Fe-ni-cr alloy having excellent corrosion resistance, weldability and oxidation resistance and method for producing the same

Abstract

The object is to provide a Fe-Ni-Cr alloy for a sheathed heater jacket pipe which is excellent in oxidation resistance, corrosion resistance, and weldability, and which can be used in a humid high-temperature environment. The alloy for solving the object is a Fe-Ni-Cr alloy consisting of, in mass%, C: 0.001 to 0.050%, Si: 0.18 to 1.00%, Mn: 0.20 to 0.80%, P: 0.030% or less, S: 0.0001 to 0.0020%, Ni: 12 to 21%, Cr: 18 to 24%, Mo: 0.20 to 1.50%, Cu: 0.30% or less, Al: 0.10 to 0.70%, Ti: 0.10 to 0.70%, N: 0.002 to 0.015%, B: 0.0001 to 0.0010%, O: 0.0002 to 0.0030%, Ca: 0.002% or less, REM: 0.0010 to 0.0150% of one or two or more of La, Ce, and Y, and the balance of Fe and inevitable impurities, and satisfies formulae 1 and 2. 0.575 x Ni + 1.25 x Cr + 3.43 x Mo - 39 x P - 5.3 x Al - 64 1 x REM - 1018 x O ≥ 20.0 … (Formula 1). 1.5 x Mn + 41.3 x Si + 1469 x S - 1.67 x Al - 1.34 x Ti - 150 x O - 620 x REM ≥ 5.0 … (Formula 2).

Description

TECHNICAL FIELD

[0001] The present invention relates to an Fe-Ni-Cr alloy excellent in corrosion resistance, weldability, and oxidation resistance, which is most suitable for a sheathed heater, and a method for producing the same. BACKGROUND

[0002] A sheathed heater using a nickel-chromium alloy wire is used as a heat source for an electric cooker, an electric water heater, and the like. The sheathed heater is generally a sheath pipe made of a metal material in a ring shape by welding the end portions of a strip-shaped plate, and a nickel-chromium alloy wire and magnesium oxide powder are filled therein to form a sealed structure which generates heat by energization. Since the sheathed heater can be heated only by energization without using fire, it is highly safe, and its demand has been increasing in recent years.

[0003] However, the product life of the sheathed heater is greatly affected by the structure thereof and the oxidation resistance, corrosion resistance, and presence or absence of surface defects of the welded portion of the metal sheath pipe. In the case where a hole is formed in the metal sheath pipe or cracking occurs, the nickel-chromium alloy wire inside thereof can be broken, and the function as a heating element can be lost.

[0004] Among the characteristics that affect the product life of the sheathed heater, the oxidation resistance is important. In the sheathed heater, the surface temperature of the sheath pipe is increased by energization of the nickel-chromium alloy wire, but in the case where a metal having low oxidation resistance is used, oxidation of the metal proceeds at a very fast rate with respect to the increase in the surface temperature, and in addition, the oxide film thereof is easily peeled off. The oxidation and peeling of the oxide film can cause the metal portion of the sheath pipe to be thinned, resulting in a hole or cracking.

[0005] In addition, the corrosion resistance is also one of the important characteristics that affect the product life of the sheathed heater. Even in the case of a metal material excellent in oxidation resistance, corrosion proceeds from an early stage of use in a humid environment, until the sheath pipe is holed or broken, if the corrosion resistance is low.

[0006] Generally, a heater used in a humid environment is not directly used for various household appliances requiring high-temperature use with oxidation resistance. However, in order to meet the market demand for cost reduction, attempts to reduce the design and manufacturing costs by the same concept are being made, and various materials corresponding thereto are required.

[0007] Further, the surface state of the welded portion also has an influence on the product life of the sheath heater. In the sheath heater product, a thin tube is manufactured by continuously welding a band-shaped plate material after being wound into a ring shape. The unevenness of the welded portion generated at the time of welding is generally polished to be removed, but it takes time to completely remove it, significantly reducing the productivity. In addition, in the case where such defects remain without being removed, it sometimes causes bending cracking at the time of product processing, and even if it does not crack at the time of processing, it reduces the corrosion resistance and oxidation resistance at the time of use, resulting in a shortened product life. Therefore, it is desirable that the surface after welding has no defects, and is in a state where polishing is almost unnecessary.

[0008] The cladding tube of the sheath heater has conventionally used Alloy 840 or Alloy 800 or the like as a high corrosion resistance material. As a metal material for improving corrosion resistance, Patent Document 1 discloses an austenitic Fe-Ni-Cr alloy for a cladding tube, which is excellent not only in corrosion resistance of the base material and the welded portion, but also in workability of the welded portion.

[0009] However, these materials are mainly aimed at improving the corrosion resistance in a wet environment, and although the corrosion resistance and workability of the welded portion are considered, the oxidation resistance evaluated particularly in a cyclic test of repeating room temperature and high temperature of around 1000°C, which has a large influence on the product life, is not considered. The characteristics of the metal cladding tube in a variety of environments requiring corrosion resistance in a wet environment and oxidation resistance in repeated high temperature and low temperature are not sufficient.

[0010] In addition, Patent Document 2 proposes an inexpensive alloy for a cladding tube of a sheath heater, which is excellent in blackening property and weldability, and is less likely to generate surface defects. These materials, while adding Al and Ti in an amount to form a black protective film excellent in emissivity, consider the influence of surface defects at the time of manufacturing, and specify the Al+Ti amount. However, although the characteristics of improving blackening property, surface defects, and emissivity are considered, the oxidation resistance capable of withstanding repeated oxidation tests at high and low temperatures is not considered, and the characteristics of the alloy are not sufficient as an alloy for sheath heater use. In addition, with respect to corrosion resistance, the influence of precipitates and the like due to the added elements is not considered, and it cannot withstand use in an atmosphere with high corrosion resistance, and the characteristics as a metal cladding tube are not sufficient.

[0011] Patent Document 3 proposes a multilayer steel material having Al, Si, Ca, Y, La, Ce added thereto, which has excellent high-temperature corrosion resistance. These materials have excellent oxidation resistance, and have excellent high-temperature corrosion resistance against chloride and hydrogen chloride gas and the like in a combustion environment. However, in other environments, such as a humid environment, the corrosion resistance is insufficient due to the adverse effects of Al, La, Ce, Y, Ca, and the like. In addition, no research has been conducted on weldability, and it is considered that the weldability is insufficient due to the adverse effects of La, Ce, Y, Ca, and the like. Furthermore, since it is a manufacturing method of sintered alloy powder, the productivity is low, and it can be said that it is difficult to apply to an alloy for a jacket heater like the present application.

[0012] Prior Art Documents

[0013] Patent Documents

[0014] Patent Document 1: Japanese Patent Application Publication No. 2014-84493,

[0015] Patent Document 2: Japanese Patent Application Publication No. 2004-197122,

[0016] Patent Document 3: Japanese Patent Application Publication No. H6-330226. SUMMARY

[0017] PROBLEMS TO BE SOLVED BY THE INVENTION

[0018] The present application was completed in view of the above problems of the prior art. An object of the present application is to provide an Fe-Ni-Cr alloy having excellent oxidation resistance, corrosion resistance, and weldability, which is required for a cladding tube of a jacket heater, which is a material that has been attempted in recent years, and which is usable in both a humid environment and a high-temperature environment.

[0019] MEANS FOR SOLVING THE PROBLEMS

[0020] The inventors repeatedly and intensively researched in order to solve the above problems. Oxidation resistance evaluated in a cyclic test from room temperature to 1000°C has a great influence on the life of a jacket heater product. Therefore, elements that improve oxidation resistance and elements that reduce oxidation resistance were investigated. As a result, it was shown that the addition of La, Ce, Y is extremely effective in order to improve oxidation resistance, and as other elements, Si, Ni, Al are also effective. On the other hand, in the case where B, N, S are contained, it was found that the improvement in oxidation resistance is lost, and the product life of the jacket heater becomes short. In particular, if the amount of B added increases, the adverse effects become significant. It was confirmed that in order to secure the oxidation resistance when La, Ce, Y are added, Si, Ni, Al, B, N, S need to be controlled.

[0021] In addition, La, Ce, Y added for improving oxidation resistance form oxide inclusions, phosphorus compounds with O, P, and decrease the corrosion resistance required for the characteristics of the sheath heater in a humid environment containing chloride ions. The cause of the decrease in corrosion resistance was investigated in detail, and as a result, it was considered that the oxygen compounds and phosphorus compounds were dissolved preferentially from the base material and became the starting point of pitting. In order to improve the corrosion resistance, it was found that in addition to La, Ce, Y, the composition balance of Ni, Cr, Mo, P, Al, Ca, O needs to be controlled in the optimum range, thereby ensuring the corrosion resistance of the base material and suppressing the formation of oxide compounds and phosphorus compounds. That is, it is important to suppress the oxygen concentration to be low by appropriately adding Al, thereby effectively dissolving La, Ce, Y in the alloy in a manner that sufficiently exerts the effectiveness of these elements. In addition, by adding Al for deoxidation, desulfurization can be performed to control the sulfur concentration within the corrected range. However, if the oxygen is too low, CaO is reduced from the molten slag in smelting, and Ca-containing oxides are formed in the alloy. It is known that this easily dissolves in a humid environment in which the sheath heater is used to become the starting point of pitting and decrease the corrosion resistance.

[0022] In the case where an Fe-Ni-Cr alloy to which La, Ce, Y is added is welded, the shape of the welded bead portion deteriorates, and unevenness occurs in the bead portion. In the case where the unevenness of the bead portion is not polished, it sometimes causes bending cracking at the time of productization processing, and even in the case where there is no cracking at the time of processing, it can decrease the oxidation resistance at the time of use or become the starting point of pitting. The correlation between the shape of the bead portion and the composition of the base material was investigated in detail, and as a result, it was found that the degree of these shape deteriorations has a correlation with the addition amounts of La, Ce, Y and Si, Mn, S, Al, Ti, O, and by adjusting the balance of these components, the shape of the bead portion can be controlled to a level where there is no problem. In addition, it was also found that by controlling the balance of the amounts of Al and Ti, the temper color coloring of the material to which REM (rare earth metal) is added can be effectively suppressed.

[0023] The present application was completed based on the above insight, and the gist thereof is as follows.

[0024] That is, the Fe-Ni-Cr alloy of the present invention is characterized by comprising, by mass%, C: 0.001-0.050%, Si: 0.18-1.00%, Mn: 0.20-0.80%, P: less than 0.030%, S: 0.0001-0.0020%, Ni: 12-21%, Cr: 18-24%, Mo: 0.20-1.50%, Cu: less than 0.30%, Al: 0.10-0.70%, and Ti: 0.10-0. The alloy consists of 70% N: 0.002–0.015%, B: 0.0001–0.0010%, O: 0.0002–0.0030%, Ca: less than 0.002%, and the total weight (REM) of any one or more of La, Ce, or Y: 0.0010–0.0150%, with the balance being Fe and unavoidable impurities, and satisfies the following relationship: Equations 1 and 2 (where the designation of each element refers to the content (mass%) of that element in the Fe-based alloy).

[0025] 0.575×Ni+1.25×Cr+3.43×Mo-39×P-5.3×Al-641×REM-1018×O≥20.0…(Equation 1)

[0026] 1.5×Mn+41.3×Si+1469×S-1.67×Al-1.34×Ti-150×O-620×REM≥5.0…(Equation 2)

[0027] In this invention, the preferred embodiment is one in which the intrinsic compound is composed of REM oxide, Ti nitride, Ti carbonitride, Ca-Al oxide, and Mg oxide, and the area fraction of any cross section of the alloy is less than 0.50%.

[0028] In this invention, a preferred embodiment is one that satisfies the following formulas 3 and 4 (where the designation of each element refers to the content (mass%) of that element in the Fe-based alloy).

[0029] Al-1.1×Ti≥0…(Equation 3)

[0030] 0.4×Si+1.7×Ni+1.1×Cr+5.4×Al+3.2×Ti+4923×REM-2425×B-744×N-1213×S≥49.0…(Equation 4)

[0031] Furthermore, the present invention relates to a method for manufacturing the aforementioned Fe-Ni-Cr alloy, characterized in that the alloy composition is adjusted by refining the alloy raw materials after melting. During refining, a mixture of oxygen and argon is blown into the molten alloy raw materials (molten alloy) to decarburize them, and the nitrogen concentration is controlled at 0.002 to 0.015%. Chromium reduction is then performed, and aluminum, limestone, and fluorite are added to the molten alloy to form a CaO-SiO2-Al2O3-MgO-F system slag. The oxygen concentration in the molten alloy is maintained at 0.0002 to 0.030%. Then, any one or more of La, Ce, or Y are added.

[0032] The sheathed heater of the present invention is characterized by having a cladding tube formed of the above-mentioned Fe-Ni-Cr alloy.

[0033] The effects of the invention

[0034] According to the present invention, an Fe-Ni-Cr alloy with excellent oxidation resistance, corrosion resistance and weldability required for use as a sheathed heater, and a method thereof are provided. Attached Figure Description

[0035] [ Figure 1 The graph shows the relationship between Formula 1 and pitting potential, where Formula 1 shows the effect of chemical composition on corrosion resistance.

[0036] [ Figure 2 The graph shows the relationship between Equation 2 and the number of weld waves, where Equation 2 shows the effect of chemical composition on weldability.

[0037] [ Figure 3 The graph shows the relationship between Al and Ti amounts and the coloration around the weld metal.

[0038] [ Figure 4 The graph shows the relationship between Equation 4 and weight change, whereby Equation 4 shows the effect of chemical composition on oxidation resistance. Detailed Implementation

[0039] Next, the embodiments of the present invention will be described in detail. It should be noted that in the following description, the units of the content of each element in the composition of the Fe-Ni-Cr alloy and the molten alloy raw material (molten alloy) are all "mass%" and are expressed only as "%" unless otherwise specified.

[0040] C: 0.001~0.050%

[0041] C in the alloy is an element that stabilizes the austenite phase. Furthermore, it enhances the alloy's strength through solid solution strengthening; therefore, to ensure strength at both room temperature and high temperatures, it needs to contain at least 0.001%. On the other hand, C forms carbides with Cr, which significantly improves corrosion resistance, creating a Cr-deficient layer in its vicinity, thus reducing corrosion resistance. Therefore, the upper limit for C content needs to be set at 0.050%. A preferred C content is 0.002–0.045%, more preferably 0.002–0.040%.

[0042] Si: 0.18~1.00%

[0043] Si in the alloy is an effective element for improving the adhesion between the oxide film and the base material, and for forming a stable film. Furthermore, it is effective in increasing the penetration depth and fluidity of the molten metal during welding, and in preventing the formation of large weld beads; therefore, a lower limit is set at 0.18%. On the other hand, excessive addition can lead to surface defects caused by inclusions; therefore, an upper limit is set at 1.00%. The preferred Si content is 0.20–0.95%, more preferably 0.22–0.90%.

[0044] Mn: 0.20–0.80%

[0045] Mn in the alloy is an element that stabilizes the austenite phase and is required for deoxidation. Furthermore, Mn fixes excess sulfur during welding, preventing weld cracking. It also reduces weld ripples on the weld surface. Therefore, a lower limit is set at 0.20%. On the other hand, excessive addition leads to decreased oxidation and corrosion resistance; therefore, an upper limit is set at 0.80%. The preferred Mn content is 0.23% to 0.75%, more preferably 0.25% to 0.70%.

[0046] P: below 0.030%

[0047] P in this alloy forms compounds with La, Ce, and Y, which are harmful elements that reduce corrosion resistance; therefore, the upper limit is set at 0.030%. Preferably, the P content is 0.026% or less, more preferably 0.022% or less.

[0048] S: 0.0001~0.0020%

[0049] S in the alloy improves the fluidity of molten metal during welding, reducing the risk of poor fusion, slag entrapment, and preventing the formation of large weld beads. However, S segregates at grain boundaries, forming low-melting-point compounds that can cause hot cracking during manufacturing, significantly reducing productivity. Furthermore, in cyclic tests involving repeated high and low temperatures, S promotes oxidation by reducing the adhesion between the oxide film and the base material, causing the oxide film to detach. Since it is also an element detrimental to oxidation resistance, the concentration range is set to 0.0001–0.0020%. A more preferred range is 0.0002–0.0015%, and a particularly preferred range is 0.0003–0.0010%.

[0050] Ni: 12-21%

[0051] Ni in the alloy is an element that stabilizes the austenite phase, improving oxidation resistance and high-temperature strength. To ensure the stability of the microstructure, oxidation resistance, and high-temperature strength, the lower limit is set at 12% or more. On the other hand, excessive addition reduces blackening properties and increases raw material costs. Therefore, the upper limit is set at 21%. The preferred range for the content is 12.5% ​​to 20.0%, and the particularly preferred range is 13.0% to 19.5%.

[0052] Cr: 18-24%

[0053] Cr in the alloy is an effective element for improving corrosion resistance in humid environments, inhibiting corrosion in high-temperature atmospheric environments, and improving oxidation resistance. Therefore, to ensure corrosion resistance and oxidation resistance, the lower limit is set at 18%. On the other hand, if the content is too high, it reduces the stability of the austenite phase, leading to a decrease in corrosion resistance; therefore, the upper limit is set at 24%. The preferred range of content is 18.5% to 23.4%, and the particularly preferred range is 19.0% to 22.5%.

[0054] Mo: 0.20–1.50%

[0055] Even small amounts of Mo added to the alloy significantly improve corrosion resistance in humid environments. Therefore, the lower limit is set at 0.20%. However, with large additions, Mo preferentially oxidizes during intermediate heat treatment in the manufacturing process of the jacketed heater, resulting in the peeling of the oxide film, which can sometimes have adverse effects. Therefore, the upper limit is set at 1.50%. The preferred range for the content is 0.25–1.40%, and the particularly preferred range is 0.30–1.30%.

[0056] Cu: below 0.30%

[0057] Cu in the alloy has the effect of stabilizing the austenite phase and softening the material's strength. However, excessive addition causes cracking during welding, therefore the upper limit is set at 0.30%. The preferred upper limit for the content is 0.25%, and the particularly preferred upper limit is 0.20%.

[0058] Al: 0.10–0.70%

[0059] Al in the alloy is an element added as a deoxidizing material, and it is an important element for controlling the oxygen concentration within the range of 0.0002 to 0.0030% according to formula (1) in this application.

[0060] 2 Al +3 O =(Al2O3)…(1)

[0061] Underlined elements represent elements in molten steel, and parentheses represent elements in slag.

[0062] By using a CaO-SiO2-Al2O3-MgO-F series slag when refining the steel of this invention, the generated alumina can be effectively absorbed, and the oxygen concentration can be controlled. In addition, by performing deoxidation, the S concentration in the molten steel is also reduced according to formula (2).

[0063] 2 Al +3 S +3(CaO)=3(CaS)+(Al2O3)…(2)

[0064] Therefore, the sulfur concentration can be controlled within the range of this invention: 0.0001 to 0.0020%.

[0065] Therefore, Al needs to be above 0.10%. Al is an element that promotes the formation of a dense film, improves oxidation resistance, and effectively inhibits discoloration during welding in materials with added REM, so it needs to be above 0.10%. On the other hand, according to formula (3), the addition of excess Al will increase the Ca concentration to more than 0.002%.

[0066] 3(CaO)+2 Al =(Al2O3)+3 Ca …(3)

[0067] Furthermore, the addition of excess Al combines with nitrogen in the alloy to form a large amount of nitrides, leading to surface defects and increased weld bead size in welds. Additionally, it forms large inclusions when combined with oxygen, resulting in reduced corrosion resistance; therefore, the upper limit is set at 0.70%. The preferred range for the content is 0.13–0.68%, and the particularly preferred range is 0.15–0.65%.

[0068] Ti: 0.10–0.70%

[0069] Ti in the alloy promotes the formation of a dense film and effectively improves oxidation resistance; therefore, the lower limit is set at 0.10%. On the other hand, excessive Ti addition combines with nitrogen in the alloy to generate a large amount of nitrides, leading to surface defects. Furthermore, excessive addition increases the weld bead size and promotes coloration of the REM-added material during welding; therefore, the upper limit is set at 0.70%. The preferred range for the content is 0.13–0.65%, and the particularly preferred range is 0.15–0.60%.

[0070] N: 0.002~0.015%

[0071] Nitrogen (N) in the alloy is an element that improves mechanical properties and also has the effect of stabilizing the austenite phase. Therefore, the lower limit of N is set to 0.002%. On the other hand, N forms nitrides with Al and Ti, which not only leads to surface defects but also reduces the effect of Al and Ti, namely oxidation resistance. Therefore, the upper limit is set to 0.015%. The preferred range is 0.003 to 0.013%, and particularly preferred is 0.004 to 0.012%. In order to control N within the range of the present invention, when CO gas is generated during the decarburization process of blowing a mixture of Ar and oxygen into the molten steel of the present invention during refining, the nitrogen in the molten steel will be reduced according to formula (4), and can be controlled within the range of the present invention.

[0072] 2 N =N2 (gas)...(4)

[0073] B: 0.0001~0.0010%

[0074] Boron (B) in the alloy improves hot workability and typically reduces hot rolling cracking and surface defects during hot working or hot forging. Therefore, the lower limit is set at 0.0001%. However, B makes the oxide scale of the alloy porous, reducing oxidation resistance. Therefore, the upper limit is set at 0.0010%. The preferred range for the content is 0.0002–0.0009%, and the particularly preferred range is 0.0003–0.0008%.

[0075] O: 0.0002~0.0030%

[0076] O in the alloy combines with Al, Ti, Si, La, Ce, and Y in molten steel to form oxides, leading to surface defects, reduced corrosion resistance, and increased weld bead size in welded seams. Therefore, the oxygen concentration is preferably as low as 0.0030% or less. Furthermore, since O combines with Al, Ti, Si, La, Ce, and Y, it impairs the beneficial effects of these elements, such as oxidation resistance; therefore, the upper limit is set at 0.0030%. To achieve this range, deoxidation can be achieved simply by controlling the Al concentration as described in this invention. On the other hand, if O in the alloy is excessively reduced, the Ca concentration will increase to over 0.002% according to formula (3). Therefore, the lower limit is set at 0.0002%. The preferred range for the content is 0.0003% to 0.0027%, and the particularly preferred range is 0.0005% to 0.0025%.

[0077] Ca: below 0.002%

[0078] In the alloy of this invention, Ca is an element mixed in from CaO in the slag, as described above. Ca tends to form Ca-Al oxide inclusions, which reduce corrosion resistance, and therefore needs to be suppressed to a low level. As mentioned above, for this purpose, the Al concentration needs to be controlled at 0.10 to 0.70%, and the oxygen concentration at 0.0002 to 0.0030%. Therefore, the Ca content needs to be below 0.002%.

[0079] Total weight (REM) of any one or more of La, Ce, and Y: 0.0010% to 0.0150%

[0080] La, Ce, and Y in the alloy improve the adhesion between the oxide film and the base alloy, significantly enhancing oxidation resistance; therefore, the lower limit is set at 0.0010%. On the other hand, La, Ce, and Y combine with O and P to form oxides and phosphorus compounds, leading to a significant reduction in corrosion resistance. Furthermore, excessive addition of La, Ce, and Y deteriorates the weld shape during welding, causing unevenness in the weld. These reductions in corrosion resistance and weldability significantly shorten the lifespan of the sheath heater used in the product, almost negating the improved oxidation resistance effect. Therefore, the upper limit is set at 0.0150%. A preferred range is 0.0015–0.0140%, and a particularly preferred range is 0.0020–0.0130%.

[0081] Equation 1: 0.575×Ni + 1.25×Cr + 3.43×Mo - 39×P - 5.3×Al - 641×REM - 1018×O ≥ 20.0

[0082] For elements affecting corrosion resistance in materials with added REM, their influence is expressed by regression analysis. As mentioned above, La, Ce, and Y form oxygen-based inclusions and phosphorus compounds, which can lead to pitting corrosion initiation points and reduced corrosion resistance. Therefore, it is necessary to suppress the formation of oxygen-based inclusions and phosphorus compounds, and ensure the corrosion resistance of the base material through Ni, Cr, and Mo. Furthermore, even without La, Ce, and Y, corrosion resistance is reduced due to Al oxide inclusions; therefore, as mentioned above, the Al content should be reduced within the possible range. Therefore, a lower limit of 20.0 or more is set. Preferably, it is 23.0 or more, and more preferably 26.0 or more.

[0083] Equation 2: 1.5×Mn + 41.3×Si + 1469×S - 1.67×Al - 1.34×Ti - 150×O - 620×REM ≥ 5.0

[0084] For the elements in the alloy containing REM that affect the surface roughness of the weld, their influence is expressed by regression analysis. Si and S are elements that improve the penetration depth and melt flow during welding, while Mn has the effect of preventing weld cracking. By adding these elements, not only is post-weld cracking reduced, but the surface roughness of the weld is also reduced. On the other hand, Al and Ti form oxides and nitrides, while La, Ce, and Y form oxides and phosphorus compounds with oxygen and phosphorus, thereby increasing the surface roughness of the weld. Therefore, the lower limit for these elements is set to 5.0 or more. Preferably, it is 7.0 or more, and more preferably 10.0 or more.

[0085] Composition and area fraction of compounds

[0086] The alloy contains inherent compounds including REM oxides, Ti nitrides, and Ti carbonitrides. The REM oxides contain less than 1% of Ni, P, Si, and S elements. In addition, although in trace amounts, it contains Al-Ca oxides and Mg oxides. These compounds become the starting point for corrosion, therefore the cross-sectional area fraction needs to be drastically reduced to below 0.50%. Even less is preferred, more preferably below 0.40%, and more preferably below 0.30%. To achieve this, along with the aforementioned control of Al concentration, the oxygen concentration needs to be controlled within an appropriate range, as well as the Ca concentration. That is, if the Al concentration is controlled at 0.10–0.70% and the oxygen concentration at 0.0002–0.0030%, La, Ce, and Y are effectively dissolved in the alloy, fully exerting their effects, and the Ca concentration does not increase, thus not impairing corrosion resistance. Furthermore, regarding the N concentration, as described above, when CO gas is generated during the decarburization process by blowing in a mixture of Ar and oxygen, the nitrogen in the molten steel is controlled within the range of 0.002 to 0.015% as per this invention, and the Ti concentration is controlled within 0.10 to 0.70%, thereby minimizing Ti nitrides and Ti carbonitrides. In other words, for compounds formed in the alloy, the characteristics required by this application can be satisfied by controlling the concentrations of Al, Ti, N, and O within the range of this invention.

[0087] Equation 3: Al-1.1×Ti≥0

[0088] A formula for controlling the coloring, i.e., the tempering color, of weld seams in alloys with added REM. By adjusting the constituent elements of the oxide film, its color is made lighter, making it easier to remove later. Therefore, it needs to be set to 0 or higher. Preferably, it is 0.02 or higher, more preferably 0.05 or higher.

[0089] Equation 4: 0.4×Si + 1.7×Ni + 1.1×Cr + 5.4×Al + 3.2×Ti + 4923×REM - 2425×B - 744×N - 1213×S ≥ 49.0

[0090] For the elements affecting oxidation resistance in the alloy of the sheath heater, their influence is expressed by regression analysis. Si, Ni, Cr, Al, La, Ce, and Y improve oxidation resistance, as evaluated by repeated cyclic tests at room temperature and high temperatures around 1000°C. On the other hand, high B content results in a porous oxide scale in the alloy, thus increasing the oxidation rate at high temperatures and promoting scale growth and peeling. N combines with Al, which contributes to improved oxidation resistance, to form AlN, reducing the effect of Al. Furthermore, S promotes oxidation by reducing the adhesion between the oxide film and the base material, causing the oxide film to detach. Therefore, the lower limit for these elements is set to 49.0 or more. Preferably, it is 50.0 or more, more preferably 51.0 or more.

[0091] In the Fe-Ni-Cr alloy of the present invention, which exhibits excellent oxidation resistance, corrosion resistance, and weldability, the balance other than the aforementioned components is Fe and unavoidable impurities.

[0092] The special provisions for the limiting formula of Equation 1 above are as follows.

[0093] Various alloys with Fe-20%Ni-20%Cr-0.5%Mo as the basic composition, with various modifications to the addition of Ni, Cr, P, Mo, Al, REM, etc., were melted in a vacuum furnace. After hot forging, 8mm thick × 80mm wide hot-forged plates were produced. The resulting hot-forged plates were solution-treated at 1200℃ for 10 minutes, and after surface grinding, cold-rolled to a thickness of 2mm. Then, they underwent solution treatment at 1050℃ for 1 minute. The plates were then cut into 20mm × 30mm pieces, and the surface was finished by #600 wet grinding to produce test pieces. Using the obtained test pieces, the test solution was prepared by adding 10ml / l acetic acid to a 3.5% NaCl solution at 70℃. The pitting potential was measured according to JIS G0577, and the result was plotted in the anodic polarization curve corresponding to a current density of 100μA·cm. -2 The potential is used as the pitting potential. The electrochemical measurement system uses the HZ-5000 manufactured by Beidou Electric Co., Ltd. The reference electrode is a saturated calomel electrode (Sat.KCl, hereinafter referred to as SCE, all potentials are based on SCE and are expressed only in V), and the counter electrode is Pt. For example, 0.07V means 0.07V vs. SCE.

[0094] It was found that the pitting potential obtained in the pitting corrosion test increased with the addition of Ni, Cr, and Mo, but decreased significantly with the addition of La, Ce, and Y. Furthermore, the content of P and O also had an adverse effect. Observation of the test pieces revealed that La, Ce, and Y oxide inclusions and phosphorus compounds were abundant, acting as the starting point for pitting corrosion. Additionally, although the effect of La on reducing the pitting potential was smaller than that of La, Ce, and Y, the large amount of Al reacting with O to form spinel-based inclusions, which also became the starting point for pitting corrosion, leading to a decrease in corrosion resistance. Therefore, it was found that controlling the content of La, Ce, Y, P, O, and Al is effective in preventing a decrease in corrosion resistance. Furthermore, the effectiveness of Ni, Cr, and Mo in improving the corrosion resistance of the base material was confirmed, indicating that a balance with these elements is necessary.

[0095] Based on the above experimental results, the influence of added elements on the corrosion resistance of the sheath heater alloy was clarified. The compositional relationship, expressed in Equation 1, was obtained through multiple regression analysis. It was determined that if the sufficient corrosion resistance of the sheath heater is set to 0.07V, then by making the value of Equation 1 above 20.0, oxides and phosphorus compounds can be suppressed, preventing the reduction in corrosion resistance caused by La, Ce, and Y.

[0096] The special provisions for the limiting formula in Equation 2 above are as follows.

[0097] Various alloys with Fe-20%Ni-20%Cr-0.5%Mo as the basic composition, with variations in the addition amounts as described above, were melted in a vacuum furnace. After hot forging, 8mm thick × 80mm wide hot-forged plates were produced. The resulting hot-forged plates were solution-treated at 1200℃ for 10 minutes, then ground and cold-rolled to a thickness of 2mm, followed by solution treatment at 1050℃ for 1 minute. The specimens were adjusted to dimensions of 2mm thick × 80mm wide × 200mm long, and the oxide film generated by the heat treatment was removed by shot peening to prepare test pieces. Overlay welding tests were conducted on the surface of the test pieces under the conditions of an arc length of 2mm, Ar gas flow rate of 10L / min, welding speed of 300mm / min, and welding current of 100A. The weld seam of the test piece was selected near the weld termination point where the weld was sufficiently stable. The number of weld ripples (wavy bumps) with a height of 0.3 mm or higher within a 30 mm weld length was measured using a color 3D laser microscope (KEYENCE, VK-9710, 100x magnification) for evaluation. The reason for using 0.3 mm as the height threshold is that the presence of weld ripples above this height requires additional time for removal through grinding.

[0098] The above experiments clarified the influence of added elements on the weldability of the sheath heater alloy, identifying that Si, Mn, and S reduce weld beads, while Al, Ti, O, and REM increase them. Based on these results, for cases with more than three weld beads, the influence on the composition was determined by multiple regression analysis, yielding Equation 2. It was found that to achieve a level where weld beads can be removed without additional grinding time (below 10), Equation 2 needs to be set to 5.0 or higher. Therefore, it can be evaluated as having sufficient weldability.

[0099] The special provisions for the limiting formula in Equation 3 above are as follows.

[0100] Various alloys with Fe-20%Ni-20%Cr-0.5%Mo as the basic composition, with variations in the addition amounts as described above, were melted in a vacuum furnace. After hot forging, 8mm thick × 80mm wide hot-forged plates were produced. The resulting hot-forged plates were solution-treated at 1200℃ for 10 minutes, then ground and cold-rolled to a thickness of 2mm, followed by solution treatment at 1050℃ for 1 minute. The specimens were adjusted to dimensions of 2mm thick × 80mm wide × 200mm long, and the oxide film generated by the heat treatment was removed by shot peening to prepare test pieces. Overlay welding tests were conducted on the surface of the test pieces under the conditions of an arc length of 2mm, Ar gas flow rate of 10L / min, welding speed of 300mm / min, and welding current of 100A. The weld seam was selected near the weld termination point where the weld was sufficiently stable, and the presence of discoloration in the weld and surrounding area within a 40mm weld length was observed and evaluated. The weld was observed using a stereomicroscope (KEYENCE, VK-9710, 100x magnification). The weld width was evaluated as the ratio of the colored area around the weld; a ratio of 1.5 or less was considered good. This is because if the weld is not colored by oxidation around it, grinding can be done only at the weld itself. However, if the coloration extends to the surrounding area, grinding is required to reach that area. Therefore, the amount of grinding needs to be increased, which means increasing the time required to remove the color through grinding.

[0101] The above experiments clarified the influence of Al and Ti on weld coloration, demonstrating that controlling the addition amounts of Al and Ti is effective in suppressing coloration near the weld seam of materials with added REM. That is, to ensure appropriate grinding time, the Al content only needs to be slightly higher than the Ti content, such as... Figure 3 As shown, it was determined that adding more than 1.1 times the amount of Ti and more Al can effectively control the problem.

[0102] The special provisions for the limiting form of Equation 4 above are as follows.

[0103] Various alloys with Fe-20%Ni-20%Cr-0.5%Mo as the basic composition, with variations in the addition amounts as described above, were melted in a vacuum furnace. After hot forging, 8mm thick × 80mm wide hot-forged plates were produced. The resulting hot-forged plates were solution-treated at 1200℃ for 10 minutes, then surface-ground and cold-rolled to a thickness of 2mm. They were then solution-treated again at 1050℃ for 1 minute. The plates were then cut into 10mm × 80mm pieces, surface-finished by #400 wet grinding, and U-shaped bent to create test pieces simulating the bent portion of a sheathed heater. These were subjected to repeated oxidation tests at 1100℃ for 40 minutes followed by 20 minutes at room temperature. For the test pieces after 200 cycles, the mass change after removing the weight of the peeled oxide scale was evaluated by dividing the surface area before the test. The surface area before the test was determined by measuring the dimensions before bending.

[0104] The above experiments clarified the influence of added elements on the oxidation resistance of the sheath heater alloy, demonstrating that the addition of Si, Ni, Cr, Al, Ti, and REM is effective, but the increase of B, N, and S worsens the oxidation resistance. Based on these results, when the weight change is within 20 mg / cm³, 2 Within the range above where significant changes can be identified, the contribution of the added elements, determined through multiple regression analysis, is shown in Equation 4. By setting the value above 49.0, the weight change that can be judged as sufficient oxidation resistance for a sheathed heater is less than 100 mg / cm³. 2 .

[0105] If the Fe-Ni-Cr alloy for a sheathed heater of the present invention has excellent oxidation resistance, corrosion resistance and weldability, then a Fe-Ni-Cr alloy with excellent oxidation resistance evaluated by repeated high and low temperature cyclic oxidation tests, excellent corrosion resistance in humid environments, and smooth weld seams during welding can be obtained.

[0106] Example

[0107] The embodiments of the present invention will be described next, but the present invention is not limited to these embodiments as long as it does not depart from its spirit.

[0108] (Examples 1-27, Comparative Examples 1-16)

[0109] Manufacturing of Fe-Ni-Cr alloys

[0110] First, scrap iron, nickel, chromium, molybdenum, and other raw materials are melted in a 60t electric furnace. Decarburization is then achieved by blowing a mixture of oxygen and Ar gas through AOD (argon-oxygen decarburization) or VOD (vacuum oxygen decarburization), while controlling the nitrogen concentration at 0.002–0.015%. It should be noted that the AOD and VOD furnaces are lined with dolomite (MgO-CaO refractory). Then, ferrosilicon alloy and aluminum are added, followed by Cr reduction. Limestone, fluorite, and aluminum are then added to the molten alloy to form a CaO-SiO2-Al2O3-MgO-F system slag, which is then deoxidized and desulfurized. Finally, an Fe-Ni alloy containing any one of lanthanum (La), cerium (Ce), or yttrium (Y) is added. Then, using the adjusted composition of the molten raw materials, Fe-Ni-Cr alloy slabs with a thickness of 200 mm and a width of 1200 mm are cast by continuous casting. The chemical composition of the resulting alloy is shown in Table 1. It should be noted that the values ​​outside the scope of claim 1 in Tables 1 and 2 are enclosed in parentheses.

[0111] [Table 1]

[0112]

[0113] The surface of the obtained slab was ground by 3-5 mm, heated to 1000-1300℃, and then hot-rolled into a 3 mm thick hot-rolled strip. After annealing and pickling, it was cold-rolled into a 1 mm thick alloy strip, and then further annealed and pickled. The resulting cold-rolled annealed strip was used as a test piece. The composition of each sample and the values ​​obtained from the above formulas (1) to (4) are shown in Table 2.

[0114] The obtained samples were evaluated as follows, and the results are shown in Table 2.

[0115] (1) Corrosion resistance evaluation

[0116] Each obtained sample was cut into 20mm × 30mm pieces, and the surface was finished by #600 wet grinding to prepare test pieces. Using the obtained test pieces, the test solution was a 3.5% NaCl solution at 70℃ with 10 ml / L acetic acid added. The pitting potential was determined according to JIS G 0577, and the value corresponding to a current density of 100 μA·cm in the anodic polarization curve was plotted. -2 The potential is denoted as pitting potential. Test pieces with a pitting potential V below 0.07V are rated as poor (D), test pieces with a pitting potential above 0.07V but below 0.10V are rated as acceptable (C), test pieces with a pitting potential above 0.10V but below 0.15V are rated as good (B), and test pieces with a pitting potential above 0.15V are rated as excellent (A).

[0117] (2) Weldability evaluation

[0118] Each obtained sample was adjusted to a thickness of 2 mm × 80 mm × 200 mm. The oxide film generated during heat treatment was removed by shot peening to prepare test pieces. Two test pieces were subjected to surface surfacing welding tests under the conditions of an arc length of 2 mm, Ar gas flow rate of 10 L / min, welding speed of 300 mm / min, and welding current of 100 A. Near the weld termination point where the weld seam of the test piece was sufficiently stable, the number of weld ripples (wavy, uneven surfaces) with a height of 0.3 mm or more within a 30 mm weld seam length was measured. The number of weld ripples in a total of 60 mm across the two test pieces was used for evaluation. Test pieces with 10 or more weld ripples were rated as poor (D), those with 5 or more but less than 10 were rated as acceptable (C), those with 2 or more but less than 5 were rated as good (B), and those with less than 2 were rated as excellent (A).

[0119] (3) Colorimetric evaluation

[0120] Each obtained sample was adjusted to a thickness of 2 mm × 80 mm wide × 200 mm long. The oxide film generated during heat treatment was removed by shot peening to prepare test pieces. Two test pieces were subjected to surface surfacing welding tests under the conditions of an arc length of 2 mm, Ar gas flow rate of 10 L / min, welding speed of 300 mm / min, and welding current of 100 A. The weld seam near the sufficiently stable weld termination point was selected, and a 40 mm weld seam length was used as the evaluation object. In both test pieces, the ratio of the weld width to the colored area around the weld seam within a total of 80 mm was used for evaluation. Test pieces with a ratio of 1.5 or higher were rated as poor (D), those with a ratio of 1.3 or higher but lower than 1.5 were rated as acceptable (C), those with a ratio of 1.1 or higher but lower than 1.3 were rated as good (B), and those with a ratio lower than 1.1 were rated as excellent (A).

[0121] (4) Evaluation of oxidation resistance

[0122] Each obtained sample was cut into 20mm × 30mm pieces, and the surface was finished by #400 wet grinding to prepare test pieces. Repeated oxidation tests were conducted, with one cycle consisting of 1100℃ × 40 minutes and room temperature × 20 minutes. For the test pieces after 200 cycles, the mass change after removing the weight of the peeled oxide scale was divided by the surface area before the test (mg / cm²). 2 The evaluation was conducted. The surface area before the test was determined by measuring the dimensions before bending. The mass reduction was within 100 mg / cm². 2 The above test tablets were rated as poor (D), 60 mg / cm². 2 Above and below 100 mg / cm 2The test tablet was rated as qualified (C), 30 mg / cm³. 2 Above and below 60 mg / cm 2 The test tablet was rated as good (B), below 30 mg / cm³. 2 The test piece was rated as excellent (A).

[0123] (5) Identification of compounds and area ratio

[0124] The obtained 1mm thick annealed material was cut to prepare an embedded sample with an observable cross-section. Wet grinding was then performed, followed by polishing to create a mirror-like finish for observation. The sample was observed using FE-SEM and analyzed with the accompanying EDS to identify the compounds. The area ratio was calculated from the SEM image observed at ×8000 magnification. The observation area was set to 0.05mm². 2 For blocky compounds, the lengths of two sides were measured, and an approximation of a circle was made to calculate its area. For linear compounds, the long and short sides were measured, and the area was calculated as a rectangle. Because the short side varies greatly, measurements were taken at five locations, and the average value was taken as the short side of the compound. Therefore, the area of ​​the compound relative to the observed area was calculated as 0.05 mm². 2 The evaluation is based on the area ratio. An area ratio of 0.50% or less is a preferred embodiment.

[0125] [Table 2]

[0126]

[0127] According to Table 2, in Examples 1 to 27 conforming to the present invention, good results were observed in corrosion resistance, weldability, and oxidation resistance. All are suitable for sheathed heaters.

[0128] In contrast, Comparative Examples 1-16 were rated D in their respective tests because they do not meet the scope of the present invention. In particular, once corrosion of the heater occurs, it progresses rapidly, and there is a risk of electric shock in humid environments. Furthermore, if weldability is lacking, the heater itself becomes impossible to manufacture; therefore, corrosion resistance and weldability are of particular importance, and since these characteristics cannot be obtained, they are classified as comparative examples. Similarly, regarding oxidation resistance, since it is also related to the lifespan of the product equipped with the heater, it is classified as a comparative example since these characteristics cannot be obtained. On the other hand, since colorability does not significantly reduce the characteristics of the heater itself, even if the rating is D, it is classified as an example as long as corrosion resistance, weldability, and oxidation resistance do not receive a D rating.

[0129] In Comparative Example 1, since Cr, which is most effective for corrosion resistance, does not meet the scope of this invention or Equation 1, its corrosion resistance rating is D. Furthermore, because Al has a higher scope than the invention and a stronger deoxidizing effect, forming Ca oxides, this also contributes to the reduced corrosion resistance.

[0130] Comparative Example 2 does not satisfy Equation 2, and generates a large number of weld waves during welding, resulting in a D rating.

[0131] In Comparative Example 3, because Al has a higher range than the invention, Al nitrides are formed. In addition, due to the strong deoxidation effect, Ca oxides are formed, and the corrosion resistance is rated as D.

[0132] In Comparative Example 4, because Al is lower than the scope of the invention, deoxidation is weak and oxides are abundant. Furthermore, because Ti is also higher than the scope of the invention, a large amount of Ti nitrides are formed. Moreover, neither Equation 1 nor Equation 2 is satisfied, and both corrosion resistance and weldability are rated D.

[0133] In Comparative Example 5, the Si content is lower than the scope of the invention, thus failing to satisfy Equation 2, resulting in a weldability rating of D. Furthermore, the N content is higher than the scope of the invention, leading to the formation of Ti nitrides, which results in both corrosion resistance and oxidation resistance ratings of D.

[0134] Comparative Example 6 does not satisfy Equation 1 because Ni and Cr are lower than the limits of the invention, and its corrosion resistance rating is D. Although it satisfies Equation 4, its Cr content is low, so its oxidation resistance rating is also D.

[0135] Although Comparative Example 7 satisfies Equation 1, Mo has a lower range than the invention, and its corrosion resistance is rated as D.

[0136] In Comparative Example 8, because the levels of P and S are higher than those in the invention, and Al is lower, resulting in weaker deoxidation, while O is higher than those in the invention, the levels of phosphorus compounds, sulfur compounds, and oxides increase, leading to a corrosion resistance rating of D. Furthermore, because Al is lower than those in the invention, the oxidation resistance rating is also D.

[0137] Comparative Example 9 does not satisfy Equation 1 because the Cr content is lower than that of the invention. In addition, the amount of REM added is higher than that of the invention, so the REM oxide content increases and the corrosion resistance rating is D.

[0138] In Comparative Example 10, since Si is lower than the scope of the invention, it does not satisfy Equation 2, so the weldability evaluation is D.

[0139] In Comparative Example 11, the Mn content was lower than that of the invention, resulting in weld cracking and a weld evaluation of D.

[0140] In Comparative Example 12, the levels of Ti and N are higher than those in the invention, resulting in the formation of Ti nitrides, and the corrosion resistance is rated as D.

[0141] Comparative Example 13 has a lower Si content and a higher Al content than the invention, and does not satisfy Equation 2, so its weldability is rated as D. Additionally, its oxidation resistance is also rated as D because the REM addition is less than the invention's range.

[0142] In Comparative Example 14, since the Si content is lower than the scope of the invention, Equation 2 is not satisfied, so the weldability evaluation is D. Furthermore, since the addition of B exceeds the scope of the invention, the oxidation resistance evaluation is also D.

[0143] In Comparative Example 15, Al has a lower tolerance than the invention's limit, resulting in weaker deoxidation, while O has a higher tolerance than the invention's limit. Therefore, an oxide is formed, and the corrosion resistance rating is D. Furthermore, because Al has a lower tolerance than the invention's limit, its oxidation resistance rating is also D.

[0144] Comparative Example 16 has a lower Ti content than the invention, resulting in a fragile oxide film and thus an oxidation resistance rating of D.

[0145] Industrial availability

[0146] The present invention provides a Fe-Ni-Cr alloy for sheathed heaters with excellent oxidation resistance, corrosion resistance and weldability, and a method for manufacturing the same.

Claims

1. An Fe-Ni-Cr alloy, characterized in that, By mass%, the composition is as follows: C: 0.001–0.050%, Si: 0.18–1.00%, Mn: 0.20–0.80%, P: less than 0.030%, S: 0.0001–0.0020%, Ni: 12–21%, Cr: 18–24%, Mo: 0.20–1.50%, Cu: less than 0.30%, Al: 0.10–0.70%, Ti: 0.10–0.70%, N: 0.0 The alloy consists of 0.02-0.015%, B: 0.0001-0.0010%, O: 0.0002-0.0030%, Ca: less than 0.002%, and the total weight of any one or more of La, Ce, or Y (REM: 0.0010-0.0150%), with the balance being Fe and unavoidable impurities, and satisfies the relationship of Equations 1 and 2 below, where the designation of each element refers to the mass percentage content of that element in the Fe-based alloy: 0.575×Ni+1.25×Cr+3.43×Mo-39×P-5.3×Al-641×REM-1018×O≥20.0…(Equation 1), 1.5×Mn+41.3×Si+1469×S-1.67×Al-1.34×Ti-150×O-620×REM≥5.0…(Equation 2).

2. The Fe-Ni-Cr alloy according to claim 1, characterized in that, The intrinsic compounds consist of REM oxides, Ti nitrides, Ti carbonitrides, Ca-Al oxides, and Mg oxides, and the area fraction of any cross section of the alloy is less than 0.50%.

3. The Fe-Ni-Cr alloy according to claim 1 or 2, characterized in that, The following relationships are satisfied: Equations 3 and 4, where the designation of each element refers to the mass percentage content of that element in the Fe-based alloy. Al-1.1×Ti≥0…(Equation 3), 0.4×Si+1.7×Ni+1.1×Cr+5.4×Al+3.2×Ti+4923×REM-2425×B-744×N-1213×S≥49.0…(Equation 4).

4. A method for manufacturing an Fe-Ni-Cr alloy, which is the method for manufacturing an Fe-Ni-Cr alloy according to claim 1 or 2, characterized in that, The alloy composition is adjusted by refining the alloy raw materials after melting them. In the refining process, a mixture of oxygen and argon is blown into the molten alloy raw material to decarburize it, and the nitrogen concentration is controlled at 0.002-0.015%. Chromium reduction is then carried out. Then, aluminum, limestone and fluorite are added to the molten alloy to form a CaO-SiO2-Al2O3-MgO-F system slag, and the oxygen concentration in the molten alloy is kept at 0.0002-0.0030%. Then, any one or more raw materials selected from La, Ce or Y are added.

5. A sheathed heater, characterized in that, It has a coated tube formed of the Fe-Ni-Cr alloy according to claim 1 or 2.

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