A low-carbon 5Mn medium manganese steel with a yield strength of 1.2 GPa level and a preparation method thereof

Abstract

The application discloses a kind of 1.2GPa grade yield strength low-carbon 5Mn medium manganese steel and preparation method thereof, belong to the technical field of changing ferrous metal physical structure.The application uses critical zone annealing, secondary cold rolling, flash annealing+quenching and short-time tempering continuous heat treatment process, so that the medium manganese steel forms multiphase structure composed of deformed ferrite, martensite and residual austenite, effectively activates multiple strengthening mechanism and phase transformation induced plasticity TRIP effect;Flash annealing simultaneously realizes the reservation of high-density defects introduced by secondary cold rolling and the construction of metastable residual austenite, and the yield strength of the medium manganese steel is improved from 681MPa to 1195MPa;The tensile strength is improved from 986MPa to 1648MPa, while maintaining a good elongation of 16.2%.Compared with the conventional flash annealing process, the application method obtains more excellent mechanical properties at a lower annealing temperature, effectively reduces the production cost and helps the industrialization of the process.

Description

Technical Field

[0001] This invention belongs to the field of technology for altering the physical structure of ferrous metals, and relates to the heat treatment technology of high-strength steel. Specifically, it discloses a low-carbon 5Mn medium-manganese steel with a yield strength of 1.2 GPa and its preparation method. Background Technology

[0002] Achieving vehicle weight reduction and lightweighting has been a focal point of the automotive industry's development over the past decade. Although steel-aluminum hybrid bodies have gradually become mainstream in recent years, and the use of lightweight alloys such as aluminum and magnesium alloys has gradually increased, steel materials still possess irreplaceable advantages in some components requiring high strength and toughness. Therefore, achieving lightweighting of steel itself and improving its specific strength and specific stiffness to achieve weight reduction without sacrificing safety has become the focus of steel research and development. Against this backdrop, steel development has gone through several stages, from traditional steel to advanced high-strength steel (AHSS), from first-generation AHSS (including duplex steel and TRIP steel) to second-generation AHSS (including austenitic stainless steel and TWIP steel), and now to the third-generation AHSS. Representative materials of the third-generation AHSS include medium-manganese steel and quenched and partitioned steel (Q&P), whose initial design goal was to develop a high-strength and high-toughness steel with higher strength-ductility product than the first-generation AHSS and lower cost and processing difficulty than the second-generation AHSS.

[0003] The design concept of medium-manganese steel is based on traditional TRIP steel, which increases the content of the austenite stabilizing element Mn (Mn mass fraction is usually 3~12 wt.%) to obtain an ultrafine dual-phase structure of austenite + ferrite after annealing in the critical zone. During deformation, the phase transformation-induced plasticity (TRIP) effect triggered by the martensitic transformation of metastable austenite can provide excellent elongation and tensile strength. In 2011, Cao Wenquan et al. of the Iron & Steel Research Institute [WQ Cao, C. Wang, J. Shi, et al. Microstructure and mechanical properties of Fe-0.2C-5Mn steel processed by ART-annealing [J]. Materials Science & Engineering A, 528 (2011)6661-6666] studied low-carbon medium-manganese steel of Fe-0.2C-4.72Mn (wt.%). They obtained a dual-phase microstructure of austenite + ferrite with a volume fraction of 34% by critical zone annealing, and obtained an excellent combination of mechanical properties: tensile strength of ~960 MPa, elongation of ~45% and strength-ductility product of ~43 GPa%. Subsequently, Luo et al. [HW Luo, H. Dong. New ultrahigh-strength Mn-alloyed TRIP steels with improved formability manufactured by intercritical annealing [J]. Materials Science & Engineering A, 626 (2015) 207-212] obtained a series of steels with different morphologies and performance combinations by controlling the initial microstructure and annealing temperature of Fe-0.1~0.2C-5Mn (wt.%) steel in the critical region annealing. They achieved a tensile strength of up to ~1.2 GPa while maintaining an elongation of ~20%, outperforming commercial cold-rolled steel sheets. Low-carbon 5Mn steel has advantages in low raw material cost and processing difficulty. Further improvement in strength would provide core support for its industrialization and commercial application in key structural components of automotive bodies.

[0004] In recent years, an ultrafast heating or flash annealing process, characterized by an ultra-high heating rate (>100℃ / s), has been gradually applied to medium-manganese steel. Professor Chen Hao's team at Tsinghua University [Y. Wang, R. Ding, C. Franke, et al. Flash annealing of a chemically heterogeneous mediumMn steel [J]. Scripta Materialia, 242 (2024) 115923] designed a continuous heat treatment process of critical zone annealing + flash annealing + quenching + tempering, and applied it to low-carbon 5Mn medium-manganese steel (Fe-0.18C-4.95Mn (wt.%)), successfully increasing the yield strength of the steel by nearly double. Its ultra-high strength is attributed to the martensitic matrix formed after flash annealing and quenching, while the Mn-rich region formed by austenite during critical zone annealing is retained, resulting in 11% retained austenite in the steel after quenching, thus providing good elongation. However, the flash annealing temperature of up to 850℃ used in the above process not only significantly increases heat treatment costs but also places higher performance requirements on production equipment, thereby increasing the difficulty of industrial application of this process. If the flash annealing temperature is lowered, it is easy to lead to insufficient martensitic transformation, resulting in the inclusion of low-strength ferrite phases in the final structure of the steel, which in turn leads to a decrease in steel strength. Therefore, developing a heat treatment process that enables low-carbon medium-manganese steel to obtain higher strength and good elongation at a relatively lower flash annealing temperature (to reduce costs) has important practical significance and application value. Summary of the Invention

[0005] Purpose of the invention: To address the aforementioned problems in the prior art, this invention provides a low-carbon 5Mn medium-manganese steel with a yield strength of 1.2 GPa and its preparation method; this invention develops a low-carbon 5Mn medium-manganese steel with high yield strength, high tensile strength and good elongation by adding an additional cold rolling process between critical zone annealing and flash annealing.

[0006] The first objective of this invention is to provide a method for preparing low-carbon 5Mn medium-manganese steel with a yield strength of 1.2 GPa, comprising the following steps:

[0007] (1) Ingot smelting: Ingots are prepared by vacuum induction melting;

[0008] (2) High-temperature forging: The ingot is forged into a square slab and air-cooled to room temperature;

[0009] (3) Hot rolling in the austenitic region: The square slab is heated to 1200℃, held for 1 hour and then rolled in multiple passes. The final rolling temperature is 920℃~980℃ to ensure that the entire rolling process is in the austenitic single-phase temperature region. After rolling, it is air-cooled to room temperature to obtain a hot-rolled plate.

[0010] (4) Homogenization treatment: After shot peening, the hot-rolled plate is homogenized by holding at 1150℃~1200℃ for 24h~48h in a vacuum heat treatment furnace and protected by argon gas.

[0011] (5) Softening annealing: The hot-rolled plate after homogenization is heated to the austenite-ferrite dual-phase region (the temperature region in the phase diagram of iron-carbon alloy where austenite and ferrite coexist under thermodynamic equilibrium conditions, also known as the two-phase region or critical region) for softening annealing, and then air-cooled to room temperature.

[0012] (6) Cold rolling: The softened and annealed sheet is rolled at room temperature with a reduction of 70%;

[0013] (7) Critical zone annealing: The cold-rolled plate is heated to the austenite-ferrite dual-phase region (the temperature region in the phase diagram of iron-carbon alloy where austenite and ferrite coexist under thermodynamic equilibrium conditions, also known as the two-phase region or critical region), and then annealed in the critical zone. After that, it is water quenched to room temperature to obtain a dual-phase structure of austenite and ferrite.

[0014] (8) Secondary cold rolling: The plate after critical zone annealing is rolled at room temperature with a large reduction rate;

[0015] (9) Flash annealing: The secondary cold-rolled plate is placed in a salt bath furnace for instantaneous flash annealing. The plate is rapidly heated to the peak temperature at an ultra-high heating rate and then directly air-cooled to room temperature.

[0016] (10) Low temperature tempering: The flash annealed sample is placed in an oil bath furnace and kept warm to remove internal stress and avoid brittleness, and then air-cooled to room temperature.

[0017] The second objective of this invention is to provide a low-carbon 5Mn medium-manganese steel with a yield strength of 1.2 GPa, a tensile strength of 1.6 GPa, and good elongation (15%~20%).

[0018] The third objective of this invention is to provide an application of low-carbon 5Mn medium-manganese steel with a yield strength of 1.2 GPa, a tensile strength of 1.6 GPa, and good elongation (15%~20%) in automobile manufacturing.

[0019] Compared with the prior art, the embodiments of the present invention have the following beneficial effects:

[0020] This invention introduces an additional secondary cold rolling with a large reduction rate between critical zone annealing and flash annealing, which can retain the high density of dislocations introduced by cold rolling and obtain an appropriate amount of residual austenite, so that the steel prepared in the example has both high strength (yield strength up to 1.2 GPa, tensile strength up to 1.6 GPa) and good elongation (15~20%).

[0021] Furthermore, based on the combined process of steps (1) to (7), by introducing secondary cold rolling and combining steps (9) and (10), the sample treated with peak temperature flash annealing can achieve a yield strength increase of ~400 MPa and a tensile strength increase of approximately ~250 MPa compared to the sample treated with peak temperature flash annealing only without secondary cold rolling. Simultaneously, the strength level of this sample is also superior to that of the sample treated with flash annealing at a higher peak temperature (760~790℃) without secondary cold rolling. The method described in this invention not only effectively solves the technical problem of the excessively high temperature required by conventional flash annealing processes and reduces production costs, but also significantly improves the economic efficiency and practical value of the technical solution of this invention.

[0022] The medium-manganese steel provided by this invention has high strength while maintaining good ductility. For automotive parts, the increased material strength means that thinner and less steel can meet the service requirements, thus providing an optional solution for achieving automotive lightweighting.

[0023] The key process of this invention lies in the sequential combination of critical zone annealing, secondary cold rolling, and flash annealing. Specifically, the Mn-rich austenite formed during critical zone annealing introduces an uneven distribution of Mn; the secondary cold rolling process introduces high-density dislocations and grain refinement; the flash annealing process inversely transforms the Mn-rich region into austenite, while retaining the high-density defects introduced by cold rolling and introducing a new quenched martensite structure; ultimately, a highly refined ferrite + martensite matrix containing high-density dislocations is obtained, providing high strength to the steel, while the retained austenite can exert a TRIP effect during deformation, giving the steel good elongation, thereby achieving a synergy between strength and plasticity. Attached Figure Description

[0024] To more clearly illustrate the differences between the technical solutions and obtained mechanical properties of the present invention (embodiments) and existing heat treatment processes (comparative examples), the accompanying drawings used in the description of each embodiment and comparative example will be briefly introduced below.

[0025] Figure 1 The microstructure diagrams (EBSD) of the medium manganese steel samples obtained in Examples 1 and 2 of this invention are shown.

[0026] Figure 2The microstructure diagram (EBSD) of the medium manganese steel samples obtained in Comparative Examples 1, 2, 3, and 4 of this invention.

[0027] Figure 3 This is a tensile stress-strain curve of the medium manganese steel sample obtained in Example 1 of the present invention;

[0028] Figure 4 This is a tensile stress-strain curve of the medium manganese steel sample obtained in Example 2 of the present invention;

[0029] Figure 5 The tensile stress-strain curve of the medium manganese steel sample obtained in Comparative Example 1 of this invention is shown.

[0030] Figure 6 The tensile stress-strain curve of the medium manganese steel sample obtained in Comparative Example 2 of this invention is shown.

[0031] Figure 7 The tensile stress-strain curve of the medium manganese steel sample obtained in Comparative Example 3 of this invention is shown.

[0032] Figure 8 This is a tensile stress-strain curve of the medium manganese steel sample obtained in Comparative Example 4 of the present invention. Detailed Implementation

[0033] To further understand the purpose, content, and advantages of this invention, specific embodiments of the invention are described in detail below. However, these embodiments are not limited to the examples described below and should be freely combined according to actual circumstances. The endpoints and values ​​of the ranges disclosed herein are not limited to the precise ranges and values. For numerical ranges, endpoint values ​​of various ranges, endpoint values ​​of various ranges and individual point values, and individual point values ​​can be combined to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0034] The first aspect of this invention provides a method for preparing low-carbon 5Mn medium-manganese steel with a yield strength of 1.2 GPa, a tensile strength of 1.6 GPa, and good elongation (15~20%), comprising the following sequential steps:

[0035] (1) Ingot smelting: Ingots are prepared by vacuum induction melting;

[0036] (2) High-temperature forging: The ingot prepared in step (1) is forged into a square slab and air-cooled to room temperature;

[0037] (3) Hot rolling in the austenitic region: The square slab after air cooling in step (2) is heated to 1200℃, held for 1 hour and then rolled in multiple passes. The final rolling temperature is 920℃~980℃ to ensure that the entire rolling process is in the austenitic single-phase temperature region. Then it is air cooled to room temperature.

[0038] (4) Homogenization treatment: After shot peening, the hot-rolled plate from step (3) is subjected to homogenization treatment at 1150℃~1200℃ for 24h~48h in a vacuum heat treatment furnace, and protected by argon gas.

[0039] (5) Softening annealing: The hot-rolled plate after homogenization treatment in step (4) is heated to the austenite-ferrite dual-phase region for softening annealing, and then air-cooled to room temperature;

[0040] (6) Cold rolling: The hot-rolled plate after softening and annealing in step (5) is rolled at room temperature to obtain a cold-rolled plate, and the rolling reduction rate is controlled at 70%;

[0041] (7) Critical zone annealing: The cold-rolled plate from step (6) is heated to the austenite-ferrite dual-phase region and subjected to critical zone annealing. Then it is water-quenched to room temperature to obtain an austenite and ferrite dual-phase structure.

[0042] (8) Secondary cold rolling: The cold-rolled plate after critical zone annealing in step (7) is rolled at room temperature with a reduction rate of 70%~85%;

[0043] (9) Flash annealing: The cold-rolled plate after the second cold rolling in step (8) is placed in a salt bath furnace for instantaneous flash annealing, that is, the temperature is raised to the peak temperature of 760~790℃ and then directly air-cooled to room temperature.

[0044] (10) Low-temperature tempering: The cold-rolled plate after flash annealing in step (9) is placed in an oil bath furnace to remove internal stress, and then air-cooled to room temperature.

[0045] Based on the processes described in steps (1) to (7), by introducing secondary cold rolling and combining it with steps (9) and (10), the yield strength of the sample treated with peak temperature flash annealing is increased by approximately 400 MPa and the tensile strength is increased by approximately 250 MPa compared to the sample treated with peak temperature flash annealing only without secondary cold rolling. Furthermore, the strength level of this sample is superior to that of the sample treated with flash annealing at a higher peak temperature (760~790℃) without secondary cold rolling. The method described in this invention not only effectively solves the technical problem of the excessively high temperature required by conventional flash annealing processes and reduces production costs, but also significantly improves the economic efficiency and practical value of the technical solution of this invention.

[0046] In some preferred embodiments, the oxide scale is removed by sandblasting before hot rolling in the austenitic region, and 6 to 8 passes of hot rolling are performed using a 30T twin-roll mill to avoid cracking of the plate; the final rolling temperature is 920℃ to 980℃ to ensure that the entire rolling process is in the austenitic single-phase temperature region to obtain a uniform austenitic structure.

[0047] In some other preferred embodiments, the vacuum degree during homogenization is 7 × 10⁻⁶.-3 ~10Pa, which can effectively prevent high-temperature oxidation and decarburization. Homogenization temperature: 1150℃~1200℃, time: 24h~48h.

[0048] In some other preferred embodiments, during softening annealing, the heating rate is 10~20℃ / s, the softening annealing temperature is 640~660℃, and the annealing holding time is 2h, so that the material is fully softened to facilitate subsequent cold rolling.

[0049] In some other preferred embodiments, during critical zone annealing, the heating rate is 10~20℃ / s, the critical zone annealing temperature is 640~660℃, and the annealing holding time is 2h, so as to obtain a dual-phase structure of Mn-depleted ferrite and Mn-rich austenite, and construct a heterogeneous distribution of Mn element.

[0050] In some other preferred embodiments, the reduction rate of the second cold rolling is 70% to 85% to introduce high-density defects and refine the grains.

[0051] In some other preferred embodiments, the flash annealing is performed at a heating rate of 100~150℃ / s and a peak annealing temperature of 760~790℃ without heat preservation, in order to achieve rapid austenite inversion transformation and suppress the diffusion of Mn element.

[0052] In some other preferred embodiments, in the low-temperature tempering process, the tempering temperature is 150~200℃ and the holding time is 30min, so as to eliminate the internal stress and brittleness caused by the martensitic phase transformation during quenching.

[0053] When preparing billets for medium manganese steel, the following composition by mass percentage should be selected: C: 0.1-0.4%, Mn: 3-12%, Si: ≦1.5%, S: ≦0.005%, with the remainder being Fe and unavoidable impurities.

[0054] In the specific embodiments below: the following components by mass percentage are selected: C 0.16%, Mn 4.88%, Si 0.024%, S 0.003%, with the remainder being Fe and unavoidable impurities.

[0055] The key process of this invention lies in introducing a second cold rolling step (8) and controlling the reduction rate, thereby introducing a large number of defects and achieving significant grain refinement. The Mn-rich retained austenite formed during the critical zone annealing process is transformed into deformation-induced martensite during cold rolling, but the uneven distribution of Mn elements is still retained, and then it is reversed into austenite again in the subsequent flash annealing. At the same time, the instantaneous flash annealing allows the high-density dislocations and ultrafine grains introduced by cold rolling to be retained, the deformed ferrite + martensite matrix provides high strength, and the retained austenite imparts ductility to the steel, thus giving medium manganese steel excellent mechanical properties.

[0056] The second aspect of the present invention is to provide a low-carbon 5Mn medium-manganese steel with a yield strength of 1.2 GPa, a tensile strength of 1.6 GPa, and good elongation (15~20%).

[0057] The third aspect of the present invention is to provide the application of low-carbon 5Mn medium-manganese steel with a yield strength of 1.2GPa, a tensile strength of 1.6GPa, and good elongation (15~20%) in automobile manufacturing.

[0058] It should be noted that, unless otherwise specified, the experimental methods used in this invention are all conventional methods. The mechanical property tests performed on the examples and comparative examples were all conducted in accordance with GB / T 228.1-2021 "Metallic materials, tensile testing—Part 1: Tensile testing at room temperature".

[0059] The present invention will be further described below with reference to more specific embodiments:

[0060] Example 1

[0061] The preparation method of high-strength low-carbon 5Mn medium-manganese steel in this embodiment includes the following steps (steps and parameters not disclosed and having no impact on the results are conventional operations in the field):

[0062] (1) Vacuum induction melting: Prepare 25kg ingots with the following composition by mass percentage: C 0.16%, Mn 4.88%, Si 0.024%, S 0.003%, the remainder being Fe and unavoidable impurities;

[0063] (2) High temperature forging: The ingot is kept at 1200℃ for 1 hour to homogenize it, and then forged into a square billet. The final forging temperature is 950℃~1000℃ (the temperature can only fluctuate within the range of 950℃~1000℃ in a single experiment, and the fluctuation within this range has no effect on the results). The billet specifications are: length, width and height: 50~60mm×70~80mm×70~80mm (fluctuation within the size range has no effect on the results).

[0064] (3) Hot rolling in the austenitic region: The forging billet is heated to 1200℃ and held for 1 hour. After exiting the furnace, the oxide scale is completely removed from the top and bottom surfaces by sandblasting. Then, it is rolled in 6 passes using a 30T twin-roll mill. The final rolling temperature is between 920℃ and 980℃ (the temperature can only fluctuate within the range of 920℃ to 980℃ in a single experiment, and the fluctuation within this range has no effect on the results). This ensures that the entire rolling process is in the austenitic single-phase temperature zone. The billet is rolled into a hot-rolled plate with a thickness of 20mm and then air-cooled to room temperature.

[0065] (4) Homogenization treatment: After shot peening, the hot-rolled plate is homogenized by holding at 1200℃ for 24 hours in a vacuum heat treatment furnace under argon protection. The vacuum degree is 7×10⁻⁶.-3 Pa;

[0066] (5) Softening annealing: The hot-rolled plate was heated to the austenite-ferrite dual-phase region for softening annealing. The heating rate was 10~20℃ / s (the heating rate in a single experiment can only fluctuate within the range of 10~20℃ / s, and the fluctuation within this range has no effect on the results). The softening annealing temperature was 650℃, the annealing holding time was 2h, and then it was air-cooled to room temperature.

[0067] (6) Cold rolling: The softened sheet is rolled at room temperature with a reduction of 70%;

[0068] (7) Critical zone annealing: The cold-rolled plate is heated to the dual-phase region for critical zone annealing. The heating rate is 10~20℃ / s (the heating rate can only fluctuate within the range of 10~20℃ / s in a single experiment, and the fluctuation within this range has no effect on the results). The annealing temperature is 650℃, the annealing holding time is 2h, and then it is water quenched to room temperature to obtain a dual-phase structure of austenite and ferrite.

[0069] (8) Secondary cold rolling: The plate after the critical zone annealing is rolled at room temperature with a reduction rate of 70%;

[0070] (9) Flash annealing: The secondary cold-rolled plate is placed in a salt bath furnace for instantaneous flash annealing. The heating rate is 100~150℃ / s (the heating rate can only fluctuate within the range of 100~150℃ / s in a single experiment, and the fluctuation within this range has no effect on the results). After heating to the peak temperature of 770℃, it is not kept warm and is directly air-cooled to room temperature.

[0071] (10) Low temperature tempering: Place the flash annealed sample in an oil bath furnace and keep it warm for 30 minutes to remove internal stress and avoid brittleness. The tempering temperature is 200℃, and then air cool to room temperature.

[0072] The final microstructure is as follows Figure 1 As shown in (a), the deformed ferrite + martensite matrix has an ultrafine grain size of ~0.75 μm; Figure 3 The results are for room temperature tensile properties testing. The room temperature yield strength is ~1137MPa, the room temperature tensile strength is ~1596MPa, and the room temperature elongation at break is ~19.62%. While possessing high yield and tensile strength, it still maintains good elongation.

[0073] Example 2

[0074] The preparation method of high-strength low-carbon 5Mn medium-manganese steel in this embodiment includes the following steps (steps and parameters not disclosed and having no impact on the results are conventional operations in the field):

[0075] (1) Vacuum induction melting: Prepare 25kg ingots with the following composition by mass percentage: C 0.16%, Mn 4.88%, Si 0.024%, S 0.003%, the remainder being Fe and unavoidable impurities;

[0076] (2) High temperature forging: The ingot is kept at 1200℃ for 1 hour to homogenize it, and then forged into a square billet. The final forging temperature is 950℃~1000℃ (the temperature can only fluctuate within the range of 950℃~1000℃ in a single experiment, and the fluctuation within this range has no effect on the results). The billet specifications are: length, width and height: 50~60mm×70~80mm×70~80mm (fluctuation within the size range has no effect on the results).

[0077] (3) Hot rolling in the austenitic region: The forging billet is heated to 1200℃ and held for 1 hour. After exiting the furnace, the oxide scale is completely removed from the top and bottom surfaces by sandblasting. Then, it is rolled in 6 passes using a 30T twin-roll mill. The final rolling temperature is between 920℃ and 980℃ (the temperature can only fluctuate within the range of 920℃ to 980℃ in a single experiment, and the fluctuation within this range has no effect on the results). This ensures that the entire rolling process is in the austenitic single-phase temperature zone. The billet is rolled into a hot-rolled plate with a thickness of 20mm and then air-cooled to room temperature.

[0078] (4) Homogenization treatment: After shot peening, the hot-rolled plate is homogenized by holding at 1200℃ for 24 hours in a vacuum heat treatment furnace under argon protection. The vacuum degree is 7×10⁻⁶. -3 Pa;

[0079] (5) Softening annealing: The hot-rolled plate was heated to the two-phase region for softening annealing. The heating rate was 10~20℃ / s (the heating rate in a single experiment can only fluctuate within the range of 10~20℃ / s, and the fluctuation within this range has no effect on the results). The softening annealing temperature was 650℃, the annealing holding time was 2h, and then it was air-cooled to room temperature.

[0080] (6) Cold rolling: The softened sheet is rolled at room temperature with a reduction of 70%;

[0081] (7) Critical zone annealing: The cold-rolled plate is heated to the dual-phase region for critical zone annealing. The heating rate is 10~20℃ / s (the heating rate can only fluctuate within the range of 10~20℃ / s in a single experiment, and the fluctuation within this range has no effect on the results). The annealing temperature is 650℃, the annealing holding time is 2h, and then it is water quenched to room temperature to obtain a dual-phase structure of austenite and ferrite.

[0082] (8) Secondary cold rolling: The plate after the critical zone annealing is rolled at room temperature with 85% (reduction rate);

[0083] (9) Flash annealing: The secondary cold-rolled plate is placed in a salt bath furnace for instantaneous flash annealing. The heating rate is 100~150℃ / s (the heating rate can only fluctuate within the range of 100~150℃ / s in a single experiment, and the fluctuation within this range has no effect on the results). After heating to the peak temperature of 770℃, it is not kept warm and is directly air-cooled to room temperature.

[0084] (10) Low temperature tempering: Place the flash annealed sample in an oil bath furnace and keep it warm for 30 minutes to remove internal stress and avoid brittleness. The tempering temperature is 200℃, and then air cool to room temperature.

[0085] The final microstructure is as follows Figure 1 As shown in (b), the deformed ferrite + martensite matrix has an ultrafine grain size of ~0.78 μm; Figure 4 The room temperature tensile properties test results show that the room temperature yield strength is ~1195MPa, the room temperature tensile strength is ~1648MPa, and the room temperature fracture elongation is ~16.16%. While possessing ultra-high yield and tensile strength, it still maintains good elongation.

[0086] Comparative Example 1

[0087] The comparative method for preparing low-carbon 5Mn medium-manganese steel by critical zone annealing includes the following steps (steps and parameters not disclosed and having no impact on the results are standard practices in the field):

[0088] (1) Vacuum induction melting: Prepare 25kg ingots with the following composition by mass percentage: C 0.16%, Mn 4.88%, Si 0.024%, S 0.003%, the remainder being Fe and unavoidable impurities;

[0089] (2) High temperature forging: The ingot is kept at 1200℃ for 1 hour to homogenize it, and then forged into a square billet. The final forging temperature is 950℃~1000℃ (the temperature fluctuates within the range of 950℃~1000℃ in a single experiment, and the fluctuation within this range has no effect on the results). The billet dimensions are 50~60mm×70~80mm×70~80mm (fluctuation within the size range has no effect on the results).

[0090] (3) Hot rolling in the austenitic region: The forging billet is heated to 1200℃ and held for 1 hour. After exiting the furnace, the oxide scale is completely removed from the top and bottom surfaces by sandblasting. Then, it is rolled in 6 passes using a 30T twin-roll mill. The final rolling temperature is between 920℃ and 980℃ (the temperature fluctuates within the range of 920℃ to 980℃ in a single experiment, and the fluctuation within this range has no effect on the results). This ensures that the entire rolling process is in the austenitic single-phase temperature zone. The billet is rolled into a hot-rolled plate with a thickness of 20mm and then air-cooled to room temperature.

[0091] (4) Homogenization treatment: After shot peening, the hot-rolled plate is homogenized by holding at 1200℃ for 24 hours in a vacuum heat treatment furnace under argon protection. The vacuum degree is 7×10⁻⁶. -3 Pa;

[0092] (5) Softening annealing: The hot-rolled plate was heated to the two-phase region for softening annealing. The heating rate was 10~20℃ / s (the heating rate fluctuated within the range of 10~20℃ / s in a single experiment, and the fluctuation within this range had no effect on the results). The softening annealing temperature was 650℃, and the annealing holding time was 2h. Then it was air-cooled to room temperature.

[0093] (6) Cold rolling: The softened sheet is rolled at room temperature with a reduction of 70%;

[0094] (7) Critical zone annealing: The cold-rolled plate is heated to the dual-phase region for critical zone annealing. The heating rate is 10~20℃ / s (the heating rate fluctuates within the range of 10~20℃ / s in a single experiment, and the fluctuation within this range has no effect on the results). The annealing temperature is 650℃, the annealing holding time is 2h, and then it is water quenched to room temperature to obtain a dual-phase structure of austenite and ferrite.

[0095] The final microstructure is as follows Figure 2 As shown in (a), it is composed of equiaxed ferrite and austenite, with an austenite grain size of ~0.77μm and a ferrite grain size of ~3.28μm; Figure 5 The room temperature tensile properties test results show that the room temperature yield strength is ~681MPa, the room temperature tensile strength is ~986MPa, and the room temperature fracture elongation is ~40.11%, exhibiting excellent elongation but low yield strength.

[0096] Comparative Example 2

[0097] The preparation method of low-carbon 5Mn medium-manganese steel in this comparative example includes the following steps (steps and parameters not disclosed and having no impact on the results are standard practices in the field):

[0098] (1) Vacuum induction melting: Prepare 25kg ingots with the following composition by mass percentage: C 0.16%, Mn 4.88%, Si 0.024%, S 0.003%, the remainder being Fe and unavoidable impurities;

[0099] (2) High temperature forging: The ingot is kept at 1200℃ for 1 hour to homogenize it, and then forged into a square billet. The final forging temperature is 950℃~1000℃ (the temperature fluctuates within the range of 950℃~1000℃ in a single experiment, and the fluctuation within this range has no effect on the results). The billet dimensions are 50~60mm×70~80mm×70~80mm (fluctuation within the size range has no effect on the results).

[0100] (3) Hot rolling in the austenitic region: The forging billet is heated to 1200℃ and held for 1 hour. After exiting the furnace, the oxide scale is completely removed from the top and bottom surfaces by sandblasting. Then, it is rolled in 6 passes using a 30T twin-roll mill. The final rolling temperature is between 920℃ and 980℃ (the temperature fluctuates within the range of 920℃ to 980℃ in a single experiment, and the fluctuation within this range has no effect on the results). This ensures that the entire rolling process is in the austenitic single-phase temperature zone. The billet is rolled into a hot-rolled plate with a thickness of 20mm and then air-cooled to room temperature.

[0101] (4) Homogenization treatment: After shot peening, the hot-rolled plate is homogenized by holding at 1200℃ for 24 hours in a vacuum heat treatment furnace under argon protection. The vacuum degree is 7×10⁻⁶. -3 Pa;

[0102] (5) Softening annealing: The hot-rolled plate was heated to the two-phase region for softening annealing. The heating rate was 10~20℃ / s (the heating rate fluctuated within the range of 10~20℃ / s in a single experiment, and the fluctuation within this range had no effect on the results). The softening annealing temperature was 650℃, and the annealing holding time was 2h. Then it was air-cooled to room temperature.

[0103] (6) Cold rolling: The softened sheet is rolled at room temperature with a reduction of 70%;

[0104] (7) Critical zone annealing: The cold-rolled plate is heated to the dual-phase region for critical zone annealing. The heating rate is 10~20℃ / s (the heating rate fluctuates within the range of 10~20℃ / s in a single experiment, and the fluctuation within this range has no effect on the results). The annealing temperature is 650℃, the annealing holding time is 2h, and then it is water quenched to room temperature to obtain a dual-phase structure of austenite and ferrite.

[0105] (8) Flash annealing: The critical zone annealing sample is placed in a salt bath furnace for instantaneous flash annealing. The heating rate is 100~150℃ / s (the heating rate fluctuates within the range of 100~150℃ / s in a single experiment, and the fluctuation within this range has no effect on the results). After heating to the peak temperature of 770℃, it is not kept at the temperature, but directly air-cooled to room temperature.

[0106] (9) Low temperature tempering: Place the flash annealed sample in an oil bath furnace and keep it warm for 30 minutes to remove internal stress and avoid brittleness. The tempering temperature is 200℃, and then air cool to room temperature.

[0107] The final microstructure is as follows Figure 2 As shown in (b), it consists of equiaxed ferrite + retained austenite + a small amount of quenched martensite, with an austenite grain size of ~0.53μm and a ferrite grain size of ~2.00μm; Figure 6The room temperature tensile properties test results show that the room temperature yield strength is ~825MPa, the room temperature tensile strength is ~1402MPa, and the room temperature fracture elongation is ~18.57%. The presence of quenched martensite improves its yield strength, and the TRIP effect of higher austenite fraction leads to a significant increase in tensile strength, while also exhibiting good elongation.

[0108] Comparative Example 3

[0109] The preparation method of low-carbon 5Mn medium-manganese steel in this comparative example includes the following steps (steps and parameters not disclosed and having no impact on the results are standard practices in the field):

[0110] (1) Vacuum induction melting: Prepare 25kg ingots with the following composition by mass percentage: C 0.16%, Mn 4.88%, Si 0.024%, S 0.003%, the remainder being Fe and unavoidable impurities;

[0111] (2) High temperature forging: The ingot is kept at 1200℃ for 1 hour to homogenize it, and then forged into a square billet. The final forging temperature is 950℃~1000℃ (the temperature fluctuates within the range of 950℃~1000℃ in a single experiment, and the fluctuation within this range has no effect on the results). The billet dimensions are 50~60mm×70~80mm×70~80mm (fluctuation within the size range has no effect on the results).

[0112] (3) Hot rolling in the austenitic region: The forging billet is heated to 1200℃ and held for 1 hour. After exiting the furnace, the oxide scale is completely removed from the top and bottom surfaces by sandblasting. Then, it is rolled in 6 passes using a 30T twin-roll mill. The final rolling temperature is between 920℃ and 980℃ (the temperature fluctuates within the range of 920℃ to 980℃ in a single experiment, and the fluctuation within this range has no effect on the results). This ensures that the entire rolling process is in the austenitic single-phase temperature zone. The billet is rolled into a hot-rolled plate with a thickness of 20mm and then air-cooled to room temperature.

[0113] (4) Homogenization treatment: After shot peening, the hot-rolled plate is homogenized by holding at 1200℃ for 24 hours in a vacuum heat treatment furnace under argon protection. The vacuum degree is 7×10⁻⁶. -3 Pa;

[0114] (5) Softening annealing: The hot-rolled plate was heated to the two-phase region for softening annealing. The heating rate was 10~20℃ / s (the heating rate fluctuated within the range of 10~20℃ / s in a single experiment, and the fluctuation within this range had no effect on the results). The softening annealing temperature was 650℃, and the annealing holding time was 2h. Then it was air-cooled to room temperature.

[0115] (6) Cold rolling: The softened sheet is rolled at room temperature with a reduction of 70%;

[0116] (7) Critical zone annealing: The cold-rolled plate is heated to the dual-phase region for critical zone annealing. The heating rate is 10~20℃ / s (the heating rate fluctuates within the range of 10~20℃ / s in a single experiment, and the fluctuation within this range has no effect on the results). The annealing temperature is 650℃, the annealing holding time is 2h, and then it is water quenched to room temperature to obtain a dual-phase structure of austenite and ferrite.

[0117] (8) Flash annealing: The critical zone annealing sample is placed in a salt bath furnace for instantaneous flash annealing. The heating rate is 100~150℃ / s (the heating rate fluctuates within the range of 100~150℃ / s in a single experiment, and the fluctuation within this range has no effect on the results). After heating to the peak temperature of 810℃, it is not kept at the temperature, but directly air-cooled to room temperature.

[0118] (9) Low temperature tempering: Place the flash annealed sample in an oil bath furnace and keep it warm for 30 minutes to remove internal stress and avoid brittleness. The tempering temperature is 200℃, and then air cool to room temperature.

[0119] The final microstructure is as follows Figure 2 As shown in (c), it can be seen that a large amount of quenched martensite forms the matrix, with a small amount of retained austenite and ferrite distributed in it. The size of the retained austenite grains is only ~0.31μm. Figure 7 The room temperature tensile properties test results show that the room temperature yield strength is ~1117MPa, the room temperature tensile strength is ~1444MPa, and the room temperature fracture elongation is ~23.52%. The large amount of quenched martensite significantly improves the yield strength of medium manganese steel.

[0120] Comparative Example 4

[0121] The preparation method of low-carbon 5Mn medium-manganese steel in this comparative example includes the following steps (steps and parameters not disclosed and having no impact on the results are standard practices in the field):

[0122] (1) Vacuum induction melting: Prepare 25kg ingots with the following composition by mass percentage: C 0.16%, Mn 4.88%, Si 0.024%, S 0.003%, the remainder being Fe and unavoidable impurities;

[0123] (2) High temperature forging: The ingot is kept at 1200℃ for 1 hour to homogenize it, and then forged into a square billet. The final forging temperature is 950℃~1000℃ (the temperature fluctuates within the range of 950℃~1000℃ in a single experiment, and the fluctuation within this range has no effect on the results). The billet dimensions are 50~60mm×70~80mm×70~80mm (fluctuation within the size range has no effect on the results).

[0124] (3) Hot rolling in the austenitic region: The forging billet is heated to 1200℃ and held for 1 hour. After exiting the furnace, the oxide scale is completely removed from the top and bottom surfaces by sandblasting. Then, it is rolled in 6 passes using a 30T twin-roll mill. The final rolling temperature is between 920℃ and 980℃ (the temperature fluctuates within the range of 920℃ to 980℃ in a single experiment, and the fluctuation within this range has no effect on the results). This ensures that the entire rolling process is in the austenitic single-phase temperature zone. The billet is rolled into a hot-rolled plate with a thickness of 20mm and then air-cooled to room temperature.

[0125] (4) Homogenization treatment: After shot peening, the hot-rolled plate is homogenized by holding at 1200℃ for 24 hours in a vacuum heat treatment furnace under argon protection. The vacuum degree is 7×10⁻⁶. -3 Pa;

[0126] (5) Softening annealing: The hot-rolled plate was heated to the two-phase region for softening annealing. The heating rate was 10~20℃ / s (the heating rate fluctuated within the range of 10~20℃ / s in a single experiment, and the fluctuation within this range had no effect on the results). The softening annealing temperature was 650℃, and the annealing holding time was 2h. Then it was air-cooled to room temperature.

[0127] (6) Cold rolling: The softened sheet is rolled at room temperature with a reduction of 70%;

[0128] (7) Critical zone annealing: The cold-rolled plate is heated to the dual-phase region for critical zone annealing. The heating rate is 10~20℃ / s (the heating rate fluctuates within the range of 10~20℃ / s in a single experiment, and the fluctuation within this range has no effect on the results). The annealing temperature is 650℃, the annealing holding time is 2h, and then it is water quenched to room temperature to obtain a dual-phase structure of austenite and ferrite.

[0129] (8) Secondary cold rolling: The plate after the critical zone annealing is rolled at room temperature with a reduction rate of 70%;

[0130] (9) Secondary short-time critical zone annealing: The cold-rolled sheet is heated again to the dual-phase region for critical zone annealing. The heating rate is 10~20℃ / s (the heating rate fluctuates within the range of 10~20℃ / s in a single experiment, and the fluctuation within this range has no effect on the results). The annealing temperature is 650℃, the annealing holding time is 30min, and then it is air-cooled to room temperature to obtain a dual-phase structure of austenite and ferrite.

[0131] The final microstructure is as follows Figure 2 As shown in (d), its microstructure is similar to that of Comparative Example 1, consisting of critical region austenite and ferrite. The austenite grain size is ~0.77 μm, the same as that of Comparative Example 1, while the ferrite grain size is ~1.11 μm, which is smaller than that of Comparative Example 1. Figure 8The room temperature tensile properties test results show that the room temperature yield strength is ~815MPa, the room temperature tensile strength is ~1031MPa, and the room temperature fracture elongation is ~37.13%. The slight increase in yield strength and tensile strength may be attributed to the refinement of the ferrite matrix grains.

[0132] In this invention, all room temperature tensile properties (tensile strength, yield strength, and total elongation at break) tests were conducted in accordance with GB / T 228.1-2021 standard and were directly measured using an Instron 5985 tensile testing machine. Tensile specimens were all of the same size, with a rectangular cross-section, a clamping head wider than the gauge length, and a transition arc connecting the clamping head and the gauge length. Load-displacement curves were determined using an automatic extensometer, with the extensometer gauge length (… L e All gauge lengths are 10 mm, and the test rate is 1 mm / min. (The gauge length of the extensometer affects the total elongation at break. Different gauge lengths cannot be directly compared, so they are kept consistent. The test rate affects the strain rate and is a key factor affecting the tensile property test results, so they are kept consistent.)

[0133] The following methods were used to test the above performance:

[0134] tensile strength R m The maximum force is read from the recorded load-displacement curve and the engineering stress-strain curve. F m Corresponding stress. Yield strength. R p0.2 : Read the stress value corresponding to the occurrence of 0.2% residual plastic deformation from the recorded engineering stress-strain curve (for specimens with discontinuous yielding, such as Comparative Examples 1 and 4, read the stress value corresponding to the upper yield point). Total elongation at fracture. A t : Read the percentage of elongation at the fracture moment from the engineering stress-strain curve.

[0135] To make the differences between the examples and the comparative examples clearer, the heat treatment processes and room temperature tensile property test results of the examples and the comparative examples are shown in the table below.

[0136] Table 1

[0137]

[0138] As shown in Table 1, the embodiments of the present invention exhibit the highest combination of yield and tensile strength. Comparative Example 1 is a conventional critical zone annealed medium manganese steel, whose recrystallized ferrite matrix results in the lowest yield and tensile strength. Comparative Example 4 introduces secondary cold rolling and short-time secondary critical zone annealing on the basis of critical zone annealing. The higher dislocation density and finer grain size improve its strength compared to Comparative Example 1, but the strength of the medium manganese steel treated by the critical zone annealing process remains limited.

[0139] Both Comparative Examples 2 and 3 employed a critical zone annealing followed by flash annealing, differing only in the peak flash annealing temperature, which was 770℃ and 810℃, respectively. As the peak temperature increased, the fraction of reverse-transformed austenite generated during flash annealing increased, further transforming into martensite during subsequent quenching, thus increasing the strength of the medium-manganese steel. Therefore, in Comparative Example 3, the matrix composed of ferrite and martensite endowed it with higher yield strength (~1.1 GPa) and tensile strength (~1.4 GPa). However, this strength level depended on the higher flash annealing temperature (810℃), resulting in higher production costs.

[0140] In contrast, this invention, by introducing a secondary cold rolling process, achieves higher overall strength than Comparative Example 3 at a relatively low flash annealing temperature of only 770℃, with a yield strength reaching ~1.2 GPa and a tensile strength reaching ~1.6 GPa. The above comparison demonstrates that this invention effectively solves the technical pain point of the high temperature required by existing flash annealing processes, significantly reducing costs while further improving steel strength, thus possessing greater economic and practical value. The core heat treatment process of this invention is a sequential combination of critical zone annealing + secondary cold rolling + flash annealing, representing an innovative technology for preparing high-strength medium-manganese steel.

[0141] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for preparing low-carbon 5Mn medium-manganese steel with a yield strength of 1.2 GPa, characterized in that, Includes the following steps: (1) Ingot smelting: Ingots are prepared by vacuum induction melting; the ingots contain the following components by mass percentage: C: 0.1-0.4%, Mn: 3-12%, Si: ≦1.5%, S: ≦0.005%, the remainder being Fe and unavoidable impurities; (2) High-temperature forging: The ingot prepared in step (1) is forged into a square slab and air-cooled to room temperature; (3) Hot rolling in the austenitic region: The square slab after air cooling in step (2) is heated to 1200℃, held for 1 hour and then rolled in multiple passes. The final rolling temperature is 920℃~980℃ to ensure that the entire rolling process is in the austenitic single-phase temperature region. After rolling, it is air cooled to room temperature to obtain a hot-rolled plate. (4) Homogenization treatment: After shot peening, the hot-rolled plate from step (3) is subjected to homogenization treatment at 1150℃~1200℃ for 24h~48h in a vacuum heat treatment furnace, and protected by argon gas. (5) Softening annealing: The hot-rolled plate after homogenization treatment in step (4) is heated to the austenite-ferrite dual-phase region for softening annealing, and then air-cooled to room temperature; the austenite-ferrite dual-phase region refers to the temperature region in the iron-carbon alloy phase diagram where austenite and ferrite coexist under thermodynamic equilibrium conditions. (6) Cold rolling: The hot-rolled plate after softening and annealing in step (5) is rolled at room temperature to obtain a cold-rolled plate, and the rolling reduction rate is controlled at 70%; (7) Critical zone annealing: The cold-rolled plate from step (6) is heated to the austenite-ferrite dual-phase region and subjected to critical zone annealing. Then it is water-quenched to room temperature to obtain an austenite and ferrite dual-phase structure. (8) Secondary cold rolling: The cold-rolled plate after critical zone annealing in step (7) is rolled at room temperature with a reduction rate of 70%~85%; (9) Flash annealing: The cold-rolled plate after the second cold rolling in step (8) is placed in a salt bath furnace for instantaneous flash annealing, that is, the temperature is raised to the peak temperature of 760~790℃ and then directly air-cooled to room temperature. (10) Low-temperature tempering: The cold-rolled sheet after flash annealing in step (9) is placed in an oil bath furnace for heat preservation to remove internal stress, and then air-cooled to room temperature; In step (9), the heating rate of flash annealing is 100~150℃ / s, the peak annealing temperature is 760~790℃, and no heat preservation is required; in step (10), the heat preservation temperature is 150~200℃, and the heat preservation time is 30min.

2. The method for preparing low-carbon 5Mn medium-manganese steel with a yield strength of 1.2 GPa according to claim 1, characterized in that, The hot rolling method in step (3) includes using a 30T twin-roll mill for 6 to 8 passes of hot rolling, with a final rolling temperature of 920℃ to 980℃; And / or, in step (4), the vacuum degree is 7×10-3~10Pa during the homogenization process; And / or, in step (5), the heating rate is 10~20℃ / s, the softening annealing temperature is 640~660℃, and the annealing holding time is 2h.

3. The method for preparing low-carbon 5Mn medium-manganese steel with a yield strength of 1.2 GPa according to claim 1, characterized in that, In step (7), the heating rate is 10~20℃ / s, the annealing temperature is 640~660℃, and the annealing holding time is 2h.

4. The method for preparing low-carbon 5Mn medium-manganese steel with a yield strength of 1.2 GPa according to claim 1, characterized in that, The ingot in step (1) contains the following components by mass percentage: C 0.16%, Mn 4.88%, Si 0.024%, S 0.003%, with the remainder being Fe and unavoidable impurities.

5. A low-carbon 5Mn medium-manganese steel with a yield strength of 1.2 GPa, characterized in that, It is prepared by any one of claims 1 to 4.

6. The application of a low-carbon 5Mn medium-manganese steel with a yield strength of 1.2 GPa as described in claim 5, characterized in that, Used in automobile manufacturing.

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