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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite

Definitions

• the present invention relates to a duplex stainless steel.
• 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.
• 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).
• PRE pitting resistance equivalent
• 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.
• PREs have recently been a demand for steel materials with PREs of 40 or more, for enhancing corrosion resistance.
• 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 ⁇ phase, are likely to precipitate.
• intermetallic compounds which decrease mechanical properties and corrosion resistance, such as a ⁇ phase
• the starting material is significantly hardened, and cracking is likely to occur, which extremely decreases hot workability.
• 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 ⁇ phase and a X (chi) phase, which are brittle phases, in producing the high-corrosion-resistance duplex stainless steel.
• 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.
• Ni causes the concentration of Cr and Mo in ferritic phases, promoting the precipitation of ⁇ phases.
• a cast piece made of a steel containing a large number of ⁇ 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.
• the present inventors conducted diligent studies to solve the problems described above and consequently obtained the following findings.
• the present invention is made based on such findings, and a gist of the present invention is the following duplex stainless steel.
• 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.
• C is an element that is dissolved in austenite phases to increase strength.
• C contained in a large amount causes the precipitation of carbides, decreasing corrosion resistance.
• 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.
• the content of C is preferably 0.010% or more, and more preferably 0.015% or more.
• 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 manganese
• Mn contained in a large amount decreases corrosion resistance.
• 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.
• the content of Mn is preferably 0.20% or more, and more preferably 0.40% or more.
• P phosphorus
• P 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 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 chromium
• Cr is an element that is essential in keeping corrosion resistance.
• Cr is a ferrite stabilizing element.
• 20.0% or more of Cr needs to be contained with consideration given to phase fractions.
• Cr contained in a large amount rather leads to a decrease in corrosion resistance.
• 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 is an austenite stabilizing element.
• Ni has an effect of enhancing corrosion resistance.
• the content of Ni is set to 3.00% or more.
• 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.
• 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 molybdenum
• Mo 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 copper
• Cu is an element that is highly useful in enhancing resistance to sulfuric acid.
• 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.
• the content of Cu is preferably 0.10% or more, and preferably 0.20% or more.
• N nitrogen
• 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 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 ⁇ Al 2 O 3 ), 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.
• 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 tungsten
• W 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 ⁇ 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 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 niobium
• Nb 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 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 vanadium
• V vanadium
• 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 zirconium
• Zr zirconium
• Zr 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 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 (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 (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 manganesium
• 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 rare earth metal
• REM 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.
• 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.
• the lanthanoids are added in the form of misch metal.
• the balance is Fe and impurities.
• 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 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.
• Formula (i) shown below.
• 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 and Nieq are defined as Formulas (ii) and (iii) shown below, respectively.
• the value of Creq/Nieq is preferably 2.400 or more.
• 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.
• the solidification in the F mode in which a ferrite single phase is solidified, 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.
• 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 ⁇ phase are likely to be formed.
• 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 ⁇ X i ⁇ Md i where the meanings of symbols in Formula (iv) above are as follows.
• 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.
• 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.
• an area fraction of the ⁇ phase contained in its metal micro-structure is 2.0% or less.
• a large value of PRE and additionally a high content of Ni promote the precipitation of ⁇ phases.
• the area fraction of ⁇ phases is set to 2.0% or less.
• the area fraction of ⁇ 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 ⁇ phases, the more preferable it is. It is therefore not necessary to put a lower limit on the area fraction.
• 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 ⁇ phases, Cr 2 N and the like. 2.0% or less of Cr 2 N and the like in total is tolerable.
• area fractions of ferrite and austenite are measured in conformity to JIS Z 3119:2017 with a ferrite meter.
• 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.
• 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 ⁇ phases.
• 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 ⁇ phase area fraction. Note that ⁇ 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 ⁇ phase area fraction.
• 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.
• 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 ⁇ phases.
• 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.
• 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.
• 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 ⁇ 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 ⁇ phases.
• 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 ⁇ 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.
• 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 ⁇ phases to 2.0% or less.
• 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.
• 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.
• the hot-rolling step is preferably performed under the following conditions.
• a temperature and a duration of the heating mean an average temperature in a furnace and an in-furnace time, respectively.
• a finish rolling temperature is preferably 900 to 1110°C.
• 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.
• air cooling may be performed.
• 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.
• 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.
• 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 ⁇ phases.
• 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.
• 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 ⁇ phases.
• 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 ⁇ phase area fraction. An average value of measurement values from the five samples was taken as the ⁇ phase area fraction.
• 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/cm 2 or more were rated as good, and the impact values being less than 30.0 J/cm 2 were rated as poor.
• 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.
• 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.
• 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.
• 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 ⁇ phases.
• 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 ⁇ phase area fraction. An average value of measurement values from the five samples was taken as the ⁇ phase area fraction.
• 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 ⁇ phases in their hot-rolled materials were successfully reduced to 2.0% or less.
• 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.

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