Fe-al high-strength high-plasticity ferrite low-density steel and preparation method thereof

Abstract

The application discloses a Fe-Al series high-strength high-plasticity ferrite low-density steel and a preparation method thereof. The steel contains the following components in percentage by weight: C: 0.001-0.005%, Mn: 0.1-0.5%, Al: 6-9%, and the rest is Fe and inevitable impurities. The microstructure of the Fe-Al series high-strength high-plasticity ferrite low-density steel is equiaxed ferrite grains with bimodal distribution. The steel is prepared by optimizing the composition of the steel and matching a suitable preparation process, and the preparation process comprises smelting, ingot casting, hot rolling, warm rolling and annealing. The Fe-Al series high-strength high-plasticity ferrite low-density steel has the advantages of low density, high strength, high plasticity and excellent forming performance, and the steel has equiaxed ferrite grains with bimodal distribution and strong gamma fiber texture ({111} / / ND).

Description

Technical Field

[0001] This invention belongs to the field of metallurgical materials technology and relates to a Fe-Al based high-strength, high-ductility ferritic low-density steel and its preparation method. Background Technology

[0002] Improving fuel efficiency, reducing carbon dioxide emissions, and conserving limited resources are objective requirements of today's sustainable development strategy, making lightweight transportation a future development trend. The focus of lightweight steel is to reduce structural density without compromising other desirable properties. This can be achieved by using high-strength, high-stiffness steel to reduce the dimensions of traditional steel, or by replacing steel with aluminum and aluminum alloys, magnesium and magnesium alloys, engineering plastics, or carbon fiber composites. However, research has found that the former is limited by its own dent resistance and stiffness, while the latter has high manufacturing costs, relatively complex forming processes, and poor weldability. Based on this, a research trend has emerged aiming to develop a steel sheet that combines low density with high strength and high ductility.

[0003] Al plays a powerful role in reducing density, increasing strength, and other engineering aspects such as alloy manufacturing and machinability, making it one of the most important alloying elements in the development of low-density steel. Disordered Fe-Al alloys represent a new type of low-density steel, attracting significant interest from industry professionals due to their low density and feasibility for mass production. Early research focused on the smelting and forging methods of Fe-Al based low-density steels (Fe-Al steels belong to single-phase ferritic steels), and produced low-density steels using hot rolling / cold rolling and other machining methods. These Fe-Al alloys exhibit large grain sizes (~1 mm) under casting conditions, and their strength and ductility remain relatively low compared to other competitive structural materials. Therefore, further improvements in the room-temperature tensile properties and formability of these low-density steels are needed.

[0004] Chinese patent CN101115850B discloses a novel Fe-Al alloy and its manufacturing method. After plastic processing of an alloy containing 2-12 wt% Al, with the balance being Fe and unavoidable impurities, the cold rolling processing conditions are controlled, preferably with a reduction in cross-section of 5% or more, and more preferably 20-95%. After annealing, an Fe-Al alloy with an average grain size of less than 250 μm and a microstructure different from conventional Fe-Al alloys can be obtained; however, the elongation of the Fe-Al alloy obtained after cold rolling and annealing using this technology is only 13%.

[0005] Chinese Patent Publication No. CN104169027A discloses a method for manufacturing Fe-Al alloys, which involves casting Fe-Al alloys with Al content of 2.0-9.0 wt% and the balance consisting of Fe and impurities. To reduce the risk of cracking, a novel idea is proposed: the surface temperature of the ingot removed from the mold is not cooled to below 250°C before entering the hot forging process, thereby suppressing the occurrence of micro-cracks and preventing cracks from becoming more pronounced during hot forging. Although the ingots produced by this technology achieve high impact resistance, the mechanical properties of the Fe-Al alloys prepared by this technology are not specifically mentioned.

[0006] In view of the above, there is an urgent need to develop a Fe-Al based low-density steel and its preparation method, which can reduce the density of steel while also possessing high strength, high plasticity, and excellent formability. Summary of the Invention

[0007] To address the problem that existing Fe-Al low-density steels cannot simultaneously possess low density, good mechanical properties, and formability, this invention provides a Fe-Al high-strength, high-ductility ferritic low-density steel and its preparation method. By optimizing the steel composition and using appropriate preparation processes, a low-density, high-strength, high-ductility, and excellent formability Fe-Al high-strength, high-ductility ferritic low-density steel is obtained through smelting, ingot casting, hot rolling, warm rolling, and annealing processes. It has bimodal equiaxed ferrite grains and a strong γ-fiber texture ({111} / / ND).

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] The first aspect of the present invention provides a Fe-Al based high-strength, high-ductility, ferritic, low-density steel, comprising the following components by weight percentage: C: 0.001-0.005%, Mn: 0.1-0.5%, Al: 6-9%, with the remainder being Fe and unavoidable impurities;

[0010] The microstructure of the Fe-Al series high-strength, high-plasticity ferritic low-density steel consists of equiaxed ferrite grains with a bimodal distribution.

[0011] Preferably, the average grain size of the Fe-Al based high-strength, high-ductility ferritic low-density steel is 10–63 μm.

[0012] Preferably, in the equiaxed ferrite grains, the volume content of ferrite grains with a size of 5 to 10 μm is 10 to 20%, and the volume content of ferrite grains with a size of 40 to 60 μm is 80 to 90%.

[0013] Preferably, the Fe-Al based high-strength, high-ductility ferritic low-density steel has a γ-fiber texture ({111} / / ND).

[0014] Preferably, the Fe-Al based high-strength, high-ductility, ferritic, low-density steel has a density of less than 7.5 g / cm³. 3 The room temperature yield strength is 400-500 MPa, the tensile strength is 500-600 MPa, the elongation after fracture is 30-41%, and the yield strength ratio is 0.75-0.9.

[0015] Preferably, the average plastic strain ratio R of the Fe-Al based high-strength, high-ductility ferritic low-density steel is... m ≥1, the anisotropy degree Δr is -0.5 to 0.5.

[0016] Preferably, the average plastic strain ratio R of the Fe-Al based high-strength, high-ductility ferritic low-density steel is... m It ranges from 1 to 1.5.

[0017] A second aspect of the present invention provides a method for preparing Fe-Al based high-strength, high-ductility, ferritic, low-density steel as described in the first aspect of the present invention, comprising the following steps:

[0018] S1, vacuum induction smelting, is carried out by feeding materials according to the composition of Fe-Al system high strength high plasticity ferritic low density steel and obtaining molten steel by vacuum induction smelting.

[0019] S2, Ingot casting: The molten steel obtained in step S1 is poured into the mold. The mold is demolded within 1 hour after the pouring is completed. After demolding, the steel is slowly cooled to room temperature to obtain the billet.

[0020] S3, hot rolling, involves heating and holding the billet at a certain temperature, then performing multiple hot rolling deformation passes, and finally air cooling to room temperature to obtain a hot-rolled plate.

[0021] S4, warm rolling, involves holding the hot-rolled plate at a temperature of 100-400℃ for more than 0.5 hours and then performing multiple cold deformation rolling processes, followed by air cooling to room temperature to obtain the warm-rolled plate.

[0022] S5, Annealing, is the process of annealing warm rolled steel to obtain Fe-Al series high-strength, high-plasticity ferritic low-density steel.

[0023] Preferably, in step S1, during the vacuum induction smelting process, when the vacuum level decreases to 2×10⁻⁶... -2 After the pressure drops below Pa, argon gas is introduced and stirring is performed; and / or

[0024] In step S2, the pouring temperature of the molten steel is 1550–1600℃, and the slow cooling rate is 5–10℃ / h; and / or

[0025] In step S3, during the hot rolling process:

[0026] The heating temperature is 1150–1250℃, the heating rate is 25–35℃ / h, and the holding time is more than 3 hours; and / or

[0027] The hot rolling temperature is 900-1150℃, the reduction per pass is less than 20%, the hot rolling is performed in 8-12 passes, and the total reduction rate is 80-90%.

[0028] Preferably, the hot rolling temperature is 950–1050°C.

[0029] Preferably, in step S4, during the warm rolling process:

[0030] The warm rolling temperature is 200–350℃, and the holding time is 0.5–1.5 h; the reduction per pass in the cold deformation rolling is less than 10%, and the total deformation is 60–80%; and / or

[0031] In step S5, the annealing temperature is 750–950°C and the annealing time is 1–15 min.

[0032] Preferably, the annealing temperature is 850–900°C and the annealing time is 5–10 min.

[0033] In the compositional design of the Fe-Al based high-strength, high-ductility, ferritic, low-density steel of this invention:

[0034] Aluminum and Iron: In the Fe-Al alloy system, the addition of Al expands the lattice parameters of the steel, and its low atomic weight effectively reduces the density of the steel. Each 1% addition of Al reduces the steel density by 0.101 g / cm³. 3 Density ρ≤7.5g / cm³ 3 The addition of more than 6% Al is required, and Al significantly improves the corrosion resistance and strength of steel. However, excessive Al content promotes the formation of brittle κ phases and intermetallic compounds, which in turn reduces ductility and toughness. Therefore, the Al content of the steel in this invention is limited to 6-9%.

[0035] Carbon and manganese: C and Mn mainly play a role in solid solution strengthening. The addition of small amounts of C and Mn can improve the comprehensive properties of steel, such as strength and toughness. However, the content of both in the Fe-Al alloy system should not be too high. Too high a content will lead to an increase in the overall C equivalent, which will easily form κ-carbides, causing cracks during deformation, resulting in material fracture and reduced plasticity. Therefore, the C content is limited to 0.001-0.005%, and the Mn content is limited to 0.1-0.5%.

[0036] The Fe-Al based high-strength, high-ductility ferritic low-density steel and its preparation method provided by this invention have the following beneficial effects:

[0037] 1. The Fe-Al series high-strength and high-plasticity ferritic low-density steel and its preparation method of the present invention are prepared by vacuum induction smelting, ingot casting, hot rolling, warm rolling and annealing processes. By introducing warm rolling, optimizing cold deformation rolling and annealing processes, the ferrite grain size is refined and a large amount of space is provided for the proliferation of mobile dislocations, making its plastic deformation stage uniform and continuous. At the same time, the microstructure and internal stress are improved, so that the morphology of the ferrite grains becomes an equiaxed structure with a bimodal distribution and has a strong γ fiber texture ({111} / / ND). The Fe-Al series high-strength and high-plasticity ferritic low-density steel has an average plastic strain ratio ≥1 and an anisotropy close to 0, which can better ensure formability and processing performance.

[0038] 2. The Fe-Al based high-strength, high-ductility, ferritic, low-density steel prepared by this invention utilizes a simplified Fe-Al alloy system, reducing the steel's density by approximately 4.5%, resulting in a density of less than 7.5 g / cm³. 3 At the same time, it significantly reduces production input, lowers production costs, and expands the application scope and application scenarios of steel grades;

[0039] 3. This invention optimizes the preparation process by innovatively introducing a reasonable combination of warm rolling and annealing processes into the process of Fe-Al system high-strength and high-ductility ferritic low-density steel. By controlling the dislocation movement, equiaxed ferrite grains with bimodal distribution are obtained. Combined with a strong γ-fiber texture ({111} / / ND), the Fe-Al system high-strength and high-ductility ferritic low-density steel achieves excellent formability and realizes a balance between strength and ductility. Attached Figure Description

[0040] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0041] Figure 1 This is a schematic flowchart of the preparation method of Fe-Al based high-strength, high-plasticity ferritic low-density steel of the present invention.

[0042] Figure 2 The image shows the ferrite microstructure of the Fe-Al series high-strength, high-plasticity, ferrite-based low-density steel prepared in Example 6 of this invention.

[0043] Figure 3 In the middle, (a) is Figure 2 Distribution diagram of ferrite grains with medium size of 5-10 μm, (b) is Figure 2 Distribution diagram of ferrite grains with a medium size of 40–60 μm;

[0044] Figure 4 This is a ferrite grain size distribution diagram of the Fe-Al system high-strength, high-plasticity, low-density ferrite steel prepared in Example 6 of the present invention;

[0045] Figure 5 The texture diagram is shown for the Fe-Al series high-strength, high-plasticity ferritic low-density steel prepared in Example 6 of this invention.

[0046] Figure 6 This is an engineering stress-strain curve obtained during the tensile test of the Fe-Al system high-strength, high-plasticity ferritic low-density steel prepared in Example 6 of the present invention. Detailed Implementation

[0047] To better understand the above-mentioned technical solutions of the present invention, the technical solutions of the present invention will be further described below in conjunction with embodiments.

[0048] A survey of existing e-Al series low-density steels revealed relatively little research on Fe-Al ferritic steels, which may be related to the lower strength and plasticity of Fe-Al ferritic steels. Therefore, this invention designs alloy compositions and optimizes the preparation process to obtain Fe-Al series high-strength and high-plasticity ferritic low-density steels that combine low density, high strength, high plasticity, and excellent formability.

[0049] The Fe-Al series high-strength, high-ductility ferritic low-density steel provided by this invention comprises the following components by weight percentage: C: 0.001-0.005%, Mn: 0.1-0.5%, Al: 6-9%, with the remainder being Fe and unavoidable impurities; the microstructure of this Fe-Al series high-strength, high-ductility ferritic low-density steel is equiaxed ferrite grains with a bimodal distribution.

[0050] In the equiaxed ferrite grains, the volume content of ferrite grains with a size of 5–10 μm is 10–20%, and the volume content of ferrite grains with a size of 40–60 μm is 80–90%. This Fe-Al series high-strength, high-ductility ferritic low-density steel also has a strong γ-fiber texture ({111} / / ND), where ND is the normal direction of the rolling surface, which can improve its formability.

[0051] Generally, for ferritic low-density steel, a tensile strength > 400 MPa is considered high strength, and an elongation after fracture > 20% is considered high ductility; the Fe-Al system high-strength, high-ductility ferritic low-density steel of this invention has a density of less than 7.5 g / cm³. 3 The tensile strength is 500-600 MPa, the elongation after fracture is 30-41%, and the yield strength ratio is 0.75-0.9.

[0052] In a specific embodiment, the grain size of the Fe-Al based high-strength, high-ductility ferritic low-density steel is 10–63 μm, and its average plastic strain ratio R m ≥1, in the preferred embodiment, the average plastic strain ratio R mThe anisotropy is 1 to 1.5; its anisotropy is close to 0, and the specific anisotropy Δr is -0.5 to 0.5, which can better ensure the formability and processing performance of the steel.

[0053] Combination Figure 1 As shown, the preparation method of the above-mentioned Fe-Al series high-strength, high-ductility ferritic low-density steel adopts vacuum induction smelting, ingot casting, hot rolling, warm rolling and annealing processes, specifically including the following steps:

[0054] S1, vacuum induction smelting, is carried out by feeding materials according to the composition of Fe-Al system high strength high plasticity ferritic low density steel and obtaining molten steel by vacuum induction smelting.

[0055] Specifically, according to the above-mentioned composition of Fe-Al system high-strength, high-plasticity ferritic low-density steel, metal raw materials are obtained. The metal raw materials can be high-purity Al rods, high-purity carbon rods, high-purity iron rods, and electrolytic manganese sheets, etc. These metal raw materials need to be pretreated before smelting, such as cleaning and drying. The cleaning solution can be alcohol, and the cleaning method can be ultrasonic cleaning.

[0056] The aforementioned metal raw materials were fed into a smelting furnace and smelted using vacuum induction. When the vacuum level decreased to 2×10⁻⁶, -2 After the Pa level is below a certain point, argon gas is introduced into the smelting furnace and stirred to obtain molten steel. To avoid smelting losses caused by the low C content and its low density, Fe, Mn, and Al, which have higher densities, are first added to the crucible for smelting. After the molten steel is completely melted, C is added and the melting process continues, thereby precisely controlling the alloy composition.

[0057] S2, Ingot casting: Pour the molten steel obtained in step S1 into the mold, demold within 1 hour after pouring, and slowly cool to room temperature after demolding to obtain the billet;

[0058] Specifically, the molten steel obtained above is poured into a mold, where the temperature of the mold is about 10% of the temperature of the molten steel, and the pouring temperature of the molten steel is 1550-1600℃. The molten steel is demolded within 1 hour after pouring, and then slowly cooled to room temperature at a cooling rate of 5-10℃ / h to obtain a ferritic low-density steel billet.

[0059] S3, hot rolling, involves heating and holding the billet at a certain temperature, then performing multiple hot rolling deformation passes, and finally air cooling to room temperature to obtain a hot-rolled plate.

[0060] Specifically, after removing the riser from the aforementioned billet, the temperature is slowly increased to 1150–1250°C at a heating rate of 25–35°C / h and held for at least 3 hours to ensure uniform heating and dissolution of elements within the billet. Then, the heated billet is subjected to multi-pass hot rolling deformation using a two-roll reversible mill. The hot rolling temperature is controlled at 900–1150°C, with a reduction of less than 20% per pass, for 8–12 passes, resulting in a total reduction of 80–90%. The billet is then cooled to room temperature in air. In a specific embodiment, the hot rolling temperature is 980–1150°C, the initial rolling temperature is controlled at 1000–1150°C, and the final rolling temperature is ≤1100°C. This hot rolling process helps prevent rolling cracks.

[0061] S4, warm rolling, involves holding the hot-rolled plate at a temperature of 100-400℃ for more than 0.5 hours and then performing multiple cold deformation rolling processes, followed by air cooling to room temperature to obtain the warm-rolled plate.

[0062] Specifically, the hot-rolled plate prepared above is heated to 100–400°C in a furnace and held at that temperature for at least 0.5 hours before being directly subjected to multi-pass cold deformation rolling. Finally, it is air-cooled to room temperature to obtain a warm-rolled plate. In the multi-pass cold deformation rolling, the reduction in each pass is less than 10%, the total deformation is 60–80%, and the thickness of the obtained warm-rolled plate is 0.5–1.5 mm. In a preferred embodiment, the warm rolling temperature is 200–350°C, and the holding time after cold deformation rolling is 0.5–1.5 hours. During this process, by increasing the cold deformation rolling temperature, equiaxed ferrite grains with a bimodal distribution are formed in the steel.

[0063] S5, Annealing, is the process of annealing warm rolled steel to obtain Fe-Al series high-strength, high-plasticity ferritic low-density steel.

[0064] Specifically, the warm-rolled steel obtained above is annealed at a temperature of 750–950°C for 1–15 minutes. After annealing, it is cooled to room temperature in the furnace to obtain Fe-Al based high-strength, high-ductility, ferritic, low-density steel. In a specific embodiment, the annealing temperature is 850–900°C for 5–10 minutes.

[0065] The properties of the Fe-Al based high-strength, high-ductility, ferritic, low-density steel prepared above are as follows: room temperature yield strength of 400–500 MPa, tensile strength of 500–600 MPa, elongation after fracture of 30–41%, and density of less than 7.5 g / cm³. 3 The grain size is higher than grade 7, the average plastic strain ratio Rm is 1 to 1.5, and the anisotropy is Δr of -0.5 to 0.5.

[0066] The microstructure of this Fe-Al series high-strength, high-plasticity ferritic low-density steel consists of equiaxed ferrite grains with a bimodal distribution. The volume content of ferrite grains with a size of 5–10 μm is 10–20%, and the volume content of ferrite grains with a size of 40–60 μm is 80–90%.

[0067] This Fe-Al series high-strength, high-plasticity ferritic low-density steel has a high-density γ-fiber texture ({111} / / ND), an average plastic strain ratio ≥1, and an anisotropy close to 0.

[0068] The Fe-Al based high-strength, high-plasticity ferritic low-density steel and its preparation method are further described with specific examples.

[0069] Example

[0070] The preparation method of Fe-Al based high-strength, high-ductility ferritic low-density steel of the present invention used in Examples 1-10 is as follows:

[0071] (1) Prepare the metal raw materials according to the composition of Fe-Al series high-strength, high-ductility ferritic low-density steel shown in Table 1. After ultrasonic cleaning and drying in alcohol, the metal raw materials are smelted in vacuum induction to obtain 3 furnaces of molten steel. The composition is shown in Table 1. Specifically, Fe, Mn and Al are first added to the crucible of the smelting furnace. After they are completely melted, C raw materials are added. After all raw materials are completely melted, the vacuum induction furnace is evacuated and the vacuum degree is reduced to 2×10. -2 After the pressure drops below Pa, high-purity argon gas is introduced into the vacuum induction furnace and stirred to obtain molten steel.

[0072] (2) Pour the molten steel into the mold. The pouring temperature of the molten steel is 1550-1600℃. Demold within 1 hour after pouring. After demolding, cool slowly to room temperature at a cooling rate of 5-10℃ / h to obtain the billet.

[0073] (3) After removing the riser from the billet, cut it into several 100mm×80mm×30mm steel blocks, put them into a heating furnace, and slowly heat them to 1150-1250℃ at a heating rate of 25-35℃ / h and hold them for more than 3 hours. In Examples 1-5, the heating temperature is 1150℃ and the holding time is 6 hours; in Examples 6-10, the heating temperature is 1250℃ and the holding time is 3 hours. The heated billet is hot rolled on a two-roll reversible rolling mill. The starting and ending rolling temperatures are detailed in Table 2. Hot rolling is performed in 8-12 passes, with a reduction of less than 20% per pass, and a total reduction rate of 80-90%. The final thickness of the hot-rolled plate is 3-6mm, and it is air-cooled to room temperature.

[0074] (4) After holding the hot-rolled plate at a temperature of 100-400℃ for more than 0.5h on a two-roll mill, it is subjected to multiple cold deformation rolling. The reduction in each pass is less than 10%, and the total deformation of the warm rolling is 60-80%. The specific warm rolling temperature and the thickness of the warm-rolled plate are detailed in Table 2.

[0075] (5) The warm-rolled plate is added to the heating furnace for annealing. The specific annealing temperature and time are detailed in Table 2. After annealing, the plate is cooled in the furnace to obtain the final Fe-Al system high-strength, high-plasticity ferritic low-density steel. Its room temperature yield strength (YS), tensile strength (UTS), elongation after fracture (δ), yield ratio (YR), density, grain size, average plastic strain ratio and anisotropy are listed in Table 3.

[0076] As shown in Table 3, the Fe-Al based high-strength, high-ductility ferritic low-density steel prepared in Example 6 exhibits the highest ductility, achieving a better balance between strength and ductility. The microstructure of the Fe-Al based high-strength, high-ductility ferritic low-density steel prepared in Example 6 is shown in the figure below. Figure 2 As shown, where Figure 3 (a) shows the distribution of ferrite grains with a size of 5–10 μm, and their volume content is 10–20%. Figure 3 (b) shows the distribution of ferrite grains with a size of 40–60 μm and a volume content of 80–90%.

[0077] Figure 4 The image shows the ferrite grain size distribution of the Fe-Al high-strength, high-ductility ferritic low-density steel prepared in Example 6. The size distribution was determined using the method specified in ASTM E2627 2013. The Fe-Al high-strength, high-ductility ferritic low-density steel was subjected to EBSD experiments using a low-vacuum ultra-high resolution field emission scanning electron microscope, and the results were processed using OIM analysis software. As can be seen from the image, the ferrite grains in this Fe-Al high-strength, high-ductility ferritic low-density steel exhibit a bimodal distribution.

[0078] like Figure 5 The texture diagram of the steel shown has the highest density of γ-fiber texture ({111} / / ND); the engineering stress-strain curve of the steel plate under tensile test is shown below. Figure 6 As shown, its tensile strength is 533 MPa and its elongation after fracture is 40.1%.

[0079] The deep-drawing performance of the Fe-Al ferritic low-density steels obtained in Examples 1-10 was tested. The average plastic strain ratio was ≥1 and the anisotropy was close to 0, which ensured the formability and processing performance.

[0080] Comparative Example

[0081] According to the comparative composition in Table 1, prepare the required raw materials in the corresponding proportions and place them in the crucible of the vacuum induction furnace, then reduce the vacuum level inside the furnace to 2×10⁻⁶. -2 Below Pa, after complete melting, high-purity argon gas is introduced into a vacuum induction furnace and stirred to obtain molten steel. The molten steel is poured into a mold at a pouring temperature of 1550–1600℃. The mold is removed within 1 hour after pouring, and the ingot is slowly cooled to room temperature at a rate of 5–10℃ / h to obtain a ferritic low-density steel billet. After removing the riser, the billet is placed in a heating furnace and slowly heated to 1250℃ and held for 1 hour. After heating, it is hot-rolled at a final rolling temperature of 900℃ and air-cooled to room temperature, resulting in a steel plate thickness of 3mm. The hot-rolled plate is then cold-rolled from 3mm to 1.2mm at room temperature. Finally, the cold-rolled plate is annealed at 900℃ for 5 minutes. The specific rolling and heat treatment processes are shown in Table 2. The room temperature yield strength (YS), tensile strength (UTS), elongation after fracture (δ), yield ratio (YR), density, grain size, average plastic strain ratio, and anisotropy of the final steel plate are listed in Table 3.

[0082] Table 1. Chemical composition (wt%) of Fe-Al based high-strength, high-ductility, ferritic, low-density steel.

[0083] Ⅰ 6.1 0.001 0.48 Ⅱ 7.5 0.0025 0.10 Ⅲ 8.8 0.005 0.26 Comparative Example 6.8 0.0035 0.10

[0084] Table 2. Process parameters for preparing Fe-Al based high-strength, high-ductility, ferritic, low-density steel

[0085]

[0086] Table 3 Properties of Fe-Al based high-strength, high-ductility, ferritic, low-density steels

[0087]

[0088]

[0089] As shown in Tables 1, 2, and 3, the steel plates prepared in the comparative examples use the steel composition of this invention, although their density is less than 7.5 g / cm³. 3 However, because it does not incorporate a warm rolling process, its room temperature yield strength, tensile strength, elongation after fracture, and anisotropy are relatively poor. The Fe-Al series high-strength, high-ductility ferritic low-density steel prepared in this embodiment of the invention has the following properties: room temperature yield strength of 415–480 MPa, tensile strength of 510–590 MPa, elongation after fracture of 30–41%, and density of 7.2–7.5 g / cm³. 3The grain size is higher than grade 7 (equivalent to an average grain size between 16 and 63 μm), the average plastic strain ratio Rm is 1 to 1.5, and the anisotropy Δr is -0.5 to 0.5; the grain size grades in Table 3 are obtained according to ASTM E2627-2013. Therefore, this invention, through designing the steel composition and optimizing the preparation process, enables the Fe-Al series high-strength, high-ductility ferritic low-density steel prepared in the examples to possess characteristics such as low density, good mechanical properties, and formability.

[0090] In summary, this invention optimizes the steel composition and, with appropriate manufacturing processes, increases the warm rolling temperature during cold deformation rolling to form equiaxed ferrite grains with a bimodal distribution. In this process, dislocations in large grains accumulate at grain boundaries, amplifying stress. The more dislocations accumulate, the greater the amplification factor. Lower stress can then trigger the activation of dislocation sources in the next grain, initiating dislocation-induced deformation (the force required to activate a dislocation source is greater than the force required for dislocation movement). Conversely, smaller grains have fewer dislocations and a smaller stress amplification factor. To achieve macroscopic plastic deformation, a larger external load is required, resulting in higher yield strength. The stress may then be transferred to either large or small grains, creating a cycle that achieves a strength-plasticity balance. Simultaneously, warm rolling facilitates dislocation movement within the rolled plate, promoting dynamic recrystallization. This significantly refines the grain structure of the low-density ferrite steel, achieving both strengthening and plasticity enhancement through the synergistic effect of grain refinement and dynamic recrystallization softening. On the other hand, warm rolling and annealing processes on ferritic low-density steel can enhance the density of γ-fiber texture ({111} / / ND), improve its average plastic strain ratio and anisotropy, and improve the forming performance of Fe-Al system high-strength and high-plasticity ferritic low-density steel.

[0091] Those skilled in the art should recognize that the above embodiments are merely illustrative of the present invention and are not intended to limit the present invention. Any variations or modifications to the above embodiments that are within the spirit and essence of the present invention will fall within the scope of the claims of the present invention.

Claims

1. A Fe-Al based high-strength, high-ductility, ferritic, low-density steel, characterized in that, It includes the following components by weight percentage: C: 0.001-0.005%, Mn: 0.1-0.5%, Al: 6-9%, with the remainder being Fe and unavoidable impurities; The microstructure of the Fe-Al based high-strength, high-ductility, ferritic, low-density steel consists of equiaxed ferrite grains with a bimodal distribution. The Fe-Al series high-strength, high-plasticity ferritic low-density steel has a γ-fiber texture ({111} / / ND). The Fe-Al series high-strength, high-ductility ferritic low-density steel has an elongation after fracture of 30-41% and a yield strength ratio of 0.75-0.

9. The average plastic strain ratio R of the Fe-Al based high-strength, high-ductility, ferritic, low-density steel m ≥1, anisotropy Δr is -0.5 to 0.5; The Fe-Al based high-strength, high-ductility ferritic low-density steel is prepared by the following method, including the following steps: S1, vacuum induction smelting, involves preparing the steel according to the composition of Fe-Al series high-strength, high-ductility, ferritic, low-density steel, and obtaining the molten steel using vacuum induction smelting. During the vacuum induction smelting process, when the vacuum level is reduced to 2×10 -2 After the pressure drops below Pa, argon gas is introduced and the mixture is stirred. S2, Ingot casting: The molten steel obtained in step S1 is poured into a mold. The steel is demolded within 1 hour of pouring and then slowly cooled to room temperature to obtain a cast billet. The pouring temperature of the molten steel is 1550-1600℃, and the cooling rate of the slow cooling is 5-10℃ / h. S3, hot rolling, involves heating and holding the cast billet at that temperature, followed by multiple passes of hot rolling deformation, and then air cooling to room temperature to obtain a hot-rolled plate. During the hot rolling process: The heating temperature is 1150–1250℃, the heating rate is 25–35℃ / h, and the holding time is more than 3 hours; and / or The hot rolling temperature is 900–1150℃, the reduction per pass is less than 20%, the hot rolling is performed in 8–12 passes, and the total reduction rate is 80–90%. S4, warm rolling, involves holding a hot-rolled plate at a temperature of 100–400°C for at least 0.5 hours, followed by multiple passes of cold deformation rolling, and then air-cooling to room temperature to obtain the warm-rolled plate. During the warm rolling process: The warm rolling temperature is 200–350℃, and the holding time is 0.5–1.5 h; the reduction in each pass of the cold deformation rolling is less than 10%, and the total deformation is 60–80%. S5, Annealing, is the process of annealing warm-rolled steel to obtain Fe-Al based high-strength, high-ductility, ferritic, low-density steel. Therefore, the annealing temperature is 750–950℃, and the annealing time is 1–15 min.

2. The Fe-Al series high-strength, high-ductility ferritic low-density steel according to claim 1, characterized in that, The average grain size of the Fe-Al series high-strength, high-plasticity ferritic low-density steel is 10–63 μm.

3. The Fe-Al series high-strength, high-ductility ferritic low-density steel according to claim 1, characterized in that, In the equiaxed ferrite grains, the volume content of ferrite grains with a size of 5 to 10 μm is 10 to 20%, and the volume content of ferrite grains with a size of 40 to 60 μm is 80 to 90%.

4. The Fe-Al series high-strength, high-ductility ferritic low-density steel according to claim 1, characterized in that, The density of the Fe-Al based high-strength, high-ductility, ferritic, low-density steel is less than 7.5 g / cm³. 3 Its tensile strength is 500-600 MPa.

5. The Fe-Al series high-strength, high-ductility ferritic low-density steel according to claim 1, characterized in that, The average plastic strain ratio R of the Fe-Al based high-strength, high-ductility, ferritic, low-density steel m It ranges from 1 to 1.

5.

6. A method for preparing Fe-Al based high-strength, high-ductility, ferritic, low-density steel as described in any one of claims 1 to 1, characterized in that, Includes the following steps: S1, vacuum induction smelting, involves preparing the steel according to the composition of Fe-Al series high-strength, high-ductility, ferritic, low-density steel, and obtaining the molten steel using vacuum induction smelting. During the vacuum induction smelting process, when the vacuum level is reduced to 2×10 -2 After the pressure drops below Pa, argon gas is introduced and the mixture is stirred. S2, Ingot casting: The molten steel obtained in step S1 is poured into a mold. The steel is demolded within 1 hour of pouring and then slowly cooled to room temperature to obtain a cast billet. The pouring temperature of the molten steel is 1550-1600℃, and the cooling rate of the slow cooling is 5-10℃ / h. S3, hot rolling, involves heating and holding the cast billet at that temperature, followed by multiple passes of hot rolling deformation, and then air cooling to room temperature to obtain a hot-rolled plate. During the hot rolling process: The heating temperature is 1150–1250℃, the heating rate is 25–35℃ / h, and the holding time is more than 3 hours; and / or The hot rolling temperature is 900–1150℃, the reduction per pass is less than 20%, the hot rolling is performed in 8–12 passes, and the total reduction rate is 80–90%. S4, warm rolling, involves holding a hot-rolled plate at a temperature of 100–400°C for at least 0.5 hours, followed by multiple passes of cold deformation rolling, and then air-cooling to room temperature to obtain the warm-rolled plate. During the warm rolling process: The warm rolling temperature is 200–350℃, and the holding time is 0.5–1.5 h; the reduction in each pass of the cold deformation rolling is less than 10%, and the total deformation is 60–80%. S5, Annealing, is the process of annealing warm-rolled steel to obtain Fe-Al based high-strength, high-ductility, ferritic, low-density steel. Therefore, the annealing temperature is 750–950℃, and the annealing time is 1–15 min.

7. The method for preparing Fe-Al based high-strength, high-ductility, ferritic, low-density steel according to claim 6, characterized in that, The hot rolling temperature is 950–1050℃.

8. The method for preparing Fe-Al based high-strength, high-ductility, ferritic, low-density steel according to claim 6, characterized in that, The annealing temperature is 850–900℃, and the annealing time is 5–10 min.

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