JP2020041204A - Stainless steel and manufacturing method therefor - Google Patents

Abstract

To provide a stainless steel good in fatigue property.SOLUTION: A stainless steel contains C:0.05 mass% to 0.30 mass%, Si:1.50 mass% or less, Mn:0.10 mass% to 2.00 mass%, P:0.06 mass% or less, S:0.010 mass% or less, Ni:5.00 mass% to 7.00 mass%, Cr:15.00 mass% to 19.00 mass%, and N:0.05 mass% to 0.30 mass%, with total of contents of C and N of 0.20 mass% or more, and the balance Fe with inevitable impurities. The stainless steel has a Mdof 5 to 30, and a value of SFE of 15 or more and less than 25. It has a double-phase structure of an austenite phase and a process induction martensite phase. Further, average hardness of a surface is 550 HV or more and standard deviation of hardness is 3.5 or less.SELECTED DRAWING: None

Description

  The present invention relates to a stainless steel 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 a high density, and components used are becoming lighter, thinner and smaller.

  Further, in the metal materials used for such electronic device parts, the stress applied repeatedly and the number of times of repetition are significantly increased.

  In particular, as an input key switch such as a tact switch in an electronic device, a metal dome switch device using a thin metal plate is used. With a metal plate adapted to the metal dome switch device, a click feeling at the time of operation and durability are provided. (Fatigue properties).

  As a metal plate for a switch of this type, for example, as disclosed in Patent Literatures 1 and 2, stainless steel in which the crystal grain size is reduced to improve fatigue characteristics is known.

WO 2008/41638 JP 2004-244725 A

  In recent years, the miniaturization of electronic devices such as mobile phone terminals and home appliances has been increasingly progressing, and there has been a demand for more improved material properties, especially for the fatigue characteristics of materials that lead to longer switch life. ing.

  The present invention has been made in view of such a point, and an object of the present invention is to provide a stainless steel having good fatigue characteristics.

The stainless steel according to claim 1, C: 0.05% by mass or more and 0.30% by mass or less, Si: 1.50% by mass or less, Mn: 0.10% by mass or more and 2.00% by mass or less, P: 0.06% by mass or less, S: 0.010% by mass or less, Ni: 5.00% to 7.00% by mass, Cr: 15.00% to 19.00% by mass, and N: It contains not less than 0.05% by mass and not more than 0.30% by mass, the total content of C and N is 0.20% by mass or more, and the balance consists of Fe and unavoidable impurities, and Md ₃₀ = 551-462. When the value of Md ₃₀ represented by (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 The value of SFE represented by .2Mn + 32 is It has a multiple phase structure of an austenite phase and a work-induced martensite phase in a range of 15 or more and less than 25, and has an average hardness of 550 HV or more and a standard deviation of hardness of 3.5 or less on a steel surface.

  The stainless steel according to the second aspect is the stainless steel according to the first aspect, which contains at least one of Mo: 2.00% by mass or less and Cu: 2.00% by mass or less.

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

The method for producing stainless steel according to claim 4, wherein C: 0.05% by mass or more and 0.30% by mass or less, Si: 1.50% by mass or less, Mn: 0.10% by mass or more and 2.00% by mass. %, P: 0.06% by mass, S: 0.010% by mass, Ni: 5.00% to 7.00% by mass, Cr: 15.00% to 19.00% by mass And N: not less than 0.05% by mass and not more than 0.30% by mass, the total content of C and N is not less than 0.20% by mass, the balance is Fe and unavoidable impurities, and Md ₃₀ = 551-462 (C + N) -9.2Si-8.1Mn-29 (Ni + Cu) -13.7Cr-18.5Mo has a value of Md ₃₀ of 5 to 30 and SFE = 2.2Ni + 6Cu-1.1Cr-. S represented by 13Si-1.2Mn + 32 A stainless steel having a FE value of 15 or more and less than 25 and having a dual-phase structure of an austenite phase and a work-induced martensite phase is temper-rolled, tension-annealed after temper-rolling, and 3 kgf is applied to the stainless steel in the tension annealing. In a state where a tension of not less than / mm ^{2 and not} more than 7 kgf / mm ² is applied, the material is heated so that the temperature rise time from room temperature to 480 ° C. or more and 540 ° C. or less is 5 seconds or more and 10 seconds or less. It is.

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

  Hereinafter, the configuration of an embodiment of the present invention will be described in detail.

  In the stainless steel according to the present invention, 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 0.000% 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 to 7.00% by mass, Cr ( Chromium): 15.00% to 19.00% by mass and N (nitrogen): 0.05% to 0.30% by mass, and the total content of C and N is 0.20%. When the content is not less than mass%, the balance consists of Fe (iron) and unavoidable impurities.

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

Within the scope of the chemical _{composition, Md 30 = 551-462 (C +} N) -9.2Si-8.1Mn-29 (Ni + Cu) -13.7Cr-18.5Mo of _{Md 30} indicated by the formula of which is the austenite stability index The value is 5 or more and 30 or less.

  Further, the value of SFE represented by the equation of 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.

  Further, the value of δcal, which is an index of δ ferrite formation after heating at 1230 ° C. for 2 hours, is represented by the formula of δcal = −15−44.91C−0.88Mn−2.31Ni + 2.20Cr−1.08Cu−28.8N. 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 the element not contained.

  C and N are austenite-forming elements. If the content of these elements is too small, the amount of the δ-ferrite phase increases and the hot workability decreases. Further, C and N are useful elements for solid solution strengthening of the work-induced martensite phase. It is important that both the content of C and the content of N be 0.05% by mass or more in order to stably obtain a remarkable ductility improving action. On the other hand, if C and N are excessively contained in excess of 0.30% by mass, the steel may be excessively hardened, which may be a factor that impairs workability. Therefore, both the content of C and the content of N are set to 0.05% by mass or more and 0.30% by mass or less.

  In addition, at the time of forming a process-induced martensite phase, C + N (the total content of C and N) must be 0.20% by mass or more in order to develop sufficient ductility due to the TRIP phenomenon. Therefore, C and N are set to C + N ≧ 0.20% by mass in the above ranges of the respective contents.

  If C + N exceeds 0.40% by mass, workability due to hardening may be impaired. Therefore, it is preferable that C + N is 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 that the content be 0.30% by mass or less.

  Si is an element useful for deoxidation in steelmaking and an element contributing to solid solution strengthening. However, if added in excess of 1.50% by mass, the steel becomes hard and becomes a factor that impairs workability. In addition, since Si is a ferrite-forming element, excessive addition causes generation of a large amount of δ-ferrite phase in a high temperature range, and impairs 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 affects work hardening. In order to utilize these effects, it is necessary to add Mn at 0.10% by mass or more. On the other hand, when Mn is added in excess of 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 mixed as unavoidable impurities, and the lower the content, the better. In consideration of workability and 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 addition).

  Ni is an element effective for improving ductility and toughness. In order to exert its effect sufficiently, it is necessary to add 5.00% by mass or more of Ni. On the other hand, if Ni is added in excess of 7.00% by mass, this may cause a reduction in strength properties, and also lowers economic efficiency due to an 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 element indispensable for forming a passivation film that ensures the corrosion resistance of stainless steel, and when contained at 15.00% by mass or more, corrosion resistance can be sufficiently ensured. 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 a large amount of δ-ferrite is generated even after heating. It becomes a factor that impairs the interworkability. Therefore, the content of Cr 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 also contributes to solid solution strengthening. However, if added in excess of 2.00% by mass, Mo becomes a factor that impairs hot workability. Therefore, when adding Mo, the content is set to 2.00% by mass or less.

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

Md ₃₀ is an austenite stability index, the more the value is greater transformation from austenite phase to strain-induced martensite phase is likely to occur. Then, by the value of Md ₃₀ 5 or more, can prevent excessive generation of strain-induced martensite phase, it is possible to suppress the variation of hardness, it is possible to prevent decrease in manufacturing workability to ensure ductility. On the other hand, when Md ₃₀ exceeds 30, the amount of formation of the work-induced martensite phase in a case where bending is performed or the like becomes too large, and the hardness may be likely to vary. Therefore, in the above stainless steel, the content of each element is adjusted so that the value of Md ₃₀ is 5 or more and 30 or less.

  SFE is a stacking fault energy generation index, and the larger the value, the more difficult it is for the work hardening of the austenite phase to occur. If the SFE is less than 15, the work hardening of the austenite phase is likely to occur, and the ductility may be reduced. On the other hand, when the SFE is 25 or more, work hardening of the austenite phase becomes difficult to occur, the hardness difference between the austenite phase and the work-induced martensite phase increases, and the hardness may be likely to vary. Therefore, in the above stainless steel, the content of each element is adjusted so that SFE is 15 or more and less than 25.

  δcal is an index of δ ferrite formation after heating at 1230 ° C. for 2 hours. If it exceeds 1.0, ear cracks may easily occur during hot rolling, and mechanical properties in the final product may be reduced. It can also affect properties and fatigue properties. Therefore, the content of each element in the stainless steel is preferably adjusted so that δcal is 1.0 or less.

  The stainless steel constituted by the above chemical composition is subjected to predetermined manufacturing steps (for example, hot rolling, cold rolling, annealing, finish rolling, tension annealing, and temper rolling) described below, and the average of the stainless steel surface is obtained. A foil having a hardness of 550 HV or more and a thickness of 20 μm to 100 μm is formed.

  Such a foil-like stainless steel has a dual phase structure of an austenite phase and a work-induced martensite phase.

  Here, the greater the variation in hardness on the stainless steel surface, the more the stress acting on the portions having different hardness differs due to repeated load (bending, etc.), and the strain tends to concentrate due to the difference in the stress. , Fatigue properties are reduced.

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

  The stainless steel is suitably used as a spring material for a metal dome for a tact switch provided in an electronic device such as a mobile phone or a home appliance.

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

  First, a raw material of stainless steel is melted, and decarburization is performed by blowing oxygen into the molten steel. Then, deoxidation is performed to reduce oxygen concentration by adding Si and reacting with oxygen in the molten steel.

  In addition, a slab is formed by continuous casting, the slab is heated to 1100 to 1300 ° C., and hot-rolled to form a hot-rolled steel strip.

  The hot-rolled steel strip is repeatedly cold-rolled and annealed to a predetermined plate thickness, work-hardened the austenite phase by temper rolling after finish annealing, and transformed into a work-induced martensite to have a hardness of 550 HV. Above.

  After the temper rolling, the residual stress is removed and the shape is corrected by tension annealing (TA).

Tension annealing, in a state plus 3 kgf / mm ² or more 7 kgf / mm ² or less tension in stainless steel, heating-up period to the material temperature becomes less than the maximum temperature 480 ° C. or higher 540 ° C. from room temperature is more than 5 seconds Heat to 10 seconds or less, then cool immediately.

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

According to the above stainless steel, the components are 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 within a predetermined chemical composition range, and the average hardness of the surface is 550 HV. As described above, since the standard deviation of the hardness is 3.5 or less, even if, for example, the average crystal grain size is the same as that of SUS301 which is a conventional steel, strain concentration can hardly occur and the fatigue characteristics can be improved.

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

  Hereinafter, this example and a comparative example will be described.

  A stainless steel having a chemical composition shown in Table 1 was melted in an electric furnace, 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.

  In Table 1, steel No. Tension annealing was performed on a plurality of conditions for Test No. 1a and 1b. The same applies to 2 to 6.

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

  In addition, the strength (tensile strength) of each of the present example and the comparative example was made equal to compare the fatigue characteristics.

  In the fatigue test, a bending fatigue test called a so-called MIT test was performed according to JIS P8115. Specifically, a test piece having a length of 110 mm and a width of 15 mm was cut out from each of the above stainless steels, and the test load was set to 1 kg, the bending angle was set to 135 °, and the bending was performed using a folding fatigue tester manufactured by Toyo Seiki Seisaku-sho. 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 having a durability of 12,000 or more were evaluated as having good fatigue properties.

  As shown in Table 2, the average hardness is 550 HV or more and the standard deviation is 3.5 or less in any given range of the chemical composition. And the fatigue characteristics were good.

  In all of the comparative examples having a standard deviation of hardness of more than 3.5, the number of durability in the bending fatigue test was less than 12,000.

  In addition, in Test No. Using a stainless steel having the same composition as in Examples 1a to 6a and performing tension annealing under different conditions, Test No. 1 was a comparative example in which the standard deviation of hardness exceeded 3.5. In all of 1b to 6b, the number of durability in the bending fatigue test was less than 12,000.

Claims (4)

C: 0.05 to 0.30% by mass, Si: 1.50% by mass, Mn: 0.10 to 2.00% by mass, P: 0.06% by mass, S: 0.010% by mass or less, Ni: 5.00% by mass to 7.00% by mass, Cr: 15.00% by mass to 19.00% by mass, and N: 0.05% by mass to 0.30% by mass Containing the following, 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-18.5 Mo has a value of Md ₃₀ of 5 or more and 30 or less,
The value of SFE represented by SFE = 2.2Ni + 6Cu-1.1Cr-13Si-1.2Mn + 32 is 15 or more and less than 25,
It has a dual phase structure of an austenite phase and a work-induced martensite phase,
A stainless steel, wherein the average hardness of the steel surface is 550HV or more and the standard deviation of the hardness is 3.5 or less. The stainless steel according to claim 1, wherein the stainless steel contains at least one of Mo: 2.00% by mass or less and Cu: 2.00% by mass or less. The stainless steel according to claim 1 or 2, wherein the average hardness and the standard deviation of the surface are calculated based on hardness values measured at intervals of 400 µm with a measured load of 50 gf. C: 0.05 to 0.30% by mass, Si: 1.50% by mass, Mn: 0.10 to 2.00% by mass, P: 0.06% by mass, S: 0.010% by mass or less, Ni: 5.00% by mass to 7.00% by mass, Cr: 15.00% by mass to 19.00% by mass, and N: 0.05% by mass to 0.30% by mass Containing the following, 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-18.5 Mo has a value of Md ₃₀ of 5 or more and 30 or less,
The value of SFE represented by SFE = 2.2Ni + 6Cu-1.1Cr-13Si-1.2Mn + 32 is 15 or more and less than 25,
Temper rolling of stainless steel having a dual phase structure of austenite phase and work induced martensite phase,
Tension annealing after temper rolling,
In the tension annealing, in a state where a tension of 3 kgf / mm ² or more and 7 kgf / mm ² or less is applied to the stainless steel, a time required for the temperature of the material to rise from room temperature to 480 ° C. or more and 540 ° C. or less is 5 seconds or more and 10 seconds or less. A method for producing stainless steel, wherein the heating is carried out for less than a second.