WO2018003823A1 - Austenitic stainless steel - Google Patents
Austenitic stainless steel Download PDFInfo
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- WO2018003823A1 WO2018003823A1 PCT/JP2017/023657 JP2017023657W WO2018003823A1 WO 2018003823 A1 WO2018003823 A1 WO 2018003823A1 JP 2017023657 W JP2017023657 W JP 2017023657W WO 2018003823 A1 WO2018003823 A1 WO 2018003823A1
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
Definitions
- the present invention relates to stainless steel, and more particularly to austenitic stainless steel.
- heat-resistant steel austenitic stainless steel with increased Cr content and Ni content, or Ni base with increased Cr content
- An alloy is used.
- These heat resistant steels are austenitic stainless steels or Ni-based alloys containing about 20 to 30% by mass of Cr and about 20 to 70% by mass of Ni.
- Piping for equipment such as thermal power boilers and chemical plants is manufactured from steel pipes.
- the steel base tube is manufactured by hot working after the above-described austenitic stainless steel or Ni-based alloy is melted. Therefore, high heat workability is required for heat resistant steel.
- austenitic stainless steel generally has high deformation resistance at high temperatures and low ductility. Therefore, an austenitic stainless steel having excellent hot workability is required.
- Patent Document 1 Japanese Patent Application Laid-Open No. 2005-48284
- the stainless steel disclosed in Patent Document 1 is, in mass%, C: 0.01 to 0.6%, Si: 0.1 to 5%, Mn: 0.1 to 10%, P: 0.08%
- O (oxygen): 0.02% or less the balance being Fe and It consists of a base material having a chemical composition consisting of inevitable impurities.
- This stainless steel is provided with a Cr-deficient layer in the surface layer portion, the Cr concentration in the Cr-deficient layer is 10% or more and less than the Cr concentration of the base material, and the thickness of the Cr-deficient layer is within 20 ⁇ m.
- Patent Document 1 describes that carburization resistance and coking resistance are enhanced by forming a protective film mainly composed of a Cr 2 O 3 film.
- the main component of the protective film is a Cr 2 O 3 film. Therefore, particularly in a high-temperature carburizing environment, the function of preventing the entry of oxygen and carbon from the external atmosphere is not sufficient. As a result, internal oxidation and carburization may occur in the material.
- Patent Document 2 International Publication No. 2010/113830
- Patent Document 3 International Publication No. 2004/066788
- Patent Document 4 Japanese Patent Laid-Open No. 10-140296
- Patent Document 4 replace Cr 2 O 3 coating.
- a technique relating to a protective film is disclosed. Specifically, in these documents, a protective film mainly composed of thermodynamically stable Al 2 O 3 is formed on the surface of the heat-resistant steel as a protective film replacing the Cr 2 O 3 film.
- the casting product disclosed in Patent Document 2 is, in mass%, C: 0.05 to 0.7%, Si: more than 0% to 2.5% or less, Mn: more than 0% to 3.0% %: Cr: 15-50%, Ni: 18-70%, Al: 2-4%, Rare earth elements: 0.005-0.4%, and W: 0.5-10% and / or Mo : It has a cast body of a heat-resistant alloy containing 0.1 to 5%, the balance being Fe and inevitable impurities.
- a barrier layer is formed on the surface of the cast body in contact with the high-temperature atmosphere, and the barrier layer is an Al 2 O 3 layer having a thickness of 0.5 ⁇ m or more, and 80% by area or more of the outermost surface of the barrier layer is It is Al 2 O 3 , and Cr base particles having a Cr concentration higher than that of the base of the alloy are dispersed at the interface between the Al 2 O 3 layer and the cast body.
- Patent Document 2 describes that by adding Al, a protective film mainly composed of an Al 2 O 3 film is formed, and the carburization resistance is improved.
- the cast nickel-chromium alloy disclosed in US Pat. No. 6,057,059 contains up to 0.8% carbon, up to 1% silicon, up to 0.2% manganese, 15% -40% chromium, 0.5% -13. % Iron, 1.5% to 7% aluminum, up to 2.5% niobium, up to 1.5% titanium, 0.01% to 0.4% zirconium, up to 0.06% nitrogen, Up to 12% cobalt, up to 5% molybdenum, up to 6% tungsten, 0.019% to 0.089% yttrium, the rest being nickel.
- Patent Document 3 the addition of REM in addition to Al, nickel enhanced peeling resistance of the Al 2 O 3 is a protective coating - chrome cast alloys are obtained are described as.
- the austenitic stainless steel disclosed in Patent Document 4 is, by weight, C: 0.15% or less, Si: 0.9% or less, Mn: 0.2-2%, P: 0.04% or less, S: 0.005% or less, and S (%) and O (%) combined to 0.015% or less, Cr: 12-30%, Ni: 10-35%, Al: 1.5-5.5 %, B: 0.001 to 0.01%, N: 0.025% or less, Ca: 0 to 0.008%, Cu: 0 to 2%, Ti, Nb, Zr, V and 1 of Hf Contains 0 to 2% in total of one or more species, 0 to 3% in total of one or more of W, Mo, Co and Re, and 0 to 0.05% in total of one or more of rare earth elements The balance consists of Fe and inevitable impurities.
- the heat-resistant alloy contains up to 50% of Cr. Therefore, Cr may form as a carbide on the steel surface in a high-temperature carburizing environment such as a hydrocarbon gas atmosphere. In this case, Al 2 O 3 which is a protective film is not uniformly formed. Therefore, carburization may occur.
- the cast products and nickel-chromium cast alloys disclosed in Patent Documents 2 and 3 have a high C content, so the hot workability is significantly reduced.
- Patent Document 3 since the Ni content is high, the raw material cost is remarkably increased.
- An object of the present invention is to provide an austenitic stainless steel having excellent carburization resistance even in a high-temperature carburizing environment such as a hydrocarbon gas atmosphere, and further having excellent hot workability at the time of manufacture. .
- the austenitic stainless steel according to the present embodiment in mass%, C: 0.03 to less than 0.25%, Si: 0.01 to 2.0%, Mn: 2.0% or less, P: 0.04 %: S: 0.01% or less, Cr: 10 to less than 22%, Ni: more than 30.0% to 40.0%, Al: more than 2.5% to less than 4.5%, Nb: 0.
- the austenitic stainless steel according to the present embodiment has excellent carburization resistance even in a high-temperature carburizing environment such as a hydrocarbon gas atmosphere, and further has excellent hot workability during manufacturing.
- the present inventors investigated and examined the carburization resistance of austenitic stainless steel in a high-temperature carburizing environment and hot workability during production, and obtained the following knowledge.
- the high-temperature carburizing environment refers to an environment of 1000 ° C. or higher in a hydrocarbon gas atmosphere.
- TEE effect Cr Third Element Effect
- Cr Cr Third Element Effect
- the mechanism of the TEE effect is as follows. At the very beginning of the heat treatment step described later, Cr is first preferentially oxidized on the steel surface to form Cr 2 O 3 . For this reason, the oxygen partial pressure on the steel surface is locally reduced. Thus, Al is formed as a uniform Al 2 O 3 film in the vicinity of the surface without internal oxidation. Thereafter, oxygen was used as a Cr 2 O 3 is incorporated into the Al 2 O 3. Then, the protective coating of only Al 2 O 3 is in the heat treatment step at the end is formed.
- Cr has a TEE effect even in a high-temperature carburizing environment. That is, Cr promotes uniform formation of the Al 2 O 3 film even in a high-temperature carburizing environment. Therefore, in order to form a uniform Al 2 O 3 film, it is necessary to contain a certain amount or more of Cr.
- the Cr content is set to less than 10 to 22% in the present invention.
- (C) In the austenitic stainless steel, it is effective to make the ratio of the Cr concentration in the surface layer and the Al concentration in the surface layer appropriately smaller than the ratio between the Cr concentration other than the surface layer and the Al concentration other than the surface layer. That is, if the austenitic stainless steel satisfies the formula (1), the carburization resistance in a high-temperature carburizing environment is enhanced. 0.40 ⁇ (C Cr ′ / C Al ′) / (C Cr / C Al ) ⁇ 0.80 (1)
- the Cr concentration (% by mass) in the surface layer of the austenitic stainless steel is substituted for C Cr ′ in the formula (1).
- Al concentration (% by mass) in the surface layer of austenitic stainless steel is substituted for C Al '. Also, the C Cr Cr concentration than the surface layer of austenitic stainless steel (mass%) is substituted. Al concentration (mass%) other than the surface layer of austenitic stainless steel is substituted for C Al .
- F1 (C Cr '/ C Al ') / (C Cr / C Al ). If F1 is 0.40 or more, the TEE effect by Cr is sufficiently obtained on the steel surface in a high-temperature carburizing environment. In this case, formation of the Al 2 O 3 film is promoted. If F1 is 0.80 or less, formation of Cr carbide on the steel surface is suppressed in a high-temperature carburizing environment. Therefore, uniform Al 2 O 3 film is formed. As a result, carburization resistance is increased.
- the austenitic stainless steel according to the present embodiment completed on the basis of the above knowledge is in mass%, C: 0.03 to less than 0.25%, Si: 0.01 to 2.0%, Mn: 2.0 %: P: 0.04% or less, S: 0.01% or less, Cr: 10 to less than 22%, Ni: more than 30.0% to 40.0%, Al: more than 2.5% to 4.
- the balance has a chemical composition consisting of Fe and impurities, and satisfies the formula (1).
- the chemical composition is Ti: 0.005 to less than 0.2%, Mo: 0.01 to 0.5%, W: 0.01 to 0.5%, Cu: 0.005 to 0.5%, One or two or more selected from the group consisting of V: 0.005 to 0.2% and B: 0.0001 to 0.01 may be contained.
- the chemical composition of the austenitic stainless steel according to the present embodiment contains the following elements.
- C 0.03 to less than 0.25%
- Carbon (C) mainly bonds with Cr to form Cr carbide in the steel, and increases the creep strength when used in a high-temperature carburizing environment. If the C content is too low, this effect cannot be obtained. On the other hand, if the C content is too high, a large number of coarse eutectic carbides are formed in the solidified structure after casting of steel, and the toughness of the steel is lowered. Therefore, the C content is 0.03 to less than 0.25%.
- the minimum with preferable C content is 0.05%, More preferably, it is 0.08%.
- the upper limit with preferable C content is 0.23%, More preferably, it is 0.20%.
- Si 0.01 to 2.0% Silicon (Si) deoxidizes steel. In the case where deoxidation can be sufficiently performed with other elements, the Si content may be as small as possible. On the other hand, if the Si content is too high, the hot workability decreases. Therefore, the Si content is 0.01 to 2.0%.
- the minimum with preferable Si content is 0.02%, More preferably, it is 0.03%.
- the upper limit with preferable Si content is 1.0%.
- Mn 2.0% or less Manganese (Mn) is unavoidably contained. Mn combines with S contained in the steel to form MnS and enhances the hot workability of the steel. However, if the Mn content is too high, the steel becomes too hard and the hot workability and weldability deteriorate. Therefore, the Mn content is 2.0% or less. The minimum with preferable Mn content is 0.1%, More preferably, it is 0.2%. The upper limit with preferable Mn content is 1.2%.
- P 0.04% or less Phosphorus (P) is an impurity. P decreases the weldability and hot workability of steel. Therefore, the P content is 0.04% or less.
- the upper limit with preferable P content is 0.03%.
- the P content is preferably as low as possible.
- the lower limit of the P content is, for example, 0.0005%.
- S 0.01% or less Sulfur (S) is an impurity. S decreases the weldability and hot workability of steel. Therefore, the S content is 0.01% or less.
- the upper limit with preferable S content is 0.008%.
- the S content is preferably as low as possible.
- the lower limit of the S content is, for example, 0.001%.
- Chromium (Cr) promotes the formation of an Al 2 O 3 film during the heat treatment process and in a high-temperature carburizing environment due to the above-described TEE effect. Further, Cr combines with C in the steel to form Cr carbide in the steel, increasing the creep strength. If the Cr content is too low, these effects cannot be obtained. On the other hand, if the Cr content is too high, Cr combines with C derived from atmospheric gas (hydrocarbon gas) in a high-temperature carburizing environment, and forms Cr carbide on the steel surface. When Cr carbide is formed on the steel surface, Cr on the steel surface is locally depleted. Therefore TEE effect decreases, uniform Al 2 O 3 film is not formed.
- Chromium (Cr) promotes the formation of an Al 2 O 3 film during the heat treatment process and in a high-temperature carburizing environment due to the above-described TEE effect. Further, Cr combines with C in the steel to form Cr carbide in the steel, increasing the creep strength. If the Cr content is too low
- the Cr content is 10 to less than 22%.
- the minimum with preferable Cr content is 11%, More preferably, it is 12%.
- the upper limit with preferable Cr content is 21%, More preferably, it is 20%.
- Cr carbide is divided into Cr carbide formed in steel and Cr carbide formed on the steel surface. In the austenitic stainless steel of this embodiment, Cr carbide is formed in the steel, and Cr carbide on the steel surface is suppressed.
- Ni more than 30.0% to 40.0%
- Nickel (Ni) stabilizes austenite and increases creep strength. Ni further enhances the carburization resistance of the steel. If the Ni content is too low, these effects cannot be obtained. On the other hand, if the Ni content is too high, these effects are not only saturated, but the raw material costs are increased. Therefore, the Ni content is more than 30.0% to 40.0%.
- the minimum with preferable Ni content is 31.0%, More preferably, it is 32.0%.
- the upper limit with preferable Ni content is 39.0%, More preferably, it is 38.0%.
- Al more than 2.5% to less than 4.5%
- Aluminum (Al) forms an Al 2 O 3 film on the steel surface during the heat treatment process and in a high-temperature carburizing environment, thereby improving the carburization resistance of the steel.
- the Al 2 O 3 film is thermodynamically stable as compared with the conventionally used Cr 2 O 3 film. If the Al content is too low, these effects cannot be obtained. On the other hand, if the Al content is too high, the structural stability is lowered and the creep strength is significantly lowered. Therefore, the Al content is more than 2.5% to less than 4.5%.
- the minimum with preferable Al content is 2.55%, More preferably, it is 2.6%.
- the upper limit with preferable Al content is 4.2%, More preferably, it is 4.0%.
- the Al content means the total amount of Al contained in the steel material.
- Niobium forms an intermetallic compound (Laves phase and Ni 3 Nb phase) that becomes a precipitation strengthening phase, and precipitates and strengthens the grain boundaries and inside the crystal grains, thereby increasing the creep strength of the steel.
- Nb content is 0.01 to 3.5%.
- the minimum with preferable Nb content is 0.05%, More preferably, it is 0.1%.
- the upper limit with preferable Nb content is less than 3.2%, More preferably, it is 3.0%.
- N 0.03% or less Nitrogen (N) stabilizes austenite and is unavoidably contained.
- N content is 0.03% or less.
- a preferable upper limit of the N content is 0.01%.
- the lower limit of the N content is, for example, 0.0005%.
- Ca 0.0005 to 0.05%
- Ca fixes S as a sulfide and improves hot workability.
- the Ca content is too high, toughness and ductility are reduced. Therefore, hot workability is reduced.
- the Ca content is too high, the cleanliness is further deteriorated. Therefore, the Ca content is 0.0005 to 0.05%.
- a preferable lower limit of Ca is 0.0006%, and more preferably 0.0008%.
- the upper limit with preferable Ca content is 0.01%, More preferably, it is 0.008%.
- Mg 0.0005 to 0.05%
- Mg Magnesium (Mg) fixes S as a sulfide and improves the hot workability of steel.
- Mg content is too high, toughness and ductility are reduced. Therefore, hot workability is reduced.
- Mg content is too high, the cleanliness is further deteriorated. Therefore, the Mg content is 0.0005 to 0.05%.
- a preferable lower limit of Mg is 0.0006%, and more preferably 0.0008%.
- the upper limit with preferable Mg content is 0.01%, More preferably, it is 0.008%.
- the balance of the chemical composition of the austenitic stainless steel of this embodiment is composed of Fe and impurities.
- the impurities are those that are mixed from ore, scrap, or production environment as raw materials when industrially producing austenitic stainless steel, and are allowed within a range that does not adversely affect the present invention. Means what will be done.
- the chemical composition of the austenitic stainless steel described above may further contain Ti instead of a part of Fe.
- Titanium (Ti) is an optional element and may not be contained. When contained, Ti forms an intermetallic compound (Laves phase and Ni 3 Ti phase) that becomes a precipitation strengthening phase, and increases the creep strength by precipitation strengthening. On the other hand, if the Ti content is too high, an intermetallic compound is excessively produced, and high temperature ductility and hot workability are reduced. If the Ti content is too high, the toughness after aging for a long time further decreases. Therefore, the Ti content is 0 to less than 0.2%.
- the minimum with preferable Ti content is 0.005%, More preferably, it is 0.01%.
- the upper limit with preferable Ti content is 0.15%, More preferably, it is 0.1%.
- the chemical composition of the austenitic stainless steel described above may further include one or two selected from the group consisting of Mo and W instead of part of Fe. All of these elements are optional elements and increase the creep strength of steel.
- Mo 0 to 0.5% Molybdenum (Mo) is an optional element and may not be contained. When contained, Mo dissolves in the austenite that is the parent phase. The solid solution Mo increases the creep strength by solid solution strengthening. On the other hand, if Mo content is too high, hot workability will fall. Therefore, the Mo content is 0 to 0.5%.
- the minimum with preferable Mo content is 0.01%, More preferably, it is 0.05%.
- the upper limit with preferable Mo content is 0.4%, More preferably, it is 0.3%.
- W 0-0.5% Tungsten (W) is an optional element and may not be contained. When contained, W forms a solid solution in the austenite that is the parent phase. The solid solution W increases the creep strength by solid solution strengthening. On the other hand, if the W content is too high, the hot workability decreases. Accordingly, the W content is 0 to 0.5%.
- the minimum with preferable W content is 0.01%, More preferably, it is 0.05%.
- the upper limit with preferable W content is 0.4%, More preferably, it is 0.3%.
- the chemical composition of the austenitic stainless steel described above may further contain Cu instead of part of Fe.
- Cu 0 to 0.5% Copper (Cu) is an optional element and may not be contained. When contained, Cu stabilizes austenite. Cu further increases the strength of the steel by precipitation strengthening. On the other hand, if Cu content is too high, the ductility and hot workability of steel will fall. Therefore, the Cu content is 0 to 0.5%.
- the minimum with preferable Cu content is 0.005%, More preferably, it is 0.01%.
- the upper limit with preferable Cu content is 0.3%, More preferably, it is 0.1%.
- the chemical composition of the austenitic stainless steel described above may further contain V instead of a part of Fe.
- V 0 to 0.2%
- Vanadium (V) is an optional element and may not be contained. When contained, V forms an intermetallic compound similar to Ti and increases the creep strength of steel. On the other hand, if the V content is too high, the volume ratio of the intermetallic compound in the steel becomes excessively high, and the hot workability decreases. Therefore, the V content is 0 to 0.2%.
- the minimum with preferable V content is 0.005%, More preferably, it is 0.01%.
- the upper limit with preferable V content is 0.15%, More preferably, it is 0.1%.
- the chemical composition of the austenitic stainless steel described above may further contain B instead of a part of Fe.
- B 0 to 0.01% Boron (B) is an optional element and may not be contained. When contained, B segregates at the grain boundary and promotes precipitation of intermetallic compounds at the grain boundary. This increases the creep strength of the steel. On the other hand, if the B content is too high, the weldability and hot workability of the steel deteriorate. Therefore, the B content is 0 to 0.01%.
- the minimum with preferable B content is 0.0001%, More preferably, it is 0.0005%.
- the upper limit with preferable B content is 0.008%, More preferably, it is 0.006%.
- the austenitic stainless steel of this embodiment further satisfies the formula (1). 0.40 ⁇ (C Cr ′ / C Al ′) / (C Cr / C Al ) ⁇ 0.80 (1)
- the Cr concentration (% by mass) in the surface layer of the austenitic stainless steel is substituted for C Cr ′ in the formula (1).
- Al concentration (% by mass) in the surface layer of austenitic stainless steel is substituted for C Al '.
- the C Cr Cr concentration than the surface layer of austenitic stainless steel (mass%) is substituted.
- Al concentration (mass%) other than the surface layer of austenitic stainless steel is substituted for C Al .
- the surface layer of austenitic stainless steel means a range from the surface of austenitic stainless steel to a depth of 2 ⁇ m.
- 2 ⁇ m depth from the surface means 2 ⁇ m depth from the surface of the base material.
- the depth of 2 ⁇ m from the surface of the base material is 2 ⁇ m from the surface of the base material after removing the Al 2 O 3 coating by descaling.
- the surface of the austenitic stainless steel in the case where the austenitic stainless steel has an Al 2 O 3 coating on the surface, the Al 2 O 3 coating is removed by descaling treatment) in C Cr ′ in the formula (1).
- C Al ′ in formula (1) is the surface of austenitic stainless steel (if the austenitic stainless steel has an Al 2 O 3 coating on the surface, the mother after removing the Al 2 O 3 coating by descaling treatment) Al concentration (mass%) in a range from the surface of the material to a depth of 2 ⁇ m is substituted.
- the Cr concentration (% by mass) other than the surface layer means the average Cr concentration (% by mass) of the base material in the region other than the surface layer.
- the Al concentration (mass%) other than the surface layer means the average Al concentration (mass%) of the base material in the region other than the surface layer.
- the ratio of the Cr concentration of the surface layer to the Al concentration of the surface layer is more appropriate than the ratio of the Cr concentration of the base material to the Al concentration of the base material. small.
- the formation of the Al 2 O 3 film is promoted.
- carburization resistance is enhanced in a high-temperature carburizing environment.
- F1 (C Cr '/ C Al ') / (C Cr / C Al ).
- F1 is an index of Cr behavior.
- the ratio between the Cr concentration in the surface layer and the Al concentration in the surface layer is too large than the ratio between the Cr concentration in the base material and the Al concentration in the base material. That is, the surface layer Cr concentration C Cr ′ is too high.
- Cr carbide is formed on the steel surface in a high-temperature carburizing environment, and the formation of a uniform Al 2 O 3 film is physically hindered.
- the ratio between the Cr concentration in the surface layer and the Al concentration in the surface layer is too small than the ratio between the Cr concentration in the base material and the Al concentration in the base material. That is, the Cr concentration C Cr ′ of the surface layer is too small. In this case, the TEE effect of Cr cannot be obtained in a high-temperature carburizing environment. Therefore, uniform Al 2 O 3 film on the steel surface is not formed.
- F1 is 0.40 to 0.80.
- the minimum with preferable F1 is 0.42, More preferably, it is 0.44.
- the upper limit with preferable F1 is 0.79, More preferably, it is 0.78.
- the surface layer Cr concentration C Cr ′ and the surface layer Al concentration C Al ′ are determined by the following method.
- Austenitic stainless steel is cut perpendicular to the surface. From the surface of the cut austenitic stainless steel (when the austenitic stainless steel has an Al 2 O 3 coating on the surface, the surface of the base material after removing the Al 2 O 3 coating by descaling) to a depth of 2 ⁇ m In the range, arbitrary 5 points (measurement points) are selected.
- the Cr concentration and the Al concentration at each measurement point are measured by EDX (energy dispersive X-ray spectroscopy). Values obtained by averaging the measured values are defined as C Cr ′ (%) and C Al ′ (%).
- the surface layer Cr concentration C Cr ′ and the surface layer Al concentration C Al ′ are measured.
- the conditions for descaling austenitic stainless steel conform to JIS Z 2290 (2004).
- Analysis of the Cr concentration C Cr other than the surface layer and the Al concentration C Al other than the surface layer can be obtained by a well-known component analysis method. Specifically, the following method is used. An austenitic stainless steel is cut perpendicularly to the longitudinal direction (in the axial direction in the case of a steel pipe) to prepare a measurement surface. Drill the center of thickness of the measurement surface with a drill. Chips are generated by drilling and collected. Collect chips from four locations on the same measurement surface. When the austenitic stainless steel is a steel pipe, chips are collected from four locations at a 45 ° pitch. The collected chips are subjected to ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry) to perform elemental analysis of chemical composition. The analysis procedure by ICP-OES conforms to JIS G 1258 (2007). The average of the four measured values is defined as the Cr concentration C Cr (%) other than the surface layer and the Al concentration C Al (%) other than the surface layer.
- ICP-OES Inductive
- the austenitic stainless steel of this embodiment is provided with an Al 2 O 3 coating on the surface after the heat treatment step described later. Therefore, the austenitic stainless steel of this embodiment may have an Al 2 O 3 coating on the surface.
- the Al 2 O 3 film can be removed by a known method such as pickling after the heat treatment step or shot peening. Therefore, the austenitic stainless steel of this embodiment may be in a state where the surface Al 2 O 3 film is removed.
- the austenitic stainless steel of this embodiment has a crystal grain size of 30 to 80 ⁇ m. If the crystal grain size is 30 ⁇ m or more, the creep strength of the steel is further increased. If the crystal grain size is 80 ⁇ m or less, the grain boundary diffusion of Al is promoted and the formation of the Al 2 O 3 film is further promoted.
- the crystal grain size is determined by a microscopic test method for crystal grain size specified in JIS G 0551 (2013).
- the shape of the austenitic stainless steel according to the present embodiment is not particularly limited.
- Austenitic stainless steel is, for example, a steel pipe.
- Austenitic stainless steel pipes are used as reaction tubes for chemical plants.
- the austenitic stainless steel may be a plate, rod, wire, or the like.
- a steel pipe manufacturing method will be described as an example of a method for manufacturing the austenitic stainless steel according to the present embodiment.
- a molten steel having the above chemical composition is produced.
- a well-known degassing process is implemented with respect to molten steel as needed.
- a raw material is manufactured by casting using molten steel.
- the material may be an ingot obtained by an ingot-making method or a slab such as a slab, bloom or billet obtained by a continuous casting method.
- you may manufacture a pipe-shaped casting by the centrifugal casting method.
- Hot forging process You may manufacture a cylindrical raw material by implementing hot forging with respect to the manufactured raw material. If hot forging is carried out, the internal structure of the molten steel produced in the preparation process can be changed from a solidified structure to a uniform sized structure.
- the temperature of hot forging is 900 to 1200 ° C., for example.
- a steel base tube is manufactured by performing hot working on the material manufactured in the preparation process or the material that has been hot forged (cylindrical material). For example, a through hole is formed in the center of a cylindrical material by machining.
- a steel base tube is manufactured by performing hot extrusion on a cylindrical material in which a through hole is formed. The processing temperature for hot extrusion is, for example, 900 to 1200 ° C.
- a steel blank may be manufactured by piercing and rolling a cylindrical material (Mannesmann method or the like).
- Cold work is performed on the steel base tube after hot working to produce an intermediate material.
- the cold working is, for example, cold drawing or the like. If strain is applied to the steel surface in the cold working process, elements such as Al and Cr easily move to the steel surface. In this case, the TEE effect can be sufficiently obtained. Thereby, Cr of a steel surface layer is lacking moderately, and the austenitic stainless steel which satisfy
- the heat treatment temperature is 900-1100 ° C., and the heat treatment time is 3.0-30.0 minutes.
- the heat treatment temperature is less than 900 ° C. or the heat treatment time is less than 3.0 minutes, the TEE effect cannot be sufficiently obtained.
- the Cr concentration C Cr ′ of the steel surface layer becomes too high and does not satisfy the formula (1).
- Cr carbide is formed on the steel surface in a high-temperature carburizing environment, and a uniform Al 2 O 3 film is not sufficiently formed.
- the heat treatment temperature is 900 ° C. or higher, and the heat treatment time is 3.0 minutes or longer. If the heat treatment temperature is 900 ° C. or more and the heat treatment time is 3.0 minutes or more, the crystal grains are 30 ⁇ m or more.
- the Al concentration C Al ′ of the steel surface layer becomes too low and does not satisfy the formula (1). Therefore, a uniform Al 2 O 3 film is not sufficiently formed in a high-temperature carburizing environment. As a result, carburization resistance is reduced. Therefore, the heat treatment temperature is less than 1100 ° C. and the heat treatment time is 30.0 minutes or less. If the heat treatment temperature is less than 1100 ° C. and the heat treatment time is 30.0 minutes or less, the crystal grains will be 80 ⁇ m or less.
- the heat treatment temperature is 900 to less than 1100 ° C. and the heat treatment time is 3.0 to 30.0 minutes, a TEE effect can be obtained sufficiently and appropriately, and a steel having a chemical composition satisfying the formula (1) can be obtained. As a result, carburization resistance in a high-temperature carburizing environment is enhanced.
- ⁇ Pickling treatment may be applied to the intermediate material after heat treatment for the purpose of removing scale formed on the surface.
- pickling for example, a mixed acid solution of nitric acid and hydrochloric acid is used.
- the pickling time is, for example, 30 minutes to 60 minutes.
- shot processing may be performed on the steel surface for the purpose of descaling the steel surface and imparting strain to the steel surface with respect to the pickled intermediate material.
- the material, shape, and processing conditions of the shot grains in the shot processing are not specified, but the material, shape, and processing conditions are sufficient for exfoliating the scale of the steel surface or imparting strain to the steel surface.
- the scale is, for example, Al 2 O 3 .
- the Al 2 O 3 film can be removed by known methods such as pickling and shot processing.
- the austenitic stainless steel of this embodiment is manufactured by the above manufacturing method.
- the manufacturing method of the steel pipe was demonstrated above.
- the austenitic stainless steel of this embodiment is particularly preferably applied to a steel pipe. Therefore, Preferably, the austenitic stainless steel of this embodiment is an austenitic stainless steel pipe.
- a cylindrical ingot (30 kg) having an outer diameter of 120 mm was manufactured using the molten steel. Hot forging and hot rolling were performed on the ingot. After hot rolling, cold rolling was performed under the conditions shown in Table 2 to produce an intermediate material having a thickness of 15 mm. From the obtained intermediate material, a plate material of 8 mm ⁇ 20 mm ⁇ 30 mm was manufactured by machining for two steel types. The plate material was heat-treated at the temperature and time shown in Table 2. After the heat treatment, the plate was cooled with water to produce a test steel plate.
- a test piece for microscopic observation was produced from the central part of the cross section perpendicular to the rolling direction of the steel plate of each test number.
- the microscopic test method defined in ASTM E 112 was performed, and the crystal grain size was measured.
- the observation surface was mechanically polished and then corroded using a corrosive solution, and the crystal grain boundary on the observation surface was revealed.
- the average grain size of each field was determined. The area of each visual field is about 0.75 mm 2 .
- Test results The test results are shown in Table 2.
- test number 13 the processing rate during cold rolling was too low. Therefore, F1 was 0.35 and did not satisfy Formula (1). As a result, the amount of intrusion C was 0.51%, and the carburization resistance was low.
- test number 14 the heat treatment temperature was too low. Therefore, F1 was 1.00 and did not satisfy the formula (1). As a result, the amount of intrusion C was 0.65%, and the carburization resistance was low. In Test No. 14, the crystal grain size was 21 ⁇ m.
- test number 15 the heat treatment temperature was too high. Therefore, F1 was 0.39, and the formula (1) was not satisfied. As a result, the amount of intrusion C was 0.58%, and the carburization resistance was low. In Test No. 15, the crystal grain size was 131 ⁇ m.
- test number 16 the heat treatment time was too short. Therefore, F1 was 1.06 and did not satisfy Formula (1). As a result, the amount of intrusion C was 0.69%, and the carburization resistance was low. In Test No. 16, the crystal grain size was 22 ⁇ m.
- test number 17 the heat treatment time was too long. Therefore, F1 was 0.95 and did not satisfy the formula (1). As a result, the amount of intrusion C was 0.54%, and the carburization resistance was low. In Test No. 17, the crystal grain size was 95 ⁇ m.
- test number 19 the Cr content was too high. Therefore, formation of the Al 2 O 3 film was inhibited by Cr carbide. As a result, the amount of intrusion C was 0.60%, and the carburization resistance was low.
- test number 20 the Al content was too low. Therefore, the Al 2 O 3 film was not sufficiently formed. As a result, the amount of intrusion C was 0.83%, and the carburization resistance was low.
- test number 21 the Ni content was too low. Therefore, the amount of intrusion C was 0.52%, and the carburization resistance was low.
- test number 23 the Mg content was too high. Therefore, the drawing value was less than 60% and the hot workability was low.
- the austenitic stainless steel of the present invention can be used even in a high-temperature carburizing environment where carburization and coking are a concern, such as a hydrocarbon gas atmosphere. It is particularly suitable for use as a reaction tube steel in chemical industrial plants such as ethylene production plants.
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Abstract
Description
0.40≦(CCr´/CAl´)/(CCr/CAl)≦0.80 (1)
ここで、式(1)中のCCr´にはオーステナイト系ステンレス鋼の表層におけるCr濃度(質量%)が代入される。CAl´にはオーステナイト系ステンレス鋼の表層におけるAl濃度(質量%)が代入される。また、CCrにはオーステナイト系ステンレス鋼の表層以外のCr濃度(質量%)が代入される。CAlにはオーステナイト系ステンレス鋼の表層以外のAl濃度(質量%)が代入される。 The austenitic stainless steel according to the present embodiment, in mass%, C: 0.03 to less than 0.25%, Si: 0.01 to 2.0%, Mn: 2.0% or less, P: 0.04 %: S: 0.01% or less, Cr: 10 to less than 22%, Ni: more than 30.0% to 40.0%, Al: more than 2.5% to less than 4.5%, Nb: 0. 01 to 3.5%, N: 0.03% or less, Ca: 0.0005 to 0.05%, Mg: 0.0005 to 0.05%, Ti: 0 to less than 0.2%, Mo: 0 -0.5%, W: 0-0.5%, Cu: 0-0.5%, V: 0-0.2%, and B: 0-0.01%, with the balance being Fe And a chemical composition consisting of impurities, satisfying the formula (1).
0.40 ≦ (C Cr ′ / C Al ′) / (C Cr / C Al ) ≦ 0.80 (1)
Here, the Cr concentration (% by mass) in the surface layer of the austenitic stainless steel is substituted for C Cr ′ in the formula (1). Al concentration (% by mass) in the surface layer of austenitic stainless steel is substituted for C Al '. Also, the C Cr Cr concentration than the surface layer of austenitic stainless steel (mass%) is substituted. The C Al Al concentration than the surface layer of austenitic stainless steel (mass%) is substituted.
0.40≦(CCr´/CAl´)/(CCr/CAl)≦0.80 (1)
ここで、式(1)中のCCr´にはオーステナイト系ステンレス鋼の表層におけるCr濃度(質量%)が代入される。CAl´にはオーステナイト系ステンレス鋼の表層におけるAl濃度(質量%)が代入される。また、CCrにはオーステナイト系ステンレス鋼の表層以外のCr濃度(質量%)が代入される。CAlにはオーステナイト系ステンレス鋼の表層以外のAl濃度(質量%)が代入される。 (C) In the austenitic stainless steel, it is effective to make the ratio of the Cr concentration in the surface layer and the Al concentration in the surface layer appropriately smaller than the ratio between the Cr concentration other than the surface layer and the Al concentration other than the surface layer. That is, if the austenitic stainless steel satisfies the formula (1), the carburization resistance in a high-temperature carburizing environment is enhanced.
0.40 ≦ (C Cr ′ / C Al ′) / (C Cr / C Al ) ≦ 0.80 (1)
Here, the Cr concentration (% by mass) in the surface layer of the austenitic stainless steel is substituted for C Cr ′ in the formula (1). Al concentration (% by mass) in the surface layer of austenitic stainless steel is substituted for C Al '. Also, the C Cr Cr concentration than the surface layer of austenitic stainless steel (mass%) is substituted. Al concentration (mass%) other than the surface layer of austenitic stainless steel is substituted for C Al .
0.40≦(CCr´/CAl´)/(CCr/CAl)≦0.80 (1)
ここで、式(1)中のCCr´にはオーステナイト系ステンレス鋼の表層におけるCr濃度(質量%)が代入される。CAl´にはオーステナイト系ステンレス鋼の表層におけるAl濃度(質量%)が代入される。また、CCrにはオーステナイト系ステンレス鋼の表層以外のCr濃度(質量%)が代入される。CAlにはオーステナイト系ステンレス鋼の表層以外のAl濃度(質量%)が代入される。 The austenitic stainless steel according to the present embodiment completed on the basis of the above knowledge is in mass%, C: 0.03 to less than 0.25%, Si: 0.01 to 2.0%, Mn: 2.0 %: P: 0.04% or less, S: 0.01% or less, Cr: 10 to less than 22%, Ni: more than 30.0% to 40.0%, Al: more than 2.5% to 4. Less than 5%, Nb: 0.01 to 3.5%, N: 0.03% or less, Ca: 0.0005 to 0.05%, Mg: 0.0005 to 0.05%, Ti: 0 to 0 Less than 2%, Mo: 0 to 0.5%, W: 0 to 0.5%, Cu: 0 to 0.5%, V: 0 to 0.2%, and B: 0 to 0.01 %, The balance has a chemical composition consisting of Fe and impurities, and satisfies the formula (1).
0.40 ≦ (C Cr ′ / C Al ′) / (C Cr / C Al ) ≦ 0.80 (1)
Here, the Cr concentration (% by mass) in the surface layer of the austenitic stainless steel is substituted for C Cr ′ in the formula (1). Al concentration (% by mass) in the surface layer of austenitic stainless steel is substituted for C Al '. Also, the C Cr Cr concentration than the surface layer of austenitic stainless steel (mass%) is substituted. Al concentration (mass%) other than the surface layer of austenitic stainless steel is substituted for C Al .
本実施形態によるオーステナイト系ステンレス鋼の化学組成は、次の元素を含有する。 [Chemical composition]
The chemical composition of the austenitic stainless steel according to the present embodiment contains the following elements.
炭素(C)は主にCrと結合して鋼中にCr炭化物を形成し、高温浸炭環境での使用時におけるクリープ強度を高める。C含有量が低すぎれば、この効果が得られない。一方、C含有量が高すぎれば、鋼の鋳造後の凝固組織中に粗大な共晶炭化物を多数形成し、鋼の靭性を低下する。したがって、C含有量は0.03~0.25%未満である。C含有量の好ましい下限は0.05%であり、より好ましくは0.08%である。C含有量の好ましい上限は0.23%であり、より好ましくは0.20%である。 C: 0.03 to less than 0.25% Carbon (C) mainly bonds with Cr to form Cr carbide in the steel, and increases the creep strength when used in a high-temperature carburizing environment. If the C content is too low, this effect cannot be obtained. On the other hand, if the C content is too high, a large number of coarse eutectic carbides are formed in the solidified structure after casting of steel, and the toughness of the steel is lowered. Therefore, the C content is 0.03 to less than 0.25%. The minimum with preferable C content is 0.05%, More preferably, it is 0.08%. The upper limit with preferable C content is 0.23%, More preferably, it is 0.20%.
シリコン(Si)は鋼を脱酸する。他の元素で脱酸を十分に実施できる場合、Siの含有量は出来るだけ少なくてもよい。一方、Si含有量が高すぎれば、熱間加工性が低下する。したがって、Si含有量は0.01~2.0%である。Si含有量の好ましい下限は0.02%であり、さらに好ましくは0.03%である。Si含有量の好ましい上限は1.0%である。 Si: 0.01 to 2.0%
Silicon (Si) deoxidizes steel. In the case where deoxidation can be sufficiently performed with other elements, the Si content may be as small as possible. On the other hand, if the Si content is too high, the hot workability decreases. Therefore, the Si content is 0.01 to 2.0%. The minimum with preferable Si content is 0.02%, More preferably, it is 0.03%. The upper limit with preferable Si content is 1.0%.
マンガン(Mn)は不可避に含有される。Mnは鋼中に含まれるSと結合してMnSを形成し、鋼の熱間加工性を高める。しかしながら、Mn含有量が高すぎれば、鋼が硬くなりすぎ、熱間加工性及び溶接性が低下する。したがって、Mn含有量は2.0%以下である。Mn含有量の好ましい下限は0.1%であり、さらに好ましくは0.2%である。Mn含有量の好ましい上限は1.2%である。 Mn: 2.0% or less Manganese (Mn) is unavoidably contained. Mn combines with S contained in the steel to form MnS and enhances the hot workability of the steel. However, if the Mn content is too high, the steel becomes too hard and the hot workability and weldability deteriorate. Therefore, the Mn content is 2.0% or less. The minimum with preferable Mn content is 0.1%, More preferably, it is 0.2%. The upper limit with preferable Mn content is 1.2%.
燐(P)は不純物である。Pは鋼の溶接性及び熱間加工性を低下する。したがって、P含有量は0.04%以下である。P含有量の好ましい上限は0.03%である。P含有量はなるべく低い方が好ましい。P含有量の下限はたとえば、0.0005%である。 P: 0.04% or less Phosphorus (P) is an impurity. P decreases the weldability and hot workability of steel. Therefore, the P content is 0.04% or less. The upper limit with preferable P content is 0.03%. The P content is preferably as low as possible. The lower limit of the P content is, for example, 0.0005%.
硫黄(S)は不純物である。Sは鋼の溶接性及び熱間加工性を低下する。したがって、S含有量は0.01%以下である。S含有量の好ましい上限は0.008%である。S含有量はなるべく低い方が好ましい。S含有量の下限はたとえば、0.001%である。 S: 0.01% or less Sulfur (S) is an impurity. S decreases the weldability and hot workability of steel. Therefore, the S content is 0.01% or less. The upper limit with preferable S content is 0.008%. The S content is preferably as low as possible. The lower limit of the S content is, for example, 0.001%.
クロム(Cr)は、上述のTEE効果により、熱処理工程中及び高温浸炭環境下でAl2O3皮膜の形成を促進する。Crはさらに、鋼中のCと結合して鋼中にCr炭化物を形成し、クリープ強度を高める。Cr含有量が低すぎれば、これらの効果が得られない。一方、Cr含有量が高すぎれば、高温浸炭環境下で、Crは雰囲気ガス(炭化水素ガス)由来のCと結合し、鋼表面にCr炭化物を形成する。鋼表面にCr炭化物が形成されると鋼表面のCrが局所的に欠乏する。このためTEE効果が低下し、均一なAl2O3皮膜が形成されない。Cr含有量が高すぎればさらに、鋼表面のCr炭化物が均一なAl2O3皮膜の形成を物理的に阻害する。したがって、Cr含有量は10~22%未満である。Cr含有量の好ましい下限は11%であり、さらに好ましくは12%である。Cr含有量の好ましい上限は21%であり、さらに好ましくは20%である。本明細書において、Cr炭化物は、鋼中に形成されるCr炭化物と、鋼表面に形成されるCr炭化物とに分かれる。本実施形態のオーステナイト系ステンレス鋼では、鋼中にCr炭化物を形成させ、鋼表面のCr炭化物は抑制する。 Cr: 10 to less than 22% Chromium (Cr) promotes the formation of an Al 2 O 3 film during the heat treatment process and in a high-temperature carburizing environment due to the above-described TEE effect. Further, Cr combines with C in the steel to form Cr carbide in the steel, increasing the creep strength. If the Cr content is too low, these effects cannot be obtained. On the other hand, if the Cr content is too high, Cr combines with C derived from atmospheric gas (hydrocarbon gas) in a high-temperature carburizing environment, and forms Cr carbide on the steel surface. When Cr carbide is formed on the steel surface, Cr on the steel surface is locally depleted. Therefore TEE effect decreases, uniform Al 2 O 3 film is not formed. If the Cr content is too high, the Cr carbide on the steel surface further physically inhibits the formation of a uniform Al 2 O 3 film. Therefore, the Cr content is 10 to less than 22%. The minimum with preferable Cr content is 11%, More preferably, it is 12%. The upper limit with preferable Cr content is 21%, More preferably, it is 20%. In this specification, Cr carbide is divided into Cr carbide formed in steel and Cr carbide formed on the steel surface. In the austenitic stainless steel of this embodiment, Cr carbide is formed in the steel, and Cr carbide on the steel surface is suppressed.
ニッケル(Ni)は、オーステナイトを安定化させ、クリープ強度を高める。Niはさらに、鋼の耐浸炭性を高める。Ni含有量が低すぎれば、これらの効果が得られない。一方、Ni含有量が高すぎれば、これらの効果が飽和するだけでなく、原料コストが高くなる。したがって、Ni含有量は30.0%超~40.0%である。Ni含有量の好ましい下限は31.0%であり、さらに好ましくは32.0%である。Ni含有量の好ましい上限は39.0%であり、さらに好ましくは38.0%である。 Ni: more than 30.0% to 40.0%
Nickel (Ni) stabilizes austenite and increases creep strength. Ni further enhances the carburization resistance of the steel. If the Ni content is too low, these effects cannot be obtained. On the other hand, if the Ni content is too high, these effects are not only saturated, but the raw material costs are increased. Therefore, the Ni content is more than 30.0% to 40.0%. The minimum with preferable Ni content is 31.0%, More preferably, it is 32.0%. The upper limit with preferable Ni content is 39.0%, More preferably, it is 38.0%.
アルミニウム(Al)は、熱処理工程中及び高温浸炭環境下で鋼表面にAl2O3皮膜を形成し、鋼の耐浸炭性を高める。特に本発明にて想定している高温浸炭環境においては、従来用いられているCr2O3皮膜と比較して、Al2O3皮膜は熱力学的に安定である。Al含有量が低すぎれば、これらの効果が得られない。一方、Al含有量が高すぎれば、組織安定性が低下し、クリープ強度が著しく低下する。したがって、Al含有量は2.5%超~4.5%未満である。Al含有量の好ましい下限は2.55%であり、さらに好ましくは2.6%である。Al含有量の好ましい上限は4.2%であり、さらに好ましくは4.0%である。本発明によるオーステナイト系ステンレス鋼において、Al含有量は、鋼材中に含有する全Al量を意味する。 Al: more than 2.5% to less than 4.5% Aluminum (Al) forms an Al 2 O 3 film on the steel surface during the heat treatment process and in a high-temperature carburizing environment, thereby improving the carburization resistance of the steel. In particular, in the high-temperature carburizing environment assumed in the present invention, the Al 2 O 3 film is thermodynamically stable as compared with the conventionally used Cr 2 O 3 film. If the Al content is too low, these effects cannot be obtained. On the other hand, if the Al content is too high, the structural stability is lowered and the creep strength is significantly lowered. Therefore, the Al content is more than 2.5% to less than 4.5%. The minimum with preferable Al content is 2.55%, More preferably, it is 2.6%. The upper limit with preferable Al content is 4.2%, More preferably, it is 4.0%. In the austenitic stainless steel according to the present invention, the Al content means the total amount of Al contained in the steel material.
ニオブ(Nb)は、析出強化相となる金属間化合物(ラーベス相及びNi3Nb相)を形成して、結晶粒界及び結晶粒内を析出強化し、鋼のクリープ強度を高める。一方、Nb含有量が高すぎれば、金属間化合物が過剰に生成して、鋼の靭性が低下する。Nb含有量が高すぎればさらに、長時間時効後の靭性も低下する。したがって、Nb含有量は0.01~3.5%である。Nb含有量の好ましい下限は0.05%であり、さらに好ましくは0.1%である。Nb含有量の好ましい上限は3.2%未満であり、さらに好ましくは3.0%である。 Nb: 0.01 to 3.5%
Niobium (Nb) forms an intermetallic compound (Laves phase and Ni 3 Nb phase) that becomes a precipitation strengthening phase, and precipitates and strengthens the grain boundaries and inside the crystal grains, thereby increasing the creep strength of the steel. On the other hand, if the Nb content is too high, an intermetallic compound is excessively generated and the toughness of the steel is lowered. If the Nb content is too high, the toughness after aging for a long time further decreases. Therefore, the Nb content is 0.01 to 3.5%. The minimum with preferable Nb content is 0.05%, More preferably, it is 0.1%. The upper limit with preferable Nb content is less than 3.2%, More preferably, it is 3.0%.
窒素(N)はオーステナイトを安定化し、不可避に含有される。一方、N含有量が高すぎれば、熱処理後でも未固溶で残存する粗大な窒化物及び/又は炭窒化物が生成する。粗大な窒化物及び/又は炭窒化物は鋼の靱性を低下する。したがって、N含有量は0.03%以下である。好ましいN含有量の上限は0.01%である。N含有量の下限はたとえば、0.0005%である。 N: 0.03% or less Nitrogen (N) stabilizes austenite and is unavoidably contained. On the other hand, if the N content is too high, coarse nitrides and / or carbonitrides that remain undissolved even after heat treatment are generated. Coarse nitrides and / or carbonitrides reduce the toughness of the steel. Therefore, the N content is 0.03% or less. A preferable upper limit of the N content is 0.01%. The lower limit of the N content is, for example, 0.0005%.
カルシウム(Ca)は、Sを硫化物として固定し、熱間加工性を高める。一方、Ca含有量が高すぎれば、靱性及び延性が低下する。そのため、熱間加工性が低下する。Ca含有量が高すぎればさらに、清浄性が低下する。したがって、Ca含有量は0.0005~0.05%である。Caの好ましい下限は0.0006%であり、さらに好ましくは0.0008%である。Ca含有量の好ましい上限は0.01%であり、さらに好ましくは0.008%である。 Ca: 0.0005 to 0.05%
Calcium (Ca) fixes S as a sulfide and improves hot workability. On the other hand, if the Ca content is too high, toughness and ductility are reduced. Therefore, hot workability is reduced. If the Ca content is too high, the cleanliness is further deteriorated. Therefore, the Ca content is 0.0005 to 0.05%. A preferable lower limit of Ca is 0.0006%, and more preferably 0.0008%. The upper limit with preferable Ca content is 0.01%, More preferably, it is 0.008%.
マグネシウム(Mg)は、Sを硫化物として固定し、鋼の熱間加工性を高める。一方、Mg含有量が高すぎれば、靱性及び延性が低下する。そのため、熱間加工性が低下する。Mg含有量が高すぎればさらに、清浄性が低下する。したがって、Mg含有量は0.0005~0.05%である。Mgの好ましい下限は0.0006%であり、さらに好ましくは0.0008%である。Mg含有量の好ましい上限は0.01%であり、さらに好ましくは0.008%である。 Mg: 0.0005 to 0.05%
Magnesium (Mg) fixes S as a sulfide and improves the hot workability of steel. On the other hand, if the Mg content is too high, toughness and ductility are reduced. Therefore, hot workability is reduced. If the Mg content is too high, the cleanliness is further deteriorated. Therefore, the Mg content is 0.0005 to 0.05%. A preferable lower limit of Mg is 0.0006%, and more preferably 0.0008%. The upper limit with preferable Mg content is 0.01%, More preferably, it is 0.008%.
上述のオーステナイト系ステンレス鋼の化学組成はさらに、Feの一部に代えて、Tiを含有してもよい。 [Arbitrary elements]
The chemical composition of the austenitic stainless steel described above may further contain Ti instead of a part of Fe.
チタン(Ti)は任意元素であり、含有されなくてもよい。含有される場合、Tiは、析出強化相となる金属間化合物(ラーベス相及びNi3Ti相)を形成して、析出強化によりクリープ強度を高める。一方、Ti含有量が高すぎれば、金属間化合物が過剰に生成して、高温延性及び熱間加工性が低下する。Ti含有量が高すぎればさらに、長時間時効後の靭性が低下する。したがって、Ti含有量は0~0.2%未満である。Ti含有量の好ましい下限は0.005%であり、さらに好ましくは、0.01%である。Ti含有量の好ましい上限は0.15%であり、さらに好ましくは、0.1%である。 Ti: 0 to less than 0.2% Titanium (Ti) is an optional element and may not be contained. When contained, Ti forms an intermetallic compound (Laves phase and Ni 3 Ti phase) that becomes a precipitation strengthening phase, and increases the creep strength by precipitation strengthening. On the other hand, if the Ti content is too high, an intermetallic compound is excessively produced, and high temperature ductility and hot workability are reduced. If the Ti content is too high, the toughness after aging for a long time further decreases. Therefore, the Ti content is 0 to less than 0.2%. The minimum with preferable Ti content is 0.005%, More preferably, it is 0.01%. The upper limit with preferable Ti content is 0.15%, More preferably, it is 0.1%.
モリブデン(Mo)は任意元素であり、含有されなくてもよい。含有される場合、Moは、母相であるオーステナイトに固溶する。固溶したMoは、固溶強化によりクリープ強度を高める。一方、Mo含有量が高すぎれば、熱間加工性が低下する。したがって、Mo含有量は0~0.5%である。Mo含有量の好ましい下限は0.01%であり、さらに好ましくは0.05%である。Mo含有量の好ましい上限は0.4%であり、さらに好ましくは0.3%である。 Mo: 0 to 0.5%
Molybdenum (Mo) is an optional element and may not be contained. When contained, Mo dissolves in the austenite that is the parent phase. The solid solution Mo increases the creep strength by solid solution strengthening. On the other hand, if Mo content is too high, hot workability will fall. Therefore, the Mo content is 0 to 0.5%. The minimum with preferable Mo content is 0.01%, More preferably, it is 0.05%. The upper limit with preferable Mo content is 0.4%, More preferably, it is 0.3%.
タングステン(W)は任意元素であり、含有されなくてもよい。含有される場合、Wは、母相であるオーステナイトに固溶する。固溶したWは、固溶強化によりクリープ強度を高める。一方、W含有量が高すぎれば、熱間加工性が低下する。したがって、W含有量は0~0.5%である。W含有量の好ましい下限は0.01%であり、さらに好ましくは0.05%である。W含有量の好ましい上限は0.4%であり、さらに好ましくは0.3%である。 W: 0-0.5%
Tungsten (W) is an optional element and may not be contained. When contained, W forms a solid solution in the austenite that is the parent phase. The solid solution W increases the creep strength by solid solution strengthening. On the other hand, if the W content is too high, the hot workability decreases. Accordingly, the W content is 0 to 0.5%. The minimum with preferable W content is 0.01%, More preferably, it is 0.05%. The upper limit with preferable W content is 0.4%, More preferably, it is 0.3%.
銅(Cu)は任意元素であり、含有されなくてもよい。含有される場合、Cuはオーステナイトを安定化する。Cuはさらに、析出強化により鋼の強度を高める。一方で、Cu含有量が高すぎれば、鋼の延性及び熱間加工性が低下する。したがって、Cu含有量は0~0.5%である。Cu含有量の好ましい下限は0.005%であり、さらに好ましくは0.01%である。Cu含有量の好ましい上限は0.3%であり、さらに好ましくは0.1%である。 Cu: 0 to 0.5%
Copper (Cu) is an optional element and may not be contained. When contained, Cu stabilizes austenite. Cu further increases the strength of the steel by precipitation strengthening. On the other hand, if Cu content is too high, the ductility and hot workability of steel will fall. Therefore, the Cu content is 0 to 0.5%. The minimum with preferable Cu content is 0.005%, More preferably, it is 0.01%. The upper limit with preferable Cu content is 0.3%, More preferably, it is 0.1%.
バナジウム(V)は任意元素であり、含有されなくてもよい。含有される場合、VはTiと同様に金属間化合物を形成し、鋼のクリープ強度を高める。一方、V含有量が高すぎれば、鋼中の金属間化合物の体積率が過剰に高くなり、熱間加工性が低下する。したがって、V含有量は0~0.2%である。V含有量の好ましい下限は0.005%であり、さらに好ましくは0.01%である。V含有量の好ましい上限は0.15%であり、さらに好ましくは0.1%である。 V: 0 to 0.2%
Vanadium (V) is an optional element and may not be contained. When contained, V forms an intermetallic compound similar to Ti and increases the creep strength of steel. On the other hand, if the V content is too high, the volume ratio of the intermetallic compound in the steel becomes excessively high, and the hot workability decreases. Therefore, the V content is 0 to 0.2%. The minimum with preferable V content is 0.005%, More preferably, it is 0.01%. The upper limit with preferable V content is 0.15%, More preferably, it is 0.1%.
ボロン(B)は任意元素であり、含有されなくてもよい。含有される場合、Bは粒界に偏析して、粒界での金属間化合物の析出を促進する。これにより、鋼のクリープ強度を高める。一方、B含有量が高すぎれば、鋼の溶接性及び熱間加工性が低下する。したがって、B含有量は0~0.01%である。B含有量の好ましい下限は0.0001%であり、さらに好ましくは0.0005%である。B含有量の好ましい上限は0.008%であり、さらに好ましくは0.006%である。 B: 0 to 0.01%
Boron (B) is an optional element and may not be contained. When contained, B segregates at the grain boundary and promotes precipitation of intermetallic compounds at the grain boundary. This increases the creep strength of the steel. On the other hand, if the B content is too high, the weldability and hot workability of the steel deteriorate. Therefore, the B content is 0 to 0.01%. The minimum with preferable B content is 0.0001%, More preferably, it is 0.0005%. The upper limit with preferable B content is 0.008%, More preferably, it is 0.006%.
本実施形態のオーステナイト系ステンレス鋼はさらに、式(1)を満たす。
0.40≦(CCr´/CAl´)/(CCr/CAl)≦0.80 (1)
ここで、式(1)中のCCr´にはオーステナイト系ステンレス鋼の表層におけるCr濃度(質量%)が代入される。CAl´にはオーステナイト系ステンレス鋼の表層におけるAl濃度(質量%)が代入される。また、CCrにはオーステナイト系ステンレス鋼の表層以外のCr濃度(質量%)が代入される。CAlにはオーステナイト系ステンレス鋼の表層以外のAl濃度(質量%)が代入される。 [Regarding Formula (1)]
The austenitic stainless steel of this embodiment further satisfies the formula (1).
0.40 ≦ (C Cr ′ / C Al ′) / (C Cr / C Al ) ≦ 0.80 (1)
Here, the Cr concentration (% by mass) in the surface layer of the austenitic stainless steel is substituted for C Cr ′ in the formula (1). Al concentration (% by mass) in the surface layer of austenitic stainless steel is substituted for C Al '. Also, the C Cr Cr concentration than the surface layer of austenitic stainless steel (mass%) is substituted. Al concentration (mass%) other than the surface layer of austenitic stainless steel is substituted for C Al .
好ましくは、本実施形態のオーステナイト系ステンレス鋼の結晶粒径は30~80μmである。結晶粒径が30μm以上であれば、鋼のクリープ強度がさらに高まる。結晶粒径が80μm以下であれば、Alの粒界拡散が促進され、Al2O3皮膜の形成がさらに促進される。結晶粒径は、JIS G 0551(2013)に規定する結晶粒度の顕微鏡試験方法によって求める。 [Crystal grain size]
Preferably, the austenitic stainless steel of this embodiment has a crystal grain size of 30 to 80 μm. If the crystal grain size is 30 μm or more, the creep strength of the steel is further increased. If the crystal grain size is 80 μm or less, the grain boundary diffusion of Al is promoted and the formation of the Al 2 O 3 film is further promoted. The crystal grain size is determined by a microscopic test method for crystal grain size specified in JIS G 0551 (2013).
本実施形態のオーステナイト系耐ステンレス鋼の製造方法の一例として、鋼管の製造方法を説明する。 [Production method]
A steel pipe manufacturing method will be described as an example of a method for manufacturing the austenitic stainless steel according to the present embodiment.
上述の化学組成を有する溶鋼を製造する。溶鋼に対して、必要に応じて周知の脱ガス処理を実施する。溶鋼を用いて、鋳造により素材を製造する。素材は、造塊法によるインゴットであってもよいし、連続鋳造法によるスラブやブルーム、ビレット等の鋳片であってもよい。また、遠心鋳造法により、管形状の鋳造体を製造してもよい。 [Preparation process]
A molten steel having the above chemical composition is produced. A well-known degassing process is implemented with respect to molten steel as needed. A raw material is manufactured by casting using molten steel. The material may be an ingot obtained by an ingot-making method or a slab such as a slab, bloom or billet obtained by a continuous casting method. Moreover, you may manufacture a pipe-shaped casting by the centrifugal casting method.
製造された素材に対して熱間鍛造を実施して円柱素材を製造してもよい。熱間鍛造を実施すれば、準備工程で製造した溶鋼の内部組織を、凝固組織から均質な整粒組織へと変化させることができる。熱間鍛造の温度はたとえば、900~1200℃である。 [Hot forging process]
You may manufacture a cylindrical raw material by implementing hot forging with respect to the manufactured raw material. If hot forging is carried out, the internal structure of the molten steel produced in the preparation process can be changed from a solidified structure to a uniform sized structure. The temperature of hot forging is 900 to 1200 ° C., for example.
準備工程で製造された素材、又は熱間鍛造された素材(円柱素材)に対して熱間加工を実施して、鋼素管を製造する。たとえば、機械加工により円柱素材中心に貫通孔を形成する。貫通孔が形成された円柱素材に対して熱間押出を実施して、鋼素管を製造する。熱間押出の加工温度はたとえば900~1200℃である。円柱素材を穿孔圧延(マンネスマン法等)して鋼素管を製造してもよい。 [Hot working process]
A steel base tube is manufactured by performing hot working on the material manufactured in the preparation process or the material that has been hot forged (cylindrical material). For example, a through hole is formed in the center of a cylindrical material by machining. A steel base tube is manufactured by performing hot extrusion on a cylindrical material in which a through hole is formed. The processing temperature for hot extrusion is, for example, 900 to 1200 ° C. A steel blank may be manufactured by piercing and rolling a cylindrical material (Mannesmann method or the like).
熱間加工後の鋼素管に対して冷間加工を実施し、中間材を製造する。冷間加工はたとえば、冷間引抜等である。冷間加工工程において鋼表面に歪を付与すれば、Al及びCr等の元素が鋼表面に移動しやすくなる。この場合、TEE効果を十分に得られる。これにより、鋼表層のCrが適度に欠乏し、式(1)を満たすオーステナイト系ステンレス鋼を得ることができる。冷間加工の加工率が低すぎれば、この効果が得られない。冷間加工の加工率の上限は特に設けないが、加工率が高すぎる冷間加工は、現実的に実施が困難である。したがって、冷間加工の加工率は10~90%である。 [Cold working process]
Cold work is performed on the steel base tube after hot working to produce an intermediate material. The cold working is, for example, cold drawing or the like. If strain is applied to the steel surface in the cold working process, elements such as Al and Cr easily move to the steel surface. In this case, the TEE effect can be sufficiently obtained. Thereby, Cr of a steel surface layer is lacking moderately, and the austenitic stainless steel which satisfy | fills Formula (1) can be obtained. If the processing rate of cold working is too low, this effect cannot be obtained. There is no particular upper limit on the working rate of cold working, but cold working with a working rate that is too high is practically difficult to implement. Therefore, the working rate of cold working is 10 to 90%.
製造された中間材に対して、大気雰囲気で熱処理を実施する。大気雰囲気での熱処理により、鋼表面に均一なAl2O3皮膜が形成される。このとき、TEE効果により鋼表層のCrが適度に欠乏する。その結果、式(1)を満たすオーステナイト系ステンレス鋼を得ることができる。 [Heat treatment process]
The manufactured intermediate material is heat-treated in an air atmosphere. A uniform Al 2 O 3 film is formed on the steel surface by heat treatment in an air atmosphere. At this time, Cr in the steel surface layer is moderately lacking due to the TEE effect. As a result, an austenitic stainless steel satisfying the formula (1) can be obtained.
表1に示す化学組成を有する溶鋼を、真空溶解炉を用いて製造した。 [Production method]
Molten steel having the chemical composition shown in Table 1 was produced using a vacuum melting furnace.
各試験番号の鋼板の圧延方向と垂直な断面の中央部から顕微鏡観察用の試験片を作製した。試験片の表面のうち、上記断面に相当する表面(観察面という)を用いて、ASTM E 112に規定される顕微鏡試験方法を実施し、結晶粒径を測定した。具体的には、観察面を機械研磨後、腐食液を用いて腐食し、観察面の結晶粒界を現出させた。腐食した表面上の10視野において、各視野の平均結晶粒径を求めた。各視野の面積は、約0.75mm2である。 [Measurement of austenite grain size]
A test piece for microscopic observation was produced from the central part of the cross section perpendicular to the rolling direction of the steel plate of each test number. Using the surface corresponding to the cross section (referred to as the observation surface) among the surfaces of the test piece, the microscopic test method defined in ASTM E 112 was performed, and the crystal grain size was measured. Specifically, the observation surface was mechanically polished and then corroded using a corrosive solution, and the crystal grain boundary on the observation surface was revealed. In 10 fields on the corroded surface, the average grain size of each field was determined. The area of each visual field is about 0.75 mm 2 .
各試験番号の鋼板に対して、JIS Z 2290(2004)に準拠する条件で脱スケール処理を実施した。脱スケール処理後の鋼板を圧延方向に対して垂直に切断し、表面を含むサンプルを採取した。サンプルを樹脂に埋め込み、表面近傍の断面を含む観察面を研磨した。研磨後の観察面に対して、上述の方法を用いて表層(表面から2μm深さまでの範囲)のCr濃度CCr´及びAl濃度CAl´を求めた。 [Surface layer of Cr concentration C Cr 'and the surface layer of the Al concentration C Al' measured '
A descaling process was performed on the steel plates of each test number under conditions based on JIS Z 2290 (2004). The steel sheet after descaling was cut perpendicular to the rolling direction, and a sample including the surface was collected. The sample was embedded in resin, and the observation surface including the cross section near the surface was polished. The Cr concentration C Cr ′ and Al concentration C Al ′ of the surface layer (range from the surface to a depth of 2 μm) were determined on the observation surface after polishing using the method described above.
上述の方法により、表層以外のCr濃度CCr及び表層以外のAl濃度CAlを求めた。 [Cr concentration other than surface layer C Cr and Al concentration other than surface layer C Al measurement]
By the above-mentioned method, the Cr concentration C Cr other than the surface layer and the Al concentration C Al other than the surface layer were determined.
各試験番号の鋼板を、H2-CH4-CO2雰囲気にて1100℃×96時間保持した。浸炭後の鋼板表面を#600研磨紙で乾式手研磨して、表面のスケール等を除去した。鋼板表面から0.5mmピッチで4層分の分析切粉を採取した。得られた分析切粉について、高周波燃焼赤外吸収法にてC濃度を測定した。測定結果から、鋼に元から含有されているC濃度を差し引いて、C濃度増加量とした。4層分のC濃度増加量の平均を、侵入C量とした。 [Carburization test]
The steel plates of each test number were held at 1100 ° C. for 96 hours in an H 2 —CH 4 —CO 2 atmosphere. The steel plate surface after carburizing was dry-hand-polished with # 600 abrasive paper to remove the scale on the surface. Four layers of analytical chips were collected from the steel plate surface at a pitch of 0.5 mm. About the obtained analytical chip, C density | concentration was measured with the high frequency combustion infrared absorption method. From the measurement results, the C concentration originally contained in the steel was subtracted to obtain an increase in C concentration. The average amount of increase in C concentration for four layers was defined as the amount of intrusion C.
製造されたインゴットに対して、肉厚中央部から、直径が10mmで長さが130mmの円柱状の引張り試験片を切り出した。各引張り試験片について、引張り速度(ひずみ速度)10/sで引張り試験を実施し、熱間加工性を評価した。本発明においては、引張り試験後の絞りが、900℃において、60%以上を合格(○)、60%未満を不合格(×)とした。 [High temperature tensile test]
A columnar tensile test piece having a diameter of 10 mm and a length of 130 mm was cut out from the central portion of the manufactured ingot. Each tensile test piece was subjected to a tensile test at a tensile rate (strain rate) of 10 / s to evaluate hot workability. In the present invention, when the drawing after the tensile test is 900 ° C., 60% or more is acceptable (◯) and less than 60% is unacceptable (x).
試験結果を表2に示す。 [Test results]
The test results are shown in Table 2.
The austenitic stainless steel of the present invention can be used even in a high-temperature carburizing environment where carburization and coking are a concern, such as a hydrocarbon gas atmosphere. It is particularly suitable for use as a reaction tube steel in chemical industrial plants such as ethylene production plants.
Claims (2)
- 質量%で、
C:0.03~0.25%未満、
Si:0.01~2.0%、
Mn:2.0%以下、
P:0.04%以下、
S:0.01%以下、
Cr:10~22%未満、
Ni:30.0%超~40.0%、
Al:2.5%超~4.5%未満、
Nb:0.01~3.5%、
N:0.03%以下、
Ca:0.0005~0.05%、
Mg:0.0005~0.05%、
Ti:0~0.2%未満、
Mo:0~0.5%、
W:0~0.5%、
Cu:0~0.5%、
V:0~0.2%、及び、
B:0~0.01%を含有し、
残部がFe及び不純物からなる化学組成を有し、
式(1)を満たす、オーステナイト系ステンレス鋼。
0.40≦(CCr´/CAl´)/(CCr/CAl)≦0.80 (1)
ここで、式(1)中のCCr´にはオーステナイト系ステンレス鋼の表層におけるCr濃度(質量%)が代入される。CAl´にはオーステナイト系ステンレス鋼の表層におけるAl濃度(質量%)が代入される。また、CCrにはオーステナイト系ステンレス鋼の表層以外のCr濃度(質量%)が代入される。CAlにはオーステナイト系ステンレス鋼の表層以外のAl濃度(質量%)が代入される。 % By mass
C: 0.03 to less than 0.25%,
Si: 0.01 to 2.0%,
Mn: 2.0% or less,
P: 0.04% or less,
S: 0.01% or less,
Cr: less than 10-22%,
Ni: more than 30.0% to 40.0%,
Al: more than 2.5% to less than 4.5%,
Nb: 0.01 to 3.5%,
N: 0.03% or less,
Ca: 0.0005 to 0.05%,
Mg: 0.0005 to 0.05%,
Ti: 0 to less than 0.2%,
Mo: 0 to 0.5%,
W: 0 to 0.5%
Cu: 0 to 0.5%,
V: 0 to 0.2%, and
B: 0 to 0.01% is contained,
The balance has a chemical composition consisting of Fe and impurities,
An austenitic stainless steel that satisfies formula (1).
0.40 ≦ (C Cr ′ / C Al ′) / (C Cr / C Al ) ≦ 0.80 (1)
Here, the Cr concentration (% by mass) in the surface layer of the austenitic stainless steel is substituted for C Cr ′ in the formula (1). Al concentration (% by mass) in the surface layer of austenitic stainless steel is substituted for C Al '. Also, the C Cr Cr concentration than the surface layer of austenitic stainless steel (mass%) is substituted. Al concentration (mass%) other than the surface layer of austenitic stainless steel is substituted for C Al . - 請求項1に記載のオーステナイト系ステンレス鋼であって、
前記化学組成は、
Ti:0.005~0.2%未満、
Mo:0.01~0.5%、
W:0.01~0.5%、
Cu:0.005~0.5%、
V:0.005~0.2%、及び、
B:0.0001~0.01からなる群から選択される1種又は2種以上を含有する、オーステナイト系ステンレス鋼。
The austenitic stainless steel according to claim 1,
The chemical composition is
Ti: 0.005 to less than 0.2%,
Mo: 0.01 to 0.5%,
W: 0.01 to 0.5%
Cu: 0.005 to 0.5%,
V: 0.005 to 0.2%, and
B: An austenitic stainless steel containing one or more selected from the group consisting of 0.0001 to 0.01.
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JP2018525193A JP6614347B2 (en) | 2016-06-29 | 2017-06-28 | Austenitic stainless steel |
EP17820177.8A EP3480330A4 (en) | 2016-06-29 | 2017-06-28 | Austenitic stainless steel |
CN201780040380.3A CN109415786A (en) | 2016-06-29 | 2017-06-28 | Austenite stainless steel |
SG11201810839TA SG11201810839TA (en) | 2016-06-29 | 2017-06-28 | Austenitic stainless steel |
CA3028610A CA3028610A1 (en) | 2016-06-29 | 2017-06-28 | Austenitic stainless steel |
US16/310,613 US20190127832A1 (en) | 2016-06-29 | 2017-06-28 | Austenitic Stainless Steel |
KR1020197002265A KR102124914B1 (en) | 2016-06-29 | 2017-06-28 | Austenitic stainless steel |
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JP (1) | JP6614347B2 (en) |
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WO2020067444A1 (en) * | 2018-09-27 | 2020-04-02 | 日本製鉄株式会社 | Austenitic alloy |
JP2020168639A (en) * | 2019-04-02 | 2020-10-15 | 日本製鉄株式会社 | Welding joint and welding material used for manufacturing the welding joint |
JP2020169346A (en) * | 2019-04-02 | 2020-10-15 | 日本製鉄株式会社 | Alloy pipe |
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CN110257690B (en) * | 2019-06-25 | 2021-01-08 | 宁波宝新不锈钢有限公司 | Resource-saving austenitic heat-resistant steel and preparation method thereof |
JP2021127517A (en) * | 2020-02-14 | 2021-09-02 | 日本製鉄株式会社 | Austenitic stainless steel material |
CN111304532B (en) * | 2020-03-04 | 2021-04-27 | 湖州盛特隆金属制品有限公司 | Heat-resistant austenitic stainless steel and preparation method thereof |
JPWO2022123812A1 (en) * | 2020-12-10 | 2022-06-16 |
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