Ferritic-austenitic two-phase stainless steel member and welded structure

Abstract

This ferritic-austenitic two-phase stainless steel member has a composition including, on a mass basis, 0.100% or less of C, 1.00% or less of Si, 6.00% or less of Mn, 9.00% or less of Ni, 0.050% or less of P, 0.0300% or less of S, 18.00-32.00% of Cr, 5.00% or less of Mo, 3.00% or less of Cu, 0.006-0.050% of Ti, 0.005-0.100% of Al, 0.1000% or less of Ca, 0.1000% or less of Mg, and 0.050-0.300% of N, the remaining portion being Fe and impurities. In the ferritic-austenitic two-phase stainless steel member, the number density of inclusions having a longer diameter of 1 µm or more is 250.0 inclusions / mm2 or less. Among the inclusions, the number density of soluble inclusions in which the ratio of the Al-concentration to the total concentration of Al, Mg, Ca, and Mn is 0.2 or less is 20.0 inclusions / mm2 or less. The amount of Ti existing in the form of a solid solution is 0.002 mass% or more.

Description

Ferritic-austenitic duplex stainless steel materials and welded structures

[0001] This invention relates to ferritic-austenitic duplex stainless steel materials and welded structures.

[0002] Ferritic-austenitic duplex stainless steel (hereinafter sometimes abbreviated as "duplex stainless steel") is widely used in infrastructure applications (e.g., sluice gates and dam facilities) and plant applications (e.g., industrial water piping and heat exchangers) due to its excellent strength and corrosion resistance. In these applications, corrosion resistance is required to be maintained over long periods even in harsh corrosive environments. For example, duplex stainless steel used in sluice gates installed near river mouths contains chloride ions (Cl - Because they are exposed to environments with high concentrations of minerals, they are required to exhibit corrosion resistance over long periods even under such conditions. For this reason, many methods are used to ensure good corrosion resistance by increasing the Cr and Ni content in duplex stainless steel materials. Conventional duplex stainless steel materials with excellent corrosion resistance have been proposed, in which the composition and the number of various inclusions are controlled (for example, Patent Documents 1 to 7).

[0003] Japanese Patent Publication No. 5072285, Japanese Unexamined Patent Publication No. 2017-95794, Japanese Patent Publication No. 7004118, Japanese Unexamined Patent Publication No. 2005-36313, Japanese Unexamined Patent Publication No. 2009-7638, Japanese Unexamined Patent Publication No. 2015-59247, Japanese Unexamined Patent Publication No. 2015-110828

[0004] Various microorganisms exist in environmental waters such as rivers and oceans. Furthermore, microorganisms can proliferate in water used in plant facilities if the environmental conditions are suitable. Therefore, when duplex stainless steel is used in infrastructure or plant applications, a microbial layer called a biofilm may form on the surface of the duplex stainless steel. Duplex stainless steel with a biofilm on its surface is more susceptible to pitting corrosion than when immersed in saltwater, because the activity of microorganisms in the biofilm increases the natural potential (corrosion potential). Since the corrosion initiation point of duplex stainless steel under such conditions has not been clearly defined, it is necessary to excessively increase the Cr and Ni content to ensure sufficient corrosion resistance. Also, while it is known that inclusions such as MnS are common corrosion initiation points in general stainless steel, the type of inclusion that acts as a corrosion initiation point differs under environments with high chloride ion concentrations and increased natural potential. In fact, although the conventional duplex stainless steel materials described above control the number of various inclusions, the inclusions that initiate corrosion in environments with high chloride ion concentrations and favorable natural potential have not been identified, and there is room for improvement.

[0005] Furthermore, welding is essential for the construction of structures used in the above applications, but conventional duplex stainless steel materials experience a decrease in corrosion resistance in the heat-affected zone after welding. The main cause of this decrease in corrosion resistance in the heat-affected zone is sensitization. Duplex stainless steel materials have a deliberately high amount of nitrogen (N) added, considering the balance of each phase and corrosion resistance. Although the nitrogen in duplex stainless steel is in solid solution, when heated to approximately 600-1000°C by welding, the nitrogen, which was mainly in solid solution in the ferrite phase, generates Cr nitride (Cr2N) around the grain boundaries during cooling, and Cr depletion (sensitization) occurs around it. The decrease in corrosion resistance in the heat-affected zone is difficult to improve by adding Cr or Ni alone. In addition, while adding stabilizing elements that readily form carbonitrides, such as Ti, is effective in suppressing the formation of Cr nitrides, adding Ti causes coarse TiN to disperse and precipitate during manufacturing, reducing the toughness of the duplex stainless steel material. For this reason, it is difficult to ensure toughness, especially in low-temperature environments during winter.

[0006] The present invention was made to solve the above-mentioned problems and aims to provide a ferritic-austenitic duplex stainless steel material that can ensure corrosion resistance in harsh environments, particularly in environments with high chloride ion concentration and nourishing natural potential, has good corrosion resistance in the heat-affected zone even after welding, and also exhibits excellent toughness. Furthermore, the present invention aims to provide a welded structure that has good corrosion resistance in the base material and heat-affected zone, and also exhibits excellent toughness, in harsh environments, particularly in environments with high chloride ion concentration and nourishing natural potential.

[0007] The inventors conducted extensive research on ferritic-austenitic duplex stainless steel materials and, based on the finding that certain soluble inclusions act as corrosion initiation points under harsh environments, improved corrosion resistance under harsh environments by controlling the number density of these soluble inclusions. Furthermore, based on the finding that the number density of inclusions is related to the toughness of ferritic-austenitic duplex stainless steel materials, the inventors improved the toughness of ferritic-austenitic duplex stainless steel materials by controlling the number density of inclusions. In addition, based on the finding that the amount of Ti dissolved in solid solution is related to the corrosion resistance of the heat-affected zone when ferritic-austenitic duplex stainless steel materials are welded, the inventors improved the corrosion resistance of the heat-affected zone by controlling the amount of Ti dissolved in solid solution. The present invention was completed under the above background.

[0008] In other words, the present invention has a composition, by mass, consisting of C: 0.100% or less, Si: 1.00% or less, Mn: 6.00% or less, Ni: 9.00% or less, P: 0.050% or less, S: 0.0300% or less, Cr: 18.00 to 32.00%, Mo: 5.00% or less, Cu: 3.00% or less, Ti: 0.006 to 0.050%, Al: 0.005 to 0.100%, Ca: 0.1000% or less, Mg: 0.1000% or less, N: 0.050 to 0.300%, with the remainder being Fe and impurities, and the number density of inclusions with a major axis of 1 μm or more is 250.0 pieces / mm². 2The following conditions apply: Among the inclusions, the number density of soluble inclusions where the ratio of Al concentration to the total concentration of Al, Mg, Ca, and Mn is 0.2 or less is 20.0 particles / mm³. 2 The following relates to a ferritic-austenitic duplex stainless steel material having a Ti solid solution content of 0.002% by mass or more.

[0009] Furthermore, the present invention relates to a welded structure comprising a base material, a weld metal portion, and a heat-affected zone formed between the base material and the weld metal portion, wherein the base material is the ferritic-austenitic duplex stainless steel material.

[0010] According to the present invention, it is possible to provide a ferritic-austenitic duplex stainless steel material that can ensure corrosion resistance in harsh environments, particularly in environments with high chloride ion concentration and a noble natural potential, exhibits good corrosion resistance in the heat-affected zone even after welding, and has excellent toughness. Furthermore, according to the present invention, it is possible to provide a welded structure that exhibits good corrosion resistance in the base material and heat-affected zone, and has excellent toughness, in harsh environments, particularly in environments with high chloride ion concentration and a noble natural potential.

[0011] This is a schematic, partially enlarged cross-sectional view of a welded structure.

[0012] The embodiments of the present invention will be described in detail below. The present invention is not limited to the embodiments described below, and it should be understood that modifications, improvements, etc., made to the embodiments described below, based on the ordinary knowledge of those skilled in the art, without departing from the spirit of the invention, also fall within the scope of the present invention. In this specification, "%" in relation to components means "mass%" unless otherwise specified.

[0013] The ferritic-austenitic duplex stainless steel material (hereinafter simply referred to as "duplex stainless steel material") according to an embodiment of the present invention has a composition comprising C: 0.100% or less, Si: 1.00% or less, Mn: 6.00% or less, Ni: 9.00% or less, P: 0.050% or less, S: 0.0300% or less, Cr: 18.00 to 32.00%, Mo: 5.00% or less, Cu: 3.00% or less, Ti: 0.006 to 0.050%, Al: 0.005 to 0.100%, Ca: 0.1000% or less, Mg: 0.1000% or less, and N: 0.050 to 0.300%, with the remainder being Fe and impurities.

[0014] In this specification, "stainless steel material" means a material formed from stainless steel, and its shape is not particularly limited. Examples of shapes include plates (including strips), rods, and tubes. It may also be various types of shaped steel, such as T-beams and I-beams. Furthermore, in this specification, "ferritic-austenitic" means a material whose structure at room temperature consists mainly of two phases: a ferrite phase and an austenite phase. Therefore, "ferritic-austenitic" also includes materials that contain small amounts of phases other than the ferrite and austenite phases (e.g., a martensite phase). In addition, in this specification, "impurities" mean components that are mixed in during the industrial production of stainless steel materials due to various factors in the raw materials such as ore and scrap, and the manufacturing process, and are acceptable as long as they do not adversely affect the present invention. For example, unavoidable impurities are included. For example, the material may contain 0.02% or less of oxygen as an impurity. Furthermore, regarding the content of each element, "containing xx% or less" means that it contains xx% or less, but also more than 0% (especially above the impurity level). In addition, numerical ranges expressed using "~" in this specification mean a range that includes the numbers written before and after "~" as the lower and upper limits, respectively.

[0015] The duplex stainless steel material according to the embodiment of the present invention may further contain, if necessary, one or more elements selected from Nb: 0.100% or less, V: 1.00% or less, Co: 1.00% or less, Zr: 1.00% or less, W: 2.50% or less, REM: 0.100% or less, Sn: 0.100% or less, and B: 0.010% or less. Each component will be described in detail below.

[0016] <C: 0.100% or less> Carbon (C) is an element that greatly affects the stability of the austenite phase. If the C content is too high, it hardens and reduces workability, and sensitization occurs when subjected to heat effects such as welding, reducing the corrosion resistance of the duplex stainless steel material. For this reason, the upper limit of the C content is controlled to 0.100%, preferably 0.080%, more preferably 0.060%, even more preferably 0.040%, and particularly preferably 0.030%. On the other hand, the lower limit of the C content is not particularly limited, but from the viewpoint of reducing refining costs, it is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%. Note that the above numerical values ​​(lower limits) or numerical ranges for the C content may be any combination of such numerical values ​​or numerical ranges. Therefore, for example, the C content can be 0.001 to 0.100%, 0.001 to 0.080%, 0.001 to 0.060%, 0.001 to 0.040%, 0.001 to 0.030%, 0.003 to 0.080%, 0.003 to 0.060%, 0.003 to 0.040%, 0.003 to 0.030%, 0.005 to 0.080%, 0.005 to 0.060%, 0.005 to 0.040%, or 0.005 to 0.030%. The same applies to the numerical values ​​(upper or lower limits) or numerical ranges of the content of the following other elements.

[0017] <Si: 1.00% or less> Si is an effective element for improving the corrosion resistance of duplex stainless steel. However, if the Si content is too high, it hardens and reduces workability, and the toughness of the welded part decreases when welding is performed. Therefore, the upper limit of the Si content is controlled to 1.00%, preferably 0.90%, more preferably 0.80%, and even more preferably 0.60%. On the other hand, the lower limit of the Si content is not particularly limited, but from the viewpoint of obtaining the effect of Si, it is preferably 0.01%, more preferably 0.05%, and even more preferably 0.10%. Therefore, the Si content can be in the range having the above upper and lower limits, for example, 0.01 to 1.00%.

[0018] <Mn: 6.00% or less> Mn is an effective element for stabilizing the austenite phase. However, if the Mn content is too high, MnS, which acts as a corrosion initiation site, is easily generated, and the ratio of the ferrite phase to the austenite phase becomes unstable. Therefore, the upper limit of the Mn content is controlled to 6.00%, preferably 5.80%, more preferably 5.60%, and even more preferably 5.40%. On the other hand, the lower limit of the Mn content is not particularly limited, but from the viewpoint of obtaining the effect of Mn, it is preferably 0.01%, more preferably 0.10%, and even more preferably 0.30%. Therefore, the Mn content can be within the range having the above upper and lower limits, for example, 0.01 to 6.00%.

[0019] <Ni: 9.00% or less> Ni is an element that improves the corrosion resistance (especially crevice corrosion resistance) of duplex stainless steel materials. However, if the Ni content is too high, the ratio of ferrite phase and austenite phase becomes unstable, similar to Mn, and the manufacturing cost also increases. Therefore, the upper limit of the Ni content is controlled to 9.00%, preferably 8.50%, and more preferably 8.00%. On the other hand, the lower limit of the Ni content is not particularly limited, but from the viewpoint of obtaining the effect of Ni, it is preferably 0.10%, more preferably 0.50%, and even more preferably 1.00%. Therefore, the Ni content can be in the range having the above upper and lower limits, for example, 0.10 to 9.00%.

[0020] <P: 0.050% or less> High P content can cause a decrease in weldability and workability, so it is desirable to reduce the P content. Therefore, the upper limit of the P content is controlled to 0.050%, preferably 0.045%, and more preferably 0.040%. On the other hand, the lower limit of the P content is not particularly limited, but since reducing it incurs refining costs, it is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%. Accordingly, the P content can be within the range having the above upper and lower limits, for example, 0.001 to 0.050%.

[0021] <S: 0.0300% or less> If the sulfur content is high, inclusions that serve as corrosion initiation points (sulfide-based inclusions such as MnS) are more likely to form, and the toughness of the welded joint decreases when welding is performed. Therefore, the upper limit of the sulfur content is controlled to 0.0300%, preferably 0.0200%, and more preferably 0.0100%. On the other hand, the lower limit of the sulfur content is not particularly limited, but since reducing it incurs refining costs, it is preferably 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%. Accordingly, the sulfur content can be within the range having the above upper and lower limits, for example, 0.0001 to 0.0300%.

[0022] <Cr: 18.00-32.00%> Cr is an important element for ensuring the corrosion resistance and oxidation resistance of duplex stainless steel. However, if the Cr content is too high, it will lead to an increase in refining costs and hardening (reduced toughness) due to solid solution strengthening, resulting in reduced workability. Therefore, the upper limit of the Cr content is controlled to 32.00%, preferably 30.00%, more preferably 28.00%, and even more preferably 27.00%. On the other hand, if the Cr content is too low, sufficient corrosion resistance cannot be obtained. Therefore, the lower limit of the Cr content is controlled to 18.00%, preferably 19.00%, and more preferably 20.00%. Accordingly, the Cr content can be within the range having the above upper and lower limits, for example, 18.00-32.00%.

[0023] <Mo: 5.00% or less> Mo is an effective element for improving the corrosion resistance and oxidation resistance of duplex stainless steel materials. However, if the Mo content is too high, it can lead to a decrease in workability and an increase in manufacturing costs. Therefore, the upper limit of the Mo content is controlled to 5.00%, preferably 4.50%, and more preferably 4.00%. On the other hand, the lower limit of the Mo content is not particularly limited, but from the viewpoint of obtaining the effects of Mo, it is preferably 0.01%, more preferably 0.05%, and even more preferably 0.10%. Therefore, the Mo content can be in the range having the above upper and lower limits, for example, 0.01 to 5.00%.

[0024] <Cu: 3.00% or less> Cu is an austenite-forming element and has the effect of suppressing nitride precipitation and improving corrosion resistance. However, if the Cu content is too high, the austenite phase becomes unstable and the manufacturing cost also increases. For this reason, the upper limit of the Cu content is controlled to 3.00%, preferably 2.50%, and more preferably 2.00%. On the other hand, the lower limit of the Cu content is not particularly limited, but from the viewpoint of obtaining the effect of Cu, it is preferably 0.01%, more preferably 0.03%, and even more preferably 0.05%. Therefore, the Cu content can be in the range having the above upper and lower limits, for example, 0.01 to 3.00%.

[0025] <Ti: 0.006-0.050%> Ti is an element that affects the intergranular corrosion resistance (sensitization suppression effect) of duplex stainless steel. Ti is also an effective element for immobilizing C and N. However, if the Ti content is too high, in addition to the increase in manufacturing costs, the workability and toughness of the duplex stainless steel will decrease. For this reason, the upper limit of the Ti content is controlled to 0.050%, preferably 0.045%, and more preferably 0.040%. On the other hand, the lower limit of the Ti content is 0.006%, preferably 0.007%, and more preferably 0.008%, from the viewpoint of obtaining the effects of Ti. Therefore, the Ti content can be within the range having the above upper and lower limits, for example, 0.006-0.050%.

[0026] <Al: 0.005-0.100%> Al is an element used for desulfurization and deoxidation. In addition, Al improves oxidation resistance and facilitates the formation of oxide-based inclusions that are less likely to become corrosion initiation sites (they are less likely to dissolve in harsh environments, especially in environments with high chloride ion concentration and a high natural potential). Examples of these oxide-based inclusions include Al-based oxides such as Al2O3. However, if the Al content is too high, hardening (reduced toughness) occurs, reducing workability and increasing manufacturing costs. Therefore, the upper limit of the Al content is controlled to 0.100%, preferably 0.095%, and more preferably 0.090%. On the other hand, the lower limit of the Al content is 0.005%, preferably 0.008%, more preferably 0.010%, and even more preferably 0.020%, from the viewpoint of obtaining the effects of Al. Therefore, the Al content can be within the range of the above-mentioned upper and lower limits, for example, 0.005 to 0.100%.

[0027] <Ca: 0.1000% or less> Ca is an element used for desulfurization and deoxidation. Ca is also an effective element for improving the oxidation resistance of duplex stainless steel materials. However, if the content of these elements is too high, soluble inclusions that are prone to corrosion initiation (dissolving under harsh conditions, especially under conditions where the chloride ion concentration is high and the natural potential is noble) are more likely to form, and this leads to an increase in manufacturing costs. Therefore, the upper limit of the Ca content is controlled to 0.1000%, preferably 0.0500%, more preferably 0.0100%, and even more preferably 0.0050%. On the other hand, the lower limit of the Ca content is not particularly limited, but from the viewpoint of obtaining the effect of Ca, it is preferably 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. Therefore, the Ca content can be within the range having the above upper and lower limits, for example, 0.0001 to 0.1000%.

[0028] <Mg: 0.1000% or less> Mg is an element used for deoxidation. However, if the Mg content is too high, it leads to an increase in manufacturing costs and makes it easier for soluble inclusions that are prone to corrosion to form (dissolve under harsh conditions, especially in environments with high chloride ion concentration and a high natural potential) to form. Therefore, it is desirable to reduce the Mg content. For this reason, the upper limit of the Mg content is controlled to 0.1000% or less, preferably 0.0500% or less, more preferably 0.0100% or less, even more preferably 0.0050% or less, and particularly preferably 0.0020% or less. On the other hand, the lower limit of the Mg content is not particularly limited, but from the viewpoint of obtaining the effect of Mg, it is preferably 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. Therefore, the Mg content can be within the range having the above upper and lower limits, for example, 0.0001 to 0.1000%.

[0029] <N: 0.050 to 0.300%> N is an effective element for stabilizing the austenite phase and improving the corrosion resistance of duplex stainless steel. However, if the N content is too high, it becomes hard and its workability decreases, and sensitization occurs when subjected to heat effects such as welding, reducing corrosion resistance. Therefore, the upper limit of the N content is controlled to 0.300%, preferably 0.290%, and more preferably 0.280%. On the other hand, the lower limit of the N content is controlled to 0.050%, preferably 0.080%, and more preferably 0.100% from the viewpoint of reducing refining costs. Therefore, the N content can be within the range having the above upper and lower limits, for example, 0.050 to 0.300%.

[0030] <Nb: 0.100% or less> Nb, like Ti, affects the intergranular corrosion resistance (sensitization suppression effect) of duplex stainless steel materials and is an element effective for immobilizing C and N, but it is not an essential element, and its content may be 0%. However, if the content of Nb is too high, in addition to an increase in manufacturing cost, the workability and surface quality of duplex stainless steel materials will deteriorate. Therefore, the upper limit value of the content of Nb is controlled to 0.100%, preferably 0.080%, more preferably 0.050%. Accordingly, the content of Nb can be 0 to 0.100%. On the other hand, the lower limit value of the content of Nb is not particularly limited, but from the viewpoint of obtaining the effect of Nb, it is preferably 0.001%, more preferably 0.003%, still more preferably 0.005%. Therefore, from the viewpoint of obtaining the effect of Nb, the content of Nb can be in the range having the above upper limit and lower limit values, for example, 0.001 to 0.100%.

[0031] <V: 1.00% or less, Co: 1.00% or less, Zr: 1.00% or less> V, Co, and Zr are elements effective for improving the oxidation resistance of duplex stainless steel materials, but they are not essential elements, and the content of these elements may be 0%. However, if the content of these elements is too high, it will lead to a decrease in workability and toughness and an increase in manufacturing cost. Therefore, the upper limit value of the content of these elements is controlled to 1.00%, preferably 0.80%, more preferably 0.60% for all of them. Accordingly, the content of V, Co, and Zr can be 0 to 1.00% respectively. On the other hand, the lower limit value of the content of these elements is not particularly limited for all of them, but from the viewpoint of obtaining the effect of these elements, it is preferably 0.01%, more preferably 0.02%, still more preferably 0.03%. Therefore, from the viewpoint of obtaining the effect of these elements, the content of V, Co, and Zr can be in the range having the above upper limit and lower limit values, for example, 0.01 to 1.00%.

[0032] <W: 2.50% or less> W is an element effective in improving the corrosion resistance of duplex stainless steel materials, but is not an essential element in particular, and its content may be 0%. However, if the content of W is too high, it will lead to a decrease in workability and toughness and an increase in manufacturing cost. Therefore, the upper limit value of the content of these elements is controlled to 2.50%, preferably 2.30%, more preferably 2.00%. Thus, the content of W can be 0 to 2.50%. On the other hand, the lower limit value of the content of these elements is not particularly limited, but from the viewpoint of obtaining the effects of these elements, it is preferably 0.01%, more preferably 0.02%, still more preferably 0.03%. Therefore, from the viewpoint of obtaining the effects of W, the content of W can be in the range having the above upper limit value and lower limit value, for example, 0.01 to 2.50%.

[0033] <REM: 0.100% or less> REM (rare earth element) is an element effective in improving the oxidation resistance of duplex stainless steel materials, but is not an essential element in particular, and its content may be 0%. However, if the content of REM is too high, it will lead to an increase in manufacturing cost. Therefore, the upper limit value of the content of REM is controlled to 0.100%, preferably 0.080%, more preferably 0.050%. Thus, the content of REM can be 0 to 0.100%. On the other hand, the lower limit value of the content of REM is not particularly limited, but from the viewpoint of obtaining the effects of REM, it is preferably 0.001%, more preferably 0.002%, still more preferably 0.003%. Therefore, from the viewpoint of obtaining the effects of REM, the content of REM can be in the range having the above upper limit value and lower limit value, for example, 0.001 to 0.100%. Note that REM refers to the general term for two elements, scandium (Sc) and yttrium (Y), and the fifteen elements (lanthanoids) from lanthanum (La) to lutetium (Lu). These may be used alone or as a mixture. Also, the content of REM in this specification means the total content of these elements.

[0034] <Sn: 0.100% or less> Sn is an effective element for improving the oxidation resistance of duplex stainless steel materials, but it is not an essential element, and its content may be 0%. However, if the Sn content is too high, segregation becomes more likely, reducing manufacturability. Therefore, the upper limit of the Sn content is controlled to 0.100%, preferably 0.080%, and more preferably 0.050%. Thus, the Sn content can be 0 to 0.100%. On the other hand, the lower limit of the Sn content is not particularly limited, but from the viewpoint of obtaining the effect of Sn, it is preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%. Thus, from the viewpoint of obtaining the effect of Sn, the Sn content can be within the range having the above upper and lower limits, for example, 0.001 to 0.100%.

[0035] <B: 0.010% or less> B is an element that is effective in improving the secondary workability of duplex stainless steel material, but it is not an essential element and its content may be 0%. However, if the B content is too high, it will lead to a decrease in fatigue strength. For this reason, the upper limit of the B content is controlled to 0.010%, preferably 0.008%. Thus, the B content can be 0 to 0.010%. On the other hand, the lower limit of the B content is not particularly limited, but from the viewpoint of obtaining the effect of B, it is preferably 0.001%. Thus, from the viewpoint of obtaining the effect of B, the B content can be in the range having the above upper and lower limits, for example, 0.001 to 0.010%.

[0036] The duplex stainless steel material according to the embodiment of the present invention has a number density of 250.0 inclusions / mm² with a major diameter of 1 μm or more. 2 The following applies: Duplex stainless steel used in infrastructure and plant applications is often used outdoors, and it is required to ensure toughness in low-temperature winter environments. Since toughness in low-temperature environments tends to decrease as the number density of inclusions with a major axis of 1 μm or more increases, the number density of inclusions with a major axis of 1 μm or more is set to 250.0 pieces / mm 2By controlling the following, toughness in a low-temperature environment can be ensured. From the viewpoint of stably ensuring toughness in a low-temperature environment, the number density of inclusions having a major axis of 1 μm or more is more preferably 230.0 particles / mm 2 Hereinafter, even more preferably 200.0 particles / mm 2 Hereinafter, particularly preferably 100.0 particles / mm 2 Hereinafter. Note that the lower limit of the number density of inclusions having a major axis of 1 μm or more is not particularly limited and may be 0 particles / mm 2 since the toughness in a low-temperature environment improves as the number density decreases

[0037] The number density of inclusions having a major axis of 1 μm or more is determined by observing the surface of the duplex stainless steel material with a SEM (scanning electron microscope). Specifically, the number density of inclusions having a major axis of 1 μm or more is determined as follows. First, the surface of the duplex stainless steel material is mirror-polished, and then the surface is observed with a SEM. In the backscattered electron image of the SEM, inclusions are distinguished based on the contrast difference, inclusions having a major axis of 1 μm or more are specified, and the number thereof is determined. Then, the number density of inclusions having a major axis of 1 μm or more can be calculated by dividing the number of inclusions thus obtained by the area of the observation region. Note that the major axis of an inclusion means the length of the straight line that becomes the longest when two points on the outer periphery of the inclusion are connected in the backscattered electron image of the SEM

[0038] In the duplex stainless steel material according to the embodiment of the present invention, among inclusions having a major axis of 1 μm or more, the number density of soluble inclusions in which the ratio of the Al concentration to the total concentration of Al, Mg, Ca, and Mn is 0.2 or less is 20.0 particles / mm 2The following explains why the number density of such soluble inclusions is limited. Inclusions with a major axis of 1 μm or more tend to form pits when dissolved under harsh conditions (especially under conditions with high chloride ion concentration and a high natural potential), making them prone to acting as corrosion initiation sites. Furthermore, Al-based inclusions with high Al concentration are difficult to dissolve under harsh conditions, while Mg, Ca, and Mn-based inclusions dissolve under harsh conditions and are prone to acting as corrosion initiation sites. For this reason, inclusions with an Al concentration ratio of 0.2 or less to the total concentration of Al, Mg, Ca, and Mn are considered soluble inclusions that dissolve easily under harsh conditions, and the number density of soluble inclusions with a major axis of 1 μm or more is limited to 20.0 inclusions / mm². 2 The following limitations apply. Therefore, the number density of soluble inclusions with a major axis of 1 μm or more is 20.0 particles / mm². 2 By controlling the following, corrosion resistance can be ensured over a long period of time. From the viewpoint of stably ensuring this effect, the number density of soluble inclusions with a major axis of 1 μm or more is preferably 15.0 pieces / mm 2 More preferably, 10.0 pieces / mm 2 More preferably, 8.0 pieces / mm 2 The following applies. Note that the lower the number density of soluble inclusions with a major axis of 1 μm or larger, the better the corrosion resistance; therefore, there is no particular lower limit, and it should be 0 inclusions / mm². 2 This may also be the case. In this specification, soluble inclusions refer to inclusions that dissolve in water under harsh conditions, especially under conditions where the natural potential becomes noble. Specific examples of soluble inclusions include Mg-based oxides and sulfides, Ca-based oxides and sulfides, and Mn-based oxides and sulfides. Among these, Mn-based oxides and sulfides precipitate relatively in small numbers because they solid-solve in the austenite phase of duplex stainless steel, while Ca-based oxides and sulfides and Mg-based oxides and sulfides tend to precipitate in relatively large numbers.

[0039] Here, the number density of soluble inclusions with a major axis of 1 μm or larger can be determined by observing the surface of duplex stainless steel material with an SEM (scanning electron microscope). Specifically, the number density of soluble inclusions is determined as follows: First, the surface of the duplex stainless steel material is mirror-polished, and then the surface is observed with an SEM. In the backscattered electron image of the SEM, inclusions are distinguished by the difference in contrast, and inclusions with a major axis of 1 μm or larger are identified. Then, EDX analysis is performed on the identified inclusions with a major axis of 1 μm or larger to determine their composition (concentration of each element). Next, the ratio of the Al concentration to the total concentration of Al, Mg, Ca, and Mn is calculated, and those with a ratio of 0.2 or less are considered soluble inclusions, and their number is determined. Finally, by dividing the number of soluble inclusions obtained in this way by the area of ​​the observation region, the number density of soluble inclusions with a major axis of 1 μm or larger can be calculated.

[0040] In the duplex stainless steel material according to the embodiment of the present invention, from the viewpoint of corrosion resistance, the proportion of soluble inclusions among inclusions with a major axis of 1 μm or more is preferably 20.0% or less, more preferably 15.0% or less, and even more preferably 12.0% or less. By controlling the proportion of soluble inclusions among inclusions with a major axis of 1 μm or more to this range, the number of soluble inclusions that serve as corrosion initiation points is reduced, thereby improving corrosion resistance. The lower limit of the proportion of soluble inclusions among inclusions with a major axis of 1 μm or more is not particularly limited, as a lower proportion improves corrosion resistance, but it can be, for example, 1.0% or 2.0%. In this specification, the number ratio of soluble inclusions with a major axis of 1 μm or more refers to the ratio of the number density of soluble inclusions with a major axis of 1 μm or more to the number density of inclusions with a major axis of 1 μm or more (number density of soluble inclusions with a major axis of 1 μm or more / number density of inclusions with a major axis of 1 μm or more × 100).

[0041] The duplex stainless steel material according to the embodiment of the present invention has a Ti solid solution content of 0.002% by mass or more. By controlling the Ti solid solution content within this range, when the duplex stainless steel material is welded, the resistance to intergranular corrosion in the heat-affected zone (sensitization suppression effect) can be ensured and corrosion resistance can be improved. From the viewpoint of stably ensuring this effect, the Ti solid solution content is preferably 0.003% by mass or more, more preferably 0.004% by mass or more, and even more preferably 0.005% by mass or more. On the other hand, the upper limit of the Ti solid solution content is not particularly limited, but is preferably 0.200% by mass, more preferably 0.150% by mass, and even more preferably 0.100% by mass.

[0042] Here, the amount of Ti dissolved in duplex stainless steel is determined as follows. First, two 50 mm x 50 mm test pieces are cut from the duplex stainless steel, and the entire surface is wet-polished with #600 grit sandpaper. Next, one of these test pieces is used for component analysis to determine the Ti content. The component analysis is performed in accordance with JIS G1253:2002 "Iron and steel - Spark discharge emission spectrometry method". Next, the other test piece is electrolytically etched using the SPEED method. Electrolytic etching is performed in an electrolyte containing 10% acetylacetone at 400 mV until the electric charge reaches 5000 coulombs. After electrolytic etching, the electrolyte is filtered through a filter with a grid diameter of 0.05 μm to collect the precipitate. The mass of the precipitate is measured, and the amount of N contained in the precipitate (TiN) is determined by gas analysis. Then, the amount of Ti dissolved is calculated using the following formula. Ti solid solubility [mass%] = Ti content [g] - (N content in precipitate (TiN) [g] × 47.9 / 14)

[0043] In the embodiment of the present invention, the duplex stainless steel material preferably has a PREN value of 20.00 or higher, represented by formula (1). PREN = Cr + 3.3Mo + 16N ... (1) In the formula, each element symbol represents the content (mass%) of each element. Here, PREN (Pitting Resistance Equivalent Number) is one of the indicators that shows the corrosion resistance of duplex stainless steel material, and the higher the value, the better the corrosion resistance. In particular, if PREN is 20.00 or higher, it can be said that the material has excellent corrosion resistance under harsh conditions. From the viewpoint of stably improving corrosion resistance, PREN is more preferably 22.00 or higher, and even more preferably 24.00 or higher. The upper limit of PREN is not particularly limited, but for example, it is 50.00 or lower.

[0044] In the embodiment of the present invention, the duplex stainless steel material preferably has a pitting potential of 0.40 V vs SSE or higher, measured at 30°C. A pitting potential within this range indicates excellent corrosion resistance in harsh environments. From the viewpoint of stably ensuring corrosion resistance in harsh environments, a pitting potential of 0.42 V vs SSE or higher, measured at 30°C, is more preferably 0.45 V vs SSE or higher. Here, the pitting potential of the duplex stainless steel material can be measured by the method described later.

[0045] In the embodiment of the present invention, the duplex stainless steel material preferably has a pitting potential of 0.25 V vs SSE or higher when welded, measured at 30°C in the heat-affected zone. A pitting potential in the heat-affected zone within this range indicates excellent corrosion resistance. From the viewpoint of stably ensuring corrosion resistance in the heat-affected zone, it is more preferable that the pitting potential measured at 30°C in the heat-affected zone is 0.27 V vs SSE or higher, and even more preferable that it is 0.30 V vs SSE or higher. Here, the pitting potential in the heat-affected zone can be measured by the method described later.

[0046] The duplex stainless steel material according to the embodiment of the present invention has a Charpy impact value of 70 J / cm² measured at -20°C. 2It is preferable that the values ​​are as described above. A Charpy impact value within this range indicates excellent toughness in low-temperature environments. From the viewpoint of stably ensuring toughness in low-temperature environments, a Charpy impact value measured at -20°C of 75 J / cm² is preferable. 2 It is more preferable that the value be 80 J / cm² or higher. 2 It is even more preferable that the concentration be 90 J / cm² or higher. 2 The above is particularly preferable. Here, the Charpy impact value can be measured in accordance with JIS Z2242:2018. Specifically, the Charpy impact value can be measured by the method described later.

[0047] The type of duplex stainless steel material in the embodiment of the present invention is not particularly limited, but may be hot-rolled or cold-rolled, and the hot-rolled or cold-rolled material may be annealed and / or pickled. In the case of hot-rolled material, its thickness is generally 3 mm or more. In the case of cold-rolled material, its thickness is generally less than 3 mm.

[0048] The method for manufacturing duplex stainless steel according to the embodiment of the present invention is not particularly limited as long as it can produce duplex stainless steel having the above-described characteristics. Since the duplex stainless steel according to the embodiment of the present invention has a reduced number density of soluble inclusions that act as corrosion initiation points in harsh environments, at least a method for reducing the number density of soluble inclusions is employed. This method can be broadly divided into two parts. The first is to reduce the number of soluble inclusions (especially Mg-based and Ca-based oxides and sulfides) by controlling the S and O content in the molten stainless steel to low levels. Therefore, deoxidation is performed in the steelmaking process while reducing the S content. The second is to suppress the formation of soluble inclusions by reducing the content of Mg and Ca that form soluble inclusions. Here, Mg and Ca are added in the stainless steelmaking process when Mg and Ca derived from refractories mixed into the slag are reduced and dissolved by Al in the molten stainless steel. Therefore, by reducing the Al content and controlling the slag basicity (CaO / Al2O3 mass ratio), the dissolution of Mg and Ca into the slag is reduced, thereby suppressing the formation of soluble inclusions.

[0049] Furthermore, to suppress the decrease in corrosion resistance due to sensitization of the heat-affected zone when welding duplex stainless steel materials, Ti is added to ensure a sufficient amount of solid-solution Ti. When solid-solution Ti is heated during welding, it preferentially forms nitrides over Cr, thus suppressing sensitization. TiN tends to aggregate at high temperatures of 1500°C or higher during steelmaking, forming coarse precipitates. To suppress this, it is effective to pre-disperse and precipitate substances that can act as precipitation nuclei for TiN. This makes it possible to promote the precipitation of fine TiN. The substance that acts as a precipitation nucleus for TiN is Al oxide, which has a high melting point of 2000°C or higher. It is desirable to add Al to disperse and precipitate Al oxide, but excessive addition of Al leads to the growth and aggregation of Al oxide, and further TiN precipitation causes the formation of coarse precipitates, resulting in a significant decrease in toughness. Furthermore, Mg and Ca derived from refractories mixed into the slag are reduced and dissolved by Al in the molten steel, forming soluble inclusions that reduce the corrosion resistance of the duplex stainless steel material. For this reason, in order to ensure corrosion resistance in the heat-affected zone while maintaining toughness, it is necessary to appropriately adjust the balance of Al and Ti content. Based on the above considerations, an example of a typical manufacturing method for a duplex stainless steel material according to an embodiment of the present invention is described below.

[0050] An example of a method for producing a duplex stainless steel material according to the embodiment of the present invention is: C: 0.100% or less, Si: 1.00% or less, Mn: 6.00% or less, Ni: 9.00% or less, P: 0.050% or less, S: 0.0300% or less, Cr: 18.00 to 32.00%, Mo: 5.00% or less, Cu: 3.00% or less, Ti: 0.006 to 0.050%, Al: 0.005 to 0.100%, C In a steelmaking process for stainless steel having a composition containing a: 0.1000% or less, Mg: 0.1000% or less, N: 0.050 to 0.300%, with the remainder being Fe and impurities, the slag basicity (CaO / Al2O3 mass ratio) is controlled to 1.5 or less, and the time until the temperature drops to 1600°C or less after the addition of Ti and N (hereinafter referred to as "TiN generation time") is controlled to within 110 minutes. Specifically, Al deoxidation is performed on the stainless steel in the converter obtained by melting the raw materials, controlling the composition of the stainless steel and the slag basicity (CaO / Al2O3 mass ratio) to 1.5 or less, and controlling the TiN generation time to within 110 minutes. By controlling the slag basicity (CaO / Al2O3 mass ratio) to 1.5 or less, the activity of Mg and Ca in the slag is reduced, thereby suppressing the formation of soluble inclusions. Furthermore, by controlling the TiN formation time to within 110 minutes, excessive TiN precipitation is suppressed, thereby ensuring the desired amount of Ti solid solution. Specifically, since TiN precipitates in the high-temperature range (above 1600°C), excessive TiN precipitation can be suppressed by lowering the temperature to a temperature range where TiN does not precipitate (below 1600°C) within 110 minutes after adding Ti and N. Here, in this specification, "after adding Ti and N" means after both Ti and N have been added in the steelmaking process. Therefore, for example, if N is added after Ti is added, it means after N is added; if Ti is added after N is added, it means after Ti is added; and if Ti and N are added simultaneously, it means after Ti and N have been added.

[0051] Examples of equipment used in the steelmaking process include electric furnaces, AOD (argon oxygen decarburization furnaces), LF (ladle smelting furnaces), and CC (continuous casting machines). In the steelmaking process, the approximate composition is adjusted in the electric furnace and AOD, the composition is finely adjusted in the LF, and then casting is performed in the CC. N is added in the AOD by blowing in N2 gas or by adding nitrides. Ti is added in the electric furnace, AOD, or LF by adding Ti-containing components. However, if N and Ti are added to the molten steel and held for a long time, it will lead to excessive TiN precipitation and a corresponding decrease in the amount of Ti solid solution, as described above. Therefore, after adding Ti and N, it is necessary to keep the temperature below 1600°C, which is the temperature range in which TiN does not precipitate quickly. Accordingly, the TiN generation time should be within 110 minutes, preferably within 100 minutes.

[0052] The stainless steel obtained in the steelmaking process may further contain one or more elements selected from Nb: 0.100% or less, V: 1.00% or less, Co: 1.00% or less, Zr: 1.00% or less, W: 2.50% or less, REM: 0.100% or less, Sn: 0.100% or less, and B: 0.010% or less.

[0053] The method for manufacturing duplex stainless steel according to the embodiment of the present invention may further include a casting step, a hot rolling step, and an annealing step. Furthermore, after the annealing step, pickling and polishing treatment may be performed. The casting step is a step of casting the stainless steel obtained in the steelmaking step to obtain a cast slab. The hot rolling step is a hot rolling step of hot rolling the cast slab to obtain a hot-rolled material. The annealing step is an annealing step of annealing the hot-rolled material. The methods and conditions for the casting step, hot rolling step, and annealing step are not particularly limited and can be carried out in accordance with known methods. In the case of cold-rolled (cold-rolled annealed) duplex stainless steel, a cold rolling step and annealing treatment may be further performed after the annealing step.

[0054] The duplex stainless steel material according to the embodiment of the present invention can ensure corrosion resistance in harsh environments, particularly in environments with high chloride ion concentrations and nourishing natural potential, exhibits good corrosion resistance in the heat-affected zone even after welding, and has excellent toughness, making it suitable for various applications where these properties are required. Typically, the duplex stainless steel material according to the embodiment of the present invention can be used in various components for infrastructure applications such as sluice gates and dam facilities, and for plant applications such as industrial water piping and heat exchangers.

[0055] A welded structure according to an embodiment of the present invention includes a base material, a weld metal portion, and a heat-affected zone formed between the base material and the weld metal portion. The base material is the duplex stainless steel material described above. Figure 1 shows a schematic partially enlarged cross-sectional view of a welded structure according to an embodiment of the present invention. As shown in Figure 1, the welded structure (100) includes a base material (10), a weld metal portion (30), and a heat-affected zone (20) between the base material (10) and the weld metal portion (30). Here, "base material" means the portion that is not affected by welding. "Heat-affected zone" means the portion that is affected by heat but does not melt due to welding (also called HAZ). "Weld metal portion" means the portion that melts and resolidifies due to welding.

[0056] In the welded structure according to the embodiment of the present invention, it is preferable that the pitting potential of the heat-affected zone measured at 30°C is 0.25 V vs SSE or higher. A pitting potential of the heat-affected zone within this range indicates excellent corrosion resistance of the heat-affected zone. From the viewpoint of stably ensuring corrosion resistance of the heat-affected zone, it is more preferable that the pitting potential of the heat-affected zone measured at 30°C is 0.27 V vs SSE or higher, and even more preferable that it is 0.30 V vs SSE or higher. Here, the pitting potential of the heat-affected zone can be measured by the method described later.

[0057] A welded structure according to an embodiment of the present invention can be manufactured by using the above-mentioned duplex stainless steel material and welding the duplex stainless steel material. The welding may involve welding multiple duplex stainless steel materials together, or welding the duplex stainless steel material to a metal material of another material. The welding method is not particularly limited, and methods known in the art, such as arc welding (TIG welding, etc.), electron beam welding, laser welding, plasma arc welding, and spot welding, can be used. Furthermore, filler material may or may not be used in the welding. The welding conditions can be appropriately adjusted according to the type of welding and the composition of the duplex stainless steel material, and are not particularly limited.

[0058] The welded structure according to the embodiment of the present invention uses the above-mentioned duplex stainless steel material, and therefore exhibits good corrosion resistance in the base material and heat-affected zone, as well as excellent toughness, especially in harsh environments where the chloride ion concentration is high and the natural potential is noble.

[0059] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

[0060] Stainless steel having the composition shown in Table 1 (the remainder being Fe and impurities) was melted and refined. Specifically, Al deoxidation was performed on the stainless steel in the converter obtained by melting the raw materials, and the composition of the stainless steel and the slag basicity (CaO / Al2O3 mass ratio) were controlled to those shown in Table 1. In addition, Ti and N were added during refining, and the TiN formation time was controlled to those shown in Table 2. Next, the stainless steel was cast to obtain a slab, which was then hot-rolled to obtain a hot-rolled sheet with a thickness of 12 mm. Hot rolling was performed by heating at 1100-1250°C for 2 hours, followed by finish rolling at a finishing temperature of 900°C, and then water-cooling from a temperature of 800°C or higher. Next, the obtained hot-rolled sheet was annealed at 1000-1100°C, and then pickled by immersion in hydrofluoric acid. Next, it was polished using Si abrasive paper to obtain a duplex stainless steel sheet. The polishing was to a grit of 600 (#600).

[0061]

[0062] The following evaluations were performed on the duplex stainless steel sheets obtained above.

[0063] (Number density of inclusions with a major axis of 1 μm or more) After mirror polishing the surface of a duplex stainless steel plate, the surface was analyzed using an FE-SEM (Explorer 4, Thermo Fisher Scientific: automated particle analysis SEM) to identify inclusions with a major axis of 1 μm or more and to determine their number. In this analysis, the analysis magnification was 800x, one observation area was 125 μm × 125 μm, and the total observation area was 31.38 mm. 2 The settings were adjusted accordingly. The number density was calculated by dividing the number of inclusions with a major axis of 1 μm or more by the area of ​​the observation region. In the following, the number density of inclusions with a major axis of 1 μm or more will be abbreviated as "number density of inclusions".

[0064] (Number density of soluble inclusions with a major axis of 1 μm or more, and where the ratio of Al concentration to the total concentration of Al, Mg, Ca, and Mn is 0.2 or less) After mirror polishing the surface of a duplex stainless steel plate, the surface was examined using an FE-SEM (Explorer 4, Thermo Fisher Scientific: automated particle analysis SEM) to identify inclusions with a major axis of 1 μm or more and to perform EDX analysis of those inclusions. In this analysis, the analysis magnification was 800x, one observation area was 125 μm × 125 μm, and the total observation area was 31.38 mm. 2 The parameters were set to the specified values. The number of soluble inclusions that were 1 μm or larger in length and whose Al concentration ratio to the total concentration of Al, Mg, Ca, and Mn was 0.2 or less was determined, and the number density was calculated by dividing this number by the area of ​​the observation region. In the following, the number density of soluble inclusions that are 1 μm or larger in length and whose Al concentration ratio to the total concentration of Al, Mg, Ca, and Mn is 0.2 or less will be abbreviated as "number density of soluble inclusions".

[0065] (Percentage of soluble inclusions in inclusions with a major axis of 1 μm or more) Using the obtained number density of soluble inclusions with a major axis of 1 μm or more and the number density of inclusions, the percentage of soluble inclusions in inclusions with a major axis of 1 μm or more was calculated using the formula described above.

[0066] (Ti solid solubility) Two 50 mm x 50 mm test pieces were cut from a duplex stainless steel plate, and the entire surface was wet-polished with #600 grit sandpaper. Next, a component analysis was performed on one of these test pieces to determine the Ti content. The component analysis was performed in accordance with JIS G1253:2002 "Iron and steel - Spark discharge emission spectrometry method". Next, the other test piece was electrolytically etched using the SPEED method. Electrolytic etching was performed using constant potential electrolysis at 400 mV in an electrolyte containing 10% acetylacetone until the electric charge reached 5000 coulombs. After electrolytic etching, the electrolyte was filtered through a filter with a grid diameter of 0.05 μm to collect the precipitate. The mass of the precipitate was measured, and the N content in the precipitate (TiN) was determined by gas analysis. Then, the Ti solid solubility was calculated using the above formula.

[0067] (Corrosion resistance of duplex stainless steel sheet (base material): Pitting corrosion potential of duplex stainless steel sheet measured at 30°C) A 20 mm x 15 mm test specimen was prepared by shearing a duplex stainless steel sheet. Next, a wire was spot-welded to one end of this test specimen, and the portion other than the 10 mm x 10 mm test surface was covered with silicone resin. Then, the test surface of the test specimen was completely immersed in the test solution and left for 10 minutes, after which a potentiometric test was performed using a potentiostat. A 3.5 mass% NaCl aqueous solution was used as the test solution, and Ar was degassed at 30°C. The test was performed with a potential sweep rate of 20 mV / min, and an anode current density of 500 μA / cm² from the natural electrode potential. 2 The potential was measured until it reached a certain level, and an anodic polarization curve was obtained. The pitting potential was 100 μA / cm² on the anodic polarization curve. 2 The most noble value among the corresponding potentials was defined as the pitting potential. An Ag / AgCl electrode with saturated KCl as the internal solution was used as the reference electrode.

[0068] (Corrosion resistance of the heat-affected zone: Pitting corrosion potential measured at 30°C in the heat-affected zone) Two test pieces measuring 133 mm (rolling direction) x 65 mm (width direction) were cut from a duplex stainless steel plate, and 1 mm was ground on both sides to a thickness of 10 mm. Next, a 30° bevel was made on one end of the two test pieces in the rolling direction, and then a welded joint (welded structure) was fabricated by FCAW (flux-cored arc welding). Stainless steel arc welding flux-cored wire WEL FCW 329J4L (manufactured by Nippon Welding Rod Co., Ltd.) was used as the welding material, and CO2 was used as the shielding gas. After grinding 1 mm off the welded side surface of the obtained welded joint, a test piece measuring 15 mm (rolling direction) x 20 mm (width direction) was cut out. Next, a wire was spot-welded to one end of this test piece, and the part other than the 10 mm x 10 mm test surface was covered with silicone resin. The test surface was positioned so that the boundary between the weld metal and the duplex stainless steel plate was located in the center. Next, the test surface of the test piece was completely immersed in the test solution and left for 10 minutes, after which the test was performed using the potentiometric method with a potentiostat. The test solution and test conditions were the same as those used for the pitting potential measured at 30°C for the duplex stainless steel plate described above.

[0069] (Toughness: Charpy impact value measured at -20°C) After cutting a duplex stainless steel sheet to a thickness of 10 mm, a V-notch test specimen was prepared as specified in JIS Z2242:2018. At this time, the V-notch was made perpendicular to the rolling direction. Subsequently, a Charpy impact test was performed at -20°C in accordance with JIS Z2242:2018. The Charpy impact test was performed three times, and the average result was used.

[0070] The results of each of the above evaluations are shown in Table 2.

[0071]

[0072] As shown in Table 2, the duplex stainless steel sheets of Examples 1 to 10 had the predetermined composition, and the number density of inclusions and soluble inclusions, as well as the amount of Ti solid solution, were within the predetermined range. As a result, the duplex stainless steel sheets (base material) and heat-affected zone exhibited excellent corrosion resistance and good toughness. In contrast, the duplex stainless steel sheet of Comparative Example 1 had too little Ti, resulting in a low amount of Ti solid solution and insufficient corrosion resistance in the heat-affected zone. The duplex stainless steel sheet of Comparative Example 2 had too much Ti, resulting in a high number density of inclusions and insufficient toughness. The duplex stainless steel sheet of Comparative Example 3 had too much slag basicity (CaO / Al2O3 mass ratio) during the steelmaking process, resulting in a high number density of soluble inclusions and insufficient corrosion resistance in the base material and heat-affected zone. The duplex stainless steel sheet of Comparative Example 4 had too much sulfur, resulting in a high number density of soluble inclusions and insufficient corrosion resistance in the base material and heat-affected zone. The duplex stainless steel sheet of Comparative Example 5 had too much Al content, resulting in a high number density of inclusions and thus insufficient toughness. The duplex stainless steel sheet of Comparative Example 6 had too little Al content, resulting in a high number density of inclusions and thus insufficient toughness. The duplex stainless steel sheet of Comparative Example 7 had too little Ti solid solution content, resulting in insufficient corrosion resistance in the heat-affected zone.

[0073] As can be seen from the above results, the present invention provides a ferritic-austenitic duplex stainless steel material that can ensure corrosion resistance in harsh environments, particularly in environments with high chloride ion concentration and a noble natural potential, exhibits good corrosion resistance in the heat-affected zone even after welding, and has excellent toughness. Furthermore, the present invention provides a welded structure that exhibits good corrosion resistance in the base material and heat-affected zone, and has excellent toughness, in harsh environments, particularly in environments with high chloride ion concentration and a noble natural potential.

[0074] Furthermore, based on the above results, the present invention can be provided in the following embodiments.

[0075] (Aspect 1) The composition, by mass, consists of C: 0.100% or less, Si: 1.00% or less, Mn: 6.00% or less, Ni: 9.00% or less, P: 0.050% or less, S: 0.0300% or less, Cr: 18.00 to 32.00%, Mo: 5.00% or less, Cu: 3.00% or less, Ti: 0.006 to 0.050%, Al: 0.005 to 0.100%, Ca: 0.1000% or less, Mg: 0.1000% or less, N: 0.050 to 0.300%, with the remainder being Fe and impurities, and the number density of inclusions with a major axis of 1 μm or more is 250.0 pieces / mm 2 The following conditions apply: Among the inclusions, the number density of soluble inclusions where the ratio of Al concentration to the total concentration of Al, Mg, Ca, and Mn is 0.2 or less is 20.0 particles / mm³. 2 The following is a ferritic-austenitic duplex stainless steel material having a Ti solid solution content of 0.002% by mass or more.

[0076] (Aspect 2) The ferritic-austenitic duplex stainless steel material according to Aspect 1, further comprising one or more selected by mass from Nb: 0.100% or less, V: 1.00% or less, Co: 1.00% or less, Zr: 1.00% or less, W: 2.50% or less, REM: 0.100% or less, Sn: 0.100% or less, and B: 0.010% or less.

[0077] (Aspect 3) A ferritic-austenitic duplex stainless steel material according to Aspect 1 or Aspect 2, wherein PREN, represented by the following formula (1), is 20.0 or more. PREN = Cr + 3.3Mo + 16N ... (1) In the formula, each element symbol represents the content (mass %) of each element.

[0078] (Aspect 4) A ferritic-austenitic duplex stainless steel material according to any one of aspects 1 to 3, wherein the pitting potential measured at 30°C is 0.40 V vs SSE or higher.

[0079] (Aspect 5) The ferritic-austenitic duplex stainless steel material according to any one of aspects 1 to 4, wherein when the ferritic-austenitic duplex stainless steel material is welded, the pitting potential measured at 30°C in the heat-affected zone is 0.25 V vs SSE or higher.

[0080] (Aspect 6) Charpy impact value measured at -20°C is 70 J / cm 2 The above describes the ferritic-austenitic duplex stainless steel material according to any one of embodiments 1 to 5.

[0081] (Aspect 7) A ferritic-austenitic duplex stainless steel material according to any one of aspects 1 to 6, wherein the number ratio of soluble inclusions with a major axis of 1 μm or more is 20.0% or less.

[0082] (Aspect 8) A welded structure comprising a base material, a weld metal portion, and a heat-affected zone formed between the base material and the weld metal portion, wherein the base material is a ferritic-austenitic duplex stainless steel material according to any one of aspects 1 to 7.

[0083] (Aspect 9) The welded structure according to aspect 8, wherein the pitting potential measured at 30°C in the heat-affected zone is 0.25 V vs SSE or higher.

[0084] Furthermore, instead of embodiments 1 and 2, the following embodiment 10 may be used. (Embodiment 10) By mass, C: 0.100% or less, Si: 1.00% or less, Mn: 6.00% or less, Ni: 9.00% or less, P: 0.050% or less, S: 0.0300% or less, Cr: 18.00 to 32.00%, Mo: 5.00% or less, Cu: 3.00% or less, Ti: 0.006 to 0.050%, Al: 0.005 to 0.100%, Ca: 0. The composition consists of 1000% or less of the following: Mg: 0.1000% or less, N: 0.050-0.300%, Nb: 0-0.100%, V: 0-1.00%, Co: 0-1.00%, Zr: 0-1.00%, W: 0-2.50%, REM: 0-0.100%, Sn: 0-0.100%, B: 0-0.010%, with the remainder being Fe and impurities. The number density of inclusions with a major axis of 1 μm or more is 250.0 pieces / mm². 2 The following conditions apply: Among the inclusions, the number density of soluble inclusions where the ratio of Al concentration to the total concentration of Al, Mg, Ca, and Mn is 0.2 or less is 20.0 particles / mm³. 2 The following is a ferritic-austenitic duplex stainless steel material having a Ti solid solution content of 0.002% by mass or more.

Claims

1. The composition, by mass, consists of C: 0.100% or less, Si: 1.00% or less, Mn: 6.00% or less, Ni: 9.00% or less, P: 0.050% or less, S: 0.0300% or less, Cr: 18.00 to 32.00%, Mo: 5.00% or less, Cu: 3.00% or less, Ti: 0.006 to 0.050%, Al: 0.005 to 0.100%, Ca: 0.1000% or less, Mg: 0.1000% or less, N: 0.050 to 0.300%, with the remainder being Fe and impurities, and the number density of inclusions with a major axis of 1 μm or more is 250.0 pieces / mm². 2 The following conditions apply: Among the inclusions, the number density of soluble inclusions where the ratio of Al concentration to the total concentration of Al, Mg, Ca, and Mn is 0.2 or less is 20.0 particles / mm³. 2 The following is a ferritic-austenitic duplex stainless steel material having a Ti solid solution content of 0.002% by mass or more.

2. The ferritic-austenitic duplex stainless steel material according to claim 1, further comprising one or more elements selected by mass from Nb: 0.100% or less, V: 1.00% or less, Co: 1.00% or less, Zr: 1.00% or less, W: 2.50% or less, REM: 0.100% or less, Sn: 0.100% or less, and B: 0.010% or less.

3. A ferritic-austenitic duplex stainless steel material according to claim 1 or 2, wherein the PREN represented by the following formula (1) is 20.00 or more. PREN = Cr + 3.3Mo + 16N ... (1) In the formula, each element symbol represents the content (mass%) of each element.

4. A ferrite-austenitic duplex stainless steel material according to any one of claims 1 to 3, wherein the pitting potential measured at 30°C is 0.40 V vs SSE or higher.

5. The ferritic-austenitic duplex stainless steel material according to any one of claims 1 to 4, wherein when the ferritic-austenitic duplex stainless steel material is welded, the pitting potential measured at 30°C in the heat-affected zone is 0.25 V vs SSE or higher.

6. The Charpy impact value measured at -20°C is 70 J / cm². 2 The ferritic-austenitic duplex stainless steel material described in any one of claims 1 to 5.

7. The ferritic-austenitic duplex stainless steel material according to any one of claims 1 to 6, wherein the number ratio of soluble inclusions with a major axis of 1 μm or more is 20.0% or less.

8. A welded structure comprising a base material, a weld metal portion, and a heat-affected zone formed between the base material and the weld metal portion, wherein the base material is a ferritic-austenitic duplex stainless steel material according to any one of claims 1 to 7.

9. The welded structure according to claim 8, wherein the pitting potential measured at 30°C in the heat-affected zone is 0.25 V vs SSE or greater.

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