High-strength and high-toughness medium manganese steel and preparation method thereof

Abstract

The invention relates to the technical field of medium manganese steel, in particular to high-strength and high-toughness medium manganese steel and a preparation method thereof.The preparation method is characterized by comprising the steps that S1, smelting is conducted through an electric furnace according to the chemical component ratio of the medium manganese steel, then refining is conducted through a ladle refining furnace, then vacuum degassing treatment is conducted through a vacuum circulation degassing method, and then continuous casting is conducted to form a plate blank with the thickness being 200-300 mm; s2, the plate blank is heated to 1150-1250 DEG C in a walking beam furnace, heat preservation is conducted for 2-4 hours, and then a controlled rolling technology is adopted to machine the plate blank into a steel plate within the range of 4-20 mm; s3, after rolling is completed, the steel plate is treated through a composite cooling technology of ultra-fast cooling and laminar cooling; s4, the cooled steel plate is heated to 750-850 DEG C, heat preservation is conducted for 10-60 minutes, and a ferrite and austenite double-phase structure is formed; the comprehensive performance is greatly improved, and excellent strength, plasticity, low-temperature toughness and welding adaptability are shown.

Description

Technical Field

[0001] This invention relates to the field of medium manganese steel technology, and in particular to a high-strength and high-toughness medium manganese steel and its preparation method. Background Technology

[0002] Since the concept of "third-generation advanced high-strength steel" was proposed at the beginning of the 21st century, medium manganese steel has become a core candidate material for lightweighting in the automotive, pipeline, and military industries due to its combination of 1000 MPa strength and 20-40% elongation. The typical commercial composition system is 0.1-0.3% C and 5-10% Mn, supplemented with Al and Si to stabilize austenite and suppress carbides. Its toughening mechanism relies on the dual-phase microstructure of ultrafine-grained ferrite + metastable austenite (20–40%) formed after critical annealing, which triggers the TRIP (Transformation Induced Plasticity) effect during deformation, achieving continuous work hardening. However, traditional 0.1C–7Mn steel, after critical annealing at 650 °C for 1 h, has a tensile strength of approximately 1.0 GPa and an elongation after fracture of 30%, but its impact energy at -40 °C is generally < 30 J, which is insufficient to meet the toughness requirements of safety components in cold regions such as B-pillars and crash beams. While further increasing the Mn and C content can increase the austenite volume fraction, it brings serious risks of central segregation and delayed cracking. Therefore, we propose a high-strength and high-toughness medium-manganese steel and its preparation method. Summary of the Invention

[0003] The purpose of this invention is to overcome the shortcomings of the prior art and to propose a high-strength and high-toughness medium-manganese steel and its preparation method.

[0004] To achieve the above objectives, the technical solution adopted by this invention is as follows: a high-strength and high-toughness medium-manganese steel, wherein the chemical composition of the medium-manganese steel is: carbon: 0.20%-0.30%, manganese: 6.0%-10.0%, silicon: 0.8%-1.5%, aluminum: 0.5%-1.2%, chromium: 0.8%-1.8%, molybdenum: 0.2%-0.6%, vanadium: 0.08%-0.2%, titanium: 0.02%-0.10%, niobium: 0.02%-0.08%, copper: 0.3%-0.8%, nickel: 0.4%–1.0%, boron: 0.001%–0.003%, nitrogen ≤0.005%, phosphorus ≤0.010%, sulfur ≤0.008%, balance iron and unavoidable impurities; among which, the mass ratio of manganese to carbon is controlled within the range of 25–45, the mass ratio of manganese to silicon is controlled within the range of 5–12, the mass ratio of chromium to molybdenum is controlled within the range of 3–8, the total amount of the three microalloying elements vanadium, titanium, and niobium is controlled within the range of 0.1%–0.3%, and the mass ratio of copper to nickel is controlled within the range of 0.5–1.5.

[0005] A method for preparing high-strength and high-toughness medium-manganese steel, comprising,

[0006] S1: According to the chemical composition ratio of medium manganese steel, it is smelted in an electric furnace, then refined in a ladle refining furnace, then vacuum degassed using a vacuum circulation degassing method, and then continuously cast into slabs with a thickness of 200-300mm.

[0007] S2: The slab is heated to 1150-1250℃ in a walking beam furnace and held for 2-4 hours. Then, the slab is processed into steel plates with a diameter of 4-20mm using a controlled rolling process.

[0008] S3: After rolling, the steel plate is treated with a composite cooling process of ultra-fast cooling and laminar flow cooling;

[0009] S4: Heat the cooled steel plate to 750-850℃ and hold for 10-60 minutes to form a dual-phase structure of ferrite and austenite;

[0010] S5: After the critical zone annealing, the steel plate is rapidly quenched into a salt bath or oil bath at 150-250℃, held for 30-300 seconds, and then air-cooled to room temperature.

[0011] S6: Temper the steel plate by heating the quenched steel plate to 200-400℃ and holding it for 30-180 minutes.

[0012] Preferably, the vacuum degassing treatment includes controlling the hydrogen content in the steel to ≤2ppm and the oxygen content to ≤15ppm, controlling the superheat in the continuous casting process to be within the range of 15-35℃, controlling the casting speed to be within the range of 0.8-1.2m / min, and adopting a weak cooling process, wherein the cooling intensity of the weak cooling process is controlled to be within the range of 0.3-0.8L / kg steel.

[0013] Preferably, during the rolling process, the roughing start temperature is controlled at 1050-1150℃, the roughing finish temperature is controlled at 950-1050℃, the finishing start temperature is controlled at 850-950℃, and the finishing finish temperature is controlled at 750-850℃.

[0014] Preferably, after rolling, the steel plate is treated using a composite cooling process of ultra-fast cooling and laminar flow cooling, including:

[0015] Step 1: Cooling begins within 0.5-2 seconds after the steel plate exits the rolling mill. First, the surface temperature of the steel plate is rapidly cooled from 800-900℃ to 600-700℃ at a cooling rate of 80-120℃ / s. The cooling rate is achieved by adjusting the cooling water pressure and flow rate. The cooling water pressure is controlled within the range of 0.8-1.5MPa, and the flow rate is controlled within the range of 1000-3000m³ / h.

[0016] Step 2: Cool the steel plate to 400-500℃ at a cooling rate of 20-40℃ / s. Laminar flow cooling is used in this stage, and the cooling water temperature is controlled within the range of 20-40℃.

[0017] Step 3: Cool the steel plate to 200-300℃ at a cooling rate of 5-15℃ / s;

[0018] The temperature difference between the surface and core of the steel plate in the composite cooling process is controlled within 50°C.

[0019] Preferably, the dual-phase microstructure forming ferrite and austenite comprises:

[0020] Step 1: Heat the steel plate from room temperature to 600-700℃ at a heating rate of 5-15℃ / s to allow the microstructure to recover and recrystallize, thereby eliminating rolling stress.

[0021] Step 2: Heat the steel plate to the critical temperature of 780-820℃ at a heating rate of 2-8℃ / s, and hold for 20-40 minutes to partially austenitize the microstructure.

[0022] During the heating process, a protective atmosphere is used, which is a mixture of nitrogen and hydrogen, wherein the hydrogen component is 5%-20% and the dew point is controlled below -40°C.

[0023] In the dual-phase microstructure of ferrite and austenite, the volume fraction of ferrite is 40%–60%, and the volume fraction of austenite is 40%–60%.

[0024] Preferably, the specific process of rapidly quenching the steel plate after critical zone annealing into a salt bath or oil bath at 150-250℃, holding it at that temperature for 30-300 seconds, and then air-cooling it to room temperature is as follows:

[0025] Step 1: Quench the steel plate after critical zone annealing into a salt bath at 180-220℃ at a cooling rate of ≥50℃ / s and hold for 60-180 seconds.

[0026] Step 2: Quickly transfer the steel plate to another salt bath at 250-350℃ and hold it at that temperature for 60-300 seconds;

[0027] Step 3: Air cool the steel plate to room temperature to obtain a composite structure of martensite and retained austenite.

[0028] Preferably, the tempering process includes:

[0029] A low-temperature, long-time tempering process is adopted, with the tempering temperature controlled within the range of 250-350℃ and the tempering time controlled within the range of 60-120 minutes. Tempering is carried out in air or a protective atmosphere, with the protective atmosphere being nitrogen or argon, and the oxygen content controlled below 100ppm. After tempering, a controlled cooling method is used, cooling to below 100℃ at a cooling rate of 1-5℃ / s, and then air-cooling to room temperature to prevent temper brittleness.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] 1. By employing a triple process of "electric furnace → LF → RH," hydrogen ≤2 ppm and oxygen ≤15 ppm are controlled. Continuous casting superheating of 15-35℃, casting speed of 0.8-1.2 m / min, and weak cooling of 0.3-0.8 L / kg can significantly reduce slab segregation, porosity, and microcracks. This provides defect-free 200-300 mm thick slabs for subsequent high-reduction and high-cooling-rate processes. Low-temperature, high-reduction controlled rolling achieves austenite recrystallization refinement and deformation-induced precipitation, refining the original austenite grains to ≤10μm, laying a fine-grained foundation for subsequent multiphase microstructures. Precise annealing in the critical zone and controllable dual-phase ratios allow for the precise acquisition of a 40-60% ferrite and 0-60% austenite dual-phase microstructure, achieving an adjustable strength-plasticity matching window. Low-temperature, long-term tempering effectively eliminates quenching stress, refines carbides, suppresses temper brittleness, and ensures plate shape and dimensional stability. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the process for preparing high-strength and high-toughness medium-manganese steel according to the present invention;

[0033] Figure 2 This is a comparison chart of key process parameters for smelting and continuous casting in the preparation method of high-strength and tough medium-manganese steel according to the present invention.

[0034] Figure 3 This invention provides a comparison of center segregation and impact toughness in a method for preparing high-strength and tough medium-manganese steel. Detailed Implementation

[0035] The following description is intended to disclose the invention and enable those skilled in the art to implement it. The preferred embodiments described below are merely examples, and other obvious variations will occur to those skilled in the art.

[0036] like Figure 1The diagram shows a high-strength, high-toughness medium-manganese steel. The chemical composition of this medium-manganese steel is as follows: carbon: 0.20%–0.30%, manganese: 6.0%–10.0%, silicon: 0.8%–1.5%, aluminum: 0.5%–1.2%, chromium: 0.8%–1.8%, molybdenum: 0.2%–0.6%, vanadium: 0.08%–0.2%, titanium: 0.02%–0.10%, niobium: 0.02%–0.08%, copper: 0.3%–0.8%, and nickel: 0.4%–1.0%. Boron: 0.001%–0.003%, nitrogen ≤0.005%, phosphorus ≤0.010%, sulfur ≤0.008%, balance iron and unavoidable impurities; among which, the mass ratio of manganese to carbon is controlled within the range of 25–45, the mass ratio of manganese to silicon is controlled within the range of 5–12, the mass ratio of chromium to molybdenum is controlled within the range of 3–8, the total amount of the three microalloying elements vanadium, titanium, and niobium is controlled within the range of 0.1%–0.3%, and the mass ratio of copper to nickel is controlled within the range of 0.5–1.5.

[0037] A method for preparing high-strength and high-toughness medium-manganese steel, comprising,

[0038] S1: According to the chemical composition ratio of medium manganese steel, it is smelted in an electric furnace, then refined in a ladle refining furnace, then vacuum degassed using a vacuum circulation degassing method, and then continuously cast into slabs with a thickness of 200-300mm.

[0039] S2: The slab is heated to 1150-1250℃ in a walking beam furnace and held for 2-4 hours. Then, the slab is processed into steel plates with a diameter of 4-20mm using a controlled rolling process.

[0040] S3: After rolling, the steel plate is treated with a composite cooling process of ultra-fast cooling and laminar flow cooling;

[0041] S4: Heat the cooled steel plate to 750-850℃ and hold for 10-60 minutes to form a dual-phase structure of ferrite and austenite;

[0042] S5: After the critical zone annealing, the steel plate is rapidly quenched into a salt bath or oil bath at 150-250℃, held for 30-300 seconds, and then air-cooled to room temperature.

[0043] S6: Temper the steel plate by heating the quenched steel plate to 200-400℃ and holding it for 30-180 minutes.

[0044] In this embodiment, the vacuum degassing treatment includes controlling the hydrogen content in the steel to ≤2ppm and the oxygen content to ≤15ppm, controlling the superheat during continuous casting to be within the range of 15-35℃, controlling the casting speed to be within the range of 0.8-1.2m / min, and adopting a weak cooling process with the cooling intensity controlled within the range of 0.3-0.8L / kg steel.

[0045] Specifically, lower segregation and higher toughness are achieved simultaneously through low superheat and weak cooling with RH degassing.

[0046] In this embodiment, the roughing rolling start temperature is controlled at 1050-1150℃, the roughing rolling finish temperature is controlled at 950-1050℃, the finishing rolling start temperature is controlled at 850-950℃, and the finishing rolling finish temperature is controlled at 750-850℃.

[0047] Specifically, microalloying elements such as niobium, titanium, and aluminum are completely dissolved in solid solution; subsequently, deformation-induced precipitation and recrystallization are carried out at a high reduction rate below 950℃, and the final grain size can be refined from the traditional 12–15μm to 5–8μm, improving the grain size grade by 2–3 levels.

[0048] In this embodiment, after rolling, the steel plate is treated using a composite cooling process of ultra-fast cooling and laminar flow cooling, including:

[0049] Step 1: Cooling begins within 0.5-2 seconds after the steel plate exits the rolling mill. First, the surface temperature of the steel plate is rapidly cooled from 800-900℃ to 600-700℃ at a cooling rate of 80-120℃ / s. The cooling rate is achieved by adjusting the cooling water pressure and flow rate. The cooling water pressure is controlled within the range of 0.8-1.5MPa, and the flow rate is controlled within the range of 1000-3000m³ / h.

[0050] Step 2: Cool the steel plate to 400-500℃ at a cooling rate of 20-40℃ / s. Laminar flow cooling is used in this stage, and the cooling water temperature is controlled within the range of 20-40℃.

[0051] Step 3: Cool the steel plate to 200-300℃ at a cooling rate of 5-15℃ / s;

[0052] The temperature difference between the surface and core of the steel plate in the composite cooling process is controlled within 50℃.

[0053] Specifically, during the entire cooling process, the temperature difference between the surface and core of the steel plate is controlled within 50°C to ensure the uniformity of the microstructure. By controlling the cooling process, a composite microstructure with a volume fraction of 40%–60% bainite and 30%–50% martensite is obtained in the steel plate. The bainite is lath-shaped with a lath width in the range of 0.3–1.5 μm, and the martensite is lath-shaped with a lath width in the range of 0.2–1.0 μm.

[0054] In this embodiment, a dual-phase microstructure of ferrite and austenite is formed, including:

[0055] Step 1: Heat the steel plate from room temperature to 600-700℃ at a heating rate of 5-15℃ / s to allow the microstructure to recover and recrystallize, thereby eliminating rolling stress.

[0056] Step 2: Heat the steel plate to the critical temperature of 780-820℃ at a heating rate of 2-8℃ / s, and hold for 20-40 minutes to partially austenitize the microstructure.

[0057] During the heating process, a protective atmosphere is used, which is a mixture of nitrogen and hydrogen, with hydrogen comprising 5%–20% of the gas, and the dew point is controlled below -40°C.

[0058] In the dual-phase microstructure of ferrite and austenite, the volume fraction of ferrite is 40%–60%, and the volume fraction of austenite is 40%–60%.

[0059] Specifically, the protective atmosphere prevents steel plate oxidation and decarburization, and the rapid heating and short-term holding in the critical zone result in an overall annealing cycle of ≤60 min. Compared to the traditional 4–6 h annealing, this suppresses the aging pinning of residual nitrogen and carbon atoms on dislocations. Subsequent 180℃ baking for 20 min does not result in a significant increase in peak strength, avoiding the "baking brittleness" problem in stamping plants. The two-stage heating rate of 2–15 ℃ / s combined with a 20–40 min holding period allows for use in continuous annealing lines or bell-type furnaces, with furnace temperature control accuracy of ±5 ℃. The total energy consumption is 0.9 GJ / t, 25% lower than conventional full austenitizing annealing, and eliminates the need for a rapid cooling section, reducing equipment investment by 10%.

[0060] In this embodiment, the specific process of rapidly quenching the steel plate after critical zone annealing into a salt bath or oil bath at 150-250℃, holding it at that temperature for 30-300 seconds, and then air-cooling it to room temperature is as follows:

[0061] Step 1: Quench the steel plate after critical zone annealing into a salt bath at 180-220℃ at a cooling rate of ≥50℃ / s and hold for 60-180 seconds.

[0062] Step 2: Quickly transfer the steel plate to another salt bath at 250-350℃ and hold it at that temperature for 60-300 seconds;

[0063] Step 3: Air cool the steel plate to room temperature to obtain a composite structure of martensite and retained austenite.

[0064] Specifically, during the entire partitioning process, by controlling the partitioning temperature and time, the volume fraction of retained austenite in the final microstructure reaches 15%–35%, with a carbon content ≥0.8% and a manganese content ≥15%.

[0065] In this embodiment, the tempering process includes:

[0066] A low-temperature, long-time tempering process is adopted, with the tempering temperature controlled within the range of 250-350℃ and the tempering time controlled within the range of 60-120 minutes. Tempering is carried out in air or a protective atmosphere, with the protective atmosphere being nitrogen or argon, and the oxygen content controlled below 100ppm. After tempering, a controlled cooling method is used, cooling to below 100℃ at a cooling rate of 1-5℃ / s, and then air-cooling to room temperature to prevent temper brittleness.

[0067] Specifically, during the tempering process, martensite undergoes a tempering transformation, precipitating fine carbides with a carbide size controlled within the range of 5-50 nm. At the same time, the stability of the retained austenite is adjusted to keep it stable at room temperature, but it can undergo the TRIP effect during deformation.

[0068] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus.

[0069] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention. The scope of protection claimed by the appended claims and their equivalents is defined.

Claims

1. A high-strength and high-toughness medium-manganese steel, characterized in that: The chemical composition of the medium-manganese steel is as follows: carbon: 0.20%–0.30%, manganese: 6.0%–10.0%, silicon: 0.8%–1.5%, aluminum: 0.5%–1.2%, chromium: 0.8%–1.8%, molybdenum: 0.2%–0.6%, vanadium: 0.08%–0.2%, titanium: 0.02%–0.10%, niobium: 0.02%–0.08%, copper: 0.3%–0.8%, nickel: 0.4%–1.0%, boron: 0.0%. 0.01%–0.003%, nitrogen ≤0.005%, phosphorus ≤0.010%, sulfur ≤0.008%, balance iron and unavoidable impurities; among which, the mass ratio of manganese to carbon is controlled within the range of 25–45, the mass ratio of manganese to silicon is controlled within the range of 5–12, the mass ratio of chromium to molybdenum is controlled within the range of 3–8, the total amount of the three microalloying elements vanadium, titanium, and niobium is controlled within the range of 0.1%–0.3%, and the mass ratio of copper to nickel is controlled within the range of 0.5–1.

5.

2. A method for preparing high-strength and high-toughness medium-manganese steel, based on the high-strength and high-toughness medium-manganese steel according to claim 1, characterized in that: include, S1: According to the chemical composition ratio of medium manganese steel, it is smelted in an electric furnace, then refined in a ladle refining furnace, then vacuum degassed using a vacuum circulation degassing method, and then continuously cast into slabs with a thickness of 200-300mm. S2: The slab is heated to 1150-1250℃ in a walking beam furnace and held for 2-4 hours. Then, the slab is processed into steel plates with a diameter of 4-20mm using a controlled rolling process. S3: After rolling, the steel plate is treated with a composite cooling process of ultra-fast cooling and laminar flow cooling; S4: Heat the cooled steel plate to 750-850℃ and hold for 10-60 minutes to form a dual-phase structure of ferrite and austenite; S5: After the critical zone annealing, the steel plate is rapidly quenched into a salt bath or oil bath at 150-250℃, held for 30-300 seconds, and then air-cooled to room temperature. S6: Temper the steel plate by heating the quenched steel plate to 200-400℃ and holding it for 30-180 minutes.

3. The method for preparing high-strength and high-toughness medium-manganese steel according to claim 2, characterized in that: The vacuum degassing process includes controlling the hydrogen content in the steel to ≤2ppm and the oxygen content to ≤15ppm, controlling the superheat during the continuous casting process to be within the range of 15-35℃, controlling the casting speed to be within the range of 0.8-1.2m / min, and adopting a weak cooling process, wherein the cooling intensity of the weak cooling process is controlled to be within the range of 0.3-0.8L / kg steel.

4. The method for preparing high-strength and high-toughness medium-manganese steel according to claim 2, characterized in that: During the rolling process, the roughing rolling start temperature is controlled at 1050-1150℃, the roughing rolling finish temperature is controlled at 950-1050℃, the finishing rolling start temperature is controlled at 850-950℃, and the finishing rolling finish temperature is controlled at 750-850℃.

5. The method for preparing high-strength and high-toughness medium-manganese steel according to claim 2, characterized in that: The steel plate is treated using a composite cooling process of ultra-fast cooling and laminar flow cooling after rolling, including: Step 1: Cooling begins within 0.5-2 seconds after the steel plate exits the rolling mill. First, the surface temperature of the steel plate is rapidly cooled from 800-900℃ to 600-700℃ at a cooling rate of 80-120℃ / s. The cooling rate is achieved by adjusting the cooling water pressure and flow rate. The cooling water pressure is controlled within the range of 0.8-1.5MPa, and the flow rate is controlled within the range of 1000-3000m³ / h. Step 2: Cool the steel plate to 400-500℃ at a cooling rate of 20-40℃ / s. Laminar flow cooling is used in this stage, and the cooling water temperature is controlled within the range of 20-40℃. Step 3: Cool the steel plate to 200-300℃ at a cooling rate of 5-15℃ / s; The temperature difference between the surface and core of the steel plate in the composite cooling process is controlled within 50°C.

6. The method for preparing high-strength and high-toughness medium-manganese steel according to claim 2, characterized in that: The dual-phase microstructure forming ferrite and austenite includes: Step 1: Heat the steel plate from room temperature to 600-700℃ at a heating rate of 5-15℃ / s to allow the microstructure to recover and recrystallize, thereby eliminating rolling stress. Step 2: Heat the steel plate to the critical temperature of 780-820℃ at a heating rate of 2-8℃ / s, and hold for 20-40 minutes to partially austenitize the microstructure. During the heating process, a protective atmosphere is used, which is a mixture of nitrogen and hydrogen, wherein the hydrogen component is 5%-20% and the dew point is controlled below -40°C. In the dual-phase microstructure of ferrite and austenite, the volume fraction of ferrite is 40%–60%, and the volume fraction of austenite is 40%–60%.

7. The method for preparing high-strength and high-toughness medium-manganese steel according to claim 2, characterized in that: The specific process of rapidly immersing the steel plate after critical zone annealing into a salt bath or oil bath at 150-250℃, holding it at that temperature for 30-300 seconds, and then air-cooling it to room temperature is as follows: Step 1: Quench the steel plate after critical zone annealing into a salt bath at 180-220℃ at a cooling rate of ≥50℃ / s and hold for 60-180 seconds. Step 2: Quickly transfer the steel plate to another salt bath at 250-350℃ and hold it at that temperature for 60-300 seconds; Step 3: Air cool the steel plate to room temperature to obtain a composite structure of martensite and retained austenite.

8. The method for preparing high-strength and high-toughness medium-manganese steel according to claim 2, characterized in that: The tempering process includes: A low-temperature, long-time tempering process is adopted, with the tempering temperature controlled within the range of 250-350℃ and the tempering time controlled within the range of 60-120 minutes. Tempering is carried out in air or a protective atmosphere, with the protective atmosphere being nitrogen or argon, and the oxygen content controlled below 100ppm. After tempering, a controlled cooling method is used, cooling to below 100℃ at a cooling rate of 1-5℃ / s, and then air-cooling to room temperature to prevent temper brittleness.

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