced.

[0007] In order to make it possible to reduce the thickness, it is necessary to increase the strength and improve the fatigue characteristics. However, the demand for the increase in the strength and the improvement in the fatigue characteristics is based on conventional martensitic stainless steel, work hardening stainless steel, and the like. Precipitation hardening type stainless steels and the like have not been able to respond sufficiently. In order to increase the strength of austenitic stainless steel using age hardening and work hardening, it is necessary to set a high cold rolling rate, resulting in a significant decrease in toughness and a deterioration in the shape of thin products. I do. Further, a decrease in strength becomes apparent due to a deviation from a predetermined aging temperature.

In order to achieve both the strength and toughness required for the holding spring for electronic parts, the work hardening is increased by cold rolling, the aging treatment temperature is set high, and the toughness reduced by strong cold rolling is recovered. And a decrease in hardness due to high-temperature aging is required. The present inventors have sought to use a material having a high tempering resistance on the premise that a decrease in hardness due to high-temperature aging is small, and to find a material for a pressing spring for an electronic component from the material side.

[0009]

DISCLOSURE OF THE INVENTION The present invention has been devised based on the above-described investigation results, and a metastable austenitic stainless steel having a two-phase structure of a work-induced martensite phase and an austenite phase is excellent. It is an object of the present invention to provide a high-strength stainless steel excellent in fatigue characteristics that satisfies the required characteristics of a holding spring for an electronic component based on the knowledge that it exhibits strength and toughness.

[0010] In order to achieve the object, the stainless steel for electronic part press springs of the present invention has a C content of 0.15 mass% or less, a Si content of 1.0 to 4.0 mass%, and a Mn content of 5.0 mass%.
Hereinafter, P: 0.040% by mass or less, S: 0.010% by mass or less, Ni: 4.0 to 10.0% by mass, Cr: 12.12.
0 to 18.0% by mass, Cu: 0 to 3.5% by mass, Mo:
1.0 to 5.0% by mass, N: 0.15% by mass or less, N
b: 0.02% by mass or less; Ti: 0.015% by mass or less; O: 0.015% by mass or less; C + N: 0.10% by mass or more; Si + Mo: 3.5% by mass or more; Fe composition, Md (N) = 580- (520C
+ 2Si + 16Mn + 16Cr + 23Ni + 300N +
The Md (N) value defined as 10 Mo) is in the range of 0 to 100. Further, it is preferable that the maximum area of the nonmetallic inclusions deposited on the matrix is regulated to 40 μm ² or less.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have investigated and studied various stainless steel sheets which satisfy the required characteristics of a holding spring for electronic parts which can withstand a severe use environment. As a result, when a metastable austenitic stainless steel having a two-phase structure of a work-induced martensitic phase and an austenite phase is used instead of the conventional single-phase martensitic stainless steel, an electron with a high balance of strength and fatigue properties is obtained. It has been found that a holding spring for parts can be obtained.

As described above, stainless steel whose components and compositions are specified hardens by work hardening and transformation strengthening by work-induced martensitic transformation, and age hardens by heat treatment after cold working. In addition, due to the component design that facilitates age hardening, toughness can be recovered by aging at a relatively low temperature for a short time. The aging treatment at a relatively low temperature and for a short time is effective for preventing the formation of coarse precipitates which are the starting points of fatigue fracture, and also improves the fatigue characteristics and corrosion resistance. Stainless steel, which has high strength and excellent fatigue properties, is used as a material for other leaf springs, coil springs, inner blades, metal gaskets, and the like, as well as holding springs for electronic components, which are required to be thinner.

Next, the components, contents and the like contained in the stainless steel of the present invention will be described. C: 0.15% by mass or less Austenite-forming element, which is an alloy component that suppresses the formation of δ ferrite in a high temperature range and strengthens work-induced martensite generated in cold working. However, in the component system of the present invention having a high Si content, the solid solubility limit of C is low.
Therefore, if the C content is increased, coarse C
The r-based carbides precipitate and cause intergranular corrosion resistance and deterioration of fatigue properties. Therefore, the upper limit of the C content is set to 0.15% by mass. The preferred range of the C content is 0.05 to
0.1% by mass.

Si: 1.0 to 4.0 mass% Although it is also an alloy component added as a deoxidizing agent at the steelmaking stage, the content of Si added as a deoxidizing agent is SUS301, SU
As seen in work hardening stainless steel such as S304.
0 mass% or less. On the other hand, in the present invention, the Si content is set high, and the effect of significantly promoting the formation of a work-induced martensite phase during cold working is utilized. S
i hardens the work-induced martensite phase,
It also forms a solid solution in the austenite phase and hardens, increasing the strength after cold working. Further, in the aging treatment, age hardening is promoted by interaction with Cu. Such curing becomes remarkable when the Si content is 1.0% by mass or more. However, when an excessive amount of Si exceeding 4.0% by mass is included, high-temperature cracking is likely to occur, and troubles are likely to occur in manufacturing. The preferred range of the Si content is as follows.
0 to 3.5% by mass.

Mn: 5.0 mass% or less Mn is an alloy component for stabilizing an austenite phase, and the Mn content is determined according to the balance with other alloy components. However, when an excessive amount of Mn exceeding 5.0% by mass is contained, work-induced martensite transformation hardly occurs during cold rolling, and no transformation hardening ability can be expected. The Mn content is preferably set to 4.5% by mass or less. P: 0.040% by mass or less An alloy component having a large solid solution strengthening ability, but the upper limit of the P content is set to 0.040% by mass in order to suppress the adverse effect on toughness.

S: 0.010% by mass or less S is a component that becomes a nonmetallic inclusion that causes deterioration of fatigue characteristics. When used as an ultra-thin plate such as a pressing spring for an electronic component, the S content is reduced. It is necessary to reduce nonmetallic inclusions. Therefore, the upper limit of the S content is set to 0.010% by mass. Ni: 4.0 to 10.0 mass% Ni is an alloy component for stabilizing the austenite phase at high temperature and room temperature, but is converted to a metastable austenite phase at room temperature and is formed such that a work-induced martensite phase is formed by cold working.
The content is determined. When the Ni content is less than 4.0% by mass, a large amount of δ ferrite is generated in a high temperature range, and a martensite phase is generated in a cooling process to room temperature, so that it cannot exist as an austenite single phase. Conversely, if an excessive amount of Ni exceeding 10.0% by mass is contained, work-induced martensitic transformation hardly occurs during cold working, and transformation hardening cannot be expected. Preferably, the N content is in the range of 5.0 to 9.5% by mass.
Set the i content.

Cr: 12.0 to 18.0 mass% An alloy component essential for improving corrosion resistance, and 12.0 mass%
As described above, the effect of adding Cr on the improvement of corrosion resistance becomes remarkable. However, Cr, which is a ferrite forming element, is changed to 18.0.
When added in excess of mass%, a large amount of δ ferrite is formed in a high temperature range. The formation of δ ferrite is C, N, N
Addition of an austenite-forming element such as i, Mn, or Cu suppresses the formation of δ ferrite, but excessive addition of an austenite-forming element stabilizes the austenite phase at room temperature. As a result, no work-induced martensite transformation occurs during cold working, and high strength cannot be obtained after aging treatment. The preferred Cr content is 12.0 to 1
6.5% by mass.

Cu: 0 to 3.5 mass% is an alloy component added as necessary, and promotes age hardening during aging treatment by interaction with Si. The effect on age hardening increases with increasing Cu content. However, excessive addition of Cu exceeding 3.5% by mass lowers hot workability and causes cracking. C
The u content is preferably set in the range of 1.0 to 3.0% by mass.

Mo: 1.0 to 5.0 mass% Mo is an alloy component that has an effect of improving corrosion resistance and having a function of finely distributing carbonitrides during aging treatment. Excessive rolling strain, which adversely affects fatigue properties, can be reduced by setting the aging temperature high, but at high aging temperatures, the strain may be rapidly released. In this regard, in the component system of the present invention, rapid release of strain is regulated by the addition of Mo. Further, since precipitates that contribute to strength are formed during the aging treatment, a decrease in strength is prevented even when the aging treatment is performed in a considerably high temperature range. Such an effect is 1.0% by mass.
It becomes remarkable by the above Mo addition. However, when an excessive amount of Mo exceeding 5.0% by mass is added, δ ferrite is easily formed in a high temperature region. Also, when the Mo content increases, the deformation resistance at high temperatures increases, and the hot workability may decrease. Preferably, the Mo content is set in the range of 1.0 to 4.5% by mass.

N: 0.15% by mass or less N is an austenite-forming element and is an alloy component extremely effective in hardening the austenite phase and the martensite phase. However, since excessive addition of N causes blowholes during casting, the upper limit of the N content was set to 0.15% by mass. Nb: 0.02% by mass or less, Ti: 0.015% by mass or less
Since it is a component that generates non-metallic inclusions harmful to lower fatigue properties, the upper limits of Nb and Ti are set to 0.
It was regulated to 02% by mass and 0.015% by mass.

O: 0.015% by mass or less Oxide-based nonmetallic inclusions are formed to lower the cleanliness of the steel and adversely affect fatigue properties, breathability and bending workability. Such adverse effects can be suppressed by regulating the O content to 0.015% by mass or less. C + N: 0.10% by mass or more C and N are components exhibiting the same effect. However, in order to sufficiently exert the effect, the total content of C and N must be 0.10% by mass or more. is necessary.

Si + Mo: 3.5% by mass or more In the component system of the present invention, high strength is achieved by precipitating Mo-based precipitates during aging treatment. But M
When the o-based precipitate grows coarsely, it becomes a starting point of fatigue fracture and deteriorates fatigue characteristics. However, since the amount of Si is increased, the Mo-based precipitate does not grow coarsely and takes a fine dispersion form, thereby suppressing the adverse effect on the fatigue characteristics. Si, M
By setting the total content of o to 3.5% by mass or more,
Fine dispersion of the Mo-based precipitate is guaranteed.

Md (N) value: 0 to 100 In the present invention, the work-induced martensitic transformation at the time of cold rolling is used as one of the means for increasing the strength. Therefore, an alloy design in which the stability of the austenite phase with respect to working is adjusted so that the work-induced martensite phase is easily formed in accordance with the strain imparted by the cold rolling after the solution treatment. An Md (N) value defined by the following equation is used as an index representing the stability of the austenite phase. Md (N) = 580- (520C + 2Si + 16Mn + 16Cr + 23N
(i + 300N + 10Mo) By adjusting the Md (N) value to 0 to 100, an appropriate amount of a work-induced martensite phase is generated at the time of cold working, and high strength can be achieved by work hardening and transformation effects. On the other hand, in a steel type having an Md (N) value of less than 0, the austenite phase has high stability against cold rolling, and a work-induced martensite phase contributing to high strength is not sufficiently generated. Conversely, if the Md (N) value exceeds 100, a large amount of work-induced martensite phase is formed at a relatively low cold rolling reduction, and the fatigue properties of the product are rather deteriorated.

Non-metallic inclusions deposited in the matrix
Non-metallic inclusions precipitated in the matrix of 40 μm ² or less serve as starting points of fatigue fracture and lower the fatigue properties of the material. The effect on the deterioration of the fatigue characteristics becomes more remarkable as the size of the nonmetallic inclusion increases. Maximum area of non-metallic inclusions is 40μ
When the pressure is regulated to m ² or less, fatigue fracture due to non-metallic inclusions is reduced, and the durability of the holding spring made of an extremely thin plate is improved.

[0025]

EXAMPLE Stainless steel having the composition shown in Table 1 was melted in a vacuum melting furnace, and subjected to forging, hot rolling, and intermediate annealing to obtain a sheet thickness of 0.2 mm.
Until cold-rolled. After subjecting the cold-rolled steel strip to a solution treatment of holding at 1050 ° C. × 1 minute, water-cooled, and further, a plate thickness of 0.07 mm
Until cold-rolled. The obtained cold-rolled steel strip was aged at 500 ° C. for 1 minute.

[0026]

Specimens were cut out from each stainless steel, and nonmetallic inclusions were measured and subjected to mechanical tests and fatigue tests. In the area measurement of non-metallic inclusions, 2 cm in the rolling direction
Using 10 test pieces cut from each stainless steel plate with a length of, the maximum area ratio was obtained from image processing information obtained from the total cross-sectional area in the rolling direction. In mechanical tests, JIS Z2
Using a No. 13B test piece specified in 201, tensile strength was measured by a tensile test method specified in JIS Z2241.

In the fatigue test, the length of the parallel portion was 100 mm,
10mm, 11mm, 12m diameter specimens of 3mm width
m, 13mm, 14mm, 15mm, 17.5mm, 2
A bending-tensile fatigue test was performed in which the belt was hung on a 0 mm pulley and both ends were pulled by a belt hung on a driving pulley. The test piece was moved at a speed of 500 rpm (the number of up-and-down
0 times / min) is reciprocated, the minimum stress is the maximum surface stress applied to the outermost layer of the test KATAHIRA Gyobu under conditions to be 50 N / mm ² was varied in the range of 1392~715N / mm ^2. Bending - tensile fatigue test was repeated 10 ⁷ cycles,
The fatigue limit that does not break was determined.

As can be seen from the survey results in Table 2, the steel type N
o. 1 to 7 stainless steel (Example of the present invention) has a tensile strength of 1
And at 800 N / mm ² or more, the bending - Both fatigue limit by the tensile fatigue test 1100 N / mm ² or more, which is high, and was replete with strength and fatigue properties required for electronic components presser spring in an extremely thin. In contrast, steel grades Nos. 8 to 13
All of the stainless steels (comparative examples) exhibited a low fatigue limit of 1000 N / mm ² or less in a bending-tensile fatigue test. In addition, since steel type No. 8 has a too low Md (N) value and does not sufficiently form a work-induced martensite phase,
The strength was insufficient. Conversely, steel type with too high Md (N) value
In No. 9, since the amount of austenite contributing to ductility and toughness was insufficient, the fatigue limit was low. Steel type No. 11 having a small Mo content had insufficient strength due to insufficient age hardening, and exhibited a low fatigue limit. Steel type No.1 with excessive S content
0, steel type No. 12 with excessive Ti content, steel type No. 2 with excessive O content
Sample No. 13 was inferior in fatigue characteristics because it had a structure in which non-metallic inclusions having a maximum area exceeding 40 μm ² were dispersed.

[0030]

[0031]

As described above, the stainless steel of the present invention is made of an austenitic stainless steel whose austenite stability is appropriately adjusted and has a strength by work hardening, transformation effect and age hardening during cold rolling. And the fatigue characteristics are improved. Therefore, even an extremely thin plate is used in a wide range of fields, such as a leaf spring, a coil spring, and a metal gasket, such as a holding spring for an electronic component, which is required to be thin and excellent in strength and fatigue characteristics.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Kenichi Morimoto 4976 Nomura Minamicho, Shinnanyo-shi, Yamaguchi Prefecture F-term in the Nisshin Steel Co., Ltd. Stainless Steel Business Division (reference) 3J059 AB11 BC02 BC19 GA30

Claims (2)

[Claims] C: 0.15% by mass or less, Si: 1.0
To 4.0% by mass, Mn: 5.0% by mass or less, P: 0.0
40 mass% or less, S: 0.010 mass% or less, Ni:
4.0 to 10.0 mass%, Cr: 12.0 to 18.0 mass%, Cu: 0 to 3.5 mass%, Mo: 1.0 to 5.0.
% By mass, N: 0.15% by mass or less, Nb: 0.02% by mass or less, Ti: 0.015% by mass or less, O: 0.015%
Mass% or less, C + N: 0.10 mass% or more, Si + M
o: 3.5 mass% or more, the balance substantially has a composition of Fe, and Md (N) = 580− (520C + 2Si + 16M)
A stainless steel for a pressing spring for electronic parts, which has high strength and excellent fatigue properties, wherein the Md (N) value defined as (n + 16Cr + 23Ni + 300N + 10Mo) is in the range of 0-100. 2. The stainless steel for electronic component presser springs according to claim 1, wherein the maximum area of the nonmetallic inclusions precipitated in the matrix is 40 μm ² or less.