KR20140064941A - Duplex stainless steel, duplex stainless steel slab, and duplex stainless steel material - Google Patents

Abstract

One aspect of the two-phase stainless steel is a ferritic stainless steel having a composition of 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% The balance of Fe and inevitable impurities, and the balance of Fe and inevitable impurities, and the balance of Cr and Fe, , The ratio Ca / O of the contents of Ca and O is 0.3 to 1.0, and the formula index PI represented by the formula (1) is less than 30.
[Equation 1]

Description

DUPLEX STAINLESS STEEL, DUPLEX STAINLESS STEEL SLAB, AND DUPLEX STAINLESS STEEL MATERIAL [0001] The present invention relates to a double-phase stainless steel,

The present invention relates to an inexpensive Sn-containing two-phase stainless steel. Further, the present invention relates to a two-phase stainless steel which contains Cu and Sn in combination and is excellent in corrosion resistance and inexpensive. More particularly, the present invention relates to a two-phase stainless steel, a two-phase stainless steel casting and a two-phase stainless steel which can be used as seawater desalination equipment, transport tanks, various containers, and the like.

The present application claims priority based on Japanese Patent Application No. 2011-231352, filed on October 21, 2011, and Japanese Patent Application No. 2011-266351, filed on December 6, 2011, I use the contents here.

The general-purpose two-phase stainless steel contains Cr, Mo, Ni, and N in a large amount and has good corrosion resistance. However, since it contains expensive Mo and Ni, it is difficult to say that the alloy cost is high and the manufacturability is good. As a result, the price of steel is not so cheap, and it is hard to say that it is used in place of stainless steel of 316 series and 317 series. Further, the general-purpose two-phase stainless steel used in the present invention is a stainless steel having an average value of not less than 30 and less than 40 (mass%) represented by the formula index PI (sum of the contents of right alloy elements / PI=Cr+3.3Mo + ≪ / RTI > From the above-described circumstances, it is considered that, in these steels, a steel which exhibits corrosion resistance equivalent to that of conventional general-purpose two-phase stainless steels and which has an alloy cost lower than that of the prior art, good hot-

In recent years, alloy-saving two-phase stainless steels have been developed that reduce Cr, Ni, and Mo. Here, the alloy-saving two-phase stainless steel is a steel having corrosion resistance equivalent to SUS304 and 316L in terms of pitting resistance, and stainless steel having an internal index PI (= Cr + 3.3Mo + 16N) indexed by the content of the alloy element of less than about 30 Lt; / RTI > It is difficult to obtain the corrosion resistance equivalent to that of the general-purpose two-phase stainless steel in these steels in which the content of the alloying element useful for pitting corrosion resistance and acid resistance is reduced. However, it is considered possible to develop an improved steel using an inexpensive alternative element.

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

Further, in the patent document, a steel having a N content of 0.2% or less is targeted. N is an element that lowers the hot workability of stainless steel. Securing a desired level of hot workability of the duplex stainless steel containing 0.2% or more of N is more difficult than securing the hot workability of the duplex stainless steel containing less than 0.2% of N to a desired level. No technical literature disclosing hot workability of a two-phase stainless steel containing 0.20% or more of N and containing Sn and Cu in combination was found.

The present inventors have focused on the possibility of improving the acid resistance and pitting resistance due to Sn in the alloy-saving two-phase stainless steel. And the relationship between Sn content, corrosion resistance and hot workability was investigated. As a result, it has been found that by containing 0.01 to 0.2% of Sn, the corrosion resistance can be improved. However, it has been found that the hot-dip composition of these two-phase stainless steels containing a large amount of Sn is lowered. This increases the frequency with which the yield of the steel decreases, and a remarkable increase in cost is expected.

Further, the present inventors have focused on the possibility of improving the acid resistance and pitting resistance due to Sn and Cu in general-purpose two-phase stainless steels. The relationship between the contents of Sn and Cu, corrosion resistance and hot workability was examined in a two-phase stainless steel in which the contents of Mo and Ni were reduced and also N was contained in an amount of 0.20% or more. As a result, it has been found that the corrosion resistance can be improved by containing 0.01 to 0.2% of Sn and 0.2 to 3.0% of Cu. However, it has been found that the hot-dip galvanizing is reduced in these two-phase stainless steels containing a large amount of Sn and Cu. This increases the frequency with which the yield of the steel decreases, and a remarkable increase in cost is expected.

The inventors of the present invention have studied conventional knowledge concerning the manufacturing technology of the conventional Sn-containing two-phase stainless hot-rolled steel including the Patent Documents 1 to 4. As a result, it was found that there was a lack of knowledge on the temperature range of generation of hot brittleness due to Sn contained in the two-phase stainless steel, the relationship with the Sn content, and the relationship with the content of other elements.

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

&Quot; Effect of Cu and Ni on Hot-rolled Mild Steel "ISIJ, Vol.37, p.217-223 (1997)

DISCLOSURE OF THE INVENTION The present invention discloses the relationship between the Sn content and the hot production performance in an alloy-saving two-phase stainless steel, and finds measures to solve the above problems. Further, the present invention discloses the relationship between the Sn content and the Cu content in general-purpose two-phase stainless steels, and finds measures to solve the above problems. Accordingly, it is an object of the present invention to provide an Sn-containing two-phase stainless steel, a two-phase stainless steel casting piece and a two-phase stainless steel material having a good hot- Such a two-phase stainless steel is expected to have excellent balance between corrosion resistance and cost. As a result, the possibility of being widely used in various fields is considered to be high.

Particularly, in the second embodiment (the second embodiment), by increasing the content of N and Mn and by adding Cu and Sn in combination, it is possible to reduce the content of Ni and Mo, which are expensive elements, It is the purpose of the invention to develop the river.

DISCLOSURE OF THE INVENTION In order to solve the above problems, the inventors of the present invention have made a dissolution material having a Sn content, a content of Ca, B, a rare earth element (REM) or the like changed in the alloy- The following experiment was conducted. In addition, it is said that the content of Ca, B, rare earth element (REM) and the like improves hot melt composition.

A tensile test piece was taken from a cast piece casting a dissolving material. The tensile test piece was subjected to high temperature stretching at 1200 to 700 ° C to measure the cross-sectional shrinkage ratio (cross-sectional reduction ratio of fracture cross-section) to evaluate the high-temperature ductility. A hot-rolled steel sheet having a thickness of 12 mm was obtained by hot forging and hot rolling, and the edge cracking property was evaluated. The edge cracking property was evaluated by changing the heating temperature and the rolling temperature of the hot rolling for a part of steel, and the correlation between the heating temperature and the rolling temperature of the hot rolling and the high temperature ductility was determined.

As described in the above-mentioned Patent Documents 5 and 6, when the cross-sectional shrinkage of the cast steel piece evaluated by high-temperature tensile in a two-phase stainless steel is less than 60%, in most cases, It is known to cause significant edge cracks. As a result, a person skilled in the art often performs steel refining, casting and hot working aiming at a cross-sectional shrinkage at a high temperature of at least 60% at a high temperature. The inventors of the present invention evaluated the high temperature ductility of the alloy-saving two-phase stainless steel (base composition: 21% Cr-2% Ni-3% Mn-0.18% N) containing Sn at around 0.1% The shrinkage rate of less than 60% was clarified by several solvent experiments. The evaluation of the high temperature ductility was carried out as follows. First, a parallel portion of a round bar of 8 mm? Was heated to 1200 占 폚 using a high frequency wave. Subsequently, the temperature was lowered to the temperature at which the fracture test was carried out, and the specimen was subjected to tensile fracture at the temperature of 20 mm / sec. Then, the shrinkage ratio of the cross section was obtained. An example of the data is shown in Fig. From these results, it was considered that there is almost no possibility of practically obtaining a cheap alloy-saving two-phase stainless steel containing Sn added thereto.

The inventors of the present invention observed hot-rolled casting pieces of an alloy-saving Sn-containing two-phase stainless steel obtained by vacuum melting and casting, and observed the edge crack length that occurred at that time. As a result, it was found that Sn-containing two-phase stainless steel steels rarely have edge cracks. The hot rolling test was carried out as follows. First, the cast piece having a thickness of 90 to 44 mm was heated to 1200 deg. Subsequently, the thickness was reduced to a thickness of 12 to 6 mm through a plurality of rolling passes. The finishing rolling temperature was controlled to about 900 캜. Edge cracks occur on both sides, and the maximum crack lengths are summed to determine the edge crack length. Even when the edge crack length of the steel material is summarized to a minimum value (in FIG. 1, a minimum value is obtained at about 900 DEG C) of the cross-sectional shrinkage percentage of the high temperature ductility of the casting piece, neat correlation can not be obtained. However, as shown in Fig. 2 (summarized by the sectional shrinkage ratio of 1000 占 폚, it is clear that regardless of whether Sn is contained or not, good correlation is shown). In Fig. 2, The points of the plot correspond to the results of Sn-A and Sn-B in Fig. 1, and the points of the plot (black squirrel-cage model) are the results of other experiments Results).

The inventors of the present invention conducted solvent, casting and rolling tests by changing the content of the various elements so as to find a condition in which a steel material with small edge cracks can be reliably obtained. Then, evaluation of the high temperature ductility of the casting member and evaluation of cracks of the steel material edge after hot rolling were energetically performed. Based on the obtained knowledge, the above-mentioned experiments have led to the completion of the first embodiment of the present invention, which is described for an inexpensive Sn-containing alloy-saving two-phase stainless steel.

The requirements of the first embodiment of the two-phase stainless steel of the present invention are shown below.

(1) by mass, C: not more than 0.03%, Si: 0.05 to 1.0%, Mn: 0.1 to 7.0%, P: not more than 0.05%, S: 0.0001 to 0.0010%, Ni: 0.5 to 5.0% The content of Ca and the content of O, the content of Ca and the content of O, and the content of Ca and O, Of Ca / O is 0.3 to 1.0, and the formula index PI represented by the formula (1) is less than 30.

(In the formula (1), the symbol represents the content of the element)

(2) The duplex stainless steel according to (1), further comprising at least one 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%

(3) The two-phase stainless steel according to (1) or (2), further comprising at least one selected from 0.05 to 0.5% of V, 0.01 to 0.20% of Nb and 0.003 to 0.05% .

(4) The duplex stainless steel according to any one of (1) to (3), further comprising at least one selected from the group consisting of 0.0050% or less of B, 0.0030% or less of Mg, and 0.10% or less of REM .

In order to solve the above problems, the present inventors have also changed the content of Sn, the content of Ca, B, the rare earth element (REM), and the content of Ni in the general-purpose two-phase stainless steel to which the present invention is applied And a dissolving material to which Co was further added was prepared, and the following experiment was conducted. In addition, it is said that when Ca, B, a rare earth element (REM) or the like is contained, the hot melt composition is improved.

A tensile test piece was taken from a cast piece casting a dissolving material. The tensile test piece was subjected to high temperature stretching at 1200 to 700 ° C to measure the cross-sectional shrinkage ratio (cross-sectional reduction ratio of fracture cross-section) to evaluate the high-temperature ductility. A hot-rolled steel sheet having a thickness of 12 mm was obtained by hot forging and hot rolling, and the edge cracking property was evaluated. The edge cracking property was evaluated by changing the heating temperature and the rolling temperature of the hot rolling to a part of the steel, and the correlation between the heating temperature, the rolling temperature and the high temperature ductility of the hot rolling was obtained.

As described in the above-mentioned Patent Documents 5 and 6, when the cross-sectional shrinkage of the cast steel piece evaluated by high-temperature tensile in a two-phase stainless steel is less than 60%, in most cases, It is known to cause significant edge cracks. As a result, a person skilled in the art often performs steel refining, casting and hot working aiming at a cross-sectional shrinkage at a high temperature of at least 60% at a high temperature. However, the present inventors have evaluated the high temperature ductility of a general-purpose two-phase stainless steel (base composition: 25% Cr-4% Ni-1.2% Mo-1.5% Cu-0.25% N) casting member containing about 0.1% It has been clarified by several solvent experiments that the minimum value of the cross-sectional shrinkage ratio of the bar is less than 60%. The evaluation of the high temperature ductility was carried out as follows. First, a parallel portion of a round bar of 8 mm? Was heated to 1200 占 폚 using a high frequency wave. Subsequently, the temperature was lowered to the temperature at which the fracture test was carried out, and the specimen was subjected to tensile fracture at the temperature of 20 mm / sec. Then, the shrinkage ratio of the cross section was obtained. An example of the data is shown in Fig. From these results, it was thought that practically obtaining an inexpensive general-purpose type two-phase stainless steel containing Sn was almost unlikely.

The inventors of the present invention observed hot-rolled casting pieces of a general-purpose two-phase stainless steel obtained by vacuum melting and casting, and observed the edge crack length that occurred at that time. As a result, it was found that Sn-containing two-phase stainless steel steels rarely have edge cracks. The hot rolling test was carried out as follows. First, the cast piece having a thickness of 90 to 44 mm was heated to 1200 deg. Subsequently, the thickness was reduced to a thickness of 12 to 6 mm through a plurality of rolling passes. The finishing rolling temperature was controlled to about 900 캜. Edge cracks occur on both sides, and the maximum crack lengths are summed to determine the edge crack length. Even if the edge crack length of the steel material is summarized to a minimum value (in FIG. 3, a minimum value is obtained at about 900 DEG C) of the cross-sectional shrinkage ratio of the high temperature ductility of the casting piece, neat correlation can not be obtained. However, as shown in Fig. 4, when the sectional shrinkage percentage at 1000 캜 is summarized, it is clear that regardless of whether or not Sn is contained, it shows a good correlation. 4, the points of the plot (white circle) correspond to the results of Sn-A and Sn-B of FIG. 3, and the points of the plot (black color model) The results of examinations, whether or not containing Sn).

The inventors of the present invention conducted solvent, casting and rolling tests in which the contents of the various elements were varied in order to find out conditions under which the steel material with less edge cracks can be reliably obtained. The evaluation of the high temperature ductility of the casting member and the evaluation of cracks of the steel material edge after hot rolling were performed energetically. Based on the obtained knowledge, the above-mentioned experiments have led to the completion of the second embodiment of the present invention, which is described for an inexpensive Sn-containing two-phase stainless steel.

The requirements of the second embodiment of the two-phase stainless steel of the present invention are shown below.

(5)% by mass, C: not more than 0.03%, Si: 0.05 to 1.0%, Mn: 0.1 to 4.0%, P: not more than 0.05%, S: 0.0001 to 0.0010%, Cr: 23.0 to 28.0% From 0.01 to 0.2%, N: from 0.20 to 0.30%, Al: up to 0.05%, and Ca: from 0.0010 to 0.0040%, and the balance Fe and inevitable impurities, wherein Ni + Co is not less than 2.5%, Ca / O ratio of Ca / O is 0.3 to 1.0, and PI is 30 or more and less than 40 Two phase stainless steel.

[Equation 1]

(In the formula (1), the symbol represents the content of the element)

(6) The duplex stainless steel according to (5), further comprising at least one of Mo and no more than 2.0% and W: not more than 1.0%.

(7) The two-phase stainless steel according to (5) or (6), further comprising at least one selected from 0.05 to 0.5% of V, 0.01 to 0.15% of Nb and 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 0.0050% or less of B, 0.0030% or less of Mg, and 0.10% or less of REM .

The requirements of one form of the two-phase stainless steel casting piece and the two-phase stainless steel material of the present invention are shown below.

(9) A two-phase stainless steel casting mold having a composition according to any one of (1) to (8), wherein the shrinkage at break at 1000 占 폚 is 70% or more.

(10) A two-phase stainless steel material produced by hot working a two-phase stainless steel casting piece according to (9).

According to the aspect of the present invention, it is possible to provide a two-phase stainless steel, a two-phase stainless steel casting material having excellent corrosion resistance and a good balance of cost as a material for seawater desalination equipment, transport tanks, And a two-phase stainless steel material. As a result, the form of the present invention contributes greatly to the development of industry.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram illustrating high temperature ductility of a two-phase stainless steel containing Sn and without Sn, in relation to the first type of two-phase stainless steel (alloy-saving two-phase stainless steel).
Fig. 2 is a diagram showing the relationship between the edge crack length after hot rolling and the section shrinkage ratio at 1000 캜, in relation to the first type of two-phase stainless steel (alloy-saving two-phase stainless steel).
3 is a diagram illustrating high temperature ductility of a two-phase stainless steel casting member containing Sn and no Sn with respect to a second type of two-phase stainless steel (general-purpose two-phase stainless steel).
Fig. 4 is a graph showing the relationship between the edge crack length after hot rolling and the section shrinkage rate at 1000 캜, in relation to the second type of two-phase stainless steel (general-purpose two-phase stainless steel).

(First Embodiment)

The reason for limiting the first mode of the two-phase stainless steel of the present invention (alloy-saving two-phase stainless steel) will be described below. The content of each component represents mass%.

Further, in the present embodiment, the stainless steel casting piece means a steel in a state before casting, a hot working, a forging, or the like, and the stainless steel material means a casting piece obtained by processing the casting piece by various methods Hot rolled steel sheet, cold rolled steel sheet, steel wire, steel pipe and the like. In addition, stainless steel means a general shape as a steel such as a cast piece or a steel material. The processing includes hot and cold working.

In order to secure the corrosion resistance of stainless steel, the amount of C is limited to 0.03% or less. When C is contained in an amount exceeding 0.03%, Cr carbide is formed at the time of hot rolling, and corrosion resistance and toughness are deteriorated.

Si is added by 0.05% or more for deoxidation. However, when Si is added in an amount exceeding 1.0%, toughness is deteriorated. As a result, the upper limit of the amount of Si is limited to 1.0%. The preferable range of the amount of Si is 0.2 to 0.7%.

Mn has an effect of improving the toughness by increasing the austenite phase. Furthermore, since Mn has an effect of lowering the nitride precipitation temperature TN, it is preferable to actively add Mn in the steel material of the present embodiment. Add 0.1% or more of Mn for the toughness of the base material and the welded part. However, when Mn is added in excess of 7.0%, corrosion resistance and toughness are deteriorated. As a result, the upper limit of the Mn amount is limited to 7.0%. The Mn content is preferably 1.0 to 6.0%, more preferably 2.0 to 5.0%.

P is an element that is inevitably incorporated from the raw material and deteriorates hot workability and toughness, so that the amount of P is limited to 0.05% or less. The P content is preferably 0.03% or less.

S is an element that is inevitably incorporated from a raw material and deteriorates hot workability, toughness and corrosion resistance. Therefore, the amount of S is limited to 0.0010% or less. Further, if the amount of S is reduced to less than 0.0001%, the cost for desulfurization refining becomes high. Therefore, the amount of S is set to 0.0001 to 0.0010%. The amount of S is preferably 0.0002 to 0.0006%.

Ni contains at least 0.5% of Ni in order to stabilize the austenite structure and further improve corrosion resistance and toughness against various acids. By increasing the Ni content, it becomes possible to lower the precipitation temperature of the nitride. On the other hand, Ni is an expensive alloy and limits the amount of Ni to 5.0% or less from the viewpoint of cost in the steel of the present embodiment, which is an alloy-saving two-phase 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, Cr is contained in an amount of 18.0% or more. On the other hand, when Cr is contained in an amount exceeding 25.0%, the ferrite phase fraction increases, and toughness and corrosion resistance of the welded portion are impaired. As a result, the content of Cr was set to 18.0% or more and 25.0% or less. The content of Cr is preferably 19.0 to 23.0%.

N is an effective element for enhancing strength and corrosion resistance by being dissolved in an austenite phase. As a result, the N content is 0.10% or more. On the other hand, although the solubility limit increases with the content of Cr and Mn, if N is contained in excess of 0.30% in the steels according to the present embodiment, Cr nitride precipitates to deteriorate toughness and corrosion resistance as well as to inhibit hot- . As a result, the upper limit of the N content was set to 0.30%. The preferable N content is 0.10 to 0.25%.

Al is a deoxidizing element of the steel, and reduces oxygen in the steel as needed. As a result, Al is contained together with not less than 0.05% of Si. In the Sn-containing steel, the reduction of the amount of oxygen is essential for ensuring hot-working composition, and accordingly, it is necessary to contain Al in an amount of 0.003% or more as necessary. On the other hand, Al has a relatively large affinity to N, and when added in excess, AlN is generated to inhibit the toughness of stainless steel. The degree depends on the N content. If Al exceeds 0.05%, the decrease in toughness becomes remarkable. As a result, the upper limit of the Al content was set at 0.05%. The amount of Al is preferably 0.04% or less.

Ca is an important element for hot-forming the steel, and it is necessary to contain Ca in order to fix O and S in the steel as inclusions and to improve hot-formed composition. In the steel of the present embodiment, Ca is contained at 0.0010% or more for the purpose. Also, the addition of excessive amount decreases the pitting resistance. Therefore, the upper limit of the content of Ca was set to 0.0040%.

Sn is contained in order to improve the corrosion resistance of the steel of the present embodiment. Therefore, a Sn content of at least 0.01% is required. Further, it is preferable to contain Sn at 0.02% or more. On the other hand, Sn is an element inhibiting the hot-rolling of the steel, and in the alloy-reduced-size two-phase stainless steel to which the present embodiment is applied, the hot strength of the interface between the ferrite phase and the austenite phase is lowered, . The degree of the deterioration depends on the content of S, Ca and O. If other limitations are applied in this embodiment, if the content of Sn exceeds 0.2%, it is impossible to prevent deterioration in hot productivity, Was set at 0.2%.

The ratio Ca / O of the contents of O and Ca is an important ingredient index for improving the hot-rolled composition and corrosion resistance of the steel of the present embodiment. The lower limit of Ca / O is limited in order to improve the hot manufacturability of a Sn-containing steel. The high ductility of the Sn-containing steel is particularly lowered at a temperature of 900 DEG C or lower. If the value of Ca / O is less than 0.3, the high temperature ductility at 1000 占 폚 is also lowered, and the composition of the hot melt composition is greatly damaged. Therefore, in the steel of the present embodiment, Ca / O is limited to 0.3 or more. On the other hand, when Ca is excessively added and Ca / O exceeds 1.0, the pitting resistance is impaired. Further, when Ca is excessively increased, high temperature ductility at 1000 to 1100 占 폚 is also impaired. As a result, the upper limit of Ca / O was set at 1.0. Ca / O is preferably 0.4 to 0.8.

O is an inevitable impurity and its upper limit is not particularly specified, but it is an important element constituting an oxide which is representative of a nonmetal inclusion. The control of the composition of the oxide is very important in improving the hot productivity. If a coarse cluster shaped oxide is produced, it causes surface scratches. Therefore, it is necessary to limit the content of O to a low level. In the present embodiment, as described above, the content of O 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.

At least one selected from Mo: not more than 1.5%, Cu: not more than 2.0%, W: not more than 1.0%, and Co: not more than 2.0%, may be contained in order to additionally enhance corrosion resistance. The reasons 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 needed. In order to improve the corrosion resistance, it is preferable to add Mo of 0.2% or more. On the other hand, Mo is an element promoting intermetallic compound precipitation. In the steel of the present embodiment, the upper limit of the Mo content is set to 1.5% from the viewpoint of suppressing precipitation during hot rolling.

Cu is an element which additionally increases the corrosion resistance of stainless steel to an acid, and has an action of improving toughness. Therefore, it is recommended that Cu is contained in an amount of 0.3% or more if necessary. If Cu is contained in an amount exceeding 2.0%, εCu precipitates in excess of the solubility at the time of hot rolling, resulting in embrittlement. As a result, the upper limit of the amount of Cu was set to 2.0%. When Cu is contained, the preferable content is 0.3 to 1.5%.

W is an element which additionally improves the corrosion resistance of stainless steel similarly to Mo and can be added as needed. For the purpose of increasing the corrosion resistance in the steel of the present embodiment, the upper limit of the amount of W is set to 1.0%. The content of W is preferably 0.05 to 0.5%.

Co is an effective element for increasing the toughness and corrosion resistance of steel, and is optionally added. The content of Co is preferably 0.03% or more. If Co is contained in an amount exceeding 2.0%, an expensive effect is not obtained because it is an expensive element. For this reason, the upper limit of the amount of Co was set to 2.0%. When added, the preferable Co content is 0.03 to 1.0%.

0.05 to 0.5% of V, 0.01 to 0.20% of Nb, and 0.003 to 0.05% of Ti. These are elements having a tendency to produce nitride more than Cr. V, Nb, and Ti can be added as needed, and when added in a trace amount, the corrosion resistance tends to be improved.

The nitride and carbide formed by V are generated in the hot working and the cooling process of the steel and have an action of enhancing the corrosion resistance. For this reason, there is no sufficient confirmation, but there is a possibility of suppressing the rate of chromium nitride formation at 700 캜 or lower. In order to improve the corrosion resistance, V of at least 0.05% is contained. When V is contained in an amount exceeding 0.5%, coarse V-based carbonitrides are produced and toughness is deteriorated. As a result, the upper limit of the amount of V is limited to 0.5%. When added, the preferred V content is in the range of 0.1 to 0.3%.

Nitride and carbide formed by Nb are generated during the hot working and cooling process of the steel, and have an action of enhancing the corrosion resistance. For this reason, there is no sufficient confirmation, but there is a possibility of suppressing the rate of chromium nitride formation at 700 캜 or lower. In order to improve the corrosion resistance, Nb of 0.01% or more is contained. On the other hand, excessive addition causes precipitation as non-solid precipitates upon heating before hot rolling, and toughness is inhibited. For this reason, the upper limit of the content of Nb was set to 0.20%. The preferable range of the Nb content when added is 0.03% to 0.10%.

Ti is an element that forms oxides, nitrides, and sulfides in a very small amount to refine the coagulation of steel and the crystal grains of a high-temperature heating structure. Like V and Nb, Ti has a property of substituting a part of chromium in chromium nitride. By containing Ti in an amount of 0.003% or more, precipitates of Ti are formed. On the other hand, when Ti is contained in the two-phase stainless steel exceeding 0.05%, coarse TiN is generated and the toughness of the steel is inhibited. As a result, the upper limit of the content of Ti was set to 0.05%. A suitable content of Ti is 0.005 to 0.020%.

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 defined 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 the purpose. In both B, Mg and REM, excessive additions adversely affect 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 Mg amount is 0.0030%. The upper limit of the REM amount is 0.10%. The preferable contents are 0.0005 to 0.0030% of B, 0.0001 to 0.0015% of Mg, and 0.005 to 0.05% of REM, respectively. Here, REM is the sum of the contents of lanthanoid-based rare-earth elements such as La and Ce.

By having the features of the two-phase stainless steel of the present embodiment described above, the hot-rolled composition of the alloy-saving two-phase stainless steel containing Sn can be remarkably improved.

At the stage of casting, the shrinkage at break at 1000 캜 is 70% or more. Further, by carrying out machining including hot working on this cast piece, it is possible to obtain a two-phase stainless steel material having good yield and few surface scratches.

(Second Embodiment)

The reason for limiting the second mode (general-purpose two-phase stainless steel) of the two-phase stainless steel of the present invention will be described below. The content of each component represents mass%.

Further, in the present embodiment, the stainless steel casting piece means a steel in a state before casting, a hot working, a forging, or the like, and the stainless steel material means a casting piece obtained by processing the casting piece by various methods Hot rolled steel sheet, cold rolled steel sheet, steel wire, steel pipe and the like. In addition, stainless steel means a general shape as a steel such as a cast piece or a steel material. The processing includes hot and cold working.

In order to secure the corrosion resistance of stainless steel, the amount of C is limited to 0.03% or less. When C is contained in an amount exceeding 0.03%, Cr carbide is formed at the time of hot rolling, and corrosion resistance and toughness are deteriorated.

Si is added by 0.05% or more for deoxidation. However, when Si is added in an amount exceeding 1.0%, toughness is deteriorated. As a result, the upper limit of the amount of Si is limited to 1.0%. The preferable range of the amount of Si is 0.2 to 0.7%.

Mn has an effect of improving the toughness by increasing the austenite phase. Further, since Mn has the effect of suppressing the precipitation of nitride, it is preferable to actively add Mn in the steel of the present embodiment. Add 0.1% or more of Mn for the toughness of the base material and the welded part. However, when Mn is added in an amount exceeding 4.0%, corrosion resistance and toughness are deteriorated. As a result, the upper limit of the Mn amount is limited to 4.0%. The Mn content is preferably 1.0 to 3.5%, more preferably 2.0 to 3.0%.

P is an element that is inevitably incorporated from the raw material and deteriorates hot workability and toughness, so that the amount of P is limited to 0.05% or less. The amount of P is preferably 0.03% or less.

S is an element that is inevitably incorporated from a raw material and deteriorates hot workability, toughness and corrosion resistance. Therefore, the amount of S is limited to 0.0010% or less. Further, if the amount of S is reduced to less than 0.0001%, the cost for desulfurization refining becomes high. Therefore, the amount of S is set to 0.0001 to 0.0010%. The amount of S is preferably 0.0002 to 0.0006%.

Cr is contained in an amount of 23.0% or more in order to secure basic corrosion resistance. On the other hand, when Cr is contained in an amount exceeding 28.0%, the ferrite phase fraction increases to deteriorate toughness and corrosion resistance of the welded portion. As a result, the content of Cr was 23.0% or more and 28.0% or less. The preferable Cr content is 24.0 to 27.5%.

Ni stabilizes the austenite structure and improves corrosion resistance and toughness against various acids. Also, deterioration of hot workability due to addition of Sn and Cu is suppressed. As a result, 2.0% or more of Ni is contained. By increasing the Ni content, it becomes possible to lower the precipitation temperature of the nitride. 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 effective element for increasing the toughness and corrosion resistance of steel, and is an element for suppressing deterioration of hot workability due to the addition of Sn and Cu, and is preferably contained together with Ni. In the case of adding Co, it is preferable that Co is contained at 0.1% or more. If Co is contained in an amount exceeding 1.0%, since Co is an expensive element, a cost-effective effect is not obtained. As a result, the upper limit of the amount of Co was set at 1.0%. When added, the preferable Co content is 0.1 to 0.5%.

It is known from Non-Patent Document 1 that Ni has an action of increasing the 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 a similar element of Ni. Therefore, by increasing the sum of the contents of Ni and Co, it is considered that the deterioration of hot workability due to Cu and Sn is suppressed. The inventors of the present invention have summarized the hot workability of the steel in the present embodiment as the sum of the contents of Ni and Co and found that the edge cracking property of the steel is improved when the total amount of Ni and Co is less than 2.5% Respectively. For this reason, the range of Ni + Co is set to 2.5% or more.

Cu is an element for increasing the corrosion resistance of stainless steel to an acid, and has an action of improving toughness. In the present embodiment, Cu is added in an amount of not less than 0.01% and not less than 0.2% in order to improve corrosion resistance. When Cu is contained in an amount exceeding 3.0%, εCu precipitates in excess of solubility during hot rolling, resulting in embrittlement. For this reason, the upper limit of the amount of Cu was set to 3.0%. When Cu is contained, the preferable content is 0.5 to 2.0%.

Sn is contained in order to improve the corrosion resistance of the steel of the present embodiment. Therefore, a Sn content of at least 0.01% is required. And more preferably 0.02% or more Sn. On the other hand, Sn is an element inhibiting the hot-phase composition of steel. In the alloy-reduced-size two-phase stainless steel of the present embodiment, the hot strength of the interface between ferrite phase and austenite phase . The degree of the deterioration depends on the content of S, Ca, and O. Even if other limitations are applied in this embodiment, if the content of Sn exceeds 0.2%, it is impossible to prevent deterioration in hot- Was set at 0.2%.

N is an effective element for enhancing strength and corrosion resistance by being dissolved in an austenite phase. As a result, N is contained at 0.20% or more. Since increasing the amount of N makes it possible to reduce Ni, N is an element to be actively added. On the other hand, the upper limit of the content of N should be limited within the solubility limit of N. The solubility limit of N increases with the content of Cr and Mn. In the steel of the present embodiment, when N is contained in excess of 0.30%, Cr nitride precipitates to deteriorate toughness and corrosion resistance, as well as inhibit hot-rolled steel composition. For this reason, the upper limit of the N content was set to 0.30%. The preferred N content is 0.20 to 0.28%.

Al is a deoxidizing element of steel, and if necessary, contains Al in an amount of 0.05% or more and Al in order to reduce oxygen in the steel. In the Sn-containing steel, the reduction of the amount of oxygen is essential for ensuring hot-working composition, and accordingly, it is necessary to contain Al in an amount of 0.003% or more as necessary. On the other hand, Al has a relatively large affinity to N, and when added in excess, AlN is generated to inhibit the toughness of stainless steel. The degree depends on the N content. If Al exceeds 0.05%, the decrease in toughness becomes remarkable. For this reason, the upper limit of the Al content was set at 0.05%. The amount of Al is preferably 0.04% or less.

Ca is an important element for hot-forming the steel, and it is necessary to contain Ca in order to fix O and S in the steel as inclusions and to improve hot-formed composition. In the steel of the present embodiment, Ca is contained at 0.0010% or more for the purpose. Also, the addition of excessive amount decreases the pitting resistance. Therefore, the upper limit of the content of Ca was set to 0.0040%.

The ratio Ca / O of the contents of O and Ca is an important ingredient index for improving the hot-rolled composition and corrosion resistance of the steel of the present embodiment. The lower limit of Ca / O is limited in order to improve the hot manufacturability of a Sn-containing steel. The high ductility of the Sn-containing steel decreases particularly at a temperature of 900 DEG C or lower. If the value of Ca / O is less than 0.3, the high temperature ductility at 1000 占 폚 is also lowered, and the hot-knot composition is greatly damaged. Therefore, in the steel of the present embodiment, Ca / O is limited to 0.3 or more. On the other hand, when Ca is excessively added and Ca / O exceeds 1.0, the pitting resistance is impaired. Also, when Ca is excessive, high temperature ductility at 1000 to 1100 占 폚 is also impaired. As a result, the upper limit of Ca / O was set at 1.0. Ca / O is preferably 0.4 to 0.8.

O is an inevitable impurity and its upper limit is not particularly specified, but it is an important element constituting an oxide which is representative of a nonmetal inclusion. The control of the composition of the oxide is very important in improving the hot productivity. If a coarse cluster shaped oxide is produced, it causes surface scratches. Therefore, it is necessary to limit the content of O to a low level. In the present embodiment, as described above, the content of O 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.

Mo: 2.0% or less, and W: 1.0% or less may be further contained. These are elements which additionally increase the corrosion resistance. The reasons 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 needed. In order to improve the corrosion resistance, it is preferable to add Mo of 0.2% or more. On the other hand, Mo is an expensive element. In the steel of the present 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 which additionally improves the corrosion resistance of stainless steel and can be added if necessary. In the steel of the present embodiment, for the purpose of increasing the corrosion resistance, the upper limit of the W content is set to 1.0%. The preferred W content is 0.1 to 0.8%.

0.05 to 0.5% of V, 0.01 to 0.15% of Nb, and 0.003 to 0.05% of Ti. These are elements having a tendency to produce nitride more than Cr. V, Nb, and Ti can be added as needed, and when added in a trace amount, the corrosion resistance tends to be improved.

The nitride and carbide formed by V are generated in the hot working and the cooling process of the steel and have an action of enhancing the corrosion resistance. For this reason, there is no sufficient confirmation, but there is a possibility of suppressing the rate of chromium nitride formation at 700 캜 or lower. In order to improve the corrosion resistance, it is preferable to contain V of at least 0.05%. When V is contained in an amount exceeding 0.5%, coarse V-based carbonitrides are produced and toughness is deteriorated. As a result, the upper limit of the amount of V is limited to 0.5%. When added, the preferred V content is in the range of 0.1 to 0.3%.

Nitride and carbide formed by Nb are generated during the hot working and cooling process of the steel, and have an action of enhancing the corrosion resistance. For this reason, there is no sufficient confirmation, but there is a possibility of suppressing the rate of chromium nitride formation at 700 캜 or lower. In order to improve the corrosion resistance, it is preferable to add 0.01% or more of Nb. On the other hand, excessive addition causes precipitation as non-solid precipitates upon heating before hot rolling, and toughness is inhibited. For this reason, the upper limit of the content of Nb was set at 0.15%. The preferable range of the Nb content when added is 0.03% to 0.10%.

Ti is an element that forms oxides, nitrides, and sulfides in a very small amount to refine the coagulation of steel and the crystal grains of a high-temperature heating structure. Like V and Nb, Ti has a property of substituting a part of chromium in chromium nitride. By containing Ti in an amount of 0.003% or more, precipitates of Ti are formed. On the other hand, when Ti is contained in the two-phase stainless steel exceeding 0.05%, coarse TiN is generated and the toughness of the steel is inhibited. As a result, the upper limit of the content of Ti was set to 0.05%. A suitable content of Ti is 0.005 to 0.020%.

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 defined as follows.

B, Mg, and REM are all elements that improve the hot workability of steel, and one or more of them is preferably added for the purpose. In both B, Mg and REM, excessive addition decreases the 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 Mg amount is 0.0030%. The upper limit of the REM amount is 0.10%. The preferable contents are 0.0005 to 0.0030% of B, 0.0001 to 0.0015% of Mg, and 0.005 to 0.05% of REM, respectively. Here, REM is the sum of the contents of lanthanoid-based rare-earth elements such as La and Ce.

By having the characteristics of the two-phase stainless steel of the present embodiment described above, the hot-dip composition of the general-purpose two-phase stainless steel containing Sn can be remarkably improved.

At the stage of casting, the shrinkage at break at 1000 캜 is 70% or more. Further, by carrying out machining including hot working on this cast piece, it is possible to obtain a two-phase stainless steel material having good yield and few surface scratches.

Example

(Example 1)

An embodiment of alloy-saving two-phase stainless steel will be described below. Tables 1 to 4 show the chemical compositions of the test steels. The balance parts other than those shown in Table 1 are Fe and inevitable impurity elements. In addition, with respect to the components shown in Tables 1 to 4, the portion where the content is not described indicates that it is an impurity level. REM means a lanthanide-based rare earth element, and the content of REM indicates the sum of the elements. The underlined value in the table indicates that it is out of the range specified in the first embodiment.

For each steel, a casting piece having a thickness of 100 mm was first evaluated to evaluate the fracture cross-sectional shrinkage percentage. The evaluation was carried out as follows. First, a parallel portion of a round bar of 8 mm? Was heated to 1200 占 폚 using a high frequency wave. Subsequently, the temperature was lowered to the temperature (1000 DEG C) at which the fracture test was performed. At that temperature, tensile fracture was carried out at a speed of 20 mm / sec to determine the shrinkage ratio of the cross section. A steel having a fracture sectional shrinkage of 70% or more was evaluated as A (good), a steel having a section shrinkage of 60 to less than 70% was evaluated as B (fair), a steel having a section shrinkage of less than 60% was evaluated as C , And the results are shown in Tables 5 and 6.

The cast piece was hot-forged to form a 60 mm-thick piece of steel, which was used as a hot-rolled material. Hot rolling was performed as follows. Heated to a predetermined temperature of 1150 to 1250 占 폚 and subsequently hot-rolled under the following conditions by a two-stage mill in a laboratory. First, the pressing down was repeated to adjust the plate thickness to 25 mm. Subsequently, the steel sheet was subjected to treatment rolling from 1000 占 폚, and final finish rolling at 900 占 폚 to obtain a hot-rolled steel sheet with a final plate thickness of 12 mm and a plate width of 120 mm. The maximum value of the edge cracks generated in the right and left edge portions of the obtained hot-rolled steel sheet was measured to obtain the sum of the maximum values of the edge cracks on the right and left sides. A steel having a sum of edge cracks of less than 5 mm is evaluated as A (good), a steel having a sum of edge cracks of 5 to 10 mm is evaluated as B (fair) C (bad), and the results are shown in Tables 5 and 6.

The steel sheet was subjected to a solution heat treatment as follows. A steel sheet was inserted into the heat treatment furnace set at 1000 占 폚 to take a cracking time of 5 minutes. Subsequently, the steel sheet was 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. An immersion test of 6 h in 5% sulfuric acid was carried out on a specimen of 3 mm thickness × 25 mm width × 25 mm length. The weight before and after immersion was measured to determine the rate of decrease in weight. A steel having a corrosion rate in sulfuric acid of less than 0.3 g / m 2 · hr was evaluated as A (good), a steel having a corrosion rate in sulfuric acid of 0.3 to 1 g / m 2 · hr was evaluated as B (fair) The steel having a weight of 1 g / m < 2 > hr or more was evaluated as C (bad), and the evaluation results are shown in Tables 5 and 6.

The impact characteristics were measured using Charpy test specimens taken long in the width direction. A 2 mm V notch was machined in the rolling direction in a full size to prepare a test piece. The test was carried out at -20 DEG C using two test pieces, and the impact characteristics were evaluated by the average value of the obtained impact values. A steel having an impact value of 100 J / cm 2 or more is evaluated as A (good), a steel having an impact value of 50 to 100 J / cm 2 is evaluated as B (fair) And the evaluation results are shown in Tables 5 and 6.

In the examples shown in Tables 5 and 6, Steel Nos. 1-1 to 1-33 satisfying the conditions of the first embodiment have good hot-dip composition, corrosion resistance and impact properties. On the other hand, the steel Nos. 1-A to 1-U, which did not satisfy the conditions of the first embodiment, were inferior to those of the hot-knotting, corrosion resistance and impact properties.

As can be seen from the above examples, it is clear that the corrosion resistance is improved by the Sn addition, and the low temperature alloy composition is good and the alloy-saving duplex stainless steel can be obtained by the first embodiment.

(Example 2)

Examples of the general-purpose two-phase stainless steel will be described below. Tables 7 to 10 show the chemical compositions of the disclosed steel. The remaining parts of the components shown in Tables 7 to 10 are Fe and inevitable impurity elements. In addition, with respect to the components shown in Tables 7 to 10, the portion where the content is not described indicates that it is at the impurity level. REM means a lanthanide-based rare earth element, and the content of REM indicates the sum of the elements. The underlined value in the table indicates that it is out of the range specified in the second embodiment.

Evaluation of the fracture cross-sectional shrinkage percentage of the casting pieces, production of hot-rolled material, hot-rolling to hot-rolled material, and evaluation of edge cracks were carried out under the same conditions as in Example 1. The obtained evaluation results are shown in Tables 11 and 12.

The steel sheet was subjected to a solution heat treatment as follows. A steel sheet was inserted into the heat treatment furnace set at 1050 占 폚 to take a cracking time of 5 minutes. Subsequently, the steel sheet was 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. An immersion test was carried out on a test piece of 3 mm thickness × 25 mm width × 25 mm length for 6 h in sulfuric acid containing 2000 ppm of Cl ion at a concentration of 15% and a temperature of 40 ° C. The weight before and after immersion was measured to determine the rate of decrease in weight. A steel having a corrosion rate in sulfuric acid of less than 0.1 g / m 2 · hr was evaluated as A (good), a steel having a corrosion rate in sulfuric acid of 0.1 to 0.3 g / m 2 · hr was evaluated as B (fair) (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.

In the examples shown in Table 11 and Table 12, the general-purpose two-phase stainless steels Nos. 2-1 to 2-23 satisfying the conditions of the second embodiment had good hot-working composition, corrosion resistance and impact properties. On the other hand, the steel Nos. 2-A to 2-K and 2-M to 2-T, which did not satisfy the conditions of the second embodiment, were inferior in either one of the hot-build composition, corrosion resistance and impact properties. In addition, Comparative Example 2-L satisfies the characteristics, but because it contains a large amount of Co, it is inferior in cost. In addition, Comparative Example 2-U is S31803 steel, and both the hot-dip galvanizing, corrosion resistance, and composition are all good. However, the content of Ni and Mo is high, and the second embodiment is inferior in the aimed cost.

As can be seen from the above examples, it has been clarified that the corrosion resistance is improved by the addition of Sn and Cu and the low-cost general-purpose two-phase stainless steel is obtained with good hot-working composition by the second embodiment.

According to the first and second embodiments, it is possible to provide a cheap alloy-saving two-phase stainless steel material and a general-purpose two-phase stainless steel material improved in corrosion resistance. This two-phase stainless steel material can be used as seawater desalination equipment, transport tanks, various containers, etc., and the industry contributes greatly.

Claims (10)

% By mass,
C: 0.03% or less,
Si: 0.05 to 1.0%
Mn: 0.1 to 7.0%
P: not more than 0.05%
S: 0.0001 to 0.0010%,
0.5 to 5.0% of Ni,
Cr: 18.0 to 25.0%
N: 0.10 to 0.30%,
Al: 0.05% or less,
Ca: 0.0010 to 0.0040% and
0.01 to 0.2% of Sn,
The balance being Fe and inevitable impurities,
The ratio Ca / O of Ca / O is 0.3 to 1.0,
Wherein the formula index PI represented by the formula (1) is less than 30. 2. The two-phase stainless steel according to claim 1,
[Equation 1]

(In the formula (1), the symbol represents the content of the element) The method according to claim 1,
Mo: 1.5% or less,
Cu: 2.0% or less,
W: 1.0% or less and
And Co: 2.0% or less. The two-phase stainless steel according to claim 1, 3. The method according to claim 1 or 2,
V: 0.05 to 0.5%
0.01 to 0.20% Nb and
, And Ti: 0.003 to 0.05%. ≪ RTI ID = 0.0 > 2, < / RTI > 4. The method according to any one of claims 1 to 3,
B: 0.0050% or less,
Mg: 0.0030% or less and
And 0.10% or less of REM. The two-phase stainless steel according to claim 1, % By mass,
C: 0.03% or less,
Si: 0.05 to 1.0%
Mn: 0.1 to 4.0%
P: not more than 0.05%
S: 0.0001 to 0.0010%,
23.0 to 28.0% Cr,
Ni: 2.0 to 6.0%
Co: 0 to 1.0%
Cu: 0.2 to 3.0%
0.01 to 0.2% of Sn,
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 of not less than 2.5%, a ratio Ca / O of Ca / O of 0.3 to 1.0,
Wherein the PI represented by the formula (1) is not less than 30 and less than 40. 2. A two-
[Equation 1]

(In the formula (1), the symbol represents the content of the element) 6. The method of claim 5,
Mo: 2.0% or less and
W: 1.0% or less, and either or both of W and 1.0%. The method according to claim 5 or 6,
V: 0.05 to 0.5%
0.01 to 0.15% Nb and
, And Ti: 0.003 to 0.05%. ≪ RTI ID = 0.0 > 2, < / RTI > 8. The method according to any one of claims 5 to 7,
B: 0.0050% or less,
Mg: 0.0030% or less and
And 0.10% or less of REM. The two-phase stainless steel according to claim 1, A two-phase stainless steel casting having the composition according to any one of claims 1 to 8 and having a shrinkage at break at 1000 DEG C of not less than 70%. A two-phase stainless steel material produced by hot working a two-phase stainless steel casting piece according to claim 9.

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