Medium manganese steel with low yield ratio and high tensile strength and preparation method thereof

Abstract

The invention provides medium manganese steel with low yield ratio and high tensile strength. The medium manganese steel comprises the following chemical components in percentage by mass: 0.32%-0.36% of C, 6.8%-7.2% of Mn, 0.4%-0.8% of Si, 0.4%-0.6% of Al, 0.18%-0.22% of V, 0.003%-0.004% of B and the balance of Fe and inevitable impurities. The invention further provides a preparation method of the low-yield-ratio and high-tensile-strength medium manganese steel, the preparation method comprises the four steps of ingot casting through vacuum melting, diffusion annealing treatment, forging and two-phase region annealing treatment, the low-yield-ratio and high-tensile-strength medium manganese steel is obtained through medium manganese steel alloy component design and a proper hot working technology, the yield ratio of the low-yield-ratio and high-tensile-strength medium manganese steel is 0.31-0.33, and the tensile strength of the low-yield-ratio and high-tensile-strength medium manganese steel can be greatly improved. The yield strength is 550-600 MPa, the tensile strength is 1650-1900 MPa, and the prepared medium manganese steel further has the advantages of being large in percentage elongation after fracture, low in alloying cost and easy to prepare.

Description

Technical Field

[0001] The invention belongs to the technical field of medium manganese steel, and specifically relates to a medium manganese steel with low yield ratio and high tensile strength and a preparation method thereof. Background Art

[0002] Medium manganese steel is an alloy steel containing an appropriate amount of C and Mn, usually containing 0.3%-0.6% C and 3%-12% Mn (mass percentage), and is often used to manufacture parts with high strength and high toughness requirements. At present, most medium manganese steels are alloyed with more precious metal elements (such as Cr and Ni) and more Si and / or Al elements to obtain high strength (yield strength and tensile strength). However, precious metal elements increase production costs, more Al is prone to high-temperature ferrite phase, which reduces its hot working performance, and more Si reduces the precipitation strengthening effect of carbides. Appropriate heat treatment process is also the key to ensure the excellent performance of medium manganese steel. In terms of heat treatment process, medium manganese steel often uses quenching-partitioning treatment to adjust the organizational structure and properties of steel. Quenching can make steel obtain high hardness and strength, while partitioning helps to improve the toughness and plasticity of steel. In addition, at present, in order to obtain high strength, medium manganese steel mostly uses larger deformation, which is difficult to achieve in industrial production.

[0003] When designing the composition and heat treatment process of medium manganese steel, in order to obtain high tensile strength, its yield strength is often also high, that is, it has a higher yield strength ratio (ratio of yield strength to tensile strength), which is applicable to the weight reduction requirements of parts in the automotive industry. For some special working condition parts, medium manganese steel is required to have a smaller initial deformation resistance and a higher deformation resistance after deformation, that is, a lower yield strength and a higher tensile strength. For example, the expansion tube of the oil and gas industry requires a lower yield strength and a higher tensile strength, that is, a lower yield strength ratio. The lower yield strength makes the expansion tube easy to expand and deform, which brings convenience to the expansion construction of the expansion tube, and the higher tensile strength makes the ability to resist internal and external pressure after expansion enhanced. However, in order to meet the low yield strength, the prior art often reduces the tensile strength, and the yield strength ratio is still high. CN115198191A discloses a method for preparing medium manganese steel with a low yield strength ratio and low yield extension, with a minimum yield strength ratio of 0.60, and a tensile strength of 1120MPa under the yield strength ratio, which is relatively low in tensile strength. CN115323252A discloses an ultra-high strength and high plasticity medium manganese steel and a preparation method thereof, wherein the yield ratio is 0.73, and the tensile strength under the yield ratio is 1216MPa, which is relatively low in tensile strength. CN117551937A discloses a high-strength and plasticity Fe-Mn-Al-Nb medium manganese steel and a preparation method thereof, wherein the minimum yield ratio is 0.40, and the tensile strength under the yield ratio is 1214MPa, which is relatively low in tensile strength. CN116770185A discloses a thin-gauge cold-rolled high-strength and plasticity medium manganese steel plate for automobiles and a method for manufacturing the same, wherein the minimum yield ratio is 0.59, and the tensile strength is relatively low. The yield strength and tensile strength of the medium manganese steel provided in the document Ductile 2-GPa steels with hierarchical substructure, Science, 2023, 379: 168-173 are both relatively high, the yield ratio is 0.53, and the rolling deformation during the alloy preparation process is extremely large. In general, the current research and development of medium manganese steel lacks specific steel grades that have both low yield strength and high tensile strength as well as excellent plasticity. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a medium manganese steel with low yield strength ratio and high tensile strength and a preparation method thereof in view of the deficiencies of the above-mentioned prior art, and to optimize the organizational structure of the medium manganese steel by designing the alloy composition and selecting an appropriate heat treatment process, so as to achieve the purpose of having both low yield strength and high tensile strength as well as excellent plasticity.

[0005] The technical solution adopted by the present invention is:

[0006] A medium manganese steel with low yield ratio and high tensile strength, wherein the chemical composition mass percentage of the medium manganese steel is: 0.32%-0.36% C, 6.8%-7.2% Mn, 0.4%-0.8% Si, 0.4%-0.6% Al, 0.18%-0.22% V, 0.003%-0.004% B, and the balance is Fe and unavoidable impurities; the low yield ratio means that the yield ratio is not higher than 0.35, and the high tensile strength means that the tensile strength is not lower than 1600MPa.

[0007] Preferably, the low yield strength ratio refers to a yield strength ratio of 0.31 to 0.33, and the high tensile strength refers to a tensile strength of 1650 to 1900 MPa.

[0008] One of the medium manganese steels has the following chemical components by mass percentage: 0.35% C, 7.00% Mn, 0.5% Si, 0.5% Al, 0.20% V, 0.0035% B, and the remainder is Fe and unavoidable impurities.

[0009] One of the medium manganese steels has a chemical composition percentage by mass of: 0.32% C, 6.8% Mn, 0.4% Si, 0.4% Al, 0.18% V, 0.003% B, and the remainder is Fe and unavoidable impurities.

[0010] One of the medium manganese steels has a chemical composition percentage by mass of: 0.36% C, 7.2% Mn, 0.8% Si, 0.6% Al, 0.22% V, 0.004% B, and the remainder is Fe and unavoidable impurities.

[0011] The present invention also provides a method for preparing the medium manganese steel with low yield ratio and high tensile strength, which comprises the following steps:

[0012] S1. The alloy raw materials are proportioned according to the mass percentage of the chemical components in the medium manganese steel, and then vacuum smelted to form alloy ingots;

[0013] S2, subjecting the alloy ingot obtained in S1 to diffusion annealing treatment, wherein the diffusion annealing temperature is 1150-1200° C., and the diffusion annealing time is 4-6 hours, to obtain an ingot after diffusion annealing treatment;

[0014] S3, forging the ingot after diffusion annealing obtained in S2, with the initial forging temperature of 850°C, the final forging temperature of 630°C, the forging ratio of 6, and air cooling after forging to obtain a forging blank;

[0015] S4. The forging blank obtained in S3 is annealed in the two-phase region at a temperature of 650-680°C and a holding time of 30-90 minutes to obtain a medium manganese steel with a low yield ratio and high tensile strength.

[0016] Preferably, the diffusion annealing temperature in S2 is 1170° C. and the time is 5 hours.

[0017] Preferably, the annealing temperature in S4 is 670° C. and the holding time is 60 min.

[0018] The present invention has the following significant technical effects:

[0019] 1. The medium manganese steel of the present invention does not use relatively expensive Cr and Ni elements in the composition design, but adds a small amount of Al, Si, V and B elements on the basis of C and Mn for micro-alloying treatment. By accurately controlling the content of these elements, a medium manganese steel with low yield strength, high tensile strength and excellent plasticity is obtained through hot working.

[0020] 2. The present invention controls the final forging temperature to be relatively low during ingot forging. The lower final forging temperature makes it difficult for the austenite of the medium manganese steel to recrystallize and grow, thereby refining the austenite grains, and the smaller forging ratio is used, which is conducive to industrial production.

[0021] 3. The forging blank of the present invention is annealed in the two-phase region. During the heating and heat preservation process of the two-phase region annealing, the reverse transformation of austenite and the redistribution of elements occur, which increases the volume fraction of austenite and stabilizes it to room temperature. At the same time, the annealing treatment causes the structure to recover, reduces the stress in the structure and the dislocation density of martensite, and more residual austenite and martensite with low dislocation density result in lower yield strength.

[0022] 4. During the tensile deformation process, the retained austenite with different stabilities undergoes martensitic transformation, and the relaxation and transfer of local stress caused by the martensitic transformation make it show a gradually enhanced and discontinuous phase transformation induced plasticity effect during the longer strain hardening process, and the discontinuous phase transformation induced plasticity effect improves the strength and plasticity.

[0023] 5. The medium manganese steel prepared by the present invention has the advantages of low yield strength ratio, high tensile strength, large elongation after fracture, low alloying cost and simple preparation. Its yield strength ratio is 0.31-0.33, yield strength is 550-600MPa, tensile strength is 1650-1900MPa, elongation after fracture is 20.0-28.0%, strength-plasticity product is 36.78-46.20GPa%, and impact energy at -40°C is 20.0-27.2J.

[0024] The present invention is further described in detail below with reference to the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a graph showing the variation of the content of each element in the austenite of the medium manganese steel of Example 1 with temperature;

[0026] Figure 2 is a scanning electron microscope photograph of the microstructure of the medium manganese steel after two-phase region annealing in Example 1, where Lath-M represents lath martensite and RA represents retained austenite;

[0027] Figure 3 1 is a transmission electron microscope photo of the microstructure of the medium manganese steel after two-phase region annealing in Example 1, wherein (a) is a bright field image, (b) is a dark field image, RA represents retained austenite, and V-carbide represents carbide containing V element;

[0028] Figure 4 This is the phase distribution diagram of the medium manganese steel structure after two-phase annealing in Example 1, where the red color represents the martensite phase and the green color represents the retained austenite phase;

[0029] Figure 5 It is the engineering stress-strain curve of the medium manganese steel tensile specimen after two-phase region annealing in Example 1. DETAILED DESCRIPTION

[0030] Example 1

[0031] This embodiment is a medium manganese steel with low yield ratio and high tensile strength, and its chemical composition by mass percentage is: 0.35% C, 7.0% Mn, 0.5% Si, 0.5% Al, 0.20% V, 0.0035% B, and the remainder is Fe and unavoidable impurities.

[0032] The method for preparing medium manganese steel with low yield ratio and high tensile strength in this embodiment comprises the following steps:

[0033] S1. The alloy raw materials are proportioned according to the mass percentage of the chemical components in the medium manganese steel, and smelted in a vacuum induction furnace to obtain a 50 kg ingot with a diameter of 250 mm;

[0034] S2, cutting off the riser of the alloy ingot obtained in S1, and performing homogenization diffusion annealing treatment at a temperature of 1200° C. for 5 h to obtain an ingot after diffusion annealing treatment;

[0035] S3, heating the diffusion annealed ingot obtained in S2 to 850°C, keeping the temperature for 2h, and then forging and drawing it into a bar with a cross section of about 90mm×90mm, with the initial forging temperature of 850°C, the final forging temperature of 630°C, the forging ratio (ratio of the cross-sectional area of ​​the ingot to that of the forging blank) of 6, and air cooling after forging to obtain a forging blank;

[0036] S4. Cut a long bar with a cross section of 25 mm × 25 mm from the forging blank obtained in S3 and perform annealing in the two-phase region, i.e., the two-phase region of ferrite and austenite is in A. c1 and A c3 In the temperature range, A is calculated c1 and A c3They are approximately 470℃ and 700℃ respectively. The annealing temperature is selected to be 680℃ and the holding time is 30min to obtain medium manganese steel.

[0037] The medium manganese steel obtained in this embodiment was processed into a standard tensile specimen (5 times the specimen) with a gauge length of Φ5mm×25mm for tensile test and a 10mm×10mm×55mm "V" notch impact specimen for impact test. After testing, the yield strength of the medium manganese steel obtained in this embodiment is 566MPa, the tensile strength is 1700MPa, the elongation after fracture is 23.9%, the strength-ductility product is 40.63GPa%, the impact energy at -40℃ is 25.5J, and the yield strength ratio is 0.33.

[0038] The changes of the content of each element in austenite with temperature during the preparation process of manganese steel in this example are shown in Figure 1 .

[0039] The microstructure characterization of medium manganese steel after two-phase annealing is shown in Figure 2-4 . Figure 2 This is a scanning electron microscope photo of the microstructure of medium manganese steel after two-phase annealing. Lath-M represents lath martensite, and RA represents retained austenite. Figure 3 The transmission electron microscope photos of the microstructure of the medium manganese steel after two-phase annealing, where (a) is a bright field image, (b) is a dark field image, RA represents retained austenite, and V-carbide represents carbide containing V element; Figure 4 This is the phase distribution diagram of the medium manganese steel after two-phase annealing. The red phase is martensite and the green phase is retained austenite. Studies have shown that after two-phase annealing, the medium manganese steel undergoes a reverse transformation of austenite. XRD tests show that the volume fraction of retained austenite is 23%. Austenite grows along the original martensite laths or bundles and forms lath structures, which can obtain an alternating arrangement of retained austenite / lath martensite microstructure (see Figure 3). Under the conditions of two-phase annealing temperature of 680℃ and holding time of 30min, the partial dissolution of cementite caused the enrichment of Mn and C at the boundary of martensite lath (bundle), and the reverse transformation austenite grew along the martensite lath (bundle) near the cementite particles. These structural changes are closely related to the performance of medium manganese steel: the orderly arrangement of martensite lath bundles and austenite between laths in the organization improves plastic toughness and elongation after fracture, and its elongation after fracture reaches 23.9%; at the same time, the recovery inside martensite reduces the dislocation density, and the yield strength is mainly related to the amount of deformable phase and the composition. The tensile strength is mainly related to the stability of the retained austenite and the deformation-induced martensitic transformation during the tensile deformation. The stable retained austenite is maintained until the two-phase annealing, and the retained austenite undergoes martensitic transformation under the induction of stress, which improves the tensile strength and makes its tensile strength reach 1700MPa. The elongation after fracture is mainly related to the transformation amount of the retained austenite during the tensile deformation process. The transformation amount of the retained austenite after stretching is 69%.

[0040] Figure 5 It is the engineering stress-strain curve of the medium manganese steel tensile specimen after two-phase annealing in Example 1. This sawtooth-like tensile curve is due to the discontinuous transformation-induced plasticity (TRIP) effect exhibited by the specimen during the room temperature tensile process. The AB segment is the accumulation stage of stress and strain, in which the soft phase structure undergoes plastic deformation, and the RA with poor stability undergoes the TRIP effect; when reaching point B, stress relaxation occurs, resulting in a decrease in stress, as shown in the BC segment; due to the transformation of RA into martensite, volume expansion occurs, and adjacent grains are compressed to cause deformation. These two effects cause the specimen to produce work hardening, and the stress is slightly increased, as shown in the CD segment; martensite causes the stress to expand to the surrounding area and continue to transfer to the adjacent RA. As the stress and strain continue to increase, when a larger critical value is reached, the more stable RA is triggered to undergo the TRIP effect. The curve shows that during the tensile deformation process, the retained austenite with different stabilities undergoes martensitic transformation, and the relaxation and transfer of local stress caused by the martensitic transformation make it show a gradually enhanced and discontinuous phase transformation induced plasticity effect during the longer strain hardening process. The discontinuous phase transformation induced plasticity effect improves both strength and plasticity.

[0041] Example 2

[0042] This embodiment is a medium manganese steel with low yield ratio and high tensile strength, and its chemical composition by mass percentage is: 0.35% C, 7.0% Mn, 0.5% Si, 0.5% Al, 0.20% V, 0.0035% B, and the balance is Fe and unavoidable impurities. The preparation method comprises the following steps:

[0043] S1. The alloy raw materials are proportioned according to the mass percentage of the chemical components in the medium manganese steel, and smelted in a vacuum induction furnace to obtain a 50 kg ingot with a diameter of 250 mm;

[0044] S2, cutting off the riser of the alloy ingot obtained in S1, and performing homogenization diffusion annealing treatment at a temperature of 1150° C. for 4 hours to obtain an ingot after diffusion annealing treatment;

[0045] S3, heating the diffusion annealed ingot obtained in S2 to 850°C, keeping the temperature for 2 hours, and then forging and drawing it into a bar with a cross section of about 90mm×90mm, with the initial forging temperature of 850°C, the final forging temperature of 630°C, the forging ratio of 6, and air cooling after forging to obtain a forging blank;

[0046] S4. Long bars with a cross section of 25 mm×25 mm are cut from the forging blank obtained in S3, and subjected to two-phase zone annealing treatment at a temperature of 680° C. for a holding time of 90 min to obtain medium manganese steel.

[0047] Example 3

[0048] This embodiment is a medium manganese steel with low yield ratio and high tensile strength, and its chemical composition by mass percentage is: 0.35% C, 7.0% Mn, 0.5% Si, 0.5% Al, 0.20% V, 0.0035% B, and the balance is Fe and unavoidable impurities. The preparation method comprises the following steps:

[0049] S1. The alloy raw materials are proportioned according to the mass percentage of the chemical components in the medium manganese steel, and smelted in a vacuum induction furnace to obtain a 50 kg ingot with a diameter of 250 mm;

[0050] S2, cutting off the riser of the alloy ingot obtained in S1, and performing homogenization diffusion annealing treatment at a temperature of 1200° C. for 6 hours to obtain an ingot after diffusion annealing treatment;

[0051] S3, heating the diffusion annealed ingot obtained in S2 to 850°C, keeping the temperature for 2 hours, and then forging and drawing it into a bar with a cross section of about 90mm×90mm, with the initial forging temperature of 850°C, the final forging temperature of 630°C, the forging ratio of 6, and air cooling after forging to obtain a forging blank;

[0052] S4. Cut a long bar with a cross section of 25 mm×25 mm from the forging billet obtained in S3, and perform two-phase zone annealing treatment at a temperature of 650° C. for a holding time of 60 min to obtain medium manganese steel.

[0053] Example 4

[0054] This embodiment is a medium manganese steel with low yield ratio and high tensile strength, and its chemical composition by mass percentage is: 0.32% C, 6.8% Mn, 0.4% Si, 0.4% Al, 0.18% V, 0.003% B, and the balance is Fe and unavoidable impurities. The preparation method comprises the following steps:

[0055] S1. The alloy raw materials are proportioned according to the mass percentage of the chemical components in the medium manganese steel, and smelted in a vacuum induction furnace to obtain a 50 kg ingot with a diameter of 250 mm;

[0056] S2, cutting off the riser of the alloy ingot obtained in S1, and performing homogenization diffusion annealing treatment at a temperature of 1150° C. for 4 hours to obtain an ingot after diffusion annealing treatment;

[0057] S3, heating the diffusion annealed ingot obtained in S2 to 850°C, keeping the temperature for 2 hours, and then forging and drawing it into a bar with a cross section of about 90mm×90mm, with the initial forging temperature of 850°C, the final forging temperature of 630°C, the forging ratio of 6, and air cooling after forging to obtain a forging blank;

[0058] S4. Cut a long bar with a cross section of 25 mm×25 mm from the forging billet obtained in S3, and perform two-phase zone annealing treatment at a temperature of 680° C. for a holding time of 30 min to obtain medium manganese steel.

[0059] Example 5

[0060] This embodiment is a medium manganese steel with low yield ratio and high tensile strength, and its chemical composition by mass percentage is: 0.36% C, 7.2% Mn, 0.8% Si, 0.6% Al, 0.22% V, 0.004% B, and the balance is Fe and unavoidable impurities. The preparation method comprises the following steps:

[0061] S1. The alloy raw materials are proportioned according to the mass percentage of the chemical components in the medium manganese steel, and smelted in a vacuum induction furnace to obtain a 50 kg ingot with a diameter of 250 mm;

[0062] S2, cutting off the riser of the alloy ingot obtained in S1, and performing homogenization diffusion annealing treatment at a temperature of 1170° C. for 5 hours to obtain an ingot after diffusion annealing treatment;

[0063] S3, heating the diffusion annealed ingot obtained in S2 to 850°C, keeping the temperature for 2 hours, and then forging and drawing it into a bar with a cross section of about 90mm×90mm, with the initial forging temperature of 850°C, the final forging temperature of 630°C, the forging ratio of 6, and air cooling after forging to obtain a forging blank;

[0064] S4. Long bars with a cross section of 25 mm×25 mm are cut from the forging billet obtained in S3, and subjected to two-phase zone annealing treatment at a temperature of 670° C. for a holding time of 60 min to obtain medium manganese steel.

[0065] The performance test results of the medium manganese steel prepared in Examples 1 to 5 are shown in Table 1:

[0066] Table 1 Performance test results of medium manganese steel prepared in different embodiments

[0067] performance Example 1 Example 2 Example 3 Example 4 Example 5 Yield strength / MPa 566 570 585 550 590 Tensile strength / MPa 1700 1710 1760 1650 1900 Elongation after break / % 23.9 22.1 20.9 28.0 20.0 Strength and plasticity product / GPa% 40.63 37.79 36.78 46.20 38.00 Impact energy at -40℃ / J 25.5 27.2 23.4 20.0 26.3 Yield-to-strength ratio 0.33 0.33 0.33 0.33 0.31

[0068] The above research shows that through the appropriate design of medium manganese steel alloy composition and the appropriate selection of hot working process, the low yield strength ratio and high tensile strength of medium manganese steel can be achieved. In the design of medium manganese steel composition, the C content is selected at 0.32%-0.36%. If the C content is too high, it will inevitably lead to high yield strength and tensile strength, and the yield strength ratio may not be reduced; while the lower C content has lower tensile strength. The Mn content is selected at 6.8%-7.2%. The addition of an appropriate amount of Mn is mainly based on two considerations. One is to use Mn to improve hardenability and phase transformation induced plasticity effect, and the other is that too much Mn will make steel difficult to cold and hot process. The Si content is selected at 0.4%-0.8%. Adding an appropriate amount of Si element can ensure a higher C content in the retained austenite, but more Si reduces the precipitation strengthening effect of carbides during annealing, thereby reducing the tensile strength. The Al content is selected to be 0.4%-0.6%. The addition of Al widens the temperature range between the two-phase region, is easy to form a certain amount of retained austenite, and the expansion of the two-phase region is easy to realize the annealing process, but more Al is easy to appear high-temperature ferrite phase, which makes the sample crack during forging. V can improve the strength of the sample by precipitation strengthening, but too much V makes the austenite carbon content of the reverse transformation during annealing lower, reducing the tensile strength. Adding a small amount of B element will quickly segregate and occupy the original austenite grain boundary, prevent the segregation of Mn and increase the bonding strength of the original austenite grain boundary, if excessive addition of B will reduce the bonding strength of the original austenite grain boundary. Although the addition amount of Si, Al, V, and B in the alloy is not much, they work together, and the alloying effect is better. By accurately controlling the content of these elements in the alloy, the required mechanical properties can be obtained through hot working.

[0069] During ingot forging, the final forging temperature is controlled to be relatively low. A relatively low final forging temperature makes it difficult for the austenite of the medium manganese steel to recrystallize and grow, thereby refining the austenite grains. However, the final forging temperature cannot be too low, as a too low final forging temperature makes the alloy prone to cracking during deformation. The relatively small forging ratio adopted by the present invention is conducive to industrial production.

[0070] The forging blank is annealed in the two-phase region. During the heating and holding process of the two-phase region annealing, the reverse transformation of austenite and the redistribution of elements occur, which increases the volume fraction of austenite and stabilizes it to room temperature. At the same time, the annealing treatment causes the organization to recover, reducing the stress in the organization and the dislocation density of martensite. More retained austenite and martensite with low dislocation density result in lower yield strength. During the tensile deformation process, the retained austenite with different stabilities undergoes martensitic transformation, and the relaxation and transfer of local stress caused by martensitic transformation make it show a gradually enhanced and discontinuous phase transformation induced plasticity effect during the long strain hardening process, and the discontinuous phase transformation induced plasticity effect improves strength and plasticity.

[0071] The present invention develops a medium manganese steel with low yield ratio and high tensile strength by designing alloy components and adopting appropriate hot working technology. The medium manganese steel has a yield ratio of 0.31-0.33, a yield strength of 550-600 MPa, a tensile strength of 1650-1900 MPa, an elongation after fracture of 20.0-28.0%, a strength-ductility product of 36.78-46.20 GPa%, and an impact energy of 20.0-27.2 J at -40°C.

[0072] The above is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent change made to the above embodiment according to the technical essence of the invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. A medium manganese steel with low yield ratio and high tensile strength, characterized in that: The chemical composition mass percentage of the medium manganese steel is: 0.32%-0.36% C, 6.8%-7.2% Mn, 0.4%-0.8% Si, 0.4%-0.6% Al, 0.18%-0.22% V, 0.003%-0.004% B, and the remainder is Fe and unavoidable impurities; the low yield ratio means that the yield ratio is not higher than 0.35, and the high tensile strength means that the tensile strength is not less than 1600MPa.

2. The medium manganese steel with low yield ratio and high tensile strength according to claim 1, characterized in that: The low yield strength ratio refers to a yield strength ratio of 0.31 to 0.33, and the high tensile strength refers to a tensile strength of 1650 to 1900 MPa.

3. The medium manganese steel with low yield ratio and high tensile strength according to claim 1, characterized in that: The chemical composition mass percentage of the medium manganese steel is: 0.35% C, 7.00% Mn, 0.5% Si, 0.5% Al, 0.20% V, 0.0035% B, and the balance is Fe and unavoidable impurities.

4. The medium manganese steel with low yield ratio and high tensile strength according to claim 1, characterized in that: The chemical composition mass percentage of the medium manganese steel is: 0.32% C, 6.8% Mn, 0.4% Si, 0.4% Al, 0.18% V, 0.003% B, and the balance is Fe and unavoidable impurities.

5. The medium manganese steel with low yield ratio and high tensile strength according to claim 1, characterized in that: The chemical composition mass percentage of the medium manganese steel is: 0.36% C, 7.2% Mn, 0.8% Si, 0.6% Al, 0.22% V, 0.004% B, and the balance is Fe and unavoidable impurities.

6. A method for preparing medium manganese steel with low yield ratio and high tensile strength as claimed in any one of claims 1 to 5, characterized in that: The method comprises the following steps: S1. The alloy raw materials are proportioned according to the mass percentage of the chemical components in the medium manganese steel, and then vacuum smelted to form alloy ingots; S2, subjecting the alloy ingot obtained in S1 to diffusion annealing treatment, wherein the diffusion annealing temperature is 1150-1200° C., and the diffusion annealing time is 4-6 hours, to obtain an ingot after diffusion annealing treatment; S3, forging the ingot after diffusion annealing obtained in S2, with an initial forging temperature of 850°C, a final forging temperature of 630°C, a forging ratio of 6, and air cooling after forging to obtain a forging blank; S4. The forging blank obtained in S3 is annealed in the two-phase region at a temperature of 650-680°C and a holding time of 30-90 minutes to obtain a medium manganese steel with low yield ratio and high tensile strength.

7. The method according to claim 6, characterized in that The diffusion annealing temperature in S2 is 1170° C. and the time is 5 hours.

8. The method according to claim 6, characterized in that The annealing temperature in S4 is 670° C. and the holding time is 60 min.

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