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

Abstract

The invention relates to high-strength and high-toughness medium-manganese hot forming steel and a preparation method thereof. The medium-manganese hot forming steel comprises the following chemical components in percentage by mass: 0.12-0.17% of C, 6.0-9.0% of Mn, 0.2-0.3% of Si and the balance of Fe and inevitable impurities. The preparation method comprises the steps that a medium manganese steel cast ingot is sequentially subjected to forging, hot rolling, softening annealing, cold rolling, one-step annealing, two-step annealing, rapid heating, quenching and low-temperature tempering, and the high-strength and high-toughness medium manganese hot forming steel is obtained. Compared with the prior art, the method has the advantages that the mechanical property of the hot-formed medium manganese steel can be remarkably improved, and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of material heat treatment and processing, and in particular to a medium manganese hot forming steel with high strength and high toughness and a preparation method thereof. Background Art

[0002] Medium manganese steel is an advanced high strength steel containing a relatively high manganese element, and its manganese content is generally between 3% and 12% (there is no fixed standard). It can be formed by either cold forming or hot forming processes. When used for hot forming, a martensite matrix + a small amount of retained austenite structure with ultra-high strength can be obtained, having broad application prospects.

[0003] Currently, when applying medium manganese steel to hot forming, it generally refers to the treatment method of hot forming boron steel. Usually, a heating furnace is used to heat the steel to a temperature above the austenitizing temperature and hold for 3 - 5 minutes to fully austenitize it, and then subsequent forming and quenching steps are carried out. Although the final product has good strength, the plasticity (elongation) is usually not high.

[0004] Some units have also proposed a rapid heating method to replace the traditional furnace heating. For example, the method with the publication number CN112251679A uses salt bath heating to achieve rapid temperature rise. This method can effectively retain the element inhomogeneous distribution phenomenon, and prepare a nanostructured lamellar structure composed of alternating metastable austenite and high-strength martensite lamellae, improving the plasticity while ensuring high strength. However, this method constructs chemical inhomogeneous distribution through pearlite transformation, and is only applicable to medium manganese steel with a relatively high C content. At the same time, using a salt bath furnace for rapid temperature rise will cause environmental pollution. In addition, the above method only focuses on the heat treatment process of the material and does not combine with forming. Summary of the Invention

[0005] The purpose of the present invention is to provide a medium manganese hot forming steel with high strength and high toughness and a preparation method thereof, which can be combined with hot stamping to manufacture formed parts.

[0006] The purpose of the present invention can be achieved by the following technical solutions: A medium manganese hot forming steel with high strength and high toughness, the chemical composition of the medium manganese hot forming steel is by mass fraction: C 0.12 - 0.17%, Mn 6.0 - 9.0%, Si 0.2 - 0.3%, and the rest is Fe and inevitable impurities.

[0007] The present invention is applicable to medium manganese steel with a relatively low C content.

[0008] Preferably, the medium manganese hot forming steel has a martensite matrix + retained austenite duplex structure.

[0009] More preferably, in the medium manganese hot forming steel, the content of retained austenite is 28 - 32%.

[0010] Further preferably, in the medium manganese hot forming steel, retained austenite is mainly in the form of laths.

[0011] Further preferably, in the medium manganese hot forming steel, the average grain size of the bcc phase is 1.0 - 1.2 μm in terms of the equivalent circle diameter.

[0012] A preparation method of the above-mentioned high-strength and high-toughness medium manganese hot forming steel, wherein the medium manganese steel ingot is successively subjected to forging, hot rolling, soft annealing, cold rolling, one-step annealing, two-step annealing, rapid heating, quenching and low-temperature tempering to obtain the high-strength and high-toughness medium manganese hot forming steel.

[0013] Preferably, the preparation method of the medium manganese steel ingot includes the following steps: taking a certain weight of C, Mn, Si, and Fe elements according to the proportion by weight percentage for smelting, and pouring the molten steel after smelting into a medium manganese steel ingot. By mass fraction, the medium manganese steel ingot after smelting includes the following components: C 0.12 - 0.17%, Mn 6.0 - 9.0%, Si 0.2 - 0.3%, and the rest is Fe and unavoidable impurities.

[0014] Preferably, the forging includes heating the medium manganese steel ingot to 1100 - 1250 °C for heat preservation for 1 - 2 h, performing hot forging treatment and cooling to room temperature.

[0015] Preferably, the hot rolling includes reheating the medium manganese steel after forging (hot forging) treatment to 1100 - 1250 °C for heat preservation for 1 - 2 h, performing multi-pass hot rolling at 900 - 1200 °C, and then quenching to room temperature.

[0016] Further preferably, the total hot rolling deformation amount is 30 - 90%.

[0017] Preferably, the soft annealing includes heating the medium manganese steel after hot rolling to 600 - 700 °C for heat preservation for 0.5 - 5 h.

[0018] Preferably, the cold rolling includes cold rolling the medium manganese steel after soft annealing at room temperature.

[0019] Further preferably, the total cold rolling deformation amount is 30 - 70%.

[0020] Preferably, the one-step annealing includes heating the medium manganese steel after cold rolling to 800 - 950 °C for heat preservation for 20 - 30 min, and then cooling to room temperature.

[0021] Preferably, the two-step annealing includes reheating the medium manganese steel after one-step annealing to 600 - 700 °C for heat preservation for 10 - 20 h, and then cooling to room temperature.

[0022] Preferably, the rapid heating includes rapidly heating the medium manganese steel after two-step annealing from room temperature to 780 °C - 840 °C at a heating rate greater than 80 °C / s.

[0023] Preferably, the rapid heating uses resistance heating.

[0024] Preferably, the quenching is forming quenching or direct quenching, specifically:

[0025] If only high-strength and high-toughness medium manganese steel plates need to be prepared, direct quenching is adopted in this step. The plate after rapid heating is quickly transferred to a flat die for quenching, or directly put into a coolant for quenching;

[0026] If high-strength and high-toughness medium manganese steel parts need to be made, the plate after rapid heating needs to be transferred to a die for forming in this step, and then pressure is maintained until quenched to room temperature to complete the production of the parts.

[0027] The method of the present invention can not only prepare high-strength and high-toughness steel plates, but also directly manufacture high-strength and high-toughness parts in combination with the forming process.

[0028] Further preferably, the coolant includes water.

[0029] Preferably, the low-temperature tempering includes heating the quenched sample to 120 - 220 °C for heat preservation for 5 - 60 min, and then air-cooling to room temperature.

[0030] An application of the above high-strength and high-toughness medium manganese hot-formed steel, using the high-strength and high-toughness medium manganese hot-formed steel for the production of automotive safety parts or armor steel.

[0031] Preferably, the automotive safety parts include the B-pillar of the vehicle body and the anti-collision beam.

[0032] The synthesis mechanism of the present invention is as follows: During the annealing process of medium manganese steel, the Mn element segregates from ferrite to austenite, enabling the austenite to avoid martensitic transformation during the annealing and cooling process and stabilizing in the austenite form to room temperature. Subsequently, a duplex structure of Mn-rich austenite and Mn-poor ferrite is formed after annealing. During the rapid heating process, the medium manganese steel is heated above the Ac3 temperature, but due to the fast heating rate and no holding time, Mn hardly diffuses. After rapidly cooling from the austenite single-phase region to room temperature, the Mn-rich region still exists as austenite, and the Mn-poor region undergoes martensitic transformation. Therefore, a duplex structure of martensite + retained austenite is obtained after quenching. During the low-temperature tempering process (if it is an automotive part, this step can be directly incorporated into the baking process), C partitions from martensite to austenite and diffuses to the martensite / austenite phase interface. The former stabilizes the retained austenite, which is beneficial for improving the elongation; the latter pins at the geometrically necessary dislocations, enhancing the yield and tensile strength.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] 1. The present invention obtains a duplex structure of lamellar retained austenite and martensite by constructing a chemically inhomogeneous distribution and rapid heating method, and pins dislocations and stabilizes retained austenite by low-temperature tempering, which can significantly improve the mechanical properties of medium manganese steel in hot forming.

[0035] 2. The present invention provides a preparation method for medium manganese hot forming steel, which implements two-step annealing on cold-rolled steel to create chemical inhomogeneity, and uses rapid heating to retain the chemical inhomogeneity. After quenching, a duplex structure of martensite matrix + retained austenite is obtained. Combining low-temperature tempering strengthening further improves strength and stabilizes retained austenite, and finally obtains high-strength and high-toughness mechanical properties. The tensile strength can reach 1500 - 2000 MPa; the elongation can reach 18 - 22%. The comprehensive performance is higher than that of most medium manganese hot forming steels.

[0036] 3. The present invention uses resistance heating instead of traditional furnace heating, shortening the heating time from 3 - 5 minutes to within 20 seconds, greatly shortening the heating time and improving production efficiency; and compared with furnace heating, the energy utilization rate of using resistance heating is higher, which can reduce production energy consumption.

[0037] 4. Based on rapid heating technology, combined with material composition design and rolling process design, the present invention proposes a new preparation method for high-strength and high-toughness medium manganese hot forming steel; this process can also be combined with the hot stamping process to obtain high-strength and high-toughness medium manganese steel parts.

[0038] 5. The present invention is not only a material preparation method, but it can be naturally combined with the hot stamping forming process of ultra-high strength steel. The steps before two-step annealing are the preparation process of hot forming sheet metal; the rapid heating process required by the present invention is also applicable to the sheet metal heating in the hot stamping process; the rapid quenching treatment required by the present invention can be realized through the in-mold forming quenching process of the hot stamping process; the low-temperature tempering treatment required by the present invention can be realized through the baking treatment after the hot stamping part is formed. Therefore, the present invention can not only prepare high-strength and high-toughness steel sheet materials, but also be combined with the forming process to directly manufacture high-strength and high-toughness parts.

[0039] 6. The present invention takes into account the influence of microscopic morphology on mechanical properties. By replacing one-step annealing with two-step annealing, while completing element enrichment, the morphology of austenite is changed from equiaxed to lath-shaped. On the one hand, this improves the stability of retained austenite, and on the other hand, it weakens the plastic instability that may be caused by equiaxed tissue, such as Lüders bands, etc.

[0040] 7. The present invention can obtain medium manganese hot forming steel with high strength-plasticity product and no plastic instability. Description of the Drawings

[0041] Figure 1 It is the phase distribution diagram of Example 1 of the present invention, where the red part represents the bcc phase and the blue part represents the fcc phase;

[0042] Figure 2 It is the phase distribution diagram of Comparative Example 1, where the red part represents the bcc phase and the blue part represents the fcc phase;

[0043] Figure 3 It is the phase distribution diagram of Comparative Example 2, where the red part represents the bcc phase and the blue part represents the fcc phase;

[0044] Figure 4 It is the phase distribution diagram of Comparative Example 3, where the red part represents the bcc phase and the blue part represents the fcc phase;

[0045] Figure 5 It is the phase distribution diagram of Comparative Example 4, where the red part represents the bcc phase and the blue part represents the fcc phase;

[0046] Figure 6 It is the room temperature tensile stress-strain curve diagram of Example 1 and Comparative Examples 1-3. Detailed implementation manners

[0047] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. This embodiment is implemented on the premise of the technical solution of the present invention, and the detailed implementation manners and specific operation processes are given, but the protection scope of the present invention is not limited to the following embodiments.

[0048] A preparation method of medium manganese hot forming steel specifically includes the following steps:

[0049] Smelting: By weight percentage, a certain weight of carbon C, manganese Mn, silicon Si, and iron Fe elements are smelted in proportion, and the molten steel after smelting is cast into an ingot. By mass fraction, the material after smelting includes the following components: C 0.12-0.17%, Mn 6.0-9.0%, Si 0.2-0.3%, and the rest are Fe and inevitable impurities.

[0050] Forging: Heat the medium manganese steel ingot to 1100-1250°C and keep it warm for 1-2h, perform hot forging treatment and cool it to room temperature.

[0051] Hot rolling: Reheat the medium manganese steel after hot forging to 1100-1250°C and keep it warm for 1-2h, perform multi-pass hot rolling at 900-1200°C, the hot rolling reduction is 30-90%, and then quench it to room temperature.

[0052] Softening annealing: Heat the quenched medium manganese steel to 600-700°C and keep it warm for 0.5-5h to complete the softening annealing.

[0053] Cold rolling: Cold roll the medium manganese steel after hot rolling annealing, and the cold rolling reduction is 30-70%.

[0054] One-step annealing: Heat the medium manganese steel after cold rolling to 800 - 950 °C and hold for 20 - 30 min, then cool to room temperature.

[0055] Two-step annealing: After the medium manganese steel is cooled, heat it up again to 600 - 700 °C and hold for 10 - 20 h, then cool to room temperature.

[0056] Rapid heating: Rapidly heat the medium manganese steel after two-step annealing from room temperature to 780 - 840 °C at a heating rate greater than 80 °C / s. This stage ends after reaching the specified temperature without special heat preservation. The heating requirement can be met by methods such as resistance heating or induction heating.

[0057] Forming quenching or direct quenching: If only high-strength and high-toughness medium manganese steel plates need to be prepared, direct quenching is used in this step. The sheet material after the previous heating is quickly transferred to a flat die for quenching, or directly put into a coolant (such as water) for quenching. If high-strength and high-toughness parts need to be made, the heated specimen in this step needs to be quickly transferred to the die for forming, then pressure quenched to room temperature to complete the part production.

[0058] Low-temperature tempering: Heat the sample after die quenching to 120 - 220 °C and hold for 5 - 60 min, then air-cool to room temperature.

[0059] The following is a detailed description in combination with specific embodiments.

[0060] Example 1

[0061] This example provides a preparation method for medium manganese hot forming steel, and the specific steps are as follows:

[0062] (1) Prepare raw materials according to the ratio of hot forming medium manganese steel. Put the prepared raw materials into an electric arc furnace for high-temperature melting treatment to obtain molten steel. After the molten steel is homogenized, it is cast and hot forged to obtain a slab. By mass fraction, the composition of the medium manganese steel is: C 0.15%; Mn 7.0%; Si 0.23%; the rest are Fe and inevitable impurities.

[0063] (2) Heat the slab in step (1) to 1220 °C, hold for 2 h, and then perform 6 hot rolling operations at 1200 - 900 °C. After hot rolling, air-cool to obtain a hot-rolled steel plate, and the thickness of the hot-rolled plate is 3 mm.

[0064] (3) Heat the hot-rolled plate obtained in step (2) to 620 °C and hold for 1 h, then air-cool to room temperature to complete the softening annealing.

[0065] (4) Perform 5 cold rolling operations on the material after softening annealing in step (3), with a total deformation of 53.3% to obtain a cold-rolled plate with a thickness of 1.4 mm.

[0066] (5) Heat the cold-rolled sheet in step (4) to 850 °C, hold for 20 min, and then air-cool to room temperature. Heat it again to 620 °C and hold for 12 h to complete the reverse austenite transformation annealing, and then air-cool to room temperature to obtain the cold-rolled two-step annealed steel sheet.

[0067] It should be noted that through Gleeble measurement, the Ac3 temperature of the cold-rolled sheet is 758 °C, and holding at 850 °C for 20 min can ensure complete austenite phase transformation.

[0068] (6) Use the method of resistance heating to heat the cold-rolled two-step annealed steel obtained in step (5) to 800 °C, with a current density of 26.1 A / mm 2 , and the average heating rate is 85.6 °C / s.

[0069] (7) Quench the steel sheet after heating in step (6) to room temperature rapidly without holding.

[0070] (8) Heat the steel sheet after quenching in step (7) to 150 °C, hold for 20 min, and then air-cool to room temperature to obtain the target medium manganese steel.

[0071] The medium manganese steel obtained in this example has an elongation of 21.6%, a tensile strength of 1670 MPa, and no plastic instability at all.

[0072] Example 2

[0073] This example provides a manufacturing method for medium manganese hot-formed steel parts, and the difference from Example 1 is that:

[0074] (6) Cut the steel sheet after two-step annealing into the blanks required for the passenger car anti-collision beam, and use the method of resistance heating to heat the blanks to 800 °C, with a current density of 26.1 A / mm 2 , and the average heating rate is 85.6 °C / s.

[0075] (7) Without holding, transfer the blank after heating in step (6) to the mold with cooling channels for hot stamping forming and quenching integration for the production of the anti-collision beam by the robotic arm within 10 s. The cooling rate is 50 - 100 °C / s, hold the pressure for 8 - 10 s and then take out the mold. The temperature of the part after taking out the mold is below 250 °C, and then air-cool to room temperature to obtain the high-strength and high-toughness anti-collision beam part.

[0076] Comparative Example 1

[0077] This comparative example is basically the same as Example 1, and the difference is only that:

[0078] (6) Use the method of furnace heating to heat the cold-rolled two-step annealed steel in (5) to 800 °C at a heating rate of about 10 °C / s and hold for 4 min.

[0079] The phase distribution diagrams of Example 1 and Comparative Example 1 are as follows Figures 1 - 2 shown. It can be seen from the figure that the retained austenite content in Example 1 is significantly higher than that in Comparative Example 1. At the same time, the martensite grain size in Example 1 is significantly smaller. After two-step annealing, lath-shaped Mn-rich austenite and Mn-poor ferrite are formed. In Example 1, due to the high heating rate and no holding time, there is almost no diffusion in the Mn-rich region, which is beneficial to the formation of more retained austenite. In Comparative Example 1, due to the long holding time in the austenite single-phase region, the Mn-rich region diffuses into the Mn-poor region, and the chemical inhomogeneous distribution formed during annealing is greatly weakened, resulting in a lower retained austenite content of only about 8%, while the retained austenite content in Example 1 is as high as 30%. In addition, during the holding process in the austenite single-phase region, the original austenite grains grow, resulting in a larger martensite grain size after quenching compared to Example 1 (in terms of the equivalent circle diameter, the average bcc grain size in Example 1 is 1.1 μm; the average bcc grain size in Comparative Example 1 is 2.0 μm).

[0080] The room-temperature tensile stress-strain curves of Example 1 and Comparative Example 1 are shown in Figure 6 . It can be seen from the figure that the elongation in Example 1 is significantly higher than that in Comparative Example 1. Retained austenite exerts the TRIP effect during deformation, increasing the hardening ability and delaying necking. Since the retained austenite content in Example 1 is significantly higher than that in Comparative Example 1, more retained austenite leads to a higher elongation.

[0081] Comparative Example 2

[0082] This comparative example is basically the same as Example 1, except that

[0083] (6) The non-annealed cold-rolled medium manganese steel in (4) is heated to 830 °C by using the method of resistance heating, and the current density is 26.1 A / mm 2 , and the average heating rate is 88.9 °C / s.

[0084] The phase distribution diagrams of Example 1 and Comparative Example 2 are as follows Figure 1 、 Figure 3 shown. It can be seen from the figure that the austenite content in Example 1 is significantly higher. For Comparative Example 2, there is almost no element inhomogeneous distribution in the cold-rolled state before rapid heating, and the lack of Mn-rich regions results in its inability to obtain a high content of austenite like Example 1 by using the Mn-rich regions.

[0085] The room-temperature tensile stress-strain curves of Example 1 and Comparative Example 2 are shown in Figure 6 . It can be seen from the figure that the elongation in Example 1 is significantly higher than that in Comparative Example 2. More retained austenite in Example 1 leads to a higher elongation.

[0086] Comparative Example 3

[0087] This comparative example is basically the same as Example 1, with the only difference being that:

[0088] (5) The cold-rolled sheet in step (4) was heated to 620 °C and held for 12 h to complete the reverse austenite transformation annealing, and then air-cooled to room temperature to obtain a cold-rolled one-step annealed steel sheet.

[0089] (6) The cold-rolled one-step annealed steel obtained in step (5) was heated to 820 °C by means of resistance heating, and the current density was 26.1 A / mm 2 , and the average heating rate was 87.8 °C / s.

[0090] (8) The steel sheet after quenching in step (7) was heated to 180 °C and held for 20 min, and then air-cooled to room temperature to obtain the target medium manganese steel.

[0091] The phase distribution diagrams of Example 1 and Comparative Example 3 are as shown in Figure 1 、 Figure 4 . It can be seen from the figure that the retained austenite content of the two is close (about 30%) after heat treatment, and there are only differences in the microscopic tissue morphology. In Example 1, it is mainly lath-shaped, while in Comparative Example 3, it is mainly equiaxed.

[0092] The room temperature tensile stress-strain curves of Example 1 and Comparative Example 3 are shown in Figure 6 . It can be seen from the figure that the strength and elongation in Example 1 are slightly higher, and continuous yielding is presented without Lüder's bands. Compared with the equiaxed shape, the lath-shaped austenite has higher stability and can continuously exhibit the TRIP effect within a longer strain range during the tensile process, so the elongation is slightly higher. At the same time, compared with the lath shape, plastic instabilities such as Lüder's bands and PLC effects are more likely to occur in the equiaxed microstructure, which may be related to the fact that the lath-shaped microstructure is more likely to activate dislocations and has a smaller grain width. Therefore, continuous yielding is presented in Example 1, avoiding plastic instability phenomena.

[0093] Comparative Example 4

[0094] This comparative example is basically the same as Example 1, with the only difference being that:

[0095] (5) The order of the two-step tempering was reversed, that is, first held at 620 °C for 12 h, air-cooled to room temperature, then held at 850 °C for 20 min, and then air-cooled to room temperature.

[0096] (6) The medium manganese steel with the reversed two-step tempering order was heated to 850 °C by means of resistance heating, and the current density was 26.1 A / mm 2 , and the average heating rate was 92.2 °C / s.

[0097] The phase distribution diagrams of Example 1 and Comparative Example 4 are as shown in Figure 1 、Figure 5 As shown, it can be seen from the figure that the austenite content in Comparative Example 4 is significantly lower. For Comparative Example 4, the two-step tempering sequence is reversed, and the uneven distribution of elements obtained during the first tempering process is homogenized during the second tempering process, and a higher content of retained austenite cannot be obtained through the uneven distribution of elements during the subsequent electric heating process.

[0098] Compared with Comparative Example 4, the elongation of Example 1 is significantly higher. Since the content of retained austenite in Example 1 is higher, a higher elongation is obtained through the TRIP effect.

[0099] The above description of the embodiments is for the convenience of those of ordinary skill in the art to understand and use the invention. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative efforts. Therefore, the present invention is not limited to the above embodiments, and all improvements and modifications made by those skilled in the art without departing from the scope of the present invention according to the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. A medium manganese hot-formed steel with high strength and high toughness, characterized in that: The chemical composition of the medium manganese hot-forming steel is calculated by mass fraction as follows: C 0.12-0.17%, Mn 6.0-9.0%, Si 0.2-0.3%, and the rest is Fe and unavoidable impurities.

2. The high-strength and high-toughness medium manganese hot-formed steel according to claim 1, characterized in that: The medium manganese hot-formed steel has a dual-phase structure of martensite matrix+retained austenite.

3. A method for preparing the high-strength and high-toughness medium manganese hot-formed steel according to claim 1 or 2, characterized in that: The medium manganese steel ingot is sequentially subjected to forging, hot rolling, softening annealing, cold rolling, one-step annealing, two-step annealing, rapid heating, quenching and low-temperature tempering to obtain the high-strength and high-toughness medium manganese hot-formed steel.

4. The method for preparing high-strength and high-toughness medium manganese hot-formed steel according to claim 3, characterized in that: The preparation method of the medium manganese steel ingot comprises the following steps: taking a certain weight of C, Mn, Si and Fe elements according to weight percentage, smelting them in proportion, and casting the smelted molten steel into a medium manganese steel ingot, wherein the smelted medium manganese steel ingot comprises the following components by mass fraction: C 0.12-0.17%, Mn 6.0-9.0%, Si 0.2-0.3%, and the rest being Fe and unavoidable impurities.

5. The method for preparing high-strength and high-toughness medium manganese hot-formed steel according to claim 3, characterized in that: The forging comprises heating the medium manganese steel ingot to 1100-1250° C., keeping the temperature for 1-2 hours, hot forging and cooling to room temperature; The hot rolling comprises heating the medium manganese steel after hot forging to 1100-1250° C. and keeping the temperature for 1-2 hours, performing multiple hot rolling at 900-1200° C., with the total hot rolling deformation being 30-90%, and then quenching to room temperature.

6. The method for preparing high-strength and high-toughness medium manganese hot-formed steel according to claim 3, characterized in that: The softening annealing comprises heating the hot-rolled medium manganese steel to 600-700° C. and keeping the temperature for 0.5-5 hours; The cold rolling comprises cold rolling the medium manganese steel after softening and annealing, and the total deformation of the cold rolling is 30-70%.

7. The method for preparing high-strength and high-toughness medium manganese hot-formed steel according to claim 3, characterized in that: The one-step annealing comprises heating the cold-rolled medium manganese steel to 800-950° C. for 20-30 minutes, and then cooling to room temperature; The two-step annealing comprises heating the medium manganese steel after the one-step annealing to 600-700° C. again, keeping the temperature for 10-20 hours, and then cooling it to room temperature.

8. The method for preparing high-strength and high-toughness medium manganese hot-formed steel according to claim 3, characterized in that: The rapid heating includes rapidly heating the medium manganese steel after two-step annealing from room temperature to 780°C-840°C at a heating rate greater than 80°C / s.

9. The method for preparing high-strength and high-toughness medium manganese hot-formed steel according to claim 3, characterized in that: The quenching is forming quenching or direct quenching, specifically: If only high-strength and high-toughness medium-manganese steel plates are required, direct quenching is used in this step, and the rapidly heated plates are quickly transferred to a flat die for quenching, or directly placed in a coolant for quenching; If you need to make high-strength and high-toughness medium manganese steel parts, this step requires transferring the rapidly heated sheet to the mold for forming, and then pressurizing and quenching it to room temperature to complete the part production.

10. The method for preparing high-strength and high-toughness medium manganese hot-formed steel according to claim 3, characterized in that: The low temperature tempering includes heating the quenched sample to 120-220° C. for 5-60 minutes, and then air cooling to room temperature.

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