Heat-resistant alloy and method for producing the same

Abstract

Provided is a heat-resistant alloy with excellent mechanical properties after welding. 【Solution means】A heat-resistant alloy having a component composition containing, on a mass basis, 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 0.500%, Al: 1.00 to 1.70%, B: 0.0001 to 0.0060%, Fe: 12.0 to 18.0%, Mg: 0.0010 to 0.0300%, and O: 0.0040% or less, with the balance being Ni and inevitable impurities, and the component composition satisfying the relationship of 100Mg - 2500S ≥ -1.5.

Description

Technical Field

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

Background Art

[0002] Many Ni-based alloys are used for heat-resistant applications and strength is important. Also, since they are used as members and structures of equipment, welding is almost always performed. After welding, pickling is performed to remove the oxide scale. In some cases, stress relief annealing is performed before pickling, and then pickling is performed.

[0003] There are also cases where a heat-resistant alloy is clad with ordinary steel and used. When joined by explosion cladding or the like, stress relief annealing is also required, and in this case, pickling removal is required because an oxide scale is formed on the surface. Among heat-resistant alloys, the oxide scale of high heat-resistant alloys containing Cr in particular is dense, and long-time pickling is required for its removal.

[0004] On the other hand, although heat-resistant alloys are excellent in heat resistance, they are not alloys excellent in acid resistance. Furthermore, the solid solubility of C is not large, and it has an alloy composition that is prone to sensitization. That is, considering the above process, pickling is often performed in a sensitized state, and surface roughening may occur due to grain boundary erosion. In particular, when welding is performed, the grain boundaries of the welded part are often more eroded.

[0005] That is, a structure composed of a heat-resistant alloy will be operated in a state where the grain boundaries are eroded, and it is necessary to ensure the strength characteristics in a state where there are mechanical weaknesses such as cracks that can occur at the surface due to erosion or the like. That is, a heat-resistant alloy needs to have both grain boundary corrosion resistance and grain boundary strength.

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

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] However, the above prior art had the following problems. That is, in the alloy disclosed in Patent Document 1, although welding is considered and cracking evaluation etc. are carried out, there is no description of removing the oxidation scale after welding and the accompanying pickling operation, etc., and the surface morphology of the alloy after pickling is not considered. Similarly, the pickling property etc. of the alloy described in Patent Document 2 is not considered.

[0009] Therefore, an object of the present invention is to provide a heat-resistant alloy in which mechanical properties due to surface shape or metal structure are less likely to deteriorate even when pickling is performed after welding. Further, an object is to provide a heat-resistant alloy in which the base material is excellent in intergranular corrosion resistance.

Means for Solving the Problems

[0010] The inventors have intensively studied to solve the above problems. In particular, they focused on the relationship between the S content and intergranular corrosion resistance. In order to reduce the dissolved S as much as possible at the industrial level, it was found that fixing S with Mg is useful. At the same time, paying attention to the fact that not only the total S but also the dissolved S should be controlled, the amount of sulfide in the material was estimated, the ratio to the total S was taken, and it was found that it is effective to make the ratio within a certain value. Furthermore, after fixing S, by adding an appropriate amount of B, the intergranular corrosion resistance is improved, the strength of the grain boundary is increased, and even if some intergranular erosion occurs due to pickling, cracks are less likely to progress in the tensile test. Furthermore, although fixing S is effective even by adding Mg alone, it was found that by adding at least one of Ca and REM in addition, intergranular erosion of the bead part is suppressed and cracks are less likely to progress under tensile stress. Based on the above, the present invention has been developed.

[0011] The heat-resistant alloy according to the present invention that advantageously solves the above problems contains, on a mass basis, 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 0.500%, Al: 1.00 to 1.70%, B: 0.0001 to 0.0060%, 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%, and the balance consists of Ni and inevitable impurities, and the component composition satisfies the following relational expression (1). [Relational Expression (1)] 100Mg - 2500S ≧ -1.5 Here, the element symbols in the above formula are the contents based on the mass% of each element.

[0012] In addition, the heat-resistant alloy according to the present invention (a) The total S content (%S) based on mass% and 1 mm 2That the amount of sulfide SS obtained by multiplying the number density of sulfide in the vicinity by the square of the sulfide diameter in mm notation satisfies the following relational expression (2), (b) That the above component composition satisfies the following relational expression (3), (c) Containing at least one of Ca and REM, and the component composition satisfies the following relational expression (4), etc. become more preferable problem-solving means. [Relational Expression (2)] (%S) / SS < 100 [Relational Expression (3)] 100Mg + 200B - 2500S ≧ -1.2 Here, the element symbols in the above formula are the contents based on the mass % of each element. [Relational Expression (4)] 100Mg + 500Ca + 400REM - 2500S ≧ -1.2 Here, the element symbols in the formula are the contents based on the mass % of each element.

[0013] The method for producing a heat-resistant alloy according to the present invention that advantageously solves the above problems is any of the methods for producing a heat-resistant alloy described above. The alloy composition is adjusted by performing refining after heating and melting alloy raw materials, casting the obtained molten alloy to obtain a cast slab, hot-rolling the obtained cast slab to obtain a hot-rolled alloy plate, and optionally cold-rolling the hot-rolled alloy plate to obtain a cold-rolled alloy plate. In the refining, a mixed gas of oxygen gas and argon gas is blown into the heated and melted alloy raw materials for decarburization, the nitrogen content is controlled to 0.01 mass % or less, Cr is reduced, and then a CaO source containing aluminum and fluorite is added to the molten alloy to form a CaO-SiO2-Al2O3-MgO-F-based slag, and then a raw material containing REM is optionally added.

Effects of the Invention

[0014] Even if pickling is performed after welding, the heat-resistant alloy of this invention is less likely to deteriorate in mechanical properties due to the surface shape or the metal structure, and thus can be suitably used for structures for heat-resistant applications.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Embodiments for Carrying Out the Invention

[0016] The background leading to the development of the present invention will be described. The inventor mainly focused on reducing the content of dissolved S that promotes intergranular corrosion.

[0017] (Experiment 1) An alloy having the component composition shown in Table 1 and the balance consisting of Ni and unavoidable impurities was prepared. This alloy targeted Cr: 23% by mass and Fe: 15% by mass and contained C, Si, and Mn. In the preparation of the alloy, the relationship between the addition amount of Mg and corrosion was investigated for the cases of addition and non-addition with a target of S: 0.001% by mass. The alloy was prepared by melting the raw metals in a magnesia crucible in the atmosphere using a high-frequency induction furnace. After desulfurization with a CaO - Al2O3 - MgO - F - based slag, it was cast into a mold to obtain a 20 kg ingot. This ingot was hot-rolled to obtain a hot-rolled alloy plate. The obtained hot-rolled alloy plate was annealed, pickled, and cold-rolled to obtain a cold-rolled alloy plate with a thickness of 2 mm. The obtained cold-rolled alloy plate was annealed at 1150 °C. The compositions of C and S in Table 1 were measured using a carbon and sulfur simultaneous analyzer based on the combustion - infrared absorption method in an oxygen stream. The other elements were analyzed using a fluorescent X-ray analyzer. Note that the content of S in Table 1 is the value of "total S" including not only the S dissolved in the alloy but also the S in compounds and non-metallic inclusions. Also, the "-" at the lower limit indicates that it was not intentionally added, allowing for the presence of trace amounts that are unavoidably present.

[0018]

Table 1

[0019] The cold-rolled alloy plate after the above annealing was sampled as a test piece. The test piece was polished and subjected to the ASTM G28A intergranular corrosion test. The test results are shown in Figure 1. As the corrosion degree ratio, the corrosion degree of the Mg-free material was set as the reference and set to 1.0, and the effect of Mg addition on the corrosion degree depending on the presence or absence of S is shown in Figure 1. Figure 1 has the corrosion degree ratio on the vertical axis and the Mg content as the target element on the horizontal axis.

[0020] From the results in Fig. 1, in the material without S addition, no influence of Mg addition on intergranular corrosion resistance was observed within the test range. On the other hand, in the alloy plate containing 0.001 mass% S, it was found that the intergranular corrosion resistance became more excellent with the addition of Mg within the test range. The reason for this is considered that by adding Mg, S becomes sulfide, the solid solution S is reduced, and in particular, the content of S segregating at the grain boundaries is reduced, improving the intergranular corrosion resistance. This can also be understood from the fact that sulfides are observed in the metal structure when observing the metal structure of the alloy plate added with Mg.

[0021] From the above results, it was considered that not only reducing the total S but also reducing the solid solution S in the alloy is effective. Therefore, as an index for quantitatively indicating the content of solid solution S, the index obtained by dividing the total 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 sulfides (pieces / mm 2 ) × (Diameter of sulfides) 2 The number density of sulfides and the diameter of sulfides in the above formula can be calculated as follows.

[0022] <Identification of Sulfides> The cross-section parallel to the rolling direction was finished to a mirror surface, and particle analysis by SEM-EDS was performed. Using mapping and point analysis, in a field of view of 2000 times magnification, an area of 9 mm 2 was evaluated. 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.

[0023] <Size of Sulfides> ​The size of the sulfide was calculated as follows. Thirty random points of sulfide detectable by the above particle analysis were selected, and the range of S was detected from a mapping image at 20,000 times magnification. Separately, a secondary electron image and a composite image were taken, the region where S was detected was identified, and the range was enclosed and calculated as the equivalent circle diameter. The average value of the equivalent circle diameters of the 30 points was defined 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 the sulfide.

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

Table 2

[0027] Cold-rolled alloy plates obtained in the same manner as in Experiment 1 were butt-welded by TIG welding, and test pieces were collected so as to include the welded portion and the heat-affected zone. The test pieces were subjected to the 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 TIG welding, I-groove, welding current of 150 A, welding speed of 250 cm / min, and Ar gas was used as the shielding gas for both the front and back surfaces.

[0028] The intergranular corrosion depth of the bead and base material of the test pieces after the ASTM G28A intergranular corrosion test was measured. A reference material without added S or target elements was tested in the same way, and its corrosion depth was set to 1.0, and the corrosion depth ratio was normalized for evaluation. Note that there were test pieces in which bead defects occurred and defects remained even after cutting and polishing to the size required for the corrosion test pieces, but the corrosion test was carried out in the defective state. Even when the defective part corroded, the corrosion depth was measured.

[0029] <Corrosion of base material> The corrosion depth ratio of the base material is summarized by the B content in Fig. 3. When no S was added, the intergranular corrosion resistance of the base material of the alloy of this embodiment deteriorated with the addition of B. The inventors speculate that this is because B segregated at the grain boundaries and promoted corrosion.

[0030] On the other hand, when 0.001 mass% S was added, the addition of a small amount of B improved the intergranular corrosion resistance, while the addition of excessive B deteriorated the intergranular corrosion resistance. The inventors speculate that this mechanism is as follows. Solute S and B compete for segregation to grain boundaries, and the addition of a small amount of B reduces the amount of solute S segregated to grain boundaries, improving intergranular corrosion resistance. On the other hand, the addition of excessive B increases the amount of B segregated to grain boundaries, canceling out the effect of solute S in reducing segregation, thereby deteriorating intergranular corrosion resistance.

[0031] These results indicate that intergranular corrosion resistance can be further improved by reducing the amount of dissolved S with elements that easily form sulfides, such as Mg, and then adding an appropriate amount of B.

[0032] <Corrosion of bead area> Next, the corrosion depth ratio of the bead part is shown in Fig. 4 in relation to the total content of the additive elements. First, in the case of adding only Mg, improvement in intergranular corrosion resistance was also observed in the bead part. However, the effect saturated with addition of a certain amount or more. It is considered that due to the heat during welding, MgS, which is the generated sulfide, decomposed and the dissolved S segregated again at the grain boundaries. In addition, excessive addition of Mg may generate Mg vapor due to the welding heat because of its low boiling point, resulting in welding defects such as black spots and bulges in the bead. Corrosion progresses starting from such welding defects. Therefore, excessive addition of Mg deteriorates the corrosion resistance of the welded part.

[0033] In the alloy to which B was added in addition to Mg, an additional improvement effect in intergranular corrosion resistance was observed even in the region where the improvement effect in intergranular corrosion resistance saturated with addition of only Mg. It is speculated that this is due to the fact that when S, which redissolved due to the decomposition of the sulfide, segregates at the grain boundaries, it competes with B that also segregates at the grain boundaries, resulting in a decrease in the content of S segregating at the grain boundaries of the bead part.

[0034] Furthermore, by adding Ca or REM that forms sulfides together with Mg, an additional improvement effect in intergranular corrosion resistance was also observed in the region where the improvement effect in intergranular corrosion resistance saturated with addition of only Mg. Here, REM means rare earth elements and refers to 17 elements of Sc, Y, and lanthanoids. By containing one selected from Ca and REM in addition to Mg, the corrosion depth of the bead part became shallower. The reason for this is speculated to be that the sulfides of Ca and REM are thermodynamically more stable than the sulfide of Mg and are less likely to be decomposed by the heat during welding.

[0035] (Experiment 3) Test pieces were taken from the alloy plates and the welded specimens prepared in Experiments 1 and 2. Since an oxide scale is formed on the surface after welding, the test pieces were immersed in nitric hydrofluoric acid to completely descale the surface. The immersion conditions in nitric 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 pieces of JIS 13B were taken from the test materials before and after pickling. The test pieces were taken so that the tensile direction and the welding direction were perpendicular. Also, the test pieces were taken so that the welded part was at the center of the longitudinal direction of the tensile test piece. The tensile strength was calculated by subjecting the test pieces before pickling and the test pieces after pickling to tensile tests respectively. By normalizing with the tensile strength of the test piece before pickling, the degree of deterioration of mechanical properties due to pickling was judged. Hereinafter, the normalized tensile strength = tensile strength of the test piece after pickling / tensile strength of the test piece before pickling is called the "TS ratio". The results were plotted in Figs. 5 and 6. The symbols "◎" and "◇" in the figure 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 due to pickling has occurred.

[0037] The inclusion of excessive S significantly eroded the grain boundaries by pickling, and in the tensile test, fracture occurred at a low tensile strength at the grain boundary erosion part. Depending on the S content, the addition of Mg reduced the solid solution S, suppressed the erosion of the grain boundaries, and suppressed the decrease in the TS ratio. The plots "◎", "〇" and "×" in Fig. 5 represent the results of adding Mg alone. The horizontal axis is the S content, and the first axis (left side) of the vertical axis is the Mg content.

[0038] First, the following relational expression (1A) was found as a condition with little deterioration of mechanical properties due to pickling. [Relational expression (1A)] 100Mg - 2500S ≥ -1.5 Here, the elemental symbols in the formula are the contents based on the mass% of each element.

[0039] Also, the following relational expression (1B) was found as a condition with almost no deterioration of mechanical properties due to pickling. [Relational expression (1B)] 100Mg - 2500S ≥ -1.2 Here, the elemental symbols in the formula are the contents based on the mass% of each element.

[0040] The numbers attached to the plots in Fig. 5 are the indices of the dissolved S calculated by the above relational expression (2A). Even within the range that satisfies the above relational expression (1), there is a range where, when the following relational expression (2) is satisfied between the total S content (%S) and the amount of sulfide SS, there is almost no deterioration of the mechanical properties due to pickling, indicated by the symbol "◎". [Relational expression (2)] (%S) / SS < 100 Here, (%S) is the total S content based on mass%, and SS is the amount of sulfide obtained by multiplying the number density of sulfides per 1 mm 2 by the square of the sulfide diameter in mm notation.

[0041] The plots "◇" and "□" in Fig. 5 represent examples of the combined addition of Mg and B. The horizontal axis is the S content, and the second axis (right side) of the vertical axis represents the total amount of additives. As a condition where there is almost no deterioration of the mechanical properties due to pickling, the following relational expression (3) was found. [Relational expression (3)] 100Mg + 200B - 2500S ≥ -1.2 Here, the elemental symbols in the formula are the contents based on mass% of each element.

[0042] The improvement of intergranular corrosion resistance by the addition of B is as shown in the results of the above Experiment 2. In addition, the addition of B improves the strength of the grain boundaries. Therefore, it was speculated that even if the grain boundaries are corroded, the progress of cracks by tensile testing is suppressed.

[0043] In Fig. 6, the results of adding Ca and REM as other sulfide-forming elements together with Mg are plotted with "◇" and "□". The horizontal axis is the S content, and the second axis (right side) of the vertical axis represents the total amount of additives. As a condition where there is almost no deterioration of the mechanical properties due to pickling, the following relational expression (4) was found. [Relational expression (4)] 100Mg + 500Ca + 400REM - 2500S ≥ -1.2 Here, the elemental symbols in the formula are the contents based on mass% of each element.

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

[0045] (Component composition of heat-resistant alloy) The reasons for limiting the component composition of the heat-resistant alloy according to the present embodiment will be described below. In the following description, unless otherwise specified, "%" representing the component composition means "% by mass".

[0046] C: 0.010 to 0.100% C is an essential element for the heat-resistant alloy to ensure strength. Therefore, an addition of 0.010% or more is required. On the other hand, a large amount of addition causes sensitization during welding and deteriorates the intergranular corrosion resistance, so it is necessary to add 0.100% or less. Preferably, the C content is 0.015% or more, and preferably, the C content is 0.055% or less. More preferably, the C content is 0.020% or more, and more preferably, the C content is 0.044% or less.

[0047] Si: 0.10 to 0.50% Si is an important element having a deoxidizing action and has an effect of contributing to oxidation resistance. It is necessary to contain 0.10% or more to exert the above effect. On the other hand, excessive addition deteriorates the weldability, so the upper limit is 0.50%. Preferably, the Si content is 0.13% or more, and preferably, the Si content is 0.45% or less. More preferably, the Si content is 0.15% or more, and more preferably, the Si content is 0.40% or less.

[0048] Mn: 0.10 to 1.0% Mn is a deoxidizer, and in order to obtain its effect, it is necessary to contain 0.10% or more. On the other hand, excessive content forms MnS. MnS not only serves as a starting point for corrosion, but also dissolves more easily at high temperatures compared to MgS and CaS. Therefore, S is redissolved into the matrix at the welded part, deteriorating the intergranular corrosion resistance. Therefore, the Mn content is limited to 1.0% at the upper limit. Preferably, the Mn content is 0.20% or more, and preferably, the Mn content is 0.80% or less. More preferably, the Mn content is 0.30% or more, and more preferably, the Mn content is 0.60% or less.

[0049] P: 0.030% or less P is an element that inevitably mixes into the alloy as an impurity. It tends to segregate at the grain boundaries and deteriorates the hot workability, so it is necessary to reduce it as much as possible. Also, when a large amount of P is contained, it segregates during welding solidification, and solidification cracking is likely to occur. Also, the areas where P is concentrated are inferior in corrosion resistance. Therefore, the upper limit of the P content is set to 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 an increase in processing costs, so it is preferable that the P content has a lower limit of 0.001%.

[0050] S: 0.0020% or less S is an element that deteriorates the hot workability, deteriorates the intergranular corrosion resistance, and increases the solidification cracking susceptibility during welding. Therefore, it is better to be as low as possible within the cost limit, and it is necessary to reduce it 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 including not only the solid solution S in the alloy but also S as sulfide. The formation process of sulfide will be described later.

[0051] Cr: 21.0 - 25.0% Cr is an element that improves intergranular corrosion resistance. It is also an element that suppresses high-temperature oxidation because it forms a dense oxide scale in a high-temperature environment. Therefore, a content of 21.0% or more is required. Excessive inclusion of Cr will cause an overly large surface oxide scale to form, which instead results in poor adhesion and deteriorated oxidation resistance. In addition, since the stability of the austenite phase decreases, the upper limit of the Cr content is 25.0%. Preferably, the Cr content is 21.5% or more, and 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 Mo has the effect of increasing high-temperature strength. On the other hand, in a high-temperature environment, the oxide scale may peel off due to preferential oxidation. When including it, the content 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 improving corrosion resistance in reducing acids such as sulfuric acid. On the other hand, it may deteriorate the corrosion resistance against solutions containing oxidizing acids such as nitric acid. Therefore, when including it, 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, the Cu content is 0.40% or less. More preferably, the Cu content is 0.30% or less.

[0054] Ti: 0.050 - 0.500% Ti is an element that promotes the formation of a dense film and improves oxidation resistance. Its effect can be obtained by adding Ti at 0.050% or more. On the other hand, excessive addition causes surface defects due to the formation of a large amount of carbonitrides. Furthermore, it causes defects during welding, particularly the generation of black spots in the bead portion. Therefore, the upper limit of the Ti content was set at 0.500%. Preferably, the Ti content is 0.080% or more, and preferably, the Ti content is 0.400% 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 having a deoxidizing action. It has the function of controlling the oxygen concentration, the S content, and the contents of Mg and Ca required to fix S as sulfides. This action will be described later. Furthermore, Al is an element that promotes the formation of a dense oxide film in a high-temperature environment and improves oxidation resistance. On the other hand, excessive addition causes defects during welding. When the Al content in the alloy exceeds 1.70%, the composition of the inclusions becomes Al2O3 (alumina) and forms clusters, generating black spots during welding. For the above reasons, the Al content is in the range of 1.00 - 1.70%. Preferably, the Al content is 1.05% or more, and 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 - 0.0060% B is an element that tends to segregate at grain boundaries. When S is contained, it has the effect of reducing the S concentration at grain boundaries due to competitive segregation with S. Secondarily, it has the effect of suppressing grain boundary erosion during pickling. This effect is effectively expressed also in the alloy according to the present embodiment, for example, in a welded portion, that is, a portion that has been remelted and solidified again. Also, it is an element that increases the strength of grain boundaries. Therefore, when a tensile load is applied under the condition that grain boundaries are eroded, it has the effect of suppressing the progress of cracks. On the other hand, since adding a large amount increases the susceptibility to solidification cracking, the B content is limited to 0.006% at the upper limit. From 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 to 18.0% From the viewpoints of the corrosion resistance and heat resistance of the heat-resistant alloy, Fe may not be contained. However, since it is an element that can be sufficiently utilized even if a part of Ni is replaced with Fe, it may be used because it reduces expensive Ni. However, if Fe is contained in an amount exceeding 18.0%, the corrosion resistance of the alloy may deteriorate, so the Fe content is set to 18.0% or less. When it is contained, the Fe content is preferably 12.0% or more. Preferably, the Fe content is 13.0% or more, and 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 combines with S to immobilize S as MgS, thereby detoxifying S. To obtain this effect, it is necessary to add Mg in an amount of 0.0010% or more. On the other hand, even if Mg is added in excess of 0.0300%, the S detoxification effect saturates, and excessive addition results in a decrease in hot workability and ductility. Furthermore, it causes bulging and black spots during welding. Therefore, the Mg content is in the range of 0.0010 to 0.0300%. Preferably, the Mg content is 0.0015% or more, and preferably, the Mg content is 0.0200% or less. More preferably, the Mg content is 0.0020% or more, and more preferably, the Mg content is 0.0140% or less.

[0059] O: 0.0040% or less O is an impurity element that inevitably mixes into the alloy. It increases the number of oxide-based non-metallic inclusions and causes surface defects. Also, when it exceeds 0.0040%, the desulfurization ability weakens, the S content becomes higher than 0.0015%, and the hot workability deteriorates. Therefore, the O content is set to 0.0040% or less. However, excessive deoxidation leads to an increase in cost, so the lower limit of the O content is preferably about 0.0001%. Preferably, the O content is 0.0030% or less. More preferably, the O content is 0.0020% or less.

[0060] <Fixation of S due to formation of sulfides> The fixation of S by Mg and Ca described below occurs as follows. During the refining of the molten alloy according to this embodiment, a CaO - SiO2 - Al2O3 - MgO - F - based slag is used. Al2O3 formed by Al added as a deoxidizer can be effectively absorbed by the slag to control the oxygen concentration in the alloy. This is represented by the following reaction formula (6). In the following reaction formula, [M] represents that element M is dissolved in the molten alloy, and (R) represents that a substance of chemical formula R is contained in the slag. [Reaction formula (6)] 2[Al] + 3[O] = (Al2O3) Also, as deoxidation progresses, sulfides are formed and the S concentration in the molten alloy decreases according to the following reaction formula. [Reaction formula (7)] 2[Al] + 3(CaO) = 3[Ca] + (Al2O3) [Reaction formula (8)] 2[Al] + 3(MgO) = 3[Mg] + (Al2O3) [Reaction formula (9)] 2[Al] + 3[S] + 3(CaO) = 3(CaS) + (Al2O3) [Reaction formula (10)] 2[Al] + 3[S] + 3(MgO) = 3(MgS) + (Al2O3)

[0061] At this time, the sulfide generated not only remains in the slag but also partially remains in the molten alloy. However, as long as it exists as a sulfide, it is not harmful in this embodiment. By controlling the above-mentioned Al content, the total S content in the alloy, that is, the total content of dissolved S and S in the sulfide, can be controlled to 0.0020% or less. At the same time, the Mg content can be similarly controlled. Similarly, REM also forms sulfides, but its content range 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 above essential elements. 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 combines with S to immobilize S as CaS, thereby rendering S harmless. To obtain this effect, an addition of 0.00001% or more is required. On the other hand, even if added in excess of 0.00500%, the S harmlessification effect saturates, and excessive addition causes a decrease in hot workability and ductility. Furthermore, it causes bulging and black spots during welding. Therefore, the Ca content is preferably in the range of 0.00001 to 0.00500%. More preferably, the Ca content is 0.00003% or more, and 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 17 elements from Sc, Y to lanthanoids in the periodic table. Like Mg and Ca, REM immobilizes S as a sulfide to render S harmless. To obtain this effect, an addition of REM of 0.00001% or more is necessary. Excessive addition leads to increased costs and also deteriorates hot workability. Therefore, the REM content is preferably in the range of 0.00001 to 0.00500%. More preferably, the REM content is 0.00003% or more, and more preferably, the REM content is 0.00200% or less. Even more preferably, the REM content is 0.00004% or more, and even more preferably, the REM content is 0.00090% or less. The method of adding REM may be addition of each element alone such as La or Ce, or addition in the form of an alloy composed of a plurality of elements containing REM. From the perspective of cost, addition in the form of mischmetal or mischmetal-Ni alloy may also be used.

[0065] The heat-resistant alloy according to this embodiment consists of Ni and unavoidable impurities as the remainder of the above essential elements and the above optional elements. The unavoidable impurities are not particularly limited, and examples include non-metallic inclusions excluding sulfides. Further, if necessary, as alloying elements, a total of about 0.4% such as Co, N, W, V, Nb, Pb, etc. may be included. The heat-resistant alloy according to this embodiment needs to satisfy the following relational expression (1) in addition to satisfying the above component composition. By satisfying this relationship, the immobilization of S by sulfides can be promoted and the dissolved S can be reduced. [Relational Expression (1)] 100Mg - 2500S ≥ -1.5 Here, the element symbols in the formula are the contents based on the mass% of each element.

[0066] Preferably, it is to satisfy the following relational expression (5). [Relational Expression (5)] 100Mg - 2500S ≥ -1.2 More preferably, it is to satisfy the relationship of 3.0 ≥ 100Mg - 2500S ≥ -1.2. Even more preferably, it is to satisfy the relationship of 1.0 ≥ 100Mg - 2500S ≥ -1.0.

[0067] The heat-resistant alloy according to this embodiment preferably satisfies the following relational expression (2). As shown in the above Experiment 1, by estimating the amount of solid solution S from the total S (%S) and the amount of sulfide SS and controlling the amount, excellent properties can be obtained. [Relational Expression (2)] (%S) / SS < 100 Here, (%S) is the total S content based on mass%, and SS is the amount of sulfide obtained by multiplying the number density of sulfides per 1 mm 2 by the square of the sulfide diameter in mm notation. More preferably, it is to satisfy the relationship of (%S) / SS < 90. Even more preferably, it is to satisfy the relationship of (%S) / SS < 80.

[0068] The heat-resistant alloy according to this embodiment preferably satisfies the following relational expression (3). As shown in the above Experiment 3, the content of B in the situation where S is fixed with Mg or the like has the effect of suppressing intergranular erosion, particularly in the bead portion, during pickling, and further has the effect of improving the intergranular strength. In addition, it has the effect of suppressing the progress of cracks starting from the intergranular erosion portion when a load such as tension is applied. [Relational Expression (3)] 100Mg + 200B - 2500S ≥ -1.2 Here, the elemental symbols in the formula are the contents based on mass% of each element.

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

[0070] In addition, the heat-resistant alloy according to this embodiment may be additionally doped with at least one of Ca and REM in addition to Mg. In that case, within the range of the above component composition, it is preferable to satisfy the following relational expression (4) because erosion at the grain boundaries of the bead portion during welding can be further suppressed. [Relational expression (4)] 100Mg + 500Ca + 400REM - 2500S ≥ -1.2 Here, the elemental symbols in the formula represent the contents based on the mass% of each element.

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

[0072] (Method for manufacturing a heat-resistant alloy) Next, a method for manufacturing a heat-resistant alloy according to the present embodiment will be described. There are no particular limitations on the melting of the alloy having the above-described component composition. It is preferably carried out by the following manufacturing method. The heat-resistant alloy according to the present embodiment is preferably an austenitic Ni-Cr-Fe alloy from the above-described component composition. The heat-resistant alloy according to the present embodiment melts raw materials such as iron scraps, stainless steel scraps, ferronickel, and ferrochrome in an electric furnace to obtain a molten alloy. The obtained molten alloy is decarburized and refined by blowing a mixed gas of oxygen and a rare gas in an AOD (Argon Oxygen Decarburization) furnace or a VOD (Vacuum Oxygen Decarbutization) furnace. At the same time, quicklime, an Fe-Si alloy, Al, etc. are added to reduce the Cr oxide in the slag, and then fluorite is added to form a CaO-SiO2-Al2O3-MgO-F-based slag for deoxidation and desulfurization. Optionally, when REM is added, it is added in the form of a Ni-based alloy containing at least one of these elements, mischmetal, or a mischmetal-Ni alloy. The reason for using the CaO-SiO2-Al2O3-MgO-F-based slag is that, as described above, deoxidation and desulfurization can be effectively carried out. At the same time, in order to efficiently promote desulfurization, the slag preferably satisfies CaO / Al2O3 ≧ 0.5 and CaO / SiO2 ≧ 5 in terms of mass ratio. Also, the refractories of the AOD furnace and the VOD furnace are preferably magnesia chrome or dolomite. After refining by the above AOD furnace or the like, component adjustment and temperature adjustment are performed in the LF process, and then a slab is manufactured by a continuous casting machine. It is not limited to a continuous casting machine, and an alloy slab may be used by an ingot-making and block-rolling method. In the continuous casting process, it is preferable that the mode is not bent in the apparatus until solidification is completed after casting, particularly a mode called a vertical type. The reason is to make the distribution of precipitates more symmetrical in the plate thickness direction.

[0073] In the present embodiment, the slab or the alloy slab is hot-rolled, and cold rolling is performed as necessary to obtain a product. By such a method, it is preferable to obtain various products such as thin plates, thick plates, shaped materials, bars, and wire rods. After solution heat treatment of the hot-rolled alloy material manufactured by hot rolling, cold rolling is performed to obtain a cold-rolled alloy material, and it is preferable that the cold-rolled alloy material is made into a product through a final annealing and pickling process.

Example

[0074] Hereinafter, the present invention will be described in more detail by way of examples. However, the present invention is not limited to these examples as long as it does not exceed the gist thereof. First, raw materials such as stainless steel scraps, heat-resistant alloy scraps, and pure nickel adjusted to a predetermined ratio were melted in an electric furnace to obtain a molten alloy. The obtained molten alloy was decarburized and refined in an AOD furnace, and at the same time, quicklime, fluorite, Al, Si, etc. were added to perform desulfurization and deoxidation treatments. Thereafter, when REM was to be contained, a predetermined amount was added as a Ni-based alloy containing REM. Thereafter, in the LF process, after adjusting to various component compositions shown in Tables 3 and 4, continuous casting was performed to obtain a slab. The compositions of C and S shown in the table were measured by combustion-infrared absorption method in an oxygen stream using a carbon-sulfur simultaneous analyzer. The composition of N was measured by an inert gas-impulse heating melting method using an oxygen-nitrogen simultaneous analyzer. The components other than the above were analyzed values using fluorescent X-ray analysis. In addition, "-" in the table indicates that no intentional addition was made.

[0075] Next, the above slab was hot-rolled to obtain a hot-rolled alloy sheet, and cold rolling and heat treatment were repeated on the hot-rolled alloy sheet to obtain a cold-rolled coil with a thickness of 2 mm. The final annealing temperature was 1150 °C for 1 minute with respect to the cold-rolled coil. A plate was cut out from the cold-rolled coil and butt-welded. The welding conditions were the same as in Experiment 2. TIG welding was performed with a I-shaped 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 carried out with nitric hydrofluoric acid. The immersion conditions of nitric hydrofluoric acid were the same as in Experiment 4 and are as follows. The test piece was immersed in an aqueous solution of 1.5 M nitric acid and 1.5 M hydrofluoric acid at 50 °C for 36 hours.

[0077] From each of the plates before and after immersion in nitric hydrofluoric acid, JIS 13B tensile test pieces including the welded part were taken. The sampling method was the same as in Experiment 4, and the test pieces were processed so that the weld line was located perpendicular to the longitudinal direction at the center of the tensile test piece.

[0078] From the tensile strength ratio TS ratio before and after pickling, the mechanical properties of the test pieces after pickling were evaluated. 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, indicating that there is almost no deterioration in mechanical properties due to pickling. The symbol "〇" indicates that the TS ratio is in the range of 0.70 to 0.90, indicating that there is little deterioration in mechanical properties due to pickling. The symbol "×" indicates that the TS ratio is less than 0.70, indicating that deterioration in mechanical properties due to pickling has occurred. The TS ratio of 0.7 or more was taken as the invention example, and less than 0.7 was taken as the comparative example. The results are shown in Tables 3 and 4. The values of the above relational expressions (1) to (4) are also shown in Tables 3 and 4.

[0079]

Table 3

[0080]

Table 4

[0081] The alloys from Sample No. 1 to 28 in the table are invention examples that satisfy the conditions of this embodiment, and there is little deterioration in mechanical properties after pickling. Among them, the materials that satisfy the component composition satisfying any of the relational expressions (2) to (4) are particularly less deteriorated. On the other hand, Sample Nos. 29 to 31 do not satisfy the relational expression (1). Therefore, the mechanical properties after pickling deteriorated due to deep erosion of the grain boundaries by pickling. Sample Nos. 32 and 33 have too high an S content and do not satisfy the relational expression (1). These samples had significant erosion of the grain boundaries and deterioration of the mechanical properties after pickling. Regarding Sample No. 33, cracks also occurred in the beads. Sample No. 34 had too high a C content. Sensitization to corrosion was significant, erosion of the HAZ part due to pickling was significant, and the mechanical properties after pickling deteriorated. Sample No. 35 had too high an O content. That is, deoxidation was weak and desulfurization was weak, resulting in too high an S content, and significant erosion of the grain boundaries occurred during pickling. Sample No. 36 had too high an Al content. Bulging occurred in the welded part, which was eroded during pickling and became the crack initiation point in the tensile test. Sample No. 37 had too high an Mg content. Bulging occurred in the welded part, which was eroded during pickling and became the crack initiation point in the tensile test. Sample No. 38 had too low an Mg content. The S fixing ability decreased, and although the relational expression (1) was satisfied, the grain boundaries were eroded and the tensile properties were inferior. Sample No. 39 had too high a B content. The welded part developed solidification cracking. Sample No. 40 had too low a B content. It was inferior in hot workability, and a product with good yield could not be manufactured.

Industrial Applicability

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

Claims

1. On a mass basis, C: 0.010-0.100%, Si: 0.10 to 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 to 0.402%, Al: 1.00-1.70%, B: 0.0001 to 0.0060%, Fe: 12.0-18.0%, Mg: 0.0010 to 0.0300%, and O: 0.0040% or less Contains Optionally, Contains at least one selected from Ca: 0.00001 to 0.00500% and REM: 0.00001% to 0.00500%, The balance is Ni and unavoidable impurities. The heat-resistant alloy has a composition satisfying the following relational formula (1): [Relationship (1)] 100Mg-2500S≧-1.5 Here, the element symbols in the formula indicate the content of each element in mass %.

2. Total S content (%S) based on mass% and 1mm ２ 2. The heat-resistant alloy according to claim 1, wherein the amount of sulfides SS calculated by multiplying the number density of sulfides per unit area by the square of the sulfide diameter in mm satisfies the following relational expression (2): [Relationship (2)] (%S) / SS<100

3. The heat-resistant alloy according to claim 1, wherein the composition satisfies the following relational expression (3): [Relationship formula (3)] 100Mg+200B-2500S≧-1.2 Here, the element symbols in the formula indicate the content of each element in mass %.

4. Contains at least one of Ca and REM, The heat-resistant alloy according to claim 1, wherein the composition satisfies the following relational expression (4): [Relationship (4)] 100Mg+500Ca+400REM-2500S≧-1.2 Here, the element symbols in the formula indicate the content of each element in mass %.

5. A method for producing a heat-resistant alloy according to any one of claims 1 to 4, comprising the steps of: The alloy composition is adjusted by heating and melting the alloy raw materials, followed by refining, casting the resulting molten alloy to obtain a slab, hot rolling the resulting slab to obtain a hot-rolled alloy plate, and optionally cold rolling the hot-rolled alloy plate to obtain a cold-rolled alloy plate. In the refining, a mixed gas of oxygen gas and argon gas is blown into the heated and melted alloy raw material to decarburize it, and the nitrogen content is controlled to 0.01 mass% or less, followed by Cr reduction. Then, a CaO source containing aluminum and fluorite is added to the molten alloy to produce CaO-SiO ２ -Al ２ O ３ - A method for producing a heat-resistant alloy by forming a MgO-F based slag and then optionally adding raw materials containing REM.

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