TW201333223A - Duplex stainless steel, duplex stainless steel slab, and duplex stainless steel material - Google Patents

Abstract

One aspect of this duplex stainless steel includes: in terms of percent 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%, with the remainder being Fe and unavoidable impurities, wherein a ratio Ca/O of a Ca content to an O content is in a range of 0.3 to 1.0, and a pitting corrosion index, PI represented by the equation (1) is in a range of less than 30. PI = Cr + 3.3Mo + 16N (1)

Description

Duplex stainless steel, duplex stainless steel cast, and duplex stainless steel Field of invention

This invention relates to an inexpensive duplex stainless steel containing Sn. Further, the present invention relates to a duplex stainless steel which contains composite Cu and Sn and which has excellent corrosion resistance and is inexpensive. More specifically, the present invention relates to a duplex stainless steel, a duplex stainless steel cast, and a duplex stainless steel which can be used as a seawater desalination machine, a storage tank of a transport vessel, various containers, and the like.

The present invention is based on the priority of Japanese Patent Application No. 2011-231351, filed on Jan. 21, 2011, and the benefit of Japanese Patent Application No. 2011-266351.

Background of the invention

The general-purpose duplex stainless steel contains a large amount of Cr, Mo, Ni, and N, and thus has good corrosion resistance. However, since the alloy is expensive because it contains expensive Mo and Ni, it is difficult to say that the manufacturability is also good. As a result, the steel price is not so low, so it is difficult to say that it will be used in large quantities instead of the 316 series, the 317 series stainless steel, and the like. In addition, in the present invention, the so-called general-purpose duplex stainless steel means that the pitting index PI (the formula of the content of the alloy element on the right and the expression /PI=Cr+3.3Mo+16N) has 30 or more and less than 40 ( Mass%) The value of the duplex stainless steel. From the above, in the steels, it is considered that the corrosion resistance is the same as that of the conventional general-purpose duplex stainless steel, and the alloy cost is lower than that of the conventional one, and the heat-manufacturability is good and the manufacturing cost is low. As necessary.

On the other hand, recently, a provincial alloy type duplex stainless steel which reduces Cr, Ni, Mo, etc. has been developed. Here, the alloy-type duplex stainless steel refers to a corrosion-resistant steel having a corrosion resistance equivalent to that of SUS304 and 316L, and a pitting resistance index PI (=Cr+3.3Mo) which is indexed by the content of the alloying element. +16N) is stainless steel of less than about 30. Among these steels which reduce the content of alloying elements which are useful for pitting resistance and acid resistance, it is difficult to obtain corrosion resistance equivalent to that of general-purpose duplex stainless steel. However, it is possible to consider the development of improved steel using inexpensive alternative elements.

Regarding the duplex stainless steel containing Sn, various proposals have been made since the prior art. For example, a duplex stainless steel containing 25% or more of Cr and having Sn as a selective element and containing 0.01 to 0.1% is disclosed (see Patent Documents 1 and 2 below). Further, a provincial alloy type duplex stainless steel containing 1% or less or 0.1% of Sn is disclosed (see Patent Documents 3 and 4 below). In these patent documents, although Sn is contained and the corrosion resistance is improved, the relationship between the thermal manufacturability of the steel and the Sn content is not examined.

Further, in the above patent documents, the content of N is 0.2% or less. Steel is the object. The N system is an element which lowers the hot workability of stainless steel. Compared with the case where the duplex stainless steel contains less than 0.2% of N to ensure that the hot workability reaches a desired level, it is difficult to make the duplex stainless steel contain 0.2% or more of N to ensure that the hot workability reaches a desired level. . The hot workability of a duplex stainless steel containing 0.20% or more of N and containing composite Sn and Cu is not found in the disclosed technical literature.

The present inventors focused on the possibility of improving acid resistance and pitting resistance by using Sn in the alloy-type duplex stainless steel. Then, tune Check the relationship between the content of Sn and corrosion resistance and thermal manufacturability. As a result, it was found that there is a possibility of improving corrosion resistance by containing 0.01 to 0.2% of Sn. However, it has been learned that the inclusion of a large amount of Sn in the duplex stainless steel reduces the heat manufacturability. Therefore, the frequency of predicting a decrease in steel yield is increased, and there is a significant cost increase.

Further, the inventors of the present invention have focused on the possibility of improving acid resistance and pitting resistance by using Sn and Cu in a general-purpose alloy type duplex stainless steel. Then, in the duplex stainless steel containing 0.2% or more of N and reducing the content of Mo and Ni, the relationship between the content of Sn and Cu and the corrosion resistance and the heat manufacturability were examined. As a result, it was found that there is a possibility of improving corrosion resistance by containing 0.01 to 0.2% of Sn and 0.2 to 3.0% of Cu. However, it has been learned that the inclusion of a large amount of Sn and Cu in the duplex stainless steel reduces the heat manufacturability. Therefore, the frequency of predicting a decrease in steel yield is increased, and there is a significant cost increase.

The present inventors reviewed the related knowledge about the manufacturing technique of the Sn-containing duplex stainless steel hot-rolled steel material, which is known from Patent Documents 1 to 4. As a result, it has been found that knowledge relating to the relationship between the temperature region or the content of Sn which occurs in the hot brittleness and the content of other elements by the inclusion of Sn in the duplex stainless steel is lacking.

Advanced technical literature [Patent Literature]

Patent Document 1 Japanese Patent Publication No. Hei 3-184537

Patent Document 2 Japanese Patent Publication No. 4-072013

Patent Document 3, JP-A-2010-222593

Patent Document 4 International Publication WO2009-119895

Patent Document 5, JP-A-2002-69592

Patent Document 6 JP-A-7-118805

[Non-patent literature]

[Non-Patent Document 1] "Effect of Cu and Ni on Hot Workability of Hot-rolled Mild Steel" ISIJ, Vol.37, p.217-223 (1997)

Summary of invention

The present invention clarifies that in the alloy-type duplex stainless steel, the content of Sn is related to the thermal manufacturability, and countermeasures for solving the above problems have been found. also. The present invention is to clarify that the content of Sn and Cu is related to the thermal manufacturability in the general-purpose duplex stainless steel, and countermeasures for solving the above problems have been found. Therefore, the present invention has been made in an effort to provide a duplex stainless steel, a duplex stainless steel cast piece, and a duplex stainless steel containing Sn which are excellent in heat manufacturability and low in cost. Such duplex stainless steel systems are expected to have excellent corrosion resistance and cost balance. Therefore, the possibility of being widely used in various fields is increased.

In particular, in the second aspect (second embodiment), by increasing the content of N and Mn, adding Cu, Sn, and reducing the content of Ni and Mo in expensive elements, it is possible to develop a low-cost general purpose. Type duplex stainless steel is the object of the present invention.

In order to solve the above problems, the present inventors have focused on the alloy-type duplex stainless steel, and changed the content of Sn and Ca, B, and rare earth. The content of the element (REM) or the like was prepared into a molten material, and the following experiment was carried out. Further, the contents of Ca, B, rare earth elements (REM) and the like are said to improve the heat manufacturability.

A tensile test piece was taken by casting a cast piece of the molten material. The tensile test piece was subjected to high temperature drawing at 1200 to 700 ° C, and the pore diameter value (sectional reduction rate of the fractured section) was measured and the high temperature ductility was evaluated. Further, a hot-rolled steel sheet having a thickness of 12 mm was obtained by hot forging and hot rolling, and edge cracking property was evaluated. For some steels, the hot rolling temperature and rolling temperature are changed to evaluate the edge cracking property, and the heating temperature and rolling temperature of the hot rolling are required to be related to the high temperature ductility.

As described in the above-mentioned Patent Document 5 or Patent Document 6, in the case of a general duplex stainless steel, if the pore diameter of the cast piece evaluated by high-temperature drawing is less than 60%, in many cases, in the casting Significant edge cracking occurs in the hot rolling of the sheet. Therefore, those skilled in the art aim to set the pore diameter value in the high temperature of the cast piece to at least 60% or more, and perform refining, casting, and hot working of the steel multiple times. However, the present inventors have clearly evaluated the high-temperature ductility of a cast alloy type duplex stainless steel (base composition: 21%Cr-2%Ni-3%Mn-0.18%N) containing about 0.1% of Sn. It was found that the pore diameter values in the several melting experiments were all less than 60%. The evaluation of high temperature ductility is carried out as follows. First, the parallel portion of the 8 mmφ round bar was heated at 1200 ° C using a high frequency wave. Next, the temperature was lowered to the temperature at which the breaking test was performed, and the film was stretched and broken at this temperature and at a speed of 20 mm/sec. Next, the shrinkage ratio of the cross section was obtained. An example of its data is shown in Figure 1. It is speculated from the results that it is practical to obtain an inexpensive alloy with the addition of Sn. Duplex stainless steel is almost impossible.

The present inventors performed hot rolling of a cast piece of alloy-type duplex stainless steel containing Sn obtained by vacuum melting and casting, and observed the length of edge cracking which occurred at this time. As a result, it has been found that a duplex stainless steel containing Sn having few edge cracks is rarely present. The hot rolling test was carried out as follows. First, a cast piece having a thickness of 90 to 44 mm was heated at 1200 °C. Next, it is passed through a plurality of passes and the thickness is reduced to a thickness of 12 to 6 mm. The final rolling temperature is controlled at around 900 °C. Although the edge rupture occurs on the left and right sides, the edge rupture length can be obtained by summing their respective maximum lengths. Even if the edge rupture length of the steel is finished, the minimum value of the pore diameter of the high temperature ductility of the cast piece (the minimum value at about 900 ° C is obtained in Fig. 1) cannot be perfectly correlated. However, when the pore diameter value of 1000 ° C as shown in Fig. 2 was arranged, it was clearly found that it was not related to whether or not Sn was contained, and showed a good correlation. In addition, in FIG. 2, the point shown by ○ (white circle) corresponds to the result of Sn-A and Sn-B of FIG. 1, and the point of the figure of ◆ (黒 菱) is another experimental result (review and whether or not Contains Sn-independent experimental results).

The inventors of the present invention further changed the contents of various elements and carried out the melting, casting, and rolling experiments in order to find the conditions for obtaining the above-mentioned steel having little edge cracking. After that, the evaluation of the high-temperature ductility of the cast piece and the evaluation of the edge crack of the steel after hot rolling were actively carried out. Through the above experiments, and based on the knowledge obtained, the first aspect of the invention relating to the inexpensive stainless steel containing Sn is completed.

The first aspect of the duplex stainless steel of the present invention is shown below.

(1) A duplex stainless steel characterized by containing C: 0.03% or less, Si: 0.05 to 1.0%, Mn: 0.1 to 7.0%, P: 0.05% or less, and S: 0.0001 to 0.0010% by mass%. Ni: 0.5~5.0%, Cr: 18.0~25.0%, N: 0.10~0.30%, Al: 0.05% or less, Ca: 0.0010~0.0040%, and Sn: 0.01~0.2%, and the remaining part is Fe and The ratio of the content of Ca to O is Ca/O of 0.3 to 1.0, and the pitting index PI represented 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) The duplex stainless steel according to (1), further comprising one or more elements selected from the group consisting of Mo: 1.5% or less, Cu: 2.0% or less, W: 1.0% or less, and Co: 2.0% or less.

(3) The duplex stainless steel according to (1) or (2), further comprising one or more elements selected from the group consisting of V: 0.05 to 0.5%, Nb: 0.01 to 0.20%, and Ti: 0.003 to 0.05%. .

(4) The duplex stainless steel according to any one of (1) to (3) further comprising: B: 0.0050% or less, Mg: 0.0030% or less, and REM: 0.10% or less. element.

In order to solve the above problems, the present inventors have made the present invention a general-purpose duplex stainless steel, and to change the content of Sn, the contents of Ca, B, rare earth elements (REM), and the Ni content, and further add Co made the molten material and carried out the following experiment. Further, if Ca, B, a rare earth element (REM) or the like is contained, the heat manufacturability can be improved.

A tensile test piece was taken by casting a cast piece of the molten material. The tensile test piece was subjected to high temperature drawing at 1200 to 700 ° C, and the pore diameter value (sectional reduction rate of the fractured section) was measured and the high temperature ductility was evaluated. Further, a hot-rolled steel sheet having a thickness of 12 mm was obtained by hot forging and hot rolling, and edge cracking property was evaluated. For some steels, the hot rolling temperature and rolling temperature are changed to evaluate the edge cracking property, and the heating temperature and rolling temperature of the hot rolling are required to be related to the high temperature ductility.

As described in the above-mentioned Patent Document 5 or Patent Document 6, in the case of a general duplex stainless steel, if the pore diameter of the cast piece evaluated by high-temperature drawing is less than 60%, in many cases, in the casting Significant edge cracking occurs in the hot rolling of the sheet. Therefore, those skilled in the art aim to set the pore diameter value in the high temperature of the cast piece to at least 60% or more, and perform refining, casting, and hot working of the steel multiple times. However, the present inventors evaluated the high temperature of a cast alloy type duplex stainless steel (base composition: 25% Cr-4% Ni-1.2% Mo-1.5% Cu-0.25% N) containing about 0.1% of Sn. For ductility, it is clear that the pore size values in all of the melting experiments are less than 60%. The evaluation of high temperature ductility is carried out as follows. First, the parallel portion of the 8 mmφ round bar was heated at 1200 ° C using a high frequency wave. Next, the temperature was lowered to the temperature at which the breaking test was performed, and the film was stretched and broken at this temperature and at a speed of 20 mm/sec. Next, the shrinkage ratio of the cross section was obtained. An example of its data is shown in Figure 3. From the results, it is presumed that it is almost impossible to practically obtain an inexpensive alloy-type duplex stainless steel to which Sn is added.

The present inventors performed hot rolling of a cast alloy type duplex stainless steel obtained by vacuum melting and casting, and observed the length of the edge crack occurring at this time. It was found that there is very little edge rupture containing Sn Duplex stainless steel is rarely present. The hot rolling test was carried out as follows. First, a cast piece having a thickness of 90 to 44 mm was heated at 1200 °C. Next, it is passed through a plurality of passes and the thickness is reduced to a thickness of 12 to 6 mm. The final rolling temperature is controlled at around 900 °C. Although the edge rupture occurs on the left and right sides, the edge rupture length can be obtained by summing their respective maximum lengths. Even if the edge rupture length of the steel is finished, the minimum value of the pore diameter of the high temperature ductility of the cast piece (the minimum value at about 900 ° C is obtained in Fig. 3) cannot be perfectly correlated. However, when the pore diameter value of 1000 ° C as shown in Fig. 4 was sorted, it was clearly known that it was not related to whether or not Sn was contained, and showed a good correlation. In addition, in FIG. 4, the point shown by ○ (white circle) corresponds to the result of Sn-A and Sn-B of FIG. 3, and the point of the figure of ◆ (黒 菱) is another experimental result (review and whether or not Contains Sn-independent experimental results).

The inventors of the present invention further changed the contents of various elements and carried out the melting, casting, and rolling experiments in order to find the conditions for obtaining the above-mentioned steel having little edge cracking. After that, the evaluation of the high-temperature ductility of the cast piece and the evaluation of the edge crack of the steel after hot rolling were actively carried out. Through the above experiments, and based on the knowledge obtained, the second aspect of the present invention relating to the inexpensive stainless steel containing Sn is completed.

The second aspect of the duplex stainless steel of the present invention is shown below.

(5) A duplex stainless steel characterized by containing C: 0.03% or less, Si: 0.05 to 1.0%, Mn: 0.1 to 4.0%, P: 0.05% or less, and S: 0.0001 to 0.0010% by mass%. , Cr: 23.0~28.0%, Ni: 2.0~6.0%, Co: 0~1.0%, Cu: 0.2~3.0%, Sn: 0.01~0.2%, N: 0.20~0.30%, Al: 0.05% or less, and Ca: 0.0010~0.0040%, and the remaining part is composed of Fe and unavoidable impurities, while Ni+Co is 2.5% or more, and the content of Ca and O is The 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, and PI=Cr+3.3Mo+16N (1) (the element symbol in the formula (1) indicates the content of the element. .)

(6) The duplex stainless steel according to (5), which further contains either or both of Mo: 2.0% or less and W: 1.0% or less.

(7) The duplex stainless steel according to (5) or (6), further comprising one or more elements selected from the group consisting of V: 0.05 to 0.5%, Nb: 0.01 to 0.15%, and Ti: 0.003 to 0.05%. .

(8) The duplex stainless steel according to any one of (5) to (7), further comprising at least one selected from the group consisting of B: 0.0050% or less, Mg: 0.0030% or less, and REM: 0.10% or less. element.

The elements of the duplex stainless steel slab and the duplex stainless steel of the present invention are shown below.

(9) A duplex stainless steel slab having the composition according to any one of (1) to (8), wherein the breaking pore diameter at 1000 ° C is 70% or more.

(10) A duplex stainless steel material obtained by subjecting a duplex stainless steel slab as described in (9) to hot working.

Since the aspect of the present invention can provide steel which is used as a seawater desalination machine, a storage tank of a transport vessel, various containers and the like, which has a better corrosion resistance and an excellent cost balance, the two phases are not rust Steel, duplex stainless steel cast and duplex stainless steel. Therefore, the aspect of the present invention greatly contributes to the development of the industry.

Simple illustration

Fig. 1 is a first aspect of a duplex stainless steel (provincial alloy type duplex stainless steel), and exemplifies high temperature ductility of a duplex stainless steel containing Sn and no added Sn.

Fig. 2 is a view showing the first aspect of the duplex stainless steel (the alloy-type duplex stainless steel), and shows the relationship between the edge crack length after hot rolling and the pore diameter value at 1000 °C.

Fig. 3 is a second aspect of the duplex stainless steel (universal duplex stainless steel), and exemplifies the high temperature ductility of the duplex stainless steel slab containing Sn and no added Sn.

Fig. 4 is a second aspect of the duplex stainless steel (provincial alloy type duplex stainless steel), and shows the relationship between the edge crack length after hot rolling and the pore diameter value at 1000 °C.

Form for implementing the invention

(First embodiment)

The reason for the limitation of the first aspect (provincial alloy type duplex stainless steel) of the duplex stainless steel of the present invention will be described below. In addition, the content of each component is represented by mass %.

In the present embodiment, the term "stainless steel slab" refers to a steel in a pre-machined state after casting, hot working, or forging, and the term "stainless steel material" refers to a steel sheet or hot-rolled steel obtained by processing the slab by various methods. Plate, cold rolled steel, steel wire, steel pipe, etc. Moreover, the term "stainless steel" refers to the entire form of steel such as a cast piece or a steel material. The above processing includes heat and cold processing.

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

The Si system is added in an amount of 0.05% or more for deoxidation. However, if more than 1.0% of Si is added, the toughness is deteriorated. Therefore, the upper limit of the amount of Si is limited to 1.0%. The preferred range of the amount of Si is 0.2 to 0.7%.

Mn has an effect of improving the toughness of the Worthite iron phase. Further, in order to have an effect of lowering the nitride precipitation temperature TN, Mn is preferably positively added with Mn in the steel material of the present embodiment. Mn is added in an amount of 0.1% or more for the toughness of the base material and the welded portion. However, if Mn exceeding 7.0% is added, corrosion resistance and toughness are deteriorated. Therefore, the upper limit of the amount of Mn is limited to 7.0%. The content of Mn is preferably from 1.0 to 6.0%, more preferably from 2.0 to 5.0%.

P is derived from an element in which raw materials are inevitably mixed, and causes deterioration in hot workability and toughness, and the amount of P is limited to 0.05% or less. The amount of P is preferably 0.03% or less.

S is derived from an element in which raw materials are inevitably mixed, and causes deterioration in hot workability, toughness, and corrosion resistance, and the amount of S is limited to 0.0010% or less. Further, reducing the amount of S to less than 0.0001% increases the cost for desulfurization refining. Therefore, the S amount is set to be 0.0001 to 0.0010%. The amount of S is preferably 0.0002 to 0.0006%.

In order to stabilize the Vostian iron structure, the Ni system further improves the toughness of various acids and further improves the toughness, and Ni is contained in an amount of 0.5% or more. Due to the increase in the Ni content, the precipitation temperature of the nitride may be lowered. On the other hand, in the steel of the present embodiment which is an alloy of the Ni-based alloy, the amount of Ni is limited to 5.0% or less from the viewpoint of cost. The Ni content is preferably from 1.0 to 4.0%, more preferably from 1.5 to 3%.

In order to ensure basic corrosion resistance, Cr is contained in an amount of 18.0% or more. On the other hand, if the Cr content is more than 25.0%, the ferrite iron phase fraction is increased, and the toughness and the corrosion resistance of the welded portion are inhibited. For this reason, the Cr content is set to be 18.0% or more and 25.0% or less. A preferred Cr content is from 19.0 to 23.0%.

The N system is an element effective for improving the strength and corrosion resistance of the solid solution of the Worthite iron phase. Therefore, it is made to contain 0.10% or more of N. On the other hand, although the solid solution limit is improved based on the contents of Cr and Mn, in the steel of the present embodiment, if the N content exceeds 0.30%, the Cr nitride precipitates and the toughness is hindered. Corrosion resistance also hinders thermal manufacturability. Therefore, the upper limit of the N content is 0.30%. Preferably, the N content is from 0.10 to 0.25%.

The deoxidizing element of the Al-based steel reduces the oxygen in the steel as needed. Therefore, Si and Al are contained in an amount of 0.05% or more. Since Sn is contained in steel, the amount of oxygen is reduced in order to ensure thermal manufacturability. Therefore, it is necessary to contain 0.003% or more of Al as required. On the other hand, Al is an element having a relatively large affinity with N, and if it is excessively added, AlN is generated to hinder the toughness of the stainless steel. Although the degree depends on the N content, if Al exceeds 0.05%, the toughness is remarkably lowered. Therefore, the upper limit of the Al content Set to 0.05%. The amount of Al is preferably 0.04% or less.

The Ca system is an important element for achieving the heat manufacturability of steel, and is fixed by using O and S in the steel as an inclusion, and it is necessary to contain Ca in order to improve the heat manufacturability. In the steel of the present embodiment, Ca is contained in an amount of 0.0010% or more in order to achieve the object. Moreover, excessive addition causes a decrease in pitting resistance. For this reason, the upper limit of the content of Ca is 0.0040%.

In order to improve the corrosion resistance of the steel of the present embodiment, Sn is contained. Therefore, it is necessary to contain a minimum of 0.01% of Sn. It is preferable to contain 0.02% or more of Sn. On the other hand, Sn is an element which hinders the heat manufacturability of steel, and in the alloy element type saving type duplex stainless steel which is the object of this embodiment, the ferrite phase and the Worth are particularly reduced at 900 ° C or lower. The thermal strength of the interface of Tian Tiexiang. Although the degree of reduction depends on the contents of S, Ca, and O, in the present embodiment, even if other restrictions are added, when Sn is contained in an amount exceeding 0.2%, the decrease in heat manufacturability cannot be obtained. The Sn content is limited to 0.2% above.

The ratio Ca/O ratio of the content of O and Ca is an important component index for improving the heat manufacturability and corrosion resistance of the steel of the present embodiment. In order to improve the thermal manufacturability of the steel containing Sn, the lower limit of Ca/O is limited. The high temperature ductility of steel containing Sn is particularly lowered at temperatures below 900 °C. If the value of Ca/O is less than 0.3, the high temperature ductility at 1000 ° C is also lowered, resulting in great thermal manufacturability. Therefore, in the steel of the present embodiment, Ca/O is limited to 0.3 or more. On the other hand, excessive addition of Ca, if Ca / O becomes more than 1.0, will impair the pitting resistance. Further, if Ca is excessive, the ductility at 1000 to 1100 °C will be damaged. Therefore, Ca/O The upper limit is 1.0. Ca/O is preferably from 0.4 to 0.8.

The O system is an unavoidable impurity, and although the upper limit is not particularly set, it is an important element constituting a representative oxide of the non-metallic inclusion. The composition control of its oxide is very important for the improvement of thermal manufacturability. Moreover, if a coarse cluster-like oxide is formed, it will cause a surface defect. Therefore, it is necessary to have a low content of O. In the present embodiment, as described above, the content of O is limited by setting the ratio of the Ca content to the O content to 0.3 or more. The upper limit of the O content is preferably 0.005% or less.

In order to additionally improve the corrosion resistance, one or more elements selected from the group consisting of Mo: 1.5% or less, Cu: 2.0% or less, W: 1.0% or less, and Co: 2.0% or less may be contained as needed. The explanation is about the reasons for its restrictions.

Mo is an extremely effective element for improving the corrosion resistance of stainless steel and can be contained as needed. In order to improve corrosion resistance, it is preferable to contain 0.2% or more of Mo. On the other hand, Mo is an element which promotes precipitation of an intermetallic compound, and in the steel of this embodiment, the content of Mo is limited to 1.5% from the viewpoint of suppressing precipitation during hot rolling.

Cu is an additional element which improves the corrosion resistance to the acid of stainless steel, and in order to have an effect of improving toughness, it is recommended to contain 0.3% or more as needed. If Cu is contained in an amount exceeding 2.0%, the embrittlement of εCu is precipitated when the hot rolling exceeds the solid solubility. Therefore, the upper limit of the amount of Cu is limited to 2.0%. A preferred content of the case containing Cu is 0.3 to 1.5%.

The W system is an element which additionally increases the corrosion resistance of stainless steel, and can be added as needed. In the steel of the present embodiment, the amount of W is limited to 1.0% in order to achieve the purpose of improving corrosion resistance. Preferred W The content is 0.05~0.5%.

Co is an effective element for improving the toughness and corrosion resistance of steel, and is selected for addition. The content of Co is preferably 0.03% or more. If Co is contained in excess of 2.0%, it will not be able to achieve cost-effective effects because it is an expensive element. Therefore, the amount of Co is limited to 2.0%. The preferred Co content in the case of addition is 0.03 to 1.0%.

Further, one or more selected from the group consisting of V: 0.05 to 0.5%, Nb: 0.01 to 0.20%, and Ti: 0.003 to 0.05% may be contained. These elements are elements that have a greater tendency to form nitrides than Cr. V, Nb, and Ti can be all added depending on the demand, and when a trace amount is contained, there is a tendency to improve corrosion resistance.

The nitrides and carbides formed by V are formed during the inter-heat processing and the cooling of the steel, and have the effect of improving the corrosion resistance. Although the reason for this is not sufficiently confirmed, it is thought that there is a possibility that the formation rate of chromium nitride at 700 ° C or lower is suppressed. In order to improve the corrosion resistance, it is contained in an amount of 0.05% or more. If V is contained in excess of 0.5%, coarse V-based carbonitrides are formed and the toughness is deteriorated. Therefore, the upper limit of the amount of V is limited to 0.5%. The preferred V content in the case of addition is in the range of 0.1 to 0.3%.

The nitrides and carbides formed by Nb are formed during the inter-heat processing and the cooling of the steel, and have an effect of improving corrosion resistance. Although the reason for this is not sufficiently confirmed, it is thought that there is a possibility that the formation rate of chromium nitride at 700 ° C or lower is suppressed. In order to improve the corrosion resistance, 0.01% or more of Nb is contained. On the other hand, excessive addition will precipitate in the case of heating before hot rolling, and it will become an unsolid precipitate, and it will inhibit a toughness. Therefore, Nb The upper limit of the content is defined as 0.20%. The preferred Nb content for the addition is in the range of 0.03% to 0.10%.

Ti is an element which atomizes crystal grains in which a steel such as an oxide, a nitride or a sulfide is formed in a very small amount and a high temperature heating structure is formed. Further, similarly to V and Nb, Ti also has a property of replacing a part of chromium of chromium nitride. A precipitate of Ti is formed by containing 0.003% or more of Ti. On the other hand, if more than 0.05% of Ti is contained in the duplex stainless steel, coarse TiN will be formed to hinder the toughness of the steel. Therefore, the upper limit of the content of Ti is limited to 0.05%. A suitable content of Ti is 0.005 to 0.020%.

Further, one or more selected from the group consisting of B: 0.0050% or less, Mg: 0.0030% or less, and REM: 0.10% or less may be contained. In an attempt to further improve the hot workability, B, Mg, and REM are contained as required according to the following limitations.

B, Mg, and REM are elements which improve the hot workability of steel, and one or more types are added depending on the purpose. Excessive addition of any of B, Mg, and REM causes a decrease in hot workability and toughness. Therefore, the upper limit of the content is 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 content is B: 0.0005 to 0.0030%, Mg: 0.0001 to 0.0015%, and REM: 0.005 to 0.05%. Here, the REM is a total of the contents of the rare earth elements of La or Ce.

As described above, in the feature of the duplex stainless steel according to the present embodiment, it has been shown that the thermal manufacturability of the alloy duplex stainless steel containing Sn is remarkably improved.

In the stage of casting, the breaking aperture value at 1000 ° C is 70% on. also. By performing the processing including the hot working on the cast piece, a duplex stainless steel material having good productivity and few surface defects can be obtained.

(Second embodiment)

Hereinafter, the reason for limiting the second aspect (universal duplex stainless steel) of the duplex stainless steel of the present invention will be described. Moreover, the content of each component is represented by mass %.

In the present embodiment, the term "stainless steel slab" refers to steel in a state before processing such as casting, hot working, or forging, and the term "stainless steel" refers to processing of the slab by various methods. Steel sheet, hot rolled steel sheet, cold rolled steel sheet, steel wire, steel pipe, etc. Moreover, the term "stainless steel" means a whole form of steel as a cast piece or a steel material. The above processing includes processing of heat and cold.

In order to ensure the corrosion resistance of the stainless steel, the amount of C is limited to 0.03% or less. If C is contained in an amount of more than 0.03%, Cr carbide is formed during hot rolling, and corrosion resistance and toughness are deteriorated.

Si is added in an amount of 0.05% or more for deoxidation. However, if more than 1.0% of Si is added, the toughness is deteriorated. Therefore, the upper limit of the amount of Si is limited to 1.0%. The preferred range of the amount of Si is 0.2 to 0.7%.

The Mn system has an effect of increasing the iron phase of the Vostian and improving the toughness. Further, in order to achieve the effect of suppressing the precipitation of nitride, Mn is preferably added Mn in the steel material of the present embodiment. 0.1% or more of Mn is added for the toughness of the base material and the welded portion. However, if more than 4.0% of Mn is added, corrosion resistance and toughness are deteriorated. Therefore, the upper limit of the amount of Mn is limited to 4.0%. The preferred Mn content is from 1.0 to 3.5%, more preferably from 2.0 to 3.0%.

P is an element which is inevitably mixed with raw materials, and causes hot workability and toughness. Therefore, the amount of P is limited to 0.05% or less. The amount of P is preferably 0.03% or less.

S is derived from an inevitable element of the raw material, and also causes deterioration in hot workability, toughness, and corrosion resistance. Therefore, the amount of S is limited to 0.0010% or less. Further, reducing the amount of S to less than 0.0001% increases the cost for desulfurization refining. Therefore, the amount of S is set to be 0.0001 to 0.0010%. The amount of S is preferably 0.0002 to 0.0006%.

The Cr system contains 23.0% or more in order to ensure basic corrosion resistance. On the other hand, if Cr is contained in an amount of more than 28.0%, the ferrite-grain phase fraction is increased, and the toughness and the corrosion resistance of the welded portion are inhibited. Therefore, the content of Cr is set to be 23.0% or more and 28.0% or less. A preferred Cr content is 24.0 to 27.5%.

The Ni system stabilizes the Worthite iron structure and improves corrosion resistance and toughness for various acids. Further, the decrease in hot workability is suppressed by adding Sn and Cu. Therefore, it is made to contain 2.0% or more of Ni. By increasing the content of Ni, the precipitation temperature of the nitride 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 from 2.5 to 5.5%, more preferably from 3.0 to 5.0%.

Co is an element effective for improving the toughness and corrosion resistance of steel, and is an element which suppresses a decrease in hot workability by adding Sn and Cu, and it is preferable to contain Ni at the same time. Further, in the case of addition, it is preferred to contain Co in an amount of 0.1% or more. If Co is contained in excess of 1.0%, since Co is an expensive element, it will not be able to exhibit a cost-effective effect. Therefore, the amount of Co is limited above It is 1.0%. The preferred Co content in the case of addition is from 0.1 to 0.5%.

Non-Patent Document 1 discloses that Ni has an effect of increasing the solid solubility of Cu and suppressing the occurrence of a liquid phase having a low melting point by adding Cu and Sn. Further, Co is an element of the same family as Ni. Therefore, it is considered that the sum of the contents of Ni and Co is increased, and the decrease in hot workability is suppressed by Cu and Sn. However, the inventors of the present invention have mastered the hot workability of steel by the sum of the contents of Ni and Co, and grasped that when the total amount of Ni and Co is less than 2.5%, the steel is obtained. The edge cracking will increase. Therefore, the range of Ni + Co is set to be 2.5% or more.

Cu is an element which improves the corrosion resistance to the acid of stainless steel, and has an effect of improving toughness. In the present embodiment, in order to improve the corrosion resistance, 0.2% or more of Sn is contained in an amount of 0.2% or more. If Cu is contained in an amount exceeding 3.0%, εCu is precipitated when the hot rolling is exceeded, and embrittlement occurs. Therefore, the amount of Cu is limited to 3.0%. A preferred content in the case of containing Cu is 0.5 to 2.0%.

The Sn-containing material is used to improve the corrosion resistance of the steel of the present embodiment. Therefore, it is necessary to contain a minimum of 0.01% of Sn. Further, it is preferable to contain 0.02% or more of Sn. On the other hand, Sn is an element which hinders the heat manufacturability of steel. In the alloy element type saving type duplex stainless steel which is the object of the present embodiment, it is possible to particularly reduce the ferrite phase and the Worthfield below 900 ° C. The thermal strength of the interface of the iron phase. Although the degree of reduction depends on the contents of S, Ca, and O, in the present embodiment, even if other restrictions are added, if the content of Sn is more than 0.2%, the decrease in heat manufacturability cannot be obtained. The Sn content is limited to 0.2% above.

The N system is an element effective for improving the strength and corrosion resistance of the solid solution of the Worthite iron phase. Therefore, it is made to contain 0.20% or more of N. By increasing N, it is possible to reduce Ni, and thus N is an element that is intended to be actively added. On the other hand, the upper limit of the content of N is necessary to be limited to the solid solution limit of N. The solid solution limit of N is increased depending on the content of Cr and Mn. In the steel of the present embodiment, when N is contained in an amount of more than 0.30%, Cr nitride is precipitated to impede toughness and corrosion resistance, and thermal manufacturability is also inhibited. Therefore, the N content is limited to 0.30%. A preferred N content is from 0.20 to 0.28%.

Al is a deoxidizing element of steel, and in order to reduce the oxygen in the steel in response to demand, Si and Al together contain 0.05% or more. In the steel containing Sn, the reduction in the amount of oxygen is necessary to ensure the heat manufacturability. Therefore, it is necessary to contain 0.003% or more of Al as needed. On the other hand, Al is an element having a relatively large affinity with N, and if excessive addition occurs, AlN is generated to hinder the toughness of the stainless steel. Although the degree depends on the content of N, if it exceeds 0.05% of Al, the toughness is remarkably lowered. Therefore, the upper limit of the content of Al is limited to 0.05%. The amount of Al is preferably 0.04% or less.

The Ca system is an important element for achieving the heat manufacturability of steel, and is fixed by using O and S in the steel as an inclusion, and it is necessary to contain Ca in order to improve the heat manufacturability. In the steel of the present embodiment, Ca is contained in an amount of 0.0010% or more in order to achieve the object. Moreover, excessive addition causes a decrease in pitting resistance. For this reason, the upper limit of the content of Ca is 0.0040%.

The ratio Ca/O ratio of the content of O and Ca is an important component index for improving the heat manufacturability and corrosion resistance of the steel of the present embodiment. In order to improve The thermal manufacturability of steel containing Sn limits the lower limit of Ca/O. The high temperature ductility of steel containing Sn is particularly lowered at temperatures below 900 °C. If the value of Ca/O is less than 0.3, the high temperature ductility at 1000 ° C is also lowered, resulting in great thermal manufacturability. Therefore, in the steel of the present embodiment, Ca/O is limited to 0.3 or more. On the other hand, excessive addition of Ca, if Ca / O becomes more than 1.0, will impair the pitting resistance. Further, if Ca is excessive, the ductility at 1000 to 1100 °C will be damaged. Therefore, the above Ca/O is limited to 1.0. Ca/O is preferably from 0.4 to 0.8.

The O system is an unavoidable impurity, and although the upper limit is not particularly set, it is an important element constituting a representative oxide of the non-metallic inclusion. The composition control of its oxide is very important for the improvement of thermal manufacturability. Moreover, if a coarse cluster-like oxide is formed, it will cause a surface defect. Therefore, it is necessary to have a low content of O. In the present embodiment, as described above, the content of O is limited by setting the ratio of the Ca content to the O content to 0.3 or more. The upper limit of the O content is preferably 0.005% or less.

Furthermore, it may contain either or both of Mo: 2.0% or less and W: 1.0% or less. These are elements that can additionally improve corrosion resistance. Explain the reasons for its restrictions.

Mo is an extremely effective element for improving the corrosion resistance of stainless steel and can be contained as needed. In order to improve corrosion resistance, it is preferable to contain 0.2% or more of Mo. On the other hand, Mo is an expensive element, and in the steel of the present embodiment, the upper limit of the content of Mo is 2.0% from the viewpoint of suppressing the alloy cost.

The W system is the same element as Mo, which increases the corrosion resistance of stainless steel. It can be added as needed. In the steel of the present embodiment, the W content is limited to 1.0% for the purpose of improving corrosion resistance. A preferred W content is from 0.1 to 0.8%.

Furthermore, one or more elements selected from the group consisting of V: 0.05 to 0.5%, Nb: 0.01 to 0.15%, and Ti: 0.003 to 0.05% may be contained. These elements are elements that have a greater tendency to form nitrides than Cr. V, Nb, and Ti can be all added depending on the demand, and when a trace amount is contained, there is a tendency to improve corrosion resistance.

The nitrides and carbides formed by V are formed during the inter-heat processing and the cooling of the steel, and have the effect of improving the corrosion resistance. Although the reason for this is not sufficiently confirmed, it is thought that there is a possibility that the formation rate of chromium nitride at 700 ° C or lower is suppressed. In order to improve the corrosion resistance, it is preferred to contain 0.05% or more of V. If V is contained in excess of 0.5%, coarse V-based carbonitrides are formed and the toughness is deteriorated. Therefore, the upper limit of the amount of V is limited to 0.5%. The preferred V content in the case of addition is in the range of 0.1 to 0.3%.

The nitrides and carbides formed by Nb are formed during the inter-heat processing and the cooling of the steel, and have an effect of improving corrosion resistance. Although the reason for this is not sufficiently confirmed, it is thought that there is a possibility that the formation rate of chromium nitride at 700 ° C or lower is suppressed. In order to improve the corrosion resistance, it is preferred to contain 0.01% or more of Nb. On the other hand, excessive addition will precipitate in the case of heating before hot rolling, and it will become an unsolid precipitate, and it will inhibit a toughness. Therefore, the upper limit of the content of Nb is 0.15%. The preferred Nb content in the case of addition ranges from 0.03% to 0.10%.

Ti is used to form oxides, nitrides, and sulfides in a very small amount. The solidification of the steel and the element of the high-temperature heating of the crystal grains of the structure are atomized. Further, similarly to V and Nb, Ti also has a property of replacing a part of chromium of chromium nitride. A precipitate of Ti is formed by containing 0.003% or more of Ti. On the other hand, if more than 0.05% of Ti is contained in the duplex stainless steel, coarse TiN will be formed to hinder the toughness of the steel. Therefore, the upper limit of the content of Ti is limited to 0.05%. A suitable content of Ti is 0.005 to 0.020%.

Further, one or more selected from the group consisting of B: 0.0050% or less, Mg: 0.0030% or less, and REM: 0.10% or less may be contained. In order to further attempt to improve the hot workability, B, Mg, and REM are contained as required according to the following limitations.

B, Mg, and REM are all elements for improving the hot workability of steel, and it is preferred to add one or more kinds depending on the purpose. Excessive addition of any of B, Mg, and REM causes a decrease in hot workability and toughness. Therefore, the upper limit of the content is 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 content is B: 0.0005 to 0.0030%, Mg: 0.0001 to 0.0015%, and REM: 0.005 to 0.05%. Here, the REM is a total of the contents of the rare earth elements of La or Ce.

As described above, in the feature of the duplex stainless steel according to the present embodiment, the heat manufacturability of the general-purpose duplex stainless steel containing Sn is remarkably improved.

In the stage of casting the sheet, the breaking pore diameter at 1000 ° C is 70% or more. also. By performing the processing including the hot working on the cast piece, a duplex stainless steel material having good productivity and few surface defects can be obtained.

[Examples]

(Example 1)

Examples of the provincial alloy type duplex stainless steel are described below. Tables 1 to 4 show the chemical composition of the test steel. Further, the remainder other than the components described in Table 1 are Fe and an unavoidable impurity element. Further, regarding the components shown in Tables 1 to 4, the undocumented portions of the contents indicate the level of the impurities. REM means a lanthanide rare earth element, and the content of REM means the total of such elements. The numerical values of the additional bottom lines in the table are shown outside the range defined in the first embodiment.

All steels were first used as cast pieces with a thickness of 100 mm and evaluated for breaking pore size. The evaluation was carried out as follows. First, the parallel portion of the 8 mmφ round bar was heated at 1200 ° C using a high frequency wave. Next, the temperature was lowered to the temperature at which the breaking test was performed (1000 ° C). At this temperature, the film was stretched and broken at a speed of 20 mm/sec, and then the shrinkage ratio of the cross section was obtained. A steel having a broken pore diameter of 70% or more was evaluated as A (good), a steel having a pore diameter of 60 to less than 70% was evaluated as B (fair), and a steel having a pore diameter of less than 60% was evaluated as C (bad). The results are shown in Tables 5 and 6.

The cast piece was hot-forged to obtain a steel sheet having a thickness of 60 mm, and hot rolled to obtain a material. The hot rolling is carried out as follows. The heating was carried out at a predetermined temperature of 1150 to 1250 ° C, and secondly, hot rolling was carried out by a 2-stage rolling mill of the laboratory under the following conditions. First, press down repeatedly and adjust the plate thickness to 25mm. Next, finish rolling was performed at 1000 ° C, and final finish rolling was performed at 900 ° C to obtain a hot-rolled steel having a final sheet thickness of 12 mm and a sheet width of 120 mm. The maximum value of edge cracking occurring at the left and right edge portions of the obtained hot-rolled steel sheet was measured, and the sum of the maximum values of the left and right edge cracks was determined. The steel with the edge rupture sum of less than 5 mm was evaluated as A (good), the steel with the edge rupture of 5 to 10 mm was evaluated as B (fair), and the steel with the edge rupture of more than 10 mm was evaluated as C (f Bad), the results are shown in Tables 5 and 6.

Further, the steel sheet is subjected to a solution heat treatment as follows. The steel sheet was inserted into a heat treatment furnace set at 1000 ° C for 5 minutes of soaking time. Next, the steel sheet was taken out, and then water-cooled to normal temperature.

The corrosion resistance of the steel sheet was evaluated by the corrosion rate in sulfuric acid.

The corrosion rate in sulfuric acid was determined as follows. For the test piece of 3 mm thick × 25 mm wide × 25 mm long, a immersion test was carried out for 6 hours in boiling 5% sulfuric acid. The weight before and after the immersion was measured, and the rate of decrease in weight was determined. The corrosion rate in sulfuric acid is less than 0.3 g/m ² . The steel of hr is evaluated as A (good), and the corrosion rate in sulfuric acid is 0.3~1 g/m ² . The steel of hr is evaluated as B (fair), and the corrosion rate in sulfuric acid is 1 g/m ² . The steel above hr was evaluated as C (bad), and the evaluation results are shown in Tables 5 and 6.

The punching characteristics were measured using a Carpy test piece taken long in the width direction. In the full size, a 2 mm V shape was processed in the rolling direction to prepare a test piece. Each of the two test pieces was used, and the test was carried out at -20 ° C, and the punching characteristics were evaluated by the average value of the obtained punch values. The IMPACT is more than 100J / cm ² of the steel was evaluated as A (good), the IMPACT value of 50 ~ 100J / cm steel Evaluation ² of as B (fair), the IMPACT is insufficient steel Evaluation 50J / cm ² of of C (bad), the evaluation results are shown in Tables 5 and 6.

Compared with the examples shown in Tables 5 and 6, steel Nos. 1-1 to 1-33 satisfying the conditions of the first embodiment were excellent in heat productivity, corrosion resistance and punching characteristics. On the other hand, steel No. 1-A to 1-U which did not satisfy the conditions of the first embodiment was inferior in thermal manufacturability, corrosion resistance and punching characteristics.

As is apparent from the above-described examples, it is possible to clearly obtain the alloy-type duplex stainless steel which is excellent in corrosion resistance and is excellent in heat-manufacturability by adding Sn.

(Example 2)

Examples of general-purpose duplex stainless steels are described below. Tables 7 to 10 show the chemical composition of the test steel. Further, the remaining portions of the components described in Tables 7 to 10 are elements of Fe and an unavoidable impurity. Also, Tables 7 to 10 show The undeclared portion of the component and the content indicates the level of the impurity. REM means a lanthanide rare earth element, and the content of REM means the total of such elements. The numerical values of the additional bottom lines in the table are shown outside the range defined in the second embodiment.

The production of the cast piece, the evaluation of the breaking pore diameter of the cast piece, the production of the hot rolled material, the implementation of hot rolling of the hot rolled material, and the evaluation of edge cracking were carried out under the same conditions as in Example 1. The evaluation results obtained are shown in Tables 11 and 12.

Further, the steel sheet was subjected to a solution heat treatment as follows. The steel sheet was inserted into a heat treatment furnace set at 1050 ° C for 5 minutes of soaking time. Next, the steel sheet was taken out, and then 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 determined as follows. For the test piece of 3 mm thick × 25 mm wide × 25 mm long, an immersion test was carried out for 6 hours in sulfuric acid containing 2000 ppm of Cl ions, a concentration of 15%, and a temperature of 40 °C. The weight before and after the immersion was measured, and the rate of decrease in weight was determined. The corrosion rate in sulfuric acid is less than 0.1 g/m ² . The steel of hr is evaluated as A (good), and the corrosion rate in sulfuric acid is 0.1-0.3 g/m ² . The steel of hr is evaluated as B (fair), and the corrosion rate in sulfuric acid is more than 0.3 g/m ² . The steel above hr was evaluated as C (bad), and the evaluation results are shown in Tables 11, 12.

The punching characteristics were determined by the same conditions as in Example 1. The evaluation results obtained are shown in Tables 11 and 12.

Compared with 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 are excellent in heat productivity, corrosion resistance, and punching property. On the other hand, steel No. 2-A to 2-K and 2-M to 2-T which did not satisfy the conditions of the second embodiment were inferior in thermal manufacturability, corrosion resistance and punching characteristics. Further, in Comparative Example 2-L, although the characteristics were satisfied, since a large amount of Co was contained, it was inferior in terms of cost. Further, Comparative Example 2-U was S31803 steel, and was excellent in heat productivity, corrosion resistance, and manufacturability. However, since the Ni and Mo contents are high, they are inferior in terms of cost as the object of the second embodiment.

As can be seen from the above embodiment, it can be clarified by the second embodiment. It is obtained by adding Sn and Cu to improve corrosion resistance, and the heat-manufacturability is a low-cost general-purpose duplex stainless steel.

[Industrial availability]

According to the first and second embodiments, it is possible to provide an alloy-type duplex stainless steel material and a general-purpose duplex stainless steel material which have improved corrosion resistance and low cost. This duplex stainless steel material can be used as a seawater desalination machine, a storage tank of a transport ship, various containers, etc., and contributes greatly to the industry.

Simple illustration

Fig. 1 is a first aspect of a duplex stainless steel (provincial alloy type duplex stainless steel), and exemplifies high temperature ductility of a duplex stainless steel containing Sn and no added Sn.

Fig. 2 is a view showing the first aspect of the duplex stainless steel (the alloy-type duplex stainless steel), and shows the relationship between the edge crack length after hot rolling and the pore diameter value at 1000 °C.

Fig. 3 is a second aspect of the duplex stainless steel (universal duplex stainless steel), and exemplifies the high temperature ductility of the duplex stainless steel slab containing Sn and no added Sn.

Fig. 4 is a second aspect of the duplex stainless steel (provincial alloy type duplex stainless steel), and shows the relationship between the edge crack length after hot rolling and the pore diameter value at 1000 °C.

Claims (10)

A duplex stainless steel characterized by containing 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%, and Ni: by mass%; 0.5~5.0%, Cr:18.0~25.0%, N:0.10~0.30%, Al:0.05% or less, Ca:0.0010~0.0040%, and Sn:0.01~0.2%, and the rest is made of Fe and unavoidable The composition of impurities, the ratio of Ca to O is Ca/O of 0.3~1.0, and the pitting index PI represented by formula (1) is less than 30, PI=Cr+3.3Mo+16N (1) (Formula (1) The element symbol in the middle indicates the content of the element). The duplex stainless steel according to claim 1, further comprising one or more elements selected from the group consisting of 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 claim 1 or 2, further comprising one or more elements selected from the group consisting of 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 any one of claims 1 to 3, further comprising at least one element selected from the group consisting of B: 0.0050% or less, Mg: 0.0030% or less, and REM: 0.10% or less. A duplex stainless steel characterized by containing 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%, and Cr: by mass%; 23.0~28.0%, Ni: 2.0~6.0%, Co: 0~1.0%, Cu: 0.2~3.0%, Sn: 0.01~0.2%, N: 0.20~0.30%, Al: 0.05% or less, and Ca: 0.0010 ~0.0040%, and the rest is composed of Fe and unavoidable impurities. Ni+Co is 2.5% or more, and the ratio of Ca to O is Ca/O is 0.3 to 1.0, and the PI represented by the formula (1) is 30 or more and less than 40, and PI=Cr+3.3Mo+16N (1) (The element symbol in the formula (1) indicates the content of the element). The duplex stainless steel according to claim 5, further comprising any one or both of Mo: 2.0% or less and W: 1.0% or less. The duplex stainless steel according to claim 5 or 6, further comprising one or more elements selected from the group consisting of 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 any one of claims 5 to 7, further comprising at least one element selected from the group consisting of B: 0.0050% or less, Mg: 0.0030% or less, and REM: 0.10% or less. A duplex stainless steel slab having a composition according to any one of claims 1 to 8 and having a breaking pore diameter of at least 70% at 1000 °C. A duplex stainless steel material obtained by subjecting a duplex stainless steel slab of claim 9 to thermal processing.