Cold-worked products

Abstract

To provide cold-worked products capable of preventing the formation of a hardened surface layer.SOLUTION: A cold-worked product, composed of a material containing Mn, Fe, Ni, Cr, and inevitable impurities, satisfies the condition -(c1lnc1+c2lnc2+c3lnc3+c4lnc4)≥1.3 where c1, c2, c3, and c4 represent the molar fractions of Mn, Fe, Ni, and Cr, respectively, in the material.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This disclosure relates to cold-worked products, which are products manufactured by cold working. [Background technology]

[0002] Austenitic stainless steel is primarily used as a material for equipment and piping in nuclear power plants. It is known that when austenitic stainless steel is subjected to cold working such as normal cutting and grinding, a hardened layer is formed on its surface. For example, in the recirculation system piping of boiling water reactors, it is generally said that stress corrosion cracking (SCC) may occur if the hardness of this hardened layer exceeds 300 HV on the Vickers hardness scale, and the same is thought to be true for equipment and piping through which water circulates in pressurized water reactors. Patent Document 1 describes how to suppress the formation of a hardened layer during processing by using a finishing cutting tool with a rake angle of +29° or more, thereby suppressing the hardness (Vickers hardness) of the processed surface to less than 300 HV. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] International Publication No. 2011 / 024706 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] However, the method described in Patent Document 1 has the drawback that the hardness reduction effect may be lost when the cutting tool wears down, and furthermore, this method is only applicable to machining and is not a measure to reduce hardness in processes such as grinding.

[0005] In view of the circumstances described above, at least one embodiment of this disclosure aims to provide a cold-worked product capable of suppressing the formation of a surface hardened layer. [Means for solving the problem]

[0006] To achieve the above objective, the cold-worked product relating to this disclosure contains Mn, Fe, Ni, Cr, and unavoidable impurities. alloy consisting of A cold-worked product formed from the material, alloy If c1, c2, c3, and c4 are the mole fractions of Mn, Fe, Ni, and Cr in the material, then -(c1lnc1+c2lnc2+c3lnc3+c4lnc4)≧1.3, and c1=0.15, 0.16≦c2≦0.39, 0.27≦c3≦0.40, and 0.19≦c4≦0.29. [Effects of the Invention]

[0007] According to the cold-worked product of this disclosure, the arrangement entropy of the material is 1.3R or higher, where R is the gas constant. This significantly increases the stability of the austenite phase and suppresses the increase in hardness of the surface and interior of the cold-worked product during cold working, thereby suppressing the formation of a hardened surface layer on the surface of the cold-worked product. [Brief explanation of the drawing]

[0008] [Figure 1] This flowchart shows a method for manufacturing a cold-worked product according to one embodiment of the present disclosure. [Figure 2] This graph shows experimental results illustrating the relationship between the reduction rate due to cold working and Vickers hardness. [Figure 3] This graph shows experimental results illustrating the changes in the mole fraction of each element in the alloy before and after ion irradiation. [Modes for carrying out the invention]

[0009] The following description of cold-worked products according to embodiments of this disclosure will be based on the drawings. The embodiments described below represent one aspect of this disclosure and are not limiting, and can be modified at will within the scope of the technical idea of ​​this disclosure.

[0010] <Configuration of cold-worked product according to an embodiment of the present disclosure> A cold-worked product according to an embodiment of the present disclosure is a product manufactured from a metallic material by cold working, and is, for example, a bar material such as a bolt, a pipe material such as piping, or a plate material such as a structural steel sheet. The material constituting the cold-worked product according to an embodiment of the present disclosure is an alloy containing four components of Mn, Fe, Ni, and Cr and unavoidable impurities, and is a medium-entropy alloy in which these four components are mixed at substantially equal atomic weights (or equal molar fractions).

[0011] Furthermore, the reason why an alloy composed of four components of Mn, Fe, Ni, and Cr is preferable as the material for the cold-worked product can be explained as follows. High-entropy alloys composed of five components of Mn, Fe, Ni, Cr, and Co have been widely studied, but from the viewpoint of reducing radiation exposure in a nuclear reactor environment, alloys excluding Co, that is, alloys composed of four components of Mn, Fe, Ni, and Cr are preferable. Further, for Fe-Cr-Ni based stainless steels, in order to increase stacking fault energy (SFE), it is effective to contain Mo or Mn compared to Cr. However, since Mo is an element for stabilizing a body-centered cubic (bcc) structure, when Mo is excluded as a material for cold-worked products that do not require such a purpose, an alloy composed of four components of Mn, Fe, Ni, and Cr is preferable.

[0012] This medium-entropy alloy has a configurational entropy ΔS represented by the following formula (1) of 1.3R [J / K] or more, where R is the gas constant. ΔS=-R(c1lnc1+c2lnc2+c3lnc3+c4lnc4)···(1) Here, c1 to c4 are respectively the molar fractions of Mn, Fe, Ni, and Cr in the medium-entropy alloy.

[0013] A medium-entropy alloy whose configurational entropy ΔS is 1.3R or greater can be rephrased as a medium-entropy alloy that satisfies -(c1lnc1+c2lnc2+c3lnc3+c4lnc4)≧1.3, given only the composition of the medium-entropy alloy.

[0014] Thus, medium-entropy alloys composed of four components—Mn, Fe, Ni, and Cr—with a configuration entropy ΔS of 1.3R or higher exhibit remarkably high austenite phase stability. Consequently, cold working such medium-entropy alloys can suppress the increase in surface and internal hardness of the cold-worked product. This, in turn, can suppress the formation of a surface hardening layer on the surface of the cold-worked product.

[0015] In such medium-entropy alloys, if the valence electron concentration (VEC), which is the average number of outermost electrons of the constituent elements, is 7.9 or higher, the face-centered cubic (fcc) lattice structure of the medium-entropy alloy tends to stabilize, increasing the material's stability. Such materials have high plastic deformability and generally excellent manufacturability and workability because multiple slip systems can be active due to their crystal structure. As a result, the manufacturability and workability of cold-worked products can be improved.

[0016] Furthermore, the VEC of an alloy composed of four components, Mn, Fe, Ni, and Cr, is expressed by the following formula (2). VEC = 7c1 + 8c2 + 10c3 + 6c4 ... (2) Therefore, for medium-entropy alloys with a VEC of 7.9 or higher, it can be rephrased as a medium-entropy alloy that satisfies the condition 7c1+8c2+10c3+6c4≧7.9, based solely on the composition of the medium-entropy alloy.

[0017] Furthermore, in such a medium-entropy alloy, when the atomic radii of Mn, Fe, Ni, and Cr are r1, r2, r3, and r4 respectively, the standard deviation VAR of the atomic radii of the constituent elements is expressed by the following formula (3): _Radiusis 10 or more and 17 or less, the randomness of lattice point positions in the solid solution increases, so the material property of being hard to harden can be obtained. VAR _Radius =(r1 2 c1+r2 2 c2+r3 2 c3+r4 2 c4)-(r1c1+r2c2+r3c3+r4c4) 2 ···(3)

[0018] In order to increase the randomness of atomic arrangement, VAR _Radius is set to 10 or more. However, if VAR _Radius is excessively increased, it will adversely affect cold workability. Therefore, for the purpose of not adversely affecting cold workability, the upper limit value of VAR _Radius is set to 17. Thereby, the material property of being hard to harden without deteriorating cold workability can be obtained.

[0019] In each case where the mole fraction of Mn in the above medium-entropy alloy is 0.15, 0.16, 0.17, 0.18, 0.19, ΔS≧1.3R, VEC≧7.9, 10≦VAR _Radius ≦17, when the ranges of the respective mole fractions of Fe, Ni, and Cr are expressed using the mole fractions of other elements, the results are as shown in Table 1 below.

[0020]

Table 1

[0021] By setting the mole fractions of each constituent element in the medium-entropy alloy within the ranges shown in Table 1, the increase in hardness on the surface and inside of the cold worked product is suppressed, so the formation of a hardened surface layer on the surface of the cold worked product can be suppressed without deteriorating cold workability.

[0022] Next, for each of the above conditions of ΔS, VEC and VAR _Radius , SFE is set to 120 [mJ / m 2Adding the condition of being above ], when the mole fraction of Mn in the above-mentioned medium-entropy alloy is set to 0.15, 0.16, 0.17, 0.18, and 0.19, the range of mole fractions of Fe, Ni, and Cr in each case, expressed in terms of the mole fractions of the other elements, is as shown in Table 2 below.

[0023] Furthermore, the formula used to calculate SFE was the following formula (4) published by Yonezawa et al. in Metallurgical and Materials A, Vol. 44A, 2013. SFE=-7.1+2.8Ni+0.49Cr+2.0Mo-2.0Si+0.75Mn-5.7C-24N...(4) Here, Ni, Cr, Mo, Si, Mn, C, and N represent the mole fractions of each element. In the medium-entropy alloy described above, Mo, Si, C, and N are all zero.

[0024] [Table 2]

[0025] By setting the mole fractions of each constituent element in a medium-entropy alloy within the range shown in Table 2, the degree of segregation near the grain boundaries of the medium-entropy alloy (irradiation-induced segregation) is reduced under irradiated conditions. Since irradiation-induced segregation is considered one of the main causes of irradiation-induced stress corrosion cracking (IASCC) under irradiated conditions, this can improve IASCC resistance.

[0026] <Method for manufacturing a cold-worked product according to one embodiment of the present disclosure> Next, we will explain the manufacturing method of the materials that make up the cold-worked product. As shown in the flowchart in Figure 1, in the melting and refining process S1, Mn, Fe, Ni, and Cr are mixed in predetermined component ratios and then melted. A vacuum degassing process is then performed to reduce impurity elements, and a refined steel ingot is produced. For example, in an argon atmosphere using a non-consumable tungsten electrode arc melting, a plasma is created with argon gas, and the mixture of Mn, Fe, Ni, and Cr in a water-cooled copper mold is melted and solidified by arc heat using electrons in the plasma as a heat source, thereby obtaining the aforementioned steel ingot.

[0027] In the homogenization heat treatment process S2 following the melting and refining process S1, the material is held in a high-temperature environment (for example, held in an electric furnace at 1200°C for about 24 hours) to reduce elemental segregation during melting and promote elemental diffusion, thereby increasing the homogeneity of the steel ingot. In the next hot working process S3, plastic deformation is performed in a high-temperature environment to eliminate voids and cast structures in the steel ingot. For example, processing is performed at a temperature of about 1100°C with a cross-sectional reduction rate of 50% (ideally 25%) or more. This process aims to make the metal easier to deform by overheating, eliminate voids created in the melting and refining process S1, and improve homogeneity by destroying the cast structure. In the final solution heat treatment process S4, the material is held above the solid solution temperature and then rapidly cooled to remove the strain applied during processing and improve the stability of the structure. This results in a homogeneous structure. The cold-worked product described above is obtained by cold working the material obtained from the above processes S1 to S4. [Examples]

[0028] The alloys of Examples 1 and 2 in Table 3 below were produced using the manufacturing method described above. Example 1 is an alloy that satisfies the ranges of both Tables 1 and 2, and Example 2 is an alloy that satisfies the range of Table 1.

[0029] [Table 3]

[0030] The Vickers hardness (JIS Z 2244) was measured before and after cold working for each of Examples 1 and 2, and for 316 stainless steel as a comparative example. Figure 2 shows the relationship between the processing rate due to cold working and the Vickers hardness. The processing rate is calculated using the following formula (5) by measuring the cross-sectional area A0 before cold working and the cross-sectional area A1 after cold working for the part that is processed by cold working. Processing rate=(A0-A1) / A0×100...(5)

[0031] As shown in Figure 2, in the comparative example, the Vickers hardness exceeds 300 HV even with a processing rate of less than 20%, but in Examples 1 and 2, the Vickers hardness remains below 300 HV even with a processing rate of 80%. From this, it can be said that the material hardens less during cold working, and it has been confirmed that an effect of suppressing the formation of a hardened surface layer on the surface of cold-worked products can be obtained.

[0032] Next, ion irradiation simulating a light water reactor core was performed on each of Example 1 and Comparative Example 1. Specifically, ion irradiation tests were conducted using the combined beam material irradiation device (DuET) owned by the Institute of Advanced Energy, Kyoto University. Fe was used to induce irradiation damage. 3+ For acceleration, a tandem Cockcroft-Walton accelerator (High Voltage Engineering, HVEE Tandetron Model 4117) was used. Fe was supplied from the ion source. 3+ Extract Fe 3+ The samples were irradiated with an accelerating voltage of 6.4 MeV. The irradiation temperature was 400°C, the cumulative irradiation time was 18.5 hours, and the nominal irradiation dose was 22.8 dpa. The change in the mole fraction of each element in each alloy was measured before and after ion irradiation. Specifically, small sample pieces were extracted by focused ion beam microsampling, and the elemental concentrations near the grain boundaries were analyzed using the EDX analyzer of a field emission transmission electron microscope (Talos F200X) manufactured by FE-I Japan Co., Ltd. Figure 3 shows the change in the mole fraction of each element in each alloy before and after ion irradiation for both Example 1 and Comparative Example 1.

[0033] As shown in Figure 3, the change in the mole fraction of each element in Example 1 is smaller than that in Example 2 and Comparative Example 1. From this, it can be said that the irradiation-induced segregation is smaller in Example 1 than in Example 2 and Comparative Example 1, and therefore Example 1 has superior IASCC resistance compared to Example 2 and Comparative Example.

[0034] The contents described in each of the above embodiments can be understood, for example, as follows:

[0035] [1] A cold-worked product relating to one aspect is: A cold-worked product formed from a material containing Mn, Fe, Ni, Cr, and unavoidable impurities, If the mole fractions of Mn, Fe, Ni, and Cr in the aforementioned material are c1, c2, c3, and c4, then -(c1lnc1+c2lnc2+c3lnc3+c4lnc4)≧1.3.

[0036] According to the cold-worked product of this disclosure, the arrangement entropy of the material is 1.3R or higher, where R is the gas constant. This significantly increases the stability of the austenite phase and suppresses the increase in hardness of the surface and interior of the cold-worked product during cold working, thereby suppressing the formation of a hardened surface layer on the surface of the cold-worked product.

[0037] [2] A cold-worked product according to another embodiment is the cold-worked product of [1], 7c1+8c2+10c3+6c4≧7.9.

[0038] This configuration facilitates the stabilization of the material's face-centered cubic lattice structure, thereby increasing its overall stability. Because multiple slip systems can function within such materials due to their crystal structure, they exhibit high plastic deformability and generally superior manufacturability and processability. As a result, the manufacturability and processability of cold-worked products can be improved.

[0039] [3] A cold-worked product relating to yet another embodiment is the cold-worked product of [1], If the atomic radii of Mn, Fe, Ni, and Cr in the aforementioned material are r1, r2, r3, and r4, then 10 ≤ (r12 c1+r2 2 c2+r3 2 c3+r4 2 c4)-(r1c1+r2c2+r3c3+r4c4) 2 The value is ≤ 17.

[0040] With this configuration, it is possible to obtain material properties that make the material less prone to hardening without worsening its cold workability.

[0041] [4] A cold-worked product relating to yet another embodiment is the cold-worked product of [2], If the atomic radii of Mn, Fe, Ni, and Cr in the aforementioned material are r1, r2, r3, and r4, then 10 ≤ (r1 2 c1+r2 2 c2+r3 2 c3+r4 2 c4)-(r1c1+r2c2+r3c3+r4c4) 2 The value is ≤ 17.

[0042] With this configuration, it is possible to obtain material properties that make the material less prone to hardening without worsening its cold workability.

[0043] [5] A cold-worked product relating to yet another embodiment is the cold-worked product of [4], c1 = 0.15, 0.15 ≤ c2 ≤ 0.40, (-0.5c2 + 0.44) ≤ c3 ≤ (-c2 + 0.65), c3 ≤ 0.40, and c4 = 1 - c1 - c2 - c3.

[0044] With this configuration, it is possible to suppress the formation of a surface hardening layer on the surface of a cold-worked product without degrading its cold workability.

[0045] [6] A cold-worked product relating to yet another embodiment is the cold-worked product of [4], c1 = 0.16, 0.15 ≤ c2 ≤ 0.40, (-0.5c2 + 0.44) ≤ c3 ≤ (-c2 + 0.64), c3 ≤ 0.40, and c4 = 1 - c1 - c2 - c3.

[0046] With this configuration, it is possible to suppress the formation of a surface hardening layer on the surface of a cold-worked product without degrading its cold workability.

[0047] [7] A cold-worked product relating to yet another embodiment is the cold-worked product of [4], c1 = 0.17, 0.15 ≤ c2 ≤ 0.40, (-0.5c2 + 0.435) ≤ c3 ≤ (-c2 + 0.63), c3 ≤ 0.40, and c4 = 1 - c1 - c2 - c3.

[0048] With this configuration, it is possible to suppress the formation of a surface hardening layer on the surface of a cold-worked product without degrading its cold workability.

[0049] [8] A cold-worked product in yet another embodiment is the cold-worked product of [4], c1 = 0.18, 0.15 ≤ c2 ≤ 0.37, (-0.5c2 + 0.435) ≤ c3 ≤ (-c2 + 0.62), c3 ≤ 0.40, and c4 = 1 - c1 - c2 - c3.

[0050] With this configuration, it is possible to suppress the formation of a surface hardening layer on the surface of a cold-worked product without degrading its cold workability.

[0051] [9] A cold-worked product relating to yet another embodiment is the cold-worked product of [4], c1 = 0.19, 0.15 ≤ c2 ≤ 0.28, (-0.5c2 + 0.43) ≤ c3 ≤ (-c2 + 0.61), c3 ≤ 0.40, and c4 = 1 - c1 - c2 - c3.

[0052] With this configuration, it is possible to suppress the formation of a surface hardening layer on the surface of a cold-worked product without degrading its cold workability.

[0053]

[10] A cold-worked product according to yet another embodiment is the cold-worked product of [4], c1 = 0.15, 0.20 ≤ c2 ≤ 0.40, (-0.5c2 + 0.44) ≤ c3 ≤ (-c2 + 0.65), c3 ≤ 0.25c2 + 0.29, c3 ≤ 0.36, and c4 = 1 - c1 - c2 - c3.

[0054] With this configuration, the degree of irradiation-induced segregation of the material is reduced under irradiation conditions, thereby improving IASCC resistance.

[0055]

[11] A cold-worked product relating to yet another embodiment is the cold-worked product of [4], c1 = 0.16, 0.20 ≤ c2 ≤ 0.40, (-0.5c2 + 0.44) ≤ c3 ≤ (-c2 + 0.64), c3 ≤ 0.25c2 + 0.29, c3 ≤ 0.36, and c4 = 1 - c1 - c2 - c3.

[0056] With this configuration, the degree of irradiation-induced segregation of the material is reduced under irradiated conditions, thereby improving IASCC resistance.

[0057]

[12] A cold-worked product according to yet another embodiment is the cold-worked product of [4], c1 = 0.17, 0.19 ≤ c2 ≤ 0.39, (-0.5c2 + 0.435) ≤ c3 ≤ (-c2 + 0.63), c3 ≤ 0.2c2 + 0.302, c3 ≤ 0.35, and c4 = 1 - c1 - c2 - c3.

[0058] With this configuration, the degree of irradiation-induced segregation of the material is reduced under irradiation conditions, thereby improving IASCC resistance.

[0059]

[13] A cold-worked product relating to yet another embodiment is the cold-worked product of [4], c1 = 0.18, 0.20 ≤ c2 ≤ 0.37, (-0.5c2 + 0.435) ≤ c3 ≤ (-c2 + 0.62), c3 ≤ 0.25c2 + 0.29, c3 ≤ 0.35, and c4 = 1 - c1 - c2 - c3.

[0060] With this configuration, the degree of irradiation-induced segregation of the material is reduced under irradiated conditions, thereby improving IASCC resistance.

[0061]

[14] A cold-worked product in yet another embodiment is the cold-worked product of [4], c1 = 0.19, 0.20 ≤ c2 ≤ 0.28, (-0.5c2 + 0.43) ≤ c3 ≤ (-2c2 + 0.87), c3 ≤ 0.25c2 + 0.29, c3 ≤ 0.35, and c4 = 1 - c1 - c2 - c3.

[0062] With this configuration, the degree of irradiation-induced segregation of the material is reduced under irradiated conditions, thereby improving IASCC resistance.

[0063]

[15] Another cold-worked product is any of the cold-worked products in [1] to

[14] , The cold-worked product is a bar, tube, or plate.

[0064] This configuration makes it possible to suppress the formation of a hardened surface layer on the surface of rods, tubes, or plates.

Claims

1. A cold-worked product formed from an alloy material consisting of Mn, Fe, Ni, Cr, and unavoidable impurities, Let the mole fractions of Mn, Fe, Ni, and Cr in the alloy material be c １ , c ２ , c ３ , c ４ , then -(c １ ln c １ + c ２ ln c ２ + c ３ ln c ３ + c ４ ln c ４ ) ≧ 1.3, c １ = 0.15, 0.16 ≤ c ２ ≤0.39, 0.27 ≤c ３ ≤0.40, 0.19 ≤c ４ Cold-worked products with a coefficient of ≤0.

29.

2. 7c １ +8c ２ +10c ３ +6c ４ The cold-worked product according to claim 1, wherein the coefficient is ≥ 7.

9.

3. The cold-worked product according to claim 1 or 2, wherein the cold-worked product is a bar, a pipe, or a plate.

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