Classifications

• • 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
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • 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
• • 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
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • 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
  • C22C38/002—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
• • 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
  • C22C38/04—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
  • C22C38/18—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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni

Definitions

• the present invention generally relates to austenitic stainless steels.
• the present invention relates to an austenitic stainless steel for use in high-temperature and oxidizing environments such as in thermal power generation boiler tubes.
• Such thermal power generation boiler tubes employ austenitic stainless steels that have excellent high-temperature strengths and oxidation resistance.
• portions to be exposed to a high-temperature and highly corrosive environment employ 25Cr stainless steels typified by steel-use stainless (SUS) 310.
• SUS steel-use stainless
• the boiler tubes (steel tubes) are exposed not only to a high-temperature environment of 500°C to 700°C, but also to high-pressure steam passing through the inside of the steel tubes and thereby require excellent creep properties. Based on conventional knowledge, various attempts have been made to improve creep strength.
• Patent Literature (PTL) 1 discloses a technique of controlling the contents of Ti, Nb, Zr, Ta, and C to appropriate proportions.
• PTL 2 discloses a technique of incorporating trace amounts of oxygen and titanium (Ti) into an austenitic stainless steel to restrain the formation of duplex grains.
• PTL 3 discloses a technique of controlling the contents typically of P, Al, and V in an austenitic stainless steel further containing Cu, Nb, and N.
• PTL 4 discloses a technique of controlling the contents of Mo, W, and N in an austenitic steel.
• the boiler tubes require better toughness.
• the boiler tubes if having poor toughness, may be susceptible to crack or tear caused by impact or shock loaded upon operation and/or inspection and may burst with higher possibility.
• Si, Al, S (sulfur), and O (oxygen) in stainless steels are believed to be effective.
• existing materials for thermal power generation boiler tubes have reduced Si and Al contents within such ranges as to offer deoxidation effects so as to restrain the formation of brittle phases such as sigma ( ⁇ ) phase.
• These materials further include Mn, Ca, and Mg so as to reduce the influence of sulfur (S) (see, for example, PTL 5 and 6).
• tantalum may be added to 25Cr-20Ni and other heat-resistant stainless steels.
• PTL 7 to 11 describe that Ta and other elements such as Ti, Nb, V, Mo, W, and Re, when added selectively, may allow stainless steels to have better corrosion resistance, better hot workability, and higher high-temperature strengths due to carbon fixation.
• Kagi-Kaishaku (KA) SUS 310J1HTB and other JIS standardized steels serving as existing boiler materials contain nitrogen (N) in high contents and fail to enjoy the synergistic effect of nitrogen with another element as in the technique disclosed in PTL 4 and fail to have higher creep strengths.
• the techniques described in PTL 5 to 11 may allow stainless steels to have toughness maintained at certain high level even upon exposure to a high-temperature environment (e.g., at 500°C to 700°C) for a long time. These techniques, however, are considered not to meet the heat resistance requirement that becomes severer year by year, as described above.
• the present invention has been made under such circumstances. It is an object of the present invention to provide an austenitic stainless steel that can develop excellent creep strength in a high-temperature environment over a long term without the addition of large amounts of expensive metals such as W, Mo, and Cu. It is another object of the present invention to provide an austenitic stainless steel that can maintain excellent toughness even after exposure to a high-temperature environment for a long term. Solution to Problem
• an austenitic stainless steel when incorporated with tantalum (Ta) in a content within a specific range, can have excellent toughness even after an aging heat treatment, where the aging heat treatment simulates a use environment typically in a boiler, namely, an environment in which the stainless steel is exposed to a high temperature for a long term.
• the austenitic stainless steel containing Ta in a content within a specific range when containing Nb and Ta in contents with a specific ratio therebetween, can have higher creep strength.
• the present invention has been made based on these findings. Specifically, the present invention provides an austenitic stainless steel that contains C in a content of 0.01 to 0.15 percent by mass, Si in a content of 0.10 to 1.00 percent by mass, Mn in a content of 0.10 to 2.50 percent by mass, Ni in a content of 15.0 to 25.0 percent by mass, Cr in a content of 20.0 to 30.0 percent by mass, Nb in a content of 0.10 to 0.80 percent by mass, Ta in a content of 0.20 to 1.00 percent by mass, B in a content of 0.0005 to 0.0050 percent by mass, N in a content of 0.10 to 0.35 percent by mass, S in a content of 0.0050 percent by mass or less (excluding 0 percent by mass), and P in a content of 0.050 percent by mass or less (excluding 0 percent by mass) with the remainder including iron and inevitable impurities.
• the austenitic stainless steel according to the present invention may contain Ta in a content of 0.25 to 0.8 percent by mass to exhibit particularly excellent toughness after aging.
• the austenitic stainless steel according to the present invention may contain Nb in a content of 0.10 to 0.60 percent by mass and may have a ratio of Ta to Nb of 0.8 to 4.0 to exhibit particularly excellent creep strength.
• the austenitic stainless steel according to the present invention may further include at least one of groups (a) and (b) below.
• the resulting austenitic stainless steel can have higher high-temperature strength and/or better oxidation resistance depending on an element to be contained.
• the groups (a) and (b) are as follows:
• the austenitic stainless steel according to an embodiment of the present invention can maintain excellent toughness even after exposure to a high-temperature environment for a long term.
• the austenitic stainless steel according to the present invention can develop excellent creep strength in a high-temperature environment over a long term without the addition of large amounts of expensive metals such as Mo and Cu.
• An austenitic stainless steel contains C in a content of 0.01 to 0.15 percent by mass, Si in a content of 0.10 to 1.00 percent by mass, Mn in a content of 0.10 to 2.50 percent by mass, Ni in a content of 15.0 to 25.0 percent by mass, Cr in a content of 20.0 to 30.0 percent by mass, Nb in a content of 0.10 to 0.80 percent by mass, Ta in a content of 0.20 to 1.00 percent by mass, B in a content of 0.0005 to 0.0050 percent by mass, N in a content of 0.10 to 0.35 percent by mass, S in a content of 0.0050 percent by mass or less (excluding 0 percent by mass), and P in a content of 0.050 percent by mass or less (excluding 0 percent by mass) with the remainder including iron and inevitable impurities.
• the austenitic stainless steel according to the embodiment of the present invention has chemical compositions in Ni and Cr contents equivalent to a 25Cr-20Ni austenitic stainless steel, but includes specific chemical compositions (C, Si, Mn, Ni, Cr, Nb, Ta, B, N, S, and P) as mentioned below. Operations of these elements and reasons for specifying the content ranges of the elements are as follows.
• Carbon (C) forms carbides in a high-temperature use environment and allows the steel to have higher high-temperature strength and higher creep strength necessary as a heat-exchanger tube.
• Carbon may be contained in a content of 0.01 percent by mass or more so as to ensure certain amounts of precipitated carbides acting as a strengthening mechanism. However, carbon, if contained in an excessively high content of greater than 0.15 percent by mass, may form coarse carbides to fail to offer still further strengthening.
• the carbon content is preferably 0.03 percent by mass or more, and more preferably 0.05 percent by mass or more; and is preferably 0.10 percent by mass or less, and more preferably 0.07 percent by mass or less.
• Si has a deoxidation action in a molten steel Silicon, even if contained in a trace amount, may effectively allow the steel to have better oxidation resistance.
• the Si content may be controlled to 0.1 percent by mass or more.
• Si if contained in an excessively high content of greater than 1.0 percent by mass, may cause the formation of sigma ( ⁇ ) phase and may cause the austenitic stainless steel to have inferior toughness.
• the Si content is preferably 0.2 percent by mass or more, and more preferably 0.3 percent by mass or more; and is preferably 0.7 percent by mass or less, and more preferably 0.5 percent by mass or less.
• Manganese (Mn) has a deoxidation action in the molten steel as with Si and functionally stabilizes austenite. To allow the element to exhibit the effects effectively, the Mn content may be controlled to 0.1 percent by mass or more. However, Mn, if contained in an excessively high content of greater than 2.5 percent by mass, may adversely affect the hot workability.
• the Mn content is preferably 0.5 percent by mass or more, and more preferably 1.0 percent by mass or more; and is preferably 2.0 percent by mass or less, and more preferably 1.5 percent by mass or less.
• Phosphorus (P) is an inevitable impurity. Phosphorus, if contained in a higher content, may adversely affect the weldability. To prevent this, the phosphorus content may be controlled to 0.05 percent by mass or less. The phosphorus content may be controlled preferably to 0.04 percent by mass or less, and more preferably to 0.03 percent by mass or less.
• S Sulfur
• S Sulfur
• the sulfur content may be controlled to 0.005 percent by mass or less.
• the sulfur content may be controlled preferably to 0.003 percent by mass or less, and more preferably to 0.001 percent by mass or less.
• Nickel (Ni) functionally stabilizes austenite. To maintain the austenite phase, Ni may be contained in a content of 15 percent by mass or more. However, Ni, if contained in an excessively high content of greater than 25 percent by mass, may cause increased cost.
• the Ni content is preferably 17 percent by mass or more, and more preferably 19 percent by mass or more; and is preferably 23 percent by mass or less, and more preferably 21 percent by mass or less.
• Chromium (Cr) s essential for the austenitic stainless steel to exhibit corrosion resistance as a stainless steel.
• Cr may be contained in a content of 20 percent by mass or more. However, Cr, if contained in an excessively high content of greater than 30 percent by mass, may cause an increased amount of ferrite phase that causes reduction of high-temperature strength.
• the Cr content is preferably 22 percent by mass or more, and more preferably 24 percent by mass or more; and is preferably 28 percent by mass or less, and more preferably 26 percent by mass or less.
• Ta 0.20 to 1.00 percent by mass
• Precipitate formations are classified into precipitation at grain boundaries and precipitation inside of grains. It has been known that the particles precipitated at grain boundaries, if covering the grain boundaries, cause the steel to have inferior toughness. Tantalum (Ta), when contained, can reduce the amounts of particles precipitated at grain boundaries and allows the steel to have excellent toughness after aging. In contrast, Ta allows carbides and carbonitrides to precipitate in grains to offer precipitation strengthening and thereby makes the steel have higher creep strength. In addition, Ta is dissolved in Z phase (CrNbN) precipitated in the austenitic stainless steel and allows the steel to have still higher creep strength. To offer these effects, Ta may be contained in a content of 0.20 percent by mass or more.
• Ta if contained in an excessively high content of greater than 1.00 percent by mass, may cause the formation of excessive amounts of precipitates and cause the austenitic stainless steel to have inferior ductility and poor economic efficiency.
• the lower limit is preferably 0.25 percent by mass or more, and more preferably 0.30 percent by mass or more; and the upper limit is preferably 0.80 percent by mass or less, and more preferably 0.60 percent by mass or less.
• Boron (B) is dissolved in the steel and functionally promotes the formation of M 23 C 6 carbides, where M represents a carbide-forming element.
• M 23 C 6 carbides act as one of main strengthening mechanisms.
• the boron content may be 0.0005 percent by mass or more.
• boron if contained in an excessively high content, may cause inferior hot workability and/or inferior weldability.
• the boron content may be controlled to 0.005 percent by mass or less.
• the boron content is preferably 0.001 percent by mass or more, and more preferably 0.0015 percent by mass or more; and is preferably 0.003 percent by mass or less, and more preferably 0.0025 percent by mass or less.
• Nitrogen (N) is dissolved in the steel to cause solute strengthening and thereby allows the steel to have higher high-temperature strength.
• the element is one of important elements that bear high-temperature strength of the austenitic stainless steel according to the embodiment of the present invention.
• nitrogen may be contained in a content of 0.10 percent by mass or more.
• nitrogen if contained in an excessively high content of greater than 0.35 percent by mass, may adversely affect the hot workability.
• the nitrogen content is preferably 0.20 percent by mass or more, and more preferably 0.23 percent by mass or more; and is preferably 0.30 percent by mass or less, and more preferably 0.27 percent by mass or less.
• the austenitic stainless steel according to the embodiment of the present invention contains the elements as above, with the remainder including iron and inevitable impurities.
• impurities include Sn, Pb, Sb, As, Zn, and other low-melting-point metallic elements derived from scrap materials.
• the low-melting-point metallic elements cause the steel to have inferior strength at grain boundaries upon hot working and/or upon use in a high-temperature environment. To prevent this and to improve hot workability and embrittlement cracking resistance after long-term use, the contents of metallic elements having the low-melting-point are desirably controlled at low levels.
• the austenitic stainless steel according to the one embodiment of the present invention has been described above.
• the austenitic stainless steel according to an embodiment of the present invention when having a Ta content of 0.25 to 0.8 percent by mass, can exhibit particularly excellent toughness after aging, as described above. Reasons for this will be described below.
• Precipitate formations are classified into precipitation at grain boundaries and precipitation inside of grains, as described above.
• the precipitates formed at grain boundaries when covering the grain boundaries, cause the steel to have inferior toughness, as has been known
• the present inventors have found that Ta, when contained in the austenitic stainless steel in a content of 0.25 to 0.8 percent by mass, can particularly effectively contribute to reduction of amounts of particles precipitated at grain boundaries. This can allow the steel to exhibit excellent toughness after aging.
• Ta is particularly preferably contained in a content of 0.3 percent by mass or more.
• Ta is an expensive metal and, when added, causes increased cost.
• Ta may be contained in a content of preferably 0.6 percent by mass or less, and more preferably 0.5 percent by mass or less.
• the austenitic stainless steel contains Ta in a content within the specific range and contains other elements than Ta in appropriately controlled contents, as above.
• the austenitic stainless steel can thereby have excellent toughness (toughness value) after an aging heat treatment, where the aging heat treatment simulates a use environment typically in a boiler, namely, an environment in which the stainless steel is exposed to a high temperature for a long term. This will be concretely described in experimental examples below.
• the austenitic stainless steel according to the embodiment of the present invention is therefore advantageously usable as a material for heat-exchanger tubes typically of boilers (boiler tubes).
• the austenitic stainless steel according to the embodiment of the present invention maintains excellent toughness even after exposure to a high-temperature environment for a long term typically in a boiler, can endure shock (impact) occurring during operation and/or during inspection, and is usable as a material for a boiler tube that can maintain its reliability over a long term.
• the austenitic stainless steel according to the present invention has a Nb content of 0.10 to 0.60 percent by mass and has a ratio of the Ta content to the Nb content of 0.8 to 4.0 as described above.
• the austenitic stainless steel in this embodiment can exhibit particularly excellent creep strength. Reasons for this will be described below.
• the present inventors have found that Nb, when contained in a content of 0.10 to 0.60 percent by mass, may exhibit these effects particularly advantageously.
• Ta when contained in a ratio of Ta to Nb (Ta/Nb) within a predetermined range, may be dissolved in the Z phase in an optimum amount and allows the steel to have higher creep strength.
• the austenitic stainless steel if having a ratio Ta/Nb of less than 0.8, may include solute tantalum in a small amount and may fail to have higher creep strength in an expected manner.
• the austenitic stainless steel if having a ratio Ta/Nb of greater than 4.0, may have inferior ductility due to a large amount of solute tantalum and may suffer from poor economic efficiency.
• the ratio Ta/Nb may be controlled to the range of from 0.8 to 4.0.
• the austenitic stainless steel according to the present invention may further include at least one of groups (a) and (b) below.
• the austenitic stainless steel can have higher high-temperature strength and/or better oxidation resistance depending on an element to be contained
• the groups (a) and (b) are as follows:
• W 4 percent by mass or less and Mo: 4 percent by mass or less
• Tungsten (W) and molybdenum (Mo) offer solute strengthening and effectively allow the steel to have higher high-temperature strength. These elements, when contained according to necessity, allow the austenitic stainless steel to have higher high-temperature strength.
• tungsten if contained in an excessively high content of greater than 4 percent by mass, may form coarse intermetallic compounds to cause the austenitic stainless steel to have inferior hot ductility. To prevent this, the tungsten content is preferably 4 percent by mass or less (excluding 0 percent by mass), more preferably 3 percent by mass or less, and furthermore preferably 2 percent by mass or less.
• Mo if contained in an excessively high content of greater than 4 percent by mass, may adversely affect the hot workability.
• the Mo content is preferably 4 percent by mass or less (excluding 0 percent by mass), more preferably 3 percent by mass or less, and furthermore preferably 2 percent by mass or less.
• the tungsten content is preferably 0.1 percent by mass or more, and more preferably 0.5 percent by mass or more.
• the Mo content is preferably 0.1 percent by mass or more, and more preferably 0.5 percent by mass or more.
• these elements cause increased cost, although they exhibit the above-mentioned effects.
• the contents of these elements are preferably set depending on the necessary magnitude of strengthening and an allowable cost.
• Cu forms conformable precipitates in the steel and allows the steel to have significantly higher creep high-temperature strength and acts as one of main strengthening mechanisms in the stainless steel.
• conformable precipitates refers to precipitates having an atomic arrangement continuous with that of the parent metal.
• the Cu content is preferably 4 percent by mass or less (excluding 0 percent by mass), more preferably 3.7 percent by mass or less, and furthermore preferably 3.5 percent by mass or less.
• the Cu content is preferably 0.2 percent by mass or more, more preferably 2 percent by mass or more, and furthermore preferably 2.5 percent by mass or more.
• Rare-earth element 0.15 percent by mass or less
• Rare-earth elements usable herein are exemplified by seventeen elements including Sc and Y as well as lanthanoid elements typified by La, Ce, and Nd.
• the rare-earth elements effectively allow the stainless steel to have better oxidation resistance and can restrain the formation of oxidized scale on the inner surface of a heat-exchanger tube through which high-temperature and high-pressure steam passes.
• the rare-earth element or elements if contained in an excessively high content of greater than 0.15 percent by mass, may cause the grain boundaries to partially melt in a high-temperature environment to adversely affect the hot workability.
• the rare-earth element content is preferably 0.15 percent by mass or less, more preferably 0.1 percent by mass or less, and furthermore preferably 0.05 percent by mass or less.
• the rare-earth element content is preferably 0.01 percent by mass or more, more preferably 0.015 percent by mass or more, and furthermore preferably 0.02 percent by mass or more.
• the austenitic stainless steel when configured as above, can give a boiler heat-exchanger tube (boiler tube) having excellent toughness.
• the individual rare-earth elements may be added independently to the steel, but may be added as so-called misch metal including these elements. The misch metal, when used, contributes to lower cost for the separation of individual elements and contributes to better profitability.
• Ca and Mg contents act as desulfurizing and deoxidizing elements.
• the Ca and Mg contents are each preferably 0.005 percent by mass or less (excluding 0 percent by mass), and more preferably 0.002 percent by mass or less.
• the Ca and Mg contents are each preferably 0.0002 percent by mass or more, and more preferably 0.0005 percent by mass or more.
• Vanadium (V), titanium (Ti), zirconium (Zr), and hafnium (Hf) exhibit effects as with Nb, but when added in combination, may allow the precipitates to be still further stabilized and may effectively allow the steel to maintain high-temperature strength for a long term.
• these elements if contained in excessively high contents, may fail to be dissolved in the steel and may thereby cause the steel to include larger amounts of inclusions and to have inferior toughness.
• the contents of V, Ti, Zr, and Hf are each preferably controlled to 0.2 percent by mass or less (excluding 0 percent by mass), more preferably 0.15 percent by mass or less, and furthermore preferably 0.1 percent by mass or less.
• the contents of V, Ti, Zr, and Hf are each preferably 0.02 percent by mass or more, more preferably 0.04 percent by mass or more, and furthermore preferably 0.06 percent by mass or more.
• the contents of V, Ti, Zr, and Hf are each preferably 0.02 percent by mass or more, more preferably 0.04 percent by mass or more, and furthermore preferably 0.06 percent by mass or more.
• two or more of these elements when contained in combination, are preferably contained in a total content of 0.4 percent by mass or less.
• the austenitic stainless steel according to the embodiment of the present invention may be produced typically by melting a steel while adding individual elements to give the chemical compositions in primary refining, and sequentially performing production processes such as secondary refining according to a common procedure. Examples
• Experimental Example 1 Experimental example relating to toughness after aging
• the steels of Test Nos. 1A to 12A are steels having chemical compositions within ranges specified in the present invention for austenitic stainless steels excellent particularly in toughness after aging. These are steels according to the present invention.
• the steels of Test Nos.13A to 19A are steels having chemical compositions out of the ranges specified in the present invention for the austenitic stainless steels excellent particularly in toughness after aging. These are comparative steels. Of these steels, the steel of Test No.19A corresponds to the existing steel "KA-SUS 310J1HTB".
• the steel (Test No.19A) corresponding to "KA-SUS 310J1HTB” belongs to 25Cr-20Ni austenitic stainless steels and is a steel grade having been actually used in boiler heat-exchanger tubes (boiler tubes). Italicized and underlined numerical values in Table 1 refer to data out of the chemical composition ranges specified in the present invention. The symbol "-" in Table 1 indicates that an element in question is not added. A material for rare-earth elements to be added was a misch metal including Ce, La, and Nd. [Table 1] Test No.
• the steels of Test No.17A and Test No.18A were samples respectively having Si and S (sulfur) contents out of (greater than the upper limits of) the chemical compositions specified in the present invention.
• the results demonstrated that steels as with Test Nos. 1A to 12A can have better toughness, because these steels are designed to contain Ta in a content within the specific range specified in the present invention while controlling the contents of Si and S within the ranges specified in the present invention.
• the base metals of Test Nos. 2B, 3B, 5B, 6B, 7B, 10B, 11B, 13B, 15B, and 17B are base metals as examples having chemical compositions within the ranges specified in the present invention for austenitic stainless steels excellent particularly in creep strength.
• the base metals of Test Nos.1B, 4B, 8B, 9B, 12B, 14B, and 16B are base metals as comparative examples having chemical compositions out of the ranges specified in present invention for austenitic stainless steels excellent particularly in creep strength.
• the base metal of Test No. 8B is a base metal including a steel corresponding to the existing steel "KA-SUS 310J1HTB".
• austenitic stainless steels have the precipitation strengthening of the Nb compound (CrNbN) and tend to have better precipitation strengthening and a longer creep rupture time with an increasing amount (content) of Nb.
• Ta also contributes to precipitation strengthening as with Nb.
• the total sums of the atomic concentrations (atomic percentages) of Ta and Nb were set at approximately the same level among Test Nos.1B to 3B, among 4B to 8B, among 9B to 11B, between 12B and13B, between 14B and 15B, and between 16B to 17B. This was performed so as to distinguish the effect obtained by the total amount of Nb and Ta from the effect obtained by allowing Ta to substitute for part of Nb.
• the base metals were machined to give creep test specimens having a diameter of 6 mm, and the creep test specimens were subjected to creep rupture tests at 700°C and 189 MPa using a multi-specimen creep testing machine. Measured creep rupture times are indicated in Table 3 below. [Table 3] Test No.
• the results in Table 3 demonstrated that the base metals according to the examples having chemical compositions within the ranges specified in the present invention had longer creep rupture times and had more excellent creep strengths as compared with the base metals according to the comparative examples.
• the ranges are specified for austenitic stainless steels excellent particularly in creep strength.
• the base metals according to the comparative examples included the existing steel (Test No. 8B) or included the stainless steels having chemical compositions out of the ranges specified in the present invention.
• the creep strengths were evaluated by comparing stainless steels having total sums of the atomic concentrations of Ta and Nb at approximately the same levels.
• the base metal of Test No. 8B is a conventional boiler tube material.
• a comparison between the base metals of Test Nos. 5B to 7B (examples) and the base metal of Test No. 8B (comparative example) demonstrated that the base metals, when containing Ta substituting for part of Nb, could have more excellent creep strengths as compared with the conventional material.
• the austenitic stainless steels according to the embodiments of the present invention are suitable as materials for thermal power generation boiler tubes, and other parts and apparatuses to be used in a high-temperature and oxidizing environment.

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• 2013
  • 2013-10-29 WO PCT/JP2013/079292 patent/WO2014069467A1/ja active Application Filing
  • 2013-10-29 KR KR1020157010895A patent/KR20150060942A/ko not_active Application Discontinuation
  • 2013-10-29 EP EP13850082.2A patent/EP2915893A4/de not_active Withdrawn
  • 2013-10-29 CN CN201380055256.6A patent/CN104736735A/zh active Pending
  • 2013-10-29 CN CN201811208834.6A patent/CN109321822A/zh active Pending

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