Ferrite-based light weight steel and method for producing the same

Abstract

The application discloses a ferrite-based light steel and a preparation method thereof, and belongs to the technical field of light steel, and solves one of the problems of the cold speed sensitivity and narrow use temperature range of the light steel in the prior art. The components of the ferrite-based light steel are as follows in percentage by mass: C 0.04% to 0.12%, Mn 25.0% to 36.0%, Al 8.0% to 10.0%, P < 0.02%, S < 0.01%, Nb 0.05% to 1.0%, V 0.05% to 1.0%, Mo 0.5% to 1.5%, and W 0.5% to 1.5%; and the balance is Fe and inevitable trace impurities. The ferrite-based light steel has good strength and toughness at room temperature to -196 DEG C, has a wide application range, and has a simple preparation method.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of light steel, in particular to a ferrite-based light steel and a preparation method thereof. BACKGROUND

[0002] Light steel is an ideal energy-saving and environment-friendly material in the structural applications of automobiles, ships, pipelines, low-temperature pressure vessels, aerospace, etc. Light steel is mainly divided into four types according to the structure, i.e. ferrite light steel, austenite light steel, austenite-based dual-phase light steel, and ferrite-based dual-phase light steel. The current light steels have their own advantages and disadvantages, but none of them can meet the complex environmental requirements of low density, easy welding (cold speed insensitivity), wide temperature range, high strength, high plasticity, and high toughness matching in the future.

[0003] For example, the austenite light steel has good strength and plasticity, but has a significant ductile-brittle transition phenomenon, especially when the Al content is high, and the mechanism of the ductile-brittle transition is not clear. The austenite-based dual-phase light steel has similar performance to the austenite light steel, and the deformation incompatibility between the two phases worsens its impact toughness. The ferrite light steel has low plasticity and shows obvious brittle fracture. The traditional ferrite-based dual-phase light steel has a low Al content (<6Al) and a high austenite content, and the deformation mainly relies on the TWIP and TRIP effects of the austenite, the density reduction is limited, and there is a significant ductile-brittle transition phenomenon.

[0004] CN 114892084 A discloses a high-impact toughness high-strength austenite light steel and a manufacturing method thereof. The chemical composition is as follows in terms of weight percentage: Mn: 23-26%, Al: 6.90-8.20%, C: 0.83-0.92%, Si: 0.15-0.35, Cr: 0.05-0.14%, Cu: 0.1-0.3%, Nb: 0.01-0.04, N≤0.10%, P<0.008, S<0.002, and the balance is iron and inevitable impurities. The quenching and solid solution process requires high cooling system, which is not conducive to subsequent welding application, and cannot be applied at lower temperatures (such as -196℃).

[0005] CN 116288021 A discloses a high-strength and high-toughness dual-phase low-density steel, a preparation method and application thereof. The chemical composition is as follows in terms of weight percentage: C: 0.58-0.62%, Mn: 18-22%, Al: 8-10%, Nb: 0-0.02%, and the balance is Fe and other inevitable impurities. The structure contains about 90% austenite, the deformation is mainly austenite, no impact performance test is performed, and no low-temperature performance is provided.

[0006] Therefore, how to provide a high-strength, plastic and tough light steel that is not sensitive to cold speed and can be used in a wide temperature range has become a problem to be solved. SUMMARY

[0007] In view of the above, the present invention aims to provide a ferritic-based lightweight steel and its preparation method, in order to solve one of the problems of existing lightweight steels being sensitive to cooling rate and having a narrow operating temperature range.

[0008] The objective of this invention is mainly achieved through the following technical solutions:

[0009] This invention provides a ferritic lightweight steel, the composition of which, by mass percentage, is: C 0.04%–0.12%, Mn 25.0%–36.0%, Al 8.0%–10.0%, P < 0.02%, S < 0.01%, Nb 0.05%–1.0%, V 0.05%–1.0%, Mo 0.5%–1.5%, W 0.5%–1.5%; the balance being Fe and unavoidable trace impurities.

[0010] Furthermore, the microstructure of ferrite-based lightweight steel includes a ferrite matrix and extremely fine DO3 nano-precipitates and polygonal blocky austenite uniformly distributed on the matrix.

[0011] Furthermore, in the microstructure of ferrite-based lightweight steel, the ferrite matrix exhibits planar slip characteristics.

[0012] Furthermore, in the microstructure of ferritic lightweight steel, the volume percentage of austenite is approximately 20%–35%, and the volume percentage of ferrite is approximately 65%–80%.

[0013] Furthermore, the impact energy of ferritic lightweight steel at -196°C is above 95J.

[0014] The present invention also provides a method for preparing ferritic lightweight steel, comprising the following steps: steelmaking, slab continuous casting or ingot casting, heating, rolling or forging, cooling, solution treatment and cooling treatment after solution treatment.

[0015] Furthermore, the heating process includes: holding the billet at 1100-1200℃ for 2-4 hours to homogenize it.

[0016] Furthermore, the rolling or forging process includes hot rolling or forging at 830–1150°C to form bars or plates of different specifications.

[0017] Furthermore, the cooling process after rolling or forging employs conventional water cooling, oil cooling, or air cooling.

[0018] Furthermore, the solution treatment process includes: high-temperature solution treatment at 950–1150°C for 30–240 minutes.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0020] a) The compositional design of the ferrite-based lightweight steel of this invention employs a low C, high Mn, and high Al content, while incorporating Nb, V, Mo, and W alloys that strongly bind with C. Through compositional control, this ensures that the resulting microstructure consists of a ferrite matrix with uniformly distributed, extremely fine nano-DO3 particles and polygonal blocky austenite, thereby suppressing the precipitation of κ carbides. This guarantees the low-temperature stability and toughness of the ferrite-based lightweight steel of this invention.

[0021] (b) The ferrite-based lightweight steel of the present invention, through a low-carbon, high-alumina design and a suitable preparation method, introduces DO3 particles. By altering the deformation mechanism of ferrite, specifically through the dislocation shearing of extremely fine nano-DO3 particles, the deformation mechanism of ferrite exhibits planar slip characteristics, which greatly improves the strength, ductility, and toughness of the material. Furthermore, the uniformly distributed polygonal blocky austenite and nano-DO3 precipitates on the polygonal ferrite matrix significantly reduce the deformation incompatibility between the two phases, further improving the strength, ductility, and toughness of the material.

[0022] c) The ferrite-based lightweight steel of the present invention has a low density (e.g., 6.8–7.0 g / cm³). 3 The room temperature properties of the ferritic-based lightweight steel of the present invention are as follows: R t0.5 The ferrite-based lightweight steel of the present invention has the following properties at -196°C: Ra = 425 MPa or higher (e.g., 425–470 MPa), Rm = 740 MPa or higher (e.g., 741–755 MPa), A = 38% or higher (e.g., 38.5%–41%), and impact energy = 130 J or higher (e.g., 134–166 J). t0.5 The strength is 820 MPa or higher (e.g., 820–930 MPa), Rm is 1160 MPa or higher (e.g., 1161–1190 MPa), A is 38% or higher (e.g., 38%–41%), and impact energy is 95 J or higher (e.g., 95–122 J). The ferritic lightweight steel of this invention exhibits good strength and toughness from room temperature to -196°C, overcoming the challenge of the ductile-brittle transition in bcc structures. Compared to traditional ferritic lightweight steels, it has lower density, better low-temperature performance, and a wider operating temperature range, making it widely applicable in automotive, shipbuilding, pipeline, cryogenic pressure vessel, aerospace, and other fields.

[0023] d) The ferrite-based lightweight steel of the present invention reduces the requirements of the microstructure and properties on the cooling process by inhibiting the precipitation of κ-carbide. It is not sensitive to the cooling rate and has low requirements for the cooling process during preparation. The preparation method is simple.

[0024] Other features and advantages of the invention will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures particularly pointed out in the written description, claims, and drawings. Attached Figure Description

[0025] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0026] Figure 1 Typical metallographic structure of the ferrite-based lightweight steel obtained in Example 1;

[0027] Figure 2 The image shows a TEM bright-field image of ferrite at the uniform deformation site of the ferrite-based lightweight steel obtained in Example 1, and its properties in

[011] . δ Diffraction spots;

[0028] Figure 3 This is the dark field of the DO3 nanoprecipitated phase in Example 1;

[0029] Figure 4 The DO3 nano-precipitates and their size obtained by inverse Fourier transform in Example 1;

[0030] Figure 5 The tensile stress-strain curve for Example 1 is shown from room temperature to -196°C. Detailed Implementation

[0031] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of the present invention and, together with the embodiments of the present invention, serve to illustrate the principles of the present invention.

[0032] This invention provides a ferritic lightweight steel, wherein the composition of the ferritic lightweight steel, by mass percentage, is: C 0.04%–0.12%, Mn 25.0%–36.0%, Al 8.0%–10.0%, P < 0.02%, S < 0.01%, Nb 0.05%–1.0%, V 0.05%–1.0%, Mo 0.5%–1.5%, W 0.5%–1.5%; the balance being Fe and unavoidable trace impurities.

[0033] The following details the function and dosage selection of the components contained in this invention:

[0034] C: Too low a C content increases the difficulty of smelting, and also makes it difficult to form MC carbides, thus failing to effectively improve subsequent welding performance; while too high a C content increases the precipitation of κ-carbides in austenite and ferrite, especially resulting in the formation of large κ-carbide particles after welding, which seriously deteriorates welding performance. Therefore, this invention limits the C content to 0.04% to 0.12%.

[0035] Mn: Mn is an austenite stabilizing element that can expand the austenite phase region and obtain a certain amount of austenite. Simultaneously, a high Mn content can increase the stability of austenite at low temperatures, ensuring that the stacking fault energy of austenite at low temperatures is located within the TWIP mechanism, further improving the plasticity and toughness of austenite. However, when the Mn content is too high, it easily forms a brittle β-Mn phase, significantly reducing the material's toughness. Therefore, this invention limits the Mn content to 25%–36%.

[0036] Al: Al is a ferrite-forming element that can increase the proportion of ferrite. Al can significantly improve the strength of ferrite through solid solution strengthening. Al atoms significantly reduce the density of lightweight steel by changing the lattice parameters of steel and their low atomic mass. In addition, high Al content can precipitate nano-sized DO3 particles during cooling. Dislocation shearing of DO3 particles can change the deformation mode of bcc structure ferrite, significantly improving the strength, plasticity, and toughness of ferrite. Furthermore, high Al content can stabilize the stability of austenite at low temperatures, allowing austenite to be in the TWIP deformation mechanism at low temperatures, further improving the strength, plasticity, and toughness of the material. If the Al content is too high, austenite is difficult to stabilize, which will significantly reduce the low-temperature toughness. Therefore, this invention limits the Al content to 8% to 10%.

[0037] Nb, V, Mo, W: The low-carbon design and the addition of Nb, V, Mo, and W can greatly reduce the precipitation driving force of κ-carbides. Nb, V, Mo, and W reduce the precipitation driving force of κ-carbides after welding by forming carbides with C, while refining the grain size, ultimately achieving the goal of improving weld performance.

[0038] The present invention also provides a method for preparing the above-mentioned ferritic lightweight steel, comprising the following steps: steelmaking, slab continuous casting or ingot casting, heating, rolling or forging, cooling, solution treatment and cooling treatment after solution treatment.

[0039] Specifically, the aforementioned steelmaking, continuous casting, or ingot casting processes employ conventional smelting and pouring methods, such as induction melting, electric furnaces, or converters, and are then continuously cast or ingot cast into billets.

[0040] Specifically, the steps of the above heating process include: holding the billet at 1100-1200℃ for 2-4 hours to perform homogenization treatment.

[0041] Specifically, the steps of the above rolling or forging process include: forming bars or plates of different specifications by hot rolling or forging within the range of 830 to 1150°C, controlling the initial rolling or forging temperature at 1050 to 1150°C, and controlling the final rolling or forging temperature at 830 to 880°C.

[0042] Specifically, the cooling process after rolling or forging can be carried out using conventional water cooling, oil cooling, or air cooling.

[0043] Specifically, the steps of the above-mentioned solution treatment process include: high-temperature solution treatment at 950-1150℃ for 30-240 minutes to obtain a quasi-polygonal ferrite and austenite structure.

[0044] Specifically, the cooling process after the above-mentioned solid solution can be carried out by air cooling, oil cooling, or water cooling to precipitate DO3 nano-precipitates.

[0045] It should be noted that the parameters in the above preparation method are determined as follows:

[0046] Determination of the heating process: The liquidus temperature of the lightweight steel in this invention is between 1333 and 1555°C. Homogenization temperatures exceeding 1200°C result in abnormally long grains and potential localized melting; temperatures below 1100°C fail to achieve sufficient element homogenization. Therefore, the billet heating temperature is set between 1100 and 1200°C. To ensure effective homogenization of the microstructure and elements in the billet, the heating time is set to ≥2 hours. To avoid ineffective prolonged heating, the upper limit is set to 4 hours; that is, the heating time is controlled between 2 and 4 hours.

[0047] Determining the rolling or forging temperature: If the rolling or forging temperature is too high or too low, the microstructure will become severely unstable. Therefore, the rolling or forging temperature is set between 830 and 1150℃.

[0048] Determination of rolling or forging cooling process: Since the microstructure and properties of the steel components of this invention are not sensitive to the cooling rate, the cooling process can adopt conventional water cooling, oil cooling or air cooling.

[0049] Determination of the solution treatment process: Solution temperatures below 950℃ cannot effectively eliminate deformation incompatibility between the two phases; temperatures above 1150℃ result in abnormal grain growth and excessively low austenite content, reducing the material's strength, ductility, and toughness. Therefore, high-temperature solution treatment is performed between 950 and 1150℃. To ensure effective homogenization of the microstructure and elements in the solution-treated steel, the solution treatment time is set to ≥30 min. Excessively long solution treatment times have no significant impact on the microstructure and properties; to avoid ineffective long-term solution treatment, the upper limit is set at 240 min; that is, the solution treatment time is controlled between 30 and 240 min.

[0050] Determination of solution cooling process: Since the microstructure and properties of the steel of the present invention are not sensitive to the cooling rate, the cooling process window is relatively wide, and conventional water cooling, oil cooling or air cooling can be used.

[0051] Specifically, the microstructure of the aforementioned ferrite-based lightweight steel includes a ferrite matrix and uniformly distributed, extremely fine DO3 nano-precipitates and polygonal blocky austenite on the matrix. The ferrite matrix exhibits planar slip characteristics.

[0052] Specifically, in the microstructure of the aforementioned ferrite-based lightweight steel, the volume percentage of austenite is approximately 20% to 35%, and the volume percentage of ferrite is approximately 65% ​​to 80%.

[0053] Specifically, in the microstructure of the aforementioned ferrite-based lightweight steel, the average grain size of austenite is approximately 15.5–18.5 μm, the average grain size of ferrite is approximately 47.5–51.5 μm, and the average size of the DO3 nanoprecipitate is approximately 1.5–3 nm.

[0054] The design concept of the ferrite-based lightweight steel of the present invention is to adopt a low C, high Mn, and high Al content, while adding Nb, V, Mo, and W alloys that have strong bonding with C, and combining them with appropriate solid solution processes to obtain extremely fine DO3 nano-precipitated phase particles and polygonal blocky austenite uniformly distributed on the ferrite matrix.

[0055] The ferrite-based lightweight steel of this invention introduces DO3 particles through a low-carbon, high-aluminum design. By altering the deformation mechanism of ferrite—specifically, by introducing extremely fine nano-DO3 particles that undergo dislocation shear—the deformation mechanism of ferrite exhibits planar slip characteristics, which significantly improves the material's strength, ductility, and toughness. Furthermore, the uniformly distributed polygonal blocky austenite and DO3 nanoprecipitates on the polygonal ferrite matrix greatly reduce the deformation incompatibility between the two phases, further improving the material's strength, ductility, and toughness.

[0056] The ferritic-based lightweight steel of the present invention has a low density (e.g., 6.8–7.0 g / cm³). 3 The room temperature properties of the ferritic-based lightweight steel of the present invention are as follows: R t0.5 The ferrite-based lightweight steel of the present invention has the following properties at -196°C: Ra = 425 MPa or higher (e.g., 425–470 MPa), Rm = 740 MPa or higher (e.g., 741–755 MPa), A = 38% or higher (e.g., 38.5%–41%), and impact energy = 130 J or higher (e.g., 134–166 J). t0.5The strength is 820 MPa or higher (e.g., 820–930 MPa), the Rm is 1160 MPa or higher (e.g., 1161–1190 MPa), the A content is 38% or higher (e.g., 38%–41%), and the impact energy is 95 J or higher (e.g., 95–122 J). The ferritic-based lightweight steel of this invention exhibits good strength and toughness from room temperature to -196°C, overcoming the challenge of the ductile-brittle transition in bcc structures. Compared to traditional ferritic-based lightweight steels, it has lower density, better low-temperature performance, and a wider operating temperature range, making it widely applicable in the automotive, shipbuilding, pipeline, cryogenic pressure vessel, and aerospace industries.

[0057] The ferrite-based lightweight steel of the present invention reduces the requirements of cooling process on microstructure and properties by inhibiting the precipitation of κ-carbide. It is not sensitive to cooling rate, has low requirements for cooling process during preparation, and the preparation method is simple.

[0058] Examples 1-5

[0059] The advantages of precise control of the composition and process parameters of the steel of the present invention will be demonstrated below with specific embodiments and comparative examples.

[0060] Examples 1-5 of the present invention provide a ferritic-based lightweight steel and its preparation method. The chemical composition of the steel in Examples 1-5 is shown in Table 1.

[0061] The steel preparation methods in Examples 1-5 include: steelmaking, slab casting, heating, forging, cooling, solution treatment, and post-solution cooling. Specifically, in the heating process, the slab is held at 1100–1200℃ for 2–4 hours. In the forging process, the slab is forged into a 16mm diameter bar at 830–1150℃, with a final forging temperature of 830℃–880℃. Air cooling is used in the cooling process. In the solution treatment process, high-temperature solution treatment is performed at 950–1150℃ for 30–240 minutes. Air cooling or oil cooling is used in the solution cooling process.

[0062] The specific process parameters for Examples 1-5 are shown in Table 2; the main performance test results of the produced ferritic lightweight steel are shown in Table 3.

[0063] Table 1 Chemical composition, wt%

[0064] Examples C Mn Al P S Nb V Mo W 1 0.12 25.0 8.0 0.013 0.008 1.0 1.0 1.5 1.5 2 0.12 36.0 10.0 0.016 0.005 0.05 1.0 0.5 0.5 3 0.08 34.8 8.1 0.011 0.004 0.10 0.15 1.0 1.1 4 0.12 35.8 8.0 0.010 0.008 0.05 0.05 1.2 1.3 5 0.04 34.1 8.2 0.011 0.009 0.10 0.50 0.9 0.8

[0065] Table 2 Production Process Parameters

[0066]

[0067] Table 3 shows some performance test results.

[0068]

[0069]

[0070] Figure 1 The image shows a typical metallographic structure of the ferrite-based lightweight steel obtained in Example 1. The ferrite matrix volume percentage is approximately 72%, and the average size of the DO3 nano-precipitates can be calculated to be approximately 2 nm using Fourier transform (see...). Figure 2 The austenite is uniformly distributed on the ferrite matrix and its volume fraction is negligible, while the volume percentage of austenite is approximately 28%.

[0071] Figure 2 The image shows a TEM bright-field image of ferrite at the uniform deformation site of the ferrite-based lightweight steel obtained in Example 1, and its properties in

[011] . δ Diffraction spots, in which rhomboids are DO3 diffraction spots and rectangles are matrix ferrite diffraction spots; bright field images show that the ferrite matrix in the invented steel has planar slip characteristics.

[0072] Figure 3 The dark field test of the DO3 nano-precipitated phase obtained in Example 1 shows that the DO3 precipitated phase has an extremely fine nanoscale size.

[0073] Figure 4 The DO3 nano-precipitates and their size obtained by inverse Fourier transform in Example 1 are shown. The average size of the DO3 nano-precipitates in Example 1 is about 2 nm.

[0074] Figure 5 The figure shows the tensile stress-strain curves from room temperature to -196°C for Example 1. It demonstrates that the steel of this invention exhibits high strength and good plasticity over a wide temperature range.

[0075] The microstructures of Examples 1-5 are shown in Table 4 below.

[0076] Table 4 Microstructure of steel in the examples

[0077]

[0078]

[0079] The inventors conducted extensive experimental research during the research process, and some poorly performing solutions are now presented as comparative examples.

[0080] Comparative Example 1

[0081] This comparative example provides a lightweight steel, the composition of which is shown in Table 5 below. The preparation method is the same as that in Example 1, and will not be repeated here.

[0082] Comparative Example 2

[0083] This comparative example provides a lightweight steel, the composition of which is shown in Table 5 below. The preparation method is the same as that in Example 1, and will not be repeated here.

[0084] Comparative Example 3

[0085] This comparative example provides a lightweight steel, the composition of which is shown in Table 5 below. The solution temperature in the preparation method is 1200℃, and the rest of the preparation method is the same as in Example 2, and will not be repeated here.

[0086] The main performance test results of the comparative steel are shown in Table 6. The microstructure of the comparative steel is shown in Table 7. The comparative steel has low low-temperature impact energy and poor low-temperature toughness, and is not suitable for low-temperature applications such as low-temperature pressure vessels.

[0087] Table 5 Chemical composition, wt%

[0088] Comparative Example C Mn Al P S Nb V Mo W 1 1.00 31.0 10.0 0.011 0.008 0.10 0.11 0.11 0.11 2 0.95 30.0 8.0 0.015 0.006 0.12 0.18 0.12 0.12 3 0.12 36.0 10.0 0.016 0.005 0.05 1.00 0.50 0.50

[0089] Table 6 shows some performance test results.

[0090]

[0091] Table 7 Microstructure of Comparative Steels

[0092]

[0093] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A ferritic-based lightweight steel, characterized in that, The composition of the ferritic lightweight steel, by mass percentage, is: C 0.04%–0.12%, Mn 25.0%–36.0%, Al 8.0%–10.0%, P < 0.02%, S < 0.01%, Nb 0.05%–1.0%, V 0.05%–1.0%, Mo 0.5%–1.5%, W 0.5%–1.5%; the balance being Fe and unavoidable trace impurities. The microstructure of the ferrite-based lightweight steel includes a ferrite matrix and extremely fine DO3 nano-precipitates and polygonal blocky austenite uniformly distributed on the matrix. In the microstructure of the ferrite-based lightweight steel, the ferrite matrix exhibits planar slip characteristics. In the microstructure of the ferrite-based lightweight steel, the volume percentage of austenite is 20%–35%, and the volume percentage of ferrite is 65%–80%.

2. The ferritic-based lightweight steel according to claim 1, characterized in that, The impact energy of the ferritic lightweight steel at -196°C is above 95J.

3. A method for preparing ferrite-based lightweight steel according to claim 1 or 2, characterized in that, It includes the following steps: steelmaking, slab continuous casting or ingot casting, heating, rolling or forging, cooling, solution treatment and cooling treatment after solution treatment.

4. The preparation method according to claim 3, characterized in that, The heating process includes: holding the billet at 1100-1200℃ for 2-4 hours to homogenize it.

5. The preparation method according to claim 3, characterized in that, The rolling or forging process includes hot rolling or forging at 830–1150°C to form bars or plates of different specifications.

6. The preparation method according to claim 3, characterized in that, The cooling process after rolling or forging uses conventional water cooling, oil cooling, or air cooling.

7. The preparation method according to any one of claims 3 to 6, characterized in that, The solution treatment process includes: high-temperature solution treatment at 950–1150℃ for 30–240 minutes.

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