A multi-stage heterogeneous Fe-Mn-Al-C-Ni high strength and ductility austenitic low-density steel and a preparation method thereof

Abstract

This invention discloses a multi-level heterogeneous Fe-Mn-Al-C-Ni high-strength and ductile austenitic low-density steel and its preparation method. The microstructure of the high-strength and ductile austenitic low-density steel is a composite structure composed of multi-peaked austenite and multi-morphological B2 phase. The austenite is composed of fine recrystallized grains and coarse unrecrystallized grains, forming a heterogeneous microstructure with alternating soft and hard phase regions. The B2 phase is distributed as polygonal particles within the grains and grain boundaries in the recrystallized austenite region, while it precipitates as needle-like morphology within the unrecrystallized austenite grains. This invention innovatively constructs a multi-level heterogeneous structural system of "grain size heterogeneity + precipitate phase heterogeneity," effectively solving the problem of the mutual constraint between strength and ductility in traditional low-density steel.

Description

Technical Field

[0001] This invention relates to the field of metal material design and preparation technology, and in particular to a multi-level heterogeneous Fe-Mn-Al-C-Ni high-strength and ductile austenitic low-density steel and its preparation method. Background Technology

[0002] Fe-Mn-Al-C low-density steel has attracted much attention in lightweight structural materials due to its high specific strength, good strength and toughness, and the ability to achieve weight reductions of 8% to 20% through adjustment of aluminum content. Among them, austenitic low-density steel exhibits the most outstanding comprehensive mechanical properties, maintaining high strength while possessing good toughness. With the increasing demand for lightweight, high-performance materials in aerospace equipment, defense, automotive, and other fields, this material system shows broad application potential in reducing weight and energy consumption.

[0003] However, existing Fe-Mn-Al-C low-density steels still have significant limitations in terms of composition and microstructure control. With increasing aluminum content, coarse κ-carbides or B2 phases easily form in the microstructure, leading to a significant decrease in strength, plasticity, and work hardening ability, becoming a bottleneck restricting its engineering applications. Existing patents, such as CN113278896A, disclose a Fe-Mn-Al-C system high-strength low-density steel and its preparation method, which obtains a uniform austenitic matrix through smelting, casting, hot working, and solution treatment. However, the yield strength of the material obtained by this technology is 1050 MPa, the tensile strength is 1087 MPa, and the elongation is only 33.2%, and the morphology control of the B2 strengthening phase is insufficient. Patent CN114480984B proposes a method for preparing Ti-alloyed low-density high-strength steel, utilizing TiC precipitation strengthening to achieve a tensile strength higher than 1000 MPa, but the elongation is less than 30%, and the addition of Ti and rare earth elements increases the cost.

[0004] In recent years, synergistically improving the strength and plasticity of materials through heterogeneous structure design has become an important research direction. However, current research on low-density steel with heterogeneous structures is relatively limited. Patent CN118272728B proposes a complex process of preparing a high-strength, plasticity-enhancing low-density steel with a "double heterogeneous structure" through two cold rolling and two annealing processes. This process constructs a bimodal distribution of grain size in the austenite matrix and the B2 phase. Although it achieves a good match between strength and plasticity, its process route is cumbersome, involves many steps, consumes a lot of energy, and has a long production cycle, which is not conducive to industrialization and cost control. This proves that heterogeneous structure design is one of the effective ways to overcome the bottleneck of strength and plasticity, while process simplification and microstructure controllability are key to realizing its engineering applications. Therefore, designing and developing low-density steel with a reasonable heterogeneous structure is of great significance for promoting the application of this material in high-end fields such as aerospace. Summary of the Invention

[0005] The purpose of this invention is to address the technical deficiency in existing Fe-Mn-Al-C austenitic low-density steels where it is difficult to synergistically improve strength and plasticity. This invention provides a multi-level heterogeneous Fe-Mn-Al-C-Ni high-strength and plasticity austenitic low-density steel and its preparation method. By precisely controlling the heat treatment process, the recrystallization degree of the austenitic matrix and the precipitation behavior of the B2 phase are altered. This simultaneously constructs a multi-level heterogeneous structure within the austenitic matrix, exhibiting both "grain size heterogeneity" and "precipitate phase heterogeneity," thereby achieving an excellent balance between high strength and high plasticity. This provides a new technical solution to resolve the long-standing contradiction between strength and plasticity.

[0006] The technical solution adopted to achieve the purpose of this invention is:

[0007] A multi-level heterogeneous Fe-Mn-Al-C-Ni high-strength and ductile austenitic low-density steel comprises the following elemental composition by mass percentage: 0.8wt%≤C≤1.4wt%, 25wt%≤Mn≤35wt%, 6.5wt%≤Al≤10wt%, 3wt%≤Ni≤7wt%, with the balance being Fe;

[0008] The microstructure of the high-strength and ductile austenitic low-density steel is a composite structure consisting of multi-peaked austenite and multi-morphological B2 phase. The austenite is composed of fine recrystallized grains and coarse non-recrystallized grains, forming a heterogeneous microstructure with alternating soft and hard phase regions. The B2 phase is distributed as polygonal particles in the recrystallized austenite region within the grains and at the grain boundaries, while the B2 phase precipitates in the non-recrystallized austenite grains in a needle-like form.

[0009] In the above technical solution, the size of the recrystallized grains is 8~15μm, and the size of the non-recrystallized grains is 70~240μm.

[0010] In the above technical solution, the recrystallized grains account for 30% to 70%.

[0011] The preparation method of the multi-level heterogeneous Fe-Mn-Al-C-Ni high-strength and ductile austenitic low-density steel includes the following steps:

[0012] Step 1, Smelting: Weigh high-purity Fe, Mn, Al, Ni and C as alloy raw materials according to the chemical composition mass percentage of the multi-level heterogeneous high-strength ductile austenitic low-density steel, perform vacuum smelting, and cast into steel billets;

[0013] Step 2, Forging: Forge the steel billet into a steel ingot of the required size and air cool it to room temperature;

[0014] Step 3, Solution treatment: The steel ingot is solution treated and then immediately water-quenched to room temperature;

[0015] Step 4, hot rolling: The solution-treated steel ingot is rolled in multiple passes at 1200~900℃ to obtain hot-rolled steel plate;

[0016] Step 5, cold rolling: The hot-rolled steel sheet from step 4 is subjected to multiple cold rolling passes at room temperature to obtain a cold-rolled steel sheet;

[0017] Step 6, Annealing treatment: The cold-rolled steel sheet from step 5 is processed into tensile test blanks, and then placed at 950℃ for 10~60min, followed by immediate water quenching to room temperature to obtain multi-level heterogeneous high-strength ductile austenitic low-density steel.

[0018] In the above technical solution, in step 2, the steel billet is placed in a heating furnace at 1000~1300℃ and kept at that temperature for 7~10 hours before forging, with a final forging temperature of 950~1100℃.

[0019] In the above technical solution, in step 3, the solution treatment temperature is 1000~1300℃ and the time is 1~3 hours.

[0020] In the above technical solution, the rolling reduction in step 4 is 90%.

[0021] In the above technical solution, the rolling reduction in step 5 is 55%.

[0022] In the above technical solution, the heat preservation time in step 6 is 30 minutes.

[0023] Compared with the prior art, the beneficial effects of the present invention are:

[0024] 1. Multi-level heterogeneous synergistic reinforcement, significantly improving performance: By precisely controlling the competitive relationship between the degree of recrystallization and the precipitation behavior of the B2 phase, a multi-level heterogeneous structure containing recrystallized fine grains, non-recrystallized coarse grains, and intragranular / grain boundary B2 phases is simultaneously constructed in the microstructure. This composite structure can effectively introduce strain gradients, activate geometrically necessary dislocations, and generate a continuous back stress hardening effect during deformation, thereby significantly improving the material's strain hardening capacity and overall deformation coordination, providing a reliable microstructure basis for achieving a synergistic effect of high strength and high plasticity.

[0025] 2. Overcoming the bottleneck of strength-ductility inversion, achieving superior comprehensive performance: An innovative multi-level heterogeneous structural system of "grain size heterogeneity + precipitate phase heterogeneity" effectively solves the problem of the mutual constraint between strength and ductility in traditional low-density steel. Under optimized annealing conditions (annealing at 950℃ for 30 min), the material exhibits excellent mechanical property matching. The multi-level heterogeneous austenitic low-density steel obtained by this invention has a yield strength of 912~1046 MPa, a tensile strength of 1299~1395 MPa, a total elongation of 38%~45%, and a strength-ductility product of 51.76~58.46 GPa%, outperforming most existing similar materials and possessing significant application potential.

[0026] 3. Simple and efficient process, suitable for engineering applications: The process route adopted in this invention is simple and feasible, requiring only one hot rolling and cold rolling followed by appropriate annealing treatment to achieve a significant improvement in the strength and plasticity of the material. This process has the advantages of simple operation, resource saving, and low energy consumption, and has good prospects for industrialization. Attached Figure Description

[0027] Figure 1 These are the SEM morphology, EBSD analysis diagram, and grain size distribution diagram of the multi-level heterogeneous austenitic low-density steel of Embodiment 1 of the present invention;

[0028] Figure 2 These are the SEM morphology, EBSD analysis diagram, and grain size distribution diagram of the multi-level heterogeneous austenitic low-density steel of Embodiment 2 of the present invention;

[0029] Figure 3 These are the SEM morphology, EBSD analysis diagram, and grain size distribution diagram of the multi-level heterogeneous austenitic low-density steel of Embodiment 3 of the present invention;

[0030] Figure 4 The SEM morphology, EBSD analysis diagram, and grain size distribution diagram of the multi-level heterogeneous austenitic low-density steel of this invention are shown in the comparative example.

[0031] Figure 5 The diagram shows the engineering stress-strain curve and work hardening rate curve of the multi-stage heterogeneous austenitic low-density steel of this invention under tensile testing at room temperature. Detailed Implementation

[0032] The present invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention.

[0033] Example 1

[0034] A multi-level heterogeneous Fe-Mn-Al-C-Ni austenitic low-density steel has the following chemical composition by weight percentage: Mn 29wt.%, Al 8wt.%, C 1.2wt.%, Ni 5wt.%, with the balance being Fe and unavoidable impurities. The preparation steps are as follows:

[0035] Step 1, Smelting: Weigh high-purity Fe, Mn, Al, Ni and C as alloy raw materials according to the chemical composition mass percentage of the multi-level heterogeneous high-strength ductile austenitic low-density steel, perform vacuum smelting, and cast into steel billets;

[0036] Step 2, forging: The steel billet is placed in a heating furnace at 1200℃ and held for 10 hours before forging. The final forging temperature is 1000℃. After obtaining the steel ingot of the required size, it is air-cooled to room temperature.

[0037] Step 3, solution treatment: Place the steel ingot in a heating furnace at 1150℃ and keep it at that temperature for 2 hours for solution treatment, and then immediately water quench it to room temperature;

[0038] Step 4, hot rolling: The solution-treated steel ingot is rolled in multiple passes at 1150-950℃, with a total reduction of 90% and a final rolling temperature of 950℃, to obtain a 6mm thick hot-rolled steel plate.

[0039] Step 5, cold rolling: The hot-rolled steel sheet from step 4 is subjected to multiple cold rolling passes at room temperature, with a total reduction of 55%, to obtain a cold-rolled steel sheet;

[0040] Step 6, Annealing treatment: The cold-rolled steel sheet from step 5 is processed into tensile test blanks, then placed at 950℃ for 10 minutes and immediately water-quenched to room temperature to obtain multi-level heterogeneous high-strength ductile austenitic low-density steel.

[0041] After the above-mentioned process, the typical microstructure of low-density steel is as follows: Figure 1 As shown, its main characteristic is a composite microstructure consisting of multi-peaked austenite grains and multi-morphological B2 phases. The austenite matrix is ​​composed of fine recrystallized grains (such as...). Figure 1 (as shown in the RX zone of (a)) and coarse, unrecrystallized grains (such as...) Figure 1 The N-RX zone in (a) together form a heterogeneous microstructure with alternating soft and hard phase regions; the B2 phase also exhibits two typical morphologies and distributions: in the recrystallized austenite region, it is distributed as polygonal particles within the grain (as shown in the image). Figure 1 (b) Intragranular polygonal B2 shown) and grain boundaries (as shown) Figure 1 In (b) the intergranular polygonal B2 phase is shown, while in the unrecrystallized austenite crystals it is in the form of needle-like B2 phase (as shown in the image). Figure 1 (b) shows the precipitation of intragranular acicular B2. By controlling the recrystallization fraction, the size, distribution, morphology of austenite grains, and the precipitation size and distribution location of the B2 phase can be effectively affected, thereby synergistically optimizing the strain hardening capability of the material. Figure 1 The EBSD analysis results of (c) and (d) show that the recrystallization fraction of the sample is 34.4%, which confirms the heterogeneous characteristics of the grain size.

[0042] The room temperature tensile test results of the low-density steel prepared in this embodiment are as follows: Figure 5 As shown in the figure, its engineering stress-strain curves indicate that the material has a yield strength of 1046 MPa, a tensile strength of 1395 MPa, a total elongation of 38%, and a strength-ductility product of approximately 53.01 GPa∙%, demonstrating an excellent synergy between high strength and good ductility. This fully reflects the significant effect of the multi-level heterogeneous structure constructed in this invention in simultaneously improving strength and ductility.

[0043] Example 2:

[0044] The only difference between this embodiment and Embodiment 1 is the annealing holding time in step 6: after the tensile specimen blank is held at 950℃ for 30 minutes, it is immediately quenched in water to room temperature to obtain a multi-level heterogeneous high-strength ductile austenitic low-density steel.

[0045] After this process, the microstructure of the material is as follows: Figure 2 As shown, its austenite grains still exhibit a multi-peak distribution characteristic, and the morphology and distribution of the B2 phase are similar to those in Example 1. EBSD analysis results indicate that the recrystallization fraction of the sample under these conditions is 49.3%. By adjusting the recrystallization fraction, the degree of heterogeneity in grain size and the precipitation behavior of the B2 phase were further controlled, thereby optimizing the work hardening response while maintaining high strength. The tensile property test results ( Figure 5 As can be seen, the material prepared in this embodiment has a yield strength of 1028 MPa, a tensile strength of 1362 MPa, a total elongation of 38%, and a strength-ductility product of approximately 51.76 GPa∙%. Compared with Example 1, the material prepared in this embodiment has a slightly lower strength while maintaining the same plasticity, but its work hardening ability is slightly better than that of Example 1, which reflects the characteristic of achieving a balance of different properties through process control at the same high performance level.

[0046] Example 3:

[0047] The only difference between this embodiment and Embodiment 1 is the annealing holding time in step 6: after the tensile specimen blank is held at 950℃ for 60 minutes, it is immediately water-quenched to room temperature to obtain a multi-level heterogeneous high-strength ductile austenitic low-density steel.

[0048] After this process, the microstructure of the material is as follows: Figure 3 As shown, its austenite grains still exhibit a multi-peak distribution characteristic, and the morphology and distribution of the B2 phase are similar to those of the aforementioned examples. EBSD analysis results indicate that the recrystallization fraction of the sample under these conditions is 62.0%. By further increasing the recrystallization fraction, the grain size distribution becomes more broad, and the precipitation size and location distribution of the B2 phase also evolve accordingly, thus significantly improving plasticity while maintaining high strength. Tensile property test results ( Figure 5 As can be seen, the material prepared in this embodiment has a yield strength of 912 MPa, a tensile strength of 1299 MPa, a total elongation of 45%, and a strength-ductility product of approximately 58.46 GPa∙%. Compared with Examples 1 and 2, the material prepared in this embodiment significantly improves ductility by moderately reducing strength, thereby achieving an excellent strength-ductility balance.

[0049] Comparative example:

[0050] The only difference between this comparative example and Example 1 is the annealing process parameters in step 6: after the tensile sample blank is placed at 1000℃ for 30 minutes, it is immediately water-quenched to room temperature to obtain a multi-level heterogeneous high-strength ductile austenitic low-density steel.

[0051] After this process, the microstructure of the material is as follows: Figure 4 As shown. EBSD analysis revealed that the recrystallization fraction of the sample under these conditions reached 92.2%, indicating that the material was nearly fully recrystallized. The grain size exhibited a bimodal distribution, but the non-recrystallized region was extremely small, and the grain size heterogeneity was significantly reduced. The B2 phase mainly precipitated as coarse polygonal particles, and its distribution tended to be uniform, with no obvious heterogeneity in the morphology and distribution of the precipitated phase. Tensile property test results ( Figure 5 As can be seen, the material prepared in this comparative example has a yield strength of 780 MPa, a tensile strength of 1189 MPa, and a total elongation as high as 91%, with a strength-ductility product of approximately 108.20 GPa∙%, but its yield strength is significantly lower than that of Examples 1-3 (1046 MPa, 1028 MPa, and 912 MPa, respectively), and its overall strength is relatively low. This indicates that at excessively high annealing temperatures, although the ductility is greatly improved, the strength level of the material decreases significantly, failing to achieve the optimal synergy between strength and ductility, and the overall mechanical properties are not as well-matched as the three examples annealed at 950℃.

[0052] In summary, this invention successfully overcomes the traditional contradiction of inverted strength and plasticity in low-density steel by constructing a multi-level heterogeneous structure, achieving a synergistic improvement in both properties. The developed preparation process is stable and controllable, and flexible performance output from "high strength" to "high plasticity" can be achieved simply by adjusting the annealing process parameters, fully meeting diverse engineering application needs and providing a solid technical foundation for the industrial manufacturing and engineering application of high-performance austenitic low-density steel.

[0053] The above description is only a preferred embodiment of the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A multi-stage heterogeneous Fe-Mn-Al-C-Ni high-strength, ductile, low-density austenitic steel, characterized in that, The elemental composition includes the following mass percentages: 0.8wt%≤C≤1.4wt%, 25wt%≤Mn≤35wt%, 6.5wt%≤Al≤10wt%, 3wt%≤Ni≤7wt%, with the balance being Fe; The microstructure of the high-strength and ductile austenitic low-density steel is a composite structure consisting of multi-peaked austenite and multi-morphological B2 phase. The austenite is composed of fine recrystallized grains and coarse non-recrystallized grains, forming a heterogeneous structure with alternating soft and hard phase regions. In the recrystallized austenite region, the B2 phase is distributed as polygonal particles within the grains and at the grain boundaries, while in the non-recrystallized austenite grains, the B2 phase precipitates as needle-like B2 phase. The high-strength, high-ductility austenitic low-density steel has a yield strength of 912-1046 MPa, a tensile strength of 1299-1395 MPa, a total elongation of 38%-45%, and a strength-ductility product of 51.76-58.46 GPa%. The size of the recrystallized grains is 8~15μm, and the size of the non-recrystallized grains is 70~240μm.

2. The multi-stage heterogeneous Fe-Mn-Al-C-Ni high-strength, ductile, low-density austenitic steel as described in claim 1, characterized in that, The recrystallized grains account for 30% to 70%.

3. The multi-stage heterogeneous Fe-Mn-Al-C-Ni high-strength, ductile, low-density austenitic steel as described in claim 1, characterized in that, The recrystallized grains accounted for 49.3%.

4. The method for preparing multi-stage heterogeneous Fe-Mn-Al-C-Ni high-strength and ductile austenitic low-density steel according to any one of claims 1 to 3, characterized in that, Includes the following steps: Step 1, Smelting: Weigh high-purity Fe, Mn, Al, Ni and C as alloy raw materials according to the chemical composition mass percentage of the multi-level heterogeneous high-strength ductile austenitic low-density steel, perform vacuum smelting, and cast into steel billets; Step 2, Forging: Forge the steel billet into a steel ingot of the required size and air cool it to room temperature; Step 3, Solution treatment: The steel ingot is solution treated and then immediately water-quenched to room temperature; Step 4, hot rolling: The solution-treated steel ingot is rolled in multiple passes at 1200~900℃ to obtain hot-rolled steel plate; Step 5, cold rolling: The hot-rolled steel sheet from step 4 is subjected to multiple cold rolling passes at room temperature to obtain a cold-rolled steel sheet; Step 6, Annealing treatment: The cold-rolled steel sheet from step 5 is processed into tensile test blanks, and then placed at 950℃ for 10~60min, followed by immediate water quenching to room temperature to obtain multi-level heterogeneous high-strength ductile austenitic low-density steel.

5. The preparation method according to claim 4, characterized in that, In step 2, the steel billet is placed in a heating furnace at 1000~1300℃ and held for 7~10 hours before forging, with a final forging temperature of 950~1100℃.

6. The preparation method according to claim 4, characterized in that, In step 3, the solution treatment temperature is 1000~1300℃ and the time is 1~3 hours.

7. The preparation method according to claim 4, characterized in that, The total rolling reduction in step 4 is 90%.

8. The preparation method according to claim 4, characterized in that, The total rolling reduction in step 5 is 55%.

9. The preparation method according to claim 4, characterized in that, The heat preservation time in step 6 is 30 minutes.

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