Manufacturing method of austenitic stainless steel strip

Abstract

A method for manufacturing an austenitic stainless steel strip includes: hot rolling a raw material for hot rolling, which has the following composition by mass percent: Ni: more than 20.0% and less than 30.0%, Cr: more than 15.0% and less than 18.0%, Mo: 1.0% to 2.0%, Al: more than 3.5% and less than 5.0%, Nb+Ta: more than 1.0% and less than 2.0%, Ti+V: less than 0.3%, Si The steel strip contains less than 1.0% Mn, less than 2.0% Zr, 0.01% to 0.3% C, 0.005% to 0.045% B, and the total content of Y, La, Ce, and Hf is within the range of 0.01% to 0.5%, with the remainder being Fe and unavoidable impurities; the steel strip is cold rolled; and the steel strip is heated and held before being rapidly cooled.

Description

Technical Field

[0001] This invention relates to a method for manufacturing austenitic stainless steel strip. Background Technology

[0002] Austenitic stainless steels, primarily composed of Fe, Cr, and Ni, possess a stable austenitic structure from low to high temperatures, making them suitable for various applications requiring corrosion resistance and high-temperature strength. When used at high temperatures, not only high-temperature strength but also resistance to oxidation in oxidizing environments is crucial. Typical austenitic stainless steels contain approximately 16% or more Cr, resulting in the formation of a protective Cr oxide film (Cr₂O₃) on the surface in oxidizing environments up to approximately 700°C, thus providing oxidation resistance.

[0003] On the other hand, since Al oxide films are more stable than Cr oxide films at higher temperatures, austenitic stainless steels with better oxidation resistance have been proposed, for example, by forming a protective Al oxide film containing Al2O3 on the surface of steel by containing more than 2% Al. For example, Patent Document 1 discloses an austenitic stainless steel with high creep strength of Nb, Ta, and Al and good oxidation resistance. Patent Document 2 discloses an Al-containing austenitic stainless steel with oxidation resistance and high creep strength. Patent Document 3 discloses an Al-containing austenitic stainless steel with high Mn content. Regarding the manufacturing method, Non-Patent Document 1 discloses that experimental molten material (500g) of alumina-formed austenitic stainless steel is heated and held at 1200°C to 1250°C for 0.5 hours to 2 hours, followed by water cooling, thereby controlling the crystal grain size to 40μm to 340μm. Furthermore, Non-Patent Document 2 discloses that, in order to control the grain size of alumina-formed austenitic stainless steel, which is obtained by hot rolling or cold rolling an experimental molten material (12.7mm × 12.7mm × 76.2mm) at 1150°C to 20μm to 50μm, it is heated to 1200°C. In addition, Non-Patent Document 3 discloses that, after heating 15kg of experimental alumina-formed austenitic stainless steel produced by vacuum melting in a natural gas environment at 1093°C for 4 hours, it is hot-forged, then heated in a natural gas environment at 1093°C for 1.5 hours, hot-rolled, and then held at 1200°C for 0.25 to 0.5 hours, followed by water cooling, a nominal grain size of 50μm is obtained.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: US Patent No. 7,754,144

[0007] Patent Document 2: US Patent No. 7744813

[0008] Patent Document 3: US Patent No. 7,754,305

[0009] Non-patent literature

[0010] Non-Patent Literature 1: Oxidation of Metals (2009) 72, pp. 311-333

[0011] Non-patent literature 2: Metallurgical Transactions, A38A (2007), pp. 2737-2746

[0012] Non-patent literature 3: Materials Science and Engineering, A 590 (2014), pp. 101-115 Summary of the Invention

[0013] The problem that the invention aims to solve

[0014] Non-Patent Documents 1 to 3 describe manufacturing methods and the crystal grain size obtained by these methods, but the final heat treatment temperature determining the crystal grain size is 1200°C or higher. Crystal grain size is a microstructure factor that significantly affects creep strength; to obtain high creep strength, it is necessary to increase the crystal grain size. Therefore, it is considered necessary to set the final heat treatment temperature of the austenitic stainless steel disclosed in Non-Patent Documents 1 to 3 to 1200°C or higher. However, in mass production equipment for steel strips, manufacturing at a final heat treatment temperature of 1200°C or higher can be constrained or difficult. Furthermore, while Patent Documents 1 to 3 describe the chemical composition and microstructure of high-Al austenitic stainless steels with various chemical compositions, they do not describe manufacturing methods. Although it is believed that there is a close causal relationship between chemical composition, microstructure, properties, and manufacturing methods, the optimal manufacturing method for austenitic stainless steel with various chemical compositions is not clearly defined, leaving room for further research.

[0015] The purpose of this invention is to provide a method for manufacturing austenitic stainless steel strip, which has properties equivalent to the creep strength and oxidation resistance of existing high-Al austenitic stainless steel, and includes industrially applicable low-temperature final heat treatment conditions.

[0016] Technical means to solve the problem

[0017] The inventors have studied the chemical composition and manufacturing methods of existing high-Al austenitic stainless steels, especially the reduction of the final heat treatment temperature. The results show that when the C content is adjusted to a low level while maintaining high Cr and Al content which will help improve oxidation resistance, a large crystal grain size and high creep strength can be obtained, and a final heat treatment temperature below 1200°C can be achieved, thus realizing the present invention.

[0018] That is, the present invention is a method for manufacturing austenitic stainless steel strip, comprising: a hot rolling process, wherein the hot rolling raw material has the following composition by mass%: Ni: more than 20.0% and less than 30.0%, Cr: more than 15.0% and less than 18.0%, Mo: 1.0% to 2.0%, Al: more than 3.5% and less than 5.0%, Nb+Ta: more than 1.0% and less than 2.0%, Ti+V: less than 0.3%, Si: less than 1.0%, Mn: less than 2.0%, Zr: 0.01% to 0.3%, C: 0.005% to 0.045%, B: 0. The austenitic stainless steel strip comprises 0.001% to 0.03%, and as needed, includes one or more of Y, La, Ce, and Hf in the range of Y+La+Ce+Hf+Zr: 0.01% to 0.5%, with the remainder being Fe and unavoidable impurities; a cold rolling process, in which the hot-rolled steel strip after the hot rolling process is cold-rolled; and a melt treatment process, in which the cold-rolled steel strip after the cold rolling process is heated and held at 1000°C to 1150°C for 0.1 minutes to 30 minutes in a substantially nitrogen-free non-oxidizing environment, and then rapidly cooled at a cooling rate of 5°C / s or more, thereby obtaining an austenitic stainless steel strip with a width of 120 mm or more and a thickness of 3 mm or less.

[0019] The preferred austenite stainless steel strip has an average austenite grain size of 30μm to 100μm.

[0020] Preferably, a grinding process is included between the hot rolling process and the cold rolling process, or during the cold rolling process, wherein the grinding process removes the oxide layer and nitrided layer on the surface of the rolled steel strip.

[0021] The effects of the invention

[0022] According to the present invention, the industrial-scale manufacturability of austenitic stainless steel, which combines high creep strength with good oxidation resistance, can be significantly improved. Attached Figure Description

[0023] Figure 1 This is a graph showing the weight gain due to oxidation when the austenitic stainless steel strips of the present invention and comparative examples are heated at 1000°C for 1000 hours.

[0024] Figure 2 (a) is a cross-sectional electron image of the austenitic stainless steel strip of the present invention after being heated at 1000°C for 1000 hours. Figure 2 (b) shows the surface analysis results of Fe obtained by electron beam microanalyzer. Figure 2 (c) shows the surface analysis results of Al obtained by electron beam microanalyzer. Figure 2 (d) is the surface analysis result of O obtained by electron beam microanalyzer.

[0025] Explanation of symbols

[0026] 1: Ni coating

[0027] 2: Oxide film

[0028] 3: Metal matrix Detailed Implementation

[0029] Embodiments related to the manufacturing method of the austenitic stainless steel strip of the present invention will be described. Furthermore, the term "steel strip" in the present invention also includes steel plates produced by cutting the steel strip. First, in the present invention, a hot-rolled raw material having the composition shown below is prepared. The hot-rolled raw material can be produced using an industrially applicable melting method, such as arc melting in the atmosphere, high-frequency induction melting followed by secondary ladle melting, or induction melting in a vacuum. The obtained ingot is preferably subjected to a homogenization heat treatment at 1150°C to 1200°C for 1 hour to 100 hours to reduce compositional segregation, thereby producing a raw material for thermoplastic processing. Then, thermoplastic processing is performed by hot segment forging or hot segment rolling, thereby producing the hot-rolled raw material.

[0030] Next, the rationale for limiting the composition of the raw materials for hot rolling as specified in this invention will be explained. Furthermore, the content of each element is expressed as a percentage by mass.

[0031] <Ni: More than 20.0% but less than 30.0%>

[0032] Ni is an important element for stabilizing the austenite phase, which forms the matrix structure, in austenitic stainless steels. Furthermore, it is an important element for improving high-temperature strength by precipitating fine intermetallic compounds (NiAl) into the austenite matrix along with Al. Ni can be added in a balance with the amount of Cr, which contributes good corrosion resistance and oxidation resistance in austenitic stainless steels. In the case of the steel strip used in this invention, when Ni is 20.0% or less, the austenite phase becomes unstable and may form a ferrite phase. On the other hand, even if Ni is added to more than 30.0%, no improvement in effect can be expected, and it leads to increased costs. Therefore, Ni is set to be more than 20.0% and less than 30.0%. A preferred lower limit for Ni is 23.0%, and a preferred upper limit is 27.0%. A more preferred lower limit for Ni is 24.0%, and a more preferred upper limit is 26.0%.

[0033] <Cr: More than 15.0% but less than 18.0%>

[0034] Cr is an important element in austenitic stainless steels that contributes to corrosion resistance and oxidation resistance. When the Cr content is below 15.0%, sufficient oxidation resistance may not be achieved. On the other hand, if more than 18.0% is added, ferrite phases and σ phases may form, reducing oxidation resistance and mechanical properties. Therefore, the Cr content is set between 15.0% and 18.0%. A preferred upper limit for Cr is 17.0%, and a more preferred upper limit is 16.0%.

[0035] <Mo: 1.0%~2.0%>

[0036] Mo is an element that improves the mechanical properties and corrosion resistance of austenitic stainless steel by being dissolved in the austenitic phase of the matrix. If Mo is less than 1.0%, the improvement in mechanical properties and corrosion resistance is minimal. On the other hand, if the addition exceeds 2.0%, ferrite and σ phases are easily formed, which may reduce mechanical properties, corrosion resistance, and oxidation resistance. Therefore, the Mo content is set at 1.0% to 2.0%, with a preferred upper limit of 1.5%.

[0037] <Al: 3.5% or more but less than 5.0%>

[0038] Al is an essential element for preferentially forming a dense, protective oxide film (Al₂O₃) on the surface of steel strip in high-temperature oxidizing environments to obtain good oxidation resistance. Additionally, it is an important element for improving high-temperature strength by finely precipitating as a mesometallic compound (NiAl) into the austenitic phase of the matrix when used at high temperatures. If Al content is less than 3.5%, it is difficult to form a dense oxide film, thus potentially resulting in insufficient oxidation resistance. On the other hand, if more than 5.0% is added, there is a possibility of easy formation of a ferrite phase or excessive precipitation of mesometallic compounds, leading to deterioration of plastic workability. Therefore, Al content is set at 3.5% or more and less than 5.0%. The preferred lower limit for Al is 4.0%. Furthermore, the preferred upper limit for Al is 4.5%.

[0039] <Nb+Ta: More than 1.0% but less than 2.0%>

[0040] Nitrogen (Nb) is an important element for improving the oxidation resistance and creep strength of high-Al austenitic stainless steels. Nb enhances oxidation resistance by facilitating the formation of a dense Al oxide film on the steel strip surface and improves creep strength by precipitating Fe₂Nb, NbC, etc. Some or all of Nb can also be replaced by Ta. When the Nb+Ta content is below 1.0%, the improvement in oxidation resistance and creep strength is minimal. On the other hand, if the addition exceeds 2.0%, a large amount of coarse precipitates such as Fe₂Nb and NbC will be precipitated, potentially impairing hot workability. Therefore, the Nb+Ta content is set between 1.0% and 2.0%. The preferred lower limit for Nb+Ta is 1.3%, and the preferred upper limit is 1.9%.

[0041] <Ti+V: 0.3% or less (including 0%)>

[0042] Ti and / or V are elements that, like Nb and Ta, enhance creep strength by precipitating MC-type carbides, and may include one or both of these. If the necessary amounts of Nb and / or Ta have already been added, Ti and V may not be necessary and can be omitted. On the other hand, if Ti+V exceeds 0.3%, it may impair oxidation resistance and hot workability; therefore, Ti+V is set to 0.3% or less (including 0%).

[0043] <Si: 1.0% or less (including 0%), Mn: 2.0% or less (including 0%)>

[0044] Si and Mn can be added as deoxidizing elements, but they may not be necessary or can be omitted in applications involving induction melting in a vacuum. Even if more than 1.0% Si or more than 2.0% Mn are added, there is no further effect. Therefore, Si is set to 1.0% or less (including 0%) and Mn is set to 2.0% or less (including 0%).

[0045] <Zr: 0.01%~0.3%>

[0046] Zr is an important element for improving oxidation resistance by enhancing the adhesion of the Al oxide film formed on the surface of austenitic stainless steel strips. If Zr content is less than 0.01%, sufficient effect cannot be obtained; conversely, even adding more than 0.3% does not yield further improvement and may even increase the amount of Zr-containing MC-type carbides, reducing hot workability. Therefore, the Zr content is set between 0.01% and 0.3%. The preferred lower limit for Zr is 0.03%, and the preferred upper limit is 0.2%.

[0047] <C: 0.005%~0.045%>

[0048] C is an element that not only stabilizes the austenite phase, which forms the matrix structure, but also enhances the creep strength primarily by forming MC-type carbides with Nb. If C is less than 0.005%, sufficient effect cannot be obtained; on the other hand, if more than 0.045% is added, coarse MC-type carbides will precipitate in large quantities, reducing hot workability. Furthermore, the final melt treatment temperature used to solidify the MC-type carbides to increase the crystal size is increased, making it difficult to perform melt treatment at the low temperatures typically applicable in industry, resulting in a decrease in crystal size and a reduction in creep strength. Therefore, C is set to 0.005% to 0.045%. A preferred lower limit for C is 0.01%, and a preferred upper limit is 0.04%. A more preferred lower limit for C is 0.02%, and a more preferred upper limit is 0.035%.

[0049] <B: 0.001%~0.03%>

[0050] Boron (B) is an element that enhances the latent strength of austenitic stainless steel by segregating at the grain boundaries of austenite grains, thereby increasing grain boundary strength. If B is less than 0.001%, the effect is insufficient; conversely, if more than 0.03% is added, it reacts with alloying elements to form coarse borides, which not only fails to achieve the grain boundary strengthening effect but may also reduce hot workability. Therefore, B is set at 0.001% to 0.03%. The preferred lower limit for B is 0.005%, and the preferred upper limit is 0.02%.

[0051] <Y+La+Ce+Hf+Zr is 0.01% to 0.5% of one or more of Y, La, Ce, and Hf>

[0052] Y, La, Ce, and Hf are elements that improve oxidation resistance by enhancing the adhesion of the Al oxide film formed on the surface of austenitic stainless steel strips, and can be added together with Zr as needed. Since they are added together with Zr, the formula Y+La+Ce+Hf+Zr is specified. If the content of Y+La+Ce+Hf+Zr is less than 0.01%, a sufficient effect on improving oxidation resistance cannot be obtained. On the other hand, if the content exceeds 0.5%, a large amount of oxides and other inclusions will be formed, which may reduce hot workability and cold workability. Therefore, one or more of Y, La, Ce, and Hf, calculated as Y+La+Ce+Hf+Zr, is set at 0.01% to 0.5%.

[0053] <Remaining portion: Fe and unavoidable impurities>

[0054] The remaining portion is set as Fe, the basic constituent element of austenitic stainless steel, and of course also includes impurities. For example, if W, Cu, N, P, and S are below 1.0%, Cu below 0.5%, N below 0.03%, P below 0.040%, and S below 0.01%, then there is no particularly significant harmful effect.

[0055] Next, the reasons for limiting the manufacturing method will be explained.

[0056] <Hot Rolling Process>

[0057] In this invention, a process is performed to obtain hot-rolled steel strip by hot-rolling a raw material having the above-mentioned composition. Hot rolling is carried out by heating the raw material to a temperature that ensures hot workability and passing it through a hot rolling mill. For the purpose of solidifying and softening carbides and intermetallic compounds containing Nb, Al, Ni, etc., to ensure good hot workability, a preferred hot rolling starting temperature is 1100°C or higher. More preferably, it is 1130°C or higher. Furthermore, the upper limit of the preferred hot rolling starting temperature is less than 1200°C, which would significantly reduce grain boundary strength and cause cracking.

[0058] <Cold rolling process>

[0059] The hot-rolled steel strip is cold-rolled in a cold rolling mill to apply the cold working distortion required for further thickness reduction, high-precision dimensional adjustment, and recrystallization and grain growth through a subsequent melt treatment process, resulting in a cold-rolled steel strip with a width of 120 mm or more and a thickness of 3 mm or less. Preferably, the width of the cold-rolled steel strip is 150 mm or more, and more preferably 200 mm or more. Furthermore, the thickness of the cold-rolled steel strip is preferably 2.8 mm or less, and more preferably 2.6 mm or less. Pickling may be performed before entering the cold rolling process to substantially remove the surface oxide and nitride layers formed during hot rolling. Additionally, after the hot rolling process and / or during multiple cold rolling processes, annealing may be performed once or more to soften the steel strip in order to obtain good cold-rollability. Annealing is preferably performed in a substantially nitrogen-free, non-oxidizing gas environment to prevent the formation of an Al oxide layer and / or an Al nitride layer on the surface of the rolled steel strip.

[0060] <Melting Process>

[0061] The melt treatment process involves heating the cold-rolled steel strip after the cold rolling process to a high temperature and then rapidly cooling it to promote the solidification of alloying elements. Through recrystallization and grain growth, a relatively coarse grain size is obtained, which is necessary for achieving high creep strength. The process also softens the steel strip to facilitate part forming and welding. It is a necessary and important final heat treatment process for this steel strip. Regarding the environment for the melt treatment, to suppress the formation of oxide and / or nitride layers on the steel strip surface due to oxidation, it is designed to be carried out in a substantially nitrogen-free, non-oxidizing environment. The ambient gas is preferably a reducing gas or an inert gas such as hydrogen or argon. By using steel strip with this composition, the grain size can be coarsened and adjusted through recrystallization and grain growth at low temperatures. Therefore, melt treatment can be performed at low temperatures within the range where heat treatment can be carried out in conventional manufacturing equipment. If the melting treatment temperature is below 1000°C, the solidification of alloying elements becomes insufficient, leaving carbides and intermetallic compounds, and the hardness does not decrease sufficiently. Furthermore, due to insufficient recrystallization and grain growth, the desired large grain size cannot be obtained. On the other hand, if the temperature exceeds 1150°C, the grain size becomes too large, potentially reducing tensile ductility and impact toughness. Therefore, the melting treatment temperature is set between 1000°C and 1150°C. A preferred lower limit temperature for melting treatment is 1050°C. A preferred upper limit temperature is 1130°C. Continuous furnaces are often used in the melting treatment of cold-rolled steel strips, and the heating and holding time is relatively short. The heating and holding time tends to be shorter for thinner sheets and longer for thicker sheets, but it can be determined based on indicators such as the solidification of alloying elements, the degree of hardness reduction, and the degree of grain size growth. If the heating holding time is less than 0.1 minutes, sufficient effect cannot be obtained; on the other hand, even if it is longer than 30 minutes, further effect is difficult to obtain. Therefore, the heating holding time is set to 0.1 minutes to 30 minutes. Preferably, the upper limit of the heating holding time is 10 minutes. In addition, if the desired microstructure cannot be obtained by a single melt treatment due to equipment limitations, multiple melt treatments can be performed. To maintain the solidified state, rapid cooling is performed during the cooling process after melt treatment. Cooling methods such as water cooling, oil cooling, and air cooling are not particularly limited. If the cooling rate is slower than 5°C / s, the already solidified alloying elements may precipitate again during cooling, increasing hardness or decreasing oxidation resistance. Therefore, the cooling rate is set to 5°C / s or higher. Preferably, the cooling rate is 7.5°C / s or higher.

[0062] The average austenite grain size of the austenitic stainless steel strip after the melt treatment process significantly affects its creep strength. To obtain high creep strength, a relatively coarse grain size is required. The grain size can be mainly controlled by the final melt treatment conditions. In the case of the austenitic stainless steel strip of this invention, it can be controlled to an appropriate range through these melt treatment conditions. If the average austenite grain size is less than 30 μm, sufficient creep strength cannot be obtained. On the other hand, if it is greater than 100 μm, tensile ductility and impact toughness may decrease. Therefore, it is set to 30 μm to 100 μm. Preferably, the lower limit of the average austenite grain size is 40 μm. Furthermore, the upper limit of the average austenite grain size is preferably 80 μm.

[0063] <Grinding Process>

[0064] Because the austenitic stainless steel strip of the present invention contains a large amount of Al, it is easy to form a dense oxide layer containing Al oxides and / or a needle-like nitride layer containing Al nitrides on the surface of the steel strip through heat treatment in the atmosphere, hot rolling, etc. If the steel strip is cold-worked by cold rolling until the final melt treatment process is completed while the Al oxide layer and Al nitride layer remain on the surface of the steel strip, an uneven Al oxide layer and Al nitride layer will remain on the surface of the steel strip of the final product, and therefore it is difficult to obtain good oxidation resistance stably. Therefore, it is preferable to remove the oxide layer and nitride layer on the surface of the rolled material (steel strip). The removal method is not limited as long as the Al oxide layer and Al nitride layer remaining on the surface of the rolled material can be completely removed. Since the Al oxide layer and Al nitride layer are chemically stable, it is difficult to completely remove them by chemical removal methods, such as pickling, and it is difficult to obtain a uniform metal surface layer, but this does not preclude the application of the pickling process before cold rolling. On the other hand, mechanical removal methods, such as grinding, can remove a certain thickness and are easy to remove completely. Therefore, grinding is the preferred method for removing the oxide and nitride layers from the surface of rolled materials to obtain a metallic luster. Since it is sufficient to completely remove the oxide and nitride layers from the surface of rolled materials before the final melt heat treatment, the grinding process can be any process between the hot rolling and cold rolling processes, or within the cold rolling process.

[0065] Example

[0066] Using ingots melted and cast by vacuum induction melting, hot-rolled raw materials with a thickness of approximately 45 mm and a width of approximately 330 mm were prepared through homogenization heat treatment, hot forging, and hot rolling. The chemical composition of the hot-rolled raw materials is shown in Table 1. Here, No. 1 is the hot-rolled raw material of the present invention example, and No. 2 is the hot-rolled raw material of the comparative example. These hot-rolled raw materials were heated to 1150°C and then hot-rolled to produce hot-rolled steel strips with a thickness of 3 mm. Here, the degree of surface damage of hot-rolled raw materials No. 1 and No. 2 during the hot forging and hot rolling processes was confirmed. The results showed that, compared with No. 2, hot-rolled raw material No. 1 was better able to suppress the generation of surface damage and had good hot workability. Subsequently, a grinding process for removing the Al oxide layer and Al nitride layer on the surface of the steel strip was performed midway through the cold rolling process. Based on this, several cold rolling and annealing processes were repeated to produce cold-rolled steel strips with various thicknesses from 0.2 mm to 1.5 mm and a width of approximately 250 mm. Furthermore, the obtained cold-rolled steel strip was subjected to a melt treatment involving heating in a continuous furnace at 1100°C in a hydrogen environment for approximately 1 to 5 minutes, followed by rapid cooling at a rate of 5°C / s or higher. This resulted in austenitic stainless steel strip No. 5 of the present invention, manufactured from hot-rolled raw material No. 1, and austenitic stainless steel strip No. 7 of the comparative example, manufactured from hot-rolled raw material No. 2.

[0067] Furthermore, as a typical example of austenitic stainless steel, hot-rolled raw materials with the composition shown in Table 2, approximately 30 mm thick and 120 mm wide, were prepared by melting and casting using vacuum induction melting. Here, No. 3 and No. 4 correspond to NCF800 steel and NCF625 steel as described in Japanese Industrial Standard (JIS) G 4902, respectively. These hot-rolled raw materials were repeatedly heated to 1100°C and then hot-rolled to produce hot-rolled steel strips with a thickness of approximately 3.5 mm. Subsequently, repeated cold rolling and annealing were performed to obtain cold-rolled steel strips with a thickness of 1.5 mm. Based on this, a melt treatment was performed by heating and holding at 1150°C for 30 minutes in a vacuum furnace followed by rapid cooling to obtain austenitic stainless steel strips No. 9 and No. 10.

[0068] [Table 1]

[0069] (quality%)

[0070]

[0071] (Note) Impurity elements: P: 0.003%–0.005%, S: 0.002%, W: <0.01%, C u :<0.01%, N: 0.004%~0.006%

[0072] [Table 2]

[0073] (quality%)

[0074] No. C Si Mn Ni Cr Mo Al Ti Nb Fe Remark 3 0.02 0.30 0.74 30.88 20.31 - 0.23 0.310 - The remaining part Existing examples 4 0.07 0.30 0.26 The remaining part 21.42 8.90 0.35 0.370 3.58 3.48 Existing examples

[0075] Note: "-" indicates no impurities added (level).

[0076] Test pieces (samples) were cut from 1.5 mm thick austenitic stainless steel strips of No. 5 and No. 7. The average austenite grain size was determined by optical microscopy observation of the microstructure at the longitudinal section. Tensile tests in the rolling direction were performed at room temperature and 850°C; creep fracture tests in the rolling direction were performed at 800°C, 850°C, and 900°C; and oxidation resistance tests were performed at 1000°C. In addition, test pieces cut from 1.5 mm thick cold-rolled steel strips were subjected to melt treatment by heating and holding in a hydrogen environment at 1150°C for 5 minutes, followed by quenching at a cooling rate of 5°C / s or more by air cooling. This yielded Sample No. 6 of the present invention, manufactured from hot-rolled raw material of No. 1, and Sample No. 8 of the comparative example, manufactured from hot-rolled raw material of No. 2. Similar to No. 5 and No. 7, the average austenite grain size was determined by optical microscopy observation of the longitudinal section, and tensile tests were conducted in the rolling direction at room temperature and 850°C; creep rupture tests were conducted in the rolling direction at 800°C, 850°C, and 900°C; and oxidation resistance tests were conducted at 1000°C. For the 1.5 mm thick austenitic stainless steel strips of No. 9 and No. 10, test pieces (samples) were cut, and only oxidation resistance tests were conducted at 1000°C. The average austenite grain size is shown in Table 3, the tensile test results are shown in Table 4, the creep rupture test results are shown in Table 5, and the oxidation resistance test results are shown in Table 6.

[0077] According to Table 3, the samples of the present invention, under any melt treatment temperature of 1100°C or 1150°C, formed coarse particles with an average austenite grain size of approximately 50 μm, which is most preferably preferred. In contrast, the samples of the comparative examples, under any melt treatment temperature of 1100°C or 1150°C, formed particles with an average austenite grain size finer than 30 μm. Thus, the manufacturing method of the present invention can obtain an appropriate average austenite grain size that easily exhibits high creep strength. Furthermore, according to Table 4, the samples of the present invention, under any melt treatment temperature of 1100°C or 1150°C, had lower 0.2% endurance and tensile strength at room temperature compared to the samples of the comparative examples, but their 0.2% endurance and tensile strength at 850°C, a high-temperature environment, were equivalent to those of the samples of the comparative examples. Furthermore, as shown in Table 5, the samples of the present invention exhibited longer creep rupture times and higher creep strengths compared to the comparative samples under either of the melt treatment temperatures of 1100°C or 1150°C. The high creep strength of the steel strip manufactured using the hot-rolling raw materials of the present invention and by the method of the present invention is due to the control of a coarse average austenite grain size, which improves creep rupture strength even under relatively low melt treatment conditions such as 1100°C and 1150°C.

[0078] [Table 3]

[0079]

[0080] [Table 4]

[0081]

[0082] [Table 5]

[0083]

[0084] In the oxidation resistance test, for test pieces (specimens) of No. 5 to No. 10 with dimensions of 15 mm (w) × 15 mm (l) × 1.5 mm (t), the surface was polished to #1000 using sandpaper. Subsequently, in the atmosphere, the polished test pieces were heat-treated at 1000 °C for 100 hours to 1000 hours, and the weights before and after oxidation were measured. The results are shown in Table 6. In the specimens of the existing examples of No. 9 and No. 10, which are general austenitic stainless steels that form Cr oxide films, the oxidation weight gain was large until 500 hours. In addition, in the specimen of No. 10 during the 1000-hour heating, the oxide film peeled off due to thermal stress during cooling, and the oxidation weight gain decreased. To promote the oxidation of the metal matrix, such peeling of the oxide film must be avoided. On the other hand, in the specimens of the example of the present invention, which is a high-Al austenitic stainless steel, and the comparative examples of No. 7 and No. 8, the oxidation weight gain until 1000 hours was small, and good oxidation resistance was confirmed. In addition, according to Figure 1 It was also confirmed that the oxidation weight gain of the test pieces of No. 5 to No. 8 followed the parabolic law, the oxide film did not peel off, and the oxidation behavior was stable.

[0085] A nickel plating was applied to test piece No. 5 after 1000 hours of heating, and surface analysis of Fe, Al, and O was performed on the metal matrix and the oxide film using an electron microanalyzer. The obtained photos are shown in Figure 2 (a) to Figure 2 (d). Figure 2 (a) is a photo showing the reflected electron image in the cross-section of the specimen, Figure 2 (b) to Figure 2 (d) are photos showing the surface analysis results of Fe, Al, and O in the same observation area as Figure 2 (a), respectively. The reflected electron image was compared with the surface analysis of each element, and as a result, it was confirmed that a protective Al oxide film containing Al2O3 was formed in the specimen of the example of the present invention.

[0086] As described above, the austenitic stainless steel strip obtained by the manufacturing method of the present invention has both high creep strength and good oxidation resistance, so it is expected to improve the reliability of parts of equipment used at high temperatures, such as heat treatment furnaces, heat exchangers, and solid oxide fuel cells.

[0087] [Table 6]

[0088]

Claims

1. A method for manufacturing austenitic stainless steel strip, characterized in that, include: The hot rolling process involves hot rolling raw materials for hot rolling, wherein the raw materials for hot rolling have the following composition by weight%. composition: Ni: More than 20.0% but less than 30.0% Cr: exceeding 15.0% but below 18.0% Mo: 1.0%~2.0%, Al: 3.5% or more but less than 5.0% Nb+Ta: Above 1.0% but below 2.0% Ti+V: below 0.3% Si: below 1.0% Mn: below 2.0% Zr：0.01%～0.3%、 C：0.005%～0.045%、 B：0.001%～0.03%， Furthermore, depending on the need, the range of Y+La+Ce+Hf+Zr: 0.01% to 0.5% may include one or more of Y, La, Ce, and Hf. The remainder consists of Fe and unavoidable impurities; The cold rolling process involves cold rolling the hot-rolled steel strip following the hot rolling process; and The melt treatment process involves heating and holding the cold-rolled steel strip after the cold rolling process in a nitrogen-free, non-oxidizing environment at 1000℃~1150℃ for 0.1 minutes to 30 minutes, followed by rapid cooling at a cooling rate of 5℃ / s or higher. The method for manufacturing austenitic stainless steel strips described herein yields austenitic stainless steel strips with a width of 120 mm or more and a thickness of 3 mm or less. The average austenite grain size of the austenitic stainless steel strip obtained after the melt treatment process is 30 μm to 100 μm.

2. The method for manufacturing austenitic stainless steel strip according to claim 1, characterized in that, A grinding process is provided between the hot rolling process and the cold rolling process, or a grinding process is provided during the cold rolling process, wherein the grinding process removes the oxide layer and nitriding layer on the surface of the rolled steel strip.

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