Manufacturing method for austenitic stainless steel strip

Abstract

Provided is a method for manufacturing an austenitic stainless steel strip having both of high creep strength and satisfactory oxidation resistance.　A method for manufacturing an austenitic stainless steel strip comprises: a hot rolling step for subjecting a material to be hot-rolled to a hot rolling procedure, in which the material to be hot-rolled has a component composition that contains, in % by mass, more than 20.0% and 30.0% or less of Ni, more than 15.0% and 18.0% or less of Cr, 1.0 to 2.0% of Mo, 3.5% or more and less than 5.0% of Al, more than 1.0% and 2.0% or less of Nb+Ta, 0.3% or less of Ti+V, 1.0% or less of Si, 2.0% or less of Mn, 0.01 to 0.3% of Zr, 0.005 to 0.045% of C, 0.001 to 0.03% of B, and also contains at least one element selected from Y, La, Ce and Hf in an amount such that the content of Y+La+Ce+Hf+Zr can become 0.01 to 0.5% with the remainder comprising Fe and unavoidable impurities; a cold rolling step for subjecting the hot-rolled steel strip to a cold rolling procedure; and a solution treatment step for heating the cold-rolled steel strip, then maintaining the heated steel strip at that temperature, and then subjecting the heated steel sheet to a rapid cooling procedure.

Description

[Technical Field]

[0001] The present invention relates to a method for producing an austenitic stainless steel strip. [Background technology]

[0002] Austenitic stainless steel is primarily composed of Fe, Cr, and Ni, and has an austenitic structure that is stable from low to high temperatures. It is therefore used in a variety of applications that require corrosion resistance and high-temperature strength. When used at high temperatures, not only high-temperature strength but also oxidation resistance in oxidizing atmospheres is required. Typical austenitic stainless steels contain approximately 16% or more Cr, and demonstrate oxidation resistance by forming a protective Cr oxide film made of Cr2O3 on their surface in oxidizing atmospheres at temperatures up to about 700°C.

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

[0004] [Patent Document 1] U.S. Patent No. 7,754,144 [Patent Document 2] U.S. Patent No. 7,744,813 [Patent Document 3] U.S. Patent No. 7,754,305 [Non-patent literature]

[0005] [Non-Patent Document 1] Oxidation of Metals (2009)72, p.311-333 [Non-patent document 2] Metallurgical Transactions A 38A (2007)p.2737-2746 [Non-patent document 3] Materials Science and Engineering A 590 (2014)p.101-115 Summary of the Invention [Problem to be solved by the invention]

[0006] The aforementioned Non-Patent Documents 1 to 3 describe manufacturing methods and the crystal grain sizes obtained by these methods, but the final heat treatment temperature, which determines the crystal grain size, is 1200°C or higher in all cases. Since crystal grain size is a structural factor that significantly affects creep strength, and a large crystal grain size is required to achieve high creep strength, it is believed that the final heat treatment temperature of the austenitic stainless steels disclosed in Non-Patent Documents 1 to 3 must be 1200°C or higher. However, final heat treatment at temperatures of 1200°C or higher may be difficult or impossible to perform in mass production facilities for steel strips. Furthermore, while Patent Documents 1 to 3 describe the chemical compositions and structures of high-Al austenitic stainless steels with various chemical compositions, they do not describe manufacturing methods. While a close causal relationship is believed to exist between chemical composition, structure, properties, and manufacturing methods, the optimal manufacturing method for austenitic stainless steels with each chemical composition is unknown, and room for further study remains. An object of the present invention is to provide a method for producing an austenitic stainless steel strip that has creep strength and oxidation resistance equivalent to those of existing high-Al austenitic stainless steels, and includes industrially applicable low-temperature final heat treatment conditions. [Means for solving the problem]

[0007] The inventors have studied the chemical composition and manufacturing methods of existing high-Al austenitic stainless steels, particularly the reduction in the final heat treatment temperature. As a result, they have discovered that there is a final heat treatment temperature lower than 1200°C at which a large grain size and high creep strength can be obtained by adjusting the C content to a low level while maintaining high levels of Cr and Al, which contribute to oxidation resistance. This discovery led to the present invention.

[0008] That is, the present invention provides, in mass%, Ni: more than 20.0% and not more than 30.0%, Cr: more than 15.0% and not more than 18.0%, Mo: 1.0 to 2.0%, Al: 3.5% or more and less than 5.0%, Nb + Ta: more than 1.0% and not more than 2.0%, Ti + V: 0.3% or less (including 0%), Si: 1.0% or less (including 0%), Mn: 2.0% or less (including 0%), Zr: 0.01 to 0.3%, C: 0.005 to 0.045%, B: 0.001 to 0.03%, and if necessary, one or more of Y, La, Ce, and Hf are added to form a Y + La + Ce + Hf + Zr: 0.01 to 0.03%. The method for producing an austenitic stainless steel strip includes: a hot rolling step of hot rolling a material for hot rolling having a chemical composition containing Cr in the range of 0.5%, with the remainder being Fe and unavoidable impurities; a cold rolling step of cold rolling the hot rolled steel strip after the hot rolling step; and a solution treatment step of heating and holding the cold rolled steel strip after the cold rolling step at 1000 to 1150°C in a non-oxidizing atmosphere containing substantially no nitrogen for 0.1 to 30 minutes, and then quenching it at a cooling rate of 5°C / s or more, thereby obtaining an austenitic stainless steel strip having a width of 120 mm or more and a thickness of 3 mm or less. Preferably, the austenitic stainless steel strip has an average austenite grain size of 30 to 100 μm. Preferably, the method further comprises a polishing step for removing an oxide layer and a nitride layer on the surface of the rolled steel strip between the hot rolling step and the cold rolling step or during the cold rolling step. [Effects of the Invention]

[0009] According to the present invention, it is possible to significantly improve the industrial-scale manufacturability of austenitic stainless steels that combine high creep strength and good oxidation resistance. [Brief explanation of the drawings]

[0010] [Figure 1] It is a graph showing the oxidation weight gain when the austenitic stainless steel strips of the present invention examples and comparative examples are heated at 1000 °C up to 1000 hours. [Figure 2] (a) It is a backscattered electron image of the cross-section after heating the austenitic stainless steel strip of the present invention example at 1000 °C for 1000 hours. (b) It is the surface analysis result of Fe by an electron beam microanalyzer. (c) It is the surface analysis result of Al by an electron beam microanalyzer. (d) It is the surface analysis result of O by an electron beam microanalyzer.

Embodiments for Carrying Out the Invention

[0011] Embodiments regarding the manufacturing method of the austenitic stainless steel strip of the present invention will be described. Note that the steel strip in the present invention includes steel plates manufactured by cutting the steel strip. First, in the present invention, a hot-rolling material having the following component composition is prepared. The hot-rolling material may be prepared by an industrially applicable melting method, for example, arc melting in the atmosphere, high-frequency induction melting and subsequent secondary refining outside the furnace, or induction melting in a vacuum. The obtained ingot is preferably subjected to a homogenization heat treatment at 1150 to 1200 °C for 1 to 100 hours to reduce component segregation and used as a hot-plastic working material. Further, hot plastic working is performed by hot cogging forging or hot cogging rolling, etc., to obtain a hot-rolling material.

[0012] Next, the reasons for limiting the components of the hot-rolling material defined in the present invention will be described. Note that the content of each element is in mass%. <Ni: Exceeding 20.0% and not exceeding 30.0%> Ni is an important element that stabilizes the austenite phase, which is the matrix phase, in austenitic stainless steel. It is also an important element that improves high-temperature strength by precipitating fine intermetallic compounds (NiAl) in the matrix austenite phase together with Al. Ni is added considering the balance with the amount of Cr, which provides good corrosion resistance and oxidation resistance in austenitic stainless steel. When used in the steel strip of the present invention, if Ni is 20.0% or less, the austenite phase becomes unstable and there is a risk of generating a ferrite phase. On the other hand, even if it is added in excess of 30.0%, no improvement effect can be expected, leading to an increase in cost. Therefore, Ni is set to exceed 20.0% and be 30.0% or less. The lower limit of preferable Ni is 23.0%, and the upper limit of preferable Ni is 27.0%. More preferably, the lower limit of Ni is 24.0%, and the upper limit of Ni is 26.0%.

[0013] <Cr: exceeding 15.0% and not exceeding 18.0%> Cr is an important element that contributes to corrosion resistance and oxidation resistance in austenitic stainless steel. If Cr is 15.0% or less, there is a risk that sufficient oxidation resistance cannot be obtained. On the other hand, if it is added in excess of 18.0%, there is a risk that a ferrite phase or σ phase will be generated, reducing oxidation resistance and mechanical properties. Therefore, Cr is set to exceed 15.0% and not exceed 18.0%. The upper limit of preferable Cr is 17.0%, and more preferably, the upper limit of Cr is 16.0%. <00...​​​​​​​​​Al is an element necessary to preferentially form a dense protective oxide film (Al2O3) on the surface of the steel strip in a high-temperature oxidation atmosphere to obtain good oxidation resistance. Also, it is an important element that precipitates finely as an intermetallic compound (NiAl) in the austenite phase of the matrix during use at high temperatures, improving the high-temperature strength. If Al is less than 3.5%, it becomes difficult to form a dense oxide film, so the oxidation resistance may be insufficient. On the other hand, if more than 5.0% is added, the ferrite phase is likely to be generated, or the intermetallic compound may precipitate excessively, deteriorating the plastic workability. Therefore, Al is set to be 3.5% or more and less than 5.0%. The lower limit of preferable Al is 4.0%. Also, the upper limit of preferable Al is 4.5%.

[0016] <Nb + Ta: More than 1.0% and less than or equal to 2.0%> Nb is an important element that improves the oxidation resistance and creep strength of high-Al austenitic stainless steel. Nb improves the oxidation resistance by assisting in the formation of a dense Al oxide film formed on the surface of the steel strip, and improves the creep strength by precipitating Fe2Nb, NbC, etc. Nb can also be partially or entirely replaced by Ta. If Nb + Ta is less than or equal to 1.0%, the effects of improving oxidation resistance and creep strength are small. On the other hand, if added in excess of 2.0%, a large number of coarse precipitates such as Fe2Nb and NbC will precipitate, possibly harming the hot workability. Therefore, Nb + Ta is set to be more than 1.0% and less than or equal to 2.0%. The lower limit of preferable Nb + Ta is 1.3%, and the upper limit of preferable Nb + Ta is 1.9%.

[0017] <Ti + V: 0.3% or less (including 0%)> Ti and / or V are elements that increase the creep strength by precipitating MC-type carbides, similar to Nb and Ta, and one or both of these can be included. When the required amount of Nb and / or Ta is added, Ti and V are not necessarily required and may be not added. On the other hand, if Ti + V exceeds 0.3%, it may harm the oxidation resistance and hot workability. Therefore, Ti + V is set to be 0.3% or less (including 0%).

[0018] <Si: 1.0% or less (including 0%), Mn: 2.0% or less (including 0%)> Si and Mn are added as deoxidizing elements, but when applying induction melting in a vacuum, it is not always necessary to add them, and they may not be added. Since adding more than 1.0% of Si or more than 2.0% of Mn does not have any further effect, Si is set to 1.0% or less (including 0%) and Mn is set to 2.0% or less (including 0%).

[0019] <Zr: 0.01 - 0.3%> Zr is an important element that improves oxidation resistance by improving the adhesion of the Al oxide film formed on the surface of the steel strip of austenitic stainless steel. If Zr is less than 0.01%, a sufficient effect cannot be obtained. On the other hand, adding more than 0.3% not only does not result in any further effect, but may increase the MC-type carbides containing Zr and reduce the hot workability. Therefore, Zr is set to 0.01 - 0.3%. The preferred lower limit of Zr is 0.03%, and the preferred upper limit of Zr is 0.2%.

[0020] <C: 0.005 - 0.045%> C not only stabilizes the austenite phase, which is the matrix, but also improves the creep strength mainly by forming MC-type carbides together with Nb. If C is less than 0.005%, a sufficient effect cannot be obtained. On the other hand, adding more than 0.045% not only precipitates a large amount of coarse MC-type carbides and reduces the hot workability, but also dissolves the MC-type carbides and raises the final solution treatment temperature to increase the crystal grain size, making it difficult to perform solution treatment at a low temperature that can be normally industrially applied, reducing the crystal grain size and lowering the creep strength. Therefore, C is set to 0.005 - 0.045%. The preferred lower limit of C is 0.01%, and the preferred upper limit of C is 0.04%. More preferably, the lower limit of C is 0.02%, and more preferably, the upper limit of C is 0.035%.

[0021] <B: 0.001 - 0.03%> B is an element that improves the creep strength in austenitic stainless steels by segregating at the grain boundaries of austenite crystal grains and enhancing the grain boundary strength. If the amount of B is less than 0.001%, the effect cannot be fully obtained. On the other hand, if it is added in an amount exceeding 0.03%, it reacts with alloying elements to form coarse borides, not only resulting in no grain boundary strengthening effect but also possibly reducing the hot workability. Therefore, the amount of B is set to 0.001 - 0.03%. The lower limit of preferable B is preferably 0.005%, and the upper limit of preferable B is preferably 0.02%.

[0022] <One or more of Y, La, Ce, Hf; Y+La+Ce+Hf+Zr: 0.01 - 0.5%> Y, La, Ce, and Hf are elements that improve the oxidation resistance by enhancing the adhesion of the Al oxide film formed on the surface of the steel strip of austenitic stainless steels, and are added as required together with Zr. Since they are added together with Zr, it is only necessary to define Y+La+Ce+Hf+Zr. If Y+La+Ce+Hf+Zr is less than 0.01%, a sufficient effect on improving the oxidation resistance cannot be obtained. On the other hand, if it is added in an amount exceeding 0.5%, there is a risk of forming many inclusions such as oxides, which may reduce the hot workability and cold workability. Therefore, one or more of Y, La, Ce, Hf are set to 0.01 - 0.5% in terms of Y+La+Ce+Hf+Zr.

[0023] <The balance: Fe and inevitable impurities> The balance is made up of Fe, which is the basic constituent element of austenitic stainless steels. Of course, impurities are included. For example, W, Cu, N, P, S, etc., as long as W: 1.0% or less, Cu: 0.5% or less, N: 0.03% or less, P: 0.040% or less, S: 0.01% or less, there is no particularly significant harmful effect.

[0024] Next, the reasons for limiting the manufacturing method will be described. <Hot rolling process> In the present invention, a process is carried out in which a hot-rolled material having the above-mentioned components is hot-rolled to obtain a hot-rolled steel strip. Hot rolling is carried out by heating the material to a temperature at which hot workability can be ensured and passing it through a hot rolling mill. A preferred hot-rolling start temperature is 1100°C or higher in order to soften carbides and intermetallic compounds consisting of Nb, Al, Ni, etc. as much as possible into solid solution and ensure good hot workability. More preferably, it is 1130°C or higher. The upper limit of the hot-rolling start temperature is preferably less than 1200°C, at which point grain boundary strength is significantly reduced, causing cracks.

[0025] <Cold rolling process> The hot-rolled steel strip is then cold-rolled through a cold rolling mill to further reduce its thickness and apply the cold working strain necessary for high-precision dimensional adjustment and subsequent solution treatment for recrystallization and grain growth, resulting in a cold-rolled steel strip with a width of 120 mm or more and a thickness of 3 mm or less. The width of the cold-rolled steel strip is preferably 150 mm or more, more preferably 200 mm or more. The thickness of the cold-rolled steel strip is preferably 2.8 mm or less, even more preferably 2.6 mm or less. Prior to the cold-rolling process, the steel strip may be pickled to roughly remove the surface oxide and nitride layers formed during hot rolling. After the hot-rolling process and / or during multiple cold-rolling processes, the steel strip may be annealed one or more times to soften it and achieve good cold-rollability. Annealing is preferably carried out in a non-oxidizing gas atmosphere that is substantially free of nitrogen, in order to prevent the formation of an Al oxide layer and / or an Al nitride layer on the surface of the rolled steel strip.

[0026] <Solution treatment process> The solution treatment process involves heating the cold-rolled steel strip to a high temperature after cold rolling and then rapidly cooling it to promote solid solution of alloying elements, obtain a relatively coarse grain size necessary for high creep strength through recrystallization and grain growth, and soften the steel strip to facilitate part forming and welding. This process is a necessary and important final heat treatment for this steel strip. The solution treatment is performed in a non-oxidizing atmosphere that is substantially free of nitrogen to prevent the formation of an oxide layer and / or nitride layer on the steel strip surface due to oxidation. The atmospheric gas is preferably a reducing gas, such as hydrogen gas or argon gas, or an inert gas. Using a steel strip with this composition allows for coarsening and adjusting the grain size through recrystallization and grain growth at low temperatures, making solution treatment possible at low temperatures within the range of heat treatment possible using conventional manufacturing equipment. If the heating temperature for solution treatment is lower than 1000°C, the solid solution of alloying elements will be insufficient, resulting in residual carbides and intermetallic compounds, insufficient hardness reduction, and insufficient recrystallization and grain growth, preventing the desired coarse grain size. On the other hand, if the heating temperature exceeds 1150°C, the grain size will become too coarse, which may result in reduced tensile ductility and impact toughness. Therefore, the solution treatment temperature is set to 1000 to 1150°C. The preferred lower limit of the solution treatment temperature is 1050°C. The preferred upper limit of the solution treatment temperature is 1130°C. A continuous furnace is often used for solution treatment of cold-rolled steel strips, and the heating holding time is relatively short. The heating holding time tends to be shorter for thinner steel sheets and longer for thicker steel sheets; however, the holding time can be determined based on the solid solution of alloying elements, the degree of hardness reduction, the degree of grain size growth, etc. If the heating holding time is shorter than 0.1 minute, sufficient effect cannot be obtained, while if it is longer than 30 minutes, it is difficult to obtain a significant effect. Therefore, the heating holding time is set to 0.1 to 30 minutes. The upper limit of the heating holding time is preferably 10 minutes. Furthermore, if the desired structure cannot be obtained by one solution treatment due to equipment limitations, the solution treatment may be repeated multiple times. After the solution treatment, rapid cooling is performed to maintain the solid solution state. The cooling method may be water cooling, oil cooling, air cooling, or the like, and is not particularly limited.The cooling rate is set to 5°C / s or more because if it is slower than 5°C / s (seconds), the alloy elements that have dissolved during cooling may be reprecipitated, increasing hardness and decreasing oxidation resistance. A preferable cooling rate is 7.5°C / s or more.

[0027] The average austenite grain size of the austenitic stainless steel strip after the above-mentioned solution treatment process has a significant effect on creep strength, and must be adjusted to a relatively coarse grain size to obtain high creep strength. The grain size can be controlled mainly by the final solution treatment conditions, and in the case of the austenitic stainless steel strip of the present invention, it can be controlled within an appropriate range by the above-mentioned solution treatment conditions. If the average austenite grain size is less than 30 μm, sufficient creep strength cannot be obtained, while if it is more than 100 μm, tensile ductility and impact toughness may decrease. Therefore, the average austenite grain size is set to 30 to 100 μm. The preferred lower limit of the average austenite grain size is 40 μm. The preferred upper limit of the average austenite grain size is 80 μm.

[0028] <Polishing process> Because the austenitic stainless steel strip of the present invention contains a large amount of Al, a dense oxide layer consisting of Al oxides and / or a nitride layer consisting of needle-like Al nitrides is likely to form on the surface of the steel strip during heat treatment in air, hot rolling, or other processes. If the steel strip is cold-rolled and then subjected to a final solution treatment process while leaving the Al oxide layer or Al nitride layer on the surface, the resulting steel strip will have a non-uniform Al oxide layer or Al nitride layer on the surface, making it difficult to achieve stable and good oxidation resistance. Therefore, it is preferable to remove the oxide layer and nitride layer from the surface of the rolled material (steel strip). As long as the Al oxide layer and Al nitride layer remaining on the surface of the rolled material can be completely removed, there is no particular restriction on the removal method. Because the Al oxide layer and Al nitride layer are chemically stable, complete removal by chemical removal methods such as pickling is difficult, making it difficult to obtain a uniform metal surface texture. However, this does not preclude the application of a pickling process before cold rolling. On the other hand, mechanical removal methods such as polishing can remove a certain thickness and facilitate complete removal, so a polishing step is preferably selected as a method for removing the oxide layer and nitride layer on the surface of the rolled material to obtain metallic luster. The polishing step may be performed either between the hot rolling step and the cold rolling step or during the cold rolling step, as long as the oxide layer and nitride layer on the surface of the rolled material are completely removed before the final solution heat treatment. [Example]

[0029] Ingots were melted and cast by vacuum induction melting, and then homogenized, hot forged, and hot rolled to prepare hot-rolled steel strips approximately 45 mm thick and 330 mm wide. The chemical compositions of the hot-rolled steel strips are shown in Table 1. No. 1 is an inventive hot-rolled steel strip, and No. 2 is a comparative hot-rolled steel strip. These hot-rolled steel strips were heated to 1150°C and then hot-rolled to produce 3 mm-thick hot-rolled steel strips. The degree of surface damage during the hot forging and hot rolling processes in No. 1 and No. 2 hot-rolled steel strips was examined. Hot-rolled steel strip No. 1 exhibited less surface damage than No. 2, demonstrating superior hot workability. Subsequently, a polishing process was carried out during the cold rolling process to remove the Al oxide layer and Al nitride layer from the steel strip surface, and then cold rolling and annealing were repeated several times to produce cold-rolled steel strips of various thicknesses from 0.2 mm to 1.5 mm and widths of approximately 250 mm. The obtained cold-rolled steel strips were then heated and held in a continuous furnace in a hydrogen atmosphere at 1100°C for approximately 1 to 5 minutes, and then subjected to solution treatment by rapid cooling at a cooling rate of 5°C / s or more to obtain austenitic stainless steel strip No. 5 of the present invention produced from the hot-rolling material No. 1, and austenitic stainless steel strip No. 7 of the comparative example produced from the hot-rolling material No. 2.

[0030] Furthermore, as a conventional example of austenitic stainless steel, hot-rolled materials with the chemical compositions shown in Table 2 were prepared by vacuum induction melting and casting to a thickness of approximately 30 mm and a width of approximately 120 mm. Nos. 3 and 4 correspond to NCF800 and NCF625 steels, respectively, as specified in JIS G 4902. These hot-rolled 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, cold rolling and annealing were repeated to obtain cold-rolled steel strips with a thickness of 1.5 mm. These cold-rolled steel strips were then subjected to solution treatment by heating and holding at 1150°C for 30 minutes in a vacuum atmosphere furnace, followed by rapid cooling, to obtain austenitic stainless steel strips Nos. 9 and 10.

[0031] [Table 1]

[0032] [Table 2]

[0033] Test pieces (samples) were cut out from the 1.5 mm thick austenitic stainless steel strips of Nos. 5 and 7, and the average austenite grain size was measured by optical microscopic observation of the longitudinal cross section, and tensile tests in the rolling direction were performed at room temperature and 850°C, creep rupture tests in the rolling direction at 800, 850, and 900°C, and an oxidation resistance test at 1000°C. In addition, test pieces cut out from the 1.5 mm thick cold-rolled steel strip were subjected to solution treatment by heating and holding for 5 minutes in a hydrogen atmosphere at 1150°C, and then quenching by air at a cooling rate of 5°C / s or more, to obtain Sample No. 6, an example of the present invention produced from the No. 1 hot-rolling material, and Sample No. 8, a comparative example produced from the No. 2 hot-rolling material. As with Nos. 5 and 7, the average austenite grain size was measured by optical microscopic observation of the longitudinal section, tensile tests were performed in the rolling direction at room temperature and 850°C, creep rupture tests were performed in the rolling direction at 800, 850, and 900°C, and an oxidation resistance test was performed at 1000°C. For the 1.5 mm thick austenitic stainless steel strips Nos. 9 and 10, test pieces (samples) were identified and only the oxidation resistance test at 1000°C was performed. The average austenite grain size is shown in Table 3, the tensile test results in Table 4, the creep rupture test results in Table 5, and the oxidation resistance test results in Table 6.

[0034] Table 3 shows that the inventive samples exhibited an average austenite grain size of approximately 50 μm, which is optimal for coarse grains, regardless of whether the solution treatment temperature was 1100 or 1150°C. In contrast, the comparative samples exhibited an average austenite grain size of less than 30 μm regardless of whether the solution treatment temperature was 1100 or 1150°C. Thus, the manufacturing method of the present invention achieves an appropriate average austenite grain size that facilitates high creep strength. Table 4 also shows that the inventive samples exhibited lower 0.2% proof stress and tensile strength at room temperature than the comparative samples, regardless of whether the solution treatment temperature was 1100 or 1150°C. However, their 0.2% proof stress and tensile strength at a high temperature of 850°C were comparable. Table 5 also shows that the inventive samples exhibited longer creep rupture times and higher creep strength than the comparative samples, regardless of whether the solution treatment temperature was 1100 or 1150°C. The high creep strength of the steel strip produced by the method of the present invention using the hot rolling material of the present invention is due to the fact that the average austenite grain size is controlled to be coarse, and the creep rupture strength can be increased even when solution treatment is carried out at a relatively low temperature such as 1100°C or 1150°C.

[0035] [Table 3]

[0036] [Table 4]

[0037] [Table 5]

[0038] For the oxidation resistance test, the surfaces of test pieces (specimens) Nos. 5 to 10, measuring 15 mm (w) × 15 mm (l) × 1.5 mm (t), were polished to a polishing grade of #1000 using sandpaper. The polished test pieces were then heat-treated in air at 1000°C for 100 to 1000 hours, and their weights were measured before and after oxidation. The results are shown in Table 6. Conventional specimens Nos. 9 and 10, which are typical austenitic stainless steels that form Cr oxide films, showed a large oxidation mass gain up to 500 hours. Furthermore, specimen No. 10 showed peeling of the oxide film due to thermal stress during cooling after 1000 hours of heating, resulting in a decrease in oxidation mass gain. Such peeling of the oxide film must be avoided because it accelerates oxidation of the metal matrix. On the other hand, the high-Al austenitic stainless steels of the present invention and comparative specimens Nos. 7 and 8 showed a small oxidation mass gain up to 1000 hours, confirming their excellent oxidation resistance. Furthermore, from Figure 1, it was confirmed that the oxidation mass gain of test pieces No. 5 to 8 followed a parabolic law, there was no peeling of the oxide film, and the oxidation behavior was stable.

[0039] After heating for 1000 hours, test piece No. 5 was plated with Ni and subjected to surface analysis of Fe, Al, and O using an electron microanalyzer, targeting the metal matrix and oxide film. The resulting photographs are shown in Figure 2. Figure 2(a) is a photograph showing a backscattered electron image of the cross section of the sample, and Figures 2(b) to 2(d) are photographs showing the results of surface analysis of Fe, Al, and O in the same observation area as Figure 2(a). Comparing the backscattered electron image with the surface analysis of each element, it was confirmed that a protective Al oxide film made of Al2O3 had formed on the sample of the present invention. From the above, the austenitic stainless steel strip obtained by the manufacturing method of the present invention has both high creep strength and good oxidation resistance, and is therefore expected to improve the reliability of parts for equipment used at high temperatures, such as heat treatment furnaces, heat exchangers, and solid oxide fuel cells.

[0040] [Table 6] [Explanation of symbols]

[0041] 1 Ni plating 2. Oxide film 3 Metal base

Claims

1. In mass%, Ni: more than 20.0% and not more than 30.0%; Cr: more than 15.0% and not more than 18.0%; Mo: 1.0-2.0%, Al: 3.5% or more and less than 5.0%; Nb + Ta: more than 1.0% and not more than 2.0%; Ti + V: 0.3% or less (including 0%), Si: 1.0% or less (including 0%), Mn: 2.0% or less (including 0%), Zr: 0.01 to 0.3%, C: 0.005-0.045%, B: 0.001-0.03%, If necessary, one or more of Y, La, Ce, and Hf are contained in the range of Y+La+Ce+Hf+Zr: 0.01 to 0.5%; a hot rolling step of hot rolling a material for hot rolling having a component composition of the remainder Fe and unavoidable impurities; a cold rolling step of cold rolling the hot-rolled steel strip after the hot rolling step; a solution treatment step in which the cold-rolled steel strip after the cold rolling step is heated and held at 1000 to 1150°C for 0.1 to 30 minutes in a non-oxidizing atmosphere in which a nitride layer is not formed on the steel strip surface, and then rapidly cooled at a cooling rate of 5°C / s or more; A method for producing an austenitic stainless steel strip, characterized in that an austenitic stainless steel strip having a width of 120 mm or more and a thickness of 3 mm or less is obtained.

2. 2. The method for producing an austenitic stainless steel strip according to claim 1, wherein the austenitic stainless steel strip obtained after the solution treatment step has an average austenite grain size of 30 to 100 μm.

3. 3. The method for producing an austenitic stainless steel strip according to claim 1, further comprising a polishing step for removing an oxide layer and a nitride layer on the surface of the rolled steel strip between the hot rolling step and the cold rolling step, or during the cold rolling step.

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