Heat-resistant alloy and production method for same

Abstract

Provided is a heat-resistant alloy that does not readily experience a degradation of mechanical characteristics after welding. A heat-resistant alloy according to the present invention has a component composition that is, by mass, 0.010%–0.100% C, 0.10%–0.50% Si, 0.10%–1.0% Mn, no more than 0.030% P, no more than 0.0020% S, 21.0%–25.0% Cr, no more than 0.50% Mo, no more than 0.50% Cu, 0.050%–0.430% Ti, 1.00%–1.70% Al, 0.0001%–0.0060% B, 12.0%–18.0% Fe, 0.0010%–0.0300% Mg, and no more than 0.0040% O, the remainder being Ni and unavoidable impurities, and 100Mg-2500S being greater than or equal to -1.5.

Description

Heat-resistant alloy and method for manufacturing the same

[0001] This invention relates to a heat-resistant alloy having excellent mechanical properties and a method for manufacturing the same, and more particularly to a heat-resistant alloy with excellent weldability. In this specification, "x to y" representing a numerical range means "more than or equal to x and less than or equal to y," and includes boundary values. The unit of mass "t" represents 1000 kg.

[0002] Most nickel-based alloys are used for heat-resistant applications, where strength is crucial. Furthermore, because they are used as components and structures in equipment, welding is almost always performed. After welding, pickling is carried out to remove oxide scale. In some cases, annealing to remove weld distortion is performed before pickling.

[0003] Heat-resistant alloys are sometimes used clad with ordinary steel. When joined by explosive bonding, stress-relieving annealing is also necessary, and in this case, oxide scale forms on the surface, requiring pickling to remove it. Among heat-resistant alloys, the oxide scale of high heat-resistant alloys containing chromium is particularly dense, and its removal requires prolonged pickling.

[0004] On the other hand, heat-resistant alloys have excellent heat resistance, but they do not have excellent acid resistance. Furthermore, the solid solubility of carbon is not high, and the alloy composition is prone to sensitization. In other words, considering the above process, pickling is often performed in a sensitized state, which can lead to surface roughness caused by grain boundary erosion. In particular, when welding is performed, the grain boundaries of the weld are often more severely eroded.

[0005] In other words, structures made of heat-resistant alloys will be operated in a state where the grain boundaries are eroded, and it is necessary to ensure strength properties even when there are mechanical weaknesses on the surface that can become crack initiation points due to erosion, etc. In short, heat-resistant alloys need to possess both corrosion resistance at the grain boundaries and strength at the grain boundaries.

[0006] For example, Patent Document 1 discloses an alloy that exhibits excellent crack prevention in the HAZ and creep strength by limiting the content of elements that embrittle grain boundaries. Patent Document 2 discloses a method for precisely controlling the concentrations of Mg, Ca, S, and O in an alloy to obtain excellent surface quality and hot workability.

[0007] Japanese Patent Publication No. 2010-150593 Japanese Patent Publication No. 2009-114544

[0008] However, the above-mentioned prior art had the following problems. Specifically, although welding is considered and cracking is evaluated for the alloy disclosed in Patent Document 1, there is no description of the removal of oxide scale after welding or the associated pickling process. In addition, the surface morphology of the alloy after pickling is not considered. Similarly, the pickling properties of the alloy described in Patent Document 2 are not considered.

[0009] Therefore, the present invention aims to provide a heat-resistant alloy in which the mechanical properties, whether due to the surface shape or the metal structure, do not deteriorate easily even if pickling is performed after welding. Furthermore, the present invention aims to provide a heat-resistant alloy in which the base material exhibits excellent resistance to intergranular corrosion.

[0010] The inventor diligently conducted research to solve the above problems. In particular, he focused on the relationship between sulfur content and intergranular corrosion resistance. He found that fixing sulfur with magnesium is useful in order to reduce dissolved sulfur as much as possible at an industrial level. In addition, he focused on the fact that it is not only total sulfur that needs to be controlled, but also dissolved sulfur, and found that it is effective to estimate the amount of sulfide in the material, take the ratio with total sulfur, and keep that ratio within a certain value. Furthermore, he found that it is effective to add an appropriate amount of boron after fixing sulfur. In this case, he found that resistance to intergranular corrosion improves, the strength of the grain boundaries increases, and even if some erosion of the grain boundaries occurs due to pickling, crack propagation is less likely in tensile tests. Furthermore, he found that although the fixing of sulfur is effective with the addition of magnesium alone, it is also effective to add at least one of Ca and REM. In this case, he found that intergranular erosion of the bead is suppressed and crack propagation is less likely under tensile stress. Based on the above, the present invention was developed.

[0011] The heat-resistant alloy according to the present invention, which advantageously solves the above problems, has the following composition by mass: C: 0.010 to 0.100%, Si: 0.10 to 0.50%, Mn: 0.10 to 1.0%, P: 0.030% or less, S: 0.0020% or less, Cr: 21.0 to 25.0%, Mo: 0.50% or less, Cu: 0.50% or less, Ti: 0.050 to 0.430%, Al: 1.00 to 1.70%, B: 0.0001 to 0.0060%. The material has a component composition that contains Fe: 12.0 to 18.0%, Mg: 0.0010 to 0.0300%, and O: 0.0040% or less, and optionally contains at least one selected from Ca: 0.00001 to 0.00500% and REM: 0.00001% to 0.00500%, with the remainder being Ni and unavoidable impurities, and the component composition satisfies the following relational formula (1). [Relational formula (1)] 100Mg - 2500S ≥ -1.5 Here, the elemental symbols in the above formula represent the mass percentage content of each element.

[0012] Furthermore, the heat-resistant alloy according to the present invention has (a) total sulfur content (%S) on a mass percentage basis and 1 mm ２ A more preferable way to solve the problem is to ensure that the amount of sulfide SS, obtained by multiplying the number density of sulfides per unit area by the square of the sulfide diameter in mm, satisfies the following relation (2), (b) the above component composition satisfies the following relation (3), and (c) contains at least one of Ca and REM, and the component composition satisfies the following relation (4). [Relationship (2)] (%S) / SS < 100 [Relationship (3)] 100Mg + 200B - 2500S ≥ -1.2 Here, the element symbols in the above formula represent the mass percentage content of each element. [Relationship (4)] 100Mg + 500Ca + 400REM - 2500S ≥ -1.2 Here, the element symbols in the formula represent the mass percentage content of each element.

[0013] The present invention provides a method for manufacturing a heat-resistant alloy that advantageously solves the above problems, and is a method for manufacturing any of the above heat-resistant alloys, wherein the alloy composition is adjusted by heating and melting the alloy raw materials and then refining them, the obtained molten alloy is cast to obtain a cast slab, the obtained cast slab is hot-rolled to make a hot-rolled alloy sheet, and the hot-rolled alloy sheet is optionally cold-rolled to make a cold-rolled alloy sheet, wherein in the refining process, a mixed gas of oxygen and argon is blown into the heated and melted alloy raw materials to decarburize them, the nitrogen content is controlled to 0.01% by mass or less, Cr is reduced, and then a CaO source containing aluminum and fluorite is added to the molten alloy to form CaO-SiO ２ - Al ２ O ３ The process is characterized by forming a -MgO-F slag, and then optionally adding a raw material containing REM.

[0014] The heat-resistant alloy of this invention exhibits minimal deterioration in mechanical properties, whether due to surface morphology or metal structure, even after pickling following welding. Therefore, it can be suitably used in structures requiring heat resistance.

[0015] This graph shows the effects of S and Mg on intergranular corrosion of a heat-resistant alloy according to an embodiment of the present invention. This graph shows the effect of the index value of solid-solution S, which is the total S content divided by the amount of sulfide, on intergranular corrosion of the above heat-resistant alloy. This graph shows the effects of S and B on intergranular corrosion of the base material after welding of the above heat-resistant alloy. This graph shows the effects of Mg and composite additive elements on intergranular corrosion of the bead portion after welding of the above heat-resistant alloy. This graph shows the effects of the composition of the contained elements and the index value of solid-solution S on the tensile strength ratio before and after pickling of a test piece after welding of the above heat-resistant alloy. This graph shows the effect of the composition of the contained elements on the tensile strength ratio before and after pickling of a test piece after welding of the above heat-resistant alloy.

[0016] The background to the development of this invention will be explained. The inventor focused primarily on reducing the content of solid-solution sulfur, which promotes intergranular corrosion.

[0017] (Experiment 1) An alloy was prepared having the component composition shown in Table 1, with the remainder being Ni and unavoidable impurities. This alloy targeted Cr: 23 mass% and Fe: 15 mass% and contained C, Si, and Mn. In preparing the alloy, the relationship between the amount of Mg added and corrosion was investigated for both with and without S: 0.001 mass% added. The alloy was prepared by melting the raw metals in a magnesia crucible in air using a high-frequency induction furnace. CaO-Al ２ O ３ After desulfurization with MgO-F slag, the mixture was cast into a mold to form a 20 kg ingot. This ingot was hot-rolled to obtain a hot-rolled alloy sheet. The obtained hot-rolled alloy sheet was annealed, pickled, and cold-rolled to obtain a cold-rolled alloy sheet with a thickness of 2 mm. The obtained cold-rolled alloy sheet was annealed at 1150°C. The C and S compositions in Table 1 were measured using a carbon-sulfur simultaneous analyzer based on combustion-infrared absorption spectroscopy in an oxygen stream. Other elements were analyzed using an X-ray fluorescence analyzer. Note that the S content in Table 1 is the "total S" value, which includes not only S dissolved in the alloy but also S in compounds and nonmetallic inclusions. The lower limit "-" indicates that it was not intentionally added, and allows for trace amounts that are inevitably present.

[0018]

[0019] Cold-rolled alloy sheets after the above annealing process were taken as test specimens. The specimens were polished and subjected to the ASTM G28A intergranular corrosion test. The test results are shown in Figure 1. As a corrosion degree ratio, the corrosion degree of the material without Mg additive was set as the baseline at 1.0, and Figure 1 shows the effect of Mg addition on the corrosion degree depending on the presence or absence of sulfur. In Figure 1, the corrosion degree ratio is plotted on the vertical axis, and the Mg content as the target element is plotted on the horizontal axis.

[0020] As shown in Figure 1, in the sulfur-free material, no effect of Mg addition on intergranular corrosion resistance was observed within the test range. On the other hand, in the alloy sheet containing 0.001% by mass of sulfur, it was found that intergranular corrosion resistance improved as the amount of Mg added increased within the test range. This is thought to be because the addition of Mg converts sulfur into sulfides, reducing the amount of dissolved sulfur, and in particular, reducing the amount of sulfur segregated at the grain boundaries, thereby improving intergranular corrosion resistance. This can also be understood from the observation of sulfides within the metal structure of the alloy sheet with added Mg.

[0021] From the above results, it was considered that not only reducing all S but also reducing the dissolved S in the alloy was effective. Therefore, as an index for quantitatively indicating the content of dissolved S, the index obtained by dividing all S (%S) by the amount of sulfide SS was used for sorting. Specifically, the following relational expression (2A) was used. [Relational expression (2A)] (%S) / SS = (%S) / [number density of sulfide (number / mm ２ )×(diameter of sulfide) ２ ] The number density of sulfide and the diameter of sulfide in the above formula can be calculated as follows.

[0022] <Identification of sulfide> A cross-section parallel to the rolling direction was finished to a mirror surface, and particle analysis of SEM-EDS was performed. Using mapping and point analysis, an area of 9 mm was evaluated in a field of view of 2000 times magnification. At that time, three fields of view were randomly observed in the range of 1 mm×3 mm. When counting the number of sulfides, oxides and sulfides can be detected because their brightness is clearly different from that of the metal matrix. Only those with an S concentration of 0.5% or more were counted as sulfides by point analysis of the extracted particles. ２ <Size of sulfide> The size of the sulfide was calculated as follows. 30 points of sulfides that could be detected by the above particle analysis were randomly selected, and the range of S was detected from the mapping image of 20000 times magnification. Separately, secondary electron images and composite images were taken, the region where S was detected was identified, and the range was surrounded and calculated as the equivalent circle diameter. The average value of the equivalent circle diameters of 30 points was taken as the "sulfide diameter". By this process, even when evaluating a composite inclusion composed of sulfide and oxide, the equivalent circle diameter can be calculated only from the region of only sulfide.

[0023]

[0024] Figure 2 shows the relationship between (%S) / SS calculated by the relational expression (2A) as an index of solid solution S and the corrosion rate ratio. The corrosion rate ratio in Figure 2 is expressed with the corrosion rate of a test piece having the same composition as other elements within the range of Table 1 except that S and Mg are not added being set to 1.0. The numerical values in the figure are the values of total S expressed in mass%. From the results of Figure 2, it can be seen that the intergranular corrosion resistance depends not on the content of total S but on the content of solid solution S. That is, the intergranular corrosion resistance of the alloy according to the present embodiment can be improved by fixing S to sulfides or the like to reduce the solid solution S.

[0025] (Experiment 2) An alloy having the component composition shown in Table 2 was melted in the same manner as in Experiment 1 to produce an alloy plate. In this experiment, the addition effect of B and the addition effects of Ca and REM having the same S-fixing property as Mg were confirmed.

[0026]

[0027] The cold-rolled alloy plates obtained in the same manner as in Experiment 1 were butt-joined and TIG welded, and test pieces were collected so as to include the welded portion and the heat-affected zone. The test pieces were subjected to an ASTM G28A intergranular corrosion test. In this experiment, the relationship between the influence of each element on the intergranular corrosion resistance and welding was investigated. The welding conditions were as follows: no filler metal was used, TIG welding, I-groove, welding current was 150 A, welding speed was 250 cm / min, and Ar gas was used as the shielding gas for both the front and back surfaces.

[0028] The intergranular corrosion depths of the bead portion and the base metal portion of the test pieces after the ASTM G28A intergranular corrosion test were measured. A reference material without adding S and the target elements was similarly tested, and its corrosion depth was normalized with a corrosion depth ratio of 1.0 for evaluation. Note that there were test pieces in which bead defects occurred and defects remained even after cutting and polishing to the size for the corrosion test pieces, but the test pieces were subjected to the corrosion test in a defective state. When the defective portion corroded, it was measured as the corrosion depth.

[0029] <Corrosion of the base metal portion> The corrosion depth ratio of the base metal portion was arranged according to the B content and is shown in Figure 3. When S was not added, the intergranular corrosion resistance of the base metal of the alloy of the present embodiment deteriorated with the addition of B. The inventor speculates that this is because B segregated at the grain boundaries and promoted corrosion.

[0030] On the other hand, when 0.001% by mass of sulfur (S) was added, the intergranular corrosion resistance improved with the addition of a small amount of boron (B), while the intergranular corrosion resistance deteriorated with the addition of an excessive amount of B. The inventor speculates that this mechanism is as follows: Solid-solution sulfur (S) and boron (B) compete for segregation at the grain boundaries, and with the addition of a small amount of B, the amount of segregation of solid-solution sulfur at the grain boundaries decreases, improving intergranular corrosion resistance. Conversely, the addition of an excessive amount of B increases the amount of segregation of B at the grain boundaries, canceling out the segregation-reducing effect of solid-solution sulfur and degrading intergranular corrosion resistance.

[0031] These results suggest that by reducing the amount of dissolved sulfur (S) using elements that readily form sulfides, such as Mg, and then adding an appropriate amount of B, resistance to intergranular corrosion can be further improved.

[0032] <Corrosion of the Weld Bead> Next, Figure 4 shows the relationship between the corrosion depth ratio of the weld bead and the total content of the added elements. First, in the case of adding Mg alone, an improvement in resistance to intergranular corrosion was observed even in the weld bead. However, the effect saturated with additions above a certain amount. This is thought to be because the MgS sulfide generated by the heat during welding decomposed, and the solid-solution sulfur segregated again at the grain boundary. Furthermore, excessive addition of Mg may cause Mg vapor to be generated by the welding heat due to the low boiling point of Mg, potentially resulting in welding defects such as black spots and blisters on the weld bead. Corrosion progresses starting from such welding defects. Therefore, excessive addition of Mg deteriorates the corrosion resistance of the weld.

[0033] In alloys with added B in addition to Mg, a further improvement in intergranular corrosion resistance was observed even in the region where the improvement in intergranular corrosion resistance with Mg alone had reached saturation. This is presumed to be because, when sulfur (S), which is redissolved due to the decomposition of sulfides, segregates at the grain boundaries, it competes with B, which also segregates at the grain boundaries, resulting in a decrease in the amount of S segregated at the grain boundaries in the bead region.

[0034] Furthermore, by adding Ca and REM, which generate sulfides, along with Mg, an improvement in intergranular corrosion resistance was observed in the region where the improvement effect of Mg alone had saturated. Here, REM refers to rare earth elements, specifically Sc, Y, and 17 lanthanides. By including one element selected from Ca and REM in addition to Mg, the corrosion depth of the weld bead became shallower. This is presumed to be because the sulfides of Ca and REM are thermodynamically more stable than the sulfides of Mg and are less likely to decompose due to the heat during welding.

[0035] (Experiment 3) Test specimens were taken from the alloy plates prepared in Experiments 1 and 2 and from the welded test materials. Since oxide scale forms on the surface after welding, the test specimens were immersed in nitrate hydrofluoric acid to completely descale the surface. The immersion conditions in nitrate hydrofluoric acid were immersion in a mixed aqueous solution of 1.5 M nitric acid and 1.5 M hydrofluoric acid at 50°C for 36 hours.

[0036] Tensile test specimens conforming to JIS No. 13B were taken from the test material before and after pickling. The specimens were taken so that the tensile direction and the welding direction were perpendicular. In addition, the weld was taken so that it was in the longitudinal center of the tensile test specimen. The tensile strength was calculated by subjecting the pre-pickling and post-pickling specimens to tensile tests. The degree of deterioration of mechanical properties due to pickling was determined by normalizing with the tensile strength of the pre-pickling specimen. Hereafter, the normalized tensile strength = tensile strength of the post-pickling specimen / tensile strength of the pre-pickling specimen will be called the "TS ratio". The results are plotted in Figures 5 and 6. The symbols "◎" and "◇" in the figures indicate that the TS ratio is greater than 0.90, and that there is almost no deterioration of mechanical properties due to pickling. The symbols "〇" and "□" indicate that the TS ratio is in the range of 0.70 to 0.90, and that there is little deterioration of mechanical properties due to pickling. The symbol "×" indicates that the TS ratio is less than 0.70, and that deterioration of mechanical properties has occurred due to pickling.

[0037] Excessive sulfur content led to significant erosion of grain boundaries during pickling, resulting in fracture at low tensile strength in tensile tests at the eroded grain boundaries. Depending on the sulfur content, the addition of magnesium reduced dissolved sulfur, suppressed grain boundary erosion, and prevented a decrease in the TS ratio. The plots "◎", "〇", and "×" in Figure 5 represent the results of adding magnesium alone. The horizontal axis represents sulfur content, and the first vertical axis (left side) represents magnesium content.

[0038] First, we found the following relation (1A) as a condition under which the deterioration of mechanical properties due to pickling is minimal. [Relationship (1A)] 100Mg - 2500S ≥ -1.5 Here, the element symbols in the formula represent the mass percentage content of each element.

[0039] Furthermore, the following relation (1B) was found as a condition under which there is almost no deterioration of mechanical properties due to pickling. [Relationship (1B)] 100Mg - 2500S ≥ -1.2 Here, the element symbols in the formula represent the mass percentage content of each element.

[0040] The numbers attached to the plots in Figure 5 are indices of solid solution sulfur calculated using the above relational equation (2A). Even within the range that satisfies the above relational equation (1), there was a range where the following relational equation (2) was satisfied between the total sulfur content (%S) and the sulfide amount SS, resulting in the symbol "◎" indicating almost no deterioration of mechanical properties due to pickling. [Relational equation (2)] (%S) / SS < 100 Here, (%S) is the total sulfur content on a mass percentage basis, and SS is 1 mm ２ This is the amount of sulfide obtained by multiplying the number density of sulfides per unit area by the square of the sulfide diameter in mm.

[0041] The plots "◇" and "□" in Figure 5 represent examples of combined Mg and B addition. The horizontal axis represents the S content, and the second vertical axis (right side) represents the total amount of additives. The following relation (3) was found as a condition in which there is almost no deterioration of mechanical properties due to pickling. [Relationship (3)] 100Mg + 200B - 2500S ≥ -1.2 Here, the element symbols in the formula represent the mass percentage content of each element.

[0042] As shown in the results of Experiment 2 above, the addition of B improves resistance to intergranular corrosion. In addition, the addition of B improves the strength of the grain boundaries. Therefore, it is presumed that even if the grain boundaries are corroded, the propagation of cracks in tensile tests will be suppressed.

[0043] Figure 6 plots the results of adding Ca and REM along with Mg as other sulfide-forming elements using "◇" and "□". The horizontal axis represents the sulfur content, and the second vertical axis (right side) represents the total amount of additives. The following relation (4) was found as a condition in which there is almost no deterioration of mechanical properties due to pickling. [Relationship (4)] 100Mg + 500Ca + 400REM - 2500S ≥ -1.2 Here, the element symbols in the formula represent the mass percentage content of each element.

[0044] As obtained in Experiment 2 above, by including at least one of Ca and REM in addition to Mg, the corrosion depth at the grain boundaries due to pickling was reduced and the deterioration of the TS ratio was suppressed.

[0045] (Component composition of the heat-resistant alloy) The reasons for limiting the component composition of the heat-resistant alloy according to this embodiment are explained below. In the following explanation, unless otherwise specified, "%" representing the component composition means "mass%".

[0046] C: 0.010-0.100% Carbon (C) is an essential element in heat-resistant alloys to ensure strength. Therefore, an addition of 0.010% or more is necessary. On the other hand, excessive addition causes sensitization during welding, which deteriorates intergranular corrosion resistance, so the addition should be 0.100% or less. Preferably, the C content is 0.015% or more, and more preferably, 0.055% or less. More preferably, the C content is 0.020% or more, and more preferably, 0.044% or less.

[0047] Si: 0.10-0.50% Si is an important element with deoxidizing properties and contributes to oxidation resistance. To achieve the above effect, it is necessary to include at least 0.10% Si. On the other hand, excessive addition degrades weldability, so the upper limit is 0.50%. Preferably, the Si content is 0.13% or more, and more preferably, 0.45% or less. More preferably, the Si content is 0.15% or more, and more preferably, 0.40% or less.

[0048] Mn: 0.10-1.0% Mn is a deoxidizing agent, and to obtain its effect, it needs to be contained at a concentration of 0.10% or more. On the other hand, excessive content forms MnS. MnS not only acts as a starting point for corrosion, but it also melts more easily at high temperatures than MgS or CaS. Therefore, it causes S to redissolve into the matrix phase at the weld, degrading the resistance to intergranular corrosion. Accordingly, the Mn content should be limited to 1.0%. Preferably, the Mn content is 0.20% or more, and more preferably, 0.80% or less. More preferably, the Mn content is 0.30% or more, and more preferably, 0.60% or less.

[0049] P: 0.030% or less. P is an element that inevitably mixes into alloys as an impurity. It tends to segregate at grain boundaries and reduces hot workability, so it needs to be reduced as much as possible. Also, if a large amount of P is present, it segregates during weld solidification, making solidification cracking more likely. Furthermore, areas with high P concentration have poor corrosion resistance. For this reason, the upper limit of the P content is set at 0.030%. Preferably, the P content is 0.025% or less. More preferably, the P content is 0.020% or less. However, excessive reduction leads to increased processing costs, so it is preferable to set the lower limit of the P content at 0.001%.

[0050] S: 0.0020% or less. S is an element that degrades hot workability, degrades resistance to intergranular corrosion, and increases susceptibility to solidification cracking during welding. Therefore, it is desirable to keep the amount as low as cost allows, and it needs to be reduced to 0.0020% or less. The lower limit may be 0. Preferably, the S content is 0.0015% or less. More preferably, the S content is 0.0010% or less. Here, the S content is a value that includes not only solid-solution S in the alloy but also S as sulfides. The process of sulfide formation will be described later.

[0051] Cr: 21.0-25.0% Cr is an element that improves resistance to intergranular corrosion. It also forms a dense oxide scale in high-temperature environments, thus suppressing high-temperature oxidation. For this reason, a content of 21.0% or more is necessary. Excessive Cr content leads to excessive formation of surface oxide scale, which conversely results in poor adhesion and deterioration of oxidation resistance. In addition, it reduces the stability of the austenite phase, so the upper limit of Cr content is 25.0%. Preferably, the Cr content is 21.5% or more, and more preferably, the Cr content is 24.5% or less. More preferably, the Cr content is 22.0% or more, and more preferably, the Cr content is 24.0% or less.

[0052] Mo: 0.50% or less. While Mo has the effect of increasing high-temperature strength, it may cause peeling of oxide scale due to preferential oxidation in high-temperature environments. If included, the amount should be 0.50% or less. The lower limit of the Mo content may be 0. Preferably, the Mo content is 0.01% or more, and the Mo content is 0.40% or less. More preferably, the Mo content is 0.30% or less.

[0053] Cu: 0.50% or less. Cu is an element that contributes to improved corrosion resistance in reducing acids such as sulfuric acid. On the other hand, it can degrade corrosion resistance in solutions containing oxidizing acids, such as nitrate-hydrofluoric acid. Therefore, if Cu is included, the Cu content should be 0.50% or less. The lower limit of the Cu content may be 0. Preferably, the Cu content is 0.01% or more, and preferably 0.40% or less. More preferably, the Cu content is 0.30% or less.

[0054] Ti: 0.050-0.430% Ti is an element that promotes the formation of a dense film and improves oxidation resistance. This effect can be obtained by adding 0.050% or more Ti. On the other hand, excessive addition can cause surface defects due to the formation of a large amount of carbonitrides. Furthermore, it can cause defects during welding, especially the occurrence of black spots in the bead. For this reason, the upper limit of the Ti content was set to 0.430%. Preferably, the Ti content is 0.080% or more, and more preferably, the Ti content is 0.410% or less. More preferably, the Ti content is 0.100% or more, and more preferably, the Ti content is 0.300% or less.

[0055] Al: 1.00–1.70% Al is an important element with deoxidizing properties. It controls the oxygen concentration, the sulfur content, and the Mg and Ca content necessary for the sulfur to adhere as sulfides. This function will be described later. Furthermore, Al is an element that promotes the formation of a dense oxide film in high-temperature environments, improving oxidation resistance. On the other hand, excessive addition can lead to defects during welding. If the Al content in the alloy exceeds 1.70%, the composition of inclusions will be Al ２ O ３ It forms clusters as alumina, causing black spots during welding. For the reasons above, the Al content is in the range of 1.00 to 1.70%. Preferably, the Al content is 1.05% or more, and more preferably, the Al content is 1.65% or less. More preferably, the Al content is 1.10% or more, and more preferably, the Al content is 1.60% or less.

[0056] B: 0.0001 to 0.0060% B is an element that easily segregates at grain boundaries. When S is present, competitive segregation with S has the effect of lowering the S concentration at the grain boundaries. Secondarily, it has the effect of suppressing grain boundary erosion during pickling. This effect is also effectively exhibited in the alloy according to this embodiment, for example, in welded parts, that is, parts that have been melted and solidified again. It is also an element that increases the strength of grain boundaries. Therefore, when a tensile load is applied under conditions where the grain boundaries are eroded, it has the effect of suppressing crack propagation. On the other hand, adding a large amount increases the susceptibility to solidification cracking, so the B content is limited to 0.006%. Based on the above, the B content is in the range of 0.0001 to 0.0060%. Preferably, the B content is 0.0002% or more, and preferably, the B content is 0.0050% or less. More preferably, the B content is 0.003% or more, and more preferably, the B content is 0.0040% or less.

[0057] Fe: 12.0-18.0% Fe does not need to be included from the viewpoint of corrosion resistance and heat resistance of the heat-resistant alloy. However, since Fe is an element that can be used even if a part of Ni is replaced with Fe, it may be used to reduce the amount of expensive Ni. However, if the Fe content exceeds 18.0%, the corrosion resistance of the alloy may deteriorate, so the Fe content should be 18.0% or less. If Fe is included, it is preferable that the Fe content be 12.0% or more. Preferably, the Fe content is 13.0% or more, and more preferably, the Fe content is 17.0% or less. More preferably, the Fe content is 13.5% or more, and more preferably, the Fe content is 16.0% or less.

[0058] Mg: 0.0010 to 0.0300% Mg detoxifies sulfur by binding with it to form MgS, which fixes the sulfur in place. To obtain this effect, it is necessary to add 0.0010% or more of Mg. On the other hand, if Mg is added in amounts exceeding 0.0300%, the sulfur detoxification effect saturates, and excessive addition reduces hot workability and ductility. Furthermore, it can cause blistering and black spots during welding. Therefore, the Mg content should be in the range of 0.0010 to 0.0300%. Preferably, the Mg content is 0.0015% or more, and more preferably, 0.0200% or less. More preferably, the Mg content is 0.0020% or more, and more preferably, 0.0140% or less.

[0059] O: 0.0040% or less. O is an impurity element that is inevitably mixed into alloys. It increases the number of oxide-based nonmetallic inclusions and causes surface defects. Also, if it exceeds 0.0040%, the desulfurization ability weakens, the sulfur content becomes high, exceeding 0.0015%, and the hot workability deteriorates. For this reason, the O content should be 0.0040% or less. However, excessive deoxidation leads to increased costs, so it is preferable to set the lower limit of the O content to around 0.0001%. Preferably, the O content is 0.0030% or less. More preferably, the O content is 0.0020% or less.

[0060] <S deposition due to sulfide formation> The deposition of S by Mg and Ca, described later, occurs as follows: During the refining of the molten alloy according to this embodiment, CaO-SiO ２ - Al ２ O ３ -MgO-F type slag is used. Al formed by Al added as a deoxidizing agent. ２ O ３ By effectively absorbing the slag, the oxygen concentration in the alloy can be controlled. This is represented by the following reaction equation (6). In the following reaction equation, [M] represents the element M dissolved in the molten alloy, and (R) represents the substance with chemical formula R contained in the slag. [Reaction equation (6)] 2[Al] + 3[O] = (Al ２ O ３Furthermore, as deoxidation progresses, sulfides are formed according to the following reaction equation, and the sulfur concentration in the molten alloy also decreases. [Reaction Equation (7)] 2[Al] + 3(CaO) = 3[Ca] + (Al ２ O ３ ) [Reaction equation (8)] 2[Al] + 3(MgO) = 3[Mg] + (Al ２ O ３ ) [Reaction equation (9)] 2[Al] + 3[S] + 3(CaO) = 3(CaS) + (Al ２ O ３ ) [Reaction equation (10)] 2[Al] + 3[S] + 3(MgO) = 3(MgS) + (Al ２ O ３ )

[0061] The sulfides generated at this time remain not only in the slag but also partially in the molten alloy. However, as long as they exist as sulfides, they are not harmful in this embodiment. By controlling the Al content as described above, the total S content in the alloy, that is, the sum of the S content in the solid solution and the S content in the sulfides, can be controlled to 0.0020% or less. At the same time, the Mg content can also be controlled in the same way. Similarly, REM also forms sulfides, but the range of their content can be controlled by deoxidation and desulfurization with Al.

[0062] The heat-resistant alloy of this embodiment preferably contains the following optional elements in addition to the essential elements mentioned above. The optional element is at least one selected from Ca: 0.00001% to 0.00500% and REM: 0.00001% to 0.00500%.

[0063] Ca: 0.00001 to 0.00500% Ca detoxifies sulfur by bonding with it to form CaS, which fixes the sulfur in place. To obtain this effect, an addition of 0.00001% or more is necessary. On the other hand, adding more than 0.00500% saturates the sulfur detoxification effect, and excessive addition reduces hot workability and ductility. Furthermore, it can cause blistering and black spots during welding. Therefore, it is preferable that the Ca content be in the range of 0.00001 to 0.00500%. More preferably, the Ca content is 0.00003% or more, and even more preferably, the Ca content is 0.00200% or less. Even more preferably, the Ca content is 0.00004% or more, and even more preferably, the Ca content is 0.00130% or less.

[0064] REM: 0.00001% to 0.00500% REM (rare earth elements) is a general term for the 17 elements in the periodic table from Sc to Y and lanthanides. Like Mg and Ca, REM neutralizes sulfur by fixing it as a sulfide. To obtain this effect, it is necessary to add 0.00001% or more of REM. Excessive addition leads to increased costs and deterioration of hot workability. Therefore, it is preferable that the REM content be in the range of 0.00001% to 0.00500%. More preferably, the REM content is 0.00003% or more, and more preferably, 0.00200% or less. Even more preferably, the REM content is 0.00004% or more, and even more preferably, 0.00090% or less. REM may be added as individual elements such as La or Ce, or as an alloy consisting of multiple elements including REM. From a cost perspective, it may be added in the form of mischmetal or mischmetal-Ni alloy.

[0065] The heat-resistant alloy according to this embodiment consists of Ni and unavoidable impurities, with the remainder being the essential elements and optional elements mentioned above. The unavoidable impurities are not particularly limited, but examples include nonmetallic inclusions other than sulfides. In addition, if necessary, alloying elements such as Co, N, W, V, Nb, and Pb may be included in total at a concentration of about 0.4%. In addition to satisfying the above component composition, the heat-resistant alloy according to this embodiment must also satisfy the following relational equation (1). Satisfying this relationship promotes the adhesion of sulfur by sulfides and reduces solid-solution sulfur. [Relational equation (1)] 100Mg - 2500S ≥ -1.5 Here, the element symbols in the equation represent the mass percentage content of each element.

[0066] Preferably, the following relation (5) is satisfied. [Relationship (5)] 100Mg - 2500S ≥ -1.2 More preferably, the relationship 3.0 ≥ 100Mg - 2500S ≥ -1.2 is satisfied. Even more preferably, the relationship 1.0 ≥ 100Mg - 2500S ≥ -1.0 is satisfied.

[0067] The heat-resistant alloy according to this embodiment preferably satisfies the following relation (2). As shown in Experiment 1 above, excellent properties can be obtained by estimating the amount of dissolved sulfur from the total sulfur (%S) and sulfide amount SS, and controlling its amount. [Relationship (2)] (%S) / SS < 100 Here, (%S) is the total sulfur content on a mass percentage basis, and SS is 1 mm ２ This is the amount of sulfide obtained by multiplying the number density of sulfides per unit area by the square of the sulfide diameter in mm. More preferably, the relationship (%S) / SS < 90 is satisfied. Even more preferably, the relationship (%S) / SS < 80 is satisfied.

[0068] The heat-resistant alloy according to this embodiment preferably satisfies the following relation (3). As shown in Experiment 3 above, the inclusion of B when S is fixed with Mg, etc. has the effect of suppressing grain boundary erosion, especially in the bead area, during pickling, and further improves grain boundary strength. In addition, it has the effect of suppressing the propagation of cracks originating from the grain boundary erosion area when loads such as tension are applied. [Relationship (3)] 100Mg + 200B - 2500S ≥ -1.2 Here, the element symbols in the formula represent the mass percentage content of each element.

[0069] More preferably, the relationship 4.2 ≥ 100Mg + 200B - 2500S ≥ -1.0 is satisfied. Even more preferably, the relationship 2.1 ≥ 100Mg + 200B - 2500S ≥ -0.9 is satisfied.

[0070] Furthermore, the heat-resistant alloy according to this embodiment may also contain at least one of Ca and REM in addition to Mg. In that case, it is preferable that the following relation (4) is satisfied within the range of the above component composition, as this further suppresses erosion of the grain boundaries of the bead during welding. [Relationship (4)] 100Mg + 500Ca + 400REM - 2500S ≥ -1.2 Here, the element symbols in the formula represent the mass percentage content of each element.

[0071] More preferably, the relationship 7.5 ≥ 100 Mg + 500 Ca + 400 REM - 2500 S ≥ -1.1 is satisfied. Even more preferably, the relationship 1.0 ≥ 100 Mg + 500 Ca + 400 REM - 2500 S ≥ -1.0 is satisfied.

[0072] (Method for Manufacturing Heat-Resistant Alloys) Next, the method for manufacturing the heat-resistant alloy according to this embodiment will be described. The method for melting the alloy having the above component composition is not particularly limited. The following manufacturing method is preferred. The heat-resistant alloy according to this embodiment is preferably an austenitic Ni-Cr-Fe alloy based on the above component composition. The heat-resistant alloy according to this embodiment is obtained by melting raw materials such as iron scrap, stainless steel scrap, ferronickel, and ferrochrome in an electric furnace to obtain a molten alloy. The obtained molten alloy is decarburized by blowing a mixed gas of oxygen and noble gas in an AOD (Argon Oxygen Decarburization) furnace or a VOD (Vacuum Oxygen Decarburization) furnace. At the same time, quicklime, Fe-Si alloy, Al, etc. are added to reduce the Cr oxide in the slag, and then fluorite is added to form CaO-SiO ２ - Al ２ O ３ -MgO-F slag is formed to deoxidize and desulfurize. If REM is optionally added, it is added in the form of a Ni-based alloy, mischmetal, or mischmetal-Ni alloy containing at least one of these elements. CaO-SiO ２ - Al ２ O ３ The reason for using -MgO-F slag is, as mentioned above, that it allows for effective deoxidation and desulfurization. Simultaneously, to ensure efficient desulfurization, the slag is composed of CaO / Al slag in a mass ratio. ２ O ３ ≧0.5, CaO / SiO ２ It is preferable that the condition ≥ 5 be satisfied. Furthermore, it is preferable that the refractory material for the AOD furnace and VOD furnace be magnesium chloride or dolomite. After refining in the above AOD furnace, the composition and temperature are adjusted in the LF process, and then cast slabs are produced in a continuous casting machine. It is not limited to a continuous casting machine, and alloy pieces may also be produced by the ingot-molding-rolling method. In the continuous casting process, it is particularly preferable to use a vertical type, in which bending is not performed inside the machine until solidification is complete after casting. The reason for this is to make the distribution of precipitates more symmetrical in the thickness direction of the plate.

[0073] In this embodiment, cast slabs or alloy pieces are hot-rolled, and cold-rolled as necessary to produce the final product. This method is preferred for producing various products such as thin plates, thick plates, profiles, bars, and wires. Alternatively, the hot-rolled alloy material produced by hot-rolling is subjected to solution heat treatment, then cold-rolled to produce a cold-rolled alloy material, which is then subjected to final annealing and pickling processes to produce the final product.

[0074] The present invention will be further described in detail below with reference to examples. However, the present invention is not limited to these examples unless it exceeds the spirit of the invention. First, raw materials such as stainless steel scrap, heat-resistant alloy scrap, and pure nickel, adjusted to predetermined ratios, were melted in an electric furnace to obtain a molten alloy. The obtained molten alloy was decarburized and refined in an AOD furnace, and desulfurized and deoxidized by adding quicklime, fluorite, Al, Si, etc. After that, if REM was to be included, a predetermined amount was added as a Ni-based alloy containing REM. Then, in the LF process, the alloy was adjusted to various component compositions shown in Tables 3 and 4, and then continuously cast to form slabs. The C and S compositions shown in the tables were measured using a carbon-sulfur simultaneous analyzer by combustion in an oxygen stream-infrared absorption method. The N composition was measured using an oxygen-nitrogen simultaneous analyzer by inert gas-impulse heating and melting method. The values ​​of components other than those mentioned above were analyzed using fluorescent X-ray analysis. Note that "-" in the table indicates that no intentional additions were made.

[0075] Next, the slab was hot-rolled to form a hot-rolled alloy sheet, and the hot-rolled alloy sheet was repeatedly cold-rolled and heat-treated to form a cold-rolled coil with a thickness of 2 mm. The final annealing temperature for the cold-rolled coil was 1150°C for 1 minute. Sheets were cut from the cold-rolled coil and butt-welded. The welding conditions were the same as in Experiment 2. TIG welding was performed with an I-groove, the welding current was 150 A, the welding speed was 250 cm / min, and Ar gas was used as the shielding gas for both the front and back surfaces.

[0076] After welding, descaling was performed with nitrate-hydrofluoric acid. The immersion conditions for the nitrate-hydrofluoric acid were the same as in Experiment 4, as follows: The test specimens were immersed in aqueous solutions of 1.5 M nitric acid and 1.5 M hydrofluoric acid at 50°C for 36 hours.

[0077] Tensile test specimens conforming to JIS No. 13B, including the welded area, were taken from both the plate before and after immersion in nitrate-hydrofluoric acid. The sampling method was the same as in Experiment 4, and the specimens were processed so that the welded line was positioned perpendicular to the longitudinal direction in the center of the tensile test specimen.

[0078] The mechanical properties of the test specimens after pickling were evaluated from the tensile strength ratio (TS ratio) before and after pickling. The determination of the TS ratio was the same as in Experiment 4. In Tables 3 and 4, the symbol "◎" indicates that the TS ratio is greater than 0.90, and that there is almost no deterioration of mechanical properties due to pickling. The symbol "〇" indicates that the TS ratio is in the range of 0.70 to 0.90, and that there is little deterioration of mechanical properties due to pickling. The symbol "×" indicates that the TS ratio is less than 0.70, and that deterioration of mechanical properties has occurred due to pickling. A TS ratio of 0.7 or higher was designated as the inventive example, and a TS ratio less than 0.7 was designated as the comparative example. The results are shown in Tables 3 and 4. The values ​​of the above relational formulas (1) to (4) are also shown in Tables 3 and 4.

[0079]

[0080]

[0081] Alloys No. 1 to 28 in the table are examples of the invention that satisfy the conditions of this embodiment, and exhibit little deterioration of mechanical properties after pickling. In particular, materials with a component composition that satisfies any of the relationships (2) to (4) show especially little deterioration. On the other hand, samples No. 29 to 31 do not satisfy relationship (1). Therefore, the grain boundaries were deeply eroded by pickling, resulting in deterioration of mechanical properties after pickling. Samples No. 32 and 33 have too high a sulfur content and do not satisfy relationship (1). The grain boundaries of these samples were severely eroded, resulting in deterioration of mechanical properties after pickling. In the case of sample No. 33, cracks also occurred in the bead. Sample No. 34 had too high a carbon content. It was severely sensitized to corrosion, and the HAZ area was severely eroded by pickling, resulting in deterioration of mechanical properties after pickling. Sample No. 35 had too high an oxygen content. In short, the deoxidation and desulfurization were weak, resulting in an excessively high sulfur content, which led to significant erosion of the grain boundaries during pickling. Sample No. 36 had an excessively high aluminum content. Blistering occurred in the weld area, which was eroded during pickling and became the crack initiation point in the tensile test. Sample No. 37 had an excessively high magnesium content. Blistering occurred in the weld area, which was eroded during pickling and became the crack initiation point in the tensile test. Sample No. 38 had an excessively low magnesium content. The adhesion ability of sulfur was low, and although the relationship (1) was satisfied, the grain boundaries were eroded, resulting in poor tensile properties. Sample No. 39 had an excessively high boron content. Solidification cracking occurred in the weld area. Sample No. 40 had an excessively low boron content. It had poor hot workability, and it was not possible to manufacture products with a good yield.

[0082] Thus, the heat-resistant alloy of the present invention exhibits excellent mechanical properties after welding and pickling. It is suitably used as a material for components and structures that require welding. Therefore, it is industrially useful.

Claims

1. By mass, it contains C: 0.010-0.100%, Si: 0.10-0.50%, Mn: 0.10-1.0%, P: 0.030% or less, S: 0.0020% or less, Cr: 21.0-25.0%, Mo: 0.50% or less, Cu: 0.50% or less, Ti: 0.050-0.430%, Al: 1.00-1.70%, B: 0.0001-0.0060%, Fe: 12.0-18.0%, Mg: 0.0010-0.0300%, and O: 0.0040% or less, and optionally, A heat-resistant alloy having a component composition that contains at least one selected from Ca: 0.00001 to 0.00500% and REM: 0.00001% to 0.00500%, with the remainder being Ni and unavoidable impurities, and the component composition satisfies the following relation (1). [Relationship (1)] 100Mg - 2500S ≥ -1.5 Here, the element symbols in the formula represent the mass percentage content of each element.

2. Total sulfur content (%S) based on mass percentage, and 1 mm ２ The heat-resistant alloy according to claim 1, wherein the amount of sulfide SS, obtained by multiplying the number density of sulfides per unit area by the square of the sulfide diameter in mm, satisfies the following relation (2). [Relationship (2)] (%S) / SS < 100 3. The heat-resistant alloy according to claim 1, wherein the component composition satisfies the following relational formula (3). [Relational formula (3)] 100Mg + 200B - 2500S ≥ -1.2 Here, the element symbols in the formula represent the mass percentage content of each element.

4. The heat-resistant alloy according to claim 1, which contains at least one of Ca and REM, and whose component composition satisfies the following relational formula (4). [Relational formula (4)] 100Mg + 500Ca + 400REM - 2500S ≥ -1.2 Here, the element symbols in the formula represent the mass percentage content of each element.

5. A method for producing a heat-resistant alloy according to any one of claims 1 to 4, wherein the alloy composition is adjusted by heating and melting the alloy raw materials and then refining them, the obtained molten alloy is cast to obtain a cast slab, the obtained cast slab is hot-rolled to make a hot-rolled alloy sheet, and the hot-rolled alloy sheet is optionally cold-rolled to make a cold-rolled alloy sheet, in which, in the refining, a mixed gas of oxygen gas and argon gas is blown into the heated and melted alloy raw materials to decarburize them, the nitrogen content is controlled to 0.01% by mass or less, then Cr reduction is performed, and thereafter, a CaO source containing aluminum and fluorite is added to the molten alloy to form CaO-SiO ２ - Al ２ O ３ A method for producing a heat-resistant alloy, comprising forming a -MgO-F slag and then optionally adding a raw material containing REM.

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