EP4310214A1

EP4310214A1 - Duplex stainless steel

Info

Publication number: EP4310214A1
Application number: EP22771249.4A
Authority: EP
Inventors: Shinsuke OGYU; Hiroki Itoh; Kyohtaroh AMAFUJI; Yuuji Iwasaki
Original assignee: Nippon Steel Stainless Steel Corp
Current assignee: Nippon Steel Stainless Steel Corp
Priority date: 2021-03-15
Filing date: 2022-03-09
Publication date: 2024-01-24
Also published as: WO2022196498A1; CN116997670A; JPWO2022196498A1

Abstract

There is provided a duplex stainless steel including a chemical composition consisting of, in mass%, C: 0.10% or less, Si: 3.0% or less, Mn: 8.0% or less, P: 0.040% or less, S: 0.020% or less, Cr: 20.0 to 38.0%, Ni: 3.00 to 12.00%, Mo: 1.0 to 6.5%, Cu: 3.0% or less, N: 0.200 to 0.700%, Al: 0 to 1.0%, Sn: 0 to 1.0%, W: 0 to 6.0%, Co: 0 to 3.0%, Nb: 0 to 0.50%, Ti: 0 to 1.5%, V: 0 to 1.0%, Zr: 0 to 0.50%, Ta: 0 to 0.100%, B: 0 to 0.100%, Ca: 0 to 0.50%, Mg: 0 to 0.50%, and REM: 0 to 0.10%, with the balance: Fe and impurities, wherein PRE is 41.0 or more, Creq/Nieq is 2.360 to 2.530, an average Md value is 0.9140 or less, and an area fraction of a σ phase is 2.0% or less.

Description

TECHNICAL FIELD

The present invention relates to a duplex stainless steel.

BACKGROUND ART

Duplex stainless steels are stainless steels that have both austenite phases and ferritic phases in structures of the steels. Duplex stainless steels have excellent corrosion resistances and high strengths. Taking advantage of their high corrosion resistances, applications of duplex stainless steels are sought in various fields such as materials for petrochemical equipment, materials for pumps, and materials for chemical tanks.
For example, Patent Document 1 discloses a duplex stainless steel, a duplex stainless steel cast piece, and a duplex stainless steel material, which contain Sn, are good in producibility, and are inexpensive.
A known parameter that indicates a corrosion resistance, particularly a pitting resistance of a duplex stainless steel is pitting resistance equivalent (PRE: Cr + 3.3Mo + 16N). Components of a duplex stainless steel are commonly designed such that contents of Cr, Mo, and N are adjusted so as to increase a value of PRE. There has recently been a demand for steel materials with PREs of 40 or more, for enhancing corrosion resistance.
On the other hand, a problem with a duplex stainless steel having increased contents of Cr and Mo is that intermetallic compounds, which decrease mechanical properties and corrosion resistance, such as a σ phase, are likely to precipitate. When the σ phase and the like precipitate in a starting material, the starting material is significantly hardened, and cracking is likely to occur, which extremely decreases hot workability. In addition, even in a finished product, toughness in the vicinities of intermetallic compounds degrades, which makes it difficult to ensure a desired performance.
Patent Document 2 discloses a method for continuous casting of a high-corrosion-resistance duplex stainless steel that is made to have more excellent embrittlement resistance, castability, and hot workability while keeping high corrosion resistance by preventing the precipitation of intermetallic compounds such as a σ phase and a X (chi) phase, which are brittle phases, in producing the high-corrosion-resistance duplex stainless steel.

LIST OF PRIOR ART DOCUMENTS

PATENT DOCUMENT

Patent Document 1: JP2013-119627
Patent Document 2: JP2017-80765

SUMMARY OF INVENTION

TECHNICAL PROBLEM

However, according to Patent Document 2, a content of Ni, which contributes to stabilizing austenite phases, enhancing toughness, and preventing nitrides from precipitating, is 7.0% or less, which may fail to provide these effects sufficiently, leaving room for improvement.
However, a high content of Ni causes the concentration of Cr and Mo in ferritic phases, promoting the precipitation of σ phases. A cast piece made of a steel containing a large number of σ phases is highly likely to crack, raising a problem of difficulty in subsequent hot working.
An objective of the present invention is to provide a duplex stainless steel in which cracking due to a decrease in toughness can be prevented even when its value of PRE is high and its content of Ni is high.

SOLUTION TO PROBLEM

The present inventors conducted diligent studies to solve the problems described above and consequently obtained the following findings.

(a) To achieve the prevention of cracking in a cast piece made of duplex stainless steel, an improvement in toughness is required.
(b) A decrease in toughness can be prevented by appropriately managing a ratio between Cr equivalent (Creq) and Ni equivalent (Nieq) and controlling a solidification.
(c) The precipitation of σ phases can be prevented by controlling an Md value, which serves as an index of how likely intermetallic compounds are to be formed, such that the Md value is not more than a prescribed value, and by controlling a condition for cooling after casting.
(d) When these conditions are satisfied, the resultant duplex stainless steel is good in toughness, and thus cracking can be prevented, even when its value of PRE is high and its content of Ni is high.

The present invention is made based on such findings, and a gist of the present invention is the following duplex stainless steel.

(1) A duplex stainless steel including
- a chemical composition consisting of, in mass%:
  - C: 0.10% or less,
  - Si: 3.0% or less,
  - Mn: 8.0% or less,
  - P: 0.040% or less,
  - S: 0.020% or less,
  - Cr: 20.0 to 38.0%,
  - Ni: 3.00 to 12.00%,
  - Mo: 1.0 to 6.5%,
  - Cu: 3.0% or less,
  - N: 0.200 to 0.700%,
  - Al: 0 to 1.0%,
  - Sn: 0 to 1.0%,
  - W: 0 to 6.0%,
  - Co: 0 to 3.0%,
  - Nb: 0 to 0.50%,
  - Ti: 0 to 1.5%,
  - V: 0 to 1.0%,
  - Zr: 0 to 0.50%,
  - Ta: 0 to 0.100%,
  - B: 0 to 0.100%,
  - Ca: 0 to 0.50%,
  - Mg: 0 to 0.50%, and
  - REM: 0 to 0.10%,
- with the balance: Fe and impurities, wherein
- a value of PRE defined by Formula (i) shown below is 41.0 or more,
- a value of a ratio Creq/Nieq between Creq defined by Formula (ii) shown below and Nieq defined by Formula (iii) shown below is 2.360 to 2.530,
- an average Md value defined by Formula (iv) shown below is 0.9140 or less, and
- an area fraction of a σ phase contained in a metal micro-structure is 2.0% or less: $PRE = Cr + 3 .3Mo + 16 N$
  $Creq = Cr + 1.37 Mo + 1.5 Si + 2 Nb + 3 Ti$
  $Nieq = Ni + 0.31 Mn + 22 C + 14.2 N + Cu$
  $Average Md value = {ΣX}_{i} \cdot {(Md)}_{i}$
- where symbols of elements in Formulas (i) to (iii) above are contents (mass%) of elements, and meanings of symbols in Formula (iv) above are as follows:
  - X_i: Atomic fraction of an alloying component i
  - (Md)_i: Md value (eV) of the alloying component i.
(2) A method for producing a duplex stainless steel, the method including
- a continuous casting step of performing continuous casting on a molten steel having the chemical composition according to (1) above, wherein
- in the continuous casting step, a cast piece is subjected to first cooling to a temperature within a temperature range of 950 to 1050°C, then subjected to heat recuperation until a maximum temperature reaches 1050°C or more, and then cooled under such a condition that a holding time during which the cast piece is held within a temperature range of 900 to 1000°C is 400 s or less.
(3) The method for producing a duplex stainless steel according to (2) above, further including
- a hot-rolling step of performing hot rolling on the cast piece, wherein
- in the hot-rolling step, the cast piece is heated within a temperature range of 1150 to 1300°C for 1.5 h or more, hot rolled under such a condition that a finish rolling temperature is 900 to 1110°C, and then cooled to a temperature range of 500°C or less under such a condition that an average cooling rate for a temperature range of 800 to 500°C is 0.1 to 1.0°C/s.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a duplex stainless steel in which cracking due to a decrease in toughness can be prevented even when its value of PRE is high and its content of Ni is high can be provided.

DESCRIPTION OF EMBODIMENTS

Requirements of the present invention will be described below in detail.

1. Chemical Composition

Reasons for limiting the content of each element are as follows. In the following description, the symbol "%" for contents means "mass%".

C: 0.10% or less

C (carbon) is an element that is dissolved in austenite phases to increase strength. However, C contained in a large amount causes the precipitation of carbides, decreasing corrosion resistance. For that reason, the content of C is set to 0.10% or less, and preferably 0.050% or less. The content of C is more preferably 0.030% or less with consideration given to corrosion resistance with aging. It is not necessary to put a lower limit on the content of C. However, in order to provide the effect described above, the content of C is preferably 0.010% or more, and more preferably 0.015% or more.

Si: 3.0% or less

Si (silicon) is used as a deoxidation element. Si is added in some cases for enhancing oxidation resistance. However, Si contained in a large amount hardens the steel, degrading a workability of the steel. For that reason, the content of Si is set to 3.0% or less, preferably 2.0% or less or 1.0% or less. It is not necessary to put a lower limit on the content of Si. However, in order to provide the effects described above, the content of Si is preferably 0.10% or more, and more preferably 0.20% or more.

Mn: 8.0% or less

Mn (manganese) has effects of increasing austenite phases, increasing a solubility of nitrogen, and preventing pinhole defects and the like in production. However, Mn contained in a large amount decreases corrosion resistance. For that reason, the content of Mn is set to 8.0% or less, preferably 3.0% or less or 1.0% or less. It is not necessary to put a lower limit on the content of Mn. However, in order to provide the effects described above, the content of Mn is preferably 0.20% or more, and more preferably 0.40% or more.

P: 0.040% or less

P (phosphorus) is an element that is unavoidably mixed in the steel. P is also contained in a raw material of Cr or the like. P is thus difficult to reduce. However, P contained in a large amount decreases formability. The content of P is preferably as low as possible and thus set to 0.040% or less. The content of P is preferably 0.030% or less.

S: 0.020% or less

S (sulfur) is an element that is unavoidably mixed in the steel. S may combine with Mn to form inclusions, serving as starting points of rusting. For that reason, the content of S is set to 0.020% or less. The lower the content of S, the more corrosion resistance is enhanced. Thus, the content of S is preferably 0.010% or less, and more preferably 0.0050% or less.

Cr: 20.0 to 38.0%

Cr (chromium) is an element that is essential in keeping corrosion resistance. In addition, Cr is a ferrite stabilizing element. In order to provide a duplex micro-structure of austenite and ferrite, 20.0% or more of Cr needs to be contained with consideration given to phase fractions. However, Cr contained in a large amount rather leads to a decrease in corrosion resistance. For that reason, the content of Cr is set to 38.0% or less. The content of Cr is preferably 22.0% or more or 24.0% or more and is preferably 33.0% or less, 28.0% or less, or 27.0% or less.

Ni: 3.00 to 12.00%

Ni (nickel) is an austenite stabilizing element. In addition, Ni has an effect of enhancing corrosion resistance. For that reason, the content of Ni is set to 3.00% or more. However, Ni contained in a large amount brings about an increase in raw-material cost and may raise a problem of stress corrosion cracking or the like. For that reason, the content of Ni is set to 12.00% or less. The content of Ni is preferably 5.00% or more, more preferably 7.50% or more, and is preferably 10.00% or less.

Mo: 1.0 to 6.5%

Mo (molybdenum) is an element that enhances corrosion resistance. For that reason, the content of Mo is set to 1.0% or more. However, Mo contained in a large amount not only brings about an increase in raw-material cost but also rather leads to a decrease in corrosion resistance. For that reason, the content of Mo is set to 6.5% or less. The content of Mo is preferably 2.0% or more, more preferably 3.0% or more and is preferably 5.5% or less, more preferably 4.4% or less.

Cu: 3.0% or less

Cu (copper) is an element that is highly useful in enhancing resistance to sulfuric acid. However, Cu contained in a large amount rather leads to a decrease in corrosion resistance. For that reason, the content of Cu is set to 3.0% or less. The content of Cu is preferably 2.0% or less, and more preferably 0.90% or less. It is not necessary to put a lower limit on the content of Cu. However, in order to provide the effect described above, the content of Cu is preferably 0.10% or more, and preferably 0.20% or more.

N: 0.200 to 0.700%

N (nitrogen) is an element that is dissolved in austenite phases to increase strength and corrosion resistance, contributing to making the duplex stainless steel lean. For that reason, the content of N is set to 0.200% or more. However, N contained in a large amount causes defects and the like due to the production of blowholes, degrading a corrosion resistance of the steel. For that reason, the content of N is set to 0.700% or less. The content of N is preferably 0.240% or more and is preferably 0.450% or less.

Al: 0 to 1.0%

Al (aluminum) is an optional element and need not be contained. When contained, Al exerts effects of desulfurization and deoxidation. However, Al contained in a large amount causes the precipitation of spinel inclusions (MgO·Al₂O₃), which is hard and causes nozzle blockage, and leads to an increase in a production defect and an increase in raw-material cost. For that reason, the content of Al is set to 1.0% or less. The content of Al is preferably 0.50% or less or 0.10% or less. In order to provide the effects described above reliably, the content of Al is preferably 0.010% or more.

Sn: 0 to 1.0%

Tin (Sn) is an optional element and need not be contained. When contained, Sn increases a corrosion resistance of the steel. However, Sn is an element that hampers a workability of the steel. For that reason, the content of Sn is set to 1.0% or less. The content of Sn is preferably 0.50% or less or 0.10% or less. In order to provide the effect described above reliably, the content of Sn is preferably 0.002% or more.

W: 0 to 6.0%

W (tungsten) is an optional element and need not be contained. When contained, W increases an SCC resistance and a pitting resistance of the steel. Further, W resists producing σ phases compared with Mo. For that reason, W may be contained in lieu of a part of Mo. Even a trace amount of W contained produces the effects to some extent. However, an excessively high content of W increases a production cost. For that reason, the content of W is set to 6.0% or less. The content of W is preferably 3.0% or less, and more preferably 1.0% or less. In order to provide the effects reliably, the content of W is preferably 0.01% or more, more preferably 0.10% or more.

Co: 0 to 3.0%

Co (cobalt) is an optional element and need not be contained. When contained, Co increases a strength of the steel. Further, Co stabilizes austenite. Even a trace amount of Co contained produces the effects to some extent. However, an excessively high content of Co decreases a corrosion resistance of the steel and additionally increases production cost. For that reason, the content of Co is set to 3.0% or less. The content of Co is preferably 2.0% or less or 1.0% or less. In order to provide the effects reliably, the content of Co is preferably 0.01% or more, more preferably 0.05% or more.

Nb: 0 to 0.50%

Nb (niobium) is an optional element and need not be contained. When contained, Nb increases a strength of the steel. Even a trace amount of Nb contained produces the effect to some extent. However, an excessively high content of Nb decreases a corrosion resistance of the steel. For that reason, the content of Nb is set to 0.50% or less. The content of Nb is preferably 0.30% or less, 0.10% or less, or 0.050% or less. In order to provide the effect described above reliably, the content of Nb is preferably 0.005% or more.

Ti: 0 to 1.5%

Ti (titanium) is an optional element and need not be contained. When contained, Ti increases a strength of the steel. Even a trace amount of Ti contained produces the effect to some extent. However, an excessively high content of Ti decreases a corrosion resistance of the steel. For that reason, the content of Ti is set to 1.5% or less. The content of Ti is preferably 0.50% or less, 0.10% or less, or 0.050% or less. In order to provide the effect described above reliably, the content of Ti is preferably 0.005% or more.

V: 0 to 1.0%

V (vanadium) is an optional element and need not be contained. When contained, V increases a strength of the steel. Even a trace amount of V contained produces the effect to some extent. However, an excessively high content of V decreases a corrosion resistance of the steel. For that reason, the content of V is set to 1.0% or less. The content of V is preferably 0.80% or less, 0.50% or less, or 0.30% or less. In order to provide the effect described above reliably, the content of V is preferably 0.01% or more or 0.05% or more.

Zr: 0 to 0.50%

Zr (zirconium) is an optional element and need not be contained. When contained, Zr contributes to the enhancement of corrosion resistance. Even a trace amount of Zr contained produces the effect to some extent. However, an excessively high content of Zr saturates the effect. For that reason, the content of Zr is set to 0.50% or less. The content of Zr is preferably 0.40% or less or 0.30% or less. In order to provide the effect described above reliably, the content of Zr is preferably 0.005% or more.

Ta: 0 to 0.100%

Ta (tantalum) is an optional element and need not be contained. When contained, Ta reforms inclusions, thus enhancing corrosion resistance. However, an excessively high content of Ta leads to a decrease in ductility at normal temperature. For that reason, the content of Ta is set to 0.100% or less. The content of Ta is preferably 0.050% or less. In order to provide the effect described above reliably, the content of Ta is preferably 0.005% or more.

B: 0 to 0.100%

B (boron) is an optional element and need not be contained. When contained, B increases hot workability. Even a trace amount of B contained produces the effect to some extent. However, an excessively high content of B saturates the effect. For that reason, the content of B is set to 0.100% or less. The content of B is preferably 0.0100% or less, more preferably 0.0050% or less. In order to provide the effect reliably, the content of B is preferably 0.0001% or more, more preferably 0.0003% or more.

Ca: 0 to 0.50%

Ca (calcium) is an optional element and need not be contained. When contained, Ca exerts effects of desulfurization and deoxidation as well as preventing the production of spinel inclusions. However, Ca contained in a large amount decreases corrosion resistance and additionally increases a spatter amount during welding. For that reason, the content of Ca is set to 0.50% or less. The content of Ca is preferably 0.050% or less, more preferably 0.010% or less, and further preferably 0.0040% or less. In order to provide the effect reliably, the content of Ca is preferably 0.0010% or more, more preferably 0.0015% or more.

Mg: 0 to 0.50%

Mg (magnesium) is an optional element and need not be contained. When contained, Mg and S in the steel form the sulfide, reducing the segregation of S in grain boundaries. As a result, a corrosion resistance of the steel is increased, which contributes to the enhancement of hot workability. Even a trace amount of Mg contained produces the effect to some extent. However, if an excessively high content of Mg forms coarse oxide and sulfide, serving as starting points of pitting. As a result, a corrosion resistance of the steel is decreased. For that reason, the content of Mg is set to 0.50% or less. The content of Mg is preferably 0.050% or less, more preferably 0.010% or less, and further preferably 0.0040% or less. In order to provide the effects described above reliably, the content of Mg is preferably 0.0005% or more.

REM: 0 to 0.10%

REM (rare earth metal) is an optional element and need not be contained. When contained, REM increases a hot workability of the steel. For this reason, REM is desirably contained in a minute quantity. However, if REM is contained to excess, a corrosion resistance of the steel is decreased. For that reason, the content of REM is set to 0.10% or less. The content of REM is preferably 0.050% or less, and more preferably 0.010% or less. In order to provide the effect described above reliably, the content of REM is preferably 0.0005% or more or 0.005% or more.
Here, in the present invention, REM refers to Sc (scandium), Y (yttrium), and lanthanoids, 17 elements in total, and the content of REM means a total content of these elements. In industrial practice, the lanthanoids are added in the form of misch metal.
In the chemical composition of the duplex stainless steel according to the present invention, the balance is Fe and impurities. The term "impurities" herein means components that are mixed in the steel in producing the steel industrially from raw materials such as ores and scraps and due to various factors in the producing process, and are allowed to be mixed in the steel within their respective ranges in which the impurities have no adverse effect on the present invention.
The chemical composition of the duplex stainless steel according to the present invention needs to make the contents of the elements fall within their respective ranges described above and additionally to make a value of PRE and a value of Creq/Nieq, which are calculated by formulas shown below, fall within their prescribed ranges.

PRE: 41.0 or more

PRE is a general index that indicates a corrosion resistance of a stainless steel. PRE is calculated from a chemical composition of the steel by Formula (i) shown below. When the alloy is designed such that the value of PRE is 41.0 or more, excellent corrosion resistance can be kept. It is not necessary to put an upper limit on the value of PRE. However, an excessively high value of PRE may raise a problem of an increase in alloy cost. For that reason, the value of PRE is preferably 60.0 or less. $PRE = Cr + 3.3 Mo + 16 N$
where symbols of elements in the formula indicate contents (mass%) of the elements contained in the steel.

Creq/Nieq: 2.360 to 2.530

Creq and Nieq are defined as Formulas (ii) and (iii) shown below, respectively. When the value of Creq/Nieq is controlled to be 2.360 or more, solidification in the F mode can take place, enabling toughness to be kept. The value of Creq/Nieq is preferably 2.400 or more. On the other hand, an excessively high Creq/Nieq results in a ferritic single phase micro-structure, thus failing to provide properties of a duplex steel. For that reason, the value of Creq/Nieq is set to 2.530 or less. $Creq = Cr + 1.37 Mo + 1.5 Si + 2 Nb + 3 Ti$
$Nieq = Ni + 0.31 Mn + 22 C + 14.2 N + Cu$
where symbols of elements in the formula indicate contents (mass%) of the elements contained in the steel.
If the value of Creq/Nieq is low, and solidification in the FA mode takes place, the resultant metal micro-structure is composed mainly of vermicular ferrite, which is produced by crystallization of austenite in the middle of the solidification. A metal micro-structure composed mainly of vermicular ferrite is apt to decrease in toughness because the interfacial coherency ferrite/austenite is low, and thus a crack is likely to propagate such that phases are separated from each other at their boundaries.
In contrast, when the value of Creq/Nieq is not less than a prescribed value, the solidification in the F mode, in which a ferrite single phase is solidified, takes place. When the solidification in the F mode takes place, the resultant metal micro-structure is composed mainly of acicular ferrite, which is produced by complete solidification in the form of ferrite followed by the precipitation of austenite by solid-state transformation. A metal micro-structure composed mainly of acicular ferrite is high in properties of lattice matching at ferrite/austenite interfaces and thus is capable of preventing a decrease in toughness.

2. Md value

An Md value is one of indices of a phase stability of a multicomponent and means electron orbital energy in a d orbital of each component of an alloy. When the Md value becomes large, the phase becomes unstable, and intermetallic compounds such as a σ phase are likely to be formed. In the present invention, an average Md value defined by Formula (iv) shown below is set to 0.9140 or less from the viewpoint of preventing the precipitation of intermetallic compounds. From the viewpoint of further preventing the precipitation of intermetallic compounds, the average Md value is desirably 0.9090 or less. $The average Md value = {ΣX}_{i} \cdot {(Md)}_{i}$
where the meanings of symbols in Formula (iv) above are as follows.

X_i: Atomic fraction of an alloying component i
(Md)_i: Md value (eV) of the alloying component i.

For preventing the precipitation of intermetallic compounds, the lower the average Md value, the more preferable it is. It is therefore not necessary to put a lower limit on the average Md value. However, in a component system that is specified in the present invention, it is difficult to bring the average Md value to less than 0.8800. For that reason, the average Md value may be 0.8800 or more.
The Md value of an alloying component i can be determined by a cluster calculation (a molecular orbital calculation method performed with an aggregate (cluster) model constituted by several to several tens of atoms) (M. Morinaga et al., J. Phys. Soc. Jpn., 53 (1984), p.653). Based on an average Md value of an alloy, precipitation of intermetallic compounds can be sorted out by calculating X_i by converting the compositions in grain boundaries and the final stage of solidification that are determined from an initial composition and the partition coefficients into atomic fractions.

3. Metal Micro-Structure

In the duplex stainless steel according to the present invention, an area fraction of the σ phase contained in its metal micro-structure is 2.0% or less. As described above, a large value of PRE and additionally a high content of Ni promote the precipitation of σ phases. In particular, when an area fraction of σ phases is more than 2.0%, a degradation in toughness is prominent. For that reason, the area fraction of σ phases is set to 2.0% or less. The area fraction of σ phases is preferably 1.0% or less, more preferably 0.10% or less, and further preferably 0.05% or less. The lower the area fraction of σ phases, the more preferable it is. It is therefore not necessary to put a lower limit on the area fraction.
There are no specific restrictions on the rest of the metal micro-structure. However, when the value of Creq/Nieq is adjusted within the range described above, the metal micro-structure becomes a duplex micro-structure of ferrite and austenite, and the solidification in the F mode takes place. At this time, it is preferable for the metal micro-structure to contain 50% or less of acicular ferrite in terms of area fraction with the balance being austenite and unavoidable products. In the metal micro-structure, an area fraction of austenite is relatively high, and thus toughness can be enhanced.
The unavoidable products can contain, in addition to the σ phases, Cr₂N and the like. 2.0% or less of Cr₂N and the like in total is tolerable.
In the present invention, area fractions of ferrite and austenite are measured in conformity to JIS Z 3119:2017 with a ferrite meter. In addition, whether the ferrite is mainly composed of vermicular ferrite or acicular ferrite can be determined by micro-structure observation under an optical microscope at a magnification of x50.
Further, a sample for microscopic observation is cut in such a manner that an observation surface is set at a 5-mm depth position from a surface of a cast piece. The sample is then subjected to KOH electrolytic etching to expose σ phases. Then, microstructure images of 60 visual fields are obtained under an optical microscope at a magnification of x400. The obtained images are then subjected to binarize processing to measure a σ phase area fraction. Note that σ phases are unevenly contained in the structure. Therefore, samples are taken from five or more locations in the cast piece, and an average of measurement values from the samples is adopted as the σ phase area fraction.

4. Production Method

The duplex stainless steel according to the present invention can be produced by, for example, performing continuous casting on a molten steel having the chemical composition described above. In other words, the duplex stainless steel according to the present invention may be a cast piece. It is important to control casting conditions at this time. The present inventors first conducted the following studies about the casting conditions for preventing the precipitation of σ phases.
In a continuous casting step, the cast piece is cooled mainly through two steps including a first cooling with a water-cooled copper mold and a second cooling in which cooling spray is sprayed on surfaces of the cast piece. Of these steps, temperature histories at a 5-mm position from an outer layer of the cast piece at various water flow rates in the second cooling were investigated by heat transfer analysis. In this analysis, the casting speed was set to 1.1 m/min.
The nose temperature of a σ phase is considered to be about 900 to 1000°C. Therefore, in a cooling process after the casting, shortening a time during which the cast piece is held within a temperature range of 900 to 1000°C is effective in preventing the precipitation of σ phases.
As a result of studies, the present inventors found that the time during which an outer layer portion (at a depth of 5 mm) of the cast piece is held within the temperature range of 900 to 1000°C, which is in the vicinity of the nose temperature of a σ phase, can be minimized by performing allowing cooling, with 0 L/min set as the water flow rate in the second cooling, which is usually set to about 80 L/min in conventional practices.
Specifically, the outer layer portion of the cast piece after the casting is subjected to the first cooling to a temperature within a temperature range of 950 to 1050°C, the temperature is then increased by heat recuperation from a center portion of the cast piece until a maximum temperature reaches 1050°C or more, and the cast piece is then subjected to allowing cooling, so that a holding time during which the outer layer portion is held within the temperature range of 900 to 1000°C is brought to 400 s or less, which makes it possible to bring the area fraction of σ phases to 2.0% or less.
The shorter the holding time, the more preferable it is for the reduction of the area fraction of σ phases. The holding time is preferably 300 s or less.
The duplex stainless steel according to the present invention may be a hot-rolled material having a plate shape or a rod shape. In this case, a method for producing the duplex stainless steel according to the present invention further includes a hot-rolling step of performing hot rolling on the cast piece. There are no specific restrictions on conditions for the hot-rolling step. However, the hot-rolling step is preferably performed under the following conditions.
Before performing the hot rolling, it is preferable to heat the cast piece within a temperature range of 1150 to 1300°C for 1.5 h or more. By the heating, σ phases that have precipitated in the cast piece can be melted again. Here, a temperature and a duration of the heating mean an average temperature in a furnace and an in-furnace time, respectively.
The heated cast piece is then subjected to rough rolling and finish rolling. At this time, a finish rolling temperature is preferably 900 to 1110°C. After the finish rolling is ended, the cast piece is preferably cooled to a temperature range of 500°C or less under such a condition that an average cooling rate for a temperature range of 800 to 500°C is 0.1 to 1.0°C/s. The average cooling rate is more preferably 0.7°C/s or less. There are no specific restrictions on a method for cooling at this time. For example, air cooling may be performed.
There are no specific restrictions on a cooling rate for cooling from a temperature of 500°C or less to a room temperature. The cooling may be performed by air cooling, mist cooling, water cooling, or the like. Here, the finish rolling temperature means a surface temperature of the hot-rolled material at an outlet of a final stand of a rolling mill including a plurality of stands. The cooling rate after the finish rolling is ended refers to a cooling rate for surfaces of the hot-rolled material.
Alternatively, the duplex stainless steel according to the present invention may be a cold-rolled material that is the hot-rolled material subjected to cold rolling. The cold rolling may be performed by a conventional method. The hot-rolled material or the cold-rolled material may be further annealed to be a hot-rolled annealed material or a cold-rolled annealed material. An annealing temperature at this time is preferably set to, for example, 550 to 900°C from the viewpoint of preventing the precipitation of σ phases.
The present invention will be described below more specifically with reference to examples, but the present invention is not limited to these examples.

EXAMPLES

Columnar cast pieces having chemical compositions shown in Table 1 and having a diameter of 180 mm were produced under various production conditions. Continuous casting conditions for the cast pieces are shown in Table 2.

[Table 2]

Table 2

Test No.	Steel	Casting condition				Metal micro-structure of cast piece				Mechanical property			Metal micro-structure of hot-rolled material
		Stop tempareture of first cooling (°C)	Maximum temperature after heat recuperation (°C)	Water flow rate in second cooling (L/min)	Holding time within 900 to 1000°C (s)	Area fraction (%)			Type of ferrite	Impact value at 100°C (J/cm²)	Tensile strength at 1000°C (kgf/mm²)	Reduction of area at 1000°C (%)
						Ferrite	Austenite	a phase
						Ferrite	Austenite	a phase					Area fraction of σ phase (%)
1	a	970	1055	80	493	32	64.5	3.5	Acicular	25.6	-	-	-	Comparative example
2	a	985	1119	0	247	32	68.0	0.1	Acicular	74.7	15.6	79.1	0.3	Inventive example
3	b	978	1059	80	497	32	64.3	3.7	Acicular	21.2	-	-	-	Comparative example
4	b	980	1126	0	251	32	68.0	0.1	Acicular	72.4	15.1	72.2	0.4	Inventive example
5	c	969	1057	80	485	30	67.8	2.2	Acicular	24.3	-	-	-	Comparative example
6	c	973	1118	0	245	30	69.6	0.0	Acicular	96.9	15.4	69.4	0.5	Inventive example
7	d	964	1056	80	482	29	68.9	2.1	Acicular	27.8	-	-	-	Comparative example
8	d	982	1124	0	242	29	71.0	0.0	Acicular	101.0	15.5	67.1	0.5	Inventive example
9	e	975	1052	80	488	27	72.0	2.3	Acicular	28.9	-	-	-	Comparative example
10	c	981	1122	0	240	27	73.0	0.0	Acicular	113.9	15.1	70.1	0.4	Inventive example
11	f	979	1118	0	245	27	73.0	0.0	Vermicular	24.1	-	-	-	Comparative example
12	g	984	1121	0	259	25	72.9	2.1	Vermicular	24.9	-	-	-	Comparative example
13	h	980	1125	0	243	24	76.0	0.0	Vermicular	17.6	-	-	-	Comparative example
14	i	983	1120	0	249	33	65.5	1.5	Acicular	30.0	15.1	68.3	0.4	Inventive example
15	j	985	1123	0	246	27	71.1	1.9	Acicular	34.1	14.8	67.4	0.5	Inventive example
16	k	980	1125	0	252	34	65.0	1.0	Acicular	44.6	15.8	71.6	0.3	Inventive example

The resultant cast pieces were subjected to metal micro-structure measurement. Specifically, the measurement was performed in the following procedure. First, area fractions of ferrite and austenite were measured in conformity to JIS Z 3119:2017 with a ferrite meter. In addition, whether the ferrite was mainly composed of vermicular ferrite or acicular ferrite was determined by micro-structure observation under an optical microscope at a magnification of x50.
Further, samples for microscopic observation were cut at five locations in such a manner that an observation surface was set at a 5-mm depth position from a surface of each cast piece. The samples were then subj ected to KOH electrolytic etching to expose σ phases. Then, microstructure images of 60 visual fields were obtained under an optical microscope at a magnification of x400. The obtained images were then subjected to binarize processing to measure a σ phase area fraction. An average value of measurement values from the five samples was taken as the σ phase area fraction.
In addition, a toughness of each cast piece was evaluated. From a 5-mm position of an outer layer of each cast piece, a V-notch specimen was fabricated. A size of the specimen was set to 10 mm × 10 mm × 55 mm. The specimen was subjected to the Charpy impact test in conformity to JIS Z 2242:2005. Regarding impact properties, impact values at 100°C being 30.0 J/cm² or more were rated as good, and the impact values being less than 30.0 J/cm² were rated as poor.
Results of the measurement and the evaluation are shown in Table 2 altogether. As understood from Table 2, Test Nos. 1, 3, 5, 7, 9, and 11 to 13, in which area fractions of σ phases were more than 2.0%, or in which metal micro-structures were composed mainly of vermicular ferrite, were poor in impact value. Furthermore, in these test numbers, cracking occurred at a stage of producing their cast pieces.
In contrast, Test Nos. 2, 4, 6, 8, 10, and 14 to 16, which satisfied the specification of the present invention, caused no cracking in their resultant cast pieces, and additionally, their impact values were good. These cast pieces were further subjected to evaluation of hot workability.
From an outer layer portion of each cast piece, a specimen having a diameter of 8 mm and a length of 110 mm was cut out. Then, a temperature of the specimen was increased from the room temperature to 1250°C in 30 s, and the specimen was held for 30 s. Subsequently, the specimen was cooled to 1000°C at a cooling rate of 20°C/s and then held for 30 s. The specimen was then subjected to a tensile test for measuring its tensile strength and a reduction of area.
In addition, the cast pieces of Test Nos. 2, 4, 6, 8, 10, and 14 to 16 satisfying the specification of the present invention were hot rolled to be formed into hot-rolled materials having a diameter of 5.5 mm (wire rods). Specifically, the cast pieces were heated at 1200°C for 2 h, then hot rolled under such a condition that a finish rolling temperature was 1100°C, subsequently air cooled to 400°C under such a condition that an average cooling rate for a temperature range 800 to 500°C was 0.5°C/s, and further water cooled to a room temperature.
Then, from each of the resultant hot-rolled materials, samples for microscopic observation were cut out at five locations in such a manner that a section of the hot-rolled material perpendicular to its longitudinal direction and radial direction serves as an observation surface. The samples were then subjected to KOH electrolytic etching to expose σ phases. Then, microstructure images of 60 visual fields were obtained under an optical microscope at a magnification of x400. The obtained images were then subjected to binarize processing to measure a σ phase area fraction. An average value of measurement values from the five samples was taken as the σ phase area fraction.
As shown in Table 2, Inventive Examples of the present invention each resulted in a reduction of area at 1000°C being 60.0% or more, thus having good hot workability, and in addition, area fractions of σ phases in their hot-rolled materials were successfully reduced to 2.0% or less.

INDUSTRIAL APPLICABILITY

Claims

A duplex stainless steel comprising
a chemical composition consisting of, in mass%:
C: 0.10% or less,

Si: 3.0% or less,

Mn: 8.0% or less,

P: 0.040% or less,

S: 0.020% or less,

Cr: 20.0 to 38.0%,

Ni: 3.00 to 12.00%,

Mo: 1.0 to 6.5%,

Cu: 3.0% or less,

N: 0.200 to 0.700%,

Al: 0 to 1.0%,

Sn: 0 to 1.0%,

W: 0 to 6.0%,

Co: 0 to 3.0%,

Nb: 0 to 0.50%,

Ti: 0 to 1.5%,

V: 0 to 1.0%,

Zr: 0 to 0.50%,

Ta: 0 to 0.100%,

B: 0 to 0.100%,

Ca: 0 to 0.50%,

Mg: 0 to 0.50%, and

REM: 0 to 0.10%,

with the balance: Fe and impurities, wherein

a value of PRE defined by Formula (i) shown below is 41.0 or more,

a value of a ratio Creq/Nieq between Creq defined by Formula (ii) shown below and Nieq defined by Formula (iii) shown below is 2.360 to 2.530,

an average Md value defined by Formula (iv) shown below is 0.9140 or less, and

an area fraction of a σ phase contained in a metal micro-structure is 2.0% or less: $PRE = Cr + 3.3 Mo + 16 N$
$Creq = Cr + 1.37 Mo + 1.5 Si + 2 Nb + 3 Ti$
$Nieq = Ni + 0.31 Mn + 22 C + 14.2 N + Cu$
$Average Md value = \sum X_{i} \cdot {(Md)}_{i}$

where symbols of elements in Formulas (i) to (iii) above are contents (mass%) of elements, and meanings of symbols in Formula (iv) above are as follows:
X_i: Atomic fraction of an alloying component i

(Md)_i: Md value (eV) of the alloying component i.
A method for producing a duplex stainless steel, the method comprising
a continuous casting step of performing continuous casting on a molten steel having the chemical composition according to claim 1, wherein

in the continuous casting step, a cast piece is subjected to first cooling to a temperature within a temperature range of 950 to 1050°C, then subjected to heat recuperation until a maximum temperature reaches 1050°C or more, and then cooled under such a condition that a holding time during which the cast piece is held within a temperature range of 900 to 1000°C is 400 s or less.
The method for producing a duplex stainless steel according to claim 2, further comprising
a hot-rolling step of performing hot rolling on the cast piece, wherein

in the hot-rolling step, the cast piece is heated within a temperature range of 1150 to 1300°C for 1.5 h or more, hot rolled under such a condition that a finish rolling temperature is 900 to 1110°C, and then cooled to a temperature range of 500°C or less under such a condition that an average cooling rate for a temperature range of 800 to 500°C is 0.1 to 1.0°C/s.