DUAL-PHASE STAINLESS STEEL WIRE AND DUAL-PHASE STAINLESS STEEL WIRE

Abstract

A duplex stainless steel wire rod having an aspect ratio of 1 μm or more and containing inclusions selected from one or more of S and O, a chemical composition in mass % of C: 0.005 to 0.10 %, Si: 0.2 to 2.0 %, Mn: 6.0 % or less, P: 0.04 % or less, S: 0.0050 % or less, Ni: 2.0 to 14.0 %, Cr: 18.0 to 28.0 %, Mo: 0.05 to 5.50 %, Cu: 2.0 % or less, N: 0.10 to 0.35 %, O: 0.0001 to 0.0060 %, Al: 0.010 to 0.2 %, Ca: 0.0025 % or less, Mg: 0.0100 % or less, arbitrary elements, balance: Fe and impurities, a DF value of 20 to 60 %, and a number ratio of Ca-Al-Mg-based oxides of 75 % or more.

Description

Technical Field

[0001] This invention relates to duplex stainless steel wire and duplex stainless steel wire. Background Technology

[0002] Duplex stainless steel is sometimes used for welding depending on the application. Various welding methods exist, but in cases of welding duplex stainless steel, gas metal arc welding (also known as GMA welding) is sometimes used.

[0003] GMA welding is a welding method that uses welding materials such as welding rods (welding wires) as electrodes, and forms weld metal from molten welding materials and molten base materials. In this case, duplex stainless steel is usually used as the welding material.

[0004] Duplex stainless steel used in welding materials requires machining to reduce wire diameter during manufacturing. Therefore, good hot workability is required. To improve hot workability and reduce the sulfur concentration in the steel, thorough deoxidation and desulfurization are necessary. Therefore, Al is used in deoxidation.

[0005] On the other hand, during deoxidation with Al, MgO·Al₂O₃ spinel is easily formed as an inclusion. This MgO·Al₂O₃ spinel becomes coarse within the dipping nozzle during continuous casting, easily causing nozzle clogging and reducing manufacturability. Therefore, the presence of Ca is effective in suppressing the formation of MgO·Al₂O₃ spinel. However, if Ca is present in duplex stainless steel used for welding, it results in spattering of metal particles and slag, reducing weldability.

[0006] Therefore, Patent Document 1 discloses a duplex stainless steel that controls the Ca content, ferrite phase ratio, inclusion composition, etc. in the steel, suppresses the formation of MgO·Al2O3 spinel, suppresses nozzle clogging, etc., and ensures hot workability and weldability.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 2018-171640 Summary of the Invention

[0010] The problem the invention aims to solve

[0011] However, in the duplex stainless steel disclosed in Patent Document 1, the range of Ca content is specified very narrowly in order to suppress nozzle clogging and ensure hot workability and weldability. Therefore, in actual operation, the load used to adjust the chemical composition is sometimes large, resulting in reduced manufacturability. In addition, from a compositional point of view, there is room for further improvement in suppressing nozzle clogging. Therefore, the following problem exists: it is difficult to suppress nozzle clogging while ensuring manufacturability, and at the same time, achieve good hot workability and weldability.

[0012] The objective of this invention is to solve the aforementioned problems, and to provide a duplex stainless steel wire that can suppress nozzle clogging while ensuring manufacturability, and has good hot workability and weldability.

[0013] Solution for solving the problem

[0014] The present invention was made to solve the above-mentioned problems, and its essence lies in the following duplex stainless steel wire.

[0015] (1) A duplex stainless steel wire having a major diameter of 1 μm or more and containing one or more inclusions selected from S and O.

[0016] The chemical composition of the duplex stainless steel wire, expressed in % by mass, is as follows:

[0017] C: 0.005~0.10%

[0018] Si: 0.2-2.0%

[0019] Mn: below 6.0%

[0020] P: below 0.04%

[0021] S: below 0.0050%

[0022] Ni: 2.0–14.0%

[0023] Cr: 18.0–28.0%

[0024] Mo: 0.05–5.50%

[0025] Cu: less than 2.0%

[0026] N: 0.10–0.35%

[0027] O: 0.0001~0.0060%

[0028] Al: 0.010–0.2%

[0029] Ca: below 0.0025%

[0030] Mg: less than 0.0100%

[0031] REM: 0–0.01%

[0032] Ta: 0-0.10%

[0033] B: 0~0.0020%

[0034] W: 0–2.0%

[0035] V: 0~1.00%

[0036] Ti: 0~0.005%

[0037] Nb: 0–0.10%

[0038] Sn: 0-0.10%

[0039] Sb: 0~0.30%

[0040] Co: 0-1.00%

[0041] Zr: 0~0.0050%

[0042] Ga: 0-0.01%

[0043] Balance: Fe and impurities,

[0044] The DF value calculated by the following formula (i) is 20%–60%.

[0045] When an oxide containing CaO, Al2O3, and MgO, whose sum satisfies equation (ii) below, and whose ratio of Al2O3 to MgO satisfies equation (iii) below, is identified as a Ca-Al-Mg oxide,

[0046] The proportion of the Ca-Al-Mg oxides relative to the inclusions is 75% or more.

[0047] DF value=7.2×(Cr+0.88Mo+0.78Si+2.2Ti+2.3V)-8.9×(Ni+0.03Mn+0.72Cu+22C+21N)-44.9…(i)

[0048] CaO+Al2O3+MgO+ROx+Ta2O5≥90%…(ii)

[0049] Al2O3 / MgO≤1.25…(iii)

[0050] In formula (i), the element symbols represent the content (mass%) of each element in the steel, which is zero if the element is not present. In formula (ii), ROx represents REM oxide. In formulas (ii) and (iii), CaO, Al2O3, MgO, ROx, and Ta2O5 represent the mass% of their respective oxides, which is zero if the element is not present.

[0051] (2) The duplex stainless steel wire according to (1) above, wherein the chemical composition contains, in mass %, a variety selected from...

[0052] REM: 0.0005~0.01%

[0053] Ta: 0.001~0.10%

[0054] B: 0.0001~0.0020%

[0055] W: 0.05–2.0%

[0056] V: 0.01~1.00%

[0057] Ti: 0.001~0.005%

[0058] Nb: 0.02–0.10%

[0059] Sn: 0.02-0.10%

[0060] Sb: 0.01~0.30%

[0061] Co: 0.07~1.00%

[0062] Zr: 0.0001~0.0050%, and

[0063] Ga: 0.0001% to 0.01% or more of the following.

[0064] (3) The duplex stainless steel wire described in (1) or (2) above is used as a welding material.

[0065] (4) A duplex stainless steel wire that uses the duplex stainless steel wire described in (1) or (2) above.

[0066] The effects of the invention

[0067] According to the present invention, duplex stainless steel wire that can suppress nozzle clogging while ensuring manufacturability and has good hot workability and weldability can be obtained. Detailed Implementation

[0068] The inventors of this invention have studied duplex stainless steel wire suitable for welding materials and obtained the following insights (a) to (c).

[0069] (a) To ensure hot workability, deoxidation with Al results in the formation of MgO·Al2O3 spinel, which causes nozzle clogging. Adding Ca is effective in suppressing the formation of this MgO·Al2O3 spinel, but the presence of Ca easily leads to sputtering and reduces weldability.

[0070] (b) Therefore, in addition to methods for increasing the Ca content, the inventors of the present invention also investigated whether the formation of MgO·Al2O3 spinel could be suppressed. The results showed that by increasing the activity of MgO in the slag and decreasing the activity of Al2O3, MgO was preferentially formed, rather than MgO·Al2O3 spinel.

[0071] (c) The formation of MgO can suppress nozzle clogging and ensure hot workability and weldability. Furthermore, it is effective in practical applications because it does not require very strict control of the Ca content.

[0072] One embodiment of the present invention is based on the above-described understanding. The elements of this embodiment will now be described in detail.

[0073] 1. Chemical composition of duplex stainless steel wire

[0074] The reasons for the elemental limitations in duplex stainless steel wire are as follows. It should be noted that in the following explanation, the "%" for content refers to "mass %".

[0075] C: 0.005~0.10%

[0076] Carbon (C) is an impurity present in steel and a strong austenite-forming element. Furthermore, a low C content significantly reduces the DF value (described later). Therefore, the C content is set to 0.005% or more, preferably 0.010% or more, and more preferably 0.015% or more. However, if C is present in excess, it combines with Cr during welding and reheating to form carbides. As a result, the toughness and corrosion resistance of the weld metal decrease. Therefore, the C content is set to 0.10% or less. The C content is preferably 0.08% or less, and more preferably 0.025% or less.

[0077] Si: 0.2–2.0%

[0078] Silicon (Si) is an element that acts as a deoxidizer during the smelting of duplex stainless steel. Therefore, the Si content is set to 0.2% or more. Preferably, the Si content is 0.3% or more, more preferably 0.35% or more, and even more preferably 0.4% or more. However, if Si is present in excess, the hot workability decreases. Therefore, the Si content is set to 2.0% or less. Preferably, the Si content is 1.2% or less, more preferably 1.0% or less, and even more preferably 0.6% or less.

[0079] Mn: below 6.0%

[0080] Manganese (Mn) is a deoxidizer and improves hot workability by fixing sulfur (S) to MnS. It also acts as a stabilizer for austenite and controls the DF value (described later). However, excessive Mn content leads to refractory loss during smelting and a decrease in the corrosion resistance of the finished product. Therefore, the Mn content is set to 6.0% or less. Preferably, the Mn content is 5.0% or less, more preferably 2.0% or less, and even more preferably 1.0% or less. On the other hand, to obtain the above-mentioned effects, the Mn content is preferably 0.5% or more, more preferably 0.7% or more.

[0081] P: below 0.04%

[0082] Phosphorus (P) is an impurity element in steel that reduces toughness, hot workability, and corrosion resistance. Therefore, the P content is set to 0.04% or less, preferably 0.03% or less. While P is preferably minimized, excessive reduction increases the refining load or raw material costs. Therefore, the P content is preferably set to 0.005% or more.

[0083] S: below 0.0050%

[0084] Sulfur (S) is an impurity element in steel that reduces toughness, hot workability, and corrosion resistance. Therefore, the S content is set to 0.0050% or less, preferably 0.0025% or less. S content should be minimized as much as possible, but excessive reduction increases the refining load or raw material costs. Therefore, the S content is preferably set to 0.0002% or more.

[0085] Ni: 2.0–14.0%

[0086] Ni (Ni) is an element that helps stabilize austenite. It also improves the corrosion resistance and toughness of weld metals and inhibits hot-working cracks. Therefore, the Ni content is set to 2.0% or more. Preferably, the Ni content is 4.0% or more, more preferably 6.0% or more. However, Ni is an expensive element, and excessive content leads to increased costs. Therefore, the Ni content is set to 14.0% or less. Preferably, the Ni content is 12.0% or less, more preferably 11.0% or less.

[0087] Cr: 18.0–28.0%

[0088] Chromium (Cr) is a fundamental element in stainless steel and contributes to improving the corrosion resistance of weld metal. Therefore, the Cr content is set to 18.0% or more. Preferably, the Cr content is 20.0% or more, more preferably 22.0% or more, and even more preferably 24.0% or more. However, Cr promotes the formation of the σ phase in the weld, reducing the toughness of the weld. Therefore, the Cr content is set to 28.0% or less. Preferably, the Cr content is 27% or less.

[0089] Mo: 0.05–5.50%

[0090] Mo (Mo) is an effective element for improving corrosion resistance. Additionally, it improves strength through solid solution strengthening. Therefore, the Mo content is set at 0.05% or more. Preferably, the Mo content is 0.10% or more, more preferably 3.00% or more, and even more preferably more than 4.00%. However, if Mo is present in excess, hot workability decreases. Therefore, the Mo content is set at 5.50% or less, preferably 4.50% or less.

[0091] Cu: below 2.0%

[0092] Copper (Cu) is an element that helps stabilize austenite, but if it is present in excess, its hot workability decreases. Therefore, the Cu content is set to 2.0% or less, preferably 1.0% or less. On the other hand, in order to obtain the above-mentioned effect, the Cu content is preferably set to 0.05% or more, more preferably 0.5% or more, and even more preferably more than 0.5%.

[0093] N: 0.10–0.35%

[0094] Nitrogen (N) is an element that helps stabilize austenite. In addition, N improves corrosion resistance and strength. Therefore, the N content is set at 0.10% or more, preferably 0.20% or more. However, if N is present in excess, surface defects may occur due to reduced hot workability. Therefore, the N content is set at 0.35% or less, preferably 0.30% or less.

[0095] O: 0.0001~0.0060%

[0096] Oxygen (O) is the main element contained in inclusions that cause nozzle clogging. Furthermore, the lower the O concentration in the steel, the higher the solubility of magnesium (Mg), which is desirable to reduce. Therefore, the O content is set to 0.0060% or less. The O content is preferably set to 0.0030% or less, and more preferably 0.0015% or less. While O is preferably minimized, excessive reduction increases the load during refining. Therefore, the O content is set to 0.0001% or more.

[0097] Al: 0.010–0.2%

[0098] Al (aluminum) has a deoxidizing effect on steel. However, insufficient deoxidation leads to incomplete desulfurization, resulting in rusting starting with sulfides and reduced hot workability. Furthermore, adequate deoxidation is important to control inclusions by reducing MgO in the slag, thereby increasing the Mg concentration in the molten steel. Therefore, the Al content is set to 0.010% or more. However, excessive Al content reduces workability. Therefore, the Al content is set to 0.2% or less, preferably 0.12% or less.

[0099] Ca: below 0.0025%

[0100] Ca (calcium) has the effect of suppressing the formation of MgO·Al2O3 spinel, which causes nozzle clogging during casting. However, if it is present in excess, it promotes sputtering during welding. Therefore, the Ca content is set to 0.0025% or less. The Ca content is preferably set to 0.0020% or less, more preferably 0.0015% or less. On the other hand, in order to obtain the above-mentioned effect, the Ca content is preferably set to 0.0010% or more.

[0101] Mg: less than 0.0100%

[0102] Magnesium (Mg) is an element that constitutes MgO. However, if Mg is present in excess, it is easy for inclusions to form excessively, which can lead to a decrease in properties. Therefore, the Mg content is set to 0.0100% or less, preferably 0.0030% or less. On the other hand, in order to obtain the above-mentioned effect, the Mg content is preferably set to 0.0001% or more, more preferably 0.0005% or more.

[0103] In addition to the elements mentioned above, the range shown below may also include one or more elements selected from REM and Ta. The rationale for each element's limitation will be explained.

[0104] REM: 0–0.01%

[0105] REM (Rare Earth Metal) has a high affinity for O, thus reducing the amount of MgO·Al2O3 spinel, a cause of nozzle clogging. Furthermore, due to its high affinity for S, it improves hot workability by fixing S. Therefore, it can be included as needed. However, if REM is present in excess, nozzle clogging caused by REM oxides can easily occur. Therefore, the REM content is set to 0.01% or less. The REM content is preferably set to 0.009% or less. On the other hand, to obtain the above-mentioned effects, the REM content is preferably set to 0.0005% or more.

[0106] Here, REM refers to a total of 17 elements, including Sc, Y, and the lanthanides. The REM content mentioned above refers to the total content of these elements. In industry, REM is mostly added in the form of mixed rare earth metals.

[0107] Ta: 0~0.10%

[0108] Ta (tantalum) has a high affinity for O, thus reducing the amount of MgO·Al2O3 spinel that causes nozzle clogging. Therefore, it can be included as needed. However, excessive Ta content leads to decreased ductility and toughness at room temperature. Therefore, the Ta content is set to 0.10% or less. The Ta content is preferably set to 0.08% or less. On the other hand, to obtain the above-mentioned effects, the Ta content is preferably set to 0.001% or more.

[0109] In addition to the elements mentioned above, the following range may also contain one or more elements selected from B, W, V, Ti, Nb, Sn, Sb, Co, Zr, and Ga. The reasons for these limitations will be explained.

[0110] B: 0~0.0020%

[0111] Boron (B) is an element that improves the strength of grain boundaries and thus enhances processability. Therefore, it can be included as needed. However, excessive B content reduces processability due to decreased ductility. Therefore, the B content is set to 0.0020% or less, preferably 0.0010% or less. On the other hand, to obtain the aforementioned effects, the B content is preferably 0.0001% or more, more preferably 0.0005% or more.

[0112] W: 0–2.0%

[0113] Tungsten (W) further enhances corrosion resistance. Therefore, it can be included as needed. However, W is a very expensive element, and even if it is included in excess, the effect is not commensurate with the increase in alloy cost. Therefore, the W content is set to 2.0% or less. The W content is preferably set to 1.0% or less. On the other hand, in order to obtain the above-mentioned effect, the W content is preferably set to 0.05% or more, and more preferably 0.25% or more.

[0114] V: 0~1.00%

[0115] Vanadium (V) improves corrosion resistance. Furthermore, it is an element that affects the ferrite phase ratio, and may be included as needed to adjust the DF value described later. However, excessive V content reduces toughness. Therefore, the V content is set to 1.00% or less. Preferably, the V content is 0.20% or less. On the other hand, to obtain the aforementioned effects, the V content is preferably 0.01% or more, and more preferably 0.10% or more.

[0116] Ti: 0~0.005%

[0117] Titanium (Ti) is an element that affects the ferrite phase ratio, and it can be included as needed to adjust the DF value described later. However, if Ti is present in excess, TiN will form before casting, promoting nozzle clogging. Therefore, the Ti content is set to 0.005% or less. On the other hand, to obtain the above-mentioned effect, the Ti content is preferably set to 0.001% or more.

[0118] Nb: 0–0.10%

[0119] Niobium (Nb) improves formability and corrosion resistance. Therefore, it can be included as needed. However, excessive Nb content reduces toughness. Therefore, the Nb content is set to 0.10% or less, preferably 0.05% or less. On the other hand, to obtain the above-mentioned effects, the Nb content is preferably set to 0.02% or more.

[0120] Sn: 0–0.10%

[0121] Sn (tin) further enhances corrosion resistance. Therefore, it can be included as needed. However, excessive Sn reduces toughness. Therefore, the Sn content is set to 0.10% or less. On the other hand, to obtain the above-mentioned effect, the Sn content is preferably set to 0.02% or more.

[0122] Sb: 0~0.30%

[0123] Antimony (Sb) further enhances corrosion resistance. Therefore, it can be included as needed. However, if Sb is present in excess, it promotes the formation of TiN, potentially causing surface defects. Therefore, the Sb content is set to 0.30% or less, preferably 0.10% or less. On the other hand, to obtain the above-mentioned effects, the Sb content is preferably set to 0.01% or more.

[0124] Co: 0~1.00%

[0125] Co (cobalt) further enhances corrosion resistance. Therefore, it can be included as needed. However, Co is a very expensive element, and excessive Co content increases alloy costs and saturates its effect. Therefore, the Co content is set to 1.00% or less. The Co content is preferably set to 0.80% or less. On the other hand, to obtain the above-mentioned effect, the Co content is preferably set to 0.07% or more, more preferably 0.10% or more, and even more preferably 0.25% or more.

[0126] Zr: 0~0.0050%

[0127] Zirconium (Zr) has the effect of immobilizing sulfur (S), which can improve corrosion resistance and hot workability. Therefore, it can be included as needed. However, Zr has a very high affinity for sulfur, and if it is present in excess, coarse sulfides will form in the molten steel, which will actually reduce corrosion resistance. Therefore, the Zr content is set to 0.0050% or less. The Zr content is preferably set to 0.0040% or less. On the other hand, in order to obtain the above-mentioned effects, the Zr content is preferably set to 0.0001% or more.

[0128] Ga: 0~0.01%

[0129] Gallium (Ga) improves corrosion resistance. Therefore, it can be included as needed. However, excessive Ga content increases alloy costs. Therefore, the Ga content is set to 0.01% or less. Preferably, the Ga content is 0.005% or less. On the other hand, to obtain the above-mentioned effect, the Ga content is preferably set to 0.0001% or more.

[0130] In the chemical composition of this embodiment, the balance is Fe and impurities. Here, "impurities" refers to components that may be introduced during the industrial manufacturing of steel due to various factors in raw materials such as ores and waste, as well as in the manufacturing process, and are permissible to the extent that they do not adversely affect this embodiment.

[0131] 2. DF value: 20-60%

[0132] In the steel wire of this embodiment, the DF value is controlled within a predetermined range. The DF value is a numerical value that serves as an indicator of the ferrite phase content in the steel, and is calculated by the following formula (i). It should be noted that formula (i) is calculated by regressing the relationship between the ferrite phase content obtained when manufacturing with varying composition ranges of various elements and the content of each element.

[0133] If the DF value is less than 20%, the amount of ferrite phase cannot be sufficiently ensured, resulting in reduced hot workability. Therefore, the DF value is set to 20% or more, preferably 30% or more. On the other hand, if the DF value exceeds 60%, the hot workability actually decreases. Therefore, the DF value is set to 60% or less, preferably 55% or less.

[0134] DF value=7.2×(Cr+0.88Mo+0.78Si+2.2Ti+2.3V)-8.9×(Ni+0.03Mn+0.72Cu+22C+21N)-44.9…(i)

[0135] In equation (i), the symbols of each element represent the content (mass%) of each element contained in the steel, which is zero if the element is not contained.

[0136] 3. Inclusions

[0137] 3-1. Ca-Al-Mg series oxides

[0138] The duplex stainless steel wire of this embodiment has a major diameter of 1 μm or more and contains one or more inclusions selected from S and O. In addition, among the inclusions, oxides containing CaO, Al2O3 and MgO, whose sum satisfies the following formula (ii), and whose Al2O3 to MgO ratio satisfies the following formula (iii), are identified as Ca-Al-Mg oxides.

[0139] CaO+Al2O3+MgO+ROx+Ta2O5≥90%…(ii)

[0140] Al2O3 / MgO≤1.25…(iii)

[0141] In addition, ROx in formula (ii) represents REM oxide, and CaO, Al2O3, MgO, ROx and Ta2O5 in formulas (ii) and (iii) represent the mass percentage of their respective oxides, which is zero if they are not present.

[0142] In the steel wire of this embodiment, the aforementioned Ca-Al-Mg oxides are actively formed. Here, Ca-Al-Mg oxides are oxides that satisfy formula (ii), meaning that the total amount of CaO, Al2O3, MgO, ROx, and Ta2O5 is 90% or more by mass. It should be noted that if formula (ii) is not satisfied, it indicates the presence of a large amount of lower oxides such as SiO2 and MnO, which contribute to the formation of surface and internal defects during processing, and is therefore undesirable. Ca-Al-Mg oxides are typically composed mostly of oxides of Ca, Al, and Mg, but in cases containing, for example, oxides of REM (ROx, Ta, etc.) may also be included. X () and / or oxides of Ta. This is because oxides of REM and Ta are generally unlikely to be a cause of the defects described above. Additionally, S may also be included.

[0143] Furthermore, Ca-Al-Mg oxides satisfy equation (iii), meaning the ratio of Al2O3 to MgO is 1.25 or less. This is because if the ratio of Al2O3 to MgO is 1.25 or less, MgO is more likely to form as a crystalline phase during continuous casting compared to MgO·Al2O3 spinel.

[0144] 3-2. The proportion of Ca-Al-Mg oxides

[0145] The proportion of Ca-Al-Mg oxides relative to the aforementioned inclusions is 75% or more. This is because if the proportion of Ca-Al-Mg oxides is less than 75%, a crystalline phase mainly composed of MgO will not be formed, but rather a crystalline phase mainly composed of MgO·Al2O3 spinel, which easily leads to nozzle clogging. The proportion of Ca-Al-Mg oxides is preferably 85% or more, and more preferably 90% or more.

[0146] 3-3. Methods for observing and determining the number ratio of Ca-Al-Mg system oxides

[0147] For steel wire, the cross-section perpendicular to the drawing direction is used as the observation surface and is polished using mirror finishing. This observation surface is analyzed by SEM-EDS, and at least 100 inclusions with a major diameter of 1 μm or more containing O and / or S are randomly selected. This is taken as the parent group, and the elements contained in the compounds of the parent group are analyzed by SEM-EDS. Inclusions containing O and / or S that satisfy the requirements of equations (ii) and (iii) above are identified as Ca-Al-Mg oxides. The content of each element in the inclusion is determined, and using the determined elemental analysis values, Ca, Al, and Mg are all CaO, Al2O3, and MgO, respectively, as the CaO, Al2O3, and MgO content (mass%) in the oxide. Furthermore, in the case of Ca-Al-Mg oxides, the proportion of the 100 inclusions relative to the parent group is calculated as the percentage of Ca-Al-Mg oxides (%).

[0148] It should be noted that in JIS G 0555:2020, which is used as a general method for evaluating inclusions, even when two or more inclusions exist separately, they are sometimes considered as a single inclusion depending on their type and distance. However, in this embodiment, they are considered as individual inclusions. Furthermore, the major axis of an inclusion refers to the length of the longest straight line connecting two points on the outer periphery of the inclusion.

[0149] Furthermore, regarding the observation surface, the reason for using a section perpendicular to the drawing direction is that the magnitude of the direction parallel to the drawing direction is significantly affected by the reduction rate, making it difficult to evaluate it consistently. On the other hand, the magnitude of the direction perpendicular to the drawing direction does not change significantly during rolling. Therefore, it is assumed that the magnitude of the major axis of inclusions in the section perpendicular to the drawing direction reflects the result of inclusion (oxide) control during the steelmaking stage, which is a key point of this embodiment. Moreover, this also applies when the steel is drawn into wire.

[0150] 4. Steel wire

[0151] The aforementioned steel wire is processed into steel wire through drawing and other processes. The steel wire inherits the characteristics of the aforementioned steel wire, specifically, its chemical composition and the proportion of Ca-Al-Mg oxides. Therefore, if the steel wire satisfies the chemical composition and the proportion of Ca-Al-Mg oxides of the steel wire of this embodiment described above, it can be presumed to be a steel wire using the steel wire of this embodiment.

[0152] 5. Applications

[0153] The steel wire and steel wire of this embodiment are suitable for use as welding materials such as welding rods.

[0154] 6. Manufacturing method

[0155] A preferred manufacturing method for the duplex stainless steel wire and duplex stainless steel wire of this embodiment will be described. The duplex stainless steel wire and duplex stainless steel wire of this embodiment can be stably manufactured, for example, by the following manufacturing method.

[0156] First, it is preferable to melt the main raw material, such as iron ore, and perform a primary refining process. The conditions for melting and primary refining are not particularly limited; conventional methods are acceptable. Next, a secondary refining process is preferred. In the steel wire rod of this embodiment, Al is used as a deoxidizer to form the aforementioned chemical composition of the steel wire rod. At this time, it is preferable to add Al to the molten steel in two stages. This will be explained in detail below. In the initial stage of the secondary refining, the first addition of Al is performed for deoxidation. Preferably, this deoxidation treatment brings the O concentration in the molten steel to 0.006% or less. Furthermore, prior to the first addition of Al, pre-deoxidation with Si, Mn, etc., may be performed as needed.

[0157] Then, in order to minimize the crystallization of MgO·Al2O3 spinel during casting, and to primarily control the crystals to be MgO, it is necessary to increase the Mg concentration in the molten steel relative to the Al concentration. This is because, as a result, MgO becomes stable, making it difficult for MgO·Al2O3 spinel to form.

[0158] Therefore, in order to increase the Mg concentration in the molten steel and make the ratio of Ca-Al-Mg oxides within the range that satisfies this embodiment, it is preferable to increase the activity of MgO in the slag, while decreasing the activity of Al2O3. Specifically, the activity of MgO, based on pure solid MgO, is preferably set to 0.9 or more, and the activity of Al2O3, based on pure solid Al2O3, is preferably set to 0.05 or less.

[0159] At this point, the activities of MgO and Al2O3 in the slag can be determined simply by measuring the composition of the slag and calculating using thermodynamic datasets and commercial thermodynamic calculation software. If the activities of MgO and Al2O3 in the slag do not meet the above-mentioned range, the activities can be adjusted using the following methods. To increase the activity of MgO, it is preferable to add CaO and / or MgO to the molten steel, for example. Similarly, to decrease the activity of Al2O3, it is preferable to add CaO to the molten steel, for example.

[0160] It should be noted that while methods such as adding Mg alloys or Ti to increase the Mg concentration in the molten steel are also considered, it is difficult to adjust the chemical composition to the range required for this embodiment, or nozzle clogging may easily occur. Therefore, as described above, it is preferable to keep the activity of MgO and Al2O3 in the slag within the aforementioned range. As a result, the Mg concentration in the molten steel can be stably increased, and the Ca-Al-Mg oxide system, which is essential in this embodiment, can be fully formed. Next, it is preferable to add Al a second time in the latter half of the secondary refining process to adjust the composition of the molten steel. In addition, the conditions for secondary refining are not particularly limited except for the conditions mentioned above. Conventional methods can be used to adjust the chemical composition to meet the range required for the steel wire rod of this embodiment.

[0161] Next, the obtained molten steel is preferably continuously cast to form a steel billet. The obtained steel billet is preferably heated in a range of, for example, 1000–1300°C and then hot-rolled. Furthermore, the rolling end temperature is preferably set in the range of, for example, 900–1100°C, and the total reduction of area is preferably set to 90.0% or more. After this hot rolling, a steel wire with a duplex stainless steel microstructure is produced. It should be noted that the diameter of the steel wire is not particularly limited at this stage, but is generally preferably set in the range of 5.0–6.0 mm. It should also be noted that annealing, pickling, etc., may be performed as appropriate.

[0162] Furthermore, it is preferable to draw the obtained steel wire into steel wire. The drawing process can be repeated multiple times, and heat treatment can be performed during the drawing process as needed. The heat treatment temperature is preferably set in the range of 1000–1100°C, as long as the wire achieves a duplex stainless steel structure after heat treatment. Pickling or other appropriate pickling processes can also be performed. It should be noted that the diameter of the steel wire is not particularly limited, but is generally preferably set in the range of 0.5–1.5 mm.

[0163] The steel wire and steel wire of this embodiment will be described in more detail below through examples, but this embodiment is not limited to these examples.

[0164] Example

[0165] Molten steel with the chemical composition shown in Table 1 was poured into a tundish and cast into sheets using a mold. It should be noted that during the steelmaking process, deoxidation was performed using Al in the secondary refining stage. In this case, Al was added in two stages. After the first Al addition, the O concentration in the molten steel was brought to below 0.006%, and the slag composition was controlled to adjust the activities of MgO and Al₂O₃ in the slag. The activities in the slag for each example are shown in Table 2, which is calculated using thermodynamic calculation software based on pure solids. Then, the second Al addition was performed. It should be noted that the chemical composition in Table 1 is the chemical composition after the adjustment of the composition range of each element is complete.

[0166] [Table 1]

[0167]

[0168] The obtained slab is heated to 1100℃ and hot-rolled under conditions of a rolling start temperature of 1080℃, a rolling finish temperature of 1050℃, and a total reduction of area of ​​99.9%, thereby producing steel wire with a diameter of 5.5mm. Next, the obtained steel wire is subjected to one wire drawing process, followed by strand heat treatment at 1000-1100℃, and then two wire drawing processes, thereby producing duplex stainless steel wire with a diameter of 1.0mm.

[0169] Then, the following steps are used to evaluate the ease of nozzle clogging during the manufacturing stage, the hot workability, and the weldability when steel wire is used as the welding material.

[0170] (Nozzle clogged)

[0171] Nozzle clogging during casting is determined by whether or not there is blockage on the inner wall of the nozzle after casting. Cases where casting is completed successfully are recorded as "good," and cases without a tendency to clog are recorded as "excellent." Conversely, cases where nozzle clogging occurs, preventing continuous casting, are recorded as "bad."

[0172] (Hot workability)

[0173] Regarding hot workability, the presence or absence of surface defects in the hot-rolled steel wire is confirmed. The absence of defects is evaluated as good hot workability and recorded as "Good". Conversely, the presence of surface defects with a depth of 0.15 mm or more is evaluated as poor hot workability and recorded as "Poor".

[0174] (Welding workability)

[0175] Weldability was evaluated by the presence or absence of spatter during welding. Specifically, in a copper trapping chamber, flat welding was performed on a duplex stainless steel plate (NSSC2120, manufactured by Nippon Steel Stainless Steel Co., Ltd.) using electrodes made of the steel wire used as the test material, and the weld bead was formed for 1 minute. The test conditions were set as follows: welding current 150A, voltage 24V, welding speed 30cm / min, shielding gas 100% CO2 (20l / min), and no preheating. No spatter was observed in the chamber during the weld bead formation, which was considered good weldability and recorded as "Good". Conversely, spatter was considered poor weldability and recorded as "Poor". The results are summarized in Table 2 below.

[0176] [Table 2]

[0177] Table 2

[0178]

[0179] Underlined: indicates a deviation from the requirements of this implementation method.

[0180] * indicates a deviation from the preferred manufacturing conditions.

[0181] As shown in Table 2, symbols B1 to B15 satisfy the chemical composition and Ca-Al-Mg oxide requirements of this embodiment, thus preventing nozzle clogging and exhibiting good hot workability and weldability. On the other hand, symbols b1 to b8 do not satisfy at least one of the chemical composition and Ca-Al-Mg oxide requirements of this embodiment. Therefore, this results in at least one of nozzle clogging, reduced hot workability, and reduced weldability.

[0182] Symbol b1, due to the low activity of MgO, does not meet the requirements for a Ca-Al-Mg system oxide, resulting in nozzle clogging. Symbol b2, due to the high activity of Al2O3, also does not meet the requirements for a Ca-Al-Mg system oxide, resulting in nozzle clogging.

[0183] Symbol b3 exhibits poor hot workability due to its low DF value. Furthermore, evaluation of weldability using the obtained steel wire revealed sputtering due to its high Ca content. Symbol b4 suffers from reduced hot workability due to its high S concentration. Additionally, the high O concentration leads to high concentrations of SiO2 and MnO, failing to meet the requirements for a Ca-Al-Mg oxide system. However, the low content of MgO·Al2O3 spinel, which is a cause of nozzle clogging, prevented nozzle blockage.

[0184] Symbol b5 exhibits reduced hot workability due to its low DF value. Symbol b6, due to its high Al2O3 activity, fails to meet the requirements for a Ca-Al-Mg oxide system in this embodiment, resulting in nozzle clogging. Symbols b7 and b8, due to their low MgO activity or high Al2O3 activity, also fail to meet the requirements for a Ca-Al-Mg oxide system in this embodiment, leading to nozzle clogging.

Claims

1. A duplex stainless steel wire having an aspect ratio of 1 μm or more and containing one or more inclusions selected from S and O, the chemical composition of the duplex stainless steel wire containing one or more of, in mass %, C: 0.005 to 0.10%, Si: 0.2 to 2.0%, Mn: 6.0% or less, P: 0.04% or less, S: 0.0050% or less, Cr：18.0～28.0％、 Ni: 2.0 to 14.0%, Mo: 0.05 to 5.50%, N：0.10～0.35％、 O：0.0001～0.0060％、 Al：0.010～0.2％、 Cu: 2.0% or less, Ca: 0.0025% or less, Mg: 0.0100% or less, REM: 0 to 0.01%, B：0～0.0020％、 W：0～2.0％、 V：0～1.00％、 Ta: 0 to 0.10%, Ti: 0 to 0.005%, Nb: 0 to 0.10%, Sn: 0 to 0.10%, Sb: 0 to 0.30%, Zr：0～0.0050％、 Co: 0 to 1.00%, Ga: 0 to 0.01%, the balance: Fe and impurities, a DF value calculated from the following (i) formula is 20 to 60%, of the inclusions, the Ca-Al-Mg oxide is identified as a Ca-Al-Mg oxide when the sum of CaO, Al203, and MgO satisfies the following (ii) formula, and the ratio of Al203to MgO satisfies the following (iii) formula, the number ratio of the Ca-Al-Mg oxide to the inclusions is 75% or more, DF value = 7.2 x (Cr + 0.88Mo + 0.78Si + 2.2Ti + 2.3V) - 8.9 x (Ni + 0.03Mn + 0.72Cu + 22C + 21N) - 44.9... (i) CaO + Al203+ MgO + ROx + Ta205≥ 90%... (ii) wherein Al203 / MgO≤ 1.25... (iii) 2. The duplex stainless steel wire of claim 1, wherein, the respective element symbols in the (i) formula represent the mass % content of the respective elements contained in the steel, and are zero in the case of not being contained, and in the (ii) formula, ROx represents REM oxide, and in the (ii) and (iii) formulas, CaO, Al203, MgO, ROx, Ta205 represent the mass % of the respective oxides, and are zero in the case of not being contained. the chemical composition contains one or more of, in mass %, REM: 0.0005 to 0.01%, B：0.0001～0.0020％、 W：0.05～2.0％、 V：0.01～1.00％、 Ta: 0.001 to 0.10%, Ti: 0.001 to 0.005%, Nb: 0.02 to 0.10%, Sn: 0.02 to 0.10%, Sb: 0.01 to 0.30%, Co: 0.07 to 1.00%, Zr: 0.0001 to 0.0050%, and Ga: 0.0001 to 0.01%.

3. The duplex stainless steel wire according to claim 1 or claim 2, which is for welding material use.

4. A duplex stainless steel wire using the duplex stainless steel wire according to claim 1 or claim 2.

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