JP2005307296A - Austenitic stainless steel foil for spring having spring property and durability and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide austenitic stainless steel foil for a spring for an electronic equipment component having excellent spring properties and durability, and to provide its production method. <P>SOLUTION: In the austenitic stainless steel foil as a stainless steel strip for a spring of JIS Z 4313 SUS301-CPS, a yield point is exhibited in a stress-strain diagram on a tensile test based on JIS Z 2241. In the production method, after annealing, successively to cold rolling at a total draft of 20 to 35%, heating treatment is performed at 350 to 700°C, more preferably, at 350 to 500°C. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an austenitic stainless steel foil used for spring applications and a method for producing the same. More particularly, the present invention relates to an austenitic stainless steel foil for springs having high strength and excellent durability (fatigue properties) used for electronic device parts, for example, a disc spring of a mobile phone, and a method for producing the same.

  Generally, steel materials used for spring applications are required to have high strength characteristics in order to exhibit high fatigue strength, and high strength martensitic stainless steels and metastable austenitic stainless steels are used. Martensitic stainless steel is a material hardened by rapid cooling from a high-temperature austenite state and martensitic transformation. On the other hand, metastable austenitic stainless steel is a material that exhibits an austenite phase in the solution treatment state and is given high strength by forming work-induced martensite by subsequent cold rolling. The metastable austenitic stainless steel does not require a quenching-tempering heat treatment step unlike martensite stainless steel when generating martensite, and therefore has an advantage that the fluctuation of characteristics due to the influence of the heat treatment temperature is small.

  However, if metastable austenitic stainless steel is simply used as the steel type, desired strength and hardness as spring austenitic stainless steel foil and higher fatigue characteristics cannot be obtained. In order to achieve a high strength material and a high fatigue property, it is practically indispensable to design a plastic working property of a metal that realizes a high durability and to set a heat treatment condition to achieve it.

  Patent Documents 1 to 4 and the like are known as prior art relating to the metallographic control and manufacturing method of austenitic stainless steel for springs. Patent Document 1 describes that a two-phase structure of austenite and martensite is formed by cold rolling after solution treatment, Patent Documents 2 and 3 describe the cold rolling rate, and Patent Document 4 describes a rolling roll. The relationship between properties and surface morphology is described. As described above, the prior art relates to the metal structure after cold rolling of the metastable austenitic stainless steel, the cold rolling rate, and the properties of the rolling roll. However, the disclosure of the prior art relating to the metal structure of the excellent austenitic stainless steel foil for springs that achieves both high strength and high spring characteristics and excellent durability (fatigue characteristics), which is the subject of the present invention, and the manufacturing technology that realizes it Absent.

JP 2002-173742 A JP-A-8-134595 JP-A-8-165519 JP-A-3-204102

Austenitic stainless steel foils for springs are required to have high strength, high spring characteristics, and excellent durability (fatigue characteristics). In order to improve the durability, it is common to increase the cold rolling rate to increase the strength.However, if the cold rolling rate cannot be changed significantly, or if the high strength is If it is not always effective to improve durability, development of a method different from the conventional method is indispensable.

  Accordingly, an object of the present invention is to solve such problems, and to provide a high-strength austenitic stainless steel foil excellent in spring characteristics and durability suitable for a disc spring or the like used for a mobile phone button, and its manufacture. A method is provided.

  For this purpose, the present invention was completed as a result of examining the heat treatment technology after cold rolling of the foil material, and as a technology for imparting plastic processing characteristics to the foil material to suppress the settling of the foil material, It has been found that by controlling the heating temperature after cold rolling, the spring characteristics and durability of the metastable austenitic stainless steel foil are significantly improved. The gist is as follows. That is, the objective of this invention is achieved by the austenitic stainless steel foil for springs which was excellent in the spring characteristic and durability as described in the following (1)-(2), and its manufacturing method.

  (1) Austenitic stainless steel foil for springs, which is a stainless steel strip for springs of JIS Z 4313 SUS301-CSP, must have a yield point in the stress-strain diagram when a tensile test is performed based on JIS Z 2241. Austenitic stainless steel foil for springs with excellent spring characteristics and durability.

  (2) After annealing, the austenitic stainless steel foil subjected to cold rolling with a total rolling reduction of 20% to 35% is heat-treated at 350 to 700 ° C., and the spring characteristics according to (1) And manufacturing method for austenitic stainless steel foil for springs with excellent durability.

  The austenitic stainless steel foil for springs of the present invention realizes high durability and is an extremely excellent austenitic stainless steel foil that can be used for a disc spring of a mobile phone, etc., and its industrial value is remarkably large. It will be.

  The present invention was completed as a result of studying the heat treatment technology after cold rolling of austenitic stainless steel foils for springs, and the spring characteristics and durability are remarkably improved by controlling the plastic working characteristics of the foil material. A telephone phone spring material is provided.

JISZ4313 SUS301-CPS Stainless Steel Band for Spring Next, the austenitic stainless steel foil targeted by the present invention will be described. The austenitic stainless steel foil for springs in the present invention is a stainless steel band for springs of JIS Z 4313 SUS301-CSP, and is used for spring applications having a Vickers hardness of 430 or more and 530 or less. For foil.

There is a yield point in the stress-strain diagram. In the plastic working characteristics of the austenitic stainless steel foil material for springs in the present invention, the yield point is shown in the stress-strain diagram when a tensile test is performed based on JIS Z2241. Is expressed. FIG. 1 shows an example of a stress-strain diagram. Foil material that does not exhibit a yield point has a narrow elastic deformation range, so that when used as a spring, the strain that causes the plastic deformation range is easily deformed, and the spring characteristics deteriorate, so the foil material that exhibits a yield point. Limited to.

Total rolling reduction is 20% to 35%
In the present invention, the cold rolling rate after annealing for the purpose of removing processing strain and solidifying the austenitic stainless steel foil for springs is a cold rolling method in which the total rolling reduction is 20% to 35%. . The Vickers hardness of the austenitic stainless steel foil for springs targeted by the present invention is 430 or more and 530 or less, and generally corresponds to a cold rolling rate of 20% or more and 35% or less.

Heat treatment at 350 to 700 ° C. In the heat treatment after cold rolling of the austenitic stainless steel foil for spring in the present invention, the heating temperature is 350 to 700 ° C. And more preferably, it is 350 degreeC-500 degreeC. When the heating temperature is higher than 350 ° C., a yield point appears in the stress-strain diagram mainly due to strain aging caused by carbon diffusion. If it is less than 350 degreeC, a yield point cannot be provided as a plastic processing characteristic of foil material, and it is inferior to a spring characteristic. Above 500 ° C. and below 700 ° C., the yield point is present and the spring characteristics are excellent. However, high strength due to strain aging becomes remarkable, but local deformation tends to occur, and durability begins to deteriorate. If it exceeds 700 ° C, the yield point does not appear, so the spring characteristics are poor. Therefore, the temperature at which the best characteristics are obtained from the viewpoint of durability is 350 ° C. or more and 500 ° C. If the temperature exceeds 500 ° C., durability deteriorates, but durability required as a spring material can be obtained, and the upper limit temperature is 700 ° C. As mentioned above, in this invention, the heating temperature after cold rolling was limited to 350 to 700 degreeC. More preferably, it is 350 degreeC-500 degreeC. FIG. 2 shows a relationship diagram between the heating temperature and the durability (number of times to breakage) in a fatigue test. When the heating temperature exceeds 500 ° C., the durability is lowered. FIG. 3 shows the relationship between the heating temperature and the metal structure showing carbide precipitation. At over 500 ° C. where the durability starts to deteriorate, precipitation of carbides is remarkable and it is considered that the strain aging is accelerated. In a more preferable heating temperature range of 350 ° C. to 500 ° C., there is little precipitation of carbides. For observation of the metal structure, a sample was prepared by 10% oxalic acid electrolytic etching. The conditions were room temperature and 30 mA / cm ² for 6 minutes.

  Hereinafter, the present invention will be specifically described with reference to Examples.

  Table 1 shows chemical components of the austenitic stainless steel foil material for springs. The components were analyzed by sampling a test piece from the product plate. The bulk component concentration is measured by gas analysis for C, S, N, and O (N and O are inert gas melting-heat conduction measurement methods, and C and S are combustion in an oxygen stream-infrared absorption method). About other elements, it implemented with the fluorescent-X-ray-analysis apparatus (SHIMADZU, MXF-2100).

A 0.3 mm cold-rolled steel strip of spring austenitic stainless steel shown in Table 1 was made into a 82 μm-thick foil strip by cold rolling, and then bright annealed at 1150 ° C. for solution treatment. Further, it was cold-rolled to a thickness of 60 μm with high-speed steel. Then, the heating temperature was changed under the conditions shown in Table 2 in an atmosphere of 100% hydrogen, and continuous annealing was performed to obtain a foil product.
Table 2 shows the method for producing the foil material, the presence / absence of yield point, spring characteristics and durability evaluation results. As for the spring characteristics, a spring limit value (Kb) of 800 MPa or more is indicated by ◯, and less than 800 MPa is indicated by ×. The durability is indicated by ◯ when the number of times the foil broke in the bending fatigue test is 2500 or more, and x when less than 2500, and by ◎ when the number of breaks is 3000 or more and particularly long life. The foil material in which the yield point appears in the stress-strain diagram of the tensile test by the manufacturing method of the present invention has good durability. In Comparative Examples 5 and 6, the heating temperature after cold rolling deviates from the lower limit value of the scope of the claims, and in Comparative Example 7, the heating temperature deviates from the upper limit value of the scope of the claims, and the yield point does not appear, Spring characteristics are inferior. Inventive Example 4 has a yield point and good spring characteristics, but has a high 0.2% proof stress and is slightly inferior in durability to Inventive Examples 1 to 3.

  Whether or not a yield point appears in the stress-strain diagram when the tensile test was performed was determined by a test method of JIS Z 2241. The yield phenomenon of a metal material is a phenomenon in which plastic deformation occurs during a test when there is no increase in force when a tensile test is performed. In the present invention, it is determined that a yield point exists when a decrease in stress is observed in the stress-strain diagram regardless of an increase in strain. As the tensile test piece, a JIS No. 13B test piece defined in JIS Z 2201 was used. In the present invention, the presence or absence of the yield point is determined by a stress-strain diagram by a tensile test, but any verification method may be used as long as it is a detectable method.

  The spring limit value (Kb) was taken from a direction parallel to the rolling direction and measured using a moment type tester of JIS H 3130.

  The durability of the foil material was evaluated by a bending resistance fatigue test (manufactured by Tester Sangyo Co., Ltd.) by repeated bending. The upper part of the strip-shaped (60 μm thickness, 210 mm long, 15 mm wide) foil test piece is fixed to the chuck and 300 g of load is applied to the lower part. The number of times when the foil broke, given at a rate of 175 cycles / min, was used as a durability index for fatigue characteristics.

  The present invention is a technique applicable to a spring material used for electronic device parts such as a disc spring of a mobile phone.

It is an example of a stress-strain diagram when a tensile test is performed based on JIS Z 2241. It is a figure which shows the relationship between the heating temperature and the durability (the number of times to failure) in a fatigue test. It is a figure which shows the relationship between heating temperature and the metal structure which shows carbide precipitation.

Claims (2)

  Austenitic stainless steel foil for springs, which is a stainless steel strip for springs of JIS Z 4313 SUS301-CSP, characterized in that there is a yield point in the stress-strain diagram when a tensile test is performed based on JIS Z 2241. Austenitic stainless steel foil for springs with excellent spring characteristics and durability.   The spring characteristics and durability according to claim 1, wherein the austenitic stainless steel foil subjected to cold rolling with a total rolling reduction of 20% to 35% is heat-treated at 350 to 700 ° C after annealing. A method for producing an austenitic stainless steel foil for springs with excellent resistance.

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