Ferritic austenitic two-phase stainless steel

Abstract

To provide a ferritic austenitic two-phase stainless steel excellent in hot ductility and hot drawing.SOLUTION: A ferritic austenitic two-phase stainless steel has a composition comprising, on a mass basis, 0.001-0.050% of C, 0.01-0.50% of Si, 1.0-3.5% of Mn, 0.050% or less of P, 0.0300% or less of S, 1.5-3.0% of Ni, 19.6-23.0 of Cr, 0.01-1.00% of Mo, 0.01-1.00% of Cu, 0.010-0.120% of N, and the balance Fe with impurities. The ferritic austenitic two-phase stainless steel has a DF value expressed by the following formula(1): DF=7.2(Cr+0.88Mo+0.78Si)-8.9(Ni+0.03Mn+0.72Cu+22C+21 N)-44.9 (1) of 60.0-80.0,(where a symbol of element expresses a content (mass%) of each element), and an A value expressed by the following formula (2): A=1.38×DF-136.5×N (2) of 55.0 or more, (where DF is a value calculated by the formula (1), and N expresses a content (mass%) of N).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a ferritic-austenitic duplex stainless steel. [Background technology]

[0002] Ferritic-austenitic duplex stainless steel has excellent corrosion resistance and high strength, and is therefore used in building materials and structural materials. However, ferritic-austenitic duplex stainless steels often have poor hot workability due to the difference in temperature dependence between the ferrite and austenite phases in the high-temperature range. To improve this hot workability, attempts have been made to control trace elements (e.g., Patent Documents 1 to 3). However, the total elongation (hot ductility) at high temperatures (e.g., above 950°C) is often insufficient.

[0003] Furthermore, from the perspective of reducing the cost of ferritic-austenitic duplex stainless steel, there is also a need for lean ferritic-austenitic duplex stainless steels that use fewer alloying elements. Lean ferritic-austenitic duplex stainless steels often contain large amounts of N to ensure the stability of the austenite phase while reducing the amount of expensive Ni. N is known to increase the strength of the ferritic and austenitic phases and improve the corrosion resistance of the austenitic phase, and is therefore actively used. However, adding a large amount of N increases the ratio of the austenite phase (i.e., increases the interface between the ferrite phase and the austenite phase) and hardens the austenite phase, which reduces hot workability and often leads to reduced productivity due to issues such as edge breakage during hot rolling.

[0004] On the other hand, ferritic-austenitic duplex stainless steels are known to exhibit a superplastic phenomenon in which the ferrite and austenite phases mutually inhibit grain growth, maintaining fine recrystallized grains during high-temperature deformation. Attempts have been made to induce this superplastic phenomenon by controlling the ratio of each phase, the amount of solute N, and the precipitation of the σ phase (see, for example, Patent Documents 4 to 6). However, these attempts sometimes result in insufficient hot drawing. Insufficient hot drawing can result in reduced yields and increased susceptibility to surface defects during hot working, such as hot rolling, of the billet, leading to reduced productivity. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2020-59900 A [Patent Document 2] Japanese Patent Application Publication No. 2-258956 [Patent Document 3] Japanese Unexamined Patent Publication No. 56-142855 [Patent Document 4] Japanese Patent Application Publication No. 8-41594 [Patent Document 5] Japanese Patent Application Publication No. 8-13093 [Patent Document 6] Japanese Patent Application Publication No. 61-210158 Summary of the Invention [Problem to be solved by the invention]

[0006] As described above, conventional ferritic-austenitic duplex stainless steels have good hot ductility or good hot drawing ability, but have not been able to improve both hot ductility and hot drawing ability. Therefore, an object of the present invention is to provide a ferritic-austenitic duplex stainless steel that is excellent in hot ductility and hot drawing ability. [Means for solving the problem]

[0007] In order to solve the above problems, the inventors have conducted extensive research into lean ferritic-austenitic duplex stainless steels. As a result, they have discovered that by reducing the N content and appropriately adjusting the types and amounts of added elements, the ratio of ferrite phase can be increased, thereby improving hot drawing ability, and that by appropriately controlling the ratio of ferrite phase to austenite phase, superplasticity can be achieved, thereby improving hot ductility, and have completed the present invention.

[0008] That is, the present invention provides a steel sheet having a composition, on a mass basis, containing C: 0.001 to 0.050%, Si: 0.01 to 0.50%, Mn: 1.0 to 3.5%, P: 0.050% or less, S: 0.0300% or less, Ni: 1.5 to 3.0%, Cr: 19.6 to 23.0%, Mo: 0.01 to 1.00%, Cu: 0.01 to 1.00%, N: 0.010 to 0.120%, and the balance being Fe and impurities; The following formula (1): DF=7.2(Cr+0.88Mo+0.78Si)-8.9(Ni+0.03Mn+0.72Cu+22C+21N)-44.9 (1) (wherein the element symbols represent the content (mass%) of each element) is 60.0 to 80.0, The following formula (2): A=1.38×DF-136.5×N (2) (where DF is the value calculated by the above formula (1), and N represents the N content (mass%)) is 55.0 or more. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a ferritic-austenitic duplex stainless steel having excellent hot ductility and hot drawing ability. DETAILED DESCRIPTION OF THE INVENTION

[0010] The following is a detailed description of the embodiments of the present invention. The present invention is not limited to the following embodiments, and it should be understood that modifications and improvements made to the following embodiments based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention are also within the scope of the present invention. In this specification, the "%" designation for components means "% by mass" unless otherwise specified.

[0011] A ferritic-austenitic duplex stainless steel (hereinafter simply referred to as "duplex stainless steel") according to an embodiment of the present invention has a composition containing C: 0.001 to 0.050%, Si: 0.01 to 0.50%, Mn: 1.0 to 3.5%, P: 0.050% or less, S: 0.0300% or less, Ni: 1.5 to 3.0%, Cr: 19.6 to 23.0%, Mo: 0.01 to 1.00%, Cu: 0.01 to 1.00%, N: 0.010 to 0.120%, with the balance being Fe and impurities.

[0012] Here, in this specification, the term "stainless steel" is a concept that includes not only stainless steel products but also raw materials for manufacturing stainless steel products. The raw materials are not particularly limited, but may include continuously cast steel billets (CC billets), blooms, billets, beam blanks, and other steel billets obtained by blooming. Examples of stainless steel materials include rolled materials such as hot-rolled and cold-rolled materials. The shape of the stainless steel material is not particularly limited, but may include plate (including strip), bar, and tube shapes, as well as various types of shaped steel with cross sections such as T-shaped and I-shaped. In this specification, "ferritic-austenitic" refers to a metal structure that is primarily composed of two phases, ferrite and austenite, at room temperature. Therefore, "ferritic-austenitic" also includes metals that contain small amounts of phases other than ferrite and austenite (for example, martensite). Furthermore, in this specification, "impurities" refers to components that are mixed in during the industrial production of stainless steel due to various factors in raw materials such as ores and scraps, and in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention. For example, impurities include unavoidable impurities. An example of an impurity is O. The O content is, for example, 0.0001 to 0.0070%. Regarding the content of each element, "including xx% or less" means that the content is xx% or less, but includes an amount exceeding 0% (particularly, exceeding the impurity level).

[0013] The duplex stainless steel according to the embodiment of the present invention may further contain one or more selected from Nb: 0.010 to 0.500%, Ti: 0.010 to 0.500%, V: 0.01 to 0.50%, W: 0.05 to 0.50%, Co: 0.01 to 0.30%, B: 0.0002 to 0.0050%, Sn: 0.010 to 0.500%, Al: 0.010 to 0.050%, Mg: 0.0002 to 0.0100%, Ca: 0.0002 to 0.0100%, Ta: 0.050% or less, Ga: 0.050% or less, Zr: 0.01 to 0.50%, and REM: 0.0002 to 0.0100%, as needed. Each component will be described in detail below.

[0014] <C:0.001~0.050%> C is an element that has a significant effect on the stability of the austenite phase. If the C content is too high, ductility (workability) may decrease, or the precipitation of Cr carbides may be promoted, causing intergranular corrosion. Therefore, the C content is set to 0.050% or less, preferably 0.048% or less, and more preferably 0.045% or less. From the viewpoint of corrosion resistance, a low C content is preferable, but reducing the C content too much increases costs. Therefore, the C content is set to 0.001% or more, preferably 0.002% or more, and more preferably 0.005% or more.

[0015] <Si:0.01~0.50%> Si is added as a deoxidizing element and is also a useful element for improving oxidation resistance. However, if the Si content is too high, it will harden and the ductility will decrease. Therefore, the Si content is set to 0.50% or less, preferably 0.45% or less, more preferably 0.40% or less. Also, if the Si content is excessively reduced, the cost during steelmaking will increase. Therefore, the Si content is set to 0.01% or more, preferably 0.02% or more, more preferably 0.05% or more.

[0016] <Mn: 1.0 - 3.5%> Mn is an element that plays an important role in concentrating in the austenite phase and stabilizing the austenite phase. However, if the Mn content is too high, in addition to ductility, corrosion resistance and hot workability will also decrease. Therefore, the Mn content is set to 3.5% or less, preferably 3.4% or less, more preferably 3.3% or less. Also, if the Mn content is excessively reduced, the cost during steelmaking will increase. Therefore, the Mn content is set to 1.0% or more, preferably 1.1% or more, more preferably 1.2% or more.

[0017] <P: 0.050% or less> P is an element contained in raw materials such as Cr. If the P content is high, the formability will decrease. Therefore, the P content is set to 0.050% or less, preferably 0.048% or less, more preferably 0.045% or less. On the other hand, although a lower P content is preferred, there is a limit to reducing the P content. The lower limit of the P content is generally 0.001%, preferably 0.002%, more preferably 0.003%.

[0018] <S: 0.0300% or less> S is an element contained in various raw materials. S combines with Mn to form inclusions and may become the starting point of rust. Therefore, the lower the S content, the better the corrosion resistance. Therefore, the S content is set to 0.0300% or less, preferably 0.0250% or less, more preferably 0.0200% or less. On the other hand, there is a limit to reducing the S content. The lower limit of the S content is generally 0.0001%, preferably 0.0005%.

[0019] <Ni:1.5~3.0%> Ni is an austenite-forming element and is an important element for adjusting the stability of the austenite phase. Ni also has the effect of suppressing nitride precipitation and improving corrosion resistance. To achieve these effects, the Ni content is set to 1.5% or more, preferably 1.6% or more, more preferably 1.7% or more, and even more preferably 1.8% or more. On the other hand, if the Ni content is too high, not only will the raw material cost increase, but the high proportion of austenite phase may also cause problems such as stress corrosion cracking. Therefore, the Ni content is set to 3.0% or less, preferably 2.9% or less, and more preferably 2.8% or less.

[0020] <Cr:19.6~23.0%> Cr is an element necessary for ensuring corrosion resistance. To achieve this effect, the Cr content is set to 19.6% or more, preferably 19.8% or more, and more preferably 20.0% or more. On the other hand, if the Cr content is too high, it can cause cracks during hot working and increase the cost of the refining process. Therefore, the Cr content is set to 23.0% or less, preferably 22.9% or less, and more preferably 22.8% or less.

[0021] <Mo:0.01~1.00%> Mo is an element that improves corrosion resistance. To achieve this effect, the Mo content is set to 0.01% or more, preferably 0.02% or more, and more preferably 0.03% or more. On the other hand, if the Mo content is too high, the raw material cost increases. Therefore, the Mo content is set to 1.00% or less, preferably 0.95% or less, and more preferably 0.90% or less.

[0022] <Cu:0.01~1.00%> Like Mn and Ni, Cu is an austenite-forming element and has the effect of suppressing nitride precipitation and improving corrosion resistance. To achieve these effects, the Cu content is set to 0.01% or more, preferably 0.05% or more, and more preferably 0.10% or more. On the other hand, if the Cu content is too high, it will not only increase raw material costs but also reduce hot workability. Therefore, the Cu content is set to 1.00% or less, preferably 0.95% or less, and more preferably 0.90% or less.

[0023] <N:0.010~0.120%> Like C, N is an element that has a significant effect on the stability of the austenite phase. N is also an element that dissolves in solid solution to improve corrosion resistance. To achieve these effects, the N content is set to 0.010% or more, preferably 0.020% or more. On the other hand, if the N content is too high, ductility decreases and corrosion resistance also decreases due to the precipitation of Cr nitrides. Therefore, the N content is set to 0.120% or less, preferably 0.110% or less, and more preferably 0.100% or less.

[0024] <Nb:0.010~0.500%> Nb forms nitrides (NbN) and carbides (NbC) and has the effect of improving workability. To achieve this effect, the Nb content is set to 0.010% or more, preferably 0.013% or more, and more preferably 0.015% or more. On the other hand, if the Nb content is too high, ductility decreases. Therefore, the Nb content is set to 0.500% or less, preferably 0.300% or less, and more preferably 0.200% or less.

[0025] <Ti:0.010~0.500%> Like Nb, Ti also forms nitrides (TiN) and carbides (TiC) and has the effect of improving workability. To achieve this effect, the Ti content is set to 0.010% or more, preferably 0.015% or more, and more preferably 0.020% or more. On the other hand, if the Ti content is too high, ductility decreases. Therefore, the Ti content is set to 0.500% or less, preferably 0.300% or less, and more preferably 0.200% or less.

[0026] <V:0.01~0.50%> V forms nitrides and has the effect of improving workability. To achieve this effect, the V content is set to 0.01% or more, preferably 0.03% or more, and more preferably 0.05% or more. On the other hand, if the V content is too high, ductility and hot workability will decrease. Therefore, the V content is set to 0.50% or less, preferably 0.45% or less, and more preferably 0.40% or less.

[0027] <W:0.05~0.50%> W is an element effective in improving corrosion resistance. To achieve this effect, the W content is set to 0.05% or more, preferably 0.06% or more, and more preferably 0.08% or more. On the other hand, if the W content is too high, ductility decreases. Therefore, the W content is set to 0.50% or less, preferably 0.45% or less, and more preferably 0.40% or less.

[0028] <Co:0.01~0.30%> Co is an element effective in increasing high-temperature strength and improving hot workability. To achieve these effects, the Co content is set to 0.01% or more, preferably 0.02% or more, and more preferably 0.05% or more. On the other hand, if the Co content is too high, toughness decreases. Therefore, the Co content is set to 0.30% or less, preferably 0.25% or less, and more preferably 0.20% or less.

[0029] <B:0.0002~0.0050%> B is an element that segregates at grain boundaries to improve hot workability. To achieve this effect, the B content is set to 0.0002% or more, preferably 0.0010% or more, and more preferably 0.0015% or more. On the other hand, if the B content is too high, corrosion resistance will be significantly reduced. Therefore, the B content is set to 0.0050% or less, preferably 0.0045% or less, and more preferably 0.0040% or less.

[0030] <Sn:0.010~0.500%> Sn is an element that improves corrosion resistance. To exert this effect, the content of Sn is set to 0.010% or more, preferably 0.015% or more, more preferably 0.020% or more. On the other hand, if the content of Sn is too high, the hot workability will deteriorate. Therefore, the content of Sn is set to 0.500% or less, preferably 0.450% or less, more preferably 0.400% or less.

[0031] <Al: 0.010~0.050%> Al is an element effective for desulfurization and deoxidation. To exert these effects, the content of Al is set to 0.010% or more, preferably 0.015% or more, more preferably 0.020% or more. On the other hand, if the content of Al is too high, it will cause an increase in manufacturing defects and raw material costs. Therefore, the content of Al is set to 0.050% or less, preferably 0.048% or less, more preferably 0.045% or less.

[0032] <Mg: 0.0002~0.0100%> Mg is an element that not only has the effect of deoxidation but also refines the solidification structure. To exert these effects, the content of Mg is set to 0.0002% or more, preferably 0.0005% or more, more preferably 0.0010% or more. On the other hand, if the content of Mg is too high, it will lead to an increase in raw material costs. Therefore, the content of Mg is set to 0.0100% or less, preferably 0.0095% or less, more preferably 0.0090% or less.

[0033] <Ca: 0.0002~0.0100%> Ca is an element effective for desulfurization and deoxidation. To exert these effects, the content of Ca is set to 0.0002% or more, preferably 0.0005% or more, more preferably 0.0010% or more. On the other hand, if the content of Ca is too high, hot work cracks are likely to occur and the corrosion resistance also deteriorates. Therefore, the content of Ca is set to 0.0100% or less, preferably 0.0095% or less, more preferably 0.0090% or less.

[0034] <Ta: 0.050% or less> Ta is an element that improves corrosion resistance by modifying inclusions. However, if the content of Ta is too high, it will cause a decrease in room-temperature ductility and toughness. Therefore, the content of Ta is 0.050% or less, preferably 0.045% or less, more preferably 0.040% or less. On the other hand, the lower limit of the content of Ta is not particularly limited, but in order to exhibit the effect of Ta, it is preferably 0.001%, more preferably 0.003%.

[0035] <Ga: 0.050% or less> Ga is an element that contributes to improving corrosion resistance and suppressing hydrogen embrittlement. However, if the content of Ga is too high, the workability will decrease. Therefore, the content of Ga is 0.050% or less, preferably 0.040% or less, more preferably 0.030% or less. On the other hand, the lower limit of the content of Ga is not particularly limited, but in order to exhibit the effect of Ga, it is preferably 0.001%, more preferably 0.003%.

[0036] <Zr: 0.01 - 0.50%> Zr has an action similar to Nb and Ti and is an element that improves oxidation resistance. In order to exhibit those effects, the content of Zr is 0.01% or more, preferably 0.02% or more. On the other hand, if the content of Zr is too high, it will cause an increase in raw material cost in addition to a decrease in ductility. Therefore, the content of Zr is 0.50% or less, preferably 0.40% or less, more preferably 0.30% or less.

[0037] <REM: 0.0002 - 0.0100%> REM (rare earth) is an element effective in improving hot workability. In order to exhibit this effect, the content of REM is 0.0002% or more, preferably 0.0003% or more, more preferably 0.0004% or more. On the other hand, if the content of REM is too high, it will impair manufacturability and cause cost increase. Therefore, the content of REM is 0.0100% or less, preferably 0.0095% or less. REM is a general term for Sc, Y, and 15 elements from La to Lu (lanthanoids). These elements can be used alone or in combination of two or more as REM.

[0038] In the duplex stainless steel according to the embodiment of the present invention, the DF value represented by the following formula (1) is 60.0 to 80.0, preferably 62.0 to 79.0, and more preferably 64.0 to 78.5. DF=7.2(Cr+0.88Mo+0.78Si)-8.9(Ni+0.03Mn+0.72Cu+22C+21N)-44.9 (1) In formula (1), the element symbols represent the content (mass %) of each element. Here, the DF value is an index representing the amount of ferrite phase. Therefore, 100-DF is the amount of austenite phase. It should be noted that the DF value is an index determined based on the content of elements, and therefore does not coincide with the amount of austenite phase actually measured. By controlling the DF value within the above range, the amount of ferrite phase can be controlled to be suitable for improving both hot ductility and hot reduction. If the DF value is outside the above range, the hot ductility and / or hot reduction will be reduced.

[0039] In the duplex stainless steel according to the embodiment of the present invention, the value of A represented by the following formula (2) is 55.0 or more, preferably 60.0 or more, more preferably 70.0 or more, and even more preferably 80.0 or more. A=1.38×DF-136.5×N (2) In formula (2), DF is the value calculated by the above formula (1), and N represents the N content (mass %). Here, the value of A is an index related to hot drawing ability. That is, the value of A specifies the N content and DF value (amount of ferrite phase) effective for improving hot drawing ability, and is an index obtained experimentally. By controlling the value of A within the above range, excessive hardening of the austenite phase is suppressed and the frequency of interfaces between the ferrite phase and the austenite phase is reduced, thereby improving hot drawing ability. If the value of A is outside the above range, the hot drawing ability will decrease. The upper limit of the A value is not particularly limited, but is preferably 150.0 or less, more preferably 130.0 or less, and even more preferably 110.0 or less.

[0040] The duplex stainless steel according to the embodiment of the present invention preferably has a fracture reduction of area at 900°C of 75% or more, more preferably 76% or more, and even more preferably 78% or more. By controlling the reduction of area at fracture at 900°C within the above range, it is possible to suppress a decrease in yield and the occurrence of surface defects when the steel slab is subjected to hot working such as hot rolling, thereby improving productivity. Here, the reduction in area at break at 900°C can be measured by the method in the examples described later.

[0041] When the duplex stainless steel according to the embodiment of the present invention is a rolled material, the average grain size of the ferrite phase is preferably 5.0 to 90.0 μm, more preferably 7.0 to 80.0 μm, and even more preferably 8.0 to 70.0 μm. By controlling the average grain size of the ferrite phase within the above range, grain growth of the austenite phase can be suppressed, effectively causing the superplastic phenomenon, and improving the hot drawing ability. Here, the average grain size of the ferrite phase in the duplex stainless steel can be determined by EBSD (electron backscatter diffraction) measurement, specifically by the method described in the examples below.

[0042] When the duplex stainless steel according to the embodiment of the present invention is a rolled material, the total elongation at 950°C is preferably 200% or more, more preferably 220% or more, and even more preferably 300% or more. If the total elongation at 950°C is within the above range, it can be said that the hot ductility is good, and therefore, the occurrence of edge breakage during hot rolling can be suppressed, and productivity can be improved. The upper limit of the total elongation at 950°C is not particularly limited, but is typically 1000% or less, preferably 900% or less, and more preferably 800% or less. Here, the total elongation at 950°C can be measured by the method described in the Examples below.

[0043] When the duplex stainless steel according to the embodiment of the present invention is a rolled material, the rolled material may be a hot-rolled material or a cold-rolled material. In addition, the hot-rolled material or the cold-rolled material may be annealed or pickled. When the duplex stainless steel according to the embodiment of the present invention is a rolled material, its thickness is not particularly limited and can be adjusted appropriately depending on the application, but is generally 5.0 mm or less.

[0044] The method for producing a duplex stainless steel according to an embodiment of the present invention is not particularly limited as long as it is a method capable of producing a duplex stainless steel having the above-described characteristics. For example, when the duplex stainless steel according to the embodiment of the present invention is a billet for producing various stainless steel materials, it can be produced by vacuum melting raw materials so as to have the above-mentioned composition. Furthermore, when the duplex stainless steel according to the embodiment of the present invention is a hot-rolled material, it can be produced by hot-rolling the above-mentioned billet. Furthermore, when the duplex stainless steel according to the embodiment of the present invention is a cold-rolled material, it can be produced by hot-rolling the hot-rolled material and then cold-rolling it. Annealing may be performed after hot rolling, or finish annealing may be performed after cold rolling.

[0045] In hot rolling, the temperature immediately after the final pass is set to 950°C or higher, and then the material is cooled to 800°C at a cooling rate of 20°C / sec or higher. By performing hot rolling under these conditions, the crystal grains of the ferrite phase can be coarsened. There are two main reasons why ferrite grains become finer. The first is recrystallization during hot rolling or subsequent annealing due to the accumulation of hot-rolling strain. The lower the hot-rolling temperature, the more strain accumulates, inducing recrystallization and facilitating ferrite grain refinement. Therefore, it is necessary to raise the temperature to the temperature immediately after the final pass, where this is less likely to occur. The second is the suppression of ferrite grain growth due to the formation of austenite. When austenite precipitates at the ferrite grain boundaries, the movement of the ferrite grain boundaries slows down, suppressing grain growth. Since the austenite phase decreases with increasing temperature, peaking at around 900°C, the higher the temperature, the easier it is for ferrite grains to grow. On the other hand, increasing the heating temperature of the slab before hot rolling or shortening the heat dissipation time by increasing the rolling speed is effective in maintaining a high hot-rolling temperature, but these methods increase fuel costs and manufacturing difficulties. Taking these circumstances into consideration, the lower limit of the temperature immediately after the final pass of hot rolling is set to 950°C.

[0046] The annealing after hot rolling is not particularly limited, but preferably involves holding the steel at a temperature of 1030 to 1150°C for 10 seconds or more, and then cooling to 400°C or less at a cooling rate of 20°C / second or more. By performing annealing under these conditions, carbides and nitrides precipitated during cooling after hot rolling are sufficiently solid-dissolved, and the precipitation of carbides and nitrides during the cooling process after annealing can be suppressed. In addition, the proportion of austenite phase can be relatively reduced, which reduces the suppression of grain growth of the ferrite phase and makes it easier to coarsen the grains of the ferrite phase.

[0047] The conditions for cold rolling are not particularly limited, but a rolling ratio of 40 to 90% is preferred. The reason for setting the rolling ratio at 40% or more is that carbides and precipitates are crushed or extended to increase their surface area, thereby facilitating solid solution during heat treatment. The reason for setting the rolling ratio at 90% or less is to prevent edge breakage due to excessive rolling. From the viewpoint of stably obtaining this effect, a rolling ratio of 85% or less is more preferred. When cold rolling is performed two or more times, intermediate annealing may be performed between each cold rolling. When intermediate annealing is performed, the conditions may be the same as those for annealing after hot rolling.

[0048] Although the conditions for the finish annealing are not particularly limited, it is preferable to hold the final temperature of 1040 to 1120°C for 5 seconds or more, and then cool at a cooling rate of 30°C / second or more. By performing the finish annealing under such conditions, it becomes possible to suppress the precipitation of carbides and nitrides during heating, complete recrystallization, dissolve carbides and nitrides, control the proportion of the austenite phase, suppress fluctuations in the proportion of the austenite phase during cooling, and suppress the reprecipitation of carbides and nitrides.

[0049] The duplex stainless steel sheet according to the embodiment of the present invention is excellent in both hot ductility and hot drawing, and therefore is easy to hot work. This makes it possible to suppress the occurrence of edge breaks and surface defects during hot rolling, thereby improving productivity. Therefore, this duplex stainless steel sheet can be used in various applications where these properties are required. [Example]

[0050] The present invention will be described in detail below with reference to examples, but the present invention should not be construed as being limited to these examples.

[0051] <Test 1> Using a 150 kg vacuum induction furnace, raw materials were vacuum melted to obtain the steel composition shown in Table 1 (the balance being Fe and impurities), and the molten metal was poured into a flat mold to produce an ingot weighing 40 to 45 kg. This ingot was subjected to solution heat treatment by heating to 1050 to 1150°C.

[0052] [Table 1]

[0053] Next, a round bar test piece with a diameter of 8 mm and a length of 110 mm in the longitudinal direction was taken from near the surface layer of the ingot. Next, the central region of the taken round bar test piece (a 15 mm region centered on the center of the longitudinal length) was heated to 1180°C in a vacuum atmosphere and held for 1 minute, and then the round bar test piece was cooled to 900°C at a rate of 20°C / min. Next, the round bar test piece at 900°C was subjected to a strain rate of 1.3 / s. -1 A tensile test was carried out at this temperature to fracture the round bar test specimens. The area of ​​the fracture surface of the fractured round bar test specimens was divided by the cross-sectional area of ​​the round bar test specimens before the tensile test, and the result was multiplied by 100 to calculate the reduction in area at fracture. The results are shown in Table 2.

[0054] <Test 2> In Test 2, the ingot obtained in Test 1 was bloomed to form a slab, and then a cold-rolled and annealed sheet was produced by sequentially carrying out a hot rolling process, an annealing process, a cold rolling process, and a finish annealing process. In the hot rolling process, the temperature immediately after the final pass (referred to as "final pass temperature") was set to the temperature shown in Table 2, and the material was cooled to 800°C by water cooling (cooling rate of 20°C / sec or more). In the annealing step after the hot rolling step, the steel sheet was held within the above-mentioned temperature and time ranges, and then cooled to 400°C or lower by water cooling (cooling rate of 20°C / sec or higher). The cold rolling step was carried out to obtain cold-rolled sheets at the rolling ratios shown in Table 2. The cold rolling was carried out once or twice. In the finish annealing step, the steel sheet was held within the above-mentioned temperature and time ranges, and then cooled to 400°C or less by water cooling (cooling rate of 30°C / sec or more). The thickness (sheet thickness) of the obtained cold-rolled annealed sheet is shown in Table 2.

[0055] [Table 2]

[0056] The cold-rolled and annealed sheets obtained above were subjected to the following evaluations.

[0057] <Average grain size of ferrite phase (α phase) in duplex stainless steel sheet> After cutting test specimens from the cold-rolled and annealed sheets, the thickness cross section parallel to the rolling direction was mirror-polished and subjected to EBSD (electron backscatter diffraction) measurements. EBSD measurements were performed using a scanning electron microscope with the measurement software TSL OIM Data Collection 7 (TSL Solutions, Inc.) over a 300 μm square area at the center of the test specimen's thickness direction, with a step size of 0.7 μm. The average area of ​​the ferrite phase (BCC) crystal grains was calculated using the area fraction method from the data obtained from this EBSD measurement. The diameter of a circle with the same area as the average area of ​​the ferrite phase crystal grains thus obtained was taken as the average grain size of the ferrite phase.

[0058] <Total elongation at 950℃> Plate-shaped test pieces with a total length of 120 mm, a parallel portion length of 35 mm, and a parallel portion width of 10.5 mm were cut from the cold-rolled annealed sheet so that the rolling direction was parallel. Next, the cut plate-shaped test pieces were heated to 950°C and held for 10 minutes, and then subjected to strain rate of 1.5 × 10 6 until fracture. -3 / s -1 The tensile test was carried out at 100°C until the plate-shaped test piece was broken. The total elongation (%) was calculated from the crosshead displacement of the tensile tester from the start of the test to the time of breakage.

[0059] The evaluation results are shown in Table 3.

[0060] [Table 3]

[0061] As shown in Table 3, in Examples 1 to 6, the composition, DF and A values ​​of the cold-rolled annealed sheets (duplex stainless steel sheets) were controlled within predetermined ranges, and therefore, both the hot ductility and hot drawing properties were excellent. In contrast, in Comparative Example 1, the Cr content was low and the values ​​of DF and A were low, so the hot drawing was insufficient. In Comparative Example 2, the contents of Mn, Ni, and Cr were low and the DF value was high, so the hot ductility was insufficient. In Comparative Example 3, the N content was high and the DF and A values ​​were low, so the hot drawing was insufficient. In Comparative Example 4, the contents of Si, Ni, Cr, Mo, and N were high and the content of Mn was low, so that the hot drawing was insufficient. In Comparative Example 5, the contents of Si, Mn, and N were high and the values ​​of DF and A were low, so that the hot drawing was insufficient. In Comparative Example 6, the contents of Si, Ni, Cr and N were high, and therefore the hot drawing was insufficient. In Comparative Example 7, the values ​​of DF and A were low, and therefore the hot drawing was insufficient.

[0062] As can be seen from the above results, the present invention can provide a ferritic-austenitic duplex stainless steel that is excellent in hot ductility and hot drawing ability.

Claims

1. The composition includes, on a mass basis, C: 0.001 to 0.050%, Si: 0.01 to 0.50%, Mn: 1.0 to 3.5%, P: 0.050% or less, S: 0.0300% or less, Ni: 1.5 to 3.0%, Cr: 19.6 to 23.0%, Mo: 0.01 to 1.00%, Cu: 0.01 to 1.00%, N: 0.010 to 0.120%, and the balance being Fe and impurities; The following formula (1): DF=7.2(Cr+0.88Mo+0.78Si)-8.9(Ni+0.03Mn+0.72Cu+22C+21N)-44.9... (1) (wherein the element symbols represent the content (mass%) of each element) is 60.0 to 80.0, The following formula (2): A=1.38×DF-136.5×N...(2) (wherein DF is the value calculated by the above formula (1), and N represents the N content (mass%)) is 55.0 or more.

2. The ferritic-austenitic duplex stainless steel according to claim 1, further comprising, on a mass basis, one or more selected from Nb: 0.010 to 0.500%, Ti: 0.010 to 0.500%, V: 0.01 to 0.50%, W: 0.05 to 0.50%, Co: 0.01 to 0.30%, B: 0.0002 to 0.0050%, Sn: 0.010 to 0.500%, Al: 0.010 to 0.050%, Mg: 0.0002 to 0.0100%, Ca: 0.0002 to 0.0100%, Ta: 0.050% or less, Ga: 0.050% or less, Zr: 0.01 to 0.50%, and REM: 0.0002 to 0.0100%.

3. 3. The ferritic-austenitic duplex stainless steel according to claim 1, wherein the fracture reduction of area at 900°C is 75% or more.

4. The ferritic-austenitic duplex stainless steel according to claim 1 or 2, which is a rolled material.

5. The ferritic-austenitic duplex stainless steel according to claim 4, wherein the average particle size of the ferrite phase is 5.0 to 90.0 μm.

6. The ferritic-austenitic duplex stainless steel according to claim 4, having a total elongation at 950°C of 200% or more.

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