High corrosion-resistant ni-cr-mo-n alloy having excellent phase stability

Abstract

The Ni-Cr-Mo-N alloy of the present application contains, in mass%, Ni: 22.0% or more, Cr: 22.0% or more, Mo: 5.0% or more, N: 0.180% or more, Si, Al, Mn, the balance consisting of Fe and inevitable impurities, satisfies the following formulas (1) to (3), and in the cross-sectional structure after holding at 950°C for 30 minutes, the area ratio of the phase determined by EBSD is 1.0% or less. σ The Ni-Cr-Mo-N alloy of the present application has excellent corrosion resistance even in the case of exposure to a temperature range of 700 to 1000°C like precipitation σ of the phase. Cr + 3.3 x Mo + 16 x N ≥ 43.0 … (1) 7.3 x Mo - Ni ≤ 21.0 … (2-1) 1.3 x Cr - Ni ≤ 5.7 … (2-2) 1.6 x Si + 0.99 x Mn + 2.2 x Al ≤ 0.95 … (3)

Description

[0001] This application is a divisional application of the application filed on July 27, 2021, with application number "202180050115.X" and invention title "Highly Corrosion-Resistant Ni-Cr-Mo-N Alloy with Excellent Phase Stability". Technical Field

[0002] This invention relates to Ni-Cr-Mo-N alloys that maintain excellent corrosion resistance even when high-temperature heating processes are performed in manufacturing processes involving brazing of components such as sheathed heaters, heat exchangers, and automotive exhaust system parts, or components that require post-weld heat treatment (PWHT) after welding. Background Technology

[0003] Alloys containing high levels of Cr, Mo, and Ni exhibit excellent corrosion resistance and can be used in harsh corrosive environments; brazing is frequently employed in their manufacturing processes. In such cases, to melt and penetrate the brazing material, the temperature is raised to above 900°C and held for a certain period. If alloys with high Cr and Mo content are held at this temperature, precipitation occurs. s Phases can sometimes cause a decrease in corrosion resistance, becoming a problem. Therefore, in the absence of precipitation... s The phase is processed at a higher temperature, such as 1150°C. However, if brazing is performed at such a temperature, heat-induced deformation or cooling-induced deformation or new residual strain may occur during high-temperature holding. Alloys that can be brazed at lower temperatures, i.e., those that are difficult to precipitate, are required. s A phase that can minimize precipitation in an alloy.

[0004] In addition, most alloys are assembled by welding. Residual strain from welding leads to deformation or cracking over time, and stress corrosion cracking; therefore, it is generally removed through a heat treatment known as PWHT. To relieve stress, heat treatment is often performed at around 600–900°C. However, in alloys containing large amounts of Cr and Mo, maintaining these temperatures can lead to the precipitation of Cr carbides. s The precipitation of the phase reduces corrosion resistance. Therefore, it is carried out at higher temperatures, such as above 1150°C. Here, the problem is the same as in the case of brazing, where precipitation should be difficult to occur even when PWHT is applied. s A phase that can minimize precipitation in an alloy.

[0005] As a matter of control sExisting technologies for phase precipitation, such as those described in Patent Document 1, propose controlling the precipitation of steels containing Cu, La, and Ce by specifying hot rolling conditions such as heating temperature, holding time, and number of heating cycles. s Steel with a phase weight of less than 1% by volume and its manufacturing method. The aim is to improve elongation and bending properties perpendicular to the rolling direction. s The phasor density was determined based on observations of the C-section according to JIS G 0555. Although attention was paid to... s Phase control was considered, but the impact on corrosion resistance was not taken into account.

[0006] In Patent Document 2, for steel containing Cu, it is proposed to control the thickness of the central portion of the plate by means of an area ratio. s The relationships between constituent elements with a phase content of less than 1%, the relationships controlling corrosion formation, and the combinations of relationships controlling corrosion occurrence, thereby controlling corrosion. s Steels with excellent corrosion resistance are preferred. The focus is on materials that have undergone solution heat treatment; materials that have undergone processes such as brazing or PWHT are not considered. s Phase precipitation, corrosion resistance.

[0007] In Patent Document 3, for steel containing Cu, it is also proposed to control the composition using a formula consisting of Fe, Cr, Mo, Ni, and Cu, thereby achieving a concentration of Cu within 0.1 mm of the surface layer. m Within a depth of m s Steel with a phase content of less than 1% by area. While the aim is to ensure corrosion resistance after brazing, the corrosion resistance is sometimes insufficient in environments primarily containing chlorides, due to the assumption of special environments containing sulfuric acid and hydrochloric acid. Although used for control... s One of the key aspects of this phase is the addition of Cu, but Cu is a very expensive element, and the improvement in corrosion resistance in chloride-containing environments is not commensurate with the cost. Furthermore, the brazing was performed at 1150°C; to reduce deformation and residual strain, it is desirable to perform it at a lower temperature.

[0008] Patent document 4 proposes a method to suppress the presence of [something] in steel ingots. s The precipitation of certain phases improves the resistance to crevice corrosion and hot workability of austenitic stainless steel. Since the object is a steel ingot, the aim is to suppress the precipitation of certain phases in the solidification structure. s The object is a plate formed by phase generation and thermal processing, so the brazing process is not considered.

[0009] Existing technical documents Patent documents Patent Document 1: Japanese Patent Application Publication No. 2002-322545, Patent Document 2: WO2016 / 076254, Patent Document 3: Japanese Patent Application Publication No. 2018-172709, Patent document 4: Japanese Patent No. 3512304. Attached Figure Description

[0010] [ Figure 1 [This indicates that the evaluation of the alloy was obtained through EBSD.] s Electron microscope images of the phase determination results.

[0011] [ Figure 2 ] indicates component pair s The diagram shows the effect of phase precipitation. (a) shows the relationship between Ni and Mo, and (b) shows the relationship between Ni and Cr.

[0012] [ Figure 3 [ represents the ratio of Si, Mn, and Al] s A graph showing the effects of phase precipitation area ratio and corrosion resistance.

[0013] [ Figure 4 [This is a schematic diagram of the test piece used in the brazing performance evaluation.]

[0014] [ Figure 5 The figure shows the effect of Si, Mn, and Al content on the wettability of Ni brazing materials. Summary of the Invention

[0015] The problem that the invention aims to solve The present invention was made in view of the above-mentioned problems in the prior art, and its object is to provide a solution even when exposed to precipitates. s Ni-Cr-Mo-N alloys exhibit excellent corrosion resistance even when exposed to temperatures ranging from 700 to 1000°C.

[0016] Methods for solving problems The inventors conducted repeated and in-depth research to solve the aforementioned problems. As a result, by adjusting the alloy composition and satisfying equations (1) to (3), they discovered that after holding at 950℃ for 30 minutes... s Alloys with a phase content of less than 1.0% by area ratio exhibiting excellent corrosion resistance.

[0017] That is, the high corrosion-resistant Ni-Cr-Mo-N alloy with excellent phase stability of the present invention is an alloy containing, by mass percent, Ni: 22.0% or more, Cr: 22.0% or more, Mo: 5.0% or more, N: 0.180% or more, Si, Al, Mn, and the balance being Fe and unavoidable impurities. Its characteristic is that, satisfying the following formulas (1) to (3), the cross-sectional microstructure after holding at 950°C for 30 min, as determined by EBSD... s The area ratio of the phase is less than 1.0%.

[0018] Cr+3.3×Mo+16×N≥43.0…(1) 7.3×Mo-Ni≤21.0 …(2-1) 1.3×Cr-Ni≤5.7 …(2-2) 1.6×Si+0.99×Mn+2.2×Al≤0.95 …(3) In this invention, the preferred composition is as follows: C is 0.001–0.030%, Si is 0.02–0.30%, Mn is 0.02–0.40%, P is 0.005–0.050%, S is 0.0001–0.0030%, Ni is 22.0–38.0%, Cr is 22.0–28.0%, Mo is 5.0–8.0%, Cu is 0.02–0.50%, N is 0.180–0.250%, and Al is 0.005–0.100%.

[0019] In this invention, the preferred method is to satisfy the following formula (4), where B is 0.0005 to 0.0050% and O is ≤35ppm.

[0020] 0.20≤1.6×Si+0.99×Mn+2.2×Al≤0.95 …(4) The high corrosion-resistant component of the present invention is characterized in that the above alloy is brazed using Ni brazing material at a temperature of 1000°C or higher in an inert gas atmosphere.

[0021] The sheathed heater of the present invention is characterized in that it has a cladding tube made of the above-mentioned alloy and a joint formed of Ni brazing material.

[0022] Invention Effects According to the present invention, even when exposed to substances such as precipitates s Even in temperature ranges like those, the reduction in corrosion resistance can be suppressed. Therefore, a high corrosion-resistant alloy can be provided that is suitable for manufacturing processes such as brazing, PWHT for welded parts, and annealing to remove residual strain after cladding with steel. Detailed Implementation

[0023] The inventor firstly s A quantitative evaluation method for the phase was studied. s Quantitative methods for phase identification generally employ the point method, exemplified by ASTM E562. This method involves identifying the phase of an etched metallic microstructure based on the intersections of the grid lines provided with the microscope. sA method for evaluating the proportion of phase overlap. The evaluation results are influenced by the etching results used for observation, and are considered relative to the true... s The amount of phase precipitation may contain an error of a few percent. Furthermore, there is also the possibility of small and minor variations. s When the phase and lattice intersections do not coincide, it is not preferable for evaluations involving small quantities of less than 1% by area ratio. Therefore, the inventors have evaluated the phase using field emission scanning electron microscopy and backscattered electron diffraction (EBSD) methods, which allow for highly precise measurements and provide reliable results through crystal structure determination. s The area ratio of the phase.

[0024] As an example of evaluation, the results obtained from EBSD determination of an Fe-0.01%C-23%Cr-35%Ni-7.48%Mo-0.22%N-0.04%Cu alloy are shown below. Figure 1 middle. s The phase area ratio was determined by the following method. Small pieces cut perpendicular to the rolling direction from a 2mm cold-rolled sheet subjected to a 950℃×60min heat treatment were electrolytically ground using a Struers "Tenupol-5" microscope. The backscattered electron diffraction apparatus (TSL SOLUTIONS, "EBSD Analysis Software OIMAnalysis 7.3") attached to a field emission scanning electron microscope (JEOL Ltd., "JSM-7001F") was used to measure the area ratio in the 80-square-meter region. m m×240 m m, step size 0.2 m The result of m measurement was obtained s Phase area ratio. s The size of the phase is determined by scanning electron microscopy composition images in two orthogonal directions, and the average value is taken as the particle size.

[0025] According to this method, 1 can be reliably captured. m Smaller than m s The phase may not be correctly evaluated due to defects such as scratches, foreign body attachments, and misidentification of non-metallic inclusions that would be problematic in optical microscopy. It should be noted that the smallest unit that can be confirmed in this measurement is determined based on the composition image obtained from the scanning electron microscope. s Phase is 0.1 m m×0.3 m Approximately m. Figure 1 The average particle size is 1.4. m m. Furthermore, under conditions of arbitrary temperature and time holding within the range of 850–1000℃, for… sAn investigation was conducted to determine whether phase precipitation occurred fastest at 950°C. Considering the heating and cooling of the brazed components, maintaining around 950°C typically lasts approximately 15–20 minutes, with a maximum of 30 minutes. Therefore, it can be concluded that even with a holding time of 950°C for 30 minutes, as long as precipitation does not occur, the properties will not deteriorate.

[0026] <Experiment 1> To form by brazing s Alloys with minimal phase precipitation are considered to require no precipitation, even when brazing at around 1050°C. s The precipitation of phases was observed. Therefore, the relationship between the microstructure and composition after holding at 1050℃ was investigated, focusing on Ni, Cr, and Mo as the main elements. The experiment used a high-frequency induction furnace to melt 20 kg of an alloy with N=0.225%, varying within the range of Ni: 18–36%, Cr: 20–29%, and Mo: 5.5–8.0%. Then, a 2 mm cold-rolled sheet was obtained by hot forging, annealing, and cold rolling. The cold-rolled sheet underwent solution heat treatment at 1100℃ for 1 min, followed by forced air cooling. Subsequently, the cold-rolled annealed sheet underwent aging heat treatment at 1050℃ for 60 min, followed by EBSD. s Phase area ratio determination. Performed via EBSD. s The phase area ratio was determined using the method described above.

[0027] The experimental results are shown in Figure 2 The representation of components s The diagram illustrates the effects of phase precipitation. (a) shows the relationship between Ni and Mo amounts, and (b) shows the relationship between Ni and Cr amounts. s Phases below 1.0% are plotted as 〇, and those above 1.0% are plotted as ●. This figure shows that in order to suppress [the condition] at 1050℃... s For phase precipitation, the amounts of Ni and Mo, and Ni and Cr added need to be in the ratio shown by the dashed line, that is, added in a manner that satisfies the following formula.

[0028] 7.3 × Mo-Ni ≤ 21.0 1.3 × Cr - Ni ≤ 5.7 <Experiment 2> To delay as much as possible s Phase precipitation inhibits corrosion resistance degradation; experiments were conducted focusing on Si, Mn, and Al. It is known that Cr, Mo, Ni, and N... sPhase precipitation has a significant impact, but if corrosion resistance is the primary concern, the control using these elements has its limits. Therefore, as other elements, the three elements mentioned above are of particular interest. The experiment used a high-frequency induction furnace to melt 20 kg of an alloy with the basic composition of Fe-23.5%Cr-25.5%Ni-6.0%Mo-0.22%N-0.2%Cu, where the contents of Si, Mn, and Al varied within the ranges of ~0.50%, ~0.60%, and ~0.20%, respectively. Subsequent sample conditioning methods were the same as in Experiment 1, involving aging heat treatment of 2 mm thick cold-rolled annealed sheets at 950 °C for 30 min, followed by EBSD analysis. s The phase area ratio was determined and corrosion resistance was evaluated. The EBSD test method is as described above. Corrosion resistance was evaluated by performing an immersion test in an aqueous solution of ferric chloride and hydrochloric acid as specified in ASTM G48 Method D, and determining the Critical Crevice Corrosion Temperature (CCT). Test pieces were cut from cold-rolled sheets that had undergone aging heat treatment, measuring 25mm × 50mm. The entire surface was wet-polished with SiC 120 abrasive paper, degreased with acetone, and then used for testing. The test solution was 600ml per sample, and the corrosion was assessed by observing whether a corrosion layer formed at a depth of 25mm. m Evaluation was conducted on crevice corrosion exceeding μm. To also evaluate smaller crevice corrosion... s To mitigate the effects of the phase, the holding time was set to 100 hours, longer than the specified time. Crevice formation was assessed using Teflon (registered trademark) multiclevis clips, Ti bolts, and nuts, with a tightening torque of 0.28 N·m. The reason for choosing the crevice corrosion test is that, in the sheathed heater, scale adheres to the boundary between the substrate and the Ni brazing material, and crevice corrosion has occurred in this area.

[0029] The experimental results are shown in Figure 3 The representation of Si, Mn, and Al quantities s The graph shows the effects of phase precipitation area ratio and corrosion resistance. The horizontal axis is obtained through regression analysis. s The results show the relationship between phase area ratio and the amounts of Si, Mn, and Al. It can be seen that by controlling the content of these elements, the effects of [unspecified factors] can be suppressed. s Phase formation. At this point, corrosion resistance is... s The phase does not deteriorate when the amount is small, but it significantly decreases when the amount exceeds a certain level, indicating that the initial characteristics cannot be displayed. s The phase area fraction is approximately 1.0%. It can be seen that in order to... s The phase content is 1.0% or less, and preferably the relationship between Si, Mn, and Al is 0.95 or less. Furthermore, sThe average particle size of the phase is 2.3. m m, confirming that if the area ratio increases, the particle size also increases with the number of particles.

[0030] In addition, Si, Mn, and Al are easily oxidized elements. They can be oxidized by trace amounts of oxidizing gases such as O2, H2O, and CO contained in the brazing atmosphere, which may affect the wettability of the brazing material. Therefore, the wettability and the influence of chemical composition on the brazing material were investigated for the above alloys. The test pieces were the above 2mmt plates that had undergone solution heat treatment. They were cut into (1) 10mmw×100mml and (2) 20mmw×100mml, and the entire surface was wet-polished with SiC 120 polishing paper and degreased with acetone. Figure 4 As shown, it is used to form a T-shaped test piece that is vertically erected (1) in the center of the width of (2) and fixed by spot welding. When forming the test piece, the average roughness Ra of the joint surface is adjusted to below 1.6 to ensure constant fit. About 1g of brazing material is placed at one end of the test piece, and brazing is performed at 1020°C in a hydrogen atmosphere. The "wetting length" of the brazing material flow is evaluated. The length is 100mm when it extends to the opposite side where the brazing material is placed. The brazing material used is nickel brazing material BNi-7 (14Cr-10P-Ni). The brazing process is performed in a bridge brazing furnace with a total capacity of 12m and a linear speed of 1m / min.

[0031] The evaluation results are shown in Figure 5 The figure shows the effect of Si, Mn, and Al content on the wettability of Ni brazing materials. As the content of these elements increases, the wetting length shortens, indicating an impediment to wettability. Therefore, it is desirable to extend the wetting length as much as possible. If the threshold of more than half of the wetting length extending to 60 mm is taken as the threshold, the upper limits for Si, Mn, and Al are approximately 0.30%, 0.40%, and 0.10%, respectively. Furthermore, during the evaluation process, experiments were conducted on alloys containing trace amounts of B (Mn=0.24%, B=0.025%), and the results showed an effect on improving wettability. It is speculated that this is because B sublimates from the alloy during heating in the inert gas atmosphere of brazing, temporarily suppressing the oxidizing atmosphere. This was found to be effective in ensuring more stable wettability.

[0032] Next, the reasons for limiting the composition and relationships of each element in this invention will be explained. Hereinafter, % represents mass%. In this invention, as shown in claim 1, it must be configured to contain Ni: 22.0% or more, Cr: 22.0% or more, Mo: 5.0% or more, N: 0.180% or more, Si, Al, Mn, with the balance consisting of Fe and unavoidable impurities, and satisfying formulas (1) to (3). However, in the following description, the preferred range shown in claim 2 and below will also be explained.

[0033] C: 0.001~0.030% C is an effective element for stabilizing the FCC phase (face-centered cubic structure), suppressing... s The precipitation of the phase is still important for ensuring strength. Therefore, at least 0.001% needs to be added. However, if it is excessively present, the precipitation of Cr carbides becomes easy, which deteriorates the corrosion resistance. Therefore, the upper limit is set at 0.030%. The preferred lower limit of the content is 0.002%, the more preferred lower limit is 0.005%, the preferred upper limit is 0.025%, and the more preferred upper limit is 0.020%.

[0034] Si: 0.02~0.30% Si is an important element with deoxidizing properties. Therefore, at least 0.02% needs to be added. However, an excess of Si will promote… s The precipitation of the phase can also easily form an oxide scale, which deteriorates the wettability of the brazing material. Therefore, the upper limit is set at 0.30%. The preferred lower limit of the content is 0.07%, the more preferred lower limit is 0.09%, the preferred upper limit is 0.25%, and the more preferred upper limit is 0.23%.

[0035] Mn: 0.02~0.40% Mn is an important element with deoxidizing properties, stabilizing the FCC phase and increasing the solubility of N. Therefore, it is essential for inhibiting the precipitation of carbonitrides and nitrides. Thus, at least 0.02% needs to be added. However, excessive addition can promote… s The precipitation of the phase reduces corrosion resistance. The formation of MnS becomes the starting point for pitting corrosion, further deteriorating corrosion resistance. It also easily forms oxide scale, worsening the wettability of the brazing material. Therefore, the upper limit is set at 0.40%. The preferred lower limit for the content is 0.06%, a more preferred lower limit is 0.07%, the preferred upper limit is 0.35%, and a more preferred upper limit is 0.30%.

[0036] P: 0.005~0.050% P is an element that inevitably mixes into the alloy as an impurity, but in this invention it exists at the grain boundaries and thus delays the process. sThe element causing the precipitation of the phase. To achieve this effect, at least 0.005% needs to be added. However, when the content exceeds 0.050%, corrosion resistance and hot workability are significantly deteriorated. Therefore, the content of P is set to 0.005-0.050%. The preferred lower limit of the content is 0.010%, the more preferred lower limit is 0.012%, the preferred upper limit is 0.040%, and the more preferred upper limit is 0.035%.

[0037] S: 0.0001~0.0030% Sulfur (S) is an unavoidable impurity element in alloys, reducing hot workability and forming sulfides that become the starting point for pitting corrosion, thus negatively impacting corrosion resistance. Therefore, a very low S content is preferable. Thus, the upper limit is set at 0.0030%. However, since S improves the fluidity of the melt during molten metal formation, it is an element that contributes to good weldability. From the viewpoint of obtaining good weldability, a content of 0.0001% or higher is required. The preferred lower limit for the content is 0.0002%, a more preferred lower limit is 0.0003%, a preferred upper limit is 0.0020%, and a more preferred upper limit is 0.0015%.

[0038] Ni: 22.0~38.0% Ni is an element that stabilizes the FCC phase and suppresses... s The precipitation of equivalent intermetallic compounds is an important element for improving resistance to pitting corrosion and surface corrosion. Therefore, at least 22.0% needs to be added. However, if the Ni content exceeds 38.0%, it leads to increased resistance to hot deformation and increased cost. Furthermore, compared to promoting… s The addition of Cr and Mo precipitates in the phase has an optimal amount. Therefore, the Ni content is set to 22.0%–38.0%. The preferred lower limit of the content is 23.0%, the more preferred lower limit is 24.0%, the preferred upper limit is 37.7%, and the more preferred upper limit is 37.5%.

[0039] Cr: 22.0~28.0% Cr is an essential element for improving resistance to pitting corrosion and intergranular corrosion, with its primary function being resistance to crevice corrosion. It also increases nitrogen solubility, thus inhibiting nitride formation. However, excessive Cr content can promote… s The precipitation of the Cr phase actually deteriorates the corrosion resistance. Therefore, the Cr content is set to 22.0%–28.0%. The preferred lower limit of the content is 22.5%, the more preferred lower limit is 23.0%, the preferred upper limit is 27.5%, and the more preferred upper limit is 27.0%.

[0040] Mo: 5.0–8.0% Like Cr and N, Mo is an element that improves resistance to pitting and crevice corrosion. However, excessive Mo content significantly promotes... s The precipitation of the phase deteriorates the corrosion resistance. Therefore, the Mo content is set in the range of 5.0% to 8.0%. The preferred lower limit of the content is 5.1%, the more preferred lower limit is 5.2%, the preferred upper limit is 7.9%, and the more preferred upper limit is 7.8%.

[0041] Cu: 0.02–0.50% Cu is an element that stabilizes the FCC phase and helps improve acid resistance. To achieve this effect, a content of 0.02% or more is required. However, excessive addition increases costs and degrades hot workability, so the upper limit is set below 0.50%. Therefore, its content is set at 0.02% to 0.50%. The preferred lower limit of the content is 0.04%, the more preferred lower limit is 0.05%, the preferred upper limit is 0.45%, and the more preferred upper limit is 0.40%.

[0042] N: 0.180~0.250% N is an element that stabilizes the FCC phase and inhibits... s It is an effective element for the precipitation of phases. Furthermore, like Cr and Mo, it significantly improves resistance to pitting and crevice corrosion; moreover, like C, it is an important element for ensuring strength. Therefore, at least 0.18% needs to be added. However, excessive addition promotes the precipitation of carbonitrides and nitrides, leading to a decrease in corrosion resistance. Therefore, it should not exceed 0.250%. The preferred lower limit of the content is 0.185%, the more preferred lower limit is 0.190%, the preferred upper limit is 0.235%, and the more preferred upper limit is 0.230%.

[0043] Al: 0.005~0.100% Al is an important element with deoxidizing properties. Furthermore, in the presence of CaO-SiO2-Al2O3-MgO slag, it promotes desulfurization through deoxidation, making it crucial for stabilizing the yield of boron in refining. However, in cases of excess content, promoting desulfurization... s The precipitation of the phase easily forms an oxide scale, which deteriorates the wettability of the brazing material. Therefore, the Al content is set to 0.005–0.100%. The preferred lower limit of the content is 0.015%, the more preferred lower limit is 0.025%, the preferred upper limit is 0.095%, and the more preferred upper limit is 0.090%.

[0044] Cr+3.3×Mo+16×N≥43.0…(1) To ensure corrosion resistance in chloride-containing environments, a certain amount or higher of Cr, Mo, and N needs to be added. The effects of Mo and N are compared with the effect of Cr to determine the coefficient of their relative effectiveness. For use in harsh environments, a coefficient of at least 43.0 is required. Preferably, it is 44.0 or higher, and more preferably 50.0 or higher.

[0045] 7.3×Mo-Ni≤21.0 …(2-1) 1.3×Cr-Ni≤5.7 …(2-2) When brazing at around 1050℃, it is important that no precipitation occurs in this temperature range. s The FCC phase exhibits high stability. The metal microstructure at 1050℃ is primarily composed of Ni, Cr, and Mo, determined by the balance of these elements. Ni is the element that stabilizes the FCC phase, while the other two promote the formation of the ferrite phase. s Elements involved in phase formation. In suppressing s In the case of phases, it is necessary to ensure equilibrium and optimize their amounts. The former represents the relationship between Mo and Ni, and the latter between Cr and Ni. To effectively suppress... s The former needs to be 21.0 or less, preferably 20.0 or less, and more preferably 19.5 or less. Similarly, the latter needs to be 5.7 or less, preferably 5.6 or less, and more preferably 5.5 or less.

[0046] 0.20≤1.6×Si+0.99×Mn+2.2×Al≤0.95 …(3) For control s The precipitation rate is an important relationship. Precipitation can be suppressed by reducing any one of the elements Si, Mn, and Al. The effect of Mn is used as a benchmark, and the effects of Si and Al are compared to determine the coefficient relative to the effect of Mn. Brazing is performed at 1050℃ to suppress precipitation. s The precipitation of the phase needs to be at least 0.95 or less. Preferably, it is 0.93 or less, and more preferably 0.90 or less.

[0047] Furthermore, Si, Mn, and Al are elements with deoxidizing properties. Reducing the content of these elements leads to poor deoxidation, increased inclusions, and consequently, decreased corrosion resistance. Hot workability also deteriorates. Therefore, it is appropriate to ensure that the addition amount is 0.20 or higher. The preferred lower limit of the relationship is 0.30 or higher, and more preferably 0.35 or higher.

[0048] s Phase area ratio: below 1.0% Precipitation during heat treatments such as brazing and PWHT sWhile this process can degrade corrosion resistance, the degradation is limited for extremely small quantities and sizes. This can be demonstrated by utilizing EBSD. s The precise quantification of phase area ratio and its determination through corrosion testing are crucial. To ensure that the reduction in corrosion resistance is within acceptable limits, s The phase area fraction should not exceed 1.0%. Preferably it is 0.8%, more preferably 0.7% or less. Furthermore, s Larger phase growth indicates a poorer degree of Cr and Mo deficiency in the surrounding layer. Therefore, to ensure corrosion resistance, it is preferable to... s The size is relatively small. In this invention, its size is considered to be an average of 2.5. m less than m. Preferably 2 m m, more preferably 1.5 m m.

[0049] B: 0.0005~0.0050% B is one of the key elements constituting this invention and is believed to exist at grain boundaries. It has a delay effect. s The precipitation of the phase and its volatilization before the brazing material melts during the brazing process inhibit oxidation of the alloy surface. Hot workability is also improved, contributing to higher yield. Therefore, at least 0.0005% needs to be added. However, excessive B content leads to deterioration of hot workability and cracking during welding, so excessive addition must be avoided. Therefore, the upper limit is set at 0.0050%. The preferred lower limit of the content is 0.0007%, a more preferred lower limit is 0.0008%, a preferred upper limit is 0.0035%, and a more preferred upper limit is 0.0032%.

[0050] O: ≤35ppm O is an impurity element that inevitably mixes into the alloy during melting and deteriorates hot workability. Therefore, elements such as Si, Mn, and Al are added to the melt for deoxidation and should be reduced. In this invention, in order to suppress... s Phase precipitation limits the amount of these elements that can be added. Therefore, it is necessary to maintain the elements permitted in this invention and their addition amounts, in combination, to sufficiently reduce oxygen levels. Thus, the upper limit should be set at 35 ppm. A preferred upper limit is 33 ppm, and a more preferred upper limit is 30 ppm.

[0051] The balance of the high corrosion-resistant Ni-Cr-Mo-N alloy of the present invention, other than the aforementioned components, consists of Fe and unavoidable impurities. Here, "unavoidable impurities" refers to components that inevitably mix in during the industrial manufacture of stainless steel for various reasons, and refers to components that are permissible within a range that does not adversely affect the effectiveness of the present invention.

[0052] Next, the manufacturing method of the high corrosion-resistant Ni-Cr-Mo-N alloy involved in this invention will be described.

[0053] The manufacturing method of the alloy of the present invention is not particularly limited, but it is preferably manufactured by the following method. First, raw materials such as Ni alloy scraps, iron or stainless steel scraps, ferrochrome alloys, ferronickel alloys, pure nickel, and metallic chromium are melted in an electric furnace. Then, in an AOD furnace or VOD furnace, while decarburizing and refining by blowing oxygen and argon, quicklime, fluorite, Al, Si, etc. are added for desulfurization and deoxidation treatment. The composition of the slag in this treatment is preferably adjusted to a CaO-Al2O3-SiO2-MgO-F system. In addition, in order to effectively desulfurize, the slag preferably satisfies CaO / Al2O3≥2 and CaO / SiO2≥3. In addition, the refractory material of the AOD furnace or VOD furnace is preferably a magnesium-chromium alloy or dolomite. After refining in the above-mentioned AOD furnace, etc., the composition and temperature are adjusted by the LF process, and then continuous casting is performed to manufacture slabs. Then, hot rolling is performed, and cold rolling is performed as needed to produce thick plates or thin plates such as hot-rolled alloy plates and cold-rolled alloy plates. Example

[0054] The present invention will now be described in more detail through examples. However, the present invention is not limited to these examples without departing from its spirit.

[0055] First, pure iron, pure Ni, pure Cr, and pure Mo were melted in a 500kg vacuum melting furnace and directly cast into molds in a vacuum to produce alloy ingots (samples 1-4). These were then hot-forged to obtain alloy plates with a thickness of 8mm. In addition, raw materials such as iron filings, stainless steel filings, and ferrochrome alloys were melted in a 60-ton electric furnace (samples 5-28). Then, in the AOD process, oxygen and argon were blown in for decarburization and refining. Next, quicklime, fluorite, Al, and Si were added for desulfurization and deoxidation. Finally, ingots were cast using a continuous casting machine to obtain slabs. The chemical composition is shown in Table 1. It should be noted that chemical components other than C, S, and N were analyzed using fluorescence X-ray analysis. Furthermore, N was analyzed using an inert gas-pulse heating melting method, and C and S were analyzed using an oxygen stream combustion-infrared absorption method. It should be noted that "---" in the table indicates no intentional addition. Although B and O were not intentionally added, they were analyzed. The values ​​in the table are the results; an analysis value recorded as 0.0000 indicates that it is below the analysis limit.

[0056] Then, the slab was hot-rolled according to conventional methods to obtain a hot-rolled alloy sheet with a thickness of 8.0 mm. Next, after solution heat treatment, the hot-rolled alloy sheet was cold-rolled, followed by final annealing and pickling to obtain a cold-rolled strip with a thickness of 2.0 mm. The solution heat treatment was performed by holding at 1150°C for 1 minute followed by water cooling. Then, the cold-rolled strip was subjected to aging heat treatment at 950°C for 30 minutes. The aging heat-treated material was then subjected to the method described below. s Quantitative evaluation of phases.

[0057] < s Determination of phase area ratio > For plates that have undergone the above heat treatment, s Determination of phase area ratio. Details are as follows, the same as in Experiment 1.

[0058] • Sample collection direction: Samples should be collected from a direction perpendicular to the rolling direction; • Electrolytic grinding apparatus: Struers Co., Ltd., “Tenupol-5”; • EBSD determination: Backscattered electron diffraction apparatus (TSL SOLUTIONS Co., Ltd., “EBSD analysis software OIM Analysis 7.3”) attached to a field emission scanning electron microscope (JEOL Ltd., “JSM-7001F”); • Measurement area: 80 m m×240 m m; • Step size: 0.2 m m.

[0059] < s Phase particle size > s The particle size of the phase was the same as in Experiment 1, and the size was determined based on the composition image obtained by scanning electron microscopy at 5000x magnification.

[0060] <Corrosion Resistance Evaluation Test> For plates that have undergone solution heat treatment and those that have undergone aging heat treatment, an immersion test in ferric chloride solution as specified in ASTM G48 (Method D) is performed under the following conditions to determine the critical crevice corrosion initiation temperature (CCT). The degree of reduction in corrosion resistance is compared based on the difference between the two. In the evaluation, materials in the same state as solution heat-treated materials are rated ◎ if they reach the CCT, 0 if the CCT decreases by 5°C, △ if the CCT decreases by 10°C, and × if the CCT decreases by more than 15°C. Furthermore, solution heat-treated materials with a CCT below 35°C are rated ×× due to their low corrosion resistance, regardless of the test results for aging heat-treated materials.

[0061] • Test piece: 25mm width × 50mm length × 2mm thickness; • Test solution: 6% FeCl3 + 1% HCl aqueous solution; • Test solution volume: 600ml per test piece; • Surface grinding: Perform wet grinding on the entire surface using #120 SiC polishing paper; • Test duration: 100 hours; • Gap forming clamp: Multiclevis (マルチクレビス) made of Teflon (registered trademark), tightening torque 0.28 N·m.

[0062] <Evaluation of the wettability of brazing materials> Grinding was performed during solution heat treatment, followed by assembly by welding to create T-shaped test pieces. Evaluation was based on the extension of the brazed material's "wetting length" after brazing. Details are as follows: Brazing was performed in a 12m bridge brazing furnace in an atmosphere of 100% hydrogen, with the furnace saturation temperature set at 1020°C. The linear velocity was 1m / min. Extensions exceeding 80mm were rated ◎, those exceeding 70mm but less than 80mm were rated 〇, and those exceeding 60mm but less than 70mm were rated △. Extensions less than 60mm were rated ×.

[0063] • Test pieces: (1) 10mm width × 100mm length × 2mm thickness, (2) 20mm width × 100mm length × 2mm thickness; • Surface grinding: Perform wet grinding on the entire surface using #120 SiC polishing paper; Assembly: By spot welding, (1) is vertically erected and fixed at the center of the width of (2); • Brazing material: Nickel brazing material BNi-7 (14Cr-10P-Ni); • Brazing material coating amount: 0.5g at one end, applied in granular form to the corner of plate (1).

[0064] <Evaluation of Non-metallic Inclusions> Test pieces were cut from 2 mm cold-rolled sheets that had undergone solution heat treatment, and cleanliness was determined according to JIS G0555 (2003). The cleanliness was evaluated by the "total cleanliness" of B-series and C-series inclusions, represented by Al2O3 and MnO·SiO2C, respectively. A cleanliness level below 0.05% was rated ◎, above 0.05% but below 0.20% was rated 〇, above 0.20% but below 0.40% was rated △, and above 0.40% was rated ×.

[0065] • Test piece: A 2mm thick section was cut from the plate, parallel to the direction of travel. The total observation area is 300mm². 2 ; • Refinement during evaluation: utilizing 1 m Mirror polishing with diamond polishing paste and polishing wheel with a particle size of m; • A point algorithm based on optical microscope observation with 16 vertical and horizontal grid lines. Observation magnification is 400x.

[0066] <Overall Evaluation> A comprehensive evaluation is conducted for the three evaluation criteria mentioned above: corrosion resistance, wettability, and non-metallic inclusions. The comprehensive evaluation is set as follows: ◎ = 3 points, 〇 = 2 points, △ = 1 point, × = 0 points, and the scores are summed. For the alloy of this invention, excellent corrosion resistance is the most important factor; therefore, regardless of other evaluation results, any evaluation with an × is rated as unacceptable. Furthermore, because corrosion resistance should be evaluated with greater emphasis than other items, the score is doubled for the comprehensive evaluation. In the corrosion resistance evaluation, any score without an × and exceeding 3 points but below 4 points is rated acceptable; exceeding 4 points but below 7 points is rated good; exceeding 7 points but below 9 points is rated excellent; and exceeding 9 points is rated superior.

[0067] [Table 1] Examples 1-7 of the invention satisfy the component range and formulas (1)-(3) specified in claim 1 of the present invention, and s The phase area ratio is good, so the overall evaluation is acceptable to excellent. Invention Examples 8 to 18 also meet the alloy composition specified in claim 2, so they fall into the range of good to excellent. Invention Examples 19 to 24 also meet the composition range of B and O and formula (4), so the overall evaluation is excellent to excellent. The corrosion resistance of Invention Example 19 is slightly poor, which is thought to be because, although O is within the range, it is as high as 31 ppm.

[0068] On the other hand, since Comparative Example 25 does not satisfy Equation (1), Comparative Example 26 does not satisfy Equation (3), Comparative Example 27 does not satisfy Equation (2-1), and Comparative Example 28 does not satisfy Equation (2-2), their corrosion resistance is poor.

[0069] In Comparative Example 29, since Cr is outside the range and Mn is also low, although Equations (1), (2-1), (2-2), and (3) are satisfied, Cr nitrides are precipitated, resulting in poor corrosion resistance. In addition, wettability may also be poor due to the obstruction caused by the precipitated nitrides.

Claims

1. A Ni-Cr-Mo-N alloy, comprising, by mass percent, C: 0.001~0.030%, Si: 0.02~0.30%, Mn: 0.28~0.40%, P: 0.005~0.050%, S: 0.0001~0.0030%, Ni: 22.0~less than 33.4%, Cr: 22.0~28.0%, Mo: 5.0~8.0%, Cu: 0.03~0.50%, N: 0.180~0.250%, Al: 0.024~0.100%, with the balance being Fe and unavoidable impurities, characterized in that, The microstructure of the cross-section after holding at 950℃ for 30 min satisfies the following equations (1) to (3), with B being 0.0005~0.0050% and O being ≤30ppm, as determined by EBSD. σ The phase area ratio is below 1.0%. Cr+3.3×Mo+16×N≥43.0…(1) 7.2×Mo-Ni≤21.0 …(2-1) 1.3×Cr-Ni≤5.7 …(2-2) 0.20≤1.6×Si+0.99×Mn+2.2×Al≤0.95 …(3).

2. A Ni-Cr-Mo-N alloy, characterized in that, It is an alloy containing, by mass percent, the following components: C: 0.001–0.030%, Si: 0.02–0.30%, Mn: 0.28–0.40%, P: 0.005–0.050%, S: 0.0001–0.0030%, Ni: 33.4–38.0%, Cr: 22.0–28.0%, Mo: 5.0–8.0%, Cu: 0.03–0.50%, N: 0.180–0.250%, Al: 0.024–0.100%, with the balance being Fe and unavoidable impurities. The microstructure of the cross-section after holding at 950℃ for 30 min satisfies the following equations (1) to (3), with B being 0.0005~0.0050% and O being ≤30ppm, as determined by EBSD. σ The phase area ratio is below 1.0%. Cr+3.3×Mo+16×N≥43.0…(1) 7.2×Mo-Ni≤21.0 …(2-1) 1.3×Cr-Ni≤5.7 …(2-2) 0.20≤1.6×Si+0.99×Mn+2.2×Al≤0.95 …(3).

3. A highly corrosion-resistant component, characterized in that, The alloy described in claim 1 or 2 is brazed using Ni brazing material at a temperature above 1000°C in an inert gas atmosphere.

4. A sheathed heater, characterized in that, A tube having a cladding made of the alloy of claim 1 or 2, having a joint formed of Ni brazing material.

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