Stirring blade for secondary battery electrode paste

Abstract

To provide a stirring blade for secondary battery electrode paste that has corrosiveness necessary for the production of an electrode layer used in a secondary battery, corrosion resistance and wear resistance necessary for the stirring of paste composed of high-viscosity active substance, and excellent toughness and fatigue strength.SOLUTION: A stirring blade for secondary battery electrode paste is made of martensitic stainless steel comprising, in mass%, Cr: 15.00-16.00%, Mo: 1.50-1.90%, V: 0.20-0.40%, C: 0.37-0.45%, Si: 0.60% or less, Mn: 0.60% or less, P: 0.030% or less, S: 0.010% or less, and Al: 0.10% or less, and comprising, in mass ppm, N: 1600-2500 ppm and O: 50 ppm or less, with the balance being Fe and inevitable impurities.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an agitating blade for paste used in manufacturing electrodes such as positive and negative electrodes of secondary batteries such as nickel-metal hydride batteries and lithium-ion batteries. [Background technology]

[0002] The electrode layer of a secondary battery is produced by applying a paste-like active material to a current collector made of metal foil and drying it. This paste-like active material is produced by kneading hard metal oxides such as cobalt oxide, manganese oxide, lithium oxide, and nickel oxide with various materials such as carbon black, organic solvents, and binders. Therefore, a mixing blade is required to mix and knead these various materials into a paste. However, the raw materials being kneaded contain binders in addition to the hard metal oxides mentioned above, so the viscosity increases when they are stirred, and the stirring blades are required to have wear resistance, toughness, high fatigue strength, etc. Furthermore, since the paste contains corrosive substances, the stirring blades must also be corrosion resistant. Furthermore, in terms of ease of manufacturing the stirring blade, a material with excellent bending workability is preferred. In particular, if Cu is mixed into the paste, it may affect the battery characteristics, so it is preferable to minimize the amount of Cu mixed in as much as possible.

[0003] Up until now, austenitic stainless steels such as SUS304 have been used from the viewpoint of corrosion resistance, but their wear resistance is insufficient (Patent Document 1). Furthermore, from the viewpoint of wear resistance, the use of ferritic or martensitic stainless steel has been proposed, but the corrosion resistance is insufficient (Patent Documents 2 and 3). Precipitation hardening stainless steel such as SUS630 is said to have a good balance between corrosion resistance and wear resistance, and Cu is added as a strengthening element. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-89527 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-186113 [Patent Document 3] Japanese Patent Application Publication No. 2019-063770 Summary of the Invention [Problem to be solved by the invention]

[0005] In view of the above background art, an object of the present invention is to provide an agitating blade for secondary battery electrode paste, which has excellent corrosion resistance and abrasion resistance, as well as good toughness and fatigue strength, required for agitating paste made of a corrosive, highly viscous active material necessary for producing electrode layers used in secondary batteries. [Means for solving the problem]

[0006] The stirring blade for secondary battery electrode paste according to the present invention is characterized in that it is made of martensitic stainless steel containing, in mass%, Cr: 15.00 to 16.00%, Mo: 1.50 to 1.90%, V: 0.20 to 0.40%, C: 0.37 to 0.45%, Si: 0.60% or less, Mn: 0.60% or less, P: 0.030% or less, S: 0.010% or less, and Al: 0.10% or less, and, in ppm by mass, N: 1600 to 2500 ppm, O: 50 ppm or less, and the balance being Fe and unavoidable impurities.

[0007] The detailed reasons for setting the above-mentioned range of components will be described later, but in the present invention, by employing martensitic stainless steel and controlling the N content within the range of 1600 to 2500 ppm by mass, the surface hardness becomes 57 HRC or more on the Rockwell C scale, and excellent wear resistance is achieved. More preferably, it is 59 HRC or higher.

[0008] In the present invention, the martensitic stainless steel may have an average precipitated carbide particle size of 4 μm or less when observed by microstructural observation using an optical microscope. In martensitic stainless steel, high strength is achieved by the precipitation of carbides during quenching, but if large particles of carbide precipitate, they may become the starting point for deterioration of toughness and fatigue strength. Therefore, in the present invention, it is preferable to control the average particle size of precipitated carbides to 4 μm or less. For example, when the microstructure is observed with an optical microscope, the maximum particle size of carbides appearing in the observation field of the optical microscope is preferably 5 μm or less. In addition, by reducing the C content to less than that of commercially available SUS440C, ductility is improved and bending workability is excellent.

[0009] In the present invention, it is preferable that the martensitic stainless steel has a Cu content of 0.01 mass % or less as an unavoidable impurity. This is because if Cu is mixed into the secondary battery electrode paste, it may affect the battery characteristics. [Effects of the Invention]

[0010] The stirring blade for secondary battery electrode paste according to the present invention employs martensitic stainless steel, and by setting the N content in the range of 1600 to 2500 mass ppm and restricting the C content to the range of 0.37 to 0.45 mass%, it is possible to achieve both wear resistance and corrosion resistance, and ductility is also improved.

[0011] Furthermore, by controlling the particle size of the carbides in the microstructure, the toughness and fatigue strength are improved compared to existing martensitic stainless steels such as SUS440C and SUS420J2, and the blades do not break, bend, or chip even during long periods of stirring, allowing the material to withstand long periods of use. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a perspective view of an embodiment of an agitation blade according to the present invention. [Figure 2] An example of a microstructure photograph of the martensitic stainless steel used in the present invention is shown. [Figure 3] The chemical composition of the material used for evaluation is shown. [Figure 4] A schematic diagram of the bending test is shown. [Figure 5] The visual observation results of the bent portion after the bending test are shown. [Figure 6] The heat treatment conditions of each test piece are shown. [Figure 7] The measurement results of the surface hardness of each test piece are shown. [Figure 8] The weight change of each test piece before and after the wear test is shown. [Figure 9] The corrosion test results of each test piece are shown.

Embodiments for Carrying Out the Invention

[0013] First, the reason for setting the content range of the components of the martensitic stainless steel used for the stirring blade according to the present invention will be explained. Except for N and O, all the following are in mass %. <Cr component> Cr (chromium) is in the range of 15.00 to 16.00%. If it is less than 15.00%, sufficient corrosion resistance cannot be obtained as martensitic stainless steel. Also, if it exceeds 16.00%, large carbides will precipitate in the structure and the toughness will decrease. Mo (molybdenum) is in the range of 1.50 to 1.90%. If it is less than 1.50%, the corrosion resistance will decrease, and at the same time, sufficient tempering secondary hardening cannot be obtained. Also, if it exceeds 1.90%, large carbides will precipitate in the structure and the toughness will decrease. <V component> V (vanadium) is in the range of 0.20 to 0.40%. If it is less than 0.20%, sufficient tempering secondary hardening cannot be obtained. Moreover, when it exceeds 0.40%, large carbides precipitate in the structure, leading to a decrease in toughness. <N component> N (nitrogen) shall be in the range of 1600 - 2500 ppm by mass ppm. When it is less than 1600 ppm, sufficient corrosion resistance and hardened and tempered hardness cannot be obtained. Moreover, when it exceeds 2500 ppm, there is a risk of blow holes (casting nests) occurring inside the material. <C component> C (carbon) shall be in the range of 0.37 - 0.45%. When it is less than 0.37%, the amount of hard carbides formed in the structure decreases, and as a result, the hardness after hardening and tempering cannot be ensured. Moreover, when it exceeds 0.45%, the amount of carbides formed in the structure increases, and the solid solution amounts of Cr, Mo, etc. in the matrix phase decrease, so the corrosion resistance decreases. Moreover, large carbides precipitate in the structure, also reducing toughness and fatigue strength. <O component> O (oxygen) shall be 50 ppm or less by mass ppm. When it exceeds 50 ppm, it forms coarse oxides with elements such as Mg, Al, Si, Ca in the structure, reducing toughness and fatigue strength. <Si, Mn component> Si (silicon) shall be 0.60% or less. When it exceeds 0.60%, the hot and cold workability as martensitic stainless steel decreases, and toughness also decreases. Moreover, for Mn (manganese): when it is 0.60% or less, when it exceeds 0.60%, the hot and cold workability as martensitic stainless steel decreases. <Al component> Moreover, Al (aluminum) shall be 0.10% or less. When it exceeds 0.10%, it combines with O (oxygen) in the structure to form coarse oxides, reducing toughness and fatigue strength. <P, S component> P (phosphorus) shall be 0.030% or less. When it exceeds 0.030%, segregation occurs at the grain boundaries, reducing toughness. Also, for S (sulfur): when it exceeds 0.010%, sulfides are formed in the structure, resulting in a decrease in corrosion resistance and toughness. <Cu component> In the present invention, the Cu component is preferably minimized as much as possible, and it is advisable to keep it at least 0.01% or less. <Other components> In the present invention, other components are treated as inevitable impurities. For example, Ni, W, Co, etc., and they are kept at 0.01% or less.

[0014] An embodiment of the stirring blade 1 for the secondary battery electrode paste made of martensitic stainless steel according to the present invention is shown in FIG. 1. Note that the stirring blade of the present invention is not limited to the form in which the peripheral edge is bent in the vertical direction of the drawing as shown in FIG. 1. As long as bending is performed during the manufacturing process from a plate material, it may be in any form such as a propeller type, a turbine type, a paddle type, etc.

[0015] When manufacturing the stirring blade 1 shown as an example in FIG. 1 by bending, a material with the chemical composition shown as the invention material in the table of FIG. 3 is cast, then heated at 1050°C, quenched, and a sub-zero treatment and a tempering treatment at 180°C are performed. After performing the pretreatment by a conventional method, a photograph (500 times magnification) of the microstructure taken with an optical microscope is shown in FIG. 2. As shown in FIG. 2, it can be seen that carbide particles are dispersed and present in the structure. The particle size of the carbide is in the range of 1 to 4 μm, and the average is approximately 1.5 μm.

[0016] <Bending test> A bending test was performed using the steel type (hereinafter referred to as the invention material) of the stirring blade for the secondary battery electrode slurry of the present invention and a plate material of a commercially available stainless steel (hereinafter referred to as the comparative material), and the bending processing characteristics were evaluated. For commercially available stainless steels, austenitic stainless steel SUS304 was designated "Comparative Material 1," martensitic stainless steel SUS420J2 was designated "Comparative Material 2," martensitic stainless steel SUS440C was designated "Comparative Material 3," and precipitation hardening stainless steel SUS630 was designated "Comparative Material 4." The chemical compositions of the test pieces of the inventive material and comparative materials 1 to 4 are shown in FIG. 3, and a schematic diagram of the bending test state is shown in FIG. For the bending test, test pieces of five steel types, namely the invention material and comparative materials 1 to 4, having the chemical compositions shown in Figure 3, were cut into plate materials of predetermined dimensions (width 10 mm x thickness 1.8 mm x length 100 mm), and a three-point bending test was performed using an Amsler tensile testing machine as shown in Figure 4. The evaluation method was as follows: the above-mentioned test piece was placed in a special jig, and then a load was applied from above the test piece at a rate of 5 mm / min. The test was completed when the test piece was bent at approximately 90°. After the bending test, each test piece was removed from the Amsler tensile tester and visually inspected for cracks, wrinkles, etc. at the inner and outer bent portions of the test piece. Figure 5 shows the results of visual inspection of each test piece after the test.

[0017] As a result of visually observing each test piece, as shown in Figure 5, no abnormalities such as cracks or wrinkles were found in the bent parts (the load-bearing side and the back side) of the inventive material and comparative material 2. On the other hand, in each of the test pieces of comparative materials 1 and 4, many wrinkles were observed in the bent portion, especially on the back side of the load bearing surface. This is because when this material is used to manufacture stirring blades that require bending, there is a risk that the material will break at the bent part due to the occurrence of wrinkles, making it unsuitable as a material for stirring blades. The test piece of comparative material 3 broke during the bending test, and the test was terminated at that point. From the results of the bending tests, it was found that among the inventive material and comparative materials 1 to 4, the inventive material and comparative material 2 had the best bending processability and were therefore suitable as materials for manufacturing stirring blades that require bending.

[0018] <Wear test> Next, the stirring blades shown in Figure 1 were manufactured using the inventive material and comparative materials 1 to 4, which have the chemical compositions shown in Figure 3. Using these stirring blades, stirring tests were conducted in a slurry (paste-like substance) that will be used as an electrode material for a secondary battery, and the wear resistance of various test pieces was evaluated. The physical properties of the slurry and the stirring conditions used in this stirring test were as follows: Note that comparative material 3, which broke at the processed part during bending in the TP production, and comparative material 4, in which many wrinkles were found at the processed part, were not used in this test. Testing machine: Homo Disper 2.5 type, manufactured by Primix Slurry composition: LiNi 0.8 Co 0.15 Al 0.05 O268.0 parts by mass Acetylene black 2.5 parts by mass Polyvinylidene fluoride 2.0 parts by mass NMP 27.5 parts by mass Slurry volume: 500cc Slurry temperature: 25~30℃ Slurry viscosity: 3000cp Stirring speed: 3000 rpm Mixing time: 21 hours

[0019] Of the inventive materials shown in Figure 3, the one subjected to the heat treatment conditions described above is called Inventive Material 1, and the one subjected to a variation of this heat treatment condition is called Inventive Material 2. The heat treatment conditions for Comparative Materials 1 to 4 are shown in Figure 6, and the surface hardness (unit: HRC) of each of these test pieces is shown in Figure 7. For evaluation of this wear test, the weight of the stirring blade of each test piece is measured before the test, and after the test is completed, the stirring blade is washed with water, dried, and then the weight of the stirring blade is measured again. The difference in weight of the stirring blade before and after the test was calculated, and the weight loss was divided by the weight before the test, and the resulting ratio was compared and evaluated as a percentage (%). Figure 8 shows the weight change of each test piece before and after the abrasion test. Regarding the evaluation of abrasion resistance, as shown in FIG. 8, the weight loss rates of Inventive Materials 1 and 2 were 0.02 to 0.03%. In contrast, the weight loss rate of Comparative Material 1 was 0.16%. From the above results, it is clear that the inventive material has superior abrasion resistance against slurry.

[0020] <Salt spray test> Next, a salt spray test (in accordance with JIS Z2371) was carried out on each test piece of the inventive material and comparative materials 1 to 4 shown in FIG. 3 to evaluate corrosion resistance to salt water. Each test piece was cut to the specified dimensions (diameter φ25 mm x thickness 10 mm) and exposed in a salt spray tester. The test pieces were then removed 6 hours, 24 hours, 96 hours, and after the end of the test (192 hours) and the condition of the test pieces was checked. The test pieces were evaluated by calculating the percentage of rust that had developed on the surface of the test piece at each of the aforementioned times (rust rate). That is, a relative evaluation was made on a five-point scale: "1" if the rust rate on the surface of the test piece was less than 10%, "2" if the rust rate was 10% or more but less than 25%, "3" if the rust rate was 25% or more but less than 50%, "4" if the rust rate was 50% or more but less than 75%, and "5" if the rust rate was 75% or more. Fig. 9 shows the evaluation results of the degree of corrosion for each test piece. As shown in Figure 9, the corrosion level of the inventive material and comparative materials 1 and 4 was "1" until the end of the test. In contrast, the corrosion levels of the other comparative materials 2 and 3 were "4" or "5." From these test results, the corrosion resistance of the inventive material in salt water was equivalent to that of austenitic stainless steel (comparative material 1) and precipitation hardening stainless steel (comparative material 4). [Explanation of symbols]

[0021] 1 stirring blade

Claims

1. In mass%, Cr: 15.00 to 16.00%, Mo: 1.50 to 1.90%, V: 0.20 to 0.40%, C: 0.37 to 0.45%, Si: 0.60% or less, Mn: 0.60% or less, P: 0.030% or less, S: 0.010% or less, Al: 0.10% or less, Cu: 0.01% or less, A stirring blade for secondary battery electrode paste, characterized in that it is made of martensitic stainless steel containing, in ppm by mass, N: 1600 to 2500 ppm, O: 50 ppm or less, and the remainder being Fe and unavoidable impurities.

2. 2. The stirring blade for secondary battery electrode paste according to claim 1, wherein the surface hardness is 57 HRC or more on the Rockwell C scale.

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