Method for improving forming performance of high-chromium and high-molybdenum ferritic stainless steel

Abstract

The invention relates to high-chromium and high-molybdenum ferritic stainless steel, in particular to a method for improving the formability of the high-chromium and high-molybdenum ferritic stainless steel. The novel method for preparing the high-chromium and high-molybdenum ferritic stainless steel is provided for solving the problems that the surface wrinkling defect exists in the forming process, and the surface roughness is increased in the existing preparation method for preparing the high-chromium and high-molybdenum stainless steel. The method sequentially comprises the following steps of smelting, casting, forging, hot rolling and rough rolling, hot rolling, cooling and finish rolling, curling, uncoiling, solution annealing, acid pickling, cold rolling and recrystallization annealing. The problems of alternate distribution of coarse / fine grains and alternate distribution of alpha, eta and gamma fiber textures in a solid solution structure and a cold rolling annealing structure are solved, recrystallized grains which are small in grain size and are uniformly distributed, and the gamma fiber textures are strong and are uniformly distributed are formed, and the formability of the material is improved.

Description

Technical Field

[0001] This invention relates to the field of high-chromium and high-molybdenum ferritic stainless steel technology, specifically to a method for improving the formability of high-chromium and high-molybdenum ferritic stainless steel. Background Technology

[0002] High-chromium, high-molybdenum ferritic stainless steel is a nickel-saving type of stainless steel. It possesses excellent comprehensive properties such as high corrosion resistance, high strength, good thermal conductivity, and low expansion, making it an economical key basic material for use in highly corrosive environments such as seawater and concentrated sulfuric acid. However, high-chromium, high-molybdenum ferritic stainless steel is highly susceptible to σ-phase embrittlement (precipitation and embrittlement of σ-phase, χ-phase, and Laves phase) during its preparation process. This not only deteriorates the material's ductility and toughness but also reduces its corrosion resistance, severely restricting the large-scale production and widespread application of this type of material. To address the brittleness of the σ phase, a "high-temperature rapid cooling" process is commonly used to avoid the precipitation of brittle phases. For example, in the prior art, patent number 201110363876.9, entitled "A High-Cr Ferritic Stainless Steel and Its Manufacturing Method," high-Cr ferritic stainless steel is prepared by hot rolling of steel ingots after holding at 1080-1120℃ (high-temperature holding is conducive to element diffusion and avoids element segregation, which leads to the easy precipitation of brittle phases and causes brittleness risk), high-temperature final rolling at 950-1000℃, and hot-rolled strip annealing at 1050-1150℃. Although high-temperature final rolling avoids the temperature range where brittle phases easily precipitate, the high final rolling temperature causes dynamic recovery during the rolling process of the plate, reducing deformation energy storage and recrystallization driving force. Therefore, the hot-rolled strip needs to be annealed at high temperature to complete recrystallization, which results in large grain size in the finished plate and deteriorates the material's plasticity and toughness. The existing technology has patent number 202210381116.9, which is entitled "A Hot Continuous Rolling Method for High-Chromium and High-Molybdenum Ferritic Stainless Steel". It uses a method of segmented heating of continuously cast billets and holding at 1230-1250℃, rough rolling at 1150-1170℃, finish rolling at 1030-1050℃, and finish rolling at 880-900℃ to prepare high-chromium molybdenum ferritic stainless steel. Although this method solves the problem of brittle phase precipitation, the "high temperature technology" leads to coarse microstructure after solution annealing, which deteriorates the plasticity and toughness of the material. At the same time, high-temperature finish rolling has high requirements for industrial equipment, and temperature control during the production process is difficult, making production implementation difficult. The existing technology, patent number 202011559796.6, entitled "A Method for Preparing High-Chromium, High-Molybdenum Ferritic Stainless Steel," avoids σ-phase precipitation and solves the grain coarsening problem through pre-precipitation of the Laves phase. However, the finished steel exhibits an alternating distribution of fine-grained and coarse-grained regions in terms of grain size, and an alternating distribution of α, η fiber textures and γ fiber textures in terms of texture. (See...) Figure 13 and Figure 14According to the Hall-Page effect, the existence of this phenomenon leads to differences in deformation behavior at different locations during the stretching process, which deteriorates the surface quality of the product during the forming process, causing defects such as cracking and surface wrinkling. It also increases the surface roughness of the product, seriously affecting its appearance, and the added polishing process will increase manufacturing costs. Summary of the Invention

[0003] In order to solve the problem that the finished steel in the prior art is prone to alternating distribution of coarse / fine grains and alternating distribution of α, η and γ fiber textures, which leads to surface wrinkling during the forming process, the present invention provides a method to improve the forming performance of high-chromium and high-molybdenum ferritic stainless steel. This method solves both the problem of grain coarsening in the finished steel and the problem of surface wrinkling.

[0004] This invention is achieved using the following technical solution: A method for improving the formability of high-chromium, high-molybdenum ferritic stainless steel includes the following steps: 1) Smelting: The weight percentages of each element in the smelting raw materials are as follows: 25.0%≤Cr≤30.0%, 2.0%≤Mo≤4.5%, 1.0%≤Ni≤4.0%, C≤0.015%, N≤0.010%, 0.15%≤Nb≤0.45%, 0.05%≤Ti≤0.35%, Mn≤0.3%, Si≤1.0%, S≤0.005%, P≤0.005%, O≤0.004%, and satisfy Cr+3.3×Mo≥35%, 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) Casting: The molten steel obtained in step 1) is poured into a billet, which is then slowly cooled to room temperature and then ground. 3) Forging: Heat the ground billet obtained in step 2) to 1180-1280℃, with a heating rate ≤10℃ / min, an initial forging temperature ≥1120℃, a final forging temperature ≥960℃, and air cool to room temperature after forging; 4) Hot rolling roughing: After holding the forging billet obtained in step 3) at 1100-1200℃, hot continuous rolling or multi-pass roughing is carried out. The total reduction rate of hot continuous rolling or roughing is ≥60%. When carrying out multi-pass roughing, if the exit temperature of the hot rolled plate after a certain pass is lower than 1000℃, it is heated to 1100-1200℃ and held for 5-20 minutes before the next pass of roughing is carried out. If the exit temperature of the hot rolled plate after a certain pass is higher than 1000℃, the next pass of roughing is carried out directly. 5) Hot-rolled cooling finishing rolling: The rough-rolled plate obtained in step 4) is cooled to 750-860℃ and subjected to multi-pass cooling rolling, with a single-pass reduction rate ≥10% and a total reduction rate ≥60%. 6) Curled: The finished rolled plate obtained in step 5) is rolled into a hot rolled plate coil (the temperature of the hot rolled plate obtained by the cooling rolling method has been reduced to the coiling temperature of 500-600℃, so there is no need to cool the finished rolled plate, and it can be rolled directly). 7) Unwinding and solution treatment: Unwind the hot-rolled coil obtained in step 6) and perform solution treatment at a heating temperature of 1000-1100℃ for 5-60 minutes, followed by rapid cooling. 8) Pickling: The solution plate obtained in step 7) is then acid-washed. 9) Cold rolling: The pickled solution plate obtained in step 8) is subjected to multi-pass room temperature cold rolling, with a single-pass reduction rate of <10% and a total cold rolling reduction rate of 60-90%. 10) Recrystallization annealing: The cold-rolled sheet obtained in step 9) is subjected to recrystallization annealing at a temperature of 950-1090℃ for 0.5-5 min, and then cooled to room temperature.

[0005] Explanation of the principle: ① Adding Cr and Mo can improve pitting corrosion resistance. When pitting occurs, Cr forms a passivation film on the steel surface, while Mo exists as MoO2-4, adhering to the surface and inhibiting further pitting corrosion. A pitting corrosion equivalent model (Cr% + 3.3 × Mo%) ≥ 35% is required to improve the alloy's pitting corrosion resistance. The low C and N content is due to their low solubility, making them prone to forming Cr after high-temperature heating. 23 Harmful compounds such as C6 are detrimental to the plastic deformation of materials. Meanwhile, lower C and N contents can increase the plastic strain ratio (r value) of high-chromium, high-molybdenum ferritic stainless steel, thereby improving the steel's resistance to wrinkling. Adding a certain amount of Ni and Nb can reduce the content of C and N interstitial atoms in the steel and form stable niobium and titanium carbonitriding compounds (TiN, Ti / Nb(C,N)). The C in the matrix is ​​consumed, thus slowing down the degradation of Cr. 23 The formation of C6 improves the plastic deformation capacity of steel; the added C+N elements, not exceeding 0.025%, weaken the formation of coarse Ti / Nb(C,N) and TiN particles, thereby reducing the intergranular corrosion of ferritic stainless steel; the added Ni elements are used to improve the toughness of ferritic stainless steel. ②Step 2) A cooling rate of ≤20℃ / h can provide sufficient time for element diffusion, avoiding element segregation which can lead to the easy precipitation of carbonitrides and cause brittleness risk; ③ Step 3) Heating the forging billet at high temperature and slowly increasing the temperature can avoid large internal thermal stress and cracks caused by the material, while allowing the elements to dissolve fully; using a final forging temperature ≥960℃ can avoid the segregation of Nb and Ti elements, thereby weakening the formation of large-sized TiN and Ti / Nb(C,N) particles. ④ Step 4) Hold the forging billet at 1100-1200℃ to allow recrystallization and restore its plasticity. At the same time, the high temperature holding facilitates element diffusion and avoids element segregation, which can lead to the easy precipitation of brittle phases. Intermediate heating and holding cause recrystallization inside the previous hot-rolled plate, forming new fine recrystallized grains and restoring the plasticity of the previous hot-rolled plate. Controlling the high hot rolling exit temperature ensures that the dissolved Laves phase does not precipitate excessively, providing compositional assurance for the precipitation of Laves phase during the finishing rolling process in step 5). ⑤ Step 5) Finish rolling is performed using a cooling rolling method, with a finishing rolling temperature of 750-860℃. During the rolling process, the plate undergoes a cooling process, with the actual temperature being 550-700℃. This avoids the nose temperature (800-850℃) for σ phase precipitation, delaying the precipitation of brittle σ phase. During finish rolling, the lower temperature facilitates cross-slip and dislocation slip, increasing the number of shear-deformed grains and improving the uniformity of plate deformation. Simultaneously, finish rolling can increase the amount of deformation without precipitating σ phase, obtaining a large number of shear bands and other deformed microstructures. The structure improves deformation energy storage; in addition, fine rolling can make the subgrains generated by the dynamic recovery of α and η fiber structure during rough rolling smaller, and the recrystallized grains formed by subgrain polymerization during solid solution are finer. The driving force required for grain growth is larger, and the Laves phase pinning subgrain boundaries has a significant effect; the fine rolling temperature and the Laves phase precipitation temperature (600-900℃) intersect, and the shear band and subgrain boundaries provide nucleation sites for Laves phase precipitation, promoting the precipitation of nanoscale Laves phase, and playing a role in pinning grain boundaries and refining grains during recrystallization during solid solution; ⑥ Step 7) Solution treatment induces recrystallization in the deformed microstructure of the hot-rolled plate. During finishing rolling, the shear deformation grains, which constitute a large proportion, undergo shear band nucleation. The recrystallized grains inherit the γ orientation of the fibrous microstructure, resulting in small recrystallized grains that are primarily γ oriented. The smaller α and η fiber microstructures, which dynamically recover, generate fine subgrains that aggregate, resulting in small recrystallized grains that are primarily α and η oriented. The similar grain sizes formed by the two nucleation methods reduce the alternating distribution of coarse / fine grains and the alternating distribution of α, η, and γ fiber textures. Simultaneously, the Laves phase precipitation during finishing rolling acts as a grain boundary pinning agent and grain refinement agent. Deformation bands, shear bands, and other deformation microstructures provide nucleation sites, promoting recrystallization. ⑦ Step 8) Remove the oxide scale generated by the solution treatment to prevent it from affecting the surface quality of the cold-rolled stainless steel sheet; ⑧ Step 9) Using a large reduction rate ensures that the fine and uniform equiaxed grains formed by solution treatment undergo large deformation during cold rolling, increasing the number of shear-deformed grains and further breaking the grain size advantage; a small reduction per pass can reduce the non-uniformity of intergranular deformation, and multi-pass rolling can improve the uniformity of intragranular deformation, reducing the phenomenon of alternating thickness and alternating distribution of internal microstructures (deformation bands, shear bands) along the normal thickness of the cold-rolled deformed structure; ⑨ Step 10) Recrystallization annealing causes recrystallization in the cold-rolled sheet, restoring the material's plasticity and toughness. The majority of shear-deformed grains undergo shear band nucleation, and the recrystallized nuclei inherit the γ orientation of the fibrous structure, resulting in small recrystallized grains primarily γ-oriented. The minority of α and η fibrous structures undergo subgrain aggregation nucleation. During cold rolling, the large reduction rate results in a small thickness of the deformation band along the normal direction, thus forming small recrystallized grains primarily α and η-oriented. Similar to solution treatment, the recrystallized grain sizes formed by the two nucleation methods are similar, reducing the alternating distribution of coarse / fine grains and the alternating distribution of α, η, and γ fibrous textures. Furthermore, the generated γ texture has high strength. Simultaneously, during low-temperature annealing, nanoscale Laves phases precipitate, pinning grain boundaries and subgrain boundaries, refining the grains. During high-temperature annealing, although the Laves phase disappears and the pinning effect disappears, the recrystallization driving force increases, increasing the number of recrystallization nuclei. Due to competition among the recrystallization nuclei, the grains are refined.

[0006] Furthermore, in step 1), a vacuum induction furnace is used for melting.

[0007] Furthermore, in step 2), the cooling rate of the billet is ≤20℃, which provides sufficient time for element diffusion and avoids element segregation that could lead to the easy precipitation of carbonitrides and cause brittleness risk.

[0008] Furthermore, in step 3), the ground billet is heated and held at that temperature for 1-4 hours.

[0009] Furthermore, in step 4), during multi-pass rough rolling, the single-pass reduction rate is ≥15%.

[0010] Furthermore, in step 5), if the exit temperature of the hot-rolled plate after a certain pass of finishing rolling is lower than 650°C, it is heated to 750-860°C before the next pass of finishing rolling is performed to improve the quality of finishing rolling.

[0011] Furthermore, in step 7), the rapid cooling temperature is reduced to ≤10℃ and the rapid cooling rate is ≥10℃ / s.

[0012] Furthermore, in step 10), water cooling is used to bring the temperature to room temperature, and the cooling rate is controlled at 100-200℃ / s.

[0013] The beneficial effects of this invention are as follows: The hot rolling process employs a two-step method of rough rolling and cooling finish rolling. Firstly, it avoids the nose temperature (800~850℃) for σ phase precipitation, delaying the precipitation of the brittle σ phase. Secondly, cooling finish rolling increases the number of shear-deformed grains, leading to an increase in the number of shear band nuclei during solution treatment, resulting in smaller recrystallized grains that are primarily γ-oriented. Simultaneously, it makes the subgrains generated by the dynamic recovery of α and η fiber structures during rough rolling even smaller, and the recrystallized grains formed by subgrain polymerization during solution treatment even finer. The recrystallized grain sizes formed by the two nucleation methods are similar, reducing the alternating distribution of coarse / fine grains and the alternating distribution of α, η, and γ fiber textures. Cold rolling uses a large reduction rate to break the grain size advantage, causing large deformation of fine, uniform equiaxed grains and increasing the number of shear-deformed grains. The cooling finish rolling and recrystallization annealing processes precipitate nanoscale Laves phases, which can pin grain boundaries and subgrain boundaries, further refining the grains. The preparation process of this invention achieves synergistic regulation of microstructure and precipitation, solving the problems of alternating distribution of coarse / fine crystals and alternating distribution of α, η and γ fiber textures, thereby improving mechanical and forming properties. Attached Figure Description

[0014] 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.

[0015] 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.

[0016] Figure 1 This is a schematic diagram of the nucleation mechanism (subcrystalline polymerization) during the solid solution process; Figure 2 This is a schematic diagram of the nucleation mechanism (shear band nucleation) during solid solution processing; Figure 3 This is a schematic diagram of nanoscale Laves phase pinning grain boundaries and subgrain boundaries during the solid solution process; Figure 4 The IPF and texture diagram of the hot-rolled fine-rolled plate obtained by this invention; Figure 5 IPF and texture diagrams of hot-rolled plates obtained for comparison; Figure 6 The IPF and texture diagram of the solid solution plate obtained by this invention; Figure 7 IPF and texture diagrams of solid solution plates obtained for comparison; Figure 8 The IPF and texture diagram of the cold-rolled sheet obtained by this invention; Figure 9 The IPF and texture diagram of the cold-rolled sheet obtained in Comparative Example 1; Figure 10 The image shows the IPF and texture diagram of the cold-rolled recrystallized annealed sheet of Embodiment 1 of the present invention; Figure 11 The IPF and texture diagram of the cold-rolled recrystallized annealed sheet of Embodiment 2 of the present invention are shown below; Figure 12 The IPF and texture diagram of the cold-rolled recrystallized annealed sheet of Embodiment 3 of the present invention are shown below. Figure 13 The IPF and texture diagrams of the cold-rolled recrystallized annealed sheet in Comparative Example 1 are shown. Figure 14 The IPF and texture diagrams are for Comparative Example 2, showing the cold-rolled recrystallized annealed sheet. Detailed Implementation

[0017] 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.

[0018] 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.

[0019] 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.

[0020] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0021] A method for improving the formability of high-chromium, high-molybdenum ferritic stainless steel includes the following steps: 1) Smelting: The weight percentages of each element in the smelting raw materials are as follows: 25.0%≤Cr≤30.0%, 2.0%≤Mo≤4.5%, 1.0%≤Ni≤4.0%, C≤0.015%, N≤0.010%, 0.15%≤Nb≤0.45%, 0.05%≤Ti≤0.35%, Mn≤0.3%, Si≤1.0%, S≤0.005%, P≤0.005%, O≤0.004%, and satisfy Cr+3.3×Mo≥35%, with the remainder being Fe and unavoidable impurities. After the raw materials are prepared according to the above element ratios, they are smelted in a vacuum induction furnace to obtain molten steel. 2) Casting: The molten steel obtained in step 1) is poured into a billet, which is then slowly cooled to room temperature at a cooling rate of ≤20℃ and then ground. 3) Forging: Heat the ground billet obtained in step 2) to 1180-1280℃, hold for 1-4 hours, with a heating rate ≤10℃ / min, initial forging temperature ≥1120℃, final forging temperature ≥960℃, and air cool to room temperature after forging. 4) Hot rolling roughing: After holding the forging billet obtained in step 3) at 1100-1200℃, hot continuous rolling or multi-pass roughing is carried out. The total reduction rate of hot continuous rolling or roughing is ≥60%. When carrying out multi-pass roughing, the single-pass reduction rate is ≥15%. If the exit temperature of the hot rolled plate after a certain pass is lower than 1000℃, it is heated to 1100-1200℃ and held for 5-20 minutes before the next pass of roughing is carried out. If the exit temperature of the hot rolled plate after a certain pass is higher than 1000℃, the next pass of roughing is carried out directly. 5) Hot-rolled cooling finishing rolling: The rough-rolled plate obtained in step 4) is cooled to 750-860℃ and subjected to multi-pass cooling rolling. If the exit temperature of the hot-rolled plate after a certain pass finishing rolling is lower than 650℃, it is heated to 750-860℃ and then subjected to the next pass finishing rolling. The single-pass reduction rate is ≥10%, and the total reduction rate is ≥60%. 6) Curled: The finished rolled plate obtained in step 5) is rolled into a hot rolled plate coil (the temperature of the hot rolled plate obtained by the cooling rolling method has been reduced to the coiling temperature of 500-600℃, so there is no need to cool the finished rolled plate, and it can be rolled directly). 7) Unwinding and solution treatment: Unwind the hot-rolled coil obtained in step 6) and perform solution treatment. The heating temperature is 1000-1100℃ and the holding time is 5-60min. Then, the temperature is rapidly cooled to ≤10℃ and the rapid cooling rate is ≥10℃ / s. 8) Pickling: The solution plate obtained in step 7) is then acid-washed. 9) Cold rolling: The pickled solution plate obtained in step 8) is subjected to multi-pass room temperature cold rolling, with a single-pass reduction rate of <10% and a total cold rolling reduction rate of 60-90%. 10) Recrystallization annealing: The cold-rolled sheet obtained in step 9) is subjected to recrystallization annealing at a temperature of 950-1090℃ and a holding time of 0.5-5min. Then, it is cooled to room temperature by water at a rate of 100-200℃ / s.

[0022] In specific implementation, the chemical composition of the specific embodiments and comparative examples of the present invention is shown in Table 1, the main process parameters of the specific embodiments of the present invention are shown in Table 2, and the main process parameters of the comparative examples are shown in Table 3.

[0023] Table 1 Chemical composition of various embodiments and comparative examples of the present invention

[0024] Table 2 Main parameters of the preparation process in the embodiments of the present invention

[0025] Table 3 Main parameters of the comparative example preparation process

[0026] The room temperature tensile test was conducted according to GB / T 228.1-2021 standard. A CMT5105-SANS microcomputer-controlled electronic universal testing machine was used to test the room temperature tensile properties of the specimens. The tensile rate was controlled at 1 mm / min using an electronic extensometer. Two parallel samples were set for each group of specimens. The tensile strength, yield strength, and elongation after fracture were tested, and the average and standard deviation of the measured values ​​were calculated. Simultaneously, the CMT5105-SANS microcomputer-controlled electronic universal testing machine was used for tensile testing, with the tensile rate controlled at 1 mm / min using an electronic extensometer and the tensile strain at 15%. After tensile deformation, the surface roughness of the tensile specimens perpendicular to the tensile direction was measured using an MFT-4000 multifunctional material surface performance tester. The results of the room temperature tensile properties and surface roughness are shown in Table 4.

[0027] Table 4

[0028] Comparing the experimental data of the embodiments of the present invention with those of the comparative examples, it can be seen that the yield strength, tensile strength, and elongation after fracture of Examples 1-4 are all greater than those of Comparative Examples 1 and 2, the average grain size is close to that of Comparative Examples 1 and 2, and the surface roughness of Examples 1-4 is smaller than that of Comparative Examples 1 and 2, indicating that the steel prepared by this method has excellent mechanical properties and formability. This is because the cooling precision rolling method increases the number of shear deformation grains without the same σ precipitation. Shear band nucleation occurs during solution treatment, resulting in small recrystallized grains that are mainly γ-oriented. It also makes the subgrains generated by the dynamic recovery of α and η fiber structures during rough rolling smaller, and subgrain polymerization occurs during solution treatment, resulting in small recrystallized grains that are mainly α and η-oriented. The recrystallized grain sizes formed by the two nucleation methods are similar, which reduces the alternating distribution of coarse / fine grains and the alternating distribution of α, η and γ fiber textures. During cold rolling, a large reduction rate is used to induce large deformation in fine, uniform equiaxed grains, increasing the number of shear-deformed grains and further breaking the grain size advantage. The nucleation mechanism during cold rolling and recrystallization annealing is the same as that during solution treatment. The nanoscale Laves phase precipitated during cooling finishing rolling and recrystallization annealing pins grain boundaries and subgrain boundaries, further refining the grains. Through synergistic control of microstructure and precipitation, the alternating distribution of coarse / fine grains and the alternating distribution of α, η, and γ fiber textures are resolved, improving mechanical and forming properties.

[0029] 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 improving the formability of high-chromium, high-molybdenum ferritic stainless steel, characterized in that, Includes the following steps: 1) Smelting: The weight percentage of each element in the smelting raw materials is as follows: 25.0%≤Cr≤30.0%, 2.0%≤Mo≤4.5%, 1.0%≤Ni≤4.0%, C≤0.015%, N≤0.010%, 0.15%≤Nb≤0.45%, 0.05%≤Ti≤0.35%, Mn≤0.3%, Si≤1.0%, S≤0.005%, P≤0.005%, O≤0.004%, and satisfies Cr+3.3×Mo≥35%, with the remainder being Fe and unavoidable impurities. After the raw materials are prepared according to the above element ratios, smelting is carried out to obtain molten steel. 2) Casting: The molten steel obtained in step 1) is poured into a billet, which is then slowly cooled to room temperature and then ground. 3) Forging: Heat the ground billet obtained in step 2) to 1180-1280℃, with a heating rate ≤10℃ / min, an initial forging temperature ≥1120℃, a final forging temperature ≥960℃, and air cool to room temperature after forging; 4) Hot rolling roughing: After holding the forging billet obtained in step 3) at 1100-1200℃, hot continuous rolling or multi-pass roughing is carried out. The total reduction rate of hot continuous rolling or roughing is ≥60%. When carrying out multi-pass roughing, if the exit temperature of the hot rolled plate after a certain pass is lower than 1000℃, it is heated to 1100-1200℃ and held for 5-20 minutes before the next pass of roughing is carried out. If the exit temperature of the hot rolled plate after a certain pass is higher than 1000℃, the next pass of roughing is carried out directly. 5) Hot-rolled cooling finishing rolling: The rough-rolled plate obtained in step 4) is cooled to 750-860℃ and subjected to multi-pass cooling rolling, with a single-pass reduction rate ≥10% and a total reduction rate ≥60%. 6) Curled: The fine-rolled plate obtained in step 5) is rolled into a hot-rolled plate coil; 7) Unwinding and solution treatment: Unwind the hot-rolled coil obtained in step 6) and perform solution treatment at a heating temperature of 1000-1100℃ for 5-60 minutes, followed by rapid cooling. 8) Pickling: The solution plate obtained in step 7) is then acid-washed. 9) Cold rolling: The pickled solution plate obtained in step 8) is subjected to multi-pass room temperature cold rolling, with a single-pass reduction rate of <10% and a total cold rolling reduction rate of 60-90%. 10) Recrystallization annealing: The cold-rolled sheet obtained in step 9) is subjected to recrystallization annealing at a temperature of 950-1090℃ for 0.5-5 min, and then cooled to room temperature.

2. The method for improving the formability of high-chromium, high-molybdenum ferritic stainless steel according to claim 1, characterized in that, In step 3), the heating rate of the billet is ≤10℃ / min, and the forging temperature is ≥1120℃.

3. The method for improving the formability of high-chromium, high-molybdenum ferritic stainless steel according to claim 2, characterized in that, In step 1), a vacuum induction furnace is used for melting.

4. The method for improving the formability of high-chromium, high-molybdenum ferritic stainless steel according to claim 3, characterized in that, In step 2), the cooling rate of the billet is ≤20℃.

5. The method for improving the formability of high-chromium, high-molybdenum ferritic stainless steel according to claim 4, characterized in that, In step 3), the ground billet is heated and held at that temperature for 1-4 hours.

6. The method for improving the formability of high-chromium, high-molybdenum ferritic stainless steel according to claim 5, characterized in that, In step 4), during multi-pass rough rolling, the single-pass reduction rate is ≥15%.

7. The method for improving the formability of high-chromium, high-molybdenum ferritic stainless steel according to claim 6, characterized in that, In step 5), if the exit temperature of the hot-rolled plate after a certain pass of finishing rolling is lower than 650°C, it is heated to 750-860°C before the next pass of finishing rolling is performed.

8. The method for improving the formability of high-chromium, high-molybdenum ferritic stainless steel according to claim 7, characterized in that, In step 7), the rapid cooling temperature is reduced to ≤10℃ and the rapid cooling rate is ≥10℃ / s.

9. A method for improving the formability of high-chromium, high-molybdenum ferritic stainless steel according to claim 8, characterized in that, In step 10), water cooling is used to bring the temperature to room temperature, and the cooling rate is controlled at 100-200℃ / s.

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