JP4834292B2 - Super duplex stainless steel with excellent corrosion resistance, embrittlement resistance, castability and hot workability with reduced formation of intermetallic compounds - Google Patents

Description

The present invention relates to a high corrosion resistance duplex stainless steel, and more particularly, a brittle sigma (σ) phase and a chi (χ) phase produced during the production (casting, hot rolling or welding) of a high corrosion resistance duplex stainless steel. The present invention relates to a super duplex stainless steel having more excellent embrittlement resistance, castability and hot workability while maintaining high corrosion resistance by suppressing the formation of intermetallic compounds such as.

  Duplex stainless steel is a combination of an austenite (γ) phase that provides excellent workability and a ferrite (α) phase that provides excellent corrosion resistance, and is at least 1.7 times stronger than austenitic stainless steel. In addition to the above, it is very excellent in pitting corrosion resistance and stress corrosion cracking resistance, and has attracted attention in recent years. Pitting resistance equivalemt index “PREW =% Cr + 3.3 (% Mo + 0.5% W) + 30% N” is about 46, SAF2507 (UNS S32750), UR52N + (UNS 32550), ZERON100 ( Commercial super duplex stainless steels such as UNS 32760) have been used in various applications since the 1990s. The quality of duplex stainless steel has been improved by the development of refining methods, and its usage has been steadily increasing in various application fields in recent years.

  However, PREW46 grade super duplex stainless steel is the main constituent element of sigma (σ) phase and chi (χ) phase, which lowers mechanical properties and corrosion resistance compared to general-purpose steel grades such as PREW38 grade SAF2205. It contains a large amount of Cr, Mo, W, and there is a problem that these precipitated phases are easily generated during production or use. In fact, embrittlement due to precipitation phases was observed in the cooling after continuous casting of duplex stainless steel, slow cooling after hot rolling, slow cooling of the heat-affected zone after welding, and slow cooling of the ingot center after casting. ing. Of the alloy elements added, Mo added for improving local corrosion and stress corrosion cracking properties is not only expensive, but also promotes the formation of sigma (σ) phase and 475 ° C brittleness. Since it is an element, its addition amount is limited. The sigma (σ) phase is a very brittle intermetallic compound produced at a high temperature of 650 to 1000 ° C. A sigma phase exceeding about 1 vol% drastically reduces the impact toughness and corrosion resistance of the duplex stainless steel.

  Therefore, many researches and developments have been conducted so far to suppress the formation of such a sigma phase during production or use. However, when such conventional research and development is classified and analyzed, there are the following problems.

1) When 1-3% Al is added to ferritic stainless steel containing 39% Cr or when Al and Nb are added in combination, the sigma phase formation rate is delayed and the sigma phase formation temperature range It is reported that the precipitation rate of the sigma phase is delayed and the deposition rate of the sigma phase is delayed (K. Permachandra et al, Materials Science and Technology, Vol. 8, p. 437 (1992)). However, this is not an example applied to a duplex stainless steel in which austenite and ferrite coexist.

2) When Zr is added to stainless steel, the sigma phase formation rate decreases. However, alloying elements such as Al and Zr reduce the fraction of the austenite phase as a strong ferrite-forming element, and form other forms of intermetallic compounds containing nitrogen, reducing corrosion resistance and mechanical properties. (MB Cotrie et al, Metallurgical and Materials Transaction 28A (1997) 2477).

3) When Sn is added to ferritic stainless steel containing 43 to 46% Cr, Sn precipitates at sigma phase nucleation sites such as grain boundaries and grain boundary triple points, and the sigma phase formation rate decreases. However, due to the low melting point of Sn (232 ° C.), cracks may occur when ferritic stainless steel is exposed to high temperatures of 232 ° C. or higher. It is not the case applied to duplex stainless steel (Costa et al, Physica Status Solidi, A 139 (1993) 83).

4) Okamoto et al., DP3W (UNS S39274), a super duplex stainless steel containing 3% Mo + 2% W, is a commercial super duplex stainless steel SAF 2507 containing 3.8% Mo. Compared to UR52N + and ZERON 100, it is announced that the precipitation rate of the sigma phase is delayed by the addition of W during the aging heat treatment at 850 ° C. for 10 minutes. However, when hot-rolling large ingots and slabs, or when casting large castings, there is a problem that corrosion resistance and mechanical properties deteriorate due to precipitation of brittle chi (χ) phase and sigma (σ) phase. ^{(H. Okamoto et al, 4 th} International Conferences on Duplex Stainless Steels, (1994) Paper 91 and U.S. Pat. No. 5,298,093).

In particular, the above-mentioned US Pat. No. 5,298,093 has the effect of not accelerating the formation of intermetallic compounds even when a large amount of W (1.5% to 5.0%) is added to improve corrosion resistance. And is characterized by positively adding W. On the other hand, in order to fix S and O and improve hot workability, 0.02% or less of Ca, or 0.02% or less of Mg, or 0.02% or less of B, and a total amount of 0.02% or less. It is said that at least one element selected from the group consisting of 2% or less of REM can be added, but when Ca, B, Mg and REM are added in excess of the upper limit, these alloy elements It is disclosed that oxides and sulfides are formed in large amounts, and non-metallic inclusions such as oxides and sulfides act as pitting corrosion occurrence sites, leading to a decrease in corrosion resistance.

U.S. Pat. No. 5,733,387 is 0.03% or less C, 1.0% or less Si, 2.0% or less Mn, 0.04% or less P, 0.004% or less S, 2.0% or less Cu, 5.0-8.0% Ni, 22-27% Cr, 1.0-2.0% Mo, 2.0-5.0% W And a duplex stainless steel comprising 0.13 to 0.30% N, a predetermined amount of at least one element selected from the group consisting of Ca, Ce, B and Ti, and the balance being iron. ing. The patent seeks to improve corrosion resistance by increasing the amount of W while keeping the content of Mo that causes the formation of intermetallic compounds in a low range. However, as is apparent from the PREW equation described later, the effect of Mo for improving pitting corrosion resistance is twice that of W, so it is ineffective to reduce the Mo content. .

On the other hand, in order to suppress the formation of brittle intermetallic compounds, rapid cooling during the heat treatment of the duplex stainless steel is essential. This means that when duplex stainless steel is cooled from the heat treatment temperature, it will pass the precipitation temperature of the intermetallic compound, but if the cooling rate in this temperature range is not fast enough, the intermetallic compound will precipitate rapidly. It is to do. Thus, when an intermetallic compound precipitates at a high temperature during slow cooling, the duplex stainless steel becomes very brittle and the corrosion resistance is greatly reduced. Therefore, another conventional technique for suppressing the precipitation of intermetallic compounds is to control the cooling process during heat treatment to suppress the formation of intermetallic compounds .

The Japanese Patent Publication JP 5-271776, during the heat treatment of duplex stainless steel, quenched at a rate exceeding the cooling rate intermetallic compound is precipitated to the lower limit temperature of the precipitation temperature zone of intermetallic compounds, followed intermetallic it is possible to suppress the precipitation of intermetallic compounds are disclosed by way of holding 5 minutes at a temperature range exceeding 200 ° C. below the deposition temperature range of the compound.

Japanese Patent Publication No. Sho 62-6615 provides a method for suppressing intermetallic compounds when duplex stainless steel is produced as a machine part in a cast state. That is, when manufacturing machine parts made of duplex stainless steel, products are usually manufactured by injecting molten steel into a sand mold and solidifying it, and then leaving it to room temperature. However, when manufacturing castings with super duplex stainless steel, in which intermetallic compounds are very likely to precipitate, part of the ferrite phase transforms into a sigma phase and an austenite phase during the process of cooling to room temperature after casting. Embrittlement due to sigma phase appears. In order to suppress the precipitation of such sigma phase, the above Japanese Patent Gazette describes that the cast steel obtained by injecting molten steel of duplex stainless steel into a sand mold or mold and then solidifying it is removed from the mold at 1000 ° C. or higher. Discloses a method of rapid cooling. This is because when the stainless steel is cooled from the heat treatment temperature, it passes the precipitation temperature of the sigma phase, but if the cooling rate in this temperature range is not fast enough, the sigma phase will precipitate rapidly. Thus, when the sigma phase precipitates during the cooling process, the stainless steel becomes very brittle and the corrosion resistance is greatly reduced.

  However, the method based on the addition of the third alloy element and the control of the cooling process during the heat treatment as described above could not suppress the sigma phase to a satisfactory level in the super duplex stainless steel that is the object of the present invention. .

DISCLOSURE OF THE INVENTION The present invention increases the diffusion and precipitation rate of very brittle intermetallic compounds by adding appropriate amounts of Ba, Y, Ce, La, Nd, Pr, Ta, Zr, Ti and the like having large atomic radii. By delaying and using a small amount of RE complex compound or Ba oxide, the diffusion rate of Cr, Mo, Si, W is further prevented, thereby reducing the precipitation rate of intermetallic compounds and reducing the amount of precipitation. The purpose is to reduce, prevent embrittlement, and improve corrosion resistance.

Further, the present invention is a mixture of MM (Misch metal: rare earth metals from atomic number 57 to 71) with proper preliminary deoxidation by a usual method using Ti, Mg, Ca, Al and Ca + Al, and 50% The above-mentioned Ce, predetermined amounts of La, Nd and Pr, trace amounts of Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc and Fe of 1% or less are defined. Hereinafter, in the detailed description and examples of the present invention, as an example, the main component is 51% Ce-26% La-15.5% Nd-5.5% Pr and the balance is Pm, Sm, Eu. , Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc and MM composed of Fe of 1% or less were used, hereinafter abbreviated as “MM”) and / or by adding Y A that adversely affects the properties of steel And to prevent a single product of the ₂ O _3, MnS non-metallic inclusions.

  In addition, the present invention can control the solubility product in the molten steel of the MM and / or Y rare earth metal element (REM, represented by “RE” in the following compound formulas) within a specific range, so The rare earth metal composite compound “RExOy or (RE, Al) xOy + RExOyS + RExSy” having a diameter of 5 μm or less is intentionally generated to provide a heterogeneous nucleation site during formation of dendrites during solidification, thereby finely solidifying the solidified structure. It aims at improving mechanical characteristics, physical characteristics, and corrosion resistance by controlling segregation of solute elements such as Cr, Mo, W, Ni, Mn, Si and the like at the same time.

Therefore, the present invention remarkably suppresses the formation of intermetallic compounds containing a sigma phase in duplex stainless steel by adding a new alloy element not recognized in the prior art, and significantly improves the production yield in mass production. With the goal.

Furthermore, the present invention significantly reduces the yield during casting and hot working by greatly reducing the precipitation rate of intermetallic compounds containing a sigma phase, improving the resistance to embrittlement, and significantly reducing the occurrence of cracks. The purpose is to improve.

  In addition, by suppressing the precipitation of sigma (σ) phase and chi (χ) phase, which greatly reduces corrosion resistance and mechanical properties in the cast state, and when equipment parts are essentially welded in the various application fields mentioned above In the welding heat-affected zone, the purpose is to further improve the durability of the equipment by greatly improving the corrosion resistance and mechanical properties by controlling these precipitation phases.

  Here, the gist of the present invention is as follows.

(1) By weight, Cr: 21.0% to 38.0%, Ni: 3.0% to 12.0%, Mo: 1.5% to 6.5%, W: 0 to 6.5 %, Si: 3.0% or less, Mn: 8.0% or less, N: 0.2% to 0.7%, C: 0.1% or less, Ba: 0.0001 to 0.6%, remaining Consists of iron and inevitable impurities,
The pitting resistance equivalent index PREW (Pitting Resistance Equivalent) defined by the following formula (1) satisfies 40 ≦ PREW ≦ 67, and the corrosion resistance, brittleness resistance, castability and heat with suppressed formation of intermetallic compounds. Super duplex stainless steel with excellent hot workability.
PREW = wt% Cr + 3.3 (wt% Mo + 0.5 wt% W) +30 wt% N -------- (1)

(2) The steel further contains MM and / or Y in a total amount of 0.0001 to 1.0%. Corrosion resistance, brittleness resistance, and castability in which the formation of the intermetallic compound according to (1) is suppressed. Super duplex stainless steel with excellent hot workability.

(3) Corrosion resistance, embrittlement resistance, castability and hot workability in which formation of the intermetallic compound according to (2) is suppressed, wherein the Ba is added in a range of 0.001 to 0.2%. Excellent super duplex stainless steel.

(4) By weight, Cr: 21.0% to 38.0%, Ni: 3.0% to 12.0%, Mo: 1.5% to 6.5%, W: 0 to 6.5 %, Si: 3.0% or less, Mn: 8.0% or less, N: 0.2% to 0.7%, C: 0.1% or less, and MM and / or Y is 0 in total 0.0001-1.0%, and the balance consists of iron and inevitable impurities,
The pitting corrosion resistance equivalent index PREW defined by the following formula (1) satisfies 40 ≦ PREW ≦ 67, and is excellent in corrosion resistance, brittleness resistance, castability and hot workability with suppressed formation of intermetallic compounds. Super duplex stainless steel.
PREW = wt% Cr + 3.3 (wt% Mo + 0.5 wt% W) +30 wt% N -------- (1)

(5) The value of the relational expression [MM and / or Y + Al] · [O + S] of the solubility product of MM and / or Y and Al, O and S in steel is 0.001 × 10 ⁻⁵ to 30, Corrosion resistance, embrittlement resistance, castability and hot, in which formation of the intermetallic compound according to any one of (2) to (4) is in the range of 000 × 10 ⁻⁵ [%] ² Super duplex stainless steel with excellent workability.

(6) Formation of the intermetallic compound according to (5), wherein the relational value of the solubility product is in the range of 1 × 10 ^{−5 to} 5,000 × 10 ⁻⁵ [%] ² in the case of a cast product. Super duplex stainless steel with excellent corrosion resistance, brittleness resistance, castability and hot workability.

(7) The metal according to (5), wherein the relational expression value of the solubility product is in the range of 0.1 × 10 ^{−5 to} 2,000 × 10 ⁻⁵ [%] ² in the case of a hot-worked product. Super duplex stainless steel with excellent corrosion resistance, embrittlement resistance, castability and hot workability with reduced formation of intermetallic compounds .

(8) Corrosion resistance according to any one of (2) to (4), in which the total amount of MM and / or Y is 0.01% to 0.6%, and formation of intermetallic compounds is suppressed. Super duplex stainless steel with excellent embrittlement resistance, castability and hot workability.

(9) The total amount of MM and / or Y is 0.2% to 0.5%, and the corrosion resistance, brittleness resistance, castability and the formation of intermetallic compounds according to (8) are suppressed. Super duplex stainless steel with excellent hot workability.

(10) The steel is further Ca: 0.5% or less, Mg: 0.5% or less, Al: 1.0% or less, Ta: 0.5% or less, Nb: 0.5% or less, Ti: (1) to (4) containing one or more elements selected from the group consisting of 1.5% or less, Zr: 1.0% or less, Sn: 1.0% or less, and In: 1.0% or less. The super duplex stainless steel excellent in corrosion resistance, embrittlement resistance, castability and hot workability in which the formation of the intermetallic compound according to any one of the above is suppressed.

(11) The steel further contains B: 0.1% or less. Corrosion resistance, embrittlement resistance, casting in which formation of an intermetallic compound according to any one of (1) to (4) is suppressed. Super duplex stainless steel with excellent heat and hot workability.

(12) The formation of the intermetallic compound according to any one of (1) to (4), wherein the steel further contains at least one of Cu: 3.0% or less and Co: 3.0% or less. Super duplex stainless steel with excellent corrosion resistance, brittleness resistance, castability and hot workability.

(13) [PREW (γ) −PREW (α)] value (where PREW (γ) and PREW (α) are pitting corrosion resistances of the austenite phase and the ferrite phase, respectively), which is the corrosion resistance balance between the austenite phase and the ferrite phase. Equivalent index ( hereinafter the same) is in the range of -5 to 10, and the corrosion resistance, brittleness resistance, and casting in which the formation of the intermetallic compound according to any one of (1) to (4) is suppressed. Super duplex stainless steel with excellent heat and hot workability.

(14) The volume fraction of the ferrite phase in the steel composition is 20 to 70% by volume, and the volume fraction of the austenite phase is 30 to 80% by volume, (1) to ( 4) Super duplex stainless steel excellent in corrosion resistance, embrittlement resistance, castability and hot workability in which the formation of the intermetallic compound according to any one of 4) is suppressed.

Best Mode for Carrying Out the Invention The inventors have shown that thin laboratory-scale preforms made with optimal alloy designs are capable of mass production, even though they can significantly improve corrosion resistance and mechanical properties. In order to increase the yield of thick cast products and hot-worked products, and to improve the corrosion resistance and mechanical properties of these products, these conditions must be met. As a result of earnest research on the behavior of intermetallic compounds such as sigma (σ) phase and chi (χ) phase that have a decisive influence on the corrosion resistance, embrittlement resistance, castability and hot workability of the products of I found the facts.

That is, the present inventors are Fe, Cr, Mo, Ni, which are basic alloy elements constituting a duplex stainless steel containing Cr, Mo, Si, and W, which are known to easily form intermetallic compounds . Addition of alloying elements such as Ba, MM (Ce, La, Nd, Pr) and / or Y, which have a much larger atomic radius compared to atomic radii such as W, Mn, Si, etc. In particular, austenite is formed by filling the atomic vacancies in which the atoms of the alloy elements possessed act as diffusion paths for Cr, Mo, Si, and W, which are elements constituting the sigma (σ) phase and chi (χ) phase, etc. and the boundary of the ferrite phase, by filling the grain vacancy of the ferrite phase, it can only be reduced production rate of these intermetallic compounds at 1000 ° C. to 650 ° C. temperature range , Atoms having a larger atomic radius alloying elements found atomic radius is relatively small Cr, Mo, Si, that can prevent the diffusion of W (blocking) to reduce the deposition rate of the intermetallic compound.

In addition, an alloy element having a large atomic radius is free energy that thermodynamically generates an oxide or an oxysulfide as compared with Fe, Cr, Mo, W, Ni, Mn, and Si. Is extremely low, it is possible to form fine and uniform oxides and oxysulfides having a diameter of 5 μm or less at the time of steel making. Thus, it was found that the diffusion rate of Cr, Mo, Si, and W can be further prevented to reduce the deposition rate of intermetallic compounds .

  In general, MnS non-metallic inclusions themselves have a lower corrosion resistance than matrix metals and act as a starting point for corrosion, but rare earth non-metallic inclusions themselves have much better corrosion resistance than matrix metals. And found that it does not act as a starting point for corrosion.

That is, the present invention relates to Fe (1.24 Å) (the number in parentheses represents an atomic radius), Cr (1.25 Å), Mo (1.36 Å), which are main alloy elements of conventional duplex stainless steel. ), W (1.37 Å), Ni (1.25 Å), Mn (1.12 Å), Si (1.17 Å), Ba (2.18 Å) having a large atomic radius is 0.0001% to By adding 0.6%, it is an important feature to positively suppress the formation of intermetallic compounds by the mechanism described above.

Furthermore, the present invention provides Fe (1.24 Å), Cr (1.25 Å), Mo (1.36 Å), W (1.37 Å), Ni (which are the main alloy elements of the conventional duplex stainless steel described above. MM (Ce: 1.83 Å, La: 1.88 Å, Nd: 1.82 Å, Pr: 1) compared to 1.25Å), Mn (1.12Å), Si (1.17Å) .83Å main element and the like and a small amount of Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc and Fe of 1% or less) and / or Y (1.82Å) Another feature is that the formation of an intermetallic compound is positively suppressed by the above-described mechanism by adding a silane or optionally with the addition of Ba. Here, in order to sufficiently exhibit such an effect, [MM and / or Y + Al] · [O + S], which is a relational expression of solubility products of MM and / or Y, Al, O, and S in steel, is set to 0. It is limited to the range of 001 × 10 ⁻⁵ to 30,000 × 10 ⁻⁵ .

  In addition to this, Ca (1.97 、), Mg (1.6Å), Al (1.43Å), Ta (1.43Å), Nb (1.43 の) having a larger atomic radius than the alloy elements. ), Ti (1.47 Å), Zr (1.62 Å), Sn (1.51 Ｉｎ), In (1.68 Å) when an appropriate amount of at least one alloy element is added, the sigma (σ) phase and chi ( It is possible to further improve the above-described effect relating to suppressing the formation of the χ) phase.

  Further, in addition to the alloy element having a large atomic radius, when B is added, which has an atomic radius that is extremely smaller than Fe, Cr, Mo, W, Ni, Mn, and Si and can fill the space between them, B Can further reduce the rate of formation of precipitated phases such as sigma phase and chi phase along with elements having a large atomic radius.

  In addition, any one or more alloy elements of Cu and Co for the purpose of improving acid resistance and strength can be further added.

  Below, the reason for limiting the role and chemical composition range of the alloy element added to the duplex stainless steel according to the present invention will be described.

Chromium (Cr): 21.0% to 38.0%
Chromium is the most important basic element for maintaining the corrosion resistance of stainless steel, and in order to ensure the minimum corrosion resistance, it is necessary to add 12% or more, but in the alloy of the present invention, two phases of austenite ferrite are required. Since the structure has to be obtained, the chromium equivalent (Cr _eq ) and nickel equivalent (Ni _eq ) defined by the following formula and the austenite / ferrite phase ratio determined thereby are considered to be 21% or more of chromium. Must be included. In order to produce a duplex stainless steel with a balance of C, N, Ni, Mo, W, Si, Mn and Cu, the upper limit is limited to 38%. A more preferable range is 24% to 28%.
Cr _eq =% Cr + 2% Si + 1.5% Mo + 0.75% W + 5% V + 5.5% A1 + 1.75% Nb + 1.5% Ti ---- (2)
Ni _eq =% Ni + 0.5% Mn + 30% C + 0.3% Cu + 25% N +% Co -------- (3)
Austenitic phase fraction (volume%) = 100− [55 × (Cr _eq / Ni _eq ) −66.1] ----- (4)
Ferrite phase fraction (volume%) = 55 × (Cr _eq / Ni _eq ) −66.1 -------- (5)

  Furthermore, the preferred range of the phase ratio for maximizing the corrosion resistance of the duplex stainless steel according to the present invention was obtained by the examples of the present invention described later, but the volume fraction of the ferrite phase is 20 to 70% by volume. (The volume fraction of the austenite phase was 30% to 80% by volume).

Nickel (Ni): 3% to 12%
Nickel is a useful element that increases the overall corrosion resistance in relation to corrosion resistance as an austenite stabilizing element, so it is necessary to contain at least 3% or more. It should be added in consideration of the relationship between the chromium equivalent and the nickel equivalent, and is limited to 3.0 to 12.0% in consideration of the relationship with the phase ratio and the expensive material. A more preferable range is 6% to 9%.

Molybdenum (Mo): 1.5% to 6.5%
Molybdenum, together with chromium, is an important element for maintaining the corrosion resistance of the alloy of the present invention, and acts to stabilize the ferrite phase. In the alloy of the present invention, since it is necessary to obtain an austenite ferrite two-phase structure, it is necessary to contain 1.5% or more of molybdenum in consideration of a chromium equivalent, a nickel equivalent, and a phase ratio. In particular, when combined with copper, the corrosion resistance is greatly improved in a high concentration sulfuric acid (SO ₄ ²⁻ ) and hydrochloric acid (Cl ⁻ ) environment. It is a representative element that forms intermetallic compounds such as sigma phase, which is very useful for mechanical properties and corrosion resistance in the annealed state, but has an adverse effect when performing aging heat treatment, hot rolling or welding. Therefore, in view of chromium equivalent, nickel equivalent, corrosion resistance and phase stability, the amount is limited to 6.5% or less. As can be seen from the PREW formula, the effect of Mo for improving pitting corrosion resistance is twice that of W, so that the more preferable content of Mo for ensuring pitting corrosion resistance is 2% or more.

Tungsten (W): 0 to 6.5%
Tungsten is an alloying element of the same family that has similar chemical properties to molybdenum as a ferrite stabilizing element. After improving the corrosion resistance in high agricultural grade SO ₄ ^2- and Cl ^- ion environment, performing sensitizing heat treatment or welding, delaying the precipitation rate of brittle sigma phase and chi-phase to improve the corrosion resistance and mechanical properties. It is a useful element to improve. However, tungsten is an expensive alloy element and, when added in a large amount, promotes the formation of intermetallic compounds. Therefore, in view of phase stability, mechanical properties, and corrosion resistance, the content of tungsten is made 6.5% or less. Restrict. A more preferable range is 4.0% or less.

Silicon (Si): 3% or less Silicon is an element that stabilizes the ferrite structure, which has a deoxidizing effect during melting and refining, and increases the fluidity of molten steel during the production of cast products by increasing acid resistance. It is an element that reduces defects. If added over 3%, the precipitation rate of very brittle intermetallic compounds is increased and the ductility of the steel is reduced. Considering the corrosion resistance, 3.0% or less is preferable. A more preferable range is 1.0% or less.

Manganese (Mn): 8% or less Manganese is an austenite stabilizing element that can replace expensive nickel, and is an element that increases the solidity of nitrogen and increases deformation resistance at high temperature. When trying to improve the corrosion resistance by increasing the nitrogen content, the proper amount of manganese is an essential element. Although it has a deoxidizing effect at the time of melting and refining, when it is added in a large amount, the corrosion resistance is lowered and the formation of a very brittle intermetallic compound is promoted, so the upper limit is limited to 8% or less. A more preferable range is 1.0% to 3.0%.

Nitrogen (N): 0.2% to 0.7%
Nitrogen is a useful element that improves resistance to pitting corrosion, and its effect is about 30 times or more that of chromium. As a strong austenite stabilizing element, it is one of the most important elements related to corrosion resistance. When present at the same time as molybdenum, the corrosion resistance is greatly improved by the synergy effect. When the carbon content is lowered for the purpose of improving intergranular corrosion resistance, nitrogen can be added to obtain mechanical property compensation. In addition, the formation of chromium carbide is suppressed, and the tensile strength and yield strength are increased without reducing elongation. It should be added in consideration of the balance with C, Cr, Ni, Mo and W and the ratio of austenite / ferrite phase. In consideration of corrosion resistance, 0.2% or more is preferable, but when it is added in a large amount exceeding 0.7%, castability (blow hole, shrinkage) and rollability may be deteriorated. A more preferable range is 0.32% to 0.45%.

Carbon (C): 0.1% or less Carbon is a very important element for stabilizing the austenite phase and maintaining mechanical strength. However, if added in a large amount, carbide precipitates and the corrosion resistance deteriorates. Therefore, it is limited to 0.1% or less, preferably 0.05% or less, but considering the aging corrosion resistance, 0.03% or less is more preferable.

PREW value: 40 to 67
As described above, in addition to limiting the contents of Cr, Mo, W and N, the present invention is characterized in that the PREW value defined by the following formula is limited to a range of 40 to 67.

  PREW = wt% Cr + 3.3 (wt% Mo + 0.5 wt% W) +30 wt% N -------- (1)

If the lower limit value is not reached, sufficient corrosion resistance cannot be ensured. If the upper limit value is exceeded, intermetallic compounds are likely to be formed. Preferably, the PREW value is 45 or more.

  Furthermore, the preferred range of the corrosion resistance balance “PREW (α) -PREW (γ)” for maximizing the corrosion resistance of the duplex stainless steel according to the present invention is −5 to 10 according to the examples of the present invention described later. I found out.

Barium (Ba): 0.0001% to 0.6%
As described above, barium is one of the most important elements in the present invention, and has other atomic elements (Fe, Cr, Mo, W, Ni, Mn, Barium, which has a remarkably large atomic radius compared to Si, etc.) acts as a barrier to prevent the diffusion of very brittle intermetallic compound- forming elements, reducing the diffusion rate and precipitation rate to reduce the amount of precipitation. There is a remarkable effect. In addition, it can be combined with solid atoms or oxygen to form an oxide to delay the precipitation rate of σ and χ phases. In order to obtain these effects, a maximum of 0.6% or less is necessary. If it exceeds 0.6%, not only is it not economical, but a large amount of barium precipitates at the grain boundary, lowering the grain boundary strength at high temperature and offsetting the improvement effect of hot cracking susceptibility. Limit to 6%. On the other hand, if it is less than 0.0001%, the addition effect cannot be expected.

  Further, when a third element having a large atomic radius such as MM and / or Y, or Zr, Ta, In, Mg, Ti is added in combination, a preferable addition amount of Ba for achieving the above effect is 0.001. % To 0.2%.

Misch metal (MM) and / or Y: 0.0001% to 1.0%
Misch metal (as described above, in the detailed description and examples of the present invention, as an example, 51% Ce-26% La-15.5% Nd-5.5% Pr main elements and Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc and MM composed of Fe of 1% or less are used.) And / or Y is combined with barium in the present invention. It is one of the most important alloy elements in the present invention that can be added without adding barium. In the present invention, when MM and / or Y is added, the independent formation of Al ₂ O ₃ and MnS non-metallic inclusions which adversely affect the properties of the steel is prevented, and RExOy having a diameter of 5 μm or less or (RE , Al) xOy + RExOyS + RExSy Rare earth complex compound is produced, acting as an inhomogeneous nucleation site during solidification, refining and densifying the solidified structure, and at the same time controlling segregation of solute elements as much as possible. Improve mechanical properties and corrosion resistance.

Further, Y, MM (Ce, La, Nd, Pr, etc.), Ba, Zr, Ti, etc., which have a large atomic radius remaining in the atomic state in the molten steel, which is a major feature of the present invention, are very brittle intermetallic compounds. It is very effective in delaying the diffusion and precipitation rate. In addition, it is an extremely important element for improving weldability, high temperature oxidation resistance, machinability, high temperature workability, etc., and is limited to 0.0001 to 1.0%. Addition of a large amount exceeding 1.0% is not only economical, but also adversely affects various properties. The addition effect of 0.0001% or less cannot be expected from the above-mentioned addition effect.

In the steel of the present invention, the rare earth composite compound (RExOy or (RE, Al) xOy + RExOyS + RExSy) acts as a heterogeneous nucleation site of dendritic crystals during solidification, thereby making the solidification structure finer and denser. The effect of finely uniforming the segregation region of the solute element, and Y, MM (Ce, La, Nd, Pr, etc.), Ba, Zr, Ti, etc. of the intermetallic compound forming elements Cr, Mo, Si, W In order to maximize the above-mentioned properties by suppressing the diffusion and reducing the precipitation rate of intermetallic compounds, etc., the relationship between the solubility products of MM and / or Y in the molten steel, Al, O and S in the steel The formula value [MM and / or Y + Al] · [O + S] is limited to the range of 0.001 × 10 ^{−5 to} 30,000 × 10 ⁻⁵ [%] ² . When the relational value of the solubility product is less than 0.001 × 10 ⁻⁵ , the present invention pursues the control of solidified structure and the effect of suppressing the segregation of solute elements and the formation of intermetallic compounds , When the relational value of the solubility product exceeds 30,000 × 10 ⁻⁵ [%] ² , the production of rare earth composite compounds is excessive, and the mechanical properties, physical properties, and corrosion resistance of the steel are deteriorated. The relational value of the solubility product is 1 × 10 ^{−5 to} 5,000 × 10 ⁻⁵ in the case of a cast product, and 0.1 × 10 ^{−5 to} 2,000 × 10 ^{− in} the case of a hot-work product. ⁵ [%] It is preferable to limit to ² .

  Moreover, the preferable range of the misch metal addition amount is 0.01 to 0.6%, and the more preferable range is 0.2 to 0.5%.

Calcium (Ca): 0.5% or less Calcium is a deoxidizing element and is a useful element that improves embrittlement resistance and decreases high temperature deformation resistance and cutting resistance. When added in a large amount, the cleanliness of the steel is reduced and the corrosion resistance is lowered, so the content is preferably made 0.5% or less.

Aluminum (Al), Oxygen (O) and Sulfur (S)
Aluminum is an effective element for improving oxidation resistance and embrittlement resistance as a ferrite stabilizing element. Moreover, since it has a deoxidation effect when added to steel, it is an effective element that increases the cleanliness of the steel and reduces the high temperature deformation resistance. Therefore, it is preferably added at 1.0% or less.

  In addition, oxygen (O) and sulfur (S) are unavoidably contained in the steel, but these cause cracks during the solidification process and, after completion of the product, cause ductility of the material by reducing ductility. Therefore, it is preferable to reduce the content if possible. In the case of casting products, it is preferable to suppress oxygen to 200 ppm or less and sulfur to 50 ppm or less, and in the case of processed products, it is preferable to suppress oxygen to 100 ppm or less and sulfur to 20 ppm or less.

Titanium (Ti): 1.5% or less Titanium has a deoxidizing effect during melting and refining, and forms titanium sulfide to improve machinability. In order to improve intergranular corrosion resistance, it can be added in consideration of the relationship with the carbon content. In order to improve the corrosion resistance after sensitizing heat treatment in an environment containing chlorine ions, it can be added at a maximum of 1.5% or less.

Magnesium (Mg): 0.5% or less, Tantalum (Ta): 0.5% or less, Niobium (Nb): 0.5% or less, Zirconium (Zr): 1.0% or less, Tin (Sn): 1 0.0% or less, indium (In): 1.0% or less As already clarified by the present inventors, Ca (1.97１), Al (which has a larger atomic radius than Fe, Cr, Mo, W) 1.43 以外), Ti (1.47Å), Mg (1.6Å), Ta (1.43Å), Nb (1.43Å), Zr (1.62Å), Sn (1.51Å), In since (1.68Å) is effective to inhibit the formation of σ phase and χ phase, Mg0.5% or less, Ta0.5% or less, Nb0.5% or less, Z r 1.0% or less, Sn can be added within a range of 1.0% or less and In 1.0% or less.

  When the alloy element exceeds the above range, not only is it not economical, but it causes grain boundary embrittlement and impairs castability and hot workability.

Boron (B): 0.1% or less Boron has the effect of reducing embrittlement resistance and reducing deformation resistance at high temperatures, and suppresses hot welding cracks during welding. When combined with nitrogen, boron nitride having a low melting point is formed, and the machinability is improved. Boron has an atomic radius much smaller than that of Fe, Cr, Mo, W, Ni, Mn, and Si, and fills minute gaps. Therefore, when coexisting with an alloy element having a large atomic radius, the precipitation rate of the σ phase and the χ phase can be further reduced. The content is preferably 0.1% or less.

Copper (Cu): 3% or less Copper is an austenite stabilizing element and acts as an element useful for corrosion resistance. Especially when combined with molybdenum, the corrosion resistance in concentrated sulfuric acid and hydrochloric acid oxidation environments is greatly improved, and the corrosion current density, passivation current density, critical current density and hydrogen exchange current density are related to the anodic polarization characteristics. And the hydrogen overvoltage is increased to improve the corrosion resistance. Furthermore, copper induces a substitutional solid solution strengthening effect and increases tensile strength and yield strength. If the phase ratio and the appropriate ratio of elements such as chromium and molybdenum do not match, the resistance to pitting corrosion is weakened. It is an important element that improves the workability by reducing the work hardening rate. Addition of 3% or more induces red hot brittleness, so it is limited to 3% or less.

Cobalt (Co): 3.0% or less Cobalt is an element that can replace Ni as an austenite stabilizing element and is effective in improving corrosion resistance and strength, but is expensive. It is limited to a maximum of 3.0% or less from the balance of phase ratio and corrosion resistance between phases.

Example 1: Production of Inventive Steel and Experimental Method The optimum alloy design method and production method according to the present invention are as follows. The alloy design method includes the pitting resistance equivalent index PREW of the formula (1), [PREW (γ) −PREW (α)] with respect to the corrosion resistance balance between phases, the chromium equivalent Cr _eq of the formula (2), the formula (3) Table 2 shows the results obtained by suitably combining alloy design factors such as nickel equivalent Ni _eq .

  The steel of the present invention calculates the chromium equivalent and the nickel equivalent based on the formulas (2) and (3), determines the composition, and then has Fe, Cr, Mo, Ni, W having purity of commercial grade. , Cu, Si, Mn, Fe—Cr—N, etc., steels having the composition of the claims of the present invention are melted using a high frequency induction furnace, and then Ti, Mg, Al, Ca deoxidation or Al + Ca composite deoxidation is used. Deoxidation was performed by a conventional method. The test material for the cast product was melted in the atmosphere, and the test material for the hot-rolled product was melted in a magnesia crucible in a vacuum and nitrogen gas atmosphere. [PREW (γ) −PREW (α)] values relating to the corrosion resistance balance between the phases in Table 2, as can be seen from Table 4, Cr, Mo, which constitute the austenite (γ) and ferrite (α) phases, This is obtained by analyzing the W and N components and then substituting them into the pitting resistance equivalent index PREW of equation (1).

As another embodiment, the present inventors intend that the molten steel produced with the components within the scope of the claims of the present invention is subjected to preliminary deoxidation by a conventional method using Al, Ca deoxidation or Al + Ca complex deoxidation, and then Ba and / or MM and / or Y is added to produce a suitable amount of Ba oxide or rare earth composite compound (RexOy or (RE, Al) xOy + RExOyS + RExSy). And / or Y + Al] · [O + S] = 0.001 × 10 ^{−5 to} 30,000 × 10 ⁻⁵ .

  Thereafter, the molten metal is poured into a plate-like refractory mold to produce a plate-like cast product (thickness 9 mm) weighing 25 kg, and the hot-rolled sheet product is poured into a preheated square steel mold and weighed 30 kg. An ingot was prepared. The ingot for hot rolling was processed into an appropriate size by grinding or machining, and then soaked at 1250 ° C. and hot rolled to a thickness of 6 mm. The solidification heat treatment was performed in the range of 1050 to 1150 ° C. for both the cast product and the hot rolled product. The chemical composition of the invention steel subjected to solidification heat treatment is shown in Table 1 in comparison with the comparative steel and the conventional steel.

  Microstructure, X-ray diffraction test, anodic polarization test, critical pitting temperature, critical crevice corrosion temperature and mechanical properties were measured in order to evaluate the properties of the solidified heat treatment material and aging heat treatment at 850 ° C. for 30 minutes. .

In order to observe the microstructure, after polishing to 2000 with SiC polishing paper and finish polishing using alumina, Murakami aqueous solution (30 g K ₃ Fe (CN) ₆ +30 g KOH + distilled water 100 ml) is used. Then, after etching at 80 ° C., it was ultrasonically cleaned in acetone and distilled water, and then observed with an optical microscope.

  In order to confirm the σ phase and χ phase precipitated by aging heat treatment at 850 ° C. for 30 minutes, an X-ray diffraction test was performed. The apparatus used was Rikagu D / MAX-B, which was analyzed at an acceleration voltage of 35 kv and a current of 35 mA, and a Ni filter was used as the Cu target. As a result of observing the structure, the specimen having the largest number of precipitated phases was analyzed at a rate of 12 ° / min. At 30 ° to 120 °, and then 1 in the range of 40 ° to 50 ° where the peak of the precipitated phase was concentrated. Precise analysis was performed at a speed of ° / min.

  The anodic polarization test was performed using ASTM G5 and a degassed 0.5N HCl + 1.0N NaCl mixed solution at 50 ° C. and 70 ° C. at a scanning rate of 1 mV / sec.

  The critical pitting temperature was measured according to ASTM G 48A-92, and the critical crevice temperature was measured according to ASTM G 48D.

  In order to measure the hardness of the material, after polishing to No. 600, it was measured by C-scale using a Rockwell hardness meter.

Example 2: Comparison of microstructure of aging material FIGS. 1A to 1F show invention steel 4 (FIG. 1A), invention steel 10 (FIG. 1B), invention steel 36 (FIG. 1C), which were aged at 850 ° C. for 30 minutes, comparison Steel 47 (FIG. 1D), conventional commercial super duplex stainless steel UR 52N + (FIG. 1E) and SAF 2507 (FIG. 1F), brittle intermetallics , sigma (σ) phase and chia that reduce corrosion resistance and mechanical properties It is a photograph of the microstructure showing the degree of precipitation of the (χ) phase. In the figure, the bright part is the “austenite phase” and the dark part indicates that the “ferrite phase” was decomposed into “sigma phase + austenite phase” during the aging heat treatment. The order of the degree of precipitation of intermetallic compounds is “invention steel 4 = invention steel 10 = invention steel 36 << conventional steel UR 52N + <conventional steel SAF 2507 << comparative steel 47”, and invented steels 4, 10, 36 are: Compared with the conventional steel UR 52N +, SAF 2507 and comparative steel 47, it was confirmed that precipitation of intermetallic compounds and the like was remarkably reduced, and the embrittlement resistance was greatly improved.

Example 3 X-ray Diffraction Analysis Tests FIGS. 2A-2D show invention steel 4 (FIG. 2A), comparative steel 47 (FIG. 2B), conventional commercial super duplex stainless steel UR 52N + that was aged at 850 ° C. for 30 minutes. (FIG. 2C), SAF 2507 (FIG. 2D), X-ray diffraction analysis test results showing the degree of precipitation of brittle intermetallic compounds , sigma (σ) phase and chi (χ) phase that reduce corrosion resistance and mechanical properties It is. Inventive steel 4 does not precipitate σ phase at all and χ phase precipitates only slightly compared to comparative steel 47, conventional steel UR 52N +, and SAF 2507, and it can be seen that the embrittlement resistance is greatly improved. .

Example 4: Comparison of Macrostructure and Microstructure in Casting State FIGS. 3A to 3D are produced by the techniques relating to the control of solidified structure, the control of solute element segregation, and the control of the formation of intermetallic compounds of the present invention in the cast state. 3 shows the macrostructure (FIGS. 3A and 3B) and the microstructure (FIGS. 3C and 3D) in the center of the ingot (φ110 mm × L550 mm) of the inventive invention steel 10 and the comparative steel 47. FIG.

Invented steel 10 (0.09%) in which MM (Ce, La, Nd, Pr) in molten steel and the solubility product [MM + Al] · [O + S] of Al, O, and S are 352.0 × 10 ⁻⁵ [%] ² MM, 0.02% Al, 0.025% O, 0.007% S) macrostructure (FIG. 3A) is a comparative steel 47 (0.015%) in which MM and Al are not added and the solubility product is zero. O, 0.007% S) is a fine equiaxed crystal structure in which the growth of columnar crystals is suppressed compared to the macro structure (FIG. 3B), has a dense solidified structure, and has a V segregation portion and a reverse V It was confirmed to have a good structure with almost no segregation part.

Furthermore, the microstructure of Invention Steel 10 (FIG. 3C) is a sigma (σ) phase and chi (χ) phase , which are intermetallic compounds that reduce the corrosion resistance and mechanical properties compared to the microstructure of comparative steel 47 (FIG. 3D). Precipitation was remarkably suppressed, and it was confirmed that the austenite and ferrite phases were reduced in size.

Example 5: Anodic Polarization Test Results in Cast State FIG. 4 shows the anodic polarization test results of Invention Steels 4, 10, 26, and 36 and Comparative Steel 47 in a cast state without solidification heat treatment. is there. The order of pitting corrosion resistance is as follows.

Invention Steel 10> Invention Steel 4> Invention Steel 36 ≧ Invention Steel 26> Comparison Steel 47

Example 6: Critical Pitting Temperature and Crevice Corrosion Temperature Test Results FIG. 5 shows solidified heat-treated invention steels 4, 10, 26, 36 and conventional commercial super duplex stainless steels UR 52N +, SAF 2507, The critical pitting temperatures of ZERON 100, conventional general-purpose duplex stainless steel SAF 2205, conventional commercial super austenitic stainless steel SR-50A, and conventional general-purpose austenitic stainless steel AISI 316L are shown in comparison. The higher the critical pitting temperature, the better the pitting resistance. Therefore, the corrosion resistance levels of the inventive steel and the conventional steel can be expressed as follows.

Invention Steel 10 = Invention Steel 26 = Invention Steel 36> Conventional Steel SR-50A> Invention Steel 4> Conventional Steel UR 52N + = Conventional Steel ZERON 100> Conventional Steel SAF 2507> Conventional Steel SAF 2205> Conventional Steel AISI 316L

  Inventive steels 10, 26 and 36 have a pitting corrosion resistance far superior to that of commercial super duplex stainless steels UR 52N +, SAF 2507 and ZERON 100, and conventional steel SR-50A which is an expensive super austenitic stainless steel. Better corrosion resistance. Since the steel of the present invention has a significantly higher critical pitting corrosion temperature than the comparative steel and the conventional steel, the critical crevice corrosion temperature is also high as shown in Table 2, indicating that the crevice corrosion resistance has been greatly improved. The critical pitting temperature and critical crevice corrosion temperature values are shown in detail in Table 2.

Example 7: Anodic Polarization Test Results of Solidification Heat Treated Materials FIGS. 6A to 6C show the results of anodic polarization tests (FIG. 6A) of inventive steels 4, 10, 26, and 36 subjected to solidification heat treatment and conventional commercial supermarkets. Results of anodic polarization test of duplex stainless steel UR 52N +, SAF 2507, ZERON 100 (FIG. 6B) and anodic polarization test of conventional commercial super austenitic stainless steel AL-6XN, SR-50A, 254SMO (FIG. 6C). It is a comparison. The order of pitting corrosion resistance of the steel of the present invention and the conventional steel is as follows.

Invention Steel 26 = Invention Steel 36 = Conventional Steel SR-50A> Invention Steel 10> Invention Steel 4 ≧ Conventional Steel AL-6XN> Conventional Steel 254SMO ≧ Conventional Steel UR52N + = SAF 2507 = ZERON 100

  Since the steel of the present invention shown in Example 6 has a very high critical pitting temperature and critical crevice corrosion temperature compared to the comparative steel and the conventional steel, the pitting corrosion potential by the anodic polarization test is also high (Table 2). It can be seen that these three test results tend to match.

Example 8: Anodic Polarization Test Results of Aged Material (850 ° C. × 10 Minutes) FIGS. 7A and 7B are anodic polarization test results of invention steels 4, 10, 26 and 36 subjected to aging heat treatment at 850 ° C. for 10 minutes (FIG. 7A). And comparison results of the anode polarization test of conventional commercial super duplex stainless steel UR 52N +, SAF 2507, and ZERON 100 (FIG. 7B). The order of pitting resistance of each steel is as follows.

Invention Steel 4 = Invention Steel 10 = Invention Steel 26> Invention Steel 36> Conventional Steel ZERON 100> Conventional Steel SAF 2507> Conventional Steel UR 52N +

From this result, the steels of the present invention 4, 10, and 26 have a delayed precipitation rate of sigma (σ) phase and chi (χ) phase, which are intermetallic compounds , during the aging heat treatment from the conventional steels UR 52N +, SAF 2507, and ZERON 100. It was confirmed that the pitting corrosion resistance was improved.

Example 9: Results of Anodic Polarization Test and Hardness Measurement of Aged Material (850 ° C. × 30 Minutes) FIGS. 8A and 8B are anodic polarization tests of steels of the present invention 4, 10, 26, and 36 that were aging heat treated at 850 ° C. for 30 minutes. The result (FIG. 8A) and the anodic polarization test result (FIG. 8B) of conventional super duplex stainless steel UR 52N +, SAF 2507, and ZERON 100 are shown in comparison. The pitting corrosion resistance ranking of each steel is as follows.

Invention Steel 10> Invention Steel 4> Invention Steel 36 = Invention Steel 26 = Conventional Steel SAF 2507 = Conventional Steel ZERON 100> Conventional Steel UR 52N +

From this result, the inventive steels 4 and 10 are pitting corrosion due to the delay of precipitation rate of the sigma (σ) phase and chi (χ) phase, which are intermetallic compounds, during the aging heat treatment from the conventional steels UR 2N +, SAF 507, and ZERON 100. It was confirmed that the resistance was greatly improved, and the inventive steels 36 and 26 exhibited pitting corrosion resistance equal to or higher than that of the conventional steel.

Hardness values, such as the present invention steels is aging 30 minutes at 850 ℃ (H _A) from KataHiroshi of heat-treated invention hardness value such as steel (H _S.A.) the minus change of the hardness value ([Delta] H = a _H a _{-H S.A.)} shown in Table 2. In general, the more hard and brittle sigma (σ) and chi (χ) phases, the greater the ΔH value, thereby greatly reducing the corrosion resistance and strength, elongation, and impact strength values. As shown in Table 2, the ΔH value of the steel of the present invention was measured as small as about 0.1 to 3.7 due to the delay in the precipitation rate of the intermetallic compound, whereas the comparative steel was 10.3 to 16. 2. The conventional steel was 5.6 to 6.2. Therefore, it was confirmed that the steel of the present invention has significantly improved brittleness resistance compared to the comparative steel and the conventional steel.

Example 10: Mechanical properties Table 3 shows the yield strength, tensile strength, and elongation after a solid solution heat treatment at 1130 ° C. of a cast material melted in the air by induction, followed by a tensile test. In the steel of the present invention, the strength and elongation are improved simultaneously by strengthening the interstitial solid solution by adding high nitrogen and fixing the grain boundaries with fine Ba and rare earth oxides or oxysulfides of 5 μm or less as well as the strength. It can be seen that the mechanical properties are superior to those of the comparative steel.

Example 11 Properties of Hot Rolled Products Table 5 shows critical pitting corrosion temperature, mechanical properties and hot workability for hot rolled sheet material after melt casting in vacuum and nitrogen atmosphere. . It can be seen that the mechanical properties of the hot-rolled product having a healthy microstructure are improved by 10% or more compared to the invention steel melt-cast in the atmosphere, and the corrosion resistance is the same.

  It was confirmed that the hot workability was good with less cracking at the edge during hot rolling compared to the comparative material.

INDUSTRIAL APPLICABILITY The present invention allows diffusion and precipitation of brittle intermetallic compounds by solidifying Ba, Y, Ce, La, Nd, Pr, Ta, Zr, Ti, etc. having large atomic radii in an appropriate amount in an atomic state. By slowing down the speed, the fine RE complex compound or Ba oxide further inhibits the diffusion of Cr, Mo, Si, W, thereby reducing the precipitation rate of intermetallic compounds and reducing the amount of precipitation. Prevention and corrosion resistance can be improved.

In addition, the present invention adversely affects various properties of steel by adding appropriate MM and / or Y while performing appropriate preliminary deoxidation by a usual method using Ti, Mg, Ca, Al and Ca + Al. Prevents single formation of Al ₂ O ₃ and MnS non-metallic inclusions. For this purpose, the solubility equation [MM and / or Y + Al] · [O + S] = 0.001 × 10 ^{−5 to} 30,000 × 10 ⁻⁵ [%] ² found by the present inventors is used. When intentionally forming RexOy or (RE, Al) xOy + RExOyS + RExSy complex compound with a diameter of 5 μm or less in molten steel to form dendritic crystals during solidification, a heterogeneous nucleation site is supplied to form a solidified structure. By controlling the segregation of solute elements such as Cr, Mo, W, Ni, Mn, and Si at the same time as miniaturization and densification, mechanical characteristics, physical characteristics, and corrosion resistance can be improved.

Therefore, the present invention remarkably suppresses the formation of intermetallic compounds including a sigma phase in the duplex stainless steel by adding a new alloy element that has not been recognized in the prior art. It becomes possible to provide a manufacturing method capable of improving the yield.

Furthermore, the present invention greatly reduces the precipitation rate of such an intermetallic compound containing a sigma phase to improve embrittlement resistance and significantly reduce the occurrence of cracks, thereby reducing the occurrence of cracking and hot working. The yield can be greatly improved.

  In addition, by significantly suppressing the precipitation of σ phase and χ phase, etc., which greatly reduces the corrosion resistance and mechanical properties in the cast state, in the future, when joining equipment parts in various application fields described above, By controlling these precipitated phases in the affected area, the corrosion resistance and mechanical properties can be greatly improved, and the durability of the equipment can be further improved.

FIG. 1A is a photograph of the microstructure of invention steel 4 that was aged at 850 ° C. for 30 minutes. FIG. 1B is a photograph of the microstructure of invention steel 10 that has been aged at 850 ° C. for 30 minutes. FIG. 1C is a photograph of the microstructure of invention steel 36 that was aged at 850 ° C. for 30 minutes. FIG. 1D is a photograph of the microstructure of comparative steel 47 that was aged at 850 ° C. for 30 minutes. FIG. 1E is a photograph of the microstructure of a conventional steel UR52N + that has been aged at 850 ° C. for 30 minutes. FIG. 1F is a photograph of the microstructure of a conventional steel SAF2507 that has been aged at 850 ° C. for 30 minutes. FIG. 2A is a diagram showing an X-ray diffraction test result of Invention Steel 4 that has been subjected to an aging heat treatment at 850 ° C. for 30 minutes. FIG. 2B is a diagram showing an X-ray diffraction test result of comparative steel 47 that has been subjected to an aging heat treatment at 850 ° C. for 30 minutes. FIG. 2C is a diagram showing an X-ray diffraction test result of a conventional steel UR52N + that has been subjected to an aging heat treatment at 850 ° C. for 30 minutes. FIG. 2D is a diagram showing an X-ray diffraction test result of a conventional steel SAF2507 that has been subjected to an aging heat treatment at 850 ° C. for 30 minutes. FIG. 3A is a photograph showing the macro structure of the central part of the ingot (φ110 mm × L550 mm) of Invention Steel 10. FIG. 3B is a photograph showing the macro structure of the central portion of the ingot (φ110 mm × L550 mm) of the comparative steel 47. FIG. 3C is a photograph showing the microstructure of the central part of the ingot (φ110 mm × L550 mm) of Invention Steel 10. FIG. 3D is a photograph showing the microstructure of the central part of the ingot (φ110 mm × L550 mm) of the comparative steel 47. FIG. 4 is a diagram comparing the results of an anodic polarization characteristic test in a degassed 0.5 ° HCl + 1.0N NaCl solution of the present invention steel and a conventional steel in a cast state. FIG. 5 is a diagram showing a comparison of the critical pitting temperature test results of the steel of the present invention and the conventional steel in a 6% FeCl ₃ solution. FIG. 6A is a graph showing the results of an anodic polarization property test in a degassed 0.5 ° HCl + 1.0N NaCl solution at 70 ° C. of the steel of the present invention that has been heat-treated at 1130 ° C. FIG. FIG. 6B is a graph showing the results of an anodic polarization characteristic test in a degassed 70 ° C. 0.5N HCl + 1.0N NaCl solution of a conventional super duplex stainless steel subjected to solidification heat treatment at 1130 ° C. FIG. FIG. 6C is a graph showing the results of an anodic polarization property test in a degassed 70 ° C. 0.5N HCl + 1.0N NaCl solution of a conventional superaustenitic stainless steel subjected to a solidification heat treatment at 1130 ° C. FIG. FIG. 7A is a graph showing the results of an anodic polarization property test in a degassed 50 ° C. 0.5N HCl + 1.0N NaCl solution of the steel of the present invention that has been aged at 850 ° C. for 10 minutes. FIG. 7B is a graph showing the results of an anodic polarization property test in a degassed 50 ° C. 0.5N HCl + 1.0N NaCl solution of a conventional super duplex stainless steel that has been aged at 850 ° C. for 10 minutes. FIG. 8A is a graph showing the results of an anodic polarization property test in a degassed 50 ° C. 0.5N HCl + 1.0N NaCl solution of the steel of the present invention that has been aged at 850 ° C. for 30 minutes. FIG. 8B is a graph showing the results of an anodic polarization property test in a degassed 50 ° C. 0.5 N HCl + 1.0 N NaCl solution of a conventional super duplex stainless steel that has been heat-treated at 850 ° C. for 30 minutes.

Claims (10)

By weight, Cr: 21.0% to 38.0%, Ni: 3.0% to 12.0%, Mo: 1.5% to 6.5%, W: 0 to 6.5%, Si : 3.0% or less, Al: 1.0% or less, Mn: 8.0% or less, N: 0.2% to 0.7%, C: 0.1% or less, Ba: 0.0001 to 0 .6%, and B: 0.1% or less, Cu: 3.0% or less, Co: 3.0% or less, and the remainder is composed of iron and inevitable impurities,
The pitting corrosion resistance equivalent index PREW defined by the following formula (1) satisfies 40 ≦ PREW ≦ 67, and is excellent in corrosion resistance, brittleness resistance, castability and hot workability with suppressed formation of intermetallic compounds. Super duplex stainless steel.
PREW = wt% Cr + 3.3 (wt% Mo + 0.5 wt% W) +30 wt% N -------- (1) The steel further contains MM and / or Y in a total amount of 0.0001 to 1.0%, where the MM is La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, The total amount of Ho, Er, Tm, Yb, Lu and Sc,
The value of the relational expression [MM and / or Y + Al] · [O + S] of the solubility product of MM and / or Y and Al, O and S in steel is 0.001 × 10 ^{−5 to} 30,000 × 10. ^-5 [%] is in the ^second range, the corrosion resistance of the formation of intermetallic compound of claim 1 is suppressed, embrittlement-resistant, castability and hot workability excellent super duplex stainless steel.   The super excellent in corrosion resistance, embrittlement resistance, castability and hot workability in which the formation of the intermetallic compound according to claim 2 is suppressed, wherein the Ba is in the range of 0.001 to 0.2%. Duplex stainless steel. The relational value of the solubility product is in the range of 1 × 10 ^{−5 to} 5,000 × 10 ⁻⁵ [%] ² in the case of a cast product, and the formation of the intermetallic compound according to claim 2 or 3. Super duplex stainless steel with excellent corrosion resistance, brittleness resistance, castability and hot workability. The relation values of solubility product is, in the case of hot working product, the range of ^{0.1 × 10 -5 ~2,000 × 10 -5} [%] 2, between a metal according to claim 2 or 3 Super duplex stainless steel excellent in corrosion resistance, embrittlement resistance, castability and hot workability with suppressed compound formation. The total amount of the MM and / or Y is 0.01% to 0.6%, and the corrosion resistance, brittleness resistance, castability, and hotness in which formation of the intermetallic compound according to claim 2 or 3 is suppressed. Super duplex stainless steel with excellent workability. The total amount of the MM and / or Y is 0.2% to 0.5%, corrosion resistance, brittleness resistance, castability, and hot workability with suppressed formation of intermetallic compounds according to claim 6 . Excellent super duplex stainless steel. The steel is further Ca: 0.5% or less, Mg: 0.5% or less, Ta: 0.5% or less, Nb: 0.5% or less, Ti: 1.5% or less, Zr: 1.0 % or less, Sn: 1.0% or less, and in: containing one or more elements selected from the group consisting of 1.0% or less, according to claim 1 to 3, 6, 7 any one of Super duplex stainless steel with excellent corrosion resistance, embrittlement resistance, castability and hot workability with reduced formation of intermetallic compounds. [PREW (γ) −PREW (α)] value (where PREW (γ) and PREW (α) are the corrosion resistance balance of the austenite phase and the ferrite phase, respectively) say.) is in the range of -5～10 claim 1-3, 6, 7 or corrosion formation of intermetallic compounds according to one item is suppressed, the embrittlement-resistant, castability and hot Super duplex stainless steel with excellent hot workability. The volume fraction of the ferrite phase in the set positions of the steel, 20 to 70% by volume%, volume fraction of the austenite phase is 30 to 80% by volume%, claim 1-3, 6, A super duplex stainless steel excellent in corrosion resistance, embrittlement resistance, castability and hot workability in which the formation of the intermetallic compound according to any one of 7 is suppressed.

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