WO2013058274A1 - Duplex stainless steel, duplex stainless steel slab, and duplex stainless steel material - Google Patents

Abstract

An embodiment of this duplex stainless steel contains, by mass%, 0.03% or less of C, 0.05 to 1.0% of Si, 0.1 to 7.0% of Mn, 0.05% or less of P, 0.0001 to 0.0010% of S, 0.5 to 5.0% of Ni, 18.0 to 25.0% of Cr, 0.10 to 0.30% of N, 0.05% or less of Al, 0.0010 to 0.0040% of Ca, and 0.01 to 0.2% of Sn, the remainder comprising Fe and unavoidable impurities. The ratio Ca/O between the amounts of Ca and O is 0.3 to 1.0. The pitting index (PI) represented by equation (1) is less than 30. PI = Cr + 3.3Mo + 16N (1)

Description

Duplex stainless steel, duplex stainless steel cast, and duplex stainless steel

The present invention relates to an inexpensive Sn-containing duplex stainless steel. The present invention also relates to a duplex stainless steel that contains Cu and Sn in combination, is excellent in corrosion resistance, and is inexpensive. Specifically, the present invention relates to a duplex stainless steel, duplex stainless steel slab, and duplex stainless steel material that can be used as a seawater desalination device, tanks for transport ships, various containers, and the like.
This application claims priority based on Japanese Patent Application No. 2011-231352 filed in Japan on October 21, 2011 and Japanese Patent Application No. 2011-266351 filed in Japan on December 6, 2011. The contents are incorporated herein.

General-purpose duplex stainless steel contains a large amount of Cr, Mo, Ni, and N and has good corrosion resistance. However, since expensive Mo and Ni are contained, it is difficult to say that the alloy cost is high and the productivity is good. As a result, the price of the steel material is not so low, and it is difficult to say that it is often used instead of 316 series, 317 series stainless steel and the like. The general-purpose duplex stainless steel referred to in the present invention is a pitting corrosion index PI (indicated by the mathematical sum of the contents of the right alloy element / PI = Cr + 3.3Mo + 16N) of 30 or more and less than 40 (mass%). It means a duplex stainless steel having a value of degree. In view of the above circumstances, these steels exhibit corrosion resistance equivalent to that of conventional general-purpose duplex stainless steels, have lower alloy costs than conventional ones, have good hot productivity, and low production costs. Is considered necessary.

On the other hand, recently, an alloy-saving duplex stainless steel with reduced Cr, Ni, Mo, etc. has been developed. Here, the alloy-saving duplex stainless steel is a steel having a pitting corrosion resistance equivalent to SUS304, 316L, and a pitting corrosion index PI (= Cr + 3.3Mo + 16N) indexed by the content of the alloy element. ) Refers to stainless steel of less than about 30. In these steels in which the content of alloy elements useful for pitting corrosion resistance and acid resistance is reduced, it is difficult to obtain corrosion resistance equivalent to that of general-purpose duplex stainless steel. However, it is considered possible to develop improved steel using inexpensive alternative elements.

Various proposals have been made regarding duplex stainless steel containing Sn. For example, a duplex stainless steel containing 25% or more of Cr and 0.01 to 0.1% of Sn as a selective element is disclosed (see Patent Documents 1 and 2 below). Further, an alloy-saving duplex stainless steel containing 1% or less or 0.1% Sn is disclosed (see Patent Documents 3 and 4 below). Although these patent documents aim at improving the corrosion resistance by containing Sn, the relationship between the hot manufacturability of steel and the Sn content has not been studied.

Further, in the above-mentioned patent document, the steel whose N content is 0.2% or less is targeted. N is an element that reduces the hot workability of stainless steel. To ensure the hot workability of the duplex stainless steel containing 0.2% or more of N at a desired level, the hot workability of the duplex stainless steel containing less than 0.2% of N is desired. It is more difficult than securing it to a standard. There is no technical document disclosing the hot workability of the duplex stainless steel containing 0.20% or more of N and further containing Sn and Cu in combination.

The inventors focused on the possibility of improving acid resistance and pitting corrosion resistance by Sn in alloy-saving duplex stainless steel. Then, the relationship between the Sn content, corrosion resistance and hot manufacturability was investigated. As a result, the possibility of improving corrosion resistance was found by containing 0.01 to 0.2% Sn. However, it has been found that the hot productivity decreases in these duplex stainless steels containing a large amount of Sn. For this reason, the frequency with which the yield of steel materials decreases increases, and a significant cost increase is expected.

In addition, the present inventors have focused on the possibility of improving acid resistance and pitting corrosion resistance by Sn and Cu in general-purpose duplex stainless steel. And in the duplex stainless steel in which the contents of Mo and Ni are reduced and N is 0.20% or more, the relationship between the contents of Sn and Cu, the corrosion resistance, and the hot manufacturability was investigated. As a result, the possibility of improving corrosion resistance was found by containing 0.01 to 0.2% Sn and 0.2 to 3.0% Cu. However, it has been found that the hot productivity is lowered in these duplex stainless steels containing a large amount of Sn and Cu. For this reason, the frequency with which the yield of steel materials decreases increases, and a significant cost increase is expected.

The present inventors examined the conventional knowledge about the manufacturing technology of conventional Sn-containing duplex stainless steel hot rolled steel materials including Patent Documents 1 to 4. As a result, it has been found that there is little knowledge about the relationship between the temperature range of hot embrittlement due to Sn contained in the duplex stainless steel, the relationship with the Sn content, and the relationship with the content of other elements.

JP-A-3-158437 JP-A-4-072013 JP 2010-222593 A International Publication WO2009-119895 JP 2002-69592 A Japanese Patent Laid-Open No. 7-118805

The present invention clarifies the relationship between Sn content and hot manufacturability in alloy-saving duplex stainless steel and finds a solution to solve the above problems. The present invention also clarifies the relationship between Sn and Cu contents and hot manufacturability in general-purpose duplex stainless steels and finds measures for solving the above problems. Accordingly, an object of the present invention is to provide a Sn-containing duplex stainless steel, duplex stainless steel cast, and duplex stainless steel material that are favorable and inexpensive in hot productivity. Such a duplex stainless steel is expected to have an excellent balance between corrosion resistance and cost. For this reason, it is considered that the possibility of wide use in each field is increased.

In particular, in the second aspect (second embodiment), the contents of Ni and Mo, which are expensive elements, are increased by increasing the contents of N and Mn and by adding Cu and Sn in combination. It is an object of the invention to develop an inexpensive general-purpose duplex stainless steel that saves energy.

In order to solve the above problems, the inventors changed the Sn content and the contents of Ca, B, rare earth elements (REM), etc., for the alloy-saving duplex stainless steel targeted by the present invention. A melting material was prepared and the following experiment was performed. In addition, it is said that content of Ca, B, rare earth elements (REM), etc. improves hot productivity.

A tensile test piece was collected from a slab cast from a melting material. The tensile test piece was subjected to high-temperature tension at 1200 to 700 ° C., and the drawing value (cross-sectional reduction rate of the fracture surface) was measured to evaluate high-temperature ductility. Moreover, the hot rolled steel plate of 12 mm thickness was obtained by hot forging and hot rolling, and the ear cracking property was evaluated. For some steels, the hot cracking heating temperature and the rolling temperature were changed to evaluate the ear cracking properties, and the correlation between the hot rolling heating temperature, the rolling temperature, and the high temperature ductility was obtained.

As described in Patent Document 5 and Patent Document 6 described above, generally, in a duplex stainless steel, when the drawing value of a slab evaluated by high-temperature tension falls below 60%, in many cases, the casting It is known that hot cracking of pieces causes significant ear cracks. For this reason, engineers in this field often perform steel refining, casting, and hot working with the goal of at least 60% or higher drawing value at the slab. However, the high temperature of the alloy-saving duplex stainless steel (base composition: 21% Cr-2% Ni-3% Mn-0.18% N) slab containing about 0.1% of Sn by the present inventors. When the ductility was evaluated, it was revealed by several melting experiments that the drawing value was less than 60%. Evaluation of high temperature ductility was performed as follows. First, the parallel part of an 8 mmφ round bar was heated to 1200 ° C. using high frequency. Next, the temperature was lowered to a temperature at which a break test was performed, and the sample was pulled and broken at a speed of 20 mm / sec. And the contraction | shrinkage percentage of the cross section was calculated | required. An example of the data is shown in FIG. From this result, it was considered that there was almost no hope to obtain an inexpensive alloy-saving duplex stainless steel added with Sn.

The present inventors hot-rolled a slab of alloy-saving Sn-containing duplex stainless steel obtained by vacuum melting and casting, and observed the length of the ear cracks generated at that time. As a result, it has been found that there are rarely Sn-containing duplex stainless steel materials with few ear cracks. The hot rolling experiment was performed as follows. First, a slab of 90 to 44 mm thickness was heated to 1200 ° C. The thickness was then reduced to 12-6 mm through multiple rolling passes. The finish rolling temperature was controlled to about 900 ° C. Ear cracks occur on the left and right, and the maximum length of each was added to obtain the ear crack length. Even if the ear crack length of the steel material was arranged with the minimum value of the hot ductility drawing value of the slab (the minimum value was obtained at about 900 ° C. in FIG. 1), a clean correlation could not be obtained. However, as shown in FIG. 2, when the aperture value was arranged at 1000 ° C., it became clear that a good correlation was exhibited regardless of whether or not Sn was contained. In FIG. 2, the points on the circle (white circles) plot correspond to the results of Sn-A and Sn-B in FIG. 1, and the points on the ◆ (black rhombus) plot indicate the results of other experiments (Sn (Experimental results examined regardless of whether or not it is contained).

The present inventors conducted melting, casting, and rolling experiments with various element contents varied in order to find out the conditions for reliably obtaining a steel material with few ear cracks. And evaluation of the hot ductility of a slab and evaluation of the steel material ear crack after hot rolling were performed energetically. Through the above experiments, based on the obtained knowledge, the first aspect of the present invention in which the inexpensive Sn-containing alloy-saving duplex stainless steel was clearly specified was completed.

The requirements of the 1st aspect of the duplex stainless steel of this invention are shown below.
(1) By mass%, C: 0.03% or less, Si: 0.05 to 1.0%, Mn: 0.1 to 7.0%, P: 0.05% or less, S: 0.0001 To 0.0010%, Ni: 0.5 to 5.0%, Cr: 18.0 to 25.0%, N: 0.10 to 0.30%, Al: 0.05% or less, Ca: 0 .0010 to 0.0040%, and Sn: 0.01 to 0.2%, the balance is made of Fe and inevitable impurities, and the Ca / O content ratio Ca / O is 0.3 to 1 0.0, and the pitting corrosion index PI shown by the formula (1) is less than 30.
PI = Cr + 3.3Mo + 16N (1)
(The element symbol in the formula (1) indicates the content of the element.)
(2) Further, it contains at least one selected from Mo: 1.5% or less, Cu: 2.0% or less, W: 1.0% or less, and Co: 2.0% or less. The duplex stainless steel according to (1).
(3) It further contains at least one selected from V: 0.05 to 0.5%, Nb: 0.01 to 0.20%, and Ti: 0.003 to 0.05%. The duplex stainless steel according to (1) or (2), which is characterized.
(4) Further, it contains one or more selected from B: 0.0050% or less, Mg: 0.0030% or less, and REM: 0.10% or less (1) to (3 ) Duplex stainless steel according to any one of the above.

In order to solve the above-mentioned problems, the present inventors have made a general-purpose duplex stainless steel targeted by the present invention with a Sn content, a content of Ca, B, a rare earth element (REM), etc., and a Ni content. In addition to changing the content, a melting material to which Co was further added was produced, and the following experiment was performed. In addition, when Ca, B, rare earth elements (REM), etc. are contained, it is said that hot productivity improves.

A tensile test piece was collected from a slab cast from a melting material. The tensile test piece was subjected to high-temperature tension at 1200 to 700 ° C., and the drawing value (cross-sectional reduction rate of the fracture surface) was measured to evaluate high-temperature ductility. Moreover, the hot rolled steel plate of 12 mm thickness was obtained by hot forging and hot rolling, and the ear cracking property was evaluated. For some steels, the hot cracking heating temperature and the rolling temperature were changed to evaluate the ear cracking properties, and the correlation between the hot rolling heating temperature, the rolling temperature, and the high temperature ductility was obtained.

As described in Patent Document 5 and Patent Document 6 described above, generally, in a duplex stainless steel, when the drawing value of a slab evaluated by high-temperature tension falls below 60%, in many cases, the casting It is known that hot cracking of pieces causes significant ear cracks. For this reason, engineers in this field often perform steel refining, casting, and hot working with the goal of at least 60% or higher drawing value at the slab. However, the present inventors have found that a general-purpose duplex stainless steel containing about 0.1% Sn (base composition: 25% Cr-4% Ni-1.2% Mo-1.5% Cu-0.25). % N) When the high temperature ductility of the slab was evaluated, it was revealed by several melting experiments that the minimum value of the drawing value was less than 60%. Evaluation of high temperature ductility was performed as follows. First, the parallel part of an 8 mmφ round bar was heated to 1200 ° C. using high frequency. Next, the temperature was lowered to a temperature at which a break test was performed, and the sample was pulled and broken at a speed of 20 mm / sec. Then, the shrinkage ratio of the cross section was obtained. An example of the data is shown in FIG. From this result, it was considered that there was little hope to obtain an inexpensive general-purpose duplex stainless steel added with Sn practically.

The inventors of the present invention hot rolled a slab of general-purpose duplex stainless steel obtained by vacuum melting and casting, and observed the length of the ear cracks generated at that time. As a result, it was discovered that there are rarely Sn-containing duplex stainless steel materials with few ear cracks. The hot rolling experiment was performed as follows. First, a slab of 90 to 44 mm thickness was heated to 1200 ° C. The thickness was then reduced to 12-6 mm through multiple rolling passes. The finish rolling temperature was controlled to about 900 ° C. Ear cracks occur on the left and right, and the maximum length of each was added to obtain the ear crack length. Even if the ear crack length of the steel material was arranged with the minimum value of the drawing value of the hot ductility of the slab (the minimum value was obtained at about 900 ° C. in FIG. 3), a clean correlation could not be obtained. However, as shown in FIG. 4, when the aperture value was arranged at 1000 ° C., it became clear that a good correlation was exhibited regardless of whether or not Sn was contained. In FIG. 4, dots in the circle (white circles) correspond to the results of Sn-A and Sn-B in FIG. 3, and points in the ◆ (black rhombus) plot indicate other experimental results (Sn (Experimental results examined regardless of whether or not it is contained).

The present inventors conducted melting, casting, and rolling experiments in which various element contents were further changed in order to find out the conditions under which the above-mentioned steel material with few ear cracks could be obtained with certainty. And the hot ductility evaluation of the slab and the steel ear crack evaluation after hot rolling were performed energetically. Through the experiments described above, the second aspect of the present invention, which was clearly shown for inexpensive Sn-containing duplex stainless steel, was completed based on the obtained knowledge.

The requirements for the second aspect of the duplex stainless steel of the present invention are shown below.
(5) By mass%, C: 0.03% or less, Si: 0.05 to 1.0%, Mn: 0.1 to 4.0%, P: 0.05% or less, S: 0.0001 To 0.0010%, Cr: 23.0 to 28.0%, Ni: 2.0 to 6.0%, Co: 0 to 1.0%, Cu: 0.2 to 3.0%, Sn: Contains 0.01 to 0.2%, N: 0.20 to 0.30%, Al: 0.05% or less, and Ca: 0.0010 to 0.0040%, the balance being Fe and inevitable impurities Ni + Co is 2.5% or more, the Ca / O content ratio Ca / O is 0.3 to 1.0, and the PI represented by the formula (1) is 30 or more and less than 40. Duplex stainless steel characterized by that.
PI = Cr + 3.3Mo + 16N (1)
(The element symbol in the formula (1) indicates the content of the element.)
(6) Furthermore, any one or both of Mo: 2.0% or less and W: 1.0% or less are contained, The duplex stainless steel according to (5).
(7) It further contains at least one selected from V: 0.05 to 0.5%, Nb: 0.01 to 0.15%, and Ti: 0.003 to 0.05%. The duplex stainless steel according to (5) or (6), characterized in that it is characterized.
(8) Further, it contains one or more selected from B: 0.0050% or less, Mg: 0.0030% or less, and REM: 0.10% or less (5) to (7 ) Duplex stainless steel according to any one of the above.

The requirements for one aspect of the duplex stainless steel slab and duplex stainless steel material of the present invention are shown below.
(9) A duplex stainless steel slab having the composition according to any one of (1) to (8) and having a fracture drawing value at 1000 ° C. of 70% or more.
(10) A duplex stainless steel material produced by hot working the duplex stainless steel cast according to (9).

According to an aspect of the present invention, duplex stainless steel having improved corrosion resistance and excellent balance with cost compared to steel conventionally used as a material for seawater desalination equipment, tanks for transport ships, various containers, etc. A duplex stainless steel slab and a duplex stainless steel material can be provided. For this reason, the aspect of the present invention greatly contributes to industrial development.

It is a figure which illustrates high temperature ductility of the duplex stainless steel which contains Sn and does not contain Sn in relation to the first aspect (alloy-saving duplex stainless steel) of the duplex stainless steel. It is a figure which shows the relationship between the ear crack length after a hot rolling, and the aperture value at 1000 degreeC in relation to the 1st aspect (alloy saving type duplex stainless steel) of a duplex stainless steel. It is a figure which illustrates high temperature ductility of the duplex stainless steel slab containing Sn and Sn-free in connection with the second aspect of the duplex stainless steel (general-purpose duplex stainless steel). It is a figure which shows the relationship between the ear crack length after hot rolling, and the aperture value at 1000 degreeC in connection with the 2nd aspect (general-purpose type duplex stainless steel) of duplex stainless steel.

(First embodiment)
Below, the reason for limitation of the 1st aspect (alloy saving type duplex stainless steel) of the duplex stainless steel of this invention is demonstrated. In addition, content of each component shows the mass%.

In the present embodiment, the stainless steel slab means steel in a state after casting and before being subjected to processing such as hot working or forging, and the stainless steel material means that the slab is processed by various methods. It means a steel slab, a hot rolled steel plate, a cold rolled steel plate, a steel wire, a steel pipe, etc. Stainless steel means all forms of steel such as slabs and steel materials. The above processing includes hot and cold processing.

In order to ensure the corrosion resistance of stainless steel, the C content is limited to 0.03% or less. When C is contained exceeding 0.03%, Cr carbide is generated during hot rolling, and corrosion resistance and toughness deteriorate.

Si is added at 0.05% or more for deoxidation. However, when Si exceeds 1.0%, toughness deteriorates. Therefore, the upper limit of Si content is limited to 1.0%. A preferable range of the amount of Si is 0.2 to 0.7%.

Mn has the effect of increasing the austenite phase and improving toughness. Further, since Mn has the effect of lowering the nitride precipitation temperature TN, it is preferable to add Mn positively in the steel material of this embodiment. 0.1% or more of Mn is added for the toughness of the base metal and the weld. However, when Mn is added exceeding 7.0%, corrosion resistance and toughness deteriorate. Therefore, the upper limit of the amount of Mn is limited to 7.0%. The Mn content is preferably 1.0 to 6.0%, more preferably 2.0 to 5.0%.

Ｐ P is an element inevitably mixed from the raw material, and the P content is limited to 0.05% or less in order to deteriorate hot workability and toughness. The amount of P is preferably 0.03% or less.

S is an element that is inevitably mixed from the raw material, and also degrades hot workability, toughness, and corrosion resistance, so the S amount is limited to 0.0010% or less. Moreover, reducing the amount of S to less than 0.0001% increases the cost for desulfurization refining. For this reason, the S content is determined to be 0.0001 to 0.0010%. The amount of S is preferably 0.0002 to 0.0006%.

Ni stabilizes the austenite structure and contains 0.5% or more of Ni in order to improve corrosion resistance to various acids and toughness. By increasing the Ni content, the nitride precipitation temperature can be lowered. On the other hand, Ni is an expensive alloy, and the amount of Ni is limited to 5.0% or less from the viewpoint of cost in the steel of the present embodiment intended for alloy-saving duplex stainless steel. The Ni content is preferably 1.0 to 4.0%, more preferably 1.5 to 3%.

¡In order to secure basic corrosion resistance, 18.0% or more of Cr is included. On the other hand, if the Cr content exceeds 25.0%, the ferrite phase fraction increases, and the toughness and the corrosion resistance of the weld zone are impaired. Therefore, the Cr content is set to 18.0% or more and 25.0% or less. A preferable Cr content is 19.0 to 23.0%.

N is an effective element that improves the strength and corrosion resistance by dissolving in the austenite phase. For this purpose, 0.10% or more of N is contained. On the other hand, the solid solution limit increases depending on the Cr and Mn contents. However, in the steel of this embodiment, if N is contained in excess of 0.30%, Cr nitride is precipitated to inhibit toughness and corrosion resistance. As a result, hot manufacturability is impaired. For this reason, the upper limit of N content was 0.30%. A preferable N content is 0.10 to 0.25%.

Al is a deoxidizing element of steel and reduces oxygen in the steel as necessary. For this reason, Al is contained together with 0.05% or more of Si. In the Sn-containing steel, the reduction of the oxygen amount is essential to ensure hot productivity, and for this purpose, it is necessary to contain 0.003% or more of Al as necessary. On the other hand, Al is an element having a relatively large affinity with N, and if added excessively, AlN is generated and inhibits the toughness of stainless steel. The degree depends on the N content, but when Al exceeds 0.05%, the toughness is significantly lowered. For this reason, the upper limit of Al content was set to 0.05%. The amount of Al is preferably 0.04% or less.

Ca is an important element for the hot manufacturability of steel, and it is necessary to contain Ca in order to fix O and S in the steel as inclusions and improve the hot manufacturability. . In the steel of this embodiment, 0.0010% or more of Ca is contained for that purpose. Moreover, excessive addition reduces pitting corrosion resistance. Therefore, the upper limit of the Ca content is set to 0.0040%.

Sn is contained in order to improve the corrosion resistance of the steel of this embodiment. Therefore, it is necessary to contain at least 0.01% Sn. Furthermore, it is preferable to contain 0.02% or more of Sn. On the other hand, Sn is an element that hinders the hot manufacturability of steel. In the alloy element type saving duplex stainless steel targeted by the present embodiment, the heat at the interface between the ferrite phase and the austenite phase particularly at 900 ° C. or lower. Decrease the strength. The degree of the reduction depends on the contents of S, Ca, and O, but even if other limitations in the present embodiment are added, if Sn is contained exceeding 0.2%, the hot productivity is lowered. Therefore, the upper limit of the Sn content is set to 0.2%.

The ratio Ca / O of the content of O and Ca is an important component index for improving the hot manufacturability and corrosion resistance of the steel of this embodiment. In order to improve the hot manufacturability of the Sn-containing steel, the lower limit of Ca / O is limited. The hot ductility of the Sn-containing steel decreases particularly at a temperature of 900 ° C. or less. When the value of Ca / O is less than 0.3, the hot ductility at 1000 ° C. is also lowered, and hot productivity is greatly impaired. For this reason, in the steel of this embodiment, Ca / O is limited to 0.3 or more. On the other hand, when Ca is added excessively and Ca / O exceeds 1.0, pitting corrosion resistance is impaired. Further, when Ca becomes excessive, the high temperature ductility at 1000 to 1100 ° C. is also impaired. For this reason, the upper limit of Ca / O was set to 1.0. Ca / O is preferably 0.4 to 0.8.

O is an inevitable impurity, and its upper limit is not particularly defined, but is an important element constituting an oxide that is representative of non-metallic inclusions. Controlling the composition of the oxide is very important for improving hot manufacturability. In addition, when a coarse cluster-like oxide is generated, it causes surface defects. For this reason, it is necessary to restrict | limit the content of O low. In this embodiment, as described above, the O content is limited by setting the ratio of the Ca content and the O content to 0.3 or more. The upper limit of the O content is preferably 0.005% or less.

In order to further increase the corrosion resistance, 1 is selected from Mo: 1.5% or less, Cu: 2.0% or less, W: 1.0% or less, and Co: 2.0% or less as necessary. It may contain seeds or more. The reason for the limitation will be described.

Mo is a very effective element that additionally enhances the corrosion resistance of stainless steel, and can be contained as required. In order to improve the corrosion resistance, it is preferable to contain 0.2% or more of Mo. On the other hand, Mo is an element that promotes precipitation of intermetallic compounds, and the upper limit of the Mo content is set to 1.5% from the viewpoint of suppressing precipitation during hot rolling in the steel of this embodiment.

Cu is an element that additionally increases the corrosion resistance of stainless steel to acids, and has the effect of improving toughness. Therefore, it is recommended to contain 0.3% or more as necessary. When Cu is contained exceeding 2.0%, εCu precipitates exceeding the solid solubility during hot rolling to cause embrittlement. For this reason, the upper limit of the amount of Cu was made into 2.0%. A preferable content when Cu is contained is 0.3 to 1.5%.

W, like Mo, is an element that additionally improves the corrosion resistance of stainless steel, and can be added as necessary. For the purpose of improving the corrosion resistance in the steel of this embodiment, the upper limit of the W amount is set to 1.0%. A preferable W content is 0.05 to 0.5%.

Co is an element effective for enhancing the toughness and corrosion resistance of steel and is selectively added. The Co content is preferably 0.03% or more. When Co is contained in excess of 2.0%, an effect commensurate with the cost cannot be exhibited because it is an expensive element. For this reason, the upper limit of the amount of Co was set to 2.0%. The preferable Co content when added is 0.03 to 1.0%.

Further, it may contain one or more selected from V: 0.05 to 0.5%, Nb: 0.01 to 0.20%, and Ti: 0.003 to 0.05%. These are elements that have a greater tendency to form nitrides than Cr. V, Nb, and Ti can be added as necessary, and when contained in a very small amount, the corrosion resistance tends to be improved.

Nitride and carbide formed by V are generated during the hot working and cooling of the steel material, and have the effect of enhancing the corrosion resistance. Although sufficient confirmation has not been made for this reason, there is a possibility of suppressing the generation rate of chromium nitride at 700 ° C. or lower. In order to improve this corrosion resistance, 0.05% or more of V is contained. When V is contained exceeding 0.5%, coarse V-based carbonitrides are produced and toughness is deteriorated. Therefore, the upper limit of the V amount is limited to 0.5%. When V is added, the V content is preferably in the range of 0.1 to 0.3%.

Nitrides and carbides formed by Nb are generated during the hot working and cooling of the steel material, and have the effect of increasing the corrosion resistance. Although sufficient confirmation has not been made for this reason, there is a possibility of suppressing the generation rate of chromium nitride at 700 ° C. or lower. In order to improve the corrosion resistance, 0.01% or more of Nb is contained. On the other hand, excessive addition causes precipitation as an undissolved precipitate during heating before hot rolling, thereby inhibiting toughness. For this reason, the upper limit of the Nb content is set to 0.20%. When Nb is added, the preferable range of Nb content is 0.03% to 0.10%.

Ti is an element that forms oxides, nitrides, and sulfides in a very small amount, and solidifies the steel and refines the crystal grains of the high-temperature heating structure. Further, like V and Nb, Ti also has a property of substituting for a part of chromium in the chromium nitride. When Ti is contained in an amount of 0.003% or more, Ti precipitates are formed. On the other hand, when Ti is contained in the duplex stainless steel exceeding 0.05%, coarse TiN is generated and the toughness of the steel is inhibited. For this reason, the upper limit of the Ti content is set to 0.05%. A preferable content of Ti is 0.005 to 0.020%.

Furthermore, you may contain 1 or more types chosen from B: 0.0050% or less, Mg: 0.0030% or less, and REM: 0.10% or less. In order to further improve the hot workability, B, Mg, and REM to be contained as necessary are limited as follows.

B, Mg, and REM are all elements that improve the hot workability of steel, and one or more of them are added for that purpose. In any of B, Mg, and REM, excessive addition conversely decreases hot workability and toughness. For this reason, the upper limit of the content was defined as follows. The upper limit of the amount of B is 0.0050%. The upper limit of the amount of Mg is 0.0030%. The upper limit of the amount of REM is 0.10%. The preferred contents are B: 0.0005 to 0.0030%, Mg: 0.0001 to 0.0015%, and REM: 0.005 to 0.05%, respectively. Here, REM is the total content of lanthanoid rare earth elements such as La and Ce.

As described above, by having the characteristics of the duplex stainless steel of the present embodiment described above, the hot productivity of the alloy-saving duplex stainless steel containing Sn can be remarkably improved.
At the slab stage, the fracture drawing value at 1000 ° C. is 70% or more. In addition, by performing processing including hot working on the slab, it is possible to obtain a duplex stainless steel material with high yield and low surface flaws.

(Second Embodiment)
Below, the reason for limitation of the 2nd aspect (general-purpose type duplex stainless steel) of the duplex stainless steel of this invention is demonstrated. In addition, content of each component shows the mass%.

In the present embodiment, the stainless steel slab means steel in a state after casting and before being subjected to processing such as hot working or forging, and the stainless steel material means that the slab is processed by various methods. It means a steel slab, a hot rolled steel plate, a cold rolled steel plate, a steel wire, a steel pipe, etc. Stainless steel means all forms of steel such as slabs and steel materials. The above processing includes hot and cold processing.

In order to ensure the corrosion resistance of stainless steel, the C content is limited to 0.03% or less. When C is contained exceeding 0.03%, Cr carbide is generated during hot rolling, and corrosion resistance and toughness deteriorate.

Si is added at 0.05% or more for deoxidation. However, when Si exceeds 1.0%, toughness deteriorates. Therefore, the upper limit of Si content is limited to 1.0%. A preferable range of the amount of Si is 0.2 to 0.7%.

Mn has the effect of increasing the austenite phase and improving toughness. Further, since Mn has an effect of suppressing the precipitation of nitride, it is preferable to positively add Mn to the steel material of this embodiment. 0.1% or more of Mn is added for the toughness of the base metal and the weld. However, when Mn is added exceeding 4.0%, corrosion resistance and toughness deteriorate. Therefore, the upper limit of the amount of Mn is limited to 4.0%. The Mn content is preferably 1.0 to 3.5%, more preferably 2.0 to 3.0%.

Ｐ P is an element inevitably mixed from the raw material, and the P content is limited to 0.05% or less in order to deteriorate hot workability and toughness. The amount of P is preferably 0.03% or less.

S is an element that is inevitably mixed from the raw material, and also degrades hot workability, toughness, and corrosion resistance, so the S amount is limited to 0.0010% or less. Moreover, reducing the amount of S to less than 0.0001% increases the cost for desulfurization refining. For this reason, the S content is determined to be 0.0001 to 0.0010%. The amount of S is preferably 0.0002 to 0.0006%.

¡Cr is contained 23.0% or more in order to ensure basic corrosion resistance. On the other hand, when Cr is contained exceeding 28.0%, the ferrite phase fraction is increased, and the toughness and the corrosion resistance of the welded portion are inhibited. Therefore, the Cr content is set to 23.0% or more and 28.0% or less. A preferable Cr content is 24.0 to 27.5%.

Ni stabilizes the austenite structure and improves corrosion resistance and toughness against various acids. Furthermore, the deterioration of hot workability due to the addition of Sn and Cu is suppressed. For this reason, 2.0% or more of Ni is contained. By increasing the Ni content, the nitride precipitation temperature can be lowered. On the other hand, since Ni is an expensive alloy, the amount of Ni is limited to 6.0% or less. The Ni content is preferably 2.5 to 5.5%, more preferably 3.0 to 5.0%.

Co is an element effective for enhancing the toughness and corrosion resistance of steel, and is an element that suppresses the decrease in hot workability due to the addition of Sn and Cu, and is desirably contained together with Ni. Moreover, when adding, it is preferable to contain Co 0.1% or more. When Co is contained exceeding 1.0%, Co is an expensive element, so that an effect corresponding to the cost cannot be exhibited. For this reason, the upper limit of the amount of Co is set to 1.0%. The preferred Co content when added is 0.1 to 0.5%.

It is known from Non-Patent Document 1 that Ni has the effect of increasing the solid solubility of Cu and suppressing the generation of a liquid phase having a low melting point due to the addition of Cu and Sn. Co is an element of the Ni family. For this reason, it is thought that the fall of the hot workability by Cu and Sn is suppressed by making the sum of content of Ni and Co high. When the present inventors arranged the hot workability of the steel targeted in the present embodiment by the sum of the contents of Ni and Co, when the total amount of Ni and Co is less than 2.5%, the steel material It was understood that the ear cracking property of was increased. For this reason, the range of Ni + Co was set to 2.5% or more.

Cu is an element that enhances the corrosion resistance of stainless steel to acids, and has the effect of improving toughness. In this embodiment, in order to improve corrosion resistance, 0.2% or more of Cu is contained together with 0.01% or more of Sn. When Cu is contained exceeding 3.0%, εCu precipitates exceeding the solid solubility during hot rolling, and embrittlement occurs. For this reason, the upper limit of the amount of Cu was made into 3.0%. A preferable content when Cu is contained is 0.5 to 2.0%.

Sn is contained in order to improve the corrosion resistance of the steel of this embodiment. Therefore, it is necessary to contain at least 0.01% Sn. Furthermore, it is preferable to contain 0.02% or more of Sn. On the other hand, Sn is an element that hinders the hot productivity of steel. In the alloying element type saving duplex stainless steel targeted by the present embodiment, the interface between the ferrite phase and the austenite phase particularly at 900 ° C. or less. Reduce hot strength. The degree of the reduction depends on the contents of S, Ca, and O. However, even if other restrictions in this embodiment are added, if Sn is contained in excess of 0.2%, the hot productivity decreases. Therefore, the upper limit of the Sn content is set to 0.2%.

N is an effective element that improves the strength and corrosion resistance by dissolving in the austenite phase. For this purpose, 0.20% or more of N is contained. Increasing the amount of N makes it possible to reduce Ni. Therefore, N is an element to be actively added. On the other hand, the upper limit of the N content needs to be limited within the limit of the solid solution of N. The solid solubility limit of N increases depending on the Cr and Mn contents. In the steel of this embodiment, when N is contained in excess of 0.30%, Cr nitride is precipitated to inhibit toughness and corrosion resistance, and hot productivity is inhibited. For this reason, the upper limit of N content was 0.30%. A preferable N content is 0.20 to 0.28%.

Al is a deoxidizing element of steel, and if necessary, Al is contained together with 0.05% or more of Si in order to reduce oxygen in the steel. In the Sn-containing steel, the reduction of the oxygen amount is essential to ensure hot productivity, and for this purpose, it is necessary to contain 0.003% or more of Al as necessary. On the other hand, Al is an element having a relatively large affinity with N, and if added excessively, AlN is generated and inhibits the toughness of stainless steel. The degree depends on the N content, but when Al exceeds 0.05%, the toughness is significantly lowered. For this reason, the upper limit of the Al content is set to 0.05%. The amount of Al is preferably 0.04% or less.

Ca is an important element for the hot manufacturability of steel, and it is necessary to contain Ca in order to fix O and S in the steel as inclusions and improve the hot manufacturability. . In the steel of this embodiment, 0.0010% or more of Ca is contained for that purpose. Moreover, excessive addition reduces pitting corrosion resistance. Therefore, the upper limit of the Ca content is set to 0.0040%.

The ratio Ca / O of the content of O and Ca is an important component index for improving the hot manufacturability and corrosion resistance of the steel of this embodiment. In order to improve the hot manufacturability of the Sn-containing steel, the lower limit of Ca / O is limited. The hot ductility of the Sn-containing steel decreases particularly at a temperature of 900 ° C. or less. When the value of Ca / O is less than 0.3, the hot ductility at 1000 ° C. is also lowered, and the hot productivity is greatly impaired. For this reason, in the steel of this embodiment, Ca / O is limited to 0.3 or more. On the other hand, when Ca is added excessively and Ca / O exceeds 1.0, pitting corrosion resistance is impaired. Further, when Ca becomes excessive, the high temperature ductility at 1000 to 1100 ° C. is also impaired. For this reason, the upper limit of Ca / O was set to 1.0. Ca / O is preferably 0.4 to 0.8.

O is an inevitable impurity, and its upper limit is not particularly defined, but is an important element constituting an oxide that is representative of non-metallic inclusions. Controlling the composition of the oxide is very important for improving hot manufacturability. In addition, when a coarse cluster-like oxide is generated, it causes surface defects. For this reason, it is necessary to restrict | limit the content of O low. In this embodiment, as described above, the O content is limited by setting the ratio of the Ca content and the O content to 0.3 or more. The upper limit of the O content is preferably 0.005% or less.

Furthermore, you may contain any one or both among Mo: 2.0% or less and W: 1.0% or less. These are elements that additionally enhance the corrosion resistance. The reason for the limitation will be described.
Mo is a very effective element that additionally increases the corrosion resistance of stainless steel, and can be contained as necessary. In order to improve the corrosion resistance, it is preferable to contain 0.2% or more of Mo. On the other hand, Mo is an expensive element. In the steel of this embodiment, the upper limit of the Mo content is set to 2.0% from the viewpoint of suppressing the alloy cost.
W, like Mo, is an element that additionally improves the corrosion resistance of stainless steel, and can be added as necessary. In the steel of this embodiment, the upper limit of the W content is set to 1.0% for the purpose of improving the corrosion resistance. A preferable W content is 0.1 to 0.8%.

Further, it may contain one or more selected from V: 0.05 to 0.5%, Nb: 0.01 to 0.15%, and Ti: 0.003 to 0.05%. These are elements that have a greater tendency to form nitrides than Cr. V, Nb, and Ti can be added as necessary, and when contained in a very small amount, the corrosion resistance tends to be improved.

The nitrides and carbides formed by V are generated during hot working and cooling of the steel material, and have the effect of enhancing the corrosion resistance. Although sufficient confirmation has not been made for this reason, there is a possibility of suppressing the generation rate of chromium nitride at 700 ° C. or lower. In order to improve the corrosion resistance, it is desirable to contain 0.05% or more of V. When V is contained exceeding 0.5%, coarse V-based carbonitrides are produced and toughness is deteriorated. Therefore, the upper limit of the V amount is limited to 0.5%. When V is added, the V content is preferably in the range of 0.1 to 0.3%.

Nitrides and carbides formed by Nb are generated during the hot working and cooling of the steel material, and have the effect of increasing the corrosion resistance. Although sufficient confirmation has not been made for this reason, there is a possibility of suppressing the generation rate of chromium nitride at 700 ° C. or lower. In order to improve the corrosion resistance, it is desirable to contain 0.01% or more of Nb. On the other hand, excessive addition causes precipitation as an undissolved precipitate during heating before hot rolling, thereby inhibiting toughness. For this reason, the upper limit of the Nb content is set to 0.15%. When Nb is added, the preferable range of Nb content is 0.03% to 0.10%.

Ti is an element that forms oxides, nitrides, and sulfides in a very small amount, and solidifies the steel and refines the crystal grains of the high-temperature heating structure. Further, like V and Nb, Ti also has a property of substituting for a part of chromium in the chromium nitride. When Ti is contained in an amount of 0.003% or more, Ti precipitates are formed. On the other hand, when Ti is contained in the duplex stainless steel exceeding 0.05%, coarse TiN is generated and the toughness of the steel is inhibited. For this reason, the upper limit of the Ti content is set to 0.05%. A preferable content of Ti is 0.005 to 0.020%.

Furthermore, you may contain 1 or more types chosen from B: 0.0050% or less, Mg: 0.0030% or less, and REM: 0.10% or less. In order to further improve the hot workability, B, Mg, and REM to be contained as necessary are limited as follows.

B, Mg, and REM are all elements that improve the hot workability of steel, and it is desirable to add one or more of them for that purpose. In any of B, Mg, and REM, excessive addition conversely decreases hot workability and toughness. For this reason, the upper limit of the content was defined as follows. The upper limit of the amount of B is 0.0050%. The upper limit of the amount of Mg is 0.0030%. The upper limit of the amount of REM is 0.10%. The preferred contents are B: 0.0005 to 0.0030%, Mg: 0.0001 to 0.0015%, and REM: 0.005 to 0.05%, respectively. Here, REM is the total content of lanthanoid rare earth elements such as La and Ce.

As described above, by having the characteristics of the duplex stainless steel of the present embodiment that has been described, the hot productivity of general-purpose duplex stainless steel containing Sn can be remarkably improved.
At the slab stage, the fracture drawing value at 1000 ° C. is 70% or more. In addition, by performing processing including hot working on the slab, it is possible to obtain a duplex stainless steel material with high yield and low surface flaws.

Example 1
Examples of alloy-saving duplex stainless steel will be described below. Tables 1 to 4 show the chemical compositions of the test steels. The balance other than the components listed in Table 1 is Fe and inevitable impurity elements. In addition, regarding the components shown in Tables 1 to 4, the portion where the content is not described indicates an impurity level. REM means a lanthanoid rare earth element, and the content of REM indicates the total of these elements. A numerical value with an underline in the table indicates that it is outside the range defined in the first embodiment.

For each steel, first, a slab having a thickness of 100 mm was used, and the fracture drawing value was evaluated. Evaluation was performed as follows. First, the parallel part of an 8 mmφ round bar was heated to 1200 ° C. using high frequency. Next, the temperature was lowered to a temperature at which a break test was performed (1000 ° C.). At that temperature, the film was pulled and broken at a speed of 20 mm / second to obtain the shrinkage ratio of the cross section. A steel with a fracture drawing value of 70% or more is evaluated as A (good), a steel with a drawing value of 60 to less than 70% is evaluated as B (fair), and a steel with a drawing value of less than 60% is C (bad). The results are shown in Tables 5 and 6.
The slab was hot forged into a steel piece having a thickness of 60 mm, and this was used as a hot rolled material. Hot rolling was performed as follows. It was heated to a predetermined temperature of 1150 to 1250 ° C., and then hot-rolled by the two-stage rolling mill in the laboratory under the following conditions. First, the reduction was repeated to adjust the plate thickness to 25 mm. Subsequently, finish rolling was performed from 1000 ° C., final finish rolling was performed at 900 ° C., and the steel sheet was rolled to a final plate thickness of 12 mm and a plate width of 120 mm to obtain a hot-rolled steel plate. The maximum value of the ear crack generated in the left and right ears of the obtained hot rolled steel sheet was measured, and the sum of the maximum values of the left and right ear cracks was obtained. Steel with an ear crack sum of less than 5 mm is evaluated as A (good), steel with an ear crack sum of 5 to 10 mm is evaluated as B (fair), and steel with an ear crack sum of more than 10 mm is evaluated as C ( The results are shown in Tables 5 and 6.

Furthermore, solution heat treatment was applied to this steel sheet as follows. The steel plate was inserted into a heat treatment furnace set at 1000 ° C., and a soaking time of 5 minutes was taken. The steel plate was then extracted and then water cooled to room temperature.

The corrosion resistance of the steel sheet was evaluated by the corrosion rate in sulfuric acid.
The corrosion rate in sulfuric acid was measured as follows. A 6-hour immersion test was performed on a test piece having a thickness of 3 mm, a width of 25 mm, and a length of 25 mm in boiling 5% sulfuric acid. The weight before and after immersion was measured, and the rate of weight reduction was determined. A steel having a corrosion rate in sulfuric acid of less than 0.3 g / m ² · hr is evaluated as A (good), and a steel having a corrosion rate in sulfuric acid of 0.3 to 1 g / m ² · hr as B (fair) Evaluation was made, and steels having a corrosion rate in sulfuric acid of 1 g / m ² · hr or more were evaluated as C (bad), and the evaluation results are shown in Tables 5 and 6.

Impact characteristics were measured using Charpy specimens taken long in the width direction. A full size 2 mm V notch was processed in the rolling direction to produce a test piece. The test was performed at −20 ° C. using two test pieces each, and the impact characteristics were evaluated by the average value of the obtained impact values. Impact value and evaluate the 100 J / cm ² than the steel and A (good), the impact value and evaluate the 50 ~ 100J / cm ² steel and B (fair), impact value of 50 J / cm ² less than the steel C (bad) was evaluated, and the evaluation results are shown in Tables 5 and 6.

From the examples shown in Tables 5 and 6, Steel Nos. 1-1 to 1-33 that satisfy the conditions of the first embodiment have good hot manufacturability, corrosion resistance, and impact characteristics. On the other hand, steel Nos. 1-A to 1-U that do not satisfy the conditions of the first embodiment are inferior in any of hot manufacturability, corrosion resistance, and impact characteristics.

As can be seen from the above examples, it was clarified that according to the first embodiment, the addition of Sn improves the corrosion resistance, and provides an inexpensive alloy-saving duplex stainless steel with good hot productivity.

(Example 2)
Examples of general-purpose duplex stainless steel will be described below. Tables 7 to 10 show the chemical compositions of the test steels. The balance of the components described in Tables 7 to 10 is Fe and inevitable impurity elements. In addition, regarding the components shown in Tables 7 to 10, the portion where the content is not described indicates an impurity level. REM means a lanthanoid rare earth element, and the content of REM indicates the total of these elements. A numerical value with an underline in the table indicates that it is outside the range defined in the second embodiment.

鋳 Under the same conditions as in Example 1, slab production, evaluation of the slab break drawing value, production of hot rolled material, hot rolling of the hot rolled material, and evaluation of ear cracks were performed. The obtained evaluation results are shown in Tables 11 and 12.

Furthermore, solution heat treatment was applied to this steel sheet as follows. The steel plate was inserted into a heat treatment furnace set at 1050 ° C., and a soaking time of 5 minutes was taken. The steel plate was then extracted and then water cooled to room temperature.

The corrosion resistance of the steel sheet was evaluated by the corrosion rate in sulfuric acid.
The corrosion rate in sulfuric acid was measured as follows. A test piece having a thickness of 3 mm, a width of 25 mm, and a length of 25 mm was subjected to a 6-hour immersion test in sulfuric acid containing 2000 ppm of Cl ions, having a concentration of 15%, and a temperature of 40 ° C. The weight before and after immersion was measured, and the rate of weight reduction was determined. A steel having a corrosion rate in sulfuric acid of less than 0.1 g / m ² · hr is evaluated as A (good), and a steel having a corrosion rate in sulfuric acid of 0.1 to 0.3 g / m ² · hr is B (fair). The steel having a corrosion rate in sulfuric acid exceeding 0.3 g / m ² · hr was evaluated as C (bad), and the evaluation results are shown in Tables 11 and 12.

The impact characteristics were measured under the same conditions as in Example 1. The obtained evaluation results are shown in Tables 11 and 12.

From the examples shown in Tables 11 and 12, the general-purpose duplex stainless steel Nos. 2-1 to 2-23 satisfying the conditions of the second embodiment have good hot manufacturability, corrosion resistance and impact properties. . On the other hand, steel Nos. 2-A to 2-K and 2-M to 2-T that do not satisfy the conditions of the second embodiment are inferior in any of hot manufacturability, corrosion resistance, and impact characteristics. Comparative Example 2-L satisfies the characteristics but is inferior in cost because it contains a large amount of Co. Comparative Example 2-U is S31803 steel, which has good hot manufacturability, corrosion resistance, and manufacturability. However, the contents of Ni and Mo are high, and the cost of the second embodiment is inferior.

As can be seen from the above examples, it was clarified that according to the second embodiment, corrosion resistance is improved by addition of Sn and Cu, and a general-purpose duplex stainless steel with good hot productivity and low cost can be obtained.

According to the first and second embodiments, it is possible to provide an inexpensive alloy-saving duplex stainless steel material and general-purpose duplex stainless steel material with improved corrosion resistance. This duplex stainless steel material can be used for seawater desalination equipment, tanks for transport ships, various containers, etc., and has a great industrial contribution.

Claims (10)

1. % By mass
   C: 0.03% or less,
   Si: 0.05 to 1.0%,
   Mn: 0.1 to 7.0%,
   P: 0.05% or less,
   S: 0.0001 to 0.0010%,
   Ni: 0.5 to 5.0%,
   Cr: 18.0-25.0%,
   N: 0.10 to 0.30%
   Al: 0.05% or less,
   Ca: 0.0010 to 0.0040%, and Sn: 0.01 to 0.2%,
   The balance consists of Fe and inevitable impurities,
   The Ca / O content ratio Ca / O is 0.3 to 1.0,
   A duplex stainless steel having a pitting corrosion index PI represented by the formula (1) of less than 30.
   PI = Cr + 3.3Mo + 16N (1)
   (The element symbol in the formula (1) indicates the content of the element.)
2. Furthermore,
   Mo: 1.5% or less,
   Cu: 2.0% or less,
   The duplex stainless steel according to claim 1, comprising at least one selected from W: 1.0% or less and Co: 2.0% or less.
3. Furthermore,
   V: 0.05-0.5%
   The duplex stainless steel according to claim 1 or 2, comprising at least one selected from Nb: 0.01 to 0.20% and Ti: 0.003 to 0.05%. .
4. Furthermore,
   B: 0.0050% or less,
   The duplex stainless steel according to any one of claims 1 to 3, comprising one or more selected from Mg: 0.0030% or less and REM: 0.10% or less.
5. % By mass
   C: 0.03% or less,
   Si: 0.05 to 1.0%,
   Mn: 0.1 to 4.0%,
   P: 0.05% or less,
   S: 0.0001 to 0.0010%,
   Cr: 23.0-28.0%,
   Ni: 2.0-6.0%,
   Co: 0 to 1.0%,
   Cu: 0.2 to 3.0%,
   Sn: 0.01 to 0.2%,
   N: 0.20-0.30%,
   Al: 0.05% or less, and Ca: 0.0010-0.0040%,
   The balance consists of Fe and inevitable impurities,
   Ni + Co is 2.5% or more, and the Ca / O content ratio Ca / O is 0.3 to 1.0,
   (1) PI shown by a formula is 30 or more and less than 40, Duplex stainless steel characterized by the above-mentioned.
   PI = Cr + 3.3Mo + 16N (1)
   (The element symbol in the formula (1) indicates the content of the element.)
6. Furthermore,
   The duplex stainless steel according to claim 5, wherein either one or both of Mo: 2.0% or less and W: 1.0% or less is contained.
7. Furthermore,
   V: 0.05-0.5%
   The duplex stainless steel according to claim 5 or 6, comprising at least one selected from Nb: 0.01 to 0.15% and Ti: 0.003 to 0.05%. steel.
8. Furthermore,
   B: 0.0050% or less,
   The duplex stainless steel according to any one of claims 5 to 7, comprising one or more selected from Mg: 0.0030% or less and REM: 0.10% or less. .
9. A duplex stainless steel slab having the composition according to any one of claims 1 to 8, wherein a fracture drawing value at 1000 ° C is 70% or more.
10. A duplex stainless steel material produced by hot working the duplex stainless steel slab according to claim 9.

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