JP7116648B2 - Stainless steel plate and manufacturing method thereof - Google Patents

Description

TECHNICAL FIELD The present invention relates to a stainless steel plate having excellent fatigue properties and a method for producing the same.

2. Description of the Related Art Electronic devices such as mobile phone terminals and personal computers are mounted at high density, and the parts used are becoming lighter, thinner and smaller.

In addition, in the metal materials used for the parts of such electronic devices, the current situation is that the increase in stress and the number of repeated loads are remarkable.

In particular, metal dome switch devices made of thin metal plates are used as input key switches such as tact switches in electronic equipment. (fatigue properties) are required.

As a metal plate for this type of switch, stainless steel having improved fatigue characteristics by reducing the crystal grain size is known, for example, as disclosed in Patent Documents 1 and 2.

WO2008/41638 JP 2004-244725 A

In recent years, electronic devices such as mobile phone handsets and home appliances are becoming more and more miniaturized, so there is a greater demand than ever to improve the properties of materials, especially the fatigue properties of materials that lead to longer switch life. ing.

SUMMARY OF THE INVENTION The present invention has been made in view of these points, and an object of the present invention is to provide a stainless steel sheet having good fatigue properties.

The stainless steel sheet according to claim 1 has C: 0.05% by mass or more and 0.30% by mass or less, Si: 1.50% by mass or less (not including no additives) , Mn: 0.10% by mass. 2.00% by mass or less, P: 0% by mass or more and 0.06% by mass or less, S: 0% by mass or more and 0.010% by mass or less, Ni: 5.00% by mass or more and 7.00% by mass or less, Cr : 15.00% by mass or more and 19.00% by mass or less and N: 0.05% by mass or more and 0.30% by mass or less, and the total content of C and N is 0.20% by mass or more, The balance consists of Fe and unavoidable impurities, and the value of Md ₃₀ represented by Md ₃₀ =551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo is 5 or more and 30 Below, the SFE value indicated by SFE = 2.2Ni + 6Cu-1.1Cr-13Si-1.2Mn + 32 is 15 or more and less than 25, has a multiphase structure of an austenite phase and a deformation-induced martensite phase, and has a steel plate surface has an average hardness of 550 HV or more and a standard deviation of hardness of 3.5 HV or less.

The stainless steel sheet according to claim 2 is the stainless steel sheet according to claim 1, wherein Mo: 0% by mass or more and 2.00% by mass or less and Cu: 0% by mass or more and 2.00% by mass or less It contains at least one.

The stainless steel sheet according to claim 3 is the stainless steel sheet according to claim 1 or 2, wherein the surface average hardness and standard deviation are hardness values measured at intervals of 400 μm under a measurement load of 50 gf. It is calculated based on

The method for manufacturing a stainless steel plate according to claim 4 comprises: C: 0.05% by mass or more and 0.30% by mass or less; Si: 1.50% by mass or less (not including no additives) ; 10% by mass or more and 2.00% by mass or less, P: 0% by mass or more and 0.06% by mass or less, S: 0% by mass or more and 0.010% by mass or less, Ni: 5.00% by mass or more and 7.00% by mass Below, Cr: 15.00% by mass or more and 19.00% by mass or less and N: 0.05% by mass or more and 0.30% by mass or less are contained, and the total content of C and N is 0.20% by mass In the above, the balance consists of Fe and unavoidable impurities, and the value of Md ₃₀ represented by Md ₃₀ =551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo is 5 or more and 30 or less, SFE=2.2Ni+6Cu-1.1Cr-13Si-1.2Mn+32, SFE value of 15 or more and less than 25, stainless steel having a multiphase structure of an austenite phase and a deformation-induced martensite phase The sheet is skin-pass rolled, and tension annealing is performed after the skin-pass rolling. In the tension annealing, the temperature of the material reaches the maximum from room temperature when a tension of 3 kgf/mm ² or more and 7 kgf/mm ² or less is applied to the stainless steel plate . After heating so that the temperature rise time is 5 seconds or more and 10 seconds or less until the temperature reaches 480 ° C. or higher and 540 ° C. or lower , by cooling immediately, the average hardness of the steel plate surface is 550 HV or higher, and the hardness standard A stainless steel plate having a deviation of 3.5HV or less is obtained .

According to the present invention, in a predetermined chemical composition range, the value of Md ₃₀ is 5 or more and 30 or less, the value of SFE is 15 or more and less than 25, and it has a dual phase structure of an austenite phase and a deformation-induced martensite phase. However, since the average hardness of the surface is 550 HV or more and the standard deviation of hardness is 3.5 HV or less, fatigue characteristics can be improved.

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

The stainless steel according to the present invention has C (carbon): 0.05% by mass or more and 0.30% by mass or less, Si (silicon): 1.50% by mass or less, Mn (manganese): 0.10% by mass or more 2 .00% by mass or less, P (phosphorus): 0.06% by mass or less, S (sulfur): 0.010% by mass or less, Ni (nickel): 5.00% by mass or more and 7.00% by mass or less, Cr ( Chromium): 15.00% by mass or more and 19.00% by mass or less and N (nitrogen): 0.05% by mass or more and 0.30% by mass or less, and the total content of C and N is 0.20 % by mass or more, and the balance consists of Fe (iron) and unavoidable impurities.

Moreover, at least one of Mo (molybdenum): 2.00% by mass or less and Cu (copper): 2.00% by mass or less is contained as necessary.

Within the above chemical composition range, the austenite stability index Md ₃₀ =551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr- _18.5Mo . The value is 5 or more and 30 or less.

Also, the value of SFE represented by the formula SFE=2.2Ni+6Cu-1.1Cr-13Si-1.2Mn+32, which is a stacking fault energy generation index, is 15 or more and less than 25.

Furthermore, the value of δcal shown by the formula δcal=−15−44.91C−0.88Mn−2.31Ni+2.20Cr−1.08Cu−28.8N, which is an indicator of δ ferrite formation after heating at 1230° C. for 2 hours. is preferably 1.0 or less.

Each of the above formulas is based on the content of each element, and the value of the content (% by mass) of each element is substituted, and 0 is substituted for elements that are not contained.

C and N are austenite forming elements, and if the content of these elements is too small, the amount of δ ferrite phase formed increases and the hot workability deteriorates. Also, C and N are elements useful for solid-solution strengthening of the strain-induced martensitic phase. It is important that both the C content and the N content be 0.05% by mass or more in order to stably obtain a remarkable effect of improving ductility. On the other hand, if the C and N content exceeds 0.30% by mass, the steel may be excessively hardened, impairing workability. Therefore, the content of C and the content of N are both set to 0.05% by mass or more and 0.30% by mass or less.

In addition, in order to develop sufficient ductility due to the TRIP phenomenon when forming the deformation-induced martensite phase, the content of C+N (the total content of C and N) must be 0.20% by mass or more. Therefore, C and N are defined as C+N≧0.20% by mass within the respective content ranges described above.

In addition, when C+N exceeds 0.40% by mass, there is a possibility that workability due to hardening may be hindered. Therefore, C + N is preferably 0.40% by mass or less, both the C content and the N content are 0.05% by mass or more and 0.15% by mass or less, and C + N is 0.20% by mass or more. It is more preferable to make it 0.30% by mass or less.

Si is an element useful for deoxidation in steelmaking and an element that contributes to solid solution strengthening. However, if it is added in excess of 1.50% by mass, it hardens the steel and impairs workability. In addition, since Si is a ferrite-forming element, excessive addition causes formation of a large amount of δ-ferrite phase in a high temperature range, impairing hot workability. Therefore, the content of Si is set to 1.50% by mass or less (not including no addition).

Mn is a useful austenite-forming element that can replace the action of Ni. In addition, it is an element that is useful for solid-solution strengthening of steel and also affects work hardening. In order to utilize these actions, it is necessary to add 0.10% by mass or more of Mn. On the other hand, when Mn is added in an amount exceeding 2.00% by mass, it becomes a factor that impairs hot workability. Therefore, the content of Mn is set to 0.10% by mass or more and 2.00% by mass or less.

P and S are included as unavoidable impurities, and their content is preferably as low as possible. Considering workability, other material properties, and adverse effects on manufacturability, the content of P is set to 0.06% by mass or less (including no addition), and the content of S is set to 0.010% by mass. % or less (including no additives).

Ni is an element effective in improving ductility and toughness. In order to exhibit the action sufficiently, it is necessary to add 5.00% by mass or more of Ni. On the other hand, if Ni is added in an amount exceeding 7.00% by mass, the strength characteristics are lowered, and the economic efficiency is also lowered due to the increase in cost. Therefore, the content of Ni is set to 5.00% by mass or more and 7.00% by mass or less.

Cr is an essential element for the formation of a passive film that ensures the corrosion resistance of stainless steel. On the other hand, since Cr is a ferrite-forming element, if it is added in excess of 19.00% by mass, the heating temperature before hot rolling becomes a two-phase region of (γ + δ), and even after heating, a large amount of δ ferrite is generated, causing heat It becomes a factor that impairs workability. Therefore, the Cr content is set to 15.00% by mass or more and 19.00% by mass or less.

Mo is an element useful for improving corrosion resistance and contributing to solid-solution strengthening. Therefore, when adding Mo, the content shall be 2.00 mass % or less.

Cu is an element that suppresses work hardening due to the formation of a strain-induced martensite phase, and is added for the purpose of adjusting Md ₃₀ , SFE and δcal. On the other hand, adding more than 2.00% by mass of Cu leads to deterioration of hot workability. Therefore, when adding Cu, the content is made 2.00% by mass or less.

_Md30 is an austenite stability index, and the larger the value, the easier the transformation from the austenite phase to the deformation-induced martensite phase occurs. By setting the value of Md ₃₀ to 5 or more, it is possible to prevent excessive generation of deformation-induced martensite phase, suppress variations in hardness, and secure ductility to prevent deterioration of manufacturing workability. On the other hand, if Md ₃₀ exceeds 30, the amount of deformation-induced martensite phase generated in the case of bending work or the like is excessively increased, and there is a possibility that variations in hardness are likely to occur. Therefore, in the above stainless steel, the content of each element is adjusted so that the value of Md30 is 5 or more and ₃₀ or less.

SFE is a stacking fault energy production index, and the larger the value, the less work hardening of the austenite phase occurs. If the SFE is less than 15, work hardening of the austenite phase is likely to occur, which may reduce ductility. On the other hand, when the SFE is 25 or more, work hardening of the austenite phase is difficult to occur, the difference in hardness between the austenite phase and the work-induced martensite phase becomes large, and there is a possibility that the hardness tends to vary. Therefore, in the above stainless steel, the content of each element is adjusted so that the SFE is 15 or more and less than 25.

δcal is a δ ferrite formation index after heating at 1230° C. for 2 hours. Properties and fatigue properties may also be affected. Therefore, it is preferable to adjust the content of each element in the stainless steel so that δcal is 1.0 or less.

The stainless steel having the chemical composition described above undergoes predetermined manufacturing processes (for example, hot rolling, cold rolling, annealing, finish rolling, tension annealing, and temper rolling) to be described later, and the average of the stainless steel surface is It has a hardness of 550 HV or more and a foil shape with a plate thickness of 20 μm to 100 μm.

Such foil-shaped stainless steel has a multi-phase structure of an austenite phase and a strain-induced martensite phase.

Here, the greater the variation in hardness on the surface of the stainless steel, the greater the difference in the stress that acts on parts with different hardness due to repeated loads (bending, etc.), and the difference in stress makes it easier for strain to concentrate. , the fatigue properties decrease.

Specifically, when the measurement load is 50 gf, the measurement interval is in the micro range (eg, 400 μm interval), and the hardness is measured at multiple locations (eg, 30 locations), the average hardness is 550 HV or more, and the standard If the deviation is larger than 3.5 HV and there is variation in hardness, there is a possibility that strain concentration cannot be effectively suppressed. Therefore, the above stainless steel should have an average hardness of 550 HV or more and a standard deviation of hardness of 3.5 HV or less.

The above stainless steel is suitably used as a spring material such as a metal dome for a tact switch provided in electronic devices such as mobile phones and home electric appliances.

Next, a method for manufacturing the above stainless steel will be described.

First, raw materials for stainless steel are melted, 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.

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

The hot-rolled steel strip is repeatedly cold-rolled and annealed to obtain a predetermined thickness, and the austenite phase is work-hardened and work-induced martensitic transformation is performed by skin-pass rolling after finish annealing, resulting in a hardness of 550 HV. That's it.

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

Tension annealing is performed by applying a tension of 3 kgf/mm ^{2 or more and 7 kgf/mm 2 or} less to the stainless steel, and the temperature rise time from room temperature to the maximum temperature of 480 ° C or more and 540 ° C or less is 5 seconds or more. Heat for 10 seconds or less, then immediately cool.

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

According to the above stainless steel, the composition is adjusted so that the value of Md ₃₀ is 5 or more and 30 or less and the value of SFE is 15 or more and less than 25 in a predetermined chemical composition range, and the average hardness of the surface is 550 HV. As described above, since the standard deviation of hardness is 3.5 HV or less, for example, even if the average crystal grain size is about the same as that of conventional steel SUS301, strain concentration is less likely to occur, and fatigue characteristics can be improved.

In addition, by performing tension annealing under predetermined conditions after skin pass rolling, residual stress can be appropriately removed and variations in hardness can be suppressed, so fatigue characteristics can be improved.

Examples and comparative examples will be described below.

A stainless steel having a chemical composition shown in Table 1 was melted in an electric furnace, and subjected to casting, hot rolling, cold rolling, annealing and temper rolling to produce a stainless steel foil having a thickness of 40 μm.

Further, tension annealing was performed under the conditions shown in Table 2, hardness, tensile strength and elongation were measured, and a fatigue test was performed.

Steel No. in Table 1. For Test No. 1, tension annealing was performed under a plurality of conditions. 1a and 1b, and steel No. The same is true for 2 to 6.

The hardness was measured at 30 points per stainless steel with a measurement load of 50 gf and a measurement interval of 400 μm.

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

As a fatigue test, a bending fatigue test commonly called MIT test was performed according to JIS P 8115. Specifically, a test piece with 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 2.0 mm and a bending speed of 175 times/min.

In this bending fatigue test, those with endurance times of 12,000 or more were evaluated as having good fatigue characteristics.

As shown in Table 2, in the predetermined chemical composition range, all of the examples having an average hardness of 550 HV or more and a standard deviation of 3.5 HV or less have a durability of 12000 times in the bending fatigue test. As described above, the fatigue property was good.

All of the comparative examples in which the standard deviation of hardness exceeded 3.5 HV had less than 12,000 endurance times in the bending fatigue test.

In addition, test No. of this example. Using a stainless steel having the same composition as 1a to 6a, tension annealing was performed under different conditions, and Test No., which is a comparative example, had a hardness standard deviation exceeding 3.5 HV . All of 1b to 6b had a durability of less than 12,000 times in the bending fatigue test.

Claims (4)

C: 0.05% by mass or more and 0.30% by mass or less Si: 1.50% by mass or less (not including no additives) Mn: 0.10% by mass or more and 2.00% by mass or less P: 0% by mass % or more and 0.06 mass % or less, S: 0 mass % or more and 0.010 mass % or less, Ni: 5.00 mass % or more and 7.00 mass % or less, Cr: 15.00 mass % or more and 19.00 mass % and N: 0.05% by mass or more and 0.30% by mass or less, the total content of C and N is 0.20% by mass or more, and the balance consists of Fe and unavoidable impurities,
Md ₃₀ =551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo with a value of Md ₃₀ of 5 or more and 30 or less,
SFE = 2.2Ni + 6Cu - 1.1Cr - 13Si - 1.2Mn + 32 with an SFE value of 15 or more and less than 25,
Having a dual-phase structure of an austenite phase and a strain-induced martensite phase,
A stainless steel sheet having an average hardness of 550 HV or more and a standard deviation of hardness of 3.5 HV or less on the surface of the steel sheet . The stainless steel plate according to claim 1, containing at least one of Mo: 0 mass% to 2.00 mass% and Cu: 0 mass% to 2.00 mass%. 3. The stainless steel plate according to claim 1, wherein the surface average hardness and standard deviation are calculated based on hardness values measured at intervals of 400 μm under a measurement load of 50 gf. C: 0.05% by mass or more and 0.30% by mass or less Si: 1.50% by mass or less (not including no additives) Mn: 0.10% by mass or more and 2.00% by mass or less P: 0% by mass % or more and 0.06 mass % or less, S: 0 mass % or more and 0.010 mass % or less, Ni: 5.00 mass % or more and 7.00 mass % or less, Cr: 15.00 mass % or more and 19.00 mass % and N: 0.05% by mass or more and 0.30% by mass or less, the total content of C and N is 0.20% by mass or more, and the balance consists of Fe and unavoidable impurities,
Md ₃₀ =551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo with a value of Md ₃₀ of 5 or more and 30 or less,
SFE = 2.2Ni + 6Cu - 1.1Cr - 13Si - 1.2Mn + 32 with an SFE value of 15 or more and less than 25,
Pass-rolling a stainless steel sheet having a multi-phase structure of an austenite phase and a strain-induced martensite phase,
Tension annealing is performed after temper rolling,
In the tension annealing, in a state in which a tension of 3 kgf/mm ^{2 or more and 7 kgf/mm 2 or} less is applied to the stainless steel plate , the temperature of the material rises from room temperature to the maximum temperature of 480° C. or more and 540° C. or less. A stainless steel plate having an average hardness of 550 HV or more and a standard deviation of hardness of 3.5 HV or less is obtained by immediately cooling after heating for 5 seconds or more and 10 seconds or less.
A method for manufacturing a stainless steel plate , characterized by: