duplex stainless steel

Abstract

A duplex stainless steel has the following chemical composition (by mass%): C: less than 0.10%, Si: less than 3.0%, Mn: less than 8.0%, P: less than 0.040%, S: less than 0.020%, Cr: 20.0–38.0%, Ni: 3.00–12.00%, Mo: 1.0–6.5%, Cu: less than 3.0%, N: 0.200–0.700%, Al: 0–1.0%, Sn: 0–1.0%, W: 0–6.0%, Co: 0–3.0%. %, Nb: 0-0.50%, Ti: 0-1.5%, V: 0-1.0%, Zr: 0-0.50%, Ta: 0-0.100%, B: 0-0.100%, Ca: 0-0.50%, Mg: 0-0.50%, REM: 0-0.10%, balance: Fe and impurities, wherein PRE is above 41.0, Creq / Nieq is 2.360-2.530, average Md value is below 0.9140, and σ phase area fraction is below 2.0%.

Description

Technical Field

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

[0002] Duplex stainless steel is a type of stainless steel that contains both austenitic and ferritic phases in its microstructure. Due to its excellent corrosion resistance and high strength, duplex stainless steel is increasingly being used in various fields, including petrochemical equipment materials, pump materials, and chemical storage tank materials, leveraging its superior corrosion resistance.

[0003] For example, Patent Document 1 discloses a Sn-containing duplex stainless steel with good thermal manufacturability and low cost, a duplex stainless steel billet, and duplex stainless steel products.

[0004] As a parameter indicating the corrosion resistance, especially pitting corrosion resistance, of duplex stainless steel, PRE (Pitting Resistance Equivalent: Cr + 3.3Mo + 16N) is known. Composition design typically involves adjusting the contents of Cr, Mo, and N to increase the PRE value. In recent years, for the purpose of improving corrosion resistance, steels with a PRE of 40 or higher are required.

[0005] On the other hand, in duplex stainless steels with increased Cr and Mo content, there is a problem of the easy precipitation of intermetallic compounds such as the σ phase, which reduces mechanical properties and corrosion resistance. When this σ phase precipitates, the billet hardens significantly, making it prone to cracking and drastically reducing hot workability. Furthermore, even in the final product, the toughness around the intermetallic compounds deteriorates, making it difficult to ensure the desired performance.

[0006] Patent document 2 discloses a method for manufacturing highly corrosion-resistant duplex stainless steel by suppressing the precipitation of intermetallic compounds such as the σ phase and X phase, which are embrittlement phases, and continuously casting highly corrosion-resistant duplex stainless steel that has better embrittlement resistance, castability, and hot workability while maintaining high corrosion resistance.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 2013-119627

[0010] Patent Document 2: Japanese Patent Application Publication No. 2017-80765 Summary of the Invention

[0011] The problem the invention aims to solve

[0012] However, in Patent Document 2, since the Ni content, which helps stabilize the austenitic phase, improve toughness, and suppress nitride precipitation, is less than 7.0%, there is a concern that these effects may not be fully achieved, and there is still room for improvement.

[0013] However, with high Ni content, the enrichment of Cr and Mo in the ferrite phase promotes the precipitation of the σ phase. This results in billets containing a large amount of σ phase being highly prone to cracking and difficult to hot work later.

[0014] The purpose of this invention is to provide a duplex stainless steel that can suppress cracks caused by reduced toughness even when the PRE value is high and the Ni content is high.

[0015] Solution for solving the problem

[0016] The inventors conducted in-depth research to solve the above-mentioned problems, and the following insights were obtained as a result.

[0017] (a) To prevent billet cracks caused by duplex stainless steel, improved toughness is required.

[0018] (b) By properly managing the ratio of Cr equivalent (Creq) to Ni equivalent (Nieq), the solidification mode can be controlled, which can suppress the reduction of toughness.

[0019] (c) By controlling the Md value, which is an indicator of the ease of formation of intermetallic compounds, below a specified value, and by controlling the cooling conditions after casting, the precipitation of the σ phase can be suppressed.

[0020] (d) By satisfying these conditions, even with high PRE values ​​and high Ni content, duplex stainless steels are obtained with good toughness and can suppress cracking.

[0021] This invention is based on the above insights and its main content is duplex stainless steel.

[0022] (1) A duplex stainless steel having a chemical composition, in mass percent, of the following:

[0023] C: Below 0.10%

[0024] Si: below 3.0%

[0025] Mn: below 8.0%

[0026] P: below 0.040%

[0027] S: Below 0.020%

[0028] Cr: 20.0–38.0%

[0029] Ni: 3.00~12.00%

[0030] Mo: 1.0–6.5%

[0031] Cu: below 3.0%

[0032] N: 0.200~0.700%

[0033] Al: 0-1.0%

[0034] Sn: 0-1.0%

[0035] W: 0–6.0%

[0036] Co: 0-3.0%

[0037] Nb: 0-0.50%

[0038] Ti: 0-1.5%

[0039] V: 0~1.0%

[0040] Zr: 0~0.50%

[0041] Ta: 0~0.100%

[0042] B: 0~0.100%

[0043] Ca: 0-0.50%

[0044] Mg: 0-0.50%

[0045] REM: 0–0.10%

[0046] Balance: Fe and impurities,

[0047] A PRE value defined by equation (i) below is 41.0 or higher.

[0048] The ratio of Creq, as defined by equation (ii) below, to Nieq, as defined by equation (iii) below, is 2.360 to 2.530.

[0049] The average Md value defined by equation (iv) below is 0.9140 or less.

[0050] The area fraction of the σ phase in the metallographic structure is less than 2.0%.

[0051] PRE=Cr+3.3Mo+16N ··· (i)

[0052] Creq=Cr+1.37Mo+1.5Si+2Nb+3Ti··· (ii)

[0053] Nieq=Ni+0.31Mn+22C+14.2N+Cu···(iii)

[0054] Average Md value = ΣX i ·(Md) i ···(iv)

[0055] In equations (i) to (iii) above, the element symbols represent the content (mass%) of each element, and the symbols in equation (iv) above have the following meanings:

[0056] X i The atomic fraction of alloy component i.

[0057] (Md) i Md value (eV) of alloy composition i.

[0058] (2) A method for manufacturing duplex stainless steel, comprising a continuous casting process for continuously casting molten steel having the chemical composition described in (1) above,

[0059] In the continuous casting process, the billet is cooled to a temperature range of 950 to 1050°C in one step, then reheated to a maximum temperature of 1050°C or higher, and then cooled under the condition that the residence time in the temperature range of 900 to 1000°C is less than 400 seconds.

[0060] (3) The method for manufacturing duplex stainless steel according to (2) above further includes a hot rolling process for hot rolling the cast billet.

[0061] In the hot rolling process, the billet is heated for more than 1.5 hours in the temperature range of 1150 to 1300°C, and then hot rolled at a final rolling temperature of 900 to 1110°C. Then, it is cooled to a temperature range below 500°C at an average cooling rate of 0.1 to 1.0°C / s in the temperature range of 800 to 500°C.

[0062] The effects of the invention

[0063] According to the present invention, it is possible to obtain duplex stainless steel that can suppress cracks caused by reduced toughness even when the PRE value is high and the Ni content is high. Detailed Implementation

[0064] The features of the present invention will be described in detail below.

[0065] 1. Chemical composition

[0066] The reasons for the limitations of each element are as follows. It should be noted that in the following explanation, "%" referring to content means "mass %".

[0067] C: Below 0.10%

[0068] Carbon (C) is an element that dissolves in the austenitic phase to increase strength. However, when present in large quantities, corrosion resistance decreases due to the precipitation of carbides. Therefore, the C content is set to 0.10% or less, preferably 0.050% or less. Considering aging corrosion resistance, the C content is more preferably 0.030% or less. It should be noted that although it is not necessary to set a lower limit for the C content, it is preferable to have 0.010% or more, more preferably 0.015% or more, to achieve the aforementioned effects.

[0069] Si: below 3.0%

[0070] Si is used as a deoxidizing element and is sometimes added to improve oxidation resistance. However, high levels can lead to hardening of the steel and deterioration of its machinability. Therefore, the Si content is set to 3.0% or less, preferably 2.0% or less or 1.0% or less. It should be noted that although it is not necessary to set a lower limit for the Si content, it is preferable to have 0.10% or more, and more preferably 0.20% or more, to achieve the aforementioned effects.

[0071] Mn: below 8.0%

[0072] Mn has the effects of increasing the austenite phase, improving the solid solubility of nitrogen, and suppressing bubble defects during manufacturing. However, a high Mn content reduces corrosion resistance. Therefore, the Mn content is set to 8.0% or less, preferably 3.0% or less or 1.0% or less. It should be noted that although it is not necessary to set a lower limit for the Mn content, it is preferable to have 0.20% or more, and more preferably 0.40% or more, to obtain the above-mentioned effects.

[0073] P: below 0.040%

[0074] Phosphorus (P) is an element that inevitably mixes into steel, and since it is also present in raw materials such as chromium (Cr), it is difficult to reduce. High P content reduces formability. Lower P content is preferred, and it is set to 0.040% or less. Preferably, the P content is 0.030% or less.

[0075] S: below 0.020%

[0076] Sulfur (S) is an element that inevitably mixes into steel, and it sometimes combines with manganese (Mn) to form inclusions, which become the basis for rust formation. Therefore, the S content is set to 0.020% or less. Since the lower the S content, the better the corrosion resistance, the S content is preferably 0.010% or less, and more preferably 0.0050% or less.

[0077] Cr: 20.0–38.0%

[0078] Cr is an element required to ensure corrosion resistance. Furthermore, Cr is a ferrite stabilizing element; to obtain a dual-phase structure of austenite and ferrite, a Cr content of 20.0% or more is necessary, considering the specific ratio. However, a high Cr content can actually lead to a decrease in corrosion resistance. Therefore, the Cr content is set to 38.0% or less. The Cr content is preferably 22.0% or more or 24.0% or more, and more preferably 33.0% or less, 28.0% or less, or 27.0% or less.

[0079] Ni: 3.00~12.00%

[0080] Ni is an austenite stabilizing element. Additionally, Ni improves corrosion resistance. Therefore, the Ni content is set to 3.00% or more. However, a high Ni content leads to increased raw material costs and may cause problems such as stress corrosion cracking. Therefore, the Ni content is set to 12.00% or less. The Ni content is preferably 5.00% or more, more preferably 7.50% or more, and most preferably 10.00% or less.

[0081] Mo: 1.0–6.5%

[0082] Mo is an element that improves corrosion resistance. Therefore, the Mo content is set to 1.0% or more. However, a high Mo content not only increases raw material costs but also reduces corrosion resistance. Therefore, the Mo content is set to 6.5% or less. The Mo content is preferably 2.0% or more, more preferably 3.0% or more, and more preferably 5.5% or less, more preferably 4.4% or less.

[0083] Cu: below 3.0%

[0084] Cu is a very effective element for improving sulfuric acid resistance. However, a high Cu content can actually lead to a decrease in corrosion resistance. Therefore, the Cu content is set to 3.0% or less. Preferably, the Cu content is 2.0% or less, more preferably 0.90% or less. It should be noted that while it is not necessary to set a lower limit for the Cu content, to achieve the aforementioned effects, the Cu content is preferably 0.10% or more, more preferably 0.20% or more.

[0085] N: 0.200~0.700%

[0086] Nitrogen (N) is an element that dissolves in the austenite phase to improve strength and corrosion resistance, thus helping to conserve alloying resources. Therefore, the N content is set to 0.200% or more. However, when the N content is high, defects such as porosity occur, which deteriorate the corrosion resistance of the steel. Therefore, the N content is set to 0.700% or less. The N content is preferably 0.240% or more, and more preferably 0.450% or less.

[0087] Al: 0–1.0%

[0088] Al can be any element, or it can be absent. When present, Al performs desulfurization and deoxidation. However, when Al is present in large quantities, hard spinel-based inclusions (MgO·Al2O3), which can cause nozzle clogging, will precipitate. Furthermore, this leads to increased manufacturing defects and higher raw material costs. Therefore, the Al content is set to 1.0% or less. The Al content is preferably 0.50% or less or 0.10% or less. To reliably achieve the aforementioned effects, the Al content is preferably 0.010% or more.

[0089] Sn: 0–1.0%

[0090] Sn can be any element, or it can be absent. When present, Sn improves the corrosion resistance of steel. However, Sn is an element that hinders the workability of steel. Therefore, the Sn content is set to 1.0% or less. The Sn content is preferably 0.50% or less or 0.10% or less. To reliably obtain the above-mentioned effects, the Sn content is preferably 0.002% or more.

[0091] W: 0–6.0%

[0092] W can be any element, or it can be absent. When present, W improves the steel's resistance to SCC and pitting corrosion. Furthermore, W is less likely to form the σ phase compared to Mo. Therefore, W can be included to replace a portion of Mo. Even a small amount of W can achieve the aforementioned effects to some extent. However, excessively high W content increases production costs. Therefore, the W content is set to 6.0% or less. Preferably, W is 3.0% or less, more preferably 1.0% or less. To reliably achieve the aforementioned effects, the W content is preferably 0.01% or more, more preferably 0.10% or more.

[0093] Co: 0-3.0%

[0094] Co can be any element, or it can be absent. When present, Co increases the strength of the steel. Co also stabilizes austenite. Even a small amount of Co can achieve these effects to some extent. However, excessively high Co content not only reduces the corrosion resistance of the steel but also increases production costs. Therefore, the Co content is set to 3.0% or less. The Co content is preferably 2.0% or less or 1.0% or less. To reliably achieve the above effects, the Co content is preferably 0.01% or more, more preferably 0.05% or more.

[0095] Nb: 0–0.50%

[0096] Nitrogen (Nb) can be any element or may be absent. When present, Nb increases the strength of the steel. Even a small amount of Nb can achieve this effect to some extent. However, excessively high Nb content reduces the corrosion resistance of the steel. Therefore, the Nb content is set to 0.50% or less. Preferably, the Nb content is 0.30%, 0.10%, or 0.050% or less. For optimal results, the Nb content is preferably 0.005% or more.

[0097] Ti: 0–1.5%

[0098] Ti can be any element, or it can be absent. When present, Ti increases the strength of the steel. Even a small amount of Ti can achieve this effect to some extent. However, excessively high Ti content reduces the steel's corrosion resistance. Therefore, the Ti content is set to 1.5% or less. Preferably, the Ti content is 0.50%, 0.10%, or 0.050% or less. For optimal results, the Ti content is preferably 0.005% or more.

[0099] V: 0~1.0%

[0100] V can be any element, or it can be absent. When present, V increases the strength of the steel. Even a small amount of V can achieve this effect to some extent. However, excessive V content reduces the corrosion resistance of the steel. Therefore, the V content is set to 1.0% or less. Preferably, the V content is 0.80%, 0.50%, or 0.30% or less. To reliably achieve the aforementioned effects, the V content is preferably 0.01% or more, or 0.05% or more.

[0101] Zr: 0~0.50%

[0102] Zr can be any element, or it can be absent. When present, Zr helps improve corrosion resistance. Even a small amount of Zr can achieve the above-mentioned effect to some extent. However, the effect will saturate when the Zr content is too high. Therefore, the Zr content is set to 0.50% or less. The Zr content is preferably 0.40% or less or 0.30% or less. To reliably achieve the above-mentioned effect, the Zr content is preferably 0.005% or more.

[0103] Ta: 0~0.100%

[0104] Ta can be any element, or it can be absent. When present, Ta improves corrosion resistance through modification of inclusions. However, excessive Ta content leads to decreased ductility at room temperature. Therefore, the Ta content is set to 0.100% or less. The Ta content is preferably 0.050% or less. To reliably achieve the above-mentioned effects, the Ta content is preferably 0.005% or more.

[0105] B: 0~0.100%

[0106] B can be any element, or it may be absent. When present, B improves heat workability. Even a small amount of B is sufficient to achieve the aforementioned effects to some extent. However, if the B content is too high, the aforementioned effects will saturate. Therefore, the B content is set to 0.100% or less. The B content is preferably 0.0100% or less, more preferably 0.0050% or less. To reliably achieve the aforementioned effects, the B content is preferably 0.0001% or more, more preferably 0.0003% or more.

[0107] Ca: 0–0.50%

[0108] Ca can be any element, or it may be absent. When present, Ca, in addition to desulfurization and deoxidation, also prevents the formation of spinel-based inclusions. However, a high Ca content not only reduces corrosion resistance but also increases spatter during welding. Therefore, the Ca content is set to 0.50% or less. The Ca content is preferably 0.050% or less, more preferably 0.010% or less, and even more preferably 0.0040% or less. To reliably achieve the above-mentioned effects, the Ca content is preferably 0.0010% or more, more preferably 0.0015% or more.

[0109] Mg: 0–0.50%

[0110] Mg can be any element or may be absent. When present, Mg forms sulfides with sulfur (S) in the steel, reducing S segregation towards grain boundaries. As a result, the steel's corrosion resistance is improved, and its hot workability is also enhanced. Even a small amount of Mg can achieve these effects to some extent. However, excessively high Mg content can lead to the formation of coarse oxides or sulfides, becoming the starting point for pitting corrosion. Consequently, the steel's corrosion resistance decreases. Therefore, the Mg content is set to 0.50% or less. Preferably, the Mg content is 0.050% or less, more preferably 0.010% or less, and even more preferably 0.0040% or less. To reliably achieve the aforementioned effects, the Mg content is preferably 0.0005% or more.

[0111] REM: 0–0.10%

[0112] REM can be any element, or it may be absent. When present, REM improves the hot workability of steel. Therefore, a trace amount of REM is desirable. However, since excessive REM content reduces the corrosion resistance of steel, the REM content is set to 0.10% or less. The REM content is preferably 0.050% or less, more preferably 0.010% or less. To reliably achieve the above-mentioned effects, the REM content is preferably 0.0005% or more.

[0113] In this invention, REM refers to a total of 17 elements, including Sc, Y, and the lanthanides, and the aforementioned REM content refers to the total content of these elements. It should be noted that the lanthanides are added industrially in the form of mixed rare earth alloys.

[0114] In the chemical composition of the duplex stainless steel of the present invention, the balance is Fe and impurities. Here, "impurities" refers to components that are mixed in during the industrial manufacturing of steel due to various factors such as raw materials (ore, waste, etc.) and manufacturing processes, and are permissible within the scope that do not adversely affect the present invention.

[0115] In addition to ensuring that the content of each element in the duplex stainless steel of the present invention is within the above-mentioned range, the chemical composition of the stainless steel also requires that the PRE value calculated by the formula shown below and the Creq / Nieq value are within the specified range.

[0116] PRE: 41.0 or above

[0117] PRE is a conventional indicator of the corrosion resistance of stainless steel, calculated based on the steel's chemical composition using formula (i) below. Excellent corrosion resistance can be ensured by designing alloys with a PRE value of 41.0 or higher. While it is not necessary to set an upper limit on the PRE value, excessively high values ​​may lead to increased alloy costs. Therefore, a PRE value of 60.0 or lower is preferred.

[0118] PRE=Cr+3.3Mo+16N ··· (i)

[0119] In the above formula, the element symbols represent the content (mass%) of each element contained in the steel.

[0120] Creq / Nieq: 2.360~2.530

[0121] Creq and Nieq are defined by equations (ii) and (iii) below, respectively. By controlling the value of Creq / Nieq to be above 2.360, F-mode solidification can be achieved, ensuring toughness. The value of Creq / Nieq is preferably above 2.400. On the other hand, if Creq / Nieq is too high, it will result in a ferrite single-phase structure, and the characteristics of a two-phase steel cannot be obtained. Therefore, the value of Creq / Nieq is set to 2.530 or below.

[0122] Creq=Cr+1.37Mo+1.5Si+2Nb+3Ti··· (ii)

[0123] Nieq=Ni+0.31Mn+22C+14.2N+Cu ··· (iii)

[0124] In the above formula, the element symbols represent the content (mass%) of each element contained in the steel.

[0125] When the Creq / Nieq value is low and solidification occurs in the FA mode, a metallographic structure dominated by vermicular ferrite is formed during solidification, resulting from the crystallization of austenite. In this vermicular ferrite-dominated metallographic structure, due to the low interfacial compatibility between the ferrite and austenite phases, cracks easily propagate through phase boundary detachment, thus tending towards reduced toughness.

[0126] In contrast, when the Creq / Nieq value is above a specified value, solidification occurs in the F-mode, where ferrite solidifies as a single phase. In the F-mode, a metallographic structure is formed where austenite precipitates through a solid-state phase transformation after complete ferrite solidification, primarily composed of acicular ferrite. This acicular ferrite-dominated metallographic structure exhibits high interfacial compatibility between ferrite and austenite, effectively suppressing any reduction in toughness.

[0127] 2. Md value

[0128] The Md value is one of the indicators of phase stability in a multi-component system, representing the electron orbital energy of each component in the alloy in the d orbitals. Furthermore, the higher the Md value, the less unstable the phase, and the easier it is for intermetallic compounds such as the σ phase to form. In this invention, from the perspective of suppressing the precipitation of intermetallic compounds, the average Md value defined by the following formula (iv) is set to 0.9140 or less. From the perspective of further suppressing the precipitation of intermetallic compounds, it is ideal to be 0.9090 or less.

[0129] Average Md value = ΣX i ·(Md) i ···(iv)

[0130] The symbols in equation (iv) above have the following meanings.

[0131] X i Atomic fraction of alloy component i

[0132] (Md) i Md value (eV) of alloy composition i

[0133] To control the precipitation of intermetallic compounds, a lower average Md value is preferable, thus no lower limit needs to be set. However, in the composition system specified in this invention, it is difficult to set the average Md value to be less than 0.8800. Therefore, the average Md value can be greater than 0.8800.

[0134] The Md value of alloy composition i can be obtained through cluster calculations (a molecular orbital calculation method using a model of an aggregate (cluster) of several to tens of atoms) (M. Morinaga et al., J. Phys. Soc. Jpn., 53 (1984), p. 653). The average Md value of the alloy is calculated by converting the grain boundaries obtained from the initial composition and segregation ratio, and the composition of the final solidified portion into atomic fractions. i This allows for the processing of intermetallic compound precipitation.

[0135] 3. Metallographic structure

[0136] In the duplex stainless steel of this invention, the area fraction of the σ phase in the metallographic structure is 2.0% or less. As mentioned above, in addition to the PRE value, a high Ni content promotes the precipitation of the σ phase. Especially when the area fraction of the σ phase exceeds 2.0%, the deterioration of toughness becomes significant. Therefore, the area fraction of the σ phase is set to 2.0% or less. The area fraction of the σ phase is preferably 1.0% or less, more preferably 0.10% or less, and even more preferably 0.05% or less. The lower the area fraction of the σ phase, the better; therefore, there is no need to set a lower limit.

[0137] There are no particular limitations regarding other metallographic structures. However, by adjusting the Creq / Nieq values ​​to the range described above, a dual-phase structure of ferrite and austenite is achieved, and F-mode solidification occurs. In this case, it is preferable to have acicular ferrite comprising less than 50% by area, with the balance being austenite and other unavoidable byproducts. In the aforementioned metallographic structure, the area ratio of austenite is relatively high, thus improving toughness.

[0138] As an unavoidable byproduct, in addition to the aforementioned σ phase, it may also contain Cr2N, etc. A total content of Cr2N, etc., below 2.0% is permissible.

[0139] In this invention, the area ratios of ferrite and austenite were measured according to JIS Z 3119:2017 using a ferrite analyzer. Furthermore, whether the ferrite matrix is ​​predominantly worm-like or acicular ferrite can be determined by observing the microstructure using an optical microscope at 50x magnification.

[0140] Furthermore, after cutting out a sample for microscopic observation at a depth of 5 mm from the surface of the cast billet, the σ phase was exposed by KOH electrolytic etching. Then, microstructure images of 60 fields of view were acquired using an optical microscope at 400x magnification. The obtained images were then binarized, and the σ phase area ratio was measured. It should be noted that, because the σ phase is unevenly distributed in the microstructure, samples were collected from at least 5 locations on the cast billet, and the average value obtained from the measurements of each sample was used as the σ phase area ratio.

[0141] 4. Manufacturing method

[0142] The duplex stainless steel of the present invention can be manufactured, for example, by continuous casting of molten steel having the above-described chemical composition. That is, the duplex stainless steel of the present invention can be a cast billet. In this case, the control of the casting conditions becomes very important. The inventors first conducted the following research on casting conditions for suppressing the precipitation of the σ phase.

[0143] In the continuous casting process, the billet is cooled primarily through two processes: primary cooling using a water-cooled copper casting mold and secondary cooling by spraying cooling mist onto the billet surface. Specifically, heat transfer analysis was conducted to investigate the temperature history at a depth of 5 mm on the billet surface under various variations in the water volume during secondary cooling. The casting speed was set at 1.1 m / min.

[0144] The nose point for the precipitation of the σ phase is considered to be around 900–1000℃. Therefore, during the cooling process after casting, shortening the residence time in the temperature range of 900–1000℃ is effective in suppressing the precipitation of the σ phase.

[0145] The study found that by setting the water flow rate in the secondary cooling process, which is usually set to around 80 L / min, to 0 L / min for natural cooling, the surface layer (5 mm depth) of the cast billet can remain in the temperature range of 900–1000 °C near the precipitation nose of the σ phase for the shortest time.

[0146] Specifically, after the surface of the cast billet is cooled to a temperature range of 950 to 1050°C in one go, the maximum temperature is raised to above 1050°C by reheating starting from the center of the billet. Then, by natural cooling, the residence time in the temperature range of 900 to 1000°C is set to less than 400 seconds, and the area ratio of the σ phase can be set to less than 2.0%.

[0147] To reduce the area ratio of the σ phase, it is preferable to have a shorter residence time, preferably less than 300 s.

[0148] Furthermore, the duplex stainless steel of the present invention can be a hot-rolled material in the form of a plate or a rod. In this case, the method for manufacturing the duplex stainless steel of the present invention further includes a hot rolling process of hot rolling the above-mentioned cast billet. There are no particular limitations on the conditions in the hot rolling process; for example, it is preferred to carry out the process under the following conditions.

[0149] Before hot rolling, it is preferable to heat the above-mentioned billet at a temperature range of 1150–1300°C for at least 1.5 hours. This allows the σ phase precipitated in the billet to dissolve again. Here, the temperature and time mentioned above refer to the average temperature in the furnace and the time spent in the furnace, respectively.

[0150] The heated billet is then subjected to rough rolling and final rolling. The final rolling temperature is preferably 900–1110°C. After final rolling, the billet is preferably cooled to below 500°C at an average cooling rate of 0.1–1.0°C / s within a temperature range of 800–500°C. More preferably, the average cooling rate is 0.7°C / s or less. There are no particular limitations on the cooling method used; air cooling is acceptable.

[0151] There are no particular restrictions on the cooling rate during the cooling process from below 500°C to room temperature; cooling can be achieved through air cooling, spray cooling, or water cooling. Here, the final rolling temperature refers to the surface temperature of the hot-rolled material at the exit of the final stand of a multi-stand rolling mill. Furthermore, the cooling rate after final rolling refers to the cooling rate at the surface of the hot-rolled material.

[0152] Furthermore, the duplex stainless steel of the present invention can be a cold-rolled material obtained by cold rolling the aforementioned hot-rolled material. Cold rolling can be performed using conventional methods. Alternatively, the aforementioned hot-rolled or cold-rolled material can be annealed to produce annealed hot-rolled or annealed cold-rolled material. In this case, from the perspective of suppressing the precipitation of the σ phase, the annealing temperature is preferably set to, for example, 550–900°C.

[0153] The present invention will be described in more detail below through embodiments, but the present invention is not limited to these embodiments.

[0154] Example

[0155] Cylindrical ingots with a diameter of 180 mm and the chemical composition shown in Table 1 were manufactured under various manufacturing conditions. The continuous casting conditions for each ingot are shown in Table 2.

[0156] [Table 1]

[0157]

[0158] [Table 2]

[0159]

[0160] Using the obtained cast billet, the metallographic structure was determined through the following steps. First, the area ratios of ferrite and austenite were measured according to JIS Z3119:2017 using a ferrite analyzer. Furthermore, whether the ferrite matrix consisted of vermicular or acicular ferrite was determined by microstructural observation using an optical microscope at 50x magnification.

[0161] Subsequently, samples for microscopic observation were cut from five locations at a depth of 5 mm from the surface of the cast billet, and the σ phase was exposed by KOH electrolytic etching. Then, microstructure images of 60 fields of view were acquired using an optical microscope at 400x magnification. The obtained images were binarized, and the σ phase area ratio was measured. The average value of the measurements from the five samples was then taken as the σ phase area ratio.

[0162] Next, the toughness of each cast billet was evaluated. V-notch test pieces were prepared at a position 5 mm from the surface of each billet. The test pieces were 10 mm × 10 mm × 55 mm in size and were prepared for Charpy impact testing according to JIS Z 2242:2005. Regarding impact characteristics, the impact value at 100 °C was 30.0 J / cm. 2 The above conditions are defined as good, with a value less than 30.0 J / cm³. 2 The situation is defined as undesirable.

[0163] These results are presented together in Table 2. Table 2 clearly shows that in tests No. 1, 3, 5, 7, 9, and 11–13, where the area fraction of the σ phase exceeded 2.0% or the metallographic structure consisted mainly of worm-like ferrite, the impact values ​​were unsatisfactory. Furthermore, these tests resulted in cracking during the billet manufacturing stage.

[0164] On the other hand, in tests No. 2, 4, 6, 8, 10 and 14-16 as specified in this invention, the obtained ingots did not develop cracks, and the impact values ​​were also good. The hot workability of these ingots was further evaluated.

[0165] Test pieces with a diameter of 8 mm and a length of 110 mm were cut from the surface of each cast billet. The temperature was then increased from room temperature to 1250 °C over 30 seconds and held for 30 seconds. Next, the temperature was cooled to 1000 °C at a rate of 20 °C / s and held for 30 seconds. Tensile tests were then performed to determine the tensile strength and reduction of area.

[0166] Furthermore, for the billets that meet the requirements of Tests No. 2, 4, 6, 8, 10 and 14-16 specified in this invention, hot rolling is performed to form hot-rolled material (wire) with a diameter of 5.5 mm. Specifically, the billets are heated at 1200°C for 2 hours, then hot-rolled at a final rolling temperature of 1100°C, followed by air cooling to 400°C at an average cooling rate of 0.5°C / s within a temperature range of 800-500°C, and further water-cooled to room temperature.

[0167] Subsequently, five microscopic specimens were cut from the obtained hot-rolled material, with cross-sections perpendicular to the length and diameter directions serving as observation surfaces. The σ phase was then exposed through KOH electrolytic etching. Next, 60 microstructure images were acquired using an optical microscope at 400x magnification. The images were binarized, and the σ phase area ratio was measured. The average value of the measurements from the five samples was then taken as the σ phase area ratio.

[0168] As shown in Table 2, in this example of the invention, the reduction of area at 1000°C is 60.0% or more, exhibiting good hot workability, and furthermore, the area fraction of the σ phase in the hot-rolled material can be suppressed to below 2.0%.

[0169] Industrial availability

[0170] According to the present invention, it is possible to obtain duplex stainless steel that can suppress cracks caused by reduced toughness even when the PRE value is high and the Ni content is high.

Claims

1. A duplex stainless steel having a chemical composition, by mass percent, of C: less than 0.10%, Si: below 3.0% Mn: below 8.0% P: below 0.040% S: Below 0.020% Cr：20.0～38.0％、 Ni: 7.50~12.00% Mo: 1.0–6.5% Cu: below 3.0% N：0.200～0.700％、 Al：0～1.0％、 Sn: 0-1.0% W：0～1.0％、 Co: 0-3.0% Nb: 0-0.50% Ti: 0-1.5% V：0～1.0％、 Zr：0～0.50％、 Ta: 0~0.100% B：0～0.100％、 Ca: 0-0.50% Mg: 0-0.50% REM: 0–0.10% Balance: Fe and impurities, A PRE value defined by equation (i) below is 41.0 or higher. The ratio of Creq, as defined by equation (ii) below, to Nieq, as defined by equation (iii) below, is 2.360 to 2.

530. The average Md value defined by equation (iv) below is 0.9140 or less. The area fraction of the σ phase in the metallographic structure is less than 2.0%. PRE=Cr+3.3Mo+16N · · · (i) Creq=Cr+1.37Mo+1.5Si+2Nb+3Ti · · · (ii) Nieq=Ni+0.31Mn+22C+14.2N+Cu···(iii) Average Md value = ΣX i ·(Md) i ···(iv) in, The element symbols in equations (i) to (iii) above represent the mass percentage of each element, and the symbols in equation (iv) above have the following meanings: X i Atomic fraction of alloy component i (Md) i Md value (eV) of alloy composition i.

2. A method for manufacturing duplex stainless steel, comprising a continuous casting process for continuously casting molten steel having the chemical composition of claim 1. In the continuous casting process, the billet is cooled to a temperature range of 950 to 1050°C in one step, then reheated to a maximum temperature of 1050°C or higher, and then cooled under the condition that the residence time in the temperature range of 900 to 1000°C is less than 400 seconds.

3. The method for manufacturing duplex stainless steel according to claim 2, further comprising a hot rolling process for hot rolling the cast billet. In the hot rolling process, the billet is heated for more than 1.5 hours in the temperature range of 1150 to 1300°C, and then hot rolled at a final rolling temperature of 900 to 1110°C. Then, it is cooled to a temperature range below 500°C at an average cooling rate of 0.1 to 1.0°C / s in the temperature range of 800 to 500°C.

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