A warm rolled medium manganese steel with excellent low temperature toughness and a method of making the same

Abstract

The application belongs to the technical field of metal material heat treatment, and particularly relates to a warm-rolled medium-manganese steel with excellent low-temperature strength and toughness and a preparation method thereof, the preparation method comprising the following steps: S1, obtaining a cast ingot, and performing homogenization annealing on the cast ingot to obtain an annealed cast ingot; S2, performing rough hot rolling on the annealed cast ingot, and performing water cooling to room temperature to obtain a rough hot-rolled plate; S3, performing ferrite zone warm rolling on the rough hot-rolled plate, and performing air cooling to room temperature to obtain a warm-rolled plate; S4, performing bainite zone tempering on the warm-rolled plate to obtain a tempered warm-rolled plate, and the cooling mode of the bainite zone tempering is air cooling; and S5, performing dual-phase zone annealing on the tempered warm-rolled plate to obtain the warm-rolled medium-manganese steel, and the cooling mode of the dual-phase zone annealing is air cooling. The application adopts a homogenization annealing, rough hot rolling, ferrite zone warm rolling, bainite zone tempering and dual-phase zone annealing coupling process to prepare the warm-rolled medium-manganese steel with excellent low-temperature strength and toughness, breaks through the problem that the low-temperature strength and toughness of the medium-manganese steel do not match, and widens the application field of the medium-manganese steel.

Description

Technical Field

[0001] This application relates to the field of heat treatment technology for metallic materials, specifically to a warm-rolled medium-manganese steel with excellent low-temperature strength and toughness and its preparation method. Background Technology

[0002] Medium-manganese steel possesses advantages such as low cost, excellent weldability, and a superior room-temperature strength-ductility combination, making it highly anticipated in specialized fields such as aerospace, marine, petroleum, and chemical industries. However, its relatively poor low-temperature strength and toughness combination limits its application and promotion in components used in some low-temperature environments. Considering the multiphase microstructure of medium-manganese steel and the special characteristics of its low-temperature service environment, preparing high-volume-fraction, highly stable austenite is the core approach to overcoming the mismatch between its low-temperature strength and toughness. First, the softer austenite phase facilitates the transfer and propagation of micro-strain during deformation, preventing premature microcracks or even fracture. Second, high-volume-fraction austenite can undergo explosive martensitic transformation during deformation, ensuring the continuity of plastic deformation. Finally, fine-grained austenite exhibits high stability and can still undergo martensitic transformation in the later stages of deformation to provide additional work hardening, thereby promoting plastic deformation. Although processes such as quenching-partitioning, quenching-partitioning-tempering, and quenching-partitioning-annealing-tempering have been used to control the microstructure of medium-manganese steel and improve its room temperature strength and toughness, the problem of the mismatch between its low-temperature strength and toughness remains difficult to overcome. Therefore, optimizing existing processing techniques to solve the problem of the low-temperature strength-toughness mismatch in medium-manganese steel is urgently needed. Summary of the Invention

[0003] To solve the above-mentioned technical problems, this application provides a method for preparing warm-rolled medium-manganese steel with excellent low-temperature strength and toughness, comprising the following steps: S1, obtaining an ingot, and performing homogenization annealing on the ingot to obtain an annealed ingot; S2, performing rough hot rolling on the annealed ingot and water cooling to room temperature to obtain a rough hot-rolled plate; S3, performing warm rolling on the rough hot-rolled plate in the ferrite region and air cooling to room temperature to obtain a warm-rolled plate; S4, performing tempering on the warm-rolled plate in the bainite region to obtain a tempered warm-rolled plate, wherein the cooling method for tempering in the bainite region is air cooling to room temperature; S5, performing dual-phase annealing on the tempered warm-rolled plate to obtain warm-rolled medium-manganese steel, wherein the cooling method for annealing in the dual-phase region is air cooling to room temperature.

[0004] As a preferred embodiment of the method for preparing a warm-rolled medium-manganese steel with excellent low-temperature strength and toughness as described in this application, in step S5, the chemical composition of the warm-rolled medium-manganese steel includes: C: 0.30-0.55wt%, Mn: 8.5-11.0wt%, Al: 2.0-3.0wt%, V: 0.4-0.6wt%, with the balance being Fe and unavoidable impurities.

[0005] As a preferred embodiment of the method for preparing a warm-rolled medium-manganese steel with excellent low-temperature strength and toughness as described in this application, before step S1, the method further includes S0: preparing raw materials according to the composition of the warm-rolled medium-manganese steel, adding the raw materials sequentially into the furnace cavity of a vacuum induction melting furnace in order of increasing melting point, melting at a temperature of 1400-1500℃, casting in a mold after melting and air-cooling to room temperature to obtain the ingot.

[0006] As a preferred embodiment of the method for preparing a warm-rolled medium-manganese steel with excellent low-temperature strength and toughness as described in this application, in step S1, the homogenization annealing temperature is 1120-1150℃, the homogenization annealing time is 3-5h, and the cooling method for homogenization annealing is air cooling.

[0007] As a preferred embodiment of the method for preparing a warm-rolled medium-manganese steel with excellent low-temperature strength and toughness as described in this application, in step S2, the initial rolling temperature of the rough hot rolling is 1020-1050℃, the final rolling temperature of the rough hot rolling is 950-1000℃, the rough hot rolling is divided into 3-4 passes, and the total deformation of the rough hot rolling is 70%-75%.

[0008] As a preferred embodiment of the method for preparing a warm-rolled medium-manganese steel with excellent low-temperature strength and toughness as described in this application, in step S3, the holding temperature of the warm rolling in the ferrite region is 400-500℃, the holding time of the warm rolling in the ferrite region is 20-30min, the final rolling temperature of the warm rolling in the ferrite region is 350-450℃, the warm rolling in the ferrite region is divided into 2-3 passes, and the total deformation of the warm rolling in the ferrite region is 40%-50%.

[0009] As a preferred embodiment of the method for preparing a warm-rolled medium-manganese steel with excellent low-temperature strength and toughness as described in this application, in step S4, the holding temperature for tempering in the bainitic region is 180-250℃, and the holding time for tempering in the bainitic region is 60-150min.

[0010] As a preferred embodiment of the method for preparing a warm-rolled medium-manganese steel with excellent low-temperature strength and toughness as described in this application, in step S5, the holding temperature of the two-phase annealing is 620-680℃, and the holding time of the two-phase annealing is 30-80min.

[0011] This application also provides a warm-rolled medium-manganese steel with excellent low-temperature strength and toughness, which is prepared by the above-described method for preparing warm-rolled medium-manganese steel with excellent low-temperature strength and toughness.

[0012] As a preferred embodiment of the excellent low-temperature strength and toughness of warm-rolled medium manganese steel described in this application, the warm-rolled medium manganese steel has a yield strength of 530-620 MPa, a tensile strength of 855-980 MPa, an elongation after fracture of 20.5%-25.5%, and an impact absorption energy of 40-50 J under low-temperature deformation conditions of -40℃. The matrix structure of the warm-rolled medium manganese steel includes 62-71 vol% austenite and 29-38 vol% bainite + martensite + ferrite + precipitates, wherein the precipitates include nano-cementite and vanadium carbide particles.

[0013] The beneficial effects of this application are as follows:

[0014] This application proposes a method for preparing warm-rolled medium-manganese steel with excellent low-temperature strength and toughness. The method employs a coupled process of homogenization annealing, rough hot rolling, warm rolling in the ferrite region, tempering in the bainite region, and annealing in the two-phase region. By holding the steel in the ferrite region, the coarse austenite is accelerated to decompose into ferrite and cementite. The plastic deformation of the warm rolling further refines the ferrite and cementite, resulting in a large amount of fine-grained ferrite and nano-cementite. Holding the steel in the bainite region promotes further decomposition of the coarser austenite, refining the matrix structure and yielding fine bainite and nano-cementite particles. Annealing in the two-phase region reverses the phase transformation of the fine-grained ferrite, bainite, and nano-cementite formed during the warm rolling and tempering in the ferrite region into a large amount of fine-grained austenite, ensuring both high stability and sufficient volume fraction of austenite. This process produces warm-rolled medium-manganese steel with excellent low-temperature strength and toughness, overcoming the challenge of the mismatch between low-temperature strength and toughness in medium-manganese steel and broadening its application areas. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0016] Figure 1 This is a schematic diagram of the SEM and EBSD microstructure of the warm-rolled medium-manganese steel prepared in Example 1 of this application;

[0017] Figure 2 This is a schematic diagram of the TEM microstructure of the warm-rolled medium-manganese steel prepared in Example 1 of this application;

[0018] Figure 3 This is a schematic diagram of the SEM microstructure of the warm-rolled medium-manganese steel prepared in Comparative Example 1 of this application.

[0019] Figure 4The diagram shows the engineering stress-strain curves of the warm-rolled medium-manganese steels prepared in Examples 1-4 and Comparative Examples 1-7 of this application.

[0020] The realization of the purpose, functional features and advantages of this application will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0021] The technical solutions in the embodiments will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0022] This application provides a method for preparing warm-rolled medium-manganese steel with excellent low-temperature strength and toughness, comprising the following steps:

[0023] S1. Obtain an ingot and perform homogenization annealing on the ingot to obtain an annealed ingot;

[0024] The homogenization annealing temperature is 1120-1150℃, the homogenization annealing time is 3-5 hours, and the homogenization annealing cooling method is air cooling; specifically, the homogenization annealing temperature is any one or a range between 1120℃, 1130℃, 1140℃, and 1150℃; the homogenization annealing time is any one or a range between 3 hours, 4 hours, and 5 hours.

[0025] S2. The annealed ingot is subjected to rough hot rolling and water cooling to room temperature to obtain a rough hot rolled plate.

[0026] The initial rolling temperature of the roughing hot rolling is 1000-1050℃, the final rolling temperature of the roughing hot rolling is 950-1000℃, the roughing hot rolling is divided into 3-4 passes, and the total deformation of the roughing hot rolling is 70%-75%. Specifically, the initial rolling temperature of the roughing hot rolling is any one or any two of 1000℃, 1010℃, 1020℃, 1030℃, 1040℃, and 1050℃; the final rolling temperature of the roughing hot rolling is any one or any two of 950℃, 960℃, 970℃, 980℃, 990℃, and 1000℃; and the total deformation of the roughing hot rolling is any one or any two of 70%, 71%, 72%, 73%, 74%, and 75%.

[0027] S3. The rough hot-rolled plate is subjected to ferrite zone warm rolling and air cooling to room temperature to obtain a warm-rolled plate;

[0028] The holding temperature for the ferrite region warm rolling is 400-500℃, and the holding time is 20-30 minutes; the final rolling temperature for the ferrite region warm rolling is 350-450℃; the ferrite region warm rolling is performed in 2-3 passes, and the total deformation is 40%-50%; specifically, the holding temperature for the ferrite region warm rolling is any one or any two of 400℃, 450℃, 500℃, and 550℃; the iron... The holding time for warm rolling in the ferrite region is any one of 20 min, 25 min, or 30 min, or any two of these values; the final rolling temperature for warm rolling in the ferrite region is any one of 350℃, 400℃, or 450℃, or any two of these values; and the total deformation for warm rolling in the ferrite region is any one of 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, or 50%, or any two of these values.

[0029] S4. The warm rolled plate is tempered in the bainitic region to obtain a tempered warm rolled plate. The cooling method for the bainitic region tempering is air cooling to room temperature.

[0030] The holding temperature for tempering the bainitic region is 180-250℃, and the holding time for tempering the bainitic region is 60-150 min; specifically, the holding temperature for tempering the bainitic region is any one or a range between any two of 180℃, 190℃, 200℃, 210℃, 220℃, 230℃, 240℃, and 250℃; the holding time for tempering the bainitic region is any one or a range between any two of 60 min, 70 min, 80 min, 90 min, 100 min, 110 min, 120 min, 130 min, 140 min, and 150 min.

[0031] S5. The tempered warm-rolled plate is subjected to two-phase annealing to obtain warm-rolled medium manganese steel. The cooling method for the two-phase annealing is air cooling to room temperature.

[0032] The holding temperature for the two-phase annealing is 620-680℃, and the holding time for the two-phase annealing is 30-80 minutes; specifically, the holding temperature for the two-phase annealing is any one or any two of 620℃, 630℃, 640℃, 650℃, 660℃, 670℃, and 680℃; and the holding time for the two-phase annealing is any one or any two of 30 minutes, 40 minutes, 50 minutes, 60 minutes, 70 minutes, and 80 minutes.

[0033] The chemical composition of the warm-rolled manganese steel includes: C: 0.30-0.55wt%, Mn: 8.5-11.0wt%, Al: 2.0-3.0wt%, V: 0.4-0.6wt%, with the balance being Fe and unavoidable impurities. The warm-rolled manganese steel exhibits a yield strength of 530-620MPa, a tensile strength of 855-980MPa, an elongation after fracture of 20.5%-25.5%, and an impact energy of 40-50J under low-temperature deformation conditions of -40℃. The matrix structure of the warm-rolled manganese steel comprises: 62-71vol% austenite and 29-38vol% bainite + martensite + ferrite + precipitates, wherein the precipitates include nano-cementite and vanadium. Carbide particles; specifically, the volume percentage of austenite in the matrix of the warm-rolled manganese steel is any one or any two of 62 vol%, 63 vol%, 64 vol%, 65 vol%, 66 vol%, 67 vol%, 68 vol%, 69 vol%, 70 vol%, and 71 vol%, and the volume percentage of bainite + martensite + ferrite + precipitates in the matrix of the warm-rolled manganese steel is any one or any two of 29 vol%, 30 vol%, 31 vol%, 32 vol%, 33 vol%, 34 vol%, 35 vol%, 36 vol%, 37 vol%, and 38 vol%.

[0034] Before step S1, the process further includes S0: preparing raw materials according to the composition of the warm-rolled manganese steel, adding the raw materials sequentially into the furnace cavity of a vacuum induction melting furnace in order of increasing melting point, melting at a temperature of 1400-1500℃, casting in a mold after melting and air-cooling to room temperature to obtain the ingot; specifically, the melting temperature is any one or any two of 1400℃, 1420℃, 1440℃, 1460℃, 1480℃, and 1500℃.

[0035] The technical solution of this application will be further described below with reference to specific embodiments.

[0036] The ingots in each embodiment and comparative example were prepared by the following method: raw materials were prepared according to the composition of warm rolled manganese steel, and added to the furnace cavity of a vacuum induction melting furnace in order of increasing melting point. The melting temperature was 1400-1500℃. After melting, the ingots were cast in a mold and air-cooled to room temperature to obtain ingots.

[0037] The chemical composition of warm-rolled manganese steel is shown in Table 1. Table 1 shows the chemical composition of warm-rolled manganese steel in each example and comparative example.

[0038] Table 1

[0039]

[0040] The low-temperature tensile test conditions for the warm-rolled medium-manganese steel prepared in each embodiment and comparative example are as follows: the warm-rolled medium-manganese steel is rapidly cooled to the deformation temperature in an electronic universal tensile testing machine equipped with a cooling device, and then subjected to low-temperature tensile deformation after holding at the temperature. The rapid cooling rate is 20℃ / s, the deformation temperature is -40℃, the holding time is 15min, and the low-temperature tensile deformation rate is 0.5mm / min.

[0041] Example 1

[0042] A method for preparing warm-rolled medium-manganese steel with excellent low-temperature strength and toughness, the specific steps of which are as follows:

[0043] S1. Obtain the ingot, and perform homogenization annealing on the ingot to obtain an annealed ingot. The homogenization annealing temperature is 1130℃, the annealing time is 4h, and the cooling method is air cooling.

[0044] S2. After rough hot rolling and water cooling to room temperature, the annealed ingot is used to obtain a rough hot rolled plate. The initial rolling temperature of the rough hot rolling is 1030℃, the final rolling temperature is 980℃, the rough hot rolling is divided into 3 passes, and the total deformation is 72%.

[0045] S3. The rough hot-rolled plate is subjected to ferritic zone warm rolling and air cooling to room temperature to obtain a warm-rolled plate; the holding temperature of the ferritic zone warm rolling is 500℃, the holding time is 25min, the final rolling temperature is 450℃, the ferritic zone warm rolling is divided into 2 passes, and the total deformation is 50%;

[0046] S4. The warm rolled plate is tempered in the bainitic region to obtain a tempered warm rolled plate. The cooling method for tempering in the bainitic region is air cooling to room temperature. The holding temperature for tempering in the bainitic region is 200℃ and the holding time is 120min.

[0047] S5. The tempered warm-rolled plate is subjected to two-phase annealing to obtain warm-rolled medium manganese steel. The cooling method for two-phase annealing is air cooling to room temperature, the holding temperature for two-phase annealing is 650℃, and the holding time is 60min.

[0048] Please see Figure 1 and Figure 2 , Figure 1 These are schematic diagrams of the SEM and EBSD microstructures of the warm-rolled medium-manganese steel prepared in Example 1 of this application. Figure 2 This is a schematic diagram of the TEM microstructure of the warm-rolled medium-manganese steel prepared in Example 1 of this application. Figure 1 (a) represents the distribution of austenite, bainite + martensite + ferrite in medium manganese steel. Figure 1 (b) represents the cementite particle distribution in medium manganese steel. Figure 1 The (c) indicates the phase distribution of medium manganese steel (FCC represents austenite, BCC represents bainite + martensite + ferrite). Figure 2 This is represented by the vanadium carbide particle distribution in medium-manganese steel; based on Figure 1 Statistical analysis of the results in (c) shows that the matrix structure of the medium manganese steel prepared in Example 1 includes 68 vol% austenite and 32 vol% bainite + martensite + ferrite, as well as high-density nano-cementite and vanadium carbide particles. The medium manganese steel prepared in Example 1 has a yield strength of 600 MPa, a tensile strength of 975 MPa, an elongation after fracture of 22%, and an impact absorption energy of 45 J under low temperature deformation conditions of -40℃.

[0049] Example 2

[0050] A method for preparing warm-rolled medium-manganese steel with excellent low-temperature strength and toughness, the specific steps of which are as follows:

[0051] S1. Obtain the ingot, and perform homogenization annealing on the ingot to obtain an annealed ingot. The homogenization annealing temperature is 1150℃, the annealing time is 5h, and the cooling method is air cooling.

[0052] S2. After rough hot rolling and water cooling to room temperature, the annealed ingot is used to obtain a rough hot rolled plate. The initial rolling temperature of the rough hot rolling is 1050℃, the final rolling temperature is 1000℃, the rough hot rolling is divided into 3 passes, and the total deformation is 75%.

[0053] S3. The rough hot-rolled plate is subjected to ferritic zone warm rolling and air cooling to room temperature to obtain a warm-rolled plate; the holding temperature of the ferritic zone warm rolling is 500℃, the holding time is 20min, the final rolling temperature is 450℃, the ferritic zone warm rolling is divided into 2 passes, and the total deformation is 50%;

[0054] S4. The warm rolled plate is tempered in the bainitic region to obtain a tempered warm rolled plate. The cooling method for tempering in the bainitic region is air cooling to room temperature. The holding temperature for tempering in the bainitic region is 250℃ and the holding time is 60min.

[0055] S5. The tempered warm-rolled plate is subjected to two-phase annealing to obtain warm-rolled medium manganese steel. The cooling method for two-phase annealing is air cooling to room temperature, the holding temperature for two-phase annealing is 680℃, and the holding time is 30min.

[0056] The matrix structure of the medium manganese steel prepared in Example 2 includes 71 vol% austenite and 29 vol% bainite + martensite + ferrite, as well as high-density nano-cementite and vanadium carbide particles. The medium manganese steel prepared in Example 2 has a yield strength of 530 MPa, a tensile strength of 855 MPa, an elongation after fracture of 25.5%, and an impact absorption energy of 48 J under low temperature deformation conditions of -40℃.

[0057] Example 3

[0058] A method for preparing warm-rolled medium-manganese steel with excellent low-temperature strength and toughness, the specific steps of which are as follows:

[0059] S1. Obtain the ingot and perform homogenization annealing on the ingot to obtain an annealed ingot. The homogenization annealing temperature is 1120℃, the annealing time is 3h, and the cooling method is air cooling.

[0060] S2. After rough hot rolling and water cooling to room temperature, the annealed ingot is used to obtain a rough hot rolled plate. The initial rolling temperature of the rough hot rolling is 1020℃, the final rolling temperature is 950℃, the rough hot rolling is divided into 3 passes, and the total deformation is 70%.

[0061] S3. The rough hot-rolled plate is subjected to ferritic zone warm rolling and air cooling to room temperature to obtain a warm-rolled plate; the holding temperature of the ferritic zone warm rolling is 400℃, the holding time is 30min, the final rolling temperature is 350℃, the ferritic zone warm rolling is divided into 2 passes, and the total deformation is 40%;

[0062] S4. The warm rolled plate is tempered in the bainitic region to obtain a tempered warm rolled plate. The cooling method for tempering in the bainitic region is air cooling to room temperature. The holding temperature for tempering in the bainitic region is 180℃ and the holding time is 150min.

[0063] S5. The tempered warm-rolled plate is subjected to two-phase annealing to obtain warm-rolled medium manganese steel. The cooling method for two-phase annealing is air cooling to room temperature, the holding temperature for two-phase annealing is 620℃, and the holding time is 80min.

[0064] The matrix structure of the medium manganese steel prepared in Example 3 includes 62 vol% austenite and 38 vol% bainite + martensite + ferrite, as well as high-density nano-cementite and vanadium carbide particles. The medium manganese steel prepared in Example 3 has a yield strength of 620 MPa, a tensile strength of 935 MPa, an elongation after fracture of 20.5%, and an impact absorption energy of 40 J under low temperature deformation conditions of -40℃.

[0065] Example 4

[0066] A method for preparing warm-rolled medium-manganese steel with excellent low-temperature strength and toughness, the specific steps of which are as follows:

[0067] S1. Obtain the ingot, and perform homogenization annealing on the ingot to obtain an annealed ingot. The homogenization annealing temperature is 1150℃, the annealing time is 5h, and the cooling method is air cooling.

[0068] S2. After rough hot rolling and water cooling to room temperature, the annealed ingot is used to obtain a rough hot rolled plate. The initial rolling temperature of the rough hot rolling is 1050℃, the final rolling temperature is 1000℃, the rough hot rolling is divided into 4 passes, and the total deformation is 75%.

[0069] S3. The rough hot-rolled plate is subjected to ferrite zone warm rolling and air cooling to room temperature to obtain a warm-rolled plate; the holding temperature of the ferrite zone warm rolling is 450℃, the holding time is 20min, the final rolling temperature is 400℃, the ferrite zone warm rolling is divided into 3 passes, and the total deformation is 45%;

[0070] S4. The warm rolled plate is tempered in the bainitic region to obtain a tempered warm rolled plate. The cooling method for tempering in the bainitic region is air cooling to room temperature. The holding temperature for tempering in the bainitic region is 250℃ and the holding time is 60min.

[0071] S5. The tempered warm-rolled plate is subjected to two-phase annealing to obtain warm-rolled medium manganese steel. The cooling method for two-phase annealing is air cooling to room temperature, the holding temperature for two-phase annealing is 680℃, and the holding time is 30min.

[0072] The matrix structure of the medium manganese steel prepared in Example 4 includes 70 vol% austenite and 30 vol% bainite + martensite + ferrite, as well as high-density nano-cementite and vanadium carbide particles. The medium manganese steel prepared in Example 4 has a yield strength of 535 MPa, a tensile strength of 871 MPa, an elongation after fracture of 24%, and an impact absorption energy of 50 J under low temperature deformation conditions of -40℃.

[0073] Comparative Example 1

[0074] The difference from Example 1 is that steps S3, S4 and S5 are not performed.

[0075] Please see Figure 3 , Figure 3 This is a schematic diagram of the SEM microstructure of the warm-rolled medium-manganese steel prepared in Comparative Example 1 of this application. Figure 3 The austenite and ferrite + martensite distribution of the medium manganese steel is represented by the matrix structure of the prepared medium manganese steel, which includes 89 vol% austenite and 11 vol% ferrite + martensite. The medium manganese steel prepared in Comparative Example 1 has a yield strength of 480 MPa, a tensile strength of 660 MPa, an elongation after fracture of 13.9%, and an impact absorption energy of 32 J under low temperature deformation conditions of -40℃.

[0076] Comparative Example 2

[0077] The difference from Example 1 is that steps S4 and S5 are not performed.

[0078] The matrix structure of the medium manganese steel prepared in Comparative Example 2 consists of 26 vol% austenite and 74 vol% ferrite + martensite. The medium manganese steel prepared in Comparative Example 2 has a yield strength of 720 MPa, a tensile strength of 810 MPa, an elongation after fracture of 10.6%, and an impact absorption energy of 33 J under low temperature deformation conditions of -40℃.

[0079] Comparative Example 3

[0080] The difference from Example 1 is that step S5 is not performed.

[0081] The matrix structure of the medium manganese steel prepared in Comparative Example 3 consists of 22 vol% austenite and 78 vol% bainite + martensite + ferrite. The medium manganese steel prepared in Comparative Example 3 has a yield strength of 696 MPa, a tensile strength of 803 MPa, an elongation after fracture of 8.9%, and an impact absorption energy of 29 J under low temperature deformation conditions of -40℃.

[0082] Comparative Example 4

[0083] The difference from Example 1 is that the heat preservation temperature in step S3 is 380°C.

[0084] The matrix structure of the medium manganese steel prepared in Comparative Example 4 includes 52 vol% austenite and 48 vol% bainite + martensite + ferrite, as well as low-density nano-cementite and vanadium carbide particles. The medium manganese steel prepared in Comparative Example 4 has a yield strength of 640 MPa, a tensile strength of 840 MPa, an elongation after fracture of 17.5%, and an impact absorption energy of 30 J under low temperature deformation conditions of -40℃.

[0085] Comparative Example 5

[0086] The difference from Example 1 is that the heat preservation temperature in step S4 is 160°C.

[0087] The matrix structure of the medium manganese steel prepared in Comparative Example 5 includes 40 vol% austenite and 60 vol% ferrite + martensite, as well as low-density nano-cementite and vanadium carbide particles. The medium manganese steel prepared in Comparative Example 5 has a yield strength of 651 MPa, a tensile strength of 810 MPa, an elongation after fracture of 15.5%, and an impact absorption energy of 29 J under low temperature deformation conditions of -40℃.

[0088] Comparative Example 6

[0089] The difference from Example 1 is that the heat preservation temperature in step S5 is 700°C.

[0090] The matrix structure of the medium-manganese steel prepared in Comparative Example 6 includes 68 vol% austenite and 32 vol% bainite + martensite + ferrite, as well as low-density nano-cementite and vanadium carbide particles. The medium-manganese steel prepared in Comparative Example 6 has a yield strength of 540 MPa, a tensile strength of 841 MPa, an elongation after fracture of 27.5%, and an impact energy of 44 J under low-temperature deformation conditions of -40℃.

[0091] Comparative Example 7

[0092] The difference from Example 1 is that the heat preservation temperature in step S5 is 600°C.

[0093] The matrix structure of the medium-manganese steel prepared in Comparative Example 7 includes 56 vol% austenite and 44 vol% bainite + martensite + ferrite, as well as low-density nano-cementite and vanadium carbide particles. The medium-manganese steel prepared in Comparative Example 7 has a yield strength of 554 MPa, a tensile strength of 816 MPa, an elongation after fracture of 19.5%, and an impact absorption energy of 36 J under low temperature deformation conditions of -40℃.

[0094] Please see Figure 4 , Figure 4 The following are engineering stress-strain curves of the warm-rolled medium-manganese steels prepared in Examples 1-4 and Comparative Examples 1-7 of this application; Figure 4 (a) shows the engineering stress-strain curves of the warm-rolled medium-manganese steels prepared in Examples 1-4. Figure 4 (b) The engineering stress-strain curves of the warm-rolled medium manganese steel prepared in Comparative Examples 1-7 are shown in Table 2. The statistical results of the mechanical properties of the warm-rolled medium manganese steel prepared in Examples 1-4 and Comparative Examples 1-7 are shown in Table 2.

[0095] Table 2

[0096]

[0097] As can be seen from the above embodiments and comparative examples: Embodiment 1, combined with Comparative Example 1, shows that rough hot rolling alone leads to a lower dislocation density and a larger grain size, reducing the yield strength. Large-grained austenite has low mechanical stability and undergoes phase transformation at the beginning of tensile deformation, resulting in poor plasticity and impact toughness. Embodiment 1, combined with Comparative Example 2, shows that although the rolled plate after rough hot rolling and warm rolling introduces a higher density of dislocations while refining the grains, increasing the yield strength, the limited martensitic transformation effect provided by the small amount of austenite during tensile deformation results in poor plasticity and impact toughness. Embodiment 1, combined with Comparative Example 3, shows that tempering in the bainitic region without annealing in the two-phase region can decompose the ferrite in the rolled plate into fine bainite, while also forming a small amount of tempered martensite. In the subsequent deformation stage, the limited martensitic transformation effect provided by the small amount of austenite during tensile deformation results in poor plasticity and impact toughness. Continuous heat treatment also reduces the yield strength. Strength; Example 1 combined with Comparative Example 4 shows that a lower holding temperature for warm rolling is difficult to promote ferrite phase transformation, affecting the final grain refinement effect and austenite volume fraction, thus affecting the mechanical properties of the rolled plate; Example 1 combined with Comparative Example 5 shows that a lower holding temperature for bainite tempering leads to austenite phase transformation into martensite, reducing the plasticity and impact toughness of the rolled plate; Example 1 combined with Comparative Example 6 shows that a higher holding temperature for two-phase annealing can accelerate the austenite reverse phase transformation kinetics, and the higher volume fraction of austenite formed is beneficial to the improvement of plasticity and impact toughness, but excessively high holding temperatures accelerate dynamic recovery, reduce dislocation density and increase grain size, and reduce yield strength; Example 1 combined with Comparative Example 7 shows that a lower holding temperature for two-phase annealing is difficult to drive carbon to fully distribute into austenite, reducing austenite stability, reducing plasticity, impact toughness and tensile strength, and continuous heat treatment also accelerates recovery, reduces dislocation density, and leads to a worse yield strength. Based on the above embodiments and comparative examples, this application proposes a warm-rolled medium-manganese steel with excellent low-temperature strength and toughness and its preparation method. The method employs a coupled process of homogenization annealing, rough hot rolling, warm rolling in the ferrite region, tempering in the bainite region, and annealing in the two-phase region. The prepared warm-rolled medium-manganese steel exhibits a yield strength of 530-620 MPa, a tensile strength of 855-980 MPa, an elongation after fracture of 20.5%-25.5%, and an impact energy of 40-50 J at -40℃. This overcomes the problem of the mismatch between the low-temperature strength and toughness of medium-manganese steel and broadens its application fields.

[0098] This application proposes a method for preparing warm-rolled medium-manganese steel with excellent low-temperature strength and toughness. The method employs a coupled process of homogenization annealing, rough hot rolling, warm rolling in the ferrite region, tempering in the bainite region, and annealing in the two-phase region. By holding the steel in the ferrite region, the coarse austenite is accelerated to decompose into ferrite and cementite. The plastic deformation of the warm rolling further refines the ferrite and cementite, resulting in a large amount of fine-grained ferrite and nano-cementite. Holding the steel in the bainite region promotes further decomposition of the coarser austenite, refining the matrix structure and yielding fine bainite and nano-cementite particles. Annealing in the two-phase region reverses the phase transformation of the fine-grained ferrite, bainite, and nano-cementite formed during the warm rolling and tempering in the ferrite region into a large amount of fine-grained austenite, ensuring both high stability and sufficient volume fraction of austenite. This process produces warm-rolled medium-manganese steel with excellent low-temperature strength and toughness, overcoming the challenge of the mismatch between low-temperature strength and toughness in medium-manganese steel and broadening its application areas.

[0099] The above description is only a preferred embodiment of this application and does not limit the patent scope of this application. All equivalent structural transformations made using the content of this application's specification under the inventive concept of this application, or direct / indirect applications in other related technical fields, are included within the patent protection scope of this application.

Claims

1. A method for producing a warm-rolled medium-manganese steel having excellent low-temperature toughness, characterized by, The method comprises the following steps: S1, obtaining an ingot, homogenizing the ingot to obtain an annealed ingot; S2, rough hot rolling the annealed ingot and water cooling to room temperature to obtain a rough hot rolled plate; S3, ferrite zone warm rolling the rough hot rolled plate and air cooling to room temperature to obtain a warm rolled plate; S4, bainite zone tempering the warm rolled plate to obtain a tempered warm rolled plate, the cooling mode of the bainite zone tempering being air cooling to room temperature; S5, dual-phase zone annealing the tempered warm rolled plate to obtain a warm rolled medium manganese steel, the cooling mode of the dual-phase zone annealing being air cooling to room temperature; In the step S3, the holding temperature of the ferrite zone warm rolling is 400-500℃, the holding time of the ferrite zone warm rolling is 20-30min, the final rolling temperature of the ferrite zone warm rolling is 350-450℃, the ferrite zone warm rolling is divided into 2-3 passes, and the total deformation of the ferrite zone warm rolling is 40%-50%; In the step S4, the holding temperature of the bainite zone tempering is 180-250℃, and the holding time of the bainite zone tempering is 60-150min; The yield strength of the warm rolled medium manganese steel under the condition of low temperature deformation at-40℃ is 530-620MPa, the tensile strength is 855-980MPa, the elongation after fracture is 20.5%-25.5%, the impact energy is 40-50J, the matrix structure of the warm rolled medium manganese steel comprises 62-71vol% of austenite and 29-38vol% of bainite+martensite+ferrite+precipitated phase, and the precipitated phase comprises nanometer cementite and vanadium carbide particles.

2. The method of producing a warm-rolled medium-manganese steel having excellent low-temperature toughness according to claim 1, characterized by, In the step S5, the chemical composition of the warm rolled medium manganese steel comprises C: 0.30-0.55wt%, Mn: 8.5-11.0wt%, Al: 2.0-3.0wt%, V: 0.4-0.6wt%, and the balance being Fe and inevitable impurities.

3. The method of producing a warm-rolled medium-manganese steel having excellent low-temperature toughness according to claim 1, characterized by, Before the step S1, the method further comprises S0, preparing raw materials according to the composition of the warm rolled medium manganese steel, adding the raw materials to the furnace cavity of a vacuum induction melting furnace in the order from low to high melting point, the melting temperature being 1400-1500℃, and pouring the raw materials in a mold after melting to obtain the ingot and air cooling to room temperature.

4. The method of producing a warm-rolled medium-manganese steel having excellent low-temperature toughness according to claim 1, characterized by, In the step S1, the homogenizing annealing temperature is 1120-1150℃, the homogenizing annealing time is 3-5h, and the cooling mode of the homogenizing annealing is air cooling.

5. The method of producing a warm-rolled medium-manganese steel having excellent low-temperature toughness according to claim 1, characterized by, In the step S2, the initial rolling temperature of the rough hot rolling is 1020-1050℃, the final rolling temperature of the rough hot rolling is 950-1000℃, the rough hot rolling is divided into 3-4 passes, and the total deformation of the rough hot rolling is 70%-75%.

6. The method of producing a warm-rolled medium-manganese steel having excellent low-temperature toughness according to claim 1, characterized by, In the step S5, the holding temperature of the dual-phase zone annealing is 620-680℃, and the holding time of the dual-phase zone annealing is 30-80min.

7. A warm rolled medium manganese steel with superior low temperature toughness, characterized in that, The warm rolled medium manganese steel prepared by the method has excellent low temperature strength and toughness.

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