Ultrafine lamellar martensite-austenite medium manganese steel, method for manufacturing same, and use thereof

Abstract

This invention discloses an ultrafine lamellar martensitic-austenitic medium-manganese steel, its preparation method, and its applications. By mass percentage, the medium-manganese steel comprises: C: 0.15–0.25%; Mn: 6.0–8.0%; Al: 0–1.5%; the balance being Fe, and unavoidable impurities. The ultrafine lamellar martensite and ultrafine lamellar austenite in the medium-manganese steel are distributed alternately, continuously, or semi-continuously along the rolling direction, forming a multiphase structure. The ultrafine lamellar austenite is Mn-rich retained austenite, and the ultrafine lamellar martensite is Mn-poor martensite. Mn is unevenly distributed in the multiphase structure in a layered or banded manner. At least a portion of the ultrafine lamellar austenite has a lamellar width of 10–500 nm, and at least a portion of the ultrafine lamellar martensite is distributed in the form of lamellar bundles or lamellar blocks. The ultrafine lamellar martensitic-austenitic medium-manganese steel has promising application prospects in fields such as advanced high-strength steel plates for automobiles, structural reinforcements, or high-strength, tough, and impact-resistant components.

Description

Technical Field

[0001] This invention belongs to the field of alloy materials technology, particularly to the field of advanced high-strength steel and metal materials technology, specifically relating to ultrafine lamellar martensitic-austenitic manganese steel and its preparation method and application. Background Technology

[0002] As an important representative of third-generation advanced high-strength steel, medium-manganese steel typically achieves a high balance between strength and plasticity by controlling the multiphase microstructure, including ferrite, martensite, and retained austenite. Among these, the retained austenite can undergo a transformation-induced plasticity effect during deformation, thereby improving the material's work hardening ability and delaying local instability. This is a crucial microstructure basis for the excellent comprehensive properties of medium-manganese steel.

[0003] Existing methods for controlling the microstructure of medium-manganese steel mainly include two-phase annealing, reverse phase transformation annealing, rapid heating, hot stamping, and combinations thereof. Patent CN202110952683.0 describes a heterogeneous lamellar microstructure consisting of overlapping Mn-rich retained austenite and Mn-poor martensite through pearlite transformation and two-stage heat treatment. While these methods have a positive effect on improving the properties of medium-manganese steel, they still have at least the following shortcomings:

[0004] Firstly, the formation of pearlite precursor structure depends on the steel having a high carbon content of 0.20–0.45 wt.%, which deteriorates weldability;

[0005] Secondly, the formation of heterogeneous lamellar structures depends on pearlite precursor structures and long-term isothermal treatment, resulting in low preparation efficiency and high requirements for initial structure and elemental distribution processes.

[0006] Third, the formation of heterogeneous lamellar structures depends on a two-stage heating process, which requires a specific heating rate and makes the preparation process complex.

[0007] Therefore, how to obtain medium-manganese steel with both high strength and good plasticity under low carbon content and simple processing conditions is an urgent problem to be solved in this field. Summary of the Invention

[0008] In view of this, some embodiments disclose ultrafine lamellar martensitic-austenitic manganese steel, comprising, by mass percentage: C: 0.15-0.25%; Mn: 6.0-8.0%; Al: 0-1.5%; balance Fe, and unavoidable impurities;

[0009] In medium manganese steel, ultrafine lamellar martensite and ultrafine lamellar austenite are distributed alternately, continuously or semi-continuously along the rolling direction to form a multiphase structure; the ultrafine lamellar austenite is Mn-rich retained austenite, and the ultrafine lamellar martensite is Mn-poor martensite; Mn element is unevenly distributed in the multiphase structure in a layered or banded manner.

[0010] At least some of the ultrafine lamellar austenite has a lamellar width of 10–500 nm, and at least some of the ultrafine lamellar martensite is distributed in the form of lamellar bundles or lamellar blocks.

[0011] Furthermore, in some embodiments of the ultrafine lamellar martensite-austenite manganese steel, the volume fraction of ultrafine lamellar austenite in the multiphase structure is 5-35%.

[0012] Some embodiments disclose ultrafine lamellar martensitic-austenitic medium manganese steel, with a yield strength of 900-1600 MPa, a tensile strength of 1500-2200 MPa, and an elongation after fracture of 5-40%.

[0013] On the other hand, some embodiments disclose a method for preparing manganese steel in ultrafine lamellar martensite-austenite, including the following steps:

[0014] S1. Initial microstructure control: Medium manganese steel is heated to the austenitic region and held at that temperature, then cooled to room temperature to obtain medium manganese steel with an initial microstructure of martensite.

[0015] S2. Two-phase annealing: Medium manganese steel with an initial martensitic microstructure is heated to the two-phase region for annealing to obtain a dual-phase microstructure of Mn-depleted ferrite and Mn-rich austenite. S3. Rolling and oil quenching: The medium manganese steel after annealing in the two-phase region is directly rolled with a large reduction, and then directly oil quenched. The Mn-depleted ferrite in the two-phase structure is transformed into ultrafine lamellar ferrite, and the Mn-rich austenite is transformed into ultrafine lamellar austenite. S4. Short-time austenitization: Oil-quenched medium manganese steel is placed in a salt bath for rapid heating and short-time austenitization treatment, so that at least part of the ultrafine lamellar ferrite undergoes reverse phase transformation into Mn-depleted ultrafine lamellar austenite. S5. Cooling: Cool the medium manganese steel after short-time austenitization to room temperature, so that at least part of the Mn-depleted ultrafine lamellar austenite is transformed into ultrafine lamellar martensite, while retaining part of the Mn-rich ultrafine lamellar austenite, to obtain medium manganese steel with ultrafine lamellar martensite and ultrafine lamellar austenite structure.

[0016] Furthermore, in some embodiments of the preparation method of ultrafine lamellar martensitic-austenitic manganese steel, in step S1, the holding temperature of the austenitic region is 950-1050℃, and the holding time is 60-180min.

[0017] In some embodiments of the method for preparing manganese steel in ultrafine lamellar martensite-austenite, in step S2, the two-phase annealing temperature is located at A. C1 ~A C3 The annealing time is between 30 and 300 minutes.

[0018] In some embodiments of the preparation method of ultrafine lamellar martensitic-austenitic manganese steel, in step S3, the large reduction rolling is warm rolling, the rolling temperature is 400-750°C, and the total reduction is 40-85%.

[0019] In some embodiments of the preparation method of manganese steel in ultrafine lamellar martensite-austenite, in step S4, the temperature of short-time austenitization is 780-880°C and the holding time is 10-120s.

[0020] On the other hand, some embodiments disclose the application of ultrafine lamellar martensitic-austenitic medium manganese steel, which is used to manufacture advanced high-strength steel sheets, structural reinforcements or high-strength, tough and impact-resistant components for automobiles.

[0021] The ultrafine lamellar martensitic-austenitic manganese steel and its preparation method disclosed in this invention have at least the following beneficial technical effects:

[0022] (1) An ultrafine lamellar martensite / austenite multiphase microstructure was formed, which is different from conventional homogeneous martensite / austenite microstructure and ordinary duplex microstructure. In this microstructure, both martensite and austenite are distributed in lamellar or lamellar bundles, and the non-uniform distribution of Mn element is preserved, which is conducive to increasing the phase interface density, strengthening the interphase coordinated deformation and promoting stable work hardening behavior.

[0023] (2) An ultrafine lamellar ferrite / austenite precursor structure was constructed by a synergistic process of "two-phase region annealing - high-pressure warm rolling - oil quenching". The lamellar ferrite was then transformed into lamellar austenite by short-time salt bath austenitization, which avoided the disappearance of Mn distribution caused by long-term element homogenization and enabled the target structure to be obtained in a short processing time.

[0024] (3) High-pressure warm rolling is the key step to obtain the final ultrafine lamellar martensite / austenite structure. Under the same salt bath temperature and holding time, the rolled example showed higher yield strength and tensile strength compared with the unrolled example, indicating that the ultrafine lamellar precursor structure formed by warm rolling significantly improved the strengthening effect of the final structure.

[0025] (4) The process route has strong controllability. By adjusting the salt bath temperature and holding time, the amount of lamellar austenite retention, the amount of martensite transformation and the strength-plasticity matching relationship can be controlled to varying degrees, thereby meeting the performance requirements of different high-strength and high-toughness components. Attached Figure Description

[0026] Figure 1 This is a diagram illustrating the heating mechanism of the method for preparing ultrafine lamellar martensitic-austenitic manganese steel in Example 1.

[0027] Figure 2SEM images of the ultrafine lamellar ferrite and ultrafine lamellar austenite precursor microstructures obtained after two-phase region annealing, rolling and oil quenching in Example 1;

[0028] Figure 3 This is a phase distribution diagram of the precursor structure obtained after two-phase region annealing, rolling, and oil quenching in Example 1;

[0029] Figure 4 SEM images of the ultrafine lamellar martensite and ultrafine lamellar austenite microstructures obtained after short-time salt bath austenitization in Example 1;

[0030] Figure 5 This is a STEM-EDS Mn elemental distribution map of the STEM-BF image of the tissue obtained after short-term austenitization in a salt bath in Example 1.

[0031] Figure 6 The mechanical properties and engineering stress-strain curves of manganese steel in the examples and comparative examples are shown. Detailed Implementation

[0032] The term "embodiment" used herein, as an example, is not necessarily to be construed as superior to or better than other embodiments. Performance testing in these embodiments of the invention, unless otherwise specified, employs conventional testing methods in the art. It should be understood that the terminology used in these embodiments is merely for describing particular implementations and is not intended to limit the scope of the disclosure of these embodiments.

[0033] Unless otherwise stated, the technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the embodiments of this invention pertain; other experimental methods and technical means not specifically noted in the embodiments of this invention refer to experimental methods and technical means commonly used by one of ordinary skill in the art.

[0034] The terms “basic” and “approximately” as used herein are used to describe small fluctuations. For example, they can mean less than or equal to ±5%, such as less than or equal to ±2%, such as less than or equal to ±1%, such as less than or equal to ±0.5%, such as less than or equal to ±0.2%, such as less than or equal to ±0.1%, such as less than or equal to ±0.05%. Numerical data presented or expressed in range format herein are used for convenience and brevity only, and should therefore be interpreted flexibly to include not only the explicitly listed values ​​that define the range, but also all independent values ​​or subranges contained within that range. For example, a numerical range of “1–5%” should be interpreted to include not only the explicitly listed values ​​from 1% to 5%, but also the independent values ​​and subranges within the indicated range. Thus, this numerical range includes independent values ​​such as 2%, 3.5%, and 4%, and subranges such as 1%–3%, 2%–4%, and 3%–5%, etc. This principle also applies to ranges that list only one value. Furthermore, this interpretation applies regardless of the width of the range or the characteristics described.

[0035] In this document, including in the claims, conjunctions such as "comprising," "including," "with," "having," "containing," "involving," and "accommodating" are understood to be open-ended, meaning "including but not limited to." Only the conjunctions "consisting of" and "composed of" are closed conjunctions.

[0036] To better illustrate the content of this invention, numerous specific details are provided in the following detailed embodiments. Those skilled in the art should understand that the invention can be practiced even without certain specific details. In the embodiments, some methods, means, instruments, and devices well-known to those skilled in the art are not described in detail, in order to highlight the main points of the invention.

[0037] Without conflict, the technical features disclosed in the embodiments of the present invention can be combined arbitrarily, and the resulting technical solution belongs to the content disclosed in the embodiments of the present invention.

[0038] In some embodiments, the ultrafine lamellar martensite-austenite medium manganese steel, by mass percentage, comprises: C: 0.15–0.25%; Mn: 6.0–8.0%; Al: 0–1.5%; the balance being Fe, and unavoidable impurities; the ultrafine lamellar martensite-austenite multiphase structure of the medium manganese steel is formed by the alternating, continuous, or semi-continuous distribution of ultrafine lamellar martensite and ultrafine lamellar austenite along the rolling direction; the ultrafine lamellar austenite is Mn-rich retained austenite, and the ultrafine lamellar martensite is Mn-poor martensite, with Mn element unevenly distributed in the multiphase structure in a layered or banded manner; at least a portion of the ultrafine lamellar austenite has a lamellar width of 10–500 nm, and at least a portion of the ultrafine lamellar martensite is distributed in the form of lamellar bundles or lamellar blocks. Typically, manganese steel in ultrafine lamellar martensite-austenite has a yield strength of 900–1600 MPa, a tensile strength of 1500–2200 MPa, and an elongation after fracture of 5–40%.

[0039] Carbon (C) is an important element for ensuring martensitic strength and stabilizing austenite. When the C content is too low, the martensitic strengthening effect is insufficient, and the stability of retained austenite decreases. When the C content is too high, it easily increases quenching brittleness and reduces welding and forming adaptability. Therefore, some embodiments control the C content at 0.15–0.25 wt.%. Mn is a key element for stabilizing austenite and forming a chemically inhomogeneous distribution. When the Mn content is too low, it is difficult to retain sufficient austenite after short-term austenitization and cooling. When the Mn content is too high, smelting, segregation, and welding problems are exacerbated. Therefore, some embodiments control the Mn content at 6.0–8.0 wt.%. Al can regulate phase transformation temperature and microstructure stability, and helps reduce density and improve some process adaptability. When the Al content is too low, its regulatory effect is insufficient. When the Al content is too high, it easily affects smelting and inclusion control. Therefore, embodiments of the present invention control the Al content at 0.5–1.5 wt.%.

[0040] In some embodiments, the volume fraction of ultrafine lamellar austenite in the multiphase microstructure is 5-35%.

[0041] In some preferred embodiments, the volume fraction of ultrafine lamellar austenite in the multiphase microstructure is 10–30%.

[0042] In some embodiments, the preparation method of manganese steel in ultrafine lamellar martensite-austenite includes the following steps:

[0043] S1. Initial microstructure control: Medium manganese steel is heated to the austenitic region and held at that temperature, then cooled to room temperature to obtain medium manganese steel with an initial microstructure of martensite; in some embodiments, the holding temperature in the austenitic region is 950-1050℃ and the holding time is 60-180min.

[0044] In some preferred embodiments, the austenitic region is held at 1000°C for 120 minutes.

[0045] S2. Two-phase annealing: Medium-manganese steel with an initial martensitic microstructure is heated to the two-phase region for annealing to obtain a dual-phase microstructure of Mn-depleted ferrite and Mn-rich austenite; in some embodiments, the two-phase annealing temperature is located at A. C1 ~A C3 The annealing time is between 30 and 300 minutes.

[0046] In some preferred embodiments, the two-phase annealing temperature is 600–750°C.

[0047] S3. Rolling and oil quenching: The medium manganese steel after annealing in the two-phase region is directly rolled with a large reduction, and then directly oil quenched. The Mn-depleted ferrite in the two-phase structure is transformed into ultrafine lamellar ferrite, and the Mn-rich austenite is transformed into ultrafine lamellar austenite. Usually, direct oil quenching means that after rolling, the medium manganese steel is placed in an oil medium and cooled to room temperature without intermediate heat preservation treatment.

[0048] In some embodiments, the high-reduction rolling is warm rolling, with a rolling temperature of 400–750°C and a total reduction of 40–85%. In some preferred embodiments, the total reduction of the high-reduction rolling is 50–80%.

[0049] S4. Short-time austenitization: The oil-quenched medium-manganese steel is placed in a salt bath for rapid heating and short-time austenitization treatment, so that at least part of the ultrafine lamellar ferrite undergoes reverse phase transformation into Mn-depleted ultrafine lamellar austenite. Typically, rapid salt bath heating and short-time austenitization treatment are used to restrict the long-range diffusion of Mn elements, so that the uneven distribution of Mn elements in the ultrafine lamellar ferrite and ultrafine lamellar austenite precursor microstructure formed in step S3 is retained in the final microstructure.

[0050] In some embodiments, the short-time austenitization temperature is 780–880°C, and the holding time is 10–120 seconds.

[0051] In some preferred embodiments, the temperature and holding time for short-time austenitization are 800°C for 50–70 s, or 830°C for 50–70 s, or 850°C for 30–50 s.

[0052] S5. Cooling: The medium-manganese steel after short-time austenitization is cooled to room temperature, causing at least some of the Mn-depleted ultrafine lamellar austenite to transform into ultrafine lamellar martensite, while retaining some Mn-rich ultrafine lamellar austenite, resulting in medium-manganese steel with both ultrafine lamellar martensite and ultrafine lamellar austenite microstructures. The final microstructure of the medium-manganese steel is a multiphase microstructure comprising ultrafine lamellar martensite and ultrafine lamellar austenite, with both austenite and martensite exhibiting distinct lamellar morphologies, and the lamellar structure extending along the rolling direction. Because short-time austenitization restricts long-range diffusion of Mn, the Mn concentration difference between Mn-rich austenite and Mn-depleted ferrite / austenite in the precursor microstructure is retained in the final microstructure, ultimately forming an ultrafine lamellar martensite / austenite multiphase structure with Mn element unevenly distributed in lamellar or banded patterns.

[0053] In the preparation process of the medium-manganese steel disclosed in this invention, an ultrafine lamellar ferrite / austenite precursor microstructure is first constructed. Then, through short-time austenitization, this precursor microstructure undergoes a selective phase transformation. After annealing in the two-phase region, the initial martensite microstructure forms Mn-depleted ferrite and Mn-rich austenite, with Mn enriched in the austenite. Subsequently, during high-reduction warm rolling, both ferrite and austenite are elongated and refined along the rolling direction, forming ultrafine lamellar ferrite and ultrafine lamellar austenite. During the short-time austenitization process in the salt bath, at least some of the lamellar ferrite undergoes a reverse phase transformation and transforms into Mn-depleted ultrafine lamellar austenite. However, due to the short salt bath treatment time, Mn is difficult to fully homogenize, and the spatial distribution of Mn-rich and Mn-depleted austenite regions in the precursor microstructure is preserved. Upon cooling to room temperature, the relatively Mn-depleted austenite becomes unstable and transforms into ultrafine lamellar martensite; the Mn-rich austenite region remains as ultrafine lamellar austenite. This results in a lamellar multiphase microstructure where ultrafine lamellar martensite and ultrafine lamellar austenite are coupled. This multiphase microstructure provides high strength through the ultrafine lamellar martensite and sustained work hardening capability through phase transformation and interphase coordinated deformation of the lamellar austenite during deformation. Simultaneously, the high-density lamellar phase interface enhances heterogeneous deformation constraint and improves back stress strengthening, thereby improving the strength-ductility balance.

[0054] In some embodiments, ultrafine lamellar martensitic-austenitic manganese steel is used in advanced high-strength steel plates for automobiles, structural reinforcements, or high-strength, tough, and impact-resistant components.

[0055] The technical details are further illustrated below with reference to the embodiments.

[0056] Example 1

[0057] In Example 1, the composition of the medium manganese steel by mass percentage is: C: 0.20%, Mn: 7.0%, Al: 1.0%, with the balance being Fe and unavoidable impurities.

[0058] In Example 1, as Figure 1As shown, the preparation method of medium manganese steel includes the following steps:

[0059] S1. Hold the medium manganese steel at 1000℃ for 120 min and cool it to room temperature to obtain the initial martensitic structure.

[0060] S2. The initial martensitic structure is heated to the two-phase region for annealing to obtain a dual-phase structure of Mn-depleted ferrite and Mn-rich austenite; the annealing temperature in the two-phase region is located at A. C1 ~A C3 The annealing time is 30 minutes.

[0061] S3. After annealing in the two-phase region, the material is directly subjected to high-reduction warm rolling, followed by direct oil quenching to obtain an ultrafine lamellar ferrite and ultrafine lamellar austenite precursor structure; the rolling temperature is 750℃, and the total reduction is 85%;

[0062] S4. Place the oil-quenched sample in an 800℃ salt bath for 50 seconds to perform short-time austenitization treatment.

[0063] S5. Cool to room temperature to obtain medium manganese steel with ultrafine lamellar martensite and ultrafine lamellar austenite microstructure.

[0064] The medium manganese steel in Example 1 underwent performance testing and microstructure characterization, and the results are as follows: Figure 2 , Figure 3 , Figure 4 , Figure 5 As shown in the figure; microstructure characterization revealed that after rolling and oil quenching, Example 1 formed an ultrafine lamellar ferrite and ultrafine lamellar austenite precursor microstructure; after short-time salt bath austenitization, the final microstructure retained a distinct lamellar morphology, forming ultrafine lamellar martensite and ultrafine lamellar austenite. Mechanical property test results showed that the yield strength, tensile strength, uniform elongation, and total elongation of Example 1 were 1323 MPa, 1937 MPa, 7.58%, and 9.43%, respectively.

[0065] Comparative Example 1

[0066] The preparation method of the manganese steel in Comparative Example 1 is the same as that in Example 1;

[0067] The difference between Comparative Example 1 and Example 1 is that after annealing in the two-phase region, the high-reduction warm rolling and direct oil quenching treatment are not performed, but instead a short-time austenitizing treatment in a salt bath at 800°C for 50 seconds is performed directly.

[0068] Test results show that Comparative Example 1 failed to form the same lamellar multiphase structure of alternating ultrafine lamellar martensite and ultrafine lamellar austenite as Example 1. Its yield strength, tensile strength, uniform elongation and total elongation were 665 MPa, 1280 MPa, 31.17% and 36.89%, respectively, which were significantly lower than the strength level of Example 1.

[0069] The mechanical property test results of Example 1 and Comparative Example 1 show that, under the same salt bath conditions, the yield strength and tensile strength of the material are significantly improved after high-reduction warm rolling treatment.

[0070] Example 2

[0071] In Example 2, the preparation method of medium manganese steel is the same as in Example 1;

[0072] The difference between Example 2 and Example 1 is that the salt bath short-time austenitizing treatment in step S4 is held at 800°C for 70 seconds.

[0073] The results of the microstructure and performance tests showed that the manganese steel obtained in Example 2 retained the lamellar multiphase microstructure characteristics, possessing ultrafine lamellar martensite and ultrafine lamellar austenite. The yield strength, tensile strength, uniform elongation, and total elongation of Example 2 were 1414 MPa, 2075 MPa, 5.99%, and 6.24%, respectively.

[0074] Comparative Example 2

[0075] In Comparative Example 2, the preparation method of medium manganese steel is the same as in Example 2;

[0076] The difference between Comparative Example 2 and Example 2 is that after annealing in the two-phase region, the high-reduction warm rolling and direct oil quenching treatment are not performed, but instead a short-time austenitizing treatment in a salt bath at 800°C for 70 seconds is performed directly.

[0077] Test results show that the yield strength, tensile strength, uniform elongation and total elongation of Comparative Example 2 are 739 MPa, 1423 MPa, 11.09% and 11.14%, respectively.

[0078] The results of Example 2 and Comparative Example 2 show that the lamellarity and strength of the medium manganese steel in Comparative Example 2 are lower than those in Example 2. This proves that high-pressure warm rolling and the resulting ultra-fine lamellar ferrite / austenite precursor structure are necessary process steps to obtain the target structure and improve the performance of this invention. The ultra-fine lamellar precursor structure formed by high-pressure warm rolling can significantly improve the strength of the final material.

[0079] Example 3

[0080] In Example 3, the preparation method of medium manganese steel is the same as in Example 1;

[0081] The difference between Example 3 and Example 1 is that the salt bath short-time austenitizing treatment in step S4 is held at 830°C for 50 seconds.

[0082] The results of the microstructure and performance tests showed that the manganese steel obtained in Example 3 retained the characteristics of lamellar multiphase microstructure, possessing ultrafine lamellar martensite and ultrafine lamellar austenite. The yield strength, tensile strength, uniform elongation, and total elongation of Example 3 were 1529 MPa, 2001 MPa, 8.78%, and 13.40%, respectively.

[0083] Comparative Example 3

[0084] In Comparative Example 3, the preparation method of medium manganese steel is the same as in Example 3;

[0085] The difference between Comparative Example 3 and Example 3 is that after annealing in the two-phase region, the high-reduction warm rolling and direct oil quenching treatment are not performed, but instead a short-time austenitizing treatment in a salt bath at 830°C for 50 seconds is performed directly.

[0086] Test results show that the yield strength, tensile strength, uniform elongation and total elongation of Comparative Example 3 are 976 MPa, 1827 MPa, 11.35% and 11.55%, respectively.

[0087] The test results of Example 3 and Comparative Example 3 show that after warm rolling, the material retains a certain degree of plasticity while the yield strength and tensile strength are significantly improved. The lamellarity and strength of the manganese steel in Comparative Example 3 are lower than those in Example 3, proving that high-pressure warm rolling and the resulting ultra-fine lamellar ferrite / austenite precursor structure are necessary process steps to obtain the target structure and improve the performance of this invention.

[0088] Example 4

[0089] In Example 4, the preparation method of medium manganese steel is the same as in Example 1;

[0090] The difference between Example 4 and Example 1 is that the salt bath short-time austenitizing treatment in step S4 is held at 830°C for 70 seconds.

[0091] The results of the microstructure and performance tests showed that the manganese steel obtained in Example 4 retained the characteristics of lamellar multiphase microstructure, possessing ultrafine lamellar martensite and ultrafine lamellar austenite. The yield strength, tensile strength, uniform elongation, and total elongation of Example 4 were 1431 MPa, 2021 MPa, 8.32%, and 12.58%, respectively.

[0092] Comparative Example 4

[0093] In Comparative Example 4, the preparation method of medium manganese steel is the same as in Example 4;

[0094] The difference between Comparative Example 4 and Example 4 is that after annealing in the two-phase region, the high-reduction warm rolling and direct oil quenching treatment are not performed, but instead a short-time salt bath austenitizing treatment with a holding temperature of 830°C for 70 seconds is performed directly.

[0095] Test results show that the yield strength, tensile strength, uniform elongation and total elongation of Comparative Example 4 are 1160 MPa, 1971 MPa, 8.83% and 8.92%, respectively.

[0096] The test results of Example 4 and Comparative Example 4 show that the lamellarity and strength of the manganese steel in Comparative Example 4 are lower than those in Example 4, proving that high-reduction warm rolling and the resulting ultrafine lamellar ferrite / austenite precursor structure are necessary process steps to obtain the target microstructure and improve the properties of this invention. This indicates that the ultrafine lamellar precursor structure formed by high-reduction warm rolling plays a key role in improving the final microstructure and mechanical properties.

[0097] Example 5

[0098] In Example 5, the preparation method of medium manganese steel is the same as in Example 1;

[0099] The difference between Example 5 and Example 1 is that the salt bath short-time austenitizing treatment in step S4 is held at 850°C for 30 seconds.

[0100] The results of the microstructure and performance tests showed that the manganese steel obtained in Example 5 retained the lamellar multiphase microstructure characteristics, possessing ultrafine lamellar martensite and ultrafine lamellar austenite. The yield strength, tensile strength, uniform elongation, and total elongation of the high manganese steel in Example 5 were 1442 MPa, 1968 MPa, 6.21%, and 6.88%, respectively.

[0101] Comparative Example 5

[0102] In Comparative Example 5, the preparation method of medium manganese steel is the same as in Example 5;

[0103] The difference between Comparative Example 5 and Example 5 is that, after annealing in the two-phase region, the high-reduction warm rolling and direct oil quenching treatment are not performed, but instead a short-time salt bath austenitizing treatment is performed with a holding temperature of 850°C for 30 seconds.

[0104] Test results show that the yield strength, tensile strength, uniform elongation and total elongation of Comparative Example 5 are 936 MPa, 1676 MPa, 8.18% and 8.29%, respectively.

[0105] The test results of Example 5 and Comparative Example 5 show that the degree of lamellarization and strength of the manganese steel in Comparative Example 5 are lower than those in Example 5, proving that high-pressure warm rolling and the resulting ultra-fine lamellar ferrite / austenite precursor structure are necessary process steps to obtain the target structure and improve the performance of this invention; after warm rolling, the material strength is significantly improved.

[0106] Example 6

[0107] In Example 6, the preparation method of medium manganese steel is the same as in Example 1;

[0108] The difference between Example 6 and Example 1 is that the salt bath short-time austenitizing treatment in step S4 is held at 850°C for 50 seconds.

[0109] The results of the microstructure and performance tests showed that the manganese steel obtained in Example 6 retained the characteristics of lamellar multiphase microstructure, possessing ultrafine lamellar martensite and ultrafine lamellar austenite. The yield strength, tensile strength, uniform elongation, and total elongation of the manganese steel in Example 6 were 1360 MPa, 1940 MPa, 4.83%, and 4.83%, respectively.

[0110] Comparative Example 6

[0111] In Comparative Example 6, the preparation method of medium manganese steel is the same as in Example 6;

[0112] The difference between Comparative Example 6 and Example 6 is that after annealing in the two-phase region, the high-reduction warm rolling and direct oil quenching treatment are not performed, but instead a short-time austenitizing treatment in a salt bath at 850°C for 50 seconds is performed directly.

[0113] Test results show that the yield strength, tensile strength, uniform elongation and total elongation of Comparative Example 6 are 1056 MPa, 1918 MPa, 7.57% and 7.63%, respectively.

[0114] The test results of Example 6 and Comparative Example 6 show that the high-reduction warm rolling in Comparative Example 6 can improve the strengthening level of the final microstructure. The lamellarity and strength level of the microstructure are lower than those of Example 6, which proves that high-reduction warm rolling and the ultrafine lamellar ferrite / austenite precursor microstructure formed therefrom are necessary process steps to obtain the target microstructure and performance improvement of this invention.

[0115] Performance Comparison Explanation

[0116] Table 1 Summary of mechanical properties of the examples and comparative examples

[0117]

[0118] Table 1 and Figure 6 The mechanical properties and engineering stress-strain curves of the manganese steel obtained in the examples and comparative examples are shown. Figure 6In the figures, curves marked with "WR" correspond to examples that underwent two-phase annealing, high-reduction warm rolling, direct oil quenching, and short-time salt bath austenitization; curves without "WR" correspond to comparative examples that did not undergo high-reduction warm rolling. Examples 1-6 that underwent warm rolling exhibited yield strengths of 1323-1529 MPa, tensile strengths of 1937-2075 MPa, uniform elongation of 4.83-8.78%, and total elongation of 4.83-13.40%. Comparative examples 1-6, on the other hand, exhibited yield strengths of 665-1160 MPa, tensile strengths of 1280-1918 MPa, uniform elongation of 7.57-31.17%, and total elongation of 7.63-36.89%. Overall, the yield strength and tensile strength of the manganese steel in the examples were significantly higher than those of the manganese steel in the comparative examples under the same salt bath conditions.

[0119] The above results demonstrate that, under the same salt bath temperature and holding time, simple short-time austenitization is insufficient to fully obtain the manganese steel of the present invention with a multiphase microstructure of ultrafine lamellar martensite and ultrafine lamellar austenite. However, through high-reduction warm rolling after two-phase annealing, an ultrafine lamellar ferrite / austenite precursor microstructure can be pre-constructed, and this lamellar characteristic can be inherited during subsequent short-time austenitization and cooling in the salt bath, thereby significantly improving the yield strength and tensile strength of the material. Under the conditions of 830℃×50s and 830℃×70s, the total elongation of the manganese steel in the examples is also better than that of the comparative example, indicating that the preparation method disclosed in the present invention can simultaneously improve strength and plasticity under partial salt bath conditions. Under the partial conditions of 800℃ and 850℃, although the elongation decreases, excellent comprehensive strength and plasticity matching can still be obtained.

[0120] The technical solutions and technical details disclosed in the embodiments of this invention are merely illustrative of the inventive concept of this invention and do not constitute a limitation on the technical solutions of the embodiments of this invention. Any conventional changes, substitutions, or combinations made to the technical details disclosed in the embodiments of this invention have the same inventive concept as this invention and are within the protection scope of the claims of this invention.

Claims

1. An ultra-fine sheet martensitic-austenitic medium manganese steel, characterized in that, By mass percentage, it includes: C: 0.15–0.25%; Mn: 6.0–8.0%; Al: 0–1.5%; the balance being Fe, and unavoidable impurities; The ultrafine lamellar martensite and ultrafine lamellar austenite in the medium manganese steel are distributed alternately, continuously or semi-continuously along the rolling direction to form a multiphase structure; the ultrafine lamellar austenite is Mn-rich retained austenite, the ultrafine lamellar martensite is Mn-poor martensite, and the Mn element is unevenly distributed in lamellar or banded form in the multiphase structure. At least a portion of the ultrafine lamellar austenite has a lamellar width of 10–500 nm, and at least a portion of the ultrafine lamellar martensite is distributed in the form of lamellar bundles or lamellar blocks.

2. The ultrafine lamellar martensitic-austenitic medium-manganese steel according to claim 1, characterized in that, In the multiphase microstructure, the volume fraction of the ultrafine lamellar austenite is 5-35%.

3. The ultrafine lamellar martensitic-austenitic medium-manganese steel according to claim 1, characterized in that, The medium manganese steel has a yield strength of 900-1600 MPa, a tensile strength of 1500-2200 MPa, and an elongation after fracture of 5-40%.

4. A method for preparing ultrafine lamellar martensitic-austenitic manganese steel according to any one of claims 1 to 3, comprising the steps of: S1. Initial microstructure control: Medium manganese steel is heated to the austenitic region and held at that temperature, then cooled to room temperature to obtain medium manganese steel with an initial microstructure of martensite. S2. Two-phase annealing: Medium manganese steel with an initial martensitic microstructure is heated to the two-phase region for annealing to obtain a dual-phase microstructure of Mn-depleted ferrite and Mn-rich austenite. S3. Rolling and oil quenching: The medium manganese steel after annealing in the two-phase region is directly rolled with a large reduction, and then directly oil quenched. The Mn-depleted ferrite in the two-phase structure is transformed into ultrafine lamellar ferrite, and the Mn-rich austenite is transformed into ultrafine lamellar austenite. S4. Short-time austenitization: Oil-quenched medium manganese steel is placed in a salt bath for rapid heating and short-time austenitization treatment, so that at least part of the ultrafine lamellar ferrite undergoes reverse phase transformation into Mn-depleted ultrafine lamellar austenite. S5. Cooling: Cool the medium manganese steel after short-time austenitization to room temperature, so that at least part of the Mn-depleted ultrafine lamellar austenite is transformed into ultrafine lamellar martensite, while retaining part of the Mn-rich ultrafine lamellar austenite, to obtain medium manganese steel with ultrafine lamellar martensite and ultrafine lamellar austenite structure.

5. The preparation method according to claim 4, characterized in that, In step S1, the holding temperature in the austenitic region is 950–1050℃, and the holding time is 60–180 min.

6. The preparation method according to claim 4, characterized in that, In step S2, the two-phase region annealing temperature is between A C1 ~ A C3 , and the annealing time is 30-300 min.

7. The preparation method according to claim 4, characterized in that, In step S3, the large reduction rolling is warm rolling, with a rolling temperature of 400-750℃ and a total reduction of 40-85%.

8. The preparation method according to claim 4, characterized in that, In step S4, the short-time austenitizing temperature is 780–880°C, and the holding time is 10–120 seconds.

9. The application of the ultrafine lamellar martensitic-austenitic manganese steel according to any one of claims 1 to 3, characterized in that, The medium manganese steel is used to manufacture advanced high-strength steel plates, structural reinforcements, or high-strength, tough, and impact-resistant components for automobiles.

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