ES2768088T3 - Duplex stainless steel, duplex stainless steel sheet and duplex stainless steel material - Google Patents

Abstract

A duplex stainless steel consisting of, in 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%; Sn: 0.01% to 0.2%; optionally one or more selected from Mo: 1.5% or less, Cu: 0.3% to 2.0%, W: 0.05% to 1.0%, Co: 2.0% or less, V: 0.05% to 0.5%, Nb: 0.01% to 0.20%, Ti: 0.003% to 0.05%, B: 0.0005% to 0.0050%, Mg: 0.0001% at 0.0030%, and REM: 0.005 to 0.10%; and the remainder being Fe and unavoidable impurities, wherein a Ca / O ratio of the amounts of Ca and O is in a range of 0.3 to 1.0, and a pitting index PI shown by formula (1) is in a range less than 30, PI = Cr + 3,3Mo + 16N (1), where the chemical symbols in formula (1) indicate the amounts of the elements.

Description

DESCRIPTION

Duplex stainless steel, duplex stainless steel plate and duplex stainless steel material

Technical field

The present invention relates to an inexpensive Sn containing duplex stainless steel. Furthermore, the present invention relates to a cheap duplex stainless steel containing a combination of Cu and Sn and which is excellent in corrosion resistance. In detail, the present invention relates to a duplex stainless steel, a duplex stainless steel sheet (a cast steel of a duplex stainless steel), and a duplex stainless steel material that can be used in a seawater desalination unit , tank for a transport ship, various types of containers, or the like.

Background of the Art

A general purpose duplex stainless steel contains a high amount of Cr, Mo, Ni and N, and has favorable corrosion resistance. However, as a result of containing Mo and Ni, which are expensive, the cost of alloying is high and the manufacturing capacity is not favorable. As a result, the price of steel material is not cheap, and duplex stainless steel is not widely used in place of grade 316 stainless steel or grade 317 stainless steel. Here, the referenced general purpose duplex stainless steel in the present invention indicates duplex stainless steel having the PI pitting index (represented by the following formula which is the sum of the amounts of the alloying elements: PI = Cr + 3.3Mo + 16N) of about 30 or more at less than 40 (mass%). From the circumstances described above, in such steels, it is considered that there is a need for steels where the cost of alloying is lower than that of the related art and the manufacturing costs are inexpensive, and have a manufacturing capacity in hot favorable while exhibiting the same level of corrosion resistance as the related general purpose duplex stainless steel.

Furthermore, recently, an alloy saver type duplex stainless steel has been developed in which the amounts of Cr, Ni, Mo and the like are reduced. Here, the alloy saver type duplex stainless steel indicates a stainless steel that exhibits a pitting resistance equivalent to that of SUS 304 and 316L, and where the pitting index (PI = Cr + 3.3Mo + 16N), which it is indexed by the amounts of the alloying elements, it is approximately in a range less than 30. In these steels where the quantities of alloying elements that are effective for pitting resistance and acid resistance are low, it is difficult Obtain the same level of corrosion resistance as that of general-purpose duplex stainless steel. However, it is considered that it is possible to develop improved steels using inexpensive alternative elements.

Various types of Sn containing duplex stainless steels have been proposed in the related art. For example, duplex stainless steels are described that contain 25% or more of Cr and contain 0.01% to 0.1% of Sn as the selected element (see Patent Documents 1 and 2 described below). In addition, alloy sparing type duplex stainless steels containing 1% or less or 0.1% Sn are described (see Patent Documents 3 and 4 described below). In the Patent Documents, an object is to improve corrosion resistance by means of the amount of Sn; however, the relationship between the hot manufacturing ability of the steel material and the amount of Sn was not investigated.

Furthermore, in the Patent Documents described above, the subject is a steel where the amount of N is in a range of 0.2% or less. N is an element that reduces the hot malleability of stainless steel. Ensuring a desired level of hot malleability of a duplex stainless steel containing 0.2% or more of N is more difficult than ensuring a desired level of hot malleability of a duplex stainless steel containing less than 0.2% of N The technical literature describing the hot malleability of a duplex stainless steel containing 0.20% or more of N and also containing a combination of Sn and Cu will not be found.

The present inventors focused on the possibility of improving acid resistance and pitting resistance by using Sn in an alloy saver duplex stainless steel. Next, the present inventors investigated the relationship between the amount of Sn and the corrosion resistance and the heat build ability. As a result, it was found that it was possible to improve the corrosion resistance with 0.01% to 0.2% Sn content. However, it was learned that hot fabrication capacity decreased in duplex stainless steels containing a large amount of Sn. For this reason, the frequency of decreases in the performance of the steel material will increase, and a significant increase in cost is predicted.

Furthermore, the present inventors focused on the possibility of improving acid resistance and pitting resistance by using Sn and Cu in general purpose duplex stainless steel. So, with respect to duplex stainless steel where the amounts of Mo and Ni are reduced and containing 0.20% or more of N, the present inventors investigated the relationship between the amounts of Sn and Cu, the corrosion resistance and the hot manufacturing capacity. As a result, it was found that it was possible to improve the corrosion resistance by being 0.01% to 0.2% Sn and 0.2% to 3.0% Cu. However, it was learned that hot build capacity decreased in duplex stainless steels that contained a large amount of Sn and Cu. For this reason, the frequency of decreases in the performance of the steel material will increase, and a significant increase in cost is predicted.

The present inventors investigated knowledge of the related art relating to manufacturing techniques for a related prior art Sn-containing hot-rolled duplex stainless steel material from Patent Documents 1 to 4. As a result, it was found There was little knowledge regarding the relationship between the temperature range where hot embrittlement occurs due to the Sn that is included in duplex stainless steel and the amount of Sn and the relationship to the amounts of other elements.

Prior Art Document

Patent Documents

Patent Document 1: Unexamined Japanese Patent Application, First Publication No. H3-158437

Patent Document 2: Unexamined Japanese Patent Application, First Publication No. H4-072013

Patent Document 3: Unexamined Japanese Patent Application, First Publication No. 2010-222593

Patent Document 4: PCT International Publication No. WO2009-119895, also published as EP2258885 A1

Patent Document 5: Unexamined Japanese Patent Application, First Publication No. 2002-69592

Patent Document 6: Unexamined Japanese Patent Application, First Publication No. H7-118805

Non-Patent Document

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

Description of the Invention

Problems to be solved by the invention

The present invention finds a measure to solve the problems described above by clarifying the relationship between the amount of Sn and the capacity of hot manufacturing in a duplex stainless steel of alloy-saving type. Furthermore, the present invention finds a measure to solve the problems described above by clarifying the relationship between the amounts of Sn and C u and the capacity of hot manufacturing in a duplex stainless steel for general use. Because of this, the object of the present invention is to provide a duplex stainless steel containing Sn, a cast steel of a duplex stainless steel, and a duplex stainless steel material that are inexpensive and have a favorable hot build capacity. Such duplex stainless steel is expected to have an excellent balance between corrosion resistance and cost. For this reason, the likelihood that duplex stainless steel is widely used in various fields is considered to be high.

In particular, an object of a second aspect (a second embodiment) of the invention is to develop an inexpensive general purpose duplex stainless steel where the amounts of Ni and Mo, which are expensive elements, are reduced by increasing the amounts of N and Mn and adding a combination of Cu and Sn.

Means of solving problems

In order to solve the problems described above, for the alloy saver type duplex stainless steel which is the subject of the present invention, the present inventors prepared cast materials where the amount of Sn and the amounts of Ca, B, earth elements Rare (REM) or the like were changed, and performed the following experiments. Here, amounts of Ca, B, rare earth elements (REM) or the like are said to enhance hot build ability.

Tensile test pieces of cast steels were collected and cast from the cast materials. A high temperature tensile test was performed at a temperature of 1,200 to 700 ° C with respect to the tensile test pieces, and high temperature ductility was evaluated by measuring the area reduction (reduction ratio in cross section of the fracture surface). Furthermore, a hot-rolled steel plate with a plate thickness of 12 mm was obtained by hot forging and hot rolling, and the edge cracking resistance was evaluated. Edge cracking resistance was evaluated by changing the heating temperature and the rolling temperature of the hot rolling with respect to a part of the steel, and a correlation of the heating temperature and the rolling temperature of the rolling was determined. hot with high temperature ductility.

As described in Patent Documents 5 and 6 described above, generally, in duplex stainless steels, it is known that significant edge cracking is generated in hot rolling of cast steel in most cases where reduction area of cast steel, which is evaluated by a high-temperature tensile test, falls below 60%. For this reason, engineers in this field often subject the steels to refining, casting, and hot working in order to adjust the area reduction of the cast steel at high temperatures to be in the range of 60% or more. Here, when the present inventors evaluated the high temperature ductility of alloy saver duplex stainless steel cast steel (composition base: 21% Cr -2% Ni - 3% Mn - 0.18% N) containing about 0.1% Sn, it was clear that all area reductions fell below 60% in various fusion experiments . The evaluation of high temperature ductility was performed as follows. First, a parallel section of a round bar was heated from 8 to 1,200 ° C using a high frequency. The temperature was then lowered to a temperature to perform a break test, and a tensile break was performed at a rate of 20 mm / second at this temperature. Then, the shrink ratio of the cross section was determined. An example of the data is shown in FIG. 1. From these results, it was considered that there was almost no hope of obtaining an economical alloy saver duplex stainless steel with Sn added in practice.

The present inventors observed an edge cracking length that was generated when a cast saver type duplex stainless steel cast alloy steel, obtained by vacuum melting and casting, was subjected to hot rolling. As a result, it was found that there is seldom a cast steel of a duplex stainless steel containing Sn in which a number of edge cracks is small. The hot rolling experiments were carried out as follows. First, a cast steel with a thickness of 90 to 44 mm was heated to 1,200 ° C. Next, the thickness of the cast steel was reduced to a thickness of 12 to 6 mm by a plurality of rolling passes. The finish rolling temperature was controlled to be approximately 900 ° C. Edge cracking was generated on the left and right sides and maximum lengths on both sides were totaled to obtain the edge cracking length. Even when the cracking length at the edges of the steel material was viewed as being related to the minimum value (the minimum value is obtained at approximately 900 ° C in FIG. 1) of the ductility area reduction to high temperature of the cast steel, it was not possible to obtain a clear correlation. However, when the edge cracking length was viewed as being related to area reduction at 1,000 ° C as shown in FIG. 2, it was clear that a good correlation is exhibited, regardless of whether Sn is contained or not. Here in FIG. 2, the points represented by or (open circles) correspond to the results of Sn-A and Sn-B in FIG. 1, and the points represented by ♦ (black diamonds) are the other results of the experiment (the results of the experiment examined regardless of whether Sn is contained or not).

The present inventors performed melting, casting and rolling experiments while also changing the amounts of various elements, in order to find the conditions to reliably obtain a cast steel with little edge cracking as described above. Thereafter, the high temperature ductility evaluation of the cast steel and the evaluation of cracking at the edges of the steel material after hot rolling were actively performed. The first aspect of the present invention where economical Sn containing alloy duplex stainless steel saver type is specified was completed based on the findings obtained by the above experiments.

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

(1) A duplex stainless steel consisting of, in 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: 001% to 0.2%, optionally one or more selected from Mo: 1.5% or less, Cu: 0.3% to 2.0%, W: 0.05% to 1.0%, 'Co: 2.0% or less, V: 0.05% to 0.5%, Nb: 0.01% to 0.20%, Ti: 0.003% to 0.05%, B: 0.0005% to 0.0050%, Mg: 0.0001% to 0.0030%, and REM: 0.005% to 0.10%, and the rest being Fe and unavoidable impurities, where a Ca / O ratio of the amounts of Ca and O is in a range of 0.3 to 1.0, and a PI sting index shown by formula (1) is in a range of less than 30.

PI = Cr + 3.3Mo + 16N (1)

(The chemical symbols in formula (1) indicate the quantities of the elements).

(2) Duplex stainless steel according to (1), which further includes one or more selected Mo: 1.5% or less, Cu: 0.3% to 2.0%, W: 0.05% to 1, 0%, and Co: 2.0% or less.

(3) Duplex stainless steel according to (1) or (2), which also includes one or more selected from V: 0.05% to 0.5%, Nb: 0.01% to 0.20%, and Ti : 0.003% to 0.05%.

(4) Duplex stainless steel according to any one of (1) to (3), which also includes one or more selected from B: 0.0005% to 0.0050%, Mg: 0.0001% to 0.0030% , and REM: 0.005% to 0.10%.

Furthermore, in order to solve the problems described above, regarding the general purpose duplex stainless steel which is the subject of the present invention, the present inventors prepared molten materials where the amount of Sn, the amounts of Ca, B, elements of Rare earths (REM) and the like and the amount of Ni were changed, and where Co was added, and the following experiments were performed. Here, hot build capacity is said to be enhanced by containing Ca, B, rare earth elements (REM), and the like.

Tensile test pieces were collected from a cast steel that was cast from the molten materials. The tensile test pieces were subjected to a high temperature tensile test at a temperature of 1,200 to 700 ° C, and high temperature ductility was evaluated by measuring the area reduction (reduction ratio in section transverse fracture surface). Furthermore, a hot-rolled steel plate with a plate thickness of 12 mm was obtained by hot forging and hot rolling, and the edge cracking resistance was evaluated. The edge cracking resistance was evaluated by changing the heating temperature and the rolling temperature of the hot rolling with respect to a part of the steel, and a correlation of the heating temperature and the rolling temperature of the rolling was determined. hot with high temperature ductility.

As described in Patent Documents 5 and 6 described above, generally, on duplex stainless steels, it is known that significant edge cracking is generated in hot rolling of cast steel in most cases where the reduction of Cast steel area, which is evaluated by a high-temperature tensile test, falls below 60%. For this reason, engineers in this field often subject the steels to refining, casting, and hot working in order to adjust the area reduction of the cast steel at high temperatures to be in a range of 60% or more. Here, when the present inventors evaluated the high temperature ductility of general purpose cast steel of a duplex stainless steel (base composition: 25% Cr - 4% Ni -1.2% Mo -1.5% Cu - 0.25 % N) containing about 0.1% Sn, it was clear that the minimum values of all area reductions fell below 60% in various fusion experiments. The evaluation of high temperature ductility was performed as follows. First, a parallel section of a $ 8mm round bar was heated to 1,200 ° C using a high frequency. The temperature was then lowered to a temperature to perform a break test, and a tensile break was performed at a rate of 20 mm / second at this temperature. Then, the shrink ratio of the cross section was determined. An example of the data is shown in FIG. 3. From these results, it was considered that there was almost no hope of obtaining an inexpensive general purpose duplex stainless steel with Sn added in practice.

The present inventors observed an edge cracking length that was generated when a cast steel of a general purpose duplex stainless steel, obtained by vacuum melting and casting, was subjected to hot rolling. As a result, it was found that there is rarely a duplex stainless steel material containing Sn in which a number of edge cracks is small. The hot rolling experiments were carried out as follows. First, a cast steel with a thickness of 90 to 44 mm was heated to 1,200 ° C. Next, the thickness of the cast steel was reduced to a thickness of 12 to 6 mm by a plurality of rolling passes. The finish rolling temperature was controlled to be approximately 900 ° C. Edge cracking was generated on the left and right sides and maximum lengths on both sides were totaled to obtain the edge cracking length. Even when the cracking length at the edges of the steel material was regarded as being related to the minimum value (the minimum value is obtained at approximately 900 ° C in FlG. 3) of the ductility area reduction to high temperature of the cast steel, it was not possible to obtain a clear correlation. However, when the edge cracking length was viewed as being related to the area reduction at 1,000 ° C as shown in FIG. 4, it was clear that a good correlation is exhibited, regardless of whether Sn is contained or not. Here in FIG. 4, the points represented by or (open circles) correspond to the results of Sn-A and Sn-B in FIG. 3, and the points represented by ♦ (black diamonds) are the other results of the experiment (the results of the experiment examined regardless of whether Sn is contained or not).

The present inventors performed melting, casting and rolling experiments while changing the amounts of various elements in order to find the conditions to reliably obtain a steel material with little edge cracking as described above. Thereafter, the high temperature ductility evaluation of the cast steel and the evaluation of cracking at the edges of the steel material after hot rolling were actively performed. The second aspect of the present invention where inexpensive Sn containing duplex stainless steel is specified was completed based on the findings that were obtained by the above experiments.

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

(5) A duplex stainless steel consisting of, in mass%: C: 0.03% or less; Si: 0.05% to 1.0%; Mn: 0.1% to 4.0%; P: 0.05% or less; S: 0.0001% to 0.0010%; Cr: 23.0% to 28.0%; Ni: 2.0% to 6.0%; Co: 0% to 1.0%; Cu: 0.2% to 2.0%; Sn: 0.01% to 0.2%; N: 0.20% to 0.30%; Al: 0.05% or less; and Ca: 0.0010% to 0.0040%, optionally one or more selected from Mo: 0.2% to 2.0%, W: 0.1% to 1.0%, V: 0.05% to 0.5%, Nb: 0.01% to 0.15%, Ti: 0.003% to 0.05%, B: 0.0005% to 0.0050%, Mg: 0.0001% to 0.0030% , and REM: 0.005% to 0.10%; and with the balance Fe and unavoidable impurities, where Ni + Co is in a range of 2.5% or more and a Ca / O ratio of the amounts of Ca and O is in a range of 0.3 to 1.0 , and the PI shown by formula (1) is in a range of 30 or more and less than 40.

PI = Cr + 3.3Mo + 16N (1)

(The chemical symbols in formula (1) indicate the quantities of the elements).

(6) Duplex stainless steel according to (5), which further includes any one or both of Mo: 0.2% to 2.0%, and W: 0.1% to 1.0%.

(7) Duplex stainless steel according to (5) or (6), which also includes one or more selected from V: 0.05% to 0.5%, Nb: 0.01% to 0.15%, and Ti: 0.003% to 0.05%.

(8) Duplex stainless steel according to any one of (5) to (7), which also includes one or more selected from B: 0.0005% to 0.0050%, Mg: 0.0001% to 0.0030% , and REM: 0.005% to 0.10%.

The requirements for a cast steel aspect of the duplex stainless steel and the duplex stainless steel material of the present invention are shown below.

(9) A cast steel of a duplex stainless steel having a composition according to any one of (1) to (8), wherein the cast steel is a steel in a state after casting and before processing, and a reduction of Area fracture at 1,000 ° C is in a range of 70% or more.

(10) A duplex stainless steel material that is obtainable by hot working cast steel from duplex stainless steel according to (9), wherein the stainless steel material is a semi-finished product, a hot-rolled steel plate, a cold rolled steel, steel wire, or steel pipe.

Effects of the Invention

In accordance with one aspect of the present invention, it is possible to provide a duplex stainless steel, a cast steel of a duplex stainless steel, and a duplex stainless steel material having improved corrosion resistance compared to a steel used in the related art as material for a seawater desalination unit, tanks for a transport ship, various types of containers or the like, plus an excellent balance with cost. For this reason, the aspects of the present invention make a significant contribution to industrial development.

Brief description of the drawings

FIG. 1 is a diagram illustrating the high temperature ductility of Sn-free and Sn-free duplex stainless steels associated with the first appearance of duplex stainless steel (an alloy saver duplex stainless steel).

FIG. 2 is a diagram showing the relationship between edge cracking length after lamination and area reduction at 1,000 ° C associated with the first appearance of duplex stainless steel (the alloy saver duplex stainless steel).

FIG. 3 is a diagram illustrating the high temperature ductility of Sn-containing duplex stainless steels and Sn-free cast steels associated with the second aspect of duplex stainless steel (a general-purpose duplex stainless steel).

FIG. 4 is a diagram showing the relationship between edge cracking length after lamination and area reduction at 1,000 ° C associated with the second aspect of duplex stainless steel (general purpose duplex stainless steel).

Embodiments of the invention

(First embodiment)

Next, a description will be given of the reasons for limiting the first aspect (the alloy saver type duplex stainless steel) of the duplex stainless steel of the present invention. Here, the amounts of the respective components are shown in terms of mass%.

Here, in the present embodiment, the cast steel of the stainless steel indicates a steel in a state after casting and before processing, such as hot working, forging or the like, and the stainless steel material indicates a semi-finished product, a plate hot rolled steel, cold rolled steel plate, steel wire or steel pipe, after processing the cast steel by various methods. Furthermore, stainless steel indicates general shapes for a steel such as cast steel, a steel material, and the like. The processing described above includes hot and cold processing.

In order to ensure the corrosion resistance of stainless steel, the amount of C is limited to be in a range of 0.03% or less. When more than 0.03% C is contained, the corrosion resistance and toughness degrade due to the generation of Cr carbides during hot rolling.

0.05% or more of Si is added for deoxidation. However, when more than 1.0% Si is added, the toughness degrades. Therefore, the upper limit for the amount of Si is limited to 1.0%. The preferable range for the amount of Si is in a range of 0.2% to 0.7%.

Mn has the effect of improving toughness by increasing the austenite phase. Furthermore, since Mn has the effect of lowering the precipitation temperature of TN nitrides, it is preferable to actively add Mn to the steel material of the present embodiment. For the toughness of the base material and weld sections, 0.1% or more of Mn. However, when more than 7.0% Mn is added, the corrosion resistance and toughness degrade. Therefore, the upper limit for the amount of Mn is limited to 7.0%. The amount of Mn is preferably in a range of 1.0% to 6.0%, and more preferably in a range of 2.0% to 5.0%.

P is an element that is inevitably mixed from the raw materials, and the amount of P is limited to be in a range of 0.05% or less, since P degrades the hot pliability and toughness. The amount of P is preferably in a range of 0.03% or less.

S is an element that inevitably mixes from the raw materials, and the amount of S is limited to be in a range of 0.0010% or less, since S degrades the hot pliability, toughness and resistance to corrosion. Also, reducing the amount of S to less than 0.0001% increases costs due to desulfurization refining. For this reason, the amount of S is adjusted to be in a range of 0.0001% to 0.0010%. The amount of S is preferably in a range of 0.0002% to 0.0006%.

Since Ni stabilizes the austenitic structure and improves toughness and corrosion resistance with respect to various types of acid, it contains 0.5% or more of Ni. By increasing the amount of Ni, it is possible to decrease the precipitation temperature of nitrides. On the other hand, Ni is an expensive alloy, and from a cost point of view, the amount of Ni is limited to be in a range of 5.0% or less in the steel of the present embodiment where the subject is an alloy saver duplex stainless steel. The amount of Ni is preferably in a range of 1.0% to 4.0%, and more preferably in a range of 1.5% to 3%.

In order to ensure basic corrosion resistance, it contains 18.0% or more of Cr. On the other hand, when it contains more than 25.0% of Cr, the ferrite phase fraction increases, and the toughness and the corrosion resistance of the welding sections are inhibited. For this reason, the amount of Cr is adjusted to be in a range of 18.0% or more and 25.0% or less. The amount of Cr is preferably in a range of 19.0% to 23.0%.

N is an element that is effective in increasing strength and corrosion resistance while being solubilized in solid in the austenite phase. For this reason, 0.10% or more of N is contained. On the other hand, the limit of solid solubility is increased according to the amounts of C r and Mn; however, when more than 0.30% N is contained in the steel of the present embodiment, Cr nitrides are precipitated, such that toughness and corrosion resistance are inhibited and hot workability is inhibited. For this reason, the upper limit of the amount of N is adjusted to 0.30%. The amount of N is preferably in a range of 0.10% to 0.25%.

Al is an element that deoxidizes a steel and reduces oxygen in the steel as needed. For this reason, Al is contained together with 0.05% or more of Si. In a steel containing Sn, the reduction of the amount of oxygen is essential to ensure the capacity of hot manufacturing, and for this reason, it is necessary to contain 0.003% or more of Al according to the need. On the other hand, Al is an element that has a comparatively large affinity for N, and when an excessive amount of Al is added, the toughness of stainless steel is inhibited due to the generation of AlN. The degree also depends on the amount of N; however, when the amount of Al exceeds 0.05%, the toughness is greatly decreased. For this reason, the upper limit of the amount of Al is adjusted to 0.05%. The amount of Al is preferably in a range of 0.04% or less.

Ca is an important element in the hot manufacturing capacity of steel, and it is necessary for it to be contained in Ca in order to fix S and O in the steel as inclusions and to improve the hot manufacturing capacity. In the steel of the present embodiment, 0.0010% or more of Ca is contained for this purpose. Also, adding an excessive amount of it decreases pitting resistance. For this reason, the upper limit of the amount of Ca is adjusted to 0.0040%.

Sn is contained in order to improve the corrosion resistance of the steel of the present embodiment. For this reason, it is necessary that it contains at least 0.01% Sn. Furthermore, it is preferable that 0.02% or more of Sn is contained. On the other hand, Sn is an element that inhibits the hot manufacturing capacity of steel, and decreases the hot resistance of the interface between the ferrite phase and the austenite phase, particularly at a temperature of 900 ° C or less in the alloy sparing element type duplex stainless steel which is the subject of the present embodiment. The degree of the decrease depends on the amounts of S, Ca, and O; however, when more than 0.2% Sn is contained, it is not possible to prevent the decrease in hot manufacturing capacity, even by restricting other limits in the present embodiment. Therefore, the upper limit of the amount of Sn is adjusted to 0.2%.

The Ca / O ratio of the amounts of O and Ca is an important component index for improving the hot fabrication capacity and corrosion resistance of the steel of the present embodiment. The lower Ca / O limit is limited in order to improve the hot fabrication capacity of the Sn containing steel. The high temperature ductility of Sn-containing steel is decreased, particularly at a temperature of 900 ° C or less. When the Ca / O value is in a range less than 0.3, the high temperature ductility at 1,000 ° C is also decreased and the hot manufacturing capacity is greatly impaired. For this reason, Ca / O is limited to be in a range of 0.3 or more in the steel of the present embodiment. On the other hand, when an excessive amount of Ca and Ca / O exceeds 1.0 is added, the resistance to pitting is impaired. Furthermore, when the amount of Ca is excessive, high temperature ductility at a temperature of 1,000 to 1,100 ° C is also impaired. For this reason, the upper limit of Ca / O is adjusted to be in a range of 1.0. Ca / O is preferably in a range of 0.4 to 0.8.

Or it is an inevitable impurity, and its upper limit is not particularly set; however, O is an important element that configures oxides that are representative of non-metallic inclusions. Control of the composition of the oxides is extremely important for the improvement of the capacity of hot manufacturing. In addition, surface defects are caused when thick, clustered oxides are generated. For this reason, it is necessary to limit the amount of O so that it is low. In the present embodiment, as described above, by adjusting the ratio of the amount of Ca and the amount of O to be in a range of 0.3 or more, the amount of O is limited. The upper limit of the amount of O is preferably in a range of 0.005% or less.

In order to incrementally increase corrosion resistance, one or more selected Mo may be contained: 1.5% or less, Cu: 0.3% to 2.0%, W: 0.05% to 1 , 0%, and Co: 2.0% or less, as needed. A description of the reasons for these limits will be given.

Mo is an element that is extremely effective in incrementally increasing the corrosion resistance of stainless steel, and Mo can be contained as needed. In order to improve corrosion resistance, it is preferable that it contain 0.2% or more Mo. On the other hand, Mo is an element that promotes the precipitation of intermetallic compounds, and the upper limit of the amount of Mo is adjusts to 1.5% from the standpoint of suppressing precipitation on the steel of the present embodiment during hot rolling.

Cu is an element that incrementally increases the corrosion resistance of stainless steel with respect to acid, and Cu has an effect of improving toughness; and therefore, it is recommended that it be contained 0.3% or more as needed. When more than 2.0% Cu is contained, the amount of Cu exceeds the solubility in solid; and thus £ -Cu is precipitated during hot rolling to cause embrittlement. For this reason, the upper limit of the amount of Cu is adjusted to 2.0%. In a case where Cu is contained, the amount is preferably in a range of 0.3% to 1.5%.

W is an element that incrementally increases the corrosion resistance of stainless steel in the same way as Mo, and W can be added as needed. In order to increase the corrosion resistance in the steel of the present embodiment, the upper limit of the amount of W is adjusted to 1.0%. The amount of W is preferably in a range from 0.05% to 0.5%.

Co is an element that is effective in increasing the toughness and corrosion resistance of steel and that is selectively added. The amount of Co is preferably in a range of 0.03% or more. When more than 2.0% Co is contained, an effect that is proportional to cost is not exhibited, since Co is an expensive item. For this reason, the upper limit of the amount of Co is adjusted to 2.0%. In a case where Co is added, the amount is preferably in a range from 0.03% to 1.0%.

Furthermore, one or more selected from V may be contained: 0.05% to 0.5%, Nb: 0.01% to 0.20%, and Ti: 0.003% to 0.05%. These are elements that are more likely to generate nitrides than Cr. V, Nb, and Ti can be added as needed, and there is a tendency for corrosion resistance to be improved in cases where these are contained in trace amounts.

The nitrides and carbides that are formed by V are generated in the hot work and the cooling process of the steel material, and these have the effect of increasing resistance to corrosion. The reasons for the same are not sufficiently confirmed; however, it is considered that there is a probability of suppressing the rate of chromium nitride generation at a temperature of 700 ° C or less. It is 0.05% or more V content to improve corrosion resistance. When more than 0.5% V is contained, thick V carbonitrides are generated, and toughness degrades. Therefore, the upper limit of the amount of V is limited to 0.5%. In a case where V is added, the amount is preferably in a range of 0.1% to 0.3%.

The nitrides and carbides that are formed by Nb are generated in the hot work and the cooling process of the steel material, and these have the effect of increasing resistance to corrosion. The reasons for the same are not sufficiently confirmed; however, it is considered that there is a probability of suppressing the rate of chromium nitride generation at a temperature of 700 ° C or less. Contains 0.01% or more of Nb to improve corrosion resistance. On the other hand, in the case where an excessive amount of Nb is added, Nb precipitates as non-solid solubilized precipitates during heating before hot rolling; and thus, toughness is inhibited. For this reason, the upper limit of the amount of Nb is adjusted to 0.20%. In a case where Nb is added, the amount range is preferably in a range of 0.03% to 0.10%.

Ti is an element that forms oxides, nitrides and sulfides in very small amounts, and Ti refines the crystalline grains in the solidified structure and the structure heated to a high temperature of the steel. Furthermore, in the same way as V and Nb, Ti also has the property of replacing a part of the chromium in the chromium nitrides. With an amount of Ti of 0.003% or more, Ti precipitates are formed. On the other hand, when more than 0.05% Ti is contained in the duplex stainless steel, the toughness of the steel is impaired due to the generation of coarse TiN. For this reason, the upper limit of the amount of Ti is adjusted to 0.05%. An adequate amount of Ti is in a range from 0.005% to 0.020%.

In addition, one or more selected B's may be contained: 0.0005% to 0.0050%, Mg: 0.0001% to 0.0030%, and REM: 0.005% to 0.10%. In order to achieve a further improvement in hot malleability, the B, Mg and REM to be contained as needed are limited as follows.

B, Mg and REM are all elements that improve the hot malleability of the steel, and one or more of them is added for this purpose. The addition of an excessive amount of any one of B, Mg, and REM has the opposite effect of decreasing hot malleability and toughness. For this reason, the upper limits of the above amounts are adjusted 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 REM amount is 0.10%. The preferable amounts of the respective elements are B: 0.0005% to 0.0030%, Mg: 0.0001% to 0.0015%, and REM 0.005% to 0.05%. Here, REM is the sum of the amounts of lanthanide rare earth elements such as Ce, La and the like.

By having the characteristics of the duplex stainless steel of the present embodiment described above, it is possible to greatly improve the hot manufacturing capability of the Sn containing alloy duplex stainless steel alloy.

In the cast steel stage, an area fracture reduction at 1,000 ° C is in the range of 70% or more. In addition, by subjecting cast steel to procedures that include hot working, it is possible to obtain a duplex stainless steel material with high performance and few surface defects.

(Second embodiment)

Next, a description will be given of the reasons for the limits of the second aspect (a general purpose duplex stainless steel) of the duplex stainless steel of the present invention. Here, the amounts of the respective components are shown in terms of mass%.

Here, in the present embodiment, the cast steel of the stainless steel indicates a steel in a state after casting, and before processing, such as hot work, forging or the like, and the stainless steel material indicates a semi-finished product, hot-rolled steel plate, cold-rolled steel plate, steel wire, or steel pipe, after the cast steel is processed by various methods. In addition, stainless steel indicates the general shapes for a steel such as cast steel, a steel material, and the like. The processing described above includes hot and cold processing.

In order to ensure the corrosion resistance of stainless steel, the amount of C is limited to be in a range of 0.03% or less. When more than 0.03% C is contained, the corrosion resistance and toughness degrade due to the generation of Cr carbides during hot rolling.

0.05% or more Si is added for deoxidation. However, when more than 1.0% Si is added, the toughness degrades. Therefore, the upper limit for the amount of Si is limited to 1.0%. The preferable range for the amount of Si is in a range of 0.2% to 0.7%.

Mn has the effect of improving toughness by increasing the austenite phase. Furthermore, since Mn has the effect of suppressing nitride precipitation, it is preferable to actively add Mn to the steel material of the present embodiment. For the toughness of the base material and weld sections, 0.1% or more Mn is added. However, when more than 4.0% Mn is added, the corrosion resistance and toughness degrade. Therefore, the upper limit for the amount of Mn is limited to 4.0%. The amount of Mn is preferably in a range of 1.0% to 3.5%, and more preferably in a range of 2.0% to 3.0%.

P is an element that is inevitably mixed from the raw materials, and the amount of P is limited to be in a range of 0.05% or less, since P degrades the hot pliability and toughness. The amount of P is preferably in a range of 0.03% or less.

S is an element that inevitably mixes from the raw materials, and the amount of S is limited to a range of 0.0010% or less, since S degrades hot workability, toughness and corrosion resistance. Also, reducing the amount of S to less than 0.0001% increases costs due to desulfurization refining. For this reason, the amount of S is adjusted to be in a range of 0.0001% to 0.0010%. The amount of S is preferably in a range of 0.0002% to 0.0006%.

It is 23.0% or more Cr in order to ensure basic corrosion resistance. On the other hand, when more than 28.0% Cr is contained, the ferrite phase fraction increases and the toughness and corrosion resistance of the weld sections are inhibited. For this reason, the amount of Cr is adjusted to be in a range of 23.0% or more to 28.0% or less. The amount of Cr is preferably in a range of 24.0% to 27.5%.

Nor does it stabilize the austenitic structure and improve toughness and corrosion resistance with respect to various types of acid. Furthermore, Ni suppresses a decrease in hot malleability due to the addition of Sn and Cu. For this reason, it contains 2.0% or more of Ni. By increasing the amount of Ni, it is possible to decrease the temperature of nitride precipitation. On the other hand, since Ni is an expensive alloy, the amount of Ni is limited to be in a range of 6.0% or less. The amount of Ni is preferably in a range of 2.5% to 5.5%, and more preferably in a range of 3.0% to 5.0%.

Co is an element that is effective in increasing the toughness and corrosion resistance of steel and that suppresses a decrease in hot malleability due to the addition of Sn and Cu, and it is desirable that Co be contained together with Ni. Furthermore, in a case where Co is added, it is preferable that 0.1% or more of Co be contained. When more than 1.0% of Co is contained, an effect that is proportional to cost is not exhibited, since Co it is an expensive item. For this reason, the upper limit of the amount of Co is adjusted to 1.0%. In a case where Co is added, the amount is preferably in a range of 0.1% to 0.5%.

It is known from Non-Patent Document 1 that Ni increases the solid solubility of Cu and has an effect of suppressing the generation of a liquid phase having a low melting point due to the addition of Cu and Sn. Furthermore, Co is an element that belongs to the same group as Ni. For this reason, it is considered that the decrease in hot malleability due to Cu and Sn is suppressed by increasing the sum of the amounts of Ni and Co. The present inventors learned that the cracking at the edges of the steel material increases in the case where the total amount of Ni and Co is in a range less than 2.5% when the hot malleability of the steel that is the subject of the present embodiment is arranged on the sum of the amounts of Ni and Co. For this reason, the Ni + Co range is adjusted to be in a range of 2.5% or more.

Cu is an element that increases the corrosion resistance of stainless steel with respect to acid and has an effect of improving toughness. In the present embodiment, in order to increase the corrosion resistance, 0.2% or more of Cu is contained together with 0.01% or more of Sn. When more than 3.0% Cu is contained, the amount of Cu exceeds the solubility in solid; and thus £ -Cu is precipitated during hot rolling to cause embrittlement. For this reason, the upper limit of the amount of Cu is adjusted to 2.0%. The amount is preferably in a range of 0.5% to 2.0%.

Sn is contained in order to improve the corrosion resistance of the steel of the present embodiment. For this reason, it is necessary that it contains at least 0.01% Sn. Furthermore, it is preferable that 0.02% or more of Sn is contained. On the other hand, Sn is an element that inhibits the hot manufacturing capacity of steel, and decreases the hot resistance of the interface between the ferrite phase and the austenite phase, particularly at a temperature of 900 ° C or less in the alloying element-saving duplex stainless steel which is the subject of the present embodiment. The degree of the decrease depends on the amounts of S, Ca, and O; however, when more than 0.2% Sn is contained, it is not possible to prevent the decrease in hot build capacity even by restricting other limits in the present embodiment. Therefore, the upper limit of the amount of Sn is adjusted to 0.2%.

N is an element that is effective in increasing strength and corrosion resistance while being solubilized in solid in the austenite phase. For this reason, 0.20% or more of N is contained. Since it is possible to decrease the amount of Ni by increasing the amount of N, N is an element that is desirable to actively add. On the other hand, it is necessary to limit the upper limit of the amount of N so that it is within the solubility limit of N. The solubility limit of N is increased according to the amounts of Cr and Mn. When more than 0.30% N is contained in the steel of the present embodiment, Cr nitrides precipitate, such that toughness and corrosion resistance are inhibited and hot workability is inhibited. For this reason, the upper limit of the amount of N is adjusted to 0.30%. The amount of N is preferably in a range of 0.20% to 0.28%.

Al is an element that deoxidizes a steel, and Al is contained together with 0.05% or more of Si in order to reduce the oxygen in the steel as needed. In a steel containing Sn, the reduction of the amount of oxygen is essential to ensure the capacity of hot manufacturing, and for this reason, it is necessary to contain 0.003% or more of Al according to the need. On the other hand, Al is an element that has a comparatively large affinity for N, and when an excessive amount of Al is added, the toughness of stainless steel is inhibited due to the generation of AlN. The degree also depends on the amount of N; however, when the amount of Al exceeds 0.05%, the toughness is greatly decreased. For this reason, the upper limit of the amount of Al is adjusted to 0.05%. The amount of Al is preferably in a range of 0.04% or less.

Ca is an important element in the hot manufacturing capacity of steel, and it is necessary that Ca is contained in order to fix the S and O in the steel as inclusions and to improve the hot manufacturing capacity. In the steel of the present embodiment, 0.0010% or more of Ca is contained for this purpose. Also, adding an excessive amount of it decreases pitting resistance. For this reason, the upper limit of the amount of Ca was adjusted to 0.0040%.

The Ca / O ratio of the amounts of O and Ca is an important component index for improving the hot fabrication capacity and corrosion resistance of the steel of the present embodiment. The lower Ca / O limit is limited in order to improve the hot fabrication capacity of the Sn containing steel. The high temperature ductility of Sn-containing steel is decreased, particularly at a temperature of 900 ° C or less. When the Ca / O value is in a range less than 0.3, the high temperature ductility at 1,000 ° C is also decreased and the hot manufacturing capacity is greatly impaired. For this reason, in the steel of the present embodiment, Ca / O is limited to be in a range of 0.3 or more. On the other hand, when an excessive amount of Ca and Ca / O exceeds 1.0 is added, the resistance to pitting is impaired. Furthermore, when the amount of Ca is excessive, high temperature ductility at a temperature of 1,000 to 1,100 ° C is also impaired. For this reason, the upper limit of Ca / O is adjusted to be in a range of 1.0. Ca / O is preferably in a range of 0.4 to 0.8.

Or it is an inevitable impurity, and an upper limit of it is not particularly set; however, O is an important element that configures oxides that are representative of non-metallic inclusions. Control of the composition of the oxides is extremely important for the improvement of the capacity of hot manufacturing. In addition, surface defects are caused when thick, clustered oxides are generated. For this reason, it is necessary to limit the amount of O so that it is low. In the present embodiment, as described above, by adjusting the ratio of the amount of Ca and the amount of O to be in a range of 0.3 or more, the amount of O is limited. The upper limit of the amount of O is preferably in a range of 0.005% or less.

In addition, either or both of Mo may be contained: 0.2% to 2.0% and W: 0.1% to 1.0%. These are elements that incrementally increase corrosion resistance. A description of the reasons for these limits will be given.

Mo is an element that is extremely effective in incrementally increasing the corrosion resistance of stainless steel, and Mo can be contained as needed. In order to improve the resistance to corrosion, it contains 0.2% or more of Mo. On the other hand, Mo is an expensive element, and from the point of view of suppressing the cost of the alloy in the steel herein. embodiment, the upper limit of the amount of Mo is set to 2.0%.

W is an element that incrementally increases the corrosion resistance of stainless steel in the same way as Mo, and W can be added as needed. In order to increase the corrosion resistance in the steel of the present embodiment, the upper limit of the amount of W is adjusted to 1.0%. The amount of W is preferably in a range of 0.1% to 0.8%.

Furthermore, one or more selected from V may be contained: 0.05% to 0.5%, Nb: 0.01% to 0.15%, and Ti: 0.003% to 0.05%. These are elements that are more likely to generate nitrides than Cr. It is possible to add any of V, Nb and Ti as needed, and there is a tendency for the corrosion resistance to be improved in cases where these are contained in trace amounts.

The nitrides and carbides that are formed by V are generated in the hot work and the cooling process of the steel material, and these have the effect of increasing resistance to corrosion. The reasons for the same are not sufficiently confirmed; however, it is considered that there is a probability of suppressing the rate of chromium nitride generation at a temperature of 700 ° C or less. It is desirable that 0.05% or more of V be contained in order to improve corrosion resistance. When more than 0.5% V is contained, thick V carbonitrides are generated and toughness degrades. Therefore, the upper limit of the amount of V is limited to 0.5%. In a case where V is added, the amount is preferably in a range of 0.1% to 0.3%.

The nitrides and carbides that are formed by Nb are generated in the hot work and the cooling process of the steel material, and these have the effect of increasing resistance to corrosion. The reasons for the same are not sufficiently confirmed; however, it is considered that there is a probability of suppressing the rate of chromium nitride generation at a temperature of 700 ° C or less. It is desirable that 0.01% or more of Nb be contained in order to improve corrosion resistance. On the other hand, in the case where an excessive amount of Nb is added, Nb precipitates as non-solid solubilized precipitates during heating before hot rolling; and thus, toughness is inhibited. For this reason, the upper limit of the amount of Nb is set to 0.15%. In a case where Nb is added, the amount range is preferably in a range of 0.03% to 0.10%.

Ti is an element that forms oxides, nitrides and sulfides in very small amounts, and Ti refines the crystalline grains in the solidified structure and the structure heated to a high temperature of the steel. Furthermore, in the same way as V and Nb, Ti also has the property of replacing a part of the chromium in the chromium nitrides. With an amount of Ti of 0.003% or more, Ti precipitates are formed. On the other hand, when more than 0.05% Ti is contained in the duplex stainless steel, the toughness of the steel is impaired due to the generation of coarse TiN. For this reason, the upper limit of the amount of Ti is adjusted to 0.05%. A suitable amount of Ti is in a range of 0.005% to 0.020%.

In addition, one or more selected B's may be contained: 0.0005% to 0.0050%, Mg: 0.0001% to 0.0030%, and REM: 0.005% to 0.10%. In order to achieve a further improvement in hot malleability, the B, Mg and REM to be contained as needed are limited as follows.

B, Mg and REM are all elements that improve the hot malleability of the steel, and it is desirable that one or more be added for this purpose. Adding an excessive amount of any of B, Mg, and REM has the opposite effect of decreasing hot malleability and toughness. For this reason, the upper limits of the above amounts are adjusted 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 REM amount is 0.10%. Preferable quantities of items Respective are B: 0.0005% to 0.0030%, Mg: 0.0001% to 0.0015%, and REM 0.005% to 0.05%. Here, REM is the sum of the amounts of lanthanide rare earth elements such as Ce, La and the like.

Previously, by having the characteristics of the duplex stainless steel of the present embodiment described above, it is possible to greatly improve the hot fabrication ability of the general purpose duplex stainless steel containing Sn.

In the cast steel stage, an area fracture reduction at 1,000 ° C is in the range of 70% or more. In addition, by subjecting cast steel to procedures that include hot working, it is possible to obtain a duplex stainless steel material with high performance and few surface defects.

Examples

(Example 1)

A description of examples of the alloy saver duplex stainless steel will be given below. The chemical compositions of test steels are shown in Tables 1 to 4. Here, the residue other than the components described in Table 1 is Fe and elements of unavoidable impurities. Furthermore, for the components shown in Tables 1 to 4, the portions where the amounts are not described show the levels of impurities. REM indicates rare earth elements lanthanides, and the amount of REM shows the total of these elements. The numbers that are underlined in the tables indicate values outside the ranges that are defined in the first embodiment.

For all steels, first, a cast steel with a thickness of 100 mm was prepared, and the reduction of area fracture was evaluated. The evaluation was carried out as follows. First, a parallel section of a $ 8mm round bar was heated to 1,200 ° C using a high frequency. The temperature was then decreased to a temperature (1,000 ° C) at which a burst test was performed. Tensile rupture was performed at a speed of 20 mm / second at this temperature, and the shrinkage of the cross section was measured. Steels where area fracture reduction was in a range of 70% or more were rated as A (good), steels where area reduction was in a range of 60% or more unless 70% were rated as B (acceptable), the steels where the area reduction was in an interval less than 60% were evaluated as C (bad), and the results are given in Tables 5 and 6.

The cast steel was hot forged to obtain a semi-finished product with a thickness of 60 mm, and this semi-finished product was used as a hot-rolled material. The semi-finished product was heated to a predetermined temperature of 1,150 to 1,250 ° C, and then hot lamination was performed using a two-stage lamination machine in a laboratory under the following conditions. First, a reduction was repeatedly made to adjust the plate thickness to be 25 mm. Thereafter, a finish lamination was performed starting at 1,000 ° C, and the final finish lamination was performed at 900 ° C. This rolling was carried out in such a way that the final plate thickness became 12 mm and the plate width became 120 mm to obtain a hot-rolled steel plate. The maximum lengths of the edge cracks that were generated in the left and right edge sections of the obtained hot-rolled steel plate were measured, and the sum of the maximum lengths of the edge cracks in the left and right edge sections. Steels where the sum of edge cracks was in a range less than 5 mm were evaluated as A (good), steels where the sum of edge cracks was in a range of 5 to 10 mm were evaluated as B (acceptable), steels where the sum of the cracks at the edges exceeded 10 mm were evaluated as C (bad), and the results are given in Tables 5 and 6.

Furthermore, the steel plates were subjected to a solubilization heat treatment in the following manner. The steel plate was inserted into a heat treatment oven at 1,000 ° C and heated for approximately 5 minutes. The steel plate was then removed, and then subjected to cooling in water to room temperature.

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

The corrosion rate in sulfuric acid was measured as follows. Test pieces 3mm thick * 25mm wide * 25mm long were subjected to a 6 hour immersion test in 5% boiling sulfuric acid. Weight was measured before and after immersion, and the rate of decrease in weight was calculated. Steels where the corrosion rate in sulfuric acid was in a range less than 0.3 g / m2 per hour were evaluated as A (good), steels where the corrosion rate in sulfuric acid was in a range of 0 , 3 to 1 g / m2 per hour were evaluated as B (acceptable), steels where the corrosion rate in sulfuric acid was in a range of 1 g / m2 per hour or more were evaluated as C (bad), and the evaluation results are given in Tables 5 and 6.

Impact characteristics were measured using Charpy test pieces that were taken in the width direction. Test pieces were prepared by processing full size 2mm V-notches in the rolling direction. The test was carried out at -20 ° C using two test pieces for each of the steels, and the impact characteristics were evaluated by means of the mean values of the impact values obtained. Steels where the impact value was in a range of more than 100 J / cm2 were evaluated as A (good), steels where the impact value was in a range of 50 to 100 J / cm2 were evaluated as B (acceptable ), the steels where the impact value was less than 50 J / cm2 were evaluated as C (bad), and the evaluation results are given in Tables 5 and 6.

Table 5

Table 6

From the examples shown in Table 5 and 6, the steels Nos. 1-1 to 1-33, which satisfy the conditions of the first embodiment, have a hot workability, corrosion resistance, and characteristics of favorable impact. On the other hand, the No. 1-A to 1-U steels, which do not satisfy the conditions of the first embodiment, were all inferior in hot manufacturing capacity, corrosion resistance and impact characteristics.

As seen from the previous examples, it is clear that it is possible to obtain an economical alloy saver type duplex stainless steel with a favorable hot manufacturing capacity where the corrosion resistance is improved by the addition of Sn according to the first realization.

(Example 2)

A description of examples of general purpose duplex stainless steel will be given below. The chemical compositions of the test steels are shown in Tables 7 to 10. Here, the remainder other than the components described in Tables 7 to 10 is Fe and elements of unavoidable impurities. Furthermore, for the components shown in Tables 7 to 10, the portions where the amounts are not described show the levels of impurities. REM indicates rare earth elements lanthanides, and the amount of REM shows the total of these elements. The numbers that are underlined in the tables indicate values outside the ranges that are defined in the second embodiment.

Under the same conditions as Example 1, cast steel fabrication, evaluation of area fracture reduction of cast steel, fabrication of hot rolled material, performing hot rolling with respect to rolled material was performed. hot, and evaluation of cracking at the edges. The evaluation results obtained are given in Tables 11 and 12.

Furthermore, the steel plates were subjected to a solubilizing heat treatment in the following manner. The steel plate was inserted into a heat treatment oven at 1,050 ° C and heated for approximately 5 minutes. The steel plate was then removed, and then subjected to cooling in water to room temperature.

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

The corrosion rate in sulfuric acid was measured as follows. Test pieces 3 mm thick * 25 mm wide * 25 mm long were subjected to a 6 hour immersion test in sulfuric acid including 2,000 ppm Cl ions, where the concentration was 15% and the temperature was 40 % Weight was measured before and after immersion, and the rate of decrease in weight was calculated. Steels where the rate of corrosion in sulfuric acid was in a range less than 0.1 g / m2 per hour were evaluated as A (good), steels where the rate of corrosion in sulfuric acid was in a range of 0 , 1 to 0.3 g / m2 per hour were evaluated as B (acceptable), steels where the corrosion rate in sulfuric acid was in a range other than 0.3 g / m2 per hour or more were evaluated as C (bad), and the evaluation results are given in Tables 11 and 12.

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

Table 11

Table 12

From the examples shown in Table 11 and 12, the general purpose duplex stainless steels No. 2-1 to 2-23, which satisfy the conditions of the second embodiment, have a hot workability, strength corrosion and favorable impact characteristics. On the other hand, the steels N ° 2-A to 2-K and 2-M to 2-T, which do not satisfy the conditions of the second embodiment, were inferior in hot manufacturing capacity, resistance to corrosion and characteristics of impact. Furthermore, Comparative Example 2-L satisfied the characteristics; however, since a large amount of Co was contained, Comparative Example 2-L was lower in terms of cost. In addition, Comparative Example 2-U is S31803 steel and is favorable in all of the hot build ability, corrosion resistance, and build ability. However, the amounts of Ni and Mo are high, and the comparative example 2-U is lower in terms of cost for the purpose of the second embodiment.

As seen from the previous examples, it is clear that it is possible to obtain an inexpensive general purpose duplex stainless steel with a favorable hot manufacturing capacity where the corrosion resistance is improved due to the addition of Sn and Cu according to the second embodiment.

Industrial applicability

According to the first and second embodiments, it is possible to provide an alloy saver type duplex stainless steel and a general purpose duplex stainless steel which are inexpensive and where the corrosion resistance is improved. These duplex stainless steel materials make an extremely significant contribution to industries, because it is possible to use duplex stainless steel materials in a seawater desalination unit, tanks for a transport ship, various types of containers, or the like.

Claims (10)

1. A duplex stainless steel consisting of, in 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%; Sn: 0.01% to 0.2%; optionally one or more selected from Mo: 1.5% or less, Cu: 0.3% to 2.0%, W: 0.05% to 1.0%, Co: 2.0% or less, V: 0.05% to 0.5%, Nb: 0.01% to 0.20%, Ti: 0.003% to 0.05%, B: 0.0005% to 0.0050%, Mg: 0.0001% to 0.0030%, and REM: 0.005 to 0.10%; and the rest being Faith and inevitable impurities, wherein a Ca / O ratio of the amounts of C to O is in a range of 0.3 to 1.0, and a PI sting index shown by formula (1) is in a range of less than 30, PI = Cr + 3.3Mo + 16N (1), where the chemical symbols in formula (1) indicate the quantities of the elements. 2. The duplex stainless steel according to claim 1, comprising: one or more selected from Mo: 1.5% or less, Cu: 0.3% to 2.0%, W: 0.05% to 1.0%, and Co: 2.0% or less. 3. Duplex stainless steel according to claim 1 or 2, comprising: one or more selected from 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 claims 1 to 3, comprising: one or more selected from B: 0.0005% to 0.0050%, Mg: 0.0001% to 0.0030%, and REM: 0.005 to 0.10%. 5. A duplex stainless steel consisting of, in mass%: C: 0.03% or less; Si: 0.05% to 1.0%; Mn: 0.1% to 4.0%; P: 0.05% or less; S: 0.0001% to 0.0010%; Cr: 23.0% to 28.0%; Ni: 2.0% to 6.0%; Co: 0% to 1.0%; Cu: 0.2% to 2.0%; Sn: 0.01% to 0.2%; N: 0.20% to 0.30%; Al: 0.05% or less; Ca: 0.0010% to 0.0040%, optionally one or more selected from Mo: 0.2% to 2.0%, W: 0.1% to 1.0%, V: 0.05% to 0.5%, Nb: 0.01% to 0.15%, Ti: 0.003% to 0.05%, B: 0.0005% to 0.0050%, Mg: 0.0001% to 0.0030%, and REM: 0.005 to 0.10%; and the rest being Faith and inevitable impurities, wherein Ni + Co is in a range of 2.5% or more and a Ca / O ratio of the amounts of Ca and O is in a range of 0.3 to 1.0, and PI shown by formula (1) is in a range of 30 or more to less than 40, PI = Cr + 3.3Mo + 16N (1), where the chemical symbols in formula (1) indicate the quantities of the elements. 6. The duplex stainless steel according to claim 5, comprising: either one or both of Mo: 0.2% to 2.0%, and W: 0.1% to 1.0%. 7. Duplex stainless steel according to claim 5 or 6, comprising: one or more selected from V: 0.05% to 0.5%, Nb: 0.01% to 0.15%, and Ti: 0.003% to 0.05%. 8. Duplex stainless steel according to any one of claims 5 to 7, comprising: one or more selected from B: 0.0005% to 0.0050%, Mg: 0.0001% to 0.0030%, and REM: 0.005 to 0.10%. 9. A cast steel of a duplex stainless steel having a composition according to any one of claims 1 to 8, wherein the cast steel is a steel in a state after casting and before processing, and An area fracture reduction at 1,000 ° C is in the range of 70% or more. 10. A duplex stainless steel material which is obtainable by hot working the cast steel of the duplex stainless steel according to claim 9, wherein the stainless steel material is a semi-finished product, a hot-rolled steel plate, a cold-rolled steel plate, a steel wire, or a steel pipe.