Preparation method of high-formability super ferritic stainless steel

Abstract

The invention relates to ultra-super ferritic stainless steel, in particular to high-formability ultra-super ferritic stainless steel, and particularly relates to the high-formability ultra-super ferritic stainless steel and a preparation method thereof. In order to solve the problem of layering of super ferrite stainless steel in the prior art, the invention provides the preparation method of the super ferrite stainless steel with high formability and high performance. Comprising the following steps of smelting, die casting and coping, forging cogging, hot rolling, curling, uncoiling, solid solution, hot coping, cold rolling and recrystallization annealing. According to the preparation method, the layering problem of existing super ferrite stainless steel is effectively solved.

Description

Technical Field

[0001] This invention relates to the field of super ferritic stainless steel technology, specifically to a method for preparing highly formable super ferritic stainless steel. Background Technology

[0002] Super ferritic stainless steel is a nickel-saving, highly corrosion-resistant stainless steel that also possesses good thermal conductivity, a low coefficient of linear expansion, and good mechanical properties. It has successfully replaced copper and titanium tubes in the manufacture of condensers for coastal power plants and can also replace super austenitic stainless steel and nickel-based alloys in industrial applications such as acid production, seawater desalination, and petrochemicals. However, due to the high Cr, high Mo, and Nb content of super ferritic stainless steel, various brittle intermediate phases, such as sigma(σ), chi(χ), and Laves(η), are easily formed within the steel. The formation of these intermediate phases not only severely deteriorates the material's ductility and toughness, causing embrittlement (known as σ-phase brittleness), but also reduces its formability and corrosion resistance, seriously restricting the large-scale production and widespread application of this type of product.

[0003] Existing patent research has found that the addition of Al can suppress the precipitation of brittle phases. Therefore, the problem of brittle phase precipitation can be solved by alloying with Al, and a super ferritic stainless steel has been proposed. Although super ferritic stainless steel has excellent corrosion resistance, its inherent poor formability has long severely restricted its further application, especially in parts requiring complex cold forming processes. In the existing patent (CN202211553095.0), entitled "A Super Ferritic Stainless Steel and Its Preparation Method," the annealing temperature of the cold-rolled super ferritic stainless steel sheet is reduced to 900-1150℃. During recrystallization, the selective pinning effect of the Laves phase formed at the shear band position inhibits the recrystallization of α-fibers and other oriented grains and promotes the recrystallization of γ-fiber texture, thereby improving its formability. However, the finished steel still exhibits a layering phenomenon with alternating fine and coarse grain regions, and the γ-texture is weak. The applicant's research found that the presence of layering and weak γ-texture in the microstructure of super ferritic stainless steel leads to a decrease in the plastic strain ratio of cold-rolled annealed sheets, making them prone to surface wrinkling after plastic forming. This significantly increases the surface roughness and consequently increases the cost of subsequent polishing. Summary of the Invention

[0004] In order to solve the problems of delamination and weak γ-texture in the existing super ferritic stainless steel, the present invention provides a method for preparing highly formable super ferritic stainless steel.

[0005] This invention is achieved using the following technical solution: A method for preparing a highly formable super ferritic stainless steel includes the following steps: 1) Smelting: The weight percentages of each element in the smelting raw materials are as follows: 25%≤Cr≤30.0%, 0.5%≤Mo≤4.0%, 1.5%≤Ni≤4.5%, C≤0.01%, N≤0.008%, 0.10%≤Nb≤0.30%, 0.10%≤Ti≤0.25%, 0.50%≤Al≤2.0%, Mn≤0.4%, 0.2%≤Si≤0.7%, S≤0.005%, P≤0.005%, O≤0.004%, with the remainder being Fe and unavoidable impurities. After the raw materials are prepared according to the above element ratios, they are smelted to obtain molten steel. 2) Molding and grinding: The molten steel obtained in step 1) is poured into a billet inside the furnace hood. The billet is then slowly cooled to room temperature and then ground. 3) Forging and billet preparation: The ground billet obtained in step 2) is slowly heated to 1180-1250℃, with an initial forging temperature of 1100-1200℃ and a final forging temperature of ≥950℃. It is then air-cooled to room temperature to obtain the forging billet.

[0006] 4) Hot rolling: The forging billet obtained in step 3) is held at 1050-1180℃ and then subjected to multi-pass hot rolling. The total reduction rate of hot rolling is ≥80%, and the reduction rate of a single pass is ≥15%. During hot rolling, if the exit temperature of the hot-rolled plate after a certain pass is lower than 600-800℃, it needs to be heated to 600-800℃ first, held at that temperature, and then the next pass of hot rolling is carried out. If the exit temperature of the hot-rolled plate after a certain pass is higher than 600-800℃, the next pass of hot rolling is carried out directly to obtain the hot-rolled plate. 5) Curled: After cooling the hot-rolled sheet obtained in step 4) to 500-600℃, it is rolled into a hot-rolled sheet coil. 6) Opening the book: After cooling the hot-rolled coil obtained in step 5) to room temperature, open it on an uncoiler. 7) Solid solution: Step 6) The hot-rolled coil obtained is solution-treated at a heating temperature of 850℃-1150℃ to obtain a hot-rolled solution-treated plate; 8) Hot grinding: The hot-rolled solution plate obtained in step 7) is hot-ground to remove the iron oxide scale on the surface of the hot-rolled solution plate. 9) Cold rolling: The solution-treated plate obtained in step 8) is subjected to multiple cold rolling passes, with a single-pass reduction rate of ≤10% and a total reduction rate of 60%-90%, to obtain a cold-rolled plate. 10) Recrystallization annealing: The cold-rolled sheet obtained in step 9) is subjected to recrystallization annealing at a temperature of 980-1100℃, and then rapidly cooled to room temperature after recrystallization annealing.

[0007] Explanation of the principle: ① The applicant's research found that: due to the high C and N content in existing super ferritic stainless steel, micron-sized Nb(C,N) particles are easily formed during forging. Due to the particle-induced nucleation mechanism, deformation zones with high dislocation density are generated around the micron-sized Nb(C,N) particles, which become preferential nucleation points. A large number of crystal nuclei are generated in the particle-dense areas around the micron-sized Nb(C,N) particles. After competing with each other, fine-grained areas are formed. In other areas, the particles are sparse and the crystal nuclei are relatively few, which can grow freely into coarse grains. In the microstructure, Nb(C,N) particles are usually distributed in a straight line along the rolling direction. Therefore, a layer of fine grains appears around the Nb(C,N) particles distributed in a straight line, while a layer of coarse grains appears in areas where there are no Nb(C,N) particles. The uneven distribution of particles leads to a large difference in nucleation rate, and eventually, stratification occurs. Therefore, step 1) of this application effectively suppresses the precipitation of micron-sized Nb(C,N) particles by reducing the C and N content, thereby avoiding the stratification phenomenon in the microstructure of super ferritic stainless steel caused by the particle-induced nucleation mechanism. ② In step 2), slow cooling helps improve the plasticity and toughness of the billet and prevents the casting from cracking; ③ In step 3), the billet is slowly heated, which can fully dissolve the segregated elements such as Cr, Mo, Nb, and Al formed in the billet and make them uniformly distributed. When the billet is heated to 1180-1250℃, it can dissolve the Nb (C, N) particles that may exist inside the billet. At the same time, excessively high or low temperatures during the forging process may cause element segregation. Therefore, the final forging temperature is controlled to be no lower than 950℃ to prevent the generation of micron-sized Nb (C, N) particles and avoid the stratification phenomenon caused by micron-sized Nb (C, N) particles. In addition, it also facilitates the precipitation of an appropriate amount of Laves phase, reduces forging resistance, and avoids forging cracks. ④ After rolling, super ferritic stainless steel is rolled into fibrous elongated grains, resulting in a γ-fiber oriented deformed structure dominated by shear bands and an α-fiber oriented deformed structure dominated by deformation bands. Nucleation in the shear bands occurs through grain growth, and the rolling process creates numerous nucleation points within these bands, resulting in fine grains. For the deformation bands within the grains, these grains have low deformation energy storage and nucleate during annealing via subgrain polymerization, resulting in larger grain sizes and forming coarse-grained regions in the annealed microstructure. It is known to those skilled in the art that the dynamic recovery rate of super ferritic stainless steel increases with temperature. Therefore, when the hot-rolling finishing temperature of super ferritic stainless steel is high, the faster dynamic recovery rate and lower deformation resistance lead to reduced strain retained in the stainless steel microstructure and less activation of the slip system, thus reducing the formation of shear bands. This results in a layered alternating distribution of α-fiber oriented deformed structures dominated by deformation bands and γ-fiber oriented deformed structures dominated by shear bands in the post-hot-rolled microstructure. Because the subgrain polymerization mechanism in the deformation band and the nucleation growth mechanism in the shear band are different, the microstructure after solution treatment exhibits an alternating layer of coarse and fine grains. Coarse grains experience lower deformation resistance during cold rolling, resulting in a thicker deformed microstructure dominated by deformation bands; fine grains experience higher deformation resistance after cold rolling, resulting in a thinner deformed microstructure dominated by shear bands. After recrystallization annealing, due to the different nucleation mechanisms of the shear band and deformation band, stratification eventually occurs. Therefore, in step 4), this application selects a low-temperature final rolling at 600-800℃. When the final rolling temperature is low, the slow dynamic recovery rate and high deformation resistance will result in more strain energy remaining in the ferritic stainless steel sample, thereby activating more slip systems, increasing the likelihood of shear band formation, and allowing the shear bands to cover as much of the α-fiber oriented deformed microstructure as possible. This ensures that the subsequent recrystallization process is mostly based on the nucleation growth mechanism, avoiding the grain stratification phenomenon caused by both subgrain polymerization and nucleation growth mechanisms during recrystallization. During the hot rolling heating process, submicron-sized Laves phase and micron-sized Laves phase will precipitate in the microstructure. At the same time, some of the Nb (C, N) particles generated during the hot rolling process will dissolve and the other part will be transformed into submicron-sized Laves phase and micron-sized Laves phase. The submicron-sized Laves phase will pin the grain boundaries and refine the grains during the solid solution process. In step 7), the addition of Al during solution treatment lowers the recrystallization temperature but raises the precipitation temperature of the Laves phase. Therefore, using a lower solution temperature prevents grain growth caused by high temperatures and allows for the dissolution of the micron-sized Laves phase and the appropriate precipitation of the nano-sized Laves phase. The submicron-sized Laves phase pins the grain boundaries, refining the grains and resulting in fine, uniform grains after solution treatment. This avoids the formation of shear bands and deformation bands during cold rolling due to excessive grain size differences, thus preventing grain stratification caused by subgrain polymerization and nucleation growth during recrystallization. During solution treatment, because the shear bands cover both α-fiber oriented and γ-fiber oriented deformed structures during hot rolling, and the grains generated by the shear band nucleation growth mechanism are γ-fiber oriented, the grains in the microstructure after solution treatment are predominantly γ-fiber textured. In step 9), the grains obtained after solution treatment are uniform and fine. The large reduction rate during cold rolling causes each grain to undergo large deformation, which in turn causes the shear band to cover the deformed structure with α-fiber orientation and γ-fiber orientation. This makes the subsequent recrystallization process a nucleus growth mechanism, avoiding the grain stratification phenomenon caused by the two mechanisms of subgrain polymerization and nucleus growth during recrystallization. In step 10), a large number of nanoscale Laves phases precipitate along shear bands, deformation bands, and subgrain boundaries during recrystallization. The nanoscale Laves phases pin the grain boundaries and subgrain boundaries, preventing grain growth and subgrain aggregation, resulting in uniform and fine grains during recrystallization. During recrystallization annealing, since the shear bands cover the α-fiber oriented deformation structure and γ-fiber oriented deformation structure during cold rolling, and the grains generated by the shear band nucleation and growth mechanism are γ-fiber oriented, a sharp γ-fiber texture is formed in the final recrystallization annealed structure.

[0008] Furthermore, in step 2), the billet is first cooled to ≤1000℃ in the furnace hood, removed, and then slowly cooled to room temperature at a cooling rate of ≤20℃ / h.

[0009] Furthermore, in step 3), the slow heating rate of the ground billet is 0.5-10℃ / min, and the holding time is 1-4h.

[0010] Furthermore, in step 5), the cooling time of the hot-rolled plate shall not exceed 500s.

[0011] Furthermore, in step 9), during hot grinding, the oxide layer on the surface of the hot-rolled solution plate is removed by pickling or machining.

[0012] Furthermore, in step 10), the rapid cooling rate after recrystallization annealing is ≥30℃ / s.

[0013] Furthermore, in step 7), the heat preservation time during solution treatment is 5 min to 120 min.

[0014] Furthermore, in step 10), the holding time during recrystallization annealing is 0.5-60 min.

[0015] The beneficial effects of this invention are as follows: The preparation method described in this invention avoids the generation of micron-sized Nb(C,N) particles, thereby avoiding the particle-induced nucleation mechanism caused by micron-sized Nb(C,N) particles and the resulting delamination phenomenon in the super ferritic stainless steel microstructure. The selection of low-temperature final rolling during hot rolling and the large reduction rate during cold rolling allow the shear bands to cover the α-fiber-oriented deformed structure, ensuring that the subsequent recrystallization process is entirely based on the nucleation and growth mechanism, avoiding grain delamination caused by both subgrain polymerization and nucleation and growth mechanisms during recrystallization. Simultaneously, the grains generated by the shear band nucleation and growth mechanism are γ-fiber-oriented, thus forming a sharp γ-fiber texture in the final recrystallized annealed microstructure. This texture has high strength, effectively avoiding delamination and further improving the formability of the super ferritic stainless steel. Attached Figure Description

[0016] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 SEM images showing the layering in tissues caused by particle-induced nucleation; Figure 2 A schematic diagram illustrating how large deformation during low-temperature hot rolling causes shear bands to cover most of the grains. Figure 3 A schematic diagram of submicron-sized Laves phase pinning grain boundaries during solid solution treatment; Figure 4 A schematic diagram of nanoscale Laves phase pinning subgrain boundaries during recrystallization annealing; Figure 5 This is a schematic diagram of the IPF and texture after recrystallization annealing in Example 1; Figure 6 This is a schematic diagram of the IPF and texture after recrystallization annealing in Example 2; Figure 7 This is a schematic diagram of the IPF and texture after recrystallization annealing in Example 3; Figure 8 This is a schematic diagram of the IPF and texture after recrystallization annealing in Comparative Example 1; Figure 9 This is a schematic diagram of the IPF and texture after recrystallization annealing in Comparative Example 2; Figure 10 This is a schematic diagram of the IPF and texture after recrystallization annealing in Comparative Example 3. Detailed Implementation

[0019] To better understand the above-mentioned objectives, features, and advantages of the present invention, the solutions of the present invention will be further described below. It should be noted that, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other.

[0020] In this description, it should be noted that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance. It should also be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "joint" should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to the internal connection between two components. Those skilled in the art can understand the specific meaning of the above terms according to the specific circumstances.

[0021] Many specific details are set forth in the following description in order to provide a full understanding of the invention, but the invention may also be practiced in other ways different from those described herein; obviously, the embodiments in the specification are only some embodiments of the invention, and not all embodiments.

[0022] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings. Example 1

[0023] 1) Smelting: The weight percentages of each element in the smelting raw materials are as follows: 25% Cr, 0.5% Mo, 4.5% Ni, 0.006% C, 0.005% N, 0.30% Nb, 0.12% Ti, 1.1% Al, 0.28% Mn, 0.2% Si, 0.0022% S, 0.005% P, 0.004% O, with the remainder being Fe and unavoidable impurities. After the raw materials are prepared according to the above element ratios, they are smelted to obtain molten steel. 2) Molding and grinding: The molten steel obtained in step 1) is poured into a billet in the furnace hood. The billet is cooled to 1000°C in the furnace hood, and then slowly cooled to room temperature at a cooling rate of 20°C / h. Then it is ground. 3) Forging The billet obtained in step 2) is heated to 1180°C in the furnace at a heating rate of 0.5°C / min and a holding time of 4 hours. The initial forging temperature is 1100°C and the final forging temperature is 950°C. Then it is air-cooled to room temperature. 4) Hot rolling: The forging billet obtained in step 3) is held at 1050℃ and then subjected to multi-pass hot rolling. The total reduction rate of hot rolling is 80%, and the reduction rate per pass is 15%. During hot rolling, if the exit temperature of the hot-rolled plate after a certain pass is lower than 600℃, it is heated to 600℃, held at that temperature, and then the next pass of hot rolling is performed. If the exit temperature of the hot-rolled plate after a certain pass is higher than 600℃, the next pass of hot rolling is performed directly to obtain the hot-rolled plate. 5) Curled: The hot-rolled sheet obtained in step 4) is cooled to 550°C and then rolled to form a hot-rolled sheet coil. The cooling time is 500 seconds. 6) Opening the book: After cooling the steel coil obtained in step 5) to room temperature, open it on an uncoiler; 7) Solid solution: The hot-rolled plate obtained in step 6) is solution treated to obtain a hot-rolled solution plate, wherein the heating temperature is 850℃ and the holding time is 5min. 8) Hot grinding: Remove the iron oxide scale from the surface of the hot-rolled solution plate obtained in step 7) by pickling or machining. 9) Cold rolling: The solution-treated plate obtained in step 8) is subjected to multi-pass cold rolling, with a single-pass reduction rate of 6% and a total reduction rate of 60%. 10) Recrystallization annealing: The cold-rolled sheet obtained in step 9) was subjected to recrystallization annealing at a temperature of 1030℃ for 1 min. After recrystallization annealing, it was rapidly cooled to room temperature at a cooling rate of 30℃ / s. The steel sheet prepared by the above process was then sampled and subjected to microstructure observation and tensile property testing. The results are shown in Table 1.

[0024] Table 1 Yield strength tensile strength Elongation after fracture Texture strength Plastic strain ratio 0° Plastic strain ratio 45° Plastic strain ratio 90° Plastic strain ratio weighted average roughness 558.5MPa 682MPa 25.16% 16.2 2.11 1.51 1.69 1.71 3.24 Example 2

[0025] 1) Smelting: The weight percentages of each element in the smelting raw material are as follows: 27.5% Cr, 2.5% Mo, 3.0% Ni, 0.01% C, 0.005% N, 0.20% Nb, 0.10% Ti, 0.5% Al, 0.30% Mn, 0.4% Si, 0.0021% S, 0.0016% P, 0.0023% O, with the remainder being Fe and unavoidable impurities. After the raw material is prepared according to the above element ratios, it is smelted to obtain molten steel. 2) Molding and grinding: The molten steel obtained in step 1) is poured into a billet in the furnace hood. The billet is cooled to 1000°C in the furnace hood, and then slowly cooled to room temperature at a cooling rate of 18°C / h. Then it is ground. 3) Forging The billet obtained in step 2) is heated to 1250℃ in the furnace at a heating rate of 10℃ / min and a holding time of 4h. The initial forging temperature is 1200℃ and the final forging temperature is 1000℃. Then it is air-cooled to room temperature. 4) Hot rolling: The forging billet obtained in step 3) is held at 1180℃ and then subjected to multi-pass hot rolling. The total reduction rate of hot rolling is 85%, and the reduction rate per pass is 18%. During hot rolling, if the exit temperature of the hot-rolled plate after a certain pass is lower than 800℃, it is heated to 800℃, held at that temperature, and then the next pass of hot rolling is performed. If the exit temperature of the hot-rolled plate after a certain pass is higher than 800℃, the next pass of hot rolling is performed directly. After hot rolling, the plate is water-cooled to room temperature at a cooling rate of 50℃ / min. 5) Curled: The hot-rolled sheet obtained in step 4) is cooled to 500°C and then rolled to form a hot-rolled sheet coil. The cooling time is 500 seconds. 6) Opening the book: After cooling the steel coil obtained in step 5) to room temperature, open it on an uncoiler; 7) Solid solution: The hot-rolled plate obtained in step 6) is solution-treated to obtain a hot-rolled solution-treated plate, wherein the heating temperature is 1150℃ and the holding time is 60min. 8) Hot grinding: Remove the iron oxide scale from the surface of the solid solution plate obtained in step 7) by pickling or machining. 9) Cold rolling: The solution-treated plate after hot grinding in step 8) is subjected to multi-pass cold rolling, with a single-pass reduction rate of 6% and a total reduction rate of 87.5%. 10) Recrystallization annealing: The cold-rolled sheet obtained in step 9) was subjected to recrystallization annealing at a temperature of 980℃ for 30 minutes. After recrystallization annealing, it was rapidly cooled to room temperature at a cooling rate of 30℃ / s. The steel sheet prepared by the above process was then sampled for microstructure observation and tensile property testing. The results are shown in Table 2.

[0026] Table 2 Yield strength tensile strength Elongation after fracture Texture strength Plastic strain ratio 0° Plastic strain ratio 45° Plastic strain ratio 90° Plastic strain ratio weighted average 0° roughness 550.5MPa 673.5MPa 27.67% 22.7 2.18 2.97 2.46 2.64 3.14 Example 3

[0027] 1) Smelting: The weight percentages of each element in the smelting raw material are as follows: 30% Cr, 4.0% Mo, 1.5% Ni, 0.008% C, 0.008% N, 0.10% Nb, 0.25% Ti, 2% Al, 0.30% Mn, 0.7% Si, 0.005% S, 0.0016% P, 0.0023% O, with the remainder being Fe and unavoidable impurities. After the raw material is prepared according to the above element ratios, it is smelted to obtain molten steel. 2) Molding and grinding: The molten steel obtained in step 1) is poured into a billet in the furnace hood. The billet is cooled to 1000°C in the furnace hood, and then slowly cooled to room temperature at a cooling rate of 20°C / h. Then it is ground. 3) Forging The billet obtained in step 2) is heated to 1200℃ in the furnace at a heating rate of 0.5~10℃ / min and a holding time of 2h. The initial forging temperature is 1170℃ and the final forging temperature is 970℃. Then it is air-cooled to room temperature. 4) Hot rolling: The forging billet obtained in step 3) is held at 1100℃ and then subjected to multi-pass hot rolling. The total reduction rate of hot rolling is 88%, and the reduction rate of a single pass is ≥19%. During hot rolling, if the exit temperature of the hot-rolled plate after a certain pass is lower than 700℃, it is heated to 700℃, held at that temperature, and then the next pass of hot rolling is performed. If the exit temperature of the hot-rolled plate after a certain pass is higher than 700℃, the next pass of hot rolling is performed directly. After hot rolling, the plate is water-cooled to room temperature at a cooling rate of 50℃ / min. 5) Curled: The hot-rolled sheet obtained in step 4) is cooled to 600°C and then rolled to form a hot-rolled coil. The cooling time is 500 seconds. 6) Opening the book: After cooling the steel coil obtained in step 5) to room temperature, open it on an uncoiler; 7) Solid solution: The hot-rolled plate obtained in step 6) is solution-treated to obtain a hot-rolled solution-treated plate, wherein the heating temperature is 1050℃ and the holding time is 120min. 8) Surface treatment: Remove the iron oxide scale from the surface of the solid solution plate obtained in step 7) by pickling or machining. 9) Cold rolling: The surface-treated solidification plate from step 8) is subjected to multi-pass cold rolling, with a single-pass reduction of 10% and a total reduction of 90%. 10) Recrystallization annealing: The cold-rolled sheet obtained in step 9) was subjected to recrystallization annealing at a temperature of 1100℃ for 60 min. After recrystallization annealing, it was rapidly cooled to room temperature at a cooling rate of 30℃ / s. The steel sheet prepared by the above process was then sampled for microstructure observation and tensile property testing. The results are shown in Table 3.

[0028] Table 3 Yield strength tensile strength Elongation after fracture Texture strength Plastic strain ratio 0° Plastic strain ratio 45° Plastic strain ratio 90° Plastic strain ratio weighted average 0° roughness 554.5 670 27.4 22.4 1.94 2.26 2.88 2.33 3.16 Comparative Example 1:

[0029] 1) Smelting: The weight percentages of each element in the smelting raw material are as follows: 25% Cr, 0.5% Mo, 4.5% Ni, 0.016% C, 0.013% N, 0.30% Nb, 0.12% Ti, 1.1% Al, 0.28% Mn, 0.2% Si, 0.0022% S, 0.005% P, 0.004% O, with the remainder being Fe and unavoidable impurities. After the raw material is prepared according to the above element ratios, it is smelted to obtain molten steel. 2) Molding and grinding: The molten steel obtained in step 1) is poured into a billet in the furnace hood. The billet is cooled to 1000°C in the furnace hood, and then slowly cooled to room temperature at a cooling rate of 20°C / h. Then it is ground. 3) Forging The billet obtained in step 2) is heated to 1180°C in the furnace at a heating rate of 0.5°C / min and a holding time of 1 hour. The initial forging temperature is 1100°C and the final forging temperature is 950°C. Then it is air-cooled to room temperature.

[0030] 4) Hot rolling: The forging billet obtained in step 3) is held at 1050℃ and then subjected to multi-pass hot rolling. The total reduction rate of hot rolling is 80%, and the reduction rate per pass is 15%. During hot rolling, if the exit temperature of the hot-rolled plate after a certain pass is lower than 920℃, it is heated to 920℃, held, and then the next pass of hot rolling is performed. If the exit temperature of the hot-rolled plate after a certain pass is higher than 920℃, the next pass of hot rolling is performed directly. After hot rolling, the plate is water-cooled to room temperature at a cooling rate of 50℃ / min.

[0031] 5) Curled: The hot-rolled plate obtained in step 4) is cooled to 550°C and rolled into a hot-rolled coil. The cooling time is 500 seconds.

[0032] 6) Opening the book: After cooling the steel coil obtained in step 5) to room temperature, open it on an uncoiler.

[0033] 7) Solid solution: The hot-rolled plate obtained in step 6) is solution-treated to obtain a hot-rolled solution-treated plate, wherein the heating temperature is 850℃ and the holding time is 5min.

[0034] 8) Surface treatment: The iron oxide scale on the surface of the solid solution plate obtained in step 7) is removed by pickling or machining.

[0035] 9) Cold rolling: After surface treatment in step 8), the solidified plate is subjected to multiple cold rolling passes, with a single-pass reduction rate of 6% and a total reduction rate of 60%.

[0036] 10) Recrystallization annealing: The cold-rolled sheet obtained in step 9) was subjected to recrystallization annealing at a temperature of 1030℃ for 1 min. After recrystallization annealing, it was rapidly cooled to room temperature at a cooling rate of 30℃ / s. The steel sheet prepared by the above process was then sampled for microstructure observation and tensile property testing. The results are shown in Table 4.

[0037] Table 4 Yield strength tensile strength Elongation after fracture Texture strength Plastic strain ratio 0° Plastic strain ratio 45° Plastic strain ratio 90° Plastic strain ratio weighted average roughness 500.50MPa 665.59MPa 25.18% 4.29 1.38 1.35 1.33 1.36 5.17 Comparative Example 2:

[0038] 1) Smelting: The weight percentages of each element in the smelting raw material are as follows: 30% Cr, 4.0% Mo, 1.5% Ni, 0.014% C, 0.013% N, 0.40% Nb, 0.25% Ti, 2% Al, 0.30% Mn, 0.7% Si, 0.005% S, 0.0016% P, 0.0023% O, with the remainder being Fe and unavoidable impurities. After the raw material is prepared according to the above element ratios, it is smelted to obtain molten steel. 2) Molding and grinding: The molten steel obtained in step 1) is poured into a billet in the furnace hood. The billet is cooled to 1000°C in the furnace hood, and then slowly cooled to room temperature at a cooling rate of 20°C / h. Then it is ground. 3) Forging The billet obtained in step 2) is heated to 1200℃ in the furnace at a heating rate of 0.5~10℃ / min and a holding time of 2h. The initial forging temperature is 1170℃ and the final forging temperature is 970℃. Then it is air-cooled to room temperature.

[0039] 4) Hot rolling: The forging billet obtained in step 3) is held at 1100℃ and then subjected to multi-pass hot rolling. The total reduction rate of hot rolling is 88%, and the reduction rate of a single pass is ≥19%. During hot rolling, if the exit temperature of the hot-rolled plate after a certain pass is lower than 920℃, it is heated to 920℃, held at that temperature, and then the next pass of hot rolling is performed. If the exit temperature of the hot-rolled plate after a certain pass is higher than 920℃, the next pass of hot rolling is performed directly. After hot rolling, the plate is water-cooled to room temperature at a cooling rate of 50℃ / min.

[0040] 5) Curled: The hot-rolled plate obtained in step 4) is cooled to 600°C and rolled into a hot-rolled coil. The cooling time is 500 seconds.

[0041] 6) Opening the book: After cooling the steel coil obtained in step 5) to room temperature, open it on an uncoiler.

[0042] 7) Solid solution: The hot-rolled plate obtained in step 6) is solution-treated to obtain a hot-rolled solution-treated plate, wherein the heating temperature is 1050℃ and the holding time is 40min.

[0043] 8) Surface treatment: The iron oxide scale on the surface of the solid solution plate obtained in step 7) is removed by pickling or machining.

[0044] 9) Cold rolling: After surface treatment in step 8), the solidified plate is subjected to multiple cold rolling passes, with a single pass reduction rate of 10% and a total reduction rate of 75%.

[0045] 10) Recrystallization annealing: The cold-rolled sheet obtained in step 9) was subjected to recrystallization annealing at a temperature of 1100℃ for 1 min. After recrystallization annealing, it was rapidly cooled to room temperature at a cooling rate of 30℃ / s. The steel sheet prepared by the above process was then sampled for microstructure observation and tensile property testing. The results are shown in Table 5.

[0046] Table 5 Yield strength tensile strength Elongation after fracture Texture strength Plastic strain ratio 0° Plastic strain ratio 45° Plastic strain ratio 90° Plastic strain ratio weighted average roughness 484.58MPa 648.37MPa 29.10% 7.13 0.98 1.12 1.61 1.21 7.27 Comparative Example 3:

[0047] 1) Smelting: The weight percentages of each element in the smelting raw material are as follows: 30% Cr, 4.0% Mo, 1.5% Ni, 0.019% C, 0.014% N, 0.10% Nb, 0.25% Ti, 0.30% Mn, 0.7% Si, 0.005% S, 0.0016% P, 0.0023% O, with the remainder being Fe and unavoidable impurities. After the raw material is prepared according to the above element ratios, it is smelted to obtain molten steel. 2) Molding and grinding: The molten steel obtained in step 1) is poured into a billet in the furnace hood. The billet is cooled to 1000°C in the furnace hood, and then slowly cooled to room temperature at a cooling rate of 20°C / h. Then it is ground. 3) Forging The billet obtained in step 2) is heated to 1200℃ in the furnace at a heating rate of 0.5~10℃ / min and a holding time of 2h. The initial forging temperature is 1170℃ and the final forging temperature is 970℃. Then it is air-cooled to room temperature.

[0048] 4) Hot rolling: The forging billet obtained in step 3) is held at 1100℃ and then subjected to multi-pass hot rolling. The total reduction rate of hot rolling is 88%, and the reduction rate of a single pass is ≥19%. During hot rolling, if the exit temperature of the hot-rolled plate after a certain pass is lower than 950℃, it is heated to 950℃, held at that temperature, and then the next pass of hot rolling is performed. If the exit temperature of the hot-rolled plate after a certain pass is higher than 950℃, the next pass of hot rolling is performed directly. After hot rolling, the plate is water-cooled to room temperature at a cooling rate of 50℃ / min.

[0049] 5) Curled: The hot-rolled plate obtained in step 4) is cooled to 600°C and rolled into a hot-rolled coil. The cooling time is 500 seconds.

[0050] 6) Opening the book: After cooling the steel coil obtained in step 5) to room temperature, open it on an uncoiler.

[0051] 7) Solid solution: The hot-rolled plate obtained in step 6) is solution-treated to obtain a hot-rolled solution-treated plate, wherein the heating temperature is 1050℃ and the holding time is 40min.

[0052] 8) Surface treatment: The iron oxide scale on the surface of the solid solution plate obtained in step 7) is removed by pickling or machining.

[0053] 9) Cold rolling: After surface treatment in step 8), the solidified plate is subjected to multiple cold rolling passes, with a single-pass reduction rate of 9% and a total reduction rate of 80%.

[0054] 10) Recrystallization annealing: The cold-rolled sheet obtained in step 9) was subjected to recrystallization annealing at a temperature of 1100℃ for 1 min. After recrystallization annealing, it was rapidly cooled to room temperature at a cooling rate of 30℃ / s. The steel sheet prepared by the above process was then sampled for microstructure observation and tensile property testing. The results are shown in Table 6.

[0055] Table 6 Yield strength tensile strength Elongation after fracture Texture strength Plastic strain ratio 0° Plastic strain ratio 45° Plastic strain ratio 90° Plastic strain ratio weighted average roughness 525.26 673.5 27.0 4.65 0.84 1.04 1.28 1.05 3.34 The test tables from the above embodiments and comparative examples, as well as those from the appendix... Figure 5-10 It can also be seen that the steel plate prepared by the preparation method described in this invention has significantly reduced the delamination phenomenon compared with the comparative example (in the IPF diagram in the attached figure, blue represents the deformed structure with γ-fiber orientation and red represents the deformed structure with α-fiber orientation), the γ-texture strength is significantly enhanced, the plastic strain ratio is increased, the roughness is significantly reduced, and its formability is significantly improved.

[0056] The above description is merely a specific embodiment of the present invention, enabling those skilled in the art to understand or implement the present invention. Although detailed descriptions have been provided with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments, and they should all be covered within the protection scope of the claims.

Claims

1. A method for producing a high-formability extra-super ferritic stainless steel, characterized by, The method comprises the following steps: 1) smelting: The weight percentage of each element of the smelting raw material is: 25%≤Cr≤30.0%, 0.5%≤Mo≤4.0%, 1.5%≤Ni≤4.5%, C≤0.01%, N≤0.008%, 0.10%≤Nb≤0.30%, 0.10%≤Ti≤0.25%, 0.50%≤Al≤2.0%, Mn≤0.4%, 0.2%≤Si≤0.7%, S≤0.005%, P≤0.005%, O≤0.004%, and the rest is Fe and inevitable impurities; the raw materials are smelted after being configured according to the above element proportion to obtain a molten steel; 2) mold casting and grinding: The molten steel obtained in step 1) is cast into a casting blank in a furnace cover, and the casting blank is slowly cooled to room temperature and then ground; 3) forging and blooming: The ground casting blank obtained in step 2) is slowly heated to 1180-1250℃, the open forging temperature is 1100-1200℃, the final forging temperature is ≥950℃, and then air cooling is performed to room temperature to obtain a forged blank; 4) hot rolling: The forged blank obtained in step 3) is heated and kept at 1050-1180℃, and then multi-pass hot rolling is performed, the total reduction rate of hot rolling is ≥80%, and the single-pass reduction rate is ≥15%; when the outlet temperature of the hot-rolled plate after a certain pass of hot rolling is lower than 600-800℃, the hot-rolled plate is heated to 600-800℃ first, kept for a period of time, and then the next pass of hot rolling is performed; if the outlet temperature of the hot-rolled plate after a certain pass of hot rolling is higher than 600-800℃, the next pass of hot rolling is directly performed to obtain a hot-rolled plate; 5) coiling: The hot-rolled plate obtained in step 4) is cooled to 500-600℃, and then coiling is performed to form a hot-rolled plate coil; 6) uncoiling: The hot-rolled plate coil obtained in step 5) is cooled to room temperature, and then uncoiling is performed on an uncoiler; 7) solid solution: The hot-rolled plate coil obtained in step 6) is subjected to solid solution treatment, and the heating temperature is 850℃-1150℃ to obtain a hot-rolled solid solution plate; 8) hot grinding: The hot-rolled solid solution plate obtained in step 7) is subjected to hot grinding to remove the iron oxide scale on the surface of the hot-rolled solid solution plate; 9) cold rolling: The hot-ground solid solution plate obtained in step 8) is subjected to multi-pass cold rolling, the single-pass reduction rate is ≤10%, and the total reduction rate is 60%-90% to obtain a cold-rolled plate; 10) recrystallization annealing: The cold-rolled plate obtained in step 9) is subjected to recrystallization annealing, and the annealing temperature is 980-1100℃, and then fast cooling is performed to room temperature.

2. The method of producing a high-formability extra super ferritic stainless steel according to claim 1, characterized by, In step 2), the casting blank is first cooled to ≤1000℃ in the furnace cover, and then removed and slowly cooled to room temperature at a cooling rate of ≤20℃ / h.

3. The method of producing a high-formability extra super ferritic stainless steel according to claim 2, characterized by, In step 3), the slow heating rate of the ground casting blank is 0.5-10℃ / min, and the holding time is 1-4h.

4. The method of producing a high-formability extra super ferritic stainless steel according to claim 3, characterized by, In step 5), the cooling time of the hot-rolled plate is not more than 500s.

5. The method of producing a high-formability extra super ferritic stainless steel according to claim 4, characterized by, In step 9), the hot-rolled solid solution plate is subjected to hot grinding, and the oxide scale on the surface of the hot-rolled solid solution plate is removed by pickling or mechanical processing.

6. The method of producing a high-formability extra super ferritic stainless steel according to claim 5, characterized by, In step 10), the fast cooling rate after recrystallization annealing is ≥30℃ / s.

7. The method of producing a high-formability extra super ferritic stainless steel according to claim 6, characterized by, The holding time during solid solution treatment is 5min-120min.

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