A high-strength, high-ductility, cold-rolled medium-manganese steel without localized deformation bands and its preparation method.

Abstract

This invention belongs to the field of metal material preparation technology, and relates to a high-strength, high-plasticity, cold-rolled medium-manganese steel without localized deformation bands and its preparation method. The method includes: annealing in the austenite-ferrite dual-phase region to obtain a medium-manganese steel billet with a microstructure mainly composed of manganese-rich austenite and manganese-poor ferrite regions; cold-rolling the annealed medium-manganese steel billet to obtain a cold-rolled medium-manganese steel blank composed of martensite and deformed ferrite; rapidly heating the cold-rolled medium-manganese steel billet to above the austenite transformation temperature to obtain a medium-manganese steel billet with a microstructure mainly composed of unevenly distributed manganese austenite; and quenching the medium-manganese steel billet to room temperature to obtain a finished medium-manganese steel product with a microstructure mainly composed of manganese-rich austenite and manganese-poor martensite regions. The martensitic matrix and a small amount of metastable austenite in the microstructure ensure good plasticity and strength, while a large number of dislocations suppress the generation of localized deformation bands, ensuring the uniform deformation capability of the medium-manganese steel.

Description

Technical Field

[0001] This invention relates to the field of metal material preparation technology, and in particular to a high-strength, high-ductility, cold-rolled medium-manganese steel without local deformation bands and its preparation method. Background Technology

[0002] Medium manganese steel, as an important branch of third-generation advanced high-strength steel, has a microstructure mainly composed of austenite and ferrite. Compared with hot-rolled medium manganese steel, cold-rolled medium manganese steel generally has lower energy consumption and processing costs, higher dimensional accuracy, better surface quality and thickness uniformity, and can meet the needs of high-precision products. The traditional processing technology for cold-rolled medium manganese steel involves stress-relief annealing and multi-pass cold rolling to obtain cold-rolled medium manganese steel sheets, followed by reverse transformation annealing to obtain a dual-phase microstructure of austenite and ferrite.

[0003] Existing cold-rolled medium-manganese steel has the following drawbacks: On the one hand, the TRIP effect of austenite brings excellent work hardening characteristics to medium-manganese steel, but the presence of a ferrite matrix limits its strength improvement. On the other hand, the deformation process of traditional medium-manganese steel is usually accompanied by the formation and expansion of local deformation bands (including Lüders bands and PLC bands), which not only affects the surface finish of the workpiece but may also adversely affect the material's resistance to hydrogen embrittlement. The formation and expansion of Lüders bands usually correspond to the yield plateau on the tensile curve, while the formation and expansion of PLC bands correspond to the sawtooth stress fluctuations on the tensile curve, such as... Figure 1 Red area.

[0004] Therefore, there is an urgent need in the existing technology for a cold-rolled medium-manganese steel with high strength and high plasticity that can suppress the formation of local deformation bands. Summary of the Invention

[0005] In view of the above analysis and in view of the shortcomings of the prior art, the present invention aims to provide a high-strength, high-plasticity, cold-rolled medium manganese steel without local deformation bands and its preparation method, thereby solving at least one of the problems in the prior art, such as the difficulty in simultaneously obtaining high strength and good plasticity of cold-rolled medium manganese steel and the formation and expansion of local deformation bands during the deformation process of cold-rolled medium manganese steel.

[0006] The objective of this invention is mainly achieved through the following technical solutions:

[0007] This invention discloses a method for preparing high-strength, high-ductility, cold-rolled medium-manganese steel without localized deformation bands, comprising:

[0008] Annealing in the austenite-ferrite dual-phase region yields a medium-manganese steel billet with a microstructure mainly composed of manganese-rich austenite regions and manganese-poor ferrite regions.

[0009] Cold rolling of annealed medium-manganese steel billets yields cold-rolled medium-manganese steel billets composed of martensite and deformed ferrite.

[0010] The cold-rolled medium-manganese steel billet is heated to above the austenite transformation temperature at a rate of 20℃ / s to 200℃ / s to obtain a medium-manganese steel billet whose microstructure is mainly composed of austenite with uneven manganese distribution.

[0011] Medium-manganese steel billets are quenched to room temperature to obtain finished medium-manganese steel products whose microstructure mainly consists of manganese-rich austenite regions and manganese-poor martensite regions.

[0012] Preferably, the annealing temperature for the austenitic-ferrite dual-phase annealing is 600℃~800℃.

[0013] Preferably, the austenite transformation temperature is 800℃~1000℃.

[0014] Preferably, the method for preparing the high-strength, high-ductility, cold-rolled medium-manganese steel without localized deformation bands further includes:

[0015] Before the austenitic-ferrite dual-phase annealing treatment of medium manganese steel billets, pre-annealing and cold rolling in the austenitic-ferrite dual-phase region can be performed to improve the overall reduction rate.

[0016] Preferably, the method for preparing the high-strength, high-ductility, cold-rolled medium-manganese steel without localized deformation bands includes:

[0017] Step 1: Prepare raw materials according to the design composition, and obtain medium manganese steel billets through smelting and casting;

[0018] Step 2: Heat, hold, and then forge the medium manganese steel raw material billet to prepare a medium manganese steel forging billet;

[0019] Step 3: Heat and hold the medium manganese steel forging billet, then hot roll it to obtain a hot-rolled plate;

[0020] Step 4: After stress-relief annealing, the hot-rolled plate is subjected to the first cold rolling to obtain a medium-manganese steel billet of the target thickness;

[0021] Step 5: Anneal the medium-manganese steel billet of the target thickness in the austenite-ferrite dual-phase region and air-cool it to room temperature to obtain the dual-phase annealed plate.

[0022] Step 6: Perform a second cold rolling on the two-phase annealed plate obtained in Step 5;

[0023] Step 7: Heat the second cold-rolled plate obtained in Step 6 to 800℃~1000℃ at a heating rate of 20℃ / s~200℃ / s, and quench it to room temperature at a cooling rate of 20℃ / s~40℃ / s.

[0024] Preferably, the reduction rate of the first cold rolling in step 4 is 10% to 80%.

[0025] Preferably, the reduction rate of the second cold rolling in step 6 is 10% to 80%.

[0026] Preferably, the mass content of Mn in the medium manganese steel billet is 3% to 10%.

[0027] Preferably, the chemical composition of the medium manganese steel billet is as follows by mass percentage: C 0.1%–0.3%, Mn 3%–10%, Si 0–1%, Al 0–1%, Nb 0–0.2%, Mo 0–3.0%, V 0–1.0%, Ti 0–0.5%, Ni 0–5.0%, Cu 0–5.0%, Cr 0–5.0%, with the balance being Fe and unavoidable impurities.

[0028] A high-strength, high-plasticity, cold-rolled medium-manganese steel without local deformation bands is prepared by the above method. The cold-rolled medium-manganese steel is mainly composed of two phases: martensite and austenite. The volume fraction of austenite is 10% to 40%, and the volume fraction of martensite is 60% to 90%.

[0029] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0030] (1) The present invention involves annealing the cold-rolled medium manganese steel billet in the austenite-ferrite dual-phase region, followed by cold rolling, and then rapidly heating it to above the austenite transformation temperature and immediately quenching it. On the one hand, the microstructure of the obtained medium manganese steel product is composed of martensite and austenite. The martensite matrix ensures a significant increase in its strength, and the TRIP effect of the metastable austenite ensures good plasticity. On the other hand, the rapid heating process prevents the dislocations in the deformed structure from fully recovering. Together with the shear formed by the martensite transformation, this results in a large number of mobile dislocations in the sample, which ensures the work hardening of the cold-rolled medium manganese steel, suppresses the generation of local deformation bands, and ensures the uniform deformation capability of the medium manganese steel.

[0031] (2) By adding a secondary cold rolling process between the austenite-ferrite dual-phase annealing process and the rapid heating heat treatment step, the present invention increases the number of dislocations in the finished medium manganese steel. These dislocations and the TRIP effect of austenite further suppress the generation of local deformation bands and further improve the uniform deformation capability of medium manganese steel. Attached Figure Description

[0032] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0033] Figure 1 This is a schematic diagram showing the relationship between the generation and expansion process of the PLC belt and the sawtooth stress fluctuations on the tension curve.

[0034] Figure 2aThis is a schematic diagram of the heat treatment and quenching process in step 8 of Embodiment 1 of the present invention;

[0035] Figure 2b This is a secondary electron image of the microstructure of the sample obtained in step 8 of Example 1 of the present invention;

[0036] Figure 2c This is a microstructure image quality distribution map of the sample obtained in step 8 of Embodiment 1 of the present invention;

[0037] Figure 2d This is a transmission electron microscope (TEM) image of the microstructure of the sample obtained in step 8 of Example 1 of the present invention;

[0038] Figure 3 The image shows the engineering stress-beam displacement curve of the sample obtained in step 8 of Embodiment 1 of the present invention.

[0039] Figure 4 The engineering stress-engineering strain curves of samples 1 and 2 in Example 2 of this invention are shown.

[0040] Figure 5 The engineering stress-engineering strain curve of the sample obtained in step 8 of Embodiment 3 of the present invention;

[0041] Figure 6 This is a microstructure image quality distribution diagram of the sample obtained in Comparative Example 1 of this invention;

[0042] Figure 7 The engineering stress-engineering strain curve of the sample obtained in Comparative Example 1 of this invention;

[0043] Figure 8 This is the engineering stress-engineering strain curve of the sample obtained in Comparative Example 2 of this invention. Detailed Implementation

[0044] To make the technical problems solved, the technical solutions, and the beneficial effects of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and are not intended to limit the invention.

[0045] Terminology definition:

[0046] Dislocations are a type of line defect in crystals. Through activities such as dislocation multiplication and cross slip, the density of movable dislocations can be increased, which can enhance the work hardening ability of medium manganese steel, thereby effectively suppressing the formation of local deformation bands and improving its uniform deformation ability.

[0047] On one hand, this invention discloses a method for preparing high-strength, high-ductility, cold-rolled medium-manganese steel without localized deformation bands, comprising:

[0048] The medium-manganese steel billet was annealed in the austenite-ferrite dual-phase region to obtain a medium-manganese steel billet whose microstructure mainly consists of a manganese-rich austenite region and a manganese-poor ferrite region.

[0049] Cold rolling of annealed medium-manganese steel billets yields cold-rolled medium-manganese steel billets composed of martensite and deformed ferrite.

[0050] The cold-rolled medium-manganese steel billet is heated to above the austenite transformation temperature at a heating rate of 20℃ / s to 200℃ / s to obtain a medium-manganese steel billet whose microstructure is mainly composed of austenite with uneven manganese distribution.

[0051] Medium-manganese steel billets with unevenly distributed manganese austenite are quenched to room temperature to obtain finished medium-manganese steel products whose microstructure mainly consists of manganese-rich austenite regions and manganese-poor martensite regions.

[0052] During implementation, the microstructure of the medium-manganese steel billet after cold rolling treatment forms a manganese-rich martensite region and a manganese-poor ferrite region.

[0053] After annealing in the austenite-ferrite dual-phase region, the manganese-rich martensite region in the medium-manganese steel billet is transformed into a manganese-rich austenite region, and the microstructure of the medium-manganese steel is composed of a manganese-rich austenite and manganese-poor ferrite dual phases.

[0054] The medium manganese steel billet after dual-phase annealing is rapidly heated to above the austenite transformation temperature to obtain a medium manganese steel billet dominated by austenite; at the same time, the rapid heating process limits the diffusion distance of substitutional alloying elements such as Mn, so the non-uniform distribution of Mn is retained in the high-temperature austenite.

[0055] When a medium-manganese steel billet composed of austenite with uneven manganese distribution is quenched to room temperature, the austenite in the Mn-rich region has higher stability and can be retained to room temperature, while the austenite in the Mn-depleted region transforms into martensite during the quenching process.

[0056] Compared with existing technologies, this invention rapidly heats the cold-rolled medium-manganese steel billet to above the austenite transformation temperature and then quenches it. On the one hand, the resulting medium-manganese steel product has a microstructure composed of martensite and austenite. The martensitic matrix ensures a significant increase in strength, while the TRIP effect of the metastable austenite ensures good plasticity. On the other hand, the large number of dislocations inherited in the microstructure after rapid heating and the dislocations generated during the martensitic transformation during quenching, together with the TRIP effect of austenite, ensure its work hardening ability and suppress the generation of local deformation bands.

[0057] Preferably, the method for preparing the high-strength, high-ductility, cold-rolled medium-manganese steel without localized deformation bands further includes:

[0058] Before the austenitic-ferrite dual-phase annealing treatment of medium manganese steel billets, pre-annealing and cold rolling in the austenitic-ferrite dual-phase region can be performed to improve the overall reduction rate.

[0059] Specifically, the mass content of Mn in the raw material billet of medium manganese steel is 3% to 10%.

[0060] Preferably, the chemical composition of the medium manganese steel billet is as follows by mass percentage: C 0.1%–0.3%, Mn 3%–10%, Si 0–1%, Al 0–1%, Nb 0–0.2%, Mo 0–3.0%, V 0–1.0%, Ti 0–0.5%, Ni 0–5.0%, Cu 0–5.0%, Cr 0–5.0%, with the balance being Fe and unavoidable impurities.

[0061] The specific components and functions of the manganese steel in cold rolling are as follows:

[0062] Carbon (C) is a major factor affecting the strength of steel plates and can combine with elements such as titanium (Ti), niobium (Nb), vanadium (V), and molybdenum (Mo) to form carbides, thereby improving the properties of steel. According to the strength grade and ductility / toughness requirements of the steel according to this invention, the C content is precisely controlled within the range of 0.1% to 0.3%.

[0063] Mn: As a key austenite stabilizing element, increasing the manganese content helps to improve the content and stability of metastable austenite in medium-manganese steel. However, excessively high manganese content may lead to macrosegregation in the material, affecting smelting and processing. Therefore, in this invention, the Mn content is controlled at 3% to 10% to balance strength and plasticity requirements while ensuring processing performance.

[0064] Si and Al: The addition of these two elements aims to suppress cementite precipitation, thereby increasing the content and stability of metastable austenite. However, excessive addition of Si and Al may adversely affect the surface quality of medium manganese steel; therefore, the Si and Al content is precisely controlled within the range of 0–1%.

[0065] Nb, Mo, V, Ti, Cu, and Cr: These microalloying elements enhance the strength of medium-manganese steel through mechanisms such as precipitation strengthening, solid solution strengthening, and grain refinement strengthening. However, excessive addition may impair the steel's plasticity. Therefore, according to the strength and toughness requirements of the steel grades of this invention, the addition amounts of these elements are controlled at Nb 0–0.2%, Mo 0–3.0%, V 0–1.0%, Ti 0–0.5%, Cu 0–5.0%, and Cr 0–5.0%.

[0066] Ni: The addition of nickel helps to further improve the stability of austenite, but due to its high cost, to ensure the best balance between economy and performance, Ni should be controlled at 0-5.0%. Specifically, the volume fraction of austenite is 8%-35%, and the corresponding volume fraction of ferrite is 65%-92%; the volume fraction of austenite can be 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, or 35%.

[0067] Specifically, the annealing temperature for austenite-ferrite dual-phase annealing is 600℃~800℃, for example, 600℃, 605℃, 640℃, 650℃, 698℃, 700℃, 712℃, 725℃, 745℃, 765℃, 798℃ or 800℃.

[0068] It should be noted that the annealing temperature has a significant impact on the microstructure of medium manganese steel: as the annealing temperature increases, the volume fraction of cementite decreases, the volume fraction of austenite increases, and the Mn concentration in austenite decreases. Lower annealing temperatures can lead to a low austenite volume fraction and the formation of a large amount of cementite, which is detrimental to mechanical properties. Higher annealing temperatures, on the other hand, lead to a decrease in austenite stability.

[0069] Preferably, the annealing temperature for the austenitic-ferrite duplex pre-annealing and the austenitic-ferrite duplex annealing is 600℃~680℃.

[0070] More preferably, the annealing temperature for the pre-annealing in the austenite-ferrite dual-phase region before and after cold rolling and the annealing in the austenite-ferrite dual-phase region is 630℃~660℃.

[0071] Specifically, the heating rate range for austenite-ferrite dual-phase annealing is 1℃ / s to 300℃ / s, for example, it can be 1℃ / s, 5℃ / s, 40℃ / s, 50℃ / s, 98℃ / s, 100℃ / s, 112℃ / s, 125℃ / s, 145℃ / s, 165℃ / s, 198℃ / s, 200℃ / s, 212℃ / s, 225℃ / s, 232℃ / s, 250℃ / s, 298℃ / s or 300℃ / s.

[0072] It should be noted that a lower heating rate allows the material more time for phase transformation, promoting homogenization between austenite and ferrite and reducing internal stress; a higher heating rate may lead to non-uniform phase transformation, increasing internal stress and thus affecting the material's mechanical properties.

[0073] Specifically, the cooling rate for austenite-ferrite dual-phase annealing is 5℃ / min to 500℃ / min, for example, it can be 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min, 20℃ / min, 30℃ / min, 40℃ / min, 50℃ / min, 60℃ / min, 100℃ / min, 200℃ / min, 300℃ / min, 400℃ / min or 500℃ / min.

[0074] It should be noted that slow cooling allows sufficient time for phase transformation, promoting homogenization of the microstructure and preventing cracking and deformation, but may also lead to an increase in grain size. Rapid cooling may lead to martensitic phase transformation or other unfavorable phase transformations, resulting in increased residual stress, which may cause cracking or other defects. On the other hand, appropriate heating and cooling rates help recrystallize and refine the grains, enhancing the mechanical properties of the material.

[0075] Specifically, the holding time for annealing in the austenitic-ferrite dual-phase region is 1h to 10h, for example, it can be 1h, 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h or 10h.

[0076] It should be noted that while a certain phase transformation can be achieved in a short time (e.g., 1 hour), it is insufficient to complete all phase transformations and homogenize the microstructure, potentially leading to inhomogeneity and unstable properties. Longer holding times (e.g., 10 hours) allow for more complete phase transformations, promoting grain recrystallization and growth, helping to eliminate internal stress, and improving the material's machinability. However, excessively long holding times may result in excessively large grain sizes, leading to reduced austenite stability and sample strength.

[0077] Specifically, the austenite transformation temperature is 800℃~1000℃.

[0078] For example, it could be 800℃, 805℃, 840℃, 850℃, 898℃, 900℃, 912℃, 925℃, 945℃, 965℃, 998℃, or 1000℃.

[0079] It should be noted that the austenite transformation temperature described in this invention is the temperature at which ferrite is almost completely transformed into austenite. This temperature is not only higher than the austenite transformation initiation temperature Ac1, but also even higher than the austenite transformation completion temperature Ac3. However, excessively high temperatures can lead to increased grain size, Mn diffusion, and dislocation recovery, thereby causing a decrease in the material's mechanical properties. Rapid heating austenitization restricts grain growth, Mn diffusion, and dislocation recovery, thus improving the material's performance.

[0080] Specifically, the cooling rate from quenching to room temperature is 20℃ / s to 40℃ / s, which can be 20℃ / s, 22℃ / s, 24℃ / s, 25℃ / s, 26℃ / s, 28℃ / s, 30℃ / s, 32℃ / s, 33℃ / s, 35℃ / s, 38℃ / s or 40℃ / s.

[0081] It should be noted that an excessively high quenching cooling rate can lead to a large temperature gradient within the workpiece, resulting in significant thermal stress. This stress accumulates within the workpiece, potentially causing deformation or cracking; it can also lead to microcracks within the material, especially at sharp corners or stress concentration areas; and it can significantly increase the workpiece's hardness. While this may be advantageous in some applications requiring high hardness, it can lead to increased brittleness in situations requiring a certain level of toughness.

[0082] If the quenching cooling rate is too low, austenite may first transform into ferrite or bainite, which reduces the hardness of the workpiece; this will also reduce the martensite content in the workpiece and result in insufficient strength.

[0083] Preferably, the method for preparing cold-rolled medium-manganese steel further includes:

[0084] After the dual-phase annealing treatment, the medium manganese steel billet undergoes a second cold rolling treatment and is then heated to above the austenite transformation temperature at a heating rate of 20℃ / s to 200℃ / s.

[0085] During implementation, the medium manganese steel billet after dual-phase annealing undergoes cold rolling, which generates a large number of dislocations. These dislocations cannot be fully recovered during rapid heating and are subsequently inherited by the room temperature martensite-austenite dual-phase structure of the finished medium manganese steel. The TRIP effect of austenite further suppresses the generation of local deformation bands.

[0086] Compared with the prior art, the present invention adds cold rolling between the two-phase annealing treatment and the rapid heating heat treatment, which increases the number of dislocations in the finished manganese steel. These dislocations and the TRIP effect of austenite further suppress the generation of local deformation bands.

[0087] Preferably, both the first and second cold rolling processes may include multiple cold rolling operations, rather than a single cold rolling to the target size; the reduction rates of the multiple cold rolling operations may be the same or different, and the total reduction rate after multiple cold rolling operations must be within the target range.

[0088] Specifically, the method for preparing the high-strength, high-ductility, cold-rolled medium-manganese steel without localized deformation bands includes:

[0089] Step 1: Prepare raw materials according to the design composition, and obtain medium manganese steel billets through smelting and casting;

[0090] Step 2: Heat, hold, and then forge the medium manganese steel raw material billet to prepare a medium manganese steel forging billet;

[0091] Step 3: Heat and hold the medium manganese steel forging billet, then hot roll it to obtain a hot-rolled plate;

[0092] Step 4: After stress-relief annealing, the hot-rolled plate is subjected to the first cold rolling to obtain a medium-manganese steel billet of the target thickness;

[0093] Step 5: Anneal the medium-manganese steel billet of the target thickness in the austenite-ferrite dual-phase region and air-cool it to room temperature to obtain the dual-phase annealed plate.

[0094] Step 6: Perform a second cold rolling on the two-phase annealed plate obtained in Step 5;

[0095] Step 7: Heat the second cold-rolled plate obtained in Step 6 to 800℃~1000℃ at a heating rate of 20℃ / s~200℃ / s, and quench it to room temperature at a cooling rate of 20℃ / s~40℃ / s.

[0096] It should be noted that step (1) can adopt existing material balance methods, use existing technology for manganese steel smelting raw materials, prepare raw materials according to the design composition, and obtain steel billets through smelting and casting.

[0097] It should be noted that the solidus temperature of medium manganese steel is around 1300℃~1350℃, so its maximum heating temperature should not exceed 1200℃. In step 2, the steel billet and in step 3, the forging billet are preferably heated to 1180℃~1250℃, which can ensure that the steel billet has good plasticity and low deformation resistance, while avoiding overheating and burning.

[0098] It should also be noted that when medium manganese steel is heated to 1180℃~1250℃, which is in the austenitic region, the preferred holding time for the billet in step 2 and the forging in step 3 is 60min~120min. This is because when medium manganese steel is heated in the austenitic region, it is necessary to ensure that the carbides inside the billet are fully dissolved and that the austenitic grains are homogenized. If the holding time is too short, it may lead to incomplete austenitization, which will affect the subsequent processing performance.

[0099] In addition, during the heating process of medium manganese steel billets, after the surface temperature reaches the set value, the internal temperature still needs time to conduct and homogenize; the holding time ensures that the internal temperature of the steel billet is uniform, avoiding stress concentration and uneven deformation caused by excessive temperature gradient.

[0100] In addition, in the austenitic region, austenitic grains gradually grow as the holding time increases; an appropriate holding time can control the grain growth rate, preventing excessively coarse grains and thus ensuring the mechanical properties of the material. Studies on the austenitic grain growth kinetics of medium-manganese steel show that excessively long holding times lead to grain coarsening, reducing the material's toughness and strength.

[0101] In addition, medium-manganese steel contains a certain amount of alloying elements (such as manganese), which affect the austenitization rate and grain growth behavior. Appropriate holding time can ensure sufficient diffusion and uniform distribution of alloying elements, thereby obtaining ideal microstructure and properties.

[0102] Specifically, the final rolling temperature in step 3 shall not be lower than 900℃.

[0103] It should be noted that the final rolling temperature should not be too low to ensure that the steel has sufficient plasticity and deformation capacity during the rolling process, while avoiding rolling difficulties and uneven microstructure and properties caused by excessively low temperatures. Furthermore, the final rolling temperature has a significant impact on the microstructure and properties of the steel. Higher final rolling temperatures may lead to grain growth, thereby reducing the material's strength and toughness; while appropriate final rolling temperatures can refine the grains and improve the overall performance of the material.

[0104] It should be noted that the hot rolling of medium manganese steel in step 3 is a process of heating in the austenitic region, and the final rolling temperature should not be lower than 900℃.

[0105] Specifically, the reduction rate of the first cold rolling in step 4 is 10% to 80%, for example, it can be 10%, 20%, 30%, 40%, 50%, 60%, 70% or 80%.

[0106] Specifically, the reduction rate of the second cold rolling in step 6 is 10% to 80%, for example, it can be 10%, 20%, 30%, 40%, 50%, 60%, 70% or 80%.

[0107] It should be noted that the reduction ratio directly affects the material's microstructure, properties, and the stability of the processing; a reasonable selection of the reduction ratio can improve the material's microstructure and macroscopic properties, achieving better performance in use.

[0108] It should be noted that increasing the manganese content in medium-manganese steel can increase the proportion and stability of austenite, thereby enhancing the strength and plasticity of the steel. However, high manganese content can also lead to cracking during cold rolling, posing challenges to production and processing. Existing technologies employ multiple stress-relief annealing and multi-pass cold rolling to improve the reduction rate of medium-manganese steel. In contrast, this invention significantly improves the cold rolling performance of medium-manganese steel through multiple two-phase annealing and cold rolling, while shortening the cold rolling-heat treatment production process. Preferably, steps 4 and 6 can be repeated multiple times within the operable cold rolling temperature range, depending on the required plate thickness.

[0109] On the other hand, the present invention provides a cold-rolled medium manganese steel, prepared by the above method, wherein the mass content of Mn in the cold-rolled medium manganese steel is 3% to 10%; it is mainly composed of two phases, martensite and austenite, wherein the volume fraction of austenite is 10% to 40% and the volume fraction of martensite is 60% to 90%.

[0110] Specifically, its chemical composition by mass percentage is as follows: C 0.1%–0.3%, Mn 3%–10%, Si 0–1%, Al 0–1%, Nb 0–0.2%, Mo 0–3.0%, V 0–1.0%, Ti 0–0.5%, Ni 0–5.0%, Cu 0–5.0%, Cr 0–5.0%, with the balance being Fe and unavoidable impurities.

[0111] Specifically, the volume fraction of austenite is 10%, 12%, 15%, 18%, 20%, 25%, 28%, 30%, 33%, 38%, or 40%.

[0112] To better illustrate the present invention, the following embodiments are further provided:

[0113] Example 1

[0114] This embodiment discloses a cold-rolled medium-manganese steel and a high-strength, high-plasticity cold-rolled medium-manganese steel without local deformation bands, and its preparation method, including:

[0115] Step 1: Smelt molten steel according to the set composition and cast it to obtain a steel billet. Its composition, by weight percentage, contains 0.18% C, 4.95% Mn, 0.4% Si, 0.0021% Mo, with the remainder being Fe and other unavoidable impurities.

[0116] Step 2: Heat the steel billet to 1200℃, hold for 2 hours, and then forge to obtain a forged billet;

[0117] Step 3: Hold the forging billet at 1200℃ for 2 hours, and then hot roll it through multiple passes to obtain a hot-rolled plate with a thickness of approximately 5.5 mm. The final rolling temperature should not be lower than 900℃.

[0118] Step 4: Anneal the hot-rolled plate at 630℃ for 3 hours and then air-cool it to room temperature;

[0119] Step 5: Cold roll the annealed hot-rolled sheet to a thickness of approximately 2.8 mm, with a reduction rate of approximately 49%.

[0120] Step 6: Anneal the obtained cold-rolled sheet at 630℃, isothermal for 2 hours, and then air-cool to room temperature;

[0121] Step 7: Continue to cold roll the sheet obtained in Step 6 to a thickness of 1.2 mm, with a reduction rate of approximately 57%;

[0122] Step 8: Heat the cold-rolled sheet obtained in Step 7 to 850°C at a heating rate of 100°C / s, and then rapidly cool it to room temperature at a cooling rate of 120°C / s.

[0123] The purpose of step 4 is to eliminate residual stress generated during hot rolling and to form a non-uniform distribution of Mn. The microstructure of the sample obtained in step 8 above is as follows: Figures 2a-2c As shown, the red grains are austenite, and the green area is martensite; it is composed of a martensitic matrix and austenite, with an austenite volume fraction of approximately 15%.

[0124] Figure 2d The TEM images shown indicate that the sample contains high-density dislocations in both the martensitic matrix and the austenitic matrix. The austenitic matrix also contains deformed twins and numerous stacking faults.

[0125] Figure 3 The tensile curves show that the sample has a tensile breaking strength of approximately 1950 MPa, a crossbeam displacement of approximately 2.0 mm at fracture, a gauge length of 25 mm, a tensile direction parallel to the rolling direction, and an elongation of approximately 10%. Therefore, the obtained medium-manganese steel sample possesses both high strength and high plasticity, and no local deformation bands such as Lüders bands / PLC bands are generated during the deformation process.

[0126] Quasi-static tensile tests were performed on an MTS electronic universal testing machine, with the tensile rate set to 10. -3 / s.

[0127] Example 2

[0128] This embodiment discloses a cold-rolled medium-manganese steel and a high-strength, high-plasticity cold-rolled medium-manganese steel without local deformation bands, and its preparation method, including:

[0129] Step 1: Smelt molten steel according to the set composition and cast it to obtain a steel billet. Its composition, by weight percentage, contains 0.18% C, 4.95% Mn, 0.4% Si, 0.0021% Mo, with the remainder being Fe and other unavoidable impurities.

[0130] Step 2: Heat the steel billet to 1200℃, hold for 2 hours, and then forge to obtain a forged billet;

[0131] Step 3: Hold the forging billet at 1200℃ for 2 hours, and then hot roll it through multiple passes to obtain a hot-rolled plate with a thickness of approximately 6.4 mm. The final rolling temperature should not be lower than 900℃.

[0132] Step 4: The hot-rolled plate is subjected to isothermal treatment at 450℃ for 2 hours and then air-cooled to room temperature, followed by cold rolling to a thickness of about 5mm and a reduction rate of about 22%.

[0133] Step 5: The obtained sheet is subjected to isothermal treatment at 400℃ for 1 hour and air-cooled to room temperature, and then cold-rolled to a thickness of about 4mm with a reduction rate of about 20%.

[0134] Step 6: Anneal the obtained sheet at 630℃, isothermal for 1 hour and air-cool to room temperature, then cold-roll to a thickness of about 3mm with a reduction rate of about 25%;

[0135] Step 7: Anneal the obtained sheet at 630℃, isothermal for 2 hours and air-cool to room temperature, then cold-roll to a thickness of about 1.4 mm with a reduction rate of about 53%;

[0136] Step 8: Anneal the obtained sheet at 630℃, isothermal for 2 hours and air-cool to room temperature, then cold-roll to a thickness of about 1 mm with a reduction rate of about 29%.

[0137] Step 9: Heat the cold-rolled sheet obtained in Step 8 to 850°C at a heating rate of 100°C / s, and then rapidly cool it to room temperature at a cooling rate of 25°C / s.

[0138] The microstructure of the cold-rolled sheet obtained in step 8 is deformed martensite with non-uniform Mn distribution. Following the rapid heat treatment in step 9, where deformation defects are not fully recovered, a dual-phase microstructure with high dislocation density, consisting of a martensitic matrix and metastable austenite, is generated. The volume fraction of metastable austenite is 15%.

[0139] In contrast, sample 2 was annealed and then rapidly heated. The two samples had similar microstructures, but sample 2 had a lower dislocation density. Figure 4 The tensile curves show that the tensile breaking strength of sample 1 is 1803 MPa, and the total elongation is 15.1%. The gauge length of the tensile sample used is 25 mm, and the tensile direction is parallel to the rolling direction. No local deformation bands were generated during the deformation process of sample 1, while in the later stage of deformation, sawtooth fluctuations appeared on the tensile curve of sample 2, indicating the generation of PLC bands.

[0140] Quasi-static tensile tests were performed on an MTS electronic universal testing machine, and the change in gauge length during the tensile process was recorded using a mechanical extensometer to calculate the engineering strain; the tensile rate was set to 10. -3 / s.

[0141] Example 3

[0142] This embodiment discloses a cold-rolled medium-manganese steel and a high-strength, high-plasticity cold-rolled medium-manganese steel without local deformation bands, as well as its preparation method.

[0143] 1. Steel billets are obtained by smelting molten steel according to the set composition and casting it. The composition of the billets, by weight percentage, is 0.18% C, 4.95% Mn, 0.4% Si, 0.0021% Mo, with the remainder being Fe and other unavoidable impurities.

[0144] 2. Heat the steel billet to 1200℃, hold for 2 hours, and then forge to obtain a forged billet;

[0145] 3. The forging billet is held at 1200℃ for 2 hours, and then hot-rolled in multiple passes to obtain a hot-rolled plate with a thickness of approximately 4.6 mm. The final rolling temperature is not lower than 900℃.

[0146] 4. Anneal the hot-rolled plate at 630℃ for 5 hours and then air-cool it to room temperature;

[0147] 5. Cold roll the annealed hot-rolled sheet to a thickness of approximately 2.8 mm, with a reduction rate of approximately 39%;

[0148] 6. Anneal the obtained cold-rolled sheet at 630℃, isothermal for 2 hours, and then air-cool to room temperature;

[0149] 7. Continue to cold roll the sheet obtained in step 6 to a thickness of 1.2 mm, with a reduction rate of approximately 57%;

[0150] 8. The cold-rolled sheet obtained in step 7 is heated to 850°C at a heating rate of 100°C / s and then rapidly cooled to room temperature at a cooling rate of 25°C / s.

[0151] The purpose of step 4 is to eliminate the residual stress generated during hot rolling and to form a non-uniform distribution of Mn.

[0152] Following the rapid heat treatment in step 8, where deformation defects are not fully recovered, a dual-phase microstructure with high dislocation density, consisting of a martensitic matrix and metastable austenite, is generated. The volume fraction of metastable austenite is 12%.

[0153] The tensile curve of the obtained sample is as follows Figure 5 As shown. The total elongation of the sample was 11.7%, the tensile breaking strength was 1937 MPa, the gauge length of the tensile sample used was 25 mm, the tensile direction was parallel to the rolling direction, and no Lüders band / PLC band was generated during the deformation process.

[0154] Quasi-static tensile tests were performed on an MTS electronic universal testing machine, and the change in gauge length during the tensile process was recorded using a mechanical extensometer to calculate the engineering strain; the tensile rate was set to 10. -3 / s.

[0155] Comparative Example 1

[0156] This comparative example discloses a cold-rolled medium manganese steel and a high-strength, high-plasticity cold-rolled medium manganese steel without local deformation bands and its preparation method. Compared with Example 1, step 5 of this comparative example, which involves cold rolling to the target thickness, does not include the second cold rolling step in step 7. Instead, it is obtained by reverse phase transformation annealing of the cold-rolled medium manganese steel sheet.

[0157] The microstructure of the obtained cold-rolled manganese steel is as follows Figure 6 As shown, it consists of a ferrite matrix and approximately 10% austenite by volume. Figure 6 As shown, the volume fraction of austenite obtained is 10%.

[0158] Its mechanical properties are as follows Figure 7 As shown, its fracture strength is approximately 750 MPa, and its elongation is approximately 18%. The tensile curve shows a distinct yield plateau and sawtooth fluctuations, indicating the formation of Lüders bands and PLC bands.

[0159] Comparative Example 2

[0160] This comparative example discloses a cold-rolled medium manganese steel and a cold-rolled medium manganese steel with high strength and high plasticity and no local deformation band, and its preparation method. Compared with Example 2, this comparative example adds an isothermal annealing step before rapid heating in step 9 to eliminate a large number of dislocations in the sample caused by cold rolling.

[0161] 1. Steel billets are obtained by smelting molten steel according to the set composition and casting it. The composition of the billets, by weight percentage, is 0.18% C, 4.95% Mn, 0.4% Si, 0.0021% Mo, with the remainder being Fe and other unavoidable impurities.

[0162] 2. Heat the steel billet to 1200℃, hold for 2 hours, and then forge to obtain a forged billet;

[0163] 3. The forging billet is held at 1200℃ for 2 hours, and then hot-rolled in multiple passes to obtain a hot-rolled plate with a thickness of approximately 6.4 mm. The final rolling temperature is not lower than 900℃.

[0164] 4. The hot-rolled plate is subjected to isothermal treatment at 450℃ for 2 hours and then air-cooled to room temperature, followed by cold rolling to a thickness of about 5mm;

[0165] 5. The obtained sheet material is subjected to isothermal treatment at 400℃ for 1 hour and then air-cooled to room temperature, followed by cold rolling to a thickness of approximately 4mm;

[0166] 6. Anneal the obtained sheet at 630℃, isothermal for 1 hour and air-cool to room temperature, then cold-roll to a thickness of about 3mm;

[0167] 7. Anneal the obtained sheet at 630℃, isothermal for 2 hours and air-cool to room temperature, then cold-roll to a thickness of about 1.4mm;

[0168] 8. Anneal the obtained sheet at 630℃, isothermal for 2 hours and air-cool to room temperature, then cold-roll to a thickness of about 1mm;

[0169] 9. Anneal the cold-rolled sheet obtained in step 8 at 630°C, isothermal for 2 hours and air-cool to room temperature; then heat it to 850°C at a heating rate of 100°C / s and then rapidly cool it to room temperature at a cooling rate of 25°C / s.

[0170] Following the rapid heat treatment in step 9, where deformation defects are not fully recovered, a dual-phase microstructure with high dislocation density, consisting of a martensitic matrix and metastable austenite, is generated. The volume fraction of metastable austenite is approximately 15%.

[0171] Figure 8 The results show that sample 2 has a tensile strength of 1655 MPa and a total elongation of 18.8%. Compared to the obtained sample, the tensile strength is lower and the elongation is slightly higher. Its tensile curve exhibits obvious sawtooth-like fluctuations, indicating the formation of the PLC band.

[0172] As can be seen from the above, the embodiments of the present invention can prepare martensitic-metastable austenitic dual-phase cold-rolled medium manganese steel with a tensile fracture strength of 1800MPa to 1950MPa, a total elongation of 10% to 15%, and no Lüders band / PLC band generated during the deformation process.

[0173] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for producing a high-strength high-ductility, cold-rolled medium-manganese steel having no localized deformation bands, characterized in that, The application relates to a high-strength and high-plasticity cold-rolled medium-manganese steel without local deformation bands and a preparation method thereof. The application comprises the following steps: carrying out austenite-ferrite dual-phase zone annealing to obtain a medium-manganese steel billet mainly composed of a manganese-rich austenite region and a manganese-poor ferrite region; carrying out cold rolling treatment on the annealed medium-manganese steel billet to obtain a cold-rolled medium-manganese steel billet composed of martensite and deformed ferrite; heating the cold-rolled medium-manganese steel billet to above the austenite transformation temperature at a heating rate of 20-200 DEG C / s to obtain a medium-manganese steel billet mainly composed of austenite with uneven manganese distribution; quenching the medium-manganese steel billet to room temperature to obtain a finished medium-manganese steel mainly composed of a manganese-rich austenite region and a manganese-poor martensite region; The preparation method of the high-strength and high-plasticity cold-rolled medium-manganese steel without local deformation bands comprises the following steps: Step 1, preparing raw materials according to the designed composition, smelting and casting to obtain a medium-manganese steel raw billet; Step 2, heating, holding and post-forging the medium-manganese steel raw billet to prepare a medium-manganese steel forged billet; Step 3, heating and holding the medium-manganese steel forged billet and then hot rolling to obtain a hot-rolled plate; Step 4, carrying out first cold rolling on the hot-rolled plate after stress relief annealing to obtain a medium-manganese steel billet with a target thickness; Step 5, annealing the medium-manganese steel billet with the target thickness in the austenite-ferrite dual-phase zone and then air cooling to room temperature to obtain a dual-phase zone annealed plate; Step 6, carrying out second cold rolling on the dual-phase zone annealed plate obtained in step 5; 2. The method of producing a high-strength high-ductility, cold-rolled medium-manganese steel without a localized deformation zone according to claim 1, characterized in that, Step 7, heating the second cold-rolled plate obtained in step 6 to 800-1000 DEG C at a heating rate of 20-200 DEG C / s and quenching to room temperature at a cooling rate of 20-40 DEG C / s.

3. The method of producing a high-strength high-ductility, cold-rolled medium-manganese steel without a localized deformation zone according to claim 2, characterized in that, The annealing temperature of the austenite-ferrite dual-phase zone annealing is 600-800 DEG C.

4. Process for the production of a high-strength high-ductility, cold-rolled medium-manganese steel without a localised deformation band according to claim 2 or 3, characterised in that, The austenite transformation temperature is 800-1000 DEG C. The preparation method of the high-strength and high-plasticity cold-rolled medium-manganese steel without local deformation bands further comprises the following steps:

5. The method of producing a high-strength high-ductility, cold-rolled medium-manganese steel without a localized deformation zone according to claim 1, characterized in that, Before the austenite-ferrite dual-phase zone annealing treatment of the medium-manganese steel billet, austenite-ferrite dual-phase zone pre-annealing and cold rolling can be carried out to improve the overall reduction rate.

6. The method of producing a high-strength high-ductility, cold-rolled medium-manganese steel without a localized deformation zone according to claim 5, characterized in that, The reduction rate of the first cold rolling in step 4 is 10-80%.

7. The method of producing a high-strength high-ductility, cold-rolled medium-manganese steel without a localized deformation zone according to claim 6, characterized by, The reduction rate of the second cold rolling in step 6 is 10-80%.

8. The method of producing a high-strength high-ductility, cold-rolled medium-manganese steel without a localized deformation zone according to claim 7, characterized by, The mass content of Mn in the medium-manganese steel raw billet is 3-10%.

9. A high-strength, high-ductility, cold-rolled medium-manganese steel, characterized in that, The chemical composition of the medium-manganese steel raw billet is as follows in terms of mass percentage: C 0.1-0.3%, Mn 3-10%, Si 0-1%, Al 0-1%, Nb 0-0.2%, Mo 0-3.0%, V 0-1.0%, Ti 0-0.5%, Ni 0-5.0%, Cu 0-5.0%, Cr 0-5.0%, and the balance being Fe and inevitable impurities. The cold-rolled medium-manganese steel prepared by the method in any one of claims 1-8 is mainly composed of martensite and austenite, the volume fraction of the austenite is 10-40%, and the volume fraction of the martensite is 60-90%.

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