JP4522785B2 - Stainless steel contact material - Google Patents

Description

  The present invention relates to a stainless steel contact material suitable as a wiring terminal, lead frame, connector or the like incorporated in an electric / electronic device.

  Conventionally, copper-based materials with good conductivity have been used for wiring terminals such as harnesses that are incorporated in electric / electronic devices and connect copper wires. Among copper-based materials, cold-rolled materials having low internal resistance and excellent spring properties are frequently used. Cold rolled material that is soft and has low elongation, when manufacturing small and precise parts by punching, has a small punching load applied to the machined surface and is less likely to generate burrs. It is a material suitable for punching.

  However, copper-based materials are inferior in corrosion resistance. When an electrical wiring terminal made of a copper-based material is used in an exposed state, surface oxidation proceeds to increase surface contact electrical resistance, which may change the characteristics of electrical and electronic components. An increase in surface contact electrical resistance due to surface oxidation can be suppressed by plating with Sn, Ni or the like. However, since a plating process is required, the product cost increases, and depending on the use environment, the necessary corrosion resistance may not be imparted.

Therefore, among the electrical contact materials incorporated in electrical parts and electronic parts, wiring terminals where weak current flows do not need to consider heat generation due to the internal resistance of the connected parts, so they have excellent corrosion resistance and spring properties. The use of stainless steel as a base material for wiring terminals has been studied. The present applicant also introduced stainless steel exhibiting low contact resistance required for contact materials by precipitating a second phase mainly composed of Cu or forming a Cu concentrated layer on the stainless steel surface (patent) Reference 1).
Japanese Patent Laid-Open No. 2001-89865

Stainless steel in which a second phase mainly composed of Cu is precipitated and stainless steel in which a Cu concentrated layer is formed on the surface of the steel sheet show a much lower contact resistance than normal stainless steel, but depending on the use environment In some cases, the contact resistance may increase as the period of use increases. Then, the influence which the 2nd phase and Cu concentrated layer which consist mainly of Cu have on the fall of contact resistance was investigated further. As a result, when a very thin Ni plating layer is formed on the steel plate surface in addition to the Cu-based second phase, Cu enriched layer, the contact resistance is maintained at a low level even after long-term use in a humid environment or corrosive atmosphere. I found out that
The present invention is based on the knowledge that the deterioration resistance of the low contact resistance is improved by the coexistence of the second phase mainly composed of Cu, the Cu enriched layer, and the ultrathin Ni plating layer, and the ultrathin Ni plating layer is provided. Thus, an object of the present invention is to provide a contact material that further improves the characteristics of the stainless steel introduced above.

Stainless steel contact material of the present invention is Cu: 1.0 mass% or more, Cr: stainless steel containing 9 mass% or more, or Cu: 1.0 mass% or more, Cr: 9 mass% or more, Ni: 6 mass Ferritic or austenitic stainless steel containing at least% is used as the base material.
In the base material, the second phase mainly composed of Cu is dispersed in the matrix at a ratio of 0.2% by volume or more, and the formation and growth of the passive film can be suppressed at the deposition site. Two phases are exposed. The Cu enriched layer is formed on the surface of the substrate by a combination of bright annealing, atmospheric annealing, and pickling in an atmosphere with a low dew point, but the component design and processing take into account that the diffusion coefficient differs for each component element. The mass ratio of Cu / (Si + Mn) can be adjusted to 0.5 or more depending on conditions. It is also possible to coexist the precipitation of the second phase mainly composed of Cu and the formation of a Cu concentrated layer.

  In addition to the precipitation of the second phase mainly composed of Cu and / or the generation of the Cu concentrated layer, a Ni plating layer is formed on the surface of the substrate. Film thickness: 0.05 to 0.7 μm makes the Ni plating layer not a continuous film that completely covers the surface of the steel sheet, but a portion where the base material is exposed at a number of locations. A state in which a two-phase or Cu concentrated layer coexists on the stainless steel surface together with the Ni plating layer is obtained.

Effects and embodiments of the invention

  Stainless steel in which a second phase mainly composed of Cu (hereinafter referred to as “Cu-rich phase”) is precipitated or a Cu concentrated layer is formed exhibits a much lower contact resistance than conventional stainless steel. However, by providing an extremely thin Ni plating layer on the surface of the stainless steel, an increase in contact resistance is suppressed, and the contact resistance is maintained at a low level for a long time even in a wet environment or a corrosive atmosphere. The reason why the deterioration resistance of the low contact resistance is improved by the extremely thin Ni plating layer is presumed as follows, and is supported in the examples described later.

  An extremely thin Ni plating layer having a thickness of 0.05 to 0.7 μm exhibits a surface in which the exposed portion of the base steel and the Ni plating layer are mixed without completely covering the entire surface of the steel plate. That is, the Cu rich phase 2 and the Ni plating layer 3 precipitated in the matrix coexist on the surface of the stainless steel 1 (FIG. 1). Since the standard electrode potential (25 ° C.) is +0.34 V for Cu and −0.23 V for Ni, Ni becomes a base metal with respect to Cu and is easily eluted as metal ions in a humid environment. The elution of Ni prevents surface oxidation of the Ni plating layer 3 and keeps the Cu rich phase 2 in an active state. As a result, an increase in contact resistance due to the growth of the oxide film is suppressed, and the action of the Cu rich phase 2 effective for reducing the contact resistance is sustained.

  Such an action / effect of the Ni plating layer 3 is not manifested in the Ni plating layer 3 that is thick enough to cover the entire surface of the stainless steel 1. It can be understood that it comes from. Even when the Cu enriched layer is formed on the extreme surface layer of the stainless steel 1 in place of the Cu rich phase 2, an increase in contact resistance is suppressed by the interaction between the similar Cu enriched layer and the Ni plating layer 3.

As long as the stainless steel used as a base material has a Cu-rich phase precipitated at 0.2% by volume or more, or a Cu-concentrated layer having a Cu / (Si + Mn) mass ratio of 0.5 or more is formed, ferrite Either an austenite or austenite may be used.
For example, C + N: 0.1 mass% or less, Si: 1.0 mass% or less, Mn: 1.0 mass% or less, Cr: 9.0 to 25.0 mass%, Cu: 1.0 There is stainless steel containing ~ 3.0 wt%. In the austenitic system, C + N: 0.2 mass% or less, Si: 1.0 mass% or less, Mn: 2.0 mass% or less, Cr: 9.0 to 25.0 mass%, Ni: 5.0 to 15 There is stainless steel containing 0.0 mass%, Cu: 1.0-4.0 mass%. Both ferrite and austenite can contain 1.0 mass% or less of Ti and / or Nb.

C and N promote the precipitation of a Cu-rich phase by forming chromium carbide or the like. However, excessive addition adversely affects manufacturability and corrosion resistance, so the upper limit of C + N was set to 0.1% by mass.
Si is an effective component for improving the corrosion resistance, but excessive addition deteriorates manufacturability, so the upper limit was made 1.0 mass%.
Mn improves the manufacturability and fixes S in the steel as MnS. However, excessive addition adversely affects the corrosion resistance, so that 1.0% by mass for ferrite and 2.0% by mass for austenitic. Was the upper limit of the Mn content.

Cr is an alloy component essential for ensuring the corrosion resistance of stainless steel, and is effective when the Cr content is 9.0% by mass or more. However, excessive addition of Cr exceeding 25.0% by mass decreases manufacturability.
In austenitic stainless steel, Ni is added as an essential component for stabilizing the austenitic phase, but Ni is also effective in improving corrosion resistance. The effect of addition of Ni is seen at 5.0 mass% or more, but excessive addition causes an increase in steel material cost, so 15.0 mass% was made the upper limit of the Ni content.

Cu is a component necessary for the precipitation of a Cu-rich phase that reduces the contact resistance of the surface or the formation of a Cu concentrated layer, and the effect of addition of Cu is observed at 1.0 mass% or more. However, excessive addition causes a decrease in manufacturability and corrosion resistance, so the upper limit of the Cu addition amount was set to 3.0% by mass for the ferrite type and 4.0% by mass for the austenite type.
Ti and Nb can be added as necessary. Ti and Nb both fix C and N in steel as carbonitrides, reduce C and N dissolved in the matrix, and improve manufacturability and corrosion resistance. However, the excessive addition of Ti and Nb also causes a hindrance to the manufacturability, so the upper limit was made 1.0 mass%.

Cu-rich phase: 0.2% by volume or more The Cu-rich phase is uniformly dispersed in the matrix of the ferritic stainless steel plate, and is also dispersed on the steel plate surface at the same distribution ratio. As a result of investigating the relationship between the Cu-rich phase and the surface contact electrical resistance, when the Cu-rich phase is precipitated at a rate of 0.2% by volume or more, the surface contact electrical resistance comparable to that of the conventional Ni plating material is obtained. It was found that
By performing an aging treatment for about 1 hour or more at around 800 ° C. in the process up to the final annealing in the stainless steel plate production line, a Cu rich phase is precipitated. The amount of precipitation of the Cu-rich phase can be controlled not only by heat treatment conditions such as temperature and time, but also by rolling conditions that adjust the stainless steel sheet so that the Cu-rich phase is likely to precipitate. When Cu is concentrated on the passive film or the outermost surface layer of the substrate in addition to the precipitation of the Cu-rich phase, a lower surface contact electric resistance of 1Ω or less is exhibited.

Cu concentrated layer: Cu / (Si + Mn) ≧ 0.5
The surface contact electrical resistance decreases as the Cu concentration of the outermost layer of the substrate or the passive film increases. Surface contact electrical resistance equivalent to that of a conventional Ni plating material can be achieved by forming a Cu concentrated layer in which the mass ratio Cu / (Si + Mn) of Cu to Si and Mn is 0.5 or more.
For the formation of the Cu enriched layer, a method of bright annealing a stainless steel plate in an atmosphere having a dew point of −30 ° C. or lower is employed as the final annealing. When the dew point of the annealing atmosphere is lowered, the oxidation reaction is suppressed, and the increase of the metal oxide having a high specific electric resistance is suppressed. As a result, the metal Cu or Cu oxide is concentrated on the passive film or the outermost layer. On the other hand, in an annealing atmosphere with a dew point exceeding −30 ° C., diffusion of Si, Mn, etc. from the inside of the base material to the surface layer is promoted as the oxidation of Si, Mn, etc. proceeds, and a large amount of metal oxide with high specific electric resistance is produced. The passive film or outermost layer contained in is formed.

  Instead of bright annealing, a necessary Cu concentrated layer is formed by a combination of atmospheric annealing and pickling. When a stainless steel plate is annealed to the atmosphere, a scale containing oxides such as Cr, Fe, Mn, Si, and Cu is formed on the surface of the steel plate, but a passive film is formed after the scale is removed by pickling. In pickling using a mixed acid such as hydrofluoric acid-nitric acid or sulfuric acid-nitric acid, Cu and Cu-rich phases do not elute preferentially from the stainless steel plate. The Cu concentration is maintained. The mixed acid used for the pickling does not impose any particular restrictions on the type and concentration of the acid, but generally a mixed acid of sulfuric acid, hydrofluoric acid and nitric acid having a concentration of about 10% by volume is preferable.

By electroplating a stainless steel plate on which a Cu-rich phase has precipitated and / or a Cu enriched layer has been formed, a Ni plating layer having a thickness of 0.05 to 0.7 μm is formed on the stainless steel surface. The action and effect of coexistence with the Cu-rich phase and Cu-enriched layer can be seen in the Ni plating layer with a film thickness of 0.05 μm or more, but with a thick film over 0.7 μm, the Ni plating layer coverage on the stainless steel surface , The effects of the Cu rich phase and the Cu concentrated layer are impaired, and the characteristics are deteriorated to the same extent as those of the conventional Ni plating material.
A Ni plating layer having a thickness of 0.05 to 0.7 μm can be formed by an extremely short time electric flash plating. For example, using a total chloride bath composed of nickel chloride and hydrochloric acid with a cathode current efficiency of 15 to 25%, energizing under conditions of bath temperature: 25 to 40 ° C. and current density: 0.2 to 1.5 kA / m ² By changing the time, a Ni plating layer having a required thickness is formed.

  Austenitic stainless steels A1 and A2 and ferritic stainless steels F1 and F2 having the composition shown in Table 1 were used as substrates. Prior to final annealing, each stainless steel plate was subjected to a precipitation treatment at 800 ° C. for 24 hours to precipitate a Cu-rich phase. The amount of precipitation of the Cu-rich phase is based on the ratio of the Cu-rich phase deposited on the matrix by electropolishing a specimen collected from the stainless steel plate after the final annealing and observing the metal structure with a transmission electron microscope. Asked.

  Each stainless steel plate was immersed in a total chloride bath and Ni flash plated under the conditions shown in Table 2. For comparison, SUS304 and SUS430 stainless steel with no Cu-rich phase precipitation were Ni flash plated under the same conditions. The film thickness of the Ni plating layer formed on the stainless steel surface was adjusted in the range of 0.02 to 0.7 μm depending on the plating time.

A pure gold counter electrode and a measurement terminal were brought into contact with the stainless steel plate immediately after Ni plating, and the surface contact resistance (initial value of contact resistance) was measured in a state where a load of 100 g was applied to the measurement terminal. Further, after Ni-plated stainless steel was left in an atmosphere of 60 ° C. and 93% RH for 60 days, surface contact resistance (contact resistance after deterioration test) was measured under the same conditions.
The contact resistance after the deterioration test, as seen in the investigation results in Table 3, was obtained by forming a Ni plating layer having a film thickness of 0.05 to 0.7 μm on the stainless steel on which the Cu-rich phase is precipitated. It is almost the same value as the initial value, and it can be seen that the increase in contact resistance is suppressed.

On the other hand, in the test Nos. 7 and 8 in which the Ni plating layer was too thin, the contact resistance after the deterioration test was significantly increased. In test Nos. 1 to 6 based on SUS304 and SUS430 stainless steel with no Cu-rich phase precipitation, even after the Ni plating layer was in the range of 0.05 to 0.7 μm, The contact resistance increased significantly.
From this contrast, even when exposed to a moist atmosphere by forming a Ni plating layer having a film thickness of 0.05 to 0.7 μm using stainless steel on which a Cu-rich phase is precipitated in an amount of 0.2% by volume or more as a base material. It can be confirmed that the increase in contact resistance is suppressed.

  Even when a Cu enriched layer having a Cu / (Si + Mn) mass ratio of 0.5 or more is formed in place of the precipitation of the Cu-rich phase, a Ni plating layer having a film thickness of 0.05 to 0.7 μm is similarly formed. As a result, the increase in contact resistance was suppressed. In fact, when a sample in which a Ni plating layer having a film thickness of 0.7 μm was formed on a stainless steel having a Cu / (Si + Mn) mass ratio of 0.7 and Cu concentrated on the surface layer was analyzed by GDS, The coexistence state of the Ni plating layer was confirmed (FIG. 2), and the surface state was effective for suppressing the increase in contact resistance over a long period of time.

  Contact resistance was measured for each elapsed day of the deterioration test, and changes in contact resistance were investigated. Film thickness: Stainless steel of type F1 provided with a 0.05 μm Ni plating layer increased in contact resistance to about 0.011Ω after 60 days of deterioration test, but there was no Ni plating layer even with the same steel type F1 In some cases, the initial value of the contact resistance was as high as about 0.05Ω, and the contact resistance increased to about 0.11Ω after the 60-day deterioration test (FIG. 3). Film thickness: In SUS430 stainless steel that does not precipitate Cu-rich phase even when a 0.3 μm Ni plating layer is formed, the contact resistance increases according to the elapsed days of the deterioration test, and after the 60-day deterioration test, the contact resistance increases. It rose to about 2Ω (Fig. 4).

  Furthermore, the load of the measurement terminal brought into contact with each stainless steel immediately after Ni plating and 60 days after the deterioration test was changed, and the contact resistance corresponding to the load was measured. Also from the measurement results of FIG. 5, it is found that the contact resistance is stably maintained at a low level even after the deterioration test by using the stainless steel on which the Cu-rich phase is precipitated as the base material. On the other hand, when the base material is stainless steel that does not precipitate Cu-rich phase, the contact resistance after the deterioration test is greatly increased, and the variation due to the load is large, so the contact resistance is unstable. Be redeemed.

  As described above, the contact material of the present invention is based on stainless steel with a Cu-rich phase precipitated or stainless steel with a Cu concentrated layer on the extreme surface layer, and has a film thickness of 0.05 to 0.7 μm. Since the Ni plating layer is formed, the low contact resistance is maintained for a long time even in a humid atmosphere. Moreover, since the base material is stainless steel having excellent corrosion resistance and strength, it is used as a highly reliable contact material for electrical and electronic equipment.

Schematic diagram explaining that low contact resistance is maintained by coexistence of Cu-rich phase and ultra-thin Ni plating layer Graph showing that Cu concentrated layer and Ni plating layer coexist on stainless steel surface Graph comparing the change in contact resistance of ferritic stainless steel with deposited Cu-rich phase with and without Ni plating layer Graph comparing the change in contact resistance of Ni-plated ferritic stainless steel with and without Cu-rich phase Chart comparing contact resistance measurement results for each steel grade and Ni plating layer thickness

Explanation of symbols

1: Stainless steel 2: Cu rich phase (second phase mainly composed of Cu) 3: Ni plating layer

Claims (2)

Cu: 1.0% by mass or more, Cr: 9% by mass or more, second phase mainly composed of Cu: stainless steel dispersed in a matrix at a ratio of 0.2% by volume or more, and a film thickness: A stainless steel contact material characterized in that a Ni plating layer of 0.05 to 0.7 μm is formed on the surface of the base material so that the second phase coexists with the surface of the stainless steel together with the Ni plating layer . The base material is a stainless steel containing Cu: 1.0% by mass or more, Cr: 9% by mass or more, Ni: 6% by mass or more, and the outermost layer of the base material has a mass ratio of Cu / (Si + Mn) of 0.5 or more. By forming a Ni plating layer having a film thickness of 0.05 to 0.7 μm on the substrate surface, the Cu concentration layer coexists on the stainless steel surface together with the Ni plating layer. Stainless steel contact material characterized by being in a state .

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