Al alloyed high strength ductility medium manganese steel with yield strength of 1000 MPa and preparation method thereof

Abstract

The application discloses a yield strength 1000MPa Al-alloyed high-strength and high-plasticity medium-manganese steel and a preparation method thereof, and belongs to the technical field of automobile steel.The Al and V elements are used to replace noble metal elements, the element composition of the medium-manganese steel is reasonably controlled, the VC nanometer precipitated phase is formed in a two-phase zone through two-phase zone heat preservation, the warm rolling temperature and rolling process are adjusted, and the microstructure composition and grain size of the medium-manganese steel are controlled, so that the medium-manganese steel with the austenite volume fraction of 62% to 91%, the yield strength of 1036MPa to 1130MPa, the tensile strength of 1244MPa to 1308MPa and the elongation after fracture of 48% to 64% is prepared.The preparation process flow is simple, the two-phase zone heating and direct warm rolling are adopted, the extremely simple and short process is used, the repeated high-temperature heating, multi-stage rolling and long-time annealing links are effectively shortened, the rolling deformation resistance and heat treatment energy consumption are reduced, and the production efficiency is improved.

Description

Technical Field

[0001] This invention belongs to the field of automotive steel technology, specifically relating to an Al alloyed high-strength and ductile medium-manganese steel with a yield strength of 1000MPa and its preparation method. Background Technology

[0002] In response to the urgent need for the automotive industry to undergo a profound transformation and upgrade towards energy conservation, emission reduction, and green environmental protection, the next generation of vehicles places higher demands on lightweighting and collision safety. As a crucial structural component of automobiles, the improvement of the mechanical properties of steel plays a decisive role in the lightweighting and safety of the entire vehicle. High-strength, high-ductility automotive steel can achieve overall vehicle weight reduction and lightweighting by lowering the density and wall thickness of steel materials, while ensuring load-bearing capacity and collision energy absorption safety. This significantly reduces the energy consumption of next-generation vehicles and increases their driving range. Simultaneously, its excellent formability can meet the forming requirements of complex automotive structural components, avoiding cracking and springback defects during the processing of high-strength steel, greatly improving yield and reducing manufacturing costs. Therefore, the development of ultra-high-strength, high-ductility automotive steel that combines low-density lightweighting with excellent cold formability has become a key research direction in the steel materials field and the automotive industry, possessing significant engineering application value and industrial economic benefits.

[0003] As a core representative system of the third generation of advanced high-strength automotive steel, medium-manganese steel adopts an alloying design concept with a manganese content of 3%~12% by mass. By controlling the volume fraction of austenite and room temperature stability in the microstructure, and leveraging the transformation-induced plasticity (TRIP) effect during deformation, it can effectively enhance the work hardening rate, thereby significantly improving tensile strength and elongation. This solves the industry problem of insufficient formability and collision safety of traditional high-strength steel, achieving a strength-ductility product that is significantly superior to dual-phase steel and multiphase steel systems in the first generation of advanced high-strength steel. At the same time, compared with the high-manganese alloyed twinned induced plasticity (TWIP) steel in the second generation of advanced high-strength steel, medium-manganese steel does not require the addition of a large amount of Mn element and avoids expensive precious alloying elements such as Ni and Mo, reducing production costs and making it more suitable for the economic needs of large-scale mass production of high-strength automotive steel. Among them, Al alloying can modify the microstructure and properties of medium manganese steel. The addition of Al not only effectively reduces the density of steel and achieves the goal of lightweighting, but also significantly expands the temperature range of the two-phase region, inhibits the precipitation of cementite, and optimizes the microstructure control space of austenite and ferrite, becoming the core development direction of the new generation of medium manganese steel for automobiles.

[0004] Traditional manufacturing processes for medium manganese steel mainly fall into two categories. The first is a multi-pass hot rolling + two-phase annealing process. This process thins the billet through hot rolling, and dynamic recrystallization refines the grains in the high-temperature zone. Subsequent cooling yields lath martensite and austenite. Two-phase annealing then achieves partial martensite-to-austenite reversal, as well as martensite recovery and carbon partitioning to form ferrite, ultimately resulting in a two-phase alternating layered microstructure of austenite and ferrite. While this process yields a large amount of austenite through high-temperature annealing, the austenite's stability is insufficient, and the ferrite coarsens. The tensile process results in continuous yielding with insufficient yield strength, and the poor mechanical stability of austenite degrades elongation, making it difficult to meet the core requirements of automotive lightweighting and collision safety. The second type involves a complex process route consisting of hot rolling, intermediate stress-relief annealing, multi-pass cold rolling, and two-phase annealing. This process introduces high-density dislocations and refined ferrite, austenite, and martensite deformation structures through large deformation during cold rolling. Subsequent two-phase annealing causes recrystallization and reverse phase transformation, resulting in a two-phase equiaxed structure of ferrite and austenite. Low-temperature annealing further refines the grain structure, resulting in low austenite content, high stability, and high yield strength. However, this process suffers from yield drop, long Lüders bands, poor dislocation multiplication ability in the fine-grained structure, and the inability of stable austenite to fully utilize the TRIP effect, leading to insufficient work hardening and poor elongation. High-temperature annealing coarsens the grains and increases the austenite content, but reduces stability and yield strength. Furthermore, the unstable austenite undergoes significant martensitic transformation in the early stages of deformation, resulting in insufficient TRIP effect in the later stages, thus hindering significant improvement in elongation. Therefore, the cold rolling process has inherent drawbacks such as lengthy procedures, high energy consumption, and high manufacturing costs. In particular, for the Al alloyed manganese steel system, the deformation resistance at room temperature is high, and problems such as high rolling force, edge cracking of steel plate, and breakage are prone to occur during the cold rolling process. At the same time, the dislocation strengthening and grain refinement strengthening effects introduced by cold rolling deformation will be greatly lost in the recovery and recrystallization process of subsequent two-phase annealing, and the further improvement of the strength-ductility product of the material encounters a significant bottleneck.

[0005] Chinese patent CN111321351A discloses a technical solution for applying warm rolling technology to the preparation of Fe-Mn-Al-C medium manganese steel. It adopts a two-stage warm rolling process. Although this process can complete the warm rolling deformation of medium manganese steel and achieve microstructure control, it has problems such as long preparation cycle, high energy consumption, poor microstructure controllability, yield strength of less than 1000MPa, and elongation that is difficult to exceed 50%.

[0006] Addressing the production challenges of manganese steel in Al alloying, there is an urgent need to develop a short-process, low-energy-consumption, and high-yield preparation route to obtain the ideal morphology and microstructure of austenite and ferrite, thereby achieving a breakthrough synergistic improvement in material strength and plasticity. Therefore, the design and application development of a short-process preparation technology for the integrated control of microstructure and properties of high-strength and high-plasticity manganese steel in Al alloying is of great significance and necessity, and can generate significant economic and social benefits. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the present invention aims to provide an Al-alloyed high-strength and ductile medium-manganese steel with a yield strength of 1000 MPa and its method. The invention focuses on the composition system of Al-alloyed high-strength and ductile medium-manganese steel, conducting research on the process of direct warm rolling after two-phase region heat treatment, and utilizing my country's abundant V alloy reserves to achieve a large amount of nanoscale VC precipitation, exerting a significant precipitation strengthening effect. The present invention adopts the following technical solution:

[0008] The first aspect of this invention provides an Al-alloyed high-strength and ductile medium-manganese steel with a yield strength of 1000 MPa, which is obtained by direct warm rolling after heat treatment in the two-phase region. Its chemical composition by weight percentage is: C: 0.22%~0.45%, Mn: 10.5%~11.8%, Al: 3.1%~4.5%, Si: 1.0%~1.5%, V: 0.15%~0.51%, Cr: 0.05%~0.51%, P<0.01%, S<0.01%, with the balance being Fe and other unavoidable impurities.

[0009] Furthermore, the microstructure of the medium manganese steel includes austenite, ferrite, nano-VC precipitates and a small amount of κ carbides, wherein the austenite includes micron-sized layered austenite and submicron-sized equiaxed austenite, with a volume fraction ranging from 62% to 91%; the ferrite includes micron-sized layered ferrite and submicron-sized equiaxed ferrite.

[0010] Furthermore, the medium manganese steel plate has a thickness of 2.0mm to 4.4mm, a yield strength of 1036MPa to 1130MPa, a tensile strength of 1244MPa to 1308MPa, and an elongation after fracture of 48% to 64%.

[0011] The second aspect of this invention provides a method for preparing the above-mentioned Al alloyed high-strength and ductile medium-manganese steel with a yield strength of 1000 MPa, specifically including the following process steps:

[0012] (1) Smelting and forging:

[0013] Alloy ingots were prepared by smelting in a vacuum induction furnace. The chemical composition of the alloy ingots by weight percentage was as follows: C: 0.22%~0.45%, Mn: 10.5%~11.8%, Al: 3.1%~4.5%, Si: 1.0%~1.5%, V: 0.15%~0.51%, Cr: 0.05%~0.51%, P<0.01%, S<0.01%, with the balance being Fe and other unavoidable impurities. The alloy ingots were heated in the furnace to 1000℃~1200℃ and held for 1h~3h. Then, they were forged into slabs with a thickness of 46mm~71mm and air-cooled to room temperature.

[0014] (2) Heating in the two-phase region:

[0015] The air-cooled slab is heated to 698℃~801℃ and held for 1.5h~4h for homogenization.

[0016] (3) Warm rolling:

[0017] Direct warm rolling is carried out at 683℃~784℃, and the total reduction rate during the warm rolling process is 90%~97% to roll into a warm rolled plate with a thickness of 2.0mm~4.4mm. After warm rolling, it is cooled to room temperature.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0019] 1. Medium manganese steel adopts an Al alloying design, using inexpensive Al-V elements to replace precious metals such as Ni and Mo, thereby reducing production costs;

[0020] 2. It has the characteristic of low density. Al is a lightweight element and causes lattice expansion. Appropriate addition can effectively reduce the density of steel plates and achieve the goal of lightweighting.

[0021] 3. The addition of Al significantly expands the two-phase region and effectively suppresses the precipitation of cementite, optimizing the microstructure control space during the warm rolling process. V, through combination with C, forms VC nano-precipitates in the two-phase region. By adjusting the warm rolling temperature and rolling process, the volume fraction, grain size, and morphological characteristics of ferrite, austenite, and precipitates can be controlled. Based on this, by combining C-Mn alloying and optimizing the addition of Al and Si, the thermodynamic stabilizing effect of Mn and the synergistic effect of Si and Al in suppressing cementite are utilized. Through the dual control of composition design and warm rolling process, the volume fraction of austenite is increased to 62%~91%, ultimately obtaining a heterogeneous layered microstructure composed of micron-level layered austenite-ferrite and submicron-level equiaxed recrystallized austenite-ferrite. This microstructure feature with both morphological and scale gradients provides a microstructural basis for achieving excellent strength and plasticity.

[0022] 4. It possesses high strength and plasticity. Warm rolling achieves grain refinement and dislocation accumulation, while VC nanoprecipitates exert precipitation strengthening effects. Under stress and deformation, austenite can effectively stimulate the TRIP effect, enhancing the tensile strength and elongation of the material. The prepared Al alloyed high-strength and plastic medium-manganese steel has a yield strength of 1036MPa~1130MPa, a tensile strength of 1244MPa~1308MPa, and an elongation after fracture of 48%~64%.

[0023] 5. The preparation process is simple, adopting a very short process of "two-phase zone heating + direct warm rolling", which effectively reduces repeated high-temperature heating, multi-stage rolling and long-term annealing, reduces rolling deformation resistance and heat treatment energy consumption, and improves production efficiency. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the warm rolling process of the preparation method of the present invention;

[0025] Figure 2 The image shows the XRD pattern of the Al alloyed high-strength and ductile medium-manganese steel prepared in Example 1 after warm rolling.

[0026] Figure 3 The microstructure of the Al alloyed high-strength and ductile medium-manganese steel prepared in Example 1 after warm rolling is shown in SEM.

[0027] Figure 4 The microstructure of the Al alloyed high-strength and ductile medium-manganese steel prepared in Example 1 after warm rolling under TEM.

[0028] Figure 5 The tensile curve of the Al alloyed high-strength and ductile medium-manganese steel prepared in Example 1 after warm rolling. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] The heating furnace used in the warm rolling process of this invention is a high-temperature box-type resistance furnace, model HL07-22.

[0031] The hot rolling test equipment used in this invention is the φ450mm two-roll hot rolling test machine of the DSL Laboratory of Northeastern University.

[0032] In this invention, a ZEISS Ultra 55 scanning electron microscope was used to observe microstructures.

[0033] In this invention, the XRD test was performed using a Rigaku SmarLab 9KW diffractometer.

[0034] The tensile property test in this invention uses an AI-7000-LAU10 universal testing machine.

[0035] This invention provides an Al-alloyed high-strength and ductile medium-manganese steel with a yield strength of 1000 MPa, which is obtained by direct warm rolling after heat treatment in the two-phase region. Its chemical composition by weight percentage is: C 0.22%~0.45%, Mn 10.5%~11.8%, Al 3.1%~4.5%, Si 1.0%~1.5%, V 0.15%~0.51%, Cr 0.05%~0.51%, P<0.01%, S<0.01%, with the balance being Fe and other unavoidable impurities.

[0036] According to a preferred embodiment of the present invention, the microstructure of the high-strength and ductile medium-manganese steel includes austenite, ferrite, nano-VC precipitates and a small amount of κ carbides, wherein the austenite includes micron-scale layered austenite and submicron-scale equiaxed austenite, with a volume fraction ranging from 62% to 91%; the ferrite includes micron-scale layered ferrite and submicron-scale equiaxed ferrite.

[0037] According to a preferred embodiment of the present invention, the medium manganese steel is a steel plate with a thickness of 2.0 mm to 4.4 mm, a yield strength of 1036 MPa to 1130 MPa, a tensile strength of 1244 MPa to 1308 MPa, and an elongation after fracture of 48% to 64%.

[0038] This invention also provides a method for preparing Al alloyed high-strength and ductile medium-manganese steel with a yield strength of 1000 MPa, the process being as follows: Figure 1 As shown, the specific steps include:

[0039] Step 1. Smelting and forging: Smelting and preparing the alloy ingot with the said element composition in a vacuum induction furnace, heating and holding it in the furnace, forging it into a slab, and air cooling it to room temperature;

[0040] Step 2. Two-phase heating: The air-cooled slab is kept at 698℃~801℃ for homogenization.

[0041] Step 3. Warm rolling: The homogenized slab is directly warm rolled at 683℃~784℃ to obtain a warm-rolled plate with a thickness of 2.0mm~4.4mm. After cooling to room temperature, the target high-strength and ductile medium-manganese steel is obtained.

[0042] According to a preferred embodiment of the present invention, during the smelting and forging process, the alloy ingot is heated to 1000℃~1200℃ and held for 1h~3h.

[0043] According to a preferred embodiment of the present invention, the thickness of the slab obtained by smelting and forging is 46mm~77mm.

[0044] According to a preferred embodiment of the present invention, the heating and holding time in the two-phase region is 1.5h to 4h.

[0045] According to a preferred embodiment of the present invention, the total reduction rate during the warm rolling process is 90% to 97%.

[0046] Compared with the cumbersome process of traditional hot rolling + intermediate stress-relief annealing + multi-pass cold rolling + two-phase annealing, the method of this invention solves the problems of high rolling force and easy cracking in cold rolling and coarse grains and insufficient yield strength in two-phase annealing. It optimizes the microstructure, improves the performance of medium manganese steel, and eliminates high-energy-consuming and complex processes such as hot rolling, intermediate annealing, multi-pass cold rolling and long-term two-phase annealing, shortening the production cycle and reducing resource consumption.

[0047] The preparation method provided by this invention is a holistic technical solution comprised of elemental composition design, two-phase region heating, and direct warm rolling. The steps are progressively coordinated and are not independent processes that can be arbitrarily separated or replaced. Specifically, the elemental composition, including C, Mn, Al, Si, V, and Cr, defines the stability of austenite, the range of the two-phase region, and the conditions for nano-VC precipitation. Two-phase region heating rationally regulates the austenite and ferrite phases, providing a suitable initial microstructure for subsequent deformation. Direct warm rolling further refines the microstructure, regulates the morphology of austenite and ferrite, and promotes dislocation strengthening and precipitation strengthening. Through the synergistic effect of compositional design, two-phase region heating, and direct warm rolling, Al-alloyed medium-manganese steel with a high austenite volume fraction, heterogeneous layered microstructure, and excellent strength and plasticity is ultimately obtained. Therefore, the preparation method of the present invention should be implemented as a complete process route. If any step is omitted, replaced or arbitrarily changed, the synergistic relationship between elemental distribution, austenite stabilization, VC precipitation and warm rolling microstructure control may be destroyed, making it difficult to obtain the medium manganese steel with high austenite volume fraction and excellent mechanical properties as described in the present invention.

[0048] Example 1:

[0049] This embodiment provides an Al alloyed high-strength and ductile medium-manganese steel with a yield strength of 1000 MPa and its preparation method. The preparation method of the medium-manganese steel includes the following steps:

[0050] Step 1. Smelting and Forging: Alloy ingots are smelted and prepared in a vacuum induction furnace. The composition of the alloy ingots, by mass percentage, is as follows: C: 0.42%, Mn: 11.8%, Al: 3.7%, Si: 1.2%, V: 0.51%, Cr: 0.05%, P: 0.005%, S: 0.005%, with the remainder being Fe and unavoidable impurities. The alloy ingots are heated in a furnace to the austenitizing temperature, controlled at 1150℃, and held for 3 hours. Then, they are forged into steel billets with a thickness of 46 mm within a temperature range of 1050℃ to 1150℃.

[0051] Step 2. Two-phase heating: The forged steel billet is heated to 744℃ and held for 2 hours for homogenization.

[0052] Step 3. Warm rolling: Direct warm rolling is carried out at 735℃. During the warm rolling process, the total reduction rate is 93%, and the steel billet is rolled into a warm rolled steel plate with a thickness of 3.3mm through 11 rolling passes. After warm rolling, it is cooled to room temperature.

[0053] The prepared medium-manganese steel samples were subjected to morphological analysis and the final microstructure SEM images are shown below. Figure 3 As shown, the TEM image is as follows Figure 4 As shown in the figure, the microstructure consists of austenite, ferrite, and nano-VC precipitates. XRD analysis was used to calculate the content of metastable austenite, and the peak distribution of each substance is shown below. Figure 2 As shown, and based on the integral strength calculation of austenite and ferrite, the austenite content in the medium-manganese steel of this embodiment is 91%. The prepared medium-manganese steel sample blank was ground into a mechanical property sample and subjected to mechanical property testing. The obtained engineering stress-strain curve is shown below. Figure 5 As shown, its yield strength is 1036 MPa, tensile strength is 1308 MPa, and elongation after fracture is 64%.

[0054] Example 2:

[0055] This embodiment provides an Al alloyed high-strength and ductile medium-manganese steel with a yield strength of 1000 MPa and its preparation method. The preparation method of the medium-manganese steel includes the following steps:

[0056] Step 1. Smelting and Forging: Alloy ingots are smelted and prepared in a vacuum induction furnace. The composition of the alloy ingots, by mass percentage, is as follows: C: 0.22%, Mn: 11.7%, Al: 3.9%, Si: 1.2%, V: 0.31%, Cr: 0.15%, P: 0.007%, S: 0.006%, with the remainder being Fe and unavoidable impurities. The alloy ingots are heated in a furnace to the austenitizing temperature, controlled at 1200℃, and held for 1 hour. Then, they are forged into steel billets with a thickness of 63 mm within a temperature range of 1100℃~1180℃.

[0057] Step 2. Two-phase heating: The forged steel billet is heated to 786℃ and held for 1.5 hours for homogenization.

[0058] Step 3. Warm rolling: Direct warm rolling is carried out at 775℃. During the warm rolling process, the total reduction rate is 96%, and the steel billet is rolled into a warm rolled steel plate with a thickness of 2.7mm through 9 rolling passes. After warm rolling, it is cooled to room temperature.

[0059] The prepared medium-manganese steel sample was subjected to morphological analysis. The final microstructure obtained was austenite, ferrite, and nano-VC precipitates. The content of metastable austenite was calculated by XRD analysis. Based on the integral strength of austenite and ferrite, the austenite content in the medium-manganese steel of this embodiment was calculated to be 71%. The prepared medium-manganese steel sample blank was ground into mechanical property test specimens and subjected to mechanical property testing. Its yield strength was 1079 MPa, tensile strength was 1260 MPa, and elongation after fracture was 51%.

[0060] Example 3:

[0061] This embodiment provides an Al alloyed high-strength and ductile medium-manganese steel with a yield strength of 1000 MPa and its preparation method. The preparation method of the medium-manganese steel includes the following steps:

[0062] Step 1. Smelting and Forging: Alloy ingots are smelted and prepared in a vacuum induction furnace. The composition of the alloy ingots, by mass percentage, is as follows: C: 0.38%, Mn: 11.2%, Al: 4.2%, Si: 1.4%, V: 0.44%, Cr: 0.24%, P: 0.004%, S: 0.005%, with the remainder being Fe and unavoidable impurities. The alloy ingots are heated in a furnace to the austenitizing temperature, controlled at 1200℃, and held for 2 hours. Then, they are forged into steel billets with a thickness of 54 mm within a temperature range of 1120℃~1180℃.

[0063] Step 2. Two-phase heating: The forged steel billet is heated to 767℃ and held for 3 hours for homogenization.

[0064] Step 3. Warm rolling: Direct warm rolling is carried out at 752℃. During the warm rolling process, the total reduction rate is 95%, and the steel billet is rolled into a warm rolled steel plate with a thickness of 2.9mm through 11 rolling passes. After warm rolling, it is cooled to room temperature.

[0065] The prepared medium-manganese steel sample was subjected to morphological analysis. The final microstructure obtained was austenite, ferrite, and nano-VC precipitates. The content of metastable austenite was calculated by XRD analysis. Based on the integral strength of austenite and ferrite, the austenite content in the medium-manganese steel of this embodiment was calculated to be 77%. The prepared medium-manganese steel sample blank was ground into mechanical property test specimens and subjected to mechanical property testing. Its yield strength was 1065 MPa, tensile strength was 1276 MPa, and elongation after fracture was 54%.

[0066] Example 4:

[0067] This embodiment provides an Al alloyed high-strength and ductile medium-manganese steel with a yield strength of 1000 MPa and its preparation method. The preparation method of the medium-manganese steel includes the following steps:

[0068] Step 1. Smelting and Forging: Alloy ingots are smelted and prepared in a vacuum induction furnace. The composition of the alloy ingots, by mass percentage, is as follows: C: 0.45%, Mn: 11.3%, Al: 3.1%, Si: 1.0%, V: 0.50%, Cr: 0.42%, P: 0.008%, S: 0.008%, with the remainder being Fe and unavoidable impurities. The alloy ingots are heated in a heating furnace to the austenitizing temperature, controlled at 1000℃, and held for 1.5 hours. Then, they are forged into steel billets with a thickness of 46 mm within a temperature range of 1050℃~1100℃.

[0069] Step 2. Two-phase heating: The forged steel billet is heated to 698℃ and held for 4 hours for homogenization.

[0070] Step 3. Warm rolling: Direct warm rolling is carried out at 683℃. During the warm rolling process, the total reduction rate is 90%, and the steel billet is rolled into a warm rolled steel plate with a thickness of 4.4mm through 10 rolling passes. After warm rolling, it is cooled to room temperature.

[0071] The prepared medium-manganese steel sample was subjected to morphological analysis. The final microstructure obtained was austenite, ferrite, and nano-VC precipitates. The content of metastable austenite was calculated by XRD analysis. Based on the integral strength of austenite and ferrite, the austenite content in the medium-manganese steel of this embodiment was found to be 84%. The prepared medium-manganese steel sample blank was ground into mechanical property test specimens and subjected to mechanical property testing. Its yield strength was 1051 MPa, tensile strength was 1292 MPa, and elongation after fracture was 58%.

[0072] Example 5:

[0073] This embodiment provides an Al alloyed high-strength and ductile medium-manganese steel with a yield strength of 1000 MPa and its preparation method. The preparation method of the medium-manganese steel includes the following steps:

[0074] Step 1. Smelting and Forging: Alloy ingots are smelted and prepared in a vacuum induction furnace. The composition of the alloy ingots, by mass percentage, is as follows: C: 0.29%, Mn: 10.7%, Al: 3.6%, Si: 1.3%, V: 0.48%, Cr: 0.35%, P: 0.007%, S: 0.005%, with the remainder being Fe and unavoidable impurities. The alloy ingots are heated in a furnace to the austenitizing temperature, controlled at 1200℃, and held for 2 hours. Then, they are forged into steel billets with a thickness of 58 mm within a temperature range of 1120℃~1180℃.

[0075] Step 2. Two-phase heating: The forged steel billet is heated to 736℃ and held for 3 hours for homogenization.

[0076] Step 3. Warm rolling: Direct warm rolling is carried out at 724℃. During the warm rolling process, the total reduction rate is 93%, and the steel billet is rolled into a warm rolled steel plate with a thickness of 4.1mm through 11 rolling passes. After warm rolling, it is cooled to room temperature.

[0077] The prepared medium-manganese steel sample was subjected to morphological analysis. The final microstructure obtained was austenite, ferrite, and nano-VC precipitates. The content of metastable austenite was calculated by XRD analysis. Based on the integral strength of austenite and ferrite, the austenite content in the medium-manganese steel of this embodiment was calculated to be 67%. The prepared medium-manganese steel sample blank was ground into mechanical property test specimens and subjected to mechanical property testing. Its yield strength was 1095 MPa, tensile strength was 1244 MPa, and elongation after fracture was 51%.

[0078] Example 6:

[0079] This embodiment provides an Al alloyed high-strength and ductile medium-manganese steel with a yield strength of 1000 MPa and its preparation method. The preparation method of the medium-manganese steel includes the following steps:

[0080] Step 1. Smelting and Forging: Alloy ingots are smelted and prepared in a vacuum induction furnace. The composition of the alloy ingots, by mass percentage, is as follows: C: 0.35%, Mn: 10.5%, Al: 4.5%, Si: 1.5%, V: 0.15%, Cr: 0.51%, P: 0.006%, S: 0.004%, with the remainder being Fe and unavoidable impurities. The alloy ingots are heated in a furnace to the austenitizing temperature, controlled at 1200℃, and held for 2 hours. Then, they are forged into steel billets with a thickness of 71 mm within a temperature range of 1120℃~1180℃.

[0081] Step 2. Two-phase heating: The forged steel billet is heated to 801℃ and held for 2 hours for homogenization.

[0082] Step 3. Warm rolling: Direct warm rolling is carried out at 784℃. During the warm rolling process, the total reduction rate is 97%, and the steel billet is rolled into a warm rolled steel plate with a thickness of 2.0mm through 11 rolling passes. After warm rolling, it is cooled to room temperature.

[0083] The prepared medium-manganese steel sample was subjected to morphological analysis. The final microstructure obtained was austenite, ferrite, and nano-VC precipitates. The content of metastable austenite was calculated by XRD analysis. Based on the integral strength of austenite and ferrite, the austenite content in the medium-manganese steel of this embodiment was calculated to be 62%. The prepared medium-manganese steel sample blank was ground into mechanical property test specimens and subjected to mechanical property testing. Its yield strength was 1130 MPa, tensile strength was 1261 MPa, and elongation after fracture was 48%.

[0084] Comparative Example 1:

[0085] This comparative example provides a medium-manganese steel and its preparation method, wherein the preparation method of the medium-manganese steel includes the following steps:

[0086] Step 1. Smelting and Forging: Alloy ingots are smelted and prepared in a vacuum induction furnace. The composition of the alloy ingots, by mass percentage, is as follows: C: 0.18%, Mn: 10.7%, Al: 3.6%, Si: 1.3%, V: 0.48%, Cr: 0.35%, P: 0.005%, S: 0.005%, with the remainder being Fe and unavoidable impurities. The alloy ingots are heated in a furnace to the austenitizing temperature, controlled at 1200℃, and held for 2 hours. Then, they are forged into steel billets with a thickness of 60 mm within a temperature range of 1100℃ to 1200℃.

[0087] Step 2. Two-phase heating: The forged steel billet is heated to 752℃ and held for 2.5 hours for homogenization.

[0088] Step 3. Warm rolling: Direct warm rolling is carried out at 740℃. During the warm rolling process, the total reduction rate is 94%, and the steel billet is rolled into a warm rolled steel plate with a thickness of 3.6mm through 10 rolling passes. After warm rolling, it is cooled to room temperature.

[0089] The prepared medium-manganese steel samples were subjected to morphological analysis. The final microstructure obtained was austenite, ferrite, and nano-VC precipitates. The content of metastable austenite was calculated by XRD analysis. Based on the integral strength of austenite and ferrite, the austenite content in the medium-manganese steel of this comparative example was calculated to be 51%. The prepared medium-manganese steel sample blank was ground into mechanical property test specimens and subjected to mechanical property testing. Its yield strength was 963 MPa, tensile strength was 1205 MPa, and elongation after fracture was 36%.

[0090] Comparative Example 2:

[0091] This comparative example provides a medium-manganese steel and its preparation method, wherein the preparation method of the medium-manganese steel includes the following steps:

[0092] Step 1. Smelting and Forging: Alloy ingots are smelted and prepared in a vacuum induction furnace. The composition of the alloy ingots, by mass percentage, is as follows: C: 0.29%, Mn: 9.2%, Al: 3.5%, Si: 1.4%, V: 0.45%, Cr: 0.34%, P: 0.006%, S: 0.005%, with the remainder being Fe and unavoidable impurities. The alloy ingots are heated in a furnace to the austenitizing temperature, controlled at 1200℃, and held for 2 hours. Then, they are forged into steel billets with a thickness of 59 mm within a temperature range of 1100℃ to 1200℃.

[0093] Step 2. Two-phase heating: The forged steel billet is heated to 745℃ and held for 3 hours for homogenization.

[0094] Step 3. Warm rolling: Direct warm rolling is carried out at 733℃. During the warm rolling process, the total reduction rate is 93%, and the steel billet is rolled into a warm rolled steel plate with a thickness of 3.8mm through 11 rolling passes. After warm rolling, it is cooled to room temperature.

[0095] The prepared medium-manganese steel samples were subjected to morphological analysis. The final microstructure obtained was austenite, ferrite, and nano-VC precipitates. The content of metastable austenite was calculated by XRD analysis. Based on the integral strength of austenite and ferrite, the austenite content in the medium-manganese steel of this comparative example was calculated to be 55%. The prepared medium-manganese steel sample blank was ground into mechanical property test specimens and subjected to mechanical property testing. Its yield strength was 995 MPa, tensile strength was 1242 MPa, and elongation after fracture was 41%.

[0096] Comparative Example 3:

[0097] This comparative example provides a medium-manganese steel and its preparation method, wherein the preparation method of the medium-manganese steel includes the following steps:

[0098] Step 1. Smelting and Forging: Alloy ingots are smelted and prepared in a vacuum induction furnace. The composition of the alloy ingots, by mass percentage, is as follows: C: 0.26%, Mn: 10.5%, Al: 2.7%, Si: 0.8%, V: 0.44%, Cr: 0.24%, P: 0.004%, S: 0.006%, with the remainder being Fe and unavoidable impurities. The alloy ingots are heated in a furnace to the austenitizing temperature, controlled at 1200℃, and held for 2 hours. Then, they are forged into steel billets with a thickness of 42mm within a temperature range of 1100℃~1200℃.

[0099] Step 2. Two-phase heating: The forged steel billet is heated to 695℃ and held for 3.5 hours to perform homogenization treatment.

[0100] Step 3. Warm rolling: Direct warm rolling is carried out at 681℃. During the warm rolling process, the total reduction rate is 90%, and the steel billet is rolled into a warm rolled steel plate with a thickness of 4.2mm through 10 rolling passes. After warm rolling, it is cooled to room temperature.

[0101] The prepared medium-manganese steel samples were subjected to morphological analysis. The final microstructure obtained was austenite, ferrite, and nano-VC precipitates. The content of metastable austenite was calculated by XRD analysis. Based on the integral strength of austenite and ferrite, the austenite content in the medium-manganese steel of this comparative example was calculated to be 68%. The prepared medium-manganese steel sample blank was ground into mechanical property test specimens and subjected to mechanical property testing. Its yield strength was 948 MPa, tensile strength was 1256 MPa, and elongation after fracture was 44%.

[0102] Comparative Example 4:

[0103] This comparative example provides a medium-manganese steel and its preparation method, wherein the preparation method of the medium-manganese steel includes the following steps:

[0104] Step 1. Smelting and Forging: Alloy ingots are smelted and prepared in a vacuum induction furnace. The composition of the alloy ingots, by mass percentage, is as follows: C: 0.28%, Mn: 10.8%, Al: 3.6%, Si: 1.2%, V: 0.35%, Cr: 0.25%, P: 0.005%, S: 0.005%, with the remainder being Fe and unavoidable impurities. The alloy ingots are heated in a furnace to the austenitizing temperature, controlled at 1200℃, and held for 2 hours. Then, they are forged into steel billets with a thickness of 48 mm within a temperature range of 1100℃ to 1200℃.

[0105] Step 2. Single-phase heating: Heat the forged steel billet to 1150℃ and hold for 2 hours to perform homogenization treatment.

[0106] Step 3. High-temperature rolling: Direct rolling is carried out at 1138℃. During the hot rolling process, the billet undergoes a total reduction rate of 95% and is rolled in 9 passes to form a hot-rolled steel plate with a thickness of 2.6mm. After rolling, it is cooled to room temperature.

[0107] The prepared medium-manganese steel samples were subjected to morphological analysis. The final microstructure obtained was austenite and ferrite. The content of metastable austenite was calculated by XRD analysis. Based on the integral strength of austenite and ferrite, the austenite content in the medium-manganese steel of this comparative example was calculated to be 52%. The prepared medium-manganese steel sample blank was ground into mechanical property test specimens and subjected to mechanical property testing. Its yield strength was 962 MPa, tensile strength was 1285 MPa, and elongation after fracture was 38%.

[0108] Comparative Example 5:

[0109] This comparative example provides a medium-manganese steel and its preparation method, wherein the preparation method of the medium-manganese steel includes the following steps:

[0110] Step 1. Smelting and Forging: Alloy ingots are smelted and prepared in a vacuum induction furnace. The composition of the alloy ingots, by mass percentage, is as follows: C: 0.28%, Mn: 11.2%, Al: 3.1%, Si: 0.9%, V: 0.30%, Cr: 0.21%, P: 0.006%, S: 0.005%, with the remainder being Fe and unavoidable impurities. The alloy ingots are heated in a furnace to the austenitizing temperature, controlled at 1200℃, and held for 2 hours. Then, they are forged into steel billets with a thickness of 58 mm within a temperature range of 1100℃ to 1180℃.

[0111] Step 2. First two-phase heating and first warm rolling: The forged steel billet is heated to 690℃ and held for 3 hours for homogenization treatment; then it is warm rolled at 676℃ for the first time. The cumulative reduction rate during the warm rolling process is 48%. After 6 passes of rolling, an intermediate billet with a thickness of 30.2mm is obtained and cooled to room temperature after rolling.

[0112] Step 3. Second two-phase region heating and second warm rolling: The above intermediate billet is reheated to 688℃ and held for 2 hours; then, a second warm rolling is performed at 674℃. During the warm rolling process, the reduction rate relative to the intermediate billet is 87%. After 6 rolling passes, a warm rolled steel plate with a thickness of 3.8mm is finally rolled. The total reduction rate of the whole process is 93%, and the total number of rolling passes is 12. After warm rolling, it is cooled to room temperature.

[0113] The prepared medium-manganese steel samples were subjected to morphological analysis. The final microstructure obtained was austenite, ferrite, and nano-VC precipitates. The content of metastable austenite was calculated by XRD analysis. Based on the integral strength of austenite and ferrite, the austenite content in the medium-manganese steel of this comparative example was found to be 78%. The prepared medium-manganese steel sample blank was ground into mechanical property specimens and tested. Its yield strength was 932 MPa, tensile strength was 1256 MPa, and elongation after fracture was 48%.

Claims

1. A high-strength, high-ductility, medium-manganese steel with Al alloying and a yield strength of 1000 MPa, characterized in that, The medium manganese steel is obtained by direct warm rolling after heat preservation in the two-phase region; The elemental composition of the medium manganese steel, by mass percentage, includes: C: 0.22%~0.45%, Mn: 10.5%~11.8%, Al: 3.1%~4.5%, Si: 1.0%~1.5%, V: 0.15%~0.51%, Cr: 0.05%~0.51%, P<0.01%, S<0.01%, with the balance being Fe and other unavoidable impurities; the elongation after fracture of the medium manganese steel is 48%~64%. The microstructure of the medium manganese steel comprises austenite with a volume fraction of 62% to 91%.

2. The Al alloyed high-strength and ductile medium-manganese steel with a yield strength of 1000 MPa according to claim 1, characterized in that, The microstructure of the medium manganese steel also includes ferrite, nano-VC precipitates, and κ carbides.

3. The Al alloyed high-strength and ductile medium-manganese steel with a yield strength of 1000 MPa according to claim 2, characterized in that, Austenite includes micron-scale layered austenite and submicron-scale equiaxed austenite, while ferrite includes micron-scale layered ferrite and submicron-scale equiaxed ferrite.

4. The Al alloyed high-strength and ductile medium-manganese steel with a yield strength of 1000 MPa according to claim 1, characterized in that, The medium manganese steel plate has a thickness of 2.0mm to 4.4mm, a yield strength of 1036MPa to 1130MPa, and a tensile strength of 1244MPa to 1308MPa.

5. A method for preparing Al alloyed high-strength and ductile medium-manganese steel with a yield strength of 1000 MPa as described in any one of claims 1 to 4, characterized in that, Includes the following steps: S1. Smelting and forging: Alloy ingots are prepared by smelting according to the elemental composition of medium manganese steel in a vacuum induction furnace. The alloy ingots are heated and held in the furnace and then forged into slabs. The slabs are air-cooled to room temperature. S2. Two-phase region heating: The air-cooled slab is heated to the two-phase region and then held at that temperature for homogenization. S3. Warm rolling: The homogenized slab is directly warm rolled at 683℃~784℃ and then cooled to room temperature to obtain the target Al alloyed high-strength and ductile medium-manganese steel.

6. The method for preparing Al alloyed high-strength and ductile medium-manganese steel with a yield strength of 1000 MPa according to claim 5, characterized in that, In S2, the temperature in the two-phase region is 698℃~801℃, and the holding time is 1.5h~4h.

7. The method for preparing Al alloyed high-strength and ductile medium-manganese steel with a yield strength of 1000 MPa according to claim 5, characterized in that, In S1, the alloy ingot is heated to 1000℃~1200℃ in the furnace and held for 1h~3h.

8. The method for preparing Al alloyed high-strength and ductile medium-manganese steel with a yield strength of 1000 MPa according to claim 5, characterized in that, In S1, the slab thickness is 46mm~71mm.

9. The method for preparing Al alloyed high-strength and ductile medium-manganese steel with a yield strength of 1000 MPa according to claim 5, characterized in that, In S3, the total reduction rate during the warm rolling process is 90%~97%.

Images