High manganese steel having excellent yield strength and very low temperature impact toughness and method of manufacturing the same

Abstract

The application provides a high manganese steel with excellent yield strength and extremely low temperature impact toughness and a manufacturing method thereof, and belongs to the technical field of steel materials. The high manganese steel contains the following elements in percentage by mass: C 0.21-0.61%, Mn 22.5-28.9%, Si 0.15%-0.2%, N≤0.01%-0.03%, Cr 0-6.0%, S≤0.005%, P≤0.01% and Ti≤0.03% including, V 0.03-0.2%, Nb 0-0.3%, and the balance of Fe and inevitable impurities. The high manganese steel has nanoscale precipitates with a local stacking fault energy gradient, and the local stacking fault energy reduction caused by the nanoscale precipitates is 0.5-3 mJ / m 2 2 under the condition of extremely low temperature of-269 DEG C, which promotes TWIP effect, improves crack propagation absorption work in the process of extremely low temperature impact, and excellent extremely low temperature impact toughness is obtained. The application obtains the high manganese steel with excellent room temperature yield strength and extremely low temperature impact toughness by optimizing the alloying composition of the high manganese steel and adopting a combined process of smelting-casting-forging-heat rolling.

Description

Technical Field

[0001] This invention belongs to the field of steel materials technology, specifically relating to a high-manganese steel with excellent yield strength and extremely low-temperature impact toughness and its manufacturing method. Background Technology

[0002] Hydrogen energy, due to its high calorific value, zero emissions, and renewable characteristics, is considered the most promising clean energy source for the future. Currently, liquid hydrogen storage has outstanding potential for large-scale hydrogen energy storage and transportation due to its high hydrogen density and high transportation efficiency. However, liquid hydrogen needs to be stored stably in an ultra-low temperature environment close to -253°C for a long time. The toughness of some traditional steels decreases sharply under this environment, which places stringent requirements on the materials of storage tanks and transportation equipment.

[0003] Currently, the materials widely used in liquid hydrogen storage tanks are mainly austenitic stainless steel (such as 304L and 316L) and 9Ni steel. However, these materials have high costs, making large-scale promotion difficult. Therefore, it is necessary to develop steels that are low-cost and have good low-temperature toughness. Among them, all-austenitic high-manganese steel has attracted much attention due to its excellent low-temperature mechanical properties and significant price advantage.

[0004] Chinese patent CN110724872 A discloses an austenitic wear-resistant steel with ultra-low temperature impact toughness. Its composition (wt.%) is: C: 0.15~0.42%, Si: 0.15~0.60%, Mn: 25~33%, P: <0.045%, S: <0.03%, Cr: 2~5%, Ni: ≤3%, Mo: ≤0.5%, V: 0.30~0.50%, Cu: ≤0.5%, Al ≤2%, with the remainder being iron and unavoidable impurities. Furthermore, when the Cr content is in the range of 2~5%, the Mn and C contents must satisfy n+35C=37~40%. This steel grade can significantly improve the ultra-low temperature impact toughness of high-manganese steel at -196℃.

[0005] Chinese Patent CN 110114491 A discloses a high-manganese steel with excellent low-temperature toughness and yield strength. The high-manganese steel contains, by weight %, C: 0.3~0.6%, Mn: 20~25%, Mo: 0.01~0.3%, Al: less than 3%, and contains other unavoidable impurities and the balance Fe. The high-manganese steel is composed of austenite with a grain size of less than 50 μm and has excellent low-temperature toughness and yield strength.

[0006] Chinese patent CN 118756060 A discloses a high-strength and high-toughness high-manganese steel plate for ultra-low temperature applications. Its chemical composition and mass percentages are: C 0.04~0.10%, Mn 21.0~25.5%, Si 0.25~0.35%, P<0.008%, S<0.003%, O<0.003%, Ti0.02~0.05%, W0.01~0.05%, with the balance being Fe and unavoidable impurities. The high-manganese steel plate has a stacking fault energy of 40~45 mJ / m at -196℃. 2 The high-manganese steel plate has a yield strength >520MPa, tensile strength >900MPa, elongation after fracture >50%, and low-temperature impact toughness (AKV) >200J at -196℃.

[0007] Chinese Patent CN 118360548 A discloses a high-manganese steel square ingot for welding materials and its preparation method. The chemical composition and mass percentage of the high-manganese steel square ingot are as follows: C: 0.35-0.45%, Si: 0.95-1.05%, Mn: 24.5-26.5%, Ni: 3.5-4%, Cr: 1.6-1.8%, W: 1.9-2.2%, Nb: 0.045-0.055%, V: 0.045-0.055%, Ti: 0.045-0.055%, Mo: 0.2-0.28%, P≤0.004%, S≤0.004%, N: 0.003-0.006%, O≤0.0015%, with the balance being Fe and unavoidable impurities. The high-manganese steel square ingot has a yield strength ratio ≥0.53, elongation ≥40%, hardness ≥180HV, and ingot cross-sectional dimensions controlled between 45mm×45mm and 50mm×50mm. The metallographic structure is austenite and free of mixed crystals.

[0008] Chinese Patent CN 119663111 A discloses a welding wire steel, welding wire, and preparation method for welding high-manganese steel at ultra-low temperatures, as well as a welding method thereof. The steel comprises the following components by mass percentage: C 0.05–0.1%; Si 0.5–0.6%; Mn 24%–25%; P ≤0.01%; S ≤0.01%; Al 0.05%–0.1%; Ni 0.12–0.2%; N 0.02–0.06%; Ti 0.08%–0.15%; V 0.08%–0.15%; Nb 0.08%–0.15%; Mo 0.05%–0.1%; with the balance being Fe and unavoidable impurities. The welding wire steel meets at least one of the following characteristics: (1) tensile strength ≥ 800 MPa; (2) yield strength ≥ 435 MPa; (3) impact energy at -196℃ ≥ 68 J.

[0009] The main design concept of the aforementioned patents focuses on high-manganese steel for use at -196℃, but in-depth research has not been conducted on its performance at extremely low temperatures of -269℃. Furthermore, traditional materials for cryogenic storage tanks suffer from drawbacks such as the expensive addition of Cr and Ni elements and complex processing techniques. Summary of the Invention

[0010] To address the aforementioned technical problems, this invention proposes a high-manganese steel with excellent yield strength and extremely low-temperature impact toughness, and its manufacturing method. Through composition and process design, nanoscale precipitates with local stacking fault energy gradients are obtained, enabling the high-manganese steel plate to possess both excellent room-temperature yield strength and -269℃ extremely low-temperature impact toughness.

[0011] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0012] On one hand, the present invention provides a high-manganese steel with ultra-low temperature impact toughness, wherein the high-manganese steel comprises the following elements by mass percentage: C 0.21-0.61%, Mn 22.5-28.9%, Si 0.15-0.2%, N≤0.01%-0.03%, Cr 0-6.0%, S≤0.005%, P≤0.01% including Ti≤0.03%, V 0.03-0.2%, Nb 0-0.3%, with the balance being Fe and unavoidable impurities. The high-manganese steel has nanoscale precipitates with local stacking fault energy gradients.

[0013] Furthermore, the microstructure of the high-manganese steel mainly consists of austenite phase and nanoscale precipitates. At an extremely low temperature of -269℃, the reduction in local stacking fault energy caused by the nanoscale precipitates is between 0.5 and 3 mJ / m. 2 .

[0014] Optionally, the nanoscale precipitates have a size of 10-30 nm and are dispersed in an ellipsoidal shape.

[0015] In the aforementioned high-manganese steel, the main alloying elements, in descending order of their roles, are Mn, C, Cr, Ti, and V, as detailed below:

[0016] Manganese: Manganese is a key alloying element in high-manganese steel. It can stabilize the austenite structure, keeping the austenite stable at room temperature and even lower temperatures. The manganese content also affects the stacking fault energy of high-manganese steel; as the manganese content increases, the stacking fault energy also increases, thus affecting the deformation mechanism of high-manganese steel. In this invention, the Mn content of the high-manganese steel is controlled at 22.5% to 28.9%.

[0017] Carbon: Carbon is one of the main elements constituting high-manganese steel. Carbon can significantly improve solid solution strengthening and yield strength. With a high manganese content, an appropriate amount of carbon can work with manganese to regulate the stacking fault energy of high-manganese steel, thereby increasing its strength while maintaining high toughness. The high-manganese steel of this invention has a C content of 0.21% to 0.61%. It should be noted that the sum of the contents of Mn and C should satisfy the general formula: 28 ≤ Mn + 10C ≤ 30. Therefore, when Mn takes the upper limit, C should take the lower limit; conversely, when Mn takes the lower limit, C should take the upper limit. For example, when the mass percentage of Mn is 22.5%, the percentage of C is 0.61%; when the mass percentage of Mn is 28.9%, the percentage of C is 0.21%.

[0018] Chromium (Cr): Cr can enhance room temperature yield strength through solid solution strengthening and by influencing stacking fault energy, but its effect on extremely low temperature impact toughness is more complex. One of chromium's core roles is to regulate the stacking fault energy of austenite. Lower stacking fault energies promote the formation of planar defects such as stacking faults and twins during deformation, which can lead to a sustained high work hardening rate, indirectly contributing to strength and providing more energy dissipation pathways for crack propagation, theoretically benefiting low-temperature toughness. However, the addition of chromium can also tend to form chromium carbides. During hot rolling and subsequent cooling, improper process control can lead to the formation of coarse carbides (such as Cr). 23 C6) precipitates along grain boundaries, becoming a source of brittle cracks and severely impairing impact toughness, especially under extremely low temperature conditions. The chromium content of the high-manganese steel in this invention is controlled at 0~6.0%.

[0019] Titanium: Adding Ti is a powerful method for grain refinement and inclusion modification. The core mechanism of Ti is the preferential formation of high-melting-point TiN particles in the liquid or high-temperature austenite region. These particles effectively pin grain boundaries, strongly inhibiting austenite grain growth during hot rolling, thereby significantly improving both strength and toughness through grain refinement. Furthermore, the fine TiC particles precipitated during cooling provide precipitation strengthening, further contributing to strength improvement. Crucially, Ti can also modify harmful inclusions, such as transforming elongated MnS into spherical or composite inclusions, reducing their cutting effect on the matrix, which is essential for protecting ultra-low temperature impact toughness. However, if the addition amount is too high (e.g., exceeding 0.05%), the excess Ti will combine with C to form coarse TiC particles. These particles are prone to becoming microcrack initiations under stress, severely deteriorating impact toughness, especially ultra-low temperature toughness. Therefore, the Ti content should be strictly limited to 0.01%-0.05%, and precisely matched to the nitrogen content in the steel. The high-manganese steel of this invention has a titanium content ≤0.03%.

[0020] Vanadium: Vanadium's core role is primarily achieved through its precipitation behavior during hot rolling and subsequent cooling. On one hand, vanadium refines the austenite grains after hot rolling and produces a significant precipitation strengthening effect through the precipitation of fine V(C,N), thereby effectively improving room temperature yield strength. This strength improvement typically increases with increasing vanadium content. On the other hand, for extremely low temperature impact toughness, the key lies in controlling the morphology and distribution of vanadium carbide precipitation. During water cooling or controlled cooling after hot rolling, if the cooling is properly controlled, fine precipitates can pin grain boundaries and refine the microstructure, which is beneficial to toughness. However, if improper processing leads to coarse precipitates or excessive precipitation at grain boundaries, these can become crack initiation points, severely impairing toughness. To balance excellent room temperature yield strength and -269℃ extremely low temperature impact toughness, the vanadium content in this invention is controlled between 0.03% and 0.2%.

[0021] On the other hand, the present invention provides a method for manufacturing the high-manganese steel with excellent yield strength and extremely low-temperature impact toughness, comprising the following steps:

[0022] S1. Smelting process: Select pure metals according to the mass percentage of high manganese steel composition, perform vacuum induction melting, and cast into billets.

[0023] S2. Forging process: The billet is homogenized and then forged after being taken out of the furnace;

[0024] S3. Hot rolling process: The forging billet is reheated and then rolled in two stages after exiting the furnace. The first stage of rolling is rough rolling in the recrystallization temperature range, and the second stage of rolling is finish rolling at 100~200℃ above the final rolling temperature.

[0025] Optionally, in step S1, the content of elements affecting inclusions is controlled as follows: P, O≤0.01%, S≤0.005%, N≤0.01%, H≤0.0002, to prevent excessive inclusions from damaging the performance of the steel plate.

[0026] Optionally, in step S1, non-volatile raw materials such as pure iron and pure carbon are loaded into a crucible, vacuumed to a low vacuum, melted by electricity and heated to above 1550-1600℃; elements such as manganese and silicon are added in batches to avoid volatilization; after alloying, deoxidation treatment is performed, and the temperature is adjusted for casting.

[0027] Optionally, in step S2, the homogenization treatment temperature is 980~1100℃, and the time is 1-2 hours; subsequently, the steel is forged after removal from the furnace, and after each forging, it needs to be reheated in the furnace for 20 minutes to control the temperature between 950~1020℃. Homogenization and forging within this temperature range ensures the dense microstructure of the designed steel. If the temperature is too high, forging hot cracking may occur in high-manganese steel; if the temperature is too low, the requirements for forging equipment are too high, making large-scale production difficult. Specifically, the number of forgings is usually 5-7 times, and the forging time depends on the surface temperature of the billet. The time for each forging should be as short as possible (about 0.5-3 minutes from removal from the furnace to forging). If the temperature is below 950℃ after a single forging, it needs to be reheated in the furnace; if it is still within the required range, continuous forging can be performed.

[0028] Optionally, in step S3, the slab heating temperature is controlled between 1150 and 1230°C, the rough rolling temperature is controlled between 1120 and 980°C, the finish rolling temperature is controlled 100 to 200°C above the final rolling temperature, the final rolling temperature is controlled between 740 and 860°C, the cumulative reduction rate is 76%, and the slab is water-cooled to room temperature after rolling. Among the above parameters, the heating temperature of the hot rolling process ensures the solid solution of precipitated elements and prevents excessive growth of undissolved precipitates; the rough rolling temperature range ensures the generation of strain-induced micron-sized precipitates and inhibits grain growth; the finish rolling temperature range ensures the applied dislocation density enables rapid nucleation of nano-sized precipitates during the cooling stage.

[0029] The present invention also provides the application of the high manganese steel as a liquid hydrogen storage tank, wherein the high manganese steel as a steel plate has a yield strength ≥400MPa, an impact toughness KV2 ≥100J at -269℃, and a crack initiation absorption energy / crack propagation energy <1.

[0030] Compared with the prior art, the present invention has the following advantages:

[0031] 1. Compared with existing patents, this invention provides a reasonable ratio of C and Mn elements, while adding microalloying elements such as Ti, V, and Nb. Through microalloying treatment, the grain size of the austenite phase after hot rolling is significantly refined, alleviating the control range of the final rolling temperature of high manganese steel and reducing the difficulty in preparing high manganese steel to obtain excellent room temperature yield strength and extremely low temperature impact toughness.

[0032] 2. The stacking fault energy of high manganese steel is 18~40 mJ / m 2 Within a certain range, a deformation twinning mechanism occurs, and the lower the stacking fault energy, the stronger the twinning deformation capability. At extremely low temperatures of -269℃, the dislocation slip motion of atoms is restricted. By adding microalloying elements, nanoscale precipitates reduce the local stacking fault energy, promoting twinning deformation near the precipitates. This helps to hinder crack propagation, increase energy dissipation, and improve the ultra-low temperature impact toughness of high-manganese steel.

[0033] 3. This invention employs a combined process of smelting—diffusion annealing—high-temperature forging—temperature-controlled hot rolling. For high-manganese steel (manganese content above 22%), its as-cast microstructure suffers from severe segregation, coarse grains, and numerous internal defects, making it difficult to achieve a uniform and stable microstructure using traditional processing techniques. This invention effectively improves the as-cast segregation and coarse grain problems by subjecting the ingot to diffusion annealing at 1050℃ and then repeatedly forging it at ≥900℃, resulting in a more uniform and refined microstructure, significantly improving purity and surface quality. Furthermore, by heating and holding the ingot at 1130–1180℃ and employing a reasonable final rolling temperature, the sheet metal achieves a stable austenitic microstructure and excellent mechanical properties, while simultaneously increasing yield and production efficiency and reducing manufacturing costs. Attached Figure Description

[0034] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0035] Figure 1 This is a flowchart of the manufacturing method of high manganese steel with excellent yield strength and extremely low temperature impact toughness according to the present invention.

[0036] Figure 2 The image shows the metallographic structure of the hot-rolled product from Example 1.

[0037] Figure 3 Microstructure characterization of steel plate samples taken in Example 1 after impact at -269℃.

[0038] Figure 4 Load-deflection curves of steel plates prepared using the same process (green line) without precipitate addition (blue line) after being subjected to an impact at -269°C.

[0039] Figure 5 This is an HRTEM image of nanoscale precipitates with an atomic distribution gradient contained within twins, as shown in Example 1. Detailed Implementation

[0040] To make the technical problems, technical solutions and advantages of the present invention clearer, a detailed description will be given below in conjunction with the accompanying drawings and specific embodiments.

[0041] Figure 1 This is a flowchart illustrating the manufacturing method of high-manganese steel with excellent yield strength and extremely low-temperature impact toughness according to the present invention. Detailed explanation follows:

[0042] First, the addition amounts of microalloying elements such as Ti, V, and Nb in the high-manganese steel are limited by phase equilibrium and stacking fault energy thermodynamic calculations. Taking Ti as an example, when the addition amount is greater than 0.03% (by mass), its solution temperature has exceeded the upper limit of the homogenization temperature. During subsequent hot rolling, undissolved precipitates will coarsen, thereby impairing low-temperature impact toughness.

[0043] Secondly, the control of inclusions in the smelting process mainly refers to controlling them within the range of P, O≤0.01%, S≤0.005%, N≤0.05%, and H≤0.0002%, because excessive amounts of these elements will damage the microstructure and properties of high manganese steel plates.

[0044] The homogenization temperature during forging needs to be controlled at 980~1100℃ for 1-2 hours, and the forging temperature should be controlled between 950~1120℃. Homogenization and forging at this temperature can ensure the dense structure of the designed steel. If the temperature is too high, it will cause forging hot cracks in high manganese steel, while low temperature places too high demands on the forging equipment.

[0045] The heating temperature of the hot-rolled slab is controlled between 1150 and 1230℃, the roughing rolling temperature is controlled between 1120 and 980℃, the finishing rolling temperature is controlled between 100 and 200℃ above the final rolling temperature, and the final rolling temperature is controlled between 740 and 860℃, with a cumulative reduction rate of 76%. After rolling, the slab is water-cooled to room temperature. The heating temperature of the hot rolling process ensures the solid solution of precipitated elements and prevents excessive growth of undissolved precipitates. The roughing rolling temperature range ensures the formation of strain-induced micron-sized precipitates and inhibits grain growth. The finishing rolling temperature range needs to be controlled between 100 and 200℃ above the final rolling temperature to ensure that the applied dislocation density allows for rapid nucleation of nano-sized precipitates during the cooling stage without growth.

[0046] The steel plate characteristics refer to high-manganese steel plates manufactured through a combination of composition and process design, exhibiting excellent room-temperature yield strength and extremely low-temperature impact toughness. The room-temperature yield strength originates from dislocation, precipitation, and grain refinement strengthening influenced by microalloying design and final rolling temperature control. The extremely low-temperature impact toughness stems from the local stacking fault energy gradient present in nanoscale precipitation, which induces a stronger twinning deformation mechanism through reduced stacking fault energy, thereby increasing impact energy. Furthermore, the ratio of crack initiation absorption work to crack propagation work is < 1.

[0047] The high-manganese steel prepared by the above process exhibits excellent room temperature yield strength and -269℃ ultra-low temperature impact energy. The specific performance test results are as follows:

[0048] The room temperature yield strength test shall be conducted in accordance with GB / T 228.1-2021 Metallic materials - Tensile testing - Part 1: Room temperature test method;

[0049] The -269℃ ultra-low temperature impact energy test refers to "GB / T 229-2020 Metallic Materials Charpy Pendulum Impact Test Method" and "GB / T 19748-2019 Metallic Materials Charpy V-Notch Pendulum Impact Test Instrumented Test Method".

[0050] The present invention will be further described in detail below with reference to specific embodiments.

[0051] Example 1

[0052] A high-manganese steel with excellent yield strength and extremely low-temperature impact toughness is manufactured by the following steps:

[0053] (1) Smelting process: According to the chemical composition of the molten steel, the mass percentage of each element is: 0.41%C, 24.98%Mn, 3.34Cr, 0.021Ti, 0.18Si, 0.0035P, 0.0010S, 0.0082Al, 0.0084N, with the balance being Fe and unavoidable impurities.

[0054] Select high-purity raw materials according to the target chemical composition. First, non-volatile raw materials, such as iron and carbon, are loaded into the crucible of a vacuum induction furnace, and the furnace is evacuated to a low vacuum state. Power is supplied to heat the furnace charge, melting and superheating it to 1550-1600℃. While molten, volatile or easily oxidized elements such as manganese, chromium, and silicon are added in batches to reduce burn-off and ensure uniform composition. Afterward, trace amounts of the precipitation element Ti are added. Approximately 50 kg of molten iron is added, and after all alloying elements have been added and completely melted, the mixture is thoroughly stirred. Refining is performed under vacuum or by introducing inert gas to reduce gas content. Deoxidizers such as aluminum are added for final deoxidation to purify the molten steel. The temperature of the molten steel is adjusted to a suitable casting temperature (usually 50-100℃ above the liquidus), and poured into a preheated mold under a protective atmosphere. Solidification yields a billet. After removing risers and other defects, a bullet-shaped billet weighing approximately 35 kg is obtained.

[0055] (2) Forging process: The above-mentioned billet is placed in a heat treatment furnace at 1070℃ for homogenization treatment for 1.5h, and then taken out of the furnace for forging. The forging temperature is controlled between 950~1020℃, and a total of 6 forgings are carried out. Between each forging, the billet is returned to the furnace at 1077℃ for 20min. After the billet is forged into a 50mm square billet, it is air-cooled to room temperature.

[0056] (3) Hot rolling process: The forging billet is reheated to 1147℃ and held for 2 hours. Then it is rolled in two stages after exiting the furnace. The first stage rolling (rough rolling) temperature is in the austenite recrystallization temperature range, i.e., 1000℃~1120℃, the initial rolling temperature is 1120℃, the single reduction is 15~20%, and the cumulative reduction rate is 40%; the second stage (finish rolling) is in the austenite non-recrystallization temperature range, i.e., 760℃~980℃. Based on the fact that it is about 100-200℃ higher than the final rolling temperature, the final rolling temperature is controlled at 861℃, the initial rolling temperature is 973℃, the single reduction is 25~335%, and the cumulative reduction rate is 63%. Then it is water cooled to room temperature.

[0057] Figure 2 The image shows the metallographic structure of the product after hot rolling in step (3) of Example 1. It contains austenite, twins, and titanium nitride precipitates. The precipitates shown are micron-sized and are generated during the rough rolling stage of the hot rolling process. They indirectly inhibit grain growth and produce fine-grained strengthening.

[0058] Figure 3 Microstructure characterization of the steel plate obtained in Example 1 after impact at -269℃ was performed. The ellipse marked the region as austenite, and the nanoscale Ti(C,N) precipitates were covered by twins, proving that the localized reduction of stacking faults in the nanoscale precipitates can induce a twinning deformation mechanism.

[0059] Figure 4 The load-deflection curves are shown for samples of steel plates prepared using the same process (green line) and without precipitates (blue line) after impact at -269°C. This indicates that the crack propagation energy containing Ti nano-scale precipitates accounts for more than 50% of the total impact energy, while the high-manganese steel without precipitates exhibits high crack initiation absorption energy and low crack propagation energy, with a ratio greater than 1. This suggests that the material bears a large load before crack propagation, losing the stress release mechanisms such as TWIP occurring in local low-level stacking fault energy regions. Therefore, the crack propagation stage occurs rapidly, reducing the absorbed energy.

[0060] Figure 5 This is an HRTEM image of nanoscale precipitates with an atomic distribution gradient contained within twins in Example 1. The image reflects changes in atomic concentration based on atomic contrast. Combined with stacking fault energy calculations, the stacking fault energy is higher in the region without precipitates and lower in the region with nanoscale precipitates. The induced deformed twins contain nanoscale precipitates, and stacking faults that do not further develop into twins are shown at the twin boundaries, confirming the phenomenon of decreasing gradient stacking fault energy near nanoscale precipitates. That is, the stacking fault energy is lowest in the precipitate core (inner layer) and higher in the epitaxial (matrix) layer, forming a stacking fault energy gradient in between.

[0061] Based on the C and N element fractions consumed by the nanoscale precipitates, and combined with the formula for calculating stacking fault energy, the resulting reduction in local stacking fault energy caused by nanoscale precipitates at -269℃ in high-manganese steel plates is approximately 1.8 mJ / m. 2 .

[0062] Example 2

[0063] A high-manganese steel with excellent yield strength and extremely low-temperature impact toughness is manufactured by the following steps:

[0064] Step (1) smelting process and step (2) forging process are the same as in Example 1. The difference in step (3) hot rolling process is:

[0065] The forged billet was heated to 1155℃ and held for 2 hours. It was then subjected to a two-stage rolling process. The first stage (rough rolling) was conducted at the austenite recrystallization temperature range, i.e., 1000℃~1120℃, with a single reduction of 15~20% and a cumulative reduction of 40%. The second stage (finish rolling) was conducted at the austenite non-recrystallization temperature range, i.e., 760℃~950℃. The initial rolling temperature was set 100℃ above the final rolling temperature, i.e., 932℃, and the final rolling temperature was 807℃. The single-pass reduction was 25~335%, and the cumulative reduction was 63%. The billet was then water-cooled to room temperature.

[0066] Based on the C and N element fractions consumed by the nanoscale precipitates, and combined with the formula for calculating stacking fault energy, the resulting reduction in local stacking fault energy caused by nanoscale precipitates at -269℃ in high-manganese steel plates is approximately 2.5 mJ / m. 2 .

[0067] Example 3

[0068] A high-manganese steel with excellent yield strength and extremely low-temperature impact toughness is manufactured by the following steps:

[0069] Step (1) smelting process and step (2) forging process are the same as in Example 1. The difference in step (3) hot rolling process is:

[0070] The forged billet was reheated to 1150℃ and held for 2 hours. It was then subjected to a two-stage rolling process. The first stage (rough rolling) was conducted at the austenite recrystallization temperature range, i.e., 1000℃~1120℃, with a single reduction of 15~20% and a cumulative reduction of 40%. The second stage (finish rolling) was conducted at the austenite non-recrystallization temperature range, i.e., 760℃~950℃. The initial rolling temperature was set 150℃ above the final rolling temperature, i.e., 903℃, and the final rolling temperature was 747℃. The single-pass reduction was 25~335%, and the cumulative reduction was 63%. The billet was then water-cooled to room temperature.

[0071] Based on the C and N element fractions consumed by the nanoscale precipitates, and combined with the formula for calculating stacking fault energy, the resulting reduction in local stacking fault energy caused by nanoscale precipitates at -269℃ in high-manganese steel plates is approximately 2.9 mJ / m. 2 .

[0072] Example 4

[0073] A high-manganese steel with excellent yield strength and extremely low-temperature impact toughness is manufactured by the following steps:

[0074] Step (1) The smelting process is the same as in Example 1, except that 0.05% V is added to replace Ti.

[0075] Step (2) forging process and step (3) hot rolling process are the same as in Example 1.

[0076] Based on the C and N element fractions consumed by the nanoscale precipitates, and combined with the formula for calculating stacking fault energy, the resulting reduction in local stacking fault energy caused by nanoscale precipitates at -269℃ in high-manganese steel plates is approximately 0.5 mJ / m. 2 .

[0077] Example 5

[0078] A high-manganese steel with excellent yield strength and extremely low-temperature impact toughness is manufactured by the following steps:

[0079] Step (1) The smelting process is the same as in Example 1, except that 0.12% V is added to replace Ti.

[0080] Step (2) forging process and step (3) hot rolling process are the same as in Example 1.

[0081] Based on the C and N element fractions consumed by the nanoscale precipitates, and combined with the formula for calculating stacking fault energy, the resulting reduction in local stacking fault energy caused by nanoscale precipitates at -269℃ in high-manganese steel plates is approximately 0.7 mJ / m. 2 .

[0082] Example 6

[0083] A high-manganese steel with excellent yield strength and extremely low-temperature impact toughness is manufactured by the following steps:

[0084] Step (1) The smelting process is the same as in Example 1, except that 0.22% V is added to replace Ti.

[0085] Step (2) forging process and step (3) hot rolling process are the same as in Example 1.

[0086] Based on the C and N element fractions consumed by the nanoscale precipitates, and combined with the formula for calculating stacking fault energy, the resulting reduction in local stacking fault energy caused by nanoscale precipitates at -269℃ in high-manganese steel plates is approximately 1.1 mJ / m.2 .

[0087] Comparative Example 1

[0088] The high manganese steel with high yield strength and extremely low temperature impact toughness and its manufacturing method are similar to those in Example 1, except that the addition of Ti element is eliminated in the smelting process of step (1).

[0089] Since Ti affects the nanoscale precipitates produced in the hot rolling process of step (3), the precipitation strengthening and the local stacking fault energy gradient effect caused by the nanoscale precipitates are weakened, resulting in a decrease in the final yield strength and impact energy. At the same time, the ratio of crack initiation absorption work to crack propagation work is greater than 1, indicating that the energy to resist crack propagation also decreases.

[0090] Comparative Example 2

[0091] The high-manganese steel with high yield strength and extremely low temperature impact toughness and its manufacturing method are similar to those in Example 1, except that the final rolling temperature of the hot rolling process in step (3) is about 920°C. Because high-manganese steel will undergo high-temperature recovery and recrystallization at this temperature, the dislocation density will decrease, which will further affect the generation of Ti-containing nanoscale precipitates, resulting in a decrease in room temperature yield strength and a decrease in impact energy at -269°C.

[0092] Comparative Example 3

[0093] The high-manganese steel with high yield strength and extremely low temperature impact toughness and its manufacturing method are similar to those in Example 5, except that the final rolling temperature of the hot rolling process in step (3) is 920°C. Because high-manganese steel will undergo high-temperature recovery and recrystallization at this temperature, the dislocation density will decrease, resulting in a decrease in room temperature yield strength. At the same time, the amount of V-nano-scale precipitates will decrease, thus reducing the proportion of crack propagation energy.

[0094] Comparative Example 4

[0095] The high-manganese steel with high yield strength and extremely low temperature impact toughness and its manufacturing method are similar to those in Example 6, except that the final rolling temperature of the hot rolling process in step (3) is about 800°C. At this temperature, the high-manganese steel will generate a large number of dislocation densities, which will significantly improve the room temperature yield strength. However, a large number of precipitates will nucleate and grow, causing cracks to initiate at the precipitates and the matrix, reducing the impact energy.

[0096] The hot-rolled final rolling temperature, room temperature yield strength, and extremely low temperature impact energy values ​​of the steels manufactured in the above embodiments and comparative examples are shown in Table 1.

[0097] Table 1

[0098]

[0099] As shown in Table 1, the steels obtained by Examples 1-6 using the elemental composition range and manufacturing method of the present invention have high strength and high and low temperature toughness.

[0100] The high-manganese steel obtained in Example 1 contains trace amounts of Ti. Rolling at a relatively high final rolling temperature (861℃), i.e., deformation in the non-recrystallization zone, introduces trace dislocations into the austenite phase. Since Ti readily forms precipitates with N and C elements at high temperatures, these precipitates nucleate at dislocation sites. A water-cooling process prevents the precipitates from coarsening, resulting in a diffusely distributed nanoscale precipitate with a gradient in N and C element composition. Therefore, the stacking fault energy also exhibits a gradient characteristic. In the -269℃ impact energy absorption, the localized low stacking fault energy induces a TWIP effect near the precipitates, releasing the impact load stress. This results in a crack initiation absorption energy / crack propagation energy ratio of less than 1.

[0101] The main difference between the high-manganese steels prepared in Examples 2 and 3 lies in the different final rolling temperatures of the hot rolling process. More dislocations are introduced into the austenite phase in the non-recrystallized zone, which causes a certain degree of coarsening after the precipitates nucleate and precipitate. This is reflected in the improvement of yield strength and the reduction of impact energy.

[0102] The main difference between the high-manganese steels prepared in Examples 4, 5, and 6 lies in the amount of V added during the hot rolling process. Higher V content results in more precipitates, but also carries the risk of coarsening. Through a similar hot rolling process, the yield strength of the prepared high-manganese steel is significantly improved, while the impact energy decreases only slightly. It still maintains the ability to resist crack propagation with a crack initiation absorption energy / crack propagation energy ratio of less than 1, thus possessing high impact absorption energy.

[0103] Compared to Example 1, Comparative Example 1 did not involve the input of Ti material in the smelting process. On the one hand, the lack of Ti-containing precipitates would result in a lack of inhibition of recrystallization, thereby coarsening the grains and reducing mechanical properties. On the other hand, the lack of nanoscale precipitates would lead to a lack of TWIP effect generated by local low stacking fault regions to release stress during the deformation of high-manganese steel, resulting in a decrease in impact energy at -269°C.

[0104] Compared with Example 1, Comparative Example 2 increased the final rolling temperature. Because high-manganese steel undergoes high-temperature recovery and recrystallization at this temperature, the dislocation density decreases, further affecting the formation of Ti-containing nanoscale precipitates. This leads to a decrease in room temperature yield strength and a reduction in impact energy at -269°C, while the crack initiation absorption energy / crack propagation energy ratio is greater than 1.

[0105] Compared with Example 5, Comparative Example 3 increased the final rolling temperature. Since the solid solution temperature of 0.12V element is low, the precipitation temperature of V(C,N) precipitates is about 1000°C. The high final rolling temperature cannot promote the nucleation and precipitation of nanoprecipitates by increasing dislocations, but instead coarsens the grains, thus reducing the yield strength.

[0106] Compared with Example 6, Comparative Example 4 reduced the final rolling temperature. Since the solid solution temperature of 0.22V element is relatively high, the precipitation temperature of V(C,N) precipitates is about 1100°C. The low final rolling temperature promotes the nucleation and growth of precipitates by increasing the number of dislocations, resulting in a decrease in total impact absorption energy. At the same time, the crack initiation absorption energy / crack propagation energy is greater than 1.

[0107] This invention provides a high-manganese steel with excellent yield strength and extremely low-temperature impact toughness. Its compositional design involves alloying to control the stacking fault energy of the high-manganese steel, utilizing precipitation strengthening to increase yield strength, and employing nanoscale precipitation to reduce local stacking fault energy, inducing the TWIP effect to enhance crack propagation absorption energy during extremely low-temperature impact, thus achieving excellent extremely low-temperature impact toughness. A combined diffusion annealing-high-temperature forging-controlled hot rolling process is used to obtain a stable and uniform austenitic microstructure, thereby improving the room-temperature yield strength and extremely low-temperature impact toughness of the high-manganese steel.

[0108] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A high-manganese steel with ultra-low temperature impact toughness, characterized in that, The high-manganese steel comprises, by mass percentage, the following elements: C 0.21-0.61%, Mn 22.5-28.9%, Si 0.15-0.2%, N≤0.01%-0.03%, Cr 0-6.0%, S≤0.005%, P≤0.01% including Ti≤0.03%, V 0.03-0.2%, Nb 0-0.3%, with the balance being Fe and unavoidable impurities. The high-manganese steel has nanoscale precipitates with local stacking fault energy gradients.

2. The high-manganese steel with ultra-low temperature impact toughness according to claim 1, characterized in that, The microstructure of the high-manganese steel includes austenitic phase and nanoscale precipitates.

3. The high-manganese steel with ultra-low temperature impact toughness according to claim 2, characterized in that, The nanoscale precipitates have a size of 10~30nm and are diffusely distributed in an ellipsoidal shape.

4. The high-manganese steel with ultra-low temperature impact toughness according to claim 3, characterized in that, The local stacking fault energy reduction caused by nanoscale precipitates in the high-manganese steel at an extremely low temperature of -269℃ ranges from 0.5 to 3 mJ / m. 2 .

5. The method for manufacturing high-manganese steel with excellent yield strength and extremely low-temperature impact toughness according to any one of claims 1 to 4, characterized in that, Includes the following steps: S1. Smelting process: Select pure metals according to the mass percentage of high manganese steel composition, perform vacuum induction melting, and cast into billets. S2. Forging process: The billet is homogenized and then forged after being taken out of the furnace; S3. Hot rolling process: The forging billet is reheated and then rolled in two stages after exiting the furnace. The first stage of rolling is rough rolling in the recrystallization temperature range, and the second stage of rolling is finish rolling at 100~200℃ above the final rolling temperature.

6. The method for manufacturing high-manganese steel with excellent yield strength and extremely low-temperature impact toughness according to claim 5, characterized in that, In step S1, the content of elements affected by inclusions is controlled as follows: P, O≤0.01%, S≤0.005%, N≤0.01%, H≤0.0002.

7. The method for manufacturing high-manganese steel with excellent yield strength and extremely low-temperature impact toughness according to claim 5, characterized in that, In step S2, the homogenization treatment temperature is 980~1100℃ and the time is 1-2h; then it is taken out of the furnace for forging, and after each forging, it is put back into the furnace for 20min to keep the temperature controlled between 950~1020℃.

8. The method for manufacturing high-manganese steel with excellent yield strength and extremely low-temperature impact toughness according to claim 5, characterized in that, In step S3, the slab heating temperature is controlled at 1150~1230℃, the rough rolling temperature is controlled between 1120~980℃, the finishing rolling temperature is controlled 100~200℃ above the final rolling temperature, the final rolling temperature is controlled at 740~860℃, the cumulative reduction rate is 76%, and the slab is water-cooled to room temperature after rolling.

9. The use of the high-manganese steel according to any one of claims 1 to 4 or the high-manganese steel prepared by the method according to any one of claims 5 to 8 as a liquid hydrogen storage tank.

10. The application according to claim 9, characterized in that, The high-manganese steel used as the steel plate has a yield strength ≥400MPa and an impact toughness KV2 ≥100J at -269℃, wherein the crack initiation absorption energy / crack propagation energy <1.

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