Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/16—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of other metals or alloys based thereon
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment of ferrous alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C19/00—Alloys based on nickel or cobalt
  • C22C19/03—Alloys based on nickel or cobalt based on nickel
  • C22C19/05—Alloys based on nickel or cobalt based on nickel with chromium
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D7/00—Modifying the physical properties of iron or steel by deformation
  • C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C19/00—Alloys based on nickel or cobalt
  • C22C19/03—Alloys based on nickel or cobalt based on nickel
  • C22C19/05—Alloys based on nickel or cobalt based on nickel with chromium
  • C22C19/051—Alloys based on nickel or cobalt based on nickel with chromium and Mo or W
  • C22C19/055—Alloys based on nickel or cobalt based on nickel with chromium and Mo or W with the maximum Cr content being at least 20% but less than 30%
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C30/00—Alloys containing less than 50% by weight of each constituent
  • C22C30/02—Alloys containing less than 50% by weight of each constituent containing copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/10—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of nickel or cobalt or alloys based thereon

Definitions

• the present invention relates to an austenitic heat resistant alloy and a method for producing the same.
• 18-8 austenitic stainless steels such as SUS304H, SUS316H, SUS321H, and SUS347H, have been used as materials for apparatuses.
• Patent Documents 1 to 4 disclose austenitic steel excellent in high temperature strength and corrosion resistance.
• Patent Document 5 discloses austenitic stainless steel excellent in high temperature strength and corrosion resistance. According to Patent Documents 1 to 5, the amount of Cr is increased to 20% or more, and W and/or Mo are contained so as to enhance high temperature strength.
• Patent Document 1 JP61-179833A
• Patent Document 2 JP61-179834A
• Patent Document 3 JP61-179835A
• Patent Document 4 JP61-179836A
• Patent Document 5 JP2004-3000A
• the present invention has been made to overcome the above problems, and an objective of the present invention is to provide an austenitic heat resistant alloy and a method for producing the same which exhibits sufficient 0.2% proof stress and tensile strength at a normal temperature, and sufficient creep rupture strength at a high temperature in large-sized structural members.
• the present invention has been made to overcome the above problems, and the gist of the present invention is the following austenitic heat resistant alloy and method for producing the same.
• An austenitic heat resistant alloy having a chemical composition consisting of, in mass %:
• Al 0.30% or less
• a shortest distance from a center portion to an outer surface portion of a cross section of the alloy is 40 mm or more, the cross section being perpendicular to a longitudinal direction of the alloy,
• an austenite grain size number at the outer surface portion is ⁇ 2.0 to 4.0
• TS S tensile strength at outer surface portion.
• a method for producing an austenitic heat resistant alloy including the steps of:
• symbol “D” denotes a maximum value (mm) of a linear distance between an arbitrary point on an outer edge of a cross section of the alloy and another arbitrary point on the outer edge, the cross section being perpendicular to a longitudinal direction of the alloy.
• the working is performed one or more times in a direction substantially perpendicular to the longitudinal direction.
• the austenitic heat resistant alloy of the present invention has small variation in mechanical properties from region to region, and is excellent in creep rupture strength at a high temperature.
• C forms carbides so that C is an indispensable element for maintaining high temperature tensile strength and creep rupture strength required for an austenitic heat resistant alloy. Accordingly, it is necessary to set a content of C to 0.02% or more. However, when the C content exceeds 0.12%, not only undissolved carbides are formed, but also Cr carbides increase and hence, mechanical properties, such as ductility and toughness, and weldability deteriorate. Accordingly, the C content is set to a value ranging from 0.02 to 0.12%. The C content is preferably 0.05% or more and 0.10% or less.
• Si silicon is contained as a deoxidizing element. Further, Si is an element effective in increasing oxidation resistance, steam oxidation resistance and the like. Si is also an element which facilitates the flow of a casting material.
• a content of Si exceeds 2.0%, the formation of intermetallic compounds, such as a ⁇ phase, is promoted and hence, stability of micro-structure at a high temperature deteriorates, thus lowering toughness and ductility.
• the Si content exceeds 2.0%, weldability is also lowered. Accordingly, the Si content is set to 2.0% or less. When importance is placed on structural stability, the Si content is preferably set to 1.0% or less.
• the Si content is preferably set to 0.05% or more, and more preferably set to 0.10% or more.
• Mn manganese
• the Mn content is set to 3.0% or less.
• the Mn content is preferably 2.0% or less, and more preferably 1.5% or less. It is not necessary to set the lower limit of the Mn content.
• the Mn content is preferably set to 0.10% or more, and more preferably set to 0.20% or more.
• a content of P is set to 0.030% or less. It is preferable to reduce the P content to as much as possible.
• the P content is preferably set to 0.020% or less, and more preferably set to 0.015% or less.
• S sulfur
• a content of S is set to 0.015% or less.
• the S content is preferably set to 0.010% or less, more preferably set to 0.005% or less, and further preferably set to 0.003% or less.
• Cr chromium
• the Cr content is set to a value ranging of 20.0% or more and less than 28.0%.
• the Cr content is preferably 21.0% or more, and more preferably 22.0% or more.
• the Cr content is preferably 26.0% or less, and more preferably 25.0% or less.
• Ni nickel
• Ni nickel
• the Ni content is preferably 40.0% or more, and more preferably 42.0% or more. Further, the Ni content is preferably 50.0% or less, and more preferably 48.0% or less.
• Co cobalt
• Co makes the austenitic structure stable, and also contributes to enhancing creep rupture strength. Accordingly, Co may be contained in lieu of a part of Ni.
• the Co content is set to a value ranging from 0 to 20.0%.
• the Co content is preferably 15.0% or less.
• the Co content is preferably set to 0.5% or more.
• W tungsten
• W is dissolved in a matrix, thus not only contributing to enhancing creep rupture strength as a solid-solution strengthening element, but also precipitating as a Fe 2 W Laves phase or a Fe 7 W 6 ⁇ phase so that creep rupture strength is significantly enhanced. Accordingly, W is an important element.
• the W content is set to a value ranging from 4.0 to 10.0%.
• the W content is preferably 5.0% or more, and more preferably 5.5% or more. Further, the W content is preferably 9.0% or less, and more preferably 8.5% or less.
• Ti titanium is an element which forms carbo-nitrides, thus having an effect of enhancing creep rupture strength.
• a content of Ti is less than 0.01%, sufficient effects cannot be obtained.
• the Ti content exceeds 0.50%, ductility at a high temperature is lowered. Accordingly, the Ti content is set to a value ranging from 0.01 to 0.50%.
• the Ti content is preferably set to 0.05% or more, and more preferably set to 0.10% or more. Further, the Ti content is preferably set to 0.40% or less, and more preferably set to 0.35% or less.
• Nb niobium
• Nb has an action of forming carbo-nitrides, thus enhancing creep rupture strength.
• a content of Nb is less than 0.01%, sufficient effects cannot be obtained.
• the Nb content exceeds 1.0%, ductility at a high temperature is lowered. Accordingly, the Nb content is set to a value ranging from 0.01 to 1.0%.
• the Nb content is preferably 0.10% or more. Further, the Nb content is preferably 0.90% or less, and more preferably 0.70% or less.
• Mo molybdenum
• Mo molybdenum
• W molybdenum
• the inventors of the present invention have made studies, and found the following.
• Mo is contained in combination in an alloy which contains the amounts of W and Cr, a ⁇ phase may precipitate after long-term use and hence, creep rupture strength, ductility and toughness may be lowered. Accordingly, it is desirable to reduce a content of Mo as much as possible, and the Mo content is set to less than 0.50%. It is preferable to limit the Mo content to less than 0.20%.
• Cu copper
• Cu lowers a fusing point, thus lowering hot workability and weldability. Accordingly, it is desirable to reduce a content of Cu as much as possible, and the Cu content is set to less than 0.50%. It is preferable to limit the Cu content to less than 0.20%.
• Al is an element which is contained as a deoxidizer for molten steel.
• the Al content is set to 0.30% or less.
• the Al content is preferably 0.25% or less, and more preferably 0.20% or less.
• the Al content is preferably set to 0.01% or more, and more preferably set to 0.02% or more.
• N nitrogen
• nitrogen is an element having an action of making the austenitic structure stable, and is an element inevitably contained when an ordinary melting method is adopted.
• Ti is contained as an indispensable element
• the N content is set to less than 0.10%.
• the balance consists of Fe and impurities. It is preferable to set a content of Fe to 0.1 to 40.0%.
• impurity means a component which is mixed in industrially producing the alloy due to various causes, such as raw materials including ores or scrap, or production steps, and which is allowed to be mixed without adversely affecting the present invention.
• the austenitic heat resistant alloy of the present invention may further contain one or more kinds selected from a group consisting of Mg, Ca, REM, V, B, Zr, Hf, Ta, and Re.
• Any of Mg, Ca or REM has an action of fixing S as sulfides to enhance high temperature ductility. Accordingly, when it is desired to obtain greater high temperature ductility, one or more kinds of these elements may be positively contained within the following range.
• Mg magnesium
• Mg has an action of fixing S, which inhibits ductility at a high temperature, as sulfides, thus improving high temperature ductility. Accordingly, Mg may be contained so as to obtain this advantageous effect.
• the amount of Mg is set to 0.05% or less.
• the Mg content is more preferably set to 0.02% or less, and further preferably set to 0.01% or less.
• the Mg content is preferably set to 0.0005% or more, and more preferably set to 0.001% or more.
• Ca (calcium) has an action of fixing S, which inhibits ductility at a high temperature, as sulfides, thus improving high temperature ductility. Accordingly, Ca may be contained so as to obtain this advantageous effect. However, when a content of Ca exceeds 0.05%, cleanliness is lowered, and high temperature ductility is impaired on the contrary. Accordingly, when Ca is contained, the amount of Ca is set to 0.05% or less.
• the Ca content is more preferably set to 0.02% or less, and further preferably set to 0.01% or less.
• the Ca content is preferably set to 0.0005% or more, and more preferably set to 0.001% or more.
• REM has an action of fixing S as sulfides, thus improving high temperature ductility.
• REM also has an action of improving adhesiveness of a Cr 2 O 3 protection film on a steel surface, thus improving oxidation resistance particularly when the alloy is repeatedly oxidized.
• REM contributes to strengthening grain boundaries, thus having an action of enhancing creep rupture strength and creep rupture ductility.
• the amount of REM is set to 0.50% or less.
• the REM content is more preferably set to 0.30% or less, and further preferably set to 0.15% or less.
• the REM content is preferably set to 0.0005% or more, more preferably set to 0.001% or more, and further preferably set to 0.002% or more.
• REM indicates 17 elements in total, including Sc, Y, and the lanthanoids.
• the REM content means the total content of these elements.
• the total content of Mg, Ca and REM may be 0.6% or less. However, the total content is more preferably 0.4% or less, and further preferably 0.2% or less.
• the alloy may positively contain one or more kinds of these elements within the following range.
• V vanadium
• V has an action of forming carbo-nitrides to enhance high temperature strength and creep rupture strength. Accordingly, V may be contained so as to obtain these advantageous effects.
• a content of V exceeds 1.5%, high temperature corrosion resistance is lowered and, further, ductility and toughness deteriorate due to the precipitation of a brittle phase. Accordingly, when V is contained, the amount of V is set to 1.5% or less.
• the V content is more preferably set to 1.0% or less.
• the V content is preferably set to 0.02% or more, and more preferably set to 0.04% or more.
• B (boron) is present in carbide or in a matrix.
• B has not only an action of promoting micronization of precipitated carbide, but also an action of strengthening grain boundaries, thus enhancing creep rupture strength.
• a content of B exceeds 0.01%, ductility at a high temperature is lowered, and a fusing point is also lowered.
• the amount of B is set to 0.01% or less.
• the B content is more preferably 0.008% or less, and further preferably 0.006% or less.
• the B content is preferably set to 0.0005% or more, more preferably set to 0.001% or more, and further preferably set to 0.0015% or more.
• Zr zirconium
• Zr zirconium
• the Zr content is more preferably 0.06% or less, and further preferably 0.05% or less.
• the Zr content is preferably set to 0.005% or more, and more preferably set to 0.01% or more.
• Hf (hafnium) has an action of contributing to strengthening precipitation as carbo-nitrides, thus enhancing creep rupture strength. Accordingly, Hf may be contained so as to obtain these advantageous effects. However, when a content of Hf exceeds 1.0%, workability and weldability are impaired. Accordingly, when Hf is contained, the amount of Hf is set to 1.0% or less.
• the Hf content is more preferably set to 0.8% or less, and further preferably set to 0.5% or less.
• the Hf content is preferably set to 0.005% or more, more preferably set to 0.01% or more, and further preferably set to 0.02% or more.
• the total content of V, B, Zr, and Hf is preferably 2.6% or less, and more preferably 1.8% or less.
• Either one of Ta or Re dissolves in austenite forming a matrix, thus having an action of solid-solution strengthening. Accordingly, when it is desired to obtain greater high temperature strength and creep rupture strength due to an action of solid-solution strengthening, one or both of these elements may be positively contained within the following range.
• Ta has an action of forming carbo-nitrides, and also has an action of enhancing high temperature strength and creep rupture strength as a solid-solution strengthening element. Accordingly, Ta may be contained so as to obtain these advantageous effects. However, when a content of Ta exceeds 8.0%, workability and mechanical properties are impaired. Accordingly, when Ta is contained, the amount of Ta is set to 8.0% or less. The Ta content is more preferably set to 7.0% or less, and further preferably set to 6.0% or less. On the other hand, to obtain the advantageous effects with certainty, the Ta content is preferably set to 0.01% or more, more preferably set to 0.1% or more, and further preferably set to 0.5% or more.
• Re rhenium
• Re has an action of enhancing high temperature strength and creep rupture strength mainly as a solid-solution strengthening element. Accordingly, Re may be contained so as to obtain these advantageous effects.
• a content of Re exceeds 8.0%, workability and mechanical properties are impaired. Accordingly, when Re is contained, the amount of Re is set to 8.0% or less.
• the Re content is more preferably set to 7.0% or less, and further preferably set to 6.0%.
• the Re content is preferably set to 0.01% or more, more preferably set to 0.1% or more, and further preferably set to 0.5% or more.
• the total content of Ta and Re is preferably 14.0% or less, and more preferably 12.0% or less.
• Austenite grain size number at outer surface portion ⁇ 2.0 to 4.0
• the austenite grain size number at the outer surface portion is set to a value ranging from ⁇ 2.0 to 4.0.
• a heat-treatment temperature and holding time after hot working and a cooling method it is possible to set the grain size number at the outer surface portion to a value which falls within the range after final heat treatment.
• the austenitic heat resistant alloy according to the present invention exhibits sufficient 0.2% proof stress and tensile strength at a normal temperature, and sufficient creep rupture strength at a high temperature in large-sized structural members. That is, the present invention can obtain remarkable advantageous effects in members having a thick wall.
• the shortest distance from the center portion to the outer surface portion of a cross section is set to 40 mm or more, the cross section being perpendicular to a longitudinal direction.
• the shortest distance from the center portion to the outer surface portion is preferably 80 mm or more, and more preferably 100 mm or more.
• the shortest distance from the center portion to the outer surface portion refers to a radius (mm) of a cross section when an alloy has a columnar shape
• the shortest distance refers to a half-length (mm) of the short side of a cross section when an alloy has a quadrangular prism shape, for example.
• the heat resistant alloy according to the present invention is obtained by performing hot working, such as hot forging or hot rolling on an ingot, or a cast piece, obtained by continuous casting or the like, for example.
• hot working such as hot forging or hot rolling on an ingot, or a cast piece, obtained by continuous casting or the like, for example.
• the longitudinal direction of a heat resistant alloy substantially refers to a direction along which a top portion and a bottom portion of the ingot are connected.
• the longitudinal direction of a heat resistant alloy substantially refers to the longitudinal direction of the cast piece.
• test coupons for measuring Cr precipitates are obtained from the center portion and the outer surface portion of the cross section of an alloy specimen, the cross section being perpendicular to the longitudinal direction of the alloy specimen.
• the surface area of each test coupon is obtained and, thereafter, only the base metal of the alloy specimen is completely electrolyzed in a 10% acetylacetone—1% tetramethyl ammonium chloride—methanol solution under an electrolysis condition of 20 mA/cm 2 .
• the solution after electrolysis is performed is filtered through a 0.2 ⁇ m filter to extract precipitates as a residue.
• the extracted residue is decomposed with an acid, and is analyzed using an inductively coupled plasma emission spectrophotometer (ICP-AES) to measure a content (mass %) of Cr contained as undissolved Cr precipitate, and a value of Cr PB /Cr PS is obtained based on the measured value.
• ICP-AES inductively coupled plasma emission spectrophotometer
• a cooling speed at the time of performing heat treatment varies from region to region and hence, there is a tendency that great variations occur in mechanical properties from region to region due to the difference in the cooling speed. If there is a large difference in 0.2% proof stress and tensile strength at a normal temperature between the center portion and the outer surface portion of the large-sized structural member, there arises a problem that some regions do not satisfy the specifications.
• the austenitic heat resistant alloy of the present invention is used in a high temperature environment, thus being required to be excellent in high temperature strength, particularly, in creep rupture strength. Accordingly, 10,000-hour creep rupture strength at 700° C. in the longitudinal direction is preferably 100 MPa or more at the center portion of the heat resistant alloy of the present invention.
• Creep rupture strength is obtained by the following method. First, round bar creep rupture test coupons, described in JIS Z 2241 (2011), and having a diameter of 6 mm and a gage length of 30 mm, are cut out by mechanical processing from the center portions of the alloys parallel to the longitudinal direction. Then, a creep rupture test is performed in the atmosphere of 700° C., 750° C., and 800° C. to obtain 10,000-hour creep rupture strength at 700° C. by a Larson-Miller parameter method. The creep rupture test is performed in accordance with JIS Z 2271 (2010).
• the austenitic heat resistant alloy of the present invention can be produced by performing hot working on an ingot or a cast piece having the above chemical composition.
• processing is performed such that the longitudinal direction of the alloy in the final shape aligns with the longitudinal direction of the ingot or the cast piece forming a starting material.
• Hot working may be performed only in the longitudinal direction. However, to obtain a more uniform micro-structure at a higher working ratio, hot working may be performed one or more times in a direction substantially perpendicular to the longitudinal direction. After the hot working is performed, hot working of another method, such as hot extrusion, may be further performed when necessary.
• the alloy on which hot working was performed is heated to a heat-treatment temperature T (° C.) ranging from 1100 to 1250° C., and is held for 1000 D/T to 1400 D/T (min) within such a range.
• T heat-treatment temperature
• symbol “D” denotes the diameter (mm) of the alloy when the alloy has a columnar shape
• “D” denotes a diagonal distance (mm) when the alloy has a quadrangular prism shape, for example. That is, symbol “D” denotes the maximum value (mm) of a linear distance between an arbitrary point on the outer edge of the cross section of the alloy and another arbitrary point on the outer edge, the cross section being perpendicular to a longitudinal direction of the alloy.
• the heat-treatment temperature When the heat-treatment temperature is less than 1100° C., the amount of undissolved chromium carbide or the like increases, thus lowering creep rupture strength. On the other hand, when the heat-treatment temperature exceeds 1250° C., grain boundaries are dissolved or grains are remarkably coarsened so that ductility is lowered. Accordingly, it is more desirable to set the heat-treatment temperature to 1150° C. or above and 1230° C. or below. Further, when the holding time is less than 1000 D/T (min), undissolved chromium carbide at the center portion increases so that Cr PB /Cr PS falls outside a range defined by the present invention. On the other hand, when the holding time exceeds 1400 D/T (min), grain at the outer surface portion is coarsened so that the austenite grain size number falls outside the range defined by the present invention.
• the alloy is cooled with water. This is because when a cooling speed becomes lower, particularly at the center portion of the alloy, a large amount of undissolved Cr precipitates is generated at crystal grain boundaries or within grains so that there is a possibility that the formula (i) is not satisfied.
• the obtained ingots were processed to have a columnar shape with an outer diameter of 120 to 480 mm by hot forging, and final heat treatment was performed under conditions shown in Table 2 to obtain alloy member specimens. Alloys 1, 2 and 4 were subjected to forging in a direction substantially perpendicular to the longitudinal direction after hot forging in the longitudinal direction and before final heat treatment and, thereafter, final hot forging was further performed in the longitudinal direction.
• a test coupon for observing micro-structure was obtained from the outer surface portion of each specimen, and the cross section in the longitudinal direction was polished with emery paper and a buff. Thereafter, the test coupon was etched with a mixed acid, and optical microscopic observation was performed.
• the grain size number on an observation surface was obtained in accordance with a determination method defined by JIS G 0551 (2013) where the grain size number is determined based on crossing line segments (grain size).
• test coupons for measuring the amount of Cr precipitates were obtained from the center portion and the outer surface portion of the cross section of each specimen, the cross section being perpendicular to the longitudinal direction of the specimen.
• the surface area of each test coupon was obtained and, thereafter, only the base metal of the alloy specimen was completely electrolyzed in a 10% acetylacetone—1% tetramethyl ammonium chloride—methanol solution under an electrolysis condition of 20 mA/cm 2 . Then, the solution after electrolysis was performed was filtered through a 0.2 ⁇ m filter to extract precipitates as a residue.
• extracted residue was decomposed with an acid, and was subjected to ICP-AES measurement to measure a content (mass %) of Cr contained as undissolved Cr precipitate and, then, a value of Cr PB /Cr PS was obtained based on the measured value.
• Tensile test coupons each having a parallel portion with a length of 40 mm, were cut out by mechanical processing from the center portion and the outer surface portion of each specimen parallel to the longitudinal direction, and a tensile test was performed on these test coupons at a room temperature so as to obtain 0.2% proof stress and tensile strength.
• creep rupture test coupon having a parallel portion with a length of 30 mm, was cut out by mechanical processing from the center portion of each specimen parallel to the longitudinal direction. Then, a creep rupture test was performed in the atmosphere of 700° C., 750° C., and 800° C. to obtain 10,000-hour creep rupture strength at 700° C. by a Larson-Miller parameter method.
• the alloy A and the alloy B have substantially the same chemical composition as the alloy 1, and are formed into a final shape same as that of the alloy 1 by hot forging.
• a holding time in heat treatment falls outside the production conditions defined by the present invention. Due to such holding time, the alloy A has the result that the grain size number at the outer surface portion falls outside the range defined by the present invention, and a value of YS S /YS B and a value of TS S /TS B fall outside the range defined by the present invention. Accordingly, the alloy A has a large variation in mechanical characteristics from region to region.
• the alloy B falls outside the range defined by the present invention with respect to creep rupture strength and, as a result, creep rupture strength of the alloy B is remarkably lower than that of the alloy 1.
• Alloys C, D, and E have substantially the same chemical composition as the alloy 2, and are formed into a final shape same as that of the alloy 2 by hot forging.
• the alloy C is lower than the range defined by the present invention with respect to the heat-treatment temperature and hence, the grain size number at the outer surface portion and a value of Cr PB /Cr PS fall outside the ranges defined by the present invention. As a result, creep rupture strength of the alloy C is remarkably lower than that of the alloy 2.
• the alloy D is higher than the range defined by the present invention with respect to a heat-treatment temperature and hence, the grain size number at the outer surface portion and a value of YS S /YS B and a value of TS S /TS B fall outside the range defined by the present invention. As a result, creep rupture strength of the alloy D is remarkably lower than that of the alloy 2.
• the alloy E With regard to the alloy E, a cooling method in final heat treatment was not water cooling but was air cooling and hence, a cooling speed was remarkably low. Accordingly a value of Cr PB /Cr PS falls outside the range defined by the present invention and, as a result, creep rupture strength of the alloy E is remarkably lower than that of the alloy 2. On the other hand, the alloys 1 to 9 which satisfy all specifications of the present invention have small variation in mechanical characteristics, and favorable creep rupture strength.
• the austenitic heat resistant alloy of the present invention has small variation in mechanical properties from region to region, and is excellent in creep rupture strength at a high temperature. Accordingly, the austenitic heat resistant alloy of the present invention is preferably applicable to a large-sized structural member for a thermal power generation boiler, a chemical plant or the like which is used in a high temperature environment.

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• 2017
  • 2017-02-09 EP EP17895819.5A patent/EP3581669A4/en not_active Withdrawn
  • 2017-02-09 JP JP2018566714A patent/JP6816779B2/ja active Active
  • 2017-02-09 KR KR1020197026299A patent/KR20190117598A/ko not_active Application Discontinuation
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