Duplex stainless steel, steel plate, and method for manufacturing the steel plate

Abstract

To provide a duplex stainless steel sheet having excellent surface properties, a duplex stainless steel sheet for providing the steel sheet through hot / cold working, and a method for manufacturing the steel sheet. [Solution] This is a duplex stainless steel sheet having a rolled two-phase structure with a matrix of austenite and dispersed ferrite particles. The predetermined chemical composition satisfies 70<([P]×5000+[S]×500+[Sn]×5300+[O]×2500)<280. The duplex stainless steel that is rolled to produce the sheet has a two-phase structure with a matrix of austenite and dispersed ferrite particles, and the shape of the ferrite particles in a cross section perpendicular to the longitudinal direction satisfies 4π×S1 / L. 2 The simple average circularity defined by is 0.10 or more. The manufacturing method involves heating a slab made of steel having the same predetermined component composition and a dual-phase structure in which the shape of the ferrite phase particles in a cross section perpendicular to the longitudinal direction has a simple average circularity of 0.10 or more to a temperature between 1100°C and 1270°C, and rolling it.

Description

[Technical Field]

[0001] The present invention relates to a duplex stainless steel sheet having excellent surface properties, a duplex stainless steel for providing such a steel sheet through hot and / or cold working, and a method for producing the steel sheet. [Background technology]

[0002] Duplex stainless steels, which are made of ferrite and austenite, have excellent seawater resistance and stress corrosion cracking resistance, and also generally have high strength, and are therefore used for components of seawater condensers, heat exchangers, flue gas desulfurization systems, various chemical plant equipment, etc. Here, duplex stainless steel materials are required to have high hot workability because they are subjected to hot working such as hot rolling and hot extrusion during production.

[0003] For example, Patent Document 1 discloses a method for improving the hot workability of duplex stainless steel by controlling the balance of the contents of Cr, Mo, and N, which are indicators of pitting corrosion resistance, as well as the balance of the contents of N, S, and B to suppress embrittlement of the phase boundary between ferrite and austenite. In other words, it states that the cause of the deterioration of hot workability is thought to be the occurrence and propagation of cracks at the interface (phase boundary) between ferrite and austenite or at the grain boundary, and therefore that the balance of the contents of N, S, and B, which strengthen the phase boundary and the grain boundary, should be controlled. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2024-006717 Summary of the Invention [Problem to be solved by the invention]

[0005] Generally, stainless steel sheets that are not painted are required to have excellent surface properties. In the manufacturing process of duplex stainless steel, cracks that occur during hot working as described above have a significant impact on the surface properties. Even small cracks that do not develop into large cracks that cause problems during hot forging can affect the surface properties of the steel sheet. Therefore, duplex stainless steel materials with controlled cracks and superior surface properties have been sought.

[0006] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a duplex stainless steel sheet having excellent surface properties, a duplex stainless steel for providing such a steel sheet through hot and / or cold working, and a method for manufacturing the steel sheet. [Means for solving the problem]

[0007] The present invention relates to a duplex stainless steel sheet having a rolled duplex structure in which ferrite phase particles are dispersed in a matrix of an austenite phase, and the duplex stainless steel sheet contains, in mass%, 15.0 to 19.0% Cr, 2.0 to 7.0% Ni, 1.6 to 5.0% Mo, 0.001 to 0.200% C, 0.01 to 0.90% Si, 0.01 to 2.00% Mn, 0.010 to 0.050% P, 0.0001 to 0.0050% S, 0.01 to 1.00% Cu, and 0.01 to 1.00% Al. The composition comprises P: 0.0003 to 0.0040%, Sn: 0.0003 to 0.0040%, N: 0.001 to 0.200%, O: 0.0005 to 0.0100%, Nb: 0.05% or less, with the balance being Fe and unavoidable impurities, and the composition satisfies 70<([P]×5000+[S]×500+[Sn]×5300+[O]×2500)<280, where [M] is the mass% of element M.

[0008] These features make it possible to produce a duplex stainless steel sheet with excellent surface properties.

[0009] In the above invention, in optical observation of the surface, the number density of surface defects is 14 / 100m 2The present invention may be characterized as follows: According to such a feature, a duplex stainless steel sheet having more excellent surface properties can be obtained.

[0010] In the above invention, in the component composition, δ cal δ defined as =4.3×(1.3×[Si]+[Cr]+[Mo]+2.2×[Al]+[Nb]+0.3×[Sn])-3.9×(30×[C]+30×[N]+[Ni]+0.8×[Mn]+0.3×[Cu])-31.5 cal may be in the range of 5 to 25. According to this feature, a duplex stainless steel sheet with more excellent surface properties can be obtained.

[0011] The present invention also provides a duplex stainless steel that is rolled to provide a plate material, the duplex stainless steel comprising, by mass%, 15.0 to 19.0% Cr, 2.0 to 7.0% Ni, 1.6 to 5.0% Mo, 0.001 to 0.200% C, 0.01 to 0.90% Si, 0.01 to 2.00% Mn, 0.010 to 0.050% P, 0.0001 to 0.0050% S, 0.0001 to 0.0050% Cu, 0.01 to 1.00% Al, 0.0003 to 0.0040% Sn, 0.0003 to 0.0040% N, The alloy has a composition containing O: 0.0005 to 0.0100%, Nb: 0.05% or less, the balance being Fe and unavoidable impurities, and the composition satisfies 70<([P]×5000+[S]×500+[Sn]×5300+[O]×2500)<280, where [M] is the mass% of element M, and has a two-phase structure in which ferrite phase particles are dispersed in a matrix made of austenite phase, and in the shape of the ferrite phase particles in a cross section perpendicular to the casting direction, S1: area (mm 2 ), L: perimeter of ferrite phase particle (mm), (circularity) = 4π × S1 / L 2 The simple average of the circularity defined by is 0.10 or more.

[0012] According to this feature, a duplex stainless steel sheet having excellent surface properties can be obtained.

[0013] In the above invention, in the component composition, δ cal δ defined as =4.3×(1.3×[Si]+[Cr]+[Mo]+2.2×[Al]+[Nb]+0.3×[Sn])-3.9×(30×[C]+30×[N]+[Ni]+0.8×[Mn]+0.3×[Cu])-31.5 cal may be in the range of 5 to 25. According to this feature, a duplex stainless steel sheet with more excellent surface properties can be obtained.

[0014] Furthermore, the present invention relates to a method for producing a duplex stainless steel sheet having a rolled duplex structure in which ferrite phase particles are dispersed in a matrix of an austenite phase, the duplex stainless steel sheet comprising, by mass%, 15.0 to 19.0% Cr, 2.0 to 7.0% Ni, 1.6 to 5.0% Mo, 0.001 to 0.200% C, 0.01 to 0.90% Si, 0.01 to 2.00% Mn, 0.010 to 0.050% P, 0.0001 to 0.0050% S, 0.0001 to 0.0050% Cu, and 0.0003 to 0.0040% Al. %, Sn: 0.0003 to 0.0040%, N: 0.001 to 0.200%, O: 0.0005 to 0.0100%, Nb: 0.05% or less, the balance being Fe and unavoidable impurities, and the composition satisfies 70<([P]×5000+[S]×500+[Sn]×5300+[O]×2500)<280 when the mass% of element M is [M], and in the shape of the ferrite phase particle in the cross section perpendicular to the casting direction, S1: area (mm 2 ), L: perimeter of ferrite phase particle (mm), (circularity) = 4π × S1 / L 2 The method is characterized in that a slab made of steel having a two-phase structure in which ferrite phase particles are dispersed in a matrix made of an austenite phase, with a simple average circularity defined as 0.10 or more, is heated to a temperature between 1100°C and 1270°C and rolled.

[0015] According to this feature, a duplex stainless steel sheet having excellent surface properties can be obtained.

[0016] In the above invention, in optical observation of the surface, the number density of surface defects is 14 / 100m2 The present invention may be characterized as follows: According to such a feature, a duplex stainless steel sheet having more excellent surface properties can be obtained.

[0017] In the above invention, in the component composition, δ cal δ defined as =4.3×(1.3×[Si]+[Cr]+[Mo]+2.2×[Al]+[Nb]+0.3×[Sn])-3.9×(30×[C]+30×[N]+[Ni]+0.8×[Mn]+0.3×[Cu])-31.5 cal may be in the range of 5 to 25. According to this feature, a duplex stainless steel sheet with more excellent surface properties can be obtained. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a scatter diagram showing the relationship between the circularity of ferrite phase particles in the cross section of a slab before hot rolling and the number density of surface defects in a duplex stainless steel plate. [Figure 2] FIG. 2 is a schematic diagram showing the relationship between the degree of circularity of ferrite phase particles and cracks caused by rolling. [Figure 3] FIG. 1 is a scatter diagram showing the relationship between the PSSnO value and the circularity of ferrite phase particles. [Figure 4] 1 is a list of chemical compositions of steels used in manufacturing tests. [Figure 5] 1 is a table showing the results of a manufacturing test. DETAILED DESCRIPTION OF THE INVENTION

[0019] The present inventors have conducted extensive research in order to produce a duplex stainless steel sheet with excellent surface properties.

[0020] First, we investigated surface defects that occurred in duplex stainless steel sheets. Specifically, we collected samples containing surface defects (cracks), and observed the cross-sectional structure of the surface defect area using SEM and optical microscope. As a result, we found that the crack originated at the ferrite / austenite phase interface and propagated along the ferrite / austenite phase interface. Furthermore, no nonmetallic inclusions that could cause cracks were found in the defect area. Based on these findings, we conducted further intensive research into the relationship between the morphology of the ferrite / austenite phase interface and cracks.

[0021] The study focused on the shape of ferrite phase particles in slabs before hot rolling. Samples were cut from duplex stainless steel slabs in multiple lots, and the observation surface was a cross section perpendicular to the casting direction. The samples were embedded in resin, and the observation surface was mirror-polished. After electrolytic etching in a 10% potassium hydroxide etching solution to reveal the structure, the structure of the observation surface was observed using an optical microscope. During the structure observation, the shape of the ferrite phase particles was focused on, and the particle area and perimeter of more than 100 ferrite phase particles were measured using image analysis software. The degree of sphericity of the ferrite phase particles was then evaluated by calculating the circularity, defined by the following formula: Circularity = 4π × S1 / L 2 In the formula, S1 is the area of ​​the ferrite phase particles (mm 2 ), L is the particle perimeter (mm).

[0022] Figure 1 shows the relationship between the circularity of ferrite phase particles and the surface defect properties of duplex stainless steel sheets. Here, the circularity was measured for multiple lots of slabs before hot rolling, and the number density of surface defects was investigated for the sheets obtained after hot rolling. It was found that the greater the circularity, i.e., the closer the shape of the ferrite phase particles is to a sphere, the fewer the surface defects. In particular, by setting the above-mentioned circularity to 0.10 or more, good surface properties with few surface defects were obtained.

[0023] The relationship between the circularity of ferrite phase particles and surface quality is shown schematically in Figure 2. As mentioned above, the key to achieving good surface quality in duplex stainless steel is to suppress the propagation of cracks along the austenite / ferrite phase interface, which is unique to duplex stainless steel.

[0024] First, as shown in Fig. 1(a), a case will be described in which the circularity of ferrite phase particles 12 in the matrix of austenite phase 11 in a slab 1 before hot rolling is low, i.e., the ferrite phase particles 12 have a thin, elongated shape. As shown in Fig. 1(b), when obtaining a steel sheet 2, if fine cracks are generated during hot rolling, the cracks 15 propagate along the austenite / ferrite phase interface, becoming larger cracks 15 and deteriorating the surface quality.

[0025] In contrast, as shown in Figure 1(c), if the ferrite phase particles 12 in the slab 1 before hot rolling have a high circularity, i.e., if the ferrite phase particles 12 have a shape close to a sphere, then even if fine cracks 15 are generated during hot rolling, the cracks 15 will not propagate, and excellent surface properties can be obtained, as shown in Figure 1(d).

[0026] Cracks that occur in the steel sheet 2 after hot rolling can be removed by grinding with a grinder if they are near the surface. While this may ensure the surface quality, it reduces yield and increases process load. Furthermore, if the circularity of the ferrite phase particles 12 before hot rolling is low, even if surface cracks are removed with a grinder after hot rolling, cracks may reappear on the surface during subsequent cold rolling. As shown in the figure, if the circularity of the ferrite phase particles 12 is low, cracks along the austenite / ferrite phase interface also exist inside the steel sheet. These cracks may propagate during cold rolling and appear as surface cracks.

[0027] Next, the inventors determined the relationship between each component of duplex stainless steel and the circularity according to the above formula by organizing a large amount of data.

[0028] As a result, it was found that there is a relationship between the value calculated from the content of P, S, Sn, and O and the circularity of the ferrite phase particles, as shown in Figure 3. In particular, it was concluded that duplex stainless steel with good surface properties can be obtained by controlling the content of these elements to the following "PSSnO value" within the range of more than 70 and less than 280. PSSnO value = ([P] x 5000 + [S] x 500 + [Sn] x 5300 + [O] x 2500) Here, [M] is the mass % content of element M in the chemical composition of steel. That is, the mass % content of the corresponding element is substituted for each element symbol.

[0029] There are still many unknowns about the mechanism by which ferrite phase particles become spheroidized (the circularity of ferrite phase particles in cross section increases) when the component composition satisfies the above-mentioned PSSnO value of more than 70 and less than 280. However, it is thought that the interfacial activation elements S, Sn, and O promote the spheroidization of ferrite phase particles by acting on the ferrite / austenite phase interface. In addition, P, which has the effect of lowering the melting point, is thought to suppress the formation of a network due to the bonding of ferrite particles by forming a residual liquid phase during solidification.

[0030] As described above, it was found that by precisely controlling the contents of P, S, Sn, and O and controlling the PSSnO value to be more than 70 and less than 280, the ferrite phase particles can be made spherical and surface defects can be suppressed.

[0031] Therefore, in this example, a composition was determined that contains, by mass%, 15.0-19.0% Cr, 2.0-7.0% Ni, 1.6-5.0% Mo, 0.001-0.200% C, 0.01-0.90% Si, 0.01-2.00% Mn, 0.010-0.050% P, 0.0001-0.0050% S, 0.01-1.00% Cu, 0.0003-0.0040% Al, 0.0003-0.0040% Sn, 0.001-0.200% N, 0.0005-0.0100% O, and up to 0.05% Nb. Furthermore, this composition has a PSSnO value in the range of more than 70 and less than 280. The duplex stainless steel sheet in this example has the composition shown on the left and has a rolled two-phase structure in which ferrite phase particles are dispersed in a matrix of austenite phase, thereby providing a duplex stainless steel sheet with excellent surface properties.

[0032] Furthermore, it is also preferable that the shape of the ferrite phase particles in the cross section perpendicular to the casting direction in the slab before hot rolling has a simple average circularity of 0.10 or more. By having such a microstructure in the slab, it is possible to suppress the propagation of cracks during hot rolling as described above, and to obtain excellent surface properties in the resulting duplex stainless steel sheet.

[0033] In the above-mentioned composition, δ defined by the following formula cal It is also preferable that δ is in the range of 5 to 25. cal The value of is a calculation formula that corresponds to the volume fraction of the ferrite phase in duplex stainless steel. cal If the value of δ is small, the volume fraction of the ferrite phase will be low, and the amount of segregated elements such as P and S at the grain boundaries during solidification will increase, making cracks more likely to occur during hot rolling. cal If the value of δ is large, the volume fraction of the ferrite phase increases, which increases the area fraction of the ferrite / austenite phase interface, which is the starting point for cracks during hot rolling, and increases the surface defects of the resulting duplex stainless steel material. calIt is preferable to set the value of within the above range. In the formula, [M] is the mass % of the content of element M in the chemical composition of the steel. δ cal =4.3×(1.3×[Si]+[Cr]+[Mo]+2.2×[Al]+[Nb]+0.3×[Sn])-3.9×(30×[C]+30×[N]+[Ni]+0.8×[Mn]+0.3×[Cu])-31.5

[0034] In addition, the number density of surface defects in the obtained duplex stainless steel sheet was 14 / 100m by optical observation of the surface. 2 It is also preferable that the method for measuring the number density of surface defects is as follows: The method for measuring the number density of surface defects will be described in detail later.

[0035] <Manufacturing Test> Next, a description will be given of the results of producing a duplex stainless steel sheet by manufacturing slabs having the multiple component compositions shown in FIG. 4 using the manufacturing method described below and then hot rolling the slabs under the conditions shown in FIG.

[0036] First, raw materials such as stainless steel scrap were melted in a 60-ton electric furnace. Subsequently, oxygen refining (oxidation refining) was performed to remove unnecessary carbon using the electric furnace and an argon-oxygen smelting furnace (AOD). Limestone and fluorite were then added to produce a CaO-SiO2-AlO3-MgO-F slag. Furthermore, pure Si and / or Al were added to perform Cr reduction for deoxidation, and Ar stirring was used to desulfurize the slag. The slag was then poured into a ladle, where the temperature was adjusted, and the components, such as P, S, and Sn, were adjusted. Finally, a 150-mm-thick slab was produced by continuous casting. Samples for component analysis were collected from the tundish during continuous casting.

[0037] Here, P, S, Sn, and O have a significant effect on the circularity of ferrite phase particles in the slab before hot rolling, so their contents must be precisely controlled. To precisely control the trace amounts of P, S, and Sn, we first investigated the raw materials. As a result, we identified that the main source of P contamination was deposits on the inner surface of pipe-shaped scrap in the raw materials. We also identified that the main source of S contamination was scrap of S-containing steels, such as free-cutting steel. Furthermore, we identified that the main source of Sn contamination was certain scrap, such as that with plating. Therefore, we selected the scrap used as raw materials to prevent unexpected contamination of P, S, and Sn.

[0038] Furthermore, when the P concentration is low during the refining process of the steelmaking process, an Fe-20 wt% P alloy is added to prevent P evaporation loss and distribution loss to the slag, making it possible to precisely control the P content.

[0039] Furthermore, the S content was precisely controlled by desulfurization using slag with a composition in which the mass ratio of CaO to SiO2 during the refining process was 1.5 to 22. In this desulfurization using slag, the O concentration in the molten metal is important as well as the slag composition, and as a result, the O content can be precisely controlled. Furthermore, the S content can be precisely controlled by controlling the concentrations of the deoxidizing agents Al, Si, and Mn in the molten metal. When the S concentration is low during the refining process of steelmaking, the addition of an Fe-25 wt% S alloy prevents distribution loss to the slag when S is added, making it possible to precisely control the S content.

[0040] Furthermore, when the Sn concentration is low during the refining process of the steelmaking process, adding Ni-40 wt% Sn alloy prevents Sn evaporation loss and enables precise control of the Sn content.

[0041] Samples were taken from the obtained slabs to measure the circularity of the ferrite phase particles. Each slab was then heated at the temperature and for the heating time described above and hot-rolled to produce steel plates with the thicknesses (3.0 to 6.0 mm) shown in the figure. The steel plates were then annealed and pickled to remove surface scale. They were then cold-rolled to produce duplex stainless steel plates with a width of 1 m and a thickness of 1 mm. The duplex stainless steel plates were manufactured into coils. In this way, duplex stainless steel plates having a rolled duplex structure in which ferrite phase particles are dispersed in an austenite matrix can be obtained.

[0042] The hot rolling conditions are determined as follows. Duplex stainless steel slabs cast using a continuous casting machine are heated in a heating furnace before hot rolling. If the heating temperature is too low, the ductility during hot rolling will be insufficient, resulting in surface cracks. If the heating temperature is too high, elements such as P and S, which are generated during solidification during continuous casting, will segregate to grain boundaries, exceeding the ductility recovery temperature (Nil temperature: the temperature at which embrittlement occurs as the temperature increases), resulting in poor ductility and brittle cracks at the grain boundaries, deteriorating surface quality. Furthermore, if the heating time before hot rolling is too short, the temperature throughout the slab will be uneven, resulting in a low temperature at the center. This will result in uneven ductility throughout the thickness during hot rolling, resulting in surface and internal cracks. If the heating time is too long, a thick scale will form on the slab surface. Even if a water spray is used to remove the scale during hot rolling, the scale will remain, resulting in fine cracks and deteriorating surface quality. Cracks will also occur at the interface between the scale and the duplex stainless steel. Therefore, the heating temperature before hot rolling is preferably 1100°C to 1270°C and the heating time is preferably 2.0 to 8.0 hours, and more preferably 1150°C to 1250°C and the heating time is preferably 3.0 to 6.0 hours.

[0043] The chemical composition of the duplex stainless steel was determined by quantitative analysis using an X-ray fluorescence analyzer for the above-mentioned sample for composition analysis. The oxygen content was determined by quantitative analysis using an inert gas impulse fusion infrared absorption method.

[0044] The circularity of the ferrite phase particles was determined as follows. Test pieces were prepared so that the observation surface was a cross section perpendicular to the longitudinal direction, which is the casting direction of the slab before hot rolling. The sampling position and size of the test piece were not particularly limited. The observation surface of the prepared test piece was mirror-polished. The mirror-polished observation surface was electrolytically etched in a 10% potassium hydroxide etching solution to reveal the structure. The observation surface with the revealed structure was observed in 10 fields of view using an optical microscope. The measurement field of view was not particularly limited, but in this case, it was 1.00 mm 2 The area was observed at 100x magnification. The ferrite phase was identified from the contrast in each field of view, and the particle area and perimeter of each identified ferrite phase particle were measured, and the circularity of each particle was calculated. The circularity was measured for a total of 100 or more particles in 10 fields of view, and the simple average was calculated and recorded. Image analysis software was used to measure the particle area and perimeter.

[0045] Regarding surface defects, the surface of the duplex stainless steel sheet was visually optically observed, and the number of surface defects caused by cracks during cold or hot working was counted. The measurement range was the entire surface of the entire length of a coil 1 m wide and 100 m long. If the number of surface defects in the measurement range was 1 or less, it was evaluated as the best and marked with a "◎", if there were 2 to 5 defects, it was evaluated as excellent and marked with a "◯", if there were 6 to 9 defects, it was evaluated as good and marked with a "△", if there were 10 to 14 defects, it was evaluated as acceptable and marked with a "□", and if there were 15 or more defects, it was evaluated as unacceptable and marked with an "×". Note that the measurement range was 100 m. 2 Therefore, if the number of surface defects in this measurement range is 1, the density is 1 / 100m 2 is.

[0046] As shown in Figure 5, the duplex stainless steel sheets obtained in Examples 1 to 18 were all evaluated as "fair" or better for surface defects, and relatively good surface quality was obtained. Of these, Examples 1 to 7 were evaluated as "excellent" or better for surface defects, and particularly good surface quality was obtained.

[0047] 4 again, in Examples 8, 9, and 12, the contents of P, S, Sn, and O were relatively high, and although the PSSnO values ​​were within the range of more than 70 and less than 280, they were relatively large at 256, 242, and 275. As a result, the circularity of the ferrite phase particles was relatively low, and cracks were observed at the ferrite / austenite phase interface, which are thought to have occurred during hot rolling and cold rolling, and the surface defects were evaluated as "good."

[0048] In Examples 10 and 11, the contents of P, S, Sn, and O were relatively low, and although the PSSnO values ​​were within the range of more than 70 and less than 280, they were relatively small at 70 and 75. As a result, the circularity of the ferrite phase particles was relatively low, and cracks were observed at the ferrite / austenite phase interface, which were thought to have occurred during hot rolling and cold rolling, and the surface defects were evaluated as "good."

[0049] For Example 13, δ cal The value of 29 was large. It is thought that the volume fraction of the ferrite phase increased, increasing the area ratio of the ferrite / austenite phase interface, which is the starting point for cracks. As a result, cracks were observed at the ferrite / austenite phase interface, and the surface defect rating was given a "fair" rating.

[0050] For Example 14, δ cal The value of 3 was small. It is thought that the volume fraction of the ferrite phase was low, and the amount of elements such as P and S that segregated at the grain boundaries during solidification increased, making it easier for fine cracks to occur during hot rolling. As a result, the surface defects were evaluated as "fair."

[0051] In Example 15, the heating temperature before hot rolling was relatively high at 1280° C. Therefore, it is considered that the segregation at the grain boundaries that occurred during solidification in continuous casting exceeded the nil temperature, causing a lack of ductility and resulting in brittle cracks at the grain boundaries. As a result, the surface defects were evaluated as "fair."

[0052] In Example 16, the heating temperature before hot rolling was relatively low at 1080°C. Therefore, it is considered that cracks occurred on the surface due to insufficient ductility during hot rolling, and cracks were likely to occur at the ferrite / austenite phase interface during hot and cold rolling. As a result, the surface defects were evaluated as "fair."

[0053] In Example 17, the heating time before hot rolling was relatively short at 1.8 hours. Therefore, the soaking time for the slab was insufficient, and hot rolling was performed while the center of the slab was kept at a low temperature. This is thought to have resulted in surface and internal cracks due to uneven ductility in the thickness direction. As a result, the surface defects were evaluated as "fair."

[0054] In Example 18, the heating time before hot rolling was relatively long at 8.3 hours. As a result, a thick layer of scale formed on the slab surface, and even after spraying water to remove the scale during hot rolling, the scale remained. This is thought to have caused fine cracks and deteriorated the surface quality. As a result, the surface defects were evaluated as "fair."

[0055] On the other hand, the surface defects were evaluated as "unacceptable" in all of Comparative Examples 1 to 8. Each example will be described below.

[0056] In Comparative Example 1, although the individual contents of P, S, Sn, and O were not particularly high, the overall contents were high, resulting in a large PSSnO value of 287. Furthermore, the circularity of the ferrite phase particles in the slab before hot rolling was reduced to 0.07. This is thought to have caused frequent cracking at the ferrite / austenite phase interface during hot and cold rolling. As a result, the surface defects were evaluated as "fail."

[0057] In Comparative Example 2, although the individual contents of P, S, Sn, and O were not particularly low, the overall contents were low, resulting in a small PSSnO value of 55. Furthermore, the circularity of the ferrite phase particles in the slab before hot rolling was reduced to 0.05. This is thought to have caused frequent cracking at the ferrite / austenite phase interface during hot and cold rolling. As a result, the surface defects were evaluated as "fail."

[0058] In Comparative Examples 3 and 4, the contents of two of the individual elements P, S, Sn, and O were particularly low, resulting in small PSSnO values ​​of 43 and 57, respectively. Furthermore, the circularity of the ferrite phase particles in the slab before hot rolling was small, 0.02 and 0.05, respectively. This is thought to have caused frequent cracking at the ferrite / austenite phase interface during hot and cold rolling. As a result, the surface defects were evaluated as "fail."

[0059] In Comparative Examples 5 and 6, the contents of one and two of the elements P, S, Sn, and O were particularly high, resulting in large PSSnO values ​​of 305 and 302, respectively. Furthermore, the circularity of the ferrite phase particles in the slabs before hot rolling was reduced to 0.04 and 0.06, respectively. This is thought to have resulted in frequent cracking at the ferrite / austenite phase interface during hot and cold rolling. As a result, the surface defects were evaluated as "fail."

[0060] In Comparative Example 7, the contents of P, S, and Sn were relatively high, and the content of O was particularly high, resulting in a large PSSnO value of 304. Furthermore, the circularity of the ferrite phase particles in the slab before hot rolling was reduced to 0.04. This is thought to be why cracks frequently occurred at the ferrite / austenite phase interface during hot and cold rolling. cal The value of 0 was also large. In particular, the high O content caused surface defects caused by non-metallic inclusions. As a result, the surface defects were rated as "fail."

[0061] In Comparative Example 8, the individual contents of P, S, Sn, and O were all high, resulting in a large PSSnO value of 332, and furthermore, the circularity of the ferrite phase particles in the slab before hot rolling was reduced to 0.01. This is thought to be why cracks frequently occurred at the ferrite / austenite phase interface during hot and cold rolling. cal As a result, the surface defect was evaluated as "fail."

[0062] As described above, with the steels of Examples 1 to 18, duplex stainless steel sheets with excellent surface properties could be produced.

[0063] The ranges of the chemical composition of the steels of the present invention, including Examples 1 to 18, are determined as follows.

[0064] C is necessary to ensure the mechanical strength of the resulting duplex stainless steel sheet. However, excessive C content causes precipitation of carbides such as Cr carbide, which leads to embrittlement. Taking these factors into consideration, the C content is set within the range of 0.001 to 0.200%, preferably 0.005 to 0.150%, and more preferably 0.010 to 0.120%, by mass.

[0065] Si is an effective element for deoxidation and is necessary to obtain a sufficient deoxidation effect. However, excessive Si content can form a σ phase, which reduces corrosion resistance and can cause embrittlement. In consideration of these factors, the Si content is set within the range of 0.01 to 0.90%, preferably 0.10 to 0.80%, and more preferably 0.20 to 0.70%, by mass.

[0066] Mn is an element that is useful for deoxidation and is necessary. However, excessive Mn content reduces corrosion resistance. Taking these factors into consideration, the Mn content is set within the range of 0.01 to 2.00%, preferably 0.10 to 1.50%, and more preferably 0.20 to 1.20%, by mass.

[0067] P is an important element because it has the effect of lowering the melting point of steel and suppresses the network-like connections of the ferrite phase by forming a residual liquid phase during solidification, making the ferrite phase particles uniform and spherical. However, if it is contained in excess, it segregates at grain boundaries, deteriorating hot workability and causing cracks during hot rolling. Taking these factors into consideration, the P content is set within the range of 0.010 to 0.050%, preferably 0.012 to 0.040%, and more preferably 0.015 to 0.038%, by mass.

[0068] S is an interface-activating element and is one of the important elements that acts on the ferrite / austenite phase boundary to spheroidize ferrite phase particles. However, excessive S content causes segregation at grain boundaries, deteriorating hot workability and causing cracks during hot rolling. In consideration of these factors, the S content is set within the range of 0.0001 to 0.0050%, preferably 0.0002 to 0.0040%, and more preferably 0.0003 to 0.0030%, by mass.

[0069] Ni is necessary to ensure the toughness of the resulting duplex stainless steel sheet. However, excessive Ni content increases the cost of the material. Taking these factors into consideration, the Ni content is set within the range of 2.0 to 7.0%, preferably 3.0 to 6.5%, and more preferably 3.5 to 6.0%, by mass.

[0070] Cr forms a dense passive film on the surface of duplex stainless steel sheet and is essential for maintaining corrosion resistance. However, excessive Cr content forms carbides, reducing corrosion resistance. Taking these factors into consideration, the Cr content is 15.0 to 19.0%, preferably 15.5 to 18.0%, and more preferably 16.0 to 17.0%, by mass.

[0071] Mo has the effect of improving corrosion resistance and is therefore necessary. However, excessive Mo content increases the cost of the material. Taking these factors into consideration, the Mo content is set within the range of 1.6 to 5.0% by mass, preferably 1.8 to 4.5%, and more preferably 2.0 to 4.0%.

[0072] Cu is necessary to ensure the toughness of the resulting duplex stainless steel sheet. However, excessive Cu content will form carbides, adversely affecting corrosion resistance. Taking these factors into consideration, the Cu content is set within the range of 0.01 to 1.00%, preferably 0.02 to 0.50%, and more preferably 0.03 to 0.30%, by mass.

[0073] Al is necessary as a deoxidizing element, particularly reducing the number of oxide-based inclusions that cause surface defects. However, excessive Al content reduces MgO bricks used in refractories in the steelmaking process, such as ladles, increasing the Mg concentration in the molten metal and generating non-metallic inclusions primarily composed of MgO·Al2O3 that tend to coarsen, adversely affecting surface quality. Taking these factors into consideration, the Al content is set within the range of 0.0003 to 0.0040% by mass, preferably 0.0005 to 0.0035%, and more preferably 0.0010 to 0.0030%.

[0074] Since excessive Nb content forms carbides and causes embrittlement, it is desirable to reduce the Nb content as much as possible. Therefore, the Nb content is set to 0.05% or less, preferably 0.04% or less, and more preferably 0.03% or less, by mass percent.

[0075] Sn is an interface-activating element that acts on the ferrite / austenite phase boundary to spheroidize ferrite phase particles, making it an important element that must be precisely controlled, even in trace amounts. However, excessive Sn content causes segregation at grain boundaries, degrading hot workability and causing cracks during hot rolling. Taking these factors into consideration, the Sn content is set within the range of 0.0003 to 0.0040%, preferably 0.0005 to 0.0028%, and more preferably 0.0010 to 0.0023%, by mass.

[0076] N is necessary to ensure the mechanical strength of the resulting duplex stainless steel sheet. However, excessive N content forms Cr nitrides, which adversely affect corrosion resistance. Taking these factors into consideration, the N content is set within the range of 0.001 to 0.200%, preferably 0.005 to 0.150%, and more preferably 0.010 to 0.120%, by mass.

[0077] O is an interface-activating element and is one of the important elements that acts on the ferrite / austenite phase boundary to spheroidize ferrite phase particles. However, excessive O content increases the number of nonmetallic inclusions that cause surface defects. As described above, controlling the O concentration in molten steel also controls desulfurization during the refining process, which allows for precise control of the S concentration. Taking these factors into consideration, O is limited to a range of 0.0005 to 0.0100%, preferably 0.0010 to 0.0090%, and more preferably 0.0020 to 0.0080%, by mass.

[0078] The PSSnO value is an index for sufficiently spheroidizing ferrite phase particles. Although the mechanism for spheroidizing ferrite phase particles remains largely unknown, the interfacially activating elements S, Sn, and O promote spheroidization of ferrite phase particles by acting on the ferrite / austenite phase interface. Furthermore, P, which lowers the melting point, is thought to contribute to the spheroidization of ferrite phase particles by forming a residual liquid phase during solidification, thereby suppressing the formation of a network due to the bonding of ferrite particles. On the other hand, excessive S, Sn, and O content excessively reduces the energy of the ferrite / austenite phase interface, which in turn leads to coarsening of ferrite phase particles. Furthermore, excessive P content is thought to result in an excessive amount of residual liquid phase during solidification, leading to a heterogeneous structure. Taking these factors into consideration, the PSSnO value is set to a range of greater than 70 and less than 280, preferably greater than 80 and less than 240, and more preferably greater than 100 and less than 170.

[0079] δ cal The value of δ is an index of the volume fraction of the ferrite phase in duplex stainless steel. cal If the value of δ is small, the volume fraction of the ferrite phase is reduced, which increases the segregation of segregating elements such as P and S at the grain boundaries during solidification, making cracks more likely to occur during hot rolling. cal A large value of increases the volume fraction of the ferrite phase, increases the area ratio of the ferrite / austenite phase interface, which is the starting point of cracks, and increases the number of surface defects. cal The value of is preferably in the range of 5 to 25, more preferably in the range of 8 to 23, and further preferably in the range of 10 to 20.

[0080] While typical embodiments of the present invention and modifications based thereon have been described above, the present invention is not necessarily limited to these, and a person skilled in the art will be able to find various alternative embodiments and modifications without departing from the spirit of the present invention or the scope of the appended claims.

Claims

1. A duplex stainless steel sheet having a rolled two-phase structure in which ferrite phase particles are dispersed in a base material made of an austenite phase, In mass%, Cr: 15.0-19.0%, Ni: 2.0 to 7.0%, Mo: 1.6-5.0%, C: 0.001-0.200%, Si: 0.01-0.90%, Mn: 0.01 to 2.00%, P: 0.010-0.050%, S: 0.0001 to 0.0050%, Cu: 0.01 to 1.00%, Al: 0.0003 to 0.0040%, Sn: 0.0003 to 0.0040%, N: 0.001-0.200%, O: 0.0005-0.0100%, Including, Nb: 0.05% or less, the balance being Fe and unavoidable impurities; The component composition is as follows, where the mass % of element M is [M]: 70<([P]×5000+[S]×500+[Sn]×5300+[O]×2500)<280 A duplex stainless steel plate characterized by satisfying the above.

2. In optical observation of the surface, the number density of surface defects was 14 / 100m ２ 2. The duplex stainless steel sheet according to claim 1, wherein:

3. In the above-mentioned composition, when the mass % of the element M is [M], δ ｃａｌ =4.3×(1.3×[Si]+[Cr]+[Mo] +2.2×[Al]+[Nb]+0.3×[Sn]) -3.9×(30×[C]+30×[N]+[Ni] +0.8×[Mn]+0.3×[Cu])-31.5 δ defined as ｃａｌ The duplex stainless steel sheet according to claim 1 or 2, characterized in that the value of β is in the range of 5 to 25.

4. A duplex stainless steel that is rolled to give a plate, In mass%, Cr: 15.0-19.0%, Ni: 2.0 to 7.0%, Mo: 1.6-5.0%, C: 0.001-0.200%, Si: 0.01-0.90%, Mn: 0.01 to 2.00%, P: 0.010-0.050%, S: 0.0001 to 0.0050%, Cu: 0.01 to 1.00%, Al: 0.0003 to 0.0040%, Sn: 0.0003 to 0.0040%, N: 0.001-0.200%, O: 0.0005-0.0100%, Including, Nb: 0.05% or less, the balance being Fe and unavoidable impurities; The component composition is as follows, where the mass % of element M is [M]: 70<([P]×5000+[S]×500+[Sn]×5300+[O]×2500)<280 Fulfilling It has a two-phase structure in which ferrite phase particles are dispersed in a matrix made of austenite phase, In the shape of the ferrite phase particle in a cross section perpendicular to the longitudinal direction, S １ : Area of ​​ferrite phase particles (mm ２ ), L: perimeter length of ferrite phase particle (mm), (circularity) = 4π × S １ / L ２ A duplex stainless steel characterized in that the simple average of the circularity defined by is 0.10 or more.

5. In the above-mentioned composition, when the mass % of the element M is [M], δ ｃａｌ =4.3×(1.3×[Si]+[Cr]+[Mo] +2.2×[Al]+[Nb]+0.3×[Sn]) -3.9×(30×[C]+30×[N]+[Ni] +0.8×[Mn]+0.3×[Cu])-31.5 δ defined as ｃａｌ The duplex stainless steel according to claim 4, characterized in that the value of β is in the range of 5 to 25.

6. A method for producing a duplex stainless steel sheet having a rolled two-phase structure in which ferrite phase particles are dispersed in a substrate made of an austenite phase, comprising: In mass%, Cr: 15.0-19.0%, Ni: 2.0 to 7.0%, Mo: 1.6-5.0%, C: 0.001-0.200%, Si: 0.01-0.90%, Mn: 0.01 to 2.00%, P: 0.010-0.050%, S: 0.0001 to 0.0050%, Cu: 0.01 to 1.00%, Al: 0.0003 to 0.0040%, Sn: 0.0003 to 0.0040%, N: 0.001-0.200%, O: 0.0005-0.0100%, Including, Nb: 0.05% or less, the balance being Fe and unavoidable impurities; The component composition is as follows, where the mass % of element M is [M]: 70<([P]×5000+[S]×500+[Sn]×5300+[O]×2500)<280 Fulfilling The shape of the ferrite grains in a cross section perpendicular to the casting direction is as follows: S １ : Area of ​​ferrite phase particles (mm ２ ), L: perimeter length of ferrite phase particle (mm), (circularity) = 4π × S １ / L ２ A method for manufacturing a duplex stainless steel sheet, characterized in that a slab made of steel having a two-phase structure in which ferrite phase particles are dispersed in a substrate made of an austenite phase, with a simple average circularity defined as above being 0.10 or more, is heated to a temperature between 1100°C and 1270°C and rolled.

7. In optical observation of the surface after rolling, the number density of surface defects is 14 / 100m ２ 7. The method for producing a duplex stainless steel sheet according to claim 6, wherein the following steps are performed:

8. In the above-mentioned composition, when the mass % of the element M is [M], δ ｃａｌ =4.3×(1.3×[Si]+[Cr]+[Mo] +2.2×[Al]+[Nb]+0.3×[Sn]) -3.9×(30×[C]+30×[N]+[Ni] +0.8×[Mn]+0.3×[Cu])-31.5 δ defined as ｃａｌ The method for producing a duplex stainless steel sheet according to claim 6 or 7, characterized in that the value of the hardness is in the range of 5 to 25.

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