JP7116647B2 - austenitic stainless steel foil - Google Patents

Description

TECHNICAL FIELD The present invention relates to an austenitic stainless steel foil suitable as a material for springs.

Conventionally, metastable austenitic stainless steel has been used as a material for thin push-button switches such as tact switches provided in electronic devices such as mobile phone terminals and home electric appliances.

As this type of austenitic stainless steel, stainless steels disclosed in Patent Documents 1 to 4 are known.

Patent Literature 1 describes a stainless steel foil with excellent flatness and low residual stress.

Patent Document 2 describes a stainless steel foil having a tensile strength of 1600 N/mm ² or more and excellent fatigue properties.

Patent Document 3 describes a technique for prolonging the life of plating when nickel and silver alloys are plated on the surface.

Patent Literature 4 describes a stainless steel that exhibits a small property change after solder reflow and is excellent in click property.

JP 2001-286904 A Japanese Patent Application Laid-Open No. 2001-286905 Japanese Unexamined Patent Application Publication No. 2005-2400 Japanese Patent Application Laid-Open No. 2018-3099

In recent years, electronic devices such as mobile phone handsets and home electric appliances are becoming more and more compact, and there is a demand for improved material properties, especially fatigue properties, which lead to longer switch life.

In order to meet these demands for materials, it is first necessary to have high strength, and high strength can be achieved by increasing the rolling reduction. tends to decrease. In addition, simply increasing the strength of the material often fails to extend the life of the tactile switch due to insufficient fatigue characteristics.

Therefore, for example, as a material for springs such as tactile switches, a material having a predetermined strength and good fatigue characteristics has been demanded.

The present invention has been made in view of such points, and an object of the present invention is to provide an austenitic stainless steel foil having good fatigue properties.

In the austenitic stainless steel foil described in claim 1, C: 0.08% by mass or more and 0.15% by mass or less, Si: 0.30% by mass or more and 1.00% by mass or less, Mn: 0.50% by mass % or more and 2.00 mass % or less, P: 0.04 mass % or less, S: 0.010 mass % or less, Ni: 6.00 mass % or more and 8.00 mass % or less, Cr: 16.00 mass % or more 18.00% by mass or less, Cu: 1.00% by mass or less, N: 0.005% by mass or more and 0.15% by mass or less, Ti: 0.010% by mass or less, Al: 0.010% by mass or less, Ca : 0.005% by mass or less, and O: 35 ppm or more and 80 ppm or less, the balance being Fe and unavoidable impurities, the plate thickness being 20 µm or more and 100 µm or less, and the average number of crystal grains in the plate thickness direction being 10 or more The number of inclusions with an equivalent circle diameter of 15 μm or more contained in 0.1 g of steel is 100/0.1 g or less, and the tensile strength is 1800 N/mm ² or more.

The austenitic stainless steel foil described in claim 2 is the austenitic stainless steel foil described in claim 1, wherein Mo: 1.00 mass% or less, V: 0.50 mass% or less, and B: 0.001 mass% % or more and 0.01% by mass or less.

The austenitic stainless steel foil described in claim 3 is the austenitic stainless steel foil described in claim 1 or 2, wherein the inclusions have a C content of less than 39% by mass .

According to the present invention, within a predetermined chemical composition range, the plate thickness is 20 μm or more and 100 μm or less, the average number of crystal grains in the plate thickness direction is 10 or more, and the equivalent circle diameter contained in 0.1 g of steel material is 15 μm or more. The number of inclusions is 100/0.1 g or less, so the fatigue property is good.

A configuration of an embodiment of the present invention will be described in detail below.

The austenitic stainless steel according to the present invention contains C (carbon): 0.08% by mass or more and 0.15% by mass or less, Si (silicon): 0.30% by mass or more and 1.00% by mass or less, and Mn (manganese). : 0.50% by mass or more and 2.00% by mass or less, P (phosphorus): 0.04% by mass or less, S (sulfur): 0.010% by mass or less, Ni (nickel): 6.00% by mass or more 8 .00% by mass or less, Cr (chromium): 16.00% by mass or more and 18.00% by mass or less, Cu (copper): 1.00% by mass or less, N (nitrogen): 0.005% by mass or more and 0.15% by mass % by mass or less, Ti (titanium): 0.010 mass % or less, Al (aluminum): 0.010 mass % or less, Ca (calcium): 0.005 mass % or less, and O (oxygen): 35 ppm or more and 80 ppm or less and the balance consists of Fe (iron) and unavoidable impurities.

Further, if necessary, Mo (molybdenum): 1.00% by mass or less, V (vanadium): 0.50% by mass or less, and B (boron) 0.001% by mass or more and 0.01% by mass or less Contains at least one.

C is an austenite-generating element and is an element effective for strengthening the austenite phase and the strain-induced martensite phase. If the C content is less than 0.08% by mass, there is a possibility that the reinforcing action will not be sufficiently exhibited. On the other hand, if C is added in excess of 0.15% by mass, it may become a factor of deteriorating workability. Therefore, the content of C should be 0.08% by mass or more and 0.15% by mass or less, preferably 0.13% by mass or less.

Si is useful as a deoxidizing material for the purpose of controlling the Al content to a low level. Moreover, Si has the effect of increasing the work hardening of the austenite phase, and is an effective element for obtaining high strength by rolling. In order to exhibit these effects, it is necessary to add 0.30% by mass or more of Si. On the other hand, if Si is added in excess of 1.00% by mass, δ ferrite may be generated during hot rolling, resulting in deterioration of hot workability. Therefore, the content of Si should be 0.30% by mass or more and 1.00% by mass or less, preferably 0.40% by mass or more and 0.90% by mass or less.

Mn combines with Si and O to form soft Mn--Si--O-based inclusions that are easily broken during the hot rolling process and the cold rolling process. Also, Mn is an austenite-generating element and is an element effective for obtaining an austenite structure. In order to exhibit these effects, it is necessary to add 0.50% by mass or more of Mn. On the other hand, when Mn is added in an amount exceeding 2.00% by mass, the austenite phase is stabilized, and deformation-induced martensitic transformation is less likely to occur. Therefore, the content of Mn is 0.50% by mass or more and 2.00% by mass or less, preferably 1.50% by mass or less.

The lower the content of P and S, the better, but an unnecessarily low content may affect workability, other material properties, and manufacturability. Therefore, the content of P is set to 0.04% by mass or less (excluding non-additives), and the content of S is set to 0.010% by mass or less (including non-additives).

Ni is an austenite-forming element and is an effective element for improving ductility and toughness. In order to exhibit these effects, it is necessary to add 6.00% by mass or more of Ni. On the other hand, since Ni is a relatively expensive element, excessive addition of Ni unnecessarily increases the material cost. Therefore, the Ni content should be 6.00% by mass or more and 8.00% by mass or less, preferably 7.50% by mass or less.

Cr is an element that improves corrosion resistance, and must be added in an amount of 16.00% by mass or more to ensure sufficient corrosion resistance. On the other hand, if Cr is added in excess of 18.00% by mass, workability may deteriorate. Therefore, the Cr content should be 16.00% by mass or more and 18.00% by mass or less, preferably 17.50% by mass or less.

Cu is an effective element for stabilizing the austenite phase, but when added in excess of 1.00% by mass, workability may deteriorate. Therefore, the content of Cu is set to 1.00% by mass or less (not including non-additives).

N is an austenite-forming element and is an element effective in improving strength and ductility. In order to exhibit these effects, it is necessary to add 0.005% by mass or more of N. On the other hand, adding more than 0.15% by mass of N may cause blowholes. Therefore, the content of N is set to 0.005% by mass or more and 0.15% by mass or less.

Ti is an element that is easily oxidized and is easily combined with N to form TiN. Although TiN is smaller than oxide-based inclusions, it is a hard inclusion that is difficult to break during the hot rolling process and cold rolling process, so it can be observed as a relatively large inclusion at the foil stage. be. Therefore, the content of Ti should be 0.010% by mass or less (including no addition), preferably 0.005% by mass or less.

Al is an element that is easily oxidized and precipitates at a high temperature to form coarse inclusions. Since the inclusions are hard, they are difficult to break during the hot rolling and cold rolling processes. Therefore, the content of Al should be 0.010% by mass or less (including no addition), preferably 0.005% by mass or less.

Although Ca is softer than Al ₂ O ₃ , it is an element that forms coarse oxides because it precipitates at high temperatures. Therefore, the content of Ca should be 0.005% by mass or less (including no addition), preferably 0.003% by mass or less.

O is an element that is inevitably mixed in stainless steel, and is deoxidized using Si or Al in the steelmaking process. It is common to remain. The remaining O combines with easily oxidizable elements such as Si, Mn, Al, Ti and Ca in the steel to form oxide inclusions. If the O content is less than 35 ppm, Si and Mn are less likely to be oxidized and the ratio of Al ₂ O ₃ in inclusions increases. Al ₂ O ₃ is hard and does not stretch easily in the hot rolling process and the cold rolling process, so that it tends to remain as relatively large inclusions even at the stage of forming a foil of 20 to 100 μm. On the other hand, when the O content exceeds 80 ppm, the absolute amount of oxides increases and may accumulate to correspond to large inclusions. Therefore, the O content should be 35 ppm or more and 80 ppm or less.

Mo is an element effective in improving corrosion resistance, but is a relatively expensive element. Therefore, when adding Mo, the content shall be 1.00 mass % or less.

V is an element that forms carbides at high temperatures and improves strength by precipitation strengthening and solid solution strengthening of V itself. On the other hand, if added in excess of 0.50% by mass, the toughness of the steel material may be impaired. Therefore, when V is added, its content is made 0.50% by mass or less.

B is an element effective in improving the hot workability of stainless steel and preventing cracks in the hot rolling process. On the other hand, if B is added excessively, the hot workability may rather deteriorate. Therefore, when B is added, its content is made 0.001% by mass or more and 0.01% by mass or less.

The austenitic stainless steel having the chemical composition described above has a tensile strength of 1800 N/mm ² after undergoing predetermined manufacturing processes (for example, hot rolling, cold rolling, annealing, finish rolling and tension annealing) described later. As described above, a foil-like plate having a plate thickness of 20 μm or more and 100 μm or less is obtained.

Such a foil-shaped austenitic stainless steel has an average number of crystal grains in the sheet thickness direction of 10 or more.

The number of crystal grains in the plate thickness direction is determined by polishing the cross section parallel to the rolling direction and perpendicular to the plate thickness direction of the annealed material before final rolling, and performing an etching treatment to reveal the grain boundaries. The number of crystal grains in the plate thickness direction is measured by observing the structure with a microscope. In addition, since it is assumed that the number of crystal grains in the plate thickness direction varies depending on the measurement positions, measurements are taken at a plurality of positions (for example, 5 or more positions) and the average value is obtained.

Here, in foil-shaped austenitic stainless steel of 20 to 100 μm, hard inclusions with an equivalent circle diameter of 15 μm or more, which are difficult to break into fine pieces during rolling, adversely affect the fatigue characteristics. If it exceeds 1 g, the fatigue properties are remarkably lowered. Therefore, the number of inclusions having an equivalent circle diameter of 15 μm or more in the austenitic stainless steel should be 100/0.1 g or less. The plate thickness circle equivalent diameter means the diameter of a circle equal to the area of the inclusion.

When measuring inclusions in austenitic stainless steel, first, the stainless steel to be measured is added to a solution that dissolves the stainless steel base but does not dissolve the inclusions, such as a solution of 3.25 g of iodine dissolved in 25 mL of methanol. 0.1 g is immersed and dissolved while applying ultrasonic waves.

Next, using a membrane filter made of polytetrafluoroethylene (PTFE) with a pore size of 0.5 μm, filtration is performed while vacuuming to extract a residue mainly composed of inclusions.

In addition, it is preferable to use glassware having an inner diameter of 40 mm for filtration so that the area from which inclusions are extracted is within the range of 40 mm in diameter.

The membrane filter is subjected to platinum vapor deposition, and the shape and composition of inclusions are measured by a scanning electron microscope (SEM) and energy dispersive X-ray analysis, and the number of inclusions having an equivalent circle diameter of 15 μm or more is counted.

When measuring the number of inclusions, inclusions with a C content of 39% by mass or less in the residue extracted by the above method are used as inclusions, so that contaminants etc. that are unavoidably mixed in during the work process and inclusions It is preferable because it is possible to distinguish between

In a foil-shaped austenitic stainless steel with a thickness of 20 to 100 μm, it is considered that hard inclusions having an equivalent circle diameter of 15 μm or more, which are difficult to break during rolling, adversely affect the fatigue characteristics. Therefore, when measuring inclusions, only inclusions that satisfy at least one of an Al content of 1% by mass or more and a Ti content of 1% by mass or more are counted, thereby reducing the fatigue characteristics. This is preferable because hard inclusions can be measured more accurately.

For example, Table 1 shows the sizes and compositions of inclusions and contaminants measured as described above.

In Table 1, inclusions (No. 5 to 8) that satisfy C < 39% by mass and satisfy at least one of Al ≥ 1% by mass or Ti ≥ 1% by mass, and others ( No. 1 to 4) are regarded as contaminants. Among the inclusions classified in this way, no. Count 8 as inclusions of interest to define the number per 0.1 g.

The austenitic stainless steel is suitably used as spring materials such as metal domes for tactile switches provided in electronic devices such as mobile phones and home electric appliances.

Next, a method for producing the austenitic stainless steel will be described.

First, stainless steel raw materials are melted in an electric furnace, oxygen is blown into the molten steel for decarburization, and then Si is added to react with the oxygen in the molten steel to reduce the oxygen concentration.

In order to finally control the oxygen concentration in molten steel to 35 to 80 ppm, it is effective to control the slag basicity (CaO/SiO ₂ ) relatively low, for example, to about 1.3 to 1.5. be.

If the oxygen concentration is 35 to 80 ppm in this way, then Si and Mn are oxidized in the process up to the continuous casting process, and Mn-Si- An O-based oxide is produced.

If the oxygen concentration is reduced to less than 35 ppm, Mn is less likely to be oxidized, and Si--Al--O-based oxides, which are less likely to be split during rolling, are more likely to form. In addition, even if the oxygen concentration is 35 to 80 ppm, if the amount of Al, Ti and Ca that are unavoidably mixed in is large, their oxides will inevitably increase, so unavoidable contamination should be avoided as much as possible. As such, it is important to pay attention to the condition of the raw material and the ladle in which the molten steel is placed.

Next, the slab produced by continuous casting is heated to 1100 to 1300° C. and hot rolled to obtain a hot rolled steel strip.

A hot-rolled steel strip is repeatedly cold-rolled and annealed to obtain an annealed material with a predetermined thickness, and the austenite phase is work-hardened by finish rolling and work-induced martensite transformation is performed to produce a high-strength austenitic stainless steel. get foil.

After finish rolling, tension annealing (TA) is preferably performed to remove residual stress and correct the shape.

The steps that are particularly important for properties are the annealing temperature before finish rolling, the finish rolling reduction and the finish rolling temperature. If the annealing temperature before finish rolling is high, the grain size increases.

Specifically, in the case of a plate thickness of 20 to 100 μm, if the number of crystal grains in the plate thickness direction is small, the characteristics tend to vary. Therefore, it is preferable to appropriately set the annealing temperature and time in the range of 900 to 1100 ° C. according to the plate thickness so that the average number of crystal grains in the plate thickness direction is 10 or more. It is 950-1050°C.

The finish rolling reduction may be set in consideration of the target properties, composition and grain size. If the finish rolling temperature is low, deformation-induced martensitic transformation is likely to occur, making it easier to obtain strength. . On the other hand, if the finish rolling temperature is high, deformation-induced martensitic transformation is less likely to occur, but high strength can still be obtained by increasing the rolling reduction. Therefore, it is preferable to set the finish rolling temperature in consideration of the target properties, composition and grain size.

Tension annealing may be carried out by heat treatment at 480 to 540° C. for several seconds (eg, 5 seconds) under a tension of several kgf/mm ² (eg, 5 kgf/mm ² ). In addition, after the material temperature reaches 480 to 540° C., the material may be immediately cooled.

Next, the operation and effects of the above embodiment will be described.

According to the above austenitic stainless steel, it is foil-shaped with a plate thickness of 20 μm or more and 100 μm or less within a predetermined chemical composition range, and 100 inclusions/0. Since it is 1 g or less, deterioration of thermal fatigue characteristics due to inclusions can be suppressed. More specifically, the contents of O, Si, Mn, Al, Ca and Ti, which particularly affect the formation of inclusions, are specified to control the inclusions in the stainless steel so that they are easily broken by rolling. Therefore, when the stainless steel is rolled into a foil having a plate thickness of 20 μm or more and 100 μm or less, many fine inclusions having an equivalent circle diameter of less than 15 μm are present. Therefore, it is possible to secure a tensile strength of 1800 N/mm ² or more by rolling, and the fatigue property is also good.

Further, by setting the average number of crystal grains in the plate thickness direction to 10 or more, it is possible to suppress the occurrence of variations in properties depending on the location in the stainless steel, so that the fatigue properties can be improved.

Examples and comparative examples will be described below.

Stainless steels having chemical compositions shown in Table 2 were melted in an electric furnace, and austenitic stainless steel foils were produced by the method described above.

The content of each element in Table 2 is the value in Opppm, and the other elements are the values in mass%.

The value of Md30 in Table 2 is an austenite stability index represented by the formula Md30=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo. This formula is based on the content of each element, and the value of the content (mass%) of each element is substituted, and 0 is substituted for elements that do not contain

Further, annealing and finish rolling were performed under the conditions shown in Table 3, and the plate thickness was set to 40 μm.

Steel grades A1, A2 and B1 were annealed and finish rolled under multiple conditions as shown in Table 3.

Note that if the plate thickness is different, it is necessary to change the fatigue test conditions, making it difficult to make a relative comparison.

Moreover, in order to compare the fatigue characteristics, the strength (tensile strength) was set to be the same in both the present example and the comparative example.

Inclusions were measured by dissolving the stainless steel in a liquid of methane and iodine as described above, filtering and extracting them, and subjecting them to SEM and energy dispersive X-ray analysis. The number of inclusions having an equivalent circle diameter of 15 μm or more was counted, with inclusions satisfying at least one of the conditions and having C<39% by mass.

As a fatigue test, a bending fatigue test commonly called MIT test was performed according to JIS P 8115. Specifically, a test piece having a length of 110 mm and a width of 15 mm is cut out from each of the above stainless steels, and a folding endurance fatigue tester manufactured by Toyo Seiki Seisakusho Co., Ltd. is used to set the test load to 1 kg and the bending angle to 135°. A bending fatigue test was performed with a radius of 2.9 mm and a bending speed of 175 times/min.

In this bending fatigue test, the fatigue property was evaluated to be good if the endurance count was 10,000 or more.

As shown in Table 3, within a predetermined chemical composition range, the number of crystal grains in the plate thickness direction is 10 or more, and the number of inclusions of 15 μm or more contained in 0.1 g of steel is 100/0.1 g or less. All of the present examples had a durability of 10,000 times or more in the bending fatigue test, indicating good fatigue characteristics.

A1, which is a comparative example in which the number of inclusions of 15 μm or more contained in 0.1 g of steel material is 100/0.1 g or less in a predetermined chemical composition range, but the number of crystal grains in the plate thickness direction is less than 10 -4 and A2-2 were less than 10000 times in the bending fatigue test.

B1-1, B1-2, B1-3, B1-4, B2, B3 and B4 in which the content of at least one of Ti, Al, Ca and O is outside the range are all included in 0.1 g of steel The number of inclusions with a diameter of 15 μm or more in the sample exceeded 100/0.1 g, and the number of times of endurance in the bending fatigue test was less than 10,000 times.

Claims (3)

C: 0.08% by mass or more and 0.15% by mass or less Si: 0.30% by mass or more and 1.00% by mass or less Mn: 0.50% by mass or more and 2.00% by mass or less P: 0.04 % by mass or less, S: 0.010% by mass or less, Ni: 6.00% by mass or more and 8.00% by mass or less, Cr: 16.00% by mass or more and 18.00% by mass or less, Cu: 1.00% by mass Below, N: 0.005% by mass or more and 0.15% by mass or less, Ti: 0.010% by mass or less, Al: 0.010% by mass or less, Ca: 0.005% by mass or less, and O: 35 ppm or more and 80 ppm containing the following, with the remainder consisting of Fe and unavoidable impurities,
The plate thickness is 20 μm or more and 100 μm or less,
The average number of crystal grains in the plate thickness direction is 10 or more,
100 inclusions/0.1 g or less with an equivalent circle diameter of 15 μm or more contained in 0.1 g of steel material,
An austenitic stainless steel foil having a tensile strength of 1800 N/mm ² or more. Mo: 1.00% by mass or less, V: 0.50% by mass or less, and B: 0.001% by mass or more and 0.01% by mass or less. austenitic stainless steel foil . The austenitic stainless steel foil according to claim 1 or 2, wherein the inclusions have a C content of less than 39% by mass.