High Mn nonmagnetic steel

Abstract

The present invention provides a high-Mn nonmagnetic steel that exhibits good cold-forming properties and maintains a low relative permeability even after cold-forming. [Solution] A high-Mn nonmagnetic steel that satisfies a predetermined chemical composition, has X represented by the following formula (1) as 5.0 or greater, and has a cold compressibility of 70% or more and a relative permeability after cold heading of less than 1.02. X = Ni + 0.5Mn + 5(C + N) - 0.5Cr (1) However, Ni, Mn, C, N, and Cr represent the content of Ni, Mn, C, N, and Cr in the steel, expressed in mass percent, and elements that are not present are treated as zero.

Description

Technical Field

[0005]

[0001] The present disclosure relates to high-Mn non-magnetic steel.

Background Art

[0002] In structural steel members exposed to a magnetic field environment, when the influence of the magnetic field on the performance of the equipment body is not allowed, low-permeability steel that is not magnetized by an external magnetic field is usually used for the above structural steel members. Steel showing such low permeability is called non-magnetic steel. As non-magnetic steel, austenitic stainless steel is known, but non-magnetic steel with an increased amount of Mn that suppresses the amount of rare alloys and stabilizes the austenite structure has also been proposed.

[0003] For example, in Patent Document 1, excellent rust resistance in a wet environment, and as a tough steel for cryogenic use at 25K or lower, containing C 0.20% or less, Si 0.05 - 2.5%, Mn 16 - 35%, Cr 10 - 20%, Ni 0.1 - 8.0%, N 0.10 - 0.50%, Al 0.001 - 0.20%, S 0.003% or less, and the balance being iron and unavoidable impurities, there is shown a high-manganese tough steel for cryogenic use with excellent rust resistance.

[0004] In Patent Document 2, there is shown a non-magnetic steel wire rod having excellent cold workability and corrosion resistance obtained by hot rolling a steel having a composition of C: 0.01 - 0.50%, Si: 1.0% or less, Mn: 10 - 25%, Ni: 0.1 - 5%, Cr: 10 - 20%, N: 0.01 - 0.5% by weight, and the balance being Fe and unavoidable impurities. Further, as a method for manufacturing the non-magnetic steel wire rod, a slab or steel piece having a predetermined component composition is heated to a temperature of 1150°C or higher and lower than 1250°C, then hot rolled with a reduction ratio of 70% or higher, and after completion of hot rolling, wound at a winding temperature of 900°C or higher and 1000°C or lower, and cooled at a cooling rate of 5°C / sec or higher.

[0005] Furthermore, Patent Document 3 describes a non-magnetic drill collar steel that does not require large processing equipment after rolling or heat treatment equipment for precipitation hardening, possesses stable non-magnetic properties and high yield strength, and is also good in terms of ductility, corrosion resistance and drilling workability, characterized by containing C0.01~0.50%, Si2.0% or less, Mn10~25%, Ni0.01~5%, Cr10~20%, S0.15% or less, N0.01~0.50%, with the remainder being substantially Fe.

[0006] Furthermore, Patent Document 4 describes a roll used in a continuous casting machine, and in particular, a non-magnetic roll that can effectively apply electromagnetic force to a cast slab and obtain a strong stirring force when electromagnetic stirring is applied to the cast slab to improve the quality of the cast slab. The non-magnetic roll is characterized by being made of steel containing C: 0.15% or less, Si: 1.0% or less, sol.Al: 0.050% or less, Mn: 17.0 to 25.0%, Cr: 6.0 to 11.5%, Ni: 3.0% or less, Cu: 3.0% or less, and N: 0.05 to 0.25% such that the sum of the Ni concentration and Cu concentration is 3.0% or less, with the remainder being Fe and unavoidable impurities. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 143563 / 1983 [Patent Document 2] Japanese Patent Application Publication No. 61-037953 [Patent Document 3] Japanese Patent Application Publication No. 61-000563 [Patent Document 4] Japanese Unexamined Patent Publication No. 57-152452 [Overview of the project] [Problems that the invention aims to solve]

[0008] While various non-magnetic steels with increased Mn content have been proposed, Patent Document 1 does not consider cold-workability or relative permeability after cold-working, and these properties are not considered sufficient. Patent Document 2 requires special manufacturing conditions, such as maintaining a certain level of cooling rate after rolling, to obtain a non-magnetic wire with excellent cold-workability and corrosion resistance, which is expected to impose constraints and increase costs on wire production. Furthermore, while Patent Document 2 considers the wire drawing limit, it does not consider cracking of parts during cold-working, and although it confirms the permeability after wire drawing, it does not consider magnetism after cold-working.

[0009] Furthermore, Patent Documents 3 and 4 aim to increase strength, but do not consider cold forming properties or relative permeability after cold forming. It is believed that further research is needed to improve these properties.

[0010] This disclosure has been made in view of the above circumstances, and its purpose is to provide a high-Mn nonmagnetic steel that exhibits good cold-forming properties and can maintain a low relative permeability even after cold-forming. [Means for solving the problem]

[0011] One aspect of the present invention is: C: 0.05% by mass to 0.25% by mass, Si: more than 0 mass%, 2.50 mass% or less, Mn: more than 15.0 mass%, 25.0 mass% or less, P:0 mass%~0.030 mass%, S: 0% by mass to 0.010% by mass, Cr: 5.0% by mass ~ 20.0% by mass, N: More than 0% by mass, 0.35% by mass or less, Ni: 1.5% by mass to 3.5% by mass, B: 0.0010% by mass to 0.0100% by mass, Mo: 0% by mass to 0.60% by mass, Cu: 0% by mass to 1.0% by mass, Nb: 0% by mass to 1.0% by mass, V: 0 mass% to 1.0 mass%, Al: 0 mass% to 1.0 mass%, Ti: 0 mass% to 1.0 mass%, Pb: 0 mass% to 0.2 mass%, Bi: 0 mass% to 0.2 mass%, Ca: 0 mass% to 0.010 mass% satisfies the conditions, and the balance consists of Fe and unavoidable impurities, X represented by the following formula (1) is 5.0 or more, It is a high-Mn non-magnetic steel with a magnetic permeability ratio less than 1.02 after cold forging with a cold compression ratio of 70% or more. X = Ni + 0.5Mn + 5(C + N) - 0.5Cr (1) However, Ni, Mn, C, N, and Cr respectively represent the contents of Ni, Mn, C, N, and Cr in the steel shown in mass%, and elements not contained are taken as zero.

Effect of the Invention

[0012] According to the present disclosure, it is possible to provide a high-Mn non-magnetic steel that exhibits good cold forging properties and can maintain a low magnetic permeability ratio even after cold forging.

Brief Description of the Drawings

[0013] [Figure 1] It is a graph showing the relationship between X obtained from formula (1) and the magnetic permeability ratio after cold forging.

Mode for Carrying Out the Invention

[0014] When forming parts using the high-Mn non-magnetic steel, the high-Mn non-magnetic steel may be subjected to cold forging, which is a type of cold forging, for example, cold forging represented by axial upset forging. Also, the high-Mn non-magnetic steel contains a large amount of C, Mn, Ni, N, etc. to obtain austenite, which is a non-magnetic phase, at room temperature. When cold forging is applied to conventional high-Mn non-magnetic steels, since these elements are basically in solid solution, the deformability of the steel material is extremely low, and there is a problem that parts cannot be used even with a small amount of deformation because they crack. There is also a problem that the deformation resistance increases and the life of the die is short.

[0015] Furthermore, in conventional high-Mn non-magnetic steel, even if the specific magnetic permeability before cold rolling is low, when cold rolling is performed, austenite transforms into work-induced martensite, and the specific magnetic permeability increases. Even for parts that have transformed into work-induced martensite by cold rolling, reheating and re-austenitizing can reduce the specific magnetic permeability, but additional heat treatment is required, so an increase in component manufacturing cost is inevitable.

[0016] First, the inventors recognized these problems, and then earnestly studied to obtain a high-Mn non-magnetic steel that exhibits good cold rolling properties and can maintain a low specific magnetic permeability without additional processes such as reheating even after cold rolling. As a result, in high-Mn non-magnetic steel, to ensure good cold rolling properties, it is important to ensure the deformation ability of the steel material while keeping the added elements as low as possible, and to suppress work-induced martensite by cold rolling, that is, to stabilize austenite. Also, to maintain a low specific magnetic permeability even after cold rolling, it was noted that it is important to suppress work-induced martensite by cold rolling, that is, to stabilize austenite, which is a non-magnetic phase. And to realize ensuring the deformation ability of these steel materials, it was found that it is necessary to control each chemical component within an appropriate range and simultaneously satisfy the following [1] to [3]. [1] To ensure the deformation ability of the steel material, B is contained at 0.0010 mass% to 0.0100 mass%. [2] To stabilize austenite, C is contained at 0.05 mass% to 0.25 mass%, Mn is more than 15.0 mass% and 25.0 mass% or less, Cr is contained at 5.0 mass% to 20.0 mass%, and Ni is contained at 1.5 mass% to 3.5 mass%. [3] To stabilize austenite, in addition to [2] above, the chemical components are controlled so that X represented by the following formula (1) is 5.0 or more. X = Ni + 0.5Mn + 5(C + N) - 0.5Cr (1) However, Ni, Mn, C, N, and Cr represent the content of Ni, Mn, C, N, and Cr in the steel, expressed in mass percent, and elements that are not present are treated as zero.

[0017] In other words, high-Mn nonmagnetic steel that satisfies the specified range of each chemical component and the above [1] to [3] exhibits good cold-forming properties because the austenite is sufficiently stabilized, and can maintain a low relative permeability after cold-forming.

[0018] Below, we will first describe the chemical composition of the high-Mn nonmagnetic steel of this disclosure.

[0019] [C:0.05 mass%~0.25 mass%] Carbon (C) is an effective element for stabilizing austenite, which is a nonmagnetic phase. To achieve this effect, the C content must be 0.05 mass% or more. Preferably, the C content is 0.07 mass% or more, and more preferably 0.08 mass% or more. On the other hand, if there is an excess of C, the deformability of the steel decreases and the cold heading properties deteriorate. Therefore, the C content must be 0.25 mass% or less. Preferably, the C content is 0.244 mass% or less, and more preferably 0.240 mass% or less.

[0020] [Si: more than 0 mass%, 2.50 mass% or less] Si acts as a deoxidizing agent during melting. It also contributes to the stabilization of austenite. However, if there is an excess of Si, δ-ferrite is formed, increasing the relative permeability. Furthermore, it reduces hot ductility and causes casting defects such as surface cracking during casting. For this reason, the Si content must be 2.50 mass% or less. Preferably, the Si content is 2.30 mass% or less, more preferably 2.10 mass% or less. There is no particular lower limit for the Si content, and it is greater than 0 mass%, but to further enhance the effect of stabilizing austenite, it is preferable that the Si content be 0.10 mass% or more, more preferably 0.15 mass% or more, and even more preferably 0.20 mass% or more.

[0021] [Mn: more than 15.0 mass%, 25.0 mass% or less] Mn is an effective element for stabilizing austenite, which is a nonmagnetic phase, and to achieve this effect, the amount of Mn must be greater than 15.0 mass%. The amount of Mn is preferably 16.0 mass% or more, more preferably 17.0 mass% or more. On the other hand, if there is an excess of Mn, Mn3P compounds and coarse MnS will precipitate at the grain boundaries during casting, significantly reducing hot ductility and causing casting defects such as surface cracks. Also, if there is an excess of Mn, when bending is performed, it becomes easier for it to transform into ε-martensite, making it more prone to cracking. To prevent these problems, the amount of Mn must be 25.0 mass% or less. The amount of Mn is preferably 24.0 mass% or less, more preferably 23.0 mass% or less.

[0022] [P:0 mass%~0.030 mass%] Although phosphorus (P) is an unavoidable impurity, if present in excess, it forms Mn3P, reducing the effectiveness of manganese (Mn). Furthermore, the Mn3P generated during casting significantly reduces hot workability and causes casting defects such as surface cracks. It also impairs the weldability and bendability of the steel. For these reasons, the amount of P must be 0.030 mass% or less. Preferably, the amount of P is 0.028 mass% or less, and more preferably 0.026 mass% or less. Ideally, the amount of P should be 0 mass%, but excessive reduction leads to increased steelmaking costs, so the lower limit of the amount of P may be greater than 0 mass%, and the lower limit may be even lower, around 0.001 mass%.

[0023] [S:0 mass%~0.010 mass%] Although sulfur (S) is an unavoidable impurity, if present in excess, it precipitates in large quantities as MnS, reducing the effectiveness of manganese. Furthermore, MnS generated during casting reduces hot ductility and causes casting defects such as surface cracking. For this reason, the S content must be 0.010 mass% or less. Preferably, the S content is 0.008 mass% or less, and more preferably 0.007 mass% or less. Ideally, the S content should be 0 mass%, but excessive reduction leads to increased steelmaking costs, so the lower limit of the S content may be greater than 0 mass%, and the lower limit may be even lower, around 0.001 mass%.

[0024] [Cr:5.0 mass%~20.0 mass%] Cr is a useful element for stabilizing austenite, which is a nonmagnetic phase. To achieve this effect, the amount of Cr must be 5.0 mass% or more, preferably 5.2 mass% or more, and more preferably 5.4 mass% or more. On the other hand, if there is an excess of Cr, δ-ferrite will be formed, which will actually lead to an increase in relative permeability, so the amount of Cr must be 20.0 mass% or less. The amount of Cr is preferably 19.0 mass% or less, and more preferably 18.0 mass% or less.

[0025] [N: More than 0 mass%, 0.35 mass% or less] N is an unavoidable impurity, and there is no specific lower limit for the amount of N; it is greater than 0 mass%. N is also an effective element for stabilizing austenite, which is a non-magnetic phase. To achieve this effect, it is preferable to have an N content of 0.002 mass% or more, and more preferably 0.003 mass% or more. On the other hand, excessive N content reduces the deformability of the steel and worsens its cold heading properties. Furthermore, it makes it easier for casting defects such as blowholes to form in the steel during casting, significantly reducing the manufacturability of the steel. For this reason, the N content needs to be 0.35 mass% or less. Preferably, the N content is 0.345 mass% or less, more preferably 0.340 mass% or less, and even more preferably 0.330 mass% or less.

[0026] [Ni: 1.5% by mass to 3.5% by mass] Ni is a useful element for stabilizing austenite, which is a nonmagnetic phase, and in order to obtain this effect, the amount of Ni must be 1.5 mass% or more. Preferably, the amount of Ni is 1.6 mass% or more, and more preferably 1.8 mass% or more. On the other hand, there is no particular upper limit set for the amount of Ni, but since adding too much increases the cost of the steel, the amount of Ni should be 3.5 mass% or less. Preferably, the amount of Ni is 3.4 mass% or less, and more preferably 3.2 mass% or less.

[0027] [B:0.0010 mass%~0.0100 mass%] B is an important element for improving cold heading properties. It suppresses grain boundary embrittlement by segregating at grain boundaries before elements such as P, which significantly reduce grain boundary strength, and thus suppresses cracking during cold heading. To obtain this effect, the amount of B must be 0.0010 mass% or more, preferably 0.0015 mass% or more, and more preferably 0.0020 mass% or more. On the other hand, if B is present in excess, borides will precipitate during casting, reducing hot ductility and causing casting defects such as surface cracking. For this reason, the amount of B must be 0.0100 mass% or less, preferably 0.0080 mass% or less, and more preferably 0.0060 mass% or less.

[0028] [Remaining components: Fe and unavoidable impurities] In one preferred embodiment, the remainder consists of Fe and unavoidable impurities. As unavoidable impurities, the inclusion of trace elements (e.g., As, Sb, Sn, etc.) introduced depending on the conditions of the raw materials, materials, manufacturing equipment, etc., is permissible. Note that there are elements, such as P and S, which are generally preferable in smaller amounts and therefore unavoidable impurities, but whose composition range is separately defined as described above. In this specification, arbitrary elements within the ranges shown below are also included as unavoidable impurities. Therefore, "unavoidable impurities" in "remainder: Fe and unavoidable impurities" above is a concept that excludes elements whose composition range is separately defined, such as P and S, as well as arbitrary elements at the unavoidable impurity levels shown below. Less than 0.01 mass% of Mo, less than 0.01 mass% of Cu, less than 0.01 mass% of Nb, less than 0.005 mass% of V, less than 0.005 mass% of Al, less than 0.005 mass% of Ti, less than 0.001 mass% of Pb, less than 0.001 mass% of Bi, less than 0.0001 mass% of Ca.

[0029] The chemical composition in this embodiment can achieve a high-Mn nonmagnetic steel that exhibits good cold-forming properties and maintains a low relative permeability even after cold-forming, even without including the arbitrary elements described below. Therefore, the chemical composition in this embodiment does not need to include the arbitrary elements described below. It may also contain any other elements as long as the desired properties are maintained. The properties can be further enhanced by including the arbitrary elements described below as needed.

[0030] [Mo:0 mass%~0.60 mass%] Mo may not be included, or it may be included in the above amount of Mo as needed. Mo is an element that contributes to further stabilization of austenite. Furthermore, it inhibits the diffusion of P and improves hot ductility. From the viewpoint of exhibiting this effect, the amount of Mo may be 0.01 mass% or more. The amount of Mo is more preferably 0.05 mass% or more, even more preferably 0.10 mass% or more, and even more preferably 0.40 mass% or more. On the other hand, if it is included in excess, it will reduce the deformability of the steel and worsen the cold heading properties. Therefore, the amount of Mo needs to be 0.60 mass% or less. The amount of Mo is preferably 0.55 mass% or less, more preferably 0.50 mass% or less.

[0031] [Cu:0 mass%~1.0 mass%] Cu may not be included. Alternatively, the above amount of Cu may be included as needed. Cu is also an element that contributes to further stabilization of austenite. From the viewpoint of exhibiting this effect, the amount of Cu may be 0.01 mass% or more. On the other hand, if Cu is included in excess, it will reduce hot ductility and cause casting defects such as surface cracking during casting. Therefore, the amount of Cu must be 1.0 mass% or less, preferably 0.7 mass% or less, and more preferably 0.4 mass% or less.

[0032] [Nb:0 mass%~1.0 mass%] [V:0 mass%~1.0 mass%] Nb and V may not be included. Alternatively, for example, one or more of the above amounts of Nb and V may be included. Both Nb and V are carbide-forming elements and may be included as needed to increase the strength of the steel. To achieve this effect as needed, if Nb is included, it is preferable that the amount of Nb be 0.01% by mass or more, and if V is included, it is preferable that the amount of V be 0.005% by mass or more. On the other hand, if Nb and V are included in excess, it leads to a decrease in cold heading properties. Furthermore, it leads to austenite destabilization due to a decrease in solid solution carbon. Therefore, the content of both Nb and V should be 1.0% by mass or less, preferably 0.7% by mass or less, and more preferably 0.4% by mass or less.

[0033] [Al:0 mass%~1.0 mass%] [Ti:0 mass%~1.0 mass%] Al and Ti may not be included. Alternatively, for example, one or more of the above amounts of Al and Ti may be included. Al and Ti are nitride-forming elements and may be included as needed to increase the strength of the steel. To exert this effect as needed, if Al is included, it is preferable that the amount of Al be 0.005 mass% or more, and if Ti is included, it is preferable that the amount of Ti be 0.005 mass% or more. On the other hand, if Al and Ti are included in excess, it leads to a decrease in cold heading properties. Furthermore, it leads to austenite destabilization due to a decrease in solid dissolved nitrogen. Therefore, the content of both Al and Ti should be 1.0 mass% or less, preferably 0.7 mass% or less, and more preferably 0.4 mass% or less.

[0034] [Pb:0 mass%~0.2 mass%] [Bi:0 mass%~0.2 mass%] Pb and Bi may not be included. Alternatively, for example, one or more of the above amounts of Pb and Bi may be included. Pb and Bi are elements that improve the machinability of steel and may be included as needed. To achieve this effect as needed, if Pb is included, it is preferable that the amount of Pb be 0.001% by mass or more, and if Bi is included, it is preferable that the amount of Bi be 0.001% by mass or more. On the other hand, if Pb and Bi are included in excess, it will reduce hot ductility and cause casting defects such as surface cracks during casting. Therefore, the content of both Pb and Bi should be 0.2% by mass or less, preferably 0.1% by mass or less, and more preferably 0.05% by mass or less.

[0035] [Ca:0 mass%~0.010 mass%] Ca may not be included. Alternatively, the above amount of Ca may be included as needed. Ca is an element that improves the machinability of steel, and to exert this effect as needed, the amount of Ca may be 0.0001% by mass or more. On the other hand, if Ca is included in excess, a large amount of oxide will be generated, which can cause cracking during cold heading. Therefore, the amount of Ca must be 0.010% by mass or less, preferably 0.007% by mass or less, and more preferably 0.005% by mass or less.

[0036] [X shown in equation (1) below is 5.0 or greater] X = Ni + 0.5Mn + 5(C + N) - 0.5Cr (1) However, Ni, Mn, C, N, and Cr represent the mass percentage content of Ni, Mn, C, N, and Cr in the steel, respectively, and elements that are not present are treated as zero.

[0037] The inventors focused on C, Mn, Ni, Cr, and N as elements that suppress the transformation of austenite into martensite by cold heading (also known as "austenite stabilization"). Ni is an austenite-stabilizing element commonly used in stainless steel and the like. C and N are interstitial elements that stabilize austenite. Based on the Ni equivalent formula in Schaeffler's microstructure diagram, the inventors considered these five elements to be compatible with the high-Mn nonmagnetic steel of this disclosure, examined the coefficients of each element, and derived equation (1). In equation (1), as mentioned above, C and N are interstitial elements that stabilize austenite, and since stability can be increased even in small amounts, their coefficients are larger than those of Ni. Mn is also an element that stabilizes austenite, but its effect is weaker than that of Ni, so its coefficient is smaller than that of Ni. Furthermore, while Cr is also an element that contributes to the stabilization of austenite, it forms carbides in the rolling temperature range, reducing the amount of dissolved carbon and destabilizing austenite, hence its negative coefficient.

[0038] Furthermore, in order to ensure a low relative permeability after cold heading and to achieve excellent austenite stability, we investigated the range of X represented by equation (1). As shown in Figure 1, we found that in order to achieve a relative permeability of less than 1.02 after cold heading with a cold compressibility of 70% or more, X needs to be 5.0 or higher. X is preferably 5.1 or higher, and more preferably 5.2 or higher.

[0039] The high-Mn nonmagnetic steel in this disclosure refers to high-Mn nonmagnetic steel bars and high-Mn nonmagnetic wires. The high-Mn nonmagnetic steel in this disclosure is either a high-Mn nonmagnetic steel bar or a high-Mn nonmagnetic wire. The specific shape of the high-Mn nonmagnetic steel bars and high-Mn nonmagnetic wires is not limited and can be determined as appropriate depending on the application.

[0040] The high-Mn nonmagnetic steel of this disclosure exhibits good cold-forming properties and can maintain a low relative permeability even after cold-forming. These properties are described below. The high-Mn nonmagnetic steel of this disclosure may have a 0.2% yield strength of 500 MPa or less, further 400 MPa or less, further 300 MPa or less, and 200 MPa or more.

[0041] (1) Cold forming properties In this disclosure, "excellent cold forming properties" means that the limit compressibility in the compression test shown in the examples described later is 70% or higher.

[0042] (2) Maintaining low magnetic permeability after cold heading In this disclosure, "maintaining a low relative permeability even after cold heading" means that the relative permeability after cold heading with a cold compressibility of 70% or more is less than 1.02. More specifically, it means that the relative permeability of the steel after a compression test with a cold compressibility of 70% or more, as shown in the examples described later, is less than 1.02. The aforementioned relative permeability is preferably 1.01 or less. Furthermore, the high-Mn nonmagnetic steel of this disclosure also has a low relative permeability of less than 1.02 before cold heading.

[0043] The high-Mn nonmagnetic steel according to this embodiment can be manufactured by the following method. First, steel satisfying the aforementioned chemical composition is melted and cast. The casting method is not particularly limited, and any commonly used method may be employed. For example, ingot casting or continuous casting methods can be used. After casting, hot rolling can be performed to obtain the high-Mn nonmagnetic steel. The conditions in the hot rolling process are not particularly limited, and any commonly used method may be employed. For example, hot rolling can be performed at 850 to 1200°C. Alternatively, forging may be performed instead of hot rolling.

[0044] After casting and before hot rolling, hot bloc rolling may be performed as needed. Bloc rolling may include a soaking treatment before bloc rolling. The bloc rolling conditions are not particularly limited, and commonly used methods can be employed. For example, bloc rolling can be performed at 1000°C to 1250°C.

[0045] After hot rolling, normalizing may be performed as needed. Normalizing can be done, for example, by heating to 900°C to 1200°C and then air-cooling. [Examples]

[0046] The present disclosure will be described in more detail below with reference to examples. The present disclosure is not limited by the following examples, and can be implemented with appropriate modifications to the extent that it is consistent with the spirit described above and below, and all such modifications are included within the technical scope of the present disclosure.

[0047] (1) Sample preparation Using a small melting furnace (capacity 20-50 kg / ch), steel materials satisfying the chemical composition shown in Table 1 were melted according to a normal melting method and cast to obtain ingots. Note that "unavoidable impurities" in Table 1 may include arbitrary elements at the aforementioned unavoidable impurity levels. Also, the Mo content of less than 0.01 mass% in Sample No. 8 in Table 1 is an arbitrary element at the unavoidable impurity level. Next, the ingot was heated to a heating temperature of 1100°C, and then forged instead of hot rolling to obtain a 45 mm diameter round bar. Subsequently, the 45 mm diameter round bar was normalized by heating at a heating temperature of 1000°C for 30 minutes and then air-cooling to obtain a sample.

[0048] (2) Evaluation of cold forming properties (measurement of critical compressibility) From the obtained sample (45 mm in diameter x 500 mm in length), cylindrical test specimens measuring 10 mm in diameter x 15 mm in height were taken. Specifically, the sample was cut perpendicular to its longitudinal direction, and four cylindrical test specimens were taken from each cut surface. Multiple cylindrical test specimens were taken from multiple cut surfaces.

[0049] The obtained cylindrical specimens were used to evaluate their cold-forming properties by performing end-face-restrained compression tests with the end faces constrained by a grooved die. In the end-face-restrained compression tests, the target compression ratio was increased in 10% increments from 30% to 80%, and compression tests were performed at each target compression ratio. A new sample was used for each compression test at each target compression ratio. Three cylindrical specimens (N3) were tested for each target compression ratio. In the compression tests, the cylindrical specimens were placed in a forging press and subjected to a strain rate of 5 / sec to 10 / sec at room temperature. After compression, the surface of the cylindrical specimens was observed using a stereomicroscope to check for crack formation. The maximum target compression ratio at which no cracks were observed on the surface of two or more of the three cylindrical specimens was defined as the limit compression ratio. A limit compression ratio of 70% or higher was evaluated as having excellent cold-forming properties. The results are shown in Table 1.

[0050] (3) Measurement of relative permeability Using the relative permeability measurement samples compressed to the limiting compressibility as described in (2) above, the relative permeability was measured using a relative permeability meter (FOERSTER). The relative permeability of the samples before cold heading (as normalized) was also measured in the same manner as above, and the relative permeability of all samples, both the examples of the present invention (No. 1 to 4) and the comparative examples (No. 5 to 8), before cold heading (as normalized) was within the range of 1.00 to 1.01.

[0051] Furthermore, materials with a relative permeability of less than 1.02 after cold heading with a cold compressibility of 70% or higher were judged to have maintained a low relative permeability even after cold heading (passing grade). Note that the critical compressibility measured in (2) above for comparative examples (No. 5 to 8) were all below 70%, and therefore the relative permeability of comparative examples (No. 5 to 8) does not meet the prerequisite of the evaluation criteria, "after cold heading with a cold compressibility of 70% or higher." However, for reference, the relative permeability measured at each critical compressibility is shown in Table 1.

[0052] (4) Measurement of 0.2% proof stress From the obtained sample (45 mm in diameter x 500 mm in length), a tensile test specimen (JIS No. 14A) with a parallel section diameter of 6 mm and a parallel section length of 36 mm was taken, and a tensile test was performed in accordance with JIS Z2241 to determine the 0.2% yield strength. The results are shown in Table 1.

[0053] [Table 1]

[0054] Table 1 shows the following: Samples No. 1-4 and 9 are examples of the invention that satisfy all the requirements of the embodiments of the present invention. That is, each chemical component and the value of X are within the range, and as a result, the limit compressibility is 70% or more, the cold heading performance is excellent, and the relative permeability after cold heading with a cold compressibility of 70% or more is less than 1.02, resulting in steel that exhibits a stable low relative permeability even after cold heading.

[0055] It should be noted that Patent Document 2 also measures permeability after wire drawing. However, cold forging according to this disclosure is a different mode of processing from wire drawing, and generally the amount of strain applied to the material is greater than in wire drawing. Specifically, while the maximum strain due to wire drawing described in Patent Document 2 is less than 0.6, the average strain at a compressibility of 70% in the test piece shape corresponds to 1.40. Therefore, it can be said that this is an extremely harsh processing method for maintaining low permeability, and the evaluation criterion in this embodiment, which requires a relative permeability of less than 1.02 after cold forging with a compressibility of 70% or more, is a stricter evaluation criterion than that in Patent Document 2, etc.

[0056] On the other hand, samples No. 5-8, 10, and 11 are comparative examples that do not satisfy any of the requirements in the embodiments of the present invention.

[0057] Samples No. 5 and 6 had a small value for X, as expressed in equation (1), resulting in a low limiting compressibility and poor cold-forging properties. Furthermore, the relative permeability after cold-forging was high despite the low limiting compressibility, and the desired non-magnetic properties were not achieved.

[0058] Sample No. 7 had insufficient Mn content and a small value of X as expressed by equation (1), resulting in a low limiting compressibility and poor cold-forging properties. Furthermore, despite being a cold-forged material with a low limiting compressibility, the relative permeability after cold-forging was high, and the desired non-magnetic properties were not achieved.

[0059] Sample No. 8 satisfies equation (1), but has an excess of C and insufficient amounts of Mn, Ni, and Cr, resulting in a low limiting compressibility and poor cold-forging properties. Furthermore, the relative permeability after cold-forging was high despite the low limiting compressibility, and the desired non-magnetic properties were not obtained.

[0060] Samples No. 10 and 11 satisfy equation (1), but because the amount of B was low and the amount of P was excessive, the critical compressibility decreased, and processing was not possible even at a compressibility of 60%. [Industrial applicability]

[0061] The high-Mn nonmagnetic steel of this disclosure can be used primarily in electromagnetic components and medical equipment that utilize magnetic fields, for controlling magnetic flux paths and suppressing weak magnetic fields that hinder performance. Furthermore, it can be used as a reinforcing material for structures that require non-magnetic properties, such as nuclear magnetic resonance imaging (MRT) rooms. However, the applications of the high-Mn nonmagnetic steel of this disclosure are not limited to these applications.

Claims

[Claim 1] C: 0.05% by mass to 0.25% by mass, Si: more than 0 mass%, 2.50 mass% or less, Mn: more than 15.0 mass%, 25.0 mass% or less, P: 0% by mass to 0.030% by mass, S: 0% by mass to 0.010% by mass, Cr: 5.0% by mass to 20.0% by mass, N: more than 0 mass%, 0.35 mass% or less, Ni: 1.5% by mass to 3.5% by mass, B: 0.0010% by mass to 0.0100% by mass, Mo: 0% by mass to 0.60% by mass, Cu: 0% by mass to 1.0% by mass, Nb: 0% by mass to 1.0% by mass, V: 0% by mass to 1.0% by mass, Al: 0% by mass to 1.0% by mass, Ti: 0% by mass to 1.0% by mass, Pb: 0% by mass to 0.2% by mass, Bi: 0% by mass to 0.2% by mass, Ca: 0% by mass to 0.010% by mass It satisfies the following conditions, and the remainder consists of Fe and unavoidable impurities. The value of X shown in the following equation (1) is 5.0 or greater, A high-Mn nonmagnetic steel with a cold compressibility of 70% or more and a relative permeability of less than 1.02 after cold heading. X=Ni+0.5Mn+5(C+N)-0.5Cr (1) However, Ni, Mn, C, N, and Cr represent the mass percentage content of Ni, Mn, C, N, and Cr in the steel, respectively, and elements that are not present are treated as zero.

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