Duplex stainless steel

Abstract

The present invention provides a duplex stainless steel material which has excellent intergranular corrosion resistance. A duplex stainless steel material according to the present disclosure is composed of, in mass%, 0.030% or less of C, 0.50% or less of Si, 2.00% or less of Mn, 0.040% or less of P, 0.0010% or less of S, 26.0% to 28.0% of Cr, 6.0% to 10.0% of Ni, 0.20% to 1.70% of Mo, more than 2.00% but not more than 3.00% of W, more than 0.30% but not more than 0.40% of N, 0.020% or less of O and 0.050% or less of Al, with the balance being made up or Fe and impurities; and if the longitudinal direction and the thickness direction of three rectangular regions thereof are respectively defined as direction L and direction T, and five line segments that divide each region into six equal parts in direction L are defined as line segments LS, the average thickness TF of ferrites overlapping with 15 line segments LS is 2.50 µm to 4.50 µm, the sample standard deviation ∆TF of the ferrite thickness is 0.50 µm or less, and the average thickness TA of austenites overlapping with the line segments LS is 2.50 µm to 4.50 µm.

Description

[Technical Field]

[0001] This disclosure relates to duplex stainless steel materials. [Background technology]

[0002] In a urea plant, a type of chemical plant, urea is produced. In a urea plant, urea is produced by the following method: a mixed gas containing ammonia and carbon dioxide is heated at a high temperature of 160-230°C at a rate of 120 kg / cm³. 2 The synthesis is carried out under the high pressure described above. During this process, urea is produced by the synthesis reaction.

[0003] In the urea manufacturing process described above, an intermediate substance called ammonia carbamate is produced. Ammonia carbamate is highly corrosive and accelerates intergranular corrosion of steel. Therefore, steel used in urea plants requires excellent resistance to intergranular corrosion.

[0004] Duplex stainless steel has excellent corrosion resistance. For this reason, duplex stainless steel is used as a steel material for urea production plants. Duplex stainless steel for urea production plants is proposed, for example, in Japanese Patent Publication No. 2003-301241 (Patent Document 1) and Japanese Patent Publication No. 2011-127186 (Patent Document 2).

[0005] The duplex stainless steel material disclosed in Patent Document 1 attempts to improve corrosion resistance from the standpoint of chemical composition. Specifically, it reduces the content of Cu, which accelerates the corrosion rate. Furthermore, it reduces the content of Mo, which promotes the formation of the σ phase, and includes W, a ferrite-stabilizing element that replaces Mo and does not promote the formation of the σ phase. Patent Document 1 states that this improves the corrosion resistance of the duplex stainless steel material.

[0006] The duplex stainless steel material disclosed in Patent Document 2 attempts to improve corrosion resistance from the viewpoint of chemical composition. Specifically, it defines a relational expression for the content of Nd, P, S, Al, and Mo, and by ensuring that this relational expression falls within a predetermined range, the formation of the σ phase at the interface between ferrite and austenite is suppressed. As a result, the corrosion resistance of the duplex stainless steel material is improved, as described in Patent Document 2. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2003-301241 [Patent Document 2] Japanese Patent Publication No. 2011-127186

[0008] Incidentally, duplex stainless steel used in urea production plants is sometimes welded during plant construction to form welded joints. In such welded joints, a heat-affected zone (HAZ) is formed in the base metal portion adjacent to the weld metal. Intergranular corrosion is prone to progress in the HAZ of welded joints. Therefore, even when duplex stainless steel is used in welded joints, excellent resistance to intergranular corrosion is required. [Overview of the project] [Problems that the invention aims to solve]

[0009] The purpose of this disclosure is to provide a duplex stainless steel material that has excellent resistance to intergranular corrosion, even when used in welded joints. [Means for solving the problem]

[0010] The duplex stainless steel material disclosed herein is The chemical composition is expressed in mass percent. C: 0.030% or less, Si: 0.50% or less, Mn: 2.00% or less, P: 0.040% or less, S: 0.0010% or less, Cr: 26.0~28.0%, Ni: 6.0~10.0%, Mo: 0.20~1.70%, W: more than 2.00~3.00%, N: more than 0.30~0.40%, O: 0.020% or less, Al: 0.050% or less, Cu: 0~0.30%, Co: 0~1.0%, Ti: 0~0.300%, Nb: 0~0.300%, Ca: 0~0.010%, Mg: 0~0.010%, B: 0~0.010%, and the balance being Fe and impurities, defining the longitudinal direction of the duplex stainless steel material as L direction and the thickness direction of the duplex stainless steel material as T direction, in a cross section including the L direction and T direction of the duplex stainless steel material, specifying three rectangular regions at a 100 mm pitch in the L direction, each specified region being a rectangle of 200 µm in the L direction and 200 µm in the T direction, in each rectangular region, when five line segments extending in the T direction, arranged at equal intervals in the L direction of the rectangular region and dividing the rectangular region into six equal parts in the L direction are defined as line segments LS, the average thickness TF of each ferrite overlapping with the 15 line segments LS of the three rectangular regions is 2.50~4.50 µm, and the sample standard deviation ΔTF of the ferrite thickness is 0.50 µm or less, the average thickness TA of each austenite overlapping with the 15 line segments LS is 2.50~4.50 µm. [Advantageous Effects of Invention]

[0011] The duplex stainless steel material according to the present disclosure has excellent intergranular corrosion resistance even when formed into a welded joint. [Brief Description of Drawings]

[0012] [Figure 1] Figure 1 is a schematic diagram of a longitudinal section of a duplex stainless steel material, including the longitudinal direction (L direction) and the thickness direction (T direction). [Figure 2] Figure 2 is a schematic diagram of the longitudinal cross-section of a duplex stainless steel material when it is welded together to form a welded joint. [Figure 3] Figure 3 is a longitudinal cross-sectional view of a duplex stainless steel material to illustrate a method for identifying a rectangular area TP for measuring the average ferrite thickness TF, the sample standard deviation ΔTF of the ferrite thickness, and the average austenite thickness TA of the duplex stainless steel material when the duplex stainless steel material is a steel pipe. [Figure 4] Figure 4 is a schematic diagram of the rectangular region TP in Figure 3. [Figure 5] Figure 5 is an enlarged view of the area within the rectangular region in Figure 4 that overlaps with the line segment LS1. [Modes for carrying out the invention]

[0013] The inventors of this invention investigated duplex stainless steel materials that exhibit excellent resistance to intergranular corrosion, even when used in welded joints. As a result, the inventors obtained the following findings.

[0014] First, the inventors investigated duplex stainless steel materials that have excellent resistance to intergranular corrosion even when used in welded joints, from the perspective of chemical composition, similar to Patent Documents 1 and 2. As a result, the inventors concluded that if the chemical composition of the duplex stainless steel material satisfies the following characteristic 1, then excellent resistance to intergranular corrosion can be obtained even when used in welded joints. (Feature 1) The chemical composition, in mass%, is as follows: C: 0.030% or less, Si: 0.50% or less, Mn: 2.00% or less, P: 0.040% or less, S: 0.0010% or less, Cr: 26.0~28.0%, Ni: 6.0~10.0%, Mo: 0.20~1.70%, W: greater than 2.00~3.00%, N: greater than 0.30~0.40%, O: 0.020% or less, Al: 0.050% or less, Cu: 0~0.30%, Co: 0~1.0%, Ti: 0~0.300%, Nb: 0~0.300%, Ca: 0~0.010%, Mg: 0~0.010%, B: 0~0.010%, and the remainder being Fe and impurities.

[0015] Therefore, the inventors further investigated means to obtain sufficient resistance to intergranular corrosion even when duplex stainless steel materials are welded to form welded joints. As mentioned above, when duplex stainless steel materials are welded to form welded joints, intergranular corrosion is particularly likely to occur in the heat-affected zone (HAZ). Therefore, the inventors observed the metal structure in the HAZ and the metal structure in the base material parts other than the HAZ when duplex stainless steel materials were used as welded joints. As a result, the inventors obtained the following findings.

[0016] The microstructure of duplex stainless steel is substantially composed of ferrite and austenite. Specifically, in a cross-section including the longitudinal direction (L direction) and the thickness direction (T direction) of duplex stainless steel (hereinafter also referred to as a longitudinal section), as shown in Figure 1, ferrite F and austenite A are layered together.

[0017] When duplex stainless steel materials are welded together to form a welded joint, the microstructure of the heat-affected zone (HAZ) also consists of ferrite and austenite. Comparing the microstructure of HAZ where intergranular corrosion occurred with that of HAZ where intergranular corrosion was not observed, ferrite and austenite of similar size were observed in both the HAZ with and without intergranular corrosion. However, as shown in Figure 2, in the HAZ where intergranular corrosion occurred, a large number of Cr nitrides and secondary austenite MA were also formed within the ferrite F. Secondary austenite refers to austenite that is significantly finer than the austenite A mentioned above. The investigation revealed that such Cr nitrides and secondary austenite MA are formed during welding.

[0018] Based on the above findings, the inventors considered that the factors reducing the intergranular corrosion resistance of the HAZ of welded joints in duplex stainless steel materials are chromium nitrides and secondary austenites generated during welding. Therefore, the inventors investigated means to suppress the generation of chromium nitrides and secondary austenites during welding. As a result, the inventors obtained the following findings.

[0019] As mentioned above, Cr nitrides and secondary austenite are formed from ferrite during welding. Therefore, in layered ferrite and austenite, if the thickness of the ferrite (i.e., the length in the T direction) is large, the distance over which the nitrogen dissolved in the ferrite during welding diffuses into the austenite during the cooling process becomes longer. Furthermore, since the cooling time during welding is short, the time available for diffusion is also short. For this reason, Cr nitrides and secondary austenite are likely to be formed during welding.

[0020] Furthermore, if there is variation in the thickness of ferrite in the L-direction of the longitudinal section of duplex stainless steel, it is thought that Cr nitride and secondary austenite are more likely to be formed during welding in the wider (thicker) portions of the ferrite extending in the L-direction.

[0021] As described above, the inventors of the present invention believe that narrowing the width (thickness) of the ferrite and reducing the variation in the width (thickness) of the ferrite is effective in suppressing the amount of Cr nitride and secondary austenite generated during welding.

[0022] Therefore, we conducted further investigations into the microstructure of duplex stainless steel. As a result, we found that if the microstructure of duplex stainless steel satisfies the following characteristics, the intergranular corrosion resistance of the HAZ (High-Area Zone) when used as a welded joint is significantly improved. (Feature 2) The longitudinal direction of the duplex stainless steel material is defined as the L direction, and the thickness direction is defined as the T direction. In a cross-section of the duplex stainless steel material that includes the L and T directions, three rectangular regions are identified at 100 mm intervals in the L direction. Each identified region is a rectangle with dimensions of 200 μm in the L direction and 200 μm in the T direction. Within each rectangular region, five line segments extending in the T direction, arranged at equal intervals in the L direction of the rectangular region, and dividing the rectangular region into six equal parts in the L direction are defined as line segments LS. In this case, the following (1) to (3) are satisfied. (1) The average thickness TF of each ferrite overlapping with the 15 line segments LS of the three rectangular regions is 2.50 to 4.50 μm. (2) The sample standard deviation ΔTF of the ferrite thickness is 0.50 μm or less. (3) The average thickness TA of each austenite that overlaps with the 15 line segments LS is 2.50 to 4.50 μm.

[0023] Based on the above findings, the duplex stainless steel material according to this embodiment has the following configuration.

[0024] [1] The first configuration of duplex stainless steel is The chemical composition is expressed in mass percent. C: 0.030% or less, Si: 0.50% or less, Mn: 2.00% or less, P: 0.040% or less, S: 0.0010% or less, Cr: 26.0~28.0%, Ni: 6.0~10.0%, Mo: 0.20~1.70%, W: over 2.00~3.00%, N: more than 0.30~0.40%, O: 0.020% or less, Al: 0.050% or less, Cu: 0~0.30%, Co: 0~1.0%, Ti: 0~0.300%, Nb: 0~0.300%, Ca: 0~0.010%, Mg: 0~0.010%, B: 0~0.010%, and, The remainder consists of Fe and impurities. The longitudinal direction of the duplex stainless steel material is defined as the L direction, and the thickness direction of the duplex stainless steel material is defined as the T direction. In a cross-section of a duplex stainless steel material including the L and T directions, three rectangular regions are identified at 100 mm intervals in the L direction, and each identified region is a rectangle measuring 200 μm in the L direction and 200 μm in the T direction. In each rectangular region, When line segments extending in the T direction, arranged at equal intervals in the L direction of a rectangular region, and dividing the rectangular region into 6 equal parts in the L direction, are defined as line segments LS, The average thickness TF of each ferrite overlapping with the 15 line segments LS of the three rectangular regions is 2.50 to 4.50 μm, and the sample standard deviation ΔTF of the ferrite thickness is 0.50 μm or less. The average thickness TA of each austenite layer overlapping with the 15 line segments LS is 2.50–4.50 μm.

[0025] [2] The second configuration of the duplex stainless steel material is A duplex stainless steel material having a first configuration, The chemical composition is, Cu: 0.01~0.30%, Co: 0.1~1.0%, Ti: 0.001~0.300%, Nb: 0.001~0.300%, Ca: 0.001~0.010%, Mg: 0.001~0.010%, and, B: 0.001~0.010%, It contains one or more elements selected from the group consisting of the following.

[0026] [3] The third configuration of duplex stainless steel is, A duplex stainless steel material having a first or second configuration, It is a seamless steel pipe.

[0027] The duplex stainless steel material of this embodiment will be described in detail below. Unless otherwise specified, the "%" in relation to elements refers to mass percentage.

[0028] [Features of the duplex stainless steel material of this embodiment] The duplex stainless steel material of this embodiment satisfies the following features 1 and 2. (Feature 1) The chemical composition, in mass%, is as follows: C: 0.030% or less, Si: 0.50% or less, Mn: 2.00% or less, P: 0.040% or less, S: 0.0010% or less, Cr: 26.0~28.0%, Ni: 6.0~10.0%, Mo: 0.20~1.70%, W: greater than 2.00~3.00%, N: greater than 0.30~0.40%, O: 0.020% or less, Al: 0.050% or less, Cu: 0~0.30%, Co: 0~1.0%, Ti: 0~0.300%, Nb: 0~0.300%, Ca: 0~0.010%, Mg: 0~0.010%, B: 0~0.010%, and the remainder being Fe and impurities. (Feature 2) The longitudinal direction of the duplex stainless steel material is defined as the L direction, and the thickness direction is defined as the T direction. In a cross-section of the duplex stainless steel material that includes the L and T directions, three rectangular regions are identified at 100 mm intervals in the L direction. Each identified region is a rectangle with dimensions of 200 μm in the L direction and 200 μm in the T direction. Within each rectangular region, five line segments extending in the T direction, arranged at equal intervals in the L direction of the rectangular region, and dividing the rectangular region into six equal parts in the L direction are defined as line segments LS. In this case, the following (1) to (3) are satisfied. (1) The average thickness TF of each ferrite overlapping with the 15 line segments LS of the three rectangular regions is 2.50 to 4.50 μm. (2) The sample standard deviation ΔTF of the ferrite thickness is 0.50 μm or less. (3) The average thickness TA of each austenite that overlaps with the 15 line segments LS is 2.50 to 4.50 μm. Features 1 and 2 are described below.

[0029] [(Feature 1) Regarding chemical composition] The chemical composition of the duplex stainless steel material of this embodiment contains the following elements.

[0030] C: 0.030% or less Carbon (C) is inevitably present; that is, the C content is greater than 0%. C forms carbides, which increase the strength of the steel. However, if the C content exceeds 0.030%, Cr carbides are formed at the grain boundaries. In this case, even if the content of other elements is within the range of this embodiment, the intergranular corrosion resistance of the steel decreases. Therefore, the C content is 0.030% or less. A low carbon content is preferable. However, excessive reduction of the carbon content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit for the carbon content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferred upper limit for the C content is 0.028%, more preferably 0.025%, more preferably 0.022%, and still more preferably 0.020%.

[0031] Si:0.50% or less Silicon (Si) is inevitably present. In other words, the Si content is greater than 0%. Si deoxidizes steel during the steelmaking stage in the steel manufacturing process. On the other hand, if the Si content exceeds 0.50%, Si will segregate at the grain boundaries. In this case, even if the content of other elements is within the range of this embodiment, the grain boundary corrosion resistance of the steel will decrease. Therefore, the Si content is 0.50% or less. The preferred lower limit for the Si content is 0.01%, more preferably 0.02%, and even more preferably 0.05%. The preferred upper limit for the Si content is 0.45%, more preferably 0.40%, even more preferably 0.38%, and even more preferably 0.35%.

[0032] Mn: 2.00% or less Manganese (Mn) is inevitably present; that is, the Mn content is greater than 0%. Mn deoxidizes steel during the steelmaking stage of the steel manufacturing process. Furthermore, Mn is an austenite-forming element and stabilizes the austenite in the steel. Even a small amount of Mn content will provide some degree of the above effects. However, if the Mn content exceeds 2.00%, Mn will segregate at the grain boundaries along with impurities such as P and S. In this case, even if the content of other elements is within the range of this embodiment, the corrosion resistance of the steel in high-temperature environments will decrease. Therefore, the Mn content is 2.00% or less. The preferred lower limit of the Mn content is 0.01%, more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit for the Mn content is 1.60%, more preferably 1.40%, more preferably 1.20%, more preferably 1.00%, more preferably 0.90%, more preferably 0.80%, and more preferably 0.70%.

[0033] P:0.040% or less Phosphorus (P) is an impurity, and the P content is greater than 0%. If the P content exceeds 0.040%, P will segregate at the grain boundaries, even if the content of other elements is within the range of this embodiment. As a result, the intergranular corrosion resistance of the steel decreases. Therefore, the P content is 0.040% or less. A low phosphorus (P) content is preferable. However, excessive reduction of the P content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the P content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferred upper limit for the P content is 0.035%, more preferably 0.030%, more preferably 0.025%, more preferably 0.020%, and still more preferably 0.015%.

[0034] S: 0.0010% or less Sulfur (S) is an impurity, and the S content is greater than 0%. If the S content exceeds 0.0010%, S will segregate at the grain boundaries, even if the content of other elements is within the range of this embodiment. As a result, the intergranular corrosion resistance of the steel decreases. Therefore, the sulfur content is 0.0010% or less. A low sulfur (S) content is preferable. However, excessive reduction of the S content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit for the S content is 0.0001%, and more preferably 0.0002%. The preferred upper limit for the S content is 0.0009%, more preferably 0.0007%, and even more preferably 0.0005%.

[0035] Cr: 26.0~28.0% Chromium (Cr) dissolves in the steel to enhance its resistance to intergranular corrosion. Furthermore, Cr stabilizes ferrite in the steel, further improving its resistance to intergranular corrosion. If the Cr content is less than 26.0%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Cr content exceeds 28.0%, a sigma (σ) phase will form in the steel, even if the content of other elements is within the range of this embodiment. The σ phase reduces the intergranular corrosion resistance of the steel. Therefore, the Cr content is 26.0-28.0%. The preferred lower limit for the Cr content is 26.1%, more preferably 26.2%, more preferably 26.3%, and still more preferably 26.4%. The preferred upper limit for the Cr content is 27.9%, more preferably 27.8%, more preferably 27.7%, and still more preferably 27.6%.

[0036] Ni: 6.0~10.0% Nickel (Ni) stabilizes austenite in steel. In other words, Ni stabilizes the two-phase structure of ferrite and austenite. Therefore, the intergranular corrosion resistance of the steel is improved. If the Ni content is less than 6.0%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Ni content exceeds 10.0%, the proportion of austenite in the steel becomes excessively high, even if the content of other elements is within the range of this embodiment. In this case, the intergranular corrosion resistance of the steel decreases. Therefore, the Ni content is 6.0-10.0%. The preferred lower limit for the Ni content is 6.2%, more preferably 6.3%, even more preferably 6.4%, and even more preferably 6.5%. The preferred upper limit for the Ni content is 9.5%, more preferably 9.0%, even more preferably 8.5%, and even more preferably 8.0%.

[0037] Mo: 0.20~1.70% Molybdenum (Mo) enhances the intergranular corrosion resistance of steel materials. If the Mo content is less than 0.20%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mo content exceeds 1.70%, a σ phase will be formed even if the content of other elements is within the range of this embodiment. In this case, the intergranular corrosion resistance of the steel material will decrease. Therefore, the Mo content is 0.20-1.70%. The preferred lower limit for the Mo content is 0.30%, more preferably 0.40%, and even more preferably 0.50%. The preferred upper limit for the Mo content is 1.60%, more preferably 1.50%, more preferably 1.40%, more preferably 1.30%, more preferably 1.20%, more preferably 1.10%, and more preferably 1.00%.

[0038] W: More than 2.00~3.00% Tungsten (W) enhances the intergranular corrosion resistance of steel materials. If the W content is 2.00% or less, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the W content exceeds 3.00%, the σ phase will be formed even if the content of other elements is within the range of this embodiment. In this case, the intergranular corrosion resistance of the steel material will decrease. Therefore, the W content is between 2.00% and 3.00%. The preferred lower limit of the W content is 2.01%, more preferably 2.02%, more preferably 2.05%, more preferably 2.08%, more preferably 2.10%, and more preferably 2.12%. The preferred upper limit for the W content is 2.90%, more preferably 2.80%, more preferably 2.70%, more preferably 2.60%, more preferably 2.50%, more preferably 2.40%, and more preferably 2.30%.

[0039] N: More than 0.30~0.40% Nitrogen (N) stabilizes austenite in steel. In other words, N stabilizes the two-phase structure of ferrite and austenite. Therefore, the intergranular corrosion resistance of the steel is increased. If the N content is 0.30% or less, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the N content exceeds 0.40%, the hot workability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the N content is between 0.30% and 0.40%. The preferred lower limit for the N content is 0.31%, and more preferably 0.32%. The preferred upper limit for the N content is 0.39%, more preferably 0.38%, and even more preferably 0.37%.

[0040] O: 0.020% or less Oxygen (O) is an impurity, and the O content is greater than 0%. If the O content exceeds 0.020%, an excess of oxides will be generated in the steel. In this case, even if the content of other elements is within the range of this embodiment, the intergranular corrosion resistance of the steel will decrease. Therefore, the O content is 0.020% or less. A low oxygen content is preferable. However, excessive reduction of the oxygen content significantly increases manufacturing costs. Therefore, considering industrial productivity, the preferred lower limit of the oxygen content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferred upper limit for the O content is 0.018%, more preferably 0.016%, and even more preferably 0.014%.

[0041] Al: 0.050% or less Aluminum (Al) is inevitably present; that is, the Al content is greater than 0%. Al deoxidizes steel. However, if the Al content exceeds 0.050%, an excessive amount of oxides will be generated in the steel. In this case, even if the content of other elements is within the range of this embodiment, the intergranular corrosion resistance of the steel will decrease. Therefore, the Al content is 0.050% or less. The preferred lower limit for the Al content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit for the Al content is 0.045%, more preferably 0.040%, even more preferably 0.038%, and even more preferably 0.036%. In the chemical composition of the duplex stainless steel material of this embodiment, the Al content refers to the content of "acid-soluble Al," that is, sol.Al.

[0042] The remainder of the chemical composition of the duplex stainless steel material according to this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are introduced during the industrial production of duplex stainless steel material from raw materials such as ore, scrap, or the manufacturing environment, and are not intentionally included, and are acceptable within a range that does not adversely affect the duplex stainless steel material according to this embodiment.

[0043] [Optional Elements] The chemical composition of the duplex stainless steel material in this embodiment is further as follows: Cu: 0~0.30%, Co: 0~1.0%, Ti: 0~0.300%, Nb: 0~0.300%, Ca: 0~0.010%, Mg: 0~0.010%, and, B: 0~0.010%, It may contain one or more elements selected from the group consisting of the following. The following describes these arbitrary elements.

[0044] [Group 1: Regarding Cu and Co] The chemical composition of the duplex stainless steel material according to this embodiment may further include one or more elements selected from the group consisting of Cu and Co in place of a portion of Fe. These elements are all optional and may not be included. If included, Cu and Co enhance the intergranular corrosion resistance of the steel material.

[0045] Cu: 0~0.30% Copper (Cu) is an optional element and does not need to be included. In other words, the Cu content may be 0%. When copper (Cu) is present, i.e., when the Cu content is greater than 0%, the Cu strengthens the passivation film, improving the corrosion resistance of duplex stainless steel. Furthermore, Cu stabilizes the austenite. Even a small amount of Cu can provide some of the above effects. However, if the Cu content exceeds 0.30%, corrosion of the duplex stainless steel material will be accelerated in the corrosive environment of a urea production plant, even if the content of other elements is within the range of this embodiment. Therefore, the copper content is 0-0.30%. The preferred lower limit for the Cu content is 0.01%, and more preferably 0.05%. The preferred upper limit for the Cu content is 0.29%, more preferably 0.27%, more preferably 0.25%, and still more preferably 0.22%.

[0046] Co: 0~1.0% Cobalt (Co) is an optional element and does not need to be included. In other words, the Co content may be 0%. When Co is present, i.e., when the Co content is greater than 0%, Co strengthens the passivation film and improves the corrosion resistance of duplex stainless steel. Furthermore, Co stabilizes the austenite. Even a small amount of Co will provide some of the above effects. However, if the Co content exceeds 1.0%, the manufacturing cost will increase drastically, even if the content of other elements is within the range of this embodiment. Therefore, the Co content is 0-1.0%. The preferred lower limit of the Co content is 0.1%, more preferably 0.2%, and even more preferably 0.3%. The preferred upper limit for the Co content is 0.9%, more preferably 0.8%, and even more preferably 0.7%.

[0047] [Group 2: Regarding Ti and Nb] The chemical composition of the duplex stainless steel material according to this embodiment may further include one or more elements selected from the group consisting of Ti and Nb in place of a portion of Fe. Any of these elements are optional and may not be included. If included, Ti and Nb increase the strength of the steel material.

[0048] Ti: 0~0.300% Titanium (Ti) is an optional element and does not need to be included. In other words, the Ti content may be 0%. When Ti is present, that is, when the Ti content is greater than 0%, Ti forms carbonitrides, increasing the strength of the steel. Furthermore, by forming carbonitrides, Ti suppresses the formation of chromium carbonitrides. Therefore, the intergranular corrosion resistance of the steel is improved. Even if only a small amount of Ti is present, the above effects can be obtained to some extent. However, if the Ti content exceeds 0.300%, even if the content of other elements is within the range of this embodiment, the strength of the steel becomes too high and the toughness of the steel decreases. Therefore, the Ti content is 0-0.300%. The preferred lower limit of the Ti content is 0.001%, more preferably 0.005%, more preferably 0.010%, more preferably 0.015%, and more preferably 0.020%. The preferred upper limit for the Ti content is 0.250%, more preferably 0.200%, more preferably 0.150%, more preferably 0.100%, more preferably 0.090%, more preferably 0.080%, and more preferably 0.070%.

[0049] Nb: 0~0.300% Niobium (Nb) is an optional element and does not need to be included. In other words, the Nb content may be 0%. When Nb is present, that is, when the Nb content is greater than 0%, Nb forms carbonitrides, increasing the strength of the steel. Furthermore, by forming carbonitrides, Nb suppresses the formation of Cr carbonitrides. Therefore, the intergranular corrosion resistance of the steel is improved. Even if only a small amount of Nb is present, the above effects can be obtained to some extent. However, if the Nb content exceeds 0.300%, even if the content of other elements is within the range of this embodiment, the strength of the steel becomes too high and the toughness of the steel decreases. Therefore, the Nb content is 0-0.300%. The preferred lower limit of the Nb content is 0.001%, more preferably 0.005%, more preferably 0.010%, more preferably 0.015%, and more preferably 0.020%. The preferred upper limit for the Nb content is 0.250%, more preferably 0.200%, more preferably 0.150%, more preferably 0.120%, more preferably 0.110%, and more preferably 0.100%.

[0050] [Group 3: Regarding Ca, Mg, and B] The chemical composition of the duplex stainless steel material according to this embodiment may further contain one or more elements selected from the group consisting of Ca, Mg, and B in place of a portion of Fe. These elements are all optional and may not be included. If included, Ca, Mg, and B enhance the hot workability of the steel material.

[0051] Ca: 0~0.010% Calcium (Ca) is an optional element and does not need to be included. In other words, the Ca content may be 0%. When calcium (Ca) is present, that is, when the Ca content is greater than 0%, the Ca neutralizes the sulfur (S) in the steel by fixing it as sulfides, thereby improving the hot workability of the steel. Even a small amount of Ca can provide some degree of this effect. However, if the Ca content exceeds 0.010%, even if the content of other elements is within the range of this embodiment, the oxides in the steel will coarseen, and the toughness of the steel will decrease. Therefore, the Ca content is 0-0.010%. The preferred lower limit for the Ca content is 0.001%, and more preferably 0.002%. The preferred upper limit for the Ca content is 0.009%, more preferably 0.008%, more preferably 0.007%, more preferably 0.006%, and more preferably 0.005%.

[0052] Mg: 0~0.010% Magnesium (Mg) is an optional element and does not need to be included. In other words, the Mg content may be 0%. When magnesium (Mg) is present, that is, when the Mg content is greater than 0%, the Mg neutralizes the sulfur (S) in the steel by fixing it as sulfides, thereby improving the hot workability of the steel. Even a small amount of Mg can provide some degree of the above effect. However, if the Mg content exceeds 0.010%, even if the content of other elements is within the range of this embodiment, the oxides in the steel will coarseen, and the toughness of the steel will decrease. Therefore, the Mg content is 0-0.010%. The preferred lower limit for the Mg content is 0.001%, and more preferably 0.002%. The preferred upper limit for the Mg content is 0.009%, more preferably 0.008%, more preferably 0.007%, more preferably 0.006%, and more preferably 0.005%.

[0053] B: 0~0.010% Boron (B) is an optional element and does not need to be included. In other words, the B content may be 0%. When B is present, that is, when the B content is greater than 0%, B suppresses the segregation of S at grain boundaries in the steel, thereby improving the hot workability of the steel. Even if only a small amount of B is present, the above effect can be obtained to some extent. However, if the B content exceeds 0.010%, boron nitride (BN) will be formed, even if the content of other elements is within the range of this embodiment, and the toughness of the steel will decrease. Therefore, the B content is 0-0.010%. The preferred lower limit for the B content is 0.001%, and more preferably 0.002%. The preferred upper limit for the B content is 0.009%, more preferably 0.008%, more preferably 0.007%, more preferably 0.006%, more preferably 0.005%, and more preferably 0.004%.

[0054] [Metal structure] The microstructure of the duplex stainless steel material according to this embodiment consists of ferrite and austenite. In this specification, "consisting of ferrite and austenite" means that the amount of phases other than ferrite and austenite is negligibly small. The microstructure of the duplex stainless steel material according to this embodiment may contain trace amounts of precipitates, inclusions, etc., in addition to ferrite and austenite. The area ratio of precipitates and inclusions in the microstructure of the duplex stainless steel material according to this embodiment is negligibly low compared to the area ratio of ferrite and austenite.

[0055] In the metallographic structure of the duplex stainless steel material according to this embodiment, the ferrite area ratio is 35-55%. The preferred lower limit of the ferrite area ratio is 37%, and more preferably 39%. The preferred upper limit of the ferrite area ratio is 53%, and more preferably 51%.

[0056] [Method for measuring ferrite area ratio] In this embodiment, the area ratio of ferrite in the duplex stainless steel material can be determined by the following method.

[0057] In this embodiment, the longitudinal direction of the duplex stainless steel material is defined as the L direction, and the thickness direction is defined as the T direction. Specifically, when the duplex stainless steel material is a steel pipe, the pipe axis direction (rolling direction) is the L direction, and the wall thickness direction is the T direction. When the duplex stainless steel material is a steel plate, the longitudinal direction (rolling direction) is the L direction, and the plate thickness direction is the T direction. When the duplex stainless steel material is a steel bar, the central axis direction (longitudinal direction) is the L direction, and the radial direction is the T direction.

[0058] A test specimen is taken from the center of the thickness of the duplex stainless steel material, including a longitudinal section on the surface that includes the L-direction and T-direction. If the duplex stainless steel material is a steel pipe, the test specimen is taken from the center of the wall thickness. If the duplex stainless steel material is a steel plate, the test specimen is taken from the center of the plate thickness. If the duplex stainless steel material is a steel bar, the test specimen is taken from the center of the section perpendicular to the L-direction.

[0059] The surface of the test specimen corresponding to the longitudinal section (the plane including the L-direction and T-direction) is defined as the observation surface. There are no particular limitations on the size of the test specimen; it is sufficient if an observation surface of 5mm in the L-direction × 5mm in the T-direction is obtained.

[0060] The observation surface of the test specimen is polished to a mirror finish. The mirror-polished observation surface is electrolytically etched in a 30% sodium hydroxide etchant to reveal the microstructure. The revealed observation surface is observed in 10 fields of view using an optical microscope. The area of ​​the observation field is not particularly limited, but for example, 4.00 × 10⁻⁶ 4 μm 2 (Magnification 500x).

[0061] In each field of view, ferrite and austenite are identified based on contrast. The area ratios of the identified ferrite and austenite are determined. The method for determining the area ratios of the identified ferrite and austenite is not particularly limited and any well-known method may be used. For example, the area ratios of ferrite and austenite can be determined by image analysis. In this embodiment, the arithmetic mean of the ferrite area ratios obtained in all observed fields of view is defined as the ferrite area ratio (%). The ferrite area ratio is rounded to the first decimal place. The austenite area ratio (%) is calculated using the following formula. Austenite area fraction = 100 - Ferrite area fraction

[0062] [(Feature 2) Regarding the average ferrite thickness TF, sample standard deviation ΔTF, and average austenite thickness TA] The longitudinal direction of the duplex stainless steel material is defined as the L direction, and the thickness direction of the duplex stainless steel material is defined as the T direction. In a cross-section of the duplex stainless steel material including the L and T directions, three rectangular regions are identified at 100 mm intervals in the L direction. Each identified region is a rectangle with dimensions of 200 μm in the L direction and 200 μm in the T direction. Within each rectangular region, five line segments extending in the T direction, arranged at equal intervals in the L direction of the rectangular region, and dividing the rectangular region into six equal parts in the L direction are defined as line segments LS. In this case, the duplex stainless steel material of this embodiment satisfies the following (1) to (3). (1) The average thickness TF of each ferrite overlapping with the 15 line segments LS of the three rectangular regions is 2.50 to 4.50 μm. (2) The sample standard deviation ΔTF of the ferrite thickness is 0.50 μm or less. (3) The average thickness TA of each austenite that overlaps with the 15 line segments LS is 2.50 to 4.50 μm. The following details Feature 2.

[0063] [Measurement methods for average ferrite thickness TF, sample standard deviation of ferrite thickness ΔTF, and average austenite thickness TA] The average ferrite thickness TF, the sample standard deviation of ferrite thickness ΔTF, and the average austenite thickness TA can be measured by the following method.

[0064] A test specimen is taken from the center of the thickness of the duplex stainless steel material, with the surface including longitudinal sections in the L and T directions. Figure 3 is a longitudinal cross-sectional view illustrating the sampling location of test specimens when the duplex stainless steel material is a steel pipe. In Figure 3, the dashed line C1 is the pipe axis. The dashed line L1 is a dashed line passing through the center of the wall thickness of the steel pipe. Referring to Figure 3, when the duplex stainless steel material is a steel pipe, in the cross-section (longitudinal section) that includes the L and T directions, three rectangular regions TP are identified in the center of the wall thickness, with P=100 mm pitch in the L direction. Three test specimens are then taken, each containing one of the rectangular regions TP on its surface. The rectangular regions TP are 200 μm in the L direction and 200 μm in the T direction. Similarly, when the duplex stainless steel material is a steel sheet, in a longitudinal section including the L direction (rolling direction) and the T direction (thickness direction), three rectangular regions TP are identified in the center of the thickness, at 100 mm intervals in the L direction. Then, three test specimens are taken that include each rectangular region TP on their surface. Similarly, when the duplex stainless steel material is a steel bar, in a longitudinal section including the L direction (central axis direction) and the T direction (radial direction), three rectangular regions TP are identified at the radial center (i.e., the central axis) and at 100 mm intervals in the L direction. Then, three test specimens are taken that include each rectangular region TP on their surface. The surface of the test specimen that includes the rectangular region TP is defined as the observation surface. The size of the test specimen is not particularly limited; it must be large enough to include the rectangular region TP.

[0065] The observation surface of the test specimen is polished to a mirror finish. The mirror-polished observation surface is electrolytically etched in a 30% sodium hydroxide etchant to reveal the microstructure. The rectangular region TP of the observation surface is observed at 500x magnification using an optical microscope.

[0066] Figure 4 is a schematic diagram of the rectangular region TP. Referring to Figure 4, the rectangular region is a rectangle with dimensions of 200 μm in the L direction and 200 μm in the T direction. Within the rectangular region TP, five line segments LS are arranged at equal intervals in the L direction, dividing the rectangular region TP into six equal parts in the L direction. In this case, each line segment LS overlaps with the ferrite F and austenite A within the rectangular region TP.

[0067] Figure 5 is an enlarged view of the region of the rectangular area TP in Figure 4 that overlaps with line segment LS1. Referring to Figure 5, line segment LS1 overlaps with ferrite F1 to F5 and austenite A1 to A5. Here, the length of ferrite F1 that overlaps with line segment LS1 is defined as the thickness TF1 of ferrite F1, and thickness TF1 is determined. Similarly, thicknesses TF2 to TF5 of ferrite F2 to F5 that overlap with line segment LS1 are determined. Similarly, thicknesses TA1 to TA6 of austenite A1 to A6 that overlap with line segment LS1 are determined.

[0068] Similarly, for the other four line segments LS in Figure 4, the length over which ferrite F overlaps with line segment LS is defined as the thickness of ferrite F. The length over which austenite A overlaps with line segment LS is defined as the thickness of austenite A.

[0069] Using the method described above, the thickness TF of each ferrite F and the thickness TA of each austenite A that overlap with the 15 line segments of the three rectangular regions TP are determined. The arithmetic mean of all the determined ferrite thicknesses is taken as the average ferrite thickness TF (μm). Based on all the determined ferrite thicknesses and the average ferrite thickness TF, the sample standard deviation ΔTF (μm) is calculated. Furthermore, the arithmetic mean of all the calculated austenite thicknesses is defined as the average austenite thickness TA (μm).

[0070] [Regarding features (1) to (3) of Feature 2] The average thickness TF of ferrite, the sample standard deviation ΔTF of the ferrite thickness, and the average thickness TA of austenite obtained from the above measurements satisfy the following conditions (1) to (3). (1) The average thickness TF of each ferrite overlapping with the 15 line segments LS of the three rectangular regions is 2.50 to 4.50 μm. (2) The sample standard deviation ΔTF of the ferrite thickness is 0.50 μm or less. (3) The average thickness TA of each austenite that overlaps with the 15 line segments LS is 2.50 to 4.50 μm.

[0071] If the average ferrite thickness TF exceeds 4.50 μm, the ferrite thickness is too large. In this case, when duplex stainless steel is welded to form a welded joint, Cr nitride and secondary austenite are likely to form in the HAZ. Therefore, the intergranular corrosion resistance of the welded joint decreases. If the average ferrite thickness TF is 4.50 μm or less, the ferrite thickness is sufficiently small. Therefore, sufficient intergranular corrosion resistance can be obtained even when a welded joint is formed.

[0072] The lower limit of the average thickness TF of the ferrite is not particularly limited. For example, the lower limit of the average thickness TF of the ferrite is 2.50 μm.

[0073] The preferred upper limit for the average thickness TF of the ferrite is 4.45 μm, more preferably 4.40 μm, and even more preferably 4.35 μm. The preferred lower limit for the average thickness TF of the ferrite is 2.55 μm, and more preferably 2.60 μm.

[0074] Furthermore, if the average thickness TF of ferrite is 2.50 to 4.50 μm, the average thickness TA of austenite will also be 2.50 to 4.50 μm.

[0075] The preferred upper limit for the average thickness TA of the austenite is 4.45 μm, more preferably 4.40 μm, and even more preferably 4.35 μm. The preferred lower limit for the average thickness TA of the austenite is 2.55 μm, and more preferably 2.60 μm.

[0076] Furthermore, regarding ferrite, the sample standard deviation ΔTF of ferrite thickness is 0.50 μm or less. As mentioned above, even if the average thickness TF of ferrite is sufficiently small, if the variation in ferrite thickness in the L direction of the duplex stainless steel material is large, there will be locally thicker portions of ferrite extending in the L direction. In this case, Cr nitride and secondary austenite are likely to be formed in these locally thicker portions during welding. Therefore, when used as a welded joint, resistance to intergranular corrosion decreases.

[0077] If the sample standard deviation ΔTF of the ferrite thickness is 0.50 μm or less, the variation in ferrite thickness in the L direction is sufficiently small. Therefore, assuming that conditions (1) and (3) above are met, sufficient resistance to intergranular corrosion can be obtained when a welded joint is formed.

[0078] The preferred upper limit of the sample standard deviation ΔTF is 0.48 μm, more preferably 0.45 μm, and even more preferably 0.43 μm.

[0079] [Effects of the duplex stainless steel material of this embodiment] The duplex stainless steel material of this embodiment satisfies features 1 and 2. Therefore, even when welded joints are formed using the duplex stainless steel material of this embodiment, excellent resistance to intergranular corrosion can be obtained.

[0080] [Regarding resistance to intergranular corrosion] In the duplex stainless steel material of this embodiment, sufficient resistance to intergranular corrosion can be obtained when used as a welded joint if the corrosion rate obtained from the ASTM A262 PracticeC nitric acid corrosion test on a welded joint using the duplex stainless steel material of this embodiment as the base material is 0.100 g / m².2 This means it is less than or equal to / h.

[0081] [Regarding the evaluation method for resistance to intergranular corrosion] The intergranular corrosion resistance of the duplex stainless steel material of this embodiment can be evaluated by the following method.

[0082] First, a welded joint made of duplex stainless steel is manufactured. Specifically, a pair of duplex stainless steel materials is prepared. A groove is formed at the end of the prepared duplex stainless steel materials. The groove shape is a U-groove with a groove angle of 20°. The grooves of the pair of duplex stainless steel materials are butt-jointed and welded. A welding material is prepared whose chemical composition satisfies the above-mentioned characteristic 1. Using the prepared welding material, the pair of duplex stainless steel materials are welded by automatic gas tungsten arc welding (GTAW). The heat input at this time is set to 0.5 to 4.00 kJ / mm. A test piece containing the weld in the center is taken from the manufactured duplex stainless steel welded joint. Specifically, a test piece extending perpendicular to the direction of extension of the weld in the duplex stainless steel welded joint is taken. The size of the test piece is 2 mm thick × 10 mm wide × 40 mm long. The test piece is taken so that the weld metal is positioned at the center of the longitudinal direction of the test piece. Furthermore, the test specimens should be taken such that the maximum width of the weld metal in the longitudinal direction of the specimen is 25 mm or less.

[0083] Using the collected test specimens, an ASTM A262 Practice C nitric acid corrosion test will be performed. Specifically, a test solution will be prepared, which is an aqueous solution of 65% by mass of nitric acid. The test specimens will be immersed in the boiling test solution for 48 hours (first immersion test). After the test is complete, a new test solution will be prepared, and the immersion test will be performed in the same manner as the first time. Specifically, the test specimens will be removed from the test solution used in the first immersion test, and immersed in the test solution for the second immersion test for 48 hours. This immersion test will be repeated 10 times (from the 1st to the 10th test).

[0084] Before and after each immersion test (1st to 10th), the mass of the test specimen is measured, and the difference (mass loss) is calculated. Based on the mass loss, for each immersion test, the mass loss per unit area per unit time of the test specimen (hereinafter referred to as unit mass loss, in units of g / m²) is calculated. 2 Calculate the corrosion rate (g / m³). The arithmetic mean of the unit mass loss over the 10 measurements (1st to 10th) is used to determine the corrosion rate (g / m³). 2 Let's use / h).

[0085] The resulting corrosion rate was 0.100 g / m 2 If the value is less than / h, it is determined that sufficient resistance to intergranular corrosion has been achieved when using a welded joint.

[0086] [Regarding the shape of the duplex stainless steel material in this embodiment] As described above, the duplex stainless steel material of this embodiment may be a steel pipe, a steel plate, or a steel bar. Preferably, the duplex stainless steel material of this embodiment is a steel pipe. More preferably, the duplex stainless steel material of this embodiment is a seamless steel pipe.

[0087] [Applications of the duplex stainless steel material of this embodiment] The duplex stainless steel material of this embodiment is widely applicable to applications requiring resistance to intergranular corrosion. In particular, the duplex stainless steel material of this embodiment is suitable for steel materials in urea manufacturing plants. However, the applications of the duplex stainless steel material of this embodiment are not limited to the above-mentioned range.

[0088] [Manufacturing method for duplex stainless steel materials] An example of a method for manufacturing the duplex stainless steel material of this embodiment will be described. The method for manufacturing the duplex stainless steel material described below is just one example for manufacturing the duplex stainless steel material of this embodiment. Therefore, the duplex stainless steel material having the above-described configuration may be manufactured by a manufacturing method other than the one described below. However, the manufacturing method described below is a preferred example of a method for manufacturing the duplex stainless steel material of this embodiment.

[0089] An example of the method for producing a duplex stainless steel material according to the present embodiment includes the following steps. (Step 1) Material preparation step (Step 2) Hot working step (Step 3) Cold working step (Step 4) Solution treatment step

[0090] Main production conditions in the above steps 1 to 4 are as follows. (Condition 1) Area reduction ratio R1 in the hot working step: 60% or more (Condition 2) Cooling rate CR1 after hot working: 50°C / sec or more (water cooling) (Condition 3) Area reduction ratio R2 in the cold working step: 60% or more (Condition 4) FA defined by formula (A): 150 to 500 FA=R1 / 100×K CR ×R2 / 100×T1×(t1 / 60) 0.5 (A) Here, T1 in formula (A) is the solution treatment temperature (°C) in the solution treatment step, and t1 is the holding time (minutes) at the solution treatment temperature T1. Further, K CR is assigned a value of "1.2" when the cooling rate CR1 immediately after completion of hot working is 50°C / sec or more (water cooling), and is assigned a value of "0.8" when the cooling rate CR1 immediately after completion of hot working is less than 50°C / sec (air cooling). Each step will be described below.

[0091] [(Step 1) Material preparation step] In the material preparation process, a material having a chemical composition satisfying characteristic 1 is prepared. The material may be an ingot, a slab, a bloom, or a billet. When manufacturing the material, the material is manufactured by the following method: Molten steel having a chemical composition satisfying characteristic 1 is manufactured. An ingot is manufactured using the manufactured molten steel by the ingot-making method. A slab, a bloom, or a billet (cylindrical material) may be manufactured using the manufactured molten steel by the continuous casting method. A billet may be manufactured by hot working on the manufactured ingot, slab, or bloom. For example, a cylindrical billet may be manufactured by hot forging on an ingot, and this billet may be used as the material (cylindrical material). In this case, the temperature of the material immediately before the start of hot forging is not particularly limited, but for example, it is 1000 to 1300°C. The method of cooling the material after hot forging is not particularly limited.

[0092] [(Process 2) Hot working process] In the hot working process, the materials prepared in the material preparation process are subjected to hot working to produce intermediate steel materials. The intermediate steel materials may be steel pipes, steel plates, or steel bars.

[0093] When the intermediate material is a steel pipe, the following processing is carried out in the hot working process. First, a cylindrical material is prepared. A through hole is formed along the central axis of the cylindrical material by machining. Hot extrusion, such as the Eugène Séjournet method, is performed on the cylindrical material with the through hole to produce the intermediate material (seamless steel pipe). The temperature of the material immediately before hot extrusion is not particularly limited. For example, the heating temperature of the material immediately before hot extrusion is 1000 to 1300°C.

[0094] When the intermediate material is steel sheet, the hot working process uses, for example, one or more rolling mills equipped with a pair of work rolls. Steel sheets are manufactured by hot rolling of materials such as slabs using a rolling mill. The heating temperature of the material during hot rolling is, for example, 1000 to 1300°C.

[0095] When the intermediate material is steel bars, the hot working process includes, for example, a rough rolling process and a finish rolling process. In the rough rolling process, the material is hot-worked to produce billets. The rough rolling process uses, for example, a bloc mill. The bloc mill performs bloc rolling on the bloom to produce billets. If a continuous rolling mill is located downstream of the bloc mill, the billets after bloc rolling may be further hot-rolled using the continuous rolling mill to produce smaller billets. In a continuous rolling mill, for example, horizontal stands with a pair of horizontal rolls and vertical stands with a pair of vertical rolls are arranged alternately in a row. The material temperature immediately before the rough rolling process is not particularly limited, but is, for example, 1000 to 1300°C. In the finish rolling process, the billets are first heated. After heating, the billets are hot-rolled using a continuous rolling mill to produce steel bars. The heating temperature in the heating furnace during the finishing rolling process is not particularly limited, but is, for example, 1000 to 1200°C.

[0096] The intermediate steel material is rapidly cooled immediately after hot working. Specifically, the intermediate steel material is water-cooled immediately after hot working. Water cooling results in a cooling rate CR1 of 50°C / second or more for the intermediate steel material. Water cooling cools the intermediate steel material to room temperature. By performing water cooling on the intermediate steel material, the release of strain accumulated during the hot working process is suppressed.

[0097] Furthermore, the intermediate steel material, after water cooling, is subjected to the next cold working process without any heat treatment such as annealing. This prevents the release of strain accumulated during the hot working process.

[0098] [(Process 3) Cold working process] In the cold working process, cold working is performed on the intermediate steel material produced in the hot working process. If the intermediate steel material is a steel pipe or bar, the cold working is cold drawing or cold Pilger rolling. If the intermediate steel material is a steel plate, the cold working is, for example, cold rolling. By performing the cold working process, further strain is accumulated in the intermediate steel material before solution treatment. As a result, during solution treatment, fine austenite precipitates using the accumulated strain as nuclei, and as a result, ferrite with less thickness variation in the L direction is obtained.

[0099] Furthermore, descaling treatment may be performed on intermediate steel materials after the hot working process before cold working. Descaling treatment is not required. If descaling treatment is performed, it may be, for example, shot blasting and / or pickling.

[0100] [(Step 4) Solution treatment process] In the solution treatment process, the intermediate steel material after the cold working process is subjected to solution treatment. In the solution treatment, precipitates are dissolved. In this embodiment, the solution treatment further forms ferrite with less variation in thickness in the L direction due to the strain accumulated in the intermediate steel material during the hot working and cold working processes. The solution treatment temperature T1 is 1000 to 1200°C. The holding time t1 at the solution treatment temperature T1 is 1.00 to 50.00 minutes.

[0101] [Regarding Conditions 1-4] In the manufacturing process described above, each step is carried out in such a way that the following conditions are met. (Condition 1) Surface reduction ratio R1 in the hot working process: 60% or more (Condition 2) Cooling rate after hot working CR1: 50℃ / sec or higher (water cooling) (Condition 3) Area reduction ratio R2 in the cold working process: 60% or more (Condition 4) FA defined by equation (A): 150~500 FA = R1 / 100 × K CR ×R2 / 100×T1×(t1 / 60) 0.5 (A) Here, in equation (A), T1 is the solution temperature (°C) in the solution treatment process, and t1 is the holding time (minutes) at the solution temperature T1. Also, K CR If the cooling rate CR1 immediately after the completion of hot working is 50°C / second or higher (water cooling), "1.2" is substituted; if the cooling rate CR1 immediately after the completion of hot working is less than 50°C / second (air cooling), "0.8" is substituted. The following explains each condition.

[0102] [Regarding Condition 1] The reduction ratio R1 (%) in the hot working process is defined by the following formula. Area reduction ratio R1 = (1 - Cross-sectional area perpendicular to the longitudinal direction of the intermediate steel material after hot working / Cross-sectional area perpendicular to the longitudinal direction of the material) × 100 If the reduction ratio R1 is less than 60%, the strain accumulated in the intermediate steel is insufficient. Therefore, the duplex stainless steel material after manufacturing cannot satisfy characteristic 2. Thus, the reduction ratio R1 must be 60% or more.

[0103] [Regarding Condition 2] The cooling rate CR1 of the intermediate steel material immediately after hot working should be 50°C / second or higher. This cooling rate is achieved by water cooling. If the intermediate steel material is air-cooled, the cooling rate CR1 will be slower than 50°C / second. If air cooling is performed instead of water cooling on the intermediate steel material immediately after hot working, the strain accumulated in the intermediate steel material due to hot working will be released during cooling. Therefore, the amount of strain required to exhibit the metallic structure of characteristic 2 in the solution treatment will be insufficient. Accordingly, the cooling rate CR1 should be 50°C / second or higher.

[0104] [Regarding Condition 3] The reduction ratio R2 (%) in the cold working process is defined by the following formula: Area reduction ratio R2 = (1 - Cross-sectional area perpendicular to the longitudinal direction of the intermediate steel material after cold working / Cross-sectional area perpendicular to the longitudinal direction of the intermediate steel material before cold working) × 100 If the reduction ratio R2 is less than 60%, the strain accumulated in the intermediate steel is insufficient. Therefore, the duplex stainless steel material after manufacturing cannot satisfy characteristic 2. Thus, the reduction ratio R2 must be 60% or more.

[0105] [Regarding Condition 4] In the manufacturing process described above, the FA defined by equation (A) is between 150 and 500. FA = R1 / 100 × K CR ×R2 / 100×T1×(t1 / 60) 0.5 (A) Here, in equation (A), T1 is the solution temperature (°C) in the solution treatment process, and t1 is the holding time (minutes) at the solution temperature T1. Also, K CR If the cooling rate CR1 immediately after the completion of hot working is 50°C / second or higher (water cooling), "1.2" is substituted; if the cooling rate CR1 immediately after the completion of hot working is less than 50°C / second (air cooling), "0.8" is substituted.

[0106] FA is a manufacturing condition for the metal structure of duplex stainless steel to satisfy characteristic 2. Among FA is "R1 / 100×K CR "×R2 / 100" is a factor related to the amount of strain accumulated, and "T1×(t1 / 60) 0.5 This is a factor that causes austenite precipitation during solution treatment. If FA is between 150 and 500, sufficient strain has accumulated in the intermediate steel material before solution treatment, and the conditions for solution treatment are also appropriate. Therefore, the microstructure of the duplex stainless steel material after manufacturing satisfies characteristic 2.

[0107] Through the above manufacturing process, a duplex stainless steel material that satisfies features 1 and 2 can be produced.

[0108] The effects of the duplex stainless steel material of this embodiment will be further explained in detail below with reference to examples. The conditions in the following examples are just one example of conditions adopted to confirm the feasibility and effects of the duplex stainless steel material of this embodiment. Therefore, the duplex stainless steel material of this embodiment is not limited to this one example of conditions. [Examples]

[0109] Duplex stainless steel pipes (seamless steel pipes) having the chemical compositions shown in Tables 1-1 and 1-2 were manufactured.

[0110] [Table 1-1]

[0111] [Table 1-2]

[0112] In Table 1-2, "-" indicates that the corresponding element content is 0% in terms of the significant figures (number to the least significant digit) specified in the embodiment. In other words, it means that the corresponding element content is 0% when rounded to the least significant digit as specified in the embodiment described above. For example, the Cu content of test number 1 was "0"% when rounded to the third decimal place. The remainder of the elements other than those listed in Tables 1-1 and 1-2 consisted of Fe and impurities.

[0113] Specifically, first, in the material preparation process, blooms having the chemical compositions shown in Tables 1-1 and 1-2 were produced. Cylindrical materials (round billets) were produced by hot forging the blooms. The heating temperature of the blooms during hot forging was 1100-1250°C. The round billets were allowed to cool to room temperature after hot forging.

[0114] In the hot working process, steel pipes (seamless steel pipes), which are intermediate steel materials, were manufactured by hot extrusion of round billets. The heating temperature of the round billets during hot working was 1100-1200°C. The reduction ratio R1 (%) during hot working is shown in Table 2.

[0115] [Table 2]

[0116] The intermediate steel material was cooled to room temperature immediately after hot working. The cooling rate CR1 (°C / sec) is shown in Table 2. Cold working was performed on the cooled intermediate steel material without any annealing treatment. Specifically, the intermediate steel material was cold-worked using a Pilger rolling mill. The surface area reduction ratio R2 (%) during cold working is shown in Table 2.

[0117] A solution treatment process was performed on the intermediate steel material after the cold working process. In the solution treatment process, the solution treatment temperature T1 was set to 1000 to 1200°C, and the holding time t1 at the solution treatment temperature T1 was set to 1.00 to 50.00 minutes. The FA values ​​for each test number are shown in Table 2. Through the above manufacturing process, duplex stainless steel material (seamless steel pipe) was produced.

[0118] [Evaluation Test] The following evaluation tests were performed on each test number of the manufactured duplex stainless steel material. (Test 1) Ferrite area ratio measurement test (Test 2) Measurement test of average ferrite thickness TF, sample standard deviation ΔTF, and average austenite thickness TA (Test 3) Evaluation test of intergranular corrosion resistance when using welded joints The following explains Exams 1 through 3.

[0119] [(Test 1) Ferrite Area Ratio Measurement Test] Based on the method described in the above-mentioned [Method for Measuring Ferrite Area Ratio], the ferrite area ratio of the duplex stainless steel material for each test number was determined. As a result, in all test numbers, the microstructure consisted of ferrite and austenite, and the ferrite area ratio was 35-55%.

[0120] [(Test 2) Measurement test of average ferrite thickness TF, sample standard deviation ΔTF, and average austenite thickness TA] Based on the above-described [measurement method for average ferrite thickness TF, sample standard deviation of ferrite thickness ΔTF, and average austenite thickness TA], the average ferrite thickness TF, sample standard deviation ΔTF, and average austenite thickness TA were determined for the duplex stainless steel material for each test number. The results are shown in Table 2.

[0121] [(Test 3) Evaluation test of intergranular corrosion resistance when using welded joints] Welded joints of duplex stainless steel for each test number were manufactured using the method described in [Regarding the Evaluation Method for Intergranular Corrosion Resistance] above. Then, using the method described in [Regarding the Evaluation Method for Intergranular Corrosion Resistance] above, nitric acid corrosion tests according to ASTM A262 Practice C were performed on test specimens taken from the welded joints to determine the corrosion rate (g / m²). 2 The corrosion rate ( / h) was calculated. The calculated corrosion rates are shown in Table 2.

[0122] [Evaluation Results] Referring to Tables 1-1, 1-2, and 2, the duplex stainless steel materials for test numbers 1 to 13 satisfied features 1 and 2. Therefore, the corrosion rate of these duplex stainless steel materials when used in welded joints was 0.100 g / m². 2 The value was less than / h, and sufficient resistance to intergranular corrosion was achieved.

[0123] On the other hand, in test number 14, the reduction ratio R1 during the hot working process was less than 60%. Therefore, the duplex stainless steel material did not meet characteristic 2. As a result, the corrosion rate when used as a welded joint was 0.100 g / m 2 The value exceeded / h, and sufficient resistance to intergranular corrosion could not be obtained.

[0124] In test number 15, the cooling rate CR1 after hot working was less than 50°C / second. Therefore, the FA value was less than 150. Consequently, the duplex stainless steel material did not meet characteristic 2. As a result, the corrosion rate when used as a welded joint was 0.100 g / m². 2 The value exceeded / h, and sufficient resistance to intergranular corrosion could not be obtained.

[0125] In test number 16, the reduction ratio R2 during the cold working process was less than 60%. Therefore, the duplex stainless steel material did not meet characteristic 2. As a result, the corrosion rate when used as a welded joint was 0.100 g / m 2 The value exceeded / h, and sufficient resistance to intergranular corrosion could not be obtained.

[0126] In tests 17 through 19, although the reduction ratio R1, cooling rate CR1, and reduction ratio R2 were appropriate, the FA value was less than 150. Therefore, the duplex stainless steel material did not meet characteristic 2. As a result, the corrosion rate when used as a welded joint was 0.100 g / m². 2 The value exceeded / h, and sufficient resistance to intergranular corrosion could not be obtained.

[0127] In test numbers 20 and 21, although the reduction ratio R1, cooling rate CR1, and reduction ratio R2 were appropriate, the FA value exceeded 500. Therefore, the duplex stainless steel material did not meet characteristic 2. As a result, the corrosion rate when used as a welded joint was 0.100 g / m 2 The value exceeded / h, and sufficient resistance to intergranular corrosion could not be obtained.

[0128] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure.

Claims

1. Duplex stainless steel material, The chemical composition is expressed in mass percent. C: 0.030% or less, Si: 0.50% or less, Mn: 2.00% or less, P: 0.040% or less, S: 0.0010% or less, Cr: 26.0-28.0%, Ni: 6.0 to 10.0%, Mo: 0.20-1.70%, W: more than 2.00 to 3.00%, N: more than 0.30 to 0.40%, O: 0.020% or less, Al: 0.050% or less, Cu: 0 to 0.30%, Co: 0 to 1.0%, Ti: 0-0.300%, Nb: 0-0.300%, Ca: 0-0.010%, Mg: 0 to 0.010%, B: 0-0.010%, and, The remainder consists of Fe and impurities. The longitudinal direction of the duplex stainless steel material is defined as the L direction, and the thickness direction of the duplex stainless steel material is defined as the T direction. In the cross-section of the duplex stainless steel material including the L direction and the T direction, three rectangular regions are identified at 100 mm intervals in the L direction, and each identified region is a rectangle measuring 200 μm in the L direction and 200 μm in the T direction. In each of the aforementioned rectangular regions, When line segments extending in the T direction, arranged at equal intervals in the L direction of the rectangular region, and dividing the rectangular region into six equal parts in the L direction, are defined as line segments LS, The average thickness TF of each ferrite overlapping with the 15 line segments LS of the three rectangular regions is 2.50 to 4.50 μm, and the sample standard deviation ΔTF of the ferrite thickness is 0.50 μm or less. The average thickness TA of each austenite overlapping with the 15 line segments LS is 2.50 to 4.50 μm. Duplex stainless steel material.

2. A duplex stainless steel material according to claim 1, The aforementioned chemical composition is Cu: 0.01-0.30%, Co: 0.1 to 1.0%, Ti: 0.001 to 0.300%, Nb: 0.001-0.300%, Ca: 0.001-0.010%, Mg: 0.001-0.010%, and, B: 0.001 to 0.010%, It contains one or more elements selected from the group consisting of, Duplex stainless steel material.

3. A duplex stainless steel material according to claim 1 or claim 2, The aforementioned duplex stainless steel material is a seamless steel pipe. Duplex stainless steel material.

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