Austenitic stainless steel plates and springs

Abstract

The present invention is an austenitic stainless steel which contains, on a mass basis, from 0.01% to 0.2% of C, 2% or less of Si, 3% or less of Mn, 0.035% or less of P, 0.03% or less of S, from 6% to 14% of Ni, from 20% to 26% of Cr, 3% or less of Mo, from 0.01% to 3% of Cu, 1% or less of Ti, 0.2% or less of Al, 0.1% or less of Ca, from 0.1% to 0.25% of N and 0.008% or less of O, with the balance being made up of Fe and impurities, wherein the E value obtained by formula (1) is -17.0 or more and the F value obtained by formula (2) is 0 or more.　(1): E = -0.33 × SFE + 0.25 × R In formula (1), SFE = 25.7 + 2Ni + 410C – 0.9Cr – 77N – 13Si – 1.2 Mn (wherein the atomic symbols represent the contents (mass%) of the respective elements); and R represents the average crystal grain size (μm). 　(2): F = 1003O – 211Al – 158Ca – 79Ti In formula (2), the atomic symbols represent the contents (mass%) of the respective elements.

Description

[Technical Field]

[0001] The present invention relates to an austenitic stainless steel and a spring. [Background technology]

[0002] SUS316L, a type of austenitic stainless steel, has good corrosion resistance in corrosive environments such as seawater and saltwater, and is also easy to process, so it is used in a variety of applications such as household goods, building materials, and automobile parts. However, SUS316L does not have sufficient corrosion resistance in some applications, and it contains a large amount of expensive elements (such as Mo and Ni), which makes the product price high.

[0003] Patent Document 1 proposes an austenitic stainless steel having three layers on its surface, in order from the surface to the inner layer: an outermost layer containing nitrides of Fe and Cr in a γ phase, a layer made of an S phase in which N is supersaturated in solid solution at 10% by mass or more, and a carbon-enriched layer, as an austenitic stainless steel having improved corrosion resistance while minimizing the use of expensive elements. However, the austenitic stainless steel described in Patent Document 1 requires complex surface treatment to form the predetermined layer on the surface, which results in problems such as low manufacturing efficiency and additional costs due to the surface treatment.

[0004] On the other hand, SUS329J1, a type of austenitic-ferritic duplex stainless steel, has better corrosion resistance than SUS316L in corrosive environments such as seawater and saltwater, and its content of expensive elements (Ni) can be reduced, making it suitable for a variety of applications such as water tanks and chemical plants.However, SUS329J1 is less machinable than SUS316L, making it difficult to apply to applications requiring complex forming processes.

[0005] Furthermore, when austenitic stainless steel is used as a spring material, improved fatigue properties are required. In particular, with the recent trend toward miniaturization of electronic devices such as mobile phone terminals and home appliances, improved fatigue properties are required for spring materials such as metal domes for tactile switches installed in electronic devices in order to extend their lifespan (Patent Document 2). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-188417 [Patent Document 2] Japanese Patent Publication No. 2020-41203 Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention has been made to solve the above problems, and has an object to provide an inexpensive austenitic stainless steel that has a higher level of corrosion resistance than SUS316L, better workability than SUS329J1, and good fatigue properties. Another object of the present invention is to provide a spring containing this austenitic stainless steel. [Means for solving the problem]

[0008] The present inventors first investigated the composition of austenitic stainless steels based on the composition of SUS316L. Reducing the Ni content broadens the stable temperature range of the ferrite phase (hereinafter referred to as the "α phase"), which can result in a two-phase structure consisting of the austenite phase (hereinafter referred to as the "γ phase") and the α phase, even at room temperature. The presence of the α phase within the γ phase reduces workability due to the difference in strength between them. Therefore, to ensure a balance between the α phase and the γ phase, it is necessary to add γ-forming elements (e.g., Mn, Cu, N). However, increasing the Mn or Cu content reduces corrosion resistance, while increasing the N content increases hardness and reduces workability. Furthermore, the deterioration of fatigue properties is due to the presence of coarse alumina-based inclusions that are difficult to break apart even during rolling and remain in the austenitic stainless steel. Therefore, it is important to reduce the amount of these coarse inclusions. Therefore, the present inventors optimized the composition of austenitic stainless steels to improve workability, corrosion resistance, and fatigue properties at low cost. Furthermore, the inventors have produced and analyzed austenitic stainless steels having various compositions, and as a result have discovered that the average grain size (R) and stacking fault energy (SFE) are closely related to the workability of austenitic stainless steel, and that the balance of the contents of Al, Ca, Ti, and O is closely related to the fatigue properties of austenitic stainless steel. Under the above circumstances, the inventors have found that the above problems can be solved by controlling the composition of the austenitic stainless steel, the E value, which is defined by the average grain size (R) and stacking fault energy (SFE) as parameters, and the F value, which is defined by the contents of Al, Ca, Ti, and O as parameters, and have thus completed the present invention.

[0009] That is, the present invention provides a steel sheet containing, by mass, 0.010 to 0.200% C, 2.00% or less Si, 3.00% or less Mn, 0.035% or less P, 0.030% or less S, 6.00 to 14.00% Ni, 20.0 to 26.0% Cr, 3.00% or less Mo, 0.01 to 3.00%, 1.000% or less Ti, 0.200% or less Al, 0.1000% or less Ca, 0.1000% or less N, and 0.0080% or less O, with the balance being Fe and impurities, and having an E value represented by the following formula (1) of -17.0 ~8.0 and the F value represented by the following formula (2) is 0 or more. The number of inclusions with a diameter of 15 μm or more was 1.0 / mm 2 is Austenitic stainless steel board is. E = -0.33 × SFE + 0.25 × R (1) In formula (1), SFE is 25.7+2Ni+410C-0.9Cr-77N-13Si-1.2Mn (each element symbol represents the content (mass%) of each element), and R is the average crystal grain size (μm). F=1003O-211Al-158Ca-79Ti (2) In formula (2), each element symbol represents the content (mass %) of each element.

[0010] The present invention also provides the austenitic stainless steel. board It is a spring including: [Effects of the Invention]

[0011] The present invention can provide an inexpensive austenitic stainless steel that has a higher level of corrosion resistance than SUS316L, better workability than SUS329J1, and good fatigue properties. The present invention also can provide a spring that includes this austenitic stainless steel. DETAILED DESCRIPTION OF THE INVENTION

[0012] The following is a detailed description of the embodiments of the present invention. The present invention is not limited to the following embodiments, and it should be understood that modifications and improvements made to the following embodiments based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention are also within the scope of the present invention. In this specification, the "%" designation for components means "% by mass" unless otherwise specified.

[0013] An austenitic stainless steel according to an embodiment of the present invention contains C: 0.010 to 0.200%, Si: 2.00% or less, Mn: 3.00% or less, P: 0.035% or less, S: 0.0300% or less, Ni: 6.00 to 14.00%, Cr: 20.0 to 26.0%, Mo: 3.00% or less, Cu: 0.01 to 3.00%, Ti: 1.000% or less, Al: 0.200% or less, Ca: 0.1000% or less, N: 0.100 to 0.250%, O: 0.0080% or less, with the balance being Fe and impurities. In this specification, the term "impurities" refers to components that are mixed in during the industrial production of austenitic stainless steel due to various factors in raw materials such as ores and scraps, and in the production process, and are acceptable within a range that does not adversely affect the present invention. In addition, in this specification, "austenitic stainless steel" refers not only to stainless steel having a metal structure of a single austenite phase, but also to stainless steel having a metal structure containing an austenite phase and 5 volume % or less of other phases (e.g., ferrite phase).

[0014] Moreover, the austenitic stainless steel according to the embodiment of the present invention may further contain B: 0.0001 to 0.0100%. Furthermore, the austenitic stainless steel according to the embodiment of the present invention may further contain one or more selected from Mg: 0.0001% to 0.1000% and REM: 0.0001% to 0.1000%. Furthermore, the austenitic stainless steel according to an embodiment of the present invention may further contain one or more selected from Nb: 0.001% to 1.000%, V: 0.001% to 1.000%, Zr: 0.001% to 1.000%, W: 0.001 to 1.000%, Co: 0.001 to 1.000%, Hf: 0.001 to 1.000%, Ta: 0.001 to 1.000%, and Sn: 0.001 to 0.100%. Hereinafter, each component will be described in detail.

[0015] <C: 0.010 to 0.200%> If the content of C is too high, the corrosion resistance of the austenitic stainless steel will decrease. Therefore, the upper limit value of the content of C is controlled to 0.200%, preferably 0.150%, more preferably 0.100%. On the other hand, if the content of C is too low, it will lead to an increase in refining cost. Therefore, the lower limit value of the content of C is controlled to 0.010%, preferably 0.015%, more preferably 0.020%. In this specification, "corrosion resistance" means the corrosion resistance in a corrosive environment containing NaCl such as seawater or salt water.

[0016] <Si: 2.00% or less> If the content of Si is too high, the workability of the austenitic stainless steel will decrease. Therefore, the upper limit value of the content of Si is controlled to 2.00%, preferably 1.98%, more preferably 1.95%. On the other hand, the lower limit value of the content of Si is not particularly limited, but preferably 0.01%, more preferably 0.05%, still more preferably 0.10%.

[0017] <0000​​​

[0018] <P: Below 0.035%> If the content of P is too high, the workability of austenitic stainless steel will deteriorate. Therefore, the upper limit value of the content of P is controlled at 0.035%, preferably 0.034%, more preferably 0.033%. On the other hand, the lower limit value of the content of P is not particularly limited, but is preferably 0.001%, more preferably 0.005%, still more preferably 0.010%.

[0019] <S: Below 0.0300%> If the content of S is too high, the manufacturability of austenitic stainless steel will deteriorate. Therefore, the upper limit value of the content of S is controlled at 0.0300%, preferably 0.0250%, more preferably 0.0200%. On the other hand, the lower limit value of the content of S is not particularly limited, but is preferably 0.0001%, more preferably 0.0003%, still more preferably 0.0005%.

[0020] <Ni: 6.00 - 14.00%> Ni, like Mn, is an austenite phase (γ-phase) forming element. Since Ni is expensive, if the content is too high, it will lead to an increase in manufacturing cost. Therefore, the upper limit value of the content of Ni is controlled at 14.00%, preferably 11.00%, more preferably 10.00%. On the other hand, if the content of Ni is too low, the workability of austenitic stainless steel will deteriorate. Therefore, the lower limit value of the content of Ni is controlled at 6.00%, preferably 6.20%, more preferably 6.50%.

[0021] <Cr: 20.0 - 26.0%> If the content of Cr is too high, the formation of intermetallic compounds (σ-phase) will be promoted, so the workability of austenitic stainless steel will deteriorate. Therefore, the upper limit value of the content of Cr is controlled at 26.0%, preferably at 25.8% or less. On the other hand, if the content of Cr is too low, sufficient corrosion resistance cannot be obtained. Therefore, the lower limit value of the content of Cr is controlled at 20.0%, preferably 20.5%.

[0022] <Less than 3.00% Mo> Since Mo is expensive, if the content of Mo is too high, it will lead to an increase in manufacturing cost. Therefore, the upper limit of the content of Mo is controlled at 3.00%, preferably 2.00%, more preferably 1.00%. On the other hand, the lower limit of the content of Mo is not particularly limited, but preferably 0.001%, more preferably 0.002%, still more preferably 0.01%.

[0023] <Cu: 0.01 - 3.00%> If the content of Cu is too high, the corrosion resistance of austenitic stainless steel will decrease. Therefore, the upper limit of the content of Cu is controlled at 3.00%, preferably 2.50%. On the other hand, if the content of Cu is too low, the workability of austenitic stainless steel will decrease. Therefore, the lower limit of Cu is controlled at 0.01%, preferably 0.10%, more preferably 0.15%.

[0024] <Ti: 1.000% or less> Ti is an element added to fix C in steel and improve intergranular corrosion resistance. If the content of Ti is too high, the workability of austenitic stainless steel will decrease, and at the same time, the amount of inclusions generated will increase, resulting in a decrease in the fatigue properties of austenitic stainless steel. Therefore, the upper limit of the content of Ti is controlled at 1.000%, preferably 0.800%, more preferably 0.500%, still more preferably 0.100%, particularly preferably 0.010%. On the other hand, the lower limit of the content of Ti is not particularly limited, but from the perspective of obtaining the effect of Ti, it is preferably 0.001%, more preferably 0.005%, still more preferably 0.010%.

[0025] <Al: 0.200% or less> If the Al content is too high, the amount of inclusions generated increases, and the fatigue properties of austenitic stainless steel deteriorate. Therefore, the upper limit value of the Al content is controlled to 0.200%, preferably 0.100%, more preferably 0.050%, and still more preferably 0.020%. On the other hand, the lower limit of the Al content is not particularly limited, but is preferably 0.0001%, more preferably 0.0002%, and still more preferably 0.001%.

[0026] <Ca: 0.1000% or less> Ca is an element added to improve hot workability. If the Ca content is too high, the amount of inclusions generated increases, and the fatigue properties of austenitic stainless steel deteriorate. Therefore, the upper limit value of the Ca content is controlled to 0.1000%, preferably 0.0500%, more preferably 0.0100%, and still more preferably 0.0050%. On the other hand, the lower limit value of the Ca content is not particularly limited, but from the viewpoint of obtaining the effect of Ca, it is preferably 0.0001%, more preferably 0.0005%, and still more preferably 0.0010%.

[0027] <N: 0.100 - 0.250%> If the N content is too high, the workability of austenitic stainless steel deteriorates. Therefore, the upper limit value of the N content is controlled to 0.250%, preferably 0.230%, and more preferably 0.220%. On the other hand, if the N content is too low, the corrosion resistance of austenitic stainless steel cannot be sufficiently obtained. Therefore, the lower limit value of the N content is controlled to 0.100%, preferably 0.110%.

[0028] <O: 0.0080% or less> O causes the formation of alumina (Al2O3)-based inclusions. Because alumina-based inclusions are hard, they are difficult to break apart by rolling and remain as coarse inclusions (diameter 15 μm or more), which reduces the fatigue properties of austenitic stainless steel. That is, if the O content is too high, the amount of alumina-based inclusions formed increases, resulting in a deterioration in the fatigue properties of austenitic stainless steel. Therefore, the upper limit of the O content is controlled to 0.0080% (80 ppm), preferably 0.0070%, and more preferably 0.0060%. On the other hand, the lower limit of the O content is not particularly limited, but is preferably 0.0010%, more preferably 0.0020%, and even more preferably 0.0030%.

[0029] <B:0.0001~0.0100%> B is an element that is added as needed to improve workability. If the B content is too high, the corrosion resistance of the austenitic stainless steel will decrease. Therefore, the upper limit of the B content is controlled to 0.0100%, preferably 0.0060%, and more preferably 0.0040%. On the other hand, if the B content is too low, the manufacturability of the austenitic stainless steel will decrease. Therefore, the lower limit of the B content is controlled to 0.0001%, and preferably 0.0010%.

[0030] <Mg:0.0001~0.1000%> Mg is an element that is added as needed to improve hot workability. From the viewpoint of obtaining the effects of Mg, the lower limit of the Mg content is controlled to 0.0001%, preferably 0.0005%, and more preferably 0.0010%. On the other hand, if the Mg content is too high, the amount of inclusions generated increases, degrading the quality. Therefore, the upper limit of the Mg content is controlled to 0.1000%, preferably 0.0500%, and more preferably 0.0100%.

[0031] <REM:0.0001~0.1000%> REM is an element added as needed to improve hot workability. From the viewpoint of obtaining the effects of REM, the lower limit of the REM content is controlled to 0.0001%, preferably 0.0005%, and more preferably 0.0010%. Furthermore, since REM is expensive, if the REM content is too high, the manufacturing cost will increase. Therefore, the upper limit of the REM content is controlled to 0.1000%, preferably 0.0500%, and more preferably 0.0100%.

[0032] <Nb:0.001~1.000%> Nb is an element that is added as needed to fix C in steel and improve intergranular corrosion resistance. From the viewpoint of obtaining the effects of Nb, the lower limit of the Nb content is controlled to 0.001%, preferably 0.005%, and more preferably 0.010%. Furthermore, if the Nb content is too high, the workability of austenitic stainless steel will decrease. Therefore, the upper limit of the Nb content is controlled to 1.000%, preferably 0.800%, and more preferably 0.500%.

[0033] <V:0.001~1.000%> V is an element that is added as needed to fix C in steel and improve intergranular corrosion resistance. From the viewpoint of obtaining the effects of V, the lower limit of the V content is controlled to 0.001%, preferably 0.005%, and more preferably 0.010%. Furthermore, if the V content is too high, the workability of the austenitic stainless steel will decrease. Therefore, the upper limit of the V content is controlled to 1.000%, preferably 0.800%, and more preferably 0.500%.

[0034] <Zr:0.001~1.000%> Zr is an element that is added as needed to fix C in steel and improve intergranular corrosion resistance. From the viewpoint of obtaining the effects of Zr, the lower limit of the Zr content is controlled to 0.001%, preferably 0.005%, and more preferably 0.010%. On the other hand, if the Zr content is too high, the workability of the austenitic stainless steel will decrease. Therefore, the upper limit of the Zr content is controlled to 1.000%, preferably 0.800%, and more preferably 0.500%.

[0035] <W:0.001~1.000%> W is an element that is added as needed to improve high-temperature strength and corrosion resistance. From the viewpoint of obtaining the effects of W, the lower limit of the W content is controlled to 0.001%, preferably 0.005%, and more preferably 0.010%. Furthermore, if the W content is too high, the workability of the austenitic stainless steel decreases and the manufacturing cost increases. Therefore, the upper limit of the W content is controlled to 1.000%, preferably 0.800%, and more preferably 0.500%.

[0036] <Co:0.001~1.000%> Co is an element that is added as needed to improve corrosion resistance. From the viewpoint of obtaining the effects of Co, the lower limit of the Co content is controlled to 0.001%, preferably 0.005%, and more preferably 0.010%. Furthermore, if the Co content is too high, the workability of the austenitic stainless steel decreases and the manufacturing cost increases. Therefore, the upper limit of the Co content is controlled to 1.000%, preferably 0.800%, and more preferably 0.500%.

[0037] <Hf:0.001~1.000%> Hf is an element added as needed to fix C in steel and improve intergranular corrosion resistance. From the viewpoint of obtaining the effects of Hf, the lower limit of the Hf content is controlled to 0.001%, preferably 0.005%, and more preferably 0.010%. Furthermore, if the Hf content is too high, the workability of austenitic stainless steel will decrease. Therefore, the upper limit of the Hf content is controlled to 1.000%, preferably 0.800%, and more preferably 0.500%.

[0038] <Ta:0.001~1.000%> Ta is an element that is added as needed to fix C in steel and improve intergranular corrosion resistance. From the viewpoint of obtaining the effects of Ta, the lower limit of the Ta content is controlled to 0.001%, preferably 0.005%, and more preferably 0.010%. Furthermore, if the Ta content is too high, the workability of the austenitic stainless steel will decrease. Therefore, the upper limit of the Ta content is controlled to 1.000%, preferably 0.800%, and more preferably 0.500%.

[0039] <Sn:0.001~0.100%> Sn is an element that is added as needed to improve corrosion resistance. From the viewpoint of obtaining the effects of Sn, the lower limit of the Sn content is controlled to 0.001%, preferably 0.005%, and more preferably 0.010%. Furthermore, if the Sn content is too high, the manufacturability of austenitic stainless steel decreases. Therefore, the upper limit of the Sn content is controlled to 0.100%, preferably 0.050%, and more preferably 0.010%.

[0040] The austenitic stainless steel according to the embodiment of the present invention has an E value represented by the following formula (1) of −17.0 or more, preferably −10.0 or more. E = -0.33 × SFE + 0.25 × R (1) In formula (1), SFE is 25.7+2Ni+410C-0.9Cr-77N-13Si-1.2Mn (each element symbol represents the content (mass%) of each element), and R is the average crystal grain size (μm). By controlling the E value to -17.0 or higher, elongation of 40% or more can be ensured, improving the workability of austenitic stainless steel. In particular, by setting the E value to -10.0 or higher, elongation of 45% or more can be ensured, allowing it to be worked into various shapes. The upper limit of the E value is not particularly limited, but from the viewpoint of stably ensuring good processability, it is preferably 10.0, more preferably 9.0, and even more preferably 8.0.

[0041] In this specification, the term "average grain size" refers to the average grain size of the metal structure of the austenitic stainless steel, and means the average value of the grain size calculated by the intercept method described below. The average grain size can be adjusted in the manufacturing process of austenitic stainless steel by controlling conditions such as the reduction ratio in hot rolling or cold rolling, and annealing conditions (annealing temperature, heating rate, cooling rate, heating time (annealing time)). Although it is difficult to generalize because other conditions also have an effect, generally, increasing the reduction ratio tends to decrease the average grain size. Furthermore, increasing the annealing temperature or lengthening the heating time tends to increase the average grain size. Furthermore, increasing the heating rate or cooling rate tends to decrease the average grain size. The average crystal grain size is not particularly limited, but is preferably 5 μm or more from the viewpoint of workability of the austenitic stainless steel. Furthermore, when the austenitic stainless steel is a plate material, the average crystal grain size is preferably half the plate thickness or less (for example, 150 μm or less when the plate thickness is 300 μm).

[0042] The austenitic stainless steel according to the embodiment of the present invention has an F value represented by the following formula (2) of 0 or more, preferably 0.1 or more, and more preferably 0.3 or more. F=1003O-211Al-158Ca-79Ti (2) In formula (2), each element symbol represents the content (mass %) of each element. The F value is an index that indicates the amount of inclusions with a diameter of 15 μm or more that affect fatigue properties. By setting the F value to 0 or more, the number of inclusions with a diameter of 15 μm or more is reduced to 1.0 / mm 2 Below It is possible to control the number of inclusions with a diameter of 15 μm or more to 1.0 / mm 2 If it is below, The inclusions are dispersed, improving fatigue properties. Note that the number of inclusions with a diameter of 15 μm or more is 0 / mm 2 is preferable, but for example, 0.01 pieces / mm 2 It can be more than that. The upper limit of the F value is not particularly limited, but from the viewpoint of stably ensuring good fatigue properties, it is preferably 10.0, more preferably 8.0, and even more preferably 6.0. Here, the number density (number / mm 2 ) is an austenitic steel It can be measured by observing the cross section of the stainless steel with a FE-SEM (field emission scanning electron microscope).

[0043] The austenitic stainless steel according to the embodiment of the present invention preferably has a pitting potential of 0.70 V or more. If the pitting potential is in this range, it can be said to have a higher level of corrosion resistance than SUS316L. The upper limit of the pitting potential is not particularly limited, but is, for example, 2.00 V, and preferably 1.50 V. The pitting potential can be measured by the method described below, and the potential is based on Ag / AgCl.

[0044] The austenitic stainless steel according to the embodiment of the present invention may be in any shape as long as it has the above-described characteristics. For example, the austenitic stainless steel may be in the form of various plate materials such as a hot-rolled plate, a hot-rolled annealed plate, a cold-rolled plate, or a cold-rolled annealed plate, but is preferably a cold-rolled annealed plate from the viewpoint of manufacturability.

[0045] The austenitic stainless steel according to the embodiment of the present invention can be produced by using a method known in the art, except for producing a stainless steel having the above-described composition. Specifically, when the austenitic stainless steel is a cold-rolled annealed sheet, it can be produced as follows. First, a stainless steel having the above-described composition is produced by melting and forging or casting, and then hot-rolling to obtain a hot-rolled sheet. Next, the hot-rolled sheet is appropriately annealed, pickled, and cold-rolled to obtain a cold-rolled sheet. Next, the cold-rolled sheet is appropriately annealed and pickled to obtain a cold-rolled annealed sheet. The conditions for each step are not particularly limited and may be adjusted appropriately depending on the composition of the stainless steel.

[0046] The austenitic stainless steel according to the embodiment of the present invention, which has the above-described characteristics, has better workability than SUS329J1 and higher corrosion resistance than SUS316L in corrosive environments containing NaCl, such as seawater or saltwater, and can therefore be used as a material for various components used in more severe corrosive environments. For example, the austenitic stainless steel according to the embodiment of the present invention is suitable for use in household appliances, building components (structural members), automobile parts, electronic components, water tanks, chemical plants, and the like. In particular, the austenitic stainless steel according to the embodiment of the present invention can also have improved fatigue properties, making it particularly suitable for use as a spring material for spiral springs, springs for electronic components, and the like. Furthermore, this austenitic stainless steel can reduce manufacturing costs by reducing the content of expensive elements such as Mo and Ni.

[0047] The spring according to the embodiment of the present invention includes the above-mentioned austenitic stainless steel. Because the spring according to the embodiment of the present invention includes the above-mentioned austenitic stainless steel, it has good workability and corrosion resistance, and can also improve fatigue characteristics. Therefore, the life of the spring can be extended. The type of spring is not particularly limited, but a leaf spring is preferable. [Example]

[0048] The present invention will be described in detail below with reference to examples, but the present invention should not be construed as being limited to these examples.

[0049] 30 kg of stainless steel having the composition shown in Table 1 was vacuum melted and forged into a 30 mm thick plate. This was then heated at 1230°C for 2 hours and hot-rolled to a 4 mm thick plate to obtain a hot-rolled plate. The hot-rolled plate was then annealed and pickled to obtain a hot-rolled annealed plate, which was then cold-rolled to a thickness of 0.3 mm or 1.0 mm to obtain a cold-rolled plate. The cold-rolled plate was then annealed, water-cooled, and pickled to obtain a cold-rolled annealed plate. Steel No. K is a steel grade equivalent to SUS329J1, and Steel No. M is a steel grade equivalent to SUS316L, which contains large amounts of Ni and Mo.

[0050] [Table 1]

[0051] The cold-rolled and annealed sheets obtained above were subjected to the following evaluations.

[0052] <Average grain size> The grain size of the metallographic structure was evaluated using cold-rolled and annealed sheets obtained by cold-rolling to a thickness of 0.3 mm, followed by annealing, water cooling, and pickling. Specifically, 15 mm × 20 mm test pieces were cut from the widthwise center of the cold-rolled and annealed sheets and subjected to sensitization heat treatment at 700 °C for 30 minutes. The cross section of the cold-rolled and annealed sheets parallel to the rolling direction (L direction) was mirror-polished and electrolytically etched with oxalic acid. The etched surface was then measured by optical microscopy. For optical microscopy, five fields of approximately 240 μm × 320 μm within the etched surface were observed, and the number of grains was calculated using the intercept method to determine the average grain size. A line 320 μm long was drawn in each field, and the number of grains intersected by this line was counted. The grain size in that field was calculated as "line length (320 μm) / number of grains." The average of the five fields was calculated as the average grain size. Note that grains at the ends of the line were counted as half.

[0053] <Ratio of ferrite phase> The ferrite phase ratio was evaluated using a cold-rolled annealed sheet obtained by cold-rolling to a thickness of 0.3 mm, followed by annealing, water cooling, and pickling. Five 50 mm × 50 mm test pieces were cut from the center of the width direction of the cold-rolled annealed sheet, and the five cut test pieces were stacked and the amount of ferrite phase (α phase) was measured using a ferrite scope (FERITESCOPE (registered trademark) FMP30 manufactured by Fischer). Measurements were performed at three random locations on the surface of the test piece, and the average value was used as the result.

[0054] <Number density of inclusions with a diameter of 15 μm or more> The number density of inclusions with a diameter of 15 μm or more was evaluated using a cold-rolled annealed sheet obtained by cold-rolling to a thickness of 1.0 mm, followed by annealing, water cooling, and pickling. Specifically, a 15 mm × 30 mm test piece was cut from the center of the width direction of the cold-rolled annealed sheet, embedded in resin so that the cross section (C cross section) in the thickness direction perpendicular to the rolling direction of the cold-rolled annealed sheet was exposed, and the C cross section was mirror-polished. Next, using a FE-SEM (SU5000) manufactured by Hitachi High-Tech Corporation, the center of the C cross section of the test piece was observed at a magnification of 200x with 60 fields of view (observation area: approximately 18.4 mm). 2 ) The number of inclusions with a diameter of 15 μm or more was counted. Inclusions with a diameter of 15 μm or more were defined as those for which the square root of the product of the major axis length a and the minor axis length b (a×b) was 15 μm or more. The number of inclusions thus obtained was divided by the observation area to determine the number density (pieces / mm) of inclusions with a diameter of 15 μm or more. 2 ) was calculated.

[0055] <Workability> The workability was evaluated using cold-rolled and annealed sheets obtained by cold-rolling to a thickness of 0.3 mm, followed by annealing, water cooling, and pickling. The workability was evaluated in accordance with the tensile test method specified in JIS Z2241:2011. Specifically, No. 13B test pieces were cut from the center of the width direction of the cold-rolled and annealed sheets, and a tensile test was performed at a tension speed of 20 mm / min to measure the elongation (%). In this evaluation, samples that achieved an elongation of 40%, which allows workability into various shapes, were evaluated as pass (A), and samples with an elongation of less than 40%, were evaluated as fail (B).

[0056] <Corrosion resistance> Corrosion resistance was evaluated using cold-rolled and annealed sheets obtained by cold-rolling to a thickness of 1.0 mm, followed by annealing, water cooling, and pickling. Corrosion resistance was measured in accordance with JIS G0577:2014. A 15 mm × 20 mm test piece was cut from the center of the cold-rolled and annealed sheet in the width direction and then wet-polished with #600 polishing paper. Next, the electrode surface (exposed portion) of the test piece was insulated with silicone resin except for the electrode surface to obtain a test piece for pitting potential measurement. Next, the test piece for pitting potential measurement was immersed in a 3.5% NaCl solution at 30°C that had been thoroughly degassed with Ar, and potentiodynamic anodic polarization was performed at 20 mV / min from the natural potential to measure the pitting potential. The pitting potential was measured at a current of 100 μA / cm. 2 The potential when the current flows is taken as the potential. In the evaluation, a pitting potential of 0.7 V vs. Ag / AgCl (hereinafter, all potentials are based on Ag / AgCl) or higher was evaluated as pass (A), and a potential of less than 0.7 V was evaluated as fail (B).

[0057] <Fatigue properties> The fatigue properties were evaluated using cold-rolled and annealed sheets obtained by cold-rolling to a thickness of 1.0 mm, followed by annealing, water cooling, and pickling. Specifically, a test piece with a width of 30 mm, a length of 90 mm, and a rounded section with a radius of 4.25 mm at both ends in the width direction was cut out from the center of the cold-rolled and annealed sheet, with the minimum sheet width of the rounded section being 20 mm. The length direction of the test piece was the rolling direction. Next, a maximum stress of 650 MPa, a cycle rate of 1500 cpm, fully reversed swing, and a test stop count of 1 × 10 were applied. 7A plane bending fatigue test was conducted under the condition of 1 × 1 cycle. 0 7 If the number of times of durability is 1 × 10, it is considered as pass (A). 7 Those that were less than this were deemed to have failed (B).

[0058] The results of the above evaluations are shown in Table 2.

[0059] [Table 2]

[0060] As shown in Tables 1 and 2, the cold-rolled annealed sheets (austenitic stainless steels) of Examples 1 to 10 satisfied the specified composition, E value, and F value, and therefore exhibited good results in all aspects of workability, corrosion resistance, and fatigue strength. On the other hand, the cold-rolled annealed sheet (austenitic stainless steel) of Comparative Example 1 had an E value outside the range, and therefore did not have sufficient workability. The cold-rolled and annealed sheet of Comparative Example 2 (austenitic-ferritic duplex stainless steel; SUS329J1) had an excessively low Ni content, resulting in a two-phase structure of α and γ phases and insufficient workability. Furthermore, the F value of this cold-rolled and annealed sheet was outside the specified range, and therefore the fatigue properties were also insufficient. The cold-rolled annealed sheet (austenitic stainless steel) of Comparative Example 3 had insufficient corrosion resistance because the content of N was too low. Furthermore, this cold-rolled annealed sheet also had insufficient fatigue properties because the F value was outside the specified range. The cold-rolled annealed sheet of Comparative Example 4 (austenitic stainless steel; SUS316L) does not have sufficient corrosion resistance, and contains a large amount of Ni and Mo, which increases the manufacturing cost. The cold-rolled annealed sheet (austenitic stainless steel) of Comparative Example 5 had an excessively low Cr content and therefore did not have sufficient corrosion resistance. The cold-rolled annealed sheet (austenitic stainless steel) of Comparative Example 6 had an excessively low Cr content and an excessively high O content, and therefore did not have sufficient corrosion resistance. The cold-rolled steel sheets (austenitic stainless steel) of Comparative Examples 7 to 9 had F values outside the predetermined range, and therefore did not have sufficient fatigue properties.

[0061] As can be seen from the above results, the present invention can provide an inexpensive austenitic stainless steel that has a higher level of corrosion resistance than SUS316L, is more workable than SUS329J1, and has good fatigue properties.

Claims

1. An austenitic stainless steel sheet comprising, by mass, 0.010 to 0.200% C, 2.00% or less Si, 3.00% or less Mn, 0.035% or less P, 0.0300% or less S, 6.00 to 14.00% Ni, 20.0 to 26.0% Cr, 3.00% or less Mo, 0.01 to 3.00%, 1.000% or less Ti, 0.200% or less Al, 0.1000% or less Ca, 0.100 to 0.250% N, and 0.0080% or less O, with the balance being Fe and impurities, having an E value expressed by the following formula (1) of -17.0 to 8.0, an F value expressed by the following formula (2) of 0 or more, and having 1.0 or less inclusions with a diameter of 15 μm or more per mm. E=-0.33×SFE+0.25×R (1) In formula (1), SFE is 25.7 + 2Ni + 410C - 0.9Cr - 77N - 13Si - 1.2Mn (each element symbol represents the content (mass%) of each element), and R is the average crystal grain size (μm). F=1003O-211Al-158Ca-79Ti (2) In formula (2), each element symbol represents the content (mass %) of each element.

2. The austenitic stainless steel sheet according to claim 1, further comprising, on a mass basis, 0.0001 to 0.0100% B.

3. 3. The austenitic stainless steel sheet according to claim 1, further comprising, on a mass basis, one or more selected from Mg: 0.0001 to 0.0100% and REM: 0.0001 to 0.0100%.

4. 4. The austenitic stainless steel sheet according to claim 1, further comprising, on a mass basis, one or more selected from Nb: 0.001 to 0.500%, V: 0.001 to 0.500%, Zr: 0.001 to 0.500%, W: 0.001 to 0.500%, Co: 0.001 to 0.500%, Hf: 0.001 to 0.500%, Ta: 0.001 to 0.500%, and Sn: 0.001 to 0.010%.

5. The austenitic stainless steel sheet according to any one of claims 1 to 4, wherein the contents are, on a mass basis, Al: 0.020% or less, Ca: 0.0001 to 0.0050%, and Ti: 0.001 to 0.010%.

6. The austenitic stainless steel sheet according to any one of claims 1 to 5, having a pitting potential of 0.70 V or more.

7. A spring comprising the austenitic stainless steel sheet according to any one of claims 1 to 6.

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