Method for preparing superplastic heterogeneous dual-phase fine-grained medium manganese steel and prepared medium manganese steel

Abstract

The present invention provides a method for preparing superplastic heterogeneous dual-phase fine-grained medium manganese steel and the prepared medium manganese steel, and the specific steps are: according to the chemical composition of the medium manganese steel configured with a C content of 0.1-0.3%, a Mn content of 4%-12%, a Si content of less than 3%, an Al content of less than 4%, and the rest of the Fe element and impurities, smelting and casting into an ingot, and forging the ingot, air cooling to room temperature and then cutting to obtain a plate blank; the plate blank is sequentially subjected to small deformation warm rolling, small deformation cold rolling and superplastic deformation processes to prepare superplastic heterogeneous dual-phase fine-grained medium manganese steel. The material organization of the medium manganese steel is composed of austenite and ferrite, and the austenite includes coarse-grained austenite and fine-grained austenite, and ferrite accounts for 40-50% in the material organization of the medium manganese steel, and the grain size range of the coarse-grained austenite is 10-20μm, and the grain size range of the fine-grained austenite is 1-5μm. The present invention prepares heterogeneous dual-phase fine-grained medium manganese steel with superplastic deformation mechanical properties through a process, thereby broadening the advanced automobile sheet forming process method.

Description

Technical Field

[0001] The invention relates to the technical field of automobile steel materials, and in particular to a method for preparing superplastic heterogeneous dual-phase fine-grained medium manganese steel and the prepared medium manganese steel. Background Art

[0002] Due to the rapid development of the automobile industry, automobile steel materials are constantly being improved and optimized. In the iterative process of automobile steel, medium manganese steel plate has become one of the representatives of the third generation of advanced automobile steel. Its excellent comprehensive mechanical properties and low proportion of alloy elements make it a representative of low-cost and high-performance excellent metal materials.

[0003] Medium manganese steel containing 5%-12% Mn content generally presents a dual-phase structure of ferrite (BCC) and austenite (FCC) at room temperature. At the same time, depending on the differences in element ratio, heat treatment process and processing technology, the proportion of the two-phase structure and the grain size are different. In this type of steel, austenite has moderate stability at room temperature, and TRIP / TWIP effect can occur during room temperature deformation, thereby effectively improving its comprehensive mechanical properties in the cold deformation process, thus becoming a typical representative of advanced automotive steel.

[0004] However, due to the disadvantages of cold deformation process, such as springback, wear, large deformation resistance, easy cracking during deformation, and difficulty in realizing super-complex forming, the application of medium manganese steel sheet is actually greatly limited. Based on the above problems, some scholars proposed to use superplastic deformation to realize the forming process of metal materials. Superplasticity refers to the phenomenon that the material shows abnormally high plasticity without necking and fracture under certain internal conditions such as microstructure, grain size and crystal structure and external conditions such as deformation temperature and rate. This hot deformation method can effectively avoid the above problems, and at the same time provide ideas for realizing one-step forming of multi-step cold deformation process or realizing more difficult structural parts. At the same time, in the research process of medium manganese steel, the heterogeneous structure of medium manganese steel triggers different deformation mechanisms of TWIP / TRIP due to the difference in grain size, and the deformation mechanism is carried out in sequence due to the difference in grain size, which can further improve the comprehensive mechanical properties of the sheet. Therefore, through experimental verification, a method for preparing superplastic heterogeneous dual-phase fine-grained medium manganese steel is proposed, which provides guidance for promoting the hot forming process of medium manganese steel automotive sheet. Summary of the invention

[0005] In view of the problems existing in the prior art, the present invention provides a method for preparing superplastic heterogeneous dual-phase fine-grained medium manganese steel and the prepared medium manganese steel. Firstly, a certain weight of C, Mn, Si, Al and Fe elements are respectively taken for smelting and cast into ingots, then the ingots are heated to a temperature at which the columnar grains inside the ingots are broken and refined, and the ingots are forged while being kept warm, and medium manganese steel slabs are obtained by wire cutting; finally, the medium manganese steel slabs are subjected to small deformation warm rolling, small deformation cold rolling and superplastic deformation processes in sequence to prepare superplastic heterogeneous dual-phase fine-grained medium manganese steel, so that the material structure of the medium manganese steel can achieve superplastic deformation, thereby providing a solution for the superplastic deformation process of automobile steel.

[0006] The present invention provides a method for preparing superplastic heterogeneous dual-phase fine-grained medium manganese steel, comprising the following steps:

[0007] S1. According to the weight percentage, a certain weight of C, Mn, Si, Al and Fe elements are respectively taken for smelting, and the smelted molten steel is cast into an ingot, wherein the content of C element is 0.1-0.3%, the content of Mn element is 4%-12%, the content of Si element is less than 3%, the content of Al element is less than 4%, and the rest is Fe element and unavoidable impurities;

[0008] S2, heating the ingot obtained in step S1 to a temperature at which the columnar grains inside the ingot are broken and refined, and forging the ingot at this temperature to obtain a medium manganese steel slab containing austenite and ferrite dual-phase structure and a thickness of 4-5 mm, and air-cooling the medium manganese steel slab to room temperature at a certain cooling rate;

[0009] S3, using wire cutting to cut the medium manganese steel slab obtained in step S2 to obtain a medium manganese steel slab with a thickness of 4-5 mm;

[0010] S4, heating the medium manganese steel slab obtained in step S3 to 300° C.-400° C. and keeping the temperature for 1 hour to obtain a heat-treated medium manganese steel slab;

[0011] S5, subjecting the medium manganese steel slab obtained in step S4 to single-pass small deformation warm rolling, so that 80-85% of the austenite in the medium manganese steel slab undergoes martensitic transformation, while retaining 15-20% of the stable forged coarse-grained austenite structure, to obtain a medium manganese steel warm-rolled slab with a thickness of 3-4 mm, and air-cooling it to room temperature at a cooling rate of 20° C. to 30° C. / s;

[0012] S6, subjecting the medium manganese steel warm-rolled slab obtained in step S5 to single-pass small deformation cold rolling to adjust the content ratio of coarse-grained austenite and transformed martensite, so that the ratio of coarse-grained austenite to fine-grained austenite is in the range of 1:9-2:8, and at the same time, the medium manganese steel warm-rolled slab maintains a certain dislocation density, so as to obtain a medium manganese steel cold-rolled slab with a thickness of 2-3 mm;

[0013] S7. Cut the medium manganese steel cold-rolled slab obtained in step S6 to obtain a superplastic tensile deformation specimen. In order to make the dislocation density in step S6 disappear and maintain the grain size of coarse-grained austenite, fine-grained austenite and ferrite structure, the superplastic tensile deformation specimen is placed in a closed insulation furnace, and the superplastic tensile deformation specimen is tested at different superplastic deformation temperatures and different strain rates.

[0014] Preferably, in step S2, the temperature for crushing and refining columnar grains inside the ingot is 1150°C, the forging temperature is 1000°C-1200°C, the holding time is 1.5-2.5h, and the cooling rate is 20°C-30°C / s.

[0015] Preferably, in step S5, the warm rolling temperature is 300-400°C, and the warm rolling deformation is 20-30%.

[0016] Preferably, in step S6, the cold rolling deformation rate is 0.5 m-2 m / s, and the cold rolling deformation amount is 20-30%.

[0017] Preferably, in the tensile deformation test of step S7, the heating rate is 25°C / min, the deformation temperature is 650°C-750°C, and the strain rate is 10 -2 -10 -3 / s, the holding time is 3-5min, and the cooling rate is 20℃~30℃ / s.

[0018] Preferably, the chemical composition of the medium manganese steel further includes: a Nb element content of 0-0.3% and a Cu element content of 0.5-2.0%.

[0019] Another aspect of the present invention provides a medium manganese steel prepared based on the method for preparing superplastic heterogeneous dual-phase medium manganese steel, wherein the material structure of the medium manganese steel consists of austenite and ferrite, the austenite includes coarse-grained austenite and fine-grained austenite, the ferrite accounts for 40-50% of the material structure of the medium manganese steel, the grain size range of the coarse-grained austenite is 10-20μm, and the grain size range of the fine-grained austenite is 1-5μm.

[0020] Compared with the prior art, the present invention has the following advantages:

[0021] 1. The medium manganese steel prepared by the present invention has a strong-plastic product. The C element with a content of 0.1-0.3% can improve the stability of austenite in the medium manganese steel; the Mn element with a content of 4%-12% can broaden the austenite phase region, strengthen the matrix, and improve the stability of austenite; the Si element with a content of less than 3% can promote the formation of austenite and increase the content of residual austenite at room temperature; the Al element with a content of less than 4% can significantly reduce the density of the medium manganese steel. The formation of austenite mainly depends on the stacking fault energy. The Al element can increase the stacking fault energy of austenite, so that the weight of automobile steel is reduced, the deformation resistance is small, and it is not easy to crack or rebound after deformation, thereby reducing the cost of automobile steel.

[0022] 2. In the present invention, under the sequential rolling process of warm rolling and cold rolling based on small deformation, the medium manganese steel structure prepared presents a heterogeneous dual-phase structure of coarse-grained austenite and fine-grained austenite with a certain dislocation density, and the other is a ferrite phase. In this kind of organization, the bimodal grain size during room temperature deformation causes the austenite to trigger different deformation mechanisms, namely, TRIP and TWIP effects, thereby improving the ductility of superplastic deformation.

[0023] 3. The Al and Mn elements added in the present invention have different proportions in the dual-phase austenite and ferrite, and the solute drag effect can effectively suppress grain growth, thereby ensuring the requirement of fine grain size of medium manganese steel in superplastic deformation.

[0024] 4. The warm rolling process of the present invention can reduce the deformation resistance during the rolling process of steel, ensure the surface quality of steel, have small deformation resistance, smooth surface, high production efficiency, and warm rolling can ensure the refinement of grain size. The small deformation cold rolling process can ensure the surface quality of the test steel is intact, and can further refine the grains, and keep the test steel able to store certain dislocation defects caused by cold rolling deformation. The reverse phase transformation process of the deformed martensite in the test steel is realized during the superplastic deformation process, thereby realizing the appearance of fine-grained austenite grains, while ensuring that the grain size does not grow and controlling the proportion of austenite and ferrite at 55%-60% to 40%-45%.

[0025] 5. The superplastic deformation process temperature and strain rate of the present invention can trigger the grain boundary sliding mechanism of the superplastic deformation of the fine-grained dual-phase medium-manganese steel, thereby generating the superplastic deformation behavior of the fine-grained dual-phase medium-manganese steel.

[0026] 6. The present invention controls the ratio of superplastic deformation materials and the rolling process, regulates the temperature range of superplastic deformation materials, makes the reverse phase transformation temperature, superplastic deformation temperature and recrystallization temperature coincide in three dimensions, eliminates the cumbersome critical annealing process, shortens the superplastic preparation process, eliminates the critical annealing process, and realizes innovative process simplification for the preparation of superplastic medium manganese steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 A comparison diagram of bimodal austenite grain sizes in the method for preparing superplastic heterogeneous dual-phase fine-grained medium manganese steel according to the present invention;

[0028] Figure 2A and 2B They are respectively microstructure morphology diagrams after forging and after rolling in the method for preparing superplastic heterogeneous dual-phase fine-grained medium manganese steel of the present invention;

[0029] Figure 3A and 3B The superplastic deformation at 650 / 700 / 750°C for 10 -2 / 10 -3 s -1 Superplastic engineering stress-strain curve at strain rate;

[0030] Figure 4 In the method for preparing superplastic heterogeneous dual-phase fine-grained medium manganese steel of the present invention, the -3 s -1 Engineering stress-strain curve of superplastic deformation;

[0031] Figure 5 In the method for preparing superplastic heterogeneous dual-phase fine-grained medium manganese steel of the present invention, the -3 s -1 Microstructure morphology at the initial stage of deformation;

[0032] Figure 6 The engineering stress-strain curve diagram under 700°C superplastic deformation in the method for preparing superplastic heterogeneous dual-phase fine-grained medium manganese steel of the present invention;

[0033] Figure 7 This is a microstructure morphology diagram under superplastic deformation at 700°C in the method for preparing superplastic heterogeneous dual-phase fine-grained medium manganese steel of the present invention;

[0034] Figure 8 A simulated phase diagram of thermodynamic calculation in the method for preparing superplastic heterogeneous dual-phase fine-grained medium manganese steel according to the present invention;

[0035] Fig. 9 The present invention is a process flow chart of the method for preparing superplastic heterogeneous dual-phase fine-grained medium manganese steel. DETAILED DESCRIPTION

[0036] In order to fully describe the technical content, objectives and effects of the present invention, the following will be described in detail with reference to the accompanying drawings.

[0037] The main principle of the method for preparing superplastic heterogeneous dual-phase fine-grained medium manganese steel is: cooling after forging, through high-temperature forging process and subsequent moderate cooling rate, the cast columnar grains in the medium manganese steel plate blank are broken, and the internal grains are refined, so that the medium manganese steel plate blank contains austenite and ferrite dual-phase structure, and the residual stress is released, which is convenient for warm rolling treatment. At the same time, warm rolling of the medium manganese steel plate blank at 300℃-400℃ can ensure that the medium manganese steel plate blank has a low rolling deformation resistance and a good flatness on the surface of the medium manganese steel plate blank. At the same time, the low deformation makes the stability of austenite after forging have a certain difference, so that 80-85% of the austenite undergoes martensitic phase transformation during the warm rolling process, while preserving 15-20% of the stable coarse-grained austenite structure after forging. The subsequent cold rolling deformation process regulates the content ratio of coarse-grained austenite and transformed martensite, and at the same time, the accumulated medium manganese steel sheet maintains a certain dislocation density, which provides a role for the subsequent superplastic deformation and keeps the coarse-grained austenite ratio at about 15-20%. Figure 1 As shown. The material structure of medium manganese steel is made of coarse-grained austenite, fine-grained austenite and ferrite, so that a certain proportion of dislocation defects can be maintained in the middle of the medium manganese steel cold-rolled plate billet. In the subsequent superplastic deformation, due to the heating time and the holding time before superplastic deformation, the dislocation density will gradually disappear, which can keep the grain size of coarse-grained austenite, fine-grained austenite and ferrite structure basically unchanged to a certain extent, thereby ensuring the conditions for superplastic deformation, that is, the small grain size requirement, and the selection of superplastic deformation temperature and strain rate can trigger the grain boundary sliding deformation mechanism of austenite and ferrite heterogeneous dual-phase fine-grained medium manganese steel, and the effect of coarse-grained austenite can trigger the triggering sequence of the grain boundary sliding mechanism, so that the medium manganese steel material can maintain the grain boundary sliding deformation mechanism in a longer strain range, thereby achieving higher ductility, that is, the superplastic deformation performance is better, as shown in Fig. 9 As shown, the following steps are included:

[0038] S1. According to the weight percentage, a certain weight of C, Mn, Si, Al and Fe elements are respectively taken for smelting, and the smelted molten steel is cast into ingots.

[0039] Specifically, the C element can improve the stability of austenite in medium manganese steel. Generally, the mass fraction of C increases by 1%, and the transformation temperature Ms of martensite decreases by 423°C. However, if the C content is too high, the welding performance of medium manganese steel will deteriorate. Therefore, the C content in the manganese steel of the present invention is 0.1-0.3%. The Mn element can widen the austenite phase region, strengthen the matrix, and improve the stability of austenite. However, if the Mn content is too high, segregation will occur, making it difficult to homogenize the composition of the diffusion annealing. Therefore, the content of the Mn element in the medium manganese steel is 4%-12%. The Si element can promote the formation of austenite and increase the content of residual austenite at room temperature. The Al element has a similar effect to the Si element. At the same time, the Al element can significantly reduce the density of the medium manganese steel, so that the quality of the automobile steel is reduced. The formation of austenite mainly depends on the stacking fault energy. The Al element can increase the stacking fault energy of the austenite. However, too much Al content will cause the nozzle to be blocked during the continuous casting process, affecting production. Therefore, the content of the Al element in the manganese steel of the present invention is less than 4%, the content of the Si element is less than 3%, and the rest is the Fe element and unavoidable impurities.

[0040] Furthermore, the chemical composition of the medium manganese steel also includes: the content of Nb element is 0-0.3% and the content of Cu element is 0.5-2.0%.

[0041] S2. The ingot obtained in step S1 is heated to 1150° C., a temperature at which the columnar grains inside the ingot are broken and refined, and the ingot is forged at this temperature to obtain a medium manganese steel slab containing austenite and ferrite dual-phase structure and a thickness of 4-5 mm, and the medium manganese steel slab is air-cooled to room temperature at a certain cooling rate to release residual stress.

[0042] Specifically, in the forging process of step S2, the forging temperature is 1000°C-1200°C, the holding time is 1.5-2.5h, and the cooling rate is 20°C-30°C / s.

[0043] S3. Wire cutting is performed on the medium manganese steel slab obtained in step S2 to obtain a medium manganese steel slab with a thickness of 5 mm.

[0044] S4. The medium manganese steel slab obtained in step S3 is heated to 300° C.-400° C. and kept warm for 1 hour to obtain a medium manganese steel slab with low rolling deformation resistance and good surface flatness.

[0045] S5. The medium manganese steel slab obtained in step S4 is subjected to single-pass small deformation warm rolling, so that 80-85% of the austenite in the medium manganese steel slab undergoes martensitic phase transformation, while retaining 15-20% of the stable forged coarse-grained austenite structure, to obtain a medium manganese steel warm-rolled slab with a thickness of 3-4 mm, and air-cooled to room temperature at a cooling rate of 20°C to 30°C / s.

[0046] Specifically, in the single-pass small deformation warm rolling process, the warm rolling temperature is 300-400° C., and the warm rolling deformation is 20-30%.

[0047] S6. The medium manganese steel warm-rolled slab obtained in step S5 is subjected to single-pass small deformation cold rolling to adjust the content ratio of coarse-grained austenite and transformed martensite, so that the ratio of coarse-grained austenite to fine-grained austenite is in the range of 1:9-2:8, and at the same time, the medium manganese steel warm-rolled slab maintains a certain dislocation density, so as to obtain a medium manganese steel cold-rolled slab with a thickness of 2-3 mm.

[0048] Specifically, in a single-pass small deformation cold rolling process, the cold rolling deformation rate is 0.5m-2m / s, and the cold rolling deformation amount is 20-30%.

[0049] S7. Cut the medium manganese steel cold-rolled slab obtained in step S6 to obtain a superplastic tensile deformation specimen. In order to make the dislocation density in step S6 disappear and maintain the grain size of coarse-grained austenite, fine-grained austenite and ferrite structure, the superplastic tensile deformation specimen is placed in a closed insulation furnace, and the superplastic tensile deformation specimen is tested at different superplastic deformation temperatures and different strain rates.

[0050] Specifically, in the tensile deformation test, the heating rate of the holding furnace is 25°C / min, the deformation temperature of the holding furnace is 650°C-750°C, and the strain rate is 10 -2 -10 -3 / s, the holding time of the holding furnace is 3-5min, and the cooling rate is 20℃~30℃ / s.

[0051] In a preferred embodiment of the present invention, the material structure of the medium manganese steel is composed of austenite and ferrite, the austenite includes coarse-grained austenite and fine-grained austenite, ferrite accounts for 40-50% of the material structure of the medium manganese steel, the grain size range of the coarse-grained austenite is 10-20μm, and the grain size range of the fine-grained austenite is 1-5μm.

[0052] The following is a further description of a method for preparing superplastic heterogeneous dual-phase fine-grained medium manganese steel and the prepared medium manganese steel in combination with an embodiment of the present invention:

[0053] The preparation process of this specific embodiment is achieved as follows:

[0054] S1. Take 0.30% by weight of C, 11.23% by weight of Mn, 2.05% by weight of Si, 3.21% by weight of Al, Fe elements and inevitable impurities for smelting, and cast the smelted molten steel into a 20kg ingot.

[0055] S2. The ingot obtained in step S1 is first cooled at a certain cooling rate and then heated to 1150°C at a temperature at which the columnar grains inside the ingot are broken and refined, and the ingot is forged at a temperature of 1000°C-1200°C. At the same time, the grains inside the ingot are refined to obtain a medium manganese steel slab containing austenite and ferrite dual-phase structure and a thickness of 4-5 mm. The medium manganese steel slab is air-cooled to room temperature at a certain cooling rate to release residual stress for easy warm rolling treatment.

[0056] S3. Wire cutting is performed on the medium manganese steel slab obtained in step S2 to obtain a medium manganese steel slab with a thickness of 5 mm.

[0057] S4. The medium manganese steel slab obtained in step S3 is heated to 300°C-400°C and kept warm for 1 hour to obtain a medium manganese steel slab with low rolling deformation resistance and good surface flatness. At the same time, the lower deformation amount makes the austenite stability have certain differences after forging, so that part of the austenite undergoes martensitic phase transformation during the warm rolling process, while preserving part of the stable coarse-grained austenite structure after forging.

[0058] S5, heating the medium manganese steel slab obtained in step S4 to 350°C for 1h, and then performing single-pass small deformation warm rolling, with a warm rolling deformation of 25%, so that 80-85% of the austenite in the medium manganese steel slab undergoes martensite transformation, while retaining 15-20% of the stable forged coarse-grained austenite structure, grinding the surface layer to obtain a medium manganese steel warm-rolled slab with a thickness of 3.5mm, and air-cooling to room temperature at a cooling rate of 20°C-30°C / s. Figure 1 As shown, the medium manganese steel has a coexistence structure of coarse-grained austenite, fine-grained austenite and ferrite, which can achieve the preservation of a certain proportion of dislocation defects in the middle of the medium manganese steel cold-rolled plate.

[0059] S6. The medium manganese steel warm-rolled slab obtained in step S5 is subjected to single-pass small deformation cold rolling with a cold rolling deformation amount of 20% to adjust the content ratio of coarse-grained austenite and transformed martensite so that the ratio of coarse-grained austenite to fine-grained austenite is in the range of 1:9-2:8, and at the same time, the medium manganese steel warm-rolled slab maintains a certain dislocation density to provide an effect for subsequent superplastic deformation, thereby obtaining a medium manganese steel cold-rolled slab with a thickness of 2.8 mm.

[0060] S7. Cut the medium manganese steel cold-rolled slab obtained in step S6, polish it with 1000-grit sandpaper to obtain a superplastic tensile deformation specimen, put the superplastic tensile deformation specimen into a closed insulation furnace, and test the superplastic tensile deformation specimen using a Zwick universal tensile testing machine equipped with a high-temperature deformation temperature box at different superplastic deformation temperatures and different strain rates.

[0061] In the superplastic tensile deformation test, three groups of superplastic tensile deformation specimens were taken in the same test, and the test results were taken as the average value. The deformation temperatures were 650℃, 700℃ and 750℃ respectively, and the strain rate was 10 -2 s -1 and 10 -3 s -1 . Since heating takes time and the holding time before superplastic deformation is 3-5 minutes, the dislocation density will gradually disappear, which can keep the grain size of coarse-grained austenite, fine-grained austenite and ferrite structure basically unchanged to a certain extent, thereby ensuring the conditions for superplastic deformation - the small grain size requirement, and the selection of superplastic deformation temperature and strain rate can trigger the grain boundary sliding deformation mechanism of austenite and ferrite heterogeneous dual-phase fine-grained medium manganese steel, and the effect of coarse-grained austenite can trigger the triggering sequence of the grain boundary sliding mechanism, so that the medium manganese steel material can maintain the grain boundary sliding deformation mechanism in a longer strain range, thereby achieving higher ductility, that is, the superplastic deformation performance is better.

[0062] In this specific embodiment, the heat treatment furnace used is a high temperature furnace. The superplastic tensile test results obtained by the above preparation method are as follows: Figure 3A , Figure 3B As shown in Table 1:

[0063] Table 1. Test results of superplastic properties of heterogeneous dual-phase fine-grained medium manganese steel

[0064] Deformation temperature(℃) Strain rate (s-1) Peak stress (MPa) Elongation after break (%) 650℃ 0.01 262.83 199.58％ 650℃ 0.001 172.99 251％ 700℃ 0.01 137.94 479.25％ 700℃ 0.001 107.29 1001％ 750℃ 0.01 95.71 416.38％ 750℃ 0.001 73.93 829.61％

[0065] It can be seen from Table 1 that after small deformation warm rolling and small deformation cold rolling, the superplastic deformation process is carried out at 700℃ and 10 -3 s -1 The medium manganese steel prepared at a strain rate of has the best superplastic mechanical properties, with a ductility of more than 1000%.

[0066] The microstructure of the samples before and after superplastic deformation was determined. The equipment for microscopic characterization technology was a ZEISS Sigma 500 scanning electron microscope equipped with an electron backscatter diffraction analysis system (EBSD). Figure 2A , Figure 2B and Figure 5 As shown in the figure. Microstructure analysis shows that the forged structure shows obvious coarse grain distribution, and there is no obvious grain refinement and evolution of austenite grains towards two grain size distributions. With the action of small deformation warm rolling and cold rolling, the microstructure presents a two-phase double-peak structure grain morphology with a certain dislocation structure. With the start of the superplastic deformation test, the grain size in the test steel has a small grain growth dislocation structure that gradually disappears.

[0067] Figure 8Thermo-calc thermodynamic calculation software is used to simulate the phase diagram. It can be seen from the phase diagram that the Ac3 temperature of the experimental steel, that is, the temperature at which all ferrite is transformed into austenite, is about 800°C, indicating that above 800°C, all ferrite will be transformed into austenite structure, that is, at 800°C, the ferrite structure can undergo sufficient phase change reaction to produce austenite structure.

[0068] Comparative Example 1:

[0069] S1. Take 0.09% by weight of C, 10.34% by weight of Mn, 0.33% by weight of Si, 0.35% by weight of Mo, 0.09% by weight of V, 0.14% by weight of Cr element, balance Fe and inevitable impurities for smelting, and cast the smelted molten steel into 20kg ingot.

[0070] S2. The ingot obtained in step S1 is first cooled at a certain cooling rate and then heated to 1150°C at a temperature at which the columnar grains inside the ingot are broken and refined, and the ingot is forged at a temperature of 1000°C-1200°C. At the same time, the grains inside the ingot are refined to obtain a micro-alloyed medium manganese steel slab containing austenite and ferrite dual-phase structure and a thickness of 4-5 mm. The micro-alloyed medium manganese steel slab is air-cooled to room temperature at a certain cooling rate to release residual stress for easy warm rolling treatment.

[0071] S3. Wire cutting is performed on the microalloyed medium manganese steel slab obtained in step S2 by wire cutting to obtain a microalloyed medium manganese steel slab with a thickness of 5 mm.

[0072] S4. The microalloyed medium manganese steel slab obtained in step S3 is heated to 350°C-400°C and kept warm for 1 hour to obtain a microalloyed medium manganese steel slab with low rolling deformation resistance and good surface flatness. Meanwhile, the lower deformation amount makes the austenite stability have certain differences after forging, so that part of the austenite undergoes martensitic phase transformation during the warm rolling process, while preserving part of the stable coarse-grained austenite structure after forging.

[0073] S5. The microalloyed medium manganese steel slab obtained in step S4 is first heated to 350° C. and kept warm for 1 hour, and then subjected to single-pass small deformation warm rolling with a warm rolling deformation of 25%, so that 80-85% of the austenite in the microalloyed medium manganese steel slab undergoes martensitic phase transformation, while retaining 15-20% of the stable post-forging coarse-grained austenite structure, and after grinding the surface layer, a microalloyed medium manganese steel warm-rolled slab with a thickness of 3.5 mm is obtained, and then air-cooled to room temperature at a cooling rate of 20° C. to 30° C. / s.

[0074] S6. The microalloyed medium manganese steel warm-rolled slab obtained in step S5 is subjected to single-pass small deformation cold rolling with a cold rolling deformation amount of 20% to adjust the content ratio of coarse-grained austenite and transformed martensite so that the ratio of coarse-grained austenite to fine-grained austenite is within the range of 1:9-2:8, and at the same time, the microalloyed medium manganese steel warm-rolled slab maintains a certain dislocation density to provide an effect for subsequent superplastic deformation, thereby obtaining a microalloyed medium manganese steel cold-rolled slab with a thickness of 2.8 mm.

[0075] S7. Cut the microalloyed medium manganese steel cold-rolled slab obtained in step S6, polish it with 1000-grit sandpaper to obtain a superplastic tensile deformation specimen, put the superplastic tensile deformation specimen into a closed insulation furnace, and test the superplastic tensile deformation specimen using a Zwick universal tensile testing machine equipped with a high-temperature deformation temperature box at different superplastic deformation temperatures and different strain rates.

[0076] Three groups of samples were taken for the same test, and the test results were taken as the average value. The deformation temperatures were 700℃ and 800℃ respectively, and the strain rate was 10 -2 s -1 , 10 -3 s -1 The superplastic deformation test results are as follows: Figure 4 As shown, compared with the specific embodiment, the addition of microalloying elements can significantly refine the grain size, but the hot deformation mechanical properties of the superplastic prepared plate are not excellent, which shows that the addition of microalloying elements may inhibit the superplastic mechanical properties, which shows that the heterogeneous structure in the present invention plays an important role in the superplastic preparation process.

[0077] Comparative Example 2:

[0078] S1. 0.32% by weight of C, 11.23% by weight of Mn, 3.21% by weight of Al, 2.05% by weight of Si, the remainder of Fe and inevitable impurities are smelted respectively, and the smelted molten steel is cast into a 20 kg ingot.

[0079] S2. The ingot obtained in step S1 is first cooled at a certain cooling rate and then heated to 1200°C at a temperature at which the columnar grains inside the ingot are broken and refined, and the ingot is forged at a temperature of 1000°C-1200°C. At the same time, the grains inside the ingot are refined to obtain a medium manganese steel slab containing austenite and ferrite dual-phase structure and a thickness of 4-5 mm. The medium manganese steel slab is air-cooled to room temperature at a certain cooling rate to release residual stress for easy warm rolling treatment.

[0080] S3. The medium manganese steel slab obtained in step S2 is subjected to wire cutting by wire cutting to obtain a medium manganese steel slab with a thickness of 12 mm.

[0081] S4. The medium manganese steel slab obtained in step S3 is heated to 350°C-400°C and kept warm for 1 hour to obtain a medium manganese steel slab with low rolling deformation resistance and good surface flatness. At the same time, the lower deformation amount makes the austenite stability have certain differences after forging, so that part of the austenite undergoes martensitic phase transformation during the warm rolling process, while preserving part of the stable coarse-grained austenite structure after forging.

[0082] S5. The medium manganese steel slab obtained in step S4 is first heated to 350° C. and kept warm for 1 hour, and then subjected to single-pass small deformation warm rolling with a warm rolling deformation of 50%, so that 80-85% of the austenite in the medium manganese steel slab undergoes martensitic phase transformation, while retaining 15-20% of the stable post-forging coarse-grained austenite structure, and after grinding the surface layer, a medium manganese steel warm-rolled slab with a thickness of 6 mm is obtained, and air-cooled to room temperature at a cooling rate of 20° C. to 30° C. / s.

[0083] S6. The medium manganese steel warm-rolled slab obtained in step S5 is subjected to single-pass small deformation cold rolling with a cold rolling deformation amount of 50% to adjust the content ratio of coarse-grained austenite and transformed martensite so that the ratio of coarse-grained austenite to fine-grained austenite is in the range of 1:9-2:8, and at the same time, the medium manganese steel warm-rolled slab maintains a certain dislocation density to provide an effect for subsequent superplastic deformation, thereby obtaining a medium manganese steel cold-rolled slab with a thickness of 3 mm.

[0084] S7. Cut the medium manganese steel cold-rolled slab obtained in step S6, polish it with 1000-grit sandpaper to obtain a superplastic tensile deformation specimen, put the superplastic tensile deformation specimen into a closed insulation furnace, and test the superplastic tensile deformation specimen using a Zwick universal tensile testing machine equipped with a high-temperature deformation temperature box at different superplastic deformation temperatures and different strain rates.

[0085] The same test was conducted with three groups of specimens, and the test results were averaged. The deformation temperature was 700°C and the strain rate was 10 -3 s -1 The polished medium manganese steel cold-rolled slab specimens were corroded with copper chloride corrosion solution for 1 minute. After the corrosion was completed, they were cleaned with alcohol and observed by SEM. Figure 6 As shown in Figure 2, the microstructure before superplastic deformation is as follows: Figure 7 As shown, in comparison, the superplastic properties of non-heterogeneous fine-grained manganese steel are lacking in ductility compared to heterogeneous structure organization, which shows that the heterogeneous structure in the present invention plays an important role in the superplastic preparation process.

[0086] The embodiments described above are only descriptions of the preferred implementation modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for preparing superplastic heterogeneous dual-phase fine-grained medium manganese steel, It is characterized in that It includes the following steps: S1. According to the weight percentage, a certain weight of C, Mn, Si, Al and Fe elements are respectively taken for smelting, and the smelted molten steel is cast into an ingot, wherein the content of C element is 0.1-0.3%, the content of Mn element is 4%-12%, the content of Si element is greater than or equal to 2.05% and less than 3%, the content of Al element is greater than or equal to 3.21% and less than 4%, and the rest is Fe element and impurities; S2, heating the ingot obtained in step S1 to a temperature at which the columnar grains inside the ingot are broken and refined, and forging the ingot at this temperature to obtain a medium manganese steel slab containing austenite and ferrite dual-phase structure and a thickness of 4-5 mm, and air-cooling the medium manganese steel slab to room temperature at a certain cooling rate; S3, using wire cutting to perform wire cutting on the medium manganese steel slab obtained in step S2 to obtain a medium manganese steel slab with a thickness of 4-5 mm; S4, heating the medium manganese steel slab obtained in step S3 to 300° C.-400° C. and keeping the temperature for 1 hour to obtain a heat-treated medium manganese steel slab; S5, subjecting the medium manganese steel slab obtained in step S4 to single-pass small deformation warm rolling, so that 80-85% of the austenite in the medium manganese steel slab undergoes martensitic transformation, while retaining 15-20% of the stable forged coarse-grained austenite structure, to obtain a medium manganese steel warm-rolled slab with a thickness of 3-4 mm, and air-cooling to room temperature at a cooling rate of 20°C-30°C / s; the warm rolling temperature is 300-400°C, and the warm rolling deformation is 20-30%; S6, subjecting the medium manganese steel warm-rolled slab obtained in step S5 to single-pass small deformation cold rolling to adjust the content ratio of coarse-grained austenite and transformed martensite, so that the ratio of coarse-grained austenite to fine-grained austenite is in the range of 1:9-2:8, and at the same time, the medium manganese steel warm-rolled slab maintains a certain dislocation density, so as to obtain a medium manganese steel cold-rolled slab with a thickness of 2-3 mm; the cold rolling deformation rate is 0.5 m-2 m / s, and the cold rolling deformation amount is 20-30%; S7, cutting the medium manganese steel cold-rolled slab obtained in step S6 to obtain a superplastic tensile deformation specimen, in order to make the dislocation density in step S6 disappear and maintain the grain size of coarse-grained austenite, fine-grained austenite and ferrite structure, the superplastic tensile deformation specimen is placed in a closed insulation furnace, and the superplastic tensile deformation specimen is tested at different superplastic deformation temperatures and different strain rates; in the tensile deformation test, the heating rate is 25°C / min, the deformation temperature is 650°C-750°C, the strain rate is 10 -3 -10 -2 / s, the holding time is 3-5min, and the cooling rate is 20℃~30℃ / s.

2. The method for preparing superplastic heterogeneous dual-phase fine-grained medium manganese steel according to claim 1, It is characterized in that In step S2, the temperature at which the columnar grains inside the ingot are broken and refined is 1150°C, the forging temperature is 1000°C-1200°C, the holding time is 1.5-2.5h, and the cooling rate is 20°C-30°C / s.

3. A medium manganese steel prepared by the method for preparing superplastic heterogeneous dual-phase fine-grained medium manganese steel according to any one of claims 1 to 2, It is characterized in that The material structure of the medium manganese steel includes austenite and ferrite, the austenite includes coarse-grained austenite and fine-grained austenite, the ferrite accounts for 40-50% of the material structure of the medium manganese steel, the grain size range of the coarse-grained austenite is 10-20μm, and the grain size range of the fine-grained austenite is 1-5μm.

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