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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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  • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • 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
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • 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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • 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
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with 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
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • 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
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • 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
  • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite

Definitions

• the present invention relates to austenitic stainless steel.
• Patent Literature 1 discloses an 18 Cr - based austenitic stainless steel superior in high temperature strength as well as superior in steam oxidation resistance.
• Patent Literature 2 discloses an austenitic stainless steel superior in high temperature corrosion thermal fatigue cracking resistance.
• Patent Literature 3 discloses a heat-resistant austenitic stainless steel superior in high temperature strength and cyclic oxidation resistance.
• Patent Literature 4 discloses an austenitic stainless steel exhibiting superior toughness even after exposure to a high temperature environment for a prolonged period of time.
• Patent Literature 5 discloses a high strength austenitic stainless steel with a creep rupture strength at 800°C for 600 hours of 100 MPa or more.
• Patent Literature 6 discloses a method for securing a high temperature strength (a method of adding a large amount of N) by which a large amount of nitrogen (N) is added for utilizing solid solution strengthening and nitride precipitation strengthening so as to make up low strength of a low carbon stainless steel.
• Patent Literature 7 relates to an austenitic stainless steel consisting of, in percent by mass, C: 0.05-0.15 %, Si: not more than 2 %, Mn: 0.1-3%, P: 0.05-0.30 %, S: not more than 0.03 %, Cr: 15-28 %, Ni: 8-55 %, Cu: 0-3.0 %, Ti: 0.05-0.6 %, REM: 0.001-0.5 %, sol. Al: 0.001-0.1%, N: not more than 0.03 %, and the balance being Fe and incidental impurities.
• Stress corrosion cracking of stainless steel may occur because a crystal grain boundary becomes susceptible to selective corrosion due to precipitation of a Cr - based carbide or generation of a zone with a low Cr concentration (Cr depleted zone) in the vicinity of a crystal grain boundary.
• Patent Literature 6 (a method of adding a large amount of N) is a method devised for replacing the aforementioned conventional methods.
• the method of adding a large amount of N is a method by which a large amount of N is added for utilizing solid solution strengthening and nitride precipitation strengthening so as to make up low strength of a low carbon stainless steel.
• Patent Literature 6 the method of adding a large amount of N
• a large amount of nitride is formed against expectation to cause stress corrosion cracking, or sufficient high temperature strength cannot be obtained in a high temperature range of 700°C or higher
• An object of the invention is to provide an austenitic stainless steel, which is an 18 Cr - based austenitic stainless steel securing superior high temperature strength and superior stress corrosion cracking resistance.
• the means for achieving the object includes the following aspects.
• an austenitic stainless steel which is an 18 Cr - based austenitic stainless steel securing superior high temperature strength and superior stress corrosion cracking resistance, is provided.
• a numerical range expressed by "x to y" herein includes the values of x and y in the range as the lower and upper limit values, respectively.
• C carbon
• C content The content of another element may be expressed similarly.
• An austenitic stainless steel of the embodiment (hereinafter also referred to as "the steel of the embodiment") is an austenitic stainless steel with a chemical composition consisting of in terms of mass%: 0.05 to 0.13% of C, 0.10 to 1.00% of Si, 0.10 to 3.00% of Mn, 0.040% or less of P, 0.020% or less of S, 17.00 to 19.00% of Cr, 12.00 to 15.00% of Ni, 2.00 to 4.00% of Cu, 0.01 to 2.00% of Mo, 2.00 to 5.00% of W, 2.50 to 5.00% of 2Mo+W, 0.01 to 0.40% of V, 0.05 to 0.50% of Ti, 0.15 to 0.70% of Nb, 0.001 to 0.040% of Al, 0.0010 to 0.0100% of B, 0.0010 to 0.0100% of N, 0.001 to 0.20% of Nd, 0.002% or less of Zr, 0.001% or less of Bi, 0.010% or less of Sn, 0.010% or less of Sb
• the chemical composition of the steel of the embodiment includes 17.00 to 19.00% of Cr.
• the steel of the embodiment belongs to the 18 Cr - based austenitic stainless steel.
• superior high temperature strength and superior stress corrosion cracking resistance may be secured without depending on the low carbon addition method, the stabilizing heat treatment method, the method of adding a large amount of Cr, and the method of adding a large amount of N, which are conventional methods.
• grain boundary purification and strength improvement may be achieved by adding Nd and B combinedly at the above respective contents, and further by adjusting the effective M content Meff in the above range.
• purity refinement is achieved by limiting the contents of Zr, Bi, Sn, Sb, Pb, and As, which are impurities (hereinafter also collectively referred to as "6 impurity elements"), in the above ranges.
• a carbide and a Laves phase precipitate preferentially around a nitride and on a nitride at a crystal grain boundary to impair the high temperature strength and corrosion resistance.
• a nitride when a nitride is present, both precipitation of a fine carbide, and precipitation of a fine and stable Laves phase become difficult, and the high temperature strength is not improved.
• a coarse Zr nitride precipitation of a fine carbide, and precipitation of a fine and stable Laves phase become more difficult, and therefore N and Zr are reduced to the extent as possible.
• N is not an impurity element but a useful element, and controlled in a very low content range (specifically, from 0.0010 to 0.0100%).
• both of precipitation strengthening with a fine carbide and precipitation strengthening with a fine and stable Laves phase may be achieved effectively.
• the high temperature strength can be secured and the metallic structure can be stabilized in a temperature range of 700°C or higher.
• enhancement of the strength can be achieved without depending on precipitation strengthening with a nitride, and stabilization of the metallic structure can be achieved without forming a brittle phase, etc.
• the technique has not been known conventionally.
• C is an essential element for formation of a carbide, and stabilization of an austenitic structure, as well as improvement of high temperature strength and stabilization of a metallic structure at a high temperature.
• stress corrosion cracking can be prevented without utilizing strengthening by addition of N, or without reducing C.
• the C content is 0.05% or more, which is because, when the C content is less than 0.05%, improvement of high temperature creep strength, and stabilization of a metallic structure at a high temperature becomes difficult.
• the C content is preferably 0.06% or more.
• the C content exceeds 0.13%, a coarse Cr carbide precipitates at a crystal grain boundary, which may cause stress corrosion cracking or welding cracking to reduce toughness. Therefore, the C content is 0.13% or less, and is preferably 0.12% or less.
• Si is an element which functions as a deoxidizing agent during steel making, and prevents steam oxidation at a high temperature.
• the Si content is 0.10% or more, and is preferably 0.20% or more.
• the Si content is 1.00% or less, and is preferably 0.80% or less.
• Mn is an element which makes S harmless by forming MnS with S as an impurity element to contribute to improvement of a hot workability, as well as to stabilization of a metallic structure at a high temperature.
• the Mn content is 0.10% or more, and is preferably 0.20% or more.
• the Mn content is 3.00% or less, and is preferably 2.60% or less.
• P is an impurity element, which disturbs workability and weldability.
• the P content exceeds 0.040%, the workability and weldability decrease remarkably. Therefore, the P content is 0.040% or less, and is preferably 0.030% or less, and more preferably 0.020% or less.
• the P content is as low as possible, and may be even 0%.
• the P content may be 0.001% or more from the viewpoint of production cost.
• S is an impurity element, which disturbs workability, weldability, and stress corrosion cracking resistance.
• the S content exceeds 0.020%, the workability, weldability, and stress corrosion cracking resistance decrease remarkably. Therefore the S content is 0.020% or less.
• the S content is added at 0.020% or less, and is preferably added at 0.010% or less.
• the S content is as low as possible, and may be even 0%.
• the S content may be 0.001% or more from the viewpoint of production cost.
• Cr is a major element of an 18 Cr - based austenitic stainless steel, which contributes to improvement of oxidation resistance, steam oxidation resistance, and stress corrosion cracking resistance, as well as to stabilization of the strength or metallic structure with a Cr carbide.
• the Cr content is 17.00% or more.
• the Cr content is preferably 17.30% or more, and more preferably 17.50% or more.
• the Cr content exceeds 19.00%, a large amount of Ni becomes necessary for maintaining the stability of an austenitic structure, and further a brittle phase is formed to decrease high temperature strength or toughness. Therefore, the Cr content is 19.00% or less.
• the Cr content is preferably 18.80% or less, and more preferably 18.60% or less.
• Ni is an element to form austenite, and as a major element of an 18 Cr - based austenitic stainless steel contributes to improvement of high temperature strength and workability as well as to stabilization of a metallic structure at a high temperature.
• the Ni content is 12.00% or more.
• the Ni content is preferably 12.50% or more.
• the Ni content is 15.00% or less, and is preferably 14.90% or less, more preferably 14.80% or less, and further preferably 14.50% or less.
• Cu is an element, which precipitates as a fine Cu phase that is stable at a high temperature, to contribute to improvement of high temperature strength.
• the Cu content is 2.00% or more, and is preferably 2.20% or more, and more preferably 2.50% or more.
• the Cu content is 4.00% or less, and is preferably 3.90% or less, more preferably 3.80% or less, and further preferably 3.50% or less.
• Mo is an essential element for improvement of the corrosion resistance, high temperature strength, and stress corrosion cracking resistance. Further, Mo is an element that contributes to formation of a Laves phase that is stable at a high temperature for a long time period of time and a carbide, through a synergistic effect with W to be added combinedly.
• the Mo content is 0.01% or more, and is preferably 0.02% or more.
• the Mo content is 2.00% or less, and is preferably 1.80% or less, more preferably 1.50% or less, and further preferably 1.30% or less.
• W is an essential element for improvement of the corrosion resistance, high temperature strength, and stress corrosion cracking resistance. Further, it is an element to contribute to precipitation of a Laves phase stable at a high temperature for a long time period of time and a carbide, through a synergistic effect with Mo to be added combinedly. Further, W is slower in terms of diffusion at a high temperature than Mo, and therefore it is an element to contribute to stable maintenance of the strength at a high temperature for a long period of time.
• the W content is 2.00% or more, and is preferably 2.10% or more.
• the W content is 5.00% or less, and is preferably 4.90% or less, more preferably 4.80% or less, and further preferably 4.70% or less.
• 2Mo+W (Wherein Mo represents a Mo content, and W represents a W content. The same holds hereinbelow.) is less than 2.50%, the synergistic effect of the combined addition cannot be obtained adequately. Therefore, 2Mo+W is 2.50% or more, and is preferably 2.60% or more, more preferably 2.80% or more, and further preferably 3.00% or more.
• 2Mo+W exceeds 5.00%, the strength or toughness decreases, and the stability of a metallic structure is also decreased at a high temperature. Therefore 2Mo+W is 5.00% or less, and is preferably 4.90% or less.
• V is an element contributing to improvement of high temperature strength by forming a fine carbide together with Ti and Nb.
• the V content is less than 0.01%, the addition effect is not obtained adequately. Therefore, the V content is 0.01% or more, and is preferably 0.02% or more.
• the V content exceeds 0.40%, the strength or stress corrosion cracking resistance decreases. Therefore, the V content is 0.40% or less, and is preferably 0.38% or less.
• Ti is an element contributing to improvement of high temperature strength by forming a fine carbide together with V and Nb, and contributing also to improvement of stress corrosion cracking resistance through suppression of precipitation of a Cr carbide at a crystal grain boundary by fixing C.
• the inventors have found, with respect to an 18 Cr - based austenitic stainless steel, that an advantageous action effect of a fine Ti carbide can be obtained by controlling the N content at a very low level, and that, specifically, a fine Laves phase precipitates using a fine Ti carbide as a nucleus, as a result of which the high temperature strength of the steel is enhanced remarkably.
• the Ti content is less than 0.05%, the addition effect is not obtained adequately, and therefore, the Ti content is 0.05% or more.
• the Ti content is preferably 0.10% or more.
• the Ti content exceeds 0.50%, a clumpy precipitate is precipitated to decrease the strength, toughness, and stress corrosion cracking resistance. Therefore, the Ti content is 0.50% or less, and is preferably 0.45% or less.
• Nb is an element contributing to improvement of high temperature strength by forming a fine carbide together with V and Ti, and contributing also to improvement of stress corrosion cracking resistance through suppression of precipitation of a Cr carbide at a crystal grain boundary by fixing C.
• Nb is, similar to Ti, an element contributing to improvement of the high temperature strength due to precipitation of a fine Laves phase.
• the Nb content is 0.15% or more, and is preferably 0.20% or more.
• the Nb content is 0.70% or less, and is preferably 0.60% or less.
• Al is an element which functions as a deoxidizing element in steel making to purify a steel.
• the Al content is less than 0.001%, purification of a steel cannot be achieved adequately. Therefore, the Al content is 0.001% or more, and is preferably 0.002% or more.
• the Al content exceeds 0.040%, a large amount of nonmetallic inclusion is formed to decrease the stress corrosion cracking resistance, high temperature strength, workability, toughness, and stability of a metallic structure at a high temperature. Therefore, the Al content is 0.040% or less, and is preferably 0.034% or less.
• B is an element for achieving securance of superior high temperature strength and superior stress corrosion cracking resistance by combined addition with Nd, which is an important element in the steel of the embodiment. Therefore, B is an essential element.
• B is an element not only to contribute to improvement of the high temperature strength through segregation at a crystal grain boundary, but also to contribute to formation of a carbide, micronization of a Laves phase, and stabilization of a metallic structure, which are effective for improvement of the high temperature strength.
• B is an element, which makes N (present in the steel of the embodiment at 0.0010 to 0.0100%) harmless as BN, and contributes to improvement of the high temperature strength and stress corrosion resistance.
• the B content is less than 0.0010%, residual B, which has not been consumed as a nitride, namely B able to contribute to improvement of the high temperature strength and stress corrosion resistance cannot be secured.
• the B content is less than 0.0010%, a synergistic effect (to be described below) of combined addition with Nd (and securance of effective M content) is not obtained, so that the high temperature strength and stress corrosion cracking resistance are not improved. Therefore the B content is 0.0010% or more, and is preferably 0.0015% or more.
• the B content exceeds 0.0100%, a boron compound is formed to decrease the workability, weldability, and high temperature strength. Therefore the B content is 0.0100% or less, and is preferably 0.0080% or less, and more preferably 0.0060% or less.
• N nitrogen
• a nitride disturbs stress corrosion cracking resistance, and therefore N is not added actively.
• N is not added actively, but is allowed only in a small content range, which is different from the prior art.
• the N content is 0.0010% or more, and is preferably 0.0020% or more, and more preferably 0.0030% or more.
• the N content exceeds 0.0100%, a nitride is formed to decrease the high temperature strength and stress corrosion cracking resistance. Therefore the N content is 0.0100% or less, and is preferably 0.0090% or less, more preferably 0.0080% or less, and further preferably 0.0070% or less.
• Nd is an element to improve remarkably the high temperature strength and stress corrosion cracking resistance through a synergistic effect (described below) of combined addition with B.
• the stress corrosion cracking resistance is improved by micronizing a carbide and a Laves phase effective for improvement of the high temperature strength, by securing the long term stability, and further by strengthening a crystal grain boundary through combined addition of Nd and B.
• the Nd content is 0.001% or more, and is preferably 0.002% or more, and more preferably 0.005% or more.
• the Nd content is 0.20% or less, and is preferably 0.18% or less, more preferably 0.15% or less, and further preferably 0.10% or less.
• the Nd content is preferably in a range of from 0.002 to 0.15%, and more preferably from 0.005 to 0.10%.
• Zr, Bi, Sn, Sb, Pb, As, and O are treated as impurity elements for the sake of securance of superior characteristics of the steel of the embodiment, and the contents of the elements are limited.
• scraps such as alloy steel are used mainly.
• the scraps contain, although at low contents, Zr, Bi, Sn, Sb, Pb, and As (6 impurity elements), which get mixed in a stainless steel (product) inevitably.
• Zr is ordinarily not contained. However Zr may get mixed from scraps, etc. and/or a facility for melting, etc. contaminated by production of another alloy to form an oxide and a nitride.
• the nitride functions as a nucleus for precipitation of a precipitate such as a Laves phase.
• Zr is a harmful element in terms of high temperature strength and stress corrosion cracking resistance. Therefore in a relational expression (Formula (1)) introduced for the sake of securance of superior high temperature strength and superior stress corrosion cracking resistance, a term of "-1.6 ⁇ Zr" expressing a negative action effect has been added.
• the upper limit of the Zr content is set at 0.002% which is close to the analytical limit (0.001%).
• the Zr content is preferably 0.001% or less.
• the Zr content may be 0%. However, Zr may occasionally get mixed inevitably at 0.0001% or so. Therefore, from the viewpoint of production cost, the Zr content may be 0.0001% or even more.
• Bi is an element which is ordinarily not contained. However, Bi may get mixed from scraps, etc. and/or a facility for melting, etc. contaminated by production of another alloy, and disturbs high temperature strength and stress corrosion cracking resistance.
• the upper limit of the Bi content is set at 0.001% which is the analytical limit.
• the Bi content may be 0%. However, Bi may occasionally get mixed inevitably at 0.0001% or so. Therefore, from the viewpoint of production cost, the Bi content may be 0.0001% or even more.
• Sn, Sb, Pb, and As are elements, which easily get mixed from scraps, etc. and/or a facility for melting, etc. contaminated by production of another alloy, and are hardly removed in a refining process.
• the upper limits of the Sn content and the Sb content are set at 0.010% respectively.
• the Sn content and the Sb content are preferably 0.005% or less respectively.
• Pb content and the As content are set at 0.001% respectively.
• Pb and As are preferably 0.0005% or less respectively.
• Any of the Sn content, the Sb content, the Pb content, and the As content may be 0%.
• the elements may inevitably get mixed at 0.0001% or so. Therefore from the viewpoint of production cost, the content of any of the elements may be 0.0001% or even more.
• the total content of the 6 impurity elements in the steel of the embodiment is 0.020% or less.
• the total content of the 6 impurity elements is preferably 0.015% or less, and more preferably 0.010% or less.
• the total content of the 6 impurity elements is preferably as low as possible. Therefore, the lower limit of the total content of the 6 impurity elements is 0%.
• O (oxygen) remaining inevitably after refining a molten steel is an element used as an index of the content of a nonmetallic inclusion.
• the O content is 0.0090% or less, and is preferably 0.0080% or less, more preferably 0.0070% or less, and further preferably 0.0050% or less.
• the O content may be 0%. However, O may occasionally remain after refining inevitably at 0.0001% or so. Therefore, from the viewpoint of production cost, the O content may be 0.0001% or even more.
• the chemical composition of the steel of the embodiment may include one or more of Co, Ca, or Mg, and/or one or more of lanthanoid elements except Nd, Y, Sc, Ta, Hf, or Re.
• Any of the elements is an optional element, and therefore the contents thereof may be respectively 0%.
• the Co content is 0.80% or less, and is preferably 0.60% or less.
• a steel of the embodiment is not required to contain Co (namely, the Co content may be 0%), however from the viewpoint of further stabilization of a metallic structure and improvement of high temperature strength, Co may be contained.
• the Co content is preferably 0.01% or more, and more preferably 0.03% or more.
• Ca is an optional element, and the Ca content may be 0%.
• Ca may be added as a finishing element for deoxidation. Since the steel of the embodiment contains Nd, it is preferable that the same is deoxidized by Ca in a refining process. When the steel of the embodiment contains Ca, from the viewpoint of obtaining more effectively a deoxidation effect, the Ca content is preferably 0.0001% or more, and more preferably 0.0010% or more.
• the Ca content exceeds 0.20%, the amount of a nonmetallic inclusion increases to lower the high temperature strength, stress corrosion cracking resistance, and toughness. Therefore the Ca content is 0.20% or less, and is preferably 0.15% or less.
• Mg is an optional element, and the Mg content may be 0%.
• Mg is an element, which contributes to improvement of high temperature strength or corrosion resistance by addition of a small amount thereof.
• the Mg content is preferably 0.0005% or more, and more preferably 0.0010% or more.
• the Mg content exceeds 0.20%, the strength, toughness, corrosion resistance, and weldability are lowered. Therefore the Mg content is 0.20% or less, and is preferably 0.15% or less.
• Y, Sc, Ta, Hf, Re and lanthanoid elements other than Nd namely, La, Ce, Pr, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu
• the total content of the elements may be 0%.
• the steel of the embodiment contains one or more of the elements, the total content of the elements is preferably 0.001% or more, and more preferably 0.005% or more.
• the total content of Y, Sc, Ta, Hf, Re and lanthanoid elements other than Nd exceeds 0.20%, the amount of a nonmetallic inclusion increases to lower the strength, toughness, corrosion resistance, and weldability. Therefore the total content is 0.20% or less, and is preferably 0.15% or less.
• a remainder excluding the aforementioned elements from the chemical composition of the steel of the embodiment is Fe and impurities.
• the impurities referred to herein mean one or more of elements other than the aforementioned elements.
• the contents of the elements (impurities) other than the aforementioned elements are preferably limited to 0.010% or less respectively, and more preferably to 0.001% or less.
• an effective M content Meff defined by the following Formula (1) is from 0.0001 to 0.250%.
• Effective M content Meff Nd + 13 ⁇ B ⁇ 11 ⁇ N / 14 ⁇ 1.6 ⁇ Zr wherein in Formula (1), each element symbol represents the content (mass%) of each element.
• the effective M content Meff is an index defining a quantitative relationship between Nd and B, which are essential for improvement of high temperature strength and stress corrosion cracking resistance.
• Formula (1) defining an effective M content Meff is a relational expression discovered by the inventors from the viewpoint of securance of superior high temperature strength and superior stress corrosion cracking resistance.
• Formula (1) is basically a relational expression, in which to the content of Nd to function effectively for securance of superior high temperature strength and superior stress corrosion cracking resistance, the content of B also to function effectively is added, and the content of Zr to function harmfully against securance of superior high temperature strength and superior stress corrosion cracking resistance is subtracted.
• N is reduced to the extent as possible so as to suppress formation of a nitride in order to secure superior high temperature strength and superior stress corrosion cracking resistance.
• Nd is an element that functions effectively similarly as B for securing superior high temperature strength and superior stress corrosion cracking resistance.
• the impurity element Zr by forming a nitride and an oxide, functions to reduce a synergistic effect of combined addition of Nd and B.
• Nd and B necessary for obtaining superior high temperature strength and superior stress corrosion cracking resistance and the limited amount of Zr being harmful to securance of superior high temperature strength and superior stress corrosion cracking resistance can be quantified by an effective M content Meff defined by Formula (1) (specific examples will be described in Examples in detail).
• the effective M content Meff is 0.0001 % or more, and is preferably 0.001% or more, more preferably 0.002% or more, and further preferably 0.010% or more
• the effective M content Meff may take a negative value.
• the effective M content Meff exceeds 0.250%, the improvement effect on high temperature strength and stress corrosion cracking resistance according to the effective M content Meff is saturated, and the economy declines, and moreover the strength, toughness, workability, and weldability decrease. Therefore, the effective M content Meff is 0.250% or less, and is preferably 0.200% or less, and more preferably 0.150% or less.
• the metallic structure of the steel of the embodiment is preferably a coarse grain metallic structure from the viewpoint of improvement of high temperature strength (for example, high temperature creep strength between 700°C and 750°C).
• the ASTM grain size number of the metallic structure thereof is preferably 7 or less.
• the metallic structure of the steel of the embodiment is a coarse grain structure with an ASTM grain size number of 7 or less, a suppression effect on grain boundary sliding in creep, change in a metallic structure by element diffusion through a crystal grain boundary, and formation of precipitation site for an ⁇ phase can be conceivably obtained.
• the metallic structure of the steel of the embodiment is a coarse grain structure with an ASTM grain size number of 7 or less.
• the ASTM grain size number of the metallic structure of the steel of the embodiment is preferably 7 or less, and more preferably 6 or less.
• the lower limit of the ASTM grain size number of a metallic structure is preferably 3.
• a steel of the embodiment is superior in high temperature strength (especially, creep rupture strength) as described above.
• the creep rupture strength at 700°C and 10,000 hours of the steel of the embodiment is preferably 140 MPa or more.
• 700°C is a temperature higher than an actual usage temperature.
• the creep rupture strength at 700°C and 10,000 hours of 140 MPa or more means that the high temperature characteristic is remarkably superior.
• a high temperature strength at which the creep rupture strength is 140 MPa or more at 700°C and 10,000 hours is a high temperature strength that is remarkably superior to a 347H steel (18 Cr-12Ni-Nb), which is used widely in the world as a conventional 18 Cr - based austenitic stainless steel (see, for example, Inventive Steels 1 to 20, and Comparative steel 21 in Table 3 below).
• a creep rupture strength less than 140 MPa may be easily achievable by extension of the conventional art, however it is difficult to achieve a creep rupture strength of 140 MPa or more by mere extension of the prior art.
• a creep rupture strength of 140 MPa or more at 700°C, which is higher than an actual service temperature, and 10,000 hours (superior high temperature strength) can be attained by fine precipitation of a carbide and a Laves phase, the Laves phase precipitates during creep, by means of optimization of the chemical composition, optimization of the effective M content Meff by the Nd content and the B content, higher degree of purification by limiting the amount of impurity elements, etc.
• a steel of the embodiment may be a heat-treated steel plate or a heat-treated steel tube or pipe.
• the heating temperature of the heat treatment is preferably from 1050 to 1250°C, more preferably from 1150°C to 1250°C.
• quenching for example, water cooling
• air cooling for example, water cooling
• the heat-treated steel plate or the heat-treated steel tube or pipe is obtained for example by preparing a steel plate or a steel tube or pipe having an chemical composition of the aforementioned steel of the embodiment, then heating the prepared steel plate or the prepared steel tube or pipe at, for example, from 1050 to 1250°C (preferably from 1150°C to 1250°C), and thereafter cooling the same.
• the steel plate or the steel tube or pipe having the chemical composition (a steel plate or a steel tube or pipe before a heat treatment) may be prepared according to an ordinary method.
• a steel tube or pipe having the chemical composition may be prepared, for example, by casting a molten steel having the chemical composition to form a steel ingot or a steel billet, and performing at least one kind of a processing selected from the group consisting of hot extrusion, hot rolling, hot forging, cold drawing, cold rolling, cold forging, and cutting, on the obtained steel ingot or steel billet.
• the steel of the embodiment is a material steel suitable for, for example, a heat-resistant and pressure-resistant heat exchanger tube or a pipe for a boiler, a chemical plant, or the like; a heat-resistant forged product; a heat-resistant steel bar; or a heat-resistant steel plate.
• the steel of the embodiment is a material steel especially suitable for a heat-resistant and pressure-resistant heat exchanger tube to be placed inside a boiler (for example, a heat-resistant and pressure-resistant heat exchanger tube with an outer diameter of from 30 to 70 mm, and a thickness of from 2 to 15 mm), or a pipe of boiler (for example, a pipe with an outer diameter of from 125 to 850 mm, and a thickness of from 20 to 100 mm).
• a boiler for example, a heat-resistant and pressure-resistant heat exchanger tube with an outer diameter of from 30 to 70 mm, and a thickness of from 2 to 15 mm
• a pipe of boiler for example, a pipe with an outer diameter of from 125 to 850 mm, and a thickness of from 20 to 100 mm.
• Steels 1 to 20 are Inventive Steels which are examples of the invention (hereinafter also referred to as “Inventive Steels 1 to 20" respectively), and Steels 21 to 30 are Comparative Steels which are comparative examples (hereinafter also referred to as “Comparative Steels 21 to 30" respectively).
• Comparative Steel 21 is a general-purpose steel 347H (18Cr-12Ni-Nb) and is a standard material for comparison between the prior art and Inventive Steels 1 to 20.
• melt-producing Inventive Steels 1 to 20 as a Fe source, high purity Fe obtained by smelting in a blast furnace and a converter and secondary refining by a vacuum oxygen degassing process was used, and as an alloy element, a high purity alloy element analyzed in advance was used. Further, before melt-producing any of Inventive Steels 1 to 20, the furnace for melt-producing Inventive Steels 1 to 20 was washed adequately, and special care was taken so as to prevent contamination with impurities.
• the 6 impurity elements specifically, Zr, Bi, Sn, Sb, Pb, and As
• the O content, the N content and the like were limited, and the Nd content and the B content were regulated within an appropriate range.
• melt-producing Comparative Steels 23 to 30 the high purity Fe source was used also. Further, in melt-producing Comparative Steels 23 to 30, the chemical compositions were adjusted as follows.
• melt-producing Comparative Steels 21, 23, 24, 27, and 29 at least one of the 6 impurity elements and O (oxygen) was added intentionally.
• melt-producing Comparative Steels 21 to 23, 25, 27, and 28 at least one of B or Nd was not added.
• a numerical value represents the content of each element (mass%).
• An underlined numerical value is a value outside the range of the chemical composition of the embodiment.
• Meff was calculated according to Formula (1).
• the Meff was calculated by regarding the Zr content as 0%.
• Sub-total (X) shows the total content (mass%) of the 6 impurity elements (specifically, Zr, Bi, Sn, Sb, Pb, and As). In this regard, for an element with a content of less than 0.001% (written as " ⁇ 0.001" in Table 2), the sub-total (X) was calculated by regarding the content of the element as 0%.
• a steel having an chemical composition shown in Table 1 and Table 2 was melted by vacuum melting and cast to obtain a 50 kg- steel ingot.
• a heat treatment at 1200°C was performed on the test material by heating the test material to 1200°C, then keeping test material at 1200°C for 15 min, and thereafter cooling the test material with water.
• the ASTM grain size of the test material after the heat treatment was measured according to ASTM E112. A measurement position of an ASTM grain size was near the central part in the thickness direction of a longitudinal cross-section of the test material.
• a creep rupture test piece with a size of 6 mm ⁇ and a length of the parallel portion of 30 mm was cut out from the heat-treated test material, whose longitudinal direction was the longitudinal direction of the test piece.
• the creep rupture test piece was subjected to a long term creep rupture test at 700°C for 10,000 hours or longer, and a creep rupture strength (MPa) at 700°C and 10,000 hours was measured as a high temperature strength.
• a corrosion test piece with a width of 10 mm ⁇ a thickness of 4 mm ⁇ a length of 40 mm was sliced out from the heat-treated test material.
• the sliced out corrosion test piece is hereinafter called a "base material”.
• the base material was subjected to a thermal aging treatment at 650°C for 10 hours.
• a Strauss test (ASTM A262, Practice E: Sensitization evaluation) was performed on the base material after the thermal aging treatment, and presence or absence of a crack with a depth of 100 ⁇ m or more was examined.
• a corrosion test piece with a width of 10 mm ⁇ a thickness of 4 mm ⁇ a length of 40 mm was sliced out from the heat-treated test material.
• the sliced-out test piece was heated at 950°C for 25 sec using a Greeble tester (Joule heating in vacuum).
• a weld HAZ equivalent material i.e. a weld heat affected zone equivalent material was obtained by blowing He for cooling after the heating.
• a thermal aging treatment and a Strauss test were conducted on the obtained weld HAZ equivalent material similarly as the stress corrosion cracking test on the base material, and presence or absence of a crack with a depth of 100 ⁇ m or more was examined.
• the high temperature strengths of Inventive Steels 1 to 20 were superior strengths of 147 MPa or more, which were approx. 1.5 times or more higher than the high temperature strength of Comparative Steel 21 (general-purpose steel 347H).
• Comparative Steels 21 to 30 were as low as 137 MPa or less, which were inferior to the high temperature strengths of Inventive Steels 1 to 20.
• Comparative Steel 24 The reason behind the low high temperature strength of Comparative Steel 24 is presumed that Nd and B were consumed as an oxide or a nitride respectively and fine precipitation strengthening did not develop.
• test material that had not been subjected to the aforementioned heat treatment at 1200°C was subjected to a heat treatment at 1125°C by heating the test material to 1125°C, then keeping test material at 1125°C for 15 min, and thereafter cooling the test material with water.
• test material having received the heat treatment at 1125°C With respect to the test material having received the heat treatment at 1125°C, an ASTM grain size measurement, a stress corrosion cracking test on a base material, and a stress corrosion cracking test on a weld HAZ equivalent material were conducted similarly as the test material having received the heat treatment at 1200°C.
• the metallic structures of test materials having received the heat treatment at 1200°C with respect to Inventive Steels 1, 10, and 17, and Comparative Steels 21 and 23 were coarse grain structures with an ASTM grain size number of 7 or less.

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