Fe-Ni ALLOY, ALLOY PIPE, AND METHOD FOR PRODUCING THE SAME

Abstract

To provide an Fe-Ni alloy with which it is possible to easily obtain exceptional rust resistance in processing for a welded pipe such as LNG transportation piping, an alloy pipe in which the Fe-Ni alloy is used, and a method for producing the same.SOLUTION: An Fe-Ni alloy has a component composition containing, in mass%, C: 0.020-0.040%, Si: 0.10-0.30%, Mn: 0.25-0.45%, P: 0.0025-0.0050%, S: 0.0001-0.0012%, Ni: 35.5-36.5%, Cr: 0.03-0.22%, Cu: 0.01-0.10%, Al: 0.0005-0.010%, Ti: 0.0005-0.010%, N: 0.0005-0.0050%, Mg: 0.0003-0.0020%, and Ca: 0.0003-0.0015%, the balance being Fe and unavoidable impurities. An Fe-Ni alloy pipe is composed of the alloy. In a production method therefor, a welded pipe is obtained from a sheet material composed of the alloy, and a surface oxide coating is imparted by heat treatment.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to an Invar Fe-Ni alloy having excellent low-temperature toughness, an alloy pipe using the same, and a manufacturing method thereof, and more particularly to an Fe-Ni alloy, an alloy pipe, and a manufacturing method thereof suitable for use in piping for transporting LNG. [Background technology]

[0002] Invar Fe-Ni alloys containing 30-50wt% Ni have a thermal expansion coefficient that can be controlled by adjusting the composition, and are widely used as electronic component materials. They have also been proposed for use in structural components, and for example, Fe-Ni alloys containing 36% Ni have a small thermal expansion coefficient and excellent low-temperature toughness, and are therefore used in LNG transport piping.

[0003] For example, Patent Document 1 discloses an Invar Fe-Ni alloy for LNG transport piping, which has a composition containing, in mass %, C: 0.001-0.1%, Si: 0.5% or less, Mn: 0.1-1%, Ni: 35-40%, Ti: 0.1-0.5%, and Nb: 0.1-0.5% in Fe. Welding is essential for application to structural members, but the document mentions that welding of Invar steel is difficult, and that the effects of precipitation strengthening elements on the toughness and reheat cracking resistance of the weld metal are contradictory, so the amounts of Ti and Nb added should be controlled.

[0004] Non-Patent Document 1 also describes measures to prevent stress corrosion cracking (SCC) based on the assumption that Fe-Ni alloys applied to LNG piping will be exposed to a corrosive environment both during construction and operation. Here, it is stated that sandblasting was performed to impart compressive residual stress and improve paint adhesion, and zinc-rich paint was applied to impart cathodic corrosion protection, and that remarkable results were obtained. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 11-364468 [Non-patent literature]

[0006] [Non-Patent Document 1] Stress corrosion cracking of Invar alloys and its prevention measures; Susumu Hongo, Shuji Yamamoto, Kiyoko Takeda, Akira Takashima, Takehito Yamakawa, Hideaki Yuki, Hideo Kobayashi; 2003, Vol. 52, No. 6, p. 308-315 Summary of the Invention [Problem to be solved by the invention]

[0007] Incidentally, Invar Fe-Ni alloy is considered to be very susceptible to rusting. For example, in the manufacturing process and construction process of LNG transport piping, it may be stored for several days to several months depending on the work, but if it is left for several days, rust will occur and repairs will be required. Non-Patent Document 1 describes a rust prevention treatment by painting after processing Invar Fe-Ni alloy into pipe material, but adding such a rust prevention treatment process by painting increases the manufacturing cost. Therefore, there is a demand for Invar Fe-Ni alloy itself as a structural material to be resistant to rust.

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an Fe-Ni alloy that can easily provide excellent rust prevention properties in manufacturing and processing, particularly in processing for welded pipes such as LNG transport piping, an alloy pipe using the alloy, and a manufacturing method thereof. [Means for solving the problem]

[0009] The Fe-Ni alloy according to the present invention is characterized by having a composition containing, by mass%, C: 0.020-0.040%, Si: 0.10-0.30%, Mn: 0.25-0.45%, P: 0.0025-0.0050%, S: 0.0001-0.0012%, Ni: 35.5-36.5%, Cr: 0.03-0.22%, Cu: 0.01-0.10%, Al: 0.0005-0.010%, Ti: 0.0005-0.010%, N: 0.0005-0.0050%, Mg: 0.0003-0.0020%, Ca: 0.0003-0.0015%, and the balance being Fe and unavoidable impurities.

[0010] According to this feature, a strong surface oxide film can be easily formed by heat treatment, and an alloy product having excellent rust prevention properties can be obtained.

[0011] In the above-mentioned invention, the component composition is 0.15≦Si+Cr≦0.40 (1) 0.003≦2×Al+Ti≦0.017 (2) According to this feature, an alloy product having better rust resistance can be obtained.

[0012] In the above-mentioned invention, the component composition is Al≧Ti (3) and may further comprise one or more of Mo: 0.01-0.10%, Co: 0.01-0.25%, W: 0.01-0.05%, and B: 0.0001-0.0010%. According to such a feature, an alloy product having better rust prevention properties can be obtained.

[0013] Moreover, an Fe-Ni alloy tube according to the present invention is characterized in that it is made of the above-mentioned Fe-Ni alloy.

[0014] According to such characteristics, a strong surface oxide film can be easily formed by heat treatment, and the Fe-Ni alloy pipe, in particular, an Fe-Ni alloy pipe for LNG transport piping, has excellent rust prevention properties.

[0015] The above-mentioned invention may be characterized in that the welded pipe is made of the above-mentioned Fe-Ni alloy together with the welded portion. According to this characteristic, a strong surface oxide film can be easily formed by heat treatment, and the Fe-Ni alloy pipe, particularly the Fe-Ni alloy pipe for LNG transport piping, has excellent rust prevention properties.

[0016] The above-mentioned invention may be characterized in that the surface oxide film is made of a composite oxide of Fe, Si, Mn, and Cr and has a thickness of 2 to 30 μm. With this characteristic, the Fe-Ni alloy pipe, particularly the Fe-Ni alloy pipe for LNG transport piping, has excellent rust prevention properties.

[0017] The method for producing an Fe-Ni alloy pipe according to the present invention is characterized in that a plate material made of the above-mentioned Fe-Ni alloy is prepared, the plate material is processed into a tubular shape and welded to form a welded pipe, and then heated and held at 750 to 900°C to provide a surface oxide film having a thickness of 2 to 30 μm and made of a composite oxide of Fe, Si, Mn, and Cr. Alternatively, a plate material and a filler material made of the above-mentioned Fe-Ni alloy are prepared, the plate material is processed into a tubular shape and welded using the filler material to form a welded pipe, and then heated and held at 750 to 900°C to provide a surface oxide film having a thickness of 2 to 30 μm and made of a composite oxide of Fe, Si, Mn, and Cr. According to these characteristics, an Fe-Ni alloy pipe having excellent rust resistance, particularly an Fe-Ni alloy pipe for LNG transport piping, can be obtained. [Brief description of the drawings]

[0018] [Figure 1] 1 is a list of the composition of alloys used in the test. [Diagram 2] 1 is a table showing the alloys used in tests for weldability and rust resistance and the test results. [Diagram 3] FIG. 2 is a diagram for explaining a method for evaluating the appearance of a weld bead. [Figure 4] 1 is a table showing additional test results regarding weldability. [Diagram 5]1 is a table showing additional test results regarding rust prevention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] As described above, the Invar Fe-Ni alloy is very susceptible to rusting. Therefore, the inventors of the present application have conducted extensive research, focusing mainly on the relationship between the formation of a surface oxide film during annealing and rust prevention. As a result, it has been found that in order to improve rust prevention by the surface oxide film formed by heat treatment, it is important to control the composition, adhesion, and thickness of the surface oxide film, and this requires control of the chemical components of the alloy, particularly the trace components. It has been found that the rust prevention can be improved by adopting the following alloy composition.

[0020] That is, the Fe-Ni alloy in this example contains, in mass %, C: 0.020-0.040%, Si: 0.10-0.30%, Mn: 0.25-0.45%, P: 0.0025-0.0050%, S: 0.0001-0.0012%, Ni: 35.5-36.5%, Cr: 0.03-0.22%, Cu: 0.01-0.10%, Al: 0.0005-0.010%, Ti: 0.0005-0.010%, N: 0.0005-0.0050%, Mg: 0.0003-0.0020%, and Ca: 0.0003-0.0015%. By using such a composition, a strong surface oxide film can be easily formed by heat treatment, and an alloy product obtained by using this alloy can be provided with excellent rust resistance. In addition, an Fe-Ni alloy pipe manufactured by using this alloy can easily form a strong surface oxide film by heat treatment, and can obtain excellent rust resistance when processed into a welded pipe such as a pipe for transporting LNG.

[0021] In addition, it is preferable that this component composition further satisfies the following relational expressions (1) and (2). Relational expression (1) specifies the contents of Si and Cr in order to densify and stabilize the surface oxide film while maintaining a low thermal expansion coefficient. Relational expression (2) specifies the contents of Al and Ti in order to form a dense and adhesive surface oxide film while maintaining a low thermal expansion coefficient. 0.15≦Si+Cr≦0.40 (1) 0.003≦2×Al+Ti≦0.017 (2)

[0022] Furthermore, this component composition is expressed by the following formula (3): Al≧Ti (3) and preferably contains, in mass%, one or more of Mo: 0.01-0.10%, Co: 0.01-0.25%, W: 0.01-0.05%, and B: 0.0001-0.0010%. In the relational formula (3), the contents of Al and Ti, which are useful for forming a dense and adhesive surface oxide film, are specified to prevent a decrease in penetration and crack sensitivity during welding. The contents of Mo, Co, W, and B are specified as optional added elements that improve the quality and manufacturability of the Fe-Ni alloy in this embodiment, as will be described in detail later. In the relational formula of the amount of these elements, the element symbols mean the numerical values ​​representing the contents of the elements in mass%.

[0023] The manufacturing method of the Fe-Ni alloy pipe is, for example, as follows. First, the raw material is melted and refined, a slab is obtained by continuous casting, and a plate material made of an alloy having a predetermined composition is obtained by hot rolling. After forming a groove at the end of the plate material by machining and forming it into a tube, a welded pipe is obtained by longitudinal seam welding using a predetermined filler metal (described later). Then, as an annealing heat treatment, the plate material is heated and held at 750 to 900°C to form a surface oxide film of 2 to 30 μm thick made of a composite oxide of Fe, Si, Mn, and Cr. Such a surface oxide film provides excellent rust prevention. It is preferable to use an alloy having the same composition as the alloy pipe as the filler metal.

[0024] Here, the purpose of the above-mentioned annealing heat treatment is to remove residual strain after welding, but also to form a surface oxide film to obtain excellent rust prevention. Regarding the temperature of the annealing heat treatment, if the holding temperature is too low, annealing will be insufficient and the thickness of the surface oxide film will be thin. On the other hand, if the holding temperature of the annealing heat treatment is too high, the loss due to oxidation will be large, leading to a decrease in yield. Furthermore, the surface oxide film will become too thick, reducing its uniformity, and uniform cooling will not be obtained, making it easy to cause partial peeling. Therefore, the holding temperature of the annealing heat treatment is 750 to 900°C, preferably 780 to 880°C, and more preferably 800 to 870°C.

[0025] The holding time of the annealing heat treatment can be appropriately selected depending on the dimensions of the material, such as the plate thickness, and the heat treatment temperature. The holding time is required to be at least a certain time so as to form a uniform surface oxide film. On the other hand, if the holding time is too long, the surface oxide film becomes too thick, reducing uniformity and causing partial peeling. For this reason, the holding time of the annealing heat treatment is 2 to 30 minutes, preferably 4 to 25 minutes, and more preferably 5 to 20 minutes. Regarding cooling after heating and holding, it is preferable to cool the entire material uniformly, and it is preferable to adopt gentle cooling such as furnace cooling or air cooling.

[0026] The surface oxide film obtained in the above manner is also formed on the inner surface of the pipe after processing into a welded pipe, and can ensure the rust prevention required for a pipe product for transporting liquids, etc. The surface oxide film is composed of a composite oxide layer mainly containing Fe oxide and also containing Si, Mn, Cr, or Al and Ti, and is set to a thickness of 2 to 30 μm, preferably 5 to 25 μm, and more preferably 7 to 20 μm in consideration of uniformity and stability.

[0027] [Weldability and rust resistance tests] Next, a material plate made of a specified Fe-Ni alloy is prepared, and a weldability test using a welded test plate cut from the material plate and a rust prevention test using a welded pipe obtained by welding the material plate are described.

[0028] <Production of blanks> Material plates made of alloys having the respective compositional compositions of Alloys 1 to 18 and Alloys 101 to 118 shown in Fig. 1 were prepared. In detail, raw materials such as scrap and Ni master alloys were put into an electric furnace and melted, and then decarburized by oxygen blowing in AOD (Argon Oxygen Decarburization) and / or VOD (Vacuum Oxygen Decarburization). Then, Al and limestone were put in to reduce Cr, and further limestone and fluorite were put in to form CaO-SiO 2 -Al 2 O 3 A -MgO-F slag was formed, which was then deoxidized and desulfurized. The molten alloy was then cast in a continuous casting machine to form a slab, which was then hot rolled to form a rolled plate with a thickness of 20 mm, width of 2000 mm, and length of 10000 mm. The rolled plate was heat treated at 960°C for 10 minutes, water-cooled in a cooling tank, and the oxide film on the surface was removed by pickling or polishing to produce a blank plate for use as a welded pipe material. A portion of this blank was cut out and used for the weldability test described below.

[0029] Regarding the composition of each alloy in FIG. 1, the column "Formula 1" shows the value of (Si+Cr), the column "Formula 2" shows the value of (2×Al+Ti), and the column "Formula 3" shows the value of (Al-Ti). In other words, when "Formula 1" is within the range of 0.15 to 0.40, the relational formula (1) is satisfied. When "Formula 2" is within the range of 0.003 to 0.017, and "Formula 3" is a positive value, the relational formulas (2) and (3) are satisfied.

[0030] <Welded pipe manufacturing> Next, welded pipes of Examples 1 to 18 and Comparative Examples 1 to 18 shown in FIG. 2 were manufactured (for example, Example 1 shows that the blank plate obtained by the above-mentioned manufacturing method using the alloy 1 shown in FIG. 1 was welded using a filler metal made of the alloy 1 shown in FIG. 1). The blank plate was cut to a width of 785 mm and a length of 10,000 mm, and the ends in the width direction were machined to provide U-grooves or X-grooves, and formed into a tubular shape of an outer diameter of 276.4 mm and a length of 10,000 mm. Then, using a predetermined filler metal, welded pipes were made by longitudinal seam welding at a maximum of 25 KJ / cm using plasma and TIG welding machines.

[0031] Here, the filler metal is a wire for TIG and plasma welding, and is made from a slab during the manufacturing process of the above-mentioned blank plate. That is, the slab is hot forged in the temperature range of 1200 to 1000°C to form a 100 mm rectangular steel billet, which is then hot drawn in the temperature range of 1200 to 900°C to form a wire rod with a wire diameter of 9.5 mm. This wire rod is repeatedly cold drawn and softened annealed in the temperature range of 1050 to 950°C to finally form a wound solid wire with a wire diameter of 1.6 to 1.2 mm.

[0032] <Weldability test> In the weldability test, a welded test plate was cut out from the base plate, and the resistance to weld hot cracking was evaluated by a Varestraint test, and the soundness was evaluated by observing the properties of the welded portion.

[0033] -Welding hot crack resistance- In the Varestraint test, a part of the blank was cut out and machined to cut out a Varestraint test welded test plate with a thickness of 10 mm x length of 120 mm x width of 50 mm, and solidification cracking and reheat cracking were observed by a Varestraint test using a double bead method. In detail, the first bead was formed by TIG welding under the condition of a heat input of 25 kJ / cm, and then the second bead was formed so as to overlap the first bead under the same heat input condition. In the middle of forming the second bead, a bending strain equivalent to 2 to 5% was suddenly applied to the test piece, and the cracks that occurred were observed with a microscope and the crack lengths were measured, and the total crack length was measured by adding up the lengths of all the cracks formed in the second bead. Meanwhile, for the reheat cracks, the total crack length that occurred near the reheated part of the first bead was measured.

[0034] Regarding the evaluation of solidification cracking, if the total crack length was within 1 mm, it was evaluated as excellent with a "◎", if it was between 1 mm and 2 mm or less, it was evaluated as good with a "○", if it was between 2 mm and 3 mm or less, it was evaluated as passable with a "△", and if it exceeded 3 mm, it was evaluated as poor with an "X" in the respective columns in Figure 2. Meanwhile, regarding reheat cracking, if there was no reheat cracking, it was evaluated as excellent with a "◎", if the total crack length was 0.3 mm or less, it was evaluated as good with a "〇", if it was 0.5 mm or less, it was evaluated as passable with a "△", and if it exceeded 0.5 mm, it was evaluated as poor with an "X" in the respective columns in Figure 2.

[0035] In addition, the alloys evaluated this time generally have high hot crack resistance, and the difference was not clear when the strain in the Varestraint test was set to about 2%. Therefore, a strain of 5%, which is the maximum value of the test machine performance, was added.

[0036] - Properties of welded joints - In observing the properties of the welds, the penetration depth and weld bead width were measured and the soundness of the welds was evaluated. A portion of the material plate was cut out and processed into a 10 mm thick welding test plate for observing the welds, and TIG bead-on welding was performed with a welding current of 200 A and a weld line of 150 mm. The welds were cut and the macrostructure of the cross section was observed, and the bead penetration depth was measured with a microscope. If the penetration depth was 3.0 mm or more from the plate surface, it was evaluated as excellent and marked with a "◎", if it was 2.5 mm or more but less than 3.0 mm, it was evaluated as good and marked with a "○", if it was 2.0 mm or more but less than 2.5 mm, it was evaluated as acceptable and marked with a "△", and if it was less than 2.0 mm, it was evaluated as unacceptable and marked with an "×" in the respective columns in Figure 2.

[0037] As shown in Fig. 3, the minimum weld bead width W min and the maximum value W max The difference ΔW was calculated and marked in the respective columns in Figure 2. If the difference ΔW was less than 0.25 mm, it was evaluated as excellent and marked with a "◎", if it was between 0.25 mm and 0.45 mm, it was evaluated as good and marked with a "〇", if it was between 0.45 mm and 0.65 mm, it was evaluated as acceptable and marked with a "△", and if it was 0.65 mm or more, it was evaluated as poor and marked with an "×".

[0038] <Rust prevention test> In the rust prevention test, a pure water spray test, an exposure test in a factory, and an investigation of the surface oxide film were conducted to evaluate the rust prevention properties of the welded pipes.

[0039] -Rust prevention / pure water spray test- The rust prevention properties of welded pipes were evaluated by a pure water spray test. For example, the salt spray test is known as a method for evaluating the rust prevention properties of stainless steels, but it was expected that the rust prevention properties of Fe-Ni alloys as shown in Figure 1 would cause a lot of rust and make it difficult to clearly distinguish the differences between test materials. Therefore, a "pure water spray test" was conducted by replacing the test liquid of the salt spray test with pure water. Except for the use of distilled water as the test liquid, the test was the same as the salt spray test specified in JIS. In other words, the test temperature was set at 50°C, and after three spray cycles, each cycle consisting of 2 hours of spray + 1 hour of rest, the spray test specimens were observed and the rust prevention properties were evaluated. For the spray test specimens, welded pipes were cut to a thickness of approximately 20 mm x width 25 mm x length 10 mm. To compare the presence or absence of a surface oxide film, two test specimens were prepared: one in which a surface oxide film was formed by performing the annealing heat treatment described above after welding, and the other in which the surface oxide film was removed by pickling. The holding temperature (heat treatment temperature) and holding time of the annealing heat treatment for forming the oxide film are as shown in FIG.

[0040] The sprayed test specimens were observed by measuring the area of ​​rust that had developed in the evaluation area using a digital microscope (Keyence Corporation / VHX-7000). In detail, the color tolerance was set to 5 using the digital microscope's "automatic area calculation" function, and the rust area was calculated by binarizing the rust and other areas by color. The rust area rate was calculated by dividing the rust area by the area of ​​the evaluation area, and the rust prevention properties were evaluated using the RN (rating number) corresponding to the rust area rate. If the RN was 9.9 or more, it was evaluated as excellent and marked with "◎", if it was less than 9.9 and 9.5 or more, it was evaluated as good and marked with "〇", if it was less than 9.5 and 9.0 or more, it was evaluated as acceptable and marked with "△", and if it was less than 9.0, it was evaluated as poor and marked with "×" in the respective columns in Figure 2.

[0041] -Rust prevention / in-factory exposure test- Furthermore, the rust prevention property of the welded pipe was also evaluated by an exposure test. For each of the welded pipe with a surface oxide film formed and the welded pipe with the surface oxide film removed, they were stored in the factory's product storage area for 20 days to observe the occurrence of rust and evaluate the rust prevention property. The factory's product storage area used was a normal product storage area within a manufacturing factory that is not exposed to the wind and rain environment but has no humidity control or the like. The observation of the occurrence of rust was carried out visually. For the outer peripheral side, it was directly observed visually, and for the inner peripheral side, it was observed visually through a camera. The number of rust spots was measured within a range of 500×500 mm of the observation area, and the total was obtained. If the number of rust spots was 100 or less, it was evaluated as excellent and marked with "◎"; if it exceeded 100 and was 150 or less, it was evaluated as good and marked with "〇"; if it exceeded 150 and was 200 or less, it was evaluated as acceptable and marked with "△"; if it exceeded 200, it was evaluated as defective and marked with "×" in each column of Figure 2.

[0042] -Observation of Rust Prevention Property / Surface Oxide Film- Furthermore, the surface oxide film of the welded pipe as it was after annealing heat treatment was observed to evaluate the rust prevention property. A film observation sample including the cross-section of the surface oxide film was cut out from the welded pipe after annealing heat treatment, and the composition, adhesion, and thickness of the surface oxide film were observed by EDS (Energy Dispersive Spectroscopy) attached to the FE-SEM. In order to prevent the peeling and falling off of the surface oxide film due to the preparation of the film observation sample, the small piece of film observation sample was copper-plated, and the cross-section was cut out at a position about 5 mm away from the position to be observed, and after embedding, it was polished up to the position to be observed. The polishing was carried out only wet, and fine-grained polishing paper was selected and the work was done slowly and carefully. The polished cross-section was observed over a length of 1000 μm. If there was no peeling of the surface oxide film, it was evaluated as excellent and marked with "◎"; if the peeled part was less than 15 μm, it was evaluated as good and marked with "〇"; if the peeled part was 15 μm or more and less than 30 μm, it was evaluated as acceptable and marked with "△"; if peeling occurred in a wider range than this, it was evaluated as defective and marked with "×" in each column of Figure 2. Also, if the thickness of the surface oxide film was in the range of 2 - 30 μm, it was evaluated as good and marked with "〇"; among them, those with a thickness of 5 - 20 μm were evaluated as excellent and marked with "◎"; those outside the range of 2 - 30 μm were evaluated as defective and marked with "×" in each column of Figure 2.

[0043] <Evaluation of the Soundness and Rust Prevention Property of the Welded Joint> As shown in FIG. 2, in the weldability test of Examples 1 to 18, no solidification cracking or reheat cracking occurred or was slight, the bead shape and penetration were also excellent, and the soundness of the welds were all rated as "fair" or higher. It is believed that such excellent results were obtained by reducing the contents of S and P and further optimizing the contents of Ca, Mg, and Cu. In addition, in the rust prevention test and film observation of Examples 1 to 18, a good surface oxide film was formed by the annealing heat treatment, and all were rated as "fair" or higher for rust prevention. It is believed that the surface oxide film was properly formed by controlling the contents of Si, Cr, Al, and Ti, and excellent results were obtained. In both the pure water spray test and the in-factory exposure test, when the surface oxide film was removed, both were rated as "poor". From this, it is also necessary to have a surface oxide film in order to obtain excellent rust prevention.

[0044] In addition, in Examples 1 to 3, alloys 1 to 3 (see FIG. 1) that do not satisfy all of the relational expressions (1) to (3) were used, but the rust prevention evaluation was all acceptable when the surface oxide film was present. In Examples 4 to 6, alloys 4 to 6 (see FIG. 1) that do not satisfy the relational expression (3) were used, but the rust prevention evaluation was good. Furthermore, in Examples 7 to 11, alloys 7 to 11 that satisfy all of the relational expressions (1) to (3) were used, but the rust prevention evaluation was excellent. In addition, in Example 12, alloy 12 that does not satisfy the relational expression (1), in Examples 13 and 14, alloys 13 and 14 that do not satisfy the relational expression (2), and in Example 15, alloy 15 that does not satisfy the relational expression (3), all of them were lower in the rust prevention evaluation than in Examples 7 to 11. From these, it was concluded that higher rust prevention can be obtained by satisfying the relational expressions (1) to (3).

[0045] In Examples 1 to 18, the blank for the welded pipe and the filler metal used for seam welding are made of alloys having the same composition. Therefore, it is considered that the soundness of the welded portion described above would be evaluated in the same way even if the welded portion was jointly welded without using a filler metal.

[0046] On the other hand, in the weldability test of Comparative Example 1, the penetration depth was evaluated as poor. This is thought to be due to the low C content.

[0047] In the weldability test of Comparative Example 2, the penetration depth was evaluated as poor. In the rust prevention test, even though a surface oxide film was formed, both the peeling and thickness of the surface oxide film were evaluated as poor. And, both the pure water spray test and the in-factory exposure test were evaluated as poor. This is thought to be due to the low Si content.

[0048] In the weldability test of Comparative Example 3, the reheat cracking and bead width were evaluated as poor, presumably because the Si content was too high.

[0049] In the weldability test of Comparative Example 4, solidification cracking was evaluated as poor, presumably because the Mn content was low.

[0050] In the weldability test of Comparative Example 5, the reheat crack and bead width were evaluated as poor. In the rust prevention test, the peeling and thickness of the surface oxide film were both evaluated as poor. And, the pure water spray test and the in-factory exposure test were both evaluated as poor. This is thought to be due to the high Mn content.

[0051] In the weldability test of Comparative Example 6, solidification cracking and reheat cracking were evaluated as poor. This is considered to be due to the high P content.

[0052] In the weldability test of Comparative Example 7, the solidification cracking and reheat cracking were evaluated as poor. This is considered to be due to the high S content.

[0053] In the weldability test of Comparative Example 8, solidification cracking and reheat cracking were evaluated as poor. This is considered to be due to the high Cu content.

[0054] In the rust prevention test of Comparative Example 9, both the peeling and thickness of the surface oxide film were evaluated as poor, and both the pure water spray test and the in-factory exposure test were evaluated as poor. This is believed to be due to the low content of Al.

[0055] In the weldability test of Comparative Example 10, the penetration depth and bead width were evaluated as poor. This is considered to be due to the high Al content.

[0056] In the weldability test of Comparative Example 11, the reheat crack and bead width were evaluated as poor. In the rust prevention test, the peeling of the surface oxide film was evaluated as poor, and the pure water spray test and the in-factory exposure test were also evaluated as poor. This is thought to be due to the low Ti content.

[0057] In the weldability test of Comparative Example 12, solidification cracking, reheat cracking, penetration depth, and bead width were all evaluated as poor. This is believed to be due to the high Ti content.

[0058] In the weldability test of Comparative Example 13, the solidification cracking and reheat cracking were evaluated as poor. In the rust resistance test, the in-factory exposure test was evaluated as poor. This is considered to be due to the low Mg content.

[0059] In the weldability test of Comparative Example 14, the bead width was evaluated as poor. This is considered to be due to the high Mg content.

[0060] In the weldability test of Comparative Example 15, the solidification cracking and reheat cracking were evaluated as poor. In the rust resistance test, the in-factory exposure test was evaluated as poor. This is believed to be due to the low Ca content.

[0061] In the weldability test of Comparative Example 16, the penetration depth and bead width were evaluated as poor. This is considered to be due to the high Ca content.

[0062] In the rust prevention test of Comparative Example 17, both the peeling and thickness of the surface oxide film were evaluated as poor, and both the pure water spray test and the in-factory exposure test were evaluated as poor. This is thought to be due to the low Cr content. In addition, the EDS analysis of the surface oxide film did not show any Cr content.

[0063] In the weldability test of Comparative Example 18, solidification cracking, reheat cracking, and bead width were evaluated as poor. This is considered to be due to the low N content.

[0064] In this way, with Alloys 1 to 18, as shown in Examples 1 to 18, a strong surface oxide film can be easily formed by heat treatment, and excellent rust prevention properties can be imparted.

[0065] [Additional Test 1] As shown in Figure 4, welding tests a to d were carried out when different alloys were selected for the welding test plate and the filler metal. As a result, cracks occurred in the welds of all of the welding test plates a to d, and stable beads could not be obtained.

[0066] [Additional Test 2] As shown in Figure 5, we carried out rust prevention tests a to o for welded pipes in which different alloys were selected for the base plate and filler metal, that is, evaluation of rust prevention when the holding temperature and holding time of the annealing heat treatment were changed. The results showed that, for example, when the holding temperature was too high (see k and n), the oxide film became too thick, peeling or porosity occurred, and rust occurred frequently. Also, when the holding temperature was too low (see j and o), the surface oxide film became thin and incomplete, and rust occurred even in small scratches caused by the manufacturing process, reducing the rust prevention properties.

[0067] As described above, based on Additional Tests 1 and 2, in Examples 1 to 18, which are welded pipes in which the same alloy was selected for the base plate and filler metal, a strong surface oxide film can be formed by heat treatment, and excellent rust resistance can be easily obtained.

[0068] The ranges of the composition of the alloys of the present invention, including alloys 1 to 18, are set as follows: First, the essential additive elements will be described.

[0069] Ni is an important element for controlling the thermal expansion coefficient of Fe-Ni alloy. In order to form a uniform surface oxide film that enhances rust prevention while keeping the thermal expansion coefficient small, it is necessary to regulate the content more precisely. For this reason, Ni is in the range of 35.5 to 36.5%, preferably 35.6 to 36.4%, more preferably 35.7 to 36.3%, by mass%.

[0070] C is an element necessary for ensuring mechanical strength, and is also necessary for making the joint strength of the welded portion equivalent to that of the base metal when used as a filler metal. It is also necessary for ensuring the penetration during welding. On the other hand, if it is contained excessively, carbides are generated, which may cause deterioration of the low-temperature toughness of the weld metal, especially in the case of multi-layer welding. Therefore, in order to obtain excellent mechanical strength, low-temperature toughness, and a low thermal expansion coefficient, the content of C must be strictly limited. In consideration of these, C is in the range of 0.020 to 0.040%, preferably in the range of 0.022 to 0.037%, and more preferably in the range of 0.025 to 0.035%, by mass%.

[0071] Silicon is an effective element as a deoxidizer and for maintaining good penetration during welding. It also promotes densification of the oxide layer by annealing and stably forms a surface oxide film. On the other hand, excessive content of silicon deteriorates reheat cracking resistance and increases the thermal expansion coefficient. In consideration of these, the silicon content is within the range of 0.10 to 0.30%, preferably within the range of 0.12 to 0.28%, and more preferably within the range of 0.14 to 0.26%, by mass%.

[0072] Mn is a solid solution strengthening element, which contributes to improving the mechanical strength of the base material and is also an effective element as a deoxidizer. It also forms MnS to promote the fixation of S, improving hot workability and weld crack resistance. On the other hand, if it is contained in excess, MnS precipitates, which affects the weld hot crack resistance and also deteriorates the surface properties. It also increases the thermal expansion coefficient and makes the surface oxide film excessively thick. In consideration of these, Mn is in the range of 0.25 to 0.45%, preferably 0.26 to 0.43%, and more preferably 0.27 to 0.40% by mass.

[0073] P is an important element that should be contained in a certain amount to ensure mechanical strength. However, if contained in excess, it segregates at grain boundaries, reducing hot workability and causing hot welding cracks, so the upper limit is strictly limited. Taking these factors into consideration, P is in the range of 0.0025 to 0.0050%, preferably 0.0027 to 0.0047%, and more preferably 0.0030 to 0.0045%, by mass%.

[0074] S is an element necessary for ensuring good penetration during welding. However, excessive content is particularly detrimental to resistance to hot welding cracking. In consideration of this, the S content is in the range of 0.0001 to 0.0012%, preferably 0.0002 to 0.0011%, and more preferably 0.0003 to 0.0010%, by mass%.

[0075] Cr is a solid solution strengthening element that contributes to securing mechanical strength and is an important element for improving rust resistance. In addition, like Si, it is an element that stabilizes the surface oxide film, and forms a complex oxide film on the surface together with Si, promoting densification of the surface oxide film and further improving rust resistance. On the other hand, if contained in excess, the thermal expansion coefficient increases. In consideration of these, the Cr content is within the range of 0.03 to 0.22%, preferably within the range of 0.05 to 0.20%, and more preferably within the range of 0.08 to 0.18%, by mass%.

[0076] Cu is an element that contributes to improving the stability of the FCC phase. On the other hand, if it is contained in excess, the thermal expansion coefficient tends to increase and the susceptibility to hot cracking during welding increases. In consideration of these, the Cu content is in the range of 0.01 to 0.10%, preferably in the range of 0.02 to 0.09%, and more preferably in the range of 0.03 to 0.08%, by mass%.

[0077] Al plays a role as a deoxidizer, and when an appropriate amount of Al is left in the alloy, it effectively forms a complex oxide film having rust-preventive properties, which is dense and stabilized. On the other hand, if it is contained in an excessive amount, it increases the thermal expansion coefficient, deteriorates the weld penetration, and increases the viscosity of the molten metal during welding, which deteriorates the bead shape. In consideration of these, the Al content is within the range of 0.0005 to 0.010%, preferably within the range of 0.0010 to 0.008%, and more preferably within the range of 0.002 to 0.007%, by mass%.

[0078] Ti is an essential element for effectively forming a dense surface oxide film with good adhesion. On the other hand, if it is contained in excess, the thermal expansion coefficient increases and weld cracks become more likely to occur. It also increases the viscosity of the molten metal during welding, deteriorating the bead shape. Taking these factors into consideration, the Ti content is within the range of 0.0005 to 0.010%, preferably within the range of 0.0007 to 0.008%, and more preferably within the range of 0.001 to 0.007%, by mass%.

[0079] N is a solid solution strengthening element and contributes to ensuring mechanical strength. On the other hand, excessive content of N forms nitrides, which leads to the occurrence of surface defects. In addition to promoting the occurrence of defects such as hot cracking, N increases the viscosity of the molten metal during welding, which deteriorates the bead shape. In consideration of these, N is in the range of 0.0005 to 0.0050%, preferably 0.0007 to 0.0040%, and more preferably 0.0008 to 0.0035%, by mass%.

[0080] Mg has an effect as a deoxidizer and also a desulfurization action, and can reduce the effect of S on welding hot cracking resistance. On the other hand, if added in excess, it increases the oxide concentration, deteriorating low-temperature toughness and inviting deterioration of the weld bead appearance. In consideration of these, the Mg content is within the range of 0.0003 to 0.0020%, preferably within the range of 0.0005 to 0.0017%, and more preferably within the range of 0.0007 to 0.0015%, by mass%.

[0081] Ca has an effect as a deoxidizer and also a desulfurization action, and can reduce the effect of S on welding hot cracking resistance. On the other hand, if it is contained excessively, it increases the oxide concentration, degrades low-temperature toughness, makes the arc unstable, and greatly deteriorates the surface quality of the bead. In consideration of these, Ca is in the range of 0.0003 to 0.0015%, preferably in the range of 0.0004 to 0.0013%, and more preferably in the range of 0.0005 to 0.0011%, by mass%.

[0082] Regarding the relational expression (1) (0.15≦Si+Cr≦0.40), Cr and Si are elements necessary for exhibiting excellent rust prevention properties, and they stabilize and densify the surface oxide film. On the other hand, excessive content of these elements increases the thermal expansion coefficient. In consideration of this, it is preferable that Si+Cr is regulated to be within the range of 0.15 to 0.40%, preferably within the range of 0.17 to 0.38%, and more preferably within the range of 0.20 to 0.35%, by mass%.

[0083] Regarding the relational expression (2) (0.003≦2×Al+Ti≦0.017), both Al and Ti are elements necessary for forming a dense and adhesive surface oxide film. On the other hand, if contained excessively, it increases the thermal expansion coefficient and deteriorates the weld penetration. Considering these, in order to form a more stable surface oxide film, it is preferable to contain more Al, which has a relatively large effect and a small adverse effect, and to further restrict the total amount of these elements. Therefore, the total content of 2×Al+Ti is specified, and 2×Al+Ti is within the range of 0.003 to 0.017%, preferably within the range of 0.005 to 0.016%, and more preferably within the range of 0.007 to 0.015%, by mass%.

[0084] Next, the optional added elements will be described.

[0085] Mo is an element that effectively increases the mechanical strength of Fe-Ni alloys, and is also a useful element for ensuring the mechanical strength of the base metal and welded metal parts and improving rust resistance. On the other hand, if it is contained in excess, it increases the thermal expansion coefficient and promotes the occurrence of weld cracks. In consideration of these, Mo can be added arbitrarily within the range of 0.01 to 0.10%, preferably within the range of 0.02 to 0.09%, and more preferably within the range of 0.04 to 0.08%, by mass%.

[0086] Co, like Ni, is an element that maintains the thermal expansion coefficient of the Fe-Ni alloy low. It has the effect of forming a highly adhesive surface oxide film and can reduce the effect of S on welding hot cracking resistance. On the other hand, excessive content of Co reduces machinability and makes it difficult to stabilize the quality of the weld groove processing surface. In consideration of these, Co can be added arbitrarily within the range of 0.01 to 0.25%, preferably within the range of 0.02 to 0.23%, and more preferably within the range of 0.04 to 0.20%, by mass%.

[0087] W is an element that effectively increases the mechanical strength of Fe-Ni alloys and improves their rust resistance. On the other hand, excessive content of W increases the thermal expansion coefficient and promotes the occurrence of weld cracks. In consideration of these, W can be added at any amount within the range of 0.01 to 0.05%, preferably 0.01 to 0.04%, and more preferably 0.02 to 0.04%, by mass%.

[0088] B is a useful element for improving hot workability. On the other hand, if it is contained in excess, it makes it easier for cracks to occur in the weld bead during pipe manufacturing and solidification cracks to occur during the base metal manufacturing process. It also makes the surface oxide film porous, which has a negative effect on rust resistance. In consideration of these, B can be added arbitrarily within the strictly limited range of 0.0001 to 0.0010%, preferably 0.0001 to 0.0009%, and more preferably 0.0001 to 0.0007%, by mass%.

[0089] Regarding the relational expression (3) (Al≧Ti), Al and Ti effectively generate a complex oxide having rust-preventive properties and form a dense and well-adherent surface oxide film, which helps to ensure rust-preventive properties in the atmosphere. On the other hand, excessive inclusion of either of them has adverse effects such as deteriorating weld penetration and crack sensitivity. In particular, Ti is more likely to cause these adverse effects. Taking these into consideration, it is preferable to make Al≧Ti. Preferably, Al>Ti.

[0090] Although the representative embodiments of the present invention and modified examples based thereon have been described above, the present invention is not necessarily limited to these, and a person skilled in the art will be able to find various alternative embodiments and modified examples without departing from the spirit of the present invention or the scope of the appended claims.

Claims

1. In mass percent, C: 0.020-0.040%, Si: 0.10-0.30%, Mn: 0.25-0.45%, P: 0.0025-0.0050%, S: 0.0001 to 0.0012%, Ni: 35.5-36.5%, Cr: 0.03-0.22%, Cu: 0.01 to 0.10%, Al: 0.0005-0.010%, Ti: 0.0005 to 0.010%, N: 0.0005 to 0.0050%, Mg: 0.0003 to 0.0020%, Ca: 0.0003-0.0015% and the balance being Fe and unavoidable impurities.

2. The composition of the components is 0.15≦Si+Cr≦0.40 (1) 0.003≦2×Al+Ti≦0.017 ... (2) The Fe-Ni alloy according to claim 1, which satisfies the following relationship:

3. The composition of the components is Al≧Ti (3) and Mo: 0.01~0.10%, Co: 0.01-0.25%, W: 0.01-0.05%, B: 0.0001-0.0010% 2. The Fe-Ni alloy according to claim 1, further comprising one or more of the following:

4. The composition of the components is Al≧Ti (3) and Mo: 0.01~0.10%, Co: 0.01-0.25%, W: 0.01-0.05%, B: 0.0001-0.0010% The Fe-Ni alloy according to claim 2, characterized in that it contains one or more of the following:

5. A welded pipe, comprising an Fe-Ni alloy according to any one of claims 1 to 4.

6. 6. The Fe-Ni alloy tube according to claim 5, characterized in that it has a surface oxide film made of a composite oxide of Fe, Si, Mn and Cr and having a thickness of 2 to 30 μm.

7. A welded pipe, characterized in that it is made of an Fe-Ni alloy according to any one of claims 1 to 4 together with a weld.

8. 8. The Fe-Ni alloy tube according to claim 7, characterized in that it has a surface oxide film made of a composite oxide of Fe, Si, Mn and Cr and having a thickness of 2 to 30 μm.

9. A method for manufacturing an Fe-Ni alloy tube, comprising the steps of: preparing a plate material made of the Fe-Ni alloy according to any one of claims 1 to 4; processing and welding the plate material into a tubular shape to form a welded tube; heating and holding the same at 750 to 900°C; and providing a surface oxide film having a thickness of 2 to 30 μm and made of a composite oxide of Fe, Si, Mn, and Cr.

10. A method for manufacturing an Fe-Ni alloy tube, comprising the steps of: preparing a plate material and a filler material made of the Fe-Ni alloy according to any one of claims 1 to 4; processing the plate material into a tubular shape; welding the plate material with the filler material to form a welded tube; heating and holding the tube at 750 to 900°C; and providing a surface oxide film having a thickness of 2 to 30 μm and made of a composite oxide of Fe, Si, Mn, and Cr.

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