JP2015137419A - Austenite stainless steel weld joint - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an austenite stainless steel weld joint having excellent steam oxidation resistance.SOLUTION: An austenite stainless steel weld joint has a chemical composition containing, by mass%, C:0.2% or less, Si:2.0% or less, Mn:0.1 to 3.0%, Cr:14 to 28%, Ni:6 to 30%, N:0.3% or less, Mo:0 to 5%, W:0 to 10%, Ta:0 to 5%, Cu:0 to 5%, V:0 to 1.0%, Nb:0 to 1.5%, Ti:0 to 0.5%, Ca:0 to 0.02%, Mg:0 to 0.02%, Al:0 to 0.3%, Zr:0 to 0.5%, B:0 to 0.02%, rare earth elements of 0 to 0.1% and the balance Fe with inevitable impurities. The stainless steel weld joint has further a surface which a blast treatment by a projection material is conducted. Further a crystal grain size number at a depth position of 50 μm from a surface of a welding heat affected zone is larger than that at a thickness central part of a base material part.

Description

  The present invention relates to a welded joint, and more particularly to an austenitic stainless steel welded joint.

  In recent years, power plants have been required to reduce the total amount of carbon dioxide emissions. Therefore, the power plant is required to generate power with high efficiency. For example, in a thermal power generation boiler, high-temperature and high-pressure steam is required for highly efficient power generation.

When steam is increased in temperature and pressure, the wall temperature of boiler superheater tubes, reheater tubes, and the like rises. Therefore, steel materials (steel pipes) used for such boilers are required to have resistance to high-temperature oxidation by steam (hereinafter referred to as steam oxidation resistance) as well as high-temperature strength.
Patent Documents 1 to 14 propose a technique for improving the steam oxidation resistance of a steel material. The techniques proposed in these documents are classified into the following five.

(1) Technology for forming a cold-worked layer by peening In Patent Documents 1 to 3, peening is performed on the inner surface of the steel pipe to increase the steam oxidation resistance. Specifically, in Patent Document 1, after the final heat treatment is performed on the steel pipe, peening by particle spraying is performed on the inner surface of the steel pipe. In patent document 2, a peening process is implemented with respect to a steel pipe, and a 10 micrometer or more processed layer is formed. In patent document 3, heat processing is implemented with respect to the steel pipe taken out from the existing boiler. Chemical cleaning is performed on the heat-treated steel pipe to remove scale on the inner surface. After chemical cleaning, shot blasting is performed on the inner surface of the steel pipe to form a cold-worked layer.

(2) Technology for improving the adhesion of the scale In Patent Documents 4 and 5, the adhesion of the scale on the inner surface of the steel pipe is increased to improve the steam oxidation resistance. Specifically, in Patent Document 4, an austenitic stainless steel pipe containing a rare earth element is prepared. Solution treatment is performed on this steel pipe. Peening is performed by spraying particles on the inner surface of the solution-treated steel pipe. In patent document 5, the steel pipe containing 9-28 mass% Cr is prepared. The maximum height of the inner surface of the steel pipe after cold working is set to 15 μm or more. Further, the difference between the Vickers hardness of the inner surface layer of the steel pipe and the Vickers hardness of the central thickness portion is set to 100 or more. These treatments increase the adhesion of the scale.

(3) Technology for performing cold work with high workability In Patent Documents 6 to 9, cold work with high workability is performed on a steel pipe to increase the steam oxidation resistance of the steel pipe. Specifically, in Patent Document 6, cold working with a high workability is performed on austenitic stainless steel containing 16 to 20 wt% Cr. In the steel pipe after cold working, the Cr concentration in the vicinity of the inner surface of the steel pipe is 14% by weight or more. Furthermore, the hardness at a depth of 100 μm from the inner surface of the steel pipe is either 1.5 times or more of the average hardness of the base material or Hv300 or more. In patent document 7, cold work with a high workability is implemented with respect to the steel pipe containing 8-28 mass% Cr. Thereby, a processed layer with high hardness is formed on the inner surface of the steel pipe. In Patent Documents 8 and 9, cold working with a high degree of work is performed on the steel pipe. In Patent Document 8, the metal structure at a depth of 10 to 20 μm from the inner surface of the steel pipe after cold working has a subgrain (small-angle grain boundary and large-angle grain boundary) structure having a volume ratio of 0.3% or more. In Patent Document 9, the average dislocation density obtained by XRD measurement is 3.0 × 10 ¹⁴ / m ² or more in the metal structure on the inner surface of the steel pipe after cold working.

(4) Technology for implementing a solution treatment after forming a cold-worked layer In Patent Documents 10 to 13, after forming a cold-worked layer on the inner surface of a steel pipe, the solution treatment is carried out to prevent water vapor resistance of the steel pipe. Increase oxidation. Specifically, in Patent Document 10, a solution treatment is performed on an austenitic stainless steel pipe. Cold processing such as shot processing, grinder processing, or polishing processing is performed on the inner surface of the steel pipe after the solution treatment. Solution treatment is performed again on the steel pipe after the cold working. In Patent Document 11, cold working at a working rate of 20% or more is performed on an austenitic stainless steel pipe. A solution heat treatment is performed on the steel pipe after cold working at a heating rate of 2.9 ° C./second or less. In Patent Document 12, a crystal grain size No. 1 is formed on the inner surface of an austenitic iron alloy tube. A fine-grained layer that is finer than 7 and has a thickness of 30 μm or more is formed. Cold-working of 20% or more is performed on the steel pipe on which the fine grain layer is formed. Recrystallize the steel pipe after cold working. In patent document 13, cold work is implemented with respect to an austenitic stainless steel pipe so that the hardness in the depth position of 20 micrometers from a steel pipe inner surface may become Hv320 or more. Solution treatment is performed on the steel pipe after cold working.

(5) Technology for maintaining a fine grain structure even after solution treatment In Patent Document 14, the steam oxidation resistance of a steel pipe is improved by maintaining the fine grain structure even after a solution treatment. Specifically, the total content of C and N in the steel pipe is set to 0.15%. In this case, even after the solution treatment, the steel pipe inner surface layer has a grain size number of No. It has a fine grain structure of 7 or more.

JP-A-49-135822 JP 52-8930 A JP-A-63-54598 JP-A-6-322489 JP 2006-307313 A International Publication No. 2008/023410 JP 2009-68079 A International Publication No. 2011/155296 International Publication No. 2013/001956 JP-A-53-114722 JP 54-138814 A JP-A-55-58329 JP 58-39733 A JP 58-133352 A

  By the countermeasures of Patent Documents 1 to 14 described above, the steam oxidation resistance of the austenitic stainless steel material is increased. However, when welding is performed on an austenitic stainless steel material to form a welded joint, the steam oxidation resistance may be reduced. In particular, when heat treatment is performed on a steel material (welded joint) after welding, the steam oxidation resistance of the weld heat affected zone (HAZ) decreases.

  An object of the present invention is to provide an austenitic stainless steel welded joint having excellent steam oxidation resistance.

  The austenitic stainless steel welded joint according to this embodiment is in mass%, C: 0.2% or less, Si: 2.0% or less, Mn: 0.1-3.0%, Cr: 14-28%, Ni: 6 to 30%, N: 0.3% or less, Mo: 0 to 5%, W: 0 to 10%, Ta: 0 to 5%, Cu: 0 to 5%, V: 0 to 1.0 %, Nb: 0 to 1.5%, Ti: 0 to 0.5%, Ca: 0 to 0.02%, Mg: 0 to 0.02%, Al: 0 to 0.3%, Zr: 0 -0.5%, B: 0-0.02%, and rare earth elements: 0-0.1%, with the balance having a chemical composition consisting of Fe and impurities. The stainless steel welded joint further has a surface that has been subjected to a blast treatment with a projection material. Furthermore, the crystal grain size number at the 50 μm depth position from the surface of the weld heat affected zone is larger than the crystal grain size number at the center of the thickness of the base material portion.

  The austenitic stainless steel welded joint according to the present embodiment has excellent steam oxidation resistance.

  The present inventors investigated the oxide scale generated on the weld heat affected zone after heat-treating the austenitic stainless steel welded joint whose inner surface was shot blasted as usual. As a result, the present inventors obtained the following knowledge.

The oxide scale on the inner surface of the weld heat affected zone was thicker than the oxide scale on the inner surface of the base metal portion, which is a portion other than the weld heat affected zone. The oxidation scale (hereinafter referred to as HAZ scale) of the weld heat affected zone includes an outer layer scale, an inner layer scale, and chromia (Cr ₂ O ₃ ). The outer scale was mainly Fe ₃ O ₄ . The inner layer scale was a spinel type oxide formed under the outer layer scale and containing Fe and Cr. The chromia was formed at the interface between the inner layer scale and the weld heat affected zone. The chromia was not formed continuously, but was formed intermittently (discontinuously).

  On the other hand, the oxide scale of the base material portion (hereinafter referred to as the base material scale) also includes the outer layer scale, the inner layer scale, and chromia, like the HAZ scale. However, chromia was continuously formed on the base material scale.

  As described above, in the base material scale, chromia was continuously formed, and thus the steam oxidation resistance was high. On the other hand, in the HAZ scale, since chromia was formed discontinuously, the steam oxidation resistance was lower than that of the base material.

  The inventors investigated the difference in chromia formation between the weld heat affected zone and the base metal. As a result, the crystal grains at a depth of 50 μm from the inner surface of the weld heat affected zone were larger than the crystal grains at the center of the thickness of the base metal portion. Furthermore, coarse Cr carbide was also precipitated on the grain boundary near the inner surface of the weld heat affected zone.

From the above results, the following matters can be estimated. A working layer is formed on the surface of the austenitic stainless steel material by blasting such as shot blasting. The crystal grains in the processed layer are fine. However, if a steel material having a processed layer is welded to produce a welded joint and then heat treatment is performed, a weld heat affected zone is formed in a part of the processed layer. In this case, during the heat treatment after welding or during the cooling process after heat treatment, the crystal grains in the weld heat affected zone become coarse, and coarse Cr carbide precipitates at the crystal grain boundaries. The formation of coarse Cr carbide reduces Cr in the steel. Furthermore, when the crystal grains are coarse, it is difficult to supply Cr to the surface. Therefore, chromia (Cr ₂ O ₃ ) is formed discontinuously in the steam oxidation environment.

  If the crystal grain at a depth of 50 μm from the surface of the weld heat affected zone is smaller than the crystal grain at the center of the thickness of the base metal portion, coarse Cr carbide is unlikely to precipitate at the crystal grain boundary. Furthermore, if the crystal grains are fine, there are a large number of grain boundaries, so Cr is likely to be supplied to the inner surface through the grain boundaries. Therefore, even in the weld heat affected zone, chromia is easily formed continuously, and the steam oxidation resistance is improved.

  The austenitic stainless steel welded joint according to the present embodiment completed based on the above knowledge is in mass%, C: 0.2% or less, Si: 2.0% or less, Mn: 0.1 to 3.0%. Cr: 14-28%, Ni: 6-30%, N: 0.3% or less, Mo: 0-5%, W: 0-10%, Ta: 0-5%, Cu: 0-5% , V: 0 to 1.0%, Nb: 0 to 1.5%, Ti: 0 to 0.5%, Ca: 0 to 0.02%, Mg: 0 to 0.02%, Al: 0 to 0% It contains 0.3%, Zr: 0 to 0.5%, B: 0 to 0.02%, and rare earth elements: 0 to 0.1%, and the balance has a chemical composition composed of Fe and impurities. The stainless steel welded joint further has a surface that has been subjected to a blast treatment with a projection material. Furthermore, the crystal grain size number at the 50 μm depth position from the surface of the weld heat affected zone is larger than the crystal grain size number at the center of the thickness of the base material portion.

  Preferably, the crystal grain size number at a depth of 50 μm from the surface of the weld heat affected zone is 9 or more.

  The austenitic stainless steel welded joint has N: 0.005-0.3%, Mo: 0.1-5%, W: 0.1-10%, Ta: 0-5%, Cu: 0.1 -5%, V: 0.01-1.0%, Nb: 0.01-1.5%, Ti: 0.01-0.5%, Ca: 0.0001-0.02%, Mg: 0.0001 to 0.02%, Al: 0.0001 to 0.3%, Zr: 0.0001 to 0.5%, B: 0.0001 to 0.02%, and rare earth element: 0.0001 You may contain 1 type, or 2 or more types selected from the group which consists of -0.1%.

  Hereinafter, the austenitic stainless steel welded joint of this embodiment will be described in detail. “%” Of the content of each element means “mass%”.

[Chemical composition]
The chemical composition of the austenitic stainless steel welded joint according to the present embodiment contains the following elements.

C: 0.2% or less Carbon (C) is inevitably contained, and increases the strength and creep strength of steel. However, if the C content is too high, undissolved carbide remains after the solution treatment, and the strength decreases. If the C content is too high, mechanical properties such as toughness are further deteriorated. Therefore, the C content is 0.2% or less. The minimum with preferable C content is 0.01% or more, More preferably, it is 0.02%. The upper limit with preferable C content is less than 0.2%, More preferably, it is 0.16%, More preferably, it is 0.12%.

Si: 2.0% or less Silicon (Si) is inevitably contained. Si deoxidizes steel. Si further increases the steam oxidation resistance of the steel. However, if the Si content is too high, the weldability and hot workability of the steel deteriorate. Therefore, the Si content is 2.0% or less. The minimum with preferable Si content is 0.1%, More preferably, it is 0.12%. The upper limit with preferable Si content is less than 2.0%, More preferably, it is 1.5%, More preferably, it is 0.8%.

Mn: 0.1 to 3.0%
Manganese (Mn), like Si, deoxidizes steel. Further, Mn combines with S to form MnS and enhances hot workability. If the Mn content is too low, the above effect cannot be obtained. On the other hand, if the Mn content is too high, the steel becomes brittle. Therefore, the Mn content is 0.1 to 3.0%. The minimum with preferable Mn content is higher than 0.1%, More preferably, it is 0.2%, More preferably, it is 0.5%. The upper limit with preferable Mn content is less than 3.0%, More preferably, it is 2.5%, More preferably, it is 2.0%.

Cr: 14 to 28%
Chromium (Cr) increases the high temperature strength of the steel. Further, Cr forms an oxide (Cr ₂ O ₃ ) on the surface of the steel in a steam oxidation environment, thereby improving the resistance to steam oxidation. If the Cr content is too low, the above effect cannot be obtained. On the other hand, if the Cr content is too high, the toughness and hot workability of the steel decrease. Therefore, the Cr content is 14 to 28%. The minimum with preferable Cr content is higher than 14%, More preferably, it is 15%, More preferably, it is 16%. The upper limit with preferable Cr content is less than 28%, More preferably, it is 27%, More preferably, it is 26%.

Ni: 6-30%
Nickel (Ni) stabilizes the austenite structure in the steel. Ni further increases the creep strength. If the Ni content is too low, the above effect cannot be obtained. On the other hand, if the Ni content is too high, the above effect is saturated and the manufacturing cost increases. Therefore, the Ni content is 6-30%. The minimum with preferable Ni content is higher than 6%, More preferably, it is 7%, More preferably, it is 8%. The upper limit with preferable Ni content is less than 30%, More preferably, it is 25%, More preferably, it is 21%.

N: 0.3% or less Nitrogen (N) is inevitably contained. N may not be actively contained. When N is positively contained, N dissolves in the steel and increases the strength of the steel. N further combines with other elements to form nitrides and precipitation strengthens the steel. However, if the N content is too high, the toughness and weldability of the steel will decrease. Therefore, the N content is 0.3% or less. When N is positively contained in order to strengthen steel, the minimum with preferable N content is 0.005%, More preferably, it is 0.01%. The upper limit with preferable N content is less than 0.3%, More preferably, it is 0.28%, More preferably, it is 0.27%. When N is not actively contained, the N content is less than 0.005%.

  The balance of the chemical composition of the austenitic stainless steel welded joint according to the present embodiment is composed of Fe and impurities. Here, an impurity is a thing mixed from the ore as a raw material, a scrap, or a manufacturing environment, when manufacturing steel materials industrially.

  The austenitic stainless steel welded joint of this embodiment may further contain one or more selected from the group consisting of Mo, W, Ta, and Cu. These elements are optional elements, and all increase the high temperature strength of the steel.

Mo: 0 to 5%,
W: 0-10%
Ta: 0 to 5%
Cu: 0 to 5%
Molybdenum (Mo), tungsten (W), tantalum (Ta), and copper (Cu) are all optional elements and may not be contained. When included, these elements increase the high temperature strength of the steel. However, if the content of these elements is too high, the weldability and hot workability of the steel deteriorate. Therefore, the Mo content is 0 to 5%, the W content is 0 to 10%, the Ta content is 0 to 5%, and the Cu content is 0 to 5%. The minimum with preferable Mo content is 0.1%, More preferably, it is 0.15%. The upper limit with preferable Mo content is less than 5%, More preferably, it is 4.5%. The minimum with preferable W content is 0.1%, More preferably, it is 0.15%. The upper limit with preferable W content is less than 10%, More preferably, it is 8%. The minimum with preferable Ta content is 0.1%, More preferably, it is 0.15%. The upper limit with preferable Ta content is less than 5%, More preferably, it is 4.5%. The minimum with preferable Cu content is 0.1%, More preferably, it is 0.15%. The upper limit with preferable Cu content is less than 5%, More preferably, it is 4%.

  The austenitic stainless steel welded joint of the present embodiment may further contain one or more selected from the group consisting of V, Nb, and Ti. All of these elements are optional elements, and precipitate strengthen the steel.

V: 0 to 1.0%
Nb: 0 to 1.5%,
Ti: 0 to 0.5%
Vanadium (V), niobium (Nb), and titanium (Ti) are all optional elements and may not be contained. When contained, all of these elements combine with C and N to form carbonitrides and precipitation strengthen the steel. However, if the content of these elements is too high, the workability of the steel decreases. Therefore, the V content is 0 to 1.0%, the Nb content is 0 to 1.5%, and the Ti content is 0 to 0.5%. The minimum with preferable V content is 0.01%, More preferably, it is 0.03%. The upper limit with preferable V content is less than 1.0%, More preferably, it is 0.8%. The minimum with preferable Nb content is 0.01%, More preferably, it is 0.03%. The upper limit with preferable Nb content is less than 1.5%, More preferably, it is 1.0%. The minimum with preferable Ti content is 0.01%, More preferably, it is 0.03%. The upper limit with preferable Ti content is less than 0.5%, More preferably, it is 0.4%.

  The austenitic stainless steel welded joint of this embodiment may further contain one or more selected from the group consisting of Ca, Mg, Al, Zr, B, and rare earth elements (REM). All of these elements are optional elements, and enhance the strength, workability, and steam oxidation resistance of the steel.

Ca: 0 to 0.02%,
Mg: 0 to 0.02%,
Al: 0 to 0.3%,
Zr: 0 to 0.5%,
B: 0 to 0.02%,
REM: 0 to 0.1%
Calcium (Ca), magnesium (Mg), aluminum (Al), zirconium (Zr), boron (B), and rare earth element (REM) are all optional elements and may not be contained. When contained, all of these elements increase the strength, workability and steam oxidation resistance of the steel. However, if the content of these elements is too high, the workability and weldability of the steel are reduced. Therefore, the Ca content is 0 to 0.02%, the Mg content is 0 to 0.02%, the Al content is 0 to 0.3%, and the Zr content is 0 to 0.5%. %, B content is 0-0.02%, REM content is 0-0.1%. As used herein, the Al content is sol. It means the content of Al (acid-soluble Al). In addition, REM in this specification contains at least one of Sc, Y, and lanthanoid (La of atomic number 57 to Lu of 71), and the REM content is the total content of these elements. Means.

The minimum with preferable Ca content is 0.0001%, More preferably, it is 0.0005%. The upper limit with preferable Ca content is less than 0.02%, More preferably, it is 0.015%. The minimum with preferable Mg content is 0.0001%, More preferably, it is 0.0005%. The upper limit with preferable Mg content is less than 0.02%, More preferably, it is 0.015%. The minimum with preferable Al content is 0.0001%, More preferably, it is 0.0005%. The upper limit with preferable Al content is less than 0.3%, More preferably, it is 0.2%. The minimum with preferable Zr content is 0.0001%, More preferably, it is 0.0005%. The upper limit with preferable Zr content is less than 0.5%, More preferably, it is 0.4%. The minimum with preferable B content is 0.0001%, More preferably, it is 0.0005%. The upper limit with preferable B content is less than 0.02%, More preferably, it is 0.015%. The minimum with preferable REM content is 0.0001%, More preferably, it is 0.0005%. The upper limit with preferable REM content is less than 0.1%, More preferably, it is 0.07%.
More preferably, the total content of these elements (Ca, Mg, Al, Zr, B and REM) is 0.8% or less.

[Microstructure, etc.]
The austenitic stainless steel welded joint having the above-described chemical composition is a joint obtained by welding austenitic stainless steel materials having the above-described chemical composition, such as a steel pipe welded joint and a steel plate welded joint. The austenitic stainless steel welded joint includes a base material portion and a weld heat affected zone. In this specification, the base material portion (Base Material Zone) means a base material portion that is not affected by the heat of welding heat. The chemical composition of the weld metal of the austenitic stainless steel welded joint is not particularly limited.

  The austenitic stainless steel welded joint according to the present embodiment further has a surface that has been subjected to a blast treatment with a projectile. Examples of the blasting process using the projecting material include shot peening, shot blasting, and sand blasting. The projection material is, for example, particles, and the particles are steel, cast steel, stainless steel, glass, silica sand, alumina, amorphous, zirconia, and the like. The shape of the projection material may be spherical, or may be a cut wire, a round cut wire, or a grid. In the blast treatment, for example, the projection material may be sprayed onto the steel material surface using compressed air, centrifugal force generated by an impeller (impeller type), high-pressure water, ultrasonic waves, or the like.

A processed layer is formed on the blasted surface. Since the crystal grains of the processed layer are fine, it is easy to supply Cr to the surface. Therefore, when the processed layer is formed by blasting, a chromia (Cr ₂ O ₃ ) film is easily formed on the surface, and the steam oxidation resistance is improved. In the following description, the surface on which the processed layer is formed by blasting is referred to as “processed surface”.

  Among the heat affected zone (Heat Affected Zone), the crystal grain at the 50 μm depth position (referred to as HAZ surface layer portion) from the processed surface of the steel is the central portion of the thickness of the steel (base material). It is finer than the crystal grains at the center). More specifically, the crystal grain size number based on JIS G0551 (2005) in the HAZ surface layer portion is larger than the crystal grain size number in the center portion of the base material. Here, when the austenitic stainless steel welded joint is a steel plate, the thickness center portion of the base material portion means the plate thickness center portion. When the austenitic stainless steel welded joint is a steel pipe, the thickness center portion of the base material portion means the thickness center portion.

  The grain size number is determined by the following method. The above-mentioned austenitic stainless steel welded joint is cut perpendicular to the machining surface. Among the cut surfaces, arbitrary four visual fields are determined at a depth position (HAZ surface layer portion) of 50 μm from the processed surface in the weld heat affected zone. In each field of view, an austenite grain size number is obtained by carrying out a microscopic test method for grain size as defined in JIS G0551 (2005). Specifically, a sample including each visual field is collected. The visual field of the sample is corroded using a well-known corrosive liquid (Glyceria, Curling reagent, Marble reagent, aqua regia, nitric acid-hydrochloric acid-glycerin mixed reagent, etc.), and a grain boundary appears. For each corroded field surface, determine the grain size number for each field. The grain size number in each field of view is determined by comparison with the grain size standard diagram defined in 7.1.2 of JIS G0551 (2005). The average of the grain size numbers of the four fields of view is defined as the grain size number of the HAZ surface layer portion.

  The crystal grain size number at the center of the base material is also determined in the same manner as the crystal grain size number at the HAZ surface layer. Specifically, four arbitrary visual fields in the central portion of the thickness of the base material portion are determined. By the above-described method, the visual field of the sample including the visual field is corroded, and the crystal grain size number of each visual field is obtained. The average of the obtained crystal grain size numbers is defined as the crystal grain size number at the center of the base material.

In the austenitic stainless steel welded joint of the present embodiment, as described later, welding is performed after blasting, and even if heat treatment is performed, the crystal grains in the HAZ surface layer portion are larger than the crystal grains in the central portion of the base material. small. Therefore, many crystal grain boundaries exist in the HAZ surface layer portion. The crystal grain boundary becomes a path for supplying Cr to the surface. Since the vicinity of the processing surface of the weld heat affected zone is finer than the center of the base material, it is easy to supply Cr to the surface. Therefore, chromia (Cr ₂ O ₃ ) is easily generated continuously on the processed surface of the weld heat affected zone, and the steam oxidation resistance is improved.

  Preferably, the HAZ surface layer portion has a crystal grain size number of 9 or more. In this case, the HAZ surface layer portion has sufficient grain boundaries for supplying Cr to the inner surface, so that the steam oxidation resistance is further improved.

[Production method]
An example of the manufacturing method of the above-mentioned austenitic stainless steel welded joint will be described. The manufacturing method of this embodiment includes a step of preparing a steel material (steel pipe, steel plate, etc.) (preparation step), a step of performing a blasting process on the surface of the steel material to form a processed layer (processed layer forming step), And a step (welding step) of manufacturing a welded joint by performing welding on the steel material on which the processed layer is formed, and a step (heat treatment step) of performing a heat treatment on the welded joint. Hereinafter, each step will be described.

[Preparation process]
A material having the above chemical composition is prepared. The raw material may be a slab manufactured by a continuous casting method (including round CC). Moreover, the steel piece manufactured by hot-working the ingot manufactured by the ingot-making method may be sufficient. It may be a steel piece manufactured from a slab.

  The prepared material is charged into a heating furnace or a soaking furnace and heated. Subsequently, the heated material is hot worked to produce an austenitic stainless steel material. When manufacturing an austenitic stainless steel pipe, for example, the Mannesmann method is performed as hot working. Specifically, the material is pierced and rolled with a piercing machine to form a raw pipe. Subsequently, the base tube is further rolled by a mandrel mill or a sizing mill. Hot extrusion may be performed as hot working, or hot forging may be performed. If necessary, the hot-worked element tube may be subjected to softening heat treatment and then cold-worked. The cold working is, for example, cold rolling or cold drawing. The material may be hot worked to produce an austenitic stainless steel sheet. An austenitic stainless steel material is manufactured by the above process.

[Processed layer formation process]
The surface of the prepared austenitic stainless steel material is subjected to blasting using a projection material to form a processed layer. In the case of an austenitic stainless steel pipe, for example, a blast process is performed on the inner surface of the steel pipe to form a processed layer. In the case of an austenitic stainless steel plate, a blasting process is performed on one surface to form a processed layer. In short, a processed layer is formed by blasting on the steel surface exposed to the steam oxidation environment.

  Preferably, the maximum depth of the processed layer formed by blasting is ¼ or less of the thickness (wall thickness, plate thickness, etc.) of the steel material. This is because if the depth of the processed layer is larger than ¼ of the thickness of the steel material, the mechanical characteristics are deteriorated. When a processed layer is formed by cold rolling or cold drawing, the maximum depth of the processed layer tends to be 1/4 or more of the steel thickness. Therefore, in the austenitic stainless steel welded joint of the present embodiment, a processed layer is formed on the surface of the steel material by blasting.

[Welding process]
A weld joint is manufactured by performing welding on the austenitic stainless steel material after the processing layer forming step. The welding method is not particularly limited. Examples of the welding method include TIG welding, MIG welding, MAG welding, covering arc welding, and laser welding.

[Heat treatment process]
Heat treatment is performed on the welded joint after welding. By heat treatment, the stress introduced into the steel material by welding is removed.

  A preferable heat treatment temperature in the heat treatment after welding is 800 to 1150 ° C. If the heat treatment temperature is too low, removal of residual stress generated by welding becomes insufficient. On the other hand, if the heat treatment temperature is too high, the crystal grains in the HAZ surface layer portion become coarse and become larger than the crystal grains in the central portion of the base material. In this case, the steam oxidation resistance is lowered. Therefore, a preferable heat treatment temperature is 800 to 1150 ° C. The upper limit with preferable heat processing temperature is less than 1150 degreeC, More preferably, it is 1130 degreeC, More preferably, it is 1100 degreeC.

  A preferable soaking time for the heat treatment after welding (time for holding at the heat treatment temperature) is 5 to 240 minutes. If the soaking time is too short, the residual stress is not sufficiently removed. On the other hand, if the soaking time is too long, the crystal grains in the HAZ surface layer portion become coarse and become larger than the crystal grains in the central portion of the base material. In this case, the steam oxidation resistance is lowered. Therefore, a preferable soaking time is 5 to 240 minutes. A preferable lower limit of the soaking time is 10 minutes. The preferable upper limit of the soaking time is 210 minutes, more preferably 180 minutes.

  Through the above manufacturing process, the austenitic stainless steel welded joint according to the present embodiment is manufactured.

  Molten steel having the chemical composition shown in Table 1 was produced.

  With reference to Table 1, the chemical composition of the steel numbers 1-9 was in the range of the chemical composition of this embodiment. On the other hand, the Cr content of Steel No. 10 was too low. In addition, the N content of steel numbers 1, 5 to 10 was less than 0.005%, which was an impurity level.

  Molten steels having chemical compositions of steel numbers 1 to 8 and 10 were produced by vacuum melting. Using these molten steels, 180 kg ingots were produced. Hot forging and hot extrusion were performed on the ingot to produce a steel pipe having an outer diameter of 110 mm and a wall thickness of 12 mm.

  The surface scale was removed from the steel pipe of steel number 1. Thereafter, a solution treatment was performed on the steel pipe of steel number 1.

  Cold rolling was performed on the steel pipes having steel numbers 2 to 8, and 10 to produce a steel pipe having an outer diameter of 50.8 mm and a wall thickness of 8 mm. And the solution treatment was implemented on the manufactured steel pipe on the same conditions as steel number 1.

  Molten steel having a chemical composition of steel number 9 was produced by vacuum melting. Using this molten steel, a 50 kg ingot was produced. The ingot was hot forged and hot rolled to produce a steel plate having a thickness of 10 mm. The steel sheet was cold rolled to produce a steel sheet having a thickness of 3 mm. The solution treatment was performed on the steel sheet after cold rolling under the same conditions as in steel number 1. If necessary, a plurality of steel pipes with steel numbers were manufactured.

Shot blasting was performed on the inner surface of the steel pipe of each steel number under the same conditions. For steel plate No. 9, shot blasting was performed on one surface of the steel plate. Then, the steel pipes of the same steel number or steel plates were welded, and the welded joint was manufactured. Welding was performed by TIG welding. The chemical composition of the weld metal of each steel number was within the range of the chemical composition of the austenitic stainless steel pipe of this embodiment.
The heat treatment of test numbers 1 to 23 shown in Table 2 was performed on each welded joint.

  Table 2 shows the heat treatment temperature (° C.) and the soaking time (minutes) of the heat treatment carried out with each test number. After the heat treatment, the following test was performed.

[Microscopic examination of grain size]
The crystal grain size number was determined by the above-described method in the HAZ surface layer portion of the weld joint of each test number and the center portion of the base material. Diluted aqua regia was used as the corrosive liquid. Table 2 shows the crystal grain size numbers of the HAZ surface layer part and the center part of the base material.

[Steam oxidation test]
From the weld joint of each test number, a test piece including a welded portion and including a processed surface (shot blasted surface) of a pipe inner surface or a steel plate was collected. The test piece was inserted into a horizontal tubular furnace while being suspended from a jig. In the heating furnace, the test piece was soaked at 600 ° C. for 200 hours to carry out an oxidation test. The atmosphere in the heating furnace during the test was a water vapor atmosphere having a dissolved oxygen content of 100 ppb. After soaking, the specimen was furnace cooled to room temperature (25 ° C.). A test piece cooled to room temperature was embedded in the resin. Cutting was performed perpendicular to the processed surface, and mirror polishing was performed on the cross section. After polishing, the cross section of the oxide scale formed in the weld heat affected zone was observed with an optical microscope. In each test number, the oxide scale thickness of the weld heat-affected zone was measured in any four visual fields. The average value of the measured values was defined as the oxide scale thickness (μm) for that test number.

[Test results]
With reference to Table 2, in the test numbers 1, 2, 4-9, 12-16, and 19-22, the chemical composition was appropriate. Furthermore, the crystal grain size number of the HAZ surface layer portion was larger than the crystal grain size number of the center portion of the base material. Therefore, in these test numbers, the thickness of the oxide scale generated in the steam oxidation test was 20 μm or less, and excellent steam oxidation resistance was obtained.

  In particular, in the test numbers 1, 4 to 8, 12 to 16, and 19 to 22, the crystal grain size number of the HAZ surface layer portion was 9 or more. Therefore, in these test numbers, compared with test numbers 2 and 9, the thickness of the oxide scale was thin and was 15 μm or less.

  On the other hand, in the test numbers 3, 10 and 17, although the chemical composition was appropriate, the heat treatment temperature was too high. For this reason, the crystal grain size number of the HAZ surface layer portion was smaller than the crystal grain size number of the center portion of the base material. As a result, the thickness of the oxide scale exceeded 20 μm, and the steam oxidation resistance was low.

In test numbers 11 and 18, although the chemical composition was appropriate, the soaking time of the heat treatment was too long. For this reason, the crystal grain size number of the HAZ surface layer portion was smaller than the crystal grain size number of the center portion of the base material. As a result, the thickness of the oxide scale exceeded 20 μm, and the steam oxidation resistance was low.
In test number 23, the Cr content was too low. Therefore, the thickness of the oxide scale exceeded 20 μm, and the steam oxidation resistance was low.

  The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

  The austenitic stainless steel welded joint according to this embodiment can be widely applied as a steel material used in an environment where steam oxidation resistance is required. In particular, it is suitable as a steel material used in boilers, piping, waste heat recovery boilers, heat exchangers and the like of power generation facilities.

Claims (4)

An austenitic stainless steel welded joint,
% By mass
C: 0.2% or less,
Si: 2.0% or less,
Mn: 0.1 to 3.0%
Cr: 14 to 28%,
Ni: 6-30%,
N: 0.3% or less,
Mo: 0 to 5%,
W: 0-10%
Ta: 0 to 5%
Cu: 0 to 5%,
V: 0 to 1.0%
Nb: 0 to 1.5%,
Ti: 0 to 0.5%,
Ca: 0 to 0.02%,
Mg: 0 to 0.02%,
Al: 0 to 0.3%,
Zr: 0 to 0.5%,
B: 0-0.02% and
Rare earth element: containing 0-0.1%, the balance being Fe and impurities chemical composition;
A surface subjected to blasting with a projectile material,
An austenitic stainless steel welded joint, wherein a crystal grain size number at a position of 50 μm depth from the surface of the weld heat affected zone is larger than a crystal grain size number at a thickness central portion of the base material portion. The austenitic stainless steel welded joint according to claim 1,
An austenitic stainless steel welded joint having a grain size number of 9 or more at a depth of 50 μm from the surface of the weld heat affected zone. The austenitic stainless steel welded joint according to claim 1 or 2,
N: 0.005-0.3%,
Mo: 0.1 to 5%,
W: 0.1 to 10%
Ta: 0.1 to 5%,
Cu: 0.1 to 5%,
V: 0.01-1.0%
Nb: 0.01 to 1.5%,
Ti: 0.01 to 0.5%,
Ca: 0.0001 to 0.02%,
Mg: 0.0001 to 0.02%,
Al: 0.0001 to 0.3%,
Zr: 0.0001 to 0.5%,
B: 0.0001 to 0.02%, and
Rare earth element: An austenitic stainless steel welded joint containing one or more selected from the group consisting of 0.0001 to 0.1%. The austenitic stainless steel welded joint according to any one of claims 1 to 3,
The austenitic stainless steel welded joint is a steel pipe,
The austenitic stainless steel welded joint, wherein the surface is an inner surface of the steel pipe.