Ferritic-austenitic duplex stainless steel material

Abstract

Disclosed is a ferritic-austenitic duplex stainless steel material which has a composition that contains, 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, 0.050-0.300% of N, 0.005-0.100% of Al, 0.1000% or less of Ca, and 0.1000% or less of Mg, with the balance being made up of Fe and impurities. In the ferritic-austenitic duplex stainless steel material, the number density of soluble inclusions in each of which the length is 1 µm or more and the proportion of the Al concentration to the total concentration of Al, Mg, Ca, and Mn is 0.2 or less is 10.0 per mm2 or less.

Description

Ferritic-austenitic duplex stainless steel

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

[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 chromium, 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. As conventional duplex stainless steel materials with excellent corrosion resistance, duplex stainless steel materials having a predetermined composition and controlling the number of various inclusions have been proposed (for example, Patent Documents 1 to 6).

[0003] 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. 2017-95794, Japanese Unexamined Patent Publication No. 2015-110828

[0004] Various microorganisms exist in environmental waters such as rivers and oceans. Furthermore, microorganisms can proliferate in the water used in plant facilities if the environmental conditions are suitable. For this reason, 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 formed on its surface is more susceptible to pitting corrosion than duplex stainless steel immersed in saltwater because the natural potential (corrosion potential) becomes more noble due to the activity of microorganisms in the biofilm.

[0005] The corrosion initiation points of duplex stainless steel materials under the above-mentioned environmental conditions have not been clearly identified, and therefore, in order to ensure sufficient corrosion resistance, it is necessary to excessively increase the content of Cr and Ni. Furthermore, although it is known that inclusions such as MnS tend to act as corrosion initiation points in general stainless steel materials, the type of inclusion that acts as a corrosion initiation point differs in environments where the chloride ion concentration is high and the spontaneous potential is noble. In fact, although the conventional duplex stainless steel materials described above control the number of various inclusions, the inclusions that act as corrosion initiation points in environments with high chloride ion concentration and noble spontaneous potential have not been clarified, and there is room for improvement.

[0006] This 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 over a long period of time in harsh environments, particularly in environments with high chloride ion concentrations and favorable natural potentials.

[0007] The inventors of this invention have conducted extensive research on ferritic-austenitic duplex stainless steel materials and have found that certain soluble inclusions act as corrosion initiation points in environments with high chloride ion concentrations and noble spontaneous potential. The inventors have then discovered that by controlling the composition and the number density of these soluble inclusions, it is possible to obtain a ferritic-austenitic duplex stainless steel material that can ensure long-term corrosion resistance in environments with high chloride ion concentrations and noble spontaneous potential. Furthermore, the inventors have found that such ferritic-austenitic duplex stainless steel materials can be manufactured by controlling the slag basicity during the stainless steel melting process. This invention was completed against this background.

[0008] In other words, the present invention relates to a material having a composition by mass 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, N: 0.050 to 0.300%, Al: 0.005 to 0.100%, Ca: 0.1000% or less, Mg: 0.1000% or less, with the remainder being Fe and impurities, having a major axis of 1 μm or more, and a number density of soluble inclusions of 0.2 or less, where the ratio of Al concentration to the total concentration of Al, Mg, Ca, and Mn is 0.2 or less. 2 The following pertains to ferritic-austenitic duplex stainless steel materials.

[0009] According to the present invention, it is possible to provide a ferritic-austenitic duplex stainless steel material that can ensure corrosion resistance over a long period of time in harsh environments, particularly in environments with high chloride ion concentrations and favorable natural potentials.

[0010] 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.

[0011] In this specification, numerical ranges expressed using "~" mean a range that includes the numbers before and after "~" as the lower and upper limits, respectively, unless otherwise specified. Furthermore, in this specification, numerical ranges preceded by "greater than" or "less than" mean a range that does not include the number as the lower or upper limit. Also, for numerical ranges described in steps in this specification, the upper limit of one step may be replaced with the upper limit of another step described numerical range or the value shown in the example. Similarly, for numerical ranges described in steps in this specification, the lower limit of one step may be replaced with the lower limit of another step described numerical range or the value shown in the example. Furthermore, a numerical range may be a combination of any of the upper and lower limits described in this specification. In this specification, "%" in relation to ingredients means "mass%" unless otherwise specified.

[0012] 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, N: 0.050 to 0.300%, Al: 0.005 to 0.100%, Ca: 0.1000% or less, Mg: 0.1000% or less, with the remainder being Fe and impurities.

[0013] Herein, "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. The cross-sectional shape may also be various types of steel, such as T-shapes and I-shapes. Furthermore, "ferritic-austenitic" in this specification means a material whose metallic structure at room temperature consists mainly of two phases: a ferrite phase and an austenite phase. Therefore, "ferritic-austenitic" also includes materials that contain phases other than the ferrite and austenite phases (for example, a martensite phase). Furthermore, "impurities" in this specification means components that are mixed in during the industrial production of stainless steel material due to raw materials such as ore and scrap, and various factors in the manufacturing process, and are acceptable as long as they do not adversely affect the present invention. For example, unavoidable impurities are also included in impurities.Regarding the content of each element, "containing xx% or less" means that it is xx% or less, but contains more than 0% (especially above the impurity level).

[0014] The duplex stainless steel material according to the embodiment of the present invention may further contain, if necessary, one or more elements selected from Ti: 0.100% or less, Nb: 0.100% or less, V: 1.00% or less, W: 1.00% or less, Co: 1.00% or less, Zr: 1.00% 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.

[0015] <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%. Therefore, the C content can be within the range having the above upper and lower limits, for example, 0.001 to 0.100%.

[0016] <Si: 1.00% or less> 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 since Si is an effective element for improving the corrosion resistance of duplex stainless steel material, it is preferably 0.01%, more preferably 0.05%, and even more preferably 0.10%. Accordingly, the Si content can be in the range having the above upper and lower limits, for example, 0.01 to 1.00%.

[0017] <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%.

[0018] <Ni: 9.00% or less> If the Ni content is too high, the ratio of the ferrite phase and the 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 since Ni is an element that improves the corrosion resistance (especially crevice corrosion resistance) of duplex stainless steel materials, 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%.

[0019] <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%.

[0020] <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%.

[0021] <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 of the above upper and lower limits.

[0022] <Mo: 5.00% or less> If the Mo content is too high, it leads to a decrease in processability 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 since Mo is an effective element for improving the corrosion resistance and oxidation resistance of duplex stainless steel materials, 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%.

[0023] <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%.

[0024] <N: 0.050-0.300%> N is an effective element for 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.

[0025] <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 noble 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 0.080%, and more preferably 0.070%. On the other hand, the lower limit of the Al content is controlled to 0.005%, preferably 0.010%, and more preferably 0.015%, from the viewpoint of obtaining the effects of Al. Therefore, the Al content can be within the range having the above upper and lower limits.

[0026] <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%.

[0027] <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%.

[0028] <Ti: 0.100% or less, Nb: 0.100% or less> Ti and Nb are elements that affect the intergranular corrosion resistance (sensitization suppression effect) of duplex stainless steel materials, but they are not particularly essential elements, and the content of these elements may be 0%. Also, Ti and Nb are effective elements for immobilizing C and N. However, if the content of Ti and Nb is too high, in addition to the increase in manufacturing costs, the workability and surface quality of the duplex stainless steel material will decrease. For this reason, the upper limit of the Ti and Nb content is controlled to 0.100%, preferably 0.080%, and more preferably 0.050%, respectively. Accordingly, the Ti and Nb content may be 0 to 0.100%, respectively. On the other hand, the lower limit of the Ti and Nb content is not particularly limited, but from the viewpoint of obtaining the effects of these elements, it is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%. Therefore, from the viewpoint of obtaining the effects of these elements, the content of Ti and Nb is within the range having the above-mentioned upper and lower limits, for example, 0.001 to 0.100%.

[0029] <V: 1.00% or less, W: 1.00% or less, Co: 1.00% or less, Zr: 1.00% or less> V, W, Co, and Zr are effective elements for improving the oxidation resistance of duplex stainless steel materials, but they are not particularly essential elements, and their content 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 costs. For this reason, the upper limit of the content of these elements is controlled to 1.00%, preferably 0.80%, and more preferably 0.60%. Accordingly, the content of V, W, Co, and Zr may be 0 to 1.00% each. On the other hand, the lower limit 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%, and even more preferably 0.03%. Therefore, from the viewpoint of obtaining the effects of these elements, the content of V, W, Co, and Zr should be within the range of the above-mentioned upper and lower limits, for example, 0.01 to 1.00%.

[0030] <REM: 0.100% or less> REM (rare earth elements) are effective in improving the oxidation resistance of duplex stainless steel materials, but they are not particularly essential elements, and their content may be 0%. However, if the REM content is too high, it will lead to an increase in manufacturing costs. Therefore, the upper limit of the REM content is controlled to 0.100%, preferably 0.080%, and more preferably 0.050%. Accordingly, the REM content may be 0 to 0.100%. On the other hand, the lower limit of the REM content is not particularly limited, but from the viewpoint of obtaining the effect of REM, it is preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%. Accordingly, from the viewpoint of obtaining the effect of REM, the REM content is within the range having the above upper and lower limits, for example, 0.001 to 0.100%. REM refers to the two elements scandium (Sc) and yttrium (Y), and the 15 elements from lanthanum (La) to lutetium (Lu) (lanthanides). These can be used individually or as mixtures.

[0031] <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 may 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 is within the range having the above upper and lower limits, for example, 0.001 to 0.100%.

[0032] <B: 0.010% or less> B is an element effective in improving the secondary workability of the duplex stainless steel material, but is not an essential element, and its content may be 0%. However, if the content of B is too high, it will cause a decrease in fatigue strength. Therefore, the upper limit value of the content of B is controlled to 0.010%, preferably 0.008%. Therefore, the content of B may be 0 to 0.010%. On the other hand, the lower limit value of the content of B is not particularly limited, but from the viewpoint of obtaining the effect of B, it is preferably 0.001%. Therefore, from the viewpoint of obtaining the effect of B, the content of B is in the range having the above upper limit value and lower limit value, for example, 0.001 to 0.010%.

[0033] The duplex stainless steel material according to the embodiment of the present invention has a major axis of 1 μm or more, and the number density of soluble inclusions having a ratio of Al concentration to the total concentration of Al, Mg, Ca, and Mn of 0.2 or less is 10.0 pieces / mm 2 or less. The reason for limiting the number density of such soluble inclusions is as follows. Inclusions with a major axis of 1 μm or more are likely to act as corrosion initiation points because they form pits when dissolved in a severe environment (especially an environment with a high chloride ion concentration and a noble natural potential). In addition, Al-based inclusions with a high Al concentration are difficult to dissolve in a severe environment, while Mg, Ca, and Mn-based inclusions dissolve in a severe environment and are likely to act as corrosion initiation points. Therefore, inclusions with a ratio of Al concentration to the total concentration of Al, Mg, Ca, and Mn of less than 0.2 are defined as soluble inclusions that are easily dissolved in a severe environment, and the number density of soluble inclusions with a major axis of 1 μm or more is limited to 10.0 pieces / mm 2 or less. Therefore, by controlling the number density of this soluble inclusion to 10.0 pieces / mm 2 or less, corrosion resistance can be ensured over a long period. From the viewpoint of stably ensuring this effect, the number density of soluble inclusions is preferably 9.0 pieces / mm 2 or less, more preferably 8.0 pieces / mm 2 or less, and even more preferably 7.0 pieces / mm 2 or less.The following applies. Note that the lower the number density of soluble inclusions, the better the corrosion resistance, so there is no particular lower limit. In this specification, soluble inclusions refer to inclusions that dissolve in water under harsh conditions, particularly under conditions where the natural potential is 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.

[0034] Here, the number density of soluble inclusions can be determined by observing the surface of duplex stainless steel material with a scanning electron microscope (SEM). Specifically, the number density of soluble inclusions can be 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, the number of soluble inclusions obtained in this way can be divided by the area of ​​the observation region to calculate the number density of soluble inclusions. The major axis of an inclusion refers to the length of the longest straight line obtained by connecting two points on the outer perimeter of the inclusion in the backscattered electron image obtained by SEM.

[0035] The duplex stainless steel material according to an embodiment of the present invention preferably has a PREN represented by the formula (1) of 20.00 or more. 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 indexes indicating the corrosion resistance of the duplex stainless steel material, and it can be said that the higher the value, the better the corrosion resistance. In particular, if PREN is 20.00 or more, it can be said that the corrosion resistance is excellent in a severe environment. From the viewpoint of stably enhancing the corrosion resistance, PREN is more preferably 22.00 or more, and still more preferably 25.00 or more. The upper limit of PREN is not particularly limited, but for example, it is 50.00 or less.

[0036] The duplex stainless steel material according to an embodiment of the present invention preferably has a number density of inclusions having a major diameter of 1 μm or more of 200 pieces / mm 2 or less. Here, the inclusions that affect the toughness in a low-temperature environment include the above-mentioned soluble inclusions and other inclusions. The duplex stainless steel material used for infrastructure applications and plant applications is often used outdoors, and it is required to ensure toughness in the low-temperature environment in winter. Since the toughness in a low-temperature environment tends to decrease as the number density of inclusions having a major diameter of 1 μm or more increases, by controlling the number density of inclusions having a major diameter of 1 μm or more to 200 pieces / mm 2 or less, the toughness in a low-temperature environment can be ensured. From the viewpoint of stably ensuring the toughness in a low-temperature environment, the number density of inclusions having a major diameter of 1 μm or more is more preferably 150 pieces / mm 2 or less, and still more preferably 100 pieces / mm 2 or less. The lower limit of the number density of inclusions having a major diameter of 1 μm or more is not particularly limited because the lower the number density, the better the toughness in a low-temperature environment.

[0037] The number density of inclusions with a major axis of 1 μm or more can be determined by observing the surface of duplex stainless steel material with a scanning electron microscope (SEM), similar to the number density of soluble inclusions. Specifically, the number density of inclusions with a major axis of 1 μm or more can be 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, inclusions with a major axis of 1 μm or more are identified, and their number is determined. Then, by dividing the number of inclusions obtained in this way by the area of ​​the observation region, the number density of inclusions with a major axis of 1 μm or more can be calculated.

[0038] 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, the pitting potential measured at 30°C is more preferably 0.45 V vs SSE or higher, even more preferably 0.50 V vs SSE or higher, and particularly preferably 0.70 V vs SSE or higher. Here, the pitting potential can be measured by the method described later.

[0039] 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. 2 It 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 should be 80 J / cm². 2 It is more preferable that the concentration be greater than or equal to 100 J / cm². 2 It is even more preferable that the above is true, and 110 J / cm 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.

[0040] In the duplex stainless steel material according to the embodiment of the present invention, from the viewpoint of ensuring corrosion resistance over a long period of time in harsh environments, particularly in environments with high chloride ion concentration and a noble natural potential, it is preferable that the number ratio of soluble inclusions with a major axis of 1 μm or more is 20.0% or less. By controlling the number ratio of soluble inclusions, which are likely to act as corrosion initiation sites, to 20.0% or less among inclusions with a major axis of 1 μm or more, in other words, by controlling the number ratio of Al-based inclusions, which are less likely to act as corrosion initiation sites, to more than 80.0%, corrosion resistance in harsh environments can be ensured over a long period of time. 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 proportions of ferrite phase (α) and austenite phase (γ) in the duplex stainless steel material according to the embodiment of the present invention are not particularly limited, but the total proportion of ferrite and austenite phases is, for example, 90% or more, and typically 95% or more. Furthermore, the proportion of ferrite phase in the duplex stainless steel material is not particularly limited, but is preferably 35 to 65%, more preferably 40 to 60%, and even more preferably 45 to 55%. The proportions of ferrite and austenite phases can be measured by magnetic induction. For example, they can be measured using a ferrite scope manufactured by Helmut Fischer GmbH or a SEM equipped with an OIM (orientation imaging microscopy) system.

[0042] The type of duplex stainless steel material in the embodiment of the present invention is not particularly limited, but may be hot-rolled (hot-rolled and annealed) or cold-rolled (cold-rolled and annealed). In the case of hot-rolled (hot-rolled and annealed) material, its thickness is generally 3 mm or more. In the case of cold-rolled (cold-rolled and annealed) material, its thickness is generally less than 3 mm.

[0043] 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 serve 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 melting 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 molten stainless steel when Mg and Ca derived from refractories mixed into the slag during the stainless steel melting process dissolve into the molten stainless steel. Therefore, by controlling the slag basicity (CaO / Al2O3 mass ratio), the activity of Mg and Ca in the slag is reduced, thereby suppressing the formation of soluble inclusions. Based on the above, an example of a typical manufacturing method for a duplex stainless steel material according to an embodiment of the present invention is described below.

[0044] An example of a method for producing a duplex stainless steel material according to an embodiment of the present invention includes controlling the slag basicity (CaO / Al2O3 mass ratio) to 1.5 or less in a process of melting stainless steel having 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, N: 0.050 to 0.300%, Al: 0.005 to 0.100%, Ca: 0.1000% or less, Mg: 0.1000% or less, with the remainder being Fe and impurities. Specifically, Al deoxidation is performed on the stainless steel obtained in the converter by melting the raw materials, and the composition of the stainless steel and the slag basicity (CaO / Al2O3 mass ratio) are controlled as described above. 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.

[0045] The stainless steel obtained in the melting process may further contain one or more elements selected from Ti: 0.100% or less, Nb: 0.100% or less, V: 1.00% or less, W: 1.00% or less, Co: 1.00% or less, Zr: 1.00% or less, REM: 0.100% or less, Sn: 0.100% or less, and B: 0.010% or less.

[0046] 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 melting 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 sheet. The annealing step is an annealing step of annealing the hot-rolled sheet. The methods and conditions of the casting step, hot rolling step, and annealing step are not particularly limited and can be carried out in accordance with known methods. In addition, when the duplex stainless steel material is to be cold-rolled (cold-rolled and annealed) material, a cold rolling step and annealing treatment may be further performed after the annealing step.

[0047] The duplex stainless steel material according to the embodiment of the present invention can ensure corrosion resistance over long periods of time in harsh environments, particularly in environments with high chloride ion concentrations and favorable natural potentials, and can therefore be used in 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.

[0048] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

[0049] Stainless steel having the composition shown in Table 1 (the remainder being Fe and impurities) was melted down. 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. 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).

[0050]

[0051] The following evaluations were performed on the duplex stainless steel sheets obtained above.

[0052] (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 material, 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 approximately 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".

[0053] (Number density of inclusions with a major axis of 1 μm or more) After mirror polishing the surface of a duplex stainless steel material, 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 approximately 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".

[0054] (Percentage of soluble inclusions with a major axis of 1 μm or more) Using the number density of soluble inclusions and the number density of inclusions obtained by the method described above, the percentage of soluble inclusions with a major axis of 1 μm or more was calculated by dividing the number density of soluble inclusions by the number density of inclusions.

[0055] (Corrosion resistance: Pitting corrosion potential measured at 30°C) A 20 mm x 15 mm test specimen was prepared by shearing a duplex stainless steel material. 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 the 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.

[0056] (Toughness: Charpy impact value measured at -20°C) After cutting a duplex stainless steel material 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.

[0057] The results of each of the above evaluations are shown in Table 2.

[0058]

[0059] As shown in Table 2, the duplex stainless steel sheets of Examples 1 to 9 have a predetermined composition and a number density of soluble inclusions of 10.0 pieces / mm². 2 The duplex stainless steel sheets of Examples 1 to 9 exhibited excellent corrosion resistance and good toughness. Furthermore, the number of soluble inclusions among inclusions with a major axis of 1 μm or more was 20.0% or less. In contrast, the duplex stainless steel sheets of Comparative Examples 1 and 3 had too much sulfur content, resulting in a high number density of soluble inclusions and reduced corrosion resistance. The number of soluble inclusions among inclusions with a major axis of 1 μm or more was also too high. The duplex stainless steel sheets of Comparative Examples 2 and 4 had too high a slag basicity (CaO / Al2O3 mass ratio) during the melting process, resulting in a high number density of soluble inclusions and reduced corrosion resistance. The number of soluble inclusions among inclusions with a major axis of 1 μm or more was also too high. The duplex stainless steel sheet of Comparative Example 5 had too much sulfur and algae content, resulting in a high number density of soluble inclusions and reduced corrosion resistance. Furthermore, this duplex stainless steel sheet had a high number density of inclusions and insufficient toughness.

[0060] As can be seen from the above results, the present invention provides a ferritic-austenitic duplex stainless steel material that can ensure corrosion resistance over a long period of time in harsh environments, particularly in environments with high chloride ion concentrations and favorable natural potentials.

[0061] Furthermore, based on the above results, the present invention can be provided in the following embodiments.

[0062] (Aspect 1) A composition comprising, 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, N: 0.050 to 0.300%, Al: 0.005 to 0.100%, Ca: 0.1000% or less, Mg: 0.1000% or less, with the remainder being Fe and impurities, having a major axis of 1 μm or more, and a number density of soluble inclusions of 0.2 or less, where the ratio of Al concentration to the total concentration of Al, Mg, Ca, and Mn is 0.2 or less. 2 The following are ferritic-austenitic duplex stainless steel materials.

[0063] (Aspect 2) The ferritic-austenitic duplex stainless steel material according to Aspect 1, further comprising one or more selected from Ti: 0.100% or less, Nb: 0.100% or less, V: 1.00% or less, W: 1.00% or less, Co: 1.00% or less, Zr: 1.00% or less, REM: 0.100% or less, Sn: 0.100% or less, and B: 0.010% or less, on a mass basis.

[0064] (Aspect 3) A ferritic-austenitic duplex stainless steel material according to Aspect 1 or 2, wherein 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.

[0065] (Aspect 4) Number density of inclusions with a major axis of 1 μm or more is 200 pieces / mm 2 A ferritic-austenitic duplex stainless steel material according to any one of the following embodiments 1 to 3.

[0066] (Aspect 5) A ferritic-austenitic duplex stainless steel material according to any one of aspects 1 to 4, wherein the pitting potential measured at 30°C is 0.40 V vs SSE or higher.

[0067] (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.

[0068] (Aspect 7) A ferritic-austenitic duplex stainless steel material according to any one of aspects 1 to 6, wherein the proportion of soluble inclusions in the inclusions having a major axis of 1 μm or more is 20.0% or less.

Claims

1. The composition, by mass, includes 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, N: 0.050 to 0.300%, Al: 0.005 to 0.100%, Ca: 0.1000% or less, Mg: 0.1000% or less, with the remainder being Fe and impurities, and the number density of soluble inclusions is 10.0 particles / mm³, with a major axis of 1 μm or more, and the ratio of Al concentration to the total concentration of Al, Mg, Ca, and Mn being 0.2 or less. 2 The following are ferritic-austenitic duplex stainless steel materials.

2. The ferritic-austenitic duplex stainless steel material according to claim 1, further comprising one or more selected by mass from Ti: 0.100% or less, Nb: 0.100% or less, V: 1.00% or less, W: 1.00% or less, Co: 1.00% or less, Zr: 1.00% 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. The number density of inclusions with a major axis of 1 μm or more is 200 particles / mm². 2 The ferritic-austenitic duplex stainless steel material according to any one of claims 1 to 3, which is as follows:

5. A ferritic-austenitic duplex stainless steel material according to any one of claims 1 to 4, wherein the pitting potential measured at 30°C is 0.40 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 in the inclusions having a major axis of 1 μm or more is 20.0% or less.

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