Classifications

• • 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
  • 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
• • 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
• • 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 a Ni-based heat resistant alloy and a method for producing the same.
• a Fe-based alloy such as austenitic stainless steel
• a Ni-based alloy which utilizes the precipitation of ⁇ ' phase or the like.
• welding is inevitably applied to pipes for boilers and chemical industrial plants and hence, the alloy is also required to possess excellent weldability.
• the Patent Document 1 discloses an austenitic heat resistant alloy which is excellent in both of weld crack resistance and toughness in a heat affected zone (HAZ), and is also excellent in creep strength at a high temperature.
• Patent Document 1 JP4697357B
• the present invention has been made to overcome the above problems, and an objective of the present invention is to provide a Ni-based 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 Ni-based heat resistant alloy and method for producing the same.
• the Ni-based 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 carbon
• the austenitic structure makes the austenitic structure stable, and forms fine carbides at grain boundaries, thus enhancing creep strength at a high temperature. Accordingly, it is necessary to set a content of C to 0.005% or more. However, when the C content is excessively increased, carbides are coarsened, and a large amount of carbides precipitates and hence, ductility at the grain boundaries is lowered, thus causing lowering of toughness and creep strength. Accordingly, the C content is set to 0.15% or less.
• the C content is preferably 0.01% or more. Further, the C content is preferably 0.12% or less, and more preferably 0.10% or less.
• Si silicon is contained as a deoxidizing element. Further, Si is an element effective in enhancing corrosion resistance and oxidation resistance at a high temperature. However, when a content of Si exceeds 2.0%, stability of the austenite phase is lowered, thus causing lowering of toughness and creep strength. Accordingly, the Si content is set to 2.0% or less. The Si content is preferably 1.5% or less, and more preferably 1.0% or less. It is not particularly necessary to set the lower limit of the Si content. However, when the Si content is excessively reduced, deoxidation effect cannot be sufficiently obtained, thus deteriorating cleanliness of the alloy, and causing an increase in production cost. Accordingly, the Si content is preferably 0.02% or more, and more preferably 0.10% or more.
• Mn manganese
• Si silicon
• Mn content is set to 3.0% or less.
• the Mn content is preferably 2.5% or less, more preferably 2.0% or less, and further preferably 1.5% or less. It is not necessary to set the lower limit of the Mn content.
• the Mn content is preferably 0.02% or more, more preferably 0.10% or more, and further preferably 0.15% or more.
• P phosphorus
• the alloy is an impurity.
• P is an element which segregates at the crystal grain boundary of the HAZ during welding, thus increasing liquation cracking susceptibility, and adversely affecting toughness after long-term use. For this reason, it is preferable to reduce a content of P as much as possible. However, excessive reduction of the P content causes an increase in steel production cost. Accordingly, the P content is set to 0.030% or less. The P content is preferably 0.020% or less.
• S sulfur
• S is contained in the alloy as an impurity.
• S is an element which segregates at the crystal grain boundary of the HAZ during welding, thus increasing liquation cracking susceptibility, and adversely affecting toughness after long-term use. For this reason, it is preferable to reduce a content of S as much as possible.
• the S content is set to 0.010% or less.
• the S content is preferably 0.005% or less.
• N nitrogen
• a content of N is set to 0.030% or less.
• the N content is preferably 0.020% or less, and more preferably 0.015% or less. It is not particularly necessary to set the lower limit of the N content.
• excessive reduction of the N content causes an increase in production cost. Accordingly, the N content is preferably 0.0005% or more, more preferably 0.001% or more, and further preferably 0.005% or more.
• O oxygen
• a content of O is set to 0.030% or less.
• the O content is preferably 0.020% or less, more preferably 0.010% or less, and further preferably 0.005% or less. It is not particularly necessary to set the lower limit of the O content. However, excessive lowering of the O content causes an increase in production cost. Accordingly, the O content is preferably 0.001% or more.
• Ni nickel
• Ni is an element effective in obtaining the austenitic structure, and is an indispensable element for ensuring structural stability after long-term use. Further, Ni is bonded to Al, Ti, and Nb, thus forming the fine intermetallic compound phase and hence, Ni also has an action of increasing creep strength.
• Ni is an expensive element and hence, if the Ni content exceeds 60.0%, cost is increased. Accordingly, the Ni content is set to 40.0 to 60.0%.
• the Ni content is preferably 42.0% or more, more preferably 45.0% or more, and further preferably 48.0% or more, and the Ni content is preferably 58.0% or less.
• Co is an austenite former in the same manner as Ni. Co increases stability of the austenite phase, thus contributing to enhancing creep strength. To obtain such an advantageous effect, it is necessary to set a content of Co to 0.01% or more. However, Co is an extremely expensive element and hence, if the Co content exceeds 25.0%, cost is significantly increased. Accordingly, the Co content is set to 0.01 to 25.0%.
• the Co content is preferably 0.1% or more, more preferably 2.0% or more, and further preferably 8.0% or more. Further, the Co content is preferably 23.0% or less, and more preferably 21.0% or less.
• Cr chromium
• Cr is an indispensable element for ensuring oxidation resistance and corrosion resistance at a high temperature.
• the Cr content is set to 15.0% or more to less than 28.0%.
• the Cr content is preferably 17.0% or more, and more preferably 19.0% or more.
• the Cr content is preferably 26.0% or less, and more preferably 24.0% or less.
• Mo molybdenum
• W tungsten
• the atomic weight of W is larger than the atomic weight of Mo. Accordingly, to obtain substantially the same advantageous effect as Mo, W is required to be contained with an amount larger than that of Mo and hence, W is disadvantageous in terms of cost and ensuring stability of the phase. Accordingly, the W content is set to less than 4.0%. Mo and W are not required to be contained in combination. When Mo or W is contained in a single form, either one of the Mo content or the W content is preferably 0.1% or more.
• B (boron) segregates at grain boundaries during use, thus strengthening the grain boundaries, and causing carbide at the grain boundaries to be finely dispersed and hence, creep strength is enhanced. Accordingly, B is an element necessary for enhancing creep strength. In addition to the above, B segregates at the grain boundaries, thus enhancing a sticking force and hence, B also has an effect of contributing to the improvement of toughness. To obtain these advantageous effects, it is necessary to set a content of B to 0.0005% or more.
• the B content is set to 0.0005 to 0.006%.
• the B content is preferably 0.001% or more, and preferably 0.005% or less.
• any of Al (aluminum), Ti (titanium) or Nb (niobium) is an element which is bonded to Ni, and finely precipitates as intermetallic compound within grains, thus enhancing creep strength at a high temperature.
• a content of each of Al, Ti, and Nb is set to 3.0% or less.
• the content of each element is preferably 2.8% or less, and more preferably 2.5% or less.
• Rare earth metal has strong affinity to P, and forms compound combined with P which has high fusing point and is stable even at a high temperature. Accordingly, REM has an action of fixing P, thus removing adverse effects of P on liquation cracking and toughness in the HAZ. REM is also an element which precipitates as carbide, thus contributing to enhancing high temperature strength. Accordingly, REM may be contained when necessary. However, when a content of REM is excessively increased, and exceeds 0.1%, in addition to that the effect of reducing adverse effects caused by P is saturated, a large amount of REM precipitates as carbides, thus causing lowering of toughness on the contrary. Accordingly, the REM content is set to 0.1% or less. The REM content is preferably 0.08% or less, and more preferably 0.06% or less. To obtain the advantageous effects, the REM content is preferably 0.001% or more, more preferably 0.005% or more, and further preferably 0.01% or more.
• REM indicates 17 elements in total, including Sc, Y, and the lanthanoids.
• the REM content means the total content of these elements.
• Mg (magnesium) has strong affinity to S, and has an action of increasing hot workability. Mg also has an action of reducing both of generation of liquation cracking and lowering of toughness, which are caused by S, in the HAZ. Accordingly, Mg may be contained when necessary. However, excessive addition of Mg causes lowering of cleanliness due to bonding to oxygen. Particularly when a content of Mg exceeds 0.02%, cleanliness is remarkably lowered, thus deteriorating hot workability on the contrary. Accordingly, the Mg content is set to 0.02% or less. The Mg content is preferably 0.01% or less. However, to obtain the advantageous effects, the Mg content is preferably 0.0001% or more, more preferably 0.0005% or more, and further preferably 0.001% or more.
• Ca (calcium) has strong affinity to S, and has an action of increasing hot workability. Ca also has an action of reducing both of generation of liquation cracking and lowering of toughness, which are caused by S, in the HAZ. Accordingly, Ca may be contained when necessary. However, excessive addition of Ca causes lowering of cleanliness due to bonding to oxygen. Particularly when a content of Ca exceeds 0.02%, cleanliness is remarkably lowered, thus deteriorating hot workability on the contrary. Accordingly, the Ca content is set to 0.02% or less. The Ca content is preferably 0.01% or less. However, to obtain the advantageous effects, the Ca content is preferably 0.0001% or more, more preferably 0.0005% or more, and further preferably 0.001% or more.
• either of Mo or W is an element which is dissolved in the austenitic structure forming a matrix, thus contributing to enhancing creep strength at a high temperature.
• the total content of Mo and W is required to satisfy the formula (i).
• the value of the middle side in the formula (i) is preferably 1.0 or more to 10.0 or less. 1.0 ⁇ 4 ⁇ Al + 2 ⁇ Ti + Nb ⁇ 12.0
• Either of P or B is an element which segregates at grain boundaries of the HAZ near the fusion boundary due to heat cycle during welding, thus lowering the fusing point and increasing liquation cracking susceptibility in the HAZ.
• B strengthens the grain boundaries on the contrary. Accordingly, P adversely affects toughness, but B reduces the lowering of toughness on the contrary.
• Cr is an element which affects segregation behavior of P and B at the grain boundaries, and indirectly affects properties of P and B. That is, the degree of effect of B on liquation cracking in the HAZ becomes more conspicuous with larger Cr content.
• toughness in the HAZ after long-term use is significantly and adversely affected by P. However, in the case where the P content and the B content are substantially equal to each other, there is a tendency that toughness is lowered more with smaller Cr content.
• the formula (iii) is required to be satisfied.
• the value of the left side in the formula (iii) is preferably 0.030 or less.
• the lower limit of the value of the left side in the formula (iii) is not particularly limited. However, the lower limit of the value of the left side may be set to a value close to 0.0015 which can be obtained when the content of P as an impurity is extremely low, Cr is 15.0%, and B is 0.0005%.
• the balance consists of Fe and impurities.
• 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.
• 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.
• the grain size number is determined based on crossing line segments (grain size) defined by JIS G 0551 (2013).
• a production process for a Ni-based alloy by properly adjusting 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 Ni-based 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.
• (Al+Ti+Nb) PB /(Al+Ti+Nb) PS it is not necessary to set the lower limit value of (Al+Ti+Nb) PB /(Al+Ti+Nb) PS .
• (Al+Ti+Nb) PB /(Al+Ti+Nb) PS is preferably set to 1.0 or more.
• the precipitate obtained by the extraction residue analysis is undissolved ⁇ ' phase contained in the alloy.
• the extraction residue analysis is performed by the following procedure. First, test coupons for measuring ⁇ ' phase 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 heat resistant alloy is completely electrolyzed in a 1% tartaric acid - 1% ammonium sulfate aqueous solution under an electrolysis condition of 20 mA/cm 2 . Then, 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 subjected to ICP-AES measurement to measure contents (mass%) of Al, Ti and Nb contained as undissolved ⁇ ' phase and, then, the value of (Al+Ti+Nb) PB /(Al+Ti+Nb) PS is obtained based on the measured values.
• Ni-based 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, it is necessary that 10,000-hour creep rupture strength at 700°C in the longitudinal direction is 150 MPa or more at the center portion of the 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 Ni-based heat resistant alloy of the present invention is produced by performing hot working on an ingot or a cast piece having the 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 1070 to 1220°C, and is held for 1150 D/T to 1500 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 1070°C, the amount of undissolved ⁇ ' phase increases, thus lowering creep rupture strength. On the other hand, when the heat-treatment temperature exceeds 1220°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 1100°C or above, and it is more preferable to set the heat-treatment temperature to 1200°C or below. Further, when the holding time is less than 1150 D/T (min), ⁇ ' phase at the center portion increases and hence, (Al+Ti+Nb) PB /(Al+Ti+Nb) PS falls outside a range defined by the present invention. On the other hand, when the holding time exceeds 1500 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 ⁇ ' phase is generated mainly within grains so that there is a possibility that the formula (iv) is not satisfied.
• the obtained ingots were processed to have a columnar shape with an outer diameter of 200 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, 3 and 5 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 ⁇ ' phase 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 heat resistant alloy was completely electrolyzed in a 1% tartaric acid - 1% ammonium sulfate aqueous 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.
• 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.
• round bar creep rupture test coupon described in JIS Z 2241 (2011), and having a diameter of 6 mm and a gage 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 alloys 1 to 8 are Inventive Examples of the present invention.
• the alloy composition, the grain size number, (Al+Ti+Nb) PB /(Al+Ti+Nb) PS , YS S /YS B , TS S /TS B , and creep rupture strength of the alloys 1 to 8 fall within ranges defined by the present invention so that the alloys 1 to 8 have small variation in mechanical characteristics, and favorable creep rupture strength.
• 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 the value of (Al+Ti+Nb) PB /(Al+Ti+Nb) PS 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 value of (Al+Ti+Nb) PB /(Al+Ti+Nb) PS and the grain size number at the outer surface portion 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.
• creep rupture strength of the alloy D is remarkably lower than that of the alloy 2.
• 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 (Al+Ti+Nb) PB /(Al+Ti+Nb) 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 3.
• the alloys F, G, H are Comparative Examples where the chemical composition falls outside the specification of the present invention.
• the alloy F is an example where the W content is higher than the specification
• the alloy G is an example where the value of the middle side in the formula (i) is higher than the specification
• the alloy H is an example where the value of the middle side in the formula (ii) is lower than the specification. Accordingly, as a result, creep rupture strength is low in these examples.
• the Ni-based heat resistant alloy according to 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 Ni-based heat resistant alloy of the present invention is preferably applicable to a large-sized structural member for a boiler, a chemical plant or the like which is used in a high temperature environment.

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• 2018
  • 2018-02-15 EP EP18753655.2A patent/EP3584335A4/de not_active Withdrawn
  • 2018-02-15 KR KR1020197026436A patent/KR20190117605A/ko not_active Application Discontinuation
  • 2018-02-15 JP JP2018568607A patent/JP6819700B2/ja active Active
  • 2018-02-15 CA CA3053741A patent/CA3053741A1/en not_active Abandoned
  • 2018-02-15 WO PCT/JP2018/005298 patent/WO2018151222A1/ja unknown
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