Wire rod

Abstract

To provide a wire rod which can suppress breakage during bending and maintain non-magnetic properties even after bending.SOLUTION: Provided is a wire rod having a chemical composition consisting of, by mass%, C: 0.50 to 1.10%, Si: 0.10 to 1.00%, Mn: 10.00 to 18.00%, P: 0.050% or less, S: 0.050% or less, and N: 0.005 to 0.035%, and the balance being Fe and impurities; wherein, in a cross section perpendicular to a length direction of the wire rod, the austenite structure has an area ratio of 98.00% or more and less than 100.00%, the carbide in the center has an area ratio of 0.05 to 2.00%, and the carbon segregation degree of the center segregation portion is 1.20 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to wire rods. [Background technology]

[0002] Steel materials used in magnetic fields, such as those found in linear motor car related equipment, or in weak magnetic fields, such as those found in medical equipment and geomagnetic measuring devices, are required to have non-magnetic properties (low magnetic permeability) to suppress energy loss and noise caused by the excitation of induced currents.

[0003] Patent Document 1 discloses a non-magnetic steel that achieves high strength, high yield strength, and low magnetic permeability, and that is excellent in bending workability at room temperature and also in bending workability at low temperatures. The non-magnetic steel contains 0.8 to 1.2% C (meaning % by mass; the same applies hereinafter to chemical components), 0.1 to 0.6% Si, more than 13% but not more than 20% Mn, 0.001% or more but less than 0.02% Al, 0.040% or less (not including 0%) P, 0.045% or less S (not including 0%), and 0.025 to 0.05% N, with the balance being iron and unavoidable impurities, with 99.0% or more by area of ​​the microstructure being an austenite structure and with an austenite grain size number of 8.0 to 10.5.

[0004] Patent Document 2 discloses a non-magnetic steel that exhibits high strength and low magnetic permeability, and further has excellent bending workability, particularly bending workability at low temperatures, even when a decarburized layer is present. The non-magnetic steel has a chemical composition, in mass %, of C: 0.8 to 1.2%, Si: 0.1 to 0.6%, Mn: more than 13% but less than 20%, Al: 0.001% or more but less than 0.02%, P: more than 0% but 0.040% or less, S: more than 0% but 0.045% or less, and N: 0.025 to 0.05%, with the balance being iron and unavoidable impurities, and the hydrogen concentration in the steel being kept to 4.5 mass ppm or less. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-177662 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-179395 Summary of the Invention [Problem to be solved by the invention]

[0006] An object of the present disclosure is to provide a wire that is capable of suppressing breakage during bending and maintaining non-magnetic properties even after bending. [Means for solving the problem]

[0007] The means for solving the above problems include the following aspects. <1> In mass%, C: 0.50~1.10%, Si: 0.10 to 1.00%, Mn: 10.0 to 18.0%, P: 0.050% or less, S: 0.050% or less, and N: 0.005 to 0.035%; The balance is Fe and impurities, In a cross section perpendicular to the length of the wire, The austenite structure has an area ratio of 98.00% or more and less than 100.00%, The area ratio of carbides in the center is 0.05 to 2.00%, A wire rod having a carbon segregation ratio of 1.20 or less in the central segregation region. <2> In mass%, C: 0.50~1.10%, Si: 0.10 to 1.00%, Mn: 10.0 to 18.0%, P: 0.050% or less, S: 0.050% or less, and N: 0.005 to 0.035%; Furthermore, it contains one or more selected from the group consisting of the following first, second, and third groups: The balance is Fe and impurities, (Group 1) Cu: 0.40% or less, and Ni: 0.40% or less, one or two selected from the group consisting of (Group 2) Cr:2.50% or less, Mo: 1.00% or less V: 0.25% or less, Ti: 0.100% or less, Al: 0.100% or less, and Nb: 0.050% or less, one or more selected from the group consisting of (Group 3) Sn: 0.050% or less, B: 0.0050% or less, Bi: 0.20% or less, Pb: 0.09% or less, and Ca: 0.0100% or less, one or more selected from the group consisting of In a cross section perpendicular to the length of the wire, The austenite structure has an area ratio of 98.00% or more and less than 100.00%, The area ratio of carbides in the center is 0.05 to 2.00%, A wire rod having a carbon segregation ratio of 1.20 or less in the central segregation region. <3> The maximum thickness of the carbide is 0.50 μm or less <1> or <2> The wire rod described in <4> The grain size of the austenite grains in the austenite structure is 8 μm or more and 14 μm or less. <1> ~ <3> 1. The wire according to claim 1 , [Effects of the Invention]

[0008] According to the present disclosure, a wire is provided that is capable of suppressing breakage during bending and maintaining non-magnetic properties even after bending. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram showing an example of carbides that become the starting points of cracks when bending a wire rod. DETAILED DESCRIPTION OF THE INVENTION

[0010] An embodiment that is an example of the present disclosure will be described. In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits. Furthermore, when the numerical values ​​before and after "to" are followed by "greater than" or "less than," the numerical range does not include these numerical values ​​as the lower or upper limit. In the numerical ranges described in stages in this specification, the upper limit value of a certain numerical range may be replaced by the upper limit value of another numerical range described in stages, or may be replaced by a value shown in an example. In the numerical ranges described in stages in this specification, the lower limit value of a certain numerical range may be replaced by the lower limit value of another numerical range described in stages, or may be replaced by a value shown in an example. The content of an element in a chemical composition may be expressed simply as "amount" (for example, C amount, Si amount, etc.). With respect to the content of elements in the chemical composition, "%" means "mass %." Furthermore, the term "process" does not only refer to an independent process, but also includes processes that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.

[0011] The inventors of the present disclosure have conducted extensive research to find a wire rod that is both non-magnetic and suppresses cracking during bending, and as a result, have discovered the following findings. (a) The high Mn content allows the utilization of a pure stable austenite structure. However, it is known that metastable austenite undergoes strain-induced martensitic transformation during cold working, making it difficult to maintain low magnetic properties after bending. (b) We investigated high carbon and high manganese components to find a component that can maintain low magnetic properties without causing deformation-induced martensitic transformation after bending, and arrived at a component that maintains low magnetic properties even after bending. (c) On the other hand, with high carbon content, carbides are formed in the center, which become the starting point for cracks during bending. Therefore, in order to suppress cracks during bending, it was effective to keep the carbide area ratio below 2%. (d) Furthermore, when there is variation in the solute carbon concentration, such as a high concentration in the center, bending workability deteriorates. The precipitation of carbides is effective in reducing variation in the solute carbon concentration. (e) The wire rod according to the present disclosure was developed as a result of satisfying the conditions for maintaining bending workability and non-magnetism after bending.

[0012] [Wire rod] The wire rod according to the present disclosure will be described.

[0013] <Chemical composition> The chemical composition (content of each element) of the wire according to the present disclosure will be described.

[0014] C: 0.50 to 1.10% Carbon (C) stabilizes austenite and suppresses deformation-induced martensitic transformation after bending, thereby achieving sufficient non-magnetic properties. If the C content is less than 0.50%, the above effect cannot be fully achieved. On the other hand, if the C content exceeds 1.10%, excessive carbides precipitate. The precipitated carbides become the starting point for cracks during bending, resulting in insufficient workability. Therefore, the C content is 0.50 to 1.10%. The lower limit of the C content is preferably 0.60%, more preferably 0.70%, and even more preferably 0.80%. The upper limit of the C content is preferably 1.05%, more preferably 1.03%, and even more preferably 1.00%.

[0015] Si: 0.10 to 1.00% Silicon (Si) has the effect of deoxidizing steel. Si also increases the strength of steel through solid solution strengthening. If the Si content is less than 0.10%, the above effect cannot be fully achieved. On the other hand, if the Si content exceeds 1.00%, bending workability decreases. Therefore, the Si content is 0.10 to 1.00%. The lower limit of the Si content is preferably 0.15%, more preferably 0.18%, and even more preferably 0.20%. The upper limit of the Si content is preferably 0.90%, more preferably 0.80%, and even more preferably 0.70%.

[0016] Mn: 10.0 to 18.0% Manganese (Mn) stabilizes austenite and suppresses deformation-induced martensitic transformation, thereby achieving sufficient non-magnetic properties. If the Mn content is less than 10.0%, the above effects cannot be fully achieved. On the other hand, if the Mn content exceeds 18.0%, the strength of the steel material increases excessively, resulting in a decrease in the workability of the steel material. Therefore, the Mn content is 10.0 to 18.0%. The lower limit of the Mn content is preferably 11.0%, more preferably 11.5%, and even more preferably 12.0%. The upper limit of the Mn content is preferably 17.0%, more preferably 16.5%, and even more preferably 16.0%.

[0017] P:0.050% or less Phosphorus (P) is an impurity. P segregates at the grain boundaries of austenite crystal grains and reduces bending workability. If the P content of wire rod is 0.050% or less, the reduction in workability is suppressed, and the target properties can be obtained by satisfying other requirements. The upper limit of the P content is preferably 0.045%, more preferably 0.040% or less. The lower limit of the P content is not limited and is preferably 0% (i.e., no P is contained), but from the viewpoint of reducing the dephosphorization cost, it may be more than 0% or may be 0.001% or more.

[0018] S: 0.050% or less Sulfur (S) reduces workability. If the S content of wire rod is 0.050% or less, the target properties can be obtained while satisfying other requirements. The upper limit of the S content is preferably 0.040%. The lower limit of the S content is not limited, but may be more than 0% or may be 0.001% or more from the viewpoint of reducing the desulfurization cost.

[0019] N: 0.005 to 0.035% Nitrogen (N) stabilizes austenite and suppresses deformation-induced martensitic transformation, thereby achieving sufficient non-magnetic properties. N also increases the strength of steel through solid solution strengthening. If the N content is less than 0.005%, the above effects cannot be fully achieved. On the other hand, if the N content exceeds 0.035%, defects such as blowholes tend to occur in the steel. This reduces the manufacturability of the steel. Therefore, the N content is set to 0.005 to 0.35%. The lower limit of the N content is preferably 0.007%, more preferably 0.010%, and even more preferably 0.012%. The upper limit of the N content is preferably 0.030%, more preferably 0.024%, and even more preferably 0.020%.

[0020] The wire rod according to the present disclosure may contain one or more of Cu, Ni, Cr, Mo, V, Ti, Al, Nb, Sn, B, Bi, Pb, and Ca as optional elements in place of a portion of Fe. These optional elements may be absent or may be contained within the following ranges. When these optional elements are contained, the lower limit of the content may be greater than 0%. Furthermore, these optional elements are divided into the following first to third groups in terms of their effects.

[0021] [Group 1] Cu and Ni The chemical composition of the wire according to the present disclosure may further contain one or two elements selected from the first group described above in place of a portion of Fe. These elements are optional elements, and all of them improve the non-magnetic properties of the steel material and increase the toughness of the steel material. Each element will be described below.

[0022] Cu: 0.40% or less Copper (Cu) is an optional element, i.e., the Cu content may be 0%. Cu is an element that stabilizes austenite and improves the non-magnetic properties of steel. From the viewpoint of this function, the Cu content may be more than 0%, 0.05% or more, 0.10% or more, or 0.20% or more. On the other hand, if the Cu content exceeds 0.40%, the hot workability of the wire rod decreases. Therefore, the Cu content is 0.40% or less. The Cu content is preferably 0.35% or less.

[0023] Ni: 0.40% or less Nickel (Ni) is an optional element, i.e., the Ni content may be 0%. Ni is an element that stabilizes austenite and improves the non-magnetic properties of steel. From the viewpoint of this function, the Ni content may be more than 0%, 0.05% or more, 0.10% or more, or 0.20% or more. On the other hand, if the Ni content exceeds 0.40%, bending workability decreases. Therefore, the Ni content of the wire rod is 0.40% or less. The Ni content is more preferably 0.35% or less.

[0024] [Group 2] Cr, V, Ti, Mo, Al and Nb The chemical composition of the wire rod according to the present disclosure may further contain, in place of a portion of Fe, one or more elements selected from the above-mentioned second group. These elements are optional elements, and all of them increase the strength of the steel material. Each element will be described below.

[0025] Cr:2.50% or less Chromium (Cr) is an optional element, i.e., the Cr content may be 0%. Cr is an element that forms carbides and increases the strength of steel materials through precipitation strengthening. Cr also stabilizes austenite and improves the non-magnetic properties of steel materials. This improves the strength and non-magnetic properties of wire rods. In view of this effect, the Cr content may be more than 0%, 0.02% or more, or 0.05% or more. On the other hand, if the Cr content exceeds 2.50%, coarse Cr carbides are formed, which reduces the bending workability of the wire rod. Therefore, the Cr content is 2.50% or less. From the viewpoint of bending workability, the Cr content is preferably 2.00% or less.

[0026] Mo: 1.00% or less Molybdenum (Mo) is an optional element, i.e., the Mo content may be 0%. Mo is an element that forms carbides and increases the strength of steel materials through precipitation strengthening. This increases the strength of wire rods. From this perspective, the Mo content of wire rods may be more than 0% or may be 0.02% or more. On the other hand, if the Mo content of the wire rod exceeds 1.00%, the above effect saturates and the manufacturing cost of the wire rod increases. Therefore, the Mo content is preferably in the range of 0.02 to 1.00%, and more preferably 0.04 to 0.90%.

[0027] V: 0.25% or less Vanadium (V) is an optional element, i.e., the V content may be 0%. V is an element that forms carbides and nitrides and increases the strength of steel materials through precipitation strengthening. This increases the strength of wire rods. From the viewpoint of this effect, the V content may be more than 0%, 0.002% or more, or 0.005% or more. On the other hand, if the V content exceeds 0.25%, the amount of carbides or carbonitrides increases, resulting in a decrease in bending workability, so the V content is set to 0.25% or less, and preferably 0.15% or less.

[0028] Ti:0.100% or less Titanium (Ti) is an optional element, that is, the Ti content may be 0%. Ti is an element that forms carbides and nitrides and increases the strength of steel materials through precipitation strengthening. This increases the strength of wire rods. From the viewpoint of this effect, the Ti content may be more than 0%, 0.005% or more, or 0.007% or more. On the other hand, if the Ti content exceeds 0.100%, the amount of carbides or carbonitrides increases, resulting in a decrease in bending workability, so the Ti content is set to 0.100% or less, preferably 0.050% or less.

[0029] Al: 0.100% or less Aluminum (Al) is an optional element, i.e., the Al content may be 0%. Al is an element that forms nitrides and refines austenite grains through a pinning effect. Refining austenite grains increases the strength of the steel material. Al may be added to reduce the amount of oxygen in the wire rod. In view of this effect, the Al content may be more than 0%, 0.005% or more, or 0.030% or more. On the other hand, if the Al content exceeds 0.100%, excessive Al nitrides are formed. In this case, the amount of solute N in the steel material decreases, the stability of austenite decreases, and the non-magnetic properties deteriorate. Therefore, the Al content is 0.100% or less, preferably 0.050% or less, and more preferably 0.035% or less.

[0030] Nb: 0.050% or less Niobium (Nb) is an optional element, that is, the Nb content may be 0%. Nb is an element that forms nitrides and refines austenite grains through a pinning effect. Refining austenite grains increases the strength of the steel. In view of this effect, the Nb content may be more than 0%, 0.002% or more, or 0.005% or more. On the other hand, if the Nb content exceeds 0.050%, the amount of carbides or carbonitrides increases, resulting in a decrease in bending workability, so the Nb content is set to 0.050% or less, and preferably 0.030% or less.

[0031] [Group 3] Sn, B, Bi, Pb, and Ca The chemical composition of the wire rod according to the present disclosure may further contain, in place of a portion of Fe, one or more elements selected from the above-mentioned third group. These elements are optional elements, and all of them improve the bending workability of the steel material. Each element will be described below.

[0032] Sn: 0.050% or less Tin (Sn) is an optional element, i.e., the Sn content may be 0%. Sn is an element that improves the bending workability of steel materials. From the viewpoint of this function, the Sn content may be more than 0%, 0.002% or more, or 0.005% or more. On the other hand, if the Sn content exceeds 0.050%, the hot workability deteriorates. Therefore, the Sn content is 0.050% or less. From the viewpoint of further improving the hot workability, the Sn content is preferably 0.040% or less.

[0033] B: 0.0050% or less Boron (B) is an optional element, that is, the B content may be 0%. B is an element that segregates at grain boundaries to increase grain boundary strength, thereby improving the bending workability of steel. From the viewpoint of this effect, the B content may be more than 0%, 0.0001% or more, or 0.0005% or more. If the B content exceeds 0.0050%, coarse carbonitrides are likely to be formed in the wire rod, which may result in a deterioration in the non-magnetic properties of the wire rod. Therefore, the B content is 0.0050% or less. From the viewpoint of further reducing the deterioration in the non-magnetic properties of the wire rod, the B content is preferably 0.0040% or less.

[0034] Bi:0.20% or less Bismuth (Bi) is an optional element, i.e., the Bi content may be 0%. Bi is an element that refines the dendritic structure during solidification. This refines inclusions and improves the bending workability of the steel material. From the viewpoint of this effect, the Bi content may be more than 0%, 0.002% or more, or 0.005% or more. On the other hand, if the Bi content exceeds 0.20%, the hot workability of the steel material decreases. Therefore, the Bi content is 0.20% or less. From the viewpoint of further improving the hot workability, the Bi content is preferably 0.10% or less.

[0035] Pb: 0.09% or less Lead (Pb) is an optional element, i.e., the Pb content may be 0%. Pb is an element that improves the bending workability of steel materials. From the viewpoint of this function, the Pb content may be more than 0%, 0.002% or more, or 0.005% or more. On the other hand, if the Pb content exceeds 0.09%, the hot workability deteriorates. Therefore, the Pb content is 0.09% or less. From the viewpoint of further improving the hot workability, the Pb content is preferably 0.05% or less.

[0036] Ca:0.0100% or less Calcium (Ca) is an optional element, i.e., the Ca content may be 0%. Ca dissolves in MnS and has the effect of finely dispersing the MnS. Finely dispersing the MnS suppresses cracking during bending and allows the wire to be processed into a high-strength wire. From the viewpoint of this effect, the Ca content may be more than 0%, 0.0002% or more, or 0.0005% or more. On the other hand, even if the Ca content exceeds 0.0100%, the effect saturates. Furthermore, the formation of oxides actually reduces the bending workability. For this reason, the Ca content is 0.0100% or less. From the viewpoint of further improving the bending workability of the wire rod, the Ca content is preferably 0.0050% or less.

[0037] <Metal structure> Next, the metal structure of the wire rod according to the present disclosure will be described. Unless otherwise specified, the area ratio and the like regarding the metal structure of the wire rod refer to values ​​in a cross section (sometimes referred to as a "transverse section") perpendicular to the length direction of the wire rod.

[0038] Area ratio of austenite structure: 98.00% or more and less than 100.00% The austenite phase is non-magnetic. By setting the area ratio of the austenite structure to 98.00% or more, sufficient non-magnetic properties can be obtained. However, if the area ratio of the austenite structure is 100.00%, workability decreases. Therefore, the area ratio of the austenite structure is 98.00 to less than 100.00%. Other structures besides the austenite structure (total of 2.00% or less) include ferrite and cementite, which are ferromagnetic at room temperature.

[0039] Carbon segregation ratio in the central segregation area: 1.20 or less If the degree of carbon segregation is high in the center where the cooling rate is slow during the wire manufacturing process, excessive carbides precipitate at the grain boundaries. These carbides become the starting point for cracks during bending. Therefore, if the degree of carbon segregation in the center segregation region is high, sufficient bending workability cannot be obtained. Therefore, the degree of carbon segregation in the center segregation region of the wire is 1.20 or less.

[0040] Carbide area ratio in the center: 0.05 to 2.00% During the wire manufacturing process, the carbides that precipitate are greatest in the center, where the cooling rate is slow, and so are referred to as the center. When a large amount of carbides precipitate in the center, they often form carbides that cover the grain boundaries. In this case, the carbides tend to be thick and can easily become crack initiation points during processing, resulting in insufficient workability. Figure 1 shows an example of carbides (white area) that can become crack initiation points during bending. The presence of such plate-shaped carbides in the center of the wire makes them prone to becoming crack initiation points during bending. On the other hand, if the area ratio of carbides is less than 0.05%, workability decreases. This is thought to be because the precipitation of part of the solute carbon in the center reduces the variation in solute carbon and improves workability. Therefore, to obtain sufficient workability, the area ratio of carbides in the center is 0.05 to 2.00%. The area ratio of carbides in the center is preferably 0.07 to 1.90%, and more preferably 0.10 to 1.80%.

[0041] Maximum thickness of carbide: 0.50 μm or less Among carbides, thick cementite precipitated at grain boundaries is particularly likely to be the starting point for cracks during processing. By keeping the maximum thickness of the carbides preferably to 0.50 μm or less, workability can be further improved.

[0042] Austenite grain size in austenite structure: 8 μm to 14 μm The grain size of austenite affects strength. If the grain size of the austenite grains in the austenite structure is 8 μm or more, the increase in nucleation frequency is suppressed, grain boundary carbides are less likely to form, and workability is improved. If the grain size of the austenite grains in the austenite structure is 14 μm or less, the grains do not become coarse, and workability is improved. Therefore, the grain size of the austenite grains in the austenite structure is preferably 8 to 14 μm.

[0043] <Measurement method> Next, the method for measuring the metal structure will be described.

[0044] (carbon segregation in the central segregation area) To determine the degree of carbon segregation, measurements are performed using an electron probe micro analyzer (EPMA). The EPMA measures an area of ​​D / 2 mm x D / 2 mm, centered within a radius of 0.5 mm from the center of the wire's cross section, with a beam diameter of 10 μm and measurement intervals of 10 μm, where D is the wire diameter. The carbon concentration of a 50 μm x 50 μm area with high carbon concentration is calculated from the obtained data. The ratio of the carbon concentration in a 150 μm x 150 μm area to the carbon concentration in the entire D / 2 mm x D / 2 mm area is taken as the degree of segregation. Five locations with high carbon concentration are selected, and the average of these segregation degrees is taken as the degree of carbon segregation in the central segregation area. The 150 μm square area is selected as the area with the highest carbon concentration within the measurement range. However, if there are any contaminants or holes on the measurement surface, these will indicate a very high carbon concentration and are not included in the calculation.

[0045] (Carbide in the center) The cross section of steel (wire rod) is polished. After polishing, it is washed with alcohol or the like and immediately etched with picral. If the surface is washed with water after polishing or if time passes, etching will no longer be possible. Etching with picral allows carbides to be more clearly distinguished. An area within a radius of 0.5 mm from the center position is photographed at 100x magnification (1.1 mm x 0.8 mm) using an SEM. The carbides are binarized using image analysis software (e.g., Image-J), and the area ratio is calculated from the ratio to the total number of pixels. Five cross sections are observed, and the average is taken as the carbide area ratio.

[0046] (area ratio of austenite structure) The cross section of the steel (wire rod) is polished. After polishing, it is washed with alcohol or similar and immediately etched with picral. If the surface is washed with water after polishing or if time passes, etching will no longer work. The austenite structure ratio (area ratio) is determined by observing five cross sections in one field of view at 100x magnification using an SEM, centered on the surface region (within 1.5 mm from the surface), the middle region (within 1 mm centered at 1 / 4 D from the surface, assuming the wire diameter is D), and the center region (within 1 mm centered at 1 / 2 D from the surface). Using the same method as above, the area ratio of carbide, as well as the area ratios of ferrite, pearlite, bainite, and martensite, is determined, and these are subtracted from 100% to calculate the area ratio of austenite structure.

[0047] (Maximum thickness of carbide) Observe the area within a radius of 0.5 mm from the center of the cross section using an SEM at 100x magnification to identify the area with the thickest carbide, and then photograph the carbide at 10,000x magnification. The thickness of the carbide is measured using image analysis software (e.g., Image-J).

[0048] (Austenite grain size) Polish the cross section of steel (wire). After polishing, clean with alcohol etc. and immediately etch with picral. If you wash with water after polishing or if time passes, etching will become difficult. The area within a radius of 0.5 mm from the center of the cross section is observed at 500x magnification using an SEM, and the particle size is calculated using quadrature.

[0049] <Wire rod manufacturing method> The method for producing the wire according to the present disclosure is not particularly limited, but an example of a suitable production method will be described below.

[0050] (Soaking treatment) Steel having the above-mentioned chemical composition is cast, and after casting, the cast piece is subjected to a soaking treatment (heat treatment to reduce segregation) at 1250°C for 10 hours or more.

[0051] (blooming) The cast slab is heated to 1000-1250°C, and then rolled into a billet, after which it is allowed to cool.

[0052] (wire rod rolling) The billets are rolled to obtain rolled wire rod. The heating temperature of the billets before rolling is 1000°C to 1250°C, and the holding time is more than 10 to 150 minutes. If the heating temperature of the billets before rolling is less than 1000°C, the carbides formed in the billets do not dissolve, and the amount of carbides tends to be high.

[0053] The finish rolling temperature is set to 780° C. or higher. If the finish rolling temperature is lower than 780° C., the amount of carbides that precipitate in the center portion tends to increase.

[0054] By cooling the center after finish rolling at 5°C / sec or more, the area ratio of carbides in the center can be set to 0.05 to 2.00%. To achieve a cooling rate of 5°C / sec or more in the center, the cooling rate in the surface layer must be 7.0°C / sec or more. If the cooling rate in the surface layer is less than 7.0°C / sec, the amount of carbides that precipitate in the center tends to be high. If the cooling rate in the surface layer exceeds 25.0°C / sec, the amount of carbide precipitated in the center tends to be excessively low. Each temperature is measured using a radiation thermometer.

[0055] Through the above steps, it is possible to produce a wire rod according to the present disclosure in which carbide segregation in the center portion is suppressed. Note that when producing a steel billet by continuous casting, center segregation may be suppressed by soft reduction.

[0056] The wire diameter (diameter) of the wire according to the present disclosure is not particularly limited, and is, for example, 5.0 mm or more and 10.0 mm or less.

[0057] <Application> The applications of the wire rod according to the present disclosure are not particularly limited. The wire rod according to the present disclosure is capable of suppressing breakage during bending and maintaining non-magnetic properties even after bending, and is therefore suitable as a steel material used in magnetic fields such as those found in strong magnetic fields such as those found in linear motor car-related equipment, and weak magnetic fields such as those found in medical equipment and geomagnetic measuring devices. If the wire rod according to the present disclosure is applied to these applications, energy loss and noise generation due to the excitation of induced current can be effectively suppressed. [Example]

[0058] The wire rod of the present disclosure will be described in more detail below with reference to examples, although these examples do not limit the wire rod of the present disclosure.

[0059] Example 1 Steel materials having the chemical compositions (unit: mass%) shown in Table 1 were prepared, and wire rods were manufactured by the methods (conditions) shown in Table 2. Note that the notation "-" in Table 1 indicates that the content of the element in question is at the impurity level, and it can be determined that the element is not substantially contained. The same applies to "-" in Table 4 described below. The remainder of the chemical compositions in Tables 1 and 4 is Fe and impurities.

[0060] [Table 1]

[0061] [Table 2]

[0062] After casting, the slab was subjected to soaking heat treatment at 1250°C for 12 hours.

[0063] The metal structure of the produced wire rod was measured by the method described above. The wire was also bent under the following conditions, and if no breaks or cracks occurred, the bending workability was judged to be sufficient (Y), and if breaks or cracks occurred, the bending workability was judged to be insufficient (N). The bending was performed by three-point bending in accordance with JIS Z2248. The presence or absence of breaks at a bending angle of 180° and the presence or absence of cracks on the surface was checked. Regarding bending workability, if there were no breaks and no abnormalities in the surface properties, the bending workability was judged to be sufficient (Y), and if surface defects such as breaks or cracks occurred, the bending workability was judged to be insufficient (N).

[0064] The magnetic properties after bending were evaluated based on the relative magnetic permeability. The relative magnetic permeability was measured using a vibrating sample type automatic magnetization measuring device (BHV-50, manufactured by Riken Denshi Co., Ltd.). At room temperature in the atmosphere, the specimen was magnetized from a demagnetized state to a maximum magnetic field of 15 kOe, then to a maximum magnetic field of -15 kOe in the negative direction, and again to a maximum magnetic field of 15 kOe. The relative magnetic permeability was calculated using the magnetic field strength and magnetic polarization in the range of 5 kOe or more. In this disclosure, a relative magnetic permeability of less than 1.10 was considered "non-magnetic."

[0065] The results are shown in Table 3. The γ fraction refers to the area ratio of the austenite structure, and the γ grain size refers to the grain size of the austenite grains. The underlined values ​​indicate values ​​outside the scope of the present disclosure. In addition, in the "non-magnetic" category, "Y" means that the relative magnetic permeability is less than 1.10, and "N" means that the relative magnetic permeability is 1.10 or more.

[0066] [Table 3]

[0067] The wires Nos. 1-1A to 1-6A satisfy the requirements of the present disclosure, and are both non-magnetic and suppress the occurrence of cracks and breakage during bending. The wire rods Nos. 1-1B to 1-5B had excessive or insufficient amounts of carbides, and cracks and breakage occurred during bending.

[0068] Steels 2 to 15 having the chemical compositions shown in Table 4 were used to produce wire rods in the same manner as under production condition A, and the wires were evaluated in the same manner as described above. The results are shown in Table 5. The underlines in Tables 4 and 5 indicate values ​​outside the scope of the present disclosure.

[0069] [Table 4]

[0070] [Table 5]

[0071] The wire rods Nos. 2A to 11A satisfy the requirements of the present disclosure, and exhibit both good bending workability and non-magnetism after bending. No. 12B had an excessively high C content, which resulted in a large amount of carbide precipitation, poor workability, and cracks occurring during bending. No. 13B had an insufficient C content, and the stability of the austenite was low, so the wire rod did not become nonmagnetic after bending. No. 14B had an excessively high Si content, which resulted in poor workability and caused cracks during bending. No. 15B had an excessive Mn content, which resulted in poor workability and caused cracks during bending. No. 16B had an insufficient Mn content, and the stability of the austenite in the wire was low, so the wire did not become non-magnetic after bending. No. 17B had an excessively high N content, which resulted in poor workability and caused cracks during bending. No. 18B had an insufficient N content, and the stability of the austenite was low, so the wire rod did not become nonmagnetic after bending.

Claims

1. In mass%, C: 0.50 to 1.10%, Si: 0.10-1.00%, Mn: 10.0-18.0%, P: 0.050% or less, S: 0.050% or less, and N: 0.005 to 0.035%; The balance has a chemical composition of Fe and impurities, In a cross section perpendicular to the length of the wire, The austenite structure has an area ratio of 98.00% or more and less than 100.00%, The area ratio of carbides in the center portion is 0.05 to 2.00%, A wire rod having a carbon segregation degree of 1.20 or less in a central segregation portion.

2. In mass%, C: 0.50 to 1.10%, Si: 0.10-1.00%, Mn: 10.0-18.0%, P: 0.050% or less, S: 0.050% or less, and N: 0.005 to 0.035%; Furthermore, it contains one or more selected from the group consisting of the following first, second, and third groups: The balance has a chemical composition of Fe and impurities, (Group 1) Cu: 0.40% or less, and Ni: 0.40% or less, one or two selected from the group consisting of (2nd group) Cr: 2.50% or less, Mo: 1.00% or less, V: 0.25% or less, Ti: 0.100% or less, Al: 0.100% or less, and Nb: 0.050% or less, one or more selected from the group consisting of (Group 3) Sn: 0.050% or less, B: 0.0050% or less, Bi: 0.20% or less, Pb: 0.09% or less, and Ca: 0.0100% or less, one or more selected from the group consisting of In a cross section perpendicular to the length of the wire, The austenite structure has an area ratio of 98.00% or more and less than 100.00%, The area ratio of carbides in the center portion is 0.05 to 2.00%, A wire rod having a carbon segregation degree of 1.20 or less in a central segregation portion.

3. 3. The wire according to claim 1, wherein the maximum thickness of the carbide is 0.50 μm or less.

4. 3. The wire rod according to claim 1, wherein the grain size of the austenite grains in the austenite structure is 8 μm or more and 14 μm or less.

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